Study of Bake-hardening behaviour 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 diffusion of carbon atoms to the dislocation along with drying of paint. The present study was intended to investigate the bake-hardening (BH) behaviour of ultra low carbon (0.0027wt % C)BH220 steel with different pre-strain values and to justify the work by dislocation density measurements. the specimen were subjected to tensile pre-strain of 2%, 4%, 6% and 8% at room temperature followed by bake hardening at predefined temperature of 170C for 20 minutes. Specimens with a particular pre-strain and prestrain 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 difraction technique. The results reveals that in case of quasi static strain rate tests, pre-straining increases the yeild strength compared to the as received steel in the range of 20-40%, where as in case of same prestrain and bake hardening combination it is in the range of 30-60%. Similar increase in ultimate tensile strength and bake bardening 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.