Effect of ferrite formation on abnormal austenite grain coarsening in low-alloy steels during the hot rolling process

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 1998-05, Vol.29 (5), p.1375-1381
• Main Authors: ASAHI, H, YAGI, A, UENO, M
• Format: Article
• Language: English
• Subjects: Applied sciences; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Metals. Metallurgy; Other heat and thermomechanical treatments; Physics; Treatment of materials and its effects on microstructure and properties
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-998-0352-2

Abnormal coarsening of austenite ( gamma ) grains occurred in low-alloy steels during a seamless pipe hot-rolling process. Often, the grains became several hundred micrometers in diameter. This made it difficult to apply direct quenching to produce high-performance pipes. The phenomenon of grain coarsening was successfully reproduced using a thermomechanical simulator, and the factors which affected grain coarsening were clarifed. The mechanism was found to be basically strain-induced grain growth which occurred during reheating at approx930 deg C. Furthermore, once a pipe temperature decreased to the dual-phase region after the minimal hot working and prior to the reheating process, the grain coarsening was more pronounced. It was understood that the formation of ferrite along grain boundaries had the role of reducing the migration of grain boundaries into neighboring grains, leaving a strain-free, recrystallized region behind. This abnormal grain coarsening was found to be effectively prevented by an addition of Nb, the content of which varied depending on the C content. The effect of the Nb addition was confirmed by an in-line test.