Direct-quenched and tempered low-C high-strength structural steel: The role of chemical composition on microstructure and mechanical properties

The direct quenching of low-carbon steels after thermomechanical processing on hot strip mills is able to produce both strong and tough coiled plate without the need for subsequent tempering. The process is energy and time efficient with relatively low emissions when compared to conventional reheating, quenching and tempering. For some applications, however, it is desirable to combine direct quenching with tempering, and, bearing in mind the form of the semi-finished product, it is of interest to study the effect of tempering whole coils in a bell furnace. Here, the effects of boron, carbon, titanium, vanadium and tempering temperature on the microstructure, crystallography and mechanical properties of direct-quenched steels has been studied with the aid of simulated bell furnace heating and cooling cycles. All steels contained (in wt.%) 0.2Si-1Mn-1Cr-0.65Mo-0.03A1, while there were two levels of C (0.095 /0.140), V (0 /0.08), Ti (0 /0.025) and B (0 /0.0015). Tempering was performed with peak temperatures at 180 and 570 degrees C. The paper reveals several possible alloying and processing routes to strong and tough low-C steel. Carbon controls the strength and toughness, while titanium and boron affects the grain size of coarsest grains (d(90%)), Vanadium has a strong effect on strength retention during tempering at 570 degrees C: an addition of 0.08 wt% vanadium increases yield strength by 70 MPa and ultimate tensile strength by 100 MPa. The removal of boron from the steel is shown to have a huge impact not only on the microstructure but also on the impact toughness.