JPH08206787A - Continuous casting method of slab of large section and mold for casting - Google Patents

Abstract

PURPOSE: To cast a slab of large section by forming the heat insulation layer of the metal or ceramics with the heat conductivity lower than that of copper on an inner wall of a mold at the lower and upper part of the meniscus to prevent surface flaws due to the inclination. CONSTITUTION: The region to form the heat insulation layer 2 is from the position of 35-45mm below the meniscus level 3 to the position of >=20mm above the meniscus level 3. At the part of <35mm below the meniscus level 3, the cooling is intense and the inclination of the solidified shell becomes large while at the part of >45mm, the growth of the solidified shell becomes slow and the breakout is generated. This phenomenon is similar to the position of 20mm from the upper end part of the heat insulation layer 2. The substance composing the heat insulation layer 2 is the metal or ceramics such as Ni and Fe with the 1/3-1/10 heat conductivity of copper, i.e. The slab of large section where surface flaws are prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has a thickness of 600 to 1, for example.
TECHNICAL FIELD The present invention relates to a method for continuously casting a large cross-section slab having a cross-sectional size of 000 mm and a width of 700 to 3000 mm, and a casting mold for carrying out this method, and particularly to a large-section slab having excellent surface quality. The present invention relates to a technique for continuously casting steel.

[0002]

2. Description of the Related Art In casting a large cross-section slab as described above, continuous casting has previously been difficult due to dimensional constraints and the like. It is generally carried out, and various techniques have been proposed from such a viewpoint.

On the other hand, the widespread use of continuous casting operations in the steel manufacturing industry has greatly contributed to cost rationalization in terms of yield improvement, energy saving and labor saving. For these reasons, in recent years, it has been attempted to apply the continuous casting method even when casting a large-section slab as described above.

In the production of such a large-section slab by the continuous casting method, the casting speed is set to be much slower than in the case of producing a normal slab from the viewpoint of preventing bulging of the slab. Is common. For example, in the case of a large cross-section slab having a cross-sectional size of 700 mm × 1400 mm, the casting speed is about 0.10 m / min.

[0005]

However, in the low-speed casting as described above, there is a phenomenon called so-called collapse of the solidified shell, in which the solidified shell collapses inward from the surface of the slab at the contact portion between the molten steel meniscus and the mold. There is a problem of manifestation. When this phenomenon becomes apparent,
If the surface of the slab becomes uneven, and if the degree of this unevenness becomes severe, it will cause surface defects during forging and rolling, and will impair the quality of the product.

In the present continuous casting method, good lubricity between the cast piece and the mold is also an important requirement. As a means for achieving the lubricity, a lubricant called mold flux is used for the cast piece. The casting is performed while the molten metal is present between the mold and the mold. However, if the melting of the mold flux is insufficient, the lubricity between the slab and the mold is not maintained well, and sticking occurs between the slab and the solidified shell on the surface of the slab and the so-called breakout occurs. There's a problem. As a technique for solving such a problem, a technique such as Japanese Patent Laid-Open No. 57-109952 has been proposed. In this technique, the electrode is immersed in a flux layer formed on the molten steel surface in the mold, and the flux is melted by resistance heat accompanying energization, and a molten falax layer having a predetermined thickness is secured on the molten steel surface. Is. According to this technique, from the viewpoint of preventing the occurrence of breakout due to insufficient melting of the mold flux, it is considered that a temporary effect can be obtained.
It is also said that this technique achieves the effect of preventing surface defects due to unmelted flux.

However, even this technique is still insufficient from the viewpoint of preventing the occurrence of surface defects due to a phenomenon called collapse of the solidified shell, and it is desired to establish a technique for eliminating such inconvenience. It is the actual situation.

The present invention has been made in order to solve the technical problems in the prior art, and the purpose thereof is a phenomenon called collapse of a solidified shell when continuously casting a large-section slab (hereinafter referred to as "falling down"). , Which is simply referred to as "falling phenomenon"), and a casting mold for carrying out this method, which is capable of effectively preventing the occurrence of surface defects caused by the above, and has a simpler structure. To achieve that.

[0009]

According to the method of the present invention which achieves the above object, molten steel is supplied to a copper or copper alloy mold whose upper and lower sides are opened, and a slab is continuously drawn from the lower part of the mold to obtain a large size. In casting a cross-section cast slab, 1/3 to 1/10 of copper in the mold inner wall surface portion extending from the lower 35-45 mm position of the meniscus portion of the molten steel in the mold to the upper 20 mm or more position of the meniscus portion. The method for continuous casting of large-section cast slabs is characterized in that a heat retaining layer made of metal or ceramics having the above-mentioned thermal conductivity is formed and operated. In this method, it is also effective to coat the surface of the molten steel supplied to the mold with a heat-generating mold flux for operation.

On the other hand, the casting mold of the present invention which has achieved the above-mentioned object is a mold made of copper or a copper alloy whose upper and lower sides are open, and the lower parts 35 to 4 of the meniscus portion in the molten steel in the mold.
1/3 to 1 of copper on the inner wall surface of the mold extending from the position of 5 mm to the position of 20 mm or more above the meniscus
The gist is that a heat insulating layer made of metal or ceramics having a thermal conductivity of / 10 is formed.

[0011]

First, the present inventors examined the cause of the collapse phenomenon in the prior art. A casting mold in a conventional casting apparatus is generally made of copper or a copper alloy (hereinafter, may be represented by copper). However, the casting speed is slow when casting a large cross-section slab using a mold made of such a material with high thermal conductivity (copper thermal conductivity is 3.8 J / cm · s · k). Together with this, by the time the molten steel sequentially supplied reaches the contact portion with the mold, the molten steel previously supplied is cooled by the mold, so that the contact portion between the meniscus portion of the molten steel and the mold, It was considered that the solidified shell was formed to have a relatively thick wall, which caused the collapse phenomenon as described above.

In order to reduce the above-mentioned collapse phenomenon,
Although it is conceivable to raise the casting temperature of the molten steel, raising the casting temperature more than necessary leads to deterioration of the slab quality such as increased segregation, which is limited. Therefore, the present inventors considered that the above-mentioned collapse phenomenon could be prevented by appropriately controlling the cooling strength of the meniscus portion in the molten steel in the mold, and examined its specific configuration from various angles. As a result, for casting, a heat insulating layer made of a substance (metal or ceramics) having a thermal conductivity of 1/3 to 1/10 that of copper is formed in a predetermined region of the inner wall surface of the mold corresponding to the meniscus portion of molten steel. The present invention has been completed based on the finding that it is possible to manufacture a large-section cast slab having excellent surface quality while effectively preventing the occurrence of the collapse phenomenon by operating using a mold.

FIG. 1 is a schematic explanatory view showing the vicinity of a mold of a constitutional example of a casting apparatus for carrying out the present invention. In the figure, 1 is a rectangular copper mold and 2 is a heat insulating layer composed of a Ni plating layer. ,
Reference numeral 3 indicates a meniscus level, and 4 indicates an exothermic mold flux. Thus, in a predetermined region of the inner wall surface of the mold corresponding to the meniscus portion of molten steel, 1/3 to 1/10 of copper
A substance having a thermal conductivity of (Ni has a thermal conductivity of 0.9 J
/ Cm · s · k) by using a casting mold in which a heat retaining layer is formed, it is possible to prevent a relatively thick solidified shell from being formed in the meniscus portion, and to effectively cause the collapse phenomenon. It is prevented. Moreover, the method of the present invention can be implemented with a relatively simple device configuration.

In the present invention, as described above, the substance forming the heat retaining layer must have a thermal conductivity of 1/3 to 1/10 that of copper. If the thermal conductivity of this material is greater than 1/3 of the thermal conductivity of copper, the solidified shell will grow faster and the slab surface quality will deteriorate due to the phenomenon of collapse. Also, the thermal conductivity of this material is less than that of copper.
When it is smaller than / 10, the growth of the solidified shell is delayed, and the above-mentioned problems such as breakout occur.
Specific materials for forming the heat insulating layer include metals such as Fe and ceramics such as boron nitride (BN), in addition to Ni described above.

As described above, the region for forming the heat retaining layer is the lower portion 35 to 45 of the meniscus portion in the molten steel in the mold.
It is necessary to form the inner wall surface of the mold extending from the position of mm to the position of 20 mm or more above the meniscus portion. The lower end of the heat retaining layer is located at the lower portion 35 to 45 mm of the meniscus portion, because when the lower portion of the meniscus portion is less than 35 mm, the influence of the mold becomes large, cooling becomes strong, the collapse of the solidification shell becomes large, and the meniscus becomes large. Lower part 45
If it exceeds mm, the growth of the solidified shell is slowed and breakout is likely to occur. The reason why the upper end of the heat retaining layer is located at a position 20 mm or more above the meniscus is that if it is less than 20 mm, the influence of the mold becomes large, cooling becomes strong, and collapse of the solidified shell becomes large. The means for forming the heat retaining layer in the above region is not particularly limited, and other than the above-described plating, a structure that allows the above substance to be fitted into a predetermined position of the mold may be used. The thickness of the heat insulating layer is not particularly limited, but in order to exert the function as the heat insulating layer,
The thickness is preferably 3 mm or more, and considering the heat load, it is preferably 10 mm or less.

In carrying out the present invention, as shown in FIG. 1, it is also effective to coat the surface of the molten steel supplied to the mold with an exothermic mold flux for operation. An example of such an exothermic mold flux is a normal mold flux containing metal Si, metal Al, or the like. That is, on the surface of the molten steel supplied to the mold,
By coating and operating the above exothermic mold flux, the heat generated by the oxidation reaction is applied to the meniscus portion of the molten steel, and the surface quality of the cast slab is further improved.

The present invention will be described in more detail with reference to the following examples. However, the following examples are not intended to limit the present invention, and any modification of the present invention can be made without departing from the spirit of the preceding and following paragraphs. It is included in the technical scope.

[0018]

EXAMPLE The present invention was carried out by using the casting apparatus having the structure shown in FIG. 1, a large-section cast slab having a section of 700 mm × 1400 mm was manufactured, and the occurrence of surface defects was investigated. At this time, the formation region of the heat insulation layer made of the Ni plating layer was the inner wall surface of the mold extending from the position of 40 mm below the meniscus to the position of 20 mm above the meniscus in the molten steel in the mold, and the casting speed was set to 0. 1m / min,
The test was performed with and without the exothermic mold flux. In addition, as a comparative example, the occurrence of surface defects in a slab having the same cross-sectional shape produced by a conventional casting apparatus equipped with a mold having no heat insulation layer was also investigated. Regarding the occurrence of surface defects, the number of occurrences of surface defects was measured.

The results are shown in FIG. In addition, the number of surface defects generated is 1 after the number of surface defects generated by the conventional method is measured by measuring the number of surface defects generated after rolling the slab manufactured by each method.
It was expressed as an index when 00 was set. As is clear from these results, the method of the present invention has smaller irregularities on the surface of the slab than the conventional method, and the number of surface defects after rolling is remarkably improved.

[0020]

EFFECT OF THE INVENTION The present invention is configured as described above, and when continuously casting a large-section slab, it is possible to effectively prevent the occurrence of surface flaws due to the phenomenon of collapse. Moreover, it was possible to achieve it with a simpler device configuration.

[Brief description of drawings]

FIG. 1 is a plan view showing the vicinity of a mold of a structural example of a casting apparatus for carrying out the present invention.

FIG. 2 is a bar graph showing a surface flaw generation index after rolling, comparing the method of the present invention with the conventional method.

[Explanation of symbols]

 1 Copper mold 2 Heat insulation layer 3 Meniscus level 4 Exothermic mold flux

Claims (3)

[Claims] 1. A molten steel is supplied to a copper or copper alloy mold whose upper and lower sides are opened, and a slab is continuously drawn from the lower part of the mold to cast a large-section slab. A heat insulating layer made of metal or ceramics having a thermal conductivity of 1/3 to 1/10 that of copper is formed on the inner wall surface of the mold extending from the lower portion 35 to 45 mm to the upper portion 20 mm or more of the meniscus portion. A continuous casting method of a large-section slab, which is characterized in that 2. The continuous casting method according to claim 1, wherein the molten steel supplied to the mold is coated with an exothermic mold flux for operation. 3. A copper or copper alloy mold having open upper and lower parts, the lower part 35 of the meniscus portion of molten steel in the mold.
1/3 of copper on the inner wall surface of the mold extending from the position of ~ 45 mm to the position of 20 mm or more above the meniscus.
A casting mold, wherein a heat insulating layer made of metal or ceramics having a thermal conductivity of 1/10 is formed.