Unexpected Effect of Nb Addition as a Microalloying Element on Mechanical Properties ofδ-TRIP Steels

摘要

The concept of microalloying was applied to theδ-TRIP (transformation-induced plasticity)steel to inves-tigate the feasibility of increasing the mechanical properties and understanding the effect of microalloying on the morphology and structure of the steel.A hot rolledδ-TRIP steel with three different contents of Nb (0,0.03, 0.07 mass%)was subjected to the microstructural and mechanical examination.The high Al and Si concentration in these steels guaranteed the presence of the considerableδ-ferrite phase in the microstructure after the casting and the subsequent hot rolling.The obtained results showed that Nb dramatically affects the microstructure,the dynamic re-covery and recrystallization behavior,as well as the grain shape and thus the stability of austenite after the thermo-mechanical process of hot rolling.The results also revealed an unexpected effect of Nb on the mechanical properties. The addition of Nb to theδ-TRIP steel led to a significant decrease in the ultimate strength (from 1 144 to 917 MPa) and an increase in ductility (from 24% to 28%).These unconventional results could be explained by the change in the steel microstructure.The work-hardening behaviors of all samples exhibit three stages of the work-hardening rate evolution.At the stage 2,the work-hardening rate of the studied steels increased,being attributed to the TRIP effect and the transformation of austenite to martensite.