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

Abstract

Provided is an austenitic stainless steel material having a composition comprising, on a mass basis, 0.200% or less C, 4.00% or less Si, 5.00% or less Mn, 4.00-10.00% Ni, 12.00-18.00% Cr, 3.50% or less Cu, 1.00-5.00% Mo, and 0.200% or less N, with the total amount of C and N being 0.100% or greater, and the remainder comprising Fe and impurities, the value of Md30 indicated in equation (1) (1): Md30 = 551 - 462(C+N) - 9.2Si - 8.1Mn - 29(Ni + Cu) - 13.7Cr - 18.5Mo (where the element symbols indicate the content (mass%) of each element) being from -40.0ºC to 0ºC. The austenitic stainless steel also has a metal structure in which the dislocation density of the processing-induced martensite phase is 8.0×1016 m-2 or less, the dislocation density of the residual austenite phase is from 1.5×1016 to 6.0×1016 m-2, and the content of the processing-induced martensite phase is 20-55 vol%.

Description

[Technical field]

[0001] The present invention relates to an austenitic stainless steel material, a manufacturing method thereof, and a leaf spring. [Background technology]

[0002] As communication devices such as smartphones and precision equipment such as personal computers become smaller and more powerful, the structural and functional parts used in these devices are becoming thinner and lighter. For this reason, materials used in these parts are required to have excellent workability (ductility) and high strength. In particular, parts such as leaf springs that are exposed to repeated stresses are required to have properties that allow them to withstand repeated stresses (sag resistance). Here, "sag resistance" refers to the property of being able to withstand "sag," which is the state in which a material does not completely return to its original shape due to minute deformation when used repeatedly under elastic stress.

[0003] As a material having both high strength and high ductility, Patent Document 1 describes a material containing, by mass%, 0.05-0.15% C, 0.05-1% Si, 2% or less Mn, 16-18% Cr, 4-11% Ni, 2.5-3.5% Mo, and one or two selected from the group consisting of 0.1-3.5% Al and 0.1-3.5% Ti, with the balance being Fe and unavoidable impurities, having a two-phase structure of α' phase and γ phase, and having a YS (0.2% proof stress) of 1400-1900 N / mm 2 Therefore, a stainless steel material (metastable austenitic stainless steel strip or plate) that satisfies YS (0.2% yield strength) × EL (elongation) of 21,000 to 48,000 has been proposed.

[0004] Patent Document 2 proposes a stainless steel material (high-strength stainless steel with excellent toughness) containing, in mass%, C: 0.10% or less, Si: 1.0 to 3.0%, Mn: 2.0% or less, Ni: 4.0 to 9.0%, Cr: 12.0 to 18.0%, Mo: 1.0 to 5.0%, N: 0.15% or less, C and N so as to satisfy the relationship C+N≧0.10%, with the balance being Fe and unavoidable impurities. [Prior art documents]

Patent Document

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Although the stainless steel materials described in Patent Documents 1 and 2 have both high strength and high ductility, the sag resistance required for parts (especially leaf springs, etc.) used in communication equipment and precision equipment has not been studied.

[0007] 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, high ductility, and excellent sag resistance, and a method for producing 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

[0008] The present inventors have found that the above problems can be solved by controlling the composition and metal structure of the austenitic stainless steel material, and have completed the present invention.

[0009] That is, the present invention is An austenitic stainless steel material used for a leaf spring, Based on mass, containing C: 0.200% or less, Si: 4.00% or less, Mn: 5.00% or less, Ni: 4.00 to 10.00%, Cr: 12.00 to 18.00%, Cu: 3.50% 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 being 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) (where the element symbols represent the content (% by mass) of each element) Md 30 has a composition where the value is from -40.0 to 0 °C, the dislocation density of the strain-induced martensite phase is 8.0×10 16 m -2 or less, the dislocation density of the retained austenite phase is 1.5×10 16 to 6.0×10 16 m -2 and has a metal structure where the content of the strain-induced martensite phase is 20 to 55% by volume and having a thickness of 0.2 mm or less, is an austenitic stainless steel material.

[0010] Also, the present invention A method for manufacturing the austenitic stainless steel material, contains, by mass basis, C: 0.200% or less, Si: 4.00% or less, Mn: 5.00% or less, Ni: 4.00 to 10.00%, Cr: 12.00 to 18.00%, Cu: 3.50% 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 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) (where the element symbols represent the content (% by mass) of each element) Md 30 has a composition where the value is from -40.0 to 0 °C Cold After solution treating the rolled material, the dislocation density of the strain-induced martensite phase is 8.0×10 16 m -2 or less, the dislocation density of the retained austenite phase is 1.5×10 16 to 6.0×10 16 m -2 and cold rolling is performed at a rolling ratio such that the content of the strain-induced martensite phase becomes 20 to 55% by volume until the thickness becomes 0.2 mm or less, including the step of Side is a method.

[0011] Furthermore, the present invention is a leaf spring including the above-described austenitic stainless steel material.

Effects of the Invention

[0012] 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, the present invention can provide a leaf spring having high strength, excellent dimensional accuracy, and a long service life.

Modes for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be specifically described. It should be understood that the present invention is not limited to the following embodiments, and modifications and improvements can 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 also fall within the scope of the present invention. In addition, in this specification, the “%” indication regarding components means “mass %” unless otherwise specified.

[0014] The austenitic stainless steel material according to the embodiment of the present invention contains C: 0.200% or less, Si: 4.00% or less, Mn: 5.00% or less, Ni: 4.00 to 10.00%, Cr: 12.00 to 18.00%, Cu: 3.50% 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, "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, it may be various shaped steels such as T-shaped and I-shaped in cross-sectional shape. Further, "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 austenitic stainless steel materials, and are those allowed within a range that does not adversely affect the present invention. For example, inevitable impurities that are difficult to remove such as P and S are also included in these impurities.

[0015] In addition, the austenitic stainless steel material according to an embodiment of the present invention can 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 0.0150% as required. Furthermore, the austenitic stainless steel material according to an embodiment of the present invention can further contain one or more selected from Ti: 0.0001 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% as required. Hereinafter, each component will be described in detail.

[0016] <C: 0.200% or less> C is an interstitial element and contributes to the hardening by work hardening and heat treatment. Also, C is an element that stabilizes the austenite phase and is effective in 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%, and still more preferably 0.020%.

[0017] <Si: 4.00% or less> 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. However, 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 content of Si is too high, it will cause a reduction in cold workability. Therefore, the upper limit value of the content of Si is set to 4.00%, preferably 3.50%, more preferably 3.00%. On the other hand, the lower limit value of the content of Si is not particularly limited, but it is preferably set to 1.00%, more preferably 1.20%, and still more preferably 1.50%.

[0018] <Mn: 5.00% or less> Mn is an element that forms oxide 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 content of Mn is set to 5.00%, preferably 4.00%, more preferably 3.00%. On the other hand, the lower limit value of the content of Mn is not particularly limited, but it is preferably set to 0.01%, more preferably 0.05%, and still more preferably 0.10%.

[0019] <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 a metastable austenite phase is formed at room temperature and a martensite phase is induced during cold rolling. If the content of Ni is too low, a δ-ferrite phase is generated at high temperature and a 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 content of Ni is set to 4.00%, preferably 4.50%, more preferably 5.00%. On the other hand, if the content of Ni is too high, it becomes difficult to induce a martensite phase during cold rolling. Therefore, the upper limit value of the content of Ni is set to 10.00%, preferably 9.50%, more preferably 9.00%.

[0020] <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 content of Cr is set to 12.00%, preferably 12.50%, more preferably 13.00%. On the other hand, if the content of Cr is too high, the cold workability deteriorates. Therefore, the upper limit value of the content of Cr is set to 18.00%, preferably 17.50%, more preferably 17.00%.

[0021] <Cu: 3.50% or less> Cu is an element that has the effect of hardening stainless steel during heat treatment. However, if the content of Cu is too high, the hot workability deteriorates and causes cracking. Therefore, the upper limit value of the content of Cu is set to 3.50%, preferably 3.00%, more preferably 2.00%. On the other hand, the lower limit value of the content of Cu is not particularly limited, but is preferably 0.01%, more preferably 0.02%, still more preferably 0.03%.

[0022] <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. In recent years, considering the use in structural parts and functional parts (especially leaf springs) where improvement in corrosion resistance and sag resistance is required, 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 leads to an increase in manufacturing cost. Also, the δ-ferrite phase and α-ferrite phase are 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%.

[0023] <N: 0.200% or less> N is an austenite-forming element. Also, N is an element extremely effective for hardening the austenite phase and martensite phase. However, if the N content is too high, it causes blowholes during casting. Therefore, the upper limit value of the N content is set to 0.200%, preferably 0.150%, more preferably 0.100%. On the other hand, the lower limit value of the N content is not particularly limited, but is preferably 0.001%, preferably 0.010%.

[0024] <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 sufficiently 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%.

[0025] <Al: 0.100% or less> Al has a higher oxygen affinity than Si and Mn. If the Al content is too high, it is likely to form coarse oxide inclusions that serve as 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%, and even more preferably 0.030%. On the other hand, the lower limit value of the Al content is not particularly limited, but if the Al content is excessively low, it will lead to an increase in manufacturing cost. Therefore, it is preferably set to 0.0001%, more preferably 0.0003%, and still more preferably 0.0005%.

[0026] <O: 0.010% or less> If the O content is too high, it is likely to form coarse inclusions with a particle size exceeding 5 μm. Therefore, the upper limit value of the O content is preferably set to 0.010% and preferably 0.008%. On the other hand, the lower limit value of the O content is not particularly limited, but if the O content is too low, it becomes difficult to oxidize Mn, Si, etc., and the ratio of Al2O3 in the inclusions increases. Therefore, the lower limit value of the O content is preferably set to 0.001% and more preferably 0.003%.

[0027] <V: 0.0001 - 0.500%> V is an element that has the effect of enhancing the age hardening property during the heating of the heat treatment performed after cold rolling. From the perspective of obtaining this effect sufficiently, the lower limit value of the V content is preferably set to 0.0001% and more preferably 0.0010%. On the other hand, if the V content is too high, it will lead to an increase in manufacturing cost. Therefore, the upper limit value of the V content is preferably set to 0.500%, more preferably 0.400%, and still more preferably 0.300%.

[0028] <B: 0.0001 - 0.0150%> If the B content is too high, it will cause a factor for the deterioration of workability due to the formation of borides. Therefore, the upper limit value of the B content is preferably set to 0.0150% and more preferably 0.0100%. On the other hand, the lower limit value of the B content is not particularly limited, but it is preferably set to 0.0001% and more preferably 0.0002%.

[0029] <Ti: 0.0001~0.500%> Ti is an element that forms carbonitrides, 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 Ti content is preferably set to 0.0001%, more preferably 0.001%. On the other hand, if the Ti content is too high, the amount of C and N in solid solution will decrease, and carbides will precipitate unevenly in non-uniform sizes and localize, which may inhibit the recrystallized grain growth. Also, since Ti is expensive, it leads to an increase in manufacturing cost. Therefore, the upper limit value of the Ti content is preferably set to 0.500%, more preferably 0.400%, and even more preferably 0.300%.

[0030] <Co: 0.010~0.500%> Co is an element that improves the resistance to crevice corrosion. From the viewpoint of exerting such an effect, the lower limit value of the Co content is preferably set to 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 to 0.500%, more preferably 0.100%.

[0031] <Zr: 0.010~0.100%> Zr is an element with a high affinity for C and N, and it 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 the workability. From the viewpoint of exerting such an effect, the lower limit value of the Zr content is preferably set to 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 to 0.100%, more preferably 0.050%.

[0032] <Nb: 0.010~0.100%> Nb is an element with a high affinity for C and N. It precipitates as carbide or nitride during hot rolling, reducing the dissolved C and N in the matrix phase and having the effect of improving workability. From the perspective of exerting such an effect, the lower limit 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 harden and the ductility will decrease. Therefore, the upper limit of the Nb content is preferably set at 0.100%, more preferably 0.050%.

[0033] <Mg: 0.0005~0.0030%> Mg forms Mg oxide together with Al in the molten steel and acts as a deoxidizer. From the perspective of exerting such an effect, the lower limit of the Mg content is preferably set at 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 will decrease. Therefore, the upper limit of the Mg content is preferably set at 0.0030%, more preferably 0.0020%.

[0034] <Ca: 0.0003~0.0030%> Ca is an element that improves hot workability. From the perspective of exerting the effect of Ca, the lower limit of the Ca content is preferably set at 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 will decrease. Therefore, the upper limit of the Ca content is preferably set at 0.0030%, more preferably 0.0020%.

[0035] <Y: 0.010~0.200%> Y is an element that reduces the viscosity of the molten steel and improves cleanliness. From the perspective of exerting the effect of Y, the lower limit of the Y content is preferably set at 0.010%, more preferably 0.020%. On the other hand, if the Y content is too high, the effect of Y will saturate and the workability will decrease. Therefore, the upper limit of the Y content is preferably set at 0.200%, more preferably 0.100%.

[0036] <Ln: 0.001 to 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 perspective of exerting the effect of such Ln, the lower limit value 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 saturates, and surface defects occur during hot rolling, resulting in a decrease in manufacturability. Therefore, the upper limit value of the Ln content is preferably set to 0.100%, more preferably 0.050%.

[0037] <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 perspective of exerting the effect of such 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%.

[0038] <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 perspective of exerting the effect of such 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%.

[0039] <Pb: 0.010 to 0.100%> Pb is an element effective in improving machinability. From the viewpoint of exerting the effect of such 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 lowering the melting point of grain boundaries and reducing the bonding strength of grain boundaries, leading to liquation cracking based on grain boundary melting. Therefore, the upper limit value of the Pb content is preferably set to 0.100%, more preferably 0.090%.

[0040] <W: 0.010~0.500%> W has the effect of improving high-temperature strength without impairing ductility at room temperature. From the viewpoint of exerting the effect of such W, the lower limit value of the W content is preferably set to 0.010%, more preferably 0.020%. On the other hand, if the W content is too high, coarse eutectic carbides are generated, causing a decrease in ductility. Therefore, the upper limit value of the W content is preferably set to 0.500%, more preferably 0.450%.

[0041] <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 is (the higher the temperature), 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 content (% by mass) of each element.

[0042] Md 30 If it is too low, the stability of the austenite phase increases, making it difficult to transform the austenite phase into a strain-induced martensite phase by cold rolling, and it is impossible to sufficiently increase the strength. Therefore, Md 30The lower limit value is set to -40.0°C. On the other hand, when Md 30 is too high, the austenite phase becomes unstable, and the amount of the strain-induced martensite phase transformed by cold rolling increases, and it becomes difficult to control the dislocation density of each phase described below within a desired range, so that desired ductility and dent resistance cannot be obtained. Therefore, the upper limit value of Md 30 is set to 0°C.

[0043] The austenitic stainless steel material according to the embodiment of the present invention has a dislocation density of the strain-induced martensite phase of 8.0×10 16 m -2 or less, preferably 7.5×10 16 m -2 or less, and a dislocation density of the retained austenite phase of 1.5×10 16 ~6.0×10 16 m -2 , preferably 2.0×10 16 ~5.5×10 16 m -2 . When the dislocation density of the retained austenite phase is less than 1.5×10 16 m -2 , the dislocation movement in the phase cannot be sufficiently suppressed. Therefore, the stress relaxation rate decreases, and desired dent resistance cannot be obtained. Also, when the dislocation density of the strain-induced martensite phase exceeds 8.0×10 16 m -2 , and when the dislocation density of the retained austenite phase exceeds 6.0×10 16 m -2 , the strength of the austenitic stainless steel material becomes too high, and thus desired ductility cannot be obtained. The lower limit value of the dislocation density of the strain-induced martensite phase is not particularly limited and may be 0m -2 . Here, the dislocation density is the total length of dislocations contained in a unit volume of crystal. Usually, when cold rolling is performed, a part of the moved dislocations accumulates in the material, so the dislocation density increases. The dislocations thus accumulated interact with subsequent dislocations to inhibit the movement of dislocations. Therefore, as the dislocation density increases, the strength and dent resistance improve.

[0044] In the metallographic structure of the austenitic stainless steel material according to an embodiment of the present invention, the content of the strain-induced martensite phase is 20 to 55% by volume, preferably 25 to 50% by volume. When the content of the strain-induced martensite phase is less than 20% by volume, the strength and sag resistance of the austenitic stainless steel material decrease. Further, when the content of the strain-induced martensite phase exceeds 55% by volume, the ductility of the austenitic stainless steel material decreases. Here, the content of the strain-induced martensite phase can be measured using a method known in the art. For example, it may be measured using a ferrite scope or the like.

[0045] The austenitic stainless steel material according to an embodiment of the present invention has a tensile strength (TS) of preferably 1400 MPa or more, more preferably 1450 MPa or more. By controlling the tensile strength within such a range, the strength of the austenitic stainless steel material can be ensured. The upper limit value of the tensile strength is not particularly limited, but is generally 2100 MPa, preferably 2050 MPa. Here, the tensile strength of the austenitic stainless steel material can be measured in accordance with JIS Z2241:2011.

[0046] The austenitic stainless steel material according to an embodiment of the present invention has an elongation at break (EL) of preferably 2.0% or more, more preferably 3.0% or more. By controlling the elongation at break within such a range, the ductility of the austenitic stainless steel material can be ensured. The upper limit value of the elongation at break is not particularly limited, but is generally 20.0%, preferably 15.0%. Here, the elongation at break 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 a Vickers hardness of preferably 420 HV or more, more preferably 450 HV or more. By controlling the Vickers hardness within such a range, the strength of the austenitic stainless steel material can be ensured. The upper limit value of the Vickers hardness is not particularly limited, but is generally 600 HV, preferably 580 HV.

[0048] 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 preferably 1.00% or less, more preferably 0.90% or less, and still more preferably 0.80% 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 180 seconds after applying the stress of σ1. By controlling the stress relaxation rate within the above range, the sag resistance of the austenitic stainless steel material can be ensured. The lower limit value of the stress relaxation rate is not particularly limited, but is generally 0%, preferably 0.10%, and 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.

[0049] The thickness of the austenitic stainless steel material according to an embodiment of the present invention is not particularly limited, but is preferably 0.2 mm or less, more preferably 0.15 mm or less. By controlling the thickness to such a value, the weight reduction of various components can be achieved. The lower limit value of the thickness may be adjusted according to the application and is not particularly limited, but is generally 0.01 mm.

[0050] The manufacturing method of the austenitic stainless steel material according to an embodiment of the present invention is not particularly limited, but for example, it can be manufactured by solution heat-treating a rolled material having the above composition and then cold-rolling it. The rolled material is not particularly limited as long as it has the above composition, and those manufactured by using known methods in the relevant technical field can be used. As the rolled material, a hot-rolled material or a cold-rolled material can be used, but a cold-rolled material with a small thickness is preferred. The hot-rolled material can be manufactured by melting stainless steel having the above composition, forging or casting it, and then performing hot rolling and, if necessary, aging treatment. Further, the cold-rolled material can be manufactured by performing cold rolling on the hot-rolled material. Note that after each rolling, annealing, pickling, etc. may be appropriately performed as needed.

[0051] The conditions for the solution treatment (solutionizing 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 treatment can be performed by heating the rolled material to 1000 - 1200 °C, holding it, and then rapidly cooling.

[0052] The cold rolling after the solution treatment is performed to cause processing strain in the rolled material, transform a part of the austenite phase into a strain-induced martensite phase, and increase the dislocation density of the strain-induced martensite phase and the retained austenite phase. The rolling reduction of the cold rolling is such that the dislocation density of the strain-induced martensite phase is 8.0×10 16 m -2 Hereinafter, the dislocation density of the retained austenite phase is 1.5×10 16 ~6.0×10 16 m -2 and the content of the strain-induced martensite phase is 20 - 55 vol%. By performing cold rolling at such a rolling reduction, an austenitic stainless steel sheet having high strength, high ductility, and excellent sag resistance can be obtained. Note that the specific rolling reduction of the cold rolling may be appropriately adjusted according to the composition of the rolled material, etc., but generally it is 20 - 80%, preferably 25 - 70%.

[0053] After cold rolling, heat treatment can be performed as necessary. By performing heat treatment, it is possible to reduce the dislocation density of the strain-induced martensite phase increased by cold rolling, making it easier to control the dislocation density of the strain-induced martensite phase within a predetermined range. Specific conditions for the heat treatment may be appropriately adjusted according to the composition of the rolled material, etc., but the heating temperature is preferably 100 to 600 °C, more preferably 200 to 500 °C, and the heating time is preferably 0.5 to 5 hours, more preferably 1 to 3 hours.

[0054] The austenitic stainless steel material according to the 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 the embodiment of the present invention is suitable for use in leaf springs.

Examples

[0055] 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.

[0056] 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, and then cold rolling and annealing were repeated on the hot-rolled annealed plate to thin it and obtain a cold-rolled material.

[0057]

Table 1

[0058] 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, cold rolling was performed at the rolling ratios shown in Table 2 to finally obtain a thickness of 0.2 mm. Further, for Test Nos. 18 to 22, heat treatment was performed under the conditions shown in Table 2 after cold rolling. The following evaluations were performed on the austenitic stainless steel material obtained as described above.

[0059] (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 performed at three arbitrary locations on the surface of the test piece, and the average value was taken as the result. In Table 2, the strain-induced martensite phase is represented as "M phase".

[0060] (Dislocation density of strain-induced martensite phase and retained austenite phase) The dislocation density was calculated by performing line profile analysis on the shape of the diffraction peak measured by X-ray diffraction. In the structure with introduced dislocations, lattice strain occurs around the dislocations, and small-angle grain boundaries, cell structures, etc. develop due to the arrangement of the dislocations. By capturing these with X-rays, the dislocation density can be calculated. The test pieces were cut out from the austenitic stainless steel material at arbitrary positions and were those subjected to mechanical polishing and chemical polishing. X-ray diffraction was performed on the surface structure of this test piece, and the dislocation density ρ was calculated from the single diffraction peaks of {111} of the retained austenite phase and {110} of the strain-induced martensite phase. The following formula (3) was used to calculate the dislocation density ρ. ρ = [3(2π) 1 / 2 <ε 2 > 1 / 2 / Db ··· (3) In the formula, <ε 2 > represents the root mean square strain, D represents the crystallite size, and b represents the Burgers vector. Also, the root mean square strain <ε used in formula (3) 2> and the crystallite size D was determined from the following formula (4). -lnA(L) / L = 1 / D + [-1 / (2D 2 ) + 2π 2 <ε 2 >h0 2 / a 2 L ··· (4) In the formula, lnA(L) is the logarithm of the Fourier coefficient of the line profile of each diffraction peak, L is the Fourier length, h0 2 =h 2 +k 2 +l 2 (h, k, l are the plane indices of the diffraction peak used), and a is the lattice constant. With the vertical axis being -lnA(L) / L and the horizontal axis being L, D was obtained from the y-intercept 1 / D of the plot, and <ε 2 ) + 2π 2 <ε 2 >h0 2 / a 2 was used to obtain <ε 2 . Also, an X-ray diffractometer (manufactured by Rigaku Corporation) was used as the analyzer, and a Cu dry bulb was used as the target. In Table 2, the retained austenite phase is represented as the γ phase.

[0061] (0.2% proof stress, tensile strength (TS), and elongation at break (EL)) A JIS 13B test piece was cut out from the austenitic stainless steel material, and measurements were carried out in accordance with JIS Z2241:2011 using this test piece.

[0062] (Vickers hardness) A test piece was 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 any 5 locations, and the average value was taken as the result. In Table 2, the Vickers hardness is abbreviated as "hardness".

[0063] (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.

[0064] The above evaluation results are shown in Table 2.

[0065]

Table 2

[0066] As shown in Table 2, it was confirmed that the austenitic stainless steel materials (Examples of the present invention) of Test Nos. 2 to 7, 9 to 15, and 19 to 21 are excellent in tensile strength (TS), elongation at break (EL), and stress relaxation rate. On the other hand, for the austenitic stainless steel material (Comparative Example) of Test No. 1, since the amount of the strain-induced martensite phase and the dislocation density of the retained austenite phase were too low, the tensile strength (TS) and the stress relaxation rate were insufficient. For the austenitic stainless steel material (Comparative Example) of Test No. 8, since the amount of the strain-induced martensite phase was too large, the elongation (EL) was insufficient. For the austenitic stainless steel material (Comparative Example) of Test No. 16, since it was a steel type with a low Md 30 and high stability, the amount of the strain-induced martensite phase was small, and the tensile strength (TS) and the stress relaxation rate were insufficient. For the austenitic stainless steel material (Comparative Example) of Test No. 17, since it was a steel type with a high Md 30 and low stability, the amount of the strain-induced martensite phase was large, and the elongation (EL) was insufficient. For the austenitic stainless steel material (Comparative Example) of Test No. 18, since the dislocation density of the retained austenite phase was too low, the tensile strength (TS) and the stress relaxation rate were insufficient. For the austenitic stainless steel material (Comparative Example) of Test No. 22, since the amount of the strain-induced martensite phase and the dislocation density of the retained austenite phase were too low, the tensile strength (TS) and the stress relaxation rate were insufficient. This is considered to be because annealing and reverse transformation occurred due to too high heat treatment temperature.

[0067] 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. Further, according to the present invention, it is possible to provide a leaf spring having high strength, excellent dimensional accuracy, and a long life.

Claims

Claim 1: An austenitic stainless steel material used for a leaf spring, comprising: by mass, C: 0.200% or less, Si: 4.00% or less, Mn: 5.00% or less, Ni: 4.00 - 10.00%, Cr: 12.00 - 18.00%, Cu: 3.50% or less, Mo: 1.00 - 5.00%, N: 0.200% or less, with the total amount of C and N being 0.100% or more, the balance being 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, The dislocation density of the strain-induced martensite phase is 8.0×10 16 m -2 or less, the dislocation density of the retained austenite phase is 1.5×10 16 to 6.0×10 16 m -2 , and has a metal structure in which the content of the strain-induced martensite phase is 20 to 55% by volume. An austenitic stainless steel material with a thickness of 0.2 mm or less.

2. The austenitic stainless steel material according to Claim 1, further comprising one or more selected from the group consisting of, by mass, Al: 0.100% or less, O: 0.010% or less, V: 0.0001 - 0.500%, and B: 0.0001 - 0.0150%.

3. The austenitic stainless steel material according to Claim 1 or 2, further comprising one or more selected from the group consisting of, by mass, Ti: 0.0001 - 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%, and W: 0.010 - 0.500%.

4. The austenitic stainless steel material according to any one of Claims 1 to 3, having a tensile strength (TS) of 1400 MPa or more and an elongation at break (EL) of 2.0% or more.

5. The austenitic stainless steel material according to any one of Claims 1 to 4, having a stress relaxation rate represented by the following formula (2) of 1.00% 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 180 seconds after applying the stress of σ1.

6. A method for manufacturing the austenitic stainless steel material according to Claim 1, comprising: by mass, C: 0.200% or less, Si: 4.00% or less, Mn: 5.00% or less, Ni: 4.00 - 10.00%, Cr: 12.00 - 18.00%, Cu: 3.50% or less, Mo: 1.00 - 5.00%, N: 0.200% or less, with the total amount of C and N being 0.100% or more, the balance being 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 (mass %) of each element) 30 After solution-treating a cold-rolled material having a composition in which the value of is from -40.0 to 0 °C, the dislocation density of the strain-induced martensite phase is 8.0 × 10 16 m -2 or less, the dislocation density of the retained austenite phase is 1.5 × 10 16 to 6.0 × 10 16 m -2 and cold rolling to a thickness of 0.2 mm or less at a rolling ratio such that the content of the strain-induced martensite phase is 20 to 55% by volume.

7. The method according to claim 6, further comprising a step of performing heat treatment after the cold rolling.

8. A method for manufacturing an austenitic stainless steel material according to claim 2, The cold-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 0.0150% by mass. The method according to claim 6 or 7.

9. A method for manufacturing an austenitic stainless steel material according to claim 3, The cold-rolled material further contains one or more selected from Ti: 0.0001 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. The method according to any one of claims 6 to 8.

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

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