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1 Many different bolts are employed in automobiles for different purposes and uses, and their strength generally ranges from 700 N/mm2 to 1200 N/mm2. Automobiles face the issue of making improvements in fuel economy as an environmental measure, and there is consequently a requirement to lighten component parts. The creationof higher-strength bolts is an important factor in achieving lighter weight. Increasing the strength, however, can also bring about an increased incidence of delayed fracture, and the conventional solution used to require the application of special steels such as expensive maraging steel. The present development addressed this issue by focusing on high carbon steel rod, which had been considered less susceptible to delayed fracture, although heading was also considered to be difficult. Heading techniques were therefore devised that made it possible not only to form bolts from this material, but also to provide satisfactory strength.

Increasing use of aluminum in automotive components has led to lower fuel consumption and enhanced performance of automotive designs. From a manufacturing standpoint, aluminum provides the additional advantage of utilizing same processes as steel. Performance and durability of painted aluminum cars, however, is dependent on proper optimization of process conditions. As part of an extensive study of factors influencing corrosion resistance of painted aluminum, the present study deals with the influence of pretreatment and coating variables and the interaction of alloy composition with zinc phosphate and electrocoat. Interfacial analysis of corrosion products indicates the relative influence of alloying elements on stability of the metal/phosphate/electrocoat interface. As a result, guidelines and recommendations on aluminum processing in an automotive manufacturing floor have been developed.

This paper summarizes a laboratory study aimed at revealing the effect of chemical elements. Si, Cr, Mo and V, on the sag resistance and developing new steels with all the characteristics required for suspension coil spring in comparison with AISI 9260. It was found that as far as sag resistance is concerned an optimum silicon content exist (1.5 silicon) and chromium deteriorates sag resistance and molybdenum and vanadium improve it. Further studies showed that Si-Cr-V steels are the best spring steels available, particularly 1.5Si-0.5Cr-G.2V steel has all the required characteristics, e.g., good sag resistance, good decarburizing resistance and sufficient hardenability.

In order to simulate the perforation corrosion of an automobile, hem model samples made of various kinds of coated steel sheets were set inside of the door outer panel and the door was exposed in the open air with once a week 5% salt spraying. After two years, the model samples were disassembled for investigation. Perforation corrosion occurred most severely just above the lapped portion of outer and inner specimens. Red rust samples taken from the hem model and from a field vehicle run in Detroit did not show clear pearks of spectra by X-ray diffraction analysis, whearas the one formed by a conventional cyclic corrosion test showed peaks of Fe3O4. These facts indicate that this new test well simulates the corrosion environment of an actual automobile hem portion. Corrosion resistance of many kinds of coated steel sheets was studied by this new test method. Perforation depth became smaller as a coating weight of pure zinc electroplated steel sheet increased.

A thin organic coated steel sheet (HI-SUPER-DN) has been recently developed for automotive body panel applications. The steel sheet consists of a Zn-Ni alloy-plated layer, a chromate film and a thin organic coated layer, which is composed of colloidal silica, organic polymer and an organic additive. Corrosion resistance, formability of the coated layer, weidability and electropaintability of the steel sheet required for automotive body panel were studied. An increase in the colloidal silica in the organic coated layer improves corrosion resistance. However, the formability of the organic coated layer deteriorates with such as increase in the colloidal silica. We have optimized the ratio of colloidal silica to organic polymer so as to balance both the corrosion resistance and the formability of the organic coated layer. Futhermore, the bake-hardenability of the steel sheet was maintained by a relatively low temperature baking of the chromate film and the organic coated layer.

Mo-free 1.6-GPa bolt was developed for a Variable Compression Turbo (VC-Turbo) engine, which is environment friendly and improves fuel efficiency and output. Mo contributes to the improvement of delayed fracture resistance; therefore, the main objective is to achieve both high strength and delayed fracture resistance. Therefore, Si is added to the developed steel to achieve high strength and delayed fracture resistance. The delayed fracture tests were performed employing the Hc/He method. Hc is the limit of the diffusible hydrogen content without causing a delayed fracture under tightening, and He is the diffusible hydrogen content entering under a hydrogen-charging condition equivalent to the actual environment. The delayed fracture resistance is compared between the developed steel and the SCM440 utilized for 1.2-GPa class bolt as a representative of the current high-strength bolts.