KR-102964149-B1 - Resistance spot welded joint and the resistance spot welding method

Abstract

The present invention aims to provide a resistance spot welded joint and a resistance spot welding method. The present invention comprises a resistance spot welded joint in which two or more steel plates, including at least one high-strength steel plate, are resistance spot welded, wherein the high-strength steel plate has a composition in which, in mass%, C: 0.05 to 0.6%, Si: 0.1 to 2.0%, Mn: 1.5 to 4.0%, and P: 0.10% or less, and the remainder is Fe and unavoidable impurities, wherein the hardness of the nugget softest region is 90% or less of the hardness of the heat-affected zone, and furthermore, the microstructure of the nugget softest region has tempered martensite, and furthermore, the average number density of carbides with a grain size of 100 nm or more in the nugget softest region is 10 or more per 5 μm 2 of the plate cross-section.

Inventors

• 엔도 레이코
• 다카시마 가츠토시
• 마츠다 히로시

Assignees

• 제이에프이 스틸 가부시키가이샤

Dates

Publication Date

20260512

Application Date

20220721

Priority Date

20210819

Claims (5)

1. A resistance spot welded joint having a resistance spot welded portion formed by resistance spot welding two or more steel plates, including at least one high-strength steel plate, The above high-strength steel plate, in mass %, C : 0.05 ∼ 0.6 ％, Si : 0.1 ~ 2.0 %, Mn : 1.5 ~ 4.0 %, and P: 0.10% or less It has a component composition containing, and the remainder containing Fe and unavoidable impurities, Two points on the boundary of the nugget intersecting the overlapping surface of the steel plate are designated as the first end and the second end, the length of the line segment X connecting the first end and the second end is D (mm), and the positions on the line segment X from the first end and the second end toward the center of the nugget are designated as points a and b. When the region within the nugget where the distance L (mm) from the first end to point a and from the second end to point b satisfies the relationship of Equation (1) with respect to the length D (mm) of the line segment X is defined as the nugget softening region, In at least one of the above-mentioned overlapping surfaces, the hardness of the nugget softening region is 90% or less of the hardness of the heat-affected zone formed to surround the nugget, and In addition, the hardness of the most softened region of the nugget is the softest compared to the hardness inside the nugget excluding the most softened region of the nugget and the hardness of the heat-affected zone, and In addition, the organization of the above-mentioned nugget softening region has tempered martensite, In addition, a resistance spot welded joint in which the average number density of carbides with a particle size of 100 nm or more in the above-mentioned nugget softening region is 10 or more per 5 μm 2 of plate cross-section. 0 < L ≤ 0.10 × D … (1) However, if there is a gap between the steel plates in the above-mentioned overlapping surface, two points on the boundary of the nugget located in the middle of the gap and intersecting with a straight line Y parallel to the above-mentioned overlapping surface are designated as the first end and the second end.
2. In Article 1, The above component composition of the above high-strength steel plate is additionally, in mass %, Cu: 0.8% or less, Ni: 1.0% or less, Mo: 1.0% or less, Cr: 1.0% or less, Nb : 0.080 % or less, V : 0.50% or less, Ti : 0.20 % or less, B : 0.005% or less, Al : 2.0% or less, and Ca: 0.005% or less A resistance spot weld joint containing one or more types selected from.
3. In Article 1, The above high-strength steel plate is a resistance spot welded joint having a plating layer on the surface of the steel plate.
4. In Article 2, The above high-strength steel plate is a resistance spot welded joint having a plating layer on the surface of the steel plate.
5. A resistance spot welding method for a resistance spot welded joint as described in any one of claims 1 to 4, When forming the resistance spot weld by clamping a plate set comprising two or more overlapping steel plates, including at least one of the high-strength steel plates, with a pair of welding electrodes and applying current while applying pressure, As the above current, A main current application process for forming a nugget by applying current at a value of I m (kA), and A cooling process forming the cooling time t c (ms) represented by Equation (2), and Next, a heating process in which the resistance spot weld is energized with a current value I t (kA) as shown in Equation (3) for an energization time t t (ms) as shown in Equation (4), and A resistance spot welding method for a resistance spot weld joint having a heat treatment process after tempering, wherein the resistance spot weld part is energized for a energizing time t tm (ms) as shown in Equation (6) with a current value I tm (kA) as shown in Equation (5). 800 ≤ t c … (2) 1.1 × I m ≤ I t ≤ 2.0 × I m … (3) 100 < t t ≤ 200 … (4) I tm < I t … (5) 300 ≤ t tm ＜ 3500 … (6)

Description

Resistance spot welded joint and the resistance spot welding method The present invention relates to a resistance spot welded joint and a resistance spot welding method. Recently, various high-strength steel sheets are being applied to automobile bodies in order to improve fuel efficiency through weight reduction and to ensure crash safety. In addition, resistance spot welding is mainly used as a joining method for components on automobile assembly lines. The joint strength of a resistance spot weld is evaluated by tensile shear strength (TSS), which is the tensile strength in the shear direction, and cross tension strength (CTS), which is the tensile strength in the peeling direction. In a resistance spot weld, TSS tends to increase with the tensile strength of the base material, but CTS is said to decrease when the tensile strength of the base material is 780 MPa or higher. When CTS decreases, the fracture mode of the resistance spot weld (weld) transitions from plug fracture, where ductile fracture occurs in the base material or heat-affected zone (HAZ) surrounding the resistance spot weld, to interfacial fracture or partial plug fracture, where brittle fracture occurs within the nugget. Causes of CTS decrease include, for example, brittle fracture occurring due to hardening of the nugget end after rapid cooling. Therefore, to resolve this problem, various studies are being conducted on the post-transmission method, which involves re-transmissioning after the initial transmission. As a technology for solving the above problem, examples include Patent Documents 1 to 3. Patent Document 1 specifies a welded portion in which the nugget end is tempered. Specifically, Patent Document 1 discloses that in the outer layer region of the nugget, the microstructure consists of a dendrite structure with an average arm spacing of 12 μm or less, the average particle size of the carbides included in the microstructure is 5 nm to 100 nm, and the number density of the carbides is 2 × 10⁶ /mm² or more. Patent Document 2 discloses that in a resistance-welded joint of a resistance-welded steel plate made of a specific composition, the molten solidified zone (nugget) and the heat-affected zone have a structure consisting mainly of tempered martensite or tempered bainite. Patent Document 3 discloses a resistance spot welded joint that specifies the hardness of the outer side of the nugget and the microstructure within the nugget. The resistance spot welded portion of this resistance spot welded joint is conditioned on the condition that the microstructure within the nugget is an equiaxed martensitic microstructure and that a softening zone with lower hardness than the base material exists on the outer side of the nugget. FIG. 1 is a cross-sectional view schematically showing the area around the resistance spot weld of a resistance spot weld joint related to one embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing the area around the resistance spot weld of a resistance spot weld joint related to one embodiment of the present invention. FIG. 3 is a cross-sectional view schematically showing the area around the resistance spot weld of a resistance spot weld joint related to one embodiment of the present invention. FIG. 4 is a cross-sectional view illustrating an example of the resistance spot welding method of the present invention. FIG. 5 is a schematic diagram illustrating a method for measuring the hardness of a nugget end in the present invention. FIG. 6 is a schematic diagram illustrating a method for measuring the average number density of carbides at the nugget end in the present invention. FIG. 7 is a diagram illustrating an example of a current flow pattern of the resistance spot welding method of the present invention. The present invention will be described below. Furthermore, the present invention is not limited to this embodiment. [Resistance Spot Welded Joint] First, the resistance spot weld joint of the present invention will be described with reference to FIGS. 1 to 3. FIGS. 1 to 3 show, as an example, cross-sectional views in the plate thickness direction of the resistance spot weld portion and its surroundings in the resistance spot weld joint of the present invention. FIG. 1 is the case where the number of overlapping steel plates is 2, FIG. 2 is the case where the number of overlapping steel plates is 2 and there is also a plate gap between the steel plates, and FIG. 3 is the case where the number of overlapping steel plates is 3. The present invention is a resistance spot welded joint having a resistance spot welded portion formed by resistance spot welding a plurality of overlapping steel plates. The overlapping steel plates include at least one high-strength steel plate as described below. The number of the plurality of steel plates is not particularly limited and may be two or more. Furthermore, while there is no specific upper limit for the number of the plurality o