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A new variable compression turbo (VC-Turbo) engine, which has a multi-link system for controlling the compression ratio from 8:1 to 14:1, requires high axial force for fastening the multi-links because of high input loads and the downsizing requirement. Therefore, it was necessary to develop a 1.6-GPa tensile strength bolt with plastic region tightening. One of the biggest technical concerns is delayed fracture. In this study, quenched and tempered alloy steels were chosen for the 1.6-GPa tensile strength bolt.

In recent years, the temperature of automobile exhaust gas is on a rising trend due to lowering pollutant emissions and improving fuel economy, and exhaust gas temperature reaches as high as 1173K in the case of diesel engine cars. Against this background, Ni-resist D-5S cast iron has been chosen extensively as a turbine housing material for the diesel engine cars. But, Ni-resist D-5S has become a material of great cost volatility due to high Nickel content of 35 mass%, which price is expensive and unstable. On the contrary Ferritic cast steels, which possesses favorable thermal fatigue properties and good material cost stability, are considered to be promising substitutions for the Ni-resist D-5S. However conventional ferritic cast steels have relatively high melting points, which cause poor castability.

It was found that pitting resistance of gears is strongly influenced by resistance to temper softening of carburized steel. The investigation about the influence of chemical compositions on hardness after tempering revealed that silicon, chromium and molybdenum are effective elements to improve resistance to temper softening and pitting resistance. Considering the production of gears, molybdenum is unfavorable because it increases hardness of normalized or annealed condition. Developed new steel contains about 0.5 mass% of silicon and 2.7 mass% chromium. The new steel has excellent pitting resistance and wear resistance. Fatigue and impact strength are equivalent to conventional carburized steels. Cold-formability and machinability of the new steel are adequate for manufacturing gears because of its ordinary hardness before carburizing. The new steel has already been put to practical use in automatic transmission gears. Application test results are also reported.

Engine valve development trends are to first, reduce the costly metal content and secondly, increase strength or reduce weight. These developments can be used to reduce valve cost or fuel consumption or increase power. The authors developed a new strain age hardening type alloy, NCF2415C, which has both good cold forgeability and heat resistance. Its chemical composition is Fe-24Ni-15Cr-2.2Ti-1.5Al-0.5Nb-0.02C-.006B-2Cu. This new alloy and the establishment f cold forging technology made it possible to develop cold forged exhaust valves having durability equal or better than the conventional hot forged exhaust valves.

For the purpose of saving natural resources and energy, the requirements of vehicle weight-saving have been increasing continuously. As for Automotive Suspension Coil Spring, its weight-saving has been achieved by increasing the design stress. Since the increase of design stress requires higher fatigue life and sag resistance, the strength of spring is usually increased. However, in case of the conventional spring steel, the high strength over σB=1900MPa can dramatically reduce the corrosion fatigue life of spring, to decrease the reliability of spring at the actual usage. In this paper, newly developed spring steel material, satisfying higher strength and corrosion fatigue life simultaneously, is proposed, and its application of Automotive Suspension Coil Spring under the appropriate spring manufacturing processes in introduced.

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.

Honda has developed, in collaboration with Hitachi Metals and Daido Steel, two types of low-nickel heat-resistant alloys for exhaust valves which are more cost effective than the conventional nickel alloys. They are NCF4015 that contains approximately 40% nickel and NCF3015 with approximately 30% nickel content. The two types of new alloys were developed based on our unique alloy design concept. Both alloys feature superb high-temperature strength and are capable of maintaining favorable material properties, even after an high-temperature exposure. The NCF4015 is compatible with the conventional Inconel 751 and 60Ni alloys in terms of high-temperature strength. The NCF3015 falls slightly behind the two metals, but overwhelms the 21-4N (SUH35) in high-temperature strength. The exhaust valves made of the two alloys developed have been used for mass production engines.

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 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.