| In the background of lightweight vehicles,it has been the main research direction of automotive steel how to obtain high-strength and high-plasticity,which makes materials thinner and lighter to reduce vehicle weight and improve vehicle safety.The third generation of ultra-high strength Q&P steel possesses high strength and plasticity and has broad application prospects.However,the industrial Q&P steel integrated process is more complicated,and the basic research is still not perfect.Therefore,it is of great scientific and engineering significance to regulate microstructure and mechanical property under integrated process.Based on the above background and the basic theoretical research of Q&P steel,the regular Fe-Mn-Si system is used as the experimental materials.Three cooling methods of quenching,air cooling and coiling after hot rolling are designed to obtain different initial microstructures after hot rolling.Temper brittleness is studied to prevent crack after cold rolling.Hot-rolled Q&P treatment and cold-rolled Q&P treatments of different initial microstructures are used to explore inheritance of microstructure and mechanical property and link and difference between microstructure and property after hot-rolled Q&P and cold rolled Q&P treatment is established.The phase transformation temperature and phase composition in the microstructure are measured by dilatometer device.The tensile experiment is used to characterize the mechanical properties.Microstructure image is characterized by OM and SEM and element distribution is characterized by EPMA.The content of residual austenite and its carbon are characterized by XRD.The grain size and aspect ratio of retained austenite are characterized by EBSD.The evolution of retained austenite before and after tensile is used to analyze the inheritance behavior from multi-perspective and multidimensional The main results are as follows:(1)The temper brittleness of hot-rolled annealed materials is studied.The results showed that the strength decreased and the plasticity increased with the increase of tempering temperature.Compared with the tempering of 500s,the strength of the material after 5h tempering decreased and the plasticity increased.At a tempering temperature of 600?,segregation of Mn and Si occurred in the center of the material.The mechanical properties at different tempering temperatures indicated that the material had no obvious temper brittleness.(2)Q&P treatment with different initial microstructures after hot rolling,the retained austenite in the microstructure was blocky and acicular,while the retained austenite microstructure of the cold-rolled Q&P treatment was mostly acicular and all martensite was lath.(3)Q&P treatment with different initial microstructures After hot-rolling,the fraction of phase in the structure was quite different.The initial microstructure of ferrite pearlite had higher comprehensive mechanical properties,more content of retained austenite and carbon in retained austenite.smaller the grain size and larger aspect ratio.(4)There is little difference between the mechanical properties and the retained austenite state after cold rolling Q&P treatment with different initial microstructures.Compared with the microstructure after hot rolling Q&P treatment,The tensile strength was higher,the uniform elongation was higher,the retained austenite grains are finer,the martensite laths are finer,and the overall mechanical properties are better after cold rolling Q&P treatment.(5)Comparing the state of retained austenite in the microstructure before and after tensile,the results showed that the retained austenite grain size distribution was mainly concentrated in the small size range before tensile,and the aspect ratio is relatively average.After tensile,the content of retained austenite is greatly reduced,and the remaining retained austenite grains were mostly small size,and the large-sized grains were almost converted and the average grain size is lowered. |
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