Ferrite-austenitic duplex stainless steel material, its manufacturing method, and corrosion-resistant member

Abstract

To provide a ferrite-austenite two-phase system stainless steel material which has smooth and glossy surface and has excellent corrosion resistance and fatigue characteristics.SOLUTION: A ferrite-austenite two-phase system stainless steel material 10 has a composition comprising, by mass, 0.001 to 0.150% C, 0.20 to 5.00% Si, 6.00% or less Mn, 0.050% or less P, 0.0300% or less S, 1.00 to 9.00% Ni, 15.00 to 30.00% Cr, 5.00% or less Mo, 2.00% or less Cu, 0.400% or less N, and 3.500% or less Al, wherein Si+2Al is 1.20% or more, and the balance Fe and inevitable impurities. The ferrite-austenite two-phase system stainless steel material 10 has a ratio D2 / D1 of a number density D2 of an inclusion 13 on a surface B to a number density D1 of the inclusion 13 in a parent phase 11 at a position C where a depth from the surface B is 1 / 4 of the thickness of 0.50 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a ferritic-austenitic duplex stainless steel material, a method for producing the same, and a corrosion-resistant member. [Background technology]

[0002] Stainless steel materials have various excellent properties such as corrosion resistance, and are therefore used in a wide range of applications such as 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 through the following process. For example, hot-rolled steel plates (thick plates) are manufactured by continuously casting molten iron, which is made by dissolving stainless steel raw materials, into slabs, hot-rolling the slabs, and then annealing and pickling them. Cold-rolled steel plates (thin plates) are manufactured by cold-rolling the hot-rolled steel plates, followed by annealing and pickling them. In these stainless steel manufacturing processes, pickling is performed to remove oxide scale formed on the surface of the stainless steel material. Hereinafter, removing 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. Therefore, in a typical descaling process, mechanical pretreatment using a scale breaker or shot blasting is performed 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). Another known chemical pretreatment method is to immerse the stainless steel material in a salt bath. [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, conventional descaling processes using pickling result in a whitening of the stainless steel surface due to the surface irregularities created by pretreatment and the surface roughness caused by pickling, resulting in a loss of luster and a reduction in design appeal. Cold-rolled steel requires a higher level of design appeal (smoothness and gloss) than hot-rolled steel, making it difficult to achieve the desired design appeal using conventional descaling processes using pickling. Furthermore, the thick oxide scale formed on the surface of ferritic-austenitic duplex stainless steels makes it difficult to create cracks through mechanical pretreatment and prone to uneven pickling. Furthermore, for stainless steels with high Si and Al contents, the internal oxide layer containing SiO2 and Al2O3 formed at the interface between the oxide scale and the stainless steel is chemically stable, making oxide scale removal even more difficult. Furthermore, removing the oxide scale exposes inclusions on the surface of stainless steel, which can reduce the corrosion resistance and fatigue properties of the stainless steel. To solve these problems, it is conceivable to polish the surface after the descaling process. However, polishing the surface until it is smooth increases the amount of grinding required, reducing yield, and also reduces corrosion resistance and design properties due to polishing burns and entrapment of polishing debris. Furthermore, since inclusions are also present inside the stainless steel material, polishing the surface after the descaling process will only expose new inclusions on the surface of the stainless steel material. Therefore, this method cannot be said to be effective.

[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 fatigue properties, a method for manufacturing the same, 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 irradiating it with laser light under specific conditions, it is possible to descale the material while maintaining surface smoothness and gloss, and to dissolve surface inclusions, thereby improving corrosion resistance and fatigue properties. Based on this discovery, the inventors produced and investigated various ferritic-austenitic duplex stainless steel materials. As a result, they discovered that a ferritic-austenitic duplex stainless steel material having a specified composition and a specific range of the ratio D2 / D1 of the number density D2 of inclusions at the surface to the number density D1 of a specified inclusion in the matrix at a depth of one-quarter of the thickness from the surface can solve the above-mentioned problems, thereby completing 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 0.150%, Si: 0.20 to 5.00%, 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: 0.400% or less, Al: 3.500% or less, Si+2Al being 1.20% or more, and the balance being Fe and impurities; The ratio D2 / D1 of the number density D2 of inclusions on the surface to the number density D1 of inclusions having a circle equivalent diameter of 0.5 to 10 μm in the matrix at a position where the depth from the surface is 1 / 4 of the thickness is 0.50 or less. and the number density D2 of the inclusions on the surface is 75 pieces / mm 2 Below is the It is a ferritic-austenitic duplex stainless steel material.

[0009] The present invention also provides a method for producing the ferritic-austenitic duplex stainless steel material, comprising the steps of: By mass, The aforementioned a descaling step of irradiating a cold-rolled steel material having a composition with a laser beam to remove oxide scale formed on the surface of the cold-rolled steel material, The laser light irradiation is a method carried out under conditions that allow melting of a region from the interface between the parent phase and the oxide scale in the cold-rolled steel material to a depth of 0.50 to 10.00 μm.

[0010] Furthermore, the present invention is a corrosion-resistant member comprising the ferritic-austenitic duplex stainless steel material. [Effects of the Invention]

[0011] 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 fatigue properties, a method for producing the same, and a corrosion-resistant component using the same. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic cross-sectional view of a ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention. [Figure 2] 10A and 10B are schematic cross-sectional views for explaining the difference between descaling by laser light irradiation and descaling by a conventional method. DETAILED DESCRIPTION OF THE INVENTION

[0013] 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.

[0014] A ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention has a composition containing C: 0.001 to 0.150%, Si: 0.20 to 5.00%, 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: 0.400% or less, and Al: 3.500% or less, with Si+2Al being 1.20% or more, 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 shape include plate (including strip), rod, and tube. The cross-sectional shape may also be various shaped steels such as T-shaped and I-shaped. 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.

[0015] 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 ferritic-austenitic duplex stainless steel material according to an 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%, Sn: 0.001 to 0.500%, and REM: 0.200% or less. Each component will be described in detail below.

[0016] <C:0.001~0.150%> If the C content is too high, not only will it become hard and the workability will decrease, but sensitization will occur when it is affected by heat such as welding, and the corrosion resistance of the ferritic-austenitic duplex stainless steel will decrease. Therefore, the upper limit of the C content is controlled to 0.150%, preferably 0.100%, more preferably 0.080%, and still more preferably 0.060%. On the other hand, if the C content is too low, it will lead to deterioration of workability due to a decrease in the stability of the austenite phase and an increase in refining costs. Therefore, the lower limit of the C content is controlled to 0.001%, preferably 0.002%, more preferably 0.005%, and still more preferably 0.010%.

[0017] <Si: 0.20 - 5.00%> If the Si content is too high, the workability of the ferritic-austenitic duplex stainless steel will decrease due to hardening. Therefore, the upper limit of the Si content is controlled to 5.00%, preferably 4.00%, more preferably 3.80%, and still more preferably 3.50%. On the other hand, if the Si content is too low, the heat resistance of the ferritic-austenitic duplex stainless steel will decrease. Therefore, the lower limit of the Si content is controlled to 0.20%, preferably 1.00%, more preferably 1.50%, and still more preferably 2.5%.

[0018] <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 the 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%.

[0019] <P: 0.050% or less> If the content of P is too high, the corrosion resistance and workability of the ferrite-austenite duplex stainless steel will deteriorate. Therefore, the upper limit value of the content of P is controlled to 0.050%, preferably 0.030%, more preferably 0.020%, and still more preferably 0.010%. On the other hand, the lower limit value of the content of P is not particularly limited, but is preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%.

[0020] <S: 0.0300% or less> If the content of S is too high, the hot workability will decrease, resulting in a reduction in the manufacturability of the ferrite-austenite duplex stainless steel and having an adverse effect on the corrosion resistance. Therefore, the upper limit value of the content of S is controlled to 0.0३00%, preferably 0.0100%, more preferably 0.0050%, and still more preferably 0.0010%. On the other hand, the lower limit value of the content of S is not particularly limited, but is preferably 0.0001%, and more preferably 0.0002%.

[0021] <Ni: ۱.00 - 9.00%> Ni, like Mn, is an austenite phase (γ phase) forming element. Since Ni is expensive, if the content is too high, it will lead to an increase in the manufacturing cost. Therefore, the upper limit value of the content of Ni 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 value of the content of Ni is preferably 1.00%, more preferably 2.00%, still more preferably 5.00%, and still more preferably 5.50%.

[0022] <Cr: 15.00 - 30.00%> If the Cr content is too high, it will lead to an increase in refining costs, harden due to solid solution strengthening, and reduce the workability of ferritic-austenitic duplex stainless steel. Therefore, the upper limit 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 of the Cr content is controlled to 15.00%, preferably 18.00%, more preferably 20.00%, and still more preferably 22.00%.

[0023] <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 costs. Therefore, the upper limit 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 of the Mo content is not particularly limited, but preferably 0.10%, more preferably 0.50%, and still more preferably 1.00%.

[0024] <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 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 of the Cu content is not particularly limited, but preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%.

[0025] <N: 0.400% 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 the ferritic-austenitic duplex stainless steel. Therefore, the upper limit value of the content of N is controlled to 0.400%, preferably 0.300%, more preferably 0.280%, and still 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 0.01%, preferably 0.05%, and more preferably 0.10%.

[0026] <Al: 3.500% 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 3.500%, preferably 3.000%, more preferably 2.000%, and still more preferably 1.500%. On the other hand, the lower limit value of the content of Al is not particularly limited, but is preferably 0.001%, more preferably 0.010%, and still more preferably 0.100%.

[0027] <Si + 2Al: 1.20% or more> The ferritic-austenitic duplex stainless steel according to an embodiment of the present invention targets those with high contents of Si and Al. Specifically, Si + 2Al (each element symbol represents the content of each element) is 1.20% or more, preferably 1.30% or more. The upper limit value of Si + 2Al is not particularly limited, but is preferably 10.00%, more preferably 8.00%, and still more preferably 5.00%.

[0028] <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%.

[0029] <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%.

[0030] <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%.

[0031] <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%.

[0032] <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 will decrease. Therefore, the upper limit of the Zr content is controlled to 1.000%, preferably 0.200%, more preferably 0.050%.

[0033] <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%.

[0034] <Ca:0.0001~0.0100%> Ca is an element that improves the hot workability of ferritic-austenitic duplex stainless steel materials and is added as necessary. Also, Ca is an element that improves the intergranular oxidation resistance by forming sulfides and suppressing the grain boundary segregation of S. From the viewpoint of obtaining the effect of Ca, the lower limit value of the Ca content is controlled to 0.0001%, preferably 0.0003%. On the other hand, if the Ca content is too high, the number of inclusions increases, leading to a decrease in workability. Therefore, the upper limit value of the Ca content is controlled to 0.0100%, preferably 0.0050%.

[0035] <B: 0.0001~0.0080%> B is an element that improves the hot workability of ferritic-austenitic duplex stainless steel materials and is added as necessary. Also, B is an element that improves the secondary workability of ferritic-austenitic duplex stainless steel materials by grain boundary strengthening. From the viewpoint of obtaining the effect of B, the lower limit value of the B content is controlled to 0.0001%, preferably 0.0003%, more preferably 0.0005%. On the other hand, if the B content is too high, it causes a decrease in weldability and fatigue strength. Therefore, the upper limit value of the B content is controlled to 0.0080%, preferably 0.0040%, more preferably 0.0025%.

[0036] <Sn: 0.001~0.500%> Sn is an element that improves corrosion resistance and high-temperature strength and is added as necessary. From the viewpoint of obtaining the effect of Sn, the lower limit value of the Sn content is controlled to 0.001%, preferably 0.002%. On the other hand, if the Sn content is too high, it forms a low melting point phase and the hot workability of ferritic-austenitic duplex stainless steel materials deteriorates. Therefore, the upper limit value of the Sn content is controlled to 0.500%, preferably 0.100%, more preferably 0.050%.

[0037] <REM: 0.200% or less> Like B and Ca, REM (rare earth elements) are elements that improve the hot workability of ferritic-austenitic duplex stainless steels and are added as needed. REM also improves corrosion resistance by forming sulfides that are difficult to dissolve and suppressing the formation of MnS, which acts as a corrosion starting point. However, too much REM content increases manufacturing costs. Therefore, the upper limit of the REM content is controlled to 0.200%, preferably 0.100%. While the lower limit of the REM content is not particularly limited, it is preferably 0.001%, more preferably 0.010%, in order to obtain the effects of REM. REM is a collective term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). These may be used alone or as a mixture.

[0038] In a ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention, the ratio D2 / D1 of the number density D2 of inclusions having an equivalent circle diameter of 0.5 to 10 μm at the surface (e.g., surface B in FIG. 1 ) to the number density D1 of inclusions having an equivalent circle diameter of 0.5 to 10 μm in the matrix (e.g., position C in FIG. 1 ) at a depth of one-quarter of the thickness from the surface (e.g., surface B in FIG. 1 ) is 0.50 or less, preferably 0.49 or less, and more preferably 0.48 or less. By controlling the inclusion number density ratio D2 / D1 within this range, the proportion of inclusions present at the surface can be reduced compared to inclusions present in the matrix, thereby improving the corrosion resistance and fatigue properties of the ferritic-austenitic duplex stainless steel material. The lower limit of the ratio D2 / D1 of the number densities of inclusions is not particularly limited because the smaller the ratio, the greater the effect of improving corrosion resistance and fatigue properties. For example, the lower limit is 0.01.

[0039] The number density D1 of inclusions with a circle-equivalent diameter of 0.5 to 10 μm in the matrix at a depth of 1 / 4 of the thickness from the surface of a ferritic-austenitic duplex stainless steel material (hereinafter referred to as "number density D1 of inclusions in the matrix") can be calculated as follows. First, arbitrary locations (10 or more fields of view) at a depth of 1 / 4 of the thickness from the surface of the ferritic-austenitic duplex stainless steel material are photographed using an electron microscope. Next, the photographed images are binarized to separate inclusions with a circle-equivalent diameter of 0.5 to 10 μm (black) from the matrix (white). The black areas are counted to determine the number of inclusions. The number of inclusions thus obtained is divided by the area of ​​the observation field to calculate the number density D1 of inclusions in the matrix. Similarly, the number density D2 of inclusions with an equivalent circle diameter of 0.5 to 10 μm on the surface of a ferritic-austenitic duplex stainless steel material (hereinafter referred to as "number density D2 of inclusions on the surface") can be calculated by photographing any location (10 or more fields of view) on the surface of the ferritic-austenitic duplex stainless steel material with an electron microscope and dividing the number of inclusions obtained in the same manner as above by the area of ​​the observed fields of view. Here, since most of the inclusions have an equivalent circle diameter in the range of 0.5 to 10 μm, the number of inclusions in this size range was counted and defined as the number densities D1 and D2 of the inclusions. The equivalent circle diameter means the diameter of a circle when the area of ​​each observed black region is converted into a circle having the same area.

[0040] The electron microscope used was a Schottky scanning electron microscope SU5000 manufactured by Hitachi High-Tech Corporation. The inclusions were identified using an EDX detector EMAX3.3SP2 manufactured by Oxford Instruments, which was attached to the electron microscope. The image processing software (AZtecSteel) manufactured by Oxford Instruments, was used.

[0041] The number density D1 of the inclusions in the matrix is, for example, 80 to 300 pieces / mm 2 That's about it.

[0042] The number density D2 of the inclusions on the surface is preferably 75 pieces / mm 2 Less than or equal to 60 pieces / mm 2 By controlling the number density D2 of the surface inclusions within this range, the amount of inclusions present on the surface of the ferritic-austenitic duplex stainless steel material can be reduced, thereby enabling stable improvements in the corrosion resistance and fatigue strength of the ferritic-austenitic duplex stainless steel material. The lower limit of the number density D2 of the inclusions on the surface is not particularly limited because the smaller the value, the greater the effect of improving corrosion resistance and fatigue properties. For example, 2 is.

[0043] The inclusions are non-metallic inclusions (especially sulfide-based and oxide-based inclusions), and specific examples thereof include MnS, TiN, TiC, NbN, NbC, CaO, SiO2, Al2O3, and complex compounds thereof.

[0044] The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention preferably has a molten and solidified layer on the surface. As an example, a schematic cross-sectional view of a ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention is shown in FIG. As shown in Figure 1, a ferritic-austenitic duplex stainless steel material 10 comprises a matrix 11 and a molten and solidified layer 12 formed on a surface A of the matrix 11. The matrix 11 contains inclusions 13. The molten and solidified layer 12 may also contain inclusions 13, but there are few inclusions 13 exposed on a surface B of the molten and solidified layer 12. This is because the inclusions 13 exposed on the surface B have been solid-dissolved by descaling with a laser beam.

[0045] The thickness of the molten solidified layer is not particularly limited, but is preferably 0.50 to 10.00 μm, more preferably 1.00 to 10.00 μm, and even more preferably 2.00 to 10.00 μm. If the thickness of the molten solidified layer is less than 0.50 μm, the inclusions are likely to be insufficiently solid-dissolved. Therefore, the inclusions exposed on the surface increase, and the corrosion resistance and fatigue properties of the ferritic-austenitic duplex stainless steel material tend to deteriorate. On the other hand, if the thickness of the molten solidified layer exceeds 10.00 μm, the surface becomes rough, making it difficult to obtain a smooth and glossy surface.

[0046] The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention preferably has a surface arithmetic mean roughness Ra of 0.01 to 0.40 μm, more preferably 0.01 to 0.30 μm, and even more preferably 0.01 to 0.20 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.

[0047] The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention has a surface 60-degree specular gloss Gs(60°) of preferably 200 to 750%, more preferably 300 to 750%, and even more preferably 400 to 750%. By controlling the surface 60-degree specular gloss Gs(60°) 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.

[0048] The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention is preferably a ferritic-austenitic duplex cold-rolled stainless steel material, and more preferably a ferritic-austenitic duplex cold-rolled stainless steel sheet. In this specification, the cold-rolled steel material and cold-rolled steel sheet after the oxide scale has been removed are referred to as "ferritic-austenitic duplex cold-rolled stainless steel material" and "ferritic-austenitic duplex cold-rolled stainless steel sheet," and the cold-rolled steel material and cold-rolled steel sheet before the oxide scale has been removed are referred to as "cold-rolled steel material" and "cold-rolled steel sheet." Hereinafter, a case will be described in which the ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention is a ferritic-austenitic duplex cold-rolled stainless steel sheet. The thickness (sheet thickness) of the ferritic-austenitic duplex stainless cold rolled steel sheet according to one embodiment of the present invention is not particularly limited, but is preferably less than 3 mm, more preferably 2.5 mm or less, and even more preferably 2 mm or less. The lower limit is, for example, 0.1 mm, and may be 0.3 mm.

[0049] The ferritic-austenitic dual-phase stainless steel cold-rolled sheet according to one embodiment of the present invention can be produced by a method known in the art, except for a descaling step in which a cold-rolled steel sheet having the above-described composition is irradiated with laser light to remove oxide scale formed on the surface.

[0050] The method for producing a cold-rolled steel sheet having the above composition is not particularly limited, but it can be produced, for example, as follows. First, stainless steel having the above composition is melted and forged or cast to obtain a steel slab. Next, the steel slab is hot-rolled, annealed, and then descaled. The descaling method is not particularly limited, and can be carried out using pickling, polishing, or laser light. Next, the descaled hot-rolled steel sheet is cold-rolled and annealed to obtain a cold-rolled steel sheet. The conditions for each step are not particularly limited, and can be adjusted appropriately depending on the composition of the stainless steel, etc.

[0051] The cold-rolled steel sheet is irradiated with laser light under conditions that allow melting of the region from the interface between the matrix and oxide scale in the cold-rolled steel sheet to a depth of 0.50 to 10.00 μm, preferably 1.00 to 10.00 μm, and more preferably 2.00 to 10.00 μm. Irradiation with laser light under these conditions removes oxide scale while maintaining surface smoothness and gloss, and thermally dissolves surface inclusions to form a molten, solidified layer on the surface, improving corrosion resistance and fatigue properties. If the irradiation conditions allow melting only to a region less than 0.50 μm deep from the surface of the cold-rolled steel sheet, the inclusions are not sufficiently dissolved, and the inclusions exposed on the surface cannot be sufficiently reduced, resulting in a deterioration in the corrosion resistance and fatigue properties of the ferritic-austenitic duplex stainless cold-rolled steel sheet. Furthermore, if the irradiation conditions allow melting of the region from the surface of the cold-rolled steel sheet to a depth of more than 10.00 μm, the surface of the ferritic-austenitic duplex stainless cold-rolled steel sheet becomes rough, preventing the achievement of a smooth, glossy surface.

[0052] Here, a schematic cross-sectional view is shown in FIG. 2 to explain the difference between descaling by laser light irradiation and descaling by a conventional method. As shown in Fig. 2(a), a cold-rolled steel material (cold-rolled steel sheet) 20 has an oxide scale 21 formed on the surface of a matrix 11. In addition, inclusions 13 are present at an interface D between the matrix 11 and the oxide scale 21 (on the surface of the matrix 11). When descaling (removal of oxide scale 21) is performed by conventional methods (pickling and / or polishing), inclusions 13 present at the interface D between the matrix 11 and the oxide scale 21 are exposed at the surface E, as shown in FIG. 2(b). Furthermore, even if the surface layer of the matrix 11 is removed along with the oxide scale 21 in order to remove the inclusions 13 present at the interface D between the matrix 11 and the oxide scale 21, the inclusions 13 present inside the matrix 11 end up being exposed at the surface E. Therefore, it is difficult to reduce the inclusions 13 exposed at the surface E using conventional methods.

[0053] In contrast, when descaling is performed by irradiating a laser beam, as shown in FIG. 2(c), the oxide scale 21 is removed and the inclusions 13 present at the interface D between the matrix 11 and the oxide scale 21 are dissolved. As a result, a molten solidified layer 12 is formed on the surface of the matrix 11, reducing the number of inclusions 13 exposed at the surface B. The reduction in the number of inclusions 13 occurs because the inclusions 13 near the surface melt when the molten solidified layer 12 is formed, become diluted with matrix components, and rapidly solidify before reprecipitation. In addition, the increase in temperature also causes the inclusions 13 to dissolve in the areas near the molten solidified layer 12 that were not directly melted, resulting in a reduction in the size and number of the inclusions 13. Even if the inclusions are not completely melted and rendered harmless, partial melting reduces the circle-equivalent diameter of the inclusions 13, improving corrosion resistance and fatigue strength.

[0054] The conditions for the laser irradiation may be adjusted depending on the device used, taking into consideration the following points. (Type of laser light) When continuous wave laser light is used, the energy required to remove oxide scale increases, the required power increases, and the range in which heat is generated is too large, making it difficult to control the thickness of the molten and solidified layer. Therefore, pulsed laser light, which can apply heat instantaneously, is preferred. (wavelength) Generally, the reflectivity of a material to light is wavelength-dependent, and selecting a wavelength with low reflectivity increases the heat input, making it easier for oxide scale to evaporate. Therefore, by selecting a wavelength with high reflectivity for the parent phase and low reflectivity for the oxide, it is possible to selectively evaporate and remove oxide scale without melting the parent phase more than necessary. (pulse width) The pulse width represents the time that one pulse is irradiated, and the narrower the pulse width, the more instantaneous heating occurs. With a narrow pulse width, ablation occurs before the heat input by the laser is transmitted to the surrounding area, lowering the ablation threshold and reducing the thermal impact on the parent phase. When laser irradiation is performed under conditions that cause the parent phase to melt, the narrower the pulse width, the more rapid cooling occurs and the more suppressed the reprecipitation of molten inclusions. However, the pulse width is primarily determined by the performance of the oscillator, and since devices capable of oscillating with short pulse widths are expensive, it is preferable to select a short pulse width within the specifications of the device. (oscillation frequency) The shorter the pulse width, the higher the oscillation frequency can be, and the higher the oscillation frequency, the more pulses are emitted per unit time, improving the rate at which oxide scale is removed. Therefore, it is preferable to select a high oscillation frequency within the specifications of the device.

[0055] (scan frequency) The scanning frequency represents the speed at which the pulse irradiation position moves in the planar direction. The higher the scanning frequency, the faster the oxide scale removal rate. However, if the scanning frequency is too high, gaps will form between the pulse irradiation positions, leaving oxide scale behind and reducing the descaling rate. Therefore, it is preferable to increase the scanning frequency within a range that allows the descaling rate to 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 oxide scale. (Fluence) Oxide scale can be vaporized and removed by irradiating it with laser light with a fluence exceeding the ablation threshold of the oxides that make up the oxide scale. The higher the fluence, the thicker the oxide scale that can be removed; however, if the fluence is too high, not only the oxide scale but also the parent phase will be vaporized and removed. Pulsed lasers have less thermal impact than continuous-wave lasers, but the higher the fluence, the greater the heat input to the parent phase, resulting in larger melted and heat-affected zones. Therefore, the fluence can be adjusted within a range that does not melt the parent phase more than necessary, taking into account the balance between the characteristics of the oxide scale to be removed (thickness, structure, composition, etc.) and the heat input to the parent phase. If the fluence distribution within the beam spot varies, the average fluence can be used for control.

[0056] The ferritic-austenitic duplex stainless steel material according to the embodiment of the present invention, having the above-described characteristics, has excellent corrosion resistance and can be used as a corrosion-resistant member. Furthermore, since this ferritic-austenitic duplex stainless steel material also has excellent fatigue properties, it is suitable for use in corrosion-resistant members that require fatigue properties. Furthermore, since this ferritic-austenitic duplex stainless steel material has a smooth and glossy surface and excellent designability, it is suitable for use in corrosion-resistant members that require designability.

[0057] A corrosion-resistant member according to an embodiment of the present invention includes the above-described ferritic-austenitic duplex stainless steel material. The ferritic-austenitic duplex stainless steel material used for this corrosion-resistant member may be processed into various shapes by methods known in the art. The corrosion-resistant member according to the embodiment of the present invention may further include members other than the above-mentioned ferritic-austenitic duplex stainless steel material. Examples of corrosion-resistant members include, but are not limited to, automobile parts, building parts, kitchen utensils, and the like. [Example]

[0058] 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.

[0059] 30 kg of stainless steel having the composition of steel grades A to E shown in Table 1 (the remainder being Fe and impurities) was vacuum melted and forged into a 30 mm thick billet, which was then heated at 1230°C for 2 hours, hot rolled to a thickness of 3 mm, and annealed at 1100°C for 5 minutes in air to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was cut into a 50 mm (rolling direction) x 50 mm (width direction) by processing and then subjected to pickling and descaling. The pickling and descaling was performed by immersing the hot-rolled annealed sheet in a hydrofluoric-nitric acid aqueous solution containing 50 g / L of hydrofluoric acid and 150 g / L of nitric acid in a constant-temperature bath held at 60°C for 240 to 1320 seconds, followed by immediate rinsing with running water and air drying. The specific immersion times were 240 seconds for steel type A, 1200 seconds for steel type B, 1320 seconds for steel type C, 1200 seconds for steel type D, and 1200 seconds for steel type E. The hot-rolled steel sheets were then cold-rolled from 3 mm to 1 mm in thickness and annealed at 1100°C for 5 minutes in an air atmosphere to obtain cold-rolled steel sheets. The obtained cold-rolled steel sheets were used in the following examples and comparative examples.

[0060] [Table 1]

[0061] Examples 1 to 5 Cold-rolled steel sheets having the various steel compositions were subjected to a descaling process by irradiating them with laser light. The laser beam was irradiated using a commercially available device (LaserClear50A manufactured by IHI Inspection & Measurement Co., Ltd.). The cold-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 pulsed laser beam was scanned from above the cold-rolled steel sheet in the sheet width direction at a constant speed, irradiating it once. The scan width per scan was 25 mm. The pulsed laser beam irradiation conditions were as follows: Wavelength: 1085nm Pulse width: 220ns Oscillation frequency: 60kHz Scan frequency: 100Hz Laser beam diameter: 90 μm Average fluence: 8J / cm 2

[0062] (Comparative Example 1) A cold-rolled steel sheet having the composition of steel type A was subjected to a descaling process by pickling. The pickling was carried out as follows: A hydrofluoric acid / nitric acid aqueous solution containing 30 g / L of hydrofluoric acid and 100 g / L of nitric acid was kept at 60°C in a thermostatic bath, and the cold-rolled steel sheet was immersed for 600 seconds, after which it was immediately rinsed with running water and air-dried.

[0063] (Comparative Example 2) The cold-rolled steel sheet after the descaling process obtained in Comparative Example 1 was subjected to belt grinding using SiC abrasive paper (grit #400) and water-soluble grinding oil. The grinding depth was 20 μm from the surface.

[0064] The cold-rolled steel sheets (ferritic-austenitic dual-phase stainless steel cold-rolled sheets) obtained in the above examples and comparative examples after the descaling process were evaluated as follows.

[0065] (Inclusion density D1, D2) Test pieces measuring 50 mm square were cut from the center of the width and length of the ferritic-austenitic duplex stainless cold-rolled steel sheets obtained in the above examples and comparative examples, and random locations on the surface were photographed at 200x magnification using a Schottky scanning electron microscope SU5000 manufactured by Hitachi High-Tech Corporation. The surface of this test piece was also polished to remove 1 / 4 of the thickness (250 μm), exposing the parent phase at a depth of 1 / 4 of the thickness from the surface, and then random locations on this exposed surface were photographed in the same manner as above. The photographs were taken in a 0.48 mm × 0.64 mm (0.3072 mm) area. 2 ) was considered as one field of view, and 10 fields of view were analyzed. Next, using analysis software (AZtecSteel) manufactured by Oxford Instruments, the captured images were binarized and separated into inclusions (black) with a circle equivalent diameter of 0.5 to 10 μm and the matrix (white). The image analysis conditions were as follows: Resolution: 4096 Minimum detection size: 8 pixels (0.5 μm) Inclusion color threshold: 47~24057 Anything less than 8 pixels was excluded as noise. Next, the number of inclusions was determined by counting black areas with a circle-equivalent diameter of 0.5 to 10 μm in the binarized image, and the number of inclusions obtained was divided by the area of ​​the observation field to calculate the number densities D1 and D2 of the inclusions. The results of the number densities D1 and D2 of the inclusions were taken as the average values ​​of the results in each field of view. Furthermore, based on the calculated number densities D1 and D2 of the inclusions, the ratio of the number densities of the inclusions D2 / D1 was calculated.

[0066] (Thickness of the melted and solidified layer) A ferritic-austenitic dual-phase stainless steel cold-rolled sheet was cut in the thickness direction (direction perpendicular to the surface) parallel to the rolling direction, and the compositional image of the cut surface was observed using an electron microscope up to 10,000 times. The melted and solidified layer was identified based on the difference in contrast, and its thickness was measured. The thickness was measured at 10 random locations, and the average value was used as the result.

[0067] (surface roughness measurement) The arithmetic mean roughness (Ra) of the surface of the ferritic-austenitic duplex stainless steel cold-rolled steel sheet was measured using a contact surface roughness meter (Surfcom 2800, manufactured by Tokyo Seimitsu Co., Ltd.) in accordance with JIS B0601:2013. The arithmetic mean roughness (Ra) was measured at five locations, excluding the area from the edge to 5 mm, over a reference length of 4 mm, and the average value was used as the evaluation result. The measurement locations were spaced at least 5 mm apart.

[0068] (Gloss measurement) The 60° specular gloss Gs(60°) of the surface of ferritic-austenitic cold-rolled stainless steel sheets was measured using a glossmeter (PG-1M, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS Z8741:1997. Measurements of the 60° specular gloss Gs(60°) were taken at five locations, excluding the area within 5 mm from the edge, and the average value was used as the evaluation result. The measurement locations were at least 5 mm apart.

[0069] (Corrosion resistance test) Corrosion resistance tests were conducted in accordance with JASO M609 and M610. Specifically, a salt-dry-wet cycle test was performed, in which salt spray, drying, and wetting were repeated. Ferritic-austenitic duplex stainless steel cold-rolled steel sheets were subjected to a cycle of spraying a 5% NaCl solution (2 hours at 35°C), drying (4 hours at 30% relative humidity and 60°C), and wetting (2 hours at 95% relative humidity and 50°C). After each cycle, the steel sheets were rinsed with water and dried. The surfaces of the ferritic-austenitic duplex stainless steel cold-rolled steel sheets were then inspected and the rust area ratio calculated. The number of cycles at which the rust area ratio reached 10% or more was defined as the number of cycles at which corrosion occurred. The rust area ratio was calculated using the following procedure. After the salt-dry-wet cycle test, the surface of the ferritic-austenitic duplex stainless cold-rolled steel sheet was photographed, and the percentage of the area of ​​the rusted area within a 25mm x 25mm central area, excluding the edges, was determined. The rust area was determined by binarizing the surface photograph of the ferritic-austenitic duplex stainless cold-rolled steel sheet 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, the improvement rate in the number of cycles to corrosion occurrence for the ferritic-austenitic duplex stainless cold-rolled steel sheet of Comparative Example 2 was used as the standard, and the improvement rate in the number of cycles to corrosion occurrence for the ferritic-austenitic duplex stainless cold-rolled steel sheets of Examples 1 to 5 and Comparative Example 1 was calculated. A calculated improvement rate in the number of cycles to corrosion occurrence of 20% or more was marked "Good," a rate of 10% or more but less than 20% was marked "Good," and a rate of less than 10% was marked "Poor."

[0070] (Fatigue property test) The fatigue property test was conducted in accordance with JIS Z2275:1978, using a plane bending fatigue test. Specifically, a test piece was cut from a ferritic-austenitic dual-phase stainless steel cold-rolled sheet, measuring 30 mm in width and 90 mm in rolling direction, and a 30 mm radius rounded section was formed on both ends of the width direction to obtain a test piece. This test piece was attached to a plane bending tester and subjected to 10 repetitions. 7 The fatigue test was carried out using two or more test pieces for each stress step, and the time strength was measured. In this evaluation, the improvement rates of the long-term strength of the ferritic-austenitic duplex stainless cold-rolled steel sheets of Comparative Example 2 were calculated based on the fatigue strength of the ferritic-austenitic duplex stainless cold-rolled steel sheet of Comparative Example 2. A calculated improvement rate of long-term strength of 10% or more was marked "Good", and one less than 10% was marked "Poor". The results of the above evaluations are shown in Table 2.

[0071] [Table 2]

[0072] As shown in Table 2, the ferritic-austenite duplex stainless cold-rolled steel sheets of Examples 1 to 5, which had an inclusion number density ratio D2 / D1 of 0.50 or less, had a surface arithmetic mean roughness Ra of 0.01 to 0.40 μm and a 60-degree specular gloss Gs(60°) of 200 to 750%, confirming that they had smooth and glossy surfaces. Furthermore, the ferritic-austenite duplex stainless cold-rolled steel sheets of Examples 1 to 5 had improved corrosion resistance and fatigue properties compared to the ferritic-austenite duplex stainless cold-rolled steel sheet of Comparative Example 2. In contrast, the ferritic-austenitic duplex stainless cold-rolled steel sheet of Comparative Example 1 was descaled by pickling, resulting in an inclusion number density ratio D2 / D1 exceeding 0.50. As a result, the ferritic-austenitic duplex stainless cold-rolled steel sheet of Comparative Example 1 did not exhibit sufficient improvements in corrosion resistance and fatigue properties compared to the ferritic-austenitic duplex stainless cold-rolled steel sheet of Comparative Example 2.

[0073] 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 and fatigue properties, a method for manufacturing the same, and a corrosion-resistant component made from the same. [Explanation of symbols]

[0074] 10 Ferrite-austenitic duplex stainless steel 11 Mother phase 12 Melted and solidified layer 13 Inclusions 20 Cold rolled steel 21 Oxide scale

Claims

1. The composition contains, on a mass basis, C: 0.001 to 0.150%, Si: 0.20 to 5.00%, 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: 0.400% or less, and Al: 3.500% or less, with Si+2Al being 1.20% or more, and the balance being Fe and impurities; The ratio D2 / D1 of the number density D2 of inclusions on the surface to the number density D1 of inclusions having a circle equivalent diameter of 0.5 to 10 μm in the parent phase at a position where the depth from the surface is 1 / 4 of the thickness is 0.50 or less, and the number density D2 of the inclusions on the surface is 75 pieces / mm 2 The following is a ferritic-austenitic duplex stainless steel material.

2. The ferritic-austenite duplex stainless 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%.

3. The ferritic-austenitic duplex stainless steel material according to claim 1 or 2, further comprising, on a mass basis, one or more selected from Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0080%, Sn: 0.001 to 0.500%, and REM: 0.200% or less.

4. The ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 3, wherein the arithmetic mean roughness Ra of the surface is 0.01 to 0.40 µm.

5. The ferrite-austenitic duplex stainless steel material according to any one of claims 1 to 4, wherein the 60-degree specular gloss Gs (60 °) of the surface is 200 to 750%.

6. The ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 5, which has a molten and solidified layer on the surface.

7. The ferritic-austenitic duplex stainless steel material according to claim 6, wherein the thickness of the molten and solidified layer is 0.50 to 10.00 μm.

8. A method for producing the ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 3, comprising: A descaling process includes irradiating a cold-rolled steel material having a composition according to any one of claims 1 to 3 with laser light to remove oxide scale formed on the surface of the cold-rolled steel material, The laser beam is irradiated under conditions that allow the region of the cold-rolled steel material to melt from the interface between the parent phase and the oxide scale to a depth of 0.50 to 10.00 μm.

9. A corrosion-resistant member comprising the ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 7.

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