Effect of thermomechanical controlled processing on the microstructure and mechanical properties of Fe-C-Mn-Si multiphase steels

Abstract

In this paper, a three step cooling (first water cooling, subsequent air cooling and second water cooling stage) pattern on the runout table (ROT) was conducted in a laboratory hot rolling mill, and effect of thermomechanical controlled processing (TMCP) on the microstructure and mechanical properties of Fe-C-Mn-Si multiphase steels was investigated. The results showed that the microstructure containing polygonal ferrite, granular bainite and a significant amount of the stable retained austenite can be obtained through three step cooling on the ROT after hot rolling. The first water cooling stage made high density dislocation remain in the austenite, the second water cooling stage avoided pearlite formation, and it is necessary to hold about 10s during intermediate air cooling on the ROT, which contributes to enhancement the stability of retained austenite. Excellent mechanical properties were obtained through TMCP due to the transformation of retained austenite into martensite during straining (transformation induced plasticity) for the present Fe-C-Mn-Si multiphase steels, and the specimen 3 shows the highest value of total elongation (37%) and the balance of strength and ductility (30488 MPa%). © 2006 ISIJ.