Effect of rolling temperature on microstructure and mechanical properties of 18%Mn TWIP/TRIP steels

The microstructure and mechanical properties of Fe-18Mn-0.6C and Fe-18Mn-0.4C steels subjected to rolling with a total reduction of 60% at temperatures of 773 K to 1373 K were investigated. Warm rolling at temperatures of 773-1073 K resulted in flattened grain structure with the transverse grain size of 7 µm, whereas hot rolling at 1073-1373 K was accompanied by the dynamic recrystallization leading to almost equiaxed grains, the size of which increased with temperature. Internal stresses and corresponding dislocation densities increased as the rolling temperature decreased. The steel with higher carbon content exhibited finer grains and higher dislocation density after rolling. A decrease in the rolling temperature from 1373 K to 773 K resulted in a significant increase in the yield strength from about 300-400 MPa to 850-950 MPa, while ultimate tensile strength increased from 1000-1100 MPa to 1200-1300 MPa (the higher strength corresponds to higher carbon content). On the other hand, the total elongation decreased from approx. 85% in the Fe-18Mn-0.6C steel and from 65% in the Fe-18Mn-0.4C steel to approx. 30% in the both steels as the rolling temperature decreased from 1373 K to 773 K. The difference in the tensile behavior at room temperature was attributed to the variation in the deformation mechanisms. Namely, mechanical twinning operated in the both steels during tension, whereas ε-martensite formation took place in the Fe-18Mn-0.4C steel.