JPH06106313A - Immersion nozzle for continuous casting - Google Patents

Abstract

PURPOSE:To prevent a blistering and enable an inclusion to float and separate by arranging an inner hole body of gas blowing part in the powder line erosion part and non erosion part of a main body peripheral and keeping SiO content of the inner hole body under the prescribed value. CONSTITUTION:An inner body is arranged as divided corresponding to the powder line erosion part and non erosion part of a main body peripheral and a content of SiO2 is kept under 5wt.%. Because each Ar flow is independently changed, by reducing Ar flow of a lower inner hole body 1b with a flow of a upper inner hole body 1b increased, the gas blowing pressure, which is applied on the powder line part 3 with keeping total flow constant, is reduced. As Ar flow required for preventing nozzle clogging is secured, the pressure applied on the powder line 3 is reduced, preventing crack generation. Quality of the steel plate produced by this method is stable, improving yield.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas injection type immersion nozzle used for casting molten steel from a tundish into a mold in continuous casting of steel.

[0002]

2. Description of the Related Art At present, in continuous casting, an immersion nozzle is used to supply molten steel from a tundish into a mold without oxidizing it. As the material of the immersion nozzle, alumina and carbon are mainly used.
Those containing about 0 wt% silica are the mainstream. In such a submerged nozzle, alumina and metal ingots deposited in steel adhere to the inner wall of the nozzle as the casting time elapses, and when violent, the nozzle may be clogged and the casting may be stopped.

As one of means for solving this problem, for example, as shown in Japanese Patent Publication No. 58-3467,
It is known to insert a porous cylindrical refractory (inner hole body) that is concentric with the inner hole of the immersion nozzle into the main body of the immersion nozzle and blow Ar or another inert gas from the inner wall of the porous refractory. However, the Ar gas blown by this method is partially trapped at the solidification interface while floating in the mold, and remains as bubbles in the slab.

The larger the bubbles, the more they are not pressure-bonded after hot rolling or cold rolling, and appear as swelling defects on the surface of the steel sheet. Here, the blistering defect is a defect that appears on the surface of the steel sheet after hot rolling or cold rolling, and is raised to a width of 1 to 4 mm and a length of several mm, or these raised portions of several mm are continuous in a dot shape to 300 mm. It is a string that has been crossed over. This blistering defect is particularly caused in the steel type in which Ti is added to fix the solid solution carbon in the product as a precipitate in an extremely low carbon steel in which the carbon concentration in the steel sheet is reduced as much as possible, for example, in an extremely low carbon steel having a carbon concentration of 50 ppm or less. It often occurs, causing a significant decrease in product yield.

Therefore, in order to surely prevent the immersion nozzle from being blocked and to prevent the occurrence of blistering defects, a mixed gas of Ar and the residual N ₂ which is limited to 4 Nl or less per ton of molten steel is used, and the inside of the slab is used. Based on gas bubbles trapped in
A method of reducing the number of pinholes of mmφ or more to 10 or less per ton (Japanese Patent Laid-Open No. 62-38747) has been reported and is effective.

[0006]

However, when nitrogen gas is blown from the immersion nozzle, the casting speed becomes particularly high, and when nitrogen bubbles are brought deep into the slab and the contact time with the molten steel becomes long, the molten steel is Nitrogen concentration in molten steel increases more than it already exists. This nitrogen component may impair the workability and formability of the thin steel sheet,
It is preferably as low as possible. Therefore, in the present day when further improvement of steel material properties is desired, when the nitrogen concentration is increased beyond the current level, the amount of the additive alloy is increased to secure the material, and the cost for refining is not increased. Absent.

In view of these problems, the present invention secures a gas blowing flow rate necessary for preventing nozzle clogging, and
An object of the present invention is to provide a continuous casting immersion nozzle capable of casting a stable steel plate material without swelling defects, without any significant increase in refining cost and without damaging the material of the steel material.

[0008]

DISCLOSURE OF THE INVENTION According to the present invention, in a gas-blowing-type continuous casting immersion nozzle, an inner hole body corresponding to a gas-blowing portion is divided in correspondence with a powder line melting portion and a non-melting portion on the outer periphery of the main body. The present invention relates to the immersion casting nozzle for continuous casting, characterized in that the SiO ₂ content of the inner hole body is 5 wt% or less.

[0009]

The inventors of the present invention have positively injected Ar gas from the dipping nozzle to prevent nozzle clogging, and then cast hot-rolled and cold-rolled steel sheet slabs that do not lead to blistering defects. We have continued to research and develop a dipping nozzle for continuous casting. The most common alumina graphite nozzle will be described in detail below.

The larger the bubbles trapped in the slab, the more easily they will lead to swelling defects after hot rolling and cold rolling.
Therefore, the inventors of the present invention have considered that A
It is considered that the coarsening of the bubble diameter of r is the cause of the occurrence of blistering defects, and in particular, a detailed investigation was carried out on the immersion nozzle cast from the ultra-low carbon steel containing Ti, which has a high defect occurrence rate.

The composition of the pores in the immersion nozzle is silica 2
6 wt%, graphite 24 wt%, alumina 50 wt%. In an At gas blowing test in water, the average pore diameter of the unused inner pores was 0.3 mm, whereas the average pore diameter of the inner pores cast from the ultra-low carbon steel containing Ti had an average pore diameter of 2.
It was as high as 0 mm.

Further, in order to clarify the cause of the coarsening of the bubble diameter, the inner hole portion was cut out from the dipping nozzle, the structure of the molten steel contact surface was observed, and the surface analysis by EPMA was performed. As a result, the silica contained in the inner pores is
It was found that the Ar bubble diameter is coarsened by being indirectly reduced by i and disappearing from the tissue.

Here, the indirect reaction is (1),
Graphite in which silica coexists in the refractory by the reactions represented by the formulas (2) and (3) SiO ₂ + C = SiO (gas) + CO (gas) in the refractory (1) refractory / molten steel interface 2SiO ( Gas) + Ti = TiO ₂ +2 Si (2) 2CO (gas) + Ti = TiO ₂ +2 C (3) to generate SiO gas and CO gas, which are reduced by Ti in the molten steel Is. Therefore, in order to prevent the blistering defect that occurs in the ultra-low carbon steel containing Ti, the silica content in the inner pore body is reduced,
It is effective to suppress the reaction of formula (1).

However, since the porosity and the pore diameter of the submerged nozzle inner body not containing silica do not increase with the disappearance of silica, the Ar gas blowing pressure is kept high. For this reason, in the latter half of continuous casting, in which melting of the outer circumference of the nozzle body due to the powder progresses, vertical cracks occur in the outer circumference of the nozzle due to inability to withstand the gas blowing pressure.

Therefore, the inventors of the present invention conducted a study on a method for reducing the load on the powder melt-damaged portion on the outer periphery of the nozzle body to prevent the occurrence of cracks. As a result, as shown in FIG. It has been found that vertical cracks on the outer circumference of the nozzle can be completely prevented by providing the divided parts corresponding to the powder line melted part and the non-melted part on the outer circumference of the main body. Here, in the following description, the inner hole body corresponding to the powder line melted portion on the outer periphery of the main body is referred to as the lower inner hole body 1b, and the inner hole body corresponding to the non-melted portion is referred to as the upper inner hole body 1a.

FIG. 2 shows the shape of a conventional nozzle. Ar gas blowing pressure ΔP (kgf / cm ² ) of immersion nozzle
Is the inner pore thickness L (cm), Ar gas flow rate Q (cm ³ /
min / cm ² ), porosity ε, pore diameter d (cm) and A
It is given by the equation (4) as a function of the viscosity of r gas μ (kgf / cm ² · s).

On the other hand, the pressure P _max that the nozzle body 2 can withstand
(Kgf / cm ² ) is the stress σ _max (kgf /
cm ² ) and is calculated by the equation (5). Where a
(Cm) and b (cm) are the inner diameter and the outer diameter of the nozzle body 2, respectively.

Since the porosity ε and the pore diameter d are not increased in the nozzle in which the silica of the inner pore body 1 is reduced, the Ar gas blowing pressure ΔP is kept high as can be seen from the equation (4). On the other hand, powder line 3
Since the outer diameter b of the nozzle is reduced due to melting loss, the strength P _max of the nozzle body 2 becomes small as can be seen from the equation (5).

As a result, the condition of ΔP> P _max is continuously satisfied in the latter half of casting, and vertical cracking occurs in the nozzle. In the conventional nozzle, since the Ar gas blowing pressure ΔP also decreases due to the increase in the porosity and the pore diameter due to the disappearance of silica, under the current usage conditions, the condition for cracking in the latter half of the casting ΔP> P
_It never meets _max .

However, as shown in FIG. 1, if the inner hole body is divided and provided corresponding to the powder line melting portion and the non-melting portion on the outer periphery of the main body, each Ar flow rate can be changed independently. Increase the flow rate of the upper inner hole body 1a to increase the A of the lower inner hole body 1b.
If the flow rate of r is reduced, the gas injection pressure applied to the powder line section 3 can be reduced while keeping the overall flow rate constant. Therefore, even if the strength of the powder line 3 is reduced due to melting loss in the latter half of continuous casting, the Ar required for preventing nozzle clogging is prevented.
After securing the flow rate, the pressure applied to the powder line 3 can be reduced to prevent cracking. On the contrary, in the upper inner hole body 1a, the Ar gas flow rate increases and the gas injection pressure increases, but since there is no melting loss on the outer circumference of the nozzle at this portion, there is no decrease in nozzle strength and there is no problem of cracking.

From the above results, according to the present invention, it is possible to prevent the occurrence of cracks, suppress the expansion of the pore diameter, and stably blow in fine air bubbles. it can.

In the present invention, it is preferable that silica is not contained in the inner pore body from the viewpoint of preventing blistering defects, but if necessary, it may be added in an amount of 5 wt% or less. This is because if the silica content is 5 wt% or less, the reaction rate becomes very slow, and further, even if all the silica disappears, the porosity and the pore diameter do not increase as the bubble diameter increases.

Alumina has a role of imparting corrosion resistance,
The preferable compounding ratio to the inner pore body is 30 to 80 wt%.
If it is less than 30 wt%, the corrosion resistance is insufficient, and if it exceeds 80 wt%, the coefficient of thermal expansion becomes large and the spalling resistance decreases.

Since graphite is excellent in moldability and spalling resistance, graphite is added to the inner pores within a range not deteriorating the corrosion resistance. The blending range of graphite in the inner pores is preferably about 5 to 40 wt%. If it is less than 5 wt%, the formability and spalling resistance are lowered, and if it exceeds 40 wt%, the corrosion resistance is lowered due to oxidation of graphite and elution into molten steel. Further, the high thermal conductivity may cause nozzle clogging.

Although the basic constituent components of the submerged nozzle inner hole body are as described above, other materials already known as additives to the nozzle material are contained within a range not impairing the effects of the present invention. May be. Examples of the material thereof include silicon carbide, zirconia, zircon, and various metal powders. It is desirable to use the same material for the nozzle body when constructing the nozzle inner hole using a refractory composed of these constituents, but for the part that does not come into contact with the molten steel, the conventional composition containing silica is used. Materials can also be used,
It is also possible to interpose a material intermediate between the two.

The above description relates to the most common alumina graphite material as the material of the immersion nozzle, but in the case of a combination of graphite and other components such as zirconia, spinel, calcia, magnesia etc. as the material of the inner pore body. However, the present invention can obtain the same effect.

[0027]

EXAMPLES The present invention will be described below with reference to Examples and Comparative Examples.

[0028]

[Table 1]

15 wt% of a phenol resin was added as a resin binder to the raw material-containing materials shown in Table 1 by external coating, and the mixture was kneaded.
Molded into a nozzle shape as shown in FIGS. 1 and ^{2 with} a pressure of cm ² .
The powder line refractory is zirconia 75w
The zirconia graphite material containing t% and C in an amount of 25 wt% was unified, and the other parts were all the components shown in Table 1. Furthermore, this molded body is reduction-fired at a temperature of 1200 ° C., and a gas blowing type immersion nozzle for continuous casting (outer diameter 185 mm
φ, inner diameter 90 mmφ, discharge hole diameter 70 mmφ, discharge hole angle 35 °).

Using the immersion nozzle thus obtained, a carbon concentration of 0.08 wt% of Ti and a carbon concentration of 30 ppm
Of ultra low carbon steel was cast for 400 minutes. At this time, the flow rate of Ar gas blown was fixed at 6 Nl per ton of molten steel. Further, in the nozzle in which the inner hole body is divided into upper and lower parts corresponding to the powder line melted part and the non-melted part on the outer periphery of the main body, the cracking limit pressure of the powder line part should not exceed 1.5 kg / cm ^2. The casting was performed by controlling the Ar flow rate of the lower inner hole body. In both the example of the present invention and the comparative example, the casting size is 2
45 mm × width 1500 mm, cut into length 8500 mm to make one coil unit. This slab is hot-rolled and cold-rolled by a conventional method, and finally has a thickness of 0.7 mm and a width of 150.
A 0 mm coil cold-rolled steel sheet was used. The evaluation of the immersion nozzle refractory for blistering defect prevention was carried out by visually observing the bubble diameter obtained by an Ar blowing test in water and the inspection line after cold rolling (the number of blistering defects generated per coil ( It was evaluated by the blistering defect index). Further, the occurrence of cracks in the immersion nozzle was evaluated using the time at which the immersion nozzle was cracked as an index. Table 2 shows the quality evaluation results of the examples and comparative examples.

[0031]

[Table 2]

As shown in Table 2, in the embodiment, the inner hole body is divided into upper and lower parts corresponding to the powder line melting portion and the non-melting portion on the outer periphery of the main body, and the silica content is 5 wt% or less. By doing so, the occurrence of cracks due to the Ar gas blowing pressure did not occur, and swelling defects could always be prevented in a stable manner. On the other hand, Comparative Example 1 had a high silica content in the inner pore body,
The Ar bubble diameter was enlarged and a swelling defect was generated. But,
Since the porosity and the pore diameter increased due to the disappearance of silica in the inner pores, no crack was generated even in the shape in which the inner pores were not divided. Further, in Comparative Example 2, since the inner hole body was not divided and provided, the strength decreased in the latter half of casting and cracks occurred in the inner hole body. Therefore, the casting was stopped 320 minutes after the start of casting.
However, since the silica content of the inner pores was 5 wt% or less, no blistering defect occurred.

[0033]

As described above, according to the immersion nozzle for continuous casting of the present invention, the corrosion resistance and the spalling resistance are ensured and the fine air bubbles are stably blown from the inner hole. You can Therefore, it is possible not only to prevent the blistering defect but also to more efficiently prevent the floating separation of inclusions and the nozzle blockage due to bubbles. Due to the above effects, the quality of the steel sheet produced by the continuous casting method is very stable and the yield is remarkably improved.

[Brief description of drawings]

FIG. 1 shows a vertical cross section of the structure of an immersion nozzle according to an embodiment of the present invention.

FIG. 2 shows a vertical cross section of a structure of a dipping nozzle of a comparative example.

[Explanation of symbols]

 1 Immersion nozzle inner hole body 1a Lower inner hole body 1b Upper inner hole body 2 Immersion nozzle body 3 Powder line part

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuhiro Sakamoto 5-3 Tokai-cho, Tokai-shi, Aichi New Nippon Steel Co., Ltd. Nagoya Steel Works

Claims (1)

[Claims] 1. A gas injection type continuous casting dipping nozzle, wherein an inner hole body corresponding to a gas injection part is divided and arranged corresponding to a powder line melting part and a non-melting part on the outer periphery of the main body, and the inner hole is formed. The SiO ₂ content of the body is 5 wt%
An immersion nozzle for continuous casting, characterized in that: