JP2004243352A - Continuous casting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method that can effectively suppress generation of internal flaws of a cast billet. <P>SOLUTION: In the continuous casting method, molten steel is cast in a bottomlessly-shaped mold 2 comprising a movable and/or a fixed mold, in which a cast billet 7 with only the surface solidified by cooling is pulled out downward from the bottom of the mold, and in which the cast billet is sprayed with water on the side in a secondary cooling zone 10. A tapered cast billet is made to be cast in which at least a pair of oppositely facing inner sides of the mold are separated little by little from each other during the casting, thereby making both sides gradually flared upward. In addition, a passing time of the cast billet through the secondary cooling zone is quickened by increasing the descending speed of the cast billet during the casting, namely, the casting speed, so that a temperature difference between the surface and the center of the cast billet is suppressed to 350°C or below. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a continuous casting method in which molten steel is cast into a bottomless mold, and a slab whose surface is solidified only by cooling is vertically lowered from the bottom of the mold.
[0002]
[Prior art]
[Patent Document 1]
JP 2000-288705 A
As is well known, impurities are condensed in the center of the thickness of a steel slab obtained by continuous casting, and internal defects called zaku, porosity, etc. may occur. As disclosed in Patent Literature 1 and the like, a technique is known in which a slab is lightly or strongly reduced by a reduction roll or continuously forged by an anvil.
[0004]
[Problems to be solved by the invention]
However, in this conventional method, the equipment tends to be large-scale and high in cost, and if conditions such as the amount of reduction and the time of reduction are not always appropriate, there is a possibility that indentation flaws or internal cracks may be induced. In some cases, it is difficult to apply the method to tool steel having a high material strength.
[0005]
Accordingly, an object of the present invention is to provide a continuous casting method capable of suppressing the occurrence of internal defects without applying pressure reduction or the like to a slab.
[0006]
[Means for Solving the Problems]
Therefore, the continuous casting method according to the present invention is a method of casting molten steel in a bottomless mold composed of a movable mold and a movable mold or a fixed mold, and casting a slab in a state in which only the surface is solidified by cooling at the bottom of the mold. A continuous casting method in which water is sprayed on the side surface of the slab in a secondary cooling zone while being withdrawn downward, wherein at least a pair of opposed inner surfaces of the mold are gradually separated from each other during casting. Is upwardly tapered so that a tapered slab that gradually expands is cast, and the speed at which the slab descends during casting, that is, the casting speed is increased to increase the surface and center of the slab. The time required for the slab to pass through the secondary cooling zone is accelerated so that the temperature difference between the slab and the slab is kept at 350 ° C. or less.
Further, the present invention is characterized in that, in the continuous casting method, the taper angle of the slab is set in a range of 1.0 to 1.8 degrees.
Further, the present invention is characterized in that in the continuous casting method, the casting speed is set so that the time required for the slab to pass through the secondary cooling zone is 30 minutes or less.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a longitudinal sectional view of a main part of a continuous casting apparatus, 1 is a tundish containing molten steel such as structural steel and tool steel, 2 is a bottomless mold, and 3 is provided at the bottom of the tundish. And a immersion nozzle for casting molten steel in the tundish into the mold. As shown in the horizontal sectional view of FIG. 2, the mold 2 comprises a pair of fixed dies 4, 4 and movable dies 5, 5, which are opposed to each other. Molten steel is cast into a space surrounded by the four dies. Numerals 6 and 6 denote hydraulic cylinders having rods connected to the movable dies 5 and 5, and when the hydraulic cylinders 6 and 6 are operated, the movable dies 5 and 5 move in the horizontal direction, and the two dies are mutually connected. It is configured to be able to approach and separate from. Then, the cast molten steel is primarily cooled in the casting mold 2, and only the surface portion is solidified to form an outer shell, and the slab 7 descends vertically from the bottom.
[0008]
Reference numerals 8, 8, ... denote guide rolls provided immediately below the mold 2 to support the vertically falling slabs 7 from the surroundings to prevent bulging. The secondary cooling 10 is provided with a plurality of nozzles 9, 9,... Opposed to the periphery of the slab 7, and cooling water is directly sprayed from the nozzles onto the slab 7. It is a belt.
[0009]
Reference numeral 11 denotes a lifting platform that supports the lower end of the cast piece 7, 12 denotes a wire that horizontally supports the lifting platform, 13 denotes a variable-speed winch provided at one end of the wire, and 14 and 15 denote the wires. The lifting platform 11 is vertically movable by the operation of the winch. For this reason, the lift 11 is lowered by the winch 13 feeding out the wire 12, and the cast piece 7 supported by the lift can be lowered at a predetermined speed. Therefore, by controlling the wire feeding speed of the winch 13, a desired lowering speed, that is, a casting speed can be set.
[0010]
During the casting, as shown in an enlarged view in FIG. 3, the fluid pressure cylinders 6 and 6 are operated to separate the movable molds 5 and 5 from each other, thereby slightly facing the inner surfaces of the mold. The tapered slabs 7 are gradually separated from each other so as to be cast upwardly. The moving speed of the movable dies 5 and 5 and the lowering speed of the slab 7 (hereinafter referred to as the casting speed) are set such that the taper angle α of the slab 7 is in the range of 0.5 to 3.0 degrees. Speed).
In this embodiment, the mold 2 is described as being composed of a pair of movable dies and a pair of fixed dies. However, as shown in FIG. 9, the mold 2 is composed of two pairs of movable dies 5a to 5c, each of which is a fluid. It may be movable by the operation of the pressure cylinders 6a to 6c.
[0011]
The casting speed of the slab 7 is set so that the time required for the slab to be extracted from the mold 2 and pass through the secondary cooling zone is 30 minutes or less. This speed greatly increases the casting speed, which was conventionally extremely low (conventionally, 0.1 m / min or less).
Further, as shown in FIG. 4, the solidification profile of the slab, the ratio (H / D) of the boundary height H between the liquidus line and the solidus line to the width D (H / D) is a measure of the occurrence of backpacking. As shown in FIG. 7, when the horizontal axis represents the taper angle of the slab and the vertical axis represents this ratio in a graph, it can be seen that the greater the taper angle, the smaller the risk of occurrence of zags.
[0012]
As described above, in the present invention, by increasing the casting speed, the temperature difference between the surface and the central portion of the slab is suppressed to 350 ° C. or less. By increasing the casting speed and reducing the temperature difference between the surface of the slab and the central portion, the maximum value of the thermal strain at the central portion of the slab can be reduced. In the graph of FIG. 5, the steel type is S48C, the casting speed is three, and the horizontal axis represents the temperature difference, and the vertical axis represents the maximum value of the thermal strain. In the graph of FIG. 6, the horizontal axis also indicates the secondary cooling zone passage time, and the vertical axis indicates the maximum value of thermal strain. Increasing the casting speed in this manner tends to suppress the temperature difference and thermal distortion of the slab, and can suppress the development of internal defects due to thermal distortion starting from the Zaku defect. In particular, the passage time in the secondary cooling zone is desirably 30 minutes or less when the ambient temperature is room temperature.
[0013]
Further, FIG. 8 shows the product of the casting speed of the slab and the taper angle on the horizontal axis, and the oxidized length (depth of internal defect) from the end surface of the slab on the vertical axis. It can be seen that setting the casting speed to an appropriate value as described above is effective for preventing the occurrence of internal defects.
[0014]
【The invention's effect】
As described above, the continuous casting method according to the present invention allows the tapered slab to be cast by gradually separating the opposite inner surfaces of the mold during casting so that both sides face upward. In addition, the slab is lowered so that the temperature difference between the surface and the center of the slab can be suppressed to 350 ° C. or less by increasing the speed of lowering the slab during casting, ie, increasing the casting speed. By increasing the time required to pass through the next cooling zone, there is a beneficial effect that the occurrence of internal defects can be reduced or prevented.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view of a main part of a continuous casting apparatus showing an embodiment of the present invention.
FIG. 2 is a horizontal sectional view of a main part of FIG.
FIG. 3 is an enlarged view of a main part of FIG. 1;
FIG. 4 is an explanatory view showing a solidification profile of a slab.
FIG. 5 is a graph showing a maximum value of thermal strain of a slab.
FIG. 6 is a graph showing the maximum value of the thermal strain of a slab.
FIG. 7 is a graph showing a guideline for occurrence of a zigzag in a slab.
FIG. 8 is a graph showing an oxidation length from an end face of a slab.
FIG. 9 is a horizontal sectional view showing another embodiment of the mold.
[Explanation of symbols]
2 mold 5 movable mold 6 fluid pressure cylinder 7 slab 8 guide roll 9 nozzle 10 secondary cooling zone 11 lift 13 winch

Claims (3)

Molten steel is cast into a bottomless mold composed of a movable mold and a movable mold or a fixed mold, and a slab whose surface is solidified only by cooling is drawn downward from the bottom of the mold and is cooled in a secondary cooling zone. A continuous casting method of spraying water on the side surface of the slab, wherein a tapered shape in which both side surfaces face upward by gradually separating at least a pair of opposed inner surfaces of the mold during casting. The temperature difference between the surface and the center of the slab is suppressed to 350 ° C. or less by making the slab cast and lowering the speed of the slab during casting, ie, increasing the casting speed. A continuous casting method characterized by accelerating the time required for the slab to pass through the secondary cooling zone. 2. The continuous casting method according to claim 1, wherein the taper angle of the slab is set within a range of 0.5 to 3.0 degrees. 3. The continuous casting method according to claim 1, wherein the casting speed is set so that the time required for the slab to pass through the secondary cooling zone is 30 minutes or less.

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