Effect Of Rare Earth Ce And Nb On Microstructure And Properties Of 65Si2CrV Valve Spring Steel

With the improvement of energy conservation and environmental protection requirements,valve springs,which are important parts of automobile engines,have gradually developed in the direction of light weight,high stress and high fatigue.Therefore,the valve spring steel as a raw material is required to have high strength and toughness.However,the current research on valve spring steel is mainly focused on improving its strength,and less research on improving its toughness.In order to achieve the purpose of improving its toughness,based on literature research and early technology accumulation,this research takes valve spring steel 65Si2CrV as the research object,and explores the influence rules and action mechanism of rare earth Ce and Nb on its inclusions,microstructures and mechanical properties.This experiment uses a 30 kg vacuum induction furnace to smelt 7 furnaces of experimental steel.The test results of the experimental steel show that an appropriate amount of rare earth Ce can play a role in purifying molten steel in the steel,and the lowest mass fractions of oxygen,sulfur,phosphorus,and arsenic can be reduced to 0.00086%,0.0012%,0.0060%,and 0.0026%,respectively.Without the addition of rare earth Ce,the types of inclusions in steel are mainly MnS,MgO·Al2O3 and MgO·Al2O3 inclusions coated with MnS.When the mass fraction of Ce is 0.01%and 0.04%,the inclusions change to rare earth-containing When MnS,MgO·Al2O3 and other inclusions of Ce are added to the mass fraction of Ce added to 0.07%,the inclusions become Ce-O-S,Ce-S inclusions,and Ce-Pcontaining rare earth Ce also appears.As-type inclusions,when the rare earth Ce content is 0.09%,the inclusions in the steel are changed to Ce-O,Ce-O-P-type inclusions.The experimental results of the structure of the rare earth-treated steel show that as the Ce mass fraction of the rare earth increases,the pearlite layer spacing of the valve spring steel is first refined and then coarsened.When the rare earth Ce mass fraction is 0.04%,the pearlite layer spacing of the experimental steel reaches the finest.With the increase of the rare earth Ce content,the quenching and tempering of valve spring steels tend to be refined first and then coarsened.When the mass fraction of rare earth Ce is 0.04%,the quenched martensite structure becomes the most refined because the austenite grain size becomes smaller,and the tempered bainite also reaches the most refined level.The related experimental results of adding Nb steel structure show that the pearlite layer spacing of Nb steel is more fine;The quenching structure of Nb free steel and Nb containing steel is a mixture of fine acicular martensite and lath martensite,but the proportion of fine acicular martensite increases in the quenching structure of Nb containing steel.Tempered tissues are all tempered corpus.On the whole,the quenched and tempered structure of Nb-free steel is significantly thicker than that of Nb-containing steel.The main reason is that the precipitation of Nb-containing precipitates at and near the grain boundaries will inhibit the recrystallization of grains,which results in that the austenite grains of Nb-containing steels are finer than those of Nb-free steels.In this way,the quenched and tempered structure is refined.The experimental results related to the mechanical properties of rare earth Ce-treated steel show that the strength,plasticity,and impact toughness of valve spring steel first increase and then decrease as the mass fraction of rare earth Ce increases.When the mass fraction of Ce added is 0.04%,the mechanical properties of valve spring steel are optimal.The improvement of mechanical properties is mainly due to the fine mass of rare earth Ce can refine the structure.The modification of rare earth Ce to modified inclusions also affects the mechanical properties of steel.The experimental results of mechanical properties related to the addition of Nb steel show that as the Nb content increases,the room temperature impact toughness,strength and plasticity of valve spring steel are improved.When the mass fraction of Nb is 0.0269%,the mechanical properties of valve spring steel reach the highest.The improvement in mechanical properties is mainly due to the refinement of the structure by Nb.