JPH01289543A - Continuous casting method for steel - Google Patents

Abstract

PURPOSE:To reduce the surface flaw generation rate of a slab by arranging in opposition a permanent magnet or electromagnet by making the magnetic flux direction in the thickness direction of a slab at two places in specified range from the upper end of a short side face mold. CONSTITUTION:A permanent magnet or an electromagnetic 23 is arranged at the positions of two places in the range of 50-300mm and that of 500-650mm from the upper end of relatively opposing short side molds 1. In this case, the magnetic flux direction of the magnet 23 is made to direct the thickness direction of a slab. The magnetic flux direction of the magnet 23 is thus faced in the slab thickness direction respectively at the upper side and lower side of the collision point of the discharging flow of a continuous casting and the short side mold 1, the movement energy of a upward reverse flow 11 is attenuated and the wave motion of a molten metal level is restrained. A downward invasion flow 12 is reduced at its depth by receiving a brake force and the floating of an inclusion is facilitated. Consequently the generation rate of the surface flaw of a slab is reduced because the molten metal level wave motion is restrained and the inclusion is reduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention is a method of producing slabs with good surface quality by adjusting the flow of molten steel in a mold and making it easier for inclusions to float in continuous slab casting. This invention relates to a continuous casting method.

[Prior Art] FIG. 6 is a diagram showing the main part of the molten metal surface of the mold of a continuous slab casting machine. FIG. 7 is a schematic diagram of the surface wave motion. The conventional technology will be explained with reference to this figure. Molten metal 8 in mold 1
There is mold powder 5 on the surface of the mold powder 5, which has the functions of preventing oxidation and keeping the molten metal 8 warm, lubricating the space between the solidified shell 9 and the mold 1, and adsorbing nonmetallic inclusions. This mold powder 5
The molten metal surface 8 side is in a molten state due to the heat of the molten metal, and the atmosphere side is turned into powder and covers the molten metal 8 surface.

The molten powder 6 flows between the solidified shell 9 and the mold 1 and plays the role of a lubricant. Therefore, since the molten powder 6 is consumed, in order to maintain a constant thickness of the mold powder 5,
The molten powder 6 is replenished in an amount corresponding to the consumed amount.

As shown in FIG. 7, a discharge hole 3 provided at the tip of a submerged nozzle 2 vertically provided in the middle of the mold 1 opens opposite to the short side of the mold 1. The molten metal is discharged into the mold through the discharge hole 3. The discharge flow 4 of the molten metal is injected into the mold in a V-shape in the direction of the short side of the mold 1.

The discharge flow 4 of the molten metal collides with the short side and is divided into two upper and lower flow reversed flows 11 and intrusion flows 12, and the reversed flow 11 rising along the solidified shell 9 on the short side faces is formed by the upper short side of the mold 1. This causes the hot water level fluctuation 10 near the side surface. This hot water surface wave was measured with an eddy current distance meter 15, and the voltage value was filtered to remove high frequency components (here, frequency components of 10 Hz or higher), and then measured with a millivolt meter. The installation position of this eddy current distance meter 15 is as shown in FIG.
mm, 50 mm from the hot water surface. FIG. 8 is a diagram showing changes in the hot water level over time for about 1 minute. In about 1 minute,
The maximum change in the hot water level was defined as the hot water surface wave height of 10 h.

Up arrow means upward direction, upward arrow indicates downward direction,
FIG. 9 is a diagram showing the relationship between the wave height of the hot-molten metal surface and the surface defects of the hot-rolled sheet. As is clear from this figure, the height of the water surface wave is 10
is in the range of 4 mm to 8 mm, the incidence of surface defects on the hot rolled sheet is low, and when this level fluctuation is large, the molten powder 6 is
Due to fluctuations in the molten metal, it gets caught up in the molten metal and becomes suspended. The molten powder 6 caught in the molten metal is mixed with the molten metal and the molten powder 6.
It floats due to the difference in specific gravity of the shell, but some of the solidified shell 9
captured by

On the other hand, when the melt level fluctuation is small, there is little supply of new molten metal to the molten metal surface, so the meltability of the mold powder 5 is also poor.Therefore, the molten powder 6 has poor ability to dissolve and adsorb inclusions in the molten metal, and It is thought that the substances are trapped in the solidified shell 9 and become internal defects in the slab. The appropriate range of the surface wave height 10 shown here is 4 mm to 8 mm, which is a value obtained from experience in continuous casting operations, and the shape of the immersion nozzle 2 and the discharge of the immersion nozzle 2 should be adjusted to fall within this range. The angle, clogging of the immersion nozzle 2, width of the mold 1, etc. were regulated.

However, in order to improve the productivity of recent continuous casting machines, 1) multi-continuous casting technology that performs continuous casting for several charges with one tundish and immersion nozzle, 2) changing the mold width during casting, 3) Operating conditions have changed, such as the long production speed changing from low to high speed. As a result, operating conditions were created that could not be satisfied with the shape and discharge angle of the discharge hole of the submerged nozzle that were suitable for the initial operating conditions, and it became impossible to control the height of the fluid level fluctuation within the optimal range. As a technology to control the height of fluctuation in the hot water level, 1) A method of applying a brake to the discharge flow using a DC magnetic field (*
1: In conventional method 1), two pairs of DC magnets are installed in the cooling box on the long side of the mold, and a DC magnetic field is applied to the flow discharged from the immersed nozzle to induce the induction generated in the flowing molten metal. The flow of the molten metal is controlled by an electromagnetic force generated in the opposite direction to the flow of the molten metal using an electric current and a DC magnetic field.

2) A method of applying a DC magnetic field to the front hot water surface position (*2: hereinafter referred to as conventional method 2), placing a DC magnetic field at the hot water level position,
By applying a DC magnetic field horizontally to the hot water surface, the height of the hot water level fluctuation within the magnetic field is controlled.

For example, (*1) Nagai et al. = 68. Iron and Steel (1982), 52
70 Suzuki et al.: 68. Iron and Steel (1982), 592 (
*2) Near Kokane 2. Iron and Steel (1986), 371
8 [Problems to be Solved by the Invention] However, in conventional method 1, a direct current magnetic field is applied perpendicularly to the discharge flow between the immersed nozzle and the short side surface to apply a brake to the fluid. Since the fluid diffuses after being discharged from the tube, it is necessary to apply a DC magnetic field over a wide range. For this reason, the equipment becomes large and the cost increases, or it is difficult to increase the magnetic flux density.

In conventional method 2, a direct current magnetic field is applied directly to the hot water surface waves, so the wave suppression effect is large, but the most intense position of the hot water surface waves is within 100 mm from the short side surface. In order to apply a DC magnetic field to this position, it is most effective to install a magnetic field generator on the long side and select the DC magnetic flux direction in the thickness direction of the slab. The equipment will be large-scale as it will need to be moved.

This invention was made in view of the circumstances, and
The equipment is simple, and there is no need to move the electromagnet.By incorporating a permanent magnet or electromagnet into the short side surface and applying a brake to the fluid flowing along the solidified shell on the short side surface,
Suppresses surface vibrations, prevents powder entrainment, and
It is an object of the present invention to provide a method for manufacturing a sound slab by reducing the penetration depth and making it easier for inclusions to float.

[Means for Solving the Problems] The continuous casting method for steel of the present invention is a continuous casting method in which molten steel is cast from a tundish into a mold through an immersion nozzle.
Permanent magnets or electromagnets are installed at two positions, one in the mm range and the other in the 500-650 mm range, so that the magnetic flux direction is in the thickness direction of the slab, and casting is performed while applying a DC magnetic field to the molten steel. shall be.

[Operation] This invention provides magnetic flux by installing one or more permanent magnets or electromagnets above (between the collision point and the scene) and below (between the collision point and the lower end of the mold) the collision point of the discharge flow in continuous casting. It is installed on the back side of the short side mold so that the direction is in the slab thickness direction.

By applying a perpendicular magnetic field to the more upward reversal flow, a braking force is generated, attenuating the upward kinetic energy of the reversal flow and suppressing surface waves. On the other hand, the intrusion flow that flows downward along the short-side solidified shell pushes the inclusions riding on the intrusion flow deep into the strand, making it difficult to float and separate them to the surface of the molten metal. As a result, the electromagnetic force acts as a braking force against the intruding flow, making it possible to reduce the depth of the intruding flow. As a result, inclusions do not enter deep into the strands and easily float up. Electromagnets cannot be installed less than 50mm from the top of the short side mold.
More than 300 mm to less than 500 mm from the top of the short side mold is where the discharge flow of the submerged nozzle collides with the mold wall, and reverse flow and intrusion flow cannot be controlled. Since the flow velocity becomes slower, the electromagnetic force generated becomes smaller and the braking effect becomes less effective.

[Embodiments] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

First, the concept of fluid flow when an electromagnetic force is applied to a fluid will be explained. When the magnetic flux direction of a DC magnetic field is applied perpendicularly to a fluid that is a good conductor, an electromotive force E based on equation (1) is generated in the fluid.

E=VxB=VY-Bz ■ = Velocity of fluid (m/d) B: Magnetic flux density Bz: Magnetic flux density in the thickness direction Eddy current I flows in the fluid due to this electromotive force, and the interaction between eddy current I and the applied magnetic field Therefore, body force F acts in the opposite direction to the direction of fluid movement.

F=-IXB--σVY-B22 σ: Specific electrical resistance of fluid According to equation (1), the magnitude of body force depends on VY and Bz2.

In continuous casting of molten steel, in the case of low-speed casting, VY is small, so the braking force acting on the molten steel is small, but the higher the speed of casting, the larger VY becomes, so the braking force acts more strongly. FIG. 1 is a schematic diagram of a short-side mold copper plate portion according to an embodiment of the present invention, in which (a) is a perspective view and (b) is a plan sectional view.

1 is a copper plate on the short side of the mold, 22 is a holding plate for the copper plate on the short side of the mold, 23 is a permanent magnet or an electromagnet, and 24 is a cooling groove. That is, the dimensions of the copper plate 1 on the short side of the mold are 30 mm to 40 m thick.
m, width is 228 mm, and length is 950 mm. Width: 5 mm, depth 1 in the length direction on the back side of the copper plate 1 on the short side of the mold
A cooling groove 24 for cooling the short side of the mold of 5 mm is cut.

Cooling water is flowing into the cooling grooves 24. On the back side of the copper plate 1 on the short side of the mold, a press plate 22 made of stainless steel and made of a copper plate on the short side of the mold prevents leakage of cooling water. A permanent magnet 23 (magnetic pole size 50
A groove was cut into which a mold (mm x 50 mm) could be installed, and the magnetic poles (N pole and S pole) were arranged in the width direction of the copper plate 21 on the short side of the mold. The permanent magnets 23 are attached at positions 250 mm and 650 mm from the upper end of the copper plate 1 on the short side of the mold in the length direction of the copper plate 1 on the short side of the mold. The magnetic flux density of the permanent magnet 23 is measured at the surface of the copper plate 21 on the short side of the mold.

(Example 1) FIG. 2 is a cross-sectional view of a short-side mold copper plate portion according to an example of the present invention, in which (a) is a plan cross-sectional view, and (b) is an X-Y cross-sectional view of (a>). 250mm and 65mm from the top of copper plate 1 on the short side of the mold
A horseshoe-shaped permanent magnet 23 is incorporated at the 0 mm position, and the magnetic pole size of the permanent magnet 23 is 50 mm x 50 mm square. A permanent magnet 23 with a magnetic flux density in the range of 500 to 6000 gauss is incorporated into the copper plate 1 on the short side of the mold, and casting is performed, and an eddy current distance meter is installed at a position 50 mm from the surface of the copper plate 1 on the short side of the mold and 5 Q mm from the surface of the molten metal. 15 was installed to measure the water surface wave, and after removing the high-frequency components as described above, the change in the maximum hot water level for one minute was measured 20 times, and the maximum value was calculated as the maximum hot water surface wave height ( max, H).

The casting conditions at this time were that the slab dimensions were 1000mm wide,
The thickness is 220 mm, the drawing speed is 2.5 m/min, and the discharge hole angle of the immersion nozzle 2 is 25 degrees downward.

FIG. 3 is a diagram showing the relationship between the magnetic flux density of the permanent magnet and the maximum height of the liquid surface wave.

As is clear from this figure, the magnetic flux density of the permanent magnet is 13 mm in order to achieve the maximum surface wave height of 4 to 8 mm within the appropriate range.
It is necessary to apply 00 to 5000 Gauss.

(Example 2) FIG. 4 is a diagram showing the relationship between drawing speed and maximum surface wave height. The casting conditions at this time were that the mold of Example 1 was used, the magnetic flux density of the permanent magnet 23 was 3000 Gauss, the slab dimensions were 1000 mm in width and 220 mm in thickness, and the discharge hole angle of the immersion nozzle 2 was 25 degrees downward. In this figure, ○ indicates the case where a permanent magnet was not used, and . In the above cases, the maximum surface wave height of the appropriate range will be exceeded, but if a permanent magnet is used, 1.2 m/min ~
In the drawing speed range of 2.5 m/min, the maximum surface wave height is sufficiently controlled within the appropriate range.

(Example 3) Using the mold of Example 1, the casting conditions were: the magnetic flux density of the permanent magnet 23 was 2000 Gauss, the drawing speed was 2, and Qm/mi.
n, the slab dimensions are 1000 mm in width and 220 mm in thickness, and the discharge hole angle of the immersion nozzle 2 is 25 degrees downward. FIG. 5 is a graph showing the inclusion index in the thickness direction of the slab. In this figure, the upper surface is the upper surface of the slab, and the lower surface is the upper surface of the slab. The calculation method of the inclusion index is as follows.

Inclusion index: Observe 60 points with an optical microscope at 400x magnification, count the number of alumina inclusions, set the maximum value (inclusions at 115 positions in the conventional method) as 1.0, and index other positions. It became. In this figure, O indicates the case where a permanent magnet was not used, and . indicates the case where a permanent magnet was used.As is clear from this figure, the inclusion distribution shows the same tendency without a magnetic field and with a magnetic field. The peak value has decreased to 1/2 or less. This shows that the depth of the intruding flow becomes shallower due to the application of a magnetic field, promoting the floating of inclusions.

Table 1 summarizes the operating conditions under which the hot rolled sheet surface defect incidence was investigated.

As is clear from this table, in this invention using permanent magnets, the hot rolled sheet surface defect incidence rate is reduced by 115 to 1/10 compared to the conventional method which does not use permanent magnets.

In this invention, a permanent magnet was used as an example, but the same result was obtained when an electromagnet was used.

[Effects of the Invention] According to the present invention, as configured as above, the discharge flow from the immersion nozzle collides with the short side solidified shell, and then the discharge flow rises along the short side solidified shell. It is divided into a reverse flow and a descending intrusion flow.The flow velocity of the reverse flow and the intrusion flow is suppressed to reduce the surface waves in the mold that occur due to the reverse flow, and the flow is directed downward along the short side solidified shell. By installing the above-mentioned device, the depth of the intruding flow can be made shallow because the electromagnetic force acts as a braking force on the intruding flow. As a result, inclusions do not enter deep into the strands and float easily, reducing the incidence of surface flaws.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of the short side mold copper plate part of an embodiment of the present invention, Fig. 2 is a sectional view of the short side mold copper plate part of an embodiment of the present invention, and Fig. 3 shows the magnetic flux density of the permanent magnet and the maximum Figure 4 is a graph showing the relationship between the height of the hot water surface waves; Figure 4 is a graph showing the relationship between the withdrawal speed and the maximum height of the hot water surface waves; Figure 5 is a graph showing the inclusion index in the thickness direction of the slab; The figure shows the main parts of the molten metal surface of the mold of a continuous slab casting machine, Figure 7 is a schematic diagram of the molten metal surface waves, and Figure 9 is a graph showing the relationship between the height of the molten metal surface waves and hot-rolled sheet surface defects. It is a diagram. 1...Copper plate on the short side of the mold, 22...Press plate for the copper plate on the short side of the mold, 23...Permanent magnet or electromagnet, 24...
cooling groove.

Claims (1)

[Claims] In a continuous casting method in which molten steel is cast into a mold from a tundish through an immersion nozzle, a range of 50 to 300 mm from the upper end of opposing short side molds and a range of 500 to 650 m
Continuous casting of steel, characterized in that permanent magnets or electromagnets are installed at two positions in the range of m so that the magnetic flux direction is in the thickness direction of the slab, and casting is performed while applying a DC magnetic field to the molten steel. Method.