Cooling Process for Precipitation Strengthening of Vanadium in Ferrite

Abstract

The precipitation of vanadium takes place mainly in ferrite by interphase precipitation or nucleation on dislocation line, which makes sense for the industry production due to the precipitation strengthening. The objective is to analysize the cooling process of V-steels to exert the precipitation strengthening of vanadium. The steels with 0.09%C-0.055%N/0.0107%N/0.0168%N/0.0193%N-0.08%V/0.085V steel are the researched steel grades. Using solid solubility products model and thermodynamic equation, the full solid solution temperature, nucleation rate curve and PTT curve of precipitation process are calculated. The effect of nitrogen on the precipitation behaviour of V(C,N) in γ and the precipitation of V(C,N) in α are simulated. Based on the calculation results the trial process is determined. The laboratorial trials are carried out with ultra fast cooling. The precipitate particles are observed by TEM. The solid solution amount increases monotonously and the size of precipitate particle decreases with the nitrogen content. The solid solution temperature of 0.055%N, 0.0107%N, 0.0168%N and 0.0193%N are 977.0°C, 1028.0°C, 1062.3 and 1078.9°C respectively. The laboratorial trial results shows that the tensile strength is improved about 100 MPa due to the precipitation strengthening. The relationship between the coiling temperature and the strength is parabolic curve downward and the relationship between the coiling temperature and the elongation is parabolic curve upward. This calculation can determine both the proper nitrogen content and the optimal cooling process. The trial results proves this method is feasible and efficiency.