27 August 1999, Tokyo: Mitsubishi Motors Corporation announces the development of a new lightweight and high-strength steel sheet that will enable a significant weight reduction in chassis parts and a consequent improvement in fuel consumption. The company developed the new 590MPa(MegaPascal)-grade high strength steel sheet containing retained austenite for chassis parts working together with Nippon Steel Corporation, Sumitomo Metal Industries, Ltd., and Kobe Steel Ltd. An industry first, the new steel sheet will make its debut in the new PAJERO series that is to be launched in the very near future.

Exploiting the outstanding formability and welded joint fatigue strength of retained austenite steel sheet, in the new 590MPa-grade steel sheet the sulfur and retained austenite content is controlled to realize improved stretchability, and to enable it to undergo stretch flanging, in which high degrees of localized flexing and stretching are generated. This has resulted in a dramatic increase in the hole expansion ratio-an indicator of stretch flange formability-and enabled, for the first time anywhere, the use of retained austenite sheet steel in chassis parts, many of which are difficult to form. This, in turn, makes possible a dramatic reduction in the weight of sheet metal chassis parts.

Generally, the greater the tensile strength of the steel, the more it is possible to reduce weight by using thinner gauges of steel. For this reason, the use of high-strength steel sheet in automobiles is growing in order to reduce weight. However, steel sheet formability deteriorates with increasing tensile strength, as evidenced by cracking and splitting during pressing and also softens by the heat effect during welding which results in the fatigue of welded joints. This puts a limit on the strength of steel that can be used in forming parts, and to date most chassis sheet metal parts have been made using hot rolled steel sheet of between 370MPa- and 400MPa-grade.

In the new 590MPa-grade high strength steel sheet containing retained austenite, sulfur content has been lowered from 0.006% to 0.002%, while the retained austenite content has been reduced from between 8% and 12%, to between 4% and 8%. This has made it possible to provide the stretchability required for the pressing process while increasing the hole expansion ratio to over 80%, and also to use the new steel sheet in the manufacture of hard formability chassis parts requiring stretch flanging, something that was not possible in the past. In addition, the steel's high silicon and manganese content prevents softening during welding and realizes a much higher welded joint fatigue strength-so important in chassis parts- than before.

The use of the new 590MPa-grade high strength steel sheet in some 80 chassis parts in the new PAJERO series to be launched shortly has reduced the weight of the parts by some 13%, a total weight reduction of 14kg.

The on-going development and application of many kinds of new materials and processing methods that contribute to conservation of the environment and that please customers is a way of life at Mitsubishi Motors. The company will extend the use of the new chassis steel, with its outstanding formability, to more of its model series, and will continue its efforts to reduce vehicle weight and to make its products more competitive.