The Study On Genetic Rules Of Microstructure And Mechanical Properties In Quenching And Partitioning Steel

Nowadays,Lightweight automobile has become the development trend of the automotive industry.Automotive high strength steel is to be developed to have high strength and high ductility.The third generation of advanced high strength steel-Q&P steel,its comprehensive performance of high strength and high ductility can greatly reduce vehicle weight,while enhancing the ability of automotive anti-collision to improve the car safety factor with wide proepect of application.At present,Q&P steel is mainly quenched and tempered after hot rolling.In the final Q&P treatment,it is of great significance to achieve online Q&P heat treatment as well as reduce the cost and energy consumption and to study the influence of different initial microstructures on Q&P steel.Based on the above background,this paper focuses on the genetic behavior of different initial microstructures under the same Q&P treatment and their effects on the microstructure and mechanical properties of the materials.Based on 0.4C-1.5Mn-1.5Si,three different materials of initial microstructure were obtained by changing the heat treatment process after hot rolling.Ms points,laboratory cold rolling and rolling,mechanical properties testing and microstructure characterization were studied.The genetic behavior of the initial microstructure during Q&P processing and its effect on the mechanical properties of the material.Below are several research findings:(1)Based on the material Q&T-CR,the effect of Qt on Q&P steel was studyed.The results show that the optimum properties obtained by quenching at 230℃ are 1503.3MPa,16.07%elongation and 24.16GPa·%,respectively,which are better than those obtained by quenching at 200℃ and quenching at 260℃.(2)Based on the material Q&T-CR,the effect of partitioning temperature on the Q&P steel was studyed.After the comparison between 50s and 500s partitioning in 400℃,50s is better.With the longer partitioning time,martensite carbon will separate out,dislocation will recovery and tension strength will reduce.Austenite carbon atoms precipitated out to become carbides,the stability will decrease,also the elongation will decrease.Therefore,we can determine the composition of the optimal Q&P treatment process:900℃ isothermal 5min,230℃ quenching,400℃ partitioning with 50s.(3)Based on Q&T-HR,AC-HR and Coil-HR three different initial microstructure,using EBSD,SEM and other means to get Ms.The results showed that regardless of the initial tissue type and morphology,the austenitized austenite will become equiaxial austenite,but the elemental segregation of the alloying elements inherits the elemental distribution of the original structure.The isothermal 5min is not enough to homogenize the alloying elements after quenching in Q&P,residual austenite is retained to room temperature based on the alloying element segregation of the original structure to form retained austenite with different morphology,size,distribution and carbon content.At the same time,the different microstructural features affect the mechanical properties of the material,resulting in differences in performance.The hereditary phenomenon of this tissue is of great significance for the design of post-hot rolling heat treatment and Q&P process design.(4)Based on three groups of hot rolled and cold rolled plates(Q&T-HR and Q&T-CR,AC-HR and AC-CR,Coil-HR and Coil-CR),The organization is obviously refined,and the austenite grain size is smaller during austenization.During the Q&P process,alloying elements in the cold-rolled material diffuse more rapidly and have a smaller and more uniform retained austenite size at room temperature.The grain refinement caused by cold rolling provides fine grain strengthening to the material and the strength is improved,while the uniform distribution of retained austenite improves the plasticity of Q&P steel,the elongation is improved and the comprehensive mechanical properties are more excellent,but the grain boundary angle distribution did not change significantly after cold rolling deformation.Therefore,the micro structure and mechanical properties of Q&P steel obtained by Q&P heat treatment are still different from those of Q&P treated hot rolled plate.