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The influence of chemical compositions and forging conditions on mechanical properties of forged bainitic steels were studied. Manganese and chromium are useful to produce bainite structure while carbon and vanadium are good to control the mechanical properties of the steels. One of the compositions is 0.25 % C - 2.1 % Mn - 0.7 % Cr - 0.15 % V of which tensile strength is 1000 MPa and impact value (2 mm U type notched specimen) is 50 J/cm2 for 100 mm diameter bars. Bainitic steels have lower fatigue limit in the case of smooth specimen than ferrite-pearlite microalloyed steels but have higher fatigue limit in the case of notched specimen.

Based on fundamental study about the toughness of hot rolled microalloyed steels, a new steel composed of 0.4 % carbon, 1.10 % manganese, 0.5 % chromium and 0.15 % vanadium was developed. The steel shows high toughness and high proof stress after well-controlled hot rolling. The newly designed steering racks for passenger cars using the steel bar have properties equivalent to the racks of conventional quenched and tempered carbon steels.

Based on fundamental research about the influence of chemical composition on rolled bar hardness, hardenability, case hardenability, cold formability, and mechanical properties, a new case hardening steel has been developed which can be cold forged without spheroidizing annealing. The steel contains boron and the Si and Mn contents are less than conventional steels. The steel shows fatigue strength equivalent to the conventional steels and better toughness and machinability.

The cold forging process is one of the most popular in the manufacture the automotive parts such as gears and shafts, cold forging saves material and machining costs by near-net shape the principle of forming. However, abnormal austenite grain growth sometimes occurs when the cold forged parts are heated for surface carburizing without a prior normalizing process. The size of the coarse grains can be large, sometimes ASTM Grain Size Number -2 to -4. The abnormal grain growth may cause post-carburizing distortion and is harmful to both fracture toughness and fatigue strength of the parts [1]. The purpose of our research was to develope new steels which would keep the fine grains during the carburizing treatment without normalizing. First, we studied the influence of elements on the grain growth property of case hardening steels and Naiobum (Nb) was selected as the element to control the grain growth. Secondly, we developed an ultra fine grain steel containing a small amount of Nb.

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).

Influence of lead, sulfur and oxygen contents on various properties of steels for machine structural use has been studied. Lead and sulfur improve machinability but deteriorate rolling contact fatigue strength and cold workability. Sulfur is more harmful for cold workability than lead. Oxygen is harmful for machinability, fatigue strength, rolling contact fatigue strenth and cold-workability. Based on the fundamental studies, high quality leaded steel ULO-M has been developed. ULO-M contains ultra low oxygen and an optimum amount of lead to improve the machinability without lowering fatigue strenth and cold workability. ULO-M can be applied to high strenth parts of automobiles and other machines. The good chip breakability of ULO-M helps the operation of automated and unmanned machine shops.