JPH08176638A - Refining of stainless steel - Google Patents

Abstract

PURPOSE: To provide a method for refining a stainless steel capable of improving a Cr reducing rate. CONSTITUTION: This method for refining the stainless steel comprises adding Cr2 O3 -contg. recycled slag generated in the decarburization period for smelting the stainless steel and carbon-contg. material into a furnace, then executing the rough decarburization heating up of molten iron and reduction of Cr2 O3 , then subjecting the molten iron to finish reducing, desulfurizing, slag flowing, component adjusting and finish decarburizing. The Cr reducing rate from the recycled slag is improved while slopping is suppressed by adding the carbon- contg. material into the slag in the end period of the rough decarburization heating up. As a result, the slopping is suppressed during the rough decarburization and the Cr reducing rate from the Cr2 O3 -contg. recycled slag in the end period of the rough decarburization period is improved with one unit of the furnace.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is directed to argon-oxygen decarburization (A
OD) In steelmaking furnaces such as furnaces and converters, hot metal, scrap,
The present invention relates to a method for producing stainless steel using ferroalloy, recycled slag, carbonaceous material and the like.

[0002]

2. Description of the Related Art The most typical process for smelting stainless steel is to use scrap or ferro-alloy such as Fe-Cr and Fe-Ni as the main raw materials, melt the raw materials in an electric furnace to obtain a crude molten metal, and then Decarburization and reduction refining are performed in an AOD furnace or VOD furnace, 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.

As reported in Iron and Steel, 1985, vol.71,180, the hot metal was charged into the bottom blowing converter without using an electric furnace.
Scrap or ferroalloy is added to the specified components during or before decarburization blowing so that it becomes the component of stainless steel, and after the decarburization process, ferro-alloy such as Fe-Si is added to the reduction process. There is also a process of transferring, then tapping and continuously casting.

There are other stainless steel melting processes that use chrome ores. For example, Iron and Steel, 1985, vol.71,107.
The method reported in 2 is to charge the hot metal into the AOD furnace, add chromium ore and coke, and perform so-called smelting reduction.
After that, the slag is removed and normal decarburization refining is performed.

However, the above method has the following problems.

(1) A large amount of Si for reducing chromium oxide
(In most cases Fe-Si) needs to be added, resulting in high cost.

(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 has a high fluidity, so that the refractory is eroded.

As a method for solving these problems, for example, in Japanese Examined Patent Publication No. 4-38806, the chromium-containing slag at 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. A process to do is proposed. According to this method, the reduction step using Fe-Si can be omitted, so that the above problems are solved. In the method for smelting stainless steel disclosed in JP-B-2-232312, the conditions under which the chromium-containing slag remaining in the decarburizing furnace can be reduced and recovered by [C] in the next charge of stainless steel crude molten metal are as follows. C] is -5% and the temperature is 1500 ° C or higher.

In the Japanese Patent Application No. 5-336863, the present inventors recycled the decarburization end-stage slag to the same furnace and reduced the chromium oxide in the slag with [C] at the time of decarburization heating of the crude melt of the next charge. We proposed a process of recovering and adding a Si-containing alloy at the final stage of reduction to improve the reduction rate of chromium.

[0011]

The method disclosed in Japanese Examined Patent Publication No. 4-38806 cannot be realized without having two furnaces, a smelting reduction furnace and a decarburization furnace. The method disclosed in JP-B-2-232312 reduces chromium oxide in slag with [C] in the molten metal during decarburization blowing, but [C] becomes low at the end of decarburizing blowing. The [C] in the molten metal has a low ability to recover the chromium by reducing the precharged 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, the final decarburizing slag is simply added to the furnace and the [C] in the molten metal is used to oxidize the added slag. It only reduces chromium, its reduction rate is slow, and the reduction rate of chromium is still insufficient. That is, even though it is necessary to raise the temperature immediately after the start of 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, and when adding chromium oxide-containing slag and coke before the start of blowing or immediately after the start of blowing, the heat removal due to the addition of these causes the temperature to rise to the above reduction temperature. Time will increase. Furthermore, since coke does not exist in the slag at the final stage of coarse decarburization, 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, but In the state where decarburization proceeds to a low concentration of 2% or less of [C], the reduction rate decreases, and the reduction rate of Cr ₂ O ₃ by [C] becomes insufficient. Moreover,
From the middle to the final stage of coarse decarburization, slag slagification progresses and undissolved Cr ₂ O ₃ solid particles are present in the slag.
It is a condition that sloping easily occurs, and it is necessary to suppress this sloping also from the viewpoint of improving the reduction rate of chromium from Cr ₂ O ₃ which is a chromium source.

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

[0014]

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

After adding the chromium oxide-containing recycled slag and the carbon-containing material generated during the decarburization stage during the melting of stainless steel into the furnace, the rough hot decarburization of molten pig iron and the reduction of chromium oxide are carried out. In the refining method of stainless steel, which is then subjected to finish reduction, desulfurization, slag, component adjustment and finish decarburization, carbon-containing material is added to the slag at the end of the coarse decarburization temperature-blown process, and sloping is performed. A method for refining stainless steel, which comprises improving the reduction rate of chromium from chromium oxide-containing recycled slag while suppressing the above.

The above-mentioned "end stage of the crude decarburization temperature rising blowing step" means that [C] in the molten metal has reached the range of 1.0 to 0.3% by weight, and the molten metal temperature has reached the range of 1600 to 1750 ° C. The 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 ~ 50mm coke.

A desirable range of the amount of the carbon-containing material added is 9 to 180 kg / slag ton in terms of carbon content.

[0019]

The apparatus for realizing the method of the present invention may be a single steelmaking furnace such as a normal AOD furnace or a converter. The hot metal produced by the usual method is charged into these furnaces, and the crude decarburization temperature rise and Cr
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 flow chart illustrating an example of the method of the present invention. In the case shown in the figure, before the crude decarburizing temperature rising blowing of the dephosphorized hot metal, the Cr ₂ O ₃ -containing slag and the initial coke generated by the precharge finishing decarburization are added, and the Coke grains are added to the slag in the final stage of coarse decarburization temperature rising blowing to suppress sloping, and the carbon in the coke grains causes Cr ₂ O in the slag.
Crude decarburization and temperature increase are performed while reducing and recovering Cr from ₃ . Then, in the same furnace, one or both of Si-containing alloy and Al-containing alloy is added to perform finish reduction and desulfurization, after which slag is allowed to flow out, and then Fe-Cr is added to adjust the components, and After finishing decarburization, steel is tapped. The in-furnace slag after the finish decarburization is, for example, left in the furnace as it is and recycled to the next charge of coarse decarburization temperature rising blowing, and Cr is recovered from Cr ₂ O ₃ generated by the finish decarburization as much as possible.

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

By the above-mentioned method, Cr ₂ O ₃ can be reduced by carbon-containing materials such as carbon in hot metal or coke, and the addition of a reducing agent such as metallic Si during the finish reduction period can be omitted or the amount added can be reduced. it can.

Desirable conditions in the above method will be described below.

The carbonaceous material is added additionally at the final stage of the coarse decarburizing temperature rising blowing step (hereinafter simply referred to as "crude decarburizing") so that [C] in the melt is in the range of 1.0 to 0.3% by weight and the melt temperature is 1600 ~
It is the time when the temperature reaches the range of 1750 ° C. As the carbon-containing material, coke, char or anthracite can be used, but it is preferable to use coke having a particle size range of 0.1 to 50 mm. A desirable range of the amount of the carbon-containing material added is 9 to 180 kg / slag ton in terms of pure carbon.

Next, the function 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 when charged into the furnace is at most about 1300 ° C., and when adding Cr ₂ O ₃ -containing recycled slag and, for example, coke before the start of crude decarburization or immediately after the start of crude decarburization, slag and Due to the heat removal due to the addition of coke, the heating time to the slagging temperature (1500 ° C or higher) becomes long.

FIG. 2 is a graph showing the relationship between the molten metal temperature transition and the Cr reduction rate during rough decarburization. As shown in the figure, since the reduction does not proceed so much at the initial stage of temperature increase, it is not necessary that a large amount of carbon-containing material such as coke grains be present in the furnace. Also, even after raising the temperature to the reduction temperature (slag 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 is rapidly lowered, the reduction temperature cannot be maintained, and the reduction rate is reduced. Therefore, when the temperature of the molten metal reaches 1500 ° C or higher, additional addition of carbon-containing material, preferably continuous addition of Cr
It is better to promote the reduction. In the case of continuous addition, the temperature decrease can be suppressed.

On the other hand, adding an appropriate amount of carbon-containing material such as coke grains at the final stage of rough decarburization is effective in suppressing sloping, as described later in Examples. this is,
Since the coke and the slag have poor wettability, CO bubbles easily coalesce at the interface between the slag and the coke and easily come out of the slag.By adding coke to the slag,
This is because the reduction proceeds more 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 content in the slag is low, most of the coke added before the start of the crude decarburization is contained in the slag. It does not exist, and the reduction of Cr ₂ O ₃ with carbon is only due to [C] in the molten metal. Therefore, this reduction reaction occurs only at the interface between the slag and the molten metal, and the reduction of [C] in the molten metal Since the reduction rate also decreases with this, it is a general idea to add a carbon-containing material such as coke into the slag at this time to generate an interface between the slag and the carbon-containing material to cause a reduction reaction. It is extremely advantageous for improving the Cr reduction rate and reduction rate.

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

Among the above-mentioned carbon-containing materials, the particle size range is from 0.1 to
The reason why it is desirable to use 50 mm coke is as follows.

The addition is carried out from the furnace into the slag, but in order to form the interface between the slag and the coke, it is necessary to select the conditions such that the slag disperses 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 further the coke is deposited on the upper surface of the slag, and it is not possible to secure a sufficient interfacial area between the slag and the coke.

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 3) occur.

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

It is not expected that the heat is efficiently applied to the slag and the reduction rate is improved by the uniform dispersion in the slag.

The amount of coke added to increase the interfacial area increases, which is uneconomical in cost and heat.

When the coke is added, the coke penetrates the slag layer and sinks on the surface of the molten metal. Therefore, the coke is melted in the molten metal before being dispersed in the slag, and the above interface cannot be efficiently generated.

FIG. 5 is a graph showing the relationship between the coke particle size and the Cr reduction rate at the final stage of crude decarburization. As shown,
If it exceeds 50 mm, the Cr reduction rate will decrease.

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

A desirable coke addition amount range is 9 to 180 kg / slag ton in terms of pure carbon. The amount of coke added is determined from the economical point of view. That is, the addition rate of coke improves the Cr reduction rate, but if it is less than 9 kg / slag ton, the effect of coke addition is too small.

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

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

A large amount of coke is discarded after finishing reduction, which is uneconomical.

Even after the reduction of Cr ₂ O _{3, a} large amount of coke remains and finally enters the molten metal, which unnecessarily increases the amount of decarburization during the finishing decarburization period. Leads to an extension of.

[0046]

【Example】

(Comparative example) 65 tons of dephosphorized hot metal was charged into the top-bottom blowing converter, and recycled slag (weight% composition: T.Cr = 20%, T.Fe = 3%) recovered after finishing decarburization of the precharge. , 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 quick lime to the furnace, blow bottom
Ar gas 17 Nm ³ / min, oxygen from top blowing lance 146 Nm ³
While blowing at / min, 40 minutes of rough decarburization was performed. At the final stage of rough decarburization, [Cr] in the molten metal was 1.2% by weight, [C] was 0.8% by weight, and the reduction rate of Cr from the recycled slag was 65%.

Thereafter, Fe--Si (weight% composition: Si = 75%,
The balance Fe) 243 kg, quick lime 900 kg, CaF ₂ 275 kg are added,
Finished reduction was performed. In the latter half of rough decarburization, the upper surface of the slag reached the vicinity of the furnace opening, and sloping occurred somewhat frequently.

The temperature after the addition of Fe--Si was 1632 ° C., the slag basicity was 1.8, the molten metal [Cr] was 1.75% by weight, and the Cr reduction rate from the recycled slag was 95%. After that, the slag is discharged and high carbon Fe-Cr (weight% composition: Cr = 60%, Si = 2.7
%, C = 6%, the balance Fe) 21 tons, and quick lime 1.8 tons were added for decarburization. The temperature after 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 blowing converter, 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
After adding 4000 kg of coke, 4000 kg of coke and 228 kg of quick lime into the furnace, the bottom blown Ar gas was 17 Nm ³ / min and the oxygen from the top blown lance was 14
Coarse decarburization was performed while blowing at 6 Nm ³ / min, and after 34 minutes from the start of blowing, [C] in the melt reached 1.9% by weight, and when the melt temperature reached 1700 ° C, coke (particle size 30-40 mm) An additional 480 kg was added. After that, rough decarburization was further continued for about 6 minutes.
1.67% by weight of [Cr] in the molten metal at the end of blowing and 0.8 in [C]
% By weight, and the reduction rate of Cr from recycled slag was 90%.

After the additional addition of coke, the slag thickness is 2400
It decreased to mm, and sloping hardly occurred.

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

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

The temperature after finishing decarburization was 1700 ° C., the slag basicity was 1.5, and the content of [Cr] in the molten metal was 13% by weight.

In the examples of the present invention, sloping hardly occurred, the Cr reduction rate in the rough decarburizing period was improved from 65% to 90%, and the amounts of Fe-Si and quick lime added were reduced.

[0055]

EFFECTS OF THE INVENTION According to the method of the present invention, sloping during rough decarburization is suppressed, and Cr is used in one furnace in the final stage of rough decarburization.
It is possible to improve the Cr reduction rate from the ₂ O ₃ -containing recycled slag.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a relationship between a molten metal temperature transition 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 rate when coke is added or not in the final stage of rough decarburization.

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

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

Claims (1)

[Claims] Claims: 1. After adding a chromium oxide-containing recycled slag and a carbon-containing material generated in the decarburization stage during melting of stainless steel to a furnace, rough hot decarburization blowing of hot metal and reduction of chromium oxide In the refining method of stainless steel to be subjected to finish reduction, desulfurization, slag, component adjustment and finish decarburization, a carbon-containing material is further added to the slag at the end of the coarse decarburization temperature-blown step. A method for refining stainless steel, which comprises increasing the reduction rate of chromium from a chromium oxide-containing recycled slag while suppressing sloping.