TW202509249A - Coated Steel - Google Patents

Abstract

A coated steel material, wherein the coating layer comprises, by mass%, Al: 10.0% to 40.0%, Mg: 5.0% to 12.5%, Ti: 0% to 1.0%, Zr: 0% to 1.0%, and the remainder: 50.0% or more of Zn and impurities; the total of Ti and Zr is 0.001% or more, and the diffraction intensity obtained from the X-ray diffraction measurement result of the coating layer satisfies the following formula (1). I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al }≧0.40…(1) In formula (1), I(200) _Al is the diffraction intensity of Al (200), I(111) _Al is the diffraction intensity of Al (111), I(220) _Al is the diffraction intensity of Al (220), and I(311) _Al is the diffraction intensity of Al (311).

Description

Coated Steel

Field of Invention The present invention relates to a coated steel material. This case claims priority based on Special Application No. 2023-131415 filed in Japan on August 10, 2023, and the contents thereof are cited herein.

Background of the invention Plated steel has been used in the fields of building materials and civil engineering due to its excellent corrosion resistance. In particular, molten Zn-Al-Mg plated steel has been widely used in the aforementioned fields due to its excellent corrosion resistance. Molten Zn-Al-Mg plated steel is sometimes processed into various shapes before being made into various final products.

In the molten Zn-Al-Mg coating, MgZn ₂ phase, [Al/Zn/MgZn ₂ ] ternary eutectic structure and Al primary crystals are formed as representative phase structures. Al primary crystals are considered to be a kind of Al dendritic crystal structure containing Zn. Among these phase structures, MgZn ₂ phase and [Al/Zn/MgZn ₂ ] ternary eutectic structure are considered to have lower plastic deformation ability. Although the plastic deformation ability of Al primary crystals is considered to be higher than that of MgZn ₂ phase and [Al/Zn/MgZn ₂ ] ternary eutectic structure, it is still far from sufficient. Therefore, the coating of the molten Zn-Al-Mg coating will lack workability. Therefore, the coating of molten Zn-Al-Mg plated steel has the following problems: it cannot track the processing deformation, the coating will crack in the processed part, and the corrosion resistance of the processed part is worse than that of the flat surface.

Patent document 1 discloses a technique of precipitating _MgZn2 in Al primary crystals and inhibiting the corrosion of Al primary crystals. The purpose of Patent document 1 is to improve the corrosion resistance of the processed part by inhibiting the corrosion rate of Al primary crystals, but it has never examined the improvement of the processability of the coating itself.

[Prior technical literature] [Patent literature] [Patent literature 1] Japanese Patent Publication No. 2005-336546

Summary of the invention Problem to be solved by the invention The present invention is made in view of the above situation, and its problem is to provide a plated steel material with excellent corrosion resistance and excellent workability of the plated layer.

Means to Solve the Problem In order to solve the above problem, the following structure is adopted. [1] A coated steel material comprising: a steel material and a coating layer on the steel material; the coating layer having an average chemical composition, in mass%, comprising: Al: 10.0% to 40.0%, Mg: 5.0% to 12.5%, Ti: 0% to 1.0%, Zr: 0% to 1.0%, Si: 0% to 5.00%, Ca: 0% to 3.00%, Y: 0% to 0.50%, La: 0% to 0.50%, Ce: 0% to 0.50%, Sr: 0% to 0.50%, Sn: 0% to 3.00%, Bi: 0% to 1.00%, In: 0% to 1.00%, B: 0% to 1.00%, P: 0% to 0.50%, Cr: 0% to 0.25%, V: 0% to 0.25%, Ni: 0% to 1.0%, Co: 0%~0.25%, Nb: 0%~0.25%, Cu: 0%~1.0%, Mn: 0%~0.25%, Mo: 0%~0.25%, W: 0%~0.25%, Ag: 0%~1.00%, Li: 0%~0.50%, Na: 0%~0.05%, Ba: 0%~0.25%, K: 0%~0.05%, Fe: 0%~5.0%, Sb: 0%~0.50%, Pb: 0%~0.50%, Remainder: 50.0% or more of Zn and impurities; The total amount of Ti and Zr in the above-mentioned coating is 0.001% or more; The diffraction intensity obtained from the X-ray diffraction measurement result of the above-mentioned coating shall satisfy the following formula (1): I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al }≧0.40…(1) Wherein, in formula (1), I(200) _Al is the diffraction intensity of Al (200), I(111) _Al is the diffraction intensity of Al (111), I(220) _Al is the diffraction intensity of Al (220), and I(311) _Al is the diffraction intensity of Al (311). [2] The coated steel material as described in [1], wherein the total content of Ti and Zr in the coating layer is 0.020% or more. [3] The coated steel material according to [1] or [2], wherein in a cross section of the coating layer in the thickness direction, the ratio of the total area of the Al phase having an equivalent circular diameter of 20 μm or less to the total area of the Al phase is 50% or more.

Effect of the invention According to the present invention, a plated steel material having excellent corrosion resistance and excellent workability of the plated layer can be provided.

Implementation form of the present invention Form for implementing the invention When bending the coated steel, if cracks are generated in the coating layer and the base iron of the steel is exposed, the coating layer will sacrifice the corrosion protection function in the surrounding area, the constituent elements of the coating layer will be eluted, and the coating layer itself will be partially consumed. Therefore, the corrosion resistance of the coating layer near the processing part is sometimes reduced compared to the coating layer outside the processing part. In order to prevent this situation, cracks in the coating layer of the processing part must be prevented. As far as the coating layer is concerned, the greater the plastic deformation ability of the coating layer, the more difficult it is to crack.

The phase structure of the Zn-Al-Mg based coating contains a plurality of phases/structures, for example, MgZn ₂ phase, [ternary eutectic structure of Al/Zn/MgZn ₂ ] and Al primary crystals. Al primary crystals have a crystal structure of Al dendrites containing Zn. MgZn ₂ phase and [ternary eutectic structure of Al/Zn/MgZn ₂ ] are considered to have low plastic deformation ability and low workability. On the other hand, it is known that the workability of Al changes with the orientation state of the crystals. Therefore, if the orientation state of the Al crystals mainly contained in the Al primary crystals can be controlled, it can be expected that the workability of the coating as a whole can be improved and cracks in the coating in the processed portion can be reduced.

After conducting research with the purpose of controlling the crystal orientation of the Al phase to improve the workability of the coating, the inventors came to the conclusion that if the coating contains one or both of Zr and Ti, the proportion of the (100) plane among the Al crystal orientation planes on the surface of the coating will increase, thereby improving the workability of the coating.

Furthermore, Zr and Ti form compounds with Al in the plating layer, specifically, intermetallic compounds such as Al ₃ Zr and Al ₃ Ti. Furthermore, if Si is present, Al _2.7 Si _0.3 Zr and Al _2.5 Si _0.5 Ti are formed. These intermetallic compounds act as solidification nuclei for Al primary crystals, and refine Al primary crystals. The result is that the workability of the plating layer is further improved.

Steel structures made from this type of coated steel will have excellent workability and corrosion resistance.

The following describes the coated steel material according to the embodiment of the present invention. The plated steel material of the present embodiment comprises: a steel material and a plated layer on the surface of the steel material; the average chemical composition of the plated layer comprises, by mass%, Al: 10.0% to 40.0%, Mg: 5.0% to 12.5%, Ti: 0% to 1.0%, Zr: 0% to 1.0%, Si: 0% to 5.00%, Ca: 0% to 3.00%, Y: 0% to 0.50%, La: 0% to 0.50%, Ce: 0% to 0.50%, Sr: 0% to 0.50%, Sn: 0% to 3.00%, Bi: 0% to 1.00%, In: 0% to 1.00%, B: 0% to 1.00%, P: 0% to 0.50%, Cr: 0% to 0.25%, V: 0% to 0.25%, Ni: 0%~1.0%, Co: 0%~0.25%, Nb: 0%~0.25%, Cu: 0%~1.0%, Mn: 0%~0.25%, Mo: 0%~0.25%, W: 0%~0.25%, Ag: 0%~1.00%, Li: 0%~0.50%, Na: 0%~0.05%, Ba: 0%~0.25%, K: 0%~0.05%, Fe: 0%~5.0%, Sb: 0%~0.50%, Pb: 0%~0.50%, Remainder: 50.0% or more of Zn and impurities; the total amount of Ti and Zr in the coating is 0.001% or more; the diffraction intensity obtained from the X-ray diffraction measurement results of the coating shall satisfy the following formula (1). The steel surface here refers to the interface between the coating and the steel. The coating on the steel surface refers to the coating on the steel.

I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al }≧0.40…(1)

Wherein, in formula (1), I(200) _Al is the diffraction intensity of Al (200), I(111) _Al is the diffraction intensity of Al (111), I(220) _Al is the diffraction intensity of Al (220), and I(311) _Al is the diffraction intensity of Al (311).

Furthermore, the total content of Ti and Zr in the plating layer is preferably 0.020% or more.

In addition, in the coated steel material of the present embodiment, in a cross section in the thickness direction of the coating layer, the ratio of the total area of the Al phase having an equivalent circular diameter of 20 μm or less to the total area of the Al phase is preferably 50% or more.

In the following description, the "%" symbol for the content of each element in the chemical composition means: "mass %". In addition, the numerical range expressed by "~" means: the range covered by the numerical values written before and after "~" as the lower limit and upper limit. In addition, the numerical range when the numerical values written before and after "~" are marked as "greater than" or "less than" means: the range does not include these numerical values as the lower limit or upper limit.

The so-called "corrosion resistance" refers to the property that the coating itself is difficult to corrode. Since the Zn-based coating has a sacrificial anti-corrosion effect on the steel, the coating will corrode and rust before the steel corrodes; after the rusted coating disappears, the steel will corrode and produce red rust. This is the corrosion process of the plated steel plate. The so-called "processability" refers to the property that the coating is difficult to crack when the plated steel is bent.

As shown in FIG. 1 , the coated steel material 1 of this embodiment has a steel material 11. The shape of the steel material 11 is not particularly limited, and one example of the steel material 11 is a steel plate. In addition, the steel material 11 may also be a base steel material that has been formed, such as a steel pipe, civil engineering and construction materials (grate, corrugated steel pipe, drainage ditch cover, anti-sand plate, bolt, wire mesh, guardrail, cut-off wall, etc.), home appliance components (air conditioner outdoor unit housing, etc.), automobile parts (chassis components, etc.), etc. The forming process is, for example, various plastic processing techniques such as pressing, roll forming, and bending.

The material of the steel material 11 is not particularly limited. The steel material 11 can be various steel materials such as general steel, Ni pre-plated steel, aluminum killed steel, ultra-low carbon steel, high carbon steel, various high-tensile steels, and local high-alloy steels (steel containing strengthening elements such as Ni and Cr). Furthermore, regarding the steel material 11, the conditions such as the manufacturing method of the steel material and the manufacturing method of the steel plate (hot rolling method, pickling method, cold rolling method, etc.) are not particularly limited. Furthermore, the steel material 11 can also be a steel material formed with a metal film or alloy film of less than 1 μm such as Zn, Ni, Sn or these alloys.

Next, the coating layer 12 is described. The coated steel material 1 of this embodiment has a coating layer 12 disposed on the surface of the steel material 11. When the steel material 11 is a steel plate, the coating layer 12 is disposed on at least one side of the steel plate and the other side opposite to the one side. In addition, the coating layer 12 may be disposed on the end surface between one side and the other side of the steel plate.

The plating layer 12 of this embodiment is mainly composed of a Zn-Al-Mg alloy layer due to the chemical composition described below. The Zn-Al-Mg alloy layer is composed of a Zn-Al-Mg alloy. The so-called Zn-Al-Mg alloy means a ternary alloy containing Zn, Al and Mg. Compared with a general Zn plating layer, a Zn-Al-Mg alloy layer formed by adding alloy elements such as Al and Mg to Zn will improve corrosion resistance. For example, even if the thickness is about half of a general Zn plating layer, the Zn-Al-Mg alloy layer will have the same corrosion resistance as a Zn plating layer. Therefore, the coating layer of this embodiment also has corrosion resistance equal to or higher than that of the Zn coating layer.

Furthermore, the coating layer 12 of the coated steel material 1 of the present embodiment may also contain a Fe-Al based interface alloy layer (hereinafter referred to as Al-Fe alloy layer) between the steel material 11 and the Zn-Al-Mg alloy layer. Furthermore, the Al-Fe alloy layer is an interface alloy layer located between the steel material and the Zn-Al-Mg alloy layer.

The plating layer of this embodiment can be a single layer structure of a Zn-Al-Mg alloy layer, or a laminated structure containing a Zn-Al-Mg alloy layer and an Al-Fe alloy layer. If it is a laminated structure, the Zn-Al-Mg alloy layer can be used as a layer constituting the surface of the plating layer. However, although an oxide film of the constituent elements of the plating layer is formed on the outermost surface of the plating layer and the thickness is less than about 1μm, it is relatively thin relative to the overall thickness of the plating layer, and therefore, it can be ignored from the perspective of the main body of the plating layer.

Generally speaking, the thinner the coating, the better the workability. However, if corrosion causes the coating to be consumed, the thicker the coating, the easier it is to ensure corrosion resistance. Therefore, the overall thickness of the coating should be set to 10~70μm. In addition, the overall thickness of the coating is affected by the coating conditions, so the overall thickness of the coating is not limited to the range of 10~70μm. Regarding the overall thickness of the coating, it is affected by the viscosity and specific gravity of the coating bath in the general melt coating method. Then, the overall thickness of the coating can be adjusted by the withdrawal speed of the steel material (coated master plate) and the strength of wiping.

The Al-Fe alloy layer is formed on the surface of the steel (specifically, between the steel and the Zn-Al-Mg alloy layer), and in terms of structure, it is a layer with Al ₅ Fe ₂ phase as the main phase. The Al-Fe alloy layer is formed by mutual atomic diffusion between the base iron (steel) and the plating bath. When the manufacturing method uses the melt plating method, the Al-Fe alloy layer is easily formed in the plating layer containing the Al element. Since the plating bath contains Al above a certain concentration, the Al ₅ Fe ₂ phase will be formed the most. However, atomic diffusion takes time, and there are also parts with high Fe concentrations near the base iron. Therefore, the Al-Fe alloy layer sometimes contains a small amount of AlFe phase, Al ₃ Fe phase, Al ₂ Fe phase, etc. locally. In addition, since the plating bath contains a certain concentration of Zn, the Al-Fe alloy layer also contains a small amount of Zn.

When Si is contained in the coating, Si is particularly easy to be introduced into the Al-Fe alloy layer, and sometimes an Al-Fe-Si intermetallic compound phase is formed. As for the intermetallic compound phases that can be identified, there is an AlFeSi phase, and there are α, β, q1, q2-AlFeSi phases as isomers. Therefore, these AlFeSi phases are sometimes detected in the Al-Fe alloy layer. The Al-Fe alloy layer containing these AlFeSi phases is also called an Al-Fe-Si alloy layer.

Next, the average chemical composition of the coating is explained. When the coating is a single-layer structure of the Zn-Al-Mg alloy layer, the average chemical composition of the entire coating is the average chemical composition of the Zn-Al-Mg alloy layer. In addition, when the coating is a multilayer structure of the Al-Fe alloy layer and the Zn-Al-Mg alloy layer, it is the average chemical composition of the Al-Fe alloy layer and the Zn-Al-Mg alloy layer combined.

In general melt plating, since the reaction to form the plating layer is basically completed in the plating bath, the chemical composition of the Zn-Al-Mg alloy layer is almost the same as that of the plating bath. In addition, in the melt plating method, the Al-Fe alloy layer is formed and grown immediately after immersion in the plating bath. Then, the Al-Fe alloy layer completes the formation reaction in the plating bath, and its thickness is mostly thin enough compared to the Zn-Al-Mg alloy layer. Based on this, as long as no special heat treatment such as heating alloying treatment is performed after plating, the average chemical composition of the entire plating layer will be substantially the same as the chemical composition of the Zn-Al-Mg alloy layer, and the components of the Al-Fe alloy layer can be ignored.

The chemical composition of the coating of the present embodiment contains: Zn, other alloying elements, and impurities. In addition, the chemical composition of the coating of the present embodiment may also be composed of Zn, other alloying elements, and impurities. The chemical composition of the coating is described in detail below. In addition, although the elements whose lower limit of concentration is described as 0% are not required to solve the problem of the coated steel of the present embodiment, any element allowed to be contained in the coating for the purpose of improving characteristics, etc.

Al: 10.0%~40.0% Al is an element constituting the main body of the plating layer, the same as Zn. Although the sacrificial anti-corrosion effect of Al is small, the inclusion of Al in the plating layer will improve the corrosion resistance of the planar part. In addition, when Al is not present, Mg cannot be stably maintained in the plating bath, so Al is contained in the plating bath as an indispensable element in manufacturing.

The Al content is set to 10.0% or more, which is a necessary content for containing a large amount of Mg described later, and a necessary content for ensuring workability. If the content is less than this, it will be difficult to build a bath to make a coating bath, and it will become difficult to ensure the workability of the coating layer and the corrosion resistance. In addition, the Al content is set to 40.0% or less, because the sacrificial corrosion protection effect of Al on steel is weak, and if the content is greater than this, it will become impossible to obtain sufficient sacrificial corrosion protection, so the upper limit is set to 40.0% or less.

Mg: 5.0%~12.5% Mg has a sacrificial anti-corrosion effect and is an element that improves the corrosion resistance of the coating. Containing a certain amount of Mg will form a MgZn ₂ phase in the coating. The higher the Mg content in the coating, the more MgZn ₂ phases are formed. It is known that the MgZn ₂ phase will adopt a structure called Laves' phases, and it is known that its hardness is high. The Mg content is set to 5.0% or more, which is the concentration required to exert corrosion resistance; when it is less than 5.0%, sufficient corrosion resistance cannot be obtained. In addition, the MgZn ₂ phase cannot be fully formed in the coating, and the corrosion resistance of the coating itself will also decrease. When the Mg content is excessive, it becomes difficult to manufacture the coating layer and the workability of the coating layer decreases, so the upper limit is 12.5% or less. Preferably, the Mg content is 6.0% to 8.0%.

Ti: 0%~1.0% Zr: 0%~1.0% When one or both of Ti and Zr are contained in the coating bath, intermetallic compounds of Al ₃ Zr and Al ₃ Ti are formed respectively. Also, when Si is present, Al _2.7 Si _0.3 Zr and Al _2.5 Si _0.5 Ti are formed. These have good lattice matching with the Al phase in the crystal structure and act as solidification nuclei of Al primary crystals. When the intermetallic compound containing Ti and Zr acts as the solidification nuclei of Al primary crystals, on the surface of the coated steel material, the (100) plane of Al will be preferentially oriented parallel to the coating layer surface. When the total concentration of Ti and Zr reaches 0.001% or more, the (100) plane of Al will begin to form parallel to the surface of the coating layer; as the concentration of Ti and Zr increases, the orientation of the (100) plane tends to increase. Therefore, the lower limit of the total concentration of Ti and Zr is 0.001% or more, preferably 0.020% or more.

When the total concentration of Ti and Zr reaches 0.020% or more, the Al primary crystals will begin to be refined, and the Al primary crystals will tend to be refined as the concentration increases. When the total concentration of Ti and Zr is 0.5%, the grain refinement effect is saturated. The refinement of the crystal structure contributes to the improvement of workability. Based on this point, in order to refine the Al primary crystals, the total concentration of Ti and Zr is preferably 0.020% or more. On the other hand, when the concentration of Ti and Zr becomes high, the preparation of the plating bath tends to become difficult; and when Ti and Zr are greater than 1.0%, they tend to produce a large amount of scum, often cause unplating, and deteriorate the appearance and corrosion resistance. Therefore, the concentrations of Ti and Zr are set to be less than 1.0% respectively. The total concentration of Ti and Zr is 0.001% to 2.0%, preferably 0.001 to 1.5% or 0.010% to 0.5%, and more preferably 0.1 to 0.5%.

Furthermore, when one or both of Ti and Zr are contained in the plating bath, the precipitation of the Mg ₂ Zn ₁₁ phase tends to be suppressed, and the workability is improved.

Si: 0%~5.00% Although Si is an optional additive element, when Si is contained in the plating bath, a single Si phase or _Mg2Si will precipitate in the plating layer; when Ca is further contained, Al-Ca-Si compounds will precipitate. These Si or Si-based compounds precipitate on the surface of the plating layer, thereby having the effect of improving water resistance and water flow resistance. In addition, Si is brought into the Al-Fe alloy layer to form an Al-Fe-Si phase, which inhibits the growth of the Al-Fe alloy layer, thereby having the effect of improving bending workability and also improving plating adhesion. Si should preferably contain 0.05% or more. In addition, when Si is greater than 5.00%, a large amount of scum will be generated and unplated areas will often appear. Therefore, the Si concentration is set to 5.00% or less. The Si concentration is preferably 0% to 5.00%, 0.05% to 3.00%, 0.05% to 1.0%, or 0.10% to 0.50%.

Element Group A Ca: 0%~3.00% Y: 0%~0.50% La: 0%~0.50% Ce: 0%~0.50% Sr: 0%~0.50% Although Ca, Y, La, Ce, and Sr in Element Group A are arbitrarily added elements, these elements are easily oxidized in the atmosphere. If they are present in the plating bath, a dense oxide film will be formed on the bath surface, which has the effect of preventing Mg from oxidizing. Through the above effect, the Mg concentration will be stable, and the desired composition of the plated steel plate can be easily manufactured. In order to properly exert this effect, the content of these elements is set to be greater than 0%, preferably 0.01% or more. In addition, the content of each element has an upper limit. When the content is greater than the upper limit, it tends to become difficult to build the plating bath. In addition, the amount of scum and the like attached will increase, and the appearance and corrosion resistance will tend to deteriorate.

Therefore, Ca is set to 0% to 3.00%, preferably greater than 0% and less than 2.00%, more preferably greater than 0.01% and less than 2.00%, and more preferably greater than 0.01% and less than 1.50%. Furthermore, Ca may be less than 1.00%, less than 0.60%, or less than 0.50%.

Furthermore, Y, La, Ce, and Sr are set to 0%~0.50%, preferably greater than 0% and less than 0.50%, and more preferably greater than 0.01% and less than 0.50%. In addition, each element can be greater than 0.01%, less than 0.40%, or less than 0.30%. Element group A will form compounds with Al and Zn in the coating structure. Take Ca as an example. When Ca is greater than 0%, preferably greater than 0.01%, an Al-Ca-Zn compound will be formed; when Si is further present, an Al-Ca-Si compound will be easily formed. Furthermore, when Y, La, Ce, and Sr are contained, a compound in which Ca of the above compounds is replaced by each element will be formed.

Element group B Sn: 0%~3.00% Bi: 0%~1.00% In: 0%~1.00% Although the elements of element group B are arbitrarily added elements, these elements have the function of improving corrosion resistance. However, these elements tend to bond with Mg more than with Zn, and the effect of containing Mg becomes smaller, so there is an upper limit to the content of these elements. When it is greater than the upper limit, scum and the like will adhere more and corrosion resistance will tend to deteriorate. Therefore, Sn is set to 0~3.00%, preferably greater than 0% and less than 3.00%. Sn can also be: more than 0.01%, more than 0.05%, less than 2.50%, less than 2.00%, and less than 1.50%. Bi is set to 0%~1.00%, preferably greater than 0% and less than 1.00%. Bi can also be greater than 0.01%, greater than 0.05%, less than 0.80%, less than 0.50%, and less than 0.40%. In is set to 0%~1.00%, preferably greater than 0% and less than 1.00%. In can also be greater than 0.01%, greater than 0.05%, less than 0.80%, less than 0.50%, and less than 0.40%.

Element group C B: 0%~1.00% P: 0%~0.50% B and P of element group C are semi-metallic elements. Although these elements are arbitrarily added elements, B acts as a solidification nucleus of Al primary crystals in the form of _AlB2 , and improves the workability of Al primary crystals by refining Al primary crystals. However, its effect is not large and is not as good as the effects of Ti and Zr. Although P has no effect of refining Al primary crystals, it improves corrosion resistance. The content of each element has an upper limit. When the content is greater than the upper limit, scum and the like will adhere more, and the appearance and corrosion resistance will tend to deteriorate. Therefore, B is set to 0%~1.00%, preferably greater than 0% and less than 0.50%, and more preferably greater than 0% and less than 0.10%. B may be 0.01% or more or 0.05% or more. P is set to 0% to 0.50%, preferably greater than 0% and less than 0.50%, more preferably greater than 0% and less than 0.01%. P may be 0.001% or more, or 0.005% or more.

Element group D Cr: 0%~0.25% V: 0%~0.25% Ni: 0%~1.0% Co: 0%~0.25% Nb: 0%~0.25% Cu: 0%~1.0% Mn: 0%~0.25% Mo: 0%~0.25% W: 0%~0.25% Ag: 0%~1.00% Li: 0%~0.50% Na: 0%~0.05% Ba: 0%~0.25% K: 0%~0.05% Fe: 0%~5.0% Element group D is both a metal element and an optional additive element. These elements are introduced into the coating to improve corrosion resistance. The content of each element has an upper limit. When the content is greater than the upper limit, the adhesion of slag, etc. tends to increase. Accordingly, Cr, V, Co, Nb, Mn, Mo, W, and Ba are set to 0% to 0.25%, preferably greater than 0% and less than 0.25%, greater than 0.01% and less than 0.20%, or greater than 0% and less than 0.10%. Ni, Cu, and Ag are set to 0 to 1.0%, preferably greater than 0% and less than 1.0%, greater than 0% and less than 0.5%, greater than 0% and less than 0.20%, or greater than 0% and less than 0.10%. Ni, Cu, and Ag can be greater than 0.01%, or greater than 0.05%. Li is set to 0% to 0.50%, preferably greater than 0% and less than 0.10%. Li can also be greater than 0.01%. Na and K are set to 0% to 0.05%, preferably greater than 0% and less than 0.03%. Na and K can also be more than 0.01%. In addition, Fe is sometimes inevitably contained in the coating. The reason is that when manufacturing the coating, it sometimes diffuses from the base iron into the coating. Therefore, the Fe content is 0%~5.0%, and it can also be: greater than 0% and less than 2.0%, greater than 0% and less than 1.5%, greater than 0% and less than 1.2%, or greater than 0% and less than 1.0%. Fe can also be: more than 0.1%, more than 0.3%, more than 0.5%, or less than 0.9%.

Element group E Sb: 0%~0.50% Pb: 0%~0.50% Sb and Pb of element group E are optional additive elements and are elements with similar properties to Zn. Therefore, by containing these elements, it is easy to form zinc patterns on the coating surface. However, if excessive amounts are contained, corrosion resistance may be reduced. Therefore, Sb and Pb are set to 0%~0.50%, respectively, preferably: greater than 0% and less than 0.50%, greater than 0% and less than 0.40%, or greater than 0% and less than 0.10%. Sb and Pb can be greater than 0.01% or greater than 0.05%, respectively.

Remaining part: Zn: 50.0% or more and impurities Zn is a low melting point metal that exists as the main phase of the coating layer on the steel. Zn is an element necessary to ensure corrosion resistance and obtain sacrificial corrosion protection for steel. When the Zn content is less than 50.0%, the main body of the metal structure of the Zn-Al-Mg alloy layer will be the Al phase, and the Zn phase used to exhibit sacrificial corrosion protection will be insufficient. Therefore, the Zn content is set to 50.0% or more. It is preferably set to 60.0% or more or 70.0% or more. In addition, the upper limit of the Zn content is: the amount of the remaining part excluding non-Zn elements and impurities. The Zn content can also be 85.0% or less.

Furthermore, impurities in the plating layer refer to components contained in the raw materials or components mixed in during the manufacturing process, as well as components contained in the range that do not affect the effect of the present invention other than the above-mentioned optional added elements. For example, due to mutual atomic diffusion between the steel material (base iron) and the plating bath, a small amount of components other than Fe may be mixed into the plating layer as impurities.

In addition, the coating layer of this embodiment does not exclude the inclusion of elements other than the elements listed above as long as they do not affect the effects of the present invention. The so-called not affecting the effects of the present invention means that the evaluation of the corrosion resistance evaluation and the processability evaluation described below is A or higher.

In addition, the total content of Al, Mg and Zn in the plating layer of this embodiment is preferably 83.7% or more, and may be 90.0% or more, or 94.7% or more.

The average chemical composition of the coating is determined by using an acid containing an inhibitor that can inhibit the corrosion of the base iron (steel) to peel and dissolve the coating to obtain an acid solution. The obtained acid solution is then measured by ICP emission spectrometry or ICP-MS to obtain the chemical composition. There is no particular limitation on the type of acid as long as it can dissolve the coating. The area and weight before and after peeling can also be measured in advance to obtain the coating adhesion amount (g/ ^m2 ).

Next, the structure of the coating is explained. The proportion of the phases contained in the coating has a great influence on the performance of the coating. Even for coatings with the same composition, the phases or structures contained in the metal structure will change due to the manufacturing method, and the performance will be different. The metal structure of the coating can be easily confirmed by using a scanning electron microscope (SEM-EDS) equipped with an energy dispersive X-ray analyzer. In any vertical section (thickness direction) of the coating after mirror finishing, for example, a backscattered electron image is obtained, which can confirm the general metal structure state of the coating. The thickness of the coating layer of this embodiment is about 10~70μm, so it is appropriate to confirm its metal structure with a field of view of 500~5000 times in SEM. For example, when confirming a coating layer with a thickness of 25μm at a magnification of 2000 times, each field of view can confirm the cross section of the coating layer in an area of 25μm (coating thickness) × 40μm (SEM field of view width) = 1000μm ^2. In the case of this embodiment, the SEM field of view of the coating layer may observe a local field of view, so in order to obtain the average information of the coating layer, it is sufficient to select 25 points of the field of view from any cross section to create the average information. That is, the metal structure in a total field of view of 25,000 μm ² is observed to determine the constituent phases or the area ratio and size of the metal structure of the coating layer.

Backscattered electron images obtained by SEM are suitable for easily identifying the phases or structures contained in the coating. Elements with a small atomic number, such as Al, are imaged darker, while elements with a large atomic number, such as Zn, are imaged whiter, so the ratio of these structures can be easily read.

In the EDS analysis, the phase composition is confirmed by precise pinpoints, and the phases can be identified by reading roughly the same component phases from the element distribution analysis. The use of EDS analysis means that phases with roughly the same composition can be identified by performing element distribution analysis. If phases with roughly the same composition can be identified, the area of the crystalline phase in the observation field can be known. If the area is known, the equivalent circular diameter can be calculated. The area ratio of each phase in the observation field can also be calculated. The area ratio of a specific phase in the coating is equivalent to the volume ratio of the phase in the coating.

The coating layer of this embodiment contains: MgZn2 _phase , Al primary crystals containing Al phase, and [ternary eutectic structure of Al/Zn/ _MgZn2 ]. In addition, the coating layer may further contain a residual structure. The Al primary crystals are composed of Al phase alone, or in a form in which the Al phase occupies the center of the dendrite and the Al-Zn phase occupies the periphery.

Regarding the content ratio of the phases and structures in the coating layer, in any vertical section (thickness direction) of the coating layer, the area ratio when observed by a scanning electron microscope observation field is preferably as follows: the area ratio of the MgZn2 _phase is 15% to 50%, the total area ratio of the Al phase and the Al-Zn phase is 15% to 70%, the area ratio of the [ _ternary eutectic structure of Al/Zn/MgZn2] is 0% to 60%, and the area ratio of other phases is 0% to 10%. In addition, if the coating layer of this embodiment satisfies the above chemical composition and the above formula (1), the corrosion resistance and processability will be excellent. Therefore, the area fraction of the phase and structure in the coating layer is not limited to the above range.

MgZn ₂ phase The MgZn ₂ phase of this embodiment is the region where Mg reaches 16 mass% (±5%) and Zn reaches 84 (±5%) in the coating. The MgZn ₂ phase is usually photographed as a gray color intermediate between Al and Zn in the backscattered electron image of the SEM. In the backscattered electron image of the SEM, the MgZn ₂ phase can be clearly distinguished from the Al phase, the Al-Zn phase, and the [ternary eutectic structure of Al/Zn/MgZn ₂ ], etc.

If the proportion of the MgZn ₂ phase in the plating layer increases, the corrosion resistance will be improved. On the other hand, the MgZn ₂ phase, which is called the Lafranc phase, sometimes reduces the workability of the plating layer. Therefore, from the perspective of corrosion resistance, the more MgZn ₂ phases, the better, but from the perspective of ensuring workability, an upper limit can also be set. In order to achieve a balance between corrosion resistance and workability, the proportion of the MgZn ₂ phase in the plating layer can be greater than 15% and less than 50% by area. The area ratio of the MgZn ₂ phase can be greater than 20%, greater than 25%, less than 45%, or less than 40%.

In the present embodiment, the Al phase constituting the primary crystal of Al is the region in the coating layer where the Al content is greater than 40% by mass. Although the Al phase may also contain Zn, the Zn content is less than 60% by mass. The Al phase can be clearly distinguished from other phases and structures in the backscattered electron image of the SEM. That is, the Al phase mostly appears as the darkest in the backscattered electron image of the SEM. In the present embodiment, the Al phase will take the following forms in any cross section: various forms such as block or round/flat shapes and tree-like cross sections. In addition, in the present embodiment, the Al contained in the [ternary eutectic structure of Al/Zn/MgZn ₂ ] does not include the Al phase.

The area ratio of the Al phase in the coating layer is not particularly limited, but may be 5% or more, 10% or more, 20% or more, or 25% or more. In addition, the Al phase may be 60% or less, 55% or less, or 50% or less.

Compared to the MgZn ₂ phase and the [ternary eutectic structure of Al/Zn/MgZn ₂ ], the Al phase has excellent plastic deformability. It is known that the Al phase adopts a face-centered cubic structure as its crystal structure, which is orientation-dependent on processing. During bending processing, if the preferred orientation on the surface of the coating is the (100) plane, the plastic deformability of the coating will be improved, and the coating will extend along with the processing, so the coating cracks in the processed part will become less. In addition, the development tendency of the preferred orientation will vary with the Al content of the coating; if the Al content is low and the Zn content is high, the (110) plane will be the preferred orientation, and as the Al content increases, the preferred orientation will change to the (100) plane.

The preferred orientation of the Al phase in the coating surface can be deduced by performing X-ray diffraction. For example, when measuring X-ray diffraction with a Cu radiation source at an output of 50KV-300mA, the (111) plane appears at around 38.47°, the (200) plane appears at around 44.74°, the (220) plane appears at around 65.13°, and the (311) plane appears at around 78.23°, based on the peak of 2θ. The (200) plane is parallel to the (100) plane, and the (220) plane is parallel to the (110) plane. Therefore, when evaluating the preferred orientation in the coating surface by X-ray diffraction, the (100) plane can be used instead of the (200) plane, and the (110) plane can be used instead of the (220) plane.

Whether the (100) plane appears as a preferential orientation on the surface of the coating layer can be confirmed by comparing the peak intensity of the (200) plane with the peak intensity of other orientation planes; the higher the peak intensity of the (200) plane, the more it can be said that the (100) plane appears as a preferential orientation. In this embodiment, when the diffraction intensity obtained from the X-ray diffraction measurement results of the coating layer satisfies the following formula (1), the (100) plane will be the preferential orientation for the surface of the coating layer, and the processability of the coating layer can be improved.

I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al }≧0.4…(1)

In formula (1), I(200) _Al is the diffraction intensity of Al (200), I(111) _Al is the diffraction intensity of Al (111), I(220) _Al is the diffraction intensity of Al (220), and I(311) _Al is the diffraction intensity of Al (311). In the coating composition of the present invention, the value of formula (1) may be less than 0.7.

I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al } as defined by formula (1) is determined as follows. First, the surface of the coating layer is mechanically polished and chemically polished as necessary to make the surface of the coating layer mirror-finished. Next, for example, an X-ray diffraction device (Rigaku Corporation (model RINT-TTR III), using X-ray output 50kV, 300mA, copper target, goniometer TTR (horizontal goniometer), Kβ filter slit width 0.05mm, long side limiting slit width 2mm, receiving slit width 8mm, receiving slit 2 open, and scanning speed 5deg./min, spacing width 0.01deg, scanning axis 2θ (5~90°) as measurement conditions, to carry out X-ray diffraction measurement. Then, the diffraction intensity (4 The diffraction intensity of Al(111) is the maximum intensity in the range of 4.74°±0.20°, the diffraction intensity of Al(220) is the maximum intensity in the range of 65.13°±0.20°, and the diffraction intensity of Al(311) is the maximum intensity in the range of 78.23°±0.20°. The diffraction intensity is the intensity after deducting the background intensity. From the obtained diffraction intensity, I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al } is calculated.

Furthermore, in any vertical section (thickness direction) of the coating layer, the total area of the Al phase with an equivalent circular diameter of 20 μm or less in the coating layer relative to the total area of all Al phases is preferably 50% or more. That is, in the section in the thickness direction of the coating layer, the total area of the Al phase with an equivalent circular diameter of 20 μm or less relative to the total area of the Al phase is preferably 50% or more. In this way, the Al phase with an equivalent circular diameter of 20 μm or less accounts for more than 50% of the total area of the Al phase, whereby the Al primary crystals are further refined and the workability of the coating layer can be further improved. When the Al primary crystals are refined, the workability of the coating layer tends to be improved, so in the thickness direction of the coating layer, the ratio of the total area of the Al phase with an equivalent circular diameter of 20 μm or less to the area of the total Al phase may be 70% or more. The upper limit may be 100%.

The method for measuring the area ratio of the Al phase with an equivalent circular diameter of less than 20μm is to extract the Al phase in a predetermined observation field through image extraction, measure the area of each Al phase, and deduce the equivalent circular diameter. The specific measurement method is described below.

The observation field is defined as a total of 25,000 μm ² for confirming the phase and structure of the above-mentioned coating layer. In the EDS analysis of the observation field, the phase composition is confirmed with precise points, and the roughly identical component phases are read out from the contrast of the SEM image and the element distribution analysis to identify the Al phase. Since the Al phase is a light element, it can be grasped as a black area in the backscattered electron image of the SEM. The element distribution analysis can also be performed in the EDS analysis to identify the Al phase. The identification method of the Al phase is not limited to the above, and EPMA distribution analysis, for example, can also be used. Then, the Al phase in the observation field is extracted by image processing, and the area of each is measured. If the area is grasped, the equivalent circular diameter of each Al phase can be calculated. Then, the Al phase with an equivalent circular diameter of less than 20 μm is extracted and its total area is calculated. Then, the area ratio (%) of the Al phase with an equivalent circular diameter of less than 20 μm relative to the total area of the Al phase in the observation field is calculated.

Al-Zn phase The Al-Zn phase in the present embodiment is a phase containing Al and more than 60 mass % of Zn. The Al-Zn phase is an aggregate of a fine Zn phase with a particle size of about 1 μm (hereinafter referred to as the fine Zn phase) and a fine Al phase with a particle size of less than 1 μm (hereinafter referred to as the fine Al phase). In the molten coating, Al has a structure that is different from the crystalline structure at room temperature (e.g., 25°C), and can dissolve a large amount of Zn phase, and exists in the form of a high-temperature stable phase containing about 60% of the Zn phase. On the other hand, at room temperature, the Zn phase content in the high-temperature stable phase is greatly reduced, and Al and Zn are balanced and separated, and become present in the form of an Al-Zn phase containing a fine Al phase and a fine Zn phase. That is, the Al-Zn phase is a phase containing a fine Zn phase at a ratio of 60 mass % or more. Since the Al-Zn phase is different in properties from the Al phase and Zn phase contained in the coating layer, it can be distinguished in the reflected electron SEM image and wide-angle X-ray diffraction. In the wide-angle X-ray diffraction, it has a unique diffraction peak as Al _0.403 Zn _0.597 (JCODF # 00-052-0856), Al _0.7 1Zn _0.29 (PDF # 00-019-0057), etc. Accordingly, in this embodiment, the phase with an Al component of 15 to 40 mass % and a Zn component of 60 to 85 mass % is defined as the Al-Zn phase.

As mentioned above, since the Al-Zn phase is a collection of fine Al phase and Zn phase, its plastic deformability is better than that of the MgZn ₂ phase, [Al/Zn/MgZn ₂ ternary eutectic structure]. On the other hand, in terms of corrosion resistance, it is worse than that of the MgZn ₂ phase, [Al/Zn/MgZn ₂ ternary eutectic structure] and Al phase.

The area ratio of the Al-Zn phase is not particularly limited, but may be 5% or more or 10% or more. Furthermore, the area ratio of the Al-Zn phase may be 20% or less, or 15% or less.

In the present invention, the total area ratio of the primary Al crystal, that is, the Al phase and the Al-Zn phase, may be 15% or more and 70% or less. The total area ratio of the Al phase and the Al-Zn phase may be 20% or more, 25% or more, or 30% or less, or 65% or less, 60% or less, 55% or less, or 50% or less.

[Al/Zn/MgZn ₂ ternary eutectic structure] [Al/Zn/MgZn ₂ ternary eutectic structure] is a eutectic structure composed of Al phase, MgZn ₂ phase and Zn phase. In the reflected electron SEM image, it can be clearly distinguished from the MgZn ₂ phase contained in the main phase of the coating layer or the above-mentioned Al phase.

The [ternary eutectic structure of Al/Zn/MgZn ₂ ] containing the Zn phase is allowed to exist to a certain extent, thereby ensuring the sacrifice of corrosion resistance and improving the end surface corrosion resistance. On the other hand, water resistance and water flow resistance will be reduced. Therefore, if water resistance/water flow resistance is considered, the [ternary eutectic structure of Al/Zn/MgZn ₂ ] can also be 60 area% or less. It can be 40 area% or less, 35 area% or less, or 30 area% or less. The lower limit of the area ratio of the [ternary eutectic structure of Al/Zn/MgZn ₂ ] is not particularly limited, and can be set to 0%, or can be set to 5 area% or more, 10 area% or more, or 15 area% or more.

Mg ₂ Zn ₁₁ phase The Mg ₂ Zn ₁₁ phase in the present embodiment is a region where Mg is 5 mass% (±3%) and Zn is 93 (±4%). The Mg ₂ Zn ₁₁ phase is often photographed as a gray color intermediate between Al and Zn in the backscattered electron image of the SEM, and is brighter than the MgZn ₂ phase. Since the Mg ₂ Zn ₁₁ phase has even less plastic deformation ability than the MgZn ₂ phase, it is not advisable to allow it to precipitate in the plated structure. That is, the Mg in the plated structure should preferably exist in the form of: MgZn ₂ phase, [ternary eutectic structure of Al/Zn/MgZn ₂ ], or the Mg ₂ Si phase described later. Specifically, in terms of X-ray diffraction intensity, the ratio of the diffraction intensity of the (322) plane of _{the Mg2Zn11} phase (the maximum intensity in the range of 43.60°±0.20°; hereinafter referred to as I(322) _Mg2Zn11 ) to the diffraction intensity of the (201) plane of the MgZn2 _phase (the maximum intensity in the range of 41.30°±0.20°; hereinafter referred to as I(201) _MgZn2 ) may satisfy I(322) _Mg2Zn11 /I(201) _MgZn2 /≦0.2, or preferably may satisfy less than 0.1. The composition of the coating bath and the cooling rate after being lifted out of the coating bath affect the precipitation of the Mg ₂ Zn ₁₁ phase. The coated steel sheet made in this embodiment satisfies I(322) _Mg2Zn11 /I(201) _MgZn2 /≦0.2, and the proportion of the Mg ₂ Zn ₁₁ phase in the coating structure is less than 1 area %.

The above phases and structures constitute the main phase of the coating, and they account for more than 90% of the coating area. On the other hand, the coating contains elements other than Zn, Mg and Al, thereby forming other metal phases. For example, Si forms Mg ₂ Si phases, and Ca forms Al-Zn-Ca phases. Representative components of the remaining structure sometimes include Mg ₂ Si phases, AlZnCa phases, AlCaSi phases, etc. Among them, although they have the effect of improving weldability and corrosion resistance, their effects are not significant. From the composition of the coating, it is difficult to set the total of the above to be greater than 10 area%, so it can also be less than 10 area%.

Next, the case where the coated steel material of this embodiment is manufactured by the melt plating method is described. The coated steel material of this embodiment can be manufactured by either the immersion plating method (batch method) or the continuous plating method.

The size, shape, and surface morphology of the steel material to be plated are not particularly limited. Even general steel, high-tensile steel, stainless steel, etc., can be used as long as they are steel materials. Steel strips for general structural use are the best. Surface finishing can also be performed by shot blasting, grinding, etc. in advance; it is also not a problem to make a metal film or alloy film of less than 3g/ ^m2 such as Ni, Fe, Zn, Sn, plating, etc. attached to the surface before plating. In addition, as for the pre-treatment of steel materials, it is advisable to fully clean the steel materials by degreasing and pickling.

After the steel surface is fully heated/reduced with reducing gases such as _H2 , the steel is immersed in a plating bath that has been prepared to a predetermined composition. For high-tensile steel, the gas environment during annealing is generally humidified, and internal oxidation methods are used to ensure plating adhesion for high-Si and Mn steels. Through such treatment, unplated and less-defective-looking plating steel can be plated like ordinary steel. Although such steel has a fine grain size steel surface or internal oxide film layer on the base iron side, it does not affect the performance of the present invention.

Regarding the composition of the coating layer, if the melt coating method is used, it can be controlled by the composition of the coating bath. Regarding the coating bath, a predetermined amount of pure metals are mixed and an alloy of the coating bath composition is prepared by, for example, a melting method in an inert gas environment.

The steel material with reduced surface is immersed in a plating bath maintained at a predetermined concentration to form a plating layer with a composition roughly equivalent to that of the plating bath. If the immersion time becomes long or it takes a long time until solidification is completed, the formation of the interface alloy layer will become more active, so the Fe concentration may also increase. However, if the temperature is less than 500℃, the reaction with the plating layer will rapidly slow down, so the Fe concentration contained in the plating layer is generally suppressed to less than 5.0%.

In order to form a molten plating layer, the plating bath should be kept at a temperature of 450℃~550℃. Then, the reduced steel should be immersed for a few seconds. On the surface of the reduced steel, Fe sometimes diffuses into the plating bath and reacts with the plating bath to form an interface alloy layer at the interface between the plating layer and the steel. The interface alloy layer is mainly an Al-Fe intermetallic compound layer (Al-Fe alloy layer). If an interface alloy layer (Al-Fe alloy layer) is formed, the steel below the interface alloy layer (Al-Fe alloy layer) and the plating layer above will be more firmly bonded due to metal chemistry.

After the steel is immersed in the coating bath for a predetermined time, it is lifted out of the coating bath and, while the metal attached to the surface is still molten, it is wiped with _N2 to adjust the coating to the predetermined thickness. The coating thickness is preferably adjusted to 10~70μm. When converted to the coating adhesion amount, it will be 40~450g/ ^m2 (single side).

After adjusting the amount of coating, the molten metal is solidified. The cooling method during the solidification of the coating can be carried out by blowing nitrogen, air or hydrogen/helium mixed gas, water mist cooling, or water flooding. Preferably, water mist cooling is preferred, and water mist cooling containing water in nitrogen is preferred. The cooling speed can be adjusted by the water content ratio.

In this embodiment, if the coating solidification conditions of the normal working conditions are used, such as cooling at an average cooling rate of 5-20°C/second when the coating bath temperature is between 150°C, the structure may not be controlled and the predetermined performance may not be met. Therefore, the cooling step of the coating layer of this embodiment is described below.

Average cooling rate between bath temperature and 380°C: greater than 20°C/sec and less than 50°C/sec. Between bath temperature and 380°C, the Al phase will precipitate in the form of primary crystals, followed by the precipitation of the MgZn ₂ phase. Even within the coating composition range of the present invention, if the degree of supercooling is large, it will tend to grow in orientation planes other than the (100) plane of the Al phase. Therefore, if the Al phase is to preferentially grow in the (100) plane, the average cooling rate must be at least less than 50°C/sec. On the other hand, when the cooling rate is less than 20°C/sec in terms of the average cooling rate, the primary Al crystals will coarsen, and the workability tends to decrease. Therefore, in the region between bath temperature and 380°C, the average cooling rate must be greater than 20°C/sec and less than 50°C/sec.

Average cooling rate between 380℃ and 300℃: 5℃/sec or more and less than 15℃/sec. Between 380℃ and 300℃, Al-Zn phase precipitates from the liquid phase and a ternary eutectic reaction of Zn-Al-MgZn ₂ occurs, the liquid phase disappears and the coating is completely solidified. In the temperature range between 380℃ and 300℃, if the degree of undercooling is large, Mg ₂ Zn ₁₁ phase may be formed. If Mg ₂ Zn ₁₁ phase precipitates, the workability tends to deteriorate. Therefore, the average cooling rate between 380℃ and 300℃ is preferably set to less than 15℃/sec. On the other hand, when the average cooling rate is slower than 5℃/sec, recrystallization of the Al phase occurs and the proportion of orientation planes other than (100) plane increases, so the workability tends to deteriorate. Accordingly, the average cooling rate is preferably set to be 5°C/second or more and less than 15°C/second, and more preferably set to be 5°C/second or more and 10°C/second or less.

Average cooling rate between 300℃ and 150℃: greater than 10℃/sec and less than 20℃/sec In the temperature range between 300℃ and 150℃, the fine Zn phase taken into the Al-Zn phase will be rapidly spit out from the Al-Zn phase. Therefore, if the temperature is cooled slowly in this temperature range, the proportion of the Al phase in the Al primary crystal will increase. In particular, this tendency is strengthened when the Al concentration is high. If the cooling rate between 300 and 150℃ is less than 20℃/sec, the Al-Zn phase will separate into the Al phase and the Zn phase. On the other hand, if the cooling rate is less than 10℃/sec, the ternary eutectic structure will undergo grain growth and form coarse MgZn ₂ phase and Mg ₂ Zn ₁₁ phase, and the workability will tend to deteriorate. Accordingly, in the temperature range between 300°C and 150°C, cooling is preferably performed at an average cooling rate greater than 10°C/second and less than 20°C/second.

Temperature area below 150℃ During the solidification process, the cooling rate of the temperature area below 150℃ will not affect the constituent phases in the coating layer, so there is no need to limit the cooling conditions and it can be cooled naturally.

After cooling, the coating layer can be subjected to various chemical conversion treatments and coating treatments. In addition, in order to further improve the corrosion resistance, touch-up paint or thermal spraying can be applied to the welded parts and processed parts.

The plated steel material of this embodiment can also form a film on the plated layer. The film can be formed in one layer or two or more layers. The types of films directly above the plated layer can be, for example, chromate films, phosphate films, and chromate-free films. The chromate treatment, phosphate treatment, and chromate-free treatment used to form these films can be performed by known methods. However, chromate treatment will most likely cause the weldability on the surface of the plated layer to deteriorate. Therefore, in order to fully exert the effect of improving the weldability in the plated layer, its thickness should be set to less than 1 μm in advance.

There are the following types of chromate treatment: electrolytic chromate treatment in which a chromate film is formed by electrolysis, reactive chromate treatment in which a film is formed by reaction with the material and then the remaining treatment liquid is washed away, and coating chromate treatment in which a treatment liquid is applied to the object and dried without washing to form a film. Any of these treatments can be used.

The electrolytic chromate treatment may be exemplified by electrolytic chromate treatment using the following components: chromic acid, silica sol, resin (phosphoric acid, acrylic resin, vinyl ester resin, vinyl acetate acrylic emulsion, carboxylated styrene butadiene emulsion, diisopropylamine modified epoxy resin, etc.), and hard silica.

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

Chromium-free treatment is particularly suitable because it has no environmental burden. Chromium-free treatment includes the following treatments: electrolytic type chromium-free treatment that forms a chromium-free film by electrolysis, reactive type chromium-free treatment that forms a film by reacting with the material and then rinsing the remaining treatment liquid, and coating type chromium-free treatment that forms a film by applying the treatment liquid to the object and drying it without washing. Any treatment can be used.

In addition, there may be one or more layers of organic resin films on the film directly above the coating layer. The organic resin is not limited to a specific type, and examples thereof include polyester resins, polyurethane resins, epoxy resins, acrylic resins, polyolefin resins, or modified products of these resins. The modified products referred to herein are resins obtained by reacting the reactive functional groups contained in the structures of these resins with other compounds (monomers or crosslinking agents, etc.), wherein the other compounds have functional groups in their structures that can react with the functional groups.

As for such organic resin, one or more organic resins (unmodified) may be mixed, or one or more organic resins obtained by modifying at least one other organic resin in the presence of at least one organic resin may be mixed. In addition, the organic resin film may contain any coloring pigment or rustproof pigment. A water-based material that is dissolved or dispersed in water may also be used.

<Corrosion resistance evaluation> The corrosion resistance of the coating can be evaluated through the accelerated corrosion test provided by JASO and others. Specifically, the number of cycles (time) until red rust is generated is compared. If the number of cycles until red rust is generated is long, the corrosion resistance is evaluated to be good, and if it is short, the corrosion resistance is evaluated to be poor. Since the number of cycles until red rust is generated also varies depending on the amount of coating adhered, the amount of adhered coating of the coated steel materials used for comparison should be the same.

<Evaluation of workability> Bending is performed and the number of cracks in the coating layer of the processed part is measured to evaluate the workability of the coating layer. The fewer cracks in the coating layer, the better the workability. If the coating layer has no cracks, the consumption of the coating layer due to corrosion protection can be suppressed, so the corrosion resistance of the processed part becomes equal to that of the flat part. [Example]

The master plate of the plated steel is cut from a cold-rolled steel plate with a thickness of 0.8 mm to a size of 180 mm × 100 mm. They are all SS400 (general steel). Using a batch melt plating simulator (manufactured by Rhesca), a K thermocouple is installed on a part of the steel plate, and annealing is performed at 800°C in an _N2 reducing gas environment containing 5% _H2 to fully reduce the surface of the steel plate. After that, it is immersed in the plating bath for 3 seconds, then lifted up, and wiped with _N2 gas to make the plating thickness become 20μm (±1μm). The plating thickness is the same on the front and back. After being lifted up from the plating bath, the plated steel is manufactured according to the following cooling conditions A~E.

Condition A: After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 380°C is 30°C/s, the average cooling rate between 380°C and 300°C is 5°C/s, and the average cooling rate between 300°C and 150°C is 15°C/s. Cooling is performed below 150°C.

Condition B (comparison condition): After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 150°C is set at 20°C/s. Below 150°C, the material is allowed to cool.

Condition C (comparison condition): After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 150°C is set at 2°C/s. Below 150°C, the steel material is allowed to cool.

Condition D (comparison condition): After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 150°C is set at 60°C/s. Below 150°C, the material is allowed to cool.

Condition E (comparison condition): After the steel material is taken out of the plating bath, the average cooling rate between the bath temperature and 380°C is set at 30°C/s, the average cooling rate between 380°C and 300°C is set at 1°C/s, and the average cooling rate between 300°C and 150°C is set at 15°C/s. Cooling is performed below 150°C.

The average chemical composition of the coating is determined in the following manner. An acid containing an inhibitor that can inhibit the corrosion of the base iron (steel) is used to peel and dissolve the coating to obtain an acid solution. Then, the obtained acid solution is measured by ICP emission spectrometry or ICP-MS to obtain the average chemical composition of the coating. The results are listed in Table 1A to Table 1F.

The method for determining the area ratio of the phases and structures (MgZn ₂ phase, Al phase, Al-Zn phase, [Al/Zn/MgZn ₂ ternary eutectic structure], and the remaining structure) in the coating layer is as described above, to expose the coating layer in a thickness direction section perpendicular to the steel surface, and to confirm the metal structure at a field of view of 500 to 5000 times. Specifically, the metal structure in a field of view of a total of 25000μm ² is observed to determine the area ratio of the constituent phases or structures of the coating layer metal structure. In terms of confirmation of each phase, the phase composition is confirmed by precise punctuation in EDS analysis, and the phase is identified by reading out roughly the same component phase from the element distribution analysis. By performing elemental distribution analysis, phases of roughly the same composition can be identified.

In addition, regarding the area ratio of the Al phase with an equivalent circular diameter of less than 20μm, the phase composition is confirmed by precise marking in the EDS analysis of the above-mentioned observation field of ^25000μm2 , and the Al phase is identified by reading roughly the same component phase from the element distribution analysis. Then, the Al phase in the observation field is extracted by image processing, and the area of each crystal is measured, and the equivalent circular diameter of each crystal is calculated by calculation. Then, the Al phase with an equivalent circular diameter of less than 20μm is extracted and its total area is calculated. Then, the area ratio (%) of the Al phase with an equivalent circular diameter of less than 20μm relative to the total area of the Al phase in the observation field is calculated.

Furthermore, from the X-ray diffraction pattern of the coating surface measured using Cu-Kα radiation at an X-ray output of 50 kV and 300 mA, I(200) _Al /{(I(111) _Al +I(220) _{Al +I(200) Al +} I(311) _Al } specified by the above formula (1) was determined. The measurement method is as follows. First, the coating surface is mechanically polished and chemically polished as needed to make the coating surface mirror-finished. Next, an X-ray diffraction device (Rigaku Corporation (Model RINT-TTR III), using X-ray output 50kV, 300mA, copper target, goniometer TTR (horizontal goniometer), Kβ filter slit width 0.05mm, long side limiting slit width 2mm, receiving slit width 8mm, receiving slit 2 open, and scanning speed 5deg./min, spacing width 0.01deg, scanning axis 2θ (5~90°) as measurement conditions, to carry out X-ray diffraction measurement. Then, the diffraction intensity (4 The diffraction intensity of Al(111) was the maximum intensity in the range of 4.74°±0.20°), the diffraction intensity of Al(220) was the maximum intensity in the range of 65.13°±0.20°, and the diffraction intensity of Al(311) was the maximum intensity in the range of 78.23°±0.20°. The diffraction intensity is the intensity after deducting the background intensity. From the obtained diffraction intensity, I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al } was calculated. The results are shown in Table 2A and Table 2B.

(Corrosion resistance evaluation) The test piece was cut into 50×100mm and the end faces were not coated and sealed. A composite cycle test was performed according to JASO M609 and M610 to evaluate corrosion resistance. Specifically, the salt-dry-wet repeated test was repeated with salt water spraying, drying, and wetting. Regarding the salt-dry-wet test, the test material is sprayed with a 5% NaCl aqueous solution (35°C, 2 hours), dried (relative humidity 30%, temperature 60°C, 4 hours) and wet (relative humidity 95%, temperature 50°C, 2 hours) as one cycle, and washed and dried after each cycle. After that, the surface of the test material is observed and the area ratio of red rust is calculated. The area ratio of red rust is calculated by taking a photograph of the surface of the test material after the salt-dry-wet test, and the photograph is processed by binary value through image analysis, and the area of each pixel is calculated, and then the number of pixels in the rusted part is calculated. The area ratio of red rust is calculated by the following formula.

Area ratio of red rust (%) = Area of rusted part (mm ² ) / Area of the entire observation area (mm ² ) × 100

The area ratio of red rust is 5% or more, which is considered to be red rust. The corrosion resistance is evaluated as follows. "B" is considered unqualified, and "A" to "S" are considered qualified. The results are listed in Table 3A and Table 3B.

B: Those with rust observed in less than 200 cycles. A: Those with rust in 200 cycles. AA: Those with rust in more than 200 to less than 350 cycles. AAA: Those with rust in more than 350 to less than 500 cycles. S: Those with no rust in 500 cycles.

(Processability evaluation) The test material was cut into a size of 30mm (C direction) × 100mm (L direction) and subjected to 5t180° bending processing. That is, when the test material was bent, 5 plates of the same thickness as the test material were clamped and bent 180°. After that, the number of cracks in the 30mm×1.6mm range of the top of the bend was counted through a stereo microscope at 40 times, and the judgment was made as follows. The results are listed in Table 3.

B: more than 30 cracks A: more than 20 cracks and less than 30 cracks AA: more than 10 cracks and less than 20 cracks AAA: more than 5 cracks and less than 10 cracks S: less than 5 cracks

As shown in Table 1A to Table 3B, the chemical composition and metal structure of the coating layers of No. 3 to 43, 53 and 54 (Examples) of the present invention are properly controlled, and both corrosion resistance and processability are excellent. They do not contain Mg ₂ Zn ₁₁ phase.

In the comparative example No. 1, the Al content and Mg content of the molten-deposited layer are insufficient, and the Ti and Zr contents are 0%. In addition, I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al } is less than 0.40. Therefore, in No. 1, both corrosion resistance and workability are insufficient.

In the comparative example No. 2, the Al content of the molten-coated layer is insufficient, and the Ti and Zr contents are 0%. I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al } becomes less than 0.40. Therefore, in No. 2, the workability is insufficient.

In the comparative example No. 44, the amount of Al and the amount of Mg in the melt-plated layer are excessive. Therefore, in No. 44, both the corrosion resistance and the workability are insufficient.

In the comparative example No. 45, the amount of Al in the melt-plated layer is excessive, and the corrosion resistance of No. 45 is insufficient.

In the comparative example No. 46, the Ti and Zr contents of the molten-coated layer were 0%. In addition, I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al } became less than 0.40. Therefore, in No. 46, the workability was insufficient.

In the comparative example No. 47, the amount of Ca in the melt-plated layer was excessive. In addition, the manufacturing conditions were outside the range of appropriate conditions. In No. 47, both the corrosion resistance and the workability were insufficient.

The manufacturing conditions of Comparative Examples No. 48, 49, and 50 are outside the range of appropriate conditions. In addition, I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al } becomes less than 0.40. Therefore, in No. 48, 49, and 50, the workability is reduced.

In the comparative example No. 51, the amount of Zr in the melt-plated layer is excessive. Therefore, in No. 51, the corrosion resistance is insufficient.

In the comparative example No. 52, the amount of Ti in the molten-metal coating was excessive. Therefore, in No. 52, the corrosion resistance was insufficient.

[Table 1A]

[Table 1B]

[Table 1C]

[Table 1D]

[Table 1E]

[Table 1F]

[Table 2A]

[Table 2B]

[Table 3A]

[Table 3B]

Industrial Applicability The present invention can provide a coated steel material having excellent corrosion resistance and excellent workability of the coating layer, and in this respect has industrial applicability.

1: Plated steel 11: Steel 12: Plated layer

FIG. 1 is a schematic cross-sectional view of a coated steel material according to an embodiment of the present invention.

1: Plated steel

11:Steel

12: Plating layer

Claims (3)

A coated steel material, comprising: a steel material, and a coating layer on the steel material; the average chemical composition of the coating layer comprises, by mass%, Al: 10.0%~40.0%, Mg: 5.0%~12.5%, Ti: 0%~1.0%, Zr: 0%~1.0%, Si: 0%~5.00%, Ca: 0%~3.00%, Y: 0%~0.50%, La: 0%~0.50%, Ce: 0%~0.50%, Sr: 0%~0.50%, Sn: 0%~3.00%, Bi: 0%~1.00%, In: 0%~1.00%, B: 0%~1.00%, P: 0%~0.50%, Cr: 0%~0.25%, V: 0%~0.25%, Ni: 0%~1.0%, Co: 0%~0.25%, Nb: 0%~0.25%, Cu: 0%~1.0%, Mn: 0%~0.25%, Mo: 0%~0.25%, W: 0%~0.25%, Ag: 0%~1.00%, Li: 0%~0.50%, Na: 0%~0.05%, Ba: 0%~0.25%, K: 0%~0.05%, Fe: 0%~5.0%, Sb: 0%~0.50%, Pb: 0%~0.50%, Remainder: 50.0% or more of Zn and impurities; The total amount of Ti and Zr in the above-mentioned coating is 0.001% or more; The diffraction intensity obtained from the X-ray diffraction measurement result of the above-mentioned coating shall satisfy the following formula (1): I(200) _Al /{(I(111) _Al +I(220) _Al +I(200) _Al +I(311) _Al }≧0.40…(1) Wherein, in formula (1), I(200) _Al is the diffraction intensity of Al (200), I(111) _Al is the diffraction intensity of Al (111), I(220) _Al is the diffraction intensity of Al (220), and I(311) _Al is the diffraction intensity of Al (311). The coated steel material of claim 1, wherein the total content of Ti and Zr in the coating layer is 0.020% or more. The coated steel material of claim 1 or claim 2, wherein, in a cross section of the coating layer in the thickness direction, a ratio of a total area of Al phases having an equivalent circular diameter of 20 μm or less to a total area of the Al phases is 50% or more.

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