Ferrite austenite duplex stainless steel sheet and method for producing same

Abstract

Provided are a ferrite austenite duplex stainless steel sheet having exceptional corrosion resistance and abrasive wear resistance, and a method for producing the same. A ferritic austenite duplex stainless steel sheet according to the present invention has a prescribed chemical composition and is configured such that: the PREN_Mn value calculated as Cr + 3.3(Mo + 0.5W) + 16N − Mn is 35.0 or lower; the area fraction of an austenite phase at a position of 0.5 mm in the sheet thickness direction from the surface of the steel sheet is 30-70%; the Md30 value calculated as Md30 value (°C) = 551 − 462(C + N) − 9.2Si − 8.1Mn − 13.7Cr − 29(Ni + Cu) − 18.5Mo − 68Nb from the composition in the austenite phase at the position of 0.5 mm in the sheet thickness direction from the surface of the steel sheet is −150°C or higher; the average KAM value of the austenite phase at the position of 0.5 mm in the sheet thickness direction from the surface of the steel sheet is 1.0° or higher; the difference in ferric chloride CPT between the steel sheet and the steel sheet subjected to solid-solution heat treatment is 10°C or lower; and the difference in critical pitting corrosion resistance temperature between the steel sheet and the steel sheet subjected to heat treatment is 15°C or lower.

Description

Ferrite-austenitic duplex stainless steel sheet and its manufacturing method

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

[0002] Duplex stainless steel is a stainless steel that has both austenite and ferrite phases in its structure. Compared to austenitic stainless steels, which generally have the same corrosion resistance, duplex stainless steels have a low Ni content, making them low alloying costs and high strength. Therefore, they have attracted attention as materials that can achieve both strength and corrosion resistance at low cost.

[0003] In structures such as rivers and dams, if ordinary steel is used in areas that come into contact with rainwater and river water, corrosion will occur in those areas, so painting or plating is applied, or stainless steel is used. In particular, sliding parts in gates, etc., experience friction when the gate is opened and closed, and lining materials come into contact with stones and gravel, which significantly accelerates the deterioration of paint and plating. For this reason, stainless steel, which has corrosion resistance regardless of painting or plating, is widely used.

[0004] For sliding parts, SUS304N2, which contains nitrogen (N) to enhance hardness, is widely used due to its wear resistance. However, river facilities with gates, such as sluice gates at river mouths, often experience severe corrosion due to the inflow of seawater, which significantly increases the chloride ion concentration in the river water. In such locations, SUS304N2, which has a low Cr content, may not be able to provide the desired corrosion resistance. Therefore, there is a demand for a replacement with high-strength, highly corrosion-resistant duplex stainless steel.

[0005] In addition, in environments where friction occurs between hard objects such as sliding parts, or in environments where lining materials come into contact with stones or gravel, a type of wear called abrasive wear may occur. Abrasive wear is a wear phenomenon in which the surface is scraped off by foreign matter interposed between the friction surfaces, and occurs when there is a large difference in hardness between the two rubbing surfaces, and when rough protrusions are present on the harder surface, or when a hard solid foreign matter is interposed between the friction surfaces. Abrasive wear resistance and surface hardness do not necessarily coincide, so when applied to applications where abrasive wear occurs, it is important that the abrasive wear resistance is good as well as the surface hardness.

[0006] Patent Documents 1 and 2 are PRE W,Mn The present invention discloses a method for producing duplex stainless steel having excellent wear resistance and weld corrosion resistance and a surface hardness of HBW 230 or more, by hot rolling duplex stainless steel having a viscosity of 24.0 or more and 34.0 or less at a finish rolling temperature of 1000 to 800°C and a cooling rate of 1°C / s or more in the 800 to 600°C range.

[0007] Patent Document 3 discloses a method for producing a ferritic-martensitic duplex stainless steel having excellent abrasive wear resistance and a Brinell hardness of 300 or more.

[0008] JP 2020-100872 A JP 2021-075771 A Japanese Patent No. 6621419 A

[0009] Patent Documents 1 and 2 disclose methods for producing duplex stainless steels having good surface hardness and weld corrosion resistance by low-temperature finishing. However, these documents refer to surface hardness but do not refer to abrasive wear resistance.

[0010] Patent Document 3 reports that a ferrite-martensite dual-phase structure provides good abrasive wear resistance. However, the steel described in Patent Document 3 has a minimum chromium content of 11.5 to 12%, making it difficult to use in severe corrosive environments such as river facilities.

[0011] Other techniques relating to stainless steels with good abrasive wear resistance are also related to martensitic or austenitic stainless steels, and no techniques relating to ferritic-austenitic duplex stainless steel sheets have been disclosed.

[0012] An object of the present invention is to provide a ferritic-austenitic duplex stainless steel sheet having good abrasive wear resistance and corrosion resistance, and a method for producing the same.

[0013] The present inventors produced steel sheets from duplex stainless steels having various chemical compositions under different rolling conditions and evaluated factors that affect the abrasive wear resistance of the steel sheets. In particular, they focused on the ease of deformation-induced martensitic transformation of the austenite phase and investigated factors that improve abrasive wear resistance, resulting in the following findings.

[0014] (a) Abrasive wear processing causes the austenite phase in duplex stainless steel to transform into martensite, which hardens the material and improves its abrasive wear resistance.

[0015] (b) Therefore, by optimizing the Md30 value calculated from the composition of the austenite phase and preliminarily leaving cold working strain in the surface layer of the steel sheet to facilitate deformation-induced martensite transformation, it is possible to improve abrasive wear resistance.

[0016] The present invention has been made based on the above findings and includes the following aspects.

[0017] [1] A ferritic-austenitic duplex stainless steel plate, the chemical composition of which is, in mass%, C: 0.050% or less, Si: 2.00% or less, Mn: 0.5 to 6.0%, P: 0.0500% or less, S: 0.0500% or less, N: 0.08 to 0.30%, Cr: 17.0 to 30.0%, Ni: 0.1 to 8.0%, Mo: 0.1 to 3.5%, Cu: 0 to 3.0%, Nb: 0 to 0.100%, Sn: 0 to 1.00%, W: 0 to 1.00%, V: 0 to 1.00%, Ti: 0 to 0.050%, B: 0 to 0.0050%, the ferritic-austenitic duplex stainless steel plate contains Ca: 0-0.0050%, Mg: 0-0.0050%, Al: 0-0.0500%, and REM: 0-0.500%, with the balance being Fe and impurities; the PREN_Mn value calculated by the following formula 1 is 35.0 or less; the area fraction of the austenite phase at a position 0.5 mm in the plate thickness direction from the surface of the ferritic-austenitic duplex stainless steel plate is 30-70%; the Md30 value calculated by the following formula 3 from the composition in the austenite phase at a position 0.5 mm in the plate thickness direction from the surface of the ferritic-austenitic duplex stainless steel plate is -150°C or more; the average KAM value of the austenite phase at a position 0.5 mm in the plate thickness direction from the surface of the ferritic-austenitic duplex stainless steel plate is 1.0° or more; a ferric chloride CPT test in accordance with ASTM G48E method is carried out on the ferritic-austenitic duplex stainless steel plate and a sample obtained by subjecting the ferritic-austenitic duplex stainless steel plate to a solution heat treatment at 1050°C for 5 minutes, and the difference in ferric chloride CPT between the ferritic-austenitic duplex stainless steel plate and the solution heat treated sample is 10°C or less;A test piece having dimensions of 8φ×110L is taken from the ferritic-austenitic duplex stainless steel plate at a position one-quarter of the plate thickness, and is heated to 1360°C at 30°C / s by high-frequency heating with a soaking zone of 15 mm and inert gas cooling, followed by 5-second soaking, followed by cooling at 40°C / s to 900°C, immediately cooling at 11°C / s to 600°C, and immediately cooling at 2°C / s to 300°C. A critical pitting temperature (electrochemical CPT) test based on JIS G0590 is conducted on a cross section of the sample cut in half through the soaking zone, and the difference in critical pitting temperature between the ferritic-austenitic duplex stainless steel plate and a heat-treated ferritic-austenitic duplex stainless steel plate is 15°C or less. PREN_Mn value=Cr+3.3(Mo+0.5W)+16N-Mn (Equation 1) Md30 value (°C)=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo-68Nb (Equation 2) However, the element symbols in the above equation 1 represent the content (mass%) of each element contained in the steel, and the element symbols in equation 3 represent the content (mass%) of each element contained in the austenite phase, and if an element is not contained, 0 is substituted.

[0018] [2] The ferritic-austenitic duplex stainless steel sheet according to [1], wherein the chemical composition contains, in mass%, one or more selected from Cu: 0.1 to 3.0%, Nb: 0.010 to 0.100%, Sn: 0.03 to 1.00%, W: 0.01 to 1.00%, and V: 0.01 to 1.00%.

[0019] [3] The ferritic-austenitic duplex stainless steel sheet according to [1] or [2], wherein the chemical composition contains, in mass%, one or more selected from Ti: 0.005 to 0.050%, and B: 0.0003 to 0.0050%.

[0020] [4] The ferritic-austenitic duplex stainless steel sheet according to any one of [1] to [3], wherein the chemical composition contains, in mass%, one or more elements selected from Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Al: 0.0030 to 0.0500%, and REM: 0.005 to 0.500%.

[0021] [5] The ferritic-austenitic duplex stainless steel sheet according to any one of [1] to [4], wherein the DF value calculated by the following formula 3 is 70 or less, and the Md30_γ value calculated by the following formula 4 is -150°C or more. DF value = 7.2 × (Cr + 0.88Mo + 0.78Si) - 8.9 × (Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9 ... (Formula 3) Md30_γ value = 702 - 175 × C - 190N - 19.7Si - 8.5Mn - 27.0Cr - 16.8Ni - 21.0Cu - 31.1Mo - 74.8Nb ... (Formula 4) Note that the element symbols in the above formulas 3 and 4 represent the content (mass%) of each element contained in the steel, and 0 is substituted if the element is not contained.

[0022] [6] A method for producing a ferritic-austenitic duplex stainless steel sheet according to any one of [1] to [5], wherein the chemical composition, in mass%, is: C: 0.050% or less, Si: 2.00% or less, Mn: 0.5 to 6.0%, P: 0.0500% or less, S: 0.0500% or less, N: 0.08 to 0.30%, Cr: 17.0 to 30.0%, Ni: 0.1 to 8.0%, Mo: 0.1 to 3.5%, Cu: 0 to 3.0%, Nb: 0 to 0.100%, Sn: 0 to 1.00%, W: 0 to 1.00%, V: 0 to 1.00%, Ti: 0 to 0.050%, B: 0 to 0.0050%, A method for producing a ferritic-austenitic duplex stainless steel sheet, comprising: hot rolling a steel containing Ca: 0-0.0050%, Mg: 0-0.0050%, Al: 0-0.0500%, and REM: 0-0.500%, with the balance being Fe and impurities, at a rolling reduction of 10% to 40% at 950°C or less to obtain a hot-rolled steel sheet; air-cooling the hot-rolled steel sheet at 830°C or higher for 10 seconds or more, and then cooling the hot-rolled steel sheet at a cooling rate of 1.0°C / s or more in a temperature range from 800°C to 600°C; and cold-straightening the hot-rolled steel sheet under conditions such that the 0.2% proof stress after cold-straightening is 10 MPa or more higher than that before cold-straightening.

[0023] According to the present invention, it is possible to obtain an alloy-saving duplex stainless steel sheet that is a ferritic-austenitic duplex stainless steel sheet having excellent abrasive wear resistance.

[0024] Hereinafter, each requirement of the ferritic-austenitic duplex stainless steel sheet (hereinafter sometimes simply referred to as "steel sheet") of the present invention will be described in detail.

[0025] <1. Chemical Composition of Steel Sheet> Each element in the chemical composition of the steel sheet of the present invention will be described. In the following description, unless otherwise specified, "%" for the content of each element means "mass %."

[0026] (C: 0.050% or less) C is an element that dissolves in the austenite phase to increase strength. However, if a large amount of C is contained, the strength of the steel sheet increases and workability deteriorates. Furthermore, C promotes the precipitation of Cr carbides, which causes intergranular corrosion. From the viewpoint of corrosion resistance, it is preferable to keep the C content low, and the C content is set to 0.050% or less. The C content may be 0.045% or less, 0.040% or less, 0.035% or less, 0.030% or less, 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less. In the ferritic-austenitic duplex stainless steel sheet of the present invention, the inclusion of C is not essential, and the C content may be 0%. Since lowering the C content significantly increases costs in existing steelmaking equipment, the C content may be 0.001% or more, 0.002% or more, 0.003% or more, or 0.005% or more.

[0027] (Si: 2.00% or less) Si is sometimes used as a deoxidizing element or added to improve oxidation resistance. However, a large amount of Si hardens the steel sheet, deteriorating its toughness and workability. Therefore, the Si content is set to 2.00% or less. The Si content may be 1.90% or less, 1.80% or less, 1.60% or less, 1.50% or less, 1.40% or less, 1.30% or less, 1.20% or less, 1.10% or less, or 1.00% or less. In the ferritic-austenitic duplex stainless steel sheet of the present invention, the inclusion of Si is not essential, and the Si content may be 0%. Reducing the Si content to an extremely low level increases the cost of refining the steel, so the Si content may be 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.10% or more.

[0028] (Mn: 0.5 to 6.0%) Mn has the effect of increasing the austenite phase and increasing the solid solubility of nitrogen, thereby suppressing bubble defects and the like during manufacturing. Therefore, the Mn content is set to 0.5% or more. The Mn content may be 0.6% or more, 0.8% or more, 1.0% or more, 1.5% or more, 2.0% or more, or 2.5% or more. If a large amount of Mn is contained, corrosion resistance and hot workability decrease. Therefore, the Mn content is set to 6.0% or less. The Mn content may be 5.5% or less, 5.0% or less, 4.8% or less, 4.5% or less, 4.3% or less, 4.1% or less, or 4.0% or less.

[0029] (P: 0.0500% or less) P is an element that is inevitably mixed into steel and is also contained in raw materials such as Cr. However, if a large amount of P is contained, formability decreases. Therefore, the P content is set to 0.0500% or less. The P content may be 0.0480% or less, 0.0450% or less, 0.0400% or less, 0.0350% or less, or 0.0300% or less. The lower the P content, the more preferable it is, and it may be 0%. In consideration of refining costs, the P content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0030] (S: 0.0500% or less) S is an element that is inevitably mixed into steel, and may combine with Mn to form inclusions, which may become the starting point for rusting. Furthermore, since the lower the S content, the better the corrosion resistance, so the S content is set to 0.0500% or less. The S content may be 0.0450% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0150% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The lower the S content, the more preferable it is, and it may be 0%. In consideration of refining costs, the S content may be 0.0001% or more, 0.0002% or more, or 0.0004% or more.

[0031] (N: 0.08 to 0.30%) N is an element that dissolves in the austenite phase to increase strength and corrosion resistance, contributing to alloy saving. To achieve this effect, the N content is set to 0.08% or more. The N content may be 0.09% or more, 0.10% or more, 0.12% or more, 0.15% or more, 0.16% or more, or 0.18% or more. On the other hand, N is also an element that greatly affects the precipitation of chromium nitrides. If N is contained in a large amount, the amount of chromium nitride precipitation in the steel plate or weld material increases, resulting in a decrease in corrosion resistance. Therefore, the N content is set to 0.30% or less. The N content may be 0.28% or less, 0.27% or less, 0.25% or less, 0.23% or less, or 0.22% or less.

[0032] (Cr: 17.0 to 30.0%) Cr is an element necessary for ensuring corrosion resistance. Therefore, the Cr content is set to 17.0% or more. The Cr content may be 17.5% or more, 18.0% or more, 19.0% or more, 20.0% or more, 20.5% or more, 21.0% or more, or 21.5% or more. On the other hand, if Cr is contained in a large amount, the amount of chromium nitride precipitated in the steel plate or weld material increases, reducing corrosion resistance and increasing the risk of hot working cracking. Therefore, the Cr content is set to 30.0% or less. The Cr content may be 29.0% or less, 28.0% or less, 27.0% or less, 25.0% or less, or 24.5% or less.

[0033] (Ni: 0.1 to 8.0%) Ni is an austenite stabilizing element and has the effect of improving corrosion resistance. Therefore, the Ni content is set to 0.1% or more. The Ni content may be 0.3% or more, 0.5% or more, 0.7% or more, 1.0% or more, 1.5% or more, 2.0% or more, 2.5% or more, or 2.8% or more. On the other hand, if Ni is contained in a large amount, the raw material cost increases. Therefore, the Ni content is set to 8.0% or less. The Ni content may be 7.5% or less, 7.0% or less, 6.0% or less, 5.0% or less, 4.0% or less, or 3.0% or less.

[0034] (Mo: 0.1 to 3.5%) Mo has the effect of improving corrosion resistance. For this reason, the Mo content is set to 0.1% or more. The Mo content may be 0.2% or more, 0.3% or more, 0.4% or more, 0.5% or more, 0.6% or more, 0.8% or more, 0.9% or more, or 1.0% or more. On the other hand, Mo is a very expensive element, and adding a large amount of it will increase costs. For this reason, the Mo content is set to 3.5% or less. The Mo content may be 3.2% or less, 3.0% or less, 2.5% or less, 2.0% or less, or 1.5% or less.

[0035] In addition to the above elements, one or more elements selected from Cu, Nb, Sn, W, and V may be contained as needed to improve corrosion resistance. Although the effects of the present invention can be obtained without containing these elements, further effects can be obtained by containing these elements. These elements will be described below.

[0036] (Cu: 0 to 3.0%) Cu is an element that is very effective in improving sulfuric acid resistance, so it may be contained as needed. The inclusion of Cu is not essential, and the Cu content may be 0%. The effect of Cu can be obtained even with a small amount of Cu, but to ensure the effect, the Cu content is preferably 0.01% or more. The Cu content may be 0.05% or more, 0.1% or more, 0.2% or more, 0.5% or more, 0.7% or more, or 0.8% or more. A large amount of Cu content increases raw material costs and deteriorates hot workability. Therefore, the Cu content is set to 3.0% or less. The Cu content may be 2.8% or less, 2.5% or less, 2.2% or less, 2.0% or less, 1.8% or less, or 1.5% or less.

[0037] (Nb: 0 to 0.100%) Nb may be added as needed because it has the effect of suppressing the precipitation of chromium nitrides by forming a compound with N. The inclusion of Nb is not essential, and the Nb content may be 0%. The effect of Nb can be obtained even with a small amount of Nb added, but to ensure the effect, the Nb content is preferably 0.001% or more. The Nb content may be 0.002% or more, 0.005% or more, 0.010% or more, 0.015% or more, 0.020% or more, 0.030% or more, or 0.040% or more. If Nb is added in large amounts, the workability of the steel sheet decreases. Therefore, the Nb content is set to 0.100% or less. The Nb content may be 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[0038] (Sn: 0 to 1.00%) Sn is an element that improves corrosion resistance, so it may be contained as needed. The inclusion of Sn is not essential, and the Sn content may be 0%. The effect of Sn can be obtained even with a small amount of Sn, but to ensure the effect, the Sn content is preferably 0.001% or more. The Sn content may be 0.01% or more, 0.03% or more, 0.05% or more, 0.08% or more, 0.15% or more, 0.25% or more, or 0.30% or more. If a large amount of Sn is contained, hot workability deteriorates. Therefore, the Sn content is set to 1.00% or less. The Sn content may be 0.95% or less, 0.90% or less, 0.80% or less, 0.70% or less, or 0.60% or less.

[0039] (W: 0 to 1.00%) W is an element that improves corrosion resistance, so it may be contained as needed. The inclusion of W is not essential, and the W content may be 0%. The effect of W can be obtained even with a small amount of W, but to ensure the effect, the W content is preferably 0.001% or more. The W content may be 0.01% or more, 0.02% or more, 0.05% or more, 0.10% or more, 0.15% or more, or 0.18% or more. If W is contained in a large amount, the load during rolling increases, making manufacturing defects more likely to occur. Therefore, the W content is set to 1.00% or less. The W content may be 0.90% or less, 0.80% or less, 0.70% or less, 0.60% or less, or 0.50% or less.

[0040] (V: 0 to 1.00%) V is an element that improves corrosion resistance, so it may be contained as needed. The inclusion of V is not essential, and the V content may be 0%. The effect of V can be obtained even with a small amount of V, but to reliably obtain the effect, the V content is preferably 0.001% or more. The V content may be 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.07% or more. If a large amount of V is contained, the load during rolling increases, making manufacturing defects more likely to occur. Therefore, the V content is set to 1.00% or less. The V content may be 0.90% or less, 0.80% or less, 0.60% or less, 0.50% or less, or 0.40% or less.

[0041] Furthermore, in addition to the above elements, one or more elements selected from Ti and B may be contained as necessary from the viewpoint of improving hot workability and formability. Although the effects of the present invention can be obtained without containing these elements, further effects can be obtained by containing these elements. These elements will be described below.

[0042] (Ti: 0 to 0.050%) Like Nb, Ti has the effect of preventing coarsening of the weld heat affected zone and further forming fine equiaxed crystals in the solidification structure, so it may be contained as needed. The inclusion of Ti is not essential, and the Ti content may be 0%. The effect of Ti can be obtained even with a small amount of Ti, but to ensure the effect, the Ti content is preferably 0.001% or more. The Ti content may be 0.002% or more, 0.003% or more, 0.005% or more, 0.007% or more, 0.008% or more, or 0.010% or more. If Ti is contained in a large amount, uniform elongation and local elongation decrease. Therefore, the Ti content is set to 0.050% or less. The Ti content may be 0.045% or less, 0.040% or less, 0.035% or less, 0.030% or less, or 0.025% or less.

[0043] (B: 0 to 0.0050%) B has the effect of improving hot workability and may be added as needed. The inclusion of B is not essential, and the B content may be 0%. The effect of B can be obtained even with a small amount of B, but to ensure the effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0003% or more, 0.0005% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. If B is added in a large amount, corrosion resistance will significantly deteriorate. Therefore, the B content may be 0.0045% or less, 0.0040% or less, 0.0035% or less, or 0.0030% or less.

[0044] Furthermore, in addition to the above elements, one or more elements selected from Ca, Mg, Al, and REM may be contained as necessary from the viewpoint of deoxidation and desulfurization during refining. Although the effects of the present invention can be obtained without containing these elements, further effects can be obtained by containing these elements. These elements will be described below.

[0045] (Ca: 0 to 0.0050%) Ca may be added as needed for desulfurization and deoxidation. The addition of Ca is not essential, and the Ca content may be 0%. The effect of Ca can be obtained even with a small amount of Ca added, but to ensure the effect, the Ca content is preferably 0.0001% or more. The Ca content may be 0.0002% or more, 0.0005% or more, 0.0008% or more, or 0.0010% or more. If a large amount of Ca is added, hot working cracks are more likely to occur and corrosion resistance is reduced. Therefore, the Ca content is set to 0.0050% or less. The Ca content may be 0.0045% or less, 0.0035% or less, 0.0030% or less, or 0.0025% or less.

[0046] (Mg: 0 to 0.0050%) Mg not only has the effect of deoxidizing but also of refining the solidification structure, so it may be added as needed. The inclusion of Mg is not essential, and the Mg content may be 0%. The effect of Mg can be obtained even with a small amount of Mg, but to ensure the effect, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0002% or more, 0.0004% or more, 0.0007% or more, 0.0010% or more, or 0.0015% or more. If a large amount of Mg is added, the cost of the steelmaking process increases. Therefore, the Mg content is set to 0.0050% or less. The Mg content may be 0.0045% or less, 0.0040% or less, or 0.0030% or less.

[0047] (Al: 0 to 0.0500%) Al may be added as needed for desulfurization and deoxidation. The inclusion of Al is not essential, and the Al content may be 0%. Although the effect of Al can be obtained even with a small amount of Al, to ensure the effect, the Al content is preferably 0.0001% or more. The Al content may be 0.0010% or more, 0.0020% or more, 0.0030% or more, 0.0040% or more, 0.0080% or more, 0.0120% or more, or 0.0140% or more. A large amount of Al content increases manufacturing defects and raw material costs. Therefore, the Al content is set to 0.0500% or less. The Al content may be 0.0480% or less, 0.0450% or less, 0.0400% or less, 0.0350% or less, or 0.0300% or less.

[0048] (REM: 0 to 0.500%) REM (rare earth element) has the effect of improving hot workability and may be added as needed. The addition of REM is not essential, and the REM content may be 0%. The effect of REM can be obtained even with a small amount of REM added, but to ensure the effect, the REM content is preferably 0.001% or more. The REM content may be 0.005% or more, 0.010% or more, 0.015% or more, 0.020% or more, 0.040% or more, 0.070% or more, 0.100% or more, 0.150% or more, 0.200% or more, or 0.240% or more. If a large amount of REM is added, manufacturability is impaired and costs increase. Therefore, the REM content is set to 0.500% or less. The REM content may be 0.485% or less, 0.470% or less, 0.450% or less, 0.420% or less, 0.400% or less, or 0.380% or less.

[0049] REM is a collective term for 17 elements, including Sc, Y, and 15 elements (lanthanoids) from La to Lu, and the REM content refers to the total content of these elements. Industrially, lanthanoids are added in the form of misch metals.

[0050] The balance of the chemical composition of the steel sheet of the present invention is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial steel production due to various factors in raw materials such as ores and scraps, and in the manufacturing process, but do not adversely affect the present invention, i.e., do not adversely affect abrasive wear resistance.

[0051] (PREN_Mn Value: 35.0 or Less) The steel sheet according to the embodiment of the present invention needs to have a PREN_Mn value calculated by the following formula 1 within a predetermined range.

[0052] PREN_Mn value=Cr+3.3(Mo+0.5W)+16N−Mn (Equation 1) The element symbols in the above equation represent the content (mass%) of each element contained in the steel, and 0 is substituted if the element is not contained.

[0053] The PREN_Mn value is an index showing the pitting corrosion index (PRE) of duplex stainless steel, taking into account the adverse effects of Mn and the effects of W. A higher value indicates better corrosion resistance. As can be seen from Equation 1, the PREN_Mn value increases with increasing Cr and Mo content. On the other hand, increasing the Cr and Mo content increases the likelihood of sigma phase (σ phase) precipitation. The sigma phase is an intermetallic compound in which Cr, Mo, etc. are concentrated in Fe. Even a few percent of its precipitation significantly reduces the hot ductility and the toughness and corrosion resistance of the material. Furthermore, increasing the Cr and Mo content increases alloy costs, increases the N content, and reduces the Mn content, resulting in problems such as the generation of nitrogen bubbles. Therefore, the PREN_Mn value is set to 35.0 or less. The PREN_Mn value may be 34.0 or less, 33.0 or less, 32.0 or less, 30.0 or less, or 27.0 or less. In consideration of corrosion resistance equivalent to SUS304, the PREN_Mn value may be 18.0 or more, 19.0 or more, or 20.0 or more.

[0054] (DF Value) The steel sheet according to the embodiment of the present invention preferably has a DF value calculated by the following formula 2 of 70 or less.

[0055] DF value=7.2×(Cr+0.88Mo+0.78Si)−8.9×(Ni+0.03Mn+0.72Cu+22C+21N)−44.9 (Equation 2) The element symbols in the above equation represent the content (mass%) of each element contained in the steel, and 0 is substituted if the element is not contained.

[0056] The DF value is an index of the ferrite phase area fraction calculated from the composition of the entire steel sheet. As represented by Equation 2, elements that stabilize the ferrite phase have a positive coefficient, and elements that stabilize the austenite phase have a negative coefficient. If the DF value exceeds 70, when the steel sheet is manufactured using the manufacturing method described below, the area fraction of the austenite phase at a position 0.5 mm from the surface of the steel sheet in the thickness direction may be less than 30%, and the larger the DF value, the higher the possibility. Therefore, the DF value is preferably 70 or less. The DF value may be 68 or less, 65 or less, 62 or less, or 60 or less.

[0057] When manufactured within the range of manufacturing conditions described below, the austenite phase fraction of the steel plate can be predicted to some extent from the steel plate's composition. On the other hand, since the improvement in abrasive wear resistance is due to the austenite phase area fraction being 30% or more at a position 0.5 mm from the surface of the steel plate in the plate thickness direction, a steel plate having a composition with a DF value of more than 70 may have an austenite phase area fraction of 30% or more at a position 0.5 mm from the surface of the steel plate in the plate thickness direction by some method. In other words, it is not essential that the DF value be 70 or less.

[0058] (Md30_γ Value) In the steel sheet according to the embodiment of the present invention, the Md30_γ value calculated by the following formula 3 is preferably −150° C. or higher.

[0059] Md30_γ value = 702 - 175 × C - 190N - 19.7Si - 8.5Mn - 27.0Cr - 16.8Ni - 21.0Cu - 31.1Mo - 74.8Nb ... (Equation 3) The element symbols in the above formula represent the content (mass%) of each element contained in the steel, and 0 is substituted if the element is not contained.

[0060] The Md30_γ value is a prediction formula for the Md30 value in the austenite phase of a steel sheet. If this value is less than -150°C, when a steel sheet is manufactured under the manufacturing conditions described in this patent, the Md30 value of the austenite phase, which will be described later, is likely to be less than -150°C, so it is preferably -150°C or higher. The Md30_γ value may be -140°C or higher, -130°C or higher, -120°C or higher, -110°C or higher, or -100°C or higher.

[0061] When manufactured within the range of manufacturing conditions described below, the Md30 value of the austenite phase of the steel plate can be predicted to some extent from the steel plate's composition. On the other hand, since the improvement in abrasive wear resistance is due to the austenite phase's Md30 value being -150°C or higher at a position 0.5 mm in the plate thickness direction from the surface of the steel plate, a steel plate having a composition with an Md30_γ value of less than -150°C may have an austenite phase's Md30 value at a position 0.5 mm in the plate thickness direction from the surface of the steel plate increased to -150°C or higher by some method. In other words, it is not essential that the Md30_γ value be -150°C or higher.

[0062] 2. Characteristics of the Austenite Phase In the present invention, the austenite phase near the surface of the steel sheet is specified, rather than the entire steel sheet, because only the austenite phase transforms into hard martensite by working, while the ferrite phase does not.

[0063] (The area fraction of the austenite phase at a position 0.5 mm from the surface of the steel plate in the thickness direction is 30 to 70%) In the ferritic-austenite duplex stainless steel plate of the present invention, the area fraction of the austenite phase at a position 0.5 mm from the surface of the steel plate in the thickness direction (hereinafter sometimes referred to as the "austenite phase fraction") is 30 to 70%. If the area fraction of the austenite phase is less than 30%, the amount of deformation-induced martensitic transformation will be small, and good abrasive wear resistance will not be obtained. The area fraction of the austenite phase may be 35% or more, or 40% or more. As long as the Md30 value of the austenite phase described below is within an appropriate range, the higher the area fraction of the austenite phase, the more effective the abrasive wear resistance will be due to deformation-induced martensitic transformation. However, a high area fraction of the austenite phase will deteriorate manufacturability. Furthermore, from the viewpoint of properties other than abrasive wear resistance, such as corrosion resistance or strength, the area fraction is set to 70% or less. The area fraction of the austenite phase may be 65% or less, or 60% or less.

[0064] The area fraction of the austenite phase can be measured by image analysis. Specifically, a cross section of the sample perpendicular to the rolling direction is embedded in resin, and a contrast difference between the ferrite phase and the austenite phase is created by etching using a KOH solution. Then, a microstructure photograph is taken with a width of 200 μm in the thickness direction, centered at a position 0.5 mm from the surface of the steel sheet in the thickness direction, i.e., from 0.4 to 0.6 mm from the surface of the steel sheet, and the image is binarized to measure the area fraction of the austenite phase. The measurement range is 400 μm in the direction perpendicular to the thickness direction, from 0.4 to 0.6 mm from the surface of the steel sheet.

[0065] <Md30 value calculated from the composition of the austenite phase at a position 0.5 mm from the surface of the steel plate in the plate thickness direction: −150° C. or higher> The Md30 value is an index that represents the ease with which the austenite phase becomes strain-induced martensite, and is calculated by the following formula 4.

[0066] Md30 value (°C) = 551 - 462 (C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29 (Ni + Cu) - 18.5Mo - 68Nb ... (Equation 4) The element symbols in Equation 4 represent the content (mass%) of each element contained in the austenite phase, and if an element is not contained, 0 is substituted.

[0067] Only the austenite phase transforms into hard martensite through processing. If the Md30 value is less than -150°C, the austenite phase becomes stable, and processing-induced martensitic transformation does not occur during processing due to abrasive wear, and the effect of improving abrasive wear resistance cannot be obtained. Therefore, the Md30 value is set to -150°C or higher. The Md30 value may be -140°C or higher, -130°C or higher, -120°C or higher, -110°C or higher, or -100°C or higher. The higher the Md30 value, the more likely processing-induced martensitic transformation occurs. In order to adjust properties other than abrasive wear resistance, such as corrosion resistance and workability, and the ferrite phase ratio, the Md30 value may be 50°C or lower, 40°C or lower, or 30°C or lower.

[0068] The Md30 value of the austenite phase is determined by EPMA (Electron Probe Micro Analyzer) measurement. A cross section of the sample perpendicular to the rolling direction is embedded and polished with colloidal silica, and a map analysis of each of the elements C, N, Si, Mn, Cr, Ni, Cu, Mo, and Nb is performed over a 200 μm width in the thickness direction (0.4 to 0.6 mm from the surface of the steel sheet) centered 0.5 mm from the surface. The measurement area for the map analysis can be determined based on the grain size so as to include a sufficient number of grains, and the measurement range is 400 μm from the surface of the steel sheet to a position 0.4 to 0.6 mm from the surface of the steel sheet, in the direction perpendicular to the thickness direction. The beam diameter and step size of the measurement conditions are 1 μm or less and 1 μm or less, respectively, to ensure sufficient separation of the austenite phase and the ferrite phase.

[0069] Of the obtained measurement points, data for which the N (nitrogen) value is equal to or greater than the nitrogen content of the entire steel sheet × 2 ÷ (austenite phase fraction) are considered to be precipitated nitrides and are excluded. Taking advantage of the fact that the distribution of N differs greatly between the ferrite phase and the austenite phase, the measurement points from the largest N value when arranged in descending order to the measurement point ranked by the number of measurement points × austenite phase fraction are considered to be the austenite phase, and the Md30 value of the austenite phase of the steel sheet is calculated using the average value of the measured values ​​for each element in the austenite phase. The austenite phase fraction is the value measured by the image analysis described above.

[0070] (Average KAM value of the austenite phase at a position 0.5 mm from the surface of the steel sheet in the sheet thickness direction: 1.0° or more) The KAM (Kernel Average Misorientation) value is the average value of the crystal orientation difference between the target measurement point and its surrounding measurement points. The average KAM value of the austenite phase can be determined by EBSD (Electron Backscatter Diffraction) measurement.

[0071] Specifically, a cross section of the sample perpendicular to the rolling direction is embedded in resin and polished with colloidal silica. EBSD measurements are then performed over a 200 μm width in the thickness direction (0.4 to 0.6 mm from the surface of the steel sheet) centered at a position 0.5 mm from the surface of the steel sheet in the thickness direction, and a range of 400 μm in the direction perpendicular to the thickness direction. The measurement step size is 1 μm or less. Using the obtained measurement data, analysis is performed with a KAM analysis threshold of 5° and a grain boundary determination of 15°, and the calculated average KAM value is taken as the average KAM value of the γ phase 0.5 mm below the surface.

[0072] The KAM value is the average misorientation value between the six neighboring pixels for each pixel. By performing this calculation without crossing the grain boundary, a strain distribution map based on local orientation changes within the grain can be obtained. The average KAM value is the average of the KAM values ​​for each pixel.

[0073] If the average KAM value is less than 1.0°, sufficient strain cannot be imparted to the austenite in the surface layer, making it difficult for deformation-induced martensitic transformation to occur when abrasive wear occurs. The average KAM value may be 1.1° or more, 1.2° or more, 1.3° or more, 1.5° or more, or 1.8° or more. The larger the average KAM value, the more likely deformation-induced martensitic transformation will occur when abrasive wear occurs, so no upper limit is specified. Considering that an increase in residual strain increases strength and reduces workability, the average KAM value may be 4.0° or less, 3.8° or less, 3.5° or less, 3.2° or less, or 3.0° or less.

[0074] 3. Corrosion Resistance The ferritic-austenitic duplex stainless steel sheet of the present invention, which is produced by the production method described below and has the aforementioned chemical composition and structure, has excellent base metal corrosion resistance and HAZ corrosion resistance. Specifically, a ferric chloride CPT test based on ASTM G48E was conducted on the ferritic-austenitic duplex stainless steel sheet of the present invention and on samples of the ferritic-austenitic duplex stainless steel sheet subjected to solution heat treatment at 1,050°C for 5 minutes. The difference between the ferric chloride CPT of the ferritic-austenitic duplex stainless steel sheet and the ferritic chloride CPT of the ferritic-austenitic duplex stainless steel sheet subjected to solution heat treatment is 10°C or less. The ferric chloride CPT refers to the pitting corrosion initiation temperature (°C) evaluated based on ASTM G48E.

[0075] Furthermore, a test piece having dimensions of 8φ×110L was taken from a position one-quarter of the plate thickness of a ferritic-austenitic duplex stainless steel plate, and the test piece was heated to 1360°C at 30°C / s by high-frequency heating with a soaking zone of 15 mm and inert gas cooling, and then soaked for 5 seconds, cooled to 900°C at 40°C / s, immediately cooled to 600°C at 11°C / s, and immediately cooled to 300°C at 2°C / s. A critical pitting temperature (electrochemical CPT) test based on JIS G0590 was carried out on a cross section obtained by halving the soaking zone of the test piece. When the critical pitting temperature of the ferritic-austenitic duplex stainless steel plate and the ferritic-austenitic duplex stainless steel plate that had been subjected to the above heat treatment was tested, the difference in critical pitting temperature was 15°C or less.

[0076] <4. Manufacturing method of steel sheet> The ferritic-austenite duplex stainless steel sheet of the present invention can be manufactured by hot rolling steel having the above-described chemical composition at a rolling reduction of 10% to 40% at 950°C or less to form a hot-rolled steel sheet, air-cooling the obtained hot-rolled steel sheet at 830°C or higher for 10 seconds or more, and then cooling at a cooling rate of 1.0°C / s or more in the temperature range from 800°C to 600°C, and then cold-straightening the hot-rolled steel sheet under conditions such that the 0.2% proof stress after cold straightening is 10 MPa or more higher than before cold straightening.

[0077] To improve abrasive wear resistance, deformation-induced martensitic transformation must occur early after wear. This is because wear simultaneously adds strain and simultaneously removes the surface. If the material is removed before the strain required for deformation-induced martensite to form is reached, the desired improvement in abrasive wear resistance cannot be achieved. To achieve this, it is important to not only adjust the Md30 value of the austenite phase to an appropriate value, but also to impart processing strain to the steel sheet surface in advance. While residual hot-working strain promotes deformation-induced martensitic transformation, it also promotes the precipitation of nitrides and other substances that degrade corrosion resistance during rolling and cooling. Therefore, the processing temperature and processing rate during hot rolling must be limited. Furthermore, to obtain good corrosion resistance without heat treatment, it is necessary to suppress precipitation of nitrides and other substances during manufacturing.

[0078] Specifically, hot rolling is performed at a rolling reduction rate of 10% to 40% at 950°C or less, followed by air cooling at 830°C or higher for 10 seconds or more, and then cooling in the temperature range from 800°C to 600°C at a cooling rate of 1.0°C / s or more.

[0079] Since there is a limit to the amount of hot working strain that can be introduced during rolling, it is necessary to omit heat treatment to leave the hot working strain and then impart cold working strain to the steel. Specifically, the solution heat treatment after cooling following hot rolling is omitted to leave the hot working strain, and cold straightening is then performed to impart cold working strain such that the 0.2% proof stress increases by 10 MPa or more after cold straightening.

[0080] (Hot rolling with a reduction rate of 10% to 40% at 950°C or less) In order to impart working strain to the surface layer of the steel sheet in advance, hot rolling with a reduction rate of 10% to 40% at 950°C or less is carried out. Here, the reduction rate at 950°C or less is a value calculated by (h1-h2) / h1, where h1 is the plate thickness before the first rolling at 950°C or less and h2 is the plate thickness of the steel sheet. If the reduction rate at 950°C or less is less than 10%, the amount of hot working strain remaining in the steel sheet will be reduced, and the amount of cold correction required to impart the desired working strain to the austenite phase in the surface layer of the steel sheet will be excessive, making production impossible or significantly increasing production costs. The reduction rate at 950°C or less may be 12% or more, 14% or more, or 15% or more. If the reduction rate at 950°C or less exceeds 40%, the residual strain from hot working increases, the hot rolling time becomes longer, and the temperature of the steel sheet decreases, resulting in the precipitation of nitrides that deteriorate corrosion resistance during rolling and cooling. The reduction rate at 950°C or less may be 35% or less, 32% or less, or 30% or less. Since the recovery of the austenite phase is sufficiently slow at a rolling temperature of 950°C or less, no lower limit for the rolling temperature is specified, but rolling may be performed at 850°C or higher, taking into account the rolling reaction force and the transportation time to a water cooling device after rolling. The rolling temperature may be 860°C or higher, 870°C or higher, 880°C or higher, 890°C or higher, or 900°C or higher.

[0081] (Air cooling at 830°C or higher for 10 seconds or more) The hot-rolled steel sheet is subjected to air cooling at 830°C or higher for 10 seconds or more. Since the ferrite phase recovers at a lower temperature than the austenite phase, air cooling at 830°C or higher for 10 seconds or more can recover only the ferrite phase without recovering the austenite phase. If a large amount of strain remains in the ferrite phase, the strain can accelerate diffusion or become a precipitation site, causing chromium carbonitride to precipitate in the weld HAZ, resulting in deterioration of corrosion resistance. The air cooling time may be 12 seconds or more, 15 seconds or more, 20 seconds or more, 30 seconds or more, or 60 seconds or more. Since the more the ferrite phase recovers, the more corrosion resistance improves, so there is no upper limit to the air cooling time. However, the air cooling time may be, for example, 300 seconds or less, 240 seconds or less, or 180 seconds or less, taking into account manufacturing efficiency.

[0082] (Cooling rate of 1.0°C / s or more in the temperature range from 800°C to 600°C) Generally, the precipitation of chromium nitride, which is a major factor in reducing the corrosion resistance of resource-saving dual-phase stainless steel, varies somewhat depending on the composition, but in order to shorten the time spent in the temperature range where chromium nitride precipitates between 800°C and 600°C, cooling is carried out at a cooling rate of 1.0°C / s or more in the temperature range from 800°C to 600°C.

[0083] (Cold straightening in which the 0.2% proof stress increases by 10 MPa or more before and after straightening without solution heat treatment) The obtained steel sheet is subjected to cold straightening without solution heat treatment to impart cold working strain. Cold straightening is performed under conditions such that the difference in 0.2% proof stress before and after straightening is 10 MPa or more. If the difference in 0.2% proof stress before and after straightening is less than 10 MPa, sufficient working strain cannot be imparted to the surface layer of the steel sheet, resulting in inferior abrasive wear resistance. The difference in 0.2% proof stress before and after straightening may be 11 MPa or more, 12 MPa or more, 13 MPa or more, or 15 MPa or more. The larger the cold working strain, the more likely it is that working-induced martensitic transformation will occur when abrasive wear occurs, so no upper limit is specified for the difference in 0.2% proof stress before and after straightening. Taking into consideration that an increase in residual strain increases strength and reduces workability, the difference in 0.2% proof stress before and after straightening may be 50 MPa or less, 48 ​​MPa or less, 45 MPa or less, 42 MPa or less, or 40 MPa or less.

[0084] The method of cold straightening does not matter as long as the 0.2% proof stress increases by 10 MPa or more after cold straightening. Examples of cold straightening methods include leveler straightening. Note that controlling the increase in strength (0.2% proof stress) as an indicator of the degree of cold straightening is a common manufacturing method.

[0085] By the above-mentioned manufacturing method, it is possible to obtain a duplex stainless steel sheet having the above-mentioned austenite phase and having good abrasive wear resistance and corrosion resistance.

[0086] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0087] <Sample No. 1-1> Steel was melted to produce a 200 mm thick billet having the chemical composition of Composition No. 1 in Table 1. The obtained billet was heated at 1180°C for 60 minutes, hot rolled at a reduction of 25% to 950°C or less, air-cooled at 830°C for 39 seconds, and then cooled from 800°C to 600°C at a rate of 3.2°C / s to obtain a hot-rolled steel plate having a thickness of 30 mm.

[0088] The obtained hot-rolled steel sheet was subjected to cold straightening such that the difference in 0.2% proof stress before and after the cold straightening was 38 MPa. The difference in 0.2% proof stress before and after the cold straightening was confirmed by conducting a tensile test on the samples before and after the cold straightening.

[0089] For the tensile test, JIS No. 13B tensile test pieces with a plate thickness of 1 mm were taken from positions 0.1 to 1.1 mm from the surface of the steel plate, and the tensile test was carried out in accordance with JIS Z 2241: 2022 to measure the 0.2% proof stress. The difference in the obtained 0.2% proof stress was taken as the difference in proof stress due to cold straightening.

[0090] The area fraction of the austenite phase at a depth of 0.5 mm from the surface, the Md30 value, and the average KAM value were measured for the cold-straightened samples, and an abrasive wear test was conducted.Furthermore, the cold-straightened samples and the same samples that had been subjected to solution heat treatment were subjected to a ferric chloride CPT test.

[0091] (Austenite Phase Structure) The area fraction of the austenite phase at a position 0.5 mm from the surface in the sheet thickness direction was measured by embedding a cross section of the sample perpendicular to the rolling direction in resin, creating a contrast between the ferrite phase and the austenite phase by etching with a KOH solution, taking a structure photograph of an area of ​​400 μm in the direction perpendicular to the sheet thickness direction, with a width of 200 μm (position 0.4 to 0.6 mm from the surface) in the sheet thickness direction, centered at a position 0.5 mm from the surface, and binarizing the structure photograph image.

[0092] The Md30 value of the austenite phase 0.5 mm from the surface was determined by embedding a cross section of the sample perpendicular to the rolling direction in resin and polishing with colloidal silica, and performing EPMA map analysis with a beam diameter of 1 μm and a step size of 1 μm over an area of ​​400 μm in the direction perpendicular to the thickness direction, with a width of 200 μm in the thickness direction (0.4 to 0.6 mm from the surface) centered at a position 0.5 mm from the surface, and using an EPMA beam diameter of 1 μm and a step size of 1 μm.The average composition of the austenite phase was determined by separating the obtained measured values ​​into austenite phase and ferrite phase based on the N content and the area fraction of the austenite phase, and the Md30 value of the austenite phase was then determined from the above-mentioned formula 4.

[0093] It should be noted that the steel sheet structure may contain precipitates other than nitrides and inclusions that are unavoidably mixed in during the steelmaking stage. However, in the samples used in this study, the total area fraction of these substances was less than 0.1% at most, and their influence on the calculation of the austenite phase composition was negligible.

[0094] The average KAM value of the austenite phase at a position 0.5 mm from the surface was calculated by embedding a cross section of the sample perpendicular to the rolling direction in resin and polishing with colloidal silica, and then performing EBSD measurement with a step size of 1 μm over an area of ​​400 μm in the direction perpendicular to the plate thickness direction, with a width of 200 μm in the plate thickness direction (positions 0.4 to 0.6 mm from the surface), centered at a position 0.5 mm from the surface, and performing KAM analysis using the obtained measurement data, with a threshold value of 5° and a grain boundary determination of 15°.

[0095] (Wear Resistance) Wear resistance was evaluated by an abrasive wear test. The abrasive wear test was performed using a 55 mm x 20 mm test piece including the outermost surface of the steel plate, with a steel grid TGC-50 as the wear particles, a test temperature of room temperature, a particle speed of 200 g / min, a particle weight of 12.0 kg, an NBR adhesive foil with an outer diameter of 224 x width of 12 mm, a load of 100 N (10.2 kgf), a rotation speed of 50 rpm (sliding speed 0.59 m / s), and a test time of 60 min. The abrasive wear amount was calculated from the wear weight and wear volume of the test piece, and the average value of n = 2 was calculated and evaluated.

[0096] The abrasion resistance was evaluated by comparing it with the abrasive wear amount when a similar test was conducted on SUS304N2, and the abrasive wear amount of SUS304N2 was 6.5 mm. 3 / kg or less was judged to have good abrasion resistance and was rated as "○", 3 / kg was judged to have low abrasion resistance and was rated as "x".

[0097] (Corrosion resistance of base material) The corrosion resistance of the base material was evaluated by the difference in ferric chloride CPT from a sample that had undergone solution heat treatment. Specifically, a ferric chloride CPT test in accordance with ASTM G48E was carried out on a sample after cold straightening and a sample that had undergone solution heat treatment at 1,050°C for 5 minutes, and the difference in ferric chloride CPT between the sample that had undergone solution heat treatment and the sample after cold straightening was taken as the base material CPT difference.

[0098] Regarding the corrosion resistance of the base material, if the difference in base material CPT between the sample after cold straightening and the sample that had been subjected to solution heat treatment was 10°C or less, the corrosion resistance was judged to be good and the evaluation was given as "○", and if the difference in CPT was more than 10°C, the corrosion resistance was judged to be low and the evaluation was given as "×".

[0099] (HAZ corrosion resistance) The HAZ corrosion resistance was evaluated by the electrochemical CPT difference between the steel sheet after cold straightening and a sample that had been subjected to heat treatment simulating the HAZ of SAW welding. Specifically, a test piece with an 8φ×110L length was taken from the t / 4 portion of the cold straightened sample, and the test piece was heated to 1,360°C at 30°C / s by high-frequency heating with a soaking zone of 15 mm and inert gas cooling, soaked for 5 seconds, cooled to 900°C at 40°C / s, immediately cooled to 600°C at 11°C / s, and then immediately cooled to 300°C at 2°C / s. A critical pitting temperature (electrochemical CPT) test based on JIS G0590 was performed on a cross section obtained by splitting the soaking zone in half of the sample. The difference between the critical pitting temperature of the cold straightened sample and the critical pitting temperature of the sample after cold straightening was taken as the HAZ CPT difference.

[0100] Regarding HAZ corrosion resistance, if the difference in HAZ CPT between the cold-straightened sample and the solution-heat-treated sample was 15°C or less, the corrosion resistance was judged to be good and rated as "○", and if the difference in CPT was more than 15°C, the corrosion resistance was judged to be poor and rated as "×".

[0101] <Sample Nos. 1-2 to 16-3> Samples were prepared in the same manner as sample No. 1-1, except that the chemical composition of the slab, the rolling reduction at 950°C or less, the air-cooling time at 830°C or more, the cooling rate from 800°C to 600°C, and the 0.2% difference in proof stress before and after cold straightening were as shown in Tables 1 and 2, and the structure, wear resistance, and corrosion resistance of the surface layer of the steel plate were evaluated. The results are shown in Table 3, as with sample No. 1-1. Note that for samples in which the 0.2% difference in proof stress before and after cold straightening was 0, cold straightening was not performed.

[0102] The evaluation results for Samples 1-1 to 16-3 are shown in Table 3. When the evaluations of both wear resistance and corrosion resistance were "○", it was determined that the problems to be solved by the present invention were solved. In Table 3, "γ phase ratio" means the austenite phase ratio (%) measured by image analysis, "γ phase Md30" means the Md30 value (°C) calculated from the composition of the austenite phase measured by EPMA, and "γ phase KAM" means the average KAM value of the austenite phase measured by EBSD.

[0103]

[0104]

[0105]

[0106] In sample No. 1-4, the air cooling time above 830°C was short, which is thought to have caused precipitation of chromium nitride during the welding simulation heat treatment, resulting in a decrease in HAZ corrosion resistance.

[0107] It is believed that chromium nitride precipitated in sample No. 2-3 because the cooling rate between 800° C. and 600° C. was slow, resulting in a decrease in the corrosion resistance of the base material.

[0108] In sample No. 3-3, the reduction rate at 950°C or less was small, so the hot working strain remaining in the steel sheet was small, and it is thought that even when cold straightening was performed such that the difference in 0.2% proof stress was 20 MPa, the desired working strain was not imparted to the austenite phase in the surface layer of the steel sheet. As a result, the average KAM value of the austenite phase at a position 0.5 mm from the surface of the steel sheet in the sheet thickness direction was small, and the wear resistance was reduced.

[0109] In sample No. 4-4, the air cooling time above 830°C was short, which is thought to have caused precipitation of chromium nitride during the welding simulation heat treatment, resulting in a decrease in HAZ corrosion resistance.

[0110] In sample No. 5-3, the difference in 0.2% proof stress before and after cold straightening was small, which is thought to be why sufficient processing strain could not be imparted to the surface layer of the steel sheet. As a result, the average KAM value of the austenite phase at a position 0.5 mm from the surface of the steel sheet in the sheet thickness direction became small, and the wear resistance was reduced.

[0111] In sample No. 6-3, the reduction ratio at 950°C or less was large, which increased the residual strain from hot working and lengthened the hot rolling time, lowering the temperature of the steel sheet, and presumably causing nitrides to precipitate during rolling and cooling. As a result, the corrosion resistance of the base material was reduced.

[0112] In sample No. 7-3, the reduction ratio at 950°C or less was small, so the hot working strain remaining in the steel sheet was small, and it is thought that even when cold straightening was performed such that the difference in 0.2% proof stress was 20 MPa, the desired working strain was not imparted to the austenite phase in the surface layer of the steel sheet. As a result, the average KAM value of the austenite phase at a position 0.5 mm from the surface of the steel sheet in the sheet thickness direction was small, and the wear resistance was reduced.

[0113] In sample No. 8-3, the difference in 0.2% proof stress before and after cold straightening was small, which is thought to be why sufficient processing strain could not be imparted to the surface layer of the steel sheet. As a result, the average KAM value of the austenite phase at a position 0.5 mm from the surface of the steel sheet in the sheet thickness direction became small, and the wear resistance was reduced.

[0114] In sample No. 9-3, the difference in 0.2% proof stress before and after cold straightening was small, which is thought to be why sufficient processing strain could not be imparted to the surface layer of the steel sheet. As a result, the average KAM value of the austenite phase at a position 0.5 mm from the surface of the steel sheet in the sheet thickness direction became small, and the wear resistance was reduced.

[0115] In sample No. 10-4, the air cooling time above 830°C was short, which is thought to have caused precipitation of chromium nitride during the welding simulation heat treatment, resulting in a decrease in HAZ corrosion resistance.

[0116] In sample No. 11-3, the reduction ratio at 950°C or less was small, so the hot working strain remaining in the steel sheet was small, and it is thought that even when cold straightening was performed such that the difference in 0.2% proof stress was 20 MPa, the desired working strain was not imparted to the austenite phase in the surface layer of the steel sheet. As a result, the average KAM value of the austenite phase at a position 0.5 mm from the surface of the steel sheet in the sheet thickness direction was small, and the wear resistance was reduced.

[0117] It is believed that chromium nitride precipitated in sample No. 12-3 because the cooling rate between 800° C. and 600° C. was slow, resulting in a decrease in the corrosion resistance of the base material.

[0118] In sample No. 13-3, the reduction ratio at 950°C or less was large, which increased the residual strain from hot working and lengthened the hot rolling time, lowering the temperature of the steel sheet, and presumably causing nitrides to precipitate during rolling and cooling. As a result, the corrosion resistance of the base material was reduced.

[0119] In sample numbers 15-1 to 15-3, the Md30_γ value was less than −150° C., which is thought to be why the Md30 value of the austenite phase at a position 0.5 mm from the surface of the steel plate in the plate thickness direction was low. As a result, the wear resistance was reduced.

[0120] In sample numbers 16-1 to 16-3, the DF value was over 70, which is thought to be why the area fraction of the austenite phase at a position 0.5 mm from the surface of the steel plate in the plate thickness direction was low. As a result, the wear resistance was low.

[0121] On the other hand, the other samples were examples of the present invention, and duplex stainless steel sheets having both good corrosion resistance and abrasive wear resistance were obtained.

[0122] According to the present invention, a ferritic-austenitic duplex stainless steel sheet having excellent corrosion resistance and abrasive wear resistance can be obtained, which is extremely useful industrially. The duplex stainless steel sheet of the present invention can be used for sliding parts of structures and parts where friction occurs. For example, it can be used as a lining material for gates such as sluice gates, dams, and rivers.

Claims

1. A ferritic-austenitic duplex stainless steel plate, the chemical composition of which is, in mass%, C: 0.050% or less, Si: 2.00% or less, Mn: 0.5 to 6.0%, P: 0.0500% or less, S: 0.0500% or less, N: 0.08 to 0.30%, Cr: 17.0 to 30.0%, Ni: 0.1 to 8.0%, Mo: 0.1 to 3.5%, Cu: 0 to 3.0%, Nb: 0 to 0.100%, Sn: 0 to 1.00%, W: 0 to 1.00%, V: 0 to 1.00%, Ti: 0 to 0.050%, B: 0 to 0.0050%, the ferritic-austenitic duplex stainless steel plate contains Ca: 0-0.0050%, Mg: 0-0.0050%, Al: 0-0.0500%, and REM: 0-0.500%, with the balance being Fe and impurities; the PREN_Mn value calculated by the following formula 1 is 35.0 or less; the area fraction of the austenite phase at a position 0.5 mm in the plate thickness direction from the surface of the ferritic-austenitic duplex stainless steel plate is 30-70%; the Md30 value calculated by the following formula 2 from the composition in the austenite phase at a position 0.5 mm in the plate thickness direction from the surface of the ferritic-austenitic duplex stainless steel plate is -150°C or more; the average KAM value of the austenite phase at a position 0.5 mm in the plate thickness direction from the surface of the ferritic-austenitic duplex stainless steel plate is 1.0° or more; a ferric chloride CPT test in accordance with ASTM G48E method is carried out on the ferritic-austenitic duplex stainless steel plate and a sample obtained by subjecting the ferritic-austenitic duplex stainless steel plate to a solution heat treatment at 1050°C for 5 minutes, and the difference in ferric chloride CPT between the ferritic-austenitic duplex stainless steel plate and the solution heat treated sample is 10°C or less;A test piece having dimensions of 8φ×110L is taken from the ferritic-austenitic duplex stainless steel plate at a position one-quarter of the plate thickness, and is heated to 1360°C at 30°C / s by high-frequency heating with a soaking zone of 15 mm and inert gas cooling, followed by 5-second soaking, followed by cooling at 40°C / s to 900°C, immediately cooling at 11°C / s to 600°C, and immediately cooling at 2°C / s to 300°C. A critical pitting temperature (electrochemical CPT) test based on JIS G0590 is conducted on a cross section of the sample cut in half through the soaking zone, and the difference in critical pitting temperature between the ferritic-austenitic duplex stainless steel plate and a heat-treated ferritic-austenitic duplex stainless steel plate is 15°C or less. PREN_Mn value=Cr+3.3(Mo+0.5W)+16N-Mn (Equation 1) Md30 value (°C)=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo-68Nb (Equation 2) However, the element symbols in the above equation 1 represent the content (mass%) of each element contained in the steel, and the element symbols in equation 2 represent the content (mass%) of each element contained in the austenite phase, and if an element is not contained, 0 is substituted.

2. A ferritic-austenitic duplex stainless steel sheet according to claim 1, wherein the chemical composition contains, in mass%, one or more elements selected from Cu: 0.1 to 3.0%, Nb: 0.010 to 0.100%, Sn: 0.03 to 1.00%, W: 0.01 to 1.00%, and V: 0.01 to 1.00%.

3. A ferritic-austenitic duplex stainless steel sheet according to claim 1 or 2, wherein the chemical composition contains, in mass%, one or more elements selected from: Ti: 0.005 to 0.050%, and B: 0.0003 to 0.0050%.

4. A ferritic-austenitic duplex stainless steel sheet according to claim 1 or 2, wherein the chemical composition contains, in mass%, one or more elements selected from Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Al: 0.0030 to 0.0500%, and REM: 0.005 to 0.500%.

5. A ferritic-austenitic duplex stainless steel sheet according to claim 1 or 2, wherein the DF value calculated by the following formula 3 is 70 or less, and the Md30_γ value calculated by the following formula 4 is -150°C or more: DF value = 7.2 × (Cr + 0.88Mo + 0.78Si) - 8.9 × (Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9 ... (Formula 3) Md30_γ value = 702 - 175 × C - 190N - 19.7Si - 8.5Mn - 27.0Cr - 16.8Ni - 21.0Cu - 31.1Mo - 74.8Nb ... (Formula 4) Note that the element symbols in formulas 3 and 4 above represent the content (mass%) of each element in the steel, and 0 is substituted if the element is not present.

6. A method for producing a ferritic-austenitic duplex stainless steel sheet according to claim 1, wherein the chemical composition, in mass %, is: C: 0.050% or less, Si: 2.00% or less, Mn: 0.5 to 6.0%, P: 0.0500% or less, S: 0.0500% or less, N: 0.08 to 0.30%, Cr: 17.0 to 30.0%, Ni: 0.1 to 8.0%, Mo: 0.1 to 3.5%, Cu: 0 to 3.0%, Nb: 0 to 0.100%, Sn: 0 to 1.00%, W: 0 to 1.00%, V: 0 to 1.00%, Ti: 0 to 0.050%, B: 0 to 0.0050%, A method for producing a ferritic-austenitic duplex stainless steel sheet, comprising: hot rolling a steel containing Ca: 0-0.0050%, Mg: 0-0.0050%, Al: 0-0.0500%, and REM: 0-0.500%, with the balance being Fe and impurities, at a rolling reduction of 10% to 40% at 950°C or less to obtain a hot-rolled steel sheet; air-cooling the hot-rolled steel sheet at 830°C or higher for 10 seconds or more, and then cooling the hot-rolled steel sheet at a cooling rate of 1.0°C / s or more in a temperature range from 800°C to 600°C; and cold-straightening the hot-rolled steel sheet under conditions such that the 0.2% proof stress after cold-straightening is 10 MPa or more higher than that before cold-straightening.

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