Effect Of ART-Q&P Processes On Microstructure And Mechanical Properties Of Low Carbon Silicon Manganese Steel

Automobile lightweight is an inevitable trend in the development of automobile industry.The application of the 3rd generation Advanced High Strength Steel(AHSS)with high strength and plasticity is an effective technique for automotive lightweight.The regulation of retained austenite has an important effect on the mechanical properties of AHSS.By combining the Austenite Reverted Transformation(ART)proces with the conventional Q&P process,the Austenite Reverted Transformation and Quenching and Partitioning(ART-Q&P)processes was put forward.The chemical composition of the steel used in this study was 0.17C-2.32Mn-1.63 Si.The stability of retained austenite was improved by manganese partitioning in the intercritical and carbon partitioning at low temperature,so as to obtain automobil steel with high production of strength and elongation?In this paper,the phase change and element partitioning in the ART-Q&P processes was analyzed,and the effects of reverted transformation and carbon partitioning parameters on microstructure and mechanical properties were studied by optical microscope,scanning electron microscopy and tensile testing machine.The relationship between the volume fraction of retained austenite and mechanical properties was investigated by X-ray diffractometer.The ART-Q&P processes was compared with other AHSS heat treatment proceses.The specific research results are as follows:(1)Austenite double stabilization model was proposed to analyze the element partitioning and interfacial migration in the process,to explore the influence of temperature and time in the intercritical on microstructure and properties,and to predict the microstructure at room temperature.(2)The microstructure of ART-Q&P steel was consisted of martensite,ferriteand retained austenite.The martensite laths were irregular in arrangement,with pale white at the boundary and dark gray inside.The retained austenite was film-like or fine blocky.With the increased of reverrted transformation temperature,the retained austenite content increased first and then decreases,martensite content increased and ferrite content decreased.The tensile strength and yield strength of the experimental steel increased,and the elongation was decrease.The production of strength and plastic increases first and then decreases.(3)With the prolonged holding time in interfacial,the volume fraction of austenite increases gradually,and manganese element was partitioning from ferrite to austenite and reaches equilibrium.If the insulation is too long,the grains grow up,and manganese element in the austenite are homogenized,reducing the partitioning effect.The tensile strength of ART-Q&P steel first increased and then decreased.The elongation first increased and then decreased,and then increased slightly after 30 min of insulation.When the holding time was 30 min,the homogenization of austenite improved the plasticity to a certain extent.The variation law of product of strength and elongation is consistent with that of retained austenite.(4)When the partitioning time is short,lath martensite content was higher and the laths were clear.With the extension of the partitioning time,the martensite was disordered,the laths became shorter and thicker,the content of plate martensite increased,some of the laths of martensite appeared coarsening,and the boundary was fuzzy.Some fine carbides precipitate out when carbon partitioning was too long.With the increase of carbon partitioning time,the retained austenite content first increased and then decreased.The carbon partitioning was completed in 60 s.(5)The improvement of mechanical properties of carbon and Manganese partitioning in ART-Q&P processes and I-Q&P processes was better than the carbon partitioning of traditional Q&P processes and Manganese partitioning of the ART-Q&T processes.Moreover,the elements partitioning effect of ART-Q&P processes is higher than that of I-Q&P processes.The ferrite grains were needle-like or blocky,and more fine,while the ferrite grains are mainly blocky in the I-Q&P steel.The comprehensive mechanical properties of ART-Q&P steel was the best,and theproduction of strength and elongation is 19.79GPa·%.