Method Of Controlling The Exhaust Gas Flow Volume In An Oxygen Top-blowing Converter

Abstract

A method for correctly judging the carbon value of a steel bath by controlling an exhaust gas flow produced from an oxygen top-blowing converter, wherein the exhaust gas flow is controlled so that the momentary flow volume of the exhaust gas flowing through an exhaust gas conduit may coincide with the variation with the lapse of time of a dry gas flow volume in a standard state predetermined in response to blowing conditions and a decarburizing velocity is calculated from the said exhaust gas flow volume and the analysis value of the exhaust gas.

Description

This invention relates to a method for exactly judging the carbon value of a molten steel by controlling the volume of flow of an exhaust gas produced from an oxygen top-blowing converter according to a predetermined program.

The present invention has as an object to provide a method for controlling the gas flow volume for the purpose of exactly judging the carbon value of the steel bath, while still taking into account any lag in the analysis of the gas.

The subject matter of the present invention will be explained with reference to the accompanying drawings, in which:

FIGS. 1a-1c are schematic illustrations of various states of closure of a furnace mouth part and an exhaust gas collecting device in an oxygen top-blowing converter, FIG. 1a showing the parts in a perfectly sealed state, FIG. 1b showing the state wherein slag or metal is deposited on the furnace mouth part and some clearance exists, and FIG. 1c showing the state wherein a large amount of slag or metal is deposited on the furnace mouth part and the clearance is almost a maximum;

FIG. 2 is a graph with curves showing the relations between the blowing time and the produced gas F.sub.B and entering air F.sub.A for the states shown in FIGS. 1a-1c, respectively;

FIG. 3 is a graph with curves showing the blowing time and the variation of CO in percent (.eta.) in the exhaust gas with the lapse of time in FIGS. 1a-1c, respectively;

FIG. 4 is a schematic view of an exhaust gas flow volume controlling apparatus according to the present invention;

FIG. 5 is a diagram showing the relation between the gas flow volume based on a conventional furnace pressure controlling method and the end point molten steel carbon in percent;

FIG. 6 is a relative diagram representing the operation in FIG. 5 with the decarburizing velocity and end point molten steel carbon in percent;

FIG. 7 is a diagram showing the relations between the gas flow volume based on the operating method of the present invention and the end point molten steel carbon in percent; and

FIG. 8 is a diagram showing the relations between the decarburizing velocity and the end point molten steel carbon in percent when the exhaust gas information based on the operating method of the present invention is utilized.

A method of recovering a CO-containing gas produced from an oxygen top-blowing converter is already known.

When recovering CO-containing exhaust gas, it is, of course, necessary to elevate the CO content by preventing the outside air from entering into the exhaust gas as far as possible. On the other hand, when utilizing the exhaust gas as a source of information for a steel making operation, particularly for the judging of the carbon value of a steel bath, the exhaust gas should be regarded as a primary means for detecting the state of the reaction in the furnace.

Today, the gas information from an apparatus for recovering the exhaust gas of an oxygen top-blowing converter has come to perform a very important role for the dynamic control of the carbon content in a steel bath.

That is to say, the variation of the actual flow volume of the exhaust gas produced by the decarburizing reaction within a converter or particularly the exhaust gas flow volume pattern in the final period of the blowing has a very close correlation with the carbon value of the steel bath. Therefore, by utilizing this fact, the carbon value of the final bath can be determined.

Now, in a conventional apparatus for recovering the exhaust gas of an oxygen top-blowing converter the furnace pressure control is carried out for the purpose of maintaining the quality of the recovered gas, that is, for preventing the dilution of the gas being recovered by air entering through the furnace mouth part.

In this method, as well known, a hood and movable skirt are provided between the furnace mouth of a converter and a gas conduit and an exhaust gas flow volume regulating device such as, for example, a damper is positioned in the gas conduit so that the pressure in the hood can be detected and the opening of said damper can be adjusted so as to give a determined pressure.

According to this method, the skirt for intercepting the entering air must be perfectly closed. That is, in order to attain the expected object, a strict air-tightness of the furnace mouth is required. However, in fact, due to the slopping during the blowing, slag or metal is often deposited on the furnace mouth part. In most cases, it is impossible to perfectly shield the skirt. Therefore, the furnace pressure control in the strict sense of the word is no longer performed.

This shall be concretely described with reference to FIGS. 1 and 2. When the furnace mouth and the skirt can be sealed as in the case of a, the gas flowing through the gas conduit consists of only the exhaust gas F.sub.B produced from the decarburizing reaction and the pressure within the furnace can be controlled to a determined one. However, when the skirt is somewhat opened on account of the deposition of slag or the like as in the case b, the outside air enters through this clearance, and the gas flowing through the gas conduit is the volume of the exhaust gas F.sub.B of the case a with the addition of the entering air as is shown in FIG. 2 and produces a determined pressure in a so-called diluted state.

Further, when the skirt has opened greatly as in the case c, a large amount of air enters and the variation of the total volume of the gas with the lapse of time becomes a straight line. However, in this case, too, the furnace pressure is controlled as determined.

As is evident from this fact, it is hard to close the skirt perfectly. The furnace pressure control is performed only with the outside air being sucked-in, though the inherent furnace pressure control has a its object the elimination of any entering air.

As a result, as is shown in FIG. 3, the time required for recovering the exhaust gas is shorter in the cases of b and c than in the case of a, for a determined limit of percent of CO in the recovered exhaust gas. Moreover, the gas treating equipment becomes unnecessarily large on account of the increase in the total exhaust gas flow volume.

What is more important is that, when utilizing the exhaust gas produced in the above-mentioned operating method as an information source for the decarburizing reaction, the following must be taken into consideration; namely that such an operating method includes a time lag peculiar to the apparatus used for the so-called dynamic control of the converter, particularly a time lag in obtaining the analysis value of the exhaust gas composition.

That is, as regards the exhaust gas analyzer system, there is an analysis time lag (quantitatively usually more than 10 seconds) which is a combination of the time lag due to the movement of the gas from the furnace mouth part to the gas analyzer and the time lag in analyzing the gas, which has arrived at the analyzer, and converting it to an electrical signal.

Therefore, the discrepancy in time due to such analysis time lag is fatal to the converter operation, in which the blowing time is very short. It is very difficult to judge the carbon value of a steel bath in the presence of such a time lag. Further, what is undesirable in addition to such lag in the analysis is that the volume of blown air is indefinite in carrying out the furnace pressure control.

As above-mentioned, it has been ascertained that there exists a very intimate correlation between the flow volume of the gas generated from the converter or the decarburizing velocity and the carbon value of the steel bath. However, in the conventional furnace pressure control the gas flow contains a large volume of air entrained therein. Consequently, even if such a gas flow is used for the dynamic control, by which the carbon content of the molten bath is to be judged, there will be obtained only a result of low accuracy, because the amount of entrained air is so large and the volume so indefinite that, even under the same blowing conditions various patterns of the flow volume with the lapse of time will be generated.

In carrying out the furnace pressure control the following must also be taken into consideration, namely that, on account of a characteristic inherent in the meter it is hardly possible to control the flow volume controlling device such as a damper or the like, unless the deviation of the actual pressure from the set value (of the pressure) exceeds a certain amount, the flow volume of the gas measured by the meter, particularly in the final period of the blowing of the converter, does not show a stable value but instead shows a large fluctuation.

Therefore, the dynamic control of the converter based on such inaccurate gas information naturally has a low precision.

FIG. 5 shows relations between the exhaust gas flow volume (including entering air) and the carbon in the end point molten steel when a conventional furnace pressure control was carried out under a certain blowing condition. The fluctuation is very large therein. FIG. 6 shows the relations between the decarburizing velocity and the carbon in the end point molten steel graphed on the basis of the above relations. The fluctuation is likewise very large therein.

This is caused by a lag of the analysis of CO+CO.sub.2 by a gas analyzer, which constitutes a decarburization speed-meter, as above-mentioned. This trouble would be solved, if the lag of the analysis of CO+CO.sub.2 could be lessened, which is, however, not possible on account of the inherent characteristic of the analyzer.

As the decarburizing velocity, which is correlated with the carbon value of a steel bath, is originally based on the exhaust gas flow volume and the analysis value thereof, the carbon value of the steel bath can be determined only by measuring the exhaust gas flow volume, if said analysis value could be set to a determined value by regulating the amount of air sucked-in.

In order to achieve the above-mentioned object the present invention brings the correlation between the carbon value of the steel bath and the gas flow volume into conformity with that of the past charges, while lessening fluctuations of the exhaust gas flow volume in the final period of the blowing by regulating the amount of air sucked-in during said period. More particularly the present invention is characterized by a method comprising the steps of sucking an exhaust gas produced from an oxygen top-blowing converter through an exhaust gas conduit having a suction fan, regulating a gas flow volume controlling device provided in the exhaust gas conduit so that the momentary flow volume value of the exhaust gas flowing through said exhaust gas conduit will coincide with the variation over a period of time of a dry gas flow volume in a standard state predetermined in response to blowing conditions and calculating the decarburizing velocity from said exhaust gas flow volume and the analysis value of the exhaust gas, thereby making it possible to judge the carbon value of a steel bath. The present invention contemplates at the same time carrying out the recovery operation of the exhaust gas produced from an oxygen top-blowing converter smoothly by restricting the entry of the outside air, blowing the indicated volume of the outside air in the initial and final periods of blowing respectively so that it is mixed with the gas in the furnace for combustion, thereby avoiding the danger of explosion, and solving the problem of surging accompanying the reduction of the gas volume by the suction fan or omitting the step of diluting the gas with an inert gas.

Referring to FIG. 4, 1 is a converter into which oxygen is introduced while melting pig iron. A hood 2 for collecting an exhaust gas produced by oxygen blowing is mounted above the converter. A vertically movable skirt 2' is arranged between the converter furnace mouth 1' and said hood 2.

The hood 2 is connected to an exhaust gas conduit 3 which in turn is connected to a gas flow meter 13, which is a flow volume measuring throttling mechanism, and suction fan 5 through a venturi tube 6 and dust collector 12. A damper 4 to control the gas flow volume which flows through the exhaust gas conduit 3 is positioned ahead of the gas flow meter 13. Here an ordinary PA venturi tube is used. However, the device is not limited to the venturi but any type of device which can control the flow volume will do.

7 is a damper operating cylinder operated by a regulator 8. 17 is a gas analyzer, 18 is a flow meter and 19 is a calculator for integrating the measured values from the flow meter and the analyzer.

The volume Fo.sub.2 of oxygen blown into the converter is supplied into a controller 9 as a signal from a signal generating means 16. Meanwhile the momentary coefficient .alpha.(t) and combustion coefficient K (in other words, the volume of air sucked in), as will be described later, are indicated by a multiplier 10. The signal representing Fo.sub.2 as multiplied by these coefficients is supplied to the controller 9.

The actual exhaust gas is measured by the flow meter 13 and a signal representative thereof is introduced into said controller 9 and an instruction to cancel the difference between them is transmitted to the regulator 8 to regulate the controlling damper 4. This difference pressure is converted to a dry gas flow volume at a standard state by correcting the humidity, pressure and temperature with a corrector 13' by using signals from the pressure gauge 14 and thermometer 15.

In this way, the exhaust gas analysis, exhaust gas volume and volume of oxygen introduced for a past charge are retained by the multiplier 10. Therefore, the coefficients over a period of time can be simply determined and their values can be memorized.

Speaking in more concrete terms, in carrying out the present invention by using the above-mentioned apparatus, it is noted that, for instance, in case a fixed volume of oxygen is all consumed to produce CO, and amount of exhaust gas double the volume of the oxygen from the reaction 2C + O.sub.2 => 2CO, is produced.

Here the coefficient .alpha. in the formula representing the exhaust gas volume Q = .alpha.(t).sup.. Fo.sub.2 (wherein Fo.sub.2 is the volume of blown in oxygen) is 2, and the exhaust gas volume is represented by Q = 2.sup.. F..sub.2. Actually this coefficient .alpha.(t) varies with the variation of the gas composition throughout the entire blowing period. However, it can be determined from past charges.

In the initial and final periods of the blowing, the exhaust gas volume is so small that there is a danger of a gas explosion due to the surging by the suction fan and the presence of residual O.sub.2. Therefore, it is also an object of the present invention to determine a combustion coefficient K so that the exhaust gas volume will be Q = .alpha..sup.. Fo.sub.2.sup.. K so as to burn CO by sucking in a certain amount of air.

More particularly, for example, when the oxygen feed volume at a given moment is Fo.sub.2 and it is all consumed to produce CO, the total exhaust gas volume is Q = 2Fo.sub.2.sup.. K as described above.

Therefore, if the combustion rate is .lambda.(in fact, .lambda. is 0.1 to 0.5), 2 Fo.sub.2 (1-.lambda.) is converted to CO, 2 Fo.sub.2 .lambda. is converted to CO.sub.2 and the amount of N.sub.2 in the sucked-in air is 2 Fo.sub.2.sup.. .lambda./2 .times. 79/21.

Therefore, the total exhaust gas volume Q is the sum of these amounts and therefore is

Q = 2 Fo.sub.2 (1 + 2.lambda.).

That is to say, (1+2.lambda.) is a combustion coefficient K. Needless to say that the actual oxygen feed volume is determined based on numerical values from the past charges.

As is evident from this, the method of the present invention is characterized by operating an apparatus, wherein an exhaust gas produced in an oxygen top-blowing converter is sucked through an exhaust gas conduit having a suction fan, while carrying out an exhaust gas flow volume program Q = .alpha.(t).sup.. Fo.sub.2.sup.. K established by determining a produced gas coefficient .alpha.(t) over a period of time and a combustion coefficient K for CO in the produced gas based on an oxygen feed volume Fo.sub.2 (or oxygen feeding velocity) determined from the past charges, by regulating gas flow volume controlling device provided in the exhaust gas conduit on the basis of said program. Needless to say, it is not a departure from the present application to make .alpha.(t) in this case constant. As a result, the carbon content of the steel bath can be judged with a high precision.

An example of the operation of the present invention will be described in the following.

Refining conditions: Molten pig charge : 140 tons Introduced oxygen : 25,000 Nm.sup.3 /hr. Blowing time : 17.0 minutes Converter capacity : 180 tons

Under the above-mentioned conditions, the coefficients .alpha. and K of the multiplier were set as shown in Table 1 and oxygen blowing was carried out.

The data on operation of a conventional furnace ##SPC1## ##SPC2##

controlling method under the same operating conditions are shown for comparison in Table 2.

As is evident from the above comparison of the method of the present invention and the conventional method, in the case of the present invention the curve showing a reduction in the exhaust gas flow volume in the final period of the blowing shows a more gradually decreasing tendency than in the case of the conventional method and is very accurate, because the exhaust gas flow volume in the method of the present invention is based on a predetermined pattern of the flow volume with the lapse of time and is particularly suitable for judging the molten steel composition in a converter, because not only the time during which the exhaust gas flow volume is reduced, but also the volume thereof coincide well with their actual values.

Shown in FIGS. 7 and 8 are the results of carrying out the operating method of the present invention on 125 charges on the basis of various blowing conditions and the gas information was utilized for the judgment of the carbon value in the steel bath.

FIG. 7 has exhaust gas flow volumes (.times.10.sup.2 Nm/hr) plotted on the ordinate and carbon contents (in percent) in the end point steel plotted on the abscissa.

FIG. 8 has decarburizing velocities (in kg/min) plotted on the ordinate and carbon contents (in percent) in the end point steel plotted on the abscissa.

The correlations have much smaller fluctuations than in the conventional method, the data for which is shown in FIGS. 5 and 6, as can be recognized in FIGS. 7 and 8.

Further, as is evident by comparing Tables 1 and 2, when the recovery limit of the valuable CO gas is equal to 60 percent, the gas recovering time was 6 to 7 minutes in the conventional method but was 10 minutes in the present invention and the average of the CO concentration in the recovered gas rose greatly to 67 percent in the present method from 62 percent in the conventional method. Further, the maximum exhaust gas flow volume during the entire blowing period was reduced to 60,000 Nm.sup.3 /hr in the present method from 76,000 Nm.sup.3 /hr in the conventional method. It is evident that the entering air volume was reduced as seen from the amount of N.sub.2.

Thus, according to the present invention, the treatment control of the exhaust gas of an oxygen top-blowing converter, which has been heretofore considered to be very difficult, can be carried out easily and precisely so that, as described above, the carbon value of the steel bath in the final period of the blowing can be correctly judged, the recovered gas concentration can be elevated, the volume of the gas to be treated can be reduced (the equipment can be made smaller) and the amount of outside air sucked-in can be controlled in the initial and final periods of the blowing, consequently CO becomes CO.sub.2 and this CO.sub.2 can be utilized. In this way, by the method of the present invention a safe operation of controlling the flow volume of an exhaust gas from an oxygen top-blowing converter can be established.

Claims

What is claimed is:

1. A method of operating an oxygen top blowing converter having an apparatus for recovering converter exhaust gas which includes a gas flow volume control means in an exhaust gas conduit, wherein molten iron is refined by continuously blowing a certain amount of oxygen into the molten iron in the converter through a lance and the carbon content of the molten steel in the converter is judged from exhaust gas information from the apparatus for recovering converter exhaust gas, which apparatus draws the exhaust gas discharged from the converter mouth into the apparatus by means of a suction blower provided in the exhaust gas conduit in the apparatus, and the exhaust gas is recovered, comprising the steps of regulating the gas flow volume control device provided in the exhaust gas conduit so that the total amount of exhaust gas flowing through said exhaust gas conduit, including air drawn into the conduit through the space between the converter mouth and the start of the conduit coincides with the desired pattern of the variation of the flow volume of a dry gas in a standard state previously obtained from the results of various heats carried out in the past under the same blowing conditions as those in the present heat for regulating the drawing-in of air into the exhaust gas caused by a changed positional relationship of the converter mouth and the start of the conduit due to the presence of slag and the like which are different from the positional relationship in previous heats for reproducing the exhaust gas conditions of previous heats suitable for judging the carbon content of the molten steel in the converter, measuring the exhaust gas flow volume and analyzing the exhaust gas for CO and CO.sub.2 and then detecting the decarburizing velocity therefrom at each moment in the final period of blowing, during which the volume of exhaust gas flowing through said exhaust gas conduit decreases sharply, and judging the carbon content of the molten steel by comparing the detected value of the decarburizing velocity in the final blowing period of the heat with a graphically representable correlative relation between the decarburizing velocity and the carbon content of molten steels during the final period of blowing obtained from previous heats carried out under the same blowing conditions.

2. A method for controlling the volume of exhaust gas flow in an oxygen top-blowing converter, comprising sucking the exhaust gas produced from the oxygen top-blowing converter through an exhaust gas conduit by means of a suction fan, controlling the gas flow volume so that the momentary value of the volume of flow of the exhaust gas flowing through said exhaust gas conduit coincides with an exhaust gas flow volume program G= (t).sup. . Fo.sub.2.sup.. K determined by a generated gas coefficient (t) over a period of time for a redetermined oxygen feed volume Fo.sub.2 and a determined combustion coefficient K for CO in the gas produced in the converter, analyzing the exhaust gas, and calculating a decarburizing velocity from said volume of flow of exhaust gas and the analysis of the exhaust gas and comparing said decarburizing velocity with decarburizing velocities of past heats to judge the carbon content of the melt.