JPH08187553A - Mold for continuous casting applying electromagnetic force and continuous casting method - Google Patents

Abstract

PURPOSE: To obtain a cast slab having extremely little surface defect by making the shape of a corner part in a polygonal mold into a circular-arc shape and specifying this radius. CONSTITUTION: Molten steel is poured into the mold 1 from a ladle through a tundish 8 and an immersion nozzle 2. The mold 1 is the polygonal mold and a coil 4 is arranged so as to surround the outer periphery, and a high frequency induction electromagnetic force is impressed through the coil. Further, the shape of corner part in the polygonal mold 1 is formed in a circular-arc shape and the radius γ is regulated by the following formula (I). The formula I: γ>=3×(2/σμω)<1/2> . In the formula I, σ is electric conductivity of the molten steel, μis magnetic permeability of the molten steel and ω is frequency of the impressed high frequency. The cross section of the polygonal mold is formed into substantially a square or a rectangular shape. Thus, the improvement of the yield and the reduction of the production cost, etc., can be achieved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to continuous casting of metal such as steel, and more particularly to a continuous casting mold for improving the lubrication of the mold and the slab and producing a slab with few surface defects.

[0002]

2. Description of the Related Art In the technique of casting metal such as steel, as disclosed in Japanese Patent Laid-Open No. 52-32824, a magnetic field is applied from the outside of the mold to the vicinity of the meniscus in order to improve the surface properties and workability of the slab. The method of applying an electromagnetic force to the initially solidified shell in the mold by applying the molten metal to the molten metal is disclosed. In order to increase the electromagnetic force efficiency when a magnetic field is applied therein, a method of applying a magnetic field from the outside of the cold crucible mold to more effectively exert an electromagnetic force on the solidification shell in the mold is disclosed (for example, , Materials and Processes, VOL.6 (1993) -6).

Although low frequency and high frequency can be used as the type of magnetic field applied by these methods, it is desirable to apply the high frequency magnetic field intensively to the surface of the solidified shell near the meniscus. Generally, the skin depth L that a high-frequency magnetic field penetrates is calculated by L = (2 / σμω) ^1/2 . Here, σ represents the conductivity of the molten metal, μ represents the magnetic permeability of the molten metal, and ω represents the angular frequency of the applied high frequency.

[0004]

However, this high frequency magnetic field is applied to another billet having a rectangular or square mold section,
When applied to a polygonal mold such as a bloom or a slab, the magnetic field concentrates too much on the corners (hereinafter also referred to as corners) of the mold, and the surface quality of the corners of the mold deteriorates. When a slab having a poor surface quality at the corner is rolled, a flaw caused by the corner is generated.

[0005]

In view of such circumstances, the inventors of the present invention cast a molten metal into a polygonal shape such as a slab and apply a high-frequency magnetic field from the outside of the casting mold to cast the molten metal. The radius r of the circular arc at the corner of is set as follows. r ≧ 3 × (2 / σμω) ^1/2

Here, σ is the conductivity of the molten metal, μ is the permeability of the molten metal, and ω is the angular frequency of the applied high frequency.
The inventors have found that the quality of the corner portion of the slab can be improved by carrying out continuous casting in which such a polygonal mold is used, and have made the following invention.

(1) The invention of claim 1 provides a continuous casting mold having the following features. (A) A polygonal mold for continuous casting of molten metal, which is provided with a coil for applying a high frequency electromagnetic force so as to surround the outer periphery of the mold, and (b) a corner part of the polygonal mold. A continuous casting mold whose shape is an arc and whose radius r is as in the following formula. r ≧ 3 × (2 / σμω) ^1/2 Here, σ is the conductivity of the molten metal, μ is the permeability of the molten metal, and ω is the angular frequency of the applied high frequency.

(2) The invention of claim 2 provides the continuous casting mold according to claim 1, wherein the polygonal mold has a substantially square or rectangular cross section.

(3) The invention of claim 3 provides a continuous casting method including the following steps. (A) A polygonal mold for continuous casting of molten metal, in which a coil for applying a high-frequency electromagnetic force is provided so as to surround the outer periphery of the mold, and the shape of a corner portion of the polygonal mold is a circle. Prepare a continuous casting mold that is arc-shaped and has a radius r as shown below: r ≧ 3 × (2 / σμω) ^1/2 where σ is the conductivity of the molten metal and μ is the permeability of the molten metal. The magnetic susceptibility and ω are the angular frequencies of the applied high frequency. (B) Molten metal is injected into the mold for continuous casting.

[0010]

[Function] When the electromagnetic coil is arranged outside the mold near the meniscus in the mold and a high frequency magnetic field is applied near the meniscus of the molten metal in the mold, an induced current is generated in the molten metal, and this induced current and the applied magnetic field A Lorentz force, that is, an electromagnetic force (hereinafter, also referred to as an electromagnetic force) is generated in the direction of repulsion with the coil due to the interaction.

At the same time, Joule heat is also generated by the above-mentioned induced current. The Lorentz force and the Joule heat are used to control the initial solidification of the meniscus to improve the surface properties of the slab, or to improve the lubrication of the mold and solidification shell by improving the powder consumption, thereby improving operating troubles. is there.

However, it is desirable that the strength of the applied magnetic field be uniform around the meniscus, but billets, blooms, slabs, etc. have 3 to 6 angles, usually 4 angles. When a high frequency magnetic field is applied, the magnetic field concentrates on the corners. Due to the Lorentz force based on this concentrated magnetic field, the inward bending of the solidified shell at the corner of the cast piece and the solidification delay due to excessive Joule heat occur, which disturbs the oscillation mark and deepens the oscillation mark. Etc., the surface quality deteriorates.

Therefore, the shape of the corner portion of the polygonal mold of the present invention is arcuate, and the radius r thereof is expressed by the following equation (1). r ≧ 3 × (2 / σμω) ¹ /2-(1) where σ is the conductivity of the molten metal, μ is the permeability of the molten metal, and ω is the angular frequency of the applied high frequency.

By casting using a mold having such a radius r, the path of the induced current is kept smooth even at the corners, and the electromagnetic force is not concentrated at the corners. The surface quality becomes good, and the surface quality of the slab can be improved. Here, the arc shape does not have to be a strict arc and may be slightly deformed. It may be substantially arcuate.

In the present invention, the reason why the low frequency magnetic field of less than 1000 c / s is not applied is that not only the magnetic pressure in the case of the low frequency magnetic field but also the instability of the molten metal surface for generating a large stirring force is promoted. There is a reason. On the other hand, when a high frequency magnetic field is applied, the stirring force is sufficiently small and only the effects of magnetic pressure and induction Joule heat can be expected.

The shape of the mold of the present invention may be any shape as long as it has a triangular to hexagonal cross-sectional shape, usually a square,
It can be applied to a rectangular mold such as a rectangle. It can also be applied to a dogbone-shaped mold. As a mold,
It may be a commonly used integral type or assembling type mold, or a cold crucible type mold as described in Examples. Usually, these molds are copper or copper alloy molds that are water-cooled inside.

[0017]

EXAMPLE An example of the mold of the present invention is shown in FIG. Molten steel is poured into a mold 1 from a ladle through a tundish 8 and a dipping nozzle 2. In this embodiment, a mold having a structure in which a cold crucible type slit shown in FIG. 2 is cut over the entire length of the mold (inside dimension: short side 180 mm, long side 400 m)
m) is used, the coil has 4 turns, and the coil is connected to a power source, so that the timing of applying the current can be changed in accordance with the vibration timing of the mold.

The slit 7 portion shown in FIG. 2 is a normal space and is extremely narrow so that molten steel does not enter the slit 7, but it is desirable to insert, for example, a refractory into the slit 7 portion. . The high frequency oscillator of the power supply has a frequency of 3 KHz and 300 KW and a maximum coil current value of 8000 A.

In the case of molten steel, the electric conductivity σ = 5 × 10 ⁶ (A / V · m) and the magnetic permeability μ = μ ₀ × μ _r = 4π × 10 ⁻⁷ × 1 (V · sec)
/ A · m), and when the frequency of the power source is 3 KHz, the angular frequency of the high frequency is ω = 2π × 3 × 10 ³ (1 / sec). Substituting this into equation (1), r = It becomes 12.3 × 10 ⁻³ (m) = 12.3 (mm).

Therefore, the radius r of the corner portion is set to 14 mm. A perspective view of such a mold is shown in FIG. 2, and a plan view thereof is shown in FIG.
Shown in Next, a preferable casting method using such a mold will be described. FIG. 4 shows an aspect of a method of applying a high frequency current to the electromagnetic coil on the outer periphery of the mold. In this method, the timing of mold vibration and the current application to the coil outside the mold can be variously changed.

In FIG. 4, (a) is a mold vibration wave type, in which the period in which the mold descending speed is faster than the casting speed in one cycle is a negative strip (hereinafter referred to as NS) period, and the mold descending speed is slower than the casting speed. Is referred to as a positive strip (hereinafter referred to as PS) period. Further, (I) to (IV) show modes of current application. This is because all the current application patterns have strength, but the magnetic field is stronger than that applied continuously. (I) has a weak coil current in the PS period,
(II) the coil current was applied with a strong NS period, (II) the coil current was applied only during the NS period, and (III) the coil current was applied with a strong PS period and a weak NS period.
V) indicates that the coil current is applied only in the PS period.

The steel type cast in the examples was carbon steel having a carbon concentration of 0.1%, and the molten steel superheating temperature in the tundish was adjusted to be 25 ° C. in each example. The powder used has low viscosity, which is advantageous for lubrication as shown in Table 1.
A low melting point powder was used, and the powder consumption was calculated from the thickness of the powder adhering to the surface of the slab by taking out the slab in the mold at the end of casting, cooling it, and then taking it out.

Table 2 shows that the coil current value is applied in the pattern of (I) to (IV) shown in FIG. 4 under the condition of a constant value of 5000 A, the mold vibration is a positive sign and a non-sign, and the distortion of the non-sine waveform. The ratio is a result of investigating the vibration condition of the mold and the oscillation mark depth of the slab corresponding to the corner portion of the mold when 40% is adopted.

Here, the distortion rate (%) of the non-sine waveform =
(t ₂ -t ₁₎ is a _{× 100 / t 1, t 1} : time from the displacement zero at a sine waveform to a maximum displacement, t _2: the time from the displacement zero at a non-sinusoidal up to the displacement, t
₂ > t ₁ .

The oscillation mark depth is indicated by a value D1 obtained by subtracting the oscillation mark depth at the corner of the slab from the oscillation mark depth at the center of the long side of the slab.
In Table 2, the corners of the mold under the same casting conditions
The results of the examination of the oscillation mark depth in the absence of the mark are shown as a comparison.

When a high frequency electromagnetic force is applied, by providing an arc portion having a radius shown by the formula (1) at the corner portion of the mold, the oscillation mark depth is reduced and a slab having a good surface quality can be obtained. It was In particular, the effect was remarkable in the application patterns (I) and (II).

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

As described above, by using the mold for continuous casting in which the shape of the corner portion of the mold according to the present invention is arcuate, and by appropriately selecting the timing of applying the electromagnetic force, Stable powder lubrication can be ensured even during high speed casting, and cast pieces with extremely few surface defects can be obtained without operating problems. As a result, slabs that can be carelessly rolled can be stably manufactured, and the effects such as improvement in yield and reduction in manufacturing cost are very large.

[Brief description of drawings]

FIG. 1 is a schematic view of an example of continuous casting using the mold of the present invention.

FIG. 2 is a perspective view of the shape of the mold according to the present invention.

FIG. 3 is a plan view of a mold according to the present invention.

FIG. 4 is a diagram showing various aspects of mold vibration and application timing of coil current.

[Explanation of symbols]

 1 Mold 2 Immersion Nozzle 3 Solidification Shell 4 Coil 5 Molten Metal 6 Powder 7 Slit 8 Tundish

Claims (3)

[Claims] 1. A continuous casting mold having the following features. (A) A polygonal mold for continuous casting of molten metal, which is provided with a coil for applying a high frequency electromagnetic force so as to surround the outer periphery of the mold, and (b) a corner part of the polygonal mold. A continuous casting mold whose shape is an arc and whose radius r is as in the following formula. r ≧ 3 × (2 / σμω) ^1/2 Here, σ is the conductivity of the molten metal, μ is the permeability of the molten metal, and ω is the angular frequency of the applied high frequency. 2. The continuous casting mold according to claim 1, wherein the cross section of the polygonal mold is substantially square or rectangular. 3. A continuous casting method comprising the following steps. (A) A polygonal mold for continuous casting of molten metal, in which a coil for applying a high-frequency electromagnetic force is provided so as to surround the outer periphery of the mold, and the shape of a corner portion of the polygonal mold is a circle. Prepare a continuous casting mold with an arc shape and a radius r as shown below: r ≧ 3 × (2 / σμω) ^1/2 where σ is the conductivity of the molten metal and μ is the permeability of the molten metal. The magnetic susceptibility and ω are the angular frequencies of the applied high frequency. (B) Molten metal is injected into the mold for continuous casting.