WO2025225565A1 - Plated steel material - Google Patents

Abstract

Provided is a plated steel material having a plating layer disposed on the surface of the steel material, wherein: the plating layer is a Zn-based plating layer that has an average chemical composition containing 4.0-70.0 mass% of Al and 0.3-12.5 mass% of Mg, with the balance consisting of Zn and impurities; the metal structure of the plating layer includes at least a Zn phase, an Al phase, and a MgZn₂ phase; and when an average KAM value is measured with respect to a rectangular measurement region of 125 μm × 300 μm set on the surface of the plating layer, by means of EBSD with a measurement interval of 0.25 μm/step, the average KAM value (K_Zn) of the Zn phase is in the range of 0.30-1.70°, the average KAM value (K_Al) of the Al phase is in the range of 0.20-1.00°, and the average KAM value (K_MgZn2) of the MgZn₂ phase is in the range of 0.20-1.00°.

Description

Plated steel

The present invention relates to a plated steel material having excellent durability.
This application claims priority based on Japanese Patent Application No. 2024-069483, filed on April 23, 2024, the contents of which are incorporated herein by reference.

One example of a steel sheet with a coating layer made of a Zn alloy is hot-dip zinc alloy-plated steel sheet. Because the Zn alloy coating layer has excellent durability, hot-dip zinc alloy-plated steel sheet is used in a wide range of applications, including automobiles, building materials, and home appliances.

The Zn alloy that forms the plating layer of hot-dip zinc alloy-plated steel sheet acts as a sacrificial anticorrosive for steel, protecting the underlying steel in atmospheric corrosive environments. Furthermore, the metal structure of Zn alloys containing Al and Mg is primarily composed of phases with high Al concentrations and phases with high Zn concentrations, and it is believed that Al improves the dissolution resistance of the plating layer, while Zn exerts a sacrificial anticorrosive effect.

In other words, the corrosion protection performance of hot-dip zinc alloy-plated steel sheets is thought to develop in stages as described in (1) and (2) below.

That is, (1) when the entire surface of the base steel is covered with a plating layer (Zn alloy), the dissolution rate of the plating layer is low, and corrosion protection due to the barrier effect of the plating layer is maintained.
Furthermore, (2) when the coating layer is partially removed and the underlying steel is exposed, the coating layer dissolves preferentially, maintaining the effect of sacrificial corrosion protection of the underlying steel for a long period of time. The sacrificial corrosion protection effect improves the corrosion resistance of the steel, for example, at the cut end surface of a hot-dip zinc alloy coated steel sheet.

In light of the above, in order to improve the corrosion resistance of hot-dip zinc alloy-plated steel sheets, it is necessary to control the chemical composition and metal structure of the Zn alloy that makes up the plating layer so that the plating layer achieves both high levels of dissolution resistance and sacrificial corrosion protection.

Patent Document 1 is an example that focuses on controlling the plating structure. Patent Document 1 focuses on thermal spraying as a method other than hot-dip galvanizing, and discloses a coated steel sheet coated with a coating layer of Zn alloy particles containing Mg. Thermal spraying is a coating method that does not require immersion in molten metal, and has advantages such as a wide range of melting points for metals or alloys that can be coated. However, there are concerns about reduced adhesion and productivity for coating layers that contain particles with a particle size of less than 5 μm.

International Publication No. 2017/057638

The present invention was made in consideration of the above circumstances, and aims to provide a plated steel material that can improve both the corrosion resistance of flat surfaces and the corrosion resistance of edge surfaces.

In order to solve the above problems, the present invention employs the following configuration.
[1] A steel material;
a plating layer disposed on at least a portion of the surface of the steel material;
and
the plating layer is a Zn-based plating layer having an average chemical composition of 4.0 mass % or more and 70.0 mass % or less of Al, 0.3 mass % or more and 12.5 mass % or less of Mg, and the balance being Zn and impurities;
The metal structure of the plating layer contains at least a Zn phase, an Al phase, and an MgZn ₂ phase,
When the average KAM value was measured by the EBSD method with a measurement interval of 0.25 μm/step in a rectangular measurement area of 125 μm × 300 μm set on the surface of the plating layer,
The average KAM value (K _Zn ) of the Zn phase is in the range of 0.30 to 1.70°,
The average KAM value (K _Al ) of the Al phase is in the range of 0.20 to 1.00°,
The plated steel material has an average KAM value (K _MgZn2 ) of _{the MgZn2} phase in the range of 0.20 to 1.00°.
[2] The plated steel material according to [1], wherein the plating layer further contains, in an average composition, an element listed in either or both of the following Group A and Group B:
[Group A] Si: 0.0001 to 2% by mass.
[Group B] 0.0001 to 2 mass% in total of one or more of Ni, Ti, Sr, Fe, Sb, Pb, Sn, Ca, Co, V, Nb, Cu, Mn, B, Bi, In, Cr, Y, La, Ce, and REM.
[3] The plated steel material according to [1] or [2], which satisfies the relationships of the following formulas (1) and (2):
0.15≦ _KAl / _KZn ≦1.40…(1)
0.20≦K _MgZn2 /K _Zn ≦1.40…(2)
In the formulas (1) and (2), K _Zn is the average KAM value of the Zn phase, K _Al is the average KAM value of the Al phase, and K _{MgZn 2} is the average KAM value of the MgZn ₂ phase.
[4] When an EBSD method is used to measure a rectangular measurement area of 30 μm × 60 μm set on the surface of the plating layer at a measurement interval of 0.05 μm/step, and an area surrounded by grain boundaries having an average crystal orientation misorientation of 15° or more is defined as a crystal grain, and the average crystal grain size is measured using the Number method,
The average grain size of the Zn phase is in the range of 0.10 to 2.00 μm,
the average grain size of the Al phase is in the range of 0.10 to 5.00 μm,
The plated steel material according to any one of [1] to [3], wherein the average crystal grain size of the MgZn _two- phase is in the range of 0.10 to 2.00 μm.
[5] The plated steel material according to any one of [1] to [4], wherein, when an X-ray diffraction measurement is performed on the surface of the plated layer, the half width at half maximum of the X-ray diffraction peak of the (102) plane of _Zn , the half width at half maximum of the X-ray diffraction peak of the (111) plane of Al, and the half width at half maximum of the X-ray diffraction peak of the (201) plane of MgZn2 satisfy the relationships of the following formulas (3) and (4):
0.5≦ _BAl / _BZn ≦1.8…(3)
0.4≦B _MgZn2 /B _Zn ≦1.4…(4)
In the formulas (3) and (4), B _Zn is the half-width of the X-ray diffraction peak of the (102) plane of Zn, B _Al is the half-width of the X-ray diffraction peak of the (111) plane of Al, and B _MgZn2 is the half-width of the X-ray diffraction peak of the (201) _plane of MgZn2.
[6] Steel material;
a plating layer disposed on at least a portion of the surface of the steel material;
and
the plating layer is a Zn-based plating layer having an average chemical composition of 4.0 mass % or more and 70.0 mass % or less of Al, 0.3 mass % or more and 12.5 mass % or less of Mg, and the balance being Zn and impurities;
The metal structure of the plating layer contains at least a Zn phase, an Al phase, and an MgZn ₂ phase,
When the average KAM value was measured by the EBSD method with a measurement interval of 0.25 μm/step in a rectangular measurement area of 125 μm × 300 μm set on the surface of the plating layer,
The average KAM value (K _Zn ) of the Zn phase is in the range of 0.30 to 1.70°,
The average KAM value (K _Al ) of the Al phase is in the range of 0.20 to 1.00°,
The average KAM value (K _MgZn2 ) of the _MgZn2 phase is in the range of 0.20 to 1.00°,
When a rectangular measurement area of 30 μm × 60 μm set on the surface of the plating layer was measured by an EBSD method with a measurement interval of 0.05 μm/step, and an area surrounded by grain boundaries with an average crystal orientation misorientation of 15° or more was defined as a crystal grain, the average crystal grain size was measured using the Number method.
The average grain size of the Zn phase is in the range of 0.10 to 2.00 μm,
the average grain size of the Al phase is in the range of 0.10 to 5.00 μm,
The average grain size of the MgZn _two- phase is in the range of 0.10 to 2.00 μm,
A plated steel material, wherein when an X-ray diffraction measurement is performed on the surface of the plated layer, the half width of the X-ray diffraction peak of the (102) plane of Zn, the half width of the X-ray diffraction peak of the (111) plane of _Al , and the half width of the X-ray diffraction peak of the (201) plane of MgZn2 satisfy the relationships of the following formulas (1) to (4):
0.15≦ _KAl / _KZn ≦1.40…(1)
0.20≦K _MgZn2 /K _Zn ≦1.40…(2)
0.5≦ _BAl / _BZn ≦1.8…(3)
0.4≦B _MgZn2 /B _Zn ≦1.4…(4)
In the formulas (1) and (2), K _Zn is the average KAM value of the Zn phase, K _Al is the average KAM value of the Al phase, and K _{MgZn 2} is the average KAM value of the MgZn ₂ phase.
In addition, in formulas (3) and (4), B _Zn is the half-width of the X-ray diffraction peak of the (102) plane of Zn, B _Al is the half-width of the X-ray diffraction peak of the (111) plane of Al, and B _MgZn2 is the half-width of the X-ray diffraction peak of the (201) _plane of MgZn2.
[7] The plated steel material according to [6], wherein the plating layer further contains, in an average composition, an element listed in either or both of the following Group A and Group B:
[Group A] Si: 0.0001 to 2% by mass.
[Group B] 0.0001 to 2 mass% in total of one or more of Ni, Ti, Sr, Fe, Sb, Pb, Sn, Ca, Co, V, Nb, Cu, Mn, B, Bi, In, Cr, Y, La, Ce, and REM.

The present invention provides plated steel that can improve both flat surface corrosion resistance and edge surface corrosion resistance.

Schematic diagram illustrating the average KAM value.

The inventors conducted extensive research to improve the corrosion resistance of plated steel materials with a plating layer containing Al, Mg, and Zn.

The hot-dip coating layer, which contains 4.0 to 70.0 mass% Al, 0.3 to 12.5 mass% Mg, and the remainder Zn and impurities, contains at least a Zn phase primarily composed of Zn, an Al phase primarily composed of Al, and an _MgZn2 phase. The Zn phase and _MgZn2 phase exhibit sacrificial corrosion protection, protecting the steel substrate by forming corrosion products when the coating layer corrodes. Loss of sacrificial corrosion protection results in corrosion of the steel substrate and the formation of red rust. Meanwhile, the Al phase is passivated by forming aluminum oxidation products, thereby exhibiting barrier properties and improving the flat corrosion resistance of the coating layer. However, passivation can also lead to the formation of white rust. The inventors focused on the average KAM value during their investigation into suppressing the formation of both white rust and red rust to achieve both sacrificial corrosion protection and flat corrosion resistance. The average KAM value allows for evaluation of the amount of strain in each phase. A smaller average KAM value indicates a smaller amount of local strain.

The inventors discovered that when the average KAM value of a phase making up a plating layer is small, that phase is relatively resistant to dissolution during corrosion; conversely, when the average KAM value is large, that phase is relatively resistant to dissolution during corrosion. Based on this, it was expected that, in a plating layer containing Al, Mg, and Zn, by making the amount of local strain in the Al phase relatively small, dissolution of the Al phase during corrosion would be suppressed, thereby suppressing the occurrence of white rust and improving corrosion resistance at flat surfaces. Furthermore, by making the amount of local strain in the Zn phase relatively large, dissolution of the Zn phase would be promoted, exerting sacrificial corrosion protection capabilities, suppressing corrosion of the base steel, and further improving corrosion resistance at edge surfaces.

Therefore, we investigated ways to control the local strain of each phase contained in the hot-dip coating layer and found that by subjecting the coated steel material to temper rolling a predetermined time after the end of coating and then heat treating it, it is possible to introduce strain according to the hardness of each phase constituting the coating layer and control the amount of strain for each phase. By controlling the amount of strain for each phase so that the KAM value of the Al phase is small while the KAM value of the Zn phase is maintained higher than the KAM value of the MgZn _two- phase, we were able to suppress the formation of red rust and white rust in the early stages of corrosion and successfully achieve both flat surface corrosion resistance and edge corrosion resistance.

Furthermore, it was found that when the average grain size of each phase in the plating layer is within a preferred range, the combined effect of strain control and grain refinement further improves corrosion resistance in flat areas.

Hereinafter, a plated steel material according to an embodiment of the present invention will be described.
The plated steel material of this embodiment has a steel material and a plating layer disposed on at least a part of the surface of the steel material, and the plating layer is a Zn-based plating layer having an average chemical composition of 4.0 mass% to 70.0 mass% Al, 0.3 mass% to 12.5 mass% Mg, and the balance being Zn and impurities, and the metal structure of the plating layer includes at least a Zn phase, an Al phase, and an _MgZn2 phase, and when the average KAM value is measured by an EBSD method with a measurement interval of 0.25 μm/step in a rectangular measurement area of 125 μm × 300 μm set on the surface of the plating layer, the average KAM value of the Zn phase (K _Zn ) is in the range of 0.30 to 1.70°, the average KAM value of the Al phase (K _Al ) is in the range of 0.20 to 1.00°, and the average KAM value of the MgZn2 _phase (K _MgZn2 ) is in the range of 0.20 to 1.00°.

Furthermore, the plated steel material of this embodiment preferably satisfies the relationships of the following formulas (1) and (2).
0.15≦ _KAl / _KZn ≦1.40…(1)
0.20≦K _MgZn2 /K _Zn ≦1.40…(2)
In the formulas (1) and (2), K _Zn is the average KAM value of the Zn phase, K _Al is the average KAM value of the Al phase, and K _{MgZn 2} is the average KAM value of the MgZn ₂ phase.

Furthermore, in the plated steel material of this embodiment, when the average grain size is measured by the Number method using an EBSD method with a measurement interval of 0.05 μm/step on a rectangular measurement area of 30 μm × 60 μm set on the surface of the plating layer, and when regions surrounded by grain boundaries having an average crystal orientation misorientation of 15° or more are defined as crystal grains, it is preferable that the average grain size of the Zn phase is in the range of 0.10 to 2.00 μm, the average grain size of the Al phase is in the range of 0.10 to 5.00 μm, and the average grain size of the MgZn _two- phase is in the range of 0.10 to 2.00 μm.

Furthermore, in the plated steel material of this embodiment, when X-ray diffraction measurement is performed on the surface of the plating layer, the half width of the X-ray diffraction peak of the (102) plane of Zn, the half width of the X-ray diffraction peak of the (111) plane of _Al , and the half width of the X-ray diffraction peak of the (201) plane of MgZn2 preferably satisfy the relationships of the following formulas (3) and (4):
0.5≦ _BAl / _BZn ≦1.8…(3)
0.4≦B _MgZn2 /B _Zn ≦1.4…(4)
In the formulas (3) and (4), B _Zn is the half-width of the X-ray diffraction peak of the (102) plane of Zn, B _Al is the half-width of the X-ray diffraction peak of the (111) plane of Al, and B _MgZn2 is the half-width of the X-ray diffraction peak of the (201) _plane of MgZn2.

In the following explanation, the "%" used to indicate the content of each element in the average chemical composition means "mass %." Furthermore, numerical ranges expressed using "~" mean a range that includes the numbers before and after "~" as the lower and upper limits. Furthermore, when the numbers before and after "~" are followed by "greater than" or "less than," the numerical range does not include these numbers as the lower or upper limit.

Furthermore, the flat surface corrosion resistance refers to the property of the plating layer itself that makes it difficult for the plating layer to corrode.
The corrosion resistance of the end surface refers to the property of inhibiting corrosion of the steel material at the exposed portion of the steel material (for example, the cut end surface of a plated steel material).
Sacrificial corrosion protection refers to the property of inhibiting corrosion of the base steel at locations where the steel is exposed (for example, the cut end surface of a plated steel, cracks in the plating layer during processing, and locations where the steel is exposed due to peeling of the plating layer).
Furthermore, the term "plated layer" refers to a plated film produced by a so-called hot-dip plating process.

The steel material to be plated will now be explained.
The shape of the steel material is not particularly limited. Examples of the steel material include steel plates, as well as formed steel materials such as steel pipes, civil engineering and construction materials (fences, corrugated pipes, drainage ditch covers, sand-flying prevention plates, bolts, wire mesh, guardrails, water-stop walls, etc.), home appliance components (casings for air conditioner outdoor units, etc.), and automobile parts (suspension components, etc.). Various plastic processing techniques, such as press working, roll forming, and bending, can be used for the forming process. The material of the steel material is not particularly limited. Various types of steel can be used, such as general steel, Al-killed steel, ultra-low carbon steel, high carbon steel, various high-tensile steels, and some high-alloy steels (steels containing strengthening elements such as Ni and Cr). The steel material may be, for example, a steel plate.
The steel material is not particularly limited in terms of the conditions for the method of manufacturing the steel material and the method of manufacturing the steel sheet (hot rolling method, pickling method, cold rolling method, etc.).
The steel may be pre-plated steel.

The plating layer placed on the surface of the steel material may be placed on at least a portion of the steel material. If the steel material is a steel plate, the plating layer may be formed on one side in the plate thickness direction, or on both sides in the plate thickness direction. The plating layer may also be formed on the end face of the steel plate.

The plating layer is mainly composed of a Zn—Al—Mg alloy layer due to its chemical composition, which will be described later. The plating layer may also include an Fe—Al-based interfacial alloy layer, which is located between the steel material and the Zn—Al—Mg alloy layer and contains Fe and Al as its main components. That is, the plating layer may have a single-layer structure of the Zn—Al—Mg alloy layer, or a laminated structure including the Zn—Al—Mg alloy layer and the Fe—Al-based interfacial alloy layer. In the case of a laminated structure, the Zn—Al—Mg alloy layer is preferably a layer that forms the surface of the plating layer.
It should be noted that an oxide film of the plating layer constituent elements is formed on the surface of the plating layer to a thickness of about 50 nm, but since its thickness is small compared to the overall thickness of the plating layer, this oxide film is not considered to constitute the main part of the plating layer.

There are no particular limitations on the upper and lower limits of the coating weight of the plating layer, but for example, the coating weight of the plating layer is preferably 10 to 400 g/ ^m2 per side. The coating weight of the plating layer is correlated with corrosion resistance, with a thicker coating layer providing better corrosion resistance. On the other hand, applying a thicker coating layer requires a large amount of metal, which increases costs. Therefore, the coating weight of the plating layer is preferably 400 g/ ^m2 or less. Furthermore, if the coating weight of the plating layer is less than 10 g/ ^m2 , corrosion resistance decreases, so the coating weight of the plating layer should be 10 g/m2 or ^more .

Next, we will explain the average chemical composition of the plating layer.

Al: 4.0 to 70.0% by mass
Al forms an Al phase in the Zn-Al-Mg alloy layer of the plating layer. The Al phase has a small amount of strain, is energetically stable, and is less likely to dissolve. This suppresses the formation of white rust during corrosion, maintains barrier properties against the steel material, and improves flat surface corrosion resistance. To achieve this effect, the Al content must be 4.0% by mass or more. On the other hand, if the Al content exceeds 70.0% by mass, the Zn content becomes relatively low, resulting in a decrease in sacrificial corrosion protection and further a decrease in end surface corrosion resistance. Therefore, the Al content is set to 4.0 to 70.0% by mass. The Al content may be 5.0% by mass or more, 6.0% by mass or more, 10.0% by mass or more, 15.0% by mass or more, or 20.0% by mass or more. The Al content may also be 60.0% by mass or less or 55.0% by mass or less.

Mg: 0.3 to 12.5% by mass
Mg is an element that enhances the corrosion resistance of the coating layer. In particular, Mg forms an _MgZn2 phase in the Zn-Al-Mg alloy layer of the coating layer, improving the sacrificial corrosion protection of the coating layer and further improving the corrosion resistance of the edge surface. To achieve this effect, the Mg content must be 0.3% by mass or more. On the other hand, if the Mg content exceeds 12.5% by mass, the effect of improving corrosion resistance saturates and the workability of the coating layer may deteriorate. Furthermore, manufacturing problems such as increased dross generation in the coating bath may occur. Therefore, the Mg content is set to 12.5% by mass or less. The Mg content may be 1.0% by mass or more or 3.0% by mass or more. The Mg content may also be 10.0% by mass or less, 8.0% by mass or less, or 6.0% by mass or less.

Balance: Zn and impurities Zn is an element that forms a Zn phase and an MgZn _two- phase in the Zn-Al-Mg alloy layer of the plating layer, improving the sacrificial corrosion protection of the plating layer, and is therefore included as the balance. If the Zn content decreases relatively due to an increase in the Al and Mg contents, the sacrificial corrosion protection of the plating layer will decrease, so the Al content and Mg content must be limited to 70.0 mass% or less and 12.5 mass% or less, respectively. The Zn content is more preferably 40.0 mass% or more, and may be 60.0 mass% or more, 80.0 mass% or more, 90.0 mass% or more, or 92.0 mass% or more.

Impurities refer to components contained in raw materials or components mixed in during the manufacturing process, but not intentionally added. Examples of impurities include oxygen and Fe. The maximum amount of impurities is 1% or less, and preferably 0.1% or less.

The plating layer of this embodiment may further contain, in average composition, one or two elements selected from the group consisting of Group A and Group B below.
[Group A] Si: 0.0001 to 2% by mass.
[Group B] 0.0001 to 2 mass% in total of one or more of Ni, Ti, Sr, Fe, Sb, Pb, Sn, Ca, Co, V, Nb, Cu, Mn, B, Bi, In, Cr, Y, La, Ce, and REM.

The plating layer may contain 0.0001 to 2 mass% Si as an element of Group A, on average. Si is an element effective in improving the adhesion of the plating layer. By including 0.0001 mass% or more of Si in the plating layer, the effect of improving the adhesion of the plating layer is realized, so it is preferable to include 0.0001 mass% or more of Si. However, even if the plating layer contains Si at a content of more than 2 mass%, the effect of improving plating adhesion saturates, so even if Si is included in the plating layer, the Si content should be 2 mass% or less. From the perspective of plating adhesion, the Si content in the plating layer may be 0.0010 to 1 mass%, or 0.0100 to 0.8 mass%.

The plating layer may contain, as elements of group B, one or more of the following elements in an average composition: Ni, Ti, Sr, Fe, Sb, Pb, Sn, Ca, Co, V, Nb, Cu, Mn, B, Bi, In, Cr, Y, La, Ce, and REM, in a total amount of 0.0001 to 2 mass%. The inclusion of these elements can further improve the corrosion resistance of the plating layer. REM here refers to one or more rare earth elements with atomic numbers 59 to 71 in the periodic table.

To identify the average chemical composition of the plating layer, the plating layer is stripped and dissolved using an acid containing an inhibitor that suppresses corrosion of steel to obtain an acid solution. The resulting acid solution is then measured using ICP atomic emission spectroscopy or ICP-MS to obtain the chemical composition. The chemical composition is measured as the average chemical composition. There are no particular restrictions on the type of acid, as long as it can dissolve the plating layer. By measuring the area and weight before and after stripping, the coating mass (g/m ² ) of the plating layer can also be obtained simultaneously. For example, a 10% hydrochloric acid solution containing 0.06 mass % of an inhibitor (Ivit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) can be used as the acid containing the inhibitor.

Next, the structure of the plating layer will be described. The plating layer of this embodiment contains at least a Zn phase, an Al phase, and an MgZn ₂ phase.

The average KAM values described below are the results of measurements taken using the EBSD method with a measurement interval of 0.25 μm/step in a rectangular measurement area of 125 μm x 300 μm set on the surface of the plating layer. The measurement area may be set at any position on the surface of the plating layer.

The Al phase is a phase that primarily contains Al. In this embodiment, the Al phase has smaller local strain and a smaller average KAM value than the other phases that make up the plating layer, making it energetically stable and less likely to dissolve during corrosion. This enhances the barrier properties of the plating layer against the steel material and improves the corrosion resistance of flat surfaces. Furthermore, because it is less likely to dissolve during corrosion, no corrosion products are produced, and the occurrence of white rust is suppressed.

The average KAM value (K _Al ) of the Al phase must be in the range of 0.20 to 1.00°. If the average KAM value (K _Al ) of the Al phase exceeds 1.00°, the Al phase will be more likely to dissolve during corrosion, producing corrosion products that will lead to the formation of white rust, and the corrosion resistance of flat surfaces will be reduced. Therefore, the upper limit of the average KAM value (K _Al ) is set to 1.00° or less. On the other hand, there is no need to particularly limit the lower limit of the average KAM value (K _Al ) of the Al phase, but setting it below 0.20° makes it difficult to manufacture the plating layer, so the lower limit is set to 0.20° or more.

The Al phase preferably contains 65 mass% or more of Al. The Al phase may also contain Zn in addition to Al. The Al phase may also have an Al content of 100%.

The Al phase in this embodiment may exist as a single phase in the coating layer. Alternatively, the Al phase may form a eutectic structure together with other phases. For example, the Al phase may be contained as an Al phase constituting a ternary eutectic structure of Zn/Al/MgZn ₂ .

The Zn phase is a phase that primarily contains Zn. In this embodiment, the Zn phase has larger local strain and a higher average KAM value than the other phases that make up the plating layer, and therefore dissolves preferentially during corrosion to form corrosion products. This improves the sacrificial corrosion protection of the plating layer against the steel material. Furthermore, when the steel substrate of the steel material is exposed through processing, the generated corrosion products cover the steel substrate, improving the corrosion resistance of the edge surface. This also suppresses the formation of red rust.

The average KAM value (K _Zn ) of the Zn phase must be in the range of 0.30 to 1.70°. If the average KAM value (K _Al ) of the Zn phase exceeds 1.70°, the Zn phase will be prone to excessive dissolution during corrosion, resulting in a decrease in flat surface corrosion resistance. On the other hand, if the average KAM value (K _Zn ) of the Zn phase is less than 0.30°, preferential dissolution of the Zn phase will be difficult during corrosion, resulting in a decrease in sacrificial corrosion protection and edge corrosion resistance. Therefore, the average KAM value (K _Zn ) of the Zn phase is set to be in the range of 0.30 to 1.70°.

The Zn phase preferably contains 70% or more by mass of Zn. The Zn phase may contain Al in addition to Zn. The Zn phase may also have a Zn content of 100%.

The Zn phase in this embodiment may exist as a single phase in the coating layer. Alternatively, the Zn phase may form a eutectic structure together with other phases. For example, the Zn phase may be contained as a Zn phase constituting a ternary eutectic structure of Zn/Al/MgZn ₂ .

The _MgZn2 phase is a region in the coating layer where Mg is 16 mass% (±5%) and Zn is 84 mass% (±5%). The _MgZn2 phase dramatically improves the corrosion resistance of the coating layer. That is, the _MgZn2 phase dissolves Mg during corrosion, and Mg2 ⁺ rapidly migrates to the steel substrate, forming corrosion products on the steel substrate. This improves, for example, the corrosion resistance around cross-cut areas where scratches reach the steel substrate, the corrosion resistance in processed areas where cracks in the coating layer are likely to occur, and the corrosion resistance of the cut end surfaces. The MgZn2 phase of this embodiment has smaller local strain and a lower average KAM value than the Zn phase. Therefore, during corrosion, the Zn phase preferentially dissolves, followed by the _MgZn2 phase, improving flat surface corrosion resistance and sacrificial corrosion protection.

The average KAM value (K _MgZn2 ) of the MgZn ₂ phase must be in the range of 0.20° to 1.00°. If the average KAM value (K _MgZn2 ) of the MgZn ₂ phase exceeds 1.00°, dissolution occurs together with the Zn phase during corrosion, resulting in a decrease in flat surface corrosion resistance. On the other hand, if the average KAM value (K _MgZn2 ) of the MgZn ₂ phase is less than 0.20°, preferential dissolution of the Zn phase during corrosion becomes difficult, resulting in a decrease in sacrificial corrosion protection and edge corrosion resistance. Therefore, the average KAM value (K _MgZn2 ) of the MgZn ₂ phase is set to be in the range of 0.20° to 1.00°.

The MgZn ₂ phase in this embodiment may exist as a single phase in the coating layer. Alternatively, the MgZn ₂ phase may form a eutectic structure with other phases. For example, the MgZn ₂ phase may be contained as a ternary eutectic structure of Zn/Al/MgZn ₂ .

Furthermore, it is preferable that the plating layer of this embodiment satisfy the relationships in the following formulas (1) and (2).

0.15≦ _KAl / _KZn ≦1.40…(1)
0.20≦K _MgZn2 /K _Zn ≦1.40…(2)

In the formulas (1) and (2), K _Zn is the average KAM value of the Zn phase, K _Al is the average KAM value of the Al phase, and K _{MgZn 2} is the average KAM value of the MgZn ₂ phase.

Since K _Al /K _Zn is in the range of 0.15 to 1.40, dissolution of the Al phase into the Zn phase during corrosion is less likely to proceed, suppressing the occurrence of white rust and maintaining barrier properties, further improving flat surface corrosion resistance. Furthermore, dissolution of the Zn phase is prioritized, further improving sacrificial corrosion protection and edge corrosion resistance. Furthermore, since K _MgZn2 /K _Zn is in the range of 0.20 to 1.40, dissolution of the Zn phase is prioritized over the _MgZn2 phase during corrosion, further improving sacrificial corrosion protection and edge corrosion resistance.

In this embodiment, the KAM value (Kernel Average Misorientation) is the average value of the misorientation between a pixel of interest and adjacent pixels when analyzing the individual crystal orientations of the crystal grains that make up a polycrystalline metal structure using electron backscattering diffraction (ESBD). It is often used as a parameter indicating local strain and is generally expressed, for example, by the following formula (A) (Source: Journal of the Japan Institute of Metals, Vol. 74, No. 7 (2010) pp. 467-474). This KAM value allows the degree of local strain to be determined. The larger the value, the greater the local strain.

In equation (1), α _ij is the crystal orientation difference between measurement point i and measurement point j, and n is the number of adjacent pixels.

Furthermore, as shown in Figure 1, the average of the orientation differences α1 to α6 between the central hexagonal pixel and its six adjacent pixels is calculated using the following formula (B), and this value is taken as the KAM value of the central pixel. The average of the KAM values of each pixel in the measurement area is taken as the average KAM value. The possible values for the average KAM value are in the range of 0 to 5°, with values closer to 0° indicating smaller local strain and values closer to 5° indicating larger local strain.

Next, the average crystal grain size of each phase will be described. When the average crystal grain size is measured by the Number method using an EBSD method with a measurement interval of 0.05 μm/step on a rectangular measurement area of 30 μm × 60 μm set on the surface of the coating layer, with the crystal grain defined as a region surrounded by grain boundaries with an average crystal orientation misorientation of 15° or more, the average crystal grain size of the Zn phase is preferably in the range of 0.10 to 2.00 μm, the average crystal grain size of the Al phase is preferably in the range of 0.10 to 5.00 μm, and the average crystal grain size of the MgZn _biphase is preferably in the range of 0.10 to 2.00 μm. Furthermore, by setting the average crystal grain size of the Zn phase and the MgZn _biphase to 2.0 μm or less and the average crystal grain size of the Al phase to 5.0 μm or less, the distance between the anode and cathode during corrosion is reduced, and the pH change of the solution near the coating layer is reduced, thereby improving flat surface corrosion resistance. The measurement interval in the EBSD method is 0.05 μm, and the lower limit of the average crystal grain size of each phase is 0.05 μm. The lower limit of the crystal grain size of each phase is not particularly limited, but as described above, it is sufficient that each phase is 0.1 μm or more.

Here, the method for measuring the average KAM value and the average crystal grain size will be described.
First, the surface of the plating layer is polished to a thickness of several microns using #1500 silicon carbide paper, then polished to a mirror finish using a liquid containing alcohol and diamond powder with a particle size of 1 to 6 microns dispersed therein, and then final polishing is performed using colloidal silica. In this way, the measurement sample is prepared.

The measurement sample is observed using a scanning electron microscope. The scanning electron microscope is equipped with an EBSD analyzer and an EDS measurement device. For example, the scanning electron microscope can be a Hitachi High-Technologies field emission scanning electron microscope (FE-SEM: SU-70), the EBSD analyzer can be DigiView (TSL Solutions), and the EDS measurement device can be Octan elect super (Ametec).

The measurement area for the KAM value is a 125 x 300 μm area on the surface of the plating layer. The measurement interval is 0.25 μm steps. On the other hand, the measurement area for the average crystal grain size is a rectangular area of 30 μm x 60 μm on the surface of the plating layer, with measurement intervals of 0.05 μm/step. For both the KAM value and average crystal grain size measurements, the acceleration voltage is 15 kV, the working distance is 15 mm, and secondary electron images are taken with a scanning electron microscope.

Next, an EBSD analyzer is used to perform EBSD analysis on each measurement region at an analysis speed of 200 to 300 points per second. The software "OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to calculate the average crystal grain sizes of the Al phase, Zn phase, and MgZn _two- phase. For the crystal orientation information obtained by EBSD analysis, regions surrounded by grain boundaries with an average crystal orientation difference of 15° or more are defined as crystal grains using the Number method, and the average crystal grain size is calculated. When calculating the average crystal grain size, the measurement targets are crystal grains with a grain size of 0.25 μm or more.

Next, the average KAM value for each phase is measured under the following conditions using the software "OIM Analysis (registered trademark)."

Nearest neighbor: 1st (If it is 1st, the azimuth difference is calculated using the nearest neighbor pixel.)
・Perimeter only
- If there is a difference in direction of 5° or more, all differences are specified as 5°.
・Maximum orientation difference: within 5°.

Next, the half-width of the X-ray diffraction peak will be described. The larger the half-width of the X-ray diffraction peak, the smaller the crystallite size of the phase tends to be. On the other hand, when a metal is subjected to severe processing and strain is introduced, many dislocations occur in the structure, and the crystallite size becomes smaller. Therefore, the half-width of the X-ray diffraction peak can be regarded as a parameter representing the degree of strain. In other words, the larger the half-width of the X-ray diffraction peak, the greater the strain applied. In this embodiment, when X-ray diffraction measurement is performed on the surface of the plating layer, the half-width of the X-ray diffraction peak of the (102) plane of Zn, the half-width of the X-ray diffraction peak of the (111) plane of Al, and the half-width of the X-ray diffraction peak of the (201) _plane of MgZn2 preferably satisfy the relationships of the following equations (3) and (4).

0.5≦ _BAl / _BZn ≦1.8…(3)
0.4≦B _MgZn2 /B _Zn ≦1.4…(4)

In the formulas (3) and (4), B _Zn is the half-width of the X-ray diffraction peak of the (102) plane of Zn, B _Al is the half-width of the X-ray diffraction peak of the (111) plane of Al, and B _MgZn2 is the half-width of the X-ray diffraction peak of the (201) _plane of MgZn2.

Since B _Al /B _Zn ≦ is in the range of 0.5 to 1.8, dissolution of the Al phase into the Zn phase during corrosion is less likely to proceed, suppressing the generation of white rust and maintaining barrier properties, further improving flat surface corrosion resistance. Furthermore, dissolution of the Zn phase is prioritized, further improving sacrificial corrosion protection and edge corrosion resistance. Furthermore, since B _MgZn2 /B _Zn is in the range of 0.4 to 1.4, dissolution of the Zn phase is prioritized over the MgZn2 _phase during corrosion, further improving sacrificial corrosion protection and edge corrosion resistance.

The half-width of X-ray diffraction peaks is measured as follows. A test piece measuring 20 mm x 20 mm square on the surface of the coating layer is cut from the plated steel material, and an X-ray diffraction image of the coating layer surface of the test piece is obtained. X-ray diffraction measurements are performed under the measurement conditions shown in Table 1 below, and the X-ray diffraction peaks of the Zn (102) plane, the _Al (111) plane, and the MgZn2 (201) plane are identified. The diffraction peak position 2θ and half-width B are determined by fitting with a Lorentz function after removing the background and Kα2 radiation. The instrument-specific half-width is determined by interpolation using the half-width of a standard sample ( _LaB6 ). The true half-width is determined by subtracting the "instrument-specific half-width" from the "sample half-width." An example of the X-ray diffraction device is the Ultima III X-ray diffraction device manufactured by Rigaku Corporation, and an example of the analysis software is Expert High Score Plus manufactured by Spectris.

Next, we will explain the manufacturing method of the plated steel product of this embodiment. The plated steel product of this embodiment can achieve the effects described above as long as it has the characteristics described above, regardless of the manufacturing method, but it can also be manufactured using a manufacturing method that includes the following steps.

(I) A plating step in which a plating layer is formed on the surface of the steel material by hot dip plating.
(II) A temper rolling step in which the steel material on which the plating layer has been formed (plated steel material) is temper rolled.
(III) A heat treatment step of heat treating the plated steel material after temper rolling.

Below, preferred conditions for each step are explained.

[Plating process]
In the plating process, a steel material such as a steel sheet is immersed in a plating bath containing Zn to form a plating layer on the surface. Conventional methods can be used to ensure sufficient plating adhesion. The composition of the plating bath can be adjusted depending on the chemical composition of the desired plating layer. After the steel material is removed from the plating bath, the coating weight of the plating layer can be adjusted as needed by wiping. The cooling conditions after removal from the plating bath are not particularly limited up to 300°C, and any cooling conditions can be used. Strain can be introduced into the plating layer by quenching the plating layer between 300°C and 150°C or less. Specifically, strain can be introduced into the plating layer by setting the average cooling rate of the plating layer between 300°C and 150°C or less to 15.0°C/s. Around 300°C is a temperature range in which the diffusion rate of atoms in the plating begins to decrease. Quenching the plating from this temperature range forcibly stops atomic diffusion and allows strain to remain in each plating phase. If the average cooling rate exceeds 35.0°C/s, the strain in the Zn phase, Al phase, and MgZn _two- phase increases, making it impossible to adjust the KAM value to a predetermined value.

[Temper rolling process]
Next, the coating layer formed on the surface of a steel material such as a steel sheet is subjected to temper rolling. The start time of temper rolling is between 30.0 and 120.0 seconds after the end of cooling in the coating process. In this embodiment, in order to impart sufficient strain to each phase constituting the coating layer, temper rolling is performed using rolls with a relatively small arithmetic mean roughness Ra under conditions that result in a relatively large elongation.

Specifically, temper rolling begins between 30.0 and 120.0 seconds after the end of cooling in the plating process, and is performed using rolls with an arithmetic mean roughness Ra of the roll surface in the range of 1.5 to 2.5 μm, under conditions that result in an elongation of 1.8 to 2.5%.

By setting the start time of temper rolling to be between 30.0 and 120.0 seconds after the end of cooling in the plating process, the average KAM value (K _MgZn2 ) of the MgZn ₂ phase can be controlled to be in the range of 0.20 to 1.00°.

The end of cooling in the plating process, which is the basis for the start time of temper rolling, is the point at which the spraying of cooling medium such as cooling gas onto the plating layer has finished. Furthermore, the start time of temper rolling is the point at which the temper rolling rolls come into contact with the plating layer. Note that if the post-plating cooling equipment and temper rolling equipment are arranged in succession, and the plated steel passes through the cooling equipment and temper rolling equipment in succession, the start time of temper rolling will be constant regardless of the location of the plated steel.

Furthermore, by setting the elongation of the plated steel material during temper rolling to 1.8 to 2.5%, it becomes possible to impart strain to the Al phase, Zn phase, and MgZn _two- phase according to the hardness of each phase. By rapidly cooling the plated layer at 300°C or less and then allowing a time interval of 30.0 to 120.0 seconds after quenching, skin-passing is performed so as to achieve a relatively large elongation, thereby making it possible to introduce sufficient strain into each phase.

Furthermore, by setting the roll surface roughness Ra in the range of 1.5 to 2.5 μm, it is possible to impart relatively uniform strain to the surface of the plating layer, making it possible to adjust to the specified KAM value.

[Heat treatment process]
Next, the coating layer formed on the surface of a steel material such as a steel sheet and subjected to temper rolling is subjected to heat treatment. In this embodiment, in order to recover to some extent the strain of each phase constituting the coating layer, the heat treatment is performed by setting the maximum temperature to a relatively low value, holding the temperature for a relatively short time, and controlling the cooling rate after the holding time to within an appropriate range.

Specifically, the heat treatment process is performed under conditions of a maximum temperature of 100 to 150°C, a holding time of 0.5 to 10.0 seconds, and an average cooling rate of 10.0 to 25.0°C/s during cooling after the holding time. The cooling end temperature is 50°C or less. By setting the maximum temperature to 100 to 150°C, it is possible to recover the strain of the Zn phase, which contains a large amount of Zn, a metal with a low melting point. Furthermore, by performing heat treatment, it is possible to control the strain state of the Zn phase without significantly changing the strain state of the Al phase or MgZn ₂ phase. Furthermore, by setting the holding time to 0.5 to 10.0 seconds and setting the average cooling rate during cooling after the holding time to 10.0 to 25.0°C/s, excessive recovery of the strain of the Zn phase can be prevented, and the strain state can be controlled.

As described above, by controlling the cooling conditions after coating and the conditions for temper rolling, larger strain is applied to each phase of the coating layer compared to when cooling after coating and temper rolling are performed under conventional conditions, and strain corresponding to the hardness of each phase is introduced, making it possible to control the strain in the Al phase, Zn phase, and MgZn _two- phase. Furthermore, by performing heat treatment after temper rolling, the average KAM value of each phase can be adjusted. In this way, the coated steel material of this embodiment can be produced.

After the production of plated steel, various chemical conversion treatments and painting processes may be performed. However, various chemical conversion treatments and painting processes require a drying or baking process. In other words, after the heat treatment for strain adjustment described above, the steel may be subjected to further heat treatment for a different purpose (chemical conversion treatment, painting, etc.). In this case, it is recommended to adjust the heat treatment conditions so that the KAM values of each phase of the plating layer do not deviate from the range of this application. For example, the heat treatment performed after the heat treatment for strain adjustment may have a maximum temperature of less than 100°C and a holding time of 0.5 to 10 seconds.

In the plated steel material of this embodiment, a coating may be formed on the plating layer. One or more layers may be formed as the coating. Examples of types of 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 using known methods.

There are three types of chromate treatment: electrolytic chromate treatment, which forms a chromate film through electrolysis; reactive chromate treatment, which forms a film by using a reaction with the material and then washes away excess treatment solution; and paint-on chromate treatment, which applies the treatment solution to the object to be coated and then dries it without rinsing with water to form a film. Any of these treatments may be used.

Examples of electrolytic chromate treatments include those using chromic acid, silica sol, resin (phosphoric acid, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene latex, diisopropanolamine-modified epoxy resin, etc.), and hard silica.

Examples of phosphate treatments include zinc phosphate treatment, zinc calcium phosphate treatment, and manganese phosphate treatment.

Chromate-free treatments are particularly suitable as they place no burden on the environment. Chromate-free treatments include electrolytic chromate-free treatments, which form a chromate-free film through electrolysis; reactive chromate-free treatments, which form a film by utilizing a reaction with the material and then wash away excess treatment liquid; and paint-on chromate-free treatments, which apply a treatment liquid to the substrate and then dry it without rinsing 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 other organic resin. The organic resin film may also contain any coloring pigment or anti-rust pigment. Water-based resins that have been dissolved or dispersed in water can also be used.

Furthermore, it is possible to add a design by utilizing the uneven pattern on the surface of the plating layer, or by applying a plating layer of Cr, Ni, Au, etc. and then painting it. Furthermore, to further improve corrosion resistance, touch-up paint or thermal spraying treatments may be used on welded or processed areas.

Examples will be described below.
As the steel material, a hot-rolled steel plate having a thickness of 4.5 mm and satisfying JIS G 3193:2019 was prepared.
This steel sheet was subjected to hot-dip galvanization to form a coating layer having the chemical composition shown in Tables 3A and 3B. The concentration (content) of impurities in the coating layer was 0.1% or less. In the hot-dip galvanization method, the average cooling rate from 300°C to 150°C was as shown in Tables 4A and 4B. Cooling was performed by spraying cooling gas. Next, the steel sheet on which the coating layer was formed was subjected to temper rolling (skin pass) under the conditions shown in Tables 4A and 4B after the time shown in Tables 4A and 4B had elapsed since the end of cooling. Furthermore, after the skin pass, heat treatment was performed. As shown in Tables 4A and 4B, the heat treatment was performed under conditions of a maximum temperature of 90 to 160°C, a holding time of 0.1 to 15.0 seconds, and an average cooling rate of 8.0 to 30.0°C/second when cooling to 50°C after the holding time had elapsed.

The end of cooling in the plating process, which is the basis for the start time of temper rolling, was defined as the point at which spraying of cooling gas onto the plating layer finished. Furthermore, the start time of temper rolling was defined as the point at which the temper rolling rolls came into contact with the plating layer. In this example, the post-plating cooling equipment and temper rolling equipment were arranged in series, and the plated steel material passed through the cooling equipment and temper rolling equipment in succession, so the start time of temper rolling was consistent regardless of the location of the plated steel material.

The coating weight of the plating layer was 135 g/m ² per side on both the front and back sides of the plated surface.

In this manner, plated steel products Nos. 1-1 to 1-37 and 2-1 to 2-23 were manufactured.

The average KAM value of the Zn phase, Al phase, and MgZn2 _phase contained in the coating layer, the average crystal grain size, the half-width of the X-ray diffraction peak of the Zn (102) plane, the half-width of the X-ray diffraction peak of the Al (111) plane, and the half-width of the X-ray diffraction peak of the _MgZn2 (201) plane were measured for the obtained plated steel materials. The respective measurement methods and measurement conditions were as follows.

(Average KAM value, average crystal grain size)
The surface of the plating layer was polished using #1500 silicon carbide paper, then polished to a mirror finish using a liquid containing alcohol and diamond powder with a particle size of 1 to 6 μm dispersed therein, and then further polished with colloidal silica. In this way, the measurement sample was prepared.

The measurement samples were observed using a scanning electron microscope. The scanning electron microscope used was equipped with an EBSD analysis device and an EDS measurement device. The scanning electron microscope used was a Hitachi High-Technologies field emission scanning electron microscope (FE-SEM: SU-70). The EBSD analysis device used was DigiView (TSL Solutions). The EDS measurement device used was Octan elect super (Ametec).

The measurement area for the KAM value was a 125 x 300 μm area on the surface of the plating layer. The measurement interval was 0.25 μm/step. Meanwhile, the measurement area for the average crystal grain size was a rectangular area of 30 μm x 60 μm on the surface of the plating layer, with a measurement interval of 0.05 μm/step. For both the KAM value and average crystal grain size measurements, the acceleration voltage was 15 kV, the working distance was 15 mm, and secondary electron images were taken with a scanning electron microscope.

Next, EBSD analysis was performed on the same field of view at an analysis speed of 200 to 300 points/second using an EBSD analyzer. The average crystal grain sizes of the Al phase, Zn phase, and MgZn _two- phase were calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. For the crystal orientation information obtained by EBSD analysis, regions surrounded by grain boundaries with an average crystal orientation difference of 15° or more were defined as crystal grains using the Number method, and the average crystal grain size was calculated. When calculating the average crystal grain size, the measurement targets were crystal grains with a grain size of 0.25 μm or more. The results are shown in Tables 6A and 6B.

Next, the average KAM value for each phase was measured under the following conditions using the software "OIM Analysis (registered trademark)." The results are shown in Tables 5A and 5B.

Nearest neighbor: 1st (If it is 1st, the azimuth difference is calculated using the nearest neighbor pixel.)
・Perimeter only
- If there is a difference in direction of 5° or more, all differences are specified as 5°.
・Maximum orientation difference: within 5°.

(Fat width of X-ray diffraction peak)
Test specimens were cut from the plated steel material so that the surface of the plating layer measured 20 mm x 20 mm square, and X-ray diffraction images of the test specimen's plating layer surface were obtained. X-ray diffraction measurements were performed under the conditions shown in Table 2 below, and the X-ray diffraction peaks of the Zn (102) plane, the _Al (111) plane, and the MgZn2 (201) plane were identified. The diffraction peak positions 2θ and half-widths B were determined by fitting with a Lorentz function after removing the background and Kα2 radiation. The instrument-derived half-widths were determined by interpolation using the half-width of a standard sample ( _LaB6 ). The true half-widths were determined by subtracting the "instrument-derived half-widths" from the "sample half-widths." A Rigaku Ultima III X-ray diffractometer was used, and Spectris Expert High Score Plus analysis software was used. The results are shown in Tables 6A and 6B.

The average chemical composition of the plating layer was determined by preparing an acid solution by stripping and dissolving the plating layer with an acid containing an inhibitor that suppresses corrosion of the steel, and then measuring the acid solution using ICP atomic emission spectroscopy and ICP-MS. The inhibitor-containing acid used was a 10% hydrochloric acid solution with 0.06% by mass of inhibitor (Ivit 710K, manufactured by Asahi Chemical Industry Co., Ltd.).

(Flat surface corrosion resistance)
The flat surface corrosion resistance was evaluated as follows. The obtained plated steel material was cut into test pieces with a surface size of 100 mm x 50 mm for the plating layer, and the plating layer of the test pieces was subjected to a salt spray test in accordance with JIS Z 2371 (2015) for up to 96 hours. The flat surface corrosion resistance was evaluated based on the corrosion weight loss of the test pieces after the test. The evaluation criteria for flat surface corrosion resistance are shown below. "SS", "S", "AA", and "A" were considered to be pass. The results are shown in Tables 6A and 6B.

SS: 0.020 (g/m ² /hr) or less S: More than 0.020 to 30 (g/m ² /hr) AA: More than 0.030 to 0.040 (g/m ² /hr) A: More than 0.040 to 0.10 (g/m ² /hr) or less B: More than 0.10 to 0.50 (g/m ² /hr) or less C: more than 0.50 (g/m ² /hr)

(Corrosion resistance of end face)
The obtained plated steel material was cut using an electric shear so that the surface of the plating layer had a size of 100 mm x 50 mm, leaving a burr on the surface. This formed a cut end surface with a portion bearing the plating layer and a portion where the end surface of the steel material was exposed, and these were used as test specimens. Note that no non-plated portion was formed on the surface side of the plating layer. An exposure test was conducted on the cut plated steel material, and the area ratio of red rust on the end surface after 50 days was determined. The exposure conditions were as follows. The plated steel material sample was tilted 30° from the horizontal and placed facing south, so that the treated cut end surface was on top and the plated portion of the side end surface (100 mm x 4.5 mm) was on top. After exposure, the sample was evaluated as follows based on the ratio of the area where red rust had developed to the area where the plating layer was not formed. A grade of SS, S, AA, or A was determined to have excellent end surface corrosion resistance. The results are shown in Tables 6A and 6B.

SS: 70% or less S: More than 70%, 80% or less AA: More than 80%, 90% or less A: More than 90%, 100% or less B: More than 100%, 115% or less C: More than 115%

As shown in Tables 3A to 6B, the average chemical composition of the coating layer and the average KAM value of each phase of Nos. 1-1 to 1-37 satisfied the ranges of the present invention, and both flat surface corrosion resistance and edge corrosion resistance were excellent. The average grain sizes of the Zn phase, Al phase, and _MgZn2 phase of No. 1-1 all fell outside the preferred ranges (average grain size of the Zn phase: 0.10 to 2.00 μm, average grain size of the Al phase: 0.1 to 5.0 μm, average grain size of the MgZn2 _phase : 0.10 to 2.00 μm). Furthermore, the half-width ratios of Nos. 1-1 to 1-24 all fell outside the preferred ranges (0.5≦B _Al /B _Zn ≦1.8, 0.4≦B _MgZn2 /B _Zn ≦1.4). Therefore, the values are not listed in Table 6A.

In Nos. 2-1 to 2-4, the average chemical composition of the plating layer was outside the range of the present invention, and therefore the average KAM value of either the Zn phase, Al phase, or MgZn ₂ phase was outside the range of the present invention, resulting in inferior flat surface corrosion resistance and edge surface corrosion resistance.

In No. 2-5, the average chemical composition of the plating layer satisfied the range of the present invention, but the average cooling rate between 300°C and 150°C was outside the preferred conditions, resulting in the average KAM value of the Al phase being outside the range of the present invention and resulting in inferior corrosion resistance in both the flat surface and edge surfaces.

In No. 2-6, the average chemical composition of the coating layer satisfied the range of the present invention, but the average cooling rate between 300°C and 150°C was outside the preferred condition, so the average KAM values of the Zn phase, Al phase, and MgZn _two- phase were outside the range of the present invention, and both the flat surface corrosion resistance and the edge surface corrosion resistance were inferior.

In No. 2-7, temper rolling was performed within 30 seconds from the end of cooling in the coating process, so the average KAM value of the MgZn ₂ phase was outside the range of the present invention, and both the flat surface corrosion resistance and the end surface corrosion resistance were inferior.

In No. 2-8, temper rolling was performed 120 seconds after the end of cooling in the coating process, so the average KAM value of the MgZn _two- phase was outside the range of the present invention, and both the flat surface corrosion resistance and the end surface corrosion resistance were inferior.

In Nos. 2-9 and 2-10, the average chemical composition of the coating layer satisfied the range of the present invention, but the elongation of the temper rolling was outside the preferred condition, so the average KAM value of the MgZn _two- phase was outside the range of the present invention, and both the flat surface corrosion resistance and the edge surface corrosion resistance were inferior.

In Nos. 2-11 and 2-12, the average chemical composition of the coating layer satisfied the range of the present invention, but the roll roughness in temper rolling was outside the preferred conditions, so the average KAM value of the Zn phase or MgZn _two- phase was outside the range of the present invention, and both the flat surface corrosion resistance and the edge surface corrosion resistance were inferior.

For Nos. 2-13 to 2-18, the average chemical composition of the plating layer satisfied the range of the present invention, but the heat treatment conditions were outside the preferred range, resulting in the average KAM value of the Zn phase or Al phase being outside the range of the present invention, and both the flat surface corrosion resistance and the edge corrosion resistance being inferior.

In No. 2-19, the average chemical composition of the coating layer satisfied the range of the present invention, but because temper rolling and heat treatment were not performed, the average KAM values of the Zn phase and the MgZn _two- phase were outside the range of the present invention, and both the flat surface corrosion resistance and the edge surface corrosion resistance were inferior.

In No. 2-20, the average chemical composition of the plating layer satisfied the range of the present invention, but because heat treatment was not performed, the average KAM value of the Zn phase was outside the range of the present invention, resulting in inferior corrosion resistance in both the flat surface and edge surfaces.

In No. 2-21, the average chemical composition of the coating layer satisfied the range of the present invention, but because temper rolling was not performed, the average KAM value of the MgZn _two- phase was outside the range of the present invention, and both the flat surface corrosion resistance and the end surface corrosion resistance were inferior.

In No. 2-22, the average chemical composition of the coating layer satisfied the range of the present invention, but the average cooling rate between 300°C and 150°C was outside the preferred conditions, and heat treatment was not performed, so the average KAM values of the Al phase and MgZn _two- phase were outside the range of the present invention, and both the flat surface corrosion resistance and the edge surface corrosion resistance were inferior.

In No. 2-23, the average chemical composition of the coating layer satisfied the range of the present invention, but the average cooling rate between 300°C and 150°C was outside the preferred conditions, and temper rolling was not performed, so the average KAM values of the Zn phase, Al phase, and MgZn _two- phase were outside the range of the present invention, and both the flat surface corrosion resistance and the edge surface corrosion resistance were inferior.

In addition, the average grain size and half width ratio of the Zn phase, Al phase, and MgZn _two- phase of Nos. 2-1 to 2-23 were not measured because the average KAM value was outside the preferred range.

This has industrial applicability in that it can provide plated steel that can improve both flat surface corrosion resistance and edge surface corrosion resistance.

Claims (7)

Steel and
a plating layer disposed on at least a portion of the surface of the steel material;
and
the plating layer is a Zn-based plating layer having an average chemical composition of 4.0 mass % or more and 70.0 mass % or less of Al, 0.3 mass % or more and 12.5 mass % or less of Mg, and the balance being Zn and impurities;
The metal structure of the plating layer contains at least a Zn phase, an Al phase, and an MgZn ₂ phase,
When the average KAM value was measured by the EBSD method with a measurement interval of 0.25 μm/step in a rectangular measurement area of 125 μm × 300 μm set on the surface of the plating layer,
The average KAM value (K _Zn ) of the Zn phase is in the range of 0.30 to 1.70°,
The average KAM value (K _Al ) of the Al phase is in the range of 0.20 to 1.00°,
The plated steel material has an average KAM value (K _MgZn2 ) of _{the MgZn2} phase in the range of 0.20 to 1.00°. The plated steel material according to claim 1 , wherein the plating layer further contains, in an average composition, an element selected from the group A and/or B below:
[Group A] Si: 0.0001 to 2% by mass.
[Group B] 0.0001 to 2 mass% in total of one or more of Ni, Ti, Sr, Fe, Sb, Pb, Sn, Ca, Co, V, Nb, Cu, Mn, B, Bi, In, Cr, Y, La, Ce, and REM. The plated steel material according to claim 1, which satisfies the relationships of the following formulas (1) and (2):
0.15≦ _KAl / _KZn ≦1.40…(1)
0.20≦K _MgZn2 /K _Zn ≦1.40…(2)
In the formulas (1) and (2), K _Zn is the average KAM value of the Zn phase, K _Al is the average KAM value of the Al phase, and K _{MgZn 2} is the average KAM value of the MgZn ₂ phase. When a rectangular measurement area of 30 μm × 60 μm set on the surface of the plating layer was measured by an EBSD method with a measurement interval of 0.05 μm/step, and an area surrounded by grain boundaries with an average crystal orientation misorientation of 15° or more was defined as a crystal grain, the average crystal grain size was measured using the Number method.
The average grain size of the Zn phase is in the range of 0.10 to 2.00 μm,
the average grain size of the Al phase is in the range of 0.10 to 5.00 μm,
2. The plated steel material according to claim 1, wherein the MgZn _two- phase has an average crystal grain size in the range of 0.10 to 2.00 μm. _{2. The plated steel material according to claim 1} , wherein, when an X-ray diffraction measurement is performed on a surface of the plated layer, the half width at half maximum of the X-ray diffraction peak of the (102) plane of Zn, the half width at half maximum of the X-ray diffraction peak of the (111) plane of Al, and the half width at half maximum of the X-ray diffraction peak of the (201) plane of MgZn2 satisfy the relationships of the following formulas (3) and (4):
0.5≦ _BAl / _BZn ≦1.8…(3)
0.4≦B _MgZn2 /B _Zn ≦1.4…(4)
In the formulas (3) and (4), B _Zn is the half-width of the X-ray diffraction peak of the (102) plane of Zn, B _Al is the half-width of the X-ray diffraction peak of the (111) plane of Al, and B _MgZn2 is the half-width of the X-ray diffraction peak of the (201) _plane of MgZn2. Steel and
a plating layer disposed on at least a portion of the surface of the steel material;
and
the plating layer is a Zn-based plating layer having an average chemical composition of 4.0 mass % or more and 70.0 mass % or less of Al, 0.3 mass % or more and 12.5 mass % or less of Mg, and the balance being Zn and impurities;
The metal structure of the plating layer contains at least a Zn phase, an Al phase, and an MgZn ₂ phase,
When the average KAM value was measured by the EBSD method with a measurement interval of 0.25 μm/step in a rectangular measurement area of 125 μm × 300 μm set on the surface of the plating layer,
The average KAM value (K _Zn ) of the Zn phase is in the range of 0.30 to 1.70°,
The average KAM value (K _Al ) of the Al phase is in the range of 0.20 to 1.00°,
The average KAM value (K _MgZn2 ) of the _MgZn2 phase is in the range of 0.20 to 1.00°,
When a rectangular measurement area of 30 μm × 60 μm set on the surface of the plating layer was measured by an EBSD method with a measurement interval of 0.05 μm/step, and an area surrounded by grain boundaries with an average crystal orientation misorientation of 15° or more was defined as a crystal grain, the average crystal grain size was measured using the Number method.
The average grain size of the Zn phase is in the range of 0.10 to 2.00 μm,
the average grain size of the Al phase is in the range of 0.10 to 5.00 μm,
The average grain size of the MgZn _two- phase is in the range of 0.10 to 2.00 μm,
A plated steel material, wherein when an X-ray diffraction measurement is performed on the surface of the plated layer, the half width of the X-ray diffraction peak of the (102) plane of Zn, the half width of the X-ray diffraction peak of the (111) plane of _Al , and the half width of the X-ray diffraction peak of the (201) plane of MgZn2 satisfy the relationships of the following formulas (1) to (4):
0.15≦ _KAl / _KZn ≦1.40…(1)
0.20≦K _MgZn2 /K _Zn ≦1.40…(2)
0.5≦ _BAl / _BZn ≦1.8…(3)
0.4≦B _MgZn2 /B _Zn ≦1.4…(4)
In the formulas (1) and (2), K _Zn is the average KAM value of the Zn phase, K _Al is the average KAM value of the Al phase, and K _{MgZn 2} is the average KAM value of the MgZn ₂ phase.
In addition, in formulas (3) and (4), B _Zn is the half-width of the X-ray diffraction peak of the (102) plane of Zn, B _Al is the half-width of the X-ray diffraction peak of the (111) plane of Al, and B _MgZn2 is the half-width of the X-ray diffraction peak of the (201) _plane of MgZn2. The plated steel material according to claim 6 , wherein the plating layer further contains, in an average composition, an element selected from the group A and/or B below:
[Group A] Si: 0.0001 to 2% by mass.
[Group B] 0.0001 to 2 mass% in total of one or more of Ni, Ti, Sr, Fe, Sb, Pb, Sn, Ca, Co, V, Nb, Cu, Mn, B, Bi, In, Cr, Y, La, Ce, and REM.