Improving strength-toughness of low carbon bainitic microalloyed steel via tailoring isothermal quenching process and niobium microalloying

This study focuses on improving the strength-toughness properties of low carbon bainitic steel through the optimization of the isothermal quenching process and the adoption of a microalloying strategy. The impact of Nb microalloying on the microstructure and mechanical properties was thoroughly investigated. The optimal isothermal quenching process for achieving a balance between strength and toughness is as follows: the steels were austenitized at 900°C for 30 minutes and then subjected to an isothermal treatment at 450°C for 10 minutes. The resulting microstructure consisted mainly of granular bainite (GB) and lath bainite (LB), with GB accounting for 60% of the structure. After the optimized isothermal quenching process, the ultimate tensile strength and yield strength of the Nb microalloyed steel increased to 1210 MPa and 693 MPa, respectively. The impact energy, KU2, was measured at 41.8 J. The study revealed that Nb microalloying refines the bainite structure by reducing activation energy and increasing the nucleation rate. This refinement was observed in the reduced length and thickness of the bainitic laths, as well as the increased dislocation density. Additionally, fine composite precipitated particles (Nb, V)C with an average diameter of 8.2 nm were dispersed throughout the microstructure. The addition of Nb significantly enhanced the effects of fine grain strengthening, precipitation strengthening, and dislocation strengthening in low carbon bainitic microalloyed steel. As a result, the strength was greatly improved without sacrificing toughness, thanks to the combined actions of these three strengthening mechanisms.