| The development of advanced automotive steel with high strength and plasticity is one of the important ways to reduce the weight of automobiles.The third generation of medium manganese steel for automobiles(with Mn content of 3.5-12wt%)developed in recent years,through the effective use of the TRIP/TWIP effect of its metastable austenite component,comprehensively improves the formability and mechanical properties.Current research hotspots in the field of lightweight automotive steel research and development.Among them,critical annealing of hot-rolled and(or)cold-rolled manganese steel in an appropriate temperature range to adjust the metastable austenite phase ratio,stability and element distribution is the most critical process way to control performance.In this paper,a 11 Mn type medium manganese steel with a nominal composition of Fe-11Mn-3Al-2Si-0.3C is taken as the research object,and the microstructure of the hot-rolled/cold-rolled mediummanganese steel under different critical annealing processes is systematically studied.The evolution characteristics and the response law of mechanical properties have explored the strengthening and toughening mechanism under different process conditions.The main results include:(1)After the critical annealing treatment of the hot-rolled manganese steel,the microstructure is mainly composed of the interphase arrangement of lath ferrite and reverse transformation austenite,and a small amount of ?-ferrite.With the increase of the annealing temperature,the width of the lath structure of the hot-rolled medium manganese steel increases,and the mechanical properties first increase and then decrease.Among them,the medium manganese steel used in the hot rolling test can obtain the best annealing process at 850?-30 min Mechanical properties.At this time,the yield strength and tensile strength are 315 MPa and 1327 MPa,respectively,the elongation at break is 14.6%,and the strong plastic product is about 19.4GPa·%.(2)The cyclic cold rolling critical annealing process is proposed to obtain the austenite ferrite dual phase structure,and the fine equiaxed ferrite is distributed on the austenite matrix.During the cyclic rolling process,the microstructure changes from lath-like structure to equiaxed structure,and the fracture mode changes from brittle fracture to ductile fracture.For cold-rolled manganese steel,with the increase of annealing temperature,the microstructure is mainly composed of equiaxed ferrite and reverse transformation austenite,and with the increase of annealing temperature,the microstructure of cold-rolled manganese steel Obvious growth occurs.Among them,the mechanical properties of cold-rolled manganese steel first increase and then decrease with the increase of annealing temperature.The best mechanical properties can be obtained under 800?-30 min annealing process,and its yield strength and tensile strength are 570.32 MPa and 1105.62 MPa,the elongation is76.87%,and the strong plastic product is 84.99GPa·%.(3)The plastic deformation of medium-manganese steel with equiaxed fine-grain dualphase structure is mainly controlled by the interaction of the TRIP and TWIP effects of metastable austenite.Martensite and deformation twins appear at the beginning of deformation.As the amount of deformation increases,deformation twins begin to cross.??martensite nucleates at the twin crosses,and a large amount of ?? martensite appears after fracture.Appears,due to the continuous activation of martensite and deformation twins under low strain and the large number of deformation-induced martensite under high strain,the cold-rolled manganese steel has ultra-high mechanical properties?... |
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