Control Of Microstructure Evolution And Mechanical Properties Of ART-Annealed Medium Manganese Steel

With the development of automobile industry and the more stringent environmental protection policy,automotive steels with high strength and plasticity which can effectively reduce the body weight and improve the safety performance have attracted more and more attention.In this paper,two medium manganese steels of 0.2C-5Mn-1.5Si and 0.2C-5Mn-Si-1Al were developed to be studied and ART(austensite reversed transformation)heat treatment was used.The effect of annealing process on the microstructure evolution and properties of the steels was studied.Based on the research,main work and conclusions are present as follows:(1)The two-phase region temperature range and the volume fraction of austensite as well as the concentration of elements in austensite under different annealing temperature of 0.2C-5Mn-1.5Si steel was calculated by Thermo-calc.The calculated optimal intercritical annealing temperature of 0.2C-5Mn-1.5Si steel was about 655℃.(2)The Ac1 and Ac3 temperature and static CCT curve of the 0.2C-5Mn-1.5Si steel was obtained by thermal expansion method.The Ac1 and Ac3 was 605℃ and 775℃,respectively.A fully martensitic microstructure can be obtained when the cooling rate is greater than 0.5℃/s.(3)The hot rolling annealing experiment of 0.2C-5Mn-1.5Si steel was carried out.The effects of annealing temperature,annealing time and cooling rate on the microstructure and properties of the experimental steel were studied.Experimental steel annealing at 650℃ for 4h obtained the best mechanical properties:the largest PSE was 37.32GPa·%,the volume fraction of retained austenite was 35.4%.Compared with air cooling condition,the mechanical properties of furnace cooling steel decreased significantly.After the thermal expansion test at 650℃ for 4h,the critical cooling rate of all the retained austenite retained in the room temperature was 0.2℃/s.(4)The cold rolling annealing experiment on 0.2C-5Mn-1.5Si steel was carried out.The effects of annealing temperature,annealing time on the microstructure and properties of the experimental steel were studied.Cold-rolled experimental steel obtained the best mechanical properties at 660℃ for 30min,and the largest PSE was 38.23GPa·%.(5)Excellent mechanical properties of tensile strength of 1070 MPa and total elongation of 40%of 0.2C-5Mn-1.5Si steel were obtained after austenitizing annealing for a short time and followed by 5min ART annealing process.Besides,the luders band strain disappeared.(6)One-step and two-step annealing experiments of 0.2C-5Mn-1Si-1Al steel were carried out,the results show that excellent mechanical properties were obtained in both processes,the PSE was 41.76GPa·%and 48.22GPa·%,respectively.Two-step annealing process obtained relatively smaller packets and more active TRIP(Transformation Induced Plasticity)effect during deformation,which increased the tensile strength(120MPa).