JP7817543B2 - Resistance spot welded joint and manufacturing method thereof - Google Patents

Description

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

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 sheets, for example, high-strength steel sheets with a tensile strength of 980 MPa or more, are resistance spot welded, 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 sheets.
In order to improve the CTS when spot welding high-tensile steel, it has been proposed to perform post-current welding after forming a nugget by main current welding. Two types of post-current welding have been reported: current welding for tempering and current welding for alleviating solidification segregation.

For example, Patent Document 1 proposes that high joint strength can be achieved by setting the nugget diameter and post-heat conditions within specified ranges. In this case, the post-heat conditions assume a post-heat current value that is larger than the main current value for forming the nugget, and pulsed current is used.
Furthermore, Patent Document 2 proposes a manufacturing method that can prevent the generation of spatter and obtain an appropriate nugget diameter and joint strength in a plate assembly that includes a high plate thickness ratio.
Patent Document 3 proposes that high joint strength be achieved in spot-welded joints of high-strength steel plates by specifying the carbon equivalent of the nugget, the HAZ (heat-affected zone) structure around the nugget, and post-energization conditions.

JP 2010-115706 A JP 2010-240739 A Patent No. 5987982

When resistance spot welding overlapping high-tensile steel sheets, if post-heat treatment is performed to improve the joint strength (CTS), the post-heat effect varies, and the joint strength after post-heat treatment also tends to vary.

The present disclosure therefore aims to provide a resistance spot-welded joint and a manufacturing method thereof that has improved joint strength and reduced variability in joint strength compared to spot-welded joints made by resistance spot welding overlapping high-strength steel plates with a tensile strength of 980 MPa or more using only a single current.

The gist of the present disclosure to achieve the above object is as follows.
<1> A plate assembly in which a plurality of steel plates are overlapped, including two adjacent high-strength steel plates each having a tensile strength of 980 MPa or more, and a nugget that joins the plurality of steel plates in the plate assembly and has a nugget diameter that is equal to or greater than the minimum nugget diameter Dmin (mm) defined by the following formula (1) at a position that was the plate interface between the two high-strength steel plates,
A resistance spot welded joint, wherein the nugget contains crystal grains having an aspect ratio of 1.0 or more and 1.7 or less.
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates by the plate thickness ratio of each steel plate to the total thickness of the two high-strength steel plates, and t represents the average plate thickness (mm) of the two high-strength steel plates.
<2> The resistance spot welded joint according to <1>, wherein, in a cross section of the plate assembly in the plate thickness direction passing through the center of the nugget, a portion of the fusion boundary of the nugget that corresponds to a position that was the plate interface between the two high-strength steel plates is defined as a nugget edge, and an area ratio of iron-based carbides is 0.3% or more in an observation region of 100 μm square in the vicinity of the nugget edge within the nugget.
<3> The resistance spot welded joint according to <2>, wherein in the observation region, a total area of iron-based carbides having a circle-equivalent grain size of 30 nm or more and an aspect ratio of 3 or more accounts for 40% of the total area of all precipitates.
<4> The resistance spot welded joint according to any one of <1> to <3>, wherein, in a cross section of the plate assembly in the plate thickness direction passing through the center of the nugget, a portion of the fusion boundary of the nugget that corresponds to a position that was the plate interface between the two high-strength steel plates is defined as a nugget edge, and the average Vickers hardness in a 1000 μm square measurement region within the nugget near the nugget edge is 20 Hv or more lower than an estimated Vickers 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, each element symbol represents the content of each element 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 considered to be the average chemical composition of the nugget.
<5> In a cross section in the plate thickness direction of the plate assembly passing through the center of the nugget, a portion of the fusion boundary of the nugget corresponding to a position that was a plate interface between the two high-strength steel plates is defined as a 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,
In a 100 μm square observation region in the nugget near the end of the nugget, the P concentration and the Mn concentration are measured at 10,000 points at 1 μm intervals along each of the directions perpendicular to the plate thickness direction, and the measurement points where the P concentration is twice or more the average P content of the average chemical composition of the nugget are defined as P-enriched portions, and the measurement points where the Mn concentration is twice or more the average Mn content of the average chemical composition of the nugget are defined as Mn-enriched portions.
<4> The resistance spot welded joint according to any one of <1> to <4>, wherein a P-enriched portion area ratio, which is the number of the P-enriched portions out of a total of 10,000 measurement points, and a Mn-enriched portion area ratio, which is the number of the Mn-enriched portions out of a total of 10,000 measurement points, are each 0.5% or less.
<6> The resistance spot welded joint according to any one of <1> to <5>, wherein the nugget diameter at a position of the nugget that was the plate interface between the two high-strength steel plates is 1.2 × Dmin or more.
<7> A method for producing a resistance spot welded joint according to any one of <1> to <6>,
a first current application step of applying a current I 1 (kA) to a plate assembly including two adjacent high-strength steel plates each having a tensile strength of 980 MPa or more, by sandwiching the plate assembly between a pair of electrodes in the plate thickness direction and applying pressure to the plate assembly, thereby forming a nugget having a nugget diameter equal to or larger than _a minimum nugget diameter Dmin (mm) defined by the following formula (1) at the plate interface between the two high-strength steel plates;
a cooling step of cooling the nugget by stopping the current flow between the pair of electrodes after the first current flow step;
a second current application step of applying a current I ₂ (kA) between the pair of electrodes to heat the nugget after the cooling step;
A method for manufacturing a resistance spot welded joint, comprising:
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates by the plate thickness ratio of each steel plate to the total thickness of the two high-strength steel plates, and t represents the average plate thickness (mm) of the two high-strength steel plates.
<8> The method for manufacturing a resistance spot welded joint according to <7>, wherein at least one of upslope current flow and downslope current flow is performed in the second current flow step.
<9> In the cooling step, the time for which the current supply is stopped is 0.4 seconds or more,
<8> The method for manufacturing a resistance spot _{welded joint according to <8>, wherein in the second current application step, current is applied so that the current ratio (I 2max /} I ₁ ) of the maximum current value I _2max (kA) of the second current application step to the current value I 1 (kA) of the first current application step is 0.50 to 0.80, and at least one of an upslope of 25 kA/sec or more and a downslope of −25 kA/sec or less is applied.
<10> In the cooling step, the time for which the current supply is stopped is 0.08 seconds or more,
<8> The method for manufacturing a resistance spot _{welded joint according to <8>, wherein in the second current application step, current is applied so that the current ratio (I 2max /} I ₁ ) of the maximum current value I _2max (kA) of the second current application step to the current value I 1 (kA) of the first current application step is 0.70 to 0.95, and at least one of an upslope of 25 kA/sec or more and a downslope of −25 kA/sec or less is applied.
<11> The method for manufacturing a resistance spot welded joint according to any one of <7> to <10>, further comprising, before the first current passing step, calculating the minimum nugget diameter Dmin using the formula (1).

This disclosure provides a resistance spot welded joint and a method for manufacturing the same that have improved joint strength and reduced variability in joint strength compared to spot welded joints made by resistance spot welding overlapping high-strength steel plates with a tensile strength of 980 MPa or more using only a single current.

FIG. 1 is a diagram for explaining the derivation of calculation formula 1 for the minimum nugget diameter Dmin. 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 high-strength steel plates are stacked. FIG. FIG. 3 is an enlarged schematic view showing the vicinity of the nugget end shown in FIG. 2 . 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 two high-strength steel plates and 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.
In addition, in this disclosure, a "resistance spot welded joint" may be referred to as a "spot welded joint" or simply as a "joint."

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.
However, when post-current treatment is performed, problems such as contact between the electrode and the steel sheet during post-current treatment can cause variations in the post-current treatment effect and variations in joint strength after post-current treatment.

Therefore, the inventors conducted experiments and studies and found that by specifying the minimum nugget diameter at the sheet interface of high-strength steel sheets, it is possible to obtain a stable post-heating effect and improve the joint strength (CTS). The reason for this is not clear, but is presumed to be as follows.
High-tensile steel has low nugget toughness, so cross-tensile tests result in interface fracture. Therefore, it is necessary to induce plug fracture to improve joint strength (CTS). One method for inducing plug fracture is to apply post-current, which improves nugget toughness.
However, there have been few studies investigating how the appropriate post-heat current range changes depending on the nugget diameter. Therefore, as a result of the research conducted by the present inventors, it was found that increasing the nugget diameter at the position that was the sheet interface of the high-strength steel sheet widens the appropriate post-heat current range. This is because, as the nugget diameter increases, the plug rupture strength becomes lower than the interfacial rupture strength over a wide range of post-heat current values. Therefore, it is believed that by specifying a minimum nugget diameter and having a nugget diameter that is equal to or greater than this, a wide range of post-heat current values can be achieved, resulting in a stable, high CTS.

[Spot welded joints]
The resistance spot welded joint according to the present disclosure will be described in detail below. The resistance spot welded joint according to the present disclosure includes a plate assembly in which multiple steel plates are overlapped, including two adjacent high-strength steel plates each having a tensile strength of 980 MPa or more, and a nugget that joins the multiple steel plates in the plate assembly and has a nugget diameter of at least a minimum nugget diameter Dmin (mm) defined by the following formula (1) at a position that was the plate interface between the two high-strength steel plates, and in which crystal grains with an aspect ratio of 1.0 to 1.7 are present within the nugget.
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates by the plate thickness ratio of each steel plate to the total thickness of the two high-strength steel plates, and t represents the average plate thickness (mm) of the two high-strength steel plates.

<Board set>
The plate assembly of the spot-welded joint according to the present disclosure is a plate assembly in which a plurality of steel plates are stacked together, including two adjacent high-strength steel plates, each having a tensile strength of 980 MPa or more. By including adjacent high-strength steel plates having a tensile strength of 980 MPa or more, high tensile strength can be ensured. The number of steel plates constituting the plate assembly may be two or three or more. In the case of three or more steel plates, all may be high-strength steel plates having a tensile strength of 980 MPa or more (sometimes referred to as "high-strength steel plates" in the present disclosure), or steel plates having a tensile strength of less than 980 MPa may be included.
Furthermore, when the plate assembly contains three or more high-strength steel plates and there are two or more surfaces where the high-strength steel plates are overlapped, it is sufficient that the nugget diameter and crystal grain aspect ratio specified in the present disclosure are satisfied on at least one overlapping surface, and it is preferable that the nugget diameter and crystal grain aspect ratio specified in the present disclosure are satisfied on all surfaces where the high-strength steel plates are overlapped.

Figure 2 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 high-strength steel plates 1A and 1B. Figure 3 is a schematic diagram showing an enlarged view of the vicinity of the nugget end shown in Figure 2. Two steel plates 1A and 1B are joined to form an elliptical nugget 13 whose major axis is the area that was previously the plate interface 15.

The sheet combination of the spot welded joint 10 according to the present disclosure may include steel sheets 1A, 1B with a tensile strength of 980 MPa or more, or may include a steel sheet with a tensile strength of less than 980 MPa in addition to the two adjacent high-strength steel sheets 1A, 1B. 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.

The chemical composition and metal structure of each high-strength steel plate 1A, 1B in the spot-welded joint 10 according to the present disclosure are not limited, and desired elements may be selected so that the tensile strength is 980 MPa or greater. To achieve high strength, it is preferable that the high-strength steel plates 1A, 1B have a C content of 0.20 mass% or greater, 0.25 mass% or greater, or 0.30 mass% or greater.

The thickness of each of the high-strength 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.
Fig. 4 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 a plate assembly in which three overlapping high-strength steel plates 1A, 1B, and 1C are joined together. In the spot-welded joint 20 shown in Fig. 4, the three high-strength steel plates 1A, 1B, and 1C are joined by a nugget 13 having an elliptical cross section.
The following mainly describes a spot-welded joint formed by spot-welding two high-strength 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 a position where multiple steel plates included in the sheet assembly are spot-welded, thereby joining all of the steel plates together.
When viewed in cross section in the thickness direction, the shape of the nugget 13 is usually a substantially 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 2 and 4, but is not limited to this shape.

(minimum nugget diameter)
The nugget 13 has a nugget diameter at the position that was the plate interface between the two high-strength steel plates 1A, 1B that is equal to or greater than the minimum nugget diameter Dmin defined by the following formula (1).
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates 1A, 1B by the plate thickness ratio of each steel plate 1A, 1B to the total thickness of the two high-strength steel plates 1A, 1B, and t represents the average plate thickness (mm) of the two high-strength steel plates 1A, 1B.

By specifying the minimum nugget diameter at the interface (lap surface) between two high-strength steel plates based on the chemical composition and plate thickness calculated using Equation 1 above, resistance spot welded joints with nugget diameters greater than this can achieve a stable post-heat effect, significantly improving CTS.

Here, the above-mentioned formula 1 for calculating the minimum nugget diameter Dmin is derived from the following experiments and ideas.
The coefficients for the carbon content and nitrogen content were determined by considering the minimum nugget diameter at which the plug fractured during a single current application. The plot used here was (carbon content, minimum nugget diameter) = (0%, 4 mm), (0.08%, 4.2 mm), (0.21%, 5 mm). As shown in Figure 1, the calculated coefficient was 4.9, but since the slope became smaller as the carbon content increased, the coefficient was set at a slightly smaller value of 4.2.
The coefficients for P and S were set to the same level as for C, as they affect toughness to the same extent. For other elements, the coefficients were determined taking into account the coefficients of the Ms point formula and their effect on cementite growth. For example, Cr suppresses cementite growth and inhibits softening.

The nugget diameter at the location (overlap surface) that was the sheet interface 15 between the two high-strength steel sheets 1A and 1B is preferably 1.2 x Dmin or more. Having a nugget diameter of 1.2 x Dmin or more provides the joint with the additional benefits of ensuring a high CTS and a wide range of appropriate current ratios.

(Crystal grains with an aspect ratio of 1.0 to 1.7)
The spot-welded joint 10 according to the present disclosure has crystal grains in the nugget 13 with an aspect ratio of 1.0 to 1.7. When a spot-welded joint is produced using only a single current, solidification progresses toward the center of the molten pool, resulting in elongated crystal grains with an aspect ratio of more than 1.7. In contrast, when a post-current is applied appropriately, the nugget is reheated, causing recrystallization within the nugget, resulting in an aspect ratio of 1.0 to 1.7.
The post-current application causes the presence of crystal grains in the nugget with an aspect ratio of 1.0 or more and 1.7 or less, i.e., grains with a shape in which the aspect ratio of each grain is relatively small, which makes the nugget resistant to forces in the direction of peeling the high-strength steel sheet and improves joint strength. Note that it is sufficient that crystal grains with a shape in which the aspect ratio of each grain is relatively small exist in the nugget, and the location of such crystal grains is not particularly limited. For example, the central portion of the nugget may be remelted by the post-current application, and crystal grains with a shape in which the aspect ratio of each grain is relatively small may exist in a position other than the central portion.

Here, the aspect ratio of the crystal grains in the nugget is specified as follows.
In an image showing the prior austenite grain boundaries in a cross section (nugget cross section) in the plate thickness direction passing through the center of the nugget, 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 plate thickness direction passing through the center of the nugget, and the cut surface is etched with sodium dodecylbenzenesulfonate. The aspect ratio of the prior austenite grain is measured using an optical microscope along the plate interface between the two high-strength steel plates to be measured in an observation region R2 having an observation area of 1000 μm square. Here, as shown in Figure 2, the observation area R2 of the prior austenite grains is a 1000 µm square extending from the nugget end 13E to the center of the nugget along the position that was the plate interface 15 of each high-strength steel plate 1A, 1B of the nugget 13, with one side in the plate thickness direction and symmetrical with respect to the plate interface 15.
As shown in FIG. 2, the nugget 13 is symmetrical in a concentric ellipse. Therefore, by sequentially measuring from one end 13E of the nugget 13 to the center, it is possible to confirm whether or not crystal grains with an aspect ratio of 1.0 or more and 1.7 or less are present.

If there are even trace amounts of prior austenite grains with an aspect ratio of 1.0 to 1.7 in any observation region within the nugget, it can be understood that post-heat treatment has been performed, and the proportion of such grains is not particularly limited. In other words, while there may be crystal grains with aspect ratios exceeding 1.7 in the nugget, to ensure a high CTS, it is preferable that prior austenite grains with aspect ratios of 1.0 to 1.7 account for 50% or more by number, more preferably 60% or more by number, and even more preferably 70% or more by number. Note that when post-heat treatment is performed, the aspect ratios of the prior austenite grains in the observation region are often similar, and there is rarely any significant variation in the aspect ratios of the prior austenite grains within the observation region.

(iron-based carbide)
In a cross section of the plate assembly in the plate thickness direction passing through the center of the nugget 13, the portion of the fusion boundary of the nugget 13 that corresponds to the position of the plate interface 15 between the two high-strength steel plates 1A and 1B is defined as the nugget end, and it is preferable that the area ratio of iron-based carbides is 0.3% or more in an observation region R1 of 100 μm square near the nugget end within the region including the nugget end within the nugget.
Here, iron-based carbides are, for example, Fe _2-3 C, and in the present disclosure, those containing 50 at% or more of Fe and 25 at% or more of C are considered to be iron-based carbides. The area ratio of iron-based carbides is a value measured by performing picral corrosion on the nugget cross section and observing it with an SEM. By performing post-energization (tempering energization) for the purpose of tempering, the area ratio of iron-based carbides becomes 0.3% or more in the observation region R1 near the nugget edge, and the joint can achieve high values in peel strength, mainly CTS.

In addition, in the observation region R1, it is preferable that the total area of iron-based carbides having a circle-equivalent grain size of 30 nm or more and an aspect ratio of 3 or more accounts for 40% of the total area of all precipitates.
When tempering current is advanced in the nugget 13, the area ratio of iron-based carbides having a circle-equivalent grain size of 30 nm or more and an aspect ratio of 3 or more to the area of all precipitates in the observation region R1 near the end of the nugget becomes 40% or more, and the nugget can have a higher CTS.

(Vickers hardness)
It is preferable that the average Vickers hardness in a 1000 μm square measurement region in the nugget near the end of the nugget is 20 Hv or more lower than the estimated Vickers 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, each element symbol represents the content of each element when the weighted average obtained by multiplying the chemical composition 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 is considered to be the average chemical composition of the nugget.

The spot-welded joint 10 according to the present disclosure is subjected to post-energization (tempering energization) for the purpose of tempering, so that the average Vickers hardness near the nugget edge is 20 HV or more lower than the Vickers hardness calculated using the HV estimation formula, and the joint achieves high values for peel strength, mainly CTS.

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 a 1000 μm square region R2 cannot be secured near the nugget end, the Vickers hardness is measured at 10 points in a region within 2000 μm from the nugget end 13E, and the average value is taken as the average Vickers hardness.

(P concentration and Mn concentration near the nugget edge)
The P and Mn concentrations are measured at 10,000 points at 1 μm intervals along the thickness direction and the direction perpendicular to the thickness direction in an observation region R1 near the nugget edge, and the measurement points where the P concentration is at least twice the average P content of the average chemical composition of the nugget are defined as P-enriched regions, and the measurement points where the Mn concentration is at least twice the average Mn content of the average chemical composition of the nugget are defined as Mn-enriched regions. The P-enriched region area ratio, which is the number of P-enriched regions out of a total of 10,000 measurement points, and the Mn-enriched region area ratio, which is the number of Mn-enriched regions out of a total of 10,000 measurement points, are each preferably 0.5% or less. Each enriched region can be measured using an EPMA (electron probe microanalyzer).
Here, the average chemical composition of the nugget is calculated as the weighted average content (mass %) of the two high-strength steel plates 1A and 1B.

If the area ratios of P-enriched areas and Mn-enriched areas in the nugget edge region are each 0.5% or less, it can be assumed that no segregation of these elements has occurred, resulting in a resistance spot welded joint with improved CTS.

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]
Next, a method for manufacturing a spot-welded joint according to the present disclosure will be described. While the method for manufacturing a spot-welded joint according to the present disclosure 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").

A method for manufacturing a spot welded joint according to the present disclosure includes a first current application step of applying a current of I ₁ (kA) to a plate assembly including two adjacent high-strength steel plates each having a tensile strength of 980 MPa or more, while sandwiching the plate assembly between a pair of electrodes in a plate thickness direction and applying pressure to the plate assembly, thereby forming a nugget having a nugget diameter equal to or greater than a minimum nugget diameter Dmin (mm) defined by the following formula (1) at the plate interface between the two high-strength steel plates:
a cooling step of cooling the nugget by stopping the current flow between the pair of electrodes after the first current flow step;
a second current application step of applying a current I ₂ (kA) between the pair of electrodes to heat the nugget after the cooling step;
Includes.
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates by the plate thickness ratio of each steel plate to the total thickness of the two high-strength steel plates, and t represents the average plate thickness (mm) of the two high-strength steel plates.

<First energization process>
First, in the first current application step, a plate assembly consisting of multiple overlapping steel plates, including two adjacent high-strength steel plates each having a tensile strength of 980 MPa or more, is sandwiched between a pair of electrodes in the plate thickness direction and pressurized while a current of I ₁ (kA) is applied thereto, thereby forming a nugget at the plate interface between the two high-strength steel plates having a nugget diameter equal to or greater than the minimum nugget diameter Dmin defined by the following formula (1):
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates by the plate thickness ratio of each steel plate to the total thickness of the two high-strength steel plates, and t represents the average plate thickness (mm) of the two high-strength steel plates.

To form a nugget with a minimum nugget diameter Dmin or larger at the sheet interface between two high-strength steel sheets, the actual current conditions that result in the minimum nugget diameter or larger can be determined by a preliminary test or simulation. The actual current conditions are a sequence of current values and pressure forces.
The preliminary test can be performed, for example, by keeping the applied pressure constant and varying the current value. The simulation can also be performed using commercially available resistance welding simulation software, such as SORPAS (SCSK Corporation), by inputting the current value and applied pressure and checking the nugget diameter for those conditions.

In the first current application process, it is preferable to set the current value I 1 (kA) and the current application time t ₁ (ms) so that a nugget having a nugget diameter equal to or larger than the minimum nugget diameter Dmin is formed at the plate interface between the high-strength steel plates _1A and 1B by spot welding, and which joins all of the steel plates that make up the plate assembly.
5 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. 5, 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 restrictions on the welding conditions as long as a nugget diameter equal to or larger than the minimum nugget diameter Dmin is formed. The current value _I1 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 .
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.

<Cooling process>
After the first current application step, the current between the pair of electrodes is stopped to cool the nugget. In the cooling step, at least the edge of the nugget must have undergone martensitic transformation. Since the temperature gradient within the nugget is not large, if martensitic transformation occurs at the edge of the nugget, it is likely that martensitic transformation also occurs in the center of the nugget. To induce martensitic transformation at least at the edge of the nugget, the edge of the nugget must be cooled to below the Ms point. The Ms point can be calculated from the sheet assembly using the following formula. In the formula, (% element symbol) represents the weighted average of the content (mass %) of each element in each steel sheet included in the sheet assembly multiplied by the thickness ratio of each steel sheet to the total thickness of the sheet assembly, i.e., the average chemical composition of the nugget described above.
Ms point = 550-361×(%C)-39×(%Mn)-35×(%V)-20×(%Cr)-17×(%Ni)-10×(%Cu)-5×(%Mo+%W)+15×(%Co)+30(%Al)
.
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 it is preferable that the cooling time _tc1 be 400 ms or more.
If the cooling time _tc1 is less than 400 ms, there is a risk that the nugget edge will not solidify 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.

<Second energization process>
After the cooling step, the nugget is heated by passing a current I ₂ (kA) between the pair of electrodes.
In the second current application step, it is preferable to perform at least one of upslope current application and downslope current application. As the nugget diameter increases, the nugget edge may be located outside the electrode. Therefore, by applying upslope or downslope current application, the electrode and the steel sheet become more intimately attached to each other, facilitating heat treatment of the nugget edge.

It is preferable that the time for which current application is suspended in the cooling step is 0.4 seconds (=20 cycles) or more, that post-current application is performed in the second current application step so that the current ratio (I _2max /I ₁ ) of the maximum current value I _2max (kA) in the second current application step to the current value I ₁ (kA) in the first current application step is 0.50 to 0.80, and that at least one of an upslope of 25 kA/sec (=0.5 kA/cycle) or more and a downslope of −25 kA/sec (=−0.5 kA/cycle) or less is applied.
In the second current application step, at least the edge of the nugget is heated to below point _A1 . The occurrence of tempering in the second current application step can be confirmed by observing the vicinity of the edge of the nugget in a hardness test.
Alternatively, it is also preferable that in the cooling step, the time for which current application is suspended is 0.08 seconds (=4 cycles) or more, and in the second current application step, post-current application is performed so that the current ratio (I _2max /I ₁ ) of the maximum current value I _2max (kA) in the second current application step to the current value I 1 ₍ kA) in the first current application step is 0.70 to 0.95, and that at least one of an upslope of 25 kA/sec (=0.5 kA/cycle) or more and a downslope of −25 kA/sec (=−0.5 kA/cycle) or less is applied.
In the second current application step, at least the nugget edge is heated to a temperature lower than the melting point. The occurrence of solidification segregation relaxation in the second current application step can be confirmed by observing the vicinity of the nugget edge by EPMA measurement. The upper limit of the second current application time _t2 is preferably 2500 ms or less to avoid remelting up to the nugget edge.

The second current can be applied in any pattern. An upslope or downslope pattern is preferred. This stabilizes contact between the electrode and the steel sheet, allowing the electrode shoulder to contact the steel sheet and facilitate tempering of the nugget edge.

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.
The manufacturing method of a spot welded joint according to the present disclosure may include a calculation step of calculating the minimum nugget diameter Dmin using formula (1) based on the chemical composition of each high-strength steel plate 1A, 1B before the first current application step.
Also, 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 re-melt the nugget.

Also, for example, as shown in FIG. 6, the nugget may have a shape in which two nuggets 13A, 13B formed between two adjacent steel plates are joined, with the thickness of one steel plate 1D, located on the outer side of three steel plates, thinner than the thickness of the other two high-strength steel plates 1A, 1B. In such a spot-welded joint 30, for example, if steel plates 1A, 1B are high-strength steel plates of 980 MPa or more and steel plate 1D is less than 980 MPa, similar to the spot-welded joint 10 shown in FIG. 2, it is sufficient that the nugget diameter is equal to or greater than the minimum nugget Dmin calculated by Equation 1 and that prior austenite grains with an aspect ratio of 1.0 to 7.0 are present in the portion 13B of the nugget joining the high-strength steel plates 1A, 1B.

The following describes examples of spot-welded joints and manufacturing methods thereof according to the present disclosure. Note that the spot-welded joints and manufacturing methods thereof according to the present disclosure are not limited to the following examples.

Example 1
[Manufacturing of two-plate resistance spot welded joints]
Various plate combinations were prepared by combining the steel plates shown in Table 1 as shown in Tables 2A to 2D, and spot welding was performed on each plate combination to produce various spot-welded joints.
Table 1 lists the thickness, tensile strength, and chemical component contents (mass%) of the steel plates, as well as the minimum nugget diameter Dmin calculated by Equation 1, and the Vickers hardness (referred to as "nugget hardness") near the nugget edge where two steel plates are spot-welded together.

[evaluation]
The produced spot-welded joints were measured for the aspect ratio of the prior austenite grains in the nugget, the area ratio of the P and Mn enriched portions near the nugget edge, the average Vickers hardness HV, and the like, as described above.

The CTS of each spot-welded joint was also measured in accordance with JIS Z 3137:1999, "Specimen dimensions and test methods for cross-tensile tests on resistance spot- and projection-welded joints."

(CTS increase amount)
The difference between the CTS of each spot-welded joint and the CTS of the corresponding spot-welded joint subjected to only a single current (first current) was divided by the CTS of the corresponding spot-welded joint subjected to only a single current (first current) to calculate the increase (%), and the increase was evaluated according to the following criteria.
×: 10% or less △: More than 10% but less than 20% 〇: 20% or more

(CTS Variation Evaluation)
In addition, five spot-welded joints were produced under the same conditions for each example, and the CTS of each was measured and evaluated for CTS variation according to the following criteria.
×: ±1.8 kN or more △: More than ±1.0, less than ±1.8 kN ◯: 1.0 kN or less

(Overall Judgment)
Based on the evaluation of CTS and CTS variation, the following judgments were made.
×: If there is even one ×△: If one is △ and the other is 〇〇: If both are 〇〇

Tables 2A to 2D show the sheet assembly, welding conditions, and evaluation results. Note that the "cooling time" for the second current conditions refers to the cooling time after the first current process and before the start of the second current process, and the "current ratio" refers to the current ratio of the current value in the second current process to the current value in the first current process. Furthermore, the "Vickers hardness reduction (Hv) relative to the base material" refers to the reduction in estimated Vickers hardness in the nugget edge region, calculated using the weighted average content of the chemical components of each steel sheet included in the sheet assembly according to the sheet thickness.

Example 2
[Manufacturing of three-plate resistance spot welded joints]
Various plate assemblies were prepared by combining the steel plates shown in Table 1 as shown in Table 3, and spot welding was performed on each plate assembly to produce various spot-welded joints, which were evaluated in the same manner as in Example 1. Note that a cross tensile test was performed so that a peel load was applied to the plate interface between steel plate 1 and steel plate 2 of each three-plate assembly. The actual nugget diameter was the length of the position at the plate interface between steel plate 1 and steel plate 2.

In the examples of the invention, all had nugget diameters equal to or greater than the minimum nugget diameter specified in the present disclosure, crystal grains with aspect ratios of 1.0 to 1.7 were present within the nugget, the CTS increase was high, and CTS variation was suppressed.
On the other hand, the comparative example did not satisfy the plate assembly or current application conditions defined in the present disclosure, and at least one of the CTS increase amount and the CTS was insufficient.

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 (11)

The present invention relates to a plate assembly including a plurality of overlapping steel plates, the plurality of overlapping steel plates having a C content of 0.21% by mass or more and a Si content of 0.38% by mass or more, and two adjacent high-strength steel plates each having a tensile strength of 980 MPa or more, and a nugget that joins the plurality of steel plates in the plate assembly and has a nugget diameter that is equal to or greater than the minimum nugget diameter Dmin (mm) defined by the following formula (1) at a position that was the plate interface between the two high-strength steel plates,
A resistance spot welded joint, wherein the nugget contains crystal grains having an aspect ratio of 1.0 or more and 1.7 or less.
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates by the plate thickness ratio of each steel plate to the total thickness of the two high-strength steel plates, and t represents the average plate thickness (mm) of the two high-strength steel plates. The resistance spot welded joint described in claim 1, wherein the nugget edge is defined as the portion of the nugget fusion boundary that corresponds to the position of the interface between the two high-strength steel plates in a cross section of the plate assembly in the plate thickness direction passing through the center of the nugget, and the area ratio of iron-based carbides is 0.3% or more in an observation area of 100 μm square within the nugget near the nugget edge. 3. The resistance spot welded joint according to claim 2, wherein in the observation region, a total area of iron-based carbides having a circle-equivalent grain size of 30 nm or more and an aspect ratio of 3 or more relative to the total area of all precipitates is 40% or more . 4. The resistance spot welded joint according to claim 1, wherein a portion of the fusion boundary of the nugget corresponding to a position that was an interface between the two high-strength steel plates in a cross section of the plate assembly in the plate thickness direction that passes through the center of the nugget is defined as a nugget end portion, and the average Vickers hardness in a 1000 μm square measurement region within the nugget near the nugget end portion is 20 Hv or more lower than an estimated Vickers 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, each element symbol represents the content of each element 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 considered to be the average chemical composition of the nugget. In a cross section of the plate combination in the plate thickness direction passing through the center of the nugget, a portion of the fusion boundary of the nugget corresponding to the position of the plate interface between the two high-strength steel plates is defined as a 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,
In a 100 μm square observation region in the nugget near the end of the nugget, the P concentration and the Mn concentration are measured at 10,000 points at 1 μm intervals along each of the directions perpendicular to the plate thickness direction, and the measurement points where the P concentration is twice or more the average P content of the average chemical composition of the nugget are defined as P-enriched portions, and the measurement points where the Mn concentration is twice or more the average Mn content of the average chemical composition of the nugget are defined as Mn-enriched portions.
The resistance spot welded joint according to any one of claims 1 to 4, wherein a P-enriched area ratio, which is the number of P-enriched areas out of a total of 10,000 measurement points, and a Mn-enriched area ratio, which is the number of Mn-enriched areas out of a total of 10,000 measurement points, are each 0.5% or less. The present invention includes a plate assembly in which a plurality of steel plates are overlapped, the plate assembly including two adjacent high-strength steel plates each having a tensile strength of 980 MPa or more, and a nugget in which the plurality of steel plates are joined in the plate assembly, and when a minimum nugget diameter defined by the following formula (1) is Dmin (mm), the nugget diameter at a position that was the plate interface between the two high-strength steel plates is 1.2 × Dmin or more ,
A resistance spot welded joint , wherein the nugget contains crystal grains having an aspect ratio of 1.0 or more and 1.7 or less .
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates by the plate thickness ratio of each steel plate to the total thickness of the two high-strength steel plates, and t represents the average plate thickness (mm) of the two high-strength steel plates. A method for manufacturing a resistance spot welded joint according to any one of claims 1 to 6, comprising:
a first current-passing step of sandwiching a plate assembly including a plurality of overlapping steel plates, the plate assembly including two adjacent high-strength steel plates each having a tensile strength of 980 MPa or more , with a C content of 0.21 mass % or more and a Si content of 0.38 mass % or more, between a pair of electrodes in the plate thickness direction and applying pressure thereto while passing a current I ₁ (kA), thereby forming a nugget having a nugget diameter of at least a minimum nugget diameter Dmin (mm) defined by the following formula (1) at the plate interface between the two high-strength steel plates;
a cooling step of cooling the nugget by stopping the current flow between the pair of electrodes after the first current flow step;
a second current application step of applying a current I ₂ (kA) between the pair of electrodes to heat the nugget after the cooling step;
A method for manufacturing a resistance spot welded joint, comprising:
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates by the plate thickness ratio of each steel plate to the total thickness of the two high-strength steel plates, and t represents the average plate thickness (mm) of the two high-strength steel plates. The method for manufacturing a resistance spot welded joint according to claim 7, wherein at least one of upslope current flow and downslope current flow is performed in the second current flow step. The present invention relates to a plate assembly including a plurality of overlapping steel plates, the plurality of steel plates including two adjacent overlapping high-strength steel plates each having a tensile strength of 980 MPa or more, and a nugget that joins the plurality of steel plates in the plate assembly and has a nugget diameter that is equal to or greater than the minimum nugget diameter Dmin (mm) defined by the following formula (1) at a position that was the plate interface between the two high-strength steel plates,
A method for manufacturing a resistance spot welded joint in which crystal grains having an aspect ratio of 1.0 to 1.7 are present in the nugget, comprising:
a first current-passing step of passing a current I1 (kA) through a plate assembly including two adjacent high-strength steel plates each 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 to the plate assembly, thereby forming a nugget having a nugget diameter equal to or larger than a minimum nugget diameter Dmin (mm) defined by the following formula (1) at the plate interface between the two high-strength steel plates;
a cooling step of cooling the nugget by stopping the current flow between the pair of electrodes after the first current flow step;
a second current application process of applying a current value I2 (kA) between the pair of electrodes to heat the nugget after the cooling process;
Including,
In the cooling step, the time for which the current supply is stopped is 0.4 seconds or more,
In the second current application step, current is applied so that the current ratio (I _2max /I ₁ ) of the maximum current value I _2max (kA) of the second current application step to the current value I ₁ (kA) of the first current application step is 0.50 to 0.80, and at least one of an upslope of 25 kA/sec or more and a downslope of −25 kA/sec or less is applied .
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates by the plate thickness ratio of each steel plate to the total thickness of the two high-strength steel plates, and t represents the average plate thickness (mm) of the two high-strength steel plates. The present invention relates to a plate assembly including a plurality of overlapping steel plates, the plurality of steel plates including two adjacent overlapping high-strength steel plates each having a tensile strength of 980 MPa or more, and a nugget that joins the plurality of steel plates in the plate assembly and has a nugget diameter that is equal to or greater than the minimum nugget diameter Dmin (mm) defined by the following formula (1) at a position that was the plate interface between the two high-strength steel plates,
A method for manufacturing a resistance spot welded joint in which crystal grains having an aspect ratio of 1.0 to 1.7 are present in the nugget, comprising:
a first current-passing step of passing a current I1 (kA) through a plate assembly including two adjacent high-strength steel plates each 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 to the plate assembly, thereby forming a nugget having a nugget diameter equal to or larger than a minimum nugget diameter Dmin (mm) defined by the following formula (1) at the plate interface between the two high-strength steel plates;
a cooling step of cooling the nugget by stopping the current flow between the pair of electrodes after the first current flow step;
a second current application process of applying a current value I2 (kA) between the pair of electrodes to heat the nugget after the cooling process;
Including,
In the cooling step, the time for which the current supply is stopped is 0.08 seconds or more,
In the second current application step, current is applied so that the current ratio (I _2max /I ₁ ) of the maximum current value I _2max (kA) of the second current application step to the current value I ₁ (kA) of the first current application step is 0.70 to 0.95, and at least one of an upslope of 25 kA/sec or more and a downslope of −25 kA/sec or less is applied .
Dmin=(4.2(C+N)+0.2Mn+0.1(Si+Al)+0.05Cr+4(P+S)+0.02(Ti+Mo+Nb)+0.01V+3)×√t...Formula (1)
In formula (1), each element symbol represents a weighted average content (mass%) obtained by multiplying each chemical component of the two high-strength steel plates by the plate thickness ratio of each steel plate to the total thickness of the two high-strength steel plates, and t represents the average plate thickness (mm) of the two high-strength steel plates. The method for manufacturing a resistance spot welded joint according to any one of claims 7 to 10, further comprising a calculation step of calculating the minimum nugget diameter Dmin using formula (1) before the first current application step.