Spontaneous Reverse Transformation due to Adiabatic Deformation Heating in 0.3%C-9%Ni Steel

A new concept for ultra refinement processing in austenite grain size; Spontaneous Reverse Transformation (SRT) due to adiabatic deformation heating has been proposed. The occurrence of SRT was verified using a 1000 ton laboratory mill. 0.3wt%C-9wt%Ni steels with configuration of 50×50×300mm were heated to 550°C which is lower than cementite dissolution temperature. The microstructure was tempered martensite before rolling. Reheated steels were heavily rolled with a total reduction of 60%, 70% and 90% under a rolling speed of 50 m/min. Measured plate surface temperature rose up to 675°C which was higher than Ae3 temperature in the case of 90% rolling reduction. SRT occurred only in the mid thickness region for 70% rolling reduction, while all through the thickness for 90% rolling reduction. The microstructure of the region in which SRT occurred consisted of a fresh and fine martensite with high dislocation density and cementite was no longer observed in this region. The size of grains surrounded by high angle boundaries was roughly estimated as around 0.5 μm, which was considered to be either prior γ grain size or martensite packet size. Deformation analysis incorporated with thermal analysis revealed that SRT occurrence behavior was well explained by temperature distribution along the plate thickness caused by adiabatic deformation heating and roll chill.