High strength and low yield ratio carbon steel bar processed by thermomechanical rolling

Abstract

Adding alloying elements or refining grain size are the common means to increase the strength of steels. By refining ferrite grains down to ~1μm through thermomechanical rolling, a high strength and low yield ratio (the ratio of yield strength to tensile strength) steel bar with good impact toughness has been developed from an ingot with a similar chemical composition to that of mild steel. The steel bar consisted of large ferrite grains (1 – 15μm), small ferrite colonies (smaller than ~1μm), and martensite. The tensile strength, 0.2% offset yield strength, yield ratio, uniform tensile elongation, and elongation up to fracture of the developed steel were: 856 MPa, 478 MPa, 0.558, 11.6%, and 19.1%, respectively. The absorbed energy in Charpy impact test at -40°C was 134 J. The toughness and strength of the steel can be adjusted by varying the volume fractions of the large ferrite grains, small ferrite colonies, and martensite.