JP4595351B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a technique for producing a high-quality slab by controlling a flow by applying a moving magnetic field to molten steel in continuous casting of steel in which molten steel is supplied to a mold using an immersion nozzle.

  In continuous casting of steel, it is known that the molten steel flow state in the mold, particularly the flow in the vicinity of the molten steel surface, affects the slab quality in relation to the entrainment of mold powder and the biting. In order to produce defect-free slabs, molten steel flow control technology in the mold is important.

  Conventionally, a method of applying a magnetic field to molten steel in a mold to optimize the flow has been performed. As a method for applying a magnetic field, for example, there is a method in which a coil is placed opposite to the back surfaces of both long sides of a mold and a DC static magnetic field is applied. Patent Document 1 proposes a method in which a DC static magnetic field is applied over the entire width of the mold, and the applied strength is defined by the casting speed, nozzle discharge hole angle, discharge hole area, nozzle immersion depth, and mold width. However, the method of applying a static magnetic field as in Patent Document 1 has a problem that the flow stagnation region cannot be efficiently activated because the static magnetic field always acts as a braking force on the molten steel flow.

  Patent Document 2 proposes a method in which a moving magnetic field generating coil is arranged in the long side direction of the mold to form a swirling stirring flow in the horizontal direction so that inclusions are not captured by the solidified shell. This method is a method in which the molten steel in the mold is positively stirred and inclusions mixed in the molten steel are not solidified on the slab surface layer, but there is also a risk that powder or the like is newly mixed by stirring.

  In contrast, Patent Document 3 discloses a moving magnetic field generator of a linear moving magnetic field type, in which a magnetic field moves from a short side toward an immersion nozzle at the center of the mold. Is set so that at least one cycle of the action of the linear moving magnetic field is received while the molten steel flow from the discharge hole passes through the magnetic field action region, and the upper limit of the frequency is the attenuation of the magnetic field (skin effect) In consideration of the above, it is assumed that the influence of the magnetic field reaches the molten steel inside the mold sufficiently. Patent Document 4 states that the above-mentioned Patent Document 3 is developed and both the magnetic flux density and the frequency are increased or decreased to obtain an appropriate flow.

  In Patent Documents 3 and 4, the moving magnetic field is controlled on the assumption that the molten steel flows almost uniformly from the discharge hole toward the short side of the mold, but at the time of casting a wide material in recent years, the molten steel flow is caused in the width direction. There is a problem that it has a distribution and the expected effect is not seen.

Furthermore, recently, a method of combining a static magnetic field and a moving magnetic field has been proposed (for example, Patent Document 5). By combining various types of magnetic fields in this way, control according to the situation becomes possible. However, with this technology, the control logic becomes complicated, and there are concerns that the equipment will become larger and the power consumption will increase.
JP 7-314100 A Japanese Patent Publication No.58-49172 Japanese Patent Laid-Open No. 5-23804 Japanese Patent Laid-Open No. 10-5945 JP 2003-164948 A

The present invention has been made in view of such circumstances, and even when it is difficult to optimize the flow of molten steel in the past, such as when the width of the mold is wide, without performing complicated control, the mold during casting and to provide a continuous casting how the steel is properly controllable molten steel flow in the.

In order to solve the above-mentioned problems, the present invention is to supply molten steel into a rectangular mold having a long side and a short side via an immersion nozzle provided in the center of the mold width direction, and in the mold width direction. A plurality of electromagnetic coils arranged on both sides, an AC power supply for supplying power to the plurality of electromagnetic coils, a current value control unit for controlling a current value supplied to the electromagnetic coil, and a current supplied to the electromagnetic coil A frequency control unit that controls the frequency, and the flow of the molten steel in the mold is controlled by an electromagnetic flow control device that applies a moving magnetic field in which the moving direction of the magnetic field is the mold width direction along the long side to the molten steel in the mold. A method for continuous casting of steel to be controlled and cast,
When the molten steel flow velocity in the mold width direction in the mold ascertained in advance is faster than the average speed in the vicinity of the immersion nozzle, and slower than the average speed in the vicinity of the mold short side,
The phase of alternating current supplied to the plurality of electromagnetic coils is adjusted so that the moving direction of the moving magnetic field is from the immersion nozzle side to the short side, and the moving speed of the moving magnetic field is the molten steel in the mold width direction. Control the frequency of the current by the frequency control unit to be smaller than the average speed of the flow velocity, and control the current value supplied to the electromagnetic coil by the current value control unit to control the applied magnetic field strength,
Said to slow in the immersion nozzle near a faster part than the mold width direction of the molten steel flow velocity is the average velocity, and the flow of molten steel to accelerate the mold short side near a slower portion than its average speed Provided is a continuous casting method of steel characterized by controlling .

  According to the present invention, according to the molten steel flow distribution in the mold, specifically, the molten steel flow velocity in the mold width direction in the mold is decelerated near the average speed, for example, in the vicinity of the immersion nozzle, Since both the frequency of the applied magnetic field and the applied magnetic field intensity from the electromagnetic flow control device are controlled so as to accelerate near the average speed, for example, near the short side of the mold, adjust the moving speed of the magnetic field according to the frequency. In addition, by selectively adjusting the flow control region and the activation region, the strength of the molten steel flow is adjusted by the applied magnetic field strength, so the strength of the molten steel flow is adjusted. It becomes possible to control appropriately. Therefore, even when it is difficult to optimize the flow of molten steel in the past, such as when the width of the mold is wide, it is possible to properly control the flow of molten steel in the mold, and to produce a high-quality slab. Can do.

Embodiments of the present invention will be described below.
First, referring to FIG. 1, an electromagnetic flow control device of a type in which a magnetic field applied to molten steel is used in molten steel flow control in continuous casting of steel and moves in a direction along the long side of the mold (mold width direction) and An outline of the molten steel flow control mechanism will be described. Note that the subscripts X, Y, and Z of physical quantities shown in the respective expressions in the following description indicate those in the X, Y, and Z directions in FIG.

  As shown in FIG. 1, this type of electromagnetic flow control device 10 is generally arranged at a position where a discharge flow is discharged from an immersion nozzle 3 in a rectangular mold 2 having a long side 2a and a short side 2b. A plurality of electromagnetic coils 1 are arranged side by side (in the width direction) along the long side 2a of the rectangular mold 2, and by shifting the phase of the current flowing through the adjacent coils, a so-called linear type A moving magnetic field is generated. The immersion nozzle 3 is provided at the mold width direction center along the long side 2 a of the mold 2.

The moving speed of the magnetic field can be expressed as the following equation (1) by the pole pitch (distance from the S pole to the N pole) τ of the coil 1 and the frequency f of the magnetic field generated from the electromagnetic coil 1. In the following expressions, the subscripts X, Y, and Z of each physical quantity indicate those in the X direction, Y direction, and Z direction in FIG.
V _x = 2τf (1)
Further, the applied magnetic field B _y as shown in Figure 1, is applied in a direction passing through the mold 2 in the direction of the short side, therefore, from the Lorentz's law, the induced current is to be expressed by the following equation (2) it can.
J _z = σV _x B _y (2)
Furthermore, the electromagnetic force can be expressed by the following equation (3), which indicates that the electromagnetic force mainly works in the same direction as the magnetic field moving direction.
F _x = J _z B _y = 2τσfB _y ² (3)

However, since there is actually a molten steel flow velocity u in the mold, the effective velocity of the magnetic field should be considered relative to the molten steel flow velocity. More precisely, the above equation (3) is expressed by the following (4 ) Expression.
F _x = J _z B _y = σ (V _x −u) B _y ² = σ (2τf−u) B _y ² (4)

  In such an electromagnetic flow control device 10, when the casting speed is high and it is desired to suppress the flow of molten steel in the mold 2, the magnetic field is moved from the short side 2 b side of the mold 2 toward the immersion nozzle 3 and discharged. It acts to suppress (decelerate) the flow. On the other hand, when the casting speed is low, such as when the pan is replaced, the magnetic field is moved from the immersion nozzle 3 toward the short side 2b to accelerate the discharge flow from the immersion nozzle 3 and activate the flow of molten steel in the mold 2. The effect of promoting heat supply can be exhibited. Alternatively, the magnetic field movement direction is reversed between the front surface and the rear surface of the long side 2a of the mold 2 so that the magnetic field rotates, and the cleaning effect by swirling the molten steel can be obtained.

  FIG. 2 shows a schematic diagram of a flow pattern in a mold that has been generally observed. 2A is a horizontal sectional view of the mold, and FIG. 2B is a vertical sectional view. In this case, the molten steel flow velocity u is substantially uniform in the width direction, the above equation (4) is easy to understand, and the magnetic field is directed from the short side 2b of the mold 2 to the immersion nozzle 3 with respect to the discharge flow 4. By moving in the direction, the desired molten steel flow braking effect can be obtained. Reference numeral 5 is a mold powder, and 6 is a solidified shell. FIG. 3 shows an image of the action of the molten steel flow braking effect of the electromagnetic flow control device in this case. As shown in this figure, the molten steel flow velocity is decelerated by the moving magnetic field regardless of the position.

  However, it is known that a flow pattern as shown in FIG. 4 also appears depending on the balance of casting conditions (casting width, argon gas flow rate blown into the molten steel flow from the immersion nozzle, casting speed). 4A is a horizontal sectional view of the mold, and FIG. 4B is a vertical sectional view. In particular, such a flow pattern is often observed when casting is performed using a wide mold which is implemented as a measure for improving productivity in recent years. In such a case, in the vicinity of the immersion nozzle 3, it is desired to suppress the flow of the molten steel in order to prevent the mold powder 5 from being scraped by the molten steel flow velocity and the entrainment of the floating powder on the molten metal surface. In the vicinity of the short side 2b, it is desired to activate the flow in order to prevent the mixture of unmelted powder or the bite due to insufficient heat supply.

  In this case, if the moving speed of the magnetic field can be appropriately selected according to the flow state of the molten steel, that is, if the molten steel flow can be suppressed in the slow portion and the molten steel flow can be activated in the slow portion, the mold The molten steel flow inside can be controlled appropriately.

In order to realize this, in the present embodiment, in addition to controlling the applied magnetic field strength applied from the electromagnetic flow control device 10, the frequency of the applied magnetic field strength is controlled. That, and the strength of inhibition of the molten steel flow by the magnetic field, to adjust the intensity to activate the increase or decrease of the applied magnetic field intensity (i.e. magnetic flux density) B _y is required, whereas, shown in equation (1) Thus, since the moving speed V _x of the magnetic field is proportional to the frequency f of the applied magnetic field, the moving speed of the magnetic field can be changed by changing the frequency f, and thereby the moving speed of the magnetic field depends on the flow state of the molten steel. Can be selected appropriately.

  FIG. 5 shows a control image diagram for controlling the flow pattern of FIG. 4 using the molten steel flow control of the present embodiment. By controlling the frequency of the applied magnetic field and adjusting the moving speed of the magnetic field, the applied magnetic field strength is controlled to adjust the strength of suppressing molten steel flow and the strength of activation, as shown in FIG. The flow of molten steel near 3 is decelerated, and the flow of molten steel near the short side 2b of the mold 2 can be accelerated. The flow pattern after the control is as shown in FIG. 6A is a horizontal sectional view of the mold, and FIG. 6B is a vertical sectional view. If controlled as shown in FIG. 6, the molten steel flow 4 is formed uniformly throughout, and there is no strong flow in the vicinity of the molten metal surface, so there is no danger of entrainment of the mold powder 5 and the mold 2. Sufficient molten steel flow is generated in the vicinity of the short side 2b and sufficient heat is supplied, and there is no risk that the powder remains unmelted or no bite is generated.

  Next, a specific apparatus configuration for realizing such control will be described. FIG. 7 is a horizontal sectional view showing a continuous casting apparatus including the electromagnetic flow control apparatus 10a. As described above, the immersion nozzle 3 is arranged in the center of the rectangular mold 2 having the long side 2a and the short side 2b. As in the electromagnetic flow control device 10 described in FIG. 1, the electromagnetic flow control device 10a is disposed at a position where the discharge flow is discharged from the immersion nozzle 3 of the mold 2, and the plurality of electromagnetic coils 1 are rectangular. So as to generate a so-called linear type moving magnetic field by shifting the phase of the current flowing through the adjacent coils along the two long sides 2a of the mold 2 (in the width direction). It has become. The plurality of electromagnetic coils 1 on both sides are respectively attached to the yoke 7 and integrated. The number of electromagnetic coils 1 on one side is twelve, and twelve coils are similarly arranged on the other side, and the electromagnetic coils on both sides are provided facing each other. In addition, the arrow in a figure has shown the direction of the magnetic field in a certain moment.

Power is supplied to these electromagnetic coils 1 from the three-phase AC power supply 11 so that the phases of the adjacent electromagnetic coils 1 are shifted by 120 °. In the middle of the wiring extending from the three-phase AC power supply 11, a current value control unit 12 for controlling the current value of the AC current and a frequency control unit 13 for controlling the frequency are provided, and the AC supplied to the electromagnetic coil 1 by these. The current value and frequency of the current can be controlled. The current value control and the frequency control by the current value control unit 12 and the frequency control unit 13 can be operated by an operator according to the operating conditions . Further, the number of arrangement of the electromagnetic coils 1 is not limited to 12, and the applied current may be two-phase alternating current as well as three-phase alternating current as long as the phase of the applied current can be shifted by the adjacent electromagnetic coils. For example, the number of arrangement of electromagnetic coils is 10 and two-phase alternating current is used, and the adjacent electromagnetic coils are 90 ° out of phase.

  In the continuous casting apparatus configured as described above, molten steel stored in a tundish from a ladle (not shown) is poured into the mold 2 from the immersion nozzle 3. In controlling the molten steel flow at that time, the value of the current supplied to the electromagnetic coil 1 of the electromagnetic flow control device 10a is controlled by the current value control unit 12, thereby controlling the applied magnetic field strength generated by the electromagnetic coil 1. be able to. Further, the moving speed of the magnetic field can be controlled by controlling the frequency of the current by the frequency control unit 13 and controlling the frequency of the magnetic field applied from the electromagnetic coil 1. Therefore, by these, appropriate molten steel flow control as described above can be performed.

[Example 1]
In order to confirm the effect of the present invention, an aluminum killed steel casting experiment was conducted with a slab continuous casting machine. The mold thickness (short side) is 220 mm, the mold width (long side) is 1900 mm, and the throughput is 4 ton / min. As the immersion nozzle, a two-hole nozzle whose bottom shape is a pool type, a discharge angle of 25 degrees downward, and a nozzle inner diameter D of 90 mmφ were used. The amount of argon gas blown from the nozzle was 9 NL / min. The electromagnetic coil was a linear moving magnetic field type that was fed with a three-phase alternating current, and had a length of 2100 mm and sufficiently covered the mold length. The pole pitch τ of the electromagnetic coil was 0.28 m. The electromagnetic coil was installed so that the center was at a depth of 380 mm from the molten metal surface.

  Here, casting was performed with the frequency fixed and only the magnetic field strength changed. Subsequently, casting was performed by changing both the frequency and the magnetic field strength. In order to detect the state of the hot water surface, a thermocouple was immersed at a position 100 mm from the short side and 200 mm from the nozzle, and the temperature was measured. According to past knowledge, it is known that the higher the temperature of the thermocouple, the higher the flow rate value of the molten steel.

  The transition of the thermocouple temperature on the short side of the mold and the thermocouple temperature on the immersion nozzle side is plotted in FIG. 8 together with the frequency and magnetic field strength of the applied magnetic field. When no magnetic field is applied, the temperature on the mold short side is low and the temperature on the immersion nozzle side is high, and it is estimated that the discharge flow is sufficiently strong and does not reach the short side as shown in FIG. can do.

For this reason, the mode in which the magnetic field moves from the nozzle side to the short side is set, the frequency of the applied magnetic field is set to 2 Hz, the current value is controlled, and the applied magnetic field strength is gradually increased. Thereby, as shown in FIG. 8, it can be estimated that the temperature rises on both the mold short side and the immersion nozzle side, and the molten steel flow is accelerated. The applied magnetic field strength is about 5 × 10 ⁻² T (500 Gauss), and the temperatures on the mold short side and the immersion nozzle side are almost equal, but the temperature level is high, and it is estimated that the temperature is uniform at a high flow rate. be able to.

Next, the frequency of the applied magnetic field was set to 1 Hz, and the applied magnetic field strength was gradually decreased from about 5 × 10 ⁻² T (500 Gauss) to about 1 × 10 ⁻² T (100 Gauss). On the short side of the mold, the temperature increased (accelerated) as the applied magnetic field strength increased as in the case of 2 Hz. However, the temperature was almost constant in the vicinity of the immersion nozzle, and the effect of applying the magnetic field was not observed.

Furthermore, the frequency of the applied magnetic field was set to 0.5 Hz, and the applied magnetic field intensity was gradually increased from about 1 × 10 ⁻² T (100 Gauss). The temperature rises (acceleration effect) on the short side of the mold, and the temperature drops (deceleration effect) on the immersion nozzle side. As the applied magnetic field strength increases, both temperatures approach and are approximately equal to about 5 × 10 ⁻² T (500 Gauss). became. At this time, the molten metal surface temperature was stable at an intermediate level, the molten steel flow velocity was not too fast, and the risk of powder entrainment was estimated to be small.

  Teshima et al .: Iron and Steel, 79 (1993), p. According to 576 to 582, the trajectory of the discharge flow and the flow velocity value of the molten steel at each position can be estimated. When this is applied to the main casting experiment, the average value in the width direction of the discharge flow velocity is 41 cm / s. The discharge flow is faster than the average flow velocity within 300 mm from the center of the immersion nozzle, but is slower than the average flow velocity on the shorter side than 300 mm. When the frequency of the supply current is 2 Hz, the magnetic field moving speed is 1.12 m / s, which is faster than the molten steel discharge flow, so that it can be estimated that there was an acceleration effect in the entire mold width region. When the applied magnetic field frequency was 1 Hz, the magnetic field moving speed was 0.56 m / s, and an acceleration effect was seen on the short side of the mold, but the magnetic field moving speed was almost equal to the molten steel flow velocity near the immersion nozzle. Therefore, it is considered that neither the acceleration effect nor the deceleration effect worked because the relative speed between them was small and the generated electromagnetic force was small. When the frequency is 0.5 Hz, the magnetic field moving speed is 0.28 m / s, and the moving speed of the magnetic field is small. Therefore, in the vicinity of the immersion nozzle, the relative speed is in the direction from the short side of the mold to the immersion nozzle as shown in FIG. Therefore, a deceleration effect was obtained, but it is considered that the acceleration effect worked near the short side of the mold.

The result of investigating the defect occurrence rate of the slab after casting is shown in FIG. Here, the slab defect occurrence rate was evaluated by counting the number of slab defects with an optical inspection device provided on the exit side of the continuous casting apparatus, and evaluating the defect rate per unit area of the slab. As it is apparent from this figure, that the applied magnetic field strength at frequency 0.5Hz is ^{4 × 10 -2 ~5 × 10 -2} T (400~500Gauss) about defect rate when the supply current is low confirmed.

  From the above, it was confirmed that the flow of molten steel, which cannot be controlled only by changing the applied magnetic field intensity, can be optimized by simultaneously changing the frequency of the applied magnetic field, and a high-quality slab can be manufactured.

  According to the present invention, it is possible to optimize the molten steel flow regardless of the molten steel flow distribution in the mold. Therefore, when the width of the mold is wide, conventionally, it is difficult to optimize the molten steel flow. In addition, the molten steel flow in the mold can be appropriately controlled, and a high quality slab can be manufactured.

The schematic block diagram which shows the electromagnetic flow control apparatus of the type which the magnetic field applied to molten steel moves to the direction (mold width direction) along the long side of a casting_mold | template. The horizontal sectional view and the vertical sectional view which show the molten steel flow pattern in the mold generally observed conventionally. The image figure which shows the state at the time of applying the conventional electromagnetic flow control at the time of the flow pattern shown in FIG. The horizontal sectional view and vertical sectional view which show the molten steel flow pattern in a casting_mold | template easy to be observed when it casts using a wide casting_mold | template. The image figure which shows the control state at the time of providing the electromagnetic flow control of this invention at the time of the flow pattern shown in FIG. FIG. 6 is a horizontal sectional view and a vertical sectional view showing a flow pattern after performing the electromagnetic flow control of the present invention shown in FIG. 5 during the flow pattern shown in FIG. 4. The horizontal sectional view showing the continuous casting device concerning one embodiment of the present invention. The graph which shows the relationship between the applied magnetic field intensity by an electromagnetic flow control apparatus, the frequency of an applied magnetic field, and the mold short side temperature and immersion nozzle side temperature in the vicinity of a molten metal surface. The graph which shows the relationship between the applied magnetic field intensity by the electromagnetic flow control apparatus, the frequency of an applied magnetic field, and the defect occurrence rate of a slab.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1; Electromagnetic coil 2; Mold 2a; Long side 2b; Short side 3; Immersion nozzle 4; Discharge flow 5; Mold powder 6; Solidification shell 7; Yoke 10, 10a; Current value control unit 13; frequency control unit

Claims (1)

A plurality of electromagnetic coils arranged on both sides in the mold width direction while supplying molten steel into a rectangular mold having a long side and a short side through an immersion nozzle provided in the center of the mold width direction, and these An AC power source that feeds power to the plurality of electromagnetic coils, a current value control unit that controls a current value supplied to the electromagnetic coil, and a frequency control unit that controls the frequency of the current supplied to the electromagnetic coil, This is a continuous casting method of steel in which the flow of molten steel in a mold is controlled by an electromagnetic flow control device that applies a moving magnetic field in which the moving direction of the magnetic field is in the mold width direction along the long side to the molten steel in the mold. And
When the molten steel flow velocity in the mold width direction in the mold ascertained in advance is faster than the average speed in the vicinity of the immersion nozzle, and slower than the average speed in the vicinity of the mold short side,
The phase of alternating current supplied to the plurality of electromagnetic coils is adjusted so that the moving direction of the moving magnetic field is from the immersion nozzle side to the short side, and the moving speed of the moving magnetic field is the molten steel in the mold width direction. Control the frequency of the current by the frequency control unit to be smaller than the average speed of the flow velocity, and control the current value supplied to the electromagnetic coil by the current value control unit to control the applied magnetic field strength,
The flow of the molten steel is caused to slow down in the vicinity of the immersion nozzle, where the molten steel flow velocity in the mold width direction is faster than the average speed, and to accelerate in the vicinity of the mold short side, which is a portion slower than the average speed. A continuous casting method of steel characterized by controlling.

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