Strengthening and toughening of low-carbon high-strength steels by ausforming

摘要

The effects of ausforming on the mechanicalproperties of the martensite and the martensite/bainitestructure in low carbon high-strength steels have beeninvestigated. Ausforming is a quenching operation from work-hardened austenite and it is applied to steels with relativelyhigh hardenability. In recent years, direct-quench andtempering process gradually came into practice and enabled toquench work-hardened austenite from high temperature aboveAc3 and to apply ausforming to low-carbon high strengthsteels with relatively low hardenability. However heavydeformation deteriorates hardenability of work-hardenedaustenite heavy deformation deteriorates hardenability ofwork-hardened austenite and results in anisotropic propertiesin rolled plates. Therefore, in this investigation, deformationbelow the recrystallization temperature was restricted to notmore than 30%. The martensite transformed from work-hardened austenite deformed at temperatures below therecrystallization temperature has higher dislocation densitythan the conventional quenched martensite, and hardnessincreases proportionally with increasing deformation. Theseresults are also indicated in the case of the martensite/bainitestructure, strengthening by modified ausforming can beachieved in martensite or martensite/bainite structure. Theausformed achieved in martensite or martensite/bainitestructure. The ausformed martensite structure after temperingexhibits much lower fracture appearance transitiontemperatures than the conventionally quenched martensitestructure. The improvement in toughness of these structures ismainly due to the change of the direction of a cleavage crackat the packet of ausformed martensite laths.