Study On Microstructure Evolution And Mechanical Properties Of Strip-casting Medium Manganese Steel

As the global energy crisis and environmental pollution is increasing,in the automotive industry,it has become the meaning of the industry development that energy saving and emission reduction in the automotive industry and improving the safety of automobile should be carried out in parallel.Therefore,the medium manganese steel become the best candidate of the third generation advanced high-strength steel because of its combination of high strength,high plasticity,low alloy and low cost.In this paper,strip-casting medium manganese steel with the composition Fe-0.46C-7Mn-2.7Si-1.9A(mass%)was studied.The effects of different rolling processes and intercritical annealing processes on the final microstructure evolution and mechanical properties of experimental steel were studied by hot rolling,cold rolling,intercritical annealing and other processes.Finally,excellent mechanical properties were achieved..The main work and conclusions of this paper are present as follows:(1)The temperature range of the two-phase region was about 645?800?.The theoretical optimal annealing temperature was 710?.The as-casted strip was mainly composed of columnar grains with a length of the surface layer and the subsurface layer(parallel to the thickness direction)of 700 to 900 ?m and equiaxed crystal of center layer.The as-casted microstructure was mainly composed of martensite and ?-ferrite,and a small amount of austenite.(2)The austenitizing and intercritical annealing experiments were carried out and the effects of annealing temperature and annealing time on the microstructure evolution of as-casted strip were investigated.After austenitizing annealing,alloying elements were distributed uniformly,and the recrystallization behavior occured in the ?-ferrite.After intercritical annealing,as the temperature increased to 720?,the carbides were almost dissolved.With the temperature increasing,the martensite of the as-casted strip was gradually transformed into austenite and ferrite,and the content of austenite was gradually increased.(3)The hot rolling experiments were carried out and the effects of hot rolling temperature and reduction on the microstructure and mechanical properties of strips were studied.With the same hot rolling reduction(about 75%)and the hot rolling temperature increased,the columnar grains disappeared and the grain size increased gradually,but the phenomenon of heterogeneous grain size distribution of the central layer and the surface layer also remained.When the hot rolling temperature was increased from 700? to 900?,the tensile strength increased from 1196 MPa to 1471 MPa,while the yield strength gradually decreased from 917 MPa to 574 MPa,and the elongation decreased from 24.4%to 11.5%.(4)The one-step intercritical annealing experim ents and two-step intercritical annealing experiments of the hot-rolled tested steel were carried out and the effects of annealing temperature and time on the microstructure was studied.With one-step incritical annealing,the maximum volume fraction of retained austenite was 69.1%,there was a more active TRIP occurred during straining and the tensile strength and elongation were both improved.The maximum tensile strength,the maximum elongation and the maximum PSE was 1710MPa,38.4%and 61.9 GPa·%,respectively.Compared with one-step annealing,when the hot rolling temperature was 700? and 800?,the grain size of annealed microstructure increased significantly after two-step annealing,the volume fraction of austenite decreased and the final mechanical properties decreased.When the hot rolling temperature was 900?,after intercritical annealing,the mechanical properties were obviously improved and the PSE could reach 38.9 GPa·%.(5)The hot-rolling and cold-rolling experiments were carried out and the effects of hot rolling temperature and intercritical processes on the microstructure and mechanical properties of strips were studied.After cold rolling,the columnar grains disappeared,and the structure consisted of flattened ?-ferrite,martensite,austenite,etc.Compared with annealed samples with hot rolling,the uniformity of the microstructure was significantly improved,the austenite grain size decreased and the volume fraction of austenite was significantly reduced.The austenite stability was increased and an inactive TRIP effect was observed.Therefore,the work hardening rate decreased obviously and the maximum tensile strength was reduced by 350MPa.