Study On Microstructure Evolution And Properties Of 0.13C-5Mn-1Al Steel During Heat Treatment

As for automobile steels,while meeting the requirements of lighting weight,improving their strength and safety is the development trend of modern automobile steels.Materials with higher strength and higher elongation have also attracted more and more attention.In this paper,a third-generation medium-manganese steel for automobiles with a composition of 0.13C-5Mn-1Al is designed based on the M3 organization control ideas(multi-phase,multi-scale,and metastable).Austenite reserved transformation annealing processing(ART annealing processing)with different annealing temperatures and holding times are applied to this medium Mn steel.Microstructures and mechanical properties medium manganese steel processed under different annealing temperatures and holding times were examined by Metalloscopy(OM),scanning electron microscope(SEM),X-ray diffractometer(XRD),and tensile tester to seek the best heat treatment processing.In different temperatures annealing processing,a large amount of carbides is presented at lower annealing temperature of the matrix,and carbides disappear when the temperature increasing.The volume of reverted austenite firstly increases and then decreases with the increasing of temperature,and reaches a maximum at 640 ° C,at this time,the content of reverted austenite is about 10%.The strength and ductility are also changed with the volume of reverted austenite.The strength and ductility are the highest at 640 ?.Martensite lath becomes finer as the heat treatment temperature increases.At 670-730?,due to the increase in annealing temperature,the reverted austenite continues to grow,and the reverted austenite grows faster than the diffusion rate of carbon and manganese elements in the reverted austenite,making part of the reverted austenite transformed into martensite,new martensite is generated in the matrix,and the tensile properties of medium-manganese steel increase with the increasing of processing temperature.and reach the maximum at 730?,which is 1115 Mpa,indicating that the optimal ART annealing processing temperature for medium manganese steel is 640?.In the annealing process with different holding times,the carbides disappear with the extension of the annealing time,and the reverted austenite increases first and then decreases,and it increased to the maximum volume at 6h,about 14%.At the holding time of 6-10 h,the reverted austenite transformed into martensite.The tensile property test shows that the mechanical properties of the structure are the best when the holding time is 6h.At this time,the tensile strength is 759 Mpa,the elongation is 27.75%,and the strength and ductility are 21 Gpa ·%.Therefore,the best annealing temperature is 640 ° C and holding time is 6h.The volume fraction of reverted austenite at room temperature has a certain effect on the mechanical properties of medium manganese steel.The higher the reversed austenite content,the more obvious TRIP effect(Transformation Induced Plasticity Effect)occur and the higher the strength and ductility product.When the reversed austenite content is low,the TRIP effect is weaker and the strength and ductility product is lower.Thus,reverted austenite with more components is the focus of improving the mechanical properties of the alloy.