The Precipitation of Carbon and the Segregation of Sulfur on the Surface of Cold-rolled Low Carbon Steels

Abstract

The change of the surface composition of commercial cold-rolled low carbon Al-killed and rimmed steels was investigated by AES at temperatures between 843 K and 1043 K in vacuum. The segregation behavior of Al-killed steel and that of rimmed steel were almost the same.
At temperatures below about 950 K, the surface of the steel was covered with the precipitation layer of graphite. At temperatures above about 950 K, graphite layer dissolved into the steel, and sulfur and phosphorus segregated to the surface. As the annealing was prolonged, however, the concentration of sulfur on the surface increased and phosphorus disappeared from the surface. Finally only sulfur occupies surface sites. This change of the surface composition was reversible at the increasing or decreasing temperature.
Since this steel contains about 0.05 mass% carbon, carbon precipitates as Fe₃C below about 996 K in the bulk. However, Fe₃C decomposes into iron and carbon on the surface. Then carbon grows as graphite. This steel contains manganese which stabilizes Fe₃C. Therefore, the stability range of Fe₃C of this steel is widened. Sulfur which segregates on the surface lowers the surface free energy so that the γ phase is stabilized to a lower temperature. Therefore, both manganese in the bulk and sulfur on the surface decrease the precipitation temperature of graphite on the surface from 996 K to about 950 K.
Sulfur on the surface also decreases the precipitation rate of graphite. In the case of Al-killed steel, however, this effect is canceled by aluminum which accelerates the decomposition of Fe₃C.