JPH0959708A - Method for efficently decarburization-blowing stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a reducing material cost by executing the oxygen-blowing decarburization refining while adding ferro-chromium alloy into molten steel at the specific temp. after specifying Al2 O3 concn. in slag and C concn. in the molten steel. SOLUTION: Crude molten stainless steel is melted by using molten iron of a blast furnace and ferro-chromium alloy and then, the decarburization- refining is executed in a refining furnace. At this time, a condition just before adding or during adding the ferro-chromium alloy is regulated. That is, the molten steel temp. just before adding the ferro-chromium alloy is made to 1450-1600 deg.C by adding metallic Al or aluminum dross containing the metallic Al as the temp. raising material to raise the temp. into the molten iron charged into the refining furnace. Then, after containing 10-30% Al2 O3 concn. in the slag and 2.5-4.0% [C] concn. in the molten iron, the oxygen-blowing decarburization-refining is executed while continuously adding the ferro- chromium alloy. By this method, the effective refining of the stainless steel can be executed.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to the reduction agent basic unit required for recovery of chromium by suppressing the oxidation of chromium and the generation of splash and dust during decarburization of blown acid in the refining of stainless steel in a refining furnace. It is an efficient decarburization and blowing method for stainless steel, which is characterized in that it is possible to melt and produce stainless steel with reduced cost, stable and high yield.

[0002]

2. Description of the Related Art Blowing acid decarburizing and refining furnaces for molten steel include top blowing converters, bottom blowing converters, top bottom blowing converters (LD-OB furnaces), AODs and VODs. For decarburization refining, stainless steel scrap is used as a raw material in an electric furnace, and ferrochrome (Fe-Cr), ferro-nickel (Fe-Ni), and other ferroalloys are added to it to melt high-carbon-containing stainless crude molten steel. Alternatively, a method of melting high-carbon stainless steel by melting and reducing chromium ore using a top-and-bottom blowing converter, and then melting the product with AOD or a top-and-bottom blowing converter to decarburize the product to produce a product, ,
Japanese blast furnace manufacturers are mainly used to load de-phosphorized and desulfurized blast furnace hot metal into the upper and lower blowing converter and gradually decarburize the acid while adding alloys such as Fe-Cr from the top of the furnace. The method of carrying out the decarburization of the propellant acid in the upper-bottom blown acid converter up to the middle carbon range of 0.25 to 0.7%, and the decarburization below this is carried out by the propellant acid decarburization under reduced pressure such as VOD. It is generally done.

In any of the methods, the loss of chromium, which has become an oxide after the end of blowing acid and has been transferred to slag, can be reduced and recovered by adding a reducing material such as ferrosilicon (Fe-Si) or Al. It is widely used, but especially in the method of decarburization of bromic acid while gradually adding ferrochromium from the top and bottom of the converter using blast furnace hot metal, smelting reduction of other electric furnaces and chrome ores is performed. Compared with the method of melting crude stainless steel by smelting and then decarburizing with blown acid, the dephosphorization / desulfurization hot metal that is the base metal has a lower temperature, and a large amount of ferrochrome alloy that is the cold material is used. Due to
Oxidation loss of chromium is large, leading to an increase in the basic unit of ferrosilicon for reduction and a deterioration in the basic unit of furnace material of the refining furnace due to an increase in the amount of slag.

Generally, as a method of improving decarbonation efficiency in a refining furnace such as a converter and efficiently performing decarburization refining to a low carbon region, a bottom blowing function is added to the converter to stir a steel bath. To accelerate the supply of carbon in molten steel to the hot spot of blown acid, which is the most active reaction region (Japanese Patent Publication No. 62-1460).
No. 2), the critical carbon concentration ([% C] ^* ) that shifts from the oxygen supply rate controlling region where the decarboxylation efficiency is maintained at a high level to the carbon transfer rate controlling region in steel where the decarboxylation efficiency decreases. ,
It was attempted to prevent the decarboxylation efficiency from decreasing by shifting to the lower side and controlling the blowing acid rate such as lowering the blowing acid rate in the carbon rate-controlling region of steel in the final stage of decarburization ( Iron and Steel, 68th year (1982), p.
1946).

Further, as a refining method for crude stainless molten steel, as disclosed in Japanese Patent Publication No. 54409/1989, a non-oxidizing gas is introduced below the bath surface of the molten steel to stir the molten steel, and at the same time, the molten steel is poured onto the bath surface. A method has been proposed in which a mixed gas of oxygen and a non-oxidizing gas is sprayed and the ratio of the non-oxidizing gas to oxygen in the mixed gas sprayed on the bath surface is increased as the carbon concentration in the molten steel decreases.

[0006]

As described above, when carrying out blown acid decarburization refining of stainless crude molten steel in a refining furnace such as a converter, the efficiency of decarboxylation is improved, that is, the oxidation of chromium. As a method for suppressing loss, Japanese Patent Publication No. 62-14
No. 602, Iron and Steel, 68th year (1982), p19
46, and the method disclosed in Japanese Patent Publication No. 1-54409.

However, in these methods, improvement of decarboxylation efficiency and suppression of chromium oxidation are aimed at from the middle stage to the final stage of decarburization, and particularly, in the case of using the blast furnace hot metal in the upper and lower blowing converters, For the method of performing decarburization of blown acid while continuously adding ferrochrome from the furnace, chromium oxidation loss in the middle to end decarburization can be suppressed, but chromium oxidation in this region is On the other hand, the ratio was very small, and the measure for suppressing chromium oxidation during the introduction of ferrochrome, where most of the chromium oxidation occurred, was insufficient.

Further, the Cr ₂ O ₃ -containing slag produced in stainless steel refining has a black slagification property due to the high melting point of Cr ₂ O ₃ , that is, the liquid phase ratio is low, and therefore, the removal of the propellant is not possible. The cover effect for dust and splash generated by charcoal is low, dust and splash are scattered outside the furnace, and the ingots adhere to the vicinity of the furnace top, which lowers the molten steel yield and hinders workability. There was a problem.

Therefore, an object of the present invention is to introduce ferrochromium, which accounts for most of the chromium oxidation, in the case of blown acid decarburization refining of crude stainless steel melt in a top-and-bottom blow converter using blast furnace hot metal and ferrochrome. By suppressing the chromium oxidation loss and reducing the generation of dust and splash generated during blowing, it is possible to efficiently produce the crude stainless steel melt.

[0010]

Means for Solving the Problems The present invention is to solve the above problems, and in demelting and refining in a refining furnace in smelting of stainless crude molten steel using blast furnace hot metal and ferrochrome alloy iron, By adding aluminum aluminum (Al) or aluminum dross containing 10 to 80% of metal Al as a heat-up material to the hot metal charged in the refining furnace to raise the temperature, the molten steel temperature immediately before the addition of the ferrochrome alloy is 145
0 to 1600 ° C., and an Al ₂ O ₃ concentration in slag of 10 to 30% and a [C] concentration in molten steel of 2.5 to 4.0%, while continuously adding a ferrochrome alloy, It is an efficient decarburizing and blowing method for stainless steel, which is characterized by carrying out decarburizing and refining with blown acid.

Here, it is desirable to charge the hot metal of the next charge while leaving the decarburizing slag containing the chromium oxide generated by the precharge in the furnace, and then add the carbonaceous material and the propellant acid. To reduce the chromium content in the decarburizing slag by heating with slag to remove the chromium-recovered slag, and then to remove metal aluminum (Al) or metal Al content from 10 to 80%.
By adding the included aluminum dross as a heat-up material and raising the temperature, the molten steel temperature immediately before the addition of the ferrochrome alloy is increased to 14
50 to 1600 ° C., the Al ₂ O ₃ concentration in the slag is 10 to 30%, and the [C] concentration in the molten steel is 2.5 to 4.0%
After that, by further adding the ferrochrome alloy while carrying out the decarburizing and refining of the acid, a greater effect can be obtained.

[0012]

The present invention defines the conditions immediately before and during the addition of a ferrochrome alloy in the blown acid decarburization refining of a crude stainless steel melt described below, and sets the liquid phase ratio of the slag during the decarburization blow to a high level. By holding the slag / metal reaction interface area large, the chromium oxide (Cr ₂
It is based on promoting preferential decarburization conditions by accelerating the reduction reaction of O ₃ ) by carbon in steel, and promoting a cover effect against splash and dust generated during decarburization with propionic acid.

As the decarburization reaction mechanism in the blown acid decarburization refining, first, oxygen blown into molten steel once forms chromium oxide (Cr ₂ O ₃ ), and this Cr ₂ O ₃ is in the slag. Transition to. After that, Cr ₂ O ₃ in the slag is reduced by the carbon content in the molten steel, so that the decarburization reaction proceeds. Therefore, the general decarburization reaction is represented by the balance of chromium oxidation and reduction of chromium by C in molten steel, and the decarburization reaction rate can be represented by the following equation (1). -D [% C] / dt = (Ak / V)-([% C]-[% C] _e ) (1) However, A: effective reaction interfacial area between slag / metal k: Decarburization rate constant V: Molten steel volume [% C] _e : Equilibrium carbon concentration.

Here, in the case of blown acid decarburization refining of molten steel containing chromium, when the driving force term (([% C]-[% C] _e )) in the equation (1) becomes small, decarburization is performed. Since the amount of oxygen to be consumed becomes smaller and remains consumed for the oxidation of chromium, the amount of oxidation of chromium increases as a result. Therefore, in order to suppress chromium oxidation and secure preferential decarburization, it is important to keep this driving force term as large as possible.

Specifically, metallic aluminum (Al) or aluminum dross containing metallic Al is added as a temperature raising material to the hot metal charged in the refining furnace to raise the temperature of the propellant acid, or by pre-charging. With the decarburizing slag containing the generated chromium oxide remaining in the furnace, the hot metal of the next charge was charged, and then the carbonaceous material in the decarburizing slag was added by adding carbonaceous material and reducing the temperature by blowing acid. Immediately before adding the ferrochrome alloy by reducing the chromium content, discharging the chromium recovered slag, and then adding metallic aluminum (Al) or aluminum dross containing metallic Al as a temperature raising material to raise the temperature. Molten steel temperature of 1450 ~ 1600 ℃, and in the slag A
The l ₂ O ₃ concentration is 10 to 30%, and the [C] concentration in the molten steel is 2.
It is set to 5 to 4.0, and thereafter, the blown acid decarburization refining is performed while continuously adding the ferrochrome alloy.

The inventors of the present invention have conducted various experiments in carrying out decarburizing and refining of stainless steel with the aid of blown acid to find that even in stainless steel slag containing 30% or more of Cr ₂ O ₃ ,
It was found that the addition of l ₂ O ₃ promotes the liquid phase formation of slag. The present invention is based on this finding.

In general, Cr ₂ O ₃ -containing slag produced during decarburization of stainless steel with blown acid has a characteristic that the slag-forming property is poor, that is, the liquid phase ratio of the slag is very low. In addition to the formation of Cr ₂ O ₃ which is a high melting point oxide, this Cr ₂ O ₃ and other slag component systems such as MgO and CaO.
By reaction with, MgO · Cr ₂ O ₃ and CaO · Cr ₂
This is due to the formation of high melting point compounds such as O ₃ .
However, such a low liquid phase slag has a content of 10 to 30
The liquid phase ratio of the slag is improved by adding Al ₂ O _{3 (} %) (FIG. 1). This is because the addition of Al ₂ O _3, CaO · by formation of CaO-Al ₂ O ₃ based compounds such as 12CaO, 7Al ₂ O ₃ system is a low melting compound
This is due to the disappearance of the Cr ₂ O ₃ phase and the like.

Further, due to the effect of this slag melting,
By maintaining the slag liquid phase ratio at 70% or more, the cover effect against the generation of splash is also promoted, which makes it possible to significantly reduce the amount of splash and dust generated, which leads to a decrease in molten steel yield and the occurrence of splash. It is possible to prevent the workability from deteriorating due to the adhesion of metal to the furnace mouth (Fig. 2).

Here, the addition amount of Al ₂ O ₃ is 10 to
The reason why 30% is desirable is that the slag composition is Cr ₂ O.
_{If the} Al ₂ O ₃ concentration is less than 10% with 30% or more of ₃ included, slag slag formation is insufficient, that is, the liquid phase ratio is 70%.
Due to the following, the molten steel yield decreases due to the generation of a large amount of splashes and dust, and the workability deteriorates due to the adhesion of the furnace mouth metal, and even if it exceeds 30%, This is because there is no slag formation promoting effect, and conversely, promotion of melting loss of refractory due to an increase in saturated MgO concentration becomes a problem.

In this case, the basicity (CaO / SiO 2
_{As 2} ), the range of 2.0 to 4.0 is desirable. This is because the melting loss of the refractory material becomes a problem in the range of CaO / SiO ₂ <2.0, and in the case of CaO / SiO ₂ > 4.0, the increase of chromium oxidation due to the large amount of slag is generated. This is a problem.

Further, as conditions for suppressing chromium oxidation during the addition of the ferrochrome alloy, the [C] concentration in the molten steel immediately before the addition of the ferrochrome alloy is 2.5 to 4.0%, and metallic aluminum (Al) is used. Alternatively, it is desirable that the molten steel temperature be set to 1450 to 1600 ° C. by adding aluminum dross containing metal Al as a temperature raising material and raising the temperature.

The reason for this is that if the [C] concentration in the molten steel immediately before the addition of the ferrochrome alloy is less than 2.5%, the driving force term in the above formula (1) during the addition of the ferrochrome decreases. Insufficient decarburization, that is, increase in chrome oxidation, which in turn leads to cost increase due to increase in carbonaceous material for reduction, and conversely, even if it exceeds 4.0%, almost no further effect of suppressing chrome oxidation is recognized. If the amount of decarburization is increased, the processing time is extended, and an excessive temperature causes an abnormal temperature increase, which causes operation inhibition.

When the molten steel temperature is less than 1450 ° C., the equilibrium [C] concentration ([% C] _e ) rises even if the [C] concentration is kept in the above range. As a result, the driving force is reduced, which causes an increase in chromium oxidation. On the contrary, even if the temperature exceeds 1600 ° C., the effect of suppressing chromium oxidation is small and the melting of refractories and the amount of coolant used increase due to excessive heat.

Here, the reason why aluminum aluminum (Al) or aluminum dross containing metallic Al is most suitable as the temperature raising material is Al ₂ O ₃ produced by the oxidation of Al by propellic acid or Al _{2 in} acmidross. In addition to the fact that O ₃ content can be applied to control the above slag composition, the unit calorific value due to Al heating is larger than that of ordinary carbonaceous material heating as shown in the following equations (2) and (3). This is because the temperature can be raised in a short time and the productivity is greatly improved. _{Al + 3 / 4O 2 → 1} / 2Al 2 O 3 + 442kcal / Nm 3 -O 2 ···· (2) C + 1 / 2O 2 → CO + 196kcal / Nm 3 -O 2 ···· (3)

[0025]

EXAMPLES Tables 1 and 2 show a comparison between the present invention (Table 1), a comparative example, and an example according to a conventional method (Table 2) when using a 175 ton top-bottom blowing converter. Here, the present invention-1 is a case where the temperature [% C] before addition of the ferrochrome alloy and the temperature were adjusted by the temperature increase of the propellant acid using aluminum dross as the heat-raising agent after charging the raw material such as hot metal. With the pre-charged unreduced slag remaining, the hot metal was charged, the carbonic acid was added and the temperature was reduced while blowing acid was increased, and then the slag was treated, after which the aluminum dross was added and the acid was added. This is the case where the [% C] and temperature before the subsequent addition of the ferrochrome alloy are adjusted by carrying out. Steel type is 16% in each case
Cr steel was used, and the blow-off [C] value was [C] = 0.7%.

[0026]

[Table 1]

[0027]

[Table 2]

As is clear from Tables 1 and 2, the use of the present invention achieves a high yield by greatly suppressing the generation of splashes and dusts in blowing acid, and reduces the chromium oxidation loss. It can be seen that it can be significantly suppressed.

[0029]

According to the present invention, by using blast furnace hot metal and ferrochrome alloy, in a method of decarburizing and refining stainless steel crude molten metal in a refining furnace, splash in propionic acid, and suppressing the generation of dust, and By suppressing the oxidation loss of chromium in the ferrochrome alloy, the cost of reducing material was greatly reduced, and efficient refining of stainless steel became possible.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the Al ₂ O ₃ concentration in slag and the liquid phase ratio of slag.

FIG. 2 is a diagram showing a relationship between a liquid phase ratio of slag and a dust generation index.

Claims (2)

[Claims] 1. In the melting of smelting of stainless crude molten steel using blast furnace hot metal and ferrochromium alloy iron, when decarburizing and refining in a refining furnace, the molten iron charged into the refining furnace contains metallic aluminum (Al) or metallic Al content. By adding aluminum dross containing Al as a temperature raising material to raise the temperature, the molten steel temperature immediately before the addition of the ferrochrome alloy is set to 1450 to 1600 ° C., and the Al ₂ O ₃ concentration in the slag is set to 10 to 30% in the molten steel. Efficient decarburizing and blowing method for stainless steel, characterized by carrying out blown acid decarburizing refining while continuously adding a ferrochrome alloy after the [C] concentration is adjusted to 2.5 to 4.0%. . 2. When decarburizing and refining in a refining furnace in smelting of stainless crude molten steel using blast furnace hot metal and ferrochromium alloy iron, decarburizing slag containing chromium oxide produced by precharging is placed in the furnace. While still remaining, the hot metal of the next charge is charged, and then the carbon content in the decarburizing slag is reduced by temperature reduction with addition of carbonaceous material and blowing acid, and then the chromium recovered slag is discharged. Then, metal aluminum (Al) or aluminum dross containing 10 to 80% of metal Al content is added as a temperature raising material to raise the temperature, so that the molten steel temperature immediately before the addition of the ferrochrome alloy is 1450 to 1600 ° C., and After the Al ₂ O ₃ concentration in the slag is 10 to 30% and the [C] concentration in the molten steel is 2.5 to 4.0%, blown acid decarburization refining is performed while continuously adding the ferrochrome alloy. Stainless that is characterized by Efficient decarburization blowing method of the scan steel.