Effect of hot rolling temperature on the microstructure and mechanical properties of ultra-low carbon medium manganese steel

The effect of hot rolling temperature on microstructural evolution and mechanical properties of 0.05C-5Mn (wt%) steel was investigated. With the decrease of hot rolling temperature from 1150 degrees C to 900 degrees C, the average size of prior austenite grain was decreased from similar to 40 mu M to similar to 20 mu m, and average width of martensite lath was refined from similar to 400 nm to similar to 250 nm. The morphology of reversed austenite was changed from film-like shape to block-like shape, and the volume fraction of reversed austenite was increased from 14.75% to 16.28% because of the stimulated nucleation in refined prior austenite grains by rolling at lower temperature. The yield strength, tensile strength and the total elongation were simultaneously increased from 673 MPa, 805 MPa, and 27.5% to 704 MPa, 846 MPa, and 31%, respectively. The low temperature impact toughness was significantly enhanced from 133J to 222J at - 40 degrees C. The significant improvement in mechanical properties by lower hot rolling temperature were attributed to both the refined tempered martensite lath with higher density of high misorientation grain boundaries and the active TRIP effect of reversed austenite with suitable mechanical stability.