Development of a high-strength low-alloy steel strengthened by transformation substructure and precipitation of copper and niobium

Abstract

A high-strength low-alloy (HSLA) steel with strength level in the range 830–1030 MN/m² (120 000–150 000 lb/in²) has been developed by combining precipitation strengthening due to copper and niobium, and transformation substructure strengthening due to manganese and molybdenum. Hardness, tensile, and impact properties have been determined for different finish-rolling temperatures and for different rates of cooling. Electron microscopy has been used to follow microstructural changes and their relation to the variations in the mechanical properties. Results indicate that strengthening due to niobium is additive to that of copper and that high strength levels, coupled with adequate ductility and low transition temperatures (<−70°C), are realized under optimum processing conditions. Strength-structure relations and the effect of processing and aging conditions on these relations have been analysed qualitatively in terms of the effect of alloying additions on precipitation and transformation kinetics. Finally, it appears that there is considerable scope in extra-low-carbon HSLA steels for the utilization of higher amounts of copper, niobium, and manganese than the normal practice, and nominal additions of molybdenum.