Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Patent
1997-07-03
2000-01-25
Andrews, Melvyn
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
266225, C21C 532, C21C 546
Patent
active
060173807
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a refining method featuring excellent decarburization in a top- and bottom-blown converter and to a top-blown lance for the converter.
BACKGROUND ART
The refining reaction in a top-blown converter and in a top- and bottom-blown converter proceeds by supplying an oxygen gas from a top-blown lance to oxidize impurities such as carbon, silicon, phosphorus, etc. Furthermore, the top-blown lance usually employs a convergent-divergent nozzle having a single aperture or a plurality of apertures in order to efficiently convert the secondary pressure of the lance into kinetic energy of a jet of oxygen gas, and as a result, the stirring in a steel bath is promoted by the jet. ("Handbook of Steels", 3rd edition, separate volume II, the Japanese Association of Steels, 1982, p. 468).
In order to impart stirring force to a steel bath according to a conventional method, the top-blown lance as described above is used and the refining is carried out under a secondary pressure within a proper range of expansion of the convergent-divergent nozzle from the first period of refining up to the last period of refining, however, an optimum flow rate or a velocity of jet of oxygen gas depending upon the refining steps cannot be selected freely. At the rate determining step of supplying oxygen in the initial period of refining, therefore, when the flow rate of oxygen gas is increased to increase the rate of decarburization, the velocity of jet of oxygen gas is increased, as a result, the amount of dust and spitting increases. At the rate determining step of supplying carbon in the last period of refining, furthermore, when the flow rate of oxygen gas is decreased to prevent super oxidizing of the steel bath and increasing the iron oxide in the slag, the velocity of jet becomes so small that the temperature at a hot spot where jet impinges on the steel bath drops or the stirring force becomes insufficient, resulting in a decrease in the rate of decarburization.
In general, the following three requirements are necessary for the decarburization in the converter, i.e., 1 in a high carbon range, dust is generated less and the slag is formed quickly, 2 in an intermediate carbon range, the decarburization oxygen efficiency is high, and 3 the decarburization proceeds up to a low carbon range while suppressing the formation of iron oxide.
Among them, it has been considered that the converter dust of 1 is generated from two sources, i.e., the dust is generated from a surface (hot spot) where the top-blown oxygen impinges the steel bath, namely, is generated by vaporization of iron from the high-temperature hot spot or is generated by volumetric expansion of a molten steel which occurs when the CO gas is formed by the decarburization reaction at the hot spot.
A variety of methods have heretofore been proposed to increase the iron yield by decreasing the amount of dust generated during the blowing in the converter.
Japanese Unexamined Patent Publication (Kokai) No. 2-156012 discloses a method by which the height of the lance is increased and an inert gas is mixed into the top-blown gas in order to decrease the amount of dust formation. According to this method, the post combustion rate increases accompanying an increase in the height of the lance, and the heat transfer efficiency decreases. Therefore, melt loss increases considerably in the converter refractories. Besides, inert gas is used in large amounts, which is disadvantageous.
According to "Materials and Processes", Vol. 7, 1994, p. 229, the generating rate of dust is dependent upon a value that is obtained by dividing the oxygen supplying rate by the area of hot spot. When the supplying rate of oxygen is lowered to lower the oxygen supplying rate per a unit area of a hot spot, the productivity decreases. When a nozzle having many apertures is used to increase the area of hot spot, on the other hand, the hot spots are overlapped one upon the other causing the splash to increase. When the height of the lance is increased, furthermore, t
Inomoto Takeo
Kikuchi Shin
Kitamura Shinya
Naito Ken-ichiro
Ogawa Yuji
Andrews Melvyn
Nippon Steel Corporation
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