JP2001232450A - Method for manufacturing continuous cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably manufacture a composite slab in which feeding and controlling a molten steel is easy, which is free from any variance in the solute element concentration in the width direction of a surface layer portion of a slab, and free from any generation of powder-like defects. SOLUTION: The DC magnetic field band is applied over the entire width of the slab across the thickness of the slab by using independent upper and lower magnetic poles at an upper portion of a mold including the molten steel level in a continuous casting mold and at a center portion of the mold separated downward therefrom by a predetermined distance, the molten steel is poured using a single immersed nozzle into a pool within or above the DC magnetic field zone applied to the center portion of the mold while controlling the meniscus representative flow speed in a range of 0.1 to 0.6 m/s, and the predetermined solute element is added to the molten steel within or above the DC magnetic field zone.

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 of obtaining a multilayer slab by supplying different metals above and below a static magnetic field zone formed by a magnetic flux as a boundary 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.

In addition, it is possible to reduce inclusions in the slab,
A flow control method using an electromagnet has been proposed and has been put to practical use in order to prevent the fluctuation of the molten metal level at the time of casting and to prevent the powder from being caught in the slab. For example, Japanese Patent Application Laid-Open No. 1-105817 proposes a method in which a magnetic field is applied to a molten metal surface (meniscus) and upper and lower two stages below the mold in a direction crossing the thickness of the slab over the entire width of the mold.

[0005]

However, in the method described in Japanese Patent Publication No. 3-20295, the molten steel for the surface layer and the molten steel for the inside of the slab are secured in two separate tundishes, and the upper and lower magnetic zones are secured. Since it is necessary to perform extremely difficult control of independently supplying the amount of molten steel according to the solidification rate in each part from each tundish, stable production is difficult, and as a result, there is a problem that the product yield is reduced. . Further, in the production method described in JP-A-7-51801, molten steel from the tundish is supplied only to the lower part of the magnetic field zone so as to maintain the level of the molten metal in the mold constant, and is not supplied to the upper part of the magnetic field zone. Although the shortage corresponding to the solidification amount in the upper part of the magnetic field zone naturally flows in from the lower part of the magnetic field zone, the above-mentioned problem does not occur. Due to the influence, the gas flows almost uniformly in the width direction, so that there is a problem that the extreme concentration difference between the portion where the solute element is added and the portion far from the portion cannot be eliminated only by the stirring effect of the bubbles.

The present invention advantageously solves the above-mentioned problems, and can easily and appropriately adjust the concentration of a solute element in the surface layer of a slab. An object of the present invention is to propose a method of manufacturing a continuously cast slab which can be reduced in size and can also prevent the occurrence of powdery defects and the like.

[0007]

That is, according to the present invention, in the continuous casting of molten metal, at the upper part of the mold including the level of the molten metal in the continuous casting mold and at the center of the mold separated by a certain distance below the upper part, An independent current-controlled upper and lower two-stage magnetic pole (electromagnet) applies a DC magnetic field band across the entire width of the slab in a direction crossing the slab thickness, and controls the meniscus representative flow velocity in the range of 0.1 to 0.6 m / s. Meanwhile, while injecting molten steel using a single immersion nozzle into the DC magnetic field band applied to the center of the mold or into the pool above the DC magnetic band, the molten steel is injected into the DC magnetic band or from the DC magnetic band. Also, by adding a specific solute element to the upper molten steel, the concentration of the solute element in the molten steel in the upper pool is increased to adjust the solute element concentration in the slab surface layer portion of the continuous cast slab. Manufacturing method .

Here, the representative meniscus flow velocity is defined as
It means the flow rate of the molten steel of the meniscus at the center position in the thickness direction of the mold with a quarter width (intermediate position between the mold short side and the immersion nozzle).

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings. FIGS. 1, 2 and 3 illustrate the procedure for injecting molten steel according to the present invention. The example of FIG. 1 is a case in which a single-hole straight nozzle is used as the immersion nozzle, in which molten steel is supplied in a substantially vertical direction. The example in FIG. 2 is a case where a so-called two-hole nozzle having two discharge holes and inclining the angle of the jet from each discharge hole is used as the immersion nozzle. The example of FIG. 3 is a case where a three-hole nozzle having a main discharge hole at the bottom and two sub-holes on the side is used as the immersion nozzle. In the figure, reference numeral 1 denotes a mold, 2 denotes an immersion nozzle (2a: straight nozzle, 2b: two-hole nozzle, 2
c: three-hole nozzle), 3a and 3b are upper and lower two-stage magnetic poles (electromagnets) arranged at the top and center of the mold, respectively.
These magnetic poles 3a and 3b allow a DC magnetic field band to be independently applied over the entire width of the slab in the thickness direction of the slab. FIG. 4 is a perspective view of this state as viewed from the rear side of the short side of the mold. The number 4 indicates the height center of the magnetic pole, 4a is the height center of the upper magnetic pole 3a, and 4b is the height center of the lower magnetic pole 3b. Reference numeral 5 denotes a discharge hole of the immersion nozzle 2,
Numeral 6 indicates a jet from the immersion nozzle 2, and numeral 7 indicates a backflow (upflow) from the lower pool to the upper pool in the DC magnetic field zone. 8,
8 'is a solute element (wire) and 9 is a solidified shell. In the drawings, w represents the width of the mold, θ represents the angle of the immersion nozzle discharge hole (a downward angle with the horizontal direction being 0), and h represents the distance from the lower end of the immersion nozzle discharge hole to the height center of the lower magnetic pole. .

In the place shown in FIG. 1 or 2, most of the molten steel jet 6 supplied from the immersion nozzle 2 once flows into the lower pool of the magnetic field zone, but there is no supply source of molten steel to the upper pool. However, only a necessary portion of the molten steel once flowing into the lower pool flows back to the upper pool again. Also in FIG. 3, since the supply of the main molten steel is performed from the lower main hole, similarly, the molten steel flowing into the lower pool flows back to the upper pool again. 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 6 supplied from the immersion nozzle 2 crosses the DC magnetic field zone, the jet 6
, An induced current 10 is generated as shown in FIG. As a result, the interaction between the induced current 10 and the DC magnetic field 11 generates an electromagnetic force 12 as shown in FIG. In this way, a so-called electromagnetic braking force is generated in the jet portion in a direction opposite to that of the jet 6, but such an induced current is inevitably generated on both sides of the jet portion. And the flow in the opposite direction is likely to occur on both sides of the jet.

As a result, as shown in FIG. 1, molten steel flows into the upper pool from the lower pool in the magnetic field zone, so that the rotating flow as shown in FIGS. Since the generation and the agitation are promoted, the concentration distribution of the solute element in the molten steel in the upper pool becomes uniform. Further, since the downward jet from the nozzle is decelerated when passing through the magnetic field zone, the entrapment which causes an internal defect to fall down is reduced, and the internal quality is improved.

In the present invention, it is necessary to penetrate the magnetic field zone before the jet from the immersion nozzle diffuses. Therefore, it is important to appropriately set the nozzle position, the discharge angle, and the magnetic field application position. Therefore, as a result of various studies on this point, it was found that it is preferable to satisfy the following relationship with respect to the nozzle position, the ejection angle, and the magnetic field application position. h <(1/2) · w · tanθ --- (1) 0 <h ≦ 0.3 --- (2) where θ: downward angle (°) of the immersion nozzle discharge hole w: width direction of the mold Length (m) h: Distance (m) from lower end of immersion nozzle discharge hole to center of height of magnetic pole

Equations (1) and (2) above are geometrically important conditions for penetrating the magnetic field zone before the jet from the immersion nozzle diffuses. If the operation is carried out under the conditions satisfying the formula, the solute elements are more evenly added only to the upper pool, effectively producing a slab having a high concentration of solute elements only in the surface layer of the slab. You can.

In the present method, when the strength of the lower magnetic field was studied, if the lower magnetic field was too small, the braking effect by the magnetic field was weakened, and the molten steel in the upper pool and the lower pool might be mixed, while the strength was too strong. And, the inflow into the upper pool becomes too strong due to the ascending flow generated by the induced current around the nozzle, so that excess molten steel is supplied to the upper pool, and as a result, at a position away from the inflow position Since the molten steel in the upper pool may flow out, the applied magnetic field does not cause the mixing of the molten steel in the upper pool and the lower pool or the failure to uniformly dissolve the alloy elements.
It is important to have appropriate strength, typically 0.2-0.3 T
It is good to be about. Similarly, if the flow rate of the Ar gas injected into the nozzle is too large, the flow into the upper pool becomes too strong. Therefore, the Ar gas flow rate is desirably 20 l / min or less.

Further, as for the width (height direction) of the applied DC magnetic field band, if the width 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.

In the above example, the case where the immersion nozzle is a single-hole straight nozzle has been mainly described. However, in the present invention, it is important to locally generate a molten steel inflow portion from the lower pool to the upper pool. Therefore, even in the case of a two-hole nozzle as shown in FIG. 2 or a three-hole nozzle having a total of three discharge holes at the lower portion and both side portions as shown in FIG. If the conditions of (2) are satisfied,
It is possible to effectively generate a desired local inflow site. In order to further enhance the effect of locally forming the inflow portion and the effect of attenuating the nozzle jet, the position of the nozzle outlet is preferably located above the center of the magnetic pole.

Next, the effect of the upper magnetic field will be described.
When only the lower magnetic pole is provided, the magnetic field attenuates in a curved shape with a wide skirt, as shown in FIG. As a result, the magnetic field affects the vicinity of the meniscus, and the flow of the meniscus is uniquely determined by the strength of the lower magnetic field. Here, since the strength of the lower magnetic field is determined by the strength required for uniform dissolution of the alloy element added to the upper pool, the flow of the meniscus cannot be controlled only by using the lower one-field magnetic field.

On the other hand, it is difficult to lower the application position of the magnetic field downward without affecting the meniscus with a common-sense mold size (length: 800 to 1000 mm). The pool layer increases and the thickness of the concentrated layer on the surface due to the additional element increases. This is not preferable in consideration of the object of the present invention of not only increasing the amount of element addition to the surface layer portion but also changing the composition of only the surface layer portion.

As described above, when the flow of the meniscus is slow, there is no flow in the mold width direction after the solute element added in the upper pool is dissolved by upward flow, so that the element concentration in the width direction varies. In addition, when the flow of the meniscus is slow, powder trapping occurs due to a decrease in the temperature of the meniscus. Conversely, if the flow of the meniscus is too large, it is advantageous to homogenize the added element in the width direction, but the mold powder is caught in the meniscus,
Powder adheres to the shell and causes defects. Especially in this law,
By adding the wire to be added, the entrainment of the mold powder is likely to occur. Therefore, it is important to control the meniscus flow at the same time.

Therefore, in the present invention, the upper magnetic pole and the lower magnetic pole are separately controlled. For that purpose, it is necessary to control the currents of the lower magnetic field and the upper magnetic field independently.

Next, the upper magnetic field will be described. Generally, a lower magnetic field is sufficient as the upper magnetic field. In particular, when the width is wide, the flow rate of the meniscus becomes slow, so that the necessity of the upper magnetic field is low. When the flow rate of the meniscus is low, powder entrainment occurs due to nonuniform dissolution of the solute element in the width direction or a drop in the meniscus temperature due to the introduction of the wire, as described above. Therefore,
When the flow rate of the meniscus is low, it is necessary to flow the meniscus in order to prevent inclusions and bubbles from being trapped in the solidified shell. In this case, as shown in FIG. To reverse the direction. Then,
As shown by (d) in FIG. 9, the upper magnetic pole becomes substantially zero, so that the meniscus flow velocity does not decelerate due to the application of the magnetic field. If the throughput (injection amount) is small and the meniscus flow rate still cannot be sufficiently increased, the meniscus flow rate is maintained by making the nozzle inclination shallower than before. On the other hand, when the width is small and the throughput is large, on the contrary, the flow velocity of the meniscus becomes too large, and the above-mentioned powdery defects increase.
The upper magnetic field as shown in (c) is applied to control the meniscus flow velocity.

FIG. 8 shows a method of connecting a two-step magnetic field in the conventional method and the method of the present invention. FIG. 8A shows an example of a conventional two-step magnetic field connection, while FIG. (D) show preferred connection examples according to the present invention.
8 (b) shows that when the magnitude of the current in the upper magnetic field is smaller than that in the lower magnetic field, FIG.
When the current in the upper magnetic field is set to 0, FIG. 11D shows the case where the direction of the current in the upper magnetic field is reversed. 9A and 9B show the magnitudes of the upper and lower magnetic fields in the case of the magnetic field connection as shown in FIGS. 8A to 8D, respectively. ) Are shown in Figs.
This corresponds to the case (d). In the present invention, as shown in FIGS. 8 and 9, the flow rate of the meniscus can be appropriately controlled by adjusting the way of connection, and thus the magnitude of the magnetic field at the upper magnetic pole.

FIGS. 10 (a) and 10 (b) show the typical meniscus flow velocity, the powder defect and the element (Ni) in the surface layer portion, which were measured by immersing a strain gauge at a quarter width of the mold and at the center thickness position. 2 shows the results of an investigation on the relationship between the density distribution ratio (the ratio of the maximum density portion to the minimum density portion) over the width direction of the graph. Here, the concentration distribution was determined by analyzing a plurality of portions at a surface depth of 10 mm in the width direction.

The powder defect index was determined as follows. That is, the slab continuously cast under each condition is
Hot rolling and cold rolling under the same conditions to form a cold rolled steel sheet,
The rate of occurrence of powdery defects generated in the cold-rolled steel sheet (the ratio of the total length of powdery defects per unit length of the cold-rolled steel sheet) is measured. Then, according to the present invention, the defect occurrence rate of a cold-rolled steel sheet obtained from a slab cast under the condition of a meniscus representative flow velocity of 0.3 m / s was set as a reference value, and the relative ratio to this was defined as a powder defect index.

As is apparent from the figure, when the representative meniscus flow velocity is less than 0.1 m / s, the density difference in the width direction becomes large.
On the other hand, if it exceeds 0.6 m / s, powder defects increase. Therefore, the strength of the upper magnetic field can be adjusted by setting the meniscus flow rate to 0.1 to 0.
It is important to have a size that can be controlled within the range of 6 m / s. By setting the content in this range, it is possible to prevent the occurrence of powder defects and the variation in the concentration of the added element in the width direction.

[0027]

EXAMPLES Continuous cast slabs were produced using the continuous casting molds shown in FIGS. 1 to 3 under the following conditions.・ Inner dimensions of the mold Long side: 1 to 1.7 m, Short side: 0.26 m, Height: 1 m ・ Lower DC magnetic field Applied position: 0.5 m below the mold level in the mold (up to the center of the magnetic pole height) Applied position: 0.5 m below the mold level (up to the center of the height of the magnetic pole) Strength of applied magnetic field: 0 to 0.31 T ・ Immersion nozzle Single-hole straight nozzle (discharge angle θ = 90 °) 2-hole nozzle, 3-hole nozzle Distance from the bottom of the nozzle to the center of height of the magnetic pole: 0.2 m ・ Solute element (pure Ni wire) Supply position of pure Ni wire: mold from short side To the center in the width direction
0.1m away position Melting position of pure Ni wire: 0.05m from mold level in mold ・ Casting speed: 1-2.0m / min The strength of the applied magnetic field was measured at the center of each magnetic pole height. Value, which is the actual strength of the magnetic field when the influence of the other magnetic field is superimposed.

It is known that the growth thickness d (m) of the solidified shell in the above-mentioned continuous caster is given by the following equation (3). d = 0.022 x (L / Vc) ^0.5 --- (3) where L is the distance (m) from the level of the molten metal to the center of the height of the lower magnetic pole, and Vc is the casting speed (m / min). . Therefore, it can be seen from the above equation (3) that the thickness of the solidified shell at the boundary between the upper and lower pools is about 12 mm.

Now, a continuous casting was conducted under the above conditions while changing the casting speed, the mold width, the nozzle type, etc., and the result of examining the relationship between the Ni concentration distribution ratio in the width direction of the surface layer of the slab and the powder defect index was examined. Is shown in Table 1. Also, for comparison, an example of casting using the same two-hole nozzle (Comparative Example 1)
2) and the position of the discharge hole below the center of the magnetic pole height: 0.
1 m, discharge angle downward 90 °, Ar gas blowing amount: 3.0
The same investigation was performed for a case where a 1 / min straight nozzle was used and a one-step magnetic field was used (Comparative Example 3). In addition, the addition position of the Ni wire was the same as that of the example in Comparative Examples 1 and 2, and in Comparative Example 3, the Ni wire was vertically added to a 0.3 m portion on both sides of the immersion nozzle.

[0030]

[Table 1]

As is clear from Table 1, in Comparative Examples 1 and 2 in which the meniscus representative flow velocity deviates from the range of 0.1 to 0.6 m / s, there is a problem in the width direction variation of the concentration distribution or the powder defect. was there. Further, when casting was performed with the nozzle discharge hole provided below the magnetic field as in Comparative Example 3, problems occurred in both the variation in the concentration distribution in the width direction and the powder defect. In addition, when the concentration distribution was examined in the case of Comparative Example 3, it was found that only the Ni concentration in the portion near the Ni addition position was increased. In contrast, according to the present invention, Ni is uniformly concentrated over the entire width of the slab,
In addition, powdery defects hardly increased, and good cast pieces could be obtained.

[0032]

Thus, according to the present invention, it is possible to supply molten steel to the upper and lower pools of the magnetic field zone with one tundish and a single immersion nozzle, so that not only is molten steel supply control extremely easy, A slab having no variation in solute element concentration in the width direction of the slab surface layer can be stably manufactured, and the product yield can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of a molten steel injection procedure (when a single-hole straight nozzle is used) according to the present invention.

FIG. 2 is a schematic view showing an example of a molten steel injection procedure (when a two-hole nozzle is used) according to the present invention.

FIG. 3 is a schematic diagram showing an example of a molten steel injection procedure (when a three-hole nozzle is used) according to the present invention.

FIG. 4 is a perspective view seen from the back side of the mold.

FIG. 5 is an explanatory diagram of an induced current generated around a molten steel jet from a nozzle.

FIG. 6 is an explanatory diagram of an electromagnetic force generated around a molten steel jet from a nozzle.

FIG. 7 is a diagram showing an attenuated state of a magnetic pole when a magnetic field has only one lower stage.

FIG. 8 is a diagram showing an example of a current connection method and an example of a magnetic field direction in a conventional two-step magnetic field (a) and two-step magnetic fields (b) to (d) used in the present invention.

FIG. 9 is a diagram showing an example of the relationship between the lower and upper magnetic field in the present invention.

FIG. 10: Meniscus representative flow velocity and powder defect (a)
FIG. 4 is a diagram showing the relationship between the concentration distribution ratio (b) of the element (Ni) in the width direction in the surface layer portion.

[Description of Signs] 1 mold 2 immersion nozzle 3 magnetic pole (electromagnet) 4 center of height of magnetic pole 5 discharge hole of immersion nozzle 6 jet from immersion nozzle 7 backflow (upflow) from lower pool to upper pool of magnetic field zone 8 Solute element (wire) 9 Solidified shell 10 Induced current 11 DC magnetic field (direction of magnetic field) 12 Electromagnetic force 13 Existence range of jet from nozzle

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B22D 27/02 B22D 27/02 W 27/20 27/20 B (72) Inventor Koji Yamaguchi Chiba, Chiba 1 Kawasaki-cho, Chuo-ku F-term (reference) in Kawasaki Steel Engineering Laboratory 4E004 AA09 MB12 MB14

Claims (1)

[Claims] 1. In the continuous casting of molten metal, the upper and lower portions of the upper and lower molds, each of which includes a molten metal level in the continuous casting mold, and the upper and lower portions of which are independently controlled in current, are separated from each other by a certain distance below the upper and lower portions. A DC magnetic field zone was applied across the entire width of the slab by the magnetic poles (electromagnets) of the step across the thickness of the slab, and applied to the center of the mold while controlling the typical meniscus flow rate in the range of 0.1 to 0.6 m / s. A single immersion nozzle is used to inject molten steel into the pool in or above the DC magnetic field band, and a specific solute element is added to the molten steel in or above the DC magnetic field band. A method for producing a continuous cast slab, wherein the concentration of the solute element in the molten steel in the upper pool is increased by adding a solute element to adjust the solute element concentration in the surface layer portion of the slab.