Plated stainless steel sheets for hot stamping and hot stamp formed components

Abstract

To provide a plated stainless steel plate for hot stamping that can give sufficient corrosion resistance even when a steel plate under a coating or plating is exposed.SOLUTION: A plated stainless steel plate for hot stamping includes a martensitic stainless steel plate containing C, Si, Mn, P, S, Cr, Ni, Cu, Mo, Al, N, O, with γP represented by the following formula (1) of 80 or more and 200 or less, and C+0.5 N of 0.04% or more and 0.37% or less, and a plated layer that is disposed on the surface of the martensitic stainless steel plate and is composed of aluminum or an aluminum alloy with a thickness of 10 μm or more and 100 μm or less. Formula (1): γp=420C+470N+23 Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-52Al+189.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a plated stainless steel sheet for hot stamping and Hot stamping Regarding molded members. [Background technology]

[0002] As one of the measures to improve automobile fuel efficiency, which stems from global environmental issues, efforts are being made to reduce the weight of automobile bodies, and there is a demand for increasing the strength of steel sheets used in automobiles as much as possible. However, when steel sheets are generally made stronger to reduce automobile weight, the elongation and Lankford value decrease, and formability deteriorates. To solve this problem, Patent Document 1 discloses a technology for warm forming, utilizing the heat generated during the process to increase strength. This technology aims to increase strength by appropriately controlling the composition of the steel, holding and forming the steel in a temperature range of 200 to 850°C, and utilizing precipitation strengthening in this temperature range.

[0003] Furthermore, Patent Document 2 proposes a high-strength steel sheet that has a low yield strength during warm pressing and a high yield strength at room temperature, with the aim of improving press forming accuracy.

[0004] However, the techniques disclosed in Patent Documents 1 and 2 may have limitations on the strength that can be obtained.

[0005] On the other hand, a technique for increasing the mechanical strength of a steel sheet by coating the surface of the steel sheet with aluminum or an aluminum alloy and then subjecting the steel sheet to heat treatment in order to obtain higher strength is disclosed in Patent Document 3. This technique is variously called hot stamping, hot pressing, die quenching, etc., and is now widely used.

[0006] The technology disclosed in Patent Document 3 suppresses oxidation and decarburization using a coating material, providing high mechanical strength, high corrosion resistance, and good paintability and adhesion. Furthermore, when using hot stamping technology such as that described in Patent Document 3, it is common to apply a corrosion-resistant coating to the surface of the coating material. However, it is known that if the coating layer is damaged by mechanical damage such as chipping (small chips in the painted roof plating caused by stones being thrown up while the vehicle is running), corrosion resistance decreases and localized corrosion progresses.

[0007] On the other hand, as an example of an aluminum-plated highly corrosion-resistant steel sheet, Patent Document 4 discloses a technology for aluminum-plated stainless steel for automobile exhaust systems that contains 10% to 20% Cr and 0.5% to 6.0% Mo and has excellent corrosion resistance. However, in Patent Document 4, the chemical composition of the steel is adjusted with the aim of increasing corrosion resistance due to exhaust gas condensate in the automobile exhaust system, making it difficult to obtain high strength through high-temperature forming. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-234153 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-087183 [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-038640 [Patent Document 4] Japanese Patent Application Publication No. 05-112859 Summary of the Invention [Problem to be solved by the invention]

[0009] As described above, various steel sheets that have high strength after high-temperature forming have been developed using the technologies disclosed so far. The corrosion resistance of these steel sheets is ensured by the plating layer and the paint. However, there have been concerns that the corrosion resistance cannot be ensured at the end faces of the steel sheets that are not covered by the plating layer, or at damaged or flawed areas that occur in the paint or plating layer of parts during vehicle operation.

[0010] The present invention has been made in view of the above circumstances, and provides a plated stainless steel sheet for hot stamping, which has excellent base sheet quality and is capable of obtaining sufficient corrosion resistance, high strength, and high elongation even when the painted or plated steel sheet is exposed due to defects on the edge of the steel sheet or due to external factors such as chipping, and Hot stamping The object is to provide a molded member. [Means for solving the problem]

[0011] The present inventors have conducted various studies to further improve corrosion resistance after high-temperature forming. To obtain sufficient corrosion resistance even without painting, stainless steel with a plating layer made of aluminum or an aluminum alloy is considered desirable. However, conventional techniques have not been able to obtain high strength after high-temperature forming. Therefore, it was thought that high strength could be obtained by plating a martensitic stainless steel sheet.

[0012] However, martensitic stainless steel sheets have a high Ac1 point due to their high Cr content. Therefore, in order to harden the steel during heat treatment for hot stamping, a high heating temperature is required. However, if the heating temperature is too high, the integrity of the coating layer cannot be maintained. Furthermore, the increased heating temperature makes heat treatment difficult using existing hot stamping equipment.

[0013] Therefore, the present inventors conducted extensive research into the chemical composition of steel and have completed the present invention. A stainless steel sheet having the chemical composition according to the present invention lowers the Ac1 point of the steel, and during heating, a certain amount of austenite appears, facilitating forming. During subsequent cooling, martensitic transformation occurs, precipitating a sufficient amount of martensite, thereby enabling the formed part to have high strength after hot stamping. Furthermore, since the stainless steel sheet according to the present invention does not require an increased heating temperature during hot stamping, the integrity of the coating layer is maintained and existing hot stamping equipment can be used. The gist of the present invention is as follows.

[0014] [1] In mass %, C: 0.03% or more and 0.35% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.1% or more and 2.0% or less, P: 0.035% or less, S: 0.010% or less, Cr: 10.5% or more and 13.5% or less, Ni: 0.01% or more and 0.60% or less, Cu: 0.01% or more and 1.5% or less, Mo: 0.01% or more and 1.5% or less, Al: 0.001% or more and 0.1% or less, N: 0.010% or more and 0.08% or less, O: 0.015% or less, The balance has a composition consisting of Fe and impurities, A martensitic stainless steel sheet having a γP value of 80 or more and 200 or less, and a C+0.5N value of 0.04% or more and 0.37% or less, as represented by the following formula (1): a plating layer made of aluminum or an aluminum alloy and having a thickness of 10 μm or more and 100 μm or less on the surface of the martensitic stainless steel sheet; A plated stainless steel sheet for hot stamping, comprising: γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-52Al+189 …(1) However, the element symbols in the above formula (1) and the C+0.5N indicate the content (mass %) of each element in the martensitic stainless steel sheet. [2] The plated stainless steel sheet for hot stamping according to [1], further containing, in place of a portion of Fe, one or more of the following group A elements, group B elements, group C elements, and group D elements, in mass %: Group A elements: Sn: 0.001% or more and 0.2% or less. B group elements: one or more of Nb: 0.005% or more and 0.5% or less, Ti: 0.005% or more and 0.1% or less, V: 0.005% or more and 0.2% or less, Zr: 0.005% or more and 0.1% or less. Group C elements: B: 0.0005% or more and 0.0030% or less. D group elements: one or two of W: 0.01% or more and 0.4% or less, Ta: 0.01% or more and 0.4% or less. [3] In mass %, C: 0.03% or more and 0.35% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.1% or more and 2.0% or less, P: 0.035% or less, S: 0.010% or less, Cr: 10.5% or more and 13.5% or less, Ni: 0.01% or more and 0.60% or less, Cu: 0.01% or more and 1.5% or less, Mo: 0.01% or more and 1.5% or less, Al: 0.001% or more and 0.1% or less, N: 0.010% or more and 0.08% or less, O: 0.015% or less, The balance has a composition consisting of Fe and impurities, A martensitic stainless steel material having γp represented by the following formula (2) of 80 or more and 200 or less, and C + 0.5N of 0.04% or more and 0.37% or less, a coating layer on the surface of the martensitic stainless steel material, the coating layer being made of an alloy containing aluminum, iron, and chromium, and containing 4% by mass or more and 12% by mass or less of Cr; characterized by comprising Hot stamping Molded parts. γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-52Al+189 …(2) However, the element symbols in the above formula (2) and the C+0.5N indicate the content (mass %) of each element in the martensitic stainless steel material. [4] The steel sheet according to [3], characterized in that the cross-sectional hardness is 300Hv or more and the total elongation in a tensile test according to JIS Z 2241 is 2% or more. Hot stamping Molded parts. [5] The composition according to [3] or [4], characterized in that, instead of a part of Fe, it further contains, in mass %, one or more of the following group A elements, group B elements, group C elements, and group D elements: Hot stamping Molded parts. Group A elements: Sn: 0.001% or more and 0.2% or less. B group elements: one or more of Nb: 0.005% or more and 0.5% or less, Ti: 0.005% or more and 0.1% or less, V: 0.005% or more and 0.2% or less, Zr: 0.005% or more and 0.1% or less. Group C elements: B: 0.0005% or more and 0.0030% or less. D group elements: one or two of W: 0.01% or more and 0.4% or less, Ta: 0.01% or more and 0.4% or less. [Effects of the Invention]

[0015] By using aluminum-plated or aluminum-alloy-plated stainless steel sheets as base sheets, martensitic stainless steel sheets with a composition that achieves a hardness of 300 HV or more after quenching, the base sheet has excellent quality without edge cracks or scuffs, and after hot stamping, it becomes a press-formed component that has high strength, excellent corrosion resistance, and high elongation. The corrosion resistance is superior to existing hot stamped components, and sufficient corrosion resistance can be obtained even when the steel sheet beneath the paint or plating is exposed due to scratches on the steel sheet edge or external factors such as chipping. This has significant economic benefits, such as making it possible to omit painting depending on the usage environment.

[0016] Furthermore, the present invention Hot stamping The molded member can be suitably used for automobile parts such as suspension parts, exhaust system parts, and structural members that require strength. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram showing the relationship between the amount of C+0.5N in an aluminum-plated steel sheet and the cross-sectional hardness after hot stamping. [Figure 2] FIG. 1 is a diagram showing the relationship between the Cr content in the coating layer of an aluminum-plated steel sheet and the corrosion depth after hot stamping. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in detail below. The present inventors have been investigating the possibility of hot stamping using stainless steel sheets for the purpose of reducing the weight and improving productivity of automobile suspension parts. Martensitic stainless steel has traditionally been used for brake discs of motorcycles and bicycles, and die quenching is used for quenching. These parts are surface-polished to obtain smoothness of the brake sliding surface.

[0019] On the other hand, hot-stamped components made from martensitic stainless steel blanks could be used for automotive suspension components. However, the steel surface oxidizes during the heat treatment process of hot stamping, which compromises the corrosion resistance of the stainless steel. Furthermore, unlike brake discs, polishing press-formed parts with complex shapes is difficult, and removing the oxide scale from the steel surface by pickling is also not easy.

[0020] Plated stainless steel, which has a plating layer made of aluminum or an aluminum alloy, is a stainless steel that does not lose its corrosion resistance even when heat treated for hot stamping. However, conventional plated stainless steel is mainly used for automobile exhaust system parts and building materials, and the base metal for plating is a ferritic stainless steel with a low carbon concentration, making it difficult to increase the strength by heat treatment.

[0021] Therefore, we attempted to develop a plated stainless steel sheet for hot stamping using martensitic stainless steel as the base sheet for plating. Martensitic stainless steel, which is used as the base sheet for aluminum plating, requires a high carbon content to be strengthened by quenching. However, unlike carbon steel, martensitic stainless steel contains 10.5% or more chromium, which forms chromium carbides. However, solution formation of chromium carbides is more difficult than that of cementite, an iron carbide, and prolonged heat treatment at high temperatures is required to achieve this. The presence of undissolved chromium carbides after quenching is undesirable because it degrades the material properties after quenching. Therefore, when using martensitic stainless steel as a blank for hot stamping, the chemical composition must be adjusted to prevent the presence of coarse chromium carbides after quenching.

[0022] Furthermore, Cr contained in stainless steel narrows the austenite single-phase region. To convert the steel structure to austenite during heating for hot stamping, austenite-stabilizing elements such as Ni, Mn, Cu, C, and N must be added to expand the austenite single-phase region of stainless steel. However, because all of these austenite-stabilizing elements lower the Ms point of steel, a large amount of austenite remains in the steel structure after quenching, which can result in insufficient strength. Furthermore, formed parts with residual austenite undergo transformation during use, and the volume expansion associated with this transformation can cause defects in the formed parts, such as warping.

[0023] To solve these problems and produce aluminum-plated stainless steel suitable for hot stamping, it is necessary to adjust the composition of the martensitic stainless steel used as the base sheet for plating so that coarse Cr carbides are less likely to remain, the austenite single-phase temperature range is wide, and retained austenite is less likely to remain after quenching. Furthermore, to increase the strength of the martensite phase after heat treatment, it is effective to add elements such as Ti, Nb, and V, which form particularly hard precipitates, in appropriate amounts, taking into account the corrosion resistance of the weld. However, reducing the content of elements such as Ti, Nb, and V requires careful selection of raw materials, which increases raw material costs, so they can be reduced as needed.

[0024] Based on the above findings, the present invention has found the optimum balance of components for martensitic stainless steel for the above applications. Hereinafter, the plated stainless steel sheet for hot stamping (hereinafter referred to as plated stainless steel sheet) and Hot stamping The molded member will now be described. In the following description, the "hot stamp formed member" may be referred to as the "formed member."

[0025] <Plated stainless steel sheet for hot stamping> The plated stainless steel sheet of this embodiment contains, in mass%, C: 0.03% or more and 0.35% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.1% or more and 2.0% or less, P: 0.035% or less, S: 0.010% or less, Cr: 10.5% or more and 13.5% or less, Ni: 0.01% or more and 0.60% or less, Cu: 0.01% or more and 1.5% or less, Mo: 0.01% or more and 1.5% or less, Al: 0.001% or more and 0.1% or less, and N: 0.010% and a martensitic stainless steel sheet having a chemical composition containing: C: 0.08% or less, O: 0.015% or less, and the balance being Fe and impurities, in which γp, as shown in the following formula (1), is 80 or more and 200 or less, and C+0.5N is 0.04% or more and 0.37% or less; and a plating layer made of aluminum or an aluminum alloy, having a thickness of 10 μm or more and 100 μm or less, on the surface of the martensitic stainless steel sheet.

[0026] γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-52Al+189 …(1)

[0027] However, the element symbols in the above formula (1) and "C+0.5N" represent the content (mass %) of each element in the martensitic stainless steel sheet.

[0028] [Chemical composition] The reasons for limiting the components of the martensitic stainless steel sheet are explained below. In the following explanation, "%" indicating the content of each element means "% by mass" unless otherwise specified.

[0029] C: 0.03 to 0.35% Carbon is an element that governs quench hardness. To consistently achieve the strength and hardness required for strength components, the carbon content must be 0.03% or more. However, excessive carbon content reduces ductility and toughness and increases the amount of Cr carbide, so the carbon content must be 0.35% or less. Considering the balance between strength and ductility after quenching, it is desirable for the carbon content to be 0.04% or more and 0.20% or less.

[0030] Si: 0.05 to 1.0% Si is necessary for deoxidation during steel melting and refining, and is also effective in suppressing oxide scale formation during hot rolling. Therefore, the Si content is set to 0.05% or more. However, Si narrows the austenite single-phase temperature range and impairs hardening stability. Furthermore, excessive suppression of hot rolling scale formation makes hot rolling defects more likely to occur. Therefore, the Si content is set to 1.0% or less. To reduce the incidence of defects due to oxide inclusions, the Si content is preferably set to 0.1% or more. Furthermore, because Si narrows the austenite single-phase temperature range and impairs hardening stability, it is preferable to set the Si content to 0.6% or less.

[0031] Mn: 0.1 to 2.0% Mn is an austenite-stabilizing element, but it lowers the Ac1 point. However, if the Ac1 point is lowered too much, the upper limit of the heating temperature during quenching decreases, and the heating time becomes longer. Therefore, the Mn content is set to 2.0% or less. Considering the deterioration of corrosion resistance due to the coarsening of sulfide-based inclusions such as MnS, the Mn content is preferably set to 1.0% or less. Furthermore, Mn has the effect of fixing S in the steel as MnS and improving hot workability, so the Mn content is set to 0.1% or more. Since the S concentration can become high depending on the steel refining conditions, the Mn content is preferably set to 0.5% or more.

[0032] P:0.035% or less P is an element contained as an impurity in alloys such as molten pig iron and ferrochrome, which are raw materials. Because P is an element that is harmful to hot workability and toughness, it is set to 0.035% or less. Since P also reduces workability, it is desirable to keep it to 0.030% or less. Furthermore, excessive reduction of P requires the use of high-purity raw materials, which leads to increased costs, so the P content may be set to 0.010% or more.

[0033] S: 0.010% or less S is an element that has a small amount of solid solubility in the austenite phase and segregates at grain boundaries, promoting a decrease in hot workability. If the S content exceeds 0.010%, the effect becomes significant, so the S content is set to 0.010% or less. The lower the S content, the fewer sulfide-based inclusions there are and the better the corrosion resistance, but reducing the S content increases the desulfurization load and the manufacturing cost. Therefore, the S content may be set to 0.001% or more. The S content is preferably 0.001% to 0.008%.

[0034] Cr: 10.5 to 13.5% In order to maintain the corrosion resistance required for the main applications of martensitic stainless steel, the Cr content must be at least 10.5%. On the other hand, to prevent the formation of retained austenite after quenching, the Cr content must be 13.5% or less. To make these properties more effective, the Cr content should preferably be in the range of 12.0 to 13.0%.

[0035] Ni: 0.01 to 0.60% Like Mn, Ni is an austenite stabilizing element and also lowers the Ac1 point. However, if the Ac1 point is lowered too much, the upper limit of the heating temperature during quenching will be lowered and the heating time will be longer. Therefore, the Ni content is set to 0.60% or less. On the other hand, Ni improves ductility and toughness after quenching, and this effect can be obtained at a content of 0.01% or more, so the Ni content is set to 0.01% or more. To improve the balance of strength and ductility after quenching, the Ni content is set to 0.05% or more and 0.50% or less.

[0036] Cu: 0.01 to 1.5% Cu is often unavoidably included, such as through contamination from scrap during steel melting. However, eliminating Cu by using high-purity raw materials can promote active dissolution during pit growth, impairing corrosion resistance. Therefore, the Cu content is set to 0.01% or more. To further improve corrosion resistance, the Cu content is set to 0.03% or more. Cu is actively included to increase austenite stability, but excessive content reduces hot workability and corrosion resistance, so it is set to 1.5% or less. Furthermore, because Cu precipitation can reduce corrosion resistance, a Cu content of 1.0% or 0.5% or less is preferable.

[0037] Mo: 0.01 to 1.5% Like Cu, Mo has the effect of suppressing active dissolution and inhibiting the progression of pitting corrosion, making it an effective element for improving corrosion resistance. To achieve this effect, the Mo content must be 0.01% or more. To achieve even higher corrosion resistance, it is preferable to add 0.03% or more. However, excessive Mo increases temper softening resistance, which can hinder productivity in the manufacturing process, so the Mo content must be 1.5% or less. Assuming the corrosion prevention effect of the plating layer is maintained, a Mo content of 0.5% or less is desirable.

[0038] Al: 0.001 to 0.1% Al is an effective element for deoxidation, and its effect is greatest at 0.001% or more, so the Al content is set to 0.001% or more. However, Al increases the basicity of the slag, precipitates water-soluble inclusions (CaS) in the steel, and may reduce corrosion resistance, so the Al content is set to 0.1% or less. Furthermore, considering the reduction in abrasiveness due to alumina-based non-metallic inclusions, the Al content is preferably set to 0.01% or less. However, to obtain the deoxidizing effect in combination with Si and Mn, the Al content is preferably set to 0.005% or more.

[0039] N: 0.010% to 0.08% Like C, N has the effect of increasing quench hardness and also expands the austenite single-phase temperature range. However, unlike Ni and Mn, N does not lower the Ac1 point, so it is actively included. Since quench hardness can be adjusted with both C and N, the N content is set to 0.010% or more to ensure quench hardness. On the other hand, since a large amount of nitrogen causes pore defects during solidification, the N content is set to 0.08% or less. Furthermore, in order to refine carbonitrides during annealing and promote solution during heating for hot stamping, the N content is preferably set to 0.020% or more and 0.05% or less.

[0040] O: 0.015% or less O is generally present as oxide-based inclusions, and large inclusions, together with water-soluble sulfides, can become corrosion initiation sites and are also detrimental to ductility and toughness after quenching. Therefore, the O content is set to 0.015% or less. Inclusions that aggregate and coarsen during casting can cause scuff marks on the surface of hot-rolled steel sheets, so the O content is preferably set to 0.008% or less. While a lower O content is generally preferable, industrial removal of oxides formed in molten steel by floating them up is difficult in terms of productivity and cost, so the O content may be set to 0.003% or more.

[0041] Furthermore, in addition to the above elements, the martensitic stainless steel sheet according to this embodiment may contain one or more of the following group A elements, group B elements, group C elements, and group D elements. When these elements are contained, the upper limit may be set by using high-purity raw materials.

[0042] (Group A element) Sn:0.001~0.2% Like Mo and Cu, Sn is an element that enhances corrosion resistance by suppressing the progression of pitting corrosion, so it is desirable to add it as needed. To achieve this effect, the Sn content should be 0.001% or more. However, Sn has a small solid solubility limit in the austenite phase, and is known to cause hot-rolled cracks and defects in ordinary steel. Furthermore, long-term aging at 400 to 700°C can reduce the toughness of the steel, so it is considered desirable to reduce the Sn content as much as possible. Therefore, the Sn content is preferably 0.2% or less.

[0043] (B group elements) One or more of Nb: 0.005% or more and 0.5% or less, Ti: 0.005% or more and 0.1% or less, V: 0.005% or more and 0.2% or less, Zr: 0.005% or more and 0.1% or less.

[0044] Nb: 0.005 to 0.5% Nb is an element that raises the Ac1 point, making it possible to increase the heating temperature during hot stamping, so it is desirable to add it as needed. To achieve this effect, the Nb content should be 0.005% or more. However, since excessive Nb content increases solidification cracking susceptibility, the Nb content should preferably be 0.5% or less.

[0045] Ti: 0.005% to 0.1% Ti is an element that raises the Ac1 point, making it possible to increase the heating temperature during hot stamping, so it is desirable to add it as needed. To achieve this effect, the Ti content is set to 0.005% or more. However, if Ti is added in excess, coarse TiN precipitates in the temperature range above the hot rolling heating temperature, which can cause inclusion-based defects. Therefore, the Ti content is preferably set to 0.1% or less.

[0046] V: 0.005 to 0.2% V is an element that raises the Ac1 point, making it possible to increase the heating temperature during hot stamping, so it is desirable to add V as needed. To achieve this effect, the V content is set to 0.005% or more. However, a large amount of V may promote the formation of coarse carbides due to solidification segregation, which may reduce ductility and toughness after quenching, so the V content is preferably set to 0.2% or less.

[0047] Zr: 0.005% to 0.1% Zr is an element that raises the Ac1 point, making it possible to increase the heating temperature during hot stamping, so it is desirable to add it as needed. To achieve this effect, the Zr content should be 0.005% or more. However, if Zr is added in excess, coarse Zr(C,N) precipitates in the temperature range above the hot rolling heating temperature, which can cause inclusion-based defects. Therefore, the Zr content is preferably 0.1% or less.

[0048] (C group elements) B:0.0005%~0.0030% B improves high-temperature ductility during hot rolling and reduces yield loss due to edge cracks in hot-rolled sheets, so it may be added as needed. To achieve this effect, the B content is set to 0.0005% or more. However, excessive B content can lead to the formation of Cr2B, (Cr, Fe) 23 Precipitation of (C, B)6 impairs toughness and corrosion resistance, so the B content is preferably set to 0.0030% or less.

[0049] (D group elements) W: 0.01% or more and 0.4% or less Like Mo, W is also an element effective in improving corrosion resistance. To achieve this effect, the W content is set to 0.01% or more. To obtain even higher corrosion resistance, it is preferably set to 0.03% or more, and more preferably 0.05% or more. On the other hand, since an excessive content reduces room-temperature ductility, the upper limit is set to 0.4% or less. It may also be 0.2% or less. In the present invention, W may be added to adjust general properties, and the W content is preferably 0.05 to 0.2%.

[0050] Ta: 0.01% or more and 0.4% or less Ta combines with C and N to contribute to improving corrosion resistance and toughness, and is therefore added as needed. To achieve this effect, the Ta content is set to 0.01% or more. To obtain even higher corrosion resistance, it is preferably set to 0.03% or more, and more preferably 0.05% or more. However, a content of more than 0.4% increases costs and significantly deteriorates manufacturability, so the upper limit is set to 0.4% or less. It may also be 0.2% or less. In the present invention, Ta may be added to adjust general properties, and the Ta content is preferably 0.05 to 0.2%.

[0051] [γp: 80 or more, 200 or less] The gamma potential γp is calculated using the above formula (1). To obtain high strength through hot stamping, it is necessary to convert the steel into austenite as the main phase during heating before forming. Therefore, the gamma potential γp must be 80 or higher. Considering the change in the austenite phase fraction due to fluctuations in heating temperature, a γp of 90 or higher is desirable. On the other hand, adding large amounts of Ni, Mn, and Cu, which are elements that increase γp, increases the tempering softening resistance and lowers the Ac1 point, which is undesirable. In addition, increasing the content of C and N, which are elements that increase γp like Ni, Mn, and Cu, is also undesirable because it impairs the ductility and toughness after quenching. Therefore, as a balance of the overall components, it is necessary to keep γp at 200 or less. To obtain the ductility and toughness required for structural components, a γp of 150 or less is preferable.

[0052] [C+0.5N: 0.04% or more, 0.37% or less] In order to obtain the required hardness of 300 Hv or more in the formed part after hot stamping, it is necessary to make C + 0.5N 0.04% or more. Furthermore, in order to obtain a strength of 1200 MPa or more in the formed part after hot stamping, it is preferable to make C + 0.5N 0.10% or more. On the other hand, if the C and N contents in stainless steel are too high, coarse carbides will crystallize during solidification, impairing ductility and toughness, so it is necessary to make C + 0.5N 0.37% or less. In applications where toughness is more important, it is preferable to make C + 0.5N 0.30% or less.

[0053] <Plating layer> [Thickness of plating layer: 10 μm or more, 100 μm or less] A plating layer is provided on the surface of a martensitic stainless steel sheet. The plating layer is made of aluminum or an aluminum alloy. Aluminum plating or aluminum alloy plating prevents a decrease in corrosion resistance due to oxidation of stainless steel during hot stamping heating, and provides high corrosion resistance by sacrificially protecting or protectively coating the stainless steel base material in the neutral chloride environment to which automotive undercarriage parts are exposed. To achieve the corrosion protection life required in an automotive undercarriage environment, the plating layer must be 10 μm thick or more. Considering damage to the plating layer due to cut edges where the base material is exposed and chipping, a plating layer thickness of 15 μm or more is preferred. On the other hand, if the plating layer is too thick, defects such as peeling of the plating layer will occur during formation and hot stamping, so it is set to 100 μm or less. Considering weldability, a plating layer thickness of 50 μm or less is preferred.

[0054] The material of the plating layer is not particularly limited as long as it is aluminum or an aluminum alloy. An aluminum alloy refers to an alloy in which the element with the highest concentration among the elements contained therein is aluminum. Examples of aluminum alloys include Al-Si alloys, Zn-Al alloys, Zn-Al-Mg alloys, and Zn-Al-Mg-Si alloys. An example of an Al-Si alloy is one containing 3 to 15% Si, with the remainder being Al and impurities. The plating layer may be either an electroplated layer or a hot-dip plated layer.

[0055] < Hot Stamp Molded parts> Next, the formed member of this embodiment will be described. The formed member of this embodiment is obtained by hot stamping the above-mentioned plated stainless steel sheet for hot stamping. That is, the formed member of this embodiment is configured to include a martensitic stainless steel material having the above-mentioned chemical composition and a coating layer on the surface of the martensitic stainless steel material, which is made of an alloy containing aluminum, iron, and chromium, and contains 4% by mass to 12% by mass of Cr.

[0056] The coating layer is formed by the diffusion of Fe and Cr from the steel material into the plating layer of the martensitic stainless steel sheet due to heating during hot stamping. To improve corrosion resistance after hot stamping and reduce plating peeling during forming, the coating composition after hot stamping contains aluminum, iron, and chromium, with a Cr content of 4% by mass or more and 12% by mass or less. There are no particular restrictions on the Fe and Al contents in the coating layer. To prevent plating defects such as chipping, it is more preferable to have a Cr content of 5% by mass or more. On the other hand, since an excessively high Cr content in the coating layer makes the coating more susceptible to peeling, it is more preferable to have a Cr content of 10% by mass or less.

[0057] The cross-sectional hardness of the molded part is preferably 300Hv or more. Furthermore, the total elongation in a tensile test conforming to JIS Z 2241 is preferably 2% or more for a JIS No. 13 B test piece. The cross-sectional hardness is measured by exposing the cross-section of the stainless steel material in the thickness direction and measuring the Vickers hardness at a position 1 / 4 of the thickness from the surface of the stainless steel material. The hardness measurement is performed three times at different locations, and the average of the measured values is taken as the cross-sectional hardness. The applied load is set to 5 kgf.

[0058] <Manufacturing method> [Method of manufacturing plated stainless steel sheets for hot stamping] A stainless steel sheet having the above chemical composition and a thickness of 0.5 to 1.5 mm that has been subjected to the processes of hot rolling, cold rolling, annealing, and pickling under typical conditions is used as the base sheet for plating. The conditions for producing the stainless steel sheet to be used as the base sheet for plating are not particularly limited, but an example of the hot rolling, pickling, cold rolling, annealing, and pickling conditions is to reheat the steel billet to 1100 to 1300°C, and then hot roll it at a finishing temperature of 850 to 950°C with a reduction of 80% or more. The coiling temperature can be exemplified as being in the range of 350 to 700°C. For pickling, the type, concentration, and temperature of the acid can be appropriately selected to efficiently remove the formed scale. The cold rolling reduction is preferably 40% or more to ensure flatness. The annealing temperature can be exemplified as being in the range of 800 to 950°C.

[0059] There are no particular limitations on the method of plating the stainless steel sheet, and hot-dip plating, electroplating, vacuum deposition, cladding, and other methods are possible, with hot-dip plating being preferred.

[0060] The Al plating bath used for plating has a bath temperature of, for example, 670°C and contains 3 to 15 mass% of Si as a secondary component, preferably about 10%. The Al plating bath may also contain impurities such as Fe, Cr, and Mn that have eluted from the steel sheet. Small amounts of Mg and Zn may also be added as other elements.

[0061] After plating, it is preferable to use a gas wiping method to achieve a plating thickness of 10 to 100 μm per side in order to prevent plating peeling during forming and ensure rust prevention. Furthermore, it is preferable to perform a chemical conversion treatment after plating to suppress corrosion of the plating layer before hot stamping.

[0062] [Manufacturing method for molded parts] The formed part is prepared by heating the above-mentioned hot stamping plated stainless steel sheet to a temperature of 850°C or higher, forming it using a hot stamping die, and then rapidly cooling it. An average heating rate of 10°C / s or higher is preferred during heating to ensure the peel strength of the plating, and heating to a temperature range of 950 to 1050°C is preferred to ensure strength. When the base sheet for plating is in the ferrite phase, the atomic diffusion rate is fast, and excessive diffusion of Fe and Cr into the plating layer impairs the workability of the plating layer (coating layer). Therefore, by increasing the heating rate to 10°C / s or higher, the diffusion of Fe and Cr can be appropriately suppressed, and the Cr content of the plating layer can be kept in the range of 4 to 12 mass%.

[0063] <Experiment> The relationship between the C+0.5N content and the cross-sectional strength of formed components was investigated for various plated steel sheets, as shown in Table 1. The plated steel sheets used were commercially available aluminum-plated steel sheet (No. a) (0.22%C-1.2%Mn), aluminum-plated steel sheet (No. b) made from SUH409 steel (0.004%C-11.8%Cr), and aluminum-plated steel sheet (No. c) made from SUS432 steel (0.006C-17Cr-0.5Mo). Furthermore, aluminum-plated steel sheets (Nos. d–f) were fabricated in the laboratory by forming an aluminum coating on steel sheets made from SUS410 (0.04C-12.3Cr), SUS420J1 (13.3Cr-0.21C), and SUS420J2 (13.5Cr-0.32C) with adjusted contents of other elements. The plating thickness of these steel sheets was approximately 20 μm. Table 1 shows the chemical composition of each steel sheet.

[0064] [Table 1]

[0065] These plated steel sheets were heated to 950°C for 1 minute, then clamped between a hot stamping die and cooled at a rate of approximately 50°C / s to obtain hot stamped components. The cross-sectional hardness of the formed components was then measured. The results are shown in Figure 1.

[0066] In addition, the obtained molded parts were immersed in a chemical conversion treatment solution, and after an approximately 25 μm electrocoat coating was applied, cross-shaped scratches (cross cuts) were made in the surface coating film. These were used as samples and subjected to a combined corrosion test (CCT) in accordance with JASO M610 for 150 cycles (50 days).

[0067] The test conditions for the JASO CCT method are shown below. Salt spray (5% NaCl, 35℃, 2h) → Dry (60℃, 4h) → Wet (50℃, 2h)

[0068] The corrosion depth in the thickness direction after 150 CCT cycles was measured from the surface using a laser beam at five points per sample, and the average values were calculated. The results are shown in Figure 2 and Table 2. Table 2 also shows the Cr content in the coating layer of the molded component and the evaluation results for total elongation.

[0069] As shown in Tables 1 and 2 and Figures 1 and 2, the aluminum-plated steel sheet No. b made of SUH409 steel and the aluminum-plated steel sheet No. c made of SUS432 steel had cross-sectional hardnesses of 200 HV or less, failing to achieve the required hardness of 300 HV or more. Furthermore, the aluminum-plated steel sheet for hot stamping No. a achieved a hardness of 300 HV or more, but corrosion tests showed corrosion exceeding 100 μm. On the other hand, the aluminum-plated stainless steel sheets Nos. d to f, which were made by coating aluminum on steel sheets based on SUS410, SUS403, SUS420J1, and SUS420J2 with adjusted contents of other elements, had cross-sectional hardnesses of 300 HV or more after hot stamping, and corrosion depths of 50 μm or less were observed in corrosion tests.

[0070] [Table 2] [Example]

[0071] Next, the present invention will be described in more detail with reference to examples. Steel sheets with a thickness of 1.0 mm and having the steel compositions shown in Tables 3A and 3B were manufactured through hot rolling and cold rolling processes, and the presence or absence of edge cracks and surface scabs was evaluated. If edge cracks or surface scabs were found, the quality of the plated base sheet was deemed to be unacceptable.

[0072] These steel sheets were then used as plating base sheets and subjected to hot-dip aluminum plating or aluminum alloy plating in a non-oxidizing furnace-reduction furnace type line. Inventive example P1 was hot-dip aluminum plating, while the others were aluminum alloy plating containing 8-10% Si, 1-3% Fe, and 0.2-0.6% Cr. After plating, the plating thickness was adjusted to 10-100 μm using a gas wiping method, and the sheets were cooled and coiled.

[0073] [Table 3A]

[0074] [Table 3B]

[0075] The aluminum-plated stainless steel sheet or aluminum alloy-plated stainless steel sheet thus obtained was cut into a hot stamping sheet, which was heated to 950°C for 1 minute, then sandwiched between hot stamping flat dies and cooled at a cooling rate of 50°C / s to form a formed part. Test pieces were cut out from the formed part and subjected to cross-sectional hardness measurement, tensile testing, and analysis of the Cr concentration in the plating.

[0076] In a tensile test conforming to JIS Z 2241, a total elongation of 2% or more was deemed to have passed. A cross-sectional hardness of 300Hv or more was deemed to have passed. The cross-sectional hardness of the molded part was measured by exposing the cross-section of the stainless steel material in the thickness direction and measuring the Vickers hardness at a position 1 / 4 of the thickness from the surface of the stainless steel material. The hardness measurement was performed three times at different locations, and the average of the measured values was taken as the cross-sectional hardness. The applied load was set to 5kgf.

[0077] The obtained molded parts were immersed in a chemical conversion treatment solution and electrocoated with approximately 20 μm of paint. Then, cross-cuts were made in the surface coating and a composite corrosion test (CCT) in accordance with JASO M610 was carried out for 150 cycles (50 days).

[0078] The test conditions for the JASO CCT method are shown below. Salt spray (5%NaCl, 35℃, 2h) → Dry (60℃, 4h) → Wet (50℃, 2h)

[0079] After 150 cycles, the corrosion depth in the thickness direction of each sample was measured using a laser beam from the surface at five points, and the average value was calculated. An average corrosion depth of 50 μm or less was considered to be acceptable.

[0080] The evaluation results are shown in Tables 4A and 4B.

[0081] [Table 4A]

[0082] [Table 4B]

[0083] The formed parts P1 to P28, which were produced by hot stamping using plated steel sheets for hot stamping within the scope of the present invention, were free of edge cracks and surface scuffs and achieved the target cross-sectional hardness of 300 HV or more, total elongation of 2% or more, and average corrosion depth of 50 μm or less. Note that the coating layer after hot stamping contained aluminum and iron in addition to Cr.

[0084] On the other hand, in the comparative examples S1 to S4, the C content was less than 0.04% and the γP was also less than 80, so the cross-sectional hardness was less than 300 Hv. For S5, the total elongation was less than 2% because the C content was greater than 0.35%.

[0085] S6 had poor corrosion resistance because the Si content was less than 0.05% and the amount of Cr in the surface layer was reduced due to oxidation during hot rolling heating. In S7, the Si content exceeded 1%, so the hot-rolled scale was thin and surface scuffs occurred due to seizure with the hot-rolling rolls.

[0086] In S8, the Mn content was less than 0.1% and the S content was more than 0.01%, so edge cracks occurred during the hot rolling process. In S9, the Mn content exceeded 2.0%, and therefore corrosion originating from MnS caused poor corrosion resistance.

[0087] In S10, the P content exceeded 0.035%, which resulted in a decrease in hot workability and the occurrence of edge cracks on the end surfaces of the hot-rolled sheet. In S11, the S content exceeded 0.010%, which resulted in a decrease in hot workability and the occurrence of edge cracks.

[0088] S12 had poor corrosion resistance because the Cr content was less than 10.5%. In S13, the Cr content exceeded 13.5%, so the ferrite phase increased during heating for hot stamping, resulting in low hardness after quenching.

[0089] In S14, the Mo, Cu, and Ni contents were less than 0.01%, so the corrosion resistance was reduced. In S15, the Al content was less than 0.001% and the O content was more than 0.015%, resulting in a large amount of oxide inclusions and scabs.

[0090] S16 had poor corrosion resistance because the Cr content was less than 10.5%. S17 had poor ductility because C+0.5N exceeded 0.37%.

[0091] In S18, the plating thickness was less than 10 μm, so the amount of Cr in the plating layer was excessive, resulting in poor corrosion resistance. In the case of S19, the plating thickness exceeded 100 μm, so the amount of Cr in the plating layer was reduced, which caused peeling of the plating layer during hot stamping, and the corrosion associated with the peeling resulted in poor corrosion resistance. [Industrial Applicability]

[0092] According to the present invention, it is possible to produce surface-treated stainless steel sheets with high strength and excellent corrosion resistance with good productivity without the occurrence of hot rolling defects, and therefore the present invention contributes to the reduction in weight and the extension of the life of automobile suspension parts, exhaust system parts, fuel system parts, structural members, etc.

Claims

1. In mass%, C: 0.03% or more and 0.35% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.1% or more and 2.0% or less, P: 0.035% or less, S: 0.010% or less, Cr: 10.5% or more and 13.5% or less, Ni: 0.01% or more and 0.60% or less, Cu: 0.01% or more and 1.5% or less, Mo: 0.01% or more and 1.5% or less, Al: 0.001% or more and 0.1% or less, N: 0.010% or more and 0.08% or less, O: 0.015% or less, The balance has a composition consisting of Fe and impurities, A martensitic stainless steel plate having a γP value of 80 to 200, and a C+0.5N value of 0.04% to 0.37%, as determined by the following formula (1): a plating layer made of aluminum or an aluminum alloy and having a thickness of 10 μm or more and 100 μm or less on the surface of the martensitic stainless steel sheet; A plated stainless steel sheet for hot stamping, comprising: γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-52Al+189...(1) However, the element symbols in the above formula (1) and C+0.5N represent the content (mass %) of each element in the martensitic stainless steel sheet.

2. The plated stainless steel sheet for hot stamping according to claim 1, characterized in that it further contains, in mass %, one or more of the following group A elements, group B elements, group C elements and group D elements in place of a portion of Fe: Group A element: Sn: 0.001% or more and 0.2% or less. B group elements: one or more of Nb: 0.005% or more and 0.5% or less, Ti: 0.005% or more and 0.1% or less, V: 0.005% or more and 0.2% or less, and Zr: 0.005% or more and 0.1% or less. Group C elements: B: 0.0005% or more and 0.0030% or less. D group elements: one or two of W: 0.01% or more and 0.4% or less, and Ta: 0.01% or more and 0.4% or less.

3. In mass%, C: 0.03% or more and 0.35% or less, Si: 0.05% or more and 1.0% or less, Mn: 0.1% or more and 2.0% or less, P: 0.035% or less, S: 0.010% or less, Cr: 10.5% or more and 13.5% or less, Ni: 0.01% or more and 0.60% or less, Cu: 0.01% or more and 1.5% or less, Mo: 0.01% or more and 1.5% or less, Al: 0.001% or more and 0.1% or less, N: 0.010% or more and 0.08% or less, O: 0.015% or less, The balance has a composition consisting of Fe and impurities, A martensitic stainless steel material having γp represented by the following formula (2) of 80 or more and 200 or less, and C + 0.5N of 0.04% or more and 0.37% or less; a coating layer on the surface of the martensitic stainless steel material, the coating layer being made of an alloy containing aluminum, iron, and chromium, and containing 4% by mass or more and 12% by mass or less of Cr; A hot stamp formed member comprising: γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-52Al+189...(2) However, the element symbols in the above formula (2) and the C+0.5N indicate the content (mass %) of each element in the martensitic stainless steel material.

4. 4. The hot stamp formed member according to claim 3, wherein the cross-sectional hardness is 300 Hv or more, and the total elongation in a tensile test in accordance with JIS Z 2241 is 2% or more.

5. 5. The hot-stamped member according to claim 3 or 4, characterized in that, in place of a portion of Fe, the hot-stamped member further contains, in mass %, one or more of the following group A elements, group B elements, group C elements, and group D elements: Group A element: Sn: 0.001% or more and 0.2% or less. B group elements: one or more of Nb: 0.005% or more and 0.5% or less, Ti: 0.005% or more and 0.1% or less, V: 0.005% or more and 0.2% or less, and Zr: 0.005% or more and 0.1% or less. Group C elements: B: 0.0005% or more and 0.0030% or less. D group elements: one or two of W: 0.01% or more and 0.4% or less, and Ta: 0.01% or more and 0.4% or less.

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