JPH0892627A - Production of stainless steel - Google Patents

Abstract

PURPOSE: To dissolve problem that two furnaces of a smelting reduction furnace and a decarburizing furnace are not required and the reducing rate of chromium only by the reduction of C under an atmospheric pressure is low. CONSTITUTION: At the time of subjecting the rough molten steel of stainless steel to rough carburizing, chromium oxide-contg. slag generated in the final stage of the rough carburizing is added to the molten metal in the rough carburizing or finish decarburizing stage at least partially in the shape of powder, and by carbon in the molten metal or carbon in a carbon material to be added, chromium oxide in the chromium oxide-contg. slag is reduced and is recovered into the molten metal. The chromium oxide-contg. slag is added by an upper blowing method or an injection method.

Description

Detailed Description of the Invention

[0001]

This invention relates to an argon oxygen decarburization furnace (A
(OD furnace), steelmaking furnaces such as converters, hot metal, scrap,
The present invention relates to a method for producing stainless steel through a rough decarburizing process → finishing decarburizing process → reducing process using ferroalloy or the like.

[0002]

2. Description of the Related Art Conventionally, the most typical process for melting stainless steel is scrap, Fe--Cr, Fe--
Using ferroalloys such as Ni as the main raw material, melting them in an electric furnace and performing crude decarburization by blowing oxygen, then performing final decarburization and reduction refining in an AOD furnace or VOD furnace, etc.
The molten steel is cleaned and the temperature is controlled by blowing Ar in a steel receiving pot, and then it is put into a continuous casting machine.

Further, as described in the literature (Iron and Steel, 1985, vol.71, 180), the hot metal was charged into the bottom-blown converter without using an electric furnace, and the molten iron was removed so that it became a component of stainless steel. Scrap or ferroalloy is added as a predetermined component during or before charcoal blowing, and after the coarse decarburizing and finish decarburizing steps are finished, ferroalloy such as Fe-Si is added to shift to the reducing step. After that, there is also a process of tapping and continuously casting.

In addition, there is also a stainless steel melting process using chromium ore, for example, in the literature (Iron and Steel, 1985,
(Vol.71, 1072), the hot metal was charged into the AOD furnace, and then chromium ore and coke were charged to carry out so-called smelting reduction, after which slag was removed, and then AOD The furnace performs decarburization refining consisting of ordinary rough decarburization, finish decarburization and reduction. However, these conventional methods have the following problems.

(1) Since a large amount of Si (in many cases Fe-Si) is added, the cost becomes high. (2) Since SiO ₂ is generated as a reaction product, a large amount of CaO is needed to neutralize it. As a result, a large amount of slag is generated. (3) Since the reduction reaction of chromium oxide with silicon is an exothermic reaction, the temperature rises and the slag has a high fluidity, so that it erodes refractory materials.

Therefore, in Japanese Examined Patent Publication No. 38806/1992, in order to solve such a problem, a smelting reduction furnace and a decarburization furnace are prepared, and the chromium oxide-containing slag recovered at the final decarburization stage of the crude stainless steel melt is melted. It proposes a process of returning to the reduction furnace and reducing and recovering the chromium content.

According to this method, the reduction period using Fe-Si can be omitted, so that the above problems are solved. Further, when the chromium oxide-containing slag remaining in the decarburizing furnace is reduced and recovered by C in the next charge of molten stainless steel, it is possible if the C in the molten metal is 5% and 1500 ° C or higher.

[0008]

[Problems to be Solved by the Invention] However, Japanese Patent Publication No. 4-38
The method disclosed in Japanese Patent No. 806 has a problem that it cannot be realized without having two furnaces, a smelting reduction furnace and a decarburization furnace. When the chromium-containing slag remaining in the decarburization furnace is reduced and recovered by C in the next hot metal, Cr ₂ O ₃
There is a problem that the reduction rate is not sufficient.

[0009] On the other hand, Japanese Patent Application No.
In −146167, the slag at the end of decarburization was recycled to the same furnace to recover and recover the chromium oxide in the slag with [C] when the decarburization temperature of the next charge crude molten metal was increased, and the Si-containing alloy was added at the end of the reduction. A process to improve chromium recovery is proposed.

However, even with this method, Cr ₂ O ₃ in the slag is
Although C is reduced by C, there is a problem that the C reduction rate is slow and the Cr ₂ O ₃ reduction rate is not sufficient by simply adding the decarburization end-stage slag into the furnace and reducing it by C.

That is, the reduction temperature (15
Even if it is necessary to raise the temperature immediately after the start of blowing, the hot metal temperature at the time of pouring is at most about 1300 ° C, and chromium oxide can be added before or immediately after the start of blowing. This is because, when the contained slag is added, the heating time to the reduction temperature (1500 to 1600 ° C) becomes long due to the heat removal by adding the slag.

[0012] Further, since ordinary stainless crude molten steel decarburization is processed under atmospheric pressure, the reducing power of carbon is weak and there is a problem in reducing chromium oxide. Therefore, it is necessary to reduce the chromium oxide that has not been reduced by carbon with the Si-containing alloy, and the initial purpose of reducing the Si alloy for reduction may not be sufficiently achieved.

Here, the object of the present invention is (1) it is not necessary to have two furnaces, a smelting reduction furnace and a decarburization furnace, and (2) C at atmospheric pressure.
An object of the present invention is to provide a method for producing stainless steel, which can solve the problem that the chromium reduction rate due to only reduction is low.

[0014]

The inventors of the present invention have made various studies in order to achieve the technical problems and have obtained the following findings. When recycling and reusing slag containing chromium oxide,
The addition as a bulk causes an excessive decrease in temperature of the molten metal, and it takes time for the temperature to rise.

The addition as a powder significantly improves the reaction rate as compared with the case where it is added as a bulk. The powder can be added in a vacuum degassing furnace. It is possible to significantly reduce the compounding amount of Fe-Si for reduction.

Therefore, the gist of the present invention resides in that in the method for producing stainless steel produced through the rough decarburizing step, the final decarburizing step, and the reducing step, when the crude molten steel of the stainless steel is roughly decarburized. Chromium oxide-containing slag produced at the final stage of rough decarburization or final decarburization is added to the molten metal in the rough decarburization or final decarburization step in the form of at least a part of powder, and the carbon in the molten metal or the added carbon is added. A method for producing a stainless steel, characterized in that chromium in a chromium oxide-containing slag is reduced by carbon in a raw material and recovered in a molten metal.

According to a preferred embodiment of the present invention, the chromium oxide-containing slag in the form of powder may be added by a top blowing method or an injection method. According to another preferred embodiment, the chromium oxide-containing slag in the form of powder is
For the molten metal in the vacuum degassing refining furnace in the finishing decarburization process,
It may be added by the top blowing method or the injection method.

[0018]

Next, the operation of the present invention will be described more specifically. Reusing the slag containing chromium oxide generated when decarburizing the crude molten steel of stainless steel during decarburization after the next charge is to remove chromium oxide by the carbon in the molten metal or carbon material such as coke. Reduce and metal
It is necessary to omit the addition of Si or reduce the amount used.

In this case, in order to reduce Cr ₂ O ₃ in the final decarburizing slag with C in the metal, the reduction temperature at which the slag becomes slag is reduced.
(1500 to 1600 ℃) It is necessary to raise the temperature immediately after the start of blowing. However, as mentioned above, the hot metal temperature at the time of pouring is at most about 1300 ° C, and if chromium oxide-containing slag is added before or immediately after the start of blowing, the reduction temperature (1500 to 1600 It takes a long time to heat up to (℃).

Since the reduction does not proceed so much at this temperature rise, it is not necessary that a large amount of chromium oxide-containing slag is present in the furnace. Further, if a large amount of chromium oxide-containing slag is added after the temperature has been raised to the reduction temperature, the slag serves as a cooling agent and the temperature is rapidly lowered, so that the reduction temperature cannot be maintained.

However, since the molten steel temperature after roughly decarburizing stainless molten steel in a converter or an AOD furnace is usually 1500 ° C. or higher, immediately after adding chromium oxide-containing slag in a vacuum degassing furnace after rough decarburizing, The carbon in the molten steel makes it possible to reduce the chromium oxide in the slag. Further, in a vacuum degassing furnace, since decarburization is carried out under vacuum, the reducing power of carbon in molten steel is stronger than under atmospheric pressure, which is advantageous for reduction of chromium oxide contained in slag.

Therefore, in order to increase the temperature rising rate up to the reduction temperature and maintain the temperature even after the reduction temperature is reached, it is necessary to add the chromium oxide-containing slag continuously or in a divided manner. It is preferably added to the melt in the form of at least a part of powder.

Here, the following three methods can be adopted as the method of addition to the decarburizing furnace. (1) Chromium oxide-containing slag is continuously or dividedly supplied to the molten metal from a slag supply device installed at the upper part of the decarburization furnace. This method is the simplest in terms of equipment.

(2) The chromium oxide-containing slag is recovered and crushed, and the slag powder obtained by crushing is slag powder from the top blowing lance.
Oxygen gas or inert gas is used as a carrier to supply the molten metal.

In this method, since the slag powder penetrates into the molten steel by the blowing from the upper blowing lance, the efficiency is higher than the method of adding from the upper part of the decarburizing furnace. If a mixed gas of oxygen gas or an inert gas is used as the carrier gas, a high-temperature reaction hot spot region where oxygen gas reacts with carbon in the molten metal to generate carbon monoxide is generated, and slag powder is generated there. Supplying promotes slag slagging / melting, thus improving the reduction rate.

(3) Chromium oxide-containing slag is recovered and crushed, and the slag powder obtained by crushing is used as a carrier gas using oxygen gas or inert gas as carrier gas from tuyeres installed on the side and bottom of the decarburizing furnace. Top blowing or injection.

This method can bring about the same effect as the above (2). Further, when the oxygen gas is injected as the carrier gas, the slag powder has a cooling effect in the vicinity of the tuyere, so that the life of the tuyere can be improved at the same time. That is, in the usual oxygen injection tuyere, the blown oxygen causes an exothermic reaction with carbon, iron or chromium in the molten metal, whereas in this method the powder itself acts as a cooling agent and the chromium oxide reduction is an endothermic reaction. This is because the tuyere cooling effect doubles. The chromium oxide-containing slag thus recovered may be added during any of the above-mentioned decarburization periods, that is, at the time of so-called decarburization start or finish decarburization during vacuum degassing.

Here, in the present invention, the time for recovering the chromium oxide-containing slag is not particularly limited as long as it is the time for containing Cr oxide in the slag, but it is generally the decarburization end stage, that is, the final decarburization end stage. When performing vacuum decarburization treatment, for example, the final stage of rough decarburization in a converter. Next, the effects of the present invention will be described more specifically based on examples with reference to comparative examples.

(Comparative Example) In this example, 65 tons of dephosphorized pig iron was charged into an upper-bottom blowing converter, and rough decarburization and finish decarburization were performed.
After further reduction, the steel was tapped into a ladle, and then stainless steel was produced by continuous casting.

At this time, separately, the slag (crude: T.Cr = 20%, T.Fe = 3) recovered at the end of the rough decarburization period of stainless steel.
%, CaO / SiO ₂ = 1.4, MgO = 5%, Al ₂ O ₃ <10%), and the chromium oxide-containing slag 6000 kg, coke 4000
kg and quick lime 574 kg were added all at once in the furnace at the start of the rough decarburization process in the above manufacturing process, while bottom-blown Ar gas was blown at 17 Nm ³ / min and oxygen from the top-blown lance at 146 Nm ³ / min. , 41
Blowing for 1 minute was performed to perform rough decarburization and finish decarburization.
After that, 243 kg of Fe-Si (composition: Si = 75%, balance Fe) was added, and reduction treatment was performed.

The temperature of the crude decarburized molten metal after the addition of Fe-Si is 1632.
C, the slag basicity was 1.5, and [% Cr] = 1.65%. At the end of decarburization, the metal [% C] was 1%. After that, the slag is discharged and ferrochrome (composition: Cr = 60%, Si = 2.
21% of 7%, C = 6%, the balance of Fe) and 1.8 ton of quick lime were added for finish decarburization treatment. After this decarburization treatment, the temperature is 1700 ℃, the slag basicity is 1.5, and [% Cr] = 13%.
Met.

(Embodiment 1) In this embodiment as well, in the same manner as in the comparative example, 65 tons of dephosphorized iron was charged into an upper-bottom blowing converter to perform rough decarburization,
After finishing decarburization and reduction, tap steel in a ladle,
Stainless steel was then produced by continuous casting.

In this example, separately, the slag collected at the end of the rough decarburization period of stainless steel (crude: T.Cr = 20%,
T.Fe = 3%, CaO / SiO ₂ = 1.4, MgO = 5%, Al ₂ O ₃ <10
%), 3000 kg of the chromium oxide-containing slag, 4000 kg of coke, and 570 kg of quick lime were added into the furnace, and the mixture was stirred with Ar gas (flow rate 17 Nm ³ / min) blown from the bottom blowing tuyere. On the other hand, oxygen (flow rate 146 Nm ³ / min) was also blown from the top blowing lance to stir the molten metal, and the oxygen from the top blowing lance was used as the carrier gas to crush the chromium oxide-containing slag and to decarburize the slag at the end. While supplying (66.7kg / min)
, Blowing for 45 minutes. Then, 200 kg of Fe-Si was added.

The temperature after the addition of Fe-Si to the molten steel that has been roughly decarburized is
At 1635 ° C, the slag basicity was 1.5, and [% Cr] = 1.67%. At the end of decarburization, the metal [% C] was 1%. After that, the slag is discharged and 21 tons of ferrochrome and quicklime are added.
A final decarburization treatment was performed by adding 1.8 tons. The temperature after this finishing decarburization treatment is 1705 ℃, the slag basicity is 1.5, and [%
Cr] was 13%. Table 1 shows the refining specifications.

[0035]

[Table 1]

(Embodiment 2) In this embodiment, 80 tons of crude molten steel that has undergone rough decarburization of stainless steel in a converter is tapped into a ladle,
A stainless steel was manufactured by performing a final decarburization treatment under vacuum in a vacuum degassing furnace (a VOD furnace was used in this example).

At this time, separately prepared slag (composition: T.Cr = 20%, T.Fe = T.Fe =
3%, CaO / SiO ₂ = 1.4, MgO = 5%, Al ₂ O ₃ <10%) 60
After adding 00 kg to the above vacuum degassing furnace, it was decarburized under vacuum.

When the VOD furnace arrived, the molten steel temperature was 1620 ° C and the chromium concentration in the molten steel was 11.6%, but during the vacuum decarburization treatment in the VOD furnace
Reduction of chromium oxide (oxygen flow rate 20 Nm ³ / min, acid transfer time 30 minutes) increased to 12.7% after vacuum decarburization.

Thereafter, when Fe-Si was added to the slag to reduce almost all the chromium oxide in the slag, the chromium concentration increased to 13.1%, and then the normal treatment was performed to continuously cast the slag. At this time, the amount of Fe-Si added in the VOD furnace is the same as in the conventional method, but Fe-Si in the converter becomes undeoxidized steel.
The amount added could be reduced to zero.

(Embodiment 3) Also in this embodiment, as in Embodiment 2, 80 tons of crude molten steel after rough decarburization of stainless steel in the converter was tapped into a ladle and subjected to finish decarburization treatment under vacuum. . When the VOD furnace arrived, the molten steel temperature was 1620 ° C and the chromium concentration in the molten steel was 11.6%, but during vacuum decarburization in the VOD furnace (oxygen flow rate 20 Nm ³ / min, acid transfer time 30 minutes), the stainless steel in the converter was Slag recovered at the end of crude steel decarburization (composition: T.Cr = 20%, T.Fe = 3%, CaO / SiO ₂
= 1.4, MgO = 5%, Al ₂ O ₃ <10%) 6000 kg of pulverized material was blasted with argon gas as a carrier gas from an upper lance different from the oxygen lance.

Due to the reduction of chromium oxide during the vacuum degassing process, it increased to 12.9% after vacuum decarburization. After that, when Fe-Si was added to the molten steel to reduce almost all the chromium oxide in the slag, the chromium concentration increased to 13.1%, after which normal treatment was performed and continuous casting was performed. At this time, in the VOD furnace
Although the amount of Fe-Si added was the same as that of the conventional method, the amount of Fe-Si added in the converter could be reduced to zero because undeoxidized steel was produced in the converter.

(Embodiment 4) In this embodiment as well, in the same manner as in the comparative example, 65 tons of dephosphorized iron was charged into the upper-bottom blowing converter, and rough decarburization,
After finishing decarburization and reduction, tap steel in a ladle,
Stainless steel was then produced by continuous casting.

In this example, separately, the slag (crude: T.Cr = 20%,
T.Fe = 3%, CaO / SiO ₂ = 1.4, MgO = 5%, Al ₂ O ₃ <10
%), Ar gas blown from the bottom blowing tuyere (flow rate 17N
While stirring the molten metal at (m ³ / min), oxygen (flow rate 146 Nm ³ / min) was also blown from this top blowing lance to perform blowing for 45 minutes. Then, 200 kg of Fe-Si was added.

During this blowing, the above-mentioned chromium oxide-containing slag is used.
6000 kg, 4000 kg of coke, and 570 kg of quick lime were added to the furnace in four equal portions before blowing, 10 minutes after starting blowing, 20 minutes after starting blowing, and 30 minutes after starting blowing.

The temperature after adding Fe--Si to the molten steel that has been roughly decarburized is
At 1630 ° C, the slag basicity was 1.5, and [% Cr] = 1.65%. At the end of decarburization, the metal [% C] was 1%. After that, the slag is discharged and 21 tons of ferrochrome and quicklime are added.
A final decarburization treatment was performed by adding 1.8 tons. The temperature after this finishing decarburization treatment is 1700 ℃, the slag basicity is 1.5, [%
Cr] was 13%.

[0046]

As described above, according to the method of the present invention, since the chromium oxide-containing slag is crushed and added, the reduction rate is maintained at a high value when the chromium oxide is reduced during decarburization. It is possible to shorten the reduction time and suppress the melting loss of the refractory.

Claims (3)

[Claims] 1. A method for producing stainless steel produced through a rough decarburizing step, a finish decarburizing step, and a reducing step, wherein when crude molten steel of stainless steel is roughly decarburized, the final decarburizing step or the final decarburizing step is performed. The chromium oxide-containing slag generated during the cooling stage is added to the molten metal in the rough decarburization or finish decarburization step in the form of at least a part of powder, and the chromium oxide-containing slag is added by the carbon in the molten metal or the carbon in the added carbon material. A method for producing stainless steel, characterized in that chromium oxide therein is reduced and recovered in a molten metal. 2. The method for producing stainless steel according to claim 1, wherein the chromium oxide-containing slag in the form of powder is added by a top blowing method or an injection method. 3. The chromium oxide-containing slag in the form of powder is added to the molten metal in the vacuum degassing and refining furnace in the final decarburizing step by a top blowing method or an injection method. A method for producing the described stainless steel.