JP2718608B2 - Steel continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for steel which prevents nozzle clogging in continuous casting and enables stable production of cast pieces having excellent surface quality.

[0002]

2. Description of the Related Art In a continuous casting method of steel, molten steel is poured into a mold from a tundish through an immersion nozzle. At this time, the alumina present in the molten steel causes cohesion and adhesion to the inner surface of the nozzle, causing nozzle clogging, and eventually leading to nozzle blockage. As a result, the operation has to be interrupted and the nozzle must be replaced, and from this point productivity is a problem. Regarding such a problem, there are many disclosures particularly on the prevention of alumina inclusions. As a typical example, Japanese Patent Application Laid-Open No. 4-52056 discloses a porous material for blowing gas into an immersion nozzle when C% is 0.01% or less. Ar through the quality department
Disclosed is a method for producing a clean sheet slab by supplying a certain amount of gas to control the generation of bubbles of inert gas to float and separate inclusions in a mold.

[0003] In recent years, studies have been actively conducted on a method for removing inclusions in molten steel using fine bubbles of an inert gas or on the adhesion separation behavior of inclusions by bubbles. However, the above-mentioned nozzle clogging has not been sufficiently improved in actual continuous casting of steel. Even if the conventional inert gas blowing technique for removing inclusions is applied to prevent nozzle clogging, there are many problems in the optimization of the conditions of the immersion nozzle, with only limited effects being obtained with greatly limited effects. FIG. 3 shows a typical example of the state of adhesion of alumina in an immersion nozzle made of a conventional material. In this figure, it is shown that the amount of alumina attached in the immersion nozzle is remarkable near the discharge hole below the meniscus. Techniques for preventing alumina from adhering to such sites have not been sufficient.

[0004]

SUMMARY OF THE INVENTION The present invention is based on the above-mentioned prior art. However, due to the adhesion of alumina in the nozzle, the nozzle is finally closed and the casting cannot be continued. It is another object of the present invention to solve problems such as the possibility that molten steel leaks from under the mold and that the molten steel in the mold is disturbed to deteriorate the quality. That is, the present invention provides a continuous casting method of steel that can improve the productivity of continuous casting and improve the quality of cast products by preventing or suppressing the adhesion of alumina in the nozzle.

[0005]

It is a gist of the present invention SUMMARY OF THE INVENTION, in the injection method of the tundish in the continuous casting method of steel into the mold, Q ₁ (Nl / min inert gas blowing flow rate, respectively, from the upper nozzle ), Q ₂ from the sliding nozzle plate, Q ₃ from the immersion nozzle, and the flow rate of molten steel passing through the nozzle is Q _S (t / min),
_{Q g = 0.5 x (Q 1} + Q 2) + Q 3 (Nl / min) Equation relationship between the inert gas flow rate Q _g and Q _S are the following represented by (1), (2), (3) and ( There surrounded by the area by a straight line represented by 4), a nozzle material of the molten steel in contact with portions of the meniscus lower portion including a discharge hole whether one immersion nozzle,
C: 15~30%, CaO: 10~30 %, ZrO 2: 45~65%
Characterized by containing the following components: Q _S = 0.7 x Q _{g (however, Q g = 0.5 x (Q} 1 + Q 2) + Q 3) (1) Q S = 0.2 x Q g (2) Q g = 15 (Nl / min) (3 ) Q _S = 2 (t / min) (4)

The feature of the present invention resides in that an inert gas blowing condition which is most effective for preventing or suppressing adhesion and deposition of alumina at a refractory interface is selected. That is, by giving the inert gas blowing and casting conditions, and further by arranging the zirconia material under the meniscus including the discharge hole of the immersion nozzle, the molten steel in the discharge hole from the lower meniscus where the alumina adheres most in the immersion nozzle The effect of nozzle clogging, which is the object of the present invention, is maximized at the flow path interface.

FIG. 2 shows the relationship between Q _S and Q _{g according to} the present invention. The straight line (a) and the straight line (b) in FIG. 2 are represented by the relational expressions shown in the above formulas (1) and (2), respectively.
Further, the straight line (g) and the straight line (h) are represented by the relational expressions shown in Expressions (3) and (4), respectively. Therefore, the present invention is represented by a range surrounded by a straight line (a), a straight line (b), a straight line (g), and a straight line (h). Hereinafter, the reasons for limiting the components according to the embodiments of the present invention will be described in accordance with the individual components.

[0008]

FIG. 1 shows an example of an embodiment of the present invention. In the present embodiment, molten steel 6 is injected from an immersion nozzle 4 via an upper nozzle 2 and a sliding nozzle plate 3 between a tundish 1 and a mold 5 used in continuous casting. At this time, an inert gas is blown in the direction of the arrow from one or both of the upper nozzle 2 and the sliding nozzle plate 3 and from the immersion nozzle 4.
In addition, by disposing a zirconia lime material under the meniscus including the discharge hole of the immersion nozzle, it is possible to prevent the molten steel flow path at the discharge hole from being closed or narrowed from the lower portion of the meniscus where the alumina adheres most in the immersion nozzle. is there. In addition to the present embodiment, as an injection device to which the present invention is applied, the present invention can be applied to a device in which the flow rate of molten steel is controlled by a stopper, similarly to a device in which a sliding nozzle is used.

Next, the scope of the present invention will be described with reference to the present embodiment. First, the flow rate of an inert gas blown for preventing clogging of the immersion nozzle and stabilizing the supply of molten steel to the mold will be described. FIG. 4 is a diagram showing the relationship between the flow rate of the inert gas and the flow rate of the molten steel passing through the nozzle. The straight line (a) in FIG. 4 shows the alumina adhesion limit line on the inner wall of the nozzle. In the area (a) with respect to this straight line, alumina adheres and the nozzle is closed. The region (b) is a region in which no alumina adheres and nozzle blockage hardly occurs. The straight line (b) in FIG. 4 shows a boiling limit line due to the inert gas in the mold. The region (b) is a region where the supply of molten steel to the inner wall of the mold is stable, and the region (c) is a region where the supply of molten steel into the mold is unstable due to boiling with an inert gas.

That is, in the region (b), there is no blockage of the nozzle by the alumina, indicating that the supply of molten steel into the mold is stable. This region is favorable operating conditions for performing continuous casting. Note that the straight line (a) and the straight line (b) in the drawing are the inert gas blowing flow rate Q _g (Nl /
Min) and the flow rate of molten steel passing through the nozzle Q _S (t / min) as variables. _{Q S = 0.7 x Q g (} Nl / min) (linear (a)) (5) Q S = 0.2 x Q g (Nl / min) (linear (b)) (6) where Q _g is the following formula ( Q _g = 0.5 × (Q ₁ + Q ₂ ) + Q ₃ (Nl / min) (7) Q ₁ : Inert gas blowing flow rate (Nl /
Min) Q ₂ : Blow flow rate from sliding nozzle plate (Nl / min) Q ₃ : Blow flow rate from immersion nozzle (Nl / min)

Therefore, a region (b) which is a good operating condition for performing continuous casting is represented by the following equation (8). Q _S (Nl / _{min) ≦ 0.7 x Q g (Nl} / min) (8) and, Q _S (Nl / _{min) ≧ 0.2 x Q g (Nl} / min), _{however, Q g = 0.5 x (Q} 1 + Q ₂ ) + Q ₃ . The above conditions have been confirmed by actual continuous casting, but have also been confirmed from experimental results by the inventors as described in the following examples.

FIG. 5 shows the relationship between the alumina concentration C and the function F at the interface of the immersion nozzle. The function F is represented by Q _S / Q _g , where Q _g will be further described. Q _g is the inert gas blowing flow rate. The contribution of the inert gas blown from the portion above the sliding nozzle plate to the alumina concentration at the nozzle interface below the submerged nozzle meniscus is か ら from experimental data. Since the flow rate of the inert gas from the upper nozzle, the sliding nozzle plate, and the immersion nozzle is Q ₁ , Q ₂ , and Q ₃ , respectively, the effective inert gas injection flow rate at the lower interface of the immersion nozzle meniscus Q
_g is represented by the following equation. Q _g = 0.5 x (Q ₁ + Q ₂ ) + Q ₃

[0013] curve in FIG. 5 (c) shows the relationship between the concentration of alumina C and functions F at the nozzle surface. The straight line (d) indicates the upper limit of the concentration of alumina that produces a low-melting oxide by reaction with the zirconia lime material placed in the immersion nozzle.
From FIG. 5, the alumina concentration decreases as the function F decreases. This is because as the inert gas bubbles increase with respect to the flow rate of molten steel passing through the nozzle, the rate of alumina capture by the inert gas or the shear force acting on the nozzle interface increases, and the frequency of alumina accumulation at the nozzle interface decreases. is there.
When you below a certain concentration of alumina, the last reaction with zirconia lime in the nozzle material, but to prevent sticking to the low-melting-point oxides are generated inner wall of the nozzle this limitation alumina concentration is linear (d) .

Point A is the point F at the intersection of the straight line (c) and the straight line (d).
Indicates a value. The region (b) indicates a range of F ≦ A, and is a region where alumina does not adhere due to generation of a low melting point oxide. The region (a) shows a range of F> A, and since the concentration of alumina is high, the reaction with zirconia lime in the nozzle material has a high melting point, and the region to which alumina adheres. From the above analysis results, when the A value was examined by an actual continuous casting test, it was found that A = 0.7 as shown in FIG. That is, the condition that alumina does not adhere is Q _S ≦ 0.7 × Q _g . Next, the limit of the occurrence of boiling due to the inert gas in the mold will be described.

FIG. 6 shows the relationship between the boiling index R and the function F in the mold. In this figure, the boiling index R increases as the function F decreases, which means that as the inert gas bubbles increase relative to the flow rate of the molten steel passing through the nozzle, the inert gas density in the molten steel increases, or the inert gas bubbles increase. This promotes coalescence of bubbles and increases the levitation rate of gas. When the boiling index R exceeds a certain value, the supply of molten steel to the mold becomes unstable due to the boiling of the inert gas in the mold, and stable continuous casting becomes impossible.

This limit boiling index R is (f). Point B indicates the function F value at the intersection of the straight line (e) and the straight line (f). The region (b) in FIG. 6 shows a range of F ≧ B, and is a region where the supply of molten steel into the mold is stable. The area (c) is F
<B indicates a range in which the supply of molten steel becomes unstable due to boiling with an inert gas in the mold, and stable casting cannot be performed. From the results of such analysis, when the value of B was examined by an actual casting test, it was found that B = 0.2 as shown in FIG. That stable conditions the molten steel supplied to the mold is Q _S ≧ 0.2 x Q _g.

From the above, the region (b), which is a suitable operating condition for performing continuous casting, is represented by the above equation (8). Q _S (Nl / _{min) ≦ 0.7 x Q g (Nl} / min) and, Q _S (Nl / _{min) ≧ 0.2 x Q g (Nl} / min), _{however, Q g = 0.5 x (Q} 1 + Q 2) + and Q _3. Next, the reasons for the limits of the flow rate of the molten steel and the flow rate of the inert gas are described.

FIG. 7 shows the effect of the flow rate of the inert gas injection on the rejection of surface defects of a rolled product in a continuous vertical bending machine. From this result, it was found that the quality of the rolled product tends to deteriorate when the flow rate of the inert gas exceeds the value. That is, the inert gas reaches the inside of the mold through the immersion nozzle and floats in the mold. As the gas flow rate increases, the force for stirring the interface between the molten steel and the powder in the mold increases. As a result, the powder is entrained in the meniscus portion where the molten steel starts to solidify, generating surface flaws, and the frequency of inert gas being caught at the solidified shell interface during the ascent is increased, resulting in the presence of the powder just below the surface layer of the slab. It is considered that a bubble-like defect including the object is generated.

As described above, the flow rate of the inert gas depends on the product quality.
Is an important operating condition that affects
A flow range exists. That is, the inert gas injection flow
Quantity: Q _gIs 15 (Nl /
Min) is effective to obtain good quality.
You. In addition, the flow rate of molten steel in the nozzle
When it becomes less than the flow rate, the meniscus in the mold
The occurrence of deckle or metal beam due to insufficient flow of molten steel
Therefore, there is an appropriate molten steel flow rate range. That is,
Molten steel flow rate through nozzle: Q_SIs 2 (t / min) or more
Is effective for obtaining a stable casting. Next, the nozzle material
Regarding the quality, please refer to the
In relation to the problem, the inventors have
The results of the survey are described in detail below.

The thickness of the alumina adhered in the nozzle is remarkable near the discharge hole below the meniscus. Since the discharge hole is a branch point of the molten steel flow, a dead flow occurs above the discharge hole on the inner wall of the nozzle. For this reason, the adhesion progresses preferentially at this portion due to an increase in the frequency of accumulation of alumina at the nozzle interface or a decrease in the shearing force due to the molten steel flow. In the present invention, this portion is made of zirconia lime, which will be described in detail below.
FIG. 8 shows the relationship between the adhesion thickness in the immersion nozzle after 250 minutes of casting and the flow rate of the inert gas injection by comparing the zirconia lime immersion nozzle with the conventional material immersion nozzle. The material of the immersion nozzle used in the embodiment of FIG. 8 is as shown in Table 1. In this case, the flow rate of the molten steel passing through the nozzle is 5.2 (t / min), but Q _g <7.5 (Nl / min).
In both the conventional material immersion nozzle and the zirconia lime immersion nozzle, the adhesion thickness exceeds 10 mm, which is the danger of nozzle blockage, but when Q _g ≥ 7.5 (Nl / min), the adhesion thickness of the zirconia lime immersion nozzle is greatly reduced. Therefore, it is possible to stably continue casting for a long time within the quality limit of Q _g ≦ 15 (Nl / min). Judging from the examples in Table 1, Table 2 shows the range of immersion nozzle components that are considered suitable for application to the present invention.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

According to the present invention, the relationship between the flow rate of molten steel injected from the tundish into the mold and the flow rate of the inert gas blown into the mold is limited to a certain condition range, and the ratio of the flow rate of the inert gas from each blowing site is adjusted appropriately. By applying zirconia lime to the immersion nozzle material,
This makes it possible to prevent the immersion nozzle from being clogged by alumina, and to produce a cast slab having a stable casting state and a low product quality defect after rolling, thereby stabilizing the continuous casting operation and improving the slab quality.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing a range in a relationship between Q _g and Q _S of the present invention.

FIG. 3 is a diagram showing the amount of alumina deposited by a conventional nozzle and its position.

FIG. 4 is a diagram showing a relationship between Q _g and Q _{S according to} the embodiment of the present invention.

FIG. 5 is a diagram showing a relationship between a function F and an alumina concentration C according to the embodiment of the present invention.

FIG. 6 is a diagram showing a relationship between Q _g and a mold boiling index R according to the embodiment of the present invention.

FIG. 7 shows a vertical bending type continuous casting machine according to an embodiment of the present invention.
Is a diagram showing the relationship between Q _g and surface flaws reject rate.

FIG. 8 is a diagram showing the relationship between Q _g and the adhesion thickness of the present invention and a conventional nozzle material.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Tundish 2 ... Upper nozzle 3 ... Sliding nozzle plate 4 ... Immersion nozzle 5 ... Mold 6 ... Molten steel 7 ... Meniscus 8 ... Discharge hole 9 ... Zirconia lime material part 10 ... Porous part

Claims (1)

(57) [Claims] 1. In a method of injecting steel from a tundish into a mold in a continuous casting method of steel, an inert gas blowing flow rate is set to Q ₁ (Nl / min) from an upper nozzle, Q ₂ from a sliding nozzle plate, and from an immersion nozzle, respectively. and Q _3, when the passage of molten steel flow in the nozzle was Q _S (t / _{min), Q g = 0.5 x (} Q 1 + Q 2) + Q 3 (Nl /
Relationship between the inert gas flow rate Q _g and Q _S representing min) of the following formula (1), (2), located in (3) and (surrounded by the area by a straight line represented by 4) One or nozzle material of the portion in contact with the meniscus bottom of molten steel including a discharge hole of immersion nozzle, C: 15~30%, CaO: 10~30%, ZrO 2: 45~
Continuous casting method of steel, characterized in that it contains 65% of component. Q _S = 0.7 x Q _{g (however, Q g = 0.5 x (Q} 1 + Q 2) + Q 3) (1) Q S = 0.2 x Q g (2) Q g = 15 (Nl / min) (3 ) Q _S = 2 (t / min) (4)