Regulation Of Retained Austenite In Ultra-fine Bainite Steels And Study On The Plasticizing Mechanism Of Phase Transformation

The effects of pulsed magnetic field treatment on the retained austenite content,microstructure and properties of ultra-fine bainitic steel(Fe-0.95C-1.42Si-1.82Mn-0.93Cr-0.24Mo-1.34Co-1.05Al)are studied in this paper.Taking Fe-0.6C-0.45Mn-2.5Si-0.78 Al steel as the research object,the effects of cold rolling and hot rolling predeformation on austenite grain size,the content and morphology of retained austenite and mechanical properties and microstructure of ultra-fine bainite steel are studied.The ultra-fine bainite is treated by pulsed magnetic field and the microstructure and properties of the steel is studied.It is found that the effect of pulsed magnetic field on the bainite transformation amount and bainite beam during the incubation period of bainite transformation is not obvious compared with that of the normal isothermal treatment.When the ultra-fine bainite is treated by the pulsed magnetic field at the beginning of bainite transformation and the whole process of bainite transformation,the bainite ferrite nucleation accelerates,the bainite beam coarsening and the growth rate decrease,and the bainite transformation amount increases,bainite beam is becoming thinner and shorter.When the ultra-fine bainite is treated by the pulsed magnetic field during the incubation period and the initial period and the whole process of bainite transformation,the retained austenite content in the ultra-fine bainite steel increases compared with that of the normal isothermal treatment.When the ultra-fine bainite is treated by the pulsed magnetic field during the whole process of bainite transformation,the enrichment of carbon atoms at the front of the ?/? interface close to the retained austenite increases which results in the slower speed of growth and thickening of the bainite beam compared with that of the normal isothermal treatment.The ultra-fine bainitic steel is treated by rolling predeformtion and the microstructure and properties of the steel are studied.It is found that cold rolling predeformation refines the bainite beam and bainite ferrite of ultra-fine bainitic steel and increase the tensile strength,elongation and strength-ductility compared with non rolling and hot rolling predeformation.The content of retained austenite in ultra-fine bainite steel is related to the austenite grain size.When the austenite grain size is between 13?m and 76?m and the retained austenite content of ultrafine bainite is between 10% and 1.26%,it is easier for the TRIP effect to occur in ultra-fine bainite steel.At the same austenitizing temperature,the austenite grain and bainite ferrite of cold rolling sample are finer than hot rolling sample.When the austenite grain size is the same,the bainite beam and bainite ferrite of cold rolling sample are finer than that of hot rolling sample.The optimum predeformation and isothermal treatment process is determined as follows: the experimental steel is cold-rolled deformed.After that,the austenite grain size can reach 51.19?m with the holding time of 30 min at 950?.And then isothermal at 350? for 4h.The retained austenite content is more than 10%,the tensile strength is 1900 MPa,the elongation is 15.8% and the strength-ductility is 30.02GPa·%,austenite,bainite is fine grain toughening and TRIP effect make its comprehensive mechanical properties the best.