In order to improve the fatigue strength and the sag resistance in mediumcarbon spring steels, a novel thermomechanical processing, "aus-drawing", was developed. Aus-drawing is an aus-forming using wire drawing. Theaus-drawing process was applied to two kinds of mediumcarbon spring steelscontaining Si, SWOSC-V, and SAE9254 (JIS SUP12). Three kinds of typicalmicrostructures appeared in the aus-drawing wires depending on the drawingconditions: fully martensite structure, surface bainite structure, and fullybainite structure. It was clarified that mainly the acceleration of bainitictransformation kinetics by increase in plastic strain and additionally the heatgeneration during drawing cause the appearance of bainite. The preferentialformation of bainite near the surfacecould be explained by the relatively largeequivalent strain near the surface due toredundant shear strain during drawing.The aus-drawn wires with fully martensite structure showed superior fatiguestrength and sag resistance to those ofthe conventional oil-tempered wires after tempering and shot peening. On thecontrary, the aus-drawn wires with bainite showed inferior properties tothat of the conventional wires. Therefore,it is quite important to decrease the redundant shear strain near surface toinhibit the bainitic transformation, inorder to succeed the aus-drawing process.
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