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Weight reduction of automobiles is one of the most highlighted subjects in automobile industry from the energy saving and clean environment points of view. A typical approach for the purpose is to use high strength steel sheets as well as optimizing designs and using low density materials. It is not, however, easy to apply high strength steel sheets to automotive panels because of their strict requirement for the shape-fixability although a high dent resistance is also required. Besides the use of bake hardening steels, two different high strength steel sheets, which are a continuously annealed extra-low carbon titanium-added IF steel and a low carbon TRIP steel which contains about six volume% of austenite, were assessed for the application to automotive panels in combination with a tension-controled press-forming technique and showed as good shape-fixability as a conventional box-annealed aluminum-killed DDQ steel sheet when high blank-holding-forces were applied.

Nippon Steel Corp. has developed a new hot-rolled steel sheet which is characterized by 1.0 to 1.6% copper addition to extra low carbon steel. This steel is worked while it is in a low strength state and thereafter the worked part is heat treated to increase the strength of it. The copper added steel sheet has a capability to increase tensile strength up to 600Mpa level from 400Mpa level by being heated to the temperature range of 750 to 950K for less than 600s. When copper remains in the solid solution state, the steel sheet exhibits as much formability as the conventional steel of the same strength level. The ductility ratio is higher than that of the conventional steel of the same strength level. This offers a greater freedom in the design of spot-weld structural joints. The fatigue properties of the base metal is as good as those of the conventional steel of the same strength level, and becomes much longer when heat treated.