JP2003205349A - Method for continuously casting cast slab having little blow hole defect, and produced cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for continuously casting a cast slab having little blow hole defect in which the cast slab having excellent quality is produced by restraining catching of argon gas bubble in molten steel into the solidified shell with a simple method at a low cost and preventing the surface defect, and the quality of a steel material applying a rolling work to the cast slab can be improved, too, and to provide the produced cast slab. <P>SOLUTION: In the method for continuously casting the cast slab which pours the molten steel into a mold from an immersion nozzle arranged at the bottom part of a tundish after receiving the molten steel into the tundish from a molten steel ladle and draws out while solidifying the molten steel by cooling with the mold, the argon gas is fed into the immersion nozzle at ≥0.5 NL/min and the cast slab having little blow hole defect is produced by continuously casting under casting condition and component concentration of the molten steel satisfying the following formula Y. Y=N.Vc.D.W.sinθ/A≤90000. N=79C mass%+208848S mass%+14339N(nitrogen) mass%+1280865O(oxygen) mass%. Wherein, N is surface tension gradient value with the component concentrations in the molten steel; Vc is casting velocity (m/min); D is thickness of the cast slab (m); W is width of the cast slab (m); θ is downward angle of the immersion nozzle (deg); and A is total cross sectional area of a spouting hole in the immersion nozzle (m<SP>2</SP>). <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to continuous casting of slabs with few bubble defects by pouring molten steel into a mold through a dipping nozzle and solidifying the molten steel to produce slabs without surface and surface layer defects. A method and a manufactured slab.

[0002]

2. Description of the Related Art Conventionally, a spray nozzle is placed in a supporting segment after receiving hot water from a molten steel ladle into a tundish, pouring it into a mold through an immersion nozzle provided at the bottom of the tundish, and cooling the mold. A slab is manufactured by solidifying while cooling water is sprayed from. When pouring molten steel into the mold using this immersion nozzle, inclusions in the molten steel adhere to the inside of the immersion nozzle, blocking the discharge port and making pouring impossible, or the molten steel discharge flow is uneven. This may cause a situation in which the casting operation is interrupted and the continuation of the casting operation is hindered. In addition, the adhered inclusions are separated and mixed into the molten steel to cause defects due to the inclusions.

In order to solve this problem, argon gas is fed to the immersion nozzle to prevent the inclusions from adhering to the immersion nozzle, and the inclusions in the mold are floated by argon gas bubbles and separated from the molten steel. , Defects caused by inclusions are prevented. However, although it was possible to suppress the adhesion of inclusions in the immersion nozzle by blowing argon gas, the bubbles of the blown argon gas were captured in the solidified shell formed by the solidification of the molten steel, and the bubbles were rolled. There was a problem that it was exposed on the surface during processing and became surface defects such as linear or swollen, which impaired the quality of the manufactured steel sheet.

As a countermeasure against this, generally, the casting speed is slowed down, and the argon gas bubbles in the molten steel in the mold are levitated to prevent the bubbles from being trapped by the solidified shell. Further, as in JP-A-9-192801, JP-A-2000-202603, etc., an ordinary electromagnetic stirrer such as a moving magnetic field type is used to suppress the downward flow of the molten steel discharge flow in the mold. It promotes the floating of argon gas bubbles in the molten steel and forms the flow of molten steel that swirls along the inner wall of the mold to prevent the argon gas bubbles and inclusions near the solidified shell from adhering to the solidified shell for cleaning. Form a solidified shell,
Prevention of bubble defects, inclusion defects, etc. has been carried out.

[0005]

However, in the method of slowing down the casting speed to float up the argon gas bubbles in the molten steel in the mold, the casting speed is significantly lowered, and the productivity of the continuous casting apparatus is lowered. When the amount of molten steel per one time is large, this molten steel causes a temperature drop due to heat radiation, and the temperature of the molten steel at the end of casting deviates from the target temperature to the lower side, causing metal adhesion and clogging of immersion nozzle due to the low temperature. Will interfere with the continuation of the casting operation. Furthermore, as in JP-A-9-192801 and JP-A-2000-202603,
Using a normal electromagnetic stirrer such as a moving magnetic field type, the downward flow of molten steel in the mold is suppressed to promote downward floating of argon gas bubbles in the molten steel, and the molten steel swirls along the inner wall of the mold. In the method of forming the flow of the above, there is inclusion of powder by the upward molten steel flow, or there are inclusions and air bubbles that volume below the swirling flow, which causes new defects and installation of equipment such as electromagnetic stirring. However, there is a problem such as an increase in power consumption during use.

The present invention has been made in view of the above circumstances, and it is possible to easily and inexpensively prevent the argon gas bubbles in molten steel from being trapped in the solidified shell, prevent surface defects, and have excellent quality. An object of the present invention is to provide a method for continuously casting a cast product having a small number of bubble defects capable of producing a cast product and improving the quality of a steel material obtained by rolling the cast product, and the produced cast product.

[0007]

According to the present invention, there is provided a method for continuously casting a slab having a small number of bubble defects according to the above object. The immersion nozzle is provided at the bottom of the tundish after receiving the hot water from the molten steel ladle into the tundish. In the continuous casting method of a slab, in which the molten steel is poured into a mold from which the molten steel is cooled and the molten steel is solidified and withdrawn, 0.5N is applied to the immersion nozzle.
Argon gas of L / min or more is fed, and continuous casting is performed under the casting conditions and the component concentrations of the molten steel satisfying the following formula. Y = N ・ Vc ・ D ・ W ・ sin θ / A ≦ 90000 N = 79C mass% + 2088848S mass% + 14339
N mass% + 128865O mass%

According to this method, the casting conditions and the surface tension gradient of the molten steel are cast so as to satisfy appropriate values, so that the bubbles of argon gas blown into the molten steel in the mold are trapped inside the solidified shell. Can be suppressed. Where V
c is the casting speed (m / min), D is the thickness of the slab (m), W is the width of the slab (m), and θ is the downward angle (de) of the immersion nozzle.
g), A is the total cross-sectional area (m ² ) of the discharge port of the immersion nozzle, N
Is a surface tension gradient value calculated by the component concentration of molten steel, C is carbon mass% in molten steel, S is sulfur mass% in molten steel, N is nitrogen mass% in molten steel, and O is oxygen in molten steel. mass%
Is. When the Y value exceeds 90,000, the downward velocity of the molten steel from the discharge port of the immersion nozzle increases, and argon gas bubbles are trapped inside the solidified shell due to the surface tension gradient of the molten steel over the entire solidified shell. This increases the number of bubbles in the surface layer of the slab, and causes a bubble-like defect on the surface of the steel material obtained by rolling the slab or on the surface layer.

Furthermore, the slab with few bubble defects according to the present invention receives 0.5 g / min or more of argon gas from an immersion nozzle provided at the bottom of the tundish after receiving the hot water from the molten steel ladle into the tundish. It is manufactured by continuous casting under the casting conditions and the molten steel component concentrations that satisfy the following formula. Y = N ・ Vc ・ D ・ W ・ sin θ / A ≦ 90000 N = 79C mass% + 2088848S mass% + 14339
N mass% + 128865O mass%

Since this cast piece is cast so that the casting conditions and the surface tension gradient of the molten steel satisfy appropriate values, the bubbles of argon gas blown into the molten steel in the mold are trapped inside the solidified shell. Since this is suppressed, it is possible to manufacture a slab in which the number of bubbles trapped in the initial solidified shell (surface layer) of the slab is reduced. Then, it is possible to improve the quality by preventing linear defects, swelling defects and the like due to bubbles generated in the steel material manufactured by rolling the slab. Here, Vc is the casting speed (m / min), D is the thickness of the slab (m), W is the width of the slab (m), θ is the downward angle (deg) of the immersion nozzle, and A is the discharge of the immersion nozzle. The total cross-sectional area (m ² ) at the outlet, N is the surface tension gradient value calculated by the component concentration of the molten steel, C is the carbon mass% in the molten steel, S is the sulfur mass% in the molten steel, and N is the molten steel. Mass% of nitrogen and O are mass% of oxygen in the molten steel.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Next, referring to the attached drawings, an embodiment in which the present invention is embodied will be described to provide an understanding of the present invention. FIG. 1 is an explanatory view of a continuous casting apparatus applied to a continuous casting method of a slab with few bubble defects according to an embodiment of the present invention, and FIG. 2 is an enlarged view of a dipping nozzle portion. As shown in FIG. 1 and FIG. 2, in a continuous casting apparatus A used in a continuous casting method for a slab with few bubble defects according to an embodiment of the present invention, molten steel 2 in a tundish 1 is immersed in a mold 3. It is poured through the nozzle 4. At this time, in order to prevent nozzle clogging due to adhesion of inclusions in the molten steel to the nozzle, and to float and separate inclusions in the continuous casting mold, an argon gas supply pipe communicating with the inside of the immersion nozzle 4 10 is provided, the argon gas passes through the slit 11 of the immersion nozzle 4, and the porous refractory layer 1
From 2, it is blown into the nozzle. The molten steel 2 poured into the mold is supported outside by the injection of cooling water from a cooling nozzle (not shown) attached to the support segment 5 while being supported by the support segment 5 composed of the mold 3 and a plurality of support roll groups. Solidification progresses from the pinch roll 6
Thus, the cast piece 7 is pulled out.

Next, the continuous casting method for casting a slab with few bubble defects according to the present embodiment will be described using the continuous casting apparatus A. Molten steel 2 is received in a tundish 1 from a molten steel ladle, and a dipping nozzle 4 is provided at the bottom of the tundish 1.
The molten steel 2 is poured into a mold 3 whose interior is water-cooled and cooled by the mold 3 to solidify the molten steel. By cooling the mold 3, a solidified shell is formed, and further, solidification proceeds by the guide by the support segment 5 arranged below the mold 3 and the cooling by the spray nozzle, and the unsolidified portion is formed by using the light pressure roll segment 9. After pressing, the completely solidified slab 7 is pulled out by the pinch roll 6 at a predetermined speed. When pouring molten steel 2 into the mold 3 from the immersion nozzle 4, argon gas is fed from an argon gas supply pipe 10 communicating with the immersion nozzle 4, and a porous refractory material is provided through a slit 10 provided inside the immersion nozzle 4. Layer 11 to 0.5 NL /
Blow in more than the minute amount of argon gas.

Furthermore, the components of molten steel 2 and the casting conditions are adjusted so as to satisfy the following formula, and continuous casting of molten steel 2 is performed. Y = N · Vc · D · W · sin θ / A ≦ 90000 (1) where Vc is the casting speed (m / min), D is the thickness of the slab (m), and W is the width of the slab ( m) and θ are downward angles (deg) of the immersion nozzle, A is the total cross-sectional area (m ² ) of the discharge port of the immersion nozzle, and N is a value calculated from the component concentration of the molten steel. Further, the N value is set to a condition that satisfies the following formula obtained by using the following molten steel components. N = 79 C mass% + 208848 S mass% + 14339 N mass% + 12808 65O mass% ... (2) Here, C is carbon mass% in the molten steel, S is sulfur mass% in the molten steel, N is nitrogen mass% in the molten steel, O is the mass% of oxygen in the molten steel.

N represented by the equation (2) applied to the equation (1)
The value is the argon gas that has penetrated into the concentration boundary layer formed on the front surface of the solidified shell (solidified shell) during continuous casting due to the solid-liquid distribution during solidification of the components (elements) contained in the molten steel to the solidification sur. It shows the magnitude of the suction force. That is, as shown in the publication: “Iron and Steel” 80 (1994) p527, the speed V at which the argon gas is sucked by the surface tension gradient ∂Y / ∂X formed by the concentration gradient on the front surface of the solidification sur It is expressed by equation (3). V = − (2d / 9μ) (∂Y / ∂X) = − (2d / 9μ) (∂Y / ∂C) (∂C / ∂X ... (3) where d is the diameter of the argon gas, μ is the viscosity of molten steel, Y is the surface tension of molten steel, X is the distance from the solidification interface, ∂Y / ∂X is the surface tension gradient, and ∂Y / ∂C is the dependency term of the component concentration C of surface tension. , ∂C / ∂X are gradients of component concentrations.

Considering the steady state of solidification, if the component concentration in the bulk molten steel is Co and the average distribution coefficient is k, the component concentration at the interface between the solidified shell and the molten steel is expressed as Co / k, and the width of the concentration boundary layer is expressed. Assuming that δ is δ and a linear concentration distribution is assumed, the concentration gradient is expressed by the following equation. ∂C / ∂X = (Co / k-Co) / δ = (1-k) k · Co / δ (4) Therefore, the equation (4) can be rewritten as the equation (5). V =-(2d / 9μ) (∂Y / ∂X) =-(2d / 9μ) (∂Y / ∂C) (1-k) k · Co / δ (5)

Next, by taking out only the term relating to the component concentration on the right side of the equation (5) and summing all the constituent elements, the equation (6) is obtained. Z value = ΣY (i) · {1-k (i)} / k (i) · C (i) (6) where Y (i) = − ∂Y / ∂C (i) (mN / M / m
(ass%) is a concentration coefficient representing the influence of the i element on the surface tension Y of iron (a positive value is given when the surface tension is reduced when added), and the known publication "Materia, vol. 36".
(1997). p. 47 "and the like. k (i)
Is a well-known publication on the equilibrium partition coefficient of the i element in iron, "3rd Edition Iron and Steel Handbook I, edited by the Iron and Steel Institute of Japan (1981), p. 193".
Etc. C (i) is the mass% of the i element, and Σ is the sum of the constituent elements.

The elements having a great influence on the Z value are C (carbon), S (sulfur), N (nitrogen) and O (oxygen),
N calculated using only these elements as shown in equation (2)
Even if the value is used, the degree of trapping the argon gas bubbles can be estimated without any practical problem. However, the concentration of the component used in the calculation is independent (free) in the molten steel that affects the surface tension.
It should be noted that the concentration which is dissolved in the form of, and the concentration which is present in the form of compounds (nitride, oxide, etc.) have no effect.

Next, Vc · D · W / A × si of the equation (1)
nθ corresponds to the penetration depth of bubbles in the molten steel into the mold. That is, Vc · D · W is the injection speed into the mold (m ³ / min), Vc · D · W / A is the average cross-section injection speed (m / min) at the discharge port of the immersion nozzle, and Vc · D · W / A × sin θ represents the vertically downward velocity component. Therefore, when the trapping force of bubbles calculated by the above equation (2) is the same, it is considered that the trapping degree becomes more remarkable as the casting conditions have larger values.

Furthermore, the present inventors cast molten steel of various compositions under various casting conditions, and show the relationship between the Y value calculated by the equation (1) and the number of argon gas trapped in the slab. The results of the investigation are shown in FIG. FIG. 3 is a graph showing the relationship between the Y value and the bubble trapping index of the slab according to the present invention. As shown in FIG. 3, by setting the Y value to 90,000 or less, it is possible to prevent trapping of argon gas in the solidified shell and to prevent industrially harmless 0.1
It has been found that the level can be reduced to the level of less than the number of pieces / cm ³ . In other words, by adjusting the components and the casting conditions so that the Y value becomes 90,000 or less, it is possible to systematically reduce the defects in the foaminess.

[0020]

EXAMPLES Next, examples of a continuous casting method of cast pieces having few bubble defects and produced cast pieces will be described. With 350 tons of molten steel (molten steel ladle capacity) having the chemical composition shown in Table 1 in a continuous casting apparatus as shown in FIG. 1 in which the inner dimension of the mold is 250 mm, the blowing rate of argon gas was 5 NL / min,
Casting was performed so that the Y value was 90000 or less. After casting, the number of gases trapped in the surface layer (0 to 20 mm from the surface) of the slab was investigated by an X-ray flaw detection method, and the occurrence of bubble defects after rolling was also investigated consistently. N
Even for steel types such as D, I, and J that have large values and are easily trapped by bubbles, the Y value can be controlled by controlling the casting speed, the slab width, the dipping nozzle discharge angle, and the dipping nozzle discharge area according to the present invention. It was found that by controlling the slab to be 90,000 or less, a slab of good quality with a small number of argon gas bubbles trapped in the surface layer of the slab can be obtained industrially stably.

[0021]

[Table 1]

The embodiment of the present invention has been described above.
The present invention is not limited to the above-described embodiment, and changes in conditions and the like without departing from the spirit are all within the scope of application of the present invention. For example, the Y value described in this embodiment is set to 90,000.
At the same time, electromagnetic stirring or a stirring method for imparting a swirling flow along the inner wall of the mold may be used in combination at the same time. Further, as the argon gas fed to the immersion nozzle, a gas obtained by mixing argon gas with nitrogen gas may be used in addition to the argon gas, or an inert gas other than the argon gas may be used.

[0023]

As described above, according to the present invention, it is possible to easily and inexpensively suppress the argon gas bubbles in molten steel from being trapped in the solidified shell, prevent surface defects, and obtain excellent quality casting. A piece can be manufactured, and the quality of the steel material obtained by rolling the cast piece can be improved. In particular, since the casting conditions and the surface tension gradient of the molten steel are set to proper values, it is possible to prevent the argon gas bubbles from being trapped inside the solidified shell and prevent the bubbles from forming on the surface layer of the slab. Then, it is possible to prevent the linear defects caused by the air bubbles generated in the steel material produced by rolling the slab, the swollen defects, etc., and to improve the quality, which is an extremely excellent effect. .

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a continuous casting apparatus applied to a continuous casting method of a slab with few bubble defects according to an embodiment of the present invention.

FIG. 2 is an enlarged view of an immersion nozzle section.

FIG. 3 is a graph showing the relationship between the Y value and the bubble trapping index of the slab according to the present invention.

[Explanation of symbols]

A continuous casting machine 1 tundish 2 Molten steel 3 molds 4 immersion nozzle 5 Supporting segments 6 pinch rolls 7 slab 8 Bending back correction points 9 Light rolling roll segment 10 Argon gas supply pipe 11 slits 12 Porous refractory layer 13 Discharge port

Claims (2)

[Claims] 1. A slab for drawing molten steel from a molten steel pan into a tundish, pouring the molten steel into a mold from an immersion nozzle provided at the bottom of the tundish, and cooling the molten steel to solidify the molten steel. In the continuous casting method, the number of bubble defects is small, which is characterized in that the immersion nozzle is fed with argon gas of 0.5 NL / min or more and the casting conditions and the component concentration of the molten steel are satisfied to satisfy the following formula. Continuous casting method of slab. Y = N ・ Vc ・ D ・ W ・ sin θ / A ≦ 90000 N = 79C mass% + 2088848S mass% + 14339
N mass% + 12880865 mass% Here, Vc is the casting speed (m / min), D is the thickness of the slab (m), W is the width of the slab (m), θ is the downward angle (deg) of the dipping nozzle, A is the total cross-sectional area (m ² ) of the discharge port of the immersion nozzle, N is the surface tension gradient value depending on the component concentration of the molten steel, C is the mass% of carbon in the molten steel, S is the mass% of sulfur in the molten steel, and N is N. Is nitrogen mass% in the molten steel, and O is oxygen mass% in the molten steel. 2. A tundish is heated from a molten steel ladle, and then a tundish from a dipping nozzle provided at the bottom of the tundish is used.
A slab with few bubble defects, characterized in that it is produced by continuous casting under the casting conditions and the component concentrations of molten steel that satisfy the following formula by supplying an argon gas at 5 NL / min or more. Y = N ・ Vc ・ D ・ W ・ sin θ / A ≦ 90000 N = 79C mass% + 2088848S mass% + 14339
N mass% + 12880865 mass% Here, Vc is the casting speed (m / min), D is the thickness of the slab (m), W is the width of the slab (m), θ is the downward angle (deg) of the dipping nozzle, A is the total cross-sectional area (m ² ) of the discharge port of the immersion nozzle, N is a value calculated by the component concentration of the molten steel, C is the mass% of carbon in the molten steel, S is the mass% of sulfur in the molten steel, and N is N. Is nitrogen mass% in molten steel, O is oxygen mass% in molten steel
Is.