Study On Intercritical Annealing Process And Microstructure-mechanical Property In 3Mn-TRIP Steel

Medium Mn steel with excellent mechanical properties has a bright future in the automobile industry.However,a series of industrial technical difficulties such as segregation,rolling cracking still existed in the traditional medium manganese steel with higher Mn content(5%?12%Mn).Therefore,the research into lowering Mn content simultaneously without sacrificing mechanical properties is of significance for promoting the commercial application of medium-manganese steel.In this paper,the 3Mn-TRIP(0.27C-3.5Mn-1.3Al-1.8Si-0.2V)steel was adopted to investigate the microstructure evolution,element partitioning,austenite stability and mechanical properties of cold-rolled and hot-rolled samples subjected to different intercritical annealing processes.The conclusions of this paper are obtained as follows:(1)Based on thermodynamic simulation and thermal expansion experiments,the volume fraction of retained austenite reached the maximum value of 31%at 760?.The Ac1 and Ac3 temperatures of the experimental steel is 681? and 967?,respectively.The optimal annealing temperature range can be determined to be in the range of 700-830?.(2)After annealing at 730?,the cold rolled steel obtained the mechanical properties with yield strength of 726 MPa,tensile strength of 1020 MPa,total elongation of 35.5%and the product of strength and elongation(PSE)up to 36.2 GPa·%.The optimal annealing process is 830? 5 min+740? 5 min,and the yield strength is 738 MPa,tensile strength is 996 MPa and total elongation is 41.2%.Compared with one-step annealing process,a larger amount of retained austenite with higher stability,finer grain size and higher C/Mn content was achieved in two-step annealing steel.The precipitation of VC particles was beneficial to the increase of yield strength.The two-step annealing process exhibited excellent mechanical properties for shorter annealing time,which favors commercial production and application.(3)The effect of one-step and two-step annealing on the microstructure and mechanical properties of hot-rolled steel were investigated.Compared with one-step annealing,the finer lath space of retained austenite in two-step annealing process promoted the enrichment of C and Mn elements in austenite.The fine granular austenite is found to capable of nucleating and growthing at the prior austenite grain boundaries.The total elongation increased by about 10%,owing to the positive TRIP effect caused by larger amounts of austenite different morphologies and a wide size distribution.(4)A comparative study on the microstructure evolution and mechanical properties between hot-rolled and cold-rolled two-step-annealed steels was carried out and main results was summarized as follows.The cold-rolled steels obtained the predominantly equiaxed and marginal lath-like austenite with volume fraction of 25.4%,while mainly finer lath-like and marginal blocky austenite with amount of 28.7%was observed in the hot-rolled steels.The elemental partitioning was promoted during two-step annealing of cold-rolled samples due to the smaller interval spacing,which enhanced the stability of retained austenite and mechanical properties.Meanwhile,continuous yielding characteristics was observed in the hot-rolled steels.