Study of the high strength and low yield ratio cold forging steel

High strength and low yield ratio cold forging steel was studied by a thermomechanical simulator, a laboratory hot rolling mill and a multistage cold former. The results have shown that multiphase microstructure containing polygonal ferrite, dispersed pearlite, granular bainite and retained austenite was obtained by thermomechanical processing (TMP). The cold forging steel can be directly used to fabricate bolts from hot rolled bar due to its low yield ratio, and the mechanical properties of the finished bolts can meet the standard of class 8.8 bolts without heat treatment. This is because that the hot rolled bars after TMP possess multiphase microstructure, which contributes to an increase the mechanical properties, and the TRIP effect improves strength and ductility of the bolt.