Bake-Hardening Response of High Martensite Dual-Phase Steel with Different Morphologies and Volume Fractions

Bake-hardening behaviour of carbon steel with different martensite morphologies and volume fraction was investigated. The specimens with fibrous and bulky martensite were prestrained in tension by 4%. After this, they were unloaded and bake hardened at 180 degrees C for 10-160 mm. It was found that dual-phase steel samples which were bake hardened at 180 degrees C for 20 min showed an increase in the yield stress (YS) and ultimate tensile stress (UTS) but a decrease in ductility. Further increase in the bake-hardening time of 80 or 160 min has reduced the YS and UTS, but increased the ductility. Delta sigma (increase in stress due to bake hardening), YS and UTS values are higher for the microstructure containing fibrous martensite compared to the microstructure-containing bulky martensite. It was also observed that at a given baking temperature Delta sigma, YS and UTS increased by volume of martensite.