Grain refinement and mechanical properties of asymmetrically rolled low carbon steel

The formation of fine ferrite grains by the asymmetric rolling of low carbon steel and their mechanical properties were studied. Super-cooled low carbon austenite was deformed by asymmetric rolling at 750 °C with a roll size ratio of 1.5 and immediately cooled at various cooling rates ranging from 3 °C/s to 15 °C/s. Fine ferrite grains (∼2 μm) were formed after asymmetric rolling, preferentially at the prior austenite grain boundaries. The volume fraction of the fine ferrite grains increased with increasing rolling reduction. A ferrite plus pearlite microstructure was obtained at smaller strains and slower cooling rates. However, after heavy deformation, a fine ferrite grain structure with carbide particles dispersed at the ferrite grain boundaries was obtained and the pearlite structure was not observed even after very slow cooling, which implies that most of the ferrite grains were formed dynamically, i.e. during deformation. The yield strength of the asymmetrically rolled steel plates increased with increasing deformation; however, the yield ratio also increased with increasing rolling reduction. The best combination of strength and yield ratio was obtained by using a low level of deformation and a high cooling rate, in which case a portion of the untransformed austenite transformed to martensite.