JPH0910899A - Method for controlling completion of casting in continuous casting - Google Patents

Abstract

PURPOSE: To safely and surely prevent steam explosion caused by the leakage of molten steel and the spray water stream due to supercooling and to surely eliminate internal crack of a cast slab due to the shortage of cooling in the completion of a fast casting capable of improving productivity and quality, etc. CONSTITUTION: At the time of drawing out the cast slab after completing the casting while holding the ordinary casting speed without executing a deceleration of the casting speed and stopping and a treating work of the bottom part at the uppermost part of the cast slab when a continuous casting is completed, in the interval between S0 and S1 , at least secondary cooling water quantity is reduced to the lower limit or above where the interval crack due to the shortage of cooling is not caused and at the same time, the red-heated refractory 21 is temporarily submerged into the bottommost part. By this method, a small quantity of the leaked steel is held by the solidified shell (b) at the bottommost part and the spray water stream is interrupted by the solidified shell (b) so that a distance between the solidified shell (b) at the bottommost part and the molten metal surface (a) at the bottommost part just below a mold become a prescribed distance T.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a casting completion control method in a continuous casting machine in which molten metal in a tundish is continuously cast into a slab.

[0002]

2. Description of the Related Art In continuous casting, molten steel in a ladle is continuously cast into a mold through a tundish and cooled in a secondary cooling zone of a mold and a group of guide rolls following it to form a solidified shell. Although the solidified slab is pulled out by a pinch roll, the molten steel is no longer supplied at the end of casting, so it is necessary to finish casting with special control.

As a conventional casting end method of this type, an automatic stop method in a continuous casting machine (Japanese Patent Laid-Open No. 62-24848)
Gazette), casting end control method (JP-A-62-124056 [JP-B 3-54025]), casting operation end method of strip steel casting apparatus (JP-A-62-203652), etc. Are known. Hereinafter, these methods are referred to as conventional method 1.

In this conventional method 1, first, a predetermined deceleration pattern is used, the pouring speed is decelerated according to the weight or the level of the remaining steel in the tundish before the end of the pouring, and the tundish is stored. Casting is stopped by leaving a predetermined small amount of molten steel (see FIG. 4 (b)). next,
After the pouring is stopped, as shown in FIG. 4 (a), a coolant (metal particles, metal pieces, water, etc.) is charged to the last end portion of the molten steel remaining in the mold, that is, the so-called bottom portion, and the coolant is The so-called bottom processing work is performed to solidify the bottom portion in the mold, and then the drawing speed is appropriately increased to carry out the drawing (see FIG. 4 (b)).

However, in the conventional method 1 for ending the deceleration,
Since the casting speed is reduced before the end of casting, the productivity of the continuous casting machine is reduced, the tundish residual steel casting time is lengthened, the temperature drop of the residual molten steel becomes large, and the quality of the final slab deteriorates. Yes, the deceleration moves the crater end (tip of the unsolidified portion) position of the slab, and the center segregation deteriorates
Since bottom processing work is required, the work load is large, the quality of the final slab deteriorates due to the mixing of coolant, and the cost of purchasing the coolant is high. As a result, the slab bulging increases, and after restarting the drawing, the unsolidified molten steel may leak from the bottommost portion due to squeezing. Further, the temperature of the slab in the continuous casting machine is lowered at the end of casting, which is disadvantageous at the time of performing hot charging for hot rolling or direct roll.

As a countermeasure against this, the present applicant has previously proposed that a method for controlling high-speed pouring end in continuous casting (Japanese Patent Laid-Open No. 6-262323).
Japanese patent application [Japanese Patent Application No. 3-221702]). Less than,
This method is referred to as Conventional Method 2.

In this conventional method 2, the work of decelerating / stopping the pouring speed before the end of pouring and the bottom processing work are abolished, and the pouring is ended while maintaining the normal pouring speed, and thereafter,
The slab is pulled out by increasing the drawing speed (Fig. 4 (C)).
reference). Further, in this conventional method 2, solidification of the bottom portion is performed by the secondary cooling water immediately below the mold, and the optimal control of the secondary cooling water immediately below this mold and the optimization of the drawing speed increasing pattern after completion of the casting are performed. This prevents steel leakage from the bottom.

However, although the conventional method 2 of high-speed termination has largely solved the problems in productivity and quality of the conventional method 1 described above, in order to prevent steel leakage at the bottom, Since it is necessary to accelerate the solidification by the secondary cooling water just below the mold, there is a problem that the frequency of steel leakage at the bottom increases.
That is, if the bottom portion is cooled with the same amount of secondary cooling water as when the molten metal, that is, the heat source is being supplied from the tundish, the solidified shell is deformed due to overcooling, which causes the unsolidified molten steel to be extruded upward. However, there is a problem that steel leakage (bottom blow-up) occurs (see FIG. 2B). In addition, this secondary cooling water may penetrate into the molten powder or molten steel and cause a steam explosion.

In order to solve this problem, in Japanese Unexamined Patent Publication No. 5-269556 (Japanese Patent Application No. 4-67107), the most bottom position is S ₀ (
Between the meniscus) and S ₁ (the position where the thickness of the solidified shell is sufficient), the amount of cooling water at the bottom is reduced so that the solidified shell is not deformed, and blown up due to squeezing of unsolidified molten steel is prevented. Specifically, the meniscus S ₀
Depending on the distance from, the mold cooling water amount is suppressed to, for example, 0% to 98% at the time of normal casting, and particularly, the spray cooling water amount in the secondary cooling zone is set to a cooling water amount that does not deform the solidification shell, for example, 0% to 30% of normal casting
Is suppressed. Hereinafter, this method is referred to as Conventional Method 3.

[0010]

However, in the conventional method 3 for reducing the mold cooling water amount and the secondary cooling water amount, in order to prevent the shrinkage deformation of the solidified shell, a slab having a considerable length from the bottom position is used. On the other hand, it is necessary to accurately estimate the shell thickness and shell temperature and control the amount of cooling water so that the contraction force and the static iron pressure are balanced. Specifically, the slab length is S ₁ ≤ (T ₁ / K) ² V _0/4 [T
₁ : Mold thickness (mm), V ₀ : Casting speed (m / min), K: Constant (m
m / min ^1/2 )], S ₁ that satisfies the formula, for example, T ₁
= 250mm, V ₀ = 1.5m / min, K = 27mm / min ^1/2
Then, the maximum slab length is 32 m.

Therefore, even if a complicated calculation is performed by a computer every time according to the casting speed, the slab size, the secondary cooling pattern, and the steel type (component), the calculation error and the operation (secondary cooling water amount, water temperature, etc.) Due to variations, some supercooling may occur, and blow-up due to shrinkage of the solidified shell cannot be completely prevented. Further, if the cooling water is excessively lowered to absorb these variations, conversely the bulging between the rolls of the solidified shell may increase and internal cracking may occur.

The present invention has been made to solve the above-mentioned problems, and its purpose is to cause solidification shell deformation due to supercooling at the end of so-called high-speed casting which can improve productivity, quality and the like. It is possible to completely and reliably prevent steam explosion due to steel leakage and spray water intrusion.
Moreover, it is an object of the present invention to provide a pouring end control method capable of reliably eliminating internal cracks in a cast due to insufficient cooling.

[0013]

SUMMARY OF THE INVENTION A pouring end control method according to the present invention is a method of controlling a pouring speed at the end of pouring of a continuous casting machine in which molten metal in a tundish is continuously cast into slabs. Without slowing down or stopping and processing the bottom part, which is the last end of the slab, to control the secondary cooling water when the casting is completed and the slab is withdrawn while maintaining the normal casting speed. At the same time, the molten metal surface of the bottom portion of the uppermost end of the slab is controlled to be positioned below the upper end of the solidified shell of the bottom portion by a predetermined amount or more immediately below the mold. That is, the following controls are performed.

[0014] (1) with respect to control of the cooling water, similarly to Conventional Method 3, in order to prevent extreme supercooling shrinkage of the bottom portion, as shown in FIG. 1 (a), the meniscus S ₀ at casting ended ~
The cooling water amount of the mold 2 and the secondary cooling water amount by the spray nozzle 4 at the position S ₁ where the solidified shell thickness is sufficient are
As shown in FIGS. 1 (c) and 1 (d), Q ₃ and Q ₄ in the normal state
To Q ₁ and Q ₂ , respectively. Here, in particular, as shown in FIG. 3 (a), the secondary cooling water amount is equal to or more than the amount of water that does not cause internal cracking due to bulging due to insufficient spray cooling water, and is small even if the bottommost steel leak occurs. The reduced water volume is as follows. The mold cooling water amount may be left unchanged and only the secondary cooling water amount may be decreased.

(2) As for the position control of the molten metal surface of the bottom part, as shown in FIGS. 1 (a) and 1 (b), the molten steel A in the mold was sufficiently preheated (for example, 1200 ° C.) at the end of casting. lowering speed V _m corresponding refractory 21 to the drawing speed and the slab size
After completion of the casting, the level of the molten metal surface in the mold is maintained at the same level S ₀ as in the normal casting until the predetermined drawing length L ₁ . Then, the immersion of the refractory material 21 is stopped at the S ₂ position. As a result, with the progress of drawing, the immersed refractory material 21 is extracted from the molten steel A, and the bottom-most molten metal surface is largely lowered by the refractory material integral from the normal molten metal descent amount.
As a result, immediately below the mold, the molten metal surface a at the bottommost part is lowered by the predetermined amount T with respect to the upper end of the solidified shell b at the bottommost part.

(3) After that, the drawing speed is controlled by the same method as the conventional method 2 or the conventional method 3 to draw the slab. That is, the bottommost position is the position S ₁ where the thickness of the solidified shell is sufficient.
When it reaches, the amount of secondary cooling water is returned to the amount of secondary cooling water at the time of normal casting as needed, the drawing speed V is increased to V ₅ by a predetermined acceleration, and the cast piece is drawn at this drawing speed V _5. .

[0017]

With the above construction, the casting is completed while maintaining the steady-state casting speed (high-speed casting is completed), and as shown in FIG. Cooling water W _m and spray cooling water W in the secondary cooling zone
cooled by _s . In the conventional method 2 which accelerates solidification immediately below the mold at the end of high-speed casting, the solidified shell on the bottom side shrinks due to supercooling by stopping the supply of heat source from the immersion nozzle after the end of casting, and unsolidified molten steel is squeezed to the bottom side. However, in the present invention, since the mold cooling water amount and the spray cooling water amount in the secondary cooling zone are suppressed in the same manner as in the conventional method 3, solidification by supercooling occurs. The deformation and shrinkage of the shell are prevented, and it is possible to prevent the molten steel from rising from the bottommost portion.

However, in the conventional method 3, as shown in FIG. 3 (a), even if the spray cooling water amount is reduced to Q ₂ , it is not possible to completely prevent the steel leak at the bottom, so that the spraying is not possible. If the amount of cooling water is greatly reduced to Q _5, the shrinkage of the solidified shell described above is completely suppressed, and the steel leakage occurrence rate at the bottom becomes zero, conversely the slab surface temperature rises due to insufficient cooling water. This in turn causes internal cracking due to bulging between the rolls. On the other hand, in the present invention, as shown in FIG. 3A, even if the spray cooling water amount in the secondary cooling zone is set to the spray cooling water amount Q ₂ in which internal cracking does not occur (a small amount of leakage steel may cause 2), as shown in FIG. 2 (a), since the molten metal surface a at the bottommost portion a is lowered by a predetermined amount T from the upper end of the solidification shell b at the bottommost portion immediately below the mold, the thin solidification layer is broken by a small amount of shrinkage. The squeeze out of molten steel can be held by the solidified shell b, steel leakage into the continuous casting machine can be eliminated, and internal cracking can be reliably prevented.

Further, the spray water flow in the secondary cooling zone is blocked by the solidification shell b projecting upward from the bottommost molten metal surface a, so that the spray water flow hits the bottommost molten metal surface and invades to cause steam explosion. Can be prevented (see FIG. 3B).

[0020]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a first embodiment of the present invention; In FIG. 1 (a), the upstream side of the continuous casting machine is, in order from the top, a tundish 1 having a sliding nozzle and a dipping nozzle, a mold 2 having a cooling water channel, and a spray nozzle between a number of guide rolls 3. The molten steel A from the tundish 1 is primarily cooled by the mold 2 to form the solidified shell B on the surface, and the solidified shell B is formed by the subsequent secondary cooling zone. Is grown, and the completely solidified slab is pulled out by a pinch roll (not shown).

The control device sends a speed control unit 10 which sends an operation signal (roll pressing force / rotation speed) to the driving roll 3a and the pinch roll in the guide roll group, the cooling water W _m of the mold 2 and each spray nozzle 4. Secondary cooling water W
A cooling water control unit 11 that controls the flow rate of _s , and a process computer 1 that sends a control signal to the speed control unit 10 and the cooling water control unit 11 based on the casting speed and the drawing length L of the slab.
2 is provided.

In such a continuous casting facility, according to the present invention, the bottommost molten metal level control device 20 is installed above the mold 2. This bottom-most level control device 20 is a refractory 21 having a predetermined volume, which is immersed in the molten steel A in the mold 2, and a refractory 2 attached to the upper part of the refractory 21.
1 and a drive device 23 that supports the support member 22 so as to be vertically movable and can lower the refractory material 21 at a predetermined immersion speed.

The drive device 23 is, for example, a drive system using a motor and a screw or a motor and a rack and pinion, is controlled by the speed control unit 10, and as will be described later, fire-proof at an immersion speed V _m corresponding to the drawing speed V _0. The object 21 is lowered. The refractory 21 has a shape corresponding to the cavity cross section of the mold 2 and having a predetermined distance from the inner wall surface of the mold as described later. In the case of a slab mold, the cross section has a rectangular parallelepiped shape with a rectangular cross section. Become.

With the above-mentioned structure, the pouring end control is performed as follows.

(1) Completion of casting The casting is completed by closing the sliding nozzle of the tundish 1 while maintaining the normal casting speed without slowing or stopping the casting speed and performing the bottom treatment of the slab. To do.

(2) Suppression of Mold Cooling Water / Spray Nozzle Cooling Water When the casting is completed, the cooling water flow rate to the mold 2 and the cooling water flow rate to the spray nozzle 4 after the molten steel A is no longer supplied, and the mold 2 In order to control the immersion speed V _m of the refractory material 21 inside and the drawing speed V ₀ by the driving roll 3a, the drawing length L at the bottom position is tracked by the process computer 12.

Then, between the predetermined meniscus S ₀ and the position S ₁ at which the solidified shell thickness is sufficient, the cooling water control unit 11 causes the cooling water control unit 11 to change the thickness as shown in FIGS. 1 (c) and 1 (d).
Mold cooling water to Q _1: is controlled to (<Q ₃ normal mold cooling water at the time of casting), the spray amount of cooling water of the secondary cooling zone, the solidified shell thickness, shell temperature, the slab each unit for each spray The cooling water amount Q ₂ (<Q ₄ : spray cooling water amount during normal casting) that does not deform the solidification shell B calculated from the deformation resistance and static iron pressure of the solidification shell is controlled.

Here, specifically, S ₁ is S ₁ ≤ (T ₁ / from the mold thickness T ₁ (mm) and the casting speed V ₀ (m / min).
_{^{K) 2 V 0/4 [}} K is a ^{constant, 25~35 (mm / min 1/2)} ] is a value determined by the formula. Further, Q ₁ is a mold cooling water flow rate ≧ 4 (m / sec), and Q ₃ is the same amount as during normal casting. Q ₂ is the spray cooling water amount <Q ₄ (l / kg ・ steel),
Q ₄ is the same amount of spray water as during normal casting, 2.0 (l / kg
・ Steel) ≧ Q ₄ ≧ 0.3 (l / kg · steel).

Here, the amount of secondary cooling water differs depending on the distance from the meniscus S _{0, and is made} to be approximately the same amount or decreased as compared with the time of normal casting, and is controlled by the above calculation within the above water amount range. The reason for limiting the numerical values of S ₁ , Q ₁ and Q ₂ is Q
_{This is because, if 1} and Q ₂ are performed outside the above range, the occurrence rate of steel leakage from the bottommost portion during drawing may increase, or internal cracking due to bulging may occur (Fig. 3 (a )reference).

(3) Prevention of Steel Leakage by Bottom Level Control In the present invention, from the viewpoint of prevention of internal cracks, the calculation result of the spray cooling water amount Q ₂ is ≦ 0.3 (l / kg · steel). However, the amount of cooling water is not reduced any further, and as shown in FIG. 3 (a), the molten steel is squeezed out at this time or due to some solidification shrinkage caused by calculation error / operational variations. To solve the problem and prevent steel leakage.

That is, as shown in FIG. 1 (a), in addition to the cooling water control of (2), the red hot refractory 21 is moved to the speed V at the end of casting.
Immerse at _m . The velocity V _m (m / min) at this time is A · V _m ≦ t ₁ · W · V ₀ × 1.5 (1) Formula A: refractory cross-sectional area (mm ² ), t ₁ : slab Thickness (mm) W: Slab width (mm), V ₀ : Controlled by drawing speed (m / min). The reason for limiting the numerical values is that if the immersion speed V _m of the refractory material 21 is too fast, the meniscus rising speed in the mold is large, and the solidified shell restraint occurrence rate due to defective mold powder inflow increases.

The refractory material 21 must be preheated to 1200 ° C. or higher in advance. Furthermore, the cross-sectional area of the refractory 21 must be determined so that the interval P between the surface of the refractory 21 and the inner wall surface of the mold 2 is P ≧ 50 mm. The preheating temperature and the interval P of these refractories 21 are outside the above range,
When the refractory is dipped, the molten steel at the bottom partly solidifies and the quality deteriorates.
May cause undrawable.

Next, the refractory material 21 is immersed in the molten steel A by the immersion depth T ₂ (mm) satisfying the following equation, and then stopped at the S ₂ position, T ₂ · A = T · W · t ₁ ...... (2) Formula T: Distance of the bottommost molten metal lowering (mm) from the upper end of the solidified shell directly below the mold (mm).
The bottom-most molten metal surface a is lowered by T from the upper end of the bottom-most solidified shell b by an amount proportional to the immersion volume.

As shown in FIG. 3 (a), if the spray cooling water amount Q ₂ is not reduced below 0.3 (l / kg · steel), a small amount of leakage steel is generated due to shrinkage of the solidified shell during drawing. By deciding the immersion depth T ₂ by the formula (2) so that T ≧ 100 mm, the molten steel squeezing that breaks the thin solidified layer and ejects due to a small amount of shrinkage is held by the cast solidified shell b. It is possible to prevent leakage steel and internal cracks. Further, it is also possible to prevent the spray water flow from the spray nozzle 4 from directly impinging on the bottommost molten metal surface to enter and cause a steam explosion.

(4) When the bottommost position reaches S ₁ , the spray cooling water amount is returned to Q ₄ which is the same amount as the other stationary parts, and the drawing speed V ₀ is increased to a predetermined extraction speed V ₅ at a predetermined acceleration. Speed up. Note that the acceleration needs to be equal to or less than a predetermined value that prevents the molten steel from being squeezed out due to a change in the slab bulging amount between rolls during acceleration.

FIG. 3 (a) is a graph showing the steel leakage rate at the bottommost portion with respect to the spray cooling water amount, and in the present invention, the spray cooling water amount between S _{0 and} S ₁ is Q ₂ or more (≧
By setting 0.3 (l / kg · steel), internal cracking due to bulging can be completely eliminated, and a small amount of leaked steel due to this can be retained by the solidified shell b as described above, Steel leaks to the outside of the strand can be completely and reliably prevented. FIG. 3B is a graph showing the occurrence frequency of bottom steam explosion. In the present invention, the bottom steam explosion can be significantly reduced as compared with the conventional method 2 and the conventional method 3 by shielding the spray water flow by the solidification shell b. I understand.

[0037]

As described above, according to the present invention, when the casting is finished and the slab is pulled out while the casting speed is maintained and the normal casting speed is maintained, the secondary casting is performed without slowing down / stopping the casting speed and performing the bottom treatment work. At the same time as controlling the cooling water, by temporarily immersing the red-hot refractory material in the bottommost part, the distance from the top of the bottommost solidified shell to the bottom of the molten metal immediately below the mold becomes a predetermined amount or more. Since it is configured, the following effects are achieved.

(1) After the high speed casting is completed, the productivity, quality, etc. are greatly improved in the same manner as in the conventional methods 2 and 3, and a small amount of leaked steel is held by the bottommost solidified shell. Even if there is a calculation error of the cooling water amount or a variation in the operation, it is possible to completely and reliably prevent the steam explosion due to the steel leak and the spray water flow from the bottom.

(2) Further, by allowing a small amount of steel leakage, the amount of spray cooling water can be appropriately secured, and the occurrence of internal cracks due to insufficient cooling can be completely eliminated.

[Brief description of the drawings]

FIG. 1 is a pouring end control method according to the present invention,
(A), (b) is a schematic sectional drawing of the equipment which shows the bottom bottom level control in order, (c), (d) is a graph which shows cooling water control.

2A is a cross-sectional view showing a state of a bottommost portion just below a mold according to the present invention, and FIG. 2B is a cross-sectional view showing blowing up of an unsolidified portion in Conventional Method 2.

FIG. 3A is a graph showing the relationship between the amount of spray water and the occurrence rate of steel leak at the bottom, and FIG. 3B is a graph showing the frequency of bottom steam explosions.

4A is a schematic perspective view showing a bottom processing operation in the conventional method 1, FIG. 4B is a graph showing a casting speed pattern at the end of casting by the conventional method 1, and FIG. It is a graph which shows a casting speed pattern at the time.

[Explanation of symbols]

A ... Molten steel, a ... Bottom surface of molten metal B ... Solidified shell, b ... Bottom solidified shell W _m ... Mold cooling water, W _s ... Spray (secondary) cooling water 1 ... Tundish 2 ... Mold 3 ... Guide Rolls, 3a ... Driving rolls 4 ... Spray nozzles 10 ... Speed control unit 11 ... Cooling water control unit 12 ... Process computer 20 ... Bottom liquid level control device 21 ... Refractory material 22 ... Support member 23 ... Drive device

Claims (1)

[Claims] 1. At the end of casting of a continuous casting machine for continuously casting molten metal in a tundish to form a slab,
The secondary cooling water is used when the casting is finished and the slab is withdrawn without slowing or stopping the casting speed and processing the bottom part, which is the last end of the slab, while maintaining the normal casting speed. At the same time, the molten metal surface of the bottom of the uppermost end of the slab is controlled so as to be located below the upper end of the solidified shell of the bottom by a predetermined amount or more immediately below the mold. Termination control method.