Precipitation Behavior Of Second Phase Particle As Well As Thermodynamic And Kinetic Studies In Fine Grain And High Strength IF Steel

In recent years, oil consumption, safety and environmental protection problems have aroused more and more attention with the increasing of automobile production and auto possession. It is very important to produce lightweight and high strength automotive steel sheet. IF steel as the third generation of automotive deep drawing steel, has been widely applied to automobile production because of its good deep drawing performance.Fine grain high-strength IF steel was studied in this paper and the rolling experiments were carried out on the Ф450 hot-rolling mills and Ф160 reversing two-high cold rolling mills of North-eastern University. The different annealing process experiments were carried out in University of Science and Technology Liaoning.Based on Precipitation thermodynamics and kinetics, The thermodynamic and kinetic parameters of the experiment steel was calculated at temperatures from 1073 K to1473 K by use of a regular solution sub lattice model and a method of J-M-A. The microstructure observation was studied by optical microscop y, the second particles morphology and composition analysis were carried out on the transmission electron microscopy and EDX. The main results are as follows:1. The thermodynamics calculating results in different temperatures(1473 K-1073 K) show that mass fraction of Nb、C and N dissolved in steel decreases, equilibrium volume fraction and chemical driving force of precipitates increase with decreasing temperature. To the temperature of 1073 K, N element is basically precipitated fully, chemical driving force of precipitates reachs-2.285×10-9J·m-3. The molar fraction of C(y) in a regular solution sub lattice model from 0 chang to 0.4839. This is due to the solid solution temperature of NbN is higher tha n NbC, So in the high temperature stage, the precipitates are mainly NbN. With the decrease of temperature, NbC continuous precipitating and ω( N) rapidly decreases in in iron matrix, the ratio of NbC in precipitates increases, the ratio of NbN in precipitates decreases.2. The kinetics calculating results in different temperatures(1473 K-1073 K) show that the critical core size and nucleation energy decreases narrowed sharply from 1373 K to 1273 K. The critical core size and the critical nucleation energy decreases steadily under 1273 K. This is mainly affected by the chemical driving force(ΔGV) of precipitation nucleation. The relative nucleation rate increases quickly from 1373 K to 1273 K. The relative nucleation rate increases steadily under 1273 K. This is the result of competition between the chemical driving force(ΔGV) with atomic migration activation energy(Q).3.Choicing a suitable annealing temperature, the grains become fine and uniform. With the increase of annealing temperature, ferrite grains grow up obviously.It will be precipitation in grains of fine grain and high strength IF steel in the anneaiing process. The precipitation are tiny and large numbers, evenly distributed and almost circular when the annealing temperature is low. The number of precipitations in steel reduces and becomes bigger with the increasing annealing temperature.4. Choicing a suitable annealing time, the grains become fine and uniform. With the increase of annealing time, ferrite grains grow up obviously.It will be precipitation in grains of fine grain and high strength IF steel in the anneaiing process. The precipitation are tiny and large numbers as well as evenly distributed,and they are almost NbC with circular and oval shape when the annealing time is short. The number of precipitations in steel reduce and become bigger, they are almost NbCN with square shape with the increasing annealing time.