Metallurgical Factors Influencing Microstructure of Hot Rolled Ultra Low C Steel

Abstract

Influence of processing variables on microstructure of hot-rolled ultra low C Al-killed steel containing 30 ppm C was investigated by comparing with the results for 0.006%C and 0.02%C steels. In this study, a 150 t TMT (Thermomechanical treatment) simulator capable of multi step working at high strain rate was used.
Ferrite grain diameter increased remarkably as the amount of C decreased from 0.006% to 0.003%, while dependence of α grain size on the final working temperature changed little with C content. Cooling rate after the hot working had great effect on the grain size, especially in the range from 10 to 30°C/s in 0.003%C steel. The effect of reduction percent in final hot working on the grain size was small. Grain growth of ferrite hardly occurred even under the isothermal holding condition of 800°C×1h. It was observed in 0.003%C steel that large columnar grains elongated in the direction of thickness were formed near under the surface, it depending on the cooling rate.
Metallurgical factors controlling these characteristic microstructures in ultra low C steel were discussed in terms of as heated γ grain size, γ grain growth, γ grain refinement through recrystallization, γ/α grain size change in transformation and α grain growth at higher temperature.