Multiscale Microstructure Evolution and Subgrain Strengthening of Warm-Rolled Ultrafine Pearlite Steel

To optimize the synergy between the strength and ductility of ordinary medium-carbon steel, a pseudo-eutectic structure with ultrafine pearlite and pre-eutectic ferrite was formed by pretreatment and a multiscale lamellar structure was obtained by subsequent warm rolling. The multiscale lamellar structure is composed of cross-scale ferrite and granular or short-rod cementite. With increasing rolling temperature, the width of shear bands and the average grain size gradually increased, while the length of grain boundaries decreased. The average grain size of the steel after warm rolling at 550 & DEG;C to 600 & DEG;C is less than 0.57 & mu;m, accompanied by a large number of subgrain/grain boundaries. And most of the cementite is short rod, the tensile strength is higher than 1.2 GPa and maintain a good elongation. The analysis of subgrain boundaries strengthening using geometrically necessary dislocations (GNDs) reveals that the additional strengthening of the warm-rolled specimens is mainly derived from subgrain boundaries of 5 to 10 deg. The formation of ultrafine shear bands within the coarse grains during warm rolling induced a gradient change in the Schmid factor and GNDs, leading to enhanced uniform elongation during plastic deformation.