Determination of the Heating and Soaking Stage Length of a Galvannealing Furnace.

Abstract

In order to produce galvannealead coatings, the hot-dip galvanized sheet enters a galvannealing furnace as soon as it emerges from the zinc bath. This furnace normally comprises three stages: a heating stage, a soaking stage and a cooling stage. The length of each stage varies from plant to plant and depend on several factors such as heating and cooling method and sometimes on the space available on the galvanizing line. As is well known the main objective of such furnace is to deliver an alloyed coating with a coating iron content between 10-11 mass%, which is normally accepted as the optimum iron content for best properties.In previous works 1-12) a quantitative study was carried out to describe the isothermal kinetics of iron enrichment of the Fe-Zn coating. Furthermore, in industrial galvannealing practice, the zinc coating is subjected to a nonisothermal heating cycle. In those works it was shown how one can predict the nonisothermal kinetics of iron enrichment from the isothermal kinetics which can be fairly easily measured in the laboratory. More recently the concept of "processing window" [9][10][11][12] for the galvannealing process has been introduced. A processing window is essentially an area in a soaking temperature vs. line velocity plot within which the galvanealing cycle will result in a coating iron content between 10 and 11 mass%. Each particular galvannealing furnace will have its processing window. In those works it was shown how to calculate "processing windows" for the galvannealing process and the effect of Al content of the Zn bath, of the furnace characteristics and of the steel substrate were examined. Finally, the effect of small changes in processing variables in the controlling parameters was discussed with reference to the processing windows. Details can be found in those papers. [9][10][11][12] Those previous papers essentially focused on the "direct" problem, namely, for a given furnace and galvanized coating what would be the adequate processing window. However it might be of some use to study also the inverse problem. For a certain processing condition and galvanized sheet it might be interesting to determine the furnace configuration that could be successfully used to achieve the goal of producing a coating iron content between 10-11 mass%.In this work the determination of the furnace configuration, more specifically of the length of each stage of the galvannealing furnace as a function of the kinetics of iron enrichment of hot-dip galvanized coatings is investigated.The two hot-dip galvanized steel sheets used in this work were the same used in a previous work.12) Both were produced in zinc baths with similar Al content, 0.20 mass% (nominal) and similar coating weight, 80 g/m 2 (nominal). On one sheet the substrate was a Ti-IF steel and on the other a low carbon steel. In what follows the former will be referred to as the IF steels sheet whereas the latter will be referred to as the low carbon steel sheet. The substrate chemical analysis were (in mass%): C -0.0035; Mn -0.14; P...

An Alternative Method to Calculate Processing Variables for the Galvannealing Process.

“…Recently, the concept of "processing window" [2][3][4][5][6] for the galvannealing process has been introduced. A processing window is essentially an area in a soaking temperature vs. line velocity plot within which the galvanealing cycle of a certain zinc coated steel sheet will result in a coating iron content between 10 and 11 mass%.…”

“…In a recent work, Faria and Rios 6) showed that the following equation is valid: (2) where x and y are the furnace soaking and heating length, respectively, as indicated in Fig. 1.…”