Ferrite-austenitic duplex stainless steel hot rolled steel and corrosion-resistant components

Abstract

To provide a two-phase ferrite / austenite stainless steel material that has a smooth and glossy surface and has excellent corrosion resistance.SOLUTION: A two-phase ferrite / austenite stainless steel material having excellent corrosion resistance, has a composition consisting of, by mass, C: 0.001-0.100%, Si: 1.00% or less, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 1.00-9.00%, Cr: 15.00-30.00%, Mo: 5.00% or less, Cu: 2.00% or less, N: 0.370% or less, and Al: 0.400% or less, with Si+2Al of less than 1.20%, the balance being Fe and impurities. The two-phase ferrite / austenite stainless steel material has a surface arithmetic mean roughness Ra of 0.10-3.00 μm and a 60 degree specular glossiness Gs (60°) of 10%-100%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic-austenitic duplex stainless steel material and a corrosion-resistant member. [Background technology]

[0002] Stainless steel materials have excellent properties such as corrosion resistance, and are therefore used in a wide range of applications, including automobile parts, building parts, and kitchen utensils. Stainless steel materials are classified into steel plates, steel bars, steel strips, steel bars, steel pipes, etc. depending on their shape. Stainless steel plates, which are common stainless steel materials, are manufactured by the following process. Molten iron, which is made by melting stainless steel raw materials, is continuously cast into slabs, and the slabs are hot-rolled to obtain hot-rolled steel plates (thick plates). If necessary, cold-rolled steel plates are also manufactured by cold-rolling the hot-rolled steel plates to obtain cold-rolled steel plates (thin plates). In the manufacture of such stainless steel materials, oxide scale forms on the surface of the stainless steel material, and the oxide scale is removed by pickling. Hereinafter, the removal of oxide scale formed on the surface of the stainless steel material will be referred to as "descaling."

[0003] However, pickling alone may not be sufficient to remove oxide scale. In particular, hot rolling involves high heating temperatures and long heating times, so the oxide scale formed on the surface of stainless steel is composed of thick, stable oxides that are difficult to remove. Therefore, in a typical descaling process, mechanical pretreatment is performed using a scale breaker or shot blasting to create cracks in the oxide scale, and then the oxide scale is pickled to make it easier to remove (for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-172077 [Patent Document 2] Japanese Patent Application Publication No. 2-145785 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the descaling process, the unevenness of the surface formed by the mechanical pretreatment and the roughness of the surface caused by pickling cause the surface of the stainless steel material to turn white and lose its luster, reducing its design appeal. In particular, ferritic-austenitic duplex stainless steel material has a thick oxide scale formed on its surface, making it difficult to create cracks with mechanical pretreatment and prone to uneven pickling. On the other hand, polishing the surface after the descaling process described above may be considered to ensure the design of the stainless steel material, but polishing the surface until it is smooth increases the amount of grinding required, reducing yield, and may leave grinding oil (cooling oil) or grinding chips in the grinding marks, reducing corrosion resistance.

[0006] The present invention has been made to solve the above-mentioned problems, and aims to provide a ferritic-austenitic duplex stainless steel material having a smooth and glossy surface and excellent corrosion resistance, and a corrosion-resistant component made from the same. [Means for solving the problem]

[0007] As a result of intensive research aimed at solving the above-mentioned problems, the inventors discovered that by controlling the composition of a ferritic-austenitic duplex stainless steel material and employing a laser descaling process followed by a pickling descaling process, it is possible to improve the surface smoothness and gloss without reducing corrosion resistance. Based on this discovery, the inventors produced and investigated various ferritic-austenitic duplex stainless steel materials, and found that a ferritic-austenitic duplex stainless steel material having a predetermined composition and a surface arithmetic mean roughness Ra and 60° specular gloss Gs(60°) falling within predetermined ranges can solve the above-mentioned problems, leading to the completion of the present invention.

[0008] That is, the present invention provides a steel sheet having a composition, on a mass basis, containing C: 0.001 to 0.100%, Si: 1.00% or less, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 1.00 to 9.00%, Cr: 15.00 to 30.00%, Mo: 5.00% or less, Cu: 2.00% or less, N: 0.370% or less, and Al: 0.400% or less, in which Si+2Al is less than 1.20%, and the balance being Fe and impurities; The arithmetic mean roughness Ra of the surface is 0.10 to 3.00 μm and the 60-degree specular gloss Gs (60°) is 10 to 100%. the law of nature, In a salt-dry-wet cycle test, the rust area ratio is 1% or less after 10 cycles of spraying a 5% NaCl aqueous solution (15 minutes at 35°C), drying (1 hour at 30% relative humidity and 60°C temperature) and wetting (3 hours at 95% relative humidity and 50°C temperature). It is a ferritic-austenitic duplex stainless hot-rolled steel material.

[0009] The present invention also provides the above-mentioned ferritic-austenitic duplex stainless steel. Hot rolled It is a corrosion-resistant member that includes steel. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a ferritic-austenitic duplex stainless steel material having a smooth and glossy surface and excellent corrosion resistance, and a corrosion-resistant member using the same. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an SEM photograph of the surface of a stainless steel plate manufactured by carrying out a laser descaling process and a pickling descaling process. [Figure 2] This is a laser microscope photograph of the surface of a stainless steel plate that has been produced by pre-treatment using shot blasting followed by a pickling descaling process. [Figure 3] This is a laser microscope photograph of the surface of a stainless steel plate that was manufactured by pre-treating with shot blasting, then carrying out a pickling descaling process, and then belt polishing. DETAILED DESCRIPTION OF THE INVENTION

[0012] The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0013] A ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention has a composition containing C: 0.001 to 0.100%, Si: 1.00% or less, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 1.00 to 9.00%, Cr: 15.00 to 30.00%, Mo: 5.00% or less, Cu: 2.00% or less, N: 0.370% or less, and Al: 0.400% or less, with Si+2Al being less than 1.20%, and the balance being Fe and impurities. Here, in this specification, "stainless steel material" refers to a material formed from stainless steel, and the shape of the material is not particularly limited. Examples of the material shape include plate (including strip), rod, and tube. Various types of sectional shapes, such as T-shaped and I-shaped steel, may also be used. Furthermore, "impurities" refer to components that are mixed in during industrial production of stainless steel material due to raw materials such as ore and scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, the stainless steel material may contain 0.02% or less of O as an impurity. Furthermore, the stainless steel material may contain a total of 0.1% or less of REM (rare earth elements).

[0014] Furthermore, the ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention may further contain one or more selected from Ti: 0.001 to 0.500%, Nb: 0.001 to 1.000%, V: 0.001 to 1.000%, W: 0.001 to 1.000%, Zr: 0.001 to 1.000%, and Co: 0.001 to 1.200%. Furthermore, the ferrite-austenite duplex stainless steel material according to the embodiment of the present invention may further contain one or more selected from Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0080%, and Sn: 0.001 to 0.500%. Hereinafter, each component will be described in detail.

[0015] <C: 0.001 to 0.100%> If the C content is too high, in addition to becoming hard and reducing the workability, sensitization occurs when affected by heat such as welding, and the corrosion resistance of the ferrite-austenite duplex stainless steel material deteriorates. Therefore, the upper limit value of the C content is controlled to 0.100%, preferably 0.080%, more preferably 0.060%, and still more preferably 0.030%. On the other hand, if the C content is too low, it leads to deterioration of workability and an increase in refining cost due to a decrease in the stability of the austenite phase. Therefore, the lower limit value of the C content is controlled to 0.001%, preferably 0.002%, more preferably 0.005%, and still more preferably 0.010%.

[0016] <Si: 1.00% or less> If the Si content is too high, the material hardens and the workability of the ferrite-austenite duplex stainless steel material deteriorates. Therefore, the upper limit value of the Si content is controlled to 1.00%, preferably 0.80%, more preferably 0.70%, and still more preferably 0.60%. On the other hand, the lower limit value of the Si content is not particularly limited, but is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%.

[0017] <Mn: 6.00% or less> Mn is an austenite phase (γ-phase) forming element. If the Mn content is too high, the corrosion resistance of ferritic-austenitic duplex stainless steel will decrease. Therefore, the upper limit of the Mn content is controlled to 6.00%, preferably 4.00%, more preferably 2.00%, and still more preferably 1.00%. On the other hand, the lower limit of the Mn content is not particularly limited, but is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%.

[0018] <P: 0.050% or less> If the P content is too high, the corrosion resistance and workability of ferritic-austenitic duplex stainless steel will decrease. Therefore, the upper limit of the P content is controlled to 0.050%, preferably 0.035%, and more preferably 0.030%. On the other hand, the lower limit of the P content is not particularly limited, but is preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%.

[0019] <S: 0.0300% or less> If the S content is too high, the hot workability will decrease, resulting in a reduction in the manufacturability of ferritic-austenitic duplex stainless steel and also having an adverse effect on the corrosion resistance. Therefore, the upper limit of the S content is controlled to 0.0300%, preferably 0.0100%, more preferably 0.0050%, and still more preferably 0.0010%. On the other hand, the lower limit of the S content is not particularly limited, but is preferably 0.0001%, and more preferably 0.0002%.

[0020] <Ni: 1.00 - 9.00%> Ni is an austenite phase (γ-phase) forming element like Mn. Since Ni is expensive, if the content is too high, it will lead to an increase in manufacturing costs. Therefore, the upper limit of the Ni content is controlled to 9.00%, preferably 8.00%, more preferably 7.50%, and still more preferably 7.00%. On the other hand, from the perspective of obtaining the effect of Ni, the lower limit of the Ni content is preferably controlled to 1.00%, more preferably 2.00%, still more preferably 5.00%, and still more preferably 5.50%.

[0021] <Cr: 15.00~30.00%> If the Cr content is too high, it will lead to an increase in refining cost, and hardening due to solid solution strengthening, resulting in a decrease in the workability of ferritic-austenitic duplex stainless steel. Therefore, the upper limit value of the Cr content is controlled to 30.00%, preferably 28.00%, more preferably 26.00%, and still more preferably 25.00%. On the other hand, if the Cr content is too low, sufficient corrosion resistance cannot be obtained. Therefore, the lower limit value of the Cr content is controlled to 15.00%, preferably 18.00%, more preferably 20.00%, and still more preferably 22.00%.

[0022] <Mo: 5.00% or less> Mo is an element that improves the corrosion resistance of ferritic-austenitic duplex stainless steel. Since Mo is expensive, if the Mo content is too high, it will lead to an increase in manufacturing cost. Therefore, the upper limit value of the Mo content is controlled to 5.00%, preferably 4.50%, more preferably 4.00%, and still more preferably 3.80%. On the other hand, the lower limit value of the Mo content is not particularly limited, but preferably 0.10%, more preferably 1.00%, and still more preferably 3.0%.

[0023] <Cu: 2.00% or less> Cu is an element that improves the workability of ferritic-austenitic duplex stainless steel. If the Cu content is too high, the corrosion resistance of ferritic-austenitic duplex stainless steel will decrease, and a low melting point phase will be formed during casting, leading to a decrease in hot workability. Therefore, the upper limit value of the Cu content is controlled to 2.00%, preferably 1.60%, more preferably 1.20%, and still more preferably 1.00%. On the other hand, the lower limit value of the Cu content is not particularly limited, but preferably 0.20%, more preferably 0.30%, and still more preferably 0.50%.

[0024] <N: 0.370% or less> N is an element that improves corrosion resistance. If the content of N is too high, it will harden and reduce the workability of ferritic-austenitic duplex stainless steel. Therefore, the upper limit value of the content of N is controlled to 0.370%, preferably 0.300%, more preferably 0.280%, and even more preferably 0.260%. On the other hand, the lower limit value of the content of N is not particularly limited, but is preferably controlled to 0.100%, preferably 0.150%, and more preferably 0.200%.

[0025] <Al: 0.400% or less> Al is an element that is added as necessary for deoxidation in the refining process and improves corrosion resistance and heat resistance. If the content of Al is too high, the amount of inclusions generated will increase and the quality will deteriorate. Therefore, the upper limit value of the content of Al is controlled to 0.400%, preferably 0.100%, and more preferably 0.050%. On the other hand, the lower limit value of the content of Al is not particularly limited, but is preferably 0.001%, and more preferably 0.005%.

[0026] <Si + 2Al: less than 1.20%> The ferritic-austenitic duplex stainless steel according to the embodiment of the present invention targets those with low contents of Si and Al. Specifically, Si + 2Al (each element symbol represents the content of each element) is less than 1.20%, preferably 1.10% or less, more preferably 1.00% or less, and even more preferably 0.90% or less. The lower limit value of Si + 2Al is not particularly limited, but is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%.

[0027] <Ti: 0.001 - 0.500%> Ti is an element that combines with C and N to improve corrosion resistance and intergranular corrosion resistance, and is added as needed. To obtain the effects of Ti, the lower limit of the Ti content is controlled to 0.001%, preferably 0.005%. On the other hand, too much Ti can cause surface defects, resulting in a decrease in quality and a decrease in the workability of the ferritic-austenitic duplex stainless steel. Therefore, the upper limit of the Ti content is controlled to 0.500%, preferably 0.300%, and more preferably 0.100%.

[0028] <Nb:0.001~1.000%> Like Ti, Nb is an element that combines with C and N to improve corrosion resistance and intergranular corrosion resistance, and is added as needed. To obtain the effects of Nb, the lower limit of the Nb content is controlled to 0.001%, preferably 0.004%, and more preferably 0.010%. On the other hand, if the Nb content is too high, the workability of the ferritic-austenitic duplex stainless steel material will decrease. Therefore, the upper limit of the Nb content is controlled to 1.000%, preferably 0.600%, and more preferably 0.060%.

[0029] <V:0.001~1.000%> V is an element that improves corrosion resistance and is added as needed. To obtain the effects of V, the lower limit of the V content is controlled to 0.001%, preferably 0.010%. On the other hand, if the V content is too high, the workability of the ferritic-austenitic duplex stainless steel material will decrease. Therefore, the upper limit of the V content is controlled to 1.000%, preferably 0.200%.

[0030] <W:0.001~1.000%> W is an element that improves high-temperature strength and corrosion resistance and is added as needed. To obtain the effects of W, the lower limit of the W content is controlled to 0.001%, preferably 0.010%. On the other hand, if the W content is too high, the material becomes hard and its workability deteriorates, and the surface defects increase, degrading the surface quality of the ferritic-austenitic duplex stainless steel material. Therefore, the upper limit of the W content is controlled to 1.000%, preferably 0.300%.

[0031] <Zr:0.001~1.000%> Zr is an element that combines with C and N to improve oxidation resistance and intergranular corrosion resistance, and is added as needed. To obtain the effects of Zr, the lower limit of the Zr content is controlled to 0.001%, preferably 0.010%. On the other hand, if the Zr content is too high, the workability of the ferritic-austenitic duplex stainless steel material deteriorates. Therefore, the upper limit of the Zr content is controlled to 1.000%, preferably 0.200%, and more preferably 0.050%.

[0032] <Co:0.001~1.200%> Co is an element that improves heat resistance and is added as needed. From the viewpoint of obtaining the effects of Co, the lower limit of the Co content is controlled to 0.001%, preferably 0.010%. On the other hand, since Co is expensive, if the Co content is too high, the manufacturing cost will increase. Therefore, the upper limit of the Co content is controlled to 1.200%, preferably 0.400%.

[0033] <Ca:0.0001~0.0100%> Ca is an element that forms sulfides to reduce the adverse effects of S, and is added as needed. From the viewpoint of obtaining the effects of Ca, the lower limit of the Ca content is controlled to 0.0001%, preferably 0.0003%. On the other hand, if the Ca content is too high, the amount of inclusions generated increases, resulting in a decrease in quality. Therefore, the upper limit of the Ca content is controlled to 0.0100%, preferably 0.0050%.

[0034] <B:0.0001~0.0080%> B is an element that improves hot workability and is added as needed. From the viewpoint of obtaining the effects of B, the lower limit of the B content is controlled to 0.0001%, preferably 0.0003%, and more preferably 0.0005%. On the other hand, if the B content is too high, the corrosion resistance of the ferritic-austenitic duplex stainless steel material will decrease. Therefore, the upper limit of the B content is controlled to 0.0080%, preferably 0.0040%, and more preferably 0.0025%.

[0035] <Sn:0.001~0.500%> Sn is an element that improves corrosion resistance and high-temperature strength and is added as needed. To obtain the effects of Sn, the lower limit of the Sn content is controlled to 0.001%, preferably 0.002%. On the other hand, if the Sn content is too high, a low-melting-point phase is formed, which reduces the hot workability of the ferritic-austenitic duplex stainless steel material. Therefore, the upper limit of the Sn content is controlled to 0.500%, preferably 0.100%, and more preferably 0.050%.

[0036] The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention has a surface arithmetic mean roughness Ra of 0.10 to 3.00 μm, preferably 0.50 to 2.00 μm, and more preferably 1.00 to 1.90 μm. By controlling the surface arithmetic mean roughness Ra within this range, the smoothness of the ferritic-austenitic duplex stainless steel material can be ensured. Here, in this specification, the term "arithmetic mean roughness Ra" refers to the arithmetic mean roughness Ra measured in accordance with JIS B0601:2013.

[0037] The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention has a 60 degree specular gloss Gs(60°) of 10 to 100%, preferably 13 to 70%, and more preferably 15 to 65%. By controlling the 60 degree specular gloss Gs(60°) of the surface within this range, the gloss of the ferritic-austenitic duplex stainless steel material can be ensured. Here, in this specification, "60 degree specular gloss Gs(60°)" means the 60 degree specular gloss Gs(60°) measured in accordance with JIS Z8741:1997.

[0038] The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention has excellent corrosion resistance. Here, in this specification, "excellent corrosion resistance" means that the rust area ratio is 1% or less when subjected to 10 cycles of salt spray, drying, and wetting in a cyclic salt-dry-wet test (CCT).

[0039] The proportion of the ferrite (α) phase in the ferrite-austenitic duplex stainless steel material according to the embodiment of the present invention is not particularly limited, but is preferably 30 to 70% by volume, more preferably 40 to 60% by volume, and even more preferably 50 to 55% by volume. The proportion of the ferrite phase can be measured by a magnetic induction method, for example, using a Ferritescope manufactured by Helmut Fischer.

[0040] In the ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention, the surface preferably satisfies the following (1) and (2). (1) The root-mean-square slope RΔq is 35° or less, preferably 30° or less, and more preferably 25° or less. By controlling the root-mean-square slope RΔq of the surface within this range, the gloss of the ferritic-austenitic duplex stainless steel material can be improved. The lower limit of the root-mean-square slope RΔq is, for example, 3°. Here, in this specification, the "root mean square slope RΔq" refers to the root mean square slope RΔq measured in accordance with JIS B0601:2013.

[0041] (2) Chromanetics Index b * is 7.00 or less, preferably 6.00 or less, and more preferably 5.00 or less. * L * a * b * It is a chromanetics index that indicates the color tone from blue to yellow in the color space, and it is known that stainless steel materials will take on a yellowish color when burnt (oxide) is formed on the surface by polishing or electrolysis. Chromanetics index b * By controlling the chromanetics index b within the above range, it is possible to obtain a ferritic-austenitic duplex stainless steel material with good corrosion resistance, free of oxides that can become the starting point of corrosion. * The lower limit is, for example, 2.00. Here, in this specification, "chromanetics index b * " is the CIE-L used in the CIEDE2000 color difference formula measured in accordance with JIS Z8781-6:2017 * a * b * Chromanetics index b in color space * means.

[0042] In the ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention, the surface may further satisfy the following (3). (3) The texture aspect ratio Str is 0.50 or more, preferably 0.60 or more, and more preferably 0.70 or more. By controlling the texture aspect ratio Str within this range, a ferritic-austenitic duplex stainless steel material with a good appearance without streaks can be obtained. The upper limit of the texture aspect ratio Str is 1 by definition, but it is, for example, about 0.95. Here, in this specification, "aspect ratio Str of texture" means the aspect ratio Str of texture measured in accordance with JIS B0681-2:2018.

[0043] The thickness (plate thickness) of the ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention is not particularly limited, but is preferably 3 mm or more.

[0044] The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention can be produced by melting stainless steel having the above-described composition and using a method known in the art, except that the descaling process involves a laser descaling process (hereinafter referred to as "laser descaling") and a pickling descaling process (hereinafter referred to as "pickling descaling"). Specifically, stainless steel having the above-described composition is melted and forged or cast to produce a slab. The slab is then hot-rolled, followed by a laser descaling process and then a pickling descaling process. Annealing may be performed as appropriate before the laser descaling process.

[0045] The laser descaling process involves irradiating the oxide scale formed on the surface of a ferritic-austenitic duplex stainless steel material with laser light to vaporize and remove the oxide scale. The various conditions for the laser descaling step may be adjusted depending on the device used, taking into consideration the following points.

[0046] (Laser type) A pulsed laser is preferred because continuous wave lasers generate too much heat and tend to melt the base material (ferritic-austenitic duplex stainless steel). (wavelength) Generally, the reflectivity of a material to light is wavelength-dependent, and selecting a wavelength with low reflectivity increases heat input, making evaporation more likely. Therefore, by selecting a wavelength with high reflectivity for the base material and low reflectivity for the oxide, it is possible to selectively evaporate and remove oxide scale. (pulse width) If the pulse width is short, ablation occurs before the heat input by the laser is transmitted to the surroundings, so the ablation threshold is small. However, the pulse width is mainly determined by the performance of the oscillator, and devices that can oscillate with short pulse widths are expensive, so it is preferable to select a short pulse width within the specification range of the laser descaling equipment. (oscillation frequency) The shorter the pulse width, the higher the oscillation frequency, and the higher the oscillation frequency, the smaller the gap between pulses when scanning. Therefore, it is preferable to select a high oscillation frequency within the specifications of the laser descaling equipment.

[0047] (scan frequency) The higher the scanning frequency, the faster the line processing speed, but if it is set too high, gaps will occur between pulses and the descaling rate will decrease. Therefore, it is preferable to increase the scanning frequency within a range where the descaling rate can be maintained. (laser beam diameter) The larger the beam diameter, the wider the irradiation range, i.e., the range that can be descaled with one pulse, and the better the descaling efficiency, but the lower the energy density (fluence) of one pulse. It is preferable to increase the beam diameter within a range that maintains a fluence that can evaporate and remove scale. (Fluence) Oxide scale can be vaporized and removed by irradiating it with laser light having a fluence exceeding the ablation threshold of the oxides that make up the scale, but if the fluence is too high, not only the scale but also the base material will be vaporized and removed, resulting in significant damage to the base material. Therefore, the fluence can be adjusted taking into account the descaling rate and base material damage.

[0048] The pickling descaling process involves immersing ferritic-austenitic duplex stainless steel materials that have undergone laser descaling in a pickling bath to wash away any oxide scale that was not removed during the laser descaling process. The pickling solution used in the pickling bath is not particularly limited, but can be a solution containing one or more of the following components: nitric acid (HNO3), sulfuric acid (H2SO4), hydrofluoric acid (HF), or ferric chloride (FeCl3). A typical pickling solution is a mixture of nitric acid and hydrofluoric acid.

[0049] As a reference to illustrate the differences in surface conditions, Figures 1, 2, and 3 show SEM or laser microscope photographs of (1) the surface of a stainless steel plate manufactured by performing a laser descaling process and a pickling descaling process, (2) the surface of a stainless steel plate manufactured by performing a pretreatment by shot blasting followed by a pickling descaling process, and (3) the surface of a stainless steel plate manufactured by performing a pretreatment by shot blasting followed by a pickling descaling process and then belt polishing, respectively. Figure 1 shows SEM photographs of the surface of (1) above at (a) 100x and (b) 1000x magnification. As shown in Figure 1, this stainless steel sheet has a surface structure with many smooth areas, although pulse marks from the pulse laser can be seen on the surface. This makes it possible to control the surface roughness parameters (such as arithmetic mean roughness Ra) and 60-degree specular gloss Gs(60°) within the above ranges.

[0050] Figure 2 is a laser microscope photograph (50x magnification) of the surface of (2) above. As shown in Figure 2, this stainless steel plate has a rough surface structure that appears to be a mixture of impact marks from shot blasting and dissolution marks from pickling. As a result, the arithmetic mean roughness Ra and root mean square slope RΔq tend to be large, and the 60-degree specular gloss Gs(60°) tends to be small. Figure 3 is a laser microscope photograph (50x magnification) of the surface of (3) above. As shown in Figure 3, this stainless steel sheet has a surface structure with a streaky pattern caused by belt polishing. Therefore, the aspect ratio Str of the texture tends to be small.

[0051] The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention having the above-mentioned characteristics has excellent corrosion resistance and can be used as a corrosion-resistant member. In particular, this ferritic-austenitic duplex stainless steel material has a smooth and glossy surface and is excellent in design, making it suitable for use in corrosion-resistant members that require design. [Example]

[0052] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0053] 30 kg of stainless steel having the composition of steel types A to E shown in Table 1 (the balance being Fe and impurities) was produced by vacuum melting, forged into a 30 mm thick billet, heated at 1230°C for 2 hours, and hot rolled to a thickness of 3 mm to obtain a hot-rolled steel plate (ferritic-austenitic duplex stainless steel plate). The hot-rolled steel plate was cut into pieces measuring 50 mm (rolling direction) x 50 mm (width direction) and used in the following examples and comparative examples.

[0054] [Table 1]

[0055] Example 1 A hot-rolled steel sheet having the composition of steel type A was subjected to a laser descaling process and an acid pickling descaling process in this order. The laser descaling process was carried out using a commercially available device (LaserClear50A manufactured by IHI Inspection & Measurement Co., Ltd.). The hot-rolled steel sheet was placed on the movable stage of this device, and while moving at 0.2 m / min along the rolling direction, the hot-rolled steel sheet was scanned from above in the width direction at a constant speed, irradiating it once with a pulsed laser. The scan width per scan was 25 mm. The pulsed laser irradiation conditions were as follows: Wavelength: 1085nm Pulse width: 100ns Oscillation frequency: 120kHz Scan frequency: 100Hz Laser beam diameter: 90 μm Fluence: 6J / cm 2 Pickling descaling was performed by immersing the hot-rolled steel sheet in a hydrofluoric-nitric acid aqueous solution containing 30 g / L of hydrofluoric acid and 60 g / L of nitric acid in a thermostatic bath kept at 60°C for 90 seconds, followed by immediate rinsing with running water and air drying.

[0056] Examples 2 to 5 The hot-rolled steel sheets used had the composition shown in Table 2, and the pulse laser fluence in the laser descaling process was set to 7 J / cm. 2 The rest of the procedure was the same as in Example 1.

[0057] (Comparative Example 1) Hot-rolled steel sheets having the composition of steel type A were subjected to pretreatment by bending and unbending with a bending radius of 50 mm using a scale breaker, and shot blasting with steel shot (SB-5), followed by a pickling descaling process. The pickling descaling process was carried out as follows. First, a hydrofluoric-nitric acid aqueous solution containing 50 g / L of hydrofluoric acid and 150 g / L of nitric acid was kept at 50°C in a thermostatic bath, and the hot-rolled steel sheet was immersed for 240 seconds, after which it was immediately rinsed with running water and air-dried. Next, a hydrofluoric-nitric acid aqueous solution containing 30 g / L of hydrofluoric acid and 60 g / L of nitric acid was kept at 60°C in a thermostatic bath, and the hot-rolled steel sheet was immersed for 90 seconds, after which it was immediately rinsed with running water and air-dried.

[0058] (Comparative Example 2) The hot-rolled steel sheet after the pickling and descaling process obtained in Comparative Example 1 was subjected to belt grinding using SiC abrasive paper (number #400) and water-soluble grinding oil. The grinding depth was 20 μm from the surface.

[0059] The hot-rolled steel sheets obtained in the above examples and comparative examples were evaluated as follows.

[0060] (Ferrite phase ratio) Test specimens were cut out from the above-mentioned hot-rolled steel sheets that had been subjected to the descaling process, and the amount of ferrite phase (α phase) was measured using a ferrite scope (FERITSCOPE (registered trademark) FMP30 manufactured by Fischer). The measurement was carried out at three random locations on the surface of the test specimen, and the average value was used as the result.

[0061] (surface roughness measurement) The surface of the above hot-rolled steel sheet that had been subjected to the descaling process was measured for arithmetic mean roughness Ra and root mean square slope RΔq in accordance with JIS B0601: 2013 using a contact-type surface roughness meter (Surfcom 2800 manufactured by Tokyo Seimitsu Co., Ltd.) The reference length for measuring the arithmetic mean roughness Ra was 4 mm. Similarly, the texture aspect ratio Str of the surface of the above-mentioned hot-rolled steel sheet that had been subjected to the descaling process was measured using a 3D measuring laser microscope (LEXT OLS4100 manufactured by Olympus Corporation) in accordance with JIS B0681-2:2018. The observation magnification during measurement was 50x, and the measurement range was 3 mm × 3 mm. The arithmetic mean roughness Ra, root mean square slope RΔq, and texture aspect ratio Str were measured at five points excluding the area within 5 mm from the edge, and the average values ​​were used as the evaluation results. Note that the measurement points were spaced at least 5 mm apart.

[0062] (Gloss measurement) The 60-degree specular gloss Gs(60°) of the surface of the above hot-rolled steel sheet that had been subjected to the descaling process was measured using a glossmeter (PG-1M manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS Z8741:1997. The 60-degree specular gloss Gs(60°) was measured at five locations excluding a range of 5 mm from the end, and the average value was used as the evaluation result. Note that the measurement locations were separated by at least 5 mm.

[0063] (Chromanetics Index b * ) The surface of the above hot-rolled steel sheet that had undergone the descaling process was measured for chromanetics index b using a spectrophotometer (CM-700d manufactured by Konica Minolta, Inc.) in accordance with JIS Z 8722:2009. *The geometric conditions for the measurement were c (di: 8°), the measurement diameter was 8 mmφ, the field of view was 10°, and D65 illuminant was used as the illumination light source. Measurements were taken at five points excluding the range of 5 mm from the edge, and the average value was used as the evaluation result.

[0064] (Corrosion resistance test) The corrosion resistance test was carried out by a salt-dry-wet cyclic test, which involved repeating salt spray, drying, and wetting. The salt-dry-wet cyclic test was carried out on the above-mentioned hot-rolled steel sheets that had been subjected to the descaling process, with 10 cycles consisting of spraying with a 5% NaCl aqueous solution (at 35°C for 15 minutes), drying (at 30% relative humidity and 60°C for 1 hour), and wetting (at 95% relative humidity and 50°C for 3 hours). The hot-rolled steel sheets were then rinsed and dried, and the rust area ratio of the hot-rolled steel sheets was calculated. The rust area ratio was calculated using the following procedure. The surface of the hot-rolled steel sheet after the salt-dry-wet cycle test was photographed, and the percentage of the rusted area in a central 25mm x 25mm area excluding the edges was determined. The rusted area was determined by binarizing the photograph of the hot-rolled steel sheet surface using image analysis, calculating the area per pixel, and then counting the number of pixels in the rusted area. The rust area ratio was calculated using the following formula. Rust area rate (%) = area of ​​rusted part (mm 2 ) / Total observation area (625mm 2 ) x 100 In this evaluation, a rust area rate of 1% or less was rated as "◯" (good corrosion resistance), and a rust area rate of more than 1% was rated as "×" (poor corrosion resistance). The results of the above evaluations are shown in Table 2.

[0065] [Table 2]

[0066] As shown in Table 2, the hot-rolled steel sheets of Examples 1 to 5 had a surface arithmetic mean roughness Ra of 0.10 to 3.00 μm and a 60-degree specular gloss Gs(60°) of 10 to 100%, confirming that they had smooth and glossy surfaces. The hot-rolled steel sheets of Examples 1 to 5 also had good corrosion resistance. In contrast, the hot-rolled steel sheet of Comparative Example 1 had a rough and dull surface with an arithmetic mean roughness Ra and a 60-degree specular gloss Gs(60°) outside the above ranges. In addition, the hot-rolled steel sheet of Comparative Example 2 was polished after the pickling descaling process, and therefore had insufficient corrosion resistance.

[0067] As can be seen from the above results, the present invention can provide a ferritic-austenitic duplex stainless steel material having a smooth and glossy surface and excellent corrosion resistance, as well as a corrosion-resistant component made from the same.

Claims

1. The composition contains, on a mass basis, C: 0.001 to 0.100%, Si: 1.00% or less, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 1.00 to 9.00%, Cr: 15.00 to 30.00%, Mo: 5.00% or less, Cu: 2.00% or less, N: 0.370% or less, and Al: 0.400% or less, with Si+2Al being less than 1.20%, and the balance being Fe and impurities; The arithmetic mean roughness Ra of the surface is 0.10 to 3.00 μm and the 60-degree specular gloss Gs (60°) is 10 to 100%, A ferritic-austenitic duplex stainless hot-rolled steel material in which, in a salt-dry-wet cycle test, the rust area ratio is 1% or less after 10 cycles of spraying a 5% NaCl aqueous solution (15 minutes at 35°C), drying (1 hour at 60°C and 30% relative humidity), and wetting (3 hours at 50°C and 95% relative humidity), respectively.

2. 2. The ferrite-austenite duplex hot-rolled stainless steel material according to claim 1, wherein the surface of the ferrite-austenite duplex hot-rolled stainless steel material satisfies the following (1) and (2): (1) The root mean square slope RΔq is 35° or less. (2) Chromanetics Index b * is 7.00 or less.

3. 3. The ferrite-austenite duplex stainless hot-rolled steel material according to claim 1, further comprising, on a mass basis, one or more selected from Ti: 0.001 to 0.500%, Nb: 0.001 to 1.000%, V: 0.001 to 1.000%, W: 0.001 to 1.000%, Zr: 0.001 to 1.000%, and Co: 0.001 to 1.200%.

4. The ferrite-austenite duplex stainless steel hot-rolled material according to any one of claims 1 to 3, further comprising, on a mass basis, one or more selected from Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0080%, and Sn: 0.001 to 0.500%.

5. A corrosion-resistant member comprising the ferrite-austenite duplex hot-rolled stainless steel material according to any one of claims 1 to 4.

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