Effects of N and V on Ferrite Grain Refinement in Normalized Steel

The effect of grain refinement of steel is well known as a metallurgical method to make both strength and toughness improve. In this study, in order to use VN precipitates for α-grain refinement in normalized steel, the effect of V and N content on γ-grain size at reheating stage and α-grain size after air cooling and the mechanism of grain refinement have been investigated.The results obtained are as follows, 1)The α-grain refinement in high N-V added steel is due to the degree of γ-grain refinement by VN precipitates and high γ/α transformation ratio.2) The more the VN precipitates in γ-grain, the finer the α-grain becomes. Its effect becomes distinguished in the range of 3050 ppm N as VN and is saturated with 100 ppm N as VN.3) High γ/α transformation ratio is due to the increase of a nucleation site at γ-grain boundary. It is considered that the contribution of VN is not in lowering of transformation temperature and increasing of transformation time, but in playing role as a α-nucleation site or suppressing of α grain growth through precipitation in front of α phase.