Study of Bak-hardening of ultra-low carbon BH 220 steel at different strain rates

Abstract

Automotive process adopts bake hardening technique during the paint baking cycle at elevated temperature for enhanced strength of steel by the diffusion of Carbon atoms to the dislocations along with drying of paint. The present study was intended to investigate the bake-hardening (BH) behaviour of ultra-low carbon (0.0027 wt. % C) BH220 steel with different pre-strain values and to justify the work by dislocation density measurements. The specimen were subject to tensile pre- strain of 2%, 4%, 6% and 8% at room temperature followed by bake hardening at pre- defined temperature of 170¢ªC for 20 minutes. Specimens with a particular pre-strain and pre-strain with bake hardening were subjected to monotonic tensile load at different strain rates (0.001, 0.1 and 100/s). Dislocation density was estimated by X-ray diffraction technique. The result reveals that in case of quasi static strain rate tests, pre-straining increases the yield strength compared to the as received steel in the range of 20-40% , wheres in case of same pre-strain and bake harding combination it is in the range of 30-60%. Similar increase in ultimate tensile strength and bake hardening response was observed. Similar trend was also observed in case of high strain rate tests. This can be attributed towards pinning of dislocation by bake hardening. In dislocation density study, increase in dislocation density was observed with increase in pre-strain level and as well as in strain rates.