JP3716462B2 - Method for refining chromium-containing molten steel - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for refining chromium-containing molten steel.
[0002]
[Prior art and problems to be solved by the invention]
When refining chromium-containing steel, such as stainless steel, a mixed gas of oxygen and Ar is blown into the molten steel accommodated in the refining furnace to decarburize C in the molten steel, thereby reducing the C concentration. It has been broken. This decarburization refining is known as the AOD method.
[0003]
By the way, in the decarburization refining in the atmosphere, when the C concentration in the molten steel becomes low, the blown oxygen does not work effectively for decarburization, oxidizes chromium, and the decarburization efficiency deteriorates.
In addition, decarburization takes a long time, and a large amount of expensive Ar is consumed during this period, so that the refining cost becomes high.
[0004]
For this reason, when the C concentration in the molten steel reaches a certain level, the furnace is depressurized, and only the non-oxidizing gas such as Ar gas is blown into the molten steel under such reduced pressure to stir the molten steel and slag. The present applicant has proposed a method in which chromium oxide produced during the previous atmospheric treatment reacts with C in molten steel to continue decarburization and partially reduce chromium oxide (for example, (Kaihei 254509).
In this refining method, switching from atmospheric treatment to decompression treatment is performed when the C concentration is reduced to 0.15 to 0.05% by weight.
[0005]
This is based on the following reason.
That is, when switching from atmospheric processing to decompression processing at a stage where the C concentration is higher than this, a large amount of CO gas may be generated during decompression processing, and a CO explosion may occur. Second, chromium oxide and molten steel The reaction with C in the inside is an endothermic reaction, and when the reaction occurs vigorously, the temperature of the molten steel is greatly decreased, and the temperature of the molten steel is undesirable due to the subsequent temperature decrease due to the addition of an additive such as a reducing agent. This is because it drops to temperature.
[0006]
More specifically, the temperature of the molten steel needs to be higher than a certain temperature, for example, higher than a certain temperature with respect to the melting temperature, in order to perform subsequent casting satisfactorily at the time of steel production.
However, if the molten steel temperature is greatly reduced, the necessary temperature cannot be ensured at the time of steel production, and subsequent casting cannot be performed satisfactorily.
[0007]
For the above reasons, in the conventional method, switching from the atmospheric treatment to the decompression treatment is performed for the first time when the C concentration is reduced to 0.15% by weight, but at a stage where the C concentration is higher. If switching from atmospheric treatment to decompression treatment is possible, the refining time can be further shortened and the refining cost can be reduced, which is desirable.
[0008]
[Means for Solving the Problems]
The invention of the present application has been made to solve such problems.
Thus, the invention of the present application is that in the refining furnace, after the atmospheric treatment is performed in which the decarburization is performed by blowing a gas containing oxygen gas into the molten steel of the high chromium steel in the atmosphere, the pressure inside the furnace is reduced. In the refining method for chromium-containing molten steel that performs depressurization treatment that performs decarburization by gas injection, switching from the atmospheric treatment to the depressurization treatment reduces the C concentration in the molten steel to 0.3 to 0.05% by weight. In the decompression process, the decompression process is performed under a reduced pressure of 200 to 15 Torr while analyzing the exhaust gas, and after switching to the decompression process, a non-oxidizing gas is blown into the molten steel. Performing decarburization treatment by reacting chromium oxide in the slag and C in the molten steel while stirring the molten steel and slag, and before introducing a reducing agent for reducing chromium oxide after the decarburization treatment In the gas containing oxygen Performing blowing into the steel caused the heat generated by the oxidation reaction of chromium, so as to raise the molten steel temperature, subjected to further CO concentration of the exhaust gas in the gas analysis of the exhaust gas, O ₂ concentration and the exhaust gas temperature measurement, The increase in O ₂ concentration at that time predicts the danger of CO explosion, and the decompression process is stopped.
[0009]
[Operation and effect of the invention]
The CO explosion occurs when conditions such as CO concentration, O ₂ concentration and exhaust gas temperature in exhaust gas satisfy certain conditions.
Therefore, in the present invention, it was confirmed that the C concentration at the time of switching can be increased to C: 0.3% by performing gas refining while monitoring so that the exhaust gas composition does not satisfy the explosion condition.
[0010]
That is, according to the present invention, in a refining method in which decarburization is performed by depressurization after decarburization by blowing O ₂ in the atmosphere, switching from atmospheric treatment to decompression treatment is performed at C: 0.3% (preferably C : 0.2%) as an upper limit value, and according to the present invention, switching from atmospheric treatment to decompression treatment can be performed at an early stage, so that decarburization efficiency can be improved and refining time can be increased. It can be shortened and the refining cost can be reduced.
[0011]
In the present invention, the reason why the pressure condition in the decompression process is 200 to 15 Torr is as follows.
That is, the pressure reduction treatment cannot be effectively performed under a pressure higher than 200 Torr, and conversely, the splash of molten steel becomes intense under a pressure lower than 15 Torr.
[0012]
In the present invention, after switching to the reduced pressure treatment, non-oxidizing gas is blown into the molten steel, and in the agitation of the molten steel and slag, chromium oxide in the slag and C in the molten steel react to decarburize. In this decarburization process under reduced pressure conditions, the C concentration in the molten steel can be reduced to a desired low level, for example, about 0.04%. That is, even when about 0.3% of C is contained in the molten steel at the time of switching to the decompression process, it can be lowered to a desired low level by the subsequent decarburization process.
[0013]
However, if the C concentration in the molten steel at the beginning of the decompression process is higher than this, it is difficult to lower it to a desired low level in the decarburization process.
In other words, when switching from atmospheric treatment to reduced pressure treatment at C: 0.3%, stirring by simply injecting a non-oxidizing gas is possible without particularly supplying a decarburization source into the molten steel. It is possible to reduce the C concentration in the molten steel only to a desired low level.
[0014]
The present invention is also characterized in that prior to the introduction of the reducing agent, a gas containing oxygen is blown into the molten steel to generate heat based on the oxidation reaction of chromium, and the molten steel temperature is raised. .
Since this oxygen injection is performed after decarburization, even if oxygen is supplied to the exhaust gas, the CO concentration in the exhaust gas is already low, so there is no risk of a CO explosion, and the temperature of the molten steel is raised again. And this can be raised to the desired temperature.
[0015]
In other words, when switching from the atmospheric treatment to the decompression process is performed at a relatively high concentration stage of C: 0.3%, the large temperature drop of the molten steel that occurs during the decompression treatment is caused by blowing this oxygen gas. Therefore, the temperature of the molten steel at the time of outgoing steel can be increased to a desired temperature, and the subsequent casting can be performed satisfactorily.
[0016]
In addition, the maximum temperature reached by the molten steel during the air treatment can be lowered, and therefore the life of the refractory material in the refining furnace can be extended.
In addition, the chromium oxide produced | generated by the gas blowing containing oxygen gas after a decarburization process in a pressure reduction process is reduce | restored by the injection | throwing-in of a reducing agent after that.
[0017]
In the present invention, the CO concentration in the exhaust gas, the O ₂ concentration and the exhaust gas temperature are measured by the above gas analysis, and the risk of CO explosion is caused by the increase in the O ₂ concentration in the exhaust gas at that time. processing it stops.
As described above, the CO explosion does not occur unless conditions such as CO concentration, O ₂ concentration, and exhaust gas temperature in the exhaust gas satisfy certain conditions.
In other words, there because there is no risk of CO explosions Without supply of oxygen in the exhaust gas, where the present invention some abnormality with a rising trend of the oxygen concentration in the exhaust gas, between the main body and the lid, for example refining furnace The decompression process is stopped as an abnormality such as a leak occurs. According to the present invention, safe operation can be ensured.
[0018]
【Example】
Next, examples of the present invention will be described in detail below.
18Cr-8Ni stainless steel was melted in an electric furnace, and as shown in FIG. 2, the molten steel 10 was transferred into the refining furnace 12, and a mixed gas of O ₂ and Ar was blown from the tuyere 18 near the bottom in the atmosphere to perform decarburization. It was. At this time, as shown in FIG. 1, the ratio of O ₂ to Ar gas was switched to two stages according to the decrease in C in the molten steel.
In this process, heat is generated due to the reaction between O ₂ and C in the molten steel and the reaction with Cr, and the temperature of the molten steel 10 increases accordingly.
[0019]
When the decarburization process is performed by a conventional method, the temperature of the molten steel 10 is 1740 ° C. at the peak temperature as shown by the broken line B in FIG. (However, the C concentration at the end of the air treatment is lower than 0.3%. For example, 0.15%). In the method of this example, by reducing the total amount of O ₂ blown in the air treatment step, As indicated by A in the figure, the maximum temperature could be lowered to 1710 ° C.
The initial temperature during refining was 1525 ° C., and the amount of C was 1.5%.
[0020]
Next, when the amount of C in the molten steel 10 is reduced to 0.3%, the inside of the refining furnace 12 is sealed with a lid 14 and the inside of the furnace is reduced to 40 Torr through the duct 16 by the steam ejector 20 and the water pump 22. In addition, only Ar gas was blown from the tuyere 18 this time.
The decompression process was performed while measuring the CO concentration and O ₂ concentration of the exhaust gas in the duct 16 and the temperature of the exhaust gas with the measuring device 24.
[0021]
The molten steel 10 and the slag 26 were vigorously stirred by the gas blowing under the reduced pressure, and decarburization and reduction of the chromium oxide proceeded by the reaction between the chromium oxide in the slag 26 and C in the molten steel 10.
The entire reaction at this time was an endothermic reaction, and as a result, the temperature of the molten steel 10 decreased (see FIG. 1).
[0022]
Next, the O ₂ / Ar mixed gas was blown again into the molten steel while maintaining the reduced pressure state. The total amount of O ₂ blown at this time was 100 to 200 Nm ³ . This is an amount corresponding to a decrease in the amount of blown O ₂ gas that has been reduced as compared with the conventional air treatment. That is, blowing amount of O ₂ when viewed through the entire process is such that the conventional method and the same amount, was blowing of O ₂ in the heating step.
[0023]
Chromium is oxidized by the blowing of O ₂ , and the temperature of the molten steel 10 rises again by an exothermic reaction. The ultimate temperature at this time is equal to the temperature at the time of starting the reduction process according to the conventional method.
[0024]
Next, while maintaining the reduced pressure state, the blowing gas was switched to Ar gas alone, Fe—Si was added, the generated chromium oxide was reduced, and then steel was produced through a predetermined process. The temperature at steeling was 1680 ° C.
[0025]
As described above, the method of this example switches from atmospheric treatment to reduced pressure treatment at a relatively early stage, that is, at a stage where the C concentration in the molten steel is relatively high. It can be performed efficiently in a short time.
In addition, despite the fact that the C concentration is relatively high, switching from the atmospheric treatment to the decompression treatment, the exhaust gas analysis can reliably prevent the CO explosion, and in addition, the molten steel temperature is raised again in the decompression treatment process. Since it is heated, the temperature at the time of steel output can be finally set to a desired temperature, and the subsequent casting can be performed satisfactorily.
[0026]
Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be configured in various modifications without departing from the spirit of the present invention.
[Brief description of the drawings]
FIG. 1 is a diagram showing the temperature of a molten steel that changes during the implementation of an embodiment method of the present invention in relation to each step.
FIG. 2 is a view showing a main part of an implementation process of the method together with the apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Molten steel 12 Refining furnace 14 Cover body 16 Duct 18 Tuyere 20 Steam ejector 22 Water pump 24 Measuring apparatus 26 Slag

Claims (1)

In the refining furnace, after the atmosphere treatment to decarburize by injecting oxygen-containing gas into the high chromium-containing molten steel in the atmosphere, the inside of the furnace is switched to a reduced pressure state and decarburization is performed by gas injection. In the method for refining chromium-containing molten steel subjected to the reduced pressure treatment, the switching from the atmospheric treatment to the reduced pressure treatment is performed at a stage where the C concentration in the molten steel is reduced to 0.3 to 0.05% by weight, and the reduced pressure treatment is performed. At that time, the pressure reduction treatment is performed under a reduced pressure of 200 to 15 Torr while performing a gas analysis of the exhaust gas, and after switching to the pressure reduction treatment, a non-oxidizing gas is injected into the molten steel to stir the molten steel and slag. In the molten steel containing oxygen gas, the chromium oxide in the slag is reacted with C in the molten steel to decarburize, and after the decarburizing treatment and before the introduction of the reducing agent for reducing the chromium oxide. Blowing into Te caused the heat generated by the oxidation reaction of chromium, so as to raise the molten steel temperature,
Further, in the gas analysis of the exhaust gas, the CO concentration, the O ₂ concentration and the exhaust gas temperature of the exhaust gas are measured, the risk of CO explosion is predicted by the increase of the O ₂ concentration at that time, and the decompression process is stopped. Method for refining molten steel.

Images