Studies On Microstructure,Mechanical Properties And Weldability Of Medium-Mn Automotive Steels For Hot Stamping

The application of advanced high strength steels(AHSS)into automotive components can significantly reduce weight for improving fuel economy without sacrificing safety.Among AHSSs,the boron-alloyed 22MnB5 steel,the typical and most used steel for press-hardening(PHS)in modern automotive parts,has the high ultimate tensile strength(UTS)over 1500MPa.Despite of its high strength,the 22MnB5 PHS often has a relatively poor plasticity,i.e.,total elongation(TEL)is less than 8%,and is severely oxidized during the hot forming process in the case of no coatings for protection,the latter often requires the very expensive Al-Si coating for protection against oxidization.To solve the two problems,a novel Cr-alloyed medium-Mn steel(MMS)was designed and manufactured for the hot forming application in automobiles.It possesses a better combination of mechanical properties than 22MnB5 and enormously enhanced resistance to oxidization during press hardening.Moreover,the weldiabiltiy of developed steel together with other similar medium Mn steels were evaluated under the resistance spot welding(RSW)process,which is the dominant joining technique in automotive industries.The main results can be summarized as follows:(1)The developed novel Cr-alloyed MMS(MMCr)exhibits a much better mechanical combination of 1850 MPa UTS and 14%TEL than the conventional 22MnB5 PHS after the hot forming and the baking process.The new mechanism on improving mechanical properties was revealed as follows.First,both the low austenitization temperature that was employed for press hardening and the pinning of undissolved dispersed carbides result in the fine prior austenite grain size(PAGS),which could refine both the resultant hierarchical martensite and retained austenite(RA)in the final microstructure.This then leads to strengthening via Hall-Petch mechanism;second,a much larger fraction of austenite grains is retained in MMCr due to much higher Mn conent than 22MnB5;and then it was found via both in-situ and ex-situ synchrotron examination that the solute C atoms could be partitioned from martensite to RA during baking and segregate at geometrically necessary dislocations(GNDs),the latter had been developed near the austenite/martensite interface due to the martensite transformation during cooling.As a result,the baked RA grains could be hardened and mechanically stabilized,leading to the delayed plastical deformation.Compared to the non-baked RA grains,these hardened RA grains could bear more load partitioned during deformation until they transformed to martensite,leading to more sustainable TRIP effect and ehanced work hardening,thereof greatly improving strength and ductility at the same time.(2)The thickness of oxidization layer on the developed steel is less than 3 ?m after the hot forming at 750-810?,much smaller than about 100?m-thick oxidization layer formed on 22MnB5 PHS.In other words,the MMCr has the sufficient resistance to oxidation during hot forming without requiring the expensive Al-Si coating.Such a tremendously reduced oxidation results from both the lower soaking temperature employed and the formation of the dense Cr/Al/Si oxide band at the bottom of the oxidation layer,which results from the unique chemical composition of designed steel.Compared with 22MnB5,the higher Mn content in MMCr steel leads to a pronounced decrease in the austenitizing temperature so that a low austenitizting temperature can be employed for hot forming;and the much higher contents of Cr and Al result in the formation of the dense Cr/Al/Si oxide bands at the bottom of the oxide layer to hinder the diffusion of Fe and O and prevent further oxidation into the matrix.Both result in much higher oxidation resistance of MMCr than 22MnB5.(3)The poor weldability with the feature of the interfacial fracture across weldment is the key bottleneck in the application of MMS although they have wonderful mechanical properties.To tackcle this,the new shim-assisted RSW set-ups,including an interstitial-free steel shim at the faying interface(shim-in)and shims against the electrodes(shim-out),were developed.Experimental results show that the failure mode of the 7Mn steel welds can change from the frequent interfacial fracture in the case of no shim to the desired nugget pull-out fracture in the shim-out set-up,resulting in dramatical increases in peak loads during both the tensile-shear and cross-tension tests by 1.5 and 3 times.This is because more heat is generated due to higher resistance and more metal is melt during welding in the shim-out set-up to feed the larger nugget.In addition,the lean C and Mn contents in the shims dilute the alloying concentration in nugget,resulting in lower carbon equivalent.In contrast,the shim-in set-up does not improve weldability at all.This is because the melt of entire shim consumes more heat and less heat is produced by the shim with low resistivity,leading to many porosities and cracks in nugget without larger size.(4)The micro structural mechanism on the interfacial fracture of spot-welded MMS was studied,and the technical solution was proposed.Next,the new model with determined criterion for evaluating the weldability of AHSS is established to overcome the drawbacks of the popular carbon equivalent(CE)model.?The high Mn content of MMS resulted in the great shrinkage during solidification and cooling,which caused the micro-interspaces retained between some isolated dendrites that have not impinged near the centerline of nugget.This led to the interfacial fracture of welded joint and the deteriorated weldability.?The popular paint baking process at 170? in automobile industry could afford a sufficient thermal expansion for the solidified dendrites and effectively remove the micro-interspaces,leading to these isolated dendrites connected via the"cold welding" mechanism.?Based on the above-mentioned conclusion that the solidifying and cooling shrinkage causes many micro-interspaces retained between dendrites and then leads to the interfacial fracture of weldment,a new shrinkage-based model(?l)to evaluate the weldability of AHSS was established with the determined criterion.By using this model,the IF and PF mode of RSW nuggets can be predicted when ?l is higher or lower than the threshold value of 1.39%respectively.Compared with the current popular CE concept,the new model can evaluate and predict the weldability of steels having high alloying contents more accurately,exhibiting greater potential on the future applications than CE.