JP2001300705A - Method for continuously casting molten steel excellent in quality characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method for molten steel by which in the case of casting a slab, etc., by applying electromagnetic stirring to molten steel in a mold, defect such as scab and blister, caused by gas bubble and inclusion caught into the surface layer part of the cast slab, is prevented and the cast slab excellent in the quality characteristic is obtained in a good yield. SOLUTION: In the continuous casting method for the molten steel, in which the circular flow along the inner wall of the mold 14 is given by applying the electromagnetic stirring to the molten steel 11 in the mold 14, whirling flow is given from a meniscus 21 to the molten steel 11 at a position where thickness of the solidified shell is 20 mm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a quality characteristic for preventing defects caused by bubbles or inclusions trapped in the surface layer of a slab when casting slabs or the like by electromagnetically stirring molten steel in a mold. It relates to an excellent continuous casting method for molten steel.

[0002]

2. Description of the Related Art Conventionally, in continuous casting of molten steel, in order to prevent clogging of nozzles and immersion nozzles, an alumina gas is supplied from a porous refractory provided in the immersion nozzle into the molten steel, thereby making the alumina-based material refractory. Of the oxide and its growth are suppressed. When the molten steel is solidified, the argon gas supplied to the immersion nozzle is apt to be trapped at the solidified shell interface as air bubbles, and the thin steel sheet manufactured by performing processing such as rolling on the solidified slab is subjected to processes such as annealing. When passed through, blisters and the like are generated, leading to an increase in care and a decrease in yield. Further, in the molten steel, there is an oxide generated when a deoxidizing agent is added, or a trace amount of oxide due to erosion of a refractory. This oxide may agglomerate during casting, is captured at the interface of the solidified shell that becomes the surface layer portion of the solidified slab, if the position is shallow, in the process of performing rolling or the like, in the process of rolling or the like, Surface defects such as cracks occur, resulting in an increase in care and a decrease in product yield. Therefore, as described in JP-A-58-100955, a plurality of electromagnetic coils attached to the outer wall of a mold have been disclosed in order to prevent defects caused by air bubbles and inclusions generated in the surface layer of a slab. By applying a horizontal swirling flow along the inner peripheral wall of the mold to the molten steel using a combination of acceleration and deceleration as electromagnetic stirring conditions, the interface of the surface layer (solidified shell) is cleaned to remove bubbles and inclusions. Continuous casting to form a less solidified shell such as is disclosed.

[0003]

However, in the method described in Japanese Patent Application Laid-Open No. 58-100955, first, the flow of molten steel along the inner peripheral wall of the mold is accelerated and accelerated near the short piece of the mold. Suddenly decelerates the flow
Bubbles and inclusions accumulate at the interface of the solidified shell of the slab corresponding to the boundary between the acceleration flow and the deceleration flow, and blistering flaws occur during processing such as rolling, and inclusions shallow from the surface And become defects such as barbed flaws. In particular, the argon gas supplied into the molten steel from the immersion nozzle is bubbled and enters the lower part of the molten steel in the mold, and is captured at the interface of the solidified shell at the lower part of the mold where no swirling flow acts. When the thin steel sheet produced by rolling the cast solidified molten steel passes through the annealing process, the trapped bubbles come out on the surface of the thin steel sheet by the rolling process, so that the internal bubbles expand. As a result, the occurrence of blistering flaws becomes remarkable, leading to problems such as an increase in the care of thin steel sheets and a decrease in the yield of good products.

The present invention has been made in view of such circumstances, and when casting slabs or the like by electromagnetically stirring molten steel in a mold, blisters caused by bubbles or inclusions trapped in the solidified shell of the slab. An object of the present invention is to provide a continuous casting method of molten steel that prevents defects such as flaws and scabs, improves yield, and has excellent quality characteristics.

[0005]

According to the present invention, there is provided a method for continuously casting molten steel having excellent quality characteristics according to the present invention, in which a molten steel in a mold is electromagnetically stirred to impart a swirling flow along an inner wall of the mold. In the method for continuously casting molten steel, the swirling flow is applied to the molten steel having a solidified shell thickness of 20 mm from the meniscus. According to this method, bubbles and inclusions trapped at the interface of the solidified shell within a predetermined solidified shell thickness from the meniscus (fluid surface) are washed and removed by the action of the swirling flow of the molten steel provided by the electromagnetic coil. Can be. Then, bubbles existing in the shallow portion of the solidified shell are eliminated, and the expansion of the bubbles can be suppressed in the processing and heat treatment steps of the thin steel sheet, and blistering flaws generated on the thin steel sheet can be prevented. In addition, the inclusions are also washed and removed at the same time, and it is possible to suppress the burrs and the like caused by the inclusions. When the swirling flow is applied to molten steel at a position where the thickness of the solidified shell is greater than 20 mm, it is necessary to increase the number of electromagnetic stirring devices for applying the swirling flow, which leads to an increase in equipment cost and an increase in power consumption.

Further, the swirling flow can be divided and applied. As a result, bubbles and inclusions trapped in the solidified shell can be removed by washing, so that power consumption can be reduced and equipment can be simplified.

The thrust for electromagnetically stirring the molten steel is 5 to
It can be a 70 mm iron pole. Thereby, while promoting the floating of bubbles and inclusions, fine bubbles and inclusions can be washed by the swirling flow of molten steel to clean the interface of the solidified shell of the slab. 5mm thrust to stir molten steel
If the diameter is smaller than the iron column, the swirling flow of the molten steel is weakened, and the bubbles and inclusions trapped at the interface of the solidified shell cannot be removed by washing. On the other hand, if the thrust for stirring the molten steel exceeds the iron column of 70 mm, the swirling flow of the molten steel becomes too strong, and the air bubbles and inclusions mixed in the molten steel are prevented from floating.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention. When the molten steel is poured into the mold while blowing argon gas from the immersion nozzle into the molten steel, and the molten steel is cooled by water spraying from a cooling water nozzle attached to the mold and the slab support device to produce the slab. Bubbles of argon gas are trapped in the resulting solidified shell. These bubbles cause blisters and the like when a thin steel sheet produced by subjecting a slab to processing such as rolling is passed through an annealing furnace. Of the air bubbles mixed in the molten steel, if the air bubbles are large, the air bubbles float out of the molten steel in the mold.
It is trapped by the solidified shell and causes blistering flaws. The present inventors have found that there is a difference in blistering flaws and the like depending on the thickness of a solidified shell in which air bubbles are captured, and have completed the present invention. The details will be described below with reference to the drawings.

FIG. 1 is a side sectional view of a continuous casting apparatus applied to a continuous casting method for molten steel having excellent quality characteristics according to an embodiment of the present invention, and FIG. 2 is a plan sectional view of a mold portion of the continuous casting apparatus. FIG. As shown in FIGS. 1 and 2, a continuous casting apparatus 10 used in a continuous casting method of molten steel having quality characteristics according to an embodiment of the present invention includes a tundish 12 lined with a refractory for storing molten steel 11 in a molten metal. An immersion nozzle 15 for pouring molten steel 11 from a tundish 12 into a mold 14 through a discharge port 13, and an electromagnetic nozzle provided outside a long piece 14 a of the mold 14 for stirring the molten steel 11 in the mold 14. Coil (upper electromagnetic coil) 16a, 16b, electromagnetic coil (lower electromagnetic coil) 16c, 16d, electromagnetic coil (upper electromagnetic coil) 17a installed outside long piece 14b,
17b and an electromagnetic coil (lower electromagnetic coil) 17c, 17
d. The electromagnetic coil 16a on the long piece 14a side of the mold 14 is made to have a strong thrust, and stirring is performed by a combination of strong and weak to give a weak thrust to the electromagnetic coil 16b. The long piece 14b side is an electromagnetic coil 1 facing the electromagnetic coils 16a and 16b.
It is arranged so that a weak thrust is generated at 7a and a strong thrust is generated at the electromagnetic coil 17b. Further, the upper electromagnetic coils 16a, 16
The electromagnetic coils 16c, 16d and the electromagnetic coils 17c, 17d provided below the b, 17a, and 17b are also arranged so as to perform stirring with the same combination of strength and strength as the upper electromagnetic coil. Further, the slab 19 in which the solidified shell 18 is formed by cooling in the mold 14 is provided in the continuous casting apparatus 10, and a cooling grit 20 for supporting the solidified shell 18 and a slab supporting means provided with a cooling water nozzle (not shown). Then, the sheet is cooled while being supported, and the sheet is drawn out at a predetermined speed by a pinch roll. The upper electromagnetic coils 16a, 16b and the electromagnetic coils 17a, 1
7b, the length in the horizontal direction is longer than the long pieces 14a, 14b of the mold 14 along the long pieces 14a, 14b, and the upper end thereof is more than the meniscus (fluid surface) 21 of the molten steel 11 in the mold 14. It is arranged so as to be 60 mm below. Also, the lower electromagnetic coils 16c and 16d and the electromagnetic coil 1
7c and 17d are provided in a range from a position 600 mm from an upper end of each of the long pieces 14a and 14b of the mold 14 to a lower end of the mold 14. Reference numerals 14c and 14d are short pieces of the mold 14.

Next, a continuous casting method for molten steel having excellent quality characteristics according to an embodiment of the present invention will be described. After performing decarburization refining and decompression secondary refining using a refining furnace such as a converter, 300 tons of molten steel 11 is melted and tundish 1
2 and the stopper (not shown) provided in the tundish 12 is moved up and down.
The molten steel 11 is poured into the mold 14 from the discharge port 13 while supplying argon gas at 2 to 4 NL / min. Then, the molten steel 11 is cooled by the mold 14 to solidify the shell 18.
To form a cooling grit 20
The slab 19 is drawn out by a pinch roll at a speed of 0.6 to 2.0 m / min while increasing the thickness of the solidified shell 18 by spraying water from a cooling water nozzle of the slab support device. Electromagnetic coils 16a, 16b and electromagnetic coils 17a, 17b are respectively arranged on the upper portions of the outer sides of the long pieces 14a, 14b of the mold 14 such that the upper ends thereof are located below the meniscus 21 by 10 to 60 mm.
The thrust to be applied is changed by changing the current value (and frequency) at which a current and frequency of 300 to 800 amperes are applied to 16b, 17a, and 17b in the range of 1.0 to 10.0 Hz.
Stirring is performed by adjusting the size of the iron column to about 70 mm and forming, for example, a clockwise swirling flow (arrow in FIG. 2) along the inner wall of the mold 14. This agitation makes the electromagnetic coil 16a and the electromagnetic coil 17b a strong thrust, and agitation is performed by a combination of strong and weak to give a weak thrust to the electromagnetic coil 16b and the electromagnetic coil 17a. It can be a unidirectional flow along
The occurrence of stagnation can be eliminated. Then, bubbles of argon gas blown from the immersion nozzle 15
When the molten steel 11 in the mold 14 that has floated a great deal along with the molten steel flow from the mold is solidified initially, fine bubbles near the molten steel 11 are washed away by the swirling flow, and a clean solidified shell 18 free of bubbles and inclusions. Can be formed.

Also, the solidified shell 18 which has grown and thickened
At the interface, bubbles not affected by the above-mentioned swirling flow are captured. In addition, bubbles whose floating has been suppressed by the swirling flow of the upper electromagnetic coil tend to accumulate. Therefore, the electromagnetic coils 16c, 16d, 17c, 17d are connected to the long pieces 14a, 14b.
And a strong thrust to the electromagnetic coils 16c and 17d and a weak thrust to the electromagnetic coils 16d and 17c in the same manner as the upper electromagnetic coil described above, so that the molten steel at a position where the solidified shell thickness is 20 mm or less. 11, mold 14
A swirling flow is applied along the inner wall of the. As a result, air bubbles existing at the interface of the grown solidified shell 18 and 300 μm that accumulate in a portion that is not affected by the swirling flow of the upper electromagnetic coil
The following microbubbles are washed away by the swirling flow of the molten steel 11 formed by the lower electromagnetic coil to remove bubbles of 300 μm or less in the vicinity of the interface of the solidified shell 18 to increase the cleanliness of the solidified shell 18, and float to remove them. Bubbles that have not been removed can be driven to a position where the thickness of the solidified shell is deeper than 20 mm. In addition, since large bubbles that have been combined by being swirled by the swirling flow are lifted from the center and released from the meniscus 21 to the outside of the system, the absolute amount of fine bubbles of 300 μm or less can be reduced, and the cleanliness of the solidified shell 18 can be reduced. Can be further improved, and quality defects can be suppressed so as not to affect the material properties. In this way, the swirling flow is divided into a plurality of upper and lower stages and applied, and the molten steel 1
By agitating 1, the agitation device can be simplified, and the solidified shell 18 can be cleaned with high accuracy. In addition, by the swirling flow caused by the agitation, inclusions existing at the interface of the solidified shell 18 can be washed away, and when processing such as rolling is performed, generation of barbed flaws and the like due to the inclusions is suppressed. Can be. The slab 19 manufactured in this manner is processed into a thin steel sheet by performing processing such as rolling, and is subjected to post-processing such as annealing, and then surface-treated such as plating, and then shipped as a product.

The lower electromagnetic coil has a solidified shell thickness of S
(Mm), the solidification coefficient of the molten steel is K, and the casting speed is V (m
/ Min), the position (distance L (m) from the meniscus) can be determined by the following equation (1). S ² / K ² × V ≦ L ≦ 20 m (1) Here, the solidification coefficient K of molten steel generally used is 22
Values of ３０30 can be used. As a stirring condition of the lower electromagnetic coil, the thickness of the solidified shell is 8 to 20 mm.
When the thickness of the solidified shell is less than 8 mm, the swirling flow interferes with the swirling flow of the upper electromagnetic coil to reduce the stirring effect. On the other hand, if the thickness of the solidified shell is increased to a range of more than 20 mm, the effect of suppressing blistering flaws due to bubbles saturates, and equipment costs and power consumption increase due to an increase in the number of electromagnetic stirring devices that impart a swirling flow. For this reason, a preferable result is obtained in the range of 10 to 18 mm. Also, when the thrust agitated by the electromagnetic coil becomes smaller than 5 mm iron pole,
The swirling flow due to electromagnetic stirring is weakened, and air bubbles and inclusions trapped at the interface of the solidified shell cannot be washed or removed. on the other hand,
When the thrust exceeds the 70 mm iron column, the swirling flow becomes too strong, and the air bubbles and inclusions in the middle of the levitating are involved in the swirling flow, hindering the floating, and the bubbles and the like remaining in the molten steel at the interface of the solidified shell. Be captured. Furthermore, stagnation and drift are generated due to the swirling flow colliding with the short side of the mold, and bubbles and inclusions are captured at the interface of the solidified shell at this portion, resulting in defects such as scabs and blisters.

[0013]

Next, an embodiment of a continuous casting method of molten steel will be described. Decarburizing and refining using a converter, and performing secondary refining under reduced pressure to obtain molten steel for a thin steel sheet having a carbon concentration of 0.01% by weight.
While melting 0 tons and pouring into a tundish, the flow rate of argon gas supplied to the immersion nozzle was set to 4 L / min, and pouring was performed into a mold having a width of 1300 mm and a height of 900 mm.
Outside the opposing long pieces of this mold, two electromagnetic coils each having a coil height of 300 mm are arranged such that the upper end comes to a position 50 mm below the meniscus of molten steel, and further below the upper electromagnetic coil, As for the lower electromagnetic coil, two electromagnetic coils each having a coil height of 300 mm were arranged. Then, the thrust of the electromagnetic coil on the outside of each long piece of the mold becomes alternately strong and weak, and the casting is performed by stirring, the slab is drawn out, and the slab is rolled to produce a thin steel sheet. did. Blister flaws after annealing of thin steel sheet,
The index of occurrence of barbed scratches, the yield of good products, and the overall evaluation were investigated. Table 1 shows the results. In addition, the index of occurrence of blistering scratches and scabs is an index when the value obtained when electromagnetic stirring is performed only with the upper electromagnetic coil is 1. In the first embodiment, the upper electromagnetic coil is arranged in a range of 50 to 350 mm below the surface of the molten metal, and the lower electromagnetic coil is arranged in a range of a solidified shell thickness of 8 to 20 mm.
Rolling is performed using a slab cast at a casting speed of 1.3 m / min while applying a swirl flow to molten steel corresponding to the upper and lower positions with a thrust force of a 5 mm iron column and a weak stirring electromagnetic coil. The thin steel sheet passed through a continuous annealing furnace. As a result, the occurrence index of blistering flaws was 0.8, and the occurrence index of barbed flaws was 0.9, which was good. The yield of good products was good, and good results were obtained as a comprehensive evaluation. In the second embodiment, the upper-stage electromagnetic coil is set in a range of 50 to 350 mm below the surface of the molten metal,
The lower electromagnetic coil is arranged in the range of the solidified shell thickness of 10 to 18 mm, the thrust of the strong stirring electromagnetic coil at each position is 35 mm iron pole, and the thrust of the weak stirring electromagnetic coil is 15 mm.
Using a slab cast at a casting speed of 1.3 m / min while applying a swirl flow to molten steel corresponding to the upper and lower positions of an iron column, a rolled thin steel sheet was passed through a continuous annealing furnace. . As a result, the occurrence index of blistering flaw was 0.7 and the occurrence index of barbed flaw was 0.9, which was good. The yield of good products was extremely good, and excellent results were obtained as a comprehensive evaluation.

[0014]

[Table 1]

On the other hand, two electromagnetic coils having a height of 300 mm are arranged only on the upper stage so that the upper end thereof is located 50 mm below the surface of the molten steel, and the thrust of the strong stirring electromagnetic coil is reduced to 35 mm by an iron column and weakly. Thrust of stirring electromagnetic coil is 15mm
Using a slab cast at a casting speed of 1.3 m / min while applying a swirl flow to molten steel corresponding to the upper position as an iron column,
The rolled thin steel sheet was passed through a continuous annealing furnace. As a result, the yield of good products was poor, and poor results were obtained as a comprehensive evaluation.

The embodiments of the present invention have been described above.
The present invention is not limited to the above-described embodiment, and all changes in conditions that do not depart from the gist are within the scope of the present invention. For example, as long as the swirl flow along the inner wall can be imparted to the molten steel at the top of the mold, one or three or more electromagnetic coils for stirring the molten steel may be provided on each long piece of the mold. The mold can also be placed on the cooling grit from the top of the long piece of the mold or on the support roll immediately below the mold.

[0017]

According to the present invention, there is provided a method for continuously casting molten steel having excellent quality characteristics, wherein the molten steel in the mold is subjected to electromagnetic stirring to impart a swirling flow along the inner wall of the mold. In the above, the swirling flow is changed from the meniscus to a solidified shell thickness of 2
Since it is applied to the molten steel at the position of 0 mm, it suppresses bubbles and inclusions trapped at the interface of the solidified shell and stably prevents defects caused by bubbles and inclusions generated in the thin steel plate, particularly blistering defects. And the yield and quality of good products can be improved.

In particular, in the method for continuously casting molten steel having excellent quality characteristics according to the present invention, since the swirling flow is divided into a plurality of portions, bubbles and inclusions can be efficiently cleaned and removed, and power consumption can be reduced. Savings and simplification of equipment can be achieved.

According to a third aspect of the present invention, there is provided a continuous casting method for molten steel having excellent quality characteristics, wherein the thrust for electromagnetically stirring the molten steel is 5 to 70 mm.
Since it is an iron pillar, it avoids the occurrence of defects due to entrainment of powder and the like and fluctuations in the molten metal level, and stably removes fine bubbles and inclusions while promoting the floating of bubbles and inclusions. Thus, it is possible to reliably prevent blistering scratches and bark scratches.

[Brief description of the drawings]

FIG. 1 is a side sectional view of a continuous casting apparatus applied to a continuous casting method of molten steel having excellent quality characteristics according to an embodiment of the present invention.

FIG. 2 is a plan sectional view of a mold part of the continuous casting apparatus.

[Explanation of symbols]

10: continuous casting apparatus, 11: molten steel, 12: tundish, 13: discharge port, 14: mold, 14a: long piece, 14
b: long piece, 14c: short piece, 14d: short piece, 15: immersion nozzle, 16a: electromagnetic coil, 16b: electromagnetic coil, 16
c: electromagnetic coil, 16d: electromagnetic coil, 17a: electromagnetic coil, 17b: electromagnetic coil, 17c: electromagnetic coil, 17
d: electromagnetic coil, 18: solidified shell, 19: slab, 2
0: Cooling grit, 21: Meniscus (hot surface)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Joe Nada 1-1-1, Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works (72) Inventor Ryusuke Miura Tobata-ku, Kitakyushu-shi, Fukuoka No. 1-1, Hibata-cho, New Nippon Steel Corporation Yawata Works (72) Inventor Takashi Moroboshi 1-1, Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka F-term in Nippon Steel Corporation, Yawata Works (reference) 4E004 AA09 MB12

Claims (3)

[Claims] 1. A method for continuously casting molten steel in which a molten steel in a mold is subjected to electromagnetic stirring to impart a swirling flow along an inner wall of the mold, wherein the swirling flow is formed from a meniscus and has a solidified shell thickness of 20 mm. A continuous casting method for molten steel having excellent quality characteristics, characterized by being applied to steel. 2. The continuous casting method for molten steel having excellent quality characteristics according to claim 1, wherein the swirling flow is divided into a plurality of portions and applied. 3. The method for continuously casting molten steel having excellent quality characteristics according to claim 1 or 2, wherein the thrust for electromagnetically stirring the molten steel is a 5-70 mm iron column. Continuous casting method.