JPH0681026A - Method for preventing bumping of molten steel by controlling vacuum degree - Google Patents

Abstract

PURPOSE:To efficiently prevent bumping of molten steel by controlling the vacuum degree from starting oxygen-blowing based on a CO reaction energy evaluating index in the inner part of the molten steel given by the ratio of the vacuum degree in the operation to the vacuum degree in the equilibrium. CONSTITUTION:At the time of executing refining by top-blowing the oxygen into the molten steel under reduced pressure, the index X evaluating the CO reaction energy in the inner part of the molten steel is given with the equation X=-6X10<-3>T.lnP/P0 (T: molten steel temp. K) from the ratio P/P0 of the vacuum degree P of the operation tank and the vacuum degree P0 at the equilibrium. The vacuum degree P of the operation is controlled so that this index becomes the specific value, e.g. X <= about 30. Then the vacuum degree during the interval from the starting of oxygen blowing to the confirmation of CO generation is controlled and the index is kept to the reference at the initial stage obtd. from the composition and the temp. of the molten steel. Further, the vacuum degree during the interval to the stopping of oxygen blowing after starting the CO generation is controlled based on the index. By this method, the sudden CO reaction is restrained to prevent the bumping and to avoid the inundation phenomenon of the molten steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining molten steel in which oxygen is supplied to molten steel by top-blowing under reduced pressure for refining, and in particular, the phenomenon of overflow of molten steel due to boiling of molten steel in such reduced pressure oxygen-blown refining is avoided. Regarding technology.

[0002]

2. Description of the Related Art A refining method for molten steel, in which oxygen is supplied to molten steel by upward blowing under reduced pressure to refining, has been widely known before the application of the present application. In this vacuum refining,
When the refining atmosphere is depressurized from the atmosphere to 200 Torr, a rapid CO reaction occurs inside the molten steel due to abrupt [C] + O → CO, the molten steel boiles, and a bumping phenomenon occurs in which the molten steel overflows from the upper part of the inner lid. The overflowed molten steel solidifies in the vacuum reactor, seriously damages pots and other equipment, and causes production problems. Therefore, for the condition of [C] in steel,
It is necessary to operate at an appropriate degree of vacuum.

For example, in Japanese Examined Patent Publication No. 60-13406, the decompression rate before blowing acid is set to a certain standard or less, and after a prescribed vacuum degree is maintained, the pressure is reduced at a depressurizing rate below a certain standard until the start of blowing acid. In addition, a method of starting blowing with holding at a specified vacuum degree is disclosed, and JP-A-62-13
Japanese Patent No. 9809 discloses a method of controlling the degree of vacuum by simultaneously managing a difference between a preset degree of vacuum and a rate of decarburization rate change and an actual degree of vacuum and a rate of decarburization rate obtained from the amount of exhaust gas. There is.

However, in such a bumping prevention method by controlling the degree of vacuum, (1) it takes a long time to start the blowing acid treatment and the productivity does not increase. (2) The exhaust gas flow rate has a relatively large hunting and decarburization. It is necessary to detect the CO and CO ₂ concentrations in the exhaust gas at the same time, but there is also a time delay, so in terms of response and accuracy,
The overflow phenomenon of molten steel due to boiling cannot be completely suppressed,
(3) The control system is complicated, and a sophisticated detector is required to prevent a response delay. (4) The degree of vacuum to be preset or the decarburization rate change rate is a component condition of molten steel. However, there is a problem that it greatly changes depending on the operating conditions, etc., and that it cannot fully cope with the changes in the conditions due to variations in the operating.

Therefore, in actuality, the freeboard of the ladle is set sufficiently high to cope with the bumping phenomenon, but it is uneconomical because the depressurization equipment and other related equipment must be installed higher than necessary. .

[0006]

The problem to be solved by the present invention is to solve the above problems in controlling the degree of vacuum in the refining of molten steel by blowing oxygen under such reduced pressure. The object of the present invention is to provide a vacuum control method capable of effectively preventing the above.

[0007]

SUMMARY OF THE INVENTION The present invention is directed to C inside molten steel.
An index for evaluating the O reaction energy is given by the ratio of the vacuum degree of equilibrium and the vacuum degree of operation, and the vacuum degree of operation is controlled so that the index becomes a specific value. CO generation is confirmed from the start of blowing acid. Until the vacuum level is maintained at the above-mentioned index as an initial standard,
It is characterized in that the degree of vacuum is controlled on the basis of the above-mentioned index over the entire area of blowing acid from the start of CO generation to the stop of blowing acid.

Specifically, the initial index of the degree of vacuum is determined by the starting components ([C] and [Cr]) and the temperature, and the transition of [C] in the propellant acid is set in advance so that Give an index.

However, [Al] and [S]
Since strong deoxidizing elements such as i] are burned prior to [C], the start [C] is maintained during that period.

The initial index is maintained until after the generation of CO is confirmed,
After that, it is advanced, or [Al], [S
The index in blowing acid is given from the transition of [C] in consideration of strong deoxidizing elements such as i].

The transition of [C] is estimated from the amount of oxygen fed and the oxygen utilization efficiency for decarburization.

Blowing acid was 0.26 atm from the start of pressure reduction in the tank.
Starting at (200 torr), the pressure is reduced as quickly as possible to shorten the processing time until the degree of vacuum satisfying the initial index.

[0013]

[Operation] During the process, the minimum degree of vacuum and the CO concentration information in the exhaust gas are sufficient, and the control system becomes simple.
Responsiveness is improved.

[0014]

EXAMPLES The present invention was applied to reduce carbon in molten steel having a composition of C = 0.9% and Cr = 16% to 0.05%.

The index X at the beginning of operation was set according to the following procedure.

X = −6 × 10 ⁻³ × T · lnP / P ⁰ (1) P: vacuum degree of operation (pressure in tank) P ⁰ : pressure at equilibrium (2) T : Molten steel temperature (K) That is, the term of lnP / P ⁰ shown in the equation (1) is [C]
It becomes the main factor of the driving force of the reaction of + O → CO. As a result of the 10 kg melting furnace test, the point at which the reaction suddenly becomes large is X = 3.
Since it was 0, X ≦ 30 (preferably 20 to 1)
0) was set.

Further, in general, Cr-containing steel (stainless steel)
In the reaction between [Cr] and [C] in, the following expression (2) is established. Here, P ⁰ is CO that is originally in the equilibrium state.
, Which is considered to be the pressure of the atmosphere in which equilibrium occurs in the reduced pressure refining furnace.

[0018] [% Cr]: Cr% in steel [% C]: C% in steel As a result, P ⁰ was determined to be 1.39 atm, and X = 2.
The actual atmosphere of the vacuum furnace was set to 0 and the pressure was reduced to P = 0.23 atm for operation. About 1 minute after the depressurization was started, the pressure reached the specified level, but the concentration of propellant acid was 0.39 atm (about 300 torr).
started with r).

One minute after the start of blowing acid, CO was detected from the exhaust gas to confirm the progress of decarburization. After that, the vacuum degree was controlled so as to match the preset vacuum degree pattern. In other words, the decarboxylation efficiency at the time of stable refining can be obtained,
Therefore, from the transition of [C] and the change of temperature during processing,
The vacuum degree pattern during processing is set in advance according to [C], [Cr], and temperature so that X = 20.

FIG. 1 shows the process for each index X and Cr concentration and the occurrence state of bumping.

The following can be seen concretely from FIG.

According to this index, it is possible to obtain operating conditions for avoiding bumping for a wide range of Cr levels of 10 to 20%. Further, when X <10, the decarburizing reaction suppressing effect is large and it is not practical as refining. Furthermore, when X> 30, bumping occurs in most of the processing.

[0023]

The present invention has the following effects.

(1) It is possible to perform stable processing without overflow from arbitrary [C], [Cr] and temperature levels.

(2) Applicable with or without blowing acid.

(3) Since the vacuum degree pattern of the whole process can be set in advance from the conditions of initial [C], [Cr], and temperature, the information in the middle is only exhaust gas information for confirming the progress of the pattern, It is a simple control.

(4) Since an abnormal reaction is unlikely to occur, the accuracy of refining [C], [Cr], temperature, etc. at the end of processing is improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a bumping state of molten steel when the present invention is specifically carried out.

Continued Front Page (72) Inventor Naoki Hirashima 1-1 No. 1 Tobita-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Co., Ltd. Yawata Works

Claims (1)

[Claims] 1. An index for evaluating the CO reaction energy in molten steel is given by the ratio of the vacuum degree of equilibrium and the vacuum degree of operation, and the vacuum degree of operation is controlled so that the index becomes a specific value. The degree of vacuum is maintained at the above-mentioned index as an initial standard from the start of acid until the generation of CO is confirmed. How to prevent bumping.