Research On Annealing Process Of Low Cost Third Generation Automobile Steel By Rare Earth Microalloying

In recent years,lightweight and safety have gradually become the development trend of modern automobile steel.At the same time,new automobile steel materials with high strength and high plasticity have gradually become the focus of attention.In this paper,a rare earth experimental steel containing 9ppm Ce was designed according to the M³microstructure control theory（multiphase,multi-scale,metastable）,and a group of experimental steel with similar contents of other alloying elements but without rare earth elements was selected as a comparison.Two kinds of experimental steel A and B were treated by ART annealing and direct annealing in the two-phase region respectively.The microstructure and mechanical properties of the heat treated samples were observed by scanning electron microscopy（SEM）,X-ray diffraction（XRD）and universal tensile testing machine.The research results are as follows:After testing,The phase transformation temperature of the experimental steel without rare earth was 596℃for A_C1and 734℃for A_C3.The phase transformation temperature of the experimental steel with rare earth A_C1is 598℃,and A_C3is 751℃.It can be seen that trace rare earth elements will increase the A_C3temperature of the experimental steel and expand the two-phase region.By comparing the microstructure and mechanical properties of the experimental steel with and without rare earth,which found that the grain size of the experimental steel with rare earth is more homogeneous,and the yield strength,tensile strength and elongation are improved.The XRD results show that the diffraction peak of austenite in the experimental steel with rare earth is higher,and the austenite content in the microstructure is more.From the macroscopic texture analysis,it is found that the texture distribution of the experimental steel containing rare earth is similar to that of the experimental steel without rare earth,and the texture density level and the content of each orientation texture are similar.The favorable texture{111}component of the experimental steel with rare earth is slightly higher but the difference is small.It can be seen that the effect of trace rare earth on the texture is not obvious,that is,it has little effect on the deep drawing performance of the experimental steel,and the main effect on the mechanical properties is relatively prominent.After heat treatment with different annealing processes for the experimental steel containing rare earth,it was found that both ART annealing and direct annealing in the two-phase region obtained austenite,ferrite or new martensite after annealing.At lower annealing temperature or short holding time,white granular carbides exist in the microstructure.With the increase of annealing temperature or cooking time,the grains grow up and the carbides decrease until they completely dissolve.The content of reversed austenite increases first and then decreases,and reaches the maximum after annealing at 645℃for 15 min.With the increase of annealing temperature or cooking time,the yield strength decreases,and the tensile strength increases.However,the elongation and the product of strength and plasticity increase first and then decrease,which conforms to the variation law of austenite content,and reaches the maximum value at 645℃for 15 min.According to the comprehensive performance indicators,the optimal heat treatment condition of ART annealing is 800℃-5min quenching+645℃-15min annealing,and the optimal heat treatment condition of direct annealing in two-phase region is 645℃-15min annealing.Compared with ART annealing,the elongation of the experimental steel after direct annealing in the two-phase region is slightly lower,but the tensile strength,yield strength and strong-plastic product are higher.The optimal properties are obtained after annealing at 645℃-15min.The tensile strength is 1050 MPa,the elongation is 31.94%,the product of strength and plasticity is32.90 GPa·%,and the austenite content is 22.4%.Under the optimal heat treatment conditions,the experimental steel after direct annealing in the two-phase region has a high texture density,and the content of favorable texture{111}component is also high.It can be seen that direct annealing in the two-phase region is beneficial to the improvement of deep drawing properties of the experimental steel.