Effect of Boron Precipitation Behavior on the Hot Ductility of Boron Containing Steel
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INTRODUCTION
DURING the continuous casting process in steel production, thermal stress builds up in the slab due to rapid cooling, and mechanical stress occurs due to the straightening operation around 973 to 1173 K (700 to 900 C). In steels with poor hot ductility, the combination of these stresses can cause cracks on the slab surface.[1,2] Recently, continuous casting-direct rolling techniques have been adopted to save energy and labor. However, to operate these processes, cracking of the slab surface must be prevented. One approach to preventing this cracking is to add alloying elements to improve the steel’s hot ductility. A considerable amount of research has been conducted to improve the hot ductility of steels for preventing cracking on the slab surface, by the addition of alloying elements.[3–7] Few studies have been conducted on the effect of B on the hot ductility of steel. Addition of B to plain carbon steel improves hot ductility, because solute B atoms that segregate to grain boundaries can occupy vacancy and thus prevent formation and propagation of microcracking at grain boundaries.[8–10] However, B containing steels are susceptible to transverse cracking on the slab surface during the continuous casting process because of poor KYUNG CHUL CHO and JIN YOUNG KIM, Postdoctoral Candidates, Department of Materials Science and Engineering, DONG JUN MUN, Postdoctoral Candidate, and JAE SANG LEE, Associate Professor, Graduate Institute of Ferrous Technology, and YANG MO KOO, Professor, Department of Materials Science and Engineering and the Graduate Institute of Ferrous Technology, are with the Pohang University of Science and Technology, Pohang 790784, Korea. Contact e-mail: [email protected] JOONG KIL PARK, Team Leader, is with the Operation Technology Team, POCAST Research & Development Project Department, POSCO, Gwangyang 545-711, Korea. Manuscript submitted April 13, 2009. Article published online April 10, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
hot ductility at temperatures of 973 to 1273 K (700 to 1000 C), possibly formation of boron nitride (BN) at austenite grain boundaries.[11] This phenomenon suggests that the behavior of B precipitates significantly affects hot ductility of B containing steel. However, the effect of cooling rate (CR) on the hot ductility of B containing steel has not been studied so far. Thus, in this study, the effects of B addition on the hot ductility of low-carbon steel were studied at different CRs. The distribution of B and its precipitates were also examined to clarify the effect of CR on microstructure in B containing steel.
II.
EXPERIMENT
This study was conducted using the two types of lowcarbon steel, which contained almost the same level of elements except for B (Table I). To quantify the effect of B addition on the hot ductility of low-carbon steel, 25 ppm of B was added to the low-carbon steel. Steel ingots were prepared by laboratory vacuum induction melting and then hot rolled into 20-mm-thick plates. Hot tensile test specimens were machined from the pla
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