| Recently,medium Mn steel has received significant attention as third-generation high-strength steels because of their outstanding combination of strength and ductility with low-cost.The medium Mn steels with Mn-content of 4%?12%are characterized by the production of tensile strength and total elongation(RmxTE)of 25-60 GPa·%.The excellent mechanical properties of medium Mn steels is attributed to strengthening-plasticizing mechanism by transformation induced plasticity effect of metastable austenite.The key to obtain more retained austenite(RA)is the composition design and heat treatment process.The chemical composition of the experimental steel is Fe-0.2C-9Mn-3.5Al(wt pct).The effect of different heat treatment processes on microstructural regulation,mechanical properties and deformation behavior have been investigated,in an attempt to consolidate the experimental and theoretical bases for better structure control and further improvement of properties of the AHSS.The results are summarized as follows:(1)The ?-ferrite(?-F),intercritical ferrite(I-F)and lamellar typed ultrafine RA of the medium Mn steels processed by ART-annealed have been demonstrated.It is revealed that the duplex structure with large fraction of RA(>40%)is capable of producing steels with excellent combination of strength and ductility,i.e.,RmxTE about 30.78 GPa·%and impact absorbing energy(Akv)about 182J for steels annealed at 750? for 60min.(2)It is referred the circular quenching technology on ART-annealed process,namely,CQ-ART annealed process.It is revealed the CQ-ART annealed process are effective in obtaining ultra-refined and disordered austenite block,meanwhile,the fraction of RA slightly reaches to 52%.The CQ-ART annealed steels demonstrates a significant improvement in the RmxTE of 41.53 GPa·%and impact absorbing energy(Akv)of 222J.The excellent properties of CQ-ART annealed steels are mainly attributed to the refined prior austenite grains and enhanced TRIP effect.(3)The "Quenching and Partitioning"(QP)heat treatment are adopted for medium Mn steels.The microstructural constituents comprise of 8-F,martensite(M)and RA,but the fraction of RA(<20%)is less than ART annealed steels.QP steels is characterized by a significant improvement in Rm(>1100MPa),as compared to ART annealed steels,but less TE below 25%,RmxTE of 25.26 GPa%and Akv of 47.16J.The deterioration of mechanical properties compared to ART annealed steels,is mainly attributed to martensite matrix and a small amount of RA with inadequate TRIP effect.(4)The strain hardening behavior of two processed experimental steels have been investigated.The strain hardening rate(WHR)of ART annealed steels can be divided into four stages,namely,S?-rapid decrease,S?-slight decrease,S?-stable rise,S?-fluctuant decrease.The superior mechanical properties are attributed to the long scope of S?,which indicates the enhanced TRIP effect and excellent cooperative deformation of multiphase structure.While the QP steel is characterized by a three-stage work hardening behavior,namely,S?-rapid decrease,S?-rapid rise,S?-fluctuant decrease.The shorter rise of WHR in the S?,which is presumably associated with the TRIP effect and excellent cooperative deformation of martensite and 8-F.But the short strain scope of S? is slightly contributed to the mechanical properties. |
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