JP4062213B2 - Method for adjusting the composition of molten steel in an RH degasser - Google Patents

Description

  The present invention relates to a method for adjusting the composition of molten steel in an RH degassing apparatus. More specifically, the molten steel discharged from a converter is further processed by the RH degassing apparatus (referred to as secondary refining) to target the molten steel components. It relates to a technique for accurately adjusting the value.

  In recent years, various ladle refining methods have been developed and put to practical use as secondary refining in which molten steel produced from a converter is held in a ladle and further refined in order to meet the demand for upgrading the quality of steel materials. One of the typical examples is the RH degassing refining method, which has high refining efficiency of molten steel, and various refining treatments (for example, decarburization treatment, deoxidation treatment, dehydrogenation treatment such as dehydrogenation and nitrogen). , Desulfurization treatment, component adjustment, etc.) are widely used.

  As shown in FIG. 1, this refining method uses a cylindrical tank having two dip tubes 2 and 3 in the lower part (the inside is depressurized, and hereinafter referred to as a vacuum tank 1). This is performed by immersing the dip tube in the molten steel 5 held in the tank. That is, the inside of the vacuum chamber 1 is depressurized, and a part of the molten steel 5 is sucked into the vacuum chamber 1 through the dip tubes 2 and 3, and argon is introduced into one dip tube 2 (also referred to as the riser tube 2). And a gas 6 such as nitrogen is blown in, and the levitation force is given to the molten steel 5 in the riser pipe 2 by the principle of a gas lift pump (lifting the molten steel with gas bubbles), and the upflow of the molten steel is supplied to the other immersion pipe 3 (this The downflow of the molten steel 5 from the inside of the vacuum vessel 1 to the inside of the ladle 4 is generated via the downcomer pipe 3), and the molten steel is circulated between the inside of the ladle 4 and the inside of the vacuum vessel 1. Thus, the various refining processes described above are performed efficiently. In this refining method, in particular, in the deoxidation treatment performed by adding a deoxidizer to the molten steel 5 under reduced pressure, inclusions in the molten steel collide and coalesce due to recirculation, so that the buoyancy acting on the inclusions increases. It floats on the bath surface of the molten steel 5 and is easily trapped by the slag 9 and easily separated from the molten steel, which is effective in obtaining molten steel with less non-metallic inclusions (see, for example, Patent Document 1).

  However, when the molten steel is refined in the vacuum chamber and finally the metal and / or alloy is added to adjust the components of the molten steel to the target value, the metal and / or alloy is placed in the vacuum chamber. The metal and / or alloy was rapidly melted into the molten steel by continuing the reflux of the molten steel for a certain time after the fixed amount was added.

However, depending on the steel type, for example, Ti (specific gravity: 4.5), Zr (specific gravity: 6.5), V (specific gravity: 5.98), etc., or a metal having a specific gravity smaller than that of the molten steel or the like is included. Alloys may be added. Such metals and / or alloys are likely to remain in the vacuum chamber even if they are added, such as sticking to the inner wall of the vacuum chamber. There are things that do not. Therefore, when the molten steel obtained by such an operation is continuously cast, it becomes a steel slab that does not satisfy the component standards.
JP-A-2-25251

  In view of such circumstances, the present invention provides a component adjustment method for molten steel in an RH vacuum degassing apparatus in which the metal and / or alloy charged into the vacuum chamber is surely dissolved in the molten steel and is excellent in the component specifications. The purpose is that.

  The inventor has intensively studied to achieve the above object, and the results have been embodied in the present invention.

That is, in the present invention, a cylindrical vacuum chamber provided with two dip tubes at the lower part is arranged with respect to the molten steel held in the ladle so that the dip tube is immersed in the molten steel. In the method for adjusting the molten steel components by the RH degassing apparatus, the molten steel is circulated between the ladle and the vacuum chamber by reducing the pressure, and various metals and / or alloys are introduced into the vacuum chamber to adjust the components of the molten steel. wherein the metal and / or alloy in the vacuum chamber from the introduced until after 5 minutes at longest, the molten steel depth of the vacuum chamber such that and less than 200mm at least 100mm, normal refining RH degassing apparatus characterized by further refining the immersion depth of the dip tube into the molten steel as a molten steel depth during normal refining in which the molten steel depth in the vacuum tank is 400 to 500 mm. It is the component adjustment method of the molten steel in. In this case, the metal and / or alloy has a specific gravity lower than that of the molten steel, or the metal and / or alloy is selected from Ti, Zr, V, B, Ca and REM, and alloys thereof. It is preferable that it is a seed | species or 2 or more types. Further, in the practice of the present invention, in order to make the molten steel depth in the vacuum chamber shallower than that during normal refining, the position of the ladle disposed with respect to the vacuum chamber is relatively relative to the vacuum chamber than usual. It is preferable to perform an operation of lowering the pressure or to lower the degree of vacuum in the vacuum chamber.

  According to the present invention, almost all of the metal and / or alloy charged in the vacuum chamber is quickly transferred to the ladle and mixed with the molten steel in the ladle. This not only makes the components in the molten steel uniform, but also increases the target of the components to the target values. As a result, the amount of steel materials that satisfy the component specifications has increased compared to the conventional steel materials.

  In the following, an embodiment of the present invention will be described, including the background to the invention.

  The RH degassing apparatus to which the present invention is applied may be one generally used for secondary refining of molten steel. That is, as shown in FIG. 1, it consists of a cylindrical vacuum chamber 1 and a dip tube (one is referred to as a rising tube 2 and the other is referred to as a descending tube 3) provided in the lower portion thereof. An exhaust duct 7 connected to the vacuum exhaust system is provided. In this general RH degassing apparatus, in order to immerse the dip tubes 2, 3 in the molten steel 5 held in the ladle 4, lifting means for raising the ladle 4 (for example, a hydraulic cylinder 8) is provided. Is provided. Note that there is an RH degassing apparatus in which the ladle 4 is fixed and the vacuum chamber 1 is moved up and down. Of course, even such an apparatus has no problem in applying the present invention. Further, the lifting means may not push up the ladle directly as shown, but may push up the whole or a part of the cart holding the ladle. Further, in order to add various refining functions to the RH degassing apparatus, a gas inlet tuyere (not shown) such as oxygen or inert gas is provided on the lower side wall of the vacuum chamber, and the upper portion of the vacuum chamber. Some of them are provided with a lance (not shown) for blowing gas, fuel, refining flux or the like, but any of these functions may be provided.

  First, the inventor studied carefully whether the molten steel component deviated from the target (standard) by the conventional metal and / or alloy 11 injection. As a result, in the conventional method, it was considered that the introduced metal and / or alloy 11 was not effectively used. That is, it was considered that the metal and / or alloy 11 charged in the vacuum chamber 1 was quickly trapped in the molten steel 5 and not transferred to the ladle 4 via the downcomer 3. This is because in order for the molten steel components to coincide with the target value, it is desirable that almost all of the added amount is dissolved in the molten steel. Therefore, the inventor should avoid the phenomenon that the introduced metal and / or alloy 11 remains attached to the inner wall of the vacuum chamber 1 and does not shift to the molten steel in the ladle. We thought that the standard could be achieved, and completed the concrete means as the present invention.

  That is, the present invention arranges the cylindrical vacuum chamber 1 provided with two dip tubes 2 and 3 in the lower part of the molten steel 5 held in the ladle 4 so that the dip tube is immersed in the molten steel, The inside of the vacuum chamber is depressurized and the molten steel 5 is circulated between the ladle 4 and the vacuum chamber 1, and various metals and / or alloys 11 are introduced into the vacuum chamber to adjust the components of the molten steel 5. In the method for adjusting the composition of molten steel using a gas apparatus, the molten steel depth in the vacuum chamber 1 is set to a value from the time of normal refining until a predetermined time has elapsed after the metal and / or alloy 11 has been introduced into the vacuum chamber 1. It is shallowed and then refined as the molten steel depth during normal refining. If it does in this way, before the thrown-in metal and / or alloy adheres to the inner wall of the vacuum chamber 1, it will transfer to the molten steel 5 in the ladle 4 via the downcomer 3 accompanying the molten steel 5 rapidly. While being melted, the molten steel is uniformly melted in the molten steel while being refluxed from the molten steel in the ladle through the riser 2 into the vacuum chamber 1. As a result, almost the entire amount of the metal and / or alloy charged is dissolved in the molten steel, and the components of the molten steel become substantially the target values. In addition, the normal molten steel depth (it shows with the symbol L in FIG. 1) in the vacuum chamber at the time of refining is 400-500 mm.

  In the present invention, the depth of the molten steel once shallowed is returned to the normal depth again because the following problems occur when operation at such shallow molten steel depth is continued for a long time. First, if the depth of the molten steel bath in the vacuum chamber 1 is shallow, the bubbles of the circulation promoting gas blown into the riser 2 are blown through the bath surface before being dispersed in the molten steel, and the effect of the gas lift pump is achieved. It will be diminished. This leads to a reduction in the recirculation velocity, which leads to a deterioration in the refining efficiency. Secondly, the flow of the molten steel 5 tends to occur near the molten steel bath surface in the ladle 4, whereby the slag 9 on the molten steel bath surface in the ladle 4 is gradually melted so that the riser 2 and the vacuum chamber 1 It becomes easy to be sucked up. Third, when decarburization and refining is performed by blowing oxygen from above the molten steel bath surface, the refractory 10 of the bottom (bottom) of the vacuum chamber 1 is easily melted by the oxygen gas jet. Therefore, as a countermeasure for solving these problems, in the present invention, the operation for decreasing the immersion depth as described above is not performed continuously over the entire refining period, but a metal and / or alloy is added. After that, it was limited to a certain time, and after that, it was decided to return to the operation at the normal molten steel depth. Specifically, after reducing the pressure in the vacuum chamber 1 to 13.3 kPa (100 torr) or less, the metal and / or alloy is added, and the depth of the molten steel in the vacuum chamber 1 is set to a maximum of 5 minutes. Below 200 mm (preferably 100 mm or more), after that, the molten steel depth in the vacuum chamber is set to normal operating conditions of 400 mm or more, preferably 500 mm or more.

  In order to carry out the present invention specifically, it is necessary to adjust the depth of molten steel in the vacuum chamber 1, but the raising and lowering of the ladle 4 disposed in the vacuum chamber 1 or the raising of the vacuum chamber 1 is used. it can. Specifically, in order to make the molten steel depth (L) in the vacuum chamber shallower than that during normal refining, the position of the ladle 4 disposed with respect to the vacuum chamber 1 is more than normal with respect to the vacuum chamber 1. The relative lowering operation may be performed. Here, “relatively lowering” includes literally lowering the ladle and conversely fixing the ladle and raising the vacuum chamber. Moreover, the molten steel depth (L) in the vacuum vessel 1 depends on the pressure in the vessel and the immersion depth of the dip tube in the molten steel in the ladle. The higher the pressure in the vessel, the shallower the immersion depth. It becomes shallower. Therefore, in order to make the molten steel depth in the vacuum chamber 1 shallow, an option of setting the pressure in the chamber higher than normal (13.3 kPa) (for example, about 26 kPa) can be considered. However, increasing the pressure in the vacuum chamber 1 lowers the efficiency of the vacuum degassing refining itself, so that the operation of lowering the position of the ladle disposed in the vacuum chamber than usual is better. preferable.

  In carrying out the present invention, it goes without saying that the concentration of each component in the molten steel is known in advance, and the type and amount of metal and / or alloy to be charged are determined based on the value. Further, the metal and / or alloy may be charged from a hopper 12 provided above the vacuum chamber 1 through a known cutting means such as a rotary feeder.

  In the RH vacuum degassing apparatus for processing 260 tons of molten steel, a plurality of operations using the conventional operation and the method according to the present invention were charged, and the effects were compared.

  First, hot metal having a carbon (C) concentration of 4.3% by mass is charged into a bottom-blowing converter and decarburized and refined, and discharged to a ladle as molten steel having a C concentration of 0.03 to 0.04% by mass. Made of steel. And the aluminum slag was thrown on the slag which floated on this molten steel surface, and the said slag reforming (reduction) was performed. The ladle holding the molten steel is set in an RH vacuum degassing device, and acid feeding (oxygen gas blowing) decarburization and vacuum decarburization with stopping the acid feeding are performed in the vacuum tank, so that the carbon concentration in the molten steel is 20 ppm. Reduced. Then, after killing for 5 minutes by adding aluminum and deoxidizing, “sponge Ti” was added as the metal and / or alloy, and the operation was terminated after continuing the reflux for 4 minutes. Molten steel was continuously cast into steel slabs (slabs). The main operating conditions are as shown in Table 1. The input amount of “sponge Ti” is determined in advance so that the Ti content in the molten steel at the end of the operation is 0.01 mass%. The depth of the molten steel in the vacuum chamber was 430 mm before the addition of “Sponge Ti” and 180 mm after the addition. Further, for comparison, an operation for maintaining a normal molten steel depth of 480 mm was also performed after the introduction of “Spong Ti” (comparative example).

  The operational results of the present invention show that the mass of the molten steel cast in the continuous casting equipment (the width and thickness of the slab and the casting length in relation to the molten steel mass in the ladle before casting in the tundish during continuous casting of the molten steel. Based on the calculated mass), the samples for analysis of molten steel are taken at 10%, 30%, 50%, 70% and 90%, and the Ti concentration (symbol: [Ti]) of the sample is taken. Analyzed and evaluated.

  As a result, the molten steel obtained by carrying out the present invention had [Ti] of 0.0009 to 0.011% by mass over the entire casting time, and was almost at the target value. On the other hand, in the comparative example in which the conventional operation was performed, [Ti] was 0.004% by mass at 10% casting, 0.008% by mass at 30% casting, 0.009% by mass at 50% casting, In the casting period of one charge at 0.010 mass% at 70% casting and 0.020 mass% at 90% casting, the [Ti] in the molten steel was unstable as the casting time increased. It was.

It is a cross-sectional view explaining a general RH degassing apparatus.

Explanation of symbols

1 Vacuum tank (tubular tank)
2 Rising Pipe 3 Downfalling Pipe 4 Ladle 5 Molten Steel 6 Gas 7 Exhaust Duct 8 Hydraulic Cylinder, etc. 9 Slag 10 Refractory Material 11 Metal and / or Alloy 12 Hopper 13 Rotary Feeder

Claims (5)

With respect to the molten steel held in the ladle, a cylindrical vacuum tank provided with two dip tubes at the bottom is arranged so that the dip tube is immersed in the molten steel, and the inside of the vacuum tank is decompressed to In the method of adjusting the molten steel components in the RH degassing apparatus, in which various metals and / or alloys are introduced into the vacuum chamber and the components of the molten steel are adjusted while circulating between the ladle and the vacuum chamber.
Wherein the metal and / or alloy in the vacuum chamber from the introduced until after 5 minutes at longest, the molten steel depth of the vacuum chamber such that and less than 200mm at least 100mm, normal refining RH degassing apparatus characterized by further refining the immersion depth of the dip tube into the molten steel as the molten steel depth during normal refining in which the molten steel depth in the vacuum tank is 400 to 500 mm. Method for adjusting the composition of molten steel.   2. The method for adjusting the composition of molten steel in an RH degassing apparatus according to claim 1, wherein the metal and / or alloy has a specific gravity lower than that of the molten steel.   2. The RH degassing apparatus according to claim 1, wherein the metal and / or alloy is one or more selected from Ti, Zr, V, B, Ca and REM, and alloys thereof. Method for adjusting the composition of molten steel.   In order to make the molten steel depth in the vacuum chamber shallower than that during normal refining, the operation of lowering the position of the ladle disposed relative to the vacuum chamber relative to the vacuum chamber is performed. The component adjustment method of the molten steel in the RH degassing apparatus in any one of Claims 1-3 characterized by the above-mentioned.   The RH desorption according to any one of claims 1 to 3, wherein an operation of decreasing the degree of vacuum in the vacuum tank is performed in order to make the molten steel depth in the vacuum tank shallower than that during normal refining. Method for adjusting the composition of molten steel in a gas apparatus.

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