Study On Thermal-mechanical Process And Microstructure-properties In Medium Manganese Steel With High Performance

"High strength and weight reduction" is the main development direction for automobile industry,and it is an effective way to decrease energy consumption and increase safety performance by developing advanced high strength steel with high strength and ductility.Medium manganese steel is considered to be one of the most important advanced high strength steels because of its relatively low alloy cost and excellent mechanical properties.At present,the research of medium manganese steel mainly focuses on the controlling of two-phase microstructure composed of intercritical ferrite and austenite,the optimizing of alloy composition and austenite reversed transformation,and the revealing of microstructure-mechanical property relationship.However,the studies on microstructure evolution and mechanical properties of the medium manganese steel with Al addition containing ?-ferrite or the industrialization-friendly medium manganese steel with low Mn content are rarely reported.For the purpose,the ? ferrite-containing and low-Mn microalloying medium manganese steels are regarded as studied object.The evolution of microstructure and properties under different thermo-mechanical treatment process is systematically investigated.The typical microstructure characteristics,manganese partitioning behavior,austenite stabilization and stability,TRIP effect and work-hardeing behavior during deformation are investigated to achieve the optimal design of alloying and processing routes for a novel medium manganese steel with excellent properties,low cost and great potential of industrialisation.It also provids new methods for breaking through exsiting theories and industrialization bottleneck of medium manganese steel.The main results are as follows:(1)The microstructure evolution and mechanical property were systematically studied for hot-rolled medium manganese containing ?-ferrite under different heat treatment processes.The positive effect of ?-ferrite formation on the carbon and manganese partitioning kinetics was elucidated.The relationship between austenite grain size,morphology feature,distribution,austenite stability and mechanical property was revealed.The results showed that,in the case of two-step annealing,the formation of blocky austenite effectively improved the TRIP effect during deformation,and the work-hardeing rate was increased by continuous and progressive TRIP effects,leading to the increase of tensile strength.After two-step annealing,the tensile strength could be increased by?200 MPa.(2)The influence of intercritical rolling temperature in medium manganese steel with ?ferrite on microstructure evolution(e.g.as-hot-rolled,annealed microstructure and austenite phase),manganese partitioning behavior,TRIP effect and work-hardening behavior was systematically studied.The focus is the partitioning behavior of carbon and manganese during annealing and the different TRIP effects of austenite with different stability.The results showed that a relatively high intercritical rolling temperature promoted carbon and manganese partitioning from ? ferrite to prior austenite grain,and increased carbon and manganese content in martensite after hot rolling.In addition,the enrichment of the austenite stabilizing elements decreased Ms point of tested steel,thereby refining martensite lath.The fine martensite lath with high carbon and manganese content led to a high austenite stability after intercritical annealing.The TRIP effect of excessively stable austenite proceeded slowly,deteriorating the enhancement of strength.In contrast,a low intercritical rolling temperature led to an adequate stability and grain size,guaranteeing the positive TRIP effect.(3)The microstructure and mechanical property of cold-rolled medium manganese steel with ? ferrite under different heat treatments were studied.The effect mechanism of heat treatment on austenite stabilization,stability and behavior of strengthening and plasticity-enhancing was revealed.The results showed that quenching-tempering process promoted carbon partitioning,thereby increasing carbon enrichment in austenite and its stability.However,the formation of large blocky austenite after quenching resulted in the occurrence of TRIP effect at low strain.The austenite could continue to provide TRIP effect depending on its high enrichment of carbon and manganese and unique double-structured morphology characteristics,leading to the improvement of final mechanical property.(4)The influence of heating rate on microstructure of cold-rolled medium manganese steel with ? ferrite during reversed transformation annealing was studied.The results showed that austenite preferentially nucleated at the defects(boundaries of martensite and sub-structure),and then the nucleation of ferrite proceeded.During heating stage,large amount of deformation energy provide by high density dislocations were retained by rapid heating,thus promoting manganese partitioning and increasing austenite stability and the ductility of material.The large block-type austenite provided the TRIP effect and transformed to large blocky martensite,leading to the formation of more new dislocations.The interaction of these dislocations increased the tensile strength of the tested steel.(5)The influence of microstructure and mechanical properties of cold rolled medium manganese steel with 3wt.%manganese was systematically studied.Microstructure formation,ferrite morphology,austenite fraction,manganese partitioning behavior,TRIP effect and work-hardeing behavior under different heat treatments were revealed.The formation of double-structured austenite and the mechanism of manganese enrichment were analyzed.The results showed that high quenching temperature promoted manganese partitioning behavior during two-step annealing,thereby providing high manganese concentration for quenched matrix and further promoting the second manganese partitioning during the second annealing.In addition,martensite formed after quenching was transformed to austenite during the second annealing,thus refining austenite grains.The continuous TRIP effect caused by austenite grains with"double structure" significantly improved work-hardening behavior and increased the strength during tensile deformation.In addition,high quenching temperature avoided the formation of cementite at low temperature region,and promoted the precipitation of nanoparticles consisted of V and C,thereby strengthening ferrite and increasing yield strength from 635 MPa to 795 MPa.(6)Cryogenic rolling was applied to cold rolled medium manganese steel with 3wt.%manganese.The influence of prior microstructure with high intensity dislocations at deep temperature on microstructure evolution during intercritical annealing was studied.Nucleation of austenite and mechanism of manganese enrichment in austenite was analyzed.The results show that the application of cryogenic rolling limited the recovery of dislocations,increased dislocation intensity of as-hot-rolled microstructure and thus promoted ferrite recrystallization during annealing,where the ferrite grain was refined to 1 ?m.In addition,austenite nucleates by swallowing carbides,and the nucleation rate and volume fraction of austenite were increased.The austenite with high content and stability promoted the occurrence of TRIP effect during deformation,leading to an increase of strength by 170 MPa with little or no loss of ductility.