Structure-property relationship in a 2 GPa grade micro-alloyed ultrahigh strength steel

Ultrahigh strength steel is designed for light-weight cost-effective parts with improved safety by dint of high strength coupled with better fuel efficiency for automotive and aerospace applications. The current steel with medium carbon (0.28 wt%) and alloying elements (similar to 8%) such as Mn, Ni, Mo along with microalloying of Ti and Nb has been processed by thermo-mechanical controlled processing with different finish rolling temperature (FRT) followed by air cooling and water quenching. Microstructure of air cooled specimen consists of lower bainite with a few amount of lath martensite, whereas a mixture of lath martensite and lower bainite in prior pan-caked austenite grains has also been obtained on water quenching. It is observed that microstructure becomes gradually finer with reduction of FRT for both cooling conditions and fine-scale carbide/carbonitride precipitates, enriched with Ti/Nb along with dislocation sub-structures within bainite and martensite laths have performed as strengtheners. Ultrahigh strength (1991-2032 MPa) with lower elongation (5-7%) is obtained in water quenched samples, whereas air cooled samples show comparatively lower strength (1841-1928 MPa) with marginally higher elongation (8-9%). Different strengthening mechanisms have been quantified which cumulatively contribute to the yield strength of the current steels. (C) 2017 Elsevier B.V. All rights reserved.