Microstructure evolution and static recrystallization kinetics in hot-deformed austenite of coarse-grained Mo-free and Mo containing low-carbon CrNiMnB ultrahigh-strength steels

The static recrystallization characteristics and microstructure evolution in hot-deformed austenite were evaluated for a newly developed low-carbon CrNiMnB ultrahigh-strength steel with and without molybdenum addition. The time for 50% static recrystallization (t50%) over a wide range of strains and hot-deformation temperatures were obtained using the stress-relaxation technique on Gleeble thermomechanical simulator. Moreover, effect of deformation parameters on the size distribution and average size of prior austenite grains are investigated. A novel semi-automatic stress relaxation test reading tool with a graphical user interface was created and used successfully for the current study. The obtained results of strain´s power and the apparent activation energy are within the range stated in literature for C-Mn and microalloyed steels. Addition of molybdenum increase the power of strain and the apparent activation energy from − 1.9 to − 2.6 and 206 to 212 kJ/mol, respectively. The retardation effect of molybdenum addition was shown by a new regression equation devised for calculating t50%. The developed equations show a good agreement with the experimental data and can be used in the designing of roughing during thermomechanical processing. The deformation parameters i.e., temperature, strain and holding time have a significant effect on the size distribution and average size of prior austenite grains.