JP7737015B2 - Spot welded joint and method for manufacturing spot welded joint - Google Patents

Description

This disclosure relates to spot welded joints and methods for manufacturing spot welded joints.

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 achieve better fuel economy and reduced _CO2 emissions, as well as for higher car body 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.

However, when high-tensile steel is used in resistance spot welding, the joint strength (cross tensile strength: CTS) is likely 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 the CTS when spot welding high-tensile steel, two post-energization methods have been reported: one for tempering after forming a nugget by main energization, and the other for alleviating solidification segregation.

For example, Patent Document 1 proposes increasing the joint strength by making the inside of the nugget equiaxed and by 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.

Patent Document 4 discloses a three-stage current spot welding method in which at least one steel plate in a 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 in which a main current flow process is performed at a current value I _w (kA) under the following conditions, and a post-tempering heat treatment process is performed by cooling for a cooling time t _ct (ms), and then applying current at a current value I _t (kA) for a current flow time t _t (ms).
800≦ _tct , 0.5× _Iw ≦ _It ≦ _Iw , 500≦ _tt

Furthermore, Patent Document 5 discloses a _three-stage resistance spot welding method for a sheet assembly in which at least one steel sheet has a C content of more than 0.30% and not more than 0.70% by mass, under the following conditions: current is applied at a current value _I1 (kA), no current is applied for a time _tc1 of 16 ms or more and 200 ms or less, current is applied at a current value _I2 (kA) for a time _t2 (ms), no current is applied for a time _tc2 (ms), and current is applied at a current value I3 (kA) for a time _t3 (ms).
0.6≦I ₂ /I ₁ ≦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≦I ₃ /I ₁ ≦1.0, 200≦t ₃

JP 2013-78782 A JP 2012-187615 A Japanese Patent Application Laid-Open No. 2016-13572 International Publication No. 2019/156073 Japanese Patent Application Laid-Open No. 2021-154390

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 plate assembly including high-strength steel plates with 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.

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, 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 chemical composition of the nugget has an average P content of less than 0.005% by mass and an average Mn content of 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, an area ratio of P-enriched portions where the P concentration is twice or more the average P content is 0.5% or less and an area ratio of Mn-enriched portions where the Mn concentration is twice or more the average Mn content is 0.5% or less. <2> The resistance spot welded joint according to <1>, wherein in the observation area near the end of the nugget, the average grain size of the prior austenite grains having an aspect ratio of 1.0 to 1.7 is 90 μm or less.
<3> The resistance spot welded joint according to <1> or <2>, wherein the average Vickers hardness in a 1000 μm square measurement area in the nugget near the end of the nugget is within ±20 HV of the 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 <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 first current-passing step of forming a nugget by sandwiching a sheet set obtained by stacking two or more steel sheets, including at least one steel sheet having a tensile strength of 980 MPa or more, between a pair of electrodes in a sheet thickness direction and applying pressure thereto while passing a current I ₁ (kA);
a cooling step of cooling the nugget for a time tc1 (ms) that satisfies 800≦tc1 so that the temperature of the nugget end becomes equal to or lower than the Ms point, where the nugget end is defined as a portion 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 sum of the tensile strength after the first current passing step;
a second current application step of applying current _I2 (kA) that satisfies 0.80≦ _I2 / _I1 <1.2 and time _t2 (ms) that satisfies 200≦ _t2 so that the nugget edge becomes at or above _point A3 and below the remelting temperature after the cooling step;
A method for manufacturing a spot welded joint, comprising:

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 plate assembly including high-strength steel plates having a tensile strength of 980 MPa or more using only a single current.
Furthermore, according to the present disclosure, there is provided 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.

FIG. 1 is a diagram showing the relationship between grain size and CTS in the vicinity of the nugget edge. 10 is an image showing the EPMA measurement results of P near the nugget edge when only a single current is applied. 10 is an image showing the EPMA measurement results of P in the vicinity of the nugget edge when post-energization is performed. 10 is an image showing the EPMA measurement results of P in the vicinity of the nugget edge when post-energization is performed. FIG. 10 is a diagram showing the relationship between post-energization and CTS. 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. 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 joined together. FIG. 6 is an enlarged schematic view showing the vicinity of the nugget end shown in FIG. 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 three overlapping steel plates are formed. 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 plate assembly in which two steel plates are overlapped. FIG. 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.

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 "mass %." In addition, in this disclosure, 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, unless otherwise specified. 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.

Generally, the higher the tensile strength of a 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-current treatment is used to prevent a decrease in joint strength (cross tensile strength: CTS). This is because post-current treatment causes tempering and alleviates solidification segregation.
The present inventors conducted extensive experiments and studies to obtain spot-welded joints having 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 more. 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 the 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 the resistance spot-welded joint and suppressing the segregation of P and Mn.

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 in a state in which the solidification segregation of P and Mn is alleviated near the nugget edge.

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

Resistance spot welded joints were fabricated using two stacked steel plates of the same type (each t = 1.6 mm, total thickness: 2 t) 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 before being water quenched.

The CTS of the spot-welded joints after heat treatment was measured, and the relationship between the grain size at the nugget edge and the 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 was 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 crystal grain size became smaller. This is thought to be because the smaller grain size improves the nugget toughness.

Next, we will explain the results of spot welding using a main current (first current) on a sheet assembly of 0.2%C-1.2%Mn-0.02%P steel (1.5GPa-grade hot stamp steel sheet, t = 2.0mm), followed by 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 were either "short-time post-current" or "reverse transformation-type solidification segregation relaxation post-current" as shown in Table 2.

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 was 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 reverse transformation type solidification segregation relaxation post-energization (FIG. 2C), the area ratio of the P-enriched area was 0.3%, and it was found that the solidification segregation was dispersed (relaxed).

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 for reverse transformation type solidification segregation relaxation compared to short-time post-current treatment.

Furthermore, the austenite grain size near the edge of each nugget was measured, and it was found that with short-time post-energization, the initial solidification structure was retained, resulting in most of the grains having an aspect ratio of more than 2. On the other hand, with reverse transformation-type solidification segregation relaxation post-energization, martensitic transformation occurred during the cooling time after the main energization, and it is believed that the application of post-energization caused reverse transformation, resulting in refined grains and the creation of a structure with an aspect ratio close to 1.

Figure 4 shows the thermal history of the nugget edge when the post-heating conditions are changed. When post-heating is performed after spot welding a steel sheet assembly, in general, a short post-heating period, which alleviates solidification segregation, exposes the nugget to a high temperature of _A4 or higher, as shown by line c, 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, after the temperature drops below the _A4 point by cooling, the crystal structure changes from fcc to bcc when the temperature exceeds the _A4 point, and the crystal grains are refined, and it is thought that both solidification segregation relaxation and crystal grain refinement are achieved by cooling. However, when a steel sheet having a relatively high C content (for example, a C content of 0.17% or more) is used as a high-strength steel sheet having a tensile strength of 980 MPa or more, not only are the crystal grains refined when the temperature exceeds the _A4 point, but also remelting occurs and a liquid phase is included, making it difficult to achieve solidification segregation relaxation and the effect of crystal grain refinement.
On the other hand, in the 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 crystal grains, and by raising the temperature to a temperature not exceeding the _A4 point, remelting is prevented and segregation is relaxed, so that segregation relaxation and crystal grain refinement are achieved simultaneously, and it is thought that CTS is further improved.

These analysis results showed that in spot-welded joints of plate assemblies including high-strength steel plates with a tensile strength of 980 MPa or more, when the following conditions (I) and (II) are satisfied near the nugget end, which corresponds to the location of the plate interface, the CTS is significantly improved compared to welded joints spot-welded by 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 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 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 end of the nugget, 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.

In other words, in the spot-welded joint according to the present disclosure, the base material has a low P and Mn content to begin with, preventing solidification segregation near the nugget edge, or the base material has a relatively high P and/or Mn content, but solidification segregation is mitigated. By refining the prior austenite grain size, the toughness inside the nugget is improved, and joint strength is thought to be enhanced.

[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 joining 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, 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 is defined as the nugget edge, 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 within the nugget that is 200 μm square and near the nugget edge, the average grain size of prior austenite grains having 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 chemical composition of the nugget has an average P content of less than 0.005% by mass and an average Mn content of less than 0.5% by mass. (b) The average chemical composition of the 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; and in the observation area, the area ratio of P-enriched parts where the P concentration is twice or more than 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 than the average Mn content is 0.5% or less.

<Board set>
The spot-welded joint according to the present disclosure is a plate assembly in which two or more steel plates are stacked together, including at least one steel plate having a tensile strength (TS) of 980 MPa or more. By including a steel plate having a TS of 980 MPa or more, high tensile strength can be ensured. The number of steel plates constituting the plate assembly may be two, three, or more.

Figure 5 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 two overlapping steel plates 1A and 1B. Two steel plates 1A and 1B are joined to form an elliptical nugget 13 whose major axis is the area that was the plate interface.

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

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 of the steel plates 1A, 1B are not limited, and, for example, desired elements may be selected.
In order to increase the strength, a steel plate 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, it is more difficult to increase joint strength as the C content increases. 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 the 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.

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.

<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 plate assembly to join all of the steel plates. Fig. 7 is a schematic diagram showing an example of a cross section in the plate thickness direction passing through the center of the nugget 13 formed by spot-welding a plate 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 elliptical nugget 13.
When viewed in cross section in the thickness direction, the shape of the nugget 13 is usually a roughly ellipse 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.

(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 having an aspect ratio of 1.0 or more and 1.7 or less near the nugget end have an average grain size of 110 μm or less.
In the 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 shape with 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 a 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 or more and 1.7 or less near 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 in the vicinity of the nugget edge is not particularly limited, but may be, for example, 1 μm or more, or 10 μm or more.

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. When the aspect ratios of the prior austenite grains vary within the observation region, prior austenite grains near the nugget edge may have crystal grains with aspect ratios exceeding 1.7, but in order to ensure high strength near the nugget edge, it is preferable that the number of prior austenite grains with an aspect ratio of 1.0 to 1.7 be 50% or more. It is more preferable that the number be 60% or more, and even more preferable that the number be 70% or more.
From the viewpoint of improving the CTS, the average aspect ratio of the 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 further preferably 1.3 or less.

(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 by 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 so as to pass through the center of the nugget, 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 region R1 having an observation area of 200 μm square. 6, the observation region R1 of the prior austenite grains in the vicinity of the nugget edge is the region 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 within 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 regions for the P concentration and Mn concentration in the vicinity of the nugget edge, which will be described later.

The average grain size of prior austenite grains with an aspect ratio of 1.0 or greater and 1.7 or less is calculated by taking the diameter (equivalent circle diameter) of a circle having the same area as the approximate ellipse for each prior austenite grain in question as the grain size.

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

<P Content and Mn Content in the Vicinity of the Nugget Edge>
In the spot-welded joint according to the present disclosure, the P content and the Mn content in the vicinity of the nugget end satisfy either (a) or (b) below.

(A) The average chemical composition of the nugget has an average P content of less than 0.005% by mass and an average Mn content of 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 sheet 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.

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 content and Mn content in the entire steel plate (entire plate assembly) that is the base material are low, and even when spot welding is performed, P segregation and Mn segregation that cause a decrease in CTS hardly occur at the nugget edge. Therefore, by satisfying the above requirement (A) that the average grain size of prior austenite grains having an aspect ratio of 1.0 to 1.7 is 110 μm or less, the 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.

(b) The average chemical composition of the nugget must satisfy at least one of the following conditions: the average P content is 0.005% by mass or more, and the average Mn content is 0.5% by mass or more. In a 200 μm square observation area near the edge of the nugget, the area ratio of P-enriched areas where the P concentration is at least twice the average P content is 0.5% or less, and the area ratio of Mn-enriched areas where the Mn concentration is at least twice the average Mn content is 0.5% or less.

When the average chemical composition of the nugget satisfies at least one of the following conditions: average P content of 0.005 mass% or more and average Mn content of 0.5 mass% or more, 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, CTS decreases due to segregation of P and Mn near the nugget edge. 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 area near the nugget edge, segregation is mitigated, and a high CTS is possible.

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

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.

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 from the above estimation formula or grain refinement, the average Vickers hardness near the nugget edge may become greater than the Vickers hardness calculated from the HV estimation formula.
Note that if tempering is performed after the nugget is formed, the nugget breaks, and the CTS does not improve, or the CTS may be lower than before tempering. On the other hand, if tempering is not performed after the nugget is formed, the nugget will not break, and the Vickers hardness at the nugget edge can be made to be equivalent 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.

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.
In addition, 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.

The uses of the spot welded joints disclosed herein are not particularly limited, but they can be particularly well suited for use as vehicle body parts, for example.

[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").

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 I ₁ (kA) to a plate assembly obtained by overlapping two or more steel plates, including at least one steel plate having a tensile strength of 980 MPa or more, while sandwiching the plate assembly between a pair of electrodes in the plate thickness direction and applying pressure thereto;
a cooling step of cooling the nugget for a time tc1 (ms) that satisfies 800≦tc1 so that the temperature of the nugget end becomes equal to or lower than the Ms point, where the nugget end is defined as a portion 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 sum of the tensile strength after the first current passing step;
a second current application step of applying current _I2 (kA) that satisfies 0.80≦ _I2 / _I1 <1.2 and time t2 (ms) that satisfies 200≦t2 so that the nugget edge becomes at or above _point A3 and below the remelting temperature after the cooling step;
Includes.
Each step will be described below.

<First energization process>
First, in the first current application step, a sheet assembly consisting of two or more overlapping steel sheets, including at least one steel sheet having a tensile strength of 980 MPa or more, is sandwiched between a pair of electrodes in the sheet thickness direction and pressurized while a current of I ₁ (kA) is applied to form a nugget.

In the first current application step, it is preferable to set the current value I ₁ (kA) and the current application time t ₁ (ms) so that a nugget is formed by spot welding to join all of the steel plates that make up the sheet assembly.
8 is a schematic diagram showing 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.

In the first current application step, there are no limitations on the welding conditions as long as a desired nugget diameter is formed. When 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-energization time is defined as _t1 .
When preliminary energization (pre-energization) is performed, _I1 and _t1 do not change significantly. If it is difficult to distinguish from the upslope, it is defined as the area of I × t divided by t.
The pressure applied by the electrodes 2A, 2B to the plate assembly may be constant, variable, or pulsed, and the pressure is, for example, 3.0 to 5.0 kN.

<Cooling process>
After the first current application process, if the part of the nugget fusion boundary (nugget boundary) corresponding to the position of the plate interface between two adjacent steel plates with the highest total tensile strength is taken as the nugget end, it is cooled for a time _tc1 (ms) that satisfies 800≦ _tc1 so that the temperature of the nugget end is below the Ms point.

In the cooling process, at least the nugget edge must be transformed into martensitic. Since the temperature gradient within the nugget is not large, if martensitic transformation occurs at the nugget edge, martensitic transformation will almost certainly also occur in the center of the nugget. To cause martensitic transformation at least at the nugget edge, the nugget edge must be cooled to below the Ms point. The Ms point can be calculated from the sheet combination.
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 means 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 contained in the sheet assembly by the sheet 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).

As a means for cooling the nugget edge to the Ms point or lower, for example, the following three means can be mentioned.
(1) Apply pressure without current (2) Apply low current (3) Open the electrodes. Any of the above (1) to (3) may be used for cooling alone or in combination, but the cooling time _tc1 must be 800 ms or more.
If the cooling time _tc1 is less than 800 ms, there is a risk that the nugget edge will not be transformed into martensite before the second current application step.
There is no upper limit to the cooling time _tc1 , but since the longer the cooling time _tc1 , the lower the work efficiency, it is preferable that the cooling time _tc1 be 2000 ms or less.

The temperature at the nugget edge can be determined using 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.

Whether martensitic transformation occurred during the cooling process can be confirmed by observing the recrystallized structure (grains with an aspect ratio of 1.0 to 1.7) in the joint after post-energization. Martensite structure observation can be performed after etching with sodium dodecylbenzenesulfonate.

<Second energization process>
After the cooling step, current is applied at a current _I2 (kA) that satisfies 0.80 ? _I2 / _I1 < 1.2 for a time _t2 (ms) that satisfies 200 ? _t2 so that the nugget edge is at or above the _A3 point but 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. Point _A1 and point _A3 can be calculated from the sheet combination.
At this time, in a sheet pair having an _A4 point, it is acceptable to heat to a temperature above the _A4 point. In a sheet pair having no _A4 point temperature, that is, in a sheet pair in which remelting occurs without δ transformation, the temperature of the nugget edge is prevented from rising above the remelting temperature.

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 vicinity of 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.

For points _A1 , _A3 , and _A4 , phase diagrams are created using commercially available comprehensive thermodynamic calculation software, such as Thermo-Calc (Thermo-Calc Software AB). In this case, the components are calculated as a weighted average of the steel plate thickness, and elements not in the database are not taken into consideration.
The temperature at which the austenite single phase is formed is defined as point _A3 , and the temperature at which the δ-ferrite single phase is formed is defined as point _A4 . In addition, in the present disclosure, if a liquid phase also appears when δ-ferrite precipitates, it is determined that point _A4 does not exist.

The second current flow may have any current flow pattern. Preferably, it has an upslope or downslope to extend the high-temperature holding time and thereby increase the austenite precipitation time.

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 may be temporarily separated from the sheet assembly, or may not be separated from the sheet assembly, and no current may be applied for a time _tc2 , after which a third current application may be performed, which does not remelt the nugget edge.

Furthermore, the nugget may have a shape such that, for example, as shown in FIG. 9, the thickness of one outermost steel plate 1D of three steel plates is thinner than the thicknesses of the other two steel plates 1A and 1B, and two nuggets 13A and 13B are joined together between the two adjacent steel plates. In such a spot-welded joint 30, for example, if steel plates 1A and 1B are 980 MPa or more and steel plate 1D is less than 980 MPa, the aspect ratio, P concentration, Mn concentration, Vickers hardness, etc. of the prior austenite grains can be measured near the nugget edge of portion 13B where steel plates 1A and 1B are joined, as with the spot-welded joint 10 shown in FIG. 5.

Examples of spot welded joints and methods for manufacturing spot welded joints according to the present disclosure are described below. Note that the spot welded joints and methods for manufacturing spot welded joints according to the present disclosure are not limited to the following examples.

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 manufacture various spot-welded joints. The CTS, etc. of the manufactured spot-welded joints were evaluated.
Table 3 lists the thickness of the steel plates, tensile strength, contents (mass %) of chemical elements (C, Si, Mn, P, S, Ti, N, Cr), the remainder being Fe and impurities, the temperatures (°C) of each transformation point (Ms, _A3 , _A4 ), and the Vickers hardness near the end of the nugget where two steel plates were spot-welded together.
Furthermore, "base metal P" in Table 4 is a weighted average obtained by multiplying the P content of each steel plate included in the sheet assembly by the sheet thickness ratio of each steel plate to the total thickness of the sheet assembly, and is regarded as the average P content in the nugget. The same applies to "base metal Mn."

Table 4 lists the types of steel sheets constituting the sheet combination, the P content and Mn content of the sheet combination, and the spot welding conditions (current value I, time t, and welding pressure P). Note that although the nugget edge temperature after cooling is not listed, it has been separately confirmed by SORPAS analysis that when the pause step (cooling step) _tc1 is at a level of 120 ms, the nugget edge does not fall below the Ms point and does not undergo martensitic transformation, and when the pause step (cooling step) _tc1 is at a level of 800 ms or 1600 ms, the nugget edge falls below the Ms point and martensitic transformation occurs. In addition, although the nugget edge temperature during post-current application has also been omitted, it has been separately confirmed by SORPAS analysis that in the invention examples, when the current ratio ( _I2 / _I1 ) is 0.8 or 0.9, the maximum temperature at the nugget edge is above the _A3 point and below the remelting temperature, and when the current ratio ( _I2 / _I1 ) is 1.2, the maximum temperature at the nugget edge is above the remelting temperature.

Table 5 lists the area ratio of concentrated P and Mn 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 of the nugget edge, the presence or absence of carbides, the joint strength (CTS), and the CTS improvement rate (CTS ratio).

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-current application in the remarks column means that the temperature at 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-energization: pattern of line a in Figure 4.

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

The CTS of spot welded joints 1 to 25 was measured in accordance with JIS Z 3137:1999 "Specimen 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 joint that underwent post-energization among spot-welded joints 1 to 25 by the CTS of the corresponding spot-welded joint that underwent only single-energization. A CTS ratio of 1.50 or higher was evaluated as a significant improvement in CTS.
CTS ratio = CTS of joint with post-energization / CTS of joint with post-energization omitted (single-energization only)

In all of the inventive examples, the main current, cooling, and post-current were applied under conditions within the range of the present disclosure, and in all cases the CTS ratio was 1.50 or more, i.e., the rate of increase exceeded 50%, compared to when post-current was omitted. Furthermore, from the results of Vickers hardness measurements, it can be seen 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 on top of each other.Since the CTS is high even with single-current spot welding, no improvement in CTS is achieved.

1A, 1B, 1C, 1D Steel plates 2A, 2B Electrodes 10, 20, 30 Spot welded joint 13 Nugget 13E Nugget edge 14 Heat affected zone (HAZ)
15 Plate interface

Claims (5)

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 chemical composition of the nugget has an average P content of less than 0.005% by mass and an average Mn content of 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. The resistance spot welded joint described in claim 1, wherein the average grain size of prior austenite grains having an aspect ratio of 1.0 to 1.7 in the observation area near the nugget edge is 90 μm or less. 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. A 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%. a first current-passing step of forming a nugget by sandwiching a sheet set obtained by stacking two or more 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 a current I ₁ (kA);
a cooling step of cooling the nugget for a time tc1 (ms) that satisfies 800≦tc1 so that the temperature of the nugget end becomes equal to or lower than the Ms point, where the nugget end is defined as a portion 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 sum of the tensile strength after the first current passing step;
a second current application step of applying current _I2 (kA) that satisfies 0.80≦ _I2 / _I1 <1.2 and time _t2 (ms) that satisfies 200≦ _t2 so that the nugget edge becomes at or above _point A3 and below the remelting temperature after the cooling step;
A method for manufacturing a spot welded joint, comprising: