| With the increasingly serious energy crisis and environmental problems,it has been becoming the major concern to increase fuel efficiency,improve vehicle safety and reduce the cost for the automobile manufacturing.The PSE(the production of tensile strength and total elongation)of first generation advanced high strength steels(AHSS),such interstitial free(IF)steels and dual phase(DP)steels,are about 15GPa%,which cann't meet the performance requirements of future automobile structural parts.Although the 2nd generation of AHSS represented by twin induced plasticity(TWIP)steel can reach 60 GPa%,but the alloy cost is high as well as complex smelting technology.The medium manganese(medium Mn)steel and quenching and partitioning(Q&P)steel with excellent strength-ductility combination as well as low alloy cost have been making them attractive for automotive sheet forming as the 3nd generation of AHSS.On the basis of research process of medium Mn steel,two experimental steels with compositions of Fe-5.9Mn-1.55Si-1Al-0.055C and Fe-8.9Mn-1.54Si-1Al-0.056C(wt.%)were designed and researched for this paper,which was funded by the National Natrual Science Foundation of China.Microstructures and chemanical properties of the medium Mn steels were controlled via the technologies of cold rolling(and quenching)and annealing.The microstructures of the medium Mn steels were characterized and determined by means of optical microscopy(OM),scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),X-ray diffraction(XRD)and transmission electron microscopy(TEM).Effect of reversed austenite(?R)and dislocation density on yielding behavior in the medium Mn steel is one of key researchal points and the influence of morphology of ?-ferrite(?)on microstructures and chemanical properties of the medium Mn steel.The main contents and achievements of this paper are as follows:(1)Microstructures of Fe-5.9Mn-1.55Si-1Al-0.055C steel mainly consisted of ultra-fine recrystallized ?-ferrite(?),?R and coarse-grained ?-ferrite(?)by annealing for 660-760?.The optimal mechanical properties of sample annealed at 740? were yield strength(YS)of 824 MPa,tensile strength(TS)of 980 MPa and total elongation(TE)of 32.8%,respectively.Microstructures of Fe-8.9Mn-1.54Si-1Al-0.056C steel quenched consisted of lath-shaped tempered martensite(?'T)and ?R via annealing for 620-700?.The great mechanical properties were showed in sample annealed at 660?(WQ-660),whose YS,TS and TE were 763 MPa?1001 MPa and 30%,respectively.(2)The yielding behavior of Fe-5.9Mn-1.55Si-1Al-0.055C steel with cold-rolling and annealing was systemically investigated by means of liquid nitrogen cryogenic and adjustment of the cooling rate after annealing.Compared with the sanples annealed at 720? and 740?,the volume fraction of ?R in samples annealed and cryogenic treatmented with liquide nitrogen decreased 2.2%and 5.4%,and yield point elongation of them increased 0.9%and 0.8%,respectively.This indicated that volume fraction and mechanical stability of ?R play a role in activity of TRIP effect,further shortened yield point elongation.The samples with annealing at 760? were cooled by air,water and 10%NaCl solution,and the dislocation density of ferrite in 760A,760W and 760NaCl samples were 1.35,1.59 and 2.61(×1014 m-2),respectively.Tensile test indicated that the length of yield point elongation were 4.7%?2.5%and 1.7%,respectively,thus this showed that the higher dislocation density in ?-ferrite and ?-ferrite shortened yield point elongation.(3)The yielding behavior and yield strength of Fe-8.9Mn-1.54Si-1Al-0.056C steel were systemically investigated.Interaction between dislocation and interstitial atoms was responsible for discontinuous yielding behavior in WQ-620 sample,and active TRIP effect in AQ-620 sample improved the work hardening rate in the early stage of the tensile test and further eliminated yield point elongation.The reason why the yield strength of AQ-620 specimen was improved 27 MPa compared with WQ-620 specimen,is that the strengthening of dislocation and grain boundary.Dislocation density and equivalent grain size were 1.71×1014 m-2 and 0.92 ?m for AQ-620 specimen,whereas 1.39×1014 m-2 and 1.01 ?m for the latter.(4)It was proposed and verified that ?/?'(?-ferrite and martensite)interfaces in the medium Mn steel with ?-ferrite can be as nucleation point of ?R.the ?-ferrite influenced nucleation,content and stability of ?R during annealing.In addition,?-ferrite bore more deformation due to being considered as soft phase compared with ?-ferrite and ?R,and the coarse grained ?-ferrite contributed to dislocation movement. |
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