Comparison of Novel 1 GPa Low-Carbon, Low-Alloyed Steel Produced with Simulated and Laboratory-Scale Thermomechanical Controlled Processes

Abstract

A laboratory-scale hot-rolled Ti–Mo–V–Nb steel with 1 GPa tensile strength is produced, and its microstructure and tensile properties are characterized using advanced analysis techniques and uniaxial tensile testing. A Gleeble 3800 thermomechanical simulator is used to determine a process window for the thermomechanical controlled processing (TMCP) procedure. Although the simulated TMCP specimens are fully ferritic at coiling temperatures (CT) of 590 and 630 °C, the bainitic and mixed (bainitic + ferritic) microstructure is formed in the hot-rolled steels. The variation in the microstructure causes variations in the dislocation density through the sheet thickness, which significantly reduces the steel's ductility properties, whereas a 16% elongation is achieved with the fully bainitic microstructure. Another significant difference between the simulated TMCP and hot-rolled specimens is the precipitation behavior. No nanosized interphase-precipitated (IP) carbides are formed in the hot-rolled steel during the austenite-to-ferrite phase transformation, although the formation of the nanosized spherical IPs is observed within the polygonal ferrite grains of the simulated TMCP specimens at the CT of 630 °C. Relatively coarse (5–20 nm) spherical (V,Mo,Ti,Nb)C carbides do not strongly affect the tensile properties of the hot-rolled Ti–Mo–V–Nb steel. The results show that the dislocation and grain boundary strengthening mainly contribute to the strength properties of this steel.