Ferritic stainless steel material, method for producing the same, and corrosion-resistant member

Abstract

To provide a ferritic stainless steel material which has smooth and glossy surface and has excellent corrosion resistance and fatigue characteristics.SOLUTION: A ferritic stainless steel material 10 has a composition comprising, by mass, 0.001 to 0.150% C, 0.20 to 5.00% Si, 2.00% or less Mn, 0.050% or less P, 0.0300% or less S, less than 2.00% Ni, 11.00 to 30.00% Cr, 6.00% or less Mo, 0.60% or less Cu, 0.050% 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 austenitic 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 stainless steel material, a method for manufacturing the same, and a corrosion-resistant member.

Background Art

[0002] Since stainless steel materials are excellent in various properties such as corrosion resistance, they are used in a wide range of applications such as automotive parts, architectural parts, and kitchen utensils. Stainless steel materials are classified into steel sheets, bars, steel strips, bars, steel pipes, etc. according to their shapes. A stainless steel sheet, which is a general stainless steel material, is manufactured by the following steps. For example, a hot-rolled steel sheet (thick plate) is manufactured by continuously casting molten iron obtained by melting raw materials of stainless steel into a slab, hot-rolling the slab, and then annealing and pickling. Also, a cold-rolled steel sheet (thin plate) is manufactured by cold-rolling a hot-rolled steel sheet and then annealing and pickling. In such a manufacturing process of stainless steel materials, pickling is performed to remove the oxide scale formed on the surface of the stainless steel material. Hereinafter, removing the oxide scale formed on the surface of the stainless steel material is referred to as "descaling".

[0003] However, the oxide scale may not be sufficiently removed only by pickling. Therefore, in a general descaling process, a method is adopted in which mechanical pretreatment such as a scale breaker or shot blasting is performed to crack the oxide scale, and then pickling is performed to make it easier to remove the oxide scale (for example, Patent Documents 1 and 2). Also, a method of immersing a stainless steel material in a salt bath as a chemical pretreatment is known.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the conventional descaling process using pickling, due to the surface irregularities formed in the pretreatment and the roughening of the surface by pickling, the surface of the stainless steel material becomes white and loses its luster, resulting in a decrease in design quality. In particular, cold-rolled steel materials require a higher level of design quality (smoothness and glossiness) compared to hot-rolled steel materials. Therefore, it is difficult to obtain the desired design quality in the conventional descaling process using pickling. In addition, for ferritic stainless steel materials, since the oxide scale formed on the surface is thick, it is difficult to crack it by mechanical pretreatment, and pickling unevenness is likely to occur. Also, in the case of stainless steel materials with a high content of Si or Al, SiO 2 and Al 2 O 3 -containing internal oxide layers are chemically stable, making it even more difficult to remove the oxide scale. Furthermore, when the oxide scale is removed, inclusions are exposed on the surface of the stainless steel material, and these inclusions become factors that reduce the corrosion resistance and fatigue characteristics of the stainless steel material. Therefore, in order to solve these problems, it is conceivable to polish the surface after the descaling process. However, if the surface is polished until it becomes smooth, the amount of grinding increases, resulting in a decrease in yield, and the corrosion resistance and design quality decrease due to polishing burn and entrainment of polishing debris. Also, since inclusions are present inside the stainless steel material, even if the surface is polished after the descaling process, new inclusions will be exposed on the surface of the stainless steel material. Therefore, this means is not effective.

[0006] The present invention has been made to solve the above problems, and an object thereof is to provide a ferritic stainless steel material having a smooth and glossy surface, excellent in corrosion resistance and fatigue characteristics, a method for manufacturing the same, and a corrosion-resistant member using the same.

Means for Solving the Problems

[0007] As a result of intensive research to solve the above problems, the inventors of the present invention have found that by controlling the composition of the ferritic stainless steel material and irradiating it with a laser beam under specific conditions, descaling can be performed while ensuring the surface smoothness and gloss, and the corrosion resistance and fatigue characteristics can be improved by dissolving the inclusions on the surface. As a result of manufacturing and examining various ferritic stainless steel materials based on this finding, it has been found that a ferritic stainless steel material having a predetermined composition and in which the ratio D2 / D1 of the number density D2 of inclusions on the surface to the number density D1 of predetermined inclusions in the matrix at a position where the depth from the surface is 1 / 4 of the thickness is within a specific range can solve the above problems, and the present invention has been completed.

[0008] That is, the present invention contains, on a mass basis, C: 0.001 to 0.150%, Si: 0.20 to 5.00%, Mn: 2.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: less than 2.00%, Cr: 11.00 to 30.00%, Mo: 6.00% or less, Cu: 0.60% or less, N: 0.050% or less, Al: 3.500% or less, Si + 2Al is 1.20% or more, and the balance consists of Fe and impurities, and is a ferritic stainless steel material in which the ratio D2 / D1 of the number density D2 of inclusions on the surface to the number density D1 of inclusions having an equivalent circle 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.

[0009] Further, the present invention A method for manufacturing the ferrite stainless steel material, includes a descaling step of irradiating a cold-rolled steel material having a composition containing, on a mass basis, C: 0.001 to 0.150%, Si: 0.20 to 5.00%, Mn: 2.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: less than 2.00%, Cr: 11.00 to 30.00%, Mo: 6.00% or less, Cu: 0.60% or less, N: 0.050% or less, Al: 3.500% or less, Si + 2Al is 1.20% or more, and the balance consisting of Fe and impurities with a laser beam to remove the oxide scale formed on the surface of the cold-rolled steel materialsee , The irradiation of the laser light is performed under conditions capable of melting a region from the interface between the matrix phase and the oxide scale in the cold-rolled steel material to a depth of 0.50 to 10.00 μm. way This is the method.

[0010] Furthermore, the present invention is a corrosion-resistant member including the ferritic stainless steel material.

Advantages of the Invention

[0011] According to the present invention, it is possible to provide a ferritic stainless steel material having a smooth and shiny surface, excellent in corrosion resistance and fatigue characteristics, a method for manufacturing the same, and a corrosion-resistant member using the same.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be specifically described. The present invention is not limited to the following embodiments, and it should be understood that those obtained by appropriately making changes, improvements, etc. to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the gist of the present invention also fall within the scope of the present invention. In addition, in this specification, the “%” display regarding components means “mass %” unless otherwise specified.

[0014] The ferritic stainless steel material according to an embodiment of the present invention contains C: 0.001 to 0.150%, Si: 0.20 to 5.00%, Mn: 2.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: less than 2.00%, Cr: 11.00 to 30.00%, Mo: 6.00% or less, Cu: 0.60% or less, N: 0.050% or less, Al: 3.500% or less, Si + 2Al is 1.20% or more, and the balance consists of Fe and impurities. Here, in this specification, "stainless steel material" means a material formed from stainless steel, and its material form is not particularly limited. Examples of the material form include plate shape (including strip shape), rod shape, tubular shape, etc. Also, various shaped steels such as T-shaped and I-shaped cross-sectional shapes may be used. Further, "impurities" means components that are mixed in due to raw materials such as ores and scraps and various factors in the manufacturing process when industrially manufacturing stainless steel materials, and are allowed within a range that does not adversely affect the present invention. For example, the stainless steel material may contain О as an impurity at 0.02% or less.

[0015] Moreover, the ferritic 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%, Co: 0.001 to 1.200%. Furthermore, the ferritic 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%, REM: 0.200% or less. Hereinafter, each component will be described in detail.

[0016] <C: 0.001 to 0.150%> If the content of C is too high, in addition to becoming hard and reducing workability, sensitization occurs when it is affected by heat such as welding, and the corrosion resistance of ferritic stainless steel materials decreases. Therefore, the upper limit value of the content of C is controlled to 0.150%, preferably 0.100%, more preferably 0.060%, and still more preferably 0.040%. On the other hand, if the content of C is too low, it will lead to deterioration of workability and an increase in refining cost. Therefore, the lower limit value of the content of C 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 content of Si is too high, it will become hardened and the workability of ferritic stainless steel materials will decrease. Therefore, the upper limit value of the content of Si is controlled to 5.00%, preferably 4.00%, more preferably 3.00%, and still more preferably 2.50%. On the other hand, if the content of Si is too low, the heat resistance of ferritic stainless steel materials will decrease. Therefore, the lower limit value of the content of Si is controlled to 0.20%, preferably 0.40%, more preferably 1.00%, and still more preferably 1.50%.

[0018] <Mn: 2.00% or less> Mn is an element that improves the heat resistance of ferritic stainless steel materials. However, if the content of Mn is too high, the corrosion resistance of ferritic stainless steel materials will decrease. Also, since Mn is an austenite phase (γ-phase) forming element, it generates a γ-phase (martensite phase at room temperature) at high temperature, and the workability of ferritic stainless steel materials also decreases. Therefore, the upper limit value of the content of Mn is controlled to 2.00%, preferably 1.50%, more preferably 1.20%, and still more preferably 1.00%. On the other hand, the lower limit value of the content of Mn 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 ferritic stainless steel will deteriorate. Therefore, the upper limit 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 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 decline in the manufacturability of ferritic stainless steel and also having an adverse effect on the corrosion resistance. Therefore, the upper limit of the content of S 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 content of S is not particularly limited, but is preferably 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%.

[0021] <Ni: less than 2.00%> Ni is an element that improves the corrosion resistance of ferritic stainless steel. However, since Ni is an austenite phase (γ-phase) forming element like Mn, if its content is too high, a γ-phase (martensite phase at room temperature) will be generated at high temperatures, resulting in a decline in the workability of ferritic stainless steel. Also, since Ni is an expensive element, it also leads to an increase in manufacturing costs. Therefore, the content of Ni is controlled to less than 2.00%, preferably 1.00% or less, more preferably 0.70% or less, and still more preferably 0.50% or less. On the other hand, the lower limit of the content of Ni is not particularly limited, but is preferably 0.01%, more preferably 0.03%, and still more preferably 0.05%.

[0022] <Cr: 11.00 - 30.00%> If the Cr content is too high, it will lead to an increase in refining costs, harden by solid solution strengthening, and reduce the workability of ferritic stainless steel. Therefore, the upper limit of the Cr content is controlled at 30.00%, preferably 24.00%, more preferably 22.00%, and even more preferably 18.00%. 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 at 11.00%, preferably 13.00%, more preferably 14.00%, and even more preferably 15.00%.

[0023] <Mo: 6.00% or less> Mo is an element that improves the corrosion resistance of ferritic 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 at 6.00%, preferably 3.00%, more preferably 2.00%, and even more preferably 1.00%. On the other hand, the lower limit of the Mo content is not particularly limited, but is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%.

[0024] <Cu: 0.60% or less> Cu is an element that improves the workability of ferritic stainless steel. If the Cu content is too high, the corrosion resistance of ferritic 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 at 0.60%, preferably 0.40%, more preferably 0.20%, and even more preferably 0.10%. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%.

[0025] <N: 0.050% 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 stainless steel. Therefore, the upper limit value of the content of N is controlled to 0.050%, preferably 0.040%, more preferably 0.030%, and still more preferably 0.020%. On the other hand, the lower limit value of the content of N is not particularly limited, but is preferably controlled to 0.001%, preferably 0.005%, and more preferably 0.010%.

[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 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, more preferably 1.50% or more, and still more preferably 2.00% 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 7.00%.

[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 necessary. From the perspective of obtaining the effect of Ti, the lower limit of the Ti content is controlled to be 0.001%, preferably 0.005%. On the other hand, if the Ti content is too high, it will cause surface defects, leading to a decrease in quality, and at the same time, the workability of ferritic stainless steel will decrease. Therefore, the upper limit of the Ti content is controlled to be 0.500%, preferably 0.300%, more preferably 0.100%.

[0029] <Nb:0.001~1.000%> Nb is an element that, like Ti, combines with C and N to improve corrosion resistance and intergranular corrosion resistance, and is added as necessary. From the perspective of obtaining the effect of Nb, the lower limit of the Nb content is controlled to be 0.001%, preferably 0.004%, more preferably 0.010%. On the other hand, if the Nb content is too high, the workability of ferritic stainless steel will decrease. Therefore, the upper limit of the Nb content is controlled to be 1.000%, preferably 0.600%, more preferably 0.060%.

[0030] <V:0.001~1.000%> V is an element that improves corrosion resistance and is added as necessary. From the perspective of obtaining the effect of V, the lower limit of the V content is controlled to be 0.001%, preferably 0.010%. On the other hand, if the V content is too high, the workability of ferritic stainless steel will decrease. Therefore, the upper limit of the V content is controlled to be 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 necessary. From the perspective of obtaining the effect of W, the lower limit of the W content is controlled to be 0.001%, preferably 0.010%. On the other hand, if the W content is too high, it will harden, resulting in a decrease in workability, and at the same time, the surface defects will increase, leading to a decrease in the surface quality of ferritic stainless steel. Therefore, the upper limit of the W content is controlled to be 1.000%, preferably 0.300%.

[0032] <Zr: 0.001 to 1.000%> Zr is an element that combines with C and N to improve oxidation resistance and intergranular corrosion resistance, and is added as required. From the perspective of obtaining the effect of Zr, the lower limit of the content of Zr is controlled to 0.001%, preferably 0.010%. On the other hand, if the content of Zr is too high, the workability of ferritic stainless steel 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 to 1.200%> Co is an element that improves heat resistance and is added as required. From the perspective of obtaining the effect of Co, the lower limit of the content of Co is controlled to 0.001%, preferably 0.010%. On the other hand, since Co is expensive, if the content of Co is too high, it will lead to an increase in manufacturing cost. Therefore, the upper limit of the Co content is controlled to 1.200%, preferably 0.400%.

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

[0035] <B: 0.0001 to 0.0080%> B is an element that improves the hot workability of ferritic stainless steel materials and is added as required. Also, B is an element that improves the secondary workability of ferritic stainless steel materials by grain boundary strengthening. From the viewpoint of obtaining the effect of B, the lower limit value of the content of B is controlled to 0.0001%, preferably 0.0003%, more preferably 0.0005%. On the other hand, if the content of B is too high, it will cause a decrease in weldability and fatigue strength. Therefore, the upper limit value of the content of B 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 required. From the viewpoint of obtaining the effect of Sn, the lower limit value of the content of Sn is controlled to 0.001%, preferably 0.002%. On the other hand, if the content of Sn is too high, it will form a low melting point phase and reduce the hot workability of ferritic stainless steel materials. Therefore, the upper limit value of the content of Sn is controlled to 0.500%, preferably 0.100%, more preferably 0.050%.

[0037] <REM: 0.200% or less> REM (rare earth element) is an element that improves the hot workability of ferritic stainless steel materials in the same way as B and Ca and is added as required. Also, REM is an element that improves corrosion resistance by forming sulfides that are difficult to elute and suppressing the formation of MnS that serves as a corrosion initiation point. However, if the content of REM is too high, it will lead to an increase in manufacturing cost. Therefore, the upper limit value of the content of REM is controlled to 0.200%, preferably 0.100%. On the other hand, the lower limit value of the content of REM is not particularly limited, but from the viewpoint of obtaining the effect of REM, it is preferably 0.001%, more preferably 0.010%. Note that REM refers to the general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements from lanthanum (La) to lutetium (Lu) (lanthanoids). These may be used alone or as a mixture.

[0038] The ferrite stainless steel material according to an embodiment of the present invention has a ratio D2 / D1 of the number density D2 of inclusions having an equivalent circle diameter of 0.5 to 10 μm on 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 phase (e.g., position C in FIG. 1) at a depth of 1 / 4 of the thickness from the surface (e.g., surface B in FIG. 1) of 0.50 or less, preferably 0.49 or less, more preferably 0.48 or less. By controlling the ratio D2 / D1 of the number density of inclusions within such a range, the proportion of inclusions present on the surface can be made smaller than the inclusions present in the matrix phase, so that the corrosion resistance and fatigue characteristics of the ferrite stainless steel material can be improved. Note that the lower limit value of the ratio D2 / D1 of the number density of inclusions is not particularly limited because the smaller the value, the higher the improvement effect of corrosion resistance and fatigue characteristics. For example, it is 0.01.

[0039] The number density D1 of inclusions having an equivalent circle diameter of 0.5 to 10 μm in the matrix phase at a depth of 1 / 4 of the thickness from the surface of the ferrite stainless steel material (hereinafter referred to as "the number density D1 of inclusions in the matrix phase") can be calculated as follows. First, an arbitrary location (10 or more fields of view) at a depth of 1 / 4 of the thickness from the surface (matrix phase) of the ferrite stainless steel material is photographed with an electron microscope. Next, the photographed image is binarized to separate inclusions (black) having an equivalent circle diameter of 0.5 to 10 μm and the matrix phase (white), the number of inclusions is obtained by counting the black regions, and the number density D1 of inclusions in the matrix phase can be calculated by dividing the obtained number of inclusions by the area of the observation field of view. Similarly, the number density D2 of inclusions having an equivalent circle diameter of 0.5 to 10 μm on the surface of the ferrite stainless steel material (hereinafter referred to as "the number density D2 of inclusions on the surface") can be calculated by photographing an arbitrary location (10 or more fields of view) on the surface of the ferrite 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 observation field of view. Here, since most of the inclusions are in the range of an equivalent circle diameter of 0.5 to 10 μm, the inclusions in this size range were counted as the number, and the number densities D1 and D2 of the inclusions were obtained. 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] As the electron microscope, the Schottky scanning electron microscope SU5000 manufactured by Hitachi High-Tech Corporation can be used. Also, the identification of the inclusions can be performed with an EDX detector EMAX3.3SP2 manufactured by Oxford Instruments plc attached to the electron microscope. For image processing, analysis software (AZtecSteel) manufactured by Oxford Instruments plc can be used.

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

[0042] The number density D2 of the inclusions on the surface is preferably 75 pieces / mm 2 or less, more preferably 60 pieces / mm 2 or less. By controlling the number density D2 of the inclusions on the surface within this range, the amount of inclusions present on the surface of the ferritic stainless steel material can be reduced, so that the corrosion resistance and fatigue strength of the ferritic stainless steel material can be stably improved. Note that the lower limit value of the number density D2 of the inclusions on the surface is not particularly limited because the smaller it is, the higher the improvement effect on the corrosion resistance and fatigue characteristics. For example, it is 10 pieces / mm 2 or so.

[0043] The inclusions are non-metallic inclusions (particularly, sulfide-based and oxide-based inclusions). Specific examples thereof include MnS, TiN, TiC, NbN, NbC, CaO, SiO 2 , Al 2 O 3 and composite compounds thereof.

[0044] The ferritic stainless steel material according to an embodiment of the present invention preferably has a fusion solidification layer on its surface. Here, as an example, a schematic cross-sectional view of the ferritic stainless steel material according to an embodiment of the present invention is shown in FIG. 1. As shown in FIG. 1, the ferritic stainless steel material 10 includes a matrix phase 11 and a fusion solidification layer 12 formed on the surface A of the matrix phase 11. The matrix phase 11 contains inclusions 13. The fusion solidification layer 12 may also contain inclusions 13, but there are few inclusions 13 exposed on the surface B of the fusion solidification layer 12. This is because the inclusions 13 exposed on the surface B due to descaling by laser light are dissolved.

[0045] The thickness of the fusion solidification layer is not particularly limited, but is preferably 0.50 to 10.00 μm, more preferably 1.00 to 10.00 μm, and still more preferably 2.00 to 10.00 μm. When the thickness of the fusion solidification layer is less than 0.50 μm, the inclusions are likely to be in a state where they are not sufficiently dissolved. Therefore, the number of inclusions exposed on the surface increases, and the corrosion resistance and fatigue characteristics of the ferritic stainless steel material tend to deteriorate. On the other hand, when the thickness of the fusion solidification layer exceeds 10.00 μm, the surface becomes rough, and it becomes difficult to obtain a smooth and shiny surface.

[0046] The arithmetic mean roughness Ra of the surface of the ferritic stainless steel material according to an embodiment of the present invention is preferably 0.01 to 0.40 μm, more preferably 0.01 to 0.30 μm, and still more preferably 0.01 to 0.20 μm. By controlling the arithmetic mean roughness Ra of the surface within such a range, the smoothness of the ferritic stainless steel material can be ensured. Here, in this specification, the "arithmetic mean roughness Ra" means the arithmetic mean roughness Ra measured in accordance with JIS B0601:2013.

[0047] The ferritic stainless steel material according to an embodiment of the present invention has a 60-degree specular gloss Gs(60°) on the surface, preferably 200 to 750%, more preferably 300 to 750%, and still more preferably 400 to 750%. By controlling the 60-degree specular gloss Gs(60°) on the surface within such a range, the glossiness of the ferritic 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 stainless steel material according to an embodiment of the present invention is preferably a ferritic stainless cold-rolled steel material, and more preferably a ferritic stainless cold-rolled steel sheet. In this specification, the cold-rolled steel material and cold-rolled steel sheet after the oxide scale is removed are referred to as "ferritic stainless cold-rolled steel material" and "ferritic stainless cold-rolled steel sheet", and the cold-rolled steel material and cold-rolled steel sheet before the oxide scale is removed are referred to as "cold-rolled steel material" and "cold-rolled steel sheet". Hereinafter, the case where the ferritic stainless steel material according to an embodiment of the present invention is a ferritic stainless cold-rolled steel sheet will be described as an example. The thickness (sheet thickness) of the ferritic stainless cold-rolled steel sheet according to an embodiment of the present invention is not particularly limited, but is preferably less than 3 mm, more preferably 2.5 mm or less, and still more preferably 2 mm or less. Also, the lower limit thereof is, for example, 0.1 mm, and it may be 0.3 mm.

[0049] The ferritic stainless cold-rolled steel sheet according to an embodiment of the present invention can be manufactured by using a method known in the art, except that a descaling step of irradiating a cold-rolled steel sheet having the above composition with a laser beam and removing the oxide scale formed on the surface is performed.

[0050] The manufacturing method of the cold-rolled steel sheet having the above composition is not particularly limited. For example, it can be manufactured as follows. First, stainless steel having the above composition is melted, and a steel slab is obtained by forging or casting. Next, the steel slab is hot-rolled, annealed, and then descaled. The descaling method is not particularly limited and can be performed by pickling, polishing, or using a laser beam. Next, the hot-rolled steel sheet after descaling is cold-rolled and annealed to obtain a cold-rolled steel sheet. The conditions for each step may be appropriately adjusted according to the composition of the stainless steel and the like and are not particularly limited.

[0051] Irradiation of the laser beam on the cold-rolled steel sheet is performed under conditions capable of melting a region from the interface between the matrix phase and the 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, more preferably 2.00 to 10.00 μm. By irradiating the laser beam under such conditions, the oxide scale is removed while ensuring the surface smoothness and glossiness, and the inclusions on the surface are dissolved by the heat-affected zone to form a melt-solidified layer on the surface, thereby improving the corrosion resistance and fatigue characteristics. If the irradiation conditions are such that only a region with a depth of less than 0.50 μm from the surface of the cold-rolled steel sheet can be melted, the inclusions cannot be sufficiently dissolved, so the inclusions exposed on the surface cannot be sufficiently reduced, and the corrosion resistance and fatigue characteristics of the ferritic stainless cold-rolled steel sheet deteriorate. Also, if the irradiation conditions are such that a region with a depth exceeding 10.00 μm from the surface of the cold-rolled steel sheet is melted, the surface of the ferritic stainless cold-rolled steel sheet becomes rough, and a smooth and glossy surface cannot be obtained.

[0052] Here, Fig. 2 shows a schematic cross-sectional view for explaining the difference between descaling by laser beam irradiation and descaling by a conventional method. As shown in Fig. 2(a), in the cold-rolled steel material (cold-rolled steel sheet) 20, an oxide scale 21 is formed on the surface of the matrix phase 11. Also, inclusions 13 are present at the interface D (the surface of the matrix phase 11) between the matrix phase 11 and the oxide scale 21. When descaling (removing the oxide scale 21) is performed by a conventional method (pickling and / or polishing), as shown in Fig. 2(b), the inclusions 13 existing at the interface D between the matrix phase 11 and the oxide scale 21 are exposed on the surface E. Further, in order to remove the inclusions 13 existing at the interface D between the matrix phase 11 and the oxide scale 21, even if the surface layer of the matrix phase 11 is removed together with the oxide scale 21, since the inclusions 13 exist inside the matrix phase 11, the inclusions 13 inside the matrix phase 11 are exposed on the surface E. Therefore, it is difficult to reduce the inclusions 13 exposed on the surface E by the conventional method.

[0053] On the other hand, when descaling is performed by irradiating a laser beam, as shown in Fig. 2(c), along with the removal of the oxide scale 21, the inclusions 13 existing at the interface D between the matrix phase 11 and the oxide scale 21 are dissolved. Further, as a result, a fusion-solidification layer 12 is formed on the surface layer of the matrix phase 11, and the number of inclusions 13 exposed on the surface B is reduced. The reduction in the number of inclusions 13 occurs because the inclusions 13 near the surface are melted during the formation of the fusion-solidification layer 12, become diluted with the matrix phase components, and rapidly solidify before reprecipitation. In addition to this, in the vicinity of the fusion-solidification layer 12, solid solution of the inclusions 13 occurs due to the temperature rise even in the portions that are not directly melted, resulting in a reduction in the size of the inclusions 13 and the number of inclusions 13. Also, even when the inclusions are not completely melted and rendered harmless, the corrosion resistance and fatigue strength are improved because the inclusions 13 are partially melted and the equivalent circle diameter of the inclusions 13 becomes smaller.

[0054] The conditions of the laser to be irradiated may be adjusted in consideration of the following matters according to the apparatus to be used. (Type of laser beam) When using a continuous-wave laser beam, since the energy required for removing the oxide scale becomes large, the required power becomes large, and the range in which the thermal influence occurs is too large, making it difficult to control the thickness of the fusion-solidification layer, a pulsed laser beam that can instantaneously apply heat is preferable. (Wavelength) Generally, the reflectivity of a substance with respect to light has wavelength dependence. When selecting a wavelength with a low reflectivity, the heat input increases, and the evaporation of the oxide scale is likely to occur. Therefore, by selecting a wavelength with a high reflectivity of the matrix phase and a low reflectivity of the oxide, the oxide scale can be selectively evaporated and removed without melting the matrix phase more than necessary. (Pulse width) The pulse width represents the time during which one pulse is irradiated. The narrower the pulse width, the more instantaneous heating occurs. When the pulse width is narrow, ablation occurs before the heat input by the laser is transferred to the surroundings. As a result, the ablation threshold decreases, and the thermal influence on the matrix phase decreases. When laser irradiation is performed under conditions where melting of the matrix phase occurs, the narrower the pulse width, the more rapid cooling occurs, and the re-precipitation of the melted inclusions is suppressed. However, the pulse width is mainly determined by the performance of the oscillator, and devices capable of oscillating with a short pulse width are expensive. Therefore, it is preferable to select a short pulse width within the specification range of the device. (Oscillation frequency) The shorter the pulse width, the higher the oscillation frequency can be. The higher the oscillation frequency, the more pulses are irradiated per unit time, and the removal rate of the oxide scale improves. Therefore, it is preferable to select a high oscillation frequency within the specification range of the device.

[0055] (Scanning frequency) The scanning frequency represents the moving speed in the plane direction of the pulse irradiation position. The higher the scanning frequency, the faster the removal rate of the oxide scale. However, if it is too high, gaps are generated between the pulse irradiation positions, the oxide scale remains, and the descaling rate decreases. Therefore, it is preferable to increase the scanning frequency within the range where the descaling rate can be maintained. (Laser beam diameter) The larger it is, the wider the irradiation range, that is, the range that can be descaled with one pulse, and the better the descaling efficiency. However, the energy density (fluence) per pulse decreases. It is preferable to increase the beam diameter within the range where the fluence capable of evaporating and removing the oxide scale is maintained. (Fluence) By irradiating a laser beam having a fluence exceeding the ablation threshold of the oxide constituting the oxide scale, the oxide scale can be removed by evaporation. The higher the fluence, the greater the thickness of the oxide scale that can be removed. However, if the fluence is too high, not only the oxide scale but also the evaporation removal of the matrix phase will occur. In addition, although the pulsed laser has less thermal influence compared to the continuous wave laser, the higher the fluence, the greater the heat input to the matrix phase, and the larger the molten part and the heat affected part. Therefore, considering the balance between the characteristics (thickness, structure, composition, etc.) of the oxide scale to be removed and the heat input to the matrix phase, the fluence may be adjusted within a range where the matrix phase is not melted more than necessary. When the fluence distribution is different within the beam spot, it may be controlled using the average fluence.

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

[0057] The corrosion-resistant member according to the embodiment of the present invention includes the above ferritic stainless steel material. The ferritic stainless steel material used for this corrosion-resistant member may be processed into various shapes by a method known in the art. The corrosion-resistant member according to the embodiment of the present invention may further include members other than the above ferritic stainless steel material. The corrosion-resistant member is not particularly limited, and examples include automotive parts, architectural parts, and kitchen utensils.

Example

[0058] Hereinafter, the content of the present invention will be described in detail with reference to examples, but the present invention is not construed as being limited thereto.

[0059] 30 kg of stainless steel having the compositions of Steel Grades A to E shown in Table 1 (the balance being Fe and impurities) was melted by vacuum melting, forged into a steel slab with a thickness of 30 mm, heated at 1230 °C for 2 hours, hot-rolled to a thickness of 3 mm, and annealed at 1100 °C for 5 minutes in an air atmosphere to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was cut into 50 mm (rolling direction) × 50 mm (width direction) by processing, and then pickled to remove scale. The pickling to remove scale was carried out by holding an aqueous hydrofluoric and nitric acid solution containing 50 g / L of hydrofluoric acid and 150 g / L of nitric acid at 60 °C in a thermostatic bath, immersing the hot-rolled steel sheet for 120 to 720 seconds, and then immediately washing it with running water and allowing it to dry naturally. The specific immersion times were 240 seconds for Steel Grade A, 120 seconds for Steel Grade B, 360 seconds for Steel Grade C, 660 seconds for Steel Grade D, and 720 seconds for Steel Grade E. Thereafter, the hot-rolled steel sheet was cold-rolled from a thickness of 3 mm to 1 mm and annealed at 1100 °C for 5 minutes in an air atmosphere to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheet was used in each of the following Examples and Comparative Examples.

[0060]

Table 1

[0061] (Examples 1 to 5) A descaling process by irradiation with laser light was performed on the cold-rolled steel sheets having the compositions of the respective steel grades. The irradiation with laser light was carried out 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, it was scanned at a constant speed in the plate width direction from above the cold-rolled steel sheet to irradiate the pulsed laser light once. The scan width per pass was set to 25 mm. The irradiation conditions for the pulsed laser light were as follows. Wavelength: 1085 nm Pulse width: 220 ns Oscillation frequency: 60 kHz Scan frequency: 100 Hz Laser beam diameter: 90 μm Average fluence: 9 J / cm 2

[0062] (Comparative Example 1) A descaling process by pickling was performed on the cold-rolled steel sheet having the composition of Steel Grade A. The pickling was carried out as follows. An aqueous hydrofluoric and nitric acid solution containing 30 g / L of hydrofluoric acid and 100 g / L of nitric acid was maintained at 60°C in a thermostatic bath. After the cold-rolled steel sheet was immersed for 600 seconds, it was immediately washed with running water and air-dried.

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

[0064] The following evaluations were performed on the cold-rolled steel sheets (ferritic stainless cold-rolled steel sheets) after the descaling process obtained in the above Examples and Comparative Examples.

[0065] (Number densities D1 and D2 of inclusions) Test pieces of 50 mm square were cut out from the center parts in the width direction and length direction of the ferritic stainless cold-rolled steel sheets obtained in the above Examples and Comparative Examples. An arbitrary location on the surface was photographed at 200 times magnification using a Schottky scanning electron microscope SU5000 manufactured by Hitachi High-Tech Corporation. Also, the surface of this test piece was polished to remove 1 / 4 of the thickness (250 μm), and the matrix phase at a position with a depth of 1 / 4 of the thickness from the surface was exposed. Then, an arbitrary location on this exposed surface was photographed in the same manner as above. The photographing was performed with 0.48 mm × 0.64 mm (0.3072 mm 2 ) as one field of view and for 10 fields of view. Next, using the analysis software (AZtecSteel) manufactured by Oxford Instruments Japan K.K., the photographed images were binarized to separate inclusions (black) with an equivalent circle diameter of 0.5 to 10 μm and the matrix phase (white). The conditions for image analysis were as follows. Resolution: 4096 Minimum detectable size: 8 pixels (0.5 μm) Color threshold value of inclusions: 47 to 24057 Note that those less than 8 pixels were excluded as noise. Next, in the binarized image, the number of black regions with an equivalent circle diameter of 0.5 to 10 μm was counted to obtain the number of inclusions, and the number density D1 and D2 of the inclusions were calculated by dividing the obtained number of inclusions by the area of the observation field of view. The results of the number density D1 and D2 of the inclusions were taken as the average values of the results in each field of view. Also, based on the calculated number density D1 and D2 of the inclusions, the ratio D2 / D1 of the number density of the inclusions was calculated.

[0066] (Thickness of the fusion solidification layer) The ferritic stainless cold-rolled steel sheet was cut in the thickness direction (perpendicular to the surface) parallel to the rolling direction, and the microstructure image of the cut surface was observed up to 10,000 times using an electron microscope. The fusion solidification layer was discriminated from the difference in contrast, and its thickness was measured. The thickness was measured at 10 arbitrary locations, and the average value was taken as the result.

[0067] (Measurement of surface roughness) Regarding the surface of the ferritic stainless cold-rolled steel sheet, in accordance with JIS B0601:2013, the arithmetic mean roughness Ra was measured using a contact-type surface roughness meter (Surfcom 2800 manufactured by Tokyo Seimitsu Co., Ltd.). The arithmetic mean roughness Ra was measured at 5 locations excluding the range from the end to 5 mm with a reference length of 4 mm, and the average value was taken as the evaluation result. In addition, the distance between each measurement position was more than 5 mm.

[0068] (Measurement of glossiness) Regarding the surface of the ferritic stainless cold-rolled steel sheet, in accordance with JIS Z8741:1997, the 60-degree specular gloss Gs(60°) was measured using a gloss meter (PG-1M manufactured by Nippon Denshoku Industries Co., Ltd.). The 60-degree specular gloss Gs(60°) was measured at 5 locations excluding the range from the end to 5 mm, and the average value was taken as the evaluation result. In addition, the distance between each measurement position was more than 5 mm.

[0069] (Corrosion resistance test) The corrosion resistance test was carried out according to JASO M609 and M610, and a composite cycle test was performed. Specifically, a salt-dry-wet repeated test was carried out, which repeated salt spray, drying, and wetting. In the salt-dry-wet repeated test, for a ferritic stainless cold-rolled steel sheet, spraying with a 5% NaCl aqueous solution (for 2 hours at 35°C), drying (at a relative humidity of 30% and a temperature of 60°C for 4 hours), and wetting (at a relative humidity of 95% and a temperature of 50°C for 2 hours) were taken as one cycle. After each cycle, the steel sheet was washed with water and dried, and then the surface of the ferritic stainless cold-rolled steel sheet was observed to calculate the rusted area ratio. And the number of cycles when the rusted area ratio reached 10% or more was taken as the number of corrosion occurrence cycles. The calculation of the rusted area ratio was carried out according to the following procedure. The surface of the ferritic stainless cold-rolled steel sheet after the salt-dry-wet repeated test was photographed, and the ratio of the area of the rusted part in the central 25 mm × 25 mm range excluding the end faces was determined. The area of the rusted part was obtained by binarizing the photograph of the surface of the ferritic stainless cold-rolled steel sheet by image analysis, calculating the area per pixel, and then counting the number of pixels of the rusted part. The rusted area ratio was calculated by the following formula. Rusted area ratio (%) = Area of rusted part (mm 2 ) / Total area of the observation part (625 mm 2 ) × 100 In this evaluation, based on the number of corrosion occurrence cycles of the ferritic stainless cold-rolled steel sheet in Comparative Example 2, the improvement rates of the number of corrosion occurrence cycles of the ferritic stainless cold-rolled steel sheets in Examples 1 to 5 and Comparative Example 1 were calculated. Those with an improvement rate of the calculated number of corrosion occurrence cycles of 20% or more were marked as "〇", those with 10% or more and less than 20% were marked as "△", and those with less than 10% were marked as "×".

[0070] (Fatigue property test) The fatigue property test was carried out according to JIS Z2275:1978, and a plane bending fatigue test was performed. Specifically, a test piece was obtained by cutting out a length of 30 mm in the width direction and a length of 90 mm in the rolling direction from a ferritic stainless cold-rolled steel sheet and forming R parts with a radius of 30 mm at both ends in the width direction. This test piece was mounted on a plane bending testing machine, and the number of repetitions was 10 7The back fatigue test was conducted. In this fatigue test, the test was performed on two or more test pieces for each stress stage, and the time strength was measured. In this evaluation, based on the fatigue strength of the ferritic stainless cold-rolled steel sheet of Comparative Example 2, the improvement rates of the time strength of the ferritic stainless cold-rolled steel sheets of Examples 1 to 5 and Comparative Example 1 were calculated. Those with a calculated improvement rate of the time strength of 10% or more were marked as "〇", and those less than 10% were marked as "×". The above evaluation results are shown in Table 2.

[0071]

Table 2

[0072] As shown in Table 2, the ferritic stainless cold-rolled steel sheets of Examples 1 to 5 with a ratio D2 / D1 of the number density of inclusions of 0.50 or less had an arithmetic mean roughness Ra of the surface of 0.01 to 0.40 μm and a 60-degree specular gloss Gs(60°) of the surface of 200 to 750%, and it was confirmed that they had a smooth and shiny surface. Also, the ferritic stainless cold-rolled steel sheets of Examples 1 to 5 had improved corrosion resistance and fatigue characteristics compared to the ferritic stainless cold-rolled steel sheet of Comparative Example 2. On the other hand, for the ferritic stainless cold-rolled steel sheet of Comparative Example 1, since descaling was performed by pickling, the ratio D2 / D1 of the number density of inclusions exceeded 0.50. Therefore, the ferritic stainless cold-rolled steel sheet of Comparative Example 1 did not sufficiently improve in corrosion resistance and fatigue characteristics compared to the ferritic stainless cold-rolled steel sheet of Comparative Example 2.

[0073] As can be seen from the above results, according to the present invention, it is possible to provide a ferritic stainless steel material having a smooth and shiny surface, excellent in corrosion resistance and fatigue characteristics, a manufacturing method thereof, and a corrosion-resistant member using the same.

Explanation of Symbols

[0074] 10 Ferritic stainless steel material 11 Matrix 12 Melt-solidified layer 13 Inclusion 20 Cold-rolled steel 21 Oxide scale

Claims

1. In terms of mass basis, it contains C: 0.001 to 0.150%, Si: 0.20 to 5.00%, Mn: 2.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: less than 2.00%, Cr: 11.00 to 30.00%, Mo: 6.00% or less, Cu: 0.60% or less, N: 0.050% or less, Al: 3.500% or less, Si + 2Al is 1.20% or more, and the balance consists of Fe and impurities, The ferrite stainless steel sheet in which the ratio D2 / D1 of the number density D2 of the inclusions on the surface to the number density D1 of the inclusions 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 equivalent circle diameter of the inclusions in the matrix is 0.5 to 10 μm.

2. The ferrite stainless steel sheet according to Claim 1, further containing 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%, Co: 0.001 to 1.200% in terms of mass basis.

3. The ferrite stainless steel sheet according to Claim 1 or 2, further containing one or more selected from Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0080%, Sn: 0.001 to 0.500%, REM: 0.200% or less in terms of mass basis.

4. The ferrite stainless steel sheet 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 stainless steel sheet 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 ferrite stainless steel sheet according to any one of Claims 1 to 5, having a fusion-solidified layer on the surface layer.

7. The ferrite stainless steel sheet according to Claim 6, wherein the thickness of the fusion-solidified layer is 0.50 to 10.00 μm.

8. A method for manufacturing the ferrite stainless steel sheet according to Claim 1, On a mass basis, it contains C: 0.001 to 0.150%, Si: 0.20 to 5.00%, Mn: 2.00% or less, P: 0.050% or less, S: 0.0300% or less, Ni: less than 2.00%, Cr: 11.00 to 30.00%, Mo: 6.00% or less, Cu: 0.60% or less, N: 0.050% or less, Al: 3.500% or less, Si + 2Al is 1.20% or more, and the balance consists of Fe and impurities. It includes a descaling step of irradiating a cold-rolled steel sheet with a laser beam to remove the oxide scale formed on the surface of the cold-rolled steel sheet. The irradiation of the laser beam is performed under conditions capable of melting a region with a depth of 0.50 to 10.00 μm from the interface between the matrix phase and the oxide scale in the cold-rolled steel sheet.

9. A method for manufacturing a ferritic stainless steel sheet according to Claim 2, The cold-rolled steel sheet further contains 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%, Co: 0.001 to 1.200% on a mass basis. The method according to Claim 8.

10. A method for manufacturing a ferritic stainless steel sheet according to Claim 3, The cold-rolled steel sheet further contains one or more selected from Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0080%, Sn: 0.001 to 0.500%, REM: 0.200% or less on a mass basis. The method according to Claim 8 or 9.

11. A corrosion-resistant member containing the ferritic stainless steel sheet according to any one of Claims 1 to 7.

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