Development of 980N/mm2 Class Ultra High Strength Steel Suitable for Mechanical Joining 2001-01-3359

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. The strength of mechanical joints depends on the inner shape of joints, which is influenced by mechanical properties of the joined material.