JPS62203650A - Molten steel pouring method - Google Patents

Abstract

PURPOSE:To improve the casting slab quality by setting permissible limit of blowing gas volume to nozzle passing molten steel rate and correcting the permissible limit of blowing gas volume by calculating a correction factor from a ratio of actual measured gas volume at occurrence of a boiling to the permissible limit of blowing gas volume, to control the blowing gas volume corresponding to a changing of the operation. CONSTITUTION:The permissible limit of gas volume is set after calculating the ratio R corresponding to an operational condition, boiling limit, nozzle clogging limit and the optimum value for the steel quality. Next, the standard gas volume to be permissible limit is calculated, based on informations of casting slab speed under operation, molten steel head, casting slab size, nozzle diameter, etc. On the other hand, the actual measured gas volume is calculated from the gas volume at the time start of boiling occurrence in the passing molten steel rate under the operation. Further, the setting gas volume of the above-mentioned passing molten steel rate is calculated, based on the setting permissible limit. And, the nozzle characteristic correction factor is calculated by the actual measured gas volume and the setting gas volume to correct the standard gas volume.

Description

[Detailed description of the invention] [Industrial application field] The present invention c, t:i! Regarding the method of injecting molten steel in continuous casting, in detail, inert gas is blown into the molten steel flow during the process of injecting it into type II, and the injection is performed while removing impurities in the molten steel and preventing nozzle clogging. This invention relates to a continuous casting method for injecting molten steel to produce high quality slabs.

[Conventional technology]

As is well known, in continuous steel casting, molten steel transported in a ladle is temporarily stored in a tundish, and then injected into a mold from the tundish through a submerged nozzle.

FIG. 11 is a structural diagram showing a casting part in a general continuous casting equipment, and in the figure, l is a ladle, 2 is a tamp, and ish. The solution once stored in Tanditshu 2 [
3 is injected into the mold 5 through a submerged nozzle 4. The immersion nozzle 4 in this example includes an upper nozzle 41 attached to the bottom wall of the tundish 2, a sliding nozzle 42 attached to the bottom of the tundish 2 in contact with the nozzle 41, an n:1 slide, and a movable ing nozzle 42. (1
It consists of an injection nozzle 43 that is temporarily attached.

By the way, the molten steel 3 contains deoxidation products such as Al2O3,
Or impurities such as powder, slag, and sulfide (hereinafter referred to as
These inclusions are collectively referred to as inclusions, and if these inclusions are captured in the slab and remain, they cause various problems such as surface flaws and internal defects called slag bite. Further, among the inclusions, Af□01 and the like often adhere to and accumulate on the inner surface of the nozzle when passing through the nozzle, clogging the nozzle and hindering stable operation.

For this reason, methods have been proposed to efficiently separate the inclusions from molten steel and float them, and methods to capture the floated inclusions with powder supplied into the mold, and some of them have not been put into practical use. It has become so. For example, in Japanese Patent Publication No. 49-28569, the inclusions are effectively removed by injecting an inert gas (hereinafter simply referred to as gas) such as A gas or N2 gas into the molten steel flow in the process of being poured into the mold. A technique for levitating the gas has been disclosed and has been widely adopted in recent years. 6 in FIG. , the upper nozzle 4
1 into the molten steel flow.

Further, as shown in Japanese Utility Model Publication No. 56-48440, for example, a nozzle for effectively blowing gas (a nozzle for blowing gas such as the above-mentioned upper nozzle 41 is hereinafter referred to as
Various proposals have also been made regarding the nozzle (simply referred to as a nozzle).

However, it is often difficult to increase the floating effect of inclusions by blowing the gas and prevent clogging of the nozzle. In other words, if the amount of gas injected exceeds a certain amount, the flow rate becomes unstable, the scene inside the mold is greatly disturbed, and a phenomenon called boiling occurs, in which the molten steel no longer flows into the nozzle. .

Hereinafter, this phenomenon will be referred to as boiling or boiling phenomenon, and if such boiling phenomenon becomes severe, it may lead to breakout in extreme cases. For this reason, in the past, based on past experience, it has been common for operators to operate while constantly monitoring the state of the mold surface so as to inject gas in an area where the boiling phenomenon does not occur. However, especially when there are variations in the depth of molten steel in the tundish, casting speed, etc., or when changing the casting width, it is difficult for the operator to make appropriate adjustments and control based on his/her judgment, and the boiling phenomenon described above frequently occurs. .

On the other hand, for the purpose of reducing manufacturing costs, attempts have been made to reduce the amount of gas injected depending on the steel type (and reduce its consumption.However, with conventional methods, there was no way to quantitatively understand this. Therefore, it is normal for workers to visually judge the blowing status, and as a result, the blowing gas ■
Stable operation was difficult, as the amount of water became too low, causing nozzle clogging.

[Problem that the invention seeks to solve]

As mentioned above, the setting and control of the gas injection amount in conventional methods relies on the operator's experience and intuition.
There has been no attempt to quantitatively understand gas injection in order to stabilize boiling limits, nozzle clogging limits, and quality, and to apply it to actual operations. For this reason, steel type, size,
It is difficult to control the gas injection amount to accurately follow changes in operating conditions such as casting speed and molten steel depth in the tundish, and it also causes major troubles such as operational instability and breakouts due to the occurrence of boiling phenomena. Due to concerns, the amount of gas injected was kept low. As a result, the inclusions are not removed sufficiently, and the number of inclusions remaining in the slab varies widely, leading to instability in the quality of the slab, and as mentioned above, the number of inclusions remaining in the slab varies. Particularly for steel types that have strict quality requirements such as tinplate and thin plate materials, the residual inclusions become a major problem even if the boiling phenomenon does not occur. Ta.

Therefore, in order to avoid boiling and nozzle clogging, and to stabilize quality, operators must constantly monitor the hot water level under high temperatures, which places an extremely heavy mental and physical burden on them.

The present invention aims to solve the above-mentioned conventional problems in the inert gas blowing method during injection of molten steel, and generates the foil phenomenon by accurately following fluctuations in the operating conditions and changes in nozzle characteristics. In order to obtain the economically required quality without overheating, we ensure an appropriate blowing amount, thereby improving the quality of slabs and preventing nozzle clogging and rain.
This provides a method for making the user's eyes visible.

[Means for solving problems]

FIG. 1 is a diagram showing an example of a method of implementing the present invention in the well-known general continuous casting equipment shown in FIG. 11.

As described above, the present invention is directed to the molten steel 3 stored in the tank 2 flowing down through the immersion nozzle 4 (hereinafter referred to as molten steel flow). In the method of injecting gas into the mold 5 while blowing gas into the mold, the permissible limit of the amount of blown gas for the amount of gas passing through the nozzle! The boiling temperature is determined from the nozzle clogging occurrence limit or the quality limit, and then boiling is generated at any amount of passing molten steel during the operation, and the gas amount at the start of the boiling is actually measured, and the measured gas amount and
A nozzle characteristic correction coefficient K is determined from the ratio of the boil generation starting gas amount to the same passing molten steel amount determined from the boil generation limit among the set allowable limits, and the gas injection amount is determined by correcting the above allowable limit using the correction coefficient K. The present invention relates to a molten steel injection method characterized by controlling.

[For production]

As is well known, the gas blown from the nozzle forms fine bubbles and mixes into the molten steel, traps inclusions in the molten steel and floats to the surface. As a result, inclusions are removed from the molten steel, improving the quality of the manufactured slab. Therefore, in order to promote the effect of removing inclusions by the gas, it is necessary that the blown gas forms microscopic bubbles having a spherical shape or a shape similar to the spherical shape, and is evenly mixed into the molten steel. (Hereinafter, this will be referred to as a healthy bubble).

For this? The gas pressure injected into the steel container is set to a pressure that is higher than the total pressure of the molten steel at the relevant injection part (total pressure is the sum of the dynamic pressure due to the molten steel flow, the static pressure due to the molten steel head, and atmospheric pressure). It is extremely important to do so.

The inventors of the present invention repeatedly conducted various experiments and research in actual operation in order to obtain the above-mentioned healthy bubbles. As a result, when the amount of blown gas is expressed as the actual volume (hereinafter referred to as the actual volume) Vg under the relevant pressure and temperature, the amount of molten steel passing through, that is, the amount of molten steel passing through, as shown in equation 11, and There is a close correlation between the corresponding gas amounts, and we have invented a method to effectively solve the conventional problems by appropriately controlling the gas amount based on this correlation, and have already applied for a patent.

V g / V j! = R---------(1
) However, vg: Actual volume of blown gas (m3/5ec)
■2; 21 Amount of steel passed Volume of over-molten steel m'/5ec) The present invention aims to further improve the above invention, and even if the nozzle characteristics change during actual operation, the gas amount can be adjusted accurately in response to the change in nozzle characteristics. The present invention provides a method of correcting the amount of gas and controlling the amount of gas based on the correction.

First of all, the amount of passage w4! The correlation between it and the gas amount will be explained. The gas amount (hereinafter, in the present invention, the °C gas amount refers to the actual gas volume unless otherwise specified) is the total pressure of molten steel in the gas injection section, the depth of molten steel in the tundish, the inner diameter of the nozzle, Determine from the molten steel flow rate, etc., determine the gas flow rate necessary to generate bubbles in the molten steel on which this total pressure is acting, and then calculate the gas flow rate and the pore diameter of the gas injection part of the nozzle. It is determined from the number of pores as shown in the following equation (2).

Vg=UArx(π/4) d”xn −−−−−−
-(21 However, UAr; gas flow rate (m/5ec) d; gas injection pore diameter of the nozzle (m) n; the number of gas injection holes in the nozzle and the amount of steel passing through the nozzle are determined by the inner diameter of the nozzle and the molten steel flow rate, or From the width and thickness of the manufactured slab, and the slab drawing speed (casting speed), it can be determined by the following distance.

V6=tJ, txS=WxI)xV, --------
-+3) However, Ust; flow velocity of molten steel in the nozzle (m/5ee
) S : Nozzle opening (cross-sectional area) (d) W : Width of slab (m) D : Thickness of slab (m) y , S slab drawing speed (m/5ec) The nozzle is shown in Fig. 12, for example. As shown, there is a molten steel flow hole 2 in the center of the shaft.
1, and a gas blowing part 22 made of a porous refractory layer is provided facing the communication hole 21.
2 is connected to a gas supply system 6. As shown in FIG. 12(a), the gas injection section 22 surrounds the porous refractory layer with a dense refractory J! ! 23, or as shown in FIG. 12(b), the entire inner side surrounded by an iron skin 24 or the like is made of a porous refractory layer. The gas blowing part 22 is formed into a porous refractory layer by adjusting the packing density of the refractory so that it has a predetermined apparent porosity, but even if the nozzle has the same structure and the same specifications, It is common for the pore diameters and densities of manufactured porous refractory layers to vary slightly. Therefore, the present inventors set the characteristic conditions of the nozzle 20 to the gas blowing section 2.
2, that is, the surface area facing the communication holes 21 of the porous refractory layer is determined by variations in pore diameter and pore density per unit surface area, and the surface area, variation in pore diameter, and pore density are each within predetermined ranges. The correlation between the actual gas volume Vg for obtaining the above-mentioned sound bubbles and the passing molten steel IVJ was investigated using a reference nozzle located inside. (In the present invention, injecting gas into the molten steel flow using such a reference nozzle is referred to as reference nozzle characteristic conditions). Figure 2 shows an example of the survey results, where the vertical axis represents the actual gas volume V[,
The horizontal axis is i! Indicates the amount of molten steel Vl.

The solid line l is the lower limit necessary for the gas pressure to overcome the total pressure of molten steel and obtain healthy bubbles, and if the actual gas volume Vg is equal to or greater than the solid line β, healthy bubbles can be stably obtained. In addition, the solid line m indicates the gas required to prevent nozzle clogging.
The solid line l indicates the elapsed time from the start of use of the nozzle (cumulative casting time) and An! , 03, etc., is an amount that takes into account a correction factor that is empirically set based on the relationship with the amount of inclusion precipitation, etc. For example, in the structure shown in FIG. 12(a), the surface area is 4 ooci and the average pore diameter is 0.01.
Using a nozzle with ~0.1 mm and an average pore density of 450 pores/cal, the slab drawing speed was 1.6 m/cal on average.
Min slab width W is 580 to 1350, slab thickness D is 2
In a curved slab continuous casting equipment with a sliding nozzle and a rolling nozzle of 50 tons and a tundish capacity of 60 tons, the equation (4) below is obtained.

v, =v, (1+t/1 xlO') -
−−−−−−(4) However, ■,;Gas m (m''/5ec) necessary to prevent nozzle clogging■, ;Gas ffi (m'/5ec)t necessary for healthy bubble generation; Cumulative casting Time (sec) From this figure 2, the actual gas volume VE for the amount of molten steel passing through ■l
It was found that there is a strong correlation (hereinafter referred to as ratio R).

On the other hand, when gas is injected into the molten steel flow in the nozzle, as the amount of gas blown increases, the buoyancy increases, making it difficult for the molten steel to flow smoothly, and when this exceeds a certain limit, it stops the molten steel from flowing down. Then, the molten steel stops flowing downward, or it starts to flow down intermittently, and once this phenomenon occurs, the pressure in the molten steel flow drops rapidly, and the gas in the gas supply system flows into the molten steel flow all at once, causing the above-mentioned phenomenon. becomes more and more intense, and a boiling phenomenon occurs.

There is also a close relationship between the amount of gas during the generation of boiling and the passing melt 1ffi, as in the case of obtaining the above-mentioned sound bubbles.

The solid line X in FIG. 2 indicates the ratio R determined from the boiling limit, which is approximately the limit value at which the maximum gas injection amount can be ensured without causing the boiling phenomenon. A specific formula for the ratio R in this case can be expressed by the following formula (5), for example, based on the experimental results of the curved slab continuous casting equipment.

Vg=0.0267X vt+5.3xlO-'R=V
g/Vt = (5,3x 10-'/(W ・D ・
ν, )) +0.026F Therefore, if a gas amount that exceeds the ratio R even slightly to the amount of molten steel passing through is injected, a boiling phenomenon will begin to occur, and conversely, the nozzle clogging limit indicated by the solid line m and X If the gas amount is controlled with respect to the amount of molten steel passing through at that point in time so that it is in the range R between the boiling limits, boiling will not occur even if the operating conditions change, and the required gas amount will be maintained at an economical level. quality can be effectively obtained. In addition, as the casting time becomes longer, it is possible that an operating range will occur where the boil limit . In other words, the ratio R at whichever is smaller of boil limit X and nozzle clogging limit m is 1 bll? 11 is preferable.

As will be described later, the ratio R is the part where the total pressure in the nozzle is the lowest (hereinafter referred to as the minimum total pressure part) when the molten steel is injected, where the gas volume is maximum and the volume relationship with the molten steel is greatly influenced. In particular, it is effective to be able to set it with high precision and to perform the blowing control based on it.

The Vg, Vl, and R at this minimum total pressure location
■ Below. It is displayed as It vot +Ro.

FIG. 3 shows the boil generation situation in actual operation using the nozzle having the structure shown in FIG. , and the actual volume of gas■. 9. 2 is a chart showing an example of the results of a survey based on the relationship between In the figure, the vertical axis represents the actual gas volume ■og, and the horizontal axis represents the passing melt fg.
I amount V. t, the ○ mark indicates a normal one without boiling, and the X mark indicates boiling. The straight line a represents the lower limit at which boiling has been confirmed, and is V in this example. ,
It was expressed by the relational expression =0.029·VOI. That is, straight line a
In this case, the probability of boiling is extremely high, and this straight line a is almost the limit value that can ensure the maximum gas injection star without causing boiling. Therefore, the ratio R
It has been found that if the gas amount is controlled according to the operating conditions so that 0 (V 09/V ot) is within the range of 29/1000 or less, the maximum flow rate can be ensured without causing boiling.

By the way, since the nozzle is always in direct contact with high-temperature molten steel, it is subject to severe wear and tear.For example, in a continuous casting facility with a capacity of 160,000 tons (tundish capacity: 60 tons), the nozzle generally loses about 1 hour per month.
The lifespan of the cast is limited, and we are forced to replace each cast (
has been done. However, as described above, the characteristics of the nozzle usually vary even if the nozzle is manufactured under the same specified conditions. For example, as shown in Fig. 3 above, regardless of the nozzles having the same structure and the same specifications, the boil generation limit for a constant amount of molten steel passing through is 0.1 to 0.4 m'/s.
It can be seen that there is variation within the ec range. In order to solve such problems, it is necessary to determine the ratio R in advance for various nozzle characteristics, select the optimal ratio R based on the characteristics of the nozzle used in the operation, and control based on it. . However, in practice, the nozzle characteristics can be quickly and
In addition, it is not easy to quantitatively understand the ratio R, and it is troublesome to set the ratio R for various nozzle characteristics in advance, and there is also a concern that accuracy may be poor.

Therefore, the inventors of the present invention first determined the allowable boil limit from the ratio R1 based on the boil generation limit, that is, the correlation between the amount of molten steel passing through and the amount of gas, under the above-mentioned standard nozzle characteristic conditions, and set this as the set allowable limit.

Next, the specified nozzle is attached to the tundish and actual operation is carried out.During the operation, the amount of molten steel passing through is kept constant at an arbitrary amount, and only the amount of gas is sequentially changed to actually generate boiling. The gas amount V z at the time of starting was actually measured. On the other hand, the set boil tolerance limit (the one set from the boil generation limit after the set tolerance limit is called the set boil tolerance limit, and similarly the one set from the nozzle clogging limit is the set nozzle clogging tolerance limit, and the one set from the quality limit) Calculate the gas amount (hereinafter referred to as the set gas amount) ■r9 at which boiling is expected to start for the same amount as the amount of passing molten steel (the set value is called the set quality tolerance limit),
The measured gas it v -t and the set gas II V r
The ratio K (=V, 9/V, 9) with q was determined. This ratio K was then used to correct the set tolerance limit. That is, the ratio K was used as a nozzle characteristic correction coefficient. FIG. 4 is a diagram illustrating this correction procedure, and the solid line indicates the set boil tolerance limit.

■ Any amount of molten steel passing through. , calculate the nozzle characteristic correction coefficient K from the two measured gas quantities (■) and the set gas ht v r9, and multiply the set boil tolerance limit shown by the solid line by the nozzle characteristic correction coefficient K to obtain the correction permissible limit shown by the dashed-dotted line. is obtained.

Next, during the operation, the amount of molten steel passing through was changed, boiling was actually generated in the same manner as in the above-mentioned operation at the amount of molten steel passing through, and the gas amount v -9 at the time when boiling started was measured. As a result, it almost coincided with the correction allowable limit corrected by the nozzle characteristic correction coefficient K determined at one point of the arbitrary amount of molten steel passing through, and its reliability was proved to be high.

The reason why it is possible to correct the set boil tolerance limit as a whole using the nozzle characteristic correction coefficient K determined based on an arbitrary amount of passing molten steel is surmised as follows. That is, as shown in the schematic diagram of Fig. 5, boiling is thought to occur when microbubbles 31 blown into the molten steel flow successively coalesce and grow and reach a critical distribution state. The diameter distribution of the initial bubbles immediately after the boiling is an important factor in determining the boiling limit. The initial bubble size distribution greatly depends on the pore size distribution and density of the gas injection part of the nozzle. Therefore, by understanding the characteristics of the nozzle at any amount of molten steel passing through during actual operation and comparing it with the standard nozzle characteristic conditions determined in advance, it is possible to understand the nozzle as a relative value. During operation, it is considered that the correction allowable limit corrected by the nozzle characteristic correction coefficient K as the relative value can be used.

In addition, methods for determining the point at which boiling starts during the above-mentioned operation include, for example, a method in which an operator monitors the scene inside the mold and judges when the scene starts to become violently disturbed, or a method using an image sensor/camera, etc. A method may be adopted as appropriate, such as monitoring the scene using the camera, processing the image, and making a judgment based on the fact that the number of red dots that change when the scene is disturbed exceeds a predetermined amount.

Once the nozzle characteristic correction coefficient K is determined, among the set allowable limits, not only the boil limit but also the allowable limit set from the nozzle clogging limit described above and the allowable limit set from the quality limit described later are also subjected to the nozzle characteristic correction. It is possible to correct it by the coefficient K. In other words, the nozzle characteristic correction coefficient indicates relative characteristics such as the distribution of pore diameters of the gas blowing section 22, its density, and the blowing surface area.

The initial bubble size distribution also has a great influence on the quality limit, which aims to improve quality by trapping and floating inclusions with gas bubbles blown into the molten steel flow.

Therefore, by correcting the nozzle clogging limit and quality limit using the nozzle characteristic correction coefficient K, it becomes possible to obtain the optimum limit casting according to the characteristics of the nozzle used.

Next, an example of how to determine the quality limit will be explained.

As mentioned above, there is a problem in setting the ratio R only from the boil limit and the nozzle clogging limit for steel types with strict quality where internal defects such as slag chewing are a problem.

Figure 6 shows the number of black spots detected by the sulfur printing method and the number of A Il z Oz clusters among the inclusions obtained by taking cross-sectional samples of slabs produced in actual operation under the standard nozzle characteristic conditions. It is a chart showing an example of the results of investigating the residual amount and the relationship with the slow-biting star. Black spots are gas bubbles blown into molten steel.

This is incorporated into the slab and appears as black spots due to the sulfur print.As the amount of gas increases, the number of black spots increases, and conversely, as the amount of gas decreases, the number of black spots decreases. The example in FIG.
am) The amount per. As can be seen from Fig. 6, there is a close relationship between the number of black spots per unit cross-sectional area, the type of inclusions, and the amount of inclusions remaining in the slab.・As the number of 7 points decreases, the score of the A12os cluster increases.

In addition, for the above-mentioned steel types with strict quality standards, the allowable amount of inclusions is determined based on past experience so that, for example, the A1zO'r cluster score is below the value and the number of slag bites is below the value, depending on the steel type. . Therefore, the allowable range of the number of black spots in order to reduce the number of inclusions to a predetermined total allowable value can also be determined for each steel type. Based on this knowledge, the present inventors conducted research on the relationship between the number of black spots and the number of black spots. As a result, it was found that a relational expression as shown in the following equation (6) holds true between the two.

BSN=β(α・D, 3・V, -V C) -----
-(61 However, 133N, number of black spots (pcs/
1250 cal) D,; Gas bubble diameter (tn) at the gas injection section v, H Actual gas volume per unit amount of molten steel passing through v
, : Slab drawing speed (m/5ee) β; Proportionality constant α; Constant determined from porous hole diameter, molten steel viscosity, etc. Figure 7 shows a tin plate with a slab drawing speed Vc of 1.6 m/
Gas (
(In this example, argon gas is used) This graph shows the correlation between the number of black spots that actually appeared on slabs when manufacturing stars with various changes and the number calculated by calculation based on equation (6) above. , α is 3
．． It was set to 0.

As is clear from Fig. 7, by determining β and α for each steel type, the number of blank spots that occur in actual operation can be accurately calculated from the gas amount and slab drawing speed based on equation (6) above. It is possible to estimate

Therefore, when the gas bubble diameter D9 is determined from the slab withdrawal speed Vc and the blown gas pressure during operation under the standard nozzle characteristics and the specified conditions, the actual gas volume V to obtain the specified number of black spots is determined. The ratio R to the amount of molten steel passing through can be set.

This ratio R is also corrected by the nozzle characteristic correction coefficient K,
By controlling the gas amount based on the corrected allowable limit, even if the characteristics of the reference nozzle change, the optimal gas amount can be easily and reliably found, making it possible to carry out efficient operations. .

Next, a method for specifically controlling the gas amount will be explained.

In actual operation, the flow regulating device system (for example, the flow regulating valve 60 shown in FIG. 1) of the gas supply system 6 has a volume (°ζ Since it is necessary to control based on the standard volume (hereinafter referred to as standard volume), the pressure and temperature are corrected for the n;1 actual volume as described below (712).

V,,, = (p/pa) X (273/T)
XV, ------- (71 However, v7.; Standard volume of gas (Nm) Poi Atmospheric pressure (10336 kg
/mSP ;Gas pressure (kg/m”)T
; Gas temperature (K) y, Actual volume of H gas (m3) Changes in molten steel temperature are usually small, and the influence of such changes is extremely small compared to the influence of pressure changes. Therefore, in actual operation, molten steel temperature Ignoring the changes, it is sufficient to correct only the pressure changes.

The total pressure of the molten steel flow varies greatly from the gas injection part 41 of the submerged nozzle 4 to the lower end of the injection nozzle 43 located inside the mold 5, but the parts that have a large effect on the occurrence of the boiling phenomenon are as described above. The pressure is at its lowest point. One possible method for determining the pressure at the minimum total pressure area is to install a pressure detection device at the minimum total pressure area and directly detect it, but it is difficult to detect it at a location where extremely high temperature molten steel flows down while pulsating. There are many limitations, and it is difficult to expect highly accurate measurements. For example, it may be calculated using the well-known Bernoulli equation as shown in equation (8) below.

P= (1', +ρ・1l−(1/2)・(ρ/g)・
(U□/)2-Δp)8 However, H: Distance from the gas injection part to the molten metal surface in the tundish (molten steel head m) g: Acceleration of gravity (m/sec") To: Flow velocity resistance ρ: Specific gravity of molten steel (kg/m3) Δp; Pressure loss in the nozzle (kg/m'') N; Exponent The index N in formula (8) is 1 under ideal conditions, but in actual operation, the distance from the gas injection position to the meniscus and the There is often an error between the calculated total pressure and the measured total pressure due to the actual inner diameter of the nozzle, the flow rate of molten steel, etc. In such a case, the error can be eliminated by adjusting the index N. For example, if the distance from the gas injection position to the meniscus is 500gn,
When the nozzle inner diameter is 1101, by setting the index N to 2, the actual gas volume calculated by the above equation (8) and the above equation (7) based on this equation (8) is the value obtained from the calculation. , the value was very close to the value experimentally measured off-line, etc., and it was confirmed that there is no problem even if the above-mentioned (8) is used in practice.

As a result of the above, the optimum gas amount during operation can be determined if the flow velocity of the molten steel flow in the molten steel head I during continuous casting and the part of the nozzle with the smallest cross-sectional area, that is, the most constricted part, can be determined. The molten steel head (2) may be determined, for example, by detecting the weight of molten steel in the tundish 2 and calculating from a correlation between the molten steel weight in the tundish and the depth of molten steel in the tundish, which has been determined in advance, or by optical or electrical level detection. It can be determined by adding the molten steel depth inside the tundish, which was found by calculating the deviation from the reference level using a device, and the distance from the bottom surface of the tundish to the gas injection part. On the other hand, it is not easy to detect the flow velocity at the most constricted portion, and current measurement techniques have extremely low accuracy. Therefore, the amount of molten steel flowing down the nozzle can be detected directly using, for example, a non-contact flowmeter, or indirectly from the width, thickness, and drawing speed of the slab, and at the same time, the amount of molten steel flowing down the nozzle can be detected directly using a non-contact flow meter, or indirectly from the width, thickness, and drawing speed of the slab. It is effective to calculate the flow velocity from the actual cross-sectional area (the actual cross-sectional area is the nozzle opening multiplied by the contraction coefficient, etc.).

In FIG. 1, 7 is a speed detection device for the slab 8;
The drawing speed can be detected from the slab movement speed. 9 is a weight detection device for detecting the weight of the tanditsh 2 mentioned above;
61 is a gas flow meter.

Reference numeral 10 denotes an arithmetic and control device, which has an arithmetic and control function shown in the block diagram of FIG. 8, which will be described later.

As a result of the above, it is possible to quantitatively understand the limit of gas injection amount at which the boiling phenomenon occurs as a function of factors such as casting speed, molten steel depth, and slab size. Find the nozzle characteristic correction coefficient K at
By correcting the quantitatively determined set permissible limit using this correction coefficient, efficient gas injection control that prevents the occurrence of boiling phenomenon during the operation becomes possible. In actual operation, it is also possible to control the gas amount without boiling by using the approximate equation shown in equation (9) below, which was obtained through experiments by combining equations (6) to (8).

v, 9= (Ifo・', (0,0267 bow←D −V
C+5.3x 10-')) ・K However, the permissible limit of the amount of blown gas should be set according to the operating conditions as shown in the above K・Nozzle characteristic correction coefficient, and the nozzle characteristic correction coefficient K should be determined during actual operation. The set tolerance limit is corrected by the above-mentioned set tolerance limit, the slab size, casting speed, molten steel head, etc. during the operation are detected, and the gas amount is controlled according to the detected values so as to meet the corrected tolerance limit. It is possible to efficiently produce slabs of desired quality without causing any problems. Thus, the arithmetic and control device 10 in FIG.
For example, if the device has the functions as shown in the block diagram of FIG. 8, it will be possible to set the above-mentioned allowable limit, calculate the nozzle characteristic correction coefficient, calculate the corrected allowable limit, and control the gas amount based on the corrected allowable limit. It is also possible to automatically perform all steps 4b and the like.

That is, in FIG. 8, reference numeral 101 is an input section for inputting predetermined operating conditions, the molten steel head in operation, the slab drawing speed, etc. as necessary. 102 is an R setting section;
The ratio R is calculated according to a predetermined method according to the operating conditions, boil limit, nozzle clogging limit, quality optimum value, etc. inputted from the input section 101, and the allowable limit of the gas amount is set. (
102a is a set boil tolerance limit, 102b is a set nozzle clogging tolerance limit, and IQ2CfJ (indicates a set quality tolerance limit).

103 is an R setting pattern selection section, and the input section 1
In addition to the operating conditions from 01, an optimum tolerance limit pattern is selected from the above-mentioned tolerance limit setting range according to the location where the ratio R is set, the steel type, etc. under the operating conditions. Reference numeral 104 denotes a gas amount calculation section, which selects the R setting pattern selection section 1 based on information such as slab speed, molten steel head, slab size, nozzle diameter, etc., which are detected and input from time to time during operation.
Calculate the standard gas amount to achieve the allowable limit selected in step 03. On the other hand, 110 is a value calculation unit for calculating the nozzle characteristic correction coefficient 17K, and 110a is the actual gas amount measured from between the gases when the start of boiling is actually confirmed in any amount of molten steel passing during the operation. Find v -9, and 1
In step 10b, a set gas amount v r9 for the amount of molten steel passing through is determined based on the set boil tolerance limit 102 a, and a nozzle characteristic correction coefficient K is calculated from v29 and v19. This nozzle characteristic correction coefficient is input to the correction allowable limit calculating section 111, and the standard gas amount calculated by the calculating section 104 is corrected according to the nozzle characteristics to obtain the actually required standard gas amount. The standard gas amount determined by the correction allowable limit calculation section 111 is input to the control section 105, and the control section 105 issues a control command to the flow control valve 60 and the like, thereby automatically controlling the gas amount.

In this way, the allowable limit for gas amount is determined by the boil limit, nozzle clogging limit, and black spot (slab quality), and the actual gas amount control is to further correct it according to the nozzle characteristics, as well as to prevent operational troubles and quality. You can choose and control relationships that cause fewer problems.

〔Example〕

Example 1 In the curved slab continuous casting equipment of 5,000 tons/day,
The present invention was carried out when manufacturing A1 semi-killed steel for a section with a fin thickness of 250 mm and a width of 1200 mm.

The operating conditions in this example are as shown in Table 1,
Since nozzle clogging tends to occur frequently with this steel type, the boiling limit was used as an indicator for operation.

Table 1 Under the operating conditions, with the nozzle having the structure shown in FIG.
, 0. Old ~ 0. Calculate the average pore diameter of the range of l Hozen,
The above (5
) The ratio R was set using the following formula 01 based on the formula.

R=Vg/Vj! = (5,3Xl0-S/(W・D,
Vc)l ++1. (130---------Ol Next, a nozzle manufactured with the same specifications as the standard nozzle was installed, and when performing actual operation, the slab 1111200
mm, casting speed 1.6n+/1IIin, molten steel F
1.2m (the amount of molten steel passing through at this time is 0.48m”/m1n)
) while changing the gas amount and actually measuring the gas amount at the start of boiling, the actual measured gas htv,v was 1
It was 3.0 Nl/min. Further, the weight of the passing melt at the ratio R under the standard nozzle characteristic condition is 0.41 (+n'/1
The gas gap when Iin, that is, the set gas 'ftv,
'9 became 17.4 N 1 /min due to performance p. Therefore, the nozzle characteristic correction coefficient is 0.75 (13,
0/17.4・0.75). Therefore, during the relevant operation, the nozzle characteristic 1111 positive coefficient for each C = 0.7
5 was used to control the gas amount by sequentially correcting the gas amount based on the operating conditions calculated by the ratio R. FIG. 9 shows a control chart under the above operating conditions, where the solid line S represents the depth of the tandish self-melting steel, the solid line b2 represents the amount of molten steel passing through, and the solid line b3 represents the set gas ff1vr.
Regarding the state of change in v, actual 'i; a b 4 represents the correction allowable limit obtained by multiplying the set gas amount v - * by the K. The gas amount was controlled to approach the solid line b4 in response to changes in the amount of molten steel, gas pressure, and amount of molten steel passing through.

As a result, there were no breakouts, and nozzle clogging and quality deterioration were significantly reduced.

Example 2゜Example 1. The present invention was carried out when producing tinplate material with a thickness of 25011I and a width of 980+U in the same continuous casting equipment as in the above. The number of black spots in this tin material is 110.
/1250 cal is an acceptable range in terms of quality.

Therefore, the ratio R determined from the quality limit under the standard nozzle characteristic conditions was set as shown in the following equation 00 based on various experiments.

V, =0.025X vt +5.3X
10-'------------(11) On the other hand, in this operation, the above-mentioned Example 1. As a result of determining the nozzle characteristic correction coefficient K in the same manner as above, K was found to be 1.2. Therefore, the 00
The gas amount set by the formula was multiplied by =1.2 to obtain a corrected allowable limit value, and the gas amount was controlled based on this corrected allowable limit value.

Figure 10 shows an example of the results of investigating the variation in the number of black spots in slabs produced as a result of the operation by controlling the gas amount based on the correction allowable limit. <In the example, 81-1 is acceptable in terms of quality.
It was converged to a range of 40 pieces/1250 CrA, with very little variation. In contrast, with conventional control methods based on operator judgment, there is wide variation.
Many slabs were produced that were outside the above tolerance range, resulting in poor quality 1
There were many occurrences.

〔Effect of the invention〕

As described in detail above, by carrying out the present invention, it has become possible to accurately control the amount of inert gas blown during injection of molten steel in response to momentary changes in operation.

As a result, we were able to improve the quality of slabs and significantly reduce nozzle clogging.

[Brief explanation of drawings]

Fig. 1 is a structural diagram showing an example of implementing the present invention in general continuous casting equipment, Fig. 2 shows boiling limit, sound bubble generation limit,
A chart showing an example of the correlation between the amount of passed molten steel and the actual volume of gas when determining the limit of generation of slag. Figure 3 shows the boil generation situation under standard nozzle characteristic conditions, and the actual volume of blown gas V9 Figure 4 is a chart showing an example of the results of investigation based on the relationship between Figure 6 is a chart showing the correlation between the number of black spots in the manufactured slab and the amount of AlzO3 clusters and slag bite, and Figure 7 shows the correlation between the number of black spots actually occurring in the slab and the number calculated by calculation. FIG. 8 is a functional block diagram of the arithmetic and control device, FIG. 9 is a chart diagram showing an embodiment of the control chart based on the present invention, and FIG. A chart comparing the number of black spots in control methods, Figure 11 is a structural diagram of the casting part of a typical continuous casting equipment, and Figure 12 is a cross-sectional view of a typical gas injection nozzle. . L-ladle, 2; tundish, 3; molten copper 1.4; immersion nozzle, 41; upper nozzle, 42; slurry/feding nozzle, 43; injection nozzle, 5; mold, 6; gas supply system, 60 ;
Flow M control valve, 7; Speed detection device, 8; Slab, 9; Weight detection device, lO; Arithmetic control + device, toi; Input section,
102 if? Setting pattern selection section, 103; R setting section,
4; gas amount calculation unit, 105; control unit, 110. value calculation section, 111 i correction allowable limit calculation section, 20; nozzle, 21; melt 14 flow hole, 22; scrap injection section, 23; dense refractory layer, 24; iron shell, 30; molten steel flow, 31i Imitation small bubbles. 10; Arithmetic control unit 43; Injection nozzle 60; Flow rate control valve 61: Flow meter Molten steel flow tVj (7/width 1n) - Figure 2 VoI (m"/sea) 13 Figure A; Set boil tolerance limit - Vog - f (Vot)
B: Correction allowable limit...Vog'- to -f
(VoL) Figure 4 Number of plush spots (purchased 4250 cm2) No. 6
Number of black spots determined from diagram calculation (1 animal 4250c)
m2) Figure 7 b3! Set gas amount (Vrg) b4i correction allowable limit Fig. 9 Fig. 10 Fig. 11 Fig. 1: Ladle 2: Tandinoshie - 8: Slab 3: Molten steel 41: Upper nozzle 42 1981
°Nozzle 42 varnish sliding nozzle 5:
Mold 43z Injection nozzle 6: Gas supply system 60: Flow rate control valve procedure amendment (method) % formula % 1. Indication of the case Patent application No. 43949 of 1985 2. Name of the invention Molten steel injection method 3. Person making the amendment Relationship to the incident Patent applicant address 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (name)
(665) Nippon Steel Corporation Representative Takeshi 1) Yutaka 4, Agent Address: 4-24-3-6 Shinbashi, Minato-ku, Tokyo 105 "Column for brief explanation of drawings" of the specification subject to amendment 7. In the 3rd line of page 37 of the statement of contents of the amendment, the phrase “12th is” [1st
Figure 2 is corrected.

Claims (1)

[Claims] In a method of injecting molten steel into a continuous casting mold while injecting inert gas through an immersion nozzle, the allowable limit of the amount of blown gas relative to the amount of molten steel passing through the nozzle is determined in advance under standard nozzle characteristic conditions to determine whether boiling will occur. In addition to the limit or the boil generation limit described above, the nozzle clogging generation limit or quality limit is determined and set, and then boil is generated at any amount of passing molten steel during the operation, and the gas amount at the start point is actually measured, and the actual measurement is performed. The nozzle characteristic correction coefficient K is determined from the ratio of the gas amount and the gas amount at which boiling starts to occur for the same amount of molten steel passing through, which is determined from the boiling limit among the set allowable limits.
A molten steel injection method characterized in that the amount of gas blown is controlled by determining the permissible limit of the blown gas amount using the correction coefficient K.