Isothermal Precipitation Behavior of Vanadium Carbonitride in Bainitic Ferrite Region of Low Carbon V‐B Microalloyed Steel

In order to develop low‐cost, high‐performance automotive steel by employing V microalloying and adding trace amounts of B, combined with thermal‐mechanical controlled process, the precipitation behavior of vanadium carbonitride (V(C, N)) is investigated by thermodynamic analysis on carbonitride precipitation in the cooling process and thermal simulation experiments of isothermal precipitation in the bainitic ferrite region. The calculation results show that the nucleation of V(C, N) on the dislocation line is the main nucleation mechanism between 350°C and 700°C. Precipitation‐Time‐Temperature diagram shows C‐shaped curve, and the fastest precipitate temperature is about between 600°C and 675°C. The experimental results of low carbon V‐B microalloyed steel show 600°C is identified as a critical temperature that achieves both the fastest nucleation rate and the highest V(C, N) precipitation quantity. In V microalloyed steels, boron effectively inhibits ferrite transformation while promoting bainite formation, thereby allowing for a wider cooling rates to achieve the phase transformation strengthening effect. When the vanadium content increases to 0.175 wt%, the amount of ferrite significantly increases. Adding an excessive amount of vanadium significantly promotes the transformation of ferrite during the continuous cooling process but significantly reduces the effect of boron on the transformation of bainite.