JP2819440B2 - Method for decarburizing molten steel containing extremely low carbon chromium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decarburizing molten steel containing extremely low carbon chromium, which enables efficient refining with a small amount of chromium oxidation up to the extremely low carbon region.

[0002]

2. Description of the Related Art Chromium-containing molten iron typified by stainless steel is more susceptible to oxidation of chromium than decarburization in a region where the carbon concentration is low. Various methods for decarburizing to the carbon concentration required by the standards have been proposed. Above all, AO
D and VOD are widely used. AOD is A
This is a method of blowing oxygen gas diluted with r into the bath.
D is a method in which oxygen is blown upward under vacuum, but in any case, the partial pressure of CO gas generated by the decarburization reaction is reduced, and the decarburization reaction is prioritized over the chromium oxidation reaction. I have. Of these, in order to melt ultra-low carbon steel having a carbon concentration of 200 ppm or less, vacuum refining is indispensable. Therefore, VOD is generally used.

However, VOD is a method in which the entire ladle is placed in a vacuum vessel, or a method in which the upper part of the ladle is evacuated by covering the ladle with a lid. In addition to the problem that the operation is hindered by splash, even if the amount of gas for stirring is increased to promote decarburization while suppressing chromium oxidation, the splash due to rocking of the steel bath and bottom blow gas increases. However, there is a problem in that the operation is hindered, and further, the chromium yield and the iron yield are reduced.

On the other hand, Japanese Patent Application Laid-Open No. 61-37912 discloses a method in which molten steel in a ladle is sucked up into a vacuum chamber through a large-diameter immersion pipe, and a stirring gas is supplied from a lower part. Have been. Further, in Japanese Unexamined Patent Publication No. 1-156416, the position of the bottom-blowing nozzle is eccentric to an appropriate range with respect to the center of the immersion tube, and the above-described bubble is a floating region of the bottom-blown gas. A method for impacting an active surface is disclosed. Although these methods have solved the problem that the head space of the VOD has a small space, there is no description regarding the production of ultra-low carbon steel.
With this method alone, a large amount of chromium oxide was generated in the immersion tube, so that it was not possible to stably produce ultra-low carbon steel.

[0005]

SUMMARY OF THE INVENTION The present invention relates to the problem that the operation is hindered by the swinging or splashing of molten steel due to the narrow upper space of the VOD,
No problem that the method disclosed in Japanese Patent Application Laid-Open No. 37912 or JP-A-1-156416 has a problem that a very low carbon steel cannot be melted stably, and the chromium oxidation amount is small to an extremely low carbon region. The present invention provides a decarburization method that enables efficient refining at a low temperature.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a method for forming a Cr in a ladle.
The present invention relates to a vacuum refining method in which a dip tube is immersed in molten steel having a concentration of 5% or more, the pressure in the dip tube is reduced, and a stirring gas is supplied from a lower part of a ladle. First, when the carbon concentration is in the range of 1 to 0.02%, oxygen gas is supplied in an amount of 0.04 to 0.40 Nm per bubble active area.
³ / (Hr · cm ² ). Secondly, the cell active area is subsequently reduced to 10% of the total molten steel surface area.
Stirring is performed under the conditions described above and the oxygen spraying surface is 100% or more, and the blowing acid is stopped when the carbon concentration is 0.02% or less.
The decarburization treatment is performed by stirring under a high vacuum of rr or less.Third, further successively, the double pressure is applied to pull up the immersion tube above the molten steel surface, and the reducing alloy is charged,
It is to reduce chromium oxide generated during decarburization.

[0007]

FIG. 1 is a sectional view of a vacuum refining apparatus according to the present invention, in which a chromium-containing molten steel 4 is accommodated in a ladle 1, and a dip tube 2 is immersed and stationary in the chromium-containing molten steel 4 in the ladle 1. You. The immersion pipe 2 communicates with an exhaust pipe (not shown).
The chromium-containing molten steel 4 is sucked into the immersion pipe 2 according to the degree of vacuum in the inside. Then, an inert gas 5 is blown into the molten steel from a porous plug 3 installed at a lower portion of the ladle 1 where a lower cross section in the immersion pipe 2 is vertically downward, and an upper blowing lance installed above the immersion pipe 2 An oxygen gas 7 is supplied from 6.

In order to reduce the carbon content of the chromium-containing molten steel to extremely low carbon, oxygen gas is blown until the carbon concentration reaches 0.02% or more to decarburize the carbon steel. It is necessary to stir, but a particularly important point is to efficiently perform decarburization when stirring under high vacuum. As a result of detailed tests, the inventor has found that in order to efficiently perform decarburization at this time, the molten steel free surface area exposed to vacuum without slag is increased, and the cell active area on the free surface is increased. Is important. This is intended to remove carbon by combining carbon with oxygen contained in molten steel, to make slag containing chromium oxide in a molten state, and to decarbonize with chromium oxide in slag. Are very different.

Here, the bubble active area is obtained from a water model, a mercury model, or a result of observation with an actual machine. The bubble active area (Au) for the supplied gas is given by equation (2). An = 3.14 × (0.212 × H) ² ... (1) Au = 3.14 × (7 × Q ^0.87 ) / 2 (2) where H is blowing It is the distance (m) from the injection position to the bath surface, and Q is the gas injection amount per nozzle (Nm ³ / s).

In order to efficiently perform free surface decarburization using the bubble activated surface, the following two points are important. The amount of chromium oxide generated during oxygen gas blowing is reduced, and the generated chromium oxide is finely dispersed without coarsening. By stirring after stopping the oxygen gas blowing, the chromium oxide generated during the supply of the oxygen gas is drawn into the steel bath, and flows out of the immersion tube.

Here, the purpose is to spray the oxygen gas onto the bubble activated surface to make the generated chromium oxide finer and to promote the reduction of the chromium oxide at the blowing acid fire point. Chromium oxide is very difficult to reduce because it is in a solid state at the same time as its formation. Therefore, in order to promote the reduction, it is necessary to make chromium oxide finer and increase the contact area with [C] in the molten steel.

Further, the point indicated by is a phenomenon peculiar to vacuum refining in which a dip tube is immersed in molten steel, the pressure in the dip tube is reduced, and a stirring gas is supplied from a lower part of a ladle. The oxides produced in step (1) are entrained in the bath in a large downward flow formed by the stirring gas supplied from the lower part of the ladle, and flow out of the pipe through the lower end of the immersion pipe. Therefore, by stirring for an appropriate period of time, almost all of the chromium oxide existing in the immersion tube flows out of the immersion tube, and the surface of the molten steel in the immersion tube exposed to a vacuum is free from slag. In order to efficiently implement this effect, 0.6 Nl / (min · ton) from a position deeper than 0.5 h with respect to the molten steel height (h) in the immersion tube.
It is desirable to make the above.

Further, in order to realize these effects, the amount and form of chromium oxide generated during the supply of oxygen gas are important. In other words, in order to facilitate the entrapment of the oxide, it is important to maintain a fine state of at least less than about 1 cm. The part adheres to the wall surface of the immersion tube, and there is even a case where almost no entrainment occurs in the molten steel. In order to reduce the amount of chromium oxide produced and to make it finer, the range of the carbon concentration for supplying the oxygen gas and the flow rate of the oxygen gas spray per bubble active surface are important. In particular, the oxygen gas blowing flow rate per bubble active surface is a completely new concept. This means that in order for the sprayed oxygen gas to be consumed for decarburization without causing oxidation of chromium, local stirring of the surface of the molten steel contacted by the oxygen gas is important rather than stirring of the entire molten steel. This means that the index indicating this is not the gas flow rate but the bubble active area. That is, when the bubbles supplied from the lower part float to the surface and burst, a large amount of energy is released, and thereby a large number of fine droplets are generated on the surface of the steel bath. This serves as an effective reaction surface and causes decarburization in preference to chromium oxidation, and at the same time, plays a role of forming the resulting chromium oxide as fine particles and splitting them more finely.

More specifically, when the carbon concentration is in the range of 1 to 0.02%, oxygen gas is supplied in an amount of 0.04 to 0.4 per bubble active area.
Spraying from above at a speed of 0 Nm ³ / (Hr · cm ² ), followed by stirring under the condition that the cell active area is 10% or more of the total molten steel surface area and 100% or more of the oxygen spraying surface. . Of these, as shown in FIG. 2, when the blowing rate of oxygen gas per bubble active surface is less than 0.04 Nm ³ / (Hr · cm ² ), the oxygen supply rate is low, so that the processing time becomes longer and the productivity is increased. 0.40 Nm
If the ratio is more than ³ / (Hr · cm ² ), the oxygen supply rate is excessive, so that the amount of chromium oxide generated increases, which causes a problem that it becomes difficult to decarburize or reduce the vacuum surface in the subsequent steps. As shown in FIG. 3, the bubble active area is 10% of the total molten steel surface area.
If it is smaller, the chromium oxide generated during the supply of oxygen gas will not be finely divided, and will not be caught in the bath due to the downward flow and will not easily flow out of the immersion tube. There is a problem that surface decarburization does not easily occur because it is covered with an object film. Further, when vacuum processing is performed from the region where the carbon concentration is 1% or more, the processing time becomes extremely long, and there is a problem that productivity is significantly impaired. Conversely, oxygen gas is supplied to 0.02% or less. In that case, the amount of chromium oxide generated increases at an accelerating rate,
There is a problem that it becomes difficult to decarburize and reduce the vacuum surface after the next step. Further, if the bubble active area is less than 100% of the oxygen spraying area, a region where the chromium oxide generation site is out of the bubble active surface is generated, and the generated chromium oxide grows unitedly without miniaturization, and The reduction efficiency at the fire point deteriorates. Further, the coarsening of the chromium oxide makes it difficult to flow out of the immersion tube, and as a result, the surface decarburization after the stop of the blowing acid is inhibited.

On the other hand, in the state where the ultra-low carbon steel is melted, the chromium oxide generated during the blowing of the oxygen gas is deposited outside the immersion tube. Even if Si or Al is added, the molten steel outside the immersion tube is hardly agitated, so that chromium oxide is not reduced. Therefore, it is essential to perform a step of reducing the chromium oxide generated during the decarburization by adding the reducing alloy after raising the immersion tube above the molten steel surface by applying a double pressure. In this case, since the chromium oxide generated during the decarburization is refined, the reduction can be performed very efficiently.

Here, slag sometimes adheres to the ladle and the edge of the immersion pipe during the treatment, but since the molten metal level in the ladle rises due to double pressure, the slag adheres to the semi-molten state due to the heat of the molten steel. As a result, it resurfaces on the molten steel surface in the ladle. Therefore,
All the slag can be efficiently reduced. In addition, the decarburization rate is slow in the current VOD, and especially in the melting of extremely low carbon chromium-containing steel, the overload of decarburization smelting in the converter process and the increase in the refractory basic unit due to the extension of the processing time in the VOD and This has led to lower chromium and iron yields. This is due to the fact that the amount of bottom blown gas cannot be increased due to free board restrictions in VOD, and the effect of surface decarburization is small because the entire amount of slag during tapping from the converter is brought into VOD.

In the present invention, since there is no free board restriction, it is possible to increase the stirring force by increasing the amount of bottom blown gas, and the slag in the immersion tube is almost completely discharged. Also has no effect of slag, is advantageous for accelerating the decarburization reaction, and has a large free surface area, so the effect of surface decarburization after blowing acid is stopped is extremely large, and a very high decarburization rate compared to the conventional method. As a result, the load in the converter step is greatly reduced, and as a result, the total chromium yield and iron yield are improved.

[0018]

EXAMPLE Table 1 shows an example using a 175-ton vacuum refining apparatus. In this case, the flow rate of the bottom-blown Ar gas was set at 105 to 1000 Nl / min. First, approximately after C in Cr concentration from 10 to 19% were smelted crude molten steel suitable concentration (C ₁₎ by converter refining, and supplies to the refining vessel as shown in FIG. 1, first, the carbon concentration to C _B Decarburization and refining are performed while blowing oxygen gas upward in a vacuum atmosphere of 200 Torr.
The mixture was stirred under the following vacuum atmosphere. After that, according to the present invention, the dip tube was raised above the molten steel surface by double pressure, and then the reducing alloy was charged. At this time, the flow rate of the bottom blown gas was uniformly 200 Nl / min. Moreover, in the comparative example, an example in which the reducing alloy was charged in a state in which the dip tube was inserted without performing double pressure was also performed.

As is clear from Table 1, the present invention is excellent in processing time, chromium yield and reaching [C].

[0020]

[Table 1]

[0021]

As described above, according to the present invention, it is possible to carry out decarburization efficiently while suppressing chromium oxidation. Therefore, the present invention is extremely effective for smelting extremely low carbon chromium-containing molten steel. It is a way.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an embodiment of a vacuum decarburization method for chromium-containing molten steel according to the present invention.

[Fig. 2] Oxygen spray rate per cell active surface and processing time
It is a figure which shows the relationship of the chromium oxide production amount.

FIG. 3 is a graph showing the relationship between the ratio of (cell activation area / total surface area of molten steel) and the decarburization rate after the termination of blowing acid ([C] ≦ 200 ppm).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ladle 2 Immersion pipe 3 Porous plug 4 Cr-containing crude steel 5 Inert gas 6 Top blowing lance 7 Oxygen gas

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C21C 7/10 C21C 7/00 C21C 7/04 C21C 7/068

Claims (1)

(57) [Claims] In a vacuum refining process, wherein a dipping tube is immersed in molten steel in a ladle, the pressure in the dipping tube is reduced, and a stirring gas is supplied from a lower portion of the ladle. When the carbon concentration is in the range of 1 to 0.02%, oxygen gas is supplied in an amount of 0.04 to 0.40 Nm ³ / (Hr · cm ² ) per bubble active area.
Of spraying from above at a rate, and more than 10% of the total molten steel surface bubbles active area, and was stirred under the condition that the 100% or more of oxygen blowing plane, the吹酸carbon concentration of 0.02% or less
Stop and inactive from the lower part of the ladle under high vacuum of 5 Torr or less
Degassing treatment is performed by stirring with the supply of gas only,
A method for decarburizing molten steel containing ultra-low carbon chromium, comprising subjecting the dip tube to a pressure above the molten steel surface by applying a double pressure, charging a reducing alloy, and reducing chromium oxide generated during decarburization.