JPH0871704A - Mold for continuous casting - Google Patents

Abstract

PURPOSE: To obtain the mold for continuous casting capable of improving quality of cast slab and extending life of a mold. CONSTITUTION: A protecting film consisting of an alloy containing Co as principal component as well as Ni, Cr and an cermet containing WC carbide ceramic is formed on the inner surface of the mold base material of copper or copper alloy. A composition of the protecting film is continuously or step by step changed so that a blending ratio of the WC carbide ceramic is set to 70-90% at the lower side of mold, at which the solidified shell is formed, and to 40-70% at the upper side of mold, at which the surface of molten metal exists.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting mold for continuously producing a slab using a mold made of copper or a copper alloy.

[0002]

2. Description of the Related Art The inner surface of a continuous casting mold made of copper or a copper alloy that comes into contact with molten steel or steel after solidification has abrasion resistance and good adhesion to the base material of the mold, and does not easily peel off. A two-layer plating film plated with Ni or Co as a base (Japanese Patent Publication No. 54-37562), or a three-layer film (Japanese Patent Publication No. 52-
No. 50733) is known.

[0003]

The inner surface of a mold for continuous casting of steel comes into contact with molten steel or steel after solidification, and the steel side constantly slides down at a speed of 1 to 2 m / min. , Al ₂ O ₃ based powder material between the mold and molten steel
Is inserted from above, and the sliding and high hardness Al ₂ O
_{As a} result of sliding contact with ₃ type powder material, generally about 30μ Cr plating layer and about 1000μ after casting of 500 charges
The Ni plating layer is abraded and the copper base material is exposed.

On the other hand, in the case of continuous casting, the upper part of the mold, which is in contact with the molten steel, is intermittently moved up and down by oscillation, so that it receives thermal stress due to repeated heating and cooling. Therefore, the thermal stress is excessively generated, and the toughness of the above-mentioned Cr plating layer is inferior, so that a large number of cracks are generated in the vicinity of the contact with the bath surface after casting several hundred charges. Further, since the steel is in contact with the lower part of the mold in a solidified state, it becomes solid contact without lubrication and is rapidly worn. As described above, the mold for continuous casting is required to have crack resistance, that is, toughness in the upper portion, and wear resistance, that is, hardness in the lower portion, due to the characteristics of its use environment.

It is an object of the present invention to provide a continuous casting mold which can cope with these deterioration characteristics peculiar to the continuous casting mold.

[0006]

That is, according to the present invention, Co is mainly contained in the inner surface of a copper or copper alloy mold base material and N is used.
A protective coating composed of an alloy containing i, Cr and a cermet containing WC-based carbide ceramics is formed, and the composition of the protective coating is 70% at the lower side of the mold where the solidified shell is formed. It is characterized in that the composition ratio is set to ˜90% and the composition ratio is changed continuously or stepwise so as to decrease the composition ratio to 40% to 70% on the upper side where the molten metal surface exists. It is a continuous casting mold, and the protective coating formed on the inner surface of the mold base material is Cr ₃ C ₂ instead of WC-based carbide ceramics.
Alternatively, a cermet containing NbC or TiC can be used.

[0007]

Table 1 shows the amount of wear and the rate of occurrence of cracks when alloys mainly containing Co and containing one or more of Ni and Cr and ceramic ratios of WC carbides are changed. In the table, the amount of wear is 50 mm square, thickness 10 mm, and surface roughness Ra using a ball-on-desk wear tester.
Was measured using a thermal spraying sample of 3μ, the ball diameter of ordinary steel is 10 mm, the number is 3, the turning diameter is 38 mm, the temperature is 200 ° C corresponding to the surface temperature of the lower part of the mold, the load is 10 kg,
Although it was measured at a rotation speed of 38 rpm, it shows the reduced volume of sliding wear traces on the sample surface, and the crack generation rate also corresponds to the surface temperature of the upper part of the sprayed sample of 50 mm square and 10 mm thick. It shows the area ratio of cracks generated on the surface of the sprayed sample after repeating the process of heating the sample at 500 ° C., dropping it in water at room temperature and cooling it 50 times.

Further, FIG. 1 is a graph of the measured values. As shown in FIG. 1, the wear amount and the crack generation rate obviously correlate with the ceramic ratio of the WC-based carbides, and the ratio of the carbides. Indicates that the wear resistance is excellent in the region where the ratio is high and cracking is difficult to occur in the region where the ratio of the carbide is low. That is, if this result is applied to the protective coating on the inner surface of the continuous casting mold, it is possible to meet two different required functions. Further, since a single surface treatment called thermal spraying is used, the processing is easy, and the film can be formed at a lower cost than using a combination of several kinds of surface treatments.

[0009]

[Table 1]

[0010]

[Example] Since the lower part of the mold, which is in contact with the solidified steel (slab) of the continuous casting mold, becomes solid-solid contact and abrasion resistance is required, the WC ratio is set to 88% in consideration of this. In the upper part of the meniscus, heat stress resistance is required because contact and non-contact with molten steel are repeated, so that the WC ratio cannot be increased so much. Therefore, the WC ratio is set to 70% and the balance is set to 80% and the balance is Co A carbide cermet, which is an alloy mainly composed of, was molded on the inner surface of a copper mold as a protective film having a thickness of 100 μm by a high-speed gas jet spraying method, and then used in a continuous casting line. Conventional N for comparison
A test using a two-layer plating method of i and Cr (Japanese Patent Publication No. 54-37562) was also conducted.

As a result, the conventional two-layer plating method of Ni and Cr had an average life of 600 charges due to cracks in the vicinity of the upper bath surface and wear of the lower portion, but in the case of using the mold according to the present invention. It has become possible to extend the service life up to 1700 charges.

Further, in the same continuous casting line, the WC ratio was 65% at the upper part of the mold and 88% at the lower part of the mold, and the W in between was set.
As a result of using the above-mentioned mold provided with the thermal spray coating in which the C ratio is continuously changed, it has been confirmed that 1800 charges or more can be used.

From the viewpoint of wear resistance, the lower part of the mold, which comes into contact with the solidified steel (cast slab), has a WC ratio of 70 to 90%. If this ratio is less than 70%, the wear resistance is not sufficient, and conversely, if it exceeds 90%, the brittleness increases and the mechanical strength becomes insufficient. Further, in the upper part of the meniscus portion, from the viewpoint of stress resistance, the WC ratio is in the range of 40 to 70%. If this ratio is less than 40%, the wear resistance is too small to be the same as ordinary alloy steel, and if it exceeds 70%, the brittleness is inferior, which is not preferable. In the middle part of the mold, which is located between the liquid phase and the solid phase of steel, the WC ratio is 75
It is desirable to set it to ˜85%.

Also, instead of WC, Cr ₃ C ₂ or N
With a cermet containing bC or TiC, it was confirmed that even if the carbide ratio was changed from the lower part to the upper part, the life prolonging effect was large, and it was possible to use 1500 charges or more as a result of use in a continuous casting line.

Although an example in which the carbide ratio is changed stepwise has been described, the same effect can be obtained by continuously changing the carbide ratio. That is, it is possible to form a protective coating by thermal spraying using a method of continuously changing the compounding ratio of carbide and metal within the range of WC65 to 90% in the process of high-speed gas jet thermal spraying.

[0016]

As described above, according to the present invention, it becomes possible to maintain the inner surface of the mold in a sound condition for a long period of time, and the troubles caused by flaws in the slab are reduced. Further, the manufacturing cost of the mold inner surface protective coating according to the present invention is about 1.5 times that of the conventional plating, but the life of the mold is more than doubled and the quality improvement of the slab is also great, so it is economical. Are fully satisfied.

[Brief description of drawings]

FIG. 1 shows the ceramic ratio of WC-based carbides, the wear amount,
The figure which shows the relationship with the generation amount of a crack.

Claims (2)

[Claims] 1. The inner surface of a copper or copper alloy mold base material is Co.
To form a protective coating composed of an alloy containing Ni and Cr and a cermet containing WC-based carbide ceramics, and the composition of the protective coating is blended with the WC-based carbide ceramics on the lower side where the solidified shell of the mold is formed. Ratio 70
% To 90%, and the composition ratio on the upper side where the molten metal surface of the mold is present is changed continuously or stepwise so as to reduce the composition ratio to 40% to 70%. Mold for continuous casting. 2. The continuous casting mold according to claim 1, wherein the protective coating formed on the inner surface of the mold base is a cermet containing Cr ₃ C ₂ or NbC or TiC in place of the WC-based carbide ceramics.