F2010H018
Metallurgical Design and Fatigue Endurance of TS 780 MPa Grade ERW Steel Tube for Automotive Suspension Parts
Keywords - Suspension parts, High tensile strength steel tube, Fatigue endurance, Formability Abstract - In view of application of high strength ERW (electric resistance welded) steel tubes for automotive suspension parts, the effects of carbon content and microstructure on the balance of some practical properties, i.e., strength, elongation, low temperature toughness, fatigue endurance, and electric resistance weldability, were investigated. A laboratory vacuum-fused steels having composition of 0.05-0.14%C－0.1%Si－1.1%Mn－0.01%P－0.001%S－(Ti, Nb, Cr, Mo) were hot-rolled into 4mm thick sheets after reheating 1523 K for 1 hour. Finishing temperature was controlled 1123 K, and the sheets were air-cooled to room temperature, reheated 873 K for 1hour, and finally furnace-cooled. The balance of strength and elongation was improved with increments of carbon content accompanied by an accretion of strain hardenability. However, the bendability and Charpy impact value of weld seam decreased with increments of carbon content. Consequently, about 0.1% carbon content provided the best balance of these properties. Precipitations in several to tens of nanometer were characterized by EDXS (energy dispersive X-ray spectroscopy) as the MC type (Ti, Nb, Mo)C carbide. The fatigue endurance of the steel was numerically and experimentally examined. The microstructure was modeled by two-dimensional Voronoi-Polygons. Heterogeneous stress distributions were calculated with finite element method, taking an elastic anisotropy into consideration. The number of cycles to crack initiation was estimated on the basis of the Tanaka-Mura model. The simulation yields a relationship between crack density and number of load cycles. In the calculation, the effect of cyclic strain of the preceding crack initiation was taken into account as Miner's law. The simulation results were compared with the experimental results. The change of microscopic dislocation structure was observed by TEM (transmission electron microscopy). Based on above-mentioned results, TS 780 MPa grade ERW steel tube, having well-balanced practical properties, was successfully developed and applied to actual automotive suspension parts.
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Session: Efficient Production and Operation of Vehicles and Components