阅读说明：本技术 热冲压成型体 (Hot stamp-molded body ) 是由 小林亚畅 高桥武宽 河村保明 于 2020-05-29 设计创作，主要内容包括：本发明涉及一种热冲压成型体,其具有钢板和形成于上述钢板的至少一面的镀层,上述镀层由存在于上述镀层的表面侧且氧浓度为10质量％以上的ZnO区域、和存在于上述镀层的钢板侧且氧浓度低于10质量％的Ni-Fe-Zn合金区域构成,在上述ZnO区域中,Fe、Mn及Si的合计平均浓度超过0质量％且低于5质量％。(The present invention relates to a hot-stamped steel sheet and a plated layer formed on at least one surface of the steel sheet, wherein the plated layer is composed of a ZnO region having an oxygen concentration of 10 mass% or more present on a surface side of the plated layer and a Ni — Fe — Zn alloy region having an oxygen concentration of less than 10 mass% present on a steel sheet side of the plated layer, and a total average concentration of Fe, Mn, and Si in the ZnO region exceeds 0 mass% and is less than 5 mass%.)

1. A hot-stamped steel comprising a steel sheet and a plating layer formed on at least one surface of the steel sheet, wherein the plating layer comprises a ZnO region having an oxygen concentration of 10 mass% or more and present on the surface side of the plating layer, and a Ni-Fe-Zn alloy region having an oxygen concentration of less than 10 mass% and present on the steel sheet side of the plating layer, and the total average concentration of Fe, Mn and Si in the ZnO region exceeds 0 mass% and is less than 5 mass%.

2. The hot stamp-shaped body according to claim 1, wherein the ZnO region has a thickness of 0.5 μm or more and 3.0 μm or less.

3. The hot stamped steel according to claim 1 or 2, wherein in the Ni-Fe-Zn alloy region, respective concentrations of Zn, O, Mn, and Si decrease from a surface side of the plating layer toward a steel sheet side.

4. The hot stamped steel according to any one of claims 1 to 3, wherein the Ni-Fe-Zn alloy region is composed of a 1 st region having an Fe concentration of less than 60 mass% and a 2 nd region having an Fe concentration of 60 mass% or more in this order from the surface side of the plating layer, the Zn/Ni mass ratio in the 1 st region is in a range of 3.0 to 13.0, and the average Zn/Ni mass ratio in the 2 nd region is 0.7 to 2.0.

5. The hot stamp-shaped body according to claim 4, wherein an average Zn/Ni mass ratio in the 2 nd region is 0.8 or more and 1.2 or less.

Technical Field

The present invention relates to a hot stamped steel. More particularly, the present invention relates to a hot stamped steel having improved surface corrosion resistance.

Background

In recent years, hot stamping (hot pressing) has been widely used for forming steel sheets used for automobile members. The hot stamping method is a method in which a steel sheet is press-formed in a state heated to a temperature in the austenite region, and is quenched (cooled) by a press die simultaneously with the forming, and is one of the methods for forming a steel sheet excellent in strength and dimensional accuracy. Further, with respect to a steel sheet used for hot stamping, a plating layer such as a Zn — Ni alloy plating layer may be provided on the surface of the steel sheet (for example, patent document 1).

A hot press-formed body (also referred to as a "hot-pressed member") obtained by hot-pressing a plated steel sheet having a plating layer on the steel sheet is required to have corrosion resistance so that the surface thereof is not corroded by the surrounding environment (e.g., water).

In connection with the corrosion resistance of the hot stamp-formed body, patent documents 2 and 3 describe a hot-pressed member having a Ni diffusion region in a surface layer of a steel sheet constituting the member, and having an intermetallic compound layer corresponding to a γ phase present in an equilibrium diagram of a Zn — Ni alloy and a ZnO layer in this order on the Ni diffusion region, and a natural immersion potential of-600 to-360 mV based on a standard hydrogen electrode in an air-saturated 0.5m nacl aqueous solution at 25 ℃ ± 5 ℃. Patent document 2 teaches that when the intermetallic compound layer is provided on the hot-pressed member, excellent corrosion resistance after coating can be obtained.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2004-124207

Patent document 2: japanese patent laid-open publication No. 2011-246801

Patent document 3: japanese laid-open patent publication No. 2012-1816

Disclosure of Invention

Problems to be solved by the invention

The hot-pressed members described in patent documents 2 and 3 were studied for corrosion resistance after coating, but the corrosion resistance of the surface portion of the member when the hot-pressed member is not coated or the corrosion resistance of the surface portion of the member before coating was not studied, and it is not clear how to improve the corrosion resistance of the surface portion in an uncoated state.

Accordingly, an object of the present invention is to provide a hot stamped metal body having improved surface corrosion resistance, more specifically, improved surface corrosion resistance in an uncoated state, by a novel configuration.

Means for solving the problems

In order to achieve the above object, the present inventors have found that: in the hot stamped steel, it is effective to provide a ZnO region in the surface layer of the plating layer formed on the steel sheet and to control the concentration of Fe and the like in the ZnO region at a low level. When the concentration of Fe or the like in the ZnO region is reduced, red rust generation in the surface layer of the hot stamped steel can be suppressed, and a hot stamped steel having improved surface corrosion resistance in an uncoated state can be obtained.

The present invention for achieving the above object is as follows.

(1) A hot-stamped steel product comprising a steel sheet and a plating layer formed on at least one surface of the steel sheet, wherein the plating layer comprises a ZnO region having an oxygen concentration of 10 mass% or more and present on the surface side of the plating layer, and a Ni-Fe-Zn alloy region having an oxygen concentration of less than 10 mass% and present on the steel sheet side of the plating layer, and the total average concentration of Fe, Mn and Si in the ZnO region exceeds 0 mass% and is less than 5 mass%.

(2) A hot stamp-formed body according to (1), wherein,

the ZnO region has a thickness of 0.5 to 3.0 [ mu ] m.

(3) The hot stamp-formed body according to (1) or (2), wherein,

in the Ni-Fe-Zn alloy region, the concentrations of Zn, O, Mn and Si decrease from the surface side of the plating layer toward the steel sheet side.

(4) The hot stamped steel of any one of (1) to (3), wherein,

the Ni-Fe-Zn alloy region is composed of a 1 st region having an Fe concentration of less than 60 mass% and a 2 nd region having an Fe concentration of 60 mass% or more in this order from the surface side of the plating layer, wherein the Zn/Ni mass ratio in the 1 st region is in the range of 3.0 to 13.0, and the average Zn/Ni mass ratio in the 2 nd region is 0.7 to 2.0.

(5) A hot stamp-formed body according to (4),

the average Zn/Ni mass ratio in the 2 nd region is 0.8 or more and 1.2 or less.

Effects of the invention

According to the present invention, the concentration of Fe or the like in the ZnO region of the hot stamped metal present on the surface side of the plating layer can be controlled, red rust generation in the surface layer of the hot stamped metal can be suppressed, and a hot stamped metal having improved surface corrosion resistance can be provided.

Detailed Description

< Hot Press molded body >

The hot stamped steel of the present invention includes a steel sheet and a plating layer formed on at least one surface of the steel sheet. Preferably, the plating layer is formed on both sides of the steel sheet.

[ Steel sheet ]

The composition of the steel sheet in the present invention is not particularly limited, and may be determined in consideration of the strength of the hot stamped steel after hot stamping and the hardenability at the time of hot stamping. Hereinafter, elements that can be contained in the steel sheet of the present invention will be described. The "%" indicating the content of each element in the composition means mass% unless otherwise specified.

Preferably, the steel sheet of the present invention may contain, in mass%, C: 0.05% or more and 0.70% or less, Mn: 0.5% or more and 11.0% or less, Si: 0.05% or more and 2.50% or less, Al: 0.001% or more and 1.500% or less, P: 0.100% or less, S: 0.100% or less, N: 0.010% or less, and O: 0.010% or less.

(C: 0.05% or more and 0.70% or less)

C (carbon) is an element effective for improving the strength of the steel sheet. For example, a high strength of 980MPa or more is sometimes required for automobile members. In order to sufficiently secure the strength, the C content is preferably set to 0.05% or more. On the other hand, if C is excessively contained, workability of the steel sheet may be lowered, and therefore, it is preferable to set the C content to 0.70% or less. The lower limit of the C content is preferably 0.10%, more preferably 0.12%, still more preferably 0.15%, and most preferably 0.20%. The upper limit of the C content is preferably 0.65%, more preferably 0.60%, still more preferably 0.55%, and most preferably 0.50%.

(Mn: 0.5% or more and 11.0% or less)

Mn (manganese) is an element effective for improving hardenability at hot stamping. In order to reliably obtain this effect, the Mn content is preferably set to 0.5% or more. On the other hand, if Mn is excessively contained, Mn is segregated and the strength and the like of the molded product after hot stamping may become uneven, so the Mn content is preferably set to 11.0% or less. The lower limit of the Mn content is preferably 1.0%, more preferably 2.0%, further preferably 2.5%, further preferably 3.0%, and most preferably 3.5%. The upper limit of the Mn content is preferably 10.0%, more preferably 9.5%, further preferably 9.0%, further preferably 8.5%, most preferably 8.0%.

(Si: 0.05% or more and 2.50% or less)

Si (silicon) is an effective element for improving the strength of the steel sheet. In order to sufficiently secure the strength, the Si content is preferably set to 0.05% or more. On the other hand, if Si is excessively contained, workability may be degraded, and therefore, it is preferable to set the Si content to 2.50% or less. The lower limit of the Si content is preferably 0.10%, more preferably 0.15%, still more preferably 0.20%, and most preferably 0.30%. The upper limit of the Si content is preferably 2.00%, more preferably 1.80%, still more preferably 1.50%, and most preferably 1.20%.

(Al of 0.001% or more and 1.500% or less)

Al (aluminum) is an element that functions as a deoxidizing element. In order to obtain the effect of deoxidation, the Al content is preferably set to 0.001% or more. On the other hand, if Al is contained excessively, workability may be degraded, so it is preferable to set the Al content to 1.500% or less. The lower limit of the Al content is preferably 0.010%, more preferably 0.020%, still more preferably 0.050%, and most preferably 0.100%. The upper limit of the Al content is preferably 1.000%, more preferably 0.800%, still more preferably 0.700%, and most preferably 0.500%.

(P: 0.100% or less)

(S: 0.100% or less)

(N: 0.010% or less)

(O: 0.010% or less)

P (phosphorus), S (sulfur), N (nitrogen) and oxygen (O) are impurities, preferably small, and therefore the lower limits of these elements are not particularly limited. However, the content of these elements may be set to more than 0.000% or 0.001% or more. On the other hand, if these elements are contained excessively, there is a possibility that toughness, ductility and/or workability are deteriorated, so it is preferable to set the upper limits of P and S to 0.100%, and the upper limits of N and O to 0.010%. The upper limit of P and S is preferably 0.080%, more preferably 0.050%. The upper limit of N and O is preferably 0.008%, more preferably 0.005%.

The basic composition of the steel sheet in the present invention is as described above. Further, the steel sheet may contain at least one of the following optional elements in place of a part of the remaining Fe, if necessary. For example, the steel sheet may contain B: 0% or more and 0.0040% or less. Further, the steel sheet may contain Cr: 0% or more and 2.00% or less. Further, the steel sheet may also contain a metal selected from the group consisting of Ti: 0% or more and 0.300% or less, Nb: 0% or more and 0.300% or less, V: 0% or more and 0.300% or less, and Zr: at least one of the group consisting of 0% to 0.300%. Further, the steel sheet may also contain a material selected from the group consisting of Mo: 0% or more and 2.000% or less, Cu: 0% or more and 2.000% or less, and Ni: 0% to 2.000%. Further, the steel sheet may contain Sb: 0% or more and 0.100% or less. Further, the steel sheet may also contain a component selected from the group consisting of Ca: 0% or more and 0.0100% or less, Mg: 0% or more and 0.0100% or less, and REM: at least one of the group consisting of 0% to 0.1000%. These optional elements will be described in detail below.

(B: 0% or more and 0.0040% or less)

B (boron) is an element effective for improving the hardenability at the time of hot stamping. The B content may be 0%, but in order to reliably obtain this effect, the B content is preferably set to 0.0005% or more. On the other hand, if B is excessively contained, workability of the steel sheet may be reduced, and therefore, the B content is preferably set to 0.0040% or less. The lower limit of the B content is preferably 0.0008%, more preferably 0.0010%, and still more preferably 0.0015%. The upper limit of the B content is preferably 0.0035%, more preferably 0.0030%.

(Cr is 0% or more and 2.00% or less)

Cr (chromium) is an element effective for improving hardenability at hot stamping. The Cr content may be 0%, but in order to reliably obtain this effect, the Cr content is preferably set to 0.01% or more. The Cr content may be 0.10% or more, 0.50% or more, or 0.70% or more. On the other hand, if Cr is excessively contained, the thermal stability of the steel material may be lowered. Therefore, the Cr content is preferably set to 2.00% or less. The Cr content may be 1.50% or less, 1.20% or less, or 1.00% or less.

(Ti is 0% or more and 0.300% or less)

(Nb: 0% or more and 0.300% or less)

(V: 0% or more and 0.300% or less)

(Zr: 0% or more and 0.300% or less)

Ti (titanium), Nb (niobium), V (vanadium), and Zr (zirconium) are elements that improve the tensile strength by refining the metal structure. The content of these elements may be 0%, but in order to reliably obtain this effect, the content of Ti, Nb, V, and Zr is preferably set to 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if Ti, Nb, V, and Zr are excessively contained, the effect is saturated, and the production cost increases. Therefore, the Ti, Nb, V, and Zr contents are preferably set to 0.300% or less, and may be 0.150% or less, 0.100% or less, or 0.060% or less.

(Mo: 0% or more and 2.000% or less)

(Cu: 0% or more and 2.000% or less)

(Ni: 0% or more and 2.000% or less)

Mo (molybdenum), Cu (copper) and Ni (nickel) have the function of improving tensile strength. The content of these elements may be 0%, but in order to reliably obtain this effect, the content of Mo, Cu, and Ni is preferably set to 0.001% or more, and may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if Mo, Cu and Ni are excessively contained, the thermal stability of the steel material may be lowered. Therefore, the contents of Mo, Cu, and Ni are preferably set to 2.000% or less, and may be 1.500% or less, 1.000% or less, or 0.800% or less.

(Sb: 0% or more and 0.100% or less)

Sb (antimony) is an element effective for improving wettability and adhesion of plating. The Sb content may be 0%, but in order to reliably obtain this effect, the Sb content is preferably set to 0.001% or more. The Sb content may be 0.005% or more, 0.010% or more, or 0.020% or less. On the other hand, if Sb is excessively contained, toughness may be reduced. Therefore, the Sb content is preferably set to 0.100% or less. The Sb content may be 0.080% or less, 0.060% or less, or 0.050% or less.

(Ca of 0% or more and 0.0100% or less)

(Mg: 0% or more and 0.0100% or less)

(REM: 0% or more and 0.1000% or less)

Ca (calcium), Mg (magnesium), and REM (rare earth metal) are elements that improve toughness after hot stamping by adjusting the shape of inclusions. The content of these elements may be 0%, but in order to reliably obtain this effect, the content of Ca, Mg, and REM is preferably set to 0.0001% or more, and may be 0.0010% or more, 0.0020% or more, or 0.0040% or more. On the other hand, if Ca, Mg and REM are excessively contained, the effect is saturated and the production cost is increased. Therefore, the content of Ca and Mg is preferably set to 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, or 0.0050% or less. Similarly, the REM content is preferably set to 0.1000% or less, and may be 0.0800% or less, 0.0500% or less, or 0.0100% or less.

The remainder of the elements other than the above elements is composed of iron and impurities. Here, "impurities" are components that are mixed in due to various causes in the manufacturing process, as represented by raw materials such as ores and scrap irons, when the base steel sheet is industrially manufactured, and include components that are not intentionally added to the base steel sheet according to the embodiment of the present invention. The impurities are elements other than the above-described components, and include elements included in the base steel sheet at levels at which the characteristics of the hot stamped steel according to the embodiment of the present invention are not affected by the action and effect specific to the elements.

The steel sheet in the present invention is not particularly limited, and a general steel sheet such as a hot-rolled steel sheet or a cold-rolled steel sheet can be used. The steel sheet of the present invention may have any thickness as long as it can be subjected to a hot stamping process by forming a Zn — Ni plating layer described later on, and may have a thickness of, for example, 0.1 to 3.2 mm.

[ plating layer ]

The plating layer of the hot stamped article of the present invention is composed of a ZnO region and a Ni-Fe-Zn alloy region. The ZnO region is a region having an oxygen concentration of 10 mass% or more on the surface side of the plating layer. The remaining region of the plating layer is a Ni-Fe-Zn alloy region, that is, a Ni-Fe-Zn alloy region refers to a region having an oxygen concentration of less than 10% which exists on the steel plate side of the plating layer. Therefore, the ZnO region and the Ni-Fe-Zn alloy region are present in contact with each other, and the two regions constitute a plating layer. In the plating layer in the present invention, oxygen is taken into the plating layer at the time of hot stamping, and therefore the surface side of the plating layer has the highest oxygen concentration, and the oxygen concentration decreases as the surface side advances to the steel sheet side. Therefore, the region from the surface of the hot-stamped steel to the position where the oxygen concentration is 10 mass% is a ZnO region, and the remaining portion of the plating layer is a Ni-Fe-Zn alloy region.

The plating layer of the hot stamped steel of the present invention can be obtained by, for example, forming a Zn — Ni alloy plating layer on a steel sheet, further forming a Ni plating layer thereon, and then performing hot stamping in an oxygen atmosphere of 5 to 25%, for example, in an atmospheric pressure atmosphere. Therefore, the components contained in the plating layer in the present invention are elements (typically, Zn and Ni) contained in the Zn — Ni plating layer or the Ni plating layer before hot stamping, elements (for example, Fe, Mn, and Si) contained in the steel sheet, and O taken in at the time of hot stamping, and the balance is impurities. Here, the "impurities" include not only elements that are inevitably mixed in the production process but also elements that are intentionally added within a range that does not inhibit the corrosion resistance of the hot stamped steel of the present invention.

The concentration of each component in the plating layer in the present invention is measured by Glow Discharge analysis (GDS) which is a quantitative analysis. By quantitatively performing GDS analysis in the depth direction from the surface of the plating layer, the concentration distribution of each component in the plate thickness direction can be quantitatively determined. Therefore, the oxygen concentration distribution of the plating layer was measured by GDS to specify the position where the oxygen concentration was 10 mass%, and the ZnO region and the Ni-Fe-Zn alloy region could be distinguished. Determination conditions of GDS to determine diameter 4mm phi, Ar gas pressure: 600Pa, power: 35W, measurement time: it is only necessary to do this for 100 seconds. The device used is preferably GD-profiler2 manufactured by horiba.

The thickness of the plating layer in the present invention may be, for example, 3.0 μm or more and 20.0 μm or less per surface. The ratio of the thickness of the ZnO region in the plating layer is not particularly limited, but is preferably 1% or more and 15% or less, and more preferably 2% or more and 12% or less, from the viewpoint of ensuring the corrosion resistance of the hot stamped steel and preventing deterioration in appearance due to formation of irregularities on the surface. The thickness of the plating layer can be measured by observing the cross section of the hot stamped product of the invention with a Scanning Electron Microscope (SEM). The thickness of the plating layer may be measured by identifying the region of the plating layer by elemental analysis of quantitative analysis GDS and converting the thickness.

(ZnO region)

In the hot stamped article of the invention, the plating layer has a ZnO region having an oxygen concentration of 10 mass% or more on the surface side of the plating layer. The ZnO region is typically a region formed by bonding Zn in the Zn — Ni alloy plating layer formed before hot stamping to O in the atmosphere at the time of hot stamping, that is, by oxidation of Zn to ZnO. In the present invention, although the Ni plating layer is present on the Zn — Ni plating layer in the plated steel sheet before hot stamping, Zn relatively easily oxidized may diffuse in the Ni plating layer and reach the surface to form a ZnO region in a form attracted by O in the atmosphere at the time of hot stamping.

Depending on the conditions of hot stamping, Fe, Mn, Si, and the like, which are components of the steel sheet, may diffuse into the plating layer during hot stamping heating. If such an element, particularly Fe, is diffused in a large amount into the ZnO region of the surface layer of the hot stamped steel, there is a possibility that the Fe of the surface layer is corroded by the surrounding environment (e.g., water) to cause red rust. Therefore, in the plated steel sheet used for obtaining the hot stamped steel of the present invention, in addition to the Zn — Ni plating layer, an Ni plating layer capable of suppressing diffusion of components in the steel sheet such as Fe is further provided on the steel sheet. By the presence of the Ni plating layer, a ZnO region having a desired thickness is formed in the surface layer of the hot stamped product obtained after hot stamping, and the components derived from the steel sheet are less likely to diffuse into the ZnO region, that is, the total average concentration of Fe, Mn, and Si in the ZnO region is suppressed to be low. Therefore, the occurrence of red rust can be effectively suppressed, and a hot stamped article having improved surface corrosion resistance can be obtained. In order to obtain sufficient surface corrosion resistance, the total average concentration of Fe, Mn, and Si in the ZnO region in the present invention needs to be set to more than 0 mass% and less than 5 mass%. In the present invention, the total average concentration of Fe, Mn, and Si in the ZnO region may be in the above range, but in particular, the smaller the amount of Fe that causes red rust, the more preferable. Therefore, it is preferable that the plating layer in the present invention contains Fe: 0 to 1 mass%, Mn: 0% by mass or more and 2% by mass or less, and Si: 0 to 2 mass%. The total average concentration of these elements is preferably 4% by mass or less, more preferably 3% by mass or less, and still more preferably 2% by mass or less.

The "total average concentration of Fe, Mn, and Si" is obtained by dividing a region having an oxygen concentration of 10% or more (i.e., ZnO region) determined by quantitative analysis of GDS into 10 segments at equal intervals, reading the Fe concentration, Mn concentration, and Si concentration at the center of each segment from the GDS result, obtaining the total of the concentrations of these elements in each segment, and averaging the obtained total values of 10 Fe, Mn, and Si.

As described above, the Ni plating layer is provided on the surface side of the plated steel sheet used for obtaining the hot stamped steel of the present invention. Therefore, the diffusion of Zn from the underlying Zn — Ni plating layer can be somewhat suppressed by the Ni plating layer. Therefore, the thickness of the ZnO region in the present invention may be 3.0 μm or less, for example. When the thickness of the ZnO region is 3.0 μm or less, the formation of irregularities due to the peeling of oxides or the like in the surface layer of the hot stamped steel can be prevented, and a hot stamped steel having excellent surface appearance can be obtained. If the thickness exceeds 3.0 μm, the oxide on the surface layer of the plating layer may become brittle and fall off to form irregularities and deteriorate the appearance, and the fallen oxide may damage the press mold. On the other hand, in order to set the thickness of the ZnO region to less than 0.5 μm, it is necessary to increase the thickness of the Ni plating layer of the plated steel sheet, which is not preferable in terms of cost, and therefore the lower limit of the thickness of the ZnO region is preferably 0.5 μm. The lower limit of the thickness of the ZnO region is preferably 0.7. mu.m, more preferably 1.0. mu.m, and still more preferably 1.2. mu.m. The upper limit of the thickness of the ZnO region is preferably 2.8. mu.m, more preferably 2.5. mu.m, and still more preferably 2.2. mu.m.

The ZnO region typically has a high Zn concentration compared to the Ni concentration. For example, the Zn/Ni mass ratio in the ZnO region is 5.0 or more. The phrase "the Zn/Ni mass ratio in the ZnO region is 5.0 or more" means that the Zn/Ni mass ratio is 5.0 or more at all the positions of the ZnO region, and in the present invention, the ZnO region is divided into 10 divisions at equal intervals, the Zn concentration and the Ni concentration at the central position of each division are read from the GDS results to obtain the Zn/Ni mass ratio of each division, and it is determined whether or not all the obtained 10 Zn/Ni mass ratios are 5.0 or more. The Zn/Ni mass ratio in the ZnO region is preferably 5.5 or more, more preferably 6.0 or more, and further preferably 7.0 or more. The upper limit of the Zn/Ni mass ratio in this region is not particularly limited, and may be, for example, 30.0 or 20.0.

The reason why Zn is present in a larger amount than Ni in the ZnO region of the hot stamp-formed product is as follows: when hot stamping is performed in an oxygen atmosphere, among Ni and Zn in the Zn — Ni plating layer before hot stamping, Zn that is more easily oxidized than Ni is oxidized by O in the hot stamping atmosphere to form ZnO. Due to its easy oxidation, Zn can diffuse to the surface beyond the Ni plating layer and form ZnO. Note that Ni also slightly diffuses from the Zn — Ni plating layer and the Ni plating layer. When the Zn/Ni mass ratio is 5.0 or more, ZnO as an oxide is present in a large amount in the surface layer of the hot stamped steel, and therefore the corrosion resistance of the surface portion of the hot stamped steel is improved. If the mass ratio of Zn/Ni in the ZnO region is less than 5.0, ZnO in the surface layer may not be sufficiently formed, and thus the surface corrosion resistance may be insufficient.

The concentration of each component contained in the ZnO region in the present invention is determined by quantitative analysis of GDS as described above. The measurement was performed under the same conditions as the GDS conditions described above, with at least Zn, Ni, O, Fe, Si, and Mn being specified as target elements. The thickness of the ZnO region can be determined by quantitatively analyzing GDS to determine the range of oxygen concentration ≧ 10 mass%, and measuring the depth thereof.

(Ni-Fe-Zn alloy region)

The hot-stamped steel of the present invention has a Ni-Fe-Zn alloy region having an oxygen concentration of less than 10 mass% in contact with the ZnO region on the steel sheet side of the plating layer. Preferably, Zn, Ni, O, Fe, Mn and Si are present in the alloy region. The Ni — Fe — Zn alloy region is typically a region in which Fe in the steel sheet diffuses into the plating layer during heating in hot stamping, and Zn and Ni in the Zn — Ni plating layer before hot stamping and Ni in the Ni plating layer are alloyed with Fe diffused from the steel sheet. Further, Mn and Si in the steel sheet may diffuse into the Ni-Fe-Zn alloy region together with Fe to be alloyed.

In the Ni-Fe-Zn alloy region of the present invention, it is preferable that the concentrations of Zn, O, Mn and Si decrease from the surface side of the plating layer toward the steel sheet side. In other words, in the alloy region, the Fe concentration preferably increases from the surface side of the plating layer toward the steel sheet side. The phrase "the respective concentrations of Zn, O, Mn, and Si decrease from the surface side of the plating layer toward the steel sheet side" means that in the Ni — Fe — Zn alloy region, the concentrations of these elements decrease monotonously from the surface side of the plating layer toward the steel sheet side, that is, in the case where the concentrations of any of the listed elements are measured at arbitrary 2 positions by GDS or the like, the concentration of the position closer to the surface side of the plating layer among the 2 positions is higher than that of the other position. The reduction here is only required to be monotonous in the concentration of Zn, O, Mn and Si, and is not limited to the linearity. Only Ni has the maximum concentration slightly on the steel sheet side from the surface. When the ZnO region and the Ni-Fe-Zn alloy region are formed in the plating layer of the hot stamped steel of the present invention, typically, such concentration distributions are often present. Therefore, the Ni-Fe-Zn alloy region may be composed of the 1 st region having an Fe concentration of less than 60 mass% and the 2 nd region having an Fe concentration of 60 mass% or more in this order from the surface side of the plating layer. The distinction between the 1 st region and the 2 nd region in the Ni-Fe-Zn alloy region can be made by measuring the Fe concentration by quantitative analysis of GDS.

The Ni-Fe-Zn alloy region is a region on the steel sheet side of the plating layer, and typically, Zn contained in the Zn-Ni plating layer before hot stamping diffuses into the steel sheet at the time of hot stamping. The diffusion occurs more remarkably as it comes closer to the steel sheet. Therefore, in this alloy region, the Zn concentration may decrease from the surface side of the plating layer toward the steel sheet side. Further, since oxygen is typically an element contained in the atmosphere at the time of hot stamping, the concentration of oxygen in the plating layer of the hot stamped steel decreases as it goes from the surface side of the plating layer toward the steel sheet side. Further, Mn and Si are elements present in the steel sheet before hot stamping, but when hot stamping is performed in an oxygen atmosphere, they are easily oxidized, and thus can be preferentially diffused to the surface side of the plating layer compared with Fe. Therefore, in this alloy region, the respective concentrations of Mn and Si may decrease from the surface side of the plating layer toward the steel sheet side.

In the present invention, the Zn/Ni mass ratio in the 1 st region of the Ni-Fe-Zn alloy region is preferably in the range of 3.0 or more and 13.0 or less. More preferably, in the 1 st region, the Zn/Ni mass ratio continuously changes in a range of 3.0 or more and 13.0 or less from the surface side of the plating layer toward the steel plate side. The "mass ratio of Zn/Ni in the 1 st region is in the range of 3.0 to 13.0" means that the mass ratio of Zn/Ni is in the range of 3.0 to 13.0 at all positions of the 1 st region, and in the present invention, the 1 st region may be divided into 10 zones at equal intervals, the Zn concentration and the Ni concentration at the center position of each zone are read from the GDS results to obtain the mass ratio of Zn/Ni of each zone, and the judgment may be made by whether or not all of the obtained 10 mass ratios of Zn/Ni are 3.0 to 13.0. When the Zn/Ni mass ratio of the 1 st region is in the above range, a sufficient amount of Zn can be secured in this region, and further, the amount of Zn in the other regions can be also made sufficient. Therefore, even when a scratch is formed in the plating layer of the hot-stamped metal body, the Zn present in this region is oxidized to ZnO to form an oxide film (referred to as a "substitution corrosion prevention effect"), whereby corrosion of the scratch can be suppressed, and the corrosion resistance of the scratch of the hot-stamped metal body can be improved. If the mass ratio of Zn/Ni in the 1 st region is less than 3.0, the substitution corrosion prevention effect of Zn may not be sufficiently exhibited, and the corrosion resistance of the flaw portion may be insufficient. On the other hand, if it exceeds 13.0, there is a possibility that the corrosion resistance of the flaw portion of the whole hot stamped steel may become insufficient because Zn may be insufficient in other regions, for example, the surface layer portion of the plating layer and/or the 2 nd region. The lower limit of the Zn/Ni mass ratio in the 1 st region is preferably 3.5, more preferably 4.0, and the upper limit is preferably 12.0, more preferably 11.0, and further preferably 10.0.

In the present invention, the average Zn/Ni mass ratio in the 2 nd region of the Ni-Fe-Zn alloy region is preferably 0.7 or more and 2.0 or less. As described above, Zn in the Zn — Ni plating layer formed before hot stamping diffuses to the surface side of the plating layer and the steel sheet at the time of hot stamping, but in the hot stamped steel of the present invention, a predetermined amount of Zn remains in the 2 nd region of the Ni — Fe — Zn alloy region in contact with the steel sheet. If Zn remains in the above-described range in this 2 nd region, the substitute corrosion preventing effect of Zn can be exerted even when a flaw is formed in the plating layer or, further, in the underlying steel sheet, and therefore, the flaw portion corrosion resistance can be improved. If the average Zn/Ni mass ratio in the 2 nd region is less than 0.7, the substitution corrosion prevention effect of Zn may not be sufficiently exhibited, and the corrosion resistance of the flaw portion may become insufficient. On the other hand, if it exceeds 2.0, Zn may not be sufficiently diffused into the surface layer portion of the plating layer and/or Zn may be insufficient in the 1 st region, and corrosion resistance of the flaw portion as the whole hot stamped product may be insufficient. The average Zn/Ni mass ratio in the 2 nd region is preferably 0.8 or more. The average Zn/Ni mass ratio in the 2 nd region is preferably 1.8 or less, more preferably 1.5 or less, and still more preferably 1.2 or less. Therefore, the average Zn/Ni mass ratio in the 2 nd region is most preferably 0.8 or more and 1.2 or less.

The "average Zn/Ni mass ratio in the 2 nd region" can be obtained by dividing the region (the 2 nd region) having an Fe concentration of 60% or more in the Ni-Fe-Zn alloy region into 10 zones at equal intervals, reading the Zn concentration and the Ni concentration at the center of each zone from the GDS results to obtain the Zn/Ni mass ratio of each zone, and averaging the obtained 10 Zn/Ni mass ratios.

The thickness of the Ni-Fe-Zn alloy region can be determined by determining the range of oxygen concentration <10 mass% by quantitative analysis of GDS and measuring the depth thereof. Similarly, the thicknesses of the 1 st region (Fe concentration < 60 mass%) and the 2 nd region (Fe concentration ≧ 60 mass%) of the Ni-Fe-Zn alloy region can be determined by the Fe concentration obtained by GDS.

< method for producing Hot Press molded article >

An example of the method for producing a hot stamp-formed article of the present invention will be described below. The hot-stamped steel of the present invention can be obtained by forming a Zn — Ni plating layer and a Ni plating layer in this order on at least one surface, preferably both surfaces of a steel sheet by, for example, electroplating, and hot-stamping the obtained plated steel sheet under predetermined conditions. The obtained hot-stamped steel sheet has a plated layer composed of a ZnO region having an oxygen concentration of 10 mass% or more and a Ni-Fe-Zn alloy region having an oxygen concentration of less than 10 mass% in this order from the surface side. The ZnO region is formed by oxygen contained in the atmosphere during hot stamping being bonded to Zn in the Zn — Ni plating layer diffused in the Ni plating layer and reaching the surface, while the Ni — Fe — Zn alloy region is formed by Fe diffused into the plating layer from the steel sheet during heating during hot stamping being alloyed with Zn and Ni in the Zn — Ni plating layer and the Ni plating layer.

(production of Steel plate)

The method for producing the steel sheet for producing the hot stamped steel of the present invention is not particularly limited. For example, a steel sheet can be obtained by adjusting the composition of molten steel to a desired range, and then performing hot rolling, coiling, and further cold rolling. The thickness of the steel sheet in the present invention may be, for example, 0.1 to 3.2 mm.

The composition of the steel sheet to be used is not particularly limited, but as described above, the steel sheet preferably contains, in mass%, C: 0.05% or more and 0.70% or less, Mn: 0.5% or more and 11.0% or less, Si: 0.05% or more and 2.50% or less, Al: 0.001% or more and 1.500% or less, P: 0.100% or less, S: 0.100% or less, N: 0.010% or less, O: 0.010% or less and B: 0.0005% to 0.0040%, the remainder being composed of iron and impurities.

(formation of plating layer)

The Zn — Ni plating layer and the Ni plating layer are not particularly limited in their formation methods, but are preferably formed by electroplating. However, the plating is not limited to the plating, and sputtering, vapor deposition, or the like may be used. Hereinafter, the formation of the Zn-Ni plating layer and the Ni plating layer by electroplating will be described.

As for the Zn-Ni plating layer on the steel sheet formed by electroplating, the plating adhesion amount is preferably 25g/m per side, for example²Above 90g/m²Hereinafter, more preferably 30g/m²Above and 50g/m²The following. The Zn/Ni ratio of the Zn-Ni plating layer may be, for example, 3.0 or more and 20.0 or less, and preferably 4.0 or more and 10.0 or less. If the Zn/Ni ratio is too small, the Zn concentration remaining in the plating layer of the hot stamped product may be insufficient, and the substitute corrosion prevention effect may not be sufficiently obtained, and the corrosion resistance of the flaw portion may be insufficient. On the other hand, if the Zn/Ni ratio exceeds 20.0, diffusion of Zn from the Zn — Ni plating layer may be promoted due to a decrease in the melting point of the Zn — Ni plating layer, or the like, and further, along with this, diffusion of components in the steel sheet such as Fe is promoted, so that the ZnO region becomes too thick, or the total average concentration of Fe, Mn, and Si in the ZnO region becomes too high. In such a case, the oxide on the surface layer of the finally obtained plating layer becomes brittle and falls off to form irregularities, which may deteriorate the appearance, or Fe or the like on the surface layer may corrode due to the surrounding environment to cause red rust. Further, the bath composition used for forming the Zn — Ni plating layer is, for example, nickel sulfate 6 hydrate: 25-350 g/L, zinc sulfate 7 hydrate: 10-150 g/L, and sodium sulfate: 25-75 g/L. In addition, the current density is 10 to 100A/dm²And (4) finishing. The bath composition and current density can be adjusted as appropriate so as to obtain a desired plating deposit amount and Zn/Ni ratio. The bath temperature and bath pH may be adjusted appropriately so as not to cause scorching of the plating, for example, 40 to 70 ℃ and 1.0 to 3.0, respectively.

Further, with respect to the Ni plating layer on the steel sheet formed by electroplating, the plating adhesion amount is preferably, for example, 0.3g/m per surface²Above and 15.0g/m²Hereinafter, more preferably 0.5g/m²Above and 10.0g/m²The following. By forming the Ni plating layer in such a range of plating adhesion amount, the Ni plating layer serves as a barrier to suppress diffusion of components derived from the steel sheet into the hot stamping at the time of hot stampingIn the ZnO region of the surface layer of the molded article, the total average concentration of Fe, Mn, and Si in a desired amount can be obtained in the ZnO region. If the plating adhesion amount of the Ni plating layer becomes less than 0.3g/m²Then, the barrier function may not be sufficiently exerted, and a large amount of Fe or the like may diffuse into the ZnO region. On the other hand, if it exceeds 15.0g/m²In this case, the diffusion of Zn into the surface layer of the Zn — Ni plating layer may be excessively suppressed, and the thickness of the ZnO region may become insufficient, which is not preferable in terms of cost. The bath composition used for forming the Ni plating layer may be, for example, a strike bath or a watt bath. In addition, the current density is 5 to 50A/dm²And (4) finishing. The bath temperature and bath pH may be adjusted appropriately so as not to cause scorching of the plating, for example, 40 to 70 ℃ and 1.0 to 3.0, respectively.

The plating adhesion amount and the Zn/Ni ratio of the Zn — Ni plating layer and the plating adhesion amount of the Ni plating layer are related to the diffusion of the steel sheet components from the steel sheet to the plating layer, the formation of the ZnO region, and the like. Therefore, a desired structure of the plating layer may not be obtained only by controlling the values of the parameters within the above ranges. For example, even if the plating deposition amount of the Ni plating layer is within the above range, when the Zn/Ni ratio of the Zn — Ni plating layer is large, diffusion of Zn from the Zn — Ni plating layer and accompanying diffusion of components in the steel sheet such as Fe may be promoted due to a decrease in the melting point of the Zn — Ni plating layer, and the Ni plating layer may not necessarily exhibit a sufficient barrier function, resulting in excessive formation of a ZnO region and/or an increase in the total average concentration of Fe, Mn, and Si in the ZnO region. Further, the diffusion of these elements is also greatly affected by the heating temperature and the holding time in the hot stamping treatment described below. Therefore, even if the plating adhesion amount and the Zn/Ni ratio of the Zn — Ni plating layer and the plating adhesion amount of the Ni plating layer are the same, the characteristics of the finally obtained plating layer may vary depending on the heating temperature, the temperature increase rate, the holding time, and the like at the time of the hot stamping treatment. Therefore, in order to obtain a desired composition of the plating layer, specific values of the plating adhesion amount and the Zn/Ni ratio of the Zn — Ni plating layer and the plating adhesion amount of the Ni plating layer need to be appropriately selected in consideration of the correlation between these parameters, the conditions of the hot stamping process, and the like.

The plating adhesion amount and the Zn/Ni ratio of the formed Zn — Ni plating layer and the method of measuring the plating adhesion amount of the Ni plating layer are not particularly specified, and for example, the measurement can be performed by SEM/EDX (scanning electron microscope/energy dispersive X-ray spectroscopy) from the cross section of the steel sheet on which the Zn — Ni plating layer and the Ni plating layer are formed.

(Hot stamping treatment)

Subsequently, the steel sheet having the Zn — Ni plating layer and the Ni plating layer formed thereon is hot stamped. The heating temperature of the hot stamping is not particularly limited as long as the steel sheet can be heated to the austenite region, and is, for example, 800 ℃ to 1000 ℃, preferably 850 ℃ to 950 ℃. When the heating temperature of hot stamping is increased, the components derived from the steel sheet are more likely to diffuse, and an excessive amount of Fe or the like may diffuse into the ZnO region. The heating method of the hot stamping is not limited, and examples thereof include tapping heating, electric heating, and induction heating. The holding time after heating may be appropriately set to 0.5 minutes or more and 5.0 minutes or less. More preferably 1.0 minute or more and 4.0 minutes or less, and still more preferably 1.0 minute or more and 2.0 minutes or less. If the holding time is too long, the steel sheet component such as Fe may diffuse to the surface layer of the hot stamped steel and/or the ZnO region too much. The atmosphere during hot stamping is preferably 5 to 25% oxygen atmosphere, and may be, for example, atmospheric air. After the heat treatment, cooling (quenching) may be performed at a cooling rate in the range of 10 to 100 ℃/sec, for example.

In the plated steel sheet for obtaining the hot stamped steel of the present invention, since the Ni plating layer is formed on the surface, the Ni plating layer can slightly prevent the diffusion of Zn in the Zn — Ni plating layer of the base into the surface layer, and even when the hot stamping is performed under the atmospheric pressure atmosphere, the ZnO region of the surface layer of the obtained hot stamped steel can be prevented from being excessively thickened. Therefore, the control of the furnace environment such as dew point control in the atmosphere at the time of hot stamping is not unnecessarily performed, and a relatively thin ZnO region can be easily obtained, and the control at the time of hot stamping is simplified.

By appropriately adjusting the amount of Zn — Ni plating and the Zn/Ni ratio before hot stamping, the amount of Ni plating, and the hot stamping conditions (for example, temperature, holding time, and oxygen concentration in the atmosphere), the ZnO region and the Ni — Fe — Zn alloy region, more specifically, the 1 st region and the 2 nd region of the ZnO region and the Ni — Fe — Zn alloy region can be formed, and the concentration and thickness of each element in each region can be adjusted.

Examples

The hot stamped steel of the present invention will be described in more detail below with reference to a few examples. However, the scope of the present invention described in the claims is not intended to be limited by the specific examples described below.

(formation of plated Steel sheet)

A cold-rolled steel sheet having a thickness of 1.4mm was immersed in a plating bath having the following plating bath composition (Zn-Ni plating), and Zn-Ni plating layers were formed on both sides of the cold-rolled steel sheet by electroplating. The pH of the plating bath was set to 2.0, the bath temperature was maintained at 60 ℃ and the current density was set to 50A/dm². Next, the steel sheet on which the Zn — Ni plating layer was formed was immersed in a plating bath (strike bath) having the following plating bath composition (Ni plating), and the Zn — Ni plating layer was plated to form an Ni plating layer thereon, thereby obtaining a plated steel sheet used for hot stamping described later. The pH of the plating bath was set to 1.5, the bath temperature was maintained at 50 ℃ and the current density was set to 20A/dm². All steel sheets used contain, in mass%, C: 0.50%, Mn: 3.0%, Si: 0.50%, Al: 0.100%, P: 0.010%, S: 0.020%, N: 0.003%, O: 0.003%, and B: 0.0010%, the balance being iron and impurities.

Plating bath composition (Zn-Ni plating)

Nickel sulfate 6 hydrate: 25 to 250g/L (variable)

Zinc sulfate 7 hydrate: 10 to 150g/L (variable)

Sodium sulfate: 50g/L (fixed)

Plating bath composition (Ni plating)

Nickel chloride: 240g/L (fixed)

Hydrochloric acid: 125ml/L (fixed)

In order to obtain a desired plating deposit amount and Zn/Ni ratio in the Zn-Ni plating layer, the plating bath compositions (Nickel sulfate 6 hydrate and Zinc sulfate 7 hydrate) were adjustedConcentration of) current density, and energization time. In addition, the current density and the energization time were adjusted to obtain a desired amount of plating deposit in the Ni plating layer. The plating adhesion amount (g/m) in the Zn-Ni plating layer on the steel sheet obtained by the electroplating was measured from the cross section of the plated steel sheet by SEM-EDX²) And Zn/Ni ratio, and the amount of plating deposit (g/m) in the Ni plating layer²). The measurement results are shown in table 1. The plating adhesion amount indicates the adhesion amount per surface.

(Hot stamping treatment)

Next, the obtained plated steel sheet was subjected to hot stamping under the conditions shown in table 1. The heating was performed by furnace heating, and a 90-degree V-shaped mold was used for molding. Further, quenching is performed at a cooling rate: 30 ℃/sec, all under atmospheric atmosphere.

(quantitative analysis of plating layer GDS)

The elements contained in the plated layer of each sample obtained after the hot stamping were measured by quantitative analysis of GDS using GD-profiler2 manufactured by horiba. The measurement conditions for GDS were set to measure a diameter of 4mm phi and Ar gas pressure: 600Pa, power: 35W, measurement time: for 100 seconds, the elements to be measured were Zn, Ni, Fe, Mn, Si and O. Specifically, each sample was divided into a region having an oxygen concentration of 10 mass% or more and a region having an oxygen concentration of less than 10 mass% by GDS, and the ZnO region and the Ni — Fe — Zn alloy region were set, respectively, to determine the thickness of the ZnO region. Further, it was confirmed from the concentration distributions of Zn, O, Mn and Si in the Ni-Fe-Zn alloy region whether or not the concentrations of these elements decrease from the surface side of the plating layer toward the steel sheet side in the Ni-Fe-Zn alloy region. Next, the ZnO region thus identified was divided into 10 segments at equal intervals, the Fe concentration, Mn concentration, and Si concentration at the center of each segment were read from the GDS results to determine the total of these concentrations in each segment, and the values of the obtained total concentrations of 10 Fe, Mn, and Si were averaged to determine the total average concentration of Fe, Mn, and Si in each sample. Then, based on the obtained GDS results, the Ni-Fe-Zn alloy region was divided into a region (region 1) in which the Fe concentration was less than 60 mass% and a region (region 2) in which the Fe concentration was 60 mass% or more. The maximum value and the minimum value of the Zn/Ni mass ratio are determined from the Zn concentration and the Ni concentration in the 1 st region, and the range of the Zn/Ni mass ratio in the 1 st region is determined. Further, the 2 nd region is divided into 10 zones at equal intervals, the Zn concentration and the Ni concentration at the center position of each zone are read to obtain the Zn/Ni mass ratio, and the obtained 10 Zn/Ni mass ratios are averaged to determine the average Zn/Ni mass ratio in the 2 nd region. The total average concentration (mass%) of Fe, Mn and Si in each sample, the Zn/Ni mass ratio in the 1 st region, the average Zn/Ni mass ratio in the 2 nd region, and the thickness (μm) of the ZnO region are shown in table 2. In Table 2, "concentration distribution of Zn, O, Mn and Si in the Ni-Fe-Zn alloy region", the case where all of these elements decrease from the surface side of the plating layer toward the steel plate side in the Ni-Fe-Zn alloy region is expressed as "good", and the case where it does not decrease is expressed as "X".

(evaluation of surface Corrosion resistance)

Regarding the surface corrosion resistance, a sample for evaluation having a size of 50mm × 50mm was cut out from each sample, and the red rust area ratio of the sample after leaving in a constant temperature and humidity bath having a temperature of 70 ℃ and a humidity of 70% for 1000 hours was evaluated. Specifically, the surface of the evaluation sample after being placed in the constant temperature and humidity environment is read by a scanner. Then, an area where red rust occurred was selected by using image editing software, and the red rust area ratio was obtained. This procedure was performed for 5 samples for evaluation per 1 sample, and the "red rust area rate" was determined as the average of the obtained 5 rust area rates. The case where the red rust area ratio is < 30% is expressed as "surface portion corrosion resistance: good ", the case where the red rust area ratio is not less than 30% is expressed as" surface corrosion resistance: x. The results of evaluating the corrosion resistance of the surface portion of each sample are shown in table 2.

(evaluation of appearance)

The appearance was measured by measuring the area ratio of oxide peeling in the bent portion obtained by using a 90-degree V-die at the time of hot press forming. Specifically, the surface portion of each sample was observed by SEM to evaluate. The continuous 5 adjacent fields of view of 200 μm × 200 μm at the vertex of the bend were observed by SEM, the area ratio of oxide peeling in each field of view was calculated from the observed image, and the obtained 5 values were averaged to determine the "oxide peeling area ratio". The case where the oxide peeling area ratio was < 30% was set as "appearance: good ", the case where the oxide-peeling area ratio is not less than 30% is set as" appearance: x. The evaluation results of the appearance of each sample are shown in table 2.

(evaluation of the Corrosion resistance of the scratched portion)

After a cross cut of 70mm in the diagonal length of the steel sheet reaching the substrate was formed in another sample for evaluation of 50mm × 50mm, the JASO-CCT test (M609-91), salt water spray (5% NaCl, 35 ℃): 2 hours, drying (60 ℃, 20-30% RH): 4 hours, wet (50 ℃, 95% RH): the test piece was subjected to 180 cycles for 2 hours to evaluate the corrosion resistance of the flaw portion. If the bulge width is 2mm or less, the corrosion resistance of the flaw portion is set as follows: good, if the swelling width exceeds 2mm, the "corrosion resistance of the flaw portion: x. The evaluation results of the corrosion resistance of the flaw portions of the respective samples are shown in table 2.

TABLE 1 characteristics of plated steel sheets and hot stamping conditions

In samples Nos. 1 to 4 and 8 to 11, the total average concentration of Fe, Mn and Si in the ZnO region was more than 0 mass% and less than 5 mass%, and therefore the surface corrosion resistance was good. In addition, samples Nos. 1 to 5 and Nos. 8 to 11 have good appearance because the thickness of the oxide layer is 3.0 μm or less.

In addition, in samples No.1 to 10, the mass ratio of Zn/Ni in the 1 st region and the average mass ratio of Zn/Ni in the 2 nd region of the Ni-Fe-Zn alloy region were 3.0 or more and 13.0 or less, respectively, and therefore the bulge width was 2mm or less, and the corrosion resistance of the flaw portion was good.

In samples 5 to 7, since no Ni plating layer was formed or the amount of Ni plating layer adhered was small, the total average concentration of Fe, Mn, and Si in the ZnO region was 5 mass% or more, so that a large amount of Fe and the like was present in the surface layer of the hot stamped steel, relatively large red rust was generated, and the surface corrosion resistance was insufficient. Furthermore, in samples 6 and 7, the thickness of the ZnO region exceeded 3.0. mu.m, and a relatively large amount of oxides were peeled off from the surface layer of the hot press-formed article, resulting in insufficient appearance. Sample No.11 had insufficient corrosion resistance of the flaw portion because Ni was present in excess of Zn in the Ni-Fe-Zn alloy region and Zn, which exerts a replacement corrosion preventing effect, was insufficient. Sample No.12 had an excessively large Zn/Ni ratio of the Zn — Ni plating layer, and therefore, diffusion of Zn from the Zn — Ni plating layer was promoted due to a decrease in the melting point of the Zn — Ni plating layer, and further, diffusion of components in the steel sheet such as Fe was promoted in accordance with this, and the thickness of the ZnO region exceeded 3.0 μm, and the total average concentration of Fe, Mn, and Si in the ZnO region was also 5 mass% or more, with the result that the appearance and the surface corrosion resistance were insufficient. Further, in sample No.12, Zn is present in excess in the Ni-Fe-Zn alloy region, and as a result, Zn is insufficient in the surface layer portion, so that the corrosion resistance of the flaw portion of the whole hot stamped steel is insufficient.

Industrial applicability

According to the present invention, the components derived from the steel sheet present in the ZnO region on the surface side of the plating layer can be controlled, and a hot stamped metal having improved surface corrosion resistance can be provided, whereby an automobile member having excellent surface corrosion resistance can be provided. Therefore, the present invention can be said to be an invention having an extremely high industrial value.