Transformation Behaviour and Related Mechanical Properties of B-Added ULC- and IF Steels

Abstract

Boron may be added to modern ULC- and IF steels for various reasons. It is well known that the presence of this element has a strong effect on the kinetics of the γ/α transformation, which in its turn has a major effect on the resulting microstructures. However, the origin and the nature of the resulting microstructures in ULC-and IF steels is poorly understood. In principle, the addition of boron offers the possibility of obtaining an excellent combination of strength and ductility in deep-drawable ULC-steels. In P-bearing high strength IF-steels, B is mainly added to avoid Cold-Work-Embrittlement. In this case, the effect of B on the transformation behaviour and the resulting microstructure may easily become detrimental to the mechanical properties and further processing through cold-rolling. For both ULC-steels and IF-steels, the optimisation of properties requires a trade-off in steel composition and processing conditions. This requires knowledge of the prevailing transformation mechanism. The transformation behaviour of various B-added ULC- and IF steels was studied by dilatometric and microstructural analysis, combined with hardness testing of hot-rolled material. Identification of the various constituents of the microstructure was accomplished by comparing the results of microstructural analysis and hardness tests with literature data. Tuning of the mechanical properties also requires knowledge of the recovery and recrystallisation behaviour, which is investigated in combination with the relevant mechanical properties.