Stainless steel material, spot-welded structure and manufacturing method thereof

Abstract

To provide a stainless steel material allowing spot welding and excellent in dimensional accuracy even when subjected to blackening heating.SOLUTION: A stainless steel material includes a base having a composition of by mass, C:0.100% or less, Si:1.00% or less, Mn:0.05-1.00%, P:0.100% or less, S:0.100% or less, Cr:16.00-25.00%, Ni: 1.00% or less, Cu:1.00% or less, Mo:2.00% or less, N:0.100% or less, Al:1.00% or less, Ti: 0.08-0.50% and the remainder consisting of Fe and impurities; and a pretreatment coating formed on the surface of the base and having a thickness of 50-200 nm and a Mn molar fraction of 0.30 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a stainless steel material, a spot-welded structure, and a method for manufacturing the same. [Background technology]

[0002] Stainless steel is a material with excellent corrosion resistance, and taking advantage of its glossy silver-white surface, it is used in a variety of parts, such as interior and exterior building materials and exhaust system parts. Furthermore, in order to enhance the design of stainless steel, it is often given a color tone, typically black, using methods such as chemical coloring, painting, and oxidation treatment. For example, Patent Documents 1 and 2 describe black stainless steel in which a blackened film (oxide film) is formed on the surface of the stainless steel by oxidation treatment. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-178392 [Patent Document 2] Patent No. 6307188 Summary of the Invention [Problem to be solved by the invention]

[0004] When various parts are manufactured using stainless steel, the stainless steel is subjected to various processes such as press working and welding. For example, exhaust system parts such as mufflers are often manufactured by press working followed by arc welding such as TIG or MIG. On the other hand, spot welding is sometimes used to join exhaust system parts to the vehicle body and accessories. Furthermore, depending on the type of part, the manufacturing process itself may be performed using spot welding. Note that spot welding is a welding method in which the workpieces are sandwiched between a pair of electrodes and joined by applying pressure and passing an electric current through them, thereby locally heating and melting the workpieces.

[0005] The blackened coating of Patent Document 1 is a film formed by laminating, in order from the substrate side, a first region containing an inner oxide of one or more of Al, Ti, and Mn, a second region consisting mainly of Cr oxide (Cr2O3), and a third region consisting of a Mn-enriched layer or a Mn and Ti-enriched layer. This blackened coating is difficult to spot weld because the Cr oxide has low conductivity and current is difficult to pass through. This is also true for the blackened coating of Patent Document 2. Therefore, the blackened coatings of Patent Documents 1 and 2 are not suitable for manufacturing or joining parts using spot welding.

[0006] For the reasons described above, in order to perform spot welding, it is necessary to use a material that has not yet been subjected to blackening heat treatment, and to perform blackening heat treatment after manufacturing or joining parts by spot welding. However, blackening heat treatment requires high-temperature heat treatment in an oxidizing atmosphere (specifically, heat treatment at 1050°C or 1100°C for 3 minutes), which makes the parts prone to deformation.

[0007] The present invention has been made to solve the above problems, and aims to provide a stainless steel material that can be spot welded and has good dimensional accuracy even when subjected to blackening heat treatment. Another object of the present invention is to provide a spot-welded structure that can be manufactured by spot welding and has good dimensional accuracy, and a method for manufacturing the same. [Means for solving the problem]

[0008] As a result of intensive research conducted by the inventors to solve the above problems, they discovered that by pretreating a stainless steel material having a specific composition, it is possible to form a pretreatment coating of a predetermined thickness that is suitable for spot welding and for performing blackening heat treatment at a low temperature that is less likely to reduce dimensional accuracy, and they have completed the present invention.

[0009] That is, the present invention provides a substrate having a composition, by mass, containing C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Al: 1.00% or less, Ti: 0.08 to 0.50%, with the balance being Fe and impurities; a pretreatment film formed on the surface of the substrate, the pretreatment film having a thickness of 50 to 200 nm and a Mn fraction of 0.30 or more; It is a stainless steel material containing

[0010] The present invention also provides a spot-welded structure including a stainless steel base material and a spot weld, The stainless steel base material is a substrate having a composition containing, by mass, C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Al: 1.00% or less, Ti: 0.08 to 0.50%, with the balance being Fe and impurities; A black coating formed on the surface of the substrate, L * a * b * In the color system, the lightness index L * is 45.0 or less, Chromathetics Index a * and b * A blackened film with a value within ±5.00 A spot welded structure comprising:

[0011] Furthermore, the present invention provides a method for producing a spot-welded structure, in which the stainless steel materials are spot-welded and then blackened at a temperature of less than 800°C. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a stainless steel material that is spot weldable and has good dimensional accuracy even after blackening heat treatment. Furthermore, according to the present invention, it is possible to provide a spot-welded structure that can be manufactured by spot welding and has good dimensional accuracy, and a method for manufacturing the same. DETAILED DESCRIPTION OF THE INVENTION

[0013] The following is a detailed description of embodiments of the present invention that have been completed based on the above-mentioned viewpoints. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0014] <Stainless steel material> A stainless steel material according to an embodiment of the present invention includes a substrate and a pretreatment coating formed on the surface of the substrate. Here, in this specification, "stainless steel material" means a material formed from stainless steel, and the shape of the material is not particularly limited. Examples of the shape include a plate (including a strip), a rod, and a tube. In addition, the cross section may be a variety of shaped steel such as a T-shape or an I-shape.

[0015] The base material of the stainless steel material has a composition containing C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Al: 1.00% or less, Ti: 0.08 to 0.50%, with the remainder being Fe and impurities. In this specification, the term "impurities" refers to components that are mixed in during industrial production of stainless steel materials due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, impurities include unavoidable impurities. An example of an impurity is O.

[0016] The base material of the stainless steel material may further contain at least one selected from Nb: 0.50% or less, Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less, REM: 0.100% or less, Ca: 0.100% or less, Sn: 0.100% or less, and B: 0.0100% or less. The reasons for limiting the content of each of the above elements will be explained below.

[0017] (C: 0.100% or less) C is an element that affects the properties of stainless steel materials, such as intergranular corrosion resistance (sensitization suppression effect) and workability. However, if the C content is too high, the workability and intergranular corrosion resistance of the stainless steel material will decrease. Therefore, the upper limit of the C content is 0.100%, preferably 0.080%, and more preferably 0.060%. On the other hand, the lower limit of the C content is not particularly limited, but an excessively low C content will lead to an increase in refining costs. Therefore, the lower limit of the C content is preferably 0.0001%, and more preferably 0.0003%.

[0018] (Si:1.00% or less) Silicon is an element that improves the oxidation resistance of stainless steel materials. However, if the Si content is too high, workability and weld toughness decrease. Therefore, the upper limit of the Si content is 1.00%, preferably 0.90%, and more preferably 0.80%. On the other hand, the lower limit of the Si content is not particularly limited, but from the viewpoint of obtaining the effects of Si, it is preferably 0.005%, more preferably 0.01%, and even more preferably 0.015%.

[0019] (Mn: 0.05 to 1.00%) Mn is an element that is effective in ensuring the color tone of the oxide film (blackened film after blackening heat treatment). In particular, Mn imparts a black color tone by forming a complex oxide with Cr. However, if the Mn content is too high, MnS, which acts as a corrosion starting point, is more likely to be generated and the ferrite phase is destabilized. Therefore, the upper limit of the Mn content is 1.00%, preferably 0.95%, and more preferably 0.90%. On the other hand, if the Mn content is too low, the above effects may not be fully achieved. Therefore, the lower limit of the Mn content is 0.05%, preferably 0.055%, and more preferably 0.06%.

[0020] (P:0.100% or less) P is an element that affects the properties of stainless steel materials, such as weldability and workability. If the P content is too high, the above properties may be reduced. Therefore, the upper limit of the P content is 0.100%, preferably 0.080%, and more preferably 0.060%. On the other hand, the lower limit of the P content is not particularly limited, but an excessively low P content leads to an increase in refining costs. Therefore, the lower limit of the P content is preferably 0.001%, and more preferably 0.005%.

[0021] (S:0.100% or less) S is an element that generates MnS, which acts as a corrosion initiation site, and affects the properties of stainless steel materials, such as the toughness of welded joints. If the S content is too high, the above properties may be reduced. Therefore, the upper limit of the S content is 0.100%, preferably 0.080%, and more preferably 0.060%. On the other hand, the lower limit of the S content is not particularly limited, but an excessively low S content leads to an increase in refining costs. Therefore, the lower limit of the S content is preferably 0.0001%, and more preferably 0.0002%.

[0022] (Cr:16.00~25.00%) Cr is an element effective in improving the corrosion resistance and oxidation resistance of stainless steel materials. Cr is also an element effective in ensuring the color tone of the oxide film (blackened film after blackening heat treatment). However, if the Cr content is too high, the toughness of the stainless steel material decreases and the growth of the oxide film is inhibited, making it impossible to form an oxide film with a black color tone. Therefore, the upper limit of the Cr content is 25.00%, preferably 24.50%, and more preferably 24.00%. On the other hand, if the Cr content is too low, the above effects cannot be sufficiently obtained. Therefore, the lower limit of the Cr content is 16.00%, preferably 16.25%, and more preferably 16.50%.

[0023] (Ni: 1.00% or less) Ni is an element effective in improving the corrosion resistance and weld toughness of stainless steel materials. However, if the Ni content is too high, the ferrite phase becomes unstable and the manufacturing cost increases. Therefore, the upper limit of the Ni content is 1.00%, preferably 0.90%, and more preferably 0.80%. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of obtaining the above-mentioned effects, it is preferably 0.005%, and more preferably 0.01%.

[0024] (Cu:1.00% or less) Cu is an element effective in improving the corrosion resistance of stainless steel materials. However, if the Cu content is too high, the ferrite phase becomes unstable and the manufacturing cost increases. Therefore, the upper limit of the Cu content is 1.00%, preferably 0.90%, and more preferably 0.80%. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.005%, and more preferably 0.01%.

[0025] (Mo: 2.00% or less) Mo is an element effective in improving the corrosion resistance and oxidation resistance of stainless steel materials. However, if the Mo content is too high, it will lead to a decrease in the workability of the stainless steel material and an increase in production costs. Therefore, the upper limit of the Mo content is 2.00%, preferably 1.95%, and more preferably 1.90%. On the other hand, the lower limit of the Mo content is not particularly limited, but is preferably 0.001%, and more preferably 0.005%.

[0026] (N:0.100% or less) N is an element that affects properties such as intergranular corrosion resistance (sensitization suppression) and workability. However, if the N content is too high, the intergranular corrosion resistance and workability of the stainless steel material will decrease. Furthermore, if the N content is high, TiN will easily precipitate, reducing the amount of solute Ti in the steel and inhibiting the formation of a blackened film after blackening heat treatment. Furthermore, the formed nitrides are likely to become corrosion initiation sites and reduce corrosion resistance. Therefore, the upper limit of the N content is 0.100%, preferably 0.095%, and more preferably 0.090%. On the other hand, the lower limit of the N content is not particularly limited, but an excessively low N content will lead to increased refining costs. Therefore, the lower limit of the N content is preferably 0.001%, and more preferably 0.003%.

[0027] (Al: 1.00% or less) Al is an element effective in improving oxidation resistance. However, if the Al content is too high, the workability and toughness of the stainless steel material will decrease and the manufacturing cost will increase. Therefore, the upper limit of the Al content is 1.00%, preferably 0.95%, and more preferably 0.90%. On the other hand, the lower limit of the Al content is not particularly limited, but is preferably 0.0001%, and more preferably 0.0005%.

[0028] (Ti: 0.08 to 0.50%) Ti is an element that affects intergranular corrosion resistance (sensitization suppression effect). Ti is also an element that is effective in ensuring the color tone of the oxide film (blackened film after blackening heat treatment). In particular, Ti forms a complex oxide with Cr to impart a black color tone, and also forms Ti oxide (TiO2) on the surface to suppress peeling of the oxide film. However, if the Ti content is too high, the workability and surface quality of the stainless steel material will deteriorate. Therefore, the upper limit of the Ti content is 0.50%, preferably 0.45%, and more preferably 0.40%. If the Ti content is too low, the above effects will not be fully achieved. Therefore, the lower limit of the Ti content is 0.08%, preferably 0.085%, and more preferably 0.09%.

[0029] (Nb:0.50% or less) Nb is an element that affects properties such as intergranular corrosion resistance (sensitization suppression effect). However, if the Nb content is too high, the workability and toughness of the stainless steel material will decrease. Therefore, the upper limit of the Nb content is 0.50%, preferably 0.45%, and more preferably 0.40%. On the other hand, the lower limit of the Nb content is not particularly limited, but is preferably 0.005%, and more preferably 0.01%.

[0030] (Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less) Zr, Co, V, and W are elements effective in improving the oxidation resistance of stainless steel materials. However, if the contents of Zr, Co, V, and W are too high, the workability and toughness of the stainless steel material will decrease and the manufacturing cost will increase. Therefore, the upper limits of the contents of Zr, Co, V, and W are all 1.00%, preferably 0.80%, and more preferably 0.60%. On the other hand, the lower limits of the contents of Zr, Co, V, and W are not particularly limited, but are preferably 0.001%, and more preferably 0.003%.

[0031] (REM: 0.100% or less, Ca: 0.100% or less) REM and Ca are elements effective in improving the oxidation resistance of stainless steel materials. However, excessive REM and Ca contents lead to increased manufacturing costs of stainless steel materials. Therefore, the upper limits of the REM and Ca contents are both 0.100%, preferably 0.090%, and more preferably 0.080%. On the other hand, the lower limits of the REM and Ca contents are not particularly limited, but are preferably 0.0001%, and more preferably 0.0003%. REM is a collective term for 17 elements, including Sc, Y, and lanthanides, and refers to rare earth metals. Specific examples include La, Ce, Nd, etc., and one of these elements may be contained alone, or two or more may be contained in combination. When two or more rare earth elements are contained, the REM content refers to the total content of these rare earth elements.

[0032] (Sn:0.100% or less) Sn is an element effective in improving the corrosion resistance of stainless steel materials. However, if the Sn content is too high, Sn segregates and manufacturability decreases. Therefore, the upper limit of the Sn content is 0.100%, preferably 0.090%, and more preferably 0.080%. On the other hand, the lower limit of the Sn content is not particularly limited, but is preferably 0.001%, and more preferably 0.002%.

[0033] (B:0.0100% or less) B is an element effective in improving the secondary workability of stainless steel materials. However, if the B content is too high, the fatigue strength of the stainless steel material decreases. Therefore, the upper limit of the B content is 0.0100%, preferably 0.0090%, and more preferably 0.0080%. On the other hand, the lower limit of the B content is not particularly limited, but is preferably 0.0001%, and more preferably 0.0003%.

[0034] The metal structure of the substrate is ferritic. In this specification, "ferritic" means that the metal structure at room temperature is mainly ferritic.

[0035] Next, the pretreatment film formed on the surface of the substrate will be described. The pretreatment film is an oxide film that is thinner than the blackened film (oxide film) formed by blackening heat treatment. Therefore, stainless steel material with a pretreatment film is easier to process, such as by pressing, than stainless steel material with a blackened film, and various parts (structures) can be easily manufactured by various processes. In addition, the pretreatment film is an oxide film mainly composed of a highly conductive composite oxide of Mn and Cr, so it can also be spot welded, facilitating the manufacturing and joining of various parts by spot welding.

[0036] (Pretreatment film thickness: 50 to 200 nm) If the thickness of the pretreatment coating is too large, workability and spot weldability will be reduced. Therefore, from the viewpoint of ensuring workability and spot weldability, the upper limit of the thickness of the pretreatment coating is 200 nm, preferably 190 nm, and more preferably 180 nm. On the other hand, from the viewpoint of enabling blackening heat treatment at a low temperature of less than 800°C where deformation is unlikely to occur, the lower limit of the thickness of the pretreatment coating is 50 nm, preferably 55 nm. In this specification, the thickness of the pretreatment film refers to the depth from the surface to the point where O (oxygen) is 1 / 4 of the maximum value in the component concentration profile in the depth direction obtained using glow discharge optical emission spectroscopy (GD-OES).

[0037] (Mn fraction of pre-treatment film: 0.30 or more) To ensure spot weldability, the pretreatment coating must be primarily composed of a highly conductive composite oxide of Mn and Cr. Therefore, the lower limit of the Mn fraction of the pretreatment coating is 0.30, preferably 0.31, and more preferably 0.32. On the other hand, the upper limit of the Mn fraction of the pretreatment coating is not particularly limited, but is preferably 0.80, and more preferably 0.75. Here, in this specification, the Mn fraction of the pretreatment coating can be obtained by measuring the concentration profile of each element by glow discharge optical emission spectroscopy (GD-OES) and calculating the concentration of each element at the point where the O (oxygen) peak intensity is half of the maximum value using the following formula. Mn fraction = Mn concentration / (Fe concentration + Cr concentration + Al concentration + Si concentration)

[0038] The stainless steel material according to the embodiment of the present invention has a pretreatment coating having the above-described characteristics, and therefore can be spot welded, and has good dimensional accuracy even after blackening heat treatment. Therefore, the stainless steel material according to the embodiment of the present invention is suitable for use in spot welding.

[0039] (Method of manufacturing stainless steel materials) The method for producing a stainless steel material according to an embodiment of the present invention is not particularly limited as long as it is a method capable of producing a stainless steel material having the above-described characteristics. The stainless steel material according to an embodiment of the present invention can be produced, for example, by subjecting a rolled material having the above-described composition to pretreatment in an oxidizing atmosphere with an O2 concentration of 1% by volume or more, in which the material is soaked at 850 to 1050°C for less than 60 seconds.

[0040] If the O2 concentration in the oxidizing atmosphere is less than 1% by volume, Cr, Si, Al, etc. will be preferentially oxidized, making it impossible to form an oxide film primarily composed of a highly conductive composite oxide of Mn and Cr. In particular, from the perspective of stably forming a pretreatment film having the above-mentioned characteristics, the O2 concentration in the oxidizing atmosphere is preferably 1.5% by volume or more, and more preferably 2% by volume or more. The upper limit of the O2 concentration in the oxidizing atmosphere is not particularly limited, but is generally 20% by volume.

[0041] If the soaking temperature is less than 850°C, it is difficult to remove the strain imparted to the stainless steel material during rolling (e.g., cold rolling), and it is not possible to form a pretreatment coating with the desired thickness. If the soaking temperature exceeds 1050°C, the rapid formation of an oxide coating causes the growth of a Cr2O3 layer, making it impossible to form a pretreatment coating with the desired composition. In particular, from the viewpoint of stably forming a pretreatment coating having the above-mentioned characteristics, the soaking temperature is preferably 855 to 1045°C, and more preferably 860 to 1040°C. The soaking time may be adjusted appropriately depending on the soaking temperature, but is preferably 2 seconds or more and less than 60 seconds, and more preferably 5 to 55 seconds.

[0042] The rolled material is not particularly limited, and a hot-rolled material, a cold-rolled material, or the like can be used. The rolled material can be produced according to a method known in the art. For example, the cold-rolled material can be produced by melting stainless steel having the above composition, hot-rolling, annealing, and pickling, and then cold-rolling.

[0043] <Spot welded structure> The spot-welded structure according to the embodiment of the present invention can be produced by spot-welding the above-mentioned stainless steel materials and then subjecting them to blackening heat treatment at a temperature of less than 800°C. The above-mentioned stainless steel material has a pretreatment coating that allows spot welding, making it easy to manufacture spot-welded structures. Furthermore, because the temperature of the blackening heat treatment is less than 800°C, strain imparted to the stainless steel material during cold working is not removed, and deterioration of the dimensional accuracy of the spot-welded structure can be suppressed.

[0044] The above stainless steel material may be subjected to cold working such as pressing or roll forming before spot welding. By performing such cold working, a spot-welded structure having a desired shape can be manufactured.

[0045] The conditions for spot welding are not particularly limited, and spot welding can be performed according to conditions known in the art. For example, the above-mentioned stainless steel material is used for at least one of two workpieces to be welded, and the two workpieces are sandwiched between a pair of electrodes and current is passed through while applying pressure. Resistance heat generated at the contact surface causes the two workpieces to be locally heated and melted, thereby welding the workpieces.

[0046] The conditions for the blackening heat treatment, other than the temperature, can be those known in the art. Specifically, the blackening heat treatment can be performed in an oxidizing atmosphere. From the viewpoint of stably forming a blackened film, the temperature for the blackening heat treatment is preferably 500 to 795°C, more preferably 510 to 790°C. Furthermore, the time for the blackening heat treatment is preferably 60 to 4000 minutes, more preferably 80 to 3500 minutes.

[0047] The spot-welded structure according to the embodiment of the present invention manufactured as described above includes a stainless steel base material and a spot weld. The stainless steel base material is a base material having the above composition and a black coating formed on the surface of the base material, * a * b * In the color system, the lightness index L * is 45.0 or less, Chromathetics Index a * and b * This includes black coatings with a brightness index L within ±5.00. * , Chromathetics Index a * and b * If is within the above range, it can be said that a desired black tone is obtained. Here, in this specification, "brightness index L * " and "Chromanetics Index a * and b * " can be measured in accordance with JIS Z8722:2009.

[0048] The spot-welded structure according to the embodiment of the present invention can be manufactured by spot welding and has good dimensional accuracy. Examples of spot-welded structures include, but are not limited to, exhaust system parts such as mufflers, exterior building panels joined with mounting jigs, and the like. [Example]

[0049] 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.

[0050] Stainless steel having the composition shown in Table 1 (the balance being Fe and impurities) was melted and hot-rolled to obtain a hot-rolled sheet with a thickness of 3.0 mm. The hot-rolled sheet was then annealed at 1050°C and pickled to obtain a hot-rolled annealed sheet. The hot-rolled annealed sheet was then cold-rolled to obtain a cold-rolled sheet with a thickness of 1.0 mm. The cold-rolled sheet was then pretreated under the conditions shown in Table 2 to obtain a stainless steel sheet. During the pretreatment, the oxidizing atmosphere was controlled to a predetermined O2 concentration by adjusting the introduction ratio of O2 gas and N2 gas in the atmosphere furnace. Test pieces measuring 500 mm (rolling direction) x 100 mm (width direction) were cut from the obtained stainless steel sheet.

[0051] [Table 1]

[0052] The above test pieces were subjected to the following evaluations.

[0053] (Thickness and Mn content of pre-treatment film) A 50 mm square test piece was cut from the test piece, and the surface was degreased with acetone. The pre-treated film was then analyzed using glow discharge optical emission spectroscopy (GD-OES) in accordance with JIS K0144:2018. In GD-OES, the thickness of the pretreatment film was determined as the depth from the surface to the point where O (oxygen) was 1 / 4 of the maximum value in the obtained component concentration profile in the depth direction. In GD-OES, the Mn fraction was calculated from the concentration of each element at a point where the O (oxygen) peak intensity was half of the maximum value in the obtained component concentration profile in the depth direction using the following formula. Mn fraction = Mn concentration / (Fe concentration + Cr concentration + Al concentration + Si concentration)

[0054] (Spot weldability) Two 50 mm square welding test pieces were cut from the test piece, and the surfaces were degreased with acetone. Next, the two welding test pieces were overlapped and spot welded at the center. Spot welding was performed with an electrode diameter of φ6 mm, a pressure of 4.5 kN, a current of 5 kA, and a current flow time of 0.09 seconds. Next, the joined structure was cut using a precision cutting machine, passing through the center of the spot weld. This cross section was filled with resin, mirror-polished, and then etched with fluoronitric acid. After that, samples with a nugget formed at the spot weld were judged as ○ (good spot weldability), and samples without a nugget were judged as × (insufficient spot weldability). The results are shown in Table 2.

[0055] [Table 2]

[0056] Next, the above test pieces were subjected to a blackening heat treatment under the conditions shown in Table 3, and the following evaluations were carried out. The blackening heat treatment was carried out in an air atmosphere.

[0057] (color tone) At any five points on the blackened film formed on the surface, color tone measurements were performed in accordance with JIS Z8722:2009 using a spectrophotometer with a measurement diameter of 3 mm, and the average value was calculated using CIELAB (L * a * b * The lightness index L * , Chromathetics Index a * , b * As shown.

[0058] The conditions for measuring the above color tone were as follows: Equipment: Konica Minolta Spectrophotometer CM-700d Light source: Pulsed xenon lamp Photodetector: Dual 36-element silicon photodiode array Target mask: φ3mm Measurement: 10° field of view Auxiliary illuminant: D65 daylight, color temperature 6504K Specular reflection processing mode: SCI

[0059] (Thermal deformation: dimensional accuracy) A measurement specimen measuring 75 mm (rolling direction) x 10 mm (width direction) was cut out from the above test piece. Next, a U-shaped bending process was performed using a 300 N universal testing machine with a punch having an inner radius of 8 mm so that both legs were parallel, and the distance between both legs was measured. Next, blackening heat treatment was performed under the same conditions as above, and the distance between both legs was measured again. Note that when performing the blackening heat treatment, one leg was placed on a flat Al2O3 plate. In this evaluation, if the average difference (ΔH) between the distance between both legs before and after the blackening heat treatment was within 5 mm, it was determined that the thermal deformation was small and the dimensional accuracy was good. The results are shown in Table 3.

[0060] [Table 3]

[0061] As shown in Tables 2 and 3, the stainless steel sheets of Examples 1 to 10 had pretreatment coatings with thicknesses of 50 to 200 nm and Mn fractions of 0.30 or more, which enabled spot welding and allowed the desired black color tone to be obtained by blackening heat treatment. Furthermore, Example 8 had insufficient dimensional accuracy because the blackening heat treatment temperature was set high, but Examples 1 to 7, 9, and 10 had improved dimensional accuracy because the blackening heat treatment temperature was set low.

[0062] In contrast, the stainless steel sheet of Comparative Example 1 was not subjected to pretreatment, and therefore the desired black color tone could not be imparted by blackening heat treatment. The stainless steel sheet of Comparative Example 2 had a pretreatment soaking temperature that was too high, resulting in a pretreatment coating with a low Mn fraction and a large thickness, which resulted in insufficient spot weldability. In the stainless steel sheets of Comparative Examples 3 and 4, the O2 concentration or soaking temperature during pretreatment was too low, resulting in a pretreatment film with a low Mn fraction and small thickness, and as a result, the desired black color tone could not be imparted. In the stainless steel sheet of Comparative Example 5, the Mn content in the substrate was too low, resulting in a low Mn fraction in the pretreatment coating, and it was not possible to impart the desired black color tone. The stainless steel sheet of Comparative Example 6 had an excessively high Mn content in the substrate, and therefore was unable to impart the desired black color tone. The stainless steel sheet of Comparative Example 7 had an excessively high Al content in the substrate, resulting in a pretreatment coating with concentrated Al oxides and a low Mn fraction, which resulted in insufficient spot weldability. The stainless steel sheet of Comparative Example 8 had an excessively low Cr content in the substrate, resulting in the formation of a thick pretreatment coating, and as a result, the spot weldability was insufficient. The stainless steel sheet of Comparative Example 9 had an excessively high Cr content in the substrate, resulting in a low Mn fraction in the pretreatment coating, which resulted in insufficient spot weldability and failure to impart the desired black color tone.

[0063] As can be seen from the above results, the present invention can provide a stainless steel material that can be spot welded and has good dimensional accuracy even after blackening heat treatment. Furthermore, the present invention can provide a spot-welded structure that can be manufactured by spot welding and has good dimensional accuracy, and a method for manufacturing the same.

Claims

1. a base material having a composition containing, by mass, C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Al: 1.00% or less, Ti: 0.08 to 0.50%, with the balance being Fe and impurities; a pretreatment film formed on the surface of the substrate, the pretreatment film having a thickness of 50 to 200 nm and a Mn fraction of 0.30 or more; Stainless steel materials including.

2. The stainless steel material according to claim 1, wherein the base material further contains, by mass, at least one selected from Nb: 0.50% or less, Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less, REM: 0.100% or less, Ca: 0.100% or less, Sn: 0.100% or less, and B: 0.0100% or less.

3. The stainless steel material according to claim 1 or 2, which is used for spot welding.

4. A spot-welded structure comprising a stainless steel base material and a spot weld, The stainless steel base material is a base material having a composition containing, by mass, C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Al: 1.00% or less, Ti: 0.08 to 0.50%, with the balance being Fe and impurities; A black coating formed on the surface of the substrate, * a * b * In the color system, the lightness index L * is 45.0 or less, chromanetics index a * and b * A blackened film having a value within ±5.

00. A spot welded structure comprising:

5. 5. The spot-welded structure according to claim 4, wherein the base material further contains, on a mass basis, at least one selected from Nb: 0.50% or less, Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less, REM: 0.100% or less, Ca: 0.100% or less, Sn: 0.100% or less, and B: 0.0100% or less.

6. A method for manufacturing a spot-welded structure, comprising spot-welding the stainless steel material according to any one of claims 1 to 3, and then performing blackening heat treatment at a temperature of less than 800°C.

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