"In-situ" observation of remelting phenomenon after solidification of Fe-B alloy and B-bearing commercial steels

Abstract

It is well known that boron (B) addition to steels can cause difficulties during continuous casting. The difficulty includes surface cracks, internal half-way cracks as well as centerline problems on the cast products. In severe cases, a breakout can occur during casting of B-bearing steels. According to the studies of the effect of boron on the pseudo-binary phase diagrams of Fe-B alloys at various carbon levels by the authors, it was found that the addition of boron introduces the possibility of steel initially completely solidifying followed by a retrograde melting phenomenon in all C contents below 1 %. This retrograde melting phenomenon will most likely occur at interdendritic regions and grain boundaries at temperatures approaching 1 350°C. The remelt liquid will be retained down to temperatures approaching 1 100°C. The retention of this low melting point phase during casting is believed to be the primary cause of casting difficulties of the B- bearing commercial products. In order to confirm the modeling study of the phase diagrams and the finding of the remelting phenomenon, Confocal Scanning Laser Microscope (CSLM) studies were carried out for "in-situ" observation. These studies not only confirmed the existence of the retrograde-melting phenomenon at temperatures below 1 200°C, but also revealed that severe segregation of B in steel could cause the existence of liquid phase down to at least 1 350°C even in steels bearing as little as 10 ppm B. The present paper will summarize the observation results. © 2009 ISIJ.