JPH0342144A - Method for cooling mold for continuous casting and mold thereof - Google Patents

Abstract

PURPOSE:To prevent deformation and crack of mold copper plate and to prevent breakout of a cast slab by forcedly cooling the prescribed zone at position corresponding to meniscus level in the mold or lower and weakly cooling the upper zone from the position corresponding to the meniscus level. CONSTITUTION:In the water cooling mold 1 arranging a long side face copper plate 2 having plural cooling water passages 3 formed with slit extending in longitudinal direction, in order to forcedly cool neighborhood of the meniscus, cross sectional area of the water cooling passage 3 at the zone of lower part of about 150 mm from the meniscus level 9 is made small. Then, flowing velocity V1 of cooling water is increased so as to come to the prescribed times of the flowing velocity V2 in the cooling water passage 3 at the other part. On the surface of copper plate 2 in the upper zone of about 130 mm from the meniscus level 9, Ni plating 12 is applied. In such way, by forcedly cooling the zone just below the meniscus in the mold and weakly cooling the upper and lower zones, the development of surface defect in the cast slab is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for cooling a continuous casting mold and the mold.

(Prior art) In general, water-cooled molds for continuous casting of steel, as shown in FIG. It is configured to cool by passing high-speed cooling water into the cooling water passage 3 as shown by arrow 4. Molten steel is injected into this water-cooled mold 1 from a tundish (not shown) through an immersion nozzle 5. The copper plate 2 is cooled by cooling water 4, and slag 8 is periodically added to the top of the molten steel 6 in the mold 1 to reduce the frictional force between the copper plate 2 and the shelf, which keeps the meniscus 9 warm and floats it apart from the molten steel 6. It also plays the role of absorbing inclusions. The vicinity of the meniscus of such a water-cooled mold for continuous casting of steel is shown in an enlarged manner in FIG. In the molten layer 8a portion, the molten layer solidifies at the boundary between the copper plate 2 and the molten layer 8a, and string-like solidified slag lO called slag rim is produced. Molten slag flows through a gap 11 formed between the slag rim 10, the meniscus shelf, and the copper plate 2.

The copper plate used as the inner wall plate of such a water-cooled mold for continuous casting has the largest heat load at the meniscus level during operation, and the copper plate is easily deformed in the area where the heat load is large. When the copper plate is deformed, the gap between the shell and the copper plate at the meniscus becomes uneven, and the molten slag that flows from this area between the shell and the copper plate becomes uneven. The growth rate of the shell is determined by the amount of heat passing through the mold wall surface, and the shell thickness is thicker in areas where there is less gap between the shell and the copper plate, and thinner in areas where there is a larger gap. On the other hand, tensile stress acts on the shell due to the shrinkage force of the shell itself, as well as the shell shrinkage resistance due to the static iron pressure applied to the shell and the friction between the shell and the copper plate (correctly, the slag film). As a result, the shell easily deforms and cracks occur in the thin parts of the shell, and shell cracks are very often accompanied by locally thick slag film parts called slag streaks.

Conventionally, in the water-cooled mold for continuous casting of the above-mentioned type, for example, Japanese Utility Model Application Publication No. 49-49113 and Japanese Utility Model Application No. 59-49
- As disclosed in Japanese Patent No. 180838, a mold is provided with a plurality of slit-shaped cooling water passages extending in the vertical direction, and the cross-sectional area of the cooling water passage near the meniscus level is reduced to the cooling water passage in the portion below the meniscus level. By narrowing the cross-section of the cooling water passage near the meniscus level by making it smaller than the cross-sectional area, the flow velocity of the cooling water near the meniscus level is increased and heat transfer is improved, thereby increasing the heat removal ability of the mold at the meniscus area. , it has been proposed to prevent local deformation and cracking caused by local temperature increases in the mold.

As in the prior art described above, locally strengthening the cooling of the mold is considered to be an effective means of preventing deformation of the copper plate, but intense cooling of a wide area of the meniscus The cooling of the upper molten slag is strengthened on the wall surface, and the slag rim becomes larger, which obstructs the flow of slag between the shell and the copper plate, increases the friction between the copper plate and the slab, and increases the slag between the copper plate and the slab. Because the film layer is thin, the shell is strongly cooled and the amount of shell shrinkage increases, making it easy to crack, and uneven formation of slag rim causes uneven inflow of molten slag, resulting in surface defects. There is a problem with becoming.

Although many studies have been conducted on cooling methods for water-cooled molds, the above-mentioned problems have occurred because no means have been established to control the cooling of the copper plate.

(Problems to be Solved by the Invention) The present invention provides a controlled cooling method for eliminating the above-mentioned problems, and an upper zone corresponding to the slag layer, a zone corresponding to the meniscus portion, and the like. The aim is to provide a water-cooled mold that allows precise control of the individual cooling intensity of the lower zone.

(Means for Solving the Problems) According to the present invention, a predetermined zone of a continuous casting mold below a portion corresponding to the meniscus level is strongly cooled, and a zone above the portion corresponding to the meniscus level is turned into a predetermined zone below the portion corresponding to the meniscus level. Compared to this, it cools down slightly.

Further, according to the present invention, the cooling resistance is better than that at the meniscus level (strongly cooling the zone below the strong cooling zone below the mouth part) ().

When carrying out the present invention, it is preferable to control the flow rate of the cooling water flowing into the cooling water passages in the two strong cooling zones to be higher than the flow rate of the cooling water in the weak cooling zones.

Further, when carrying out the present invention, it is preferable that the amount of cooling water flowing into the cooling water passage in the strong cooling zone be larger than the amount of cooling water flowing in the weak cooling zone.

Furthermore, according to the present invention, the surface of the mold in contact with molten steel in the strong cooling zone and the weak cooling zone is coated with materials having different thermal conductivities by plating or the like, thereby changing the heat removal ability of the mold, thereby producing a cooling difference. I can do it.

Further, according to the present invention, a cooling difference can be created by coating the surfaces of the cooling water passages in the cooling zone and the weak cooling zone with materials having different thermal conductivities using plating or the like.

Further, according to the present invention, a plurality of horizontally extending cooling water passages are provided which are separated from each other in the longitudinal direction of the continuous casting mold, and the horizontal cooling water passages are cooled in a strong cooling zone below a portion corresponding to the meniscus level. It is preferable that the water flow passage cross-sectional area is larger than the cooling water passage cross-sectional area of the horizontal cooling water passage in the upper and lower weak cooling zones. In this case, the flow rate of the cooling water in the strong cooling zone of the meniscus portion needs to be considerably higher than the flow rate of the cooling water in the upper and lower weak cooling zones.

(Function) According to the present invention, by separately controlling and cooling the meniscus zone and the zone above the meniscus zone of the water-cooled mold, and by accurately controlling the cooling in the height direction of the mold, the molten slag above the meniscus level can be cooled. In the upper zone in contact with the layer, the cooling is moderated, thereby eliminating the formation of slag rim on the mold wall, or minimizing the formation of slag rim, thereby preventing cracking of the shell. I can do it.

In addition, by strongly cooling the zone near the meniscus where the heat load is large, local deformation of the mold is prevented and cracking of the mold wall surface is prevented.

Furthermore, in the lower zone of the mold, the heat load is small and it is no longer necessary to strongly cool the zone directly below the meniscus. For example, cooling in the lower zone 150 m below the meniscus level can be relaxed to increase the melting thickness of the slag film. This reduces the friction between the copper plate and the slab, prevents an excessive drop in the slab surface temperature, and provides good slab surface properties.

(Example) As shown in FIG. 1, a water-cooled mold 1 provided with a long side surface W4 having a plurality of cooling water passages 3 formed by longitudinally extending slits 2 was used to produce a mold with a weight of C: 0. 06%, Mn
: 0.26%, P: 0.021%, Si: 0
．． 015%, Ae: 0.020% low carbon aluminum killed steel, slab size 250 mm x 1600 mr
A slab of No. Q was continuously cast with two trowels at a casting speed of 1.5 m/min, and the occurrence of surface defects was investigated.

In this embodiment, in order to strongly cool the vicinity of the meniscus, the cross-sectional area of the cooling water passage 3 in the zone up to 150 mm below the meniscus level 9 is reduced to reduce the cooling water velocity v, 1. The flow velocity was increased to 1.7 times the velocity v2 of the cooling water in 3.

The surface of the copper plate 2 in the upper zone of 130 mrn from the meniscus level 9 was coated with Ni plating 12 with a thickness of 7 mrn.

As a result of casting by strongly cooling the zone directly below the meniscus of the mold and weakly cooling the upper and lower zones, the surface defects of the cast slab were almost eliminated as shown in FIG. 5. Also,
As shown in FIG. 6, the depth of the oscillation marks on the cast slab was 18 times greater than that of the conventional mold, and surface defects such as transverse cracks caused by the depth of the oscillation marks were significantly improved.

FIG. 2 shows another embodiment of a mold used to carry out the method of the invention. In this example, a plurality of cooling water passages 3 extending in the horizontal direction are vertically separated from each other and provided in the mold 1, and cooling water passages 3a in a strong cooling zone a in a range of about 150 an below the area equivalent to the meniscus level are provided. The cross-sectional area of the cooling water passage 3b in the upper and lower weak cooling zones and C
and 3c. The mold of this example is cooled by controlling the flow rate of the cooling water in the cooling water passage 3a to be considerably higher than the flow rate of the cooling water in the cooling water passages 3b and 3c.

(Effects of the Invention) According to the present invention, deformation and cracking of the mold copper plate can be prevented, breakout of the slab can be prevented, the depth of oncillation marks can be reduced, and high quality slabs without surface defects can be produced. It is possible to manufacture

[Brief explanation of drawings]

FIG. 1 is a partial vertical cross-sectional view showing an example of a mold according to the present invention, FIG. 2 is a partial vertical cross-sectional view showing another embodiment of the mold according to the present invention, and FIG. 3 is a partial vertical cross-sectional view showing an example of a mold according to the present invention. A schematic vertical cross-sectional view showing the state in which pieces are continuously cast; Figure 4 is an enlarged cross-sectional view of the meniscus portion shown in Figure 3; Figure 5 is an example using the mold of the present invention shown in Figure 1. A graph showing the incidence of surface cracks in slabs in comparison with conventional examples. Figure 6 is a graph showing the oscillation depth of slabs in an example using the mold of the present invention shown in Figure 1 in comparison with conventional examples. This is a graph showing. ... Mold ... Cooling water passage ... Immersion nozzle ... Shell ... Meniscus level 2 ... Copper plate 4 ... Cooling water 6 ...? Yongsteel 8...Slag lO...Slag rim

Claims (1)

[Claims] 1. For continuous casting, characterized in that a predetermined zone of the mold below the area corresponding to the meniscus level is strongly cooled, and a zone above the area corresponding to the meniscus level is cooled weakly compared to the area below the area equivalent to the meniscus level. How to cool the mold. 2. The cooling method according to claim 1, characterized in that a zone below the strong cooling zone is cooled weakly compared to the strong cooling zone. 3. The cooling method according to claim 1 or 2, characterized in that the flow rate of the cooling water flowing into the cooling water passage in the strong cooling zone is made higher than the flow rate of the cooling water in the weak cooling zone. 4. The cooling method according to claim 1 or 2, wherein the amount of cooling water flowing into the cooling water passage in the strong cooling zone is made larger than the amount of cooling water flowing in the weak cooling zone. 5. The cooling method according to claim 1 or 2, wherein the mold surface facing the molten steel and/or the surface of the cooling water passage have better heat conduction in the strong cooling zone than in the weak cooling zone. 6. A plurality of horizontally extending cooling water passages separated from each other in the vertical direction of the mold are provided, and the cooling water flow cross-sectional area of the horizontal cooling water passage in the strong cooling zone below the area equivalent to the meniscus level is divided upwardly and downwardly. A mold for continuous casting, characterized in that the cross-sectional area of the horizontal cooling water passage in the cooling zone is larger than that of the cooling water channel.