Ferritic austenitic two-phase stainless steel sheet

Abstract

This ferritic austenitic two-phase stainless steel sheet has a chemical composition containing 0.030% or less of C, 0.10-1.00% of Si, 0.30-4.00% of Mn, 0.050% or less of P, 0.0500% or less of S, 20.0-28.0% of Cr, 0.03-5.00% of Mo, 0.004-0.500% of Al, 0.070-0.300% of N, 1.0- 10.0% of Ni, and 0.03-1.5% of Cu, with the balance being Fe and impurities. The glossiness Gs (60°) is 50-100. The maximum height roughness RzL in the rolling direction is 1.0-2.2 μm. The maximum height roughness RzC in the direction perpendicular to the rolling direction is 1.0-3.0 μm. RzL and RzC satisfy the relationship RzC / RzL ≥ 1.1.

Description

Ferrite-austenite duplex stainless steel sheet

[0001] The present invention relates to a ferrite-austenite duplex stainless steel sheet. The present application claims priority based on Japanese Patent Application No. 2025-051432 filed in Japan on March 26, 2025, the content of which is incorporated herein by reference.

[0002] Stainless steel is used, for example, in construction materials, kitchen appliances, general furniture and home appliances, and automotive exhaust system components due to its high corrosion resistance, oxidation resistance and design properties. Among these, for some applications such as exterior building materials such as roofing materials and outer wall materials, and interior building materials requiring design properties, antiglare properties may be required in some cases, and stainless steel sheets with low gloss are demanded.

[0003] When a stainless steel sheet is used as a roofing material or wall material, a technology for improving antiglare properties by rolling the steel sheet with a dull roll or performing shot blasting to provide irregularities of several μm to several tens of μm on the surface is known.

[0004] For example, Patent Documents 1 and 2 disclose a technology for improving antiglare properties by roughening the steel sheet surface, in which shot blasting is performed under predetermined conditions on the surface of a cold-rolled steel sheet after a finish annealing process, and then pickling is performed.

[0005] Japanese Unexamined Patent Publication No. 2021-155834 Japanese Unexamined Patent Publication No. 2021-155835

[0006] However, when reducing the gloss of the steel sheet surface by shot blasting, shape defects such as warping of the sheet may occur due to impact caused by projection of the projection material, or corrosion resistance may deteriorate due to the projection material piercing the steel sheet surface. In particular, such problems often occur noticeably in thin sheets (for example, sheets of about 0.5 mm). Therefore, there has been a demand for a technology that can stably roughen the surface and reduce the gloss of the steel sheet surface regardless of the sheet thickness.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a ferrite-austenite duplex stainless steel sheet having a low-gloss surface excellent in antiglare properties.

[0008] [1] The ferrite-austenite duplex stainless steel sheet according to one embodiment of the present invention has a chemical composition in mass percent of: C: 0.030% or less, Si: 0.10% or more, 1.00% or less, Mn: 0.30% or more, 4.00% or less, P: 0.050% or less, S: 0.0500% or less, Cr: 20.0% or more, 28.0% or less, Mo: 0.03% or more, 5.00% or less, Al: 0.004% or more, 0.500% or less, N: 0.070% or more, 0.300% or less, Ni: 1.0% or more, 10.0% or less, and Cu: 0.03% or more, 1.5% or less, with the remainder being Fe and impurities, and conforms to JIS Z The glossiness Gs (60°) as defined in 8741:1997 is 50 to 100, the maximum height roughness RzL in the rolling direction is 1.0 to 2.2 μm, the maximum height roughness RzC in the direction perpendicular to rolling is 1.0 to 3.0 μm, and the ratio of the maximum height roughness RzL in the rolling direction to the maximum height roughness RzC in the direction perpendicular to rolling satisfies RzC / RzL ≥ 1.1. [2] The ferrite-austenite duplex stainless steel sheet described in [1] above further contains, in the above chemical composition, one or more of the following in mass%, in Ti: 0.05% or less, Nb: 0.20% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less, and the total content of Ti and Nb may be 0.02% or more and 0.20% or less. [3] The ferrite-austenite duplex stainless steel sheet described in [1] or [2] above may further contain, in mass%, one or more of the following in its chemical composition: B: 0.0050% or less, V: 0.50% or less, Zr: 0.020% or less, Ga: 0.030% or less, REM: 0.050% or less, Ta: 0.070% or less, W: 1.00% or less, Co: 1.00% or less, and Sb: 0.500% or less. [4] The ferrite-austenite duplex stainless steel sheet described in any of [1] to [3] above may have an average grain size of 0.5 to 7.5 μm. [5] The ferrite-austenite duplex stainless steel sheet described in any of [1] to [4] above may have an aspect ratio of 1.3 or more for the duplex structure consisting of the ferrite phase and the austenite phase.[6] The ferrite-austenite duplex stainless steel sheet described in any of [1] to [5] above may have a thickness of 0.1 mm or more and 3.0 mm or less.

[0009] According to the present invention, a ferrite-austenite duplex stainless steel sheet having excellent low gloss can be provided.

[0010] Figure 1 illustrates the method for calculating the aspect ratio of the two-phase structure in this embodiment.

[0011] The following describes a ferrite-austenite duplex stainless steel sheet (hereinafter sometimes referred to as duplex stainless steel sheet) according to one embodiment of the present invention. It should be noted that the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention. Furthermore, numerical limits indicated below, separated by "~", include both a lower and upper limit. Numerical values ​​indicated as "less than" or "greater than" do not include that value in the numerical range.

[0012] To solve the above problems, the inventors conducted various studies. The results of these studies are shown below.

[0013] <1. Inventors' Study> The inventors investigated a means to stably achieve low gloss on the surface of a steel sheet without using shot blasting. As a result, they found that the glossiness of the steel sheet surface can be reduced by using duplex stainless steel sheets, optimizing the heating rate of the cold-rolled sheet annealing, and performing an acid pickling process using a mixed acid solution of nitric acid and hydrofluoric acid.

[0014] In the manufacturing of stainless steel sheets, exposure to high temperatures in an air atmosphere during the cold-rolled sheet annealing process generates an oxide scale mainly composed of chromium oxides on the surface. This oxide scale is removed in the subsequent pickling process by immersion in a pickling solution or by electrolysis, resulting in an annealed and pickled sheet consisting of the stainless steel base material surface and a passive film formed thereon.

[0015] The inventors first investigated how to achieve low gloss by using duplex stainless steel. In duplex stainless steel, when the above-mentioned scale is formed, the diffusion rate of Cr differs between the ferrite phase and the austenite phase. As a result, a Cr-deficient layer is formed only in the austenite phase, where the diffusion rate of Cr is slower, directly beneath the scale (especially near the grain boundaries). Then, in the subsequent pickling process, the Cr-deficient layer is preferentially dissolved, and grain boundary dissolution marks called microgrooves are formed on the surface of the steel sheet. In other words, irregularities are created between the austenite phase where microgrooves are formed and the ferrite phase where microgrooves are not formed. The inventors investigated whether low gloss could be achieved by this. Here, in pickling duplex stainless steel, it is preferable to use a mixed acid solution of hydrofluoric acid and nitric acid (hereinafter also referred to as nitrate-hydrofluoric acid solution). However, the desired gloss level could not be obtained by simply using duplex stainless steel and pickling it with nitrate-hydrofluoric acid solution.

[0016] Therefore, the inventors further investigated whether it would be possible to increase the surface irregularities of the steel sheet and achieve a lower gloss by increasing the depth of the Cr-deficient layer and the depth of the microgrooves.

[0017] Specifically, we investigated whether the depth of the Cr-depleted layer could be increased by adjusting the heating rate and holding time during cold-rolled sheet annealing. As a result, we found that the Cr-depleted layer could be increased by setting the heating rate of cold-rolled sheet annealing to predetermined conditions. This is thought to be because the temperature range of 800°C or higher, where Cr oxides are generated and grow, can be reached quickly, promoting Cr concentration in the scale during the generation and growth of oxide scale, while the Cr concentration in the Cr-depleted layer on the base material side can be further reduced. The inventors have found that by forming a deeper Cr-depleted layer, the depth of microgrooves can be increased during the pickling process, the surface irregularities of the steel sheet can be increased, and as a result, a lower gloss can be achieved.

[0018] <2. Ferrite-Austenitic Duplex Stainless Steel Sheet> One embodiment of the present invention has been made based on the above findings. The requirements of the ferrite-austenitic duplex stainless steel sheet of this embodiment will be described in detail below. Note that the duplex stainless steel sheet of this embodiment also includes steel strips and coils wound from steel strips.

[0019] [Chemical Composition] The reasons for the limitations on each component of the duplex stainless steel sheet of this embodiment are explained below. In the following explanation, the "%" indicating the content of each element refers to "mass%" unless otherwise specified.

[0020] (C: 0.030% or less) If the C content is too high, corrosion resistance and ductility (workability) may decrease. It may also lead to intergranular corrosion and make the surface too rough. For this reason, the upper limit of the C content is 0.030% or less. Preferably, it is 0.028% or less, and more preferably 0.025% or less. On the other hand, C is an element that affects the stability of the austenite phase. In order to enjoy this effect, the C content may be 0.001% or more. Preferably, it is 0.003% or more, and more preferably 0.010% or more.

[0021] (Si: 0.10% or more, 1.00% or less) Si is necessary for deoxidation during melting and refining, and is also a useful element for improving oxidation resistance. To enjoy this effect, the Si content may be 0.10% or more. Also, Si may be mixed in from raw materials such as molten iron, and excessive reduction leads to increased costs, so it is more desirable to have a Si content of 0.20% or more. On the other hand, if the Si content is too high, it may cause a decrease in ductility and poor pickling. If poor pickling occurs, the oxide scale may not dissolve completely, causing localized depressions, and as a result the surface may become too rough. For this reason, the upper limit is set to 1.00% or less. Preferably it is 0.90% or less, more preferably 0.80% or less.

[0022] (Mn: 0.30% or more, 4.00% or less) Mn is an element included as a deoxidizing agent and also has the effect of concentrating in the austenite phase and stabilizing it. In order to enjoy these effects, the Mn content may be 0.30% or more. Preferably it is 0.40% or more, more preferably 0.50% or more. However, if the Mn content is too high, it may cause a decrease in ductility and poor pickling. If poor pickling occurs, the oxide scale will not dissolve completely, causing local depressions, and as a result the surface may become too rough. For this reason, the upper limit is set to 4.00% or less. Preferably it is 3.90% or less.

[0023] (P: 0.050% or less) P is an element that is present as an impurity in the main raw materials such as molten iron and ferrochrome. If the P content is excessively high, corrosion resistance and formability may decrease. In addition, excessive P content can lead to intergranular corrosion and the surface may become too rough. Therefore, the upper limit of the P content is set to 0.050% or less. Preferably, it is 0.040% or less. On the other hand, excessive reduction of P leads to increased costs, such as requiring the use of high-purity raw materials, so the lower limit of the P content may be 0.005% or more. The P content is more preferably 0.010% or more.

[0024] (S: 0.0500% or less) S forms sulfide-based inclusions such as MnS, which degrade the general corrosion resistance (overall corrosion and pitting corrosion) of steel plates. In addition, the surface may become too rough due to the dissolution of the precipitated sulfide-based inclusions. For this reason, a low S content is preferable, with an upper limit of 0.0500% or less. Furthermore, while corrosion resistance is better with a lower S content, excessively reducing the S content increases the desulfurization load and leads to increased manufacturing costs, so the lower limit may be 0.0001% or more. The S content is more preferably 0.0003% or more.

[0025] (Cr: 20.0% or more, 28.0% or less) Cr is an effective element for ensuring oxidation resistance and corrosion resistance. However, if the Cr content is too low, it may cause poor pickling. If poor pickling occurs, the oxide scale will not dissolve completely, causing localized depressions, and as a result the surface may become too rough. For this reason, the Cr content should be 20.0% or more. Preferably 20.2% or more, and more preferably 20.5% or more. On the other hand, if the Cr content is too high, the processability and scale removal ability may decrease. For this reason, the Cr content should be 28.0% or less. Preferably 27.8% or less, and more preferably 27.5% or less.

[0026] (Mo: 0.03% or more, 5.00% or less) Mo has the effect of improving corrosion resistance. For this reason, the Mo content is 0.03% or more. Preferably it is 0.10% or more, more preferably 0.12% or more. However, if the Mo content is excessive, the processability will decrease. Also, Mo may suppress the dissolution of the ferrite phase into the base material, causing the step difference between the ferrite phase and the austenite phase to become too large, resulting in an overly rough surface. Furthermore, since Mo is an expensive element, the manufacturing cost will increase. For this reason, the Mo content is 5.00% or less. Preferably the Mo content is 4.80% or less, more preferably 4.50% or less.

[0027] (Al: 0.004% or more, 0.500% or less) Al is included as a deoxidizing element and also as an element that improves oxidation resistance. To obtain these effects, the Al content is 0.004% or more. Preferably it is 0.010% or more, more preferably 0.013% or more. However, if Al is included in excess, nitride-based inclusions such as AlN may precipitate, and the surface may become too rough due to the dissolution of the precipitated parts. For this reason, the Al content is 0.500% or less. Preferably the Al content is 0.450% or less, more preferably 0.420% or less.

[0028] (N: 0.070% or more, 0.300% or less) N has the effect of improving pitting corrosion resistance. For this reason, the N content is 0.070% or more. Preferably it is 0.090% or more, more preferably 0.100% or more. However, if N is included in excess, the workability may decrease. Also, if N is included in excess, blowholes may occur during casting and the surface may become too rough. For this reason, the N content is 0.300% or less. Preferably the N content is 0.280% or less, more preferably 0.250% or less.

[0029] (Ni: 1.0% or more, 10.0% or less) Ni is an austenite stabilizing element and increases the hardness of the steel sheet. Ni also has the effect of improving corrosion resistance. In order to enjoy these effects, the Ni content is 1.0% or more. Preferably it is 1.3% or more, more preferably 1.5% or more. However, if Ni is included in excess, the hot workability may decrease. Also, if Ni is excessively concentrated in the austenite phase and the pickling dissolution rate of the Cr-deficient layer decreases, the surface roughness may decrease and the reduction in gloss may not be sufficient. For this reason, the Ni content is 10.0% or less. Preferably the Ni content is 9.0% or less, more preferably 8.0% or less.

[0030] (Cu: 0.03% or more, 1.50% or less) Cu has the effect of improving corrosion resistance. For this reason, the Cu content is 0.03% or more. Preferably it is 0.05% or more, more preferably 0.10% or more. However, if Cu is included in excess, the hot workability may decrease. Also, if Cu is excessively concentrated in the austenite phase and the pickling dissolution rate of the Cr-deficient layer decreases, the surface roughness may decrease and the reduction in gloss may not be sufficient. For this reason, the Cu content is 1.50% or less. Preferably the Cu content is 1.30% or less, more preferably 1.20% or less.

[0031] In the duplex stainless steel sheet according to this embodiment, the remainder of the elements other than those mentioned above consists of Fe and impurities. However, elements other than those mentioned above can also be included to the extent that they do not impair the effects of this embodiment. Here, impurities refer to components that are mixed in during the industrial production of the ferrite-austenite duplex stainless steel according to the present invention due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and are acceptable to the extent that they do not adversely affect the present invention.

[0032] In this embodiment, in addition to the elements mentioned above, one or more of Ti, Nb, Ca, and Mg may be included in place of a portion of Fe, within the ranges shown below. That is, the lower limit of the content of each of Ti, Nb, Ca, and Mg is 0%. The reasons for limiting each element are explained below.

[0033] (Ti: 0.050% or less) Ti forms nitrides (TiN) and carbides (TiC), which have the effect of improving workability. To enjoy these effects, it is preferable to have a Ti content of 0.001% or more. More preferably 0.003% or more. On the other hand, excessive Ti content may reduce ductility, so the upper limit of the Ti content should be 0.050% or less. More preferably 0.030% or less.

[0034] (Nb: 0.200% or less) Like Ti, Nb forms nitrides (NbN) and carbides (NbC), which have the effect of improving workability. To enjoy these effects, it is preferable to have an Nb content of 0.001% or more. More preferably 0.003% or more. On the other hand, excessive Nb content may reduce ductility, so the upper limit of the Nb content should be 0.200% or less. Preferably 0.180% or less, more preferably 0.150% or less.

[0035] (Total of one or more Nb and Ti: 0.015 to 0.200%) Ti and Nb are elements that combine with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, room temperature ductility, and deep drawability. If the total content of Ti and Nb is less than 0.015%, improvement in the above properties cannot be expected, so the total content of Ti and Nb is set to 0.015% or more. On the other hand, if the total content of Ti and Nb exceeds 0.200%, the processability decreases, so the total content of Ti and Nb is set to 0.200% or less. From the viewpoint of stably ensuring the above effects, the total content of Ti and Nb is preferably 0.018 to 0.180%, more preferably 0.020 to 0.160%.

[0036] (Ca: 0.0050% or less) Ca is an effective element for desulfurization and deoxidation. To obtain these effects, it may be contained in an amount of 0.0005% or more. However, excessive Ca content can lead to increased susceptibility to hot working cracks and a decrease in corrosion resistance. Therefore, the Ca content is preferably 0.0050% or less. More preferably 0.0045% or less.

[0037] (Mg: 0.0050% or less) In addition to being contained as a deoxidizing element, Mg is an element that refines the solidification structure and improves workability and toughness. To obtain these effects, it may be contained in an amount of 0.0005% or more. However, excessive Mg content may lead to a decrease in hot workability, so the Mg content is preferably 0.0050% or less. More preferably 0.0045% or less.

[0038] Furthermore, in this embodiment, in addition to the above elements, one or more of B, V, Zr, Ga, REM, Ta, W, Co, and Sb may be included in place of a portion of Fe, within the ranges shown below. In other words, the lower limit of the above elements is 0%. The reasons for limiting each element are explained below.

[0039] (B: 0.0050% or less) B may be included as needed because it has the effect of increasing grain boundary strength through grain boundary segregation and improving hot workability. To enjoy this effect, it is preferable to set the lower limit of the B content to 0.0003% or more. However, excessive B content may impair corrosion resistance, so it is preferable to set the upper limit to 0.0050% or less. More preferably, it is 0.0040% or less.

[0040] (V: 0.50% or less) V has the effect of forming fine nitrides and improving workability. Therefore, it should be included as needed. To enjoy this effect, it is preferable to set the lower limit of the V content to 0.04% or more. On the other hand, if V is included in excess, ductility and hot workability may decrease, so it is preferable to set the upper limit to 0.50% or less. More preferably, it is 0.40% or less.

[0041] (Zr: 0.020% or less) Zr is an element that improves oxidation resistance, and its inclusion is desirable as needed. To enjoy this effect, it is preferable to have 0.003% or more. On the other hand, a large amount of Zr may lead to a decrease in ductility and an increase in raw material costs, so it is preferable to limit its upper limit to 0.020% or less. More preferably 0.018% or less, and even more preferably 0.015% or less.

[0042] (Ga: 0.030% or less) Ga is an element that contributes to improving corrosion resistance and suppressing hydrogen embrittlement. To obtain these effects, it may be contained in amounts of 0.001% or more. However, excessive Ga content may lead to a decrease in processability, so the Ga content is preferably 0.030% or less. More preferably 0.015% or less.

[0043] (REM: 0.050% or less) REM (rare earth elements) has a deoxidizing effect. For this reason, it may be included as needed. To enjoy this effect, it is preferable to have a REM content of 0.005% or more. However, if REM is included in excess, the processability may decrease. For this reason, it is preferable that the REM content be 0.050% or less. More preferably, it is 0.040% or less.

[0044] Note that REM refers to a total of 17 elements including Sc, Y and lanthanoids, and the above REM content means the total content of these elements. Industrially, REM is often added in the form of misch metal.

[0045] (Ta: 0.070% or less) Ta is an element that functions to improve corrosion resistance. Therefore, it may be contained as required. To obtain this effect, the content is preferably 0.010% or more. However, excessive content of Ta may cause reduction in ductility and reduction in toughness, so the Ta content is preferably 0.070% or less, and more preferably 0.050% or less.

[0046] (W: 1.00% or less) W is an element that functions to enhance corrosion resistance. Therefore, it may be contained as required. To obtain this effect, the content is preferably 0.05% or more, more preferably 0.1% or more, and still more preferably 0.2% or more. However, excessive content of W may reduce the ductility of the steel sheet, so the W content is preferably 1.00% or less, and more preferably 0.90% or less.

[0047] (Co: 1.00% or less) Co is an element effective for enhancing high-temperature strength and improving hot workability. To obtain this effect, the Co content is preferably 0.02% or more, and more preferably 0.1% or more. However, excessive content of Co may reduce toughness, so the Co content is preferably 1.00% or less, and more preferably 0.50% or less.

[0048] (Sb: 0.500% or less) Sb is an element that functions to enhance corrosion resistance. Therefore, it may be contained as required. To obtain this effect, the content is preferably 0.005% or more, and more preferably 0.010% or more. However, excessive content of Sb may reduce hot workability, so the Sb content is preferably 0.500% or less, and more preferably 0.400% or less.

[0049] [Aspect Ratio] In the microstructure of a duplex stainless steel sheet, a ferrite phase and an austenite phase are included, but in this embodiment, it is preferable that the aspect ratio of these duplex structures is 1.3 or more. In the case of a duplex stainless steel sheet, the ferrite phase and the austenite phase are extended in the rolling direction (L direction), but by making the structure more extended in the rolling direction (L direction), the aspect ratio of these duplex structures can be made 1.3 or more. By making the duplex structure with a relatively large aspect ratio in this way, the maximum height roughness RzC in the direction perpendicular to the rolling direction, which will be described later, can be increased, and as a result, the glossiness Gs (60°) can be reduced further. More preferably, the aspect ratio of the duplex structure is 1.4 or more. There is no particular upper limit to the aspect ratio of the duplex structure, but it may be 3.0 or less.

[0050] In this embodiment, the "aspect ratio" refers to the value (Ls / Cs) obtained by dividing the number Ls of ferrite / austenite interfaces (α / γ interfaces) located on a straight line perpendicular to the rolling direction within a predetermined observation field by the number Cs of α / γ interfaces located on a straight line parallel to the rolling direction. The method for calculating the aspect ratio (Ls / Cs) will be described below.

[0051] First, to analyze the distribution of ferrite and austenite on the Z-plane of the steel plate (the plane parallel to both the longitudinal and width directions of the plate), EBSD (backscattered electron diffraction) measurements are performed. The EBSD measurement is performed using a scanning electron microscope with the measurement software "TSL OIM Data Collection 7 (TSL Solutions Co., Ltd.)" to measure a 300 μm square area on the Z-plane of the steel plate with a step size of 0.7 μm. The data obtained from this EBSD measurement is then imaged using the area fraction method to capture the ferrite phase (BCC) and austenite phase (FCC) as shown in Figure 1. Next, as shown in Figure 1, five straight lines are drawn at regular intervals along the L-direction and C-direction on the captured image. The spacing between the lines is 20 μm. For each line, the number of phase interfaces (α / γ interfaces) between the ferrite phase (α) and the austenite phase (γ) is counted in each direction, and the average of the five counts is calculated as Ls and Cs. The calculated Ls is divided by Cs to obtain Ls / Cs. This operation is repeated for three fields of view, and the average of the obtained Ls / Cs is taken as the "aspect ratio (Ls / Cs)" in this embodiment.

[0052] [Glossiness Gs(60°)] In the duplex stainless steel sheet of this embodiment, the glossiness Gs(60°) is 50 to 100. Glossiness Gs(60°) refers to 60-degree specular gloss, which is the glossiness when the angle of incidence is 60°. The lower the Gs(60°), the lower the glossiness. Therefore, the glossiness Gs(60°) should be 100 or less. Glossiness Gs(60°) is preferably 90 or less, and more preferably 80 or less. On the other hand, if the glossiness Gs(60°) is excessively low, uneven gloss may occur, impairing the uniformity of the appearance. Therefore, the glossiness Gs(60°) should be 50 or more. Glossiness Gs(60°) is preferably 55 or more, and more preferably 60 or more.

[0053] The gloss level Gs(60°) is measured using the following procedure: A gloss meter (UGV-6P, manufactured by Suga Test Instruments Co., Ltd.) is used to measure the Gs(60°) as specified in JIS Z 8741:1997 on the rolled surface. Measurements are taken at three random locations on the rolled surface, and the average of all measured values ​​is taken as the gloss level Gs(60°). Other measurement conditions should be adjusted according to JIS Z 8741:1997.

[0054] [Surface Roughness] In the duplex stainless steel sheet of this embodiment, the maximum height roughness RzL in the rolling direction is 1.0 to 2.2 μm, the maximum height roughness RzC in the direction perpendicular to rolling is 1.0 to 3.0 μm, and the ratio of the maximum height roughness RzL in the rolling direction to the maximum height roughness RzC in the direction perpendicular to rolling satisfies RzC / RzL ≥ 1.1.

[0055] The maximum height roughness Rz is an important indicator that affects glossiness. For this reason, the duplex stainless steel sheet of this embodiment is evaluated using the maximum height roughness Rz specified in JIS B 0601:2013. Furthermore, as described above, the structure of the duplex stainless steel sheet of this embodiment is extended in the rolling direction, so the roughness is specified not only in the rolling direction (L direction) but also in the direction perpendicular to the rolling direction (C direction), which is perpendicular to the rolling direction. The specific measurement method will be described later.

[0056] <Maximum height roughness RzL: 1.0 to 2.2 μm> In the duplex stainless steel sheet of this embodiment, the maximum height roughness RzL in the rolling direction is in the range of 1.0 to 2.2 μm. When RzL is 1.0 μm or more, low gloss can be achieved. RzL is preferably 1.1 μm or more, and more preferably 1.2 μm or more.

[0057] On the other hand, if RzL is too large, that is, if the rolled surface is too rough, too many irregularities (steps) will form between the ferrite phase and the austenite phase in the rolling direction, making it difficult to ensure uniformity of gloss and potentially degrading the appearance. Furthermore, if RzL is excessively large, foreign matter (water, salt water, dirt, etc.) tends to accumulate, which is undesirable from the viewpoint of corrosion resistance. For this reason, RzL should be 2.2 μm or less. Preferably, RzL should be 2.1 μm or less, and more preferably 2.0 μm or less.

[0058] <Maximum height roughness RzC: 1.0 to 3.0 μm> In the duplex stainless steel sheet of this embodiment, the maximum height roughness RzC in the direction perpendicular to the rolling direction is 1.0 to 3.0 μm. In the duplex stainless steel sheet of this embodiment, the duplex structure is in the rolling direction, so the valleys evaluated by RzC measured along the C direction are morphologically extended in the direction perpendicular to the rolling direction. In other words, a larger RzC means that the difference in dissolution between the ferrite phase and the austenite phase extending in the rolling direction increases, resulting in a larger difference in height in the roughness curve and a greater reduction in gloss. To enjoy this effect, RzC is set to 1.0 μm or more. RzC is preferably 1.1 μm or more, and more preferably 1.2 μm or more.

[0059] On the other hand, if RzC is too large, similar to RzL, excessive irregularities (steps) will form between the ferrite phase and the austenite phase in the direction perpendicular to the rolling process, making it difficult to ensure uniformity of gloss and potentially degrading the appearance. Furthermore, if RzC is excessively large, foreign matter (water, saltwater, dirt, etc.) tends to accumulate, which is undesirable from the viewpoint of corrosion resistance. For this reason, RzC should be 3.0 μm or less. RzC is preferably 2.9 μm or less, and more preferably 2.8 μm or less.

[0060] The maximum height roughness RzL and RzC are measured using the following procedure. A stylus-type surface roughness measuring instrument (Mitutoyo Corporation's "SURF TEST EXTREME SV-3000CNC") is used for the measurement, with a measurement length of 14.2 mm, a reference length of 0.8 mm, and a Gaussian filter characteristic. In this measurement, the steel plate surface (rolled surface) is measured three times each in the rolling direction and perpendicular to the rolling direction, and the average value is calculated in each direction to obtain the maximum height roughness RzL and RzC.

[0061] <RzC / RzL≧1.1> In the duplex stainless steel sheet of this embodiment, RzL and RzC satisfy RzC / RzL≧1.1. A large RzC / RzL means that the difference in dissolution rates between the ferrite phase and the austenite phase extending in the rolling direction increases, making the irregularities between the two phases greater than in the L direction. On the other hand, if RzC / RzL is small, the difference in maximum height roughness between the L direction and the C direction is small, making it difficult to obtain patterns parallel to the rolling direction (so-called streaks), and a desirable appearance may not be obtained. For these reasons, in this embodiment, RzC / RzL is set to 1.1 or higher. Preferably, RzC / RzL is 1.2 or higher, and more preferably 1.3 or higher. On the other hand, there is no particular upper limit to RzC / RzL, but it may be 3.0 or lower.

[0062] [Average grain size] In the duplex stainless steel sheet of this embodiment, the average grain size is preferably 0.5 to 7.5 μm. The smaller the average grain size, the greater the area of ​​grain boundaries per unit area, and the greater the amount of Cr-depleted layer. As a result, the amount of microgrooves also increases, the surface irregularities of the steel sheet become larger, and the glossiness can be reduced. In order to enjoy these effects, the average grain size is preferably 7.5 μm or less. More preferably 7.0 μm or less. On the other hand, if the average grain size is excessively small, recrystallization may occur, which may impair the mechanical properties. Therefore, the average grain size is preferably 0.5 μm or more. More preferably 0.8 μm or more.

[0063] The average grain size of the ferrite and austenite phases can be measured using the EBSD method. The measurement is performed at a magnification of 2000x with a step size of 0.2 μm. The obtained data is analyzed using TSL's OIM analysis software, and the results are calculated. Austenite and ferrite are determined by referring to iron FCC and iron BCC from the software's database. A grain boundary is defined as a grain boundary with an orientation difference of 15° or more, and the equivalent circle diameter is calculated. The average grain size is then determined by the arithmetic mean of the obtained equivalent circle diameters.

[0064] [Sheet Thickness] The sheet thickness of the duplex stainless steel sheet of this embodiment is preferably 0.1 mm or more and 3.0 mm or less. A sheet thickness within this range can be suitably used for outdoor building material applications such as roofing materials. Furthermore, as will be described later, the duplex stainless steel sheet of this embodiment can achieve low gloss by controlling the heating rate and pickling conditions rather than physical treatments such as shot blasting, so it can be stably applied even to thinner steel sheets. The sheet thickness of the duplex stainless steel sheet of this embodiment is preferably 1.5 mm or less, more preferably 0.8 mm or less.

[0065] <3. Method for Manufacturing Ferrite-Austenitic Duplex Stainless Steel Sheets> The stainless steel sheets of this embodiment can be reliably manufactured by, for example, the following manufacturing method.

[0066] The method for manufacturing stainless steel sheets according to this embodiment includes the following steps. Specifically, molten steel having the above chemical composition is produced in a converter or electric furnace, and then the molten steel is refined. Subsequently, it is prepared as a material for hot rolling, such as a steel billet, by a continuous casting method or ingot forming method. This material for hot rolling is subjected to hot rolling, annealing of the hot-rolled sheet, pickling, cold rolling, finish annealing, and pickling to produce a stainless steel sheet. If necessary, the annealing of the hot-rolled sheet may be omitted, or the cold rolling, finish annealing, and pickling may be repeated.

[0067] (Hot Rolling Process) The material for hot rolling described above is heated and then hot-rolled. The heating temperature of the material for hot rolling, for example, a slab, is preferably between 1100°C and 1300°C. Note that the heating temperature of the material for hot rolling refers to the surface temperature of the material at the exit of the heating furnace.

[0068] In hot rolling, rough rolling is performed in multiple passes (rolling with several reciprocations in a single stand), followed by finish rolling in one direction using multiple stands. After that, the hot-rolled sheet is wound into a coil. Here, the winding temperature is preferably 600°C or lower. If necessary, the hot-rolled sheet may also be annealed.

[0069] (Cold Rolling Process) Next, the hot-rolled sheet described above is cold-rolled to produce a cold-rolled sheet. In this cold-rolling process, multi-pass rolling is generally performed using a Zenzimir rolling mill to produce a sheet of a predetermined thickness. Mineral oil or water-soluble oil is used as a lubricant during this process. The reduction ratio and other conditions in cold rolling are not particularly limited. They can be adjusted as appropriate based on the desired sheet thickness, etc.

[0070] (Finish Annealing Process) The obtained cold-rolled sheet is subjected to finish annealing. In this embodiment, from the viewpoint of refining the crystal grains and promoting the concentration of Cr in the scale (i.e., promoting the formation of a Cr-deficient layer), the heating rate in the finish annealing is set to 50 to 800°C / s.

[0071] By increasing the heating rate to 50°C / s or higher, the depth of the Cr-depleted layer can be increased. This is thought to be because it allows the temperature range of 800°C or higher, where Cr oxide is formed and grows, to be reached more quickly, thereby promoting the concentration of Cr in the oxide during the formation and growth of the oxide scale. If the heating rate is less than 50°C / s, the temperature range of 600-800°C, where Fe oxide is formed, will be passed through for a long time, and by the time the high temperature range of 800°C or higher, where Cr oxide is formed and grows, is reached, Fe oxide will have already formed on the surface. For this reason, the heating rate in finish annealing is 50°C / s or higher. The heating rate is preferably 70°C / s or higher, and more preferably 100°C / s or higher.

[0072] On the other hand, there is no particular upper limit to the heating rate, but if the heating rate is too high, the equipment load will increase and maintenance costs may increase. For this reason, the heating rate in finish annealing should be 800°C / s or less. Preferably, the heating rate should be 700°C / s or less, and more preferably 500°C / s or less.

[0073] The temperature range in which the heating rate is controlled is preferably at least 600 to 800°C, where Fe oxides are likely to precipitate. This suppresses the formation of Fe oxides, resulting in a scale where Cr is preferentially oxidized during the subsequent annealing temperature holding process, thereby achieving a low Cr content in the Cr-deficient layer.

[0074] The annealing temperature is not particularly limited, but it is preferably 800°C or higher. If the annealing temperature is below 800°C, the formation of Fe oxides prevents a decrease in Cr content in the Cr-deficient layer, resulting in poor scale removal. As a result, the desired gloss may not be obtained. On the other hand, if the annealing temperature exceeds 1250°C, the crystal grains may coarseen, and the desired gloss may not be obtained; therefore, the annealing temperature is preferably 1250°C or lower.

[0075] The holding time at the annealing temperature should preferably be 5 seconds or more. If it is less than 5 seconds, the Cr content of the Cr-deficient layer cannot be sufficiently reduced. As a result, the grain boundaries may dissolve shallowly during pickling, and the desired gloss may not be achieved. There is no particular upper limit to the holding time. However, from the viewpoint of manufacturability, the holding time should preferably be 30 seconds or less.

[0076] By using the annealing conditions described above, the chromium-deficient layer formed in the austenite phase can be made deeper. By forming a deeper chromium-deficient layer, the depth of the microgrooves can be increased in the subsequent pickling process, the surface irregularities of the steel sheet can be made larger, and as a result, a lower gloss can be achieved.

[0077] (Finish Annealed Steel Sheet Pickling Process) The steel sheet is pickled after finish annealing. It is preferable to use a mixed acid solution containing two types of acid, nitric acid and hydrofluoric acid, for pickling. The preferred concentration range for the mixed acid is 20 to 200 g / L for nitric acid and 10 to 100 g / L for hydrofluoric acid. The temperature of the mixed acid solution is preferably 20°C to 90°C.

[0078] By using the above solution, the Cr-deficient layer formed during finish annealing can be preferentially dissolved. When used in combination with the annealing conditions in this embodiment, microgrooves corresponding to the depth of the Cr-deficient layer are formed, achieving the desired low gloss finish.

[0079] Furthermore, in pickling, if the immersion time is too short, the chromium-deficient layer may not be sufficiently dissolved, resulting in an undesirable low gloss level. For this reason, an immersion time of 3 seconds or more is preferable.

[0080] The stainless steel sheet according to the present invention will be described in more detail below with reference to examples, but the embodiments are not limited to these examples.

[0081] By the method described above, the ferrite-austenite duplex stainless steel sheet of this embodiment can be manufactured.

[0082] As described above, this embodiment provides a ferrite-austeniite duplex stainless steel sheet with excellent low gloss. In particular, the ferrite-austeniite duplex stainless steel sheet of this embodiment has excellent anti-glare properties and can therefore be suitably used as a material for exterior building materials such as roofing materials and exterior wall materials, as well as interior building materials where aesthetic appeal is required.

[0083] The following are examples of the present invention. The conditions in these examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to the conditions used in the following examples. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention. In the table below, underlined items indicate that they are outside the scope of the present invention.

[0084] <Example 1> Using a slab having the chemical composition shown in Table 1, hot rolling and hot-rolled sheet annealing were performed using a conventional method to obtain a hot-rolled sheet. Subsequently, cold rolling, finish annealing, and pickling were performed in sequence to produce a duplex stainless steel sheet (test material) with a thickness of 0.5 mm.

[0085] The conditions for finish annealing were set to a heating rate of 50-750°C / s at 600-800°C, an annealing temperature of 1050-1150°C, and a holding time of 5-15 seconds. For pickling after finish annealing, a mixed acid solution of hydrofluoric acid (20-80 g / L) + nitric acid (30-150 g / L) was used, with the temperature of the mixed acid solution set to 40-60°C. The immersion time during pickling was adjusted between 3 and 15 seconds. For other manufacturing conditions not specifically mentioned, the preferred manufacturing conditions described in the specification were adopted.

[0086] By the above method, test materials for the present invention examples B1 to B17 and comparative examples b1 to b10, as shown in Table 2, were obtained.

[0087] For the obtained test materials, the aspect ratio of the two-phase structure, the surface roughness of each (RzC, RzL, and RzC / RzL), the average grain size, and the gloss were measured using the procedure described above.

[0088] For measuring surface roughness, a stylus-type surface roughness measuring instrument (Mitutoyo Corporation's "SURF TEST EXTREME SV-3000CNC") was used. The measurement length was 14.2 mm, the reference length was 0.8 mm, and the filter characteristic was set to Gauss.

[0089] Furthermore, the aspect ratio of the two-phase structure was determined by the following method. To analyze the distribution of ferrite and austenite on the Z-plane of the obtained test material, EBSD (backscattered electron diffraction) measurements were performed. EBSD measurements were performed using a scanning electron microscope with the measurement software "TSL OIM Data Collection 7 (TSL Solutions Co., Ltd.)" to measure a 300 μm square area on the Z-plane of the steel plate with a step size of 0.7 μm. From the data obtained from this EBSD measurement, the ferrite phase (BCC) and austenite phase (FCC) were imaged using the area fraction method as shown in Figure 1.

[0090] Next, as shown in Figure 1, five straight lines were drawn at regular intervals along the L and C directions on the captured image. The interval between the lines was 20 μm. For each line, the number of phase interfaces (α / γ interfaces) between the ferrite phase (α) and the austenite phase (γ) was counted in each direction, and the number of ferrite / austenite interfaces (α / γ interfaces) present on the line perpendicular to the rolling direction, Ls, and the number of α / γ interfaces present on the line parallel to the rolling direction, Cs, were determined. From the obtained Ls and Cs, the aspect ratio Ls / Cs was calculated. This operation was repeated for three fields of view, and the average of the obtained Ls / Cs was defined as the "aspect ratio of the two-phase structure (Ls / Cs)".

[0091] The results of these various measurements are shown in Table 2.

[0092]

[0093]

[0094] As shown in Table 2, all of the present invention examples B1 to B17 had aspect ratios, RzL, RzC, RzC / RzL, and average grain size within the range that satisfies the requirements of this embodiment, and exhibited good low gloss. Therefore, all of the present invention examples that satisfy the requirements of this embodiment can be evaluated as duplex stainless steel sheets having surface properties suitable for use as exterior building materials or interior building materials, for example.

[0095] On the other hand, comparative examples b1 to b10, which did not satisfy the requirements of this embodiment, failed to obtain a good low gloss because at least one of the chemical composition, RzL, RzC, or RzC / RzL did not satisfy the requirements of this embodiment. In comparative example b1, the C content was too high, which led to intergranular corrosion and made the surface too rough, causing both RzL and RzC to fall outside the range of this embodiment. As a result, a good low gloss could not be obtained. In comparative example b2, the Si content was too high, resulting in poor pickling and localized depressions where the oxide scale did not dissolve completely, resulting in a surface that was too rough, causing both RzL and RzC to fall outside the range of this embodiment. As a result, a good low gloss could not be obtained. In comparative example b3, the Mn content was too high, similar to comparative example b2, resulting in poor pickling and localized depressions where the oxide scale did not dissolve completely, causing a surface that was too rough, causing both RzL and RzC to fall outside the range of this embodiment. As a result, a good low gloss could not be obtained. In Comparative Example b4, the P content was too high, leading to intergranular corrosion and excessive surface roughness, causing both RzL and RzC to fall outside the range of this embodiment. As a result, a good low gloss was not obtained. In Comparative Example b5, the S content was too high, causing excessive surface roughness due to the dissolution of precipitated sulfide inclusions, causing both RzL and RzC to fall outside the range of this embodiment. As a result, a good low gloss was not obtained. In Comparative Example b6, the Cr content was too high, resulting in poor pickling and localized depressions where the oxide scale did not dissolve completely, resulting in excessive surface roughness, causing both RzL and RzC to fall outside the range of this embodiment. As a result, a good low gloss was not obtained. In Comparative Example b7, the Mo content was too high, excessively suppressing the dissolution of the ferrite phase by Mo. This resulted in an excessively large step difference between the ferrite phase and the austenite phase, causing excessive surface roughness, causing both RzL and RzC to fall outside the range of this embodiment. As a result, a good low gloss was not obtained. In comparative example b8, the Al content was too high, causing nitride-based inclusions to precipitate. The dissolution of these precipitates made the surface too rough, and both RzL and RzC fell outside the scope of this embodiment.As a result, a good low gloss was not obtained. In comparative example b9, the N content was too high, causing blowholes to form during casting. Also, the Ni content was too low, resulting in a large difference in dissolution rates between the ferrite phase and the austenite phase, which, combined with the blowholes, made the surface too rough. As a result, both RzL and RzC fell outside the range of this embodiment, and a good low gloss was not obtained. In comparative example b10, the Ni and Cu content was too high, causing excessive concentration of Ni and Cu in the austenite phase, which reduced the pickling dissolution rate of the Cr-deficient layer and decreased the surface roughness. As a result, both RzL and RzC fell outside the range of this embodiment, and a good low gloss was not obtained.

[0096] <Example 2> Using steels No. A17, A3, and A15 from the chemical compositions shown in Table 1, duplex stainless steel sheets (test specimens) were prepared under the manufacturing conditions shown in Table 3. Other conditions were the same as in <Example 1>. The sheet thickness was 0.5 mm.

[0097] By the above method, test materials for the present invention examples C1 to C5 and comparative examples c1 to c2, as shown in Table 3, were obtained.

[0098] The obtained test material was measured for the aspect ratio of the two-phase structure, the surface roughness of each surface, the average grain size, and the glossiness using the same procedure as in Example 1.

[0099] The results of these various measurements are shown in Table 3.

[0100]

[0101] As shown in Table 3, in all of the present invention examples C1 to C5, the heating rate during finish annealing was within the preferred range for this embodiment, and therefore the aspect ratio of the two-phase structure, RzL, RzC, RzC / RzL, and average grain size all satisfied the requirements of this embodiment, resulting in good low gloss.

[0102] On the other hand, in Comparative Example c1, the heating rate was too low, resulting in insufficient Cr enrichment, and there was almost no difference between the dissolution rates of the austenite phase and the ferrite phase during the pickling process. As a result, neither RzL nor RzC satisfied the requirements of this embodiment, and a good low gloss could not be obtained. Comparative Example c2 is an example in which shot blasting was performed on the surface of the steel sheet between finish annealing and pickling. In Comparative Example c2, the shot blasting was carried out under general conditions for stainless steel. The projection by the shot blasting was too strong, and RzL did not satisfy the requirement. As a result, the gloss was not within an appropriate range. Furthermore, since there was no anisotropy in the irregularities after shot blasting, the RzC / RzL ratio decreased. As a result, no pattern parallel to the rolling direction was observed in the visual appearance evaluation.

[0103] According to the above embodiment of the present invention, a ferrite-austenitic duplex stainless steel sheet having a low-gloss surface with excellent anti-glare properties can be provided. This makes it applicable to exterior building materials such as roofing materials and exterior wall materials, as well as interior building materials where aesthetic appeal is required.

Claims

1. The chemical composition, in mass%, contains C: 0.030% or less, Si: 0.10% or more, 1.00% or less, Mn: 0.30% or more, 4.00% or less, P: 0.050% or less, S: 0.0500% or less, Cr: 20.0% or more, 28.0% or less, Mo: 0.03% or more, 5.00% or less, Al: 0.004% or more, 0.500% or less, N: 0.070% or more, 0.300% or less, Ni: 1.0% or more, 10.0% or less, and Cu: 0.03% or more, 1.5% or less, with the remainder being Fe and impurities, and the gloss level Gs (60°) as defined in JIS Z 8741:1997 is 50 to 100. A ferritic-austenite duplex stainless steel sheet having a maximum height roughness RzL in the rolling direction of 1.0 to 2.2 μm, a maximum height roughness RzC in the direction perpendicular to rolling of 1.0 to 3.0 μm, and satisfying the ratio RzC / RzL ≥ 1.1 between the maximum height roughness RzL in the rolling direction and the maximum height roughness RzC in the direction perpendicular to rolling.

2. The ferrite-austenite duplex stainless steel sheet according to claim 1, wherein the chemical composition further contains, by mass%, one or more of the following: Ti: 0.05% or less, Nb: 0.20% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less, and the total content of Ti and Nb is 0.02% or more and 0.20% or less.

3. The ferrite-austenite duplex stainless steel sheet according to claim 1 or 2, wherein the chemical composition further contains, by mass%, one or more of the following: B: 0.0050% or less, V: 0.50% or less, Zr: 0.020% or less, Ga: 0.030% or less, REM: 0.050% or less, Ta: 0.070% or less, W: 1.00% or less, Co: 1.00% or less, and Sb: 0.500% or less.

4. A ferrite-austenite duplex stainless steel sheet according to claim 1 or 2, wherein the average grain size is 0.5 to 7.5 μm.

5. A ferrite-austenite duplex stainless steel sheet according to claim 1 or 2, wherein the aspect ratio of the duplex structure consisting of a ferrite phase and an austenite phase is 1.3 or more.

6. A ferrite-austenite duplex stainless steel sheet according to claim 1 or 2, wherein the sheet thickness is 0.1 mm or more and 3.0 mm or less.

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