Effects of hot deformation on austenite transformation in low carbon Mo-Nb and C-Mn steels

The 015C-15 Mn and 007C-03 Mo-0055 Nb steels were subjected to hot rolling to determine the effects on transformation to ferrite and to hot torsion to measure strength. After 25 or 50% reduction at 830°C, the acceleration during isothermal transformation in the range 700-600°C is much greater for C-Mn than for Mo-Nb; delays of 5-10 s before cooling reduce the acceleration markedly. In continuous cooling, three passes of 20% reduction raise the temperature of austenite decomposition, most noticeably at the higher rate 9 K s-1. At this rate, 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 Ar3 enhanced ferrite formation more in C-Mn in the range 780-720°C than in Mo-Nb over 750-690°C. The stronger Mo-Nb in torsion (900-1100°C, 01-5 s-1) exhibits an activation energy of 353 kJ mol-1 compared with 316 kJ mol-1 for C-Mn.