Ferritic / austenitic two-phase stainless steel sheet

Abstract

The ferrite / austenitic two-phase stainless steel sheet has a chemical composition containing, in mass%, 0.002-0.05% of C, 0.10-2.00% of Si, 0.10-5.00% of Mn, 0.040% or less of P, 0.030% or less of S, 20.0-30.0% of Cr, 1.0-10.0% of Ni, 0.1-5.0% of Mo, 0.08-0.30% of N, and the remainder being Fe and impurities, the average aspect ratio of ferrite grains in a cross section including the rolling direction and the sheet thickness direction being 0.30 or more, and the average crystal grain size of the ferrite grains being 10.0 [mu] m or less.

Description

Technical Field

[0001] The present invention relates to a ferrite / austenite duplex stainless steel plate.

[0002] This application claims priority based on Japanese Patent Application No. 2023-020271 filed in Japan on February 13, 2023, the contents of which are incorporated herein by reference. Background Art

[0003] Ferritic / austenitic stainless steel has a lower nickel content than austenitic stainless steel, making it more economical and therefore sometimes used as an alternative to austenitic stainless steel. Furthermore, ferritic / austenitic stainless steel is used in a wide range of applications, including chemical plant components and heat exchangers, due to its high strength and excellent corrosion and fatigue resistance.

[0004] However, ferritic / austenitic stainless steel has lower ductility than austenitic stainless steel, and therefore various technologies for improving ductility have been studied.

[0005] For example, Patent Document 1 discloses a technique for improving the elongation, that is, the ductility, of ferrite / austenitic stainless steel by heating the steel to a ferrite single phase region (α single phase region) during hot rolling.

[0006] Furthermore, Patent Document 2 discloses a ferrite / austenitic stainless steel in which the elongation is improved by utilizing a work-induced transformation of the austenite phase by regulating the volume fraction of the austenite phase and the C+N content in the austenite phase.

[0007] As described above, conventional techniques for improving the ductility of ferrite / austenitic stainless steel utilize control of hot rolling conditions and work-induced transformation of the austenite phase.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 11-50143

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2006-183129 Summary of the Invention

[0012] Problems to be solved by the invention

[0013] However, as in Patent Document 1, if the steel is heated to the α single phase region during hot rolling, the steel may soften significantly, making production difficult. In addition, in recent years, from the perspective of alloy cost (economic efficiency), so-called reduced alloying element duplex stainless steels have been developed in ferritic / austenitic stainless steels, which have reduced Ni, Mo, etc. and contain Mn, and are used as practical steels. Compared with general-purpose duplex stainless steels, this alloying element-reduced duplex stainless steel has a lower ratio of ferrite phase (α phase) at high temperatures. Therefore, if heating to the α single phase region as described in Patent Document 1 is performed, the steel may soften more significantly, making production even more difficult.

[0014] Furthermore, if the austenite phase (γ phase) constituting ferrite / austenitic stainless steel contains a stable γ phase that does not undergo a strain-induced transformation, the technology described in Patent Document 2 cannot be applied. In particular, alloying element-saving duplex stainless steels contain a stable γ phase that does not undergo a strain-induced transformation. A stable γ phase that does not undergo a strain-induced transformation can be expected to improve ductility due to TRIP (Transformation Induced Plasticity). Therefore, applying the technology described in Patent Document 2 is even more difficult.

[0015] Furthermore, ferritic / austenitic stainless steel is manufactured in the ferrite / austenite dual phase region within the temperature range of the commonly used steel plate manufacturing process. Therefore, the microstructure of ferritic / austenitic stainless steel tends to be elongated in the rolling direction, resulting in frequent anisotropy of elongation, or ductility. In particular, when ferritic / austenitic stainless steel sheets are stretched in a direction parallel to the rolling direction (L direction) and perpendicular to the rolling direction (C direction), as in bulging processing, there is the issue of premature cracking.

[0016] Furthermore, when ferritic / austenitic stainless steel is used in applications such as chemical plant components and heat exchangers, high corrosion resistance is also required because corrosion such as crevice corrosion and stress corrosion cracking is expected.

[0017] As described above, it has been extremely difficult with conventional technologies to reduce the anisotropy of elongation (ductility) and thereby improve formability in ferrite / austenitic stainless steel having a stable γ phase.

[0018] The present invention has been made to solve such a problem, and an object of the present invention is to provide a ferrite / austenite duplex stainless steel sheet having low anisotropy of elongation (ductility) and excellent formability.

[0019] Means for solving problems

[0020] In response to the above-mentioned problems, the present inventors have newly recognized that, in ferritic / austenitic stainless steels having a stable γ phase that does not exhibit strain-induced transformation, the aspect ratio of the grains significantly affects the ductility and anisotropy thereof. Furthermore, they have newly recognized that, in addition to adjusting the composition, controlling the heat treatment conditions after cold rolling is effective in controlling the aspect ratio.

[0021] Furthermore, the inventors have found that, generally speaking, controlling the grain size and aspect ratio of the ferrite phase, rather than the austenite phase having excellent ductility, is effective in reducing the ductility and its anisotropy.

[0022] The present invention has been made based on the above findings and the gist of the present invention for the purpose of solving the above problems is as follows.

[0023] [1] The chemical composition of the ferrite / austenitic duplex stainless steel sheet according to one embodiment of the present invention comprises, in mass %, the following:

[0024] C: 0.002~0.05%,

[0025] Si: 0.10-2.00%,

[0026] Mn: 0.10~5.00%,

[0027] P: 0.040% or less,

[0028] S: 0.030% or less,

[0029] Cr: 20.0~30.0%,

[0030] Ni: 1.0-10.0%,

[0031] Mo: 0.1~5.0%,

[0032] N: 0.08~0.30%,

[0033] Cu: 0-2.0%,

[0034] Nb: 0-0.50%,

[0035] Ti: 0-0.50%,

[0036] V: 0~0.50%

[0037] W: 0~0.50%,

[0038] Co: 0-0.50%,

[0039] B: 0~0.0050%,

[0040] Sn: 0-0.50%,

[0041] Al: 0-0.50%,

[0042] Mg: 0-0.010%,

[0043] Ca: 0~0.0100%,

[0044] Ta: 0~0.050%,

[0045] Ga: 0~0.050%,

[0046] Zr: 0-0.50%, and

[0047] Rare earth elements: 0-0.010%,

[0048] The rest: Fe and impurities,

[0049] The average aspect ratio of ferrite grains in a cross section including the rolling direction and the plate thickness direction is 0.30 or more,

[0050] The average crystal grain size of ferrite grains is 10.0 μm or less.

[0051] [2] The ferrite / austenite duplex stainless steel sheet according to [1], wherein the chemical composition may also contain, in mass %, the following:

[0052] Mn: 0.10-2.00%,

[0053] Cr: 21.0-30.0%,

[0054] Ni: 3.0-10.0%,

[0055] Mo: 2.0~5.0%.

[0056] [3] The ferrite / austenitic duplex stainless steel sheet according to [2] above, wherein the PI value represented by the following formula (A) may be 32.0 or greater.

[0057] PI value = Cr + 3.3Mo + 16N (A)

[0058] The symbols of the elements in the above formula (A) represent the content (mass %) of the elements contained in the steel, and are set to zero when the elements are not contained.

[0059] [4] The ferrite / austenitic duplex stainless steel sheet according to [1], wherein the chemical composition may also contain, in mass %, the following:

[0060] Mn: 1.00~5.00%,

[0061] Cr: 20.0-25.0%,

[0062] Ni: 1.0-6.0%,

[0063] Mo: 0.1-2.0%,

[0064] Cu: 0.1-2.0%,

[0065] N: 0.08~0.20%.

[0066] [5] The ferrite / austenitic duplex stainless steel sheet according to [1] above, wherein the PI value represented by the following formula (A) may be 25.0 or greater.

[0067] PI value = Cr + 3.3Mo + 16N (A)

[0068] The symbols of the elements in the above formula (A) represent the content (mass %) of the elements contained in the steel, and are set to zero when the elements are not contained.

[0069] [6] The ferrite / austenitic duplex stainless steel sheet according to any one of [1] to [5], wherein the difference between the elongation at break in a direction parallel to the rolling direction and the direction perpendicular to the rolling direction in a tensile test may be 1.0% or less.

[0070] Effects of the Invention

[0071] According to the present invention, it is possible to provide a ferrite / austenite duplex stainless steel sheet having low anisotropy of elongation (ductility) and excellent formability. DETAILED DESCRIPTION

[0072] Hereinafter, embodiments of the present invention will be described in detail.

[0073] (Chemical Composition)

[0074] First, the chemical composition of the ferrite / austenitic duplex stainless steel sheet (hereinafter also referred to simply as "stainless steel sheet" or "steel sheet") according to this embodiment will be described in detail. It should be noted that, unless otherwise noted, the percentages of element contents in this specification refer to mass %.

[0075] C: 0.002~0.05%

[0076] Carbon (C) is an element that has a significant impact on the stability of the austenite phase. On the other hand, if C is contained excessively, the elongation of the steel plate may decrease. In addition, if C is contained excessively, the precipitation of Cr carbides may be promoted, leading to the occurrence of intergranular corrosion. Therefore, the C content is set to 0.05% or less. Preferably, it is 0.04% or less. In addition, from the perspective of corrosion resistance, the C content is preferably reduced, but excessive reduction of the C content will lead to a significant increase in refining costs, so the C content is preferably 0.002% or more.

[0077] Si: 0.10~2.00%

[0078] Silicon (Si) is a useful deoxidizing element. It is also effective in improving oxidation resistance. However, excessive Si content can sometimes lead to hardening of the steel sheet, resulting in reduced elongation. Therefore, the Si content is set to 2.00% or less. It is preferably 1.50% or less, and more preferably 1.00% or less. Excessively reducing the Si content increases the cost of steel refining, so the Si content is set to 0.10% or more. It is preferably 0.20% or more.

[0079] Mn: 0.10~5.00%

[0080] Manganese (Mn) is an element useful as a deoxidizing element during dissolution and refining. Furthermore, Mn concentrates in the austenite phase and plays an important role in stabilizing the austenite phase. However, if a large amount of Mn is contained, the uniform elongation may be reduced, and corrosion resistance and hot workability may also be reduced. Therefore, the Mn content is set to 5.00% or less. Preferably, it is 4.50% or less. However, if the Mn content is excessively reduced, it may lead to an increase in refining costs, so the Mn content is set to 0.10% or more. The Mn content is preferably 0.30% or more, and more preferably 0.50% or more.

[0081] The Mn content may be 2.00% or less. By setting the Mn content to 2.00% or less, a decrease in workability and corrosion resistance can be stably suppressed.

[0082] The Mn content may be 1.00% or more. From the viewpoint of stabilizing the austenite phase, the Mn content may be 1.00% or more, and more preferably 1.20% or more.

[0083] P: 0.040% or less

[0084] Phosphorus (P) is also contained in raw materials such as Cr and is therefore an unavoidable element. However, large amounts of P can reduce formability. The lower the P content, the better, and it is set to 0.040% or less. It is preferably 0.03% or less. However, excessively reducing the P content increases refining costs, so the lower limit of the P content can be set to 0.001% or more.

[0085] S: 0.030% or less

[0086] Sulfur (S) is an element that inevitably contaminates. S sometimes combines with Mn to form inclusions, which can become the starting point of rust. Therefore, the lower the S content, the better, and it is set to 0.030% or less. Lower S content improves corrosion resistance, so it is preferably set to 0.025% or less. However, excessively reducing the S content increases refining costs, so the lower limit of the S content can be set to 0.0001% or more.

[0087] Cr: 20.0~30.0%

[0088] Chromium (Cr) is an element necessary to ensure corrosion resistance. Cr also stabilizes the ferrite phase, concentrating in the ferrite phase and suppressing grain growth during annealing and the reduction in the aspect ratio of ferrite grains. To achieve these effects, the Cr content is set to 20.0% or more, preferably 20.5% or more. However, large amounts of Cr can cause hot working cracking and increase refining costs. Therefore, the upper limit of the Cr content is set to 30.0% or less. The Cr content is preferably set to 26.0% or less.

[0089] The Cr content may be 21.0% or more. By setting the Cr content to 21.0% or more, the effect of suppressing the reduction in the aspect ratio of ferrite grains can be more stably obtained.

[0090] The Cr content may be 25.0% or less. By setting the Cr content to 25.0% or less, hot working cracking can be more stably suppressed. The Cr content is more preferably 24.0% or less.

[0091] Ni: 1.0-10.0%

[0092] Nickel (Ni) is an austenite stabilizing element and is an important element for adjusting the stability of the austenite phase. Ni also has the effect of suppressing the precipitation of nitrides and improving corrosion resistance. Therefore, the Ni content is set to 1.0% or more, preferably 1.5% or more. On the other hand, if the Ni content exceeds 10.0%, it will lead to an increase in raw material costs. In addition, the austenite phase ratio may increase, which may cause problems such as stress corrosion cracking. Therefore, the Ni content is set to 10.0% or less, preferably 8.0% or less. The Ni content is more preferably 7.0% or less.

[0093] The Ni content may be 3.0% or more. By setting the Ni content to 3.0% or more, the austenite phase can be further stabilized, and the corrosion resistance can be further improved. The Ni content is more preferably 3.5% or more.

[0094] The Ni content may be 6.0% or less. By setting the Ni content to 6.0% or less, hot working cracking can be more stably suppressed. The Ni content is more preferably 5.5% or less.

[0095] Mo: 0.1~5.0%

[0096] Molybdenum (Mo) is an element that improves corrosion resistance. To maximize its effectiveness, the Mo content is set to 0.1% or higher. The Mo content is preferably set to 0.2% or higher, and more preferably 0.3% or higher. However, if the Mo content exceeds 5.0%, the raw material cost increases significantly, so the Mo content is set to 5.0% or lower. The Mo content is preferably 4.0% or lower, and more preferably 3.5% or lower.

[0097] The Mo content can also be set to 2.0-5.0%. Mo also contributes to improving corrosion resistance and is effective for stabilizing the ferrite phase. Specifically, Mo concentrates in the ferrite phase, suppressing grain growth during annealing and stably preventing a decrease in the aspect ratio of ferrite grains. To effectively exert this effect, the Mo content is preferably set to 2.0% or more, more preferably 2.5% or more.

[0098] On the other hand, as mentioned above, Mo is an expensive, rare element, and excessive inclusion can lead to an excessive increase in raw material costs. Therefore, considering the balance between corrosion resistance and raw material costs, the Mo content can be set to 0.1-2.0%. By setting the Mo content to 2.0% or less, good corrosion resistance can be maintained while suppressing increases in raw material costs. A more preferred Mo content is 1.8% or less.

[0099] N: 0.08~0.30%

[0100] Nitrogen (N) is an element that has a significant impact on the stability of the austenite phase. Furthermore, N has the effect of improving corrosion resistance when present in solid solution. When the N content is less than 0.08%, it may lead to coarsening of ferrite grains. Therefore, the N content is set to 0.08% or more, preferably 0.10% or more. However, if the N content exceeds 0.30%, it may lead to a decrease in elongation at break and a decrease in corrosion resistance due to the precipitation of Cr nitrides. Therefore, the N content is set to 0.30% or less, preferably 0.20% or less.

[0101] PI value: 25.0 or above

[0102] The PI value is a general indicator of the pitting corrosion resistance of a stainless steel plate, and is calculated from the average composition of the steel plate using the following formula (A). In order to obtain good pitting corrosion resistance, the PI value is preferably 25.0% or more. In addition, when the PI value is lower than 32.0, corrosion occurs in an environment containing many halogen ions such as chloride ions, which may become the starting point of crevice corrosion and stress corrosion cracking depending on the structure or stress state. Therefore, the lower limit of the PI value is more preferably 32.0 or more, and more preferably 32.5 or more. There is no need to specify the upper limit of the PI value, but since the hardening of the steel plate impairs the workability and increases the alloy cost, it is preferably 42.0 or less.

[0103] PI value = Cr + 3.3 × Mo + 16 × N (A)

[0104] In addition, the symbol of each element in formula (A) means the average content (mass %) in the entire steel plate, and when the element is not contained, it is substituted into zero.

[0105] The above is the basic chemical composition of the ferrite / austenite duplex stainless steel sheet of this embodiment. The remainder of the above chemical composition is iron and impurities.

[0106] The ferrite / austenitic duplex stainless steel plate of this embodiment may further optionally contain one or more of Cu, Nb, Ti, V, W, Co, B, Sn, Al, Mg, Ca, Ta, Ga, Zr, and rare earth elements to further improve workability, corrosion resistance, and hot workability, or to perform deoxidation and desulfurization during refining. The lower limit of the content of these elements is 0% or more, and preferably exceeds 0%.

[0107] Cu: 2.0% or less

[0108] Copper (Cu) is an austenite stabilizing element and has the effect of improving corrosion resistance by suppressing the precipitation of nitrides. To achieve these effects, the Cu content can also be set to 0.1% or more. The Cu content is preferably 0.2% or more, more preferably 0.3% or more. However, if the Cu content exceeds 2.0%, it sometimes leads to an increase in cost and deteriorates hot workability. Therefore, the Cu content is preferably 2.0% or less, more preferably 1.8% or less.

[0109] Nb: 0.50% or less

[0110] Niobium (Nb) forms nitrides (NbN) and carbides (NbC), improving workability. Therefore, the Nb content can be set to 0.01% or more. However, if the Nb content exceeds 0.5%, ductility may be reduced. Therefore, the Nb content is preferably 0.50% or less. The Nb content is more preferably 0.30% or less, and even more preferably 0.20% or less.

[0111] Ti: 0.50% or less

[0112] Titanium (Ti), like Nb, forms nitrides (TiN) and carbides (TiC), improving workability. Therefore, the Ti content can be set to 0.01% or more. However, if the Ti content exceeds 0.50%, ductility may be reduced, so the Ti content is preferably 0.5% or less. The Ti content is more preferably 0.30% or less, and even more preferably 0.20% or less.

[0113] V: 0.50% or less

[0114] Vanadium (V) forms nitrides, improving workability. Therefore, the V content can be set to 0.01% or more. However, if the V content exceeds 0.50%, ductility and hot workability may be reduced. Therefore, the V content is preferably 0.50% or less, and more preferably 0.40% or less.

[0115] W: 0.50% or less

[0116] Tungsten (W) is an element that improves corrosion resistance. To achieve this effect, it is preferably set to 0.05% or more. However, if the W content exceeds 0.50%, workability may be reduced. Therefore, the W content is preferably 0.50% or less, and more preferably 0.40% or less.

[0117] Co: 0.50% or less

[0118] Cobalt (Co) has the effect of improving high-temperature strength and hot workability. Therefore, the Co content is preferably set to 0.01% or more. However, if the Co content exceeds 0.50%, there is a possibility of reducing toughness. Therefore, the Co content is preferably 0.50% or less, and more preferably 0.30% or less.

[0119] B: 0.0050% or less

[0120] Boron (B) is an element that segregates at grain boundaries and improves hot workability. To maximize this effect, the B content is preferably set to 0.0002% or more. However, if the B content exceeds 0.0050%, corrosion resistance may deteriorate significantly. Therefore, the B content is preferably 0.0050% or less, and more preferably 0.0030% or less.

[0121] Sn: 0.50% or less

[0122] Tin (Sn) is an element that improves corrosion resistance. To maximize its effect, the Sn content is preferably set to 0.01% or more, more preferably 0.03% or more. However, if the Sn content exceeds 0.50%, hot workability may deteriorate. Therefore, the Sn content is preferably 0.50% or less, more preferably 0.40% or less.

[0123] Al: 0.50% or less

[0124] Aluminum (Al) may be contained to some extent for desulfurization and deoxidation. The aforementioned effects are achieved when the Al content is 0.01% or greater, so the Al content is preferably 0.01% or greater. However, if the Al content exceeds 0.50%, there is a risk of increased manufacturing defects and increased raw material costs, so the Al content is preferably 0.50% or less.

[0125] Mg: 0.010% or less

[0126] Magnesium (Mg) not only has a deoxidizing effect but also refines the solidified structure. To achieve these effects, the Mg content is preferably 0.0002% or more. However, Mg content exceeding 0.01% increases steelmaking costs, so the Mg content is preferably 0.010% or less.

[0127] Ca: 0.0100% or less

[0128] Calcium (Ca) may be contained to a certain extent for desulfurization and deoxidation. The above effects are achieved when the Ca content is 0.0001% or more, so the Ca content is preferably 0.0001% or more. However, if the Ca content exceeds 0.0100%, hot working cracking and corrosion resistance may be reduced. Therefore, the Ca content is preferably 0.0100% or less, and more preferably 0.0050% or less.

[0129] Ta: 0.050% or less

[0130] Tantalum (Ta) is an element that improves corrosion resistance by modifying inclusions and can be contained as needed. To maximize this effect, the Ta content is preferably 0.0002% or greater. However, if the Ta content exceeds 0.050%, it may lead to a decrease in room temperature ductility and toughness. Therefore, the Ta content is preferably 0.050% or less, and more preferably 0.030% or less.

[0131] Ga: 0.050% or less

[0132] Gallium (Ga) is an element that improves corrosion resistance and suppresses hydrogen embrittlement and can be contained as needed. To achieve this effect, the Ga content is preferably 0.0002% or more. However, if the Ga content exceeds 0.050%, workability may be reduced. Therefore, the Ga content is preferably 0.050% or less, and more preferably 0.030% or less.

[0133] Zr: 0.50% or less

[0134] Zirconium (Zr) is an element that has similar effects to those of Nb and Ti and improves oxidation resistance. To achieve these effects, the Zr content is preferably 0.01% or greater. However, if the Zr content exceeds 0.50%, it may not only reduce workability but also increase raw material costs. Therefore, the Zr content is preferably 0.50% or less, and more preferably 0.30% or less.

[0135] Rare earth elements: less than 0.010%

[0136] Rare earth elements (REM) are elements that improve hot workability. The above effects are achieved when the REM content is 0.0002% or greater, so the REM content can be set to 0.0002% or greater. However, if the REM content exceeds 0.010%, manufacturability is impaired and costs increase, so the upper limit of the REM content is preferably set to 0.010% or less. A more preferred REM content range is 0.0005% to 0.008%.

[0137] Here, rare earth elements (REM) are a collective term for scandium (Sc) and yttrium (Y) and 15 elements (lanthanides) from lanthanum (La) to lutetium (Lu). As used herein, "rare earth elements" refer to one or more elements selected from these rare earth elements, and "rare earth element content" refers to the total amount of rare earth elements.

[0138] The stainless steel sheet of the present embodiment contains Fe and impurities (including inevitable impurities) in addition to the above-mentioned elements. However, in addition to the above-described elements, further elements may be contained within a range that does not impair the effects of the present invention.

[0139] Furthermore, scrap raw materials are often used in the production of stainless steel sheets. Consequently, various impurity elements are often inevitably incorporated into stainless steel sheets, making it difficult to define the content of these impurity elements in a general manner. Therefore, the term "impurities" in this embodiment refers to elements present in amounts that do not impair the effects of the present invention.

[0140] (Metallic Structure)

[0141] The ferrite / austenitic dual-phase stainless steel sheet of this embodiment has a metal structure in which the average aspect ratio of ferrite grains in a cross section perpendicular to the rolling width direction is 0.3 or more and 1.0 or less, and the average crystal grain size of the ferrite grains is 10 μm or less. It should be noted that the "cross section perpendicular to the rolling width direction" herein refers to a cross section (L cross section) that includes the rolling direction and the plate thickness direction.

[0142] The present inventors have discovered that, in ferrite / austenitic duplex stainless steel sheets having a composition that is less susceptible to strain-induced transformation, the elongation (ductility) and anisotropy thereof are strongly influenced by the grain size and aspect ratio of the ferrite phase, not the austenite phase, which generally exhibits excellent ductility, and that controlling these grains is effective.

[0143] <Crystal Grain Size>

[0144] The average grain size of the ferrite grains of the stainless steel plate of this embodiment is 10.0 μm or less. The grains of the ferrite phase are easily grown in the rolling direction due to the pinning effect of the austenite phase. If the average grain size of the ferrite grains exceeds 10.0 μm, the aspect ratio becomes large, resulting in anisotropy of ductility, so it is set to 10.0 μm or less. It is preferably 8.0 μm or less, more preferably 7.5 μm or less, and even more preferably 5.0 μm or less. On the other hand, if the grain size of the ferrite grains is too fine, it is possible that the steel plate will be hardened due to strengthening due to grain refinement. Therefore, the average grain size of the ferrite grains is preferably 1.0 μm or more.

[0145] The average crystal grain size of ferrite grains can be measured as follows.

[0146] First, a test piece with a length of 20 mm and a width of 10 mm is cut from the steel plate, and the cross section of the test piece parallel to the rolling direction and perpendicular to the surface of the steel plate is electrolytically polished. Afterwards, the phase in the range of 400 μm × 400 μm in the central part of the plate thickness is identified by backscattered electron diffraction (EBSD). The data obtained by EBSD is classified into ferrite grains (BCC phase) and austenite grains (FCC phase) for each crystal grain, and their boundaries are set as crystal grain boundaries. It should be noted that when grains with the same crystal structure are adjacent to each other, the portion where the crystal orientation difference at adjacent measuring points is 15° or more is regarded as a crystal grain boundary. The average grain size of the ferrite grains (BCC phase) is calculated by the quadrature method.

[0147] (Average aspect ratio)

[0148] In the stainless steel plate of the present embodiment, the average aspect ratio of the ferrite grains in the cross section perpendicular to the rolling width direction (i.e., the cross section including the rolling direction and the plate thickness direction; L cross section) is greater than 0.30 and less than 1.00. The average aspect ratio of the ferrite phase affects the anisotropy of the elongation (ductility). When the average aspect ratio is less than 0.30, the strain is concentrated on the ferrite / austenite phase boundary due to the lack of crystal grain boundaries in the rolling direction. As a result, the ductility in the direction perpendicular to the rolling direction (i.e., the plate width direction) decreases, and the anisotropy of the ductility becomes larger. Therefore, the average aspect ratio of the ferrite grains in the L cross section is set to be greater than 0.30. Preferably, it is greater than 0.35, and more preferably, it is greater than 0.40. When the average aspect ratio is greater than 0.50, the anisotropy of the ductility can be further improved. The upper limit of the average aspect ratio is not particularly limited, but can be less than 0.60, and can be less than 0.55.

[0149] The "aspect ratio" in this embodiment is defined as the value obtained by dividing the length of the longest diameter (long diameter) of each ferrite grain in a cross section perpendicular to the rolling width direction by the length of the largest diameter (minor diameter) perpendicular to the rolling width direction (minor diameter / long diameter). In other words, the upper limit of the aspect ratio in this specification is 1.

[0150] The average aspect ratio of ferrite grains is determined by the following method.

[0151] The length of the longest diameter (long side) and the length of the maximum diameter (short side) perpendicular to the longest diameter of the ferrite grains identified by EBSD were measured, and the value obtained by dividing the short side by the long side, i.e., the aspect ratio, was calculated. The aspect ratio of all ferrite grains in an area of ​​400 μm × 400 μm in the center of the plate thickness was calculated, and the average value of these was set as the average aspect ratio of the ferrite grains.

[0152] In the metal structure of the duplex stainless steel sheet according to the present embodiment described above, the specific ratio of the ferrite phase to the austenite phase is not particularly limited. For example, the ratio of the ferrite phase may be set to 35 to 65 area %.

[0153] (elongation at break)

[0154] According to the present embodiment, a ferrite / austenitic stainless steel sheet with excellent formability can be obtained. Here, the so-called "excellent formability" in the present embodiment means that the difference between the elongation at break in the direction parallel to the rolling direction and the direction perpendicular to the rolling direction in the tensile test (hereinafter also referred to as the difference in elongation at break) is small, that is, the anisotropy of the elongation at break is small. If the ferrite / austenitic stainless steel sheet of the present embodiment is used, the difference in elongation at break can be suppressed to less than 1.0%. If the difference in elongation at break exceeds 1.0%, in addition to the increased risk of cracking during processing, it is also necessary to consider the direction of plate collection during processing, which may lead to reduced productivity and reduced yield during processing. However, according to the present embodiment, since the difference in elongation at break can be suppressed to less than 1.0%, a ferrite / austenitic stainless steel sheet with excellent formability can be provided.

[0155] (Manufacturing Method)

[0156] The manufacturing method of the ferrite / austenitic stainless steel sheet of this embodiment will be described. Regardless of the manufacturing method, the ferrite / austenitic stainless steel sheet of this embodiment can achieve its effects as long as it has the above-mentioned characteristics. However, in order to obtain ductility with low anisotropy, it is necessary to control the metal structure as described above. This metal structure can be achieved by combining the chemical composition of the steel with appropriate manufacturing conditions. Specifically, this metal structure can be achieved by a manufacturing method including the following steps (I) to (V).

[0157] (I) A heating step of heating a steel slab having a predetermined chemical composition.

[0158] (II) A hot rolling step of hot-rolling the heated steel slab to obtain a hot-rolled steel sheet.

[0159] (III) A hot-rolled sheet annealing step of annealing the hot-rolled steel sheet to obtain a hot-rolled annealed sheet.

[0160] (IV) A cold rolling step of cold-rolling the hot-rolled annealed sheet to obtain a cold-rolled steel sheet.

[0161] (V) A final annealing step of setting the average heating rate from 400°C to 1000°C to 100°C / s or more and holding the cold-rolled steel sheet after the cold rolling step at a soaking temperature of 1050 to 1150°C for annealing.

[0162] Hereinafter, preferred conditions in each step will be described.

[0163] (Heating process)

[0164] As a raw material for hot rolling, a steel billet obtained by continuous casting is used. The steel billet used can also be produced by an ingot casting method or a thin slab casting method. The composition of the steel billet can be any steel having the component composition described above. In the heating process, heating is performed before hot rolling. The heating temperature before hot rolling is set to 1150°C to 1250°C. If the heating temperature is lower than 1150°C, edge cracks gradually occur during hot rolling, so it is set as the lower limit. In addition, if the heating temperature exceeds 1250°C, the steel billet is deformed in the heating furnace or becomes prone to defects during hot rolling, so the heating temperature is set to 1250°C or less. A more preferred range of the heating temperature is 1180°C to 1220°C.

[0165] (Hot rolling process)

[0166] In the hot rolling step, the heated steel slab is hot rolled to obtain a hot-rolled steel sheet.

[0167] The hot rolling process includes rough rolling and finish rolling. During finish rolling, the final product temperature (rolling end temperature) is preferably controlled to be above 950°C to suppress edge cracking of the sheet. More preferably, the final product temperature is set to above 980°C. It should be noted that the upper limit of the final product temperature is not particularly limited, but can be set to below 1050°C.

[0168] (Hot-rolled plate annealing process)

[0169] In the hot-rolled sheet annealing step, the hot-rolled steel sheet is annealed to obtain a hot-rolled annealed sheet. Note that the hot-rolled sheet annealing may be performed as needed and may be omitted.

[0170] The annealing temperature of the hot-rolled steel sheet after the hot rolling step is set to 1050° C. to 1150° C. A more preferred range is 1050° C. to 1100° C. The annealing time in the hot-rolled steel sheet annealing step can be set to, for example, 5 to 300 seconds.

[0171] (Cold rolling process)

[0172] In the cold rolling process, the hot-rolled annealed sheet is cold-rolled to obtain a cold-rolled steel sheet. Before the cold rolling process, the oxide scale on the surface of the hot-rolled steel sheet may be removed by pickling.

[0173] The cold rolling reduction during the cold rolling process is set to 50% to 80%. Cold rolling may be performed only once or twice or more. For example, when cold rolling is performed twice, intermediate annealing may be performed between each cold rolling step. When intermediate annealing is performed, the reduction must be set to 50% to 80% during all cold rolling steps.

[0174] Here, the "cold rolling reduction ratio" is a value (%) calculated as {(sheet thickness before cold rolling - sheet thickness after cold rolling) / sheet thickness before cold rolling} × 100. However, when intermediate annealing is performed, the sheet thickness after intermediate annealing is used as the "sheet thickness before cold rolling."

[0175] When intermediate annealing is performed, the intermediate annealing temperature can be set to 1050° C. to 1150° C., similar to the annealing of the hot-rolled sheet after hot rolling. A more preferred range of the intermediate annealing temperature is 1050° C. to 1100° C.

[0176] (Final annealing process)

[0177] In the final annealing step, the cold-rolled steel sheet after the cold rolling step is subjected to final annealing with an average heating rate from 400°C to 1000°C set to 100°C / second or more and a soaking temperature set to 1050-1150°C to obtain a final product sheet (ferrite / austenitic stainless steel sheet).

[0178] During the heating process of the final annealing step, the average heating rate from 400°C to 1000°C is set to 100°C / second or more. When the average heating rate during the final annealing is lower than 100°C / second, the strain introduced during cold rolling may be recovered during the heating process. If the strain is recovered, the recrystallized grains generated during annealing become recrystallized grains that continue the rolling structure during cold rolling, so the aspect ratio of the ferrite grains becomes smaller. On the other hand, it is believed that when the average heating rate during the heating process is higher than 100°C / second, the strain recovery during heating can be suppressed, so the frequency of recrystallization nucleation increases, and as a result, the aspect ratio of the grains can be increased. The average heating rate is preferably higher than 200°C / second, and more preferably higher than 400°C / second.

[0179] The reason why the anisotropy of elongation (ductility) in the tensile direction decreases as the aspect ratio increases is not clear, but is presumed to be the following.

[0180] If it is considered that the ferrite phase (α phase) with lower ductility than the austenite phase (γ phase) restricts the ductility during tension, then it is considered that if the aspect ratio of the α phase becomes larger, not only the heterophase interface between the α grains and the γ grains elongated in the rolling direction, but also the grain boundaries of the α phase become stress concentration sources during tensile loading, resulting in an improvement in the anisotropy of elongation.

[0181] Here, the "average heating rate" in the present embodiment is set to a value obtained by dividing the temperature increase range of the steel plate from 400°C to 1000°C by the time required for heating from 400°C to 1000°C.

[0182] The soaking temperature (final annealing temperature) is set to 1050°C to 1150°C. If the final annealing temperature is lower than 1050°C, the strain during cold rolling may remain. If the strain remains, it will become unrecrystallized grains, so it is possible to become a hard structure. Therefore, the final annealing temperature is set to 1050°C or above, preferably 1070°C or above. On the other hand, if the final annealing temperature exceeds 1150°C, grain growth in the rolling direction occurs, the aspect ratio decreases, and anisotropy of elongation in the tensile direction occurs. Therefore, the final annealing temperature is set to below 1150°C, preferably below 1130°C.

[0183] The soaking time (holding time) for hot-rolled sheet annealing, intermediate annealing, and final annealing can be appropriately set. Typically, it is 300 seconds or less. Preferably, it is 120 seconds or less, and more preferably, it is 60 seconds or less. It should be noted that, from the perspective of suppressing the residual strain, the lower limit of the soaking time in the final annealing process can be set to 5 seconds or more.

[0184] (Cooling process)

[0185] The average cooling rate from the end of the soaking process to 400°C in the final annealing process can be set to more than 30°C / second. Since the temperature range from the end of the soaking process to 400°C is a temperature range that is sensitive to grain growth, the cooling rate of this temperature range is preferably increased. If the average cooling rate of the temperature range from the end of the soaking process to 400°C is lower than 30°C / second, grain growth may occur during the cooling process, the aspect ratio may decrease, and anisotropy of ductility during stretching may occur. Therefore, the average cooling rate of the temperature range from the end of the soaking process to 400°C is preferably set to 30°C / second. It should be noted that, if productivity and pickling properties are taken into consideration, the average cooling rate is preferably 40 to 60°C / second. The cooling method can be appropriately selected from air-water cooling, water cooling, etc.

[0186] Here, the "average cooling rate" is set to a value obtained by dividing the temperature drop of the steel plate from the start of cooling (end of soaking) to 400°C by the required time required for cooling from the start of cooling to 400°C.

[0187] The ferrite / austenite duplex stainless steel sheet of this embodiment can be manufactured by the manufacturing method described above. The thickness of the stainless steel sheet of this embodiment is not particularly limited. From the perspective of manufacturability, it is preferably 0.2 mm to 3.0 mm.

[0188] According to the ferrite / austenite stainless steel sheet of the present embodiment, it is possible to obtain a ferrite / austenite stainless steel sheet having low anisotropy of elongation and excellent formability.

[0189] Example

[0190] In order to confirm the effects of the present invention in detail, the following experiments were conducted. It should be noted that this embodiment is only one embodiment of the present invention, and the present invention is not limited to the following configuration.

[0191] Steels with the chemical compositions shown in Tables 1A and 1B were melted to form steel slabs, which were then hot-rolled to a thickness of 5 mm. The heating temperature before hot rolling was set at 1230°C, and the final finish temperature (rolling end temperature) of hot rolling was set at 980-1020°C. Subsequently, the hot-rolled sheets were annealed (annealing temperature: 1100°C, annealing time: 60 seconds), cold-rolled, intermediate annealed (annealing temperature: 1100°C), and finally cold-rolled to produce final cold-rolled steel sheets with a thickness of 0.5 mm. It should be noted that each cold-rolling step was performed so that the reduction ratio was 50-80%.

[0192] The resulting cold-rolled steel sheets were subjected to final annealing by varying the average heating rate, annealing temperature (soaking temperature), and annealing time (soaking time) as shown in Tables 2A and 2B. The final annealed cold-rolled steel sheets were then cooled in a temperature range up to 400°C at an average cooling rate of 50°C / second to obtain finished steel sheets (ferritic / austenitic stainless steel sheets).

[0193] From the resulting steel product, No. 13B test pieces specified in JIS Z 2241 were collected so that the direction parallel to the rolling direction (L direction) became the longitudinal direction. Similarly, from the resulting steel product, No. 13B tensile test pieces specified in JIS Z 2241 were collected so that the direction perpendicular to the rolling direction (C direction) became the longitudinal direction. Tensile tests were conducted using each No. 13B test piece to measure the elongation at break.

[0194] Furthermore, test pieces 20 mm long and 10 mm wide were cut from the steel plates. A cross-section of the test pieces, parallel to the rolling direction and perpendicular to the steel plate surface, was electrolytically polished. Electron backscatter diffraction (EBSD) was then used to investigate the identification of each phase, the grain size of each phase, and the average aspect ratio of the ferrite grains (BCC phase).

[0195] Specifically, first, the phases in the 400 μm × 400 μm area in the center of the plate thickness were identified. The data obtained by EBSD were classified into ferrite grains (BCC phase) and austenite grains (FCC phase) for each crystal grain, and their boundaries were regarded as crystal grain boundaries. It should be noted that when grains with the same crystal structure are adjacent to each other, the location where the crystal orientation difference between adjacent measurement points is 15° or more is regarded as a crystal grain boundary. The average crystal grain size of the ferrite grains (BCC phase) was calculated by the quadrature method.

[0196] Furthermore, the longest diameter length (long side) and the longest diameter length (short side) of the ferrite grains identified by EBSD were measured, and the value obtained by dividing the short side by the long side, i.e., the aspect ratio, was calculated. The aspect ratio of all ferrite grains in an area of ​​400 μm × 400 μm in the center of the plate thickness was calculated, and the average value of these was taken as the average aspect ratio of the ferrite grains.

[0197] Tables 2A and 2B show the production conditions and their measurement results. Note that the underlined elements in Tables 1A, 2B, 2A, and 2B indicate values ​​outside the scope of the present invention or the preferred production conditions and properties of the present invention.

[0198] When the average heating rate during the final annealing step is less than 100°C / s, the difference in elongation between the L and C directions exceeds 1.0%. In contrast, when the average heating rate is 100°C / s or higher, the difference in elongation between the L and C directions is 1.0% or lower, improving the anisotropy of elongation. In this case, the average aspect ratio of the α phase with an elongation difference of 1.0% or lower satisfies 0.30 or higher, and the average grain size of the α phase satisfies 10.0 μm or lower.

[0199] On the other hand, Steel Nos. A15, A16, B16, and B17 are examples of chemical compositions outside the invention range. Despite final annealing at an average heating rate of 400°C / s, these steels failed to meet the average aspect ratio of the α phase and the average grain size of the α phase, resulting in a significant difference in elongation.

[0200] Steel No. A15 has an excessive Mn content, while Steel No. B17 has an excessive C content. Consequently, the proportion of the γ phase increases excessively in both cases, restricting the growth of the α phase in the sandwiching direction between the γ phases. Consequently, the aspect ratio of the ferrite grains is not satisfied, and the difference in elongation increases.

[0201] Steel No. A16 has an excessively low N content, while Steel No. B16 has an excessively high Mn content. This excessively increases the proportion of the α phase, reduces the pinning force of the γ phase, and coarsens ferrite grains. As a result, the α phase / α phase interface decreases, increasing the difference in elongation.

[0202]

[0203]

[0204] [Table 2A]

[0205]

[0206] [Table 2B]

[0207]

[0208] Industrial applicability

[0209] The ferrite / austenite duplex stainless steel sheet disclosed herein has low anisotropy of elongation (ductility) and excellent formability, and therefore has high industrial applicability.

Claims

1. A ferrite / austenite duplex stainless steel plate, wherein: The chemical composition contains in mass %: C：0.002～0.05％、 Si: 0.10-2.00%, Mn: 0.10~5.00%, P: 0.040% or less, S: 0.030% or less, Cr：20.0～30.0％、 Ni: 1.0-10.0%, Mo: 0.1~5.0%, N：0.08～0.30％、 Cu: 0-2.0%, Nb: 0-0.50%, Ti: 0-0.50%, V：0～0.50％、 W：0～0.50％、 Co: 0-0.50%, B：0～0.0050％、 Sn: 0-0.50%, Al：0～0.50％、 Mg: 0-0.010%, Ca: 0~0.0100%, Ta: 0~0.050%, Ga: 0~0.050%, Zr: 0-0.50%, and Rare earth elements: 0-0.010%, The rest: Fe and impurities, The average aspect ratio of ferrite grains in a cross section including the rolling direction and the plate thickness direction is 0.30 or more, The average crystal grain size of ferrite grains is 10.0 μm or less.

2. The ferrite / austenite duplex stainless steel plate according to claim 1, wherein: The chemical composition contains, in mass %, the following: Mn: 0.10-2.00%, Cr：21.0～30.0％、 Ni: 3.0-10.0%, Mo: 2.0~5.0%.

3. The ferrite / austenite duplex stainless steel plate according to claim 2, wherein: The PI value represented by the following formula (A) is 32.0 or more, PI value = Cr + 3.3Mo + 16N (A) The symbols of the elements in the formula (A) represent the content of the elements in the steel, and the unit is mass %. If not present, it is set to zero.

4. The ferrite / austenite duplex stainless steel plate according to claim 1, wherein: The chemical composition contains, in mass %, the following: Mn: 1.00~5.00%, Cr：20.0～25.0％、 Ni: 1.0-6.0%, Mo: 0.1-2.0%, Cu: 0.1-2.0%, N：0.08～0.20％。 5. The ferrite / austenite duplex stainless steel plate according to claim 1, wherein: The PI value represented by the following formula (A) is 25.0 or more, PI value = Cr + 3.3Mo + 16N (A) The symbols of the elements in the formula (A) represent the content of the elements in the steel, and the unit is mass %. If not present, it is set to zero.

6. The ferrite / austenite duplex stainless steel plate according to any one of claims 1 to 5, wherein The difference between the elongation at break in the direction parallel to the rolling direction and the direction perpendicular to the rolling direction in the tensile test is 1.0% or less.

Images