JP3158912B2 - Stainless steel refining method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an argon-oxygen decarburization (A
OD) In steelmaking furnaces such as furnaces and converters, hot metal, scrap,
The present invention relates to a method for melting stainless steel using alloyed iron, recycled slag, carbon material, and the like.

[0002]

2. Description of the Related Art The most typical process for smelting stainless steel is to use scrap or alloyed iron such as Fe-Cr or Fe-Ni as a main raw material, and melt the raw material in an electric furnace to form a coarse molten metal. Decarburization and reduction smelting are performed in an AOD furnace or a VOD furnace, etc., and after tapping, Ar is blown in a steel receiving pan to clean the molten steel and control the temperature, and then to a continuous casting machine.

[0003] As reported in Iron and Steel, 1985, vol. 71, 180, hot metal is charged into a bottom-blowing converter without using an electric furnace,
Scrap or ferroalloys are added during or before decarburization so as to become a component of stainless steel to make predetermined components, and after the decarburization process, ferroalloys such as Fe-Si are put into the reduction process. There is also a process of shifting, then tapping and continuous casting.

[0004] There are other stainless steel smelting processes using chromium ore. For example, Iron and Steel, 1985, vol. 71, 107
In the method reported in 2, the molten iron is charged into an AOD furnace, chromium ore and coke are charged, and so-called smelting reduction is performed.
Thereafter, slag is removed and ordinary decarburization refining is performed.

However, the above method has the following problems.

(1) To reduce chromium oxide, a large amount of Si
(Often Fe-Si) must be added, which increases costs.

(2) Since SiO ₂ is generated as a reduction 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 Si is an exothermic reaction, the temperature rises and the slag is rich in fluidity, so that refractories are eroded.

As a method for solving these problems, for example, Japanese Patent Publication No. 4-38806 discloses that chromium-containing slag at the final stage of decarburization of a molten stainless steel is returned to a smelting reduction furnace to reduce and recover chromium. A process has been proposed. According to this method, the above problem can be solved because the reduction step using Fe—Si can be omitted. In the method for smelting stainless steel disclosed in Japanese Patent Publication No. 2-232312, the conditions under which the chromium-containing slag remaining in the decarburization furnace can be reduced and recovered by [C] in the next charge of the stainless steel crude melt are as follows: C] is 5% and the temperature is 1500 ° C. or higher.

The inventors of the present invention disclosed in Japanese Patent Application No. 5-336863 that the last stage of decarburization slag was recycled to the same furnace, and the chromium oxide in the slag was reduced with [C] at the time of decarburization of the next charge of the molten metal. A process to improve the chromium reduction rate by adding Si-containing alloy at the end of reduction was proposed.

[0011]

The method disclosed in Japanese Patent Publication No. 4-38806 cannot be realized without two smelting reduction furnaces and decarburization furnaces. In the method disclosed in Japanese Patent Publication No. 2332312, the chromium oxide in the slag is reduced by [C] in the molten metal during decarburization blowing, but at the end of decarburization blowing, [C] decreases. [C] in the molten metal has a low ability to recover chromium by reducing the pre-charged chromium oxide-containing slag remaining in the decarburization furnace, and cannot achieve a desired high chromium reduction rate.

In the method disclosed in Japanese Patent Application No. 5-336863, coke is initially added, but similarly, slag at the end of decarburization is simply added to the furnace, and oxidation in the added slag is performed by [C] in the molten metal. It only reduces chromium, the rate of reduction is slow, and again the chromium reduction rate is not sufficient. In other words, although it is necessary to raise the temperature immediately after the decarburization blowing to the reduction temperature (1500 to 1600 ° C) at which the slag turns into slag,
The hot metal temperature at the time of pouring is at most about 1300 ° C. When slag and coke containing chromium oxide are added before or immediately after blowing, the temperature rises to the above-mentioned reduction temperature due to the heat removal caused by these additions. The time gets longer. Furthermore, since coke does not exist in the slag at the end of the crude decarburization, the reduction of chromium oxide (hereinafter also referred to as Cr ₂ O ₃ ) in the slag by [C] occurs only at the metal-slag interface. In the state where decarburization has progressed to a low concentration of 2% or less in [C], the reduction rate decreases, and the reduction rate of Cr ₂ O ₃ by [C] becomes insufficient. Moreover,
During the middle to late stages of coarse decarburization, slagification of slag progresses and undissolved Cr ₂ O ₃ solid particles are present in slag,
The conditions are such that slopping is likely to occur, and it is necessary to suppress the slopping also from the viewpoint of improving the chromium reduction rate from Cr ₂ O _{3 as} a chromium source.

An object of the present invention is to provide a method for refining stainless steel that can improve the chromium reduction rate from Cr ₂ O ₃ -containing recycled slag in the final stage of crude decarburization using one furnace. .

[0014]

The gist of the present invention resides in the following method for refining stainless steel.

After adding the chromium oxide-containing recycled slag generated during the decarburization period and the carbonaceous material during the melting of stainless steel to the furnace, the hot metal is subjected to rough decarburization heating blowing and reduction of the chromium oxide. In the refining method of stainless steel, which performs finish reduction, desulfurization, slag, component adjustment, and finish decarburization, carbon-containing material is further added to slag at the end of the rough decarburization heating and blowing step, and slopping is performed. A method for refining stainless steel, characterized by improving the chromium reduction rate from chromium oxide-containing recycled slag while suppressing chromium oxide.

The term "late stage of the rough decarburization heating and blowing step" means that [C] in the molten metal has reached the range of 1.0 to 0.3% by weight and the temperature of the molten metal has reached the range of 1600 to 1750 ° C. Time point.

The above-mentioned "carbon-containing material" means a carbon-containing material such as coke, char or anthracite, and has a particle size range of 0.1.
It is desirable to use ~ 50 mm of coke.

The preferable range of the amount of the carbon-containing material is 9 to 180 kg / slag ton in terms of pure carbon.

[0019]

The apparatus for realizing the method of the present invention may be any one ordinary steel making furnace such as an AOD furnace or a converter. The hot metal produced by the usual method is charged into these furnaces,
Performs ₂ O ₃ reduction recovery, finish reduction and desulfurization, sludge, component adjustment and finish decarburization.

The method of the present invention will be described with reference to FIG. FIG. 1 is a flowchart illustrating an example of the method of the present invention. In the case shown in the figure, before the dephosphorization of the hot metal subjected to the decarburization, the slag containing Cr ₂ O ₃ and the initial coke generated by the final decarburization of the pre-charging were added before the hot de-carburization of the hot metal. Coke grains are added to the slag at the end of coarse decarburization heating and blowing to suppress slopping, and the carbon in the coke grains reduces the Cr ₂ O in the slag.
Crude decarburization and temperature rise are performed while reducing and recovering Cr from ₃ . Next, in the same furnace, one or both of the Si-containing alloy and the Al-containing alloy are added and subjected to finish reduction and desulfurization.After that, the slag is discharged, and then Fe-Cr is added to adjust the components. After finishing decarburization, tapping is performed. The slag in the furnace after the finish decarburization is left in the furnace as it is, for example, and recycled in the next charge of rough decarburization heating and blowing to recover as much Cr as possible from Cr ₂ O ₃ generated in the finish decarburization.

The amount of the recycled slag and initially added coke may be determined according to the Cr content and the temperature of the hot metal and the target molten metal.

According to the above-described method, it is possible to reduce Cr ₂ O ₃ by carbon-containing material such as carbon or coke in hot metal and to omit addition of a reducing agent such as metal Si or to reduce the amount of addition during the finish reduction period. it can.

Desirable conditions in the above method will be described below.

The carbon-containing material is added at the end of the rough decarburization heating / blowing step (hereinafter simply referred to as coarse decarburization) at a time when [C] in the molten metal is in the range of 1.0 to 0.3% by weight, 1600 ~
This is the point in time when the temperature reached the range of 1750 ° C, respectively. Coke, char, or anthracite can be used as the carbon-containing material, but it is preferable to use coke having a particle size range of 0.1 to 50 mm. A desirable range of the carbon-containing material addition amount is 9 to 180 kg / slag ton in terms of pure carbon.

Next, the operation and effect of the method of the present invention and the reason why the above conditions are desirable will be described.

The crude To reduce Cr ₂ O ₃ in Cr ₂ O ₃ containing recycled slag in the molten metal in the [C] to decarburization end, crude decarburization to a temperature at which the slag is slag formation (1500 ° C. or higher) It is necessary to heat up immediately after the start. However, the hot metal temperature at the time of being charged into the furnace is at most about 1300 ° C., and when Cr ₂ O ₃ -containing recycled slag and, for example, coke are added before or immediately after coarse decarburization, slag and Due to the heat removal caused by the coke addition, the time required for heating up to the slagging temperature (1500 ° C or more) becomes longer.

FIG. 2 is a graph showing the relationship between the transition of the molten metal temperature and the rate of Cr reduction during rough decarburization. As shown in the figure, since the reduction does not proceed so much in the early stage of the temperature rise, it is not necessary for the carbon-containing material such as coke particles to be present in a large amount in the furnace. In addition, even after heating to the reduction temperature (slagging temperature, 1500 ° C or higher),
For example, when a large amount of coke is added, the coke serves as a coolant, so that the slag temperature sharply decreases and the reduction temperature cannot be maintained, thereby reducing the reduction rate. Therefore, when the temperature of the molten metal reaches 1500 ° C. or higher, additional carbon-containing material is added, desirably,
The reduction should be promoted. In the case of continuous addition, a decrease in temperature can be suppressed.

On the other hand, the addition of an appropriate amount of carbon-containing material such as coke particles at the end of rough decarburization is effective in suppressing sloping as described later in Examples. this is,
Because the coke and the slag have poor wettability, CO bubbles coalesce at the interface between the slag and the coke, making it easier to pass through the slag, and by adding coke to the slag,
This is because the reduction proceeds further and the slag becomes a composition that is difficult to form.

Further, at the end of the rough decarburization, when the content of [C] in the molten metal is 2% or less and the Cr ₂ O ₃ in the slag is low, the coke added before the start of the rough decarburization is almost completely contained in the slag. Absent, the reduction of Cr ₂ O ₃ by carbon is due only to [C] in the molten metal, and therefore, this reduction reaction occurs only at the interface between the slag and the molten metal, and the [C] in the molten metal decreases. At the same time, the reduction rate is also reduced.At this time, adding a carbon-containing material such as coke into the slag to generate an interface between the slag and the carbon-containing material to cause a reduction reaction is a general summary. This is extremely advantageous for improving the Cr reduction rate and the reduction rate.

FIG. 3 is a diagram showing the relationship between [C] in the molten metal and the Cr reduction ratio when coke is added or not at the end of crude decarburization. The horizontal axis shows the rough decarburization time, and the broken line shows the transition in the molten metal [C]. As shown in the figure, if coke is further added at the end of crude decarburization, the Cr reduction rate is improved even at low [C]. Therefore, it is desirable that the content of [C] in the molten metal at the end of crude decarburization to which the carbon-containing material is added be in the range of 1.0 to 0.3% by weight.

Of the carbon-containing materials described above, the particle size range is 0.1 to
It is desirable to use 50 mm coke for the following reasons.

The addition is carried out from the furnace into the slag. In order to form an interface between the slag and coke, it is necessary to select conditions such that the slag is dispersed and stays in the slag. If the coke particle size is less than 0.1 mm, the scattering loss at the time of addition is large, and the coke is further deposited on the upper surface of the slag, and a sufficient interfacial area between the slag and the coke cannot be secured.

FIG. 4 is a diagram showing the relationship between the coke particle size and the scattering loss rate during addition in the furnace. As shown in the figure, the loss increases when the particle size is less than 0.1 mm.

On the other hand, if the coke particle size exceeds 50 mm, the following problems (1) to (4) occur.

It is not possible to improve the efficiency of increasing the interfacial area between slag and coke per coke added weight.

It is not possible to improve the efficiency of heat application to the slag and the reduction rate by uniformly dispersing the slag in the slag.

The amount of coke added required to increase the interfacial area increases, which is costly and thermally uneconomical.

When coke is added, coke penetrates the slag layer and sinks on the surface of the molten metal, so that the coke is dissolved in the molten metal before being dispersed in the slag, and the above-mentioned interface cannot be efficiently formed.

FIG. 5 is a graph showing the relationship between the coke particle size and the Cr reduction rate at the end of the rough decarburization. As shown in the figure,
If it exceeds 50 mm, the Cr reduction rate decreases.

When coke is dispersed and retained in the slag, the coke reacts with the top-blown oxygen to reach a high temperature. As a result, not only a high reduction reaction rate is obtained at the interface between the slag and the coke, but also the high-temperature When the coke comes into contact with the slag, the temperature of the entire slag increases, and the reduction rate at the interface increases.

A desirable range of the coke addition amount is 9 to 180 kg / slag ton in terms of pure carbon. The amount of coke to be added is determined from the viewpoint of economy. That is, although the Cr reduction rate is improved by adding coke, the effect of adding coke is too small at less than 9 kg / slagton.

On the other hand, if the addition amount of the carbon-containing material exceeds 180 kg / slagton, it becomes inappropriate for the following reasons.

The slag temperature decreases, and conversely, the reduction rate decreases.

After the finish reduction, a large amount of coke is discarded, which is uneconomical.

Even after the reduction of Cr ₂ O _{3, a} large amount of coke remains and eventually enters the molten metal, and the amount of decarburization during the final decarburization period unnecessarily increases. Will lead to an extension.

[0046]

【Example】

(Comparative Example) 65 tons of dephosphorized hot metal was charged into an upper-bottom blower, and recycled slag collected at the end of the pre-charging finish decarburization (weight% composition: T.Cr = 20%, T.Fe = 3%) , CaO = 27%,
_{SiO 2 = 18%, MgO =} 10%, Al 2 O 3 = 10% or less) 6000 kg,
After adding 4000 kg of coke and 228 kg of quicklime into the furnace, bottom blowing
Ar gas 17 Nm ³ / min, oxygen from the top blowing lance 146 Nm ³
While blowing at / min, crude decarburization was performed for 40 minutes. At the end of crude decarburization, [Cr] in the molten metal was 1.2% by weight, [C] was 0.8% by weight, and the rate of reduction of Cr from recycled slag was 65%.

Thereafter, Fe—Si (weight% composition: Si = 75%,
The remaining Fe) 243 kg, quick lime 900 kg, CaF ₂ 275 kg were added,
Finish reduction was performed. In the latter half of the rough decarburization, the upper surface of the slag reached the vicinity of the furnace port, and slopping occurred somewhat frequently.

The temperature after the addition of Fe—Si was 1632 ° C., the slag basicity was 1.8, [Cr] in the molten metal was 1.75% by weight, and the rate of reduction of Cr from recycled slag was 95%. Thereafter, the slag is discharged, and high-carbon Fe-Cr (weight% composition: Cr = 60%, Si = 2.7%)
%, C = 6%, balance Fe) 21 tons and quick lime 1.8 tons were added to finish decarburization. The temperature after the finish decarburization was 1700 ° C., the slag basicity was 1.5, and the content of [Cr] in the molten metal was 13% by weight.

(Example of the present invention) 65 tons of dephosphorized hot metal was charged into an upper-bottom blower, and slag (weight% composition: T.Cr = 20%, T.Fe = 3%, CaO = 27
3%, SiO ₂ = 18%, MgO = 10%, Al ₂ O ₃ 10% or less) 6000
kg, 4000 kg of coke, and 228 kg of quicklime were added into the furnace, then the bottom-blown Ar gas was 17 Nm ³ / min, and the oxygen from the top-blown lance was 14
Crude decarburization is performed while blowing at 6 Nm ³ / min, and when [C] in the melt reaches 1.9% by weight and the melt temperature reaches 1700 ° C 34 minutes after the start of blowing, coke (particle size: 30 to 40 mm) An additional 480 kg was added. Thereafter, the crude decarburization was further continued for about 6 minutes.
At the end of blowing, [Cr] is 1.67% by weight, [C] is 0.8
% By weight, and the rate of reduction of Cr from recycled slag was 90%.

After additional coke addition, the slag thickness is 2400
mm, and almost no slopping occurred.

Thereafter, 200 kg of Fe-Si (weight% composition: Si = 75%, balance Fe), 768 kg of quicklime, and 275 kg of CaF ₂ were added to finish reduction. The temperature after the addition of FeSi was 1632 ° C., the slag basicity was 1.8, [Cr] in the molten metal was 1.75% by weight, and the reduction ratio of Cr from recycled slag was 95%.

Thereafter, the slag is discharged and the high carbon Fe-Cr
(Weight% composition: Cr = 60%, Si = 2.7%, C = 6%, balance
Fe) 21 tons and 1.8 tons of quicklime were added to finish decarburization.

The temperature after the decarburization was 1,700 ° C., the slag basicity was 1.5, and the content of [Cr] in the molten metal was 13% by weight.

In the example of the present invention, almost no slopping occurred, the Cr reduction rate in the rough decarburization stage was improved from 65% to 90%, and the addition amounts of Fe-Si and quicklime were reduced.

[0055]

According to the method of the present invention, slopping during crude decarburization is suppressed, and Cr is removed at the end of crude decarburization in one furnace.
The Cr reduction rate from the ₂ O ₃ -containing recycled slag can be improved.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating an example of the method of the present invention.

FIG. 2 is a diagram showing a relationship between a transition of a molten metal temperature and a Cr reduction rate during rough decarburization.

FIG. 3 is a diagram showing the relationship between [C] in the molten metal and the Cr reduction ratio in the case of the presence or absence of coke addition at the end of crude decarburization.

FIG. 4 is a diagram showing the relationship between the coke particle size and the scattering loss rate during addition in a furnace.

FIG. 5 is a diagram showing the relationship between coke particle size and Cr reduction rate at the end of crude decarburization.

Claims (1)

(57) [Claims] (1) After adding chromium oxide-containing recycled slag and carbonaceous material generated during the decarburization period during melting of stainless steel into a furnace, rough decarburization heating and blowing of hot metal and reduction of chromium oxide are performed. After that, in the refining method of stainless steel to be subjected to finish reduction, desulfurization, sludge, sludge, component adjustment and finish decarburization, the carbon-containing material is further added to the slag at the end of the rough decarburization heating and blowing step. A method for refining stainless steel, comprising improving the chromium reduction rate from chromium oxide-containing recycled slag while suppressing slopping.