Anomaly in Melting Behavior and Mechanical Properties of Carbon Steels Near Their Solidus Temperature

Abstract

Incipient melting of grain boundaries or dendrite boundaries at temperatures below the melting point of bulk matrix makes carbon steels fail in a brittle manner, leading to the occurrence of surface and internal cracks in continuously cast strand during the casting or hot rolling. The incipient melting temperature for carbon steels was measured by a conforcal laser scanning microscope, and their brittle behavior was studied at elevated temperatures by hot tensile test machine. It was found that steels containing less than 0.2 mass% C fracture at about 30K below the calculated solidus temperatures, whereas steels containing more than 0.3 mass% C fracture at about 20K above the calculated solidus temperatures on heating. These fracture temperatures are found nearly equal to the zero strength temperature (ZST). Such anomalous fracture behavior is shown in good agreement with the results of “in-situ” observation of melting occurring at the grain boundaries in the low carbon steels and at solute enriched interdendritic region in the high carbon steels. The reason for the incipient melting at the grain boundaries in low carbon steels is confirmed to be due to the segregation of impurity elements such as P and S at the grain boundaries. An experimental equation for estimating the premelting temperatures and some measures for preventing the occurrence of the cracks near the solidus temperatures are derived from these observations.