Influence of Cooling Rate on the Hot Ductility of Low Carbon V–N Steel

Low ductility is the intrinsic cause of cracking in slab, and cooling rate is a key factor affecting the hot ductility of steel. This article focuses on low carbon V-N steel, and explores the growth characteristics of carbonitride precipitation and microstructure at different cooling rates through thermal simulation tensile experiments. The results show that increasing the cooling rate can improve the ductility in the third brittle zone. The cooling rate increases from 0.5°C/s to 7°C/s, the minimum of the reduction of area increases from 38.1% to 38.9%, 39.8%, 44.9%, and 48.4%, and the third brittle temperature zone shrinks from 725 ~ 860°C to 725 ~ 820°C. Increasing the cooling rate has little effect on austenite grain size, but it will significantly reduce the proeutectoid ferrite formed at austenite grain boundary. With the cooling rate increasing, due to the accelerated transition from austenite to ferrite, the average thickness of the proeutectoid ferrite film decreases from 38μm to 4μm, and no proeutectoid ferrite is formed at some grain boundaries. The cooling rate ≤5°C/s, the carbonitrides are distributed in a chain like manner at austenite grain boundary, and the size is large. However, when the cooling rate is ≥5°C/s, the time for the microstructure to remain in the austenite temperature zone and the high fluidity of the γ/α interface cause some carbonitrides to precipitate in the ferrite matrix, with small and uniform distributions in the original austenite grains.