Microstructure evolution and carbide precipitation behavior of microalloyed TS800TB steel during hot rolling and coiling processes

A Nb–Ti–Mo–V–Cr complex microalloying has been applied to improve the strength and formability of TS800TB microalloyed low carbon steel for automobile torsion beam applications. The effects of microalloying element and coiling temperature on microstructure evolution, carbide precipitation behavior, mechanical property and formability have been studied. Meanwhile, the influence of microalloying element and coiling temperature on carbide precipitation behavior has been analyzed from the aspects of thermodynamic and kinetic. The results suggest that the coarse ferrite grains and the low temperature transformation structures of lath ferrite and martensite are the main causes for the cold bending cracking of Nb–Ti–Mo–V–Cr bearing hot rolled plate. Microstructure characterization for carbide precipitation behavior in Nb–Ti–Mo–V–Cr bearing steel indicates that the precipitation of Nb was inhibited during hot rolling with addition of Mo, V, and Cr in steel. The precipitation kinetics shows that the fastest precipitation temperatures of Ti(C,N) and (Nb,Ti)C in austenite are 1150 °C and 920 °C, respectively. Hence the (Nb,Ti)C carbides are not fully precipitated in hot rolled plate due to the lower precipitation temperature and the fast laminar cooling after finish rolling. The fastest precipitation temperatures of (Nb,Ti)C and VC type carbides in ferrite are 640 °C and 740 °C, respectively. Therefore, the carbides containing Nb, Ti, V, Mo, and Cr elements precipitated during tempering process at the temperature between 650 °C and 730 °C. Meanwhile, the number of fine carbides with size less than 10 nm increases significantly with the onset of precipitation of carbides containing V and Mo. Since the Mo significantly enhances the nucleation rate of carbides by reducing the interfacial energy and strongly inhibits their coarsening. Carbides containing Ti, Nb, V, Mo, and Cr elements in hot rolled plate not only nucleate and precipitate on the previously precipitated Ti(C,N) and (Nb,Ti)C particles, but also nucleate and precipitate independently in the matrix during tempering. The results of this study indicate that precipitation strengthening and dislocation strengthening are two major strengthening mechanisms of microalloyed TS800TB steels. The precipitation strengthening increases with increasing tempering temperature due to the precipitation of small size carbides. However, due to the reduction of dislocation density, the overall strength decreases.