Effect of isothermal holding temperature on the precipitation hardening in Vanadium-microalloyed steels with varying carbon and nitrogen levels

Combined effect of Carbon and Nitrogen levels and isothermal holding temperature on the microstructure, precipitation and the tensile properties of Vanadium microalloyed steels with 0.05 weight percent Vanadium were studied. Two different Vanadium steels, one having higher Carbon and lower Nitrogen content, HCLN steel, and the other having lower Carbon and higher Nitrogen content, LCHN steel, were prepared and subjected to isothermal holding treatment over a temperature range of 500 to 750 degree Celsius, after hot-deformation. Maximum precipitation strengthening from fine Vanadium carbonitride precipitates has been found at intermediate isothermal holding temperatures i.e. 600 to 650 degree Celsius in both the steels. In spite of the significantly smaller fraction of pearlite and bainite, coarser average ferrite grain size and lower interaction of precipitation and dislocation in the ferrite matrix, the yield strength of LCHN steel was close to HCLN steel. This can be attributed to the higher precipitation strengthening ranging from 20 to 50 MPa resulted from the finer Vanadium precipitates in LCHN steel than that of HCLN steel.