Effect Of Rolling And Heat Treatment On Microstructure And Properties Of 72B Flat Steel Wire For Coil Spring

Scroll spring is a kind of widely used planar spring. 72 B flat steel wire is the raw material for scroll spring which used to automobile seat, During spiral winding process, cracking appears on the surface of spring or in some cases the spring will fracture, affecting the performance of the spring, which lead to the scroll spring batch scrap. The reasons why the cracking appeared on the scroll spring were studied in this thesis. The effect of rolling process on microstructure and mechanical properties of flat steel wire were analyzed by means of metallographic analysis, mechanical performance test and flat wire tensile deformation hardening model establishmented, the influences on microstructure and mechanical properties of 72 B flat steel wire through annealing treatment and quenching and tempering process were compared.With the increase of rolling passes, the internal grain refined and narrowed, and core part of wire had the max squeeze deformation and the maximum hardness value. The work hardening model has been established. The Holloman index equation is modified using the Ludwik model, which leads to the fitting degree increasing from 0.937 to 0.99.The coefficient of strain K and strain hardening index n were used to quantitatively describe the variation of tensile performance of flat wire during rolling process.The effect of heat treatment on mechanical properties and microstructure of flat steel wire is studied, which show the annealing or quenching and tempering heat treatment were used to eliminate work hardening caused by cold processing, improve the follow-up processing performance, achieve optimal match of strength and plasticity. After annealing treatment, the strength and Vickers hardness of steel wire decreased gradually, a gradual increase in plasticity. The optimum temperature is 350 ℃ for the annealing process. The elongation rate of wire is 8% and the strength is 1322 MPa, and best to meet product requirements. The strength of the flat steel wire decreased and plasticity was slightly increased after oil quenching and tempering treatment. The elongation rate of wire is 4.2% and the strength is 1412 MPa after quenching at 810℃ and tempering at 350℃, which is the optimum quenching and tempering process.Flat steel wire mechanical properties are improved at 350 ℃ annealing, and 810℃ quenching and 350℃ tempering, effectively solve the problem of the crack and fracture of flat steel wire in the process of coil spring. According to the performance requirements of flat steel wire to decide what kind of heat treatment process.Can choose 350 ℃ annealing process to reduce production costs for low strength and coil weight, conversely,can choose 810℃ quenching and 350℃ tempering process.