JP3310884B2 - Electromagnetic casting of steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic field casting method for continuously casting steel while applying an electromagnetic force.

[0002]

2. Description of the Related Art In a continuous casting method of steel, an attempt is made to improve the surface properties of a slab by applying an electromagnetic force of a high-frequency magnetic field to an initially solidified portion of the continuous slab, and by a pinch force of the electromagnetic force and its heating effect. It has been done. As an example of a mold used for this electromagnetic field casting, FIG. 1 shows a cross-sectional explanatory view of an electromagnetic field mold disclosed in Japanese Patent Application Laid-Open No. 4-178247. Mold wall 3 of water-cooled mold 1
Are provided with vertical slits 4 at predetermined intervals, so that the high-frequency magnetic field applied by the high-frequency coil 2 on the outer periphery of the mold easily penetrates the slab.

[0003] The applicant of the present application has been studying a method of electromagnetically casting steel using the above-mentioned electromagnetic field mold.
No. 7150 discloses an electromagnetic field casting method capable of high-speed casting, in which powder slag is easily inserted between a mold and a solidified shell. An electromagnetic field casting method for preventing inclusion defects due to lubricant entrainment by applying a frequency of 5 to 20 kHz has already been disclosed.

[0004] The advantages of such an electromagnetic field casting method include the following. The pinch force generated by the electromagnetic field acts on the initial solidification shell and acts in the direction of reducing the static pressure of the molten steel, which can eliminate the adverse effects caused by mold vibration and reduce the possibility of forming irregularities called oscillation marks on the casting surface. The surface quality of the piece is good. The pinch force due to the electromagnetic field expands the inflow path of the powdery lubricant supplied between the mold and the initial solidified shell, so that the frictional force between the mold and the initial solidified shell is reduced and the slab surface quality is improved. . Since the molten metal surface rises due to the electromagnetic force, and the initial solidification starts below the surface of the molten metal due to the heating effect of the electromagnetic force, the influence of the fluctuation of the molten metal surface due to external causes is less likely to reach the initial solidified shell, and the surface quality of the cast slab is improved. Due to the heating effect and the pinch force, the initial solidified shell does not protrude to the surface of the molten metal, so that pinholes and inclusions can be prevented from being trapped under the protruding solidified shell.

However, in the slab cast by the conventional method, inclusions accumulate in the surface layer due to the action of the applied electromagnetic force, and defects are observed in the surface layer after rolling, so that there is room for improvement. Had left. That is, since inclusions that accumulate in the surface layer cause flaws and the like during rolling, development of an electromagnetic field casting method that does not accumulate inclusions in the surface layer has been demanded.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is intended to produce a slab having no surface layer defects due to the accumulation of inclusions and having good surface properties. It is intended to provide an electromagnetic field casting method.

[0007]

The present invention solves the above-mentioned problems. In the present invention, a longitudinal slit is formed at a predetermined interval in a mold wall, and an electromagnetic coil is installed on the outer periphery thereof to apply an electromagnetic force to an initially solidified shell. In the electromagnetic field casting method, the length of the short side of the slab dimension is L (cm), the penetration depth of the magnetic force lines is δ (cm),
The magnetic permeability is μ and the electric conductivity of the molten metal is σ (Ω ^-1 / cm)
When the frequency f (Hz) is expressed by the following equations (1) and (2)
And casting at a controlled frequency of 250 kHz or less.

[0008]

(Equation 2)

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reason why inclusions accumulate on the surface layer of a slab by the conventional electromagnetic field casting method is considered as follows. That is, the high-frequency magnetic field easily acts on molten steel having high electric conductivity, but hardly acts on inclusions having low electric conductivity, so that the pinch force of the high-frequency magnetic field acting on molten steel is apparently high for inclusions. It works as a force to discharge from the molten steel to the outside. For this reason, the inclusions are collected on the surface layer of the slab and cause flaws in the rolling process.

Accordingly, the present inventors have proposed the application of an electromagnetic field from the viewpoint of reducing the depth of penetration of electromagnetic force by a high-frequency magnetic field, thereby reducing the range of inclusions collected by the action of the electromagnetic force. The conditions were reviewed, and the present invention was reached.

It is known that the penetration depth δ of the electromagnetic force into the molten metal is represented by the following equation (2), and the frequency f
It is understood that the penetration depth δ (cm) can be reduced by increasing (Hz).

[0012]

(Equation 3)

FIG. 2 shows the frequency f and the penetration depth of the electromagnetic force when μ is 1 of the magnetic permeability in vacuum and σ is 7.22 × 10 ³ (Ω ^-1 / cm) of the molten steel. 6 is a graph showing the relationship of the length δ. As the frequency increases, the penetration depth becomes shallower, especially in the range of about 20 kHz or less, the effect is very large, and when it exceeds 50 kHz, the effect gradually becomes saturated, but the penetration depth becomes shallower as the frequency becomes higher. I understand.

Therefore, from the viewpoint of preventing the accumulation of inclusions in the surface layer, it is desirable to increase the frequency as much as possible to reduce the penetration depth of the electromagnetic force and to reduce the amount of inclusions moving to the surface layer by the electromagnetic force. .

However, as the frequency increases, heat due to high-frequency heating concentrates on the mold wall surface on the side in contact with the molten steel, and the mold temperature rises locally. Therefore, there is an upper limit to the frequency increase, which is determined by the heat-resistant temperature of the mold. For example,
In case of copper or copper alloy mold, heat resistance temperature is 350 ℃
And the upper limit of the frequency needs to be 250 kHz, and preferably 200 kHz or less.

Incidentally, it is known that the surface layer portion corresponding to a thickness of about 0.7% of the slab size is oxidized in the process of cooling the slab and falls off as scale. Therefore, if the thickness is within the range, there is no problem even if the inclusions are accumulated. Therefore, in order to prevent the accumulation of inclusions, if the lower limit of the permeation depth is set according to the size of the slab and the frequency is set as low as possible in a range exceeding the lower limit, excessive heat is applied to the mold. It is possible to suppress an increase in power supply capacity due to an increase in frequency without applying a load. For this purpose, the lower limit of the frequency may be set so as to satisfy the following equations (1) and (2). In addition, the following equation (1):
It was obtained from the results of the examples described below and the like, and shows that the penetration depth of the electromagnetic force can be increased as the cast slab size increases without increasing the amount of inclusions included in the surface layer.

[0017]

(Equation 4)

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the following examples do not limit the present invention, and all modifications and alterations that do not depart from the spirit of the present invention will be described below. Is included.

[0019]

EXAMPLE 1 Using a billet continuous casting machine having a cross section of 150 mm square, electromagnetic field casting was performed with changing the frequency under the following casting conditions, and the interposition at the surface layer of the slab after the scale dropped in the cooling process. The number of objects was measured. Table 1 shows the components of the carbon steel used for casting.

[0020]

[Table 1]

Casting is performed at a casting speed of 0.7 to 3.0 m / min. The magnetic flux density in the mold is the air-core magnetic flux density at the center of the coil where the magnetic flux density is highest, and the casting speed is 0.7 m / min. / Min, 300 gauss, casting speed 3.
380 gauss at 0 m / min, frequency is 1 to
It was changed in the range of 300 kHz. FIG. 3 shows the result of examining the relationship between the frequency, the number of inclusions in the slab surface layer (0 to 5 mm depth from the surface), and the mold temperature.

The frequency at which the temperature of the mold exceeds the heat-resistant temperature of copper and copper alloy molds of 350 ° C. is 250 kHz, and the number of inclusions decreases rapidly up to about 80 kHz, and becomes almost zero above 80 kHz. It is an acceptable amount. Therefore, when the thickness of the slab is 150 mm, it is understood that the frequency range of 80 to 250 kHz should be adopted.

Example 2 Next, a similar casting test was carried out using the steel types shown in Table 1 above using a 300 mm square mold. Casting speed 0.7m /
min, and the magnetic flux density in the mold is the air-core magnetic flux density at the center of the coil where the magnetic flux density is highest, which is 300
Gauss was. The results are shown in FIG.

The frequency at which the mold temperature exceeds the heat-resistant temperature of 350 ° C. of the copper and copper alloy mold is 250 kHz, and the number of inclusions decreases rapidly up to about 20 kHz, and becomes almost zero or an allowable amount above 20 kHz. It has become. Therefore, when the thickness of the slab is 300 mm, 20
It can be seen that a frequency of ~ 250 kHz should be employed.

From these results, accumulation of inclusions can be prevented.
The lower limit of the frequency range is obtained, and the frequency f is calculated by the above equation (2).
To calculate the penetration depth δ, this penetration depth
It can be seen that there is a positive correlation between δ and the slab size L.
won. The equation (1) indicates that the penetration depth δ and the slab size
L is a conditional expression showing the relationship of L, where δ / L is 1.4 × 10 ^-2
By performing electromagnetic field casting using the following frequency
Therefore, accumulation of inclusions can be prevented.

[0026]

As described above, the present invention can provide an electromagnetic field casting method for producing a slab having good surface properties without defects in the surface layer caused by accumulation of inclusions. It became.

[Brief description of the drawings]

FIG. 1 is an explanatory sectional view of an electromagnetic field mold in electromagnetic field casting.

FIG. 2 is a graph showing the relationship between frequency and penetration depth in electromagnetic field casting.

FIG. 3 is a graph showing the effect of frequency on the number of inclusions and the mold temperature in the surface layer of a 150 mm square cast slab.

FIG. 4 is a graph showing the effect of frequency on the number of inclusions and mold temperature in the surface layer of a 300 mm square cast slab.

[Explanation of symbols]

 Reference Signs List 1 water-cooled mold 2 electromagnetic coil 3 mold wall 4 longitudinal slit 5 immersion nozzle

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-132189 (JP, A) JP-A 8-90165 (JP, A) JP-A-7-1093 (JP, A) JP-A-6-106 246405 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B22D 11/04 311 B22D 11/11

Claims (1)

(57) [Claims] 1. An electromagnetic field casting method in which a longitudinal slit at a predetermined interval is formed in a mold wall and an electromagnetic coil is installed on an outer periphery thereof to apply an electromagnetic force to an initial solidified shell. Length is L (cm), penetration depth of magnetic field lines is δ (cm), magnetic permeability is μ, and electric conductivity of molten metal is σ
(Ω ⁻¹ / cm), the frequency f (Hz) is a value that satisfies the following formulas (1) and (2), and is controlled to 250 kHz or less. Casting method. (Equation 1)