Ferritic stainless steel sheet and laser-welded structure

Abstract

To provide a ferritic stainless steel sheet which ensures excellent corrosion resistance (including crevice corrosion resistance) and workability in the overall region including the laser-welded zone when laser welding is carried out.SOLUTION: A ferritic stainless steel sheet contains, on a mass basis, C: 0.020% or less, Si: more than 0.20% and 1.00% or less, Mn: 0.20% or less, P: 0.040% or less, S: 0.030% or less, Cr: 21.5-27.0%, Cu: 0.40% or less, Ni: 0.20-1.00%, Mo: 0.40% or less, Ti: 4(C+N)+0.10%-0.40%, Al: 0.030-0.100%, and N: 0.020% or less, with the balance being Fe and impurities. The ferritic stainless steel sheet has an area ratio of crystal grains having an aspect ratio of 5 or more of 0.20% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet and a laser-welded structure. More specifically, the present invention relates to a ferritic stainless steel sheet that is optimal for producing a laser-welded structure (particularly a tailored blank) obtained by laser welding, and a laser-welded structure obtained using the same. [Background technology]

[0002] Ferritic stainless steel sheets are used in many applications because they are cheaper than austenitic stainless steel sheets, which contain a lot of Ni, a rare metal that is expensive. In recent years, energy conservation during manufacturing has been required from the perspective of environmental conservation, and ferritic stainless steel sheets that can be manufactured without annealing or at lower annealing temperatures are in demand. Furthermore, since ferritic stainless steel sheets are often used after welding, ensuring corrosion resistance at the welds is also required. However, because carbon and nitrogen are difficult to dissolve in the ferrite phase, chromium carbonitrides precipitate at grain boundaries due to the heat effect during welding, resulting in a decrease in corrosion resistance known as sensitization. For this reason, it is known to add titanium or niobium, which bond more strongly to carbon and nitrogen than chromium.

[0003] In recent years, the tailored blank technology, in which materials are continuously welded and then pressed together to form a single piece, has begun to be used to simplify processes and reduce the number of molds required when manufacturing various parts. When ferritic stainless steel sheets are used as tailored blank materials, laser welding is often used from the perspective of productivity. In this case, even with sufficient gas shielding, it may not be possible to prevent oxidation or nitrogen penetration in the weld, and even the addition of Ti or Nb may not ensure the corrosion resistance and workability of the weld. Furthermore, tailored blanks are often joined together after forming to form parts, and crevice corrosion, which is specific to stainless steel, becomes an issue. In particular, crevice corrosion is likely to be induced when the laser welded joint of the tailored blank is located in the crevice structure of the part.

[0004] As a means for improving the crevice corrosion resistance of welded parts of ferritic stainless steel sheets, for example, Patent Document 1 discloses a method for improving the crevice corrosion resistance of welded crevice parts by using ferritic stainless steel having a predetermined composition, and achieving a tensile strength of 550 N / mm 2 The above-mentioned technology has been disclosed. Furthermore, as a means for improving the corrosion resistance of welded parts of ferritic stainless steel sheets, for example, Patent Document 2 discloses a technology for suppressing the peeling of black spots formed in the welded parts after processing, thereby ensuring corrosion resistance when processed after welding. Furthermore, as a means for improving the workability of welded parts of ferritic stainless steel sheets, for example, Patent Document 3 discloses a technique for improving the workability of welded parts by refining the solidified crystal grains of the welded parts through the combined addition of Ca and Zr. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-184732 [Patent Document 2] International Publication No. 2017 / 169377 [Patent Document 3] Special Publication No. 2010-507021 Summary of the Invention [Problem to be solved by the invention]

[0006] However, because Patent Document 1 is a technology for increasing the strength of TIG welds, there is a possibility that formability may be insufficient when applied to tailored blanks. Furthermore, while Patent Document 2 can prevent the peeling of black spots after processing, the corrosion resistance and processability of the laser welds may be insufficient. Furthermore, Patent Document 3 does not particularly address the issue of corrosion resistance of the laser welds, and there is a possibility that the corrosion resistance of the laser welds may be insufficient.

[0007] The present invention has been made to solve the above-mentioned problems, and aims to provide a ferritic stainless steel sheet that, when laser welded, has excellent corrosion resistance (including crevice corrosion resistance) and workability throughout the entire structure, including the laser weld. Another object of the present invention is to provide a laser-welded structure that has excellent corrosion resistance (including crevice corrosion resistance) and workability as a whole, including the laser-welded portion. [Means for solving the problem]

[0008] In order to solve the above problems with ferritic stainless steel sheets, the present inventors have conducted extensive research into the corrosion resistance and workability of the entire sheet, including laser welds made using N shielding gas, and have found that strict control of the composition and hot rolling conditions improves the corrosion resistance (including crevice corrosion resistance) and workability of the entire sheet, including laser welds. Specifically, the inventors have made the following findings. (1) By adding 21.5% or more Cr and adding 4(C+N)+0.10% or more Ti, and precipitating the dissolved N as TiN, Cr deficiency under the oxide film and sensitization of the weld are suppressed, thereby ensuring corrosion resistance. (2) By minimizing the addition of Nb, which acts as a solid solution strengthener for laser welds, and adding Ti at 4(C+N)+0.10% or more, and precipitating the N mixed in during laser welding as TiN, hardening of the welds can be suppressed and workability can be ensured. (3) By adding a certain amount of Si and Ni, the occurrence and progression of crevice corrosion can be suppressed. (4) By tightening the rough hot rolling conditions among the hot rolling conditions, grain size regulation is performed after rough hot rolling, and by tightening the finishing temperature and coiling temperature in the subsequent finish hot rolling, processing strain remains after finish hot rolling. Even if hot rolling annealing is omitted, corrosion resistance (including crevice corrosion resistance) and workability can be ensured by grain size regulation during cold rolling annealing.

[0009] The present invention has been completed through further investigation based on the above findings, and is exemplified as follows.

[0010] [1] By mass, it contains C: 0.020% or less, Si: over 0.20% and 1.00% or less, Mn: 0.20% or less, P: 0.040% or less, S: 0.030% or less, Cr: 21.5 to 27.0%, Cu: 0.40% or less, Ni: 0.20 to 1.00%, Mo: 0.40% or less, Ti: 4(C+N)+0.10% to 0.40%, Al: 0.030 to 0.100%, N: 0.020% or less, and the balance being Fe and impurities; A ferritic stainless steel sheet in which the area ratio of crystal grains with an aspect ratio of 5 or more is 0.20% or less.

[0011] [2] The ferritic stainless steel sheet according to [1], further containing, by mass, 0.10% or less of Nb.

[0012] [3] The ferritic stainless steel sheet according to [1] or [2], further comprising, by mass, one or more selected from V: 0.20% or less, W: 0.20% or less, Co: 0.50% or less, Zr: 0.20% or less, Sn: 0.50% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less.

[0013] [4] A laser-welded structure comprising a base material made of the ferritic stainless steel plate according to any one of [1] to [3] and a laser-welded portion.

[0014] [5] The laser-welded structure according to [4], wherein the difference between the Vickers hardness of the base material and the Vickers hardness of the laser weld is 30 HV or less.

[0015] [6] The laser-welded structure according to [4] or [5], wherein the maximum pitting depth of the laser weld in a neutral salt spray cycle test is 0.20 mm or less.

[0016] [7] The laser-welded structure according to any one of [4] to [6], wherein the maximum pitting depth of the laser-welded portion having a gap structure in a neutral salt spray cycle test is 0.40 mm or less. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a ferritic stainless steel sheet that, when laser welding is performed, has excellent corrosion resistance (including crevice corrosion resistance) and workability throughout the entire sheet including the laser welded portion. Furthermore, according to the present invention, it is possible to provide a laser-welded structure that has excellent corrosion resistance (including crevice corrosion resistance) and workability throughout the entire structure, including the laser-welded portion. DETAILED DESCRIPTION OF THE INVENTION

[0018] The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0019] (1. Ferritic stainless steel plate) A ferritic stainless steel sheet according to an embodiment of the present invention contains C: 0.020% or less, Si: more than 0.20% and 1.00% or less, Mn: 0.20% or less, P: 0.040% or less, S: 0.030% or less, Cr: 21.5 to 27.0%, Cu: 0.40% or less, Ni: 0.20 to 1.00%, Mo: 0.40% or less, Ti: 4(C+N)+0.10% to 0.40%, Al: 0.030 to 0.100%, N: 0.020% or less, with the balance being Fe and impurities.

[0020] In this specification, "ferritic" refers to a metal structure that is primarily ferrite at room temperature. Therefore, "ferritic" also includes those that contain small amounts of phases other than ferrite (e.g., austenite or martensite). However, "ferritic" does not include a multi-phase structure of ferrite and austenite, a multi-phase structure of ferrite and martensite, or a multi-phase structure of ferrite, austenite, and martensite. In addition, in this specification, the "stainless steel sheet" means a plate-shaped material formed from stainless steel, and the plate shape includes a strip shape.

[0021] In addition, in this specification, the "impurity" means a component that is mixed in due to raw materials such as ore and scrap, and various factors in the manufacturing process when industrially manufacturing a ferritic stainless steel sheet, and is allowed within a range that does not adversely affect the present invention. For example, the impurity includes inevitable impurities. Examples of the impurity include O and the like. Regarding the content of each element, including "xx% or less" means that it is xx% or less, but includes an amount exceeding 0% (especially exceeding the impurity level).

[0022] The ferritic stainless steel sheet according to an embodiment of the present invention can further contain Nb: 0.10% or less as needed. In addition, the ferritic stainless steel sheet according to an embodiment of the present invention can further contain one or more selected from V: 0.20% or less, W: 0.20% or less, Co: 0.50% or less, Zr: 0.20% or less, Sn: 0.50% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less as needed. Hereinafter, each component will be described in detail.

[0023] <C: 0.020% or less> C is an element that reduces formability and corrosion resistance. Therefore, the upper limit of the C content is 0.020%, preferably 0.018%, more preferably 0.016%. On the other hand, the lower limit of the C content is not particularly limited, but from the viewpoints of formability and corrosion resistance, a lower C content is preferable. However, reducing the C content to less than 0.002% incurs a large cost increase in existing steelmaking facilities. Therefore, the lower limit of the C content is preferably 0.002%, more preferably 0.003%.

[0024] <Si: more than 0.20% and 1.00% or less> Si is useful as a deoxidizer, improves oxidation resistance, and is an element that improves corrosion resistance (especially pitting corrosion resistance) in a neutral environment such as salt water. To ensure this effect, the lower limit of the Si content is set to more than 0.20%, preferably 0.21%, more preferably 0.22%. On the other hand, when the Si content is high, it hardens and significantly reduces the normal temperature ductility. Therefore, the upper limit of the Si content is set to 1.00%, preferably 0.95%, more preferably 0.90%.

[0025] <Mn: 0.20% or less> Mn is an element added as a deoxidizer. While it enhances scale adhesion and increases the pickling load during manufacturing, it also reduces the normal temperature ductility due to hardening. Therefore, the upper limit of the Mn content is set to 0.20%, preferably 0.18%. On the other hand, the lower limit of the Mn content is not particularly limited, but excessive reduction of the Mn content leads to an increase in refining costs. Therefore, the lower limit of the Mn content is preferably 0.01%, more preferably 0.02%.

[0026] <P: 0.040% or less> P is a solid solution strengthening element and reduces ductility and toughness. Therefore, it is desirable to reduce it as much as possible. For this reason, the upper limit of the P content is set to 0.040%, preferably 0.035%, more preferably 0.030%. On the other hand, the lower limit of the P content is not particularly limited, but excessive reduction of the P content leads to an increase in refining costs. Therefore, the lower limit of the P content is preferably 0.010%, more preferably 0.015%.

[0027] <S: 0.030% or less> S is an element that reduces elongation, has an adverse effect on formability and toughness, and also reduces corrosion resistance. Therefore, it is desirable to reduce it as much as possible. For this reason, the upper limit of the S content is set to 0.030%, preferably 0.010%, more preferably 0.005%. On the other hand, the lower limit of the S content is not particularly limited, but excessive reduction of the S content leads to an increase in refining costs. Therefore, the lower limit of the S content is preferably 0.0001%, more preferably 0.0005%.

[0028] <Cr: 21.5~27.0%> Cr is an important element effective in improving the corrosion resistance, which is a characteristic of stainless steel. In the case where the shielding gas is insufficient in the laser welded part, oxidation occurs and an oxide film mainly composed of Cr oxide is formed, and a Cr-deficient phase is generated under the oxide film. Even if a Cr-deficient phase is generated, in order to ensure the amount of Cr and obtain the corrosion resistance of the welded part, the lower limit of the Cr content is set to 21.5%, preferably 22.0%. On the other hand, Cr not only solid-solution strengthens stainless steel at room temperature, hardens it, and reduces ductility, but also significantly reduces toughness due to σ-phase and 475°C embrittlement. Since these are显著observed when the Cr content exceeds 27.0%, the upper limit of the Cr content is set to 27.0%, preferably 26.5%.

[0029] <Cu: 0.40% or less> Cu is an element that strengthens the passive film and enhances the corrosion resistance. On the other hand, Cu precipitates as a fine ε-Cu phase, hardens, and reduces the dissolution rate during pickling. Therefore, the upper limit of the Cu content is set to 0.40%, preferably 0.30%. On the other hand, the lower limit of the Cu content is not particularly limited, but excessive reduction of the Cu content requires high purity of the raw material, which leads to a decrease in productivity. Therefore, the lower limit of the Cu content is preferably 0.01%, more preferably 0.02%.

[0030] <Ni: 0.20~1.00%> Ni is an element that improves the toughness of ferritic stainless steel sheets and suppresses the progression after corrosion occurs. To ensure this effect, the lower limit of the Ni content is set to 0.20%, preferably 0.21%, more preferably 0.22%. On the other hand, Ni is an expensive element and reduces the dissolution rate during pickling. Therefore, the upper limit of the Ni content is set to 1.00%, preferably 0.90%, more preferably 0.85%.

[0031] <Mo: 0.40% or less> Mo is an element that improves corrosion resistance. On the other hand, Mo is an expensive element that hardens stainless steel through solid solution strengthening, reducing workability. Mo also reduces the dissolution rate during pickling. For this reason, the upper limit of the Mo content is set to 0.40%, preferably 0.30%. On the other hand, there is no particular restriction on the lower limit of the Mo content, but excessive reduction in the Mo content requires high-purity raw materials, which leads to reduced productivity. For this reason, the lower limit of the Mo content is preferably 0.01%, more preferably 0.02%.

[0032] <Ti:4(C+N)+0.10%~0.40%> Ti has the effect of immobilizing C and N, improving corrosion resistance, formability, and intergranular corrosion resistance of laser welds. In the present invention, Nb, which acts as a solid-solution strengthening agent during rapid cooling after laser welding, is not actively added, so Ti is an important element for immobilizing C and N. When the Ti content is 4(C + N) + 0.10% or more, TiN can be crystallized during welding even when the seal during laser welding is insufficient, and hardening of the laser weld can be suppressed during rapid cooling after laser welding. On the other hand, when the Ti content is lower than this range, TiN cannot be crystallized during cooling after laser welding, and the solid-solution N hardens the weld. Furthermore, Ti carbonitrides precipitated by immobilizing C and N become a source of strain fields during hot rolling, contributing to the disruption of the rolled elongation structure and contributing to grain regulation of cold-rolled and annealed sheets, which eliminate the need for hot-rolled annealing. Therefore, the lower limit of the Ti content is set to 4(C+N)+0.10%, preferably 4(C+N)+0.11%, and more preferably 4(C+N)+0.12%. On the other hand, if the Ti content exceeds 0.40%, it will cause a decrease in toughness and the occurrence of surface defects. Therefore, the upper limit of the Ti content is set to 0.40%, preferably 0.38%, and more preferably 0.36%.

[0033] <Al:0.030~0.100%> Al is an element added as a deoxidizing element, which can improve the corrosion resistance of the laser welded part. To obtain its effect, the lower limit of the Al content is set to 0.030%, preferably 0.032%. On the other hand, an excessive increase in the Al content not only hardens the stainless steel but may also generate slag spots during laser welding, resulting in a decrease in corrosion resistance. Therefore, the upper limit of the Al content is set to 0.100%, preferably 0.090% or less.

[0034] <N: 0.020% or less> Similar to C, N is an element that reduces formability and corrosion resistance, so its content should be as low as possible. Therefore, the upper limit of the N content is set to 0.020%, preferably 0.018%, more preferably 0.016%. On the other hand, the lower limit of the N content is not particularly limited, but an excessive reduction in the N content will lead to an increase in refining costs. Therefore, the lower limit of the N content is preferably 0.002%, more preferably 0.003%.

[0035] <Nb: 0.10% or less> Similar to Ti, Nb is an element that easily forms carbonitrides with C and N and suppresses sensitization. On the other hand, since the cooling after laser welding is rapid cooling, Nb cannot precipitate carbonitrides and remains in a solid solution state in the laser welded part, resulting in a decrease in workability due to hardening of the laser welded part. In addition, Nb raises the recrystallization temperature by precipitating finely during manufacturing. Therefore, the upper limit of the Nb content is set to 0.10%, preferably 0.09%, more preferably 0.08%. On the other hand, the lower limit of the Nb content is not particularly limited, but by adding Nb, it is possible to suppress grain growth after rough hot rolling and suppress the recovery of strain introduced during finish hot rolling. From the perspective of obtaining these effects, the lower limit of the Nb content is preferably 0.02%, more preferably 0.03%.

[0036] <V: 0.20% or less> V is an element that forms carbonitrides, similar to Nb and Ti, and like Nb, it causes hardening of the laser welded part and an increase in the recrystallization temperature. Therefore, the upper limit of the V content is set to 0.20%, preferably 0.18%, more preferably 0.16%. On the other hand, the lower limit of the V content is not particularly limited, but from the viewpoint of ensuring the above effects, it is preferably 0.01%, more preferably 0.02%.

[0037] <W: 0.20% or less> W is an element that forms carbonitrides, similar to Nb and Ti, and like Nb, it causes hardening of the laser welded part and an increase in the recrystallization temperature. Therefore, the upper limit of the W content is set to 0.20%, preferably 0.18%, more preferably 0.16%. On the other hand, the lower limit of the W content is not particularly limited, but from the viewpoint of ensuring the above effects, it is preferably 0.01%, more preferably 0.02%.

[0038] <Co: 0.50% or less> Co is an element that improves the resistance to intergranular corrosion. However, Co is an expensive element and hardens the steel by solid solution strengthening, resulting in a decrease in workability. Therefore, the upper limit of the Co content is set to 0.50%, preferably 0.40%, more preferably 0.30%. On the other hand, the lower limit of the Co content is not particularly limited, but from the viewpoint of ensuring the above effects, it is preferably 0.01%, more preferably 0.02%.

[0039] <Zr: 0.20% or less> Zr is an element that forms carbonitrides, similar to Nb and Ti, but an excessive increase in the Zr content generates intermetallic compounds and reduces the toughness of the stainless steel sheet. Therefore, the upper limit of the Zr content is set to 0.20%, preferably 0.18%, more preferably 0.16%. On the other hand, the lower limit of the Zr content is not particularly limited, but from the viewpoint of ensuring the above effects, it is preferably 0.01%, more preferably 0.02%.

[0040] <Sn: 0.50% or less> Sn is an element that does not significantly deteriorate the mechanical properties at room temperature and improves corrosion resistance. However, an excessive increase in the Sn content significantly reduces productivity. Therefore, the upper limit of the Sn content is set to 0.50% or less, preferably 0.30% or less, more preferably 0.20% or less. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of ensuring the above effects, it is preferably 0.01%, more preferably 0.02%.

[0041] <B: 0.0050% or less> B is an element that improves workability, particularly secondary workability. However, excessive addition of the B content reduces weldability and toughness. Therefore, the upper limit of the B content is set to 0.0050%, preferably 0.0030%, more preferably 0.0015%. On the other hand, the lower limit of the B content is not particularly limited, but from the viewpoint of ensuring the above effects, it is preferably 0.0002%, more preferably 0.0005%.

[0042] <Ca: 0.0100% or less> Ca is an element effective in suppressing nozzle blockage that is likely to occur during continuous casting. However, an excessive increase in the Ca content tends to cause surface defects and generates slag spots during laser welding, resulting in a decrease in corrosion resistance. Therefore, the upper limit of the Ca content is set to 0.0100%, preferably 0.0050%, more preferably 0.0030%. On the other hand, the lower limit of the Ca content is not particularly limited, but from the viewpoint of ensuring the above effects, it is preferably 0.0002%, more preferably 0.0010%.

[0043] <Mg: 0.0100% or less> Mg is an element that improves the equiaxed crystal ratio of the solidification structure and is effective in improving workability and toughness. However, an excessive increase in the Mg content reduces the toughness of the stainless steel sheet and deteriorates the surface properties, and generates slag spots during laser welding, resulting in a decrease in corrosion resistance. Therefore, the upper limit of the Mg content is set to 0.0100%, preferably 0.0050%, more preferably 0.0030%. On the other hand, the lower limit of the Mg content is not particularly limited, but from the viewpoint of ensuring the above effects, it is preferably 0.0002%, more preferably 0.0010%.

[0044] <REM:Less than 0.0100%> REM (rare earth elements) is an element that improves oxidation resistance. However, an excessive increase in the REM content reduces weldability and toughness, generates slag spots during laser welding, and leads to a decrease in corrosion resistance. Therefore, the upper limit of the REM content is set to 0.0100%, preferably 0.0080%, more preferably 0.0050%. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of ensuring the above effects, it is preferably 0.0002%, more preferably 0.0010%. Note that REM is a general term for a total of 17 elements including Sc, Y, and lanthanoids, and means rare earth metals. Specifically, La, Ce, Nd, etc. can be mentioned, and one of these can be contained alone or in combination of two or more. When two or more rare earth elements are contained, the REM content means the total content of these rare earth elements.

[0045] Next, the metallographic structure of the ferritic stainless steel sheet according to the embodiment of the present invention will be described. The ferritic stainless steel sheet according to the embodiment of the present invention has an area ratio of crystal grains with an aspect ratio of 5 or more of 0.20% or less, preferably 0.15% or less, more preferably 0.10% or less. By controlling this area ratio to 0.20% or less, it is possible to suppress a decrease in ductility due to surface undulations caused by crystal grains with a large aspect ratio during processing. When this area ratio exceeds 0.20%, the surface undulations caused by crystal grains with a large aspect ratio increase, and local deformation occurs early along the surface undulations, inducing cracks during processing. On the other hand, the lower limit of this area ratio is not particularly limited because the smaller it is, the better, and for example, it can be set to 0%. Note that the aspect ratio of crystal grains is the value obtained by dividing the long diameter of crystal grains by the short diameter. Also, the aspect ratio of crystal grains corresponds to the strain in the ferritic stainless steel sheet, and when the aspect ratio is 5 or more, a decrease in ductility due to surface undulations generated during processing becomes apparent.

[0046] The area fraction of crystal grains with an aspect ratio of 5 or more can be calculated as follows. First, an L-section (a cross section in the thickness direction perpendicular to the width direction) of a ferritic stainless steel sheet is colloidally polished, followed by electrolytic polishing to remove polishing strain. Next, the center of the sheet thickness of the polished cross section is used as the observation region, and an orientation map is obtained using electron backscatter diffraction (EBSD) with a scanning electron microscope. From the obtained orientation map, boundaries with a crystal orientation misorientation of 5° or more are defined as grain boundaries, and the aspect ratio of each crystal grain is calculated by dividing the long side of each crystal grain by the short side. The total area of crystal grains with an aspect ratio of 5 or more is then divided by the area of the observation region and multiplied by 100 to calculate the area fraction of crystal grains with an aspect ratio of 5 or more.

[0047] The ferritic stainless steel sheet according to the embodiment of the present invention is not particularly limited as long as it has the above-mentioned composition and metal structure, but is preferably a cold-rolled annealed sheet. The thickness of the ferritic stainless steel sheet is also not particularly limited, but is typically 0.1 to 4.0 mm, preferably 0.1 to 3.0 mm, and more preferably 0.1 to 2.0 mm.

[0048] (2. Manufacturing method of ferritic stainless steel sheet) The ferritic stainless steel sheet according to the embodiment of the present invention is not particularly limited as long as it can be produced with the above-described composition and metallographic structure. An example of a method for producing a ferritic stainless steel sheet according to the embodiment of the present invention will be described below. A method for producing a ferritic stainless steel sheet according to an embodiment of the present invention includes a hot rolling step including rough hot rolling and finish hot rolling, a cold rolling step, and an annealing step. After the hot rolling step, descaling may be performed by pickling or the like, and annealing after the hot rolling step is preferably omitted. After the annealing step, descaling may be performed by pickling or the like, or the shape and surface roughness of the ferritic stainless steel sheet may be adjusted by skin-pass rolling or the like.

[0049] <Hot rolling process> The hot rolling step is a step in which a slab having the above composition is roughly hot rolled and then finish hot rolled. The slab can be produced by melting steel having the above composition and using a conventional method. The slab (for example, about 200 mm thick) is heated and then roughly hot-rolled. The heating temperature of the slab is usually in the range of 1150 to 1250°C, but 1150 to 1200°C is preferred.

[0050] Rough hot rolling is performed with a final temperature of 1050°C or higher. If the final temperature of rough hot rolling is less than 1050°C, an unrecrystallized structure will remain. The upper limit of the final temperature of rough hot rolling is not particularly limited, but is typically 1150°C, preferably 1100°C. Furthermore, the total reduction rate of the rough hot rolling is not particularly limited, but is preferably 80% or more. By setting the total reduction rate to 80% or more, the amount of strain required for recrystallization after rough hot rolling can be ensured. The upper limit of the total reduction rate of the rough hot rolling is not particularly limited, but is typically 98%. The rough hot rolling may be performed in one pass or multiple passes.

[0051] Finish hot rolling is performed with an interpass time of 30 to 90 seconds between the end of rough hot rolling and the start of finish hot rolling, a final temperature of 900°C or less, and a coiling temperature of 420°C or less. By controlling the inter-pass time from the end of rough hot rolling to the start of finish hot rolling within the above range, it is possible to form fine, granular recrystallized grains in which coarse elongated grains are broken up before finish hot rolling. On the other hand, if the inter-pass time from the end of rough hot rolling to the start of finish hot rolling is outside the above range, an unrecrystallized structure will result or excessive grain growth will occur after recrystallization. If the final temperature of finish hot rolling exceeds 900°C, the working strain introduced during finish hot rolling will be recovered, and elongated grains will likely remain in the structure after cold rolling annealing. The lower limit of the final temperature of finish hot rolling is not particularly limited, but is typically 800°C. If the coiling temperature exceeds 420°C, toughness will decrease due to 475°C embrittlement promoted by the strain introduced during finish hot rolling, and manufacturability will be significantly reduced. The lower limit of the coiling temperature is not particularly limited, but is typically 250°C. Furthermore, the finish hot rolling can be performed in multiple passes, but the interval between each pass is preferably 5 seconds or less. If the interval between passes exceeds 5 seconds, the working strain introduced during the finish hot rolling will be recovered, and elongated grains may be more likely to remain in the structure after cold rolling annealing.

[0052] <Cold rolling process> The cold rolling step is a step of cold rolling the hot-rolled sheet obtained in the hot rolling step. The conditions for cold rolling are not particularly limited, and the cold rolling can be carried out in accordance with conventional methods. Although cold rolling is preferably performed once, it may be performed two or more times with intermediate annealing in between in terms of productivity and surface quality. The conditions for intermediate annealing are not particularly limited, and the cold rolling may be performed in accordance with conventional methods. The total reduction rate of the one or more cold rolling passes is not particularly limited, but is preferably 60% or more, more preferably 70% or more, and the upper limit of the total reduction rate is typically 90%.

[0053] <Annealing process> The annealing process is a process in which the cold-rolled sheet obtained in the cold-rolling process is annealed at a temperature of 850 to 950°C. If the annealing temperature is less than 850°C, an unrecrystallized structure is formed, which not only hardens but also leaves elongated grains. On the other hand, if the annealing temperature exceeds 950°C, recrystallized grains are obtained, but the crystal grain size becomes coarse. Therefore, when laser welding is performed, the solidification structure of the laser weld becomes coarse, and the toughness of the laser weld is reduced. Furthermore, the coarsening of the crystal grain size softens the base material, increasing the hardness difference with the laser weld, and also resulting in reduced workability after laser welding. From the viewpoint of stably ensuring a predetermined crystal structure, the annealing temperature is preferably 860 to 920°C. The annealing time is not particularly limited, but is typically 1 to 10 minutes.

[0054] (3. Laser-welded structures) A laser-welded structure according to an embodiment of the present invention includes a base material made of a ferritic stainless steel plate and a laser weld. This laser-welded structure can be suitably used as a tailored blank. In this specification, the term "base material" refers to a portion that is not affected by laser welding. The term "laser welded portion" refers to both the heat-affected zone (also called HAZ), which is not melted by the laser welding but is thermally affected, and the weld metal portion that melts and re-solidifies due to the laser welding.

[0055] In the laser-welded structure according to the embodiment of the present invention, the difference between the Vickers hardness of the base material and the Vickers hardness of the laser weld is preferably 30 HV or less, more preferably 25 HV or less. If this difference is 30 HV or less, the toughness of the base material and the laser weld can be said to be comparable, ensuring overall workability including the laser weld. On the other hand, if this difference exceeds 30 HV, the laser weld is hard and therefore difficult to deform when the laser-welded structure is processed. This causes strain to concentrate around the laser weld, leading to early cracking around the laser weld. Furthermore, if the laser weld itself is too hard, it may lack toughness and crack. The lower limit of this difference is not particularly limited and may be 0 HV. In this specification, the Vickers hardness means a value measured by the method described in the examples below.

[0056] In the laser-welded structure according to the embodiment of the present invention, it is preferable that the maximum pitting depth of the laser weld in a neutral salt spray cycle test is 0.20 mm or less. If the maximum pitting depth in this test is 0.20 mm or less, the corrosion resistance of the laser weld can be ensured. The lower limit of the maximum pitting depth in this test is not particularly limited and may be 0 mm. In this specification, the maximum pitting depth in a neutral salt spray cycle test of a laser welded portion means the maximum pitting depth measured by the method described in the Examples section below.

[0057] In the laser-welded structure according to the embodiment of the present invention, it is preferable that the maximum pitting depth of the laser weld having a crevice structure in a neutral salt spray cycle test is 0.40 mm or less. If the maximum pitting depth in this test is 0.40 mm or less, the crevice corrosion resistance of the laser weld having a crevice structure can be ensured. The lower limit of the maximum pitting depth in this test is not particularly limited and may be 0 mm. In this specification, the maximum pitting depth in a neutral salt spray cycle test of a laser welded portion having a crevice structure means a value measured by the method described in the Examples section below.

[0058] The laser-welded structure according to the embodiment of the present invention can be manufactured by using the above-mentioned ferritic stainless steel plate as a material and performing laser welding with a predetermined irradiation energy. When laser welding ferritic stainless steel materials of different thicknesses together, the irradiation energy is controlled based on the thickness of the thinner one. Laser welding may also be used to weld multiple ferritic stainless steel materials together, or to weld ferritic stainless steel materials to metal materials of other materials. The laser used for laser welding is not particularly limited, and known lasers such as fiber lasers, CO2 lasers, YAG lasers, and semiconductor lasers can be used. [Example]

[0059] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0060] Stainless steel having the composition shown in Table 1 (the balance being Fe and impurities) was vacuum melted and cast into a 200 mm thick slab. The slab was then heated at 1200°C for 2 hours and then roughly hot rolled at the final temperature shown in Table 2 (total reduction of 81%, 8 passes). Next, finish hot rolling (6 passes) was performed with the interpass time from the end of rough hot rolling to the start of finish hot rolling (abbreviated as "interpass" in Table 2), final temperature, and coiling temperature shown in Table 2, to obtain a hot-rolled sheet with a thickness of 4.0 mm. The sheet passing speed during finish hot rolling was adjusted so that the interpass time between each pass was 3 seconds or less. The hot-rolled sheet was then pickled and cold-rolled at a total reduction of 80% to obtain a cold-rolled sheet with a thickness of 0.8 mm. The cold-rolled sheet was then annealed for 2 minutes at the temperature shown in Table 2 to obtain a cold-rolled annealed sheet. For Test No. 22, annealing was performed at 901°C for 1 minute between finish hot rolling and pickling, and for Test No. 25, annealing was performed at 925°C for 1 minute between finish hot rolling and pickling.

[0061] [Table 1]

[0062] [Table 2]

[0063] The cold-rolled and annealed sheets obtained above were subjected to the following evaluations.

[0064] (area ratio of crystal grains with aspect ratio of 5 or more) The area ratio of crystal grains with an aspect ratio of 5 or more was determined as described above. Specifically, the L-section (thickness direction cross section perpendicular to the sheet width direction) of the cold-rolled annealed sheet was colloidally polished and then electropolished to remove polishing strain. Next, the center of the sheet thickness (400 μm × 2000 μm) of the polished cross section was used as the observation region, and an orientation map was obtained by electron backscatter diffraction (EBSD) using a scanning electron microscope. From the obtained orientation map, boundaries with a crystal orientation misorientation of 5° or more were defined as grain boundaries, and the aspect ratio of each crystal grain was calculated by dividing the long side of each crystal grain by the short side. The area ratio of crystal grains with an aspect ratio of 5 or more was then calculated by dividing the total area of crystal grains with an aspect ratio of 5 or more by the area of the observation region and multiplying the result by 100.

[0065] Next, a laser-welded structure was produced using the above cold-rolled annealed sheet. Specifically, the laser-welded structure was produced as follows. First, two test pieces (180 mm long in the rolling direction × 50 mm long in the width direction) were taken from the cold-rolled annealed sheet, and the end faces of both pieces in the rolling direction were butted together and laser-welded. A fiber laser was used for the laser welding, with a laser output of 2.0 kW, a welding speed of 5 m / min, and a flow rate of shielding gas (N2) of 30 L / min. The laser-welded structures obtained above were evaluated as follows.

[0066] (Difference between the Vickers hardness of the base material and the Vickers hardness of the laser weld) Five samples were obtained by cutting the laser-welded structure at 30 mm intervals so that the cross section perpendicular to the welding direction would be the observation surface. Next, the sample was embedded in resin so that the cross section was exposed, and then polished and electrolytically etched. The Vickers hardness of the base material and the laser welded section in the cross section was then measured. The Vickers hardness was measured using a Vickers hardness tester in accordance with JIS Z2244:2009. The test load in this measurement was HV0.2 (1.96 N). The Vickers hardness of the laser weld was measured at a grid of nine points with a pitch of 200 μm, centered on the center of the laser weld. The average of the seven measured values, excluding the maximum and minimum values, was calculated. This measurement was performed on five samples, and the average was used as the measurement result. Similarly, the Vickers hardness of the base material was measured at a grid of nine points with a 200 μm pitch, centered at a position 10 mm away from the center of the laser weld in the width direction, and the average of the seven measured values excluding the maximum and minimum values was calculated. This measurement was performed on five samples, and the average was used as the measurement result. Based on the Vickers hardness results of the base material and the laser weld obtained as described above, the difference between them was calculated. In this evaluation, if the difference in Vickers hardness was 30 HV or less, it could be determined that the overall workability, including the laser weld, was good.

[0067] (Maximum pitting depth in a neutral salt spray cycle test on laser welded parts) A 100 mm (welding direction) x 50 mm (perpendicular to the welding direction) sample was cut from the laser-welded structure, centered on the laser weld. The edges of the sample were then sealed and subjected to a neutral salt spray cycle test (CCT). The neutral salt spray cycle test involves multiple cycles of spraying a 5% by weight NaCl solution (35°C, 2 hours), drying (60°C, 1 hour, 30% RH), and wetting (50°C, 3 hours, 95% RH). After 90 cycles, the rust on the sample was removed, and the pitting depth of the laser weld was measured using a microscope's focal depth method. In this evaluation, if the maximum pitting depth of the laser weld (maximum pitting depth) was 0.20 mm or less, which is 1 / 4 of the plate thickness, the overall corrosion resistance, including the laser weld, was determined to be good.

[0068] (Maximum pitting depth in a neutral salt spray cycle test of laser welded parts with gap structures) A 100 mm (welding direction) × 50 mm (perpendicular to the welding direction) sample was cut from the laser-welded structure, centered on the laser weld. A 30 mm × 30 mm ferritic stainless steel plate (made of the same material as the cold-rolled annealed sheet used in the laser-welded structure) was then placed on top of the center of this sample and spot-welded to create a sample with a crevice structure. This sample was subjected to a neutral salt spray cycle test (CCT) under the same conditions as above, after which the 30 mm × 30 mm ferritic stainless steel plate was removed and the pitting depth of the laser weld was measured. In this evaluation, if the maximum pitting depth of the laser weld (maximum pitting depth) was 0.40 mm or less, which is half the plate thickness, the overall crevice corrosion resistance, including the laser weld, was determined to be good.

[0069] The evaluation results are shown in Table 3. In Table 3, the area ratio of crystal grains with an aspect ratio of 5 or more is abbreviated as "area ratio of crystal grains," and the difference between the Vickers hardness of the base material and the Vickers hardness of the laser weld is abbreviated as "Vickers hardness difference."

[0070] [Table 3]

[0071] As shown in Table 3, the cold-rolled annealed sheets (ferritic stainless steel sheets) of Test Nos. 1 to 13 had compositions and area ratios of crystal grains with aspect ratios of 5 or more that were in appropriate ranges, and the results for Vickers hardness difference and maximum pitting depth were also good. Therefore, these cold-rolled annealed sheets had excellent overall workability and corrosion resistance (including crevice corrosion resistance) including the laser welds. In contrast, the cold-rolled annealed sheets of Test Nos. 14 to 20 had inappropriate compositions, and therefore had poor results in one or more of the area ratio of crystal grains with an aspect ratio of 5 or more, the Vickers hardness difference, and the maximum pitting depth. Therefore, these cold-rolled annealed sheets had insufficient overall workability and corrosion resistance (including crevice corrosion resistance) including the laser welds. The cold-rolled annealed sheets of Test Nos. 21 to 25 were manufactured under inappropriate conditions, resulting in poor results for the area ratio of crystal grains with an aspect ratio of 5 or more. Therefore, these cold-rolled annealed sheets had poor workability because the base material had surface unevenness caused by crystal grains with a large aspect ratio, which led to early necking and cracking.

[0072] As can be seen from the above results, the present invention can provide a ferritic stainless steel sheet that, when laser welded, has excellent corrosion resistance (including crevice corrosion resistance) and workability throughout the entire structure, including the laser weld. Furthermore, the present invention can provide a laser-welded structure that has excellent corrosion resistance (including crevice corrosion resistance) and workability throughout the entire structure, including the laser weld.

Claims

1. On a mass basis, the steel sheet contains C: 0.020% or less, Si: more than 0.20% and 1.00% or less, Mn: 0.20% or less, P: 0.040% or less, S: 0.030% or less, Cr: 21.5 to 27.0%, Cu: 0.40% or less, Ni: 0.20 to 1.00%, Mo: 0.40% or less, Ti: 4(C+N)+0.10% to 0.40%, Al: 0.030 to 0.100%, N: 0.020% or less, and the balance being Fe and impurities, A ferritic stainless steel sheet in which the area ratio of crystal grains with an aspect ratio of 5 or more is 0.20% or less.

2. The ferritic stainless steel sheet according to claim 1, further comprising, on a mass basis, 0.10% or less of Nb.

3. 3. The ferritic stainless steel sheet according to claim 1, further comprising, on a mass basis, one or more selected from V: 0.20% or less, W: 0.20% or less, Co: 0.50% or less, Zr: 0.20% or less, Sn: 0.50% or less, B: 0.0050% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less.

4. A laser-welded structure comprising a base material made of the ferritic stainless steel plate according to claim 1 or 2 and a laser-welded portion.

5. 5. The laser-welded structure according to claim 4, wherein the difference in Vickers hardness between the base material and the laser weld is 30 HV or less.

6. The laser-welded structure according to claim 4, wherein the maximum pitting depth of the laser weld in a neutral salt spray cycle test is 0.20 mm or less.

7. 5. The laser-welded structure according to claim 4, wherein the laser-welded portion having a gap structure has a maximum pitting depth of 0.40 mm or less in a neutral salt spray cycle test.

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