JP2013154369A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method which does not need expensive computing equipment, moreover, does not receive the influence of a facility status varying from hour to hour, and can suppress the generation of surface crack of a cast slab within a continuous casting machine.SOLUTION: This continuous casting method of a cast slab using a continuous casting machine having a bending zone which bends the cast slab while water-cooling the cast slab and a straightening zone which straightens the bent cast slab into a flat plate while water-cooling the bent cast slab is characterized in that: when measuring the surface temperature of the cast slab in the bending zone and the straightening zone with a radiation thermometer which detects the thermal radiation light radiated from the surface of the cast slab, the influence of scattering by the interface of moisture and air in the light path of the thermal radiation light between the surface of the cast slab and the radiation thermometer is suppressed, while making the surface temperature of the cast slab in the bending zone and the straightening zone measured by the thermal radiation meter 800°C or higher, the surface temperature variation of the cast slab in the bending zone and the straightening zone measured by the thermal radiation meter is made to be less than 75°C/min.

Description

  The present invention relates to a method for continuously casting a slab, and more particularly to a method for suppressing the occurrence of transverse cracks on the surface of a slab in a secondary cooling zone of a continuous casting machine.

  When cracks occur on the surface of the steel slab, care is required, and the slab manufacturing efficiency decreases. This crack also causes defects in the product. Therefore, it is important to avoid surface cracks when continuously casting a slab. The surface cracks of the slab include vertical cracks that break along the casting direction and horizontal cracks that crack along the direction perpendicular to the casting direction. Longitudinal cracks often start from within the mold, and transverse cracks often occur due to strain experienced by the slab when passing through a bending band or straightening band in a continuous casting machine.

In a bending band or a straightening band in a continuous casting machine, the slab is subjected to a tensile strain at a low speed of about 10 ⁻⁴ s ⁻¹ . When the steel slab is subjected to this low-speed strain, carbonitrides such as Al and Nb are precipitated at the crystal grain boundaries of the surface structure, and there is a large strength difference between the crystal grain boundaries and the vicinity. Cracks occur due to the occurrence of. This phenomenon occurs in a temperature range of 600 to 900 ° C. (hereinafter referred to as “embrittlement temperature range”) in which high temperature ductility is reduced. Therefore, conventionally, as a method of suppressing the occurrence of surface cracks, by controlling the amount of secondary cooling water, when the temperature of the slab is in the embrittlement temperature range, it is prevented from passing through the bending band and the correction band. Has been applied.

  In order to carry out this method, it is necessary to grasp the temperature of the slab in the bending band and the straightening band in the continuous casting machine. Examples of the method for measuring the temperature of the slab include measurement using a radiation thermometer, an infrared thermograph, and the like.

  In Patent Document 1, as a method for early detection of occurrence of abnormal water splashing on a slab in continuous casting, when the slab is straightened by a pinch roll, the transition of the slab surface temperature is measured, A method for detecting a significant temperature drop is disclosed. However, in continuous casting machines that cast slabs such as slabs with a relatively large cross-sectional area, a large amount of steam and a water film on the surface of the slab are generated. It is very difficult to measure.

  From this, it has been attempted to avoid the occurrence of bending and straightening distortion in the embrittlement temperature range by adjusting the cooling water amount in the secondary cooling zone based on the surface temperature of the slab calculated by heat transfer solidification calculation. It was. An example of the heat transfer coagulation calculation method is the method disclosed in Patent Document 2.

  However, in an actual machine of a continuous casting machine, even if casting is performed with a cooling water amount based on heat transfer solidification calculation, surface cracks of the slab may occur depending on the state of the equipment. In the actual machine, unexpected situations such as clogging of the secondary cooling nozzle and dripping water on the slab surface in the continuous casting machine may occur.In this case, the surface of the slab calculated by heat transfer solidification calculation This is because there is a difference between the temperature and the actual surface temperature of the slab.

  In Patent Document 3, as a method for accurately measuring the surface temperature of a temperature-measured material during water cooling, air is jetted from a radiation thermometer toward the surface of the temperature-measured material, and the air is measured between the temperature-measured material. A method is disclosed in which a column is formed, and thermal radiation light emitted from the surface of the temperature-measured material through the air column is detected by a radiation thermometer. Hereinafter, such a radiation thermometer capable of forming an air column is referred to as an air column thermometer.

  When measuring the surface temperature of a temperature-measuring material during water cooling by detecting thermal radiation with a radiation thermometer, the heat radiation emitted from the surface of the temperature-measurement material is spray cooling water, dripping water, steam, etc. Scattering is caused by the disturbance of, and an error due to this occurs.

  Scattering of thermal radiation due to this disturbance occurs when an interface between moisture such as water droplets or steam and air exists in the optical path of thermal radiation to be detected by a radiation thermometer. Then, due to such a change in the optical path, the area or position of the measurement area on the surface of the slab to be measured by the radiation thermometer is changed, or thermal radiation light from an area other than the measurement area is detected. Therefore, a measurement error occurs.

  However, according to the method disclosed in Patent Document 3, errors due to these disturbances can be suppressed. That is, the air column thermometer disclosed in the same document can suppress the influence of scattering caused by the interface between water and air such as water droplets and steam existing in the optical path of the thermal radiation to be detected by the slab. It is a thermometer. This document discloses an example in which a surface temperature measuring device using a radiation thermometer is arranged in a cooling device in which a large amount of cooling water is used in a thick steel plate production line.

JP 2000-246212 A Japanese Patent Laid-Open No. 10-291060 JP 2011-53047 A

  Thus, when casting a slab having a relatively small cross-sectional area, the surface temperature of the slab obtained by actual measurement, and when the influence of steam or water film is large, the surface of the slab calculated by heat transfer solidification calculation A method has been developed in which the amount of secondary cooling water is controlled based on the temperature to avoid bending and straightening distortion in the embrittlement temperature region and to suppress the occurrence of surface cracks that occur during continuous casting.

  However, in the case of casting a steel material having a relatively high strength, such conventional techniques sometimes cannot suppress the occurrence of surface cracks in the slab. Specifically, the surface crack of the slab occurred even though the bending / correction strain was received while avoiding the embrittlement temperature range. As a result of investigations by the inventors, it was concluded that this surface crack was caused by thermal distortion caused by variations in cooling of the slab, that is, temporal or spatial (positional) temperature differences that occur in the cooling process of the slab. Reached.

  Based on this concept, it is not sufficient to avoid the occurrence of strain in the embrittlement temperature range in order to suppress the occurrence of surface cracks in the slab, and at the same time, the occurrence of thermal strain due to cooling variations It is necessary to suppress.

  In order to suppress the occurrence of thermal distortion, it is necessary to accurately know the degree of variation in cooling, so it is necessary to measure the surface temperature of the slab with high accuracy. That is, in the measurement using a radiation thermometer, an infrared thermograph, or the like, the surface temperature of the slab cannot be measured with sufficiently high accuracy due to the influence of steam in the continuous casting machine or dripping water on the surface of the slab. Moreover, in the prediction by heat transfer solidification calculation, an expensive calculation device is required, and it is difficult to consider the influence of the equipment state that changes every moment.

  The present invention has been made in view of such problems, and does not require an expensive calculation device, and is not affected by equipment conditions that change from moment to moment, and the surface of a slab in a continuous casting machine. It aims at providing the continuous casting method which can suppress generation | occurrence | production of a crack. In the present invention, the surface crack of the slab that is the object of suppressing the occurrence is a lateral crack having a length of 1 mm or more.

1. Examination of method for measuring surface temperature of slab As described above, it is necessary to measure the surface temperature of the slab with high accuracy in order to suppress the occurrence of thermal strain due to variations in cooling of the slab. Therefore, the present inventors are a thermometer that can suppress the influence of scattering due to the interface between water and air such as water droplets or steam existing in the optical path of the thermal radiation light to be detected by the slab, as described above. The air column thermometer disclosed in Japanese Patent Application Laid-Open No. H10-228707 was used.

  Then, the present inventors measured the surface temperature of the slab with higher accuracy, and conducted an experiment simulating an actual continuous casting machine in order to control the temperature of the slab in the continuous casting machine based on the temperature. Went. In this experiment, an air purge condition that minimizes the influence of scattering of thermal radiation by the interface between moisture and air in the optical path between the slab surface to be measured by the air column thermometer and the thermometer, and the temperature The optimization of the distance between the meter and the slab, the amount of spray water and the spray position were studied.

  FIG. 1 is a schematic diagram of an experimental apparatus used for studying air purge conditions and the like. This experimental apparatus includes a transparent acrylic plate 11, two spray nozzles 12 that spray spray water 13 toward the acrylic plate 11, and an air column thermometer 14 disposed between the spray nozzles 12. The air column thermometer 14 includes a light receiving portion at the tip, and can form an air column between the air column thermometer 14 and the surface of the acrylic plate 11 by jetting air 15 from the tip. The distance D1 between the two spray nozzles 12 was 457 mm, and the distance D2 from the spray nozzle 12 to the acrylic plate 11 was 160 mm. The air column thermometer 14 can move in the direction of arrow A perpendicular to the acrylic plate 11, and the distance from the acrylic plate 11 can be adjusted.

  Further, a camera 16 is disposed on the opposite side of the surface of the acrylic plate 11 from which the spray water 13 and air 15 are jetted so as to face the acrylic plate 11, and the spray water 13 to the acrylic plate 11 that looks like a cast piece and The injection status of the air 15 can be confirmed.

  Using the apparatus shown in FIG. 1, the light emitted from the light emitting device (not shown) is irradiated to the air column thermometer 14 through the acrylic plate 11 and is regarded as thermal radiation, and the light detection status of the air column thermometer 14 is investigated. did. As a result, it was found that by adjusting the flow rate of the air 15 and the distance between the air column thermometer 14 and the acrylic plate 11, it is possible to secure the optical path without blocking the spray water 13.

  Based on this knowledge, a test was performed on an actual continuous casting machine.By adjusting the air flow rate and the distance between the air column thermometer and the cast piece, the spray water in the continuous casting machine was not shut off. It was also found that the surface temperature of the slab can be measured stably by securing the optical path of the heat radiation light.

2. Conditions for generating surface cracks in slab Next, conditions for generating surface cracks in continuous cast slabs were investigated. Specifically, the relationship between the occurrence position of the surface crack and the surface temperature of the slab measured by the measurement method using the air column thermometer was investigated.

  As a result, it was found that surface cracks frequently occur near the corners of the slab. Furthermore, when the relationship between the position where the surface crack near the corner occurred and the surface temperature when passing through the bent part and the straightened part at that position was investigated, it was found that the surface temperature variation was large in time or space. It was found that many surface cracks occurred.

  This was caused by at least one of the fact that the surface temperature of the slab was in the embrittlement temperature range when the slab was subjected to bending / correction strain and that the thermal stress increased due to the large temperature change. It depends. Therefore, it is considered that the occurrence of surface cracks in the slab can be suppressed by controlling the surface temperature of the slab in consideration of the high temperature properties of steel.

3. Accordingly, the present inventors investigated the high-temperature properties of steel. As a result, it was found that the amount of change in tensile strength accompanying a temperature change increases in a temperature range (900-1100 ° C.) higher than the embrittlement temperature range (600-900 ° C.). From this, it is considered that cracks occur due to an increase in strength difference at sites where the temperature variation of the slab surface is large, and when the temperature of the slab surface is controlled so that this difference in strength decreases, the occurrence of cracks occurs. Was suppressed.

  That is, the surface temperature of the slab is measured with high accuracy in the bending zone and straightening zone of the secondary cooling zone of the continuous casting machine, and the temporal or spatial temperature difference of this slab surface is measured. Based on the above, the amount of secondary cooling water is controlled, the slab is cooled so that the surface temperature when the slab passes through the bending zone and the straightening zone avoids the embrittlement temperature region, and further the temporal or spatial temperature By making the difference equal to or less than the temperature difference at which surface cracks occur, the surface cracks of the slab could be suppressed.

4). SUMMARY OF THE INVENTION The present invention has been made on the basis of the above findings, and the gist thereof is a continuous casting method of cast slabs shown in the following (1) to (3).

(1) A method for continuously casting a slab using a continuous casting machine having a bending band for bending the slab while cooling with water and a correction band for correcting the curved slab into a flat plate while cooling with water, the bending part and the When the temperature of the surface of the slab in the straightening band is measured by a radiation thermometer that detects thermal radiation emitted from the surface of the slab, thermal radiation between the surface of the slab and the radiation thermometer While suppressing the influence of scattering due to the interface between moisture and air in the optical path of light, the temperature of the surface of the slab in the bent portion and the correction band measured by the radiation thermometer is 800 ° C. or more, A method for continuously casting a slab, characterized in that a temperature change of the surface of the slab in the bent portion and the correction band measured by the radiation thermometer is less than 75 ° C / min.

(2) By injecting air from the radiation thermometer toward the surface of the slab and forming an air column between the radiation thermometer and the surface of the slab, the surface of the slab and the The continuous casting method for a slab according to (1), wherein the influence of scattering by the interface between moisture and air in the optical path of thermal radiation light between the radiation thermometer is suppressed.

(3) In at least one of the bent portion and the correction band, the casting measured with the radiation thermometer in a range from a position of 20 mm to a position of 120 mm from one end in the width direction to the center in the width direction of the slab. The continuous casting method for a slab according to (1) or (2), wherein the temperature gradient in the width direction of the slab on the surface of the piece is 0.75 ° C./mm or less.

  According to the slab continuous casting method of the present invention, the surface temperature of the slab can be controlled with high accuracy by controlling the amount of secondary cooling water based on the surface temperature of the slab measured with high accuracy. Therefore, the occurrence of thermal strain in the casting direction or the slab width direction can be reduced, the occurrence of surface cracks in the slab can be reliably suppressed, and a slab excellent in surface quality can be produced. .

It is the schematic of the experimental apparatus used for examination of air purge conditions etc. It is a block diagram of the vertical bending type continuous casting machine which can apply the continuous casting method of the slab of this invention. It is an enlarged view of the part which has arrange | positioned the air column thermometer in a continuous casting machine. It is a figure which shows the relationship between tensile temperature and a cross-sectional shrinkage rate (RA). It is a figure which shows the relationship between tensile temperature and tensile strength (TS). It is a figure which shows the relationship between the surface temperature of a slab, and the surface crack generation density, The figure (a) is a figure about a bending zone passage temperature, The figure (b) is a figure about a correction zone passage temperature. It is a figure which shows the relationship between temperature fluctuation | variation (DELTA) Tc of a slab surface, and surface crack generation density, The figure (a) is a figure about the fluctuation | variation of bending-band passage temperature, The figure (b) is a figure about the fluctuation | variation of correction | amendment band passage temperature. It is. It is a figure which shows the relationship between the temperature gradient (DELTA) Tw of slab width direction, and a surface crack generation density, The figure (a) is a figure about a bending zone passage temperature, The figure (b) is a figure about a correction zone passage temperature. It is a figure which shows the surface crack generation density of each Example.

1. FIG. 2 is a configuration diagram of a vertical bending type continuous casting machine to which the continuous casting method of the present invention can be applied. The molten steel accommodated in the ladle is once supplied to the tundish which is an intermediate container. A sliding gate capable of adjusting the flow rate of the molten steel is provided at the bottom of the tundish. The molten steel supplied to the tundish passes through the sliding gate and is supplied into the mold 21 shown in the figure via the immersion nozzle. The ladle, tundish, sliding gate and immersion nozzle are not shown.

  The molten steel 22 supplied into the mold 21 is cooled by the mold 21, solidified, and continuously drawn out as a slab 23 with an unsolidified portion held inside. A plurality of pairs of slab support rolls 24 including a support roll, a guide roll, and a pinch roll are arranged below the mold 21. A nozzle (not shown) for injecting water or mist is arranged between the individual slab support rolls 24 to constitute a secondary cooling zone. In the secondary cooling zone, the slab 23 is pulled out while being cooled by water or mist sprayed from each nozzle. Thus, the slab 23 pulled out while being cooled is cut into a predetermined length by a torch car (not shown).

  In the above-described drawing process, the slab 23 is discharged from the mold 21, passes through the bending band 25, and then passes through the correction band 26. In the bending band 25, the slab 23 is bent from the vertical direction to a constant arc direction, receives compression strain on the top side, and tensile strain on the ground side. In the straightening band 26, the slab 23 is bent back in the horizontal direction from the constant arc direction in the bending band 25, receives tensile strain on the top side, and compressive strain on the ground side. Both the bending band 25 and the correction band 26 are included in the secondary cooling band.

  In the continuous casting machine shown in FIG. 2, an air column thermometer 27 is disposed in each of the bending band 25 and the correction band 26. The air column thermometer 27 is a radiation thermometer, and has a light-receiving portion for heat radiation light at the tip, and injects air from the tip to form an air column between the air column thermometer 27 and the surface of the slab 23. Can be formed.

  FIG. 3 is an enlarged view of a portion where an air column thermometer is arranged in a continuous casting machine. The air column thermometer 27 is disposed between the slab support rolls 24 and is disposed so as to detect thermal radiation emitted from the surface of the slab 23. By using the air column thermometer 27, while suppressing the influence of the scattering by the interface between the water and the air in the optical path of the heat radiation light between the surface of the slab 23 and the air column thermometer 27, the slab 23 The surface temperature can be measured.

  Since the surface crack of the slab 23 is likely to occur at a position where it receives tensile strain, the air column thermometer 27 is preferably arranged on the ground side in the bending band 25 and on the top side in the correction band 26. Of the surface cracks in the slab, lateral cracks are likely to occur in the vicinity of the corner of the slab, so the air column thermometer 27 is preferably disposed in the vicinity of the corner of the slab.

  A plurality of air column thermometers 27 are preferably arranged in at least one of the bending band 25 and the correction band 26 in the width direction of the slab. Thereby, the temperature gradient in the width direction of the slab can be measured.

2. The continuous casting method of the slab of this invention The continuous casting method of the slab of this invention using the above-mentioned continuous casting machine is demonstrated. In the continuous casting method of the slab of the present invention, the surface temperature of the slab 23 in the bending band 25 and the straightening band 26 measured by the air column thermometer 27 is set to 800 ° C. or more, and is measured by the air column thermometer 27. The change in the surface temperature of the slab 23 in the bending band 25 and the straightening band 26 is set to less than 75 ° C./min. The surface temperature of the slab 23 can be controlled by adjusting the amount of water or mist injection (secondary cooling water amount) in the secondary cooling zone.

  As a method for adjusting the secondary cooling water amount, in addition to adjusting the secondary cooling water amount divided in the casting direction, there is a method of combining width cutting cooling that appropriately stops only the secondary spray cooling water amount at the end of the slab.

  According to the continuous casting method of the slab of the present invention, it is possible to avoid the slab 23 from being subjected to tensile strain in the brittle temperature range (600 to 900 ° C.) in the bending band 25 and the straightening band 26, and the casting direction and casting. Since the thermal strain received in the half width direction can be reduced, the occurrence of surface cracks in the slab in the secondary cooling zone can be suppressed.

  Further, in at least one of the bending band 25 and the correction band 26, the width direction of the slab on the surface of the slab in the range from the position of 20 mm to the position of 120 mm from one end in the width direction of the slab 23 to the center in the width direction. The temperature gradient is preferably 0.75 ° C./mm or less. The surface temperature of the slab in this range can be measured by arranging a plurality of air column thermometers 27 in at least one of the bending band 25 and the correction band 26 in the width direction of the slab.

3. Contents of examination of surface temperature range of slab defined in the present invention The reason why the surface temperature of a slab in a bending band and a straightening band is defined in the present invention will be described. In order to study the surface temperature range of the slab in order to complete the present invention, the present inventors conducted tests as follows.

3-1. Casting condition of slab The vertical bending type continuous casting machine shown in FIG. 2 was used as a continuous casting machine. The continuous casting machine had a vertical part of 2.5 m, a bending radius of 9.4 m, a machine length of 28 m, 7-point bending, and 6-point correction. The cast slab was slab, the size was 300 mm thick, the width was 2300 m, and the casting speed was 0.80 m / min.

  The steel used for casting was in the form of slabs in mass%, C: 0.02 to 0.50%, Si: 0.04 to 0.60%, Mn: 0.50 to 2.50%, P : 0.050% or less, S: 0.01% or less, Nb: 0.04% or less, and the balance was Fe and inevitable impurities. This is a steel type used for thick steel plates.

3-2. High temperature tensile test In order to grasp the embrittlement temperature range of this steel, a test piece was taken from the cast slab and the high temperature tensile test and the high temperature ductility were evaluated. The test piece was once melted in an Ar gas atmosphere at 1550 ° C., cooled to the test temperature, held at the test temperature (tensile temperature), and then subjected to a tensile test at a strain rate of 3.3 × 10 ⁻⁴ .

The high temperature ductility was evaluated by the cross-sectional shrinkage rate of the tensile test piece, and the high temperature strength was evaluated by the maximum tensile strength at the time of the tensile test. The cross-sectional shrinkage rate of the tensile test piece was calculated using the following formula (1).
R. A. = S1 / S2 × 100 (1)
Here, R.A. A. [%]: Cross-sectional shrinkage ratio of tensile test piece, S1: Cross-sectional area of tensile test piece after breakage, S2: Cross-sectional area of tensile test piece before breakage.

  4 is a graph showing the relationship between the tensile temperature and the cross-sectional shrinkage ratio (RA). From this figure, it can be seen that in this steel type, the temperature range (embrittlement temperature range) where the high temperature ductility is reduced is 700 to 800 ° C., and the cross-sectional shrinkage rate in the embrittlement temperature range is 20% or less. In the embrittlement temperature range, it is considered that cracks are easily generated on the surface of the slab.

  FIG. 5 is a diagram showing the relationship between the tensile temperature and the tensile strength (TS). From the figure, it can be seen that the lower the tensile temperature, the higher the tensile strength. Further, there is a tendency that the gradient of decrease in tensile strength with respect to the tensile temperature increases from around the tensile temperature exceeding 900 ° C.

3-3. Relationship between the temperature of the slab surface and cracks The slab was cast by changing the surface temperature of the slab in various ways using the above continuous casting machine, and the relationship between the temperature of the slab surface and cracks was examined using this slab. . The surface temperature of the slab was controlled by adjusting the amount of secondary cooling water.

  In the bending zone and the straightening zone that overlap with the secondary cooling zone of the continuous casting machine, it is difficult to accurately measure the surface temperature of the slab because it is affected by water or mist. Therefore, the above-mentioned air column thermometer that can suppress the influence of water or mist was used.

  This air column thermometer is installed at a position of 20 mm and 120 mm in the width direction from the corner of the lower part (ground side) of the slab of the bending band of the continuous casting machine, and the surface temperature of the slab passing through the bending band (hereinafter, "Bending band passage temperature") was measured continuously. Similarly, this air column thermometer is installed at the position of 20 mm and 120 mm in the width direction from the upper corner (top side) of the slab of the straightening strip of the continuous casting machine, and the surface of the slab passing through the straightening strip. The temperature (hereinafter also referred to as “correction zone passing temperature”) was continuously measured. These positions were measured, because the temperature difference between the position of 20 mm and the position of 120 mm in the width direction from the corner of the slab is most likely to be the largest. This is because there were many cracks.

  Evaluation of the bending band passing temperature and the straightening band passing temperature was performed using the absolute value, temperature variation ΔTc [° C./min], and temperature gradient ΔTw [° C./mm] in the slab width direction as indexes. The temperature variation ΔTc was the maximum temperature variation per minute. The temperature gradient ΔTw in the width direction of the slab was obtained by dividing by 100 the value obtained by subtracting the temperature measured at the position of 20 mm from the temperature measured at the position of 120 mm in the width direction from the lower corner of the slab.

  Evaluation of cracks on the surface of the slab was performed using the surface crack generation density as an index. The surface crack generation density was defined as the number of cracks having a length of 1 mm or more per 1 m in the casting direction of the slab.

3-3-1. FIG. 6 is a diagram showing the relationship between the surface temperature of the slab and the surface crack generation density. FIG. 6A shows the bending band passing temperature. (B) is a figure about the correction band passage temperature. From FIG. 5A, it can be seen that when the bending band passage temperature is 670 to 800 ° C., the surface crack generation density is high. The temperature range in which the surface crack generation density is high generally coincides with the embrittlement temperature range (700 to 800 ° C.) that has been clarified by evaluating the high temperature ductility in the high temperature tensile test described above. Furthermore, it was confirmed that the surface crack generation density was high even when the bending band passing temperature was 800 ° C. or higher. The same tendency was confirmed for the correction band from FIG.

  Thus, it was found that in both the bending band and the straightening band, the surface temperature of the slab passing therethrough is 670 to 800 ° C., and the peak of surface crack generation density exists in two temperature ranges of 800 ° C. or higher. . The peak existing at 670 to 800 ° C. is mainly caused by a decrease in hot ductility, and the peak existing at 800 ° C. or higher is not related to the hot ductility. It is thought to be due to the temperature gradient in the vicinity.

3-3-2. Relationship between Temperature Fluctuation ΔTc and Surface Crack Generation Density Therefore, the relationship between temperature fluctuation ΔTc and surface crack generation density was investigated when the bending band passing temperature and the correction band passing temperature were 800 ° C. or higher.

  FIG. 7 is a diagram showing the relationship between the temperature fluctuation ΔTc on the slab surface and the surface crack generation density. FIG. 7A shows the fluctuation of the bending band passing temperature, and FIG. 7B shows the correction band passing temperature. It is a figure about a fluctuation | variation. From the figure, it can be seen that both the bending band passing temperature and the straightening band passing temperature have a higher surface crack generation density as the temperature fluctuation ΔTc is larger, and no surface cracks are observed when the temperature fluctuation ΔTc is 75 ° C./min or less. Recognize.

3-3-3. Relationship between temperature gradient ΔTw in the slab width direction and surface crack generation density Further, when the bending band passing temperature and the straightening band passing temperature are 800 ° C. or higher, the temperature gradient ΔTw in the slab width direction and the surface crack generation density The relationship was investigated.

  FIG. 8 is a diagram showing the relationship between the temperature gradient ΔTw in the slab width direction and the surface crack generation density. FIG. 8A shows the bending band passing temperature, and FIG. 8B shows the correction band passing temperature. FIG. From the figure, the bending crack passage temperature and the straightening band passage temperature are both higher as the temperature gradient ΔTw is larger, and the surface crack generation density is higher, and when the temperature gradient ΔTw is 0.75 ° C./mm or less, the occurrence of surface cracks is not recognized. I understand that.

3-4. Summary From the above results, the surface temperature of the slab is measured with high accuracy, the amount of secondary cooling water is controlled based on the measured temperature, and the surface temperature of the slab is controlled with high accuracy. It was found that the occurrence of surface cracks in the slab can be suppressed by setting the bending band passing temperature and the straightening band passing temperature to 800 ° C. or higher and the temperature fluctuation ΔTc to 75 ° C./min or lower. It was also found that the occurrence of surface cracks in the slab can be further suppressed by setting the temperature gradient ΔTw in the slab width direction to 0.75 ° C./mm or less.

  In order to confirm the effect of the continuous casting method of the slab of the present invention, the following tests were performed and the results were evaluated.

1. Test conditions Using the continuous casting machine shown in FIG. 2, the slab of the steel type used for the thick steel plate used in the examination of the surface temperature range of the above slab was continuously cast. As for casting conditions, in Examples 1 and 2, the bending band passing temperature and the correction band passing temperature of the slab were set to 800 ° C. or more, and the temperature fluctuation ΔTc was set to 75 ° C./min or less. Further, in Invention Example 1, the temperature gradient ΔTw in the slab width direction was made larger than 0.75 ° C./mm, and in Invention Example 2, it was made 0.75 ° C./mm or less. In the comparative example, the bending band passing temperature and the straightening band passing temperature of the slab were less than 800 ° C. The casting conditions other than this were the same as the above-described examination of the surface temperature range of the slab.

2. Test Results FIG. 9 is a diagram showing the surface crack generation density of each example. As shown in the figure, the surface crack generation density was 3.0 pieces / m in the comparative example, whereas it was as low as 1.0 pieces / m in the present invention example 1, and none in the present invention example 2. there were.

  According to the slab continuous casting method of the present invention, the surface temperature of the slab can be controlled with high accuracy by controlling the amount of secondary cooling water based on the surface temperature of the slab measured with high accuracy. Therefore, the occurrence of thermal strain in the casting direction or the slab width direction can be reduced, the occurrence of surface cracks in the slab can be reliably suppressed, and a slab excellent in surface quality can be produced. .

11: Acrylic plate, 12: Spray nozzle, 13: Spray water, 14: Air column thermometer, 15: Air, 16: Camera, 21: Mold, 22: Molten steel, 23: Cast slab, 24: Slab support roll, 25: Bending band, 26: Straightening band, 27: Air column thermometer

Claims (3)

A continuous casting method of a slab using a continuous casting machine having a bending band for bending the slab while cooling with water and a correction band for correcting the curved slab into a flat plate while cooling with water,
When measuring the temperature of the surface of the slab in the bent portion and the straightening band with a radiation thermometer that detects thermal radiation emitted from the surface of the slab, the surface of the slab, the radiation thermometer, Suppresses the influence of scattering by the interface between moisture and air in the optical path of thermal synchrotron radiation,
The temperature of the surface of the slab in the bent part and the correction band measured by the radiation thermometer is 800 ° C. or more, and the slab in the bent part and the correction band measured by the radiation thermometer. A method for continuously casting a slab characterized in that the temperature change of the surface of the slab is less than 75 ° C./min.   By injecting air from the radiation thermometer toward the surface of the slab and forming an air column between the radiation thermometer and the surface of the slab, the surface of the slab and the radiation thermometer 2. The continuous casting method for a slab according to claim 1, wherein the influence of scattering due to an interface between moisture and air in the optical path of the thermal radiation light between the two is suppressed.   The surface of the slab measured by the radiation thermometer in a range from one end in the width direction of the slab to the center in the width direction from a position of 20 mm to a position of 120 mm in at least one of the bent portion and the correction band. The continuous casting method for a slab according to claim 1 or 2, wherein a temperature gradient in a width direction of the slab is set to 0.75 ° C / mm or less.

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