Theoretical interpretation of the decarburization mechanism in convective oxygen steelmaking
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I.
INTRODUCTION
T H E decarburization mechanism is an important issue in the process design of steelmaking. Although numerous studies have been attempted to interpret the decarburization mechanism, there still remains a gap in understanding between laboratory results and the plant scale tests. In the real operation, conflicting arguments on controlling steps still exist. Also, the high-speed decarburization characteristics of the oxygen steelmaking process, which is incomparably faster than the open hearth, have not yet been fully explained. The reason for these uncertainties is that most of the studies have depended mainly on experiments, while theoretical understandings have been deficient. Furthermore, many cold model experiments have been conducted with a lack of theoretical bases with regard to the chemical similarity between real and model systems. Because the conditions of a reaction system can change the reaction kinetics itself, improper choice of a model system and its conditions might induce erroneous results. The purpose of this work is not only to understand the decarburization phenomena theoretically and to explain the existing data but also to make suggestions for future model experiments on decarburization reaction. The decarburization mechanism is treated mainly from a macroscopic point of view, which will most likely be useful in design application.
J.H. ZONG, formerly Graduate Student at Seoul National University, is Postdoctoral Associate with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA. J.K. YOON, Professor, is with the Department of Metallurgical Engineering, Seoul National University, Seoul, Korea. This paper is a part of the dissertation of Dr. Zong in partial fulfillment of his Ph.D. degree. Manuscript submitted July 19, 1988. METALLURGICAL TRANSACTIONS B
II.
PREVIOUS WORKS
Open hearth steetmaking, in the past, often used the concept of excess oxygen to describe decarburization kinetics. [~ = k•
= k Q
[11
where Keq is an equilibrium constant of the reaction C + O ~ CO, AQ is concentration of excess oxygen, and k is a constant. However, it is improper to apply this concept to modem convective oxygen steelmaking, because mixing states and contacting schemes are much different. After the advent of the Linz-Donawitz converter (LD converter), many experimental studies t3-81 on the direct decarburization between gas and molten iron were conducted, and apparent rate equations were expressed as following equa~ 12] tions despite some disagree m ent~. d_C - kG" a" P o z
dt
[2]
in the high C regime, and dC -
dt
- / ~ L - a" C
[31
in the low C regime. Baker et al. 131 and Robertson and Jenkins m reported from a levitated iron drop in oxygen atmosphere that the rate of decarburization was controlled by 02 diffusion in gas film for a high-carbon regime while controlled by carbon diffusion in the liquid phase for a low-carbon regime (1 wt pct C below), as shown in Figure 1. Similar results were obtained in a high-frequency furna
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