JP2001150104A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method having the peculiarity, in which the bulging is eliminating by forming a cast billet cross sectional shape as a circle in a bending type continuous casting method and a cast billet drawn- out locus is composed of a circular arc part having a fixed curving ratio and a straight line part, and a structure for supporting and guiding, is not provided at the intermediate part by supporting and guiding the cast slab with a mold at the upstream side and with pinch rolls on the downstream side of the circular arc part, and in the case of generating a breakout, the flowing-out molten steel is not welded between the cast billet and the fixed structure in the caster by setting spray nozzles at the distant place. SOLUTION: Even in the case of generating the breakout, after once stopping the drawing-out, the opened part of the breakout is welded and closed with the casting stream, and the casting is immediately returned back to the ordinary casting. Therefore, the damage caused by breakout is minimized and the high rate casting can easily be executed because of the minimum damage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for steel, and more particularly to a high-speed casting method.

[0002]

2. Description of the Related Art High speed casting has been pursued as an effective means for increasing productivity in a continuous casting method of steel. The first condition under which high-speed casting is performed stably is that the machine length (= from casting surface to casting slab) is sufficient to increase the length of the molten core (= distance from casting surface to solidification end point) due to speeding up. Second, overcoming quality problems such as center segregation due to high speed, deterioration of core defects such as shrinkage holes and porosity, and increase in surface and internal cracks due to speeding up. Countermeasures against work accidents such as breakouts, which increase in number.

The first condition is a problem in mechanical design and has no particular difficulty. Even if space is limited in existing equipment, it can be solved by the method presented in, for example, JP-A-10-180424. In other words, this is a method in which the cut position on the unsolidified slab is partially reduced in advance to enclose the molten core, and solidification is terminated after cutting.

Various methods have been implemented or proposed as measures against the second quality problem. For example, a core defect such as center segregation can be solved by a method disclosed in Japanese Patent Application Laid-Open No. Hei 7-144262.

[0005] The problem is the third work accident. In particular, although the speed increases, the cracks on the surface and inside of the slab increase, but depending on the situation, cracks may grow, causing holes under the mold and causing molten steel to flow out (breakout). As countermeasures, 1) optimization of the mold shape, especially the taper in the drawing direction, 2) improvement of the lubrication method and lubricant between the mold and the slab,
3) Improvement is being promoted by means such as a breakout prediction device. 4) On the other hand, some continuous casters for small-section billets have a structure that does not substantially have a slab supporting and guiding structure. In this case as well, if a breakout occurs, the casting of the strand is stopped, but the processing of the slab after the accident, the repair of the casting machine, the replacement of parts, etc. are extremely easy, so it is practically an effective breakout measure It has become.

[0006] The above-described conventional methods for preventing breakout have the following problems. The optimization of the first mold shape and the improvement of the second lubricity are effective in preventing breakout, but do not change the large loss when it occurs.

The third method, a breakout predicting device, detects a phenomenon in which a part of the slab skin is welded to the mold and the outer skin is cut off by restraint and drawing. The pouring and pulling are interrupted, and the casting is resumed after self-repair by resolidification of the divided portion. Therefore, the damage is only the disposal of the slab and the effect is large. However, although this method is effective for restrictive breakout, it has a problem that it is not useful for breakout caused more frequently than that caused by vertical cracks.

With regard to the fourth method, the slab supporting and guiding structure is omitted because it has no significant significance when casting a billet having a small cross section at a normal drawing speed. As a result, it helps in recoverability after a breakout and reduces losses. When this method is applied to blooms or billets with large cross-sections, bulging (swelling of the slab due to the internal pressure of the molten steel) occurs, especially at high speed casting, resulting in deterioration in quality due to internal cracks and an increase in breakout. Problems arise. Therefore, the application of the method is limited.

[0009]

DISCLOSURE OF THE INVENTION The present invention does not induce the above-mentioned deterioration in quality against an increase in breakout which is particularly problematic when high-speed casting is performed in continuous casting of bloom and billet. It is intended to provide a method for minimizing the damage and thereby to make high speed casting easier.

[0010]

In order to solve the above problems, the inventor has utilized the advantages of the conventional method, found a simple method for overcoming the disadvantages, and constructed the following invention.

According to a first aspect of the present invention, in continuous casting of steel bloom or billet, a slab drawing locus is a so-called curved type consisting of an arc portion having a constant curvature and a subsequent straight portion, and the slab cross section is circular or octagonal. Or a shape similar to these,
In the arc portion, the slab is supported and guided only by the upstream end mold and the pinch roll arranged near the lowermost point of the arc, and a slab support / guide structure is substantially provided between the mold and the pinch roll. And a tip of a spray nozzle for cooling the slab is arranged at a distance from the slab surface substantially equal to or more than the slab diameter.

In a second aspect of the present invention, when a breakout occurs during casting in the continuous casting method according to the first aspect,
This is a continuous casting method characterized by continuing the casting again after the interruption of the casting and the drawing for seconds.

The third invention is defined by the formula (1) by applying the first or second invention to an ordinary continuous casting machine designed so that the solidification end point is near the pinch roll. A continuous casting method characterized in that drawing is performed at a speed of 1.5 to 4.0 times the drawing speed V. V = 4k ² L / D ² --- (1) where, V; drawing speed (cm / min) k; solidification constant 2-3 cm / ３min L; distance between casting surface and pinch roll group outlet (cm) ) D; Slab diameter (cm)

In a fourth aspect of the present invention, the arc portion of the slab drawing locus in the first and second aspects has a 3/4 circumference, and is melted at a position approximately 1.4 m higher than the casting surface beyond the 1/2 circumference. The core is separated from the slab solidified shell by gravity to form a hollow slab,
Thereafter, it is a continuous casting method characterized by applying pressure rolling and drawing as a solid slab.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments will be described below with reference to the drawings. In FIG. 1, molten steel 1 is cast into a mold 2, and an outer shell 4 of a slab 3 is formed in the mold 2. The cross-sectional shape of the mold 2 is a circle, an octagon, or a shape similar thereto. Normally, a foot roller for restraining the outer shape of the drawn slab is provided directly below the mold, but the present invention does not have such a structure for supporting and guiding the slab,
A wear-resistant sliding guide 5 is provided in close contact with the sliding guide.

The slab 3 drawn from the mold 2 enters the secondary cooling zone 6, and the mold 2 has the same curvature as the curvature in the drawing direction in the same manner as in ordinary curved continuous casting. ,
Therefore, the slab 3 is pulled out from the vertical and horizontally gradually without difficulty at the same curvature from below the mold 2. Then it is straightened at the appropriate site.

The secondary cooling zone usually comprises a structure for supporting and guiding the slab and a spray cooling device for cooling the slab. In the present invention, in the arc portion, the mold 2 at the upstream end and the vicinity of the lowest point of the arc are provided. The slab 3 is formed only by the pinch rolls 9 arranged.
To support and guide the slab between the mold and pinch roll.
The guide structure is not substantially held. On the other hand, the spray cooling device 7 is arranged such that the tip of the spray nozzle 8 is separated from the surface of the slab 3 by substantially the slab diameter or more, as specifically shown in the cross section of FIG. 1AA ′.

One or more pinch rolls 9 are provided near the lowest point of the drawing locus to pull out the cast slab 3 and simultaneously straighten it if necessary. FIG. 1 BB ′
As shown in the cross section, a horn-shaped caliber 10 is provided. Light reduction is applied to the slab having a circular cross section if necessary, so that the cross sectional shape becomes closer to a square from a circle.

The reason is that the slab cross-section specification is usually a square, and a circle has a problem that it is difficult to handle due to rolling or the like in the subsequent steps. This problem can be solved by modifying the cross-sectional shape so that it slightly approaches the square shape by slightly reducing the pressure. That is, the present invention also has an object of producing a slab having a square cross section from a circular cross section which is advantageous for high speed casting.

When a breakout occurs during casting using the continuous casting machine having the above-described structure, casting and drawing are immediately stopped. Molten steel that has flowed out of the slab is scattered or adheres to the slab surface, but at the end of the spill the momentum diminishes and the openings are often self-welded closed. Number 10
When the casting is gradually resumed after the interruption of the casting for 2 seconds, the unsealed opening is also closed while the molten steel is fixed. When the casting surface reaches a normal level, drawing is resumed. Therefore, after a stop of about one minute, normal casting is restored.

In continuous casting of steel, the drawing speed is usually set so that the tip of the molten core inside the slab is in front of the slab cutting device 11 and in the vicinity of the pinch roll 9, and the value is regarded as the equipment capacity. This is because the molten steel flows out at the time of cutting if it goes beyond the cutting device 11. On the other hand, in actual operation, if there is a problem such as breakout or a quality problem, the drawing speed is suppressed in consideration of the facility capacity.

In the third aspect, the drawing speed is increased to several times the normal speed. In order to prevent molten steel from flowing out of the cutting portion, the molten steel 12 is closed by a transfer 17 to the downstream side of the cutting device 11 or a press 17 as shown in FIG.
For example, means such as 0428 is adopted. Even if a breakout occurs, pouring is resumed and continued after pouring is interrupted for about one minute by the above-described operation method.

The fourth invention will be described with reference to FIG. In the curved continuous casting apparatus, the arc portion of the slab drawing locus is set to 3/4 circumference. If the drawing speed is set to be relatively excessive with respect to the machine length and the slab diameter, the molten core 12 is extended downstream and extends beyond the 1/2 circumference and at a position Q point higher than the casting surface C by about 1.4 m. Follow up to. The height of the point Q = the casting surface + 1.4 m is the height of the static iron pressure corresponding to the atmospheric pressure.
At point Q, the molten core 12 is separated from the solidified shell 13 by gravity, and the slab 3 is drawn as a hollow slab 14. The inner surfaces of the hollow slab 14 are pressed against each other by a pressure rolling mill 15 to form a solid slab 16. Such a continuous casting method is described in detail in JP-A-7-144262.

In the continuous casting method as well, the slab is supported and guided up to a quarter of the circumference only by the mold 2 on the upstream side and the pinch roll 9 on the downstream side, and no supporting or guiding structure is provided. Absent. Therefore, even if a breakout occurs, there is no welding between the slab and the fixed structure, and the cast slab is immediately repaired and casting is resumed.

The operation, grounds and effects of the above invention will be described below.

The reason why the cross-sectional shape of the mold 2 is limited to a circle, an octagon, or a similar shape in the first invention is to prevent bulging. That is, bulging is a swelling phenomenon of the solidified shell due to the internal pressure of the molten steel. In the case of a square cross section, the material mechanics is attributed to the problem of deflection in the case of an equally distributed load of a beam fixed at both ends. The longer the side of the slab, the smaller the shell thickness, and the higher the internal pressure, the more easily it bends. In such a case, the slab support roller is indispensable. On the other hand, in a circular cross section, the internal pressure is received by the tangential strength of the pipe wall, and is not expanded by a static iron pressure of about several meters because it is strong. In actual continuous casting, bulging is hardly observed in a circular section.

The reason why the slab withdrawal locus is limited to a constant curvature is to omit a structure for supporting and guiding the slab such as guide rollers, and the reason for limiting the slab to the curved type is that the vertical type has a flow out during breakout. This is because the molten steel is welded to the pinch roll disposed below.

In the arc portion of the slab drawing locus, the slab is supported and guided only by the mold at the upstream end and the pinch roll disposed near the lowest point of the arc, and the slab is supported between the mold and the pinch roll. The reason why the guide structure is not substantially held is to prevent molten steel flowing out from welding the slab to the structure and making it impossible to pull out.

The reason for disposing the tip of the spray nozzle for cooling the slab almost apart from the slab surface by the slab diameter is also to prevent welding between the slab and the fixed structure. If there is no structure such as a slab support roller, welding can be avoided if the diameter is about the slab diameter.

The basis, reason, and action of the second invention are 0020
As described above in the section.

The basis of the specified drawing speed in the ordinary continuous casting machine designed so that the solidification end point is near the pinch roll in the third invention is as follows. That is, since the solidification is completed before the cutting device, the product of the required solidification time t and the speed V becomes the distance L between the casting surface and the pinch roll group outlet. On the other hand,
The time t is approximately obtained from the solidification rule (3).
Equation (1) is derived from equations (2) and (3). t × V = L --- (2) D / 2 = k√t (3) V = 4k ² L / D ² --- (1) 1.5 to 4.0 times the specified drawing speed The reason for the drawing at a speed of less than 1.5 is that if it is less than 1.5, the effect of high speed casting, which is the original purpose, is insufficient, and if it exceeds 4.0, the molten core diameter exceeds half of the outer diameter and various problems occur. Because.

Regarding the fourth invention, Japanese Patent Application Laid-Open No. 7-14442
The invention of No. 62 has two major effects, that is, the elimination of the core defect of the slab and the dramatic improvement of the casting efficiency. As a result, the casting speed may become extremely high. In such a case, there is a problem that the time required to pass through the mold becomes short, the initial solidification becomes uneven, and breakout easily occurs. When the first invention and the second invention are applied to the continuous casting method, the effects of the continuous casting method can be maximized.

[0032]

When the present invention is applied to the curved continuous casting, the arc portion constituting the secondary cooling zone has substantially no structure for supporting and guiding the slab. The maintenance and maintenance of the casting machine is simplified, which is extremely advantageous in cost.

Even if a breakout occurs immediately below or below the mold, the molten steel does not weld the slab to the structure of the casting machine. It is possible to return to casting.

Since the loss due to the breakout is extremely small, the test and operation of high speed casting can be easily performed. As a result, the technology and skills of high-speed casting will progress steadily.

[0035]

[Brief description of the drawings]

FIG. 1 illustrates a continuous casting method for carrying out the first and second inventions.

FIG. 2 illustrates a continuous casting method embodying the third invention.

FIG. 3 illustrates a continuous casting method embodying the fourth invention.

[Explanation of symbols]

1: molten copper 2: mold 3: slab 4: outer skin
5: Guide 6: Secondary cooling zone 7: Spray device 8: Spray nozzle 9: Pinch roll 10: Caliber
11: Cutting device 12: Melted core 13: Solidified shell
14: Hollow slab 15: Pressure welding rolling mill 16: Solid slab 17: Press

Claims (4)

[Claims] In a continuous casting of steel bloom or billet, a slab drawing locus is a so-called curved type consisting of an arc portion having a constant curvature and a subsequent straight portion, and a slab cross-sectional shape is a circle or an octagon or these. In the arc portion, the slab is supported and guided only by the mold at the upstream end and the pinch roll arranged near the lowermost point of the arc, and the slab is supported and guided between the mold and the pinch roll. A continuous casting method characterized by substantially not holding the structure described above, and disposing the tip of a spray nozzle for cooling the slab almost apart from the slab surface by the diameter of the slab or more. 2. The continuous casting method according to claim 1, wherein when a breakout occurs during the casting, the casting is continued for a few tens of seconds, and then the casting is continued after the interruption of the drawing. 3. An ordinary continuous casting machine designed so that the solidification end point is near the pinch roll is drawn at a speed of 1.5 to 4.0 times the drawing speed V defined by the formula (1). The continuous casting method according to claim 1 or 2, wherein: V = 4k ² L / D ² --- (1) where, V; drawing speed (cm / min) k; solidification constant 2-3 cm / ３min L; distance between casting surface and pinch roll group outlet (cm) ) D; Slab diameter (cm) 4. An arc portion of a slab drawing locus is defined as a 3/4 circumference, and a molten core is separated from a slab solidified shell by gravity at a position higher than a 1/2 circumference and about 1.4 m above a casting surface. The continuous casting method according to claim 1, wherein the slab is drawn into a hollow slab and then subjected to pressure rolling to be drawn as a solid slab.