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Suitability of ultra high strength steels (UHSS) for mechanical joining was investigated. When mechanical joining was applied to conventional 980N/mm2-class UHSSs having dual phase microstructure (ferrite and martensite), surface cracking occurred. Additionally cleaving inside mechanical joints was also observed in cases involving joining of dual phase steel by tools with larger clearance than designed. This cleaving causes a serious decrease in joint strength. Resistance to surface cracking and inside cleaving depends on the metallurgical structure of UHSS. Single martensite phase is superior for preventing the occurrence of defects in mechanical joining. This is because of its excellent local formability, which can be evaluated by the hole-expanding test using a machined hole specimen. Besides superior resistance to cracking and inside cleaving, single martensite phase steel exhibited higher mechanical joint strength than dual phase steel in cross tensile test.

A new 5xxx-series aluminum alloy, which was designed to have an excellent paint-bake-hardenability with practically no room-temperature aging, has been developed for automobile body sheets. The alloy contains 2∼3%Mg with additional amounts of Cu and Si. Yield strength in the as-shipped condition is very low, 80∼100MPa, but it can be increased by as much as about 60MPa through 2% stretch and paint-bake at 170°C. Such an excellent response to paint-bake was revealed to result from Guinier-Preston-Bagaryatskii(GPB) zone formation and enhanced by the optimum addition of Si. Density of vacancies in the alloy was reduced through a special annealing process to suppress room-temperature aging. Press-forming for fender outer and engine hood inner panels was done, and the alloy was demonstrated to have formability superior to 6xxx-series alloys and Al-4.5%Mg alloys, due to the reduced initial yield strength which also suppressed susceptibility to spring back.