The Efect Of Nb,Mo,V Microalloying On Microstructure And Mechanical Properties In ?-TRIP Steel

?-TRIP steel is one of 3^rd generation advanced high strength steel?AHSS?.Due to the addition of 3-6%Al element,?-TRIP steel has the advantages of low density,great mechanical properties,formability and weldability.Its excellent properties meet the purpose of lightweight of automotive and has the potential to become the industrial application for modurn automobile steel.But at present,the tensile strength of?-TRIP steel is less than ideal because of the coarse delta-ferrite phase in the microstructure.The further application of?-TRIP steel is limited.Therefore,how to improve the strength of?-TRIP steel without losing its plasticity is the key point to the development and application of?-TRIP steel in the future.However,microalloying,as a commonly used method to strengthen the steel,has become an ideal way to achieve this goal.This thesis comprehensively studies the effect of Nb,Mo,V micro-alloying on the microstructure and mechanical properties of?-TRIP steel by adding 0.07wt.%Nb,0.12wt.%Mo or 0.05wt.%Nb,0.2wt.%V microalloy into the referecnce steel?Fe-0.41C-1.6Mn-4.1Al-0.62Si?.After significantly improving the mechanical properties of?-TRIP steel,the strengthening mechanism of Nb,Mo,V microalloying in the?-TRIP steel had analyzed by using SEM,TEM,XRD and EBSD.The mechanical property of?-TRIP steel is greatly improved through the reasonable design for alloy composition and heat treatment.The Nb/Mo adding?-TRIP steel gets the tensile strength of 1044 MPa and the elongation of 22.4%.While the Nb/V adding?-TRIP steel has the tensile strength of1028MPa and the elongation of 26.7%.The product of strength and elongation is 13.8%and 33.6%higher than that of non-microalloying?-TRIP steel,respectively.The microstructure of the studied steels both contain delta-ferrite,bainite,alpha-ferrite and retained austenite phases.But after adding the Nb/Mo or Nb/V microalloy elements,the size of grain is refined,and the volume fraction of retained austenite increases slightly.Also,a mass of nano-size Nb/Mo or Nb/V carbides are precipitated in the ferrite matrix and grain boundary.The fracture mechanism of experimental steels aslo changes after adding Nb,Mo and V elements.A large number of dimples appear in fracture morphology,which is the feature of ductile fracture?microporous aggregated fracture?.The?-TRIP steel without microalloy element occurs the mixed fracture of ductile fracture and quasi-cleavage fracture.After plastic deformation,the volume fraction of retained austenite is significantly disreased both in the Nb/Mo adding and Nb/V adding?-TRIP steel.The metastable block retained austenite almost disappears in the microstructure and transforms to the martensite phase.By this Transformation induced plasticity effect?TRIP effect?,the ductility of the material is greatly increased.Additionally,many carbides precipiated particles?about 20nm?nail the dislocation and hinger dislocation movement during the deformation.The new dislocation rings are formed by Owowan mechanism leading to the increase of dislocation density.Thus this apparently enhances the strength of the steel.The microstructure characterization and mechanical properties show the major strengthening mechanism of both Nb/Mo and Nb/V steels is the transformation-induced plasticity?TRIP?effect of retained austenite as well as the carbides playing an important role on the fine-grain strengthening and second-phase strengthening by pinning grain boundaries or dislocations.