High Strength Formable Sheet Steel

High strength low alloy steels with minimum yield strength values in the 50-80 Ksi range are experiencing increased usage in automotive applications for reasons of improved safety and weight reduction. These steels are usually supplied in the hot rolled condition although markets are developing for thinner, cold rolled steels. The increased strength in these materials is usually obtained by grain refinement and precipitation by means of micro-alloying with Cb, V or Ti and by controlled mill processing. The most desirable combination of properties is the development of high strength in the formed part and optimum formability compatible with this capacity for strengthening in the as-shipped steel.

This paper deals with the developments at Jones and Laughlin leading to a product, VAN-QN, which has a superior combination of formability and strength in the formed part. Previous descriptions of this type of product have been made by Japanese researchers and M. Rashid of GMC. This product is obtained by careful heat treatment of cold rolled or hot rolled steel such that a microstructure consisting of fine ferrite grains with a controlled dispersion of martensitic or lower bainitic areas is obtained. This product is characterized by general yielding at a relatively low level (50-60 Ksi) with a rapid rate of work hardening combined with a large capacity for deformation that results in a high tensile strength (80-100 Ksi) and a high degree of strengthening in the formed part.

Property-structure relationships are presented for the engineering properties of most interest. Considerable description is made of the formability characteristics of this steel and how formability can be optimized.