The Mechanism of Hot Ductility Loss and Recovery in Nb-Bearing Low Alloy Steels
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I.
INTRODUCTION
THE studies on problems of transverse surface cracking during continuous casting have been conducted by many researchers.[1–7] Following their studies, the hot ductility highly depends on the alloying elements such as Ti, Nb, Al, V, and B. Among these elements, Nb is especially known as an element most detrimental to the hot ductility. It has been shown that Nb forms Nb(C,N) carbo-nitrides dynamically and finely during continuous casting. The hot ductility in the austenite single-phase region decreases abruptly as the size and the interparticle spacing of the carbo-nitride particles decrease. The mechanism for explaining the hot ductility loss is divided into two groups: the micro-void coalescence mechanism[2–7] and the grain boundary sliding mechanism.[2] During continuous casting, the Nb(C,N) particles formed in the matrix strengthen the austenite matrix, and the precipitates formed concurrently at grain boundaries result in the precipitate-free zone along the austenite grain boundaries (AGBs). As a result, the strain resulting from the external stress during casting is mainly concentrated in the softer precipitate-free zone, causing the decohesion at the interface of the Nb(C,N) precipitates and the AGB. Finally, the intergranular fracture occurs through the continual coalescence of the micro-voids formed at the AGB.[5–7] In the grain boundary sliding mechanism, the finely formed precipitates pin the AGB, allowing the cracks to join up and resulting in the final cracking.[2] Meanwhile, it has been reported that the S segregated to the AGB accelerates the decohesion process.[8,9] However, although many researches on the hot ductility loss mechanism[2–7] and the recovery mechanism[2] which is based on the coarsening of the precipitates have been performed, the mechanisms on the hot ductility are not still clear.
M.H. KANG, Ph.D. Candidate, J.S. LEE, Y.M. KOO, S.-J. KIM, and N.H. HEO, Professors, are with the Graduate Institute of Ferrous Technology (GIFT), Pohang Institute of Science and Technology (POSTECH), Pohang 790-784, Korea. Contact e-mail: [email protected] Manuscript submitted January 9, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
In this study, the mechanism of hot ductility loss and recovery is understood in the light of the competition between the matrix strengthening by Nb(C,N) precipitates and the grain boundary strengthening arising from the formation of MnS particles. C-Mn-Nb low alloy steels with four different compositions were prepared. Changes in reduction of area (RA) with holding time in the temperature range at which surface cracking is observed during continuous casting of this class of steels were investigated, using a thermo-mechanical simulator. The grain boundary segregation behavior of sulfur, which is changed with test condition, was also investigated, using Auger electron spectroscopy (AES).
II.
EXPERIMENTAL PROCEDURE
Ingots of 30 kg were hot-rolled to plates of 12 mm thickness after holding at 1423 K (1150 °C) for 2 hours. The chemical compositions of the pr
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