Thermomechanical and thermodynamic behavior of deformed austenite in four different steel grades

In this work, the deformation behavior and thermodynamic aspect of four different steel grades were analyzed. This group of alloys includes a plain C–Mn, and three Nb-microalloyed steels grades. Two of the microalloyed steels are pipeline grades: a modified X-70 grade, and a typical X-80 steel. These materials were previously identified to produce dynamically transformed ferrite (combined with recrystallized grains) during physical simulation of hot rolling in the single austenite phase field of their phase diagrams. For C–Mn and microalloyed steels, hot deformation at a stain of 0.4 and strain rate of 1 s−1 generates 15 and 25 vol% of ferrite, respectively. In addition, the X-70 and X-80 steels yield 19 and 8 vol% of ferrite, respectively, after deformation strains of 0.4 and 0.2, and strain rate of 1 s−1. All the deformation was applied at least 53 °C above their respective Ae3 temperatures. The concept of transformation softening was applied to explain the occurrence of dynamic transformation during hot torsion tests. Here the driving force to dynamic transformation (DT) was taken by measuring the difference between the critical stress to DT in the austenite phase, and the yield stress of the ferrite that takes its place. The obstacle energy consists of the free energy between the parent and product phases and the shear accommodation and lattice dilatation work associated with phase transformation of austenite to ferrite. It is shown here that, for all the selected materials, the calculated driving force is higher than the energy obstacles, which thermodynamically explains the occurrence of DT.