Effect of the thermal cycle on the hot ductility and fracture mechanisms of a C–Mn steel

Transverse cracking on the surface of continuously cast steel products has been one of the main problems of this stage in steelmaking for many years. The incidence of this problem has been found in microalloyed steels as well as in some plain carbon steels containing residual elements. In this work, the hot ductility and fracture mechanisms of a C–Mn steel containing 0.6%Cu and 0.053%Sn as residual elements have been evaluated. To simulate the thermo-mechanical conditions of the straightening operation, tensile tests were carried out at temperatures ranging from 700 to 1100 °C with an initial strain rate of 5 × 10−3 s−1. Specimens were subjected to three different reheating temperatures prior to the hot ductility test, including 1100 °C, 1330 °C and melting. After each test, the reduction in area of the samples tested to fracture was used as a measure of the hot ductility. The fracture surfaces were then examined by scanning electron microscopy. The widest and deepest ductility trough was obtained for the specimens tested after melting; for these conditions brittle fractures are interdendritic showing very low ductility. After reheating at 1330 °C, fracture features showed intergranular fracture combined with some plastic deformation corresponding to the test temperature. Reheating at 1100 °C produced a finer microstructure and the fracture features showed a mixture of intergranular with some interdendritic features. Also, ductile behaviours were associated with void coalescence. The different results obtained depending on the thermal cycle can be attributed to the presence of the residual elements in the steel through different segregation and precipitation patterns.