Third generation high strength low alloy steels with improved toughness

Abstract

On-line thermo mechanical controlled processing (TMCP) was conducted to develop the third generation high strength low alloy (HSLA) steel with high toughness economically. The ultra-low carbon content ensured a high level of upper shelf energy while ultrafine lath martensitic structure transformed from super-thin pancaked austenite during controlled rolling and cooling. The reduction of martensite block size decreased ductile-to-brittle transition temperature (DBTT) and compensated the strength loss due to carbon reduction. Consequently, the excellent balance of strength and toughness values was obtained as 950–1060 MPa for yield strength, 180 J for Charpy V-notch impact absorbed energy at −30°C, which is much superior to that of traditional martensitic steel. Two mechanisms for the refinement of lath martensite block were proposed: One is the austenite grain refinement in the direction of thickness, and the other is the reduction in the fraction of sub-block boundaries with small misorientation and the increase in the fraction of block boundaries with large misorientation, possibly due to austenite hardening.