Ferritic-austenitic duplex stainless steel material

Abstract

Provided is a ferritic-austenitic duplex stainless steel material containing: 0.001-0.050 mass% of C; 0.01-0.50 mass% of Si; 1.0-4.5 mass% of Mn; at most 0.050 mass% of P; at most 0.030 mass% of S; 1.5-3.5 mass% of Ni; 19.624.0 mass% of Cr; 0.01-1.00 mass% of Mo; 0.01-1.20 mass% of Cu; and 0.010-0.090 mass% of N, with C+N being less than 0.130 mass%, and the remainder comprising Fe and impurities. The ferritic-austenitic duplex stainless steel material has an Md value, of 50.0-150.0 °C, represented by formula (1), and has a metal structure having 25-49 volume% of an austenitic phase. In addition, the Md value, of the austenite phase, represented by formula (1) is 35.0-100.0 °C.　Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo ... (1) In the formula, the element symbol represents the content (mass%) of each element.

Description

Ferrite-austenitic duplex stainless steel material

[0001] The present invention relates to a ferritic-austenitic duplex stainless steel material.

[0002] Ferrite-austenitic duplex stainless steel materials have excellent corrosion resistance and high strength, and are therefore used as building materials, structural materials, and the like. However, ferrite-austenitic duplex stainless steel materials have lower ductility than general-purpose austenitic stainless steel materials such as SUS304, limiting their application to applications requiring workability. Furthermore, from the perspective of cost reduction, there is also a need for lean (alloy-reduced) ferrite-austenitic duplex stainless steel materials that reduce alloying elements. Therefore, development of lean ferrite-austenitic duplex stainless steel materials with excellent ductility is underway.

[0003] For example, Patent Document 1 proposes a ferritic-austenitic duplex stainless steel material containing, in mass%, 0.05% or less C, 1% or less Si, 2 to 8% Mn, 0.1% or less P, 0.02% or less S, 15 to 23% Cr, 4% or less Mo, 3.0% or less Ni, 2% or less Cu, 0.05 to 0.3% N, and the balance being Fe and unavoidable impurities, in which the Cr equivalent and Ni equivalent satisfy a predetermined relationship. It is stated that the ductility of this duplex stainless steel material can be improved by optimizing the Cr equivalent and Ni equivalent. Patent Document 2 proposes a ferritic-austenitic duplex stainless steel material containing, by mass%, 0.08% or less C, 0.7 to 1.1% Si, 2.4 to 3.5% Mn, 17.9 to 20.7% Cr, 0.05 to 1.15% Ni, 0.18 to 0.3% N, and 0.4 to 2.8% Cu, with the remainder consisting of Fe and unavoidable impurities, and having a pitting potential predicted by a predetermined formula of 360 to 440 mV. It is stated that the ductility of this duplex stainless steel material can be improved by optimizing the contents of alloying elements such as Ni, Si, Mn, and Cu.

[0004] Japanese Patent Application Laid-Open No. 2012-126992

[0005] In both of the ferritic-austenitic duplex stainless steel materials described in Patent Documents 1 and 2, strength is increased by reducing the Ni content while increasing the Mn and N contents. However, increasing the N content can result in excessively high strength of the ferritic-austenitic duplex stainless steel material, which can reduce ductility. Therefore, an object of the present invention is to provide a ferritic-austenitic duplex stainless steel material that is softer and has higher ductility than conventional ferritic-austenitic duplex stainless steel materials.

[0006] The present inventors have conducted extensive research into lean ferritic-austenitic duplex stainless steel materials to solve the above-mentioned problems, and as a result, have discovered the following (1) to (3): (1) By reducing the C and N contents, it is possible to soften the austenite phase while maintaining the corrosion resistance of the duplex stainless steel material. (2) By controlling the Md content of the duplex stainless steel material and the austenite phase within a predetermined range, it is possible to increase the stability of the austenite phase and achieve high ductility through the TRIP (transformation-induced plasticity) effect. (3) By reducing the content of austenite-forming elements (e.g., C, N, Ni), it is possible to reduce alloying while suppressing an excessive TRIP effect and achieving softening. Based on these findings, the present inventors have discovered that the above-mentioned problems can be solved by controlling the austenite phase ratio and Md content of the ferritic-austenitic duplex stainless steel material, as well as the composition and Md content of the ferritic-austenitic duplex stainless steel material, and have completed the present invention.

[0007] That is, the present invention provides a steel sheet having a composition, on a mass basis, of C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 4.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.5%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, and N: 0.010 to 0.090%, in which C+N is less than 0.130%, and the balance is Fe and impurities, and is expressed by the following formula (1): Md=551−462(C+N)−9.2Si−8.1Mn−29(Ni+Cu)−13.7Cr−18.5Mo ... (1) (wherein the element symbols represent the content (mass%) of each element) is 50.0 to 150.0°C, and the austenite phase has a metallographic structure in which the austenite phase has 25 to 49 volume %; and the austenite phase has an Md value of 35.0 to 100.0°C, as represented by the above formula (1).

[0008] According to the present invention, it is possible to provide a ferritic-austenitic duplex stainless steel material that is softer and has higher ductility than conventional ferritic-austenitic duplex stainless steel materials.

[0009] Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements to the following embodiments, as appropriate, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the present invention, also fall within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0010] A ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention (hereinafter simply referred to as "duplex stainless steel material") contains C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 4.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.5%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, N: 0.010 to 0.090%, with C+N being less than 0.130%, and the balance being Fe and impurities.

[0011] Here, in this specification, "stainless steel material" refers to a material formed from stainless steel, and the shape of the material is not particularly limited. Examples of the shape include plate (including strip), rod, and tube. Various sectional shapes, such as T-shaped and I-shaped, are also acceptable. Furthermore, in this specification, "ferritic-austenitic" refers to a material whose metal structure at room temperature is primarily two-phase, consisting of ferrite and austenite. Therefore, "ferritic-austenitic" also encompasses materials containing small amounts of phases other than ferrite and austenite (e.g., martensite). Furthermore, in this specification, "impurities" refer to components that are mixed in during the industrial production of stainless steel materials due to various factors, such as raw materials such as ores and scrap, and manufacturing processes, and are acceptable within a range that does not adversely affect the present invention. For example, impurities include unavoidable impurities. An example of an impurity is O. The O content is, for example, 0.0001 to 0.0070%. Regarding the content of each element, "xx% or less" means that the content is xx% or less, but includes an amount exceeding 0% (particularly, exceeding the impurity level).

[0012] The duplex stainless steel material according to the embodiment of the present invention may further contain one or more selected from Nb: 0.010 to 0.500%, Ti: 0.01 to 0.50%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, and REM: 0.0002 to 0.0100%, as needed. Each component will be described in detail below.

[0013] <C: 0.001 to 0.050%> C is an element that significantly affects the stability of the austenite phase. If the C content is too high, ductility (workability) may decrease, or the precipitation of Cr carbides may be promoted, resulting in intergranular corrosion. Therefore, the C content is set to 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less, even more preferably 0.35% or less, and particularly preferably 0.030% or less. Furthermore, from the viewpoint of corrosion resistance, a low C content is preferable, but reducing the C content too much leads to increased costs. Therefore, the C content is set to 0.001% or more, preferably 0.002% or more, and more preferably 0.005% or more.

[0014] <Si: 0.01 to 0.50%> Si is added as a deoxidizing element and is also useful for improving oxidation resistance. However, if the Si content is too high, hardening occurs and ductility decreases. Therefore, the Si content is set to 0.50% or less, preferably less than 0.50%, more preferably 0.45% or less, and even more preferably 0.40% or less. Furthermore, excessive reduction of Si increases the cost of smelting. Therefore, the Si content is set to 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more.

[0015] <Mn: 1.0 to 4.5%> Mn is an element that plays an important role in concentrating in the austenite phase and stabilizing the austenite phase. However, if the Mn content is too high, not only ductility but also corrosion resistance and hot workability decrease. Therefore, the Mn content is set to 4.5% or less, preferably 4.0% or less, and more preferably 3.5% or less. Furthermore, excessive reduction of Mn increases the cost of smelting. Therefore, the Mn content is set to 1.0% or more, preferably 1.1% or more, and more preferably 1.2% or more.

[0016] <P: 0.050% or less> P is an element contained in raw materials such as Cr. If the P content is high, formability decreases, so the P content is set to 0.050% or less, preferably 0.045% or less, and more preferably 0.040% or less. On the other hand, a lower P content is preferable, but there is a limit to how much the P content can be reduced. The lower limit of the P content is generally 0.001%, preferably 0.002%, and more preferably 0.003%.

[0017] <S: 0.030% or less> S is an element contained in various raw materials. S combines with Mn to form inclusions, which can become the starting point for rust, so the lower the S content, the better the corrosion resistance. Therefore, the S content is set to 0.030% or less, preferably 0.025% or less, and more preferably 0.020% or less. On the other hand, there is a limit to how much the S content can be reduced. The lower limit of the S content is generally 0.0001%, preferably 0.0005%.

[0018] <Ni: 1.5 to 3.5%> Ni is an austenite-forming element and is an important element for adjusting the stability of the austenite phase. Ni also has the effect of suppressing nitride precipitation and improving corrosion resistance. To achieve these effects, the Ni content is set to 1.5% or more, preferably 1.6% or more, more preferably 1.7% or more, and even more preferably 1.8% or more. On the other hand, if the Ni content is too high, not only will the raw material cost increase, but the high proportion of austenite phase may also cause problems such as stress corrosion cracking. Therefore, the Ni content is set to 3.5% or less, preferably 3.4% or less, and more preferably 3.0% or less.

[0019] <Cr: 19.6 to 24.0%> Cr is an element necessary for ensuring corrosion resistance. To achieve this effect, the Cr content is set to 19.6% or more, preferably 20.0% or more, and more preferably 20.4% or more. On the other hand, if the Cr content is too high, it can cause hot working cracks and increase the cost of the refining process. Therefore, the Cr content is set to 24.0% or less, preferably 23.5% or less, and more preferably 23.0% or less.

[0020] <Mo: 0.01 to 1.00%> Mo is an element that improves corrosion resistance. To achieve this effect, the Mo content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the Mo content is too high, the raw material cost increases. Therefore, the Mo content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[0021] <Cu: 0.01 to 1.20%> Like Mn and Ni, Cu is an austenite-forming element and has the effect of suppressing nitride precipitation and improving corrosion resistance. To achieve these effects, the Cu content is set to 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, if the Cu content is too high, not only will the raw material cost increase, but hot workability will also decrease. Therefore, the Cu content is set to 1.20% or less, preferably 1.00% or less, and more preferably 0.80% or less.

[0022] <N: 0.010 to 0.090%> Like C, N is an element that has a significant effect on the stability of the austenite phase. N is also an element that improves corrosion resistance by dissolving in solid solution. In order to exert these effects, the N content is set to 0.010% or more, preferably 0.020% or more. On the other hand, if the N content is too high, ductility decreases and corrosion resistance also decreases due to the precipitation of Cr nitrides. Therefore, the N content is set to 0.090% or less, preferably 0.080% or less, and more preferably 0.075% or less.

[0023] <C+N: Less than 0.130%> If the total content of C and N is high, corrosion resistance decreases due to sensitization, and ductility decreases due to high strength. Therefore, the total content of C and N is set to less than 0.130%, preferably less than 0.120%, and more preferably 0.110% or less. The lower limit of the total content of C and N is not particularly limited, but is preferably 0.010%, preferably 0.020%, and more preferably 0.030%.

[0024] <Nb: 0.010 to 0.500%> Nb forms nitrides (NbN) and carbides (NbC) and has the effect of improving workability. To exert this effect, the Nb content is set to 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, if the Nb content is too high, ductility decreases. Therefore, the Nb content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.200% or less.

[0025] <Ti: 0.01 to 0.50%> Like Nb, Ti also forms nitrides (TiN) and carbides (TiC) and has the effect of improving workability. To exert this effect, the Ti content is set to 0.01% or more, preferably 0.015% or more, and more preferably 0.02% or more. On the other hand, if the Ti content is too high, ductility decreases. Therefore, the Ti content is set to 0.50% or less, preferably 0.30% or less, and more preferably 0.20% or less.

[0026] <V: 0.01 to 0.50%> V forms nitrides and has the effect of improving workability. To exert this effect, the V content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the V content is too high, ductility and hot workability decrease. Therefore, the V content is set to 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

[0027] <W: 0.05 to 0.50%> W is an element effective in improving corrosion resistance. To achieve this effect, the W content is set to 0.05% or more, preferably 0.08% or more, and more preferably 0.10% or more. On the other hand, if the W content is too high, ductility decreases. Therefore, the W content is set to 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

[0028] <Co: 0.01 to 0.30%> Co is an element effective in increasing high-temperature strength and improving hot workability. To achieve these effects, the Co content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the Co content is too high, toughness decreases. Therefore, the Co content is set to 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less.

[0029] <B: 0.0002 to 0.0050%> B is an element that segregates at grain boundaries to improve hot workability. To achieve this effect, the B content is set to 0.0002% or more, preferably 0.0010% or more, and more preferably 0.0015% or more. On the other hand, if the B content is too high, corrosion resistance will decrease significantly. Therefore, the B content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0030] <Sn: 0.010 to 0.500%> Sn is an element that improves corrosion resistance. To achieve this effect, the Sn content is set to 0.010% or more, preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, if the Sn content is too high, the hot workability decreases. Therefore, the Sn content is set to 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.

[0031] <Al: 0.010 to 0.050%> Al is an element effective for desulfurization and deoxidation. To achieve these effects, the Al content is set to 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, if the Al content is too high, it will increase manufacturing defects and raw material costs. Therefore, the Al content is set to 0.050% or less, preferably 0.045% or less, and more preferably 0.040% or less.

[0032] <Mg: 0.0002 to 0.0100%> Mg is an element that not only deoxidizes but also has the effect of refining the solidification structure. To achieve these effects, the Mg content is set to 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the Mg content is too high, it leads to an increase in raw material costs. Therefore, the Mg content is set to 0.0100% or less, preferably 0.0095% or less, and more preferably 0.0090% or less.

[0033] <Ca: 0.0002 to 0.0100%> Ca is an element effective for desulfurization and deoxidation. To achieve these effects, the Ca content is set to 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the Ca content is too high, hot work cracking becomes more likely to occur and corrosion resistance also decreases. Therefore, the Ca content is set to 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.

[0034] <Ta: 0.050% or less> Ta is an element that improves corrosion resistance by modifying inclusions. However, if the Ta content is too high, it will lead to a decrease in room temperature ductility and toughness. Therefore, the Ta content is set to 0.050% or less, preferably 0.045% or less, and more preferably 0.040% or less. On the other hand, the lower limit of the Ta content is not particularly limited, but in order to exert the effects of Ta, it is preferably 0.001%, more preferably 0.003%.

[0035] <Ga: 0.050% or less> Ga is an element that improves corrosion resistance and suppresses hydrogen embrittlement. However, if the Ga content is too high, workability decreases. Therefore, the Ga content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less. On the other hand, the lower limit of the Ga content is not particularly limited, but in order to exert the effects of Ga, it is preferably 0.001% and more preferably 0.003%.

[0036] <Zr: 0.01 to 0.50%> Zr is an element that has an effect similar to that of Nb and Ti and also improves oxidation resistance. To exert these effects, the Zr content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the Zr content is too high, it not only reduces ductility but also increases raw material costs. Therefore, the Zr content is set to 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

[0037] <REM: 0.0002 to 0.0100%> REM (rare earth) is an element effective in improving hot workability. To achieve this effect, the REM content is set to 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the REM content is too high, it impairs manufacturability and increases costs. Therefore, the REM content is set to 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less. REM is a collective term for Sc, Y, and 15 elements (lanthanoids) from La to Lu. These elements can be used alone or in combination of two or more types as REM.

[0038] In the duplex stainless steel material according to an embodiment of the present invention, the value of Md, as represented by the following formula (1), is 50.0 to 150.0°C, preferably 55.0 to 140.0°C, more preferably 60.0 to 130.0°C, and even more preferably 70.0 to 120.0°C. Md = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 29 (Ni + Cu) - 13.7 Cr - 18.5 Mo ... (1) In formula (1), the element symbols represent the content (mass%) of each element. Here, Md is an index representing the stability of the austenite phase. The larger the value of Md (higher temperature), the more unstable the austenite phase. If the value of Md is less than 50.0°C, the stability of the austenite phase is too high, making it difficult to transform the austenite phase into a strain-induced martensite phase, and the desired strength and ductility cannot be obtained. On the other hand, if the value of Md exceeds 150.0°C, the amount of the strain-induced martensite phase transformed from the austenite phase increases, resulting in an excessively high strength and making it impossible to obtain the desired ductility.

[0039] The duplex stainless steel material according to the present invention has a metallographic structure in which the austenite phase is 25 to 49 volume %, preferably 25 to 47 volume %, more preferably 25 to 40 volume %, even more preferably 26 to 38 volume %, and particularly preferably 28 to 37 volume %. If the austenite phase is less than 25 volume %, the proportion of ferrite phase is high, making it difficult to obtain the desired ductility. On the other hand, if the austenite phase is more than 49 volume %, the strength becomes excessively high, making it difficult to obtain the desired ductility. Furthermore, by controlling the austenite phase to 40 volume % or less, it becomes easier to control the average grain size of the ferrite phase. Herein, the proportion of austenite phase in a duplex stainless steel material can be determined using electron backscatter diffraction (EBSD). Specifically, EBSD measurement is performed using a mirror-polished specimen of a thickness cross section of the duplex stainless steel material parallel to the rolling direction. Using the data obtained from this EBSD measurement, a phase ratio map can be created using analysis software, and the ferrite and austenite phases can be separated to determine the proportion of the austenite phase.

[0040] The austenite phase has an Md value represented by the above formula (1) of 35.0 to 100.0°C, preferably 40.0 to 95.0°C, more preferably 50.0 to 90.0°C, even more preferably 52.0 to 85.0°C, and particularly preferably 53.0 to 80.0°C. If the Md value of the austenite phase is less than 35.0°C, it becomes difficult to transform the austenite phase into a stress-induced martensite phase, making it difficult to obtain the desired strength and ductility. On the other hand, if the Md value of the austenite phase exceeds 100.0°C, the amount of stress-induced martensite phase transformed from the austenite phase increases, resulting in excessively high strength and making it difficult to obtain the desired ductility. Herein, the content of each element in the austenite phase used to calculate the Md of the austenite phase can be measured using an EPMA (electron probe microanalyzer). Specifically, a mirror-polished specimen of a thickness-wise cross section of a duplex stainless steel material parallel to the rolling direction is used for qualitative analysis using an EPMA. Because C and N tend to concentrate in the austenite phase, qualitative mapping of C or N is performed on the entire cross section to identify the austenite phase. Then, quantitative analysis of C, N, Si, Mn, Cr, Ni, Cu, and Mo is performed at approximately the center of the austenite phase, with the electron beam avoiding impact on the ferrite phase. Quantitative analysis is performed at three or more points, and the average value is used as the content of each element.

[0041] In the duplex stainless steel material according to the embodiment of the present invention, the average grain size of the ferrite phase is preferably 7.0 μm or more, more preferably 7.1 μm or more, and even more preferably 7.2 μm or more. If the average grain size of the ferrite phase is less than 7.0 μm, it is difficult to obtain the desired ductility. The upper limit of the average grain size of the ferrite phase is not particularly limited, but is typically 20.0 μm, preferably 18.0 μm, and more preferably 15.0 μm. Herein, the average grain size of the ferrite phase in the duplex stainless steel material can be determined by EBSD measurement. Specifically, EBSD measurement is performed using a sample in which a cross section of the duplex stainless steel material in the thickness direction parallel to the rolling direction is mirror-polished. From the data obtained by this EBSD measurement, the area of ​​the crystal grains of the ferrite phase (BCC) can be determined by the area fraction method.

[0042] In the duplex stainless steel material according to the embodiment of the present invention, the DF value, as expressed by the following formula (2), is preferably 50.0 to 80.0, more preferably 54.0 to 80.0, even more preferably 60.0 to 80.0, particularly preferably 63.0 to 78.0, and most preferably 65.0 to 75.0. DF = 7.2 (Cr + 0.88 Mo + 0.78 Si) - 8.9 (Ni + 0.03 Mn + 0.72 Cu + 22 C + 21 N) - 44.9 (2). In formula (2), the element symbols represent the content (mass%) of each element. Here, DF is an index representing the amount of ferrite phase. Therefore, 100 - DF is the amount of austenite phase. However, it should be noted that because DF is an index determined based on the element content, it does not coincide with the amount of austenite phase actually measured. If the DF value is less than 50.0, the strength becomes excessively high, making it difficult to obtain the desired ductility, whereas if the DF value exceeds 80.0, the proportion of ferrite phase becomes high, making it difficult to obtain the desired ductility.

[0043] The duplex stainless steel material according to the embodiment of the present invention preferably has a tensile strength of 800 MPa or less, more preferably 790 MPa or less, and even more preferably 780 MPa or less. A tensile strength within this range can be said to be softer than conventional duplex stainless steel materials, and therefore the desired ductility can be ensured. The lower limit of the tensile strength is not particularly limited, but is generally 500 MPa, preferably 550 MPa. The tensile strength of the duplex stainless steel material can be measured in accordance with JIS Z2241:2011.

[0044] The duplex stainless steel material according to the embodiment of the present invention preferably has a uniform elongation of 30.0% or more, more preferably 31.0% or more, and even more preferably 32.0% or more. A uniform elongation within this range can be said to have superior ductility compared to conventional duplex stainless steel materials. The upper limit of the uniform elongation is not particularly limited, but is generally 50.0%, preferably 48.0%, and more preferably 45.0%. The uniform elongation of the duplex stainless steel material can be measured in accordance with JIS Z2241:2011. The uniform elongation is determined as the permanent elongation at the maximum tensile load.

[0045] The duplex stainless steel material according to the embodiment of the present invention has a viscosity of 3.3 × 10 -4 ~8.3 x 10 -3 s -1 When a tensile test is performed at a strain rate of 100 s, the ratio of the n-value in the 15-20% strain range to the n-value in the 20-25% strain range is preferably 0.80 or less, and more preferably 0.79 or less. An n-value ratio within this range can be said to provide excellent ductility under typical processing conditions. The lower limit of the n-value ratio is not particularly limited, but is generally 0.01, preferably 0.10. Furthermore, the strain rate affects processing heat generation, thereby varying the magnitude of the TRIP effect. For example, a high strain rate increases processing heat, thereby reducing the TRIP effect and decreasing ductility. The n-value of duplex stainless steel materials can be measured in accordance with JIS Z2241:2011.

[0046] The duplex stainless steel material according to the embodiment of the present invention preferably has a 0.2% yield strength of 480 MPa or less, more preferably 470 MPa or less. If the 0.2% yield strength is within this range, the duplex stainless steel material can be said to be soft. The lower limit of the 0.2% yield strength is not particularly limited, but is generally 300 MPa, preferably 350 MPa. The 0.2% yield strength of the duplex stainless steel material can be measured in accordance with JIS Z2241:2011.

[0047] The duplex stainless steel material according to the embodiment of the present invention may be a hot-rolled material or a cold-rolled material, and the hot-rolled material or the cold-rolled material may be subjected to annealing or pickling.

[0048] The thickness of the duplex stainless steel material according to the embodiment of the present invention is not particularly limited and may be adjusted appropriately depending on the application, but is generally 5.0 mm or less, preferably 4.0 mm or less, and more preferably 3.0 mm or less. When the duplex stainless steel material is in the form of a rod, the thickness refers to the equivalent circle diameter of the cross section. When the duplex stainless steel material is in the form of a shaped steel, the thickness refers to the thickness at any point on the cross section.

[0049] The duplex stainless steel material according to the embodiment of the present invention can be, for example, one of the following two aspects A and B.

[0050] <Aspect A> [A1] A composition comprising, on a mass basis, C: 0.001 to 0.050%, Si: 0.01% or more but less than 0.50%, Mn: 1.0 to 4.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.5%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, and N: 0.010 to 0.090%, wherein C+N is less than 0.130%, and the balance is Fe and impurities, and is expressed by the following formula (1): Md=551−462(C+N)−9.2Si−8.1Mn−29(Ni+Cu)−13.7Cr−18.5Mo ... (1) (wherein the element symbols represent the content (mass%) of each element) has a metal structure in which the Md value, represented by the formula (1), is 50.0 to 150.0°C, and the austenite phase has a metal structure in which 25 to 49% by volume, and the austenite phase has a Md value, represented by the formula (1), of 35.0 to 100.0°C.

[0051] [A2] The duplex stainless steel material according to [A1], further comprising one or more selected from, on a mass basis, Nb: 0.010 to 0.500%, Ti: 0.01 to 0.50%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, and REM: 0.0002 to 0.0100%.

[0052] [A3] The duplex stainless steel material according to [A1] or [A2], wherein the value of DF, represented by the following formula (2): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (2) (wherein the element symbols represent the content (mass%) of each element), is 50.0 to 80.0.

[0053] [A4] The duplex stainless steel material according to any one of [A1] to [A3], which satisfies at least one of the following properties (a) and (b): (a) a tensile strength of 800 MPa or less; and (b) a uniform elongation of 30.0% or more.

[0054] <Aspect B> [B1] A composition comprising, on a mass basis, C: 0.001 to 0.040%, Si: 0.01 to 0.50%, Mn: 1.0 to 4.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.5%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, and N: 0.010 to 0.080%, wherein C+N is less than 0.120%, and the balance is Fe and impurities, and is expressed by the following formula (1): Md=551−462(C+N)−9.2Si−8.1Mn−29(Ni+Cu)−13.7Cr−18.5Mo ... (1) (wherein the element symbols represent the content (mass%) of each element) has a metal structure in which the value of Md, represented by the formula (1), is 50.0 to 150.0°C, and the austenite phase is 25 to 40% by volume, wherein the value of Md, represented by the formula (1) above, of the austenite phase is 35.0 to 100.0°C, and the average particle size of the ferrite phase is 7.0 μm or more.

[0055] [B2] The duplex stainless steel material according to [B1], further comprising one or more selected from, on a mass basis, Nb: 0.010 to 0.500%, Ti: 0.01 to 0.50%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, and REM: 0.0002 to 0.0100%.

[0056] [B3] The duplex stainless steel material according to [B1] or [B2], wherein the value of Md represented by the formula (1) of the austenite phase is 50.0 to 90.0 ° C.

[0057] [B4] The duplex stainless steel material according to any one of [B1] to [B3], wherein the value of DF, represented by the following formula (2): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (2) (wherein the element symbols represent the content (mass%) of each element), is 60.0 to 80.0.

[0058] [B5] 3.3×10 -4 ~8.3 x 10 -3 s -1 The duplex stainless steel material according to any one of [B1] to [B4], wherein when a tensile test is conducted at a strain rate of 15 to 20%, the n-value ratio of the n-value in the 20 to 25% strain range to the n-value in the 15 to 20% strain range is 0.80 or less.

[0059] The method for producing a duplex stainless steel material according to an embodiment of the present invention is not particularly limited as long as it is a method capable of producing a duplex stainless steel material having the above-described characteristics. Hereinafter, an example of a method for producing a duplex stainless steel material according to an embodiment of the present invention (particularly, the duplex stainless steel materials of Aspects A and B) will be described. The duplex stainless steel materials of Aspects A and B can be produced by vacuum melting a stainless steel having the above-described composition to form a steel slab, followed by hot rolling and annealing, and then cold rolling and finish annealing. In this production method, controlling the heat treatment conditions is particularly key. Specific production methods for each aspect will be described.

[0060] <Aspect A>

[0061] In duplex stainless steel, controlling the austenite phase ratio and Md content within a predetermined range requires controlling the heat treatment (annealing) conditions (heating rate, ultimate temperature, holding time, and cooling rate). All of these conditions affect the state of carbon and nitrogen solid solution. Furthermore, the ultimate temperature affects the thermodynamic variation of the austenite content. Furthermore, controlling the ultimate temperature and holding time aims to fully recrystallize the entire structure. Because the solid solution of carbon and nitrogen affects the austenite phase ratio (amount of formation) and Md content, if they are present in large amounts as carbides or nitrides, it becomes impossible to control the austenite phase ratio and Md content within the predetermined range. Therefore, in the heat treatment (annealing) process, it is necessary to suppress the precipitation of carbides and nitrides during heating and cooling while maintaining the ultimate temperature to fully dissolve undissolved carbides and nitrides. Furthermore, because the concentration of elements (particularly carbon and nitrogen) that constitute the austenite phase changes depending on the austenite phase ratio, resulting in variations in Md content, it is also necessary to adjust the ultimate temperature to control the austenite phase ratio.

[0062] The hot rolling conditions are not particularly limited and may be performed according to conventional methods. Annealing after hot rolling involves heating at a rate of 20°C / sec or more, holding at a target temperature of 1050 to 1150°C for 10 seconds or more, and then cooling to 400°C or less at a cooling rate of 20°C / sec or more. Annealing is performed under these conditions to sufficiently dissolve carbides and nitrides precipitated during cooling after hot rolling and to suppress precipitation of carbides and nitrides during the cooling process after annealing. In particular, if the target temperature is lower than 1050°C, the solid solution of carbides and nitrides becomes insufficient, resulting in an excessively high proportion of austenite phase. On the other hand, if the target temperature is higher than 1150°C, although sufficient carbides and nitrides are solid-dissolved, the proportion of austenite becomes too low. Furthermore, a certain amount of carbon and nitrogen also dissolves in the ferrite phase, which has a small solid solubility limit, and precipitates may form during cooling, potentially deteriorating corrosion resistance.

[0063] The conditions for cold rolling are not particularly limited, but a rolling reduction of 50 to 90% is preferable. The reason for setting the rolling reduction to 50% or more is that carbides and precipitates are crushed or extended to increase their surface area, thereby facilitating solid solution during heat treatment. The reason for setting the rolling reduction to 90% or less is to prevent edge breakage due to excessive rolling. From the viewpoint of stably obtaining this effect, a rolling reduction of 85% or less is more preferable. When cold rolling is performed two or more times, intermediate annealing may be performed between each cold rolling. When intermediate annealing is performed, the conditions may be similar to the annealing conditions after hot rolling.

[0064] The conditions for finish annealing are to raise the temperature at a rate of 20°C / sec or more, hold the reached temperature of 1040 to 1120°C for 5 seconds or more, then cool to 850°C or less at a cooling rate of 30°C / sec or more, and then cool to 400°C or less at a cooling rate of 20°C / sec or more. Finish annealing is performed under these conditions in order to suppress precipitation of carbides and nitrides during heating, complete recrystallization, form solid solutions of carbides and nitrides, control the proportion of austenite phase, suppress fluctuations in the proportion of austenite phase during cooling, and suppress reprecipitation of carbides and nitrides.

[0065] <Aspect B> In duplex stainless steel materials, in order to control the average particle size of the ferrite phase within a predetermined range, it is necessary to control the hot rolling conditions (temperature immediately after the final pass, cooling rate). Furthermore, in order to control the austenite phase ratio and Md within a predetermined range, it is necessary to control the heat treatment (annealing) conditions (heating rate, ultimate temperature, holding time, and cooling rate). All of these conditions affect the solid solution state of carbon and nitrogen. Furthermore, the ultimate temperature affects the thermodynamic fluctuation of the austenite amount. Furthermore, controlling the ultimate temperature and holding time also aims to sufficiently recrystallize the entire structure. Since the solid solution of carbon and nitrogen affects the austenite phase ratio (amount produced) and Md, if they are present in large amounts as carbides or nitrides, it becomes impossible to control the austenite phase ratio and Md within the predetermined range. Therefore, in the heat treatment (annealing) process, it is necessary to suppress the precipitation of carbides and nitrides during heating and cooling, while sufficiently dissolving undissolved carbides and nitrides by holding at the ultimate temperature. Furthermore, the concentration of elements (particularly carbon and nitrogen) constituting the austenite phase changes depending on the proportion of the austenite phase, and the resulting Md fluctuates, so it is also necessary to control the proportion of the austenite phase by adjusting the ultimate temperature.

[0066] Hot rolling involves setting the temperature immediately after the final pass to 1030°C or higher, and then cooling to 800°C at a cooling rate of 20°C / s or higher. Hot rolling under these conditions allows the ferrite grains to be coarsened and controlled within a predetermined range. There are two main reasons why the ferrite grains become smaller. The first is recrystallization during hot rolling or subsequent annealing due to the accumulation of hot rolling strain. The lower the hot rolling temperature, the more strain accumulates, inducing recrystallization and making the ferrite grains finer. Therefore, it is necessary to raise the temperature to the temperature immediately after the final pass, where this is less likely to occur. The second is the suppression of ferrite grain growth due to the formation of austenite. When austenite precipitates at the ferrite grain boundaries, the movement of the ferrite grain boundaries slows, suppressing grain growth. Since the austenite phase decreases with increasing temperature, peaking at around 900°C, the higher the temperature, the more likely the ferrite grains grow. On the other hand, in order to maintain a high temperature in hot rolling, it is effective to increase the heating temperature of the slab before hot rolling or to increase the rolling speed to shorten the heat dissipation time, but this increases fuel costs and makes manufacturing more difficult. Therefore, taking these circumstances into consideration, the lower limit of the final pass temperature in hot rolling was set to 1030°C.

[0067] Annealing after hot rolling involves a temperature increase rate of 20°C / sec or more, holding the final temperature of 1080 to 1150°C for 10 seconds or more, and then cooling to 400°C or less at a cooling rate of 20°C / sec or more. Annealing is performed under these conditions to sufficiently dissolve the carbides and nitrides precipitated during cooling after hot rolling and to suppress precipitation of carbides and nitrides during the cooling process after annealing. Furthermore, the proportion of austenite phase is relatively reduced, reducing the inhibition of grain growth of the ferrite phase and facilitating coarsening of the ferrite phase grains. In particular, if the final temperature is lower than 1080°C, the solid solution of carbides and nitrides becomes insufficient, and the proportion of austenite phase becomes too high. Furthermore, if the final temperature is higher than 1150°C, although the carbides and nitrides are sufficiently dissolved, the proportion of austenite becomes too low. Furthermore, a certain amount of carbon and nitrogen dissolve in the ferrite phase, and precipitates may form during cooling in the ferrite phase, which has a small solubility limit, and this may deteriorate corrosion resistance.

[0068] The cold rolling conditions are not particularly limited, but a rolling reduction of 50 to 80% is preferable. The rolling reduction of 50% or more is because the surface area of ​​precipitates such as carbides is increased by crushing or extending them, thereby facilitating solid solution during heat treatment. On the other hand, a rolling reduction of 80% or less is intended to prevent edge breakage due to excessive rolling. It is also intended to prevent the structure of the finish-annealed material from becoming too fine due to the accumulation of rolling strain. As mentioned above, excessive strain induces recrystallization, which results in finer crystal grains, and this must be avoided. From the viewpoint of stably obtaining these effects, a rolling reduction of 75% or less is more preferable. When cold rolling is performed two or more times, intermediate annealing may be performed between each cold rolling. When intermediate annealing is performed, the conditions may be similar to the annealing conditions after hot rolling.

[0069] The conditions for finish annealing are a heating rate of 20°C / sec or more, a temperature of 1000 to 1150°C, held for 5 seconds or more, followed by cooling to 850°C or less at a cooling rate of 30°C / sec or more, and then cooling to 400°C or less at a cooling rate of 20°C / sec or more. Finish annealing is performed under these conditions to suppress precipitation of carbides and nitrides during heating, complete recrystallization, solid solution of carbides and nitrides, control the proportion of austenite phase, suppress fluctuations in the proportion of austenite phase during cooling, and suppress reprecipitation of carbides and nitrides. Furthermore, by controlling the structure up to cold rolling, the finish annealing results in a structure with coarse ferrite phase crystal grains.

[0070] The duplex stainless steel material according to the embodiment of the present invention is softer and more ductile than conventional ferritic-austenitic duplex stainless steel materials. Therefore, this duplex stainless steel material can suppress springback caused by excessively high strength during forming, and also has good shape fixability. Furthermore, this duplex stainless steel material has higher strength and superior corrosion resistance than general-purpose austenitic stainless steel materials such as SUS304. Therefore, this duplex stainless steel material can be used in a variety of applications where these properties are required.

[0071] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0072] Example of Aspect A A cold-rolled annealed sheet was produced as a duplex stainless steel material. Specifically, stainless steel having the composition of the steel type shown in Table 1 (the balance being Fe and impurities) was vacuum melted to form a steel slab, which was then hot-rolled and annealed according to a conventional method. The annealing was performed at a heating rate of 30°C / s, with the final temperature of 1100°C held for 180 seconds, followed by cooling to 400°C or less at a cooling rate of 25°C / s. The annealed hot-rolled sheet was then cold-rolled at a rolling reduction of 80% and then finish-annealed to obtain a cold-rolled annealed sheet with a thickness of 1.0 mm. The final annealing was performed at a heating rate of 30°C / s, with the final temperature of 1080°C held for 30 seconds, followed by cooling to 850°C or less at a cooling rate of 30°C / s, and then cooling to 400°C or less at a cooling rate of 25°C / s. In Table 1, the values ​​of Md and DF were calculated based on the content of each element. Among the steel types shown in Table 1, No. 1-M and 1-N are existing duplex stainless steel materials.

[0073]

[0074] The cold-rolled and annealed sheets obtained above were subjected to the following evaluations.

[0075] <Proportion of austenite phase (γ phase) in duplex stainless steel material> After cutting out a test specimen from the cold-rolled annealed sheet, the thickness direction cross section parallel to the rolling direction was mirror-polished and subjected to EBSD (electron backscatter diffraction) measurement. EBSD measurement was performed using a scanning electron microscope with measurement software TSL OIM Data Collection 7 (TSL Solutions Co., Ltd.), measuring a 200 μm square area at the center of the thickness direction of the test specimen with a step size of 0.3 μm. Next, a phase ratio map was created using analysis software TSL OIM Analysis 7 (TSL Solutions Co., Ltd.) for the data obtained by the EBSD measurement, and the ferrite phase and austenite phase were separated. Then, the proportion of the austenite phase in the entire observation area was determined.

[0076] <Md in the Austenite Phase (γ Phase)> Test specimens were cut from the cold-rolled annealed steel sheets, and the cross sections in the thickness direction parallel to the rolling direction were mirror-polished and subjected to component analysis using an electron probe microanalyzer (EPMA). Specifically, because C and N tend to concentrate in the austenite phase, qualitative mapping of C or N was performed on the entire cross section to identify the austenite phase. Next, C, N, Si, Mn, Cr, Ni, Cu, and Mo were quantitatively analyzed at approximately the center of the austenite phase, with the electron beam not irradiating the ferrite phase. The measurement area was approximately 2 μm square, and measurements were taken at three or more points on each test specimen. The average values ​​were used to determine the content of each element. The EPMA measurements were performed under conditions of an acceleration voltage of 15 kV, a current of 0.2 μA, and a step size of 0.15 μm. The Md in the austenite phase was calculated based on the content of each element thus obtained.

[0077] <Tensile Strength and Uniform Elongation> JIS No. 13B test pieces were cut out from the cold-rolled annealed sheet so that the parallel portion was in the rolling direction, and these test pieces were subjected to tensile tests in accordance with JIS Z2241:2011. The tensile tests were carried out in an air atmosphere at room temperature (25°C) and a tension speed of 3 mm / min. In the tensile tests, the maximum strength achieved was defined as the tensile strength, and the elongation up to the tensile strength was defined as the uniform elongation. In this evaluation, if the tensile strength was 800 MPa or less, it could be said that the steel was softened, and if the uniform elongation was 30.0% or more, it could be said that the ductility was excellent.

[0078] The results of the above evaluation are shown in Table 2.

[0079]

[0080] As shown in Table 2, in Examples 1-1 to 1-7, the composition and Md of the cold-rolled annealed sheet (duplex stainless steel material) as well as the austenite phase ratio and Md were controlled within a predetermined range, and therefore both tensile strength and uniform elongation were good. In contrast, in Comparative Example 1-1, the Ni content was too low and the N and C+N contents were too high, resulting in excessively high strength and insufficient ductility. In Comparative Example 1-2, the Cr content was too high, resulting in a high ferrite phase ratio and insufficient ductility. In Comparative Example 1-3, the Ni content was too high and the Cr content was too low, resulting in a high austenite phase ratio, which made the material prone to work hardening and resulted in excessively high strength. In Comparative Example 1-4, the contents of Ni, Cr, and Sn were too low, while the content of Al was too high, and the Md values ​​of the cold-rolled annealed sheet and the austenite phase were too high. As a result, the amount of the deformation-induced martensite phase increased, resulting in excessively high strength and insufficient ductility.

[0081] Comparative Example 1-5 had too high a content of Si and Cr, too low a content of Ti, and the Md value of the cold-rolled annealed sheet was too low, and the proportion of the ferrite phase was too high, resulting in insufficient ductility. Comparative Examples 1-6 and 1-7 were existing duplex stainless steel materials, and in particular had too high a content of N. Furthermore, in Comparative Examples 1-6 and 1-7, the Md values ​​of the cold-rolled annealed sheet and the austenite phase were too low, resulting in excessively high strength and insufficient ductility. Comparative Example 1-8 had too low an Md value of the austenite phase, resulting in insufficient ductility.

[0082] Example of Aspect B A cold-rolled annealed sheet was produced as a duplex stainless steel material. The cold-rolled annealed sheet was produced by the following steps: hot rolling, annealing, cold rolling, and finish annealing. Specifically, a stainless steel having the composition shown in Table 3 (the balance being Fe and impurities) was first melted by vacuum melting to produce a steel slab. This steel slab was then hot-rolled to produce a 5 mm-thick hot-rolled sheet. In the hot-rolling step, the temperature immediately after the final pass was set to the temperature shown in Table 4, and the slab was cooled to 800°C by water cooling (cooling rate of 20°C / sec or more). In the annealing step after the hot-rolling step, the temperature was increased to 25°C / sec, the annealed sheet was held at the final temperature of 1100°C (annealing temperature) for 30 seconds, and then cooled to 400°C or less by water cooling (cooling rate of 20°C / sec or more). Cold rolling was performed at the reduction ratio shown in Table 4 to produce a cold-rolled sheet having the thickness shown in Table 4. In the final annealing step, the temperature was increased at a rate of 30°C / sec, and the steel was held at the final temperature (annealing temperature) shown in Table 4 for 30 seconds, and then cooled to 400°C or less by water cooling (cooling rate of 30°C / sec or more). In Table 3, the values ​​of Md and DF were calculated based on the contents of each element.

[0083]

[0084] The cold-rolled and annealed sheets obtained above were subjected to the following evaluations.

[0085] <Proportion of austenite phase (γ phase) in duplex stainless steel material> Determined in the same manner as in the Examples of Aspect A. <Md of austenite phase (γ phase)> Determined in the same manner as in the Examples of Aspect A.

[0086] <Average particle size of ferrite phase (α phase) in duplex stainless steel material> After cutting out a test specimen from the cold-rolled annealed sheet, the thickness direction cross section parallel to the rolling direction was mirror-polished and subjected to EBSD (electron backscatter diffraction) measurement. EBSD measurement was performed using a scanning electron microscope and measurement software TSL OIM Data Collection 7 (TSL Solutions Co., Ltd.), measuring a 200 μm square region at the center of the thickness direction of the test specimen with a step size of 0.3 μm. For the data obtained by this EBSD measurement, the area of ​​the crystal grains of the ferrite phase (BCC) was determined by the area fraction method.

[0087] <n-value ratio, 0.2% yield strength, and uniform elongation> JIS No. 13B test specimens were cut from the cold-rolled annealed sheets so that the parallel portion was aligned with the rolling direction. These test specimens were used to perform tensile tests in accordance with JIS Z2241:2011. The tensile tests were performed in an air atmosphere at room temperature (25°C) and a tension speed of 10 mm / min. In the tensile tests, the elongation up to the maximum attainable strength (tensile strength) was defined as the uniform elongation. The n-value was determined by measuring the relationship between stress σ and strain ε from 0.2% yield strength to the maximum load point. The true stress and true strain were calculated from these measurements and plotted on a logarithmic scale with strain (lnε) on the horizontal axis and stress (lnσ) on the vertical axis. The slope of the plotted line was defined as the n-value. The strain rate was as shown in Table 3. In these evaluations, a specimen with an n-value ratio of 0.80 or less and a uniform elongation of 30.0% or more can be considered to have excellent ductility. Furthermore, if the 0.2% yield strength is 480 MPa or less, it can be said to be soft.

[0088] The evaluation results are shown in Table 5.

[0089]

[0090] As shown in Table 5, in Examples 2-1 to 2-7, the composition and Md of the cold-rolled annealed sheet (duplex stainless steel material) as well as the austenite phase proportion and Md were controlled within a predetermined range, and therefore the results for the n-value ratio, 0.2% proof stress, and uniform elongation were all good. In contrast, in Comparative Example 2-1, the austenite phase proportion was too low, resulting in insufficient ductility. In Comparative Example 2-2, the austenite phase Md was too high, resulting in a small average particle size of the ferrite phase and insufficient softening. In Comparative Example 2-3, the amount of C+N was high due to the excessive N content. Furthermore, the austenite phase proportion was high and the austenite phase Md was too low, resulting in insufficient softening and ductility. In Comparative Example 2-4, the N content was too high and the Md of the cold-rolled annealed sheet was too low, resulting in high austenite phase stability and difficulty in transforming to the deformation-induced martensite phase. Furthermore, the austenite phase Md was too low, resulting in insufficient ductility. In Comparative Example 2-5, the proportion of the austenite phase was too low, resulting in a high proportion of the ferrite phase and insufficient ductility. In Comparative Example 2-6, the Ni content was high and the Cr content was low. In addition, the proportion of the austenite phase was high, resulting in insufficient softening. In addition, the Md of the austenite phase was too low, resulting in insufficient ductility.

[0091] As can be seen from the above results, the present invention can provide a ferritic-austenitic duplex stainless steel material that is softer and has higher ductility than conventional ferritic-austenitic duplex stainless steel materials. That is, the present invention can provide a ferritic-austenitic duplex stainless steel material that is softer and has higher ductility than conventional ferritic-austenitic duplex stainless steel materials by adopting the following configurations [1] to

[10] .

[0092] [1] A composition comprising, on a mass basis, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 4.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.5%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, and N: 0.010 to 0.090%, wherein C+N is less than 0.130%, and the balance is Fe and impurities, and is expressed by the following formula (1): Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo ... (1) (wherein the element symbols represent the content (mass%) of each element) has a metal structure in which the Md value, represented by the formula (1), is 50.0 to 150.0°C, and the austenite phase has a metal structure in which 25 to 49 volume %; and the austenite phase has a Md value, represented by the formula (1), of 35.0 to 100.0°C.

[0093] [2] The ferritic-austenitic duplex stainless steel material according to [1], further comprising one or more selected from, by mass, Nb: 0.010 to 0.500%, Ti: 0.01 to 0.50%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, and REM: 0.0002 to 0.0100%.

[0094] [3] On a mass basis, C: 0.001 to 0.040%, N: 0.010 to 0.080%, C + N is less than 0.120%, and the austenite phase is 25 to 40% by volume. [1] or [2] Ferrite-austenitic duplex stainless steel material.

[0095] [4] The ferritic-austenitic duplex stainless steel material according to any one of [1] to [3], wherein the Md value of the austenite phase is 50.0 to 90.0 ° C.

[0096] [5] The ferritic-austenitic duplex stainless steel material according to any one of [1] to [4], wherein the average particle size of the ferrite phase is 7.0 μm or more.

[0097] [6] The ferritic-austenitic duplex stainless steel material according to any one of [1] to [5], wherein the DF value represented by the following formula (2): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (2) (wherein the element symbols represent the content (mass%) of each element) is 50.0 to 80.0.

[0098] [7] The ferritic-austenitic duplex stainless steel material according to [6], wherein the DF value is 60.0 to 80.0.

[0099] [8] The ferritic-austenitic duplex stainless steel material according to any one of [1] to [7], having a tensile strength of 800 MPa or less.

[0100] [9] The ferritic-austenitic duplex stainless steel material according to any one of [1] to [8], having a uniform elongation of 30.0% or more.

[0101]

[10] 3.3 x 10 -4 ~8.3 x 10 -3 s -1 [9] The ferritic-austenitic duplex stainless steel material according to any one of [1] to [9], wherein when a tensile test is carried out at a strain rate of 15 to 20%, the n-value ratio of the n-value in the 20 to 25% strain range to the n-value in the 15 to 20% strain range is 0.80 or less.

Claims

1. By mass, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 4.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.5%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, N: 0.010 to 0.090%, C + N is less than 0.130%, and the balance consists of Fe and impurities, The following formula (1): Md = 551 - 462(C + N) - 9.2Si - 8.1Mn - 29(Ni + Cu) - 13.7Cr - 18.5Mo... (1) (wherein, the element symbols represent the contents (mass%) of the respective elements), and the value of Md is 50.0 to 150.0 °C, It has a metal structure in which the austenite phase is 25 to 49% by volume, A ferritic-austenitic two-phase stainless steel sheet, wherein the value of Md represented by the above formula (1) of the austenite phase is 35.0 to 100.0 °C.

2. By mass, further containing one or more selected from Nb: 0.010 to 0.500%, Ti: 0.01 to 0.50%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, REM: 0.0002 to 0.0100%, the ferritic-austenitic two-phase stainless steel sheet according to Claim 1.

3. By mass, C: 0.001 to 0.040%, N: 0.010 to 0.080%, C + N is less than 0.120%, and the austenite phase is 25 to 40% by volume, the ferritic-austenitic two-phase stainless steel sheet according to Claim 1 or 2.

4. The ferritic-austenitic two-phase stainless steel sheet according to Claim 3, wherein the value of Md of the austenite phase is 50.0 to 90.0 °C.

5. The ferritic-austenitic two-phase stainless steel sheet according to Claim 3, wherein the average particle diameter of the ferrite phase is 7.0 μm or more.

6. The following formula (2): DF = 7.2(Cr + 0.88Mo + 0.78Si) - 8.9(Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9... (2) The ferrite-austenite duplex stainless steel material according to claim 1 or 2, wherein the DF value represented by (where the element symbols represent the content (% by mass) of each element) is 50.0 to 80.

0.

7. The following formula (2): DF = 7.2(Cr + 0.88Mo + 0.78Si) - 8.9(Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9... (2) The ferrite-austenite duplex stainless steel material according to claim 3, wherein the DF value represented by (where the element symbols represent the content (% by mass) of each element) is 50.0 to 80.

0.

8. The ferrite-austenite duplex stainless steel material according to claim 7, wherein the DF value is 60.0 to 80.

0.

9. The ferrite-austenite duplex stainless steel material according to claim 1 or 2, wherein the tensile strength is 800 MPa or less.

10. The ferrite-austenite duplex stainless steel material according to claim 1 or 2, wherein the uniform elongation is 30.0% or more.

11. 3.3×10 -4 ~8.3×10 -3 s -1 The ferritic-austenitic duplex stainless steel material according to claim 3, wherein when a tensile test is carried out at a strain rate of 3.3×10 -4 to 8.3×10 -3 s -1 , the n-value ratio of the n-value in the 15 to 20% strain range to the n-value in the 20 to 25% strain range is 0.80 or less for the n-value in the strain range of 20 to 25%.