Coated steel sheet for spot-welding member and method for manufacturing spot-welding member

Abstract

To provide a coated steel sheet for a spot-welding member that is excellent in delayed fracture resistance, and to provide a method for manufacturing a spot-welding member using the steel sheet.SOLUTION: A coated steel sheet for a spot-welding member includes: a base steel sheet; and a coating film provided on at least one of the surfaces of the base steel sheet, where the base steel sheet has a component composition having C content of 0.07 mass% or more, the coating film includes Ti and / or a Ti compound, and a coating weight per surface of the coating film as Ti is 10-2000 mg / m2.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a coated steel sheet for spot-welded components and a method for manufacturing spot-welded components. [Background technology]

[0002] Cold-rolled steel sheets are used inside automobile bodies, which are not exposed to corrosive environments, due to their thickness accuracy and low cost. In recent years, efforts have been made to increase the strength of cold-rolled steel sheets used in automobiles in order to reduce CO2 emissions from automobiles and ensure safety.

[0003] However, it is known that as the strength of steel increases, it becomes more susceptible to a phenomenon known as delayed fracture. Sensitivity to delayed fracture is particularly pronounced in cold-rolled steel sheets (high-strength steel sheets) with a tensile strength of 1180 MPa or more. Delayed fracture refers to the phenomenon in which a material, in this case a steel sheet, suddenly fractures without any apparent plastic deformation after a certain period of time has passed under a static load stress (a load stress below the tensile strength).

[0004] It is known that delayed fracture of steel sheets is caused by residual stresses generated when the steel sheets are pressed into a predetermined shape and hydrogen embrittlement of the steel sheets at stress concentration points. The hydrogen that causes delayed fracture is thought to be hydrogen that has penetrated and diffused into the steel sheets from the external environment, typically hydrogen that is generated during corrosion of the steel sheets and penetrates and diffuses into the steel sheets.

[0005] In order to prevent delayed fracture in high-strength steel sheets, for example, Patent Document 1 reports a technology in which a coating containing an anionic compound having a pH buffering property in the range of pH 3 to 6.5 in the case of 0.1 N Na salt is formed on the surface of a steel sheet having a tensile strength of 1180 MPa or more, thereby significantly suppressing hydrogen penetration into the steel sheet. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-188707 Summary of the Invention [Problem to be solved by the invention]

[0007] It was expected that the steel plate proposed in Patent Document 1 would be able to reduce the amount of hydrogen that penetrates into the steel plate from the external environment, and therefore would be able to suppress delayed fracture of steel plates used in corrosive environments. However, when welded components were manufactured using the steel plate described in Patent Document 1, it became clear that delayed fracture sometimes occurred in the welded parts.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a coated steel sheet for spot-welded components having excellent delayed fracture resistance, and a method for manufacturing spot-welded components using the steel sheet. [Means for solving the problem]

[0009] As a result of extensive research, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention. [1] A coated steel sheet for spot-welded components comprising a substrate steel sheet and a coating provided on at least one surface of the substrate steel sheet, The substrate steel sheet has a chemical composition in which the C content is 0.07% by mass or more, the coating contains Ti and / or a Ti compound, The coating has a Ti coating weight of 10 to 2000 mg / m per side. 2 That is, Coated steel plate for spot welding components. [2] A manufacturing method of a spot-welded component in which two or more steel plates are overlapped to manufacture a spot-welded component, At least one of the two or more steel plates is the coated steel plate for spot-welded components according to [1], A method for manufacturing a spot-welded component, comprising overlapping the coated steel sheet for spot-welded components with another steel sheet so that the other steel sheet is positioned on the coating side of the coated steel sheet for spot-welded components, and spot welding the resulting steel sheet to manufacture a spot-welded component. [Effects of the Invention]

[0010] According to the present invention, a coated steel sheet for spot-welded components having excellent delayed fracture resistance can be obtained. The coated steel sheet for spot-welded components can be used to obtain predetermined spot-welded components. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram schematically showing a test piece for evaluating delayed fracture resistance used in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0012] As described above, the present inventors have conducted extensive research into the phenomenon of delayed fracture occurring in the welded portion of welded components, and have found that, during spot welding, hydrogen is generated by adsorbed moisture and oil present on the surface of the steel sheet, and the generated hydrogen is absorbed as hydrogen atoms into the molten portion of the steel sheet when current is applied, causing delayed fracture in the welded portion of the steel sheet.

[0013] Specifically, delayed fracture at spot welds is thought to occur through the following mechanism. First, the part heated by current flow during spot welding (the nugget part) rises to a temperature above the melting point of the steel sheet. This heating generates hydrogen from the adsorbed moisture and oil present on the steel sheet surface. The generated hydrogen remains inside the nugget and accumulates locally during the cooling process after spot welding, specifically in areas where hydrogen tends to accumulate, such as grain boundaries inside the nugget. It is thought that delayed fracture then occurs when stress is applied to the weld part during or after welding.

[0014] In particular, when hydrogen accumulates at grain boundaries, the bonding strength between the grain boundaries is significantly reduced, resulting in a fracture mode characteristic of delayed fracture called intergranular fracture. That is, in order to suppress delayed fracture due to hydrogen generated during welding, it is desirable to reduce the amount of hydrogen that is generated or to neutralize the hydrogen that has penetrated.

[0015] As mentioned above, hydrogen generated during welding is moisture and oil adsorbed on the steel sheet. To reduce this, significant changes to the manufacturing process are required, such as introducing a degreasing process before welding or performing welding indoors under humidity control. Therefore, the inventors have conceived that it is important to neutralize the hydrogen that penetrates into the steel sheet in order to prevent delayed fracture caused by hydrogen generated during welding. Specifically, they have discovered that hydrogen generated during spot welding can be neutralized by adjusting the C content of the steel sheet to 0.07 mass% or more and forming a coating on the steel sheet surface containing Ti and / or a Ti compound in a predetermined amount of Ti.

[0016] The present invention has been made based on the above findings. A coated steel sheet for welded components and a method for manufacturing a welded component according to one embodiment of the present invention will be specifically described below. Note that the following description shows an example of a preferred embodiment of the present invention, and the present invention is not limited thereto.

[0017] (1) Coated steel sheets for spot welding components In the present invention, when two or more steel sheets are spot welded, at least one of them must be a coated steel sheet for spot-welded components. In one embodiment, the coated steel sheet for welded components comprises a base steel sheet and a coating provided on at least one surface of the base steel sheet. The base steel sheet and the coating will be described below.

[0018] [Substrate steel plate] (C content: 0.07% by mass or more) The substrate steel sheet of the present invention has a chemical composition with a C content of 0.07% by mass or more. By setting the C content of the steel sheet to 0.07% by mass or more, TiC is generated by the Ti and / or Ti compounds in the coating formed on the steel sheet due to the heat during spot welding, and the C in the steel sheet, thereby achieving the effect of improving delayed fracture resistance. Therefore, the C content of the substrate steel sheet is set to 0.07% by mass or more. The C content is preferably 0.10% by mass or more, and more preferably 0.15% by mass or more. There is no upper limit to the C content for achieving the effects of the present invention, but from the viewpoints of press formability and manufacturability, the C content is preferably 0.40% by mass or less. Other examples of the composition of the base steel sheet include: Si: 0-2.5 mass%, Mn: 1-3 mass%, P: 0-0.05 mass%, S: 0-0.005 mass%, with the balance being Fe and unavoidable impurities; and further, those containing one or more elements selected from Cu, Ti, V, Al, Cr, etc. The base steel sheet preferably used in the present invention may have any composition and structure, except for the C content, as long as it has the desired tensile strength. In order to improve various properties such as mechanical properties, for example, solid solution strengthening by adding interstitial solid solution elements such as C and N and substitutional solid solution elements such as Si, Mn, P and Cr, precipitation strengthening by carbonitrides such as Ti, Nb, V and Al, chemical composition modification such as adding strengthening elements such as W, Zr, Hf, Co, B, Cu and rare earth elements, strengthening by recovery annealing at a temperature where recrystallization does not occur or partial recrystallization strengthening where unrecrystallized regions are left without complete recrystallization, strengthening by transformation structure such as forming a single phase of bainite or martensite or a composite structure of ferrite and these transformed structures, and strengthening by the Hall-Petch equation: σ=σ0+kd where d is the ferrite grain size. -1 / 2 The structural or structural modifications such as grain refinement strengthening expressed by (where σ is stress, σ0, k is material constant) and processing strengthening by rolling or the like can be carried out alone or in combination.

[0019] Furthermore, it is preferable to use a high-strength cold-rolled steel sheet as the above-mentioned base steel sheet, and commercially available high-strength cold-rolled steel sheets include, but are not limited to, JFE-CA1180, JFE-CA1370, JFE-CA1470, JFE-CA1180SF, JFE-CA1180Y1, JFE-CA1180Y2 (all manufactured by JFE Steel Corporation), SAFC1180D (manufactured by Nippon Steel & Sumitomo Metal Corporation), etc.

[0020] (tensile strength) The higher the tensile strength of the substrate steel sheet, the more likely it is that delayed fracture will occur, and if it is 1180 MPa or more, delayed fracture is more likely to occur. Therefore, the tensile strength of the substrate steel sheet is preferably 1180 MPa or more. The tensile strength of the substrate steel sheet is more preferably 1320 MPa or more, at which point the problem of delayed fracture becomes more pronounced. On the other hand, steel sheets with low tensile strength are inherently less likely to suffer from delayed fracture. Although the effects of the present invention are also achieved in steel sheets with low tensile strength, applying the present invention to steel sheets with a tensile strength of less than 1180 MPa is industrially undesirable because it leads to increased costs.

[0021] Furthermore, there are no particular limitations on the rolling method used to manufacture the base steel sheet, and either a hot-rolled steel sheet or a cold-rolled steel sheet may be used as the base steel sheet. However, since high-strength cold-rolled steel sheets are widely used in the automotive field, it is preferable to apply the present invention to high-strength cold-rolled steel sheets (tensile strength of 1180 MPa or more).

[0022] (plate thickness) Although the upper limit of the thickness of the substrate steel sheet is not particularly limited, the thicker the substrate steel sheet, the larger the nugget volume of the weld. Therefore, in order to improve delayed fracture resistance, it becomes necessary to increase the Ti deposition amount of the coating, which leads to increased costs. Therefore, the upper limit of the thickness is preferably 2.5 mm or less, and more preferably 2.0 mm or less. In addition, the lower limit of the thickness of the substrate steel sheet is also not particularly limited, but it is preferably 0.8 mm or more, which is the thickness of a typical high-strength steel sheet, and more preferably 1.2 mm or more.

[0023] Generally, zinc-based plated steel sheets are used for high-strength automotive components to enhance corrosion resistance. However, zinc-based plating generates a large amount of hydrogen during the corrosion process, which adversely affects delayed fracture resistance. For this reason, it is not preferable to apply zinc-based plating to the base steel sheet of the present invention.

[0024] [Coating] The coating of the present invention contains Ti and / or a Ti compound. The coating has a Ti deposition amount of 10 to 2000 mg / m2 per side. 2 In addition to Ti and / or a Ti compound, an organic resin and / or an inorganic compound may be contained within a range that does not impair the effects of the present invention.

[0025] (Ti and / or Ti compounds) In the present invention, the coating formed on the surface of the substrate steel sheet contains Ti and / or Ti compounds, which are then mixed with the molten substrate steel sheet in the nugget during welding. "Containing Ti and / or Ti compounds" refers to a coating containing either Ti or Ti compounds, or both. In this case, Ti oxides, such as TiO2, or Ti combine with C contained in the substrate steel sheet to form TiC, which then disperse in the nugget. TiC has traditionally been called a hydrogen trapping site and is known to trap hydrogen around TiC. Hydrogen in steel can be broadly divided into diffusible hydrogen, which can move relatively freely within the steel, and non-diffusible hydrogen, which is stable and does not move at room temperature. It is known that diffusible hydrogen contributes to delayed fracture, and reducing diffusible hydrogen is important for improving delayed fracture resistance. The hydrogen trapped by TiC becomes non-diffusible hydrogen. In other words, by retaining TiC in the nugget, the diffusible hydrogen generated during welding can be made non-diffusible, thereby neutralizing the diffusible hydrogen that contributes to delayed fracture, thereby suppressing delayed fracture. By applying the coated steel sheet for spot-welded components of the present invention, it is possible to suppress the amount of diffusible hydrogen in the welded component after spot welding to 0.25 ppm or less. While it is believed that a similar effect can be achieved by incorporating Ti into the steel sheet, a high Ti content in the steel sheet is required to achieve the same effect as that of the present invention, which is undesirable from the standpoints of cost and manufacturability.

[0026] When spot welding is performed on a steel sheet with Ti and / or Ti compounds present, the spot welded joint is heated to a high temperature, and it is believed that Ti and / or Ti compounds combine with C contained in the steel sheet to form TiC. Therefore, the form in which Ti exists on the steel sheet is not limited. Examples of Ti compounds include TiO2, TiCl4, Ti(SO4)2, and TiC. The Ti mentioned above is metallic Ti. When Ti is attached as metallic Ti, since Ti is an easily oxidizable element, the surface of the metallic Ti that comes into contact with the air will exist as at least an oxide of Ti. Note that the metallic Ti inside, which does not come into contact with the air, remains metallic Ti.

[0027] Delayed fracture in welds depends on the size of the nugget; the larger the nugget, the less stress it exerts, making it less likely to occur. Therefore, we evaluated the amount of Ti deposition that can suppress delayed fracture at nugget diameters where delayed fracture is likely to occur, specifically, at 4√t or less (where t is the thickness of the steel sheet). As a result, we found that the amount of Ti deposition per side in the coating formed on the steel sheet surface is 10mg / m 2 Therefore, the amount of Ti deposited on each side of the coating formed on the steel sheet surface is 10 mg / m 2 More than 50 mg / m is preferable. 2 More preferably, 100 mg / m 2 On the other hand, if the amount of Ti deposited increases, the electrical conductivity decreases, making spot welding impossible. Therefore, the amount of Ti deposited on each side of the coating is set to 2000 mg / m 2 Preferably, the amount of Ti deposited on one side of the coating is 1500 mg / m 2 or less, more preferably 1000 mg / m 2 The components in the coating other than Ti are not particularly limited, since the desired delayed fracture resistance can be obtained if the amount of Ti deposited in the coating can be controlled.

[0028] The amount of Ti deposited in the coating formed on the surface of the steel sheet refers only to the amount of Ti contained in the coating, and does not include the amount of Ti contained in the steel sheet itself. Therefore, as will be described later in the examples, the amount of Ti can be determined by subtracting the result of measuring Ti with fluorescent X-rays after removing the coating (by dissolving, grinding, etc.) from the result of measuring Ti with fluorescent X-rays with the coating.

[0029] The coating must be provided on at least one surface of the substrate steel sheet, but may be provided on both surfaces of the substrate steel sheet.

[0030] The method for producing the coated steel sheet for spot welding components is not particularly limited, but a coating having a desired amount of Ti can be formed by either an immersion method in which the substrate steel sheet is immersed in a solution containing Ti ions and then dried, or a vapor deposition method in which Ti is applied by vacuum deposition. The immersion method and vapor deposition method can be carried out as described in the examples below.

[0031] Spot welding using the above-mentioned coated steel sheet for spot-welded components can be performed by overlapping two surfaces provided with a coating containing Ti and / or a Ti compound (Ti-containing coating), or by overlapping one surface provided with a Ti-containing coating with another surface not provided with a Ti-containing coating. In either case, in order to prevent cracking after welding due to an insufficient amount of Ti, it is preferable to ensure that a predetermined amount of Ti is present between the two steel sheets to be overlapped before welding, and the amount of Ti present between the two steel sheets is 20 mg / m 2 It is preferable to set the concentration to 50 mg / m or more. 2 More preferably, it is set to be equal to or greater than this.

[0032] (2) Manufacturing method of welded components (spot welding) Two or more steel sheets are stacked and spot-welded to produce a spot-welded component. Specifically, at least one of the two or more steel sheets must be the coated steel sheet for spot-welded components described above. The method of overlapping the steel sheets involves overlapping the coated steel sheet for spot-welded components so that another steel sheet is positioned on the coating side of the coated steel sheet for spot-welded components, and spot welding is performed to produce a spot-welded component. The other steel sheet refers to one of the two or more steel sheets. The conditions for spot welding are not particularly limited, but spot welding is performed in a room temperature environment using chromium copper DR electrodes with a tip diameter (tip diameter) of 6 mm and a curvature radius of 40 mm for both the lower and upper electrodes, with the electrodes constantly water-cooled. By changing the welding current, it is possible to select a preferred nugget size and a current that does not generate expulsion, thereby enabling appropriate spot welding. The welding current is not particularly limited, but may be, for example, 3 to 20 kA. Furthermore, the nugget diameter may be 2.5√t to 6.0√t (where t is the thickness (mm) of the base steel sheet), but is not particularly limited. [Example]

[0033] As the base steel sheets, cold-rolled steel sheets (Steel Sheet A, Steel Sheet B, Steel Sheet C) with a tensile strength of 1180 MPa and a thickness of 1.6 mm were prepared, each having the composition shown in Table 1: 0.05 to 0.20 mass% C, 0.4 mass% Si, 1 to 2 mass% Mn, with the remainder being Fe and unavoidable impurities. In addition, a cold-rolled steel sheet (Steel Sheet AA) with the same composition and thickness as Steel Sheet A but adjusted to a tensile strength of 1300 MPa was also prepared. The prepared cold-rolled steel sheets were immersed in toluene and subjected to ultrasonic cleaning for 5 minutes, and then a coating was formed on the surface of the cold-rolled steel sheet.

[0034] The coating was formed by either the immersion method, in which the substrate was immersed in a solution containing Ti ions and then dried, or the vapor deposition method, in which Ti was applied by vacuum deposition, to form a coating with the desired amount of Ti attached.

[0035] The immersion method involved immersing the steel sheet in a Ti-containing solution at room temperature for 10 seconds, then passing it through a rubber roll immediately after removal to adjust the moisture content of the surface, and then leaving it in an oven at 100°C for 30 seconds to retain the Ti component on the steel sheet surface. The amount of Ti component formed on the steel sheet surface was adjusted by changing the contact pressure between the rubber roll and the steel sheet. The titanium-containing solution used here was either (a) a colloidal solution consisting of TiO2 or (b) a solution containing titanium tetraisopropoxide.

[0036] The deposition method for applying the Ti by vacuum deposition is not particularly limited, but the method was carried out by placing a steel sheet and a Ti substrate in the same chamber, evacuating the chamber using a rotary pump, and then irradiating the Ti substrate with an electron beam to evaporate Ti and form a film on the surface of the steel sheet.

[0037] Measurement method for the amount of Ti deposited on the coating The amount of Ti in the coating was measured using a fluorescent X-ray analyzer (manufactured by Hitachi High-Tech Science Corporation). Specifically, several levels of known amounts of Ti were attached to a Ti-free substrate, and the intensity obtained was determined by measuring with fluorescent X-rays for each Ti amount. A calibration curve was created from the various Ti amounts and the obtained intensities, and the amount of Ti that resulted in a specified intensity was calculated as the coating amount based on the calibration curve. Here, the steel sheet before the Ti coating was applied was also measured in the same way, and the intensity of the steel sheet before application was corrected to zero, thereby excluding the amount of Ti contained in the steel sheet. The type of Ti compound can be identified by its bonding state using XPS, and by extending the sputtering time, it is also possible to identify the type of compound inside.

[0038] The coated steel sheet was cut to a size of 125 mm (long side) x 30 mm (short side) and bent 90 degrees at a position 75 mm along the long side to create two test specimens. Two test specimens were then overlapped on the 50 mm x 30 mm surface with the bend facing outward. A nugget diameter of 3.8 mm was created by spot welding 25 mm from the edge of the 50 mm x 30 mm surface to obtain a test specimen for delayed fracture evaluation. Spot welding was performed at room temperature, and the electrodes were constantly water-cooled. Both the lower and upper electrodes used for welding were chromium-copper DR-type electrodes with a tip diameter of 6 mm and a curvature radius of 40 mm. The test specimen for delayed fracture evaluation is shown in Figure 1. Reference numeral 1 denotes the test specimen, and reference numeral 2 denotes the weld.

[0039] Note that test piece 1, which could not be joined even when the spot welding conditions were changed, was deemed unweldable.

[0040] In actual parts, stress is applied to the weld 2 during welding or during processing after welding, and delayed fracture occurs due to hydrogen trapped in the weld 2. Here, to apply stress to the weld 2 of the delayed fracture evaluation test piece, a tensile test was performed on the welded delayed fracture evaluation test piece at a tensile speed of 0.001 mm / s within one hour after welding. During the tensile test, a load drop occurs due to cracking in the nugget. The load at which the load drop occurred was defined as the nugget cracking stress (Sna). For comparison, a welded delayed fracture evaluation test piece was left at room temperature for one week to remove the hydrogen (diffusible hydrogen) present in the nugget. The nugget cracking stress obtained was defined as Sna'. The ratio of these values ​​(Sna / Sna') was used to determine the delayed fracture characteristic value (DF-Value) of the weld 2. If delayed fracture occurs due to hydrogen inside the nugget, cracking occurs at a low load during the tensile test, resulting in a low DF-Value. On the other hand, if the hydrogen inside the nugget is neutralized, the DF-Value approaches 1.

[0041] Here, in the present invention, the DF-Value was classified as follows: if it was rated as 3 or 2, it was determined that the delayed fracture suppression effect was achieved, and if it was rated as 1, it was determined that the delayed fracture suppression effect was not achieved. <Delayed fracture prevention effect> Rating 3: 0.8 < DF-Value ≦ 1.0 Rating 2: 0.6 < DF-Value ≦ 0.8 Rating 1: DF-Value ≦ 0.6 The results obtained are shown in Table 2.

[0042] Nos. 1 to 7 are examples in which the amount of Ti deposited on the coating was varied, and within the range of the present invention, a suitable effect of inhibiting delayed fracture was obtained. No. 2 is an example in which the amount of Ti deposited is less than the range of the present invention, and no effect of inhibiting delayed fracture was obtained, and No. 7 is an example in which the amount of Ti deposited is too high, and welding itself was not possible.

[0043] Nos. 14 and 15 are examples in which the type of solution was changed by the immersion method, but are within the scope of the present invention, and it is clear that a favorable delayed fracture suppression effect was obtained in both cases.

[0044] Nos. 8 to 10 are examples in which the coating was formed by a deposition method different from the coating formation method described above. It is clear that within the scope of the present invention, a suitable delayed fracture suppression effect was obtained.

[0045] No. 1 and Nos. 11 to 13 are examples in which the chemical composition and tensile strength of the substrate steel sheet were changed. Even when the Ti coating amount was the same, it was found that when the C content of the substrate steel sheet was 0.07 mass% or more, the delayed fracture suppression effect was obtained, but in No. 12, where the C content was less than 0.07 mass%, the delayed fracture suppression effect was not obtained. [Explanation of symbols]

[0046] 1 test piece 2 Welded parts

[0047] [Table 1]

[0048] [Table 2]

Claims

1. A coated steel sheet for spot-welded components comprising a substrate steel sheet and a coating provided on at least one surface of the substrate steel sheet, The substrate steel sheet has a chemical composition in which the C content is 0.07 mass% or more, the coating contains Ti and / or a Ti compound, The amount of Ti deposited on one side of the coating is 10 to 2000 mg / m ２ That is, Coated steel plate for spot welding components.

2. A manufacturing method of a spot-welded component in which two or more steel plates are overlapped to manufacture a spot-welded component, At least one of the two or more steel plates is the coated steel plate for spot-welded components according to claim 1, A method for manufacturing a spot-welded component, comprising overlapping the coated steel sheet for spot-welded components with another steel sheet so that the other steel sheet is positioned on the coating side of the coated steel sheet for spot-welded components, and spot welding the resulting steel sheet to manufacture a spot-welded component.

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