An Investigation Of A New Kind Of Extremely High Strength Automobile Plate Spring Steel

With the development of lightweight and high-performance in automobile industry,and extensive use of leaf spring in heavy truck,much high requirement is needed for the mechanical properties of the plate spring steel.Therefore,there is an urgent need for increasing the strength and extending the lifetime of the plate spring steel.The goal of this research is to develop extremely high strength automobile plate spring steel.Considering the existing researches in automobile plate spring steel at home and abroad and the influences of alloying elements and heat treatment process on the mechanical properties of the spring steel,a new type of extremely high strength automobile plate spring steel has been designed in this thesis.A detailed investigation has been made on the hot working technology and heat treatment process,and newly automobile plate spring steel has been developed to meet the performance requirement.The main contents and results obtained in this thesis are summarized as follows.Firstly,a thermo-dynamical simulation was made by JMatPro software on the properties of the spring steels with different chemical compositions.In order to meet the requirement for the mechanical properties of the experimental steels,an optimal chemical composition of the experimental steel have been designed(0.5%C?1.0%Si?0.9%Mn?1.0%Cr?0.1%V?0.03%Nb?0.0015%B).Secondly,the static CCT diagram of the experimental steel was measured in thermal dilatometer by using thermal expansion and metallographic methods.The phase transformation of undercooled austenite under different cooling conditions was studied using hardness measurement and microstructural analysis.The result indicates that the full martensitic microstructure could be obtained when the experimental steel was cooled at 5?/s.The flow stress curve of the experimental steel deformed at elevated temperatures is obtained by a single-pass compression experiment in thermo-mechanical simulator.The influences of deformation parameters on the dynamic recrystallization and flow stress were also studied,while the parameter Z and activation energy Q for hot compressive deformation of experimental steel have been obtained.The flow stress equation has been established.Finally,two-stage hot rolling schedule was employed to obtain the experimental steel plate in a laboratory ?450 mm hot rolling mill.The microstructure of the experimental steel after hot rolling consisted of ferrite and pearlite.After comparing pearlite interlamellar spacing at different finishing temperatures,finishing temperature was obtained at 800?.A detailed experimental study has been finally performed for the quenching and tempering in order to obtain the optimal heat treatment schedule and ensure the mechanical properties.The results showed that the quenching process should be 920?×30min and the tempering process is 410?×60min.In such a case,a comprehensively optimized mechanical performance of the experimental steel could be obtained.The tensile strength is 1890MPa and the yield strength is 1700MPa,while the elongation is 11.3%and reduction ratio of area is 35.7%.