Experimental Study On Performance Control Of High Quality Coil Spring For Car Seat Belts

Spring steels as a kind of materials are widely used in the manufacturing of mechanical equipment parts in many fields, for example aerospace, automotive, railway, etc. Because the working conditions of spring steel is very hard, the performance of spring steel is very strict. The main factors to influence the design of spring steels are the fatigue resistance and the ability of decreasing. They have close relationship with the inherent quality(chemical component, purity), surface quality(crack, pit, and decarburization) and microstructure. So, the spring steels do not only have high strength and good toughness, but also have certain hardenability, lower decarburization sensitivity and good processing performance.In this paper, the spring steel Super6 and SK5 that often used to produce the safety belts of automobile were studied. First, the law of decarburization of spring steel Super6 and SK5 was studied to avoid decarburization on the surface of steels in the process of hot rolling. The heating systems of two kinds of steels during the process of hot rolling were simulated to analysis the influence of the heating temperature and holding time on the decarburized layer depth. The results show that the sensitivity of decarburization of Super6 is significantly higher than that of SK5. The spring steel Super6 should reduce the heating time above 700 ℃, heat rapidly during 900 ℃ to 1100 ℃, minimize the holding time above 1100 ℃, and the temperature of hot coiling is under 700 ℃. And the spring steel SK5 should reduce the heating time under 1100 ℃, heat rapidly above 1100 ℃, and the temperature of hot coiling is under 950 ℃.For steady increasing the fatigue life of spring steel Super6 and SK5, the system of heating treatment should be establishing reasonably. The CCT curves of the spring steel Super6 and SK5 were simulated by the JMatPro software. According to the forecast results of JMatPro software, orthogonal experiment plans about heat treatment were designed. The influences of different systems of heating treatment on the mechanical properties(yield strength, tensile strength and elongation) were analyzed by range analysis in order to obtain the best system of heating treatment of two kinds of spring steels. The results shows that the optimal heat treatment process for the spring steel Super6 is 880 ℃/30 min quenching and oil-cooling, 430 ℃/60 min tempering and air-cooling, and the optimal heat treatment process for the spring steel SK5 is 850 ℃/30 min quenching and oil-cooling, 400 ℃/40 min tempering and air-cooling.The microstructures of two kinds of spring steels were compared to find the reasons of the differences of fatigue life between the two kinds of spring steel. The results show that the microstructures of two kinds of spring steel after heat treatment are both tiny tempered troostite. The tempered troostite in SK5 spring steel is smaller, the distributions of carbide are more uniform and the density of dislocation is greater than that in Super6 spring steel. Otherwise, there are some big size particles of dissolve carbon nitride in Super6 steel. And the density of deformation texture and recrystallization texture are both greater than that of spring steel SK5, especially the textures in the direction of {113} <110> and {331} <110> are enhanced clearly.The different microstructures of two kinds of spring steels lead to different performance of fatigue.The results shows that both of the two kinds of spring steels after optimal heat treatment meet the comprehensive mechanical properties requirements, and the fatigue life of the spring steel SK5(≥40 million) is longer than the spring steel Super6(≥10 million).