JP3388686B2 - Flow control method in continuous casting strand - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to flow control in a strand in continuous casting, and in particular, to maintain a soft contact state by using an electromagnetic force obtained when an electromagnetic coil is energized, and to maintain a good subcutaneous property. The present invention relates to a method for controlling flow in a continuously cast strand for

[0002]

2. Description of the Related Art In continuous casting, powder is generally added to the upper surface of a molten metal pool in a mold. The powder melted by the heat from the molten metal flows into the gap by the relative motion of the vertically oscillating mold wall and the solidified shell that is pulled out at a constant speed. The dynamic pressure generated when the molten powder flows in deforms the meniscus or the tip of the solidified shell. Since this deformation is repeated in the cycle of mold oscillation, periodic wrinkles called oscillation marks are formed on the surface of the slab.

As a method for controlling the initial solidification, for example, there is a method described in Japanese Patent Laid-Open No. 52-32824, in which a molten metal is poured together with a lubricant into a water-cooled mold that vibrates at a constant cycle, and continuously. In the continuous casting method by pulling out downwards, alternating current is continuously applied to the electromagnetic coil provided around the mold, the electromagnetic force generated in the alternating electromagnetic field is used to raise the molten metal in a convex shape, and the surface of the slab is cast. The properties are improved.

According to the method disclosed in Japanese Unexamined Patent Publication No. 52-32824, an electromagnetic coil continuously applies an electromagnetic force to the molten metal in the mold to improve the surface quality of the slab. However, the applied electromagnetic field not only changes the meniscus shape, but also heats the molten metal to be solidified in the mold, and initial solidification may not always proceed stably.

As a method for solving the above problems, the present inventors have previously proposed the method described below (PCT International Publication No. WO95 / 26243).

A continuous casting apparatus to which this method is applied is
The one shown in FIG. 6 was used. That is, on the upper part of the mold 1, the meniscus 10 inside the mold is surrounded,
An annular electromagnetic coil 2 is embedded (or installed outside the mold 1). A pouring nozzle 3 attached to a tundish (not shown) is arranged in the center of the upper portion of the mold 1. Further, a power supply device 4 is connected to the electromagnetic coil 2, and a waveform generator 5 is connected to the power supply device 4.

The power supply device 4 operates in accordance with the output signal of the waveform generator 5, and the electromagnetic coil 2 receives an exciting current 6 (for example, an alternating current having a frequency of 60 Hz and a peak value of 3,000 A) of 0.5.
Pulse waveform of the second cycle is superimposed) is applied. When casting is performed in this state, an induction current 8 is generated in the molten metal 7 by the electromagnetic coil 2, and thereby an electromagnetic force 9 is generated. This electromagnetic force 9 acts on the meniscus 10 formed on the lower side of the powder 11 to cause the solidification at the initial stage of continuous casting to proceed regularly without the use of a lubricant, whereby the characteristics of the surface of the slab and the casting Stability can be improved.

Here, the alternating current passing through the electromagnetic coil 2 is stepwise as shown in FIG. That is, as shown in FIG. 7A, when the large current conduction period is t ₁ and the small current conduction period is t ₂ , a large current conduction for applying the electromagnetic force required to change the meniscus shape is set. A combination of small current energization having a different function from changing the meniscus shape before and after, or a large current energization for applying an electromagnetic force necessary to change the meniscus shape as shown in FIG. 7B. A small-current energization is provided to obtain a function different from the function of changing the meniscus shape later, and a non-energization period (t
_Depending on the energization method of _off ), continuous energization or a rectangular wave with a predetermined cycle (the energization period is t _on ). By applying such a waveform, the initial solidification instability of the molten metal is suppressed, and the effect of improving lubrication and the effect of improving the surface property of the slab can be stably obtained.

Further, the ratio [t ₁ / (t ₁ + t) of the large-current energization time contributing to meniscus deformation to the in-cycle energization time.
₂ ), or t ₁ / (t ₁ + t ₂ + t _off )] is 0.2 or more,
It is preferably 0.8 or less. By this selection, the effect of improving the lubrication between the mold wall and the solidified shell and the effect of improving the surface property of the slab can be maximized. Here, the lower limit of the ratio comes from the required energization time for changing the meniscus shape and promoting the powder inflow, and the upper limit of the ratio gives disturbance to the meniscus (or flux) or prevents heat generation. It is set from the small-current energizing time required for the operation.

[0010]

However, in the above-mentioned conventional example, since the electromagnetic coil is continuously driven in the method disclosed in Japanese Patent Laid-Open No. 52-32824, the flow velocity in the strand is apt to become too fast, In this case, the surface properties of the slab are not improved.

In addition, PCT international publication number WO95 / 26
In No. 243, there is a problem that intermittent swelling of the molten metal occurs depending on the flow velocity in the strand, and as a result, entrainment of the powder occurs, so that the surface quality deteriorates.

In other words, it has been difficult with the prior art to satisfy both the improvement of the surface texture and the reduction of the amount of subcutaneous inclusions at the same time.

The present invention has been made in view of the above-mentioned problems. In the continuous cast strand, the molten steel flow rate in the strand can be optimized to improve the surface properties and prevent the inclusions from being trapped. It is intended to provide a flow control method.

[0014]

A method for realizing the object of the invention according to the present application is, as described in claim 1, a predetermined energization mode for a solenoidal coil installed so as to surround an upper portion of a mold. In the continuous cast strand in which a pulsed current is applied to generate a vertical rotating flow at a flow velocity of 0.1 m / s to 0.3 m / s in the solidified shell intervening region of the molten steel metal in the mold. There is a flow control method.

According to this method, a desired static pressure is applied to the molten steel metal to cause the molten steel metal in the mold to softly contact the inner wall of the mold,
At the same time, a pulsed electric current is applied to form a vertical rotating flow at an optimum speed. This makes it possible to obtain a slab with few inclusion defects while maintaining the soft contact casting state.

As a concrete method for realizing the object of the invention according to the present application, as described in claim 2, in the energization mode, an energization period is 20% or more and 70% or less, and a corresponding idle period is combined. There is a flow control method in a continuous casting strand using a waveform having one cycle.

According to this method, the cross-sectional flow velocity of the solidified shell and the inclusion index in the surface layer of the slab can be satisfactorily formed, and the improvement of the surface properties and the prevention of inclusion inclusion can be achieved at the same time.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an explanatory view showing the principle of a flow control method in a continuously cast strand according to the present invention.

In the present invention, in order to improve the surface texture, a pulsed current is applied to the electromagnetic coil, and 0.1 m / m
A vertical flow velocity of s to 0.3 m / s is applied to the solidified shell in the coil height range. The reason for setting the flow velocity range will be described with reference to FIG.

FIG. 1A shows the relationship between the electromagnetic pressure and the energized state of the electromagnetic coil, and FIG. 1B shows the relationship between the molten steel flow velocity in the strand and the energized state of the electromagnetic coil. In order to control the static pressure and clean the surface properties, it is necessary to apply a predetermined electromagnetic pressure, but as shown in FIG. 1A, the electromagnetic pressure is proportional to the energized state of the electromagnetic coil. . When the electromagnetic coil is not energized (OFF), the electromagnetic pressure becomes 0, and the surface quality deteriorates. In addition, 100% continuous energization (A
When the C current is applied, the electromagnetic pressure is maximized and the surface quality is similarly deteriorated.

In order to prevent trapping of inclusions,
It is necessary to optimize the molten steel flow velocity (flow velocity in the vertical direction) in the strand. As shown in FIG. 1B, the flow velocity in the vertical direction is minimum when the electromagnetic coil is not energized. In addition, the flow velocity is maximized during continuous energization, and inclusions are caught in the molten steel flow and captured. This causes deterioration of product quality.

The electromagnetic pressure and the flow velocity in the vertical direction are such that the energization to the electromagnetic coil is cyclic as shown in FIG. 2, that is, the energization period becomes longer (pause) when the ratio of the ON period and the OFF period is changed. As the period gets shorter), the electromagnetic pressure and flow velocity gradually increase. That is, the electromagnetic pressure and the flow velocity can be arbitrarily set by changing the energization ratio.

If the static pressure and the flow velocity in the vertical direction are to be simultaneously controlled by one electromagnetic coil, it is necessary to define the energization condition that balances both. Therefore, the present inventors have found that the ratio of the energized period a to the non-energized period b, that is, (a
÷ b) × 100 (%) was set to a predetermined change ratio between 0 and 100%, and the static pressure and the flow velocity in the vertical direction were observed each time. This determination can be performed by video shooting the rising height of the molten metal surface and analyzing the image content.

As a result, the vertical flow velocity is 10 to 30c.
When the m / s and the electromagnetic pressure were 30 to 40 mmFe, the surface properties were optimized, and the static pressure control in which the molten steel in the mold was kept in soft contact with the inner wall could be achieved at the same time. Then, when the flow velocity in the vertical direction at this time was measured with a strain gauge, it was 0.1 m / s to 0.3 m / s.
From this, the vertical flow velocity is 0.1 m / s to 0.3
The object of the present invention can be achieved if the range is m / s.

(Embodiment 1) The present inventors performed casting using a mold having the structure shown in FIG. 3 and a continuous casting facility using an electromagnetic coil. In the continuous casting equipment, the lower end of the injection nozzle 13 connected to a tundish (not shown) is immersed in the upper part of the mold 12. Molten metal 1 in mold 12
An electromagnetic coil 15 is embedded in the mold so as to surround the molten metal surface at a predetermined depth. This electromagnetic coil 15
Thus, the circulation 16 having the flow velocity according to the present invention is formed in the vertical direction in the vicinity of the inner wall of the mold inside thereof.

Casting was performed under the following conditions. Materials used: Austenitic stainless steel billet cast strand: 150 mm square casting speed: 2 m / min Injection nozzle: Downward straight coil height: 200 mm Coil position: Meniscus swelling height at its upper end: 40 mm Energizing current: a = 70%, b = 30% Then, as a comparative example, when the electromagnetic coil 15 is not provided (test), when the electromagnetic coil 15 is continuously energized (test), the electromagnetic coil 15 is energized for an energization period a = 7.
When the solidified shell cross-section flow velocity (cm / s) and the inclusion index of the surface layer 10 mm of the slab were measured for three cases of 0% and energization period b = 30% (test),
The results shown in FIGS. 4 and 5 were obtained.

As is apparent from FIG. 4, the cross-flow velocity of the solidified shell exceeds 40 cm / s in the test by continuous energization, whereas it is about 20 cm in the pulse energization according to the present invention.
/ S, indicating that the trapping of inclusions is reduced. In the test in which the electromagnetic coil 15 is not provided, the cross-sectional flow velocity of the solidified shell is slower than that of the present invention. This is the same as when the electromagnetic coil 15 is not energized,
This is because, as shown in (a) of FIG. 1, the condition is that the flow velocity becomes the slowest.

Next, as is clear from FIG. 5, in the test by continuous energization, the influence of inclusion of powder-type (lubricant) inclusions is significant, and the inclusion index has an extremely large value. On the other hand, in the present invention, only the slag system brought in from the tundish is involved as inclusions, and the inclusion index is shown to be extremely small as a result of the inclusion of inclusions being suppressed due to the existence of a certain flow velocity.
Although the inclusion index of the test in which the electromagnetic coil 15 is not provided is lower than that of the test, the effect of trapping the slag-based inclusions appears due to the insufficient flow velocity. The result is significantly worse than the test.

As described above, according to the present invention, it is possible to simultaneously improve the surface properties and prevent inclusions from being trapped.

[0031]

As described above, according to the present invention as set forth in claim 1, a pulse current is applied to the electromagnetic coil in a predetermined energizing mode, and the flow velocity is 0.1 m / s to 0.3 m / s.
Since a vertical rotating flow is generated in the solidified shell intervening region of the molten steel metal in the mold, it is possible to obtain a slab with few inclusion defects while maintaining the soft contact casting state. That is, it becomes possible to improve the surface properties and prevent inclusions from being captured.

According to the present invention as set forth in claim 2, the energization period is 2
Since the current-carrying mode has a waveform of 0% or more and 70% or less, and the corresponding rest period is one cycle in total, it is possible to satisfactorily form the solidified shell cross-sectional flow velocity and the inclusion index of the surface layer of the cast piece, and improve the surface quality. Also, it is possible to achieve both prevention of trapping of inclusions.

[Brief description of drawings]

FIG. 1 is an explanatory view showing the principle of a flow control method in a continuously cast strand according to the present invention, (a) showing the relationship between electromagnetic pressure and the energization state of an electromagnetic coil, and (b) showing the molten steel flow velocity in the strand. And the energized state of the electromagnetic coil.

FIG. 2 is a waveform diagram showing a waveform of a current passed through an electromagnetic coil in the present invention.

FIG. 3 is a front cross-sectional view showing a schematic configuration of a continuous casting facility used to obtain the embodiment of the present invention.

FIG. 4 is a flow velocity characteristic diagram showing cross-sectional flow velocity characteristics of a solidified shell when the present invention is carried out.

FIG. 5 is an inclusion index characteristic diagram showing inclusion indexes in a surface layer of a cast product when the present invention is carried out.

FIG. 6 is a front sectional view showing a schematic configuration of a conventional continuous casting facility.

FIG. 7 is a waveform diagram showing an AC current waveform that is applied to an electromagnetic coil of the equipment of FIG.

[Explanation of symbols]

12 Mold 13 injection nozzle 14 Molten metal 15 Electromagnetic coil 16 times

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-8-197213 (JP, A) JP-A-8-155611 (JP, A) JP-A-8-90183 (JP, A) JP-A-7- 96360 (JP, A) JP 4-13445 (JP, A) JP 64-83348 (JP, A) JP 8-187557 (JP, A) JP 8-155608 (JP, A) JP-A-5-329595 (JP, A) JP-A-55-106664 (JP, A) JP-A-5-23803 (JP, A) JP-A-52-32824 (JP, A) International Publication 95/26243 ( WO, A1) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/115 B22D 11/11 B22D 11/04

Claims (2)

(57) [Claims] 1. A pulse-shaped current in a predetermined energization mode is applied to a solenoid coil installed so as to surround the upper part of the mold, and the flow velocity is 0.1 m / s to 0.3 m / s.
A method for controlling flow in a continuous casting strand, characterized in that a vertical rotating flow is generated in a solidified shell intervening region of molten steel metal in a mold. 2. The continuous casting strand according to claim 1, wherein the energization mode has a waveform in which an energization period is 20% or more and 70% or less, and a corresponding rest period is one cycle in total. Flow control method.