JPH1085914A - Continuous casting method of molten metal - Google Patents

Abstract

(57) [Summary] [Problem] When an electromagnetic force is applied in a pulse shape, molten metal also repeats intermittent swelling and entrains powder.
The surface properties deteriorate. SOLUTION: An alternating current of which amplitude or waveform is periodically changed is applied to an electromagnetic coil arranged around the mold wall or embedded in the mold wall to apply an electromagnetic force to the molten metal in the mold. In the continuous casting apparatus for performing casting while changing the meniscus shape, the alternating current includes a large current energizing period t _a and a small current energizing period t _b , and an energizing time having one cycle of t _a + t _b. against a proportion of large current period t _a is 0.2 to 0.8, and the energizing time of a small current conduction period t _b is less than 0.6 seconds. This energization reduces fluctuations in the surface height of the molten metal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the control of an electromagnetic coil arranged to surround a mold wall in order to improve the surface properties of a slab, and more particularly to the control of a molten metal for improving the surface properties of a slab. The present invention relates to a continuous casting method.

[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 due to the relative motion of the mold wall vibrating up and down and the solidified shell drawn at a constant speed. The meniscus and the tip of the solidified shell are deformed by the dynamic pressure generated when the molten powder flows. Since this deformation is repeated at the cycle of the mold oscillation, periodic wrinkles called oscillation marks are formed on the slab surface.

A method for controlling the initial solidification is described in, for example, Japanese Patent Application Laid-Open No. 52-32824, in which a molten metal is injected together with a lubricant into a water-cooled mold that vibrates at a constant cycle.
In a continuous casting method by continuously drawing down, by continuously applying an alternating current to an electromagnetic coil provided around the mold, by using the electromagnetic force generated by the alternating electromagnetic field to raise the molten metal to a convex shape In addition to promoting the flow of lubricant, the surface properties of the slab are improved.

Japanese Patent Application Laid-Open No. 64-83348 discloses that when an electromagnetic force is applied to a molten metal in a mold by an electromagnetic coil, an alternating magnetic field is applied in a pulsed manner to intermittently apply the electromagnetic force. In addition, there has been proposed a method for further improving the surface properties in powder casting.

According to the method disclosed in JP-A-52-32824, the surface properties of a slab are improved by continuously applying an electromagnetic force to a molten metal in a mold by an electromagnetic coil. However, the applied electromagnetic force magnetic field not only changes the meniscus shape, but also generates an excessive flow in the molten metal, and the meniscus shape fluctuates unstable, so that the initial solidification may not always proceed stably.

According to the method disclosed in JP-A-64-83348, an intermittent electromagnetic force is applied to a molten metal in a mold by an electromagnetic coil, so that a powder between a solidified shell and a mold wall is provided. Is further promoted, and the surface properties of the slab are improved. However, a sudden ON / OFF energization pattern may generate a wave on the surface of the molten metal pool. These waves remain during the non-energization period, and as a result, disturb the molten metal pool meniscus, hinder the effect of the application of the electromagnetic force, and in extreme cases, cause the powder to be trapped in the solidified shell.

As a solution to the above problem, the present inventors have previously proposed a method described below.

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

The power supply 4 operates in accordance with the output signal of the waveform generator 5, and an exciting current 6 (for example, a pulse waveform having a cycle of 0.5 seconds superimposed on an alternating current having a frequency of 60 Hz and a peak value of 3000 A) is applied to the electromagnetic coil 2. ). When casting is performed in this state, an induced current 8 is generated in the molten metal 7 by the electromagnetic coil 2, thereby generating an electromagnetic force 9.
The electromagnetic force 9 acts on the meniscus 10 formed on the lower side of the powder 11, promotes the flow of the lubricant, and regularly repeats the solidification in the early stage of continuous casting, thereby improving the properties of the slab surface. And casting stability is improved.

Here, the alternating current applied to the electromagnetic coil 2 has a step shape as shown in FIG. That is, as shown in FIG. 6A, when the large current energization period is t ₁ and the small current energization period is t ₂ , the large current energization for applying the electromagnetic force necessary to change the meniscus shape is performed. A combination of a small current supply having a function different from that of changing the meniscus shape before and after, or a large current supply for applying an electromagnetic force necessary for changing the meniscus shape as shown in FIG. A small current supply for obtaining a function different from that for changing the meniscus shape later is provided, and after applying one or more pairs of these, a non-current application period (t
_off ), a rectangular wave of a predetermined cycle (the energizing 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 between the surface of the slab and the inner surface of the mold wall and the effect of improving the surface properties of the slab can be stably obtained.

Furthermore, the ratio [t ₁ / (t ₁ + t) of the large current energizing time which contributes to the meniscus deformation to the energizing time within the cycle.
₂ ) or t ₁ / (t ₁ + t ₂ + t _off )] is 0.2 or more,
It is preferred to be 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 properties of the slab can be maximized. Here, the lower limit of the ratio comes from the required energization time for changing the shape of the meniscus to promote powder inflow, and the upper limit of the ratio gives a disturbance of the meniscus (or flux) or causes Is set based on the small current energizing time required to prevent excessive flow.

[0012]

However, in the above conventional example, there is a problem that the application of the electromagnetic force in a pulse form causes the molten metal to repeat intermittent swells, entrain the powder, and deteriorate the surface properties. .

In view of the above problems, an object of the present invention is to provide a continuous casting method of molten metal capable of preventing intermittent swelling of molten metal and improving surface properties.

[0014]

A method for realizing the object of the present invention according to the present invention is as follows.
A solenoid type electromagnetic coil is disposed or buried in the mold wall so as to surround the mold wall, and an alternating current having a large current conduction period and a small current conduction period in which the amplitude or waveform is periodically changed is supplied to the electromagnetic coil. In the continuous casting apparatus, which applies an electromagnetic force to the molten metal in the mold to perform casting while changing the meniscus shape, a total of one cycle consisting of one large current conduction period and one small current conduction period The ratio of the large current conduction period to the conduction time is 0.2
To 0.8, and the energizing time during the small current energizing period is set to 0.6 second or less.

According to this method, the ratio of the large current energizing period to the total energizing time of one cycle is set to 0.2 to 0.8, thereby improving the lubrication between the mold wall and the solidified shell and improving the slab. Acting to maximize the effect of improving the surface properties of the surface, the small current conduction period with the conduction time of 0.6 seconds or less is close to the response time to the electromagnetic force of the molten metal near the mold wall, Reduces variations in surface height. Therefore, powder entrapment is prevented, and the surface properties are improved.

According to a second aspect of the present invention, a solenoid type electromagnetic coil is provided so as to surround a mold wall or is embedded in the mold wall, and the amplitude or waveform is determined. An alternating current having a periodically changed large current energizing period and a small current energizing period is applied to the electromagnetic coil to apply an electromagnetic force to the molten metal in the mold, and continuous casting is performed while changing the meniscus shape. In the casting apparatus, the large-current energizing period occupies a total period of one cycle including one or a plurality of combinations of one large-current energizing period and one small-current energizing period and one non-energizing period. Continuation of molten metal, wherein the ratio is 0.2 to 0.8, and the total time between the one large-current energizing period and the next large-current energizing period is 0.6 seconds or less. For casting method .

According to this method, the lubrication improving effect between the mold wall and the solidified shell and the effect of improving the surface properties of the slab are maximized. By setting the total time to the energization period to be 0.6 seconds or less, the response time to the electromagnetic force of the molten metal in the vicinity of the mold wall is close to the response time, and the variation in the surface height of the molten metal is reduced. Therefore, powder entrapment is prevented, and the surface properties are improved.

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a waveform diagram showing an energization waveform in the continuous casting method of the present invention.

As shown in FIG. 1, according to the present invention, when an alternating current having a step-like cycle is applied, the energizing period having a larger amplitude (or waveform) is t _a , and the energizing period having _a smaller amplitude is t _a . _{When b} , t _a / (t _a + t _b ) = 0.2 to
0.8 is set, and t _b is set to t _b ≦ 0.6 seconds.

The response time of the molten metal to the electromagnetic force is 0.
1 to 0.6 seconds, the energization period t _b having the smaller amplitude (or waveform) is limited to 0.6 seconds or less, so that as shown in FIG. While the fluctuation of the metal surface height (fluctuation between the molten metals 12a and 12b) was large, in the present invention, as shown in FIG.
The amount of fluctuation is reduced (fluctuation between the molten metals 13a and 13b). As a result, powder entrainment is prevented, and the surface properties can be improved. Also, as described above, t
By setting _a / (t _a + t _b ) = 0.2 to 0.8,
It functions to maximize the effect of improving lubrication between the mold wall and the solidified shell and the effect of improving the slab surface properties.

FIG. 3 shows the characteristics of the fluctuation of the molten metal level according to the embodiment of the present invention, and FIG. 4 shows the characteristics of the average surface roughness.
The conditions in this case are as follows. Casting conditions: high carbon chromium bearing steel "SUJ2" template Size: 160 mm × 160 mm, height 800mm mold oscillation: stroke 6 mm, cycle 150cpm drawing speed: 1 m / min _{lubricants: C-Ca-SiO 2 -AlO} 3 -Na system , Viscosity 1 poise Electromagnetic coil: Inner dimension 260 mm x 260 mm, Outer dimension 340 mm x 340 mm, Height 100 mm Electromagnetic coil position: The upper end of the coil corresponds to the level of the molten metal (100 mm from the upper end of the mold) Magnetic field condition: Single phase AC 60 Hz, 1200 Gauss (Maximum effective value) Under the above conditions, the energization period is determined by applying an electromagnetic force / pulse (t _a = 1.0 sec, t _b = 1.0 sec) and applying an electromagnetic force / pulse (t _a = 0.7 Second, t _b = 0.7 sec), electromagnetic force / pulse application (t _a = 0.5 sec, t _b = 0.5 sec), electromagnetic force / pulse application (t _a = 0.1 sec, t _b = 0.1 second), the molten metal level variation characteristics were measured. As a result, as shown in FIG. 3, in the case of (1) and (2), the variation of the molten metal level was 5 mm or less, and the effectiveness of setting t _b to 0.6 seconds or less was confirmed.

The energization period is (i) no electromagnetic force;
(Ii) electromagnetic force / continuous application, (iii) electromagnetic force / pulse application (t _a = 1.0 second, t _b = 1.0 second), (iv) electromagnetic force / pulse application (t _a = 0.7) Second, t _b = 0.7 second),
(V) Electromagnetic force / pulse application (t _a = 0.5 sec, t _b = 0.
5 seconds), (vi) electromagnetic force / pulse application (t _a = 0.1)
(Second, t _b = 0.1 second), the average surface roughness characteristics were measured. As a result, as shown in FIG. 4, (iii)
In the cases of (v) and (iv), although a considerable improvement (average surface roughness of 100 μm or less) is observed, (v) and (v)
In the case of i), the average surface roughness was 30 μm or less. As described above, by setting t _b to 0.6 seconds or less, the average surface roughness can be reduced.

(Another Embodiment of the Invention) Another embodiment of the present invention will be described with reference to FIG. FIG.
As shown in (b), when the large current conduction period is t ₁ and the small current conduction period is t ₂ , the meniscus is applied after the large current conduction for applying the electromagnetic force necessary to change the meniscus shape. A continuous current or a rectangular wave having a predetermined period (current period) is provided by applying a small current to obtain a function different from the shape change, and applying a current of at least one pair of these currents and then a non-current period (t _off ). Is set to t _on ), and the ratio [t ₁ / (t ₁ + t ₂ + t _off )] of the large current energizing time contributing to meniscus deformation to the energizing time in the cycle is set to 0.2 or more and 0.8 or less, and The total time between one large current conduction period and the next large current conduction period is set to 0.6 seconds or less.

In FIG. 6B, one combination of a large current conduction period and a small current conduction period and one non-conduction period are included in one cycle.
There may be two or more combinations of "large current conduction period-small current conduction period" in one cycle, such as small current conduction period-large current conduction period-small current conduction period-non-conduction period. . However, in that case, the total time between one large current conduction period and the next large current conduction period is set to 0.6 seconds or less.

By applying such a waveform, the initial solidification instability of the molten metal is suppressed, and the lubrication improving effect and the slab surface property improving effect can be stably obtained.

Further, by selecting the energizing time and the non-energizing time, the effect of improving the lubrication between the mold wall and the solidified shell and the effect of improving the surface properties of the slab can be maximized. Here, the lower limit of the ratio comes from the required energization time for changing the shape of the meniscus to promote powder inflow, and the upper limit of the ratio gives a disturbance of the meniscus (or flux), or causes a change in the molten metal. Is set based on the small current energizing time required to prevent excessive flow.

[0028]

As described above, the present invention according to the first aspect of the present invention comprises a large current conducting period and a small current conducting period.
Since the ratio of the large current energizing period to the total energizing time of the cycle is 0.2 to 0.8 and the energizing time of the small current energizing period is 0.6 seconds or less, the surface height of the molten metal is reduced. As a result, the powder is prevented from being entangled and the surface properties are improved.

The invention according to claim 2 is characterized in that one or a plurality of combinations of one large current conduction period and one small current conduction period and a total period of one cycle of one non-current conduction period are provided. Therefore, the ratio of the large current conduction period is 0.2 to 0.8, and the total time between the one large current conduction period and the next large current conduction period is 0.6 seconds or less. As a result, fluctuations in the surface height of the molten metal are reduced, so that powder entrainment is prevented and the surface properties are improved.

[Brief description of the drawings]

FIG. 1 is a waveform diagram showing an energization waveform in a continuous casting method of the present invention.

FIGS. 2A and 2B show the state of occurrence of fluctuations in the surface height of molten metal when a pulse-like current is applied to an electromagnetic coil; FIG. 5 shows a surface height variation during energization according to the method of the present invention.

FIG. 3 is a characteristic diagram showing a molten metal level variation characteristic according to the present invention.

FIG. 4 is a characteristic diagram showing an average surface roughness characteristic according to the present invention.

FIG. 5 is a front sectional view showing an example of a continuous casting device provided with an electromagnetic coil.

6A and 6B show an example of an alternating current applied to the electromagnetic coil of FIG. 5, wherein FIG. 6A is a current waveform diagram when the current value is changed to two types, and FIG. 6B is a current waveform diagram when the current value is changed to two types. FIG. 7 is a current waveform diagram when an off period is provided.

[Explanation of symbols]

 Reference Signs List 1 mold 2 electromagnetic coil 5 waveform generator 7 molten metal 10 meniscus

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Eiichi Takeuchi 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division

Claims (2)

[Claims] An alternating current having a large current energizing period and a small current energizing period in which an amplitude or a waveform is periodically changed is provided by arranging or embedding a solenoid type electromagnetic coil around a mold wall. In a continuous casting apparatus for applying an electromagnetic force to the molten metal in the mold by energizing the electromagnetic coil and performing casting while changing a meniscus shape, one continuous current supply period and one small current supply period are used. The ratio of the large current energizing period to the total energizing time of one cycle is 0.2 to 0.8, and the energizing time of the small current energizing period is 0.6 seconds or less. A continuous casting method for molten metal. 2. An alternating current having a large current conduction period and a small current conduction period in which a solenoid type electromagnetic coil is disposed so as to surround the mold wall or is embedded in the mold wall to periodically change the amplitude or waveform. In a continuous casting apparatus that applies an electromagnetic force to the molten metal in the mold by energizing the electromagnetic coil and performs casting while changing a meniscus shape, one continuous current supply period and one small current supply period are performed. A ratio of the large current energizing period to 0.2 to 0.8 with respect to a total time of one cycle including one or a plurality of combinations and one non-energizing period; , The total time between the next high current
A continuous casting method for molten metal, characterized in that the time is less than seconds.