Method for determining surface defects in a cast slab, apparatus for determining surface defects in a cast slab, and method for manufacturing a cast slab

Abstract

This invention provides a method for determining surface defects in a cast slab that can determine surface defects in a cast slab caused by the flow state of molten steel within a mold produced by a continuous casting machine. [Solution] A method for determining surface defects in a cast slab cast by a continuous casting machine, comprising: an estimation step of estimating the flow velocity distribution of molten steel in a mold using the operating conditions of the continuous casting machine and temperature data of the molten steel in the mold; an identification step of identifying a representative value of the discharge flow velocity difference of molten steel discharged from each of a pair of discharge ports of an immersion nozzle and a representative value of the molten steel surface flow velocity in the mold using the flow velocity distribution; and a determination step of determining surface defects in the cast slab based on the representative value of the discharge flow velocity difference and the representative value of the molten steel surface flow velocity.

Description

Technical Field

[0001] The present invention relates to a method for determining surface defects of a slab, an apparatus for determining surface defects of a slab, and a method for manufacturing a slab. Background Art

[0002] A continuous caster is an apparatus for manufacturing a slab by pouring molten steel from a ladle into a tundish, cooling the molten steel in a mold, and continuously drawing the cooled steel out from the lower part of the mold. The quality of a slab manufactured by a continuous caster may be degraded due to inclusions, cracks, segregation and the like. For this reason, techniques for preventing quality degradation of slabs manufactured by continuous casters have been developed.

[0003] For example, oxides generated by oxidation of molten steel and inclusions remaining near the surface of a slab caused by entrainment of slag, mold flux and the like into molten steel become starting points of cracks during processing or cause linear flaws on the surface of the slab. In particular, when high-speed casting is performed with a continuous caster, the discharge flow from discharge ports of an immersion nozzle that supplies molten steel to a mold becomes unstable, causing a phenomenon called uneven flow in which the discharge flows from the left and right discharge ports become non-uniform. When uneven flow occurs, slag and mold flux are easily entrained into molten steel, which easily causes surface defects of the slab.

[0004] Regarding surface defects of slabs, techniques focusing on the relationship with the flow state of molten steel in a mold have been developed conventionally. Patent Document 1 discloses a method for estimating surface defects of a slab, in which a plurality of molten steel level meters for measuring molten steel level are provided at predetermined positions on the upper part of the mold. According to Patent Document 1, when at least one detection value of a molten steel level meter exceeds a threshold set according to the operating conditions of the continuous caster, it is possible to estimate that surface defects will occur in the slab.

[0005] Patent Document 2 discloses a method for controlling molten steel flow within a mold, in which molten steel level gauges are provided on both sides of an immersion nozzle that supplies molten steel into the mold, and the measured molten steel level values ​​are subjected to a Fast Fourier Transform to calculate a frequency spectrum. According to Patent Document 2, the calculated frequency spectrum can be used to determine whether or not molten steel flow is occurring within the mold, as well as the direction and degree of such flow. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-104221 [Patent Document 2] Japanese Patent Application Publication No. 3-294053 [Patent Document 3] Japanese Patent Publication No. 2016-16414 [Patent Document 4] Japanese Patent Publication No. 2017-159363 [Overview of the project] [Problems that the invention aims to solve]

[0007] The technology disclosed in Patent Document 1 detects the molten metal level near the long and short sides of a mold. However, measuring the molten metal level near the long and short sides of a mold makes it difficult to estimate the flow state of the molten steel near the immersion nozzle or inside the mold. Therefore, the technology disclosed in Patent Document 1 has the problem of not being able to predict the behavior of mold flux entrapment into the molten steel.

[0008] The technology disclosed in Patent Document 2 estimates the flow deviation behavior using molten metal level gauges installed on both sides of the immersion nozzle. However, it is difficult to estimate the flow state of the molten steel inside the mold based solely on information regarding the height of the molten steel surface. For this reason, the technology disclosed in Patent Document 2 also has the problem of not being able to predict the behavior of mold flux being drawn into the molten steel.

[0009] The present invention has been made to solve the above problems, and its objective is to provide a method for determining surface defects in a cast slab and a device for determining surface defects in a cast slab that can determine surface defects in a cast slab caused by the flow state of molten steel in a mold using a continuous casting machine. Another objective of the present invention is to provide a method for manufacturing a cast slab that can produce a cast slab with reduced surface defects. [Means for solving the problem]

[0010] The means to solve the above problems are as follows: [1] A method for determining surface defects in a cast slab cast by a continuous casting machine, comprising: an estimation step of estimating the flow velocity distribution of molten steel in a mold using the operating conditions of the continuous casting machine and temperature data of the molten steel in the mold; an identification step of identifying a representative value of the difference in discharge flow velocity of molten steel discharged from each of a pair of discharge ports of an immersion nozzle and a representative value of the surface flow velocity of molten steel at the surface of the molten steel in the mold, using the flow velocity distribution; and a determination step of determining surface defects in the cast slab based on the representative value of the difference in discharge flow velocity and the representative value of the surface flow velocity. A method for determining surface defects in a cast slab, including [the specified method]. [2] The method for determining surface defects in a cast slab according to [1], wherein in the estimation step, the flow velocity distribution is continuously estimated, and in the identification step, the maximum value among the discharge velocity differences obtained from a plurality of flow velocity distributions continuously estimated in the estimation step is used as a representative value of the discharge velocity difference, and the maximum value of the molten metal surface velocity in the plurality of flow velocity distributions is used as a representative value of the molten metal surface velocity. [3] The method for determining surface defects in a cast slab according to [1] or [2], wherein in the determination step, it is determined that a surface defect occurs in the cast slab if both the representative value of the discharge flow velocity difference and the representative value of the molten metal flow velocity exceed a first threshold set for each. [4] The method for determining surface defects in a cast slab according to [1] or [2], wherein in the determination step, it is determined that no surface defects occur in the cast slab if both the representative value of the discharge flow velocity difference and the representative value of the molten metal flow velocity are less than a second threshold set for each. [5] A device for determining surface defects in a cast slab, comprising: an estimation unit that estimates the flow velocity distribution of molten steel in a mold using the operating conditions of the continuous casting machine and temperature data of the molten steel in the mold; an identification unit that uses the flow velocity distribution to identify a representative value of the difference in discharge flow velocity of molten steel discharged from each of a pair of discharge ports of an immersion nozzle and a representative value of the surface flow velocity of molten steel in the mold; and a determination unit that determines surface defects in the cast slab based on the representative value of the difference in discharge flow velocity and the representative value of the surface flow velocity. [6] The estimation unit continuously estimates the flow velocity distribution, The device for determining surface defects in a cast slab according to [5], wherein the specific unit takes the maximum value of the discharge velocity difference obtained from a plurality of flow velocity distributions continuously estimated by the estimation unit as the representative value of the discharge velocity difference, and the maximum value of the molten metal surface velocity in the plurality of flow velocity distributions as the representative value of the molten metal surface velocity. A method for manufacturing a cast slab using the method for determining surface defects in a cast slab described in [7] [3], wherein, for a cast slab in which it is determined in the determination step that a surface defect has occurred, the operating conditions of the continuous casting machine are changed so that the representative value of the molten metal flow velocity becomes smaller. [Effects of the Invention]

[0011] By using the method and apparatus for determining surface defects in a cast slab according to the present invention, it becomes possible to determine surface defects in a cast slab caused by the flow state of molten steel inside the mold. By using the method for manufacturing a cast slab according to the present invention, it becomes possible to manufacture a cast slab with reduced surface defects. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a schematic diagram showing an example of the configuration of a continuous casting facility including a device for determining surface defects in a cast slab according to this embodiment. [Figure 2] Figure 2 is a schematic perspective view showing the mold. [Figure 3] Figure 3 is a schematic diagram of the long side of the mold showing the placement of thermocouples. [Figure 4]FIG. 4 is a schematic cross-sectional view of a mold showing the flow state of molten steel in the mold. [Figure 5] FIG. 5 is a schematic diagram showing a configuration example of an apparatus for determining surface defects of a cast slab. [Figure 6] FIG. 6 is a diagram showing boundary conditions set for a turbulence model used for estimating the flow velocity distribution of molten steel in a mold. [Figure 7] FIG. 7 is a diagram showing an example of an estimation result of a flow velocity distribution of molten steel. [Figure 8] FIG. 8 is a schematic cross-sectional view of a mold showing a region for specifying a representative value ΔV of a discharge flow velocity difference. [Figure 9] FIG. 9 is a schematic top view of a mold showing a region for specifying a representative value VS of a molten steel surface flow velocity. [Figure 10] FIG. 10 is a flow diagram showing a flow of a method for determining surface defects of a cast slab according to the present embodiment. [Figure 11] FIG. 11 is a schematic cross-sectional view of a mold showing the flow state of molten steel generated in the mold. [Figure 12] FIG. 12 is a schematic cross-sectional view of a mold showing the flow state of molten steel generated in the mold. [Figure 13] FIG. 13 is a schematic cross-sectional view of a mold showing the flow state of molten steel generated in the mold. [Figure 14] FIG. 14 is a schematic cross-sectional view of a mold showing the flow state of molten steel generated in the mold. [Figure 15] FIG. 15 is a schematic diagram of a mold showing reference regions RV1 and RV2 for discharge flow velocity and a reference region RS for molten steel surface flow velocity set in an example. [Figure 16] FIG. 16 is a graph showing the presence or absence of surface defects, the representative value ΔV of the discharge flow velocity difference, and the representative value VS of the molten steel surface flow velocity for a cast slab produced in the present example. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in detail below through embodiments of the present invention. The following embodiments are preferred examples of the present invention, and the present invention is not limited in any way by these embodiments. Figure 1 is a schematic diagram showing an example of the configuration of a continuous casting equipment 100 including a slab surface defect determination device 60 according to this embodiment.

[0014] As shown in Figure 1, the continuous casting equipment 100 includes a continuous casting machine 10 and a slab surface defect detection device 60. The continuous casting machine 10 includes a mold 12, a tundish 14 installed above the mold 12, multiple pairs of slab support rolls 16 arranged in a row below the mold 12, and multiple secondary cooling devices 18. A sliding nozzle 22 for adjusting the flow rate of molten steel 20 is installed at the bottom of the tundish 14, and an immersion nozzle 24 is installed on the lower surface of the sliding nozzle 22.

[0015] Molten steel 20 is poured into the mold 12 through an immersion nozzle 24. The molten steel 20 poured into the mold 12 is cooled and solidified by the mold 12, forming a solidified shell 26. This forms a cast slab 30 with the solidified shell 26 as its outer shell and an unsolidified layer 28 made of molten steel 20 inside. The top surface of the molten steel 20 is covered with mold flux. The mold flux covering the top surface of the molten steel 20 will be explained later.

[0016] Below the mold 12, there are multiple pairs of slab support rolls 16, including a support roll 32, a guide roll 34, and a pinch roll 36. Of these, the pinch roll 36 supports the slab 30 and also acts as a drive roll for pulling out the slab 30. The slab 30 is pulled out by the pinch roll 36 at a predetermined casting speed. At that time, in order to ensure mass balance, the opening of the sliding nozzle 22 is adjusted according to the pulling speed of the slab 30.

[0017] Multiple secondary cooling devices 18, each equipped with a spray nozzle (not shown), such as a water spray nozzle or an air mist spray nozzle, are provided in the gaps between adjacent slab support rolls 16 in the casting direction. The slab 30 is cooled as it is drawn out by the cooling water sprayed from the spray nozzles of the multiple secondary cooling devices 18, reducing the internal unsolidified layer 28 and allowing the solidified shell 26 to grow. Subsequently, the slab 30 is properly cooled, and the solidification of the unsolidified layer 28 progresses until the unsolidified layer 28 is completely solidified. In this way, the slab 30 is manufactured using the continuous casting machine 10.

[0018] Figure 2 is a schematic perspective view showing the mold 12. The mold 12 of the continuous casting machine 10 has a pair of opposing mold long sides 40 and a pair of opposing mold short sides 42 sandwiched between the mold long sides 40. In this embodiment, the mold long side 40 on the front side is designated as the F side, and the mold long side 40 on the back side is designated as the B side.

[0019] An immersion nozzle 24 is inserted into the rectangular internal space formed by a pair of mold long sides 40 and a pair of mold short sides 42. The mold long sides 40 and mold short sides 42 are made of mold plates made of copper alloy, and a backup plate (not shown) is installed on the back of these mold plates. The back surfaces of the mold plates that make up the mold long sides 40 and mold short sides 42 that are in contact with the molten steel 20 are provided with grooves that form cooling water channels, and the cooling water channels are formed when the backup plate is installed on these surfaces. The pair of mold long sides 40 and the pair of mold short sides 42 are cooled by passing cooling water through the cooling water channels.

[0020] Figure 3 is a schematic diagram of the long side 40 of the mold showing the placement of thermocouples 44. As shown in Figure 3, multiple thermocouples 44 are embedded in the long side 40 of the mold on both the front surface (F surface) and the back surface (B surface) that are at both ends in the thickness direction (direction perpendicular to the plane of the paper) of the cast slab. Each thermocouple 44 measures the temperature of the molten steel 20 at its respective installation location. In this embodiment, for example, two rows of thermocouples 44 are embedded in the height direction and 10 thermocouples in the width direction on the F surface and B surface of the long side 40 of the mold. In addition, a coil (not shown) that generates a stirring magnetic field to rotate the molten steel 20 inside the mold 12 may be installed in the mold 12.

[0021] Figure 4 is a schematic cross-sectional view of a mold 12 showing the flow state of molten steel 20 within the mold. The cross-sectional view shown in Figure 4 is a cross-sectional view taken by cutting through a plane passing through the central axis of the immersion nozzle 24 and parallel to the F-plane or B-plane of the long side 40 of the mold.

[0022] The upper surface of the molten steel 20 inside the mold 12 is covered with molten mold flux 46 (hereinafter referred to as "mold flux 46"). The mold flux 46 flows between the mold 12 and the molten steel 20 and acts as a lubricant. The flow of the mold flux 46, which acts as a lubricant, between the mold 12 and the molten steel 20 prevents the mold 12 from sticking to the solidified shell 26.

[0023] A pair of discharge ports 48a and 48b are formed at the tip of the immersion nozzle 24, and molten steel 20 is discharged into the mold 12 from these discharge ports 48a and 48b. The pair of discharge ports 48a and 48b are each positioned to discharge the molten steel 20 toward the width direction of the cast slab 30. By arranging the immersion nozzle 24 in this way, the molten steel 20 discharged from discharge port 48a forms a discharge flow toward one short side of the mold 12. The molten steel 20 discharged from discharge port 48b forms a discharge flow toward the other short side of the mold 12.

[0024] In the example shown in Figure 4, the molten steel 20 is discharged from the discharge ports 48a and 48b slightly downward (30° downward from the horizontal) relative to the horizontal direction. The discharge flow of the molten steel 20 discharged from the discharge ports 48a and 48b collides with the solidified shell 26 formed on the short side of the cast slab 30, changing the flow direction of the molten steel 20 and creating upward and downward flows. The upward flow (reverse flow) of the molten steel 20 generated in the mold 12 rises along the solidified shell 26 on the short side of the cast slab 30, forming a surface flow along the surface (molten surface) of the molten steel 20 in the mold 12. Furthermore, the surface flow of the molten steel 20 forms a flow toward the interior of the molten steel 20 near the immersion nozzle 24. Inside the mold 12, the flow state of the molten steel 20 as shown in Figure 4 is formed, and the mold flux 46 and inclusions 50 covering the molten steel 20 may be drawn into the molten steel 20, causing surface defects in the cast slab 30.

[0025] Refer to Figure 1 again. The surface defect detection device 60 of the cast slab determines surface defects in the cast slab 30 caused by the flow state of the molten steel 20 in the mold 12. The surface defect detection device 60 of the cast slab acquires temperature data from a plurality of thermocouples 44 and uses this temperature data to estimate the flow velocity distribution of the molten steel 20 in the mold 12. Using the estimated flow velocity distribution of the molten steel 20, the surface defect detection device 60 identifies a representative value of the discharge flow velocity difference of the molten steel 20 discharged from a pair of discharge ports 48a and 48b, and a representative value of the molten steel surface flow velocity in the mold 12.

[0026] The surface defect detection device 60 for the cast slab determines surface defects in the cast slab 30 using the identified representative values. The surface defect detection device 60 determines that surface defects occur in the cast slab 30 if both the representative value of the discharge flow velocity difference of the molten steel 20 and the representative value of the molten steel 20 surface flow velocity exceed the first threshold value set for each. The surface defect detection device 60 also determines that no surface defects occur in the cast slab 30 if both the representative value of the flow velocity difference of the molten steel 20 and the representative value of the molten steel 20 surface flow velocity are less than the second threshold value set for each. Preferably, the surface defect detection device 60 outputs the determination result of surface defects in the cast slab 30 to a terminal 80 owned by the operator.

[0027] Next, the surface defect detection device 60 for casting slabs will be described. Figure 5 is a schematic diagram showing an example configuration of the surface defect detection device 60 for casting slabs. The surface defect detection device 60 for casting slabs is a general-purpose computer such as a workstation or personal computer. The surface defect detection device 60 for casting slabs has a control unit 62, an input unit 64, an output unit 66, a storage unit 68, and a communication unit 70. The control unit 62 is, for example, a CPU, and functions as an estimation unit 72, a identification unit 74, and a determination unit 76 by executing a program stored in the storage unit 68.

[0028] The input unit 64 is, for example, a keyboard, a touch panel integrated with a display, etc. The output unit 66 is, for example, an LCD or CRT display, etc. The storage unit 68 is, for example, an information recording medium such as an updatable flash memory, a hard disk that is built-in or connected via a data communication terminal, a memory card, etc., and a device for reading and writing them. The storage unit 68 stores programs, data, and calculation formulas for realizing each function of the surface defect determination device 60 of the cast slab. The storage unit 68 also stores a turbulence model used to estimate the velocity distribution of the molten steel 20, boundary conditions, reference regions (RV1, RV2, RS), and threshold values ​​used to determine surface defects of the cast slab 30, etc.

[0029] The communication unit 70 includes at least one of a communication module that supports wired communication and a communication module that supports wireless communication. The slab surface defect determination device 60 communicates with a thermocouple 44 and a process computer (not shown) that controls the operation of the continuous casting machine 10 via the communication unit 70.

[0030] Next, the processes performed by the estimation unit 72, the identification unit 74, and the determination unit 76 will be described. The estimation unit 72 acquires temperature data from the thermocouple 44 via the communication unit 70. The estimation unit 72 also acquires the operating conditions of the continuous casting machine 10 from the process computer via the communication unit 70. Using the temperature data and the operating conditions of the continuous casting machine 10, the estimation unit 72 estimates the flow velocity distribution, which is the flow velocity of the molten steel 20 at each position in the mold 12.

[0031] The estimation unit 72 estimates the velocity distribution of the molten steel 20 in the mold 12 using known physical models described in Patent Documents 3 and 4. The estimation unit 72 estimates the velocity distribution of the molten steel 20 using a turbulence model (standard k-ε model) with the operating conditions of the continuous casting machine 10 as input conditions. The operating conditions of the continuous casting machine 10 used as input conditions include, for example, the casting speed, the dimensions of the cast slab 30 (width, thickness), the coil current of the stirring magnetic field, and the discharge angles from the discharge ports 48a and 48b of the immersion nozzle 24.

[0032] Figure 6 shows the boundary conditions set for the turbulence model used to estimate the velocity distribution of molten steel 20 in the mold 12. The inlet of the turbulence model is given a velocity of molten steel 20 corresponding to the mass flow according to the casting rate. The outlet of the turbulence model is given free outlet boundary conditions, which assume that there are no gradients of various physical quantities in the flow direction. The inner wall of the mold 12 is assumed to be a solid wall moving at a velocity equal to the casting rate.

[0033] The estimation unit 72 corrects the velocity distribution of the molten steel 20, which was estimated using a turbulence model, by using temperature data measured by thermocouples 44 installed on the long side 40 of the mold, in order to improve the accuracy of the estimation of the velocity distribution of the molten steel 20. The correction method is also the method described in Patent Documents 3 and 4. Specifically, the estimation unit 72 calculates the temperature distribution of the molten steel 20 in the mold 12 by converting the velocity distribution of the molten steel 20 calculated by the turbulence model into a temperature distribution. The estimation unit 72 corrects the velocity distribution of the molten steel 20 by comparing the temperature corresponding to the position of each thermocouple 44 in the calculated temperature distribution with the temperature data obtained from the thermocouples 44 and compensating for the error.

[0034] According to the compensation methods described in Patent Documents 3 and 4, the difference between the temperature distribution estimated using the turbulence model and the temperature distribution measured by the thermocouple 44 is considered to originate from shape changes such as blockage due to deposits on the immersion nozzle 24 (boundary conditions near the immersion nozzle 24). Therefore, the difference (error) between the estimated temperature distribution and the temperature distribution measured by the thermocouple 44 is compensated for on the turbulence model by applying an external force that causes a perturbation of the flow state near the immersion nozzle 24. This makes it possible to estimate the flow velocity distribution of the molten steel 20 in the mold 12 with high accuracy.

[0035] Figure 7 shows an example of the estimated flow velocity distribution of molten steel 20. Figure 7(a) shows the region in which the flow velocity distribution of molten steel 20 was estimated within the mold 12. Figure 7(b) shows the estimated flow velocity distribution of molten steel 20 at the molten surface. Figures 7(c) to (e) show the estimated flow velocity distribution of molten steel 20 on the front side (F side) of the mold 12, the central thickness surface of the mold 12, and the back side (B side) of the mold 12. The estimation unit 72 outputs the estimated flow velocity distribution of molten steel 20 to the identification unit 74. When the identification unit 74 obtains the flow velocity distribution of molten steel 20, it uses the flow velocity distribution to identify the representative value ΔV of the discharge flow velocity difference of molten steel 20 discharged from the discharge ports 48a and 48b, and the representative value VS of the molten surface flow velocity of molten steel 20 at the molten surface.

[0036] Figure 8 is a schematic cross-sectional view of the mold 12 showing the region for identifying the representative value ΔV of the discharge velocity difference. The identification unit 74 first sets reference regions RV1 and RV2 in the velocity distribution data obtained from the estimation unit 72 and extracts the velocity distribution of the molten steel 20 in these regions. Next, the identification unit 74 determines the maximum velocity in reference region RV1 and the maximum velocity in reference region RV2, and identifies the absolute value of the difference between these maximum velocities as the representative value ΔV of the discharge velocity difference.

[0037] The specific unit 74 may determine the average flow velocity in reference region RV1 and the average flow velocity in reference region RV2 instead of the maximum flow velocity, and specify the absolute value of the difference between these average flow velocities as the representative value ΔV of the discharge flow velocity difference. However, the faster the flow velocity, the more likely the molten steel 20 is to entrain mold flux 46 and inclusions 50. For this reason, the absolute value of the difference in maximum flow velocity correlates more strongly with surface defects than the difference in average flow velocity. For this reason, it is preferable to use the absolute value of the difference in maximum flow velocity in each reference region RV1 and RV2 as the representative value ΔV of the discharge flow velocity difference.

[0038] The widthwise length of the cast slab 30 in reference areas RV1 and RV2 is preferably set to 0.2 to 0.4 × W relative to the width dimension W of the cast slab 30. Furthermore, the casting direction length of reference areas RV1 and RV2 is preferably set to 1.0 to 2.0 × D relative to the casting direction length D of the discharge port 48a. Reference areas RV1 and RV2 are predetermined and stored in the storage unit 68 via the input unit 64.

[0039] Figure 9 is a schematic top view of the mold 12 showing the region for identifying the representative value VS of the molten metal flow velocity. The identification unit 74 sets a reference region RS in the velocity distribution data obtained from the estimation unit 72 and identifies the maximum velocity of the molten steel 20 in the reference region RS as the representative value VS of the molten metal flow velocity. Alternatively, the identification unit 74 may identify the average velocity of the molten steel 20 in the reference region RS as the representative value VS of the molten metal flow velocity instead of the maximum velocity. However, the faster the velocity of the molten steel 20, the more likely it is to entrain mold flux 46 and inclusions 50. For this reason, the maximum velocity has a higher correlation with surface defects than the average velocity. For this reason, it is preferable to use the maximum velocity of the molten steel 20 in the reference region RS as the representative value VS of the molten metal flow velocity.

[0040] The reference region RS may be the molten metal surface, or it may be a range extending from the molten metal surface to a predetermined depth (for example, 10 to 20 mm below the molten metal surface). Since turbulence occurs in the molten metal flow velocity near the long side 40 and short side 42 of the mold, it is preferable to set the reference region RS to a range smaller than the size of the cast slab 30. For example, it is preferable to set the reference region RS to a range 20 to 100 mm away from the long side 40 and short side 42 of the mold.

[0041] Similarly, since turbulence occurs in the molten metal flow velocity near the immersion nozzle 24, it is preferable to set the reference region RS in a range away from the vicinity of the immersion nozzle 24. The reference region RS is preferably set in a range 20 to 100 mm away from the side of the immersion nozzle 24. When the identification unit 74 identifies the representative value ΔV of the discharge flow velocity difference and the representative value VS of the molten metal flow velocity, it outputs these representative values ​​to the determination unit 76. The reference region RS is also predetermined and stored in the storage unit 68 via the input unit 64.

[0042] When the determination unit 76 obtains the representative value ΔV of the discharge flow velocity difference and the representative value VS of the molten metal surface flow velocity from the identification unit 74, it reads out the first threshold (TJ1, TS1) and the second threshold (TJ2, TS2) from the storage unit 68. TJ1 is the first threshold for the representative value ΔV of the discharge flow velocity difference, and TS1 is the first threshold for the representative value VS of the molten metal surface flow velocity. TJ2 is the second threshold for the representative value ΔV of the discharge flow velocity difference, and TS2 is the second threshold for the representative value VS of the molten metal surface flow velocity.

[0043] The determination unit 76 compares a representative value ΔV of the discharge flow velocity difference with a first threshold value (TJ1) for that representative value. The determination unit 76 also compares a representative value VS of the discharge surface flow velocity with a first threshold value (TS1) for that representative value. The determination unit 76 determines that surface defects occur in the manufactured cast slab 30 if both the representative value ΔV of the discharge flow velocity difference and the representative value VS of the discharge surface flow velocity exceed the first threshold values ​​(TJ1, TS1).

[0044] Furthermore, the determination unit 76 compares a representative value ΔV of the discharge flow velocity difference with a second threshold (TJ2) for that representative value. The determination unit 76 also compares a representative value VS of the molten metal surface velocity with a second threshold (TS2) for that representative value. The determination unit 76 determines that no surface defects occur in the manufactured cast slab 30 if both the representative value ΔV of the discharge flow velocity difference and the representative value VS of the molten metal surface velocity are less than the second thresholds (TJ2, TS2).

[0045] The first threshold (TJ1, TS1) is predetermined based on the conditions under which surface defects occurred in the cast slab 30 in the past operating history of the continuous casting machine 10, and is stored in the storage unit 68 via the input unit 64. The second threshold (TJ2, TS2) is predetermined based on the conditions under which surface defects did not occur in the cast slab 30 in the past operating history of the continuous casting machine 10, and is stored in the storage unit 68 via the input unit 64.

[0046] The determination unit 76 may display the determination result on the output unit 66, or it may output the determination result to a terminal 80 owned by the operator via the communication unit 70. Preferably, the operator who owns the terminal 80 is the operator who manages the continuous casting machine 10. By displaying the determination result on the terminal 80 of the operator who manages the continuous casting machine 10, the operator can accurately grasp the quality of the cast slabs 30 being manufactured, and take measures such as changing the operating conditions of the continuous casting machine 10 according to the determination result.

[0047] Furthermore, it is preferable that the operator who owns the terminal 80 is the operator who manages the quality of the manufactured cast slabs 30. By displaying the judgment result on the terminal 80 of the operator who manages the quality of the cast slabs 30, the operator can accurately grasp the quality of the manufactured cast slabs 30, and based on the judgment result, can quickly determine whether or not maintenance work is necessary when transporting the cast slabs 30 to the next process.

[0048] Furthermore, the determination unit 76 may output the determination result to the process computer that controls the continuous casting machine 10 via the communication unit 70. By outputting the determination result to the process computer, the process computer can automatically set the processing for the manufactured slabs 30 according to the determination result, for example, whether to add or omit finishing work on the slabs 30.

[0049] Furthermore, if the determination unit 76 determines that surface defects will occur in the cast slab 30 being manufactured, it may change the continuous casting conditions used by the estimation unit 72 to estimate the flow velocity distribution of the molten steel 20. If the determination unit 76 determines that surface defects will occur, it may, for example, reduce the amount of molten steel 20 discharged from the immersion nozzle 24 and slow down the casting speed. As a result, the flow velocity at the molten surface slows down, the representative value VS of the flow velocity at the molten surface becomes smaller, and cast slabs 30 with reduced surface defects can be manufactured.

[0050] In this case, the determination unit 76 may output the modified continuous casting conditions to the estimation unit 72, causing it to estimate the flow velocity distribution of the molten steel 20 in the mold 12 after the modification of the continuous casting conditions, and causing the identification unit 74 to identify a representative value ΔV of the discharge flow velocity difference and a representative value VS of the molten metal flow velocity after the modification of the continuous casting conditions. The determination unit 76 uses the identified representative value ΔV of the discharge flow velocity difference and the representative value VS of the molten metal flow velocity after the modification of the continuous casting conditions to determine whether or not surface defects occur in the manufactured slab 30. The determination unit 76 repeats this process until it is determined that no surface defects occur in the manufactured slab 30.

[0051] The determination unit 76 may identify the continuous casting conditions used when it is determined that no surface defects occur in the manufactured slab 30, and output these continuous casting conditions to the process computer via the communication unit 70. Preferably, the process computer then changes the continuous casting conditions of the continuous casting machine 10 to the identified continuous casting conditions. This makes it possible to manufacture slabs 30 using the continuous casting machine 10 while suppressing the occurrence of surface defects.

[0052] Figure 10 is a flowchart showing the flow of the method for determining surface defects in a cast slab according to this embodiment. The method for determining surface defects in a cast slab according to this embodiment will be explained using Figure 10. The flow shown in Figure 10 starts, for example, when the cast slab surface defect determination device 60 acquires information from the process computer that the manufacturing of the cast slab 30 has started.

[0053] First, the estimation unit 72 estimates the flow velocity distribution of the molten steel 20 in the mold 12 using temperature data and the operating conditions of the continuous casting machine 10 (step S101). This process is the estimation step in the method for determining surface defects in a cast slab according to this embodiment. The estimation unit 72 outputs the flow velocity distribution of the molten steel 20 to the identification unit 74.

[0054] When the identification unit 74 acquires the flow velocity distribution of the molten steel 20, it uses the flow velocity distribution of the molten steel 20 to identify a representative value ΔV of the discharge flow velocity difference and a representative value VS of the molten metal surface flow velocity (step S102). This process is the identification step in the method for determining surface defects of a cast slab according to this embodiment. The identification unit 74 outputs the identified representative value ΔV of the discharge flow velocity difference and the representative value VS of the molten metal surface flow velocity to the determination unit 76.

[0055] When the determination unit 76 obtains a representative value ΔV of the discharge velocity difference and a representative value VS of the molten metal surface velocity, it reads a first threshold value (TJ1, TS1) from the storage unit 68. The determination unit 76 determines that surface defects will occur in the manufactured slab 30 if both the representative value ΔV of the discharge velocity difference and the representative value VS of the molten metal surface velocity exceed the first threshold value (TJ1, TS1). The determination unit 76 also reads a second threshold value (TJ2, TS2) from the storage unit 68. The determination unit 76 determines that no surface defects will occur in the manufactured slab 30 if the obtained representative value ΔV of the discharge velocity difference and the representative value VS of the molten metal surface velocity are less than the second threshold value (TJ2, TS2) (step SS103). These processes constitute the determination steps in the method for determining surface defects in a slab according to this embodiment.

[0056] The determination unit 76 determines whether the production of the cast slab 30 by the continuous casting machine 10 is continuing (step S104). The determination unit 76 determines that the production of the cast slab 30 is continuing if, for example, it has not received information from the process computer that the production of the cast slab 30 has ended (step S104: Yes). In this case, the determination unit 76 returns to step S101 and repeatedly performs the surface defect determination process of the cast slab 30. On the other hand, the determination unit 76 determines that the production of the cast slab 30 is not continuing if it has received information from the process computer that the production of the cast slab 30 has ended (step S104: No). In this case, the determination unit 76 terminates the flow shown in Figure 10.

[0057] As described above, the method for determining surface defects in a cast slab according to this embodiment estimates the flow velocity distribution of the molten steel 20 in the mold 12, and uses the representative value ΔV of the discharge flow velocity difference and the representative value VS of the molten metal flow velocity, which are identified using the said flow velocity distribution, to determine surface defects in the cast slab 30. This makes it possible to accurately determine surface defects in the cast slab 30 that are caused by the flow state of the molten steel 20 in the mold 12.

[0058] The larger the representative value VS of the molten metal flow velocity, the greater the amount of mold flux 46 and inclusions 50 that are incorporated. Therefore, by using the representative value VS of the molten metal flow velocity, it becomes possible to determine the surface defects of the cast slab 30. On the other hand, if the casting speed is changed or the width dimension of the cast slab 30 being manufactured is changed, the representative value VS of the molten metal flow velocity may fluctuate even if the amount of mold flux 46 and inclusions 50 incorporated is small. For this reason, if the surface defects of the cast slab 30 are determined using only the representative value VS of the molten metal flow velocity as an indicator, it may be possible to mistakenly determine that a cast slab 30 that does not have surface defects has surface defects.

[0059] Furthermore, the larger the representative value ΔV of the discharge velocity difference, the greater the flow deviation within the mold 12, which increases the amount of mold flux 46 and inclusions 50 incorporated into the molten steel 20. Therefore, by using the representative value ΔV of the discharge velocity difference, it becomes possible to determine surface defects in the cast slab 30 caused by the flow deviation within the mold 12.

[0060] On the other hand, when the casting speed of the continuous casting machine 10 is high, the flow deviation does not necessarily become large, so even if the representative value ΔV of the discharge flow velocity difference is small, the representative value VS of the molten metal flow velocity may become large. For this reason, if surface defects of the cast slab 30 are judged using only the representative value ΔV of the discharge flow velocity difference as an indicator, it may be possible to mistakenly judge that a cast slab 30 with surface defects does not have surface defects.

[0061] Furthermore, if a coil for applying a stirring magnetic field is installed in the mold 12, the representative value VS of the molten metal surface velocity may not be large even if the representative value ΔV of the discharge flow velocity difference is large. In the method for determining surface defects of a cast slab according to this embodiment, the determination is made using the representative value ΔV of the discharge flow velocity difference and the representative value VS of the molten metal surface velocity. This reflects the entrapment behavior of the mold flux 46 and inclusions 50 into the molten steel 20, enabling the determination of surface defects of the cast slab 30 with high accuracy.

[0062] Furthermore, the uneven flow of molten steel 20 that occurs within the mold 12 during the operation of the continuous casting machine 10 can change periodically not only when one discharge port 48a is blocked, but also due to the difference in discharge volume from the pair of discharge ports 48a and 48b. In other words, the uneven flow within the mold 12 can change periodically, which can cause surface defects in the cast slab 30.

[0063] Figure 11 is a schematic cross-sectional view of a mold 12 showing the flow state of molten steel 20 generated within the mold. Figure 11 shows the flow state of molten steel 20 when there is no difference in the flow velocity of the molten steel 20 discharged from the discharge ports 48a and 48b. The molten steel 20 discharged from the discharge ports 48a and 48b generates discharge flow, reverse flow, and surface flow. The surface flow flowing on the molten surface collides with the outer surface of the immersion nozzle 24, generating a settling flow. Such discharge flow, reverse flow, surface flow, and settling flow form a loop-shaped flow, and the loop-shaped flows on the left and right sides within the mold 12 form a vortex. When such a vortex is formed, the center of the vortex becomes a negative pressure.

[0064] Figure 12 is a schematic cross-sectional view of a mold 12 showing the flow state of molten steel 20 generated within the mold. Figure 12 shows a state in which the flow velocity of molten steel 20 discharged from one outlet 48a is faster than the flow velocity of molten steel 20 discharged from the other outlet 48b. For example, if a nozzle blockage occurs in the other outlet 48b, as shown in Figure 12, the flow velocity of molten steel 20 from one outlet 48a will increase, and the flow velocity of molten steel 20 will become uneven on the left and right sides.

[0065] As the flow velocity of molten steel 20 from the discharge port 48a increases, the flow velocities of the reverse flow, surface flow, and settling flow also increase. As a result, the negative pressure caused by the vortex formed on the discharge port 48a side becomes greater than the negative pressure caused by the vortex formed on the discharge port 48b side. Consequently, the vortex size becomes smaller on the discharge port 48a side than on the discharge port 48b side, and the flow velocity of the loop-shaped flow increases.

[0066] Figure 13 is a schematic cross-sectional view of a mold 12 showing the flow state of molten steel 20 occurring within the mold. Figure 13 shows a state in which the vortex formed on one discharge port 48a side is smaller than the vortex formed on the other discharge port 48b side. As the vortex formed on one discharge port 48a side shrinks and the flow velocity of the loop-shaped flow increases, the flow velocity of the settling flow also increases. Since the settling flow becomes a flow that crosses the discharge port of the immersion nozzle 24, the settling flow obstructs the flow of molten steel 20 discharged from discharge port 48a. In other words, the discharge of molten steel 20 from discharge port 48a is greatly obstructed compared to discharge port 48b.

[0067] Since the flow rate of molten steel 20 supplied from the immersion nozzle 24 is constant, the discharge velocity and flow rate of molten steel 20 from the other discharge port 48b, where the discharge of molten steel 20 is not obstructed, increase compared to the discharge port 48a, where the discharge of molten steel 20 is obstructed. As a result, even if the flow velocity of the discharge flow on the side of one discharge port 48a increases for some reason, the loop-shaped flow of the discharge flow, counterflow, surface flow and settling flow forms a vortex, and the negative pressure generated at the center of the vortex causes the flow velocity of the discharge flow on the other discharge port 48b to also increase thereafter.

[0068] Figure 14 is a schematic cross-sectional view of a mold 12 showing the flow state of molten steel 20 generated within the mold. Figure 14 shows a state in which, due to the mechanism described above, the flow velocity of molten steel 20 discharged from outlet 48b is faster than the flow velocity of molten steel 20 discharged from outlet 48a. In this case, the negative pressure caused by the vortex formed on the outlet 48b side is greater than the negative pressure caused by the vortex formed on the outlet 48a side, so the loop-shaped flow on the outlet 48b side is drawn more towards the center of the vortex. As a result, the vortex formed on the outlet 48b side shrinks, and the flow velocity and flow rate of the settling flow on the outlet 48b side increase.

[0069] Thus, even if the flow velocity of the molten steel 20 discharged from one outlet 48a becomes faster than the other due to some reason, the flow velocity of the molten steel 20 discharged from the other outlet 48b will also increase afterward. For this reason, the flow velocities of the molten steel 20 discharged from outlets 48a and 48b alternately increase and decrease. In other words, alternating flow deviations occur within the mold 12 at a frequency corresponding to the change in the flow state of the molten steel 20. The periodic flow deviations of the molten steel 20 occurring within the mold 12 are estimated to occur at a frequency of 20 to 40 seconds, based on the fluctuation period of the molten steel level at outlets 48a and 48b.

[0070] Such periodic deviations result in an unsteady flow of molten steel 20 within the mold 12. When such an unsteady flow occurs, the representative value ΔV of the discharge velocity difference and the representative value VS of the molten metal surface velocity, which are identified from the velocity distribution at one point estimated by the estimation unit 72, may not represent a flow state that is likely to cause surface defects in the cast slab 30.

[0071] Considering the case where periodic flow deviations occur in the molten steel 20 within the mold 12, it is preferable that the estimation unit 72 continuously estimates the flow velocity distribution of the molten steel 20 at intervals of, for example, 5 to 15 seconds. Then, it is preferable that the identification unit 74 identifies the maximum value of the discharge flow velocity difference obtained from the multiple continuously estimated flow velocity distributions as the representative value ΔV of the discharge flow velocity difference, and identifies the maximum value of the molten surface flow velocity obtained from the multiple flow velocity distributions as the representative value VS of the molten surface flow velocity.

[0072] Specifically, a predetermined time (for example, 240 to 900 seconds) is set, and during the operation of the continuous casting machine 10, the estimation unit 72 continuously acquires multiple velocity distributions of the molten steel 20 in the mold 12 over the predetermined time. The identification unit 74 identifies the maximum value of the discharge velocity difference and the maximum value of the molten metal surface velocity obtained from the multiple velocity distributions as the representative value ΔV of the discharge velocity difference and the representative value VS of the molten metal surface velocity. The representative value ΔV of the discharge velocity difference and the representative value VS of the molten metal surface velocity identified in this way are representative values ​​that reflect a flow state in which surface defects are more likely to occur than when identified from a single velocity distribution. Therefore, by using these representative values, even when an unsteady flow occurs, it becomes possible to determine surface defects in the cast slab 30 that reflect the unsteady entrapment behavior of mold flux 46 and inclusions 50 into the molten steel 20 that occurs over the predetermined time.

[0073] In this embodiment, the casting surface defect determination device 60 is shown to have an estimation unit 72, a specification unit 74, and a determination unit 76, but it is not limited to this. The estimation unit 72, the specification unit 74, and the determination unit 76 may be configured by separate computing devices. Furthermore, the estimation unit 72, the specification unit 74, and the determination unit 76 may each be configured by separate computing devices.

[0074] Furthermore, the slab surface defect detection device 60 may be comprised of another device, such as a process computer that controls the operation of the continuous casting machine 10. By composing the slab surface defect detection device 60 with a process computer, the operating conditions of the continuous casting machine 10 can be easily obtained. [Examples]

[0075] An example of determining surface defects in a cast slab 30 manufactured using the continuous casting machine 10 shown in Figure 1 will be described. In this example, a cast slab 30 with a width of W790 to 1950 mm and a thickness of H260 mm was manufactured using the continuous casting machine 10 at a casting speed of 0.5 to 2.3 m / min, and the surface defects of the manufactured cast slab 30 were determined using the surface defect determination method for cast slabs according to this embodiment.

[0076] In the slab surface defect detection device 60, a program for estimating the velocity distribution of molten steel 20 was created in advance based on the estimation method using a turbulence model described in Patent Document 3. Then, the operating conditions of the continuous casting machine 10 were obtained from the process computer, and the velocity distribution of molten steel 20 in the mold 12 was estimated using the temperature data measured by thermocouples 44 embedded in the mold 12 and the program. In this embodiment, 20 thermocouples 44 were embedded in the upper layer and 38 in the lower layer (41 in total) on the F and B surfaces of the mold 12, and temperature data was measured.

[0077] The estimation unit 72 estimated the velocity distribution of the molten steel 20 every 10 seconds while casting one slab, estimating the velocity distribution 24 to 90 times per slab 30. The identification unit 74 used the 24 to 90 velocity distributions estimated by the estimation unit 72 to identify the representative value ΔV of the discharge velocity difference and the representative value VS of the molten metal surface velocity.

[0078] Figure 15 is a schematic diagram of the mold showing the reference regions RV1 and RV2 for discharge flow velocity and the reference region RS for molten metal flow velocity set in the embodiment. Figure 15(a) is a schematic cross-sectional view of the mold 12 showing the reference regions RV1 and RV2 for discharge flow velocity, and Figure 15(b) is a top view of the mold 12 showing the reference region RS for molten metal flow velocity.

[0079] As shown in Figure 15(a), in this embodiment, reference regions RV1 and RV2 were set to a range from 488 mm away from the top surface of the mold 12 in the casting direction to 288 mm away in the casting direction. The width of the cast slab 30 was set to a range of 800 mm on each side in the width direction, excluding the area of ​​the immersion nozzle 24. For each estimated flow velocity distribution, the difference in discharge flow velocity between the average flow velocity within the reference region RV1 and the average flow velocity within the reference region RV2 was calculated. This operation was performed for the estimated flow velocity distributions of 24 to 90, and the absolute value of the maximum value with the largest discharge flow velocity difference was taken as the representative value ΔV.

[0080] As shown in Figure 15(b), in this embodiment, the reference regions RS1 and RS2 were set to a range of 18.5 mm in the thickness direction of the cast slab. The width direction of the cast slab 30 was set to a range of 800 mm on both the left and right sides, excluding the area of ​​the immersion nozzle 24. For each estimated flow velocity distribution, the maximum flow velocity within the reference regions RS1 and RS2 was identified. This operation was performed for the estimated flow velocity distributions of 24 to 90, and the maximum value among these maximum flow velocities was taken as the representative value VS of the molten metal flow velocity.

[0081] Figure 16 is a graph showing the presence or absence of surface defects, the representative value of the discharge velocity difference ΔV, and the representative value of the molten metal surface velocity VS for the cast slab 30 produced in this embodiment. The horizontal axis of Figure 16 represents the representative value of the discharge velocity difference ΔV (cm / s), and the vertical axis represents the representative value of the molten metal surface velocity (cm / s). Black plots indicate cast slabs 30 in which surface defects were confirmed. White plots indicate cast slabs 30 in which no surface defects were confirmed.

[0082] As shown in Figure 16, surface defects occurred in all nine cast slabs 30 in which the representative value ΔV of the discharge velocity difference exceeded TJ1 and the representative value VS of the molten metal flow velocity exceeded TS1, relative to the first threshold (TJ1, TS1) for determining whether surface defects occurred in the cast slab 30. Furthermore, the range in which the representative value ΔV of the discharge velocity difference exceeded TJ1 and the representative value VS of the molten metal flow velocity exceeded TS1 did not include any cast slabs 30 in which surface defects did not occur. From these results, it was confirmed that the occurrence of surface defects in the manufactured cast slab 30 can be determined by whether the representative value ΔV of the discharge velocity difference and the representative value VS of the molten metal flow velocity exceed the first threshold (TJ1, TS1). If it is determined that surface defects occur in the cast slab 30 manufactured by the continuous casting machine 10, operational measures can be taken quickly, such as adding a maintenance process for the cast slab 30 between the continuous casting machine 10 and the next process (e.g., hot rolling).

[0083] On the other hand, regarding the second threshold (TJ2, TS2) for determining that no surface defects occur in the cast slab 30, none of the 13 cast slabs 30 in which the representative value ΔV of the discharge velocity difference was less than TJ2 and the representative value VS of the molten metal flow velocity was less than TS2 showed any surface defects. Furthermore, no cast slabs 30 in which surface defects occurred were included in the range where the representative value ΔV of the discharge velocity difference was less than TJ2 and the representative value VS of the molten metal flow velocity was less than TS2. From these results, it was confirmed that it is possible to determine whether no surface defects occur in the cast slab 30 produced by the continuous casting machine 10 based on whether the representative value ΔV of the discharge velocity difference and the representative value VS of the molten metal flow velocity are less than the second threshold (TJ2, TS2). When it is determined that no surface defects occur in the cast slab 30 produced by the continuous casting machine 10, operational measures can be taken quickly, such as omitting the maintenance process for the cast slab 30 between the continuous casting machine 10 and the next process (e.g., hot rolling).

[0084] Furthermore, in the range below the first threshold and above the second threshold, there was a mixture of cast slabs 30 with surface defects and cast slabs 30 without surface defects. It is thought that this range also includes surface defects caused by factors other than the inclusion of mold flux 46 and inclusions 50 into the molten steel 20. [Explanation of Symbols]

[0085] 10 Continuous casting machines 12 molds 14 Tan Dish 16 Cast slab support rolls 18 Secondary cooling device 20 Molten steel 22 Sliding Nozzles 24 Immersion nozzles 26 Solidified Shell 28 Unsolidified layer 30 cast slabs 32 Support Roles 34 Guide Roll 36 Pinch Roll 40 Long side of mold 42 Short side of mold 44 Thermocouples 46 Mold Flux 48a, 48b outlet 50 inclusions 60. Apparatus for determining surface defects in cast slabs 62 Control Unit 64 Input section Output section of 66 68 Storage section 70 Communications Department 72 Estimation part 74 Specific part 76 Judgment section 80 devices 100 Continuous Casting Equipment

Claims

1. A method for determining surface defects in a cast slab cast by a continuous casting machine, An estimation step in which the flow velocity distribution of molten steel in the mold is estimated using the operating conditions of the continuous casting machine and the temperature data of the molten steel in the mold, A selection step in which, using the flow velocity distribution, a representative value of the difference in discharge flow velocity of molten steel discharged from each of the pair of discharge ports of the immersion nozzle and a representative value of the surface flow velocity of molten steel at the surface of the molten steel in the mold are identified. A determination step in which surface defects of the cast slab are determined based on the representative value of the discharge flow velocity difference and the representative value of the molten metal flow velocity, A method for determining surface defects in a cast slab, including [the specified method].

2. In the estimation step described above, the flow velocity distribution is continuously estimated, The method for determining surface defects in a cast slab according to claim 1, wherein in the specified step, the maximum value among the discharge velocity differences obtained from a plurality of velocity distributions continuously estimated in the estimation step is used as a representative value of the discharge velocity difference, and the maximum value of the molten metal surface velocity in the plurality of velocity distributions is used as a representative value of the molten metal surface velocity.

3. The method for determining surface defects in a cast slab according to claim 1 or claim 2, wherein in the determination step, it is determined that a surface defect occurs in the cast slab if both the representative value of the discharge flow velocity difference and the representative value of the molten metal flow velocity exceed a first threshold set for each.

4. The method for determining surface defects in a cast slab according to claim 1 or 2, wherein in the determination step, it is determined that no surface defects occur in the cast slab if both the representative value of the discharge flow velocity difference and the representative value of the molten metal flow velocity are less than a second threshold set for each.

5. A device for determining surface defects in a cast slab, which determines surface defects in a cast slab cast by a continuous casting machine, An estimation unit that estimates the flow velocity distribution of molten steel in a mold using the operating conditions of the continuous casting machine and temperature data of the molten steel in the mold, A specification unit that uses the flow velocity distribution to identify a representative value of the difference in discharge flow velocity of molten steel discharged from each of the pair of discharge ports of the immersion nozzle and a representative value of the surface flow velocity of molten steel in the mold, A determination unit that determines surface defects in a cast slab based on the representative value of the discharge flow velocity difference and the representative value of the molten metal flow velocity, A device for determining surface defects in cast slabs, having the following features.

6. The estimation unit continuously estimates the flow velocity distribution, The device for determining surface defects in a cast slab according to claim 5, wherein the identifying unit takes the maximum value of the discharge velocity difference obtained from a plurality of flow velocity distributions continuously estimated by the estimation unit as the representative value of the discharge velocity difference, and the maximum value of the molten metal surface velocity in the plurality of flow velocity distributions as the representative value of the molten metal surface velocity.

7. A method for manufacturing a cast slab using the method for determining surface defects in a cast slab according to claim 3, A method for manufacturing a cast slab, comprising changing the operating conditions of the continuous casting machine so that the representative value of the molten metal flow velocity decreases for a cast slab that is determined to have surface defects in the determination step.

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