Study On Deformation And Strengthening Behavior Of C-Si-Mn(C≤0.25)-Low Carbon And Low Alloy Duplex Steel

Ferrite(F)-Martensite(M)dual phase steels are widely used in the production of large automotive parts due to their high initial work hardening rate,good welding properties,high tensile strength and high ductility.In order to further improve the safety performance of vehicles and effectively reduce energy consumption,a large number of researchers are constantly exploring and further improving the comprehensive mechanical properties of dual phase steel.In recent years,with the introduction of heterogeneous materials,it provides a new way to improve the comprehensive mechanical properties of materials.By means of the deformation coordination of soft and hard phases,heterogeneous materials can induce extra geometry needs dislocation,improve the plasticity of materials,and further strengthen the deformation resistance of materials by inducing the back stress,so as to improve the strength of materials without sacrificing ductility.Moreover,grain refining is also an effective way to improve the strength and ductility of the materials.Therefore,the combination of grain refining and heterogeneous structural materials is expected to further improve the comprehensive mechanical properties of the materials.In this paper,low carbon and low alloy steel is selected to prepare fine crystal F-M heterogeneity dual phase steel through the process of heat treatment and deformation strengthening.The microstructure evolution during deformation of heterogeneous structural materials and the microstructure and mechanical behavior evolution after deformation strengthening treatment of heterogeneous structural dual phase steel are studied.The main research contents and conclusions of this work are as follows:(1)By controlling the critical annealing temperature and holding time,a kind of F-M dual phase steel with ultrafine grain was successfully prepared.The ferrite grain size and martensite grain size were 3.8 μm and 1.7 μm respectively under critical annealing at 780 ℃.Martensite morphology is fibrous and surrounded by ferrite.The tensile test showed that the tensile strength was 960 MPa and the uniform elongation was 17.3 %.The excellent mechanical properties were attributed to the heterogeneous deformation induced strengthening(HDI stress strengthening)and the ultrafine grain size structure during the deformation process.(2)In-situ EBSD and transmission electron microscopy(TEM)were used to analyze the structural and substructural evolution of dual phase steels during deformation.In the process of deformation,dislocation occurs in the soft phase region,and the deformation is mainly dominated by ferrite.At the F-M interface,due to the incongruity of deformation of the two phases,geometric necessary dislocation is generated to maintain the geometric continuity,so as to coordinate the deformation of the two phases.(3)Through the warm rolling test at 300 ℃,the nanoscale heterogeneous dual phase steel was successfully prepared by large plastic deformation treatment.The lamination thickness was20 nm,the tensile strength was 2.09 GPa,and the uniform elongation was retained.(4)By means of rotational accelerated shot peening(RASP)process,a nanocrystalline layer was introduced into the surface of the duplex steel after critical annealing treatment,and the effect of gradient structure on the microstructure and mechanical properties of the duplex steel was studied.In this work,by modifying the surface gradient structure layer/sample thickness,the material strength was improved while retaining the plasticity of the original material.The yield strength and tensile strength are about 10 % higher than before deformation.Through the test of the back stress,the contribution of the dual phase structure and gradient structure to the deformation resistance is calculated,more than 50 % deformation resistance comes from the HDI stress.