Ferritic-austenitic duplex stainless steel material and method for producing the same

Abstract

To provide a high-strength, low-cost ferritic-austenitic duplex stainless steel material.SOLUTION: A ferritic-austenitic duplex stainless steel material has a composition including, on a mass basis, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 3.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 23.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.00%, and N: 0.010 to 0.090%, the balance being Fe and impurities, DF: 60.0 to 80.0, Md: 90.0 to 150.0, as shown in the formula below, and the martensite phase being 1 to 30 vol.%. Equation (1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9. Equation (2): Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo. (Symbol of element represents content mass%)SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic-austenitic duplex stainless steel material and a method for producing the same. [Background technology]

[0002] Ferrite-austenitic duplex stainless steels have excellent corrosion resistance and strength, and are used in a variety of applications, including building and structural materials. In recent years, as their applications have expanded, high strength, with a 0.2% yield strength of 600 MPa or more, has been required of ferrite-austenitic duplex stainless steels. Addition of austenite-forming elements such as Ni and Cu, and N, is known as a method for increasing the strength of ferritic-austenitic duplex stainless steels.

[0003] For example, Patent Document 1 proposes a duplex stainless steel material containing, by weight, 0.020% or less C, 0.30 to 1.50% Si, 0.50 to 1.50% Mn, 0.040% or less P, 0.015% or less S, 1.0 to 3.0% Cu, 3.00 to 5.00% Ni, 22.00 to 28.00% Cr, 2.00 to 5.00% Mo, 0.12 to 0.25% N, 0.50% or less W, 0.0100% or less Ca, and 0.0100% or less B, with the balance being Fe and impurity elements, having a ferrite-austenite two-phase structure, with the area fraction of the ferrite phase being 40 to 70%. It is stated that this duplex stainless steel material has a 0.2% yield strength of 600 MPa or more.

[0004] Patent Document 2 proposes a duplex stainless steel material having a chemical composition that satisfies a predetermined formula, containing, by mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 0.10 to 9.00%, P: 0.040% or less, S: 0.0010% or less, Cr: 20.0 to 32.0%, Ni: 3.5 to 10.0%, Mo: 0.5 to 5.0%, Cu: 0.5 to 6.0%, V: 0.01% or more but less than 0.10%, B: 0.0010 to 0.0050%, N: less than 0.150%, and O: 0.0001 to 0.0070%, with the balance being Fe and impurities; a microstructure consisting of 30 to 80% by volume of ferrite and the balance being austenite; and a yield strength of 655 MPa or more.

[0005] Furthermore, Patent Document 3 describes a chemical composition, in mass %, of C: 0.04% or less, Si: 0.10 to 0.90%, Mn: 0.20 to 0.70%, P: 0.040% or less, S: 0.010% or less, Cu: 1.00 to 3.00%, Ni: 4.00 to 8.00%, Cr: 28.00 to 35.00%, Mo: 0.50 to 1.40%, V: 0.03 has been proposed a duplex stainless steel material having a yield strength of 550 MPa or more, containing 0.05 to 0.50%, 0.0005 to 0.0040%, 0.0005 to 0.0040%, 0.0040% to 0.20%, 0.350% to 0.700%, 0.030% or less, 0.05 to 0.50%, 0.0005 to 0.004 ...40% to 0.0040%, and the balance being Fe and impurities. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-171743 [Patent Document 2] Patent No. 7364955 [Patent Document 3] Patent Publication No. 2021-155774 Summary of the Invention [Problem to be solved by the invention]

[0007] In order to reduce costs, it is necessary to reduce the amount of alloying elements in ferritic-austenitic duplex stainless steel materials. However, Patent Document 1 is expensive because it contains large amounts of expensive elements such as Cu, Ni, and Mo. In addition, the amount of N is also large, which may reduce productivity. Furthermore, Patent Document 2 also contains a large amount of Ni, and also contains relatively large amounts of Mn, Mo, and Cu, resulting in high costs. Furthermore, Patent Document 3 also contains large amounts of Cu, Ni, and Cr, which makes it expensive.In addition, the amount of N is also large, which may reduce productivity.

[0008] The present invention aims to provide a high-strength, low-cost ferritic-austenitic duplex stainless steel material and a method for producing the same. [Means for solving the problem]

[0009] The present inventors have conducted extensive research into ferritic-austenitic duplex stainless steel materials in order to solve the above-mentioned problems, and as a result have come to the following realization. The strength of ferritic-austenitic duplex stainless steel can be improved by solid solution strengthening through the addition of alloying elements (e.g., Ni) and by grain refinement through a mixed structure of ferritic and austenitic phases. To achieve high strength through solid solution strengthening, the amount of Ni and other elements must be increased. Lean (reduced alloy) ferritic-austenitic duplex stainless steel sheets with low amounts of Ni and other elements are also available, but these require increased amounts of Cu and Mn in addition to N. Therefore, while reducing the amount of expensive elements such as Ni to keep costs down, we attempted to compensate for the reduced effect of solid solution strengthening by other methods. As a result, we found that by utilizing stress-induced transformation, high strength can be achieved even when the amount of expensive elements such as Ni is reduced.

[0010] Based on the above findings, the inventors discovered that the above problems can be solved by controlling the composition of the ferritic-austenitic duplex stainless steel material (including the values ​​of DF and Md) and the proportions of the austenite phase and martensite phase within appropriate ranges, and thus completed the present invention.

[0011] That is, the present invention provides a steel sheet containing, by mass, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 3.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 23.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.00%, N: 0.010 to 0.090%, and the balance 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) (wherein the element symbols represent the content (mass%) of each element) is 60.0 to 80.0, The following formula (2): Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) (wherein the element symbols represent the content (mass%) of each element) is 90.0 to 150.0°C, The present invention relates to a ferritic-austenitic duplex stainless steel material having a martensite phase of 1 to 30 volume %.

[0012] The present invention also provides a steel sheet containing, by mass, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 3.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 23.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.00%, N: 0.010 to 0.090%, and the balance 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) (wherein the element symbols represent the content (mass%) of each element) is 60.0 to 80.0, The following formula (2): Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) The present invention relates to a method for producing a ferritic-austenitic duplex stainless steel material, in which a cold-rolled material having an Md value of 90.0 to 150.0°C, as shown in the formula (wherein the element symbols represent the content (mass%) of each element), is finish-annealed, and then subjected to a stress-induced transformation treatment so that the martensite phase accounts for 1 to 30% by volume. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a high-strength, low-cost ferritic-austenitic duplex stainless steel material and a method for producing the same. DETAILED DESCRIPTION OF THE INVENTION

[0014] The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0015] A ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention (hereinafter simply referred to as "duplex stainless steel material") has a composition containing C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 3.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 23.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.00%, N: 0.010 to 0.090%, with the balance being Fe and impurities.

[0016] Here, in this specification, "stainless steel material" means a material formed from stainless steel, and the shape of the material is not particularly limited. Examples of the shape include a plate (including a strip), a rod, and a tube. In addition, the cross section may be a variety of shaped steel such as a T-shape or an I-shape. In this specification, "ferritic-austenitic" refers to a metal structure that includes a two-phase structure consisting primarily of ferrite and austenite at room temperature. Therefore, "ferritic-austenitic" also includes metals that include phases other than ferrite and austenite (especially martensite). Furthermore, in this specification, "impurities" refer to components that are mixed in during industrial production of stainless steel materials due to various factors in raw materials such as ores and scraps, and in the manufacturing process, 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, "including xx% or less" means that the content is xx% or less, but includes an amount exceeding 0% (particularly, exceeding the impurity level).

[0017] The duplex stainless steel material according to an embodiment of the present invention may further contain one or more selected from Nb: 0.010 to 0.500%, Ti: 0.010 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%, as needed. Each component will be described in detail below.

[0018] <C:0.001~0.050%> C is an element that has a significant effect on 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, causing intergranular corrosion. Therefore, the C content is set to 0.050% or less, preferably 0.045% or less, and more preferably 0.040% or less. Furthermore, from the viewpoint of corrosion resistance, a low C content is preferable, but reducing the C content too much leads to an increase in costs. Therefore, the C content is set to 0.001% or more, preferably 0.002% or more, and more preferably 0.005% or more.

[0019] <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, the steel will become hard and its ductility will decrease. Therefore, the Si content is set to 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less. Furthermore, if the Si content is reduced too much, the cost of smelting will increase. Therefore, the Si content is set to 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more.

[0020] <Mn:1.0~3.5%> Mn is an element that plays an important role in enriching in the austenite phase and stabilizing the austenite phase. However, if the Mn content is too high, in addition to ductility, corrosion resistance and hot workability also decrease. Therefore, the Mn content is set to 3.5% or less, preferably 3.3% or less, more preferably 3.0% or less. Also, if the Mn content is excessively reduced, the cost during steelmaking increases. Therefore, the Mn content is set to 1.0% or more, preferably 1.1% or more, more preferably 1.2% or more.

[0021] <P: 0.050% or less> P is an element contained in raw materials such as Cr. If the P content is high, the formability decreases. Therefore, the P content is set to 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less. On the other hand, although it is preferable that the P content is low, there is a limit to reducing the P content. The lower limit value of the P content is generally 0.001%, preferably 0.002%, more preferably 0.003%.

[0022] <S: 0.030% or less> S is an element contained in various raw materials. S combines with Mn to form inclusions and may become the starting point of rust. Therefore, 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, more preferably 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 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 raw material costs increase, but the proportion of austenite phase will increase, which may cause problems such as stress corrosion cracking. Therefore, the Ni content is set to 3.0% or less, preferably 2.8% or less, more preferably 2.6% or less, and even more preferably 2.5% or less.

[0024] <Cr:19.6~23.0%> Cr is an element necessary for ensuring corrosion resistance. To achieve this effect, the Cr content is set to 19.6% or more, preferably 19.8% or more, and more preferably 20.0% or more. On the other hand, if the Cr content is too high, it can cause cracks during hot working and increase the cost of the refining process. Therefore, the Cr content is set to 23.0% or less, preferably 22.5% or less, and more preferably 22.3% or less.

[0025] <Mo:0.01~1.00%> 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.

[0026] <Cu:0.01~1.00%> 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, it will not only increase raw material costs but also reduce hot workability. Therefore, the Cu content is set to 1.00% or less, preferably 0.90% or less, and more preferably 0.80% or less.

[0027] <N:0.010~0.090%> Like C, N is an element that has a significant effect on the stability of the austenite phase. N is also an element that dissolves in solid solution to improve corrosion resistance. To achieve 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, not only will ductility decrease, but corrosion resistance will also decrease due to the precipitation of Cr nitrides. Therefore, the N content is set to 0.090% or less, preferably 0.085% or less.

[0028] <Nb:0.010~0.500%> Nb forms nitrides (NbN) and carbides (NbC) and has the effect of improving workability. To achieve this effect, the Nb content is set to 0.010% or more, preferably 0.013% or more, and more preferably 0.015% 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.

[0029] <Ti:0.010~0.500%> Like Nb, Ti also forms nitrides (TiN) and carbides (TiC) and has the effect of improving workability. To achieve this effect, the Ti content is set to 0.010% or more, preferably 0.011% or more, and more preferably 0.012% or more. On the other hand, if the Ti content is too high, ductility decreases. Therefore, the Ti content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.200% or less.

[0030] <V:0.01~0.50%> V forms nitrides and has the effect of improving workability. To achieve 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 will decrease. Therefore, the V content is set to 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

[0031] <W:0.05~0.50%> 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.

[0032] <Co:0.01~0.30%> 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 0.02% 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.

[0033] <B:0.0002~0.0050%> 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 be significantly reduced. Therefore, the B content is set to 0.0050% or less, preferably 0.0045% or less, and more preferably 0.0040% or less.

[0034] <Sn:0.010~0.500%> Sn is an element that improves corrosion resistance. To exert this effect, the Sn content is set to 0.010% or more, preferably 0.020% or more, more preferably 0.025% or more. On the other hand, if the Sn content is too high, the hot workability will deteriorate. Therefore, the Sn content is set to 0.500% or less, preferably 0.450% or less, more preferably 0.400% or less.

[0035] <Al: 0.010~0.050%> Al is an element effective for desulfurization and deoxidation. To exert these effects, the Al content is set to 0.010% or more, preferably 0.015% or more, more preferably 0.020% or more. On the other hand, if the Al content is too high, it will cause an increase in manufacturing defects and raw material costs. Therefore, the Al content is set to 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less.

[0036] <Mg: 0.0002~0.0100%> Mg is an element that not only deoxidizes but also has the effect of refining the solidification structure. To exert these effects, the Mg content is set to 0.0002% or more, preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, if the Mg content is too high, it will lead to an increase in raw material costs. Therefore, the Mg content is set to 0.0100% or less, preferably 0.0095% or less, more preferably 0.0090% or less.

[0037] <Ca: 0.0002~0.0100%> Ca is an element effective for desulfurization and deoxidation. To exert these effects, the Ca content 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 work cracking is likely to occur and the corrosion resistance also deteriorates. Therefore, the Ca content is set to 0.0100% or less, preferably 0.0080% or less, more preferably 0.0050% or less.

[0038] <Ta: 0.050% or less> Ta is an element that improves corrosion resistance by modifying inclusions. However, if the content of Ta is too high, it will cause a decrease in room-temperature ductility and toughness. Therefore, the content of Ta is 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less. On the other hand, the lower limit of the content of Ta is not particularly limited, but in order to exert the effect of Ta, it is preferably 0.001%, more preferably 0.003%.

[0039] <Ga: 0.050% or less> Ga is an element that contributes to improving corrosion resistance and suppressing hydrogen embrittlement. However, if the content of Ga is too high, the workability will decrease. Therefore, the content of Ga is 0.050% or less, preferably 0.040% or less, more preferably 0.030% or less. On the other hand, the lower limit of the content of Ga is not particularly limited, but in order to exert the effect of Ga, it is preferably 0.001%, more preferably 0.003%.

[0040] <Zr: 0.01 - 0.50%> Zr has an action similar to Nb and Ti and is an element that improves oxidation resistance. In order to exert these effects, the content of Zr is 0.01% or more, preferably 0.02% or more. On the other hand, if the content of Zr is too high, it will cause an increase in raw material cost in addition to a decrease in ductility. Therefore, the content of Zr is 0.50% or less, preferably 0.40% or less, more preferably 0.30% or less.

[0041] <REM: 0.0002 - 0.0100%> REM (rare earth) is an element effective in improving hot workability. In order to exert this effect, the content of REM is 0.0002% or more, preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, if the content of REM is too high, it will impair manufacturability and cause cost increase. Therefore, the content of REM is 0.0100% or less, preferably 0.0095% or less, more preferably 0.0090% or less. REM is a general term for Sc, Y, and 15 elements from La to Lu (lanthanoids). These elements can be used alone or in combination of two or more as REM.

[0042] 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 78.0, and more preferably 61.5 to 76.0. 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 content (%) of each element. Here, DF is an index representing the amount of ferrite phase. Therefore, 100-DF is the total amount of austenite and martensite phases. However, it should be noted that DF is an index determined based on the element content, and therefore does not correspond to the amount of austenite and martensite phases actually measured. By controlling the DF value within the above range, the amount of ferrite phase is appropriate, and high strength can be achieved even with reduced alloying.

[0043] In the duplex stainless steel material according to an embodiment of the present invention, the value of Md represented by the following formula (2) is 90.0 to 150.0°C, preferably 95.0 to 140.0°C, more preferably 98.0 to 135.0°C, and even more preferably 100.0 to 130.0°C. Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) In formula (2), the element symbols represent the content (%) of each element. Here, Md is an index representing the stability of the austenite phase. The larger the Md value (higher temperature), the more unstable the austenite phase. By controlling the Md value within the above range, it is possible to adjust the stability of the austenite phase so that high strength can be achieved even with reduced alloying.

[0044] In the duplex stainless steel material according to the embodiment of the present invention, the martensite phase is 1 to 30 volume %, preferably 1 to 20 volume %, more preferably 2 to 15 volume %, and even more preferably 3 to 12 volume %. By controlling the martensite phase within such a range, high strength can be achieved even with reduced alloying. Herein, the proportion of martensite in a duplex stainless steel material can be determined by EBSD measurement. Specifically, EBSD measurement is performed on a mirror-polished specimen of a thickness cross-section parallel to the rolling direction of the duplex stainless steel material to identify the bcc phase (ferrite and martensite phases) and the fcc phase (austenite phase). Next, the bcc phase data obtained by this EBSD measurement is converted into an IQ (Image Quality) image using OIM analysis software to identify the ferrite and martensite phases, and the proportion of martensite can be determined. Note that an IQ image is an image analysis method that indicates clarity. The martensite phase has a more complex internal structure than the ferrite phase and therefore has lower clarity, so it appears dark in the IQ image. On the other hand, the ferrite phase has a simpler internal structure than the martensite phase and therefore has higher clarity, so it appears bright in the IQ image. Therefore, the proportion of martensite can be calculated by binarizing the IQ image, calculating the area of ​​the martensite phase, dividing it by the total area, and multiplying the result by 100.

[0045] The duplex stainless steel material according to the embodiment of the present invention can achieve high strength by transformation from the austenite phase to the martensite phase, but it is desirable to control the amount of austenite phase in order to achieve a moderate level of strength. Specifically, in order to achieve a 0.2% yield strength of 600 MPa or more, it is preferable to set the amount of austenite phase to 38% by volume or less, and to avoid excessively high strength, it is preferable to control the amount of austenite phase to 1% by volume or more. The amount of austenite phase is more preferably 5 to 38% by volume, even more preferably 10 to 38% by volume, and particularly preferably 20 to 38% by volume. Here, in this specification, the proportion of austenite phase in a duplex stainless steel material can be calculated by measuring the total proportion of phases with magnetic crystal structures (ferrite phase and martensite phase) by magnetic induction method and subtracting this total proportion from 100. For example, a Ferritescope manufactured by Helmut Fischer GmbH can be used to measure the total proportion of the ferrite phase and martensite phase.

[0046] The duplex stainless steel sheet according to the embodiment of the present invention preferably has a 0.2% yield strength of 600 to 1600 MPa, more preferably 650 to 1500 MPa, and even more preferably 800 to 1400 MPa. If the 0.2% yield strength is 600 MPa or more, the duplex stainless steel sheet can be said to have high strength. Furthermore, if the 0.2% yield strength is 1600 MPa or less, the load during manufacturing and processing of the duplex stainless steel material can be reduced. Here, the 0.2% yield strength of the duplex stainless steel sheet 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 can 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 method for producing 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 producing a duplex stainless steel material having the above-described characteristics. An example of a method for manufacturing a duplex stainless steel material according to an embodiment of the present invention will be described below. The duplex stainless steel material according to the embodiment of the present invention can be produced by finish-annealing a cold-rolled material having the above-described composition, and then performing a stress-induced transformation treatment so that the martensite phase is 1 to 30% by volume. The cold-rolled material having the above composition is not particularly limited, but can be produced by vacuum melting stainless steel having the above composition to form a steel slab, followed by hot rolling, annealing, and then cold rolling.

[0050] Although the hot rolling is not particularly limited, it is preferable to set the temperature immediately after the final pass to 950° C. or higher and then cool the material at a cooling rate of 20° C. / sec or higher to 800° C. By performing hot rolling under such conditions, the crystal grains of the ferrite phase can be easily coarsened. There are two main reasons why ferrite grains become finer. 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 facilitating ferrite grain refinement. 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 down, suppressing grain growth. Since the austenite phase decreases with increasing temperature, peaking at around 900°C, the higher the temperature, the easier it is for ferrite grains to grow. On the other hand, increasing the heating temperature of the slab before hot rolling or shortening the heat dissipation time by increasing the rolling speed is effective in maintaining a high hot-rolling temperature, but these methods increase fuel costs and manufacturing difficulties. Therefore, taking these circumstances into consideration, it is preferable to set the lower limit of the final pass temperature of hot rolling to 950°C.

[0051] The annealing temperature after hot rolling is not particularly limited, but is preferably maintained at a target temperature of 1030 to 1150°C for 10 seconds or more, followed by cooling to 400°C or less at a cooling rate of 20°C / second 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, which alleviates the inhibition of grain growth of the ferrite phase and facilitates coarsening of the ferrite phase grains. In particular, if the target temperature is lower than 1030°C, the dissolution of carbides and nitrides is insufficient, and the proportion of austenite phase becomes too high. Furthermore, if the target temperature is higher than 1150°C, although the carbides and nitrides are sufficiently dissolved, the proportion of austenite phase 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.

[0052] The cold rolling conditions are not particularly limited, but a rolling reduction of 40 to 90% is preferred. The rolling reduction of 40% 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, the rolling reduction of 90% 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. 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 the same as those for annealing after hot rolling.

[0053] The conditions for the finish annealing are not particularly limited, but it is preferable to hold the final temperature of 1000 to 1150°C for 5 seconds or more, and then cool at a cooling rate of 50°C / second or more. The reason for performing the finish annealing under these conditions is to suppress the precipitation of carbides and nitrides during heating, complete recrystallization, solid-solution of carbides and nitrides, control the proportion of the austenite phase, suppress fluctuations in the proportion of the austenite phase during cooling, and suppress the reprecipitation of carbides and nitrides. Furthermore, by controlling the structure up to cold rolling, the ferrite phase grains become coarse during the finish annealing.

[0054] The stress-induced transformation treatment is not particularly limited as long as it is a method that results in a martensite phase of 1 to 30% by volume, but it is preferable to perform temper rolling at a rolling reduction of 2 to 50%. By performing temper rolling under these conditions, it is possible to generate a martensite phase of 1 to 30% by volume through stress-induced transformation. From the viewpoint of stably ensuring this effect, the rolling reduction of temper rolling is more preferably 3 to 45%, and even more preferably 3 to 40%.

[0055] The duplex stainless steel material according to the embodiment of the present invention has higher strength and lower cost than conventional duplex stainless steel materials. In particular, the duplex stainless steel material according to the embodiment of the present invention can achieve high strength, with a 0.2% yield strength of 600 MPa or more, despite requiring less alloying. Therefore, this duplex stainless steel material can be used in a variety of applications requiring high strength and low cost. [Example]

[0056] 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.

[0057] (Examples 1 to 5 and Comparative Examples 1 to 5) A stainless steel having the composition shown in Table 1 (the balance being Fe and impurities) was melted by vacuum melting to obtain a steel slab. This steel slab was then hot-rolled to obtain a 5 mm-thick hot-rolled sheet. In the hot-rolling, the temperature immediately after the final pass was set to the temperature shown in Table 2, and the sheet was cooled to 800°C by water cooling (cooling rate of 20°C / sec or more). The hot-rolled sheet was then annealed by holding it at the final temperature (annealing temperature) shown in Table 2 for 30 seconds, and then cooled to 400°C or less by water cooling (cooling rate of 20°C / sec or more) to obtain a hot-rolled annealed sheet. The hot-rolled annealed sheet was then cold-rolled at the rolling reduction shown in Table 2 to obtain a cold-rolled sheet. The cold-rolled sheet was then finish-annealed by holding it at the final temperature (annealing temperature) shown in Table 2 for 30 seconds, and then cooled to 400°C or less by water cooling (cooling rate of 30°C / sec or more) to obtain a cold-rolled annealed sheet. Next, the cold-rolled annealed sheet was subjected to temper rolling at the rolling ratio shown in Table 2 to obtain a temper-rolled sheet (duplex stainless steel sheet). Note that in Comparative Example 1, temper rolling was not performed. In Table 1, the values ​​of DF and Md were calculated based on the above formulas using the contents of each element.

[0058] [Table 1] [Table 2]

[0059] The temper rolled sheets obtained above were evaluated as follows.

[0060] <Proportion of martensite phase in duplex stainless steel> After cutting the specimens from the temper-rolled sheets, the thickness cross sections parallel to the rolling direction were mirror-polished and subjected to EBSD (electron backscatter diffraction) measurements. EBSD measurements were performed using a scanning electron microscope (SEM) with the software TSL OIM Data Collection 7 (TSL Solutions, Inc.) over a 200 μm square area at the center of the specimen's thickness direction, with a step size of 0.3 μm. The data obtained from the EBSD measurements were then analyzed using the software TSL OIM Analysis 7 (TSL Solutions, Inc.) to generate an IQ (Image Quality) image, which identified the ferrite, austenite, and martensite phases. The area of ​​the identified martensite phase was then divided by the total area of ​​the observation region and multiplied by 100 to calculate the proportion of martensite.

[0061] <0.2% yield strength> JIS No. 13B test pieces were cut out from the cold-rolled annealed sheets so that the parallel part was in the rolling direction, and these test pieces were used to perform tensile tests in accordance with JIS Z2241: 2011. The tensile tests were carried out in an air atmosphere at room temperature (25°C) at a tension rate of 10 mm / min, and the stress intensity was measured at a strain of 0.2%.

[0062] The evaluation results are shown in Table 3.

[0063] [Table 3]

[0064] As shown in Tables 1 to 3, in Examples 1 to 5, the composition (including the values ​​of DF and Md) of the temper rolled plate (duplex stainless steel material) and the proportion of martensite phase were within the specified range, so the 0.2% yield strength was 600 MPa or more (high strength was achieved).

[0065] In contrast, in Comparative Example 1, temper rolling was not performed, so the proportion of martensite phase could not be controlled within a predetermined range, and the 0.2% yield strength was less than 600 MPa (strength could not be increased). In Comparative Example 2, the rolling ratio of the temper rolling was too low, so the proportion of the martensite phase could not be controlled within a predetermined range, and the 0.2% yield strength was less than 600 MPa (it was not possible to increase the strength). Comparative Example 3 has a 0.2% yield strength of 600 MPa or more, but is expensive due to its composition containing large amounts of elements such as Ni and Mo. Furthermore, in Comparative Example 3, the proportion of martensite phase could not be controlled within a predetermined range even after temper rolling. Comparative Example 4 has a 0.2% yield strength of 600 MPa or more, but is expensive because it has a composition containing large amounts of elements such as Mn and Si. Comparative Example 5 has a 0.2% yield strength of 600 MPa or more, but is expensive because it has a composition containing large amounts of elements such as Ni and Cr. In Comparative Example 6, the DF value was too low, so the proportion of the martensite phase could not be controlled within a predetermined range, and the 0.2% yield strength exceeded 1600 MPa (workability could not be ensured). In Comparative Example 7, the Md value was too low, so the 0.2% yield strength was less than 600 MPa (strength could not be increased).

[0066] As can be seen from the above results, the present invention can provide a high-strength, low-cost ferritic-austenitic duplex stainless steel material and a method for producing the same.

Claims

1. The alloy contains, on a mass basis, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 3.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 23.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.00%, N: 0.010 to 0.090%, and the balance 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) (wherein the element symbols represent the content (mass%) of each element) is 60.0 to 80.0, The following formula (2): Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) (wherein the element symbols represent the content (mass%) of each element) is 90.0 to 150.0°C, A ferritic-austenitic duplex stainless steel material having a martensite phase of 1 to 30% by volume.

2. The ferritic-austenitic duplex stainless steel material according to claim 1, further comprising, on a mass basis, one or more selected from Nb: 0.010 to 0.500%, Ti: 0.010 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%.

3. The ferritic-austenitic duplex stainless steel material according to claim 1 or 2, wherein the 0.2% yield strength is 600 to 1600 MPa.

4. The alloy contains, on a mass basis, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 3.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 23.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.00%, N: 0.010 to 0.090%, and the balance 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) (wherein the element symbols represent the content (mass%) of each element) is 60.0 to 80.0, The following formula (2): Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo... (2) (wherein the element symbols represent the content (mass%) of each element) is 90.0 to 150.0°C, is finish-annealed, and then is subjected to a deformation-induced transformation treatment so that the martensite phase is 1 to 30% by volume.

5. 5. The method for producing a ferritic-austenitic duplex stainless steel material according to claim 4, wherein the cold-rolled material further contains one or more selected from Nb: 0.010 to 0.500%, Ti: 0.010 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%.

6. 6. The method for producing a ferritic-austenitic duplex stainless steel material according to claim 4, wherein the deformation-induced transformation treatment is temper rolling carried out at a rolling ratio of 2 to 50%.

7. The method for producing a ferritic-austenitic duplex stainless steel material according to claim 4 or 5, wherein the cold-rolled material is obtained by cold-rolling a hot-rolled annealed material at a rolling reduction of 40 to 90%.

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