JP2001321903A - Method for producing continuously cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably produce a duplex-layer cast slab in which the supplying control of molten steel into the upper and the lower pool parts having different solute element concentrations with DC magnetic field zone as the boundary, is extremely easy and also, the foundation of the solute element concentration on the surface layer part of the cast slab is quite small. SOLUTION: When the molten steel is poured by using an immersion nozzle in the state of impressing the DC magnetic field zone over the whole width of the cast slab in the thickness direction of the cast slab at a fixed lower position from the molten steel surface level in a continuous casting mold, at least two spouting holes in the upper and the lower parts are arranged in the immersion nozzle. Further, the lower part spouting hole is disposed so as to satisfy the following inequality (1) and the supplying speed of the molten steel from the upper part spouting hole is smaller than the solidification speed in the upper pool part and also, a specified solute element is added to the molten steel at the upper pool part. 0<h<(1/2).w.tan θ...(1), wherein, θ: the downward angle ( deg.) of the lower spouting hole, w: the length (m) in the width direction of the mold and h: the distance (m) from the lower spouting hole center to the height center of a magnetic pole.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a continuous cast slab having a gradient composition in which the concentration of a specific solute element is higher in a surface layer portion than in the inside of the slab.

[0002]

2. Description of the Related Art Conventionally, various methods have been proposed for producing a slab having a composition different from that of a surface layer in a surface layer portion by continuous casting. For example, Japanese Patent Publication No. 3-20295 discloses that a direct magnetic flux is applied over the entire width of a slab in a direction perpendicular to the casting direction at a position below a fixed distance from a molten metal level in a continuous casting mold. A method for manufacturing a multilayer slab by supplying different metals above and below a static magnetic field zone formed by a magnetic flux is described.

[0003] Japanese Patent Application Laid-Open No. Hei 7-51801 discloses that molten steel is vertically injected together with gas into a mold for continuous casting.
A DC magnetic field is applied over the entire width in the mold above the molten steel injection position to reduce the upward flow of the molten steel, and an element different from the molten steel component is added to the molten steel above the DC magnetic field application position. A method for producing a multi-layered steel sheet comprising forming a surface layer of an alloy steel on a steel surface by using the molten steel at an upper portion as an alloy molten steel by levitation stirring of the injected gas is described.

Further, Japanese Patent Application Laid-Open No. 8-257692 discloses an immersion nozzle having a nozzle discharge hole above and below while forming a braking zone by applying a DC magnetic field to the entire width of a mold below a certain distance from a meniscus. By injecting molten steel of a constant composition and using a wire to continuously supply alloying elements to the molten steel pool above the braking zone and stirring by molten steel injection flow, casting with a uniform alloy element concentration in the surface layer is performed. A method for making a piece is disclosed.

[0005]

However, the method disclosed in Japanese Patent Publication No. 3-20295 is an extremely complicated process of separately refining the molten steel for the surface layer and the molten steel for the inside of the slab, so that a production obstacle is caused. It is extremely difficult to control the amount of molten steel in accordance with the solidification rate in the upper and lower parts of the magnetic field zone independently from each of the Danishes, making stable production difficult, resulting in lower product yield. There was a problem of doing.

In this respect, in the manufacturing method described in Japanese Patent Application Laid-Open No. Hei 7-51801, only one kind of molten steel is supplied from the dandysh, and the supply is performed so that the level of the molten metal in the mold is kept constant only in the lower part of the magnetic field zone. Therefore, the shortage of the solidification amount at the upper part of the magnetic field zone naturally flows from the lower part of the magnetic field zone, and the strict control as described above is not required. However, in this case, the molten steel flow from the lower part to the upper part of the magnetic field zone slowly flows under the influence of the DC magnetic field. There was a problem that the density difference could not be eliminated.

Further, the manufacturing method described in Japanese Patent Application Laid-Open No. 8-257692 supplies the same molten steel to the upper and lower pools from one nozzle having discharge holes above and below the magnetic field zone. The complicated process of preparing molten steel separately is not required. However, in this method, since the ratio of the supply amount of molten steel to the upper and lower pools is controlled by adjusting the ratio of the inner diameters of the upper and lower discharge holes, non-metallic inclusions in the molten steel adhere to the discharge holes. Therefore, if the supply ratio of molten steel to the lower pool is slightly reduced, the boundary between the upper and lower molten steels having different compositions is out of the magnetic field zone, the alloy component in the upper pool flows out to the lower pool, and the product There is a disadvantage that the yield is significantly reduced. Conversely, if the flow rate to the upper pool is reduced or the casting speed has to be reduced due to problems during operation, molten steel with less alloy component will flow into the upper pool from the lower pool. . At this time, the molten steel flowing from the lower part to the upper part rises along both ends in the width direction of the mold due to the influence of the injection flow from the lower discharge hole, so that the alloy component at both ends of the slab decreases, After all, there is a problem that the product yield is remarkably reduced.

The present invention advantageously solves the above-mentioned problems, and not only facilitates the supply control of molten steel to the upper and lower pools, but also simply and appropriately adjusts the concentration of solute elements in the surface layer of the slab. It is an object of the invention to propose an advantageous method for producing continuous cast slabs.

[0009]

That is, the gist of the present invention is as follows. 1. At the time of continuous casting of the molten metal, a DC magnetic field was applied across the entire width of the slab in a direction crossing the thickness of the slab at a position below a predetermined distance in the casting direction from the level of the molten metal in the continuous casting mold. When injecting molten steel into the molten steel pool in the zone or above the DC magnetic field zone using the immersion nozzle, at least two upper and lower discharge holes are provided in the immersion nozzle, and the lower discharge hole is expressed by the following formula (1). Satisfactorily arranged, the supply speed of molten steel from the upper discharge hole is made smaller than the speed consumed by solidification in the molten steel pool above the height center of the DC magnetic field zone, and Alternatively, by adding a specific solute element to the molten steel above the DC magnetic field zone, the concentration of the solute element in the molten steel in the upper pool is increased to adjust the solute element concentration in the surface layer of the slab. Ream Manufacturing method of continuous cast slab. 0 <h <(1/2) · w · tan θ --- (1) where θ: downward angle of the lower discharge hole (°) w: length of the mold in the width direction (m) h: Distance from the center of the lower discharge hole to the center of the magnetic pole height (m)

[0010] 2. 2. The method for producing a continuous cast slab according to claim 1, wherein the upper discharge hole uses an immersion nozzle designed so as to satisfy the following equation (2). Note h ′> (1/2) · w · tan θ ′ --- (2) where θ ′: downward angle of upper discharge hole (°) w: length of mold in width direction (m) h ': Distance from the center of the upper discharge hole to the center of the magnetic pole height (m)

3. The above 1 or 2 wherein the upper discharge hole and the lower discharge hole use an immersion nozzle designed so as to satisfy the following formulas (3) and (4).
A method for producing the continuous cast slab according to the above. 0 <h ≦ 1.5 V · sin θ --- (3) d ≦ 0.5 --- (4) where h: distance (m) from the center of the lower discharge hole to the center of the height of the magnetic pole V: lower discharge hole Θ: Downward angle of the lower discharge hole (°) d: Distance from the center of the upper discharge hole to the center of the lower discharge hole (m)

4. The method for producing a continuous cast slab according to the above 1, 2 or 3, wherein a supply speed of the molten steel from the upper discharge hole uses an immersion nozzle designed so as to satisfy the following formula (5). . 0.3 · Q ≦ Q '≦ 0.9 · Q --- (5) Where, Q': supply speed of molten steel supplied from upper discharge hole
(ton / min) Q: Consumption rate of molten steel solidified in molten steel pool above the center of magnetic pole height (ton / min)

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings. FIG. 1 schematically shows an example of an injection procedure of molten steel according to the present invention. In this example, a nozzle having a single lower discharge hole and two upper discharge holes is used as the immersion nozzle, and molten steel supplied from the lower discharge hole flows out almost vertically. In the figure, reference numeral 1 denotes a mold, reference numeral 2 denotes an immersion nozzle, and reference numeral 3 denotes a magnetic pole. The magnetic pole 3 allows a DC magnetic field zone to be applied in the thickness direction of the slab over the entire width of the slab. Reference numeral 4 indicates the height center of the magnetic pole. Also, 5
Is the lower discharge hole of the immersion nozzle 2, 6a and 6b are the upper discharge holes of the immersion nozzle 2, respectively, 7 indicates the jet from the lower discharge hole 5, and 8a and 8b indicate the jet from the upper discharge holes 6a and 6b. ,
Reference numeral 9 indicates a backflow from the lower pool to the upper pool in the DC magnetic field zone. 10 is a solute element (wire), 11 is a position where the solute element 10 is added, and 12 is a solidified shell. In the drawing, w is the width of the mold, θ and θ ′ are the angles of the lower and upper discharge holes 5 and 6 of the immersion nozzle 2 (downward angles with the horizontal direction being 0), and h is the center of the lower discharge hole. , The distance from the center of the upper discharge hole to the center of the height of the magnetic pole, d is the distance from the center of the upper discharge hole to the center of the lower discharge hole, and A is the distance from the level of the metal surface in the mold to the magnetic pole. Is the distance to the height center.

In FIG. 1, the molten steel jet 7 supplied from the lower discharge hole 5 of the immersion nozzle 2 once flows into the lower pool of the magnetic field zone, but the molten steel jet 7 supplied from the upper discharge hole 6 to the upper pool. Is lower than the consumption rate Q of the molten steel solidified and consumed in the upper pool, only the shortage of the molten steel once flowing into the lower pool in the upper pool naturally flows back to the upper pool. Will be. Therefore, in the present invention, as in the method described in Japanese Patent Application Laid-Open No. Hei 7-51801, there is no problem regarding the control of the supply speed of molten steel.

In the present invention, since the molten steel jet 7 supplied from the lower discharge hole 5 of the immersion nozzle 2 crosses the DC magnetic field zone, the induced current 13 around the jet 7 as shown in FIG.
Occurs. As a result, the interaction between the induced current 13 and the DC magnetic field 14 generates an electromagnetic force 15 as shown in FIG. As described above, a force in a direction opposite to the direction of the jet 7, that is, an electromagnetic braking force is generated in the jet portion 16.
Is inevitably generated on both sides of the jet 16, a similar force is generated on both sides, and a flow in the opposite direction is likely to occur on both sides of the jet 16.

As shown in FIG. 4, as a result, the inflow of the molten steel from the lower pool to the upper pool in the magnetic field zone occurs only at both sides of the jet 16. As described above, the inflow of molten steel from the lower pool to the upper pool occurs only in a specific area such as on both sides of the jet part 16 from the lower discharge hole 5 and gathers on both sides of the nozzle. However, since the upper discharge hole 6 is present therein, the molten steel flowing from the lower pool is drawn into the jet 8 from the upper discharge hole 6 and is flushed with the molten steel supplied from the upper discharge hole 6 toward both ends of the mold. While being added, it is uniformly mixed with the added alloy.

Accordingly, the solute element concentration distribution in the mold according to the present invention is as shown in FIG. 5, and the resulting slab is as shown in FIG. In FIG. 5, reference numeral 17 denotes a region where the solute element is concentrated in the mold, 18 denotes a region where the degree of the concentration of the solute element is low, 19 denotes a region where the solute element is not concentrated, and FIG. 20 is the solute element concentrated portion in the slab surface layer, 21 is the solute element concentration transition layer of the slab, the portion where the solute element concentration is low, and 22 is the inner layer of the slab without solute element concentration Part.

As described above, in the present invention, the inflow site of molten steel from the lower pool to the upper pool is limited to a specific area on both sides of the jet part, and the inflow molten steel is in the vicinity of the jet from the upper discharge hole and the nozzle. Even if non-metallic inclusions in the molten steel adhere to the upper discharge hole due to the merge, the flow rate ratio from the upper discharge hole is reduced. Does not change, so the solute element concentration distribution in the upper pool does not change. Conversely, even when the flow rate ratio from the lower discharge hole is reduced, since the molten steel originally flows from the lower part, only the flow rate is reduced, and the solute element concentration distribution in the upper pool does not change. Furthermore, in the present invention, since the molten steel supplied to the lower pool is supplied from the upper part of the magnetic field zone, the molten steel is decelerated when passing through the magnetic field zone, and the entrainment of non-metallic inclusions causing internal defects below is also possible. It decreases and the internal quality improves.

For comparison, in the case where molten steel is supplied by arranging discharge ports of immersion nozzles in the upper and lower pools of the magnetic field zone as in the method described in JP-A-8-257692, FIG. 7 shows the result of examining the flow of molten steel when the supply ratio was increased. As shown in the figure, in this method, the position of the inflow from the lower portion is concentrated at both ends of the mold due to the effect of the strong jet 7 'from the lower discharge hole 5' (see FIG. 8). In the solute element concentration distribution, as shown in FIG. 9, a region where the degree of concentration of the solute element is low appears at both ends of the mold, and as a result, as shown in FIG. A surface layer having a low alloy concentration is generated.
Conversely, if the supply ratio of molten steel to the upper part increases, the solute in the upper pool flows out to the lower pool, and the solute concentration in the surface layer decreases.

The above problem can be avoided if the outflow from the upper and lower discharge holes can be controlled with high accuracy. However, it is actually extremely difficult to control the outflow from the nozzle with high accuracy. It is difficult, and the amount of outflow from the nozzle fluctuates to some extent due to nozzle clogging or drift in the mold. Therefore, it cannot be said that it is practically impossible to precisely control the concentration of the surface layer portion by the method described in JP-A-8-257692.

As described above, in the present invention, it is necessary to appropriately set the lower discharge hole so that a backflow easily occurs around the molten steel jet supplied to the lower pool. Therefore, as a result of various studies on this point, it has been found that it is necessary to satisfy the following relationship between the positions of the upper and lower ejection holes, the ejection angles, and the application positions of the magnetic field. First, the lower discharge hole needs to satisfy the relationship of the following expression (1), and more preferably, the relationship of the following expression (3). 0 <h <(1/2) · w · tan θ --- (1) 0 <h ≦ 1.5 V · sin θ --- (3) where θ: downward angle (°) w of the lower discharge hole : Length in the width direction of the mold (m) h: Distance from the center of the lower discharge hole to the center of the height of the magnetic pole (m) V: Average flow velocity of the discharge flow in the lower discharge hole (m / s)

Here, the reason that the expression (1) is necessary is that if this condition is not satisfied, the jet collides with the wall surfaces at both ends before the jet sufficiently penetrates the magnetic field zone, and the jet flows from the lower pool sufficiently. This is because it is not possible to cause the backflow of water. Also
The reason that the formula (3) is preferable is that the jet is attenuated in inverse proportion to the distance from the ejection hole, so that when the lower ejection hole is farther from the magnetic pole, the jet is diffused before penetrating the magnetic field band, and also below the center of the magnetic pole. This is because the backflow generated when the discharge hole is provided is decelerated by the magnetic field above the center of the magnetic field, and the backflow cannot be sufficiently caused. Where V
Is obtained by dividing the amount of molten steel flowing out from the lower discharge hole (m ³ / s) by the discharge flow cross-sectional area (m ² ). It is necessary to design the shape of the discharge hole so that the jet does not contact the long-side solidified surface in the upper pool.

On the other hand, since it is necessary to prevent the molten steel flow from the upper discharge hole from flowing out to the lower pool, the following equation (2)
In order to allow the molten steel flowing from the lower pool to sufficiently reach the solidified surface in the upper pool, the molten steel flow from the upper discharge hole should be sufficiently drawn.
It is desirable to satisfy the following equation (4). h ′> (1/2) · w · tan θ ′ --- (2) d ≦ 0.5 --- (4) where θ ′: downward angle (°) of the upper discharge hole w: width of the mold Direction length (m) h ': Distance from the center of the upper discharge hole to the center of the height of the magnetic pole (m) d: Distance from the center of the upper discharge hole to the center of the lower discharge hole (m)

In addition, the supply speed of molten steel from the upper discharge port is determined by taking into account fluctuations in the supply ratio of molten steel from the upper and lower discharge ports.
It must be set lower than the speed consumed by coagulation in the upper pool. However, if the supply rate of molten steel is less than 0.3 times the consumption rate of molten steel in the upper pool, even if the above condition (4) is satisfied,
In some cases, a jet velocity sufficient to draw in the molten steel supplied from the lower pool and the added solute elements and mix the two may not be obtained. Therefore, the supply speed Q '(ton / min) of the molten steel supplied from the upper discharge port and the consumption speed Q (ton / min) of the molten steel solidified in the upper molten steel pool satisfy the relationship of the following equation (5). It is preferred that 0.3 ・ Q ≦ Q ′ ≦ 0.9 ・ Q --- (5)

FIG. 11 shows the results of a study on the transition of the ratio between the surface Ni concentration and the internal Ni concentration when Q '/ Q is variously changed. In this example, the ratio between the surface Ni concentration and the internal Ni concentration is controlled to be 10. As shown in the figure, when Q ′ / Q exceeds 0.9, the Ni concentration in the surface layer and the internal Ni concentration is controlled.
The concentration ratio decreases. The reason for this is that the supply ratio of molten steel from the upper and lower discharge holes fluctuates as described above, so that Q ′ /
This is because when Q exceeds 0.9, molten steel flows from the upper pool to the lower pool.

FIG. 12 shows the influence of Q '/ Q on the variation of the Ni concentration (expressed by the ratio of the maximum Ni concentration and the minimum Ni concentration) in the surface layer portion obtained by sampling from a plurality of portions of the surface layer portion. The result of the examination is shown. The closer this ratio is to 1, the smaller the variation in solute concentration in the surface layer is, but the Q '/ Q exceeds 0.9,
When Q '/ Q is less than 0.3, this variation increases. When Q '/ Q exceeds 0.9, the concentration ratio becomes large because the molten steel flows out from the upper pool to the lower pool to cause local flow, while Q' / Q becomes 0.3. The lower the concentration, the higher the concentration ratio is because a jet velocity sufficient for circulating mixing in the upper pool cannot be obtained.

In particular, when the operation is performed under the conditions satisfying the above-mentioned expressions (1) to (5), a uniform slab without lowering the concentration of solute elements in the surface layer of the slab is obtained. It was determined that it could be manufactured with a high yield.

In the above example, the lower discharge hole faces downward.
Although only the figure in the case of a single hole of 90 ° has been described, in the present invention, it is important to locally generate the inflow site of molten steel from the lower pool to the upper pool, and therefore, as shown in FIG. 13, Even in the case where the number of the lower discharge holes is two as used in ordinary continuous casting, a desired local inflow portion can be generated if the condition of the above-mentioned formula (1) is satisfied. In order to further enhance the effect of locally forming the inflow portion and the effect of attenuating the jet from the lower discharge hole, it is preferable that the position of the lower discharge hole be above the center of the magnetic pole.

Here, with respect to the strength of the applied magnetic field, if the strength is too small, the braking effect by the magnetic field is weakened, and the molten steel in the upper pool and the lower pool may be mixed. Inflow is too strong,
Unnecessary molten steel is supplied to the upper pool, and as a result, the molten steel in the upper pool may flow out at a position distant from the inflow position. It is important that the strength is appropriate so as not to cause mixing of the alloying elements or to cause poor uniform dissolution of the alloying elements.
Similarly, if the flow rate of Ar gas injected into the nozzle is too large, the flow of Ar gas into the upper pool becomes too strong,
It is desirable that the flow rate of Ar gas is 20 liter / min or less, since a bubble defect is easily generated.

Regarding the width (height direction) of the applied DC magnetic field band, if it is too small, the braking effect is not sufficient, while if it is too large, the power supply capacity or coil size required to generate the magnetic field is large. Since the equipment cost increases, the width of the magnetic pole in the height direction is 0.1 to
It is preferable that the thickness is about 0.5 m.

[0031]

EXAMPLE 1 A continuous cast slab was produced using the continuous casting mold shown in FIG. 1 under the following conditions. -Inside diameter of mold Long side (w): 0.4 m, Short side: 0.11 m-DC magnetic field application position (distance from mold level in mold to center of magnetic pole height) A: 0.347 m Strength of applied magnetic field: 0.3 T Magnetic pole height: 0.15 m ・ Immersion nozzle Nozzle inner diameter: 40 mm Upper discharge hole: 2 holes (hole size 10 × 10 mm) Discharge angle θ '= 0 ° (horizontal) Lower discharge hole: single hole (hole size) 28 mmφ) Discharge angle θ = 90 ° (vertical downward) ・ Immersion depth of lower discharge hole (from the level in the mold to the lower end of the lower discharge hole): 0.34 m ・ Immersion depth of the upper discharge hole (from the level in the mold to the upper part) To the center of the discharge hole): 0.177 m ・ Distance h from the center of the lower discharge hole to the center of the height of the magnetic pole h:
0.007 m ・ Distance h 'from center of upper discharge hole to center of height of magnetic pole:
0.170 m ・ Casting speed Vc: 1.6 m / min (Casting amount: 0.49 t / min) ・ Molten steel supply speed Q 'from the upper discharge hole: 0.76Q (Consumption speed of molten steel solidified above the height center of the magnetic pole) Q of 0.76
・ Solute element (pure Ni wire) Supply position of pure Ni wire (horizontal distance from upper discharge hole to both ends): 0.1 m Melting position of pure Ni wire (distance in height direction to upper discharge hole center) ： 0.12m Wire feeding speed ： 3.5 kg / min

It is known that the growth thickness d (m) of the solidified shell in the continuous caster is given by the following equation (6). d = 0.022 x (A / Vc) ^0.5 --- (6) Here, A is the distance (m) from the level of the molten metal to the center of the height of the magnetic pole, and Vc is the casting speed (m / min). Therefore, from the above equation (6), it can be seen that the thickness of the solidified shell at the boundary between the upper and lower pools is about 10.2 mm. As a result, Q =
0.112 t / min. On the other hand, Q 'is known to be 17.5% of the total throughput from the water model and the like, and Q' = 0.0853 t / min. Therefore, Q '= 0.76 Q
Becomes

Example 2 A continuous cast slab was produced using the continuous casting mold shown in FIG. 1 under the following conditions.・ Inner diameter of the mold Long side (w): 0.4 m, Short side: 0.11 m ・ Position of DC magnetic field (distance from mold level in mold to center of height of magnetic pole) A: 0.347 m Strength of applied magnetic field: 0.3 T ・ Immersion nozzle Nozzle inner diameter: 40 mm Upper discharge hole: 2 holes (hole size 10 × 10 mm) Discharge angle θ '= 0 ° (horizontal) Lower discharge hole: single hole (hole size 28 mmφ) Discharge angle θ = 90 ° (vertically downward) ・ Immersion depth of lower discharge hole (from the level in the mold to the lower end of the lower discharge hole): 0.290 m ・ Immersion depth of the upper discharge hole (from the level in the mold to the center of the upper discharge hole): 0.127 m ・ Distance h from the center of the lower discharge hole to the center of the magnetic pole height h:
0.057 m ・ Distance h 'from center of upper discharge hole to center of height of magnetic pole:
0.22 m ・ Casting speed Vc: 1.2 m / min (Casting amount: 0.37 t / min) ・ Molten steel supply speed Q 'from the upper discharge hole: 0.63 Q (Consumption speed of molten steel solidified above the height center of the magnetic pole) 0.63 of Q
・ Solute element (pure Ni wire) Supply position of pure Ni wire (horizontal distance from upper discharge hole to both ends): 0.1 m Melting position of pure Ni wire (distance in height direction to upper discharge hole center) : 0.05m Wire feeding speed: 3.6kg / min

As for the growth thickness d (m) of the solidified shell in the above-mentioned continuous caster, it can be seen from the above equation (6) that the solidified shell thickness at the boundary between the upper and lower pools is about 11.8 mm. As a result, Q = 0.0965 t / min. On the other hand, Q 'is known to be 16.5% of the total throughput from the water model and the like, and Q' = 0.0611 t / min, and therefore Q '= 0.63Q.

For comparison, a continuous cast slab was manufactured under the condition that the lower discharge hole was provided below the magnetic field zone (the method disclosed in Japanese Patent Application Laid-Open No. 8-257692). The casting conditions were as follows: ・ DC magnetic field application position (distance from the level of the molten metal in the mold to the center of the magnetic pole height) A: 0.347 m Strength of the applied magnetic field: 0.3 T ・ Immersion nozzle Nozzle inner diameter: 40 mm Upper discharge hole : 2 holes (hole size 12.2 x 12.2mm) Discharge angle θ '= 0 ° (horizontal) Lower discharge hole: Single hole (hole size 28mmφ) Discharge angle θ = 90 ° (vertical downward) ・ Lower discharge hole Immersion depth (from the mold level in the mold to the lower end of the lower discharge hole): 0.547 m ・ Immersion depth in the upper discharge hole (from the mold level in the mold to the center of the upper discharge hole): 0.3 m ・ Casting speed Vc: 1.6 m / min (・ Casting rate: 0.49 t / min) ・ Molten steel supply speed Q ′ from upper discharge hole = Q (same as consumption speed Q of molten steel solidifying above the center of magnetic pole height) ・ Magnet pole height from lower discharge hole Distance to center h: -0.2
m (0.2 m below the center of the magnetic pole) ・ Distance h 'from the center of the upper discharge hole to the center of the magnetic pole height:
0.047 m 2, and other conditions of addition of Ni were the same as in Example 1.

With respect to the slab continuously cast under each of the above conditions, the results of examining the incidence of defects (surface concentration defects and internal defects) were compared.
See Figure 15. Regarding surface layer density defects, those having a ratio of the maximum Ni concentration to the minimum Ni concentration in the surface layer portion of 1.20 or more were determined to be defective. As shown in FIG. 14, it can be seen that in the case of the present invention, the variation in the surface layer concentration is extremely small as compared with the comparative example, and the occurrence rate of defective products can be greatly reduced. Further, as shown in FIG. 15, according to the present invention, it can be seen that internal defects of the slab due to inclusion of inclusions can be significantly reduced.

Example 3 A continuous cast slab was manufactured using the continuous casting mold shown in FIG. 1 under the following conditions.・ Inner diameter of mold Long side w = 1.2 m Short side: 0.26 m Height: 0.9 m ・ DC magnetic field application position (distance from mold level in mold to center of magnetic pole height) A: 0.60 m Magnetic pole height: 0.2 m Strength of applied magnetic field: 0.3 T ・ Immersion nozzle Nozzle inner diameter: 90 mm Upper discharge hole: 2 holes (hole size 21 × 30 mm) Lower discharge hole: 2 holes (hole size 49 mmφ) ・ Lower discharge hole Distance h from center to pole height center:
0.10m ・ Distance h 'from center of upper discharge hole to center of height of magnetic pole:
0.30 m ・ Casting speed Vc: 1.6 m / min (Casting volume: 3.5 t / min) ・ Molten steel supply speed Q 'from the upper discharge hole: 0.7 Q (Consumption speed of molten steel solidifying above the center of the magnetic pole height) Q of 0.7
・ Solute element (pure Ni wire) Supply position of pure Ni wire (horizontal distance from upper discharge hole): 0.3 m Melting position of pure Ni wire (distance in height direction to center of upper discharge hole): 0.1 to 0.2 m wire feeding speed: 15 kg / min

Under the above-mentioned casting conditions, continuous casting was carried out by changing the angle of the discharge hole of the nozzle in various ways, and the influence thereof was investigated.・ Lower discharge hole: 2 holes, discharge angle θ = 0 ° (horizontal), 5 °, 10 °, 20 °, 60 ° (downward) ・ Upper discharge hole: 2 holes, discharge angle θ '=-10 ° (upward) 10 °), 0 ° (horizontal), 25
°, 30 °, 60 ° (downward)

FIG. 16 shows the obtained results. Here, ◎ in the figure indicates the dispersion index of the Ni concentration in the surface layer (maximum Ni concentration / minimum).
Ni concentration) is less than 1.05,
Less than 1.10, Δ indicates 1.10 or more, less than 1.20, and X indicates 1.20 or more (defective). As is clear from the figure, when the above equation (1) is satisfied, the variation in the solute concentration in the surface layer is extremely small, and when the above equation (2) is satisfied, the variation is further reduced. You can see that.

[0040]

As described above, according to the present invention, not only is it very easy to control the supply of molten steel to the upper and lower pools having different solute element concentrations around the magnetic field zone as a boundary, but also the solute element concentration in the surface layer of the slab. Can be manufactured stably with very little variation in the product, and the yield of products can be significantly improved. Further, since the molten steel is supplied only above the magnetic field zone, inclusions are not involved below the magnetic field zone, and defects inside the slab can be greatly reduced.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of a procedure for injecting molten steel according to the present invention (when a lower discharge hole is a vertically downward single hole).

FIG. 2 is an explanatory diagram of an induced current generated around a molten steel jet from a nozzle in the present invention.

FIG. 3 is an explanatory diagram of an electromagnetic force generated around a molten steel jet from a nozzle in the present invention.

FIG. 4 is a view showing a distribution of molten steel flowing from a lower pool to an upper pool in a magnetic field zone according to the present invention.

FIG. 5 is a diagram showing a concentration distribution of a solute element in a mold according to the present invention.

FIG. 6 is a diagram showing a solute element concentration distribution in a cross section perpendicular to a casting direction of a slab in the present invention.

FIG. 7 is a schematic diagram illustrating an example of a molten steel injection procedure according to a comparative example (when the flow rate of molten steel from an upper discharge hole is reduced).

FIG. 8 is a diagram showing a distribution of molten steel flowing from a lower pool to an upper pool in a magnetic field zone in a comparative example.

FIG. 9 is a diagram showing a concentration distribution of a solute element in a mold in a comparative example.

FIG. 10 is a diagram showing a solute element concentration distribution in a cross section perpendicular to a casting direction of a slab in a comparative example.

FIG. 11 is a graph showing the transition of the ratio between the surface Ni concentration and the internal Ni concentration when Q ′ / Q is variously changed.

FIG. 12 is a graph showing the influence of Q ′ / Q on variation of Ni concentration in a surface layer.

FIG. 13 is a schematic view showing another example of the injection procedure of molten steel according to the present invention (when the lower discharge hole is a two-hole type).

FIG. 14 is a diagram comparing the occurrence rates of Ni concentration defects in the surface layer of a slab in an example of the present invention and a comparative example.

FIG. 15 is a diagram showing a comparison between the rate of occurrence of internal defects in a slab in an example of the present invention and a comparative example.

FIG. 16 is a diagram showing a comparison of the variation in Ni concentration in the surface layer of a slab in an example of the present invention and a comparative example.

[Explanation of symbols]

Reference Signs List 1 mold 2 immersion nozzle 3 magnetic pole 4 magnetic pole height center 5 lower discharge hole of immersion nozzle 6 upper discharge hole of immersion nozzle 7 jet from lower discharge hole 8 jet from upper discharge hole 9 lower pool of DC magnetic field zone to upper part Backflow into the pool 10 Solute element (wire) 11 Addition position of solute element 12 Solidification shell 13 Induced current 14 DC magnetic field (direction of magnetic field) 15 Electromagnetic force 16 Jet section 17 Area where solute element is concentrated in mold 18 In mold Area where the concentration of solute elements is low in the area 19 Area where no solute elements are concentrated in the mold 20 Surface layer of the slab (the part where the solute element is concentrated) 21 Transition layer of the solute element concentration of the slab 22 Part of cast slab (part where no solute element is concentrated)

 ──────────────────────────────────────────────────の Continuing on the front page (72) Koji Yamaguchi 1st Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Co., Ltd. (72) Shuji Takeuchi 1st Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki 4E004 AA09 MB13 NB01 NC01 NC10

Claims (4)

[Claims] In a continuous casting of a molten metal, a DC magnetic field zone is applied across the entire width of a slab in a direction crossing the thickness of the slab at a position below a level of a molten metal in a continuous casting mold by a predetermined distance in a casting direction. At the time of injecting molten steel into the molten steel pool in the DC magnetic field band or above the DC magnetic field band using an immersion nozzle, at least two upper and lower discharge holes are provided in the immersion nozzle, and the lower discharge hole is Arranged so as to satisfy the formula (1), the supply speed of molten steel from the upper discharge hole is made smaller than the speed consumed by solidification in the molten steel pool above the height center of the DC magnetic field zone, By adding a specific solute element to the molten steel in or above the DC magnetic field band, the concentration of the solute element in the molten steel in the upper pool is increased to adjust the solute element concentration in the surface layer of the slab. To do Method for producing a continuously cast slab characterized. 0 <h <(1/2) · w · tan θ --- (1) where θ: downward angle of the lower discharge hole (°) w: length of the mold in the width direction (m) h: Distance from the center of the lower discharge hole to the center of the magnetic pole height (m) 2. The method for producing a continuous cast slab according to claim 1, wherein said upper discharge hole uses an immersion nozzle designed to satisfy the following equation (2). Note h ′> (1/2) · w · tan θ ′ --- (2) where θ ′: downward angle of upper discharge hole (°) w: length of mold in width direction (m) h ': Distance from the center of the upper discharge hole to the center of the magnetic pole height (m) 3. The upper discharge hole and the lower discharge hole,
3. The method for producing a continuous cast slab according to claim 1, wherein a dipping nozzle designed to satisfy the following equations (3) and (4) is used. 0 <h ≦ 1.5 V · sin θ --- (3) d ≦ 0.5 --- (4) where h: distance (m) from the center of the lower discharge hole to the center of the height of the magnetic pole V: lower discharge hole Θ: Downward angle of the lower discharge hole (°) d: Distance from the center of the upper discharge hole to the center of the lower discharge hole (m) 4. The continuous immersion nozzle according to claim 1, wherein the immersion nozzle is designed so that the supply speed of the molten steel from the upper discharge hole satisfies the following expression (5). A method for producing cast slabs. 0.3 · Q ≦ Q '≦ 0.9 · Q --- (5) Where, Q': supply speed of molten steel supplied from upper discharge hole
(ton / min) Q: Consumption rate of molten steel solidified in molten steel pool above the center of magnetic pole height (ton / min)