Study On Microstructure Control And Hydrogen Embrittlement Susceptibility Of Al-Containing Medium Mn Steels With High Product Of Strength To Ductility

Recently,medium-Mn steel with excellent combination of strength and ductility as well as economical efficiency and manufacturing feasibility has become a research hotspot of the third generation of advanced high strength automotive steel.How to further improve the comprehensive mechanical properties especially the manufacturability and safety service of this kind of steel are becoming the key points to its successful applications.In this dissertation,microstructural evolution,mechanical properties and hydrogen embrittlement(HE)behavior of medium-Mn steels containing 0?3(wt.%)Al designed based on the traditional 0.2C-5Mn steel subjected to different processing procedures including hot rolling(HR),cold rolling(CR)and warm rolling(WR)plus intercritical annealing(IA)were systematically investigated.Emphasis was paid to the effects of Al content,IA parameters and WR process on the microstructural evolution and mechanical properties of the experimental steels.Furthermore,the possibility of lowering the HE susceptibility of the tested steel was also explored.The main results are listed as following:The microstructural evolution and mechanical properties of the experimental steel after HR plus different IA processes were studied.The results show that the IA temperature of the experimental steel increases with an increase in Al content,which in turn significantly shortened the IA treatment duration and thus,was in favor of mass industrial production.The stability of retained austenite(RA)was also enhanced with increasing A1 content and thus,excellent mechanical properties could be obtained.Although the strength levels of the 1Al steel(1 wt%Al)and 3A1 steel(3 wt%Al)decreased slightly compared with the tensile properties of 0Al steel(Ultimate tensile strength(UTS)of 1008 MPa,total elongation(TEL)of 36.2%and the product of UTS and TEL(UTS × TEL)of 36.5 GPa%),their ductility were significantly improved and therefore,the values of UTS × TEL as high as 49.2 GPa%and 56.2 GPa·%could be obtained,respectively,which is much higher than those of medium Mn steels of similar Mn contents.The results of the study of the microstructural evolution and mechanical properties of 3A1 steel after CR plus different IA treatments show that the RA size of the CR steel coarsened and gradually changed its morphology from mainly lamellar to mainly equiaxed with increasing IA temperature and a duplex microstructure consisting of multi-scale RA could be obtained at 730 ?,which possesses suitable mechanical stability and thus presents sustained TRIP effect during tensile deformation.This kind of sustainable TRIP effect and the cooperative deformation of ferrite are responsible for the superior mechanical properties of the tested 3A1 steel.3A1 steel exhibited UTS of 1062 MPa,TEL of 58.2%and UTS × TEL of 62 GPa%after IA treatment at 730 ?and then tempering at 200 ?.The tempering treatment at 200 ? caused an increase of the carbon content in the RA and thus made a further enhancement of its tensile properties.The microstructural evolution and mechanical properties of 3A1 steel subjected to WR process with different deformation amount(0?90%)after IA treatment were studied.The results show that the laths of ferrite and RA not only gradually rotated their longitudinal axes to the rolling direction but also were refined with increasing rolling thickness reduction,and thus a well developed lamellar structure of bimodal size distributed lathy ferrite and RA was obtained after heavy rolling reduction.Though the strength increases while the ductility decreases with increasing rolling thickness reduction,an excellent combination of strength and ductility was obtained through a wide process window of WR.It is thus proposed that WR is a promising way to simplify the traditional multi-stage rolling and annealing processes of Al-containing medium Mn steels,especially to overcome the CR difficulty of medium Mn steels with high carbon content.The HE behavior of 3A1 steel subjected to both CR and WR processes with different microstructural characteristics obtained through changing IA temperature and time was investigated by carrying out slow strain rate tensile(SSRT)test and hydrogen thermal analysis on electrochemically hydrogen charged samples.It was found that the HE susceptibility of the experimental steel increased with the increase of IA temperature and time.Unlike that of typical ductile fracture characteristic of uncharged sample,the hydrogen-charged sample shows mixed fracture characteristics of ductile intragranular and brittle interface decohesion fracture mode in the crack initiation region.The susceptibility to HE was correlated mainly to volume content and the mechanical stability of RA.The HE susceptibility of 3A1 steel gradually decreases with the increase of WR thickness reduction,and it is expected that WR is an effective way to lower the HE susceptibility of medium Mn steel.