Austenitic stainless steel material, its manufacturing method, and leaf spring

Abstract

An austenitic stainless steel material according to the present invention contains a prescribed amount of a prescribed element and has: a composition represented by formula (1) below, in which the value of Md30 is between -40 and 0; a metallic structure including 25-35 vol% of a processing-induced martensite phase; a tensile strength (TS) of 1,450 MPa or greater; and a breaking elongation (EL) of 12.0% or greater, wherein TS×EL is 24,000 or greater, and the stress relaxation rate represented by formula (2) below is 1.20% or less.　(1): Md30 = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo (in this formula, element symbols denote the content (mass%) of each element); (2): the stress relaxation rate = (σ1 - σ2) / σ1 (in this formula, σ1 is the stress at less than 0.2% yield strength, and σ2 is the stress after the stress of σ1 is applied for 200 seconds)

Description

Technical Field

[0001] The present invention relates to an austenitic stainless steel material, a method for manufacturing the same, and a leaf spring.

Background Art

[0002] With the miniaturization and high performance of communication devices such as smartphones and precision devices such as personal computers, the thinning and weight reduction of structural parts and functional parts used in these devices have been progressing. Therefore, the materials used for these parts are required to have excellent workability (ductility) and high strength. In particular, parts such as leaf springs that are exposed to repeated stress are required to have characteristics (resistance to sagging) that can withstand repeated stress. Here, "resistance to sagging" means the property of withstanding "sagging" that does not completely return to the original shape due to minute deformation when repeatedly used under elastic stress.

[0003] Conventionally, metastable austenitic stainless steel materials such as SUS301 have been used as materials for structural parts and functional parts. This metastable austenitic stainless steel material can be strengthened to high strength by quenching and tempering rolling, but its ductility is not sufficient.

[0004] As an austenitic stainless steel material having both high strength and high ductility, for example, in Patent Document 1, in mass%, C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5% to 3.5%, and one or two selected from the group of Al: 0.1% to 3.5% and Ti 0.1% to 3.5% are contained, and the balance is composed of Fe and inevitable impurities, and has a predetermined two-phase structure composed of a processing-induced martensite phase (α' phase) and an austenite phase (γ phase), and the 0.2% proof stress (YS) is 1400 N / mm 2 ~1900 N / mm 2 , and a metastable austenitic stainless steel strip or steel sheet having YS×EL of 21000 to 48000 has been proposed.

[0005] As for the material used for the spring component, in Patent Document 2, in terms of weight percentage, it contains C: 0.08% or less, Si: 3.0% or less, Mn: 4.0% or less, Ni: 4.0 to 10.0%, Cr: 13.0 to 20.0%, N: 0.06 to 0.30%, O: 0.007% or less, and the amounts of C, Si, Mn, Ni, Cr, and N are adjusted so that the M value according to the formula M = 330 - (480 × C%) - (2 × Si%) - (10 × Mn%) - (14 × Ni%) - (5.7 × Cr%) - (320 × N%) is 40 or more, and the balance consists of Fe and impurities inevitably mixed in, and a stainless steel excellent in spring characteristics and fatigue characteristics of the processed part has been proposed.

[0006] Also, in Patent Document 3, in terms of mass percentage, it contains C ≤ 0.15%, Si ≤ 4.0%, 4.0% ≤ Mn ≤ 10.0%, P ≤ 0.10%, S ≤ 0.010%, 2.0% ≤ Ni ≤ 6.0%, 16.0% ≤ Cr ≤ 18.0%, 0.05% ≤ N ≤ 0.20%, and the balance consists of Fe and unavoidable impurities, and Md 30 Md according to Mn = 551 - 62(%C + %N) - 29(%Ni + %Cu) + 4.8%Si - 19.1%Mn - 13.7%Cr - 18.5%Mo 30 satisfies -35 ≤ Md 30 Mn ≤ 0, and an austenitic stainless steel for springs has been proposed, which is characterized in that a tensile strength of 1320 MPa or more is imparted by cold rolling.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] Although the austenitic stainless steel material described in Patent Document 1 has both high strength and high ductility, the sag resistance required for functional parts such as leaf springs has not been studied. Although the stainless steel described in Patent Document 2 is said to have good formability, it does not satisfy the workability (ductility) required for various parts used in communication equipment and precision equipment. In fact, the elongation of the stainless steel described in the examples of Patent Document 2 is 4.0 to 7.3%, and it cannot be said that the ductility is sufficient. Since the austenitic stainless steel described in Patent Document 3 is finish-rolled by quenching and tempering and no low-temperature heat treatment is performed, it cannot be said that the sag resistance is sufficient.

[0009] The present invention has been made to solve the above problems, and an object thereof is to provide an austenitic stainless steel material having high strength and high ductility and excellent sag resistance, and a method for manufacturing the same. Another object of the present invention is to provide a leaf spring having high strength, excellent dimensional accuracy, and a long life.

Means for Solving the Problems

[0010] The present inventors have found that the above problems can be solved by controlling the composition, metal structure, tensile strength (TS), elongation at break (EL), TS×EL, and stress relaxation rate of the austenitic stainless steel material, and have completed the present invention.

[0011] That is, the present invention contains, on a mass basis, C: 0.200% or less, Si: 1.00 to 3.50%, Mn: 5.00% or less, Ni: 4.00 to 10.00%, Cr: 12.00 to 18.00%, Cu: 3.500% or less, Mo: 1.00 to 5.00%, N: 0.200% or less, the total amount of C and N is 0.100% or more, the balance is composed of Fe and impurities, and the following formula (1): Md 30 = 551 - 462(C + N) - 9.2Si - 8.1Mn - 29(Ni + Cu) - 13.7Cr - 18.5Mo ··· (1) Md represented by (wherein, the element symbol represents the content (% by mass) of each element) 30 has a composition in which the value of 30 is -40.0 to 0 °C, has a metallic structure containing 25 to 35% by volume of a strain-induced martensite phase, has a tensile strength (TS) of 1450 MPa or more, an elongation at break (EL) of 12.0% or more, and TS×EL of 24000 or more, and the stress relaxation rate represented by the following formula (2): Stress relaxation rate = (σ1 - σ2) / σ1 ··· (2) (wherein, σ1 is a stress less than the 0.2% proof stress, and σ2 is the stress 200 seconds after applying the stress of σ1) is 1.20% or less, and is an austenitic stainless steel material.

[0012] Further, the present invention contains, on a mass basis, C: 0.200% or less, Si: 1.00 to 3.50%, Mn: 5.00% or less, Ni: 4.00 to 10.00%, Cr: 12.00 to 18.00%, Cu: 3.500% or less, Mo: 1.00 to 5.00%, N: 0.200% or less, the total amount of C and N is 0.100% or more, the balance is composed of Fe and impurities, and the following formula (1): Md 30 = 551 - 462(C + N) - 9.2Si - 8.1Mn - 29(Ni + Cu) - 13.7Cr - 18.5Mo ··· (1) Md represented by (wherein, the element symbol represents the content (% by mass) of each element) 30 After solution-treating a rolled material having a composition in which the value of 30 is -40.0 to 0 °C, cold-rolling is performed at a rolling ratio sufficient to generate a strain-induced martensite phase of 25 to 35% by volume, and then at a temperature of 100 to 200 °C, the following formula (3): P = T(log t + 20) ··· (3) (wherein, T is the temperature (K) and t is the time (h)) is a method for producing an austenitic stainless steel material in which the value of P satisfies 7000 to 9400 by performing heat treatment.

[0013] Furthermore, the present invention is a leaf spring including the above-described austenitic stainless steel material.

Advantages of the Invention

[0014] According to the present invention, it is possible to provide an austenitic stainless steel material having high strength, high ductility, and excellent sag resistance, and a method for manufacturing the same. Further, according to the present invention, it is possible to provide a leaf spring having high strength, excellent dimensional accuracy, and a long life.

Embodiment for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be specifically described. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements may be appropriately added to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the gist of the present invention, and such modified and improved embodiments are also within the scope of the present invention. In addition, in this specification, the “%” indication regarding components means “mass %” unless otherwise specified.

[0016] The austenitic stainless steel material according to the embodiment of the present invention contains C: 0.200% or less, Si: 1.00 to 3.50%, Mn: 5.00% or less, Ni: 4.00 to 10.00%, Cr: 12.00 to 18.00%, Cu: 3.500% or less, Mo: 1.00 to 5.00%, N: 0.200% or less, the total amount of C and N is 0.100% or more, and the balance consists of Fe and impurities. Here, in this specification, the “stainless steel material” means a material formed from stainless steel, and its material form is not particularly limited. Examples of the material form include plate shape (including strip shape), rod shape, tubular shape, etc. Also, various shaped steels such as T-shaped and I-shaped cross-sectional shapes may be used. Further, the “impurities” mean components that are mixed in due to raw materials such as ores and scraps and various factors in the manufacturing process when industrially manufacturing the austenitic stainless steel material, and are allowed within a range that does not adversely affect the present invention. For example, inevitable impurities such as P and S that are difficult to remove are also included in these impurities.

[0017] In addition, the austenitic stainless steel material according to the embodiment of the present invention may further contain one or more selected from Al: 0.100% or less, O: 0.010% or less, V: 0.0001 to 0.500%, and B: 0.0001 to 0.015%. Furthermore, the austenitic stainless steel material according to the embodiment of the present invention may further contain one or more selected from Ti: 0.010 to 0.500%, Co: 0.010 to 0.500%, Zr: 0.010 to 0.100%, Nb: 0.010 to 0.100%, Mg: 0.0005 to 0.0030%, Ca: 0.0003 to 0.0030%, Y: 0.010 to 0.200%, Ln: 0.001 to 0.100%, Sn: 0.001 to 0.500%, Sb: 0.001 to 0.500%, Pb: 0.010 to 0.100%, and W: 0.010 to 0.500%. Hereinafter, each component will be described in detail.

[0018] <C: 0.200% or less> C is an interstitial element and contributes to work hardening and high strength by heat treatment. Also, C is an element that stabilizes the austenite phase and is effective for maintaining non-magnetism. However, if the content of C is too high, it will cause hardening and reduce the cold workability. Therefore, the upper limit value of the content of C is set to 0.200%, preferably 0.100%, more preferably 0.090%. On the other hand, the lower limit value of the content of C is not particularly limited, but from the viewpoint of refining cost, it is preferably set to 0.010%, more preferably 0.015%, still more preferably 0.020%.

[0019] <Si: 1.00 to 3.50%> Si is an element used as a deoxidizer for stainless steel in the steelmaking process. Also, Si has the effect of improving the age hardening property in the heat treatment after cold rolling. From the viewpoint of fully obtaining these effects, the lower limit value of the Si content is set to 1.00%, preferably 1.20%, more preferably 1.50%. On the other hand, since Si has a large solid solution strengthening effect and has the effect of reducing the stacking fault energy and improving the work hardening property, if the Si content is too high, it will become a factor in reducing the cold workability. Therefore, the upper limit value of the Si content is set to 3.50%, preferably 3.20%, more preferably 3.00%.

[0020] <Mn: 5.00% or less> Mn is an element that forms oxide-based inclusions as MnO. Also, Mn has a small solid solution strengthening effect and is an austenite-forming element, and has the effect of suppressing the processing-induced martensite transformation. Therefore, the upper limit value of the Mn content is set to 5.00%, preferably 4.00%, more preferably 3.00%. On the other hand, the lower limit value of the Mn content is not particularly limited, but is preferably set to 0.01%, more preferably 0.05%, still more preferably 0.10%.

[0021] <Ni: 4.00 - 10.00%> Ni is an element contained to obtain an austenite phase at high temperature and room temperature. It is necessary to contain Ni so that it becomes a metastable austenite phase at room temperature and the martensite phase is induced during cold rolling. If the Ni content is too low, the δ ferrite phase is generated at high temperature and the martensite phase is generated during the cooling process to room temperature, and it cannot exist as a single austenite phase. Therefore, the lower limit value of the Ni content is set to 4.00%, preferably 4.50%, more preferably 5.00%. On the other hand, if the Ni content is too high, it becomes difficult to induce the martensite phase during cold rolling. Therefore, the upper limit value of the Ni content is set to 10.00%, preferably 9.50%, more preferably 9.00%.

[0022] <Cr: 12.00~18.00%> Cr is an element that improves corrosion resistance. From the viewpoint of ensuring suitable corrosion resistance for structural parts and functional parts (especially leaf springs), the lower limit value of the Cr content is set to 12.00%, preferably 12.50%, more preferably 13.00%. On the other hand, if the Cr content is too high, the cold workability will decrease. Therefore, the upper limit value of the Cr content is set to 18.00%, preferably 17.50%, more preferably 17.00%.

[0023] <Cu: 3.500% or less> Cu is an element that has the effect of hardening stainless steel during heat treatment. However, if the Cu content is too high, the hot workability will decrease, which may cause cracking. Therefore, the upper limit value of the Cu content is set to 3.500%, preferably 3.000%, more preferably 2.000%. On the other hand, the lower limit value of the Cu content is not particularly limited, but is preferably set to 0.010%, more preferably 0.020%, still more preferably 0.030%.

[0024] <Mo: 1.00~5.00%> Mo is an element effective for improving the corrosion resistance of austenitic stainless steel materials. Also, Mo is an element effective for suppressing the release of strain generated during cold rolling. Considering the use in structural parts and functional parts (especially leaf springs) where improvement in corrosion resistance and sag resistance is required in recent years, the lower limit value of the Mo content is set to 1.00%, preferably 1.30%, more preferably 1.50%. On the other hand, since Mo is expensive, if the Mo content is too high, it will lead to an increase in manufacturing cost. Also, the δ-ferrite phase and α-ferrite phase will be generated at high temperatures. Therefore, the upper limit value of the Mo content is set to 5.00%, preferably 4.50%, more preferably 4.00%.

[0025] <N: 0.200% or less> N is an austenite-forming element. Also, N is an element that is extremely effective in hardening the austenite phase and the martensite phase. However, if the content of N is too high, it will cause blowholes during casting. Therefore, the upper limit value of the content of N is set to 0.200%, preferably 0.150%, more preferably 0.100%. On the other hand, the lower limit value of the content of N is not particularly limited, but is preferably set to 0.001%, preferably 0.010%.

[0026] <Total amount of C and N: 0.100% or more> C and N are elements that give a similar hardening effect. From the viewpoint of fully exerting such a hardening effect, the lower limit value of the total amount of C and N is set to 0.100%, preferably 0.120%, more preferably 0.140%.

[0027] <Al: 0.100% or less> Al has a higher oxygen affinity than Si and Mn. If the content of Al is too high, it is likely to form coarse oxide inclusions that become the starting points of internal cracks during cold rolling. Therefore, the upper limit value of the Al content is preferably set to 0.100%, more preferably 0.080%, still more preferably 0.050%, still more preferably 0.030%. On the other hand, the lower limit value of the Al content is not particularly limited, but excessive low Al content leads to an increase in manufacturing cost, so it is preferably set to 0.0001%, more preferably 0.0003%, still more preferably 0.0005%.

[0028] <O: 0.010% or less> If the content of O is too high, it is likely to form coarse inclusions with a particle size exceeding 5 μm. Therefore, the upper limit value of the content of O is preferably set to 0.010%, preferably 0.008%. On the other hand, the lower limit value of the content of O is not particularly limited, but if the content of O is too low, it becomes difficult to oxidize Mn, Si, etc., and the ratio of Al in the inclusions 2 O 3 becomes high. Therefore, the lower limit value of the content of O is preferably set to 0.001%, more preferably 0.003%.

[0029] <V: 0.0001 to 0.500%> V is an element that has the effect of enhancing age hardenability during the heating of the heat treatment performed after cold rolling. From the viewpoint of sufficiently obtaining this effect, the lower limit value of the content of V is preferably set to 0.0001%, more preferably 0.001%. On the other hand, if the content of V is too much, it will lead to an increase in manufacturing cost. Therefore, the upper limit value of the content of V is preferably set to 0.500%, more preferably 0.400%, and even more preferably 0.300%.

[0030] <B: 0.0001 to 0.015%> If the content of B is too much, it will cause a factor of deterioration in workability due to the formation of borides. Therefore, the upper limit value of the content of B is preferably set to 0.015%, more preferably 0.010%. On the other hand, the lower limit value of the content of B is not particularly limited, but is preferably set to 0.0001%, more preferably 0.0002%.

[0031] <Ti: 0.010 to 0.500%> Ti is a carbonitride forming element, fixes C and N, and suppresses the decrease in corrosion resistance caused by sensitization. From the viewpoint of exerting such an effect, the lower limit value of the content of Ti is preferably set to 0.010%, more preferably 0.011%. On the other hand, if the content of Ti is too much, the solid solution amount of C and N will decrease, and it may precipitate unevenly with non-uniform sizes as carbides and inhibit the recrystallized grain growth. Also, since Ti is expensive, it will lead to an increase in manufacturing cost. Therefore, the upper limit value of the content of Ti is preferably set to 0.500%, more preferably 0.400%, and even more preferably 0.300%.

[0032] <Co: 0.010 to 0.500%> Co is an element that improves intergranular corrosion resistance. From the perspective of exerting such an effect, the lower limit value of the Co content is preferably set at 0.010%, more preferably 0.020%. On the other hand, if the Co content is too high, the austenitic stainless steel material will be hardened and the ductility will decrease. Therefore, the upper limit value of the Co content is preferably set at 0.500%, more preferably 0.100%.

[0033] <Zr: 0.010~0.100%> Zr is an element with a high affinity for C and N, and precipitates as carbide or nitride during hot rolling, reducing the dissolved C and dissolved N in the matrix phase and having the effect of improving workability. From the perspective of exerting such an effect, the lower limit value of the Zr content is preferably set at 0.010%, more preferably 0.020%. On the other hand, if the Zr content is too high, the austenitic stainless steel material will be hardened and the ductility will decrease. Therefore, the upper limit value of the Zr content is preferably set at 0.100%, more preferably 0.050%.

[0034] <Nb: 0.010~0.100%> Nb is an element with a high affinity for C and N, and precipitates as carbide or nitride during hot rolling, reducing the dissolved C and dissolved N in the matrix phase and having the effect of improving workability. From the perspective of exerting such an effect, the lower limit value of the Nb content is preferably set at 0.010%, more preferably 0.020%. On the other hand, if the Nb content is too high, the austenitic stainless steel material will be hardened and the ductility will decrease. Therefore, the upper limit value of the Nb content is preferably set at 0.100%, more preferably 0.050%.

[0035] <Mg: 0.0005~0.0030%> Mg forms Mg oxide together with Al in the molten steel and acts as a deoxidizer. From the viewpoint of exerting such an effect, the lower limit of the Mg content is preferably set to 0.0005%, more preferably 0.0008%. On the other hand, if the Mg content is too high, the toughness of the austenitic stainless steel material decreases. Therefore, the upper limit of the Mg content is preferably set to 0.0030%, more preferably 0.0020%.

[0036] <Ca: 0.0003~0.0030%> Ca is an element that improves hot workability. From the viewpoint of exerting the effect of this Ca, the lower limit of the Ca content is preferably set to 0.0003%, more preferably 0.0005%. On the other hand, if the Ca content is too high, the toughness of the austenitic stainless steel material decreases. Therefore, the upper limit of the Ca content is preferably set to 0.0030%, more preferably 0.0020%.

[0037] <Y: 0.010~0.200%> Y is an element that reduces the viscosity of the molten steel and improves cleanliness. From the viewpoint of exerting the effect of such Y, the lower limit of the Y content is preferably set to 0.010%, more preferably 0.020%. On the other hand, if the Y content is too high, the effect of Y becomes saturated and the workability decreases. Therefore, the upper limit of the Y content is preferably set to 0.200%, more preferably 0.100%.

[0038] <Ln: 0.001~0.100%> Ln (lanthanoid: elements with atomic numbers 57 to 71 such as La, Ce, Nd) is an element that improves high-temperature oxidation resistance. From the viewpoint of exerting the effect of such Ln, the lower limit of the Ln content is preferably set to 0.001%, more preferably 0.002%. On the other hand, if the Ln content is too high, the effect of Ln becomes saturated and surface defects occur during hot rolling, resulting in a decrease in manufacturability. Therefore, the upper limit of the Ln content is preferably set to 0.100%, more preferably 0.050%.

[0039] <Sn: 0.001 to 0.500%> Sn is an element effective in improving workability by promoting the generation of deformation bands during rolling. From the viewpoint of exerting such an effect of Sn, the lower limit value of the Sn content is preferably set to 0.001%, more preferably 0.003%. On the other hand, if the Sn content is too high, the effect of Sn saturates and the workability deteriorates. Therefore, the upper limit value of the Sn content is preferably set to 0.500%, more preferably 0.200%.

[0040] <Sb: 0.001 to 0.500%> Sb is an element effective in improving workability by promoting the generation of deformation bands during rolling. From the viewpoint of exerting such an effect of Sb, the lower limit value of the Sb content is preferably set to 0.001%, more preferably 0.003%. On the other hand, if the Sb content is too high, the effect of Sb saturates and the workability deteriorates. Therefore, the upper limit value of the Sb content is preferably set to 0.500%, more preferably 0.200%.

[0041] <Pb: 0.010 to 0.100%> Pb is an element effective in improving machinability. From the viewpoint of exerting such an effect of Pb, the lower limit value of the Pb content is preferably set to 0.010%, more preferably 0.020%. On the other hand, if the Pb content is too high, there is a concern of causing deterioration of hot workability such as liquation cracking based on grain boundary melting by lowering the melting point of the grain boundary and reducing the binding force of the grain boundary. Therefore, the upper limit value of the Pb content is preferably set to 0.100%, more preferably 0.090%.

[0042] <W: 0.010 to 0.500%> W has the effect of improving the high-temperature strength without impairing the ductility at room temperature. From the viewpoint of exerting such an effect of W, the lower limit value of the content of W is preferably set to 0.010%, more preferably 0.020%. On the other hand, if the content of W is too high, coarse eutectic carbides are generated, causing a decrease in ductility. Therefore, the upper limit value of the content of W is preferably set to 0.500%, more preferably 0.450%.

[0043] <Md 30 :-40.0~0℃> Md 30 represents the temperature (°C) at which 50% of the structure transforms into martensite when a strain of 0.30 is applied to the single-phase austenite (γ). Therefore, the higher the Md 30 , the more unstable the austenite is. Md 30 is represented by the following formula (1). Md 30 =551 - 462(C + N) - 9.2Si - 8.1Mn - 29(Ni + Cu) - 13.7Cr - 18.5Mo ··· (1) In the formula, the element symbols represent the contents (mass%) of the respective elements.

[0044] Md 30 If Md is too low, the degree of stabilization of the austenite phase increases, making it difficult to transform the austenite phase into a strain-induced martensite phase by cold rolling, so that high strength cannot be sufficiently achieved. Therefore, the lower limit value of Md 30 is set to -40.0 °C, preferably -39.0 °C, more preferably -38.0 °C. On the other hand, if Md 30 is too high, the austenite phase becomes unstable, and the amount of the strain-induced martensite phase transformed by cold rolling increases, resulting in a decrease in ductility. Therefore, the upper limit value of Md 30 is set to 0 °C, preferably -3.0 °C, more preferably -5.0 °C.

[0045] The austenitic stainless steel material according to the embodiment of the present invention has a metal structure including a strain-induced martensite phase. If the amount of the strain-induced martensite phase is too small, the strength of the austenitic stainless steel material will decrease. Therefore, the lower limit value of the content of the strain-induced martensite phase is set to 25% by volume, preferably 26% by volume. On the other hand, if the amount of the strain-induced martensite phase is too large, the properties such as the ductility of the austenitic stainless steel material will decrease. Therefore, the upper limit value of the content of the strain-induced martensite phase is set to 35% by volume, preferably 34% by volume. Here, the content of the strain-induced martensite phase can be measured using a method known in the art. For example, it can be measured using a ferrite scope or the like.

[0046] The austenitic stainless steel material according to an embodiment of the present invention has a tensile strength (TS) of 1450 MPa or more, preferably 1460 MPa or more, more preferably 1470 MPa or more. By controlling the tensile strength within such a range, the strength of the austenitic stainless steel material can be ensured. Note that the upper limit value of the tensile strength is not particularly limited, but is generally 2500 MPa, preferably 2300 MPa, more preferably 2000 MPa. Here, the tensile strength of the austenitic stainless steel material can be measured in accordance with JIS Z2241:2011.

[0047] The austenitic stainless steel material according to an embodiment of the present invention has an elongation at break (EL) of 12.0% or more, preferably 13.0% or more, more preferably 14.0% or more. By controlling the elongation at break within such a range, the ductility of the austenitic stainless steel material can be ensured. Note that the upper limit value of the elongation at break is not particularly limited, but is generally 50.0%, preferably 40.0%, more preferably 30.0%. Here, the elongation at break of the austenitic stainless steel material can be measured in accordance with JIS Z2241:2011.

[0048] The austenitic stainless steel material according to an embodiment of the present invention has a tensile strength (TS) × elongation at break (EL) of 24,000 or more, preferably 24,100 or more, more preferably 24,200 or more. By controlling TS×EL within such a range, it is possible to secure a balance between the strength and ductility of the austenitic stainless steel material. Note that the upper limit value of TS×EL is not particularly limited, but is generally 50,000, preferably 45,000, more preferably 40,000.

[0049] The austenitic stainless steel material according to an embodiment of the present invention has a Vickers hardness of preferably 350 HV or more, more preferably 400 HV or more. By controlling the Vickers hardness within such a range, it is possible to secure the strength of the austenitic stainless steel material. Note that the upper limit value of the Vickers hardness is not particularly limited, but is generally 650 HV, preferably 600 HV.

[0050] The austenitic stainless steel material according to an embodiment of the present invention has a stress relaxation rate represented by the following formula (2) of 1.20% or less, preferably 1.19% or less, more preferably 1.18% or less. Stress relaxation rate = (σ1 - σ2) / σ1 ··· (2) In the formula, σ1 is the stress less than the 0.2% proof stress, and σ2 is the stress 200 seconds after applying the stress of σ1. By controlling the stress relaxation rate within the above range, it is possible to secure the sag resistance of the austenitic stainless steel material. Note that the lower limit value of the stress relaxation rate is not particularly limited, but is generally 0%, preferably 0.10%, more preferably 0.20%. Here, the 0.2% proof stress of the austenitic stainless steel material can be measured in accordance with JIS Z2241:2011.

[0051] The thickness of the austenitic stainless steel material according to the embodiment of the present invention is not particularly limited, but is preferably 0.20 mm or less, more preferably 0.15 mm or less, and still more preferably 0.10 mm or less. By controlling the thickness to such a value, it is possible to reduce the thickness and weight of various components. The lower limit of the thickness may be adjusted according to the application and is not particularly limited, but is generally 0.01 mm or more.

[0052] The austenitic stainless steel material according to the embodiment of the present invention can be manufactured by solution heat-treating a rolled material having the above composition, then cold-rolling it, and then performing a heat treatment. The rolled material is not particularly limited as long as it has the above composition, and a material manufactured by a method known in the art can be used. As the rolled material, a hot-rolled material or a cold-rolled material can be used, but a cold-rolled material having a small thickness is preferable. The hot-rolled material can be manufactured by melting and forging or casting a stainless steel having the above composition and then hot-rolling it. The cold-rolled material can be manufactured by cold-rolling the hot-rolled material. After each rolling, annealing, pickling, etc. may be appropriately performed as necessary.

[0053] The conditions for the solution heat treatment (solution treatment) of the rolled material are not particularly limited and may be appropriately set according to the composition of the rolled material. For example, the solution heat treatment can be performed by heating the rolled material to 1000 to 1200 °C, holding it, and then rapidly cooling it.

[0054] The cold rolling after the solution heat treatment is performed at a rolling ratio sufficient to generate a 25 to 35% by volume of work-induced martensite phase. By performing cold rolling, processing strain is generated in the rolled material, and a part of the austenite phase can be transformed into a work-induced martensite phase. Also, by performing cold rolling at the above rolling ratio, an austenitic stainless steel material having a good balance between strength and ductility can be obtained.

[0055] The heat treatment after cold rolling is carried out for the purpose of diffusing and dissolving C and N dissolved in the work-induced martensite phase into the austenite phase. The crystal structure of the work-induced martensite phase is a body-centered cubic structure, while the crystal structure of the austenite phase is a face-centered cubic structure. The solubility limit of C and N in the face-centered cubic structure is higher than that in the body-centered cubic structure. Since the work-induced martensite phase is a phase formed by transformation from the structure that was the austenite phase by cold rolling, it has a body-centered cubic structure but is in a state where C and N are supersaturated and dissolved. In such a state, the ductility of austenitic stainless steel materials does not improve sufficiently. Therefore, by performing heat treatment after cold rolling, C and N supersaturated and dissolved in the work-induced martensite phase are diffused and dissolved into the austenite phase with a high solubility limit. Since C and N are austenite stabilizing elements, the degree of stabilization of the austenite phase increases by diffusing and dissolving into the austenite phase, making it possible to achieve both high strength and high ductility by promoting the TRIP (Transformation Induced Plasticity) effect.

[0056] Also, the heat treatment after cold rolling contributes to improving the sag resistance. Sag is caused by the strain introduced into the rolled material by cold rolling and the like, but by performing heat treatment after cold rolling, the strain is reduced, so the sag resistance can be improved.

[0057] In order to obtain the above effects, the heat treatment after cold rolling is carried out at a temperature of 100 to 200 °C and under the condition that the value of P represented by the following formula (3) satisfies 7000 to 9400. The temperature is preferably 110 to 190 °C, more preferably 120 to 180 °C. Also, the value of P is preferably 7200 to 9300, more preferably 7400 to 9000. P = T(log t + 20) ··· (3) In the formula, T is the temperature (K) and t is the time (h). By performing heat treatment under the above conditions, it is possible to improve the sag resistance while achieving both high strength and high ductility. When the heat treatment temperature exceeds 200°C and the value of P exceeds 9400, precipitates are generated in the processing-induced martensite during heat treatment. As a result, high strength can be achieved, but the ductility is significantly reduced. Also, when the heat treatment temperature is less than 100°C and the value of P is less than 7000, C and N that are supersaturated and dissolved in the processing-induced martensite phase cannot be sufficiently diffused and dissolved into the austenite phase.

[0058] The austenitic stainless steel material according to an embodiment of the present invention has high strength and high ductility and is excellent in sag resistance. Therefore, it can be used for various parts that require thin-wall and weight reduction, such as structural parts and functional parts in communication devices such as smartphones and precision devices such as personal computers. In particular, the austenitic stainless steel material according to an embodiment of the present invention is suitable for use in leaf springs.

Examples

[0059] Hereinafter, the content of the present invention will be described in detail with reference to examples, but the present invention is not construed as being limited thereto.

[0060] 30 kg of stainless steel having the composition shown in Table 1 was melted by vacuum melting, forged into a plate with a thickness of 30 mm, heated at 1230°C for 2 hours, and hot-rolled to a thickness of 4 mm to obtain a hot-rolled material. Next, the hot-rolled plate was annealed and pickled to obtain a hot-rolled annealed plate. Then, cold rolling and annealing were repeated on the hot-rolled annealed plate to reduce the thickness, and finally cold-rolled to a thickness of 0.2 to 1 mm to obtain a cold-rolled material.

[0061]

Table 1

[0062] Next, the cold-rolled material obtained above was subjected to a solution treatment in which it was held at 1050°C for 10 minutes and then rapidly cooled. Next, it was cold-rolled at the rolling ratios shown in Table 2, and then heat-treated under the conditions shown in Table 2 to obtain an austenitic stainless steel material. For Test Nos. 2 and 5, cold rolling was used as the finishing process and heat treatment was not performed. The following evaluations were performed on the austenitic stainless steel material thus obtained.

[0063] (Amount of strain-induced martensite phase) Test pieces were cut out from the austenitic stainless steel material, and the amount of strain-induced martensite was measured using a ferrite scope (FERITESCOPE MP30E-S manufactured by Fischer). The measurement was carried out at three arbitrary locations on the surface of the test piece, and the average value was taken as the result. In Table 2, the amount of strain-induced martensite phase is expressed as the "amount of M phase".

[0064] (0.2% proof stress, tensile strength (TS), and elongation at break (EL)) JIS 13B test pieces were cut out from the austenitic stainless steel material, and measurements were carried out in accordance with JIS Z2241:2011 using these test pieces.

[0065] (Vickers hardness) Test pieces were cut out from the austenitic stainless steel material, and the Vickers hardness was determined in accordance with JIS Z2244:2009 using a Vickers hardness tester. The test force was 294.2 N. The Vickers hardness was determined at five arbitrary locations, and the average value was taken as the result. In Table 2, the Vickers hardness is abbreviated as "hardness".

[0066] (Stress relaxation rate) The stress relaxation rate was determined based on the above formula (2). σ1 was set to 300 MPa. The tensile speed until σ1 reached 300 MPa was set to 0.5 mm / second.

[0067] The above evaluation results are shown in Table 2.

[0068]

Table 2

[0069] As shown in Table 2, the austenitic stainless steel materials (Examples of the present invention) in Test Nos. 3 to 4, 8 to 12, and 15 all had good results in terms of tensile strength (TS), elongation at break (EL), TS×EL, and stress relaxation rate, and were confirmed to have high strength, high ductility, and excellent sag resistance. On the other hand, the austenitic stainless steel materials (Comparative Examples) in Test Nos. 1 and 2 had insufficient tensile strength (TS) because the amount of the strain-induced martensite phase was too small. In addition, the austenitic stainless steel material in Test No. 2 had a high stress relaxation rate because heat treatment was not performed after cold rolling. The austenitic stainless steel material (Comparative Example) in Test No. 5 had a low TS×EL because heat treatment was not performed after cold rolling. The austenitic stainless steel materials (Comparative Examples) in Test Nos. 6 and 7 had a decreased elongation at break (EL) and a lowered TS×EL because the amount of the strain-induced martensite phase was too large. The austenitic stainless steel materials (Comparative Examples) in Test Nos. 13 and 14 did not have an appropriate composition and the amount of the strain-induced martensite phase was also outside the range, resulting in a decreased elongation at break (EL) and TS×EL. The austenitic stainless steel materials (Comparative Examples) in Test Nos. 16 to 18 had a decreased elongation at break (EL) and TS×EL because the P value and temperature of the heat treatment were too high.

[0070] As can be seen from the above results, according to the present invention, it is possible to provide an austenitic stainless steel material having high strength, high ductility, and excellent sag resistance, and a method for manufacturing the same. In addition, according to the present invention, it is possible to provide a leaf spring having high strength, excellent dimensional accuracy, and a long service life.

Claims

1. By mass, C: 0.200% or less, Si: 1.00 - 3.50%, Mn: 5.00% or less, Ni: 4.00 - 10.00%, Cr: 12.00 - 18.00%, Cu: 3.500% or less, Mo: 1.00 - 5.00%, N: 0.200% or less, the total amount of C and N is 0.100% or more, the balance consists of Fe and impurities, and the following formula (1): Md 30 = 551 - 462(C + N) - 9.2Si - 8.1Mn - 29(Ni + Cu) - 13.7Cr - 18.5Mo... (1) Md represented by (wherein the element symbols represent the content (% by mass) of each element) 30 having a composition in which the value of is from -40.0 to 0 °C, It has a metallographic structure containing 25 - 35% by volume of work-induced martensite phase, The tensile strength (TS) is 1450 MPa or more, the elongation at break (EL) is 12.0% or more, TS×EL is 24000 or more, and the following formula (2): Stress relaxation rate = (σ1 - σ2) / σ1... (2) (wherein σ1 is the stress less than the 0.2% proof stress, and σ2 is the stress 200 seconds after applying the stress of σ1) and the stress relaxation rate shown by this is 1.20% or less, an austenitic stainless steel sheet.

2. By mass, it further contains one or more selected from Al: 0.100% or less, O: 0.010% or less, V: 0.0001 - 0.500%, B: 0.0001 - 0.015%, the austenitic stainless steel sheet according to Claim 1.

3. By mass, it further contains one or more selected from Ti: 0.010 - 0.500%, Co: 0.010 - 0.500%, Zr: 0.010 - 0.100%, Nb: 0.010 - 0.100%, Mg: 0.0005 - 0.0030%, Ca: 0.0003 - 0.0030%, Y: 0.010 - 0.200%, Ln: 0.001 - 0.100%, Sn: 0.001 - 0.500%, Sb: 0.001 - 0.500%, Pb: 0.010 - 0.100%, W: 0.010 - 0.500%, the austenitic stainless steel sheet according to Claim 1 or 2.

4. The austenitic stainless steel sheet according to any one of Claims 1 to 3, having a thickness of 0.20 mm or less.

5. The austenitic stainless steel sheet according to any one of Claims 1 to 4, which is used for a leaf spring.

6. By mass, C: 0.200% or less, Si: 1.00 - 3.50%, Mn: 5.00% or less, Ni: 4.00 - 10.00%, Cr: 12.00 - 18.00%, Cu: 3.500% or less, Mo: 1.00 - 5.00%, N: 0.200% or less, the total amount of C and N is 0.100% or more, the balance consists of Fe and impurities, and the following formula (1): Md 30 = 551 - 462(C + N) - 9.2Si - 8.1Mn - 29(Ni + Cu) - 13.7Cr - 18.5Mo... (1) Md represented by (where the element symbols represent the content (% by mass) of each element) 30 After solution-treating a rolled material having a composition in which the value of is from -40.0 to 0 °C, cold rolling is performed at a rolling reduction sufficient to produce a 25 to 35% by volume of work-induced martensite phase, and then at a temperature of 100 to 200 °C, the following formula (3): P = T(log t + 20) ... (3) A method for manufacturing an austenitic stainless steel material, which performs a heat treatment such that the value of P represented by (where T is the temperature (K) and t is the time (h)) satisfies 7000 to 9400.

7. The method for manufacturing an austenitic stainless steel material according to claim 6, wherein the rolled material further contains one or more selected from Al: 0.100% or less, O: 0.010% or less, V: 0.0001 to 0.500%, and B: 0.0001 to 0.015% by mass basis.

8. The method for manufacturing an austenitic stainless steel material according to claim 6 or 7, wherein the rolled material further contains one or more selected from Ti: 0.010 to 0.500%, Co: 0.010 to 0.500%, Zr: 0.010 to 0.100%, Nb: 0.010 to 0.100%, Mg: 0.0005 to 0.0030%, Ca: 0.0003 to 0.0030%, Y: 0.010 to 0.200%, Ln: 0.001 to 0.100%, Sn: 0.001 to 0.500%, Sb: 0.001 to 0.500%, Pb: 0.010 to 0.100%, and W: 0.010 to 0.500% by mass basis.

9. A leaf spring including the austenitic stainless steel material according to any one of claims 1 to 5.

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