JP3173325B2 - How to make stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an argon oxygen decarburization furnace (A
The present invention relates to a method for refining stainless steel in a steelmaking furnace such as an OD furnace, a converter, an electric furnace, and the like, in which stainless steel is manufactured using hot metal, scrap, alloyed iron, and the like.

[0002]

2. Description of the Related Art The most typical process for smelting stainless steel is to melt scrap or ferroalloys such as Fe-Cr and Fe-Ni in an electric furnace and then use an AOD furnace or an AOD furnace.
It consists of decarburizing and reducing and refining in a VOD furnace, etc., and after buffing, injecting Ar gas into a steel receiving pan to clean and control the temperature of the molten steel, and then the obtained molten steel is cast into a continuous casting machine. I will put it on.

[0003] References ("Iron and Steel" 1985, vol. 71, 180)
As shown in, the dephosphorized hot metal is charged into the bottom-blowing converter without using an electric furnace, and scrap or alloyed iron is added during or before decarburization blowing so as to become a component of stainless steel. After completion of the decarburization step, there is also a process in which ferrous alloys such as Fe-Si are charged and the process proceeds to the reduction step, and then the steel is cast and continuously cast.

[0004] There is also a stainless steel smelting process using chromium ore. For example, in the literature (Iron and Steel 1985, vol.7
In 1, 1072), dephosphorized hot metal is charged into an AOD furnace, then chromium ore and coke are charged, so-called smelting reduction is performed, and then slag is removed and ordinary decarburization refining is performed.

[0005] However, these conventional methods have the following problems. (1) Addition of a large amount of Si (often used in the form of Fe-Si) increases costs. (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 is rich in fluidity, so that the refractory is eroded.

In view of this, Japanese Patent Publication No. 4-38806 proposes a process in which chromium-containing slag in the final stage of decarburization of crude stainless steel is returned to a smelting reduction furnace to reduce and recover chromium. It is stated that the above problem can be solved because the reduction period using Fe—Si can be omitted.

The chromium-containing slag remaining in the decarburization furnace is
Regarding the case of reducing and recovering with C in the next charge of stainless steel crude molten steel, it is possible if C in the molten metal is 5% and 1500 ° C. or more.

Japanese Patent Publication No. 4-38806 proposes a process in which chromium-containing slag in the final stage of decarburization of a molten stainless steel is returned to a smelting reduction furnace to reduce and recover chromium. It is stated that the above problem can be solved because the reduction period using Fe—Si can be omitted. In the case where chromium-containing slag remaining in the decarburization furnace is reduced and recovered with C in the next charge of stainless steel molten steel, it is possible if C in the molten metal is 5% and 1500 ° C. or higher.

[0009]

However, this method has a problem that it cannot be realized without two smelting reduction furnaces and decarburization furnaces. In addition, the chromium-containing slag remaining in the decarburization furnace is replaced with C in the next charge of stainless steel crude molten steel.
In the case of reducing and recovering by C, 5% of C in the molten metal
It is possible if the temperature is higher than 00 ° C. However, decarburization furnace slag composition CaO: 20%, MgO: 18 %, SiO 2: 5%, Cr 2 O 3: has a 55% without slag of even 1500 ° C. or higher in the composition, The rate of reduction is expected to be very slow. This is because Al ₂ O ₃ having a function of lowering the melting point is not contained in the slag in a large amount exceeding 10%.

[0010] The present invention relates to (% Al ₂ O ₃ )
It is an object of the present invention to provide a method for refining stainless steel in which reduction is performed promptly without increasing the amount of refractory material and refractory erosion is suppressed.

In addition, the inventors of the present invention disclosed in Japanese Patent Application No. Hei 5-336863, 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. We propose a process to improve the chromium recovery rate by recovering and adding a small amount of Si-containing alloy at the end of reduction.

In the method proposed by the present inventors, an alloy containing Si is added at the end of the reduction of Cr ₂ O _{3 in} slag by C to improve the chromium recovery rate. By simply adding and reducing the contained alloy, Si inevitably contained in the chromium alloy charged during the decarburization period is oxidized during decarburization, and the amount of quicklime to neutralize the generated SiO ₂ cannot be reduced .

Here, a specific object of the present invention is to (1) eliminate the need for having two furnaces, a smelting reduction furnace and a decarburization furnace, and (2) eliminate the problem of chromium oxide unreduction by only C reduction. (3) An object of the present invention is to provide a method for refining stainless steel capable of reducing the amount of slag-making agent required for the decarburization period.

[0014]

SUMMARY OF THE INVENTION Here, the gist of the present invention is to receive a molten metal made of dephosphorized iron into a converter or to prepare a step of melting the molten metal in an electric furnace. A rough decarburization and heating step of roughly decarburizing and raising the temperature of the molten metal in the furnace, and performing a rough desulfurization and a reduction of chromium oxide by introducing Si into the obtained molten metal to finish-reduce to a stainless steel coarse molten steel. Coarse desulfurization process, sludge process for discharging generated slag,
A method for producing stainless steel, comprising: a finish decarburization of the stainless steel crude molten steel, a finish decarburization step of forming the stainless steel molten steel, and a tapping step of performing the tapping of the stainless steel molten steel after generating a slag containing Cr ₂ O _3. The slag containing chromium oxide having an Al ₂ O ₃ content of 10% or less generated in the finishing decarburization step of the stainless steel crude molten steel is recycled and used in the crude decarburization / heating step after the next charge, When chromium oxide is reduced and recovered in the molten metal by carbon in the molten metal or carbon in the carbon-containing material added at the time of crude decarburization, the oxidization is performed at a slag basicity of 0.9 to 1.5 and a reduction temperature of 1500 to 1650 ° C. After the chromium is reduced and recovered in the molten metal, an Si-containing alloy (including an alloy containing Si and Cr) is added at the end of the reduction in the rough decarburization / heating step, and the oxidization remaining in the slag by the Si. Chromium is recovered in the molten metal After, through the flow slag process, a refining process of stainless steel, characterized by further performing decarburization finished by adding Cr-containing alloy and lime.

[0015]

Next, the operation of the present invention will be described more specifically with reference to the accompanying drawings.

FIG. 1 is a process chart of a method for producing stainless steel according to the present invention. In the present invention, a process for preparing a molten metal which has been subjected to a pretreatment as in the prior art, ie, a chromium-free dephosphorized molten metal, is also provided. 10.Decarburization of dephosphorized pig iron or electric furnace melting and decarburization heating to reduce chromium contained in slag12.Addition of Si (including alloy containing Si and Cr) The final reduction / crude desulfurization step 14 for further reducing the crude chromium oxide and the crude desulfurization, the slagging step 16 for discharging the generated slag, and the decarburization refining step 18 after the addition of ferrochrome are melted, After passing through the tapping step 22, the steel is finally subjected to desulfurization in an out-of-furnace desulfurization step 24 to obtain stainless steel.

According to the present invention, the chromium oxide-containing slag 20 obtained in the finishing decarburization step 18 of the molten stainless steel after the addition of ferrochrome is made of dephosphorized iron or electric furnace molten crude molten metal containing almost no chromium. In the decarburization heating step 12, the chromium oxide contained is reduced by being recycled. That is, the chromium oxide is recycled and reduced in the next charge rough decarburization heating step. As a form of recycling, the molten metal from the preparation step 10 may be received in the tapping step 22 with the chromium oxide-containing slag remaining in the furnace.

As described above, in the present invention, the slag containing chromium oxide generated when decarburizing a crude molten steel of stainless steel is recycled and used at the time of rough decarburization after the next charge. It is necessary to reduce chromium oxide by a carbon-containing material such as coke or the like, to omit the addition of metal Si, or to reduce the amount used.

According to the present invention, Cr ₂ O in the last stage of decarburization slag
To reduce ₃ with C or carbon material in the molten metal, the reduction temperature
(1500-1650 ° C), the slag must be slag. Therefore CaO-SiO ₂ -Cr ₂ O ₃ system phase melting range at reduced temperatures to determine the view of that wide, C / Si = 0.
In the range of 5 to 1.5, at 1500 to 1650 ° C, the slag basicity is set to 0.9 to 1.5 from the viewpoint of improving the reduction rate.

FIG. 2 is a graph showing the relationship between basicity CaO / SiO ₂ and reduction rate. The relationship between the refractory erosion rate and slag basicity in the case of reducing the slag Cr ₂ O ₃ in a state in which the 1,520 to 1,650 ° C. in AOD furnace shown in FIG. From this, it can be seen that the basicity needs to be 0.9 or more and 1.5 or less from the viewpoint of suppressing refractory material damage. Preferably it is 1.0 to 1.3.

The relationship between the reduction rate and the refractory erosion rate and the temperature when chromium oxide in slag was reduced in an AOD furnace was examined. However, the slag basicity was 1.11 to 1.22. The results are shown in FIG. 4 and FIG. From this, the reduction temperature is set to 1500 to 16 in order to suppress the refractory erosion and increase the reduction rate.
It can be seen that the temperature needs to be 50 ° C.

FIG. 6 shows the relationship between the Al ₂ O ₃ concentration in the slag and the reduction rate. This shows that at a basicity of 2.4, when (% Al ₂ O ₃ ) <10%, the reduction rate is significantly reduced. However(%
Even when Al ₂ O ₃ ) <10, when the basicity is 1.2, the reduction rate is maintained at a relatively high value.

In the above method, a Si-containing alloy (including a Si- and Cr-containing alloy) is added to the final stage of Cr ₂ O ₃ reduction in the final stage of pre-charging decarburization slag with [C] in the depigmented pig iron,
Cr ₂ O ₃ remaining in the slag can be reduced and recovered by the Si in the alloy to increase the chromium yield. At the same time, the progress of desulfurization can also be expected. Further, when the alloy containing Si and Cr is added in the rough decarburization and temperature raising step, the amount of the chromium-containing alloy added during the decarburization after the Cr reduction can be reduced.
For this reason, the amount of SiO ₂ generated during the decarburization stage is reduced by the oxidation of Si inevitably contained in the chromium-containing alloy, and the amount of quicklime added for neutralizing SiO ₂ for the purpose of suppressing the refractory melting damage can be reduced. The embodiments of the present invention include both the above-mentioned (a) the method for residual slag in a furnace and (b) the method for recycling discharged slag.

[0024]

【Example】

(Example 1) Dephosphorized pig iron 70T prepared in advance was charged into an upper-bottom blow converter, and Ar gas was blown at 45 Nm ³ / min from the bottom blow tuyere and oxygen was blown at 350 Nm ³ / min from the top blow lance. Slag collected at the end of the final decarburization period of stainless steel in the previous charge (Composition: T.Cr = 20%, T.Fe = 3%, CaO / SiO ₂ = 1.
6, MgO = 5%, Al ₂ O ₃ ≦ 10%) 6000kg, coke
9800kg was added and blowing was performed for about 35 minutes, and the reduction behavior of chromium oxide was investigated. Where slag basicity and (% Al ₂ O ₃ )
Quicklime, silica sand and Al ₂ O ₃ were added as needed to change the temperature. Also, the processing temperature was changed. The results are summarized in Table 1. However, the refractory erosion index is usually expressed as an index with the refractory erosion rate during blowing in the AOD furnace as 1.

[0025]

[Table 1]

As described above, the higher the reduction temperature, the higher the reduction rate, but the higher the refractory erosion rate. It can also be seen that the reduction rate decreases when the slag basicity is less than 0.9 or more than 1.5.

However, as an evaluation criterion, a reduction rate of 0.4
% / min or more and a refractory erosion rate index of 1 or less were regarded as excellent. The reduction rate and the reduction rate were calculated by the following equations. (1) The reduction rate (k) is determined by the (0th-order) rate equation.

[0028]

(Equation 1)

(% Cr ₂ O ₃ ): Cr ₂ O ₃ concentration in slag (%)
, T: time (min) (2) The reduction rate is obtained by the following equation.

[0030]

(Equation 2)

(Conventional method) Dephosphorized pig iron 65T prepared in advance is charged into an upper-bottom blow converter, and Ar gas is supplied from the bottom blow tuyere at 17 Nm ³ / min.
, O ₂ gas 63 Nm ³ / min, oxygen from the top blowing lance 83 Nm
Slag collected at the end of the decarburization stage of stainless steel while blowing ³ / min (Composition: T.Cr = 20%, T.Fe = 3%, CaO / SiO ₂
= 1.5, MgO = 5%, Al ₂ O ₃ ≦ 10%), 6000 kg of quicklime and 4,000 kg of coke were added, and blowing was performed for 41 minutes. The basicity at the end of blowing was 1.65. Then Fe-
243 kg of Si (composition: Si = 75%, balance Fe) was added.

The temperature after the crude decarburization and the addition of Fe—Si was 1632 ° C., the slag basicity was 1.5, and [% Cr] = 1.65%. At the end of the crude decarburization, the [% C] in the molten metal was 1%. After that, the slag is discharged and ferrochrome (composition: Cr = 60%, C = 6%, Si
= 2.66%, the balance Fe) was added to 21T and quick lime 1.8T, and decarburization treatment was performed. The temperature after the decarburization treatment was 1700 ° C., the slag basicity was 1.5, and [% Cr] = 1.3.

(Example 2) Dephosphorized pig iron 65T prepared in advance was charged into an upper-bottom blow converter, and Ar gas was supplied from the bottom blow tuyere at 17 Nm ³ / mi.
n, O ₂ gas 63 Nm ³ / min, oxygen from the top blowing lance 83 N
The slag collected at the end of the final decarburization stage of stainless steel at the previous charge while blowing m ³ / min (Composition: T.Cr = 20
%, T.Fe = 3%, CaO / SiO ₂ = 1.5, MgO = 5%, Al ₂ O ₃
<10%) 6000 kg, quicklime 570 kg, coke 4500 kg
Was added and blowing was performed for 45 minutes. Thereafter, 6,600 kg of ferrochrome was added to carry out crude desulfurization.

After the crude decarburization was completed, the temperature after the crude desulfurization by adding ferrochrome was 1628 ° C., the slag basicity was 1.5, and [% Cr] =
7.8%. [% C] in the melt at the end of the crude decarburization and crude desulfurization was 1.1%.

After that, the slag is discharged, and ferrochrome is
Finish decarburization treatment was performed by adding 4.4 T and quick lime 1.3 T.
Temperature after decarburization treatment is 1712 ° C, slag basicity is 1.5, [% C
r] = 12.9. Table 2 shows the amount of quicklime added during the decarburization period in the conventional method and the method of the present invention in Example 2.

[0036]

[Table 2]

[0037]

According to the present invention, during the decarburization after the next charge, when reducing chromium oxide in slag having a high melting point and low Al ₂ O ₃ concentration, the reduction rate can be maintained at a high value. The reduction time can be reduced and the refractory erosion can be suppressed, and continuous casting can be performed.

During the rough decarburization after the next charge, the terminal decarburized slag containing a large amount of Cr ₂ O ₃ is recycled, Cr ₂ O ₃ is reduced by C in the dephosphorized pig iron, and Si and Cr Cr ₂ O ₃ remaining in the slag with Si in the alloy by adding an alloy containing
By reducing and recovering chromium, it is possible to improve the chromium yield, simultaneously proceed with reduction and desulfurization, and reduce the amount of slag-making agent in the decarburization period.

[Brief description of the drawings]

FIG. 1 is a process diagram of a method according to the present invention.

FIG. 2 is a graph showing the relationship between reduction rate and basicity.

FIG. 3 is a graph showing the relationship between refractory erosion rate and basicity.

FIG. 4 is a graph showing a relationship between a reduction rate and a temperature.

FIG. 5 is a graph showing the relationship between refractory erosion rate and temperature.

FIG. 6 is a graph showing the relationship between the reduction rate and (% Al ₂ O ₃ ).

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C21C 1 / 02,5 / 28,7 / 00 C21C 7 / 04,7 / 064,7 / 068

Claims (1)

(57) [Claims] 1. A step of (i) receiving a molten metal made of dephosphorized iron into a converter or melting the molten metal in an electric furnace; (ii) roughly decarburizing and raising the molten metal in the converter or the electric furnace. (Iii) Crude desulfurization step of heating the crude molten steel; (iii) Crude desulfurization and reduction of chromium oxide by introducing Si content into the obtained molten metal to make crude stainless steel coarse molten steel; (iv) Generated slag (V) finish decarburization of the stainless steel crude molten steel to give a stainless steel molten steel; and (vi) discharge of the stainless steel molten steel after the generation of the slag containing Cr ₂ O _3. A method for producing stainless steel comprising a tapping process for producing steel, wherein Al ₂ O generated in a finish decarburization process (v) of a stainless steel crude molten steel
_{3 The} slag containing chromium oxide having a content of 10% or less is recycled and used in the crude decarburization / heating step (ii) after the next charge, and the carbon in the molten metal or the carbon-containing material added at the time of crude decarburization is used. When the chromium oxide is reduced by carbon and recovered in the molten metal, the slag basicity is 0.9 to 1.5 and the reduction temperature is 1500 to
After reducing the chromium oxide at 1650 ° C, the crude decarburization
At the end of reduction in the temperature raising step (ii), an alloy containing Si and Cr is added, and the chromium oxide remaining in the slag is recovered in the molten metal by the Si. A method for refining stainless steel, comprising adding a Cr-containing alloy and lime in a charcoal step (v) to perform decarburization.