Effect of starting microstructure on austenite grain sizes developed after reheating of HSLA steel

Abstract

Austenite grain structure has been studied over a wide range of soaking temperatures (950–1250°C) in a high strength low alloy steel containing Nb, Ti and V for different starting conditions, namely as cast, hot rolled and thermomechanical controlled rolled. Mixed (or bimodal) austenite grain structures were developed as a result of abnormal grain growth at different soaking temperatures in as cast slab (∼1200°C) and in rolled plates (∼1000°C), which has hardly been reported earlier. Higher fraction of finer precipitates, such as NbC and VC (<200 nm in size), were present in the rolled plate as compared to as cast slab, where the precipitates were coarser and complex in nature. Rapid dissolution of fine VC precipitates was primarily responsible for the observed abnormal grain growth in rolled plates at lower soaking temperature (∼1000°C). Coarse precipitates of complex nature, such as (Nb,Ti,V)(C,N) and (Nb,Ti)(C,N), remained stable up to ∼1150°C and restricted the formation of abnormal grain growth in as cast steel.

Keywords:

• Austenite grain size
• Soaking treatment
• As cast structure
• Rolled plates
• Abnormal grain growth
• Bimodal grain size distribution
• Microalloyed steel
• Precipitate pinning

Acknowledgements

The authors acknowledge the financial support from Council of Scientific and Industrial Research (CSIR) and Tata Steel R&D, Jamshedpur. The authors are also grateful to Dr G. K. Dey and S. Neogy from the Materials Science Division of Bhabha Atomic Research Centre, Mumbai for their experimental support behind the work.