Remarkable improvement in resistance spot weldability of medium-Mn TRIP steel by paint-baking heat treatment

Abstract

In this study, a remarkable improvement in resistance spot weldability of medium-Mn transformation-induced plasticity (MT) steel through a paint-baking heat treatment was addressed using dissimilar resistance spot weldments between MT and dual phase (DP) steels with various soaking temperatures. Experimental results showed that the cross-tension strength in the dissimilar MT/DP weldment increased up to 128% when the soaking temperature was more than 110 degrees C, and was at its highest at 210 degrees C. The improved cross-tension strength is attributed to the failure mode change from a pull-out to a partial pull-out failure. The brittle-to-ductile transition of martensitic matrix in coarse-grained heat-affected zone in MT steel was also considered to be a factor. Austenite reversion and carbide precipitation occurred in the martensitic matrix during the paint-baking heat treatment, whereas there was no significant change in alloying element segregation at the prior austenitic grain boundary. The driving force for the tempering was discussed based on the calculated carbon diffusion distance.