Plated steel sheet

Abstract

This plated steel sheet is provided with a steel substrate, a plating layer formed on the surface of this steel substrate, and an oxidized film formed on the surface of this plating layer. The chemical composition of the plating layer includes, by mass%, 0.000010 to 4.0% Sc, and, in a cross section in the direction of thickness of this plating layer, the surface area percentage of Sc-containing intermetallic compound phase having an equivalent circular diameter of 5.0 μm or smaller is 0.1 to 10.0%, and the thickness of the oxidized film is 10 nm or greater.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet. This application claims priority based on Japanese Patent Application No. 2022-003724 filed in Japan on January 13, 2022, and incorporates its content herein by reference.

Background Art

[0002] In recent years, in order to protect the environment and prevent global warming, it has been demanded to suppress the consumption of chemical fuels. Such a demand is not an exception even for automobiles, which are indispensable for daily life and activities as a means of transportation. In response to such a demand, in automobiles, improvement of fuel efficiency by weight reduction of the vehicle body and the like have been studied. Since most of the structures of automobiles are formed of iron, particularly steel sheets, reducing the thickness of this steel sheet to reduce the weight has a great effect on weight reduction of the vehicle body. However, if the thickness of the steel sheet is simply reduced to reduce the weight of the steel sheet, there is a concern that the strength as a structure may decrease and the safety may decrease. Therefore, in order to reduce the thickness of the steel sheet, it is required to increase the mechanical strength of the steel sheet used so as not to reduce the strength of the structure. Therefore, research and development have been conducted on steel sheets that can maintain or increase the mechanical strength even when made thinner than the steel sheets used previously by increasing the mechanical strength of the steel sheet. Such demands for steel sheets are made not only in the automobile manufacturing industry but also in various manufacturing industries.

[0003] Generally, materials having high mechanical strength tend to have low shape freezing property in forming processes such as bending, and when processing into a complex shape, the processing itself becomes difficult. As one means of solving this problem with formability, the so-called "hot press method (hot stamping method, high-temperature press method, die quenching method)" can be mentioned. In this hot press method, the material to be formed is once heated to a high temperature, and press working is performed on the material softened by heating to form it, and then cooled after forming or simultaneously with forming.

[0004] According to this hot press method, the material is first heated to a high temperature to be softened and then press-worked in the softened state, so that the material can be easily press-worked. Therefore, by this hot press working, a press-molded product that combines good shape freezing property and high mechanical strength can be obtained. Especially when the material is steel, the mechanical strength of the press-molded product can be enhanced by the quenching effect due to the cooling after forming.

[0005] However, when this hot press method is applied to a steel plate, for example, by heating to a high temperature of 800 °C or higher, iron etc. on the surface oxidizes to generate scale (oxide). Therefore, after the hot press working, a process (descaling process) for removing this scale is required, resulting in a decrease in productivity. Also, for members etc. that require corrosion resistance, since it is necessary to perform rust prevention treatment or metal coating on the member surface after processing, a surface cleaning process and a surface treatment process are required, and productivity also decreases.

[0006] As an example of a method for suppressing such a decrease in productivity, it is considered to improve corrosion resistance and omit the descaling process by applying a coating such as plating to the steel plate before hot stamping. Generally, various materials such as organic materials and inorganic materials are used as the coating on the steel plate. Among them, for steel plates, zinc-based plating with a sacrificial corrosion prevention effect is widely applied from the viewpoints of its corrosion prevention performance and steel plate production technology.

[0007] For example, Patent Document 1 discloses a hot press steel plate member having a chemical composition containing, by mass, C: 0.30% or more and less than 0.50%, Si: 0.01% or more and 2.0% or less, Mn: 0.5% or more and 3.5% or less, Sb: 0.001% or more and 0.020% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% or more and 1.00% or less, and N: 0.01% or less, with the balance being Fe and unavoidable impurities. The average crystal grain size of the prior austenite grains is 8 μm or less, the volume fraction of martensite is 90% or more, and the amount of solid solution C is 25% or less of the total C amount. It has a microstructure, a tensile strength of 1780 MPa or more, and further has an Al-based plating layer or a Zn-based plating layer on the surface. Patent Document 1 discloses that by applying a plating layer to the surface of the steel plate, oxidation of the steel plate surface during hot pressing can be prevented, and furthermore, the corrosion resistance of the hot press steel plate member can be improved.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] As described above, by forming a plating layer containing Zn (zinc-based plating layer) on the surface of the steel plate, oxidation of the steel plate surface during hot pressing is prevented, and furthermore, the corrosion resistance of the steel member after hot pressing has been improved. Such a plated steel plate having a zinc-based plating layer is heated to a high temperature such as in hot stamping or welding, and a part of Zn evaporates, and the remaining Zn (zinc) alloyizes with Fe diffused from the steel plate serving as the base material. The Fe-Zn alloy formed by alloying in such a state is likely to generate red rust at an early stage of corrosion. However, Patent Document 1 does not discuss red rust. Therefore, an object of the present invention is to provide a plated steel sheet having a plated layer containing Zn, which can ensure excellent rust resistance even when exposed to high temperatures such as hot stamping and welding.

Means for Solving the Problems

[0010] The inventors of the present invention conducted studies to obtain a plated steel sheet that can ensure excellent rust resistance even when exposed to high temperatures such as hot stamping and welding. As a result, it was found that even in a plated layer containing Zn, by containing a predetermined amount of Sc and forming an intermetallic compound containing Sc, the rust resistance is improved.

[0011] The present invention has been made in view of the above findings. The gist of the present invention is as follows. [1] A plated steel sheet according to an aspect of the present invention includes a base steel sheet, a plated layer formed on the surface of the base steel sheet, and an oxide film formed on the surface of the plated layer. The chemical composition of the plated layer is, by mass%, Sc: 0.000010 to 4.0%, Al: 0 to 93.0%, Fe: 0 to 15.0%, Si: 0 to 20.0%, Mg: 0 to 3.0%, Ca: 0 to 3.0%, La: 0 to 0.5%, Ce: 0 to 0.5%, Y: 0 to 0.5%, Cr: 0 to 1.0%, Ti: 0 to 1.0%, Ni: 0 to 1.0%, Co: 0 to 0.25%, V: 0 to 0.25%, Nb: 0 to 1.0%, Cu: 0 to 1.0%, Mn: 0 to 1.0%, Sr: 0 to 0.5%, Sb: 0 to 0.5%, Pb: 0 to 0.5%, B: 0 to 0.5%, Li: 0 to 1.0%, Zr: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 0.5%, Ag: 0 to 1.0%, P: 0 to 0.5%, Sn: 0 to 1.0%, Bi: 0 to 1.0%, In: 0 to 1.0%, and the balance: 7.0% or more of Zn and impurities. The total content of Mg, Ca, La, Ce, Y, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Sr, Sb, Pb, B, Li, Zr, Mo, W, Ag, P, Sn, Bi, and In is 0 to 5.0%. In the cross-section in the thickness direction of the plated layer, the area ratio of the intermetallic compound phase having a circular equivalent diameter containing Sc of 5.0 μm or less is 0.1 to 10.0%, and the thickness of the oxide film is 10 nm or more. The plated steel sheet described in [2][1] may contain, in terms of mass%, Al: 19.0 to 93.0% and Sc: 0.00050 to 0.30% in the plating layer. The plated steel sheet described in [3][1] contains, in terms of mass%, Al: 19.0 to 93.0% and Sc: 0.010 to 0.30% in the plating layer, and the area ratio of the intermetallic compound phase may be 1.0 to 10.0% in the cross-section in the thickness direction of the plating layer.

Effects of the Invention

[0012] According to the above aspect of the present invention, there is provided a plated steel sheet having a plating layer containing Zn, which can ensure excellent rust resistance even when exposed to high temperatures such as hot stamping and welding.

Brief Description of the Drawings

[0013]

Figure 1

Modes for Carrying Out the Invention

[0014] A plated steel sheet according to an embodiment of the present invention (the plated steel sheet according to the present embodiment) will be described. As shown in FIG. 1, the plated steel sheet 1 according to the present embodiment includes a base steel sheet 10, a plating layer 20 formed on the surface of the base steel sheet 10, and an oxide film 30 formed on the surface of the plating layer. Further, in the plated steel sheet 1 according to the present embodiment, the chemical composition of the plating layer 20 contains, in terms of mass%, Sc: 0.000010 to 4.0%, and may contain Al, Fe, and Si as necessary. Furthermore, as necessary, one or more selected from the group consisting of Mg, Ca, La, Ce, Y, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Sr, Sb, Pb, B, Li, Zr, Mo, W, Ag, P, Sn, Bi, and In may be further contained in a total range of 5.0% or less. The balance of the chemical composition consists of Zn and impurities. Further, in the plated steel sheet 1 according to the present embodiment, in the cross section in the thickness direction of the plating layer 20, an intermetallic compound phase 21 having a circle equivalent diameter containing Sc of 5.0 μm or less exists at a predetermined area ratio. Further, the thickness of the oxide film 30 is 10 nm or more. Hereinafter, the reasons for each limitation will be described.

[0015] [Base metal steel sheet] In the plated steel sheet 1 according to the present embodiment, the plating layer 20 is important, and the type of the base metal steel sheet 10 is not particularly limited and may be determined according to the product to be applied, the required strength, the plate thickness, etc. As the base metal steel sheet, for example, a hot-rolled mild steel sheet described in JIS G3131:2018 or a cold-rolled steel sheet described in JIS G3141:2021 can be used.

[0016] [Plating layer] In the plated steel sheet 1 according to the present embodiment, a plating layer 20 is provided on at least a part of the surface of the base metal steel sheet 10. In FIG. 1, the plating layer 20 is formed on one side of the base metal steel sheet 10, but it may be formed on both sides.

[0017] [Chemical composition] The chemical composition of the plating layer 20 of the plated steel sheet 1 according to the present embodiment will be described. Hereinafter, % regarding the content of each element means mass%.

[0018] Sc: 0.000010 to 4.0% Sc is an important element in the plating layer 20 of the plated steel sheet 1 according to the present embodiment. In a plated steel sheet having a zinc-based plating layer, when heated to a high temperature such as hot stamping or welding, a part of Zn evaporates. However, when the plating layer 20 contains 0.000010% or more of Sc, the evaporation of Zn at high temperature is suppressed. By suppressing the evaporation of Zn, a decrease in the Zn content in the Zn-Fe alloy formed on the surface by heating to a high temperature is suppressed, and the rust resistance is improved after high-temperature heating. Conventionally, even if Sc was contained in trace amounts as an impurity in the raw material, it was removed by refining. Therefore, Sc was hardly contained in the plating layer of conventional plated steel sheets. Even when it was rarely mixed as an impurity, it was confirmed that the Sc content was 0.000004% (0.04 ppm) or less. On the other hand, the present inventors newly found that the evaporation of Zn is suppressed by the inclusion of Sc at 0.000010% (0.10 ppm) or more. Although the mechanism by which the evaporation of Zn is suppressed when heated to a high temperature by the inclusion of Sc is not yet clear, in an environment containing oxygen such as air on the surface of the plating layer after plating, a thin oxide film containing Zn oxide (and Al oxide when Al is contained) is formed. It is assumed that Sc moves into this oxide film when the temperature rises due to heating or the like, and modifies the oxide film, thereby suppressing the evaporation of Zn. If the Sc content in the plating layer is less than 0.000010%, the above effect cannot be obtained. Therefore, the Sc content is set to 0.000010% or more. The Sc content is preferably 0.000050% or more, 0.00010% or more, 0.00025% or more, or 0.00050% or more, and more preferably 0.010% or more, 0.025% or more, or 0.050% or more. On the other hand, even if the Sc content exceeds 4.0%, the effect saturates and the cost increases. Therefore, the Sc content is set to 4.0% or less. If necessary, the Sc content may be 2.0% or less, 1.00% or less, or 0.60% or less. Also, if an attempt is made to increase the Sc content to more than 0.30%, it may be difficult to prepare the plating bath, so the Sc content may be set to 0.30% or less. As described later, the above effect is remarkable when most of the Sc exists as an intermetallic compound phase having a diameter equivalent to a predetermined circle.

[0019] Al: 0 to 93.0% Al is an element effective for improving corrosion resistance in a plating layer containing aluminum (Al) and zinc (Zn). Also, Al contributes to the formation of an alloy layer (Al-Fe alloy layer) and is an element effective for improving plating adhesion. The Al content may be 0%, but in order to sufficiently obtain the above effects, Al may be contained. When obtaining the above effects, the Al content is preferably 5.0% or more, more preferably 10.0% or more or 15.0% or more. Also, Al is an element that forms a strong oxide film on the surface of the plating layer and, when Sc is contained at the same time, has the effect of suppressing the evaporation of Zn. When obtaining this effect, the Al content is preferably 19.0% or more, 25.0% or more, or 30.0% or more. On the other hand, when the Al content exceeds 93.0%, the Zn content becomes too low and the rust resistance decreases. Therefore, the Al content is set to 93.0% or less. The Al content is preferably 90.0% or less, 85.0% or less, 80.0% or less, 75.0% or less, or 70.0% or less. The Al content is more preferably 65.0% or less, 60.0% or less, or 55.0% or less.

[0020] Fe: 0 to 15.0% Fe may be contained in the plating layer by diffusing from the plating base plate into the plating layer during production. Especially in the case of hot dip plating, it may contain up to 15.0%, but if the Fe content is 15.0% or less, the influence on rust resistance is small. Therefore, the Fe content is set to 15.0% or less. The Fe content is preferably 12.0% or less, 10.0% or less, 8.0% or less, 6.0% or less, 4.0% or less, or 2.0% or less.

[0021] Si: 0 to 20.0% Si is an element that has the effect of suppressing the formation of an overly thick alloy layer formed between the steel plate and the plating layer when forming a plating layer on the steel plate, and enhancing the adhesion between the steel plate and the plating layer. Also, when contained together with Mg, it forms a compound with Mg and contributes to the improvement of corrosion resistance after painting. Therefore, it may be contained. The Si content may be 0%, but when obtaining the above effects, the Si content is preferably 0.05% or more. The Si content is more preferably 0.5% or more, and even more preferably 1.0% or more. On the other hand, when the Si content exceeds 20.0%, a large amount of Si is contained in the intermetallic compound containing Sc, and the melting point of the intermetallic compound phase containing Sc increases. In this case, even when the plated steel sheet is exposed to a high temperature, the intermetallic compound containing Sc does not melt. As a result, the effect of suppressing Zn evaporation by Sc cannot be sufficiently obtained. Therefore, the Si content is set to 20.0% or less. From the viewpoint of the workability of the plating layer, the Si content may be 17.0% or less, 13% or less, 10.0% or less, 8.0% or less, 6.0% or less, 4.0% or less, or 2.5% or less.

[0022] The chemical composition of the plating layer of the plated steel sheet 1 according to the present embodiment may be Zn and impurities other than the above elements. However, for the purpose of improving various properties or as impurities, one or more selected from the group consisting of Mg, Ca, La, Ce, Y, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Sr, Sb, Pb, B, Li, Zr, Mo, W, Ag, P, Sn, Bi, and In may be contained in the following ranges respectively and in a total range of 5.0% or less. Since these elements may not be contained, the lower limit of the content of these elements is 0%. The Zn content of the plated steel sheet according to the present embodiment is 7.0% or more as a range in which the effect of improving the red rust resistance can be clearly obtained. The Zn content is preferably 8.0% or more, 10.0% or less, 15.0% or more, 18.0% or more, 21.0% or more, 25.0%, 30.0% or more, or 35.0% or more. The upper limit of the Zn content is 100%. If necessary, the Zn content may be 95.0% or less, 90.0% or less, 85.0% or less, 81.0% or less, 75.0% or less, 70.0% or less, 65.0% or less, 60.0% or less, or 55.0% or less.

[0023] Mg: 0 to 3.0% Mg is an element having the effect of enhancing the corrosion resistance of the plating layer. Therefore, it may be contained. On the one hand, if the Mg content exceeds 3.0%, the workability of the plating layer will decrease. In addition, manufacturing problems such as an increase in the amount of dross generated in the plating bath will occur. Therefore, the Mg content is set to 3.0% or less. The Mg content may be 2.0% or less, 1.0% or less, 0.5% or less, or 0.2% or less.

[0024] Ca: 0 to 3.0% When Ca is contained in the plating layer, it is an element that reduces the amount of dross that is easily formed during plating operation with an increase in the Mg content and improves plating productivity. Therefore, Ca may be contained. On the one hand, when the Ca content is high, Ca-based intermetallic compounds such as the CaZn 11 phase are generated in the plating layer, and the corrosion resistance decreases. Therefore, the Ca content is set to 3.0% or less. The Ca content may be 2.0% or less, 1.0% or less, 0.5% or less, or 0.2% or less.

[0025] La: 0 to 0.5% Ce: 0 to 0.5% Y: 0 to 0.5% When the La content, Ce content, and Y content become excessive, the viscosity of the plating bath increases, and it may be difficult to prepare the plating bath itself. Therefore, the La content, Ce content, and Y content are each set to 0.5% or less. The La content, Ce content, and Y content may each be 0.2% or less or 0.1% or less.

[0026] Cr: 0 to 1.0% Ti: 0 to 1.0% Ni: 0 to 1.0% Co: 0 to 0.25% V: 0 to 0.25% Nb: 0 to 1.0% Cu: 0 to 1.0% Mn: 0 to 1.0% Sr: 0 to 0.5% Sb: 0 to 0.5% Pb: 0 to 0.5% B: 0 to 0.5% Li: 0 to 1.0% Zr: 0 to 1.0% Mo: 0 to 1.0% W: 0 to 0.5% Ag: 0 to 1.0% P: 0 to 0.5% These elements replace Al, Zn, etc. in the plating layer and have the effect of improving the corrosion resistance on the acid side by shifting the potential to the noble side. Therefore, they may be included. On the other hand, when these elements become excessive, there is a concern that intermetallic compounds composed of these elements are formed and the corrosion resistance on the acid side and / or the alkali side deteriorates. Therefore, the content of Cr, Ti, Ni, Nb, Cu, Mn, Li, Zr, Mo, Ag is 1.0% or less respectively, the content of Co, V is 0.25% or less respectively, and the content of Sr, Sb, Pb, B, W, P is 0.5% or less respectively. The content of Cr, Ti, Ni, Nb, Cu, Mn, Li, Zr, Mo, Ag may be 0.5% or less, 0.3% or less, or 0.2% or less respectively. The content of Co, V may be 0.10% or less, 0.05% or less, or 0.03% or less respectively. The content of Sr, Sb, Pb, B, W, P may be 0.2% or less or 0.1% or less respectively.

[0027] Sn: 0 to 1.0% Sn is an element that increases the Mg elution rate in the plating layer containing Zn, Al, Mg. It is also an element that forms an intermetallic compound that significantly improves the acid and alkali corrosion resistance of the plating. Therefore, it may be included. On the other hand, when the Mg elution rate increases, the corrosion resistance of the flat part deteriorates. Also, the corrosion resistance on the acid side deteriorates significantly. Therefore, the Sn content is set to 1.0% or less. The Sn content may be 0.5% or less, 0.3% or less, or 0.2% or less.

[0028] Bi: 0 to 1.0% In: 0 to 1.0% Bi and In are elements that form intermetallic compounds that improve the alkali corrosion resistance. Therefore, they may be included. On the one hand, when the Bi content and the In content each exceed 1.0%, the corrosion resistance on the acid side deteriorates significantly. Therefore, the Bi content and the In content should each be 1.0% or less. The Bi content and the In content may each be 0.5% or less, 0.3% or less, or 0.2% or less.

[0029] For Mg, Ca, La, Ce, Y, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Sr, Sb, Pb, B, Li, Zr, Mo, W, Ag, P, Sn, Bi, and In described above, even if the content of each element is within the range described above, if the total content exceeds 5.0%, the corrosion resistance on the acid side and / or the alkali side deteriorates, or the viscosity of the plating bath increases, and it may be difficult to form the plating bath itself. Therefore, the total content of these elements should be 0 to 5.0%.

[0030] The chemical composition of the plating layer 20 is measured by the following method. First, an acid solution for peeling and dissolving the plating layer is obtained by using an acid containing an inhibitor for suppressing the corrosion of the subway (steel material) (for example, an acid obtained by adding 1% of Hibiron (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) to 10% hydrochloric acid). Next, the chemical composition of the plating layer 20 can be obtained by measuring the obtained acid solution by ICP analysis.

[0031] <Structure> In the plating layer 20 of the plated steel sheet 1 according to the present embodiment, in the cross-section in the thickness direction of the plating layer, the area ratio of the intermetallic compound phase having a circle equivalent diameter (circle equivalent diameter) containing Sc of 5.0 μm or less is 0.1 to 10.0%. Intermetallic compound phases with an equivalent circle diameter of 5.0 μm or less can come into contact with the surrounding η-Zn phase and act more effectively to suppress the evaporation of Zn compared to the case where they are dissolved in the α-Al phase or when the equivalent circle diameter is large. If the area ratio of the intermetallic compound phase containing Sc with an equivalent circle diameter of 5.0 μm or less is 0.1% or more, the above effects can be sufficiently obtained and the rust resistance is improved. The area ratio of the intermetallic compound phase containing Sc with an equivalent circle diameter of 5.0 μm or less is more preferably 1.0% or more. The lower limit of the equivalent circle diameter of the target intermetallic compound phase is not limited, but from the perspective of measurement accuracy, it is preferable to target intermetallic compound phases of 0.1 μm or more. On the other hand, to make the area ratio of the intermetallic compound phase containing Sc with an equivalent circle diameter of 5.0 μm or less exceed 10.0%, a Sc content of more than 4.0% is required, which increases the cost. Therefore, the area ratio of the intermetallic compound phase containing Sc with an equivalent circle diameter of 5.0 μm or less is set to 10.0% or less. The area ratio of the intermetallic compound phase containing Sc with an equivalent circle diameter of 5.0 μm or less can be controlled by the Sc content and the cooling conditions after plating, which will be described later.

[0032] The area ratio of the intermetallic compound phase containing Sc with an equivalent circle diameter of 5.0 μm or less in the cross-section of the plating layer (the region within the range of an equivalent circle diameter of 5.0 μm or less and with Sc of 3.0 mass% or more) can be obtained by the following method. Five samples are taken so that the cross-section in the thickness direction of the plating layer can be observed. For these samples, a rectangular range of 100 μm in the thickness direction and 500 μm in the direction perpendicular to the thickness direction is regarded as one field of view, and a total of 5 fields of view are photographed at a magnification of 1500 times using EDS to obtain a mapping image. At that time, the spot diameter may be 1 - 10 nm, the voltage may be 15 kV, and the current may be 10 nA. From the Sc element distribution image of the obtained mapping image, using the "Analyze" function of the image analysis software "ImageJ", the area ratio of the region within the range of an equivalent circle diameter of 5.0 μm or less and with Sc of 3.0 mass% or more to the observed field of view is obtained respectively. The average of the 5 fields of view is calculated and taken as the area ratio of the intermetallic compound phase containing Sc with an equivalent circle diameter of 5.0 μm or less.

[0033] <Coating amount> The coating amount per side of the plating layer (hereinafter simply referred to as the coating amount, which means the coating amount per side) is 20 - 160 g / m 2 is preferable. If the coating amount of the plating layer is less than 20 g / m 2 it may not be possible to obtain sufficient corrosion resistance. On the other hand, if the coating amount exceeds 160 g / m 2 the adhesion of the plating layer may decrease and the plating may peel off. The coating amount of the plating layer may be 30 g / m 2 or more, 40 g / m 2 or more, 50 g / m 2 or more or 60 g / m 2 or more, and may also be 140 g / m 2 or less, 120 g / m 2 or less, 100 g / m 2 or less or 90 g / m 2 or less.

[0034] The coating amount can be measured by the following method. A 30 mm × 30 mm sample is taken from the plated steel sheet, and the plating layer is stripped and dissolved with an acid containing an inhibitor (for example, an acid obtained by adding 1% of Hibiron (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.) to 10% hydrochloric acid) that suppresses the corrosion of the base metal (steel material). After the stripping and dissolution, the weight change of the plated steel sheet is measured, and the coating amount is calculated from the result.

[0035] [Oxide film] In the plated steel sheet according to this embodiment, an oxide film exists on the surface of the plating layer, and its thickness is 10 nm or more. The oxide film can suppress the evaporation of Zn when the plated steel sheet is at a high temperature, such as during hot stamping. If the thickness of the oxide film is less than 10 nm, it is not possible to suppress the evaporation of Zn, and as a result, the resistance to red rust decreases. The thickness of the oxide film is preferably 11 nm or more, more preferably 12 nm or more. The upper limit of the thickness of the oxide film is not limited, but the thickness of the oxide film may be 50 nm or less, 30 nm or less, or 20 nm or less. This oxide film is an oxide film containing Zn oxide (and Al oxide when Al is contained) formed by exposing the plated layer after plating to an environment containing oxygen such as air.

[0036] The thickness of the oxide film is determined by a method using a high-frequency glow discharge optical emission spectrometer (GDS). Specifically, the surface of the test piece is set in an Ar atmosphere, and while generating glow plasma, the surface is analyzed in the depth direction while sputtering. Elements are identified from the emission spectral wavelengths peculiar to the elements emitted when the atoms are excited in the glow plasma, and the emission intensity of the identified elements is estimated. The data in the depth direction is estimated from the sputtering time. By previously obtaining the relationship between the sputtering time and the sputtering depth using a standard sample, the sputtering time is converted into the sputtering depth. The sputtering depth converted from the sputtering time is defined as the depth from the surface. The obtained emission intensity is converted into mass% by preparing a calibration curve. Measure the depth from the outermost surface to the position where the O content becomes 5 mass% measured in this way. This is done at three locations, and the average of the depths at the three locations where the O content becomes 5 mass% is taken as the thickness of the oxide film.

[0037] The thickness of the plated steel sheet according to this embodiment is not limited, but considering the applications to which it is applied, etc., it is preferably 0.5 to 3.0 mm.

[0038] The plated steel sheet according to this embodiment has the above-described characteristics, and as a result, it is excellent in rust resistance even when heated to a high temperature by welding or hot stamping.

[0039] [Manufacturing method] The plated steel sheet according to this embodiment can be manufactured through a step of forming a plated layer containing Sc (plated layer forming step), regardless of the manufacturing method, as long as it has the above-described characteristics, and the effects can be obtained. Hereinafter, preferred conditions will be described.

[0040] <Plated layer forming step> The method for forming the plating layer is not limited, but the molten plating method is exemplified. In the case of the molten plating method, a method including (I) to (III) can be adopted. (I) Immerse a steel sheet (plating base plate) in a molten plating bath containing Sc. (II) Pull up the steel sheet from the plating bath and control the adhesion amount with wiping gas or the like. (III) Cool to room temperature.

[0041] (I) Immersion in the plating bath By immersing the steel sheet in a molten plating bath containing Sc, a plating layer is formed on the surface of the steel sheet. The chemical composition of the plating bath contains, for example, 0.000010 to 4.0% of Sc, and further contains Al, Si, Mg, Ca, La, Ce, Y, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Sr, Sb, Pb, B, Li, Zr, Mo, W, Ag, P, Sn, Bi, In according to the chemical composition of the finally obtained plating layer, and the balance may be Zn and impurities. The steel sheet before immersion in the plating bath may be subjected to a heat reduction treatment. For example, the steel sheet before immersion in the plating bath is heated and reduced on the surface of the plating base plate at 800 °C in an N2-5%H2 gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less, air-cooled with N2 gas, and after the immersion plate temperature reaches the bath temperature + 20 °C, it is immersed in the plating bath. The immersion time in the plating bath is preferably about 1 to 10 seconds.

[0042] (II) Pulling up After immersing the steel sheet in the plating bath in (I), pull up the steel sheet from the plating bath and control the plating adhesion amount with wiping gas such as N2 gas.

[0043] (III) Cooling Cool the steel sheet with the plating adhesion amount controlled to room temperature. By this cooling, a fine intermetallic compound phase containing fine Sc is crystallized. Specifically, by setting the average cooling rate from the bath temperature to (the bath temperature - 50 °C) to 5 °C / second or less and the average cooling rate from (the bath temperature - 50 °C) to 100 °C to 20 °C / second or more, the formation of an intermetallic compound having a circle equivalent diameter containing Sc of 5.0 μm or less is promoted. When the average cooling rate from the bath temperature to (the bath temperature - 50°C) exceeds 5°C / second, Sc is in a state of being dissolved in the α-Al phase. Sc dissolved in the α-Al phase does not crystallize as an intermetallic compound phase. Therefore, in order not to dissolve Sc in the α-Al phase, the average cooling rate from the bath temperature to (the bath temperature - 50°C) is set to 5°C / second or less. After that, during the cooling from the bath temperature to (the bath temperature - 50°C), Sc that did not dissolve in the α-Al phase is concentrated in the Zn-based liquid phase, and by cooling at an average cooling rate of 20°C / second or more from (the bath temperature - 50°C) to 100°C, it crystallizes as a fine intermetallic compound phase. When the average cooling rate from (the bath temperature - 50°C) to 100°C is less than 20°C / second, coarse intermetallic compounds crystallize, and the number density of Sc-containing intermetallic compounds with a circle equivalent diameter of 5.0 μm or less decreases.

[0044] Conventionally, for plated steel sheets, a technique of attaching an acidic solution to the surface to form a pattern part has also been proposed. Although the detailed mechanism is unknown, in the plated steel sheet according to this embodiment, when an acidic solution is applied, the surface oxide film dissolves and Zn evaporation cannot be prevented, so the acidic solution is not attached.

[0045] The steel sheet used in the plating layer forming step is not limited, and any known hot-rolled steel sheet or cold-rolled steel sheet may be used.

[0046] By hot stamping the plated steel sheet according to this embodiment obtained in this way, a hot stamping molded product can be obtained. This hot stamping molded product is excellent in rust resistance. The conditions for obtaining the hot stamping molded product may be known conditions. For example, the plated steel sheet according to this embodiment may be heated to 900°C, held for 100 seconds, and then rapidly cooled simultaneously with molding using a mold.

Examples

[0047] Examples of the present invention are shown below. The examples shown below are examples of the present invention, and the present invention is not limited to the examples described below.

[0048] As the plating substrate, a steel sheet with a thickness of 1.6 mm containing 0.2% by mass of C and 1.3% by mass of Mn was used. After cutting the plating substrate into pieces of 100 mm × 200 mm, plating was carried out using a batch-type molten plating test apparatus. When forming the plating layer, before immersing the plating substrate in the plating bath, the surface of the plating substrate was heated and reduced at 800 °C in an N2-5%H2 gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less, air-cooled with N2 gas, and after the immersion plate temperature reached the bath temperature + 20 °C, it was immersed in the plating bath at the bath temperature shown in Table 1 for about 3 seconds. After immersion in the plating bath, it was pulled up at a pulling speed of 20 - 200 mm / second. During pulling out, the amount of plating adhesion was controlled to the value shown in Table 1 using N2 wiping gas. After pulling out the steel sheet from the plating bath, it was cooled from the plating bath temperature to room temperature under the conditions shown in Table 2. As a result, plated steel sheets numbered 1 - 34 were manufactured. For No. 34, an acidic solution was applied. In each process, the plate temperature was measured using a thermocouple spot-welded to the center of the plating substrate.

[0049] Samples with a size of 30 mm × 30 mm were taken from the obtained plated steel sheets, and these samples were immersed in a 10% HCl aqueous solution containing 1% Hibiron (A - 6) to strip the plating layer by pickling. Then, the chemical composition of the plating layer was measured by ICP analysis of the elements eluted in the aqueous solution. The chemical composition of the plating layer was as shown in Table 1. The A value in Table 1 is the total content of Mg, Ca, La, Ce, Y, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Sr, Sb, Pb, B, Li, Zr, Mo, W, Ag, P, Sn, Bi, and In.

[0050] Also, for the plated steel sheet, the area ratio of the intermetallic compound phase containing Sc with an equivalent circle diameter of 5.0 μm or less (the region where the equivalent circle diameter is 5.0 μm or less and Sc is 3.0% by mass or more) in the cross-section of the plating layer was measured by the method described above. Also, the thickness of the oxide film was measured using GDS by the method described above.

[0051] Next, the obtained plated steel sheet was heat-treated assuming hot stamping. Specifically, the plated steel sheet was heated by inserting it into a heating furnace with the furnace temperature set at 900°C, and the temperature of the plated steel sheet was maintained at that temperature for 100 seconds after it reached the furnace temperature - 10°C. Then, it was taken out of the furnace and the plated steel sheet was sandwiched between flat dies at a temperature around room temperature and rapidly cooled to near room temperature.

[0052] The rust resistance of the plated steel sheet after heating and rapid cooling (after hot stamping) was evaluated by the following method. That is, a sample with a size of 50×100 mm was taken from the plated steel sheet after heating and rapid cooling, and Zn phosphate treatment was carried out according to the SD5350 system (a standard manufactured by Nippon Paint Industrial Coatings Co., Ltd.). Then, electrodeposition coating (PN110 Power Nix Gray: a standard manufactured by Nippon Paint Industrial Coatings Co., Ltd.) was carried out to a thickness of 20 μm, and baking was performed at a baking temperature of 150°C for 20 minutes. After that, a cut reaching the base metal was introduced in the center of the sample, and it was subjected to a composite cycle corrosion test according to the 8.1 neutral salt spray cycle test method of JIS H 8502:1999, and the number of cycles until rust occurred from the cut part was measured. When the number of rust occurrence cycles exceeded 180 cycles, it was rated as "AAA"; when it was more than 120 cycles and less than or equal to 180 cycles, it was rated as "AA"; when it was between 60 and 120 cycles, it was rated as "A"; when it was less than 60 cycles, it was rated as "B". If it was [AAA], "AA" or "A", it was judged to have excellent rust resistance.

[0053] As can be seen from Table 1 and Table 2, in the invention examples where the chemical composition of the plating layer, the area ratio of the intermetallic compound phase with a circle equivalent diameter containing Sc of 5.0 μm or less, and the thickness of the oxide film were within the scope of the present invention, the rust resistance was excellent. On the other hand, when Sc was not contained in the plating layer (No. 1) or when the Sc content was low (No. 7), the area ratio of the intermetallic compound phase with a circle equivalent diameter containing Sc of 5.0 μm or less was low, and the rust resistance was not sufficient. In addition, when the Al content in the plating layer was excessive (No. 6), the area ratio of the intermetallic compound phase with a circular equivalent diameter containing Sc of 5.0 μm or less was low, and the rust resistance was not sufficient. In addition, when the Si content in the plating layer was excessive (No. 28), the rust resistance was not sufficient. This is because a large amount of Si is contained in the intermetallic compound containing Sc, which raises the melting point of the intermetallic compound phase containing Sc. Even when the plated steel sheet was exposed to high temperatures, the intermetallic compound containing Sc did not melt, and it is considered that the Zn evaporation suppressing effect was not sufficiently exerted.

[0054] In addition, even when the chemical composition of the plating layer was within the preferable range, when the cooling conditions after immersion in the plating bath were not preferable conditions (No. 15, No. 24, No. 31), the area ratio of the intermetallic compound phase with a circular equivalent diameter containing Sc of 5.0 μm or less was low, and the rust resistance was not sufficient.

[0055] In addition, when coated with an acidic solution (No. 34), the thickness of the oxide film was thin, and the rust resistance was not sufficient.

[0056]

Table 1

[0057]

Table 2

Industrial Applicability

[0058] According to the present invention, there is provided a plated steel sheet having a plating layer containing Zn, which can ensure excellent rust resistance even when exposed to high temperatures such as hot stamping and welding.

Explanation of Symbols

[0059] 1 Plated steel sheet 10 Steel sheet 20 Plating layer 21 Sc-containing intermetallic compound phase 30 Oxide film

Claims

1. A base steel plate, a plating layer formed on the surface of the base steel plate, and an oxide film formed on the surface of the plating layer, characterized in that: The chemical composition of the plating layer is in mass%, Sc: 0.000010 to 4.0%, Al: 0 to 93.0%, Fe: 0 to 15.0%, Si: 0 to 20.0%, Mg: 0 to 3.0%, Ca: 0 to 3.0%, La: 0 to 0.5%, Ce: 0 to 0.5%, Y: 0 to 0.5%, Cr: 0 to 1.0%, Ti: 0 to 1.0%, Ni: 0 to 1.0%, Co: 0 to 0.25%, V: 0 to 0.25%, Nb: 0 to 1.0%, Cu: 0 to 1.0%, Mn: 0 to 1.0%, Sr: 0 to 0.5%, Sb: 0 to 0.5%, Pb: 0 to 0.5%, B: 0 to 0.5%, Li: 0 to 1.0%, Zr: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 0.5%, Ag: 0 to 1.0%, P: 0 to 0.5%, Sn: 0 to 1.0%, Bi: 0 to 1.0%, In: 0 to 1.0%, and the balance: 7.0% or more of Zn and impurities, the total content of Mg, Ca, La, Ce, Y, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Sr, Sb, Pb, B, Li, Zr, Mo, W, Ag, P, Sn, Bi, and In is 0 to 5.0%, in the cross-section in the thickness direction of the plating layer, the area ratio of the intermetallic compound phase containing Sc and having a circle equivalent diameter of 5.0 μm or less is 0.1 to 10.0%, the thickness of the oxide film is 10 nm or more, a plated steel sheet characterized by the above.

2. The chemical composition of the plating layer is in mass%, Al: 19.0 to 93.0%, and Sc: 0.00050 to 0.30%, characterized in that it contains the plated steel sheet according to claim 1.

3. The chemical composition of the plating layer is in mass%, Al: 19.0 to 93.0%, and Sc: 0.010 to 0.30%, characterized in that it contains in the cross-section in the thickness direction of the plating layer, the area ratio of the intermetallic compound phase is 1.0 to 10.0%, the plated steel sheet according to claim 1.

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