2007 Volume 47 Issue 11 Pages 1638-1646
The thermomechanical parameters have a great influence on the restoration processes of the steels and, consequently, on the grain size and morphology during and after deformation. At higher deformation temperatures the ferritic grain refinement may occur by dynamic recrystallization of austenite (DRX) and at lower temperatures, the operating mechanism might be dynamic strain-induced austenite–ferrite transformation (DSIT). In the present work, the effect of thermomechanical parameters on DSIT was investigated. In this study, after soaking at 1200°C, the samples were cooled to 1100°C submitted to hot torsion deformation to decrease the austenite grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation, at strain rates of 1 s−1 and 5 s−1 and total strains of 1.5, 2.5 and 3.5. The steels were deformed by torsion in a ‘Gleeble’ thermomechanical simulator and water cooled immediately after deformation. In the steel without Nb, recrystallization occurred before enough deformation could be accumulated to induce ferrite formation, so DSIT would only occurred at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallization allowing DSIT ferrite to form at higher temperatures than the steel without Nb, 850°C. The dynamic recrystallization of austenite at 850°C was retarded by the Nb addition thus allowing the occurrence of DSIT at this temperature. Variations in strain rate and amount of strain did not significantly change the dynamic mechanism of grain refinement, but did change the critical strain to start DRX and DSIT, as well as the fraction of the strain-induced transformed ferrite.
