JPH11140524A - Method for decarburize-refining chromium-containing molten iron - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for feeding oxygen which can drastically reduce the oxygen gas flowing rate while keeping the intensity of the oxygen jet and to improve yield of chromium and the reducing of consumption of inert gas, etc., at the time of decarburize-refining chromium-containing molten iron to the low carbon range by using a top-blown lance. SOLUTION: At the time of executing the decarburize-refining by blowing oxygen gas into the chromium-containing molten iron in a refining vessel from the top-blown lance, the top-blown lance having two or more systems 3a, 3b which can independently control the flowing rates or the presses of the oxygen gas and 2-10 of the total cross sectional area ratio of nozzle throat parts at the max. and the min. systems in the total cross sectional area of the nozzle throat part, is used. Then, in the range of >=1.5 wt.% [C] in the molten iron, the oxygen gas is fed from the whole systems, and in the range of <=0.7 wt.% [C], the oxygen gas is fed from the whole or a part of the systems except the max. system of the total cross sectional area of the nozzle throat part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for performing decarburization refining by blowing an oxygen-containing gas onto a chromium-containing molten iron in a refining vessel such as a converter or an AOD furnace using an upper blowing lance. .

[0002]

2. Description of the Related Art Molten stainless steel is produced by decarburizing and refining chromium-containing molten iron in a converter or an AOD furnace. To shorten the decarburization time, an upper-blowing lance with a high oxygen gas supply rate is used. It is desirable. For this reason, converters having a higher acid feed rate than the AOD furnace (particularly, top and bottom blown converters) are often used, or attempts have been made to increase the acid feed rate by attaching an upper blow lance to the AOD furnace.

[0003] When decarburizing and refining chromium-containing molten iron with top-blown oxygen, chromium is easily oxidized as compared with other alloying elements, and in the presence of chromium, the activity of carbon and oxygen is reduced. There are problems such as a decrease in decarburization and a reduction in decarburization speed.

Further, slag containing Cr ₂ O ₃ which is an oxide of chromium has a high solid phase ratio (C deposited as a solid).
The amount of r ₂ O ₃ is large), and the reduction of chromium oxide by carbon in the molten iron is difficult to progress, which makes decarburization and refining of the chromium-containing molten iron difficult.

In order to prevent the oxidation of chromium, a method of reducing the flow rate of oxygen gas at the end of decarburization is generally used. However, when the flow rate of oxygen gas decreases, the jet strength decreases. The high temperature and agitation of the collision part (so-called fire point) of the jet become insufficient.

In addition, when the oxygen gas flow rate is reduced, the lance tip is simply brought closer to the molten iron surface. In this method, the metal lance adheres to the lance due to the splash from the molten iron surface, or the lance comes close to a high temperature fire point. Problems such as shortening of the service life occur.

[0007] Therefore, for the purpose of suppressing the chromium oxidation by preferentially proceeding decarburization while maintaining the stirring of the iron bath by the jet, oxygen and oxygen are supplied from the top blowing lance during the decarburization refining of the chromium-containing molten iron. A number of methods have been proposed for spraying a mixed gas with a non-oxidizing gas to lower the carbon concentration in the molten iron and reduce the mixing ratio of oxygen gas (JP-A-58-13).
0216, Tokuhei 1-54409, Tokuhei 2-43
No. 803).

Japanese Patent Application Laid-Open No. 1-132714 discloses that when simultaneously blowing oxygen gas and non-oxidizing gas onto molten iron in the same manner as described above, an upper blowing lance having a plurality of gas injection holes is used to supply oxygen gas and non-oxidizing gas. Discloses a method of blowing a non-oxidizing gas from another gas injection hole.

Further, Japanese Patent Application Laid-Open No. 9-41018 discloses that, when decarburization is performed by blowing oxygen gas onto a chromium-containing molten metal from an upper blowing lance having a plurality of gas blowing holes at the tip, the gas blowing holes are connected to the lance shaft. Or a plurality of main holes provided in the vicinity of the lance and a plurality of main holes provided in the vicinity of the lance, and when the [C] concentration in the molten metal is 1 wt% or more, the amount of acid supplied from the sub-hole is reduced to a plurality of main holes. Discloses a method of decarburizing and refining chromium-containing molten steel in which blowing is performed by increasing the amount of supplied acid.

[0010]

A method of spraying a mixed gas from the same system as described above (Japanese Patent Laid-Open No. 58-130)
No. 216), there is a problem that the flash point temperature is reduced even if the jet strength can be maintained, and the amount of non-oxidizing gas necessary for maintaining the jet strength increases.

As the non-oxidizing gas, an inert gas such as argon, nitrogen or the like is usually used. However, argon is expensive and not economical. .

On the other hand, in the method disclosed in Japanese Patent Application Laid-Open No. 1-132714, there is only one oxygen gas supply system, and if the oxygen gas flow rate is reduced, the intensity of the jet from the injection hole cannot be maintained. Further, if the flow rate of the inert gas from the other injection holes is increased by the amount corresponding to the decrease in the flow rate of the oxygen gas, the flow rate becomes large, which is undesirable.

Further, the method disclosed in Japanese Patent Application Laid-Open No. 9-41018 aims to promote secondary combustion by flowing oxygen for decarburization from the main hole and oxygen for secondary combustion from the subhole. Therefore, the present invention does not have a technical idea of connecting these to separate oxygen gas supply systems and independently controlling the oxygen flow rate, and greatly reducing the oxygen gas flow rate while maintaining the jet strength. It cannot be applied to the purpose.

The present invention has been made in view of the above-mentioned problems of the prior art, and when decarburizing and refining chromium-containing molten iron to a low-carbon region, a non-oxidizing gas is not used or the amount thereof is minimized. Accordingly, it is an object of the present invention to provide a method for feeding an acid from an upper blowing lance which can significantly reduce the flow rate of oxygen gas while maintaining the strength of the oxygen jet.

[0015] In this way, when producing molten stainless steel, the yield of chromium is improved, the amount of inert gas used is reduced, and wear of refractories caused by chromium oxidation is reduced.
The purpose is to contribute to the reduction of decarburization costs.

[0016]

In order to solve the above-mentioned problems, the present inventors have focused on disposing a plurality of oxygen gas supply systems capable of independently controlling the flow rate or pressure in a single lance. . As an example of a converter steelmaking method having a plurality of acid supply systems in a single lance, there is a method disclosed in Japanese Patent Publication No. 60-31882.

In this arrangement, a plurality of large-diameter stirring nozzle holes and small-diameter slag-promoting nozzle holes are alternately arranged on the tip surface of a single lance, and both are individually oxygen-supplying systems capable of adjusting the flow rate. And attempts to independently control the acid transfer conditions.

However, this method is intended to enhance the dephosphorization in the ordinary converter blowing, and in the decarburization of the chromium-containing molten iron, the present invention is intended to greatly reduce the oxygen flow rate while maintaining the jet strength. It cannot be applied to the purpose.

The present inventors have a plurality of independent acid supply systems in a single lance, use all the systems during the peak period of decarburization, and use a part of each system appropriately in each stage of decarburization. If so, it was found that the above-mentioned problem could be solved and decarburization refining with less chromium oxidation loss was possible.

The gist of the present invention based on this finding is as follows: (1) In performing decarburization refining by blowing oxygen gas from a top blowing lance onto chromium-containing molten iron in a refining vessel, the flow rate or pressure of oxygen gas is independently controlled. It has two or more gas pipes that can be controlled, and the ratio of the total cross-sectional area of the nozzle throat part to the maximum and the minimum of the total cross-sectional area of the nozzle throat part is 2-1.
Using an upper-blowing lance that is 0, acid is fed from all of the above systems in a range where the carbon concentration [C] in the molten iron is 1.5% by weight or more, and a range where [C] is 0.7% by weight or less. The method for decarburizing and refining chromium-containing molten iron is characterized in that the acid is fed from all or some systems other than the system having the largest total nozzle throat section cross-sectional area.

(2) When [C] is in the range of 0.7 to 1.5% by weight, the oxygen gas flow rate of the system having the largest total cross-sectional area of at least the nozzle throat portion is reduced or stopped stepwise or continuously. And the non-oxidizing gas is mixed with the system, and the flow rate of the non-oxidizing gas is increased as the oxygen gas flow rate is reduced. It is.

(3) As the above-mentioned upper-blowing lance, an upper-blowing lance in which the nozzle hole at the tip of the lance is located farther from the center axis of the lance as the system having a larger total cross-sectional area of the nozzle throat portion is used. Characteristic preceding paragraph (1) or (2)
And the method for decarburizing and refining chromium-containing molten iron described in 1. above.

(4) A non-oxidizing gas of a predetermined flow rate or more is supplied to the system to which the supply of oxygen gas has been stopped in order to prevent metal from entering the nozzle hole at the tip of the lance. The method for decarburizing and refining chromium-containing molten iron according to any one of 1) to (3).

[0024]

1 and 2 are explanatory views showing examples of an upper blowing lance used in the present invention. FIG. 1 shows a case where there are two oxygen gas pipes, and FIG. 2 shows a case where there are three oxygen gas pipes. 1 (a) and 2 (a) are schematic plan views of the tip of the lance, and FIGS. 1 (b) and 2 (b) show the structure of the tip of the lance and the outline of the oxygen gas pipe in the AA cross section. Show.

In FIG. 1, the nozzle hole 1a at the tip of the upper blowing lance is open to the gas passage 2a.
a (hereinafter referred to as A system). The three nozzle holes 1b are open to the gas passage 2b, and are supplied with oxygen from an oxygen gas pipe 3b (hereinafter, referred to as a B system).

Similarly, in FIG. 2, the nozzle holes 1a, 1b
Is supplied from the A system and the B system, three nozzle holes 1c are opened in the gas passage 2c, and oxygen gas piping 3c (hereinafter, C
Acid).

In FIGS. 1 and 2, the nozzle hole 1a
Is one, and each of the nozzle holes 1b and 1c is three, but the number of nozzle holes is not limited to this.

The method for decarburizing and refining chromium-containing molten iron of the present invention comprises:
It is characterized by having two or more oxygen gas pipes in a single lance. The upper limit of the number of lines of the oxygen gas piping is not particularly limited. However, if the number of lines is four or more, the structure of the top blowing lance becomes too complicated.

Also, in the present invention, the total cross-sectional area of the nozzle throat portion is the largest (B system in FIGS. 1 and 2) and the smallest (A system in FIGS. 1 and 2). Ratio (total cross-sectional area of throat portion of nozzle 1b / nozzle 1
An upper blowing lance having a total cross-sectional area of the throat portion a of 2 to 10 is used.

According to the present invention, when the above-described oxygen gas pipe and the upper blowing lance are arranged to perform decarburization refining of chromium-containing molten iron in a refining vessel such as a converter or an AOD furnace, the carbon concentration in the molten iron [C ] In the range of 1.5% by weight or more (hereinafter referred to as the peak period of decarburization), acid is fed from all the systems, and [C] is 0.7%.
In the range of weight% or less (hereinafter referred to as the final stage of decarburization), the acid feeding of the system having the largest total cross-sectional area of the nozzle throat portion is stopped, and the acid feeding is performed from all or some other systems. I do.

That is, at least in the final stage of decarburization, in the example of FIG. 1, the acid is supplied from only the A system, and in the example of FIG. 2, the acid is supplied from the A system, the C system, or both.

The decarburization reaction of the chromium-containing molten iron depends on the blowing conditions, but usually shifts from oxygen gas supply control to carbon supply control at [C] 0.7% to 1.5% by weight. Therefore, in order to suppress the oxidation of chromium, it is necessary to drastically reduce the acid supply rate at the end of decarburization, and it is desirable that the acid supply rate be 1/2 to 1/10 of the acid supply rate at the peak of decarburization.

In the method of the present invention, while maintaining the intensity of the oxygen jet, the acid supply rate at the end of decarburization is reduced to 1/2 to 1/1 / the maximum during decarburization.
10, which makes it possible to form a high-temperature fire point and sufficiently agitate slag and metal even at the end of decarburization, minimize chromium oxidation loss and reduce low-carbon Decarburization refining can be performed up to the area.

The reason why the ratio of the maximum system to the minimum system is set to 2 to 10 in the total area ratio of the nozzle throat portion is that if the ratio is less than 2 times, the variable width of the flow rate becomes small and the effect is small, and the ratio is over 10 times. This is because the acid supply rate at the end of decarburization becomes too low.

According to the first aspect of the present invention, the acid is fed from all the systems at least in the range of not less than 1.5% by weight of [C], and the nozzle is supplied at least in the range of not more than 0.7% by weight of [C]. The acid may be fed from all or a part of the system except the system having the largest total cross-sectional area of the throat, and [C] 0.7 to
It does not limit the acid feeding method in the range of 1.5% by weight.

However, in the range of 0.7 to 1.5% by weight of [C], the decarburization rate decreases with the decrease of the [C] concentration. In the range C], it is desirable to reduce the acid feeding rate stepwise or continuously.

According to the second aspect of the present invention, in the range [C], the oxygen gas flow rate of the system having the largest total cross-sectional area of at least the nozzle throat portion (at least the system B in the examples of FIGS. 1 and 2). Is reduced or stopped stepwise or continuously, a non-oxidizing gas is mixed into the system, and the flow rate of the non-oxidizing gas is increased as the oxygen gas flow rate is reduced.

The reason why the non-oxidizing gas is mixed with the reduction of the oxygen gas flow rate is to stir the slag and the metal while maintaining a certain jet strength, and to prevent the metal from entering the nozzle hole of the lance. It is.

For this purpose, it is not necessary to increase the jet strength as much as the intensity of the oxygen gas jet at the peak of decarburization, and the gas flow rate may be about 1/10 or more of the peak of decarburization. Also, by mixing and spraying a non-oxidizing gas, the CO partial pressure on the surface of the molten iron is reduced, and decarburization is promoted.
This has the effect of further reducing chromium oxidation loss.

In the present invention, it is desirable to use an upper blowing lance in which the nozzle hole at the tip of the lance is located at a position farther from the center axis of the lance as the system has a larger total sectional area of the nozzle throat portion. For example, in the example of FIG. 2, since the total cross-sectional area of the nozzle throat portion is larger in the order of the nozzle holes 1a, 1c, and 1b, the nozzle throat portions are arranged at positions away from the lance center axis in this order.

The reason why such an arrangement is desirable is that, at the end of decarburization, when the acid supply of the system having the largest total cross-sectional area of the throat is stopped, it is desirable that the fire point is located near the center of the cross section of the refining vessel. It is preferable to increase the number of nozzle holes in a system having a larger total cross-sectional area of the throat portion, and the arrangement of the nozzle holes is easier at a position farther from the lance center axis.

In the method of the present invention, the acid supply from some of the nozzle holes is stopped at a later stage of the decarburization, particularly at a later stage of the decarburization. At this time, droplets of molten iron enter the nozzle holes. The nozzle may be blocked.

Therefore, it is desirable to supply a non-oxidizing gas of a certain flow rate or more to the system to which the supply of the oxygen gas is stopped. The flow rate of the non-oxidizing gas required to prevent such intrusion of the metal is 100 Nm / sec at the gas flow rate at the nozzle outlet.
The oxygen gas flow rate at the peak stage of decarburization.
It may be about / 10.

[0044]

EXAMPLE The decarburization and refining of chromium-containing molten iron was performed using a 6-ton top-bottom blow converter with the method of the present invention and the conventional method in which the oxygen gas piping was one system, and the chromium yield and the use of inert gas were used. The amounts were compared.

The embodiment is a case where decarburization is carried out under the following conditions by the method of the present invention using an upper blowing lance and two oxygen gas pipes as shown in FIG. In [C], blowing control was performed by predicting the sublance measurement during blowing and a dynamic model.

Nozzle throat diameter: A system 9 mm × 1 B system 9 mm × 4 (radiation angle 20 °) Acid supply condition A system is O ₂ 240 Nm ³ / h in all stages of decarburization, and B system is as follows. Was changed as follows.

[C] 1.5% or more: O ₂ 960 Nm ³ / h [C] 1.0 to 1.5%: O ₂ 760 Nm ³ / h [C] 0.7 to 1.0%: O ₂ 360 + Ar 50 N
m ³ / h [C] 0.7% or less: Ar 120 Nm ³ / h.

[0048] Comparative Example 1, the lance nozzle hole number, pore size the same as the embodiment, the oxygen-flow is not mixed with an inert gas to one system, in case of blown above only O _2, the oxygen-flow amount Changed as follows.

[C] 1.5% or more: O ₂ 1200 Nm ³ / h [C] 1.0 to 1.5%: O ₂ 1000 Nm ³ / h [C] 0.7 to 1.0%: O ₂ 600 Nm ³ / h [C] 0.7% or less: O ₂ 240 Nm ³ / h.

Comparative Example 2 has the same top-blowing lance as Comparative Example 1, but has a single acid supply system, and after the peak period of decarburization, O
_When a mixed gas of ₂ and Ar was supplied, the flow rates of O ₂ and Ar were changed as follows.

[C] 1.5% or more: O ₂ 1200 Nm ³ / h [C] 1.0 to 1.5%: O ₂ 1000 Nm ³ / h [C] 0.7 to 1.0%: O ₂ 600 + Ar 50 N
m ³ / h [C] 0.7% or less: O ₂ 240 + Ar 120
Nm ³ / h.

In both Examples and Comparative Examples 1 and 2, the components of the chromium-containing molten iron before decarburization were about 4% [C] and about 0.7% [Si].
%, [Cr] was about 16%, and the molten iron temperature was about 1350 ° C., and the molten iron was decarburized to [C] about 0.2% under the above-described acid feeding conditions. In each of Examples and Comparative Examples 1 and 2, the flow rate of bottom blown Ar during decarburization was kept constant at 100 Nm ³ / h, and the lance height was kept constant at 0.8 m.

As a result of performing decarburization blowing for each of the two heats under the above conditions, the average value of the Cr yield was 81% and the average value of the specific unit of Ar gas was 14.5 Nm ³ / t in the example.

On the other hand, in Comparative Example 1, the amount of Ar gas used was 1
Although it was 0.3 Nm ³ / t (only bottom blown Ar), the average value of Cr yield was remarkably reduced to 72%. Comparative Example 2
Although the average value of the Cr yield was relatively high at 76%, the average value of the Ar gas intensity was 16.5 Nm ³ /
It has increased to t.

[0055]

According to the present invention, it is possible to greatly change the acid feeding speed while maintaining the strength of the oxygen jet of the top blowing lance. As a result, in the decarburization and refining of chromium-containing molten iron, even if the acid supply rate at the end of decarburization is greatly reduced, a high-temperature fire point is formed and the slag and metal are sufficiently agitated. Decarburization and refining to low-coal area was possible with the minimum possible.

According to the method of the present invention, it is possible to improve the yield of chromium, reduce the amount of inert gas used, reduce the wear of refractories caused by chromium oxidation, etc., when producing molten stainless steel. Cost can be reduced.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an example of an upper blowing lance used in the present invention, showing a case where an oxygen gas pipe has two systems.

FIG. 2 is an explanatory view showing another example of the upper blowing lance used in the present invention, showing a case where the oxygen gas piping has three systems.

[Explanation of symbols]

 1a, 1b, 1c: Nozzle hole at the tip of the upper blowing lance 2a, 2b, 2c: Gas passage of the upper blowing lance 3a, 3b, 3c: Oxygen gas pipe

Claims (4)

[Claims] When degassing and refining is performed by blowing oxygen gas from a top blowing lance onto chromium-containing molten iron in a refining vessel,
It has two or more gas pipes that can independently control the flow rate or pressure of oxygen gas, and the ratio of the total cross-sectional area of the nozzle throat part of the maximum and the minimum of the total cross-sectional area of the nozzle throat part is 2 to 10. When the carbon concentration [C] in the molten iron is in the range of 1.5% by weight or more, the acid is fed from all the systems using a blowing lance, and when the [C] is in the range of 0.7% by weight or less, the nozzle throat section is used. A method for decarburizing and refining chromium-containing molten iron, wherein the acid is fed from all or some systems other than the system having the largest total cross-sectional area. 2. When [C] is in the range of 0.7 to 1.5% by weight, at least the oxygen gas flow rate of the system having the largest total cross-sectional area of the nozzle throat portion is reduced or stopped stepwise or continuously. 2. The method for decarburizing and refining chromium-containing molten iron according to claim 1, wherein a non-oxidizing gas is mixed into the system, and the flow rate of the non-oxidizing gas is increased as the oxygen gas flow rate decreases. 3. The upper-blowing lance, wherein the nozzle hole at the tip of the lance is located at a position farther from the center axis of the lance as the system having a larger total cross-sectional area of the nozzle throat portion is used. The method for decarburizing and refining chromium-containing molten iron according to claim 1 or 2. 4. A non-oxidizing gas of a predetermined flow rate or more is supplied to a system in which the supply of oxygen gas has been stopped in order to prevent metal from entering the nozzle hole at the tip of the lance. 4. The method for decarburizing and refining chromium-containing molten iron according to any one of claims 1 to 3.