Research On Microstructural Control And Strengthening-toughening Mechanismof Novel Heterogeneous Cold Rolled Medium Manganese Steel

Lightweight and high strength alloy steel has become the research mainstream of material field and automobile enterprise,so that developing a new generation of AHSS with improved balance between strength and formability is of utmost importance.In recent years,the third generation of high strength steel represented by medium manganese steel?Mn:4-12%?has been booming.The key is how to obtain metastable retained austenite?RA?with various morphologies.In this paper,the heterogeneous structure of retained austenite is obtained by adjusting rolling reduction and heat treatment,starting from controlling the influence factors of morphology,content and stability of retained austenite.Moreover,it is a huge challenge to further get the properties of medium Mn steel close to or even surpass that of twin induced plasticity?TWIP?steel in this field.In this paper,two thermo-mechanical treatment processes,namely,ART annealing and intercritical annealing plus low temperature tempering?IT?,were carried out in the cold-rolled Fe-0.2C-8Mn-1.5Al-0.04Ce steel,respectively.The effects of the two process parameters?austenitizing temperature and time,annealing temperature and time?on the microstructure and mechanical properties of medium Mn steel were studied by means of SEM,TEM,XRD and EBSD.The TRIP behavior and the strengthening-toughening mechanism of medium Mn steel with heterostructure were analyzed by means of ACTEM-EDS and EBSD.The main conclusions are as follows:?1?The microstructure of cold rolled medium manganese steel treatment by ART is composed of retained austenite and ferrite.The cold rolled specimen,austenitized at a relatively high temperature that above AC₃,exhibits ferrite and austenite duplex phase structure with lamellar and equiaxed morphologies and obtain outstanding mechanical property,that tensile strength is 1265.85MPa,total elongation is 23.86%,and the PSE is43.5GPa%.?2?When the austenitizing/semi austenitizing temperature of ART is lower than AC₃,the austenite grain exhibits lamellar and blocky morphology.With an increase in semi austenitizing temperature,the lamellar austenite grains gradually transform into blocky morphology.When the austenitizing temperature is higher than AC₃,the main morphology of austenite and ferrite grain is lamellar.In addition,the relationship between retained austenite content and austenitizing/semi austenitizing temperature display irregularly and the austenite content change in the ranges from 25%to 34%.?3?The microstructure of cold rolled medium manganese steel after IT treatment is composed of retained austenite and ferrite.The sample annealed at 680? exhibits ferrite and austenite duplex phase structure with multiple morphologies?granular shape,blocky shape,and lath-like?and dispersive grain size.The heterogeneous material has 1.27 GPa ultimate tensile strength,total elongation of 54.5%and the PSE of 69.3GPa%,which is superior to current medium Mn steels with similar compositions and even surpass that of twin induced plasticity?TWIP?steel?4?After annealing at 680?for different time and tempering at low temperature,the microstructure of medium manganese steel treated by it process is composed of austenite and ferrite.With the increase of annealing time?5 min-120 min?,the content of austenite in the steel increases continuously,and the change of grain morphology is significant.The grain size changes from multi morphology grain to near equiaxed grain gradually.However,the yield strength,tensile strength and total elongation of the sample increase first and then decrease with the increase of annealing time and the best annealing time is10 min.?5?The various morphology austenite in the heterogeneous structure medium steel exhibits different mechanical stability.Thus,the martensite transformation in the present heterogeneous steel occurs in a stepwise manner,namely,the granular-shape austenite with a low Mn content preferentially transform martensite ahead of the blocky-shape and lath-like austenite with high Mn content during tensile deformation.The cooperation of sustained TRIP effect over a broad strain regime lead to continually high WHR and decent mechanical property.