JPH11285795A - Production of continuously casting slab having high cleanliness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the quality equivalent to that of a stationary cast portion even in the joint portion of slab by operating either one of an electromagnetic stirring device or electromagnetic braking device when at least one among three conditions; a change in the back pressure of gaseous Ar to be blown from an immersion nozzle, a fluctuation in the molten metal surface of the molten steel in a casting mold and a difference in the molten metal surface level of the molten steel on the short sides of the casting mold; is judged to be abnormal. SOLUTION: The reference for judging the abnormality of the setting conditions is preferably specified to <=0.3 kg/cm<2> in the back pressure of the gaseous Ar to be blown from the immersion nozzle, >=10 mm quantity of the fluctuation in the molten metal surface of the molten metal in the casting mold and >=12 mm difference in the molten metal surface level of the molten steel on the short sides of the casting mold. The electromagnetic stirring device 4, which has a discharge port 3 facing downward 25 deg. from horizontal and impresses an electromagnetic field in a horizontal direction by the AC magnetic field to the portion of the molten steel meniscus 8 in the casting mold 1, is disposed at the immersion nozzle 2. The electromagnetic braking device 5 by a uniform DC magnetic field for braking a discharge stream 6 is disposed lower than the nozzle discharge port. As a result, the inclusions in and on the slabs are simultaneously decreased and the slabs having the excellent quality are obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting slabs, which reduces inclusions in the surface layer of a solidified shell starting to solidify from the inside of a mold and simultaneously reduces inclusions in the inner layer, thereby improving cleanliness and surface quality. The present invention relates to a method for casting a slab excellent in properties.

[0002]

2. Description of the Related Art In ordinary continuous casting, as shown in FIG. 5, an immersion nozzle 2 having two discharge ports 3 oriented in a short side direction is disposed at the center of a mold 1, and molten steel is placed in the mold 1. The discharge flow 6 collides with the short side surface of the mold and reverses in the upward direction to become an upward flow 13, and the other branches into a downward flow 14 in a downward direction. Some of the inclusions held in the molten steel by the discharge flow 6 float on the surface of the molten steel and are removed, but the rest is carried to the deep part of the molten steel by the downward flow 14, and during the floating process, the solidified shell 9 is removed. And remain in the slab. The surface inclusions trapped in the solidified shell lead to defects called slivers in the product.

On the other hand, in the upward flow 13, in the vicinity of the meniscus 8, a reverse flow 15 is generated from the short sides on both sides toward the nozzle. The presence of such a flow in the vicinity of the meniscus 8 provides a cleaning effect of inclusions by the flow, and suppresses generation of surface flaws due to trapping of inclusions on the surface layer. On the other hand, if the flow of the meniscus 8 is too strong,
Defects increase due to the involvement of continuous casting powder. In the center of the width, the reverse flow velocity near the meniscus 8 becomes slow,
The cleaning effect is not obtained, inclusions are trapped, and surface flaws are generated.

In order to solve such a problem, an electromagnetic braking device for stopping the flow of molten steel is installed below the mold so that the discharge flow 6 from the immersion nozzle collides with the short side of the mold and flows along the same. A damping magnetic field acts on the descending flow 14 to attenuate it, and an electromagnetic stirrer for stirring the molten steel is installed above the mold to apply a moving magnetic field to the reversal flow 15 of the meniscus 8, and the meniscus is forced by the stirring. 8
Many methods and apparatuses have been proposed for producing a slab having few inclusions both inside and on the surface of the slab by promoting the flow of the slab (for example, JP-A-5-177317, JP-A-7-17773).
No. 11224).

[0005]

In order to reduce the inclusions in the slab, the use of the electromagnetic stirring device and / or the electromagnetic braking device in the conventional method described above requires the inclusion of the slab in the slab. It has been possible to be an effective means for suppressing the occurrence of blemishes. However, applying an electromagnetic field over the entire period during continuous casting has the necessity for steel grades with strict quality characteristics, but does not need to be applied to low grade steel grades with strict quality characteristics, and However, when casting a steel type in which inclusions are relatively small, unnecessarily clean steel is produced, which unnecessarily increases the amount of electric power and leads to an increase in cost. In addition, operating the electromagnetic device for a long time shortens the life of the equipment and increases the frequency of maintenance, which is not a preferable operation method.

Further, blindly operating the electromagnetic stirrer or the electromagnetic braking device does not ensure that the effect of the electromagnetic field is properly received, but the electromagnetic field is applied in accordance with the required product quality. And there must be appropriate conditions for that in the casting operation. In addition, there is a value that should be a reference when setting conditions,
The operation is not always continued with the reference value constantly within the allowable range, and the work condition fluctuates due to various factors. Work is performed to determine whether the status is normal or abnormal, and to link it to an action when the value is determined to be abnormal.

An object of the present invention is to provide a method of manufacturing a continuous cast slab which is suitable for securing a quality comparable to that of a steady cast part, not only in the above-mentioned continuous casting but also in a continuous slab joint part in continuous casting. It is intended for.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to achieve the above object, and its gist lies in the following means. (1) In a continuous casting method in which an electromagnetic stirrer is installed in a portion corresponding to a molten steel meniscus in a mold and an electromagnetic braking device is installed in a portion where molten steel exists below a immersion nozzle in the mold, at least one of the following conditions is judged to be abnormal. A method for producing a continuously cast slab with high cleanliness, wherein one of an electromagnetic stirring device and an electromagnetic braking device is operated when the casting is performed. Condition: Back pressure change of Ar gas blown from immersion nozzle Condition: Fluctuation of molten steel level in the mold Condition: Difference in molten steel level on the short side of mold

(2) In a continuous casting method in which an electromagnetic stirrer is installed in a portion corresponding to a molten steel meniscus in a mold and an electromagnetic braking device is installed in a portion where molten steel exists below a immersion nozzle in the mold, at least one of the following conditions is satisfied. A method for producing a continuously cast slab with high cleanliness, wherein both of an electromagnetic stirring device and an electromagnetic braking device are operated when it is determined that there is an abnormality. Condition: Back pressure change of Ar gas blown from immersion nozzle Condition: Fluctuation of molten steel level of molten steel in mold Mold: Difference in molten steel level of molten steel on short side of mold (3) Abnormal setting condition in (1) and (2) A method for producing a continuous cast slab with high cleanliness, wherein the criteria for judging the following are the following values. Condition: back pressure of Ar gas blown from immersion nozzle ≦ 0.3
kg / cm ² Condition: Fluctuation level of molten steel in mold in mold ≧ 10 mm Condition: Difference in molten steel level in molten steel on short side of mold ≧ 12 mm

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventors of the present invention operated an electromagnetic stirrer for stirring molten steel in the upper part of a mold and an electromagnetic brake for braking a flow of molten steel discharged from an immersion nozzle in a lower part of the mold. Various experiments were repeated for the casting conditions to be performed, and as a result of trial and error, the factors affecting the inclusions remaining in the slab were successfully determined. That is,
Although many casting conditions have a greater or lesser effect, it has been found that inclusions can be reduced by specifying factors having a large contribution rate and maintaining the conditions within an appropriate range.

Hereinafter, the reasons for the conditions found by the present invention will be described in detail. First, under the conditions, in the immersion nozzle that supplies molten steel to the mold, Ar gas is injected from outside through the thick portion of the nozzle, and is blown into the molten steel flowing out of the nozzle from an appropriate position on the inner surface of the nozzle, and inclusions are formed. It prevents adhesion to the inner surface of the nozzle. The back pressure of this Ar gas is maintained at a pressure of about 1.0 to 2.0 kg / cm ^{2 in the} initial stage of the injection, and gradually decreases as the casting is continued, but usually about 0.3 kg / cm ² . After calm down, keep that state.

However, when the back pressure becomes abnormal when the immersion nozzle is abnormal, that is, when a minute crack or the like is generated in the nozzle, air from outside is sucked through the portion,
A situation occurs in which the back pressure of the blown Ar gas cannot maintain the steady value of 0.3 kg / cm ² . If casting is continued in this state, the quality of the target slab cannot be guaranteed due to the inclusions, and the slab must be taken for measures such as downgrade. Therefore, in the present invention, the above-mentioned value 0.3 kg / cm ² is used as a reference value for abnormality determination in the casting operation, and when the back pressure falls below this value, an electromagnetic field is applied to the molten steel during casting, and the casting by inclusions is performed. The effect on the piece is minimized.

FIG. 2 shows the relationship between the fluctuation of the back pressure in the pipe and the amount of inclusions remaining in the slab (inclusion index, hereinafter the same) when a fixed amount of Ar gas is blown into the immersion nozzle. The comparison is shown by the presence or absence of the application of an electromagnetic field. In the figure, the solid line indicates the maximum value of the inclusion index when the electromagnetic field is applied, and the dotted line indicates the maximum value of the inclusion index when the application of the electromagnetic field is desired (hereinafter the same as each drawing). As is clear from the figure, the residual amount of inclusions in the slab was 0.3 kg / Ar gas back pressure.
When it is less than ² cm ² , it rapidly increases and the slab inclusion index becomes 1
As described above, it is shown that the value can be reduced by applying an electromagnetic field to maintain the slab inclusion index at 1 or less. Therefore, in the present invention, when the back pressure of the Ar gas becomes less than 0.3 kg / cm ² , it is determined that an abnormality has occurred in the casting operation. (The same applies to each figure below).

Here, the inclusion index on the vertical axis in FIG. 2 is a value empirically obtained by analysis by the present inventors based on requests from users for cleanliness of steel sheets over the past several years.
In the case of a general steel sheet, the inclusion index 1 is a value adopted as a passing level of the quality (cleanliness) of the slab. The application of the electromagnetic field simultaneously activated the electromagnetic stirring device in the upper part of the mold and the electromagnetic braking device in the lower part of the mold (the same applies to FIGS. 3 and 4).

Next, the condition is as follows. Usually, the level of the molten steel in the mold is measured by a level sensor, and the level of the molten steel is controlled within a certain range. We are taking measures to prevent this from happening. However, there are cases where the level of the molten metal rises and falls due to various factors, and measures to investigate the cause may not be in time. Therefore, in the present invention, it is determined that an abnormality has occurred in the casting operation when the above and below fluctuation values of the molten metal surface become 10 mm or more. Therefore, this value is used as a reference value for abnormality determination, and when the value exceeds this value, an electromagnetic field is applied to the molten steel during casting to prevent the slab quality deterioration due to inclusions as in the above condition. In this case, the inclusions caused by the fluctuation of the molten metal level are often mold flux-based inclusions added to the surface of the molten steel due to the flow of the molten steel.

FIG. 3 shows, similarly to FIG. 2, the relationship between the fluctuation of the molten metal level in the mold and the amount of inclusions remaining in the slab. The level change is a level change measured continuously by a level sensor, and is expressed as the difference between the maximum value and the minimum value while the slab of the product passes through the mold. As is clear from the figure, the remaining amount of the inclusions in the cast slab was such that
Although the value has increased rapidly as described above, the value can be suppressed by applying an electromagnetic force.

Finally, as a condition, the discharge port of the immersion nozzle for supplying the molten steel into the mold has left and right openings on both sides of the short side of the mold as described later. As the amount of casting increases, this discharge port is blocked by the adhesion of inclusions such as Al ₂ O ₃ . If this blockage condition progresses evenly at the left and right discharge ports, it does not cause much problem. However, it is not always the case that the blockages are always equally closed. When the area becomes smaller, the flow of molten steel is more divergent toward the side with less blockage and larger opening, and as a result, the upward flow on the short side of the mold also increases, causing the molten metal to swell (the opposite is true on the opposite side). The level of the molten metal drops due to the small upward flow.)

As described above, in the mold, the molten steel surface is deviated on the left and right sides on the short side of the mold, resulting in imbalance. When the value becomes 12 mm or more, it is determined that an abnormality has occurred in the casting operation. . An electromagnetic field is applied to the molten steel during casting from the point in time when a level difference greater than this value occurs to prevent the influence of inclusions on the slab quality as in the case of the conditions. As a method of measuring this deviation, for example, the difference can be detected by a temperature change of a thermocouple embedded near the short side of the mold.

FIG. 4 shows the relationship between the difference in the level of the molten metal on the short side of the mold and the amount of inclusions remaining in the slab, as in FIGS. The difference in the level of the molten metal at the position was continuously specified from the temperature distribution of the thermocouple in the mold, and expressed by the maximum value during the time when the slab passed through the mold.
As is clear from the figure, the amount of inclusions in the cast slab increases when the deviation of the molten metal level becomes 12 mm or more, but it can be seen that the adverse effect can be reduced by applying an electromagnetic force.

The setting of the conditions for reducing the influence of inclusions in the slab and the reasons for the above have been described above. For the application of the electromagnetic field, the stirring of the molten steel flow in the upper part of the mold and the molten steel in the lower part of the mold are described. There are two ways of braking the discharge flow, and by working these two together, the respective effects can be obtained at the same time. It is desirable to secure the inclusion index of 1.0 or less. It is possible to enjoy a certain effect even by applying such an electromagnetic field and to reduce the number of rejected cast slabs, and it is not always necessary to operate both at the same time.

It is conceivable to use an electromagnetic stirrer which operates according to the above conditions and an electromagnetic field generator which is an electromagnetic braking device, but there is no causal relationship between the conditions and the electromagnetic field generator. Instead, it may be appropriately selected and performed according to various conditions at the time of casting. Also,
The operation of the electromagnetic field generating device is required only during a period deviating from the condition reference value. When each condition returns to within the reference value, the application of the electromagnetic field is stopped, so that unnecessary consumption of power can be avoided. . If multiple casters are used, one power supply can be shared by multiple casters, and the timing at which the power supply is connected to the electromagnetic field generator can be shifted so that significant equipment cost savings can be achieved. Have.

[0022]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples to clarify the effects of the present invention. The molten steel used in the examples is Al-Si killed steel.
This molten steel was applied to a vertical bending type continuous casting apparatus as shown in FIG. The immersion nozzle 2 has an inner diameter of 95 mm and a diameter of 200 mm.
It has a discharge port 3 of 25 mm downward from horizontal, from which molten steel flow is discharged to the short side of the mold 1. An electromagnetic stirrer 4 for applying a horizontal electromagnetic field by an AC magnetic field to a molten steel meniscus 8 in a mold 1 and a corresponding portion was provided. The molten steel discharge flow rate flowing from the discharge port 3 of the immersion nozzle 2 in the mold 1 to the short side of the mold 1 was 0.68 m / sec per one side of the discharge port. An electromagnetic braking device 5 for applying a braking electromagnetic force by a uniform DC magnetic field for braking the molten steel discharge flow 6 was disposed at a position 0.5 m (H) below the nozzle discharge port 3. The results of casting molten steel under such conditions are shown in Table 1 together with Comparative Examples.

[0023]

[Table 1]

As is evident from Table 1, among the examples of the present invention, Experiment Nos. 1 (lower back pressure), 2 (large level change), 3 (large level difference), which were out of the conditions regulated by the present invention. Regarding that, when both the electromagnetic stirrer and the electromagnetic braking device were activated, no rejected slabs (inclusion index exceeding 1.0) were generated, and good cast slabs could be obtained. did it. Further, in both Experiment Nos. 4 and 5, the fluctuations in the molten metal level deviated from the reference values of the present invention, so that either the electromagnetic braking device or the electromagnetic stirring device was operated. In this case, the number of occurrences of slab rejection was only two cast pieces, and the rejection rates were 6.9% and 7.1%, respectively. Further, in Experiment No. 6, all the conditions were deviated, but as a result of operating only the electromagnetic braking device, the number of rejected slabs was 4 (incidence rate 1
2.9%).

On the other hand, in Experiment Nos. 7 (lower back pressure), 8 (large level change) and 9 (large level difference) in the comparative examples, both the electromagnetic stirrer and the electromagnetic brake were activated. The number of rejected slabs was 11 (rejection rate 34.4%, the same applies hereinafter), 12 (40%), 1
The number was greatly increased to four (42.4%).

[0026]

According to the present invention, each condition of the back pressure of the Ar gas blown from the immersion nozzle into the molten steel in the mold, the fluctuation amount of the molten steel surface in the mold, and the level difference of the molten steel surface on the short side of the mold. When one or more of the slabs is determined to be abnormal, inclusions inside the slab and inclusions on the slab surface can be simultaneously reduced by applying an electromagnetic field, and a slab with excellent surface and internal quality can be obtained. It can be manufactured, and there is a great effect that contributes to the production of slabs in continuous casting.

[Brief description of the drawings]

FIG. 1 is a schematic side view for explaining an outline of the present invention.

FIG. 2 is a diagram showing the relationship between the back pressure of Ar gas blown from an immersion nozzle and the inclusion index of a slab depending on whether or not an electromagnetic field is applied.

FIG. 3 is a diagram showing the relationship between the molten metal level variation of the molten steel in the mold and the inclusion index of the slab depending on whether or not an electromagnetic field is applied.

FIG. 4 is a diagram showing a relationship between a difference in molten metal level of molten steel on a short side of a mold and an inclusion index of a slab depending on whether an electromagnetic field is applied or not.

FIG. 5 is a schematic side view illustrating a flow state of molten steel in a conventional continuous casting mold.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Immersion nozzle 3 Discharge port 4 Electromagnetic stirring device 5 Electromagnetic braking device 6 Discharge flow 7 Stirring flow 8 Meniscus 9 Solidification shell 13 Upflow 14 Downflow 15 Reverse flow

Continued on the front page (72) Inventor Kiyoshi Shigematsu 1 Nishinosu, Oita, Oita City, Oita Prefecture Nippon Steel Corporation Oita Works

Claims (3)

[Claims] In a continuous casting method in which an electromagnetic stirrer is installed in a portion corresponding to a molten steel meniscus in a mold and an electromagnetic braking device is installed in a portion where molten steel is present below an immersion nozzle in the mold, at least one of the following conditions is abnormal: A method of producing a continuously cast slab with high cleanliness, wherein one of an electromagnetic stirring device and an electromagnetic braking device is actuated when it is determined. Condition: Back pressure change of Ar gas blown from immersion nozzle Condition: Fluctuation of molten steel level in the mold Condition: Difference in molten steel level on the short side of mold 2. A continuous casting method in which an electromagnetic stirrer is installed in a portion corresponding to a molten steel meniscus in a mold and an electromagnetic braking device is installed in a portion where molten steel is present below an immersion nozzle in the mold, wherein at least one of the following conditions is abnormal: A method for producing a continuously cast slab with high cleanliness, characterized in that both the electromagnetic stirring device and the electromagnetic braking device are operated when it is determined that: Condition: Back pressure change of Ar gas blown from immersion nozzle Condition: Fluctuation of molten steel level in the mold Condition: Difference in molten steel level on the short side of mold 3. A method for producing a continuously cast slab with high cleanliness, wherein a criterion for judging that the set conditions in claim 1 and 2 are abnormal is the following value. Condition: back pressure of Ar gas blown from immersion nozzle ≦ 0.3
kg / cm ² Condition: Fluctuation level of molten steel in mold in mold ≧ 10 mm Condition: Difference in molten steel level in molten steel on short side of mold ≧ 12 mm