A New Microalloyed, Multi-Phase Steel for High Strength Forging Applications

Forged components to be used in high strength applications have traditionally been heat treated after forging. This processing route unfortunately suffers from many technical and economical shortcomings. The first attempt to overcome these difficulties led to the development of medium carbon microalloyed steels for bar applications in the early 1970's. While these steels did not require heat treatment, their strengths were limited. Furthermore, the notch toughness of these steels was rather poor. The limitation on strength and toughness have hindered their acceptance as a substitute for the conventional QT steels, especially in safety critical components. In addition, these shortcomings eliminate the possibility of downsizing through redesign.

Since the tempered martensite and the microalloyed ferrite-pearlite steels have obvious limitations, an alternative microstructure had to be developed. Ideally, this new microstructure should have the ability to generate high strengths without heat treatment and offer high toughness values. It has been found that forged and quenched components made from a low-carbon steel containing Mn-Mo-Nb exhibit a multi-phase structure consisting of low-carbon martensite and lower bainite. This structure has been found to have both high strength and high toughness. This paper describes this new steel, its properties and production trials designed to illustrate its applicability to safety critical components.