Formability of Ultrafine-grained Steels

Ultrafine-grained steels have high strength but small elongation. However, M1.7 small screws with a cross recess could be successfully formed without cracking by cold heading using ultrafine-grained steels. Typically, soft steels are selected to avoid recess cracking to produce a screw with a recess. Ultrafine-grained steels exhibit good formability with respect to compression working, despite being hard steels. In this research, the formability of ultrafine-grained steels was investigated on the basis of true stress-true strain curves. Finite Element Method simulations revealed significant strain generation near the recess during screw forming. Very low carbon ultrafine-grained steels exhibit extremely high fracture stress and fracture strain. The fracture strain is larger than that generated near the recess in a screw. head. As a result, ultrafine grained steels simultaneously achieve outstanding formability and high strength. These features allowed for a reduction in CO2 emissions by half in the screw manufacturing process as it eliminates the need for quenching and tempering.