Abstract
The effects of carbon content and microstructure on the mechanical properties of high strength electric resistance welded (ERW) steel tubes for automotive suspension parts were investigated. Characteristics considered include strength, elongation, low temperature toughness, fatigue endurance, and electric resistance weldability. Laboratory vacuum-fused steels with compositions in the range of 0.05-0.14% C0.1% Si1.1% Mn0.01% P 0.001% S(Ti, Nb, Cr, Mo) were hot-rolled into 4 mm thick sheets after reheating at 1523 K for one hour. The material was finished at 1123 K, air-cooled to room temperature, reheated at 873 K for one hour, and finally furnace-cooled. The strength, elongation, and strain hardenability improved with increasing carbon content. However, the bendability and Charpy impact value of the weld seam decreased with increasing carbon content. Consequently, a carbon content about 0.1% provided the best balance of these properties. Nanometer-scale precipitates were characterized by energy dispersive X-ray spectroscopy (EDXS) to be metal (Ti, Nb, Mo) carbides. The fatigue endurance of the steel was numerically and experimentally examined. The microstructure was modeled using two-dimensional Voronoi polygons. Heterogeneous stress distributions were calculated using the finite element method, taking elastic anisotropy into consideration. The number of cycles to crack initiation was estimated based on the Tanaka-Mura model. The simulation yields a relationship between the crack density and the number of load cycles. In the calculation, the effect of cyclic strain of the preceding crack initiation was taken into account using Miners law. The simulation results were compared with the experimental results. The change of microscopic dislocation structure was observed by transmission electron microscopy (TEM). Based on the 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. In addition to developing tube materials, a new tube bending method that does not use a mandrel and a wiper, push rotary bending (PRB), was developed for thin-walled high strength steel tubes. TS 980 MPa grade steel tube with excellent formability characteristics were developed for body structural parts by the combination of cold rolled steel sheet with excellent formability and the low-strain tube production technique. KEYWORDS suspension parts, high tensile strength steel tube, fatigue endurance, formability