Spot welded joint and member for automobile
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- NIPPON STEEL CORPORATION
- Filing Date
- 2026-01-29
- Publication Date
- 2026-08-06
Smart Images
Figure JP2026003034_06082026_PF_FP_ABST
Abstract
Description
Spot welded joints and automotive components
[0001] This disclosure relates to spot-welded joints and automotive components.
[0002] Spot welding is primarily used in processes such as vehicle body assembly and parts installation. In recent years, the automotive sector has seen a rise in demand for improved fuel efficiency and CO2 reduction. ２ To achieve emissions reductions, there is a growing demand for lighter vehicle bodies and increased rigidity to improve collision safety. To meet these demands, there is a growing need to use high-strength steel plates (high-tensile steel) in vehicle bodies, components, and other parts.
[0003] However, when resistance spot welding is performed using high-strength steel plates, the joint strength (cross tensile strength: CTS) tends to decrease. Therefore, there is a need for spot-welded joints that have a high CTS even when using high-tensile steel. To improve the CTS when spot welding with high-tensile steel, two post-current procedures have been reported: one for tempering and another for mitigating solidification segregation, after forming the nugget with the initial current.
[0004] For example, Patent Document 1 describes a high-strength steel sheet in which at least one of two or more thin steel sheets has a tensile strength of 750 MPa to 1850 MPa, a carbon equivalent Ceq of 0.22 to 0.55 mass%, a microstructure in the outer nugget region consisting of a dendrite structure with an average arm spacing of 12 μm or less, an average carbide particle size of 5 nm to 100 nm, and a number density of 2 × 10⁻⁶ ６ pieces / mm ２ The above spot-welded joint has been proposed.
[0005] Patent documents 2 and 3 disclose a method for manufacturing a spot-welded joint with improved joint strength by performing a first energizing step, a cooling step, and a second energizing step under predetermined conditions on a plate assembly of two or more plates including a high-strength steel plate with a tensile strength of 980 MPa or more.
[0006] Patent Document 4 describes a spot-welded joint of a plate assembly made by stacking two or more steel plates, each containing at least one steel plate with a carbon content of 0.280% by mass or more and 0.700% by mass or less, wherein the average ratio of the major axis to the minor axis (major axis / minor axis) of prior austenite grains in the molten boundary region from the molten boundary at the nugget end to 1 mm inside is in the range of 1.0 to 1.5, and when the carbon content (mass%) of the steel plate is C, the number density of iron-based carbides with an equivalent circle diameter of 30 nm or more in the molten boundary region is 1 mm ２ 3.0 per win x 10 ６ A spot-welded joint with more than C units is disclosed.
[0007] Patent Document 5 discloses a resistance spot-welded joint using a high-strength steel plate in which retained austenite accounts for 20-50% by volume. Patent Document 6 discloses a welded joint in which a steel plate with a tensile strength of 780-1270 MPa is spot-welded as the base material. Patent Document 7 discloses a method for manufacturing a welded joint in which resistance welding is performed on an overlapping member formed by overlapping multiple steel plates with a tensile strength of 440 MPa or more, and the welded portion is heat-treated at a predetermined temperature and time.
[0008] Patent Document 1: International Publication No. 2011 / 025015 Patent Document 2: Japanese Unexamined Patent Publication No. 2023-145265 Patent Document 3: Japanese Unexamined Patent Publication No. 2023-145266 Patent Document 4: International Publication No. 2022 / 210749 Patent Document 5: Japanese Unexamined Patent Publication No. 2018-162477 Patent Document 6: Japanese Unexamined Patent Publication No. 2009-1839 Patent Document 7: Japanese Unexamined Patent Publication No. 2009-291797
[0009] This disclosure aims to provide spot-welded joints and automotive components that include thin, high-strength steel sheets with a Vickers hardness of 410 HV or higher, and in which joint strength is significantly improved by low-temperature heat treatment after nugget formation, even without tempering or solidification segregation relaxation.
[0010] The gist of the present disclosure for achieving the above object is as follows. <1> A spot welding joint including a plate stack formed by laminating a plurality of steel plates and a nugget for joining the plurality of steel plates, wherein at least one of the plurality of steel plates has a plate thickness of 1.0 mm or more and 2.3 mm or less, and the chemical composition is, in mass%, C: 0.08 to 0.35%, Mn: 1.00 to 5.00%, Si: 0.01 to 1.20%, Al: 0.001 to 0.60%, Mo: 0.001 to 1.00%, Cr: 0.001 to 2.00%, P: 0.030% or less, Ti: 0 to 0.30%, and Cu: 0 to 0.50%, the volume fraction of retained austenite is 0 to 20.0%, and the Vickers hardness is 410 HV or more. The high-strength steel plate, and when the plate thickness of the steel plate having the smallest plate thickness in the plate stack is t ｍｉｎ when, the nugget has a nugget diameter of 3.8√t ｍｉｎ or more and 6.0√t ｍｉｎ or less, and in a cross section in the plate thickness direction passing through the center of the nugget, when a portion corresponding to a plate interface including the high-strength steel plate and having the highest total Vickers hardness of two adjacent steel plates among the melting boundaries of the nugget is defined as a nugget end, in a 200-μm square end region near the nugget end in the nugget, the number density of fine carbides having an equivalent circle diameter of 30 nm or less is 30.0 / μm ２A spot welded joint that satisfies either or both of the following conditions: the Vickers hardness is within ±50 HV of the hardness calculated from the estimation formula HV below; and when the weighted average obtained by multiplying the chemical components of the plurality of steel plates by the ratio of the thickness of each steel plate to the total thickness of the plate assembly is considered as the average chemical component of the nugget, the area ratio of P-enriched areas where the P concentration is 1.5 times or more the P content of the average chemical component is 0.5% or more; and the area ratio of Mn-enriched areas where the Mn concentration is 1.5 times or more the Mn content of the average chemical component is 0.5% or more. Estimation formula HV = 217 + 1080 × (C + Si / 70 + Mn / 113 + Cr / 93 + Mo / 30) In the formula, the element symbols represent the content (mass%) of each element in the average chemical component of the nugget, and 0 is substituted if the corresponding element is not present. <2> The spot welded joint according to <1>, wherein the total content of Si and Al in the high-strength steel sheet is 1.50% by mass or less. <3> The spot welded joint according to <1> or <2>, wherein the Si content in the high-strength steel sheet is 0.01 to 0.40% by mass. <4> The spot welded joint according to any one of <1> to <3>, wherein the Vickers hardness HV of the high-strength steel sheet is 465 or higher. <5> The high-strength steel sheet has a zinc-based plating layer, and the ZnO content in the plating layer is 15 g / m ２ A spot welded joint described in any one of <1> to <4>, which is less than <6>. Automotive component including a spot welded joint described in any one of <1> to <5>.
[0011] According to this disclosure, spot welded joints and automotive components are provided, which include a thin, high-strength steel sheet with a Vickers hardness of 410 HV or higher, and whose joint strength is significantly improved by low-temperature heat treatment after nugget formation, even without tempering or solidification segregation relaxation.
[0012] This figure shows the results of a cross-sectional test of a joint formed by spot welding two stacked steel plates with a Vickers hardness of 410 HV or higher. This figure shows the results of a uniaxial tensile test (JIS No. 5). This figure shows the results of a Vickers hardness test. This figure shows the results of observations by SEM (Scanning Electron Microscope). This figure shows the results of observations by TEM (Transmission Electron Microscope). This figure shows the hardness test results with notes added. This is a schematic diagram showing an example of a cross-section in the thickness direction of a nugget formed by spot welding two stacked steel plates. This is a schematic diagram showing an example of a nugget and heat-affected zone (HAZ) formed when resistance spot welding is performed on a stacked steel plate. This is a schematic diagram showing an example of a cross-sectional section in the thickness direction of a nugget formed by spot welding three stacked steel plates. This is a schematic diagram showing another example of a cross-sectional section in the thickness direction of a nugget formed by spot welding three stacked steel plates, including one relatively thin steel plate.
[0013] The following describes an embodiment that is an example of this disclosure. In this disclosure, the "%" indication for the content of each element means "mass %". In this disclosure, a numerical range expressed using "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, unless otherwise specified. In cases where "greater than" or "less than" is attached to the numbers written before and after "~", the numerical range means a range that does not include those numbers as the lower or upper limit. In numerical ranges described in stages in this disclosure, the upper limit of one stage numerical range may be replaced with the upper limit of another stage numerical range or the value shown in the example. In numerical ranges described in stages in this disclosure, the lower limit of one stage numerical range may be replaced with the lower limit of another stage numerical range or the value shown in the example. Furthermore, the term "process" is included not only in the sense of an independent process, but also in the sense of a process that cannot be clearly distinguished from other processes, as long as the intended purpose of that process is achieved.
[0014] Generally, the higher the tensile strength of a steel plate, the lower the toughness of the welded part and the lower the joint strength. When resistance spot welding high-tensile steel materials, post-current treatment is a means to prevent a decrease in joint strength (cross-tensile strength: CTS). This is because post-current treatment causes solidification segregation relaxation or tempering. However, the inventors of the present invention conducted experiments and studies to obtain a spot-welded joint with higher joint strength without performing post-current when performing resistance spot welding on a plate assembly including high-tensile steel materials, particularly high-strength steel plates with a Vickers hardness of 410 HV or more. As a result, by reducing Si and Al in the high-strength steel plate, it becomes easier to precipitate fine carbides. After forming a nugget by the main current, by performing low-temperature heat treatment under predetermined conditions, it was found that the solidification segregation state and the decrease in hardness were suppressed, and by precipitating fine carbides, the toughness was improved and the joint strength was improved.
[0015] Here, the experimental results leading to the present disclosure will be specifically described. After fabricating a joint by spot-welding two sheets of steel plates with a thickness of 1.2 mm and a Vickers hardness of 410 HV or more, a cross-tensile test was conducted. In the case of single current with a nugget diameter of 5 mm or 6 mm, joints were fabricated in a total of two patterns respectively. In order to eliminate the influence of hydrogen on the joint strength (CTS), all joints were left for 7 days or more. Then, the results of measuring the joint strength of those subjected to low-temperature heat treatment and those not subjected to low-temperature heat treatment are shown in FIG. 1. The joint strength was measured as the average value of the joint strengths of three samples each. As a result, since the CTS was greatly improved in the joints subjected to low-temperature heat treatment, the cause was investigated.
[0016] The following reasons were considered for the improvement of CTS in the joints subjected to low-temperature heat treatment. (I) The yield strength of the steel plate changes, and the stress state at the nugget end changes. (II) The toughness of the nugget is improved.
[0017] (I) In order to investigate whether the yield strength of the steel sheet changed due to low-temperature heat treatment, JIS No. 5 was used as a test piece, and a tensile test of the steel sheet was conducted in accordance with JIS Z2241:2011. The results are shown in Fig. 2. As shown in Fig. 2, the yield strength of the steel sheet improved by performing low-temperature heat treatment. Since the stress at the end of the nugget increases when the yield strength improves, the CTS tends to decrease. Therefore, the increase in CTS shown in Fig. 1 cannot be explained by the change in the yield strength of the steel sheet due to low-temperature heat treatment.
[0018] Next, (II) the change in the toughness of the nugget was investigated. The Vickers hardness test results are shown in Fig. 3, and the SEM observation results are shown in Fig. 4. In the SEM observation, for each of the joints with and without low-temperature heat treatment, the base material (steel sheet) part and the nugget end that were not affected by the heat of spot welding were observed. As shown in Fig. 3, the Vickers hardness decreased by about 20 HV by performing low-temperature heat treatment, but no tissue change was observed in the SEM observation.
[0019] In order to conduct a more detailed tissue observation, replica TEM observations were made on the nugget ends of the joints with and without low-temperature heat treatment. The results are shown in Fig. 5. The replica TEM observation samples were electrolytically polished to 100 nm from the surface layer, and then precipitates were pressed onto a copper mesh for TEM observation. It was found that fine carbides were precipitated as indicated by the arrow by performing low-temperature heat treatment. Although fine carbides were also observed in other locations than the arrow in all samples, the number density of fine carbides was higher in the joints with low-temperature heat treatment.
[0020] The hardness test results with considerations written in are shown in Fig. 6. Although the solution carbon that deteriorates toughness is reduced by performing low-temperature heat treatment, it is considered that the hardness increases due to particle dispersion strengthening by precipitating fine carbides. From these facts, it is considered that the toughness and the joint strength could be improved without significantly reducing the hardness by low-temperature heat treatment. The spot weld joint according to the present disclosure is derived from such analysis results.
[0021] [Spot Welded Joint] The spot welded joint according to this disclosure includes a plate assembly formed by overlapping multiple steel plates and a nugget for joining the multiple steel plates. At least one of the multiple steel plates is a high-strength steel plate having a thickness of 1.0 mm or more and 2.3 mm or less, and a chemical composition of mass% of C: 0.08 to 0.35%, Mn: 1.00 to 5.00%, Si: 0.01 to 1.20%, Al: 0.001 to 0.60%, Mo: 0.001 to 1.00%, Cr: 0.001 to 2.00%, P: 0.030% or less, Ti: 0 to 0.30%, and Cu: 0 to 0.50%, with a volume fraction of retained austenite of 0 to 20.0%, and a Vickers hardness of 410 HV or more. Furthermore, in a cross-section in the thickness direction passing through the center of the nugget, if the nugget end is defined as the portion of the nugget's melting boundary that includes a high-strength steel plate and corresponds to the plate interface where the sum of the Vickers hardness of two adjacent steel plates is the highest, then in a 200 μm square edge region near the nugget end (sometimes referred to as the "nugget end region" in this disclosure), the number density of fine carbides with an equivalent circle diameter of 30 nm or less is 30.0 / μm. ２ The above conditions are met, and the Vickers hardness is within ±50 HV of the hardness calculated from the estimation formula HV below, and when the weighted average obtained by multiplying the chemical components of multiple steel plates by the ratio of the thickness of each steel plate to the total thickness of the plate assembly is considered as the average chemical component of the nugget, the area ratio of P-enriched areas where the P concentration is 1.5 times or more the P content of the average chemical component is 0.5% or more, and the area ratio of Mn-enriched areas where the Mn concentration is 1.5 times or more the Mn content of the average chemical component is 0.5% or more, is satisfied, or both. Estimation formula HV = 217 + 1080 × (C + Si / 70 + Mn / 113 + Cr / 93 + Mo / 30) In the formula, the element symbols represent the content (mass%) of each element in the average chemical component of the nugget, and 0 is substituted if the corresponding element is not present.
[0022] Figure 7 is a schematic diagram showing an example of a cross-section in the thickness direction passing through the center of a nugget 13 formed by spot welding together two overlapping steel plates 1A and 1B. The two steel plates 1A and 1B are joined together to form an elliptical nugget 13, with the portion that was the plate interface 15 as its major axis.
[0023] The plate assembly of the spot-welded joint 10 according to this disclosure may consist of steel plates 1A and 1B, all of which may have a Vickers hardness of 410 HV or higher, or it may include at least one steel plate with a Vickers hardness of 410 HV or higher, in addition to a steel plate with a Vickers hardness of less than 410 HV. If all steel plates have a Vickers hardness of 410 HV or higher, they may be the same type of steel plate with the same Vickers hardness, or they may be different types of steel plates with different Vickers hardnesses. Steel plates 1A and 1B may consist of steel plates, at least one of which may have a Vickers hardness of 410 HV or higher.
[0024] The following description will mainly focus on spot-welded joints formed by spot-welding two high-strength steel plates 1A and 1B, each having a Vickers hardness of 410 HV or higher, as shown in Figure 7.
[0025] <Plate Assembly> In the spot-welded joint according to this disclosure, at least one of the multiple steel plates included in the plate assembly is a high-strength steel plate with a thickness of 1.0 mm or more and 2.3 mm or less, and a Vickers hardness of 410 HV or more.
[0026] (High-Strength Steel Plate) Normally, if a steel plate has a Vickers hardness of 410 HV or more, its tensile strength will be 1280 MPa or more. However, a high-strength steel plate may be, for example, a steel plate with a Vickers hardness of 410 HV or more and a tensile strength of less than 1280 MPa. In this disclosure, a steel plate with a Vickers hardness of 410 HV or more is referred to as a "high-strength steel plate". A high-strength steel plate preferably has a Vickers hardness of 465 HV or more. In this disclosure, the Vickers hardness of a steel plate is measured at a position 10 mm or more away from the center of the nugget, in a region within ±1 / 4 t (t: plate thickness) from the center of the plate in the thickness direction of the plate, with a load of 200 gf according to JIS Z 2244:2024. Note that the tensile strength and Vickers hardness of a steel plate are proportional. Therefore, by measuring the Vickers hardness, the tensile strength can be estimated using the following equations A and B (see reference: Setsuo Takagi, Work Hardening in Ferritic Steel, Vol. 105 (2019), Equation (25)). Equation A: HV(GPa) = 10² × HVp Equation B: σ(GPa) = 0.32 × HVp(GPa) In equations A and B, σ represents the tensile strength and HV represents the Vickers hardness. For example, in the case of HV410, from equation A, HVp is 410 / 10² ≈ 4.02 GPa. Furthermore, using equation B, the tensile strength σ is calculated to be 0.32 × 4.0² = 1.28 GPa = 1280 MPa. Thus, from the above estimation formulas A and B, it can be estimated that the high-strength steel sheet having a Vickers hardness of 410 HV or more in this disclosure corresponds to a steel sheet having a tensile strength of 1280 MPa or more.
[0027] -Chemical Composition- High-strength steel sheets satisfy the following chemical composition in mass percent: C: 0.08-0.35%, Mn: 1.00-5.00%, Si: 0.01-1.20%, Al: 0.001-0.60%, Mo: 0.001-1.00%, Cr: 0.001-2.00%, P: 0.030% or less, Ti: 0-0.30%, and Cu: 0-0.50%. Note that Ti and Cu are optional elements and may not be included.
[0028] C: 0.08-0.35% Carbon (C) is a strengthening element that improves the tensile strength of steel. Furthermore, the higher the carbon content of the steel, the higher the carbon content of the nugget, increasing the driving force for carbide precipitation during tempering and promoting carbide precipitation. However, if the carbon content is less than 0.08%, it becomes difficult to obtain a Vickers hardness of 410 HV or higher, and fine carbides are less likely to precipitate near the nugget edges due to the low-temperature heat treatment described later. Also, if the carbon content exceeds 0.35%, the workability of the high-strength steel sheet tends to decrease, and the toughness of the welded joint also decreases significantly. Therefore, the carbon content should be between 0.08 and 0.35%. The carbon content may also be 0.10% or more, or 0.12% or more.
[0029] Mn: 1.00–5.00% Mn increases the strength of steel. A Mn content of 1.00% or more makes it easier to obtain a Vickers hardness of 410 HV or higher. However, if the Mn content exceeds 5.00%, there is a possibility of grain boundary embrittlement due to grain boundary segregation and a decrease in toughness due to ε-martensite formation. Therefore, the Mn content should be between 1.00% and 5.00%. The Mn content may also be 1.30% or more, 1.50% or more, or 1.60% or more.
[0030] Si: 0.01–1.20% Si is a strengthening element that increases the strength of steel through solid solution strengthening and microstructure strengthening. However, if the Si content exceeds 1.20%, carbide precipitation becomes difficult. Therefore, the Si content should be 1.20% or less. The Si content may be 0% (i.e., no Si is present), but it is difficult to industrially reduce the Si content to less than 0.01%. Therefore, the Si content should be 0.01–1.20%, and may also be 0.05–1.20%. The Si content may also be 1.10% or less, 0.80% or less, 0.60% or less, or 0.40% or less.
[0031] Al: 0.001-0.60% Al functions as a deoxidizing agent. On the other hand, Al is extremely easily oxidized, and if the Al content exceeds 0.60%, the number of inclusions increases, and moldability tends to decrease. Also, carbides become less likely to precipitate. Therefore, the Al content should be 0.001-0.60%. The Al content may be 0.02% or more, or 0.03% or more. The Al content may also be 0.50% or less, 0.45% or less, or 0.40% or less.
[0032] Mo: 0.001 to 1.00% Molybdenum (Mo) is an element that enhances the hardenability of steel and contributes to improving its strength. To fully obtain the above effects, it is preferable that the Mo content be 0.001% or more. The Mo content may also be 0.002% or more, 0.003% or more, 0.004% or more, or 0.010% or more. On the other hand, if the Mo content exceeds 1.00%, the ferrite transformation in the steel sheet may be suppressed, and the ductility may decrease. Therefore, the Mo content should be 1.00% or less. The Mo content may also be 0.75% or less, 0.50% or less, or 0.35% or less.
[0033] Cr: 0.001-2.00% Chromium (Cr) is an element that contributes to improving strength. To fully obtain the above effect, it is preferable that the Cr content be 0.001% or more. The Cr content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Cr content exceeds 2.00%, the weldability may decrease. Furthermore, in cold-rolled sheet annealing, the concentration of carbon in austenite is suppressed, and after holding at the annealing temperature, the cooling to room temperature promotes pearlite transformation, which may cause a decrease in strength. Therefore, the Cr content should be 2.00% or less. The Cr content may be 1.50% or less, 1.25% or less, or 1.00% or less.
[0034] P: 0.030% or less. Phosphorus (P) is an element that enhances hardenability and increases the strength of steel plates. However, if the P content exceeds 0.030%, many P-enriched areas are formed during solidification, which may reduce the toughness of the welded joint. Therefore, the P content should be 0.030% or less. The P content may also be 0.025% or less, 0.022% or less, or 0.020% or less. On the other hand, if the P content is less than 0.0001%, the above effects may not be sufficiently obtained. In addition, reducing the P content to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the P content may be 0.0001% or more. The P content may also be 0.0005% or more, 0.001% or more, or 0.002% or more.
[0035] Ti: 0-0.30% Titanium (Ti) is an element that contributes to improving the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing grain growth, and dislocation strengthening by suppressing recrystallization. To fully obtain the above effects, it is preferable that the Ti content be 0.001% or more. The Ti content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Ti content exceeds 0.300%, sufficient ductility may not be obtained. Therefore, the Ti content should be 0.30% or less. The Ti content may be 0.25% or less, 0.22% or less, or 0.20% or less.
[0036] Cu: 0-0.50% Copper (Cu) is an element that contributes to improving strength. To fully obtain the above effect, it is preferable that the Cu content be 0.001% or more. The Cu content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Cu content exceeds 0.50%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Cu content should be 0.50% or less. The Cu content may be 0.45% or less, 0.40% or less, or 0.35% or less.
[0037] Total Si and Al content: Preferably 1.50% or less. The total Si and Al content in high-strength steel sheets is 2.00% or less, but from the viewpoint of precipitating carbides, it is preferably 1.50% or less.
[0038] Remainder: Fe and impurities The remainder of the elements other than those listed above consists of Fe and impurities. Typical examples of impurities include S, O, and N. Impurities are components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap. Furthermore, impurities also include elements other than those described above, which are present at a level where their specific effects do not affect the properties of the high-strength steel sheet in this disclosure. Accordingly, the welded joints according to this disclosure may contain elements such as B, Nb, V, Ni, As, Sn, Sb, Ca, Mg, Co, Zr, and W, to the extent that they do not hinder the effects in this disclosure.
[0039] - Volume fraction of retained austenite - The high-strength steel sheet in this disclosure has a volume fraction of retained austenite (sometimes referred to as "retained γ" in this disclosure) of 0 to 20.0%. If the volume fraction of retained γ in the high-strength steel sheet that is the base material of the spot weld joint exceeds 20.0%, the toughness of the steel sheet portion may decrease, potentially reducing the CTS at the time of plug fracture. The volume fraction of retained γ in the high-strength steel sheet is preferably 19.0% or less, more preferably 15.0% or less, and even more preferably 10.0% or less. The microstructure of the high-strength steel sheet in this disclosure is not particularly limited other than having a volume fraction of retained γ within the range of 0 to 20.0%.
[0040] The volume fraction of residual γ in high-strength steel plates is determined by taking into account the Hazardous Zone (HAZ). Samples taken from a region at least 7 mm away from the nugget center (outside the HAZ) of a spot-welded high-strength steel plate are observed by EBSD (electron backscatter diffraction) under conditions of 1000x magnification, 15 eV acceleration voltage, and 0.3 μm step width. The fcc phase is defined as residual γ, and the volume fraction of residual γ is measured. The observation position in the thickness direction is not particularly limited, but similar to the measurement area for Vickers hardness, it is sufficient to observe within ±1 / 4 t (t: thickness) from the center of the plate in the cross section in the thickness direction.
[0041] - Plate Thickness - High-strength steel plates have a plate thickness of 1.0 mm or more and 2.3 mm or less. If the plate thickness of the high-strength steel plate is less than 1.0 mm, stress does not concentrate at the nugget ends and crack propagation does not occur in the nugget, making it difficult to obtain the effects of this disclosure. On the other hand, if the plate thickness exceeds 2.3 mm, the degree of stress concentration at the nugget ends is too great, making it difficult to obtain the effect of improving toughness.
[0042] The thickness of plates other than high-strength steel plates is not particularly limited, but examples include thicknesses of 0.5 to 3.5 mm. The total thickness of the plate assembly is not particularly limited, but examples include thicknesses of 1.5 to 8.0 mm.
[0043] -Plating- The steel sheets constituting the sheet metal assembly may have a plating layer formed on their surface. Examples of plating layer types include Zn-based, Zn-Fe-based, Zn-Ni-based, Zn-Al-based, Zn-Mg-based, Pb-Sn-based, Sn-Zn-based, and Al-Si-based. Examples of high-strength steel sheets equipped with a Zn-based plating layer (a plating layer with the highest Zn content among the components constituting the plating layer) include alloyed hot-dip galvanized steel sheets, hot-dip galvanized steel sheets, and electro-galvanized steel sheets. When a plating layer is formed on the surface of a high-strength steel sheet, the spot-welded joint 10 exhibits excellent corrosion resistance. When the plating layer is an alloyed zinc plating layer on the surface of the high-strength steel sheet, particularly excellent corrosion resistance is obtained, and the adhesion of the paint is also good.
[0044] Furthermore, in the case of high-strength steel sheets, if a zinc-based plating layer is formed on the surface of the base steel sheet, the amount of ZnO produced (content) in the plating layer must be 15 g / m². ２ It is preferable that the amount is less than 15 g / m². For example, in hot-stamped materials, low-temperature heat treatment is performed after spot welding, but heat treatment at around 900°C is performed before spot welding, and the amount of ZnO generated in the plated area is 15 g / m². ２ That concludes the explanation. On the other hand, when the high-strength steel plate in the welded joint according to this disclosure has a zinc-based plating layer, the low-temperature heat treatment is not as high as the heat treatment temperature before spot welding of the hot-stamped material, so the amount of ZnO produced (content) in the plating layer is 15 g / m ２ It will be less than.
[0045] The ZnO content in the plating layer is measured by the following method: A 30 mm x 30 mm analytical sample is taken from the steel plate member, the ZnO on the plating surface is removed with ammonium dichromate, the amount of ZnO is measured from the weight difference of the analytical sample before and after removal, and the value obtained by dividing this by the area of the analytical sample is the ZnO content.
[0046] By using spot-welded joints that include high-strength steel plates, high tensile strength can be ensured even for spot-welded joints. The plate assembly may consist of two or three or more steel plates. The Vickers hardness of the steel plates other than the high-strength steel plates in the assembly is not particularly limited and can be selected according to the application and required characteristics of the spot-welded joint being manufactured.
[0047] <Nugget> The nugget 13 is a weld metal formed to join all the steel plates by melting and solidifying at the locations where multiple steel plates included in the plate assembly are spot welded.
[0048] (Nugget diameter) The nugget diameter is the thickness of the thinnest steel plate in the plate assembly. ｍｉｎ In that case, the nugget diameter is 3.8√t ｍｉｎ The above 6.0√t ｍｉｎ The following applies. Note that √t ｍｉｎ t ｍｉｎ １／２ This is equivalent to: The nugget diameter is 3.8√t ｍｉｎ If the nugget diameter is less than 6.0√t, the nugget circumference is short, and the degree of stress concentration at the nugget ends is too great, making it difficult to obtain the toughness improvement effect with the method disclosed herein. On the other hand, if the nugget diameter is 6.0√t ｍｉｎ Beyond this point, the circumference of the nugget is large, making it difficult for stress to concentrate at the ends of the nugget. As a result, crack propagation does not occur in the nugget, and the effects of this disclosure are difficult to obtain.
[0049] (Number density of fine carbides near the nugget end) In a 200 μm square end region R1 near the nugget end, the number density of fine carbides (sometimes simply referred to as "fine carbides" in this disclosure) with an equivalent circle diameter of 30 nm or less is 30.0 / μm. ２This concludes the explanation. In this disclosure, the nugget end 13E is defined as the portion of the nugget 13 in a cross-section in the thickness direction passing through the center of the nugget 13, corresponding to the position of the plate interface 15 of two adjacent steel plates where the sum of the Vickers hardnesses is highest, within the melting boundary of the nugget 13, as shown in Figure 7. Furthermore, the 200 μm square end region R1 near the nugget end within the nugget is a 200 μm square region where a pair of opposite sides are in the thickness direction and symmetrical with respect to the plate interface 15, with two of the four corners located on the nugget end side lying on the melting boundary line, and the entire region R1 is included inside the nugget. Also, for example, in the case of a plate assembly of three steel plates 1A, 1B, and 1C stacked together, as shown in Figure 9 later, the 200 μm square end region R1 near the nugget end is defined as a region where one of the two corners located on the nugget end side within the 200 μm square area lies on the melting boundary line, and the entire region R1 is included inside the nugget. The number density of fine carbides in the nugget edge region R1 is 30.0 / μm ２ This allows for a reduction in the amount of dissolved carbon in the martensite without significantly altering the Vickers hardness. The number density of fine carbides is 50.0 / μm ２ Preferably, it is 70.0 / μm or more. ２ The above is more preferable. There is no particular upper limit to the number density of fine carbides in the nugget end region R1, but from the viewpoint of preventing a large decrease in Vickers hardness, 200.0 / μm is preferable. ２ The following is also acceptable: 150.0 / μm ２ The following is also acceptable.
[0050] The number density of fine carbides in the nugget edge region R1 is determined by TEM observation (magnification: 20,000x) in a cross-section in the thickness direction of the plate assembly, including the central portion of the nugget, within a 200 μm square edge region R1 near the nugget edge, where at least three fields of view are randomly selected and the total field of view area is 300 μm. ２Measurements are taken to achieve the above results. Elemental analysis of the particle portion in the TEM observation image is performed using EDS (Energy Dispersive X-ray Spectroscopy) to determine if it is carbide. Then, by approximating the carbide particle as an ellipse and measuring the major and minor axes, the area of the carbide can be calculated, and the equivalent circle diameter can be determined. In this way, "fine carbides with an equivalent circle diameter of 30 nm or less" are identified. Given the resolution of TEM observation (magnification: 20,000x), the lower limit of the equivalent circle diameter of the fine carbides identified by the above method is set at 1 nm, and carbides with an equivalent circle diameter of less than 1 nm can be ignored. The number density is calculated from the total number of fine carbides with an equivalent circle diameter of 30 nm or less observed in each field of view and the total area of the observation field, and the calculated value is considered the number density within the 200 μm square edge region R1. By this method, the number density is calculated by measuring the number of fine carbides with an equivalent circle diameter of 30 nm or less in the 200 μm square edge region R1 of the nugget edge region.
[0051] (Vickers hardness in the nugget edge region) The average Vickers hardness in the 200 μm square edge region R1 near the nugget edge is within ±50 HV of the hardness HV calculated by the following estimation formula HV. Estimation formula HV = 217 + 1080 × (C + Si / 70 + Mn / 113 + Cr / 93 + Mo / 30) In the formula, the element symbols represent the content of each element in the average chemical composition of the nugget calculated as a weighted average.
[0052] The spot-welded joint 10 according to this disclosure suppresses the decrease in hardness due to tempering, and the average Vickers hardness in the nugget end region R1 is within ±50 HV of the Vickers hardness calculated from the estimation formula HV. However, due to the effects of low-temperature heat treatment and errors from the above estimation formula HV, the average Vickers hardness in the nugget end region R1 may be greater than the Vickers hardness calculated from the estimation formula HV. If tempering is performed after nugget formation by post-energization or heater, the nugget will break, so the CTS may not improve, or the CTS may even decrease compared to before tempering. On the other hand, if tempering is performed at a low temperature after nugget formation, the nugget will not break, and the Vickers hardness at the nugget end can be the same as that of the estimation formula HV. The higher the Vickers hardness at the nugget end, the less likely plug fracture is to occur, and a high CTS can be achieved. From this viewpoint, it is preferable that the average Vickers hardness in the nugget end region R1 is within ±40 HV, ±30 HV, or ±20 HV of the hardness HV calculated by the estimation formula HV.
[0053] The Vickers hardness in the nugget edge region R1 is measured in the aforementioned 200 μm square edge region R1. In the nugget edge region R1, the Vickers hardness is measured at 10 points with a load of 300 gf, and the average value is taken as the average Vickers hardness. The Vickers hardness is measured so that all indentations are at a distance of at least four indentation sizes from the nearest nearest indentation. If, for example, the total thickness of the plate assembly is thin and a 200 μm square region R1 cannot be secured in the nugget edge region R1, the Vickers hardness is measured at 10 points in the region within 2000 μm from the nugget edge to the inside of the nugget, and the average value is taken as the average Vickers hardness.
[0054] (P-enriched and Mn-enriched areas in the nugget edge region) When the weighted average obtained by multiplying the chemical composition of each steel sheet included in the sheet assembly by the ratio of the thickness of each steel sheet to the total thickness of the sheet assembly is considered as the average chemical composition of the nugget, the P content and Mn content in the nugget edge region R1 satisfy at least one of the following (A) and (B): (A) The area ratio of P-enriched areas (sometimes simply referred to as "P-enriched areas" in this disclosure) where the P concentration is 1.5 times or more the average P content of the average chemical composition of the nugget is 0.5% or more. (B) The area ratio of Mn-enriched areas (sometimes simply referred to as "Mn-enriched areas" in this disclosure) where the Mn concentration is 1.5 times or more the average Mn content of the average chemical composition of the nugget is 0.5% or more.
[0055] Here, the "average chemical composition of the nugget" is the weighted average obtained by multiplying the chemical composition of each steel plate 1A and 1B included in the plate assembly by the thickness ratio of each steel plate to the total plate thickness of the plate assembly. Since the nugget 13 is formed by the molten and solidified state of all the steel plates included in the plate assembly, it depends on the chemical composition of each steel plate 1A and 1B. For example, if the plate assembly is composed entirely of steel plates with the same chemical composition, then the chemical composition of those steel plates becomes the chemical composition of the nugget. On the other hand, if multiple steel plates with the same thickness but different chemical compositions are joined by the nugget, the chemical composition of the nugget is obtained by adding up the individual chemical compositions and dividing by the number of steel plates. Furthermore, if multiple steel plates with different thicknesses and different chemical compositions are joined by the nugget, the weighted average obtained by multiplying the chemical composition of each steel plate included in the plate assembly by the thickness ratio of each steel plate to the total plate thickness of the plate assembly is considered to be the chemical composition of the nugget. In any case, the weighted average of the chemical composition of each steel plate 1A and 1B, taking into account each plate thickness, is considered to be the chemical composition of the nugget.
[0056] In the spot-welded joint according to this disclosure, since no solidification segregation relaxation occurs in the nugget, the area ratio of the P-enriched area and / or the area ratio of the Mn-enriched area in the nugget end region R1 is 0.5% or more. In this way, when the area ratio of either the P-enriched area or the Mn-enriched area in the nugget end region R1 is 0.5% or more, or both area ratios are 0.5% or more, the toughness of the nugget would normally decrease due to the effects of solidification segregation. However, in the spot-welded joint according to this disclosure, it is thought that the toughness of the nugget is improved because a large number of fine carbides are precipitated in the nugget end region R1.
[0057] While there is no particular upper limit to the area ratios of the P-enriched and Mn-enriched areas in the nugget end region R1, if the area ratios of the P-enriched and Mn-enriched areas in the nugget end region R1 are too high, it can cause a decrease in CTS. From the viewpoint of suppressing a decrease in CTS, it is preferable that the area ratios of the P-enriched and Mn-enriched areas in the nugget end region R1 are 10% or less, respectively.
[0058] The P concentration and Mn concentration in the nugget edge region R1 can be measured using an EPMA (electron probe microanalyzer), and the area ratio of P-enriched areas where the average P content is 1.5 times or more, and the area ratio of Mn-enriched areas where the average Mn content is 1.5 times or more, can be identified in a 200 μm square edge region R1.
[0059] (Applications) The applications of the spot welded joints relating to this disclosure are not particularly limited, but for example, they can be used in automotive components that include the spot welded joints relating to this disclosure. Automotive components that include the spot welded joints relating to this disclosure have high joint strength and can contribute to improving the safety of the vehicle body.
[0060] [Method for Manufacturing Spot Welded Joints] The method for manufacturing spot welded joints according to this disclosure is not particularly limited, but the method for manufacturing spot welded joints described below (sometimes referred to as "the method for manufacturing spot welded joints according to this disclosure") can suitably manufacture spot welded joints according to this disclosure. However, the spot welded joints according to this disclosure are not limited to spot welded joints manufactured by the method for manufacturing spot welded joints described below.
[0061] The method for manufacturing a spot-welded joint according to this disclosure involves stacking multiple steel plates, each containing at least one high-strength steel plate with a thickness of 1.0 mm or more and a Vickers hardness of 410 HV or more, and then clamping the plate assembly in the thickness direction between a pair of electrodes and applying pressure while applying a current value I １ The process includes an energizing step of applying an electric current at (kA) to form nuggets, and a low-temperature heat treatment step of heating the nuggets at 100 to 250°C for 5 to 30 minutes after the energizing step. Each step will be described below.
[0062] <Electrification Process> In the energization process, a plate assembly consisting of two or more steel plates, including at least one high-strength steel plate, is sandwiched in the thickness direction between a pair of electrodes and pressed while applying a current value I １ The nuggets are formed by applying current at (kA).
[0063] In the energizing process, the current value I is set so that a nugget is formed by spot welding to join all the steel plates that make up the plate assembly. １ (kA) and energizing time t １ It is preferable to set (ms). Figure 8 schematically shows an example of a nugget formed when an energizing process is performed on a plate assembly of two stacked steel plates. As shown in Figure 8, electrodes 2A and 2B are pressed against the plate assembly of stacked steel plates 1A and 1B so as to sandwich them in the thickness direction, and then current is passed between electrodes 2A and 2B. As a result, a nugget 13 and a heat-affected zone (so-called HAZ) 14 are formed in the energized portion between steel plate 1A and steel plate 1B, and the two steel plates are spot welded together.
[0064] In the energizing process, there are no restrictions on welding conditions as long as the desired nugget diameter is formed. In the plate assembly, the thickness of the steel plate with the smallest thickness is t ｍｉｎIf (mm), the nugget diameter is 3.8√t ｍｉｎ The above 6.0√t ｍｉｎ It is preferable to perform spot welding as follows: Current value I １ For example, the current is 5.0 to 8.0 kA, and the energizing time t １ For example, it is 120 to 600 ms. The current value can be constant, varied, or pulsed. When the current value is varied, such as in a pulsed manner, I １ This refers to the maximum value. In the case of an upslope, the energizing time including the upslope is t. １ In the case of pulsed current application, the current application time excluding the period of no current application is t １ In the case of pre-powering, the power-on process I １ and t １ The value does not change significantly. If pre-energization is difficult to distinguish from upslope, the current value of pre-energization I ０ ×Electricity time t ０ The area of t ０ The value of the preliminary current I is obtained by dividing by this value. ０ Defines the pressure P applied to electrodes 2A and 2B against the plate assembly. １ The pressure can be constant, variable, or pulsed, and the applied pressure is, for example, 3.0 to 5.0 kN.
[0065] <Low-Temperature Heat Treatment Process> After the energizing process, the nuggets are subjected to a low-temperature heat treatment by heating them at 100-250°C for 5-30 minutes. The low-temperature heat treatment can be carried out using external heating means such as a heating furnace, hot plate, or salt bath furnace. Note that the method of further energizing (post-energizing) after the nuggets have been formed by energizing is not used because it reduces the Vickers hardness.
[0066] By performing low-temperature heat treatment at the above temperature and time, spot welded joints can be manufactured in which the number density of fine carbides in the nugget end region, the Vickers hardness, and one or both of the P-enriched and Mn-enriched regions satisfy the aforementioned ranges.
[0067] Although an example of an embodiment of the spot welded joint and its manufacturing method according to the present disclosure has been described above, the spot welded joint and its manufacturing method according to the present disclosure are not limited to the above embodiment.
[0068] For example, in the case of a spot-welded joint formed by spot-welding a plate assembly of three or more stacked steel plates, there are multiple plate interfaces. The number density of fine carbides is measured at the end of the nugget corresponding to the plate interface where the sum of the Vickers hardness of two adjacent steel plates is the highest. Figure 9 is a schematic diagram showing an example of a cross-section in the thickness direction passing through the center of a nugget 13 formed by spot-welding a plate assembly of three stacked steel plates 1A, 1B, and 1C. In the spot-welded joint 20 shown in Figure 9, the three steel plates 1A, 1B, and 1C are joined by an elliptical nugget 13. The shape of the nugget 13 is usually an approximately ellipse, as shown in Figures 8 and 9, where the shorter side is in the thickness direction and the longer side is in the in-plane direction of the plate, when viewed in cross-section in the thickness direction, but it is not limited to this shape.
[0069] For example, as shown in Figure 10, the thickness of one of the three steel plates, the outermost steel plate 1D, may be thinner than the thickness of the other two steel plates 1A and 1B, and the shape may be such that two nuggets 13A and 13B formed between two adjacent steel plates are joined together. In such a spot-welded joint 30, for example, if the Vickers hardness of steel plates 1A and 1B is 410 HV or more, and the Vickers hardness of steel plate 1D is less than 410 HV, the number density of fine carbides, the Vickers hardness, and the P-enriched and Mn-enriched areas should be measured in the nugget end region R1 of the portion 13B that joins the steel plates 1A and 1B.
[0070] The following describes examples of spot-welded joints according to this disclosure. However, the spot-welded joints according to this disclosure are not limited to the following examples.
[0071] [Manufacturing of spot-welded joints using unplated steel sheets] Prepare a sheet assembly by stacking two steel sheets having the thickness, Vickers hardness, chemical composition, and microstructure shown in Table 1, and apply the spot welding conditions (current value I) shown in Table 2 to each sheet assembly. １ , time t １ Pressing force P １After forming the nuggets, they were left to stand for more than 7 days to avoid the effects of hydrogen, and then heat-treated in a heating furnace to manufacture spot-welded joints. For Nos. 29 and 30, after nugget formation, current was applied after solidification segregation relaxation. The conditions for current application after solidification segregation relaxation were a rest time of 0.20 s, a post-current application time of 0.40 s, and a post-current application current value of 0.9 times the main current application current value. Furthermore, after current application after solidification segregation relaxation, heat treatment was performed in a heating furnace under the conditions shown in Table 2. In each table, underlined text indicates that it is outside the scope of this disclosure or outside the range of preferred manufacturing conditions.
[0072]
[0073]
[0074] For the manufactured spot-welded joints, the nugget diameter, the number density of fine carbides in the nugget end region, the Vickers hardness, and the area ratios of P-enriched and Mn-enriched regions were measured in the cross-section in the thickness direction using the method described above.
[0075] Furthermore, the CTS of spot welded joints 1 to 36 was measured in accordance with JIS Z 3137:1999 "Specimen dimensions and test method for cross tensile tests of resistance spot and projection welded joints". In addition, ΔCTS was defined as the value obtained by subtracting the CTS of the spot welded joint that was not heat-treated from the CTS of spot welded joints 1 to 36. A ΔCTS of 1.5 kN or more was evaluated as a significant improvement in CTS. A ΔCTS of 2.0 kN or more was evaluated as a good improvement in CTS. ΔCTS = CTS of heat-treated joint - CTS of unheat-treated joint (spot welding with single current only) The results are shown in Table 3. Note that "nugget diameter (√t ｍｉｎ The value of ) is obtained by dividing the nugget diameter by the square root of the minimum plate thickness of each plate assembly (essentially the plate thickness shown in Table 1). For Mn-enriched and P-enriched areas, underlines are used if both are outside the scope of this disclosure. In addition, values of ΔCTS less than 1.5 kN are also underlined.
[0076]
[0077] All of the spot-welded joints in the examples met the requirements of this disclosure, and their ΔCTS was 1.5 kN or more compared to the case where heat treatment was omitted. On the other hand, the comparative examples were outside the scope of this disclosure, and the ΔCTS of all but No. 33 was less than 1.5 kN. This is thought to be due to the following: No. 1 has a high Si content, so there is little carbide precipitation with low-temperature heat treatment. No. 2 has a low C content, so it has good toughness to begin with, and in addition, the amount of carbide precipitation is small. No. 3 has a low Mn content, so a large amount of carbide had already precipitated before low-temperature heat treatment due to self-tempering, resulting in a small ΔCTS. No. 5 has a high Al content, so there is little carbide precipitation with low-temperature heat treatment. No. 6 has a thick plate, so stress tends to concentrate at the nugget end, and the amount of toughness improvement by low-temperature heat treatment is insufficient. Also, because of the high Si content, there is little carbide precipitation with low-temperature heat treatment. No. 7 has a high carbon content, which significantly reduces the toughness of the nugget, and the improvement in toughness from low-temperature heat treatment is insufficient. No. 8 has a thin plate thickness and low stress concentration at the nugget edges, resulting in a high CTS even without low-temperature heat treatment. No. 10 has a high manganese content, which causes lenticular martensite formation, so low-temperature heat treatment does not improve toughness. No. 11 has a high Al content, resulting in little carbide precipitation from low-temperature heat treatment. No. 14 has a small nugget diameter, making it prone to interfacial fracture, so low-temperature heat treatment is insufficient for improving toughness. No. 17 has a high carbon content, resulting in low nugget toughness, and low-temperature heat treatment is insufficient for improving toughness. Also, because of the high Si content, the amount of carbide precipitation is small, and the improvement in toughness from low-temperature heat treatment is also small. No. 18 has a high Al content, resulting in little carbide precipitation from low-temperature heat treatment. No. No. 20 has a thick plate, making it prone to stress concentration at the nugget ends, and the improvement in toughness from low-temperature heat treatment is insufficient. No. 21 has a large nugget diameter, causing plug fracture, so low-temperature heat treatment does not improve CTS. No. 22 has a high Si content, resulting in less carbide precipitation from low-temperature heat treatment. No. 26 has high Si and Al content, resulting in less carbide precipitation from low-temperature heat treatment. No. 28 has a thin plate, resulting in less stress concentration at the nugget ends, and a high CTS even without low-temperature heat treatment.No. 29 has a high CTS even without low-temperature heat treatment because the area ratios of both the P-enriched and Mn-enriched areas are small after energizing following solidification segregation relaxation. No. 30 also has a high CTS even without low-temperature heat treatment because the area ratios of both the P-enriched and Mn-enriched areas are small after energizing following solidification segregation relaxation. No. 33 has a high heat treatment temperature, and although an improvement in CTS is observed due to tempering, the decrease in Vickers hardness in the nugget edge region is large. No. 34 has a low heat treatment temperature, resulting in a small amount of carbide precipitation, and therefore no improvement in toughness and no improvement in CTS.
[0078] [Manufacturing of Spot Welded Joints Using Galvanized Steel Sheets] Hot-dip galvanized steel sheets (hereinafter sometimes referred to as "galvanized steel sheets") were prepared by applying hot-dip galvanizing to steel sheets having the plate thickness, Vickers hardness, chemical composition, and microstructure shown in Table 4. The "ZnO content" in Table 4 is the content in the plating layer after the heat treatment shown in Table 5 has been performed on the spot welded joints using the plated steel sheet assembly described later, and the value was measured by the method described above.
[0079]
[0080] Prepare a plate assembly by stacking two plated steel sheets as shown in Table 5, and apply the spot welding conditions (current value I) shown in Table 5 to each plate assembly. １ , time t １ Pressing force P １ After forming the nuggets, they were left to stand for more than seven days to avoid the effects of hydrogen, and then heat-treated in a heating furnace to manufacture spot-welded joints.
[0081]
[0082] For the manufactured spot-welded joints, the nugget diameter, the number density of fine carbides in the nugget end region, the Vickers hardness, and the area ratios of P-enriched and Mn-enriched regions were measured in the cross-section in the thickness direction using the method described above.
[0083] Furthermore, the CTS of the spot-welded joints was measured in accordance with JIS Z 3137:1999 "Specimen dimensions and test method for cross-tensile tests of resistance spot and projection welded joints". In addition, ΔCTS was defined as the value obtained by subtracting the CTS of the spot-welded joints that were not heat-treated from the CTS of the spot-welded joints that were not heat-treated. A ΔCTS of 1.5 kN or more was evaluated as a significant improvement in CTS. A ΔCTS of 2.0 kN or more was evaluated as a good improvement in CTS. ΔCTS = CTS of heat-treated joint - CTS of unheat-treated joint (spot welding with single current only) The results are shown in Table 6.
[0084]
[0085] All of the spot-welded joints in the examples met the requirements of this disclosure, and their ΔCTS was 1.5 kN or more compared to the case where heat treatment was omitted. No. 103 is an example using a steel plate intended for hot-stamping material, and by performing a heat treatment of 900°C on the plated steel plate before spot welding, the ZnO content in the plating layer increased, and the ZnO content in the plating layer was also high in the spot-welded joint after spot welding. The ZnO content in the plating layer was 15 g / m ２ The above is for No. 103, which is 15 g / m ２ Numbers 101 and 102, which are below the threshold, had larger ΔCTS values and showed a greater improvement in CTS.
[0086] [Manufacturing of spot-welded joints using three dissimilar steel plates] Prepare a plate assembly by stacking three steel plates having the plate thickness, Vickers hardness, chemical composition, and microstructure shown in Table 7, and apply the spot welding conditions (current value I) shown in Table 8. １ , time t １ Pressing force P １ After forming the nuggets, they were left to stand for more than seven days to avoid the effects of hydrogen, and then heat-treated in a heating furnace to manufacture spot-welded joints.
[0087]
[0088]
[0089] For the manufactured spot-welded joints, the average hardness of the first and second steel plates was HV358, and the average hardness of the second and third steel plates was HV513. Therefore, the portion of the nugget's molten boundary corresponding to the interface between the second and third steel plates was defined as the nugget end. For the nugget end, the nugget diameter, the number density of fine carbides in the nugget end region, the Vickers hardness, and the area ratios of the P-enriched and Mn-enriched regions were measured in the cross-section in the thickness direction of the plate using the method described above.
[0090] Furthermore, the CTS at the interface between the second and third steel plates of the spot-welded joint was measured in accordance with JIS Z 3137:1999 "Specimen dimensions and test method for cross tensile tests of resistance spot and projection welded joints". In addition, ΔCTS was defined as the value obtained by subtracting the CTS of the spot-welded joint from the CTS at the interface between the second and third steel plates of a spot-welded joint that did not undergo post-spot welding heat treatment. A ΔCTS of 1.5 kN or more was evaluated as a significant improvement in CTS. A ΔCTS of 2.0 kN or more was evaluated as a good improvement in CTS. ΔCTS = CTS of heat-treated joint - CTS of joint without heat treatment (spot welding with single current only). The results are shown in Table 9.
[0091]
[0092] The three-layer spot-welded joint of No. 104 also met the requirements of this disclosure, and its ΔCTS was 1.5 kN or more compared to the case where heat treatment was omitted.
[0093] The disclosure of Japanese Patent Application No. 2025-013312, filed on 29 January 2025, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard were written specifically and individually.
[0094] 1A, 1B, 1C, 1D Steel plate 2A, 2B Electrode 10, 20, 30 Spot welded joint 13 Nugget 13E Nugget end 14 Heat-affected zone (HAZ) 15 Plate interface R1 Nugget end region
Claims
1. A spot-welded joint comprising a plate assembly formed by stacking multiple steel plates and a nugget for joining the multiple steel plates, wherein at least one of the multiple steel plates is a high-strength steel plate having a thickness of 1.0 mm or more and 2.3 mm or less, and satisfying the following chemical composition in mass%, C: 0.08-0.35%, Mn: 1.00-5.00%, Si: 0.01-1.20%, Al: 0.001-0.60%, Mo: 0.001-1.00%, Cr: 0.001-2.00%, P: 0.030% or less, Ti: 0-0.30%, and Cu: 0-0.50%, with a volume fraction of retained austenite of 0-20.0%, and a Vickers hardness of 410 HV or more, wherein the thickness of the steel plate with the smallest thickness in the plate assembly is t ｍｉｎ In that case, the nugget is 3.8√t ｍｉｎ The above 6.0√t ｍｉｎ Having the following nugget diameter, and in a cross-section in the thickness direction passing through the center of the nugget, if the portion of the molten boundary of the nugget that includes the high-strength steel plate and corresponds to the plate interface where the sum of the Vickers hardness of two adjacent steel plates is highest is defined as the nugget end, then in a 200 μm square end region near the nugget end within the nugget, the number density of fine carbides with an equivalent circle diameter of 30 nm or less is 30.0 / μm. ２ A spot welded joint that satisfies either or both of the following conditions: the Vickers hardness is within ±50 HV of the hardness calculated from the estimation formula HV below; and when the weighted average obtained by multiplying the chemical components of the plurality of steel plates by the ratio of the thickness of each steel plate to the total thickness of the plate assembly is considered as the average chemical component of the nugget, the area ratio of P-enriched areas where the P concentration is 1.5 times or more the P content of the average chemical component is 0.5% or more; and the area ratio of Mn-enriched areas where the Mn concentration is 1.5 times or more the Mn content of the average chemical component is 0.5% or more. Estimation formula HV = 217 + 1080 × (C + Si / 70 + Mn / 113 + Cr / 93 + Mo / 30) In the formula, the element symbols represent the content (mass%) of each element in the average chemical component of the nugget, and 0 is substituted if the corresponding element is not present.
2. The spot welded joint according to claim 1, wherein the total content of Si and Al in the high-strength steel plate is 1.50% by mass or less.
3. The spot welded joint according to claim 1 or claim 2, wherein the Si content of the high-strength steel plate is 0.01 to 0.40% by mass.
4. The spot welded joint according to any one of claims 1 to 3, wherein the Vickers hardness of the high-strength steel plate is 465 HV or higher.
5. The high-strength steel sheet has a zinc-based plating layer, and the ZnO content in the plating layer is 15 g / m². ２ A spot welded joint according to any one of claims 1 to 4, which is less than [a certain value].
6. An automotive component comprising a spot-welded joint according to any one of claims 1 to 5.