阅读说明：本技术 镀层钢板 (Coated steel sheet ) 是由 光延卓哉 真木纯 竹林浩史 高桥武宽 德田公平 于 2020-04-16 设计创作，主要内容包括：一种化学转化处理性优异的镀层钢板,具备钢材和设置于所述钢材的表面的镀层,所述镀层以质量％计含有Al：5.00～35.00％、Mg：2.50～13.00％、Fe：5.00～35.00％、Si：0～2.00％和Ca：0.03～2.00％,余量包含Zn及杂质,在所述镀层的表面中,Fe-Al相的面积分率为0～30％,棒状的Zn/MgZn-(2)层状组织的面积分率为5～90％,块状MgZn-(2)相的面积分率为10～70％,剩余部分的面积分率为10％以下。(A plated steel sheet having excellent chemical conversion treatability, comprising a steel material and a plating layer provided on the surface of the steel material, wherein the plating layer contains, in mass%, Al: 5.00-35.00%, Mg: 2.50-13.00%, Fe: 5.00-35.00%, Si: 0-2.00% and Ca: 0.03 to 2.00%, and the balance of Zn and impurities, wherein the surface area fraction of the Fe-Al phase in the surface of the coating is 0 to 30%, and the rod-like Zn/MgZn 2 The area fraction of the lamellar structure is 5-90%, and the mass MgZn 2 The area fraction of the phase is 10 to 70%, and the area fraction of the remaining phase is 10% or less.)

1. A plated steel sheet comprising a steel material and a plating layer provided on the surface of the steel material,

the coating contains, in mass%

Al：5.00～35.00％、

Mg：2.50～13.00％、

Fe：5.00～35.00％、

Si: 0 to 2.00%, and

Ca：0.03～2.00％，

the balance of Zn and impurities are contained,

in the surface of the plating layer, the area fraction of the Fe-Al phase is 0 to 30%, and the rod-like Zn/MgZn₂The area fraction of the lamellar structure is 5-90%, and the mass MgZn₂The area fraction of the phase is 10 to 70%, and the area fraction of the remaining phase is 10% or less.

2. The plated steel sheet according to claim 1,

the plating layer contains, in mass%, Al: 10.00-30.00%.

3. The plated steel sheet according to claim 1 or 2,

the plating layer contains, in mass%, Mg: 3.00-10.00%.

4. The plated steel sheet according to any one of claims 1 to 3,

the plating layer contains 4.00% by mass or more of Mg.

5. The plated steel sheet according to any one of claims 1 to 4,

the plating layer contains, in mass%, Ca: 0.03 to 1.00 percent.

6. The plated steel sheet according to any one of claims 1 to 5,

the surface area fraction of the layered structure on the surface of the plating layer is 10 to 60%.

7. The plated steel sheet according to any one of claims 1 to 6,

the surface area fraction of Al-Zn dendrites mainly composed of an Al phase and a Zn phase in the surface of the plating layer is 5% or less.

8. The plated steel sheet according to any one of claims 1 to 7,

in the surface of the plating layer, Zn/Al/MgZn₂The area fraction of the ternary eutectic structure is 5% or less.

9. The plated steel sheet according to any one of claims 1 to 8,

the surface of the plating layer has a bulk Zn phase surface area ratio of 10% or less.

10. The plated steel sheet according to any one of claims 1 to 9,

in the surface of the plating layer, Zn/MgZn in a plate shape₂The area fraction of the lamellar structure is 10% or less.

11. The plated steel sheet according to any one of claims 1 to 10,

in the surface of the plating layer, Mg₂The area fraction of the Si phase is 10% or less.

Technical Field

The present invention relates to a plated steel sheet.

The present application claims priority based on patent application No. 2019-080288, which was filed on the date 19/4/2019, the contents of which are hereby incorporated by reference.

Background

In recent years, the development of hot dip coated steel sheets of Zn-Al-Mg has been advanced mainly in the field of building materials.

Patent document 1 discloses an Al — Zn hot-dip coated steel sheet containing, in mass%, Al: 25-90% and Sn: 0.01 to 10% and 0.01 to 10% in total of at least one plating layer selected from the group consisting of Mg, Ca and Sr.

Patent document 2 discloses a chemical conversion treated steel sheet characterized by containing Al/Zn₂A Zn-Al-Mg alloy hot-dip coated steel sheet in which the proportion of the Mg ternary eutectic structure in the outermost surface of the coating is 60 area% or more is used as a base material, and the surface of the coating is covered with a precipitation layer, a phosphate film, and a chemical conversion film, wherein the precipitation layer contains at least one selected from Ni, Co, Fe, and Mn, and the total adhesion amount of Ni, Co, and Fe is 0.05 to 5.0Mg/m²In the range of (1), the amount of Mn deposited is 0.05 to 30mg/m²The layer in the range of (1), wherein the phosphate film is composed of phosphate crystals having an average particle diameter of 0.5 to 5.0 [ mu ] m, the chemical conversion film is a film in which an oxide or hydroxide of a valve metal and a fluoride of a valve metal coexist, a base of the phosphate crystal penetrates into the plating layer and rises from the plating layer, and the chemical conversion film is an organic resin film having an interfacial reaction layer formed at an interface with the plating layer or a precipitation layer exposed between the phosphate crystals.

Patent document 3 discloses a zinc alloy-plated steel material characterized by having, on the surface of the steel material, a coating layer containing Mg: 1-10% of Al: 2-19%, Si: 0.01-2% and Fe: 2 to 75% and the balance of Zn and unavoidable impurities.

Patent document 4 discloses the following technique: to provide a zinc alloy plated steel material excellent in corrosion resistance and weldability, Mg is added to an Al-Zn plated layer.

However, when the techniques disclosed in patent documents 1 to 4 are applied to automotive applications, Al oxide is formed on the surface of the plating layer due to Al contained in the plating layer, and as a result, the chemical conversion treatability may be deteriorated. In particular, in patent document 4, a large amount of Fe — Zn phase is formed in the plating layer, which reduces the chemical conversion treatability.

Under the above circumstances, there is a demand for the development of a plated steel sheet having excellent chemical conversion treatability suitable for automotive use.

Prior art documents

Patent document

Patent document 1: japanese laid-open patent publication No. 2015-214747

Patent document 2: japanese patent No. 4579715 publication

Patent document 3: japanese laid-open patent publication No. 2009-120947

Patent document 4: japanese laid-open patent publication No. 2009-120947

Disclosure of Invention

The present invention has been made in view of the above circumstances, and an object thereof is to provide a plated steel sheet having excellent chemical conversion treatability.

In order to solve the above problems, the present invention adopts the following technical means.

That is, a plated steel sheet according to an aspect of the present invention includes a steel material and a plating layer provided on a surface of the steel material, the plating layer containing, in mass%, Al: 5.00-35.00%, Mg: 2.50-13.00%, Fe: 5.00-35.00%, Si: 0-2.00% and Ca: 0.03 to 2.00%, and the balance of Zn and impurities, wherein the surface area fraction of the Fe-Al phase in the surface of the coating is 0 to 30%, and the rod-like Zn/MgZn₂The area fraction of the lamellar structure is 5-90%, and the mass MgZn₂The area fraction of the phase is 10 to 70%, and the area fraction of the remaining phase is 10% or less.

Here, the plating layer may contain, in mass%, Al: 10.00-30.00%.

In addition, the plating layer may contain, in mass%, Mg: 3.00-10.00%.

The plating layer may contain 4.00% by mass or more of Mg.

In addition, the plating layer may contain, in mass%, Ca: 0.03 to 1.00 percent.

In addition, the surface area fraction of the layered structure on the surface of the plating layer may be 10 to 60%.

In addition, in the surface of the plating layer, an area fraction of Al — Zn dendrites mainly composed of an Al phase and a Zn phase may be 5% or less.

Further, in the surface of the plating layer, Zn/Al/MgZn₂The area fraction of the ternary eutectic structure may be 5% or moreThe following steps.

In addition, the surface area fraction of the bulk Zn phase in the surface of the plating layer may be 10% or less.

Further, in the surface of the plating layer, Zn/MgZn is in the form of plates₂The area fraction of the lamellar structure may be 10% or less.

Further, in the surface of the plating layer, Mg₂The area fraction of the Si phase may be 10% or less.

According to the above aspects of the present invention, a plated steel sheet having excellent chemical conversion treatability can be provided.

Drawings

Fig. 1 is an SEM image showing the surface structure of the plating layer according to the present embodiment.

Fig. 2 is an SEM image showing the surface texture of the plating layer according to the prior art.

Detailed Description

Hereinafter, a plated steel sheet excellent in chemical conversion treatability and a method for manufacturing the same according to the present embodiment will be described. In the present embodiment, the numerical range indicated by the term "to" means a range including numerical values described before and after the term "to" as a lower limit value and an upper limit value.

[ plated steel sheet ]

The plated steel sheet according to the present embodiment comprises a steel material and a plating layer provided on the surface of the steel material,

the coating contains, in mass%

Al：5.00～35.00％、

Mg：2.50～13.00％、

Fe：5.00～35.00％、

Si: 0 to 2.00%, and

Ca：0～2.00％，

the balance of Zn and impurities are contained,

in the surface of the plating layer, the area fraction of the Fe-Al phase is 0 to 30%, and the rod-like Zn/MgZn₂The area fraction of the lamellar structure is 5-90%, and the mass MgZn₂The area fraction of the phase is 10 to 70%, and the area fraction of the remaining phase is 10% or less.That is, in the present embodiment, the excellent chemical conversion treatability is provided by making the rod-like Zn/MgZn₂Lamellar structure and bulk MgZn₂While the phase, preferably further the Fe-Al phase, is actively generated in the coating layer, the chemical conversion treatability of the coated steel sheet is improved by suppressing the generation of phases which lower the chemical conversion treatability, such as Al-Zn dendrites and Fe-Zn phases. Moreover, the plated steel sheet according to the present embodiment contains a large amount of rod-shaped Zn/MgZn₂The layered structure can also prevent liquid metal embrittlement cracking (LME) at spot welding (excellent LME resistance is obtained).

< Steel >

The material of the steel material (base steel sheet) to be the base of the plated steel sheet is not particularly limited. Ordinary steel, pre-plated Ni steel, Al-killed steel, and partially high alloy steel can be used. The shape of the steel material is also not particularly limited.

< coating >

The plated steel sheet according to the present embodiment, which is excellent in chemical conversion treatability, has a plated layer on the surface of a steel material.

(chemical composition)

Next, the chemical composition of the plating layer will be described. In the following description, "%" represents "% by mass" unless otherwise specified.

Al：5.0～35.00％

Al is an element necessary for the plating layer to contain elements other than Zn. Other elements are inherently difficult to be contained in the Zn plating layer (Zn layer), and Mg cannot be contained at a high concentration, for example. However, by containing Al in the plating layer (Zn-based plating layer), a plating layer containing Mg can be produced. In addition, Fe dispersed in the plating layer during the alloying treatment reacts (alloys) with Al preferentially to Zn, thereby enabling formation of an Fe — Al phase (e.g., Fe) that is advantageous in post-coating corrosion resistance and LME resistance₂Al₅Phase). Further, the alloying treatment can suppress the formation of an Fe-Zn phase which degrades the corrosion resistance after coating. Further, the Fe-Al phase may not necessarily be formed in the plating layer, but when the Fe-Al phase is formed in the plating layer, the corrosion resistance and the resistance after coating can be further improvedLME property. In addition, the addition of Mg is also effective for suppressing the formation of Fe-Zn phase, and particularly the effect is exhibited by making the Mg concentration 2.50% or more. The Mg concentration is more preferably 4.00% or more.

When the Al concentration is less than 5.00%, alloy elements imparting performance to the plating layer tend to be difficult to contain in addition to Mg. Further, since Al has a low density, an Al phase having a large amount of phase is formed compared with Zn with respect to the content on the mass basis. However, when the Al concentration is less than 5.00%, most of the plating layer tends to be a Zn phase. This also leads to a significant reduction in chemical conversion treatability. From the viewpoint of chemical conversion treatability, it is not preferable that the Zn phase becomes the first phase in the plating layer.

When the Al concentration is less than 5.0%, a large amount of dross (drop) is formed in the plating bath when Mg is added, and it becomes impossible to produce a plated steel sheet. Therefore, the Al concentration is set to 5.0% or more, preferably 5.00% or more, and more preferably 10.00% or more.

On the other hand, if the Al concentration is excessively increased, the proportion of Al phase in the coating layer rapidly increases, and a rod-like Zn/MgZn required for imparting chemical conversion treatability is sometimes provided₂The proportion of the lamellar structure is decreased, and the proportion of the Fe-Al phase is excessively increased, which is not preferable. Therefore, the Al concentration is 35.00% or less, preferably 30.00% or less.

In this way, in the present embodiment, by balancing the Al concentration with the Fe concentration (adjusted to a predetermined concentration range) described later, Al is actively reacted with Fe to form an Fe — Al phase. Therefore, in the present embodiment, the amount of Al present as an Al phase is reduced by causing Al in the plating layer to be present mainly as an Fe — Al phase, and as a result, the content of dendrites mainly composed of an Al phase and a Zn phase, which cause a reduction in corrosion resistance, is reduced.

Mg：2.50～13.00％

Mg is an element required for imparting chemical conversion treatability. When Mg is added to a Zn-based plating layer, Mg forms MgZn as an intermetallic compound₂. Further, Mg also has a property of suppressing the formation of Fe-Zn phase. To is coming toThe chemical conversion treatability of the coating is sufficiently improved, and the generation of Fe-Zn phase is suppressed, and the minimum required Mg concentration is 2.50%. Therefore, the Mg concentration is set to 2.50% or more, preferably 3.00% or more, and more preferably 4.00% or more.

On the other hand, when the Mg concentration exceeds 13.00%, MgZn₂The amount of the phase is undesirably rapidly increased, which causes a loss in the plastic deformability of the plating layer and deterioration in workability. Therefore, the Mg concentration is 13.00% or less, preferably 11.00% or less, and more preferably 10.00% or less.

In this way, in the present embodiment, by adding predetermined amounts of Al and Mg to the plating layer, the formation of the Fe — Zn phase is suppressed. Therefore, in the present embodiment, the Fe — Zn phase is substantially not present in the plating layer. In particular, the Fe — Zn phase is preferably not generated as much as possible because it not only reduces the corrosion resistance after coating but also easily causes red rust when the coated surface is damaged. Further, as the kind of the Fe-Zn phase, a gamma phase, a delta phase, and a zeta phase are exemplified. In order to suppress the formation of the Fe — Zn phase, it is necessary to adjust the chemical composition of the plating layer to the composition according to the present embodiment (particularly, the Al concentration and the Mg concentration are important), and to set the alloying temperature to 440 to 480 ℃.

Fe：5.00～35.00％

If the Fe concentration is less than 5.00%, the amount of Fe is insufficient, and the amount of Fe — Al phase formed may be reduced, which is not preferable. If the Fe concentration is less than 5.00%, the area ratio of Al — Zn dendrites, which does not contribute to the improvement of the chemical conversion treatability, may exceed 5%, which is not preferable. Therefore, the Fe concentration is set to 5.00% or more, preferably 10.00% or more, and more preferably 15.00% or more.

If the Fe concentration exceeds 35.00%, the plating layer according to the present embodiment is highly likely to have no desired metal structure, and the potential may increase with an increase in the Fe component, which may cause an increase in the corrosion rate and may not maintain an appropriate sacrificial corrosion resistance for the steel material, which is not preferable. Therefore, the Fe concentration is set to 35.00% or less. Preferably 30.00% or less, more preferably 25.00% or less.

The Fe concentration is preferably 0.9 to 1.2 in terms of the Al concentration. By setting Fe/Al to the above range, Fe is easily formed₂Al₅And (4) phase(s).

When Fe/Al is less than 0.9, it becomes difficult to generate a sufficient amount of Fe₂Al₅As a result, dendrites composed of an Al phase and a Zn phase are excessively generated.

When the Fe/Al ratio exceeds 1.2, the Fe-Zn intermetallic compound phase is easily formed, and in this case, it is difficult to form Fe₂Al₅And (4) phase(s).

Si：0～2.00％

Si is an element effective for improving adhesion between the steel material and the plating layer. Therefore, the plating layer may contain Si. Since the plating layer may be made to contain no Si, the lower limit of the Si concentration is 0%. Since the effect of improving adhesion by Si is exhibited when the Si concentration in the plating layer is 0.03% or more, when Si is contained in the plating layer, the Si concentration is preferably 0.03% or more.

On the other hand, even if the Si concentration in the plating layer exceeds 2.00%, the effect of improving the adhesion by Si is saturated. Therefore, even when Si is contained in the plating layer, the Si concentration is set to 2.00% or less. The Si concentration is preferably 1.00% or less.

Ca：0.03～2.00％

Ca is an element effective for improving the chemical conversion treatability of the plated steel sheet, and therefore Ca is contained in the plating layer. The effect of improving the chemical conversion treatability of Ca is exhibited when the Ca concentration in the plating layer is 0.03% or more, and therefore the Ca concentration is 0.03% or more, preferably 0.05% or more.

On the other hand, even if the Ca concentration in the plating layer exceeds 2.00%, the effect of improving the chemical conversion treatability of Ca is saturated, and therefore even if Ca is contained in the plating layer, the Ca concentration is set to 2.00% or less. The Ca concentration is preferably 1.00% or less.

And the balance: zn and impurities

The balance of Zn and impurities except Al, Mg, Fe, Si and Ca. Here, the impurities mean elements that are inevitably mixed in during plating, and these impurities may be contained in a total of about 3.00%. That is, the content of impurities in the plating layer may be 3.00 or less.

Examples of the elements that can be contained as impurities and the concentrations of these elements include Sb: 0-0.50%, Pb: 0-0.50%, Cu: 0-1.00%, Sn: 0-1.00%, Ti: 0 to 1.00%, Sr: 0-0.50%, Ni: 0 to 1.00%, and Mn: 0 to 1.00%, etc. If the impurity element is contained in the plating layer in excess of these concentrations, it may prevent the desired characteristics from being obtained, which is not preferable.

The chemical composition of the plating layer can be measured by, for example, the following method. First, an acid solution is obtained by stripping and dissolving a plating layer using an acid containing an inhibitor (corrosion inhibitor) for inhibiting corrosion of a steel substrate (steel material). Next, the chemical composition (type and content of chemical components) of the plating layer can be obtained by measuring the obtained acid solution by ICP analysis. The type of acid is not particularly limited as long as it is an acid capable of dissolving the plating layer. In this measurement method, the chemical composition is measured as an average chemical composition of the entire plating layer to be measured. In the examples described later, the chemical composition (chemical composition) of the plating layer was measured by this method.

(organization)

The plating layer according to the present embodiment has a surface area fraction of Fe-Al phase of 0 to 30%, and rod-like Zn/MgZn₂Lamellar structure (rod-shaped Zn/MgZn)₂Lamellar structure) of 5 to 90 percent, and massive MgZn₂The area fraction of the phase is 10 to 70%, and the area fraction of the remaining phase is 10% or less.

Fig. 1 is an SEM image showing the structure of the surface of the plated layer 10 according to the present embodiment. As shown in fig. 1, on the surface of the plating layer 10 according to the present embodiment, rod-like Zn/MgZn was observed by observation using SEM₂Lamellar structure 11, hexagonal massive MgZn₂Phase 12, and Fe-Al phase 13.

Fig. 2 is an SEM image showing the structure of the surface of the plating layer 100 according to the related art. The plating layer 100 according to the related art shown in fig. 2 is formed by hot dip plating a steel material with Zn — Al — Mg system according to the related art.

As shown in FIG. 2, in the plating layer 100 according to the prior art, since no alloying treatment is performed, Al-Zn dendrites 14, Zn/Al/MgZn are present₂The ternary eutectic structure 15 occupies most of the structure, and bulk Zn phases 16 and Mg are also observed₂Si phase 17. Bulk MgZn₂Phase 18 was not hexagonal in shape, and no rod-like Zn/MgZn was observed₂Lamellar structure and hexagonal massive MgZn₂Phase, Fe-Al phase.

The structure of the plating layer according to the present embodiment will be described below.

Area fraction of Fe-Al phase: 0 to 30 percent

In the plated steel sheet according to the present embodiment, it is preferable that an Fe — Al phase be formed in the plating layer by performing an alloying step after the hot dip plating step as described later. The Fe — Al phase according to the present embodiment is a phase containing an intermetallic compound of Fe and Al, and the intermetallic compound may be Fe₂Al₅FeAl, etc.

In the plating layer according to the present embodiment, the Fe — Al phase is not exposed to the surface structure of the plating layer, and the chemical conversion treatability is excellent. Since the area fraction of the Fe-Al phase which does not impair the chemical conversion treatability is 30%, the upper limit thereof is 30%, preferably less than 20%.

Further, the Fe — Al phase is not only a structure important for chemical conversion treatability, but also a structure important for preventing liquid metal embrittlement cracking (LME) at spot welding (obtaining excellent LME resistance).

Rod-shaped Zn/MgZn₂Area fraction of lamellar structure: 5 to 90 percent

Rod-shaped Zn/MgZn₂The lamellar structure is composed of Zn phase and MgZn as intermetallic compound₂A rod-like lamellar structure composed of phases. Here, the rod-like means: Zn/MgZn₂MgZn contained in lamellar structure₂The three-dimensional shape of the phase is rod-like, and the Zn phase surrounds rod-like MgZn₂Tissue morphology around the facies. The rod-shaped Zn/MgZn₂The layered structure is treated by chemical conversion so that the plating layer according to the present embodiment can exhibit good performanceSexual aspects are important organizations. As described above, the plating layer according to the present embodiment contains Ca and is rapidly cooled at an average cooling rate of 20 ℃/sec or more after the alloying step as described later, whereby a rod-like Zn/MgZn can be formed₂A lamellar structure.

Zn/MgZn in rod form₂When the area fraction of the lamellar structure is 5% or more, excellent chemical conversion treatability can be obtained. Thus, Zn/MgZn is formed in a rod form₂The area fraction of the lamellar structure is 5% or more, preferably 10% or more.

On the other hand, Zn/MgZn in rod form₂When the area fraction of the layered structure exceeds 90%, the effect of improving the chemical conversion treatability is saturated, and unevenness due to eutectic solidification occurs on the surface of the coating layer, which is not preferable because the appearance of the coated steel sheet is deteriorated. Thus, Zn/MgZn is formed in a rod form₂The area fraction of the lamellar structure is 90% or less, preferably 70% or less, and more preferably 60% or less.

Further, Zn/MgZn in the form of a rod₂The layered structure is not only a structure important for chemical conversion treatability, but also a structure important for obtaining desired LME resistance of a plated steel sheet. Although the details of the mechanism are not clear, it is considered that Zn/MgZn is in the form of a rod₂The reason why the lamellar structure imparts excellent LME resistance is that Ca is efficiently contained in the structure.

Bulk MgZn₂Area fraction of phase: 10 to 70 percent

In order to obtain good chemical conversion treatability, it is preferable to use MgZn in a hexagonal shape in a bulk form₂The area fraction of the phase is 10% or more, and more preferably 70% or more.

On the other hand, when having a hexagonal shape of bulk MgZn₂When the area fraction of the phase exceeds 70%, the Fe-Al phase, rod-like Zn/MgZn₂The area fraction of the lamellar structure is too low to obtain good chemical conversion treatability, and therefore, bulk MgZn is formed₂The area fraction of the phase is 70% or less.

Area fraction of the remaining portion: less than 10%

In order to obtain good chemical conversion treatability, Zn/MgZn in rod form except for Fe-Al phase₂Lamellar structure and massive MgZn₂The area fraction of the structure in the remaining portion other than the phase is 10% or less, preferably 7.5% or less, and more preferably 5% or less, in terms of the total amount.

The structure contained in the remainder includes plate-like Zn/MgZn described later₂Lamellar structure, Al-Zn dendrite, Zn/Al/MgZn₂Ternary eutectic structure, bulk Zn phase, Mg₂Si is equal. These tissues contained in the remaining part will be described below.

Plate-like Zn/MgZn₂Lamellar structure: less than 10%

Plate-like Zn/MgZn₂Lamellar structure (plate-like Zn/MgZn)₂Lamellar structure) is composed of a Zn phase and MgZn as an intermetallic compound₂A lamellar structure of a plate-like structure. As described above, Zn/MgZn in rod form₂The lamellar structure is a structure required for obtaining chemical conversion treatability, but the lamellar structure is Zn/MgZn₂The lamellar structure does not contribute to the chemical conversion treatability. Therefore, from the viewpoint of obtaining good chemical conversion treatability, Zn/MgZn in a plate form is used₂The area fraction of the layered structure is 10% or less, preferably 5% or less.

Further, Zn/MgZn in the form of a rod₂Layered structure and sheet-like Zn/MgZn₂A lamellar structure, identified by differences in morphology, based on MgZn₂Whether the phase is rod-shaped or plate-shaped is identified. Here, the rod-like shape means as described above: Zn/MgZn₂MgZn contained in lamellar structure₂The three-dimensional shape of the phase is rod-like, and the Zn phase surrounds rod-like MgZn₂Tissue morphology around the facies. Plate-like means: Zn/MgZn₂MgZn contained in lamellar structure₂The three-dimensional shape of the phases is plate-like. That is, as sheet-like Zn/MgZn₂The lamellar structure has a plate-like Zn phase and a plate-like MgZn phase₂Alternately stacked tissue. Further, MgZn₂The three-dimensional shape of the phase can be obtained by cutting the metal group in the depth direction by one-side mechanical polishing, FIB processing, or the likeThe metal structure was observed on the woven surface to investigate the metal structure.

Area fraction of dendrite mainly composed of Al phase and Zn phase (Al — Zn dendrite): less than 5%

In the process of cooling from the bath temperature after the hot dip plating step described later, first, Al primary crystals (α - (Zn, Al) phases crystallized as primary crystals) are crystallized and grow in dendritic form (hereinafter referred to as Al — Zn dendrites). Then, the alloy is heated to a temperature range of 440 to 480 ℃ to perform alloying treatment, and most of Al — Zn dendrites are replaced with other structures, but some of the Al — Zn dendrites remain after the alloying treatment.

Since the Al — Zn dendrite does not exert a good influence on the chemical conversion treatability and the LME resistance, the area fraction is preferably lower. Therefore, in the plating layer according to the present embodiment, the area fraction of Al — Zn dendrites is 5% or less, and more preferably 3% or less.

"mainly" means that the dendrites contain an Al phase and a Zn phase at an area ratio of about 15% or more, and the remainder excluding the Al phase and the Zn phase may contain 5% or less of Fe, 3% or less of Mg, and 1% or less of steel constituent elements (Ni, Mn).

Zn/Al/MgZn₂Area fraction of ternary eutectic structure: less than 5%

Zn/Al/MgZn₂The ternary eutectic structure is a Zn phase, an Al phase, and MgZn finally solidified outside the Al primary crystal portion by a Zn-Al-Mg eutectic reaction₂Zn layer, Al layer, MgZn layer composed of phases₂The lamellar organization of the layers. Zn/Al/MgZn₂The ternary eutectic structure also has an effect of improving the chemical conversion treatability, but is in a form of a rod-like Zn/MgZn with Fe-Al phase₂The improvement effect is inferior to that of the lamellar structure. Therefore, Zn/Al/MgZn is preferred₂The area fraction of the ternary eutectic structure is lower. Therefore, in the plating layer according to the present embodiment, Zn/Al/MgZn is added₂The area fraction of the ternary eutectic structure is 5% or less, and more preferably 3% or less.

Bulk Zn phase: less than 10%

The bulk Zn phase is a structure that may be formed when the Mg content in the plating layer is low. When in useWhen the bulk Zn phase is formed, the film swell width tends to be large, and therefore the area fraction is preferably low, and preferably 10% or less. The bulk Zn phase is with Zn/MgZn₂A phase in which the Zn phase contained in the binary eutectic structure is opened. The bulk Zn phase has a dendritic shape and is sometimes observed as a circle in the cross-sectional structure.

Other intermetallic compound phases: less than 10%

Since other intermetallic compound phases do not exert a good influence on the chemical conversion treatability, the area fraction is preferably 10% or less, and more preferably 5% or less. Examples of the other intermetallic compound phase include Mg₂SiCaZn₁₁Phase, Al₂CaSi₂Phase, Al₂CaZn₂Are equal.

In the present embodiment, unless otherwise specified, "surface area ratio" means: for 5 different samples selected at random, the arithmetic mean value of the surface area ratios of the desired structure in the coating surface was calculated. The area fraction substantially represents a volume fraction in the plating layer.

< method for measuring area integration ratio >

The area fraction of each structure in the plating layer was determined by the following method.

First, a plated steel sheet to be measured was cut into 25(c) mm × 25(L) mm, and a surface SEM image and an element distribution image by EDS of the plated layer were obtained. The constituent structure of the coating, i.e. Fe-Al phase, rod-like Zn/MgZn₂Lamellar structure and massive MgZn₂Phase, Al-Zn dendrite, Zn/Al/MgZn₂Ternary eutectic structure, bulk Zn phase, plate-like Zn/MgZn₂Lamellar structure, Mg₂The area fraction of the Si phase and the other intermetallic compound phases was measured by imaging EDS mapping images of the surface of the plating layer at 1 field and 5 fields in total (magnification 1500 times) from 5 different samples and analyzing the images to determine the area fraction of each structure. For example, in the EDS map image, a region containing Fe, Zn, Al, Mg, and Si can be expressed in a color separation manner. Therefore, in this map image, a phase composed of Al and Fe is determined as an Fe-Al phase. In addition, in the mapping image, the bar will be movedMgZn in the form of₂The structure composed of the phase and the Zn phase surrounding the phase is judged as a rod-like Zn/MgZn₂A lamellar structure. Other phases can be judged by the same method. The area of the field of view may be, for example, 45 μm by 60 μm. The area fraction of each tissue is obtained, for example, as an arithmetic average of the area fractions of each tissue measured in each field (i.e., the area of each tissue in any field)/(the area of the field) × 100). In the examples described later, the area fraction of each tissue was measured by this method.

< Property >

The plated steel sheet according to the present embodiment has excellent chemical conversion treatability by including the steel material and the plated layer having the above-described characteristics.

The plated steel sheet according to the present embodiment has excellent LME resistance by including the steel material and the plated layer having the above-described characteristics. The chemical conversion coating applicable to the coated steel sheet according to the present embodiment is not particularly limited, and may be, for example, a Zn phosphate coating formed mainly of Hopeite (Hopeite) which is a Zn phosphate crystal.

[ method for producing plated Steel sheet ]

Next, a method for manufacturing a plated steel sheet according to the present embodiment will be described.

The method for producing a plated steel sheet according to the present embodiment includes: a hot dip coating step of performing hot dip coating by immersing the base steel sheet in a plating bath containing at least Al, Mg, Ca, and Zn; an alloying step of heating the base steel sheet subjected to the hot dip coating at 440 to 480 ℃ for 2 to 8 seconds; and a cooling step of cooling the steel sheet to 335 ℃ at an average cooling rate of 20 ℃/sec or more.

< Process of Hot Dip plating >

In the hot dip coating step, the base steel sheet is immersed in a plating bath containing at least Al, Mg, Ca, and Zn to perform hot dip coating.

In the hot dip coating step, the molten metal is formed by a so-called hot dip coating method in which a coating bath is attached to the surface of the base steel sheet, and then the base steel sheet is lifted from the coating bath to solidify the molten metal attached to the surface of the base steel sheet.

(plating bath)

The composition of the plating bath may contain at least Al, Mg, Ca, and Zn, and a plating bath in which raw materials are mixed and melted so as to have the composition of the plating layer described above may be used.

The temperature of the plating bath is preferably more than 380 ℃ and 600 ℃ or less, and may be 400 to 600 ℃.

The surface of the base steel sheet is preferably subjected to a reduction treatment by heating the base steel sheet in a reducing atmosphere before immersion in the plating bath. For example, the heat treatment is carried out at a temperature of 600 ℃ or higher, preferably 750 ℃ or higher, for 30 seconds or longer in a mixed atmosphere of nitrogen and hydrogen. The base steel sheet after the reduction treatment is cooled to the temperature of the plating bath, and then immersed in the plating bath. The immersion time may be, for example, 1 second or more. When the base steel sheet immersed in the plating bath is lifted, the amount of deposit of the plating layer is adjusted by gas wiping. The amount of adhesion is preferably 10 to 300g/m per surface of the base steel sheet²Can be in the range of 20 to 250g/m²The range of (1).

< alloying Process >

The method for producing a plated steel sheet according to the present embodiment includes an alloying step of heating the base steel sheet subjected to hot dip plating at a temperature of 440 to 480 ℃ for 2 to 8 seconds after the hot dip plating step. By the alloying step, a plating layer having a desired structure (i.e., the above-described structure of the area fraction) is formed, and excellent chemical conversion treatability can be obtained.

In the alloying step, when the heating temperature (hereinafter referred to as the alloying temperature) is lower than 440 ℃, the alloying proceeds slowly, which is not preferable. Therefore, the alloying temperature is 440 ℃ or higher.

On the other hand, when the alloying temperature exceeds 480 ℃, the alloying proceeds excessively in a short time, and therefore the alloying step cannot be controlled well, which is not preferable. For example, in the alloying step, Fe dispersed in the plating layer reacts with Al preferentially to form an Fe — Al phase than Zn, but if the alloying proceeds excessively, the remaining Fe that has not reacted with Al reacts with Zn in the plating layer to form a large amount of an Fe — Zn phase. Therefore, the alloying temperature is set to 480 ℃ or lower.

When the heating time in the alloying step (hereinafter referred to as alloying time) is less than 2 seconds, the progress of alloying is insufficient when the base steel sheet subjected to hot dip plating is heated to a temperature range of 440 to 480 ℃. Therefore, the alloying time is set to 2 seconds or more.

On the other hand, when the alloying time exceeds 8 seconds, the alloying proceeds too significantly, and therefore is not preferable. For example, as in the case of an excessively high alloying temperature, a large amount of Fe-Zn phase is generated. Therefore, the alloying time is set to 8 seconds or less.

In the alloying step, the heating method is not particularly limited, and examples thereof include heating methods such as induction heating.

< Cooling Process >

The method for producing a plated steel sheet according to the present embodiment includes a cooling step of cooling at an average cooling rate of 20 ℃/sec or more in a temperature range from the alloying temperature to 335 ℃ (hereinafter referred to as a cooling temperature range) after the alloying step.

When the coating is cooled at an average cooling rate of less than 20 ℃/sec in the cooling temperature range, a suitable structure (particularly, rod-shaped Zn/MgZn) is not formed in the coating layer₂Lamellar structure) and is therefore not preferred. Therefore, the average cooling rate in the cooling temperature range is set to 20 ℃/sec or more, preferably 25 ℃/sec or more.

The plated steel sheet according to the present embodiment can be manufactured by the above method.

The plated steel sheet according to the present embodiment has excellent chemical conversion treatability. The plated steel sheet according to the present embodiment has excellent LME resistance.

Examples

Example 1

< base steel plate >

As the base steel sheet to be plated, a cold-rolled steel sheet (0.2% C-1.5% Si-2.6% Mn) having a thickness of 1.6mm was used.

< plating bath >

Plating baths having different chemical compositions according to test No. (levels) were prepared so that the plating layers having the chemical compositions shown in table 1 were formed on the base steel sheet. The chemical composition of the coating was measured by the method described above.

< Process of Hot Dip plating >

The base steel sheet was cut into pieces of 100mm × 200mm, and then plated using a batch-type hot dip plating test apparatus. The plate temperature was measured using a thermocouple spot-welded to the central portion of the base steel plate.

Before immersion in the plating bath, in a furnace with an oxygen concentration of 20ppm or less, in the presence of N₂－5％H₂The surface of the base steel sheet was subjected to heat reduction treatment at 860 ℃ in an atmosphere of gas with a dew point of 0 ℃. Then, in N₂After the immersion plate was cooled in air to a bath temperature of +20 ℃, the plate was immersed in the plating bath having the bath temperature shown in table 1 for about 3 seconds.

After the immersion in the plating bath, the substrate is lifted at a lifting speed of 100 to 500 mm/sec. When lifting, use N₂The gas is wiped to control the amount of deposit.

< alloying Process >

After controlling the amount of deposit of the plating layer by using the wiping gas, the alloying step was performed on the plated steel sheet under the conditions of the alloying temperature and the alloying time shown in table 1. In the alloying step, an induction heating device is used.

< Cooling Process >

The plated steel sheet was cooled from the alloying temperature to 335 ℃ by cooling in the cooling temperature range under the conditions shown in table 1.

< tissue Observation >

In order to examine the structure of the plating layer, the prepared sample was cut into 25(c) mm × 25(L) mm, and a surface SEM image and an EDS-based element distribution image of the plating layer were obtained. The constituent structure of the coating layer, i.e., Fe-Al phase, rod-like Zn/MgZn₂Lamellar structure and massive MgZn₂Phase, Al-Zn dendrite, Zn/Al/MgZn₂Ternary eutectic structure, bulk Zn phase, plate-like Zn/MgZn₂Lamellar structure, Fe-Zn phase, Mg₂The area fraction of the Si phase and the other intermetallic compound phases was measured by imaging EDS mapping images of the surface of the plating layer at 1 field and 5 fields in total (magnification 1500 times) from 5 different samples and analyzing the images to determine the area fraction of each structure. The area of each field is set to 45 μm × 60 μm. The specific measurement method is as described above.

The area fraction of each structure in each example and comparative example is shown in table 2.

< chemical conversion treatability >

The chemical conversion treatability was evaluated for each of the examples and comparative examples by the following methods.

The plated steel sheets according to the examples and comparative examples produced by the above-described method were cut into pieces of 50mm × 100mm, and subjected to Zn phosphate treatment (SD5350 series: manufactured by Japan ペイント & インダストリアルコーディング).

The coating rate of the chemical conversion treated crystal was evaluated by SEM observation for the plated steel sheet subjected to Zn phosphoric acid treatment. The chemical conversion treatment crystal coverage was evaluated as "AAA" when it was 100%, as "AA" when it was 98% or more, as "a" when it was 95% or more, as "B" when it was 90% or more and less than 95%, as "C" when it was 85% or more and less than 90%, and as "D" when it was less than 85%, with respect to the surface area. Consider a above as a pass level.

Therefore, the following steps are carried out: in the examples prepared under the predetermined plating bath composition, the appropriate alloying treatment conditions and cooling conditions, the predetermined structure was obtained, and the chemical conversion treatability was excellent.

On the other hand, at the level of Al and Fe deficiency (comparison)In example 1), the dross adheres to the plated steel sheet excessively, and the chemical conversion treatability is greatly deteriorated. At the level of Mg deficiency (comparative example 2), a sufficient amount of massive MgZn could not be formed₂In addition, the remaining portion of the structure (the total of the area fractions (a) to (F)) which deteriorates the chemical conversion treatability is excessively generated (more than 10.0%) and the performance is at a disadvantage.

At the level where the alloying time is too long (comparative example 7), the Fe-Al phase is excessively generated, and the performance is at a disadvantage. At the level of no Ca addition (comparative examples 8 and 25), Zn/MgZn could not be formed₂Rod-like lamellar structure or only a very small amount of Zn/MgZn can be formed₂A rod-like lamellar structure. Moreover, the remaining tissue is excessively generated, and the performance is at a disadvantage.

At a level at which the alloying temperature is too low (comparative example 13) or at a level at which the alloying step is not performed (comparative example 14), Zn/MgZn cannot be formed₂The rod-like lamellar structure, and the remaining part of the structure is excessively generated, resulting in a disadvantage in performance. At the level where the alloying temperature is too high (comparative example 15), an excessive amount of Fe-Al phase is generated, and the performance is at a disadvantage. In the case of the level of excessively high alloying temperature, excessively long alloying time and slow cooling rate (comparative example 36), Zn/MgZn was not formed₂A rod-like lamellar structure, and an excessive amount of Fe-Zn phase (Fe-Zn phase is included as another intermetallic compound phase) is formed, resulting in a disadvantage in performance. At a level at which the cooling rate was too slow (comparative example 16), Zn/MgZn was not formed₂The rod-like lamellar structure, and the remaining part of the structure is excessively generated, whereby the performance is at a disadvantage.

In the level containing Ca excessively (comparative example 24), the remaining portion of the structure was excessively generated, and the performance was at a disadvantage. In the level of excessive Si content (comparative example 26), alloying was inhibited, and Zn/MgZn was not formed₂The rod-like lamellar structure, and the remaining part of the structure is excessively generated, whereby the performance is at a disadvantage.

In the level containing Mg excessively (comparative example 27), Zn/MgZn was not sufficiently produced₂A rod-like layered structure, and excess formation of bulk MgZn₂Phase, performance is at a disadvantage. At a level where Al and Fe are excessively contained (comparative example)35) In the case of medium, an Fe-Al phase is excessively generated, and the performance is at a disadvantage.

Example 2

Example 2 is an example in which the LME resistance was examined for several examples used in example 1. That is, the composition, structure and production conditions of the plated steel sheet used in example 2 are shown in table 1.

< LME resistance >

The plated steel sheets of several examples used in example 1 were cut into a size of 200mm × 20mm, subjected to a hot V bending test, and subjected to hot V bending at 800 ℃. The presence or absence of LME cracks was examined by observing the cross section of the processed portion after V bending, and the LME resistance was evaluated. In a V bending die having an angle of 90 degrees, a case where LME cracks did not occur even if R was 6mm was evaluated as "AAA", a case where LME cracks did not occur even if R was 8mm was evaluated as "AA", and a case where LME cracks did not occur even if R was 16mm was evaluated as "A". Consider a above as a pass level.

The results of evaluation of the LME resistance of each example are shown in table 3. The area fraction of each tissue is not shown in table 3, since it is shown in table 2.

TABLE 3

As shown in table 3, the LME resistance was good in each example. Wherein, more than 30% of rod-shaped Zn/MgZn exists₂In the examples of the layered structure, the LME resistance tends to be "AA". In the examples in which 20% or more of the Fe-Al phase was present, the LME resistance was "AAA". In addition, the LME resistance was evaluated in the same manner as in the comparative examples, and as a result, the LME resistance was B or less in all of the comparative examples.

Description of the reference numerals

10: a plated steel sheet according to the present embodiment;

11: rod-shaped Zn/MgZn₂A lamellar structure;

12: bulk MgZn₂Phase (1);

13: an Fe-Al phase;

14: Al-Zn dendrites;

15: bulk Zn;

16: an Fe-Al phase;

17：Mg₂a Si phase;

18: bulk MgZn₂Phase (not hexagonal shape).