Ferrite-austenite duplex stainless steel sheet

Abstract

A ferritic-austenitic duplex stainless steel sheet includes a chemical composition containing, in mass%, C: 0.002 to 0.05%, Si: 0.10 to 2.00%, Mn: 0.10 to 5.00%, P: 0.040% or less, S: 0.030% or less, Cr: 20.0 to 30.0%, Ni: 1.0 to 10.0%, Mo: 0.1 to 5.0%, N: 0.08 to 0.30%, and a remainder: Fe and impurities, and the average aspect ratio of ferrite grains is 0.30 or more in a section including a rolling direction and a sheet thickness direction, and the average crystal grain size of ferrite grains is 10.0 µm or less.

Description

TECHNICAL FIELD

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

[0002] The present application claims priority based on Japanese Patent Application No. 2023-020271 filed in Japan on February 13, 2023, and the contents thereof are incorporated herein.BACKGROUND ART

[0003] Ferritic-austenitic stainless steel contains a smaller amount of Ni than austenitic stainless steel and is excellent in economic efficiency, and therefore may be used as an alternative to austenitic stainless steel. Furthermore, ferritic-austenitic stainless steel has high strength and excellent corrosion resistance and fatigue resistance, and therefore is used in a wide range of industries, for example, in chemical plant members and heat exchangers.

[0004] However, ferritic-austenitic stainless steel has a lower ductility than austenitic stainless steel, and therefore various techniques have been studied to improve the ductility.

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

[0006] Patent Document 2 discloses ferritic-austenitic stainless steel improved in elongation by specifying the volume fraction of the austenite phase and the C + N content in the austenite phase and utilizing the strain-induced transformation of the austenite phase.

[0007] As described above, the conventional techniques relating to improvement in the ductility of ferritic-austenitic stainless steel utilize control of hot rolling conditions and strain-induced transformation of the austenitic phase.Citation ListPatent Document

[0008] Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H11-50143 Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2006-183129 SUMMARY OF INVENTIONTechnical Problem

[0009] However, heating to the α single phase region during hot rolling as in Patent Document 1 significantly softens the steel material, and thus may make manufacture difficult. From the viewpoint of alloy cost (economic efficiency), so-called alloy element-conserving type duplex stainless steel has been recently developed and used as practical steel among ferritic-austenitic stainless steel. In the alloy element-conserving type duplex stainless steel, the amount of Ni, Mo, and the like is reduced, and Mn is contained. In the alloy element-conserving type duplex stainless steel, the ratio of the ferrite phase (α phase) at a high temperature is lower than in general-purpose duplex stainless steel, and therefore if heating to the α single phase region is performed as described in Patent Document 1, the steel material is further significantly softened, and manufacturing may be more difficult.

[0010] In a case where the austenite phase (γ phase) included in ferritic-austenitic stainless steel is composed of a stable γ phase in which strain-induced transformation does not occur, there is a problem that the technique described in Patent Document 2 cannot be applied. In particular, the alloy element-conserving type duplex stainless steel is steel composed of a stable γ phase in which strain-induced transformation does not occur. The stable γ phase in which strain-induced transformation does not occur is expected to improve in ductility by transformation induced plasticity (TRIP). Therefore, application of the technique described in Patent Document 2 is more difficult.

[0011] Ferritic-austenitic stainless steel is manufactured in the ferrite-austenite dual phase region in the temperature range of a usually performed steel sheet manufacturing process. Therefore, the structure of ferritic-austenitic stainless steel tends to be elongated in the rolling direction, and as a result, anisotropy of elongation, that is, anisotropy of ductility often occurs. There is also a problem of work cracking caused during an early stage particularly in a case where tension is applied to a ferritic-austenitic stainless steel sheet in the direction (L direction) parallel to the rolling direction and in the direction (C direction) perpendicular to the rolling direction, as in bulging.

[0012] In a case where ferritic-austenitic stainless steel is used in chemical plant members, heat exchangers, and the like, corrosion such as crevice corrosion or stress corrosion cracking is assumed, and therefore high corrosion resistance is also required.

[0013] As described above, in the conventional techniques, it has been very difficult to reduce the anisotropy of elongation (ductility) and further improve the formability in ferritic-austenitic stainless steel having a stable γ phase.

[0014] The present invention has been made to solve such problems, and an object of the present invention is to provide a ferritic-austenitic duplex stainless steel sheet having a low anisotropy of elongation (ductility) and excellent formability.Solution to Problem

[0015] For solving the above problems, the present inventors have newly found that the aspect ratio of a crystal grain greatly affects the ductility and the anisotropy of the ductility in ferritic-austenitic stainless steel having a stable γ phase in which strain-induced transformation does not occur. Then, the present inventors have newly found that the aspect ratio is effectively controlled by adjusting components and controlling heat treatment conditions after cold rolling.

[0016] Furthermore, the present inventors have found that the control of the crystal grain size and the aspect ratio in a ferrite phase, not an austenite phase generally excellent in ductility, is effective for the ductility and reduction in the anisotropy of the ductility.

[0017] The present invention has been made by further studying the above findings, and the gist of the present invention for the purpose of solving the above problems is as follows.

[0018] [1] A ferritic-austenitic duplex stainless steel sheet according to an aspect of the present invention includes a chemical composition containing, in mass%, C: 0.002 to 0.05%, Si: 0.10 to 2.00%, Mn: 0.10 to 5.00%, P: 0.040% or less, S: 0.030% or less, Cr: 20.0 to 30.0%, Ni: 1.0 to 10.0%, Mo: 0.1 to 5.0%, N: 0.08 to 0.30%, Cu: 0 to 2.0%, Nb: 0 to 0.50%, Ti: 0 to 0.50%, V: 0 to 0.50%, W: 0 to 0.50%, Co: 0 to 0.50%, B: 0 to 0.0050%, Sn: 0 to 0.50%, Al: 0 to 0.50%, Mg: 0 to 0.010%, Ca: 0 to 0.0100%, Ta: 0 to 0.050%, Ga: 0 to 0.050%, Zr: 0 to 0.50%, a rare earth element: 0 to 0.010%, and a remainder: Fe and impurities, and the average aspect ratio of ferrite grains is 0.30 or more in a section including a rolling direction and a sheet thickness direction, and the average crystal grain size of ferrite grains is 10.0 µm or less. [2] In the ferritic-austenitic duplex stainless steel sheet described in [1], the chemical composition may contain, in mass%, Mn: 0.10 to 2.00%, Cr: 21.0 to 30.0%, Ni: 3.0 to 10.0%, and Mo: 2.0 to 5.0%. [3] The ferritic-austenitic duplex stainless steel sheet described in [2] may have a pitting index (PI) value of 32.0 or more, and the PI value is represented by a formula (A) described below. PI value = Cr + 3.3 Mo + 16 N An element symbol in the formula (A) represents a content (mass%) of a corresponding element contained in the ferritic-austenitic duplex stainless steel sheet, and is 0 in a case where the corresponding element is not contained. [4] In the ferritic-austenitic duplex stainless steel sheet described in [1], the chemical composition may contain, in mass%, Mn: 1.00 to 5.00%, Cr: 20.0 to 25.0%, Ni: 1.0 to 6.0%, Mo: 0.1 to 2.0%, Cu: 0.1 to 2.0%, and N: 0.08 to 0.20%. [5] The ferritic-austenitic duplex stainless steel sheet described in [1] may have a PI value of 25.0 or more, and the PI value is represented by a formula (A) described below. PI value = Cr + 3.3 Mo + 16 N An element symbol in the formula (A) represents a content (mass%) of a corresponding element contained in the ferritic-austenitic duplex stainless steel sheet, and is 0 in a case where the corresponding element is not contained. [6] In the ferritic-austenitic duplex stainless steel sheet described in any one of [1] to [5], the difference between a fracture elongation in a direction parallel to the rolling direction and the fracture elongation in a direction perpendicular to the rolling direction may be 1.0% or less in a tensile test. Advantageous Effects of Invention

[0019] According to the present invention, a ferritic-austenitic duplex stainless steel sheet can be provided that has a low anisotropy of elongation (ductility) and excellent formability.DESCRIPTION OF EMBODIMENTS

[0020] Hereinafter, embodiments of the present invention will be described in detail.(Chemical Composition)

[0021] First, the chemical composition of a ferritic-austenitic duplex stainless steel sheet (hereinafter, also simply referred to as a "stainless steel sheet" or a "steel sheet") according to the present embodiment will be described in detail. In the present specification, the unit "%" of the element content means "mass%" unless otherwise noted.C: 0.002 to 0.05%

[0022] Carbon (C) is an element that greatly affects the stability of an austenite phase. Meanwhile, if C is excessively contained in a steel sheet, the elongation of the steel sheet may be reduced. Furthermore, if C is excessively contained, precipitation of a Cr carbide is promoted, and grain boundary corrosion may occur. Therefore, the C content is set to 0.05% or less. The C content is preferably 0.04% or less. From the viewpoint of corrosion resistance, a low C content is preferable, but excessive reduction in the C content largely increases the refining cost, and therefore the C content is preferably 0.002% or more.Si: 0.10 to 2.00%

[0023] Silicon (Si) is a useful element as a deoxidizing element. Si is also an element effective for improving oxidation resistance. However, if Si is excessively contained in a steel sheet, the steel sheet is hardened, and the elongation may be reduced. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.50% or less, and more preferably 1.00% or less. The Si content is set to 0.10% or more because excessive reduction in the Si content increases the cost during refining of steel. The Si content is preferably 0.20% or more.Mn: 0.10 to 5.00%

[0024] Manganese (Mn) is an element useful as a deoxidizing element during melt and smelting. Mn is concentrated in an austenite phase, and has an important role in stabilizing the austenite phase. However, if a large amount of Mn is contained, the uniform elongation may be reduced, and the corrosion resistance and the hot workability may deteriorate. Therefore, the Mn content is set to 5.00% or less. The Mn content is preferably 4.50% or less. However, excessive reduction in the Mn content may increase the refining cost, and therefore 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.

[0025] The Mn content may be 2.00% or less. If the Mn content is 2.00% or less, deterioration of the workability and the corrosion resistance can be stably suppressed.

[0026] 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.P: 0.040% or less

[0027] Phosphorus (P) is also contained in a raw material such as Cr, and thus phosphorus (P) is an element inevitably mixed. However, if a large amount of P is contained, the formability deteriorates. A smaller P content is more preferable, and the P content is set to 0.040% or less. The P content is preferably 0.03% or less. However, excessive reduction in the P content increases the refining cost, and therefore the lower limit of the P content may be set to 0.001% or more.S: 0.030% or less

[0028] Sulfur (S) is an element inevitably mixed. S combines with Mn to form an inclusion, and thus may become a starting point of rusting. Therefore, a smaller S content is more preferable, and the S content is set to 0.030% or less. The lower the S content is, the more the corrosion resistance is improved, and therefore the S content is preferably 0.025% or less. However, excessive reduction in the S content increases the refining cost, and therefore the lower limit of the S content may be set to 0.0001% or more.Cr: 20.0 to 30.0%

[0029] Chromium (Cr) is an element necessary for ensuring corrosion resistance. Cr is a ferrite phase stabilizing element, and therefore Cr is concentrated in a ferrite phase, and exhibits effects of suppressing grain growth during annealing and suppressing a decrease in the aspect ratio of ferrite grains. In order to obtain these effects, the Cr content is set to 20.0% or more, and is preferably 20.5% or more. However, if a large amount of Cr is contained, hot work cracking is caused, or the cost of the refining step is increased. Therefore, the upper limit of the Cr content is set to 30.0% or less. The Cr amount is preferably 26.0% or less.

[0030] The Cr content may be 21.0% or more. If the Cr content is 21.0% or more, the effect of suppressing a decrease in the aspect ratio of ferrite grains can be more stably obtained.

[0031] The Cr content may be 25.0% or less. If the Cr content is 25.0% or less, hot work cracking can be more stably suppressed. The Cr content is more preferably 24.0% or less.Ni: 1.0 to 10.0%

[0032] Nickel (Ni) is an austenite stabilizing element, and is an important element for adjustment of the stability of an austenite phase. Furthermore, Ni suppresses precipitation of a nitride, and has an effect of improving corrosion resistance. Therefore, the Ni content is set to 1.0% or more, and is preferably 1.5% or more. Meanwhile, if the Ni content is more than 10.0%, the raw material cost is increased, and the austenite phase fraction is increased and thus problems such as stress corrosion cracking may be caused. Therefore, the Ni content is set to 10.0% or less, and is preferably 8.0% or less. The Ni content is more preferably 7.0% or less.

[0033] The Ni content may be 3.0% or more. If the Ni content is 3.0% or more, the austenite phase can be more stabilized, and corrosion resistance can be more improved. The Ni content is more preferably 3.5% or more.

[0034] The Ni content may be 6.0% or less. If the Ni content is 6.0% or less, hot work cracking can be more stably suppressed. The Ni content is more preferably 5.5% or less.Mo: 0.1 to 5.0%

[0035] Molybdenum (Mo) is an element that improves corrosion resistance. For exhibition of the effect, the Mo content is set to 0.1% or more. The Mo content is preferably 0.2% or more, and more preferably 0.3% or more. However, if the Mo content is more than 5.0%, the raw material cost is greatly increased, and therefore the Mo content is set to 5.0% or less. The Mo content is preferably 4.0% or less, and more preferably 3.5% or less.

[0036] The Mo content may be 2.0 to 5.0%. Mo acts to contribute to improvement in corrosion resistance, and is also an element effective for stabilizing a ferrite phase. Specifically, Mo is concentrated in a ferrite phase, and exhibits effects of suppressing grain growth during annealing and stably suppressing a decrease in the aspect ratio of ferrite grains. For efficient exhibition of these effects, the Mo content is preferably 2.0% or more, and more preferably 2.5% or more.

[0037] Meanwhile, as described above, Mo is an expensive rare element, and if Mo is excessively contained, the raw material cost may be excessively increased. Therefore, the Mo content may be set to 0.1 to 2.0% in consideration of the balance between the corrosion resistance and the raw material cost. If the Mo content is set to 2.0% or less, an increase in the raw material cost can be suppressed while good corrosion resistance is maintained. The Mo content is more preferably 1.8% or less.N: 0.08 to 0.30%

[0038] Nitrogen (N) is an element that greatly affects the stability of an austenite phase. Furthermore, N has an effect of improving corrosion resistance when present as a solid solution. If the N content is less than 0.08%, coarsening of ferrite grains may be caused. Therefore, the N content is set to 0.08% or more, and is preferably 0.10% or more. However, if the N content is more than 0.30%, the fracture elongation may be reduced, and the corrosion resistance may deteriorate due to precipitation of a Cr nitride. Therefore, the N content is set to 0.30% or less, and is preferably 0.20% or less.PI value: 25.0 or more

[0039] The PI value is a general index indicating the pitting corrosion resistance of a stainless steel sheet, and is calculated from the average composition of a steel sheet with the following formula (A). In order to obtain good pitting corrosion resistance, the PI value is preferably set to 25.0% or more. If the PI value is less than 32.0, corrosion occurs in an environment including a large amount of a halogen-based ion such as a chlorine ion, and under some conditions of the structure and the stress state, the corrosion may become a starting point of crevice corrosion or stress corrosion cracking. Therefore, the lower limit of the PI value is more preferably 32.0 or more, and still more preferably 32.5 or more. The upper limit of the PI value is not required to be particularly specified, but is preferably set to 42.0 or less to prevent the steel sheet from hardening and impairing the workability and to prevent the alloy cost from increasing. PI value = Cr + 3.3 × Mo + 16 × N

[0040] An element symbol in the formula (A) means the average content (mass%) in the entire steel sheet, and is substituted with zero in a case where the element is not contained.

[0041] The basic chemical composition of the ferritic-austenitic duplex stainless steel sheet of the present embodiment is as described above. The remainder of the chemical composition is iron and impurities.

[0042] The ferritic-austenitic duplex stainless steel sheet of the present embodiment can further optionally contain one or more among Cu, Nb, Ti, V, W, Co, B, Sn, Al, Mg, Ca, Ta, Ga, Zr, and a rare earth element for further improvement in the workability, the corrosion resistance, and the hot workability, or for deoxidation and desulfurization during refining. The lower limit of the amount of these elements is 0% or more, and preferably more than 0%.Cu: 2.0% or less

[0043] Copper (Cu) is an austenite stabilizing element, and has an effect of improving corrosion resistance by suppressing precipitation of a nitride. In order to obtain these effects, the Cu amount may be set to 0.1% or more. The Cu amount is preferably 0.2% or more, and more preferably 0.3% or more. However, if the Cu amount is more than 2.0%, the cost is increased, and the hot workability may deteriorate. Therefore, the Cu amount is preferably 2.0% or less, and more preferably 1.8% or less.Nb: 0.50% or less

[0044] Niobium (Nb) forms a nitride (NbN) or a carbide (NbC), and has an effect of improving workability. Therefore, the Nb amount may be set to 0.01% or more. However, if the Nb amount is more than 0.5%, the ductility may deteriorate, and therefore the Nb amount is preferably 0.50% or less. The Nb amount is more preferably 0.30% or less, and still more preferably 0.20% or less.Ti: 0.50% or less

[0045] Similarly to Nb, titanium (Ti) forms a nitride (TiN) or a carbide (TiC), and has an effect of improving workability. Therefore, the Ti amount may be set to 0.01% or more. However, if the Ti amount is more than 0.50%, the ductility may deteriorate, and therefore the Ti amount is preferably 0.5% or less. The Ti amount is more preferably 0.30% or less, and still more preferably 0.20% or less.V: 0.50% or less

[0046] Vanadium (V) forms a nitride, and has an effect of improving workability. Therefore, the V amount may be set to 0.01% or more. However, if the V amount is more than 0.50%, the ductility and the hot workability may deteriorate, and therefore the V amount is preferably 0.50% or less, and more preferably 0.40% or less.W: 0.50% or less

[0047] Tungsten (W) is an element that improves corrosion resistance, and the amount of W is preferably 0.05% or more for exhibition of the effect. However, if the W amount is more than 0.50%, the workability may deteriorate, and therefore the W amount is preferably 0.50% or less, and more preferably 0.40% or less.Co: 0.50% or less

[0048] Cobalt (Co) has an effect of increasing high-temperature strength to improve hot workability. Therefore, the Co amount is preferably 0.01% or more. However, if the Co amount is more than 0.50%, the toughness may deteriorate, and therefore the Co amount is preferably 0.50% or less, and more preferably 0.30% or less.B: 0.0050% or less

[0049] Boron (B) is an element that segregates at a grain boundary to improve hot workability. For exhibition of the effect, the B amount is preferably 0.0002% or more. However, if the B amount is more than 0.0050%, the corrosion resistance may significantly deteriorate, and therefore the B amount is preferably 0.0050% or less, and more preferably 0.0030% or less.Sn: 0.50% or less

[0050] Tin (Sn) is an element capable of improving corrosion resistance. For exhibition of the effect, the Sn amount is preferably 0.01% or more, and more preferably 0.03% or more. However, if the Sn amount is more than 0.50%, the hot workability may deteriorate, and therefore the Sn amount is preferably 0.50% or less, and more preferably 0.40% or less.Al: 0.50% or less

[0051] Aluminum (Al) may be slightly contained for desulfurization and deoxidation. Such an effect is exhibited when the Al amount is 0.01% or more, and therefore the Al amount is preferably 0.01% or more. However, if the Al amount is more than 0.50%, manufacture defects may be increased and the raw material cost may be increased, and therefore the Al amount is preferably 0.50% or less.Mg: 0.010% or less

[0052] Magnesium (Mg) has not only a deoxidation effect but also an effect of refining a solidified structure. For exhibition of these effects, the Mg amount is preferably 0.0002% or more. However, if the Mg amount is more than 0.01%, the cost in the steelmaking process is increased, and therefore the Mg amount is preferably 0.010% or less.Ca: 0.0100% or less

[0053] Calcium (Ca) may be slightly contained for desulfurization and deoxidation. Such an effect is exhibited when the Ca amount is 0.0001% or more, and therefore the Ca amount is preferably 0.0001 % or more. However, if the Ca amount is more than 0.0100%, hot work cracking may occur and the corrosion resistance may deteriorate, and therefore the Ca amount is preferably 0.0100% or less, and more preferably 0.0050% or less.Ta: 0.050% or less

[0054] Tantalum (Ta) is an element that improves corrosion resistance by modifying an inclusion, and may be contained as necessary. For exhibition of the effect, the Ta amount is preferably 0.0002% or more. However, if the Ta amount is more than 0.050%, the room-temperature ductility and the toughness may deteriorate, and therefore the Ta amount is preferably 0.050% or less, and more preferably 0.030% or less.Ga: 0.050% or less

[0055] Gallium (Ga) is an element that suppresses improvement in corrosion resistance and hydrogen embrittlement, and may be contained as necessary. For exhibition of the effect, the Ga amount is preferably 0.0002% or more. However, if the Ga amount is more than 0.050%, the workability may deteriorate, and therefore the Ga amount is preferably 0.050% or less, and more preferably 0.030% or less.Zr: 0.50% or less

[0056] Zirconium (Zr) has an action similar to that of Nb and Ti, and is also an element that improves oxidation resistance. For exhibition of these effects, the Zr amount is preferably 0.01% or more. However, if the Zr amount is more than 0.50%, the workability may deteriorate and the raw material cost may be increased, and therefore the Zr amount is preferably 0.50% or less, and more preferably 0.30% or less.Rare earth element: 0.010% or less

[0057] A rare earth element (REM) is an element that improves hot workability. This effect is exhibited when the content of a rare earth element is 0.0002% or more, and therefore the content of a rare earth element may be 0.0002% or more. However, if the content of a rare earth element is more than 0.010%, the manufacturability is impaired and the cost is increased, and therefore the upper limit of the content of a rare earth element is preferably 0.010% or less. The content range of a rare earth element is more preferably 0.0005 to 0.008%.

[0058] Here, the term "rare earth element (REM)" is a generic term for two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoids) of from lanthanum (La) to lutetium (Lu). The term "rare earth element" used in the present specification refers to one or more selected from these rare earth elements, and the "content of a rare earth element" means the total amount of rare earth elements.

[0059] The stainless steel sheet of the present embodiment includes Fe and impurities (including an inevitable impurity) in addition to the above-described elements, and can contain an element other than the above-described elements as long as an effect of the present invention is not impaired.

[0060] In manufacture of a stainless steel sheet, a scrap raw material is often used. Thus, various impurity elements are inevitably mixed into a stainless steel sheet in many cases, and the content of impurity elements is difficult to determine uniquely. Therefore, the term "impurity" in the present embodiment means an element contained in an amount such that operation and effects of the present invention are not impaired.(Metallographic Structure)

[0061] The ferritic-austenitic stainless duplex steel sheet of the present embodiment has a metallographic structure in which the average aspect ratio of ferrite grains in a section perpendicular to the rolling width direction is 0.3 or more and 1.0 or less and the average crystal grain size of ferrite grains is 10 µm or less. The term "section perpendicular to the rolling width direction" herein means a section (L-section) including the rolling direction and the sheet thickness direction.

[0062] The present inventors have found that, in a ferritic-austenitic duplex stainless steel sheet having a composition in which strain-induced transformation is less likely to occur, the elongation (ductility) and the anisotropy of the elongation are strongly affected by the crystal grain size and the aspect ratio in the ferrite phase, not the austenite phase generally excellent in ductility, and control of the crystal grain size and the aspect ratio in the ferrite phase is effective.<Crystal Grain Size>

[0063] The average crystal grain size of ferrite grains in the stainless steel sheet of the present embodiment is 10.0 µm or less. A crystal grain in the ferrite phase easily grows in the rolling direction due to the pinning effect of the austenite phase. If the average crystal grain size of ferrite grains is more than 10.0 µm, the aspect ratio is increased and the anisotropy of the ductility occurs, and therefore the average crystal grain size is set to 10.0 µm or less. The average crystal grain size is preferably 8.0 µm or less, more preferably 7.5 µm or less, and still more preferably 5.0 µm or less. Meanwhile, if the crystal grain size of ferrite grains is too small, the crystal grain refinement is strengthened, and thus the steel sheet may be hardened. Therefore, the average crystal grain size of ferrite grains is preferably 1.0 µm or more.

[0064] The average crystal grain size of ferrite grains can be measured with the following method.

[0065] First, a test piece having a length of 20 mm and a width of 10 mm is cut out from the steel sheet, and a section parallel to the rolling direction of the test piece and perpendicular to the steel sheet surface is electrolytically polished. Then, a phase in the range of 400 µm × 400 µm at the sheet thickness center portion is identified by electron backscatter diffraction (EBSD). The data obtained by EBSD is classified into ferrite grain (BCC phase) data and austenite grain (FCC phase) data for each crystal grain, and the boundary of each crystal grain is regarded as a crystal grain boundary. In a case where grains having the same crystal structure are adjacent to each other, a portion where the crystal orientation difference between the adjacent measurement points is 15° or more is regarded as a crystal grain boundary. The average crystal grain size of ferrite grains (BCC phase) is calculated with a quadrature method.(Average Aspect Ratio)

[0066] In the stainless steel sheet of the present embodiment, the average aspect ratio of ferrite grains in a section perpendicular to the rolling width direction (that is, a section including the rolling direction and the sheet thickness direction; L-section) is 0.30 or more and 1.00 or less. The average aspect ratio in the ferrite phase affects the anisotropy of the elongation (ductility). If the average aspect ratio is less than 0.30, strain concentrates at 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 (that is, the sheet width direction) deteriorates, and thus the anisotropy of the ductility is increased. Therefore, the average aspect ratio of ferrite grains in an L-section is set to 0.30 or more. The average aspect ratio is preferably 0.35 or more, and more preferably 0.40 or more. If the average aspect ratio is 0.50 or more, the anisotropy of the ductility can be more improved. The upper limit of the average aspect ratio is not particularly limited, and may be 0.60 or less, or 0.55 or less.

[0067] The "aspect ratio" in the present embodiment is defined as a value (minor axis / major axis) obtained, in a section perpendicular to the rolling width direction, by dividing the length (minor axis) of the maximum diameter orthogonal to the longest diameter of each ferrite grain by the length (major axis) of the longest diameter. That is, the upper limit of the aspect ratio is 1 in the present specification.

[0068] The average aspect ratio of ferrite grains is determined with the following method.

[0069] The length (long side) of the longest diameter of a ferrite grain identified by EBSD described above and the length (short side) of the maximum diameter orthogonal to the longest diameter are measured, and the value obtained by dividing the short side by the long side, that is, the aspect ratio is calculated. The aspect ratios of all of the ferrite grains in the range of 400 µm × 400 µm at the sheet thickness center portion are calculated, and the average of the aspect ratios is taken as the average aspect ratio of ferrite grains.

[0070] In the metallographic structure of the duplex stainless steel sheet of the present embodiment described above, the specific phase ratio between the ferrite phase and the austenite phase is not particularly limited. For example, the ratio of the ferrite phase may be 35 to 65 area%.(Fracture Elongation)

[0071] According to the present embodiment, a ferritic-austenitic stainless steel sheet excellent in formability is obtained. Here, the phrase "excellent in formability" in the present embodiment means that the difference between the fracture elongation in the direction parallel to the rolling direction and the fracture elongation in the direction perpendicular to the rolling direction (hereinafter, also referred to as the difference in fracture elongation) is small in a tensile test, that is, the anisotropy of the fracture elongation is low. According to the ferritic-austenitic stainless steel sheet of the present embodiment, the difference in fracture elongation can be suppressed to 1.0% or less. If the difference in fracture elongation is more than 1.0%, the possibility of occurrence of work cracking is increased, and consideration is necessary for the direction in which the sheet is collected during working, and thus deterioration of productivity and a decrease in yield may be caused during working. However, according to the present embodiment, the difference in fracture elongation can be suppressed to 1.0% or less, and thus a ferritic-austenitic stainless steel sheet excellent in formability can be provided.(Manufacturing Method)

[0072] A method of manufacturing the ferritic-austenitic stainless steel sheet of the present embodiment will be described. An effect of the ferritic-austenitic stainless steel sheet according to the present embodiment can be obtained, regardless of the manufacturing method, as long as the ferritic-austenitic stainless steel sheet has the above characteristics. However, control of the metallographic structure is necessary as described above for obtaining ductility with low anisotropy, and the metallographic structure can be realized by combining the chemical composition of the steel and appropriate manufacturing conditions. Specifically, the metallographic structure can be realized by a manufacturing method including the following steps (I) to (V).

[0073] (I) A heating step of heating a steel piece having a predetermined chemical composition. (II) A hot rolling step of hot-rolling the heated steel piece to obtain a hot-rolled steel sheet. (III) A hot-rolled sheet annealing step of annealing the hot-rolled steel sheet to obtain a hot-rolled annealed sheet. (IV) A cold rolling step of cold-rolling the hot-rolled annealed sheet to obtain a cold-rolled steel sheet. (V) A final annealing step of annealing the cold-rolled steel sheet after the cold rolling step by setting the average temperature rising rate from 400°C to 1000°C to 100°C / s or more and holding the soaking temperature to 1050 to 1150°C.

[0074] Hereinafter, preferable conditions in each step will be described.(Heating Step)

[0075] As a hot-rolled material, a steel piece obtained by continuous casting is used. The steel piece to be used may be manufactured with an ingot-making method or a thin slab casting method. Steel is to be used that has the above-described component composition as the composition of the steel piece. In the heating step, heating is performed prior to hot rolling. The heating temperature before hot rolling is 1150°C to 1250°C. If the heating temperature is less than 1150°C, edge cracking occurs during hot rolling, and therefore this temperature is taken as the lower limit. If the heating temperature is more than 1250°C, the steel piece is deformed in the heating furnace or a defect is easily caused during hot rolling, and therefore the heating temperature is set to 1250°C or less. A more preferable range of the heating temperature is 1180°C to 1220°C.(Hot Rolling Step)

[0076] In the hot rolling step, the heated steel piece is hot-rolled to obtain a hot-rolled steel sheet.

[0077] The hot rolling step includes rough rolling and finish rolling. In the finish rolling, the final finish temperature (rolling finishing temperature) is preferably controlled to 950°C or more from the viewpoint of suppressing edge cracking of the sheet. The final finish temperature is more preferably 980°C or more. The upper limit of the final finish temperature is not particularly limited, and may be 1050°C or less.(Hot-Rolled Sheet Annealing Step)

[0078] In the hot-rolled sheet annealing step, the hot-rolled steel sheet is annealed to obtain a hot-rolled annealed sheet. The hot-rolled sheet annealing is to be performed as necessary, and may be omitted.

[0079] The annealing temperature of the hot-rolled steel sheet after the hot rolling step is 1050°C to 1150°C. A more preferable range of the annealing temperature is 1050°C to 1100°C. The annealing time in the hot-rolled sheet annealing step may be, for example, 5 to 300 seconds.(Cold Rolling Step)

[0080] In the cold rolling step, the hot-rolled annealed sheet is cold-rolled to obtain a cold-rolled steel sheet. Before the cold rolling step, pickling may be performed to remove an oxide scale on the surface of the hot-rolled steel sheet.

[0081] The cold rolling reduction rate in the cold rolling step is 50% or more and 80% or less. The cold rolling may be performed only once, or two or more times. For example, in a case where cold rolling is performed twice, intermediate annealing may be performed between the times of cold rolling. In the case of performing intermediate annealing, the rolling reduction rate is required to be 50% or more and 80% or less in all of the times of cold rolling.

[0082] Here, the "cold rolling reduction rate" is a value (%) calculated by {(sheet thickness before cold rolling - sheet thickness after cold rolling) / sheet thickness before cold rolling} × 100. However, in the case of performing intermediate annealing, the sheet thickness after the intermediate annealing is used as the "sheet thickness before cold rolling".

[0083] In the case of performing intermediate annealing, the intermediate annealing temperature may be 1050°C to 1150°C as in annealing for the hot-rolled sheet after hot rolling. A more preferable range of the intermediate annealing temperature is 1050°C to 1100°C.(Final Annealing Step)

[0084] In the final annealing step, the cold-rolled steel sheet after the cold rolling step is subjected to final annealing at an average temperature rising rate from 400°C to 1000°C of 100°C / s or more at a soaking temperature of 1050 to 1150°C to obtain a final product sheet (ferritic-austenitic stainless steel sheet).

[0085] In the temperature rising process in the final annealing step, the average temperature rising rate from 400°C to 1000°C is set to 100°C / s or more. If the average temperature rising rate during final annealing is less than 100°C / s, the strain introduced by cold rolling may be recovered in the temperature rising process. Recovery of the strain allows the rolled structure obtained by cold rolling to remain in the recrystallized grains generated during annealing, resulting in small aspect ratios of ferrite grains. Meanwhile, if the average temperature rising rate in the temperature rising process is 100°C / s or more, it is considered that strain recovery is suppressed during temperature rising to increase the nucleus generation frequency of recrystallization and as a result, the aspect ratios of crystal grains can be increased. The average temperature rising rate is preferably 200°C / s or more, and more preferably 400°C / s or more.

[0086] The reason is not clear why an increase in aspect ratio decreases the anisotropy of the elongation (ductility) in the tensile direction, but is presumed to be as follows.

[0087] Assuming that the ferrite phase (α phase) having lower ductility than the austenite phase (γ phase) determines the ductility during tensioning, it is considered that when the aspect ratio in the α phase is increased, a stress by a tensile load concentrates not only at the heterogenous phase interface between the α grains and the γ grains elongated in the rolling direction but also at the grain boundaries in the α phase to improve the anisotropy of the elongation.

[0088] Here, the "average temperature rising rate" in the present embodiment is a value obtained by dividing the temperature rise width of the steel sheet from 400°C to 1000°C by the time required for heating from 400°C to 1000°C.

[0089] The soaking temperature (final annealing temperature) is set to 1050°C to 1150°C. If the final annealing temperature is less than 1050°C, the strain by cold rolling may remain. If the strain remains, unrecrystallized grains are formed, and thus the structure may be hardened. Therefore, the final annealing temperature is set to 1050°C or more, and is preferably 1070°C or more. Meanwhile, if the final annealing temperature is more than 1150°C, grain growth in the rolling direction occurs, the aspect ratio is decreased, and the anisotropy of the elongation in the tensile direction occurs. Therefore, the final annealing temperature is set to 1150°C or less, and is preferably 1130°C or less.

[0090] The soaking time (holding time) in each of the hot-rolled sheet annealing, the intermediate annealing, and the final annealing of the hot-rolled sheet can be appropriately set. The soaking time is typically 300 seconds or less. The soaking time is preferably 120 seconds or less, and more preferably 60 seconds or less. The lower limit of the soaking time in the final annealing step may be 5 seconds or more from the viewpoint of suppressing residual strain.(Cooling Step)

[0091] The average cooling rate from the end of soaking to 400°C in the final annealing step may be 30°C / s or more. The temperature range from the end of soaking to 400°C is sensitive to grain growth, and therefore the cooling rate in this temperature range is desirably increased. If the average cooling rate in the temperature range from the end of soaking to 400°C is less than 30°C / s, grain growth occurs in the cooling process to decrease the aspect ratio, and the anisotropy of the ductility during tensioning may occur. Therefore, the average cooling rate in the temperature range from the end of soaking to 400°C is preferably 30°C / s. In consideration of the productivity and the pickling property, the average cooling rate is desirably 40 to 60°C / s. The cooling method is to be appropriately selected from air-water cooling, water-cooling, and the like.

[0092] Here, the "average cooling rate" is a value obtained by dividing the temperature drop width of the steel sheet from the start of cooling (end of soaking) to 400°C by the time required for cooling from the start of cooling to 400°C.

[0093] The ferritic-austenitic duplex stainless steel sheet of the present embodiment can be manufactured with the above-described manufacturing method. The thickness of the stainless steel sheet of the present embodiment is not particularly limited. The thickness is preferably 0.2 mm to 3.0 mm from the viewpoint of manufacturability.

[0094] According to the ferritic-austenitic stainless steel sheet of the present embodiment, a ferritic-austenitic stainless steel sheet can be obtained that has low anisotropy of the elongation and is excellent in formability.Examples

[0095] In order to confirm an effect of the present invention in detail, the following experiment was performed. Note that the present examples illustrate an example of the present invention, and the present invention is not limited to the following configuration.

[0096] Steel having a chemical composition shown in Tables 1A and 1B was melted to form a steel piece, and the steel piece was hot-rolled until the sheet thickness reached 5 mm. The heating temperature before hot rolling was 1230°C, and the final finish temperature (rolling finishing temperature) in hot rolling was 980 to 1020°C. Next, the hot-rolled sheet was sequentially subjected to annealing (annealing temperature: 1100°C, annealing time: 60 seconds), cold rolling, intermediate annealing (annealing temperature: 1100°C), and cold rolling to prepare a cold-rolled steel sheet having a final sheet thickness of 0.5 mm. Each cold rolling was performed so that the rolling reduction rate was 50 to 80%.

[0097] The obtained cold-rolled steel sheet was subjected to final annealing in which the average temperature rising rate, the annealing temperature (soaking temperature), and the annealing time (soaking time) were changed as shown in Tables 2A and 2B. The cold-rolled steel sheet subjected to the final annealing was cooled at an average cooling rate of 50°C / s in a temperature range to 400°C to obtain a product sheet (ferritic-austenitic stainless steel sheet).

[0098] From the obtained product sheet, a No. 13B test piece specified in JIS Z 2241 was taken so that the direction (L direction) parallel to the rolling direction was the longitudinal direction. Similarly, from the obtained product sheet, a No. 13B tensile test piece specified in JIS Z 2241 was taken so that the direction (C direction) perpendicular to the rolling direction was the longitudinal direction, a tensile test was performed using each of the No. 13B test pieces, and the fracture elongation was measured.

[0099] Furthermore, a test piece having a length of 20 mm and a width of 10 mm was cut out from the steel sheet, and a section parallel to the rolling direction of the test piece and perpendicular to the steel sheet surface was electrolytically polished. Then, each phase was identified, and the crystal grain size in each phase and the average aspect ratio of ferrite grains (BCC phase) were investigated by electron backscatter diffraction (EBSD).

[0100] Specifically, first, a phase in the range of 400 µm × 400 µm at the sheet thickness center portion was identified, the data obtained by EBSD was classified into ferrite grain (BCC phase) data and austenite grain (FCC phase) data for each crystal grain, and the boundary of each crystal grain was regarded as a crystal grain boundary. In a case where grains having the same crystal structure were adjacent to each other, a portion where the crystal orientation difference between the adjacent measurement points was 15° or more was regarded as a crystal grain boundary. The average crystal grain size of ferrite grains (BCC phase) was calculated with a quadrature method.

[0101] Furthermore, the length (long side) of the longest diameter of a ferrite grain identified by EBSD and the length (short side) of the maximum diameter orthogonal to the longest diameter were measured, and the value obtained by dividing the short side by the long side, that is, the aspect ratio was calculated. The aspect ratios of all of the ferrite grains in the range of 400 µm × 400 µm at the sheet thickness center portion were calculated, and the average of the aspect ratios was taken as the average aspect ratio of ferrite grains.

[0102] Tables 2A and 2B show the manufacturing conditions and the measurement results. The underlines shown in Tables 1A, 1B, 2A, and 2B indicate that the values are out of the scope of the present invention, or out of the range of the preferred production conditions and the properties of the present invention.

[0103] In a case where the average temperature rising rate in the temperature rising process of the final annealing step is less than 100°C / s, the difference between the elongation in the L direction and the elongation in the C direction is more than 1.0%, whereas in a case where the average temperature rising rate is 100°C / s or more, the difference between the elongation in the L direction and the elongation in the C direction is 1.0% or less, and thus the anisotropy of the elongation is improved. Here, the average aspect ratio in the α phase having an elongation difference of 1.0% or less satisfied 0.30 or more, and the average crystal grain size in the α phase satisfied 10.0 µm or less.

[0104] Meanwhile, Steel Nos. A15, A16, B16, and B17 are examples in which the chemical composition is out of the scope of the invention. In the Steel Nos. A15, A16, B16, and B17, the final annealing was performed at an average temperature rising rate of 400°C / s, but the average aspect ratio in the α phase or the average crystal grain size in the α phase was unsatisfactory, resulting in a large difference in elongation.

[0105] The Steel No. A15 had an excessively large Mn content, and the Steel No. B17 had an excessively large C content. Therefore, in both cases, the ratio of the y phase was excessively increased, and the growth of the α phase sandwiched between γ phases was restricted in the sandwiching direction. As a result, the aspect ratio of ferrite grains was unsatisfactory, resulting in a large difference in elongation.

[0106] The Steel No. A16 had an excessively small N content, and the Steel No. B16 had an excessively large Mn content, and therefore the ratio of the α-phase was excessively increased, the pinning force of the γ-phase was decreased, and the ferrite grains were coarsened, resulting in a decrease in the α-phase / α-phase interface, and a large difference in elongation. [Table 1A]Steel No.Chemical composition (mass%, remainder: Fe and impurities)CSiMnPSCrNiMoCuNPIOthersA10.030.323.220.0290.00221.22.10.31.00.1825.1-A20.030.811.560.0400.00222.43.50.30.80.1726.1-A30.030.192.230.0310.00323.34.91.10.40.1729.7-A40.020.201.440.0250.00223.15.21.40.20.1730.4-A50.040.443.500.0280.00222.03.00.50.30.1826.5V: 0.05A60.030.383.610.0330.00221.01.90.40.60.1825.2Nb: 0.04, Sn: 0.08A70.040.144.810.0290.00222.04.00.40.30.1325.4Ti: 0.03, B: 0.0011, Al: 0.12A80.030.704.720.0280.00320.62.70.80.60.1525.6Mg: 0.006, Zr: 0.11A90.030.293.230.0280.00222.22.60.31.10.1725.9W: 0.21, Ca: 0.0070, REM: 0.008A100.040.551.890.0320.00323.45.21.50.40.1630.9Co: 0.30, Ta: 0.006, Ga: 0.005A110.0030.353.470.0290.00220.92.01.80.90.1829.7Nb: 0.42, Sn: 0.32, Zr: 0.36A120.031.901.810.0240.00323.25.60.91.70.0927.6Ti: 0.39, Mg: 0.009, Ta: 0.040A130.031.800.220.0300.00022.06.21.21.40.1328.0V: 0.46, B: 0.0040, Al: 0.48A140.020.422.170.0290.00229.39.60.10.80.1832.5W: 0.44, Ga: 0.037A150.030.195.290.0310.00321.12.30.30.40.2225.6V: 0.12, B: 0.0009A160.021.300.500.0290.00222.35.40.70.20.0725.7Al: 0.08, Ca: 0.0060 [Table 1B] Steel No.Chemical composition (mass%, remainder: Fe and impurities)CSiMnPSCrNiMoCuNPIOthersB10.030.220.440.0280.00224.85.83.20.00.1838.2-B20.040.300.820.0290.00225.15.02.20.00.2235.9-B30.040.190.550.0310.00322.13.23.80.00.2739.0-B40.040.321.800.0280.00225.66.53.00.00.2038.7-B50.030.240.620.0290.00325.15.92.70.20.1937.1B60.030.410.910.0300.00224.75.83.10.00.2438.8Nb: 0.05, V: 0.18B70.030.140.660.0290.00225.26.32.90.00.2138.1Al :0.09B80.040.700.540.0290.00324.05.52.80.00.2036.4Mg: 0.007, Ca: 0.0012B90.030.250.760.0280.00223.74.42.90.00.2537.3Ti: 0.11, Co: 0.18, Ta: 0.011B100.050.180.320.0320.00323.45.22.10.00.1532.7W: 0.22, B: 0.0015, REM: 0.009B110.040.230.450.0270.00324.15.22.20.00.1834.2Sn: 0.07, Ga: 0.006, Zr: 0.12B120.040.800.200.0300.00121.24.02.10.10.1831.0V: 0.42, W: 0.21, Ga: 0.041B130.051.701.300.0250.00126.76.22.50.10.2839.4W: 0.22, Co: 0.46, Sn: 0.21B140.040.900.670.0300.00225.87.04.91.80.2245.5B: 0.0043, Ta: 0.030, Zr: 0.33B150.0060.200.400.0290.00229.29.62.11.70.0937.6Nb: 0.39, Ti: 0. 34, Al: 0.46B160.030.225.100.0290.00325.23.20.30.30.1929.2B170.061.120.880.0290.00223.15.41.80.60.2232.6Al: 0.08, Zr: 0.03 [Table 2A] No.Steel No.Average temperature rising rate (°C / s)Soaking temperature (°C)Soaking time (s)Aspect of ferrite grainsGrain size of ferrite grains (µm)Difference between elongation in L direction and elongation in C direction (%)Note1A1401050600.253.52.7Comparative Example2A1401100300.284.12.3Comparative Example3A1801100300.294.21.6Comparative Example4A1801150100.265.11.8Comparative Example5A11201100300.323.80.5Example of present invention6A12001100300.393.60.3Example of present invention7A14001050600.373.20.2Example of present invention8A14001100300.404.60.1Example of present invention9A1400115050.365.30.5Example of present invention10A18001100300.374.00.1Example of present invention11A24001050300.333.20.3Example of present invention12A24001100300.344.20.4Example of present invention13A24001150300.476.60.3Example of present invention14A24001200300.2715.32.6Comparative Example15A34001100300.364.80.5Example of present invention16A340011001200.446.10.5Example of present invention17A340011002400.498.20.3Example of present invention18A44001100300.384.70.2Example of present invention19A54001100300.355.20.2Example of present invention20A64001100300.384.10.6Example of present invention21A74001100300.333.90.3Example of present invention22A84001100300.405.50.5Example of present invention23A94001100300.374.30.1Example of present invention24A104001100300.334.50.4Example of present invention25A114001100300.413.40.2Example of present invention26A124001100300.393.50.8Example of present invention27A134001100300.444.10.1Example of present invention28A144001100300.374.60.5Example of present invention29A154001100300.253.22.5Comparative Example30A164001100300.3116.32.8Comparative Example [Table 2B] No.Steel No.Average temperature rising rate (°C / s)Soaking temperature (°C)Soaking time (s)Aspect of ferrite grainsGrain size of ferrite grains (µm)Difference between elongation in L direction and elongation in C direction (%)Note31B1401100600.236.23.0Comparative Example32B1801100600.275.81.4Comparative Example33B11201100600.366.10.4Example of present invention34B12001100600.415.40.1Example of present invention35B14001100600.385.30.2Example of present invention36B18001100600.425.60.2Example of present invention37B24001050600.434.20.2Example of present invention38B24001100600.375.20.0Example of present invention39B24001150600.468.40.3Example of present invention40B24001200600.2423.02.1Comparative Example41B34001100600.384.80.1Example of present invention42B340011001200.445.00.4Example of present invention43B340011002400.385.30.8Example of present invention44B44001100600.335.60.3Example of present invention45B54001100600.446.80.1Example of present invention46B64001100600.423.80.2Example of present invention47B74001100600.384.20.1Example of present invention48B84001100600.334.40.5Example of present invention49B94001100600.414.30.2Example of present invention50B104001100600.405.80.7Example of present invention51B114001100600.376.00.3Example of present invention52B124001100600.427.20.4Example of present invention53B134001100600.464.60.4Example of present invention54B144001100600.394.20.3Example of present invention55B154001100600.495.30.3Example of present invention56B164001100600.3317.23.8Comparative Example57B174001100600.243.22.2Comparative Example INDUSTRIAL APPLICABILITY

[0107] The ferritic-austenitic duplex stainless steel sheet of the present disclosure has low anisotropy of elongation (ductility) and is excellent in formability, and thus has high industrial applicability.

Claims

1. A ferritic-austenitic duplex stainless steel sheet comprising a chemical composition containing, in mass%, C: 0.002 to 0.05%, Si: 0.10 to 2.00%, Mn: 0.10 to 5.00%, P: 0.040% or less, S: 0.030% or less, Cr: 20.0 to 30.0%, Ni: 1.0 to 10.0%, Mo: 0.1 to 5.0%, N: 0.08 to 0.30%, Cu: 0 to 2.0%, Nb: 0 to 0.50%, Ti: 0 to 0.50%, V: 0 to 0.50%, W: 0 to 0.50%, Co: 0 to 0.50%, B: 0 to 0.0050%, Sn: 0 to 0.50%, Al: 0 to 0.50%, Mg: 0 to 0.010%, Ca: 0 to 0.0100%, Ta: 0 to 0.050%, Ga: 0 to 0.050%, Zr: 0 to 0.50%, a rare earth element: 0 to 0.010%, and a remainder: Fe and impurities, wherein an average aspect ratio of ferrite grains is 0.30 or more in a section including a rolling direction and a sheet thickness direction, and an average crystal grain size of ferrite grains is 10.0 µm or less.

2. The ferritic-austenitic duplex stainless steel sheet according to claim 1, wherein the chemical composition contains, in mass%, Mn: 0.10 to 2.00%, Cr: 21.0 to 30.0%, Ni: 3.0 to 10.0%, and Mo: 2.0 to 5.0%.

3. The ferritic-austenitic duplex stainless steel sheet according to claim 2, having a pitting index (PI) value of 32.0 or more, the PI value represented by a formula (A) described below: PI value = Cr + 3.3 Mo + 16 N wherein an element symbol in the formula (A) represents a content (mass%) of a corresponding element contained in the ferritic-austenitic duplex stainless steel sheet, and is 0 in a case where the corresponding element is not contained.

4. The ferritic-austenitic duplex stainless steel sheet according to claim 1, wherein the chemical composition contains, in mass%, Mn: 1.00 to 5.00%, Cr: 20.0 to 25.0%, Ni: 1.0 to 6.0%, Mo: 0.1 to 2.0%, Cu: 0.1 to 2.0%, and N: 0.08 to 0.20%.

5. The ferritic-austenitic duplex stainless steel sheet according to claim 1, having a PI value of 25.0 or more, the PI value represented by a formula (A) described below: PI value = Cr + 3.3 Mo + 16 N wherein an element symbol in the formula (A) represents a content (mass%) of a corresponding element contained in the ferritic-austenitic duplex stainless steel sheet, and is 0 in a case where the corresponding element is not contained.

6. The ferritic-austenitic duplex stainless steel sheet according to any one of claims 1 to 5, wherein a difference between a fracture elongation in a direction parallel to the rolling direction and a fracture elongation in a direction perpendicular to the rolling direction is 1.0% or less in a tensile test.

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