Achievement of High Ductility and Ultra-high Strength of V-Nb Microalloyed Spring Steel by Austempered Multiphase Microstructure

A V-Nb microalloyed spring steel was investigated through a series of austempering heat treatments. Optimal properties were obtained after austempering at 240 °C for 2 hours because of the good coordination of multiphase martensitic/bainite, filmy retained austenite and fine carbides. The tensile strength was 2239 MPa and the total elongation was 16.1 pct. With an increase in temperature, the matrix changed from martensite to bainite. The volume fraction of retained austenite decreased from 13.7 pct at 240 °C to 9.6 pct at 280 °C and then increased to a maximum of 19.2 pct at 360 °C. The tensile strength remained above 1500 MPa and the impact energy increased to 30 J. MC carbides formed in martensite lathes and ɛ- or θ-type carbides with acicular nanoscale were precipitated in bainite lathes at a low temperature. MC- and M₃C-type precipitated in bainite lathes at high temperature. Samples that were subjected to a low isothermal temperature had a high strain-hardening rate before necking, whereas samples at a high isothermal temperature could exert a TRIP effect for a long time. After stretching to fracture, a hierarchical nanotwinned structure and many shear cells formed, which enhanced the austempered sample ductility and toughness at 360 °C.