In Situ Observation of the MnS Precipitation Behavior in High-Sulfur Microalloyed Steel Under Different Cooling Rates

In situ observation of the precipitation behavior of MnS in a high-sulfur microalloyed steel has been conducted by means of a confocal laser scanning microscopy (CLSM) with cooling rates increased from 50 to 400 °C/min. Differential scanning calorimetry analysis and thermodynamic calculation were carried out prior to and following the CLSM experiments using scanning electron microscopy (SEM) and X-ray energy-dispersive spectrometer. The results suggested that the initial MnS precipitate temperature decreased from 1440.0 °C to 1429.6 °C and the final temperature reduced from 1413.7 °C to 1381.6 °C as the cooling rate increased from 50 to 400 °C/min in the CLSM test. Three typical MnS morphologies of small angular, globular and larger dendrite cluster MnS were observed in the melting surface. Furthermore, with an increase in the supersaturation of MnS and a significant reduction of the local solidification time when the cooling rate increased from 50 to 400 °C/min, the large clusters MnS at interdendritic regions became thinner and more small angular MnS precipitates formed at dendrite crystals.