Microstructure-mechanical property relationships in isothermally transformed vanadium steels

The relationships between the interphase precipitation reaction and the mechanical properties of an Fe-0.2C-l.0V-0.5Mn steel were studied after isothermal transformation in the temperature range 600 °C to 750 °C. The strength and room temperature toughness of the transformed steel are found to be determined by the austenitization temperature, vanadium carbide solubility, volume fraction of VC available for precipitation, size of the precipitates, and ferrite grain size. Yield strength increments due to precipitation are predicted by Melander's model for critical resolved shear stress, when all the available carbide precipitated as interphase VC. For lower austenitization temperatures, yield strength increments are modeled by a bimodal distribution of undissolved and interphase (or matrix) precipitates. Six classifications of VC morphologies are identified in the transformed microstructures, but one of these, the "fibrous" VC morphology, could not be associated with degradation of toughness as suggested by Mishima. The impact transition temperatures are approximated by regression analyses for bainitic steels. The results show that both strength and toughness can be simultaneously optimized in this steel and suggest that microstructures with strength and toughness levels equivalent to those of quenched and tempered steels can be produced in vanadium steels by the direct decomposition of austenite.