Research On Microstructures And Mechanical Properties And Plasticity-enhancing And Strengthening Mechanisms Of Warm-processed Nb-Mo Medium Mn Steels

With the increasing demand of mechacnial properties of advanced high strength steels(AHSS)from the automotive industry,it is of great significance to accelerate its research and industrial application.As one typical candidate among the 3rd generation AHSS,Medium Mn steel has attracted more attention in view of the combination of high strength(?1 GPa)and good ductility(?30%),together with the lower production and alloy costs.Until now,medium Mn steels were mainly manufactured using hot/cold rolling plus intercritical annealing(IA),and are characterized by a multi-scale,ultra-fine and complex phase microstructure.The excellent mechanical properties of medium Mn steels were mainly related to the coordinated deformation of ultra-fine microstructure and the enhanced TRIP effect.In order to overcome the springback effect and mold wear of ultra-high strength medium Mn steels during the cold working and stamping process,the warm/hot working technology has attracted more and more attention.In this work,the influence of microalloying addition,warm rolling(WR)reduction ratio and IA temperature on the microstructure and mechanical properties of a typical medium Mn steel with a multiple Nb-Mo addition was studied.Moreover,the plasticity-enhancing mechanism and roles of microalloying elements of AHSS were also investigated.This paper also investigated the influence of the stamping process parameters on the surface quality,microstructures and mechanical properties of U shaped parts in the medium Mn steel fabricated by warm/hot stamping and quenching&partitioning(Q&P).In addition,the microstructure evolution during the stamping process and deformation behavior of U shaped parts at different forming temperatures were also investigated.The main achievements are summarized below:(1)The microstructures and mechanical properties affected by the rolling reduction ratio and IA temperature of WRed medium Mn steel were investigated.With the increase of rolling reduction ratio from 25%to 95%,the lamellar microstructure was changed to equiaxed and lamellar mixed microstructure of the WRed medium Mn steel after IA.An increase in IA temperature from 590? to 710? led to an increase in the fraction of equiaxed microstructure.The WRed medium Mn steel treated at 650? for 60 min has 1340 MPa ultimate tensile strength(UTS)and 44%elongation(EI).(2)The transformation behavior of retained austenite greatly relied on the tensile deformation temperature.An increase in tensile deformation temperature from-50? to 150?improved the mechaincal stability of retained austenite.The deformation of the WRed medium Mn steel was dominated by the TRIP effect when the tensile deformation temperature ranges from-50? to 25?,and the deformation was dominated by TRIP+TWIP effect when the tensile deformation temperature ranges from 50? to 150?.(3)Three kinds of medium Mn steels with different contents of Nb and Mo were designed to systematiclly investigate the influence of Nb/Mo addition on the microstructures and mechanical properties of the WRed medium Mn steels.The addition of Nb and Nb-Mo can increase the strength of the medium Mn steel without sacrificing its elongation.The microalloying addition can decrease the volume fraction of retained austenite(V?)of WRed medium Mn steel after shorter time annealing,but increase the mechanical stability of retained austenite.The volume fraction and mechanical stability of retained austenite of Nb/Mo bearing medium Mn steels after longer time annealing are higher.The Mo addition to the Nb bearing steels can increase the density of(Nb_xMo_1-x)(C,N)and reduce the mean size of precipitates by lowering the interfacial energy between the precipitates and ferrite marix,and inhibiting the coarsening rate of(Nb_xMo_1-x)(C,N).The optimal precipitation temperature of(Nb_xMo_1-x)(C,N)in this work is around 600?.(4)The microstructure and mechanical properties of the U shaped parts in the Nb-Mo bearing WRed medium Mn steel at different stamping temperatures were investigated.No matter the stamping temperature in single-phase austenite(760?)or dual-phase(700?)zone,the U shaped parts exhibit excellent surface quality,dimensional accuracy and mechanical properties.At 700?,the U shaped parts have 1424 MPa UTS and 22%elongation.At 760?,the U shaped parts present 1717 MPa UTS and 16%elongation.(5)The Q&P treatment was applied to the warm stamping process of medium-Mn steels,and its influence on the microstructures and mechanical properties was investigated.The Q&P treatment greatly increases the yield strength of the U shaped parts by the formation of twinned martensite and some amount of carbides.In addition,the Vy is higher with the Q&P treatment for the higher C concentration in austenite.The U shaped parts exhibit the average values of product of UTS and elongation?29 GPa·%at 720? and?23 GPa·%at 800?,which are two times higher than those of 22MnB5 steel.