Spot welded joints and automotive components

Abstract

At least one of the steel plates 1A and 1B is a high-strength steel plate with a thickness of 1.0 mm or more and 2.3 mm or less, satisfying the specified chemical composition, having a volume fraction of retained austenite of 0 to 20.0%, and a Vickers hardness of 410 HV or more, and the thickness of the steel plate with the smallest thickness in the plate assembly is t min In that case, 3.8√t min The above is 6.0√t min The nugget has the following nugget diameters, and in the vicinity R1 of the nugget edge, which corresponds to the plate interface where the sum of the Vickers hardness of two adjacent steel plates, including a high-strength steel plate, is the highest, the number density of carbides with an equivalent circular diameter of 30 nm or less is 30.0 / μm. 2 The above describes a spot welded joint 10 and its applications that satisfy the following conditions: Vickers hardness is within ±50 HV of the hardness calculated from the estimation formula HV, and the area ratio of the P-enriched area is 0.5% or more, and the area ratio of the Mn-enriched area is 0.5% or more, or both.

Description

[Technical Field]

[0001] This disclosure relates to spot-welded joints and automotive components. [Background technology]

[0002] Spot welding is primarily used in processes such as vehicle body assembly and parts installation. In recent years, the automotive sector has seen an increasing demand for lighter vehicle bodies to achieve better fuel efficiency and reduced CO2 emissions, as well as 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 and 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 CTS when spot welding with high-tensile steel, it has been reported that after forming the nugget with the initial current, two subsequent currents are applied: one for tempering and another for mitigating solidification segregation.

[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 layer 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 × 10⁻¹⁶ 6 pieces / mm 2 The above spot-welded joint has been proposed.

[0005] Patent documents 2 and 3 disclose a method for manufacturing spot-welded joints 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 having more than C units is disclosed.

[0007] Patent Document 5 discloses a resistance spot welding joint using a high-strength steel plate in which retained austenite accounts for 20-50% by volume fraction. Patent document 6 discloses a welded joint in which a steel plate with a tensile strength of 780 to 1270 MPa is spot-welded as the base material. Patent Document 7 discloses a method for manufacturing a welded joint, which involves resistance welding to an overlapping member formed by overlapping multiple steel plates having a tensile strength of 440 MPa or more, and then heat-treating the welded portion at a predetermined temperature and time. [Prior art documents] [Patent Documents]

[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 [Overview of the project] [Problems that the invention aims to solve]

[0009] The present disclosure provides a spot weld joint and an automotive member including a thin high-strength steel sheet having a Vickers hardness of 410 HV or more, in which the joint strength is significantly improved by low-temperature heat treatment after nugget formation even without tempering or alleviation of segregation,

Means for Solving the Problems

[0010] The gist of the present disclosure for achieving the above object is as follows. <1> A spot weld joint including a stack of a plurality of steel sheets and a nugget joining the plurality of steel sheets, At least one of the plurality of steel sheets has a thickness of 1.0 mm or more and 2.3 mm or less, and the chemical components are, 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: 0.0 thirty% 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 it is a high-strength steel sheet having a Vickers hardness of 410 HV or more, When the thickness of the steel sheet having the smallest thickness in the stack is t min The nugget has a nugget diameter of 3.8√t min or more and 6.0√t min or less, In a cross section in the plate thickness direction passing through the center of the nugget, when a portion corresponding to the plate interface including the high-strength steel sheet and having the highest total Vickers hardness of two adjacent steel sheets among the melting boundaries of the nugget is defined as the nugget end, in a 200-μm square end region near the nugget end in the nugget, <000009​​​​​​When the weighted average obtained by multiplying the chemical composition of each of the plurality of steel sheets by the sheet thickness ratio of each steel sheet to the total sheet thickness of the sheet stack is regarded as the average chemical composition of the nugget, the area ratio of the P enrichment part where the P concentration is 1.5 times or more the P content of the average chemical composition is 0.5% or more, and the area ratio of the Mn enrichment part where the Mn concentration is 1.5 times or more the Mn content of the average chemical composition is 0.5% or more, a spot welding joint satisfying one or both of these conditions. Estimated formula HV = 217 + 1080×(C + Si / 70 + Mn / 113 + Cr / 93 + Mo / 30) In the formula, the element symbol means the content (mass%) of each element in the average chemical composition of the nugget, and 0 is substituted when the corresponding element is not contained. <2> The spot welding joint according to <1>, wherein the total content of Si and Al in the high-strength steel sheet is 1.50 mass% or less. <3> The spot welding joint according to <1> or <2>, wherein the content of Si in the high-strength steel sheet is 0.01 to 0.40 mass%. <4> The spot welding joint according to any one of <1> to <3>, wherein the Vickers hardness HV of the high-strength steel sheet is 465 or more. <5> The high-strength steel sheet has a zinc-based plating layer, and the content of ZnO in the plating layer is less than 1 2 g / m 2 The spot welding joint according to any one of <1> to <4>. <6> An automotive member including the spot welding joint according to any one of <1> to <5>.

Advantages of the Invention

[0011] According to the present disclosure, there are provided a spot welding joint and an automotive member including a thin high-strength steel sheet having a Vickers hardness of 410 HV or more, in which the joint strength is significantly improved by low-temperature heat treatment after nugget formation even without tempering or relaxation of solidification segregation.

Brief Description of the Drawings

[0012] [Figure 1]This figure shows the results of a cross-tensile test on a joint formed by spot welding together two layers of steel plates with a Vickers hardness of 410 HV or higher. [Figure 2] This figure shows the results of a uniaxial tensile test (JIS No. 5). [Figure 3] This figure shows the results of the Vickers hardness test. [Figure 4] This figure shows the results of observations made using a scanning electron microscope (SEM). [Figure 5] This figure shows the results of observations made using a TEM (transmission electron microscope). [Figure 6] This figure shows the hardness test results with the analysis noted. [Figure 7] This is a schematic diagram showing an example of a cross-section in the thickness direction of a nugget formed by spot welding together two overlapping steel plates. [Figure 8] This diagram schematically shows an example of a nugget and heat-affected zone (HAZ) formed when resistance spot welding is performed on a plate assembly consisting of two overlapping steel plates. [Figure 9] This is a schematic diagram showing an example of a cross-section in the thickness direction of a nugget formed by spot welding together three stacked steel plates. [Figure 10] This schematic diagram shows another example of a cross-section in the thickness direction of a nugget formed by spot welding together three steel plates, including one relatively thin steel plate. [Modes for carrying out the invention]

[0013] The following describes an embodiment that is an example of this disclosure. In this disclosure, the percentage (%) for the content of each element means "mass %". In this disclosure, numerical ranges expressed using "~" mean the range that includes the numbers before and after the "~" as the lower and upper limits, respectively, unless otherwise specified. In addition, numerical ranges that include "greater than" or "less than" before and after the "~" mean the range that does not include those numbers as the lower or upper limit. In the numerical ranges described in stages in this disclosure, the upper limit of one stage of numerical range may be replaced with the upper limit of another stage of numerical range or with a value shown in the examples. Also, in the numerical ranges described in stages in this disclosure, the lower limit of one stage of numerical range may be replaced with the lower limit of another stage of numerical range or with a value shown in the examples. Furthermore, the term "process" includes not only independent processes, but also any process that cannot be clearly distinguished from other processes, as long as its intended purpose is achieved.

[0014] Generally, the higher the tensile strength of a steel plate, the lower the toughness of the weld and the lower the joint strength. When resistance spot welding high-tensile steel, post-electric treatment is used to prevent a decrease in joint strength (cross tensile strength: CTS). This is because post-electric treatment causes solidification segregation relaxation or tempering. However, the inventors conducted numerous experiments and studies to obtain spot-welded joints with higher joint strength without post-current application when resistance spot welding is performed on plate assemblies including high-tensile steel, particularly high-strength steel plates with a Vickers hardness of 410 HV or higher. As a result, they found that reducing the Si and Al content in the high-strength steel plates facilitates the precipitation of fine carbides, and after forming a nugget by current application, applying low-temperature heat treatment under predetermined conditions suppresses solidification segregation and hardness reduction, thereby improving toughness and joint strength by precipitation of fine carbides.

[0015] Here, we will specifically explain the experimental results that led to this disclosure. Two steel plates, each 1.2 mm thick and with a Vickers hardness of 410 HV or higher, were stacked and spot-welded to create a joint, after which a cross tensile test was performed. For single-current applications with nugget diameters of 5mm or 6mm, two different types of connectors were fabricated. To eliminate the effect of hydrogen on joint strength (CTS), all joints were left standing for more than 7 days. The joint strength was then measured for joints that had undergone low-temperature heat treatment and those that had not, and the results are shown in Figure 1. Joint strength was measured for three samples of each type and the average value was used. As a result, the CTS of the joints that underwent low-temperature heat treatment improved significantly, and the cause of this improvement was investigated.

[0016] The following reasons were considered to be the cause of the improved CTS in joints that underwent low-temperature heat treatment. (I) The yield strength of the steel plate changes, and the stress state at the end of the nugget changes. (II) The toughness of the nuggets is improved.

[0017] (I) 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 specimen, and a tensile test was performed on the steel sheet in accordance with JIS Z2241:2011. The results are shown in Figure 2. As shown in Figure 2, the yield strength of the steel sheet improved with low-temperature heat treatment. As the yield strength improves, the stress at the nugget end increases, so the CTS tends to decrease. Therefore, the improvement in CTS shown in Figure 1 cannot be explained by the change in yield strength of the steel sheet due to low-temperature heat treatment.

[0018] Next, (II) the change in toughness of the nugget was investigated. The Vickers hardness test results are shown in Figure 3, and the SEM observation results are shown in Figure 4. In the SEM observation, the base material (steel plate) portion unaffected by the heat during spot welding and the nugget end were observed for both joints that had undergone low-temperature heat treatment and joints that had not undergone heat treatment. As shown in Figure 3, the Vickers hardness decreased by about 20 HV after low-temperature heat treatment, but no microstructural changes were observed in the SEM observation.

[0019] To perform further detailed microstructural observations, replica TEM observations were performed on the ends of each nugget of joints that had undergone low-temperature heat treatment and those that had not. The results are shown in Figure 5. For the replica TEM observation samples, the surface was electropolished to 100 nm, and then the precipitates were pressed onto a copper mesh for TEM observation. It was found that the low-temperature heat treatment resulted in the precipitation of fine carbides, as indicated by the arrows. Although fine carbides were also observed in areas other than those indicated by the arrows in both samples, the number density of fine carbides was higher in the joints that had undergone low-temperature heat treatment.

[0020] Figure 6 shows the hardness test results with the analysis noted. Although low-temperature heat treatment reduces the dissolved carbon that degrades toughness, it is thought that the hardness increases due to particle dispersion strengthening by precipitation of fine carbides. From these findings, it can be concluded that low-temperature heat treatment improved toughness and joint strength without significantly reducing hardness. The spot welded joints described herein are derived from the results of such analysis.

[0021] [Spot welded joints] The spot-welded joint relating 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 with a thickness of 1.0 mm to 2.3 mm, and its chemical composition, in mass%, satisfies the following conditions: 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 higher. Furthermore, in a cross-section in the thickness direction passing through the center of the nugget, if the portion of the nugget's melting boundary 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 the highest is defined as the nugget end, then in a 200 μm square end region near the nugget end within the nugget (which may be 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 That's all. The Vickers hardness is within ±50 HV of the hardness calculated using the following estimation formula HV. 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 nugget satisfies either or both of the following conditions: 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. Estimated formula HV=217+1080×(C+Si / 70+Mn / 113+Cr / 93+Mo / 30) In the formula, the element symbols represent the mass percentage of each element in the average chemical composition of the nugget; if an element is not present, 0 is substituted.

[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 former 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 has a Vickers hardness of 410 HV or higher.

[0024] The following description will primarily 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] <Board group> In the spot-welded joint relating 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, a steel sheet with a Vickers hardness of 410 HV or higher will have a tensile strength of 1280 MPa or higher. However, a high-strength steel sheet may be, for example, a steel sheet with a Vickers hardness of 410 HV or higher but a tensile strength of less than 1280 MPa. In this disclosure, a steel sheet with a Vickers hardness of 410 HV or higher is referred to as a "high-strength steel sheet." A high-strength steel sheet preferably has a Vickers hardness of 465 HV or higher. The Vickers hardness of the steel plate in this disclosure is measured at a position at least 10 mm away from the center of the nugget, in a region within ±1 / 4t (t: plate thickness) from the center of the plate in the thickness direction of the plate, under a load of 200 gf according to JIS Z 2244:2024. Furthermore, the tensile strength and Vickers hardness of steel plates 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)). HV(GPa) = 10² × HVp Equation A σ(GPa) = 0.32 × HVp(GPa) Equation B In equations A and B, σ represents tensile strength and HV represents Vickers hardness. For example, in the case of HV410, from equation A, HVp is 410 / 102 ≈ 4.02 GPa. Furthermore, from equation B, the tensile strength σ is calculated to be 0.32 × 4.02 = 1.28 GPa = 1280 MPa. Thus, from the above estimation equations A and B, it can be estimated that a high-strength steel sheet with a Vickers hardness of 410 HV or higher in this disclosure corresponds to a steel sheet with a tensile strength of 1280 MPa or higher.

[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 formation. However, if the carbon content is less than 0.08%, it becomes difficult to achieve a Vickers hardness of 410 HV or higher, and fine carbides are less likely to precipitate near the nugget edges during the low-temperature heat treatment described later. Also, if the carbon content exceeds 0.35%, the workability of high-strength steel sheets 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 higher, or 0.12% or higher.

[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 can be 0% (i.e., no Si is present), but it is difficult to achieve a Si content of less than 0.01% industrially. Therefore, the Si content should be between 0.01% and 1.20%, or even between 0.05% and 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. However, 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 to 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: 0.001~1.00% 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%, weldability may decrease. Furthermore, in cold-rolled sheet annealing, the concentration of carbon in austenite is suppressed, and after holding at the annealing temperature, 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 sheets. 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 through suppression of 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 content of Si and Al: Preferably 1.50% or less The total Si and Al content in high-strength steel sheets is 2.00% or less, but it is preferably 1.50% or less from the viewpoint of precipitation of carbides.

[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 described 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 welded 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. Furthermore, the microstructure of the high-strength steel sheet in this disclosure is not particularly limited, except that the volume fraction of residual γ is 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 Hazard-Area 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 / 4t (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 thickness of 1.0 mm or more and 2.3 mm or less. If the 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 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] While there are no particular limitations on the plate thickness other than high-strength steel plates, examples include thicknesses of 0.5 to 3.5 mm. The total thickness of the boards in the framing is not particularly limited, but examples include 1.5 to 8.0 mm.

[0043] -plating- The steel sheets constituting the plate assembly may have a plating layer formed on their surface. Examples of plating layers 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 its constituent components) 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 30mm x 30mm analytical sample is taken from the steel plate, the ZnO on the surface of the plating is removed using 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 in 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 that make up the plate assembly is not particularly limited and can be selected according to the application and required characteristics of the spot-welded joint being manufactured.

[0047] <Nuggets> 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 is 6.0√t min The following applies. Note that √t min is 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 edge region R1 near the end of a nugget, 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 2 That's all. 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 Vickers hardness 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 contained within 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 contained within 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 edge 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, with at least 3 random fields of view and a total field of view area of ​​300 μm.2 Measurements 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. Note that, 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 end 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 using the following estimation formula HV. Estimated 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 has suppressed hardness reduction 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. Furthermore, 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 using post-electricity or a heater, the nugget will break, resulting in no improvement in CTS, or even a decrease in CTS 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 equivalent to that of the estimated formula HV. The higher the Vickers hardness at the nugget end, the less likely plug fracture is to occur, and the higher the CTS can be achieved. From this viewpoint, it is preferable that the average Vickers hardness in the nugget end region R1 be within ±40HV, ±30HV, or ±20HV of the hardness HV calculated by the estimated formula HV.

[0053] The Vickers hardness in the nugget end region R1 is measured in the aforementioned 200 μm square end region R1. Ten Vickers hardness measurements are taken in the nugget end region R1 under a load of 300 gf, and the average value is taken as the average Vickers hardness. The Vickers hardness measurements are performed such that all indentations are at a distance of at least four indentation sizes from the nearest nearest indentation. For example, if the total thickness of the plate assembly is thin and a 200 μm square area R1 cannot be secured at the nugget edge region R1, the Vickers hardness should be measured at 10 points within a region of 2000 μm from the nugget edge to the inside of the nugget, and the average value should be taken as the average Vickers hardness.

[0054] (P-enriched and Mn-enriched areas in the nugget end region) When 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 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 (which may be 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 components of the nugget is 0.5% or more. (B) The area ratio of Mn-enriched areas (which may be 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 components of the nugget is 0.5% or more.

[0055] Here, the "average chemical composition of the nugget" is a weighted average obtained by multiplying the chemical composition of each steel plate 1A and 1B included in the plate assembly by the ratio of the thickness of each steel plate to the total 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 will be the chemical composition of the nugget. On the other hand, when multiple steel plates of the same thickness but with different chemical compositions are joined together by a 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, when multiple steel plates of different thicknesses and chemical compositions are joined by a nugget, the chemical composition of the nugget is considered to be the weighted average obtained by multiplying the chemical composition of each steel plate included in the plate assembly by the ratio of the thickness of each steel plate to the total thickness of the plate assembly. In any case, the weighted average of the chemical composition of each steel plate 1A and 1B, taking into account the thickness of each plate, 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 areas 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, the toughness of the nugget is considered to be improved by the precipitation of numerous fine carbides in the nugget end region R1.

[0057] While there is no particular upper limit to the area ratios of P-enriched and Mn-enriched areas in the nugget end region R1, excessively high area ratios of P-enriched and Mn-enriched areas in the nugget end region R1 can cause a decrease in CTS. From the viewpoint of suppressing a decrease in CTS, it is preferable that the area ratios of 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 EPMA (electron probe microanalyzer), allowing for the identification of the area percentage of P-enriched regions where the P content is 1.5 times or more the average P content, and the area percentage of Mn-enriched regions where the Mn content is 1.5 times or more the average Mn content, within a 200 μm square edge region R1.

[0059] (Application) 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 the spot welded joint according to this disclosure is not particularly limited, but the method for manufacturing the spot welded joint described below (sometimes referred to as "the method for manufacturing the spot welded joint according to this disclosure") can suitably manufacture the spot welded joint according to this disclosure. However, the spot welded joint according to this disclosure is not limited to a spot welded joint manufactured by the method for manufacturing the spot welded joint described below.

[0061] The method for manufacturing a spot-welded joint according to this disclosure comprises an energizing step of forming a nugget by applying current I1 (kA) to a plate assembly made by stacking multiple steel plates, including at least one high-strength steel plate having a thickness of 1.0 mm or more and a Vickers hardness of 410 HV or more, while clamping it in the thickness direction with a pair of electrodes and applying pressure, and After the energizing process, the nuggets undergo a low-temperature heat treatment process in which they are heated at 100-250°C for 5-30 minutes. This includes the following. Each step will be explained below.

[0062] <Electrification process> As part of the energizing process, a plate assembly consisting of two or more steel plates, including at least one high-strength steel plate, is sandwiched between a pair of electrodes in the thickness direction and subjected to pressure while an electric current of I1 (kA) is applied to form a nugget.

[0063] In the energizing process, it is preferable to set the current value I1 (kA) and energizing time t1 (ms) so that a nugget is formed by spot welding to join all the steel plates constituting the plate assembly. Figure 8 schematically shows an example of a nugget formed when an energizing process is performed on a plate assembly consisting of two stacked steel plates. As shown in Figure 8, electrodes 2A and 2B are pressed against the plate assembly, which consists of two stacked steel plates 1A and 1B, sandwiching 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 at the energized portion between steel plates 1A and 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 plate assembly, the thickness of the steel plate with the smallest thickness is t min If (mm), the nugget diameter is 3.8√t min The above is 6.0√t min It is preferable to perform spot welding in the following manner. The current value I1 is, for example, 5.0 to 8.0 kA, and the energizing time t1 is, for example, 120 to 600 ms. The current value can be constant, variable, or pulsed. When the current value is varied, such as in a pulsed manner, I1 refers to the maximum value. In the case of an upslope, the energizing time including the upslope is defined as t1, and in the case of pulsed energizing, the energizing time excluding the time without energizing is defined as t1. When pre-energizing is performed, I1 and t1 of the energizing process do not change significantly. If pre-energizing is difficult to distinguish from upslope energizing, the pre-energizing current I0 can be defined by dividing the area of ​​the pre-energizing current value I0 × energizing time t0 by t0. The pressure P1 applied by electrodes 2A and 2B to the plate assembly 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 undergo a low-temperature heat treatment by heating them at 100-250°C for 5-30 minutes. Low-temperature heat treatment can be performed using external heating means such as heating furnaces, hot plates, or salt bath furnaces. However, the method of forming nuggets by applying electricity and then applying electricity again (post-electricity) for tempering is not used because it reduces 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 three or more stacked steel plates, there are multiple plate interfaces. The number density of fine carbides is measured at the nugget end of 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, which is formed by spot welding together three overlapping 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 of the plate. However, 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 higher, 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. [Examples]

[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] Two stacked steel plates with the thickness, Vickers hardness, chemical composition, and microstructure shown in Table 1 were prepared. Nuggets were formed on each plate stack using the spot welding conditions (current I1, time t1, pressure P1) shown in Table 2. After leaving them undisturbed for at least 7 days to avoid hydrogen exposure, spot-welded joints were manufactured by heat treatment in a furnace. For samples No. 29 and 30, current was applied after solidification segregation relaxation following nugget formation. The conditions for current application after solidification segregation relaxation were a pause 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 initial current application value. Furthermore, after current application after solidification segregation relaxation, heat treatment was performed in a furnace under the conditions shown in Table 2. In each table, underlining indicates that the information is outside the scope of this disclosure or outside the scope of preferred manufacturing conditions.

[0072] [Table 1]

[0073] [Table 2]

[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 testing of resistance spot and projection welded joints". Furthermore, the CTS of spot welded joints 1-36 was subtracted from the CTS of the spot welded joint that did not undergo heat treatment to obtain ΔCTS. A ΔCTS of 1.5kN or higher was evaluated as a significant improvement in CTS. A ΔCTS of 2.0kN or higher was evaluated as a good improvement in CTS. ΔCTS = CTS of heat-treated joints - CTS of joints that were not heat-treated (only spot welding with single current applied) 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. Additionally, values ​​of ΔCTS less than 1.5kN are underlined.

[0076] [Table 3]

[0077] All of the spot-welded joints in the examples met the requirements of this disclosure, and their ΔCTS was 1.5kN or more compared to cases where heat treatment was omitted. On the other hand, in the comparative example, which was outside the scope of this disclosure, the ΔCTS values ​​other than No. 33 were less than 1.5 kN. This can be explained as follows. No. 1 has a high Si content, resulting in less carbide precipitation during low-temperature heat treatment. No. 2 has a low carbon content, which gives it inherently good toughness, and in addition, it produces less carbide precipitates. In No. 3, due to the low Mn content, a large amount of carbides had already precipitated due to self-tempering before low-temperature heat treatment, resulting in a small ΔCTS. Because No. 5 has a high Al content, it produces less carbide precipitation during low-temperature heat treatment. In No. 6, due to its thicker plate thickness, stress tends to concentrate at the nugget edges, and the amount of toughness improvement achieved by low-temperature heat treatment is insufficient. Also, because of its high Si content, carbide precipitation is minimal with low-temperature heat treatment. In No. 7, the high carbon content significantly reduces the toughness of the nugget, and the improvement in toughness achieved by 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. In sample No. 10, the high Mn content resulted in the formation of lens martensite, and therefore, low-temperature heat treatment did not improve its toughness. No. 11 has a high Al content, resulting in less carbide precipitation during low-temperature heat treatment. For No. 14, the nugget diameter is small and prone to interfacial fracture, making toughness improvement by low-temperature heat treatment insufficient. No. 17 has a high carbon content, resulting in low nugget toughness, and low-temperature heat treatment is insufficient for toughness improvement. Furthermore, its high silicon content leads to low carbide precipitation, resulting in minimal toughness improvement from low-temperature heat treatment. No. 18 has a high Al content, resulting in less carbide precipitation during low-temperature heat treatment. Because No. 20 has a thicker plate, stress tends to concentrate at the ends of the nugget, and the amount of toughness improvement achieved by low-temperature heat treatment is insufficient. No. 21 has a large nugget diameter, which causes plug breakage, so the low-temperature heat treatment does not improve CTS. No. 22 has a high Si content, resulting in less carbide precipitation during low-temperature heat treatment. No. 26 has a high content of Si and Al, resulting in less carbide precipitation during low-temperature heat treatment. No. 28 has a thin plate thickness, resulting in less stress concentration at the nugget edges and a high CTS even without low-temperature heat treatment. No. 29 exhibits high CTS even without low-temperature heat treatment because, after solidification segregation relaxation and subsequent energization, the area ratios of both the P-enriched and Mn-enriched regions are small. No. 30 exhibits high CTS even without low-temperature heat treatment because, after solidification segregation relaxation and subsequent energization, the area ratios of both P-enriched and Mn-enriched regions are small. No. 33 shows an improvement in CTS due to the high heat treatment temperature and tempering, but the decrease in Vickers hardness in the nugget edge region is significant. In No. 34, the low heat treatment temperature resulted in less carbide precipitation, which prevented improvement in toughness and thus did not improve the CTS (Coefficient of Saturation).

[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 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 was performed on spot-welded joints using the plate assembly of galvanized steel sheets described later, and the value was measured using the method described above.

[0079] [Table 4]

[0080] Two plated steel sheets, as shown in Table 5, were stacked to form a nugget on each plate assembly using the spot welding conditions (current value I1, time t1, pressure P1) shown in Table 5. After leaving the assembly undisturbed for at least 7 days to avoid hydrogen exposure, spot welded joints were manufactured by heat treatment in a heating furnace.

[0081] [Table 5]

[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 testing of resistance spot and projection welded joints". Furthermore, ΔCTS was defined as the value obtained by subtracting the CTS of the spot-welded joints without heat treatment from the CTS of the spot-welded joints that were not heat-treated. A ΔCTS of 1.5kN or higher was evaluated as a significant improvement in CTS. A ΔCTS of 2.0kN or higher was evaluated as a good improvement in CTS. ΔCTS = CTS of heat-treated joints - CTS of joints that were not heat-treated (only spot welding with single current applied) The results are shown in Table 6.

[0084] [Table 6]

[0085] All of the spot-welded joints in the examples met the requirements of this disclosure, and their ΔCTS was 1.5kN or more compared to cases where heat treatment was omitted. Note that No. 103 is an example using steel plates intended for hot stamping. By performing a heat treatment of the plated steel plate at 900°C 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 is 15 g / m². 2 Compared to No. 103, which is above 15g / m 2 Numbers 101 and 102, which were below the threshold, had larger ΔCTS values ​​and showed a greater improvement in CTS.

[0086] [Manufacturing of spot-welded joints using three different types of steel plates] Three stacked steel plates with the thickness, Vickers hardness, chemical composition, and microstructure shown in Table 7 were prepared. After forming nuggets under the spot welding conditions (current I1, time t1, pressure P1) shown in Table 8, the plates were left to stand for more than 7 days to avoid the effects of hydrogen, and then heat-treated in a heating furnace to produce spot-welded joints.

[0087] [Table 7]

[0088] [Table 8]

[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 cross-tensile strength (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 testing of resistance spot and projection welded joints." Furthermore, 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 the spot-welded joint, where no heat treatment was performed after spot welding. A ΔCTS of 1.5kN or higher was evaluated as a significant improvement in CTS. A ΔCTS of 2.0kN or higher was evaluated as a good improvement in CTS. ΔCTS = CTS of heat-treated joints - CTS of joints that were not heat-treated (only spot welding with single current applied) The results are shown in Table 9.

[0091] [Table 9]

[0092] The triple-layer spot-welded joint of No. 104 also met the requirements of this disclosure, and its ΔCTS was 1.5kN 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 described specifically and individually. [Explanation of Symbols]

[0094] 1A, 1B, 1C, 1D steel plate 2A, 2B electrode 10, 20, 30 spot welded joints 13 Nuggets 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 overlapping multiple steel plates and a nugget for joining the multiple steel plates, At least one of the aforementioned 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 percent: 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. In the aforementioned plate assembly, the thickness of the steel plate with the smallest thickness is t. ｍｉｎ In that case, the nugget is 3.8√t ｍｉｎ The above 6.0√t ｍｉｎ The nugget diameters are as follows: In a cross-section in the thickness direction passing through the center of the nugget, if the portion of the nugget's molten boundary 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 the 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 That's all. The Vickers hardness is within ±50 HV of the hardness calculated using the following estimation formula HV. A spot-welded joint that satisfies either or both of the following conditions: 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 than 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 than the Mn content of the average chemical component is 0.5% or more. Estimated formula HV=217+1080×(C+Si / 70+Mn / 113+Cr / 93+Mo / 30) In the formula, the element symbols represent the mass percentage content of each element in the average chemical composition 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 claim 1 or claim 2, wherein the Vickers hardness of the high-strength steel plate is 465 HV or more.

5. The spot welded joint according to claim 3, wherein the Vickers hardness of the high-strength steel plate is 465 HV or more.

6. 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 claim 1 or claim 2, which is less than [amount missing].

7. The spot welded joint according to claim 3, wherein the high-strength steel plate has a zinc-based plating layer, and the ZnO content in the plating layer is less than 15 g / m².

8. The spot welded joint according to claim 4, wherein the high-strength steel plate has a zinc-based plating layer, and the ZnO content in the plating layer is less than 15 g / m².

9. The spot welded joint according to claim 5, wherein the high-strength steel plate has a zinc-based plating layer, and the ZnO content in the plating layer is less than 15 g / m².

10. Automotive component comprising a spot-welded joint as described in claim 1.

11. An automotive component comprising the spot-welded joint described in Claim 6.

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