| Developing the third generation of automobile steel with high strength and high plasticity to meet the lightweight requirements of automobile is one of the effective ways to achieve the goal of“carbon neutrality and emission peak”.At present,the third generation automobile steel represented by QP steel and medium manganese steel mainly improves its plasticity by increasing the residual austenite content.However,there are many problems such as high alloy content,high processing cost,smelting and production difficulties when using this method.In order to overcome the single plasticizing mechanism of retained austenite in the third generation of automobile steel,the 24Mn2Cr Nb automobile steel is studied in this paper.According to the actual situation of the existing continuous annealing production line in enterprise,the microstructure and composition phases in steel and size distribution of second-phase particles are changed by adjusting the heating temperature and aging temperature of continuous annealing treatment.Through the coordination of a variety of strengthening and toughening mechanisms,the 24Mn2Cr Nb experimental steel not only has high strength（≥1200 MPa）but also has high strength plastic product.The results show that:1)For the continuous annealing sample,the main microstructure phases of the experimental steel are ferrite matrix and martensite island.When the two-phase region（800～860℃）is treated,the ferrite mainly includes the residual ferrite and the ferrite formed by bainite transformation during heating.When the heating temperature is increased to 900℃,it is mainly the ferrite formed by bainite shear transformation.EBSD results show that the volume fraction of retained austenite in the experimental steel treated by continuous annealing is low,and the maximum is about 2.5%.Therefore,it can be considered that the strengthening and toughening mechanism of automobile steel in this paper is different from that of traditional automobile steel.The microstructure composed of ferrite and martensite islands provides good plasticity and strength.2)The change of the second phase particles is related to the aging process.When aging at 250℃,due to the low temperature,the particle type is cementite and the shape is mostly rod-like particles.With the increase of aging temperature,when the aging temperature is reached to 450℃,the diffusion rate of C and Cr increases,forming a large number of granular Cr23C6particles.The precipitation of second phase particles is beneficial to the dispersion strengthening of steel to improve the strength of steel.At the same time,the interaction between the existence of the second-phase particles and dislocations can generate potential FR dislocation sources.When plastic deformation occurs in small martensite laths,the plasticity can be improved.This is one of the mechanisms for good strength and plasticity of automobile steel studied in this article.The room temperature tensile test results show that when the continuous heating temperature is 830℃,and the aging temperature is250℃,the comprehensive performance of the experimental steel is the best,with the tensile strength of 1355 MPa,the yield strength of 816 MPa,the elongation of 17.5%,and the product of strength and plasticity of 23.7 GPa·%.3)The effect of hot forming process on the microstructure and properties of experimental steel was simulated.With the increase of heating temperature（890～950℃）,the martensite lath grew gradually.When the heating temperature was950℃,the martensite laths of 20Mn2Cr Nb and 24Mn2Cr Nb experimental steels were the coarsest.The original dimensions of 20Mn2Cr Nb and 24Mn2Cr Nb experimental steels were calculated.At the same heating temperature,with the increase of carbon content,the average original austenite grain size of 24Mn2Cr Nb steel is smaller,and the tensile strength is significantly increased.The maximum increase can achieve more than 260 MPa,and the fine grain strengthening effect is more obvious.EBSD results of 24Mn2Cr Nb steel show that when the heating temperature increases from910°C to 950°C,the average size increases from 0.9267μm to 1.0584μm.Due to the coarsening of martensite structure,the strength and plasticity decrease. |
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