JP2000210759A - Casting method using twin-drum type continuous casting machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably produce a cast metal by controlling drum reaction force based on molten steel temp. and molten steel height varied during casting, thereby preventing the occurrence of thickness variation and breakout of the cast metal. SOLUTION: In this method for producing the cast metal by a twin-drum type continuous casting machine having a pair of counter drums 1, 1' and side weirs pushed to both end surfaces thereof, the target drum reaction force value is corrected from the difference between a reference molten steel temp. and the actual molten steel temp. measured by a thermometer 5. Further, a molten steel static pressure value is calculated from the actual value of the molten steel surface height measured by a molten steel surface level meter 8 to correct the target drum reaction force, then the practical rolling reduction force applied on a solidified shell at a drum kissing point is calculated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method of casting a thin slab having a predetermined plate thickness by a twin-drum continuous casting machine, based on a molten steel temperature and a molten metal level which fluctuate during casting.
The present invention relates to a method of constantly casting a thin slab having a constant thickness while dynamically changing a target drum reaction force so as not to generate an over-solidified state and an unsolidified state in a molten steel pool.

[0002]

2. Description of the Related Art A method of manufacturing a thin slab by a twin-drum continuous caster, a characteristic of casting in a so-called twin-drum strip caster, is that a solidified shell formed on a drum peripheral surface is
An object of the present invention is to control the solidification state of the shell based on the "drum reaction force" generated when the shell is pressed at the drum kiss point (the minimum gap position between the drums). However, this drum reaction force fluctuates due to various factors during casting, and the thickness of the cast thin slab fluctuates in the longitudinal direction of casting, and a target predetermined plate thickness cannot be obtained, or the gap between drums fluctuates. There is a problem that breakout occurs.

[0003] To solve this problem, for example,
In Japanese Patent Application Laid-Open No. 58-176063, a starter plug is inserted between drums, and the drum is rotated before the level of molten metal stored on the starter plug reaches a predetermined height during continuous casting. A method of pulling out a piece has been proposed.
Japanese Patent Application Laid-Open No. SHO 61-140361 proposes a method of detecting the reaction force of a roll (drum) and moving the immersion nozzle up and down based on the detected value to appropriately control the level of molten metal between the rolls. ing. However, even in these proposals, the problem of the occurrence of breakout and the generation of hot bands due to the fluctuation of the gap between the drums has not yet been solved, and it has occurred accompanying the crimping of the shell at the drum kiss point described later. There was no proposal for a casting method aimed at controlling the heat balance in the molten steel pool.

[0004]

The solidification state of the casting shell in the twin-drum strip caster is determined not only by the solidification state determined by the heat transfer and thermodynamic equilibrium conditions between the drum and the molten steel interface, but also by the deformation behavior of the solidified shell. The “drum reaction force” governs the solidification state of the twin-drum strip caster. It can be said that the difference from the solidification form of molten steel occurring in the mold of a normal continuous casting machine is exactly that the solidification state is controlled by the drum reaction force. In other words, in a normal continuous casting machine, all molten steel solidified in the mold is discharged as a slab, whereas in the casting with the twin-drum strip caster, the amount of metal discharged from the drum as a slab is Not all of the solidified material in the molten steel pool, only the part where the compressive strength has reached a certain level or more is regarded as solidified, and it is discharged from the space between the drums as a slab through a semi-solid rolling phenomenon at the drum kiss point. Because This means that, in casting on a twin-drum strip caster, the molten steel in a semi-solid state with a low solid fraction that is not considered to be solidified in the semi-solid rolling process is always left in the molten steel pool at a constant speed. Means the so-called squeeze phenomenon, and at the same time means that the remaining and accumulated molten steel affects the thermal equilibrium state in the pool. Furthermore, if the remaining semi-solid layer is not re-melted in the molten steel pool, it will remain and accumulate in the pool with the passage of time, and if casting is performed for a long time, the temperature of the entire pool will decrease, and eventually the entire molten steel will decrease. Means to solidify.

[0005] In the casting of a twin-drum strip caster, the present inventors have found that a semi-solid state molten steel having a low solid phase ratio, which is not considered to have been solidified in a semi-solid rolling process, is always kept at a constant speed in a molten steel pool. That the remaining and accumulating molten steel is affecting the thermal equilibrium state in the pool, and that if the remaining semi-solidified layer is not re-melted in the molten steel pool, If casting and remaining for a long time in the pool as the process progresses, the temperature of the entire pool will decrease and eventually the entire molten steel will solidify. The relationship was investigated. As a result, the thicker the cast plate thickness, the less the molten steel in the semi-solid state remaining in the pool, and therefore, the temperature of the molten steel to be supplied into the pool to re-melt it and continue casting is relatively low. I found the fact that I could. In other words, when the temperature of the molten steel supplied into the pool is constant (of course, the liquidus temperature _TL or more), the temperature drop in the molten steel pool due to squeezing near the kiss point is small. For example, it is difficult to ensure solidification uniformity in the entire width direction of the drum, especially poor solidification at the slab edge (semi-solidified molten steel seepage).
It can be said that it can be said that there is a tendency to easily occur. Therefore, in order to realize a stable solidification state, the temperature of the molten steel supplied to the drum is strictly set to be higher when the casting plate thickness is small, and to be lower by several degrees Celsius when the casting plate thickness is large.
It was also found that it was necessary to control within the range.

[0006] Further, the present inventors have continued the investigation,
Increasing the drum reaction force squeezes the throughput (the mass velocity of the slab passing through the drum, derived from the drum velocity x casting thickness x casting width, or, in other words, mass flow (mass velocity)). It has been found that the rate of occurrence of molten steel in a semi-solid state becomes large. Therefore, when the reaction force of the drum is increased, it is necessary to increase the temperature of the molten steel supplied from the immersion nozzle in order to balance the temperature in the pool. This means, in other words,
When the temperature of the molten steel supplied into the pool is constant, increasing the reaction force of the drum increases the occurrence of semi-solidified molten steel that is squeezed, which means that the temperature of the molten steel in the pool is reduced. In addition, considering the load of refining, if it is attempted to make the molten steel temperature as low as possible, it can be said that the drum reaction force should be as low as possible. However, low reaction force casting (for example, 2000
(kg / m or less) to reduce the hysteresis of the drum pressure reduction system, and to reduce the temporal change in the amount of thermal expansion (thermal crown) of the drum to maintain the widthwise uniformity of shell pressure bonding at the drum kiss point. It is necessary to take measures such as adding a function of compensating for the static pressure of the molten steel to the drum reaction force detection circuit.

[0007]

SUMMARY OF THE INVENTION According to the present invention, as a result of the above experimental results and theoretical analysis supporting the results, it has been found that, in order to realize casting stabilization in a twin-drum strip caster, the drum reaction force value during casting is reduced. Casting control method that compensates for the variation in molten steel temperature by correcting the static pressure component of molten steel to obtain the pressure reaction force of the solidified shell at the drum kiss point, and changing the target reaction force according to the temperature level of the supplied molten steel. The gist is as follows.

1) In a method of producing thin cast slabs using a twin-drum continuous caster having a pair of opposed drums and side dams pressed against both end surfaces thereof, a temperature of molten steel in a tundish or a pool between a pair of opposed drums. A casting method using a twin-drum continuous casting machine, wherein a drum reaction force is changed according to a change in molten steel temperature in the inside. 2) In a method for producing thin cast slabs using a twin-drum continuous caster having a pair of opposed drums and side weirs pressed against both end surfaces thereof, the temperature of molten steel in a tundish or the molten steel in a pool between a pair of opposed drums. A casting method using a twin-drum continuous casting machine, comprising measuring a temperature and controlling a substantial rolling force applied to a solidified shell at a drum kiss point in accordance with a change in temperature of the molten steel.

3) When producing thin cast slabs using a twin-drum continuous caster having a pair of opposed drums and side weirs pressed against both end surfaces thereof, the temperature of molten steel in a tundish or the inside of a pool between a pair of opposed drums. In the method of measuring the molten steel temperature of, and controlling the substantial rolling force applied to the solidified shell at the drum kiss point according to the temperature change of the molten steel,
The measured value of the molten steel temperature is compared with a target value, and if the measured temperature of the molten steel is higher than the target value, the rolling force is changed to be larger.If the measured temperature of the molten steel is lower than the target value, the reduction is performed. A casting method using a twin-drum type continuous casting machine, wherein the molten steel temperature in the pool between the pair of drums is controlled to be within a predetermined range by changing the force so as to be small.

[0010] 4) At least one of the drums receives a drum reaction force measured by a load cell in contact with an axle box at both ends of the drum. The casting method using a twin-drum continuous caster according to the above 2) or 3), wherein a substantial rolling force applied to the solidified shell is obtained by subtracting a static pressure of molten steel.

5) Any one of the above 2) to 4), wherein the substantial reduction force applied to the solidified shell is changed and the casting speed is changed to control the thickness of the cast plate to be constant.
The casting method using the twin-drum continuous casting machine described in the paragraph. 6) The twin-roll type as described in any one of 2) to 5) above, wherein a pressure control cylinder and a servo valve for position control are used as a device for changing a substantial rolling force applied to the solidified shell. A casting method using a continuous casting machine.

[0012]

Next, embodiments of the present invention will be described in detail. From the experience of actual operation by the present inventor, in order to ensure casting stability in a twin-drum strip caster, the molten steel temperature must be strictly controlled to be surely within an allowable range, or the allowable range itself should be expanded. There is a need. Next, a method of expanding the allowable range of the molten steel temperature by controlling the drum reaction force will be described.

The case where the thickness of the thin cast slab to be cast is 4.5 mm will be described as an example. Fig. 1 shows the relationship between the excess temperature of molten steel and the drum reaction force and drum rotation speed. The horizontal axis indicates the excess temperature of molten steel (necessary for re-melting with the molten steel that supplies the semi-solidified molten steel remaining in the molten steel pool. The vertical axis represents the drum reaction force and the drum rotation speed. When the target drum reaction force is 2500 kg / m based on the operation results, the optimum temperature condition in the pool where the slab edge does not exude and the generation of hot bands is small is at the temperature of the molten steel in the tundish at 1520 ° C. (A). Suppose there is. However, in reality, the temperature process capability at the molten steel preparation stage varies,
Since the temperature of the molten steel changes as the casting progresses or when the ladle is replaced during continuous casting, the temperature in the pool must be maintained at a constant temperature in accordance with the change in the molten steel temperature. The temperature of the molten steel in the pool is always affected by the cooling direction due to the squeeze of the semi-solidified molten steel due to the rolling phenomenon at the drum kiss point, and the magnitude of the cooling amount, that is, the squeeze volume velocity is the magnitude of the drum reaction force. Because the target drum reaction force is changed according to the change in the molten steel temperature in the tundish or the molten steel temperature in the pool between the pair of drums, the operation to maintain the molten steel temperature in the pool is performed. .

Incidentally, the stable casting state is point A shown in FIG.
That is, if it is assumed that the operation is performed at 1250 ° C. in the molten steel temperature in the tundish and 2500 kg / m in the target drum reaction force, when the molten steel temperature in the tundish rises by + 5 ° C. in this state, the molten steel temperature in the pool also increases. There is a concern that molten steel may ooze due to poor solidification at the edge. Then, when it actually comes out, the target drum reaction force is raised to the point B (3250 kg / m) shown in FIG. The drum speed at this time is 31.5 mpm to 3
It drops to 0.5 mpm. By performing such an operation, + 5 ° C. of the molten steel temperature is offset by an increase in the squeeze amount of the semi-solidified molten steel due to an increase in the drum reaction force, and the molten steel temperature in the pool is kept constant. In addition, when the molten steel temperature is lowered by -5 ° C and there is a risk of hot bands occurring due to the adhesion of metal to the side dam, the target drum reaction force is lowered to point C (1500 kg / m) shown in Fig. 1. Let it. At this time, the casting speed increases from 31.5 mpm to 32.4 mpm. By performing such an operation, the −5 ° C. portion of the molten steel temperature is offset by the decrease in the squeeze amount of the semi-solidified molten steel due to the decrease in the drum reaction force, and the molten steel temperature in the pool is kept constant. .

As described above, by changing the reaction force of the drum, the variation in the temperature of the molten steel supplied into the pool can be offset. The range is when the thickness of the thin slab to be cast is 4.5 mm and the drum reaction force is ± 1000 kg /
It is considered that the variation in the temperature of the molten steel can be compensated from about + 7 ° C. to about −5 ° C. by changing the range of m. However, the operation for lowering the drum reaction force needs to be performed after taking measures to prevent the occurrence of breakout and the like, as described above.

Next, a method of controlling a drum reaction force in a casting method using a twin-drum continuous casting machine according to the present invention will be described with reference to FIG. FIG. 2 is a schematic diagram showing a schematic configuration of the drum reaction force control. In the twin-drum continuous casting machine shown in FIG. 2, a pair of opposed drums 1 and 1 'and a pair of drums 1 and 1' are cast in a molten steel pool 2 in an upper space around a drum kiss point K. A ladle (not shown) for supplying molten steel and a tundish 4 (this may be a mold) are provided. A molten steel thermometer 5 for measuring the molten steel temperature is immersed in the tundish or the mold, continuously measures the molten steel temperature, and the measured molten steel temperature actual value is transmitted to the drum reaction force change amount calculating device 6. Then, a comparison with a preset drum reaction force target value accumulated in the arithmetic unit 7 is performed to calculate a necessary drum reaction force change amount. A level gauge 8 is installed in the molten steel pool 2 to measure the level of the molten steel stored in the molten steel pool 2. The surface level and the molten steel static pressure value based on the surface level are calculated and transmitted to the arithmetic unit 7 and stored. Further, the load cell 10 is in contact with the axle box (chock) (here, the drum 1) at both ends of at least one of the drums.
To measure the drum reaction force and transmit it to the arithmetic unit 7 in the same manner.
Stored. Based on these information, the drum reaction force correction value is calculated by the arithmetic unit 7, and this value is sent to the rolling position control unit 15. The rolling position control device 15 changes the drum position by driving the rolling cylinder 13 connected to the servo valve 11 in order to achieve the drum reaction force correction value as a target. At this time, changing the drum position causes a "shift" from the rolling target position. The rotation of the drive motor M required to correct the "shift" is adjusted and corrected in real time by the drum speed control device 14. By doing so, the drum reaction force and the casting plate thickness are controlled to be constant. That is, for example, when the molten steel temperature in the tundish is 12
If the temperature of the molten steel rises by + 5 ° C at 50 ° C, the target drum reaction force is raised to the optimum value, the casting speed (drum rotation speed) is reduced, and the + 5 ° C portion of the molten steel temperature is the drum reaction force. Operation is offset by the increase in the amount of squeeze of semi-solidified molten steel accompanying the rise in temperature, while if the molten steel temperature is lowered by -5 ° C, the target drum reaction force is lowered and the casting speed is reduced. The temperature is raised to −5 ° C. of the molten steel temperature, offset by the decrease in the squeeze amount of the semi-solidified molten steel due to the decrease in the drum reaction force, and control is performed to keep the molten steel temperature in the pool constant.

[0017]

As described above, in the method of casting a thin slab having a predetermined thickness by the twin-drum continuous caster according to the present invention, the temperature of the molten steel and the height of the molten metal that fluctuate during casting are reduced. Casting while dynamically changing the target value of the drum reaction force prevents breakouts and fluctuations in the thickness of the cast plate, and enables stable production of thin cast pieces.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between molten steel excess temperature, drum reaction force, and drum rotation speed.

FIG. 2 is a schematic diagram showing a schematic configuration of drum reaction force control in a twin-roll thin cast slab continuous casting machine according to the present invention.

[Explanation of symbols]

 1, 1 '... drum 2 ... molten steel pool 4 ... tundish (mold) 5 ... molten steel thermometer 6 ... reaction force change amount calculation device 7 ... arithmetic unit 8 ... level gauge 9 ... static pressure converter 10 ... load cell 11 ... Servo valve 12 ... Drum position detector 13 ... Cylinder for control of rolling position 14 ... Drum speed controller 15 ... Control device for rolling position K ... Drum kiss point

Claims (6)

[Claims] 1. A method for producing thin cast slabs by a twin-drum continuous caster having a pair of opposed drums and side dams pressed against both end surfaces thereof, wherein a temperature of molten steel in a tundish or a pool between a pair of opposed drums is provided. A casting method using a twin-drum continuous casting machine, wherein a drum reaction force is changed according to a change in molten steel temperature in the inside. 2. A method for producing thin cast slabs using a twin-drum continuous caster having a pair of opposed drums and side dams pressed against both end surfaces thereof, wherein a temperature of molten steel in a tundish or a pool between opposed pair of drums is provided. A casting method using a twin-drum continuous caster, wherein the temperature of the molten steel in the inside is measured, and the substantial rolling force applied to the solidified shell at the drum kiss point is controlled according to the temperature change of the molten steel. 3. A method for manufacturing thin cast slabs using a twin-drum continuous caster having a pair of opposed drums and side dams pressed against both end surfaces thereof, the temperature of molten steel in a tundish or the inside of a pool between opposed pair of drums. In the method of measuring the molten steel temperature of, and controlling the substantial rolling force applied to the solidified shell at the drum kiss point according to the temperature change of the molten steel,
The measured value of the molten steel temperature is compared with a target value, and if the measured temperature of the molten steel is higher than the target value, the rolling force is changed to be larger.If the measured temperature of the molten steel is lower than the target value, the reduction is performed. A casting method using a twin-drum type continuous casting machine, wherein the molten steel temperature in the pool between the pair of drums is controlled to be within a predetermined range by changing the force so as to be small. 4. A molten steel surface height of a pool between a pair of opposed drums is obtained from a drum reaction force measured by a load cell in contact with an axle box at both ends of the drums, which is received by at least one of the drums. 4. The casting method using a twin-drum continuous caster according to claim 2, wherein a substantial rolling force applied to the solidified shell is obtained by subtracting a static pressure of molten steel. 5. The method according to claim 2, wherein a substantial rolling force applied to the solidified shell is changed, and a casting speed is changed to control the thickness of the cast sheet to be constant. A casting method using a twin-drum continuous casting machine. 6. A dual-purpose device according to claim 2, wherein a pressure control cylinder and a servo valve for position control are used as a device for changing a substantial rolling force applied to the solidified shell. Casting method using a drum type continuous casting machine.