Specialized metallurgical processes – compositions for use therei – Processes – Process control responsive to sensed condition
Reexamination Certificate
1999-09-20
2001-09-04
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Process control responsive to sensed condition
Reexamination Certificate
active
06284016
ABSTRACT:
TECHNICAL FIELD
The art of this invention relates to a pressurized converter steelmaking method capable of blowing molten steel with high productivity, high yield, and a low degree of superoxidation.
BACKGROUND ART
The ultimate aim of converter refining is to blow molten steel having a low degree of superoxidation with high productivity and high yield. The decarburization behavior in converter refining is divided into a period I in which the decarburization rate is determined by a flow rate of oxygen supplied to molten iron in a region where the molten iron has a high carbon concentration, and a period II in which the decarburization rate is determined by a mass transfer rate of carbon in molten iron in a region where the molten iron has a low carbon concentration.
To improve the productivity in converter refining, it is required to increase the decarburization rate in the period I which occupies a large part of the refining time. For that purpose, the flow rate of oxygen supplied to molten iron requires to be increased in principle. However, the oxygen flow rate in a general top-and-bottom blowing converter has an upper limit of about 4 (Nm
3
/ton/min). If the oxygen flow rate is increased beyond the upper limit value, violent splashes would come out, the amount of dust would be increased, and a phenomenon called slopping would occur. The occurrence of those phenomena reduces yield of molten steel, increases deposition of skull onto the top of the converter, and increases the amount of waste slag beneath the converter. Accordingly, problems of prolonging a non-blowing time such as taken to remove the skull and clean the ground beneath the converter and lowering the productivity are rather encountered.
There are known several techniques of pressurizing a converter for the purpose of increasing the oxygen flow rate and suppressing the occurrence of dust during the period I. As described below, however, any of the known techniques is not sufficient to provide satisfactory operating conditions.
For achieving improved yield of molten steel, it is needed not only to reduce the occurrence of dust and splashes during the period I, but also to suppress the iron oxidation loss occurred during the period II. In the region where molten steel has a low carbon concentration during the period II, an iron oxidation loss occurs because the molten steel is brought into a superoxidated state, and iron is oxidized and released into slag. Superoxidation of molten steel increases the amount of (T·Fe) in slag (i.e., total iron components contained in slag in the form of iron oxides or iron), and also increases an oxygen concentration in the molten steel. This gives rise to an additional problem that a large amount of deoxidizing agent is required and purity of the molten steel is considerably deteriorated due to deoxidation products generated in a large amount.
For suppressing superoxidation during the period II, it is conceivable in principle to reduce the oxygen flow rate and to increase stirring intensity. A reduction of the oxygen flow rate however prolongs the refining time, and therefore accompanies a problem that an improvement of the productivity cannot be achieved at the same time. Also, an increase of bottom-blow stirring intensity results in an increase of the stirring gas cost. An increase of the stirring gas cost can be suppressed by holding low the stirring intensity during the period I and increasing the stirring intensity only during the period II. However, due to lack of technology for remarkably changing the bottom-blown gas flow rate at the same tuyere, this method raises another problem that the wearing rate of bottom-blow tuyere bricks is increased.
Meanwhile, there are known several techniques of pressurizing the interior of a converter for the purpose of increasing the oxygen flow rate and suppressing the generation of dust. However, any of the known techniques is not sufficient to provide satisfactory operating conditions as follows.
Japanese Examined Patent Publication No. 43-9982 discloses an iron refining method comprising the steps of placing both an iron charge and a slag making component in a top blown converter, introducing oxygen through a lance positioned in the converter, causing the oxygen to flow over the surface of the iron charge located below the lance, thereby developing a refining reaction to remove carbon from iron and to generate a reactor gas, causing the reactor gas to flow from the converter to a gas collecting device, providing pressure adjusting means for controlling the gas velocity, and holding a close relation between the iron charge and the pressure adjusting means so that essentially all of the gas passes the pressure adjusting means. In addition, the pressure adjusting means is controlled so as to provide at least one atmospheric pressure within the converter when the iron charge is refined with the introduced oxygen.
The technique disclosed in the above publication is featured in that a carbon dioxide production ratio (post combustion rate) raises, and the amount of dust is reduced because a mass flow rate of the waste gas is lowered. The disclosed technique however contains no quantitative restrictions with regard to the oxygen flow rate and the relationship between impingement energy of a top-blown oxygen jet upon the bath surface and pressure, which greatly affect the post combustion rate and the amount of dust generated. Further, the disclosed technique relates to a top blown converter, and greatly differs in basic conditions from refining with a top-and-bottom blowing converter. Accordingly, a converter cannot be operated as a pressurized converter based on the disclosed invention alone.
Japanese Unexamined Patent Publication No. 2-205616 discloses a highly-efficient converter steelmaking method for refining iron materials, such as molten iron and scraps if necessary, to molten steel, wherein the interior of a converter is pressurized to 0.5 kgf/cm
2
or more, the relationship between a total amount W (t/ch) of molten pig iron and scraps both charged into the converter and an inner volume V (m
3
) of the converter shell is set to satisfy W>0.8 V or 0.8 V≧W≧0.5. V, and an oxygen flow rate U (Nm
3
/min·t) into the converter is set to satisfy U≧3.7. This publication explains that the occurrence of slopping and spitting was suppressed and high yield was obtained under pressurization.
However, the above-cited publication does not discuss the conditions for suppressing the occurrence of slopping and spitting in relation to the oxygen supply condition and the relationship between stirring intensity and the pressurizing condition. It is therefore impossible to carry out the operation of a pressurized converter based on the invention disclosed in the above-cited publication alone. In a top-and-bottom blowing converter, which is subjected to strong stirring intensity, particularly, slopping hardly occurs even at normal pressure under the conditions of the comparative example described in the above-cited publication. Thus, because of great difference in basic conditions, it is difficult to obtain the pressurized operating conditions for the top-and-bottom blowing converter from the invention disclosed in the above-cited publication.
Moreover, the above-cited publication does not explain a method for operating the converter under the condition of low carbon concentration during the period II, which is most important from the viewpoint of suppressing superoxidation and improving yield.
Japanese Unexamined Patent Publication No. 62-142712 discloses a steel- and iron-making method in a converter or a smelting reduction furnace, wherein the internal pressure of the converter or the smelting reduction furnace is set to a level higher than the atmospheric pressure, particularly in the range of 2-5 kg/cm
2
, so that the linear velocity of post combustion gas is lowered.
The invention disclosed in the above-cited publication intends to lower the velocity of rising flow of post combustion gas in slag under pressurization and to prolong a heat-exch
Kitamura Shinya
Matsuo Michitaka
Naito Ken-ichiro
King Roy
McGuthry-Banks Tima
Nippon Steel Corporation
Wenderoth , Lind & Ponack, L.L.P.
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