Factors to Determine Static Strengths of Spot-weld for High-strength Steel Sheets and Developments of High-strength Steel Sheets with Strong and Stable Welding Characteristics

摘要

Considering an application for automotive body structure, variations in TSS and CTS for high-strength steel sheets are summarized with their dependence on sheet strength and thickness, steel chemistry, and coating. In case of so-called "button fracture", both of TSS and CTS seem to be proportional to the strength and thickness of sheets and nugget diameter, but the proportional constant is not settled. Proportional constant for TSS decreases with increasing sheet thickness, but does not vary with sheet strength and existence of coating. Contrarily, proportional constant for CTS remains same with varied sheet thickness, but decreases with increasing sheet strength. Proportional constant for CTS is larger in GA steel sheets than that in cold-rolled steel sheets. Both of TSS and CTS decrease with increasing carbon content. Increasing silicon content decreases TSS, but increases CTS. Above-mentioned knowledge gives a series of high-strength steel sheets with excellent characteristics for spot-welds up to a tensile strength of 980MPa, but a large decrease in CTS cannot be avoidable if expulsion occurs in welding.