JPH08267203A - Continuous casting method - Google Patents

Abstract

PURPOSE: To prevent the development of crack in a gas blowing type immersion nozzle set with low gas flow rate and to improve and stabilize the quality of a cast slab by controlling gas back pressure so as to be lower pressure than that equivalent to the strength of the powder line part of the immersion nozzle which is regulated according to the casting time. CONSTITUTION: The target gaseous Ar back pressure equivalent to strength lower than that of the powder line part 4 in consideration of the safety factor is calculated according to the casting time from the relation between the pre-estimated casting time and the strength of the powder line part 4. The gaseous Ar back pressure 9 is read from a pressure gage fitted on the way of gaseous Ar introducing pipe 8 to the immersion nozzle 3 and the read value is compared with the target gaseous Ar back pressure, and the control of the gaseous Ar back pressure 9 is started from the point of time when both values become equal. During casting, the gas permeation characteristic value is continuously calculated and the gaseous Ar flow rate is continuously or intermittently adjusted to a value obtd. by multiplying this permeation characteristic value by the target gaseous Ar back pressure and then, the gaseous Ar back pressure 9 is controlled to the target gaseous Ar back pressure. The control is effected by the both of a automatic operation and a mannual operation.

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 made of an alumina graphite material is used to supply molten steel into a mold from a tundish without oxidizing the molten steel. With the passage of time, alumina and metal ingots deposited in the steel adhered to the inner wall of the nozzle, and in severe cases, the nozzle was clogged and the casting was sometimes stopped.

As means for solving this problem, for example,
As disclosed in JP-B-58-3467, a porous cylindrical refractory (inner hole body) that is concentric with the inner hole of the immersion nozzle.
It is known to insert into the main body of the immersion nozzle and blow Ar or another inert gas 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.

[0004]

The Ar gas blown by the above method is partially trapped at the solidification interface during floating in the mold and remains as bubbles in the slab. The larger the bubbles, the more they are not pressed after hot rolling or cold rolling, and appear as swelling defects on the surface of the steel sheet. This blister-like defect occurs particularly in extremely low carbon steel having a carbon concentration in the steel sheet as low as possible, for example, in an extremely low carbon steel having a carbon concentration of 50 ppm or less, resulting in a significant reduction in product yield. Therefore, in order to secure the quality of the slab, it has become extremely important to stably blow a fine Ar gas by reducing the pore diameter and the porosity of the submerged nozzle inner hole body to lower the air permeability. . However, by reducing the pore diameter and porosity of the submerged nozzle inner pores and lowering the air permeability, the Ar gas back pressure increases, and there arises a problem that cracks occur in the submerged nozzle powder line portion that has melted in the latter half of casting. .

In view of the above problems, it is an object of the present invention to provide a method for using a continuous casting nozzle which prevents cracking of the immersion nozzle and does not cause such a defect even when used for a long time. To do.

[0006]

In order to achieve the above object, the present invention is, in continuous casting of steel using a gas injection type immersion nozzle, more than the strength of the powder line portion of the immersion nozzle specified according to the casting time. The gist of the continuous casting method is characterized in that the gas back pressure is controlled so as to show a low pressure.

[0007]

The inventors of the present invention have clarified the crack generation mechanism of the submerged nozzle having a low air permeability by the casting test in the actual machine and the stress analysis of the submerged nozzle. FIG. 1 is a schematic diagram for explaining the usage state of the immersion nozzle, and FIG. 2 shows changes with time in the powder line strength and Ar gas back pressure of the immersion nozzle. As shown in the figure, powder 2 is used in the mold to prevent oxidation of molten steel 1 and to lubricate between the mold and the solidified shell, and the powder line 4 around the outer periphery of the immersion nozzle 3 undergoes severe local melting loss. . For this reason, as the casting time elapses, the thickness of the powder line portion 4 becomes thinner and the strength decreases.

On the other hand, although Ar gas is blown into the immersion nozzle 3 from the slit 5 of the immersion nozzle 3 through the inner hole body 6, since the Ar gas back pressure 9 acts on the powder line portion 4, the immersion is performed. When the Ar gas back pressure 9 increases due to the low ventilation of the nozzle 3, the Ar gas back pressure 9 matches the strength of the powder line portion 4 in the latter half of casting, and cracks 7 occur in the immersion nozzle 3. However, when using a highly-ventilated immersion nozzle, the Ar gas back pressure is always less than or equal to the strength of the powder line portion, so cracking does not occur.

From the above crack generation mechanism, the present inventors have
In order to prevent the occurrence of cracks in the low-gassing gas-blowing immersion nozzle, it is necessary to predict the strength of the powder line part of the immersion nozzle according to the casting time and control the Ar gas back pressure so that it is below that strength. It was found to be effective.

In order to carry out the present invention, a method of estimating the amount of melt damage in the powder line portion of the dipping nozzle and a method of estimating the strength from the amount of melt loss in the powder line portion are required according to the casting time. By combining the above methods, the relationship between the casting time and the strength of the powder line part can be finally predicted.

With respect to the above, as a result of collecting the immersion nozzle after actual casting and inspecting the amount of erosion in the powder line portion in detail, it was found that the amount of erosion amount represented by the radius decrease amount of the powder line portion is proportional to the casting time. It was clarified that it was possible to estimate the amount of erosion in the powder line part according to the casting time by defining the value obtained by dividing the radius reduction amount by the casting time as the erosion rate.

[0012] With respect to the above, at the end of casting, Ar gas was caused to flow through the immersion nozzle in a hot state, and the back pressure of Ar gas (the strength of the powder line portion) at which cracks were generated in the powder line portion was measured. By repeating this measurement, the relationship between the amount of erosion and the strength of the powder line portion can be quantified.

The method for estimating the amount of erosion and the method for estimating the strength in the powder line portion described above are merely examples.
For the method of off-line immersing the refractory in the powder line part in the powder to evaluate the melt loss rate, for the method of deciding the stress of the immersion nozzle by the finite element method etc., the method of calculating the strength of the powder line part is also considered. To be Further, the melting rate varies depending on the material of the powder line portion and the type of mold powder, and the strength varies depending on the material of the powder line portion, but it is sufficient to evaluate the melting rate and the strength under each condition in advance.

Flow rate of Ar gas blown during casting / A
The r gas back pressure (aeration characteristic value) is almost constant if there is no deterioration in the submerged nozzle inner hole, and the change is gentle even if there is deterioration. It is effective to adjust the Ar gas flow rate by utilizing the characteristics.

The specific Ar gas back pressure control method is as follows. First, from the relationship between the casting time and the powder line portion strength evaluated in advance, the target Ar gas considering the safety factor below the powder line portion strength is considered. The back pressure is calculated according to the casting time. The Ar gas back pressure is read from the pressure gauge attached in the middle of the Ar gas introduction pipe 8 to the immersion nozzle, this value is compared with the target Ar gas back pressure, and the control of the Ar gas back pressure is started from the time when both agree. To do. At the time of casting, the ventilation characteristic value is continuously calculated, and the Ar gas back pressure is adjusted to the target Ar gas back pressure by continuously or intermittently adjusting the value obtained by multiplying the ventilation characteristic value by the target Ar gas back pressure. Controlled by gas back pressure. It should be noted that the control of the present invention is fully effective even when it is automated or when a person looks at the pressure gauge and the flowmeter.

[0016]

EXAMPLES The present invention will be described below with reference to Examples and Comparative Examples. Example 1 Using the gas injection type immersion nozzle shown in FIG. 1, component C: 50 ppm, Si: 0.015%, M
n: 0.25%, P: 0.02%, S: 0.01%, A
L: 0.035%, molten steel at a temperature of 1550 ° C. (in the tundish) was cast for 400 minutes. The pre-measured dissolution rate in the powder line part of the immersion nozzle is 0.05
mm / min., and the change over time in the strength of the powder line portion at this time is as shown in FIG.

Aeration rate of immersion nozzle is 1.0kg at room temperature and back pressure
When evaluated by the flow rate of Ar gas (cold air flow rate) when the gas was added, it was found that the blistering defect can be prevented by lowering the air flow to 0.05 N liter (l) /min./cm ² or less. Therefore, the cold air flow rate is 0.04 Nl / min. / Cm ²
When the immersion nozzle was used and the flow rate of Ar gas blown was set to 5.0 Nl / min. In the initial stage of casting, the initial Ar gas back pressure was 2.0 kg / cm ² . Since the Ar gas back pressure reached 95% of the strength of the powder line section 250 minutes after the start of casting, the flow rate of Ar gas was continuously changed to the strength of the powder line section x 0.95 x ventilation characteristic value. By adjusting, the Ar gas back pressure was controlled to 95% (of the strength of the powder line portion) (target Ar gas back pressure). The change over time of the Ar gas back pressure at this time is shown by the solid line in FIG. As a result, the immersion nozzle was free from cracks and the slab quality was good.

Example 2 Using the gas injection type immersion nozzle shown in FIG. 1, component C: 50 ppm, Si: 0.0
15%, Mn: 0.25%, P: 0.02%, S: 0.
Molten steel of 01%, Al: 0.035%, and a temperature of 1550 ° C. (in the tundish) was cast for 400 minutes. The dissolution rate in the powder line portion of the immersion nozzle measured in advance was 0.05 mm / min., And the change over time in the strength of the powder line portion at this time is as shown in FIG. While using a submerged nozzle with a cold flow rate of 0.03 Nl / min. / Cm ² ,
When casting was performed with the Ar gas blowing flow rate set to 7.0 Nl / min. In the initial stage of casting, the initial Ar gas back pressure was 3.0 k.
It was g / cm ² . A when 120 minutes have passed since the start of casting
Since the r gas back pressure has reached 95% of the strength of the powder line part, the Ar gas back pressure can be continuously adjusted by (powder line part strength x 0.95 x ventilation characteristic value) thereafter. The pressure is 9 of the powder line strength.
It was controlled to 5% (target Ar gas back pressure). The change over time in the Ar gas back pressure at this time is shown by the dotted line in FIG. As a result, the immersion nozzle was free from cracks and the slab quality was good.

Comparative Example 1 Using the gas injection type immersion nozzle shown in FIG. 1, component C: 50 ppm, Si: 0.0
15%, Mn: 0.25%, P: 0.02%, S: 0.
Molten steel of 01%, Al: 0.035%, and a temperature of 1550 ° C. (in the tundish) was cast for 400 minutes. The dissolution rate in the powder line portion of the immersion nozzle measured in advance was 0.05 mm / min., And the change over time in the strength of the powder line portion at this time is as shown in FIG. While using a dipping nozzle with a cold flow rate of 0.04 Nl / min. / Cm ² ,
When casting was performed with the Ar gas blowing flow rate set to 5.0 Nl / min. In the initial stage of casting, the initial Ar gas back pressure was 2.0 k.
It was g / cm ² . At this level, the back pressure of Ar gas was not controlled and the casting was continued as a result.
At 0 minutes, the immersion nozzle cracked and the casting was stopped.

[Comparative Example 2] Using the gas injection type immersion nozzle shown in FIG. 1, component C: 50 ppm, Si: 0.0
15%, Mn: 0.25%, P: 0.02%, S: 0.
Molten steel of 01%, Al: 0.035%, and a temperature of 1550 ° C. (in the tundish) was cast for 400 minutes. The dissolution rate in the powder line portion of the immersion nozzle measured in advance was 0.05 mm / min., And the change over time in the strength of the powder line portion at this time is as shown in FIG. While using a submerged nozzle with a cold flow rate of 0.03 Nl / min. / Cm ² ,
When casting was performed with the Ar gas blowing flow rate set to 7.0 Nl / min. In the initial stage of casting, the initial Ar gas back pressure was 3.0 k.
It was g / cm ² . At this level, the control of the Ar gas back pressure was not performed, and as a result of continuing the casting as it was, 13
At 0 minutes, the immersion nozzle cracked and the casting was stopped.

[0021]

As described above, the present invention makes it possible to reliably prevent the occurrence of cracks in the gas-blown submerged nozzle having a low aeration rate, so that the quality and stability of the slab can be improved and the yield can be significantly improved. Get better. Further, various unsteady operations associated with the occurrence of cracks in the immersion nozzle can be omitted, and the operability becomes good.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining a usage state of an immersion nozzle.

FIG. 2 is a diagram showing changes over time in the powder line strength of an immersion nozzle and Ar gas back pressure.

FIG. 3 is a diagram showing changes over time in the powder line strength of an immersion nozzle and Ar gas back pressure in Examples.

[Explanation of symbols]

 1 ... Molten Steel 2 ... Powder 3 ... Immersion Nozzle 4 ... Powder Line 5 ... Slit 6 ... Inner Porous Body 7 ... Cracking 8 ... Ar Gas Introducing Tube 9 ... Ar Gas Back Pressure

Claims (1)

[Claims] 1. In the continuous casting of steel using a gas injection type immersion nozzle, the gas back pressure is controlled so as to exhibit a pressure lower than the strength of the powder line portion of the immersion nozzle specified according to the casting time. And continuous casting method.