Evaluation of phase transformations and mechanical properties in a copper bearing high-strength low-carbon steel microalloyed with Nb

Abstract

In this study, the phase transformations and mechanical properties of a Cu-bearing high-strength low-carbon (HSLC) steel microalloyed with Nb were investigated. The phase transformations were studied by dilatometry and then the microstructures were evaluated. Mechanical properties were measured after various cycles of heat treatment. The results showed that the microstructure of this steel, at the cooling rates studied here, includes a mixture of bainite and martensite. The relatively high amounts of the substitutional elements such as Ni, as well as the low carbon content, resulted in the proximity of the bainite and martensite start temperatures. The Bs and Ms temperatures were determined to be 325 and 315 °C, respectively. This led to the formation of coalesced bainite in the microstructure. The maximum hardness of the alloy was obtained at the aging temperature of 450 °C. Additionally, the optimal combination of yield strength and impact energy was obtained by a single-stage aging treatment at 575 °C for 1 h. The reason for the toughness improvement can be attributed to the formation of the precipitated austenite between the martensitic lathes during the aging process.