A Formable-Strengthenable Sheet Steel

High-strength cold-rolled sheet steels offer a potential to accomplish weight savings through gage reductions. However, the reduced formability which accompanies increased strength presents difficulties in the application of these steels for sheet metal stampings which require the formability equivalent of deep-drawing steels. A new deep-drawing sheet steel is being developed which overcomes the formability limitations of high-strength cold-rolled steels. Parts made from this steel can be uniformly strengthened to a high yield strength, up to 120,000 psi (827 MPa). Strengthening is accomplished by a 1200°F (921°K) heat treatment in a controlled atmosphere which results in internal nitridation. The nitrides formed by the alloying elements in the steel provide precipitation hardening. No distortion of the parts results from the heat treatment. The effect of heat-treating variables on tensile properties, weldability, fatigue, and toughness, are discussed.