JP7247777B2 - Steel continuous casting method - Google Patents

Description

The present invention relates to a method of continuously casting steel while controlling the flow of molten steel supplied into a mold using an electromagnetic coil device capable of both electromagnetic stirring and electromagnetic braking.

Continuous casting of steel generally uses a submerged nozzle with two outlets to feed molten steel into a mold. FIG. 4 schematically shows the flow state of molten steel in the mold in this general continuous casting method. As shown in FIG. 4, the molten steel 2 supplied into the mold 3 from the discharge port 1a of the submerged nozzle 1 collides with the short side 3a of the mold 3 and then branches into an upward flow 2a and a downward flow 2b. Among them, the ascending flow 2a becomes a horizontal flow toward the submerged nozzle 1 at the meniscus position (the molten steel surface position in the mold) 4 . In addition, 5 in FIG. 4 indicates a powder, and 6 indicates a solidified shell.

The control of the molten steel flow in the mold is extremely important in terms of operation and quality control of the slab. There are ways to make it work.

Among these methods, the method of applying electromagnetic force to the molten steel is widely used. It is divided into

The electromagnetic brake, as shown in FIG. This is a method of applying a Lorentz force F in the opposite direction to the flow of the molten steel 2 discharged into the chamber 3 . In addition, the arrow in FIG. 5 indicates the flow direction of the molten steel 2 .

The static magnetic field brakes the flow of the molten steel 2 discharged from the immersion nozzle 1 into the mold 3, suppresses uneven solidification of the solidified shell 6, and prevents the solidified shell 6 from remelting. Suppression of non-uniform solidification of the solidified shell is effective in suppressing cracks on the slab surface, and suppression of remelting of the solidified shell is effective in preventing breakout due to insufficient thickness of the solidified shell. A breakout is a trouble in which the solidified shell breaks and molten steel leaks out, making it impossible to continue continuous casting.

On the other hand, in the electromagnetic stirring, as shown in FIG. 6, an alternating current is applied to the electromagnetic coil 7 to generate a magnetic field that moves in one direction along the long side 3b of the mold 3, and the molten steel near the meniscus position 4 is stirred. 2, a Lorentz force F in one direction is applied along the long side 3 b of the mold 3 .

At this time, the direction of the moving magnetic field formed on the fixed side and the free side of the mold is opposite to each other. Form. Here, the fixed side of the mold refers to the outer side of the circular arc in a vertical bending type or bending type continuous casting machine, and the free side refers to the inner side of the circular arc, specifying the thickness direction of the mold.

The molten steel flow swirling near the meniscus position in the mold has a washing effect on inclusions such as air bubbles and powder trapped at the interface of the solidified shell. Play. The washing effect refers to the effect that bubbles and inclusions are less likely to adhere when there is flow at the solidified shell-molten steel interface.

Electromagnetic brakes and electromagnetic stirring each have their advantages and disadvantages. Electromagnetic stirring is used during low-throughput casting (low-speed casting) where the amount of supply is small.

These electromagnetic brakes and electromagnetic coil devices used for electromagnetic stirring usually have only one function.

Therefore, an electromagnetic coil device (hereinafter referred to as a dual-purpose coil device) capable of performing both functions of electromagnetic braking and electromagnetic stirring, such as those disclosed in Patent Documents 1 and 2, has been developed. was done.

By switching the current application conditions, this dual-purpose coil device can apply electromagnetic stirring at the meniscus position and electromagnetic braking at the discharge port of the submerged nozzle during continuous casting using the same coil. be.

With this dual-purpose coil device, it has become possible to enjoy the benefits of switching between electromagnetic stirring and electromagnetic braking during continuous casting, improving quality and operability, and making equipment more compact.

In continuous casting using a dual-purpose coil device, considering the advantages and disadvantages of electromagnetic braking and electromagnetic stirring, as in Patent Document 3, electromagnetic stirring and the electromagnetic brake, but there are the following problems when switching by this method.

Electromagnetic stirring accelerates or cancels the upward flow of molten steel discharged from the submerged nozzle toward the submerged nozzle to create a flow in the opposite direction, thereby forming a swirling flow in the cross section of the mold. This swirling flow increases the molten steel flow velocity at the meniscus position, prevents air bubbles and inclusions from being captured by the solidified shell, and suppresses skinning at the meniscus position that occurs when the molten steel temperature is low. Effective in preventing surface flaws.

However, in the application of electromagnetic stirring when a large amount of molten steel is supplied to the mold, when the temperature of the molten steel is high, such as immediately after continuous casting, the stirring flow promotes remelting of the solidified shell. If the redissolution of the solidified shell is accelerated, the thickness of the solidified shell becomes insufficient and the risk of breakout increases. Continuous casting refers to continuous casting of multiple heats without interrupting continuous casting. Immediately after continuous casting, the molten steel temperature is high to compensate for the drop in molten steel temperature during casting. .

On the other hand, the electromagnetic brake reduces the flow velocity of the molten steel discharged from the immersion nozzle, thereby reducing the flow velocity of the molten steel colliding with the solidified shell, mitigating cracks caused by uneven solidification, and suppressing the remelting of the solidified shell. It is effective in preventing breakout due to In particular, when the temperature of the molten steel is high, non-uniform solidification and remelting of the solidified shell become significant, so the electromagnetic brake becomes even more important.

However, when the electromagnetic brake is applied when the amount of molten steel supplied to the mold is small, when the temperature of the molten steel is low, skinning occurs at the meniscus position due to insufficient heat flux at the meniscus position, increasing inclusions and bubble defects. leads to

Japanese Patent No. 5023989 Japanese Patent No. 5023990 Japanese Patent No. 4967856

The problem to be solved by the present invention is that, when continuously casting steel using a dual-purpose coil device, if switching between electromagnetic stirring and electromagnetic braking is performed only by the amount of molten steel to be supplied, a sufficient effect may not always be obtained depending on the operating conditions. The point is that it cannot be obtained.

The steel continuous casting method of the present invention comprises:
The purpose is to optimally switch the dual-purpose coil device.
For example, as shown in FIG.
Two magnetic pole iron cores 8a, two coils 8b wound around the outer circumference of each of the magnetic pole iron cores 8a, and one coil wound around the outer circumference of the combined two magnetic pole iron cores 8a. 8c are arranged on the outer periphery of the long side 3b of the mold in the same number on each long side 3b, and (2n + 2) (n is a natural number) in total on the outer periphery of the long side 3b of the mold,
When the molten steel 2 in the mold is electromagnetically stirred, the coils 8b and 8c of all the electromagnetic coils are energized with a multiphase alternating current of three or more phases with a current phase difference of 90 degrees to 120 degrees,
When the electromagnetic brake is applied to the molten steel 2 in the mold, a DC current is applied to one coil 8c wound around the outer circumference of the two magnetic pole iron cores 8a for each electromagnetic coil. Alternatively, it is a continuous casting method of steel using a dual-purpose coil device 8 in which direct current is applied to these three coils 8b and 8c.

And in the present invention,
The chemical composition of the molten steel supplied to the mold, the degree of superheat ΔT (°C, ΔT≧0) of the molten steel in the tundish, the casting speed Vc (m/min, Vc>0), and the immersion depth of the immersion nozzle X (mm, The most important feature is to selectively switch between electromagnetic stirring or electromagnetic braking depending on X>0). In the present invention, the immersion depth of the submerged nozzle refers to the distance from the meniscus position to the center position of the discharge port of the submerged nozzle in the casting direction.

Here, the degree of superheat of the molten steel in the tundish is the difference obtained by subtracting the liquidus temperature from the temperature of the molten steel in the tundish. is an important control factor that affects

More specifically, according to the chemical composition of the molten steel supplied to the mold, the degree of superheat ΔT (°C) of the molten steel in the tundish, the casting speed Vc (m/min) and the immersion depth X (mm) of the immersion nozzle, Determine as follows.

(When the component carbon concentration of the molten steel supplied to the mold is 0.07% by mass or more and 0.18% by mass or less)
When the immersion depth X of the immersion nozzle is less than 230 mm, electromagnetic stirring is applied to the area where the following formula (1) holds, and electromagnetic braking is applied to the area where the following formula (1) does not hold (Fig. 2 (a)).
“ΔT−8≦−1.14Vc” ∩ “Vc≦1.74” (1)

In addition, when the immersion depth X of the immersion nozzle is 230 mm or more, electromagnetic stirring is applied to the area where the following formula (2) holds, and electromagnetic braking is applied to the area where the following formula (2) does not hold ( See FIG. 3(a)).
"ΔT-8≦-1.14Vc" ∩ "Vc≦1.74" or "ΔT≦40" ∩ "Vc≦1.14" or "ΔT≦33" ∩ "Vc≦1.24" (2)

(When the component carbon concentration of the molten steel supplied to the mold exceeds 0.0050% by mass and is less than 0.07% by mass)
When the immersion depth X of the immersion nozzle is less than 230 mm, electromagnetic stirring is applied to the area where the following formula (3) holds, and electromagnetic braking is applied to the area where the following formula (3) does not hold (Fig. 2 (b)).
“ΔT−12≦−1.46Vc” ∩ “Vc≦2.04” (3)

In addition, when the immersion depth X of the immersion nozzle is 230 mm or more, electromagnetic stirring is applied to the area where the following formula (4) holds, and electromagnetic braking is applied to the area where the following formula (4) does not hold ( See FIG. 3(b)).
"ΔT-12≦-1.46Vc" ∩ "Vc≦2.04" or "ΔT≦40" ∩ "Vc≦1.24" or "ΔT≦35" ∩ "Vc≦1.44" (4)

(When the component carbon concentration of the molten steel supplied to the mold is 0.0050% by mass or less)
When the immersion depth X of the immersion nozzle is less than 230 mm, electromagnetic stirring is applied to the area where the following formula (5) holds, and electromagnetic braking is applied to the area where the following formula (5) does not hold (Fig. 2 (c)).
“ΔT−40≦−3.42Vc” ∩ “Vc≦1.74” (5)

In addition, when the immersion depth X of the immersion nozzle is 230 mm or more, electromagnetic stirring is applied to the area where the following formula (6) holds, and electromagnetic braking is applied to the area where the following formula (6) does not hold ( See FIG. 3(c)).
"ΔT-40≦-3.42Vc" ∩ "Vc≦1.74" or "ΔT≦40" ∩ "Vc≦1.34" (6)

According to the continuous casting method for steel of the present invention, it is possible to stably obtain slabs of good quality while stabilizing the continuous casting operation.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining an example of the combined coil apparatus used for the continuous casting method of the steel of this invention, (a) is a horizontal sectional view, (b) is a vertical sectional view. FIG. 10 is a diagram showing the application area of electromagnetic stirring and electromagnetic braking according to the present invention when the immersion depth of the immersion nozzle is less than 230 mm, wherein the component carbon concentration of the molten steel supplied to the mold is 0.07% by mass or more in (a). , 0.18% by mass or less, (b) is more than 0.0050% by mass and less than 0.07% by mass, and (c) is 0.0050% by mass or less. FIG. 3 is a view similar to FIG. 2 showing the application area of electromagnetic stirring and electromagnetic braking according to the present invention when the immersion depth of the immersion nozzle is 230 mm or more; 1 is a longitudinal sectional view schematically showing a general flow state of molten steel in a mold in continuous casting. FIG. Fig. 2 is a diagram schematically showing the flow state of molten steel in a mold when an electromagnetic brake is applied to apply a braking force to the molten steel discharged into the mold from an immersion nozzle; (a) is a plan view and (b) is a longitudinal section. It is a plan view. FIG. 2 is a diagram schematically showing the flow state of molten steel in a mold when electromagnetic stirring is applied to stir the molten steel discharged into the mold from an immersion nozzle, (a) is a plan view, and (b) is a vertical cross-sectional view. be.

The results of tests conducted by the inventors in order to solve the problems of the present invention will be described below, and the effects of implementing the present invention will also be described.

(Continuous casting of medium carbon steel with a carbon concentration of 0.09% by mass)
A. When the immersion depth of the immersion nozzle is 220 mm When electromagnetic stirring is applied under the conditions of a casting speed Vc of 0.80 m/min and a degree of superheat ΔT of the molten steel in the tundish of 15°C, a white line on the short side of the solidified shell is observed. , confirmed the deterioration of coagulation heterogeneity. The white line is a segregation line observed when the molten steel with concentrated segregation components between dendrite branches in front of the solidified shell is washed away by the molten steel flow in the mold, forming negative segregation. It is an index for estimating the thickness of the solidified shell of

In addition, when an electromagnetic brake is applied under the conditions that the casting speed Vc is 1.17 m/min and the degree of superheat ΔT of the molten steel in the tundish is 5°C, high-frequency ultrasonic flaw detection of the cast slab surface layer The increase in inclusions such as powder was confirmed by three-dimensional mapping of the distribution and diameter of bubbles and inclusions).

B. When the immersion depth of the immersion nozzle is 230 mm When electromagnetic stirring is applied under the conditions of a casting speed Vc of 1.23 m/min and a degree of superheat ΔT of the molten steel in the tundish of 33°C, no abnormalities occur on the slab surface layer. It was confirmed.

When the degree of superheat ΔT of the molten steel in the tundish is 33° C. and the casting speed Vc is 1.23 m/min, the number of inclusions on the surface of the slab to which electromagnetic stirring is applied is significantly higher than that of the slab to which electromagnetic brake is applied. was confirmed to be reduced to

(Continuous casting of low carbon steel with a carbon concentration of 0.04% by mass)
A. When the immersion depth of the immersion nozzle is 220 mm When electromagnetic stirring is applied under the conditions of a casting speed Vc of 1.46 m/min and a degree of superheat ΔT of the molten steel in the tundish of 27°C, a white line on the short side of the solidified shell is observed. , confirmed the deterioration of coagulation heterogeneity. In particular, the solidification delay at the slab corner portion was remarkable, and longitudinal cracks occurred at the slab corner portion.

In addition, when the casting speed Vc is 1.12 m/min and the degree of superheat ΔT of the molten steel in the tundish is 8°C, when the electromagnetic brake is applied, the high-frequency ultrasonic flaw detection of the surface layer of the cast slab shows that the electromagnetic stirring is applied. An increase in inclusions such as powder was confirmed.

B. When the immersion depth of the immersion nozzle is 230 mm When electromagnetic stirring is applied under the conditions of a casting speed Vc of 1.27 m/min and a degree of superheat ΔT of the molten steel in the tundish of 34°C, no abnormalities occur on the slab surface layer. It was confirmed.

On the other hand, when the molten steel superheat is the same and the casting speed Vc is 1.33 m/min, the number of inclusions on the surface of the slab to which electromagnetic stirring is applied is significantly lower than the number of inclusions on the surface of the slab to which electromagnetic braking is applied. confirmed. In addition, we rolled slabs to which electromagnetic brakes were applied and slabs to which electromagnetic stirring was applied, and compared the number of defects caused by air bubbles and inclusions on the surface after rolling. It was confirmed that there were few defects caused by material.

(Continuous casting of ultra-low carbon steel with a carbon concentration of 0.002% by mass)
A. When the immersion depth of the immersion nozzle is 220 mm When electromagnetic stirring is applied under the conditions of a casting speed Vc of 1.25 m/min and a degree of superheat ΔT of the molten steel in the tundish of 42°C, a white line on the short side of the solidified shell is observed. , confirmed the deterioration of coagulation heterogeneity.

In addition, when an electromagnetic brake is applied under the conditions of a casting speed Vc of 1.65 m/min and a degree of superheat ΔT of the molten steel in the tundish of 34°C, high-frequency ultrasonic flaw detection of the slab surface prevents inclusions such as powder from increasing. confirmed.

B. When the immersion depth of the immersion nozzle is 230 mm When electromagnetic stirring is applied under the conditions of a casting speed Vc of 1.33 m/min and a degree of superheat ΔT of the molten steel in the tundish of 40°C, no abnormalities occur on the slab surface layer. It was confirmed.

In addition, when the casting speed Vc was set to 1.30 m/min at the same molten steel superheating degree, the slab to which the electromagnetic brake was applied and the slab to which the electromagnetic stirring was applied were rolled. The number of defects was compared. As a result, it was confirmed that the slab to which electromagnetic stirring was applied had fewer defects due to bubbles and inclusions.

From the above results, when continuously casting steel using a dual-purpose coil device, switching between electromagnetic stirring and electromagnetic braking should be performed in consideration of the casting speed Vc, the degree of superheat ΔT of the molten steel, and the immersion depth X of the immersion nozzle. contributed to the improvement of production efficiency and quality improvement.

Therefore, the inventors select electromagnetic stirring and electromagnetic braking in continuous casting using a continuous casting machine in which two dual-purpose coil devices with the following specifications are arranged on each long side of the mold, a total of four. At this time, the test was conducted by changing the carbon concentration, which is a basic component of steel, the degree of superheat ΔT of the molten steel in the tundish, the casting speed Vc, and the immersion depth X of the immersion nozzle.

(Specifications of dual-purpose coil device)
Electromagnetic force at center of mold: 3000 Gauss
Frequency: 4.0Hz
Applied current to coil (magnetomotive force): 45000A
Phase of alternating current: 3-phase alternating current with 120° phase

The continuous casting machine used was a vertical bending type continuous casting machine capable of producing slabs with a width of 700 to 1625 mm and a thickness of 250, 270 mm. In the steel oxygen refining-vacuum degassing/decarburizing process, the technical limit of carbon concentration is 0.0005% by mass.

(Steel type A)
Steel type A is a hypoperitectic steel with a carbon concentration of 0.07% by mass or more and 0.18% by mass or less, and has a characteristic that uneven solidification easily occurs, causing troubles typified by breakout. big concern. On the other hand, since it is often used in building materials and automobile underbody parts, the standards for surface inspection are not as strict as those for ultra-low carbon steel, which will be described later.

Therefore, for steel type A, the index of the effect of the invention was set to 0.5 times/year or less for the occurrence rate of breakouts and 0.4% or less for the surface failure rate. Note that the surface rejection rate refers to the percentage of the weight that does not meet the required surface quality in the surface inspection of the coil, and the required surface quality differs depending on the steel type.

Table 2 below shows the results of casting steel type A while changing the degree of superheat ΔT of the molten steel in the tundish, the casting speed Vc, and the immersion depth X of the immersion nozzle.

For steel type A, according to the method disclosed in Patent Document 3, electromagnetic stirring is applied when the supply amount Q of molten steel is less than 3 t/min (1.03 m/min when converted to casting speed Vc). When the immersion depth X of the immersion nozzle is 195 mm, the casting speed Vc is 0.80 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 5° C. or 7° C., the surface drop rate is 0.5° C. when electromagnetic stirring is applied. 3% and 0.2% were effective in improving product yield (No. 1 and No. 2 in Table 2). On the other hand, when electromagnetic brakes were applied, the surface drop rates deteriorated to 0.9% and 0.6% (No.9 and No.10 in Table 2).

For steel type A, according to the method disclosed in Patent Document 3, an electromagnetic brake is applied when the supply amount Q of molten steel is 3 t/min (1.03 m/min when converted to casting speed Vc) or more. . When the immersion depth X of the immersion nozzle is 200 mm, the casting speed Vc is 1.17 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 7°C or 15°C, the occurrence rate of breakout is 0 when the electromagnetic brake is applied. .4 times/year and 0.5 times/year (No. 13 and No. 14 in Table 2). On the other hand, when electromagnetic stirring was applied, the incidence of breakout increased to 0.9 times/year and 1.4 times/year (No. 6 and No. 7 in Table 2). In this case, the face disqualification rate also deteriorated to 0.6% and 0.5%.

However, in the case of No. 3 in Table 2, where the immersion depth X of the immersion nozzle is 195 mm, the casting speed Vc is 0.80 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 15° C., breakout occurs. The incidence increased to 1.1 times/year. In the case of No. 12 in Table 2, where the immersion depth X of the immersion nozzle is 200 mm, the casting speed Vc is 1.17 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 5°C, the surface drop rate is 0. worsened to 8%.

On the other hand, when the immersion depth X of the immersion nozzle is 230 mm and the degree of superheat ΔT of the molten steel in the tundish is 33° C., even if electromagnetic stirring is applied at a casting speed Vc of 1.23 m/min (No. 8) Also, when the immersion depth X of the immersion nozzle is 255 mm, and the degree of superheat ΔT of the molten steel in the tundish is 40°C, even if electromagnetic stirring is applied at a casting speed Vc of 1.10 m/min (Table 2 No. 4 of No. 4), both of which had a surface rejection rate of 0.2%, and were effective in improving the product yield.

That is, in the continuous casting of steel type A, when switching between electromagnetic stirring and electromagnetic braking is performed only by the supply amount Q of molten steel as in the method disclosed in Patent Document 3, depending on the degree of superheat ΔT of the molten steel in the tundish, In some cases, surface drop rates worsened and breakout rates increased.

On the other hand, when the immersion depth X of the immersion nozzle is 195 mm, 200 mm, and 205 mm, in the continuous casting of steel type A, using the degree of superheat ΔT of the molten steel in the tundish and the casting speed Vc, "ΔT -8 ≤ - 1.14Vc"∩"Vc≤1.74", when switching between electromagnetic stirring and electromagnetic braking, as shown in Table 2, the surface drop rate deteriorates and breakouts occur. rate did not increase.

That is, for No. 1 and No. 2 in Table 2 where the immersion depth X of the immersion nozzle is less than 230 mm, "ΔT - 8 ≤ -1.14 Vc" ∩ "Vc ≤ 1.74" is established, so the patent document Similar to the technique disclosed in 3, electromagnetic stirring is applied. However, when the degree of superheat ΔT of the molten steel in the tundish is 15 (° C.), the relationship “ΔT−8≦−1.14Vc”∩“Vc≦1.74” does not hold, so in the present invention, an electromagnetic brake is applied. (No. 11 in Table 2). In the case of No. 11 in Table 2, the occurrence rate of breakouts could be suppressed to 0.4 (times/year), which was effective for stable casting.

In addition, in No. 13 and No. 14 in Table 2, where the immersion depth X of the immersion nozzle is less than 230 mm, "ΔT - 8 ≤ -1.14 Vc" ∩ "Vc ≤ 1.74" does not hold. Similar to the method disclosed in 3, an electromagnetic brake is applied. However, when the degree of superheat ΔT of the molten steel in the tundish is 5°C, "ΔT - 8 ≤ -1.14 Vc" ∩ "Vc ≤ 1.74" is established, so in the present invention, electromagnetic stirring is applied (Table 2 No. 5). In the case of No. 5 in Table 2, the surface rejection rate could be suppressed to 0.2%, which was effective in improving product yield.

On the other hand, No. 4 in Table 2 when the immersion depth X of the immersion nozzle is 230 mm or more does not satisfy "ΔT - 8 ≤ -1.14 Vc" ∩ "Vc ≤ 1.74", but "ΔT ≤ 40". Since ∩“Vc≦1.14” holds, electromagnetic stirring is applied. Similarly, in No. 8 of Table 2, "ΔT - 8 ≤ -1.14Vc" ∩ "Vc ≤ 1.74" does not hold, but "ΔT ≤ 33" ∩ "Vc ≤ 1.24" holds. , apply electromagnetic stirring instead of applying an electromagnetic brake as in No.15.

(Steel type B)
Steel type B is a low-carbon steel with a carbon concentration of 0.01% by mass or more and 0.065% by mass or less. Compared to hypo-peritectic steel, non-uniform solidification is less likely to occur, so there is little concern in operation. In addition, since it is often used in building materials and automobile underbody parts, the standards for surface inspection are not as strict as those for ultra-low carbon steel, which will be described later.

Therefore, for steel type B, the index of the effect of the invention was set to 0.4 times/year or less for the incidence rate of breakouts and 0.4% or less for the surface failure rate.

Table 3 below shows the results of casting steel type B while changing the degree of superheat ΔT of the molten steel in the tundish, the casting speed Vc, and the immersion depth X of the immersion nozzle.

For steel type B, according to the method disclosed in Patent Document 3, electromagnetic stirring is applied when the supply amount Q of molten steel is less than 4 t/min (1.40 m/min when converted to casting speed Vc). When the immersion depth X of the immersion nozzle is 195 mm, the casting speed Vc is 1.12 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 8°C and 10°C, the surface drop rate is 0 when electromagnetic stirring is applied. .4% was effective in improving product yield (No. 1 and No. 2 in Table 3). On the other hand, when electromagnetic brakes were applied, the surface drop rates deteriorated to 1.2% and 0.9% (No.11 and No.12 in Table 3).

For steel type B, according to the method disclosed in Patent Document 3, an electromagnetic brake is applied when the supply amount Q of molten steel is 4 t/min (1.40 m/min when converted to casting speed Vc) or more. . When the immersion depth X of the immersion nozzle is 195 mm, the casting speed Vc is 1.65 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 10°C or 13°C, the occurrence rate of breakout is 0 when the electromagnetic brake is applied. .2 times/year and 0.4 times/year (No. 16 and No. 17 in Table 3). On the other hand, when electromagnetic stirring was applied, when the degree of superheat ΔT of the molten steel was 13°C, the breakout rate increased to 0.5 times/year, and the surface drop rate worsened to 0.5% ( No. 10 in Table 3). Further, when the degree of superheat ΔT of the molten steel was 10° C., the breakout incidence rate remained unchanged at 0.2 times/year, but the surface drop rate deteriorated to 0.5% (No .9). Also, when the immersion depth X of the immersion nozzle is 220 mm, the casting speed Vc is 1.46 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 27° C., the occurrence rate of breakout is 0 when electromagnetic stirring is applied. .5 times/year (No.7 in Table 3).

However, in the case of No. 3 in Table 3, in which the immersion depth X of the immersion nozzle is 195 mm, the casting speed Vc is 1.12 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 13° C. in steel type B, the surface is degraded. rate deteriorated to 0.5%. In the case of No. 15 in Table 3, where the immersion depth X of the immersion nozzle is 195 mm, the casting speed Vc is 1.65 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 8°C, the surface drop rate is 0. worsened to 6%.

On the other hand, when the immersion depth X of the immersion nozzle is 230 mm and the degree of superheat ΔT of the molten steel in the tundish is 34° C., even if the casting speed Vc is 1.27 m/min and 1.33 m/min, electromagnetic stirring is applied. Also, when the degree of superheat ΔT of the molten steel in the tundish is 40°C, even if electromagnetic stirring is applied at a casting speed Vc of 1.24 m/min, the surface drop rates are 0.2% and 0.3%, respectively. (No.5, No.6, No.4 in Table 3).

That is, even in the continuous casting of steel type B, when switching between electromagnetic stirring and electromagnetic braking is performed only by the supply amount Q of molten steel as in the method disclosed in Patent Document 3, depending on the degree of superheat ΔT of the molten steel in the tundish, , there were cases where the surface drop rate worsened and the incidence of breakouts increased.

On the other hand, when the immersion depth X of the immersion nozzle is 195 mm and 220 mm, in the continuous casting of steel type B, using the degree of superheat ΔT of the molten steel in the tundish and the casting speed Vc, "ΔT -12 ≤ -1. 46Vc" ∩ "Vc ≤ 2.04", when switching between electromagnetic stirring and electromagnetic braking, as shown in Table 3, the surface drop rate worsens and the breakout rate increases. never increased.

That is, for No. 1 and No. 2 in Table 3, where the immersion depth X of the immersion nozzle is less than 230 mm, "ΔT-12 ≤ -1.46 Vc" ∩ "Vc ≤ 2.04" is established. Similar to the technique disclosed in 3, electromagnetic stirring is applied. However, when the degree of superheat ΔT of the molten steel in the tundish is 13°C, "ΔT -12 ≤ -1.46 Vc" ∩ "Vc ≤ 2.04" does not hold, so in the present invention, an electromagnetic brake is applied (Table 3 No. 13). In the case of No. 13 in Table 3, the surface rejection rate could be suppressed to 0.2%, which was effective in improving product yield.

In addition, No. 13, No. 16, and No. 17 in Table 3, where the immersion depth X of the immersion nozzle is less than 230 mm, "ΔT - 12 ≤ -1.46 Vc" ∩ "Vc ≤ 2.04" does not hold. Therefore, an electromagnetic brake is applied in the same manner as the method disclosed in Patent Document 3. However, when the degree of superheat ΔT of the molten steel in the tundish is 8°C, "ΔT -12 ≤ -1.46 Vc" ∩ "Vc ≤ 2.04" is established, so in the present invention, electromagnetic stirring is applied (Table No. 8 of 3). In the case of No. 8 in Table 3, the surface rejection rate could be suppressed to 0.2%, which was effective in improving product yield.

On the other hand, No. 4 in Table 3, where the immersion depth X of the immersion nozzle is 230 mm or more, does not satisfy "ΔT - 12 ≤ -1.46 Vc" ∩ "Vc ≤ 2.04", but "ΔT ≤ 40". Since ∩“Vc≦1.24” holds, electromagnetic stirring is applied. Similarly, in No. 6 of Table 3, "ΔT - 12 ≤ -1.46Vc" ∩ "Vc ≤ 2.04" does not hold, but "ΔT ≤ 35" ∩ 'Vc ≤ 1.44' does. , applying electromagnetic stirring without applying an electromagnetic brake as in No. 14 of Table 3.

(steel grade C)
Steel type C is an ultra-low carbon steel with a carbon concentration of 0.0005% by mass or more and 0.0050% by mass or less. Due to its small size, it is prone to skinning in the mold. In addition, since it is often used as an outer panel material for automobiles, the standards for surface inspection are strict.

Therefore, for steel type C, the index of the effect of the invention was set to 0.2 times/year or less for the incidence rate of breakouts and 1.7% or less for the surface failure rate.

Table 4 below shows the results of casting steel type C while changing the degree of superheat ΔT of the molten steel in the tundish, the casting speed Vc, and the immersion depth X of the immersion nozzle.

For steel type C, according to the method disclosed in Patent Document 3, electromagnetic stirring is applied when the supply amount Q of molten steel is less than 5 t/min (1.74 m/min when converted to casting speed Vc). When the immersion depth X of the immersion nozzle is 200 mm, the casting speed Vc is 1.51 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 32° C. and 35° C., the surface drop rate is 0.5° C. when electromagnetic stirring is applied. 8% and 0.9% were effective in improving product yield (No. 4 and No. 5 in Table 4). On the other hand, when electromagnetic brakes were applied, the surface drop rate worsened to 2.2% and 1.9% (No.12 and No.13 in Table 4). Further, when the immersion depth X of the immersion nozzle is 220 mm, the casting speed Vc is 1.65 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 34° C., the surface drop rate is 0.9 when electromagnetic stirring is applied. % was effective in improving product yield (No. 7 in Table 4). On the other hand, when electromagnetic brakes were applied, the surface drop rate worsened to 1.9% (No. 15 in Table 4).

For steel type C, according to the method disclosed in Patent Document 3, an electromagnetic brake is applied when the supply amount Q of molten steel is 5 t/min (1.74 m/min when converted to casting speed Vc) or more. . When the immersion depth X of the immersion nozzle is 200 mm, the casting speed Vc is 1.74 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 35°C or 42°C, when the electromagnetic brake is applied, both breakout rates are reduced. It was possible to suppress it to 0.1 times/year, which was effective for stable casting (No. 17 and No. 18 in Table 4). On the other hand, when electromagnetic stirring was applied, the incidence of breakout increased to 0.4 times/year and 0.5 times/year (No. 9 and No. 10 in Table 4). When the degree of superheat ΔT of the molten steel was 42° C., the surface drop rate deteriorated to 2.5% (No. 10 in Table 4).

However, in the case of No. 6 in Table 4, in which the immersion depth X of the immersion nozzle is 200 mm, the casting speed Vc is 1.51 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 42° C., breakout occurs. The incidence worsened to 0.4%. In the case of No. 16 in Table 4, where the immersion depth X of the immersion nozzle is 200 mm, the casting speed Vc is 1.74 m/min, and the degree of superheat ΔT of the molten steel in the tundish is 32°C, the surface drop rate is 2. worsened to 7%.

On the other hand, when the immersion depth X of the immersion nozzle is 230 mm and the degree of superheat ΔT of the molten steel in the tundish is 40°C, even if electromagnetic stirring is applied at a casting speed Vc of 1.30 m/min, the surface drop rate is 1.0% was effective in improving product yield (No. 2 in Table 3).

That is, even in the continuous casting of steel type C, when switching between electromagnetic stirring and electromagnetic braking is performed only by the supply amount Q of molten steel as in the method disclosed in Patent Document 3, depending on the degree of superheat ΔT of the molten steel in the tundish, , there were cases where the surface drop rate worsened and the incidence of breakouts increased.

On the other hand, when the immersion depth X of the immersion nozzle is 220 mm, in continuous casting of steel type C, using the degree of superheat ΔT ° C. of the molten steel in the tundish and the casting speed Vc, "ΔT -40 ≤ -3.42 Vc "∩"Vc ≤ 1.74", when switching between electromagnetic stirring and electromagnetic braking, as shown in Table 4, the surface drop rate worsens and the breakout rate increases. never did.

That is, for No. 4 and No. 5 in Table 4, where the immersion depth X of the immersion nozzle is less than 230 mm, "ΔT-40 ≤ -3.42 Vc" ∩ "Vc ≤ 1.74" is established, so the patent document Similar to the technique disclosed in 3, electromagnetic stirring is applied. However, in the case of No. 1 and No. 6 in Table 4, where the degree of superheat ΔT of the molten steel in the tundish is 42°C, "ΔT - 40 ≤ -3.42Vc" ∩ "Vc ≤ 1.74" does not hold. , the present invention does not apply electromagnetic stirring, but applies an electromagnetic brake (No. 14 in Table 4). In the case of No. 14 in Table 4, the surface rejection rate could be suppressed to 1.1%, which was effective in improving product yield.

In addition, in No. 17 and No. 18 in Table 4, where the immersion depth X of the immersion nozzle is less than 230 mm, "ΔT - 40 ≤ -3.42 Vc" ∩ "Vc ≤ 1.74" does not hold. Similar to the method disclosed in 3, an electromagnetic brake is applied. However, when the degree of superheat ΔT of the molten steel in the tundish is 32° C., “ΔT−40≦−3.42Vc”∩“Vc≦1.74” is established, so electromagnetic stirring is applied in the present invention (Table 4 No. 8). In the case of No. 8 in Table 4, the surface rejection rate could be suppressed to 1.0%, which was effective in improving product yield.

On the other hand, in No. 2 and No. 3 in Table 4 where the immersion depth X of the immersion nozzle is 230 mm or more, "ΔT - 40 ≤ -3.42 Vc" ∩ "Vc ≤ 1.74" does not hold, Since "ΔT≦40"∩"Vc≦1.34" holds, electromagnetic agitation is applied instead of applying an electromagnetic brake as in No. 11 in Table 4.

From the above test results, when continuously casting steel using a dual-purpose coil device, by switching between electromagnetic stirring and electromagnetic braking depending on the casting speed, the immersion depth of the immersion nozzle, and the degree of superheat of the molten steel in the tundish, Even when the amount of molten steel supplied to the mold is small and the degree of superheat of the molten steel is high, such as immediately after the ladle is replaced, the occurrence rate of breakouts can be reduced. In addition, the surface drop rate can be reduced even when a large amount of molten steel is supplied to the mold at the end of casting and the degree of superheat of the molten steel is low.

Of course, the present invention is not limited to the above examples, and it goes without saying that the embodiments may be changed as appropriate within the scope of the technical idea described in each claim.

For example, the alternating current does not have to be three-phase, and if the current phase difference is from 90 degrees to 120 degrees, it may be more than that.

Moreover, the method of the present invention can be applied to any molten steel having a carbon concentration within the range defined in the claims, even if the molten steel does not have the component concentrations used in the above tests.

The above-described present invention can be applied to any type of continuous casting, such as curved type and vertical type, as long as it is continuous casting. Moreover, it can be applied not only to the continuous casting of slabs but also to the continuous casting of blooms.

2 molten steel 3 mold 3b long side 8 combined coil device 8a magnetic pole iron core 8b coil 8c coil

Claims (3)

An electromagnetic coil having two magnetic pole iron cores, two coils wound around the outer circumference of each of the magnetic pole iron cores, and one coil wound around the outer circumference of the two magnetic pole iron cores together. are arranged on the outer circumference of the long side of the mold in the same number on each long side, and (2n + 2) pieces (n is a natural number) in total on the outer circumference of the long side of the mold,
When the molten steel in the mold is electromagnetically stirred, a multiphase alternating current of three or more phases with a current phase difference of 90 degrees to 120 degrees is applied to each coil of all the electromagnetic coils,
When the electromagnetic brake is applied to the molten steel in the mold, a direct current is applied to each of the electromagnetic coils, and the coil is wound around the outer circumference of the two magnetic pole iron cores, or A continuous casting method for steel using an electromagnetic coil device for both electromagnetic stirring and electromagnetic braking, in which direct current is applied to these three coils,
The component carbon concentration of the molten steel supplied to the mold is set to 0.07% by mass or more and 0.18% by mass or less,
Assuming that the degree of superheat of the molten steel in the tundish is ΔT (°C, ΔT≧0) and the casting speed is Vc (m/min, Vc>0),
When the immersion depth X of the immersion nozzle is less than 230 mm, electromagnetic stirring is applied to the area where the following formula (1) holds, and an electromagnetic brake is applied to the area where the following formula (1) does not hold. When the immersion depth X is 230 mm or more, electromagnetic stirring is applied to the area where the following formula (2) holds, and electromagnetic braking is applied to the area where the following formula (2) does not hold. casting method.
“ΔT−8≦−1.14Vc” ∩ “Vc≦1.74” (1)
"ΔT-8≦-1.14Vc" ∩ "Vc≦1.74" or "ΔT≦40" ∩ "Vc≦1.14" or "ΔT≦33" ∩ "Vc≦1.24" (2) An electromagnetic coil having two magnetic pole iron cores, two coils wound around the outer circumference of each of the magnetic pole iron cores, and one coil wound around the outer circumference of the two magnetic pole iron cores together. are arranged on the outer circumference of the long side of the mold in the same number on each long side, and (2n + 2) pieces (n is a natural number) in total on the outer circumference of the long side of the mold,
When the molten steel in the mold is electromagnetically stirred, a multiphase alternating current of three or more phases with a current phase difference of 90 degrees to 120 degrees is applied to each coil of all the electromagnetic coils,
When the electromagnetic brake is applied to the molten steel in the mold, a direct current is applied to each of the electromagnetic coils, and the coil is wound around the outer circumference of the two magnetic pole iron cores, or A continuous casting method for steel using an electromagnetic coil device for both electromagnetic stirring and electromagnetic braking, in which direct current is applied to these three coils,
The component carbon concentration of the molten steel supplied to the mold is more than 0.0050% by mass and less than 0.07% by mass ,
Assuming that the degree of superheat of the molten steel in the tundish is ΔT (°C, ΔT≧0) and the casting speed is Vc (m/min, Vc>0),
When the immersion depth X of the immersion nozzle is less than 230 mm, electromagnetic stirring is applied to the area where the following formula (3) holds, and an electromagnetic brake is applied to the area where the following formula (3) does not hold. When the immersion depth X is 230 mm or more, electromagnetic stirring is applied to the area where the following formula (4) holds, and electromagnetic braking is applied to the area where the following formula (4) does not hold. casting method.
“ΔT−12≦−1.46Vc” ∩ “Vc≦2.04” (3)
"ΔT-12≦-1.46Vc" ∩ "Vc≦2.04" or "ΔT≦40" ∩ "Vc≦1.24" or "ΔT≦35" ∩ "Vc≦1.44" (4) An electromagnetic coil having two magnetic pole iron cores, two coils wound around the outer circumference of each of the magnetic pole iron cores, and one coil wound around the outer circumference of the two magnetic pole iron cores together. are arranged on the outer circumference of the long side of the mold in the same number on each long side, and (2n + 2) pieces (n is a natural number) in total on the outer circumference of the long side of the mold,
When the molten steel in the mold is electromagnetically stirred, a multiphase alternating current of three or more phases with a current phase difference of 90 degrees to 120 degrees is applied to each coil of all the electromagnetic coils,
When the electromagnetic brake is applied to the molten steel in the mold, a direct current is applied to each of the electromagnetic coils, and the coil is wound around the outer circumference of the two magnetic pole iron cores, or A continuous casting method for steel using an electromagnetic coil device for both electromagnetic stirring and electromagnetic braking, in which direct current is applied to these three coils,
The component carbon concentration of the molten steel supplied to the mold is set to 0.0050% by mass or less,
Assuming that the degree of superheat of the molten steel in the tundish is ΔT (°C, ΔT≧0) and the casting speed is Vc (m/min, Vc>0),
When the immersion depth X of the immersion nozzle is less than 230 mm, electromagnetic stirring is applied to the area where the following formula (5) holds, and an electromagnetic brake is applied to the area where the following formula (5) does not hold. When the immersion depth X is 230 mm or more, electromagnetic stirring is applied to the area where the following formula (6) holds, and electromagnetic braking is applied to the area where the following formula (6) does not hold. casting method.
“ΔT−40≦−3.42Vc” ∩ “Vc≦1.74” (5)
"ΔT-40≦-3.42Vc" ∩ "Vc≦1.74" or "ΔT≦40" ∩ "Vc≦1.34" (6)

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