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The purpose of our research is to omit normalizing after hot forging in nitrocarburized crankshafts. Based on fundamental studies about the influence of chemical composition on as-forged and nitrocarburized properties, the authors have developed a new nitrocarburizing steel composed of 0.3% carbon, 0.8% manganese, and 0.02% nitrogen. The newly designed crankshafts for compact cars using the steel can be in use without the normalizing and have equivalent properties to conventional crankshafts, though the treatment is an indispensable process for conventional ones.

Corresponding to the demand for light and strong connecting rods for passenger cars, the authors started the project on development of a high strength free machining microalloyed steel and the application to new light steel connecting rods. To meet the demand that the new steel has high strength and good machinability, the influence of alloying elements such as C, Mn, Cr, V, S, Pb, and Ca on fatigue strength and machinability was investigated. 1.1%Mn-0.5%Cr is the best combination to obtain the high fatigue strength. Lowering the carbon content down to 0.33% and increasing the vanadium content to control the hardness gives the high toughness which improves the fatigue strength of free machining grades with S and Pb. The finally obtained chemical composition is 0.33%C-1.05%Mn-0.5%Cr-0.12%V-0.055%S-0.20%Pb-Ca. The steel has a fatigue strength 26% higher than a conventional high strength type of microalloyed forging steel (0.39%C-1.05%Mn-0.11%V).