Fundamental study on mechanical properties of high strength steels processed by quenching and partitioning

The novel quenching and partitioning (Q& P) process offers a good alternative to produce high strength and at the same time, formable steels based on the development of a microstructure consisting of martensite and retained austenite. This research is focused on the enhancement of formability while maintaining high mechanical strength using the feature of the microstructure developed after Q& P treatments for three commercially produced steel grades. The investigation had been carried out on the Si-alloyed TRIP780 steel and the boron steel grades 22MnB5 and 27MnCrB5. The investigated steels were fully austenitized with subsequent quenching to a temperature below the martensite start temperature Ms, followed by isothermal holding at the same temperature (one-step) and by reheating to a higher temperature (two-step). In case of the Si-alloyed TRIP steel, the amount of retained austenite could be adjusted due to Q& P treatments. Whereas for the boron steels, cementite formation at high partitioning temperature and carbides precipitation at low temperature suppress the stabilization of retained austenite. The largest fraction of stable austenite and the best strength-ductility combination for the TRIP780 steel were achieved after two-step Q& P treatments. For 22MnB5 and 27MnCrB5 steels, one-step Q& P treatments could improve the combination of strength-ductility, whereas two-step Q& P treatments resulted in a considerable loss in strength without further enhancement in ductility. Therefore, the improvement in ductility of the investigated boron steels without Si addition gained after Q& P processing is mainly owing to progressive tempering of martensite.