Ferrite-austenite duplex stainless steel sheet

Abstract

To provide a ferrite-austenite duplex stainless steel sheet that enables compatibility between softening and reduction of in-plane anisotropy.SOLUTION: A ferrite-austenite duplex stainless steel sheet comprises a composition containing, on a mass basis: C: 0.001 to 0.050%; Si: 0.01 to 0.50%; Mn: 1.0 to 4.0%; P: 0.050% or less; S: 0.030% or less; Ni: 1.5 to 3.0%; Cr: 19.6 to 24.0%; Mo: 0.01 to 1.00%; Cu: 0.01 to 1.20%; and N: 0.010 to 0.090%, with the balance being Fe and impurities. The steel sheet has an average grain diameter of a ferrite phase of 5.0 μm or more, and contains an austenite phase in an amount exceeding 25 vol.% and less than 40 vol.%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a ferrite-austenite duplex stainless steel sheet.

Background Art

[0002] Ferrite-austenite duplex stainless steel sheets are used as building materials and structural materials because of their excellent corrosion resistance and high strength. On the other hand, since ferrite-austenite duplex stainless steel sheets have lower ductility than general-purpose austenitic stainless steel sheets such as SUS304, their application to uses that require workability is limited. In particular, since ferrite-austenite duplex stainless steel sheets are high-strength, there is a problem that they are difficult to process. Also, from the viewpoint of cost reduction, there is a need for lean (alloy-saving) type ferrite-austenite duplex stainless steel sheets with reduced alloying elements. Therefore, the development of lean type ferrite-austenite duplex stainless steel sheets with excellent ductility is underway.

[0003] For example, Patent Document 1 proposes a ferrite-austenite duplex stainless steel sheet that contains, in mass%, C: 0.001 to 0.1%, Cr: 17 to 25%, Si: 0.01 to 1%, Mn: 0.5 to 3.7%, N: 0.06% or more and less than 0.15%, satisfies a pitting corrosion resistance index (PI value) represented by Cr + 3Mo + 10N - Mn of more than 18%, and the balance consists of Fe and inevitable impurities, and has an austenite phase volume fraction of 15 to 50% with a ferrite phase as a matrix phase, and is excellent in corrosion resistance and workability. Also, Patent Document 2 proposes a ferrite-austenite duplex stainless steel sheet that contains, in mass%, C: 0.05% or less, Si: 1% or less, Mn: 2 to 8%, P: 0.1% or less, S: 0.02% or less, Cr: 15 to 23%, Mo: 4% or less, Ni: 3.0% or less, Cu: 2% or less, N: 0.05 to 0.3%, and the balance consists of Fe and inevitable impurities, and has excellent ductility, intergranular corrosion resistance, and strength, with Cr equivalent and Ni equivalent satisfying a predetermined relationship.

[0004] In addition, Patent Document 3 proposes a ferritic-austenitic duplex stainless steel sheet excellent in corrosion resistance and workability, which contains, by mass%, C: 0.08% or less, Si: 0.7 to 1.1%, Mn: 2.4 to 3.5%, Cr: 17.9 to 20.7%, Ni: 0.05 to 1.15%, N: 0.18 to 0.3%, Cu: 0.4 to 2.8%, with the balance being Fe and inevitable impurities, and having a pitting potential predicted by a predetermined formula of 360 to 440 mV. Furthermore, Patent Document 4 proposes a ferritic-austenitic duplex stainless steel sheet excellent in formability and corrosion resistance, which contains less than 0.04% by weight of C, 0.2 to 0.8% by weight of Si, 0.3 to 2.0% by weight of Mn, 14.0 to 19.0% by weight of Cr, 2.0 to 5.0% by weight of Ni, 4.0 to 7.0% by weight of Mo, less than 4.5% by weight of W, 0.1 to 1.5% by weight of Cu, and 0.14 to 0.23% by weight of N, with the balance being Fe and inevitable impurities, and the combined effect of the contents of Cr, Mo, and W being in the range of 20 < (Cr + Mo + 0.5W) < 23.5% by weight, and the Cr / (Mo + 0.5W) ratio being in the range of 2 to 4.75.

[0005] However, the ferritic-austenitic duplex stainless steel sheets described in Patent Documents 1 to 4 have large in-plane anisotropy. Therefore, when performing press working such as drawing, the shape of the formed product is likely to become non-uniform.

[0006] On the other hand, as a ferritic-austenitic duplex stainless steel sheet with small in-plane anisotropy, Patent Document 5 proposes a ferritic-austenitic duplex stainless steel sheet with small in-plane anisotropy, which contains, 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 inevitable impurities, having an austenite phase fraction of 40 to 90% by area ratio, the maximum intensity of the crystal orientation of the ferrite phase being 10 or less, and the hardness ratio of the austenite phase to the ferrite phase being 1.1 or more.

[0007] However, since the ferrite-austenite two-phase stainless steel plate described in Patent Document 5 has high strength, springback is likely to occur. In addition, when using this ferrite-austenite two-phase stainless steel plate, there is also a problem that the life of the mold is shortened and the load on the forming apparatus is large.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0009] The conventional ferrite-austenite two-phase stainless steel plate as described above can be softened (improve ductility) or reduce in-plane anisotropy, but it has not been possible to achieve both softening and reduction of in-plane anisotropy. Therefore, an object of the present invention is to provide a ferrite-austenite two-phase stainless steel plate capable of achieving both softening and reduction of in-plane anisotropy.

Means for Solving the Problems

[0010] As a result of continuous intensive research on ferrite-austenite two-phase stainless steel plates to solve the above problems, the present inventors have obtained the following findings. The reason why a general ferrite-austenite two-phase stainless steel plate has high strength is due to solid solution strengthening by the addition of alloying elements (e.g., Ni) and grain refinement of the ferrite phase. Therefore, by reducing the addition amount of alloying elements and reducing the amount of the austenite phase that suppresses the grain growth of the ferrite phase to make it easier to coarsen the grains of the ferrite phase, the ferrite-austenite two-phase stainless steel plate can be softened and its ductility can be improved. On the other hand, to reduce the in-plane anisotropy of the ferrite-austenite two-phase stainless steel plate, it is effective to increase the proportion of the austenite phase. However, from the perspective of softening, the proportion of the austenite phase cannot be increased. Therefore, the in-plane anisotropy can be reduced by increasing the stability of the austenite phase. Based on the above findings, the inventors of the present invention have found that the above problems can be solved by controlling the composition, the values of DF and Md, the average particle size of the ferrite phase, and the proportion of the austenite phase of the ferrite-austenite two-phase stainless steel plate, and have thus completed the present invention.

[0011] That is, the present invention has a composition containing, on a mass basis, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 4.0%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, N: 0.010 to 0.090%, with 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 contents (mass%) of the respective elements), and the value of DF 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) (In the formula, the element symbols represent the content (% by mass) of each element), and the value of Md is 80.0 to 150.0 °C, the average particle size of the ferrite phase is 5.0 μm or more, and it is a ferrite-austenite two-phase stainless steel sheet in which the austenite phase is more than 25% by volume and less than 40% by volume.

Advantages of the Invention

[0012] According to the present invention, it is possible to provide a ferrite-austenite two-phase stainless steel sheet capable of achieving both softening and reduction of in-plane anisotropy.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be specifically described. The present invention is not limited to the following embodiments, and it should be understood that those obtained by appropriately making changes, improvements, etc. to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the gist of the present invention also fall within the scope of the present invention. In addition, in this specification, the “%” display regarding components means “% by mass” unless otherwise specified.

[0014] The ferrite-austenite two-phase stainless steel sheet (hereinafter simply abbreviated as “two-phase stainless steel sheet”) according to the embodiment of the present invention contains C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 4.0%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, N: 0.010 to 0.090%, and the balance consists of Fe and impurities.

[0015] Here, in this specification, the “stainless steel sheet” means a plate material (including a strip) formed from stainless steel. In addition, in this specification, "ferrite-austenite type" means that the metal structure at room temperature is mainly composed of two phases, namely, a ferrite phase and an austenite phase. Therefore, "ferrite-austenite type" includes those that slightly contain phases other than the ferrite phase and the austenite phase (for example, a martensite phase, etc.). Furthermore, in this specification, "impurities" mean components that are mixed in due to raw materials such as ores and scraps and various factors in the manufacturing process when industrially manufacturing a stainless steel plate, and are allowed within a range that does not adversely affect the present invention. For example, impurities include inevitable impurities. Examples of impurities include O. The content of O is, for example, 0.0001 to 0.0070%. Regarding the content of each element, including "xx% or less" means that it is xx% or less, but includes an amount exceeding 0% (especially exceeding the impurity level).

[0016] The duplex stainless steel plate according to an embodiment of the present invention can further contain one or more selected from Nb: 0.010 to 0.500%, Ti: 0.01 to 0.50%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, REM: 0.0002 to 0.0100% as required. Hereinafter, each component will be described in detail.

[0017] <C: 0.001 to 0.050%> C is an element that has a great influence on the stability of the austenite phase. If the C content is too high, the ductility (workability) may decrease, or the precipitation of Cr carbides may be promoted, leading to intergranular corrosion. Therefore, the C content is set to 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less. Also, from the perspective of corrosion resistance, a lower C content is better, but reducing the C content too much will lead to an increase in cost. Therefore, the C content is set to 0.001% or more, preferably 0.002% or more, more preferably 0.005% or more.

[0018] <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 will harden and the ductility will decrease. Therefore, the Si content is set to 0.50% or less, preferably 0.45% or less, more preferably 0.40% or less. Also, excessively reducing the Si content will increase the cost during steelmaking. Therefore, the Si content is set to 0.01% or more, preferably 0.02% or more, more preferably 0.05% or more.

[0019] <Mn: 1.0~4.0%> 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, in addition to ductility, corrosion resistance and hot workability will also decrease. Therefore, the Mn content is set to 4.0% or less, preferably 3.9% or less, more preferably 3.8% or less. Also, excessively reducing the Mn content will increase the cost during steelmaking. Therefore, the Mn content is set to 1.0% or more, preferably 1.1% or more, more preferably 1.2% or more.

[0020] <P: 0.050% or less> P is an element contained in raw materials such as Cr. When the content of P is high, the formability decreases. Therefore, the content of P is set to 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less. On the other hand, it is preferable that the content of P is lower, but there is a limit to reducing the content of P. The lower limit value of the content of P is generally 0.001%, preferably 0.002%, more preferably 0.003%.

[0021] <S: 0.030% or less> S is an element contained in various raw materials. Since S combines with Mn to form inclusions and may become the starting point of rust, the lower the content of S, the better the corrosion resistance. Therefore, the content of S 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 the content of S. The lower limit value of the content of S is generally 0.0001%, preferably 0.0005%.

[0022] <Ni: 1.5 - 3.0%> Ni is an austenite-forming element and is an important element for adjusting the stability of the austenite phase. Also, Ni has the effect of suppressing the precipitation of nitrides and improving the corrosion resistance. To exert these effects, the content of Ni is set to 1.5% or more, preferably 1.6% or more, more preferably 1.7% or more, still more preferably 1.8% or more. On the other hand, if the content of Ni is too high, it will not only lead to an increase in raw material costs, but also problems such as stress corrosion cracking may occur due to the high proportion of the austenite phase. Therefore, the content of Ni is set to 3.0% or less, preferably 2.8% or less, more preferably 2.5% or less.

[0023] <Cr: 19.6 - 24.0%> Cr is an element necessary to ensure corrosion resistance. To exert this effect, the Cr content is set to 19.6% or more, preferably 19.8% or more, more preferably 20.0% or more. On the other hand, if the Cr content is too high, it will cause hot working cracks or lead to an increase in the cost of the refining process. Therefore, the Cr content is set to 24.0% or less, preferably 23.5% or less, more preferably 23.0% or less.

[0024] <Mo: 0.01~1.00%> Mo is an element that improves corrosion resistance. To exert this effect, the Mo content is set to 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more. On the other hand, if the Mo content is too high, the raw material cost will increase. Therefore, the Mo content is set to 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less.

[0025] <Cu: 0.01~1.20%> Cu is an austenite-forming element like Mn and Ni, and has the effect of suppressing the precipitation of nitrides and improving corrosion resistance. To exert these effects, the Cu content is set to 0.01% or more, preferably 0.05% or more, more preferably 0.10% or more. On the other hand, if the Cu content is too high, it will not only cause an increase in raw material cost, but also reduce hot workability. Therefore, the Cu content is set to 1.20% or less, preferably 1.00% or less, more preferably 0.80% or less.

[0026] <N: 0.010~0.090%> N, like C, is an element that has a great influence on the stability of the austenite phase. Also, N is an element that dissolves in solid solution to enhance corrosion resistance. To exert these effects, the N content is set to 0.010% or more, preferably 0.020% or more. On the other hand, if the N content is too high, the ductility will decrease and the 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.

[0027] <Nb: 0.010~0.500%> Nb forms nitrides (NbN) and carbides (NbC) and has the effect of improving workability. To exert this effect, the content of Nb is set to 0.010% or more, preferably 0.013% or more, more preferably 0.015% or more. On the other hand, if the content of Nb is too high, the ductility decreases. Therefore, the content of Nb is set to 0.500% or less, preferably 0.300% or less, more preferably 0.200% or less.

[0028] <Ti: 0.01~0.50%> Similar to Nb, Ti also forms nitrides (TiN) and carbides (TiC) and has the effect of improving workability. To exert this effect, the content of Ti is set to 0.01% or more, preferably 0.02% or more, more preferably 0.03% or more. On the other hand, if the content of Ti is too high, the ductility decreases. Therefore, the content of Ti is set to 0.50% or less, preferably 0.30% or less, more preferably 0.20% or less.

[0029] <V: 0.01~0.50%> V forms nitrides and has the effect of improving workability. To exert this effect, the content of V is set to 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more. On the other hand, if the content of V is too high, the ductility and hot workability decrease. Therefore, the content of V is set to 0.50% or less, preferably 0.45% or less, more preferably 0.40% or less.

[0030] <W: 0.05~0.50%> W is an element effective in improving corrosion resistance. To exert this effect, the content of W is set to 0.05% or more, preferably 0.08% or more, more preferably 0.10% or more. On the other hand, if the content of W is too high, the ductility decreases. Therefore, the content of W is set to 0.50% or less, preferably 0.45% or less, more preferably 0.40% or less.

[0031] <Co: 0.01~0.30%> Co is an element effective in enhancing high-temperature strength and improving hot workability. To exert these effects, the Co content is set at 0.01% or more, preferably 0.02% or more. On the other hand, if the Co content is too high, toughness will decrease. Therefore, the Co content is set at 0.30% or less, preferably 0.25% or less, more preferably 0.20% or less.

[0032] <B:0.0002~0.0050%> B is an element that segregates at grain boundaries to improve hot workability. To exert this effect, the B content is set at 0.0002% or more, preferably 0.0010% or more, more preferably 0.0015% or more. On the other hand, if the B content is too high, corrosion resistance will significantly decrease. Therefore, the B content is set at 0.0050% or less, preferably 0.0045% or less, more preferably 0.0040% or less.

[0033] <Sn:0.010~0.500%> Sn is an element that improves corrosion resistance. To exert this effect, the Sn content is set at 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, hot workability will decrease. Therefore, the Sn content is set at 0.500% or less, preferably 0.450% or less, more preferably 0.400% or less.

[0034] <Al:0.010~0.050%> Al is an element effective in desulfurization and deoxidation. To exert these effects, the Al content is set at 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 lead to an increase in manufacturing defects and raw material costs. Therefore, the Al content is set at 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less.

[0035] <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 should be 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.

[0036] <Ca: 0.0002 to 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 working cracks are likely to occur and the corrosion resistance also decreases. Therefore, the Ca content is set to 0.0100% or less, preferably 0.0080% or less, more preferably 0.0050% or less.

[0037] <Ta: 0.050% or less> Ta is an element that improves corrosion resistance by modifying inclusions. However, if the Ta content is too high, it will cause a decrease in room temperature ductility and toughness. Therefore, the Ta content is 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less. On the other hand, the lower limit value of the Ta content is not particularly limited, but to exert the effect of Ta, it is preferably 0.001%, more preferably 0.003%.

[0038] <Ga: 0.050% or less> Ga is an element that contributes to the improvement of corrosion resistance and the suppression of hydrogen embrittlement. However, if the Ga content is too high, the workability will decrease. Therefore, the Ga content is 0.050% or less, preferably 0.040% or less, more preferably 0.030% or less. On the other hand, the lower limit value of the Ga content is not particularly limited, but to exert the effect of Ga, it is preferably 0.001%, more preferably 0.003%.

[0039] <Zr: 0.01 to 0.50%> Zr has an effect similar to that of Nb and Ti and is an element that improves oxidation resistance. To exert these effects, the content of Zr is set to 0.01% or more, preferably 0.02% or more. On the other hand, if the content of Zr is too high, it will lead to an increase in raw material cost in addition to a decrease in ductility. Therefore, the content of Zr is set to 0.50% or less, preferably 0.40% or less, more preferably 0.30% or less.

[0040] <REM:0.0002~0.0100%> REM (rare earth) is an element effective in improving hot workability. To exert this effect, the content of REM is set to 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 an increase in cost. Therefore, the content of REM is set to 0.0100% or less, preferably 0.0095% or less. Note that REM is a general term for 15 elements from Sc, Y, and La to Lu (lanthanoids). As REM, these elements can be used alone or in combination of two or more.

[0041] The duplex stainless steel sheet 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 65.0 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 contents (mass%) of the respective elements. Here, DF is an index representing the amount of ferrite phase. Therefore, 100 - DF is the amount of austenite phase. However, since DF is an index determined based on the element content, it should be noted that it does not coincide with the actually measured amount of austenite phase. By controlling the DF value within the above range, it is possible to adjust the amount of ferrite phase suitable for both softening and reduction of in-plane anisotropy. If the DF value is outside the above range, softening may not be appropriate or in-plane anisotropy may increase.

[0042] For the duplex stainless steel sheet according to an embodiment of the present invention, the value of Md represented by the following formula (2) is 80.0 to 150.0 °C, preferably 55.0 to 140.0 °C, more preferably 60.0 to 130.0 °C, and still more preferably 70.0 to 120.0 °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 (% by mass) of each element. Here, Md is an index representing the stability of the austenite phase. The larger the value of Md (higher temperature), the more unstable the austenite phase. By controlling the value of Md within the above range, it is possible to adjust the stability of the austenite phase suitable for both softening and reduction of in-plane anisotropy. If the value of Md is outside the above range, softening may not be appropriate or in-plane anisotropy may increase.

[0043] For the duplex stainless steel sheet according to an embodiment of the present invention, the average grain diameter of the ferrite phase is preferably 5.0 μm or more, more preferably 5.2 μm or more, and still more preferably 5.5 μm or more. If the average grain diameter of the ferrite phase is less than 5.0 μm, the duplex stainless steel sheet will have high strength and the desired ductility (softening) cannot be obtained. The upper limit of the average grain diameter of the ferrite phase is not particularly limited, but is typically 20.0 μm, preferably 18.0 μm, and more preferably 15.0 μm. Here, in this specification, the average grain diameter of the ferrite phase in the duplex stainless steel sheet can be determined by EBSD (electron backscatter diffraction) measurement. Specifically, EBSD measurement is performed using a sample obtained by mirror-polishing a cross-section in the thickness direction parallel to the rolling direction of the duplex stainless steel sheet. For the data obtained by this EBSD measurement, the average area of the crystal grains of the ferrite phase (BCC) is determined by the Area fraction method. The diameter of a circle having the same area as the average area of the crystal grains thus obtained is defined as the average grain diameter of the ferrite phase.

[0044] The duplex stainless steel plate according to an embodiment of the present invention has an austenite phase of more than 25% by volume and less than 40% by volume, preferably 26 - 39% by volume, more preferably 27 - 38% by volume. When the austenite phase is 25% by volume or less, the proportion of the ferrite phase increases, and thus the desired ductility (softening) cannot be obtained. On the other hand, when the austenite phase is 40% by volume or more, the average particle diameter of the ferrite phase becomes small, resulting in excessive high strength and the inability to obtain the desired ductility (softening). Here, in this specification, the proportion of the austenite phase in the duplex stainless steel plate can be determined by EBSD measurement. Specifically, EBSD measurement is performed using a sample obtained by mirror-polishing a cross-section in the thickness direction parallel to the rolling direction of the duplex stainless steel plate. For the data obtained by this EBSD measurement, a phase ratio map is created using analysis software, the ferrite phase and the austenite phase are separated, and the proportion of the austenite phase can be determined.

[0045] The duplex stainless steel plate according to an embodiment of the present invention preferably has an Md value of the austenite phase of 60 - 130°C, more preferably 65 - 120°C, and even more preferably 70 - 110°C. The Md value of the austenite phase can be determined by the above formula (2) based on the elements in the austenite phase. When the Md value of the austenite phase is less than 60°C, the desired ductility may not be ensured. On the other hand, when the Md value of the austenite phase exceeds 130°C, the amount of the strain-induced martensite phase transformed from the austenite phase increases, resulting in excessive high strength and the inability to ensure the desired ductility in some cases. Here, in this specification, the content of each element in the austenite phase used for calculating Md of the austenite phase can be measured by EPMA (Electron Probe Micro Analyzer). Specifically, a sample obtained by mirror-polishing the cross-section in the thickness direction of a duplex stainless steel plate parallel to the rolling direction is used, and qualitative analysis is performed by EPMA. Since C and N have the characteristic of concentrating in the austenite phase, qualitative mapping of C or N is performed on the entire cross-section to identify the austenite phase. Then, quantitative analysis of C, N, Si, Mn, Cr, Ni, Cu, and Mo is performed at approximately the center of the austenite phase so that the electron beam does not hit the ferrite phase. The quantitative analysis is performed at three or more points, and the average value is taken as the result of the content of each element.

[0046] The duplex stainless steel plate according to an embodiment of the present invention preferably has a 0.2% proof stress of 400 to 500 MPa, more preferably 410 to 490 MPa, and still more preferably 420 to 480 MPa. If the 0.2% proof stress is within such a range, softening of the duplex stainless steel plate is appropriate, and it can be said that the ductility is excellent. Here, the 0.2% proof stress of the duplex stainless steel plate can be measured in accordance with JIS Z2241:2011.

[0047] The duplex stainless steel plate according to an embodiment of the present invention preferably has Δr represented by the following formula (3) of 0.50 or less, more preferably 0.40 or less, still more preferably 0.30 or less, and particularly preferably 0.20 or less. Δr = |(r0 + r 90 ) / 2 - r 45 | ··· (3) In formula (3), r0 is the r value in the direction parallel to the rolling direction, r 90 is the r value in the direction perpendicular to the rolling direction, and r 45 is the r value in the direction of 45° with respect to the rolling direction. Here, Δr is an index representing in-plane anisotropy. The larger Δr is, the greater the in-plane anisotropy, and conversely, the smaller Δr is, the smaller the in-plane anisotropy. Therefore, when Δr exceeds 0.50, the drawing processability deteriorates, and the shape of the molded product becomes non-uniform. Specifically, the shape of the flange remaining part of the molded product does not become constant, or a phenomenon called earing occurs where the end of the molded product undulates. Each r value (r0, r 90 and r 45 ) can be measured in accordance with JIS Z2254:2021.

[0048] The duplex stainless steel sheet according to the embodiment of the present invention may be a hot-rolled material or a cold-rolled material. Further, annealing or pickling may be performed on the hot-rolled material or the cold-rolled material.

[0049] The thickness of the duplex stainless steel sheet according to the embodiment of the present invention may be appropriately adjusted according to the application and is not particularly limited, but generally it is 5.0 mm or less, preferably 4.0 mm or less, and more preferably 3.0 mm or less.

[0050] The manufacturing method of the duplex stainless steel sheet according to the embodiment of the present invention is not particularly limited as long as it can manufacture the duplex stainless steel sheet having the above characteristics. Hereinafter, an example of the manufacturing method of the duplex stainless steel sheet according to the embodiment of the present invention will be described. The duplex stainless steel sheet according to the embodiment of the present invention can be manufactured by melting stainless steel having the above composition by vacuum melting to obtain a steel slab, then performing hot rolling and annealing, and then performing cold rolling and finish annealing.

[0051] For hot rolling, the temperature immediately after the final pass is set to 950°C or higher, and then cooling is performed to 800°C at a cooling rate of 20°C / second or higher. By performing hot rolling under such conditions, the crystal grains of the ferrite phase can be coarsened. Here, there are mainly two reasons for the refinement of the ferrite-phase crystal grains. 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 to induce recrystallization, and the easier it is for the ferrite-phase crystal grains to be refined. Therefore, it is necessary to raise the temperature to a level close to that immediately after the final pass where this is less likely to occur. The second is the suppression of the growth of ferrite-phase crystal grains due to the formation of the austenite phase. When the austenite phase precipitates in the ferrite-phase grain boundaries, the movement of the ferrite-phase grain boundaries becomes sluggish, and grain growth is suppressed. Since the austenite phase decreases as the temperature increases with a peak around 900 °C, the higher the temperature, the easier it is for the ferrite-phase crystal grains to grow. On the other hand, to maintain a high hot-rolling temperature, it is effective to increase the heating temperature of the slab before hot rolling or increase the rolling speed to shorten the heat dissipation time, but this increases fuel costs and manufacturing difficulty. Therefore, considering these circumstances, the lower limit of the temperature immediately after the final pass of hot rolling was set at 950 °C.

[0052] The annealing after hot rolling is carried out with a heating rate of 20 °C / second or more, held at a reaching temperature of 1030 - 1150 °C for 10 seconds or more, and then cooled to 400 °C or less at a cooling rate of 20 °C / second or more. Annealing is performed under such conditions to sufficiently dissolve the carbides and nitrides precipitated during the cooling after hot rolling and to suppress the precipitation of carbides and nitrides during the cooling process after annealing. Also, the proportion of the austenite phase is made relatively small to reduce the suppression of the growth of ferrite-phase crystal grains and make it easier to coarsen the ferrite-phase crystal grains. In particular, when the reaching temperature is lower than 1030 °C, the solid solution of carbides and nitrides becomes insufficient, and the proportion of the austenite phase also becomes too large. When the reaching temperature is higher than 1150 °C, although the carbides and nitrides are sufficiently solid-solved, the proportion of the austenite phase becomes too small. Furthermore, a certain amount of carbon and nitrogen also solid-solves in the ferrite phase, and there is a risk of forming precipitates during cooling in the ferrite phase with a small solid-solution limit, deteriorating the corrosion resistance.

[0053] The conditions of cold rolling are not particularly limited, but the rolling ratio is preferably 40 to 90%. Here, the rolling ratio of cold rolling means the total (total rolling ratio) of each cold rolling when cold rolling is performed two or more times. Setting the rolling ratio of cold rolling to 40% or more is to reduce in-plane anisotropy by breaking up the aggregate structure of the ferrite phase and randomizing the crystal orientation. On the other hand, setting the rolling ratio to 90% or less is to suppress ear cutting due to excessive rolling. It is also to prevent the structure of the finish annealed material from becoming too fine due to the accumulation of rolling strain. As described above, when there is a lot of strain, recrystallization is induced and the crystal grains become fine, so this is avoided. When cold rolling is performed two or more times, intermediate annealing may be performed between each cold rolling. When performing intermediate annealing, the conditions may be carried out according to the annealing conditions after hot rolling.

[0054] The conditions of finish annealing are such that the heating rate is 20°C / second or more, held at the reaching temperature of 1000 to 1100°C for 5 seconds or more, then cooled to 850°C or less at a cooling rate of 30°C / second or more, and cooled to 400°C or less at a cooling rate of 20°C / second or more. Performing finish annealing under such conditions is for suppressing the precipitation of carbides and nitrides during heating, completing recrystallization, dissolving carbides and nitrides, controlling the ratio of the austenite phase, suppressing fluctuations in the ratio of the austenite phase during cooling, and suppressing the reprecipitation of carbides and nitrides. Also, due to the microstructure control up to cold rolling, the finish annealed material has a structure in which the crystal grains of the ferrite phase are coarsened.

[0055] The duplex stainless steel sheet according to the embodiment of the present invention achieves both softening and reduction of in-plane anisotropy. Therefore, this duplex stainless steel sheet is easy to process, and in particular, when performing press working such as drawing, the shape of the molded product can be made uniform. For example, specifically, it is possible to suppress phenomena such as the shape of the flange remaining part of the molded product not becoming constant and a phenomenon called earing where the end of the molded product undulates. Also, this duplex stainless steel sheet can suppress springback caused by excessive high strength during forming processing and has good shape freezing property. Therefore, this duplex stainless steel sheet can be used in various applications where these characteristics are required.

Example

[0056] The content of the present invention will be described in detail with reference to the following examples, but the present invention is not construed as being limited thereto.

[0057] A cold-rolled annealed plate was produced as a duplex stainless steel plate. The cold-rolled annealed plate was manufactured through a hot rolling process, an annealing process, a cold rolling process, and a finish annealing process in this order. Specifically, first, 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. Next, a hot rolling process was performed on this steel slab to obtain a hot-rolled plate with a thickness of 5 mm. 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: 20 °C / second or more). The annealing process after the hot rolling process was carried out with a heating rate of 25 °C / second, held at 1100 °C (annealing temperature) shown in Table 2 for 30 seconds, and then cooled to 400 °C or lower by water cooling (cooling rate: 20 °C / second or more). The cold rolling was performed at the rolling ratio shown in Table 2 to obtain a cold-rolled plate. Note that the cold rolling was carried out once. The finish annealing process was carried out with a heating rate of 30 °C / second, held at the annealing temperature shown in Table 2 for 30 seconds, and then cooled to 400 °C or lower by water cooling (cooling rate: 30 °C / second or more). In Table 1, the values of DF and Md were calculated based on the content of each element.

[0058]

Table 1

Table 2

[0059] The following evaluations were performed on the cold-rolled annealed plate obtained above.

[0060] <Ratio of austenite phase (γ phase) in duplex stainless steel plate> After cutting out a test piece from the cold-rolled and annealed sheet, a cross-section in the thickness direction parallel to the rolling direction was mirror-polished and EBSD (electron backscatter diffraction) measurement was performed. For the EBSD measurement, using the measurement software TSL OIM Data Collection7 (TSL Solutions Co., Ltd.) with a scanning electron microscope, a region of 200 μm square at the center in the thickness direction of the test piece was measured with a step size of 0.3 μm. Next, for the data obtained by the EBSD measurement, an orientation map was created using the analysis software TSL OIM Analysis7 (TSL Solutions Co., Ltd.) to separate the ferrite phase and the austenite phase. Then, the ratio of the austenite phase occupying the entire observation region was determined.

[0061] <Md of austenite phase (γ phase)> After cutting out a test piece from the cold-rolled and annealed sheet, a cross-section in the thickness direction parallel to the rolling direction was mirror-polished and component analysis by EPMA (electron probe microanalyzer) was performed. Specifically, since C and N have the characteristic of concentrating in the austenite phase, qualitative mapping of C or N was performed for the entire cross-section to identify the austenite phase. Next, C, N, Si, Mn, Cr, Ni, Cu, and Mo were quantitatively analyzed at approximately the center of the austenite phase so that the electron beam did not hit the ferrite phase. The measurement region was a region of about 2 μm square, and more than 3 points were measured for each test piece, and the average value was taken as the result of the content of each element. Also, for the EPMA measurement, the conditions were 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 of the austenite phase was calculated.

[0062] <Average grain size of ferrite phase (α phase) in duplex stainless steel sheet> After cutting out test pieces from the cold-rolled annealed sheet, a cross-section in the thickness direction parallel to the rolling direction was mirror-polished and EBSD (electron backscatter diffraction) measurement was performed. For the EBSD measurement, using the measurement software TSL OIM Data Collection7 (TSL Solutions Co., Ltd.) with a scanning electron microscope, a region of 200 μm square at the center in the thickness direction of the test piece was measured with a step size of 0.3 μm. Regarding the data obtained from this EBSD measurement, the average area of the ferrite phase (BCC) crystal grains was determined by the Area fraction method.

[0063] <0.2% proof stress> JIS No. 13B test pieces were cut out from the cold-rolled annealed sheet so that the parallel part was in the rolling direction, and a tensile test was performed in accordance with JIS Z2241:2011 using this test piece. The tensile test was carried out under an air atmosphere at room temperature (25°C) with a tensile speed of 10 mm / min, and the stress at the time when the strain was 0.2% was measured. In this evaluation, if the 0.2% proof stress is 400 to 500 MPa, it can be said that softening is appropriate (excellent ductility).

[0064] <Δr> Three types of JIS No. 13B test pieces were cut out from the cold-rolled annealed sheet so as to be 0°, 45°, and 90° with respect to the rolling direction, and a plastic strain ratio test was performed in accordance with JIS Z2254:2021. The ratio of the width-direction strain to the thickness-direction strain of the test piece was measured at the time when the elongation was 14%. Using the thus obtained respective r values (r0, r 90 and r 45 ), Δr was calculated based on Equation (3). In this evaluation, if Δr is 0.50 or less, it can be said that the in-plane anisotropy is small.

[0065] The above evaluation results are shown in Table 3.

[0066]

Table 3

[0067] As shown in Table 3, in Examples 1 to 5, since the composition of the cold-rolled annealed sheet (duplex stainless steel sheet), the values of DF and Md, the average particle diameter of the ferrite phase, and the ratio of the austenite phase were controlled, softening was appropriate and the in-plane anisotropy was small. On the other hand, in Comparative Example 1, since the value of Md was not appropriate, the in-plane anisotropy was large. In Comparative Example 2, since the values of DF, the ratio of the austenite phase, and the average particle diameter of the ferrite phase were not appropriate, the 0.2% proof stress increased and softening was not appropriate. In Comparative Example 3, since the ratio of the austenite phase was not appropriate, the 0.2% proof stress decreased and softening was not appropriate. In Comparative Example 4, since the ratio of the austenite phase and the average particle diameter of the ferrite phase were not appropriate, softening was not appropriate. In Comparative Example 5, since the contents of Ni and Cr, the values of DF and Md, the ratio of the austenite phase, and the average particle diameter of the ferrite phase were not appropriate, softening was not appropriate. In Comparative Example 6, since the content of N, the values of DF and Md, the ratio of the austenite phase, and the average particle diameter of the ferrite phase were not appropriate, softening was not appropriate and the in-plane anisotropy was large. In Comparative Example 7, since the contents of Si, Mn, Ni, Cr, Mo, and N and the value of Md were not appropriate, softening was not appropriate and the in-plane anisotropy was large. In Comparative Example 8, since the contents of Si, Mn, and N, the values of DF and Md, the ratio of the austenite phase, and the average particle diameter of the ferrite phase were not appropriate, softening was not appropriate. In Comparative Example 9, since the contents of Si, Ni, and N, the value of Md, the ratio of the austenite phase, and the average particle diameter of the ferrite phase were not appropriate, softening was not appropriate and the in-plane anisotropy was large.

[0068] As can be seen from the above results, according to the present invention, it is possible to provide a ferrite-austenite duplex stainless steel sheet capable of achieving both softening and reduction of in-plane anisotropy.

Claims

1. By mass, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 4.0%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, N: 0.010 to 0.090%, having a composition consisting of 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 value of DF represented by (where the element symbols represent the contents (mass%) of the respective elements) 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 value of Md represented by (where the element symbols represent the contents (mass%) of the respective elements) is 80.0 to 150.0 °C, the average particle diameter of the ferrite phase is 5.0 μm or more, a ferrite-austenite two-phase stainless steel sheet in which the austenite phase is more than 25% by volume and less than 40% by volume.

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

3. The ferrite-austenite two-phase stainless steel sheet according to Claim 1 or 2, wherein the value of Md of the austenite phase is 60 to 130 °C.

4. The ferrite-austenite two-phase stainless steel sheet according to Claim 1 or 2, wherein the 0.2% proof stress is 400 to 500 MPa.

5. the following formula (3): Δr = |(r 0 + r 90 ) / 2 - r 45 |... (3) (wherein r 0 is the r value in the direction parallel to the rolling direction, r 90 is the r value in the direction perpendicular to the rolling direction, r 45 is the r value in the 45° direction with respect to the rolling direction), and Δr represented by the following formula is 0.50 or less. The ferritic-austenitic two-phase stainless steel sheet according to claim 1 or 2.

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