JP7769282B1 - Surface-treated steel - Google Patents

Abstract

This surface-treated steel material has a steel material and a plating layer containing Zn formed on at least a portion of the surface of the steel material. When the portion of the surface of the steel material where the plating layer is not formed is defined as the non-plated portion, a compound containing Zn and Mg is present in at least a portion of the non-plated portion, and at the center of a first region, which is the center between a first boundary and a second boundary in the thickness direction, the atomic ratio of Mg to Zn, Mg/Zn, is 0.090 or more, and at the center of a second region, which is the center between the second boundary and the surface of the compound in the thickness direction, the atomic ratio of Mg to Zn, Mg/Zn, is less than 0.090.

Description

The present disclosure relates to surface-treated steel products.
This application claims priority based on Japanese Patent Application No. 2024-080155, filed on May 16, 2024, the contents of which are incorporated herein by reference.

Zinc (Zn)-plated steel sheets are the most widely used surface-treated steel materials with good corrosion resistance. These zinc-plated steel sheets are used in various manufacturing industries, including the automotive, home appliance, and building materials fields. For example, in the building materials field, research has long been conducted to improve the corrosion resistance of zinc-plated steel sheets in response to the need for longer life for building materials.
Under these circumstances, studies have been conducted to improve corrosion resistance by adding Al or Mg to the zinc-based plating layer.
For example, Patent Documents 1 to 3 disclose plated steel materials that contain a certain amount of Al and Mg and achieve high corrosion resistance.

Japanese Patent Application Publication No. 2006-193791 International Publication No. 2011/001662 Japanese Patent Application Publication No. 2021-172878

The plated steel materials (zinc-based plated steel sheets) disclosed in the above Patent Documents 1 to 3 have excellent long-term flat corrosion resistance. However, when the plated steel materials are used, they may be cut to a predetermined size. In this case, the cut surfaces (cut end surfaces) are left without a plating layer. Furthermore, even on plated surfaces, there may be unplated areas, or areas where the plating layer is not formed (exposed steel sheet) due to scratches that cause the plating layer to peel off, or cracks that occur during cutting, punching, bending, drawing, or the like.
As a result of investigations by the present inventors, it was found that although the plated steel materials of Patent Documents 1 to 3 have excellent corrosion resistance in the plated portion, red rust may form in the early stages of corrosion on the cut end surfaces, unplated portions, and/or portions of the steel sheet exposed by scratches or processing after the plating layer is formed (collectively referred to as unplated portions) as described above. Therefore, there is a need for the development of technology that can suppress the formation of red rust in such unplated portions.

Taking the above background into consideration, the present disclosure aims to provide a surface-treated steel material, such as a surface-treated steel sheet, that has a plating layer containing Zn (Zn-based plating layer), in which the formation of red rust in non-plated areas is suppressed.

The inventors investigated methods for suppressing the formation of red rust in non-plated areas. As a result, they discovered that the formation of red rust can be suppressed by forming a compound consisting of two regions in the non-plated areas: a region with a high Mg concentration and a region with a high Zn concentration.

The present disclosure has been made in light of the above findings.
[1] A surface-treated steel material according to one aspect of the present disclosure is a surface-treated steel material comprising a steel material and a plating layer containing Zn formed on at least a portion of a surface of the steel material, and when a portion of the surface of the steel material on which the plating layer is not formed is defined as a non-plated portion, a compound containing Zn and Mg is present in at least a portion of the non-plated portion, and when concentration distributions of O, Mg, Al, Fe, and Zn are continuously measured by linear analysis in a thickness direction of the compound from the surface of the steel material in the non-plated portion toward the surface of the surface-treated steel material using an FE-TEM equipped with an energy dispersive X-ray analyzer, a position where the Mg concentration first becomes 0.5 atomic % or more is defined as a first boundary, and When the position on the surface side of the treated steel material where the Mg concentration first becomes less than 0.5 atomic % is defined as a second boundary, the area between the first boundary and the second boundary is a first region of the compound, and the area between the second boundary and the surface of the compound is a second region of the compound, and at the center of the first region, which is the center in the thickness direction between the first boundary and the second boundary, Mg/Zn, which is the atomic ratio of Mg to Zn, is 0.090 or more and the Mg concentration is 2.0 to 50.0 atomic %; and at the center of the second region, which is the center in the thickness direction between the second boundary and the surface of the compound, Mg/Zn, which is the atomic ratio of Mg to Zn , is less than 0.090 and the Zn concentration is 2.0 to 50.0 atomic % .
[2] In the surface-treated steel material described in [1], the chemical composition of the plating layer may include, in mass%, Zn: more than 50.0%, Al: more than 0.2% and less than 40.0%, and Mg: more than 0.2% and less than 12.5%.
[3] In the surface-treated steel material according to [1] or [2], the first region may exhibit a halo pattern in an electron beam diffraction pattern obtained by a transmission electron microscope.
[4] In the surface-treated steel material according to any one of [1] to [3], the compound may be present in an area ratio of 50% or more of the surface of the steel material in the non-plated portion.
[5] In the surface-treated steel material according to any one of [1] to [4], the thickness of the first region may be 0.05 μm or more.
[6] In the surface-treated steel material according to any one of [1] to [5], the second region may have a thickness of 0.05 μm or more.

According to the above aspect of the present disclosure, it is possible to provide a surface-treated steel material in which the formation of red rust in non-plated areas is suppressed.

1 is a schematic diagram showing an example of a surface-treated steel sheet, which is one form of a surface-treated steel material according to the present embodiment. FIG. 2 is a schematic diagram showing an example of a cross section at a position where a compound is formed on the surface of a non-plated portion of a surface-treated steel material according to the present embodiment. FIG. 1 is a diagram showing an example of a halo pattern of electron beam diffraction by FIB-TEM.

We will now explain the surface-treated steel material according to one embodiment of the present disclosure (the surface-treated steel material according to this embodiment).

As shown in FIG. 1 , the surface-treated steel material 1 according to this embodiment has a steel material 11 and a plating layer 12 containing Zn formed on at least a part of the surface of the steel material 11 .
Furthermore, in the surface-treated steel material 1 according to this embodiment, when the portion of the surface of the steel material 11 on which the plating layer 12 is not formed is defined as the non-plated portion 41, a compound 31 containing Zn and Mg is present in at least a portion of the non-plated portion 41.
1 , the surface of the surface 101 that comes into contact with the plating bath and on which the plating layer is formed is the plated surface 103, and the surface that is exposed when the steel material is cut to a predetermined size after being pulled up from the plating bath is the end surface 102. The end surface 102 is in a direction that intersects with the plated surface 103, and in many cases is in a direction that is approximately perpendicular to the plated surface 103.

<Steel>
The surface-treated steel material 1 according to this embodiment has a significant feature in the compound 31. Therefore, the steel material 11 is not particularly limited. The steel material 11 may be determined based on the product to which it is applied, the required strength, plate thickness, etc. For example, a hot-rolled steel plate (hot-rolled steel plate) described in JIS G 3131:2018, JIS G 3113:2018, etc., or a cold-rolled steel plate (cold-rolled steel plate) described in JIS G 3141:2021, JIS G 3135:2018, etc., can be used. As described above, the steel material can be a steel material other than a steel plate, such as a steel pipe, a steel wire, or various members made of steel.
Hereinafter, the surface-treated steel material according to this embodiment will be described as an example of a surface-treated steel plate (when the steel material is a steel plate).

<Plating layer>
The surface-treated steel material (surface-treated steel sheet) 1 according to this embodiment has a plating layer 12 containing Zn formed on at least a part of the surface of a steel material 11 .
In this embodiment, the Zn-based plating layer is a plating layer in which the concentration (content) of Zn is more than 50.0 mass %.
The plating layer 12 may be formed on the entire plating surface 103 (front and back surfaces (in the case of a cut plated steel sheet, this will often be the surface other than the end faces)) of the steel sheet 11 (100% in terms of area ratio), but there may also be areas (non-plated portions 41) where no plating is formed due to peeling caused by scratches or the like. The area ratio of the non-plated portions is preferably 10% or less of the entire plating surface (it may be 0%).
The plating layer 12 may also be formed on a portion of the surface 101 other than the plated surface 103 (end surface 102 in Figure 1), but in the case of a surface-treated steel sheet that has been cut to a predetermined size after the plating layer has been formed, the plating layer is often not formed on the end surface (cut end surface).

In the surface-treated steel sheet 1 according to this embodiment, the plating layer 12 is made a Zn-based plating layer and then subjected to a specific treatment, thereby suppressing the formation of red rust even in the non-plated portion 41 (improving red rust resistance).
The reason for this is not entirely clear, but it is thought that by performing a specific treatment described below, the Zn contained in the Zn-based plating layer and the Mg contained in the treatment solution in the non-plated portion 41 form a specific compound.
If the plating layer 12 is not a Zn-based plating layer, the effect of forming the compound 31 cannot be sufficiently obtained. Furthermore, even if the steel sheet 11 contains Mg, unless a specific treatment is performed, the amount of Mg contained in the steel sheet is very small and the amount of Mg eluted from the steel sheet is small, so the same effect cannot be obtained.
Furthermore, even if the Zn-based plating layer contains Mg, unless a specific treatment is carried out, the desired two-region structure is not formed, and sufficient effects cannot be obtained.

In the plating layer 12 (Zn-based plating layer), the concentrations (contents) of elements other than Zn are not limited. However, it is preferable that the chemical composition of the plating layer contains, by mass, more than 50.0% Zn, more than 0.2% but less than 40.0% Al, and more than 0.2% but less than 12.5% Mg.

Furthermore, the chemical composition of the plating layer may further include one or more of Sn, Bi, In, Ca, Y, La, Ce, Si, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Fe, Sr, Sb, Pb, and B, as necessary.
That is, the chemical composition of the plating layer is, in mass %, Al: more than 0.2% and less than 40.0%, Mg: more than 0.2% and less than 12.5%, Sn: 0% or more and 20.0% or less, Bi: 0% or more and less than 5.0%, In: 0% or more and less than 2.0%, Ca: 0% or more and 3.0% or less, Y: 0% or more and 0.50% or less, La: 0% or more and less than 0.50%, Ce: 0% or more and less than 0.50%, Si: 0% or more and less than 2.50%, Cr: 0% or more and less than 0.25%, Ti: 0% or more and 0.25% or less. %, Ni: 0% or more and less than 0.25%, Co: 0% or more and less than 0.25%, V: 0% or more and less than 0.25%, Nb: 0% or more and less than 0.25%, Cu: 0% or more and less than 0.25%, Mn: 0% or more and less than 0.25%, Fe: 0% or more and less than 5.00%, Sr: 0% or more and less than 0.50%, Sb: 0% or more and less than 0.50%, Pb: 0% or more and less than 0.50%, B: 0% or more and less than 0.50%, with the balance being more than 50.0% of Zn and impurities. In this case, excellent corrosion resistance can be obtained as a surface-treated steel sheet, including the portion where the plating layer is formed, and this is preferable.

The reasons for the preferred chemical composition of the plating layer 12 are explained below. Unless otherwise specified, the percentages for the concentration (content) of each element in the chemical composition of the plating layer are mass percentages.

[Al: more than 0.2% and less than 40.0%]
Al is an element effective in improving the corrosion resistance of a zinc-based coating layer. Therefore, Al may be contained. To fully obtain the above effect, the Al concentration is preferably more than 0.2%.
On the other hand, if the Al concentration is 40.0% or more, the sacrificial corrosion protection effect of the plating layer is reduced. Therefore, the Al concentration is preferably less than 40.0%, and more preferably less than 25.0%.

[Mg: more than 0.2% and less than 12.5%]
Mg is an element that has the effect of improving the corrosion resistance of the plating layer. To obtain the effect of improving corrosion resistance, the Mg concentration is preferably more than 0.2%. The Mg concentration is more preferably 1.0% or more, and even more preferably 3.0% or more.
On the other hand, if the Mg concentration is 12.5% or more, the effect of improving corrosion resistance will saturate and the workability of the coating layer may decrease. Furthermore, problems in manufacturing, such as an increase in the amount of dross generated in the coating bath, may occur. Therefore, it is preferable that the Mg concentration be less than 12.5%.

[Sn: 0% or more, 20.0% or less]
[Bi: 0% or more, less than 5.0%]
[In: 0% or more, less than 2.0%]
These elements contribute to improving corrosion resistance and sacrificial corrosion protection. Therefore, one or more of them may be contained. To obtain the above effects, the concentration of each element is preferably 0.05% or more, and more preferably 0.1% or more.
Of these, Sn is preferred because it is a low melting point metal and can be easily incorporated into the plating bath without impairing the properties of the plating bath.
On the other hand, if the Sn concentration exceeds 20.0%, the Bi concentration is 5.0% or more, or the In concentration is 2.0% or more, the corrosion resistance decreases. Therefore, it is preferable that the Sn concentration is 20.0% or less, the Bi concentration is less than 5.0%, and the In concentration is less than 2.0%, respectively.

[Ca: 0% or more, 3.0% or less]
Ca is an element that reduces the amount of dross that is likely to form during operation and contributes to improving the productivity of plating. Therefore, Ca may be contained. To obtain this effect, the Ca concentration is preferably 0.1% or more.
On the other hand, if the Ca concentration is high, the corrosion resistance of the flat surface of the plating layer itself tends to deteriorate, and the corrosion resistance around the weld may also deteriorate. Therefore, the Ca concentration is preferably 3.0% or less.

[Y: 0% or more, 0.50% or less]
[La: 0% or more and less than 0.50%]
[Ce: 0% or more, less than 0.50%]
Y, La, and Ce are elements that contribute to improving corrosion resistance. To obtain this effect, it is preferable to contain at least one of these elements in an amount of 0.05% or more, more preferably 0.10% or more.
On the other hand, if the concentrations of these elements are excessive, the viscosity of the coating bath increases, making it difficult to prepare the coating bath itself, and there is a concern that steel products with good coating properties cannot be produced. Therefore, it is preferable that the Y concentration be 0.50% or less, the La concentration be less than 0.50%, and the Ce concentration be less than 0.50%.

[Si: 0% or more and less than 2.50%]
Si is an element that contributes to improving corrosion resistance. Furthermore, when forming a plating layer on a steel sheet, Si is also an element that has the effect of suppressing the formation of an excessively thick alloy layer between the steel sheet surface and the plating layer, thereby enhancing the adhesion between the steel sheet and the plating layer. To obtain these effects, the Si concentration is preferably 0.10% or more. The Si concentration is more preferably 0.20% or more.
On the other hand, if the Si concentration is 2.50% or more, excessive Si precipitates in the plating layer, which not only reduces the corrosion resistance but also reduces the workability of the plating layer. Therefore, the Si concentration is preferably less than 2.50%. The Si concentration is more preferably 1.50% or less.

[Cr: 0% or more, less than 0.25%]
[Ti: 0% or more and less than 0.25%]
[Ni: 0% or more, less than 0.25%]
[Co: 0% or more, less than 0.25%]
[V: 0% or more and less than 0.25%]
[Nb: 0% or more, less than 0.25%]
[Cu: 0% or more and less than 0.25%]
[Mn: 0% or more and less than 0.25%]
These elements contribute to improving corrosion resistance. To obtain this effect, it is preferable that the concentration of one or more of these elements is 0.05% or more.
On the other hand, if the concentrations of these elements are excessive, the viscosity of the coating bath increases, which often makes it difficult to prepare the coating bath itself, and there is a concern that steel products with good coating properties cannot be produced. Therefore, it is preferable that the concentrations of each element be less than 0.25%.

[Fe: 0% or more, 5.00% or less]
Fe is mixed into the coating layer when the coating layer is produced. It may be contained up to about 5.00%, but within this range, the adverse effect on the effects of the surface-treated steel sheet according to this embodiment is small. Therefore, the Fe concentration is preferably 5.00% or less.

[Sr: 0% or more, less than 0.50%]
[Sb: 0% or more, less than 0.50%]
[Pb: 0% or more, less than 0.50%]
When Sr, Sb, and Pb are contained in the plating layer, the appearance of the plating layer changes, spangles are formed, and an improvement in metallic luster is confirmed. To obtain this effect, the concentration of one or more of Sr, Sb, and Pb is preferably 0.05% or more, and more preferably 0.10% or more.
On the other hand, if the concentrations of these elements are excessive, the viscosity of the coating bath increases, which often makes it difficult to prepare the coating bath itself, and there is a concern that steel products with good coating properties cannot be produced. Therefore, it is preferable that the concentrations of each element are each less than 0.50%.

[B: 0% or more and less than 0.50%]
When B is contained in a plating layer, it combines with Zn, Al, Mg, etc. to form various intermetallic compounds. These intermetallic compounds have the effect of improving LME resistance. To obtain this effect, the B concentration is preferably 0.05% or more, and more preferably 0.10% or more.
On the other hand, if the B concentration is excessive, the melting point of the coating will rise significantly, which may lead to deterioration in coating operability and failure to obtain a surface-treated steel sheet with good coating properties. Therefore, it is preferable that the B concentration is less than 0.50%.

[balance: Zn and impurities]
The chemical composition of the plating layer may contain Zn and impurities other than the above-mentioned elements. The Zn concentration in the plating layer 12 is preferably greater than 50.0%, more preferably 70.0% or more, and even more preferably 85.0% or more.
Impurities are elements that are mixed in during the manufacturing process. The total concentration of impurities is usually 0.5% or less, and preferably 0.1% or less.

Although there are no limitations on the coating weight of the plating layer 12, it is preferable that it be 10 g/ ^m2 or more per side to improve corrosion resistance. On the other hand, if the coating weight exceeds 250 g/ ^m2 per side, the corrosion resistance will saturate and it will be economically disadvantageous. Therefore, it is preferable that the coating weight per side be 250 g/ ^m2 or less.

The chemical composition of the plating layer can be measured by the following method.
First, the plating layer is stripped and dissolved using an acid containing an inhibitor that suppresses corrosion of the base steel (steel sheet) (for example, an acid obtained by adding 1 mass % of Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) to 10 mass % hydrochloric acid) to obtain an acid solution. Next, the obtained acid solution is measured by ICP analysis to obtain the chemical composition of the plating layer 12.

The coating weight of the plating layer can be measured by the following method.
A 30 mm x 30 mm sample is taken from the surface-treated steel sheet, and the plating layer is stripped and dissolved from this sample with an acid containing an inhibitor that suppresses corrosion of the base steel (steel material) (for example, an acid obtained by adding 1 mass % of Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) to 10 mass % hydrochloric acid), and the change in weight of the plated steel sheet after the stripping and dissolution is measured, and the coating weight is calculated from the results.

<Compound>
In the surface-treated steel sheet 1 according to this embodiment, compounds 31 containing Mg are present in at least a part of the non-plated portion 41 of the surface 101 (plated surface 103, end surface 102) of the steel sheet 11. In the surface-treated steel sheet according to this embodiment, the compounds 31 are composed of a first region R1 and a second region R2.
Here, when the concentration distributions of O, Mg, Al, Fe, and Zn were measured continuously by linear analysis in the thickness direction of the compound (usually the same as the thickness direction of the steel sheet) from the surface of the steel sheet to the surface of the surface-treated steel sheet (surface-treated steel material) at the position where the compound is formed on the surface of the steel sheet in the non-plated portion, using an FE-TEM equipped with an energy dispersive X-ray analyzer, the Mg concentration became 0.5 atomic % or more for the first time. When the position is defined as a first boundary IF1, the position on the surface side of the surface-treated steel sheet 1 from the first boundary IF1 (upper side in the case of Figure 2) where the Mg concentration becomes less than 0.5 atomic % for the first time is defined as a second boundary IF2, and the position on the surface side of the surface-treated steel sheet 1 from the second boundary IF2 (upper side in the case of Figure 2) where the Zn concentration becomes less than 0.05 atomic % for the first time is defined as the surface SFC of the compound, the area between the first boundary IF1 and the second boundary IF2 is a first region R1, and the area between the second boundary IF2 and the surface SFC of the compound is a second region R2.
As will be described later, it is believed that the mechanisms for improving corrosion resistance are different between the first region R1 and the second region R2, and that by using a two-region structure, a synergistic effect can be achieved that is superior to the case where only one region is used, thereby suppressing red rust formation.
Each one will be explained below.

[First area]
In the first region, the atomic ratio of Mg to Zn, Mg/Zn, is 0.090 or more in the center of the first region, which is the center between the first boundary and the second boundary in the thickness direction.
The above-described region with a relatively high Mg concentration has the effect of passivating the surface of the steel sheet (inhibiting the anodic reaction) due to an increase in pH near the steel surface that occurs when Mg-based corrosion products dissolve in the water in the environment.
Therefore, by having the above-described region as the lower region of the compound region (the region of the compound on the steel sheet side), an excellent effect of suppressing red rust formation can be obtained.
If the Mg/Zn ratio is less than 0.090, the Mg concentration is low and a sufficient effect cannot be obtained. Preferably, the Mg/Zn ratio is 0.090 or more and the Mg concentration is 2.0 to 50.0 atomic %. If the Mg concentration is low, the effect may be insufficient. On the other hand, even if the Mg concentration exceeds 50.0 atomic %, the effect of compound formation becomes saturated.
There is no upper limit to the Mg/Zn ratio, but since the effect of enriching Mg is saturated, the Mg/Zn ratio may be 50.00 or less.

The thickness of the first region is preferably 0.05 μm or more. If it is less than 0.05 μm, the effect of inhibiting red rust formation will be reduced. There is no upper limit to the thickness, but since an increased thickness increases the possibility of peeling, it may be 30.00 μm or less.

The first region preferably exhibits a halo pattern in the electron beam diffraction pattern of a transmission electron microscope. In this case, the amorphous nature of the first region eliminates the anisotropy of the compound, reducing the number of corrosion initiation points, thereby further improving the effect of inhibiting red rust formation.
The expression "exhibiting a halo pattern in an electron beam diffraction pattern obtained by a transmission electron microscope" means that a halo-shaped (blurred ring-shaped) contrast pattern is observed when electron beam diffraction is performed, as shown in FIG.
The halo pattern also includes a point-like diffraction pattern as shown in the photograph on the right side of FIG.

[Second area]
In the first region, the atomic ratio of Mg to Zn, Mg/Zn, is less than 0.090 at the center of the second region, which is the center between the second boundary and the surface of the compound in the thickness direction.
The region with a relatively high Zn concentration as described above has an insulating effect, an effect of preventing Mg from leaking out from the lower region where there is a lot of Mg (Mg has high solubility and easily leaks into water), and a physical protective effect (preventing corrosion factors from reaching the steel surface).
Therefore, by having the above-described region as the upper region of the compound region (region on the surface side of the compound), an excellent effect of inhibiting red rust formation can be obtained.
If the atomic ratio of Mg to Zn, Mg/Zn, is 0.090 or more, the Zn concentration is low and sufficient effects cannot be obtained. Preferably, the Mg/Zn ratio is less than 0.090 and the Zn concentration is 2.0 to 50.0 atomic %. If the Zn concentration is less than 2.0 atomic %, the effect may be insufficient. On the other hand, even if the Zn concentration exceeds 50.0 atomic %, the effect of compound formation saturates.

The thickness of the second region is preferably 0.05 μm or more. If it is less than 0.05 μm, the effect of inhibiting red rust formation will be reduced. There is no upper limit to the thickness, but since an increased thickness increases the possibility of peeling, it may be 30.00 μm or less.

The Mg concentration and Mg/Zn in the center of the first region and the Zn concentration and Mg/Zn in the center of the second region can be determined by the following method.
Test pieces are cut out from the surface-treated steel sheet at the position where the compound is formed using the cryo-FIB (Focused Ion Beam) method so that a cross section in the thickness direction can be observed. The cutting position using the FIB method is determined by SEM observation, but before SEM observation, Au is evaporated as an electron irradiation protective film, and a protective film composed of carbon is formed, and then the cutting is performed at an acceleration voltage of 40 to 5 kV. A Cu mesh is used as the sample holding mesh. The position where the compound is formed is determined by micro-XRD. That is, the position where Fe is detected in the micro-XRD, and no phase of a simple metal containing Zn, Al, or Mg or an intermetallic compound phase is detected, but a phase other than the metal layer is detected, is considered to be the position where the compound is formed.
The micro-XRD is performed using a Cr tube as the X-ray source under the conditions of an irradiation diameter of Φ300 μm, an applied voltage of 35 kV, an applied current of 25 mA, and a scan range of 20 to 120°.
The cross-sectional structure of the cut-out test piece is observed under a transmission electron microscope (TEM) at a magnification of, for example, 10,000. If the entire compound cannot be included in the field of view in the thickness direction, multiple fields of view may be used for analysis.
Using an FE-TEM equipped with an energy dispersive X-ray analyzer, the concentration distributions of O, Mg, Al, Fe, and Zn are continuously measured by linear analysis of the test piece from the surface of the steel sheet toward the surface of the coated steel sheet. As a result of the analysis, the position where the Mg concentration first becomes 0.5 atomic % or more is designated as a first boundary IF1, and the position where the Mg concentration first becomes less than 0.5 atomic % on the surface side of the coated steel sheet from the first boundary IF1 is designated as a second boundary IF2.
Quantitative analysis of Zn and Mg was performed at five or more points in the center of each region (CP1, CP2) using TEM-EDS. The elements analyzed were O, Mg, Al, Fe, and Zn.
The average values of the Zn concentration and Mg concentration at each point in each region are used as the Zn concentration and Mg concentration of each region. Observation and EDS analysis are performed at an acceleration voltage of 200 kV and a probe diameter of 1 nmΦ. For example, the transmission electron microscope used can be a JEM-2100F (manufactured by JEOL Ltd.), the EDS measurement device can be a JED-2300T (manufactured by JEOL Ltd.), and the FIB device can be an NB5000 (manufactured by Hitachi High-Technologies Corporation).

Whether the first region exhibits a halo pattern in an electron beam diffraction pattern obtained by a transmission electron microscope is determined by performing electron beam diffraction using a transmission electron microscope. The electron beam diffraction using a transmission electron microscope is performed using an FIB-TEM under the following conditions.
The specimen used in the quantitative analysis was subjected to electron beam diffraction using a probe diameter of approximately 10 nm, and the resulting diffraction pattern was observed. If any of the patterns shown in Figure 3 was observed, it was determined to exhibit a halo pattern.

The thickness of the first and second regions is determined by measuring the distance between the first and second boundaries obtained above, or the distance between the second boundary and the surface of the compound. Distance measurements are taken at three locations, and the average of these measurements is taken as the thickness of the first and second regions.

The presence of the above compounds in the non-plated areas improves corrosion resistance (improves red rust resistance), but to obtain a sufficient effect for the entire surface-treated steel sheet, it is preferable that 50% or more of the area of the non-plated areas where no plating layer is formed is covered with the compounds (a coverage rate of 50% or more). The coverage rate may be 100%.

The coverage can be determined by the following method using μ-XRF.
The non-plated area is defined as a location where Fe is detected by micro-XRD, but no phase of a simple metal containing Zn, Al, or Mg or no intermetallic compound phase is detected. Micro-XRD is performed using a Cr tube as the X-ray source, with an irradiation diameter of Φ300 μm, an applied voltage of 35 kV, an applied current of 25 mA, and a scan range of 20 to 120°.
The non-plated portion is analyzed using μ-XRF, with Mg mapping analysis performed on the non-plated portion, with Mg, Al, Fe, and Zn as the target elements, and the intensity of the μ-XRF spectrum is measured. The ratio of the area of the region with an Mg concentration of 0.5 atomic % or more to the area of the non-plated portion being measured is defined as the "compound coverage." The μ-XRF measurement conditions are as follows:
Measurement atmosphere: vacuum Tube voltage: 15 kV
Tube current: 50μA
Tube: Rh tube Scan speed: 4.00 mm ^s
X-ray spot size: 30 μm

<Manufacturing method>
The surface-treated steel sheet according to this embodiment can obtain the effects as long as it has the above-mentioned characteristics regardless of the manufacturing method, but can be manufactured by a manufacturing method including the following steps.
(I) a plating step of forming a plating layer containing Zn on a surface of a steel sheet (base steel sheet);
(II) a processing step of cutting and/or punching the steel sheet (plated steel sheet) on which the plating layer has been formed into a desired shape;
(III) A compound forming step of forming a predetermined compound in the non-plated portion.
Preferable conditions for each step will be explained.

[Plating process]
In the plating process, a steel material such as a steel sheet is immersed in a plating bath containing Zn or electroplated to form a plating layer containing Zn on the surface. The conditions for forming the plating layer are not particularly limited. A conventional method may be used so as to obtain sufficient plating adhesion.
Furthermore, the steel material to be subjected to the plating step and its manufacturing method are not limited. In the case of a surface-treated steel sheet, the steel sheet to be immersed in the plating bath can be, for example, a hot-rolled steel sheet (hot-rolled steel sheet) described in JIS G 3131:2018, JIS G 3113:2018, etc., or a cold-rolled steel sheet (cold-rolled steel sheet) described in JIS G 3141:2021, JIS G 3135:2018, etc. In addition, steel materials other than steel sheets, such as steel pipes, steel wires, and various members made of steel, can also be used.
The composition of the plating bath may be adjusted depending on the chemical composition of the plating layer to be obtained.
After the steel material is pulled out of the plating bath, the coating weight of the plating layer can be adjusted by wiping, if necessary.

[Processing process]
In the processing step, the plated steel sheet is cut and/or punched to form a desired shape. When cutting or punching is performed, an end surface without a plating layer is formed at the cut portion. An end surface is also formed at the punched portion.
In the processing step, the shape may be changed by bending, drawing, etc. In this case, non-plated portions may be generated on the plated surface.

[Compound formation process]
In the compound forming step, a predetermined compound is formed in the non-plated portion. The compound forming step includes the following first and second treatments.
(First treatment)
The non-plated portion of the steel sheet after the processing step is brought into contact with a solution containing Cl ⁻ : 1.0 to 100.0 mM, SO ₄ ²⁻ : 0.1 to 10.0 mM, Mg ²⁺ : 1.0 to 100.0 mM, and CO ₃ ²⁻ : 1.0 to 100.0 mM, having a pH of 4.5 to 7.0 and a liquid temperature of 25 to 60°C, for 1.0 to 20.0 hours.
(Second treatment)
The steel sheet after the first treatment is brought into contact with a solution containing Cl ⁻ : 1.0 to 100.0 mM, SO ₄ ²⁻ : 0.1 to 10.0 mM, Na ⁺ : 1.0 to 100.0 mM, and CO ₃ ²⁻ : 1.0 to 100.0 mM, having a pH of 4.5 to 7.0 and a liquid temperature of 25 to 60°C, for 1.0 to 20.0 hours.
In the first and second treatments, if the non-plated portion is brought into contact with the solution, the plated portion may also be brought into contact with the solution.

In the first treatment, a first region with a relatively high Mg concentration is formed by contacting the plate with a solution containing Mg ²⁺ . The amount of Mg in the first region depends on the amount of Mg in the solution, but if Mg is included in the plating, the Mg concentration in the first region becomes higher, further improving the edge corrosion resistance. As the Mg concentration in the plating increases, the Mg concentration in the first region also increases.
If the ion concentration or pH of the solution is outside the above range, sufficient effects cannot be obtained.
If the liquid temperature is less than 25° C. or the contact time is less than 1.0 hour, the reaction will not occur sufficiently, and sufficient effects will not be obtained.
On the other hand, if the liquid temperature exceeds 60° C., the compound formation rate tends to decrease. Furthermore, if the contact time exceeds 20.0 hours, the effect saturates and productivity decreases.
It is desirable that the solution used in the first treatment does not contain fluorine compounds, since these compounds reduce the Mg concentration in the compound formed on the non-plated portion.

In the second treatment, the second region having a high Zn concentration is formed by contacting the above solution that does not contain Mg ²⁺ .
If the ion concentration or pH of the solution is outside the above range, sufficient effects cannot be obtained.
If the liquid temperature is less than 25° C. or the contact time is less than 1.0 hour, the reaction will not occur sufficiently, and sufficient effects will not be obtained.
On the other hand, if the liquid temperature exceeds 60° C., the compound formation rate tends to decrease. Furthermore, if the contact time exceeds 20.0 hours, the effect saturates and productivity decreases.

After the plating process and before the compound formation process, various chemical conversion treatments and painting processes may be performed. It is also possible to apply a design by forming an additional plating layer of Cr, Ni, Au, etc., utilizing the uneven pattern on the surface of the plating layer, and then painting it.

The surface-treated steel material according to this embodiment may have a coating formed on the plating layer. One or more coatings may be formed. Examples of the coatings that may be formed directly on the plating layer include chromate coatings, phosphate coatings, and chromate-free coatings. The chromate treatment, phosphate treatment, and chromate-free treatment that form these coatings can be performed by known methods.
Chromate treatments include electrolytic chromate treatments that form a chromate film by electrolysis, reactive chromate treatments that form a film by utilizing a reaction with the material and then wash away excess treatment solution, and paint-on chromate treatments that apply a treatment solution to the substrate and dry it without rinsing with water to form a film. Any of these treatments may be used.
Examples of electrolytic chromate treatments include electrolytic chromate treatments using chromic acid, silica sol, resins (phosphoric acid, acrylic resins, vinyl ester resins, vinyl acetate acrylic emulsions, carboxylated styrene butadiene latex, diisopropanolamine-modified epoxy resins, etc.), and hard silica.
Examples of the phosphate treatment include zinc phosphate treatment, zinc calcium phosphate treatment, and manganese phosphate treatment.
Chromate-free treatments are particularly suitable because they do not place a burden on the environment. Chromate-free treatments include electrolytic chromate-free treatments that form a chromate-free film by electrolysis, reactive chromate-free treatments that form a film by utilizing a reaction with the material and then wash away excess treatment liquid, and paint-on chromate-free treatments that apply a treatment liquid to the substrate and dry it without rinsing with water to form a film. Any of these treatments may be used.

Furthermore, one or more organic resin coatings may be formed on the coating directly on the plating layer. The organic resin is not limited to a specific type, and examples include polyester resin, polyurethane resin, epoxy resin, acrylic resin, polyolefin resin, and modified versions of these resins. Here, modified versions refer to resins in which reactive functional groups contained in the structure of these resins have been reacted with other compounds (monomers, crosslinkers, etc.) that contain functional groups capable of reacting with the functional groups contained in the resin structure.

Such organic resins may be a mixture of one or more organic resins (unmodified), or a mixture of one or more organic resins obtained by modifying at least one other organic resin in the presence of at least one organic resin. The organic resin film may also contain any coloring pigment or anti-rust pigment. Water-based organic resins that have been dissolved or dispersed in water may also be used.

As the steel material, a hot-rolled steel plate having a thickness of 4.5 mm and satisfying JIS G 3131:2018 was prepared.
This steel sheet was subjected to hot-dip plating to form a Zn-based plating layer having the chemical composition shown in Tables 1 and 2. The concentration (content) of impurities in the plating layer was 0.1 mass % or less in total.
The coating weight of the plating layer was 135 g/m ² on both the front and back surfaces of the plated surface.
The obtained plated steel sheet (surface-treated steel sheet) was cut with an electric shear to form an end face having a portion with a plated layer and a portion without a plated layer (where the steel sheet was exposed). No non-plated portion was formed on the plated surface.
The end surface of this plated steel sheet was subjected to a first treatment in which it was brought into contact with a solution under the conditions shown in Tables 3 and 4. However, the solution brought into contact with sample number 2-23 (solution 2-23) was a solution containing fluoride as follows:
Cl ⁻ : 10.0 mM, SO ₄ ²⁻ : 5.0 mM, Mg ²⁺ : 20.0 mM, fluoride: 1.0 mg/L
Thereafter, the end surface was subjected to a second treatment in which it was brought into contact with a solution under the conditions shown in Tables 3 and 4. This resulted in the formation of a compound.

The obtained surface-treated steel sheets were measured in the manner described above for the coverage of the compound on the non-plated surface, the thickness of the first and second regions of the formed compound, the Mg concentration and Mg/Zn ratio in the center of the first region, the Zn concentration and Mg/Zn ratio in the center of the second region, and the presence or absence of a halo pattern in the first region. However, for examples that did not have a two-region structure, the coverage and the presence or absence of a halo pattern in the first region were not evaluated. Furthermore, for examples in which the Mg/Zn ratio in the center of the first region or the second region was outside the range of the present disclosure, the presence or absence of a halo pattern in the first region was not evaluated.
The results are shown in Tables 3 to 6.

Furthermore, an exposure test was carried out on the surface-treated steel sheets, and the area ratio of red rust on the edge surface after 90 days was determined.
The exposure conditions were as follows:
The steel plate sample was tilted 30° from the horizontal so that the treated cut end surface was at the top, and was placed facing south, and an atmospheric exposure test was carried out.
After exposure, the sample was evaluated as follows based on the ratio of the area where red rust had formed to the area where no plating layer had been formed. If the sample was rated S, AA, A, or B, it was determined that the red rust resistance was good, and if the sample was rated S, AA, or A, it was determined that the red rust resistance was excellent.
When the percentage of the area where red rust has formed exceeds 100%, red rust has formed not only in the areas where no plating layer is formed, but also in the surrounding areas.
S: More than 60%, 70% or less AA: More than 70%, 80% or less A: More than 80%, 90% or less B: More than 90%, 100% or less C: More than 100% The results are shown in Tables 5 and 6.

As can be seen from Tables 1 to 6, the surface-treated steel sheets having the specified compounds on the edge surfaces had excellent red rust resistance on the edge surfaces.
On the other hand, when the predetermined compound was not formed on the end surface, the end surface had poor red rust resistance.

This disclosure makes it possible to provide a surface-treated steel material that suppresses the formation of red rust in non-plated areas. Therefore, it has high industrial applicability.

1. Surface-treated steel sheet (surface-treated steel material)
11 Steel plate 12 Plating layer (Zn-based plating layer)
31 Compound 41 Non-plated portion 101 Surface 102 End face 103 Plated surface IF1 First boundary IF2 Second boundary R1 First region R2 Second region SFC Surface of compound CP1 Center of first region CP2 Center of second region

Claims (11)

A surface-treated steel material,
Steel and
a plating layer containing Zn formed on at least a part of a surface of the steel material;
and
When a portion of the surface of the steel material on which the plating layer is not formed is defined as a non-plated portion, a compound containing Zn and Mg is present in at least a part of the non-plated portion,
When the concentration distributions of O, Mg, Al, Fe, and Zn were continuously measured by linear analysis in the thickness direction of the compound from the surface of the steel material in the non-plated portion toward the surface of the surface-treated steel material using an FE-TEM equipped with an energy dispersive X-ray analyzer, the position where the Mg concentration first becomes 0.5 atomic % or more was defined as a first boundary, and the position where the Mg concentration first becomes less than 0.5 atomic % on the surface side of the surface-treated steel material from the first boundary was defined as a second boundary,
a first region of the compound is between the first boundary and the second boundary, and a second region of the compound is between the second boundary and a surface of the compound,
In a first region central portion, which is a central portion between the first boundary and the second boundary in the thickness direction, Mg/Zn, which is an atomic ratio of Mg to Zn, is 0.090 or more, and the Mg concentration is 2.0 to 50.0 atomic %;
In a second region central portion, which is a central portion between the second boundary and the surface of the compound in the thickness direction, Mg/Zn, which is an atomic ratio of Mg to Zn , is less than 0.090, and the Zn concentration is 2.0 to 50.0 atomic %.
A surface-treated steel material characterized by: The chemical composition of the plating layer is, in mass%,
Zn: more than 50.0%,
Al: more than 0.2% to less than 40.0%;
Mg: more than 0.2% to less than 12.5%
The surface-treated steel material according to claim 1, comprising: the first region exhibits a halo pattern in an electron diffraction pattern obtained by a transmission electron microscope;
2. The surface-treated steel material according to claim 1. the first region exhibits a halo pattern in an electron diffraction pattern obtained by a transmission electron microscope;
3. The surface-treated steel material according to claim 2. the compound is present in an area ratio of 50% or more of the surface of the steel material in the non-plated portion;
The surface-treated steel material according to any one of claims 1 to 4. The thickness of the first region is 0.05 μm or more.
The surface-treated steel material according to any one of claims 1 to 4. The thickness of the first region is 0.05 μm or more.
6. The surface-treated steel material according to claim 5. The thickness of the second region is 0.05 μm or more.
The surface-treated steel material according to any one of claims 1 to 4. The thickness of the second region is 0.05 μm or more.
6. The surface-treated steel material according to claim 5. The thickness of the second region is 0.05 μm or more.
7. The surface-treated steel material according to claim 6. The thickness of the second region is 0.05 μm or more.
8. The surface-treated steel material according to claim 7.