JP7151665B2 - Water-cooled mold for continuous casting and continuous casting method for steel - Google Patents

Description

INDUSTRIAL APPLICABILITY The present invention makes it possible to cool a slab with a uniform and high heat flux in the width direction and the casting direction of the slab even during high-speed casting with a slab withdrawal speed of 2.0 m/min or more. The present invention relates to a water-cooled mold for continuous casting, and further to a continuous casting method for steel using this water-cooled mold for continuous casting.

In continuous steel casting, molten steel is injected into the inner space of a water-cooled mold for continuous casting (hereinafter simply referred to as "mold"), and the molten steel is cooled by contact with the mold surface to form a solidified shell (solidified shell). A steel slab is manufactured by continuously drawing out a slab, which is to be the outer shell, from below the mold while being supported by slab support rolls such as support rolls and guide rolls. The mold used generally has a high thermal conductivity copper plate (copper or copper alloy mold plate) on the molten steel side, and a backup plate made of steel or the like on the opposite side of the molten steel side. The copper plate has an internal water-cooling structure in which cooling grooves (also called "cooling slits") are formed in the casting direction to serve as flow paths for cooling water. A supply channel and a drain channel for cooling water are provided on the backup plate side.

In recent years, efforts have been made to improve production efficiency, and operations are generally carried out at an extremely high speed of 2.0 m/min or more for the cast strip withdrawal speed (also referred to as “casting speed”). As the strip withdrawal speed increases, the thickness of the solidified shell becomes thinner on the exit side of the mold, and non-uniform growth is more likely to occur. As a result, longitudinal cracks occur on the surface of the cast slab, and a major operational problem called breakout occurs, in which the solidified shell breaks and molten steel leaks from the lower end of the mold.

If the above-mentioned vertical cracks occur on the surface of the slab, it is necessary to remove the surface flaws in the steelmaking process prior to hot rolling by using a scarfer or grinder, etc. Direct heating, direct rolling, etc. cannot be performed, which is a hindrance to productivity improvement.

In order to prevent the occurrence of longitudinal cracks and breakouts in such continuously cast slabs, the thickness of the solidified shell can be made to grow rapidly and uniformly in the initial stage of solidification, and the thickness can be made sufficient until reaching the exit side of the mold. It is required to realize a slab cooling technology in such a mold.

Numerous measures have been proposed to avoid longitudinal cracks and breakouts due to non-uniform cooling in the mold.

For example, Patent Document 1 discloses a water-cooled continuous casting mold for high-speed casting with a cast strip withdrawal speed of 1.3 m / min or more, in which at least the upper half of the mold surface in contact with molten steel has (1) a horizontal direction The width or diameter of the concave portion: more than 3 mm and 80 mm or less, (2) the depth of the concave portion: 100 to 1000 μm, (3) the area ratio of the concave portion: 50 to 95%. It is Patent Document 1 is a technique of weakening the heat flux near the meniscus (also referred to as the "molten steel surface in the mold") to equalize the amount of heat removal near the meniscus, thereby preventing longitudinal cracks in the cast slab.

In Patent Document 2, a metal or non-metal having thermal conductivity with a ratio to the thermal conductivity of the copper plate of the mold is in a predetermined range. For continuous casting, having a plurality of them in a region including a meniscus, and having a predetermined value of thermal resistance between the mold copper plate surface and the groove-shaped water channel (cooling slit) of the mold copper plate at the position where the dissimilar material filling part is installed. A template has been proposed. Patent document 2 periodically increases or decreases the heat flux from the solidified shell to the continuous casting mold at the initial stage of solidification, and by increasing or decreasing the heat flux, the stress and thermal stress due to steel transformation acting on the solidified shell are reduced. , to suppress the occurrence of cracks on the surface of the solidified shell.

In Patent Document 3, the grooved water channel (cooling slit) of the mold is divided into a lower part and an upper part, and the flow velocity of the cooling water in the lower part of the mold is made faster than the flow velocity of the cooling water in the upper part of the mold. A continuous casting method has been proposed in which the heat removal amount ratio (lower heat removal amount/upper heat removal amount) is set to 1.47 or more. Patent document 3 is a technique of slow cooling the upper part of the mold and intensive cooling the lower part of the mold, thereby preventing longitudinal cracks in medium carbon steel and increasing the thickness of the solidified shell.

In Patent Document 4, by reducing the flow velocity of cooling water in a grooved water channel (cooling slit) of a water-cooled mold for continuous casting to 5 m / s or less, the temperature on the copper plate side of the grooved water channel (cooling slit) is reduced to It has been proposed to increase the heat flux by bringing the heat transfer regime above the saturation temperature of the nucleate boiling heat transfer regime. Patent Document 4 discloses a technique of increasing the heat flux by shifting the normal heat transfer mode of a water-cooled mold for continuous casting from the forced convection heat transfer zone to the nucleate boiling heat transfer zone.

JP-A-9-94634 JP 2017-39165 A JP-A-10-58093 JP-A-3-81049

However, the above prior art has the following problems.

In Patent Document 1, grooves are formed on the surface of the copper plate that contacts the molten steel, and the grooved surface of the copper plate wears due to friction with the solidified shell. Therefore, as the number of times the mold is used increases, the depth of the groove decreases, and the effect of grooving cannot be obtained. Moreover, in order to fully enjoy the effect of grooving, it is necessary to limit the number of times the mold is used, which increases the production cost of the steel slab. In addition, the existence of an air layer or the like between the mold copper plate and the solidified shell due to the unevenness provided on the mold surface weakens the cooling, so there is a limit to high-speed casting.

In Patent Document 2, since the depressions on the mold copper plate surface are filled with a metal or non-metal having a predetermined thermal conductivity, the problem in Patent Document 1 is alleviated, but the strip drawing speed is reduced to 2.0 m/m. In order to increase the thickness to min or more, there is a problem that the cooling of the mold is insufficient and a sufficient solidified shell thickness cannot be secured at the lower end of the mold.

In Patent Document 3, a plurality of longitudinal groove-shaped water channels (cooling slits) extending along the casting direction are arranged side by side in the width direction of the mold, and are formed on the mold surface at the position where the cooling slits are formed. There is a problem that the amount of heat removal is different on the mold surface at the position where it is not. In addition, in order to cope with high-speed casting, the more heat is removed by increasing the flow rate of the cooling water, the greater the difference in heat removal in the width direction of the mold, and the more uniform the width and casting direction of the slab. cooling becomes difficult. Furthermore, uniform cooling is difficult during high-speed casting where the cast strip drawing speed is 2.0 m/min or more. In addition, no cooling channel is formed at the boundary between the upper and lower parts of the mold, which prevents uniform cooling in the casting direction.

As a result of confirming the method of Patent Document 4 based on the temperature measurement value of the thermocouple embedded in the mold copper plate in the actual machine, the method of Patent Document 4 is that the cooling water channel is a grooved water channel (cooling slit). Therefore, it was found that the difference in the amount of heat removal in the width direction of the mold and in the casting direction is likely to occur, and that it becomes remarkable especially when the slab withdrawal speed changes.

Specifically, when the slab withdrawal speed decreases and the amount of heat transferred to the mold decreases, the susceptibility to the generation of bubbles, which is the starting point of nucleate boiling, varies significantly depending on the location. , the heat transfer regime changes from the nucleate boiling heat transfer regime to the forced convection heat transfer regime. When the heat transfer mode changes to the forced convection heat transfer region, the amount of heat removed from the slab by the mold decreases, so the growth of the solidified shell, especially on the short sides of the slab where the discharge flow from the submerged nozzle directly collides, is insufficient. However, it was confirmed that the occurrence of bulging and breakout, in which the solidified shell on the short side of the slab expands directly below the mold, cannot be completely suppressed.

The present invention has been made in view of the above circumstances, and its object is to achieve uniformity in the width direction and casting direction of the slab even during high-speed casting where the slab withdrawal speed is 2.0 m/min or more. It is to provide a water-cooled mold for continuous casting that is capable of cooling a slab with a high heat flux and is capable of stable high-speed casting and suppression of vertical cracks in the slab using this water-cooled mold for continuous casting. It is to provide a continuous casting method that achieves both.

The gist of the present invention for solving the above problems is as follows.
[1] A mold plate made of a copper alloy in which a cooling channel is formed;
A water-cooled mold for continuous casting, comprising a backup plate attached to the mold plate so as to cover the cooling water channel,
The cooling water channel in the lower part of the mold plate is composed of a plurality of groove-shaped water channels (cooling slits) arranged in the width direction of the mold, and each of the plurality of groove-shaped water channels has a vertically long shape extending in the casting direction. ,
The cooling water channel in the upper part of the mold plate is composed of a box-shaped water channel extending in the casting direction and widening in the width direction of the mold, and the box-shaped water channel communicates with a plurality of cooling water channels in the lower part of the mold plate,
A water-cooled mold for continuous casting, characterized in that a plurality of recesses are provided on an inner wall surface of a mold plate forming a box-shaped channel in the upper part of the mold plate.
[2] The water-cooled mold for continuous casting according to [1], wherein the recesses are provided periodically at regular intervals.
[3] The recesses have the same shape selected from a hemispherical shape, a conical shape, a truncated cone shape, a cylindrical shape, a spire shape, and a pyramid shape, in the above [1] or the above [ 2], the water-cooled mold for continuous casting.
[4] The water-cooled mold for continuous casting according to any one of [1] to [3], wherein the recesses are provided in a zigzag pattern.
[5] The water-cooled mold for continuous casting according to any one of [1] to [4], wherein the recess has a depth of 0.1 to 2.0 mm.
[6] Any one of [1] to [5] above, wherein a part of the plurality of recesses is embedded with a material having a thermal conductivity different from that of the mold plate. A water-cooled mold for continuous casting according to claim 1.
[7] The above-mentioned [6], characterized in that the recess embedded with a material having a thermal conductivity different from that of the mold plate has a smooth surface with respect to the surroundings. Water-cooled mold for continuous casting.
[8] The water-cooled mold for continuous casting according to [6] or [7] above, wherein the thermal conductivity of the material is lower than the thermal conductivity of the mold plate.
[9] The recesses in which the material is not embedded are arranged in a row in the width direction of the mold, the recesses in which the material is embedded are arranged in a row in the width direction of the mold, and the recesses in which the material is not embedded are arranged in the mold. A group in which the recesses embedded with the material are arranged in a row in the width direction and a group in which the recesses in which the material is embedded are arranged in a row in the width direction of the mold are alternately arranged in the casting direction [6] ] to the water-cooled mold for continuous casting according to any one of the above [8].
[10] From [1] to [ 9] The water-cooled mold for continuous casting according to any one of the above.
[11] Any one of [1] to [10] above, wherein the flow velocity of the cooling water in the cooling water passage under the mold plate is in the range of 7.0 to 13.0 m/s. A water-cooled mold for continuous casting according to .
[12] A steel continuous casting method for continuously casting molten steel using the water-cooled mold for continuous casting according to any one of [1] to [11], wherein at least part of the casting from the start of casting to the end of casting A continuous casting method for steel, characterized in that continuous casting is performed in a period of with a cast strip drawing speed of 2.0 m / min or more.

According to the present invention, the cooling water channel in the upper part of the mold plate of the water-cooled mold for continuous casting is formed by a box-shaped water channel extending in the casting direction and widening in the width direction of the mold. Since multiple recesses are provided on the wall surface, uniform strong cooling can be achieved in the upper part of the water-cooled mold for continuous casting even under high-speed casting conditions or when the slab withdrawal speed changes. can be grown to a uniform and sufficient thickness at an early stage. As a result, it becomes possible to prevent the occurrence of longitudinal cracks and breakouts in the cast slab.

1 is a perspective view of a water-cooled mold for continuous casting; FIG. FIG. 4 is a diagram showing the relationship between the degree of superheat of the heat transfer surface of the object to be cooled and the heat flux with respect to water under atmospheric pressure. BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic which shows an example of embodiment of the water-cooling mold for continuous casting which concerns on this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic which shows an example of embodiment of the water-cooling mold for continuous casting which concerns on this invention. FIG. 4 is a schematic diagram showing another example of an embodiment of the water-cooled mold for continuous casting according to the present invention; It is a figure which shows the comparison of the mold heat removal index. FIG. 4 is a diagram showing a comparison of longitudinal crack occurrence indices on the surface of a cast slab.

The present invention will be specifically described below with reference to the accompanying drawings.

Before describing the present invention, a brief description of the continuous casting method for steel will be provided. A perspective view of a water-cooled mold for continuous casting is shown in FIG. A continuous casting water-cooled mold 1 (hereinafter also simply referred to as "mold 1") for continuously casting slab slabs includes a pair of mold long sides 2 facing each other and a pair of molds sandwiched between the mold long sides 2. short sides 3; A tundish (not shown) for containing molten steel 5 is arranged above the mold 1, and an immersion nozzle 4 is installed at the bottom of the tundish. A pair of mold long sides 2 and a pair of mold short sides 3 form a rectangular internal space in the mold 1, and an immersion nozzle 4 is inserted into the internal space. As will be described later, the sides of the mold long side 2 and the mold short side 3 that come into contact with the molten steel 5 are made of a copper alloy mold plate, and a backup plate is arranged on the back side of this mold plate.

A copper alloy mold plate forming the mold long side 2 and the mold short side 3 has a cooling water channel formed on the back side of the surface in contact with the molten steel 5. is cooling. Molten steel 5 is injected into the interior space of the mold 1 through a submerged nozzle 4 , and the molten steel 5 is cooled and solidified by the mold 1 to form a solidified shell on the contact surface with the mold 1 . A cast slab having the solidified shell as the outer shell and the unsolidified molten steel 5 as the inner part is continuously drawn downward from the mold 1 to produce a steel slab cast slab. In the mold 1, by contacting the molten steel 5 and the high-temperature slab, the surface temperature of the mold plate (temperature on the side in contact with the molten steel) rises, and the vicinity of the meniscus in the mold (surface of the molten steel in the mold) indicates the highest value. As the mold plate, a copper alloy with high deformation resistance to thermal stress and high thermal conductivity that can enhance the cooling effect of cooling water is used.

A heat transfer mode when the surface of an object to be cooled such as a mold plate installed in the water-cooled mold 1 for continuous casting is cooled with cooling water will be described. FIG. 2 shows the relationship between the degree of superheat of the heat transfer surface of the object to be cooled and the heat flux with respect to water under atmospheric pressure. Here, the degree of superheat of the heat transfer surface of the object to be cooled is a value indicating how much the heat transfer surface of the object to be cooled is superheated with respect to the saturation temperature of water. Expressed as the difference from the saturation temperature. Incidentally, the saturation temperature of water is the temperature at which water boils under a certain pressure. Thermal Engineering, P. 231, February 1, 1979, Shokabo).

As shown in FIG. 2, in general, there are four heat transfer modes in cooling with cooling water (water cooling): forced convection heat transfer, nucleate boiling heat transfer, transition boiling heat transfer, and film boiling heat transfer. classified into one. The forced convection heat transfer region is a heat transfer mode when the degree of superheat of the heat transfer surface is below a certain value (for example, 4° C. or below), and the heat transfer is due to the sensible heat of water without boiling of water. Although FIG. 2 does not show the range where the superheat of the heat transfer surface is less than 1° C., this range is also a forced convection heat transfer region.

When the superheat of the heat transfer surface exceeds a certain value (for example, 4° C.) and boiling of water (referred to as “nucleate boiling”) begins, the nucleate boiling heat transfer region is entered. The nucleate boiling heat transfer region is a form in which bubbles are generated from foaming points randomly distributed on the heat transfer surface. The flux is 10 to 100 times larger than the forced convection heat transfer region, and the heat transfer efficiency is very high. If the degree of superheating of the heat transfer surface is further increased, the bubbles will coalesce, and the coalesced bubbles will hinder heat transfer. The heat flux at the upper limit of this nucleate boiling is called the critical heat flux point.

When the heat transfer surface superheat exceeds the critical heat flux point, the transition boiling heat transfer region is entered, and the heat flux decreases as the heat transfer surface superheat increases. When the heat transfer surface superheat is further increased, the film boiling heat transfer region is entered, and the heat flux increases with the increase in the heat transfer surface superheat. In the film boiling heat transfer region, the generated bubbles grow to form an air film on the heat transfer surface, and an air layer with a lower heat transfer coefficient than water is formed on the heat transfer surface. As a result, the heat transfer efficiency decreases. In the transition boiling heat transfer region, film boiling and nucleate boiling are considered to coexist spatially and temporally.

The present invention aims to make the upper portion of the mold plate a nucleate boiling heat transfer area by slowing down the flow rate of the cooling water, and the lower portion of the mold plate a forced convection heat transfer area as in the prior art.

Next, an embodiment of the water-cooled mold for continuous casting according to the present invention will be described.

3 and 4 are schematic diagrams showing an example of an embodiment of a water-cooled mold for continuous casting according to the present invention, and FIG. 4 is a schematic view of the back surface of the mold plate that constitutes the long side of the mold (the surface on which the cooling water channel is installed). In addition, FIG. 4 shows a part of the mold plate in the width direction of the mold.

As shown in FIG. 3 , the mold long side 2 has a mold plate 6 and a backup plate 7 , and the backup plate 7 is provided with stud bolts (see FIG. not shown) or the like. The backup plate 7 is provided with a cooling water supply path 13 in its lower portion and a cooling water drainage path 15 in its upper portion. A water supply port 14 is formed on the mold plate 6 side of the cooling water supply channel 13 and spreads in the mold width direction. A drain port 16 is formed. On the back surface of the backup plate 7 (the surface on the back side of the surface in contact with the mold plate 6), two-stage water boxes (not shown) on the water supply side and the water discharge side are installed. A drainage channel 15 communicates with each water box.

As shown in FIGS. 3 and 4, the cooling water channels of the mold plate 6 are different on the top and bottom in the casting direction, and the cooling water channels on the bottom of the mold plate 6 in the casting direction are a plurality of groove-shaped water channels arranged in the width direction of the mold. 8 (cooling slits), and each of the plurality of groove-shaped channels 8 has a vertically long shape extending in the casting direction. On the other hand, the cooling water channel in the upper portion of the mold plate 6 in the casting direction is composed of a box-shaped water channel 9 extending in the casting direction and widening in the width direction of the mold. This box-shaped channel 9 communicates with a plurality of channel-shaped channels 8 in the lower part of the mold plate 6 at connection points 10 . On the back side of the mold plate 6, the groove-shaped channel 8 and the box-shaped channel 9 having such a configuration are installed in the width direction of the mold. In FIG. 4, seven groove-shaped waterways 8 communicate with one box-shaped waterway 9, but the number of groove-shaped waterways 8 to which one box-shaped waterway 9 communicates is not limited to seven. Any number may be used as long as it is plural.

By attaching the backup plate 7 to the mold plate 6, the water supply port 14 and the water discharge port 16 formed in the backup plate 7 communicate with the groove-shaped water channel 8 and the box-shaped water channel 9 formed in the mold plate 6, and the backup plate A cooling channel from 7 to the mold plate 6 and a cooling channel from the mold plate 6 to the backup plate 7 are formed. Cooling water is supplied from the cooling water supply channel 13 to the groove-shaped water channel 8 below the mold plate 6, rises inside the groove-shaped water channel 8, passes through the box-shaped water channel 9 above the mold plate 6, and enters the cooling water. It is discharged from the drainage channel 15 .

Since the upper part of the water-cooled mold 1 for continuous casting is the part in contact with the injected high-temperature molten steel 5, the range of 250 mm from the upper end of the mold including the meniscus in particular should have a high cooling capacity for high-speed casting. is desired. Therefore, in the present invention, the upper part of the mold plate 6 utilizes nucleate boiling heat transfer to enhance heat removal.

In order to generate nucleate boiling heat transfer, it is necessary to set the flow velocity of the cooling water to a low value. It is said that the heat transfer mode changes to Therefore, in the present invention, the cooling water passage in the upper part of the mold plate 6, that is, the upper part of the mold 1 is formed as a box-shaped water passage 9 extending in the casting direction and widening in the width direction of the mold to intentionally reduce the flow velocity of the cooling water. ing.

Nucleate boiling occurs starting from bubbles generated on the inner wall surface of the cooling channel of the mold plate 6, but the frequency of bubble generation depends on the roughness and shape of the inner wall surface of the cooling channel. In particular, it is known that bubbles are preferentially generated at the recesses on the cooling surface.

Therefore, in the present invention, a plurality of recesses 11 are provided on the inner wall surface of the mold plate 6 that constitutes the box-shaped channel 9 on the upper part of the mold plate (hereinafter also referred to as "the inner wall surface of the box-shaped channel") to promote the generation of bubbles. Let By providing the concave portion 11, the nucleate boiling state is maintained for a while even if the generation frequency of bubbles increases and the cast strip withdrawal speed decreases and the amount of heat transferred to the mold 1 decreases. As a result, the heat removal by the mold 1 does not decrease rapidly, and the bulging phenomenon and breakout of the slab can be suppressed.

Also, if the recesses 11 are periodically provided on the inner wall surface of the box-shaped water channel 9 at regular intervals, bubbles are generated corresponding to the positions of the recesses 11 that are provided, resulting in a strong cooling effect due to nucleate boiling heat transfer. can be enjoyed uniformly on the inner wall surface of the box-shaped waterway 9. Therefore, it is preferable to provide the concave portions 11 periodically at regular intervals.

Here, it is preferable that the recesses 11 have the same shape. By using the same shape, the recess 11 can be easily formed on the inner wall surface of the box-shaped channel 9 while suppressing variations.

As described above, bubbles are generated preferentially at the recesses 11 of the cooling surface, and the ease with which bubbles are generated depends on the shape of the recesses 11 . In the present invention, since bubbles are generated uniformly within the cooling surface of the mold to achieve uniform and strong cooling of the slab, a state in which bubbles are generated in a concentrated manner at a specific location is not desirable. Therefore, it is preferable to arrange the shapes so that all the concave portions 11 have the same bubble generating ability. That is, the shape of the concave portion 11 is preferably one and the same shape selected from a hemispherical shape, a conical shape, a truncated cone shape, a cylindrical shape, a spire shape, and a pyramid shape.

The recesses 11 formed on the inner wall surface of the box-shaped waterway 9 are arranged in a row in the width direction of the mold with the same casting direction distance from the upper end of the mold (width direction row), and this width direction row is set in the casting direction from the upper end of the mold. A group of concave portions 11 is formed by providing a plurality of steps with different distances. Here, the groups of recesses 11 forming widthwise rows are preferably arranged in a staggered arrangement with respect to groups of recesses 11 forming widthwise rows adjacent to each other in the casting direction. That is, the group of recesses 11 in the width direction row of a certain stage is formed at half the width direction pitch of the group of recesses 11 in the width direction row of the upper and/or lower stage.

By arranging the groups of recesses 11 in a staggered manner in this way, heat can be removed with little difference on the inner wall surface of the box-shaped water channel 9, particularly on the inner wall surface in the width direction of the mold. In addition, the spacing in the mold width direction between the centers of recesses 11 adjacent in the mold width direction and the spacing in the casting direction between the centers of groups of recesses 11 in rows in the width direction are both 0.5 to 0.5 times the diameter of the recesses 11. It is preferably within the range of 4.0 times.

The present inventors conducted laboratory tests to examine the relationship between the shape of the cooling surface and the ease with which bubbles are generated. It was found that air bubbles were generated. Therefore, the depth of the hemispherical, conical, truncated conical, cylindrical, spire-shaped, or pyramid-shaped recess 11 formed on the inner wall surface of the box-shaped waterway 9 can be 0.1 to 2.0 mm. preferable. When the depth of the concave portion 11 is less than 0.1 mm, it is difficult to generate air bubbles. This is because the heat transfer behavior of the inner wall surface of the mold plate 6 is affected. Here, it is preferable that the diameter of the concave portion 11 is 2 to 20 mm.

As described above, the lower part of the mold plate 6, that is, the lower part of the mold 1, is cooled by the conventional system, that is, by the grooved water channels 8 whose longitudinal direction is the casting direction. Since the mold lower part is located away from the meniscus, it does not come into direct contact with the molten steel 5, and the temperature of the solidified shell is lower than near the meniscus. In addition, the heat flux is also low, less than half near the meniscus, and even if there is variation in the heat flux in the width direction of the mold, the effect on the cooling of the slab is small. For this reason, the lower part of the mold plate 6 can be cooled by the conventional shape, that is, the groove-shaped water channel 8 in which the cooling water channel is formed into a slit.

The flow velocity of the cooling water flowing through the channel 8 below the mold plate 6 is preferably within the range of 7.0 to 13.0 m/s. By increasing the flow velocity of the cooling water to 7.0 m/s or more, the heat transfer due to the forced convection of the cooling water is promoted, and the heat removal effect can be enhanced. More desirably, when a flow velocity of 10 m/s or more is ensured, a solidified shell that does not cause deformation fracture at the lower end of the mold can be formed more stably even during high-speed casting.

On the other hand, in order to increase the flow velocity, it is necessary to reduce the cross-sectional area of the cooling water channel by narrowing the width of the grooved water channel 8. Problems such as an increase in height or difficulty in machining narrow width slits in the mold plate 6 arise. The flow velocity can also be increased by supplying a high flow rate of cooling water at high pressure, but in that case, equipment investment is required to enable an increase in the flow rate and supply pressure of the cooling water. For these reasons, the flow velocity of the cooling water flowing through the channel 8 is preferably 13.0 m/s or less.

A connection position 10 between the box-shaped channel 9 in the upper part of the mold 1 and the channel-shaped channel 8 in the lower part of the mold 1 is preferably within a range of 150 to 500 mm from the upper end of the mold. The full length of the mold 1 used for continuous casting of slab slabs is generally 800 to 1000 mm, and the meniscus position is 50 to 200 mm below the upper end of the mold. Looking at the heat flux distribution in the casting direction in the mold 1, the heat flux drops significantly at a position about 100 mm below the meniscus position (about 150 mm to 300 mm from the upper end of the mold) in the casting direction, and then gradually toward the lower end of the mold. declining. Further, below about 300 mm from the meniscus (about 350 to 500 mm from the upper end of the mold), the heat flux drops to about half of the heat flux near the meniscus. From these, in the present invention, the preferred position of the connection position 10 is set to be in the range of 150 to 500 mm from the upper end of the mold. More desirably, the position is 200 to 350 mm from the upper end of the mold.

The inventors of the present invention conducted extensive research to achieve more uniform and strong cooling, and came up with a more suitable water-cooled mold 1 for continuous casting. That is, by embedding a material having a thermal conductivity different from that of the mold plate 6 in a part of the recess 11 provided on the inner wall surface of the box-shaped water channel 9 in the upper part of the mold 1, even under high-speed casting It has been found that it is possible to achieve cooling and extremely reduce the occurrence of surface cracks and breakouts in the cast slab.

A portion in which the concave portion 11 is filled with a material having a lower thermal conductivity than the mold plate 6 made of a copper alloy has a relatively high thermal resistance. Therefore, if a portion filled with a material having a lower thermal conductivity than the mold plate 6 is provided periodically, the thermal resistance of the inner wall surface of the box-shaped waterway 9 also exhibits a distribution that periodically increases and decreases. This creates a heat flux distribution in which the heat flux from the solidified shell to the mold plate 6 periodically increases and decreases.

On the other hand, the portion where the concave portion 11 is filled with a material having a higher thermal conductivity than the copper alloy mold plate 6 has a relatively low thermal resistance, but the heat flux from the solidified shell to the mold plate 6 is periodic. A heat flux distribution that increases and decreases dynamically is formed in the same way as when a material having a lower thermal conductivity than the mold plate 6 is embedded. That is, if a portion filled with a material having a higher thermal conductivity than the mold plate 6 is provided periodically, the thermal resistance of the inner wall surface of the box-shaped waterway 9 also exhibits a distribution that periodically increases and decreases.

In this way, by periodically increasing and decreasing the heat flux in the upper part of the mold 1, the stress due to the transformation and heat shrinkage associated with solidification is dispersed in the region of low heat flux, so that surface cracks occur on the surface of the solidified shell. is suppressed.

Therefore, in the present invention, it is preferable to embed a material having thermal conductivity different from that of the mold plate 6 in part of the recess 11 provided in the inner wall surface of the box-shaped waterway 9 . Furthermore, after embedding a material having a thermal conductivity different from that of the mold plate 6 in a part of the recess 11 provided on the inner wall surface of the box-shaped waterway 9, it is ground or polished to the surroundings. A smooth surface is more preferable. As described above, the frequency of bubble generation depends on the roughness and shape of the inner wall surface of the cooling channel. From this point of view, if a material having a thermal conductivity different from that of the mold plate 6 is embedded and then ground or polished to make a smooth surface with respect to the surroundings, the inner wall surface of the box-shaped waterway 9 can be positioned It can be easily formed into a constant property without variation due to

FIG. 5 shows a schematic diagram of another example of the embodiment of the water-cooled mold for continuous casting according to the present invention. FIG. 5 shows that a portion of the recess 11 is filled with a material having a thermal conductivity different from that of the mold plate 6, and then ground or polished to form a smooth surface with respect to the surroundings (hereinafter referred to as “heterogeneous material”). It is a schematic view of the back surface of the mold plate 6 (the surface on which the cooling water channel is installed) in which a material filling portion 12'' is formed.

In particular, when a material having a lower thermal conductivity than the copper alloy mold plate 6 is regularly embedded, the dissimilar material filling portion 12 is closer to the cooling water than the copper alloy mold plate 6. The surface temperature of the surface becomes high, and it is likely to become the generation position of bubbles that are the origin of nucleate boiling.

Therefore, when a material having a thermal conductivity lower than that of the mold plate 6 is embedded in a part of the recess 11 provided on the inner wall surface of the box-shaped channel 9 on the upper part of the mold plate, bubbles that cause nucleate boiling is generated from both the concave portion 11 in which the material is not embedded and the dissimilar material-filled portion 12 in which a material having a thermal conductivity lower than that of the mold plate 6 is embedded. In other words, the number of bubble generating positions in the box-shaped water channel 9 does not change so much depending on whether or not the material with low thermal conductivity is embedded, and the high cooling capacity due to nucleate boiling is maintained as it is.

As a preferred embodiment of the present invention, a group of widthwise rows of recesses 11 not embedded with a material having a thermal conductivity different from that of the mold plate 6 and A group of rows in the width direction of the dissimilar material filled portions 12 in which a material having thermal conductivity is embedded may be alternately arranged in the casting direction.

By arranging in this way, the stress of the solidified shell generated in the group of widthwise rows of recesses 11 not filled with a material having a thermal conductivity different from that of the mold plate 6 is directly under the casting direction. Distributed in groups of widthwise rows of dissimilar material-filled portions 12 embedded with a material having a thermal conductivity lower than that of the mold plate 6 provided, the stress in the solidified shell can be relieved. By repeatedly withdrawing the solidified shell from the mold 1, uniform cooling can be achieved even under high-speed casting, and the occurrence of surface cracks and breakouts in the slab can be greatly reduced.

In the practice of the present invention, it is necessary to maintain the temperature of the mold plate 6 at a high temperature and cause nucleate boiling heat transfer. Industrial water may be used as long as the water quality can be reliably controlled.

When continuously casting the molten steel 5 using the water-cooled mold 1 for continuous casting according to the present invention having the above configuration, the cast strip withdrawal speed is set to 2.0 m / min for at least a part of the period from the start of casting to the end of casting. Continuous casting is performed as described above.

As described above, according to the present invention, the cooling water channel in the upper part of the mold plate of the water-cooled mold for continuous casting is formed by a box-shaped water channel extending in the casting direction and widening in the width direction of the mold. Since a plurality of recesses are provided on the inner wall surface, even under high-speed casting conditions or when the slab withdrawal speed changes, it is possible to achieve uniform and strong cooling in the upper part of the water-cooled mold for continuous casting. The shell can be grown uniformly and to a sufficient thickness at an early stage.

Although the above description has been made with respect to the continuous casting water-cooled mold used in continuously casting the slab slab, the application of the present invention is limited to the continuous casting water-cooled mold used in the continuous casting of the slab slab. Instead, as long as it is a water-cooled mold for continuous casting having a copper alloy mold plate and a backup plate attached to the mold plate so as to cover the cooling water channel formed in the mold plate, the application of the present invention is It is possible.

The range from the upper end of the mold to 250 mm downstream in the casting direction is a box-shaped water channel as shown in FIGS. Molten steel was continuously cast using a water-cooled mold to produce slab slabs. This is referred to as Inventive Example 1. On the other hand, a conventional example is a case in which molten steel is continuously cast using a water-cooled continuous casting mold composed of a plurality of grooved channels (cooling slits) from the upper end to the lower end of the mold to produce a slab.

In the present invention example 1 and the conventional example, the following conditions were used. In Example 1 of the present invention, a hemispherical recess having a diameter of 2.0 mm and a depth of 2.0 mm was formed on the inner wall surface of the box-shaped water channel, and the distance between the centers of the recesses adjacent to each other in the width direction of the mold was 4.0 mm. The rows were arranged in a zigzag pattern with a casting direction distance of 2.0 mm. The flow velocity of the cooling water in the cooling channels was controlled to 5.0 m/s in the range of the box-shaped channels above the mold and 10.0 m/s in the range of the groove-shaped channels below the mold. In the conventional example, the flow velocity of the cooling water in the cooling water passage was controlled to 10.0 m/s over the entire range from the upper end to the lower end of the mold.

Further, as shown in FIG. 5, a group of rows of recesses in the width direction in which a material having a thermal conductivity different from that of the mold plate is not embedded in the inner wall surface of the box-shaped water channel, and a thermal conductivity of the mold plate. A group of widthwise rows of dissimilar material filled parts embedded with metal nickel (thermal conductivity at 20 ° C.; 90 W / (m × K)) as a material with a thermal conductivity lower than the conductivity, in the casting direction Example 2 of the present invention is a case in which they are alternately installed. The installation intervals of the concave portion and the dissimilar material filled portion and the flow velocity of the cooling water in the cooling water passage in Example 2 of the present invention are the same as those of Example 1 of the present invention. The metal nickel was embedded in the recesses by plating. The mold plate is made of a copper alloy having a thermal conductivity of 360 W/(m×K) at 20° C. in each of the present invention example 1, the present invention example 2, and the conventional example.

Using the above three types of water-cooled molds for continuous casting, a slab slab of carbon steel with a slab width of 1000 to 1600 mm and a slab thickness of 260 mm and a carbon content of 0.10% by mass is produced at 2.0 m / Continuous casting was performed at a billet withdrawal speed of min. The continuous casting machine used was a two-strand slab continuous casting machine. A water-cooled mold for casting was installed, and under the same casting conditions such as slab width, slab withdrawal speed and type of mold powder used, Inventive Example 1 and Inventive Example 2 were compared with the conventional example.

FIG. 6 shows a comparison of the mold heat removal index under the casting conditions of a cast strip withdrawal speed of 2.0 m/min. The amount of heat extracted from the mold is a value calculated using the temperature difference between the inlet side water temperature (temperature in the supply side water box) and the outlet side water temperature (temperature in the drain side water box) of the mold cooling water during continuous casting and the amount of cooling water. With the value of the conventional example as the standard (1.0), the ratio with the value of the conventional example was shown as an index of the heat removal amount from the mold. As shown in FIG. 6, it was confirmed that, in Inventive Example 1 and Inventive Example 2, the heat extraction index could be improved by 15 to 25% while the amount of cooling water remained the same as in the conventional example.

In addition, FIG. 7 shows a comparison of longitudinal crack occurrence indices on the slab surface. The target steel type is medium carbon steel with a carbon content of 0.10% by mass, which has a high incidence of longitudinal cracks. The vertical crack generation rate is the value confirmed in the surface treatment process after casting, and the ratio of the vertical crack generation rate of the conventional example to the value of the conventional example is shown as the vertical crack generation index, with the vertical crack generation rate of the conventional example as the standard (1.0).

As shown in FIG. 7, it was confirmed that the longitudinal crack occurrence index was reduced to 0.2 or less in Inventive Example 1, and that the occurrence of longitudinal cracks could be suppressed to almost zero in Inventive Example 2. . In order to carry out direct heating and direct rolling of the cast slab surface without maintenance, it is necessary to keep this index at 0.3 or less due to the relationship with maintenance restrictions in the hot rolling process. It was confirmed that by carrying out the invention, the slab cast piece can be transported to the next process without maintenance in the steelmaking stage.

In addition, using the water-cooled mold for continuous casting according to the present invention, a continuous casting operation was performed for about 180 days under various steel grades and casting conditions. It was confirmed that even when the water-cooled mold for continuous casting according to the present invention is used, it can be used without breakout and operational problems. In addition, it was confirmed that the life of the mold was the same as that of the conventional water-cooled mold for continuous casting.

1 Water-cooled mold for continuous casting 2 Long side of mold 3 Short side of mold 4 Immersion nozzle 5 Molten steel 6 Mold plate 7 Backup plate 8 Grooved channel 9 Box shaped channel 10 Connection position 11 Recess 12 Different material filling part 13 Cooling water supply channel 14 Water supply Port 15 Cooling water drain 16 Drain

Claims (13)

A mold plate made of a copper alloy in which a cooling channel is formed;
A water-cooled mold for continuous casting, comprising a backup plate attached to the mold plate so as to cover the cooling water channel,
The cooling water channel in the lower part of the mold plate is composed of a plurality of groove-shaped water channels (cooling slits) arranged in the width direction of the mold, and each of the plurality of groove-shaped water channels has a vertically long shape extending in the casting direction. ,
The cooling water channel in the upper part of the mold plate is composed of a box-shaped water channel extending in the casting direction and widening in the width direction of the mold, and the box-shaped water channel communicates with a plurality of cooling water channels in the lower part of the mold plate,
A plurality of recesses are provided on the inner wall surface of the mold plate that constitutes the box-shaped waterway in the upper part of the mold plate,
A water-cooled mold for continuous casting, wherein the recess has a depth of 0.1 to 2.0 mm. A mold plate made of a copper alloy in which a cooling channel is formed;
A water-cooled mold for continuous casting, comprising a backup plate attached to the mold plate so as to cover the cooling water channel,
The cooling water channel in the lower part of the mold plate is composed of a plurality of groove-shaped water channels (cooling slits) arranged in the width direction of the mold, and each of the plurality of groove-shaped water channels has a vertically long shape extending in the casting direction. ,
The cooling water channel in the upper part of the mold plate is composed of a box-shaped water channel extending in the casting direction and widening in the width direction of the mold, and the box-shaped water channel communicates with a plurality of cooling water channels in the lower part of the mold plate,
A plurality of recesses are provided on the inner wall surface of the mold plate that constitutes the box-shaped waterway in the upper part of the mold plate,
A water-cooled mold for continuous casting, wherein each of the plurality of recesses is not filled with a material having thermal conductivity different from that of the mold plate. A mold plate made of a copper alloy in which a cooling channel is formed;
A water-cooled mold for continuous casting, comprising a backup plate attached to the mold plate so as to cover the cooling water channel,
The cooling water channel in the lower part of the mold plate is composed of a plurality of groove-shaped water channels (cooling slits) arranged in the width direction of the mold, and each of the plurality of groove-shaped water channels has a vertically long shape extending in the casting direction. ,
The cooling water channel in the upper part of the mold plate is composed of a box-shaped water channel extending in the casting direction and widening in the width direction of the mold, and the box-shaped water channel communicates with a plurality of cooling water channels in the lower part of the mold plate,
A plurality of recesses are provided on the inner wall surface of the mold plate that constitutes the box-shaped waterway in the upper part of the mold plate,
Some of the plurality of recesses are filled with a material having thermal conductivity different from that of the mold plate,
The recesses in which the material is not embedded are arranged in a row in the width direction of the mold, the recesses in which the material is embedded are arranged in a row in the width direction of the mold, and the recesses in which the material is not embedded are arranged in the width direction of the mold. A water-cooled mold for continuous casting, characterized in that a group arranged in a row and a group in which the recesses in which the material is embedded are arranged in a row in the width direction of the mold are alternately arranged in the casting direction. 4. The water cooling for continuous casting according to claim 3, characterized in that said recesses filled with a material having a thermal conductivity different from that of the mold plate have a smooth surface with respect to the surroundings. template. The water-cooled mold for continuous casting according to claim 3 or 4, characterized in that the thermal conductivity of said material is lower than that of said mold plate. The water-cooled mold for continuous casting according to any one of claims 2 to 5, wherein the depth of the recess is 0.1 to 2.0 mm. The water-cooled mold for continuous casting according to any one of claims 1 to 6, wherein the recesses are provided periodically at regular intervals. 8. The recess according to any one of claims 1 to 7, wherein the recess has one and the same shape selected from a hemispherical shape, a conical shape, a truncated cone shape, a cylindrical shape, a spire shape, and a pyramid shape. A water-cooled mold for continuous casting according to item 1. The water-cooled mold for continuous casting according to any one of claims 1 to 8, characterized in that the concave portions are provided in a zigzag pattern. Any one of claims 1 to 9, wherein the connecting position of the cooling water channel in the lower part of the mold plate and the cooling water channel in the upper part of the mold plate is within a range of 150 to 500 mm from the upper end of the mold. A water-cooled mold for continuous casting according to item 1. A steel continuous casting method for continuously casting molten steel using the water-cooled mold for continuous casting according to any one of claims 1 to 10, wherein the cooling water in the cooling water passage under the mold plate A continuous casting method for steel, characterized in that the continuous casting is performed at a flow rate within a range of 7.0 to 13.0 m/s. A steel continuous casting method for continuously casting molten steel using the water-cooled mold for continuous casting according to any one of claims 1 to 10 , wherein at least a part of the period from the start of casting to the end of casting A continuous casting method for steel, characterized in that continuous casting is performed at a cast strip drawing speed of 2.0 m/min or more. 12. The continuous casting method of steel according to claim 11, characterized in that continuous casting is performed at a cast strip withdrawal speed of 2.0 m/min or more during at least a part of the period from the start of casting to the end of casting.

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