JP3402286B2 - Continuous casting method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a C content of 0.05, which can obtain a slab of good surface quality without causing a constraining breakout. The present invention relates to a continuous casting method of steel of 0.18% by mass. [0002] In continuous casting of slab slabs, which are materials for manufacturing hot coils wound with hot-rolled steel strip, from the viewpoint of improving slab quality and ensuring productivity, A slab having a thickness of about 200 mm is cast at a speed of 1 to 2 m / min. On the other hand, in recent years, attempts have been made to obtain a cast slab closer to the thickness and shape of a product from the viewpoint of the construction cost and personnel reduction of related equipment, and continuous casting of a thin slab having a thickness of 50 to 100 mm. A method combining a method and a subsequent rolling method using a simple hot rolling facility disposed on a casting line has been put into practical use. When casting such a thin slab, 3-5 in order to approach the productivity of hot rolling equipment
Casting is performed at a high speed of m / min. When high-speed casting is performed at 3 to 5 m / min, even in a low carbon steel having low cracking susceptibility, non-uniform solidification tends to occur in the mold, so that vertical cracks are likely to occur on the surface of the slab. In particular, when hypoperitectic steel, which is originally susceptible to cracking and easily causes non-uniform solidification, is cast at such a high speed, significant vertical cracks often occur on the surface of the slab. Furthermore, during high speed casting at 3-5 m / min,
Since the amount of molten slag flowing into the gap between the mold cooling plate and the solidified shell (the melted mold powder) is reduced, lubrication is poor and the solidified shell tends to be seized on the inner wall of the mold. Therefore, a restrictive breakout is likely to occur. The following countermeasures have been proposed as countermeasures against the occurrence of vertical cracks on the slab surface and the occurrence of restrictive breakout. In JP-A-8-141713, when casting a steel having a C content of 0.08 to 0.16% by mass, the basicity (ratio CaO / SiO _{2 by} mass%) is set to 1.2 to
There has been proposed a method for preventing the occurrence of vertical cracks on the surface of a slab by using a mold powder having a high solidification temperature of 1130 ° C. or higher with a high 1.6. In this method, the solidified shell in the mold is slowly cooled by using a mold powder having a high basicity and a high solidification temperature. In JP-A-7-214266, when casting a steel having a C content of 0.08 to 0.18 mass%, the viscosity at 1300 ° C. is 2 poise or less and the solidification temperature is 1000 to 1300 ° C. A method has been proposed that uses a mold powder and adjusts the negative time of the vibration conditions of the mold to prevent vertical cracks and oscillation cracks on the surface of the slab and to prevent the occurrence of constraining breakouts. . However, even in the methods proposed in Japanese Patent Laid-Open Nos. 8-141713 and 7-214266, the conditions for the cooling water passing through the cooling water flow path provided in the cooling plate of the mold are limited. If it is not appropriate, vertical cracks are generated on the surface of the slab, or constraining breakout is likely to occur. SUMMARY OF THE INVENTION [0010] The present invention is originally intended for 3-5 m / min of steel from low carbon steel to medium carbon steel including hypoperitectic steel, which tends to cause vertical cracks on the slab surface. When casting at high speed, without generating a restrictive breakout,
It is an object of the present invention to provide a continuous casting method of steel capable of obtaining a cast slab having good surface quality with no occurrence of vertical cracks on the surface. The gist of the present invention is to cast a molten steel having a C content of 0.05 to 0.18% by mass using a mold powder having a solidification temperature α of 1000 to 1300 ° C. A continuous casting method with a thickness δ of 20 to 45 mm
Then, a mold cooling plate made of copper or a copper alloy having a thermal conductivity of γ (W / m ² · k) is used and passes through a cooling water flow path provided on the back frame side in the mold cooling plate. The linear flow velocity β of the cooling water is 5 to 10 m / second, and α
The Q value obtained from β, γ, and δ is in the continuous casting method of steel that is cast under conditions that satisfy the following expression (A). 4.6 ≦ Q ≦ 14 (A) where Q = (α × β × γ) / (δ × 10 ⁴ ) Cooling water provided on the back frame side in the cooling plate of the mold The linear flow rate β (m / sec) of the cooling water passing through the flow path is defined by the following equation (B). Cooling water linear flow velocity β = a / (b × c) (B) where a: total amount of water passing through the cooling plate (Nm ³ /
B) Number of cooling water passages arranged in the cooling plate (pieces) c: Cross-sectional area of the cooling water passage (m ² ) Although the transverse area of the cooling water passage is normally constant, When this cross-sectional area is changed depending on the position of the cooling plate in the mold width direction, (b × c) in the above equation (B) may be obtained by the following equation (C). [0014] Where bn: number of cooling water passages having the same cross-sectional area
(Pieces) cm: the number of cross-sectional areas of the cooling water flow path The thickness δ of the cooling plate of the mold is 20 to 45 mm, and the cooling water flow arranged in the cooling plate on the back frame side as will be described later In the case of a rectangular shape, the size of the flow path is preferably about 10 to 20 mm in length and about 5 to 10 mm in width. The length in the size of the cooling water passage is the direction of the thickness of the cooling plate. The mold cooling plate used in the method of the present invention is the above-described size cooling plate. The inventors have solved the above-mentioned problems of the present invention.
The problem was solved by the knowledge and measures shown below. The linear flow rate of the cooling water passing through the cooling water passage provided on the anti-melting steel side in the cooling plate of the mold is set to 5 to 10 m / sec. As a result, the molten steel and the solidified shell in the mold are properly cooled by the mold cooling plate, and the amount of molten slag flowing into the gap between the mold cooling plate and the solidified shell is optimized. Since the lubricity of the gap between the cooling plate of the mold and the solidified shell is ensured, the occurrence of a restrictive breakout can be prevented, and the cooling of the solidified shell in the mold is uniform in the width direction. Generation of vertical cracks on the slab surface can be prevented. Further, since the cooling plate does not warp due to the heat of the molten steel and the solidified shell, stable operation is possible and breakout hardly occurs. Defined by the above equation (A) obtained from the solidification temperature α of the mold powder, the linear flow rate β of the cooling water passing through the cooling water passage, the thermal conductivity γ of the cooling plate, and the thickness δ of the cooling plate. The Q value to be set is 4.6-14. FIG. 2 is a graph showing the relationship between the average thickness of the solid phase solidified by the molten slag in the gap between the mold cooling plate and the solidified shell and the Q value. FIG. 3 is a graph showing the relationship between the average thickness of the liquid phase of the molten slag in the gap between the cooling plate of the mold and the solidified shell and the Q value. All show the test results using the cooling plate of the mold used in the test of Examples described later. Casting is performed at a casting speed in the range of 1.0 to 5.0 m / min, and the total thickness of the average solid phase and the liquid phase obtained from the amount of mold powder used is calculated. The solidified slag film was collected from the sample, and the average thickness of the solid phase where the molten slag solidified was measured. The result obtained by subtracting the measured average solid phase thickness from the total thickness of the average solid phase and liquid phase obtained by calculation to obtain the average liquid phase thickness is shown. As can be seen from FIGS. 2 and 3, by setting the Q value to 4.6-14, the casting speed is 1.0-5.
In the range of 0 m / min, the average liquid phase thickness is 0.1 m.
m or more can be secured. Therefore, it is possible to prevent the solidified shell from being baked on the cooling plate of the mold, and thus it is possible to prevent the occurrence of a restrictive breakout. In addition, the average solid phase thickness is set to 0.
It can be 2 mm or more. Therefore, the effect of slow cooling of the solidified shell by mold powder is obtained,
Cooling of the solidified shell in the mold becomes uniform in the width direction, and therefore vertical cracks on the surface of the slab can be prevented. The average liquid phase or solid phase thickness means the average thickness in the mold. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram schematically showing the state of a mold used in the method of the present invention and a solidified shell in the mold. 1A is a longitudinal sectional view of the long side of one side of the mold, and is a sectional view taken along line B1-B2 of FIG. 1B. FIG. 1B is A1-A2 of FIG. It is sectional drawing of a line. In the mold 1 used in the method of the present invention, a cooling water passage 4 provided on the back frame side in the cooling plate 2 on the long side 1a side of the mold is arranged on the side in contact with the back frame 3. The cooling water passage is arranged along the casting direction as in a normal mold.
FIG. 1B shows a state in which ten cooling water passages 4 are arranged to make the drawing easier to see. Actually, as will be described later, in the case where the width of the mold is 1200 mm,
About 60 cooling water passages are preferably arranged. Molten steel 6 is injected into the mold from a tundish (not shown) via the immersion nozzle 5.
Mold powder 8 is added to the injected molten steel surface. The added mold powder is melted by the heat of the molten steel,
A molten slag 9 is formed. The molten slag flows into the gap between the cooling plate of the mold and the solidified shell 7, and the molten slag in the portion in contact with the cooling plate of the mold is solidified to become a solid phase 10 in a glass state. The molten slag in contact with the solidified shell exists as the liquid phase 11. The heat of the molten steel and the solidified shell is removed by the cooling water passing through the cooling water passage provided in the cooling plate of the mold via the liquid phase of the molten slag and the solid phase in the glass state. The steel targeted by the method of the present invention is a low carbon steel having a C content of 0.05% by mass or more and less than 0.10% by mass and a sub-carbon having a C content of 0.10 to 0.18% by mass. Medium carbon steel including peritectic steel. As mentioned above, even low-carbon steel 3
This is because when the casting is performed at a high speed of ˜5 m / min or when hypoperitectic steel is cast, vertical cracks are likely to occur on the surface of the slab, and constraining breakout is likely to occur. In the method of the present invention, the solidification temperature is 1000 to 1000.
A mold powder of 1300 ° C. is used. When the solidification temperature is less than 1000 ° C., the gap between the mold cooling plate and the solidified shell is not increased even if the linear flow rate of the cooling water passing through the cooling water passage provided in the mold cooling plate is increased. An excessive amount of molten slag flows in. If too much molten slag flows in, the cooling of the solidified shell in the mold becomes uneven in the width direction, so that vertical cracks are likely to occur on the surface of the slab. When the solidification temperature exceeds 1300 ° C.,
The mold powder added to the surface of the molten steel in the mold becomes difficult to melt, and a sintered body (slag bear) of the mold powder which is not completely melted is generated and stays in the vicinity of the meniscus of the molten steel. When a large amount of slag bear is retained, it becomes difficult for molten slag to flow into the gap between the cooling plate of the mold and the solidified shell. Therefore, the cooling of the solidified shell in the mold becomes uneven in the width direction, and vertical cracks are likely to occur on the surface of the slab. In addition, since the solidified shell is easy to seize on the cooling plate of the mold,
Restraint breakout is likely to occur. The viscosity of the mold powder at 1300 ° C. is preferably 0.4 to 2.0 poise. If it is less than 0.4 poise, the amount of molten slag flowing into the gap between the mold cooling plate and the solidified shell becomes excessive, and if it exceeds 2.0 poise, it becomes difficult for the molten slag to flow. In either case, the cooling of the solidified shell in the mold becomes uneven in the width direction, and vertical cracks are likely to occur on the surface of the slab. Further, the mass% ratio CaO / SiO ₂ of CaO to SiO ₂ of the mold powder is preferably 0.8 to 2.0. By setting the ratio CaO / SiO ₂ to 0.8 to 2.0, crystals are likely to precipitate in the process of melting and solidifying the molten slag by the cooling plate of the mold, and the solidified solidified molten slag. When there are many crystals precipitated in the phase, the heat of the molten steel and the solidified shell is difficult to be transmitted to the cooling plate. Therefore, the solidified shell is easily cooled slowly even with the same slag film thickness. Therefore, the cooling of the solidified shell in the mold becomes uniform in the width direction, and vertical cracks are less likely to occur on the surface of the slab. Copper or a copper alloy is used for the cooling plate of the mold. Copper or copper alloy may be used for a normal continuous casting mold. For example, in the case of commonly used Ag-added deoxidized copper, its thermal conductivity γ is about 355 W / m ² · k,
Further, in the case of Cr · Zr copper which is a precipitation hardening type copper alloy, 209
The value is about W / m ² · k. The thickness δ of the mold cooling plate is set to 20 to 45 mm. If the thickness of the mold cooling plate is less than 20mm,
Even if the amount of cooling water passing through the cooling water flow path disposed in the cooling plate is increased, deformation such as warpage of the cooling plate due to the heat of the molten steel and the solidified shell tends to occur. Therefore, breakout is likely to occur. If it exceeds 45mm,
This is not practical because the mold becomes larger and the vibration device of the mold becomes larger. The cross-sectional shape of the cooling water passage may be rectangular or circular. When the thickness of the cooling plate of the mold is 30 mm, the width is 1200 mm, and a rectangular cooling water passage is provided, for example, a cross-sectional size of about 10 to 20 mm in length and about 5 to 10 mm in width is preferable. Further, the number of arrangements may be approximately 30-60 in the entire width of the cooling plate. The height at which the cooling water passage is disposed may be almost the entire height of the cooling plate. As shown in FIG. 1 (b), the cooling water passage is arranged on the back frame side in the cooling plate 2 on the long side 1a side of the mold. The linear flow velocity of the cooling water passing through the cooling water passage provided on the back frame side in the cooling plate of the mold is set to 5 to 1.
0 m / sec. If the linear flow velocity of the cooling water is less than 5 m / sec, the amount of molten slag flowing into the gap between the mold cooling plate and the solidified shell becomes too large, so that the solidified shell in the mold is not cooled in the width direction. Therefore, vertical cracks are likely to occur on the slab surface. In extreme cases, breakout is likely to occur because the cooling plate warps due to the heat of the molten steel and the solidified shell. On the other hand, when the linear flow rate of the cooling water exceeds 10 m / sec, the solidified shell in the mold is cooled too much and vertical cracks are likely to occur. The solidification temperature α of the mold powder, the linear flow rate β of the cooling water passing through the cooling water passage, the thermal conductivity γ of the cooling plate
The Q value defined by the above-described equation (A) consisting of the thickness δ of the cooling plate is set to 4.6-14. When the Q value is less than 4.6, the average solid phase thickness is 0.35 mm or less at a casting speed of 1.0 m / min and 0.2 mm or less at a casting speed of 5.0 m / min. . When the average solid phase thickness is 0.2 mm or less, the effect of slow cooling the solidified shell by the mold powder becomes small, and the cooling of the solidified shell in the mold becomes uneven in the width direction. Longitudinal cracks are likely to occur. When the Q value exceeds 14,
In the range of the casting speed of 1.0 to 5.0 m / min, the average liquid phase thickness is 0.1 mm or less. When the average liquid phase thickness is 0.1 mm or less, the solidified shell is likely to be seized onto the cooling plate of the mold, so that a restrictive breakout is likely to occur. EXAMPLE A vertical bending type continuous casting machine having a vertical portion length of 1 m, a bending radius of 3.5 m, and a machine length of 15 m was used.
An 11% by mass medium carbon steel, a hypoperitectic steel, was cast at a casting speed of 3 m.
Casting at / min. Mold exit side thickness is 120mm, width is 1
A rectangular mold of 500 mm was used. The cooling plate of the mold has a thermal conductivity of 355 W /
Ag-added deoxidized copper with m ² · k or 209 W / m ²
* Cr * Zr copper of precipitation hardening type copper alloy which is k was used.
The thickness of the cooling plate of the mold was 30 mm. The cross-sectional size of the cooling water passage is 20 m long
m, width 5 mm, and the height was the total height of the cooling plate. The cooling water passage was arranged in such a manner that the portion of 5 mm in width was in contact with the back frame on the anti-melting steel side in the cooling plate on the long side of the mold. 75 cooling water passages were arranged on one cooling plate. The test was performed by changing the linear flow velocity of the cooling water passing through the cooling water passage of the cooling plate in the mold within a range of 4 to 12 m / sec. Further, four types of mold powders having a solidification temperature changed within a range of 1000 to 1300 ° C. were used. Table 1 shows the chemical composition and solidification temperature of the mold powder. In each test, 1-heat single casting was performed. One heat is about 100
ton. [Table 1] The occurrence of a constraining breakout during casting was predicted by mounting a thermocouple in the cooling plate of the mold. Specifically, it is as follows. The temperature of the cooling plate was measured by a thermocouple attached in the cooling plate on the long side on both sides of the mold. Thermocouples were placed in a total of 18 locations in the width direction at the center of the thickness of the cooling plate in one side, ie, 6 locations in the width direction and 3 locations in the casting direction. When the difference between the maximum and minimum values of the temperature measurement results by the thermocouple in the cooling plate on one side is 10 ° C or higher, a breakout prediction alarm is sounded as the solidified shell starts to be seized on the cooling plate of the mold, I made a prediction. The number of predictive warnings in each test was measured to evaluate the ease of occurrence of a restrictive breakout. A representative 10 m long slab sample was taken from each test, and the number of occurrences of restraint marks (seizure marks) on the slab surface was examined by visual inspection. Investigate. From the measured number of restraint marks generated, the number of restraint marks generated per slab length was determined. Also, from the measured length of occurrence of vertical cracks, the slab length 1
The length of the vertical crack per m was calculated | required. Furthermore, from each test, a typical length of 10 m
The slab was hot rolled and wound on a hot coil. After pickling the hot coil, visually observe the surface,
The occurrence of surface defects due to vertical cracks and restraint marks in the slab was investigated. The value obtained by dividing the weight of the surface flawed portion by the total weight of the hot coil was defined as the hot coil quality defect occurrence rate. Each test condition and each test result are shown in Table 2 and Table 3. [Table 2] [Table 3] Test No. of the example of the present invention. 1-No. In 8,
The range of each condition of the solidification temperature of the mold powder prescribed by the method of the present invention, the thickness of the cooling plate of the mold, the linear flow rate of the cooling water passing through the cooling water passage provided in the cooling plate, and the Q value Tested within. For the mold cooling plate, Ag-added deoxidized copper having a thermal conductivity of 355 W / m ² · k was used. These test Nos. 1-No. In No. 8, the number of predictive alarms for restrictive breakout was zero, and the number of slab restraint marks generated was zero. Moreover, vertical cracks did not occur on the slab surface. In addition, there were tests with a hot coil quality defect rate of 1-2%, but in most tests,
No surface flaws occurred on the hot coil. in this way,
Occurrence of constraining breakout could be prevented, and slabs and hot coils with good surface quality were obtained. Comparative Example Test No. 9 and no. 10, the linear flow velocity of the cooling water passing through the cooling water passage was increased outside the range defined by the method of the present invention, and the Q value was also tested outside the upper limit of the conditions defined by the method of the present invention. Comparative Example Test No. 11 and no. 12
Then, the linear flow velocity of the cooling water passing through the cooling water passage is within the range defined by the method of the present invention, and the Q value is defined by the method of the present invention by using a mold powder having a high solidification temperature. Tested outside the upper limit of conditions. Comparative Example Test No. In No. 13, the linear flow velocity of the cooling water passing through the cooling water passage was slowed outside the range defined by the method of the present invention, but the Q value was tested within the range defined by the method of the present invention. Test No. of the comparative example. 14
Although the linear flow rate of the cooling water passing through the cooling water flow path was increased outside the range defined by the method of the present invention, the Q value was tested within the range defined by the method of the present invention. These test Nos. 9-No. In No. 12, no constraining breakout occurred, but the number of predictive alarms for constraining breakout was 3-6 times / heat.
The number of slab restraint marks generated is 0.02 to 0.10 / m.
Met. This is because the average liquid phase thickness of the molten slag is reduced because the Q value is large, and the solidified shell is easily seized onto the cooling plate of the mold. Vertical cracks did not occur on the slab surface. However, the hot coil quality defect incidence is 5-1
At 5%, it was a slightly more frequent occurrence. This is because the restraint mark on the surface of the slab became a surface defect of the hot coil. Test No. 1 in which the linear flow rate of the cooling water was slowed down. 13
And test No. with increased linear flow velocity. In No. 14, vertical cracks occurred on the surface of the slab, and quality defects also occurred in the hot coil. Test No. of the example of the present invention. 15-No. In No. 26, Cr · Zr copper of a precipitation hardening type copper alloy having a thermal conductivity of 209 W / m ² · k was used. Other conditions are test N
o. 1-No. Same as 8. Test No. 15-No. 26, test N
o. 1-No. The test results were almost the same as the test results of No. 8, and the occurrence of constraining breakout could be prevented, and slabs and hot coils with good surface quality were obtained. Comparative Example Test No. 27 and no. 28
Then, the linear flow velocity of the cooling water passing through the cooling water flow path provided in the cooling plate is slightly slower within the range specified by the method of the present invention, or outside the lower limit of the range specified by the method of the present invention. Then, by slowing down, the Q value was tested outside the lower limit of the conditions defined by the method of the present invention. For other conditions, test no. 15-No. 26 under the same conditions. Test No. with small Q value 27 and no.
In No. 28, vertical cracks occurred on the surface of the slab, and defective quality also occurred in the hot coil. Comparative Example Test No. In No. 29, the linear flow velocity of the cooling water passing through the cooling water flow path was increased outside the range defined by the method of the present invention, but the Q value was tested within the range defined by the method of the present invention. Test No. 1 in which the linear flow rate of the cooling water was increased. 29
Then, vertical cracks occurred on the surface of the slab, and defective quality also occurred in the hot coil. As a result of the application of the method of the present invention, a high speed of about 3 to 5 m / min or more from low carbon steel to medium carbon steel including subperitectic steel, which is prone to vertical cracks, on the slab surface. Also when cast by, a slab of good surface quality can be obtained without causing a vertical crack on the surface without generating a constraining breakout.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram schematically showing the cooling process of a mold used in the method of the present invention, powder added in the mold, and molten steel. FIG. 2 is a diagram showing a relationship between an average thickness of a solid phase and a Q value. FIG. 3 is a diagram showing a relationship between an average thickness of a liquid phase and a Q value. [Description of Symbols] 1: Mold 1a: Long side of mold 2: Cooling plate 3: Back frame 4: Cooling water passage 5: Immersion nozzle 6: Molten steel 7: Solidified shell 8: Mold powder 9: Molten slag 10: Solid phase 11: Liquid phase of molten slag

─────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI B22D 11/16 B22D 11/16 A (72) Inventor Toshihiko Murakami 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries Intra (72) Inventor Masahiko Oka 4-5-33 Kitahama, Chuo-ku, Osaka, Osaka Sumitomo Metal Industries, Ltd. (56) References JP 10-263769 (JP, A) JP 10-10 58093 (JP, A) JP-A-5-318035 (JP, A) JP-A-9-94635 (JP, A) JP-A-8-47760 (JP, A) JP-A-9-136144 (JP, A) JP-A-7-214266 (JP, A) JP-A-8-141713 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/10 B22D 11/00 B22D 11/055 B22D 11/059 120 B22D 11/108 B22D 11/16

Claims (1)

(57) Claims 1. A continuous casting method for casting molten steel having a C content of 0.05 to 0.18% by mass using a mold powder having a solidification temperature α of 1000 to 1300 ° C. A mold cooling plate made of copper or copper alloy having a thickness δ of 20 to 45 mm and a thermal conductivity of γ (W / m ² · k) is used, and the back frame side in the mold cooling plate is used. The linear flow rate β of the cooling water passing through the cooling water flow path provided at 5 to 10 m / sec, and the Q value obtained from α, β, γ and δ satisfies the following formula (A) A continuous casting method for steel, characterized by casting under the following conditions. 4.6 ≦ Q ≦ 14 (A) where Q = (α × β × γ) / (δ × 10 ⁴ )