JP7723263B2 - Plated stainless steel sheets for hot stamping and hot stamp formed components - Google Patents

Description

The present invention relates to a plated stainless steel sheet for hot stamping and a hot stamped member.

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

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.

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

On the other hand, Patent Document 3 discloses a technique for increasing the mechanical strength of steel sheets by coating the surface with aluminum or an aluminum alloy and then heat treating the steel sheet to achieve even higher strength. This technique is known by various names, such as hot stamping, hot pressing, and die quenching, and is widely used today.

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 moving), corrosion resistance decreases and localized corrosion progresses.

On the other hand, as an example of an aluminum-plated highly corrosion-resistant steel sheet, Patent Document 4 discloses 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 to increase corrosion resistance due to exhaust gas condensate in automobile exhaust systems, making it difficult to achieve high strength through high-temperature forming.

Japanese Patent Application Laid-Open No. 2000-234153 Japanese Patent Application Laid-Open No. 2000-087183 Japanese Patent Application Laid-Open No. 2000-038640 Japanese Patent Application Publication No. 05-112859

As such, various steel sheets that achieve high strength after high-temperature forming have been developed using technologies that have been disclosed to date. The corrosion resistance of these steel sheets is ensured by the plating layer and paint. However, there were concerns that corrosion resistance could not be guaranteed at the edge of the steel sheet that is not covered by the plating layer, or at areas where damage or scratches occur in the paint or plating layer of parts while the vehicle is in operation.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a plated stainless steel sheet for hot stamping and a hot stamp-formed member which have excellent base sheet quality and which are capable of obtaining sufficient corrosion resistance, high strength, and high elongation even when the steel sheet beneath the coating or plating is exposed due to defects on the end faces of the steel sheet or due to external factors such as chipping .

The inventors conducted various studies to further improve corrosion resistance after high-temperature forming. To achieve sufficient corrosion resistance even without painting, stainless steel with a plating layer made of aluminum or an aluminum alloy would be desirable. However, conventional technology does not provide high strength after high-temperature forming. Therefore, they believed that high strength could be achieved by using martensitic stainless steel sheet for plating.

However, martensitic stainless steel sheets contain a large amount of Cr, resulting in a high Ac ₁ point. Therefore, in order to harden the steel during heat treatment for hot stamping, it is necessary to increase the heating temperature. However, if the heating temperature becomes too high, the integrity of the coating layer cannot be maintained. Furthermore, the increase in heating temperature makes it difficult to perform heat treatment using existing hot stamping equipment.

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 Ac ₁ point of the steel, and during heating, a certain amount of austenite phase appears, facilitating forming. During subsequent cooling, martensitic transformation occurs, precipitating a sufficient amount of martensite phase, thereby enabling high strength formed components 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.

[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 [1], further containing, 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] The hot stamp formed member according to [3], characterized in that 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] The hot stamp formed member according to [3] or [4], further containing, 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.

By using aluminum-plated stainless steel sheet or aluminum alloy-plated stainless steel sheet as the base sheet, martensitic stainless steel sheet with a composition that achieves a hardness of 300 HV or more after quenching, the base sheet quality is excellent with no cracks or scuffs, and the press-formed components have high strength, excellent corrosion resistance, and high elongation after hot stamping. The corrosion resistance is superior to existing hot-stamped components, and sufficient corrosion resistance can be achieved even when the steel sheet beneath the paint or plating is exposed due to scratches on the edge of the steel sheet or scratches caused by external factors such as chipping. This has significant economic benefits, such as making it possible to omit painting depending on the usage environment.

Furthermore, the hot stamped member according to the present invention can be suitably used for automobile suspension parts, exhaust system parts, structural members, and other parts that require high strength.

FIG. 2 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. FIG. 2 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.

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.

On the other hand, it is possible to use hot-stamped components made from martensitic stainless steel blanks for automotive suspension parts. However, the steel surface oxidizes during the heat treatment process of hot stamping, which means the corrosion resistance of the stainless steel is lost. Also, unlike brake discs, it is difficult to polish press-formed parts with complex shapes, and it is not easy to remove the oxide scale from the steel surface by pickling.

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 automotive exhaust system parts and building materials, and the base metal used for plating is a ferritic stainless steel with a low carbon concentration, making it difficult to increase its strength through heat treatment.

Therefore, we attempted to develop a plated stainless steel sheet for hot stamping using martensitic stainless steel sheet as the base sheet for plating. Martensitic stainless steel, which is used as the base sheet for aluminum plating, requires a high carbon content in order to be strengthened by quenching. However, unlike carbon steel, martensitic stainless steel contains 10.5% or more chromium, which allows the chromium in the steel to form 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. If undissolved chromium carbides remain after quenching, this is undesirable, as it degrades the material properties after quenching. Therefore, when using martensitic stainless steel sheet as a blank for hot stamping, the composition must be adjusted to prevent coarse chromium carbides from remaining after quenching.

Furthermore, the Cr contained in stainless steel is an element that narrows the austenite single-phase region. To convert the steel structure to the austenite phase during heating for hot stamping, it is necessary to add austenite-stabilizing elements such as Ni, Mn, Cu, C, and N to expand the austenite single-phase region of the stainless steel. However, because all of these austenite-stabilizing elements lower the Ms point of the 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 as parts, and the volume expansion associated with this transformation can cause defects in the formed parts, such as warping.

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 martensitic phase after heat treatment, it is effective to include appropriate amounts of Ti, Nb, V, and other elements that form particularly hard precipitates, taking into account the corrosion resistance of welds. On the other hand, reducing the content of elements such as Ti, Nb, and V requires careful selection of raw materials, which increases raw material costs, so these elements can be reduced as needed.

Based on the above findings, the present invention has discovered the optimal balance of components for martensitic stainless steel for the intended application. Hereinafter, a plated stainless steel sheet for hot stamping (hereinafter referred to as plated stainless steel sheet) and a hot-stamped member according to embodiments of the present invention will be described. In the following description, the "hot-stamped member" may also be referred to as the "formed member."

<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 component composition containing: S: 0.08% or more and O: 0.015% or less, with 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 and having a thickness of 10 μm or more and 100 μm or less, on the surface of the martensitic stainless steel sheet.

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 plate.

[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 "mass %" unless otherwise specified.

C: 0.03-0.35%
C is an element that governs quench hardness. In order to stably obtain the strength and hardness required for strength components, the C content is set to 0.03% or more. On the other hand, excessive C content reduces ductility and toughness and increases Cr carbide, so the C content is set to 0.35% or less. Considering the balance between strength and ductility after quenching, it is desirable for the C content to be 0.04% or more and 0.20% or less.

Si: 0.05-1.0%
Si is necessary for deoxidation during steel melting and refining, and is also effective in suppressing oxide scale formation during hot rolling heating. 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 heating scale formation makes hot rolling defects more likely to occur. Therefore, the Si content is set to 1.0% or less. In order to reduce the incidence of defects due to oxide-based inclusions, it is desirable to set the Si content to 0.1% or more. Furthermore, since Si narrows the austenite single-phase temperature range and impairs hardening stability, it is desirable to set the Si content to 0.6% or less.

Mn: 0.1-2.0%
Mn is an austenite stabilizing element, but it lowers the Ac ₁ point. However, if the Ac ₁ point is lowered too much, the upper limit of the heating temperature during quenching is lowered, and the heating time becomes long. 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, since Mn has the effect of fixing S in the steel as MnS and improving hot workability, the Mn content is set to 0.1% or more. Since the S concentration may become high depending on the steel refining conditions, the Mn content is preferably set to 0.5% or more.

P: 0.035% or less P is an element contained as an impurity in alloys such as molten pig iron and ferrochromium, which are raw materials. Since P is an element that is harmful to hot workability and toughness, the content is set to 0.035% or less. Since P is also an element that reduces workability, it is desirable to set the content 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.

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%.

Cr: 10.5-13.5%
In order to maintain the corrosion resistance required for the main applications of martensitic stainless steels, 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%.

Ni: 0.01~0.60%
Like Mn, Ni is an austenite stabilizing element and also an element that lowers the Ac ₁ point. However, if the Ac ₁ point is lowered too much, the upper limit of the heating temperature during quenching will be lowered and the heating time will be long. Therefore, the Ni content is set to 0.60% or less. On the other hand, Ni has the function of improving 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. In order to improve the balance between strength and ductility after quenching, the Ni content is set to 0.05% or more and 0.50% or less.

Cu: 0.01~1.5%
Cu is often unavoidably contained, for example, due to contamination from scrap during steel melting. However, if Cu is eliminated by using high-purity raw materials, it may 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 added to increase austenite stability, but excessive content reduces hot workability and corrosion resistance, so the Cu content is set to 1.5% or less. Furthermore, since corrosion resistance may be reduced due to Cu precipitation, the Cu content is preferably set to 1.0% or less or 0.5% or less.

Mo: 0.01~1.5%
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 is set to 0.01% or more. To obtain even higher corrosion resistance, it is preferable to set it to 0.03% or more. On the other hand, excessive Mo increases temper softening resistance and becomes a factor that inhibits productivity in the manufacturing process, so the Mo content is set to 1.5% or less. Assuming the corrosion prevention effect of the plating layer, it is preferable that the Mo content be 0.5% or less.

Al: 0.001-0.1%
Al is an effective element for deoxidation, and its effect is maintained 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 polishability due to alumina-based non-metallic inclusions, the Al content is preferably set to 0.01% or less. However, in order to obtain the deoxidation effect in combination with Si and Mn, the Al content is preferably set to 0.005% or more.

N: 0.010% to 0.08%
Like C, N has the effect of increasing quench hardness and is an element that expands the austenite single-phase temperature range. However, unlike Ni and Mn, N does not lower the Ac ₁ 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 from the viewpoint of ensuring quench hardness. On the other hand, since the inclusion of a large amount of nitrogen generates bubble-type 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.

O: 0.015% or less O is generally present as oxide-based inclusions, and large inclusions, together with water-soluble sulfides, become corrosion initiation sites and are also detrimental to ductility and toughness after quenching, so the O content is set to 0.015% or less. Inclusions that aggregate and coarsen during casting cause scabs on the surface of the hot-rolled sheet, so the O content is desirably set to 0.008% or less. Basically, a lower O content is preferable, but because it is difficult to industrially float and remove oxides formed in molten steel from the viewpoints of productivity and cost, the O content may be set to 0.003% or more.

Furthermore, in addition to the elements described above, 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.

(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 is set to 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 desirable to reduce the Sn content as much as possible. Therefore, the Sn content is preferably set to 0.2% or less.

(B group elements) One or more of the following: 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.

Nb: 0.005-0.5%
Nb is an element that increases the Ac ₁ point, making it possible to increase the heating temperature during hot stamping, so it is desirable to add Nb as needed. To achieve this effect, the Nb content is set to 0.005% or more. However, since an excessive Nb content increases solidification cracking susceptibility, the Nb content is preferably set to 0.5% or less.

Ti: 0.005% to 0.1%
Ti is an element that raises the Ac ₁ point, making it possible to increase the heating temperature during hot stamping, so it is desirable to add Ti 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 may precipitate in a temperature range above the hot rolling heating temperature, causing inclusion-based defects, so the Ti content is preferably set to 0.1% or less.

V: 0.005-0.2%
V is an element that increases the Ac ₁ 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 V content is set to 0.005% or more. On the other hand, since the inclusion of a large amount of V promotes the formation of coarse carbides due to solidification segregation and may reduce ductility and toughness after quenching, the V content is preferably set to 0.2% or less.

Zr: 0.005% to 0.1%
Zr is an element that raises the Ac ₁ 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 is set to 0.005% or more. However, if Zr is added in excess, coarse Zr(C,N) precipitates in a temperature range equal to or higher than the hot rolling heating temperature, which may cause inclusion-based defects. Therefore, the Zr content is preferably set to 0.1% or less.

(C group element) B: 0.0005% to 0.0030%
B may be added as needed to improve high-temperature ductility during hot rolling and reduce yield reduction due to edge cracking of the hot-rolled sheet. To achieve this effect, the B content is set to 0.0005% or more. However, excessive B content impairs toughness and corrosion resistance due to precipitation of _Cr2B and (Cr,Fe) ₂₃ (C,B) ₆ , so the B content is preferably set to 0.0030% or less.

(D group element)
W: 0.01% or more and 0.4% or less Like Mo, W is an element that is 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 desirable to set it to 0.03% or more. More preferably, it is set to 0.05% or more. On the other hand, excessive inclusion reduces room-temperature ductility, so the upper limit is set to 0.4% or less. It may be 0.2% or less. In the present invention, W may be added to adjust general properties, and the W content is desirably 0.05 to 0.2%.

Ta: 0.01% or more and 0.4% or less Ta is added as needed because it bonds with C and N to improve corrosion resistance and toughness. To achieve this effect, the Ta content is set to 0.01% or more. To obtain higher corrosion resistance, it is desirable to set it to 0.03% or more. It is more preferably 0.05% or more. However, since a content of more than 0.4% increases costs and significantly deteriorates manufacturability, 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 desirably 0.05 to 0.2%.

[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 an austenite-based primary 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, it is desirable to set γp to 90 or higher. On the other hand, containing large amounts of Ni, Mn, and Cu, which are elements that increase γp, increases the tempering softening resistance and lowers the Ac ₁ point, which is undesirable. Furthermore, 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 set γp to 200 or lower. To obtain the ductility and toughness required for a structural member, it is preferable to set γp to 150 or lower.

[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 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.

<Plating layer>
[Thickness of plating layer: 10 μm or more, 100 μm or less]
The 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, the plating layer thickness is preferably 15 μm or more. On the other hand, if the plating layer is too thick, defects such as peeling of the plating layer will occur during formation or hot stamping, so the plating layer thickness is set to 100 μm or less. Considering weldability, the plating layer thickness is preferably 50 μm or less.

There are no particular restrictions on the material of the plating layer, as long as it is aluminum or an aluminum alloy. An aluminum alloy is an alloy in which the element with the highest concentration 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 3-15% Si, with the remainder being Al and impurities. The plating layer may be either an electroplated layer or a hot-dip galvanized layer.

< 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.

The coating layer is formed by the diffusion of Fe and Cr from the steel side 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 amounts 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, if the Cr content in the coating layer is too high, peeling of the coating layer becomes more likely, so it is more preferable to keep the Cr content in the coating layer at 10% by mass or less.

The cross-sectional hardness of the molded component is preferably 300 Hv or more. Furthermore, the total elongation in a tensile test conforming to JIS Z 2241 is preferably 2% or more for a JIS No. 13B test piece. Cross-sectional hardness is measured by exposing a 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. Hardness measurements are performed three times at different locations, and the average of the measured values is used as the cross-sectional hardness. The applied load is set to 5 kgf.

<Manufacturing method>
[Method of manufacturing plated stainless steel sheet 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. 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 350 to 700°C. The type, concentration, and temperature of the acid used in the pickling 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 800 to 950°C.

There are no particular limitations on the method of plating the stainless steel sheet, and methods such as hot-dip plating, electroplating, vacuum deposition, and cladding are possible, but hot-dip plating is preferred.

The Al plating bath used for plating has a bath temperature of, for example, 670°C and contains 3 to 15 mass% Si as a secondary component, but preferably contains around 10% Si. 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 included as other elements.

After plating, it is preferable to use the gas wiping method to achieve a plating thickness of 10 to 100 μm on each side to prevent plating peeling during forming and ensure rust prevention. It is also preferable to perform a chemical conversion treatment after plating to prevent corrosion of the plating layer before hot stamping.

[Method for manufacturing molded member]
The formed member is prepared by heating the above-described plated stainless steel sheet for hot stamping to a temperature of 850°C or higher, and then rapidly cooling it while forming it using a hot stamping die. An average heating rate of 10°C/s or higher during heating is preferred 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 setting the heating rate to a high rate of 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 set to a range of 4 to 12 mass%.

<Experiment>
The relationship between the amount of C+0.5N and the cross-sectional strength of formed parts was investigated for various plated steel sheets, as shown in Table 1. Commercially available plated steel sheets were prepared: No. a aluminum-plated steel sheet (0.22% C-1.2% Mn), No. b aluminum-plated steel sheet with SUH409 steel (0.004% C-11.8% Cr) as the base material, and No. c aluminum-plated steel sheet with SUS432 steel (0.006C-17Cr-0.5Mo) as the base material. Furthermore, No. 1000C steel sheets were prepared by forming an aluminum plating layer on steel sheets with SUS410 (0.04C-12.3Cr), SUS420J1 (13.3Cr-0.21C), or SUS420J2 (13.5Cr-0.32C) as the base material, with the contents of other elements adjusted. Aluminum-plated steel sheets d to f were produced in the laboratory. The plating thickness of these steel sheets was approximately 20 μm. Table 1 shows the chemical composition of each steel sheet.

These plated steel sheets were heated to 950°C for 1 minute, then clamped between flat hot stamping dies and cooled at a cooling rate of approximately 50°C/s to perform hot stamping, yielding formed parts. The cross-sectional hardness of the formed parts was then measured. The results are shown in Figure 1.

The resulting molded components were then immersed in a chemical conversion treatment solution, and after an approximately 25 μm thick electrocoat coating was applied, cross-shaped scratches (cross cuts) were made in the surface coating. These samples were then used to carry out 150 cycles (50 days) of combined corrosion testing (CCT) in accordance with JASO M610.

The test conditions for the JASO CCT are shown below.
Salt water spray (5% NaCl, 35°C, 2h) → Drying (60°C, 4h) → Wetting (50°C, 2h)

After 150 CCT cycles, the corrosion depth in the thickness direction 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.

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 and did not 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 testing showed corrosion depths of more than 100 μm. On the other hand, the plated stainless steel sheets Nos. d to f, which were aluminum-plated steel sheets based on SUS410, SUS403, SUS420J1, and SUS420J2 with adjusted content of other elements, achieved cross-sectional hardnesses of 300 HV or more after hot stamping, and corrosion depths of 50 μm or less in corrosion testing.

Next, the present invention will be described in more detail with reference to examples.
Steel sheets having 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 scabs was evaluated. If edge cracks or surface scabs were found, the steel sheets were deemed to have failed the quality of the galvanized base sheet.

These steel sheets were then used as base metal for plating 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 plated, while the others were aluminum alloy plated 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 the gas wiping method, and the sheets were cooled and then coiled.

Cut sheets for hot stamping were cut from the aluminum-plated stainless steel sheet or aluminum alloy-plated stainless steel sheet obtained in this way, heated to 950°C for 1 minute, and then hot stamped by clamping between flat hot stamping dies and cooling at a rate of 50°C/s to produce formed components. Test pieces were cut from the formed components and subjected to cross-sectional hardness measurements, tensile tests, and analysis of the Cr concentration in the plating.

In a tensile test conforming to JIS Z 2241, a total elongation of 2% or more was considered to have passed. A cross-sectional hardness of 300 Hv or more was considered 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 used as the cross-sectional hardness. The applied load was set to 5 kgf.

The resulting molded components were immersed in a chemical conversion treatment solution and then electrocoated with a thickness of approximately 20 μm. Cross-cuts were then made in the surface coating, and a combined corrosion test (CCT) in accordance with JASO M610 was carried out for 150 cycles (50 days).

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

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.

The above evaluation results are shown in Tables 4A and 4B.

The formed parts P1 to P28, manufactured by hot stamping using plated steel sheets for hot stamping within the scope of this 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. The coating layer after hot stamping contained aluminum and iron in addition to Cr.

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.
In S5, the C content exceeded 0.35%, and therefore the total elongation was less than 2%.

In S6, 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 and heating, resulting in poor corrosion resistance.
In S7, since the Si content exceeded 1%, the hot-rolled scale was thin, and surface scabs occurred due to seizure with the hot-rolling rolls.

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.

In S10, the P content exceeded 0.035%, so the hot workability deteriorated and edge cracks occurred on the end surfaces of the hot-rolled sheet.
In S11, the S content exceeded 0.010%, so the hot workability deteriorated and edge cracks occurred.

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.

In S14, the Mo, Cu, and Ni contents were less than 0.01%, and therefore the corrosion resistance was reduced.
In S15, the Al content was less than 0.001% and the O content was more than 0.015%, so the amount of oxide-based inclusions was large and scabs were observed.

S16 had poor corrosion resistance because the Cr content was less than 10.5%.
In S17, the C+0.5N content exceeded 0.37%, and therefore the ductility was poor.

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 small, and peeling of the plating layer occurred during hot stamping, and corrosion accompanying the peeling resulted in poor corrosion resistance.

The present invention makes it possible to produce surface-treated stainless steel sheets with high strength and excellent corrosion resistance with high productivity and without the occurrence of heat-rolling defects. Therefore, the present invention contributes to reducing the weight and extending the life of automotive suspension parts, exhaust system parts, fuel system parts, structural members, and other components.

Claims (5)

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. 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. 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. 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. 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.