Mold for continuous casting and method for continuous casting of steel

Abstract

This invention provides a mold for continuous casting and a method for continuous casting of steel that can suppress the occurrence of surface cracks in the cast slab and internal cracks within the mold by appropriately setting the amount of heat removed from the mold. [Solution] A continuous casting mold for continuously producing cast slabs by cooling injected molten steel and withdrawing the solidified molten steel, wherein the interior is formed into a rectangular shape by a pair of long-side molds and a pair of short-side molds facing each other in a direction perpendicular to the casting direction of the molten steel and cooling the molten steel, and the mold includes chamfers formed to bevel the corners of the interior partitioned by the pair of long-side molds and the pair of short-side molds, and the ratio of the amount of heat removed from the first long-side molds to the amount of heat removed from the second short-side molds is 0.98 to 1.02.

Description

Technical Field

[0001] The present invention relates to a continuous casting mold for continuously casting a cast slab by cooling and solidifying molten steel poured into the mold, and a continuous casting method for steel.

Background Art

[0002] In the continuous casting of steel, the molten steel poured into the mold is cooled by the water-cooled mold, and the molten steel solidifies at the contact surface with the mold to form a solidified layer (hereinafter referred to as "solidified shell"). The solidified shell is continuously drawn downward from the mold together with the internal unsolidified layer while being cooled by water sprays or air-water sprays installed on the downstream side of the mold, and is solidified up to the center by cooling from the water sprays or air-water sprays to produce a cast slab.

[0003] If the cooling in the mold is insufficient, the thickness of the solidified shell becomes thin, and when switching to roll support on the downstream side of the mold, the solidified shell cannot withstand the static pressure of the molten steel generated from the molten steel inside the cast slab, resulting in breakout where the solidified shell breaks and molten steel leaks out. On the other hand, if the inside of the mold is subjected to intense cooling, uneven cooling tends to occur. When the cooling in the mold becomes uneven, the thickness of the solidified shell becomes non-uniform in the casting direction and the slab width direction.

[0004] Stress caused by shrinkage and deformation of the solidified shell acts on the solidified shell, and in the initial stage of solidification, this stress concentrates on the thin portion of the solidified shell, and this stress causes cracks on the surface of the solidified shell. These cracks are enlarged by subsequent external forces such as thermal stress, bending stress and straightening stress from the rolls of the continuous casting machine, and become large surface cracks. When the non-uniformity of the solidified shell thickness is large, longitudinal cracks form in the mold, and breakout where molten steel leaks from these longitudinal cracks may occur in some cases. Since cracks present in the cast slab become surface defects in the subsequent rolling step, it is necessary to repair the surface of the cast slab and remove the surface cracks at the cast slab stage.

[0005] Non-uniform solidification within the mold is caused by deformation of the solidified shell due to distortion resulting from transformation stress caused by volume contraction during the transformation to γ ​​iron (austenite) via peritectic reaction, particularly in materials with a carbon content in the range of 0.08 to 0.17 mass%. This deformation reduces the solidification thickness of the solidified shell in areas away from the inner wall of the mold, and it is thought that the concentration of stress in these areas leads to surface cracking. In particular, increasing the slab withdrawal speed increases the average heat flux from the solidified shell to the mold cooling water, and the distribution of the heat flux becomes irregular and non-uniform, which tends to increase slab surface cracking. Specifically, in slab continuous casting machines with a slab thickness of 200 mm or more, surface cracking is more likely to occur when the slab withdrawal speed exceeds 1.5 m / min.

[0006] To prevent surface cracking in steel types exhibiting the above-mentioned peritectic reaction (hereinafter referred to as "medium-carbon steel"), Patent Document 1 discloses a technique that uses mold powder with a composition that easily crystallizes, thereby increasing the thermal resistance of the mold powder layer and slowing down the solidification shell. However, the slow cooling effect of mold powder alone is not sufficient to improve non-uniform solidification, and surface cracking cannot be suppressed in steel types with a large transformation amount.

[0007] Therefore, numerous methods have been proposed to slow down the cooling of the mold itself for continuous casting. Patent documents 2 and 3 disclose a method of slow cooling by creating grooves or circular holes on the inner wall surface of the mold to form an air gap, thereby preventing surface cracking.

[0008] On the other hand, the methods disclosed in Patent Documents 2 and 3 aim to suppress the growth of the solidified shell by slow cooling, so even if surface cracking and the like can be suppressed, it may not be possible to improve productivity.

[0009] Patent Document 4 discloses a continuous casting mold in which a dissimilar metal layer is provided near the meniscus of the mold cooling plate, protrusions are provided inside the cooling grooves or holes of the casting mold, and round grooves with a difference in thermal conductivity from copper are formed on the inner wall surface of the mold. Patent Document 4 states that this makes it possible to achieve both increased and uniform solidification shell thickness while suppressing surface cracking and achieving high productivity.

[0010] However, the slab (solidified shell) located at the corner of the mold is cooled from both the long and short sides, making it inevitably colder than other parts of the slab. As a result, streak defects, which cause indentations in the slab, and associated transverse crack defects are more likely to occur near the corner of the slab. Patent document 4 does not disclose transverse cracks at the corner of the slab.

[0011] Patent Document 5 discloses a method for continuously casting steel in which chamfered portions are formed at the four corners of a mold whose cross-sectional shape in the casting direction is rectangular, and the water density for secondary cooling directly below the mold is specified. Patent Document 5 states that by forming the chamfered portions and setting the water density for secondary cooling, it is possible to suppress lateral cracking at the corners of the cast slab and to create a cast slab without internal cracking within the mold. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2005-297001 [Patent Document 2] Japanese Patent Application Publication No. 6-297103 [Patent Document 3] Japanese Patent Application Publication No. 10-296399 [Patent Document 4] Japanese Patent Publication No. 2024-47887 [Patent Document 5] Japanese Patent Publication No. 2023-158520 [Overview of the project] [Problems that the invention aims to solve]

[0013] However, the technology disclosed in Patent Document 5 does not disclose the difference in heat removal between the long and short sides of the mold. If there is a difference in heat removal between the long and short sides of the mold, a gap (air gap) may form between the solidified shell and the mold at the corners of the mold. This can cause stress to be applied to the thin areas of the solidified shell near the corners of the mold, potentially resulting in streak defects or longitudinal crack defects in the cast slab.

[0014] This invention has been made in view of these circumstances, and its purpose is to provide a mold for continuous casting and a method for continuous casting of steel that can suppress the occurrence of surface cracks in the cast slab and internal cracks in the mold by appropriately setting the amount of heat removed from the mold. [Means for solving the problem]

[0015] The gist of the present invention, which can solve the above problems, is as follows. [1] A continuous casting mold for continuously producing cast slabs by cooling and solidifying injected molten steel and then withdrawing the solidified molten steel, wherein the interior is formed rectangular by a pair of long-side molds and a pair of short-side molds facing each other in a direction perpendicular to the casting direction of the molten steel and cooling the molten steel, and the mold comprises chamfers formed to bevel the corners of the interior partitioned by the pair of long-side molds and the pair of short-side molds, wherein the chamfers are formed such that the ratio of the amount of heat removed from the pair of long-side molds to the amount of heat removed from the pair of short-side molds to the ratio of the amount of heat removed from the first to the second is 0.98 to 1.02. [2] The continuous casting mold according to [1], wherein the chamfer has a ratio b / a of the length of the pair of short side molds to the length a of the pair of long side molds, which is 3.0 to 6.0. [3] The continuous casting mold according to [2], wherein the chamfer has a length a of the pair of long side mold sides that is 4 to 6 mm long, and a length b of the pair of short side mold sides that is 12 to 36 mm long. [4] The pair of long-side molds further have, in a predetermined range on the inner surface thereof, a different-material filled portion filled with a substance having a thermal conductivity different from the thermal conductivity of the material used for the pair of long-side molds, the continuous casting mold according to any one of [1] to [3]. [5] The continuous casting mold according to any one of [1] to [4], wherein the chamfer has a triangular cross-sectional shape when cut along a plane perpendicular to the casting direction. [6] A continuous casting method for steel, comprising continuously casting hypoperitectic steel having a carbon content of 0.08 to 0.12% using the continuous casting mold according to any one of [1] to [5]. Effects of the Invention

[0016] In the present invention, a chamfer is formed at the corner portion of the inner surface of the mold such that the difference between the first heat extraction amount of the pair of long-side molds facing the direction perpendicular to the casting direction of molten steel and the second heat extraction amount of the pair of short-side molds is reduced. Thereby, the occurrence of a gap (air gap) between the solidified shell and the corner portion at the corner portion between the long-side mold and the short-side mold is suppressed, so that the occurrence of streak defects and surface crack defects in the cast slab can be suppressed. Brief Description of the Drawings

[0017] [Figure 1] FIG. 1 is a schematic perspective view showing a continuous casting mold according to an embodiment of the present invention. [Figure 2] FIG. 2 is a top view of the continuous casting mold shown in FIG. 1 as viewed from above. [Figure 3] FIG. 3 is a top view showing a chamfer provided on the continuous casting mold shown in FIG. 2. [Figure 4] FIG. 4 is a graph showing the relationship between the chamfer ratio and the stress of the solidified shell in the mold width direction. [Figure 5] FIG. 5 is a side view for explaining a different-material filled portion formed in a mold long-side cooling plate. [Figure 6] FIG. 6 is a graph showing the relationship between the chamfer and the heat extraction amount of the mold. [Figure 7] Figure 7 is a graph showing the relationship between the mold material and the presence or absence of chamfering, and longitudinal cracking of the cast slab. [Figure 8] Figure 8 is a graph showing the relationship between the mold material, the presence or absence of a chamfer, and the breakout. [Modes for carrying out the invention]

[0018] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 is a schematic perspective view showing a continuous casting mold 100 according to an embodiment of the present invention. Figure 2 is a top view of the continuous casting mold shown in Figure 1, viewed from above. Figure 3 is a top view showing a chamfer provided in the continuous casting mold shown in Figure 2.

[0019] As shown in Figures 1 and 2, the continuous casting mold 100 (hereinafter sometimes simply referred to as mold 100) for continuously casting slab slabs is formed in a rectangular shape when viewed from above. The continuous casting mold 100 is constructed by combining a pair of mold long-side cooling plates 10 that form the long side and a pair of mold short-side cooling plates 12 that form the short side. The pair of mold long-side cooling plates 10 and the pair of mold short-side cooling plates 12 cool the molten steel by allowing a cooling liquid to flow through grooves or holes formed inside them. In this embodiment, the mold long-side cooling plates 10 and the mold short-side cooling plates 12 may be made of pure copper consisting of 100% by mass of copper, or they may be a copper alloy containing 90% by mass or more of copper, with the remainder being, for example, aluminum, chromium, zirconium, etc. The pair of mold long-side cooling plates 10 are an example of a pair of long-side molds that have been included in the continuous casting mold 100, and the pair of mold short-side cooling plates 12 are an example of a pair of short-side molds that have been included in the continuous casting mold 100.

[0020] As shown in Figure 2, chamfers 16 are formed at each of the four corner sections 14 within the mold 100, which are partitioned by a pair of mold long-side cooling plates 10 and a pair of mold short-side cooling plates 12. The chamfers 16 are made of the same material as the mold 100 and are formed by creating a chamfered shape at the four corners of a rectangular cross-section perpendicular to the casting direction of the slab. The chamfers 16 are formed over the entire corner section 14 in the direction of molten steel injection or the casting direction of the slab, and are integrally formed with the mold long-side cooling plates 10 and the mold short-side cooling plates 12. For example, a rectangular parallelepiped steel plate larger than the dimensions of the pair of mold short-side cooling plates 12 can be machined to form the chamfers 16. That is, the portion corresponding to the corner section 14 is machined using a cutting tool to form the inclined surface of the chamfer 16, while the other portions are machined to a thickness based on the required cooling strength, etc. As a result, a mold short-side cooling plate 12 having a chamfer 16 is manufactured from a rectangular steel plate.

[0021] As shown in Figures 2 and 3, the chamfer 16 is formed such that the length of the long-side chamfer a, which is the length on the long-side cooling plate 10 side of the mold, is shorter than the length of the short-side chamfer b, which is the length on the short-side cooling plate 12 side of the mold. Also, as shown in Figures 2 and 3, the cross-sectional shape of the chamfer 16 when cut by a plane perpendicular to the casting direction is a right-angled triangle shape, with the long-side cooling plate 10 side being the short side, the short-side cooling plate 12 side being the long side, and the other side being the hypotenuse.

[0022] This section explains the relationship between the chamfer ratio b / a (the ratio of chamfer amount a on the long side to chamfer amount b on the short side) and the widthwise stress in the mold 100 of the solidified shell. In Figure 4, the horizontal axis represents the chamfer ratio b / a(-), and the vertical axis represents the stress index value(-), which is an index value of the widthwise stress generated in the solidified shell at the exit side of the mold 100. The stress index value is set to 100 when the chamfer ratio b / a is 0, that is, when both the long and short sides have the same amount of chamfer. Note that (-) means that it is dimensionless.

[0023] As shown in Figure 4, the stress index decreased as the chamfer ratio b / a increased. Furthermore, when the chamfer ratio b / a exceeded approximately 3.0, the decrease in the stress index with increasing chamfer ratio b / a became smaller, and the stress index did not change significantly up to a chamfer ratio b / a of approximately 6.0. Subsequently, when the chamfer ratio b / a was set to 8.0, the stress index increased significantly, and when the chamfer ratio b / a exceeded 12.0, the stress index approached a value close to 100.

[0024] Furthermore, as shown in Figure 4, the smaller the chamfer amount a on the long side, the smaller the stress index value. The stress index value was significantly reduced when the chamfer amount a on the long side was reduced from 7 mm to 5 mm. On the other hand, when the chamfer amount a on the long side was reduced from 5 mm to 4 mm, the decrease in the stress index value was only slight. From these results, it was found that by setting the chamfer ratio b / a within the range of 3.0 to 6.0 and the chamfer amount a on the long side within the range of 4 to 6 mm, the stress generated in the solidified shell at the exit side of the mold 100 can be reduced.

[0025] These results indicate that it is preferable for the chamfer amount a on the long side to be in the range of 4 to 6 mm, and the chamfer amount b on the short side to be in the range of 12 to 36 mm. Furthermore, it was found that within this range, it is preferable for the chamfer 16 to be formed such that the chamfer ratio b / a between the chamfer amount a on the long side and the chamfer amount b on the short side is 3.0 to 6.0, and more preferably 4.0 to 6.0. It was found that by forming the chamfer 16 in this way, the stress load at the corner portion 14 of the cast slab can be reduced, thereby suppressing surface cracking of the solidified shell.

[0026] The shape of the chamfer 16 is not limited to the right-angled triangular shape shown in Figure 3, as long as the chamfer amount a on the long side and the chamfer amount b on the short side are within the range of the above-mentioned chamfer ratio b / a and chamfer amount. In other words, the part corresponding to the hypotenuse of the chamfer 16 shown in Figure 3 may be curved so as to be recessed towards the corner portion 14, or conversely, curved so as to bulge out from the corner portion 14. That is, the chamfer 16 is formed such that the difference in heat removal between the mold long side cooling plate 10 and the mold short side cooling plate 12 is within the above-mentioned range.

[0027] The dissimilar material filling section 20 will now be described. Figure 5 is a side view of the mold long-side cooling plate 10 to illustrate the dissimilar material filling section 20 formed on the inner surface of the mold long-side cooling plate 10. As shown in Figure 4, the dissimilar material filling section 20 is provided in a predetermined area on the inner surface of a pair of mold long-side cooling plates 10. The dissimilar material filling section 20, although not shown in the illustration, has a groove and a layer of dissimilar metals. The groove is formed in a hemispherical shape and is formed on the mold long-side cooling plate 10 at intervals from an arbitrary position above the meniscus (molten steel surface) determined when continuously casting steel, to a position a predetermined length below the meniscus. As an example, the grooves are provided at predetermined intervals in the casting direction of the molten steel and at regular intervals different from the predetermined intervals in the horizontal direction (width direction), which is perpendicular to the casting direction, forming a regular staggered arrangement.

[0028] The dissimilar metal layer is a metal that fills each of the grooves, and the metal filling has a thermal conductivity lower than that of the mold long-side cooling plate 10. For example, the dissimilar metal layer is filled with a metal whose thermal conductivity is 80% or less of that of the mold long-side cooling plate 10. As a result, a dissimilar metal layer is periodically formed in a predetermined area on the upper part of the mold long-side cooling plate 10. The filling metal is filled into the grooves by means of plating or thermal spraying. As a result, a dissimilar metal layer is formed without any gaps between the grooves and the filling metal. The material filling the grooves may be a nonmetal, or it may be a material with a thermal conductivity higher than that of the mold long-side cooling plate 10. For example, a metal or nonmetal with a thermal conductivity of 125% or more of that of the mold long-side cooling plate 10 may be used to fill the grooves.

[0029] The resulting dissimilar material filling section 20 causes the thermal resistance of the continuous casting mold 100 in the width direction and casting direction near the meniscus of the mold's long-side cooling plate 10 to increase and decrease regularly and periodically. As a result, the heat flux from the solidified shell in the initial solidification stage near the meniscus to the continuous casting mold 100 increases and decreases regularly and periodically. That is, the heat flux distribution is made uniform, and the stress generated in the solidified shell is dispersed, so surface cracking of the solidified shell on the mold's long-side cooling plate 10 side can be suppressed. Furthermore, even with subpericrystalline steel, which is highly susceptible to surface cracking among medium-carbon steels, for example, steel with a carbon content of 0.08 to 0.12%, surface longitudinal cracking of the cast slab on the long side of the mold 100 can be suppressed.

[0030] Next, we will explain the occurrence of streak defects, which cause indentations in the cast slab near the corner portion 14 of the mold 100. Due to the relatively large difference in heat removal between the long side and the short side of the mold 100, and because the molten steel at the corner portion 14 is cooled by the mold long side cooling plate 10 and the mold short side cooling plate 12, resulting in a lower temperature, an air gap may occur between the solidified shell and the side surface of the mold 100. This air gap causes the formation of the solidified shell to be slower on the short side than on the long side, and stress is applied to the thin part of the solidified shell near the corner portion 14, which can cause streak defects or longitudinal crack defects. In particular, if the ratio of heat removal from the mold long side cooling plate 10 to the heat removal from the mold short side cooling plate 12 is greater than 1.02 and is 1.10 or higher, the occurrence of streak defects near the corner portion 14 becomes more pronounced.

[0031] The difference in heat dissipation between the long and short sides of the mold 100, which causes such defects, is more likely to occur when the combination or ratio of the length of the mold long-side cooling plate 10 side and the mold short-side cooling plate 12 side of the chamfer 16 is inappropriate. Alternatively, the above difference in heat dissipation is more likely to occur when a dissimilar material filling section 20 is provided on either the mold short-side cooling plate 12 side or the mold long-side cooling plate 10 side.

[0032] For example, if a dissimilar material filling section 20 is provided in the mold's long-side cooling plate 10, the solidified shell thickness becomes uniform on the long side of the mold 100, suppressing the air gap that occurs on that side, and an air gap occurs only on the mold's short-side cooling plate 12 side. As a result, the difference in cooling intensity between the long side and the short side becomes large, and the solidified shell is pulled from the mold's short-side cooling plate 12 side to the mold's long-side cooling plate 10 side. As a result, stress is applied to the solidified shell on the mold's long-side cooling plate 10 side near the corner 14, causing dents or cracks in the solidified shell near the corner 14. Such dents and cracks are also called streak defects or corner longitudinal cracks, and they become a quality problem as defects in the cast slab, requiring a process to remove them.

[0033] In contrast, the chamfer 16 and the dissimilar material filling section 20 in the embodiment of the present invention can suppress the occurrence of the above-mentioned problems. Specifically, because the chamfer 16 has the above dimensions and ratio, and the dissimilar material filling section 20 is arranged in a staggered pattern, the difference in heat removal between the mold long-side cooling plate 10 and the mold short-side cooling plate 12 can be reduced. As a result, surface longitudinal cracks on the solidified shell on the long side and streak defects near the corner section 14 can be suppressed. Furthermore, other defects and inhibiting factors of the cast slab, such as bleed defects where molten steel leaks when the cast slab is drawn and longitudinal crack breakouts, can also be suppressed. And, because streak defects and longitudinal crack defects of the cast slab in continuous casting are suppressed, the cast slab can be transported to the next process without any maintenance, resulting in improved productivity and reduced power consumption during processing in the heating furnace.

[0034] Next, referring to Figure 6, we will explain the relationship between the amount of heat removed from the mold 100 and the use of a chamfer 16 whose chamfer ratio b / a and long-side chamfer amount a satisfy the above range. In Figure 6, the horizontal axis represents the amount of heat removed from the mold long-side cooling plate 10 (MW / m²). 2 The vertical axis represents the amount of heat removed by the mold short-side cooling plate 12 (MW / m²). 2 ) The black circles in the figure represent the heat removal amounts when the mold 100 is provided with a chamfer 16, and the cross marks represent the heat removal amounts when the mold 100 is not provided with a chamfer 16. Each heat removal amount was calculated based on the temperature difference between the inlet and outlet temperatures of the cooling water flowing through the mold long-side cooling plate 10 and the mold short-side cooling plate 12. Furthermore, the molds 100 include both those provided with a filled mold, which is a mold 100 having a dissimilar material filling section 20, and those not provided. The dotted line in the figure is the line that passes through the point where the heat removal ratio between the mold long-side cooling plate and the mold short-side cooling plate is 1:1. The heat removal amount of the mold long-side cooling plate 10 is an example of the first heat removal amount in a pair of long-side molds, and the heat removal amount of the mold short-side cooling plate 12 is an example of the second heat removal amount in a pair of short-side molds.

[0035] As shown in Figure 6, when the chamfer 16 described above was not used, the heat removal ratio was approximately 1.03 to 1.07. On the other hand, when the chamfer 16 described above was used, the heat removal ratio was approximately 0.98 to 1.02. Since the heat removal ratio is close to 1, the occurrence of air gaps and the like in the solidified shell is suppressed. Therefore, it was found that by using the chamfer 16 described above, the occurrence of streak defects and longitudinal crack defects in the cast slab can be suppressed, regardless of whether a filled mold is used or not.

[0036] Figure 7 shows the relationship between the material of the mold 100, the presence or absence of chamfers 16, and longitudinal cracking of the cast slab. For the chamfers 16, those with a chamfer ratio b / a and chamfer amounts a and b within the above range were used. In Figure 7, the horizontal axis shows the type of mold 100 used for the mold long-side cooling plate 10 and the mold short-side cooling plate 12, and the vertical axis shows the treatment rate (%), which is the percentage of cast slabs that developed longitudinal cracks requiring processing. For the mold 100, copper molds (Cu molds) or filled molds with dissimilar material filling sections 20 were used, and chamfers 16 were provided as appropriate. The number of steel plates used in each trial is indicated at the top of the graph.

[0037] As shown in Figure 7, it was found that when using a Cu mold, the maintenance rate can be significantly reduced by using the chamfer 16. Furthermore, it was found that the maintenance rate can be significantly reduced even when a filled mold is used on the mold's long-side cooling plate 10. By using a filled mold on the mold's long-side cooling plate 10, the heat flux distribution is made uniform and the stress generated in the solidified shell is dispersed, so the occurrence of surface cracks is suppressed and the maintenance rate is reduced. In addition, by having the chamfer 16, the difference in the amount of heat removed between the long side and the short side of the mold 100 is reduced, suppressing streak defects and surface cracks in the cast slab and further reducing the maintenance rate.

[0038] Figure 8 shows the relationship between the material of the mold 100, the presence or absence of the chamfer 16, and breakout (BO). In Figure 8, the horizontal axis shows the type of mold 100 used for the mold long-side cooling plate 10 and the mold short-side cooling plate 12, and the vertical axis shows the predictive occurrence rate (%), which is the percentage of notifications indicating the possibility of breakout occurring, excluding false detections. Breakout occurrence was predicted based on multiple thermocouples embedded in the mold long-side cooling plate 10 and the mold short-side cooling plate 12 in pairs, at positions 50 mm below the meniscus and 150 mm below the meniscus in the height direction, and at predetermined intervals in the width direction. Specifically, the temperature difference between the upper and lower thermocouples that are paired in the height direction was measured, and breakout was predicted by determining that there was a possibility of breakout occurring when this temperature difference fell below a predetermined value. The mold 100 used was either a copper mold (Cu mold) or a filling mold having a dissimilar material filling section 20, and chamfers 16 were provided in them as appropriate. The number of trials (charges) for each is indicated at the top of the graph.

[0039] As shown in Figure 8, when a Cu mold was used for the short-side cooling plate 12 of the mold and a filled mold was used for the long-side cooling plate 10 of the mold, the predictable occurrence rate was 12.1%. However, by providing the chamfer 16, the predictable occurrence rate was significantly reduced to 0.3%. In other words, by providing the chamfer 16, the difference in heat removal between the long side and the short side of the mold 100 was reduced, which suppressed streak defects in the cast slab and thus reduced the possibility of breakout.

[0040] Next, a continuous casting method for steel according to an embodiment of the present invention will be described. In this continuous casting method for steel, steel is cast while supporting a cast slab drawn from a continuous casting mold 100 with multiple pairs of opposing rolls. Specifically, first, molten steel is cooled (primary cooling) by being poured into the mold 100. Then, the cast slab is drawn from the mold 100 at a predetermined drawing speed, and this cast slab is cooled (secondary cooling) while being supported by multiple pairs of rolls arranged in the casting direction to obtain a steel slab. For example, in the case of a curved continuous casting machine, at least one pair of rolls for straightening the curved cast slab is provided near the exit side of the mold 100, and the bending is straightened by these rolls and the cast slab is drawn horizontally. Then, the steel slab is rapidly cooled and solidified, and the solidified steel slab is cut to an appropriate length with a cutting machine such as a gas cutting machine to cast the desired steel. [Examples]

[0041] Next, an example of this embodiment will be described. In the example, slabs were cast under various conditions, such as the presence or absence of the dissimilar material filling section 20 on the inner wall surface of the mold 100 and the shape of the chamfer 16, and the heat removal ratio and the quality of the slabs were evaluated. Regarding the quality of the slabs, the occurrence of streak defects and surface crack defects was determined. Specifically, it was determined that these defects were defects that required post-casting maintenance, that is, defects that required surface processing of the slab surface to remove surface cracks.

[0042] In the example, a vertical bending type continuous casting machine was used for mold 100. Slab slabs were continuously cast from medium carbon steel with the following composition: C / 0.05~0.15 mass%, Si / 0.01~0.3 mass%, Mn / 0.8~1.2 mass%, P / 0.006~0.015 mass%, and S / 0.002~0.012 mass%. The casting conditions were a slab thickness of 220~300 mm, a casting width of 1400~2100 mm, and a casting speed in the range of 0.6~2.4 m / min.

[0043] Furthermore, copper molds (Cu molds) or filled molds having a dissimilar material filling section 20 in a predetermined area were used on both the long and short sides of the mold 100. Nickel (Ni), which has a thermal conductivity of 80% or less compared to other parts, was used as the filling material for the dissimilar material filling section 20. The dissimilar material filling section 20 was formed in a staggered arrangement in the range from 50 mm above the meniscus to 200 mm below the meniscus. The casting conditions and evaluation results of the example are shown in Table 1 below.

[0044] [Table 1]

[0045] In Comparative Example 1, copper molds (Cu molds) were used for both the long and short sides of the mold 100. In Comparative Examples 2 and 3, filled molds were used that had a dissimilar material filling section 20 only on the long side. Furthermore, in Comparative Examples 1 and 2, molds 100 without chamfers 16 were used. In Comparative Example 3, a mold 100 was used in which chamfers 16 with a long-side chamfer amount of 3 mm and a chamfer ratio b / a of 8.0 were formed.

[0046] In Comparative Example 1, because a filled mold was not used, the heat flux distribution was not uniform, and it was confirmed that surface cracks occurred because the stress generated in the solidified shell was not dispersed. Furthermore, in Comparative Example 1, because the chamfer 16 was not formed, the difference in heat removal amount or the heat removal amount ratio (heat removal amount from the long side mold / heat removal amount from the short side mold) was large between the long side and the short side, and it was confirmed that streak defects occurred in the cast slab.

[0047] In Comparative Examples 2 and 3, the use of a filled mold on the long side of the mold 100 prevented the occurrence of surface longitudinal crack defects in the cast slab. On the other hand, in Comparative Example 2, because the chamfer 16 was not formed in the mold 100, the heat removal ratio was large, and it was confirmed that an air gap was formed, causing streak defects in the cast slab.

[0048] Furthermore, in Comparative Example 3, a mold 100 with chamfers 16 formed was used, but both the long-side chamfer amount a and the chamfer ratio b / a were outside the above range. As a result, the heat removal ratio was also outside the above range, and it was confirmed that streak defects, resulting in cracks at the corners 14, occurred. This is thought to be because the relatively large difference in heat removal caused the molten steel or solidified shell to shrink towards the short-side cooling plate 12 of the mold, generating relatively large stresses at the corners 14 and causing streak defects.

[0049] In each of the inventive examples corresponding to the present invention, a chamfer 16 was formed on the mold 100 with a long-side chamfer amount a of 4 mm, a short-side chamfer amount b of 20 mm, and a chamfer ratio b / a of 5. In each inventive example, it was confirmed that the heat removal ratio between the long side and the short side of the mold 100 was between 0.98 and 1.02, close to 1, and that the occurrence of streak defects in the cast slab could be prevented. Furthermore, it was confirmed that even when a filled mold was used only on the long side of the mold 100 or on the entire mold 100, the heat removal ratio could be reduced by the chamfer 16, and high-quality cast slabs free from streak defects and surface longitudinal crack defects could be manufactured. [Explanation of Symbols]

[0050] 100 Continuous Casting Molds 10. Cooling plate on the long side of the mold 12. Mold short-side cooling plate 14 Corner section 16 Chamfer 20 Different substance filling section

Claims

1. A continuous casting mold for continuously producing cast slabs by injecting molten steel, cooling it, and then withdrawing the solidified molten steel, The interior is formed into a rectangular shape by a pair of long-side molds and a pair of short-side molds facing each other in a direction perpendicular to the casting direction of the molten steel, and cooling the molten steel. The interior corners, separated by the pair of long-side molds and the pair of short-side molds, are chamfered, and the facility includes chamfers formed to bevel the edges. A mold for continuous casting, wherein the ratio of the amount of heat removed from the first long-side mold to the amount of heat removed from the second short-side mold is 0.98 to 1.

02.

2. The continuous casting mold according to claim 1, wherein the chamfer has a ratio b / a of the length b of the pair of short side mold sides to the length a of the pair of long side mold sides, which is 3.0 to 6.

0.

3. The continuous casting mold according to claim 2, wherein the chamfer has a length a of the pair of long side mold sides that is 4 to 6 mm, and a length b of the pair of short side mold sides that is 12 to 36 mm.

4. The continuous casting mold according to claim 1, wherein the pair of long-side molds further have a heterogeneous material filling section in a predetermined area on the inner surface, in which a substance having a different thermal conductivity than the material used in the pair of long-side molds is filled.

5. The continuous casting mold according to claim 2, wherein the pair of long-side molds further have a heterogeneous material filling section in a predetermined area on the inner surface, in which a substance having a different thermal conductivity than the material used in the pair of long-side molds is filled.

6. The continuous casting mold according to claim 3, wherein the pair of long-side molds further have a heterogeneous material filling section in a predetermined area on the inner surface, in which a substance having a different thermal conductivity than the material used in the pair of long-side molds is filled.

7. The continuous casting mold according to claim 1, wherein the chamfer has a triangular cross-sectional shape when cut by a plane perpendicular to the casting direction.

8. The continuous casting mold according to claim 2, wherein the chamfer has a triangular cross-sectional shape when cut by a plane perpendicular to the casting direction.

9. The continuous casting mold according to claim 3, wherein the chamfer has a triangular cross-sectional shape when cut by a plane perpendicular to the casting direction.

10. The continuous casting mold according to claim 4, wherein the chamfer has a triangular cross-sectional shape when cut by a plane perpendicular to the casting direction.

11. A method for continuous casting steel, comprising continuously casting subpericrystalline steel having a carbon content of 0.08 to 0.12% using a continuous casting mold according to any one of claims 1 to 10.

Images