JPH08325620A - Stainless steel manufacturing method - Google Patents

Abstract

(57) [Abstract] [Purpose] (1) It is not necessary to have two furnaces, a smelting reduction furnace and a decarburization furnace, and (2) a stainless steel that can efficiently reduce Cr ₂ O ₃ by C. A manufacturing method is provided. [Composition] A part or all of the chromium oxide-containing slag generated when decarburizing the crude molten metal of stainless steel is left in the furnace or added during the coarse decarburizing temperature rising blowing of the decarburizing furnace to After the chromium oxide in the slag is reduced by carbon, the carbon in the carbon-containing material such as coke, char, or anthracite added during rough decarburization and recovered in the molten metal, the Si-containing alloy or
In the method of reducing and desulfurizing Cr ₂ O ₃ by adding an Al-containing alloy, in the slag as a melting point depressant (% B ₂ O ₃ ) = 0.5 to 15% during the decarburization period and / or the coarse decarburization period. So that boron oxide is added.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an argon oxygen decarburization furnace,
The present invention relates to a method for producing stainless steel using hot metal, scrap, ferroalloy, etc. in a steelmaking furnace such as a converter.

[0002]

[Prior art]

[General Prior Art] The most typical process for smelting stainless steel is to melt scrap or Fe-Cr, Fe-Ni or other alloyed iron as a main raw material in an electric furnace and then melt it in an AOD furnace or a VOD furnace. Decarburization and reduction refining are carried out, etc., after the steel is tapped, Ar is blown into the steel receiving pan to clean the molten steel and control the temperature, and then it is applied to a continuous casting machine.

Further, as described in the literature (Iron and Steel, 1985, vol.71, 180), the hot metal is charged into a bottom blowing converter without using an electric furnace, and decarburization is blown so that it becomes a component of stainless steel. Add scrap or ferroalloy to a predetermined component during smelting or before smelting, and after the decarburization process, enter ferroalloy such as Fe-Si and move to the reduction process, and then tap steel to continue. There is also a casting process.

There are other stainless steel melting processes using chromium ore. For example, literature (Iron and Steel, 1985, vol.7
In 1,1072), the hot metal was charged into the AOD furnace, and then the chromium ore and coke were charged to perform so-called smelting reduction,
This is a method in which slag is removed and then ordinary decarburization refining is performed.

However, the above conventional method has the following problems. (1) Addition of a large amount of Si (often Fe-Si) increases the cost. (2) Since SiO ₂ is generated as a reaction product, a large amount of CaO is required 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.

[Prior Art Close to the Present Invention]
38806 proposes a process of returning chromium-containing slag in the final stage of decarburization of crude stainless steel to a smelting reduction furnace to reduce and recover the chromium content. As a result, the reduction period using Fe-Si can be omitted, and the above problems are solved. Further, when the chromium-containing slag remaining in the decarburizing furnace is reduced and recovered by C in the next molten stainless steel, it is possible if the C in the molten metal is 5% and 1500 ° C or higher.

[0007]

Certainly, according to the above method, the chromium-containing slag in the final stage of decarburization of the crude stainless steel melt is returned to the smelting reduction furnace to reduce and recover the chromium content. The reduction period using Si can be omitted.

However, the method disclosed in Japanese Examined Patent Publication No. 38806/1992 cannot be realized without having two furnaces, a smelting reduction furnace and a decarburization furnace. Further, when the chromium-containing slag is reduced and recovered with C, there is a problem that the Cr ₂ O ₃ reduction rate by C in the molten metal is not sufficient. Further, when the final decarburized slag is discharged and collected outside the furnace for recycling, the collected slag may be pulverized and scattered into the atmosphere, which may cause an environmental pollution problem.

By the way, the applicant of the present patent is
As No. 146167, the slag at the end of decarburization is recycled to the same furnace, and the chromium oxide in the slag is reduced and recovered with [C] when the decarburization temperature of the next molten crude metal is raised. A process to improve the recovery rate is proposed.

According to this method, Cr ₂ O ₃ in the slag is reduced by C. However, if the end-stage decarburization slag is simply added to the furnace and reduced by C, it is possible to use the method of Japanese Patent Publication No. 38806/1992.
No. As with the size of the publication disclosed, the reduction rate is slow Cr ₂ O ₃ reduction ratio of Cr ₂ O ₃ in C there is a problem that not enough.

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 heat rising time up to the reduction temperature (1500 to 1600 ° C) becomes long due to the heat removal caused by the addition of the slag.

Therefore, it is necessary to lower the melting point of the slag and start the smelting reduction from a lower temperature. Chromium oxide, which could not be reduced by C otherwise, was added to the large amount of Si.
It is necessary to reduce with the contained alloy, and there is a possibility that the initial purpose of reduction of reducing Si alloy cannot be fully 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) Cr ₂ O ₃ by C.
It is an object of the present invention to provide a method for producing stainless steel, which can efficiently reduce the above.

[0014]

[Means for Solving the Problems] The inventors of the present invention have conducted various studies in order to solve such problems, and in the decarburizing period or the coarse decarburizing period, in the slag (% B ₂ O ₃ ) = 0.5 to 15% It was found that the efficient reduction of chromium oxide by C is realized by adding the following boron oxide as a melting point depressant, and the present invention has been completed.

Therefore, the gist of the present invention lies in that a part or all of the chromium oxide-containing slag produced when decarburizing the crude molten stainless steel in the decarburizing furnace is left in the decarburizing furnace. Alternatively, the slag is discharged, and this slag is added to the molten metal in the furnace during the coarse decarburizing temperature rising blowing of the decarburizing furnace with a different charge, carbon in the molten metal, or the carbon-containing material added during the rough decarburizing
The carbon in the coke, char, or anthracite) reduces the chromium oxide in the slag and collects it in the molten metal.
A crude decarburization period and / or decarburization period in a method of decarburizing while adding Cr-containing alloy or Al-containing alloy to reduce and desulfurize Cr ₂ O ₃ , and then adding Cr-containing alloy Is a method for producing stainless steel, characterized in that boron oxide is added so that (% B ₂ O ₃ ) in the slag becomes 0.5 to 15%.

[0016]

Now, a method for manufacturing stainless steel to which the present invention is applied will be described with reference to FIG.

First, in the preparatory step, for example, the slag containing chromium oxide that has been recycled is added to the crude stainless steel melt prepared in advance, and the rough decarburizing / heating step is performed. At this time, the chromium oxide in the slag is reduced by the carbon in the molten metal or the carbon in the carbon-containing material added during the rough decarburization,
After that, Si content and the like are added in the finishing reduction and desulfurization steps.
Then, after the slag process, a finishing decarburization process is performed and steel is tapped. Here, according to the present invention, boron oxide is added to the slag in the above-described decarburization step and / or rough decarburization step.

As is clear from the explanation of the above process diagrams, the slag containing chromium oxide produced when decarburizing the crude molten steel of stainless steel cannot be recycled for the rough decarburization after the next charge. , Carbon in molten metal or carbon-containing material such as coke reduces chromium oxide,
It is necessary to omit the addition of or to reduce the amount used.

There are two methods of slag recycling: discharging and collecting, then recharging, or leaving in the furnace. In the former case, since the melting point of Cr ₂ O ₃ -containing slag at the final stage of decarburization is high and the fluidity is extremely low, it is difficult to discharge the slag outside the furnace as it is.

Therefore, boron oxide in the slag (% B
₂ O ₃ ) = 0.5 to 15% added so that slag [composition:
(CaO) / (SiO ₂ ) = 1.0 to 2.5, 50%> (% Cr ₂ O ₃ )> 10%] If the fluidity is improved, most of the final decarburization slag can be easily discharged out of the furnace. can do. Therefore, (% B ₂ O ₃ )
The reason for limiting the addition timing and amount as described above is as follows.

If (% B ₂ O ₃ ) is less than 0.5%, the change in slag fluidity is small, and no change in the slag discharge rate is observed. (% B ₂ O ₃₎ When more than 15% of the slag discharge amount (% B ₂
The effect of O ₃ ) is almost eliminated, and the addition of a melting point depressant in excess of 15% is economically disadvantageous.

Further, adding B ₂ O _{3 in an amount} of 0.5% or more to the slag is effective for suppressing pulverization of the Cr ₂ O ₃ -containing slag discharged and collected outside the furnace, and therefore the slag is recycled. In this case, handling becomes easy and environmental problems can be avoided at the same time.

In the latter case, when the decarburization end-stage slag is left in the furnace to be poured, the sensible heat of the slag is thermally advantageous as a process. In this case, if B ₂ O ₃ is added during the decarburization period, the melting point of the slag remaining in the furnace will be lowered, and the reduction after the next charge can be efficiently performed.

Although B ₂ O ₃ may be added during the decarburization period as described above, boron oxide is added to the slag (% B ₂ O ₃ ) during the rough decarburization period.
You may add so that it may become = 0.5-15%. That is,
If B ₂ O ₃ is added during the coarse decarburization and temperature rise of the next charge,
Since the slag formation progresses from the low temperature range of less than ℃ and the reduction start time is accelerated, the reduction rate can be improved.

When the content of (% B ₂ O ₃ ) in the slag is less than 0.5%, the effect of (% B ₂ O ₃ ) on the reduction rate is negligible. (% B ₂ O ₃ ) is 0.
At 5 to 15%, addition of boron oxide causes reduction to start in a low temperature range of less than 1400 ° C, and a high reduction rate can be achieved in a relatively low temperature range of 1600 ° C or less. On the other hand, if (% B ₂ O ₃ ) exceeds 15%, the slag expands remarkably during reduction and sloping frequently occurs, so that the reduction rate decreases.

Further, in a relatively low temperature range (1450 to 1600 ° C.), Cr ₂ O
_{Since 3} can be efficiently reduced and recovered, the amount of refractory erosion during the smelting reduction period can be greatly reduced. Also, for example, during decarburization, B ₂
O ₃ is added, and B ₂ is added at the final stage of crude decarburization after the next charge.
O ₃ may be added.

[0027] By adding the Si-containing alloy or Al-containing alloy C according to the last stage of reduction in the slag Cr ₂ O _3, Cr ₂ O
It is possible to reduce and recover ₃ to almost 100% molten metal and perform desulfurization. The boron oxide may be boron oxide itself or may contain boron oxide, and examples thereof include colemanite.

[0028]

【Example】

(Example 1) in decarburization stage of the crude melt 90t of 16% Cr-containing stainless steel, boron oxide in the slag 0～5T [slag (% B ₂ O ₃₎ =
[0-40% equivalent] was added, and the Cr ₂ O ₃ -containing slag at the final stage of decarburization was discharged and collected outside the furnace. The slag composition before addition of B ₂ O ₃ was T.Cr = 20%, T.Fe = 3%, CaO / SiO ₂ = 1.4, MgO.
= 5%, Al ₂ O ₃ <10%.

FIG. 2 shows the relationship between the addition amount of B ₂ O ₃ which is the melting point depressant and the slag discharge rate. As can be seen from Fig. 1, the slag ratio (discharge rate of the final decarburization slag) improves from the addition of 0.5% of the melting point depressant, and is saturated at about 15%.

(Example 2) 6000 kg of slag, 4000 kg of coke and 4000 g of calcined lime were charged at the end of the stainless steel decarburization period in the separate melting process of stainless steel, in which dephosphorized pig iron 65T was charged into an upper-bottom blowing converter. Of 574 kg and B ₂ O ₃ in the range of 0 to 3400 kg in the furnace, and while blowing bottom-blown Ar gas at 17 Nm ³ / min and oxygen from the top-blown lance at 146 Nm ³ / min for 41 minutes. I performed the training. The composition of the above-mentioned added slag is
T.Cr = 20%, T.Fe = 3%, CaO / SiO ₂ = 1.4, MgO = 5
%, Al ₂ O ₃ <10%.

Melt temperature and Cr ₂ O ₃ during rough decarburization, heating, and reduction
FIG. 3 shows the relationship of the reduction rate with C, and FIG. 4 shows the relationship between the addition amount of (% B ₂ O ₃ ) and the reduction rate. After that, Fe-Si
243 kg of (composition: Si = 75%, balance Fe) was added.

After rough decarburization and addition of Fe-Si, the temperature was 1632 ° C, the slag basicity was 1.5, and [% Cr] = 1.65%. In addition, [% C] in the metal at the end of rough decarburization was 1%. After that, slag is discharged and ferrochrome (composition: Cr = 60%, Si = 2.7
%, C = 6%, the balance Fe) 21T, quicklime 1.8T added,
A decarburization process was performed. The temperature after decarburization was 1700 ° C, the slag basicity was 1.5, and [% Cr] = 13%.

[0033]

EFFECT OF THE INVENTION By the method according to the present invention
₂ O ₃ -containing slag can be efficiently discharged and collected, and slag collected can be suppressed from being pulverized. In addition, when the recovered slag is reduced, the reduction proceeds from a low temperature range, and a high reduction rate can be efficiently obtained. Further, melting loss of refractory can be suppressed.

[Brief description of drawings]

FIG. 1 is a method 1 of producing stainless steel according to the present invention.
It is a process drawing of one example.

FIG. 2 is a graph showing the relationship between the amount of B ₂ O ₃ added and the rate of slag.

FIG. 3 is a graph showing the relationship between the molten metal temperature and the reduction rate during the temperature increase in rough decarburization.

FIG. 4 is a graph showing the relationship between (% B ₂ O ₃ ) and the reduction rate.

Claims (1)

[Claims] 1. A part or all of the chromium oxide-containing slag produced when decarburizing a crude stainless steel melt in a decarburizing furnace is left in the decarburizing furnace or discharged, and this slag is charged separately. Of the decarburizing furnace during the coarse decarburization temperature rising blowing, added to the furnace molten metal, reducing the chromium oxide in the slag by the carbon in the molten metal, or the carbon in the carbon-containing material added during the coarse decarburizing, After recovering in molten metal, Si-containing alloy or Al-containing alloy is added to reduce and desulfurize Cr ₂ O ₃ , the slag is discharged, and then decarburization is performed while adding Cr-containing alloy. A method for producing stainless steel, characterized in that boron oxide is added to slag in a slag (% B ₂ O ₃ ) = 0.5 to 15% during the carbonization period or the decarburization period or both of them.