| High-quality wire is the basic components of high-end equipment such as automotive,energy,aviation and other materials,high-strength automotive suspension spring wire is one of the best among the wire.Although many domestic research institutes and enterprises have made some important progress in high-quality wire rod after years of efforts,there is still a big gap compared with the international advanced level.According to the performance requirements of high-strength spring steel and the effects of carbon and alloying elements on the microstructure and properties of the steel,combined with the thermodynamic calculation results of Thermo-Calc software,this paper mainly studies the influence of the content of vanadium and carbon on high strength steel,On the basis of 55SiCr spring steel,the components of high strength spring steel are optimized.9 experimental steelswere smeltedby vacuum induction furnace and molybdenum disilicide furnace.The effects of different compositions and heat treatment process on the micro structure and mechanical properties of the designed alloy steel were studied by Rockwell hardness tester,universal tensile testing machine,impact tester,optical microscope,scanning electron microscope and transmission electron microscope Law and mechanism.The results show that the addition of vanadium to steel improves the strength and toughness mainly because vanadium plays the role of fine grain strengthening and precipitation strengthening.After oil quenched at 880? for 30 minutes,the steel was tempered at 450?,and the microstructure of 1#-5#steels was tempered sorbite,and the degree of grain refinement gradually increased,the tensile strength and yield strength,the reduction of area and elongation after fracture both showed an upward trend.The tensile strength of 4#steel was 97MPa higher than that of 1#steel and the yield strength was increased by 113MPa.Compared with 7#steel,9#steel has increased in strength,but reduction of area and elongation after fracture decreased slightly,the results showed that the appropriate increase of carbon content is an effective way to improve the strength of high strength spring steelwith vanadium.The strength and toughness of the steelswere improved by the fine grain strengthening and precipitation strengthening under this tempering temperature.After tempering at 350?,the microstructures of 1#?5#steels consist of a certain amount of unlabeled lath martensite,more lamellar martensite and tempered tuffte.The larger number of untransformed martensite is the main reason for the substantial increase in strength at this tempering temperature.However,due to the addition of vanadium to refine the grains,the finer the grains are,the smaller the martensiteplate in it is,and the less the fibrous cracks are in the steel,the smaller the damage to the toughness.Under the premise of a substantial increase in strength,the reduction of areaafter fracture of 4#steel reached 38.8%,the elongation after fracture reached 10.3%,it showed thatthe toughness has been greatly improved,the dataof 5#and 4#steels are basically the same,The toughness of the steel is greatly improved by the fine grain strengthening at this tempering temperature.Through TEM,the VC generated in the austenite region is preferentially precipitated at the austenite grain boundaries,providing nucleation sites for the transformation of austenite to ferrite,refining the ferrite grains,The VC precipitated at the position plays a role of fine grain strengthening and the precipitated VC at the grain boundary has a higher precipitation temperature and a larger size of about 30 to 50 nm.Due to the high carbon content in the high-strength spring steel,after the transformation into ferrite,as there is still enough precipitating driving force to promote nucleation,the VC is precipitated in the crystal.At this time,the precipitated VC mainly plays a role of precipitation strengthening.The precipitated temperature of VC in the crystal and other types of MC precipitation is lower,so their size were smaller,about 20nm.With the increase of V content,the precipitation of nanoscale VC is more diffuse and the average distance is smaller With the contentof carbon continues to increase,the effect of precipitation strengthening is slightly improved.The optimumalloy design of high strength spring steel on the basis of 55SiCr steel is that the mass fraction of V is 0.15%.Increasing the appropriate mass fraction of C to 0.65%will further increase the strength of the steel.The tempering temperature of the 55SiCr steel added with vanadium can be appropriately reduced on the basis of the standard heat treatment tempering temperature of the 55SiCr steel so as to achieve the purpose of improving the strength. |
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