Effect of composition and isothermal holding temperature on the precipitation hardening in Vanadium-microalloyed steels

The beneficial effect of increasing the C and N contents on precipitation strengthening in V-microalloyed steels is well known. However both C and N levels cannot be increased simultaneously as that can hamper the ductility and weldability of the steel. Therefore either higher C level or higher N level needs to be maintained. The present study is undertaken to understand the effect of C and N contents on the microstructure and precipitation in flat-rolled strips of V-microalloyed steels. During industrial processing, hot-rolled strips are subjected to a coiling treatment where V-precipitation primarily occurs. Therefore, besides steel composition, the effect of coiling temperature on the evolution of microstructure and precipitation in V-microalloyed steels needs to be studied. In order to achieve these objectives, two different V steels (0.05 wt% V), one having higher C and lower N content (HCLN steel) and the other having lower C and higher N content (LCHN steel) were prepared and subjected to hot-deformation followed by isothermal holding treatments over a temperature range of 500-750 °C inside Gleeble® thermo-mechanical simulator. The purpose of isothermal holding study was to simulate the industrial coiling treatment. Simulated samples were characterized in detail and the maximum precipitation strengthening from fine V(C,N) precipitates was obtained at an intermediate range of isothermal holding temperatures (600-650 °C) in both the steels. In spite of the relatively coarser ferrite grain size and smaller fraction of pearlite and bainite, the yield strength of LCHN steel was close to HCLN steel and LCHN steel also showed superior ductility.