JP2003251438A - Method for continuously casting cast slab having little blow hole and steel material obtained by working the cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for continuously casting a cast slab having little blow hole by which the cast slab having excellent quality is produced by restraining the catch of argon gas in molten steel into a solidified shell by a simple method and at a low cost to prevent the surface defect, and the quality of a steel material obtained by applying a rolling-work to the cast slab can be improved, and to provide the steel material obtained by working the cast slab. <P>SOLUTION: In the method for continuously casting the cast slab, by which the molten steel having a composition causing crystallization of γ-phase as a primary crystal at the solidification is received into a tundish from a ladle and poured into a water-cooled mold through a nozzle to cast the cast slab, the continuous casting is performed after adjusting the component concentrations in the molten steel so that a Z value calculated with the following formula becomes ≤2,500: Z=Σy(i)×(1-k(i))/k(i)×c(i) [wherein, y(i) is a concentration factor indicating the effect of an (i) element exerted to a surface tension in the molten steel and is a positive value in the case of lowering the surface tension at the addition of the element adding time; k(i) is an equilibrium distribution factor of the (i) element in the molten steel; c(i) is a mass% of the (i) element; and Σ is the total of the constituting elements in the molten steel]. <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 a continuous casting method of a slab, which is excellent in the surface quality of the slab and the surface quality of the slab and has a small number of defects in the bubble property, and a steel product obtained by processing the 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. It is an object of the present invention to produce a slab and to provide a continuous casting method of a slab with few bubble defects capable of improving the quality of a steel product obtained by rolling the slab and a steel product obtained by processing the slab. .

[0007]

The continuous casting method for a slab with few bubble defects according to the present invention in accordance with the above-mentioned object is to prepare molten steel having a composition in which a γ phase is crystallized as a primary crystal during solidification from a ladle. In a continuous casting method of a slab, in which a molten steel is poured into a water-cooled mold through a nozzle to cast a slab, a Z value calculated from the component concentration of the molten steel by the following formula is 2500 or less. In this case, the component concentration of the molten steel is adjusted beforehand so that continuous casting is performed. Z = Σy (i) × (1-k (i)) / k (i) × c (i) (1) By this method, the surface of the concentration boundary layer formed on the inner front surface of the solidified shell. The tension gradient can be made small, and the bubbles of invading gas can be suppressed from adsorbing to the inner surface of the solidified shell, and the bubbles trapped in the solidified shell grown by cooling can be greatly reduced. When the Z value exceeds 2500, the surface tension gradient of the inner surface of the solidified shell in which the molten steel is solidified increases, and the number of bubbles adhered to the inner surface of the solidified shell increases. It should be noted that y (i) is a concentration coefficient representing the effect of the i element on the surface tension of molten steel, and a positive value is given when the surface tension is reduced when added. k (i) is the equilibrium partition coefficient of the i element in the molten steel, c (i) is the mass% of the i element, and Σ is the sum of the constituent elements in the molten steel.

Further, in the continuous casting method of a slab with few bubble defects according to the present invention, molten steel having a composition in which a γ phase is crystallized out as a primary crystal at the time of solidification is received in a tundish from a ladle, and a nozzle is used. In a continuous casting method of a cast piece in which the molten steel is poured into a water-cooled mold to cast a cast piece, the M value calculated from the component concentration of the molten steel by the following formula is 2500 or less in advance of the molten steel. Continuously casting after adjusting the component concentrations.
By this method, by adjusting the concentration of carbon (C), sulfur (S), nitrogen (N), and oxygen (O), which are some specific components, in the molten steel, the internal front surface of the solidified shell is adjusted. It is possible to reduce the surface tension gradient of the formed concentration boundary layer and suppress the adsorption of the invading gas bubbles on the inner surface of the solidified shell. When the M value exceeds 2500, the surface tension gradient of the inner surface of the solidified shell in which the molten steel is solidified increases, and the number of bubbles adhered to the inner surface of the solidified shell increases. M = 36 C mass% + 303066 S mass% + 5804 N mass% + 845197 O mass% (2) In addition, C is carbon contained in molten steel, S is sulfur contained in molten steel, N is nitrogen contained in molten steel, and O is Indicates oxygen contained in molten steel.

Further, in the continuous casting method of a slab with few bubble defects according to the present invention, it is desirable that the argon gas supplied from the nozzle into the molten steel be 0.5 NL / min or more. Thus, it is possible to suppress the adhesion of Al ₂ O ₃ -based inclusions that adhere to the tundish nozzle or the dipping nozzle, and to prevent the defects of the slab caused by the peeling of the adhered Al ₂ O ₃ -based inclusions. The inclusions floating in the mold can be floated and removed. If the supply rate of the argon gas is less than 0.5 NL / min, the effect of suppressing the Al ₂ O ₃ -based inclusions adhering to the nozzle or the dipping nozzle is reduced, and fluctuations in the molten steel discharge flow and nozzle clogging occur.

The steel material having few bubble defects according to the present invention is a molten steel having a composition in which a γ phase is crystallized as a primary crystal during solidification.
In a steel material obtained by receiving hot water from a ladle in a tundish, pouring the molten steel into a water-cooled mold through a nozzle and casting a slab and rolling the slab, the concentration of the molten steel is calculated by the following formula. The component concentration of the molten steel is adjusted in advance so that the Z value is 2500 or less, and the continuously cast slab is rolled to manufacture. This steel material is manufactured by rolling a slab that suppresses air bubbles trapped in the solidified shell, so there are few surface defects due to air bubbles that occur on the surface of the steel material, and products with good appearance are manufactured. can do. Z = Σy (i) × (1-k (i)) / k (i) × c (i) (1) where y (i) is the concentration representing the effect of the i element on the surface tension of the molten steel. The coefficient is a positive value when the surface tension is reduced when added. k (i) is the equilibrium partition coefficient of the i element in the molten steel, c (i) is the mass% of the i element, and Σ is the sum of the constituent elements in the molten steel.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments embodying the present invention will be described below with reference to the accompanying drawings to provide an understanding of the present invention. FIG. 1 is an overall view of a molten steel continuous casting apparatus according to an embodiment of the present invention. As shown in FIG. 1, in a continuous casting apparatus A used for continuous casting of molten steel according to an embodiment of the present invention, a tundish 1
The molten steel 2 therein is poured into the mold 3 through the immersion nozzle 4. At this time, argon gas is blown into the immersion nozzle 4 in order to prevent nozzle clogging due to adhesion of inclusions in the molten steel to the nozzles, and to float and separate inclusions in the continuous casting mold. 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. The solidification progresses from then, and the pinch roll 6 pulls it out as a cast piece 7.

In this way, the argon gas is trapped in the solidified shell of the cast slab and is exposed on the surface or remains on the surface layer after casting. Further, when such a slab is subjected to heat treatment / rolling, linear surface flaws and swollen surface defects caused by argon gas are likely to occur, and the quality of the steel sheet is greatly impaired. As a countermeasure for this, a method of reducing the casting speed in order to secure the floatability of the argon gas in the mold is conceivable, but this method causes a secondary problem that productivity is impaired because the casting speed is reduced. . A possible method is to adjust the nozzle discharge angle upward so that molten steel does not penetrate into the deep portion of the molten steel pool in the molten steel in the mold, but this method also facilitates the inclusion of mold flux, so surface defects caused by mold flux may occur. There is a problem of increase.

A method of preventing the capture of argon gas by electromagnetically stirring the molten steel in the mold is conceivable, but there are problems such as facility cost and electric power for installing the electromagnetic stirrer. A method of reducing the amount of argon gas blown in itself is also conceivable, but as the amount of argon gas blown, both the effect of preventing the inclusions of inclusions from the nozzle and the effect of floating inclusions in the continuous casting mold are separated. In order to satisfy the requirement, according to the study by the present inventors, 0.5 L / min or more is necessary, and it may not always be a sufficient countermeasure.

Therefore, the inventors of the present invention have found that the above problems can be solved by carrying out research on the behavior of argon gas in molten steel and using the following means. That is, the molten steel is received from the molten steel ladle into the tundish, and argon gas is added to 0.5 through a refractory molten steel supply nozzle.
Regarding the continuous casting method of steel in which the molten steel is supplied into the water-cooled mold while being blown into the molten steel in the nozzle at L / min or more,
The Z value calculated from formula (1) from the steel component concentration is 2500.
A method for continuous casting of steel, wherein the molten steel composition is adjusted in advance as follows, and then the molten steel is continuously cast. Z = Σy (i) × (1-k (i)) / k (i) × c (i) (1) where y (i) is the concentration representing the effect of the i element on the surface tension of the molten steel. Coefficient (a positive value is given to reduce the surface tension when added), k (i) is the equilibrium distribution coefficient of the i element in the molten steel, c (i) is the mass% of the i element, and Σ is the molten steel. The total sum of the constituent elements is shown.

The right side of the above equation (1) solidifies the argon gas that has penetrated into the concentration boundary layer formed on the front surface of the solidification shell during continuous casting due to solid-liquid distribution during solidification of the constituent elements contained in the steel. It represents the magnitude of the force that is sucked into the surreal. That is, the publication: "Iron and Steel" 80 (1994)
p. As indicated by 527, 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 coagulation surreal is represented by the 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 solidification shell / molten steel interface is expressed by 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 (3) 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 (1) is obtained. Z value = ΣY (i) · (1-k (i)) / k (i) · C (i) (1) 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.

Next, the present inventors cast molten steels of various compositions and investigated the relationship between the Z value calculated by the equation (1) and the number of argon gas trapped in the slab. As shown in FIG. FIG. 2 is a diagram showing a relationship between the Z value and the M value calculated from the components of the molten steel according to the embodiment of the present invention and the number of bubbles trapped in the slab. As shown in FIG. 2, the Z value is 2
It has been found that by setting it to be 500 or less, it is possible to prevent the argon gas from being trapped in the solidified shell and to be reduced to the industrially harmless level of 0.1 / cm ³ or less. In other words,
By adjusting the components so that the Z value is 2500 or less,
It will be possible to systematically reduce bubble defects. Also,
Even in steel grades where it is difficult to adjust the composition of the material, by predicting the air bubble trapping ability by using the Z value, it becomes possible to take limited measures such as low speed casting and nozzle angle change, which has the effect of improving production efficiency. can get.

For molten steel having a composition in which the γ phase crystallizes out as a primary crystal during solidification, the elements that have a large effect on the Z value are C (carbon), S (sulfur), N (nitrogen), O (oxygen)
It was found that even if the M value calculated only with these elements as shown in the equation (2) is used, it is possible to prevent trapping of the argon gas bubbles without any problems in practical use. However, the concentration of the component used in the calculation is the concentration that is dissolved alone (free) in the molten steel that affects the surface tension, and is present in the form of compounds (nitride, oxide, etc.). It should be noted that the concentration has no effect. M = 36C mass% + 303066S mass% + 5804N mass% + 84519 7O mass% (2)

[0020]

EXAMPLES Examples embodying the present invention will be described below. With 350 tons of molten steel (molten steel ladle capacity) having the chemical composition shown in Table 1, a continuous casting apparatus having an inner size of 250 mm in thickness and 1000 mm in width as shown in FIG. 1 was used, the blowing rate of argon gas was 5 NL / min, and the casting speed was 1.0 m / mi
It was cast under the condition of n. 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. Table 2 shows the results of the investigation. The steel types A, D, and G of the present invention examples are good in that the number of argon gas in the billet is small and no bubble defects occur in the rolled steel sheet.
On the other hand, steel types B, C, E, F, H, I shown as comparative examples
In the case of (1), the number of argon gas in the slab is large, and bubble defects are recognized in the rolled steel sheet. Although one 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 of the present invention are within the scope of application of the present invention.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

As described above, according to the present invention, the prevention of inclusions of inclusions on the nozzle by the argon gas and the floating separation of inclusions in the continuous casting mold, which have been problems in the conventional manufacturing technology. Since it is possible to achieve both the effect of 1) and the prevention of trapping of argon gas slabs in the solidified shell, the slabs / steel plates having excellent surface quality can be industrially manufactured at low cost and stably, and this is an extremely excellent effect.

[Brief description of drawings]

FIG. 1 is an overall view of a molten steel continuous casting apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between a Z value and an M value calculated from the components of molten steel according to an embodiment of the present invention and the number of bubbles trapped in a slab.

[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

Claims (4)

[Claims] 1. A molten steel having a composition in which a γ phase crystallizes as a primary crystal during solidification is received from a ladle in a tundish, and the molten steel is poured into a water-cooled mold through a nozzle to cast a slab. In the continuous casting method of a slab, the bubble is characterized in that the component concentration of the molten steel is adjusted in advance so that the Z value calculated by the following formula from the component concentration of the molten steel becomes 2500 or less, and the continuous casting is performed. Continuous casting method for slabs with few defects. Z = Σy (i) × (1-k (i)) / k (i) × c (i) (1) where y (i) is the concentration representing the effect of the i element on the surface tension of the molten steel. The coefficient is a positive value when the surface tension is reduced when added. k (i) is the equilibrium partition coefficient of the i element in the molten steel, c (i) is the mass% of the i element, and Σ is the sum of the constituent elements in the molten steel. 2. Molten steel having a composition in which a γ phase crystallizes out as a primary crystal during solidification is received in a tundish from a ladle, and the molten steel is poured into a water-cooled mold through a nozzle to cast a slab. In the continuous casting method for slabs, the bubble is characterized in that the component concentration of the molten steel is adjusted in advance so that the M value calculated from the component concentration of the molten steel by the following formula becomes 2500 or less, and then the continuous casting is performed. Continuous casting method for slabs with few defects. M = 36 C mass% + 303066 S mass% + 5804N mass% + 84519 7O mass% (2) Here, C is carbon contained in the molten steel, S is sulfur contained in the molten steel, N is nitrogen contained in the molten steel, O Indicates oxygen contained in molten steel. 3. The continuous casting method for a slab with few bubble defects according to claim 1, wherein the argon gas supplied from the nozzle into the molten steel is 0.5 NL / min or more. Continuous casting method with less slab. 4. Molten steel having a composition in which a γ phase crystallizes out as a primary crystal during solidification is received in a tundish from a ladle, and the molten steel is poured into a water-cooled mold through a nozzle to cast a slab. In the steel product obtained by rolling the cast slab, the cast slab continuously cast after the component concentration of the smelt is adjusted in advance so that the Z value calculated from the component concentration of the smelt by the following formula is 2500 or less. A steel material with few bubble defects characterized by being rolled. Z = Σy (i) × (1-k (i)) / k (i) × c (i) (1) where y (i) is the concentration representing the effect of the i element on the surface tension of the molten steel. The coefficient is a positive value when the surface tension is reduced when added. k (i) is the equilibrium partition coefficient of the i element in the molten steel, c (i) is the mass% of the i element, and Σ is the sum of the constituent elements in the molten steel.