Spot welding method for high-strength steel sheet excellent in joint strength

Abstract

When two high-strength steel sheets (1A, 1B) having a sheet thickness ratio of 2 to 5, inclusive, and both having a tension strength of 780 MPa to 1850 MPa, inclusive, are overlapped with each other and resistance spot welding is performed, a first energizing process that is a preliminary energization with an applied pressure of P1 kN and a welding current of I1 kA and a second energizing process that is a main energization with an applied pressure of P2 kN and a welding current of I2 kA are performed, wherein the sheet thickness ratio is defined as the ratio of the total steel sheet thickness to the thickness of a steel sheet on the thinner side (the thickness per sheet if the two steel sheets (1A, 1B) have the same thickness). The applied pressures P1 and P2 are set to a constant applied pressure of P = P1 = P2 through the first energizing process and the second energizing process. In addition, when an average sheet thickness of the steel sheets (1A, 1B) is assumed to be t mm, the applied pressure P is set in the range expressed by the formula {0.5 ≤ P ≤ 3.0t(1 / 3)}. The welding current I1 is set in the range of 30% to 90%, inclusive, of the welding current I2. The second energizing process is initiated within 0.1 s after the completion of the first energizing process.

Description

technical field

[0001] The present invention relates to a spot welding method of a high-strength steel sheet having excellent joint strength by spot welding to form a welded portion in processes such as manufacturing of automobile parts or assembling a vehicle body. Background technique

[0002] In recent years, in the automotive field, in order to reduce fuel consumption and reduce carbon dioxide gas (CO 2 ) for the purpose of reducing the weight of the vehicle body and improving crash safety, the demand for using high-strength steel sheets in the body or components is increasing. On the other hand, spot welding is mainly used in processes such as body assembly and component mounting.

[0003] Tensile strength and fatigue strength are listed as important properties of a spot-welded joint (hereinafter referred to as a spot-welded joint), but tensile strength is the most important. The tensile strength of spot-welded joints includes tensile shear strength (TSS) measured by ...

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