JP2002020816A - Method for producing low nitrogen-containing chromium steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for refining low nitrogen-containing chromium steel by which, at the time of recovering chromium in slag into molten steel, the use of expensive reducing agents such as FeSi can be suppressed, the amount of required flux to be used is reduced, the using amount of slag generated at the time of refining is suppressed, and further, the amount of expensive Ar required for reducing nitrogen can be suppressed. SOLUTION: Coarse decarburization is performed to a C content of 0.005 to 1.0 mass% under the atmospheric pressure by a refining furnace, the obtained chromium-containing molten steel is tapped in a state of being unoxidized or half-oxidized, next, vacuum degassing treatment is started under the conditions of Total (0) in the steel >=150 ppm and >=5 mass% Cr2O3 in the slag by a refining furnace under the reduced pressure, and reduction for Cr2O3 in the slag is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a low-nitrogen chromium-containing steel, in particular to suppress the use of an expensive reducing agent such as FeSi, a necessary solvent medium and an expensive Ar necessary for reducing the nitrogen. The present invention relates to a method for producing a low-nitrogen chromium-containing steel capable of suppressing the use amount of chromium.

[0002]

2. Description of the Related Art Conventionally, as a method of melting chromium-containing molten steel, molten iron obtained by charging chromium alloy iron such as ferrochrome into molten iron from a blast furnace or scrap containing chromium is melted in an electric furnace. There is a method of refining the obtained hot metal in a refining furnace such as a converter or AOD. At this time, the carbon level is refined by blowing oxygen, and the nitrogen level is reduced to a required concentration level by degassing with Ar or the like. In addition, VOD is used if necessary.

In any of the methods, at the time of the decarburization reaction, chromium burns and Cr ₂ O ₃ is generated at the end of the blowing acid, so that a reducing agent such as FeSi is generally used for the reduction. However, since a reducing agent such as FeSi is expensive, various proposals have been made to reduce it even in the past.

For example, as a measure to prevent chromium combustion, that is, to improve the decarbonization efficiency, Japanese Patent Application Laid-Open No. 7-173515 discloses that the degassing contains an Al ₂ O ₃ concentration of about 10 to 20% to reduce the liquid phase ratio of slag. And promoting activation of Cr ₂ O ₃ in the slag to promote decarburization with Cr ₂ O ₃ .

[0005] JP-A-9-59708 and JP-A-9-3517
In the publication, before adding chromium alloy iron such as ferrochrome, [% C] in molten steel is set to 2.5 to 4.0%, and the molten steel temperature is set to 1450 to 1600.
Acceleration of the decarburization reaction, provided that the temperature is raised to
A method for reducing the generation of Cr ₂ O ₃ is disclosed.

Further, as measures for reducing the concentration of the generated Cr ₂ O ₃ , Japanese Patent Application Laid-Open Nos. 8-157937 and 8-53706 regulate the amount of blowing gas and regulate the stirring strength of the generated slag and molten steel. Discloses a method for decarburizing Cr ₂ O ₃ in slag.

However, in any of the methods, it is difficult to suppress the amount of generated Cr ₂ O _{3 when} reacting at atmospheric pressure, and in low N steel, a large amount of Ar is required due to slag-metal stirring. You need to use

In the invention disclosed in Japanese Patent Application Laid-Open No. 56-139614, the use of a reducing agent is specified in a VOD under reduced pressure without using a reducing agent in a smelting furnace operated under atmospheric pressure. The components required for the low-nitrogen steel described below are not regulated.

For the purpose of denitrification, the use of a large amount of Ar for blowing is required for low nitrogen steel. Furthermore, since the steel is exposed to the atmosphere during tapping from the smelting furnace to the ladle, nitrogen absorption during tapping is also a problem.

As a countermeasure, a refining method using vacuum equipment such as VOD has been proposed. However, decarburization in high coal areas, such as decarburization of blast furnace hot metal, involves problems such as blowing up, so it is necessary to use a converter or AOD.
The fundamental problem has not been solved.

[0011]

The object of the present invention is to
[N]: To develop a cheaper and more efficient method for producing low nitrogen chromium-containing steel limited to 200 ppm or less.

More specifically, an object of the present invention is to recover chromium in slag, which is a valuable metal oxidized by oxygen blown during decarburization, in molten steel in refining of low nitrogen chromium steel. The use of expensive reducing agents such as FeSi can be suppressed, the required amount of solvent used is reduced, the amount of slag generated during refining is suppressed, and the amount of expensive Ar required for low nitrogen is suppressed. It is to develop a refining method of low nitrogen chromium-containing steel that can be used.

[0013]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have made various studies and focused on the following points.

[0014] In order to reduce the use of the reducing agent,
Cr ₂ O ₃ reduction is required, but when considered from the generated reaction _{2Cr + 3/2 O 2 →} Cr 2 O 3 in Cr ₂ O _3, the reduction of the oxygen partial pressure is effective.

[0015] However, there is a limit to the reduction under atmospheric pressure. Also, in the production of low nitrogen steel, it was necessary to use expensive Ar for blowing as a stirring gas. In addition, in the use of vacuum equipment, in the high carbon region during the decarburization period, problems such as blowing up occur, so that it is inevitable that the treatment time is prolonged due to a decrease in the acid feed rate and the like.

Here, the present inventor has noticed that the amount of nitrogen absorption in an unreduced state is extremely small, and in a high carbon region, roughly removes carbon by a converter capable of decarburizing under atmospheric pressure or a refining furnace such as AOD. After carrying out charcoal and decarburizing to a predetermined range, transfer to a vacuum facility without reduction, adjust the slag composition there, then blow in Ar gas for stirring, and add Cr _By reducing ₂ O ₃ , [N]: 200 ppm or less, preferably [N]: 100 ppm or less, even in the production of low-nitrogen steel, reducing the amount of reducing agent such as FeSi and the amount of Ar used for denitrification. Knowing that a significant reduction is possible, the present invention has been completed.

Therefore, the present invention is as follows. (1) A method for producing a low-nitrogen chromium-containing steel comprising a refining process under atmospheric pressure and a refining process under reduced pressure, wherein the C content is 0.005 to 1.0 in a refining furnace placed under atmospheric pressure. Mass de-carburization, and the resulting chromium-containing molten steel is tapped in an undeoxidized or semi-deoxidized state, and then, in a refining furnace placed under reduced pressure, Total [O] ≧ 150 ppm in the steel. and start the vacuum degassing treatment at a Cr ₂ O ₃ 5 wt% or more conditions in the slag, the production method of low nitrogen-containing chromium steel which is characterized in that the reduction of Cr ₂ O ₃ in the slag. (2) The basicity of the slag composition after crude decarburization is 1.0 ≤ CaO / SiO ₂ ≤
The method for producing a low-nitrogen chromium-containing steel according to the above (1), which is 4.0. (3) Slag composition during rough decarburization treatment is CaO + SiO ₂ + MgO ≧ 5
The method for producing a low-nitrogen chromium-containing steel according to the above (1) or (2), which is mass%.

[0018]

As described above, according to the present invention, in a high carbon region, refining is performed under atmospheric pressure, transferred to a vacuum facility without reduction, and reduced under vacuum.

The reason why the respective processing conditions are limited as described above in the present invention will be described. In the present invention, the rough decarburization treatment under atmospheric pressure may be performed by conventional Cr-containing hot metal, and there is no particular limitation. The rough decarburization treatment itself may be performed according to a conventional method.

In the present invention, such a rough decarburization treatment is performed until the C content in the molten steel becomes 0.005 to 1.0% by mass.
When decarburization under atmospheric pressure is completed in a carbon region higher than C: 1.0% by mass, decarburization is performed to C: 1.0% by mass or less because blow-up of molten steel during vacuum processing cannot be prevented.

On the other hand, according to the present invention, in the denitrification in the vacuum processing equipment after the rough decarburization, the generation of CO gas to promote the denitrification leads to a reduction in the processing time. Therefore, for example, in a steel grade of [% N] ≦ 200ppm in steel after vacuum processing,
It is necessary to leave some carbon in steel after rough decarburization. In the present invention, the lower limit at this time is defined as C: 0.005%. Preferably, the crude decarburization treatment is completed within the range of C: 0.1 to 1.0%.

In order to promote denitrification, the generation of CO gas leads to a reduction in processing time. However, as an oxygen source for generating CO gas, total [O] ≧ 150 ppm in steel and Cr in slag are used.
_By setting ₂ O ₃ ≧ 5%, preferably Cr ₂ O ₃ ≧ 15%, the unit oxygen consumption in the vacuum degassing device can be reduced.
A preferred total [O] in steel is 150-300 ppm.

According to a preferred embodiment of the present invention, the slag composition after the above-mentioned crude decarburization is adjusted to 1.0 ≦ CaO / SiO ₂ ≦
4.0. Preferably, 1.0 ≦ CaO / SiO ₂ ≦ 3.0.
This slag needs to be quickly turned into slag in order to shorten the reduction processing time, but defines the range.
In addition, desulfurization of chromium-containing steel, especially stainless steel, is carried out during the reduction period.Conventionally, two reduction treatments (desulfurization treatment) were carried out after rough decarburization, vacuum treatment, but the present invention Since (desulfurization) is performed only once, the lower limit of the basicity is set to 1.0 from the viewpoint of securing the desulfurization ability.

In another preferred embodiment of the present invention, in the slag after the rough decarburization, CaO + SiO ₂ + MgO ≧ 5% is specified, which is for protecting the furnace body of the decarburization refining furnace. A lower limit has been set. Since the temperature of molten steel of the crude decarburized slag decreases due to the heat of reduction of Cr ₂ O ₃ , all or part of the crude decarburized slag may be discharged, or the entire amount may be carried over as a source of CO gas to carry the crude molten steel. In addition, vacuum treatment may be performed.

In some cases, by adding a slag slagging agent such as an Al or Si-containing substance or fluorite, it becomes possible to make the crude decarburized slag into a slag. Can be implemented.

In the present invention, vacuum degassing is performed following the above-described rough decarburization, but the total
As long as the condition of [O] ≧ 150 ppm and Cr ₂ O ₃ in slag ≧ 5% by mass is satisfied, the slag basicity is more preferably 1.0 to 1.0%.
As long as it is 4.0, the vacuum degassing process may be the same as the conventional one using a VOD furnace, for example, and is not particularly limited.

[0027]

[Example] An AOD furnace was used as a refining furnace, and VOD was used as a vacuum processing apparatus. In this example, hot metal having the following composition was prepared, and firstly, crude decarburization was performed using AOD under atmospheric pressure, and then vacuum degassing was performed using a VOD furnace with a vacuum of 4 Torr to obtain a low nitrogen chromium-containing material. Manufacture of steel.

The total processing time of the rough decarburization and vacuum degassing is approximately 100 to 200 minutes, which is substantially the same as the conventional example. Table 1 shows the component composition after the crude decarburization.

[0029]

[Table 1] Results of C content, slag composition, degree of furnace body damage, degree of slag slag, addition of FeSi as a reducing material, amount of Ar gas injected, and amount of oxygen supply after completion of rough decarburization treatment Are summarized in Table 2. The amount of FeSi added, the amount of Ar gas blown, and the amount of oxygen supply were the same as in the conventional example.
It is indicated by an index with No. 1 as 100.

In Table 2, the furnace body erosion is indicated by a three-point scale of ：: molten steel, Δ: normal, ×: deterioration. Also, slag slag formation is indicated by a three-stage evaluation of ○: good slag formation, Δ: normal slag formation, and X: poor slag formation.

[0031]

[Table 2]

[0032]

According to the present invention, N <200 ppm
In the smelting of low-nitrogen chromium-containing steel, it is possible to reduce FeSi and Ar basic units without deteriorating the furnace body and the amount of slag slag (recovery of Cr), thereby greatly reducing the production cost.

Claims (3)

[Claims] 1. A method for producing a low-nitrogen chromium-containing steel comprising a refining step under atmospheric pressure and a refining step under reduced pressure,
Crude decarburization is performed in a refining furnace placed under atmospheric pressure to a C content of 0.005 to 1.0 mass%, and the resulting chromium-containing molten steel is undeoxidized or semi-deoxidized, and then depressurized. In the refining furnace placed below, Total [O] ≧ 150ppm in steel and Cr in slag
Start vacuum degassing under the condition of ₂ O ₃ 5% by mass or more,
A method for producing a low-nitrogen chromium-containing steel, comprising reducing Cr ₂ O _{3 in} slag. 2. The basicity of the slag composition after the crude decarburization is 1.0 ≦ Ca
_{2. The} method for producing a low nitrogen chromium-containing steel according to claim 1, wherein O / SiO ₂ ≦ 4.0. 3. The slag composition during the rough decarburization treatment is CaO + SiO ₂ +
The method for producing a low-nitrogen chromium-containing steel according to claim 1, wherein MgO ≧ 5% by mass.