EFFECTS OF HOT DEFORMATION ON AUSTENITE TRANSFORMATION IN LOW-CARBON MO-NB AND C-MN STEELS

The 0.15C-1.5Mn and 0.07C-0.3Mo-0.055Nb steels were subjected to hot l olling to determine the effects on transformation to ferrite and to ho t torsion to measure strength. After 25 or 50% reduction at 830 degree s C, the acceleration during isothermal transformation in the range 70 0-600 degrees C is much greater for C-Mn than for Mo-Nb; delays of 5-1 0 s before cooling reduce the acceleration markedly. In continuous coo ling, three passes of 20% reduction mise the temperature of austenite decomposition, most noticeably at the higher rate 9 K s(-1). At this r ate, for. the C-Mn the pearlite is refined and for Mo-Nb the promoted ferrite is refined more and enhanced over bainite than at lower rates. Intercritical rolling below Ar, enhanced ferrite formation more in C- Mn in the range 780-720 degrees C than in Mo-Nb over 750-690 degrees C . The stronger Mo-Nb in torsion (900-1100 degrees C, 0.1-5 s(-1)) exhi bits an activation energy of 353 kJ mol(-1) compared with 316 kJ mol(- 1) for C-Mn.