Influence of cooling conditions on the hot ductility of Nb-Ti-bearing steels

¹ Beijing Key Laboratory of Green Recycling and Extraction of Metals (GREM), School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China
e-mail: zhanglifeng@ustb.edu.cn
² Qian’an Steelmaking Co. Ltd., Shougang Group, Qian’an City 064404, P.R. China

Received: 10 June 2015
Accepted: 22 September 2015

Abstract

The hot ductility of Nb-Ti-bearing low-carbon steels was studied. Four cooling methods were designed to investigate the influence of cooling conditions on the hot ductility at temperatures range from 650 °C to 1005 °C. Increasing the cooling rate generally deteriorates the ductility. However, the high cooling rate RA values were significantly increased when cooling was made in two steps: samples were slow cooled at high temperature zone before being cooled to the test temperatures by fast cooling. Slow cooling guaranteed relative beneficial coarse precipitates, while fast cooling was recommended for refining γ/α transformation microstructures. Proeutectoid ferrite film has a significant influence on the hot ductility of the steel: thicker proeutectoid ferrite film resulted in a ductility deterioration when the thickness of the ferrite film was larger than 5 μm. The precipitates found were mainly (Ti_xNb_{1 − x})N with high titanium content.