Effects of deformation and cooling rate on nano-scale precipitation in hot-rolled ultra-high strength steel

Abstract

In order to control nano-scale precipitation (Nb, Ti)C in hot-rolled 780 MPa grade C-Mn steel micro-alloyed with niobium and titanium for automobile frames, the effects of deformation and cooling rate on nano-scale precipitation were investigated by using the thermal simulation experiment technology, then through the transmission electron microscopy observation and statistical analysis. The result indicated, deformation could significantly improve density of dislocation, subgrain boundary and vacancy etc in microstructure, and promote heterogeneous nucleation of precipitation, and improve nucleation rate of precipitation and decrease the average diameter of precipitation. Deformation could improve vacancy concentration and promoted vacancy nucleation. The induction period of precipitation nucleation decrease with the increase of deformation amount and strain rate, and precipitation more easily to nucleate. Precipitation nucleation driving force was mainly supersaturation of microalloy in undeformed experimental steel, and the nucleation mechanism was mainly homogeneous nucleation. However, the nucleation mechanism was mainly heterogeneous nucleation in deformed experimental steel. In one fixed experimental deformation condition, when the cooling rate below 5°C/s, there was (Nb, Ti)C-PFZ (precipitate free zone) nearby original austenitic grain boundaries or subgrain boundaries, and the width of PFZ at cooling rate of 0.5, 1, 2 and 5°C/s were 46.9, 30.2, 28.1 and 0 nm, respectively, so the width of PFZ decreased with the cooling rate increasing. When the cooling rate reached 15°C/s, the nucleation of precipitation was totally inhibited during cooling process. The number of precipitation along with the cooling rate increases gradually decreases. With the increase of cooling rates, the nucleation zone of precipitation was transferred from austenite to ferrite or bainite, and the average diameter of precipitation was refined. Due to grain boundaries or the subgrain boundaries were main traps for supersaturated vacancy, but the diffusivity of vacancy was high, which made the vacancy concentration nearby grain boundaries or the subgrain boundaries lower than critical vacancy concentration for precipitation nucleation, so precipitate could not nucleate nearby grain boundaries or subgrain boundaries. Due to the diffusivity of vacancy was affected by temperature, when the cooling rate was slow, vacancy had enough time to diffuse and annihilate, which made wide PFZ formed. Whereas, when the cooling rate was high, the diffusivity of vacancy was reducing or disappearing, so the width of PFZ was small. In orde to ensure experimental steel had higher yield strength, austenite zone precipitation and (Nb, Ti)C-PFZ nearby boundaries should be inhabited, so the cooling rate should be more than 15°C/s in the practical rolling process. © Right.