An Investigation Of Vanadium Precipitation Law And Quenchingpartitioning Process In Automobile High-Strength Steel

The application of high strength steel in automobile plays very important role in reducing dead weight,fuel consumption,energy saving and emission reduction.Vanadium microalloyed automotive high strength steel can obtain higher strength and better comprehensive properties through vanadium grain refinement and diffusion precipitation mechanism.In order to give full play to the role of vanadium in steel,the precipitation mechanism of vanadium in continuous annealing process and its effect on quench-partition process were systematically studied in this paper.The following research results have been obtained.The evolution of microstructure and properties of the steel during continuous annealing was studied by using a cold-rolled steel plate with a composition of 0.21C-1.62Si-2.07Mn-0.09 V.At the initial stage of annealing,the samples did not reach the austenitizing temperature,and the microstructure and the original cold-rolled samples were ferrite and pearlite.As the annealing process proceeds,lath tempered martensite,massive martensite,bainite,vanadium precipitates,massive secondary martensite and residual austenite begin to appear.The tensile strength of the tested steel increases first and then decreases,from 1130 MPa to 1315 MPa,then from 1315 MPa to 1183 MPa,and the elongation gradually increases from 6.8 to 15.2%.After annealing,the test steel shows good plasticity and strong plasticity.The evolution of vanadium carbide precipitates during continuous annealing was analyzed qualitatively and quantitatively.The types of precipitated carbides were Vc and V2 C.The size of vanadium carbides was concentrated in the range of 20?60nm,and the number of carbides gradually increases.Vanadium carbides were mainly distributed on the ferrite matrix,and the addition of vanadium will hinder the transformation of bainite,so that only a small amount of bainite structure could be found after holding at 380? for900 s.The relationship between microstructure and properties of test steels under one-step and two-step quench-partition processes were investigated.Useing high temperature austenitizing completely made V full solid solution in the original austenite,the microstructure of one-step process is tempered martensite,ferrite,fresh martensite,M/Aisland,retained austenite composition,with the increase of temperature distribution,ferrite volume fraction decreased,martensite volume fraction increased,the retained austenite volume fraction increased first and then decreased,and the the maximum value was 2.6% when the partition temperature was 350?;The microstructure of two-step process consists of tempered martensite,fresh martensite,M/A-island and retained austenite.With the increase of partition temperature,the volume fraction of fresh martensite gradually increases,which is represented by the increase of M/A-island,and the volume fraction of retained austenite gradually increased,and the maximum value was 3.8% when the partition temperature was 400?.The properties of the steel tested by the two-step quench-division process were obviously better than those of the one-step process.The phase transformation kinetics of one-step and two-step separation processes were studied.The experimental data of one-step process were in good agreement with the theoretical calculation.The two-step process had some deviation.