The Relationship Between Microstructure And Properties Of Bainite/Martensite Multiphase Steel Under New Heat Treatment Process

"High performance" is the primary goal and the eternal theme of research and development of steel material,and Mn series bainite / martensite multiphase steel is widely used because of its excellent performance and cost-effective.In this paper,bainite / martensite multiphase steel was taken as the research object.Based on the "compositionprocess-microstructure-property",the experimental steels were obtained through the alloy composition design and the optimization of the heat treatment process.The relationship between microstructure and performance was studied,and the prediction model of mechanical properties was built to reveal the mechanism of strength and plasticity,and strength and toughness.In this paper,four kinds of Mn-Si-Cr bainite / martensite multiphase steel with different carbon contents were designed.According to the calculated and measured CCT curves and the evolution of microstructure during continuous cooling,the quenching,quenching & partitioning,air-cooled and air cooled & partitioning heat treatment processes were designed to obtain different types and sub-structure duplex microstructure.Try to reveal the mechanism of strength and plasticity,and strength and toughness of bainite / martensite multiphase steel.The tensile tests and impact tests were carried out on the specimens with different carbon content after being treated by four heat treatment processes.Microstructure observation,X-ray diffraction(XRD)test,electron probe test and EBSD test were also carried out.Through the microstructure observation,it was found that the experimental steels treated by different heat treatment processes consisted of different types of microstructures.The results show that with the increase of carbon content,the hardness and yield strength of the experimental steel after four heat treatment all showed an increasing trend,but the toughness tended to decrease.The uniform elongation of Q&P and BQ&P steel increased with the increase of carbon content,uniform elongation of Q and BQ steel first increased and then decreased.With the increase of carbon content,the PSE of BQ&P treated steel showed an increasing trend,and the U25 experimental steel after BQ&P treated had a good plasticity and toughness matching.This is because the pre-formed bainite refinement structure,and the retained austenite formed during partition process,the size and stability of the retained austenite in the deformation and fracture process to better coordinate the TRIP effect and hinder the crack growth effect,improve Experimental steel strong plasticity and toughness.The model of predicting the carbon content and the predicting model of the yield strength under the condition of quenching and quenching-partitioning were established successfully,which enriched the mechanical properties database and theoretical system of high strength steel.In addition,the strengthening toughening mechanism of bainite/martensite multiphase steel under different tempering temperatures was also studied.The results show that,at 400 ? tempering,QPT,BQT and BQPT experimental steels have good strength and plasticity,but due to the tempering brittleness of the experimental steel tempering at 400 ?,it is difficult to obtain a good matching of strength,plasticity and toughness.In the next step,it is necessary to try to obtain better strength and plasticity at the tempering temperature which is outside the tempering brittleness temperature range,in order to achieve a good matching of strength,plasticity and toughness.