Causes of High Temperature Ductility Trough of Microalloyed Steels

Influence of continuous casting parameters and hot rolling regimes on the hot ductility in microalloyed pipeline steels and structural steels has been investigated using Gleeble 3800 system. Dependence of the hot ductility on temperature for different thermomechanical treatment conditions has been studied by physical simulation. The high temperature ductility drop occurs close to the phase and magnetic transition temperature for all the studied schedules. The elastic modulus at this temperature abnormally increases. The main reason of the ductility decrease is the elastic modulus increase. The structural factors (e.g. non-metallic inclusions on initial grain boundaries, interstitial impurities, grain size) lead to the additional decrease of the ductility. Increase of the grain boundary lengths and steel refining lead to the hot ductility improvement. The ductility trough disappears after the deformation of metal in austenitic region to decrease the austenite grain size significantly.