Specialized metallurgical processes – compositions for use therei – Processes – Process control responsive to sensed condition
Reexamination Certificate
1999-10-08
2001-03-06
Kastler, Scott (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Process control responsive to sensed condition
C266S093000, C266S099000
Reexamination Certificate
active
06197086
ABSTRACT:
CLAIM TO COPYRIGHT IN REFERENCE TO MICROFICHE APPENDIX
A portion of the disclosure of this patent application contains material which is subject to copyright protection. The copyright owner has no objection to the reproduction by anyone of the patent document as it appears in the Patent and Trademark Office files or records, but otherwise reserves all copyrights whatsoever. Software for carrying out some of the methods and systems described herein has been filed with the United States Patent and Trademark Office herewith in the form of a microfiche appendix (incorporated herein by reference) including numerous frames, one of which being a title frame. This software may be included as part of a chip or disc stored in a computer or camera according to certain embodiments of this invention. The microfiche appendix is entitled SYSTEM AND METHOD FOR MINIMIZING SLAG CARRYOVER DURING THE PRODUCTION OF STEEL and includes one (1) microfiche and five (5) frames.
BACKGROUND OF THE INVENTION
This invention relates to a system and/or method for minimizing or reducing slag carryover while tapping a basic oxygen furnace (BOF) converter or other vessel during the production/manufacture of steel. For example, in certain embodiments, this invention relates to the use of an infrared (IR) imaging detector to detect the presence of slag in a tap stream. This invention also relates to the use of a system of, and method for, stopping tap when a predetermined amount of slag is detected in the tap stream.
A long-standing problem in the steel making industry has been the inability to control or minimize the carryover of slag during the tapping of a BOF converter. Tapping is the pouring of molten metal from a BOF converter into a corresponding ladle, with the metal flowing from the converter through a taphole defined therein.
During the manufacture of steel, molten iron (known as hot metal) having impurities (e.g. C, Si, Mn, S, P, etc.) therein is typically introduced into a converter vessel known as a basic oxygen furnace (BOF). In the BOF converter, gaseous oxygen (O
2
) is injected or jetted into or onto the hot metal in order to remove the impurities to desirable levels. During this purification process, fluxes such as lime (CaO) and MgO are added into the furnace and combine with oxides such as SiO
2
, MnO, and FeO formed during the oxidation process to form molten “slag” in the converter. This slag floats on top of the molten steel in the BOF converter, because the slag's density is less than that of the molten steel.
After the oxygen is introduced into the BOF converter for an extended period of time (e.g. from about 16-25 minutes depending upon the volume of the BOF converter, the amount of molten iron and steel scrap therein, and the grade of the steel to be made) and the molten slag and steel have formed, the converter vessel is tilted and tapped. During tapping, molten steel is poured from a taphole in the side of the BOF converter into a ladle located below the same. It is during this tapping that undesirable slag carryover can occur.
When the BOF converter vessel is properly tapped, a small amount of carryover may occur at the beginning of tapping, but the slag carryover of most concern occurs at the end of tapping when most of the substantially purified molten steel has already been poured into the ladle below, and mostly slag (instead of mostly steel) remains in the BOF converter. When a typical BOF converter is tilted to a pouring position for tapping, the molten steel is poured from the taphole located in the side of the converter before most of the slag is poured, due to the different densities of the two molten materials. If the operator(s) tapping the converter does not stop tapping (or pouring) at about the precise instant when the molten slag begins to flow through the taphole, the undesirable molten slag is also poured into the ladle below on top of the already poured molten steel. When too much slag is poured into the ladle from the BOF converter, this affects the cleanliness of and reintroduces impurities such as phosphorus (P) into the steel, adversely affects the aluminum efficiency during tap, and/or prevents certain grades of steel from being made. Any attempt to remove or minimize the effect of excess slag poured into the ladle is expensive, time-consuming, and/or labor intensive. For example, if too much slag is accidentally poured into the ladle, hundreds of dollars worth of aluminum or other slag modifier(s) may have to be added to the molten ladle slag to try to minimize the levels of FeO and other unstable oxides in the slag. In summary, minimizing or reducing slag carryover from the BOF converter into the ladle is important or essential for efficient manufacturing of high quality steel.
Many techniques have been used in an effort to control the carryover of slag during the tapping of BOF converters. For example, see
Slag Carryover in Oxygen Converters: an International Review
, by Da Silva, Bergman, and Lindfors [pp. 91-95], the disclosure of which is hereby incorporated herein by reference. In this review, numerous methods for controlling the carryover of slag during BOF converter tapping are discussed. For example, it is known to use refractory plugs, metallic plugs, wooden plugs, fiber plugs, gunned clay, dart-shaped floating elements, and ball-shaped floating elements in an attempt to control or minimize slag carryover.
Certain known techniques result in the interruption of the metal pour or tap stream from the converter near the end of tapping in order to minimize slag carryover. Dart-shaped and ball-shaped floating elements are often used for this purpose. In
FIGS. 4 and 5
of the above-referenced article, the often unsatisfactory results associated with these conventional methods are illustrated. For example, dart- and ball-shaped floating elements are known to be unsuccessful when the slag is thick or viscous, and it has been found that the positioning of these floating elements inside the converter is both difficult and critical. The structure of the taphole also affects the effectiveness of these types of floating elements. As discussed in the article, some steel plants have reported that the balls sometimes close the taphole too early, which may result in the leaving of purified molten steel (affecting yield) in the converter. Accordingly, it is known in the art that while floating elements may help to minimize slag carryover, they are often inefficient and the results are unpredictable. Still further, both balls and darts are undesirably expensive.
Despite the fact that so many slag carryover prevention techniques are known, it is stated at the conclusion of the above-referenced article that “none of the methods in use today can be considered to be of universal application, since each has its limitations and can only reach the expected results if specific conditions exists.” In other words, there has existed a longstanding need in the art for a system and corresponding method for minimizing the carryover of slag during the tapping of BOF converters, which is usable in different environments by operators of different skill levels. No known technique has, to date, been found to be satisfactory in all commercial steel-making environments because many techniques are not considered to be efficient enough and others are too expensive for use with ordinary steel grades.
In view of the inefficiency and non-effectiveness of known BOF slag carryover prevention methods, many steel plants simply rely upon operators to visually detect when the slag portion of tapping is reached. Unfortunately, this method of slag carryover prevention is inefficient at best, as it is difficult for most humans to visually observe any visible difference between purified molten steel being poured from the converter taphole and molten slag being poured from the taphole [both are molten and yellow to white-hot].
Still another approach used by many in the trade to minimize slag carryover in BOF environments is the positioning of electromagnetic coils on BOF convert
Goldstein Daniel A.
Sharan Alok
Stelts Elizabeth A.
Stofanak John A.
Bethlehem Steel Corporation
Kastler Scott
Masteller, Jr. Harold I.
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