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Nissan’s variable compression turbo (VC-Turbo) engine has a multilink mechanism that continuously adjusts the top and bottom dead centers of the piston to change the compression ratio and achieve both fuel economy and high power performance. Increasing the exhaust gas recirculation (EGR) rate is an effective way to further reduce the fuel consumption, although this increases the exhaust gas condensation in the cylinder bores, causing a more corrosive environment. When the EGR rate is increased in a VC-Turbo engine, the combined effect of piston sliding and exhaust gas condensation at the top dead center accelerates the corrosive wear of the thermal spray coating. Stainless steel coating is used to improve the corrosion resistance, but the adhesion strength between the coating and the cylinder bores is reduced.

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 addition to the requirements of high power output and compactness, further reduction of weight is being required for motorcycle engines from the standpoint of fuel economy and reduction of CO2 emissions. For this purpose, it is important to reduce crankshaft weight, which is the heaviest rotating part in the engine. The crankshaft has to be strong enough to bear loads, as the demands of weight reduction are increasing. Yet, productivity has to be considered at the same time even when increasing crankshaft strength. In this report of crankshaft material studies that feature high fatigue strength, machinability and distortion correct-ability, attention is given to the fact that the amount of vanadium, which is known as an element that enhances the strength with its precipitation, accelerates deposition, dissolved in the steel depends on the heating temperature.

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.

In Europe and the U.S., fracture split connecting rods are used in many types of current engines. This process can eliminate the machining of crankshaft end and eliminate the dowel pin for positioning. The most important key for fracture split connecting rods is a reduction in the plastic deformation during the fracture splitting process. For this reason, sinter-forged materials and pearlitic steels (C70S6) are used for fracture split connecting rods because of their low ductility. Such types of steel, however, are inferior to the hot forged microalloyed steels typically used as connecting rod material in Japan in terms of buckling strength and machinability although they are easier to fracture split. On the other hand, the conventional microalloyed steels used for connecting rods in Japan are not suitable for fracture splitting. The reason is that these steels have too much ductility and associated plastic deformation for fracture splitting.

In recent years, the use of aluminum die cast parts in automobile manufacturing has increased due to greater demand for automotive weight reduction. For even wider application, it is necessary to reduce manufacturing costs and improve product quality. Finite element method (FEM) analysis suggested that a new material, featuring 50% improved thermal conductivity within the working temperature of the die compared to the conventional 5% chromium hot work tool steel AISI-H13 (H-13), would decrease thermal stress on the die surface and lower the maximum surface temperature. As a result, the reduced stress should increase the die service life with respect to heat checks. At the same time, the reduced surface temperature should increase the cooling rate of die cast products, which will in turn improve product quality due to finer structure formation.

SUS301-EH is widely used as a material for exhaust system gaskets, however, at temperatures in excess of 400°C, it can not be used as gas-seal ability of the material declines due to its reduced hardness. The following methods were found to be effective in controlling the softening of stainless steel at high temperatures: (1) The addition of a nitrogen component; (2) Stabilization of the austenite structure; (3) The addition of a molybdenum component. The addition of 0.5% nitrogen to austenitic stainless steel containing molybdenum has enabled the speed of softening at high temperatures to be significantly reduced, due to strain aging by solid nitrogen below 600°C and the combined effects of precipitation hardening and control of growth of recrystallized grains through the precipitation of fine Cr2N on the dislocations and the grain boundary above 600°C.

A free machining Pb-free steel was developed with shape controlled sulfide (SCS) for use in automobile applications such as rocker arms and crankshafts. This free-machining steel is characterized by its improved chip breakability, for which a technique that adds very small quantities of Ca and Ti to control sulfide shape was specifically applied. It was confirmed that this free-machining steel can offer almost equivalent machinability and equivalent or higher fatigue strength by comparison with Pb-added steel for use in rocker arms or crankshafts with S and Ti modifications.

As trends of automobile engine exhaust gas temperature are reducing emissions, the material for the exhaust components have been changed from ductile irons to ferritic cast alloys or stainless steel, further to austenitic cast alloys for higher performance engines. The current austenitic alloys, however, have thermal fatigue failure over 1273K. The authors developed excellent thermal fatigue resistant austenitic cast alloys, by investigating the effects of alloying elements on strength and thermal expansion, which correlate with thermal fatigue property. Developed alloys are expected to apply to exhaust components at gas temperatures over 1273K.

Ferritic stainless steel gas metal arc welding (GMAW) wires have been widely using for automotive exhaust system components made of ferritic stainless steels. In order to enhance the high temperature strength of weld metal, it is necessary to make the microstructure of weld metal finer. In this study, the effect of the chemistry of ferritic stainless steel GMAW wire on the weld metal microstructure was investigated and new ferritic stainless steel GMAW wire providing fine grain microstructure of the weld metal was developed to improve high temperature mechanical properties, oxidation resistance, corrosion resistance of the weld metal and weldabiliy of the wire.

Improving the impact strength of the differential gears is one way to reduce the size and weight of the final drive unit. Previously, we developed high-strength steel for gear use by adding molybdenum and reducing impurities such as phosphorus and sulfur. However, additional improvement of impact strength is required these days due to higher engine torque and demands for further weight reductions. Toward that end, we focused on boron, which has been used as an element for improving hardenability, and analyzed what effect its addition would have on impact strength. Useful knowledge was obtained for improving impact strength through enhancement of grain boundary toughness. Various steels were then produced experimentally and used in gear strength tests. The results made it possible to improve impact strength while reducing the content of other alloys, resulting in the development of a chromium-molybdenum-boron case hardening steel with superior cold forgeabilty.

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.

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.

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.

The application of both high strength gear steels and shot peening technology has succeeded in strengthening automotive transmission gears. This technology, though, improves mainly the fatigue strength at the tooth root, but not the pitting property at the tooth face. Therefore, demand has moved to the development of new gear steels with good pitting resistance. In order to improve pitting resistance, the authors studied super carburizing which is characterized by carbide dispersion in the case, especially processed with a plasma carburizing furnace. Firstly, the influence of the carburizing temperature and carburizing period on the carbide morphology was investigated and the optimum carburizing conditions were determined. Secondly, the fatigue strength and pitting resistance was evaluated using carbide dispersed specimens.

Although shot peening is an old technology, it has been revived in the Japanese automotive industry as a means to enhance the fatigue durability of steel components. Particular emphasis is on the application of “hard shot peening”. “Hard shot peening” is a high intensity peening technology which results in a higher magnitude of compressive residual stress and, therefore, greater fatigue resistance than conventional shot peening. The first area of development was in high performance carburizing steels suitable for hard shot peening. Desirable traits were enhanced by reducing the carburizing anomalies resulting from intergranular oxidation and by the enhancing case toughness. Further improvement of fatigue resistance has been accomplished by dual peening, first with hard shot followed by smaller diameter steel shot at a lower intensity. This paper also describes the development of long life shot media for hard shot peening.