JPH08238546A - Continuous casting method - Google Patents

Abstract

PURPOSE: To stabilize and improve the quality of a cast slab by supplying mixed gas containing a specific reducing gas in inert gas into a nozzle for supplying molten steel. CONSTITUTION: The gas is blowing by passing through the porous body 1 of the immersion nozzle from a gas introducing pipe 3 during casting. As the blowing gas, the mixed gas containing 0.1 to <1.0% the total of H2 gas and/or CO gas as the reducing gas in the inert gas is used. The gas blowing is continuously or intermittently executed. Even if the gas blowing is executed in a short time at the time of progressing the metal sticking from information, such as the variation of molten steel surface level, the opening degree of the immersion nozzle, the sufficient effect is shown to the alumina sticking. By this method, various kinds of unsteady works caused by clogging of the nozzle can be eliminated, and the good operatability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous steel casting method.

[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 continuous casting mold without oxidizing it. As a material for the immersion nozzle, a material mainly composed of Al ₂ O ₃ and C and containing about 20 wt% of SiO ₂ is mainly used. In such a dipping nozzle, the deposit of alumina and steel in steel adheres to the inner wall of the nozzle as the casting time elapses, and in severe cases, nozzle clogging may occur and casting may be stopped. Further, the nozzle clogging also disturbs the flow of molten steel in the mold, thus increasing inclusion defects due to powder entrainment.

As one of means for solving this problem, for example, as shown in Japanese Patent Publication No. 58-3467,
It is known that a porous tubular refractory (inner body) concentric with the inner hole of the immersion nozzle is inserted into the main body of the immersion nozzle, and Ar or another inert gas is blown from the inner wall of the porous refractory. This gas blowing reduces the contact area between the inner wall of the nozzle and the molten steel, further stirs the molten steel, or forcibly separates the deposit by gas bubbles, so that the growth of alumina inclusions on the inner wall of the nozzle grows. It has the effect of preventing.

Further, as another means, for example, JP-A-64
As disclosed in Japanese Patent Publication No.-40154, ZrO ₂
Attempts have been made to use nozzles made from -CaO-C based materials. By using this ZrO ₂ —CaO—C-based material for the inner wall of the nozzle, CaO in the refractory and Al ₂ O _{3 in the} molten steel are reacted with each other to form a low melt of calcium aluminate. This low melt is washed away with a molten steel flow to give a slight melting loss to prevent the adhesion.

[0005]

However, in order to reliably prevent the adhesion of alumina to the inner wall of the nozzle, it is necessary to blow a large amount of Ar gas into the molten steel. In this case, the Ar bubbles blown completely rise. Instead, they are trapped at the solidification interface in the mold and cause bubble-type defects that occur after hot rolling and cold rolling. On the other hand, various methods have been proposed for effectively injecting gas from the inner wall of the immersion nozzle, but Ar to the extent that bubble-type defects do not occur is proposed.
No technique has been developed that can reduce the flow rate and reliably prevent the adhesion of alumina on the flow rate.

On the other hand, a nozzle made of ZrO ₂ --CaO--C material is used to prevent nozzle clogging without blowing Ar gas, but the casting time is long or the cleanliness of molten steel deteriorates. In this case, it becomes impossible to supply sufficient CaO for lowering the melting point of alumina adhering to the inner wall of the nozzle, and nozzle clogging occurs in the latter half of casting. Therefore, Zr
At present, the O ₂ -CaO-C quality nozzle also does not have a reliable blocking prevention measure.

Further, in the ZrO ₂ -CaO-C quality nozzle, since inclusions once attached and deposited on the inner wall of the nozzle are washed away by the molten steel flow, there is a problem that coarse inclusions flow into the mold and deteriorate the quality of the slab. Occurs.

In view of these problems, it is an object of the present invention to provide a continuous casting method capable of casting a steel sheet material for working which is always stable and has no inclusion defects while preventing nozzle clogging. Is.

[0009]

That is, according to the present invention, in continuous casting of steel, H is used as a reducing gas in an inert gas.
Total amount of ₂ gases and / or CO gas is 0.1% or more 1.0
The continuous casting method is characterized in that a mixed gas containing less than 1% is continuously or intermittently supplied into the molten steel supply nozzle.

[0010]

In order to establish the technique of preventing the adhesion of the immersion nozzle, it is first necessary to clarify the mechanism of alumina adhesion to the inner wall of the nozzle. As a result of a detailed investigation of the immersion nozzle after casting using an actual machine, the inventors found that Al ₂ O ₃ attached to the inner wall of the nozzle was attached using FeO as a binder as shown in FIG.
It was found that they were aggregated. When Al is present in the molten steel, when the molten steel comes into contact with a gas having a high oxygen potential, for example, air, Al is preferentially oxidized in the equilibrium,
Generates Al ₂ O ₃ . However, the Al concentration may be locally reduced at the gas-metal interface, and it is fully conceivable that some Fe will be oxidized to produce FeO.

The present inventors have conducted an oxidation test of Al killed molten steel with air in a high frequency melting furnace, and
It was confirmed that not only Al ₂ O ₃ but also FeO were present at the metal interface, even though Al was present in the molten steel. Therefore, it is clear that Al ₂ O _{3 in the} molten steel adhered to the inner wall of the immersion nozzle using FeO generated by air oxidation or the like as a binder.

From the above results, the inventors of the present invention examined prevention of alumina adhesion to the inner wall of the immersion nozzle, blow a mixed gas of an inert gas and a reducing gas (H ₂ gas, CO gas) into the immersion nozzle, We invented a method to prevent alumina adhesion. That is, by blowing a mixed gas of an inert gas and a reducing gas into the immersion nozzle, the inert gas has the same anti-adhesion effect as before (effect of reducing the contact area between the nozzle inner wall and the molten steel, stirring of the molten steel). (Or the effect of forcibly exfoliating the adhered matter by gas bubbles), the reducing gas has the effect of reducing the binder FeO according to the formulas (1) and (2), and breaking the inter-particle bond of the adhered alumina. It was
This makes it possible to prevent alumina from adhering more efficiently as compared with the conventional inert gas blowing. FeO + CO = Fe + CO ₂ (1) FeO + H ₂ = Fe + H ₂ O (2)

When CO gas is used as the reducing gas,
It is considered that CO ₂ generated by the equation (1) causes reoxidation of Al in molten steel and at the same time increases the C concentration in molten steel. Conventionally, the flow rate of the inert gas blown is about 20 liters / min, and even if the total amount of CO gas is blown into the molten steel to be absorbed, the casting rate is 4 t / min.
The concentration only increases by about 2.7 ppm and the total oxygen concentration increases by about 3.6 ppm. Further, when H ₂ gas is used as the reducing gas, it is considered that the H concentration in the molten steel also increases. Even in this case, even if the entire amount is absorbed in the molten steel by injecting H ₂ gas at 20 liter / min, the casting amount is 4 t / m
In the H concentration, the problem of material deterioration does not occur only by increasing by about 0.45 ppm.

Since the effect of preventing the adhesion of alumina of the present invention is extremely high, it is not necessary to continuously blow a mixed gas of an inert gas and a reducing gas during casting, and the progress of the adhesion can be promoted based on the information such as the molten metal level change and the immersion nozzle opening degree. Sometimes just blowing for a short time has a sufficient effect on the adhesion of alumina. Further, even when the alumina is heavily adhered, the reducing gas concentration of less than 1.0% exerts a sufficient effect. Therefore, the present invention is an extremely effective method without the problem of molten steel contamination.

In the present invention, the total amount of H ₂ gas and / or CO gas in the blown gas must be 0.1% or more and less than 1.0%. This is because when the reducing gas concentration is less than 0.1%, only the same effect as the nozzle adhesion prevention effect by the conventional inert gas blowing can be obtained. On the contrary, when the reducing gas concentration is 1.0% or more, the reducing gas becomes binder FeO. SiO _{2 in} refractory for gas blowing as well as decomposing
This is to reduce the low-grade oxides represented by, etc. to reduce the corrosion resistance of the refractory.

The mixed gas of the inert gas and the reducing gas may be blown from the tundish upper nozzle, the immersion nozzle inner hole body, the sliding nozzle, etc., and the nozzle shape need not be changed. The optimum blowing gas flow rate, reducing gas concentration, and blowing time differ depending on the casting conditions and the blowing position. Therefore, the optimum blowing conditions may be set based on indexes such as the nozzle opening degree, molten metal level fluctuation, and drift.

[0017]

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

[0018]

[Table 1]

[0019]

[Table 2]

Using a gas injection type immersion nozzle having the shape shown in FIG. 2, a carbon concentration of 3 containing 0.08 wt% of Ti is 3
0 ppm ultra low carbon steel was cast for 400 minutes. The amount of molten steel passing through the immersion nozzle was 4 t / min, and gas was blown from the gas introduction pipe 3 through the immersion nozzle inner hole body 1 under the conditions of blowing in Tables 1 and 2 during casting. The intermittent blowing method means that the gas was blown under the blowing conditions shown in Table 1 only when it was determined that nozzle clogging had progressed based on the fluctuation of the molten metal surface and the nozzle opening.

In both the examples and comparative examples of the present invention, the casting dimensions were 245 mm in thickness and 1500 mm in width, and cut into lengths of 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 1500.
mm cold rolled steel sheet. Regarding the slab quality, visual inspection was performed on an inspection line after cold rolling, and the number of bubble-type defects generated per coil was evaluated. The clogging condition of the immersion nozzle was evaluated by the thickness of the alumina deposited on the inner wall immediately above the discharge hole 2 after the nozzle was recovered after casting.

As shown in Table 1, in the examples, a mixed gas containing H ₂ gas and / or CO gas as a reducing gas in an inert gas in a total amount of 0.1% or more and less than 1.0% was used in the dipping nozzle. Nozzle clogging could be reliably prevented by continuously or intermittently supplying to the nozzle. As a result, fluctuations in the molten metal surface were suppressed, and no inclusion defects due to powder entrainment occurred. Furthermore, the amount of increase in C and H concentrations and the amount of increase in total oxygen concentration in the molten steel were 1 ppm or less, and there was no molten steel contamination at all.

On the other hand, as shown in Table 2, Comparative Example 1,
In Comparative Examples 2 and 3, the total amount of H ₂ gas and / or CO gas in the blown gas was less than 0.1%, and therefore a sufficient alumina adhesion preventing effect was not obtained. Therefore, the fluctuation of the molten metal surface became large, and inclusion defects due to the inclusion of powder occurred frequently. In Comparative Example 4 and Comparative Example 5, since the total amount of H ₂ gas and / or CO gas in the blown gas was 1.0% or more, the corrosion resistance of the submerged nozzle inner hole was lowered,
Molten steel was infiltrated into the inner hole and casting was stopped.

[0024]

As described above, since the nozzle clogging can be reliably prevented by the present invention, the quality and stability of the slab can be realized and the yield can be remarkably improved. Further, various unsteady operations due to nozzle clogging can be omitted, and operability becomes good.

[Brief description of drawings]

FIG. 1 is a diagram showing an adhered state of alumina.

FIG. 2 is a diagram showing a shape of an immersion nozzle.

[Explanation of symbols]

 1 ... Inner hole body 2 ... Discharge hole 3 ... Blow-in gas introduction pipe

Claims (1)

[Claims] 1. A nozzle for supplying molten steel containing a mixed gas containing H ₂ gas and / or CO gas as a reducing gas in a total amount of 0.1% or more and less than 1.0% in a continuous casting of steel in continuous casting of steel. A continuous casting method, characterized by continuously or intermittently supplying the inside.