Ferritic-austenitic duplex stainless steel

Abstract

To provide a ferritic-austenitic duplex stainless steel material that can suppress springback and skin irritation, and reduce earring wear. [Solution] By mass, the material contains C: 0.001~0.050%, Si: 0.01~0.50%, Mn: 1.0~3.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5~3.0%, Cr: 19.6~23.0%, Mo: 0.01~1.00%, Cu: 0.01~1.00%, N: 0.010~0.090%. DF:7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 is 60.0~80.0, Md:551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo is 80.0~150.0℃, The austenite phase is 10.0 to 40.0 volume percent. Steel material in which the maximum strength of the ferrite phase crystal orientation is 10 or less.

Description

[[Technical Field]]

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

[0002] Duplex ferritic-austenitic stainless steel materials (hereinafter sometimes abbreviated as "duplex stainless steel materials") are used in construction materials and structural materials due to their excellent corrosion resistance and high strength. In recent years, with the expansion of their applications, duplex stainless steel materials have been required to have improved workability. However, since duplex stainless steel materials have high strength, while weight reduction can be achieved by thinning the material, they have the problem that springback is likely to occur during processing. In addition, in duplex stainless steel materials, since the rolling texture of the ferrite phase is significantly developed, large earing and surface roughening are likely to occur during processing (particularly during drawing). Here, earing refers to the phenomenon that the shape of the remaining flange portion of a processed product is not uniform, and the portion called the ear at the end undulates.

[0003] As a technique for reducing earing of duplex stainless steel, Patent Document 1 discloses a duplex stainless steel material with small in-plane anisotropy, which comprises, by mass%, C: 0.001 to 0.10%, Si: 0.01 to 1.0%, Mn: 2 to 10%, P ≤ 0.05%, Ni: 0.1 to 3.0%, Cr: 15.0 to 30.0%, N: 0.05 to 0.30%, with the balance being Fe and unavoidable impurities, wherein an austenite phase fraction is 40 to 90% by area fraction, the maximum intensity of crystal orientation of the ferrite phase is 10 or less, and a hardness ratio of the austenite phase to the ferrite phase is 1.1 or more. In addition, Patent Document 2 proposes a duplex stainless steel material for press forming with small earing, characterized in that a difference in 0.2% yield strength between the 0° direction and the 90° direction with respect to the rolling direction is less than 20 MPa, a roughness Rz of a steel sheet surface in the rolling direction and the 90° direction is 0.5 to 4 µm, and an earing rate represented by a predetermined formula is less than 2%. [[Prior Art Documents]] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2013-185231 [Patent Document 2] Japanese Patent Publication No. 2011-208244 [Overview of the project] [Problems that the invention aims to solve]

[0005] While the technologies described in Patent Documents 1 and 2 can reduce earring by controlling in-plane anisotropy and 0.2% yield strength anisotropy, they do not particularly address issues such as springback during processing or rough skin after processing.

[0006] The present invention was made to solve the above problems and aims to provide a ferritic-austenitic duplex stainless steel material that can suppress springback and skin irritation and reduce earring wear. [Means for solving the problem]

[0007] The inventors of this invention have diligently researched ferritic-austenitic duplex stainless steel materials and have found that the above problems can be solved by controlling the composition of the ferritic-austenitic duplex stainless steel material, the proportion of DF, Md, and the austenite phase, and the maximum intensity of the crystal orientation of the ferrite phase, thereby completing the present invention.

[0008] In other words, the present invention has a composition by mass consisting of C: 0.001~0.050%, Si: 0.01~0.50%, Mn: 1.0~3.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5~3.0%, Cr: 19.6~23.0%, Mo: 0.01~1.00%, Cu: 0.01~1.00%, N: 0.010~0.090%, with the remainder being Fe and impurities. The following equation (1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (1) The DF value shown in the formula (where element symbols represent the content (mass %) of each element) is between 60.0 and 80.0. The following equation (2): Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) The value of Md shown in the formula (where the element symbol represents the content (mass %) of each element) is 80.0 to 150.0°C. The austenite phase is 10.0 to 40.0 volume percent. This is a ferritic-austenitic duplex stainless steel material in which the maximum intensity of the ferrite phase crystal orientation is 10 or less. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a ferritic-austenitic duplex stainless steel material that can suppress springback and skin irritation, and reduce earrings. [Modes for carrying out the invention]

[0010] The embodiments of the present invention will be described in detail below. The present invention is not limited to the embodiments described below, and it should be understood that modifications, improvements, etc., made to the embodiments described below, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the invention, also fall within the scope of the present invention.

[0011] In this specification, unless otherwise specified, any "%" indication for ingredients means "mass%". In this specification, a numerical range indicated by "~" means a range that includes the numbers before and after "~" as the lower and upper limits, unless otherwise specified. In this specification, a numerical range indicated by "greater than" or "less than" means a range that does not include the number as the lower or upper limit. In the numerical ranges described stepwise in this specification, the upper limit of one stepwise numerical range may be replaced with the upper limit of another stepwise numerical range or a value shown in the examples. Also, in the numerical ranges described stepwise in this specification, the lower limit of one stepwise numerical range may be replaced with the lower limit of another stepwise numerical range or a value shown in the examples.

[0012] The ferritic-austenitic duplex stainless steel material (hereinafter simply referred to as "duplex stainless steel material") according to an embodiment of the present invention has a composition comprising C: 0.001~0.050%, Si: 0.01~0.50%, Mn: 1.0~3.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5~3.0%, Cr: 19.6~23.0%, Mo: 0.01~1.00%, Cu: 0.01~1.00%, and N: 0.010~0.090%, with the remainder being Fe and impurities.

[0013] In this specification, "stainless steel material" means a material formed from stainless steel, and its shape is not particularly limited. Examples of shapes include plates (including strips), rods, and tubes. Furthermore, it may be various types of shaped steel, such as T-shaped or I-shaped cross-sections. Among these, the stainless steel material is preferably plate-shaped. Furthermore, in this specification, "ferrite-austenitic system" refers to a two-phase structure in which the metallic structure at room temperature is mainly composed of a ferrite phase and an austenite phase. Therefore, "ferrite-austenitic system" also includes systems that contain phases other than the ferrite and austenite phases (especially the martensite phase). Furthermore, in this specification, "impurities" means components that are mixed in during the industrial production of stainless steel materials due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and which are acceptable as long as they do not adversely affect the present invention. For example, impurities include unavoidable impurities. An example of an impurity is oxygen (O). The O content is, for example, 0.0001 to 0.0070%. Regarding the content of each element, "containing xx% or less" means that it contains xx% or less, but also more than 0% (especially above the impurity level).

[0014] The duplex stainless steel material according to the embodiment of the present invention may further include, as necessary, one or more selected from Nb: 0.001 to 0.500%, Ti: 0.001 to 0.500%, 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%. The following provides a detailed explanation of each component.

[0015] <C:0.001~0.050%> Carbon (C) is an element that significantly affects the stability of the austenite phase. If the C content is too high, ductility (workability) may decrease, and the precipitation of chromium carbides may be promoted, leading to intergranular corrosion. Therefore, the C content should be 0.050% or less, preferably 0.048% or less, and more preferably 0.045% or less. Also, from the viewpoint of corrosion resistance, a lower C content is better, but reducing the C content too much will lead to increased costs. Therefore, the C content should be 0.001% or more, preferably 0.002% or more, and more preferably 0.005% or more.

[0016] <Si:0.01~0.50%> Si is added as a deoxidizing element and is also useful for improving oxidation resistance. However, if the Si content is too high, it hardens and reduces ductility. Therefore, the Si content should be 0.50% or less, preferably 0.48% or less, and more preferably 0.45% or less. Also, if the Si content is excessively reduced, the cost during smelting increases. Therefore, the Si content should be 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more.

[0017] <Mn: 1.0~3.5%> 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 will decrease. Therefore, the Mn content is set to 3.5% or less, preferably 3.4% or less, more preferably 3.3% or less. In addition, excessive reduction of Mn content increases the cost during smelting. Therefore, the Mn content is set to 1.0% or more, preferably 1.1% or more, more preferably 1.2% or more.

[0018] <P: 0.050% or less> P is an element contained in raw materials such as Cr. When the P content is high, formability deteriorates, so the P content is set to 0.050% or less, preferably 0.048% or less, more preferably 0.045% or less. On the other hand, lower P content is preferred, but there is a limit to reducing P content. The lower limit of the P content is generally 0.001%, preferably 0.002%, more preferably 0.003%.

[0019] <S: 0.030% or less> S is an element contained in various raw materials. S combines with Mn to form inclusions, which may become initiation sites for rusting, so the lower the S content, the higher the corrosion resistance. Therefore, the S content is set to 0.030% or less, preferably 0.025% or less, more preferably 0.020% or less. On the other hand, there is a limit to reducing S content. The lower limit of the S content is generally 0.001%, preferably 0.002%.

[0020] <Ni: 1.5~3.0%> 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 should be 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, it will lead to increased raw material costs, and a higher proportion of the austenite phase may cause problems such as stress corrosion cracking. Therefore, the Ni content should be 3.0% or less, preferably 2.9% or less, and more preferably 2.8% or less.

[0021] <Cr:19.6~23.0%> Cr is an element necessary to ensure corrosion resistance. To achieve this effect, the Cr content should be 19.6% or more, preferably 19.8% or more, and more preferably 19.9% ​​or more. On the other hand, if the Cr content is too high, it can lead to hot working cracks and increase the cost of the refining process. Therefore, the Cr content should be 23.0% or less, preferably 22.8% or less, and more preferably 22.5% or less.

[0022] <Mo:0.01~1.00%> Mo is an element that improves corrosion resistance. To achieve this effect, the Mo content should be 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the Mo content is too high, the raw material cost will increase. Therefore, the Mo content should be 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less.

[0023] <Cu:0.01~1.00%> Cu, like Mn and Ni, is an austenite-forming element and has the effect of suppressing nitride precipitation and improving corrosion resistance. To achieve these effects, the Cu content should be 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, it will lead to an increase in raw material costs and a decrease in hot workability. Therefore, the Cu content should be 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less.

[0024] <N:0.010~0.090%> Nitrogen (N), like carbon (C), is an element that significantly affects the stability of the austenite phase. Furthermore, N is an element that enhances corrosion resistance through solid solution. To achieve these effects, the N content should be 0.010% or more, preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, if the N content is too high, the 0.2% yield strength increases, making springback more likely during processing, and corrosion resistance also decreases due to the precipitation of Cr nitrides. Therefore, the N content should be 0.090% or less, preferably 0.085% or less.

[0025] <Nb:0.001~0.500%> Nb forms nitrides (NbN) and carbides (NbC), which improve workability. To achieve this effect, the Nb content should be 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Nb content is too high, the ductility will decrease. Therefore, the Nb content should be 0.500% or less, preferably 0.300% or less, and more preferably 0.200% or less.

[0026] <Ti:0.001~0.500%> Like Nb, Ti forms nitrides (TiN) and carbides (TiC), which improve workability. To achieve this effect, the Ti content should be 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Ti content is too high, the ductility decreases. Therefore, the Ti content should be 0.500% or less, preferably 0.300% or less, and more preferably 0.200% or less.

[0027] <V:0.01~0.50%> V has the effect of forming nitrides and improving workability. To achieve this effect, the V content should be 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the V content is too high, the ductility and hot workability will decrease. Therefore, the V content should be 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

[0028] <W:0.05~0.50%> Water (W) is an effective element for improving corrosion resistance. To achieve this effect, the W content should be 0.05% or more, preferably 0.06% or more, and more preferably 0.07% or more. On the other hand, if the W content is too high, the ductility will decrease. Therefore, the W content should be 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

[0029] <Co:0.01~0.30%> Co is an effective element for increasing high-temperature strength and improving hot workability. To achieve these effects, the Co content should be 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the Co content is too high, the toughness will decrease. Therefore, the Co content should be 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less.

[0030] <B:0.0002~0.0050%> B is an element that segregates at grain boundaries and improves hot workability. To achieve this effect, the B content should be 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, the corrosion resistance will be significantly reduced. Therefore, the B content should be 0.0050% or less, preferably 0.0045% or less, and more preferably 0.0040% or less.

[0031] <Sn:0.010~0.500%> Sn is an element that improves corrosion resistance. To achieve this effect, the Sn content should be 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, if the Sn content is too high, the hot workability will decrease. Therefore, the Sn content should be 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.

[0032] <Al:0.010~0.050%> Al is an effective element for desulfurization and deoxidation. To achieve these effects, the Al content should be 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, too much Al content leads to an increase in manufacturing defects and raw material costs. Therefore, the Al content should be 0.050% or less, preferably 0.048% or less, and more preferably 0.045% or less.

[0033] <Mg:0.0002~0.0100%> Magnesium (Mg) is an element that not only deoxidizes but also refines the coagulation structure. To achieve these effects, the Mg content should be 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, too much Mg content leads to increased raw material costs. Therefore, the Mg content should be 0.0100% or less, preferably 0.0095% or less, and more preferably 0.0090% or less.

[0034] <Ca:0.0002~0.0100%> Ca is an element effective for desulfurization and deoxidation. To exert these effects, the content of Ca is set to 0.0002% or more, preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, if the Ca content is too high, hot working cracking is prone to occur and corrosion resistance also decreases. Therefore, the content of Ca is set to 0.0100% or less, preferably 0.0095% or less, more preferably 0.0090% or less.

[0035] <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 causes a decrease in room-temperature ductility and a decrease in toughness. Therefore, the Ta content is set to 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less. On the other hand, the lower limit of the Ta content is not particularly limited, but to exert the effect provided by Ta, it is preferably 0.001%, more preferably 0.003%.

[0036] <Ga: 0.050% or less> Ga is an element that contributes to improving corrosion resistance and suppressing 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, more preferably 0.030% or less. On the other hand, the lower limit of the Ga content is not particularly limited, but to exert the effect provided by Ga, it is preferably 0.001%, more preferably 0.003%.

[0037] <Zr: 0.01~0.50%> Zr is an element that has effects similar to Nb and Ti, and also improves oxidation resistance. To exert these effects, the Zr content is set to 0.01% or more, preferably 0.02% or more. On the other hand, if the Zr content is too high, it causes a decrease in ductility and an increase in raw material cost. Therefore, the Zr content is set to 0.50% or less, preferably 0.40% or less, more preferably 0.30% or less.

[0038] <REM: 0.0002~0.0100%> Rare earth elements (REMs) are effective in improving hot workability. To achieve this effect, the REM content should be 0.0002% or more, preferably 0.0003% or more, and more preferably 0.0004% or more. On the other hand, too much REM content impairs manufacturability and increases costs. Therefore, the REM content should be 0.0100% or less, preferably 0.0095% or less. REM is a collective term for 15 elements (lanthanides) from Sc, Y, and La to Lu. These elements can be used individually or in combination of two or more as REM.

[0039] The duplex stainless steel material according to the embodiment of the present invention has a DF value represented by the following formula (1) of 60.0 to 80.0, preferably 61.0 to 79.0, and more preferably 62.0 to 78.0. DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (1) In formula (1), the element symbols represent the percentage (%) of each element. Here, DF is an index representing the amount of ferrite phase. Therefore, 100-DF represents the total amount of austenite phase. However, it should be noted that DF is an index determined based on the elemental content in the duplex stainless steel material, and therefore does not necessarily correspond to the actual measured amount of austenite phase. By controlling the value of DF within the above range, the amount of each phase is appropriate, and the strength can be controlled within the specified range.

[0040] The duplex stainless steel material according to the embodiment of the present invention has an Md value represented by the following formula (2) of 80.0 to 150.0°C, preferably 85.0 to 145.0°C, more preferably 90.0 to 140.0°C, and even more preferably 95.0 to 135.0°C. Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) In equation (2), the element symbols represent the percentage (%) of each element. Here, Md is an index representing the stability of the austenite phase. A higher Md value (higher temperature) indicates that the austenite phase is less stable. If the Md value is below 80.0°C, insufficient processing-induced martensitic transformation occurs during the manufacturing process (e.g., cold rolling), leading to an increase in the maximum intensity of the ferrite phase's crystal orientation and resulting in increased earrings and skin roughness. On the other hand, if the Md value exceeds 150.0°C, the austenite phase becomes too unstable, leading to a significant decrease in ductility.

[0041] The duplex stainless steel material according to the embodiment of the present invention has a metallic structure in which the austenite phase is 10.0 to 40.0 volume%, preferably 12.0 to 39.0 volume%, and more preferably 15.0 to 38.0 volume%. If the austenite phase is less than 10.0 volume%, the proportion of the ferrite phase becomes high, resulting in excessive softening and making it impossible to control the strength within a predetermined range. Furthermore, the maximum strength of the crystal orientation of the ferrite phase increases, leading to increased earrings and skin irritation. On the other hand, if the austenite phase exceeds 40.0 volume%, the strength becomes excessively high, reducing workability and making it impossible to suppress springback. In this specification, the proportion of the austenite phase in a duplex stainless steel material can be calculated by measuring the total proportion of phases with magnetic crystalline structures (ferrite phase and martensite phase) using the magnetic induction method and subtracting this total proportion from 100. For example, a ferrite scope manufactured by Helmut Fischer GmbH can be used to measure the total proportion of the ferrite and martensite phases.

[0042] The duplex stainless steel material according to the embodiment of the present invention has a maximum intensity of 10 or less, preferably 9 or less, and more preferably 8 or less, in the crystal orientation of the ferrite phase. Here, in this specification, "crystal orientation intensity" indicates how many times greater the diffraction intensity is compared to the case where the crystal grain orientation is random. Therefore, the higher the maximum crystal orientation intensity of the ferrite phase, the more the crystal grains of the ferrite phase are oriented in a specific direction (especially the rolling direction). If the maximum crystal orientation intensity of the ferrite phase is within the above range, the crystal grains of the ferrite phase are oriented randomly, and a texture that increases in-plane anisotropy ({100} <011> , {211} <011> Because the development of ridging is suppressed, earrings become easier to reduce. Also, since ridging is an unevenness caused by a collection of crystal grains in a specific direction that has progressed in the rolling direction, the ridging height is also reduced as the ferrite phase is randomly oriented, which makes it easier to suppress skin irritation. The lower limit of the maximum intensity of the crystal orientation of the ferrite phase is not particularly limited, but for example, it is 1.

[0043] Here, the maximum intensity of the crystal orientation of the ferrite phase can be measured by X-ray diffraction. Specifically, using an X-ray diffractometer, pole measurements are performed in the central region in the thickness direction of the duplex stainless steel material using Mo-Kα rays, and positive pole figures of (200), (310), and (211) are obtained. Next, from the obtained positive pole figures, a three-dimensional crystal orientation density function is obtained using the spherical harmonic method, and then the crystal orientation distribution of the ferrite phase in a φ2=45° cross section is determined using a texture notation called the Bunge method, and the maximum value of that orientation intensity is taken as the maximum intensity of the crystal orientation of the ferrite phase.

[0044] In the embodiment of the present invention, the duplex stainless steel material has an average particle size of the ferrite phase, preferably 5.0 μm or more, more preferably 5.5 μm or more, and even more preferably 6.0 μm or more. If the average particle size of the ferrite phase is less than 5.0 μm, the duplex stainless steel material will not soften sufficiently, resulting in reduced workability and increased susceptibility to springback. The upper limit of the average particle size of the ferrite phase is not particularly limited, but is typically 30.0 μm, preferably 28.0 μm, and more preferably 25.0 μm. In this specification, the average grain size of the ferrite phase in duplex stainless steel can be determined by EBSD (backscattered electron diffraction) measurement. Specifically, EBSD measurement is performed on a sample of duplex stainless steel in which the thickness-direction cross-section parallel to the rolling direction has been mirror-polished. The average area of ​​the crystal grains of the ferrite phase (BCC) is determined from the data obtained from this EBSD measurement by the area fraction method. The diameter of a circle with the same area as the average area of ​​the crystal grains obtained in this way is taken as the average grain size of the ferrite phase.

[0045] In the embodiment of the present invention, the duplex stainless steel material has an aspect ratio of the ferrite phase that is preferably 0.15 or higher, more preferably 0.18 or higher, even more preferably 0.20 or higher, and particularly preferably 0.23 or higher. If the aspect ratio of the ferrite phase is less than 0.15, the crystal grains of the ferrite phase tend to orient in a specific direction, which can easily lead to earrings and skin irritation. The upper limit of the aspect ratio of the ferrite phase is not particularly limited, but is typically 0.80, preferably 0.60, and more preferably 0.50. In this specification, the aspect ratio of the ferrite phase in duplex stainless steel is calculated by using the analysis software TSL OIM Data Collection 7 (TSL Solutions Co., Ltd.) on EBSD data obtained when measuring the average grain size of the ferrite phase, calculating the aspect ratio for all ferrite phase grains, and deriving the average aspect ratio by the area fraction method. The aspect ratio is calculated by approximating the ferrite phase grains as elliptical shapes to derive the major and minor axes, and then dividing the minor axis length in the ferrite grain by the ratio of the major axis length in the ferrite grain.

[0046] In the austenite phase of the duplex stainless steel material according to the embodiment of the present invention, the Md value represented by formula (2) above is preferably 0.0 to 130.0°C, more preferably 10.0 to 120.0°C, even more preferably 15.0 to 110.0°C, and particularly preferably 20.0 to 100.0°C. If the Md value of the austenite phase is less than 0.0°C, it becomes difficult to transform the austenite phase into the work-induced martensite phase, making it impossible to secure the desired ductility, and the maximum strength of the crystal orientation of the ferrite phase increases, which can easily lead to increased earrings and skin roughness. On the other hand, if the Md value of the austenite phase exceeds 130.0°C, the amount of work-induced martensite phase that transforms from the austenite phase becomes too large, resulting in excessively high strength, which may prevent the securing of the desired ductility. In this specification, the content of each element in the austenite phase used to calculate the Md content of the austenite phase can be measured by EPMA (electron probe microanalyzer). Specifically, a sample of duplex stainless steel material with a thickness-direction cross-section mirror-polished parallel to the rolling direction is used, and qualitative analysis is performed by EPMA. Since C and N are characterized by their concentration in the austenite phase, qualitative mapping of C or N is performed over the entire cross-section to identify the austenite phase. Then, C, N, Si, Mn, Cr, Ni, Cu, and Mo are quantitatively analyzed at approximately the center of the austenite phase, ensuring that the electron beam does not strike the ferrite phase. Quantitative analysis is performed at three or more points, and the average value is used as the result for the content of each element.

[0047] In the embodiment of the present invention, the duplex stainless steel material has a martensitic transformation initiation temperature Ms of the austenite phase, represented by the following formula (3), which is preferably 0°C or lower, more preferably -10.0°C or lower, even more preferably -20.0°C or lower, and particularly preferably -30.0°C or lower. Ms={3000[0.068-(C+N)]+50(0.47-Si)+60(1.33-Mn)+110[8.9-(Ni+Cu)]+75(14.6-Cr)-32}×5 / 9 · · · (3) In equation (3), the element symbols represent the mass percentage of each element in the austenite phase. Here, the martensitic transformation initiation temperature Ms of the austenite phase is an indicator of the thermal stability of the austenite phase after finish annealing. By controlling the martensitic transformation initiation temperature Ms of the austenite phase to 0°C or lower, the transformation of the austenite phase into the martensite phase during the cooling process after finish annealing can be suppressed, and the proportion of the austenite phase can be controlled within a predetermined range. The lower limit of the martensitic transformation initiation temperature Ms of the austenite phase is not particularly limited, but is typically -500°C, preferably -400°C, and more preferably -300°C. Here, the content of each element in the austenite phase used to calculate the martensitic transformation initiation temperature Ms of the austenite phase can be measured in the same manner as the content of each element in the austenite phase used to calculate Md of the austenite phase.

[0048] The duplex stainless steel material according to the embodiment of the present invention has a 0.2% yield strength of preferably 400 to 550 MPa, more preferably 410 to 530 MPa, and 420 MPa or more and less than 500 MPa. If the 0.2% yield strength is 400 MPa or more, the duplex stainless steel material can be said to have high strength. Furthermore, if the 0.2% yield strength is 550 MPa or less, springback can be suppressed during processing. Herein, in this specification, the 0.2% yield strength of duplex stainless steel refers to the 0.2% yield strength in both the 0° direction (parallel direction) and the 90° direction (perpendicular direction) with respect to the rolling direction. The 0.2% yield strength of duplex stainless steel can be measured in accordance with JIS Z2241:2011 by cutting JIS 13B test specimens from the duplex stainless steel so that the parallel portions are oriented at 0° and 90° with respect to the rolling direction.

[0049] In the duplex stainless steel material according to the embodiment of the present invention, the difference in 0.2% yield strength between the 0° direction and the 90° direction with respect to the rolling direction is preferably 20 MPa or less, more preferably 18 MPa or less, and even more preferably 15 MPa or less. Here, the difference in 0.2% yield strength between the 0° and 90° directions relative to the rolling direction is one indicator of the transition from elastic to plastic deformation. A significant difference in this range indicates the presence of areas that deform easily and areas that do not. Areas that deform easily experience a large inflow during processing (especially during deep drawing), while areas that do not deform easily experience a small inflow, which is one of the causes of earring. By keeping the difference in 0.2% yield strength between the 0° and 90° directions relative to the rolling direction within the range described above, the amount of deformation during processing can be made uniform, thereby reducing earring. Note that the difference in 0.2% yield strength between the 0° and 90° directions relative to the rolling direction should be as small as possible, even 0 MPa, but it is typically 1 or 2 MPa.

[0050] The duplex stainless steel material according to the embodiment of the present invention has a ridging height of preferably 5 μm or less, more preferably 4 μm or less. Within this range of ridging height, surface roughness can be suppressed. The lower limit of the ridging height is preferably as low as 0 μm, but is typically 1 μm. Here, the rigging height is determined by cutting a JIS No. 5 test specimen from a duplex stainless steel material such that the parallel portion is oriented at 0° with respect to the rolling direction, and using this specimen, measuring the surface roughness with a roughness meter when a 16% tensile strain is applied in the rolling direction, and defining the magnitude of the surface roughness as the rigging height.

[0051] The duplex stainless steel material according to the embodiment of the present invention may be hot-rolled or cold-rolled, and may be subjected to annealing or pickling. Among these, the duplex stainless steel material according to the embodiment of the present invention is preferably cold-rolled.

[0052] The thickness of the duplex stainless steel material according to the embodiment of the present invention is not particularly limited and can be adjusted as appropriate depending on the application, but is generally 5.0 mm or less, preferably 4.0 mm or less, more preferably 3.0 mm or less, and particularly preferably 2.0 mm or less. When the duplex stainless steel material is in the form of a rod, the thickness refers to the equivalent diameter of the cross-section. When the duplex stainless steel material is in the form of a structural steel, the thickness refers to the thickness at any point in the cross-section.

[0053] The method for manufacturing a duplex stainless steel material according to the embodiment of the present invention is not particularly limited as long as it is a method capable of manufacturing a duplex stainless steel material having the above-described characteristics. The following describes an example of a method for manufacturing duplex stainless steel according to an embodiment of the present invention. The duplex stainless steel material according to the embodiment of the present invention can be manufactured by melting stainless steel having the above composition by vacuum melting to form a steel slab, then hot rolling and annealing, followed by cold rolling and finish annealing.

[0054] Hot rolling is not particularly limited and can be carried out in accordance with conventional methods, but it is preferable to set the temperature immediately after the final pass to 950°C or higher, and then cool it down to 800°C at a cooling rate of 20°C / second or higher. By performing hot rolling under such conditions, it is easier to obtain a duplex stainless steel material having the above-mentioned characteristics.

[0055] Annealing after hot rolling is performed by heating at a rate of 20°C / second or higher, holding at a target temperature of 1100-1150°C for 10 seconds or more, and then cooling to below 400°C at a cooling rate of 20°C / second or higher. The reason for performing annealing under these conditions is to sufficiently dissolve the carbides and nitrides precipitated during cooling after hot rolling, and to suppress the precipitation of carbides and nitrides during the cooling process after annealing. Furthermore, it is to control the proportion of the austenite phase within an appropriate range, reduce the suppression of grain growth in the ferrite phase, and coarseen the average grain size of the ferrite phase, while ensuring a certain proportion of the austenite phase to utilize the work-induced martensitic transformation in the cold rolling process. In particular, if the target temperature is lower than 1100°C, the solid solution of carbides and nitrides will be insufficient, and the proportion of the austenite phase will be too high. Conversely, if the target temperature is higher than 1150°C, although the carbides and nitrides will be sufficiently dissolved, the proportion of austenite will be too low. Furthermore, a certain amount of carbon and nitrogen dissolves in the ferrite phase, and there is a risk that precipitates will form during cooling in the ferrite phase, which has a small solid solubility limit, potentially degrading its corrosion resistance.

[0056] Cold rolling begins with a first cold rolling operation using large-diameter rolls with a roll radius of 300-500 mm. By using large-diameter rolls in the initial stages of cold rolling, rolling strain is introduced to the center of the sheet thickness, causing a portion of the austenite phase to undergo a work-induced martensitic transformation at the center of the sheet thickness, and randomizing the orientation rotation of the ferrite phase during cold rolling. From the viewpoint of stably obtaining this effect, a roll radius of 350 mm or more is more preferable. Furthermore, in the first cold rolling stage, cold rolling is carried out until the total cold rolling rate reaches 50% or more. This allows a sufficient amount of martensite phase to be formed in the center of the sheet thickness during the initial stages of cold rolling. From the viewpoint of stably obtaining this effect, the cold rolling rate in the first cold rolling stage is preferably 55% or more of the total cold rolling rate, and more preferably 60% or more. After the first cold rolling, a second cold rolling process is carried out using small-diameter rolls with a diameter of 50 to 200 mm or less until the target plate thickness is reached. The use of small-diameter rolls in the second cold rolling process is to prevent defects such as oil pits from occurring on the surface of the cold-rolled material and to improve surface quality.

[0057] The conditions for cold rolling (first cold rolling and second cold rolling) are a total rolling rate of 50-90% and a rolling rate of 20% or less per cold rolling pass. The reason for setting the total cold rolling ratio to 50% or more is to impart rolling strain through cold rolling, thereby causing a portion of the austenite phase to undergo a work-induced martensitic transformation, fragmenting the layered austenite phase, and randomizing the crystal orientation rotation of the ferrite phase. In addition, crushing or spreading carbides and precipitates increases the surface area, which can promote solid solution during heat treatment. Furthermore, the reason for setting the total rolling ratio to 90% or less is to suppress edge breakage due to excessive rolling and to prevent the microstructure of the finished annealed material from becoming too fine due to the accumulation of rolling strain. This is because excessive rolling strain excessively induces recrystallization of the ferrite phase, resulting in finer grain size, and this is intended to be avoided. From the viewpoint of stably obtaining this effect, a rolling ratio of 85% or less is more preferable. Furthermore, the reason for limiting the rolling rate per cold rolling pass to 20% or less is to minimize processing heat generation during cold rolling and increase the amount of processing-induced martensitic transformation. From the viewpoint of stably obtaining this effect, a rolling rate of 18% or less per cold rolling pass is preferable, and 16% or less is more preferable. Furthermore, if cold rolling is performed two or more times, intermediate annealing may be carried out between each cold rolling step. If intermediate annealing is performed, the conditions should be the same as those for annealing after hot rolling.

[0058] The conditions for finish annealing are to heat the material at a rate of 20°C / second or higher, hold it at a target temperature of 1040-1150°C for 5 seconds or more, then cool it to 850°C or lower at a cooling rate of 30°C / second or higher, and finally cool it to 400°C or lower at a cooling rate of 20°C / second or higher. These conditions are used for finish annealing to suppress precipitation of carbides and nitrides during heating, to complete recrystallization, to ensure solid solution of carbides and nitrides, to control the proportion of the austenite phase, to suppress fluctuations in the proportion of the austenite phase during cooling, and to suppress reprecipitation of carbides and nitrides. In particular, if the target temperature is lower than 1040°C, the proportion of the austenite phase becomes too high, inhibiting the growth of recrystallized grains of the ferrite phase, which tended to become randomized during the cold rolling process. In addition, solid solution of carbides and nitrides becomes insufficient. Conversely, if the target temperature is higher than 1150°C, while carbides and nitrides are sufficiently dissolved, the proportion of austenite becomes too low. Furthermore, a certain amount of carbon and nitrogen dissolves in the ferrite phase, and there is a risk that precipitates will form during cooling in the ferrite phase, which has a small solid solubility limit, potentially degrading its corrosion resistance.

[0059] The duplex stainless steel material according to the embodiment of the present invention can suppress springback and surface roughness, and reduce earring. Therefore, this duplex stainless steel material can be used in various applications where these properties are required (for example, building materials and structural materials). [Examples]

[0060] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0061] (Examples 1-6 and Comparative Examples 1-14) Stainless steel having the composition shown in Table 1 (the remainder being Fe and impurities) was melted down by vacuum melting to form a steel slab. Next, this steel slab was hot-rolled to obtain a hot-rolled sheet of a predetermined thickness (thickness before cold rolling, shown in Table 2). In the hot-rolling process, the temperature immediately after the final pass was set to the temperature shown in Table 2, and it was cooled to 800°C by water cooling (cooling rate of 20°C / sec or more). Next, the hot-rolled sheet was held at the target temperature (annealing temperature) shown in Table 2 for 30 seconds to anneal it, and then cooled to 400°C or below by water cooling (cooling rate of 20°C / sec or more) to obtain a hot-rolled annealed sheet. Next, the hot-rolled annealed sheet was cold-rolled (first cold rolling and second cold rolling) under the conditions shown in Table 2 to obtain a cold-rolled sheet. The roll diameter of the small-diameter rolls used in the second cold rolling was set to 90 mm. Next, the cold-rolled sheet was held at the target temperature (annealing temperature) shown in Table 2 for 30 seconds for finish annealing, and then cooled to 400°C or below by water cooling (cooling rate of 30°C / second or more) to obtain a cold-rolled annealed sheet. In Table 1, the values ​​for DF and Md were calculated using the content of each element and based on the above formulas (1) and (2).

[0062] [Table 1]

[0063] [Table 2]

[0064] The cold-rolled and annealed sheets obtained above were evaluated as follows.

[0065] <Percentage of Austenite Phase (γ Phase)> At five arbitrary locations on the rolled surface of the cold-rolled and annealed sheet, the proportion of the ferrite phase (however, if the martensite phase is present, the total proportion of the ferrite phase and the martensite phase) was measured using a ferritescope (FERITSCOPE® FMP30 manufactured by Helmut Fischer AG), and the average value was taken as the proportion of the ferrite phase or the total proportion of the ferrite phase and the martensite phase. The proportion of the austenite phase was obtained by subtracting this proportion of the ferrite phase or the total proportion of the ferrite phase and the martensite phase from 100.

[0066] <Maximum intensity of crystal orientation in the ferrite phase (α phase)> Using an X-ray diffractometer (manufactured by Rigaku Denki Kogyo Co., Ltd.), pole measurements were performed in the central region in the thickness direction of the cold-rolled annealed sheet using Mo-Kα rays, and positive pole figures (200), (310), and (211) were obtained. The central region in the thickness direction of the cold-rolled annealed sheet was revealed by mechanical polishing followed by electrolytic polishing. Next, a three-dimensional crystal orientation density function was obtained from the obtained positive pole figures using the spherical harmonic method, and then the crystal orientation distribution of the ferrite phase in a φ2=45° cross section was determined using a texture notation called the Bunge method, and the maximum value of the orientation intensity was taken as the maximum crystal orientation intensity of the ferrite phase.

[0067] <Average particle size and aspect ratio of the ferrite phase (α phase)> After cutting test specimens from cold-rolled and annealed sheets, the thickness-direction cross-sections parallel to the rolling direction were mirror-polished, and EBSD (backscattered electron diffraction) measurements were performed. The EBSD measurements were performed using a scanning electron microscope with the analysis software TSL OIM Data Collection 7 (TSL Solutions Co., Ltd.), measuring a 300 μm square region in the center of the thickness-direction of the test specimen with a step size of 0.7 μm. The average area of ​​the ferrite phase (BCC) grains was determined from the data obtained from these EBSD measurements using the area fraction method. The diameter of a circle with the same area as the average grain area obtained in this way was defined as the average grain size of the ferrite phase. Furthermore, using the EBSD data obtained above, the aspect ratio was calculated for all ferrite phase grains using the analysis software TSL OIM Data Collection 7 (TSL Solutions Co., Ltd.), and the average aspect ratio was derived using the area fraction method.

[0068] <Md and martensitic transformation onset temperature Ms for the austenite phase (γ phase)> After cutting test specimens from cold-rolled and annealed sheets, the thickness-direction cross-sections parallel to the rolling direction were mirror-polished, and component analysis was performed using EPMA (electron probe microanalyzer). Specifically, since C and N are characterized by enrichment in the austenite phase, qualitative mapping of C or N was performed across the entire cross-section to identify the austenite phase. Next, C, N, Si, Mn, Cr, Ni, Cu, and Mo were quantitatively analyzed in the approximate center of the austenite phase, ensuring that the electron beam did not strike the ferrite phase. The measurement area was approximately 2 μm square, and measurements were taken at least three points on each specimen, with the average value used as the result for the content of each element. The EPMA measurements were performed under the conditions of an acceleration voltage of 15 kV, a current of 0.2 μA, and a step size of 0.15 μm. Based on the content of each element obtained in this way, the Md and Ms of the austenite phase were calculated.

[0069] <0.2% yield strength> JIS 13B test specimens were cut from cold-rolled and annealed sheets so that the parallel sections were oriented at 0° (parallel direction) and 90° (perpendicular direction) to the rolling direction. Tensile tests were then performed on these specimens in accordance with JIS Z2241:2011. The tensile tests were conducted in an atmospheric environment at room temperature (25°C) at a tensile speed of 3 mm / min. In the tensile tests, the stress at which the strain reached 0.2% was defined as the 0.2% proof stress. Furthermore, the 0.2% proof stress in the 0° direction relative to the rolling direction and the 0.2% proof stress in the 90° direction were used, and the difference between these 0.2% proof stresses was calculated. In this evaluation, if the 0.2% yield strength in the 0° and 90° directions is between 400 and 550 MPa, it can be determined that there is an effect in suppressing springback. Furthermore, if the difference in 0.2% yield strength is 20 MPa or less, it can be determined that there is an effect in reducing earring.

[0070] <Rising height> A JIS No. 5 test specimen was cut from a cold-rolled and annealed sheet so that the parallel portion was oriented at 0° to the rolling direction. Using this specimen, the surface roughness (maximum unevenness) was measured with a roughness meter when a tensile strain of 16% was applied in the rolling direction. In this evaluation, if the lysing height is 5 μm or less, it can be judged that there is an effect in suppressing skin roughness.

[0071] The evaluation results are shown in Table 3. Note that in Table 3, Comparative Example 10 was a single ferrite phase, so the Md and Ms values ​​of the austenite phase could not be calculated.

[0072] [Table 3]

[0073] As shown in Table 3, Examples 1-6 controlled the composition, DF, Md, proportion of the austenite phase, and maximum intensity of the ferrite phase crystal orientation within an appropriate range, resulting in good results for the 0.2% yield strength (0° and 90° directions), their difference, and rigning height. In contrast, in Comparative Example 1, the roll diameter during the first cold rolling was too small, resulting in a high maximum strength for the crystal orientation (the ferrite phase grains could not be randomly oriented). Consequently, the difference in 0.2% yield strength and rhyming height was insufficient, making it impossible to reduce earrings or suppress surface roughness. In Comparative Example 2, the rolling rate per cold rolling pass was too high, resulting in a large maximum strength in the crystal orientation (the ferrite phase grains could not be randomly oriented). Consequently, the 0.2% yield strength difference and rhyming height were insufficient, making it impossible to reduce earrings or suppress surface roughness. In Comparative Example 3, the total rolling ratio during cold rolling was too low, resulting in a high maximum strength in the crystal orientation (the ferrite phase grains could not be randomly oriented). Consequently, the 0.2% yield strength difference and rhyming height were insufficient, making it impossible to reduce earrings or suppress skin roughness. In Comparative Example 4, the cold rolling ratio of the first cold rolling was too low as a percentage of the total cold rolling ratio, resulting in a high maximum strength in the crystal orientation (the ferrite phase grains could not be randomly oriented). Consequently, the 0.2% yield strength difference and rhyming height were insufficient, making it impossible to reduce earrings or suppress skin roughness. In Comparative Example 5, the annealing temperature was too low during annealing after hot rolling, resulting in increased maximum strength in the crystal orientation (the ferrite phase grains could not be randomly oriented). Consequently, the 0.2% yield strength difference and rigning height were insufficient, making it impossible to reduce earrings or suppress surface roughness. In Comparative Example 6, the annealing temperature was too low during the finish annealing process, resulting in a high proportion of the austenite phase. Consequently, the 0.2% yield strength in the 90° direction was high, and the difference in 0.2% yield strengths was also large, making it impossible to suppress springback or reduce earring.

[0074] In Comparative Example 7, the DF of the cold-rolled and annealed sheet was too low, resulting in a high proportion of the austenite phase. Consequently, the 0.2% yield strength in the 90° direction was high, and springback could not be suppressed. In Comparative Example 8, the Md content of the cold-rolled and annealed sheet was too low, resulting in a high maximum intensity for the crystal orientation (the ferrite phase grains could not be randomly oriented). Consequently, the 0.2% yield strength difference and rhyming height were insufficient, making it impossible to reduce earrings or suppress skin roughness. Comparative Example 9 had excessively high Cr and N content, resulting in high maximum intensity in the crystal orientation (the ferrite phase grains could not be randomly oriented). Consequently, the 0.2% proof stress in the 90° direction was high, and the difference in 0.2% proof stress and rhythmic height were insufficient, making it impossible to suppress springback and skin irritation or reduce earrings. Comparative Example 10 had low Mn, Ni, and Cr content, and the DF and Md of the cold-rolled and annealed sheet were also outside the specified range. As a result, it was a single ferrite phase, and the maximum strength of the crystal orientation was also high (the crystal grains of the ferrite phase could not be randomly oriented). Consequently, the 0.2% yield strength was too low to secure the desired strength, and the rigning height was also high, making it impossible to suppress surface roughness. Comparative Example 11 had a high nitrogen content, and the DF and Md values ​​of the cold-rolled and annealed sheet were outside the specified range, resulting in a high proportion of the austenite phase. Consequently, the 0.2% yield strength was too high, making it impossible to suppress springback. Comparative Example 12 had high Si, Mn, and N content, and the DF and Md values ​​of the cold-rolled and annealed sheet were outside the specified range. As a result, the proportion of the austenite phase was high, and the maximum strength of the crystal orientation was also high (the crystal grains of the ferrite phase could not be randomly oriented). Consequently, the 0.2% yield strength was too high, making springback likely, and the difference in 0.2% yield strength was too large to reduce earring. Comparative Example 13 had high content of Si, Ni, Cr, and N, and the Md content of the cold-rolled and annealed sheet was also outside the specified range. As a result, the proportion of the austenite phase was high, and the maximum strength of the crystal orientation was also high (the crystal grains of the ferrite phase could not be randomly oriented). Consequently, the 0.2% yield strength was too high, making springback likely, and the difference in 0.2% yield strength was too large to reduce earring. Comparative Example 14 had high content of Si, Ni, Cr, Mo, and N, low content of Mn, and the Md content of the cold-rolled and annealed sheet was outside the specified range. As a result, the proportion of the austenite phase was high, and the maximum strength of the crystal orientation was also high (the crystal grains of the ferrite phase could not be randomly oriented). Consequently, the 0.2% yield strength was too high, making springback likely; the difference in 0.2% yield strength was too large, making earring likely; and the rigning height was also large, making it difficult to suppress surface roughness.

[0075] As can be seen from the above results, the present invention provides a ferritic-austenitic duplex stainless steel material that can suppress springback and skin irritation and reduce earring wear.

Claims

1. The composition, by mass, consists of C: 0.001-0.050%, Si: 0.01-0.50%, Mn: 1.0-3.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5-3.0%, Cr: 19.6-23.0%, Mo: 0.01-1.00%, Cu: 0.01-1.00%, N: 0.010-0.090%, with the remainder being Fe and impurities. The following formula (1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9... (1) The DF value shown in the formula (where the element symbol represents the content (mass %) of each element) is between 60.0 and 80.

0. The following formula (2): Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) The Md value shown in the formula (where the element symbol represents the content (mass %) of each element) is between 80.0 and 150.0°C. The austenite phase is 10.0 to 40.0 volume percent. A ferrite-austenitic duplex stainless steel material in which the maximum intensity of the ferrite phase crystal orientation is 10 or less.

2. The ferritic-austenitic duplex stainless steel material according to claim 1, further comprising one or more elements selected by mass from Nb: 0.001 to 0.500%, Ti: 0.001 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 0.50%, 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%.

3. The ferrite-austenitic duplex stainless steel material according to claim 1 or 2, wherein the average particle size of the ferrite phase is 5.0 μm or more.

4. The ferrite-austenitic duplex stainless steel material according to claim 1 or 2, wherein the aspect ratio of the ferrite phase is 0.15 or more.

5. The ferritic-austenitic duplex stainless steel material according to claim 1 or 2, wherein the value of Md represented by formula (2) of the austenite phase is 0.0 to 130.0°C.

6. The austenite phase is given by the following formula (3): Ms={3000[0.068-(C+N)]+50(0.47-Si)+60(1.33-Mn)+110[8.9-(Ni+Cu)]+75(14.6-Cr)-32}×5 / 9... (3) The ferrite-austenitic duplex stainless steel material according to claim 1 or 2, wherein the martensitic transformation onset temperature Ms, represented by the formula (wherein element symbols represent the content (mass%) of each element in the austenite phase), is 0°C or less.

7. A ferritic-austenitic duplex stainless steel material according to claim 1 or 2, wherein the 0.2% yield strength is 400 to 550 MPa.

8. A ferritic-austenitic duplex stainless steel material according to claim 1 or 2, wherein the difference in 0.2% yield strength between the 0° direction and the 90° direction with respect to the rolling direction is 20 MPa or less.

9. A ferritic-austenitic duplex stainless steel material according to claim 1 or 2, wherein the rigning height is 5 μm or less.

Images