| Ultra-high strength steel shows the excellent ultra-high strength and toughness,which contributes to the lightweight design of energy-saving and emission reduction of automobiles and improves the impact safety,so its proportion in the field of modern automobile manufacturing has been increasing,countries around the world are vigorously developing various High-strength steel.With the development and industrialization of automobile industry,the quality and performance of ultra-high strength steel are rigorously required,which makes the automobile steel develop towards low cost and high product of strength and ductility.In this paper,by means of optical microscope（OM）,Scanning electron microscope（SEM）,transmission electron microscope（TEM）,X-ray diffractometer（XRD）,Universal Testing Machine,Micro Vickers hardness tester,etc.,the microstructure evolution of30CrMnSi2Nb ultra-high strength steel in DQ/QT/QP process was investigated.The rule of microstructure evolution in thermoforming process and the control mechanism of microstructure on mechanical properties were analyzed in detail.The main findings and findings are as follows:（1）The growth behavior of austenite grain of 30CrMnSi2Nb steel was analyzed,the dynamic model of austenite grain growth was fitted,the effect of austenite temperature on the size of packet in martensite lath was studied.The results show that in the steel studied,the NbC is the second phase particle which impales the austenite grain boundary.The dynamic model of austenite grain growth is as follows:D1.1053=1.608×106 exp（-111940.54/8.314T）t.The relationship between the size of martensite lath packet and austenite grain is linear.（2）Martensite softening and cementite precipitation in QT process were studied in detail.QT process makes martensite packet and lath coarsened and dislocation density reduced.After tempering at 425℃for 60 seconds,the lamellar-like cementite precipitates out.（3）Martensite softening,martensite substructure size change and retained austenite in QP process were studied in detail.The size of lath Packet of martensite can be refined by QP process,and the lamellar-like retained austenite can be obtained by increasing the content of retained austenite.（4）The effects of packet size in martensite,martensite softening,cementite and retained austenite content on tensile strength,yield strength,elongation at break and yield ratio of steel were discussed.The results show that the martensite packet structure is the structural unit controlling the yield strength of quenched steel,and the martensite softening（dislocation density）determines the strength-plasticity of tempered martensite;TRIP effect appears in the retained austenite of QP steel during tensile deformation,and the retained austenite can greatly increase the plasticity and toughness of the sample.The tensile strength of the sample corresponds to the retained austenite content in the structure,and the plasticity corresponds to the carbon content in the retained austenite. |
