RESISTANCE SPOT WELDING METHOD AND METHOD FOR MANUFACTURING A RESISTANCE SPOT WELDING JOINT

Abstract

A resistance spot welding method and a method for manufacturing a resistance spot welded joint are provided. The present invention relates to a resistance spot welding method comprising holding a sheet combination between a pair of welding electrodes, the sheet combination comprising at least two overlapping steel sheets; and applying a current, along with a force, from the welding electrode pair to the sheet combination, thereby joining the at least two overlapping steel sheets. At least one of the steel sheets in the sheet combination is a high-strength steel sheet. The method includes a main current application step and a post-heating tempering treatment step, and the main current application step includes applying a current at a value of Iw (kA).The post-heating tempering treatment stage includes a cooling process and a heating process, and includes at least one transition process and one holding process. In the cooling process, a cooling time tct (ms) is provided. In the heating process, a current is applied at a value of It (kA), shown in formula (2), for a current application time tt (ms). In the transition process, the applied current is continuously reduced from the current value It (kA) to a current value Itm (kA) for a falling current application time ttma (ms). In the holding process, a current is applied to a weld at a current value Itm (kA) for a current application time ttm (ms). 0.8 × Iw = It = 1.6 × Iw ...(2).

Claims

CLAIMS 1. A resistance spot welding method characterized in that it comprises holding a sheet combination between a pair of welding electrodes, the sheet combination comprising at least two overlapping steel sheets; and applying a current, together with a force, from the pair of welding electrodes to the sheet combination, thereby joining the at least two overlapping steel sheets, wherein at least one of the steel sheets in the sheet combination is a high-strength steel sheet having a chemical composition containing, by mass %, C: 0.05 to 0.60% and Mn: 1.5 to 6.0% and having a tensile strength of 780 MPa or higher, the current application comprising a main current application stage and a post-heating tempering treatment stage, the main current application stage comprising applying a current at a current value lw (kA),The post-heating tempering treatment stage includes a cooling process and a heating process and includes at least one of the transition processes and a holding process. In the cooling process, a cooling time tct (ms) is provided, which is shown in formula (1) below. In the heating process, a current is applied at a current value lt (kA), which is shown in formula (2) below, for a current application time tt (ms), which is shown in formula (3) below. In the transition process, an applied current is continuously reduced from the current value lt (kA) to a current value Itm (kA), shown in formula (5) below, for a falling current application time tima (ms), shown in formula (4) below. In the holding process, a current is applied to a weld with the current value Itm (kA).shown in formula (5) below, during a current application time ttm (ms), shown in formula (6) below, 800 < tct ...(1) 0.8 x lw^ 1.6 x lw ...(2) 0 < tt<200 ...(3) 0 < tima < 400 ...(4) 0 < Itrn < 0.90 X lt ...(5) 400 A tt + ttma + ttm ...(6) where, in a case where the transition process is not included, ttma in formula (4) and in formula (6) is 0 ms, and in a case where the holding process is not included, Itm in formula (5) is 0 kA, and ttm in formula (6) is 0 ms., 2. The resistance spot welding method according to claim 1, further characterized in that the chemical composition of the high-strength steel sheet additionally contains, % by mass, one or two selected from a group A and a group B, wherein group A includes Si: 0.1 to 2.0% and P: 0.10% or less, and group B includes one or more selected from Cu, Ni, Mo, Cr, Nb, V, Ti, B, Al and Ca, and a total content of group B is less than or equal to 5%.

3. A method for manufacturing a resistance spot welded joint, the method characterized in that it comprises manufacturing the resistance spot welded joint using the resistance spot welding method according to claim 1 or 2.

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