Property of laser welded bake-hardening steel in tailored blanks for automobile

The behavior of bake-hardening of the laser weldment was investigated. The bake-hardening steel(BH steel) was welded with Nd:YAG laser followed by plastic deformation and subsequent heat-treatment. Then the influence of laser welding on the behavior of bake-hardening was investigated. The hardness of the laser weld metal significantly increased after welding. After the plastic deformation, both the base metal and weld metal became harder by work-hardening. The heat treatment resulted in more increment of hardness in both the base metal and weld metal by bake-hardening. The amount of bake-hardening reached a maximum value at the plastic strain of 5% or more. We modified a kinetic equation proposed for predicting the strength of a low-carbon bake-hardening steel and applied to the estimation of hardness of the base metal and weld metal. The calculated hardness values agree with the experimental data. The calculated activation energy for bake-hardenig was that for diffusion of carbon and nitrogen atoms in alpha-Fe. Thus the hardening is thought to be governed by diffusion of these solute atoms.