Effect Of Microalloying On The Mechanical Properties And Microstructure Evolution Of Twinning-Induced Plastic (TWIP) Steels

Twinning-induced plasticity(TWIP)steel features high strength,good toughness,high energy absorption capacity,and it has no FATT,yet its low yield strength has severely restricted its industrial application.Therefore,a microalloying method,i.e.,adding niobium,vanadium,and titanium into TWIP steel as micro-alloying elements,is adopted in this project to enhance the yield strength with little loss of strength and plasticity.In this study,the tensile properties of TWIP steel and micro-alloyed TWIP-MA steel with different annealing temperature and cold rolled alloy are tested at room temperature.The strain hardening of the TWIP steel and micro-alloyed TWIP-MA steel at different compression amounts are studied with the hardness testing.Also,the impact absorption energy of the micro-alloyed TWIP-MA steel at different temperatures is determined with the Charpy impact test.Moreover,Optical Microscopy(OM),X-Ray Diffractometer(XRD),Scanning Electron Microscope(SEM),Transmission Electron Microscope(TEM)and Energy Dispersive Spectrometer(EDS)are utilized to observe the structure before and after deformation.And the microstructure evolution in different deformation processions are analyzed.Meanwhile,interaction and strengthening mechanism of dislocations,twin crystals and precipitation of micro-alloy elements are discussed.The following conclusions are obtained:1.The strength and plasticity of TWIP steel have been improved by adding micro-alloying elements,which strengthens metal by refining grains.The best mechanical properties of cold rolled TWIP-MA steel was gained after annealing at 900 °C,with yield strength increased by 14.7%(55MPa)and total elongation increased by 8.7%(5.6%).The best product of strength and elongation we got reaches 53.2GPa·%.2.Twinning is the main mechanism in the compressive deformation process of TWIP steel.The precipitation of microalloying elements refines the austenite grains and suppresses the formation of twins.Meanwhile,the stress concentration,leading by precipitation phases for pinning dislocations,promotes the formation and development of twins.The trend of the hardness of TWIP-MA in compressive deformation process is as same as TWIP,with a higher hardness performance from smaller grains.3.All fractures in TWIP-MA steel from 20? to-196? are ductile,with no FATT observed.The formation and development of twins in TWIP-MA are promoted by micro-alloyed precipitation phase,refining the grain and increasing the strength of the material,thereby improving the impact resistance of the material.