DYNAMIC RECRYSTALLIZATION OF FERRITE IN A MEDIUM-CARBON STEEL WITH TEMPERED MARTENSITE STRUCTURE DURING HOT DEFORMATION

For plain carbon steels, the microduplex (alpha + theta) structure consisting of ultrafine ferrite matrix and dispersed cementite particles demonstrates a good balance between strength and ductility as compared with a normal microstructure, e.g. ferrite plus pearlite in hypoeutectoid steels or pearlite in eutectoid steels etc.. Various thermo-mechanical treatments have been developed to obtain such microduplex (alpha + theta) structure. It is commonly considered that the formation of fine equiaxed ferrite grains during thermo-mechanical treatments involving hot (warm) deformation could be attributed to dynamic recrystallization (DRX) of ferrite. However, the mechanism of DRX of ferrite during the formation of the microduplex (alpha + theta) structure as well as the effect of cementite particles on DRX of ferrite are still not well understood. In the present work, DRX of ferrite in a medium-carbon steel with different tempered martensites was investigated by hot uniaxial compression tests at 0.01 s(-1) and 700 degrees C, and the effect of cementite particles with different initial states was analyzed. The results indicate that during hot deformation the coarsening of cementite particles and DRX of ferrite took place, leading to the formation of the microduplex (alpha + theta) structure. In comparison with static tempering, the diffusions of Fe atoms and C atoms were enhanced by hot deformation and thus the coarsening kinetics of cementite particles was accelerated by 2-3 orders of magnitude. During hot deformation the sizes of cementite particles at the boundaries of ferrite grains increased and the amount of cementite particles in ferrite grains decreased. The former is beneficial to the particle stimulated nucleation under coordinated effects of several particles, and the later can reduce the drag effect of cementite particles on the migration of grain boundaries, both of them are advantageous to DRX of ferrite. With the decrease in the size of cementite particles in the initial microstructures, the critical strain for DRX of ferrite increased, but the resultant microduplex (alpha + theta) structure is fine, and more uniform.