Research On The Regularity Of Solid Solution And Precipitation Of Second-phase Particles For Fine-grain And High-strength IF Steel

In recent years,as people pay more attention to the problems of environment pollution and energy,the production of car is to develop in the direction of lightweight.As the research says,the high strength of automotive steel is an effective way to realize it.The interstitial free steel(IF steel) has both high strength and excellent deep drawing performance,so this type of steel has been applied a lot in the modern car production area.With the vigorous development of the auto industry,the requirement of the properties of IF steel is improving,so the further research of IF steel is particularly meaningful.In this paper,fine-grain and high-strength IF steel is chosen as the object of study,and the solid solution experiment has been done under the condition of different heating temperature and holding time.Then the changing rule of microstructure is observed by optical microscope.Meanwhile,the solid solution and precipitation regularity of second-phase particles and the formation mechanism of PFZ are studied by the transmission electron microscopy.The mechanical properties testing of experimental steel is carried out.The main results are as follows:1.In the heating process of fine-grain and high-strength IF steel,the second-phase particles dissolve gradually with the heating temperature rising and holding time increasing,the number of second-phase particles reduce and the size increase,the size of austenitic grain increases gradually due to the combined effect of both.In the heating process,the austenite grain growth behavior model of fine-grain and high-strength IF steel is established on the basis of non-linear regression of OM experimental data,the result is as follows:2.As the OM experiment results show,the austenite grain coarsening temperature of fine-grain and high-strength IF steel is 1050 ℃ and the austenitic grain coarsening time is 40 min.In the production,the heating temperature should be controlled between1050 ℃ and 1100 ℃,the holding time should be controlled between 30 min and 40 min.3.The TEM experiment results show that most of the undissolved second-phase particles are rich in Nb and show the shape of round or oval when the temperature is lower,and most of them are rich in Ti and show the shape of square and rectangle when the temperature is higher.When the heating temperature is 1150 ℃,almost all Nb element dissolve in iron substrate entirely.Theoretical calculation shows that the temperature of complete solid solution for the second-phase particles containing Nb is1130 ℃,the theoretical calculation result is basically consistent with the experimental phenomena.4.The experimental results show that the solid solution and growth of second-phase particles and the sweep effect that grain boundary migration caused lead to the formation of precipitation free zone just at the side of the grain boundary,and precipitation free zone form preferentially in the grain boundary through the place in the process of annealing.5.In the process of annealing,with the increase of annealing temperature,the yield strength reduce,the value of n increase,the average width of PFZ increase gradually.Therefore,it can be inferred that the average width of PFZ has an effect on the yield behavior and strain hardening behavior of the fine-grain and high-strength IF steel.