Insoluble surface carbon on steel sheet annealed in hydrogen-nitrogen atmosphere
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I.
INTRODUCTION
FOR several
years steel producers have been working to improve the surface cleanliness of box-annealed sheet. Results of various studies have shown that aftcr annealing the steel surface may be contaminated with a carbonaceous material that cannot be removed with the usual organic solvents or water-base cleaners. This so-called insoluble* surface *Throughout this paper we shall use the term insoluble carbon to refer to the presence of carbon on the steel surface in some form that prevents its removal by the usual solvents or cleaners. The term is not intended to imply the existence of both soluble and insoluble forms of elemental carbon.
carbon can adversely affect the paintability of steel sheet. Its concentration can vary widely from coil to coil or even in different areas of the same coil. Although its source is generally thought to be the residual rolling lubricant film on the steel surface as it enters the annealing operation, the factors that influence the pyrolysis of that oil and cause varying amounts of insoluble carbon to be formed have not been identified. In a preliminary study we obtained 90-centimeterdiameter semicircular sections of steel from the mid-length region of coils after cold reduction and adjacent sections from the same coils after box annealing to determine whether the composition of the residual cold-rolling lubricant film on the full-hard sheet was related to the amount of insoluble carbon observed on the surface after annealing. Analyses were made of the amount of oil, its composition (animal, vegetable, or mineral), the amount of particulates in the residual lubricant film, and the particulate composition. Although the various annealed sections displayed a wide range of insoluble surface carbon, no correlation was observed H.E. BIBER, Associate Research Consultant, R.C. TAKACS, Senior Research Engineer, and A. E. DICKEY, Senior Research Technician, are all with United States Steel Corporation, Research Laboratory, 125 Jamison Lane, Monroeville, PA 15146. Manuscript submitted June 18. 1982. METALLURGICAL TRANSACTIONS B
between the amount of surface carbon and the amount or composition of the residual lubricant film. Because the coil samples used in that study appeared to have a wide range of "susceptibility" to the formation of insoluble surface carbon, it was thought that laboratory annealing experiments with the full-hard sections might provide guidelines for annealing conditions that would suppress surface-carbon formation, even though the causative factors had not been identified. It soon became obvious that with ordinary laboratory annealing conditions* both *Single specimens suspended in a tube furnace in slow flow of 6 pct hydrogen in nitrogen with - 4 0 ~ dew point; various furnace temperatures between 538 ~ and 704 ~
"susceptible" and "nonsusceptible" steels exhibited very low amounts of insoluble surface carbon after annealing (0.001 to 0.002 grams per meter squared, g m / m 2) despite the fact that their commercially annealed counterparts exhibited widel
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