The Effect Of Intercritical Annealing And Q&P Process On Cold-rolled Medium Mn Steel

It is inevitable to reduce the weight of its body structure in the modern automobile industry.The research on a new generation of automotive lightweight 3G-AHSS(Advanced High Strength Steel)mainly focuses on:the strength and toughness much higher than the 1G-AHSS without too many alloying elements;much lower contents of alloying elements than the 2G-AHSS.Medium manganese steel as a typical representative of 3G-AHSS family,has received much attention recently,due to its excellent comprehensive mechanical properties,and relatively low manufacturing costs.In the present work,a Mo-Nb micro-alloyed Fe-6.5Mn-1.1Al(wt.%)steel with high strength and excellent ductility has been designed.The optimal intercritical annealing temperature and quenching temperature of Q&P process were determined,respectively,based on the thermal-dynamic calculation and theoretical analysis.On this basis,the effect of heat treatments(Intercritical annealing-IA,Q&P and low temperature tempering)parameters on microstructure-mechanical properties and its the mechanism of high strength-ductility were studied.The main contents and results were summarized as follows:(1)The optimal quenching temperature for the present Q&P steel was predicted?170? at which the maximum volume fraction of retained austenite was obtained,by combining the "CCE(Carbon constraint equilibrium)" model proposed by speer et al,and the modified empirical equation of martensite transformation temperature(Ms).the annealing temperature of IA process was also optimized to be?650?,based on the thermal-dynamic calculation and theoretical analysis;(2)The micro structure of cold-rolled(CR-ed)medium Mn steel processed by IA at 650? with different annealing time mainly consist of ultra-fine equiaxed ferrite,retained austenite and little martensite.The volume fraction of retained austenite at ambient increased firstly and then dropped slightly with prolonging the annealing time,peaking at 30min(?23%).The tensile testing showed that the maximum combined mechanical property was obtained at 30min,e.g.YS in excess of 1 GPa,PSE(product of UTS×El)?40 GPa.%.Moreover,all samples exhibited discontinuous phenomenon,and the YPE(yield point elongation)showed an inverse relationship with the grain sizes.(3)The cooling patterns after hot rolling(HR)and cold rolling(CR)reductions exhibited a significant influence on the microstructure-mechanical properties:as the cooling pattern varied from furnace cooling(FC)to oil quenching(OQ),the IA-ed microstructure was dramatically refined and the fraction of sub-structure increased,regardless of CR reductions,leading to a higher YS/UTS,but lower El.Compared with FC-samples,the OQ-samples showed a more significant dependence on CR reduction:as the CR reduction increased from 50%to 75%,YS/UTS of OQ-samples increased slightly,El dropped significantly;whereas the FC samples appeared to exhibit little difference in overall tensile properties in both cases.The optimal tensile properties of?1 GPa(YS)and?40 GPa.%(UTS×El)were achieved in the OQ-50%CR annealed samples at 650? for 1h.This indicates that it is feasible to improve the comprehensive mechanical properties of IA-ed medium Mn steel by changing the cooling style after hot rolling,which is highly advantageous to the industrial mass production.(4)Samples for tensile testing were cut from sheet annealed at 650? and 720?for 30 min along different direction.Tensile results showed that the experimental steels had good isotropy at both annealing temperatures.The cupping test was used to characterize the forming properties of the materials.The formability index Er was?10.2mm for the annealed steel at 650? which is much higher than?2.5mm for 720? annealed steel.This indicates that the mixed microstructure of ultrafine grain ferrite/austenite/small amount of martensite corresponding to 650? annealed samples is more conducive to the forming properties for the present CR-ed medium Mn steel.(5)As the austenization temperature decrease from 850? to 800?,the microstructural components changed from lath martensite+retained austenite to lath martensite twinning martensite+retained austenite,and the calculated volume fraction of retained austenite increased slightly from16.3%to 19.0%.Tensile testing showed that all Q&P processed samples exhibited continuous yielding behavior,and with the decrease of austenization temperature,the tensile strength of the experimental steel increases from 1220 MPa to 1400 MPa,and the elongation decreases from 13%to 8%.(6)After the typical medium Mn steel being tempered at 200? for 30 min,there were no significant changes in microstructural components,but the degree of convexity of block shaped microstructure(M/A)and the dislocation density inside the martensitic lath were reduced.The tensile YS/UTS decreased slightly,while the elongation and PSE increased by?5%and?7 GPa.%,respectively.As the tempering temperature increased to 400?,the tempered microstructure changed significantly in morphology and fraction of microstructural components.The number and size of coarser M/A island decreased dramatically,and the dislocation density inside the initial martensitic lath is further reduced.In addition,the nano-scaled carbides can be detected in the martensite matrix,which lead to a significantly drop in the volume fraction of retained austenite and inside C content.There was a distinct drop in UTS(?90 MPa),but the elongation and PSE were enhanced significantly by?10%and?13 GPa.%,respectively.