SURFACE SEGREGATION OF PHOSPHORUS, CARBON, AND SULFUR IN COMMERCIAL LOW-CARBON GRADES OF STEEL

Surface segregation behavior of solute atoms has been studied on low-c arbon steels used in producing galvannealed sheet steels for automotiv e body panel applications. Samples of cold-rolled low-carbon steels wi th different amounts of carbon and phosphorus in solution were heated in a vacuum chamber and their surface chemistries analyzed by Auger el ectron spectroscopy. For the steels studied here, one or more of the e lements carbon, phosphorus, and sulfur accumulated significantly at th e surface within a temperature window of 300 to 973 K. As the temperat ure was increased, carbon appeared on the surface first, followed by p hosphorus, and then sulfur. Each succeeding segregating element displa ced the previous one from the surface. The free solute concentration i n the bulk and the temperature were critical factors controlling the a mount of solute accumulation at the surface. Once segregated, the solu te atoms remained on the surface as the samples cooled. Carbon and/or boron in steels retarded the transport of phosphorus to the surface. T he implications of these findings in understanding the galvannealing b ehavior of these steels are discussed.