JPS58113313A - Refining method for molten steel - Google Patents

Abstract

PURPOSE:To prolong the residence time of a flux in a vacuum vessel and to improve the utilization efficiency of reaction by blowing an inert gas for returning of molten steel into a riser of a circulation type degassing device and blowing the inert gas of an amt. smaller than the amt. of said gas in a downcomer. CONSTITUTION:The bottom ends of a riser 3A and a downcomer 3B are immersed in molten steel 5, and the inside of a vacuum vessel 2 is evacuated. When an inert gas is blown into the riser 3A through a blow port 6, the molten steel 5 is sucked through the riser 3A up into the vessel 2. When a desulfurizing flux 9 is added to the vessel 2, the flux 9 and the steel 5 are stirred by the foam 10 of inerr gas floating in the vessel 2, and the reaction of desulfurization progresses. In this stage, the inert gas is blown into the downcomer 3B at the flow rate Q4 of a 0.1-0.5 range with respect to the flow rate Qu of the inert gas to be blown into the riser 3A. The outflow of the flux 9 into the downcomer 3B is blocked by the formed foam 10' and the time for the reaction of desulfurization is prolonged.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for refining molten steel using a recirculation vacuum degassing device, and in particular to a method for refining molten steel by adding a refining flux such as desulfurization flux into the vacuum chamber of the degassing device to perform desulfurization treatment, etc. This relates to a method for refining.

Recently, the demand for high-grade steel materials has been on the rise.
Accordingly, there is an increasing need to produce ultra-low sulfur steel with extremely low sulfur content. However, there are limits to temperature-controlled desulfurization in primary elliptical furnaces such as converters, so in order to obtain ultra-low sulfur steel, preliminary desulfurization treatment must be carried out in the hot metal stage before primary refining, or in the stage after secondary refining. It is necessary to perform molten steel desulfurization treatment, etc. One such desulfurization treatment method is a method using a reflux type degassing device. This method involves adding desulfurization flux to the vacuum chamber of a slough-flow degassing device.
! The molten steel flowing from the pot through the riser pipe into the vacuum tank and the inert gas bubbles that are blown into the riser pipe and float in the vacuum tank create a stirring effect in the vacuum inertia, which stirs and mixes the 7lux and molten steel, and desulfurizes. This is what we aim to do. This method is different from desulfurization treatment in the air.Since the treatment is carried out in a reduced pressure atmosphere of about 1 torr or less, degassing of hydrogen and nitrogen in the molten steel is also performed at the same time as desulfurization. It has the advantage of improving the cleanliness of rounded molten steel due to its acid effect. However, in the desulfurization treatment method using such a recirculation degassing device, the flux added to the vacuum chamber is caught in the downward flow that returns to the ladle via the downcomer pipe before it has sufficiently reacted with the molten steel. There was a problem of leakage into the ladle. In other words, the flux that has flowed into the ladle floats to the bath surface in the ladle and is no longer stirred and mixed with the molten steel, so in this case, the stirring and mixing time for the flux and the ordinary metal is shortened. , the problem arises that the utilization efficiency of 7 lux for the desulfurization reaction becomes poor.

This invention was made in view of the above circumstances, and when adding refining flux such as desulfurization flux to a vacuum chamber and processing molten steel with a recirculation vacuum degassing device, the flux remains in the vacuum chamber for a long time. The purpose of this is to ensure that the molten steel remains within the molten steel and is sufficiently utilized for the reaction with the molten steel.

In other words, as a result of various experiments and studies to achieve the above-mentioned object, the present inventors have found that in addition to injecting inert gas into the riser pipe to circulate molten steel, a small amount of inert gas is injected from the downcomer pipe. It was discovered that by blowing the flux added into the vacuum chamber, the residence time in the vacuum chamber can be significantly extended, and this invention was achieved.

Therefore, the molten steel refining method of the present invention uses an fIIfL vacuum degassing device, and when refining molten steel by adding flux to the vacuum chamber of the degassing device, the degassing device is raised. In addition to injecting inert gas for molten steel circulation into the pipe, a small amount of inert gas is also blown into the descending pipe at a rate equal to the flow rate into the rising pipe. This is a symptom. And t9, especially the ratio Q of the inert gas flow rate Qd blown into the downcomer pipe and the inert gas flow rate Qu blown into the riser pipe.
a/Qa is set to a value within the range of 01 to 0.5.

The refining method of the present invention will be explained in more detail below.

FIG. 1 shows an example of a situation in which the method of the present invention is implemented. At the lower end of the vacuum chamber 2 of the recirculation type vacuum degassing apparatus 1, there are two immersion pipes 3M, 3B1, or riser pipe 3A. A descending pipe 3B is provided, and an inert gas inlet 6 such as gas is provided in the middle of the rising pipe 3B for sucking up the molten steel 5 in the ladle 4 using the principle of an airsoft pump. It is being

In this invention, an inert gas blowing lower is also provided in the middle of the downcomer pipe 3B. Note that the vacuum chamber 2 is connected to a vacuum pump 8.

In the apparatus shown in Fig. 1, the lower ends of the ascending pipe 3A and the descending pipe 3B are immersed in the molten steel 5, the pressure inside the vacuum chamber 2 is reduced, and inert gas is blown into the three rising pipes from the inert gas inlet 6. Similar to the conventional general circulation type vacuum degassing apparatus, molten steel 5 is sucked up from the ladle 4 into the vacuum chamber 2 via the rising pipe 3A, and returns into the ladle 4 via the descending pipe 3B. When the desulfurization flux 9 is added into the vacuum chamber 2, the flow of the molten steel 5 drawn up from the ladle 4 into the vacuum chamber 2 and the inert gas bubbles that are blown into the riser pipe 3A and float inside the vacuum chamber 2. 10 stirs the 7lux 9 and the molten steel 5, and the desulfurization reaction progresses. At this time, the flux 9 tends to be caught up in the flow of the molten steel 5 flowing out from the vacuum chamber 2 to the downcomer pipe 3B, and to flow out into the ladle 4 together with the molten steel 5. Since a small amount of inert gas is blown into the inert gas bubbles 10', the upward flow of the inert gas bubbles 10' prevents the 7 lux 9 from flowing into the downcomer pipe 3B, and the residence time of the rounded flux 9 in the vacuum chamber 2 is reduced. It will be extended. As a result, the stirring and mixing time of the flux 9 and the molten steel 5 is extended, and 7 lux 9 is fully utilized for the desulfurization reaction, increasing the efficiency of using the flux for the desulfurization reaction.

In the above explanation, the inert gas flow rate Qd blown into the downcomer pipe 3B and the inert gas flow rate Q blown into the three riser pipes
If the ratio Qd/Qu with w becomes 1, molten steel will not circulate, so the ratio Qd/Qu of both needs to be smaller than 1. Here Qd/Q u.

Since the residence time in the vacuum chamber of #1 flux whose value is close to 1 increases, it is preferable that the value of Q MQ u be close to 1 from the point of view of the reaction efficiency only in the vacuum chamber due to the extension of the residence time. it is conceivable that. However, Qd/Q
If the value of u approaches 1, the flow of molten steel from the downcomer pipe 3B to the ladle will be obstructed, and the circulation speed of molten steel between the ladle and the vacuum tank will decrease, and as a result, the system as a whole will Desulfurization is delayed and it becomes difficult to perform sufficient desulfurization within a predetermined time.

On the other hand, when the value of Qd/Qa is significantly smaller than 1, that is, when the downcomer inert gas injection flow rate Qa is extremely small, the reflux velocity of molten steel increases, but the effect of injecting inert gas from the downcomer is becomes extremely small. In other words, the residence time of the flux in the vacuum chamber is shortened, and the flux easily flows into the ladle as it flows downward, and the flowing flux floats onto the ladle bath and is mixed with the molten steel. Therefore, it no longer contributes to desulfurization.

For the reasons mentioned above, there is an optimal range for the value of Qd/Qu. However, this optimum range may vary depending on the purpose of the treatment, but as a result of experiments conducted by the present inventors on desulfurization treatment using desulfurization flux, the Qd/QuO value was found to be 0. A range of 1 to 0.5 has been found to be most preferred. As for the desulfurization flux, CaO is used as the main component, and CaF is used as the desulfurization flux.
It is sufficient to use a mixture of solvents such as 2, etc., specifically, CmO50'-901, CaF210-50qb
It is sufficient to use a material to which U, O, 8i02, etc. are added as necessary.

Examples in which the present invention is applied to desulfurization treatment of molten steel will be described below.

EXAMPLE Approximately 100 tons of molten steel that had been blown in a converter was tapped into a ladle and subjected to a desulfurization treatment using a reflux degassing device shown in FIG. However, this process starts when the process starts (recirculation starts).
Approximately 1 ton of desulfurization flux was added into the vacuum chamber between the time when 5 minutes had passed and the time when 10 minutes had passed, and the treatment time was 15 to 20 minutes. Here, the desulfurization flux is CaO90fk, CaF21
A powder having a composition of 0% and having a particle size of 3 mm or less occupying about 8.0 cm was used. The inner diameter of the ascending pipe and the descending pipe are each 300mm, and the A
The r gas filQu is in the range of 800 to 12006/Fnin, while the Ar gas flow rate Qd t blown from the downcomer
It was varied in the range of iO to 8004/nin. Note that the Ar gas blowing from the downcomer was started from the time when the addition of the desulfurization flux was started, and continued until the addition of the desulfurization flux was completed and 7 lux in the vacuum chamber was exhausted. Furthermore, the molten steel used in the experiment contained 0.1 to 0.12% C,
SiO, 2-0.4S, Mn 1.2-1.3%
, P 0.012~0.016 S%AA O,02~
For aluminum-killed molten steel of 0.06%, the 8gk degree in the molten steel at the start of treatment is 0.003~O,OOS-.

Note that when desulfurization flux was added, the exhaust capacity of the vacuum pump was lowered to prevent flux from flowing into the vacuum pump system.

In the above embodiment, Ar gas is supplied from the downcomer to 0, 1≦
When blowing at a flow rate within the range of Qd/Qa≦0.5,
When Ar gas is not injected from the downcomer pipe (Q
d=0), the S concentration in molten steel at the end of treatment C18
: ], and the ratio of S concentration in molten steel [chi] at the start of treatment [ch83. /C15) The results of the investigation are shown in Figure 2. Further, in the above embodiment, the ratio Q of the Ar gas flow rate Qd blown from the downcomer to the gas flow rate Qu blown from the riser
The relationship between d/Qu and the ratio of S concentration in molten steel ([S:], /Cl8)') at the end of treatment and at the start of treatment is shown in FIG.

From Figures 2 and 3, the value of Qd/Qu is 0.1 to 0.
, 5, it is clear that the S concentration in the molten steel at the end of the treatment is lower than in the case where inert gas is not blown from the downcomer pipe (Qd%Qu=0).

On the other hand, when the Qd/QuO value exceeded 0.5, it was clear that the residence time of the flux in the vacuum chamber was significantly extended, but the reflux velocity of the molten steel decreased and the S It was confirmed that the concentration did not decrease sufficiently.

It should be noted that the method of this invention can of course be applied not only to desulfurization treatment but also to other treatments.In short, the method is applicable not only to desulfurization treatment but also to other treatments. For example, it is also applicable to slack addition treatment for the purpose of deoxidation, and in this case it is effective in reducing the oxygen concentration.

As is clear from the above description, according to the method of the present invention,
By extending the residence time of flux in the vacuum chamber,
A remarkable effect can be obtained in which the reaction utilization efficiency of slacks can be improved.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view showing the situation in which the method of the present invention is carried out, and FIGS.
FIG. 3 is a correlation diagram showing the relationship between the Ik degree ratio and the inert gas blowing flow rate ratio Qd/Qu of the downcomer and the riser. 2... Vacuum tank, 3A... Ascending pipe, 3B... Descending pipe, 4... Ladle, 5... Molten steel, 6.7... Inert gas blowing port, 9... Desulfurization flux. Applicant Kawasaki Steel Co., Ltd. Agent Patent attorney Takehito Toyota (and 1 other person) Figure 1

Claims (3)

[Claims] (1) In a method of refining molten steel by using a JJl flow degassing device and adding 7 lux into the vacuum chamber of the recirculation degassing device, a molten steel reflux is added into the riser pipe of the recirculation degassing device. A method for refining molten steel, which comprises blowing an inert gas into the downcomer pipe and a smaller amount of inert gas than the flow rate of the inert gas blown into the upper draft pipe. (2) The refining method according to claim 1, characterized in that molten steel is produced using desulfurization flux as the flux. (3) The ratio Qd/Qu of the inert gas flow rate Qd blown from the downcomer pipe and the inert gas flow rate Qu blown from the riser pipe is 0.
．． The method for refining molten steel as set forth in item 1 or 2 of the special WIF Zhao Qi range, which is within the range of 1 to 05.