Surface segregation of phosphorus, carbon, and sulfur in commercial low-carbon grades of steel
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I.
INTRODUCTION
Galvanneal coatings are zinc-iron alloy coatings produced by dipping sheet steel in a molten zinc bath and, subsequently, annealing the coated sheet steel in a furnace.[1] Galvannealed sheet steels are used extensively in the automotive industry for exposed and unexposed body panel applications.[2,3] The function of the coating is to provide extended corrosion resistance for the body panels.[4] Steel chemistry plays a critical role in determining the intermetallic composition of the galvanneal coating.[5] In particular, the phosphorus in the steel has a profound effect on the galvannealing process and the galvannealed product. Rephosphorized steels require greater galvannealing heat input in order to be satisfactorily alloyed. The phosphorus in the steel influences the evolution of the coating structure as well as the final properties of the coating.[6] It has been postulated that phosphorus, which had segregated to the sheet steel surface during recrystallization annealing, affected the kinetics of the Fe-Zn reaction as the sheet steel was subsequently dipped in a molten zinc bath of a continuous galvanizing line.[7,8] Surface chemistry changes occurring during annealing of sheet steels have been studied extensively.[9–13] Some of these studies examined the selective oxidation behavior of manganese and silicon in steels as a function of oxygen potential, time, and temperature in industrial annealing atmospheres (N2/H2 gas mixtures). Particles of manganese and silicon oxides were formed on grain boundaries intersecting the surface, as well as on free surfaces away from the boundaries, during batch annealing in N2-H2 atmospheres.[9,10,11] The annealing cycles in these studies typically ran for several hours, and the peak temperature attained was V. RANGARAJAN, Senior Engineer, formerly with the Research and Development Laboratories, Inland Steel Co., is with Bethlehem Steel Corp., Bethlehem, PA 18016. R. TONCHEFF, Staff Technician, and L.L. FRANKS, Scientist, are with the Research and Development Laboratories, Inland Steel Co., E. Chicago, IN 46312. Manuscript submitted January 6, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
around 973 K.[9,10] The oxide particles were shown to interfere with the wetting of the steel surface when steel was dipped in a molten zinc bath.[9] In other studies, the annealing experiments were extended to 1123 K, and the different factors leading to external vs internal oxidation of manganese and silicon were explained.[11] Further, an accelerating influence of boron on the external oxidation of manganese was presented, particularly for an annealing cycle very typical of the recrystallization annealing step in a continuous hot-dip coating line.[11] While selective oxidation of Mn and Si have been shown to interfere with the galvanizing reaction, this phenomenon alone is insufficient to explain the anomalous galvanizing behavior of phosphorus-bearing sheet steels. Surface segregation of elements such as carbon, phosphorus, and sulfur can also occur during these anneal
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