JP7741378B2 - Continuous casting method for steel - Google Patents

Description

The present invention relates to a method for continuous casting of steel.

Vertical bending or curved continuous casting machines are commonly used in the continuous casting of steel. In a vertical bending continuous casting machine, a vertical billet is cast into the mold, bent into a curved shape in the bending section below the mold, and straightened in the straightening section where it faces horizontally after passing through the curved section, with casting completed in the horizontal section. In a curved continuous casting machine, a curved billet is cast into the mold, passed through the curved section below the mold, and straightened in the straightening section where it faces horizontally, with casting completed in the horizontal section. Here, the top side of the billet just after passing through the horizontal section is called the "upper surface" and the bottom side is called the "lower surface." In the bending section of a vertical bending machine, compressive stress is applied to the top surface of the billet, and tensile stress is applied to the bottom surface. In both the vertical bending and curved straightening sections, tensile stress is applied to the top surface of the billet, and compressive stress is applied to the bottom surface.

Regarding the high surface temperature of a slab during continuous casting, it is known that a temperature range (brittle temperature range) exists near the austenite-to-ferrite phase transformation region where cracks are likely to occur when subjected to tensile stress. This is also known as the third region of the embrittlement temperature range. Tensile stress is applied to the lower surface of the bent section and the upper surface of the straightened section during continuous casting, so if the surface temperature of the slab in these areas is within the embrittlement temperature range, cracks are likely to occur on the surface of the slab. Cracks occur particularly near the corners of the long sides. The cracks that occur are transverse cracks that occur along the prior austenite grain boundaries.

Conventionally, bending and straightening are performed by avoiding the temperature range where ductility decreases (the embrittlement temperature range) on the higher side of the slab surface at the bending and straightening sections, thereby preventing cracks from occurring. However, because the areas around the corners of the slab are cooled from both the long and short sides, the surface temperature is lower than in other parts of the slab, and bending and straightening can inevitably occur in the embrittlement temperature range. As a result, cracks easily occur near the corners.

As shown in Figure 2, the mold of a continuous slab casting machine that casts slabs is formed by two opposing long mold sides 5 and two opposing short mold sides 6. The two short mold sides 6 are constrained by being sandwiched between the two long mold sides 5. The space enclosed by these long mold sides 5 and short mold sides 6 is the space into which molten steel is poured to form a cast piece, and is hereinafter referred to as the "casting space 3." In a typical continuous casting mold, the casting space is rectangular, and the cross-sectional shape of the cast piece is rectangular.

A known mold has overhangs 2 at the corners of the inner surface of the mold that forms the casting space 3, thereby forming chamfers 7 at the four corners of the rectangular cross-section of the casting space 3. These are also known as chamfered molds. The cross-section of a slab cast using a chamfered mold also has chamfered corners.

Patent Document 1 discloses an invention that includes two long mold sides and two short mold sides with protrusions formed on both end portions to form chamfered surfaces at the corners of the slab. By forming the protrusions (extensions) on the short mold sides, it is possible to prevent chamfered surfaces from being formed at the corners of the slab during continuous casting, which would otherwise cause a sudden drop in corner temperature, and to avoid the brittle zone in the third region during bending or proofreading (straightening) of the slab, thereby reducing the occurrence of corner cracks.

Non-Patent Document 1 describes how using a chamfered mold instead of a right-angle mold increases the temperature of the slab corner at the bottom of the mold, reduces the maximum stress at the edge at the straightening point, and effectively improves transverse corner cracking in low-alloy high-tensile steel slabs.

Patent Document 2 states that when a slab is cast using the above-mentioned chamfered mold, cooling of the slab corners is slow, which may prevent the formation of a sound slab shell (solidified shell) at the slab corners. For example, this could result in a breakout, in which the slab shell at the slab corner breaks and unsolidified molten steel leaks out, potentially leading to the interruption of casting operations, a long downtime, or even accidents such as equipment damage. It also states that by forming inclined portions on the short sides whose spacing gradually narrows in accordance with the amount of solidification shrinkage of the slab shell in the direction the slab is withdrawn, a sound slab shell can be formed at the slab corners, thereby suppressing or even preventing breakouts.

Non-patent document 2 states that breakouts occurred due to insufficient cooling of the corners of chamfered molds, so the cooling structure was redesigned.

Patent Document 3 describes continuous casting using a vertical bending type continuous casting machine, in which a mold is used that has a casting space in which the four corners of the rectangular space defined by the long and short sides of the mold are removed in the shape of a right triangle at a specified length ratio. This reduces the stress load at the corners of the slab, and, combined with the secondary cooling conditions, prevents surface cracks from occurring at the corners of the slab during bending. Before reaching the bending section from directly below the mold, the surface temperature of at least the corners of the slab is temporarily reduced to below the Ar3 point, and then the surface temperature of at least the corners is raised to above 800°C before the slab passes through the bending section at above 800°C.

Patent Document 4, in relation to bloom continuous casting, discloses a measure to reduce longitudinal crack defects that occur near the corners of bloom strands by reducing the cooling capacity near the corners of the long sides of the mold when using a normal rectangular mold rather than the chamfered mold described above.

In mold copper plates, cooling water channels are formed within the copper plate. The channels are formed as slits or through holes, and the mold copper plate is cooled by cooling water flowing through the channels. The surface of the mold copper plate's long sides, whether it is a rectangular mold or a chamfered mold, generally wears off during casting, and the worn copper plate is reused by grinding off a few millimeters of the surface. In such cases, the distance between the copper plate surface and the cooling water channels is shortened, increasing the cooling capacity of the mold copper plate.

Patent Document 5, which focuses on a standard rectangular mold without chamfering, describes a phenomenon in which grinding the long-side copper plate increases cooling capacity and causes a decrease in the surface temperature of the end of the short-side copper plate. It then cites Patent Document 4 (countermeasures against vertical corner cracks) as an example and discloses a method for adjusting the flow rate of cooling water in the cooling water flow passages on the end side of the short-side copper plate depending on the number of times the long-side copper plate is ground, with the aim of reducing corner cracks in continuous slab casting.

International Publication No. WO2013/100499 JP 2015-128776 A International Publication No. WO2016/013186 Japanese Patent Application Publication No. 03-000453 JP 2016-112589 A

P.Hu et.al. "Application of a Novel Chamfered Mold to Suppress Corner Transverse Cracking of Micro-Alloyed Steel Slabs" Metall. Res. Technol. 112, 104 (2015) G.Liu et.al. "Application of a chamfered mold to improve corner defects of HSLA during slab continuous casting" Metals 2020, 10, 1289

As is clear from the descriptions in Patent Document 1 and Non-Patent Document 1, in continuous casting, by using a continuous casting mold that forms chamfers at the four corners of the rectangular shape of the casting space, i.e., a chamfer mold, the surface temperature of the slab at the corners of the slab at bending and straightening sections during continuous casting rises above the embrittlement temperature range, preventing the occurrence of cracks (lateral cracks). However, it has been found that when long-side copper plates are ground before being used for continuous casting, cracks at the corners of the slab may not be prevented even when a chamfer mold is used.

Patent Document 5 discloses an invention that reduces corner cracks (vertical cracks) during continuous slab casting by adjusting the flow rate of cooling water in the cooling water channels on the end sides of the narrow side copper plates depending on the number of times the long side copper plates are ground, when a normal rectangular mold without chamfered portions is used, as described above. However, when a chamfered mold is used and the long side copper plates are ground, it has been found that horizontal corner cracks in the cast slab cannot be reduced even if the flow rate of cooling water in the cooling water channels on the end sides of the narrow side copper plates is reduced.

The objective of the present invention is to provide a method for continuous casting of steel that can prevent transverse cracks from occurring in the corners of a cast piece when using a continuous casting mold that forms chamfered portions at the four corners of the rectangular shape of the casting space, even when the copper plate on the long sides is ground before being used for continuous casting.

That is, the gist of the present invention is as follows.
[1] A method for continuous casting steel using a continuous casting mold having two opposing long mold sides and two short mold sides sandwiched between the long mold sides,
In the continuous casting mold, a space through which a cast piece passes is called a casting space, and a cross section of the casting space at a meniscus position in the casting direction is called a casting cross section, and the casting cross section has chamfered portions at its four corners, and the short sides of the mold have protrusions corresponding to the chamfered portions of the casting cross section on the surfaces facing the casting space at both ends that are in contact with the long sides of the mold,
A portion of the surface of the short side of the mold facing the casting space that is in contact with the long side of the mold is called the "extension end," and the temperature of the extension end at the meniscus position during continuous casting is called the "extension end temperature,"
The long sides of the mold have a long side copper plate on the side facing the casting space, and the long side copper plate has a cooling mechanism, and the temperatures of the end of the overhanging portion when a cast piece is cast under the same continuous casting conditions using the continuous casting mold incorporating each of the long side copper plates at the start of use of the long side copper plate and after the surface of the long side copper plate has been ground are defined as an initial overhanging portion end temperature and an overhanging portion end temperature after grinding, respectively,
A method for continuous casting steel, characterized in that the heat transfer coefficient between the side of the copper plate with long sides facing the casting space and the cooling mechanism is reduced after grinding the surface of the copper plate with long sides so that the difference between the temperature at the end of the initial extension portion and the temperature at the end of the extension portion after grinding is 15°C or less.
[2] A method for continuous casting steel as described in [1], characterized in that the heat transfer coefficient is reduced by reducing the flow rate of the cooling water flowing through the cooling mechanism after the surface of the long side copper plate has been ground, compared to the flow rate at the start of use of the long side copper plate .
[3] A continuous steel casting method as described in [2], characterized in that when adjusting the flow rate of the cooling water in the long side copper plate, the flow rate of the cooling water is uniformly reduced for all of the cooling water flow paths installed in the long side copper plate, or the flow rate of the cooling water is reduced for only some of the cooling water flow paths installed in the long side copper plate.

The continuous steel casting method of the present invention, when using a continuous casting mold that forms chamfered portions at the four corners of the rectangular casting space, can prevent the occurrence of transverse cracks in the corners of the cast slab by reducing the heat transfer coefficient between the side of the long-side copper plate facing the casting space and the cooling mechanism, even when the long-side copper plate is ground before being used for continuous casting.

1A and 1B are diagrams showing a continuous casting mold, in which (A) is a plan view, and (B) and (C) are enlarged views of the X portion, showing the copper plate on the long side before and after grinding, respectively. 1A and 1B are diagrams showing a continuous casting mold, in which (A) is a side cross-sectional view taken along the line AA, (B) is a plan cross-sectional view taken along the line BB, and (C) is a diagram showing a casting cross section.

The continuous casting mold 1 used in the continuous casting method for steel of the present invention will be described with reference to Figures 1 and 2. Figure 1 shows the continuous casting mold, with (A) being a plan view and (B) and (C) being enlarged views of the X portion of (A).

The continuous casting mold 1 has two opposing long mold sides 5 and two short mold sides 6 sandwiched between the long mold sides 5. The space in the continuous casting mold 1 through which the cast piece passes is called the casting space 3, and the cross section of the casting space 3 at the meniscus position 9 in the casting direction is called the casting cross section 4. The casting cross section 4 has chamfered portions 7 at its four corners. The short mold sides 6 have overhangs 2 at both ends that contact the long mold sides 5, corresponding to the chamfered portions 7 of the casting cross section 4. The portion of the surface (inner surface 15) of the short mold sides 6 that faces the casting space 3 and contacts the long mold sides 5 is called the overhang end 16, and the temperature of the overhang end 16 at the meniscus position 9 during continuous casting is called the overhang end temperature.

The mold long sides 5 and mold short sides 6 each have a long-side copper plate 10 and a short-side copper plate 14, which have a cooling mechanism 11 on the side facing the casting space 3. Both the long-side copper plate 10 and the short-side copper plate 14 are made of copper or a copper alloy, which has good thermal conductivity. The long-side copper plate 10 and the short-side copper plate 14 are supported from their backs by a back frame 13. The cooling mechanism 11 has a mechanism for circulating cooling water through the cooling water flow path 12. The cooling water flow path 12 is composed of slits 12A or through holes 12B that are provided in the long-side copper plate 10 and the short-side copper plate 14 in the casting direction. When slits 12A are used as the cooling water flow path 12, the cooling water flow path 12 is formed by the slits 12A in the long-side copper plate 10 and the back frame 13, as shown in Figure 1(B). In the example shown in Figure 1(B), for the mold short side 6, a mechanism is used to circulate cooling water through through holes 12B (facing the casting direction) provided in the short side copper plate 14, which serve as cooling water flow paths 12. Of course, slits 12A can also be used as cooling water flow paths 12 for the short side copper plate 14.

As mentioned above, the long-side copper plate 10 is made of copper or a copper alloy, which has good thermal conductivity. The inner surface 15 of the long-side copper plate 10 is the surface that comes into contact with the cast slab, so it is nickel-plated or chrome-plated to prevent wear on the inner surface 15. However, when the number of casting charges exceeds 1,500 charges, the inner surface 15 of the long-side copper plate 10 becomes worn. Continuing to use a long-side copper plate 10 with worn plating on the inner surface 15 can cause defects in the surface quality of the cast slab. Therefore, the long-side copper plate 10 is removed from the continuous casting mold 1, and the inner surface 15 is ground depending on the degree of wear on the inner surface 15 of the long-side copper plate 10. It is then plated again and reassembled into the continuous casting mold 1 for reuse.

The thickness 20 of the long-side copper plate 10 decreases depending on the number of times the long-side copper plate 10 is ground (see Figure 1). Figure 1(B) shows the long-side copper plate 10 before grinding, with a thickness 20 of 40 mm. Figure 1(C) shows the long-side copper plate 10 after grinding, with a thickness 20 of 35 mm. As mentioned above, the long-side copper plate 10 has a cooling water flow path 12 formed therein. In the long-side copper plate 10, the distance between the position closest to the inner surface 15 of the cooling water flow path 12 and the inner surface 15 (shortest inner surface distance 21) also decreases in response to the decrease in the thickness 20 of the long-side copper plate 10. As the shortest inner surface distance 21 becomes shorter, the heat transfer coefficient between the side of the long-side copper plate 10 facing the casting space 3 (inner surface 15) and the cooling mechanism 11 increases. The heat transfer coefficient here refers to the heat flux entering from the inner surface 15 of the long-side copper plate 10 divided by the difference between the temperature of the inner surface 15 of the long-side copper plate 10 and the temperature of the cooling mechanism 11 (cooling water temperature).

As mentioned above, in continuous casting, by using a mold that forms chamfered portions 7 at the four corners of the rectangular shape of the casting cross section 4, i.e., a continuous casting mold 1 (chamfered mold) that has protrusions 2 at the ends of the mold's short sides 6, the surface temperature of the slab at the corners of the slab at bending and straightening sections during continuous casting rises above the embrittlement temperature range, preventing the occurrence of cracks (transverse cracks). However, it has been found that when a copper plate with long sides 10 is ground before being used for continuous casting, it may not be possible to prevent cracks from occurring at the corners of the slab, even when a chamfered mold is used.

Here, we attempted to calculate the temperature distribution within the mold during continuous casting using heat transfer calculations. For a continuous casting mold 1 having the structure shown in Figure 1, the portion of the mold short side 6's surface (inner surface 15) facing the casting space 3 that comes into contact with the mold long side 5 is the overhang end 16, and the temperature of the overhang end 16 at the meniscus position 9 during continuous casting is defined as the overhang end temperature. As shown in Figures 1(B) and (C), the cooling mechanism 11 for the long side copper plate 10 has slits 12A as cooling water channels 12, and the cooling mechanism 11 for the short side copper plate 14 has through holes 12B as cooling water channels 12. It is assumed that there is contact resistance at the contact surfaces between the long side copper plate 10 and the short side copper plate 14.

Heat transfer calculations were performed for mold shapes for steel slabs (width: 2200 mm, thickness: 300 mm). Nos. 1 to 6 in Table 1 use chamfered molds, and No. 7 uses a rectangular mold. The chamfered shape of the chamfered mold has a long side length a = 10 mm and a short side length b = 20 mm, with a straight line connecting the base 17 of the overhang and the end 16 of the overhang. For the long-side copper plate 10, one with an initial copper plate thickness (40 mm) before grinding and one with a copper plate thickness (35 mm) after grinding were used.

A heat flux of 2.0 (MW/m ² ) simulating the temperature directly below the meniscus was applied to the inner surfaces 15 of the long-side copper plates 10 and the short-side copper plates 14. No. 1 in Table 1 is a reference, and the flow rate of the cooling water in the cooling water flow paths 12 of the long-side copper plates 10 and the short-side copper plates 14 was set to 9 (m/s). The temperature distribution in the mold in a steady state was calculated, and the temperature of the protrusion end 16 and the temperature of the central part 18 of the inner surface were calculated.

No. 7 in Table 1 is a reference example using a rectangular mold, and in the heat transfer calculation, the temperature at the corners of the rectangular mold was calculated instead of the temperature at the end of the overhanging part of the chamfered mold. For No. 7, the corner temperatures of the rectangular mold are entered in italics in the "Short side copper plate temperature/overhanging part temperature" column in Table 1.

No. 1 in Table 1, the reference sample, uses an unground copper plate (initial long-side copper plate) (thickness 20 = 40 mm) as the long-side copper plate 10, and the temperature at the end of the overhanging portion was 178°C. In Table 1 below, the difference ΔT (initial overhanging portion end temperature - overhanging portion end temperature after grinding) between the overhanging portion end temperature of No. 1 and that of each other sample is listed in the "Temperature difference ΔT" column for each sample. It was found that when a long-side copper plate 10 (long-side copper plate after grinding) with a thickness 20 of 35 mm was used in comparison with No. 1, the temperature at the end of the overhanging portion dropped to 158°C (ΔT = 20°C) (No. 2 in Table 1 below).

Patent Document 5 discloses that when a copper plate on a long side of a standard rectangular mold without chamfered portions is ground 4 mm, the temperature at the corners of the copper plate on the short sides can be restored to the temperature it would be without grinding by slowing down the flow rate of cooling water in the cooling water flow passages on the end sides of the copper plate on the short sides. Therefore, in the present invention, which uses a chamfered mold, the flow rate of cooling water in only the cooling water flow passage on the end side of the copper plate on the short sides (through hole 12Ba in Figure 1(C)) was reduced from 9 (m/s) to 4 (m/s), while the flow rate of cooling water for the other through holes 12B (through holes 12Bb to 12Bc in the range shown in Figure 1(C)) was kept at 9 (m/s). The results showed that under these conditions, the temperature at the end of the protrusion could only be raised to 161°C (ΔT = 17°C) (No. 3 in Table 1 below).

The inventors focused on the effect of adjusting the cooling water flow rate of the long-side copper plate, rather than adjusting the cooling water flow rate of the short-side copper plate. Using a long-side copper plate 10 that had been ground to a thickness 20 of 35 mm, the flow rate of the cooling water flowing through the cooling water flow path 12 of the long-side copper plate 10 was reduced from 9 m/s to 4 m/s, and it was found that the temperature at the end of the protrusion recovered to 169°C (ΔT = 9°C) (No. 4 in Table 1 below).

When adjusting the cooling water flow rate in the long-side copper plate, as described above, it is also possible to uniformly reduce the cooling water flow rate for all of the multiple cooling water flow paths installed in the long-side copper plate. In the present invention, it is also possible to reduce the cooling water flow rate for only some of the cooling water flow paths installed in the long-side copper plate. In No. 5 in Table 1, of the slits 12A that make up the cooling water flow path 12 shown in Figure 1, the cooling water flow rate was set to 4 (m/s) for only slits 12Aa and 12Ab, while the cooling water flow rate for the other slits 12A (slits 12Ac to 12Ae in the range shown in Figure 1(C)) was kept at 9 (m/s). Even in this case, it was confirmed that the temperature at the end of the protrusion recovered to 169°C (ΔT = 9°C) (No. 5 in Table 1). Furthermore, in No. 5 in Table 1, In Example 6, the cooling water flow rate was set to 2.5 m/s for slits 12Aa and 12Ab only, while the cooling water flow rate for the other slits 12A (slits 12Ac to 12Ae in the range shown in Figure 1(C)) was kept at 9 m/s. It was confirmed that the temperature at the end of the protrusion recovered to 175°C (ΔT = 3°C).

Next, as described in the Examples below, actual continuous casting of steel was performed using the conditions of Nos. 1 to 7 in Table 1, and the occurrence of transverse corner cracks in the cast slab was evaluated. As a result, transverse corner cracks were observed in Nos. 2, 3, and 7 in Table 1, while no transverse corner cracks occurred in Nos. 1, 4 to 6. These experimental results demonstrated that transverse corner cracks in cast slabs can be prevented by reducing the difference between the initial overhang end temperature and the overhang end temperature after grinding. Furthermore, regarding the heat transfer coefficient between the side of the long-side copper plate facing the casting space and the cooling mechanism, the difference ΔT between the initial overhang end temperature and the overhang end temperature after grinding can be reduced by some means after grinding the surface of the long-side copper plate. Further detailed investigation revealed that transverse cracks at the corners of cast slabs can be prevented by keeping the difference between the initial overhang end temperature and the overhang end temperature after grinding (initial overhang end temperature - overhang end temperature after grinding) ΔT at 15°C or less. It is even more preferable to keep the difference between the initial overhang end temperature and the overhang end temperature after grinding (initial overhang end temperature - overhang end temperature after grinding) ΔT at 10°C or less.

As described above, the means for increasing the temperature of the inner surface 15 by reducing the flow rate of the cooling water flowing through the cooling mechanism 11 of the long-side copper plate 10 can be said to be a means for increasing the temperature of the inner surface 15 by reducing the heat transfer coefficient between the side of the long-side copper plate 10 facing the casting space 3 (inner surface 15) and the cooling mechanism 11. Furthermore, the temperature at the end of the protrusion can be determined as a result of the heat transfer calculation, as described above.

Conversely, as a specific means for reducing the heat transfer coefficient between the side of the long-side copper plate 10 facing the casting space 3 and the cooling mechanism 11 after the surface of the long-side copper plate 10 has been ground, it can be said that a means for reducing the flow rate of the cooling water flowing through the cooling mechanism 11 after the surface of the long-side copper plate 10 has been ground, compared to the flow rate when the long-side copper plate 10 is first used, can be suitably used.

Here, we will explain the preferred conditions for the chamfered portion 7 formed on the casting cross section 4 and the protruding portion 2 on the mold short side 6 for forming the chamfered portion 7 in this invention. In Figure 2(B), the point of contact between the protruding portion 2 and the portion of the inner surface 15 of the mold short side 6 where the inner surface 15 is perpendicular to the mold long side 5 is called the "protruding portion base 17." The protruding portion end 16 is as described above. The distance between the protruding portion end 16 and the straight line extending from the portion where the inner surface 15 is perpendicular to the mold long side 5 is called the "long side side length a," and the distance between the protruding portion base 17 and the inner surface 15 of the mold long side 5 is called the "short side side length b" (see Figure 2(B)).

The following explains the preferred ranges for the long side length a and the short side length b of this invention. The present invention can be implemented effectively when the long side length a is 10 to 30 mm and the short side length b is 20 to 50 mm.

When casting steel slabs (width: 2200 mm, thickness: 300 mm) using a vertical bending type continuous casting machine, Nos. 1 to 6 in Table 1 were cast using a chamfered mold, and No. 7 was cast using a rectangular mold. The chamfered shape of the chamfered mold was such that the long side length a = 10 mm and the short side length b = 20 mm, with a straight line connecting the base 17 of the protrusion and the end 16 of the protrusion. The long-side copper plates 10 used were those with an initial thickness of 40 mm before grinding and those with a thickness of 35 mm after grinding. The secondary cooling conditions were such that no horizontal corner cracks occurred on either the upper or lower surface of the slab when casting using long-side copper plates 10 with an initial thickness of 40 mm.

Continuous casting was performed using the conditions No. 1 to No. 7 in Table 1, and the cast pieces were evaluated for the presence or absence of transverse cracks at the corners. As a result, transverse cracks were observed in No. 2, 3, and No. 7 in Table 1, while no transverse cracks were observed in No. 1, 4 to 6.

DESCRIPTION OF SYMBOLS 1 Continuous casting mold 2 Overhanging portion 3 Casting space 4 Casting cross section 5 Mold long side 6 Mold short side 7 Chamfered portion 9 Meniscus position 10 Long side copper plate 11 Cooling mechanism 12 Cooling water flow path 12A Slit 12B Through hole 13 Back frame 14 Short side copper plate 15 Inner surface 16 Overhanging portion end 17 Overhanging portion base 18 Inner surface center 20 Thickness 21 Inner surface shortest distance

Claims (3)

A method for continuously casting steel using a continuous casting mold having two opposing long mold sides and two short mold sides sandwiched between the long mold sides,
In the continuous casting mold, a space through which a cast piece passes is called a casting space, and a cross section of the casting space at a meniscus position in the casting direction is called a casting cross section, and the casting cross section has chamfered portions at its four corners, and the short sides of the mold have protrusions corresponding to the chamfered portions of the casting cross section on the surfaces facing the casting space at both ends that are in contact with the long sides of the mold,
A portion of the surface of the short side of the mold facing the casting space that is in contact with the long side of the mold is called the "extension end," and the temperature of the extension end at the meniscus position during continuous casting is called the "extension end temperature,"
The long sides of the mold have a long side copper plate on the side facing the casting space, and the long side copper plate has a cooling mechanism, and the temperatures of the end of the overhanging portion when a cast piece is cast under the same continuous casting conditions using the continuous casting mold incorporating each of the long side copper plates at the start of use of the long side copper plate and after the surface of the long side copper plate has been ground are defined as an initial overhanging portion end temperature and an overhanging portion end temperature after grinding, respectively,
A method for continuous casting steel, characterized in that the heat transfer coefficient between the side of the copper plate with long sides facing the casting space and the cooling mechanism is reduced after grinding the surface of the copper plate with long sides so that the difference between the temperature at the end of the initial extension portion and the temperature at the end of the extension portion after grinding is 15°C or less. The continuous steel casting method described in claim 1, characterized in that the heat transfer coefficient is reduced by reducing the flow rate of the cooling water flowing through the cooling mechanism after the surface of the long-side copper plate has been ground, compared to the flow rate at the start of use of the long-side copper plate. A continuous steel casting method as described in claim 2, characterized in that when adjusting the flow rate of the cooling water in the long side copper plate, the flow rate of the cooling water is uniformly reduced in all of the cooling water flow paths installed in the long side copper plate, or the flow rate of the cooling water is reduced in only some of the cooling water flow paths installed in the long side copper plate.