Duplex stainless steel plate, duplex stainless steel hot-rolled plate, and method for manufacturing duplex stainless steel plate

Abstract

The duplex stainless steel plate is a duplex stainless steel plate containing an austenite phase and a ferrite phase, having a predetermined chemical composition, and having a thickness center portion of a cross section perpendicular to the rolling direction and parallel to the thickness direction on the rolled surface, i.e., in the rolling direction. <001> The area ratio S of the texture of the ferrite phase oriented in the vertical direction of the rolling <001> Relative to <111> The area ratio S of the texture of the ferrite phase oriented in the vertical direction of the rolling <111> The ratio is the area ratio S <001> / S <111> It is 0.90~1.10.

Description

Technical Field

[0001] The present invention relates to a duplex stainless steel plate, a duplex stainless steel hot-rolled plate, and a method for producing the duplex stainless steel plate. This application claims priority based on Japanese Patent Application No. 2020-198585, filed in Japan on November 30, 2020, the contents of which are incorporated herein by reference. Background Art

[0002] Stainless steel is used in various applications as a representative corrosion-resistant material. Not only does it not cause rust or cracking due to corrosion, but in recent years, its application in applications requiring aesthetically pleasing appearance after construction has also been gaining momentum.

[0003] Therefore, for example, Patent Document 1 discloses a highly corrosion-resistant stainless steel sheet for exterior building materials having an excellent ability to prevent the occurrence of banded appearance unevenness, which is a bright annealed steel sheet or an annealed and pickled steel sheet of duplex stainless steel containing, by mass%, 16 to 35% Cr, 0.05 to 0.5% Ti, 0 to 6% Mo (including no addition), 0 to 1.0% Nb (including no addition), and 0.005 to 0.025% N, with the C content limited to 0.015% or less, wherein the brightness difference ΔL within the plate width of the steel plate surface in a direction perpendicular to the rolling direction is adjusted to 5 or less.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2000-129405 Summary of the Invention

[0007] Problems to be solved by the invention

[0008] Duplex stainless steel sheets offer superior weather resistance compared to austenitic stainless steel sheets. However, conventional duplex stainless steel sheets have long-period uneven surface roughness (surface undulation). Duplex stainless steel sheets with surface undulations can visually reveal ribbed patterns caused by these undulations. Therefore, conventional duplex stainless steels have room for improvement when aesthetically pleasing appearance is required.

[0009] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a duplex stainless steel plate and a hot-rolled duplex stainless steel plate having beautiful appearance, and a method for producing the duplex stainless steel plate.

[0010] Means for solving problems

[0011] The inventors of the present invention realized that if a large number of ferrite phase grains with a specific crystal orientation are present in a duplex stainless steel sheet, then when the duplex stainless steel sheet is deformed, for example, when mirror-polished, these grains deform differently from grains with other orientations. The inventors of the present invention then realized that surface undulations are generated by the different deformation behaviors occurring between these different grains. The inventors of the present invention thus conceived of suppressing surface undulations by making the texture of the ferrite phase random (disordered). Specifically, the inventors conceived of softening the ferrite phase by annealing the hot-rolled sheet at low temperature for a long period of time during the production of the duplex stainless steel sheet. By preferentially introducing strain into the softened ferrite phase, the distribution of the crystal orientation of the ferrite phase becomes uniform. The inventors of the present invention then conducted extensive research and arrived at the present invention.

[0012] The gist of the present invention, which was accomplished based on the above findings, is as follows.

[0013] [1] A duplex stainless steel plate according to one embodiment of the present invention is a duplex stainless steel plate containing an austenite phase and a ferrite phase, and contains, by mass%, C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50-8.00%, Cr: 18.00-28.00%, Mo: 5.00% or less, Cu: 0.05-1.50%, and N: 0.080-0.320%, with the remainder consisting of Fe and impurities, and wherein, in a cross section perpendicular to the rolling direction and parallel to the thickness direction of the rolled surface, i.e., in the direction perpendicular to the rolling direction, the plate thickness center portion is: <001> The area ratio S of the texture of the ferrite phase oriented in the vertical direction of the rolling <001> Relative to <111> The area ratio S of the texture of the ferrite phase oriented in the vertical direction of the rolling <111> The ratio is the area ratio S <001> / S <111> It is 0.90~1.10.

[0014] [2] According to the duplex stainless steel sheet described in [1] above, the surface undulation height in the rolling direction may be 0.3 μm or less.

[0015] [3] The duplex stainless steel plate according to [1] or [2] above may contain, in mass%, C: 0.030% or less, Si: 0.75% or less, Mn: 2.00-4.00%, P: 0.040% or less, S: 0.0200% or less, Ni: 1.50-2.50%, Cr: 18.00-21.50%, Mo: 0.60% or less, Cu: 0.50-1.50% and N: 0.150-0.200%, with the remainder consisting of Fe and impurities.

[0016] [4] The duplex stainless steel plate according to any one of [1] to [3] above, which may contain, in mass%, one or more selected from the group consisting of Al: 0.003-0.050%, O: 0.0070% or less, Nb: 0.005-0.20%, Ti: 0.005-0.20%, Co: 0.005-0.25%, V: 0.005-0.15%, Sn: 0.005-0.20%, Sb: 0.005-0.20%, Ga: 0.001-0.050%, Zr: 0.005-0.50%, Ta: 0.005-0.100%, and B: 0.0002-0.0050%, in place of a portion of Fe.

[0017] [5] Another embodiment of the present invention is a duplex stainless steel hot-rolled plate containing an austenite phase and a ferrite phase, which contains, in mass%, C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50-8.00%, Cr: 18.00-28.00%, Mo: 5.00% or less, Cu: 0.05-1.50% and N: 0.080-0.320%, with the remainder containing Fe and impurities, and the difference between the Vickers hardness of the above-mentioned austenite phase and the Vickers hardness of the above-mentioned ferrite phase is 50 HV or more.

[0018] [6] The duplex stainless steel hot-rolled plate according to the above [5] may contain, in mass%, one or more selected from the group consisting of Al: 0.003-0.050%, O: 0.0070% or less, Nb: 0.005-0.20%, Ti: 0.005-0.20%, Co: 0.005-0.25%, V: 0.005-0.15%, Sn: 0.005-0.20%, Sb: 0.005-0.20%, Ga: 0.001-0.050%, Zr: 0.005-0.50%, Ta: 0.005-0.100% and B: 0.0002-0.0050%, in place of a portion of Fe.

[0019] [7] Another embodiment of the present invention is a method for producing a duplex stainless steel plate, comprising the following steps: a hot rolling step of hot rolling a stainless steel raw material containing, by mass%, C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50-6.80%, Cr: 18.00-28.00%, Mo: 5.00% or less, Cu: 0.05-1.50% and N: 0.080-0.320%, with the remainder containing Fe and impurities, and coiling the hot rolled stainless steel raw material at a temperature of 500°C or more and less than 600°C; a cold rolling step of cold rolling the stainless steel raw material after the heat treatment step.

[0020] [8] The method for producing a duplex stainless steel plate according to [7], wherein the stainless steel raw material may contain, in mass%, one or more selected from the group consisting of Al: 0.003-0.050%, O: 0.0070% or less, Nb: 0.005-0.20%, Ti: 0.005-0.20%, Co: 0.005-0.25%, V: 0.005-0.15%, Sn: 0.005-0.20%, Sb: 0.005-0.20%, Ga: 0.001-0.050%, Zr: 0.005-0.50%, Ta: 0.005-0.100%, and B: 0.0002-0.0050%, in place of a portion of Fe.

[0021] Effects of the Invention

[0022] As described above, according to the present invention, a duplex stainless steel sheet and a hot-rolled duplex stainless steel sheet having beautiful appearance, and a method for producing the duplex stainless steel sheet can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This figure shows an example of an inverse pole figure crystal orientation map of a ferrite phase in a transverse direction (TD) of rolling obtained by SEM-EBSD analysis.

[0024] Figure 2 This is a graph showing an example of a roughness curve for explaining a method for measuring surface undulation height. DETAILED DESCRIPTION

[0025] <Duplex stainless steel plate>

[0026] The duplex stainless steel plate of the present embodiment is a duplex stainless steel plate containing an austenite phase and a ferrite phase, and contains, by mass%, C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50-8.00%, Cr: 18.00-28.00%, Mo: 5.00% or less, Cu: 0.05-1.50%, and N: 0.080-0.320%, with the remainder consisting of Fe and impurities. In a cross section taken in a direction perpendicular to the rolling direction and parallel to the thickness direction of the rolled surface, i.e., in the center of the plate thickness, <001> The area ratio S of the ferrite phase texture oriented perpendicular to the rolling direction <001> Relative to <111> The area ratio S of the ferrite phase texture oriented perpendicular to the rolling direction <111> The ratio is the area ratio S <001> / S <111> The duplex stainless steel sheet according to the present embodiment will be described in detail below.

[0027] [Chemical composition]

[0028] First, the chemical composition of the duplex stainless steel plate of the present embodiment will be described. Note that "%" indicating a composition means "mass %."

[0029] C: 0.080% or less

[0030] If the C content exceeds 0.080%, corrosion resistance decreases due to the precipitation of Cr carbides. Therefore, the C content is preferably lower and can be allowed to be 0.080% or less. Therefore, the C content is set to 0.080% or less. From the perspective of improving corrosion resistance, the C content is preferably 0.030% or less, and more preferably 0.025% or less. The lower limit of the C content is not particularly limited, but from the perspective of cost, the C content is preferably 0.001% or more, and more preferably 0.007% or more.

[0031] Si: 1.00% or less

[0032] Si acts as a deoxidizer and desulfurizer. If the Si content exceeds 1.00%, toughness decreases, so the Si content is set to 1.00% or less. The Si content is preferably 0.65% or less. To fully function as a deoxidizer and desulfurizer, the Si content is preferably 0.05% or more. The Si content is more preferably 0.30% or more.

[0033] Mn: 4.00% or less

[0034] Mn is a relatively inexpensive element and has the effect of increasing the amount of austenite in the stainless steel plate and, by increasing the solid solubility of nitrogen, suppressing the precipitation of chromium nitrides. On the other hand, excessive amounts can cause corrosion resistance to deteriorate. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 2.50% or less. The Mn content is preferably 0.74% or more, more preferably 0.85% or more, and even more preferably 2.00% or more.

[0035] P: 0.040% or less

[0036] P is an element inevitably contained in stainless steel sheets, but because it degrades hot workability, the P content is set to 0.040% or less. The P content is preferably 0.035% or less. The lower limit of the P content is not particularly limited, but from a cost perspective, the P content is preferably set to 0.005% or more.

[0037] S: 0.0300% or less

[0038] Like P, S is an element inevitably contained in stainless steel sheets, but it degrades hot workability, toughness, and corrosion resistance. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0200% or less. While the lower limit of the S content is not particularly limited, from a cost perspective, the S content is preferably set to 0.0001% or more. The S content is more preferably 0.0005% or more.

[0039] Ni: 1.50~8.00%

[0040] In the present invention, Ni is an element that improves the design properties of the stainless steel sheet. When the Ni content is too low, the solid-solution Ni in the austenite phase of the hot-rolled sheet decreases and softens, so the difference between the Vickers hardness of the austenite phase and the Vickers hardness of the ferrite phase in the hot-rolled sheet described later does not meet 50HV or more. Therefore, when the Ni content is too low, in the present invention, the steel sheet after cold working does not meet the requirement of "the center of the thickness of the cross section in the direction perpendicular to the rolling direction and parallel to the thickness direction in the rolling surface, i.e., the direction perpendicular to the rolling direction" described later. <001> The area ratio S of the texture of the ferrite phase oriented in the vertical direction of the rolling <001> Relative to <111> The area ratio S of the texture of the ferrite phase oriented in the vertical direction of the rolling <111> The ratio is the area ratio S <001> / S <111>is 0.90 to 1.10". That is, when the Ni content is too low, the surface undulation height in the rolling direction will not be less than 0.3 μm, and therefore the effect of improving the design properties by minimizing the surface undulation height cannot be obtained. Therefore, the Ni content is 1.50% or more. The Ni content is preferably 2.00% or more. On the other hand, if the Ni content is excessive, not only the cost increases, but also the austenite phase becomes excessive, and the hot workability decreases. Therefore, the Ni content is 8.00% or less. The Ni content is preferably 6.90% or less, more preferably 6.80% or less, and even more preferably 2.50% or less.

[0041] Cr: 18.00~28.00%

[0042] Cr is an element that improves the corrosion resistance of stainless steel. From the perspective of corrosion resistance, the Cr content is 18.00% or higher. The Cr content is preferably 20.50% or higher, and more preferably 21.00% or higher. However, Cr also increases the ferrite phase. If the stainless steel contains excessive Cr, the ferrite phase becomes excessive, and toughness deteriorates. Therefore, the Cr content is set to 28.00% or lower. The Cr content is preferably 24.50% or lower, and more preferably 21.50% or lower.

[0043] Mo: 5.00% or less

[0044] Mo has a higher corrosion resistance-enhancing effect than Cr, but it is a very expensive element. Excessive Mo content increases manufacturing costs. Furthermore, excessive Mo content leads to hardening of the stainless steel sheet and deterioration of workability. Therefore, the Mo content is set to 5.00% or less. The Mo content is preferably 3.00% or less, more preferably 2.95% or less, and even more preferably 0.60% or less. The corrosion resistance-enhancing effect of Mo is insufficient if the Mo content is less than 0.01%, so the Mo content is set to, for example, 0.01% or more. The Mo content is preferably 0.05% or more, and more preferably 0.20% or more.

[0045] Cu: 0.05-1.50%

[0046] Like Ni, Cu is an element that inhibits the dissolution of stainless steel in low pH environments. However, if the stainless steel contains excessive Cu, hot workability is significantly impaired, so the Cu content is set to 1.50% or less. The Cu content is preferably 1.40% or less. On the other hand, if the Cu content is less than 0.05%, the above-mentioned effect cannot be achieved. Therefore, the Cu content is set to 0.05% or more. The Cu content is preferably 0.60% or more, and more preferably 0.70% or more.

[0047] N: 0.080~0.320%

[0048] Nitrogen is an element that significantly improves corrosion resistance and increases the amount of austenite. To achieve this effect, the N content is 0.080% or higher. The N content is preferably 0.150% or higher, and more preferably 0.155% or higher. On the other hand, if the N content exceeds 0.320%, nitrides are formed in the steel, reducing corrosion resistance and toughness. Therefore, the N content is set to 0.320% or lower. The N content is preferably 0.200% or lower.

[0049] In the duplex stainless steel plate of the present invention, the remainder other than the above-mentioned elements is Fe and impurities. However, other elements besides the above-mentioned elements may be contained within a range that does not impair the effects of this embodiment. It should be noted that the impurities referred to herein are components that are mixed in during the industrial production of the duplex stainless steel plate of the present invention through raw materials such as ore and scrap, and various factors in the production process. These components are permitted within a range that does not adversely affect the present invention.

[0050] The basic components of the duplex stainless steel sheet according to one embodiment of the present invention have been described above. However, the duplex stainless steel sheet according to one embodiment of the present invention may also contain the following elements as appropriate to replace part of the Fe. It should be noted that the following elements do not need to be contained, so the lower limit of the content of these elements is 0%.

[0051] Al: 0.003~0.050%

[0052] Al is an element with a strong deoxidizing effect. To ensure its deoxidizing effect, the Al content is preferably 0.003% or more. More preferably, the Al content is 0.005% or more. On the other hand, Al readily forms nitrides with N, and the formation of nitrides significantly reduces toughness. Therefore, the Al content is preferably 0.050% or less. More preferably, the Al content is 0.040% or less.

[0053] O: 0.0070% or less

[0054] Excessive amounts of O in steel form oxides, reducing toughness. Therefore, the O content is preferably 0.0070% or less. More preferably, it is 0.0050% or less. While the lower limit of the O content is not particularly limited, from a cost perspective, it is preferably 0.0005% or more. The O content may also be 0.001% or more.

[0055] Nb: 0.005~0.20%

[0056] Nb is an element that fixes C and N, preventing the reduction in corrosion resistance caused by the precipitation of Cr carbides, and improving corrosion resistance. This effect is achieved when the Nb content is 0.005% or more, so the Nb content is preferably 0.005% or more. The Nb content can also be 0.01% or more. On the other hand, if the Nb content exceeds 0.20%, there is a possibility that the α phase will be hardened due to solid solution strengthening, which will reduce workability. Therefore, the Nb content is preferably 0.20% or less. The Nb content can also be 0.18% or less.

[0057] Ti: 0.005~0.20%

[0058] Ti is an element that fixes C and N, preventing sensitization caused by the precipitation of Cr carbides and improving corrosion resistance. This effect is achieved when the Ti content is 0.005% or more, so the Ti content is preferably 0.005% or more. The Ti content can also be 0.01% or more. On the other hand, if the Ti content exceeds 0.20%, it leads to hardening of the ferrite phase, reducing toughness, and there is a possibility of reducing surface roughness due to Ti-based precipitates. Therefore, the Ti content is preferably 0.20% or less. The Ti content can also be 0.18% or less.

[0059] Co: 0.005~0.25%

[0060] Co inhibits the precipitation of Cr carbides and reduces the deterioration of corrosion resistance. Co exhibits this effect when the Co content is 0.005% or greater, so the Co content is preferably 0.005% or greater. The Co content can also be 0.01% or greater. On the other hand, Co is a rare and expensive element, so the Co content is preferably 0.25% or less. The Co content can also be 0.20% or less.

[0061] V: 0.005~0.15%

[0062] V is a strong carbide-forming element. Therefore, the inclusion of V, which readily forms carbides in high-temperature regions, can suppress the precipitation of Cr carbides and prevent a decrease in corrosion resistance. The aforementioned effects are achieved when the V content is 0.005% or greater, so the V content is preferably 0.005% or greater. The V content can be 0.01% or greater, but higher V contents lead to hardening, so the V content is preferably 0.15% or less. The V content can also be 0.12% or less.

[0063] Sn: 0.005~0.20%, Sb: 0.005~0.20%

[0064] Sn and Sb are elements that improve corrosion resistance, but they are also solid solution strengthening elements for the ferrite phase. Therefore, the content of each of Sn and Sb is preferably 0.20% or less. The content of each of Sn and Sb is more preferably 0.10% or less. When the content of either Sn or Sb is 0.005% or more, the corrosion resistance is improved, so the content of each of Sn and Sb is preferably 0.005% or more. The content of each of Sn and Sb is more preferably 0.030% or more.

[0065] Ga: 0.001~0.050%

[0066] Ga is an element that contributes to improving corrosion resistance. A Ga content of 0.001% or more demonstrates a corrosion resistance-enhancing effect, so the Ga content is preferably 0.001% or more. The Ga content can also be 0.005% or more. On the other hand, a Ga content exceeding 0.050% saturates the corrosion resistance-enhancing effect, resulting in increased costs. Therefore, the Ga content is preferably 0.050% or less. The Ga content can also be 0.040% or less.

[0067] Zr: 0.005~0.50%

[0068] Zr is an element that contributes to improving corrosion resistance. A Zr content of 0.005% or more demonstrates a corrosion resistance-enhancing effect, so the Zr content is preferably 0.005% or more. The Zr content may also be 0.01% or more. On the other hand, the effect saturates when the Zr content exceeds 0.50%. Therefore, the Zr content is preferably 0.50% or less. The Zr content may also be 0.40% or less.

[0069] Ta: 0.005~0.100%

[0070] Ta is an element that improves corrosion resistance by modifying inclusions. This effect is achieved when the Ta content is 0.005% or greater. Therefore, the Ta content is preferably 0.005% or greater. The Ta content can also be 0.01% or greater. On the other hand, a Ta content exceeding 0.100% may lead to a decrease in ductility and toughness at room temperature. Therefore, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.050% or less.

[0071] B: 0.0002~0.0050%

[0072] B is an element that suppresses secondary processing embrittlement and deterioration of hot workability. Furthermore, B does not affect corrosion resistance. B exhibits the aforementioned effects when the B content is 0.0002% or greater, so the B content is preferably 0.0002% or greater. The B content may also be 0.0005% or greater. On the other hand, if the B content exceeds 0.0050%, hot workability may be deteriorated, so the B content is preferably set to 0.0050% or less. The B content is more preferably 0.0022% or less, and even more preferably 0.0020% or less.

[0073] The duplex stainless steel plate of this embodiment has the above-described chemical composition, but preferably further contains C: 0.030% or less, Si: 0.75% or less, Mn: 2.00-4.00%, P: 0.040% or less, S: 0.0200% or less, Ni: 1.50-2.50%, Cr: 20.50-21.50%, Mo: 0.60% or less, Cu: 0.50-1.50%, and N: 0.150-0.200%. The duplex stainless steel plate having this chemical composition further improves corrosion resistance.

[0074] [organize]

[0075] The duplex stainless steel plate of this embodiment has a thickness center portion in a cross section perpendicular to the rolling direction. <001> The area ratio S of the ferrite phase texture oriented perpendicular to the rolling direction <001> Relative to <111> The area ratio S of the ferrite phase texture oriented perpendicular to the rolling direction <111> The ratio is the area ratio S <001> / S <111> It is 0.90~1.10.

[0076] Here, refer to Figure 1 right <111> The area ratio S of the ferrite phase texture oriented perpendicular to the rolling direction <111> and <001> The area ratio S of the ferrite phase texture oriented perpendicular to the rolling direction <001> The calculation method of is explained below. Figure 1 This figure shows an example of the inverse pole figure crystal orientation distribution diagram of the ferrite phase in the direction perpendicular to rolling obtained by SEM-EBSD (Scanning Electron Microscope-Electron Back Scatter Diffraction Pattern) analysis.

[0077] At the center of the thickness of the plate in the cross section perpendicular to the rolling obtained by cutting at the center of the plate width, the observation magnification is set to 1000 times, and SEM images are obtained in more than 3 fields of view. Here, the so-called center of the thickness refers to the range of 2t / 5 to 3t / 5 from the surface of the steel plate in the thickness direction when the thickness of the steel plate is set to t. For each SEM image, the measurement interval is set to 1μm, and the crystal orientation of the measurement point is analyzed. The object of the crystal orientation measurement is to be implemented on the grains whose CI value (Confidence Index), which is an index representing the accuracy of the calculated crystal orientation, is 0.1 or more. Then, <111> The texture with an orientation difference of less than 15° relative to the vertical direction of rolling is set as <111> The texture of the ferrite phase is oriented perpendicular to the rolling direction. <001> The texture with an orientation difference of less than 15° relative to the vertical direction of rolling is set as <001> The texture of the ferrite phase is oriented in the direction perpendicular to the rolling direction. <101> The texture of the ferrite phase oriented perpendicular to the rolling direction and <411> The texture of the ferrite phase is oriented in the direction perpendicular to the rolling direction. Figure 1 The inverse pole figure crystal orientation distribution diagram (IPF (Inverse Pole Figure) distribution diagram) shown in FIG.

[0078] From the obtained IPF graph, <111> The equivalent circle diameter of the ferrite phase texture oriented perpendicular to the rolling direction and <001> The area of each texture is calculated by the equivalent circle diameter of the texture of the ferrite phase oriented perpendicular to the rolling direction. The area of each texture is calculated by image analysis. <111> The total area of the texture of the ferrite phase oriented in the direction and <001> The total area of the ferrite phase texture oriented in each direction is calculated. Using the calculated total area of the grains in each direction, the total area of the grains in each direction is calculated. <111> The area ratio S' of the ferrite phase texture oriented perpendicular to the rolling direction <111> and <001> The area ratio S' of the ferrite phase texture oriented perpendicular to the rolling direction <001> , calculate the area ratio S' <001> / S' <111> Calculate the area ratio S' for each field of view of the obtained SEM image <001> / S' <111> , and set their average value as the area ratio S <001> / S <111> .

[0079] Generally speaking, the grain size of the austenite phase is about several μm, and the grain size of the ferrite phase is about 10 μm. Therefore, for the SEM image obtained at a magnification of 1000 times, it can be said that the above area ratio S <001> / S <111> The degree serving as a benchmark for the degree of texture randomization showed sufficient crystal grains.

[0080] If the area ratio S <001> / S <111> If it is less than 0.90, it means that the texture of the ferrite phase is <111> The direction is strongly oriented in the vertical direction of rolling. In addition, if the area ratio S <001> / S <111> If it exceeds 1.10, it means that in the ferrite phase texture, <001> The direction is strongly oriented along the vertical direction of rolling. <111> The texture of the ferrite phase oriented along the TD (vertical to rolling direction) or <001> When a steel sheet with a ferrite phase texture oriented in the TD (vertical to the rolling direction) is deformed, these grains deform differently from other textures. This creates undulations on the surface of the steel sheet. As a result, a ribbed pattern is created on the surface of the steel sheet. Therefore, the area ratio S <001> / S <111> The area ratio S is 0.90 to 1.10. <001> / S <111> It may also be 0.92 or more or 1.00 or more. <001> / S <111> It may be 1.00 or less or 1.08 or less.

[0081] The duplex stainless steel sheet of this embodiment preferably has a surface undulation height R in the rolling direction of 0.3 μm or less. Figure 2 The surface undulation height R in the rolling direction will be described. Figure 2 This graph shows an example of a roughness curve used to illustrate the surface undulation height measurement method. The surface undulation height R in the rolling direction is calculated using a surface roughness measuring machine with a measurement interval of 0.02 mm over a 10 mm length in the rolling direction, in accordance with JIS B 0601:2013. The surface undulation height R corresponds to the maximum height undulation Wz specified in JIS B 0601:2013. The surface undulation height R in the rolling direction is measured at the center of the steel plate.

[0082] If the surface undulation height R in the rolling direction is 0.3 μm or less, streaks are less likely to be visually detected in the stainless steel sheet, resulting in a more beautiful appearance. Therefore, the surface undulation height R is preferably 0.3 μm or less. More preferably, the surface undulation height R in the rolling direction of the stainless steel sheet subjected to tensile stress and 16% strain is preferably 1.8 μm or less. If the surface undulation height R in the rolling direction of the stainless steel sheet subjected to tensile stress and 16% strain is 1.8 μm or less, streaks are less likely to be visually detected even when the stainless steel sheet is processed for practical use.

[0083] The details of the duplex stainless steel sheet manufacturing method of the present invention will be described below. However, the duplex stainless steel sheet of the present invention is manufactured by cold rolling a hot-rolled stainless steel sheet manufactured by subjecting it to a prescribed treatment. The difference ΔHV (=HVγ - HVα) between the Vickers hardness HVγ of the austenite phase and the Vickers hardness HVα of the ferrite phase of the hot-rolled stainless steel sheet manufactured during the manufacturing process of the duplex stainless steel sheet of the present invention is 50 HV or greater, preferably 60 HV or greater, and more preferably 65 HV or greater or 70 HV or greater. The upper limit of ΔHV is not particularly limited. ΔHV can be, for example, 65 HV or less, 70 HV or less, or 75 HV or less. The Vickers hardness HVγ of the austenite phase and the Vickers hardness HVα of the ferrite phase are measured in accordance with JIS Z 2244:2009 under a load of 0.01 kgf. Measurements are made at five points in each of the austenite phase and the ferrite phase at the center of the plate thickness, and the average value of each is used as a representative value.

[0084] If ΔHV is 50HV or more, strain is preferentially introduced into the ferrite phase in subsequent processes such as cold rolling, temper rolling, or processing for actual use. As a result, the area ratio S of the stainless steel plate is <001> / S <111> The ΔHV is 0.90 to 1.10. As a result, undulations on the steel sheet surface are suppressed, and the formation of streaks on the steel sheet surface is suppressed. When ΔHV is 65 or greater, the hardness difference between the austenite and ferrite phases in the hot-rolled sheet is large, resulting in finer grain size in the soft phase during cold rolling. This further reduces the occurrence of differences in deformability due to crystal orientation, making streaks less visible even when the stainless steel sheet is processed for practical use.

[0085] [Plate thickness]

[0086] The duplex stainless steel plate of this embodiment has a thickness of, for example, 0.30 mm to 2.00 mm. Alternatively, the thickness may be 0.50 mm or greater, or 0.80 mm or greater. Furthermore, the thickness may be 1.80 mm or less, or 1.50 mm or less. Within these ranges, the effect of suppressing ribbed patterns is more significantly achieved, resulting in a duplex stainless steel plate with an aesthetically pleasing appearance.

[0087] <Method for Manufacturing Duplex Stainless Steel Plate>

[0088] Next, an example of a method for producing a duplex stainless steel plate according to the present embodiment will be described.

[0089] The method for producing duplex stainless steel sheet according to this embodiment includes the following steps: a hot rolling step in which a stainless steel material having the aforementioned chemical composition is hot-rolled and coiled at a temperature of 680°C or higher; a heat treatment step in which the hot-rolled stainless steel material is heat-treated at a temperature of 500°C to less than 600°C for at least one hour; and a cold rolling step following the heat treatment step. The duplex stainless steel sheet according to this embodiment is produced, for example, by sequentially performing a steelmaking step, the aforementioned hot rolling step, the aforementioned heat treatment step, a hot-rolled sheet pickling step, the aforementioned cold rolling step, a post-cold rolling heat treatment step, and a cold-rolled sheet pickling step. The production conditions for steps other than the hot rolling and heat treatment steps are not particularly limited, and known methods can be applied. The hot rolling, heat treatment, and cold rolling steps are described below.

[0090] [Hot rolling process]

[0091] In this step, the stainless steel material having the above-mentioned chemical composition is hot rolled and coiled at a temperature of 680° C. or higher. For example, a stainless steel billet obtained by continuous casting may be used as the stainless steel material for hot rolling.

[0092] The stainless steel material is preferably heated to 1150-1250°C before hot rolling. If the heating temperature is lower than 1150°C, edge cracking may occur during hot rolling. On the other hand, if the heating temperature exceeds 1250°C, the slab may deform in the heating furnace or become more susceptible to defects during hot rolling.

[0093] After the above heating, the stainless steel material is hot rolled. The rolling reduction is preferably 50% or less. If the rolling reduction is greater than 50%, there is a possibility that the microstructure will vary depending on the rolling direction, and uniform cross-sectional (fracture) properties will not be obtained regardless of the rolling direction.

[0094] Hot rolling may be performed in a plurality of passes. When the hot rolling is performed in a plurality of passes, the reduction ratio per pass is set to 50% or less.

[0095] The coiling temperature of the stainless steel raw material after rolling is above 680°C. With respect to the ferrite phase and the austenite phase, the ferrite phase recovers and recrystallizes first. By increasing the coiling temperature of the stainless steel raw material, the ferrite phase recovers during coiling, and recrystallization is caused in a part of the ferrite phase. If the coiling temperature is lower than 680°C, the ferrite phase will not fully recover during coiling. Therefore, the coiling temperature is set to above 680°C. The coiling temperature is preferably above 700°C. On the other hand, the coiling temperature is preferably below 750°C. If the coiling temperature is below 750°C, the recovery and recrystallization of the austenite phase can be further suppressed.

[0096] [Heat treatment process]

[0097] In this step, the stainless steel material after the hot rolling step is subjected to heat treatment by being kept at a temperature of 500° C. or higher and lower than 600° C. for 1 hour or longer.

[0098] The heat treatment temperature is 500°C or higher and lower than 600°C. If the heat treatment temperature is lower than 500°C, the recovery and recrystallization of the ferrite phase are insufficient, and the ferrite phase does not soften. When the ferrite phase does not soften, strain will not be preferentially introduced into the ferrite phase in the cold rolling process of the subsequent process, and the ferrite phase becomes an oriented structure with non-randomized crystal orientation. Therefore, the heat treatment temperature is 500°C or higher. The heat treatment temperature is preferably 550°C or higher. On the other hand, if the heat treatment temperature is 600°C or higher, the austenite phase is also softened, and the ferrite phase becomes an oriented structure with non-randomized crystal orientation. Therefore, the heat treatment temperature is set to lower than 600°C. Preferably, it is 585°C or lower.

[0099] The heat treatment time is more than 1 hour. If the heat treatment time is less than 1 hour, the recovery and recrystallization of the ferrite phase are insufficient, and the ferrite phase does not soften. When the ferrite phase does not soften, strain will not be preferentially introduced into the ferrite phase in the cold rolling process of the subsequent process, and the ferrite phase becomes an oriented organization with crystal orientation not randomized. Therefore, the heat treatment time is set to more than 1 hour. On the other hand, there is no particular limit to the upper limit of the heat treatment time. However, from the aspect of grain coarsening, the heat treatment time is preferably less than 2 hours.

[0100] Through the manufacturing steps up to the heat treatment step, a hot-rolled sheet having a difference ΔHV (=HVγ-HVα) of 50 HV or more between the Vickers hardness HVγ of the austenite phase and the Vickers hardness HVα of the ferrite phase can be manufactured.

[0101] [Cold rolling process]

[0102] In this step, the stainless steel material (hot-rolled sheet according to this embodiment) that has undergone the heat treatment step is cold rolled. Cold rolling conditions are not particularly limited and may be known conditions. For example, cold rolling may be performed in a single pass or in multiple passes. The cumulative cold reduction may be set to 30% to 80%, and the cold rolling temperature may be set, for example, from room temperature to 200°C.

[0103] The stainless steel material subjected to cold rolling may be a stainless steel material subjected to a pickling treatment after a heat treatment step.

[0104] The cold rolling process introduces a large amount of rolling strain into the ferrite phase, which has been softened by the heat treatment process. This creates grains with various crystal orientations in the ferrite phase, randomizing the ferrite texture. As a result, undulations on the steel sheet surface are suppressed, and the formation of ribbed patterns on the steel sheet surface is suppressed.

[0105] The above is an explanation of the method for manufacturing the duplex stainless steel plate of the present embodiment. <001> The area ratio S of the ferrite phase texture oriented perpendicular to the rolling direction <001> Relative to <111> The area ratio S of the ferrite phase texture oriented perpendicular to the rolling direction <111> The ratio is the area ratio S <001> / S <111> Duplex stainless steel sheet with a g / m² of 0.90 to 1.10. Since the ferrite phase of the duplex stainless steel sheet of this embodiment has a random texture, the height of the surface undulation in the rolling direction of the duplex stainless steel sheet can be suppressed. As a result, the generation of visually visible striations can be suppressed.

[0106] Example

[0107] Hereinafter, the embodiment of the present invention will be described in detail while showing an embodiment. It should be noted that the embodiment shown below is merely an example of the present invention, and the present invention is not limited to the following example.

[0108] Stainless steel raw materials having the chemical composition shown in Table 1 were hot rolled at a reduction ratio of 70%, and the rolled stainless steel raw materials were coiled at the coiling temperatures shown in Table 2. Subsequently, a heat treatment process was performed at the heat treatment temperature and time shown in Table 2 to produce hot-rolled sheets. The resulting hot-rolled sheets were then cold rolled at room temperature at a reduction ratio of 80% to produce stainless steel sheets. Note that "-" in Table 1 indicates that the element was not intentionally added.

[0109] [Table 1]

[0110]

[0111] The Vickers hardness HVγ of the austenite phase and HVα of the ferrite phase of the produced hot-rolled sheet were measured at a load of 0.01 kgf in accordance with JIS Z 2244: 2009. Five points were measured in each of the austenite phase and the ferrite phase at the center of the sheet thickness, and the average value was used as a representative value.

[0112] In addition, the thickness of the plate at the center of the cross section in the rolling direction cut at the center of the plate width is calculated by the following method: <001> The area ratio S of the ferrite phase texture oriented perpendicular to the rolling direction <001> Relative to <111> The area ratio S of the ferrite phase texture oriented perpendicular to the rolling direction <111> The ratio is the area ratio S <001> / S <111> At the center of the plate thickness in the cross section perpendicular to the rolling direction, the observation magnification was set to 1000 times, and SEM images were obtained in three viewing fields. For each SEM image, the measurement interval was set to 1 μm, and the crystal orientation of the measurement point was analyzed. When the orientation difference of the crystal orientation at adjacent measurement points was within 15°, it was set as the same texture. <111> The equivalent circle diameter of the ferrite phase texture oriented perpendicular to the rolling direction and <001> The area of each texture is calculated from the equivalent circle diameter of the texture of the ferrite phase oriented in the vertical direction of rolling. <111> The total area of the texture of the ferrite phase oriented perpendicular to the rolling direction and <001> The total area of the texture of the ferrite phase oriented in the direction perpendicular to the rolling direction is calculated. Using the calculated total area of each texture, the total area of the ferrite phase with respect to the entire area of the field of view is calculated. <111> The area ratio S' of the ferrite phase texture oriented perpendicular to the rolling direction <111> and <001> The area ratio S' of the ferrite phase texture oriented perpendicular to the rolling direction <001> , calculate the area ratio S' <001> / S' <111> For each field of view of the SEM image obtained, calculate the area ratio S' <001> / S' <111> , and take their average value as the area ratio S <001> / S <111> .

[0113] The surface roughness height R in the rolling direction of the produced duplex stainless steel was measured by the following method. In accordance with JIS B 0601:2013, a surface roughness measuring machine (SV-3000CNC, manufactured by MITUTOYO Co., Ltd.) was used. A measurement interval of 0.02 mm was set for a length of 10 mm in the direction perpendicular to the rolling direction at the center of the duplex stainless steel plate. A roughness curve of the stainless steel plate surface was obtained, and the surface roughness height R in the rolling direction was measured.

[0114] Furthermore, for the stainless steel plate produced after applying tensile stress in the rolling direction to impart 16% strain, the surface undulation height R in the rolling direction was measured at the center position of the steel plate.

[0115] Appearance evaluation was performed by visual observation. Specifically, the surfaces of the manufactured stainless steel plates and those embossed with a cylindrical punch, simulating an actual process, were mirror-polished. The mirror-polished surfaces were then observed from various directions to confirm the presence of ribbed patterns. If no ribbed patterns were observed on the stainless steel plates after the above-described processing, the appearance was evaluated as extremely good (A). If no ribbed patterns were observed on the stainless steel plates before 16% strain, the appearance was evaluated as good (B). If ribbed patterns were observed on the stainless steel plates before 16% strain, the appearance was evaluated as poor (C).

[0116] The evaluation results are shown in Table 2. ε＝0 R represents the surface undulation height in the rolling direction of the stainless steel sheet without strain. ε＝16 The height of the surface undulation in the rolling direction of the stainless steel sheet after applying a tensile stress in the rolling direction to impart a strain of 16% is shown. In addition, the underlined values in Table 2 indicate values outside the range of the present invention.

[0117] [Table 2]

[0118]

[0119] The chemical composition of each of the obtained steel plates was substantially the same as that of the respective stainless steel raw materials. In addition, SEM observation of the obtained hot-rolled plates and stainless steel plates revealed that all of the steel plates were duplex stainless steel plates.

[0120] As shown in Table 2, the ΔHV of the hot-rolled sheet produced under the conditions of a coiling temperature of 680°C or higher in the hot rolling process, a heat treatment temperature of 500°C or higher and lower than 600°C in the heat treatment process, and a heat treatment time of 1 hour or longer is 50 or higher. In addition, the area ratio S of the stainless steel sheet obtained by cold-rolling the hot-rolled sheet having a ΔHV of 50 or higher (referred to as "cold-rolled sheet" in Table 2) is 50 or higher. <001> / S <111> The area ratio S is 0.90 to 1.10. <001> / S <111> The surface undulation height R in the rolling direction of the duplex stainless steel plate is 0.90 to 1.10 ε＝0 is less than 0.3 μm, and the surface undulation height R in the rolling direction is ε＝16 Also, the area ratio S <001> / S <111> The appearance evaluation results of the duplex stainless steel plates having a g / L ratio of 0.90 to 1.10 were good.

[0121] In addition, for Examples No. 1 and No. 2, the ΔHV of the hot-rolled sheets was 65 or greater, and the appearance evaluation results were extremely good (A). This is believed to be because the large ΔHV of the hot-rolled sheets resulted in finer grain sizes in the soft phase during cold rolling, making it less likely that differences in deformability due to crystal orientation would occur.

[0122] While preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the examples described above. Anyone with ordinary knowledge in the technical field to which the present invention pertains will understand that various variations and modifications are conceivable within the scope of the technical concepts described in the claims, and these are naturally understood to fall within the technical scope of the present invention.

Claims

1. A duplex stainless steel plate comprising an austenite phase and a ferrite phase. It is expressed in mass % as follows: C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50~8.00%, Cr：18.00～28.00％、 Mo: 5.00% or less, Cu: 0.05-1.50%, and N：0.080～0.320％， The rest is Fe and impurities. At the center of the plate thickness of the cross section perpendicular to the rolling direction and parallel to the plate thickness direction in the rolling surface, i.e., in the direction perpendicular to the rolling direction, <001> The area ratio S of the texture of the ferrite phase oriented in the vertical direction of the rolling <001> Relative to <111> The area ratio S of the texture of the ferrite phase oriented in the vertical direction of the rolling <111> The ratio is the area ratio S <001> / S <111> It is 0.90~1.

10. 2 . The duplex stainless steel sheet according to claim 1 , wherein the surface undulation height in the rolling direction is 0.3 μm or less.

3. The duplex stainless steel plate according to claim 1 or 2, wherein the content of the duplex stainless steel plate is calculated as follows: C: 0.030% or less, Si: 0.75% or less, Mn: 2.00~4.00%, P: 0.040% or less, S: 0.0200% or less, Ni: 1.50-2.50%, Cr：18.00～21.50％、 Mo: 0.60% or less, Cu: 0.50-1.50%, and N：0.150～0.200％， The remainder is Fe and impurities.

4. The duplex stainless steel plate according to claim 1 or 2, comprising, in mass%, one or more selected from the group consisting of Al: 0.003-0.050%, O: 0.0070% or less, Nb: 0.005-0.20%, Ti: 0.005-0.20%, Co: 0.005-0.25%, V: 0.005-0.15%, Sn: 0.005-0.20%, Sb: 0.005-0.20%, Ga: 0.001-0.050%, Zr: 0.005-0.50%, Ta: 0.005-0.100%, and B: 0.0002-0.0050%, in place of a portion of Fe.

5. The duplex stainless steel plate according to claim 3, comprising, in mass%, one or more selected from the group consisting of Al: 0.003-0.050%, O: 0.0070% or less, Nb: 0.005-0.20%, Ti: 0.005-0.20%, Co: 0.005-0.25%, V: 0.005-0.15%, Sn: 0.005-0.20%, Sb: 0.005-0.20%, Ga: 0.001-0.050%, Zr: 0.005-0.50%, Ta: 0.005-0.100%, and B: 0.0002-0.0050%, in place of a portion of Fe.

6. A duplex stainless steel hot-rolled plate, comprising an austenite phase and a ferrite phase. It is expressed in mass % as follows: C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50~8.00%, Cr：18.00～28.00％、 Mo: 5.00% or less, Cu: 0.05-1.50%, and N: more than 0.150% and 0.320% or less, The rest is Fe and impurities. The difference between the Vickers hardness of the austenite phase and the Vickers hardness of the ferrite phase is 50 HV to 75 HV.

7. The duplex stainless steel hot-rolled plate according to claim 6, wherein the Ni content is 2.00 to 8.00% by mass.

8. The duplex stainless steel hot-rolled plate according to claim 6 or 7, comprising, in mass%, one or more selected from the group consisting of Al: 0.003-0.050%, O: 0.0070% or less, Nb: 0.005-0.20%, Ti: 0.005-0.20%, Co: 0.005-0.25%, V: 0.005-0.15%, Sn: 0.005-0.20%, Sb: 0.005-0.20%, Ga: 0.001-0.050%, Zr: 0.005-0.50%, Ta: 0.005-0.100%, and B: 0.0002-0.0050%, in place of a portion of Fe.

9. A method for manufacturing a duplex stainless steel plate, comprising the following steps: The hot rolling step comprises hot rolling the following stainless steel raw material and coiling it at a temperature of 680° C. or higher, wherein the stainless steel raw material is represented by mass % as follows: C: 0.080% or less, Si: 1.00% or less, Mn: 4.00% or less, P: 0.040% or less, S: 0.0300% or less, Ni: 1.50~6.80%, Cr：18.00～28.00％、 Mo: 5.00% or less, Cu: 0.05-1.50%, and N：0.080～0.320％、 The remainder is Fe and impurities; a heat treatment step of subjecting the stainless steel material after the hot rolling step to a heat treatment at a temperature of 500° C. or higher and lower than 600° C. for 1 hour or longer; and The cold rolling step is to cold-roll the stainless steel material after the heat treatment step.

10. The method for producing a duplex stainless steel plate according to claim 9, wherein the duplex stainless steel plate contains, in mass%, one or more selected from the group consisting of Al: 0.003-0.050%, O: 0.0070% or less, Nb: 0.005-0.20%, Ti: 0.005-0.20%, Co: 0.005-0.25%, V: 0.005-0.15%, Sn: 0.005-0.20%, Sb: 0.005-0.20%, Ga: 0.001-0.050%, Zr: 0.005-0.50%, Ta: 0.005-0.100%, and B: 0.0002-0.0050%, in place of a portion of Fe.

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