Microstructural Evolution of a Multiphase Steel Microalloyed with Vanadium

Multiphase steels have improved mechanical properties and enhanced ductility compared to conventional high strength steels and therefore have the potential for contributing to automotive mass reduction and crashworthiness requirements. The mechanical properties of multiphase steels are strongly dependent of the amount, size and morphology of retained austenite, bainite and martensite. The high work hardening and uniform deformation is a consequence of the Transformation Induced Plasticity (TRIP) effect of the retained austenite during straining. Further improvements in mechanical properties (>1000 MPa UTS) can be produced by precipitation, thus, vanadium additions are being considered. In this work a detailed study of the microstructural and complex precipitation evolution during the intercritical annealing of a multiphase steel microalloyed with vanadium was undertaken. Steels samples of a base composition and two vanadium (0.06% and 0.12%) microalloyed multiphase steels were supplied by Corns at the end of the commercial production route, from two different hot mill coiling temperatures. The microstructure of these five steels was examined in detail and compared to the tensile properties. The addition of vanadium increased the amount of austenite retained at the end of the process in an almost linear fashion. The lower coiling temperature (550°C) gave higher UTS than the higher coiling temperature (640°C), as expected. The expected tensile strength (UTS) increment was produced by vanadium additions but in all cases there was a reduction in ductility. The increase in UTS appeared to be mainly a result of grain size refinement, with some contribution from precipitation hardening in the 0.12%V steel, rather than the effect of the additional retained austenite content. A laboratory based simulation of the commercial intercritical anneal and bainite transformation was constructed, comprising 7 stages, including the intercritical anneal 795 °C (20 seconds), and an isothermal bainitic transformation at 460°C for 5 seconds. Steels identical to those produced by the commercial process, but in the hot and cold rolled condition were investigated. The vanadium carbide precipitation, transformation/dissolution and overall microstructural change at each stage of the annealing process were investigated. The majority of precipitation of VC occurred during hot coiling, with the lower coiling temperature leading to a smaller VC size distribution. The VC strongly controlled the rate of ferrite recrystallisation and final ferrite grain size on heating to the intercritical anneal following the cold rolling stage. Some further precipitation occurred during heating, but the extent was small compared to that which occurred on coiling. At the end of the intercritical anneal, the vanadium addition was found to have three main effects, namely, a reduction in microstructural scale, an increase in the retained austenite content and a reduction in the carbon content in the retained austenite. A subsequent intermediate cool resulted in a reduction in retained austenite for all materials, with this mainly resulting in transformation to martensite, but also bainite for the base alloy only. The isothermal bainite transformation resulted in bainite formation, but no enhancement of the carbon content in the remaining austenite. The amount of bainite decreased with increase in vanadium addition. The final total volume fraction second phases was similar for all materials, demonstrating that the vanadium addition had altered the proportion of phases only, namely increasing austenite content and decreasing bainite content. The final vanadium carbide size distribution was determined more by the vanadium content than the coiling temperature. The carbon content of the final austenite varied with vanadium addition in the same way as the commercial alloys. The principal role of the vanadium addition is discussed