JPH09241716A - Desulfurization method for electric arc furnace molten iron for stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the costs of a slag forming agent, electric power consumption and oxide raw materials in arc furnace operation by improving the desulfurization effect of ladle refining after tapping of the electric arc furnace molten iron for stainless steels. SOLUTION: At the time of melting the stainless steel molten iron in the electric arc furnace, tapping the molten iron together with slag into a ladle after refining and subjecting the molten iron to in-ladle refining, the refining in the electric arc furnace is executed by low-basicity slag and the in-ladle refining is executed by inert gaseous stirring under a nonoxidizing atmosphere condition, by which chromium oxide is reduced by Si in the molten iron and the desulfurization reaction is accelerated. The cost of the slag forming agent by the low-basicity slag and the electric power consumption are thus reduced and the recovery of the useful metal is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for refining stainless hot metal in an electric arc furnace.

[0002]

2. Description of the Related Art In the production of stainless steel by an electric arc furnace, various ferroalloys as raw materials, scraps or oxides as additional components are melted to produce crude steel (hot metal).
After manufacturing, the intermediate product is manufactured through a finishing refining process and a casting process using a vacuum degassing furnace or an AOD furnace. When operating an electric arc furnace, make a mix plan according to the target components and compositions for each charge, and in accordance with it, the main raw materials such as scrap, ferroalloys, etc., and dust generated in the steel factory in the furnace in order. , Auxiliary materials such as scale, slag and slag forming agent are charged, and they are melted by applying electricity. At that time, molten metal as a main raw material is hot-charged, or chrome ore and Ni are used.
Oxide raw materials such as ores are also supplied together with reducing agents such as coke. When the main raw material is dissolved and the slag layer is formed, adjusting the composition thereof is an important requirement in terms of its refining action and eventually in operation. To maintain the desulfurization ability properly, it is necessary to adjust the basicity of the slag, etc.
Usually, the required amount of CaO or CaF ₂ is empirically determined and is adjusted to an appropriate range by supplying it before or during the dissolution. After the melting, a refining period is provided to some extent, and after checking the components, the molten iron for stainless steel is tapped from the electric arc furnace. In such a refining process, it is desired that the slag is quickly dissolved and stirred with molten steel to maximize its desulfurization ability. For this reason, a method has been adopted in which a bottom blowing tuyere is installed in an electric furnace, gas such as nitrogen is blown, and stirring is performed by bubbling to accelerate the dissolution or scouring reaction of the slag-making agent. There is a limit to the stirring action of the system, and it is not effective when there are structural restrictions such as a shallow bath furnace such as an electric furnace or when the refining period after the burn through cannot be sufficiently taken. Also, Japanese Patent Application Laid-Open
No. 25938 discloses a method in which a slag-forming agent is blown from a hollow electrode at the time of arc heating to melt and refine it, but the equipment cost by the hollow electrode method is high, and the proportional cost of the electrode is increased. .

Recently, in order to reduce the cost of raw materials and to effectively use the waste materials generated in factories, various kinds of raw materials such as dust, scale and slag have been used. In particular, if the oxide raw material can be reduced and used as a metal raw material, it is very advantageous in terms of raw material cost. For this reason, these various and various raw materials are added and melted.However, in the operation of such an electric furnace, pulverization loss due to the origin of these raw materials, unslagging, and variation in raw material quality are caused. Therefore, it is difficult to uniformly dissolve and adjust the slag in the furnace. The slag in such a furnace does not melt even when tapping, remains non-uniform and unreacted, or cannot be properly adjusted, so that the scouring action cannot be fully exhibited. In order to deal with such a situation, in actual operation, in consideration of safety, an excessive amount of slag-making agent is charged, but as a result, the amount of slag finally produced increases. There is also a tendency to do so, and it may exceed 100 kg / T-metal. This means that the electric power consumed for the slag other than the original melting of the metal becomes large, and the increase in the electric power cost becomes a problem.

[0004]

DISCLOSURE OF THE INVENTION In order to solve such problems, the present invention improves the desulfurization effect of slag desulfurization ability by improving the desulfurization after ladle refining of electric arc furnace for stainless steel. By maximizing the refining effect, the cost of slag-making agent, power consumption, and raw materials in arc furnace operation is reduced.

[0005]

In order to achieve the object, the present invention is to melt stainless hot metal in an electric arc furnace,
In the method of tapping the ladle with slag after refining and refining in the ladle, refining in an electric arc furnace is performed at low basicity, and refining in the ladle is agitated with an inert gas under non-oxidizing atmosphere conditions. As the above-mentioned hot metal component, [Cr] = 10 to 35%, [Si] = 0.1-1.
2%, [C] = 2.0 to 5.0%, and the slag basicity (CaO / SiO ₂ ) before tapping is 1.3 to 2.2, and the oxygen concentration in the ladle atmosphere Is maintained at 10% or less.

[0006]

When the stainless hot metal is melted by the electric arc furnace, the slag produced is not only the one added as a component in the auxiliary raw material as described above, but also the one in which the Cr or Ni ore component and the refractory of the lining are melted and lost. Etc. are included. The main slag component in the electric arc furnace is C
Examples include aO, SiO ₂ , MgO, Al ₂ O _{3 and the} like. The action of slag in the electric arc furnace is in addition to the action on refining,
It has functions such as heat retention and protection of refractories from arcs. As a result of various studies on the action of slag in the electric arc furnace, the present inventors have found that [Si] in the hot metal causes reduction reaction of chromium oxide with slag and hot metal in the ladle after tapping. It was found that the slag's original refining ability can be fully exerted by sufficient stirring under the constant conditions that contribute to it, and the amount of the slag additive added was minimized to produce the slag by adding excess slag material. The cost of slag and power consumption is reduced, economical desulfurization is possible, and quality is stabilized.

FIG. 1 shows the relationship between basicity and electric power consumption (cost) in an electric arc furnace and the relationship between basicity and hot metal [S] after tapping, which are obtained by the conventional method. From this figure, it is understood that as the basicity of the slag increases, the desulfurization ability also improves and the hot metal [S] decreases, but on the other hand, the power consumption rate also increases accordingly. However, the present inventors have shown through various research results and experiments under actual operating conditions that the desulfurization ability of slag can be maximized under certain conditions, and sufficient desulfurization can be performed at a lower basicity. Clarified that it is possible. That is, after refining in the electric furnace, when tapping into the ladle together with slag and refining in the ladle, the atmosphere in the ladle is a non-oxidizing atmosphere, and an inert gas is blown from the immersion lance to stir. 2 promotes the reaction between the slag and the hot metal, and the results are shown in FIG. 2 together with [S] after tapping, that is, by the conventional method. As shown in FIG. 2, compared to the conventional method,
It can be seen that the extremely low hot metal [S] is realized by the lower slag basicity, and the desulfurization ability is improved. Conventionally, after refining in an electric furnace, "deposited iron desulfurization" is carried out using the stirring force by the falling energy during tapping. The greatest difference from the present invention lies in the atmosphere, not to mention the difference in stirring intensity due to the difference in this stirring method. Conventionally, tapping is performed in the atmosphere, so that the atmosphere becomes an oxidative atmosphere. On the other hand, in the present invention, the oxygen concentration in the atmosphere is set to 10% or less to avoid direct contact with the atmosphere. This facilitates the reduction of chromium oxide, which is advantageous for desulfurization.

According to the studies by the present inventors, it is possible to achieve the desulfurization reaction which is the most economical and the target, when the slag basicity before tapping in this case is 1.3 to 2.2. Found out. As shown in Fig. 2, by setting the slag basicity before tapping to 1.3 to 2.2, the target "S" in the hot metal can be 100 ppm or less.
If it is less than, the target desulfurization reaction ([S] ≦ 100 p
If the basicity exceeds 2.2 and basicity exceeds 2.2, further increase in basicity will no longer improve the drainage effect, but rather will increase the cost of slag waste agent and the unit price of electricity. It will be.

In the present invention, the desulfurization reaction is promoted by stirring the hot metal and the slag. This reaction is associated with the reduction reaction of an acidic component mainly containing chromium oxide in the slag under a non-oxidizing atmosphere. Desulfurization proceeds. It is important for desulfurization reaction to reduce chromium oxide, which is the oxygen potential of the reaction system, and reduce its concentration. in this way,
The reduction reaction of chromium oxide in a non-oxidizing atmosphere is necessary from the viewpoint of desulfurization, and at the same time, it is important in reducing the cost of Cr, which is the main component of stainless steel. In this case, slag and carbon in the metal also contribute as a reducing agent for the reduction reaction, but Si in the metal is mainly used. Therefore, in order to carry out efficient desulfurization,
Must work effectively. In order for Si in the metal to efficiently act on the reduction of chromium oxide, it is important to use a non-oxidizing atmosphere. This is S in the atmosphere
i is consumed by oxidation and is not used for reduction of chromium oxide. Further, as a means for increasing the desulfurization ability, C
Although there is a method of increasing aO to increase basicity, the present invention aims to achieve the desired desulfurization by exerting the effect of decreasing the chromium oxide concentration even if the basicity is decreased.

In the present invention, Si in the hot metal can be made to act effectively by setting the oxygen concentration in the stirring atmosphere in the ladle to 10% or less. When the oxygen concentration increases, Si is oxidatively consumed by oxygen in the atmosphere during stirring, and the ratio used for the reduction reaction of slag decreases. According to the research conducted by the present inventors, the oxygen concentration in the atmosphere also affects the reduction rate of chromium oxide and thus the desulfurization reaction rate. For example, it has been found that the desulfurization reaction rate decreases remarkably as the oxygen concentration in the atmosphere increases, but this means that long-term stirring is required to obtain the target [S]. . If the stirring is carried out for a long time, the temperature of the hot metal is drastically lowered, and the stirring for a long time in an oxidizing atmosphere consumes Si in addition to the reduction reaction of the oxide in the slag, thereby increasing the Si cost. means. in this way,
Acceptable by comprehensively looking at the merit of recovery by reduction of oxides, lower basicity by improving defluxing effect, reduction of slag material, reduction of power consumption, and demerit of Si consumption and temperature decrease. Target metal [S] within possible stirring time
The Si efficiency (Si amount consumed for reduction of oxides in slag / total Si consumption amount) required to achieve the above is calculated. Fig. 3 shows the oxygen concentration in the atmosphere and [S] and Si in the metal. The relation of efficiency is shown. As shown in FIG. 3, [S] in the hot metal is reduced to 100 ppm or less in the region where the oxygen concentration is 10% or less, and the Si efficiency is 25% or more by setting the oxygen concentration to 10% or less.

As the inert gas, nitrogen, Ar or the like can be used. Nitrogen is preferable from the economical viewpoint, but Ar is used depending on the type of steel such as when nitrogen pickup is harmful. If necessary, a reducing gas such as H ₂ or CO may be mixed with the inert gas. It is preferable to use an immersion lance for stirring with an inert gas, because the stirring conditions such as gas flow rate and depth can be freely set.
For example, for a ladle with a scale of hot metal of 50 to 100 T, a gas flow rate of 200 to 1500 NL /
Although the immersion depth of the lance is preferably 50 to 200 cm, optimum conditions are set depending on the amount of slag (thickness) hot metal and the target [S]. Further, the optimum slag / metal stirring state can be adjusted by changing the lance shape, the nozzle diameter, and the number of nozzles.

In the present invention, it is important to always maintain the atmospheric oxygen concentration during stirring at 10% or less. At the start of blowing the inert gas, the inert gas is supplied as a seal gas into the ladle in advance to lower the oxygen concentration of the atmosphere inside the ladle, and then the blowing with the immersion lance is started. After the start of blowing, the atmosphere can be maintained by supplying the stirring gas and the seal gas.
In this case, in order to reliably maintain these atmospheric conditions, it is effective to provide a lid that covers the ladle, which can also reduce the supply amount of the seal gas. There is no particular limitation on the components such as Cr and C of the hot metal of the present invention,
Ordinary stainless hot metal composition [Cr] = 10 to 3
5% and [C] = 2.0 to 5.0%. [S
With regard to i], if it is less than 0.1%, it is insufficient for the reduction of chromium oxide that regulates the desulfurization action, and [Si] remaining after stirring is insufficient as a heat source in the subsequent converter refining. Also, if it exceeds 1.2%, when blowing the converter,
The amount of heat becomes excessive and the temperature rises excessively. At the same time, the basicity is reduced, causing refractory erosion and unfavorable raw material costs.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Next, a description will be given based on Examples. 9
Using a 0-ton electric arc furnace, various scraps and oxide raw materials are melted, and the target components are Cr: 18%, C: 3.5.
%, Si: 0.25% of hot metal for stainless steel was smelted for 3 charges (charge No. 1 to 3), but due to low Si operation by changing the raw material composition, it was necessary to improve the efficiency of hot metal Si consumption. . During the dissolution and after the raw materials were dropped, CaO as a desulfurizing agent was intermittently supplied into the furnace to adjust the basicity. After adjusting the metal components, a slag sample was collected and analyzed immediately before tapping, and as a result, the basicity was about 1.8. Next, tap / slag in the special ladle, put the lid on the special ladle in advance,
After filling the ladle with Ar gas, gas stirring with the immersion lance was started. The oxygen concentration of the atmosphere in the ladle during stirring is measured by a solid battery type oxygen sensor,
It was adjusted by the supply amount of the seal gas (Ar). The average concentration during stirring was 1.5 to 9.8% and 10% or less. After stirring for 10 minutes, a hot metal sample was collected, analyzed and confirmed, and then the ladle was transferred to the converter factory in the next step. As shown in Table 1, [S] after gas stirring was 39 to 45 ppm, which was a target value (≦ 100 ppm). The power consumption rate is 460 to 465 KWH / T, and the Si efficiency (Si consumed for reduction of oxides in slag / total Si consumption) estimated from the analysis value [Si] before and after stirring is 30 to 62%.
Met.

On the other hand, as a comparative example, in a 90 ton electric arc furnace, various scraps and oxide raw materials were melted, and the target components were Cr: 18%, C: 3.5% and S.
i: 0.25% of hot metal for stainless steel was melted for 3 charges (charge No. 4 to 6). However, as in the case of the embodiment, the low Si operation by changing the raw material composition improves the efficiency of hot metal Si consumption. Was needed. During the dissolution and after the raw materials were dropped, CaO as a desulfurizing agent was intermittently supplied into the furnace to adjust the basicity. After adjusting the metal components, a slag sample was collected and analyzed immediately before tapping, and as a result, the basicity was 1.8 to 1.6. next,
The iron / slag was tapped into a dedicated ladle, and the inside of the ladle was filled with Ar gas in advance, and then gas agitation by the immersion lance was started. The amount of seal gas (Ar) supplied to the ladle during stirring was intentionally reduced, and the oxygen concentration in the ladle atmosphere was measured by a solid-state oxygen sensor.
Adjusted to ~ 12%. In Charge Nos. 4 and 5, [S] after gas stirring was 62 ppm and 60 ppm, respectively, and the decrease range was smaller than that of [S] before tapping. Further, the Si efficiency was as low as 20 to 21%. This is because the desulfurization rate is reduced due to the low hot metal Si,
It is considered that desulfurization did not proceed with stirring for 10 minutes. Therefore, next, with charge No. 6, the stirring time is 2
It was extended to 0 minutes. The slag basicity before tapping is 1.60,
Metal [S] was 82 ppm. However, due to the high oxygen concentration atmosphere, the Si efficiency was lowered to 19%, and the metal [S] after stirring was 85 ppm, which was the result of re-sulfurization. As described above, in the comparative example, since the amount of ineffective consumption of Si is large, when the temperature compensation after the next converter process is taken into consideration, the hot metal “Si” becomes insufficient, and eventually, it is necessary to add the Si alloy immediately before the converter operation. Resulting in increased costs.

[0015]

[0016]

As described above, according to the method of the present invention, by adjusting the atmosphere in the ladle at the time of stirring, the efficiency of consumption of Si in the hot metal for the reduction reaction is improved, and the Cr content in the slag is increased. At the same time, the desulfurization reaction can be promoted and the target desulfurization can be achieved by the stirring process in a short time. Even after operating at a basicity lower than the level conventionally required, the target desulfurization can be achieved by fully exerting the refining ability of the slag by stirring in these non-oxidizing atmospheres after tapping. The action can be achieved. Therefore, the lime basic unit can be reduced, at the same time, the slag amount can be reduced, and the electric power basic unit can be reduced. Furthermore, for this reason, a long stirring time is not required, and the temperature decrease during that time can be suppressed to a minimum. As described above, the present invention enables cost reduction in the manufacturing method of stainless steel by an electric furnace, effectively uses an oxide raw material having a high utility value, effectively utilizes resources, and industrially develops from the standpoint of environmental protection. Can contribute to.

[Brief description of drawings]

Fig. 1 Relationship between basicity after tapping and [S] and electric power consumption by conventional method

[Fig. 2] Relation between basicity before tapping, [S] after tapping, and [S] after stirring

[Fig. 3] Oxygen concentration of the atmosphere in the ladle is [S] after stirring,
Effect on Si efficiency

Claims (2)

[Claims] 1. A method of melting stainless hot metal in an electric arc furnace, refining it with slag in a ladle after refining, and refining in a ladle, wherein refining in an electric arc furnace is performed with low basicity slag. A method for desulfurizing molten metal in an arc furnace for stainless steel, characterized in that refining in a pan is carried out by stirring an inert gas under non-oxidizing atmosphere conditions. 2. As a hot metal component, [Cr] = 10 to 35
%, [Si] = 0.1 to 1.2%, [C] = 2.0 to
5.0%, and slag basicity (CaO / SiO 2 before tapping)
₂ ) is 1.3 to 2.2, and the oxygen concentration of the atmosphere in the ladle is maintained at 10% or less.
A method for desulfurizing the hot metal of the arc furnace for stainless steel described.