TWI617671B - Titanium for hot rolling - Google Patents

Abstract

The hot rolling titanium material (1) includes a base material (1b) composed of industrial pure titanium or a titanium alloy, and a rolled surface formed on at least one of the base material (1b) and has a base material (1b). 1b) The surface layer portion (1a) with different chemical composition; the surface layer portion (1a) has a thickness of 2.0 to 20.0mm and the proportion of the total thickness is 40% or less on each side. When the surface layer portion (1a) contains When measuring the content of multiple elements at multiple points, the relationship between the average value of the increased content C _AVE relative to the base material (1b) and the increased content C ₀ of each measured position relative to the base material (1b): | C _AVE -C ₀ | / C _AVE × 100 is 40 or less. The chemical composition of the surface layer portion as an increased content relative to the base material contains one or more selected from Mo, V, and Nb, and the Mo equivalent is 8.0 to 20.0. Although this titanium material for hot rolling (1) has low cost, it has desired characteristics.

Description

Titanium for hot rolling

The present invention relates to a titanium material for hot rolling.

Titanium has excellent corrosion resistance, oxidation resistance, fatigue resistance, hydrogen embrittlement resistance, and neutron barrier properties. These characteristics can be achieved by adding various alloying elements to titanium.

Industrial pure titanium is mainly composed of an α phase having a hcp (hexagonal closest-packed lattice) structure. It is known that if a large amount of hydrogen is absorbed in the α phase, hydrides are formed and embrittlement occurs. Therefore, depending on the use environment, it may become brittle due to absorption of hydrogen, resulting in an accident of fracture. It is reported in "Processing Technology of Titanium" (Non-Patent Document 1), for example, in a plant for processing non-oxidizing acid, or urea. Accidents caused by hydrogen absorption in ammonia and hydrogen environments. Therefore, a titanium alloy material having excellent hydrogen embrittlement resistance has been proposed.

Japanese Unexamined Patent Publication No. 2013-163840 (Patent Document 1) discloses that a titanium alloy containing 50% by volume or more of β phase and 500 to 6000 ppm of hydrogen has a high elongation at break, and it is disclosed that brittleness does not occur even if a large amount of hydrogen is contained. Example.

The titanium material is usually manufactured by the method shown below. First, according to the Kroll method, titanium oxide as a raw material is chlorinated to titanium tetrachloride, and then reduced with magnesium or sodium, thereby producing a block-like sponge-like metallic titanium (sponge titanium). This sponge titanium was press-molded to form a titanium consumable electrode, and a titanium ingot was produced by vacuum arc melting using the titanium consumable electrode as an electrode. At this time, alloying elements are added as needed to produce a titanium alloy ingot. Then, the titanium alloy ingot is divided into pieces, forged, and rolled to form titanium flat slabs. The titanium flat slabs are further subjected to hot rolling, annealing, pickling, cold rolling, and vacuum heat treatment to produce titanium thin plates.

In addition, as a method for manufacturing a titanium sheet, a method of manufacturing titanium powder by dividing titanium ingots into pieces, hydropulverizing, dehydrogenating, pulverizing, and classifying, and manufacturing a titanium powder by powder rolling, sintering, and cold rolling is also known. of.

Japanese Patent Application Laid-Open No. 2011-42828 (Patent Document 2) suggests a method for manufacturing a titanium sheet, in which titanium powder is produced directly from sponge titanium instead of titanium ingots. In order to produce a titanium sheet from the obtained titanium powder, The viscous composition containing titanium metal powder, a binder, a plasticizer, and a solvent is formed into a thin plate-shaped pre-sintered compact, which is sintered to produce a sintered sheet. The sintered sheet is compacted to produce a sintered compacted sheet. The dense sheet is re-sintered to produce a titanium sheet. The fracture elongation of the sintered sheet is 0.4% or more, the density ratio is 80% or more, and the density ratio of the sintered compacted plate is 90% or more.

Japanese Patent Application Laid-Open No. 2014-19945 (Patent Document 3) discloses a method for obtaining titanium alloy powder using titanium alloy scrap or titanium alloy ingot as a raw material, and iron powder, chromium powder, or copper powder is used in the titanium alloy powder. Add an appropriate amount to form a composite powder, extrude the composite powder into a carbon steel capsule, dissolve and remove the capsules on the surface of the obtained round rod, and further perform solid solution treatment or solid solution treatment and aging treatment. The powder method produces a high-quality titanium alloy.

Japanese Patent Application Laid-Open No. 2001-131609 (Patent Document 4) discloses a method of filling a copper capsule with a sponge titanium powder, and then performing a warm extrusion process at an extrusion ratio of 1.5 or higher and an extrusion temperature of 700 ° C or lower. The outer periphery of copper is removed to produce a titanium molded body in which more than 20% of the total length of the grain boundary of the molded body is in contact with the metal.

The hot rolling material is hot rolled. When the hot rolling material is a so-called difficult-to-machine material with insufficient hot rolling properties and high thermal deformation resistance, such as pure titanium or titanium alloy, it is double-rolled as a technology for rolling it into a thin plate. The method is known. The double rolling method is a method in which a core material such as a titanium alloy having poor workability is coated with a coating material such as carbon steel, which is excellent in workability and is inexpensive, and hot rolling is performed.

Specifically, for example, the surface of the core material is coated with a release agent, and at least the upper and lower surfaces thereof are covered with a covering material, or in addition to the upper and lower surfaces, a peripheral surface is covered with a spacer material, and the periphery is welded to assemble and hot-roll. . In the double rolling, the core material of the material to be rolled is covered with a covering material and hot-rolled. Therefore, the surface of the core material is prevented from directly contacting the cooled medium (air or roller), and the temperature of the core material can be suppressed from being lowered. Even a core material with poor processability can be manufactured into a thin plate.

Japanese Patent Application Laid-Open No. 63-207401 (Patent Document 5) discloses a method for assembling a closed covered box. Japanese Patent Application Laid-Open No. 09-136102 (Patent Document 6) discloses a method of sealing a covering material under a vacuum degree of 10 ^-3 torr or more to produce a sealed covering box. Furthermore, Japanese Patent Application Laid-Open No. 11-057810 (Patent Document 7) discloses a method of coating with carbon steel (coating material) and sealing by high-energy density welding under a vacuum of ^10-2 torr or less, A hermetically sealed box was manufactured.

On the other hand, as a method of manufacturing a highly corrosion-resistant material at a low cost, a method of joining a titanium material to the surface of a material as a base material is known.

Japanese Patent Application Laid-Open No. 08-141754 (Patent Document 8) discloses a method for manufacturing a titanium-clad steel sheet, in which a steel material is used as a base material and titanium or a titanium alloy is used as a cover material, and the base material and the cover material are joined. The surface was evacuated, and then welded to assemble the assembled flat slab for rolling. The flat slab was joined by hot rolling to produce a titanium-clad steel sheet.

Japanese Patent Application Laid-Open No. 11-170076 (Patent Document 9) discloses a method in which a surface selected from pure nickel, pure iron, and carbon content of 0.01 mass is interposed on the surface of a base material steel material containing 0.03 mass% or more of carbon. After the titanium foil is laminated with an embedded material made of any low-carbon steel with a thickness of 20% or more, the laser beam is irradiated from either side of the laminated direction, so that at least the edge of the titanium foil In the vicinity, the base material steel was welded to produce a titanium-coated steel material.

Japanese Unexamined Patent Publication No. 2015-045040 (Patent Document 10) exemplifies a method in which the surface of a porous titanium raw material (sponge titanium) formed into an ingot shape is melted under vacuum using an electron beam to produce a surface portion thereof. Into a dense titanium ingot, which is hot-rolled and cold-rolled, and Produces a dense titanium material (titanium ingot) with very little energy, which includes: forming a porous titanium portion of a porous titanium raw material into an ingot shape; and covering the entire surface of the porous portion with dense titanium. Dense coating.

In Japanese Patent Application Laid-Open No. 62-270277 (Patent Document 11), it is described that the surface effect treatment of automotive engine parts is performed by thermal spraying.

[Patent Document 1] Japanese Patent Laid-Open No. 2013-163840

[Patent Document 2] Japanese Patent Laid-Open No. 2011-42828

[Patent Document 3] Japanese Patent Laid-Open No. 2014-19945

[Patent Document 4] Japanese Patent Laid-Open No. 2001-131609

[Patent Document 5] Japanese Patent Laid-Open No. 63-207401

[Patent Document 6] Japanese Patent Laid-Open No. 09-136102

[Patent Document 7] Japanese Patent Laid-Open No. 11-057810

[Patent Document 8] Japanese Patent Laid-Open No. 08-141754

[Patent Document 9] Japanese Unexamined Patent Publication No. 11-170076

[Patent Document 10] Japanese Patent Laid-Open No. 2015-045040

[Patent Document 11] Japanese Patent Laid-Open No. 62-270277

[Non-Patent Document 1] Titanium Processing Technology, edited by (Japan) Japan Titanium Association, Nikkan Kogyo Shimbun, p. 214 ~ 230, issued in November 1992

As a countermeasure against embrittlement caused by hydrogen, a product is generally subjected to a surface treatment with hydrogen absorption resistance after processing, or electrical corrosion prevention is performed. However, it will result in an increase in product processing or construction man-hours, and an increase in cost is inevitable, and titanium materials with excellent hydrogen embrittlement resistance cannot be provided at a low cost.

In addition, as in the method disclosed in Patent Document 1, in order to make 50% by volume or more of the entire material into the β phase, it is necessary to contain a large amount of high-priced additive elements, which causes an increase in cost.

In the past, when manufacturing titanium materials by hot working, titanium sponge was pressed to form titanium consumable electrodes, and titanium ingots were produced by vacuum arc melting using titanium consumable electrodes as electrodes. The titanium ingots were further divided into blocks, Forging and rolling into titanium flat slabs, titanium flat slabs are subjected to hot rolling, annealing, pickling, and cold rolling to produce titanium materials.

In this case, it is necessary to include a step of smelting titanium to produce a titanium ingot. A method for manufacturing titanium powder by powder rolling, sintering, and cold rolling is also known. The method for manufacturing titanium powder from titanium ingots still includes a step of smelting titanium.

In the method for producing a titanium material from titanium powder, even if the melting step is not performed, because the expensive titanium powder is used as a raw material, the obtained titanium material becomes very expensive. The methods disclosed in Patent Documents 5 to 6 are also the same.

In the cascade rolling, the core material covered by the covering material is a flat slab or an ingot after all, which must be passed through a melting step or using expensive titanium powder as a raw material, which cannot reduce the manufacturing cost.

According to Patent Document 10, although a dense titanium material is produced with very little energy, the dense titanium surface layer portion and the internal components formed by melting the surface of the ingot-shaped sponge titanium are specified as the same kind of pure titanium Or a titanium alloy, for example, it is not possible to uniformly and widely form a titanium alloy layer only in the surface layer portion, and to reduce manufacturing costs.

On the other hand, materials that can produce inexpensive corrosion-resistant materials and join titanium or titanium alloys to the surface of the base material are often selected as the base material. Therefore, if the titanium layer on the surface disappears, the corrosion resistance is impaired. Even if the base material is also a titanium material, as long as it is a titanium material produced through a normal manufacturing process, it is impossible to expect a complete cost improvement. Then, the present inventors thought that an alloy layer containing a specific alloying element is provided on the surface layer of a flat embryo made of industrial pure titanium or a titanium alloy to obtain a titanium material which is inexpensive and has excellent specific performance.

As in Patent Document 11, spraying is a method in which a metal, a ceramic, or the like is melted and sprayed onto the surface of a titanium material to form a film. When a film is formed by this method, pores are inevitably formed in the film. Usually, in order to avoid oxidation of the film during the spraying, the spraying is performed while being shielded with an inert gas. Such inert gas is drawn into the pores of the membrane. The pores in which the inert gas is contained in this way cannot be crimped by thermal processing or the like. In addition, in the production of titanium, a vacuum heat treatment is generally performed. During this treatment, the inert gas in the pores expands and the film may peel off. According to the experience of the present inventors, the existence rate (void ratio) of pores generated by spraying is several vol.% Or more, and it may exceed 10 vol.% Depending on the spraying conditions. In this way, titanium materials with high porosity in the film are being manufactured. There is a risk of peeling in the step, and defects such as cracking may occur during processing.

As a method for forming a film, a cold spray method is known. When a film is formed on the surface by this method, an inert high-pressure gas is also used. In this method, although the porosity may be less than 1 vol.% Depending on the conditions, it is extremely difficult to completely prevent the occurrence of pores. Moreover, as in the case of spraying, because the pores contain an inert gas, they cannot be eliminated by subsequent processing. In addition, when the heat treatment is performed in a vacuum, the inert gas in the pores expands, which may cause cracking of the film.

In order to suppress surface defects during hot rolling, as a treatment for melting and re-solidifying the surface layer of a flat embryo using an electron beam, a melt-re-solidification treatment is known. In general, the surface layer after melting and re-solidification is removed by a pickling step after hot rolling. The present inventors have paid attention to this melt-resolidification treatment. That is, the present inventors have considered that when a surface layer of a flat embryo is melted, a specific alloy element is melted and solidified with a component derived from the flat embryo, thereby forming a surface layer portion containing the specific alloy element in the flat embryo. However, the purpose is to suppress the melting and re-solidification treatment of surface defects during hot rolling, and it cannot be applied as it is to form a surface layer portion containing a specific alloy element in a flat embryo. This is because the conventional melting and re-solidification treatment is based on the premise that the formed surface layer is removed by pickling, and segregation of alloy components in the surface layer portion is not considered at all.

If there is segregation of the alloy components in the surface layer portion of the flat embryo containing a specific alloy element, the desired performance cannot be fully exhibited, or This is to cause the deterioration of the desired performance early. Therefore, the method of adding specific alloying elements is important.

The object of the present invention is to reduce the content of alloying elements (the amount of specific alloying elements that exhibit target characteristics) added to improve hydrogen embrittlement resistance, and to reduce the manufacturing cost of titanium materials, thereby achieving low cost A titanium material for hot rolling with desired characteristics.

The present invention has been developed in order to solve the above-mentioned problems, and is based on the gist of a titanium material for hot rolling described below.

(1) A titanium material for hot rolling, comprising: a base material made of industrial pure titanium or a titanium alloy; and a rolled surface formed on at least one of the base materials and having a chemical composition different from the base material. The surface layer portion; the thickness of the surface layer portion is 2.0 to 20.0 mm, and the proportion of the total thickness is 40% or less on each side. The chemical composition of the surface layer portion as an increased content relative to the base material is selected from the group consisting of One or more of Mo, V, and Nb, and the Mo equivalent calculated according to the following formula (1) is 8.0 to 20.0. When the element content contained in the surface layer portion is measured at a plurality of points, the increase relative to the base material is increased. C _AVE average sum of the contents of the measurement site for each phase to increase the content C ₀ of a base material _{relationship: | C AVE -C 0 | /} C AVE × 100 is 40 or less, Mo eq = Mo content (mass%) + V Content (mass%) / 1.5 + Nb content (mass%) / 3.6 (1).

(2) In the titanium material for hot rolling of the above (1), another surface layer portion is formed on a surface other than the rolled surface of the base material, and the other surface layer portion has the same chemical composition as the surface layer portion and Metal organization.

The titanium material for hot rolling of the present invention includes a base material composed of industrial pure titanium or a titanium alloy, and a surface layer portion having a chemical composition different from that of the base material. The entire titanium material is made of the same titanium alloy and has the same hydrogen embrittlement resistance, but can be manufactured at low cost.

1‧‧‧Titanium for hot rolling

1a, 1aa, 1ab‧‧‧‧Surface Department

1b‧‧‧Base material

2‧‧‧ titanium composite

3, 4‧‧‧ surface layer (surface layer)

5‧‧‧ inner layer

FIG. 1 is an explanatory diagram showing an example of the structure of the titanium material for hot rolling of the present invention.

FIG. 2 is an explanatory view showing another example of the structure of the titanium material for hot rolling of the present invention.

FIG. 3 is an explanatory diagram showing an example of the structure of the titanium composite material of the present invention.

FIG. 4 is an explanatory diagram showing an example of the structure of the titanium composite material of the present invention.

FIG. 5 is an explanatory diagram showing a melting and re-solidification method.

FIG. 6 is an explanatory diagram showing a melting and re-solidification method.

FIG. 7 is an explanatory diagram showing a melting and re-solidification method.

FIG. 8 is an explanatory view schematically showing bonding of a titanium rectangular cast piece (flat embryo) and a titanium plate by welding in a vacuum.

FIG. 9 is an explanatory diagram schematically showing that a titanium plate is bonded by welding not only on the surface of a titanium rectangular cast sheet (flat embryo) but also on the side surface.

The hot-rolled titanium material according to the present invention is a material for hot working (slabs such as flat embryos, mesoderms, small embryos, etc.). After hot working, cold working, heat treatment, and the like are performed as necessary to form a titanium composite material. Hereinafter, the titanium material for hot rolling of this invention is demonstrated using drawing. In the following description, "%" related to the content of each element means "mass%".

1. Hot rolled titanium

1-1. Overall composition

As shown in FIG. 1, the titanium material 1 for hot rolling of the present invention includes a base material 1 b and a surface layer portion 1 a formed on a rolled surface of the base material 1 b. In addition, the surface layer portion includes a predetermined intermediate layer (not shown). The base material 1b is made of industrial pure titanium or a titanium alloy, and the surface layer portion 1a has a chemical composition different from that of the base material 1b. As shown in FIG. 2, the hot-rolled titanium material 1 of the present invention may include surface layer portions 1 aa and 1 ab on both rolled surfaces of the base material 1 b. As such, the hydrogen embrittlement resistance of the hot-rolled titanium material 1 is ensured by the surface layer portion 1a (examples shown in FIG. 2 are 1aa and 1ab) in contact with the external environment. The The titanium material for hot rolling 1 has the same hydrogen embrittlement resistance as compared to a titanium material composed of the same titanium alloy as a whole, but can be produced at low cost.

The size in the case where the titanium material for hot rolling is a rectangular titanium cast piece is not particularly limited as long as it is a size that can be hot rolled as it is. For hot rolling, coil rolling is used to produce hot rolled coil thin and medium plates with a plate thickness of about 3 ~ 8mm. As a rectangular titanium cast sheet, the thickness can be about 50 ~ 300mm, the length is about 3000 ~ 10000m, and the width About 600 ~ 1500mm.

If the thickness of the surface layer portion is too thin, the thickness of the surface layer of the final product also becomes thin, and desired characteristics cannot be sufficiently obtained. On the other hand, if it is too thick, the cost advantage is reduced because the proportion of the titanium alloy in the total titanium composite material increases. Therefore, the thickness of the surface layer portion is set to 2.0 to 20.0 mm. The ratio of the thickness of the surface layer portion to the total thickness is 40% or less on each side.

1-2. Base material

The base material 1 is made of industrial pure titanium or a titanium alloy. Among these, by using a titanium alloy, more excellent mechanical properties (strength, ductility, etc.) than when using industrial pure titanium are obtained.

As the base material 1, JIS 1 to 4 grade industrial pure titanium among pure titanium specified by JIS can be used. That is, it contains 0.1% or less of C, 0.015% or less of H, 0.4% or less of O, 0.07% or less of N, and 0.5% or less of Fe, and the rest is Ti for industrial pure titanium. As long as JIS1 ~ 4 grade industrial pure titanium is used, it is a titanium material that has sufficient processability without cracking and can be integrated with the surface titanium alloy after hot working.

As the base material 1, an α-type, an α + β-type, and a β-type titanium alloy can be used.

Here, examples of the α-type titanium alloy include Ti-0.5Cu, Ti-1.0Cu, Ti-1.0Cu-0.5Nb, Ti-1.0Cu-1.0Sn-0.3Si-0.25Nb, and Ti-0.5Al-0.45. Si, Ti-0.9Al-0.35Si, Ti-3Al-2.5V, Ti-5Al-2.5Sn, Ti-6Al-2Sn-4Zr-2Mo, Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si, etc.

Examples of the α + β-type titanium alloy include Ti-6Al-4V, Ti-6Al-6V-2Sn, Ti-6Al-7V, Ti-3Al-5V, and Ti-5Al-2Sn-2Zr-4Mo-4Cr. , Ti-6Al-2Sn-4Zr-6Mo, Ti-1Fe-0.35O, Ti-1.5Fe-0.5O, Ti-5Al-1Fe, Ti-5Al-1Fe-0.3Si, Ti-5Al-2Fe, Ti-5Al -2Fe-0.3Si, Ti-5Al-2Fe-3Mo, Ti-4.5Al-2Fe-2V-3Mo, etc.

Examples of the β-type titanium alloy include Ti-11.5Mo-6Zr-4.5Sn, Ti-8V-3Al-6Cr-4Mo-4Zr, Ti-10V-2Fe-3Mo, Ti-13V-11Cr-3Al, Ti-15V-3Al-3Cr-3Sn, Ti-6.8Mo-4.5Fe-1.5Al, Ti-20V-4Al-1Sn, Ti-22V-4Al, etc.

The base material can be produced by a known production method such as a melting method and a powder metallurgy method, and is not particularly limited. For example, the base material can be manufactured by cutting ingots into flat embryos and small embryos by cutting and finishing. In the case of being manufactured by open-cutting, since the surface is made flat by the open-cutting, the materials containing alloy elements are easily dispersed more uniformly, and it is easy to make the element distribution of the alloy phase uniform.

On the other hand, it is also possible to use a cast directly produced during casting. Block as base material. In this case, since the cutting finishing step can be omitted, it can be manufactured at a lower cost. In addition, if the surface of the ingot is subjected to cutting finishing after being manufactured, the same effect can be expected as in the case of manufacturing by incision.

1-3. Surface layer department

(chemical composition)

In order to improve the hydrogen absorption resistance of at least one surface layer (at least a surface layer in contact with the external environment) of the titanium composite material produced by the titanium material for hot rolling of the present invention, the surface layer portion of the titanium material for hot rolling may contain the following various alloy element.

Mo equivalent: 8.0 ~ 20.0

Among them, Mo equivalent = Mo content (mass%) + V content (mass%) / 1.5 + Nb content (mass%) / 3.6.

The layer having hydrogen absorption resistance is a titanium alloy layer containing a certain range of β-stabilizing elements. The reason for the formation of the β phase is that the α phase of titanium can form hydrides even at a hydrogen concentration of 10 ppm in trace amounts. In contrast, the β phase of titanium alloys can dissolve about 1000 ppm or more of hydrogen and have a cause. It is a feature that hydrogen embrittlement does not easily occur.

When a eutectoid β stabilizing element containing Fe, Cr, or the like is contained, titanium may form a compound with these elements and cause embrittlement. However, when Mo, V, and Nb are contained in the β-stabilizing element in a range of Mo equivalent: 8.0 to 20.0, even if Fe and Cr are present at the same time, the β phase can be stabilized without forming a compound phase. Therefore, Embrittlement will occur.

Here, the lower limit of the Mo equivalent is the amount of alloy required to obtain a sufficient amount of β phase. The upper limit is because the titanium alloy is more expensive as the amount of alloy added is set to an amount that is not suitable for use based on cost.

The remainder other than the above is titanium and impurities. As impurities, they can be contained within a range that does not hinder the target characteristics. Other impurities are mainly impurity elements mixed in from waste, that is, Ta, Si, Mn, Cu, etc., plus common impurity elements such as C, N, Fe, O and H, the total amount of which is 5% or less is allowable.

2. Titanium composite material

The titanium material for hot rolling of the present invention is a material for hot working (slabs such as flat slabs, meso slabs, and small slabs). After hot working, cold working, heat treatment, and the like are performed as necessary to form a titanium composite material. In addition, the titanium composite material system includes an inner layer derived from the base material of the titanium material for hot rolling of the present invention, and a surface layer derived from the surface layer portion.

(thickness)

If the thickness of the surface layer in contact with the external environment is too thin, sufficient hydrogen absorption resistance cannot be obtained. On the other hand, when the surface layer is thick, although the hydrogen absorption resistance is not a problem, the ratio of the surface layer to the entire material increases, which increases the manufacturing cost. Therefore, the ratio of the thickness of the surface layer to the total thickness of the titanium composite (surface layer occupancy) is set to 2 to 20% on each side.

The thickness of the surface layer depends on the thickness of the surface layer portion 1a and the processing rate at the time of thermal processing performed thereafter.

(Voidage)

The porosity of the surface layer is preferably 0.1% or less. When the porosity exceeds 0.1%, when hot rolling is performed, swelling, peeling, etc. of the surface layer may occur.

The porosity can be measured by observing the cross section of the material with an optical microscope, and then easily processing the photo by image processing. Observe any 10 to 20 locations on the cross section, measure the porosity, and take the average as the overall porosity. The porosity of the material after hot rolling or cold rolling is the same as that of the titanium material for hot rolling.

(Segregation)

When measuring the content of the elements contained in the surface layer portion at a plurality of points, the relationship between the average value of the increase content C _AVE relative to the base material and the increase content C ₀ relative to the base material at each measurement site: | C _AVE- C ₀ | / C _AVE × 100 is 40 or less. The reason is that when | C _AVE -C ₀ | / C _AVE × 100 exceeds 40, the desired performance cannot be fully exhibited, or the desired performance is deteriorated earlier. | C _AVE -C ₀ | / C _AVE × 100 is preferably 20 or less.

Specific elements in the surface layer can be measured using EPMA or GDS. Specifically, any 10 to 20 locations in the surface layer portion can be measured, and the average value of the increase content of each measurement location relative to the base material is taken as the increase content C _{0 of} each measurement location, and the average value of the increase content C ₀ is taken as The average value of the increase in the surface layer part C _AVE .

(middle layer)

The surface layer has an intermediate layer near the inner layer. That is, the titanium material for hot rolling of the present invention is provided on the surface of the base material with, for example, a surface layer portion formed by a melt re-solidification treatment, and the surface layer portion is subjected to a heat treatment step during subsequent hot rolling heating and after cold rolling. Diffusion occurs at the interface between the base material and the surface layer portion. When the titanium composite material is finally finished, an intermediate layer is formed between the inner layer from the base material and the surface layer from the surface layer portion. This intermediate layer allows the inner layer and the surface layer to be strongly bonded by metal bonding. In addition, a continuous elemental gradient is generated in the intermediate layer, which can reduce the difference in strength between the inner layer and the surface layer, and can suppress cracks during processing. The thickness of the intermediate layer is preferably 0.5 μm or more.

The thickness of the intermediate layer can be measured using EPMA or GDS. More detailed measurements can be made using GDS. In the case of GDS, the surface layer can be polished to a certain extent, and then GDS analysis can be performed from the surface to the depth direction to measure the thickness of the intermediate layer. The intermediate layer refers to an increase in content relative to the base material (in the case of an element not included in the base material, the content of the element; in the case of an element contained in the base material, the content relative to the base material increases Amount) is set to C _MID , and when the average of the increase amount of the surface layer portion is set to C _AVE , the area is 0 <C _MID ≦ 0.8 × C _AVE .

3. Manufacturing method of titanium for hot rolling

3-1. Formation of surface layer portion by melting and re-solidification

The titanium material for hot rolling of the present invention melts a surface layer of a base material and melts a specific alloy element to solidify with a component derived from the base material, thereby forming a surface layer portion containing the specific alloy element in the base material. Figure 5 ~ 7 It is explanatory drawing which shows the method of melting and re-solidification.

As a method for melting and solidifying the surface of the base material of the hot rolling titanium material, laser heating, plasma heating, induction heating, electron beam heating, and the like may be used, and any method may be used. In particular, in the case of electron beam heating, since it is performed in a high vacuum, even if voids are formed in the layer during the melting and re-solidification treatment, it is a vacuum and can be crimped by subsequent rolling. Make it harmless.

Furthermore, because of its high energy efficiency, it can be melted even if it is processed over a large area, so it is particularly suitable for the manufacture of titanium composite materials. The degree of vacuum in the case of melting in a vacuum is preferably a higher degree of vacuum below 3 × 10 ^-3 Torr. In addition, the number of times of melting and re-solidifying the surface layer of the titanium material for hot rolling is not particularly limited, and the number of times can be increased as needed, as long as the thickness of the alloy layer and the amount of the added elements in the surface layer portion of the material are within the above-mentioned ranges Just fine. However, the larger the number of times, the longer the processing time and the higher the cost. Therefore, it is preferably 1 to 2 times.

The method of melting and solidifying the surface layer is performed as shown in FIG. 5 in the case of a rectangular flat embryo. In other words, the outer surface of the rectangular flat embryo 10 is irradiated with electron beams on the wide two surfaces 10A, 10B that become at least the rolling surface (the surface in contact with the hot roller) in the hot rolling step, and only the surface layer of the surface is irradiated. Melting. Here, one of the two surfaces 10A and 10B is first implemented 10A.

Here, as shown in FIG. 5, generally, the area of an electron beam irradiation area 14 of the electron beam irradiation gun 12 on the surface 10A of the rectangular slab 10 is compared with the total area of the surface 10A to be irradiated. very much In fact, generally, the electron beam irradiation gun 12 is continuously moved, or the rectangular cast sheet 10 is continuously moved, and the electron beam irradiation is generally performed. The irradiation area is to adjust the focus of the electron beam, or use an electromagnetic lens to make small rays vibrate (oscillation) to form a beam, thereby adjusting the shape and area.

Further, as shown by an arrow A in FIG. 5, the following description will be made in a form in which the electron beam irradiation gun 12 is continuously moved. The moving direction of the electron beam irradiation gun is not particularly limited, and generally it is continuously moved along the length direction (usually the casting direction D) or the width direction (usually the direction perpendicular to the casting direction D) of the rectangular cast piece 10 to The width W (diameter W in the case of a circular ray or a beam) of the aforementioned irradiation region 14 is continuously irradiated in a band shape. Furthermore, in the unirradiated strip-shaped area adjacent thereto, the electron beam is irradiated in a strip shape while the irradiation gun 12 is continuously moved in the opposite direction (or the same direction). Depending on the situation, electron beam irradiation may be performed on a plurality of areas at the same time by using a plurality of irradiation guns. FIG. 5 shows a case where rectangular rays are continuously moved along the longitudinal direction of the rectangular cast sheet 10 (usually the casting direction D).

The surface (surface 10A) of the rectangular titanium casting sheet 10 is irradiated with an electron beam by such a surface heat treatment step, and the surface of the rectangular titanium casting sheet 10 is heated to melt, as shown in the center to the left of FIG. 6, the rectangular titanium casting sheet The surface layer of the 10 face 10A will form a maximum melt at a depth corresponding to the amount of heat input. However, the depth in the direction perpendicular to the irradiation direction of the electron beam is not necessarily as shown in FIG. 7 but becomes a downwardly curved shape, that is, the central part of the electron beam irradiation is the deepest, and the closer to the end of the band, the The smaller the thickness.

In a region closer to the inner side of the slab than the molten layer 16, the temperature rises due to the influence of the heat irradiated by the electron beam, and the portion (heat-affected layer = HAZ layer) at a temperature higher than the β transformation point of pure titanium is deformed as β-phase. In such a surface layer heat treatment step, the region transformed into the β phase is also affected by the heat of the electron beam irradiation, and has a downwardly convex curved shape similar to the shape of the molten layer 16.

The surface layer of the material for hot rolling is alloyed by melting and re-solidifying with the material composed of the intended alloying element. As the material to be used at this time, one or more of powders, chips, wires, films, chips, and nets can be used. The composition and amount of the material placed before melting are set so that the component in the element-concentrated region after being melt-solidified with the surface of the material becomes the target component.

However, if the added material is too large, it may cause segregation of the alloy composition. Furthermore, if segregation of an alloy component exists, the desired performance cannot be fully exhibited, or deterioration may be caused early. Therefore, it is important for the alloy material to have a size at which melting is completed while the heated portion on the surface of the titanium base material is in a molten state. In addition, it is important to uniformly arrange the above-mentioned alloy material on the surface of the titanium base material in consideration of the shape and width of the molten portion at a specific time. However, in the case where the irradiation position is continuously moved using an electron beam, the molten portion is continuously moved and stirred together with the molten titanium and the alloy, and it is not necessary to continuously arrange the alloy material. In addition, it is of course necessary to avoid the use of alloy materials having a melting point much higher than the melting point of titanium.

After the melting and re-solidification treatment, it can be The temperature of 500 ° C was maintained for more than 1 hour. After melting and re-solidification, if it is rapidly cooled, the strain during solidification may cause fine cracks in the surface layer portion. In the subsequent hot-rolling step and cold-rolling step, this fine cracking becomes a starting point, and peeling of the surface layer portion may occur, and a thinner portion of the alloy layer may be caused in some parts to cause deterioration of characteristics. In addition, if internal oxidation is caused by fine cracks, it must be removed through a pickling step to further reduce the thickness of the alloy layer. By maintaining at the above-mentioned temperature, fine cracks on the surface can be suppressed. In addition, as long as it is maintained at this temperature, atmospheric oxidation hardly occurs even if it is maintained in the atmosphere.

A titanium material for hot rolling provided with a surface layer portion formed by melting and re-solidifying treatment on the surface of the base material is generated at the interface between the base material and the surface layer portion during the subsequent hot rolling and heating treatment steps after cold rolling. Diffusion, when the titanium composite material is finally finished, a concentration gradient of a specific element is generated between the inner layer from the base material and the surface layer from the surface layer portion to form an intermediate layer. Therefore, the intermediate layer allows the inner layer and the surface layer to be strongly bonded by metal bonding. In addition, because a continuous elemental gradient occurs in the intermediate layer, the difference in strength between the inner layer and the surface layer can be reduced, and cracks during processing can be suppressed.

In addition, in the case of alloying by the melting and re-solidification treatment, the shape of the molten portion is curved as described above, and the shape will continue in the final product. In addition, during hot rolling heating, heat treatment after hot rolling, heat treatment after cold rolling, etc., alloy elements diffuse and join from the interface with the base material after bending, and the diffusion direction of the elements is not only the depth direction, Diffusion occurs even in the width direction. Therefore, in the base material and alloy layer The gradient of the alloy element in the middle part is generated not only in the depth direction but also in the width direction. Therefore, for example, when elements with different solid solution strengthening capabilities are added, the strength difference occurs not only in the direction perpendicular to the depth direction but also in the direction parallel to the depth direction. Therefore, the concentration gradient becomes complicated and the strength difference is caused. Cracking becomes difficult to occur.

On the surface layer portion formed by melting and solidifying the surface of the base material, a titanium plate containing a predetermined alloy component can be further attached to produce a titanium material for hot rolling.

FIG. 8 is an explanatory view schematically showing that a titanium rectangular cast piece (flat embryo) 6 and a titanium plate 7 which are formed by melting and re-solidifying the surface of a base material to form a surface layer portion are bonded together by vacuum welding. FIG. 9 is an explanatory view schematically showing that not only the surface of the titanium rectangular cast sheet (flat embryo) 6 but also the titanium plates 7 and 8 are welded and bonded to the side surface. In the following description, a titanium rectangular cast piece (flat slab) 6 formed by melting and solidifying the base material surface to form a surface layer portion is referred to as "titanium flat slab 6".

As shown in FIGS. 8 and 9, titanium plates 7 and 8 containing alloy elements exhibiting characteristics are bonded to the surface layer of the titanium flat embryo 6 and then joined by a hot-roll coating method to bond the surface layer 3 of the titanium composite material. , 4 alloying. That is, after bonding the titanium plate 7 containing alloy elements to the surface of the titanium flat embryo 6 as a rolled surface, it is preferable to weld at least the surrounding by the welding portion 9 in a vacuum container, thereby The titanium flat blank 6 and the titanium plate 7 are sealed in a vacuum, and the titanium flat blank 6 and the titanium plate 7 are bonded together by rolling. It is used to weld the titanium flat embryo 6 and the titanium plate 7 together. In order to avoid the intrusion of the atmosphere between the titanium flat embryo 6 and the titanium plate 7, for example, the entire circumference is welded as shown in Figs.

Titanium is an active metal. If it is placed in the atmosphere, it will form a strong passive film on the surface. It is impossible to remove the oxidized-concentrated layer on this surface portion. However, unlike stainless steel, oxygen is easily dissolved in titanium. If it is closed in a vacuum and heated without supplying oxygen from the outside, oxygen on the surface will diffuse into the inside to generate a solid solution, so it is formed on the surface. The blunt state membrane will disappear. Therefore, the titanium flat embryo 6 and the titanium plate 7 on the surface thereof can be completely adhered by the hot-roll coating method so that no inclusions or the like are generated therebetween.

Furthermore, if a flat slab cast as such is used as the titanium flat slab 6, due to the coarse crystal grains generated during solidification, surface defects will occur in the subsequent hot rolling step. On the other hand, if the titanium plate 7 is bonded to the rolled surface of the titanium flat embryo 6 as in the present invention, the bonded titanium plate 7 has a fine structure and can suppress surface defects in the hot rolling step.

In the case of manufacturing the titanium composite material shown in FIG. 1, as shown in FIG. 8, it is preferable that the titanium plate 7 is bonded in a vacuum only on one side of the titanium flat embryo 6, and the other side of the titanium flat embryo 6 is not. The titanium plate 7 is bonded and hot-rolled.

As shown in FIG. 9, the titanium plate 7 may be bonded to not only one side of the titanium flat embryo 6 but also both sides. In this way, the occurrence of hot-rolled defects in the hot-rolling step as described above can be suppressed. In the hot rolling, the titanium flat blank 6 is usually pressed down, and at least a part of the side surface of the titanium flat blank 6 is wrapped toward the surface side of the hot-rolled sheet. Therefore, if the surface structure of the side surface of the titanium flat embryo 6 is large or there are many defects, surface defects may occur on the surfaces near the both ends in the width direction of the hot-rolled sheet. Therefore, as shown in FIG. 9, it is preferable that the side surface of the titanium flat blank 6 which becomes the edge side during hot rolling is also the same as the rolled surface. The titanium plates 8 of the same specification are welded together. In this way, it is possible to effectively prevent surface defects from occurring on the surfaces near the both ends in the width direction of the hot-rolled sheet. The welding is preferably performed in a vacuum.

During hot rolling, although the amount of wrapping on the side of the titanium flat embryo 6 is different according to the manufacturing method, it is usually about 20 ~ 30mm. Therefore, it is not necessary to fully adhere the titanium plate 8 on the side of the titanium flat embryo 6, only The portion corresponding to the wrapping amount according to the manufacturing method may be bonded to the titanium plate 8. By performing high-temperature and long-term annealing after hot rolling, components from the base material can be allowed to enter the inside of the titanium composite material. For example, heat treatment at 700 to 900 ° C. for 30 hours can be exemplified.

The method for welding the titanium flat embryo 6 and the titanium plates 7 and 8 includes electron beam welding, plasma welding and the like. In particular, electron beam welding is desirable because it can be performed under high vacuum, and it is possible to achieve high vacuum between the titanium flat embryo 6 and the titanium plates 7 and 8. The vacuum degree when the titanium plates 7 and 8 are welded in a vacuum is preferably a higher vacuum degree of 3 × 10 ^-3 Torr or less.

The welding of the titanium flat blank 6 and the titanium plate 7 does not necessarily need to be performed in a vacuum container. For example, a vacuum suction hole may be provided inside the titanium plate 7, and the titanium plate 7 and the titanium flat blank 6 are overlapped and then used. The vacuum suction hole vacuum-tightens the titanium flat embryo 6 and the titanium plate 7 while welding the titanium flat embryo 6 and the titanium plate 7, and the vacuum suction hole is closed after welding.

3-2. Base material of hot rolled titanium

The base material of the hot rolling titanium material is usually manufactured by cutting ingots into a flat embryo and a small embryo shape by opening and then cutting and finishing. In addition, in recent years, rectangular flat embryos that can be directly hot-rolled during ingot manufacturing have also been supplied. The case of hot rolling. When it is produced by open-cutting, since the surface is made flat by the open-cutting, the material containing alloy elements is easily spread more uniformly, and the element distribution of the alloy phase is easy to be uniform.

On the other hand, in the case of using an ingot which is directly manufactured into the shape of a hot rolling material as a material during casting, the cutting and finishing step can be omitted, and it can be manufactured at a lower cost. In addition, after the ingot is manufactured, the surface can be used after cutting and finishing, and the same effect can be expected as in the case of manufacturing through the open-cutting. In the present invention, as long as a stable alloy layer is formed on the surface layer, an appropriate material can be selected according to the situation. Therefore, the base material is not particularly limited.

For example, after assembling the flat embryo and welding the surroundings, it is preferably heated at 700 to 850 ° C for 10 to 30% of the joint rolling, and then heated at the β zone temperature for 3 to 10 hours to allow the base material components to diffuse to the surface layer. After rolling, hot rolling is performed. Hot rolling due to the β zone temperature reduces the deformation resistance and facilitates rolling.

4. Manufacturing method of titanium composite material

It is important to leave the alloy layer formed by the melt-resolidification treatment as the final product, and it is necessary to minimize the surface layer removal caused by rust loss and surface defects. Specifically, in consideration of the characteristics and capabilities of the equipment used in production, the following hot rolling steps are optimized and appropriately adopted to achieve this.

4-1. Heating step

When heating the hot rolling material, although the scale loss can be suppressed by heating at a low temperature for a short time, the thermal conductivity of the titanium material is low. If the hot rolling is performed at a low temperature inside the flat embryo, cracks are likely to occur inside, Therefore, the performance of the heating furnace used is optimized to minimize the occurrence of scale.

4-2. Hot rolling steps

Also in the hot rolling step, if the surface temperature is too high, a large amount of scale will be generated when the plate is passed through, and the scale loss will increase. On the other hand, if the temperature is too low, although the scale loss is small, surface defects are liable to occur and must be removed by pickling in a subsequent step. Therefore, it is desirable to perform hot rolling in a temperature range where surface defects can be suppressed. Therefore, it is advisable to perform rolling in an optimum temperature region. In addition, because the surface temperature of the titanium material is reduced during rolling, it is desirable to suppress the roll cooling during the rolling to a minimum, and to suppress the decrease of the surface temperature of the titanium material.

4-3. Pickling steps

Since the hot-rolled sheet has an oxide layer on the surface, the subsequent step includes a rust removal step to remove the oxide layer. Generally speaking, titanium is mainly used after bead blasting by using nitric acid. The oxide layer is removed by pickling the hydrofluoric acid solution. In addition, depending on the case, the surface may be ground with a grindstone after pickling. After the rust removal, the two-layer or three-layer structure composed of the inner layer and the surface layer from the base material and the surface layer portion of the hot rolling titanium material may be used.

Because the scale produced during the hot rolling step is thicker, The pre-treatment for the pickling treatment is to perform a bead treatment to remove a part of the surface rust scale and allow cracks to form on the surface. The subsequent pickling step allows the pickling solution to penetrate the cracks, and the part containing the base material is also Was removed. At this time, it is important to perform a weak beading treatment so as not to generate cracks on the surface of the base material, and it is necessary to select an optimal beading condition according to the chemical composition of the surface of the titanium material. Specifically, for example, the selection of a suitable projection material and the projection speed (which can be adjusted by using the rotation speed of the impeller) are optimized to select conditions under which cracks do not occur in the base material. The optimization of these conditions varies depending on the characteristics of the molten re-solidified layer formed on the surface of the titanium material, as long as the optimal conditions are determined separately in advance.

Hereinafter, the present invention will be described more specifically based on examples, but the present invention is not limited to these examples.

[Example 1]

In order to produce a titanium composite material 2 with a surface layer 3, 4 (containing a predetermined alloy), a flat blank as a base material is used: a Ti-5Al-1Fe alloy manufactured by vacuum arc melting is hot-forged and then subjected to cutting Production of flat embryos with a thickness of 28 ~ 143mm. The chemical composition of the titanium ingot in this embodiment is Al: 5%, Fe: 1%, and O: 0.15%. Using this titanium flat embryo, mo, V, and Nb alloy element materials are spread on the surface of the flat embryo to melt and solidify, thereby forming a region (alloy layer) in which solid alloy elements are solid-dissolved on the surface of the flat embryo at a depth of 1 to 15 mm. .

The flat slab was heated to 950 ° C, hot-rolled to a thickness of 5 mm, and then heat-treated at 700 ° C for 2 hours in the atmosphere. Next, use beads and nitric acid. Hydrofluoric acid removes rust on both the front and back surfaces.

Except for the present invention example, a titanium flat blank having no surface layer portion was similarly subjected to the steps of hot rolling, heat treatment, and rust removal to prepare a comparative example.

Each of the manufactured titanium plates was exposed to a hydrogen absorption environment, that is, 1 vol% H ₂ +99 vol% Ar atmosphere at 500 ° C. for 5 hours.

The alloy element concentration of the surface layers 3 and 4 of each titanium plate is an average value of the results of line analysis using EPMA from the surface to the lower end of the alloy-concentrated portion. The remaining parts, except for the impurity components such as O and C, are components contained in industrial pure titanium.

A V-notch impact test piece having a plate thickness (4.8 to 5.0 mm) × 10 mm × 55 mm and 2 mm was produced from each titanium plate. The longitudinal direction of the test piece is the rolling direction, and the notch direction is the plate thickness penetration direction. The hydrogen embrittlement is evaluated by using an impact value.

The titanium plate on which the molten layer was not formed on the surface layer was not exposed to a hydrogen environment, and the impact value was evaluated as 20 J / cm ² . A case where 14 J / cm ² or less, which is 30% or more lower than this value, was used as a criterion for hydrogen embrittlement and was judged to be unacceptable.

The results are summarized in Table 1.

The surface layer contains elements derived from the flat embryo (base material). However, in the "Composition of the surface layer part" in the table, the elements that are not included in the flat embryo are shown in the content; the elements that are also contained in the flat embryo are shown in the increased content when the content is not increased. In the case of "," it is indicated by "-".

No. 1 is a case where the surface layer portion (alloy-enriched layer) is not formed, and the impact value after exposure to a hydrogen environment is ¹³ J / cm ^{2 which} is lower.

Although the thickness of the surface layer portion of No. 2 is within the range of the present invention, the Mo equivalent is low, and the impact value after exposure to a hydrogen environment is also low.

Although the Mo equivalent of No. 3 is within the range of the present invention, the segregation of the surface layer portion is outside the range of the present invention, and the impact value after exposure to the hydrogen environment is low.

The thickness and Mo equivalent of the surface layer portions of Nos. 4 to 8 both satisfy the scope of the present invention, and the impact value is as high as ¹⁶ J / cm ² or more.

Mo equivalents of Nos. 9, 10, and 15 are outside the scope of the present invention.

Nos. 11 to 14 are within the scope of the present invention, and the impact value after exposure to a hydrogen environment is high.

[Example 2]

As the flat blank of the base material, a 70-mm-thick Ti-6Al-4V produced by the steps of vacuum arc melting, forging, and cutting was used. Using this titanium flat embryo, mo, V, and Nb alloy element materials are spread on the surface of the flat embryo to melt and solidify, thereby forming a region (surface layer portion) in which solid alloy elements are solid-dissolved on the surface of the flat embryo at a depth of 3 to 10 mm. .

The flat slab was heated to 950 ° C, hot-rolled to a thickness of 5 mm, and then heat-treated at 700 ° C for 2 hours in the atmosphere, followed by beading and nitric acid. Hydrofluoric acid removes rust on both the front and back surfaces.

Except for the present invention example, a titanium flat blank having no surface layer portion was similarly subjected to the steps of hot rolling, heat treatment, and rust removal to prepare a comparative example.

As in Example 1, the impact value was used to evaluate hydrogen embrittlement. The titanium plate on which the molten layer was not formed on the surface layer was not exposed to a hydrogen environment, and the impact value was evaluated as 20 J / cm ² . A case where 14 J / cm ² or less, which is 30% or more lower than this value, was used as a criterion for hydrogen embrittlement and was judged to be unacceptable.

The results are summarized in Table 2.

The surface layer contains elements derived from the flat embryo (base material). However, in the "Composition of the surface layer part" in the table, the elements that are not included in the flat embryo are shown in the content; the elements that are also contained in the flat embryo are shown in the increased content when the content is not increased. In the case of "," it is indicated by "-".

No. 1 is a case where the surface layer portion (alloy-enriched layer) is not formed, and the impact value after exposure to a hydrogen environment is lower 14 J / cm ² .

The thickness and Mo equivalent of the surface layer portions of Nos. 2 to 4 are within the range of the present invention, and the impact value after exposure to the hydrogen environment is still as high as 15 J / cm ² or more.

[Example 3]

As the flat metal of the base material, a pure titanium grade 70mm thick grade manufactured by the steps of vacuum arc melting, forging, and cutting was used. Using this titanium flat embryo, the alloy element material of Mo was dispersed on the surface of the flat embryo and melted and solidified, thereby forming a region (surface layer portion) in which solid alloy elements were solid-dissolved on the entire surface of the flat embryo at a depth of 3 to 10 mm.

The flat slab was heated to 850 ° C and hot-rolled to a thickness of 4.8 to 5.0 mm, and then annealed in a vacuum atmosphere at 600 to 650 ° C for 4 to 10 hours. Next, use beads and nitric acid. Hydrofluoric acid removes rust on both the front and back surfaces.

Each of the manufactured titanium plates was exposed to a hydrogen absorbing environment, that is, a 1% by volume H _{2 +} 99% by volume Ar atmosphere at 500 ° C. for 5 hours.

The concentration of alloying elements in the surface layers 3 and 4 of each titanium plate is used EPMA is the average of the results of line analysis from the surface to the lower end of the alloy-enriched portion. The remaining parts, except for the impurity components such as O and C, are components contained in industrial pure titanium.

A V-notch impact test piece having a plate thickness (4.8 to 5.0 mm) × 10 mm × 55 mm and 2 mm was produced from each titanium plate. The longitudinal direction of the test piece is the rolling direction, and the notch direction is the plate thickness penetration direction. The hydrogen embrittlement is evaluated by using an impact value.

The titanium layer on which the surface layer did not form a molten layer was not exposed to a hydrogen environment, and the impact value was evaluated as 2.7 J / cm ² . A case where 1.9 J / cm ² or less, which is 30% or more lower than this value, was used as a criterion for hydrogen embrittlement and was judged to be unacceptable.

The results are summarized in Table 3.

The surface layer contains elements derived from the flat embryo (base material). However, in the "Composition of the surface layer part" in the table, the elements that are not included in the flat embryo are shown in the content; the elements that are also contained in the flat embryo are shown in the increased content when the content is not increased. In the case of "," it is indicated by "-".

The thickness and Mo equivalent level of the surface layer portions of Nos. 1 to 3 are within the scope of the present invention, and the impact value after being exposed to a hydrogen environment is still as high as 15 J / cm ² or more.

Claims (2)

A titanium material for hot rolling includes a base material composed of industrial pure titanium or a titanium alloy, and a surface layer portion formed on at least one of the rolled surfaces of the base material and having a chemical composition different from the base material. ; The thickness of the surface layer part is 2.0 to 20.0mm, and the proportion of the total thickness is 40% or less on each side. The chemical composition of the surface layer part, as an increased content relative to the base material, contains selected from Mo, V And one or more of Nb, and the Mo equivalent calculated according to the following formula (1) is 8.0 to 20.0. When the element content contained in the aforementioned surface layer portion is measured at a plurality of points, the average increase content relative to the base material The relationship between the value C _AVE and the increase content C ₀ of each measurement site relative to the base material: | C _AVE -C ₀ | / C _AVE × 100 is 40 or less, Mo equivalent = Mo content (mass%) + V content (mass %) / 1.5 + Nb content (% by mass) /3.6 (1). The hot-rolled titanium material according to item 1 of the scope of patent application, wherein other surface layer portions are formed on a surface other than the rolled surface of the base material, and the other surface layer portions are the same as the surface layer portion. Chemical composition and metal structure.