| Suspension springs for automobiles are developing in the direction of lightweight,high stress and high performance.At the same time,they are required to have high reliability and safety.According to the application of spring steel under different service conditions,it is required to have good corrosion resistance,wear resistance,heat resistance and so on,in order to avoid premature fatigue fracture during service.Based on the composition of 55SiCr spring steel,combined with the influence of C and V elements on the structure and properties of spring steel,the composition of 55SiCr spring steel was optimized by means of fine-tuning of its composition,so as to improve its micro structure and mechanical properties.Firstly,the precipitation phase and precipitation temperature in spring steel are calculated and analyzed by JMatPro software when the mass fraction of elements V and C is changed.Then three experimental steels were smelted in a 30 kg vacuum induction furnace in the laboratory.Five experimental steels were smelted in the previous stage,and the standard 55SiCr steel samples were provided by a domestic steel plant.Using JMatPro software thermodynamic calculation,direct reading spectrum analyzer,Rockwell hardness tester,universal material tensile test machine,impact test machine,Olympus GX51 inverted optical microscope,scanning electron microscope and rotary bending fatigue tester,etc.The structure and mechanical properties of as-cast,forged and heat-treated experimental steels with different design compositions were comprehensively analyzed to determine the optimum composition system.The experimental results show that the structure,toughness,strength and fatigue life of the experimental steel are improved remarkably when adding 0.15%V element to the steel under the heat treatment system of holding at 880℃ for 30 min+oil quenching+ tempering at 450℃ for 120 min,air cooling to room temperature heat.Compared with the properties of the specimens provided by the factory,the tensile strength of the specimens is increased by 125 MPa,and the fatigue life limit is increased from 3.93×105 to 6.68×105.The main reason for the analysis is that the diffused VC exerts precipitation strengthening and fine grain strengthening.After heat preservation at 880℃ for 30 min,oil quenching+450℃ tempering for 120 min,air cooling to room temperature heat treatment system,The metallographic structure of the experimental.steel of 1#～5#is basically tempered torsite,and with the increase of the mass fraction of V,the grain is obviously refined.With the increase of C mass fraction to 0.65%,the strength of 7#steel and 8#steel increased again to 1765 MPa,but the elongation and section shrinkage decreased by 1.3%and 1.2%respectively after fracture..Appropriately increasing the mass fraction of the C element can continue to increase the strength and performance of the high strength spring steel.Under the heat treatment system of holding at 880℃ for 30 min+oil quenching+tempering at 350℃ for 120 min+air cooling,the metallographic structure of the experimental steel is mainly composed of lath martensite,flake martensite and tempered torsite.Under the tempering system,the experimental steel has higher hardness and strength.The main reason is that the tempered martensite structure remains intact and the amount is large at this temperature,and the martensite matrix strengthening effect is obvious.4#,5#,8#experimental steels were added with more V elements,the grains were refined,and the toughness was significantly improved.According to the comparison of tensile strength,section shrinkage and elongation after fracture of the experimental steel under the 350℃ tempering system,it is found that the comprehensive mechanical properties of the experimental steel are optimal when the V element content is about 0.15%.The fatigue test shows that the fatigue life of spring steel specimens tempered at 450℃ is significantly higher than that of the specimens tempered at 350℃.When the V element content is 0.15%,the fatigue life of the experimental steel after tempering at 350℃ and 450℃is 2.46×105 and 2.75×105 respectively,compared with the factory sample.The experimental steel has a high fatigue life limit because of its small inclusion content and size.Through the analysis of inclusions at fatigue fracture,it is found that the inclusions are mainly sulphide,sulphide-coated alumina and sulphide combined with magnesia-alumina spinel.The size of inclusions is between 2μm～10 μm,and the number of inclusions is small.Thermodynamic calculation and metallographic structure analysis of steel precipitates by JMatPro software show that acicular cementite precipitates along the plate boundary after tempering at 450℃,which hinders dislocation slip,improves the structure of slip zone and plays a certain role in strengthening.The fatigue limit ratio after tempering at 350℃ is significantly lower than that after tempering at 450℃. |
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