Spot welded joint and method for manufacturing spot welded joint

Abstract

To provide a spot welded joint in which a plate set including a high strength steel sheet having a tensile strength of 980 MPa or more is used and a joint strength is improved, and a method for manufacturing the same.SOLUTION: In a spot welded joint, a portion corresponding to the position of a plate interface 15 between two adjacent steel sheets 1A, 1B of a melting boundary of a nugget 13, which have highest total tensile strength, is made into a nugget end part 13E, an average grain diameter of old austenitic grains with an aspect ratio of 1.0 or more and 1.7 or less is 110 μm or less in vicinity of the nugget end part, and contents of P and Mn are low or solidification segregation of P and Mn is relaxed. After forming the nugget at a current value I1(kA), the nugget is cooled during 800≤tc1(ms) so that a temperature of the nugget end part becomes Ms point or less, and second electric conduction for electrically conducting at a current value I2(kA) satisfying 0.80≤I2 / I1<1.2 and during 200≤t2(ms) so that the nugget end part becomes A3 point or more and less than a re-melting temperature.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present disclosure relates to spot welded joints and methods for manufacturing spot welded joints. [Background technology]

[0002] Spot welding is primarily used in processes such as assembling car bodies and attaching parts. In recent years, the automotive industry has seen increasing demand for lighter car bodies to improve fuel efficiency and CO2 emissions, as well as higher rigidity to improve collision safety. To meet these demands, there is a growing need to use high-tensile steel (high-strength steel sheets) for car bodies, parts, etc.

[0003] However, when high-tensile steel is used in resistance spot welding, the joint strength (cross tensile strength: CTS) tends to decrease. Therefore, there is a demand for spot-welded joints that have a high CTS even when using high-tensile steel. In order to improve CTS when spot welding high-tensile steel, two post-current applications have been reported: one for tempering after forming a nugget with the main current, and one for alleviating solidification segregation.

[0004] For example, Patent Document 1 proposes increasing the joint strength by making the inside of the nugget equiaxed and creating a softened portion on the outside of the nugget to facilitate plug fracture in order to increase the resistance to crack propagation. Patent Document 2 discloses that the joint strength is improved when the high-angle grain boundaries in the nugget are smaller than 30 μm. Patent Document 3 discloses that titanium carbonitrides are precipitated in the nugget to refine the crystal grains, thereby improving the joint strength.

[0005] In Patent Document 4, at least one steel plate in the sheet assembly contains 0.08≦C≦0.3 (mass%), 0.1≦Si≦0.8 (mass%), 2.5≦Mn≦10.0 (mass%), and P≦0.1 (mass%), and a current value I w(kA) is applied, and then the post-tempering heat treatment process is performed with a cooling time of t ct After cooling in (ms), the current value I t (kA), and current flow time t t (ms) is disclosed. 800≦t ct , 0.5×I w ≦I t ≦I w , 500≦t t

[0006] In addition, in Patent Document 5, a sheet assembly in which the C content of at least one steel sheet is more than 0.30% and not more than 0.70% by mass is subjected to current flow at a current value I1 (kA) under the following conditions for a time t of 16 ms to 200 ms. c1 is de-energized, and current is applied at a current value of I2 (kA) for a time period of t2 (ms), and c2 This patent discloses a resistance spot welding method using three-stage current application in which current is not applied for a period of time t3 (ms) and current is applied at a current value of I3 (kA) for a period of time t4 (ms). 0.6≦I2 / I1≦1 1.50≦t2≦1000 3.5×10 -3 ×Ms 2 -3.3×Ms+1100 <t c2 ≦9000 Ms(℃)=561-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo] 0.4≦I3 / I1≦1.0, 200≦t3 [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-78782 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-187615 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-13572 [Patent Document 4] International Publication No. 2019 / 156073 [Patent Document 5] Japanese Patent Publication No. 2021-154390 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present disclosure is to provide a spot-welded joint having improved joint strength compared to a spot-welded joint formed by resistance spot welding a sheet assembly including high-strength steel sheets having a tensile strength of 980 MPa or more using only a single current. Another object of the present disclosure is to provide a method for manufacturing a spot-welded joint that can produce a spot-welded joint with improved joint strength compared to when a plate assembly including high-strength steel plates with a tensile strength of 980 MPa or more is resistance spot welded using only a single current. [Means for solving the problem]

[0009] The gist of the present disclosure to achieve the above object is as follows. <1> A spot welded joint including a plate assembly in which a plurality of steel plates including at least one steel plate having a tensile strength of 980 MPa or more are overlapped, and a nugget that joins the plurality of steel plates in the plate assembly, In a cross section in the plate thickness direction passing through the center of the nugget, a portion of the fusion boundary of the nugget that corresponds to the position of the plate interface between two adjacent steel plates where the sum of the tensile strengths is the highest is defined as the nugget end portion, When the weighted average obtained by multiplying the chemical composition of each steel plate included in the plate assembly by the plate thickness ratio of each steel plate to the total thickness of the plate assembly is regarded as the average chemical composition of the nugget, A resistance spot welded joint in which, in an observation area of ​​200 μm square in the vicinity of the end of the nugget, the average grain size of prior austenite grains having an aspect ratio of 1.0 to 1.7 is 110 μm or less, and which satisfies either (a) or (b) below. (A) The average P content of the average chemical composition of the nugget is less than 0.005% by mass and the average Mn content is less than 0.5% by mass. (b) The average chemical composition of the nugget satisfies at least one of an average P content of 0.005% by mass or more and an average Mn content of 0.5% by mass or more, and in the observation area, the area ratio of P-enriched parts where the P concentration is twice or more the average P content is 0.5% or less, and the area ratio of Mn-enriched parts where the Mn concentration is twice or more the average Mn content is 0.5% or less. <2> In the observation area near the nugget edge, the average grain size of the prior austenite grains having an aspect ratio of 1.0 or more and 1.7 or less is 90 μm or less. <1> 2. The resistance spot welded joint according to claim 1 . <3> The average Vickers hardness in a 1000 μm square measurement area near the end of the nugget is within ±20 HV of the hardness calculated by the following estimation formula HV. <1> or <2> 2. The resistance spot welded joint according to claim 1 . 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. <4> The steel plate having a tensile strength of 980 MPa or more has a C content of 0.30 mass% or more and 0.60 mass% or less, and a Ti content of less than 0.10 mass%. <1> ~ <3> 10. A resistance spot welded joint according to any one of claims 1 to 9. <5> a first current-passing step of forming a nugget by sandwiching a sheet set made of two or more overlapping steel sheets, including at least one steel sheet having a tensile strength of 980 MPa or more, between a pair of electrodes in the sheet thickness direction and applying pressure thereto while passing current at a current value I1 (kA); After the first current application step, when the part of the fusion boundary of the nugget that corresponds to the position of the sheet interface between the two adjacent steel sheets having the highest total tensile strength is set as the nugget edge, 800≦t c1 Time t c1 A cooling step of cooling for (ms); After the cooling process, a second current application process is performed in which current is applied at a current value I2 (kA) that satisfies 0.80≦I2 / I1<1.2 and for a time t2 (ms) that satisfies 200≦t2 so that the nugget edge is at or above the A3 point and below the remelting temperature. A method for manufacturing a spot welded joint, comprising: [Effects of the Invention]

[0010] According to the present disclosure, a spot-welded joint having improved joint strength is provided compared to a spot-welded joint obtained by resistance spot welding a sheet assembly including high-strength steel sheets having a tensile strength of 980 MPa or more using only a single current. Furthermore, the present disclosure provides a method for manufacturing a spot-welded joint that can produce a spot-welded joint with improved joint strength compared to when a plate assembly including high-strength steel plates with a tensile strength of 980 MPa or more is resistance spot-welded using only a single current. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing the relationship between the grain size and CTS in the vicinity of the nugget edge. [Figure 2A] 10 is an image showing the results of EPMA measurement of P in the vicinity of the nugget edge when only a single current is applied. [Figure 2B] 10 is an image showing the results of EPMA measurement of P in the vicinity of the edge of the nugget when post-energization is performed. [Figure 2C] 10 is an image showing the results of EPMA measurement of P in the vicinity of the edge of the nugget when post-energization is performed. [Figure 3] FIG. 10 is a diagram showing the relationship between post-energization and CTS. [Figure 4] FIG. 10 is a schematic image diagram showing the thermal history of the nugget edge when the cooling time before post-current application and the conditions of post-current application are changed. [Figure 5] FIG. 1 is a schematic diagram showing an example of a cross section in the plate thickness direction of a nugget formed by spot welding a plate assembly in which two overlapping steel plates are formed. [Figure 6] FIG. 6 is an enlarged schematic view showing the vicinity of the nugget edge shown in FIG. 5. [Figure 7] FIG. 1 is a schematic diagram showing an example of a cross section in the plate thickness direction of a nugget formed by spot welding a plate assembly in which three overlapping steel plates are formed. [Figure 8]1 is a diagram schematically showing an example of a nugget and a heat-affected zone (HAZ) formed when resistance spot welding is performed on a sheet assembly in which two steel sheets are overlapped. FIG. [Figure 9] FIG. 10 is a schematic diagram showing another example of a cross section in the plate thickness direction of a nugget formed by spot welding a plate assembly in which three steel plates, including one steel plate with a relatively thin thickness, are stacked. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment that is an example of the present disclosure will be described. In this disclosure, the "%" used to indicate the content of each element means "% by mass." In addition, in this disclosure, unless otherwise specified, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. In addition, when the numerical values ​​written before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit. In the numerical ranges described in stages in the present disclosure, the upper limit of a certain numerical range may be replaced with the upper limit of another numerical range described in stages or a value shown in an Example. Also, in the numerical ranges described in stages in the present disclosure, the lower limit of a certain numerical range may be replaced with the lower limit of another numerical range described in stages or a value shown in an Example. Furthermore, the term "process" does not only refer to an independent process, but also includes processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0013] Generally, the higher the tensile strength of the steel plate, the lower the toughness of the weld, resulting in a decrease in joint strength. When high-tensile steel is resistance spot welded, post-heat treatment is used to prevent a decrease in joint strength (cross tensile strength: CTS). This is because post-heat treatment causes tempering and alleviates solidification segregation. The present inventors conducted extensive experiments and studies to obtain spot-welded joints with higher joint strength, even when resistance spot welding is performed on sheet assemblies including high-tensile steel, particularly high-strength steel sheets of 980 MPa or higher. As a result, they found that if a spot-welded joint is produced by forming a nugget by main current application and then controlling the thermal history of the nugget edge by cooling and post-current application to specific conditions, solidification segregation at the nugget edge is alleviated and the prior austenite grain size in the nugget is reduced, thereby significantly improving joint strength. In other words, the spot-welded joint and method for manufacturing a spot-welded joint according to the present disclosure are technologies that improve joint strength by miniaturizing the prior austenite grain boundaries at the nugget end in a resistance spot-welded joint and suppressing the segregation of P and Mn.

[0014] Here, the experimental results that led to this disclosure will be described. The nugget of a spot-welded joint using high-tensile steel usually has low toughness due to the high strength and the presence of solidification segregation. For this reason, there is a method to improve nugget toughness by tempering and alleviating solidification segregation through post-heating. However, it was unclear how the prior austenite grain size affects nugget toughness. Therefore, the inventors of the present disclosure conducted the following experiment and found that joint strength can be significantly improved by refining the prior austenite grain boundaries while alleviating the solidification segregation of P and Mn near the nugget edge.

[0015] Two types of 980 MPa grade steel plates (20PS and 20F) were prepared with the chemical compositions (unit: mass%) shown in Table 1. Steel plate 20PS has a general chemical composition, while steel plate 20F has an extremely low content of P and S.

[0016] [Table 1]

[0017] Resistance spot welded joints were fabricated using two stacked steel plates of the same type (each 1.6 mm thick, total thickness: 2 mm) with a nugget diameter of 4√t. Furthermore, each resistance spot welded joint was heat treated to vary the prior austenite grain size. The heat treatment conditions were as follows: the austenitizing temperature was varied from 900°C to 1150°C, and the joint was held for 5 minutes, followed by water quenching.

[0018] The CTS of the spot-welded joints after heat treatment was measured, and the relationship between the grain size at the nugget edge and CTS for spot-welded joints using each steel plate is shown in Figure 1. In the spot-welded joint using steel plate 20PS (labeled "20PS"), the CTS increased as the grain size increased. This is because the grain size increases with increasing austenitizing temperature, and solidification segregation relaxation also occurs at the same time. Therefore, the effect of solidification segregation relaxation became greater than the effect of grain size. On the other hand, in the spot-welded joint using steel plate 20F (denoted as "20F"), the CTS improved as the grain size became smaller. This is thought to be because the smaller grain size improves the nugget toughness.

[0019] Next, we will explain the results of spot welding using a 0.2%C-1.2%Mn-0.02%P steel (1.5GPa-class hot-stamped steel sheet, thickness 2.0mm) sheet assembly with a main current (first current) and then a post-current (second current). The post-current conditions are shown in Table 2. The main current conditions were adjusted so that the nugget diameter was 4√t, and the cooling time (cooling time) after the main current and the post-current conditions were either "short-time post-current" or "post-current after reverse transformation solidification segregation relaxation" as shown in Table 2.

[0020] [Table 2]

[0021] The results of EPMA measurements of P near the nugget edge after single current and post-current application are shown in Figures 2A to 2C. In the single current application (Figure 2A), the area ratio of the P-enriched area, where the P concentration is more than double, was 0.6%, and solidification segregation occurred. On the other hand, by applying a short-time post-energization (Fig. 2B) or a post-energization after reverse transformation type solidification segregation relaxation (Fig. 2C), the area ratio of the P-enriched area was 0.3%, indicating that the solidification segregation was dispersed (relaxed).

[0022] Next, the CTS was measured for the spot-welded joints that had undergone each post-current treatment. The relationship between each post-current treatment and CTS is shown in Figure 3. As shown in Figure 3, the CTS was improved with post-current treatment after reverse transformation solidification segregation relaxation compared to short-time post-current treatment.

[0023] Furthermore, the austenite grain size near the edge of each nugget was measured, and it was found that in the case of short-time post-heating, most of the grains had an aspect ratio of more than 2 due to the initial solidification structure. On the other hand, in the case of reverse transformation type post-heating for solidification segregation relaxation, martensitic transformation occurred during the cooling time after the main heat treatment, and it is thought that the post-heating caused reverse transformation, resulting in refined grains and the creation of a structure with an aspect ratio close to 1.

[0024] Figure 4 shows the thermal history of the nugget edge when the post-heat conditions are changed. When post-heat is applied after spot welding a steel sheet assembly, a short post-heat is generally used to alleviate solidification segregation, and as shown by line c, the nugget is exposed to a high temperature above the A4 point to promote the diffusion of P and Mn, thereby improving joint strength. Furthermore, in the case of short-time current application incorporating reverse transformation, as shown by line b, the temperature drops below the A4 point due to cooling, and then exceeds the A4 point, whereby the crystal structure changes from fcc to bcc, resulting in grain refinement, and it is thought that both solidification segregation relaxation and grain refinement are achieved through cooling. However, when a high-strength steel sheet with a tensile strength of 980 MPa or more and a relatively high C content (e.g., C content: 0.17% or more) is used, not only are the grains refined once the A4 point is exceeded, but remelting also results in the inclusion of a liquid phase, making it difficult to achieve solidification segregation relaxation and eliminating the effect of grain refinement. On the other hand, in reverse transformation type solidification segregation relaxation post-current application, as shown by line a, the cooling time after the main current application to form the nugget is extended to cause martensitic transformation, and then the post-current application causes the temperature to exceed the A3 point, thereby refining the grains, and by raising the temperature to a point not exceeding the A4 point, remelting is prevented and segregation is relaxed, so that segregation relaxation and grain refinement are achieved simultaneously, and it is thought that CTS is further improved.

[0025] These analysis results showed that in spot-welded joints of plate assemblies containing high-strength steel plates with a tensile strength of 980 MPa or more, when the following conditions (I) and (II) are met near the nugget edge, which corresponds to the area that was previously the plate interface, the CTS is significantly improved compared to welded joints spot-welded with a single current. (I) Prior austenite grains having an aspect ratio of 1.0 or more and 1.7 or less have an average grain size of 110 μm or less. (II) Either (a) or (b) below is satisfied. (A) The average chemical composition of the nugget has an average P content of less than 0.005 mass % and an average Mn content of less than 0.5 mass %. (b) The average chemical composition of the nugget satisfies at least one of the following conditions: the P content is 0.005 mass% or more and the Mn content is 0.5 mass% or more, and in an observation area of ​​200 μm square near the edge of the nugget, the area ratio of P-enriched areas where the P concentration is twice or more the average P content is 0.5% or less, and the area ratio of Mn-enriched areas where the Mn concentration is twice or more the average Mn content is 0.5% or less.

[0026] In other words, in the spot-welded joint according to the present disclosure, the P and Mn contents of the base material are low to begin with, so that solidification segregation does not occur near the nugget edge, or the base material has a relatively high content of either or both of P and Mn, but solidification segregation is mitigated. By refining the prior austenite grain size, it is thought that the toughness inside the nugget is improved and the joint strength is increased.

[0027] [Spot welded joints] The resistance spot welded joint according to the present disclosure is a spot welded joint including a sheet assembly in which multiple steel sheets, including at least one steel sheet having a tensile strength of 980 MPa or more, are stacked, and a nugget that joins the multiple steel sheets in the sheet assembly, wherein, in a cross section in the sheet thickness direction passing through the center of the nugget, the nugget edge is defined as a portion of the fusion boundary of the nugget that corresponds to the position of the sheet interface between two adjacent steel sheets having the highest total tensile strength, and the weighted average obtained by multiplying the chemical compositions of each steel sheet in the sheet assembly by the thickness ratio of each steel sheet to the total thickness of the sheet assembly is defined as the average chemical composition of the nugget, in an observation region of 200 μm square within the nugget near the nugget edge, the average grain size of prior austenite grains with an aspect ratio of 1.0 to 1.7 is 110 μm or less, and either one of the following (A) or (B) is satisfied: (A) The average P content of the average chemical composition of the nugget is less than 0.005% by mass and the average Mn content is less than 0.5% by mass. (B) The average chemical composition of the nugget satisfies at least one of an average P content of 0.005% by mass or more and an average Mn content of 0.5% by mass or more, and in the observation area, the area ratio of P-enriched parts where the P concentration is twice or more of the average P content is 0.5% or less, and the area ratio of Mn-enriched parts where the Mn concentration is twice or more of the average Mn content is 0.5% or less.

[0028] <Board group> The sheet assembly of the spot welded joint according to the present disclosure is a sheet assembly in which two or more steel sheets are overlapped, including at least one steel sheet with a tensile strength (TS) of 980 MPa or more. By including a steel sheet with a TS of 980 MPa or more, high tensile strength can be ensured. The number of steel sheets constituting the sheet assembly may be two, or three or more.

[0029] 5 is a schematic diagram showing an example of a cross section in the plate thickness direction passing through the center of a nugget 13 formed by spot welding two overlapping steel plates 1A and 1B. The two steel plates 1A and 1B are joined to form an elliptical nugget 13 whose major axis is the part that was the plate interface.

[0030] The sheet combination of spot welded joint 10 according to the present disclosure may have a tensile strength of 980 MPa or more for all steel sheets 1A, 1B, or may include at least one steel sheet having a tensile strength of 980 MPa or more and a steel sheet having a tensile strength of less than 980 MPa. When all steel sheets have a tensile strength of 980 MPa or more, they may be the same type of steel sheets having the same tensile strength, or different types of steel sheets having different tensile strengths.

[0031] In the spot welded joint 10 according to the present disclosure, as long as at least one of the steel plates in the plate set made up of two or more overlapping steel plates has a tensile strength of 980 MPa or more, the chemical composition and metal structure of each steel plate 1A, 1B are not limited, and for example, desired elements may be selected. In order to increase the strength, a steel sheet having a tensile strength of 980 MPa or more preferably has a C content of 0.30 mass % or more and 0.60 mass % or less, and a Ti content of less than 0.10 mass %. Generally, the higher the C content, the more difficult it is to increase joint strength. That is, according to the present disclosure, it is possible to improve joint strength even in high-strength steel sheets with a high C content, in which the effect of alleviating solidification segregation is difficult to obtain by short-time post-energization as described above. Although there is a technique for improving CTS by using a steel sheet containing Ti to precipitate TiN and refine the grains, the spot-welded joint according to the present disclosure can achieve a high CTS even when the Ti content is low. Note that the grain refinement effect according to the present disclosure is exerted independently of the steel sheet composition, and therefore the other steel sheet compositions are not particularly limited.

[0032] The thickness of each of the steel plates 1A, 1B constituting the plate assembly is not particularly limited, but may be, for example, 0.5 to 3.5 mm. The total thickness of the plate assembly is not particularly limited, but may be, for example, 1.5 to 8.0 mm. The following mainly describes a spot welded joint formed by spot welding two steel plates 1A, 1B having a tensile strength of 980 MPa or more as shown in FIG.

[0033] <Nugget> The nugget 13 is a weld metal formed by melting and solidifying at the spot-welded positions of the multiple steel plates included in the sheet assembly to join all of the steel plates. Fig. 7 is a schematic diagram showing an example of a cross section in the sheet thickness direction passing through the center of the nugget 13 formed by spot-welding a sheet assembly in which three overlapping steel plates 1A, 1B, and 1C are stacked. In the spot-welded joint 20 shown in Fig. 7, the three steel plates 1A, 1B, and 1C are joined by the nugget 13 having an oval shape. When viewed in cross section in the thickness direction, the shape of the nugget 13 is usually approximately elliptical, with the short side in the thickness direction and the long side in the in-plane direction of the plate, as shown in Figures 5 and 7, but is not limited to this shape.

[0034] (The average grain size of prior austenite grains with an aspect ratio of 1.0 to 1.7 is 110 μm or less) In the spot welded joint 10 according to the present disclosure, the prior austenite grains near the nugget end have an aspect ratio of 1.0 or more and 1.7 or less, and the average grain size is 110 μm or less. In a spot-welded joint 10 according to the present disclosure, the prior austenite grains near the nugget edge have an aspect ratio of 1.0 or more and 1.7 or less, i.e., each grain has a relatively small aspect ratio, and the prior austenite grains are refined to have an average grain size of 110 μm or less. The prior austenite grains near the nugget edge having the above-described shape and size are resistant to forces in the direction that would peel the joined steel sheets, improving joint strength. From this viewpoint, the average grain size of prior austenite grains having an aspect ratio of 1.0 to 1.7 in the vicinity of the nugget edge is preferably 100 μm or less, more preferably 90 μm or less, and even more preferably 80 μm or less. The lower limit of the average grain size of prior austenite grains having an aspect ratio of 1.0 to 1.7 near the nugget edge is not particularly limited, but may be, for example, 1 μm or more, or 10 μm or more.

[0035] The aspect ratios of the prior austenite grains within the observation region are often approximately the same, and there are few cases where the aspect ratios of the prior austenite grains vary significantly within the observation region. If the aspect ratios of the prior austenite grains within the observation region vary, prior austenite grains near the nugget edge may contain crystal grains with aspect ratios exceeding 1.7. However, to ensure high strength near the nugget edge, it is preferable that 50% or more of the prior austenite grains have an aspect ratio of 1.0 to 1.7. This is more preferable, and even more preferable, that it is 60% or more, and even more preferable, that it is 70% or more. From the viewpoint of improving CTS, the average aspect ratio of prior austenite grains in the vicinity of the nugget edge is preferably 1.0 or more and 1.7 or less, more preferably 1.5 or less, and even more preferably 1.3 or less.

[0036] (Method for measuring aspect ratio and average grain size of prior austenite grains) In the present disclosure, the aspect ratio of the prior austenite grains in the vicinity of the nugget edge is specified as follows. In an image showing the prior austenite grain boundary near the nugget edge, the shape of each prior austenite grain is approximated as an ellipse using the least squares method. The ellipse approximation method involves calculating the minor axis of an ellipse having the major axis using the major axis and area of ​​each austenite grain. The aspect ratio of the prior austenite grain is calculated by dividing the major axis dimension by the minor axis dimension of this ellipse. Specifically, the nugget is cut in the thickness direction passing through the center of the plate, and the cut surface is etched with sodium dodecylbenzenesulfonate. The aspect ratio of the prior austenite grain in the fusion boundary region at the nugget edge is measured using an optical microscope in an observation area R1 of 200 μm square. 6, the observation area R1 of the prior austenite grains near the nugget edge is the area closest to the nugget edge 13E, which corresponds to the position of the sheet-to-sheet interface 15 between the steel sheets 1A and 1B on the fusion boundary (outline) of the nugget 13, and is 200 μm square with one side in the sheet thickness direction and symmetrical with respect to the sheet-to-sheet interface 15. The same applies to the observation areas for the P concentration and Mn concentration near the nugget edge, which will be described later.

[0037] The average grain size of prior austenite grains with an aspect ratio of 1.0 to 1.7 is calculated by taking the diameter (equivalent circle diameter) of a circle having the same area as an approximate ellipse for each prior austenite grain.

[0038] When determining the average aspect ratio of the prior austenite grains, the aspect ratio of each prior austenite grain in the observation region R1 is measured, and the average value of these is taken as the average aspect ratio. The aspect ratio of the prior austenite grain boundary can be measured in the observation region R1 near one of the nugget edges.

[0039] In the spot-welded joint according to the present disclosure, the P content and Mn content in the vicinity of the nugget edge satisfy either (a) or (b) below.

[0040] (A) The average P content of the average chemical composition of the nugget is less than 0.005% by mass and the average Mn content is less than 0.5% by mass. Here, the "average chemical composition of the nugget" is a weighted average obtained by multiplying the chemical composition of each steel plate 1A, 1B included in the sheet assembly by the thickness ratio of each steel plate to the total thickness of the sheet assembly. Since the nugget 13 is formed by melting and solidifying all the steel plates included in the sheet assembly, it depends on the chemical composition of each steel plate 1A, 1B. For example, if the sheet assembly is composed of steel plates all having the same chemical composition, the chemical composition of those steel plates will be the chemical composition of the nugget. On the other hand, when multiple steel plates with the same thickness but different chemical compositions are joined by a nugget, the chemical composition of the nugget is the sum of the chemical compositions divided by the number of steel plates. In addition, when multiple steel plates with 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 plate thickness of each steel plate to the total thickness of the plate assembly. In any case, the weighted average of the chemical compositions of the steel plates 1A and 1B taking into account their thicknesses is considered to be the chemical composition of the nugget.

[0041] In this way, when the average chemical composition of the nugget calculated based on the chemical composition of each plate thickness and the plate thickness is such that the average P content is less than 0.005 mass% and the average Mn content is less than 0.5 mass%, the P and Mn contents in the entire base steel plate (entire plate assembly) are low, and even when spot welding is performed, there is almost no P segregation or Mn segregation at the nugget edge that would cause a decrease in CTS. Therefore, by satisfying the above requirement (A) that the average grain size of prior austenite grains with an aspect ratio of 1.0 to 1.7 is 110 μm or less, CTS is improved. In this case, the average P content of the average chemical composition of the nugget is preferably 0.003 mass % or less, and more preferably 0.001 mass % or less. On the other hand, the average Mn content of the average chemical composition of the nugget is preferably 0.4 mass % or less, and more preferably 0.3 mass % or less.

[0042] (b) The average chemical composition of the nugget satisfies at least one of the following: the average P content is 0.005% by mass or more and the average Mn content is 0.5% by mass or more, and in an observation area of ​​200 μm square near the edge of the nugget, the area ratio of P-enriched areas where the P concentration is twice or more the average P content is 0.5% or less, and the area ratio of Mn-enriched areas where the Mn concentration is twice or more the average Mn content is 0.5% or less.

[0043] When the average chemical composition of a nugget satisfies at least one of the following conditions: an average P content of 0.005% by mass or more and an average Mn content of 0.5% by mass or more, CTS decreases due to the segregation of P and Mn near the nugget edge, even if the average grain size of prior austenite grains with an aspect ratio of 1.0 to 1.7 is 110 μm or less. However, if the area ratio of P-enriched areas where the P concentration is twice or more the average P content is 0.5% or less and the area ratio of Mn-enriched areas where the Mn concentration is twice or more the average Mn content is 0.5% or less in the observation region near the nugget edge, segregation is mitigated, and a high CTS is possible.

[0044] The P and Mn concentrations near the nugget edge can be measured using an EPMA (electron probe microanalyzer), and the area ratio of P-enriched areas that are more than twice the average P content and the area ratio of Mn-enriched areas that are more than twice the average Mn content can be determined in an observation area R1 of 200 μm square. From the viewpoint of improving CTS, the area ratio of each thickened portion is preferably 0.3% or less, and more preferably 0.1% or less.

[0045] It is preferable that the average Vickers hardness measured in a 1000 μm square region R2 near the edge of the nugget is within ±20 HV of the hardness HV calculated by 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 the weighted average.

[0046] The spot-welded joint 10 according to the present disclosure does not require tempering, so carbides are not formed, and the average Vickers hardness near the nugget edge is within ±20 HV of the Vickers hardness calculated from the HV estimation formula. Note that due to errors in the above estimation formula and grain refinement, the average Vickers hardness near the nugget edge may be greater than the Vickers hardness calculated from the HV estimation formula. Note that if the nugget is tempered after formation, it will break, and the CTS may not improve, or may even be lower than before tempering. On the other hand, if tempering is not performed after nugget formation, the nugget will not break, and the Vickers hardness at the nugget edge will be equal to the estimated HV. The higher the Vickers hardness at the nugget edge, the less likely plug fracture will occur, and a high CTS can be achieved.

[0047] The Vickers hardness measurement near the nugget edge is performed in a 1000 μm square region R2 inside the nugget 13, closest to the nugget edge 13E, with one side in the sheet thickness direction and symmetrical with respect to the sheet interface 15. In the measurement region R2 near the nugget edge, 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. Note that in the measurement, all indentations are assumed to be at a distance equivalent to at least four indentation sizes from the nearest indentation. If the plate thickness is small and it is not possible to secure a 1000 μm square region R2 near the nugget edge, the Vickers hardness is measured at 10 points in a region within 2000 μm from the nugget edge, and the average value is taken as the average Vickers hardness.

[0048] The use of the spot welded joint according to the present disclosure is not particularly limited, but it can be particularly suitably used as, for example, an automobile body part.

[0049] [Method for manufacturing spot welded joints] Although the method for manufacturing the spot-welded joint according to the present disclosure described above is not particularly limited, the spot-welded joint according to the present disclosure can be suitably manufactured by the method for manufacturing a spot-welded joint described below. However, the spot-welded joint according to the present disclosure is not limited to spot-welded joints manufactured by the method for manufacturing a spot-welded joint described below (hereinafter referred to as the "method for manufacturing a spot-welded joint according to the present disclosure").

[0050] The method for manufacturing a spot welded joint according to the present disclosure includes a first current application step of forming a nugget by applying a current I1 (kA) to a sheet assembly formed by overlapping two or more steel sheets, including at least one steel sheet having a tensile strength of 980 MPa or more, while sandwiching the sheet assembly between a pair of electrodes in the sheet thickness direction and applying pressure thereto; After the first current application step, when the part of the fusion boundary of the nugget that corresponds to the position of the sheet interface between the two adjacent steel sheets having the highest total tensile strength is set as the nugget edge, 800≦t c1 Time t c1 A cooling step of cooling for (ms); After the cooling process, a second current application process is performed in which current is applied at a current value I2 (kA) that satisfies 0.80≦I2 / I1<1.2 and for a time t2 (ms) that satisfies 200≦t2 so that the nugget edge is at or above the A3 point and below the remelting temperature. Includes: Each step will be described below.

[0051] <First energization process> First, in the first current application process, a sheet assembly consisting of two or more overlapping steel sheets, including at least one steel sheet with a tensile strength of 980 MPa or more, is sandwiched between a pair of electrodes in the thickness direction and pressure is applied while a current of I1 (kA) is applied to form a nugget.

[0052] In the first current application step, it is preferable to set the current value I1 (kA) and the current application time t1 (ms) so that a nugget that joins all of the steel plates that make up the sheet assembly is formed by spot welding. Fig. 8 shows a schematic diagram of an example of a nugget formed when the first current application step is performed on a sheet assembly in which two steel sheets are stacked. As shown in Fig. 8, electrodes 2A and 2B are pressed against each other so as to sandwich the sheet assembly in which steel sheets 1A and 1B are stacked in the thickness direction, and current is applied between electrodes 2A and 2B. As a result, a nugget 13 and a heat-affected zone (so-called HAZ) 14 are formed at the current-applied portion of steel sheet 1A and steel sheet 1B, and the two steel sheets are spot-welded.

[0053] In the first current application process, there are no restrictions on the welding conditions as long as the desired nugget diameter is formed. If half the total plate thickness is t (mm), the nugget diameter is preferably 4√t or more, and more preferably 5√t or more. The current value I1 is, for example, 5.0 to 7.0 kA, and the current application time t1 is, for example, 120 to 600 ms. The current value may 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 energization time including the upslope is defined as t1, and in the case of pulsed energization, the energization time excluding the non-energized time is defined as t1. When pre-energization is performed, I1 and t1 do not change significantly. If it is difficult to distinguish from upslope, it can be defined as the area of ​​I × t divided by t. The pressure applied by the electrodes 2A and 2B to the plate assembly may be constant, variable, or pulsed, and the pressure is, for example, 3.0 to 5.0 kN.

[0054] <Cooling process> After the first current application process, if the part of the nugget fusion boundary (nugget boundary) that corresponds to the position of the interface between the two adjacent steel sheets with the highest total tensile strength is taken as the nugget edge, the temperature of the nugget edge is set to 800≦t c1 Time t c1 Cool for (ms).

[0055] During the cooling process, at least the edges of the nugget must be transformed into martensitic metal. Since the temperature gradient within the nugget is not large, if martensitic transformation occurs at the edges of the nugget, it is likely that martensitic transformation will also occur in the center of the nugget. To cause martensitic transformation at least at the edges of the nugget, the edges of the nugget must be cooled below the Ms point. The Ms point can be calculated from the sheet assembly. Ms point=550-361×(%C)-39×(%Mn)-35×(%V)-20×(%Cr)-17×(%Ni)-10×(%Cu)-5×(%Mo+%W)+15×(%Co)+30(%Al) The formula for calculating the Ms point is the Ms point calculated by substituting the mass percentage (% element symbol) of each element contained in the steel plates constituting the sheet assembly. However, if at least one of the steel plates constituting the sheet assembly has a different composition from the other steel plates, the weighted average obtained by multiplying the chemical composition of each steel plate in the sheet assembly by the thickness ratio of each steel plate to the total thickness of the sheet assembly is regarded as the average chemical composition of the nugget, and the Ms point is calculated from the above formula. In addition, among the elements in the above formula, for elements that are not contained in the steel sheet, zero is substituted into the corresponding (% element symbol).

[0056] As a means for cooling the nugget edge to the Ms point or lower, for example, the following three means can be mentioned. (1) Pressurize without applying current (2) Low current (3) Open the electrode The cooling may be performed by any one of the above (1) to (3) alone or in combination. c1 must be 800ms or more. cooling time t c1 If the time is less than 800 ms, the nugget edge may not be transformed into martensite before the second current application step. cooling time t c1 However, the upper limit of the cooling time t c1 The longer the cooling time t c1 is preferably 2000 ms or less.

[0057] The temperature at the nugget edge can be determined by a simulation method. By using commercially available software such as SORPAS (SCSK Corporation), it is possible to calculate the temperature history at the nugget edge.

[0058] Whether martensitic transformation has occurred in the cooling process can be confirmed by observing the recrystallized structure (grains with an aspect ratio of 1.0 to 1.7) of the joint after post-energization. The martensite structure can be observed after etching with sodium dodecylbenzenesulfonate.

[0059] <Second energization process> After the cooling process, current is applied at a current value I2 (kA) that satisfies 0.80≦I2 / I1<1.2 and for a time t2 (ms) that satisfies 200≦t2 so that the nugget edge is at or above the A3 point and below the remelting temperature. In the second current application step, at least the nugget edge is heated to point A1 or higher, preferably point A3 or higher. The temperature of the nugget edge is as described above. Points A1 and A3 can be calculated from the sheet combination. In this case, in a sheet pair that has an A4 point, it may be heated to above the A4 point. In a sheet pair that does not have an A4 point temperature, that is, in a sheet pair that remelts without undergoing δ transformation, the temperature of the nugget edge must not exceed the remelting temperature.

[0060] The fact that segregation relaxation (and grain refinement) occurs in the second current application, rather than tempering or remelting, can be confirmed by hardness testing, SEM observation, and EPMA observation of the area near the nugget edge. The upper limit of the second current application time t2 is preferably 2500 ms or less to avoid remelting up to the nugget edge.

[0061] For points A1, A3, and A4, a phase diagram is created using commercially available comprehensive thermodynamic calculation software, such as Thermo-calc (Thermo-Calc Software AB). In this case, the composition is calculated as a thickness-weighted average of the steel plate, and elements not in the database are not taken into consideration. The temperature at which the austenite single phase is reached is defined as point A3, and the temperature at which the δ-ferrite single phase is reached is defined as point A4. Note that if a liquid phase also appears when δ-ferrite precipitates, this disclosure considers that point A4 does not exist.

[0062] The second current flow may have any current flow pattern, preferably an upslope or downslope to extend the high temperature holding time and thereby extend the austenite precipitation time.

[0063] Although an example of an embodiment of the spot welded joint and the manufacturing method thereof according to the present disclosure has been described above, the spot welded joint and the manufacturing method of the spot welded joint according to the present disclosure are not limited to the above embodiment. For example, after the second current application, the electrodes are temporarily separated from the plate assembly, or the electrodes are not separated from the plate assembly, and no current is applied for a time t c2 After the elapse of time, a third current flow may be performed which does not re-melt the nugget edge.

[0064] Furthermore, the nugget may have a shape such that, for example, one steel plate 1D located on the outer side of three steel plates has a thickness thinner than the other two steel plates 1A and 1B, and two nuggets 13A and 13B formed between the two adjacent steel plates are joined together, as shown in Fig. 9. In such a spot-welded joint 30, for example, if the steel plates 1A and 1B have a resistance of 980 MPa or more and the steel plate 1D has a resistance of less than 980 MPa, the aspect ratio, P concentration, Mn concentration, Vickers hardness, etc. of the prior austenite grains may be measured in the vicinity of the nugget edge of portion 13B where the steel plates 1A and 1B are joined, as in the spot-welded joint 10 shown in Fig. 5. [Example]

[0065] Hereinafter, examples of the spot welded joint and the method for manufacturing the spot welded joint according to the present disclosure will be described. Note that the spot welded joint and the method for manufacturing the spot welded joint according to the present disclosure are not limited to the following examples.

[0066] Various plate assemblies were prepared by combining steel plates 1 to 6 having the tensile strengths, chemical compositions, etc. shown in Table 3 as shown in Table 4, and spot welding was performed on each plate assembly to produce various spot-welded joints. The CTS, etc. of the produced spot-welded joints were evaluated. Table 3 lists the steel plate thickness, tensile strength, chemical composition (C, Si, Mn, P, S, Ti, N, Cr) content (mass %, balance is Fe and impurities), temperature (°C) of each transformation point (Ms, A3, A4), and Vickers hardness near the edge of the nugget where two steel plates were spot-welded together. In addition, "Base material P" in Table 4 is a weighted average of the P content of each steel plate in the sheet assembly multiplied by the thickness ratio of each steel plate to the total thickness of the sheet assembly, and is considered to be the average P content in the nugget. The same applies to "Base material Mn."

[0067] [Table 3]

[0068] [Table 4]

[0069] Table 4 lists the types of steel sheets that make up the sheet combination, the P content, Mn content of the sheet combination, and the spot welding conditions (current value I, time t, pressure P). Note that the nugget edge temperature after cooling is omitted, but the temperature during the resting process (cooling process) t c1 At 120 ms, the nugget edge does not fall below the Ms point and does not transform into martensite. c1 It was separately confirmed by SORPAS analysis that when the current ratio (I2 / I1) was 800 ms or 1600 ms, the nugget edge fell below the Ms point and martensitic transformation occurred. Also, although the nugget edge temperature during post-heat application was not shown, it was separately confirmed by SORPAS analysis that in the inventive examples, when the current ratio (I2 / I1) was 0.8 or 0.9, the maximum temperature at the nugget edge was above the A3 point but below the remelting temperature, and when the current ratio (I2 / I1) was 1.2, the maximum temperature at the nugget edge was above the remelting temperature.

[0070] Table 5 shows the area ratio of the P and Mn enriched areas near the nugget edge, the average aspect ratio (average aspect ratio of prior γ grains) and average grain size (average grain size of prior γ grains) of the prior austenite grains at the nugget edge, the Vickers hardness at the nugget edge, the presence or absence of carbides, the joint strength (CTS), and the CTS improvement rate (CTS ratio).

[0071] [Table 5]

[0072] The * in the table indicates the average aspect ratio and average grain size calculated from the prior γ grain size at all observation positions. The post-energization in the remarks column means that the temperature of the nugget edge will have the following thermal history. Normal short-time post-energization: Pattern of line c in Figure 4 Reverse transformation type solidification segregation relaxation post-current application: a line pattern in Figure 4

[0073] The method for measuring the aspect ratio and grain size of the prior austenite grains near the nugget edge, the area ratio of the P and Mn enriched areas, and the average Vickers hardness HV were as described above.

[0074] The CTS of spot welded joints 1 to 25 was measured in accordance with JIS Z 3137:1999 "Test piece dimensions and test method for cross tension test of resistance spot and projection welded joints." Furthermore, the CTS ratio was calculated by dividing the CTS of the spot-welded joints that underwent post-current application among spot-welded joints 1 to 25 by the CTS of the corresponding spot-welded joints that underwent only single current application. A CTS ratio of 1.50 or higher was evaluated as a significant improvement in CTS. CTS ratio = CTS of joints with post-energization / CTS of joints with post-energization omitted (single-energization only)

[0075] In all of the inventive examples, the main current, cooling, and post-current were performed under conditions within the range of the present disclosure, and in all cases the CTS ratio was 1.50 or more, i.e., the increase rate exceeded 50%, compared to when post-current was omitted. Furthermore, the results of Vickers hardness measurements show that no carbides precipitated in any of the inventive examples. On the other hand, in the comparative examples, one of the conditions was outside the range of the present disclosure, and the CTS ratio was less than 1.50. In addition, in Level 19, the current flow conditions are within the range of the present disclosure, but the plate assembly does not include steel plates with a tensile strength of 980 MPa or more, and is made up of two steel plates 5 with a tensile strength of 440 MPa stacked together.Since the CTS is high even with single-current spot welding, no improvement in CTS is achieved. [Explanation of symbols]

[0076] 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

Claims

1. A spot welded joint including a plate assembly in which a plurality of steel plates including at least one steel plate having a tensile strength of 980 MPa or more are overlapped, and a nugget that joins the plurality of steel plates in the plate assembly, In a cross section in the plate thickness direction passing through the center of the nugget, a portion of the fusion boundary of the nugget that corresponds to the position of the plate interface between two adjacent steel plates where the sum of the tensile strengths is the highest is defined as the nugget end portion, When the weighted average obtained by multiplying the chemical composition of each steel plate included in the plate assembly by the plate thickness ratio of each steel plate to the total thickness of the plate assembly is regarded as the average chemical composition of the nugget, A resistance spot welded joint, wherein, in an observation region of 200 μm square within the nugget near the end of the nugget, the average grain size of prior austenite grains having an aspect ratio of 1.0 to 1.7 is 110 μm or less, and wherein either (a) or (b) below is satisfied: (A) The average P content of the average chemical composition of the nugget is less than 0.005 mass % and the average Mn content is less than 0.5 mass % (B) The average chemical composition of the nugget satisfies at least one of an average P content of 0.005% by mass or more and an average Mn content of 0.5% by mass or more, and in the observation area, the area ratio of P-enriched parts where the P concentration is twice or more the average P content is 0.5% or less, and the area ratio of Mn-enriched parts where the Mn concentration is twice or more the average Mn content is 0.5% or less.

2. 2. The resistance spot welded joint according to claim 1, wherein in an observation region near the nugget end, the prior austenite grains having an aspect ratio of 1.0 to 1.7 have an average grain size of 90 μm or less.

3. 3. The resistance spot welded joint according to claim 1, wherein an average Vickers hardness in a 1000 μm square measurement area in the nugget near the end of the nugget is within ±20 HV of a hardness calculated by 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.

4. The resistance spot welded joint according to any one of claims 1 to 3, wherein the steel plate having a tensile strength of 980 MPa or more has a C content of 0.30 mass% or more and 0.60 mass% or less and a Ti content of less than 0.10 mass%.

5. A plate assembly in which two or more steel plates including at least one steel plate having a tensile strength of 980 MPa or more are overlapped is sandwiched between a pair of electrodes in the plate thickness direction and pressurized while applying a current value I １ a first current application step of applying current at (kA) to form a nugget; After the first current application step, when a portion of the fusion boundary of the nugget corresponding to the position of the sheet interface between the two adjacent steel sheets having the highest total tensile strength is defined as the nugget end, 800≦t ｃ１ The time t ｃ１ a cooling step of cooling for (ms); After the cooling process, the nugget end is A ３ 0.80≦I so that the temperature is equal to or higher than the remelting temperature and lower than the remelting temperature. ２ / I １ <1.

2. ２ (kA) and 200≦t ２ The time t ２ a second energizing step of energizing the electrode for (ms); A method for manufacturing a spot welded joint, comprising:

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