KR20240130745A - Titanium material - Google Patents

Abstract

This titanium material has an average nitrogen concentration and an average carbon concentration of 14.0 at% or less, and an average hydrogen concentration of 30.0 at% or less, in a range from the surface to a position where the oxygen concentration in the thickness direction from the surface is 1/3 of the maximum value as measured by glow discharge spectrometry, and a difference between the c-axis lattice constant of α-phase Ti obtained by X-ray diffraction measurement by a parallel beam method at an incident angle of 0.3 degrees at the surface and the c-axis lattice constant of α-phase Ti obtained by X-ray diffraction measurement by a focused method at the center of the plate thickness is 0.015 Å or less.

Description

Titanium material

The present invention relates to titanium material.

Titanium materials are used for construction purposes such as roofs and exterior walls of buildings because they exhibit excellent corrosion resistance in atmospheric environments. However, titanium materials used over a long period of time may discolor. Discoloration of titanium materials may be problematic from the viewpoint of aesthetics. Therefore, titanium materials with excellent discoloration resistance and suppressed discoloration have been proposed.

For example, Patent Document 1 discloses a titanium material or titanium alloy material having excellent discoloration resistance, characterized in that an oxide film having a thickness of 100 Å or less exists on the surface of a substrate, the amount of C in the surface oxide film is 30 at% or less, and the amount of C in the surface layer of the substrate under the oxide film is 30 at% or less.

Patent Document 2 discloses titanium that is unlikely to discolor in an atmospheric environment, characterized in that it has an average carbon concentration of 14 at% or less in a range of depths of 100 nm from the surface and an oxide film with a thickness of 12 to 40 nm on the outermost surface.

Patent Document 3 discloses a titanium material that is unlikely to discolor, characterized in that the amount of fluorine in the oxide film on the surface is 7 at% or less.

Patent Document 4 discloses a method for manufacturing a titanium plate, characterized in that, in order to efficiently manufacture a titanium or titanium alloy plate with little discoloration under an environment irradiated with light, a lubricant is used to make the carbon content of a carbon-concentrated layer on the surface of the titanium plate after cold rolling to be 150 mg/ ^m2 or less, and then the titanium plate is annealed in an oxidizing atmosphere, and then descaled by molten salt immersion treatment and pickling with a nitric acid aqueous solution.

Patent Document 5 discloses a titanium or titanium alloy that is unlikely to discolor in an atmospheric environment, characterized in that the titanium or titanium alloy has an average carbon concentration of 14 at% or less in a range of 100 nm in depth from the surface, an oxide film having a thickness of 12 nm or more and 30 nm or less on the surface, and an arithmetic mean height (Ra) of the titanium surface of 0.035 ㎛ or less.

Japanese Patent Publication No. 2000-1729 Japanese Patent Publication No. 2002-12962 Japanese Patent Publication No. 2002-47589 Japanese Patent Publication No. 2002-60984 Japanese Patent Publication No. 2005-272870

Miyashita Tsutomu, “Surface Roughness I Want to Review Again,” Journal of the Precision Engineering Society, Public Interest Incorporated Association Precision Engineering Society, Vol.73, No.2, 2007, p.201－205

In the technologies described in Patent Documents 1 to 5, it is said that the deterioration of the discoloration resistance of the titanium material is suppressed by lowering the carbon concentration on the surface. In addition, the evaluation of the discoloration resistance of the conventional titanium material is evaluated by the color difference after immersion in sulfuric acid of pH 3 at a temperature of 60°C for 14 days, as described in Patent Document 5, for example. However, in recent years, titanium materials having better discoloration resistance than the conventional titanium materials have been demanded.

The present invention has been made in consideration of the above problems, and has as its object the provision of a titanium material having excellent discoloration resistance that can suppress discoloration over a long period of time compared to conventional titanium materials. The present invention has as its object the provision of a titanium material that does not discolor even when immersed for a longer period of time than the evaluations described in, for example, Patent Documents 1 to 5.

The gist of the present invention, which was completed based on the above knowledge, is as follows.

[1] The titanium material according to one embodiment of the present invention has an average nitrogen concentration and an average carbon concentration of 14.0 atomic% or less and an average hydrogen concentration of 30.0 atomic% or less in a range from the surface to a position where the oxygen concentration measured in the thickness direction from the surface is 1/3 of the maximum value by glow discharge spectroscopy, respectively.

The difference between the c-axis lattice constant of the α-phase Ti obtained by X-ray diffraction measurement using the parallel beam method at an incident angle of 0.3 degrees on the surface and the c-axis lattice constant of the α-phase Ti obtained by X-ray diffraction measurement using the focusing method at the center of the plate thickness is 0.015 Å or less.

[2] The titanium material described in [1] above may have an oxide film having a thickness of 30.0 nm or less.

[3] The titanium material described in [1] or [2] above has a titanium substrate and an oxide film arranged on the surface of the titanium substrate, wherein the maximum concentration of nitride-derived nitrogen in the oxide film, when analyzed by X-ray photoelectron spectroscopy, is 2.0 to 10.0 at%, and the position where the nitride-derived nitrogen concentration in the oxide film shows a maximum value exists in a range of 2 to 10 nm from the surface of the oxide film, when converted to a sputtering rate of SiO ₂ , and the concentration of nitride-derived nitrogen existing in a range of 20 nm from a position where the oxygen concentration is 1/2 of the maximum value to the titanium substrate side is less than the maximum value of the nitride-derived nitrogen concentration in the oxide film and is 7 at% or less, and the maximum value of the nitride-derived nitrogen concentration in the oxide film is less than the maximum value of the nitride-derived nitrogen concentration in the oxide film, and the position where the nitride-derived nitrogen concentration in the oxide film is maximum It may be greater than the carbon concentration derived from carbides.

[4] The titanium material described in [1] or [2] above has a titanium substrate having a ratio of the arithmetic mean roughness Ra to the element length RSm, Ra/RSm, in a roughness curve in a direction where the arithmetic mean roughness Ra is maximum, of 0.006 to 0.015, and further having a root mean square inclination RΔq of 0.150 to 0.280, and the titanium substrate may have a kurtosis Rku of more than 3 and a skewness Rsk of more than -0.5.

[5] The titanium material described in the above [3] may have a ratio of the arithmetic mean roughness Ra to the element length RSm, Ra/RSm, in a roughness curve in a direction in which the arithmetic mean roughness Ra of the titanium substrate is maximum, of 0.006 to 0.015, and further, a root mean square inclination RΔq of 0.150 to 0.280, a kurtosis Rku of the titanium substrate exceeding 3, and a skewness Rsk of the titanium substrate exceeding -0.5.

According to the present invention, it is possible to provide a titanium material having excellent discoloration resistance that can suppress discoloration over a long period of time compared to conventional titanium materials.

FIG. 1 is a schematic enlarged cross-sectional view showing the layer configuration of a titanium material according to one embodiment of the present invention.
Figure 2 is a drawing showing an example of a change in the depth direction of a spectrum obtained by X-ray photoelectron spectroscopy of a titanium material according to the same embodiment.
Figure 3 is a drawing showing an example of the change in the depth direction of the spectrum by X-ray photoelectron spectroscopy of a typical titanium material.
FIG. 4 is a drawing showing an example of a depth-direction element concentration distribution of a titanium material according to the same embodiment and a general titanium material by X-ray photoelectron spectroscopy.
Figure 5 is a diagram showing the relationship between the maximum concentration of nitrogen derived from titanium nitride within the oxide film and the color difference ΔE*ab before and after the discoloration test.
Figure 6 is a diagram showing the relationship between the ratio of the arithmetic mean roughness Ra and the average length RSm of the contour curve elements, Ra/RSm, and the root mean square slope RΔq of the roughness curve elements and the discoloration resistance.
Figure 7 is a schematic diagram explaining the kurtosis Rku.

Hereinafter, a titanium material according to one embodiment of the present invention will be described with reference to the drawings. In addition, the dimensions and ratios of each component in the drawings do not represent the actual dimensions and ratios of each component.

In addition, the numerical ranges described below, which are separated by "within", include the lower and upper limits. The values indicated as "less than" and "greater than" are not included in the numerical range.

First, the new knowledge obtained through the review of the inventors who completed the present invention will be described in detail.

Titanium material is composed of an oxide film formed on the surface of a titanium substrate, and it is thought that discoloration of the titanium material is caused by an increase in the thickness of the oxide film due to acid rain, etc. Since the carbon concentration near the surface of the titanium material affects the increase in the thickness of the oxide film, in conventional titanium materials aimed at improving discoloration resistance, the carbon concentration on the surface is limited. The present inventors conducted studies to obtain a titanium material having excellent discoloration resistance and in which discoloration is suppressed for a longer period of time than conventional titanium materials.

First, the inventors of the present invention measured the oxygen concentration, carbon concentration, and nitrogen concentration in the thickness direction from the surface of the titanium material (in other words, the surface of the oxide film) by glow discharge spectroscopy (hereinafter referred to as "GDS"). It was found that the titanium material has an oxide film, and that the position where the oxygen concentration measured by GDS is 1/3 of the maximum value is located at a portion on the titanium substrate side near the interface between the oxide film and the titanium substrate. Hereinafter, the range from the surface of the titanium material to the position where the oxygen concentration measured by GDS in the thickness direction is 1/3 of the maximum value is referred to as the surface layer of the titanium material.

Next, the inventors of the present invention immersed titanium materials in a sulfuric acid solution at pH 3 and 60°C for 4 weeks, and evaluated discoloration resistance based on the color difference before and after immersion. When comparing titanium materials that were obviously discolored before and after immersion with titanium materials that were hardly discolored, it was found that there was a difference in the carbon concentration and nitrogen concentration measured by GDS in the titanium material before immersion. Specifically, in the case of titanium materials that were obviously discolored when immersed for 4 weeks in a sulfuric acid solution at pH 3 and 60°C, it was found that nitrogen and carbon were present in the interior of the oxide film and near the interface on the substrate side from the interface between the oxide film and the substrate in the titanium material before immersion. Previously, it was not thought that nitrogen present in the oxide film and near the interface on the substrate side from the interface between the oxide film and the substrate affected the discoloration of the titanium material. However, when titanium material is exposed to an acid rain environment for a long period of time, it is presumed that nitrogen present in and near the oxide film, like carbon, serves as a starting point for the growth of the oxide film.

In addition, as a result of the examination by the present inventors, it was found that the nitrogen concentration in and around the oxide film, like the carbon concentration, affects the discoloration resistance of the titanium material, and that the discoloration resistance is improved by limiting the nitrogen concentration. In addition, it was found that when the average nitrogen concentration and the average carbon concentration are each 14.0 at% or less in the range from the surface of the titanium material to the position where the oxygen concentration measured in the thickness direction by GDS is 1/3 of the maximum value, the discoloration resistance of the titanium material is improved compared to the prior art. The average carbon concentration in the surface layer of the titanium material can be reduced by increasing the annealing temperature or increasing the annealing time. The average nitrogen concentration can be reduced by increasing the degree of vacuum in the heat treatment.

Next, the inventors of the present invention focused on the hydrogen concentration of the surface layer of the titanium material and examined the influence of the hydrogen concentration of the surface layer of the titanium material on the discoloration resistance of the titanium material. As a result, it was found that when the average hydrogen concentration of the surface layer of the titanium material is 30.0 at% or less, the discoloration resistance is further improved. Titanium hydride is thermodynamically unstable compared to titanium oxide in an acid rain environment of the atmosphere. If the formation of titanium oxide is promoted by an increase in the hydrogen concentration in the titanium material, the discoloration resistance may be reduced. However, it is thought that when the average hydrogen concentration of the surface layer of the titanium material is 30.0 at% or less, titanium hydride does not change into titanium oxide, and the reduction in discoloration resistance is suppressed.

Next, the inventors of the present invention focused on the change in the crystal structure of Ti on the surface of titanium materials. The inventors of the present invention found that the change in the c-axis of Ti in the α-phase, which is the most closely packed hexagonal crystal, affects the discoloration resistance of titanium materials.

By examination by the present inventors, it was found that if the difference between the c-axis lattice constant of α-phase Ti obtained by X-ray diffraction measurement by the parallel beam method at an incident angle of 0.3 degrees on the surface of a titanium material and the c-axis lattice constant of α-phase Ti obtained by X-ray diffraction measurement by the focused method at the center of the plate thickness (also referred to as the center of thickness) is 0.015 Å or less, the discoloration resistance can be significantly improved. Although the penetration level of X-rays in the above-mentioned X-ray diffraction measurement differs depending on the X-ray diffraction energy, the c-axis lattice constant of each α-phase Ti can be measured by performing the X-ray diffraction measurement on the surface of the titanium material and at the center of the plate thickness of the titanium material. In the present invention, the increase in the c-axis lattice constant of α-phase Ti in the surface layer is defined as follows. That is, the difference between the lattice constant of the c-axis of the α-phase Ti obtained by X-ray diffraction measurement by the parallel beam method at an incident angle of 0.3 degrees on the surface of the titanium material and the lattice constant of the c-axis of the α-phase Ti obtained by X-ray diffraction measurement by the focused method at the center of the plate thickness is referred to as the increase in the lattice constant of the c-axis of the α-phase Ti in the surface portion. The measurement depth in the X-ray diffraction measurement by the parallel beam method at an incident angle of 0.3 degrees does not strictly match the range in the thickness direction of the surface portion measured by GDS, but it can roughly measure the increase in the lattice constant of the c-axis of the α-phase Ti in the surface portion.

It is thought that oxygen is related to the increase in the c-axis lattice constant of the α-phase Ti in the surface layer of the titanium material. If oxygen is dissolved in the α-phase Ti in the surface layer of the titanium material, the lattice constant of its c-axis increases. If the c-axis lattice constant of the α-phase Ti existing on the surface of the titanium material is larger than the c-axis lattice constant of the α-phase Ti existing in the center of the thickness, it is presumed that titanium oxide with a high defect concentration is generated by the action of acid rain, the oxide film easily grows, and the discoloration resistance deteriorates. The crystal structure of the α-phase Ti in the surface layer of the titanium material is affected by the temperature, time, and vacuum degree of the heat treatment. By increasing the vacuum degree in the heat treatment atmosphere, the amount of oxygen dissolved in the α-phase Ti in the surface layer of the titanium material decreases, and the increase in the c-axis lattice constant of the α-phase Ti in the surface layer of the titanium material is suppressed. Up to this point, the new knowledge obtained through the investigation of the present inventors has been explained.

Next, a titanium material according to an embodiment of the present invention will be described with reference to FIG. 1. FIG. 1 is a schematic enlarged cross-sectional view showing the layered configuration of the titanium material according to the embodiment. The titanium material according to the embodiment has an average nitrogen concentration and an average carbon concentration of 14.0 at% or less, an average hydrogen concentration of 30.0 at% or less, in a range from the surface to a position where the oxygen concentration in the thickness direction from the surface is 1/3 of the maximum value as measured by glow discharge spectrometry, and a difference between the c-axis lattice constant of α-phase Ti obtained by X-ray diffraction measurement by a parallel beam method at an incident angle of 0.3 degrees at the surface and the c-axis lattice constant of α-phase Ti obtained by X-ray diffraction measurement by a focused method at the center of the plate thickness is 0.015 Å or less. The titanium material according to the embodiment will be described in detail below.

(Titanium material (1))

The titanium material (1) according to the present embodiment is a titanium material having an oxide film (20) formed on the surface of a titanium substrate (10), as illustrated in FIG. 1. In other words, the titanium material (1) has a titanium substrate (10) and an oxide film (20) formed on the surface of the titanium substrate (10). The surface layer (30) is a region from the surface of the titanium material (1) (in other words, the surface of the oxide film (20)) to a position where the oxygen concentration measured by GDS is 1/3 of the maximum value in the thickness direction, and includes a part of the titanium substrate (10).

(Titanium substrate (10))

The titanium substrate (10) of the titanium material (1) is pure titanium, industrial pure titanium, or a titanium alloy. The titanium substrate (10) is, for example, pure titanium, industrial pure titanium, or a titanium alloy having a Ti content of 70 mass% or more. Hereinafter, these may be collectively referred to as “titanium.” The crystal structure of pure titanium is an α phase of a closest hexagonal crystal, and does not include a β phase of a body-centered cubic structure. Industrial pure titanium mainly consists of an α phase, and may also include a β phase depending on the chemical composition, etc. The titanium alloy may be an α-type alloy of only an α phase, or an α+β-type alloy including a β phase of a body-centered cubic structure. In addition, the titanium substrate (10) may be, for example, industrial titanium. Industrial titanium used in the titanium substrate (10) may include, for example, various types of industrial titanium plates and rods described in JIS H 4600:2012, and various types of industrial titanium rods described in JIS H 4650:2016. When processability is required, industrial pure titanium of JIS Type 1 (for example, JIS H 4600:2012) with reduced impurities is suitable. In addition, when strength is required, industrial pure titanium of JIS Types 2 to 4 can be applied to the titanium substrate (10). As titanium alloys, for example, JIS Types 11 to 23 containing trace amounts of precious metal elements such as palladium, platinum, and ruthenium to improve corrosion resistance, or JIS Type 60 containing relatively many elements such as Ti-6Al-4V alloys, 60E, 61, and 61F, may be mentioned. In addition, industrial pure titanium or its equivalent, as defined by JIS Type 1 or its equivalent ASTM Gr. 1, is mainly used in buildings.

Examples of titanium alloys mainly having the α phase include high-corrosion-resistant alloys (titanium alloys specified in JIS standards Types 11 to 13, 17, 19 to 22, and ASTM standards Grades 7, 11, 13, 14, 17, 30, and 31, and titanium alloys containing small amounts of various elements (such as Ti－Ru－Mm)), Ti－0.5Cu, Ti－1.0Cu, Ti－1.0Cu－0.5Nb, Ti－1.0Cu－1.0Sn－0.35Si－0.25Nb, etc. Mm represents misch metal.

Examples of α＋β type titanium alloys include Ti－3Al－2.5V, Ti－5Al－1Fe, and Ti－6Al－4V.

When the titanium substrate (10) contains aluminum, such as in the case of a Ti-6Al-4V alloy, corrosion resistance may deteriorate and discoloration resistance may be adversely affected. Therefore, when forming an oxide film (20) on the surface of a titanium alloy as a titanium substrate (10), it is recommended to investigate the influence of alloy elements on the intended use in advance and to appropriately adjust the composition and thickness of each layer according to the titanium substrate (10).

Titanium substrate (10) is, for example, in mass%,

Co: 0% or more and 1.0% or less,

Cr: 0% or more and 0.5% or less,

Ni: 0% or more and 1.00% or less,

Ta: 0% or more and 6.00% or less,

Al: 0% or more and 7.0% or less,

V: 0% or more and 5.0% or less,

S: 0% or more and 0.3% or less,

Cu: 0% or more and 1.50% or less,

Nb: 0% or more and 0.70% or less,

Sn: 0% or more and 1.40% or less,

Si: 0% or more and 0.55% or less,

Mo: 0% or more and 0.5% or less,

W: 0% or more and 0.5% or less,

Pd: 0% or more 0.25%,

Ru: 0% or more and 0.15% or less,

Rh: 0% or more and 0.15% or less,

Os: 0% or more and 0.15% or less,

Ir: 0% or more and 0.15% or less,

Pt: 0% or more and 0.15% or less,

REM: 0% or more and 0.10% or less,

C: 0% or more and 0.18% or less,

H: 0% or more and 0.015% or less,

O: 0% or more and 0.40% or less,

N: 0% or more and 0.05% or less, and

Fe: Contains 0% or more and 2.50% or less,

Industrial pure titanium or titanium alloy, the remainder consisting of Ti and impurities.

Here, REM is a rare earth element, and specifically, it is at least one element selected from the group consisting of Sc, Y, light rare earth elements (La, Ce, Pr, Nd, Pm, Sm, Eu), and heavy rare earth elements (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu).

In addition, the titanium substrate (10) is, for example, in mass%,

C: 0% or more and 0.10% or less,

H: 0% or more and 0.015% or less,

O: 0% or more and 0.40% or less,

N: 0% or more and 0.05% or less and

Fe: Contains 0% or more and 0.50% or less,

It is industrial pure titanium, the remainder being Ti and impurities.

In addition, the titanium substrate (10) is, for example, in mass%,

Co: 0% or more and 0.80% or less,

Pd: 0% or more and 0.25% or less,

Cr: 0% or more and 0.2% or less,

Ru: 0% or more and 0.06% or less,

Ni: 0% or more and 0.60% or less,

Ta: 0% or more and 6.0% or less,

N: 0% or more and 0.05% or less,

C: 0% or more and 0.08% or less,

H: 0% or more and 0.015% or less,

O: 0% or more and 0.35% or less and

Fe: Contains 0% or more and 0.30% or less,

It is a titanium alloy, the remainder being Ti and impurities.

In addition, the titanium substrate (10) is, for example, in mass%,

Al: 2.0% or more and 7.0% or less,

V: 1.0% or more and 5.0% or less,

S: 0% or more and 0.3% or less,

REM: 0% or more and 0.08% or less,

N: 0% or more and 0.05% or less,

C: 0% or more and 0.10% or less,

H: 0% or more and 0.015% or less,

O: 0% or more and 0.35% or less and

Fe: Contains 0% or more and 2.5% or less,

It is a titanium alloy, the remainder being Ti and impurities.

In addition, the titanium substrate (10) is, for example, in mass%,

Cu: 0.3% or more and 1.50% or less,

Nb: 0% or more and 0.70% or less,

Sn: 0% or more and 1.40% or less,

Si: 0% or more and 0.55% or less,

N: 0% or more and 0.05% or less,

C: 0% or more and 0.10% or less,

H: 0% or more and 0.015% or less,

O: 0% or more and 0.15% or less and

Fe: Contains 0% or more and 0.10% or less,

It is a titanium alloy, the remainder being Ti and impurities.

In addition, the titanium substrate (10) is, for example, in mass%,

V: 0% or more and 0.5% or less,

Ni: 0% or more and 1.00% or less,

Cr: 0% or more and 0.5% or less,

Co: 0% or more and 1.0% or less,

Mo: 0% or more and 0.5% or less,

W: 0% or more and 0.5% or less,

Pd: 0% or more and 0.15% or less,

Ru: 0% or more and 0.15% or less,

Rh: 0% or more and 0.15% or less,

Os: 0% or more and 0.15% or less,

Ir: 0% or more and 0.15% or less,

Pt: 0% or more and 0.15% or less,

REM: 0.001% or more and 0.10% or less,

N: 0% or more and 0.03% or less,

C: 0% or more and 0.18% or less,

H: 0% or more and 0.015% or less,

O: 0% or more and 0.35% or less,

Fe: 0% or more and 0.30% or less, and

The sum of Pd, Ru, Rh, Os, Ir and Pt: 0.01% or more and 0.15% or less,

It is a titanium alloy, the remainder being Ti and impurities.

Impurities are components that exist in titanium regardless of the intention of addition, and do not need to originally exist in the titanium material obtained. The term "impurity" is a concept that includes impurities mixed in from raw materials or the manufacturing environment when manufacturing titanium industrially. Examples of impurities include Cl, Na, Mg, Ca, and B. The content of each element of the impurities is preferably 0.1 mass% or less, and the total amount is preferably 0.4 mass% or less.

The titanium substrate (10) is usually a plate, a tube, a bar (straight line), or a shape in which these are appropriately processed. The titanium substrate (10) may have any shape, for example, a sphere or a rectangular parallelepiped.

(Oxide film (20))

An oxide film (20) is formed on the surface of the titanium substrate (10). The thickness of the oxide film (20) is not particularly limited, but if it exceeds 30.0 nm, the color development of the titanium material (1) may be affected by the interference effect of light. Therefore, the thickness of the oxide film (20) is preferably 30.0 nm or less. From the viewpoint of suppressing color development by the interference effect of light, the thickness of the oxide film (20) is more preferably 25.0 nm or less, and even more preferably 20.0 nm or less. The thickness of the oxide film (20) is more than 0 nm, and may be, for example, 10.0 nm or more. In addition, the discoloration resistance of the titanium material is improved by securing the thickness of the oxide film (20). Therefore, from the viewpoint of improving the discoloration resistance, the thickness of the oxide film (20) on the surface of the titanium material (1) is more preferably 12.0 nm or more.

The thickness of the oxide film (20) is measured by GDS. The measurement by GDS is performed by the following method. The measurement by GDS is performed using JOBIN YVON GD-Profiler2 manufactured by Horiba Seisakusho Co., Ltd., in a constant power mode of 35 W, the pressure of the argon gas is 600 Pa, and the discharge range is 4 mm in diameter. The measurement pitch in the measurement by GDS is 0.5 nm. In the measurement by GDS, analysis of O (oxygen), N (nitrogen), C (carbon), H (hydrogen), and Ti is performed from the surface of the titanium material (1). The concentration (atomic%) of each of the above elements is calculated by assuming that the total of the above elements is 100 atomic%. The thickness of the oxide film (20) is obtained from the oxygen concentration measured by GDS. Specifically, the distance in the thickness direction from the surface to the position where the oxygen concentration is reduced by half from the maximum value is the thickness of the oxide film (20). The average nitrogen concentration, average carbon concentration, and average hydrogen concentration are numerical arithmetic averages of the nitrogen concentration, carbon concentration, and hydrogen concentration at each measurement point.

The nitrogen concentration derived from nitride near the interface with the oxide film (20) in the titanium substrate (10) is preferably less than the maximum nitrogen concentration in the oxide film (20) measured by XPS and 7 at% or less. By the method for manufacturing the titanium material (1) according to the present embodiment described later, nitride is formed in the oxide film (20), but according to the method, the nitride formed near the interface between the titanium substrate (10) and the oxide film (20) does not exist or is extremely small. The nitrogen content of the base material, which is the titanium substrate (10), is about 0.05 to 0.07 mass%, and at most about 0.20 at%, and is an impurity level, in the value obtained by chemical analysis. Since this content is below the solid solution limit of nitrogen in titanium, nitride is not formed. Therefore, when nitride exists near the interface between the titanium substrate (10) and the oxide film (20), the nitride is formed by nitrogen diffusing from the surface to the inside during the annealing treatment in an example of the method for manufacturing the titanium material (1) according to the present embodiment. Therefore, it is preferable that the nitrogen concentration derived from the nitride near the interface with the oxide film (20) in the titanium substrate (10) is less than the maximum value of the nitrogen concentration in the oxide film (20) measured by XPS and 7 at% or less. When the nitrogen concentration derived from the nitride near the interface with the oxide film (20) in the titanium substrate (10) is less than the maximum value of the nitrogen concentration in the oxide film (20) measured by XPS and 7.0 at% or less, the growth of the oxide film is suppressed when the titanium material (1) is exposed to an acid rain environment for a long period of time, and discoloration is suppressed. The nitrogen concentration derived from nitride near the interface with the oxide film (20) in the titanium substrate (10) is more preferably less than the maximum value of the nitrogen concentration in the oxide film (20) and 3.0 at% or less. On the other hand, the lower limit of the nitrogen concentration derived from nitride near the interface with the oxide film (20) in the titanium substrate (10) is not limited. Therefore, according to an example of the method for manufacturing a titanium material (1) according to the present embodiment, the lower limit of the nitrogen concentration derived from nitride near the interface with the oxide film (20) in the titanium substrate (10) is 0 at%, but when the concentration due to peak separation of XPS (which is not zero) is also taken into account, it may be about 0.5 at%.

In addition, the vicinity of the interface with the oxide film (20) on the titanium substrate (10) refers to a range of 20 nm from the interface measured by XPS toward the titanium substrate side. In the drawing measured by XPS described below (for example, FIG. 4), the position where the oxygen concentration measured by XPS becomes 1/2 of the maximum is set as the interface. Therefore, the range of 20 nm from the position where the oxygen concentration measured by XPS becomes 1/2 of the maximum to the titanium substrate side is set as the vicinity of the interface with the oxide film (21) on the titanium substrate (10). The vicinity of the interface with the oxide film (20) on the titanium substrate (10) is a different region from the surface layer (30).

The oxide film (20) can be identified by GDS or XPS, but the thickness of the oxide film obtained by each measurement method is often not strictly consistent because the measurement methods are different. However, the definition of the oxide film is consistent in that the position where the oxygen concentration becomes 1/2 of the maximum value is considered as the oxide film in each measurement method. In this application, when measuring the nitrogen concentration derived from nitride near the interface with the oxide film (20) on the titanium substrate (10), the oxide film is measured by XPS.

The oxide film (20) preferably contains nitrogen derived from nitride. Nitrogen derived from nitride in the oxide film (20) is measured by X-ray photoelectron spectroscopy (XPS). Fig. 2 is a drawing showing an example of a change in the depth direction of a spectrum by X-ray photoelectron spectroscopy of a titanium material (1) according to the present embodiment. Fig. 3 is a drawing showing an example of a change in the depth direction of a spectrum by X-ray photoelectron spectroscopy of a general titanium material. Figs. 2(A) and 3(A) show a change in the depth direction of an N1s spectrum, Figs. 2(B) and 3(B) show a change in the depth direction of a C1s spectrum, Figs. 2(C) and 3(C) show a change in the depth direction of an O1s spectrum, and Figs. 2(D) and 3(D) show a change in the depth direction of a Ti2p spectrum. As shown in (C) of Fig. 2, in the titanium material (1) according to the present embodiment, there are cases where a clear peak derived from nitride is confirmed at a depth corresponding to the oxide film (20). On the other hand, as shown in (C) of Fig. 3, in general titanium materials, the peak derived from nitride is extremely small. Thus, it is preferable that the titanium material (1) according to the present embodiment contains a predetermined amount of nitrogen derived from nitride in the oxide film (20). The content of nitrogen derived from nitride in the oxide film (20) will be described in detail below. In addition, in the titanium material (1) according to the present embodiment, as the peak intensity of the nitride of Fig. 2 (C) increases, the intensity of the peak involving TiN of Fig. 2 (A) also increases, so it is thought that the nitride of Fig. 2 (C) is derived from the nitride of titanium.

The nitrogen content derived from nitride in the oxide film (20) (maximum nitrogen concentration) is preferably 2.0 to 10.0 at%. The nitrogen content derived from nitride in the oxide film (20) refers to the maximum nitrogen concentration derived from nitride in the oxide film (20) as measured by XPS. In a non-colorizing material, if the oxide film (20) contains 2.0 at% or more of nitrogen derived from nitride, the discoloration resistance is further improved. The reason for this is not necessarily clear, but it is thought that when nitride is present in the oxide film (20), the atomic arrangement is disturbed, the strain distribution in the oxide film (20) changes, or the conductivity changes, causing a change in the potential distribution at the nanometer level, thereby changing the function (shielding function) of the oxide film (20) for shielding the penetration of ions.

When the nitrogen content derived from nitride in the oxide film (20) is 2.0 at% or more, the shielding function is improved more reliably, and the effect of improving discoloration resistance can be obtained more reliably. The nitrogen content derived from nitride in the oxide film (20) is more preferably 4.0 at% or more, considering stability in manufacturing. On the other hand, when the nitrogen content derived from nitride in the oxide film (20) exceeds 10.0 at%, the shielding function is reduced, and in some cases the effect of improving discoloration resistance cannot be obtained, but when the nitrogen content derived from nitride in the oxide film (20) is 10.0 at% or less, the shielding function is maintained, and a higher effect of improving discoloration resistance is obtained. The nitrogen content derived from nitride in the oxide film (20) is more preferably 8.0 at% or less, considering stability in manufacturing.

In addition, the inventors of the present invention have contemplated improving the discoloration resistance by controlling the nitrogen distribution derived from nitride in the oxide film (20). Fig. 4 is a diagram showing an example of the element concentration distribution in the depth direction by X-ray photoelectron spectroscopy of a titanium material (1) according to the present embodiment (an example of the present invention) and a general titanium material (a conventional example). The sputtering depth of the horizontal axis in Fig. 4 is the depth converted into the sputtering rate of SiO ₂ . The example of the present invention of Fig. 4 is the element concentration distribution for the titanium material according to the present embodiment shown in Fig. 2. The conventional example of Fig. 4 is the element concentration distribution for the general titanium material (industrial pure titanium type 1) shown in Fig. 3.

As shown in Fig. 4, in the titanium material (1) according to the present embodiment, the nitrogen concentration derived from nitride is maximum at a position where the sputtering depth is 2 to 10 nm when converted to the sputtering rate of SiO ₂ . On the other hand, in general titanium materials, the nitrogen concentration is extremely small. Therefore, the depth at which the nitrogen concentration derived from nitride in the oxide film (20) is maximum is preferably 2 to 10 nm when converted to the sputtering rate of SiO ₂ . In addition, the upper limit of the sputtering depth is preferably 30 nm or more including the range near the interface with the oxide film (20) described above. In addition, it may be changed depending on the thickness of the oxide film (20), and is preferably about three times or more the thickness of the oxide film (20).

In addition, the inventors of the present invention investigated the effect of the nitrogen concentration at the depth where the nitrogen concentration derived from nitride in the oxide film (20) becomes maximum on the discoloration resistance. Fig. 5 is a diagram showing the relationship between the maximum value of the nitrogen concentration derived from nitride in the oxide film (20) and the color difference ΔE*ab before and after the discoloration test.

The color difference ΔE*ab was obtained by the following method. The titanium material surface was immersed in a pH 3 sulfuric acid solution at 60°C for 4 weeks, and L*a*b* was measured before and after immersion. From the differences ΔL*, Δa*, and Δb* of the brightness L* and the chromaticity a*, b* before and after immersion, respectively, obtained in accordance with JIS Z 8730:2009,

ΔE*ab=[(ΔL*) ² ＋(Δa*) ² ＋(Δb*) ² ] ^1/2

It was calculated according to the following. The smaller the color difference ΔE*ab, the smaller the degree of discoloration before and after the test, indicating excellent discoloration resistance.

As shown in Fig. 5, when the nitrogen concentration at the depth where the nitrogen concentration derived from nitride in the oxide film (20) becomes maximum is less than 2.0 at%, the shielding property is not sufficiently high, and the color difference sometimes exceeds 8. On the other hand, when the nitrogen concentration derived from nitride exceeds 10.0 at%, the shielding property deteriorates, and the color difference sometimes exceeds 8. In addition, when the nitrogen concentration at the depth where the nitrogen concentration derived from nitride in the oxide film (20) becomes maximum exceeds 10.0 at%, the color tone sometimes becomes gold or yellow. When the color tone becomes gold or yellow, the color tone changes, and therefore sometimes it is not suitable for applications where the silver color of titanium itself is required. It is presumed that this change in color tone is due to the material color of the titanium nitride becoming apparent. Therefore, the nitrogen concentration at the depth where the nitride-derived nitrogen concentration in the oxide film (20) becomes maximum is 2.0 to 10.0 atomic%.

In addition, as shown in Fig. 5, there were cases where the color difference exceeded 8 when the nitrogen concentration at the depth where the nitride-derived nitrogen concentration in the oxide film (20) becomes maximum was less than the carbide-derived carbon concentration at the same depth. This is also thought to be due to, as described above, affecting the strain distribution within the oxide film (20), changing the conductivity and affecting the nanometer-level potential distribution, etc. Therefore, it is preferable that the nitrogen concentration at the depth where the nitride-derived nitrogen concentration in the oxide film (20) becomes maximum is equal to or greater than the carbide-derived carbon concentration at the position where the nitride-derived nitrogen concentration in the oxide film (20) becomes maximum.

The calculation of the concentrations of N, C, O, and Ti derived from nitrides, carbides, and oxides in the titanium substrate (10) and oxide film (20) can be performed by using X-ray photoelectron spectroscopy and sputtering the surface of the titanium material with Ar ions. Specifically, the analysis conditions are X-ray source: mono-AlKα (hν: 1486.6 eV), beam diameter: 200 ㎛Φ (≒ analysis area), detection depth: several nm, introduction angle: 45°, sputtering conditions: Ar ⁺ , sputtering rate 4.3 nm/min. (SiO ₂ equivalent value). The SiO ₂ equivalent value is a sputtering rate obtained under the same measurement conditions using a SiO ₂ film whose thickness has been measured in advance using an ellipsometer.

The peak appearing at a binding energy position of about 393 to 408 eV is measured as the N1s peak, and N derived from organic matter is separated at about 399 to 401 eV and N derived from nitride is separated at about 397±1 eV. The peak appearing at a binding energy position of about 280 to 395 eV is measured as the C1s peak, and C derived from organic matter is separated at about 284 to 289 eV and C derived from carbide is separated at about 281.5±1 eV. The peak appearing at a binding energy position of about 525 to 540 eV is measured as the O1s peak, O derived from organic matter is separated at about 399 to 401 eV and O derived from metal oxide is separated at about 529.5 to 530.5 eV. The peak appearing at a binding energy position of 450 to 470 eV is measured as the Ti2p peak. The binding energy of the above substances is a general value and may vary depending on the charging of the measurement sample, etc. As one of the charging correction methods, a method of correction based on the peak position of the C-C bond in C derived from organic matter can be applied.

As a general interpretation method using these peaks, the element concentration and the concentration by chemical state can be interpreted using the interpretation software MultiPak. The general procedure is described below. The background is corrected based on the Shirley method. Next, the Gauss-Lorents function is used for compounds, and the asymmetric function is used for metals to fit the peaks by chemical state for each element. Then, the area ratio of the peak derived from each chemical state is multiplied by the concentration of the element (at.%), and the concentration (at.%) by chemical state is calculated. The nitrogen content derived from nitrides and the carbon content derived from carbides are obtained through this procedure. In addition, the concentration of the above-mentioned element is obtained by calculating the peak area including (without separation) all peaks related to the element for each element detected by XPS, dividing this by the sensitivity coefficient for each element, and converting it into a percentage.

Up to this point, the nitrogen content derived from nitride in the oxide film (20) has been described in detail.

(Surface layer (30))

The discoloration resistance of the titanium material (1) is improved by a decrease in the average nitrogen concentration and the average carbon concentration of the surface layer (30). The average nitrogen concentration and the average carbon concentration of the surface layer (30), which are measured in the thickness direction from the surface of the titanium material (1) by the GDS of the above method, are, from the viewpoint of discoloration resistance, 14.0 at% or less, respectively. The average nitrogen concentration of the surface layer (30) of the titanium material (1) is preferably 12.0 at% or less, more preferably 10.0 at% or less. The average carbon concentration of the surface layer (30) of the titanium material (1) is preferably 13.0 at% or less, more preferably 12.0 at% or less, and even more preferably 10.0 at% or less. The average nitrogen concentration of the surface layer (30) of the titanium material (1) may be 0 at% or 1.0 at% or more. The average carbon concentration of the surface layer (30) of the titanium material (1) may be 0 atomic% or 1.0 atomic% or more.

The discoloration resistance of the titanium material (1) is further improved by a decrease in the average hydrogen concentration of the surface layer (30). The hydrogen concentration of the surface layer (30) of the titanium material (1) is 30.0 at% or less, preferably 25.0 at% or less, more preferably 20.0 at% or less. Titanium is a metal having a high affinity for hydrogen, and the hydrogen concentration of the surface layer (30) may be 10.0 at% or more.

(Difference in c-axis lattice constant of α-phase Ti in the surface layer (30))

The crystal structure of the α-phase Ti in the surface layer (30) of the titanium material (1) affects the discoloration resistance. Specifically, when the lattice constant of the c-axis of the α-phase Ti in the surface layer (30) of the titanium material (1) increases, the discoloration resistance deteriorates. The increase in the lattice constant of the c-axis of the α-phase Ti in the surface layer (30) of the titanium material (1) is evaluated as the difference between the lattice constant of the c-axis of the α-phase Ti obtained by X-ray diffraction measurement by the parallel beam method at an incident angle of 0.3 degrees on the surface of the titanium material (1) and the lattice constant of the c-axis of the α-phase Ti obtained by X-ray diffraction measurement by the focused method at the center of the plate thickness. The increase in the lattice constant of the c-axis of the α-phase Ti in the surface layer (30) of the titanium material (1) is 0.015 Å or less, and preferably 0.010 Å or less, from the viewpoint of discoloration resistance. The increment of the c-axis lattice constant of the α phase of the titanium material (1) on the surface is preferably smaller, and may even be 0 Å. If this increment is a negative value, the cause is a measurement error, and the increment is considered to be 0 Å.

The c-axis lattice constant of α-phase Ti in the surface layer (30) of the titanium material (1) is obtained by X-ray diffraction measurement using the parallel beam method on the surface of the titanium material. For the X-ray diffraction measurement using the parallel beam method, an X-ray diffraction apparatus SmartLab manufactured by Rigaku Corporation is used, and the X-ray source is Co-Kα (wavelength λ = 1.7902 Å). For removing Kβ rays, a W/Si multilayer mirror is used on the X-ray incident side. The X-ray source load power (tube voltage/tube current) is 5.4 kW (40 kV/135 mA) each. The incident angle of the X-rays to the sample is 0.3 degree, and the diffraction angle 2θ is scanned. For the measurement, a sample cut out from the titanium material by mechanical processing to a dimension of 25 mm (length) × 50 mm (width) is used. The beam is irradiated to the sample centered on a 12.5 mm (height) x 25 mm (width) area, and measurements are taken on the surface of the sample. In addition, the cut sample is cleaned with acetone or ethanol because there is a possibility that contamination may be attached to the surface to be measured.

The crystal structure of α-phase Ti in the center of the plate thickness of a titanium material is measured by X-ray diffraction using a focusing method. The sample used for the analysis of the crystal structure of α-phase Ti in the center of the plate thickness of a titanium material is finished by mechanical polishing and electrolytic polishing so that the center of the plate thickness of the titanium material becomes the measurement surface for performing X-ray diffraction measurement. For the X-ray diffraction measurement using the focusing method, an X-ray diffraction apparatus used for the X-ray diffraction measurement using the parallel beam method can be used, and the X-ray source, Kβ ray removal filter, and X-ray source load power can be the same as those of the parallel beam method. Since the crystal structure of Ti is uniform in the center of the plate thickness, a sample can be prepared from any location in the plate width or rolling direction. Here, a sample was prepared from approximately one-quarter of the plate width to the center, and a test was performed.

The c-axis lattice constant of the α-phase Ti at the surface and the center of the plate thickness of the titanium material is calculated from the diffraction peak of the (0002) plane using software (Expert High Score Plus) manufactured by Spectris Co., Ltd. Even when the titanium substrate is of the α+β type, the c-axis lattice constant of the α-phase Ti is calculated from the diffraction peak of the α-phase Ti.

(Ra/RSm: 0.006 to 0.015)

(RΔq: 0.150 to 0.280)

In addition, the inventors of the present invention have examined in detail the relationship between the surface properties of titanium materials and discoloration resistance, and have found that the discoloration resistance of titanium materials is affected by the ratio of the arithmetic mean roughness Ra of the surface of the titanium substrate to the average length RSm of the profile curve elements, Ra/RSm, and the root mean square slope RΔq of the roughness curve elements.

The arithmetic mean roughness Ra, the mean length RSm of the profile curve elements, and the root mean square slope RΔq of the roughness curve elements can be measured by a method conforming to JIS B 0601:2013. In addition, the kurtosis Rku and the skewness Rsk described below can also be measured by a method conforming to JIS B 0601:2013.

The arithmetic mean roughness Ra of the present embodiment is the arithmetic mean roughness Ra specified in JIS B 0601:2013, and is the average of the absolute values of the total coordinate values Zj in the reference length. The arithmetic mean roughness Ra is calculated by the following equation (1).

In addition, the roughness curve which is the basis for calculating the arithmetic mean roughness Ra is a roughness curve obtained by applying a low-pass filter with a cutoff wavelength λc = 0.8 mm to the measured cross-sectional curve of the oxide film to obtain a cross-sectional curve, and further applying a high-pass filter with a cutoff wavelength λs = 2.667 ㎛ to this cross-sectional curve. In addition, the reference length of the roughness curve is set to a length equivalent to the cutoff wavelength λc, that is, 0.8 mm. λc is a filter which defines the boundary between the roughness component and the waviness component. λs is a filter which defines the boundary between the roughness component and a wavelength component shorter than it.

In the above equation (1), n is a measurement score, and Zj is the height of the jth measurement point on the roughness curve.

The average length RSm of the contour curve elements is calculated using the following equation (2).

In the above equation (2), m is a measurement score, and Xsi is the length of the contour curve element in the reference length.

The root mean square slope RΔq of the roughness curve element is calculated by the following equation (3).

In the above equation (3), N is a measurement score. (dZj/dXj) is a local slope at the jth measurement point in the roughness curve, and is defined by the following equation (4).

In the above equation (4), ΔX is a measurement interval. In the present embodiment, the measurement interval ΔX may be determined as follows. That is, the measurement interval ΔX is a value set by a surface roughness shape measuring instrument, and when the measurement length L is measured and N points of numerical data are acquired, ΔX at the measurement interval becomes L/(N-1) on average. For example, when a measurement length of 5 mm is measured using SURFCOM 1900DX made by Tokyo Seimitsu, software TIMS Ver. 9.0.3, and 25601 points of digital numerical data are acquired, ΔX becomes 5 mm/25600 points and becomes approximately 0.195 ㎛ on average.

The root mean square slope RΔq of a roughness curve element is a parameter that defines the inclination angle (local inclination dZ/dX) of the microscopic range formed by surface roughness with respect to the reference length X of the roughness curve.

The present inventors produced titanium materials with changed Ra/RSm and RΔq, and examined the effects of Ra/RSm and RΔq of the titanium substrate on discoloration resistance. Fig. 6 is a diagram showing the relationship between Ra/RSm, which is a ratio of the arithmetic mean roughness Ra of the titanium substrate to the average length RSm of the profile curve elements, and the root mean square slope RΔq of the roughness curve elements, and discoloration resistance.

Discoloration resistance can be evaluated by color difference ΔE*ab and visual observation as described above. However, in the measurement of color tone L*a*b* for evaluating color difference ΔE*ab, light is irradiated from a daylight light source provided directly above the titanium plate. Therefore, there are cases where it is different from the actual appearance. In particular, in the case of a titanium plate having a large RΔq, even if the color difference ΔE*ab is small, it may appear discolored when visually observed under sunlight. Therefore, visual observation under sunlight is also important for evaluating discoloration resistance.

In Fig. 6, "○" represents the condition where the color difference ΔE*ab was 5 or less and the proportion of people who said that discoloration was not noticeable in the sensory evaluation observed by the eye was 80% or more, and "×" represents the condition where the color difference ΔE*ab was 5 or less but the proportion of people who said that discoloration was not noticeable in the sensory evaluation observed by the eye was less than 80%. Here, in this sensory evaluation by visual observation, titanium material that was not subjected to the previous discoloration acceleration test and titanium material after this discoloration acceleration test were arranged on a flat plate, and 10 evaluators compared them from various angles under sunlight to determine whether or not there was an angle at which discoloration was noticeable. The proportion of people who said that discoloration was not noticeable was compared. In addition, this visual observation is an evaluation that assumes a condition assuming the roof or wall of an actual building, and also assumes that the color tone changes depending on the viewing angle.

As shown in Fig. 6, it was found that the titanium material according to the present embodiment has excellent discoloration resistance even at higher temperatures and in an acidic environment when the ratio of the arithmetic mean roughness Ra to the average length RSm of the contour curve elements, Ra/RSm, for the surface thereof is 0.006 to 0.015, and further, the root mean square inclination RΔq of the roughness curve elements is 0.150 to 0.280. Such a titanium material having excellent discoloration resistance even at high temperatures and in an acidic environment can further suppress discoloration over a long period of time.

When Ra/RSm is less than 0.006, the unevenness of the titanium substrate surface is small and the intervals between the unevennesses are wide. When Ra/RSm is less than 0.006, the surface of the titanium material is relatively smooth, and the color of the light that is strengthened according to the optical path difference between the light reflected from the surface of the oxide film and the light reflected from the surface of the titanium substrate may be recognized. In other words, the titanium material may discolor. When Ra/RSm is 0.006 to 0.015, the optical path difference between the light reflected from the surface of the oxide film and the light reflected from the surface of the titanium substrate is small due to the relatively large inclination of the titanium material surface, and since no light is strengthened in the visible light range, it is thought that discoloration is suppressed. Considering the mechanism by which this discoloration is suppressed, there is no reason to limit the upper limit of Ra/RSm to 0.015, but it is difficult to industrially produce unevenness having a deep and narrow valley shape such as exceeding 0.015. Therefore, it is preferable that the upper limit of Ra/RSm be 0.015, at which the effect of the present invention is clearly obtained.

When RΔq is 0.150 or more, the inclination of the finer irregularities in the oxide film is large, and the regular reflection of light irradiated on the titanium substrate surface is suppressed by this local inclination, and diffuse reflection occurs. Therefore, the intensity of the reflected light on the titanium substrate surface reflected in the direction of the light reflected from the surface of the oxide film is small. As a result, the color of the enhanced light is difficult to recognize. When RΔq is less than 0.150, the above effect does not occur, so the titanium material may appear to discolor. On the other hand, when RΔq exceeds 0.280, the color difference is small at 5 or less, but there may be an angle at which the discoloration is noticeable under sunlight. It is thought that this is because when RΔq exceeds 0.280, when the titanium material is viewed obliquely, there is an inclination that becomes the direction of regular reflection, so that the interference color due to the increase in the thickness of the oxide film is enhanced and recognized by the naked eye.

When Ra/RSm is 0.006 to 0.015 and further, the root mean square inclination RΔq of the roughness curve element is 0.150 to 0.280, since the above actions are superimposed and obtained, discoloration of the titanium material is further suppressed. Furthermore, the titanium material having the above surface state has the change in color tone, that is, discoloration, suppressed even if an oxide film has grown to several tens of nm on its surface. Therefore, it is preferable that the titanium substrate has Ra/RSm, which is the ratio of the arithmetic mean roughness Ra to the element length RSm, in the roughness curve in the direction where the arithmetic mean roughness Ra is maximum, of 0.006 to 0.015 and further, the root mean square inclination RΔq is 0.150 to 0.280. The lower limit of Ra/RSm is more preferably 0.007. Furthermore, RΔq is more preferably 0.190 or more. When RΔq is 0.190 to 0.0280, the color difference of the discoloration acceleration test becomes 6 or less, and a higher effect is obtained.

The arithmetic mean roughness Ra and the average length RSm of the profile curve elements are preferably Ra/RSm of 0.006 to 0.015 as described above, and it is more preferable that the arithmetic mean roughness Ra is 0.700 to 3.0 µm and the average length RSm of the profile curve elements is 60 to 300 µm. Setting the arithmetic mean roughness Ra to 0.700 to 3.0 µm and setting the average length RSm of the profile curve elements to 60 to 300 µm can be relatively easily realized industrially by the manufacturing method described below.

[Kurtosis Rku: over 3]

Kurtosis Rku is an index indicating the sharpness of the amplitude distribution curve. Fig. 7 is a diagram for explaining kurtosis Rku. Also, Fig. 7 is a diagram published in Tsutomu Miyashita, "Surface Roughness I Want to Review Again," Journal of the Precision Engineering Society, Public Interest Incorporated Association Precision Engineering Society, Vol. 73, No. 2, 2007, p. 205. Kurtosis Rku represents the fourth power average of Zj in a reference length that is dimensionless by the fourth power of the root mean square height Rq.

Zj is the height of the jth measurement point on the roughness curve. Rq is the root mean square height, and is expressed by the following equation (6).

Kurtosis Rku is an index representing the sharpness of the height distribution. When the kurtosis Rku is 3, as shown in Fig. 7, the height distribution is a normal distribution, and as the kurtosis Rku value decreases to less than 3, the surface becomes flatter, and as the kurtosis Sku value increases to exceed 3, the surface of the titanium material has many sharp mountains and valleys.

In the roughness curve in the direction where the arithmetic mean roughness Ra becomes maximum, it is preferable that the kurtosis Rku of the titanium substrate exceeds 3. When the kurtosis Rku exceeds 3, the unevenness of the surface of the titanium substrate is sharp, and on a surface with sharp unevenness, the component of regular reflection that makes interference color apparent in light reflected from the surface of the titanium substrate is further suppressed. As a result, even if the oxide film thickness increases, the interference color becomes even more difficult to stand out, so that discoloration of the titanium material is further suppressed.

[Why Rsk: -0.5 exceeded]

The skewness Rsk, also called the strain, is an index indicating the sharpness of the surface irregularities. The skewness Rsk is the cubed average of Z(x) in a reference length that is dimensionless by the cube of the root mean square height Rq, and is expressed by the following equation (7).

In the above equation (7), N is the number of measurement points, and Zj is the height of the jth measurement point on the roughness curve.

In the roughness curve, when the valley length is greater than the mountain length, the skewness Rsk becomes greater than 0. In other words, when the skewness Rsk is greater than 0, the ratio of the concave part in the mean line of the roughness curve is high. That is, the tips of the mountains (convex parts) in the roughness curve become sharp and pointed, and the tips of the valleys (concave parts) become wide. The mean line of the roughness curve refers to a curve representing the long-wavelength component blocked by the cutoff wavelength λc.

On the other hand, when the valley length is smaller than the mountain length, the skewness Rsk becomes smaller than 0. In other words, when the skewness Rsk is smaller than 0, the ratio of the concave part in the mean line of the roughness curve is large. That is, the tips of the mountains (convex parts) in the roughness curve become wide, and the tips of the valleys (concave parts) become sharply pointed.

If Rsk is 0, the shape of the irregularities in the roughness curve is symmetrical with respect to the mean plane.

In the roughness curve in the direction where the arithmetic mean roughness Ra is maximum, it is preferable that the skewness Rsk of the titanium substrate exceeds -0.5. When the skewness Rsk exceeds -0.5, the tip of the mountain (convex part) in the roughness curve becomes sharp, and the light reflected from the surface of the titanium substrate on the side closer to the light source, that is, on the mountain (convex part), is more likely to be scattered, so that discoloration is further suppressed. On the side far from the light source, that is, the valley (concave part), there is a shadow effect by the mountain (convex part), so it is difficult to see the interference color which is the cause of discoloration, and therefore it is estimated that it is less likely to be affected than the mountain (convex part).

Up to this point, the titanium material according to the present embodiment has been described. The thickness of the titanium material according to the present embodiment may be, for example, 0.2 mm or more, or 0.3 mm or more. In addition, the thickness of the titanium material according to the present embodiment is not particularly limited, and may be, for example, 5.0 mm or less, 3.0 mm or less, or 2.0 mm or less.

In the titanium material according to the present embodiment, the average nitrogen concentration and the average carbon concentration in a range from the surface to a position where the oxygen concentration in the thickness direction from the surface is 1/3 of the maximum value as measured by glow discharge spectroscopy are 14.0 at% or less, and the average hydrogen concentration is 30.0 at% or less, and the difference between the c-axis lattice constant of α-phase Ti obtained by X-ray diffraction measurement by a parallel beam method at an incident angle of 0.3 degrees at the surface and the c-axis lattice constant of α-phase Ti obtained by X-ray diffraction measurement by a focused method at the center of the plate thickness is 0.015 Å or less. As a result, the titanium material according to the present embodiment has superior discoloration resistance and suppresses discoloration over a long period of time compared to conventional titanium materials.

In addition, when the maximum value of the nitrogen concentration derived from nitride when analyzed by X-ray photoelectron spectroscopy in the oxide film is 2.0 to 10.0 at%, and the position where the nitrogen concentration derived from nitride in the oxide film shows a maximum value exists in a range of 2 to 10 nm from the surface of the oxide film when converted to a sputtering rate of SiO ₂ , and the nitrogen concentration derived from nitride present near the interface with the oxide film in the titanium substrate is less than the maximum value of the nitrogen concentration derived from nitride in the oxide film and 7.0 at% or less, and the maximum value of the nitrogen concentration derived from nitride in the oxide film is equal to or greater than the carbon concentration derived from carbide at the position where the nitrogen concentration derived from nitride in the oxide film becomes maximum, the discoloration resistance is excellent even at high temperatures and in an acidic environment. In addition, titanium material, which has excellent discoloration resistance even in high temperature and acidic environments, can further suppress discoloration over a long period of time.

(Method for manufacturing titanium material)

An example of a method for manufacturing a titanium material according to this embodiment will be described. However, the titanium material according to this embodiment is not limited to one manufactured by the manufacturing method described below. In addition, pure titanium and titanium alloys used for building materials such as exterior materials are often in a plate shape, so an example of a method for manufacturing a plate-shaped titanium material will be described below.

Titanium materials are manufactured by cold rolling pure titanium or a titanium alloy, and then annealing and cooling treatments are performed.

As the titanium material to be used for cold rolling, a material manufactured by a known method may be used. For example, using a sponge titanium or a master alloy for adding alloy elements as a raw material, an ingot of pure titanium or a titanium alloy having the above composition is manufactured by various melting methods such as a Haas melting method such as a vacuum arc melting method, an electron beam melting method, or a plasma melting method. Next, the obtained ingot is crushed or hot forged as necessary to form a slab. Thereafter, the slab is hot rolled to form a hot-rolled coil of pure titanium or a titanium alloy having the above composition.

In addition, the slab may be subjected to pretreatment such as cleaning or cutting as needed. In addition, in the case where it is made into a rectangular shape that can be hot rolled by the Has melting method, it may be provided for hot rolling without hot forging, etc.

Next, the hot-rolled coil is subjected to cold rolling. A lubricant is used in cold rolling, but the lubricant used may sometimes cause the surface carbon concentration of the titanium material to increase during annealing. Therefore, preferably, before performing the annealing treatment, the oil present on the surface of the titanium material is removed by alkaline degreasing, rolling using a lesser roll, coil grinding, or polishing. In addition, when a lubricant is applied to the surface of the titanium material and cold rolled, carbon is included on the surface of the titanium material due to a mechanochemical reaction. The cold titanium material subjected to the final annealing treatment may be appropriately pickled or annealed.

A final annealing treatment is performed on a titanium material after cold rolling or a titanium material after oil removal treatment. The final annealing treatment is generally a process for reducing the strain introduced into pure titanium or a titanium alloy, which is a titanium material, by cold rolling, and for softening the titanium material. In the production of the titanium material according to the present embodiment, the final annealing treatment and the subsequent cooling treatment are processes for controlling the average nitrogen concentration and average carbon concentration of the surface layer of the titanium material, the average hydrogen concentration, the lattice constant of the c-axis of the α-phase Ti of the surface layer of the titanium material, and the thickness of the oxide film. It is thought that by increasing the temperature of the final annealing treatment, nitrogen and carbon in the surface layer of the titanium material caused by impurities diffuse in the thickness direction, thereby reducing the average nitrogen concentration and average carbon concentration in the surface layer of the titanium material. It is thought that the average hydrogen concentration in the surface layer of the titanium material and the lattice constant of the c-axis of the α-phase Ti in the surface layer of the titanium material are reduced by increasing the vacuum degree of the final annealing treatment. Therefore, the final annealing treatment is performed in an inert gas (excluding nitrogen gas) atmosphere after a vacuum atmosphere has been created or in a vacuum as is. The heating temperature and atmosphere of the final annealing treatment are described below. Herein, the inert gas refers to a gas that is inert to titanium, and includes argon, helium, and neon.

The heating temperature (annealing temperature) of the final annealing treatment is 630°C or higher from the viewpoint of reducing the average nitrogen concentration and average carbon concentration of the surface layer of the titanium material. The annealing temperature is preferably 650°C or higher from the viewpoint of reducing the average carbon concentration of the surface layer of the titanium material. Although the upper limit of the annealing temperature is not specifically set, it is preferably 750°C or lower from the viewpoint of manufacturing cost. The annealing temperature referred to here is the temperature inside the heating furnace used for the annealing treatment, and is measured using a thermocouple installed in the heating furnace. The annealing time is preferably 5 hours or higher from the viewpoint of reducing the average nitrogen concentration and average carbon concentration of the surface layer of the titanium material. The annealing time is more preferably 10 hours or higher. The annealing time may be 3 hours or higher since the cold-rolled titanium material can be annealed. On the other hand, from the viewpoint of productivity, the annealing time is preferably 48 hours or lower. In addition, if the annealing time exceeds 10 hours, the crystal grain size may become too coarse, which may cause problems such as a decrease in tensile strength or wrinkles due to processing. Therefore, from the viewpoint of maintaining tensile strength, etc., the annealing time is preferably 10 hours or less. In addition, the annealing time referred to here is the time during which the temperature inside the heating furnace having the titanium material inside is maintained at the annealing temperature.

The final annealing treatment is performed in a vacuum atmosphere, an inert gas atmosphere (excluding nitrogen gas), or an inert gas atmosphere in which an inert gas excluding nitrogen is introduced after the vacuum atmosphere. The degree of vacuum of the vacuum atmosphere is, for example, 1.0× ^10-2 Pa or less. The inert gas atmosphere is preferably a noble gas atmosphere, and more preferably an Ar atmosphere. The degree of vacuum before the final annealing treatment is performed in an inert gas atmosphere is preferably 1.0×10-2 Pa or less from the viewpoints of suppressing an increase in the lattice constant of the c-axis of the α-phase Ti in the surface layer of the titanium material and reducing the average hydrogen concentration and average nitrogen concentration in the surface layer of the titanium material. The degree of vacuum before the final annealing treatment is performed in an inert gas atmosphere is more preferably ^5.0 × ^10-3 Pa or less from the viewpoint of reducing the average hydrogen concentration in the surface layer of the titanium material. In addition, the inert gas atmosphere may be an atmosphere containing, for example, 99.99 vol% or more of Ar inside the furnace. The inert atmosphere may also be an atmosphere containing 99.99 vol% or more of He (helium). The inside of the furnace may be made into a vacuum atmosphere before the start of heating of the final annealing treatment, and then the inert gas atmosphere may be changed, or the inside of the furnace may be made into a vacuum atmosphere before the start of heating, and then the inside of the furnace may be changed from a vacuum atmosphere to an inert gas atmosphere during the period from the start of heating to the start of cooling.

By performing the final annealing treatment in a vacuum atmosphere, or in an inert gas atmosphere (excluding nitrogen) into which an inert gas excluding nitrogen is introduced after the vacuum atmosphere, at the annealing temperature and annealing time described above, a state in which oxygen, nitrogen, carbon, and hydrogen are low is formed on the surface of the titanium material. In the final annealing treatment, by making the surface of the titanium material a surface state with a high degree of cleanliness, when cooling thereafter, by opening it to the air, a nitrogen gas atmosphere, or an inert gas atmosphere containing 10% or more of nitrogen gas, it is possible to generate a predetermined amount of nitride in the oxide film by the nitrogen contained in these atmospheres. In a titanium material that has been cooled in the final annealing treatment and then opened to the air, even if it is heated again in an atmosphere containing nitrogen, it is impossible to generate a predetermined nitride in the oxide film.

The nitrogen concentration of the annealing atmosphere is an inert gas of 0.005 volume% or less. In addition, in general industrial pure gases, nitrogen is less than half of the impurities, so the atmosphere of the annealing treatment in the present embodiment using the above-described inert gas is of sufficient purity.

The atmosphere of the final annealing treatment is maintained at a temperature below which temper color does not occur in the titanium material after the annealing treatment, for example, below 300°C. The annealing atmosphere may be maintained until cooling to room temperature, or the inside of the furnace may be opened to the atmosphere, etc. at a temperature below 300°C.

After the final annealing treatment, the titanium material is cooled. The cooling atmosphere may be, for example, the same atmosphere as the annealing atmosphere at the start of cooling, or, when the temperature inside the furnace is 300°C or lower, an atmosphere composed of nitrogen gas, a mixed atmosphere of argon or helium containing 10% or more of nitrogen by volume, or the atmosphere.

The cooling rate after the final annealing treatment is not particularly limited. However, in order to keep the amount of nitrogen derived from nitride existing in the oxide film within a predetermined range at 300°C or lower when opening, it is preferable to set it under the conditions described later. In addition, in order to suppress shape distortion due to thermal shrinkage of the titanium material during cooling, the cooling rate until opening (up to 300°C or lower) is preferably 50°C/min or lower, more preferably 30°C/min or lower, and furthermore, when cooling a large titanium material weighing 1 ton or more, it is preferable to set it to 1°C/min.

In the cooling after the final annealing treatment, when the temperature inside the furnace becomes 300°C or lower, it is preferable to introduce nitrogen gas into the furnace or open the furnace to the atmosphere to make the furnace a nitrogen atmosphere containing 10% by volume or more of nitrogen. The amount of nitrogen derived from nitrides present in the oxide film varies depending on the temperature, atmosphere, and the cleanliness of the titanium material surface. This is because trace amounts of carbon, oxygen, hydrogen, and nitrogen present on the surface of the titanium material compete and react on the titanium material surface, and the amount of nitrogen and titanium reacts depending on which reaction occurs preferentially, and the amount of nitrides produced also varies accordingly. However, when the temperature at the time of changing the atmosphere (when opening) is 300°C or lower, the reaction between titanium and nitrogen occurs sufficiently, and the amount of nitrides produced does not become excessive. Therefore, the maximum concentration of nitrogen derived from nitrides present in the oxide film can be set to 2.0 to 10.0 at% by the cooling treatment following the annealing treatment. On the other hand, when the temperature when changing the atmosphere is less than 200°C, the nitrogen content derived from nitride of the oxide film becomes less than 2.0 atomic%. This is because the reaction between nitrogen and titanium becomes gradual when the temperature is low. The temperature is preferably 250°C or higher, and more preferably 280°C or higher.

After the inside of the furnace is made into a nitrogen atmosphere, it is preferable that the cooling time until the temperature reaches 200°C be 1.5 hours or longer, and more preferably 2.0 hours or longer. By setting the inside of the furnace into a nitrogen atmosphere and setting the cooling time until the temperature reaches 200°C to 1.5 hours or longer, the maximum value of the nitrogen concentration derived from nitride in the oxide film when analyzed by X-ray photoelectron spectroscopy becomes 2.0 to 10.0 at%, and further, the position at which the nitrogen concentration derived from nitride in the oxide film shows the maximum value exists in a range of 2 to 10 nm from the surface of the oxide film when converted to a sputtering rate of SiO ₂ .

In addition, even if cold rolling including rolling using a dull roll at a reduction ratio of 5% or less is performed on the titanium material after cooling treatment, the characteristics of the titanium material according to the present embodiment are maintained.

In the method for manufacturing a titanium material according to the present embodiment, it is preferable to have a polishing step for polishing the surface of a titanium material using a polishing powder having a particle size distribution of #320 or less according to JIS R 6001-2:2017, and a under-rolling step for reducing the titanium material using a rolling roll having a surface roughness Ra of 0.5 µm or more so that the total reduction ratio is 0.10% or more. The polishing step is performed before the final annealing treatment, and the under-rolling step is performed after the final annealing treatment. By the polishing step and the under-rolling step, the Ra/RSm of the titanium substrate surface becomes 0.006 to 0.015, and further, the root mean square inclination RΔq of the roughness curve element becomes 0.150 to 0.280. Hereinafter, the polishing step and the under-rolling step will be described.

[Polishing process]

In this process, the surface of the titanium material is polished using abrasive powder having a particle size distribution of #320 or less in accordance with JIS R 6001-2:2017. The means for polishing the surface of the titanium material is not particularly limited, and for example, known means such as a brush roll or coil grinder can be used.

For example, when using a coil grinder, the surface of the titanium material on the plate coil is polished by the following method. With a coil line polisher, the titanium material is polished using an abrasive belt having a count of #320 or lower, such as #320, #240, #100, or #80. The abrasive powder used in the abrasive belt is preferably a count of #100 or lower. In order to obtain a more uniform polished surface, there are cases where polishing is performed multiple times with the same count or with the counts changed.

As the titanium material to be used in the polishing process, a material manufactured by a known method may be used. For example, a raw material such as sponge titanium or a master alloy for adding alloy elements is used, and by various melting methods such as a Haas melting method such as a vacuum arc melting method, an electron beam melting method, or a plasma melting method, an ingot of pure titanium or a titanium alloy having the above composition is produced. Next, the obtained ingot is pulverized or hot forged as necessary to form a slab. Thereafter, the slab is hot rolled to form a hot-rolled coil of pure titanium or a titanium alloy having the above composition. This hot-rolled coil is cold rolled, and a polishing process is performed on the titanium material after the cold rolling. The titanium material after the cold rolling to be used in the polishing process may be appropriately annealed.

In addition, the slab may be subjected to pretreatment such as polishing or cutting as needed. In addition, in the case where it is made into a rectangular shape that can be hot rolled by the Has melting method, it may be provided for hot rolling without performing smelting or hot forging.

Cold rolling conditions are not particularly limited, and can be performed under conditions that obtain the desired thickness, characteristics, etc.

[Less rolling process]

In this process, a titanium material is pressed using a rolling work roll (hereinafter referred to as a rolling roll) having an arithmetic mean roughness Ra of a surface of 0.5 µm or more. By pressing the titanium material using the rolling roll so that the total reduction ratio is 0.10% or more, a more locally inclined unevenness is provided on the surface of the titanium material. If the arithmetic mean roughness Ra of the surface of the rolling roll is too large, the uneven shape provided in advance by the polishing process may change significantly in some cases, and therefore the arithmetic mean roughness Ra of the surface of the rolling roll is preferably 2.0 µm or less.

The surface roughness of the rolling roll can be adjusted by polishing or shot blasting.

The total reduction ratio is preferably 0.10% or more in order to provide a locally inclined roughness, and preferably 0.2% or more in view of the stability of surface formation over the entire length of the coil. On the other hand, it is preferably 1.5% or less in order not to destroy the surface roughness formed by the polishing in the previous process by cold rolling and thereby eliminate the necessary roughness shape. Furthermore, in order to obtain the surface characteristics of the present invention, one pass of this cold rolling is sufficient, but in consideration of the point that the entire length of a long coil is processed into a surface as uniform as possible, the cold rolling may be performed in two or more passes. Taking that into consideration, the total reduction ratio is stipulated, and in the case of multiple passes, the total reduction ratio is set as the reduction ratio obtained from the difference in the initial and finished plate thicknesses.

Example

(Example 1)

The titanium material shown in Table 1 was cold rolled to a thickness of 0.4 mm, and then degreased using an alkaline or organic solvent, etc., to remove oil from the surface of the titanium material. In this embodiment, JIS Class 1 pure titanium (ASTM Gr. 1 equivalent), JIS Class 2 pure titanium (ASTM Gr. 2 equivalent), JIS Class 3 pure titanium (ASTM Gr. 3 equivalent), JIS Class 4 pure titanium (ASTM Gr. 4 equivalent), JIS Class 11 titanium alloy (ASTM Gr. 11 equivalent, Ti-0.15Pd), JIS Class 21 titanium alloy (ASTM Gr. 13 equivalent, Ti-0.5Ni-0.05Ru), JIS Class 17 titanium alloy (ASTM Gr. 7 equivalent, Ti-0.05Pd), Ti-Ru-Mm, Ti-3Al-2.5V, Ti-5Al-1Fe, and JIS Class 60 titanium alloy (ASTM Gr. 5 equivalent, Ti-6Al-4V) compliant with JIS H 4600:2012 were used. Mm in Ti-Ru-Mm represents misch metal.

Thereafter, in Examples 1 to 23 of the present invention, annealing treatment was performed on each titanium material under the conditions shown in Table 1. The opening temperatures shown in Table 1 are the temperatures when the inside of the furnace was opened during the cooling process after each annealing time was maintained at each annealing temperature. The temperature at this time is the temperature inside the furnace measured using a thermocouple.

In Inventive Examples 1 to 13, 19 to 23 and Comparative Examples 1 to 3, heating was initiated under a vacuum atmosphere shown in the “Vacuum Degree” of Table 1, and 99.99 vol% or more of Ar gas was introduced into the annealing furnace until the start of cooling. In the cooling process after each titanium material was maintained in the furnace at each annealing temperature for each annealing time, each annealing atmosphere was maintained up to the opening temperature shown in Table 1, and the furnace was opened when the temperature inside the furnace dropped to the opening temperature.

In Invention Examples 14 and 15, heating was initiated in an Ar atmosphere, and the Ar atmosphere was maintained until the furnace was opened.

In Examples 16 to 18 of the present invention, heating was initiated under a vacuum atmosphere as shown in the “vacuum degree” in Table 1, and the vacuum degree was maintained until the furnace was opened.

In Comparative Examples 4 and 5, the titanium material shown in Table 1 was cold rolled to a thickness of 0.4 mm, degreased using an alkali or organic solvent, etc., and then finished with nitric acid pickling without performing a final annealing treatment.

The average nitrogen concentration, average carbon concentration, and average hydrogen concentration in the surface layer were obtained by the following methods. Analysis was performed with GDS for O, N, C, H, and Ti. For the measurement, JOBIN YVON GD-Profiler2 manufactured by Horiba Seisakusho Co., Ltd. was used. The conditions of the measurement were a constant power mode of 35 W, an argon gas pressure of 600 Pa, and a discharge range of 4 mm in diameter. In the measurement by GDS, the measurement pitch was 0.5 nm.

The concentration (at.%) of each element above was calculated by setting the total of the elements above to 100 at. The range from the surface of the titanium material to the position where the oxygen concentration measured in the thickness direction by GDS was 1/3 of the maximum value was defined as the surface layer of the titanium material. The average nitrogen concentration, average carbon concentration, and average hydrogen concentration were defined as the numerical arithmetic mean values of the nitrogen concentration, carbon concentration, and hydrogen concentration at each measurement point.

The c-axis lattice constant of the α phase Ti on the surface of the titanium material was obtained by X-ray diffraction measurement using the parallel beam method. For the X-ray diffraction measurement using the parallel beam method, an X-ray diffraction apparatus SmartLab manufactured by Rigaku Corporation was used, and the X-ray source was Co-Kα (wavelength λ = 1.7902 Å). To remove Kβ rays, a W/Si multilayer mirror was used on the X-ray incident side. The X-ray source load power (tube voltage/tube current) was 5.4 kW (40 kV/135 mA), respectively. The incident angle of the X-rays to the sample was 0.3 degrees, and the diffraction angle 2θ was scanned. For the measurement sample, a titanium material with a thickness of 0.4 mm was cut into dimensions of 25 mm (length) x 50 mm (width) by machining, and the beam was irradiated with the center position of the titanium material surface, in other words, a position of 12.5 mm (length) x 25 mm (width) on the surface of the measurement sample, to conduct the measurement. In addition, since the cut sample may have contamination attached to the surface to be measured, it was cleaned using acetone.

The c-axis lattice constant of α-phase Ti in the center of the plate thickness of a titanium material was measured by X-ray diffraction using the focused method. The sample used for the analysis of the crystal structure of α-phase Ti in the center of the plate thickness of a titanium material was finished by mechanical polishing and electrolytic polishing so that the center of the plate thickness of the titanium material became the measurement plane for performing X-ray diffraction measurement. For the X-ray diffraction measurement using the focused method, the X-ray diffraction apparatus used for the X-ray diffraction measurement using the parallel beam method was used, and the X-ray source, Kβ ray removal filter, and X-ray source load power were the same as those of the parallel beam method.

The c-axis lattice constant of the α-phase Ti at the surface and center of the plate thickness of the titanium material was calculated from the diffraction peak of the (0002) plane.

The difference in the c-axis lattice constant of the α phase Ti on the surface of the titanium material was obtained from the difference between the lattice constant calculated on the surface of the titanium material and the lattice constant calculated at the center of the plate thickness.

The thickness of the oxide film was obtained by the oxygen concentration measured by the above method using GDS. Specifically, the distance in the thickness direction from the surface to the position where the oxygen concentration is reduced by half of the maximum value was taken as the thickness of the oxide film.

The nitrogen content derived from nitride contained in the oxide film was measured by the following method. That is, in the distribution of the nitride-derived nitrogen concentration in the depth direction measured by the following method, the maximum value within the oxide film was taken as the nitrogen content derived from nitride contained in the oxide film.

The depth-direction (film thickness direction) distributions of the nitrogen concentration derived from nitride and the carbon concentration derived from carbide within the oxide film, and the nitride-derived nitrogen concentration near the interface with the oxide film on the titanium substrate (20 nm from the interface toward the titanium substrate) were measured by the following methods. That is, using XPS, the surface of the titanium material was sputtered with Ar ions to measure the depth-direction concentration distribution, and the state of each element at each peak position of N1s, C1s, O1s, and Ti2p was analyzed, and the concentrations of N, C, O, and Ti derived from nitride, carbide, and oxide were calculated. The details were calculated in the above-described procedure. The analysis conditions for XPS were as follows.

Device: Albak·Pije Quantera SXM

X-ray source: mono－AlKα(hν: 1486.6 eV)

Beam diameter: 200㎛Φ (≒ analysis area)

Detection depth: several nm

Introduction angle: 45°

Sputtering conditions: Ar ⁺ , sputtering rate 4.3 nm/min. (SiO ₂ equivalent)

The SiO ₂ conversion value is the sputtering rate obtained under the same measurement conditions using a SiO ₂ film whose thickness was previously measured using an ellipsometer.

Each parameter of the surface roughness of the manufactured titanium material (arithmetic mean roughness Ra, mean length RSm of profile curve elements, root mean square slope RΔq of roughness curve elements, kurtosis Rku, skewness Rsk) was measured under the following conditions in accordance with JIS B 0601:2013.

Device: Surface roughness profile measuring instrument (SURFCOM 1900DX manufactured by Tokyo Seimitsu Co., Ltd., Analysis software: TiMS Ver.9.0.3)

Measuring instrument: Tokyo Seimitsu Corporation Shape measuring instrument (Type: DT43801)

Parameter calculation standard: JIS－01 standard

Measurement type: Roughness measurement

Cutoff type: Gaussian

Slope correction: Least squares straight line correction

Measuring distance: 5.0mm

Cutoff wavelength λc: 0.8mm

Measurement range: ±64.0㎛

Measurement speed: 0.3mm/sec

Movement/return speed: 0.6mm/sec

Reset settings: Normal measurements

Preliminary drive length: (cutoff wavelength/3)×2

Measurement interval Δx: 0.195㎛

λs cutoff ratio: 300

λs cutoff wavelength: 2.667㎛

Pickup Type: Standard Pickup

Polarity: Positive rotation

The measurement location was measured at three points in the direction where Ra was maximum, and the average value was obtained. Here, the direction where Ra was maximum was determined by measuring the roughness in four directions of 22.5°, 45°, and 90° (direction perpendicular to the rolling direction) with the direction parallel to the rolling direction being set to 0° in the case of the titanium material as a plate. In the case of titanium material rolled using a rolling roll, or when the plate surface was ground by rotating a roll embedded with abrasive grains in the rolling direction, Ra was maximum in the 90° direction perpendicular to the rolling direction.

Each of the manufactured titanium materials was immersed in a sulfuric acid solution at pH 3 and 60°C for 4 weeks, the color difference before and after immersion was calculated, and the discoloration resistance was evaluated based on the color difference value. When the color difference ΔE was 0 or more and 5 or less, the discoloration resistance was judged as extremely good (A), when it was more than 5 and 10 or less, the discoloration resistance was judged as good (B), and when it was more than 10, the discoloration resistance was judged as poor (C). In addition, the color difference ΔE before and after the test was

ΔE = ((L*2－L*1) ² + (a*2－a*1) ² + (b*2－b*1) ² )1/2.

Here, L*1, a*1, b*1 are the color measurement results before the discoloration test, and L*2, a*2, b*2 are the color measurement results after the discoloration test, and are based on the L*a*b* colorimetric method specified in JIS Z 8729.

In addition, the color difference was measured using CR400 manufactured by Konica Minolta Co., Ltd., under conditions of a measurement area with a diameter of 8 mm and a light source of D65.

In addition, a visual sensory evaluation was conducted on each titanium material manufactured. For the evaluation by visual observation, titanium materials that were not subjected to this discoloration acceleration test and titanium materials after this discoloration acceleration test were arranged on a flat plate, and 10 evaluators compared them from various angles under sunlight to determine whether there was an angle at which discoloration was noticeable. If 90% or more of the 10 evaluators judged that the discoloration was not noticeable, it was considered A＋＋. If 80% or more but less than 90% of the 10 evaluators judged that the discoloration was not noticeable, it was considered A＋. If 70% or more but less than 80% of the 10 evaluators judged that the discoloration was not noticeable, it was considered A＋. If 50% or more but less than 70% of the 10 evaluators judged that the discoloration was not noticeable, it was considered A0. If 30% or more but less than 50% of the 10 evaluators judged that the discoloration was not noticeable, it was considered B. If less than 30% of the 10 evaluators judged that the discoloration was not noticeable, it was considered C. If the evaluation was C, it was considered a failure. In addition, this visual observation is a condition assuming an actual building roof or wall, and is an evaluation that assumes that the color tone changes depending on the viewing angle.

The results are shown in Tables 2 and 3. The underlines in Tables 2 and 3 indicate that they are outside the scope of the present invention.

Inventive examples 1 to 23 had an average nitrogen concentration and an average carbon concentration of a surface layer of 14.0 at% or less, an average hydrogen concentration of 30.0 at% or less, a difference in the lattice constant of the c-axis of the α-phase Ti at the surface of the titanium material and the center of the plate thickness of 0.015 Å or less, and the color difference evaluation result of these was B or higher, and the visual sensory evaluation result was B or higher.

In Comparative Example 1, since the vacuum level during the annealing treatment was low at 1.0× ^10-1 Pa, the average nitrogen concentration in the surface layer was high. As a result, the color difference evaluation results and the visual sensory evaluation results of the titanium material of Comparative Example 1 were both failed.

In Comparative Example 2, the annealing temperature was low, so the average carbon concentration in the surface layer was high. As a result, the color difference evaluation results and the visual sensory evaluation results of the titanium material of Comparative Example 2 were both failed.

In Comparative Example 3, since the vacuum degree during the annealing treatment was low at 8.0× ^10-2 Pa, the oxygen concentration in the surface layer of the titanium material increased, and the lattice constant of the c-axis of the α-phase Ti in the surface layer increased, so it is presumed that the difference in lattice constants increased. As a result, the color difference evaluation results and the visual sensory evaluation results of the titanium material of Comparative Example 3 were both failed.

In Comparative Examples 4 and 5, the average hydrogen concentration in the surface layer was high, and the color difference evaluation results and visual sensory evaluation results of the titanium material of Comparative Example 3 were both failed.

As described above, the titanium material according to the present embodiment has excellent discoloration resistance for a long period of time even in a sulfuric acid solution of pH 3 simulating severe acid rain. The present invention is particularly effective for use in outdoor environments such as roof or wall panels, and its industrial value can be said to be very high.

(Example 2)

The titanium material shown in Table 4 was cold rolled to a thickness of 0.4 mm, degreased using an alkali or organic solvent, etc., and the oil on the surface of the titanium material was removed. Thereafter, annealing treatment was performed on each titanium material under the conditions shown in Table 4. The opening temperatures shown in Table 4 are the temperatures when the inside of the furnace was opened during the cooling process after each annealing time was maintained at each annealing temperature. The temperature at this time is the temperature inside the furnace measured using a thermocouple.

The same evaluation as in Example 1 was performed on each manufactured titanium material. The results are shown in Tables 5 and 6.

As shown in Tables 5 and 6, Examples 24 to 39 of the present invention, when analyzed by X-ray photoelectron spectroscopy in the oxide film, had a maximum value of nitride-derived nitrogen concentration of 2.0 to 10.0 at%, and the position where the nitride-derived nitrogen concentration in the oxide film shows a maximum value exists in a range of 2 to 10 nm from the surface of the oxide film when converted to a sputtering rate of SiO ₂ , and the nitride-derived nitrogen concentration present near the interface with the oxide film on the titanium substrate was less than the maximum value of the nitride-derived nitrogen concentration in the oxide film and 7 at% or less, and the maximum value of the nitride-derived nitrogen concentration in the oxide film was equal to or greater than the carbide-derived carbon concentration at the position where the nitride-derived nitrogen concentration in the oxide film becomes maximum. The color difference evaluation results for these were A, and the sensory evaluation results seen with the naked eye were all A＋＋, indicating excellent discoloration resistance.

(Example 3)

The titanium material shown in Table 7 was cold rolled to a thickness of 0.4 mm, and then the polishing process, cleaning, annealing treatment, and under-rolling process were performed in this order under the conditions shown in Tables 7 and 8. The "number of polishing passes" shown in Table 7 indicates the number of times the titanium material was passed through the line of a coil grinder consisting of three polishing stands having polishing belts arranged thereon.

In Examples 40 to 72 of the present invention, the final annealing treatment was performed on each titanium material under the conditions shown in Table 8. The opening temperatures shown in Table 8 are the temperatures when the inside of the furnace was opened during the cooling process after each annealing time was maintained at each annealing temperature. The temperature at this time is the temperature inside the furnace measured using a thermocouple.

In Inventive Examples 40 to 61 and 63 to 72, heating was initiated under a vacuum atmosphere shown in the “Vacuum Degree” of Table 8, and 99.99 vol% or more of Ar gas was introduced into the annealing furnace until the start of cooling. In the cooling process after each titanium material was maintained in the furnace at each annealing temperature for each annealing time, each annealing atmosphere was maintained up to the opening temperature shown in Table 8, and the furnace was opened when the temperature inside the furnace dropped to the opening temperature.

In Invention Example 62, heating was initiated under a vacuum atmosphere as shown in “Vacuum Degree” in Table 8, and the vacuum degree was maintained until the furnace was opened.

In Comparative Example 6, a titanium material was cold rolled to a thickness of 0.4 mm, degreased using an alkali or organic solvent, etc., and then a nitric acid pickling finish was performed without performing a final annealing treatment.

For each titanium material manufactured, the same evaluation as in Example 1 was performed. The results are shown in Tables 9 and 10. The underline in Table 9 indicates something outside the scope of the present invention.

As shown in Tables 7 to 10, by controlling the heat treatment temperature, time, heat treatment atmosphere, and cooling atmosphere, it was found that the ratio of the arithmetic mean roughness Ra to the element length RSm, Ra/RSm, in the roughness curve in the direction where the arithmetic mean roughness Ra is maximum was 0.006 to 0.015, and further, the root mean square inclination RΔq was 0.150 to 0.280, the kurtosis Rku of the titanium substrate exceeded 3, and the skewness Rsk of the titanium substrate exceeded -0.5, and the evaluation results were even more excellent. In particular, in Examples 59 to 61 of the present invention, both the average nitrogen concentration and the average carbon concentration of the surface layer are 14.0 at% or less, the average hydrogen concentration is 30.0 at% or less, the difference in the lattice constant of the c-axis of the Ti of the α phase at the surface and the center of the plate thickness of the titanium material is 0.015 Å or less, the maximum value of the nitrogen concentration derived from nitride when analyzed by X-ray photoelectron spectroscopy in the oxide film is 2.0 to 10.0 at%, the position where the nitrogen concentration derived from nitride in the oxide film shows a maximum value exists in a range of 2 to 10 nm from the surface of the oxide film when converted to a sputtering rate of SiO ₂ , and the nitrogen concentration derived from nitride present near the interface with the oxide film in the titanium substrate is less than the maximum value of the nitrogen concentration derived from nitride in the oxide film and 7 at% or less, and the nitride in the oxide film The maximum value of the nitrogen concentration derived from the oxide film was higher than the carbon concentration derived from the carbide at the position where the nitrogen concentration derived from the nitride in the oxide film was maximum. As a result, the color difference evaluation result of these was A, and the sensory evaluation result seen with the eyes was A＋＋＋, indicating excellent discoloration resistance.

1: Titanium material
10: Titanium substrate
20: Oxide film
30: Surface

Claims (5)

From the surface, the average nitrogen concentration and the average carbon concentration in the range from the surface to the position where the oxygen concentration in the thickness direction is 1/3 of the maximum value as measured by glow discharge spectroscopy are 14.0 at% or less, and the average hydrogen concentration is 30.0 at% or less,
The difference between the c-axis lattice constant of the α-phase Ti obtained by X-ray diffraction measurement using the parallel beam method at an incident angle of 0.3 degrees on the surface and the c-axis lattice constant of the α-phase Ti obtained by X-ray diffraction measurement using the concentration method at the center of the plate thickness is 0.015 Å or less.
Titanium material. In the first paragraph,
Having an oxide film with a thickness of 30.0 nm or less,
Titanium material. In paragraph 1 or 2,
Titanium substrate,
Having an oxide film disposed on the surface of the titanium substrate,
When analyzed by X-ray photoelectron spectroscopy,
The maximum concentration of nitrogen derived from nitride in the above oxide film is 2.0 to 10.0 atomic%,
The position at which the nitrogen concentration derived from the nitride in the oxide film reaches a maximum value, when converted to the sputtering rate of SiO ₂ , exists in a range of 2 to 10 nm from the surface of the oxide film,
The concentration of nitrogen derived from the nitride existing in a range of 20 nm from the position where the oxygen concentration is half of the maximum value to the titanium substrate side is less than the maximum value of the nitrogen concentration derived from the nitride in the oxide film and is 7 at% or less,
The maximum value of the nitrogen concentration derived from the nitride in the oxide film is greater than or equal to the carbon concentration derived from the carbide at the position where the nitrogen concentration derived from the nitride in the oxide film becomes maximum.
Titanium material. In paragraph 1 or 2,
A titanium substrate is provided, wherein, in a roughness curve in a direction in which the arithmetic mean roughness Ra is maximum, the ratio of the arithmetic mean roughness Ra to the element length RSm, Ra/RSm, is 0.006 to 0.015, and further, the root mean square inclination RΔq is 0.150 to 0.280.
The kurtosis Rku of the above titanium substrate is greater than 3,
The skewness Rsk of the above titanium substrate is greater than -0.5,
Titanium material. In the third paragraph,
The titanium substrate has a ratio of the arithmetic mean roughness Ra to the element length RSm, Ra/RSm, in a roughness curve in the direction where the arithmetic mean roughness Ra is maximum, of 0.006 to 0.015, and further has a root mean square inclination RΔq of 0.150 to 0.280.
The kurtosis Rku of the above titanium substrate is greater than 3,
The skewness Rsk of the above titanium substrate is greater than -0.5,
Titanium material.

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