Structural Elements Influence on Surface Segregation in IF Steels

Microstructure and segregation kinetics of impurities at separate points of the surface of ferritic steels with ultra-low carbon content are studied in a temperature range of 550-750 degrees C with the use of high-resolution scanning electron microscopy and Auger electron spectroscopy. Carbon is the main segregant below 600 degrees C. Sulphur and titanium dominate above 700 degrees C. Sulphur and phosphorus are the most active elements within the range of 600-700 degrees C. An essential variation of the surface composition of individual grains is observed at all temperatures studied. This is caused by variation of the segregation rate of sulphur. Sulphur behaviour on separate grains is dependent on the presence of titanium sulphide (carbosulphide) particles within the subsurface layer. Such particles dissolve during heating, intensify segregation flux of sulphur and titanium atoms and suppress phosphorus segregation.