JP7560725B2 - Mold for continuous casting and method for continuous casting of steel - Google Patents

Description

The present invention relates to a continuous casting mold used in continuous casting equipment for continuously casting steel, and a method for continuously casting steel using the same.

In continuous casting of molten steel, when molten steel is poured into a mold, the portion of the molten steel in contact with the mold solidifies to form a solidified shell, which is then pulled out below the mold. The solidification of the molten steel progresses further in the secondary cooling zone below the mold, and finally a continuously cast slab is formed. The side of the mold that comes into contact with the molten steel is made of a water-cooled copper plate. The continuous casting mold of a continuous casting device that casts slabs is made using two long-side copper plates and two short-side copper plates, and is assembled so that the two short-side copper plates are sandwiched between the two long-side mold plates. The width of the short-side copper plates is approximately equal to the thickness of the slab to be cast.

As the solidifying shell moves downward in the mold, it shrinks as it solidifies. Therefore, the solidifying shell, which starts to solidify at the meniscus of the molten metal in the mold, shrinks when it reaches the bottom end of the mold, and the width and thickness of the solidifying slab are smaller than those at the meniscus. In continuous casting of slabs, the width of the slab is wider than the thickness, so the amount of solidification shrinkage in the width direction of the slab is large, and gaps are likely to form between the mold and the solidified shell. If a gap forms between the mold and the solidified shell below the mold as the solidification shell shrinks, heat transfer from the solidified shell to the mold is hindered, the mold cannot be sufficiently cooled, and the solidified shell, which loses support from the mold, will expand outward, causing bulging.

For this reason, at least the short sides of the mold are tapered. By tapering, we mean narrowing the distance between the opposing short sides at the bottom end of the mold compared to the distance at the meniscus position above the mold.

If the narrow side taper amount is too small, the contact between the solidified shell and the narrow side mold plate becomes uneven, causing imbalance in cooling. As a result, internal cracks due to uneven growth of the solidified shell and vertical cracks are likely to occur on the surface of the slab corresponding to the area where the solidified thickness is particularly thin near the corner of the long side solidified shell. Also, if the narrow side taper amount is too large, the contact between the solidified shell and the narrow side copper plate becomes strong, and excessive stress is applied to the solidified shell. As a result, the solidified shell breaks, and a breakout occurs due to the shell breakage. It can also cause a decrease in the mold life due to increased friction between the solidified shell and the mold.

Conventionally, the surface of the narrow copper plate that comes into contact with the solidified shell is machined to be flat from top to bottom. This is what is known as a single taper. However, the solidification and shrinkage speed of the solidified shell is not constant at each position in the casting direction inside the mold, being fast near the meniscus and slowing down as it approaches the bottom end of the mold. Therefore, it is considered that the surface of the solidified shell that comes into contact with the narrow copper plate is not flat, but is curved such that the taper of the solidified shell decreases as it goes downward in the mold. For example, Patent Documents 1 and 2 disclose a method of casting using a mold in which the narrow copper plate has a multi-stage taper with two or more stages in the casting direction.

JP 2008-49385 A JP 2009-160645 A Patent No. 5999294

Here, in difficult-to-cast steels such as peritectic steels that undergo δ-γ transformation during solidification, the solidification shrinkage of the solidified shell is large due to the phase transformation during solidification. For this reason, it is necessary to strongly taper the topmost taper of the multi-stage taper of the mold so that it can follow the solidification shrinkage. However, it has been found that if the topmost taper of the mold in which the short-side copper plate is formed with a multi-stage taper is further strongly tapered during casting, the corner shape of the cast piece becomes sharper. Figure 6 shows examples of the cross-sectional shapes of cast pieces cast using two types of mold shapes.

First, in typical continuous casting using a mold with right-angled mold corners and a taper rate of the short-side copper plate of 1.0%/m or less, molten steel is extracted heat from both the long and short sides near the corners between the long and short-side copper plates. As a result, solidification proceeds faster near the corners than in the width centers of the long and short-side copper plates, and the corners solidify and shrink. Meanwhile, bulging occurs in the width center due to the static pressure of the molten steel, causing deformation that bulges outward. As a result, the cast slab bulges outward toward the width center, as shown in cast slab shape A in Figure 6, and the corners where solidification shrinkage is large become somewhat pointed.

In addition, in a mold with right-angled mold corners and multiple tapers, the topmost taper is strongly tapered with a taper rate of 1.1%/m or more for the short-side copper plate in order to reduce the air gap between the slab and the mold. This promotes heat removal from the mold and slab near the corners, and the corners of the slab become sharper than those of slab shape A, as in slab shape B in Figure 6.

When the corners of a slab become sharper in this way, scale tends to accumulate in the depressions of the off-corner parts, increasing the likelihood of scratches occurring during rolling. For this reason, there is a need for technology that can improve the shape of the slab while making the topmost tapered part more strongly tapered.

The present invention was made in consideration of the above problems, and the object of the present invention is to provide a continuous casting mold and a method for continuous casting of steel that can improve the shape of the cast piece while making the uppermost tapered section more strongly tapered.

In order to solve the above problems, according to one aspect of the present invention, a continuous casting mold for use in a continuous casting facility for continuously casting steel is provided, the mold comprising a pair of long-side copper plates and a pair of short-side copper plates that are sandwiched between the pair of long-side copper plates and can move along the long-side direction of the long-side copper plates, the short-side copper plates have two or more different gradients in the casting direction so that multiple taper sections are formed on the inner surface side of the mold to which molten steel is supplied, the short-side copper plates have protrusions extending in the casting direction at both ends of the short-side direction on the inner surface side, the taper rate of the taper section on the most upstream side in the casting direction is 1.1%/m or more and 3.5%/m or less, and the protrusions are formed so that the length of the contact section that comes into contact with the molten steel is shorter than the sum of the short-side length and long-side length of the mold when viewed from above, and the difference is 3 mm or more.

The distance in the casting direction from the average meniscus position to the first taper change point of the short side copper plate may be 50 mm or more and 300 mm or less.

The taper ratio of the tapered portion on the most downstream side in the casting direction may be 0.5%/m or more but less than 1.9%/m, and may be equal to or less than the taper ratio of the tapered portion on the upstream side.

The mold may be formed to have two tapered portions.

In addition, in order to solve the above problems, according to another aspect of the present invention, there is provided a method for continuously casting steel, which includes a pair of long-side copper plates and a pair of short-side copper plates that are sandwiched between the pair of long-side copper plates and can move along the long-side direction of the long-side copper plates, and the short-side copper plates have two or more different gradients in the casting direction so that multiple taper sections are formed on the inner surface side of the mold to which molten steel is supplied, and the short-side copper plates have protrusions extending in the casting direction at both ends of the short-side direction on the inner surface side, the taper rate of the taper section on the most upstream side in the casting direction is 1.1%/m or more and 3.5%/m or less, and the protrusions are formed so that the length of the contact section that comes into contact with the molten steel is shorter than the sum of the short-side length and long-side length of the mold when viewed from above, and the difference is 3 mm or more.

As explained above, the present invention makes it possible to improve the shape of the slab while making the uppermost tapered section more strongly tapered.

1 is an explanatory diagram showing a schematic configuration of a continuous casting facility according to an embodiment of the present invention. FIG. FIG. 4 is a schematic cross-sectional view showing the shape of a mold according to the embodiment. FIG. 2 is a schematic diagram showing the shape of a mold when the corner portion is a right-angle corner. FIG. 2 is a schematic diagram showing an example of a shape of a corner portion of a mold. 10A to 10C are schematic diagrams showing other examples of shapes of the corner portion of the mold. 10A to 10C are schematic diagrams showing other examples of the shape of the corner portion of the mold. 10A to 10C are schematic diagrams showing other examples of the shape of the corner portion of the mold. 1A to 1C are schematic diagrams showing examples of cross-sectional shapes of slabs cast using a mold with right-angled corners (without a protrusion) and a mold with a protrusion formed therein. 3 is a cross-sectional view taken along line II of FIG. 2. 1A to 1C are schematic diagrams showing examples of cross-sectional shapes of cast pieces cast using molds of two different shapes.

The preferred embodiment of the present invention will be described in detail below with reference to the attached drawings. Note that in this specification and the drawings, components having substantially the same functional configuration are designated by the same reference numerals to avoid redundant description.

[1. Configuration of continuous casting equipment]
First, a schematic configuration of a continuous casting facility equipped with a mold according to an embodiment of the present invention will be described with reference to Figures 1 and 2. Figure 1 is an explanatory diagram showing a schematic configuration of a continuous casting facility 1 according to this embodiment. Figure 2 is a schematic cross-sectional view showing the shape of a mold 10 according to this embodiment. Note that Figure 2 shows a state in which the mold 10 is cut at an arbitrary position in the casting direction (vertical direction).

The continuous casting equipment 1 according to this embodiment is an apparatus for continuously casting molten steel 2 using a mold 10 for continuous casting to produce a cast piece 3. The continuous casting equipment 1 shown in FIG. 1 is a vertical bending type continuous casting equipment 1, but the present invention is not limited to this example and can be applied to various other types of continuous casting equipment, such as a curved type or vertical type. The continuous casting equipment 1 includes a mold 10, a ladle 4, a tundish 5, an immersion nozzle 6, and a secondary cooling device 7.

The ladle 4 is a movable container for transporting the molten steel 2 from the outside to the tundish 5. The ladle 4 is disposed above the tundish 5, and the molten steel 2 in the ladle 4 is supplied to the tundish 5. The tundish 5 is disposed above the mold 10, and stores the molten steel 2 to remove inclusions in the molten steel 2. The submerged nozzle 6 extends downward from the lower end of the tundish 5 toward the mold 10, and its tip is immersed in the molten steel 2 in the mold 10. The submerged nozzle 6 continuously supplies the molten steel 2 from which inclusions have been removed in the tundish 5, into the mold 10.

The mold 10 is a rectangular cylindrical mold formed according to the width and thickness of the cast piece 3. As shown in FIG. 2, the mold 10 according to this embodiment is assembled using a pair of short-side copper plates 11 and a pair of long-side copper plates 13, with the pair of short-side copper plates 11 sandwiched on both sides in the short-side direction (X direction) by the inner faces 13a of the pair of long-side copper plates 13. The pair of short-side copper plates 11 are configured to be movable along the long-side direction (Y direction) of the long-side copper plates 13. In other words, the mold 10 according to this embodiment is a mold with variable width. A detailed description of the shape of the mold 10 will be given later.

The copper plates 11, 13 that make up the mold 10 are, for example, water-cooled copper plates. The molten steel 2 that comes into contact with the inner surfaces 11a, 13a of the copper plates 11, 13 is cooled, and a slab 3 is produced that includes an unsolidified portion 3b inside the solidified shell 3a. As the solidified shell 3a moves downward in the mold 10, the solidification of the unsolidified portion 3b inside progresses, and the thickness of the solidified shell 3a gradually increases. The slab 3 that includes the solidified shell 3a and the unsolidified portion 3b is pulled out from the bottom end of the mold 10.

The secondary cooling device 7 is provided in the secondary cooling zone 9 below the mold 10, and cools the slab 3 pulled out from the lower end of the mold 10 while supporting and transporting it. The secondary cooling device 7 has multiple pairs of support rolls 8 arranged on both sides of the thickness of the slab 3, and multiple spray nozzles (not shown) that spray cooling water onto the slab 3. The support rolls 8 provided in the secondary cooling device 7 are arranged in pairs on both sides of the thickness of the slab 3, and function as a support transport means that supports and transports the slab 3. By supporting the slab 3 from both sides in the thickness direction with the support rolls 8, breakout and bulging of the slab 3 during solidification in the secondary cooling zone 9 can be prevented.

The support rolls 8 form a transport path (pass line) for the slab 3 in the secondary cooling zone 9. As shown in FIG. 1, this pass line is vertical just below the mold 10 (vertical zone 9A), then curves in a curved shape (curved zone 9B), and finally becomes horizontal (horizontal zone 9C). The support rolls 8 are provided in the vertical zone 9A and consist of a support roll that supports the slab 3 immediately after it is pulled out of the mold 10, a pinch roll that is a driven roll that pulls the slab 3 out of the mold 10, and segment rolls that are provided in the curved zone 9B and horizontal zone 9C and support and guide the slab 3 along the pass line.

The slab 3 that has passed through the secondary cooling zone 9 is then cut to a predetermined length by a slab cutter (not shown) installed downstream of the horizontal zone 9C. The cut slab 3 moves over a table roll and is transported to the equipment for the next process. The overall configuration of the continuous casting equipment 1 has been described above.

2. Mold Shape
[2-1. Overview]
In the present embodiment, in the mold 10 of the continuous casting equipment 1, in order to improve the shape of the cast slab while making the uppermost taper portion strongly tapered, a protrusion protruding toward the space inside the mold is formed on the inner surface 11a of the short side copper plate 11 at the mold corner. The protrusion refers to a portion where the short side copper plate 11 is deformed to fill the right-angle corner of the mold 10.

Here, FIG. 3A shows the shape of the corner portion of the mold 10 when it is a right-angle corner. Also, FIGS. 3B to 3E show examples of the shape of the corner portion of the mold 10 according to this embodiment. Note that FIGS. 3A to 3E show one of the four corner portions of the mold 10. The shape of the protrusion 12 may be the same for all four corner portions, or may include different shapes for the four corner portions in accordance with the requirements of subsequent processes.

For example, as shown in FIG. 3B, the protrusion 12 may be formed so that the right-angle corner is filled by a straight line 12a connecting the short-side copper plate 11 and the long-side copper plate 13 when the mold 10 is viewed from above. The triangular chamfered shape shown in FIG. 3B is also called a chamfer shape. Alternatively, as shown in FIG. 3C, the protrusion 12 may be formed so that the right-angle corner is filled by a circular arc 12b connecting the short-side copper plate 11 and the long-side copper plate 13 when the mold 10 is viewed from above. In other words, the protrusion 12 may be R-shaped.

Furthermore, as shown in FIG. 3D, the protrusion 12 may be formed so that the right-angle corner is filled by two circular arcs 12c and 12d when the mold 10 is viewed from above. In this case, the circular arc 12c that contacts the long-side copper plate 13 may be formed so as to be convex toward the inside of the mold 10 and contact the long-side copper plate 13 at an angle as close to perpendicular as possible. In addition, the circular arc 12d that contacts the short-side copper plate 11 may be formed so as to be convex toward the corner side of the mold 10 and contact the short-side copper plate 11 at an angle as small as possible.

As shown in Fig. 3E, the protrusion 12 may be formed so that the right-angle corner is filled by two arcs 12e, 12g and one straight line 12f connecting the two arcs 12e, 12g when the mold 10 is viewed from above. In this case, as in Fig. 3D, the arc 12e tangent to the long-side copper plate 13 may be formed so as to be convex toward the inside of the mold 10 and to be tangent to the long-side copper plate 13 at an angle as close to perpendicular as possible. Also, the arc 12g tangent to the short-side copper plate 11 may be formed so as to be convex toward the corner side of the mold 10 and to be tangent to the short-side copper plate 11 at an angle as small as possible.

The shape of the corners of the mold 10 as shown in Figures 3D and 3E is easy to process and can suppress deformation at the corners.

By using a mold 10 having a protrusion as shown in Figures 3B to 3E, it is possible to continuously cast a bevel with a chamfer. Figure 4 shows examples of the cross-sectional shapes of the slab cast using the mold with right-angle corners (without protrusions) shown in Figure 3A and the mold with an arc-shaped protrusion as shown in Figure 3C. As shown in Figure 4, the corners of the slab cast using the mold with right-angle corners were sharpened. On the other hand, the corners of the slab cast using the mold with protrusions were chamfered because the corner shape of the mold was transferred to the slab. This confirmed that by forming a protrusion 12 on the inner surface 11a of the short-side copper plate 11 at the mold corner, the sharpening of the corners of the slab was eliminated.

On the other hand, when casting a slab with chamfered corners, providing a protrusion at the corner of the mold reduces the contact length between the molten steel and the copper plate compared to when the corner is right-angled, and reduces the amount of heat dissipated from the mold corner. As a result, the surface temperature of the slab corner increases. Providing a protrusion at the corner of the mold reduces the amount of heat dissipation, resulting in a smaller shell thickness compared to when the mold has right-angle corners.

In addition, in the early stages of solidification, when the mold has right-angle corners, gaps occur mainly near the short side of the corner, but when a protrusion is provided on the corner, gaps occur in the protrusion itself and near the short side of the corner consisting of the protrusion and the short side copper plate surface. In other words, when a protrusion is provided on the corner of the mold, the area where gaps occur is larger than in the case of right-angle corners.

For these reasons, heat transfer from the solidified shell to the mold is hindered, and the solidified shell cannot be cooled sufficiently. This is not limited to cases where a triangular protrusion is provided at the corner, but also applies to cases where a protrusion is provided at the corner of the mold as shown in Figures 3C to 3E, in which the contact length between the molten steel and the copper plate is reduced compared to when the mold has right-angle corners. Regarding this, as described above, by making the tapered portion of the copper plate on the short side strongly tapered, it is possible to promote heat transfer from the mold and cast piece near the corner. Therefore, for a multi-stage tapered mold, by making the topmost tapered portion strongly tapered and providing a protrusion inside the mold, it is possible to utilize the functions of each shape.

Furthermore, by using a mold with protrusions inside the mold, the surface temperature of the slab cast using such a mold is higher at the corners than when a mold with right-angle corners is used for casting, and corner cracks on the surface of the slab can be prevented. This is due to the following mechanism.

The molten steel poured into the mold is removed from both the long and short sides at the corners of the mold. As a result, the corners of the mold have a greater amount of heat removal than the center of the width of the long copper plate due to the effect of two-sided cooling, and the surface temperature drops. In the secondary cooling zone after leaving the mold, the temperature of the center of the long side width of the slab is about 900°C, but the temperature of the corners of the slab can be below 800°C. When the temperature of the corners of the slab falls below 800°C, cracks occur in the corners. This is because the slab is bent in the continuous casting machine in a temperature range where ductility is lost, called the embrittlement region, and tensile stress acts on it. Therefore, the effect of two-sided cooling is suppressed by providing protruding parts rather than making the corners of the mold at right angles (for example, Patent Document 3). This allows the temperature of the corners of the slab in the secondary cooling zone to be the same as that of the center of the long side width (i.e., 800°C or higher). As a result, the corners do not become overcooled compared to the center of the long side width, reducing corner cracks in the slab.

Continuous casting using a mold with a strong taper at the top and a protrusion inside the mold is also effective for difficult-to-cast steels such as peritectic steels, which experience large solidification shrinkage of the solidified shell due to phase transformation during solidification. Using a mold with this shape prevents the occurrence of vertical cracks, corner cracks, and internal cracks on the surface of the slab, and improves the cross-sectional shape of the slab to prevent scratches during rolling, thereby improving the quality of the slab during continuous casting of difficult-to-cast steels.

[2-2. Basic configuration]
The shape of the mold 10 according to this embodiment will be described with reference to Fig. 2 and Fig. 5. Fig. 5 is a cross-sectional view taken along line II in Fig. 2. In Fig. 2 and Fig. 5, In order to more clearly explain the configuration of the mold 10, the shape is shown exaggerated.

As described above, the mold 10 according to this embodiment is composed of a pair of short-side copper plates 11 and a pair of long-side copper plates 13. When the mold 10 is viewed from above in the height direction, as shown in FIG. 2, a substantially rectangular space (corresponding to the "rectangular space" of the present invention) V is formed in the mold 10 by the pair of short-side copper plates 11 and the pair of long-side copper plates 13. The inner surface 11a of the short-side copper plate 11 of the mold 10 according to this embodiment has protrusions 12 protruding into the space V at both ends in the short side direction. The protrusions 12 extend along the height direction of the mold 10 (also called the Z direction or casting direction).

Here, the short-side copper plate 11 has two or more different gradients in the casting direction so that the mold 10 has multiple taper sections and is a multi-stage taper. The multi-stage taper is formed so that the gradient (i.e., taper rate) becomes smaller as it moves downstream in the casting direction. In the initial solidification stage of the upper stage of the mold 10 (i.e., the upstream side in the casting direction), the solidified shell is thin and there is a lot of heat removal from the molten steel, so the solidification shrinkage of the solidified shell is large. For this reason, it is necessary to make the gradient of the taper relatively large. On the other hand, at the lower end of the mold 10 (i.e., the most downstream side in the casting direction), the solidified shell is thick because it has grown, and there is less heat removal from the molten steel compared to the upper stage of the mold. For this reason, the solidification shrinkage of the solidified shell is small, and the gradient of the taper is relatively small. Due to this mechanism, the gradient of the taper portion in the upper stage of the mold 10 is made large, and the gradient of the taper portion is set to gradually decrease as it moves downstream in the casting direction where solidification progresses.

If the gradient of the tapered portion of the upper stage of the mold 10, called a reverse taper, were to be small and the gradient of the tapered portion were to be gradually increased toward the downstream side in the casting direction, the following problems would likely occur. First, in the upper stage of the mold 10, the solidification shrinkage of the solidified shell is large, but the gradient of the tapered portion is small, so the air gap at the corners cannot be sufficiently reduced. This causes internal cracks and reduces the internal quality of the cast piece. On the other hand, in the lower end of the mold 10, the gradient of the tapered portion is large despite the small solidification shrinkage of the solidified shell, so the cast piece and the mold come into strong contact with each other, generating large frictional forces. The mold restraint causes wear of the mold copper plate, shortening the mold life, reducing productivity, and there is also concern about the occurrence of breakouts, in which the cast piece breaks. Therefore, the multi-stage taper is formed so that the gradient becomes smaller toward the downstream side in the casting direction.

For example, the mold 10 shown in Figures 2 and 5 has a two-stage taper because the short-side copper plate 11 has two different gradients θ ₁ and θ _2. In Figures 2 and 5, the inner surface 11a of the short-side copper plate 11 that forms the taper on the most upstream side (i.e., upward) in the casting direction is referred to as a first inclined surface 11a1, and the inner surface 11a of the short-side copper plate 11 that forms the taper on the most downstream side (i.e., downward) in the casting direction is referred to as a second inclined surface 11a2. In addition, in the mold 10, the taper formed by the first inclined surface 11a1 is referred to as a first taper portion T ₁ , and the taper formed by the second inclined surface 11a2 is referred to as a second taper portion T ₂ .

In this case, the taper ratio R of the taper section on the most upstream side in the casting direction (i.e., the first taper section _T1 ) is set to 1.1%/m or more and 3.5%/m or less. The taper ratio R [%/m] is expressed by the following formula (1).

R={(W _T −W _B )/W _V /ΔL}×100 (1)

In the above formula (1), W _T [m] is the distance between the opposing copper plates 11 on the upper side, W _B [m] is the distance between the opposing copper plates 11 on the lower side, W _V [m] is the distance between the opposing copper plates 11 on the lower side at any position in the casting direction (hereinafter also referred to as the "reference distance"), and ΔL [m] is the length in the casting direction between the upper and lower positions. Note that the distance W _T at the upper position and the distance W _B at the lower position may be distances between the upper and lower positions in the casting direction in a tapered portion having the same gradient, and the casting direction positions may be selected arbitrarily.

For example, in the first taper portion _T1 , the interval W _T at the upper position may be the interval W ₀ at the upper end of the mold 10, and the interval W _B at the lower position may be the interval W ₁ at the lower end of the first taper portion _T1 . In this case, ΔL is the length X ₁ in the casting direction of the first taper portion _T1 . Similarly, in the second taper portion _T2 , the interval W _T at the upper position may be the interval W ₁ at the upper end of the second taper portion _T2 , and the interval W _B at the lower position may be the interval W ₂ at the lower end of the mold 10. In this case, ΔL is the length X ₂ in the casting direction of the second taper portion _T2 . Here, the reference interval W _V is the interval W _M of the short side copper plates 11 at the average position of the meniscus position.

If the taper rate of the first tapered portion _T1 is less than 1.1%/m, a gap will be generated between the mold 10 and the slab at the corners of the mold 10, hindering the growth of the solidified shell. This will result in a thinner solidified shell near the corners of the slab, which will cause bulging due to the static pressure of the molten steel after it leaves the mold 10, resulting in tensile stress. At this time, the thin portion of the solidified shell has low strength, which may cause internal cracks and affect product quality. Therefore, the taper rate of the first tapered portion _T1 is set to 1.1%/m or more.

On the other hand, if the taper of the top stage is larger than 3.5%/m, the mold 10 and the cast piece come into strong contact with each other. This causes a large frictional restraining force to act on the solidified shell, which may break. If the solidified shell breaks, breakout occurs, in which unsolidified molten steel flows out to the outside. For this reason, the taper rate of the first taper portion _T1 is set to 3.5%/m or less.

Table 1 shows the results of investigating the taper ratio of the taper portion on the most upstream side in the casting direction and the occurrence of internal cracks and breakouts in the slab. Table 1 shows the results of investigating the occurrence of internal cracks and breakouts in the slab when the taper ratio of the taper portion on the most upstream side in the casting direction (i.e., the first taper portion T ₁ ) was changed when a slab with a slab width of 1250 to 2300 mm and a slab thickness of 200 to 400 mm was cast using the continuous casting equipment shown in Figures 1, 2, and 5. The shape of the protruding portion 12 of the mold 10 was a triangle with a length C of the contact portion in contact with the molten steel of 36 mm, a length C _X of the mold 10 in the short side direction of 30 mm, and a length C _Y of the mold 10 in the long side direction of 20 mm, when the mold 10 was viewed from above.

From the results in Table 1, it can be seen that in order to prevent internal cracks and breakouts from occurring in the slab, the taper ratio of the taper portion on the most upstream side in the casting direction should be 1.1%/m or more and 3.5%/m or less. The taper ratios in Table 1 were calculated using the above formula (1). At this time, the reference interval _WV was set to the interval _WM of the short side copper plates 11 at the average position of the meniscus position.

Moreover, the mold 10 according to the present embodiment is provided with a protruding portion 12. By providing the protruding portion 12, it is possible to prevent the phenomenon that the corner portion of the cast piece is supercooled compared to the central portion of the long side width. The shape of the protruding portion 12 may be triangular when the mold 10 is viewed from above, for example, as shown in FIG. 2 and FIG. 3B. However, the present invention is not limited to such an example, and the protruding portion 12 may have other shapes, for example, as shown in FIG. 3C to FIG. 3E. Each of the protruding portions 12 may be formed such that the length C of the contact portion in contact with the molten steel when the mold 10 is viewed from above is shorter than the sum of the length C _X in the short side direction and the length C _Y in the long side direction of the mold 10, and the difference is 3 mm or more. That is, the length C of the contact portion of one protruding portion 12 satisfies the following formula (2).

3 [mm]≦{(C _X +C _Y )-C} ...(2)

For example, the length _C1 of the straight line 12a of the protrusion 12 shown in Fig. 3B and the length _C1 of the arc 12b of the protrusion 12 shown in Fig. 3C are the length C of the contact portion in contact with the molten steel (C = _C1 ). In addition, in the protrusion 12 shown in Fig. 3D, the length C of the contact portion in contact with the molten steel is expressed by the sum of the length _C1 of the arc 12c and the length _C2 of the arc 12d (C = _C1 + _C2 ). In addition, in the protrusion 12 shown in Fig. 3E, the length C of the contact portion in contact with the molten steel is expressed by the sum of the length _C1 of the arc _12e , the length C2 of the arc 12g, and the length _C3 of the straight line 12f (C = _C1 + _C2 + _C3 ). When the length C of the contact portion of one protrusion 12 is expressed in this way, each of the four protrusions 12 at the corners of the mold 10 is formed so as to satisfy the above formula (2).

The relationship of the above formula (2) is based on the results of solidification analysis using a numerical simulation, in which the length C of the contact portion in contact with molten steel and the sum of the length C _X of the mold 10 in the short side direction and the length C _Y of the long side direction were changed. When the length C of the contact portion was shorter than the sum of the length C _X of the mold 10 in the short side direction and the length C _Y of the long side direction, and the difference ΔC (= (C _X + C _Y ) - C) was less than 3 mm, the surface temperature of the corner did not increase, and the effect of the protrusion 12 was not achieved. Therefore, the length C of the contact portion in contact with molten steel is set shorter than the sum of the length C _X of the mold 10 in the short side direction and the length C _Y of the long side direction, and the difference ΔC is set to 3 mm or more.

When the difference ΔC was 3 mm or more, the corner surface temperature gradually increased, but when the difference ΔC exceeded 20 mm, the increase in the corner surface temperature was saturated even if the difference ΔC was further increased. On the other hand, when the casting cross section is narrowed, the yield rate decreases (i.e., productivity deteriorates). Therefore, the difference ΔC (=(C _X +C _Y )-C) may be set to 20 mm or less.

Table 2 shows the results of investigating the quality of the slab when the difference ΔC (=(C _X +C _Y )-C) between the length C of the contact part in contact with the molten steel and the sum of the short side length C _X and the long side length C _Y of the mold 10 is changed. Table 2 shows the results of investigating the occurrence of surface cracks in the slab when the shape of the protruding part 12 of the mold used for casting a slab with a width of 1250 to 2300 mm and a thickness of 200 to 400 mm is changed using the continuous casting equipment shown in Figures 1, 2 and 5. The taper rate of the taper part on the most upstream side in the casting direction was 2.0%/m. The taper rate was calculated using the above formula (1). In this case, the reference interval W _V was set to the interval W _M of the short side copper plates 11 at the average position of the meniscus position.

The surface cracks of the slabs were evaluated as follows.
A: No surface cracks occurred. B: Almost no surface cracks occurred. C: Minor surface cracks occurred, but they were removable and did not affect the quality of the product. D: Significant surface cracks occurred, causing a problem in the quality of the product.

From the results in Table 2, it can be seen that in order to prevent the occurrence of surface cracks in the slab, the length C of the contact portion in contact with the molten steel should be shorter than the sum of the short side length C _X and the long side length C _Y of the mold 10, and the difference ΔC should be 3 mm or more. Furthermore, when the difference ΔC exceeds 20 mm, the increase in the corner surface temperature is saturated even if the difference ΔC is further increased. Therefore, from the viewpoint of suppressing a decrease in yield, the difference ΔC should be 20 mm or less.

As long as the above formula (2) is satisfied, the contact portion of the protrusion 12 may have, for example, a shape having one or more corners when the mold 10 is viewed in a plan view, a shape defined by one or more arcs (curves) as shown in FIG. 3C or FIG. 3D, or a shape defined by a combination of straight lines and arcs (curves) as shown in FIG. 3E.

Here, when a protrusion 12 is provided at the corner of the mold 10, the contact length between the molten steel and the copper plate is reduced compared to when the corner is a right angle, and the amount of heat removed from the mold corner is reduced. Therefore, the surface temperature of the slab corner rises. On the other hand, the thickness of the solidified shell is reduced compared to when the corner is a right angle due to the reduced amount of heat removed. In addition, when a protrusion is provided at the corner of the mold, the gap that occurs between the mold and the slab in the early stage of solidification occurs in a wider area compared to when the corner is a right angle. For this reason, heat removal from the solidified shell to the mold is hindered, the mold cannot be sufficiently cooled, and the solidified shell remains thin. As a result, bulging occurs, and the solidified shell of the slab cracks near the corner of the long side copper plate, where the solidified shell is thinner than the center of the long side width, and there is a possibility that an internal defect called an internal crack occurs.

In order to prevent the occurrence of internal defects, the copper plate on the short side of the mold 10 is tapered. However, if the mold 10 is provided with a strong taper with a large gradient over the entire casting direction, the frictional constraint force between the mold 10 and the slab increases, and a breakout may occur. In particular, when the mold 10 is provided with a protrusion 12, when considering only the short-side copper plate 11 on which the protrusion 12 is provided, the length C of the contact portion in contact with the molten steel is greater than the length C _X of the mold 10 in the short side direction (i.e., C>C _X ). Therefore, the contact area between the mold 10 and the slab increases, and the frictional constraint force is more likely to increase.

However, as described above, the mold 10 according to this embodiment has two or more different gradients in the casting direction on the short-side copper plate 11 so that multiple tapered portions are formed on the inner surface side of the mold 10. As a result, even if the protrusions 12 are provided, it is possible to suppress non-uniform solidification of the corners of the slab in the upper part of the mold 10, where solidification shrinkage is large, and to alleviate strong contact between the slab and the mold 10 in the lower part of the mold 10, where solidification shrinkage is small.

Furthermore, in the mold 10 according to this embodiment, the narrow side copper plate 11 is provided with a gradient such that the taper rate R of the taper portion on the most upstream side in the casting direction (i.e., the first taper portion _T1 ) is 1.1%/m or more and 3.5%/m or less. This makes it possible to avoid a situation in which the gradient of the narrow side copper plate 11 is too small, causing gaps to occur near the corners of the long side copper plate 13 and inhibiting the growth of the solidified shell. In addition, it is possible to prevent the occurrence of breakout due to rupture of the solidified shell caused by an increase in the frictional force between the mold 10 and the slab due to an excessively large gradient of the narrow side copper plate 11.

The mold 10 according to this embodiment has two or more different gradients in the casting direction on the short-side copper plate 11 so that multiple tapered sections are formed on the inner surface side of the mold 10. The multiple tapered sections are formed so that the gradient (i.e., the taper rate) becomes smaller the further downstream in the casting direction they are.

As a method of providing multiple gradients in the casting direction on the inner surface of the short-side copper plate 11 of the mold 10, for example, a method of using a parabolic taper with a curved inner surface is also available. However, because the taper ratio of the taper part on the most upstream side of the casting direction is a large value exceeding 3.5%/m, a large frictional restraint force acts on the solidified shell, which may cause the solidified shell to break. If the solidified shell breaks, it may cause operational problems such as breakouts, in which unsolidified molten steel flows out to the outside. Furthermore, in a parabolic taper mold, the taper ratio varies greatly due to fluctuations in the meniscus position. For this reason, when fluctuations in the meniscus position occur due to clogging of the submerged nozzle, etc., areas with a small taper ratio are created, and vertical cracks occur in the cast piece.

Furthermore, when performing continuous casting in succession to prevent the immersion nozzle from melting due to slag, the meniscus position is changed during casting, but with a parabolic taper mold, the range in which the taper rate is appropriate is narrow, so the number of times continuous casting can be performed in succession is reduced, and casting efficiency is reduced. For this reason, in the mold 10 of this embodiment, the short-side copper plate 11 has two or more different gradients in the casting direction so that multiple tapered portions are formed on the inner surface side of the mold 10.

In addition, a mold using a parabolic taper with a curved inner surface has a complex shape and is difficult to process, which increases the manufacturing cost. On the other hand, by forming two tapered portions _T1 and _T2 on the inner side of the mold 10 as shown in Figure 5, the shape of the mold 10 is not complicated, and it is possible to set an appropriate gradient on the short side copper plate 11 with easy processing and at low cost.

Continuous casting of steel using such a mold 10 ensures uniform solidification of the molten steel and suppresses corner cracks in the cast piece.

[2-3. Additional configuration]
By adding the following configuration to the basic configuration of the mold 10 described above, it is possible to ensure more uniform solidification of the molten steel and suppress corner cracks in the cast slab.

(a) Position of the first taper change point For example, the distance XM (hereinafter also simply referred to as "distance XM") in the casting direction from the average position _M of the meniscus position to the first taper change point P1 of the short side copper plate ₁₁ (hereinafter also referred to as "first taper change point _" ) may be 50 mm or more and 300 mm or less. Since the meniscus position changes during casting, the average position M of the meniscus position is used as the reference here. By setting the distance _XM to 50 mm or more, the range with a large taper rate can be made long. As a result, the taper of the top stage of the multi-stage taper can be made large to suppress the generation of a gap between the mold and the slab, and the effect of promoting the growth of the solidified shell can be sufficiently obtained. In other words, the thickness of the slab shell can be suppressed from becoming thin, and the occurrence of internal cracks can be suppressed. As a result, even if a small internal crack occurs when the taper rate of the first taper portion _T1 is 1.1%/m or more, the effect can be suppressed to a level that does not cause a problem in the product quality.

Furthermore, by setting the distance _XM in the casting direction from the average meniscus position M to the first taper change point _P1 to 300 mm or less, it is possible to prevent the range with a large taper rate from becoming too long and causing strong contact between the mold 10 and the slab. This makes it possible to suppress the occurrence of breakouts, in which a large frictional restraining force acts on the solidified shell and the solidified shell breaks, causing unsolidified molten steel to flow out.

Here, Table 3 shows the results of investigating the distance _XM in the casting direction from the average meniscus position M to the first taper change point _P1 and the occurrence of internal cracks and breakouts in the slab. Table 3 shows the results of investigating the occurrence of internal cracks and breakouts in the slab when the distance _XM is changed when casting a slab with a slab width of 1250 to 2300 mm and a slab thickness of 200 to 400 mm using the continuous casting equipment shown in Figures 1, 2, and 5. The taper rate of the taper part on the most upstream side in the casting direction was 2.0%/m. The taper rate was calculated using the above formula (1). At this time, the reference interval _WV was set to the interval _WM of the short side copper plates 11 at the average meniscus position. The shape of the protruding portion 12 of the mold 10 was a triangle with a length C of the contact portion in contact with the molten steel of 36 mm, a length C _X of the short side of the mold 10 of 30 mm, and a length C _Y of the long side of the mold 10 of 20 mm, when viewed from above.

From the results of Table 3, it can be seen that in order to prevent internal cracks and breakouts from occurring in the slab, the distance _XM in the casting direction from the average position M of the meniscus positions to the first taper change point _P1 of the short side copper plate 11 should be set to 50 mm or more and 300 mm or less.

When the distance _XM was 100 mm, no internal cracks were generated in the slab even if the taper rate of the first taper portion _T1 was 2.3%/m or more. Furthermore, when the distance _XM was 200 mm, no internal cracks were generated in the slab even if the taper rate of the first taper portion _T1 was 1.9%/m or more. In addition, when the distance _XM was 300 mm, no internal cracks were generated even if the taper rate of the first taper portion _T1 was 1.1%/m or more.

(b) Taper ratio of the taper portion on the most downstream side in the casting direction The taper ratio of the taper portion on the most downstream side in the casting direction may be 0.5%/m or more and less than 1.9%/m, and may be equal to or less than the taper ratio of the upstream taper portion. For example, in the mold 10 shown in FIG. 5, the second taper portion _T2 is the taper portion on the most downstream side in the casting direction. Here, the taper ratio is calculated using the above formula (1). The reference interval _WV is the interval _WM of the short side copper plate 11 at the average position of the meniscus position.

If the taper rate of the tapered section on the most downstream side in the casting direction is 0.5%/m or more, a gap is less likely to occur between the solidified shell that has solidified and shrunk and the mold 10. As a result, splashes of cooling water used to spray-cool the slab after it leaves the mold 10 and water vapor will not penetrate into the gap, and corrosion of the copper plate of the mold 10 can be suppressed.

On the other hand, if the taper rate of the tapered section on the most downstream side in the casting direction is less than 1.9%/m, the taper rate will not be too high relative to the solidification shrinkage of the slab, and strong contact between the mold 10 and the slab can be suppressed. Because the solidified shell has grown sufficiently in the lower part of the mold 10, the solidified shell rarely breaks and breaks out. However, by preventing the taper rate from becoming too high, the grown solidified shell will not come into strong contact with the copper plate of the mold 10, and wear on the surface of the copper plate of the mold 10 can be suppressed.

In this way, by setting the taper rate of the tapered section on the most downstream side in the casting direction to 0.5%/m or more and less than 1.9%/m, and equal to or less than the taper rate of the tapered section on the upstream side, the life of the copper plate of the mold 10 can be extended and the frequency of replacement of the copper plate can be reduced.

Here, Table 4 shows the results of investigating the taper ratio of the taper part on the most downstream side in the casting direction and the presence or absence of corrosion and wear of the copper plate of the mold 10 after 2000 charges. Table 4 shows the results of investigating the presence or absence of corrosion and wear of the copper plate of the mold 10 after 2000 charges by changing the taper ratio of the taper part on the most downstream side in the casting direction (i.e., the second taper part T ₂ ) when casting a slab with a slab width of 1250 to 2300 mm and a slab thickness of 200 to 400 mm using the continuous casting equipment shown in Figures 1, 2 and 5. The taper ratio of the taper part on the most upstream side in the casting direction was set to 2.0%/m. In addition, the shape of the protruding part 12 of the mold 10 was a triangle with a length C of the contact part in contact with the molten steel of 36 mm, a length C _X of the short side direction of the mold 10 of 30 mm, and a length C _Y of the long side direction of the mold 10 of 20 mm when viewed from above.

The results in Table 4 also show that by setting the taper rate of the tapered section on the most downstream side in the casting direction to 0.5%/m or more and less than 1.9%/m, and equal to or less than the taper rate of the upstream tapered section, the occurrence of corrosion and wear of the copper plate of the mold 10 can be suppressed, and the life of the copper plate of the mold 10 can be extended.

(c) Relationship between the taper ratios of the most upstream taper section and the most downstream taper section in the casting direction The taper ratio of the most upstream taper section in the casting direction may be 1.1 to 2.0 times that of the most downstream taper section in the casting direction. For example, in the mold 10 shown in Fig. 5, the first taper section _T1 is the most upstream taper section in the casting direction, and the second taper section _T2 is the most downstream taper section in the casting direction.

Table 5 shows the results of investigating the taper ratio of the taper part on the most upstream side in the casting direction relative to the taper ratio of the taper part on the most downstream side in the casting direction (hereinafter also referred to as the "taper ratio") and the occurrence of internal cracks and breakouts in the slab. Table 5 shows the results of investigating the occurrence of internal cracks and breakouts in the slab when the taper ratio was changed when casting a slab with a slab width of 1250 to 2300 mm and a slab thickness of 200 to 400 mm using the continuous casting equipment shown in Figures 1, 2, and 5. The taper ratio of the taper part on the most upstream side in the casting direction was set to 2.0%/m. The taper ratio was calculated using the above formula (1). At this time, the reference interval W _V was set to the interval W _M of the short side copper plates 11 at the average position of the meniscus position. The shape of the protruding portion 12 of the mold 10 was a triangle with a length C of the contact portion in contact with the molten steel of 36 mm, a length C _X of the short side of the mold 10 of 30 mm, and a length C _Y of the long side of the mold 10 of 20 mm, when viewed from above.

As shown in Table 5, by setting the taper ratio to 1.1 or more, the occurrence of internal cracks in the slab can be suppressed. Furthermore, by setting the taper ratio to 2.0 or less, the occurrence of breakouts can be suppressed. This shows that it is desirable to set the taper ratio to 1.1 or more and 2.0 or less (i.e., the taper ratio of the tapered portion on the most upstream side in the casting direction is 1.1 to 2.0 times the taper ratio of the tapered portion on the most downstream side in the casting direction).

Using the continuous casting equipment shown in FIG. 1, FIG. 2 and FIG. 5, a slab with a slab width of 1250 to 2300 mm and a slab thickness of 200 to 400 mm was cast. At this time, the taper ratio of the taper part (first taper part T ₁ ) on the most upstream side in the casting direction, the taper ratio of the taper part (second taper part T ₂ ) on the most downstream side in the casting direction, and the position of the first taper change point were changed, and the quality of the produced slab, the stability of operation, and the copper plate life of the mold were verified. The verification results are shown in Table 6 below. The quality of the slab, the stability of operation, and the copper plate life of the mold were evaluated based on the following evaluation criteria. The taper ratio was calculated using the above formula (1). At this time, the reference interval W _V was set to the interval W _M of the short side copper plate 11 at the average position of the meniscus position. The shape of the protruding portion 12 of the mold 10 was a triangle with a length C of the contact portion in contact with the molten steel of 36 mm, a length C _X of the short side of the mold 10 of 30 mm, and a length C _Y of the long side of the mold 10 of 20 mm, when viewed from above.

The quality of the slabs was evaluated based on the presence or absence of internal cracks in the produced slabs.
A: No internal cracks occurred. B: Almost no internal cracks occurred. C: Small internal cracks occurred, but there was no problem with the quality of the product. D: Internal cracks occurred, and there was a problem with the quality of the product.

The stability of operation was evaluated based on the occurrence of breakouts.
A: No breakouts occurred, and there were no small marks on the slab surface where the solidified shell had broken. B: Almost no breakouts occurred, and there were no small marks on the slab surface where the solidified shell had broken. C: Almost no breakouts occurred, and there were small marks on the slab surface where the solidified shell had broken, but these could be removed in a later process, and there was no problem with the quality of the product. D: Breakouts occurred.

The lifespan of the copper plate of the mold was evaluated according to the following criteria for damage caused by corrosion of the copper plate and damage caused by wear.
A: No need to replace for up to 2000 charges (Excellent productivity)
B: No replacement required until 1500 charges C: No replacement required until 1000 charges (productivity is somewhat low)
D: Replacement required after 1000 charges or less (significantly low productivity)

As shown in Table 6, in Examples 1 to 14, the taper ratio of the first taper section _T1, which is the taper section on the most upstream side in the casting direction, was 1.1%/m or more and 3.5%/m or less, so there was no problem with the quality of the slab and stable operation was performed. On the other hand, in Comparative Example 1, the taper ratio of the first taper section _T1 was smaller than 1.1%/m, so no breakout occurred, but internal cracks occurred in the slab. In Comparative Example 2, the taper ratio of the first taper section _T1 was greater than 3.5%/m, so the occurrence of internal cracks in the slab was reduced compared to Comparative Example 1, but breakouts occurred.

In Examples 4 to 6 and 8 to 14, the distance in the casting direction from the average meniscus position to the first taper change point ("first taper change point position" in Table 6) was 50 mm or more and 300 mm or less, which resulted in a further reduction in the occurrence of internal cracks and breakouts in the slab. Furthermore, in Examples 8 to 13, the taper rate of the second taper section _T2, which is the taper section on the most downstream side in the casting direction, was 0.5%/m or more and less than 1.9%/m, and was equal to or less than the taper rate of the upstream taper section, which extended the copper plate life and increased productivity.

In addition, as Comparative Examples 3 and 4, the same verification was also carried out for the case where there was only one taper portion formed inside the mold (i.e., the case of a single taper). As a result, when a strong taper with a relatively large gradient was provided as in Comparative Example 3, almost no internal cracks occurred in the slab, but the frictional restraint force between the mold and the cast piece increased, and breakouts occurred. In this way, in the case of a single taper, it is difficult to set the gradient of the taper portion large because breakouts occur, but from the results of Examples 3 and 4 and Table 1 above, it is possible to provide a taper portion with a larger gradient by providing multiple taper portions inside the mold. As a result, even if a protrusion is provided, it is possible to suppress non-uniform solidification of the corners of the cast piece in the upper part of the mold where solidification shrinkage is large, and it is possible to alleviate strong contact between the cast piece and the mold 10 in the lower part of the mold where solidification shrinkage is small. In addition, when the gradient was made smaller than that of Comparative Example 3 as in Comparative Example 4, breakouts did not occur, but internal cracks occurred in the slab.

The above describes in detail preferred embodiments of the present invention with reference to the attached drawings, but the present invention is not limited to such examples. It is clear that a person with ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

10 Mold 11 Short side copper plate 11a Inner surface of short side copper plate 12 Protrusion 12a, 12f Straight line 12b, 12c, 12d, 12e, 12g Circular arc 13 Long side copper plate 13a Inner surface of long side copper plate _T1 First taper portion _T2 Second taper portion

Claims (7)

A continuous casting mold used in a continuous casting facility for continuously casting steel, comprising:
A pair of long-side copper plates,
A pair of short-side copper plates that are sandwiched between the pair of long-side copper plates and can move along the long-side direction of the long-side copper plates;
Equipped with
The short side copper plate is
The casting mold has two or more different gradients in the casting direction so that a plurality of tapered portions are formed on the inner surface side of the mold to which molten steel is supplied,
The inner surface has protrusions extending in the casting direction at both ends in the short side direction,
The taper ratio of the taper portion on the most upstream side in the casting direction is 1.1%/m or more and 3.5%/m or less,
The taper ratios of the plurality of tapered portions are set to gradually decrease from the upstream side to the downstream side in the casting direction,
The protrusion is formed so that, when the mold is viewed in a plane, the length of the contact portion that comes into contact with molten steel is shorter than the sum of the lengths of the mold in the short side and long side directions, and the difference is 3 mm or more.
The taper ratio R [%/m] is expressed by the following formula (1).
R={(W _T −W _B )/W _V /ΔL}×100 (1)
Here, W _T [m] is the distance between the opposing copper plates on the narrow side at their upper positions, W _B [m] is the distance between the opposing copper plates on the narrow side at their lower positions, W _V [m] is the distance between the opposing copper plates on the narrow side at a predetermined position in the casting direction (reference distance), and ΔL [m] is the length in the casting direction between the upper and lower positions. Note that, in the reference distance, the casting direction position is within a range of 50 mm to 300 mm above the taper change point of the copper plate on the narrow side that is furthest upstream in the casting direction. 2. The continuous casting mold according to claim 1 , wherein the taper rate of the tapered portion on the most downstream side in the casting direction is 0.5%/m or more and less than 1.9%/m and is equal to or less than the taper rate of the tapered portion on the upstream side. The continuous casting mold according to claim 1 or 2 , wherein the mold has two of the tapered portions. The surface connecting the short side copper plate and the long side copper plate formed on the inside of the mold by the protrusion is, when the mold is viewed in plan,
A straight line connecting the short side copper plate and the long side copper plate;
A line connecting the short side copper plate and the long side copper plate and consisting of a convex arc toward the corner side of the mold;
A line connecting the short side copper plate and the long side copper plate, which connects a circular arc that is convex toward the inside of the mold and has one end in contact with the long side copper plate, and a circular arc that is convex toward the corner side of the mold and has one end in contact with the short side copper plate;
or
A line connecting the short side copper plate and the long side copper plate, the line consisting of a first arc convex toward the inside of the mold and one end of which is in contact with the long side copper plate, a second arc convex toward the corner side of the mold and one end of which is in contact with the short side copper plate, and a straight line connecting the other end of the first arc and the other end of the second arc;
The continuous casting mold according to any one of claims 1 to 3, having any one of the following shapes : A method for continuous casting of steel, comprising the steps of:
A pair of long-side copper plates,
A pair of short-side copper plates that are sandwiched between the pair of long-side copper plates and can move along the long-side direction of the long-side copper plates;
Equipped with
The short side copper plate is
The casting mold has two or more different gradients in the casting direction so that a plurality of tapered portions are formed on the inner surface side of the mold to which molten steel is supplied,
The inner surface has protrusions extending in the casting direction at both ends in the short side direction,
The taper ratio of the taper portion on the most upstream side in the casting direction is 1.1%/m or more and 3.5%/m or less,
The taper ratios of the plurality of tapered portions are set to gradually decrease from the upstream side to the downstream side in the casting direction,
The method for continuous casting of steel includes casting steel using a continuous casting mold, the protrusion being formed such that, in a plan view of the mold, the length of a contact portion that contacts molten steel is shorter than the sum of the lengths of the mold in the short side direction and the long side direction, and the difference is 3 mm or more.
The taper rate R [%/m] is expressed by the following formula (1).
R={(W _T −W _B )/W _V /ΔL}×100 (1)
Here, W _T [m] is the distance between the opposing copper plates on the narrow side at the upper position, W _B [m] is the distance between the opposing copper plates on the narrow side at the lower position, W _V [m] is the distance between the opposing copper plates on the narrow side at a predetermined position in the casting direction (reference distance), and ΔL [m] is the length in the casting direction between the upper and lower positions. The reference distance is the distance [m] between the copper plates on the narrow side at the average position of the meniscus of the molten metal in the mold. 6. The method for continuous casting of steel according to claim 5 , wherein a distance in the casting direction from an average position of the meniscus position to a first taper change point of the copper plate on the narrow side is 50 mm or more and 300 mm or less. The method for continuous casting steel according to claim 5 or 6, wherein a slab having a width of 1250 to 2300 mm and a thickness of 200 to 400 mm is continuously cast.

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