Precipitation behavior of secondary phases in automobile beam steel

The precipitation and ripening behavior of secondary phases in automobile beam steel with different nitrogen contents were investigated by the thermo-kinetic theory and the Ostwald ripening theory. It is found that increasing the content of nitrogen in the steel can promote the precipitation of V(C,N) in austenite, thus the ferrite grains are greatly refined, and the ferrite grain size is decreased to 4.7μm when the content of nitrogen increases to 4.2×10^-4. The curve of nucleation rate to temperature calculated by the thermodynamic theory shows that nitrogen can effectively expand the temperature zone of the biggest nucleation rate, and the precipitation-temperature-time curve demonstrates that the shortest incubation temperature can rise from 840℃ to 968℃ when the content of nitrogen increases from 5.5×10^-5 to 4.2×10^-4. Transmission electron microscopy observations reveal that the size of precipitated V(C,N) is effectively decreased with the increasing content of nitrogen. The calculated Ostwald ripening rate of VN in austenite indicates that the ripening rate weakens with the temperature dropping, and moreover increasing the content of nitrogen also can decrease the ripening rate to inhibit the ripening growth of the precipitated secondary phases.