Austenitic stainless steel material, its manufacturing method, and corrosion-resistant member

Abstract

To provide an austenitic stainless steel material which has smooth and glossy surface and has excellent corrosion resistance and fatigue characteristics.SOLUTION: An austenitic stainless steel material 10 has a composition comprising, by mass, 0.001 to 0.150% C, 0.20 to 5.00% Si, 2.50% or less Mn, 0.050% or less P, 0.0300% or less S, 6.00 to 20.00% Ni, 14.00 to 25.00% Cr, 3.00% or less Mo, 4.00% 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 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 an austenitic 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, building 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 produced 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 produced 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 by pickling alone. In particular, among stainless steel materials, austenitic stainless steel materials contain a large amount of Ni and thus are excellent in acid resistance. Therefore, compared with other stainless steel materials, it is difficult to remove the oxide scale formed on the surface of austenitic stainless steel materials 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 Japanese Unexamined Patent Application Publication No. 2014-172077 Patent Document 2 Japanese Unexamined Patent Application Publication No. 2-145785 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, for cold-rolled steel materials, a higher level of design quality (smoothness and glossiness) is required compared to hot-rolled steel materials. Therefore, it is difficult to obtain the desired design quality in the conventional descaling process using pickling. Further, in the case of stainless steel materials with a high content of Si or Al, SiO 2 or Al 2 O 3 -containing internal oxide layer formed at the interface between the oxide scale and the stainless steel material is 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 a factor in reducing 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 entrapment of polishing debris. In addition, since inclusions also exist 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 an austenitic 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 austenitic stainless steel materials and irradiating laser light under specific conditions, it is possible to descale while ensuring surface smoothness and gloss, and to improve corrosion resistance and fatigue characteristics by dissolving inclusions on the surface. Based on this finding, various austenitic stainless steel materials were produced and examined. As a result, it was found that an austenitic stainless steel material having a predetermined composition and a 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 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: 2.50% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 6.00 to 20.00%, Cr: 14.00 to 25.00%, Mo: 3.00% or less, Cu: 4.00% or less, N: 0.400% or less, Al: 3.500% or less, Si + 2Al is 1.20% or more, and the balance consists of Fe and impurities, and is an austenitic stainless steel plate in which 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.

[0009] In addition, the present invention is A method for manufacturing the austenitic stainless steel material, Based on mass, it contains C: 0.001 to 0.150%, Si: 0.20 to 5.00%, Mn: 2.50% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 6.00 to 20.00%, Cr: 14.00 to 25.00%, Mo: 3.00% or less, Cu: 4.00% or less, N: 0.400% 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. see , The irradiation of the laser beam is performed under conditions capable of melting a region from 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. way It is a method.

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

Effects of the Invention

[0011] According to the present invention, it is possible to provide an austenitic stainless steel sheet having a smooth and shiny surface, excellent corrosion resistance and fatigue characteristics, a manufacturing method thereof, 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 modifications, improvements, etc. 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 also fall within the scope of the present invention. In addition, in this specification, the “%” indication regarding components means “mass %” unless otherwise specified.

[0014] The austenitic 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: 2.50% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 6.00 to 20.00%, Cr: 14.00 to 25.00%, Mo: 3.00% or less, Cu: 4.00% or less, N: 0.400% 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), bar 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 austenitic stainless steel material according to an embodiment of the present invention can 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 austenitic 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. Hereinafter, each component will be described in detail.

[0016] <C: 0.001 to 0.150%> If the C content is too high, in addition to becoming hard and deteriorating workability, sensitization occurs when subjected to heat influence such as welding, and the corrosion resistance of the austenitic stainless steel material decreases. Therefore, the upper limit value 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 leads to deterioration of workability due to a decrease in the stability of the austenite phase and an increase in refining cost. Therefore, the lower limit value of the C content is controlled to 0.001%, preferably 0.002%, more preferably 0.005%, and still more preferably 0.010%.

[0017] <Si: 0.20 to 5.00%> If the Si content is too high, it hardens and the workability of the austenitic stainless steel material deteriorates. Therefore, the upper limit value 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 austenitic stainless steel material decreases. Therefore, the lower limit value of the Si content is controlled to 0.20%, preferably 1.00%, more preferably 1.50%, and still more preferably 2.50%.

[0018] <Mn: 2.50% or less> Mn is an austenite phase (γ-phase) forming element. If the Mn content is too high, the corrosion resistance of austenitic stainless steel will decrease. Therefore, the upper limit of the Mn content is controlled to 2.50%, preferably 2.00%, more preferably 1.80%, and still more preferably 1.60%. 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 P content is too high, the corrosion resistance and workability of austenitic stainless steel will decrease. Therefore, the upper limit of the P content 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 P content 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 S content is too high, the hot workability will decrease and the manufacturability of austenitic stainless steel will deteriorate. Therefore, the upper limit of the S content is controlled to 0.0300%, preferably 0.0100%, more preferably 0.0050%, and still more preferably 0.0010%. On the other hand, the lower limit of the S content is not particularly limited, but is preferably 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%.

[0021] <Ni: 6.00 - 20.00%> Ni, like Mn, is an austenite phase (γ-phase) forming element. Since Ni is expensive, if its content is too high, it will lead to an increase in manufacturing cost. Therefore, the upper limit value of the Ni content is controlled to 20.00%, preferably 18.00%, more preferably 16.00%, and still more preferably 14.00%. On the other hand, if the Ni content is too low, the corrosion resistance of the austenitic stainless steel will decrease. Therefore, the lower limit value of the Ni content is controlled to 6.00%, preferably 6.50%, more preferably 7.00%, and still more preferably 8.00%.

[0022] <Cr: 14.00~25.00%> If the Cr content is too high, it will not only cause an increase in refining cost, but also harden due to solid solution strengthening, resulting in a decrease in the workability of the austenitic stainless steel. Therefore, the upper limit value of the Cr content is controlled to 25.00%, preferably 22.00%, more preferably 21.00%, and still more preferably 20.00%. On the other hand, if the Cr content is too low, sufficient corrosion resistance cannot be obtained. Therefore, the lower limit value of the Cr content is controlled to 14.00%, preferably 15.00%, more preferably 16.00%, and still more preferably 17.00%.

[0023] <Mo: 3.00% or less> Mo is an element that improves the corrosion resistance of austenitic stainless steel. Since Mo is expensive, if the Mo content is too high, it will lead to an increase in manufacturing cost. Therefore, the upper limit value of the Mo content is controlled to 3.00%, preferably 2.50%, more preferably 2.00%, and still more preferably 1.00%. On the other hand, the lower limit value of the Mo content is not particularly limited, but is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%.

[0024] <Cu: 4.00% or less> Cu is an element that improves the workability of austenitic stainless steel. If the Cu content is too high, the corrosion resistance of the austenitic 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 4.00%, preferably 3.00%, more preferably 2.00%, and even more preferably 1.00%. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.01%, more preferably 0.04%, and even more preferably 0.20%.

[0025] <N: 0.400% or less> N is an element that improves corrosion resistance. If the N content is too high, it will harden and the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the N content is controlled to 0.400%, preferably 0.200%, more preferably 0.150%, and even more preferably 0.100%. On the other hand, the lower limit of the N content 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 added as necessary for deoxidation in the refining process and is an element that improves corrosion resistance and heat resistance. If the Al content is too high, the amount of inclusions generated will increase and the quality will deteriorate. Therefore, the upper limit of the Al content is controlled to 3.500%, preferably 3.000%, more preferably 2.000%, and even more preferably 1.500%. On the other hand, the lower limit of the Al content is not particularly limited, but is preferably 0.001%, more preferably 0.010%, and even more preferably 0.100%.

[0027] <Si + 2Al: 1.20% or more> The austenitic stainless steel material according to an embodiment of the present invention targets those with high Si and Al contents. 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 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 necessary. From the viewpoint of obtaining the effect of Ti, the lower limit value of the Ti content is controlled to 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 the austenitic stainless steel material will decrease. Therefore, the upper limit value of the Ti content is controlled to 0.500%, preferably 0.300%, and more preferably 0.100%.

[0029] <Nb: 0.001 - 1.000%> Nb, like 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 viewpoint of obtaining the effect of Nb, the lower limit value 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 austenitic stainless steel material will decrease. Therefore, the upper limit value 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 necessary. From the viewpoint of obtaining the effect of V, the lower limit value of the content of V is controlled to be 0.001%, preferably 0.010%. On the other hand, if the content of V is too high, the workability of the austenitic stainless steel material will deteriorate. Therefore, the upper limit value of the content of V 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 viewpoint of obtaining the effect of W, the lower limit value of the content of W is controlled to be 0.001%, preferably 0.010%. On the other hand, if the content of W is too high, it will become hardened and the workability will deteriorate, and at the same time, surface defects will increase and the surface quality of the austenitic stainless steel material will deteriorate. Therefore, the upper limit value of the content of W is controlled to be 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 necessary. From the viewpoint of obtaining the effect of Zr, the lower limit value of the content of Zr is controlled to be 0.001%, preferably 0.010%. On the other hand, if the content of Zr is too high, the workability of the austenitic stainless steel material will deteriorate. Therefore, the upper limit value of the content of Zr is controlled to be 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 necessary. From the viewpoint of obtaining the effect of Co, the lower limit value of the content of Co is controlled to be 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 value of the content of Co is controlled to be 1.200%, preferably 0.400%.

[0034] <Ca: 0.0001~0.0100%> Ca is an element that improves the hot workability of austenitic 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 austenitic stainless steel materials and is added as necessary. Also, B is an element that improves the secondary workability of austenitic 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 austenitic 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> REM (rare earth elements) is an element that improves the hot workability of austenitic stainless steel materials, similar to B and Ca, and is added as necessary. 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 costs. Therefore, the upper limit value of the REM content is controlled to 0.200%, preferably 0.100%. On the other hand, the lower limit value of the REM content is not particularly limited, but from the perspective 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 (lanthanoids) from lanthanum (La) to lutetium (Lu). These may be used alone or as a mixture.

[0038] In the austenitic 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 on the surface (for example, 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 (for example, position C in FIG. 1) at a depth of 1 / 4 of the thickness from the surface (for example, surface B in FIG. 1) is 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 austenitic 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 with an equivalent circle diameter of 0.5 to 10 μm in the matrix at a depth from the surface of the austenitic stainless steel material equal to 1 / 4 of the thickness (hereinafter referred to as "the number density D1 of inclusions in the matrix") can be calculated as follows. First, take pictures with an electron microscope of arbitrary locations (more than 10 fields of view) at a depth from the surface of the austenitic stainless steel material equal to 1 / 4 of the thickness (matrix). Next, perform binarization processing on the taken images to separate inclusions (black) with an equivalent circle diameter of 0.5 to 10 μm from the matrix (white), count the number of inclusions by counting the black regions, and calculate the number density D1 of inclusions in the matrix by dividing the obtained number of inclusions by the area of the observation field of view. Similarly, the number density D2 of inclusions with an equivalent circle diameter of 0.5 to 10 μm on the surface of the austenitic stainless steel material (hereinafter referred to as "the number density D2 of inclusions on the surface") can be calculated by taking pictures with an electron microscope of arbitrary locations (more than 10 fields of view) on the surface of the austenitic stainless steel material 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 with a size in this range are counted as the number, and the number densities D1 and D2 of the inclusions are used. The equivalent circle diameter means the diameter of a circle when the area of each observed black region is converted into a circle with 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 inclusions can be performed with the EDX detector EMAX3.3SP2 manufactured by Oxford Instruments plc attached to the electron microscope. For image processing, the analysis software (AZtecSteel) manufactured by Oxford Instruments plc can be used.

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

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

[0043] The inclusions are non-metallic inclusions (particularly sulfide-based or 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 austenitic stainless steel material according to an embodiment of the present invention preferably has a fusion-solidified layer on the surface layer. Here, as an example, a schematic cross-sectional view of the austenitic stainless steel material according to an embodiment of the present invention is shown in FIG. 1. As shown in FIG. 1, the austenitic stainless steel material 10 includes a matrix phase 11 and a fusion-solidified layer 12 formed on the surface A of the matrix phase 11. The matrix phase 11 contains inclusions 13. The fusion-solidified layer 12 may also contain inclusions 13, but there are few inclusions 13 exposed on the surface B of the fusion-solidified layer 12. This is because the inclusions 13 exposed on the surface B due to descaling by laser light have 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, 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 austenitic 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 tends to be difficult to obtain a smooth and shiny surface.

[0046] The austenitic stainless steel material according to the embodiment of the present invention preferably has an arithmetic mean roughness Ra of the surface of 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 austenitic 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 austenitic stainless steel material according to the embodiment of the present invention preferably has a 60-degree specular gloss Gs(60°) of the surface of 200 to 750%, more preferably 300 to 750%, and still more preferably 400 to 750%. By controlling the 60-degree specular gloss Gs(60°) of the surface within such a range, the glossiness of the austenitic stainless steel material can be ensured. Here, in this specification, the "60-degree specular gloss Gs(60°)" means the 60-degree specular gloss Gs(60°) measured in accordance with JIS Z8741:1997.

[0048] The austenitic stainless steel material according to an embodiment of the present invention is preferably an austenitic stainless cold-rolled steel material, and more preferably an austenitic 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 "austenitic stainless cold-rolled steel material" and "austenitic 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 austenitic stainless steel material according to an embodiment of the present invention is an austenitic stainless cold-rolled steel sheet will be described as an example. The thickness (plate thickness) of the austenitic 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 austenitic 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 laser beam on the cold-rolled steel sheet having the above composition to remove 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, but can be manufactured, for example, as follows. First, the stainless steel having the above composition is melted to obtain a steel slab 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. Note that the conditions of 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 cold-rolled steel sheet with a laser beam is performed under conditions that can melt a region from a depth of 0.50 to 10.00 μm, preferably 1.00 to 10.00 μm, more preferably 2.00 to 10.00 μm from the interface between the matrix phase and the oxide scale in the cold-rolled steel sheet. By irradiating the laser beam under such conditions, while ensuring the surface smoothness and glossiness, the oxide scale is removed, and the inclusions on the surface are dissolved by the heat influence 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 austenitic stainless steel 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 austenitic stainless steel 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 present at the interface D between the matrix phase 11 and the oxide scale 21 are exposed on the surface E. Also, even if the surface layer of the matrix phase 11 is removed together with the oxide scale 21 in order to remove the inclusions 13 present at the interface D between the matrix phase 11 and the oxide scale 21, since the inclusions 13 are present 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] When descaling is performed by irradiating with laser light, as shown in Fig. 2(c), in addition to removing the oxide scale 21, the inclusions 13 present at the interface D between the matrix phase 11 and the oxide scale 21 dissolve. As a result, a melt-solidified layer 12 is formed on the surface layer of the matrix phase 11, and the number of inclusions 13 exposed on the surface B decreases. The decrease in the number of inclusions 13 occurs because the inclusions 13 near the surface melt during the formation of the melt-solidified layer 12, become diluted with the matrix phase components, and rapidly solidify before reprecipitation. In addition to this, in the vicinity of the melt-solidified layer 12, solid solution of the inclusions 13 occurs due to the temperature rise even in the portions that did not directly melt, resulting in a reduction in the size and the number of the inclusions 13. Also, even when the inclusions are not completely melted and rendered harmless, the corrosion resistance and fatigue strength are improved because they 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 light) When using continuous-wave laser light, the energy required for removing the oxide scale becomes large, the required power becomes large, and the range in which the thermal effect occurs is too large, making it difficult to control the thickness of the melt-solidified layer. Therefore, pulsed laser light that can instantaneously apply heat is preferred. (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 reprecipitation 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 will occur between the pulse irradiation positions, and the oxide scale will remain, resulting in a decrease in the descaling rate. 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 parent phase will occur. In addition, although the pulsed laser has less heat influence compared to the continuous wave laser, the higher the fluence, the greater the heat input to the parent 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 parent phase, the fluence may be adjusted within a range where the parent 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 austenitic 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 austenitic 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 austenitic 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 austenitic stainless steel material. The austenitic 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 austenitic stainless steel material. The corrosion-resistant member is not particularly limited, and examples include automotive parts, architectural parts, kitchen utensils, and the like.

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 steel slabs 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 hot-rolled steel sheets. The hot-rolled steel sheets were cut into 50 mm (rolling direction) × 50 mm (width direction) by processing and then descaled by pickling. The pickling descaling was carried out by holding a hydrofluoric and nitric acid aqueous solution containing 50 g / L of hydrofluoric acid and 150 g / L of nitric acid at 60 °C in a constant-temperature bath, immersing the hot-rolled steel sheets for 240 to 1320 seconds, and then immediately washing them with running water and drying them naturally. The specific immersion times were 360 seconds for Steel Grade A, 240 seconds for Steel Grade B, 1320 seconds for Steel Grade C, 1000 seconds for Steel Grade D, and 1240 seconds for Steel Grade E. Thereafter, the hot-rolled steel sheets were cold-rolled from a thickness of 3 mm to 1 mm and annealed at 1100 °C for 5 minutes 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) A descaling process by laser light irradiation was performed on the cold-rolled steel sheets having the compositions of each steel grade. The laser light irradiation 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, and pulsed laser light was irradiated once. The scan width per pass was 25 mm. The irradiation conditions of 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: 7 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 type 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 constant temperature bath. After immersing the cold-rolled steel sheet 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 set to a depth of 20 μm from the surface.

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

[0065] (Number density D1, 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 austenitic 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 from the surface of 1 / 4 of the thickness 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 analysis software (AZtecSteel) manufactured by Oxford Instruments Co., Ltd., 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 for inclusions: 47 - 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. The results of the number density D1 and D2 of the inclusions were taken as the average value 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 austenitic stainless steel cold-rolled steel sheet was cut in the thickness direction (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 austenitic stainless steel 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 austenitic stainless steel 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 spray and drying and wetting repeated test was carried out, in which salt water spray, drying and wetting were repeated. In the salt spray and drying and wetting repeated test, for the austenitic stainless steel 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 surface was washed with water and dried, and then the surface of the austenitic stainless steel cold-rolled steel sheet was observed, and the rusted area ratio was calculated. And the number of cycles when the rusted area ratio reached 10% or more was taken as the number of corrosion occurrence cycles. The rusted area ratio was calculated according to the following procedure. The surface of the austenitic stainless steel cold-rolled steel sheet after the salt spray and drying and wetting 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 obtained. The area of the rusted part was obtained by binarizing the photograph of the surface of the austenitic stainless steel 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 austenitic stainless steel cold-rolled steel sheet in Comparative Example 2, the improvement rate of the number of corrosion occurrence cycles of the austenitic stainless steel cold-rolled steel sheets in Examples 1 to 5 and Comparative Example 1 was 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 the austenitic stainless steel 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 7A cycle 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 austenitic stainless steel cold-rolled steel sheet of Comparative Example 2, the improvement rates of the time strength of the austenitic stainless steel 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 austenitic stainless steel cold-rolled steel sheets of Examples 1 to 5 in which the ratio D2 / D1 of the inclusion number density is 0.50 or less have an arithmetic mean surface roughness Ra 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 have a smooth and shiny surface. Also, the austenitic stainless steel cold-rolled steel sheets of Examples 1 to 5 have improved corrosion resistance and fatigue characteristics compared to the austenitic stainless steel cold-rolled steel sheet of Comparative Example 2. On the other hand, for the austenitic stainless steel cold-rolled steel sheet of Comparative Example 1, since descaling was performed by pickling, the ratio D2 / D1 of the inclusion number density exceeded 0.50. Therefore, the austenitic stainless steel cold-rolled steel sheet of Comparative Example 1 did not sufficiently improve in corrosion resistance and fatigue characteristics compared to the austenitic stainless steel 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 an austenitic 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.

Explanation of Signs

[0074] 10 Austenitic stainless steel material 11 Matrix 12 Melt solidification layer 13 Inclusions 20 Cold-rolled steel sheet 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: 2.50% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 6.00 to 20.00%, Cr: 14.00 to 25.00%, Mo: 3.00% or less, Cu: 4.00% or less, N: 0.400% or less, Al: 3.500% or less, Si + 2Al is 1.20% or more, and the balance consists of Fe and impurities, 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 phase at a position where the depth from the surface is 1 / 4 of the thickness is 0.50 or less, and it is an austenitic stainless steel material.

2. In terms of mass basis, it 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%, and it is the austenitic stainless steel material according to Claim 1.

3. In terms of mass basis, it 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, and it is the austenitic stainless steel material according to Claim 1 or 2.

4. The arithmetic mean roughness Ra of the surface is 0.01 to 0.40 μm, and it is the austenitic stainless steel material according to any one of Claims 1 to 3.

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

6. It is provided with a fusion-solidified layer on the surface layer, and it is the austenitic stainless steel material according to any one of Claims 1 to 5.

7. The thickness of the fusion-solidified layer is 0.50 to 10.00 μm, and it is the austenitic stainless steel material according to Claim 6.

8. A method for manufacturing the austenitic stainless steel material according to Claim 1, Based on mass, it contains C: 0.001 to 0.150%, Si: 0.20 to 5.00%, Mn: 2.50% or less, P: 0.050% or less, S: 0.0300% or less, Ni: 6.00 to 20.00%, Cr: 14.00 to 25.00%, Mo: 3.00% or less, Cu: 4.00% or less, N: 0.400% or less, Al: 3.500% or less, Si + 2Al is 1.20% or more, and the balance consists of Fe and impurities. A descaling process is included in which a cold-rolled steel sheet is irradiated 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 from the interface between the matrix phase and the oxide scale in the cold-rolled steel sheet ranging from 0.50 to 10.00 μm.

9. A method for manufacturing an austenitic 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% based on mass. The method according to claim 8.

10. A method for manufacturing an austenitic 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 based on mass. The method according to claim 8 or 9.

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

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