JP2017030013A - Continuous casting method of double-layered cast slab and continuous casting apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuously cast double-layered slab whose surface layer part is higher than the inner layer part of the double-layered slab in concentration, a manufacturing method thereof, and a manufacturing apparatus thereof.SOLUTION: Provided are a continuous casting method of a double-layered cast slab and a continuous casting apparatus thereof, in which a tundish 2 is divided into two regions by a weir 4 having an opening 10, and while a molten steel is fed from a ladle in one of the regions, a first region 11, a prescribed element or an alloy thereof is continuously added to adjust a concentration in the other second region 12, and thereby, the molten steel with two kinds of components is retained in the tundish. The molten steel quantity to be consumed by solidification in respective molten steel pools on the upper part and the lower part of a strand sandwiching a DC magnetic field band 14 formed by a DC magnetic field generating device 8 for applying a DC magnetic field in a thickness direction to the whole width in a mold width direction is supplied into a mold by using two immersion nozzles disposed on the bottom of each chamber of the tundish.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a continuous casting method and a continuous casting apparatus for casting a multilayered slab having a concentration different from that of a slab surface layer portion.

  Attempts to produce multi-layered slabs having different surface layer and inner layer composition have been made for a long time. For example, the method disclosed in Patent Document 1 can be mentioned. In Patent Document 1, two immersion nozzles having different lengths are inserted into a pool of molten metal in a mold, each discharge port is provided at a position having a different depth, and a DC magnetic field is provided between different types of molten metal. A method for producing a multilayer slab while preventing the mixing of both metals is disclosed.

  However, since the above method uses two types of molten steel having different component compositions, the two types of molten steel are separately melted at the same timing, transported to a continuous casting process, and used as an intermediate holding container for each molten steel. Each dish needs to be prepared. In addition, since the injection flow rate is greatly different between the surface layer molten steel and the inner layer molten steel, the amount of molten steel required for each heat is greatly different, which is difficult to realize in a normal steelmaking factory.

  Thus, two methods are roughly studied as methods for more easily casting slabs having different component compositions of the surface layer and inner layer of the slab. One is to use electromagnetic braking obtained by applying a DC magnetic field having a uniform magnetic flux density distribution in the width direction of the mold in the thickness direction, to wire or powder for continuous casting above the DC magnetic field band. A method of modifying the slab surface layer by containing some alloy element and continuously supplying it has been studied.

  For example, Patent Document 2 discloses a method for adding an element into a mold using a wire or the like. In this method, a direct-current magnetic field for cutting off the molten steel in the mold is provided at least 200 mm below the meniscus portion in the mold, and a predetermined element is added to the upper molten steel or the lower molten steel, and the molten steel to which the element has been added is stirred. It is the manufacturing method of the multilayer steel plate by the continuous casting characterized by this.

  A method of adding elements to molten steel by continuously supplying powder for continuous casting containing some elements, or by supplying metal powder or metal particles that are difficult to react with powder continuously from above the powder layer For example, the method disclosed in Patent Document 3 can be cited. In this method, a continuous casting powder containing an alloy element is used, and an electromagnetic stirrer is installed in the upper part of the continuous casting mold, and a stirring flow for melting and mixing the alloy element in the horizontal section of the upper molten steel in the mold is performed. A DC magnetic field is applied in the width direction below the slab to form a DC magnetic field zone in the molten steel, and the molten steel is supplied by an immersion nozzle below the DC magnetic field zone and cast. Thus, this is a method for producing a multilayered slab in which the concentration of the alloy element slab surface layer is higher than that of the inner layer.

  Non-Patent Document 1 discloses that the surface layer / inner layer can be separated by applying a magnetic field of 0.2 to 0.3 T as a DC magnetic field.

  However, a powder layer exists in the upper part in the mold and is cooled from the surroundings, and further has a rectangular cross-sectional shape. Therefore, excessive stirring cannot be performed and it is difficult to make the concentration uniform. Moreover, since the amount of molten steel supplied to the upper part and the lower part of the strand is not controlled independently, mixing of molten steel between the upper and lower pools is unavoidable, and there is a problem that it is difficult to produce a slab having a high degree of separation.

  As a method for modifying the surface of the slab after casting, for example, in Patent Document 4, the surface layer of the slab is melted by one or both of induction heating and plasma heating, and an additive element is added to the surface layer portion of the melted slab. Alternatively, a method for modifying the surface layer of a slab characterized by adding an alloy thereof is disclosed. However, since the volume of the molten pool is small, it is difficult to achieve a uniform concentration, although the components can be added, and it is difficult to melt the entire slab at once due to the method. There are problems such as the necessity of performing melt reforming a plurality of times for the modification.

JP 63-108947 A JP-A-3-243245 JP-A-8-290236 JP 2004-195512 A

E. Takeuchi, M. Zeze, H. Tanaka, H. Harada and S. Mizoguchi: Ironmaking and Steelmaking, 24 (1997), 257.

  The present invention has been made in order to solve the above-mentioned problems. The molten steel for continuous casting is supplied with one ladle and one tundish, and the alloy element concentration of the slab surface layer is different from the inside. An object of the present invention is to provide a continuous casting method and a continuous casting apparatus for casting a multilayer continuous casting slab.

The present invention
(1) A method for producing a multi-layer slab in which the composition of the surface layer and the inner layer of the slab is different, wherein an inner layer molten steel immersion nozzle is provided at the bottom of the tundish, and further, a surface layer molten steel immersion nozzle is provided downstream thereof. And a tundish weir is installed between these two immersion nozzles. The tundish weir has an opening with an opening area ratio of 10% or more and 70% or less in the molten steel immersion part, and the thickness direction extends over the entire width in the mold width direction. A DC magnetic field generator for applying a DC magnetic field is disposed on the upper part of the strand sandwiched by the DC magnetic field zone formed by the DC magnetic field generator, and the lower molten steel pool is the lower molten steel pool. Supply molten steel from the nozzle to the upper molten steel pool, supply molten steel from the inner layer molten steel immersion nozzle to the lower molten steel pool,
The ladle molten steel injection side divided by the tundish weir is the first region, the opposite side is the second region, and a predetermined element or its alloy is continuously applied to the molten steel in the tundish on the second region side. By adding and adjusting the concentration, the ladle molten steel and two types of molten steel with different composition from the ladle molten steel are held in the tundish while the molten steel contained in the first region is immersed in the inner layer molten steel. To the lower molten steel pool, the molten steel accommodated in the second region is supplied from the surface molten steel immersion nozzle to the upper molten steel pool,
A continuous casting method for a multilayer slab, characterized in that a molten steel amount consumed by solidification in each molten steel pool is supplied from each of the two immersion nozzles into a mold.
(2) The amount of molten steel supplied to the lower molten steel pool is controlled by controlling the position of the molten steel interface within the DC magnetic field zone using the relationship between the tundish head and the sliding nozzle opening of the inner layer molten steel immersion nozzle and the molten steel flow rate. The supply amount of the molten steel whose components are adjusted in the tundish to the upper molten steel pool from the immersion nozzle for surface molten steel is controlled so that the molten metal surface level in the mold is constant. A continuous casting method for layer slabs.
(3) The continuous casting method of a multilayer cast piece according to (1) or (2), wherein a swirling flow is formed in a horizontal section in the vicinity of the molten metal surface in the mold above the DC magnetic field zone.
(4) An apparatus for producing a multi-layer slab in which the composition of the surface layer and the inner layer of the slab is different, and a submerged nozzle for inner-layer molten steel at the bottom of the tundish that holds the molten steel from the ladle, and further downstream Surface surface molten steel immersion nozzles are provided side by side at intervals shorter than the casting width, and tundish weirs are installed between the nozzles. The tundish weirs have openings with an opening area ratio of 70% or less in the molten steel immersion part. , Having a component addition device for adding components to the molten steel in the region opposite to the ladle molten steel injection divided by the tundish weir,
In the mold, an electromagnetic stirrer that forms a swirl flow in the horizontal section near the molten metal surface, and a DC magnetic field generator that applies a DC magnetic field in the thickness direction over the entire mold width direction are provided below the electromagnetic stirrer. The upper part of the strand across the formed DC magnetic field zone is the upper molten steel pool, the lower part is the lower molten steel pool, the molten steel is supplied from the surface molten steel immersion nozzle to the upper molten steel pool, and the lower molten steel is supplied from the inner layer molten steel immersion nozzle. A continuous casting apparatus for multi-layer cast slabs, characterized in that molten steel is supplied to a pool.

  According to the present invention, one ladle and one tundish supply molten steel for continuous casting, and the tundish divides the molten steel region into two, and is supplied from the ladle by adding components to one region. It is possible to stably prevent mixing with the mother molten steel in the tundish and mold while adjusting the composition to a molten steel having a composition different from the mother molten steel, and the surface layer thickness and surface layer concentration are uniform over the entire width of the slab. The component composition of the surface layer and the inner layer of the slab is different, and it becomes possible to produce a multilayer slab in which components are added to the surface layer only at a high yield.

  In this method, there are few restrictions on the components to be added, not only Ni and C but also Si, Mn, P, S, B, Nb, Ti, Al, Cu, Mo, strong deoxidation and strong desulfurization elements. Elements contained in steel such as certain Ca, Mg, and REM can be added, and by changing the surface layer component of the slab, a new function of the steel material can be achieved by a relatively simple method.

It is the figure which showed typically the apparatus and method of this invention. It is a schematic sectional drawing which shows the molten steel flow condition in a tundish, (a) is a conventional case which does not have a tundish weir with one immersion nozzle, (b) is a tundish weir between two immersion nozzles In the case of the present invention having It is a figure which shows the example of the opening shape of a tundish dam, (a) is an upper dam, (a-1) is AA arrow sectional drawing of (a-2), (b) is other opening Show shape. It is the figure which showed typically how solidified shell formation and the interface of a surface layer and an inner layer are formed when a strand is divided | segmented into two by a DC magnetic field zone. It is the result of investigating the relationship between the opening area ratio of the tundish weir opening, (a) surface layer separation degree, and (b) surface layer concentration uniformity. The result of investigating the relationship between the ratio (Q ₂ / G ₂ ) of the upper molten steel pool molten steel supply amount Q ₂ and the upper molten steel pool solidification amount G ₂ , (a) surface layer separation degree, and (b) surface layer concentration uniformity is shown. FIG. It is the figure which showed the influence of the swirl | vortex flow by an electromagnetic stirrer on the slab width direction distribution of surface layer thickness.

A preferred embodiment of the present invention will be described below with reference to FIGS. First, as disclosed in Patent Document 1, the DC magnetic field generator 8 is arranged at a predetermined position below the meniscus 17 to form a DC magnetic field zone 14. In the DC magnetic field zone 14, a DC magnetic field whose magnetic field lines are directed in the thickness direction of the slab is applied, and the magnetic flux density is made substantially uniform in the mold width direction. By forming such a DC magnetic field zone, an electromagnetic brake is applied to the molten steel that is going to pass through the DC magnetic field zone 14, and the upper molten steel pool 15 above the DC magnetic field zone 14 and the lower molten steel pool 16 below are the facts. It will be blocked. The solidified shell solidified in the upper molten steel pool 15 forms the surface layer portion 24 of the slab, and the solidified shell solidified in the lower molten steel pool forms the inner layer portion 25 of the slab. The thickness D of the solidified shell in the DC magnetic field zone 14 corresponds to the thickness of the surface layer portion of the slab. Accordingly, the height H from the meniscus at which the DC magnetic field zone 14 is arranged is determined based on the target surface thickness D, the solidification coefficient K in the mold, and the casting speed V _C.

  In addition, two immersion nozzles are installed to supply molten steel above and below the DC magnetic field zone, and the molten steel is supplied from each immersion nozzle by the amount of molten steel that solidifies in each molten steel pool. It is possible to cast slabs having different component compositions. Here, the DC magnetic field band is set to the same range as the core height of the DC magnetic field generator. If the reason is within this range, a DC magnetic field having a uniform magnetic flux density is applied.

  The magnetic flux density in the DC magnetic field zone is selected so that the interchange of molten steel between the upper molten steel pool and the lower molten steel pool can be minimized. In the production of multi-layer slabs with different composition of the surface layer and inner layer of the slab, the amount ratio of molten steel varies depending on the surface layer thickness and casting width. = 4-5, the flow rate of the inner layer is overwhelmingly large. Therefore, the molten steel flow that flows out from the discharge hole of the immersion nozzle that supplies molten steel to the lower molten steel pool accounts for a large proportion of the molten steel flow phenomenon in the mold. This discharge flow collides with the short-side solidified shell to form a lower reversal flow and an upper reversal flow. If this upper reversal flow is suppressed and the passage of the DC magnetic field zone can be suppressed, the replacement of the molten steel between the upper molten steel pool and the lower molten steel pool can be minimized. If the magnetic flux density in the DC magnetic field zone is 0.3 T (Tesla) or more, the replacement of molten steel can be sufficiently suppressed. This point is also described in Non-Patent Document 1.

  The upper limit of the magnetic flux density is preferably as high as possible, but about 1.0 T is the upper limit in forming a DC magnetic field regardless of the superconducting magnet. What is necessary is just to apply the magnetic field of a suitable magnetic flux density within the range of 0.3T-1T according to casting conditions.

In the present invention,
[1] Creating a new molten steel (hereinafter referred to as the second molten steel 22) by adjusting the components of a part of the molten steel (hereinafter referred to as the first molten steel 21) injected from the ladle 1 in the tundish 2.
[2] Holding two types of molten steel: the first molten steel 21 and the second molten steel 22 in one tundish;
[3] 3 of supplying the first molten steel 21 and the second molten steel 22 stably in the mold by the amount consumed by solidification at the positions of the lower molten steel pool 16 and the upper molten steel pool 15 in the strand, respectively. One is required.

  First, in the present invention, [1] a second molten steel 22 having a different composition from that of the first molten steel 21 is made with the tundish 2.

  As schematically shown in FIG. 2 (a), the flow in the tundish is such that a ladle injection flow 13 injected downward from the ladle 1 flows horizontally in the tundish and is provided at the bottom of the tundish. It flows out from 30 downward. Therefore, the flow tends to stagnate in the tundish region further downstream from the immersion nozzle 30 when viewed from the ladle injection flow 13, and a stagnant region 28 is formed.

  In addition, as described above, when producing a multi-layer slab in which the composition of the surface layer and the inner layer of the slab differ, the amount ratio of each molten steel varies depending on the surface layer thickness and casting width, but the slab casting conditions If so, the inner layer / surface layer = 4-5, and the flow rate of the inner layer is overwhelmingly large. Therefore, as shown in FIG. 2 (b), the order in which the immersion nozzles are arranged at the bottom of the tundish is arranged such that the inner layer molten steel immersion nozzle 5 is arranged from the ladle injection flow 13 side and the downstream layer immersion steel immersion nozzle is arranged downstream thereof. 6 is arranged, and in addition, a tundish weir 4 between the both immersion nozzles and a weir having an opening 10 in the molten steel immersion portion is provided. As shown in FIG. 1 and FIG. 2 (b), the tundish is divided into a plurality of regions by the tundish weir 4, that is, the first region 11 that receives the first molten steel 21 from the ladle, and the wire to the first molten steel 21. For example, a predetermined element or an alloy thereof is added to the second region 12 to adjust the components. In the first region 11, the ladle pouring flow 13 position and the inner layer molten steel immersion nozzle 5 are disposed, and in the second region 12, the surface layer molten steel immersion nozzle 6 is disposed. The first molten steel 21 is injected into the lower molten steel pool 16 from the inner layer molten steel immersion nozzle 5. The second molten steel 22 is injected from the surface layer molten steel immersion nozzle 6 into the upper molten steel pool 15.

  By doing in this way, while the molten steel flow from the ladle pouring flow 13 to the inner layer molten steel immersion nozzle 5 is formed in the first region 11 in the tundish, the second region partitioned by the tundish weir 4 2B, as shown in FIG. 2 (b), the region is further stagnated as compared with the case where there is no weir. As described above, a predetermined element or alloy is continuously connected to the second region 12 by a wire or the like by the component addition device 7. The second molten steel 22 is produced by adjusting the content of the components and adjusting the content. As a result, two types of molten steel: the first molten steel 21 and the second molten steel 22 can be held in one tundish.

  Furthermore, in the second region 12 in which a predetermined element or an alloy thereof is continuously added to the molten steel from the ladle (first molten steel 21) with a wire or the like and the components are adjusted, a stirring force is applied to make the concentration uniform. Plan. For this purpose, uniform mixing can be achieved by applying agitation by Ar bubbling or the like from the bottom of the tundish in the second region. More preferably, it is preferable to levitate and remove inclusions and the like involved when adding the wire if a region where the wire is added and stirring is provided and a region where the molten steel is calmed down can be provided behind the region. In this way, the second molten steel 22 to be supplied to the upper molten steel pool 15 above the mold is created in the second region 12. In addition, a density | concentration can be adjusted by adjusting the component addition amount to the 2nd molten steel 22 according to the molten steel amount supplied in a 2nd area | region. A flow rate control method for the first region and the second region will be described later.

  Next, in the present invention, [2] mixing of the first molten steel and the second molten steel in the tundish is prevented, and the two molten steels are stably held in one tundish.

  Therefore, in the present invention, the tundish weir 4 that partitions the first region 11 and the second region 12 is provided in the tundish 2. An opening 10 is provided in a portion of the tundish weir 4 below the molten metal surface 18 so that the molten steel in the first region 11 and the second region 12 can flow through the opening 10. In FIG. 3, the dot hatched portion is a weir existing portion of the molten steel immersion portion 26 of the tundish weir 4, and the blank portion below the dot hatched portion indicates the opening 10. As a method of providing the opening 10, as shown in FIG. 3A, the lower part of the weir can be opened to form a so-called upper weir. Further, various openings as shown in FIG. 3B may be provided. A value (percentage) obtained by dividing the opening cross-sectional area of the weir by the cross-sectional area of the tundish molten steel in a plane parallel to the weir at the weir arrangement position is referred to as an opening area ratio (%). By setting the opening area ratio to 70% or less, mixing of molten steel in the first region 11 and the second region 12 can be effectively suppressed, and the component added to the second region 12 is the first region 11 in the first region 11. The possibility of mixing with the molten steel 21 can be reduced. On the other hand, if the opening area ratio is too small, component non-uniformity may occur, but if the opening area ratio is 10% or more, casting can be performed without any problem.

In the present invention, as schematically shown in FIG. 4, the strand is divided into two of an upper molten steel pool 15 and a lower molten steel pool 16 by a DC magnetic field zone 14 formed over the entire mold width. The second molten steel 22 is injected from the surface layer molten steel immersion nozzle 6, and the first molten steel 21 is injected from the inner layer molten steel immersion nozzle 5 into the lower molten steel pool 16. At the position of the DC magnetic field zone 14, a solidified shell (upper molten steel pool solidified portion 24) in which the molten steel of the first molten steel pool is solidified is formed on the surface side of the slab. Let S ₂ be the cross-sectional area of the solidified shell at the DC magnetic field band position. The solidified shell cross-sectional area S ₂ becomes the surface layer portion area S ₂ of the cast after cast piece. Surface portion area S ₂ other portions of the slab surface area of the inner layer portion area S _1, the value obtained by adding the S ₁ and S ₂ is the cast strip cross-sectional area. The unit time transport amount G ₂ of the upper molten steel pool solidified portion 24 transported downward as a solidified shell from the upper molten steel pool is expressed as follows. The casting speed is V _C , and the densities of the first molten steel and the second molten steel are ρ ₁ and ρ ₂ . Then
G ₂ = ρ ₂ S ₂ V _C
In addition, the unit time transport amount G ₁ of the lower molten steel pool solidified portion 25 that is solidified in the lower molten steel pool and transported downward is:
G ₁ = ρ ₁ S ₁ V _C
It becomes. If the total casting amount is G,
G = G ₁ + G ₂
It becomes.

Next, let Q _{1 be} the amount of molten steel supplied from the inner layer molten steel immersion nozzle 5 to the lower molten steel pool 16, and Q ₂ be the amount of molten steel supplied from the surface layer molten steel immersion nozzle 6 to the upper molten steel pool 15. The total amount of molten steel Q is Q = Q ₁ + Q ₂
far. The total amount of molten steel supplied from the tundish to the mold (Q) is adjusted by the molten metal level control so that the meniscus position remains constant.
Q = G
Is secured. In the present invention, the amount of molten steel supplied to each molten steel pool from each immersion nozzle,
Q ₁ = G ₁
Q ₂ = G ₂
By preventing the mixing of the molten steel via the DC magnetic field zone, the surface layer portion of the slab is formed with the composition of the second molten steel formed in the second region of the tundish in the first region. The inner layer portion of the slab can be formed with the component of the first molten steel.

Therefore, in the present invention, [3] the amount of molten steel Q, Q ₁ , Q _{2 of} these three members is controlled so that the first molten steel and the second molten steel do not pass through the DC magnetic field zone and are mixed. To do.

  A specific control method will be described with reference to FIGS.

The solidification coefficient K (mm / min ^0.5 ) in the mold in the continuous casting apparatus to be applied is confirmed in advance. By determining the height H from the meniscus 17 to the DC magnetic field zone 14 and the casting speed V _C , the solidified shell thickness D in the DC magnetic field zone 14 becomes D = K√ (H / V _C )
It is obtained as Using solidified shell thickness D in Motoma' DC magnetic field zone, Sadamari is solidified shell cross-sectional area S ₂ of the DC magnetic field zone, the aforementioned _{G 2 = ρ 2 S 2 V} C
Since G ₂ is determined by
Q ₂ = G ₂
The molten steel injection amount Q ₂ from the submerged nozzle for molten steel may be determined so that

The magnetic field forming range of the DC magnetic field band has a width in the vertical direction with the height H from the molten metal surface as the center. Therefore, even if the balance between Q ₁ and Q ₂ slightly varies, if the boundary 27 between the upper molten steel pool 15 and the lower molten steel pool 16 is within the magnetic field range of the DC magnetic field zone 14, the position of the molten steel interface is changed to the DC magnetic field. It can be controlled within the belt, and the effects of the present invention can be fully exhibited. The distance from the molten metal surface 17 to the upper limit of the DC magnetic field zone is set as H _H , and the distance from the molten metal surface 17 to the lower limit of the DC magnetic field zone is set as H _L. When the upper and lower molten steel pool boundaries 27 are at H _H or H _L , the solidified shell thickness is D _H = K√ (H _H / V _C ), respectively.
D _L = K√ (H _L / V _C )
It becomes. Regarding the solidification amount G ₂ in the upper molten steel pool, the values when the molten steel pool boundary is at H _H or H _L are G _2H and G _2L , respectively.
G _2H / G ₂ ≈D _H / D = √ (H _H / H)
G _2L / G ₂ ≈D _L / D = √ (H _L / H)
It becomes. Then, the molten steel supply amount Q ₂ of the upper molten steel pool, if within the range of G _2H ~G _2L, can control the molten steel surface 27 located in the DC magnetic field zone, the upper molten steel pool and a lower molten steel pool Mixing of molten steel can be suppressed to obtain a sufficiently good quality.

In the situation where the amount of molten steel supplied from the tundish into the mold is Q (= G), the steel is first supplied from the ladle to the tundish at a constant casting speed V _C (casting amount per unit time = G). The molten steel amount is controlled to be constant at Q. As an injection control method for setting the amount of molten steel supplied to the tundish to Q, a method of performing injection control so that the weight change per hour is measured by measuring the ladle weight, or the molten steel head in the tundish is If it can be visually observed, any of the methods for controlling the injection so that the molten steel head is constant can be used. As a result, the molten steel head in the tundish is held at a certain height. In this state, the flow rate Q ₁ of the first molten steel supplied to the lower molten steel pool 16 is
Q ₁ = Q−Q ₂ = Q−G ₂
It is controlled to be constant. Specifically, while keeping the head in the tundish constant, the predetermined opening degree is kept constant by using a predetermined sliding nozzle 33b opening degree and flow rate table to control Q ₁ to be constant. . By this alone, the amount of molten steel Q supplied to the entire mold is insufficient. Therefore, in adjusting the flow rate of the sliding nozzle 33c of the surface layer molten steel immersion nozzle 6 for supplying the second molten steel 22 whose component has been adjusted to the upper molten steel pool 15. Then, the molten steel amount Q ₂ is controlled so that the mold surface level in the mold becomes constant. As a result, it is possible to control the total flow rate Q and the amount of molten steel Q ₁ and Q ₂ consumed above and below the strand,
Q ₂ = G ₂
It can be. Thereby, in the upper molten steel pool 15 in the mold, the supplied molten steel amount (Q ₂ ) balances the transport amount per hour (G ₂ ) discharged as a solidified shell, and the lower molten steel pool 16 supplies The amount of molten steel (Q ₁ ) and the transport amount per unit time (G ₁ ) discharged as a solidified shell are balanced. Therefore, since the molten steel flow mixed through the DC magnetic field zone does not occur, the interface between the first molten steel and the second molten steel in FIG. 1 can be stably maintained. The interface between the first molten steel and the second molten steel determined by the balance between Q ₁ and Q ₂ is controlled within the range of the DC magnetic field zone.

  At this time, problems such as the relationship between the opening of the sliding nozzle 33b used for adjusting the flow rate of the inner layer molten steel immersion nozzle 5 and the flow rate are not constant each time can be considered. Therefore, the relationship between the opening degree of the sliding nozzle 33b and the flow rate characteristic may be grasped by utilizing the casting start time, and the characteristic may be corrected. Since the composition adjustment of the second molten steel has not been completed yet at the start of casting, casting is performed only with the first molten steel via the inner layer molten steel immersion nozzle. Even in such a case, the flow rate can be corrected by adjusting the relationship between the opening of the sliding nozzle 33b and the flow rate by keeping the tundish head constant and controlling the mold surface level in the mold to be constant.

As a method for controlling the amount of molten steel supplied into the mold, or first, the relationship between the sliding nozzle 33c opening degree of the surface layer molten steel immersion nozzle 6 and the amount of molten steel supplied is obtained in advance, and the amount of molten steel supplied from the surface layer molten steel immersion nozzle 6 Q _2. There defining a sliding nozzle 33c opening so that the upper molten steel pool solidified quantity G _2, for sliding nozzle 33b flow adjustment of the inner molten steel for immersion nozzle 5, to control as the molten metal surface level in the mold is constant It is also good.

  By introducing these three methods this time, there is one ladle and one tundish, but the composition of the ingredients differs from the first molten steel supplied from the ladle by adding ingredients in the tundish. While adjusting the composition of the second molten steel, mixing with the first molten steel in the tundish can be prevented, and two immersion nozzles having different lengths are placed at different depth positions in the mold. In this way, it is possible to manufacture a multi-layer slab in which the component composition of the surface layer and the inner layer of the slab is different by preventing the mixing of the two molten steels in the mold while supplying the amount of each molten steel.

  In addition, although a strand is divided | segmented into the upper molten steel pool 15 and the lower molten steel pool 16 with the DC magnetic field zone 14, as mentioned above, the amount of molten steel supplied to the upper molten steel pool above a DC magnetic field zone is from a DC magnetic field zone. The amount of molten steel supplied to the lower molten steel pool below is also small. Therefore, in the upper molten steel pool 15, sufficient molten steel stirring may not be possible. In the present invention, as a means for uniformizing the solidification over the entire inner circumferential direction of the mold, an electromagnetic stirring device 9 is installed in the vicinity of the molten metal surface in the upper molten steel pool 15 to provide a swirl flow in the horizontal section, It is preferable to make the coagulation uniform in the circumferential direction.

As described above, in order to verify the principle of the present invention described above, a casting test was performed using a test continuous casting machine. The test continuous casting machine can cast a slab having a width of 800 mm and a thickness of 170 mm. As shown in FIG. 1, the core center of the electromagnetic stirrer 9 is installed 75 mm below the level 17 of the molten metal surface in the mold, and a swirling flow of up to 0.6 m / s is applied in the horizontal section near the molten metal surface in the mold. . In addition, a DC magnetic field generator 8 is provided that can apply a DC magnetic field having a uniform magnetic flux density distribution in the width direction around H = 400 mm below the surface of the molten metal in the thickness direction of the slab. Since the core thickness of the DC magnetic field generator 8 is 200 mm, a maximum of 0.5 T of a DC magnetic field having substantially the same magnetic flux density is applied over a range from the molten metal level of H _H = 300 mm to H _L = 500 mm. Can do. Therefore, the ratio (Q ₂ / G ₂ ) between the second molten steel supply amount Q ₂ and G ₂ is G _2H / G ₂ ≈√ (H _H /H)=0.87 to G _2L / G ₂ ≈√ ( If it is in the range of H _L /H)=1.12, the position of the molten steel interface can be controlled within the DC magnetic field zone, so that the effect of the present invention can be exhibited.

  The specifications of the tundish 2 provided above the mold 3 are as follows. The capacity is 10 tons, and the tundish 2 has two immersion nozzles, an inner layer molten steel immersion nozzle 5 and a surface layer molten steel immersion nozzle 6, and the interval between the two immersion nozzles is 400 mm. In the tundish, a tundish weir 4 was installed at an intermediate position between two submerged nozzles. An upper weir as shown in FIG. 3A was used as the tundish weir 4, and the opening area ratio was changed depending on conditions.

The discharge hole positions of the two immersion nozzles for supplying molten steel into the mold were set to 1/4 width positions with the width center in the slab width direction. Further, in the depth direction, the discharge port of the surface molten steel immersion nozzle 6 is provided above the DC magnetic field zone 14 formed by the DC magnetic field generator 8, and the discharge port of the inner layer molten steel immersion nozzle 5 is installed below. . Specifically, the discharge hole position of the surface layer molten steel immersion nozzle 6 was 150 mm from the molten metal surface level, and the discharge hole position of the inner layer molten steel immersion nozzle 5 was 550 mm from the molten metal surface level. Here, it is confirmed that the solidification coefficient K value in the mold is about 25 mm / min ^0.5 , and it is formed up to the center of the DC magnetic field generator 8 when casting at a casting speed V _C = 1 m / min. The surface layer thickness D is about 16 mm. From this surface layer thickness, the solidified shell cross-sectional area (surface layer area of the cast slab after casting) S ₂ is determined from the DC magnetic field zone position, and from this surface layer area S ₂ and the casting speed, the lower molten steel pool and the upper molten steel pool The unit-time casting amounts to be transported to the steel are determined as G ₁ and G ₂ , respectively, and the flow rates (Q ₁ and Q ₂ ) of the first molten steel and the second molten steel can be defined so as to be equal to G ₁ and G _2. .

About flow control of the 1st molten steel 21 and the 2nd molten steel 22, it casts only with the 1st molten steel 21 from the immersion nozzle 5 for inner-layer molten steel at the time of a casting start, and the immersion nozzle 5 for inner-layer molten steel for supplying required molten steel flow rate The opening degree of the sliding nozzle 33b was confirmed. Thereafter, as the tundish head is constant, after controlling the injection amount from the ladle 1 at a constant, sliding nozzle 33b opening so that the supply amount of molten steel from the submerged nozzle 5 for lining molten steel becomes Q ₁ Was controlled at a constant. Further, with respect to the second molten steel 22, the opening degree of the sliding nozzle 33 c of the immersion nozzle 6 for surface molten steel is adjusted so that the molten metal level is constant, and as a result, the supply amount of the second molten steel 22 becomes Q _2. Controlled.

The molten steel component supplied from the ladle 1 is the first molten steel 21 component, and the first molten steel 21 is low-carbon Al killed steel. The first molten steel 21 supplied from the ladle is supplied to the first region 11 of the tundish 2, and a part thereof is supplied to the second region 12 via the opening 10 of the tundish weir 4. The second molten steel 22 in the second region is a wire feeder made of iron wire (containing Ni grains inside: 420 g / m) caulked with a 0.3 mm thick mild steel plate in the second region with respect to the first molten steel 21. Was added. In casting where Q ₂ = G ₂ , the addition speed was 3 m / min. Note that adding the Ni-containing wire under these conditions corresponds to adding 0.5% Ni to the first molten steel. In the casting in which Q ₂ and G ₂ are different (Experiment 2 below), the wire addition speed was adjusted so that the Ni content in the second molten steel was 0.5%.

  In order to investigate the Ni concentration distribution in the slab, the surface layer is 8 mm from the surface (center of the surface layer thickness), and the inner layer is 40 mm from the surface (slab ¼ thickness). Analytical samples were collected from a total of 16 locations, including the front and back surfaces at the width position and the front and back surfaces at the 1/2 width position, the surface layer and the inner layer, and the concentration was investigated. As for the surface layer thickness, the sample was cut out at almost the same position where the analysis sample was collected from the region from which the analysis sample was collected, and the concentration distribution in the thickness direction was investigated by EPMA. The thickness at which the concentration of the added element was high was determined.

With respect to the obtained analysis results, the separation degree of the inner surface layer and the uniformity of the surface layer concentration were evaluated by the following indices. The slab surface layer concentration C _O , the slab inner layer concentration C _I , the ladle concentration C _L and the concentration C _T added to the tundish and the surface layer separation degree X _O and the circumferential average value C _M within the slab surface layer thickness And the density uniformity Y obtained from the standard deviation σ was obtained using the following equation.
X _O = (C _O -C _I ) / (C _T -C _L ) ---- (1)
Y = σ / C _M ---- (2)

  A specific experimental method will be described. In order to verify the principle of the present invention described above, the following three experiments were conducted.

First, as Experiment 1, an experiment was performed in which the shape and opening area ratio of the opening 10 of the tundish weir 4 were changed, and the influence on the surface layer separation degree X _O and the concentration uniformity degree Y was investigated. Note that the magnetic flux density applied to the DC magnetic field zone 14 in the mold is set to 0.4 T, the generation of molten steel passing through the DC magnetic field zone 14 is suppressed with Q ₂ = G ₂ , and the stirring flow rate by the electromagnetic stirring device 9 in the mold is The condition was 0.4 m / s. The results are shown in FIG. 5, but when the opening area ratio is less than 10% and the molten steel temperature is low, there is a problem in terms of concentration uniformity. On the other hand, when the opening area ratio exceeded 70%, the degree of separation decreased and the uniformity of concentration also decreased. This is because the first molten steel 21 and the second molten steel 22 are mixed between the first region 11 and the second region 12 of the tundish 2. Conversely, if the opening area ratio of the tundish weir is adjusted so that the opening area ratio is 10% or more and 70% or less, the surface layer separation degree X _O is 0.9 or more and 1 or less, and the concentration uniformity Y is 0.1 or less. A slab with good surface separation and uniformity of density could be obtained.

Next, as experiment 2, the flow rate balance of the first and second molten steel is changed, and the molten steel supply amount Q ₂ from the immersion nozzle 6 for surface molten steel is different from the casting amount G ₂ that solidifies in the upper molten steel pool 15. The effect on surface separation and concentration uniformity was investigated. Here, the tundish weir 4 is adjusted so that the opening area ratio is 40%, the magnetic flux density of the DC magnetic field applied to the DC magnetic field zone 14 in the mold is 0.4 T, and the stirring flow rate by the electromagnetic stirring device 9 in the mold is Casting was carried out under the condition of 0.4 m / s, with Q ₂ varied. The results are shown in Figure 6, the conditions controlled in a range of Q ₂ / G ₂ = 0.87~1.12, can be controlled molten steel surface located within DC magnetic field zone, the surface layer separability X _O 0. 9 or more and 1 or less, and the density uniformity Y was 0.1 or less, and a slab having good surface layer separation and density uniformity could be obtained. On the other hand, under the condition of Q ₂ / G ₂ <0.87, the molten steel supply amount to the upper molten steel pool is insufficient, and the molten steel moves from the lower molten steel pool 16 to the upper molten steel pool 15 through the DC magnetic field zone 14. Therefore, the additive element content of the upper molten steel pool 15 is reduced. On the contrary, under the condition of Q ₂ / G ₂ > 1.12, the molten steel supply amount to the upper molten steel pool 15 is excessive, and the molten steel passes through the DC magnetic field zone 14 from the upper molten steel pool 15 to the lower molten steel pool 16. As a result, the additive element was contained in the inner layer of the slab.

  Further, as Experiment 3, the magnetic flux density applied to the mold was 0.4 T, the surface inner layer interface 27 position was 450 mm in the braking region, and the opening of the tundish weir was adjusted so that the opening area was 40%. The molten steel pool 15 was cast while changing the stirring flow rate of the electromagnetic stirring device 9 in the mold. The surface layer thickness of the surface layer nozzle side short side part, the thickness of the inner layer nozzle side short side part, and the thickness of the surface layer part of the width center part were investigated, and the relationship with the stirring conditions was investigated.

  FIG. 7 shows the results of investigating the difference in the circumferential distribution of the surface layer thickness with and without electromagnetic stirring in the mold. Under conditions where electromagnetic stirring is not applied, the molten steel tends to stagnate on the nozzle side that supplies molten steel to the bottom, and the surface layer tends to increase in thickness, but a swirling flow of 0.3 m / s or more is applied in the vicinity of the molten metal surface. Thus, the circumferential distribution of the surface layer thickness can be made uniform, which is preferable.

  Since this method adjusts the components of the second molten steel by the method as described above, there are few restrictions on the components added to the second molten steel, and not only Ni and C but also Si, Mn, P, S, B, Nb In addition to Ti, Al, Cu, and Mo, elements contained in steel such as strong deoxidation and strong desulfurization elements such as Ca, Mg, and REM can be added. For this reason, the new function of steel materials is attained by a comparatively simple method by changing the surface layer component of a slab.

  An experiment for casting low-carbon Al killed steel was performed using the casting apparatus schematically shown in FIG. The molten steel was melted by degassing and component adjustment by secondary refining after leaving the converter. The ladle molten steel was 250 tons.

  Two immersion nozzles of an inner layer molten steel immersion nozzle 5 and a surface layer molten steel immersion nozzle 6 were provided at the bottom of the tundish 2 having a capacity of 50 t. The distance between the two immersion nozzles is 600 mm. The tundish weir 4 was installed in the middle position, and the shape of the opening and the area ratio of the opening were changed according to the conditions.

The core center of the electromagnetic stirrer 9 was installed 100 mm below the level 17 of the molten metal surface in the mold, and a swirling flow of a maximum of 0.6 m / s was formed in the horizontal molten section in the upper molten steel pool in the mold. In addition, a direct-current magnetic field generator 8 capable of applying a direct-current magnetic field having a uniform magnetic flux density distribution in the width direction 450 mm below the level of the molten metal surface is provided in the thickness direction of the slab. A maximum DC magnetic field of 0.5T can be applied. Since the direct current magnetic field generator 8 has a core thickness of 200 mm, a direct current magnetic field having substantially the same magnetic flux density can be applied at a maximum of 0.5 T over a range of 350 to 550 mm from the molten metal surface level. Therefore, the range of the DC magnetic field zone 14 is such that the height from the hot water level is H _H = 350 mm to H _L = 550 mm. Therefore, the ratio (Q ₂ / G ₂ ) between the second molten steel supply amount Q ₂ and G ₂ is G _2H / G ₂ ≈√ (H _H /H)=0.88 to G _2L / G ₂ ≈√ ( If it is within the range of H _L /H)=1.11, the position of the molten steel interface 27 can be controlled within the DC magnetic field zone 14, and thus the effect of the present invention can be exhibited.

  With respect to the position of the immersion nozzle for supplying molten steel into the mold, first, in the width direction of the slab, the position was set to a quarter width position across the width center. Moreover, about the discharge hole position of an immersion nozzle, the upper side (immersion nozzle 6 for surface layer molten steel) part and the lower side (inner layer molten steel) of the braking area | region (DC magnetic field zone 14) formed by a DC magnetic field generator in a depth direction. It was installed in the immersion nozzle 5). Specifically, the discharge hole position of the surface layer molten steel immersion nozzle 6 was set to 200 mm from the molten metal level, and the discharge hole position of the inner layer molten steel immersion nozzle 5 was set to 600 mm from the molten metal level.

The casting conditions were as follows: 1200 mm width, 250 mm thickness, casting speed 1.5 m / min. Here, it has been confirmed that the solidification coefficient K value in the mold is about 25 mm / min ^0.5 , and the surface layer thickness formed up to the center of the DC magnetic field generator 8 in the mold is about 14 mm. From this surface layer thickness, the solidified shell cross-sectional area (surface layer area of the cast slab after casting) S ₂ is determined from the DC magnetic field zone position, and from this surface layer area S ₂ and the casting speed, the lower molten steel pool and the upper molten steel pool The unit-time casting amounts to be transported to the steel are determined as G ₁ and G ₂ , respectively, and the flow rates (Q ₁ and Q ₂ ) of the first molten steel and the second molten steel can be defined so as to be equal to G ₁ and G _2. .

Regarding the flow control of the first molten steel and the second molten steel, casting was performed only with the first molten steel at the start of casting, and the opening degree of the sliding nozzle 33b of the inner layer molten steel immersion nozzle 5 for supplying the necessary molten steel flow rate was confirmed. Thereafter, the amount of injection from the ladle is controlled to be constant by adjusting the flow rate of the sliding nozzle 33a so that the tundish head is constant, and then from the immersion nozzle 5 for inner layer molten steel for the present invention example and some comparative examples. supply molten steel amount was controlled sliding nozzle opening at a constant such that the Q _1. Furthermore, about the 2nd molten steel, the sliding nozzle 33c opening degree adjustment of the immersion nozzle 6 for surface molten steel is adjusted so that the molten metal level measured with the molten metal level meter 31 may become constant, and the supply amount of the 2nd molten steel is obtained as a result. Control was performed so that Q ₂ was obtained.

The component adjustment in the tundish will be described. An iron wire (outer diameter 12 mm, C powder content: 226 g / m) caulked with a mild steel plate having a thickness of 0.3 mm was added to the second region 12 of the tundish using a wire feeder. The addition rate of the wire was set to an addition rate corresponding to adding 0.3% C to the second molten steel corresponding to the flow rate Q ₂ of the second molten steel in each example.

Next, the contents of experiments conducted to verify the principle of the present invention will be described.
◆ Experiment 1: Casting is performed under the condition that Q ₂ / G ₂ is changed by changing the flow rate balance of the first and second molten steels.
◆ Experiment 2: Casting by changing the opening shape and opening area ratio of the tundish weir.
Experiment 3: Casting is performed by changing the applied current of the electromagnetic stirring device in the mold and changing the swirling flow velocity formed near the molten metal surface.
Here, in the experiment described above, a DC magnetic field of 0.5 T was applied by the in-mold DC magnetic field generator 8.

In order to investigate the C concentration distribution in the slab, the surface layer is 7 mm from the surface (center of the surface layer thickness), and the inner layer is 60 mm from the surface (slab ¼ thickness). Analytical samples were collected from a total of 16 locations, including the front and back surfaces at the width position and the front and back surfaces at the 1/2 width position, the surface layer and the inner layer, and the concentration was investigated. As for the surface layer thickness D _R, for the region from which the analysis sample was collected, for the region from the surface to 60 mm, the sample was cut out at almost the same position where the analysis sample was collected, and the concentration distribution in the thickness direction was investigated by EPMA. did. The thickness at which the concentration of the added element was high was determined.

With respect to the obtained analysis results, the separation degree of the inner surface layer and the uniformity of the surface layer concentration were evaluated by the following indices. The slab surface layer concentration C _O , the slab inner layer concentration C _I , the ladle concentration C _L and the concentration C _T added to the tundish and the surface layer separation degree X _O and the circumferential average value C _M within the slab surface layer thickness Then, the density uniformity Y obtained from the standard deviation σ is obtained using the following equation.
X _O = (C _O -C _I ) / (C _T -C _L ) ---- (1)
Y = σ / C _M ---- (2)

  The results of Experiment 1, Experiment 2, and Experiment 3 are shown in Table 1, Table 2, and Table 3, respectively.

In Table 1, the upper weir shown in FIG. 3A is used as the tundish weir, the depth of the upper weir is adjusted, the opening area ratio is 40% of the whole, and the magnetic flux density of the DC magnetic field applied to the mold is 0.5T. The casting was performed by changing the ratio (Q ₂ / G ₂ ) between the second molten steel supply rate Q ₂ and G _{2 under} the condition that the stirring flow rate by the electromagnetic stirring device in the mold was 0.4 m / s. For the interface position (distance H _R from the meniscus to the interface), the surface thickness D _{R of the} slab is measured,
D _R = K√ (H _R / V _C )
Was calculated in reverse.

Under the conditions of the present invention 1, 2 and 3 in which the interface 27 position is controlled within the braking region by the DC magnetic field in the range of Q ₂ / G ₂ = 0.88 to 1.11, good results are obtained in both the degree of separation and uniformity. It was. On the other hand, in Comparative Examples 3 and 4 that deviated to Q ₂ / G ₂ <0.87, the interface position remained at the upper end of the DC magnetic field zone, and both the surface layer separation degree X _O and the concentration uniformity degree Y were poor. In Comparative Examples 1 and 2, which deviated from Q ₂ / G ₂ > 1.11, the interface position remained at the lower end position of the DC magnetic field zone, and relatively good surface layer separation and concentration uniformity were obtained. It was insufficient compared with.

In Table 2, the magnetic flux density applied to the mold is 0.5 T, Q ₂ / G ₂ = 1 (the table / inner layer interface position is 450 mm in the braking region), and the stirring flow rate by the electromagnetic stirring device in the mold is 0.4 m / s. Under the conditions, the tundish weir shown in FIG. 3 was used, and casting was performed while changing the opening area ratio. In the present inventions 4 to 10 in which the opening area ratio was adjusted to 10% to 70%, good results were obtained in both surface layer separation and concentration uniformity, but there was no weir (opening area ratio 100%). In Comparative Examples 9 and 10 in which the area ratio was too large, both the surface layer separation degree and the concentration uniformity were insufficient. Further, Comparative Examples 6 to 8 in which the opening area ratio was too small were unsuitable due to conditions for uniformity.

In Table 3, the magnetic flux density applied to the mold is 0.5 T, Q ₂ / G ₂ = 1 (the inner layer interface position is 450 mm in the braking zone), and the upper weir depth of the tundish shown in FIG. The casting was carried out by changing the stirring flow rate of the electromagnetic stirring device in the mold under the condition that the area ratio was adjusted to 40%. The surface layer thickness of the immersion nozzle 6 side for the surface layer molten steel, the thickness of the short side portion of the immersion nozzle 5 for the inner layer molten steel, and the thickness of the surface layer portion of the central portion of the width were investigated, and the relationship with the stirring conditions was investigated. In the invention 13 in which stirring flow by electromagnetic stirring in the mold was not applied, the surface layer thickness was not uniform, although this was not a problem in quality. On the other hand, in this invention 11 and 12 which gave the stirring flow by the electromagnetic stirring apparatus in a casting_mold | template, the surface layer thickness of the immersion nozzle 6 for surface layer molten steel 6 and the short side thickness of the immersion nozzle 5 for inner layer molten steel are substantially the same. Met. Therefore, it is preferable because the thickness of the surface layer becomes uniform in the slab circumferential direction by applying a stirring flow by electromagnetic stirring in the mold.

DESCRIPTION OF SYMBOLS 1 Ladle 2 Tundish 3 Mold 4 Tundish weir 5 Inner layer molten steel immersion nozzle 6 Surface layer molten steel immersion nozzle 7 Component addition device 8 DC magnetic field generator 9 Electromagnetic stirrer 10 Opening 11 First region 12 Second region 13 Ladle Injection flow 14 DC magnetic field zone 15 Upper molten steel pool 16 Lower molten steel pool 17 Meniscus (water surface)
18 Molten surface 20 Molten steel 21 First molten steel 22 Second molten steel 23 Solidified shell 24 Upper molten steel pool solidified portion (surface layer portion)
25 Lower molten steel pool solidification part (inner layer part)
26 Molten steel immersion portion 27 Interface 28 Stagnation region 29 Slab 30 Immersion nozzle 31 Molten surface level gauge 32 Control device 33 Flow rate adjustment device

Claims (4)

A method for producing a multi-layer slab in which the component composition of the surface layer and the inner layer of the slab is different, and an immersion nozzle for inner layer molten steel is disposed at the bottom of the tundish, and further, an immersion nozzle for surface layer molten steel is disposed downstream thereof, A tundish weir is installed between these two immersion nozzles, the tundish weir has an opening with an opening area ratio of 10% to 70% in the molten steel immersion part, and a DC magnetic field in the thickness direction over the entire width in the mold width direction. The upper part of the strand sandwiching the DC magnetic field zone formed by the DC magnetic field generator is the upper molten steel pool, the lower part is the lower molten steel pool, and the upper part from the immersion nozzle for the surface molten steel Supply molten steel to the molten steel pool, supply molten steel from the immersion nozzle for inner layer molten steel to the lower molten steel pool,
The ladle molten steel injection side divided by the tundish weir is the first region, the opposite side is the second region, and a predetermined element or its alloy is continuously applied to the molten steel in the tundish on the second region side. By adding and adjusting the concentration, the ladle molten steel and two types of molten steel with different composition from the ladle molten steel are held in the tundish while the molten steel contained in the first region is immersed in the inner layer molten steel. To the lower molten steel pool, the molten steel accommodated in the second region is supplied from the surface molten steel immersion nozzle to the upper molten steel pool,
A continuous casting method for a multilayer slab, characterized in that a molten steel amount consumed by solidification in each molten steel pool is supplied from each of the two immersion nozzles into a mold.   The amount of molten steel supplied to the lower molten steel pool is controlled by controlling the position of the molten steel interface in the DC magnetic field zone using the relationship between the tundish head and the sliding nozzle opening of the immersion nozzle for inner layer molten steel and the molten steel flow rate. 2. The multilayer slab according to claim 1, wherein the supply amount of the molten steel whose components are adjusted in step 1 is supplied from the surface layer molten steel immersion nozzle to the upper molten steel pool is controlled so that the molten metal level in the mold is constant. Continuous casting method.   3. A continuous casting method for a multilayer slab according to claim 1, wherein a swirling flow is formed in a horizontal section in the vicinity of the molten metal surface in the mold above the DC magnetic field zone. An apparatus for producing a multi-layer slab in which the composition of the surface layer and the inner layer of the slab is different, the inner layer molten steel immersion nozzle at the bottom of the tundish that holds the molten steel from the ladle, and the surface layer further downstream Molten steel immersion nozzles are provided at intervals shorter than the casting width, and tundish weirs are installed between them. The tundish weir has an opening with an opening area ratio of 70% or less in the molten steel immersion part. It has a component addition device that adds components to the molten steel in the region opposite to the ladle molten steel injection divided by the weir,
In the mold, an electromagnetic stirrer that forms a swirl flow in the horizontal section near the molten metal surface, and a DC magnetic field generator that applies a DC magnetic field in the thickness direction over the entire mold width direction are provided below the electromagnetic stirrer. The upper part of the strand across the formed DC magnetic field zone is the upper molten steel pool, the lower part is the lower molten steel pool, the molten steel is supplied from the surface molten steel immersion nozzle to the upper molten steel pool, and the lower molten steel is supplied from the inner layer molten steel immersion nozzle. A continuous casting apparatus for multi-layer cast slabs, characterized in that molten steel is supplied to a pool.

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