Combined thermo-mechanical controlled processing and dynamic carbon partitioning of low carbon Si/Al-Mn steels

A novel strategy involving thermo-mechanical controlled processing (TMCP) and quenching to obtain martensite (Q&P) or bainite (B&P) followed by dynamic carbon partitioning was applied to two low carbon steels. Effect of cooling path on the microstructural evolution was studied with particular focus on dynamic partitioning, and the impact fracture behavior was observed and discussed based on the duplex structures, especially the retained austenite (RA). It was found that majority of the carbides were present in Al contained steel subjected to B&P process, leading to low fraction of RA ~ 5.1%, while ~ 9.2 to 20.1% RA was obtained in Si contained steel subjected to Q&P and B&P process. The impact energy at 20 °C of B&P steels exceeded 100 J, which was not related to RA. However, the impact energy at 20 °C of steels subjected to Q&P process indicated a large difference with impact energy of ~ 43 J to 117.1J, which resulted from carbon content in martensite and the RA fraction. The steel quenched to 380 °C followed by furnace cooling contained 14.7% RA and 5.5% RA experienced TRIP effect with transformation product of twinned martensite, which resulted in excellent impact toughness of ~ 106.6 J. During impact process, the micro-voids preferentially formed at ferrite/hard phases interface and in ferrite. In particular, the micro-voids also formed in martensite with high carbon concentration and thus led to low impact toughness of ~ 43 J. The bainite lath, M/A (martensite-austenite) island and RA acted as barriers and thus increased the resistance to crack propagation. During low temperature impact, the blocky RA has low thermal stability and is partially transformed leading to approximately 40 J decrease at 0 °C, while the microstructure of martensite lath and film-like RA exhibits high impact energy ~104.4 J at 0 °C.