Effect of Intercritical Dislocation Multiplication to Mn Partitioning and Microstructure Evolution of Bainite in Low Carbon Steel

The volume fraction and stabilization of retained austenite at room temperature were determined by the degree of stable element partitioning of austenite. The element diffusion behaviors usually had a close relationship with crystal defects, and dislocation multiplication caused by high temperature deformation might also increase the vacancy concentration, which contributed to the diffusion of substitutional atoms and interstitial atoms. By adopting a new treatment process of intercritical deformation-hold-quenching (DIQ), the effect of Mn partitioning and the structure evolution of bainite under the deformation in intercritical area were studied. The microstructure, dislocation density and distribution of alloy elements, especially the volume fraction of retained austenite, were characterized by means of OM, SEM, TEM, EPMA and XRD. The results indicated that the grains of ferrite and the lathes of martensite were refined, the number of the block martensite was decreased, and dislocation density was increased from 0.36x10(14) m(-2) to 1.20x10(14) m(-2) after deformation. The mutual movement of dislocation slip increased vacancy concentration and the number of interstitial solute atoms, accelerated the diffusive rate of C atoms and Mn atoms from alpha phase to gamma phase, and promoted the partitioning effect of Mn element in the critical region. Eventually, the contents and areas of C and Mn enrichment were increased. By adopting the process of intercritical deformation-hold-austenitizing-quenching-partitioning in bainitic region-quenching (DI&Q&PB), the volume fraction of retained austenite was increased from 11.5% to 13.9%, and the carbon content in retained austenite was increased from 1.14% to 1.28%.