Stainless steel material and forged component

Abstract

Provided is a stainless steel material with the following chemical composition in terms of mass%. C: 0.0010 to 0.1500%, Si: 0.01 to 2.00%, Mn: 0.01 to 10.00%, P: 0.30% or less, S: 0.0001 to 0.5000%, Ni: 8.0 to 30.0%, Cr: 9.0 to 21.0%, Cu: 0.01 to 5.00%, N: 0.0010 to 0.1000%, B: 0.0001 to 0.0500%, Co: 0 to 2.50%, Mo: 0 to 3.00%, optional element(s), and balance: Fe and impurities. For this stainless steel material, the A value represented by [551 - 462(C + N) - 9.2Si - 8.1Mn - 29(Ni + Cu) -13.7Cr - 18.5Mo] is -100 or less, a condition [0.01 ≦ Co + Mo ≦ 5.50] is satisfied, and the annealed twin fraction is 0.95 or less.

Description

Stainless steel materials and forged parts

[0001] The present invention relates to stainless steel materials and forged parts.

[0002] In recent years, hydrogen has been attracting attention as a new energy source to replace fossil fuels. ２ Hydrogen is a clean energy source that does not emit carbon dioxide. For this reason, development is underway for products that use hydrogen, such as fuel-cell vehicles, and facilities such as hydrogen stations. These products and facilities use parts made from austenitic stainless steel.

[0003] Furthermore, the above-mentioned parts have complex shapes. To process these parts into shapes, they must be easy to cold forge, i.e., have cold forgeability. Austenitic stainless steel materials are prone to work hardening, which reduces their cold forgeability. On the other hand, it is known that when metal materials such as austenitic stainless steel materials are used in a hydrogen environment, hydrogen penetrates into the metal, causing embrittlement, a phenomenon known as hydrogen embrittlement. This hydrogen embrittlement is particularly likely to occur after processing such as cold forging. Therefore, there is a demand for austenitic stainless steel materials that combine cold forgeability with hydrogen embrittlement resistance after cold forging. For example, Patent Document 1 discloses a stainless steel material that is excellent in both hydrogen embrittlement resistance and cold forgeability.

[0004] International Publication No. 2023 / 105852

[0005] However, the stainless steel material disclosed in Patent Document 1 has room for improvement in terms of cold forgeability. For example, in terms of cold forgeability, it is also required to reduce the variation in hardness between parts of a part, but this point has not been fully considered. In light of the above, an object of the present invention is to provide a stainless steel material and a forged part that are excellent in cold forgeability and hydrogen embrittlement resistance.

[0006] The present invention has been made to solve the above-mentioned problems, and is summarized as the following stainless steel material and forged part.

[0007] (1) Chemical composition, in mass%, C: 0.0010 to 0.1500%, Si: 0.01 to 2.00%, Mn: 0.01 to 10.00%, P: 0.30% or less, S: 0.0001 to 0.5000%, Ni: 8.0 to 30.0%, Cr: 9.0 to 21.0%, Cu: 0.01 to 5.00%, N: 0.0010 to 0.1000%, B: 0.0001 to 0.0500%, Co: 0 to 2.50%, Mo: 0 to 3.00%, Ti: 0 to 2.0%, Nb: 0 to 2.00%, Sn: 0 to 2.5%, V: 0 to 2.0%, W: 0 to 3.0%, A stainless steel material having an annealing twin fraction of 0.95 or less, the A value of which is represented by the following formula (i) being -100 or less, the following formula (ii) being satisfied, and the following formula (ii) being: Ga: 0 to 0.05%, Sb: 0 to 2.5%, Ta: 0 to 2.5%, Mg: 0 to 0.012%, Zr: 0 to 0.012%, REM: 0 to 0.05%, Pb: 0 to 0.30%, Se: 0 to 0.80%, Te: 0 to 0.30%, Bi: 0 to 0.50%, Al: 0 to 2.0%, Ca: 0 to 0.05%, and the balance: Fe and impurities. A value = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo ... (i) 0.01 ≦ Co + Mo ≦ 5.50 ... (ii) However, each element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel material, and if the element is not contained, it is set to zero.

[0008] (2) The chemical composition is, in mass%, Ti: 0.01 to 2.0%, Nb: 0.01 to 2.00%, Sn: 0.0001 to 2.5%, V: 0.001 to 2.0%, W: 0.05 to 3.0%, Ga: 0.0004 to 0.05%, Sb: 0.01 to 2.5%, Ta: 0.01 to 2.5%, Mg: 0.0002 to 0.012%, Zr: 0.0002 to 0.012%, REM: 0.0002 to 0.05%, Pb: 0.0001 to 0.30%, Se: 0.0001 to 0.80%, Te: 0.0001 to 0.30%, The stainless steel material according to (1) above, containing one or more selected from Bi: 0.0001 to 0.50%, Al: 0.001 to 2.0%, and Ca: 0.0001 to 0.05%.

[0009] (3) The stainless steel material according to (1) or (2) above, having a compressive deformation resistance of 1000 MPa or less.

[0010] (4) Chemical composition, in mass%, is: C: 0.0010 to 0.1500%, Si: 0.01 to 2.00%, Mn: 0.01 to 10.00%, P: 0.30% or less, S: 0.0001 to 0.5000%, Ni: 8.0 to 30.0%, Cr: 9.0 to 21.0%, Cu: 0.01 to 5.00%, N: 0.0010 to 0.1000%, B: 0.0001 to 0.0500%, Co: 0 to 2.50%, Mo: 0 to 3.00%, Ti: 0 to 2.0%, Nb: 0 to 2.00%, Sn: 0 to 2.5%, V: 0 to 2.0%, W: 0 to 3.0%, A forged part made of a stainless steel material having a hardness of 100 to 500 HV1, the stainless steel material having an A value represented by the following formula (i) being -100 or less, and satisfying the following formula (ii): Ga: 0 to 0.05%, Sb: 0 to 2.5%, Ta: 0 to 2.5%, Mg: 0 to 0.012%, Zr: 0 to 0.012%, REM: 0 to 0.05%, Pb: 0 to 0.30%, Se: 0 to 0.80%, Te: 0 to 0.30%, Bi: 0 to 0.50%, Al: 0 to 2.0%, Ca: 0 to 0.05%, balance: Fe and impurities. A value = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo ... (i) 0.01 ≦ Co + Mo ≦ 5.50 ... (ii) However, each element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel material, and if the element is not contained, it is set to zero.

[0011] (5) The forged part according to (4) above, wherein the difference ΔHV between the maximum hardness and the minimum hardness is 400 HV1 or less.

[0012] (6) The forged part according to (5) above, having a relative tensile strength of 80% or more in high-pressure hydrogen at -40°C and 70 MPa.

[0013] (7) The forged part according to (5) or (6) above, having a relative reduction of 50% or more in high-pressure hydrogen at -40°C and 70 MPa.

[0014] According to the present invention, it is possible to obtain a stainless steel material and a forged part that are excellent in cold forgeability and hydrogen embrittlement resistance.

[0015] FIG. 1 is a diagram illustrating the shape of a test piece used in a tensile test for measuring relative tensile strength and relative reduction of area.

[0016] The present inventors have investigated the cold forgeability and hydrogen embrittlement resistance of stainless steel materials and have obtained the following findings.

[0017] (a) It is difficult to achieve both cold forgeability and hydrogen embrittlement resistance after cold forging. This is because a large amount of strain is introduced when performing a strong process such as cold forging, and this strain promotes hydrogen embrittlement.

[0018] (b) In order to improve cold forgeability, it is desirable to reduce deformation resistance during working. Therefore, the present inventors focused on suppressing annealing twins within crystal grains. By controlling the amount of annealing twins within a certain range, the deformation resistance during working is reduced, and the amount of strain introduced during working is also reduced, thereby improving hydrogen embrittlement resistance.

[0019] (c) In order to keep the amount of annealing twins within a certain range, it is effective to segregate solute B around grain boundaries. As a result, annealing twins occurring within grains are suppressed. In order to promote such segregation of B around grain boundaries, it is effective to perform two-stage cooling in the final annealing process. Furthermore, adding Mo and / or Co to a stainless steel material is effective in promoting the grain boundary segregation of solute B, and is also effective in improving the structural stability of the steel material and reducing deformation resistance.

[0020] An embodiment of the present invention has been made based on the above findings. Hereinafter, the requirements for the stainless steel material and forged part of this embodiment will be described in detail.

[0021] 1. Stainless Steel Material 1-1. Chemical Composition The reasons for limiting each element in the chemical composition of the stainless steel material are as follows. In the following description, "%" for the content means "% by mass."

[0022] C: 0.0010 to 0.1500% C (carbon) suppresses the formation of work-induced martensite during working and improves hydrogen embrittlement resistance. Therefore, the C content is 0.0010% or more. The C content is preferably 0.0020% or more, and more preferably 0.0050% or more. However, if C is contained in excess, the annealing twin fraction increases, and cold forgeability and hydrogen embrittlement resistance decrease. Therefore, the C content is 0.1500% or less. The C content is preferably 0.1200% or less, more preferably 0.0500% or less, and even more preferably 0.0200% or less.

[0023] Si: 0.01 to 2.00% Si (silicon) has a deoxidizing effect. Therefore, the Si content is 0.01% or more. The Si content is preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.30% or more. However, if Si is contained in excess, the annealing twin fraction increases, and the cold forgeability and hydrogen embrittlement resistance decrease. Therefore, the Si content is 2.00% or less. The Si content is preferably 1.20% or less, more preferably 0.60% or less, and even more preferably 0.50% or less.

[0024] Mn: 0.01 to 10.00% Mn (manganese) suppresses the formation of strain-induced martensite and improves hydrogen embrittlement resistance. Therefore, the Mn content is 0.01% or more. The Mn content is preferably 0.10% or more, more preferably 0.30% or more, and even more preferably 0.50% or more. However, excessive Mn content increases the annealing twin fraction, reducing cold forgeability and hydrogen embrittlement resistance. Therefore, the Mn content is 10.00% or less. The Mn content is preferably 5.00% or less, more preferably 2.50% or less, even more preferably 1.50% or less, and even more preferably 1.00% or less.

[0025] P: 0.30% or less P (phosphorus) is an impurity element contained in steel materials and reduces mechanical properties. Therefore, the P content is 0.30% or less. The P content is preferably 0.20% or less, more preferably 0.10% or less, even more preferably 0.05% or less, and even more preferably 0.03% or less. It is preferable to reduce P as much as possible, but excessive reduction increases manufacturing costs. Therefore, the P content is preferably 0.0001% or more.

[0026] S: 0.0001 to 0.5000% S (sulfur) has the effect of reducing the annealing twin fraction and improving cold forgeability and hydrogen embrittlement resistance. Therefore, the S content is 0.0001% or more. The S content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more. However, S is an element that deteriorates mechanical properties. Moreover, excessive S content increases the annealing twin fraction, thereby deteriorating cold forgeability and hydrogen embrittlement resistance. Therefore, the S content is 0.5000% or less. The S content is preferably 0.2000% or less, more preferably 0.1000% or less, even more preferably 0.0500% or less, even more preferably 0.0300% or less, and even more preferably 0.0100% or less.

[0027] Ni: 8.0 to 30.0% Ni (nickel) suppresses the formation of work-induced martensite and improves hydrogen embrittlement resistance. It also has the effect of improving cold forgeability. For this reason, the Ni content is 8.0% or more. The Ni content is preferably 10.0% or more, more preferably more than 12.0%, more preferably 13.0% or more, and even more preferably 15.0% or more. However, if Ni is contained in excess, the annealing twin fraction increases, and cold forgeability and hydrogen embrittlement resistance decrease. For this reason, the Ni content is 30.0% or less. The Ni content is preferably 27.5% or less, and more preferably 25.0% or less.

[0028] Cr: 9.0 to 21.0% Cr (chromium) suppresses the formation of strain-induced martensite and enhances hydrogen embrittlement resistance. It also has the effect of reducing the annealing twin fraction and improving cold forgeability and hydrogen embrittlement resistance. It also has the effect of enhancing corrosion resistance. Therefore, the Cr content is 9.0% or more. The Cr content is preferably 10.5% or more, more preferably 12.0% or more, and even more preferably 13.5% or more. However, excessive Cr content increases the annealing twin fraction and reduces cold forgeability and hydrogen embrittlement resistance. Therefore, the Cr content is 21.0% or less. The Cr content is preferably 19.5% or less, more preferably 17.5% or less, more preferably less than 16.0%, and even more preferably 15.0% or less.

[0029] Cu: 0.01 to 5.00% Cu (copper) suppresses the formation of deformation-induced martensite and improves hydrogen embrittlement resistance and cold forgeability. Therefore, the Cu content is 0.01% or more. The Cu content is preferably 0.10% or more, more preferably 0.50% or more, even more preferably 1.00% or more, and even more preferably 2.00% or more. However, excessive Cu content increases the annealing twin fraction, thereby reducing hydrogen embrittlement resistance and cold forgeability. Therefore, the Cu content is 5.00% or less. The Cu content is preferably 4.50% or less, more preferably 4.00% or less, even more preferably 3.50% or less, and even more preferably 3.00% or less.

[0030] N: 0.0010 to 0.1000% N (nitrogen) suppresses the formation of strain-induced martensite and improves hydrogen embrittlement resistance. Therefore, the N content is 0.0010% or more. The N content is preferably 0.0030% or more, more preferably 0.0050% or more, and even more preferably 0.0100% or more. However, excessive N content not only reduces corrosion resistance but also increases the annealing twin fraction, reducing cold forgeability and hydrogen embrittlement resistance. Therefore, the N content is 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0500% or less, and even more preferably 0.0300% or less.

[0031] B: 0.0001 to 0.0500% B (boron) reduces the annealing twin fraction and improves hydrogen embrittlement resistance and cold forgeability. Therefore, the B content is 0.0001% or more. The B content is preferably 0.0005% or more, more preferably 0.0010% or more, even more preferably 0.0020% or more, even more preferably 0.0030% or more, and even more preferably 0.0050% or more. However, excessive B content not only forms coarse B-based precipitates, but also increases the annealing twin fraction, thereby reducing hydrogen embrittlement resistance and cold forgeability. Therefore, the B content is 0.0500% or less. The B content is preferably 0.0350% or less, more preferably 0.0200% or less, even more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0032] The stainless steel material of this embodiment contains one or more elements selected from Co and Mo. That is, it is sufficient to contain at least one of them, and either one may be 0%. Therefore, the formula (ii) described below is satisfied.

[0033] Co: 0 to 2.50% Co (cobalt) has the effect of suppressing the formation of work-induced martensite and improving cold forgeability and hydrogen embrittlement resistance. Co also suppresses the precipitation of B at grain boundaries, allowing solute B to exist at grain boundaries, contributing to a reduction in the annealing twin fraction. Therefore, Co may be added as needed. However, excessive Co content reduces corrosion resistance. Therefore, the Co content is 2.50% or less. The Co content is preferably 2.00% or less, more preferably 1.50% or less, even more preferably 1.00% or less, and even more preferably 0.80% or less. On the other hand, to achieve the above effects, the Co content is preferably more than 0%, more preferably 0.01% or more, more preferably 0.03% or more, even more preferably 0.05% or more, and even more preferably 0.10% or more.

[0034] Mo: 0 to 3.00% Mo (molybdenum) has the effect of suppressing the formation of deformation-induced martensite and improving hydrogen embrittlement resistance and cold forgeability. Furthermore, Mo suppresses the precipitation of B at grain boundaries and allows solute B to exist at grain boundaries, contributing to a reduction in the annealing twin fraction. Therefore, Mo may be added as needed. However, excessive Mo not only reduces corrosion resistance but also increases the annealing twin fraction, thereby reducing cold forgeability and hydrogen embrittlement resistance. Therefore, the Mo content is 3.00% or less. The Mo content is preferably 2.80% or less, more preferably 2.50% or less, even more preferably 2.00% or less, even more preferably 1.50% or less, and even more preferably 1.00% or less. On the other hand, in order to obtain the above effects, the Mo content is preferably more than 0%, more preferably 0.01% or more, still more preferably 0.03% or more, even more preferably 0.05% or more, and still more preferably 0.10% or more.

[0035] 0.01≦Co+Mo≦5.50 (ii) In the above formula, each element symbol represents the content (mass%) of each element contained in the stainless steel material, and if the element is not contained, it is set to zero.

[0036] If the value in formula (ii), which is the total content of Co and Mo, is less than 0.01%, the formation of stress-induced martensite cannot be suppressed. Furthermore, Co and Mo have the effect of suppressing the precipitation of B at grain boundaries and allowing solute B to exist at grain boundaries, thereby contributing to a reduction in the annealing twin fraction. Therefore, the value in formula (ii) is 0.01% or more. The value in formula (ii) is preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, if the value in formula (ii) exceeds 5.50%, corrosion resistance decreases. Therefore, the value in formula (ii) is 5.50% or less. The value in formula (ii) is preferably 4.50% or less, more preferably 3.00% or less, even more preferably 2.50% or less, and even more preferably 1.50% or less.

[0037] In addition to the above elements, one or more elements selected from Ti, Nb, Sn, V, W, Ga, Sb, Ta, Mg, Zr, REM, Pb, Se, Te, Bi, Al, and Ca may be contained within the ranges shown below. In other words, the lower limit of the above elements is 0%. The reasons for limiting the content of each element will be explained below.

[0038] Ti: 0 to 2.0% Ti (titanium) has the effect of refining the structure and improving strength. Therefore, it may be added as needed. However, if Ti is contained in excess, mechanical properties such as ductility will decrease. Therefore, the Ti content is 2.0% or less. The Ti content is preferably 1.0% or less, more preferably 0.7% or less, and even more preferably 0.5% or less. On the other hand, in order to obtain the above effects, the Ti content is preferably more than 0%, more preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.1% or more.

[0039] Nb: 0 to 2.00% Nb (niobium) has the effect of refining crystal grains and improving strength. Therefore, it may be added as needed. However, if Nb is added in excess, coarse Nb-based precipitates are formed, which in turn reduces mechanical properties such as ductility. Therefore, the Nb content is 2.00% or less. The Nb content is preferably 1.00% or less, more preferably 0.70% or less, and even more preferably 0.50% or less. On the other hand, in order to obtain the above effects, the Nb content is preferably more than 0%, more preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

[0040] Sn: 0 to 2.5% Sn (tin) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if Sn is contained in excess, the effect saturates and hot workability deteriorates. Therefore, the Sn content is 2.5% or less. The Sn content is preferably 1.0% or less, and more preferably 0.20% or less. On the other hand, in order to obtain the above effect, the Sn content is preferably more than 0%, more preferably 0.0001% or more, more preferably 0.001% or more, and even more preferably 0.01% or more.

[0041] V: 0 to 2.0% V (vanadium) has the effect of immobilizing C and N and improving corrosion resistance. Therefore, it may be contained as needed. However, if excessive V is contained, coarse V-based precipitates are formed, which in turn tends to reduce corrosion resistance. For this reason, the V content is 2.0% or less. The V content is preferably 1.0% or less, more preferably 0.7% or less, and even more preferably 0.5% or less. On the other hand, in order to obtain the above effect, the V content is preferably more than 0%, more preferably 0.001% or more, more preferably 0.01% or more, and even more preferably 0.05% or more.

[0042] W: 0 to 3.0% W (tungsten) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if excessive W is contained, the effect saturates and manufacturing costs increase. Therefore, the W content is 3.0% or less. The W content is preferably 2.0% or less, and more preferably 1.5% or less. On the other hand, in order to obtain the above effect, the W content is preferably more than 0%, more preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

[0043] Ga: 0 to 0.05% Ga (gallium) has the effect of improving corrosion resistance and hydrogen embrittlement resistance. Therefore, it may be contained as needed. However, if Ga is contained in excess, workability will decrease. Therefore, the Ga content is 0.05% or less. The Ga content is preferably 0.04% or less, more preferably 0.03% or less, and even more preferably 0.02% or less. On the other hand, in order to obtain the above effect, the Ga content is preferably more than 0%, more preferably 0.0004% or more, more preferably 0.001% or more, and even more preferably 0.005% or more.

[0044] Sb: 0 to 2.5% Sb (antimony) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if Sb is contained in excess, the effect saturates and manufacturing costs increase. Therefore, the Sb content is 2.5% or less. The Sb content is preferably 1.0% or less, more preferably 0.80% or less, even more preferably 0.30% or less, and even more preferably 0.10% or less. On the other hand, in order to obtain the above effect, the Sb content is preferably more than 0%, more preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.01% or more.

[0045] Ta: 0 to 2.5% Ta (tantalum) has the effect of immobilizing C and N and improving corrosion resistance. However, if Ta is contained in excess, toughness decreases. For this reason, the Ta content is 2.5% or less. The Ta content is preferably 1.0% or less, more preferably 0.70% or less, even more preferably 0.50% or less, and even more preferably 0.20% or less. On the other hand, in order to obtain the above effect, the Ta content is preferably more than 0%, more preferably 0.01% or more, preferably 0.03% or more, and preferably 0.05% or more.

[0046] Mg: 0 to 0.012% Mg (magnesium) is an element that has a deoxidizing effect. Therefore, it may be added as needed. However, excessive Mg content can lead to the formation of coarse Mg-based inclusions, which can deteriorate the surface properties and reduce manufacturability. Therefore, the Mg content is 0.012% or less. The Mg content is preferably 0.010% or less, more preferably 0.008% or less, and even more preferably 0.005% or less. On the other hand, to obtain the above effects, the Mg content is preferably more than 0%, more preferably 0.0002% or more, more preferably 0.001% or more, and even more preferably 0.003% or more.

[0047] Zr: 0 to 0.012% Zr (zirconium) has a deoxidizing effect. Therefore, it may be contained as needed. However, if Zr is contained in excess, coarse Zr-based inclusions are formed, which in turn reduces corrosion resistance. Therefore, the Zr content is 0.012% or less. The Zr content is preferably 0.010% or less, more preferably 0.008% or less, and even more preferably 0.005% or less. On the other hand, in order to obtain the above effect, the Zr content is preferably more than 0%, more preferably 0.0002% or more, more preferably 0.001% or more, and even more preferably 0.003% or more.

[0048] REM: 0 to 0.05% REM (rare earth elements) have a deoxidizing effect. Therefore, they may be added as needed. However, excessive REM content increases production costs. Furthermore, coarse REM inclusions are formed, reducing manufacturability. Therefore, the REM content is 0.05% or less. The REM content is preferably 0.03% or less, more preferably 0.01% or less, even more preferably 0.005% or less, and even more preferably 0.003% or less. On the other hand, to obtain the above effects, the REM content is preferably more than 0%, more preferably 0.0002% or more, more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0049] REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. Industrially, REM is often added in the form of misch metal.

[0050] Pb: 0 to 0.30% Pb (lead) has the effect of improving machinability. Therefore, it may be contained as needed. However, excessive Pb content reduces corrosion resistance. Therefore, the Pb content is 0.30% or less. The Pb content is preferably 0.20% or less, more preferably 0.15% or less, even more preferably 0.10% or less, and even more preferably 0.05% or less. On the other hand, in order to obtain the above effect, the Pb content is preferably more than 0%, more preferably 0.0001% or more, more preferably 0.001% or more, and even more preferably 0.01% or more.

[0051] Se: 0 to 0.80% Se (selenium) has the effect of improving machinability. Therefore, it may be contained as needed. However, excessive Se content increases manufacturing costs. For this reason, the Se content is 0.80% or less. The Se content is preferably 0.10% or less, more preferably 0.05% or less, and even more preferably 0.01% or less. On the other hand, in order to obtain the above effect, the Se content is preferably more than 0%, more preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0052] Te: 0 to 0.30% Te (tellurium) has the effect of improving machinability. Therefore, it may be contained as needed. However, excessive Te content increases manufacturing costs. For this reason, the Te content is 0.30% or less. The Te content is preferably 0.10% or less, more preferably 0.05% or less, and even more preferably 0.03% or less. On the other hand, in order to obtain the above effect, the Te content is preferably more than 0%, more preferably 0.0001% or more, more preferably 0.001% or more, and even more preferably 0.005% or more.

[0053] Bi: 0 to 0.50% Bi (bismuth) has the effect of improving machinability. Therefore, it may be contained as needed. However, if Bi is contained in excess, corrosion resistance decreases. Therefore, the Bi content is 0.50% or less. The Bi content is preferably 0.10% or less, more preferably 0.05% or less, even more preferably 0.01% or less, and even more preferably 0.005% or less. On the other hand, in order to obtain the above effect, the Bi content is preferably more than 0%, more preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0054] Al: 0 to 2.0% Al (aluminum) has a deoxidizing effect. Therefore, it may be contained as needed. However, if Al is contained in excess, coarse Al-based precipitates are formed, reducing corrosion resistance. Therefore, the Al content is 2.0% or less. The Al content is preferably 1.0% or less, more preferably 0.5% or less, even more preferably 0.3% or less, and even more preferably 0.05% or less. On the other hand, in order to obtain the above effect, the Al content is preferably more than 0%, more preferably 0.001% or more, and even more preferably 0.01% or more.

[0055] Ca: 0 to 0.05% Ca (calcium) has a deoxidizing effect. Therefore, it may be contained as needed. However, if Ca is contained in excess, coarse Ca-based inclusions are formed, reducing manufacturability. Therefore, the Ca content is 0.05% or less. The Ca content is preferably 0.03% or less, more preferably 0.01% or less, and even more preferably 0.005% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably more than 0%, more preferably 0.0001% or more, more preferably 0.001% or more, and even more preferably 0.005% or more.

[0056] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in due to various factors in raw materials such as ore and scrap, or in the manufacturing process, during industrial production of stainless steel material, and are acceptable within a range that does not adversely affect this embodiment.

[0057] A Value In order to ensure the stability of the structure, the stainless steel material of this embodiment has an A value represented by the following formula (i) of -100 or less: A value = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo ... (i) Note that each element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel material, and if no element is contained, it is set to zero.

[0058] The A value is an index showing the stability of the structure of a stainless steel material. By setting the A value to -100 or less, the generation of deformation-induced martensite can be suppressed, and work hardening can be reduced. As a result, the stainless steel material is softened, the occurrence of twins is suppressed, and cold forgeability is improved. Furthermore, since the occurrence of cracks and twins is suppressed, hydrogen embrittlement resistance after cold forging is improved. For this reason, the A value is -100 or less, preferably -120 or less, and more preferably -150 or less. The lower limit of the A value is not particularly limited, but in the stainless steel material of this embodiment, it is usually -700.

[0059] 1-2. Annealing Twin Fraction The stainless steel material of this embodiment has an annealing twin fraction of 0.95 or less to improve cold forgeability and hydrogen embrittlement resistance after cold forging. Here, the annealing twin fraction is the proportion of crystal grains containing annealing deformation, and more specifically, the proportion of the number of crystal grains containing annealing twins to the number of measured crystal grains. Furthermore, twinning refers to a state in which regions on both sides of a certain atomic plane within a crystal are mirror-symmetric to each other.

[0060] When the annealing twin fraction is 0.95 or less, deformation during cold forging is reduced, improving cold forgeability. Furthermore, the reduced deformation during cold forging reduces the amount of introduced strain, improving hydrogen embrittlement resistance after cold forging.

[0061] Therefore, the annealing twin fraction is 0.95 or less, preferably 0.80 or less, and more preferably 0.50 or less. On the other hand, in the stainless steel material of this embodiment, the annealing twin fraction is preferably 0.01 or more. This is because the inclusion of a certain amount of annealing twins is unavoidable due to factors such as the chemical composition and manufacturing conditions.

[0062] The annealing twin fraction is measured by the following procedure: An electron backscatter diffraction (EBSD) attached to an SEM is used for the measurement.

[0063] First, if the shape of the surface perpendicular to the longitudinal direction of the steel material is a circle with a radius r, a cross section parallel to the longitudinal direction of the steel material and passing through the central axis of the steel material is cut out. On the other hand, if the shape of the surface perpendicular to the longitudinal direction of the steel material is a rectangle with a thickness t, a cross section parallel to the longitudinal and thickness directions of the steel material is cut out. Then, EBSD measurements are performed at 1-μm intervals on a 4000 μm × 8000 μm region (one field of view) centered at the r / 2 or t / 4 position of the cross section. In this case, boundaries with a crystal orientation difference of 15° or more from adjacent grains are defined as grain boundaries. Furthermore, boundaries with an orientation difference of 55 to 65° among the grain boundaries are defined as annealing twins. Measurements are performed on a total of three fields of view, and the total number of crystal grains and the total number of crystal grains containing twins are determined within the observation areas in all fields of view. The ratio of the total number of crystal grains containing twins to the total number of crystal grains is calculated as a percentage to obtain the annealing twin fraction. For this measurement, OIM Analysis software built into the EBSD can be used.

[0064] 1-3. Types and Shapes of Stainless Steel Materials Stainless steel materials are generally considered to have a Cr content of 10.5% or more and a C content of 1.2% or less, but in this embodiment, a material with a Cr content of 9.0% or more is defined as a stainless steel material. Furthermore, since the stainless steel material of this embodiment satisfies the above-mentioned A value, it is generally a steel material called an austenitic stainless steel material. The shape of the stainless steel material of this embodiment is not particularly limited and may be a steel plate, a steel bar, a wire rod, or any other shape, but is preferably a steel bar because cold forging is performed.

[0065] 1-4. Compressive Deformation Resistance 1-4-1. Compressive Deformation Resistance In the austenitic stainless steel material of this embodiment, the compressive deformation resistance is preferably 1000 MPa or less. Here, the compressive deformation resistance is an index indicating the magnitude of stress when compressive deformation is applied, and the higher the index, the lower the cold forgeability. For example, in cold forging, where the width of compressive deformation applied varies from part to part, a small compressive deformation resistance reduces the variation in hardness from part to part, making cold forging easier. In cold forging, an index such as the compressive deformation rate is sometimes used, but the compressive deformation rate and the compressive deformation resistance are different indices, and there is not necessarily a correlation between these two indices. When considering hardness variation, etc., it is preferable to use the compressive deformation resistance as an index.

[0066] If the compressive deformation resistance is 1000 MPa or less, the variation in hardness after cold forging is suppressed. Therefore, the compressive deformation resistance is preferably 1000 MPa or less, more preferably 900 MPa or less, and more preferably 800 MPa or less. The lower limit of the compressive deformation resistance is preferably 400 MPa, taking into consideration the chemical composition, metal structure, etc.

[0067] The compressive deformation resistance is measured by the following procedure: When an end-constrained compression test (processing temperature: room temperature, strain rate: 10 / s, compression ratio: 80%) is performed using a test piece of φ8 × 12 mm, the deformation resistance at a compression ratio of 80% is defined as the compressive deformation resistance, and the target compressive deformation resistance is 1000 MPa or less.

[0068] 2. Forged Parts 2-1. Chemical Composition and Hardness of Forged Parts The stainless steel material of the present embodiment described above is preferably cold forged and used for forged parts. The forged parts of the present embodiment are made of the stainless steel material of the present embodiment. That is, the chemical composition is, in mass %, C: 0.0010 to 0.1500%, Si: 0.01 to 2.00%, Mn: 0.01 to 10.00%, P: 0.30% or less, S: 0.0001 to 0.5000%, Ni: 8.0 to 30.0%, Cr: 9.0 to 21.0%, Cu: 0.01 to 5.00%, N: 0.0010 to 0.1000%, B: 0.0001 to 0.0500%, Co: 0 to 2.50%, Mo: 0 to 3.00%, Ti: 0 to 2.0%, Nb: 0 to 2.00%, Sn: 0 to 2.5%, V : 0 to 2.0%, W: 0 to 3.0%, Ga: 0 to 0.05%, Sb: 0 to 2.5%, Ta: 0 to 2.5%, Mg: 0 to 0.012%, Zr: 0 to 0.012%, REM: 0 to 0.05%, Pb: 0 to 0.30%, Se: 0 to 0.80%, Te: 0 to 0.30%, Bi: 0 to 0.50%, Al: 0 to 2.0%, Ca: 0 to 0.05%, balance: Fe and impurities, and the above-mentioned A value is -100 or less, and the stainless steel material satisfies [0.01≦Co+Mo≦5.50].

[0069] Furthermore, the forged parts of this embodiment have a hardness of 100 to 500 HV1. This is because cold forging causes work hardening and increases hardness. For this reason, the hardness of the forged parts of this embodiment is 100 HV1 or higher. Preferably, it is 150 HV1 or higher, and more preferably, it is 200 HV1 or higher. On the other hand, the forged parts of this embodiment have little variation in hardness, and work hardening of the steel is suppressed from the viewpoints of cold forgeability and hydrogen embrittlement resistance. Therefore, the hardness of the forged parts of this embodiment is 500 HV1 or lower, preferably 400 HV1 or lower, and more preferably 350 HV1 or lower. Note that the hardness of the forged parts of this embodiment refers to the average value of the maximum hardness and minimum hardness, which will be described later.

[0070] 2-2. ΔHV In the forged part of this embodiment, the difference ΔHV between the maximum hardness and the minimum hardness is preferably 400 HV1 or less. If the above-mentioned ΔHV is 400 HV1 or less, it is considered that the variation in hardness can be sufficiently reduced in cold forging. ΔHV is preferably 250 HV1 or less, and more preferably 150 HV1 or less.

[0071] The maximum and minimum hardness measurements are performed according to the following procedure. For forged parts, the higher the degree of processing, the higher the hardness tends to be. Therefore, for the forged part to be measured, first, the areas estimated to be highly processed and the areas estimated to be lowly processed are identified. Then, test specimens are taken from each area, and the cross sections of the test specimens are polished smoothly. Then, using a micro Vickers hardness tester, the hardness is measured at five points at 1 mm intervals with a test force of 1 kgf, and the average values ​​are calculated. The average values ​​of the five hardness values ​​measured at the areas estimated to be highly processed and the areas estimated to be lowly processed are defined as the maximum and minimum hardness, respectively. The hardness test is performed in accordance with JIS Z 2244-1:2024 (Vickers hardness test). "HV1" refers to the "hardness symbol" when a Vickers hardness test is performed with a test force of 9.807 N (1 kgf) (see JIS Z 2244-1:2024).

[0072] 2-3. Hydrogen embrittlement resistance of forged parts In the forged parts of this embodiment, if the relative tensile strength in high-pressure hydrogen is 80% or more, it is judged to have good hydrogen embrittlement resistance. The relative tensile strength in high-pressure hydrogen is preferably 90% or more, and more preferably 95% or more. Furthermore, if the relative reduction of area in high-pressure hydrogen is 50% or more, it is judged to have good hydrogen embrittlement resistance. The relative reduction of area in high-pressure hydrogen is preferably 60% or more, and more preferably 70% or more.

[0073] The relative tensile strength is the strain rate of 1×10 in a high-pressure hydrogen gas environment of 70 MPa at -40°C. －５ The tensile strength when tensile testing was performed at a strain rate of 1 × 10 / s in a 0.1 MPa nitrogen gas environment at -40 °C was measured. －５This refers to the value obtained by dividing the tensile strength by the tensile strength when a tensile test is conducted at 1 / s.

[0074] Similarly, the relative reduction of area is the reduction in area at a strain rate of 1×10 in a high-pressure hydrogen gas environment of -40°C and 70 MPa. －５ The reduction of area when a tensile test was performed at a strain rate of 1 × 10 / s was measured in an environment of -40 °C and 0.1 MPa nitrogen gas. －５ The relative tensile strength and relative reduction of area are obtained by the same tensile test.

[0075] FIG. 1 is a diagram illustrating the shape of a test piece used in a tensile test for measuring relative tensile strength and relative reduction of area. The hydrogen embrittlement resistance of a forged part tends to decrease as the degree of processing increases. Therefore, for the forged part to be evaluated, two test pieces having the shape shown in FIG. 1 are taken from a portion estimated to have a high degree of processing, and a tensile test is performed under the above conditions. Note that if it is not possible to take a test piece having the shape shown in FIG. 1 from the forged part, a small test piece having a similar shape can be used.

[0076] 3. Manufacturing Method The stainless steel material and forged part of this embodiment can be stably manufactured, for example, by the following manufacturing method.

[0077] 3-1. Hot working Steel having the above chemical composition is melted to produce a material for hot working. The hot working method is not particularly limited, but for example, in the case of steel bars, it is preferable to use tilt rolling as the hot working method and to use induction heating to heat the material before tilt rolling.

[0078] The conditions for hot working are not particularly limited, but for example, it is preferable to heat the material for hot working at 1000 to 1400°C. In the case of steel bars, a set temperature is set in the induction heating device that heats the material. It is preferable to specify this set temperature as 1000 to 1400°C and to set the time for passing the rolling material through the induction heating device within a range of 10 to 300 seconds. It is more preferable that the set temperature is within a range of 1000 to 1300°C and the time for passing the material is within a range of 10 to 200 seconds. Note that the set temperature of the induction heating device specifically means the output temperature within the induction heating device through which the steel material passes.

[0079] It is preferable to heat the material to the above temperature before processing, i.e., to perform hot processing. In the case of steel bars, it is common to heat the material and then perform tilt rolling. In tilt rolling, for example, three work rolls are arranged on roll axes that are twisted and tilted in the same direction around the material to be rolled, and each work roll revolves around the material to be rolled while rotating on its own axis. As a result, the material to be rolled is rolled in a spiral shape while moving forward. After tilt processing, the bar and wire are hot rolled, and if necessary, pickling or the like may be performed.

[0080] 3-2. Annealing Next, the hot-worked material is annealed. During annealing, the annealing temperature is preferably in the range of 900 to 1400°C. This is because if the annealing temperature is less than 900°C, recrystallization is unlikely to be promoted. Also, this is because B does not sufficiently dissolve in the matrix phase. For this reason, the annealing temperature is preferably 900°C or higher. On the other hand, if the annealing temperature exceeds 1400°C, the temperature is too high, causing coarse crystal grains and making the material surface prone to roughening. For this reason, the annealing temperature is preferably 1400°C or lower. Preferably, the annealing temperature is 950°C or higher and 1300°C or lower, and more preferably 1000°C or higher and 1200°C or lower.

[0081] Furthermore, the holding time at the annealing temperature during annealing is preferably 1 min to 5 h. This is because if the annealing time is less than 1 min, recrystallization is unlikely to be promoted. Also, this is because B does not sufficiently dissolve in the matrix phase. For this reason, the holding time at the annealing temperature is preferably 1 min or more. On the other hand, if the holding time at the annealing temperature is 5 h or less, the temperature is too high, the crystal grains become coarse, and roughness of the material surface is likely to occur. For this reason, the holding time at the annealing temperature is preferably 5 h or less. It is preferably 3 min or more and 3 h or less, and more preferably 10 min or more and 2 h or less.

[0082] After holding at the annealing temperature, cooling is performed at two cooling rates. In the first cooling, the cooling rate is fast to suppress precipitation of Boride (a compound containing B) and ensure a sufficient amount of solute B. In the second cooling, the cooling rate is slowed to promote segregation of solute B around grain boundaries. Hereinafter, the first cooling with a fast cooling rate will be referred to as first cooling, and the subsequent cooling with a slower cooling rate will be referred to as second cooling. Furthermore, the temperature at which the cooling rate changes, distinguishing between first and second cooling, will be referred to as the cooling change temperature T'. In other words, first cooling refers to cooling from the annealing temperature to the cooling change temperature T', and second cooling refers to cooling from the cooling change temperature T' to 200°C.

[0083] In the first cooling, the cooling rate from the annealing temperature to the cooling change temperature T' described below is set to 1100°C / h or more. The cooling rate from the annealing temperature to the cooling change temperature T' is preferably 1500°C / h or more, and more preferably 2000°C / h or more. As described above, by setting the cooling rate relatively fast, the precipitation of boride can be suppressed and a sufficient amount of solute B can be secured. The upper limit of the cooling rate from the annealing temperature to the cooling change temperature T' is not particularly limited, but is usually 200,000°C / h due to constraints such as equipment.

[0084] In the second cooling, the cooling rate from the cooling transition temperature T' to 200°C is 1000°C / h or less. The cooling rate from the cooling transition temperature T' to 200°C is preferably 500°C / h or less, more preferably 100°C / h or less. As described above, a relatively slow cooling rate promotes the segregation of solute B around grain boundaries, thereby suppressing the formation of twins and improving cold forgeability and hydrogen embrittlement resistance. The lower limit of the cooling rate from the cooling transition temperature T' to 200°C is not particularly limited, but is usually 1°C / h due to constraints such as equipment.

[0085] Here, the cooling change temperature T' is the temperature at which the cooling rate changes and must be set within a temperature range of 600°C or higher and 900°C or lower. If T' exceeds 900°C, boride is generated during the second cooling, and the amount of solute B decreases, resulting in an increase in the annealing twin fraction. The cooling change temperature T' is preferably 800°C or lower, and more preferably 750°C or lower. The cooling change temperature T' can be measured with a radiation thermometer during production.

[0086] Thereafter, the stainless steel material is cooled to about room temperature (25° C.), and if necessary, the shape of the stainless steel material may be adjusted by peeling, drawing, etc. Furthermore, manufacturing conditions not specifically described may be adjusted appropriately according to ordinary methods.

[0087] The stainless steel material obtained through the above process may be cold forged to form a desired shape to produce a forged part. The conditions for cold forging are not particularly limited, and may be any of the usual methods.

[0088] EXAMPLES The stainless steel material according to the present invention will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples.

[0089] Steels having the chemical compositions shown in Tables 1 to 3 were melted and used to produce 180 mm diameter hot-worked materials (slabs). The slabs were heated to 1130°C in a heating furnace. The rolling material was then heated using induction heating before tilt rolling. The induction heating device was set to a temperature of 1210°C and the material was passed through the induction heating device for 110 seconds. The material was then tilt-rolled, subjected to in-line heat treatment, and wire-rolled. The material was then annealed at 1130°C for 20 minutes. The material was then cooled from 730°C to 200°C, with a first cooling rate of 2000°C / h and a second cooling rate of 200°C / h, to a cooling transition temperature T'. A 30 mm diameter stainless steel material was obtained. The annealing twin fraction, compressive deformation resistance, and other properties of the resulting stainless steel material were measured using the following procedure.

[0090]

[0091]

[0092]

[0093] (Annealing Twin Fraction) A 4000 μm × 8000 μm region (one field of view) centered at the r / 2 position on a cross section of the stainless steel material parallel to the longitudinal direction and passing through the central axis of the steel material was measured at 1 μm intervals using an EBSD attached to an FE-SEM (manufactured by JEOL Ltd.). A boundary with a crystal orientation difference of 15° or more from adjacent grains was defined as a grain boundary, and a boundary with an orientation difference of 55° to 65° among the grain boundaries was defined as an annealing twin. Measurements were performed on a total of three fields of view, and the total number of crystal grains and the total number of crystal grains containing twins within the observation areas in all fields of view were determined. The ratio of the total number of crystal grains containing twins to the total number of crystal grains was calculated as a percentage to determine the annealing twin fraction. For this measurement, OIM Analysis software (Analysis 8) built into the EBSD was used.

[0094] (Compression deformation resistance) A test piece of φ8 × 12 mm was cut out from the above stainless steel material, and an end face restraint compression test (working temperature: room temperature, strain rate: 10 / s, compression ratio: 80%) was performed using the test piece. The deformation resistance at a compression ratio of 80% was then defined as the compression deformation resistance.

[0095] (Hardness and ΔHV) The above 30 mm diameter stainless steel material was subjected to cold swaging to produce a forged part with a gradient from 6 mm to 25 mm diameter. The average hardness of five points measured on a cross section at a 6 mm diameter (true strain: 3) was defined as the maximum hardness, and the average hardness of five points measured on a cross section at a 25 mm diameter (true strain: 0.4) was defined as the minimum hardness. The average of the maximum and minimum hardness was defined as "hardness," and the difference between the maximum and minimum hardness was defined as "ΔHV." The hardness was measured after the cross section was polished smoothly using a micro-Vickers hardness tester, with a test force of 1 kgf and a 1 mm pitch. The hardness test was performed in accordance with JIS Z 2244-1:2024 (Vickers hardness test).

[0096] (Relative tensile strength and relative reduction of area) The above-mentioned stainless steel material with a diameter of 30 mm was subjected to cold swaging to obtain a forged part with a diameter of 6 mm. Two tensile test pieces having the shape shown in Fig. 1 were cut out from the obtained forged parts. One of the test pieces was then subjected to strain at a strain rate of 1 x 10 in a high-pressure hydrogen gas environment at -40°C and 70 MPa. －５ The other specimen was subjected to a tensile test at a strain rate of 1 × 10 / s in a nitrogen gas environment at −40°C and 0.1 MPa, and the tensile strength and reduction of area were measured. －５ The tensile strength and reduction of area were measured at 100 / s, and the relative tensile strength and the relative reduction of area were calculated from the results.

[0097] The above results are summarized in Tables 4 and 5. In the column "Annealing twin fraction" in the tables, D was given for more than 0.95, C for 0.95 or less and more than 0.80, B for 0.80 or less and more than 0.50, and A for 0.50 or less. In the column "Compressive deformation resistance", D was given for more than 1000 MPa, C for 1000 MPa or less and more than 900 MPa, B for 900 MPa or less and more than 800 MPa, and A for 800 MPa or less. In the column "Hardness after cold working", D was given for less than 100 HV or more than 500 HV, C for 100 HV or less and 150 HV or more than 400 HV and 500 HV, B for 150 HV or less and 200 HV or more than 350 HV and 400 HV or less, and A for 200 HV or more and 350 HV and 350 HV or less. In the column "ΔHV after cold working", over 400 was rated D, 400 or less but over 250 was rated C, 250 or less but over 150 was rated B, and 150 or less was rated A. In the column "relative tensile strength", less than 80% was rated D, 80% or more but less than 90% was rated C, 90% or more but less than 95% was rated B, and 95% or more was rated A. In the column "relative reduction of area", less than 50% was rated D, 50% or more but less than 60% was rated C, 60% or more but less than 70% was rated B, and 70% or more was rated A.

[0098]

[0099] Test Nos. 1 to 39, whose chemical compositions satisfied the requirements of this embodiment, had a low annealing twin fraction and exhibited good cold forgeability and hydrogen embrittlement resistance. On the other hand, Test Nos. 40 to 57, whose chemical compositions did not meet the requirements of this embodiment, had a high annealing twin fraction, resulting in poor cold forgeability and hydrogen embrittlement resistance.

[0100] A 180 mm diameter hot work material (slab) having the chemical composition of steel type P of Example 1 was subjected to tilt rolling, in-line heat treatment, and bar rolling in the same manner as in Example 1. Subsequently, as in Example 1, annealing was performed by holding at 1230°C for 20 minutes, and the cooling transition temperature T', the cooling rate in the first cooling, and the cooling rate in the second cooling were changed as shown in Table 6, followed by cooling to 200°C, yielding a 30 mm diameter stainless steel material. The annealing twin fraction, compressive deformation resistance, hardness, ΔHV, relative tensile strength, and relative reduction of area of ​​the obtained stainless steel material were measured using the procedures of Example 1. The results are summarized below in Table 6. The standards in the table are the same as those in Example 1.

[0101]

[0102] Nos. 58 to 67, which satisfied the preferred manufacturing conditions of the present application, had a low annealing twin fraction and were good in cold forgeability and hydrogen embrittlement resistance. On the other hand, Nos. 68 to 73, which did not satisfy the preferred manufacturing conditions of the present application, had a high annealing twin fraction, resulting in poor cold forgeability and hydrogen embrittlement resistance.

Claims

1. Chemical composition, in mass%, is: C: 0.0010 to 0.1500%, Si: 0.01 to 2.00%, Mn: 0.01 to 10.00%, P: 0.30% or less, S: 0.0001 to 0.5000%, Ni: 8.0 to 30.0%, Cr: 9.0 to 21.0%, Cu: 0.01 to 5.00%, N: 0.0010 to 0.1000%, B: 0.0001 to 0.0500%, Co: 0 to 2.50%, Mo: 0 to 3.00%, Ti: 0 to 2.0%, Nb: 0 to 2.00%, Sn: 0 to 2.5%, V: 0 to 2.0%, W: 0 to 3.0%, A stainless steel material having an annealing twin fraction of 0.95 or less, the A value of which is represented by the following formula (i) being -100 or less, the following formula (ii) being satisfied, and the following formula (ii) being: Ga: 0 to 0.05%, Sb: 0 to 2.5%, Ta: 0 to 2.5%, Mg: 0 to 0.012%, Zr: 0 to 0.012%, REM: 0 to 0.05%, Pb: 0 to 0.30%, Se: 0 to 0.80%, Te: 0 to 0.30%, Bi: 0 to 0.50%, Al: 0 to 2.0%, Ca: 0 to 0.05%, and the balance: Fe and impurities. A value = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo ... (i) 0.01 ≦ Co + Mo ≦ 5.50 ... (ii) However, each element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel material, and if the element is not contained, it is set to zero.

2. The chemical composition is, in mass%, Ti: 0.01 to 2.0%, Nb: 0.01 to 2.00%, Sn: 0.0001 to 2.5%, V: 0.001 to 2.0%, W: 0.05 to 3.0%, Ga: 0.0004 to 0.05%, Sb: 0.01 to 2.5%, Ta: 0.01 to 2.5%, Mg: 0.0002 to 0.012%, Zr: 0.0002 to 0.012%, REM: 0.0002 to 0.05%, Pb: 0.0001 to 0.30%, Se: 0.0001 to 0.80%, Te: 0.0001 to 0.30%, The stainless steel material according to claim 1, containing one or more selected from Bi: 0.0001 to 0.50%, Al: 0.001 to 2.0%, and Ca: 0.0001 to 0.05%.

3. The stainless steel material according to claim 1 or 2, wherein the compressive deformation resistance is 1000 MPa or less.

4. Chemical composition, in mass%, is: C: 0.0010-0.1500%, Si: 0.01-2.00%, Mn: 0.01-10.00%, P: 0.30% or less, S: 0.0001-0.5000%, Ni: 8.0-30.0%, Cr: 9.0-21.0%, Cu: 0.01-5.00%, N: 0.0010-0.1000%, B: 0.0001-0.0500%, Co: 0-2.50%, Mo: 0-3.00%, Ti: 0-2.0%, Nb: 0-2.00%, Sn: 0-2.5%, V: 0-2.0%, W: 0-3.0%, A forged part made of a stainless steel material having a hardness of 100 to 500 HV1, the stainless steel material having an A value represented by the following formula (i) being -100 or less, and satisfying the following formula (ii): Ga: 0 to 0.05%, Sb: 0 to 2.5%, Ta: 0 to 2.5%, Mg: 0 to 0.012%, Zr: 0 to 0.012%, REM: 0 to 0.05%, Pb: 0 to 0.30%, Se: 0 to 0.80%, Te: 0 to 0.30%, Bi: 0 to 0.50%, Al: 0 to 2.0%, Ca: 0 to 0.05%, balance: Fe and impurities. A value = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo ... (i) 0.01 ≦ Co + Mo ≦ 5.50 ... (ii) However, each element symbol in the above formula represents the content (mass%) of each element contained in the stainless steel material, and if the element is not contained, it is set to zero.

5. The forged part according to claim 4, wherein the difference ΔHV between the maximum hardness and the minimum hardness is 400 HV1 or less.

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