JPH08215806A - Production of high cleanliness steel using vertical tundish - Google Patents

Abstract

PURPOSE: To supply high cleanliness molten steel into a continuous casting mold from a ladle by using a vertical tundish effective for float-up and separation of the inclusion. CONSTITUTION: The molten steel 2 is supplied into the tundish 10 having large depth in comparison with the cross sectional area from the ladle 1, and after executing an adjustment of components 11, forced stirrings 13, 14 and an adjustment of temp. 15 along the flowing direction of the molten steel flowed down in the tundish 10, the killed molten steel under straightening condition 16 is supplied into the mold 5 for continuous casting. By this method, since sufficient molten steel depth can be obtd., the coagulation and growth, and the float-up and separation of inclusions are effectively executed to obtain the molten steel having high cleanliness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of removing impurities with a tundish having a refining function to obtain high cleanliness steel poured into a continuous mold.

[0002]

2. Description of the Related Art In a continuous casting method combined with vacuum degassing, a continuous cast piece is manufactured through the following steps. That is, R
After adjusting the components such as decarburization with a vacuum degasser such as H,
Deoxidized products such as l ₂ O ₃ are floated and separated by the reflux of molten steel. The treated molten steel in the ladle is received by the tundish, continuously poured into the mold, and cast into a continuous cast piece. As a tundish used at this time, a boat-shaped tundish has been conventionally used. In recent years, the tendency to improve the quality of steel products has become stronger, and the reduction of inclusions has become one of the important issues in the steelmaking process.
However, in continuous casting, the solidification rate of molten steel in the mold is high, and inclusions may be trapped by the solidification shell before floating, and the cleanliness of the slab tends to deteriorate. Therefore, various inclusion reducing means have been proposed and adopted in the process from the vacuum degassing device to the mold through the tundish.

The means for reducing inclusions are (1) promoting the separation of inclusions by a vacuum degassing device, and (2) promoting the floating of inclusions in the tundish by installing a weir and blowing gas. (3) It is roughly classified into controlling the flow of molten steel in the mold or injecting gas to promote the floating of inclusions. In the vacuum degassing device, by setting the molten steel reflux time after deoxidation to be long, Al ₂ O
Ascent time of ₃ etc. is secured, and the source of inclusions is Al ₂ O ₃
Deoxidation products such as remain in the molten steel as much as possible. In the tundish, inclusions such as refractory baffles and filters are installed on the bottom surface and bath surface, or by introducing an inert gas such as Ar gas, inclusions are sent directly into the mold together with the molten metal. This prevents the inclusions and promotes the floating separation of inclusions. For floating separation of inclusions in the mold, a method of rectifying the flow of molten steel by the action of a static magnetic field,
For example, a method of introducing an inert gas such as Ar gas for preventing clogging of the immersion nozzle is adopted.

[0004]

The longer the reflux time of the molten steel in the vacuum degassing device, the more the separation of inclusions is promoted. However, due to the increase in processing time, for steel types such as ultra-low carbon steel that require a relatively long decarburization process, matching between the converter and the continuous casting process is hindered. Easy to do. Therefore, production management becomes difficult, and productivity, which is an advantage of continuous casting, may be impaired. Further, since the molten steel temperature decreases as the processing time increases, a large amount of heat energy is consumed to compensate for the temperature required for continuous casting. When separating inclusions by an internal weir or gas injection in a tundish, the conventional boat-type tundish does not ensure a sufficient bath depth.
The time required for floating inclusions is not enough. Therefore, fine inclusions are more likely to be attached to the mold without floating. Further, in a tundish that is horizontally long and has a large contact area with the atmosphere, reoxidation of molten steel and absorption of nitrogen by the atmosphere pose problems.

Therefore, in order to float and separate inclusions in the tundish and prevent reoxidation and nitrogen absorption, the hot water supply side and the hot water discharge side in the tundish should be connected by a vacuum degassing device. No. 3-17221) is known, but the desired effect is not obtained despite the complexity of the equipment. Further, JP-A-3-32453 discloses a method in which a plurality of tundishes are arranged in multiple stages to share component adjustment and cleaning, but this method also has a problem in that the equipment becomes complicated. is there. In the method in which the inclusions are floated and separated in the mold, it is often not possible to secure a sufficient straight part of the mold that is effective for the floatation and separation of inclusions, and since the solidification rate is fast, fine inclusions do not float. It tends to be trapped by the solidified shell. In addition, the bubbles of the inert gas supplied to promote the floating of inclusions may be trapped in the solidified shell in the same manner as the inclusions. Cause. The present invention has been devised in order to solve such a problem, using a vertical tundish instead of the conventional boat type, and components along the downward flow of molten steel generated in the tundish. By performing adjustment, removal of inclusions, temperature adjustment, etc., molten steel with a high degree of cleanliness can be sent to the continuous casting mold in the optimal state for casting, and a sound cast without defects due to inclusions can be obtained. To aim.

[0006]

In order to achieve the object, the method for producing high-cleanliness steel according to the present invention is to supply molten steel from a ladle to a tundish having a depth larger than that of a cross-sectional area, thereby forming a tundish. The composition is characterized in that after the components are adjusted, the force is agitated and the temperature is adjusted along the flow direction of the molten steel flowing through the inside, the calcined rectified molten steel is supplied to the continuous casting mold.

[0007]

The present inventors investigated and studied from various viewpoints the floating separation of inclusions in the tundish. Then, by a water model test of a tundish using a silica balloon as a simulated inclusion, it was found that the discharge rate η of the inclusions sent from the tundish to the mold is approximately represented by the following equation (1). _{η = Aexp (-V p · H} · L) ···· (1) However, A: constant V _p: Stokes terminal ascent rate of inclusions (cm / sec) H: bath depth in the tundish (cm) L: Distance from inflow port to outflow port (cm) Here, the Stokes terminal levitation speed V _p is expressed by equation (2). V _p = C _D · g (1-ρ _inc / ρ _Fe ) D ² / 18ν ··· (2) However, g: Gravitational acceleration (cm / sec ² ) ρ _inc : Inclusion density (g / cm ³ ) ρ _Fe : Molten steel density (g / cm ³ ) D: Inclusion particle size (cm) ν: Kinematic viscosity of liquid (cm ² / sec) _CD : Resistance coefficient

From equation (1), it can be seen that the inclusion discharge rate η decreases as the bath depth in the tundish increases. Further, from the formula (2), it is understood that the floating speed of inclusions is proportional to the square of the particle diameter D of inclusions, and the floating speed is higher for inclusions having larger particle diameters. Therefore, in order to efficiently float and separate inclusions in the tundish, it is necessary to increase the bath depth in the tundish and promote aggregation and coalescence of inclusions. The present inventors
As a result of repeated basic research using a cold model, it was clarified that it is effective to make the tundish vertical in order to increase the bath depth. In the vertical tundish, the forced stirring area and the sedative rectification area can be provided in a vertically divided manner. Therefore, it is possible to efficiently promote the aggregation and coalescence of inclusions in the forced stirring region and the floating separation of inclusions in the sedation rectification region.

The operation and effect of the present invention will be specifically described below with reference to the drawings. In the production of the high cleanliness steel according to the present invention, a vertical tundish having a structure as shown in FIG. 1 is used. This vertical tundish is a tundish tank 1 with a large vertical length compared to the cross-sectional area.
The molten steel 2 has a value of 0 and is supplied from the ladle 1 through the long nozzle 3. As the molten steel 2, a steel type requiring cleanliness is used, and ordinary steel represented by low carbon Al killed steel,
All steel grades, such as stainless steel, steel alloy steels, are treated according to the present invention. Molten steel 2 sent into the tundish
Is a compulsory component adjusting region into which the alloy additive material 12 is fed from the alloy additive device 11, a magnetic stirring device 13 and / or an inert gas introduction device 14 while vertically flowing down inside the tundish tank 10. The molten steel 2 is sequentially passed through the stirring region, the temperature adjusting region heated by the induction coil 15, and the calming rectification region where the molten steel 2 is calmed by the application of the static magnetic field 16, and the inclusions are separated and removed to the limit, and the components and temperature Is strictly controlled.

In the vertical tundish used in the present invention, since the ratio of the surface area to the volume of molten steel in the tundish tank 10 is small, reoxidation and nitrification of molten steel 2 due to the atmosphere can be suppressed to a low level even in the atmosphere. . However, in order to prevent reoxidation and nitrogen absorption, it is preferable to attach a lid to the tundish tank 10 in which only the insertion opening of the long nozzle 3 and the alloy addition opening are opened. As a lid for shutting the inside of the tundish tank 10 from the atmosphere, a lid provided with an inert gas inlet for maintaining the inside of the tundish tank is suitable. As a result, there is almost no contamination of the molten steel 2. For the component adjustment, any of a method of adding a massive alloy source from above, a method of injecting a powdery alloy source, a method of feeding a wire containing alloy elements with a feeder, and the like can be adopted. However, the illustrated wire feed method is preferable because the yield is constant and the fine adjustment of the components is easy.

The molten steel 2 to which the components have been added reaches a forced stirring region located below the component adjusting region and is immediately stirred,
Mix evenly. For stirring the molten steel 2, electromagnetic stirring or blowing of an inert gas is effective. When the electromagnetic stirrer 13 stirs in the bath, collisions and coalescence of inclusions frequently occur, and floating separation of the aggregated and enlarged inclusions is promoted. On the other hand, when an inert gas is blown into the molten steel 2 through a porous nozzle or the like, the inclusions are adsorbed on the surface of the gas bubbles and floated and separated together with the bubbles. During continuous casting, the supply of molten steel 2 into the tundish tank 10 is stopped as when the ladle is replaced, the molten steel level in the tank changes, and when the final molten steel is poured into the previous ladle or the next ladle is opened. There is an unsteady part where the degree of contamination of the molten steel becomes extremely high as at the start of the injection of the molten steel used. Therefore, by changing the position and strength of the magnetic poles for stirring of the electromagnetic stirring device 13 according to the molten steel level in the tundish tank 10 and the cleanliness of the molten steel 2, or by adjusting the introduction position and flow rate of the inert gas, The molten steel 2 can be provided with appropriate agitation. Therefore, the molten steel 2 having high cleanliness can be fed to the continuous casting mold 5 regardless of the steady portion or the non-steady portion.

The molten steel 2 whose components have been adjusted and stirred is induction-heated by the electric current supplied to the induction coil 15,
The temperature is adjusted. At this time, while continuously measuring the temperature of the molten steel 2 on the upstream side of the tundish tank 10, if the measurement result is fed forward to the heating device and the output current is adjusted, the target casting temperature is adjusted with high accuracy. To be done. The final step in the tundish tank 10 is sedative rectification. Here, by applying the static magnetic field 16, the braking force by the Lorentz force is applied to the locally fast flow of the molten steel 2 in the sedative rectification region, and a laminar flow state according to the casting speed is obtained. As a result, the floating of the inclusions remaining in the molten steel 2 is promoted, and the highly clean molten steel 2 is sent out to the continuous casting mold 5 through the immersion nozzle 4. In addition, almost all of the inclusions that have been flocculated and enlarged in the upper-layer side forced stirring region are floated and separated in this sedation-rectifying region. The sedated and rectified molten steel 2 is supplied to the interior of the continuous casting mold 5 with a uniform flow rate distribution without causing a drift in the immersion nozzle 4 when being supplied to the continuous casting mold 5 through the immersion nozzle 4. To be done. Also in this respect, the sedation-rectifying region is effective for continuous casting.

As means for applying the electromagnetic stirrer 13 and the static magnetic field 16, known ones are used. Further, since the tundish tank 10 has a vertical shape with a small horizontal cross-sectional area, the region to which the magnetic field is applied is small, and the energy effect is correspondingly increased. Furthermore, when using the vertical tundish tank 10 having a cylindrical cross section, the applied magnetic field efficiently acts on the molten steel 2. The cleaned molten steel 2 is
It is poured into the continuous casting mold 5 through the immersion nozzle 4 attached to the bottom of the tundish tank 10. Then, it is cast into a high cleanliness steel slab. In the vertical tundish, the molten steel 2 becomes a downward flow and moves from the ladle 1 side to the continuous casting mold 5 side. Due to this molten steel flow process, there is no molten steel that flows directly to the continuous casting mold 5 side without being sufficiently treated from the ladle 1, and the molten steel 2 that has undergone component adjustment, degassing, etc. comes into contact with the atmosphere. It does not reoxidize or absorb nitrogen. Moreover, since the molten steel 2 passes through the sedation and rectification region before pouring the molten steel 2 into the continuous casting mold 5, the floating inclusions are surely floated and separated, and the continuous casting mold has a very high cleanliness as a molten steel flow. Sent to 5.

The ratio H / D of the bath depth H and the inner diameter D in the tundish tank 1 is preferably H / D ≧ 1 from the relationship with the discharge rate η of inclusions shown in FIG. The discharge rate η of inclusions is 60% or more when H / D <1, which is about the same as that of the conventional boat tundish, and the effect of the vertical tundish is not exhibited. As described above, according to the present invention, the necessary and sufficient amount of inclusions are floated and separated in the tundish, so that the load for reducing inclusions, which has been conventionally performed in the steps before and after the separation, is reduced. For example, even if the molten steel reflux time for promoting the separation of inclusions by vacuum degassing is reduced to about half, the remaining inclusions are separated and removed in the tundish. Therefore, the processing time required for vacuum degassing can be kept to a necessary minimum, and even for steel types that require long-term decarburization processing, such as ultra-low carbon steel, it can be used as a converter before and after continuous casting. The high productivity, which is an advantage of continuous casting, is maintained without affecting the matching of
Further, since the inclusions are thoroughly removed in the tundish processing according to the present invention, in the mold to prevent the nozzle clogging, which was apt to occur mainly due to the adhesion of the inclusions to the conventional immersion nozzle, in the mold. The inert gas injection that was performed can also be omitted. Therefore, the inert gas is not trapped by the solidified shell like the inclusions, and casting defects are not generated in the manufactured continuous casting slab.

[0015]

[Examples] The composition and temperature of a low carbon steel melted in a converter were adjusted by an RH vacuum degassing apparatus, and then refluxed for 4 minutes to melt an extremely low carbon Al-killed steel melt. This molten steel was continuously cast through the vertical tundish shown in FIG. As a vertical tundish, bath depth 3m, inner diameter 1m, capacity 16
A tonne dish tank having a capacity of 10 tons was used. In the forced stirring area, a magnetic field is applied to the molten steel 2 by the electromagnetic stirring device 13 so that the rotating speed of the magnetic field is 60 rpm to generate a rotating flow, and at the same time, Ar gas is supplied from the porous nozzle at a flow rate of 100 Nl / min. I blew in. In the temperature adjustment region, an induction coil with a power capacity of 1 MW was used, and the molten steel 2 was induction-heated while continuously detecting the molten steel temperature in the upper layer region and adjusting the output of the induction coil according to the molten steel temperature. In the sedation rectification region, a static magnetic field having a maximum magnetic flux density of 0.2 Tesler was applied. Tundish to mold 5
In the meantime, 85 tons of molten steel was continuously cast for 8 pots without introducing any inert gas.

For comparison, a similar ultra low carbon Al killed steel was continuously cast by a typical manufacturing method using a conventional boat type tundish. In this case, after adjusting the composition and temperature of the low carbon molten steel produced in the converter with the RH vacuum degassing device, 8
It was refluxed for a minute to melt an extremely low carbon Al-killed molten steel. The obtained molten steel was poured into a continuous casting mold via a boat tundish, and continuously cast. The boat-type tundish used had a bath depth of 0.9 m and a tundish tank with a capacity of about 30 tons. In addition, Ar gas was blown into the casting nozzle from above during the casting at a flow rate of 10 nl / min. The casting amount was set to the same 85 tons / pot of 8 molten steel pots as in the example of the present invention. The ratio of the total oxygen content in molten steel (T. [O] _RH ) before reflux in the RH vacuum degasser to the total oxygen content in molten steel (T. [O] _TD ) at the tundish outlet was calculated as the inclusion emission rate. It was calculated as η. As shown in FIG. 3A, the calculation result shows that the discharge rate η of the example is 0.2, whereas the discharge rate η of the comparative example is as high as 0.6. From this, it was confirmed that in the example according to the present invention, the discharge amount of inclusions fed into the mold was drastically reduced to 1/3. Further, in the example, the reflux time in the RH vacuum degassing apparatus was halved from 8 minutes in the comparative example to 4 minutes, which is a disadvantageous condition for reducing inclusions by vacuum degassing, but The amount of material discharged into the mold is significantly reduced. This shows that the coagulation and enlargement of inclusions in the forced stirring region and the floating promotion of inclusions in the sedative rectification region in the vertical tundish according to the present invention were efficiently promoted.

The amount of pinholes in the continuously cast slab was measured by observing the cross section of the slab with a microscope. The pinhole index of the comparative example is set to a reference value of 1, and the pinhole index of the slabs obtained in the examples of the present invention is shown in FIG. As is clear from FIG. 3B, the slab obtained according to the present invention has 8 pinholes as compared with the slab of the comparative example.
It can be reduced by 0%. This is because Ar gas was blown in to prevent clogging of the nozzle in the comparative example, whereas in the inventive example, it was desired to introduce the gas in anticipation of the effect of reducing inclusions in the tundish. Inferred. Moreover, in the embodiment according to the present invention, although the Ar for preventing clogging is not blown in the gas, adhesion of inclusions such as Al ₂ O ₃ to the immersion nozzle at the stage when the casting for 8 pots is completed. Was not detected. Further, the temperature range of molten steel during casting could be suppressed within a range of 5 ° C., which is 1/3 of the comparative example, in comparison with 15 ° C. of the comparative example, which is a comparative example. Furthermore, the amount of change in nitrogen in the molten steel from the RH vacuum degassing process to the tundish outlet was 8 ppm in the comparative example, whereas it was 1 ppm in the inventive example.
And the amount of absorbed nitrogen could be reduced to 1/8.

[0018]

As described above, in the present invention, the processing such as component adjustment, degassing, temperature adjustment, sedation and rectification is performed along the downward flow of the molten steel flowing down the vertical tundish. Are sequentially applied to. Therefore, each treatment is efficiently performed, and the aggregation and enlargement of the inclusions are promoted, so that the floating separation of the inclusions is promoted and the molten steel with high cleanliness is obtained. In addition, the bath area exposed to the atmosphere is smaller than that of the conventional boat tundish, reoxidation and nitrogen absorption by the atmosphere are suppressed, and the components, temperature, etc. are controlled with high accuracy. In this way, according to the present invention, molten steel having an extremely high degree of cleanliness can be obtained, so that a slab cast from this molten steel becomes a high quality product free from casting defects such as pinholes.

[Brief description of drawings]

FIG. 1 is a schematic structure of a vertical tundish in which the present invention is implemented.

FIG. 2 is a graph showing the effect of the ratio between the bath depth and the inner diameter of the tundish on the discharge rate of inclusions.

FIG. 3 is a graph comparing the discharge rate (a) of inclusions and the pinhole index (b) of a slab in an example of the present invention using a vertical tundish and a comparative example using a boat tundish.

[Explanation of symbols]

1: Ladle 2: Molten steel 3: Long nozzle 4: Immersion nozzle 5: Continuous casting mold 10: Tundish tank 11: Alloy addition device 12: Alloy addition material 13: Electromagnetic stirrer 14: Inert gas introduction device
15: Induction coil 16: Static magnetic field

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Atsuo Yamamoto 11-11 Showa-cho, Kure-shi, Hiroshima Nisshin Steel Co., Ltd.

Claims (1)

[Claims] 1. A molten steel is supplied from a ladle to a tundish having a greater depth than a cross-sectional area, and after the components are adjusted along the flow direction of the molten steel flowing down in the tundish, forced stirring, and temperature adjustment, A method for producing a high-cleanliness steel, which comprises supplying the quiescent rectified molten steel to a continuous casting mold.