JP2001105108A - Method for continuously producing cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably produce a multi-layer cast slab capable of easily controlling the supply of molten steel and having no unevenness of solute element concentration over the width direction on the surface layer part of the cast slab. SOLUTION: When the molten steel is poured by using a single immersion nozzle in the state that DC magnetic field is impressed over the whole width of the cast slab in the thickness direction of the cast slab at the position aparted dounwardly in a fixed distance from the molten steel surface level in a continuous casting mold, the immersion nozzle is arranged at the position satisfying both of the following formulae (1) and (2) and also, a specified solute element is added into the position satisfying both of the following formulae (3) and (4). h<(1/2).w.tanθ...(1), 0<h<=0.3...(2), 0<z<=0.5...(3), -0.3<=y<=0.3...(4). Wherein, θis the downward angle ( deg.) of the spouting hole in the immersion nozzle, w is the length (m) in the width direction of the mold, h is the downward distance (m) from the lower end of the spouting hole in the immersion nozzle to the center in the height of a magnetic pole, z is the distance (m) from the adding position of the solute element to the center in the height of the magnetic pole and y is the distance (m) in the horizontal direction from the spouting hole in the immersion nozzle to the adding position of the solute element.

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.
Above this molten steel injection position, a DC magnetic field is applied over the entire width in the mold to reduce the ascending flow of the molten steel, and elements different from the molten steel are added to the molten steel above the DC magnetic field application position. Further, there is described a method for producing a multi-layered steel sheet, which comprises 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 floating stirring of the injected gas.

[0004]

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. Because it was 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 was difficult,
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. The deficiency with respect to the coagulation amount at the upper part of the magnetic field zone naturally flows from the lower part of the magnetic field zone, and the above-mentioned problem does not occur.
Since the molten steel flow from the lower part to the upper part of the magnetic field zone flows almost uniformly in the width direction due to the influence of the DC magnetic field, an extreme difference in concentration between the part where the solute element is added and the part distant therefrom can be obtained only by the bubble stirring effect. There was a problem that it could not be solved.

The present invention advantageously solves the above-mentioned problems, and proposes a method for producing a continuous cast slab that can easily and appropriately adjust the concentration of solute elements in the surface layer of the slab. Aim.

[0006]

That is, the gist of the present invention is as follows. 1. At the time of continuous casting of the molten metal, at a position below a certain level in the casting direction from the level of the molten metal in the continuous casting mold, while applying a DC magnetic field band across the entire width of the slab in a direction across the thickness of the slab, the DC Into the pool in the magnetic field zone or in the pool above the DC magnetic field zone, molten steel shall be injected using a single immersion nozzle, and at this time, the immersion nozzle is set to the following (1), (2)
By arranging in a position satisfying both the expressions and 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. A method for producing a continuously cast slab, which comprises adjusting the solute element concentration in the surface layer of the slab. Note h <(1/2) · w · tan θ --- (1) 0 <h ≦ 0.3 --- (2) where θ: downward angle (°) of the immersion nozzle discharge hole w: width of the mold Length in the direction (m) h: Distance from the lower end of the immersion nozzle discharge hole to the center of height of the magnetic pole (m)

[0007] 2. In the above 1, the method for producing a continuously cast slab, characterized in that the position where the solute element is added is in the upward flow of the molten steel.

[0008] 3. In the above 1 or 2, the method for producing a continuous cast slab is characterized in that the position where the solute element is added is a position that satisfies both the following expressions (3) and (4). 0 <z ≦ 0.5 --- (3) -0.3 ≤y ≦ 0.3 --- (4) where, z: distance (m) from the addition position of the solute element to the center of height of the magnetic pole y: immersion nozzle Horizontal distance (m) from discharge hole to solute element addition position

[0009]

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 single-hole straight nozzle is used as the immersion nozzle, and the molten steel is supplied in a substantially vertical direction. 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. Reference numeral 5 denotes a discharge hole of the immersion nozzle 2, 6 denotes a jet from the immersion nozzle 2, and 7 denotes a reverse flow from the lower pool to the upper pool in the DC magnetic field band. 8 is a solute element (wire), 9 is a position where the solute element 8 is added, and 10 is a solidified shell. In addition,
In the drawing, w is the width of the mold, θ is the angle of the immersion nozzle discharge hole (a downward angle with the horizontal direction being 0), h is the distance from the lower end of the immersion nozzle discharge hole to the center of the magnetic pole height, and z is the solute element. The distance from the addition position to the center of the height of the magnetic pole, and y represents the horizontal distance from the discharge hole of the immersion nozzle to the addition position of the solute element.

As shown in FIG. 1, most of the molten steel jet 6 supplied from the immersion nozzle 2 once flows into the lower pool of the magnetic field zone. However, since there is no supply source of molten steel to the upper pool, Only the necessary part of the molten steel that once flows 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 11 as shown in FIG. As a result, due to the interaction between the induced current 11 and the DC magnetic field,
An electromagnetic force 12 as shown in FIG. 3 is generated. 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 shown in FIG. 4, as a result, the inflow of molten steel from the lower pool to the upper pool in the magnetic field zone occurs only at both sides of the jet part. For comparison, FIG. 5 shows the results of a study on the flow of molten steel when molten steel is supplied to the lower pool in the magnetic field zone through an immersion nozzle as in the method described in Japanese Patent Application Laid-Open No. 7-51801. As shown in the figure, in this method, an induced current is generated inside the nozzle, but the current is shielded by the immersion nozzle, so that no reverse flow occurs.

As described above, in the present invention, since the molten steel inflow from the lower pool to the upper pool is limited to a specific region on both sides of the jet, a specific solute element is provided on the upper pool side of the inflow. The supply of solutes enables the supply of solute elements only to the upper pool, and the flow of molten steel to the lower pool generates a rotating flow as shown in FIG. 1 in the upper pool. Since the stirring also proceeds smoothly, the concentration distribution of the solute element in the molten steel in the upper pool becomes uniform. In addition, since the downward jet from the nozzle is decelerated when passing through the magnetic field zone, the entrapment that causes an internal defect is also reduced, and the internal quality is improved.

As described above, in the present invention, it is necessary to penetrate the magnetic field zone before the jet flow from the immersion nozzle diffuses, so it is important to appropriately set the nozzle position, the discharge angle, and the magnetic field application position. It is. Therefore, as a result of various studies on this point, it has been found that it is necessary to satisfy the following relationship with respect to the position of the nozzle, the discharge angle, and the position where the magnetic field is applied. 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

Further, since the solute element needs to be accurately added to the molten steel inflow portion from the lower pool to the upper pool, it is preferable to add the solute element to the ascending flow of the molten steel. It has been found that the addition under the conditions satisfying the following relationship is extremely effective. 0 <z ≦ 0.5 --- (3) −0.3 ≦ y ≦ 0.3 --- (4) where, z: distance (m) from the position where the solute element is added to the center of height of the magnetic pole y: immersion nozzle discharge Horizontal distance from hole to solute element addition position (m)

When the operation is performed under the conditions satisfying the above equations (1) to (4), the solute element is uniformly added only to the upper pool, and the solute element is added only to the surface layer of the slab. It has been found that a slab having a high concentration of can be effectively produced.

In the above example, only the case where the immersion nozzle is a single-hole straight nozzle has been 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, as shown in FIG. 6, even in the case of a two-hole nozzle or the like used in normal continuous casting, if the conditions (1) and (2) are satisfied,
It is possible to create the desired local inflow site.
Further, in order to further increase the effect of locally forming the inflow portion and the effect of damping the nozzle jet, the nozzle outlet is preferably located above the center of the magnetic pole.

If the strength of the applied magnetic field 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. The 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 to have an appropriate strength that does not cause mixing of alloying or poor dissolution of alloying elements. 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.

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.

[0020]

EXAMPLE A continuous casting mold shown in FIG. 1 was used under the following conditions.
Continuous cast slabs were produced.・ Inner diameter of the mold Long side: 1.2 m, Short side: 0.27 m ・ 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 magnetic field strength: 0.3 T ・ Immersion nozzle only Hole straight nozzle (discharge angle θ = 90 °) Distance from nozzle discharge hole to center of magnetic pole height: 0.1 m ・ Solute element (pure Ni wire) Supply position of pure Ni wire: 0.1 m on both sides of immersion nozzle Location of melting point of pure Ni wire: 0.3 ~ from mold level in mold
0.4 m ・ Casting speed: 1.6 m / min

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 (5). d = 0.022 × (L / Vc) ^0.5 --- (5) Here, L 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 (5), the solidified shell thickness at the upper and lower pool boundaries is 12
It turns out that it is about mm.

FIG. 7 shows the result of investigation on the Ni concentration distribution in the width direction of the surface layer of the slab continuously cast under the above conditions. For comparison, the position of the discharge hole is below the center of the height of the magnetic pole: 0.1 m, the discharge angle is 90 ° downward,
Fig. 8 shows the results of investigation on the Ni concentration distribution when an immersion nozzle with an Ar gas injection rate of 3.0 l / min was used and the above-mentioned Ni wire was vertically added to a 0.3 m portion on both sides of the immersion nozzle. Show. As shown in FIG. 8, in the case of the conventional example, only the Ni concentration in the portion near the Ni addition position is increased. In contrast, according to the present invention, as shown in FIG. 7, it can be seen that Ni is uniformly concentrated over the entire width of the slab.

[0023]

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

FIG. 3 is an explanatory view of an electromagnetic force generated around a molten steel jet from a nozzle.

FIG. 4 is a diagram showing an inflow distribution of molten steel 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 distribution of molten steel flowing from a lower pool to an upper pool in a magnetic field zone in a comparative example.

FIG. 6 is a schematic view showing another example (in the case of using a two-hole nozzle) of the injection procedure of molten steel according to the present invention.

FIG. 7 is a diagram showing a distribution of Ni in a width direction of a surface layer portion of a slab in an example of the present invention.

FIG. 8 is a view showing a Ni distribution in a width direction of a surface layer portion of a slab in a comparative example of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Immersion nozzle 3 Magnetic pole 4 Center of magnetic pole height 5 Injection hole of immersion nozzle 6 Jet from immersion nozzle 7 Backflow from lower pool to upper pool of magnetic field zone 8 Solute element (wire) 9 Addition position of solute element 10 Solidification Shell 11 Induced current 12 Electromagnetic force 13 Magnetic field direction 14 Existence range of jet from nozzle 15 Direction of jet from nozzle

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B22D 27/02 B22D 27/02 U

Claims (3)

[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 predetermined level in a casting direction from a molten metal level in a continuous casting mold. In this state, the molten steel shall be injected using a single immersion nozzle into the DC magnetic field zone or into the pool above the DC magnetic field zone, and at this time, the immersion nozzle is set to the following (1), (2) By arranging in a position satisfying both the expressions and 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. A method for producing a continuously cast slab, comprising adjusting the solute element concentration in the surface layer of the slab. Note h <(1/2) · w · tan θ --- (1) 0 <h ≦ 0.3 --- (2) where θ: downward angle (°) of the immersion nozzle discharge hole w: width of the mold Length in the direction (m) h: Distance from the lower end of the immersion nozzle discharge hole to the center of height of the magnetic pole (m) 2. The method for producing a continuous cast slab according to claim 1, wherein the position where the solute element is added is in the upward flow of the molten steel. 3. The method for producing a continuously cast slab according to claim 1, wherein the addition position of the solute element is a position that satisfies both the following expressions (3) and (4). 0 <z ≦ 0.5 --- (3) -0.3 ≤y ≦ 0.3 --- (4) where, z: distance (m) from the addition position of the solute element to the center of height of the magnetic pole y: immersion nozzle Horizontal distance (m) from discharge hole to solute element addition position