JP2005152954A - Method for continuously casting ultralow carbon steel slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stably obtain a slab having superior surface quality without performing slab conditioning such as scarfing even if high speed casting is applied at the casting rate of molten steel of ≥2.0 m/min. <P>SOLUTION: When an ultralow carbon steel slab having a C content of ≤0.01 mass% is produced using a continuous casting apparatus where magnetic field application devices each applying a static magnetic field over the whole width of a continuous casting mold to a direction crossing the thickness of the mold are provided at the upper part of the mold and at a position in the lower part by a fixed distance, and molten steel is fed to a space between the magnetic application devices at the upper and lower stages via an immersion nozzle, casting is performed at the casting speed of ≥2.0 m/min using an immersion nozzle satisfying the condition that the ratio(D/d) of a short side of length D of a casting space in a mold of the continuous casting apparatus to a lateral width d of immersion nozzle discharge spouts is 1.5 to 3.0, under the condition that the short side of length D of a casting space in a mold of the continuous casting apparatus is 150 to 240 mm and the immersion depth of the immersion nozzle is 200 to 350 mm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for continuously casting slabs of ultra-low carbon steel, and more particularly to a technology for producing a material slab suitable for use in automobile outer plates and the like that require strict surface quality.

  Steel sheets used for applications where deep drawing and other complicated deformation processes such as the outer plate of automobiles are applied require high formability, so the C content has been reduced as much as possible. So-called very low carbon steel (usually C content in the steel is 0.01 mass% or less) is used. Among such extremely low carbon steel plates, in particular, cold rolled steel plates for automobile outer plates are required to have a beautiful appearance in addition to the paintability.

  By the way, since the process of oxidizing and removing C in molten steel using oxygen in the refining process is indispensable for ultra low carbon steel, oxygen dissolved in the molten steel in this process is further reduced to aluminum, magnesium, titanium, etc. A step of deoxidizing with a deoxidizing agent is required. In this deoxidation step, oxygen in the molten steel combines with a deoxidizer to produce deoxidation products such as alumina, magnesia, titania, etc., which remain as nonmetallic inclusions in the molten steel.

  If such non-metallic inclusions are present in the vicinity of the surface of the slab, when the slab is hot-rolled and cold-rolled to form a thin steel plate, defects such as scabs and blisters occur on the surface of the steel plate, which is not preferable. . Besides deoxidized products, argon gas supplied to prevent clogging of mold powder added to the molten steel surface in the mold during continuous casting and immersion nozzle for supplying molten steel from the tundish into the mold It is known that even if bubbles in the molten steel are entrained in the molten steel and remain in the molten steel as bubbles alone or in combination with the deoxidized product, the same surface defects as the above deoxidized product are caused. Yes.

Therefore, in the past, unlike a continuous cast slab for a cold-rolled steel sheet in which the surface of the slab is hot-rolled without maintenance, in the case of a slab for an automobile outer sheet, the surface is subjected to about 1 to 4 mm of cutting, etc. After removing the foreign substances that cause steel sheet surface defects after hot rolling such as deoxidation product inclusions, bubbles, mold flux, etc. on the slab surface, it is subjected to hot rolling and cold rolling. It was.
However, such a slab refining process has a problem in that the yield of the slab, which is a raw material, is lowered and the process is retained.
Therefore, attempts have been made to prevent the occurrence of slab surface layer defects that cause surface defects of the steel sheet as described above at the stage of producing slabs in continuous casting equipment.

The basic idea of such an attempt is
(1) The slab has a large cross section (the slab width is restricted during rolling, so it can be handled by increasing the slab thickness) and the casting speed (m / min) is reduced to reduce the mold without sacrificing productivity. Increase the residence time of the molten steel in the inside, thereby gaining time for the deoxidation products, mold powder or bubbles and other foreign matters from the molten steel to rise in the mold,
(2) Casting with a continuous casting machine having a vertical part in order to promote floating and separation of deoxidation products, mold powder or bubbles from molten steel in the mold,
(3) Electromagnetic force provides a horizontal flow in the vicinity of the meniscus to prevent foreign matter floating in the molten steel from being trapped by the solidified shell (cleaning effect).
(4) Reduce mold powder entrainment in molten steel by making mold powder viscosity appropriate.
(5) Optimization of the oscillation (vertical vibration) conditions of the casting mold for continuous casting and generation of claw of the solidified shell formed in the mold (a phenomenon in which a part of the solidified shell falls to the molten steel side due to the oscillation) Reduce the trap amount of deoxidation products, mold powder, bubbles, etc. in this part,
(6) Electromagnetic stirring and electromagnetic brake are added to the molten steel discharge flow supplied from the immersion nozzle into the mold to optimize the flow of the molten steel, and the molten steel discharge flow with deoxidation products is deep in the mold. Prevent entry into position,
Etc. were the mainstream.

  However, in all of the above technologies, in the production of ultra-low carbon steel slabs used for automotive skins, etc., in the case of high-speed casting exceeding 2.0 m / min, slabs such as cutting are still used. High quality slabs could not be produced stably without the need for care.

By the way, the following is proposed as a technique for controlling the molten steel flow in the mold by electromagnetic force.
For example, in Patent Document 1, two upper and lower magnetic poles facing each other across the mold long side are arranged on the back side of the mold long side, and (1) a DC static magnetic field and an AC moving magnetic field are superimposed on the magnetic pole arranged on the lower side. (2) Applying a magnetic field in which a DC static magnetic field and an AC moving magnetic field are superimposed on the upper magnetic pole and applying a DC static magnetic field to the lower magnetic pole A control method is disclosed.
Patent Document 2 discloses an electromagnetic stirring method in which a static magnetic field is applied to a position surrounding a molten steel flow discharged from an immersion nozzle to reduce the flow velocity, and an electromagnetic stirring device is installed at a position downstream of the static magnetic field. Has been.
In Patent Document 3, a magnet (400 to 2000 gauss) that forms a moving magnetic field is installed on the upper part of the mold, and the moving magnetic field is applied to the rising reversal molten steel flow flowing on the surface of the meniscus, and the static magnetic field is at least 500 mm below the meniscus. A casting method and apparatus are disclosed in which a magnet (1000 to 7000 gauss) is formed and a static magnetic field is applied to the molten steel flow that descends when the discharged molten steel flow hits the short side of the mold.
In Patent Document 4, a magnet for electromagnetic stirring is installed above the lower end of the immersion nozzle, and a magnet capable of applying a moving magnetic field and a static magnetic field is placed below the lower end of the immersion nozzle. A casting method that selectively uses a magnetic field and a moving magnetic field is disclosed.
Non-Patent Document 1 discloses a technique for braking (EMLS) or accelerating (EMLA) a discharge molten steel flow by applying an AC moving magnetic field to the discharge flow from an immersion nozzle.
Patent Document 5 discloses a technique for controlling the discharge flow from an immersion nozzle by installing an electric stirrer at the upper part of the mold and an electromagnetic brake at the lower part of the mold in continuous casting of steel.
In Patent Document 6, a static magnetic field and a high-frequency magnetic field are superimposed on the entire width direction by a magnet placed on the immersion nozzle discharge hole, and a static magnetic field is acted on by a magnet placed below the discharge hole. A steel casting method is disclosed.
Patent Document 7 and Patent Document 8 propose a method for improving the surface quality of a slab by superimposing an alternating vibration magnetic field or a direct current magnetic field thereon.

Japanese Patent Laid-Open No. 10-305353 JP 61-193755 A Japanese Patent No. 2610741 JP-A-9-262651 Materials and Processes Vol.3 (1990), P.256 Japanese Unexamined Patent Publication No. 63-119959 JP-A-6-190520 Japanese Patent Laid-Open No. 2003-103348 Japanese Patent Laid-Open No. 2003-103349

  The present invention uses the electromagnetic control technology for molten steel in the mold as described above, and does not require slab maintenance such as cutting even when high-speed casting with a molten steel casting speed exceeding 2.0 m / min is applied. An object of the present invention is to propose a slab continuous casting method of ultra-low carbon steel that can stably obtain a slab having excellent surface quality.

Now, in order to achieve the above object, the inventors have conducted intensive research on the type of magnetic field applied to the molten steel in the mold, the immersion depth of the immersion nozzle, the mold shape, the shape of the immersion nozzle, etc. Under various types of magnetic field application, the casting speed, the short side length of the casting space in the continuous casting mold, the nozzle immersion depth, and the ratio D / d of the short side length D and the horizontal width d of the immersion nozzle are controlled appropriately. As a result, it was found that the intended purpose can be advantageously achieved.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. A magnetic field application device that applies a static magnetic field across the entire width of the mold in a direction transverse to the mold thickness at a position below the mold upper portion including the molten metal level in the continuous casting mold, and these two upper and lower magnetic fields. When a very low carbon steel slab having a C content of 0.01 mass% or less is produced using a continuous casting facility that supplies molten steel through an immersion nozzle between application devices,
Under the conditions of the short side length of the casting space in the mold of the above continuous casting equipment: 150 to 240 mm and the immersion depth of the immersion nozzle: 200 to 350 mm, the short side length D and the immersion nozzle discharge hole lateral width d A slab continuous casting method for ultra-low carbon steel, characterized in that casting is performed at a casting speed of more than 2.0 m / min using an immersion nozzle that satisfies a ratio D / d of 1.5 to 3.0.

2. A magnetic field application device that superimposes and applies a static magnetic field and an alternating magnetic field over the entire width of the mold in a direction transverse to the mold thickness is provided on the upper part of the mold including the molten metal surface level in the continuous casting mold, and an immersion nozzle is provided below the magnetic field application device. When making a very low carbon steel slab with a C content of 0.01 mass% or less using a continuous casting facility that supplies molten steel,
Under the conditions of the short side length of the casting space in the mold of the above continuous casting equipment: 150 to 240 mm and the immersion depth of the immersion nozzle: 200 to 350 mm, the short side length D and the immersion nozzle discharge hole lateral width d A slab continuous casting method for ultra-low carbon steel, characterized in that casting is performed at a casting speed of more than 2.0 m / min using an immersion nozzle that satisfies a ratio D / d of 1.5 to 3.0.

3. A magnetic field application device that superimposes and applies a static magnetic field and an alternating magnetic field across the entire width of the mold in the direction transverse to the mold thickness is provided on the upper part of the mold including the surface level in the continuous casting mold. A pole having a C content of 0.01 mass% or less using a continuous casting facility that includes a magnetic field application device that applies a static magnetic field over the entire width, and supplies molten steel through an immersion nozzle between the upper and lower magnetic field application devices. In making low carbon steel slabs,
Under the conditions of the short side length of the casting space in the mold of the above continuous casting equipment: 150 to 240 mm and the immersion depth of the immersion nozzle: 200 to 350 mm, the short side length D and the immersion nozzle discharge hole lateral width d A slab continuous casting method for ultra-low carbon steel, characterized in that casting is performed at a casting speed of more than 2.0 m / min using an immersion nozzle that satisfies a ratio D / d of 1.5 to 3.0.

4). 4. The ultra low carbon steel slab continuous casting method according to any one of 1 to 3 above, wherein the casting speed is 2.4 m / min or more.

5). The ultra-low carbon steel according to any one of the above 1 to 4, wherein casting is performed using an immersion nozzle in which a ratio D / d between the slab thickness D and the immersion nozzle discharge hole width d satisfies 2.1 to 2.9. Slab continuous casting method.

6). 6. The ultra-low carbon steel slag continuous casting method according to any one of the above 1 to 5, wherein the ultra-low carbon steel slab is a material for a cold-rolled steel sheet for an automobile outer plate.

According to the present invention, under the proper flow control of molten steel in a mold by a magnetic field application device, the short side length of the casting space in the mold (slab thickness when it becomes a slab) is 150 to 240 mm smaller than the conventional one. On the other hand, by setting the casting speed to 2.0 m / min, which is higher than before (preferably 2.4 m / min or more), the flow velocity of the molten steel along the long side in the mold is increased.
1) In addition to improving the cleaning effect of inclusions and bubbles that are trapped in the long-side solidified shell generated in the mold,
2) Even if the long side solidified shell itself becomes thin and some inclusions or bubbles are trapped in this shell, it is effectively removed together with the scale during slab heating before hot rolling. As a result, surface defects and internal inclusions caused by trapped bubbles, non-metallic inclusions, mold flux, etc., ideal as a material for extremely low carbon steel sheets with extremely severe surface quality, such as cold rolled steel sheets for automobile outer plates It is possible to stably manufacture a slab with few objects.
Even if the casting speed is high, the short side length is small, so that the thickness of the solidified shell on the short side can be prevented from being uneven, and short side bulging can be prevented.

The present invention will be specifically described below.
1 (a) to 1 (c) schematically show a continuous casting mold provided with a magnetic field application device suitable for use in the present invention.
Fig. 11 (a) shows the case where the magnetic field application device 1 for applying a static magnetic field over two upper and lower stages is arranged at the upper part of the mold including the molten metal level and at a position below a certain distance, respectively. When the magnetic field applying device 2 for applying a static magnetic field and an alternating magnetic field is applied only to the upper part of the mold including the molten metal surface level, FIG. 5 (c) applies an alternating magnetic field to the upper part of the mold including the molten metal surface level. This is a case where the magnetic field application device 2 is arranged, and the magnetic field application device 1 for applying a static magnetic field is disposed at a position below a certain distance.

  Of the various magnetic field application devices described above, when a magnetic field application device that applies a static magnetic field is used, the magnitude of the DC magnetic field (magnetic flux density) is preferably about 1000 to 7000 gauss. This numerical value is the same whether it is provided over the upper limit two stages or only in the lower stage.

In addition, there are two types of AC magnetic fields, an AC oscillating magnetic field and an AC moving magnetic field, but both can be used preferably in the present invention.
Here, the AC oscillating magnetic field means that, as shown in FIG. 2, an AC current having substantially the opposite phase is applied to adjacent coils, or an AC current having the same phase is reversed by reversing the coil winding direction. It is a magnetic field obtained by energizing and substantially reversing the magnetic field generated in the adjacent coil. Then, by superimposing the AC oscillating magnetic field and the DC magnetic field, local flow can be induced in the molten steel in the mold. In the figure, reference numeral 3 is a DC coil, 4 is an AC coil, 5 is a mold, and 6 is molten steel (the hatched portion is a low-speed flow region).

  On the other hand, the AC moving magnetic field is a magnetic field obtained by energizing an AC current that is shifted in phase by 360 ° / N to any adjacent N coils, and normally N = N as shown in FIG. 3 (phase difference 120 °) is used because of its high efficiency. Again, local flow can be induced in the molten steel in the mold by superimposing the AC moving magnetic field and the DC magnetic field.

When using such a magnetic field applying device that applies an alternating magnetic field, it is desirable that the magnetic flux density of the alternating magnetic field be about 100 to 1000 gauss and the frequency of the oscillating magnetic field be about 1 to 10 Hz.
Furthermore, when using a magnetic field application device that applies a static magnetic field and an alternating magnetic field in a superimposed manner, the magnitude of the direct magnetic field is preferably about 1000 to 7000 gauss, and the magnetic flux density of the alternating magnetic field is preferably about 100 to 1000 gauss.

  Now, in the present invention, continuous casting is performed under the electromagnetic flow control of molten steel using the magnetic field application device as described above. Hereinafter, the phenomenon occurring in the mold in such continuous casting will be described. These new findings will be described together with the reasons for limiting the production conditions in the present invention. Hereinafter, inclusions and bubbles are referred to as foreign matters.

(1) Decrease in foreign matter trapping locations By increasing the casting speed Vc, the formation of meniscus initial solidified shells, so-called “nails”, is remarkably suppressed. This is because the solidified shell thickness at the same depth below the molten metal surface becomes thinner as Vc increases, so the force pressed against the mold side due to the influence of the molten steel static pressure is due to the heat shrinkage of the shell depending on the solidified shell thickness. This is because the nail is larger than the force of the nail to fall to the side. In addition, as the slab thickness decreases, the absolute value of shell shrinkage in the thickness direction (= slab thickness x temperature difference x linear expansion coefficient) decreases, so that the collapse to the molten steel side is further suppressed. The fall-suppressing effect becomes even more pronounced.

(2) Suppression of foreign matter adsorption Along with solidification, a gradient of interfacial tension occurs due to segregation of solute concentrated at the solidification interface, and this force causes a phenomenon that foreign matter is easily attracted and trapped at the solidified shell interface. . For this reason, attempts have been made to reduce the concentration of S, Ti, etc., which have a particularly large influence as a solute element that increases the force for attracting and capturing foreign matter. However, there is a problem that manipulating the components leads to cost increase (lower S) and material deterioration (Ti reduction).
In the present invention, by increasing the casting speed Vc, it is possible to suppress an increase in the force for sucking / capturing foreign matter into the solidified shell. That is, in the high-speed casting as in (1), the amount of solidification of the meniscus portion is further reduced, so that the amount of segregation is also reduced, and hence the interfacial tension gradient that is a suction force for foreign matters is also reduced. As a result, the amount of foreign matter adsorbed and captured on the solidified shell side is also suppressed.

(3) Decrease in trapping thickness of foreign matter When continuous casting is performed under the conditions of the present invention, foreign matter is trapped by the shell within 20 mm below the molten metal surface, and the trapping depth becomes shallower as the casting speed increases, and the casting speed Vc> 2.0. At m / min, the capture depth h from the slab surface is 1 mm or less.
Below this depth, even if the foreign matter is trapped by the shell, the foreign matter is dropped and removed together with the oxide scale on the surface of the slab in the process of becoming a product through the subsequent hot rolling → cold rolling process. Therefore, it is possible to obtain a defect-free product without performing slab maintenance. Note that when the casting speed is 2.4 m / min or more, the claw depth is 0.7 mm or less, that is, the foreign matter capturing thickness h is also less, so the casting speed is more preferably 2.4 m / min or more.

(4) Decrease in foreign matter trapping probability The residence time of the solidified shell within 20 mm below the molten metal surface where foreign matter is easily trapped by the solidified shell becomes shorter as the casting speed increases. Therefore, even if the amount of foreign matter is the same, the probability of being trapped by the solidified shell is reduced. For example, in the case of Vc = 3.0 m / min, the probability of trapping foreign matter is halved compared to the case of Vc = 1.5 m / min.

  Therefore, in the present invention, the casting speed Vc is limited to more than 2.0 m / min, preferably 2.4 m / min or more.

(5) Nozzle immersion depth (distance from molten steel surface to top of discharge hole)
The situation of the circulating flow of molten steel in the mold changes depending on the nozzle immersion depth. In particular, since the injection speed is high under high-speed casting, it is necessary to optimize the immersion depth. In other words, if the nozzle is too shallow, the flow velocity on the surface of the molten steel increases, which facilitates flux entrainment. On the other hand, if the depth is too deep, the flow velocity on the surface of the molten steel will be too low, the cleaning effect on the solidification interface will be reduced, and the trapping of bubbles and inclusions will be promoted.

  Therefore, considering these, the optimum value of the nozzle immersion depth was examined, and it was found that the nozzle immersion depth should be 200 mm or more and 350 mm or less.

The material of such an immersion nozzle is preferably alumina-graphite, which is generally used, but is not limited to this.
As the shape of the immersion nozzle, a cylindrical nozzle (so-called straight nozzle) or a two-hole nozzle in which the tip is closed and a substantially circular discharge hole is provided in both short side directions can be generally used. The cross-sectional shape of the discharge hole is not limited to a round shape, a square shape, a rectangular shape (horizontally long or vertically long), and the maximum width d only needs to meet the conditions of the present invention described later.

(6) Optimization of the ratio D / d between the short side length D of the casting space in the mold and the horizontal width d of the submerged nozzle discharge hole The molten steel ejected from the submerged nozzle discharge port widens before colliding with the short side shell. The speed distribution of the molten steel jet colliding with the mold short side shell depends on the slab width W, the casting speed Vc and the D / d ratio. If the immersion nozzle discharge port width d is too small (D / d is too large) with respect to the short side length (slab thickness) D of the casting space in the mold, it collides with the short side shell as D, Vc, and W increase. Since the ratio of the region width of the large jet flow velocity to the slab thickness (short side width) decreases, the growth of the solidified shell is uneven and easily inhibited, and if the solidified shell becomes extremely thin, breakout occurs. On the other hand, if the submerged nozzle discharge port d is too large (D / d is too small) with respect to the short side length (slab thickness) D of the casting space in the mold, the long side side before the jet collides with the short side. If the solidified shell collides with the shell and the growth of the solidified shell on the long side is inhibited, transverse cracks and oblique cracks occur, and the solidified shell becomes extremely thin, it will also lead to a breakout. In either case, there is almost no influence of the slab width.
Further, when the D / d ratio deviates from the optimum value when the flow after the collision on the short side flows along the long side side molten metal after rising, if the molten steel flow rate is biased in the slab thickness direction, the meniscus flow rate fluctuation This also increases the amount of mold flux entrained.

Therefore, as a result of investigating the influence of the D / d ratio on the product quality, it was found that the D / d range of 1.5 to 3.8 is suitable.
In addition to the product quality, the range of 2.1 to 2.9 is more suitable when the optimum slab thickness, the immersion nozzle durability and the required flow rate are taken into account.

(7) Short-side bulging prevention (reason for upper limit on short-side length of casting space in mold)
Even if an immersion nozzle in which the ratio D / d of the short side length (slab thickness) D of the casting space in the mold defined in the present invention and the horizontal width d of the immersion nozzle satisfies the appropriate range is used, the short side length is If the thickness is too thick, when the casting speed Vc exceeds 2.0 m / min, a slab shape defect or breakout due to short side bulging occurs. In this regard, when the short side thickness is small or Vc is small, bulging due to the molten steel static pressure on the short slab after exiting the mold is suppressed, and the risk of occurrence of breakout is low.
However, when the short side length (ie, slab thickness) exceeds 240 mm, the secondary flow rate increases due to electromagnetic brake braking due to the increase in the molten steel jet velocity from the immersion nozzle discharge hole due to the increase in the slab thickness even when the casting speed is 2.4 m / min Therefore, it becomes difficult to suppress the delay in the growth of the short side shell, the short side bulging at the lower end of the mold becomes remarkable, and the risk of breakout (bulging amount ≧ 10 mm) increases.
If the short side length (ie, slab thickness) exceeds 240mm, the reverse flow or secondary flow from the short side of the molten steel jet will promote disturbance of the molten metal surface for the same reason as above. Flux entrainment and biting are likely to occur, and the increase in slab thickness tends to cause stagnation of molten steel in the meniscus area, particularly in the vicinity of the immersion nozzle, increasing slab surface defects and product defects. .

(7) Reason for the lower limit of the short side length of the casting space in the mold If the short side length (slab thickness) D of the casting space in the mold is less than 150 mm, it is not preferable for the following reasons.
That is, if the slab cross-sectional area becomes too small, the amount of fluctuation of the molten metal surface becomes large for the same variation of casting amount due to the problem of molten metal surface controllability, and the nail depth of 1 mm or more caused by the water wrinkles The occurrence frequency of (1) cannot be obtained. In addition, the mold flux is likely to be caught or bitten due to the fluctuation of the molten metal surface. Furthermore, the outer diameter of general immersion nozzles is determined by the wall thickness (20 mm or more) determined by durability, throughput: 5.4 t / min (150 mm thickness, 2200 mm width, Vc: 2.1 m / min or more) to 14.5 t / min ( 240 mm thickness, 2200 mm width, Vc: up to 3.5 m / min). Here, if the short side length (slab thickness) D is too small, the distance between the outer wall of the immersion nozzle and the long side solidified shell becomes too narrow (<20 mm), the flow between them becomes uneven, and vertical cracks occur. Cause. In extreme cases, the solidified shell contacts and adheres to the nozzle, leading to breakout. Therefore, the short side length (slab thickness) D is 150 mm (inner diameter: 70 mm + outer wall total thickness: 40 mm (20 × 2) + distance between the outer wall of the immersion nozzle and the long side solidified shell: 40 mm (20 × 2)) or more. There is a need.

  Therefore, in the present invention, the short side length (slab thickness) of the casting space in the mold is limited to the range of 150 to 240 mm.

In addition, the long side length (slab width) of the casting space in the mold is not particularly limited, and may be a length for a normal cold-rolled steel sheet (particularly automatic heavy-duty cold-rolled steel sheet). It is preferable to set the degree.
The height of the mold in the vertical direction is not specified, but a solidified shell with a thickness sufficient to prevent bulging of the slab that has passed through the mold even when cast at a casting speed of more than 2.0 m / min. Since it needs to be generated, it is preferable that the thickness is about 800 mm to 1000 mm.

Further, the steel type targeted by the present invention is a so-called ultra-low carbon steel having a C content of 0.01 mass% or less. Components other than C are not particularly specified, but are preferably suitable for deep drawing applications such as automobile outer plates. The present invention intends not to include inclusions, etc. in a thickness range that does not scale off under the surface layer of the slab, especially for steel grades for applications that dislike inclusion defects, and as a deoxidation product such as alumina in the refining process This is because the ultra-low carbon steel that easily generates non-metallic inclusions enjoys the benefits of the present invention.
Incidentally, the typical composition of ultra-low carbon steel other than C is as follows.
Si: 0.01 to 0.04 mass%, Mn: 0.08 to 0.20 mass%, P: 0.008 to 0.020 mass%, S: 0.003 to 0.008 mass%, Al: 0.015 to 0.060 mass%, Ti: 0.03 to 0.08 mass%, Nb: 0.002 to 0.017 mass%. B: 0 to 0.0007 mass%.

Converter melting-obtained by RH treatment, C: 0.0015 mass%, Si: 0.02 mass%, Mn: 0.08 mass%, P: 0.015 mass%, S: 0.004 mass%, Al: 0.04 mass% and Ti: 0.04 Continuous casting of molten steel (about 300 tons) containing a mass% with the balance of Fe and inevitable impurities using a continuous casting mold equipped with a magnetic field application device shown in Figs. 1 (a) to (c) And slab.
The production conditions at this time are shown in Table 1. As the immersion nozzle, a two-hole nozzle having a rectangular discharge hole and a downward discharge angle of 15 ° was used.
Table 2 shows the results of examining surface segregation of the slab thus obtained, the amount of nonmetallic inclusions, and surface defects caused by mold flux after cold rolling.

As for surface segregation of the slab, etching was performed after slab grinding, and the number of segregation per 1 m ² was examined by visual observation. The amount of non-metallic inclusions was measured after extracting inclusions by a slime extraction method from a 1/4 thickness position of the slab. Furthermore, the surface defect of the coil after cold rolling was visually inspected, and after collecting a defect sample, the number of defects due to mold flux was investigated by analyzing the defect portion. The surface segregation, the amount of inclusions, and the mold flux defect were indexed with the worst of all conditions as 10 and displayed as a linear ratio.

As is apparent from Table 2, according to the present invention, under appropriate electromagnetic flow control of the molten steel in the mold, the casting speed, the short side length of the casting space in the continuous casting mold, the nozzle immersion depth, and the short side length By properly controlling the ratio D / d between the width D of the submerged nozzle and the discharge nozzle discharge hole d, defects due to surface segregation, the amount of non-metallic inclusions and mold flux could be reduced.
If the intensity of the oscillating magnetic field is too strong, the flux entrainment on the surface of the molten metal will increase and the surface quality will deteriorate, and if the frequency is too high, the molten metal will not be able to follow the magnetic field and the solidification interface cleaning effect will decrease However, bubbles and inclusion defects were increased.

It is a schematic diagram of the casting mold for continuous casting provided with the suitable magnetic field application apparatus used for this invention. It is a figure which shows an example of the application procedure of an alternating oscillating magnetic field. It is a figure which shows an example of the application procedure of an alternating current magnetic field.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Magnetic field application apparatus which applies a static magnetic field 2 Magnetic field application apparatus which superimposes and applies a static magnetic field and an alternating magnetic field 3 DC coil 4 AC coil 5 Mold 6 Molten steel

Claims (6)

A magnetic field application device that applies a static magnetic field across the entire width of the mold in a direction transverse to the mold thickness at a position below the mold upper portion including the molten metal level in the continuous casting mold, and these two upper and lower magnetic fields. When a very low carbon steel slab having a C content of 0.01 mass% or less is produced using a continuous casting facility that supplies molten steel through an immersion nozzle between application devices,
Under the conditions of the short side length of the casting space in the mold of the above continuous casting equipment: 150 to 240 mm and the immersion depth of the immersion nozzle: 200 to 350 mm, the short side length D and the immersion nozzle discharge hole lateral width d A slab continuous casting method for ultra-low carbon steel, characterized in that casting is performed at a casting speed of more than 2.0 m / min using an immersion nozzle that satisfies a ratio D / d of 1.5 to 3.0. A magnetic field application device that superimposes and applies a static magnetic field and an alternating magnetic field over the entire width of the mold in a direction transverse to the mold thickness is provided on the upper part of the mold including the molten metal surface level in the continuous casting mold, and an immersion nozzle is provided below the magnetic field application device. When making a very low carbon steel slab with a C content of 0.01 mass% or less using a continuous casting facility that supplies molten steel,
Under the conditions of the short side length of the casting space in the mold of the above continuous casting equipment: 150 to 240 mm and the immersion depth of the immersion nozzle: 200 to 350 mm, the short side length D and the immersion nozzle discharge hole lateral width d A slab continuous casting method for ultra-low carbon steel, characterized in that casting is performed at a casting speed of more than 2.0 m / min using an immersion nozzle that satisfies a ratio D / d of 1.5 to 3.0. A magnetic field application device that superimposes and applies a static magnetic field and an alternating magnetic field across the entire width of the mold in the direction transverse to the mold thickness is provided on the upper part of the mold including the surface level in the continuous casting mold. A pole having a C content of 0.01 mass% or less using a continuous casting facility that includes a magnetic field application device that applies a static magnetic field over the entire width, and supplies molten steel through an immersion nozzle between the upper and lower magnetic field application devices. In making low carbon steel slabs,
Under the conditions of the short side length of the casting space in the mold of the above continuous casting equipment: 150 to 240 mm and the immersion depth of the immersion nozzle: 200 to 350 mm, the short side length D and the immersion nozzle discharge hole lateral width d A slab continuous casting method for ultra-low carbon steel, characterized in that casting is performed at a casting speed of more than 2.0 m / min using an immersion nozzle that satisfies a ratio D / d of 1.5 to 3.0.   The slab continuous casting method for ultra-low carbon steel according to any one of claims 1 to 3, wherein the casting speed is 2.4 m / min or more.   5. The ultra-low carbon according to claim 1, wherein casting is performed using an immersion nozzle in which a ratio D / d between the slab thickness D and the immersion nozzle discharge hole width d satisfies 2.1 to 2.9. Steel slab continuous casting method.   6. The ultra-low carbon steel slag continuous casting method according to claim 1, wherein the ultra-low carbon steel slab is a material for a cold rolled steel sheet for an automobile outer plate.

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