Study On Metastable Austenite Control And Mechanical Properties Of Medium Manganese Steels With High Strength And Ductility

The rapid increase in the number of motor vehicles worldwide,which aggravated energy consumption and environmental pollution.At the same time,in the face of the urgent demand for body weight reduction from green car,automobile lightweighting has become an urgent global initiative,which has accelerated the development of highstrength automotive steels.It puts forward higher requirements for automobile steels when considering the cost,formability,collision safety and other factors of automobile steel.Medium Mn steels have been widely studied because of its excellent comprehensive mechanical properties,which are considered to be one of the most potential automotive steel candidates.Meanwhile,in order to meet the continuous improvement of the comprehensive mechanical properties of automotive steels,it is still challenging to achieve the improvement of the comprehensive mechanical properties of medium Mn steels.Based on the above background,this article takes medium Mn steels as the research object,and realizes the control of microstructure and mechanical properties through different rolling and austenite revrse transformation(ART)annealing processes.The effects of different rolling and ART annealing process conditions on metastable austenite and mechanical properties were emphatically studied,and the relationship between them was revealed.The control of metastable austenite was achieved by improving the process,thereby achieving the target mechanical properties on the experimental steels.The main research content and research results of the paper are as follows:(1)Through the study of ART annealing of cold-rolled 15C5Mn1.1Al medium Mn steel,it is confirmed that there is a strong interaction between the evolution of ferrite and austenite: ferrite recrystallization promotes the increase of austenite content,and the presence of austenite will reduce the recrystallization kinetics of surrounding ferrite.The research results of the austenite stability show that while the austenite content increases during the annealing process,the austenite stability gradually decreases.The annealing temperature has a greater impact on the stability of austenite than the annealing time,and the C content in austenite is the most important factor affecting the stability of austenite.As the stability of austenite decreases,the martensite transformation during the tensile test will transition from strain control to stress control.The work hardening will be significantly improved,the tensile strength will continue to increase,but the elongation will drop sharply.(2)The 17C5 Mn hot-rolled experimental steel was prefabricated with different microstructures before cold rolling by soft annealing at different temperatures,and the effect of different microstructures before cold rolling on the microstructure and mechanical properties during ART annealing was studied.It's found that the soft annealing to form a ferrite + austenite dual phase microstructure can provide favorable kinetic conditions for the formation and growth of austenite during ART annealing process,thereby rapidly increasing austenite content.(3)Different annealing methods were used to control the heating rate,and the microstructure and mechanical properties of the experimental steel after ART annealing under different heating rates are compared.The results show that rapid heating can inhibit the recrystallization process and obtain austenite with higher stability,so that excellent mechanical properties at higher annealing temperatures can be achieved.Based on the microstructure characteristics of the experimental steel under rapid heating annealing conditions,a two-step annealing method was designed to further improve the austenite content and stability,and achieve the target tensile strength of 1500 MPa and elongation of more than 20%.(4)The 25C7 Mn experimental steel was subjected to warm rolling research under different rolling conditions.The experimental steel obtained a uniform band-like ultrafine-grained ferrite and nanocrystalline austenite dual-phase microstructure through warm rolling,and the comprehensive mechanical properties were greatly improved.The yield strength reached 1300 MPa,while the tensile strength exceeded 1600 MPa,and the total elongation was maintained at about 30%.Studies have shown that the substantial increase in the yield strength of the warm-rolled experimental steel is mainly affected by the ferrite grain refinement and the dislocation density.And the refinement of the austenite grains improves the mechanical stability of the austenite,ensuring that the strength of the experimental steel is increased while still maintaining good elongation.