EP3597328A1

EP3597328A1 - Continuous casting method for steel

Info

Publication number: EP3597328A1
Application number: EP17906929.9A
Authority: EP
Inventors: Akitoshi Matsui; Hirokazu Kondo; Naoki Kikuchi
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2017-04-25
Filing date: 2017-04-25
Publication date: 2020-01-22
Anticipated expiration: 2037-04-25
Also published as: KR102324300B1; TWI690377B; EP3597328A4; WO2018198181A1; EP3597328B1; JPWO2018198181A1; CN110573271B; CN110573271A; BR112019022263A2; BR112019022263B1; KR20190127894A; JP6278168B1; TW201838744A

Abstract

In a continuous casting method including applying an AC magnetic field to in-mold molten steel, thereby creating a swirling and stirring flow in the in-mold molten steel, an appropriate AC magnetic flux density is provided in accordance with the submergence depth of a submerged entry nozzle and the distance from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field to produce a high-quality strand.A continuous steel casting method according to the present invention is a continuous steel casting method including applying an AC magnetic field to in-mold molten steel via AC magnetic field generation devices, thereby creating a horizontal swirling and stirring flow in the in-mold molten steel, each of the AC magnetic field generation devices being placed on a back surface of a corresponding one of a pair of mold long sides, the AC magnetic field generation devices facing each other. A spacing between the mold long sides that face each other is 200 to 300 mm, a submerged entry nozzle has two discharge ports, each of the discharge ports having a discharge angle within a range of 5° in a downward direction to 50° in a downward direction, the AC magnetic field has a frequency of 0.5 Hz or greater and 3.0 Hz or less, and, in accordance with a location of a peak of the AC magnetic field, a submergence depth of the submerged entry nozzle and a magnetic flux density at the location of the peak of the AC magnetic field generated by the AC magnetic field generation devices are controlled to be within a predetermined range.

Description

Technical Field

The present invention relates to a continuous steel casting method that includes continuously casting molten steel while applying an AC magnetic field to molten steel present in a mold and controlling, via the AC magnetic field, the flow of the molten steel present in the mold.

Background Art

In recent years, quality requirements of high-quality steel sheet products, such as automotive steel sheets, steel sheets for cans, and high-functionality steel plates have become stricter. It is desired that a slab strand as produced by continuous casting be of high quality. One of the properties a slab strand (hereinafter also simply referred to as a "strand") is required to have is the property that the number of oxide-based non-metallic inclusions (hereinafter simply referred to as "inclusions") in the surface portion and the inner portion of the strand is small.
Examples of inclusions entrapped in the surface portion and the inner portion of a strand include the following: (1) deoxidation products produced in the step of deoxidizing molten steel with aluminum or the like and suspended in the molten steel; (2) bubbles of argon gas injected into the molten steel present in the tundish or the submerged entry nozzle; and (3) molding powder sprayed onto the surface of the in-mold molten steel and subsequently entrained and suspended in the molten steel. All of these inclusions form surface defects or internal defects in final products, and it is therefore important to reduce inclusions that are entrapped in the surface portion and the inner portion of a strand.
In the related art, one technique used to prevent product defects due to inclusions is to control the flow of molten steel by applying a magnetic field to the molten steel in a mold and utilizing an electromagnetic force due to the magnetic field to prevent deoxidation products, molding powder, and argon bubbles in the molten steel from being entrapped in the solidified shell. Numerous proposals have been made with regard to this technology.
For example, Patent Literature 1 discloses the following technology. An AC magnetic field is applied to a discharge flow discharged from a submerged entry nozzle submerged in the in-mold molten steel, thereby imparting a braking force or horizontal rotating force to the discharge flow in a manner such that the molten steel flow velocity at the surface of the in-mold molten steel is within a range of an inclusion-adherence critical flow velocity or more to a molding-powder entrainment critical flow velocity or less.
Patent Literature 2 discloses a method for continuously casting molten steel. In the method, the upper ends of AC magnetic field generation devices are positioned 20 to 60 mm below the surface of the in-mold molten steel, and a submerged entry nozzle having an angle of 1 to 30° in a downward direction is used, whereby the discharge flow from the submerged entry nozzle is controlled so that the discharge flow can impinge on the solidified shell at portions within a range from a center of each of the AC magnetic field generation devices to a position 450 mm downward therefrom.
Furthermore, Patent Literature 3 discloses a method for continuously casting molten steel. In the method, to impart a swirling and stirring flow to the in-mold molten steel in the mold width direction via AC magnetic field generation devices, discharge ports of a submerged entry nozzle are positioned at locations where the magnetic flux density at the discharge ports is less than or equal to 50% of the maximum magnetic flux density of the AC magnetic field generation devices.

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2003-320440
PTL 2: Japanese Unexamined Patent Application Publication No. 2000-202603
PTL 3: Japanese Unexamined Patent Application Publication No. 2001-047201

Summary of Invention

Technical Problem

Unfortunately, the related-art technologies described above pose the following problems.
Specifically, in the method of Patent Literature 1, the flow is controlled by imparting a braking force or horizontal stirring force to the discharge flow discharged from a submerged entry nozzle, in accordance with the value of the molten steel flow velocity, which is a velocity at the surface of the in-mold molten steel, and therefore the method requires an instrument for measuring or monitoring the molten steel flow velocity, which is a velocity at the surface of the in-mold molten steel. Furthermore, there is a concern that in a case where the placement locations of the AC magnetic field generation devices, which are each placed on a back surface of the mold, are changed, the accuracy of the critical flow velocity prediction formula may deteriorate. It is therefore difficult to say that the technology is a technology that is applicable regardless of at which portion of a back surface of a mold an AC magnetic field generation device is placed.
The technology of Patent Literature 2 is a technology that focuses on the location of impingement of the discharge flow discharged from the submerged entry nozzle, but the technology is limited to cases in which AC magnetic field generation devices are placed near the surface of the in-mold molten steel and is therefore not applicable to cases in which AC magnetic field generation devices are placed at a relatively low location with respect to the surface of the in-mold molten steel.
Similarly to Patent Literature 2, Patent Literature 3 is also limited to cases in which AC magnetic field generation devices are placed near the surface of the in-mold molten steel. Furthermore, the discharge ports of the submerged entry nozzle are provided at locations where the magnetic flux density is less than or equal to 50% of the maximum magnetic flux density, but, in this case, the following concern may arise. Since the discharge flow discharged from the submerged entry nozzle flows downward relative to the AC magnetic field generation devices, inclusions and the like may sink into regions below the AC magnetic field generation devices and cause internal defects in the strand.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a continuous steel casting method that enables production of a high-quality strand, which is achieved as follows. In a continuous steel casting method in which a swirling and stirring flow is created in in-mold molten steel by applying an AC magnetic field to the in-mold molten steel from AC magnetic field generation devices that are placed with mold long sides positioned therebetween, an appropriate AC magnetic flux density is provided in accordance with the distance from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field and with the submergence depth of the submerged entry nozzle.

Solution to Problem

A summary of the present invention, which is provided to solve the problems described above, is as follows.

[1] A continuous steel casting method, the method including producing a strand, the producing of the strand including pouring molten steel into a mold for continuous casting and withdrawing a solidified shell from the mold, the solidified shell being a solidified portion of the molten steel, the mold having a pair of mold long sides and a pair of mold short sides with a rectangular interior space being defined within the mold,
the method including applying an AC magnetic field to in-mold molten steel via AC magnetic field generation devices, thereby creating, via the AC magnetic field, a horizontal swirling and stirring flow in the in-mold molten steel, each of the AC magnetic field generation devices being placed on a back surface of a corresponding one of the pair of mold long sides, the AC magnetic field generation devices facing each other with the mold long sides being positioned therebetween, wherein
a spacing between the mold long sides that face each other is 200 to 300 mm,
a submerged entry nozzle has two discharge ports for pouring molten steel into the interior space, each of the discharge ports having a discharge angle within a range of 5° in a downward direction to 50° in a downward direction,
the AC magnetic field has a frequency of 0.5 Hz or greater and 3.0 Hz or less,
a distance from a surface of the in-mold molten steel to a location of a peak of the AC magnetic field is 200 mm or greater and less than 300 mm,
a submergence depth of the submerged entry nozzle (a distance from the surface of the in-mold molten steel to an upper end of the discharge ports of the submerged entry nozzle) is 100 mm or greater and less than 200 mm, and
a magnetic flux density at the location of the peak of the AC magnetic field is 0.040 T or greater and less than 0.060 T.
[2] A continuous steel casting method, the method including producing a strand, the producing of the strand including pouring molten steel into a mold for continuous casting and withdrawing a solidified shell from the mold, the solidified shell being a solidified portion of the molten steel, the mold having a pair of mold long sides and a pair of mold short sides with a rectangular interior space being defined within the mold,
the method including applying an AC magnetic field to in-mold molten steel via AC magnetic field generation devices, thereby creating, via the AC magnetic field, a horizontal swirling and stirring flow in the in-mold molten steel, each of the AC magnetic field generation devices being placed on a back surface of a corresponding one of the pair of mold long sides, the AC magnetic field generation devices facing each other with the mold long sides being positioned therebetween, wherein
a spacing between the mold long sides that face each other is 200 to 300 mm,
a submerged entry nozzle has two discharge ports for pouring molten steel into the interior space, each of the discharge ports having a discharge angle within a range of 5° in a downward direction to 50° in a downward direction,
the AC magnetic field has a frequency of 0.5 Hz or greater and 3.0 Hz or less,
a distance from a surface of the in-mold molten steel to a location of a peak of the AC magnetic field is 300 mm or greater and less than 400 mm,
a submergence depth of the submerged entry nozzle (a distance from the surface of the in-mold molten steel to an upper end of the discharge ports of the submerged entry nozzle) is 100 mm or greater and less than 300 mm, and
a magnetic flux density at the location of the peak of the AC magnetic field is 0.060 T or greater and less than 0.080 T.
[3] A continuous steel casting method, the method including producing a strand, the producing of the strand including pouring molten steel into a mold for continuous casting and withdrawing a solidified shell from the mold, the solidified shell being a solidified portion of the molten steel, the mold having a pair of mold long sides and a pair of mold short sides with a rectangular interior space being defined within the mold,
the method including applying an AC magnetic field to in-mold molten steel via AC magnetic field generation devices, thereby creating, via the AC magnetic field, a horizontal swirling and stirring flow in the in-mold molten steel, each of the AC magnetic field generation devices being placed on a back surface of a corresponding one of the pair of mold long sides, the AC magnetic field generation devices facing each other with the mold long sides being positioned therebetween, wherein
a spacing between the mold long sides that face each other is 200 to 300 mm,
a submerged entry nozzle has two discharge ports for pouring molten steel into the interior space, each of the discharge ports having a discharge angle within a range of 5° in a downward direction to 50° in a downward direction,
the AC magnetic field has a frequency of 0.5 Hz or greater and 3.0 Hz or less,
a distance from a surface of the in-mold molten steel to a location of a peak of the AC magnetic field is 400 mm or greater and less than 500 mm,
a submergence depth of the submerged entry nozzle (a distance from the surface of the in-mold molten steel to an upper end of the discharge ports of the submerged entry nozzle) is 100 mm or greater and less than 300 mm, and
a magnetic flux density at the location of the peak of the AC magnetic field is 0.080 T or greater and less than 0.100 T.
[4] A continuous steel casting method, the method including producing a strand, the producing of the strand including pouring molten steel into a mold for continuous casting and withdrawing a solidified shell from the mold, the solidified shell being a solidified portion of the molten steel, the mold having a pair of mold long sides and a pair of mold short sides with a rectangular interior space being defined within the mold,
the method including applying an AC magnetic field to in-mold molten steel via AC magnetic field generation devices, thereby creating, via the AC magnetic field, a horizontal swirling and stirring flow in the in-mold molten steel, each of the AC magnetic field generation devices being placed on a back surface of a corresponding one of the pair of mold long sides, the AC magnetic field generation devices facing each other with the mold long sides being positioned therebetween, wherein
a spacing between the mold long sides that face each other is 200 to 300 mm,
a submerged entry nozzle has two discharge ports for pouring molten steel into the interior space, each of the discharge ports having a discharge angle within a range of 5° in a downward direction to 50° in a downward direction,
the AC magnetic field has a frequency of 0.5 Hz or greater and 3.0 Hz or less, and
in accordance with a location of a peak of the AC magnetic field, a submergence depth of the submerged entry nozzle (a distance from the surface of the in-mold molten steel to an upper end of the discharge ports of the submerged entry nozzle) and a magnetic flux density at the location of the peak of the AC magnetic field generated by the AC magnetic field generation devices are determined to satisfy one of three types of conditions described below, namely conditions (A), conditions (B), and conditions (C), conditions (A): when the distance from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field is 200 mm or greater and less than 300 mm, the submergence depth of the submerged entry nozzle is 100 mm or greater and less than 200 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.040 T or greater and less than 0.060 T,
conditions (B): when the distance from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field is 300 mm or greater and less than 400 mm, the submergence depth of the submerged entry nozzle is 100 mm or greater and less than 300 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.060 T or greater and less than 0.080 T, and conditions (C): when the distance from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field is 400 mm or greater and less than 500 mm, the submergence depth of the submerged entry nozzle is 100 mm or greater and less than 300 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.080 T or greater and less than 0.100 T.

Advantageous Effects of Invention

With the present invention, a high-quality strand can be produced easily because a swirling and stirring flow is imparted to in-mold molten steel by applying an AC magnetic field with an appropriate magnetic flux density in accordance with the distance from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field and with the submergence depth of the submerged entry nozzle, so that deoxidation products, argon gas bubbles, and molding powder are inhibited from being entrapped in the solidified shell.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a diagram illustrating an example of an embodiment of the present invention, schematically illustrating a mold portion of a continuous slab casting machine.
[Fig. 2] Fig. 2 is an enlarged view of a submerged entry nozzle illustrated in Fig. 1.

Description of Embodiments

An embodiment of the present invention will now be described.
With regard to a continuous steel casting method including applying an AC magnetic field to molten steel present in a mold and creating, via the AC magnetic field, a horizontal swirling and stirring flow in the molten steel present in the mold, the present inventors conducted a test and an investigation regarding the flow status of molten steel present in the mold by using a low-melting-point alloy apparatus. In the test, a mold having a pair of mold long sides and a pair of mold short sides, which define a rectangular interior space, was used; a submerged entry nozzle having two discharge ports (hereinafter also referred to as a "2-port submerged entry nozzle") was placed in a middle portion of the interior space; a situation in which a discharge flow of the molten steel is discharged from each of the discharge ports toward a corresponding one of the mold short sides was simulated; and, the flow status of the molten steel present in the mold, particularly in cases where the location of the peak of the AC magnetic field and the submergence depth of the submerged entry nozzle were varied, was tested.
Here, the location of the peak of an AC magnetic field is as follows. When the maximum value among root mean square values of the orthogonal component of the magnetic flux density of the AC magnetic field is determined for locations positioned along the mold inner wall surfaces surrounding the interior space of a mold, the location where the maximum value is greatest is the location of the peak of the AC magnetic field, the root mean square values being periodically obtained, the orthogonal component being orthogonal to a corresponding one of the inner wall surfaces. Furthermore, the submergence depth of a submerged entry nozzle is defined as the distance from the surface (also referred to as the "meniscus") of the in-mold molten steel to the upper end of a discharge port of the submerged entry nozzle.
In the test, the placement locations of AC magnetic field generation devices, which were placed facing each other on the back surfaces of the mold long sides, were varied, and the placement location of the submerged entry nozzle, that is, the submergence depth, was varied, and, in those cases, the flow status of a low-melting-point alloy in the mold, the flow velocity distribution thereof in the mold, and the like were investigated by utilizing numerical simulation and a low-melting-point alloy apparatus, which is one-fourth the size of an actual machine. The low-melting-point alloy used was a Bi-Pb-Sn-Cd alloy (melting point: 70°C) .

The investigations revealed that appropriate application ranges for the magnetic flux density of an AC magnetic field exist in association with the location of the peak of the AC magnetic field and the submergence depth of a submerged entry nozzle. That is, it was found that the conditions for AC magnetic field application can generally be classified into three types, namely conditions (A) to (C), depending on the location of the peak of the AC magnetic field and the submergence depth of the submerged entry nozzle. The results of the investigations are shown in Table 1. Note that the location of the peak of the AC magnetic field is expressed as the distance from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field.

[Table 1]

	Distance from meniscus to AC magnetic field peak location	Frequency of AC magnetic field	Discharge angle of submerged entry nozzle	Submergence depth of submerged entry nozzle	Magnetic flux density at AC magnetic field peak location
Conditions (A)	200 mm or greater and less than 300 mm	0.5 Hz or greater and 3.0 Hz or less	Range of 5° in downward direction to 50° in downward direction	100 mm or greater and less than 200 mm	0.040 T or greater and less than 0.060 T
Conditions (B)	300 mm or greater and less than 400 mm	0.5 Hz or greater and 3.0 Hz or less	Range of 5° in downward direction to 50° in downward direction	100 mm or greater and less than 300 mm	0.060 T or greater and less than 0.080 T
Conditions (C)	400 mm or greater and less than 500 mm	0.5 Hz or greater and 3.0 Hz or less	Range of 5° in downward direction to 50° in downward direction	100 mm or greater and less than 300 mm	0.080 T or greater and less than 0.100 T

1. Conditions (A)

In cases where the location of the peak of the AC magnetic field is 200 mm or greater and less than 300 mm from the surface of the in-mold molten steel, the submergence depth of a 2-port submerged entry nozzle is 100 mm or greater and less than 200 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.040 T or greater and less than 0.060 T.
Note that the magnetic flux density is determined as follows. Magnetic flux densities at locations 15 mm away from a flat surface of a copper plate of the mold in a direction normal to the flat surface and toward the interior space are considered. The copper plate of the mold is one of the copper plates of the mold and is a plate behind which an AC magnetic field generation device is provided. The flat surface is a surface that forms the interior space. Among the magnetic flux densities in the normal direction, the magnetic flux density at the location of the peak of the magnetic flux density in the strand withdrawal direction is determined. The magnetic flux density is determined as the effective value (root mean square value) of the arithmetic mean of values measured at a desired pitch in the mold width direction. It can be assumed that the measurement pitch in the mold width direction suffices if the measurement pitch is one that sufficiently represents the spatial profile of the magnetic flux density.
If the magnetic flux density is less than 0.040 T, the swirling and stirring force is low, and as a result, it is difficult to produce the effect of clearing argon gas bubbles and deoxidation products from the solidified shell. On the other hand, if the magnetic flux density is greater than or equal to 0.060 T, the swirling and stirring force is too high, which contributes to entrainment of molding powder.
If the submergence depth of the submerged entry nozzle is less than 100 mm, the distance between the surface of the in-mold molten steel and the discharge flow is too small, which likely contributes to molten steel level fluctuation in the mold. If the submergence depth is greater than or equal to 200 mm, the long length of the main body portion of the submerged entry nozzle increases the cost of the refractory material, and also, from the standpoint of heat resistance and resistance to loading, increases the probability of damage to the submerged entry nozzle, and therefore, contrarily, there is a concern that the operating costs may increase.

2. Conditions (B)

In cases where the location of the peak of the AC magnetic field is 300 mm or greater and less than 400 mm from the surface of the in-mold molten steel, the submergence depth of a 2-port submerged entry nozzle is 100 mm or greater and less than 300 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.060 T or greater and less than 0.080 T.
The location of the peak of the AC magnetic field is a location deeper than that of conditions (A) with respect to the surface of the in-mold molten steel, and therefore a magnetic flux density greater than that of conditions (A) is necessary. That is, if the magnetic flux density is less than 0.060 T, the swirling and stirring force is low, and as a result, it is difficult to produce the effect of clearing argon gas bubbles and deoxidation products from the solidified shell. On the other hand, if the magnetic flux density is greater than or equal to 0.080 T, the swirling and stirring force is too high, which contributes to entrainment of molding powder.
If the submergence depth of the submerged entry nozzle is less than 100 mm, the distance between the surface of the in-mold molten steel and the discharge flow is too small, which likely contributes to molten steel level fluctuation in the mold. If the submergence depth is greater than or equal to 300 mm, the long length of the main body portion of the submerged entry nozzle increases the cost of the refractory material, and also, from the standpoint of heat resistance and resistance to loading, increases the probability of damage to the submerged entry nozzle, and therefore, contrarily, there is a concern that the operating costs may increase.

3. Conditions (C)

In cases where the location of the peak of the AC magnetic field is 400 mm or greater and less than 500 mm from the surface of the in-mold molten steel, the submergence depth of a 2-port submerged entry nozzle is 100 mm or greater and less than 300 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.080 T or greater and less than 0.100 T.
The location of the peak of the AC magnetic field is a location even deeper than those of conditions (A) and conditions (B) with respect to the surface of the in-mold molten steel, and therefore an even greater magnetic flux density is necessary. That is, if the magnetic flux density is less than 0.080 T, the swirling and stirring force is low, and as a result, it is difficult to produce the effect of clearing argon gas bubbles and deoxidation products from the solidified shell. On the other hand, if the magnetic flux density is greater than or equal to 0.100 T, the swirling and stirring force is too high, which contributes to entrainment of molding powder.
If the submergence depth of the submerged entry nozzle is less than 100 mm, the distance between the surface of the in-mold molten steel and the discharge flow is too small, which likely contributes to molten steel level fluctuation in the mold. If the submergence depth is greater than or equal to 300 mm, the long length of the main body portion of the submerged entry nozzle increases the cost of the refractory material, and also, from the standpoint of heat resistance and resistance to loading, increases the probability of damage to the submerged entry nozzle, and therefore, contrarily, there is a concern that the operating costs may increase.
In the conditions (A) to (C), the discharge angle of the submerged entry nozzle to be used is within the range of 5° in a downward direction to 50° in a downward direction. If the discharge angle is less than 5° in a downward direction, the AC magnetic field cannot sufficiently act on the discharge flow. On the other hand, if the discharge angle is greater than 50° in a downward direction, the downward flow of the discharge flow is too strong, and as a result, there is a concern that deoxidation products and gas bubbles may sink into deep locations with respect to the casting direction and form internal defects, which may act as crack initiation sites when the steel sheet is subjected to forming.
In the present invention, the location of the peak of the AC magnetic field is 200 mm or greater and less than 500 mm from the surface of the in-mold molten steel. If the location of the peak of the AC magnetic field is less than 200 mm from the surface of the in-mold molten steel, the submergence depth of the submerged entry nozzle needs to correspond to a location shallower than the location of the peak of the AC magnetic field in order for the AC magnetic field to act on the discharge flow discharged from the submerged entry nozzle, and therefore operational limitations arise and efficient application of the AC magnetic field cannot be accomplished. Furthermore, if the location of the peak of the AC magnetic field is a location greater than or equal to 500 mm away from the surface of the in-mold molten steel, the swirling and stirring flow is imparted in a region where the solidified shell is increased in size, and therefore the effect of clearing deoxidation products and argon gas bubbles from the solidified shell is not sufficiently obtained.
The frequency of the AC magnetic field is 0.5 to 3.0 Hz and preferably 1.0 to 2.0 Hz. If the frequency is less than 0.5 Hz, the imparting of an electromagnetic force via the AC magnetic field is too intermittent, and therefore the effect of clearing deoxidation products and argon gas bubbles from the solidified shell is unstable. On the other hand, if the frequency is greater than 3.0 Hz, the reduction in magnetic flux density due to the mold and the solidified shell increases, and it is therefore impossible to efficiently apply an AC magnetic field to the in-mold molten steel.
A specific method for implementing the present invention will now be described with reference to the drawings. Fig. 1 is a diagram illustrating an example of an embodiment of the present invention, schematically illustrating a mold portion of a continuous slab casting machine. Fig. 2 is an enlarged view of a submerged entry nozzle illustrated in Fig. 1.
In Fig. 1 and Fig. 2, reference character 1 denotes molten steel; 2, a solidified shell; 3, a surface of in-mold molten steel; 4, a discharge flow; 5, a strand; 6, a mold; 7, a water-cooled mold long side; 8, a water-cooled mold short side; 9, a submerged entry nozzle; 10, a discharge port; 11, an AC magnetic field generation device; 12, molding powder; and θ, a discharge angle of the submerged entry nozzle.
The mold 6 includes a pair of mold long sides 7, which face each other, and a pair of mold short sides 8, which face each other and are held between the mold long sides 7. The pair of mold long sides 7 and the pair of mold short sides 8 define a rectangular interior space. A pair of AC magnetic field generation devices 11 are placed on the back surfaces of the respective mold long sides 7. The AC magnetic field generation devices 11 face each other with the mold long sides 7 positioned therebetween. Here, the spacing between the mold long sides that face each other is 200 to 300 mm, the submerged entry nozzle 9 has two discharge ports 10, and the discharge angle (θ) of each of the discharge ports 10 is within the range of 5° in a downward direction to 50° in a downward direction.
The submerged entry nozzle 9 is placed in a middle portion of the rectangular interior space of the mold 6. The discharge flows 4 of the molten steel 1 are discharged from the two discharge ports 10 so that each of the discharge flows 4 flows toward one of the mold short sides 8 that a corresponding one of the discharge ports 10 faces. Thus, the molten steel 1 is poured into the interior space of the mold 6. After being poured into the interior space of the mold 6, the molten steel 1 is cooled by the mold long sides 7 and the mold short sides 8 to form the solidified shell 2. Subsequently, after a predetermined amount of molten steel 1 is poured into the interior space of the mold 6, pinch rolls (not illustrated) are driven in a state in which the discharge ports 10 are immersed in the molten steel 1 in the mold, to start withdrawing the strand 5, which includes an unsolidified portion of the molten steel 1 in the interior, with the solidified shell 2 being the outer shell. After the withdrawal is started, the strand withdrawal speed is increased to a predetermined strand withdrawal speed while controlling the location of the surface 3 of the in-mold molten steel to be a substantially fixed location. In Fig. 1, the submergence depth of the submerged entry nozzle 9 is denoted by L₁, and the distance from the surface 3 of the in-mold molten steel to the location of the peak of the AC magnetic field is denoted by L₂.
The molding powder 12 is added onto the surface 3 of the in-mold molten steel. The molding powder 12 melts and prevents the molten steel 1 from being oxidized and also flows into a space between the solidified shell 2 and the mold 6 to provide a lubricant effect. Furthermore, argon gas, nitrogen gas or a mixed gas of argon gas and nitrogen gas is injected into the molten steel 1 flowing down through the submerged entry nozzle 9, to prevent deoxidation products suspended in the molten steel from adhering to the inner walls of the submerged entry nozzle.
When the molten steel 1 is continuously cast as described above, an AC magnetic field is applied from the AC magnetic field generation devices 11 to the molten steel 1 present in the mold, thereby creating a horizontal swirling and stirring flow in the molten steel 1 present in the mold. The frequency of the AC magnetic field is 0.5 Hz or greater and 3.0 Hz or less.
When an AC magnetic field is applied, in the case where the distance (L₂) from the surface 3 of the in-mold molten steel to the location of the peak of the AC magnetic field is 200 mm or greater and less than 300 mm (conditions (A)), the submergence depth (L₁) of the submerged entry nozzle 9 is 100 mm or greater and less than 200 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.040 T or greater and less than 0.060 T.
Furthermore, in the case where the distance (L₂) from the surface 3 of the in-mold molten steel to the location of the peak of the AC magnetic field is 300 mm or greater and less than 400 mm (conditions (B)), the submergence depth (L₁) of the submerged entry nozzle 9 is 100 mm or greater and less than 300 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.060 T or greater and less than 0.080 T.
Still furthermore, in the case where the distance (L₂) from the surface 3 of the in-mold molten steel to the location of the peak of the AC magnetic field is 400 mm or greater and less than 500 mm (conditions (C)), the submergence depth (L₁) of the submerged entry nozzle 9 is 100 mm or greater and less than 300 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.080 T or greater and less than 0.100 T.
The adjustment of the magnetic flux density at the location of the peak of the AC magnetic field is carried out in the following manner. Specifically, the relationship between the electrical power supplied to the AC magnetic field generation devices 11 and the magnetic flux density at a location in the interior space of the mold 6, which is a location 15 mm away from the surface of a copper plate of the mold, at the location of the peak of the AC magnetic field is measured in advance, and the electrical power to be supplied to the AC magnetic field generation devices 11 is adjusted in a manner such that the magnetic flux density at the location of the peak of the AC magnetic field becomes a desired magnetic flux density.
As described above, with the present invention, a high-quality slab strand can be produced easily because a swirling and stirring flow is imparted to in-mold molten steel by applying an AC magnetic field with an appropriate magnetic flux density in accordance with the distance (L₂) from the surface 3 of the in-mold molten steel to the location of the peak of the AC magnetic field and with the submergence depth (L₁) of the submerged entry nozzle, so that deoxidation products, argon gas bubbles, and molding powder 12 are inhibited from being entrapped in the solidified shell 2.

EXAMPLES

A test in which approximately 300 tons of molten aluminum killed steel was continuously cast was conducted by using a continuous slab casting machine having a mold such as that illustrated in Fig. 1. In the test, the submergence depth (L₁) of the submerged entry nozzle and the distance (L₂) from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field were varied. The thickness of the slab strand was 250 mm, and the width thereof was 1000 to 2200 mm. The molten steel pouring flow rate in a steady casting period was 2.0 to 6.5 tons/min (strand withdrawal speed of 1.0 to 3.0 m/min). Furthermore, the frequency of the AC magnetic field was 1.0 Hz.
The submerged entry nozzle used was a 2-port submerged entry nozzle having a discharge angle (θ) of 25° in a downward direction. Argon gas was injected via an upper nozzle into the molten steel flowing downward through the submerged entry nozzle. The cast slab strand was subjected to hot rolling, cold rolling, and galvannealing successively. Surface defects in the galvannealed steel sheet were measured continuously using an on-line surface defect meter. Overview examination, SEM analysis, and ICP analysis of the measured defects were performed. Of the measured defects, steelmaking-caused defects (deoxidation-product-caused defects, argon-gas-bubble-caused defects, and molding-powder-caused defects) were identified, and evaluations were made based on the number of steelmaking-caused defects per 100 mm in length of the galvannealed steel sheet (product defect index).

The test results of invention examples are shown in Table 2, and the test results of comparative examples are shown in Table 3.

[Table 2]

	Slab thickness (mm)	Slab width (mm)	Submergence depth L₁ of submerged entry nozzle (mm)	Discharge angle (downward) (deg.)	Frequency of AC magnetic field (Hz)	Distance L₂ from meniscus to AC magnetic field peak location (mm)	Magnetic flux density at AC magnetic field peak location (T)	Product defect index (number/100 m)
Invention example 1	250	1000	120	25	1.0	200	0.054	0.24
Invention example 2	250	1000	150	25	1.0	250	0.040	0.21
Invention example 3	250	1000	100	25	1.0	290	0.042	0.28
Invention example 4	250	1400	180	25	1.0	200	0.052	0.26
Invention example 5	250	1400	190	25	1.0	250	0.040	0.29
Invention example 6	250	1400	110	25	1.0	290	0.058	0.34
Invention example 7	250	1800	130	25	1.0	200	0.040	0.31
Invention example 8	250	1800	140	25	1.0	250	0.046	0.26
Invention example 9	250	1800	160	25	1.0	290	0.052	0.20
Invention example 10	250	2200	110	25	1.0	200	0.058	0.25
Invention example 11	250	2200	150	25	1.0	290	0.040	0.30
Invention example 12	250	2200	190	25	1.0	250	0.048	0.33
Invention example 13	250	1000	200	25	1.0	300	0.060	0.28
Invention example 14	250	1000	100	25	1.0	350	0.068	0.26
Invention example 15	250	1000	220	25	1.0	390	0.072	0.29
Invention example 16	250	1400	120	25	1.0	300	0.076	0.22
Invention example 17	250	1400	140	25	1.0	350	0.064	0.33
Invention example 18	250	1400	250	25	1.0	390	0.078	0.20
Invention example 19	250	1800	290	25	1.0	300	0.070	0.28
Invention example 20	250	1800	160	25	1.0	350	0.062	0.34
Invention example 21	250	1800	210	25	1.0	390	0.066	0.31
Invention example 22	250	2200	290	25	1.0	300	0.074	0.30
Invention example 23	250	2200	100	25	1.0	350	0.060	0.30
Invention example 24	250	2200	230	25	1.0	390	0.078	0.27
Invention example 25	250	1000	220	25	1.0	400	0.084	0.21
Invention example 26	250	1000	140	25	1.0	450	0.080	0.30
Invention example 27	250	1000	290	25	1.0	490	0.090	0.27
Invention example 28	250	1400	230	25	1.0	400	0.098	0.33
Invention example 29	250	1400	100	25	1.0	450	0.082	0.28
Invention example 30	250	1400	120	25	1.0	490	0.086	0.25
Invention example 31	250	1800	270	25	1.0	400	0.094	0.21
Invention example 32	250	1800	150	25	1.0	450	0.090	0.23
Invention example 33	250	1800	160	25	1.0	490	0.084	0.29
Invention example 34	250	2200	180	25	1.0	400	0.080	0.32
Invention example 35	250	2200	220	25	1.0	450	0.088	0.34
Invention example 36	250	2200	260	25	1.0	490	0.096	0.26

[Table 3]

	Slab thickness (mm)	Slab width (mm)	Submergence depth L₁ of submerged entry nozzle (mm)	Discharge angle (downward) (deg.)	Frequency of AC magnetic field (Hz)	Distance L₂ from meniscus to AC magnetic field peak location (mm)	Magnetic flux density at AC magnetic field peak location (T)	Product defect index (number/100 m)
Comparative example 1	250	1000	120	25	1.0	200	0.038	0.51
Comparative example 2	250	1400	190	25	1.0	250	0.034	0.48
Comparative example 3	250	1800	140	25	1.0	250	0.036	0.53
Comparative example 4	250	2200	150	25	1.0	290	0.038	0.46
Comparative example 5	250	1000	120	25	1.0	200	0.060	0.55
Comparative example 6	250	1400	190	25	1.0	250	0.062	0.51
Comparative example 7	250	1800	140	25	1.0	250	0.064	0.50
Comparative example 8	250	2200	150	25	1.0	290	0.062	0.47
Comparative example 9	250	1000	140	25	1.0	300	0.058	0.49
Comparative example 10	250	1400	270	25	1.0	390	0.056	0.53
Comparative example 11	250	1800	180	25	1.0	350	0.058	0.52
Comparative example 12	250	2200	220	25	1.0	390	0.052	0.50
Comparative example 13	250	1000	160	25	1.0	300	0.084	0.48
Comparative example 14	250	1400	230	25	1.0	350	0.080	0.46
Comparative example 15	250	1800	270	25	1.0	350	0.082	0.48
Comparative example 16	250	2200	140	25	1.0	390	0.086	0.53
Comparative example 17	250	1000	260	25	1.0	450	0.074	0.55
Comparative example 18	250	1400	290	25	1.0	400	0.078	0.54
Comparative example 19	250	1800	100	25	1.0	490	0.072	0.49
Comparative example 20	250	2200	180	25	1.0	400	0.076	0.50
Comparative example 21	250	1000	240	25	1.0	400	0.102	0.51
Comparative example 22	250	1400	160	25	1.0	450	0.100	0.48
Comparative example 23	250	1800	280	25	1.0	490	0.104	0.51
Comparative example 24	250	2200	120	25	1.0	450	0.102	0.52
Comparative example 25	250	1000	90	25	1.0	200	0.046	0.49
Comparative example 26	250	1400	80	25	1.0	250	0.040	0.47
Comparative example 27	250	1800	80	25	1.0	250	0.048	0.48
Comparative example 28	250	2200	90	25	1.0	290	0.052	0.49
Comparative example 29	250	1000	200	25	1.0	250	0.044	0.53
Comparative example 30	250	1400	220	25	1.0	200	0.056	0.48
Comparative example 31	250	1800	200	25	1.0	250	0.048	0.55
Comparative example 32	250	2200	220	25	1.0	290	0.050	0.51

Invention Examples 1 to 12 correspond to conditions (A) of Table 1, Invention Examples 13 to 24 correspond to conditions (B) of Table 1, and Invention Examples 25 to 36 correspond to conditions (C) of Table 1. All of Invention Examples 1 to 36 had a product defect index within a range of 0.21 to 0.34 (number/100 m) and therefore had a good result.
On the other hand, in each of Comparative Examples 1 to 24, the magnetic flux density at the location of the peak of the AC magnetic filed in the test was outside the range of the present invention, and the product defect index was 0.46 to 0.55 (number/100 m) and therefore inferior.
Furthermore, in each of Comparative Examples 25 to 32, the submergence depth (L₁) of the submerged entry nozzle in the test was outside the range of the present invention, and the product defect index was 0.47 to 0.55 (number/100 m) and was therefore also inferior. Comparative Examples 25 to 32 are cases in each of which the distance (L₂) from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field corresponds to that of conditions (A) of Table 1. However, it was confirmed that, under conditions (B) or conditions (C), too, the product defect index deteriorated in the case where the submergence depth (L₁) of the submerged entry nozzle was outside the range of the present invention.
Although no description is given in this example, it was confirmed that when the thickness of a strand was within a range of 200 to 300 mm, effects comparable to those described in this example were produced. Furthermore, the shape of the submerged entry nozzle is also not limited to the conditions described in this example, and it was confirmed that when the discharge angle (θ) was within the range of 5° in a downward direction to 50° in a downward direction, comparable effects were produced.
As described above, it was confirmed that employing the continuous casting method according to the present invention enables casting of a good-quality slab strand.

Reference Signs List

1: Molten steel
2: Solidified shell
3: Surface of in-mold molten steel
4: Discharge flow
5: Strand
6: Mold
7: Mold long side
8: Mold short side
9: Submerged entry nozzle
10: Discharge port
11: AC magnetic field generation device
12: Molding powder

Claims

A continuous steel casting method, the method including producing a strand, the producing of the strand including pouring molten steel into a mold for continuous casting and withdrawing a solidified shell from the mold, the solidified shell being a solidified portion of the molten steel, the mold having a pair of mold long sides and a pair of mold short sides with a rectangular interior space being defined within the mold,
the method comprising applying an AC magnetic field to in-mold molten steel via AC magnetic field generation devices, thereby creating, via the AC magnetic field, a horizontal swirling and stirring flow in the in-mold molten steel, each of the AC magnetic field generation devices being placed on a back surface of a corresponding one of the pair of mold long sides, the AC magnetic field generation devices facing each other with the mold long sides being positioned therebetween, wherein
a spacing between the mold long sides that face each other is 200 to 300 mm,
a submerged entry nozzle has two discharge ports for pouring molten steel into the interior space, each of the discharge ports having a discharge angle within a range of 5° in a downward direction to 50° in a downward direction,
the AC magnetic field has a frequency of 0.5 Hz or greater and 3.0 Hz or less,
a distance from a surface of the in-mold molten steel to a location of a peak of the AC magnetic field is 200 mm or greater and less than 300 mm,
a submergence depth of the submerged entry nozzle (a distance from the surface of the in-mold molten steel to an upper end of the discharge ports of the submerged entry nozzle) is 100 mm or greater and less than 200 mm, and
a magnetic flux density at the location of the peak of the AC magnetic field is 0.040 T or greater and less than 0.060 T.
A continuous steel casting method, the method including producing a strand, the producing of the strand including pouring molten steel into a mold for continuous casting and withdrawing a solidified shell from the mold, the solidified shell being a solidified portion of the molten steel, the mold having a pair of mold long sides and a pair of mold short sides with a rectangular interior space being defined within the mold,
the method comprising applying an AC magnetic field to in-mold molten steel via AC magnetic field generation devices, thereby creating, via the AC magnetic field, a horizontal swirling and stirring flow in the in-mold molten steel, each of the AC magnetic field generation devices being placed on a back surface of a corresponding one of the pair of mold long sides, the AC magnetic field generation devices facing each other with the mold long sides being positioned therebetween, wherein
a spacing between the mold long sides that face each other is 200 to 300 mm,
a submerged entry nozzle has two discharge ports for pouring molten steel into the interior space, each of the discharge ports having a discharge angle within a range of 5° in a downward direction to 50° in a downward direction,
the AC magnetic field has a frequency of 0.5 Hz or greater and 3.0 Hz or less,
a distance from a surface of the in-mold molten steel to a location of a peak of the AC magnetic field is 300 mm or greater and less than 400 mm,
a submergence depth of the submerged entry nozzle (a distance from the surface of the in-mold molten steel to an upper end of the discharge ports of the submerged entry nozzle) is 100 mm or greater and less than 300 mm, and
a magnetic flux density at the location of the peak of the AC magnetic field is 0.060 T or greater and less than 0.080 T.
A continuous steel casting method, the method including producing a strand, the producing of the strand including pouring molten steel into a mold for continuous casting and withdrawing a solidified shell from the mold, the solidified shell being a solidified portion of the molten steel, the mold having a pair of mold long sides and a pair of mold short sides with a rectangular interior space being defined within the mold,
the method comprising applying an AC magnetic field to in-mold molten steel via AC magnetic field generation devices, thereby creating, via the AC magnetic field, a horizontal swirling and stirring flow in the in-mold molten steel, each of the AC magnetic field generation devices being placed on a back surface of a corresponding one of the pair of mold long sides, the AC magnetic field generation devices facing each other with the mold long sides being positioned therebetween, wherein
a spacing between the mold long sides that face each other is 200 to 300 mm,
a submerged entry nozzle has two discharge ports for pouring molten steel into the interior space, each of the discharge ports having a discharge angle within a range of 5° in a downward direction to 50° in a downward direction,
the AC magnetic field has a frequency of 0.5 Hz or greater and 3.0 Hz or less,
a distance from a surface of the in-mold molten steel to a location of a peak of the AC magnetic field is 400 mm or greater and less than 500 mm,
a submergence depth of the submerged entry nozzle (a distance from the surface of the in-mold molten steel to an upper end of the discharge ports of the submerged entry nozzle) is 100 mm or greater and less than 300 mm, and
a magnetic flux density at the location of the peak of the AC magnetic field is 0.080 T or greater and less than 0.100 T.
A continuous steel casting method, the method including producing a strand, the producing of the strand including pouring molten steel into a mold for continuous casting and withdrawing a solidified shell from the mold, the solidified shell being a solidified portion of the molten steel, the mold having a pair of mold long sides and a pair of mold short sides with a rectangular interior space being defined within the mold,
the method comprising applying an AC magnetic field to in-mold molten steel via AC magnetic field generation devices, thereby creating, via the AC magnetic field, a horizontal swirling and stirring flow in the in-mold molten steel, each of the AC magnetic field generation devices being placed on a back surface of a corresponding one of the pair of mold long sides, the AC magnetic field generation devices facing each other with the mold long sides being positioned therebetween, wherein
a spacing between the mold long sides that face each other is 200 to 300 mm,
a submerged entry nozzle has two discharge ports for pouring molten steel into the interior space, each of the discharge ports having a discharge angle within a range of 5° in a downward direction to 50° in a downward direction,
the AC magnetic field has a frequency of 0.5 Hz or greater and 3.0 Hz or less, and
in accordance with a location of a peak of the AC magnetic field, a submergence depth of the submerged entry nozzle (a distance from the surface of the in-mold molten steel to an upper end of the discharge ports of the submerged entry nozzle) and a magnetic flux density at the location of the peak of the AC magnetic field generated by the AC magnetic field generation devices are determined to satisfy one of three types of conditions described below, namely conditions (A), conditions (B), and conditions (C),
conditions (A): when the distance from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field is 200 mm or greater and less than 300 mm, the submergence depth of the submerged entry nozzle is 100 mm or greater and less than 200 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.040 T or greater and less than 0.060 T,
conditions (B): when the distance from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field is 300 mm or greater and less than 400 mm, the submergence depth of the submerged entry nozzle is 100 mm or greater and less than 300 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.060 T or greater and less than 0.080 T, and
conditions (C): when the distance from the surface of the in-mold molten steel to the location of the peak of the AC magnetic field is 400 mm or greater and less than 500 mm, the submergence depth of the submerged entry nozzle is 100 mm or greater and less than 300 mm, and the magnetic flux density at the location of the peak of the AC magnetic field is 0.080 T or greater and less than 0.100 T.