JPH07224318A - Desulfurization method of molten steel - Google Patents

Abstract

PURPOSE:To prevent resulfurization and to improve desulfurization efficiency by incorporating a desulfurizing agent to molten steel, then incorporating a flux, such as magnesia, thereto at the time of desulfurizing the molten steel by using RH degassing equipment. CONSTITUTION:The powdery flux which is composed of >=1 kinds among magnesia, alumina, calcia and zirconia and has the m.p. above the temp. for treatment with the desulfurizing agent is incorporated into the molten steel under reflux or into the molten steel in a or in both of them ladle after addition of the desulfurizing agent to form a flux layer in the lower part of the slag in the ladle, by which resulfurization from the slag is prevented. The desulfurization rate by the treatment exhibits a tendency to decrease when the content of (FeO+MnO) in the ladle slag exceeds 8%. The high desulfurization rate of nearly 80% is obtd. in the case of <=8%. The desulfurization method is applicable to ladle refining as well and the high desulfurization rate is attained by adding the powdery flux to the molten steel in the ladle after incorporating the desulfurizing agent thereto.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for desulfurizing molten steel which prevents re-sulfurization.

[0002]

2. Description of the Related Art In recent years, cleanliness steel in which impurities such as carbon and sulfur have been reduced to the utmost has been demanded in response to an increasing need for higher performance and higher quality of steel materials. Conventionally, when smelting such cleanliness steel, RH and VOD after tapping the primary refining furnace
Vacuum decarburization using degassing equipment such as LF, and then moving the ladle to ladle refining equipment such as LF, and desulfurization treatment as a separate process (hereinafter referred to as ladle refining desulfurization technology) .

In the ladle refining desulfurization technology, it is first necessary to control the composition of the ladle slag within an appropriate range (this is called slag reforming) in order to improve desulfurization efficiency and prevent re-sulfurization. Therefore, after the oxidizing slag that inevitably flows into the ladle from the primary refining furnace is discharged, the slag reforming flux is arc-melted to obtain the desired desulfurization slag composition. Then, a desulfurizing agent in powder form is blown into the molten steel to proceed desulfurization. Therefore, in this technique, the flux melting power, the damage to the ladle refractory due to the high temperature arc heat, and the increase in the processing time due to the melting are caused. Further, in order to remove the hydrogen absorbed by the flux, it is necessary to return to the degassing equipment and perform a dehydrogenation step after the desulfurization treatment is completed.

On the other hand, recently, in an RH vacuum degassing facility, a technology for producing ultra low sulfur steel after vacuum decarburizing treatment,
In other words, the process integration technology by making the RH multifunctional is disclosed. For example, in "Iron and Steel" 73 (1986), S263 issued by the Iron and Steel Institute of Japan, a J-shaped lance is arranged at the lower part of the RH rising side immersion pipe, and powdery desulfurization flux is supplied from this lance to the rising side immersion pipe. A technique of performing desulfurization treatment by injecting toward the lower part (hereinafter referred to as a bottom pipe blowing method) is also disclosed in JP-A-5-23953.
In Japanese Patent No. 4 publication, a technique for disposing desulfurization by disposing a desulfurization flux in the form of powder on a steel bath surface under reduced pressure by disposing an upper-blowing lance from the upper part in an RH degassing vacuum tank (hereinafter, (Referred to as a top blowing method in a vacuum chamber).

In the lower part of the ascending tube, a powdery desulfurization flux containing CaO or the like as a main component is blown upward from the lower part of the ascending-side dip tube from a J-type lance. Then, the flux is introduced into the vacuum tank by being guided to the rising reflux, and reacts with sulfur (hereinafter referred to as S) in the molten steel in the vacuum tank according to the following equation (1), and a part of CaS It is generated and, in the state of being suspended in molten steel, recirculates into the ladle via the downcomer pipe. CaO + S → CaS + O (1)

[0006] Other unreacted CaO reacts with S in the molten steel in the ladle to produce CaS. These CaS then float in the molten steel in the ladle and are captured by the slag in the ladle, whereby desulfurization proceeds.

The top-blown method in the vacuum tank is different from the above-mentioned bottom pipe blowing method in that the desulfurization flux is added to the surface of the steel bath under reduced pressure. Since the flux recirculates from the inside of the tank through the downcomer into the ladle, the desulfurization reaction mechanism and the desulfurization effect thereafter are the same.

In the desulfurization method using these RH equipments, since the same RH equipment can carry out the desulfurization treatment after the vacuum decarburization treatment, there is no ladle transfer from the RH equipment to the ladle refining equipment. In addition, since slag reforming is not required, there is no need for flux melting power or a dehydrogenation step, and there is an advantage that a significant reduction in processing time and reduction in manufacturing cost can be achieved.

[0009]

In the desulfurization method using the above-mentioned RH equipment, as disclosed in JP-A-5-214424, (FeO + Mn in ladle slag is disclosed.
O) content is 80% when controlled below 5%
Although a high degree of desulfurization (a definitional formula will be described later) is achieved, as the above content exceeds 5%, the desulfurization rate sharply decreases, and it is said that ultra low sulfur steel cannot be melted. The problem occurs.

The reason for this is that in the above desulfurization method, the slag in the ladle is not reformed, so the composition of the slag in the ladle remains in the undeoxidized state from the primary refining furnace, and (FeO +
The MnO) content is as high as 10% or more. Therefore, the oxygen potential of the ladle slag is high, the term on the right side of the desulfurization equilibrium reaction of the formula (1) becomes large, and the reaction of the formula (1) becomes difficult to occur. Therefore, the sulfide capacity of the slag is reduced, the S value level reached by desulfurization (referred to as desulfurization arrival S concentration) is increased, and the desulfurization rate is decreased.

At the same time, if the term on the right side of the equation (1) is larger than the metallurgical reaction equilibrium theory, the reaction in the opposite direction of the equation (1), in other words, the vulcanization reaction is likely to occur. (Fe in the slag
The higher the (O + MnO) content, the greater the amount of re-sulfurization because the reverse reaction increases. This resulfurization reaction starts immediately after the desulfurization treatment ends and continues during casting, so the S value in the mold becomes larger than the S value at the end of the desulfurization treatment.

As described above, in the desulfurization method using RH, although the treatment time is greatly shortened and the manufacturing cost is reduced by the process integration, the desulfurization ability is lowered because the slag reforming is not performed. However, the S concentration reaching desulfurization becomes high, and at the same time, re-sulfurization occurs during casting, which causes a problem that it is impossible to produce ultra low-sulfur steel.

The present invention has been proposed in order to solve the problems of the above-mentioned prior art, in which the desulfurization treatment is performed after the decarburization treatment in the RH equipment, and the process is integrated. Moreover, the present invention provides an extremely low-sulfur steel melting technology for promoting desulfurization treatment while preventing re-sulfurization from slag by a simple method without modifying ladle slag.

[0014]

According to a first aspect of the present invention, there is provided a desulfurization treatment method in which desulfurization is carried out by using an RH degassing equipment, and after the desulfurizing agent is added, the molten steel is circulated in a circulating manner or in a ladle. To both of these, powdery flux composed of one or more of magnesia, alumina, calcia, and zirconia and having a melting point of desulfurization temperature or higher was added, and the flux layer was formed at the bottom of the slag layer in the ladle. It is a method for desulfurizing molten steel, characterized in that it is formed.

According to a second aspect of the present invention, in the desulfurization treatment method of desulfurizing using a ladle refining facility, after adding a desulfurizing agent, the powdery flux is added to the molten steel in the ladle. The method for desulfurizing molten steel according to claim 1, wherein

[0016]

In the present invention, after the desulfurizing agent is added in the RH, it is powdered in the molten steel in the ladle in the lower part of the rising pipe, in the steel bath surface in the vacuum tank, in the circulating molten steel such as in the rising pipe and in the descending pipe. The flux for preventing vulcanization (hereinafter referred to as the flux for preventing vulcanization) is added, so that the flux for preventing vulcanization is mixed into the molten steel in the circulating ladle via the downcomer in a short time. Moreover, since the flux is added in powder form together with the carrier gas, it can be uniformly dispersed in the molten steel in the ladle, so that it can float in the molten steel throughout the ladle and reach the lower portion of the slag layer in the ladle.

Further, magnesia, alumina, calcia, and zirconia each have a melting point of 1700 ° C. or higher. Further, the desulfurization preventing flux is also composed of one or more of these, and since the melting point of the resulfurization preventing flux is higher than the desulfurization treatment temperature (generally 1650 ° C. or lower), it does not dissolve during the desulfurization treatment. Furthermore, CaO-described later
Since it has the property that it does not melt with the slag in the ladle, which is a SiO ₂ -Al ₂ O ₃ -MgO system, it remains as powder independently in the lower part of the slag in the ladle, and furthermore, it has a uniform thickness throughout the pot. A sulfur prevention flux layer is formed.

Further, magnesia, alumina, calcia,
The zirconia oxide has a low thermodynamic standard free energy of formation and is stable, and does not react with oxygen in the oxide and CaS (desulfurization reaction product) in the slag in the ladle. No reversion occurs as the reaction in the opposite direction of the equation does not occur. For the same reason, these oxides and Si, Mn, A
Since it does not react with solute elements in molten steel such as l, the components in the molten steel do not change. Therefore, the molten steel is not contaminated due to the formation of reaction products with solute elements.

Here, if the melting point of the vulcanization preventing flux is lower than the desulfurization temperature, a part of the vulcanization preventing flux is melted in the vacuum chamber and adheres and deposits on the surface of the refractory in the chamber. Since it becomes easier, it causes troubles such as a decrease in the RH ring flow rate. Further, the slag in the quaternary system reacts with the slag and melts to form an independent flux layer for preventing vulcanization,
Since the function as a blocking layer is not exerted, re-sulfurization cannot be prevented.

Further, during the RH desulfurization treatment, the molten steel stirring energy due to reflux is large, but compared with the VOD treatment,
Since the vertical movement between the slag and the molten steel in the ladle is extremely small, the vulcanization preventing flux layer does not break. For this reason, the flux layer can stably maintain the physical blocking function, so that the addition of a small amount of flux can prevent the vulcanization.

Further, since the flux for preventing vulcanization is added to the molten steel which is refluxed with a large amount of stirring energy after the addition of the desulfurizing agent, the flux layer is formed in the lower portion of the slag in the ladle in a short time. For this reason, there is no time margin for the reaction in the ladle slag to occur in the opposite direction of the equation (1), so that the vulcanization can be prevented.

Here, based on the test results by the inventors,
The action of the vulcanization preventing flux layer in the present invention will be specifically described. Figure 1 shows (FeO + Mn in ladle slag
O) content, in other words, the effect of varying the oxygen potential in the ladle slag on the desulfurization rate was investigated. Here, after the completion of vacuum decarburization, a small amount of a slag modifier mainly composed of CaO containing Al and metal Al for deoxidation is added to the tank to adjust the (FeO + MnO) content to 0.5 to After changing to a range of 12%, desulfurization flux was added by a top blowing method in a vacuum chamber. In the example according to the present invention (indicated by ● in the figure), after the desulfurization flux addition was completed, a desulfurization preventing flux having the composition of Example 1 shown in Table 2 described later and having a CaO content of 75% was used. It was added to the inside of the tank by using an upper blowing lance to form a blocking layer composed of the above flux layer. On the other hand, in the comparative example (indicated by a circle in the figure), the flux for preventing vulcanization is not added. Other conditions were the same as those in Table 2 described later.

From the results shown in FIG. 1, in the example in which the flux for preventing vulcanization was added, the content of (FeO + MnO) was 0.1.
In the range of 5 to 8%, a high desulfurization rate of almost 80% was achieved, but in the range of 8 to 12%, the desulfurization rate tended to be slightly lower than 80%. The reason for this can be explained as follows. In the range where the (FeO + MnO) content is 8% or less, the desulfurization ability does not decrease and a high desulfurization rate of 80% is achieved, and at the same time, the blocking effect by the flux layer formation works and the re-sulfurization is prevented. It shows that the desulfurization rate is maintained at 80%. However, if it exceeds 8%,
By the formation of the flux layer for preventing desulfurization, the desulfurization preventing effect works as in the case of 8% or less, but the desulfurization ability decreases,
It is considered that the desulfurization rate tends to slightly decrease because the S concentration reaching desulfurization increases.

On the other hand, in the comparative example in which the re-sulfurization preventing flux is not added, a high desulfurization rate of 80% is secured up to the range of (FeO + MnO) content of 2% or less, but in the range of more than 2%, The desulfurization rate is rapidly decreasing. The reason for this is that, as described above, at 2% or less, a high desulfurization rate of 80% is achieved, and at the same time, it is possible to prevent re-sulfurization.
A desulfurization rate of 80% is retained. On the other hand, if it exceeds 2%,
Although a high desulfurization rate of 80% can be obtained, it is considered that the desulfurization rate sharply decreases because the re-sulfurization rapidly increases.

In the present invention, the flux for preventing vulcanization is added from both the molten steel which recirculates and the molten steel in the ladle. By this method, the amount added per unit time can be increased and the time margin for re-sulfurization is reduced, so that a high desulfurization rate can be achieved even under conditions of high (FeO + MnO) content and high re-sulfurization rate.

Further, even if a plurality of lances are dipped in the flux for preventing vulcanization and all are blown from the molten steel in the ladle,
Since a uniform flux layer can be formed over the entire ladle, it is possible to prevent vulcanization.

The invention according to claim 2 is a desulfurization treatment method in which desulfurization is performed using a ladle refining facility, and after adding a desulfurizing agent, molten steel in which the powdery flux is added to the molten steel in the ladle It is a desulfurization method. Here, when the slag has a high viscosity at the end of the degassing process in the previous step and cannot completely remove slag, the (FeO + MnO) content becomes 2% or more even if the slag is reformed, and vulcanization is prevented. Sometimes you can't.

As an example of the present invention, even under the above-mentioned conditions, after the desulfurizing agent is added, a dip lance containing the desulfurizing agent is used, and the powdery vulcanization preventing flux is put together with the carrier gas in the ladle. By blowing the molten steel into the molten steel, the flux layer can be formed under the ladle slag, so that the vulcanization can be prevented.

[0029]

[Example] The hot metal tapped from the blast furnace was desulfurized and dephosphorized by hot metal pretreatment, and then blown in a converter to produce undeoxidized molten steel having a composition range shown in Table 1.

[0030]

[Table 1]

This undeoxidized molten steel is used in an RH degassing apparatus, and from the tuyere blown with Ar gas for circulation, 2500 to 30
It was confirmed that the carbon concentration reached 30 ppm or less by blowing Ar gas flow rate of 00 Nl / min, refluxing the molten steel, and performing vacuum decarburization treatment for about 15 minutes. After that, Al was added to the steel bath in the vacuum tank to deoxidize it, and then desulfurization treatment was started. The desulfurization method was performed by two methods: an upper blowing method in a vacuum tank and a lower blowing method of the rising pipe.

FIG. 2 shows that after desulfurizing flux is sprayed by the upper spraying method in the vacuum tank, powdery re-sulfurization preventing flux is sprayed on the steel bath surface in the tank using the same upper spraying lance, and the slag in the ladle is slag. Fig. 2 shows an implementation state of the present invention in which a flux layer is formed between the molten steel and molten steel to prevent re-sulfurization.

Here, 1 is a RH vacuum tank, 2a is an ascending pipe (immersion pipe), 2b is a descending pipe (immersion pipe), 3 is a ladle, 4 is molten steel, 5 is an upper blowing lance, and 6 is Ar for circulation. Gas blown tuyere, 7 is flux for preventing vulcanization, 8 is ladle slag, 9
Is a flux layer for preventing vulcanization.

In the ladle 3, there are about 2 desulfurized products.
There is an extremely low sulfur molten steel 4 having 50 tons of S concentration of 10 ppm or less. A flux layer 9 for preventing re-sulfurization having a uniform thickness is formed on the molten steel 4, and further, (FeO + Mn) is formed on the flux layer 9.
There is a desulfurized ladle slag 8 containing O) and CaS.

The desulfurizing flux is CaO-CaF ₂ -M.
One type of gO-based flux, and the content of each component is C
aO: 60% by weight (hereinafter abbreviated as%), CaF ₂ : 20
%, MgO: 20% flux was used. Moreover, the basic unit of the desulfurization flux was uniformly set to 4 kg / T. In the example according to the present invention, after the desulfurization flux is added,
From the same top blowing lance, 1 kg / T of the vulcanization preventing flux 7 shown in Table 2 was sprayed.

[0036]

[Table 2]

The flux composition for preventing vulcanization is composed of 1 to 3 kinds of magnesia, alumina, calcia and zirconia, and the total content is 50% by weight or more, and the melting point of the flux is 1650 ° C. or more. And

Further, the vulcanization preventing flux 7 was made into a powder form to facilitate blowing, and argon gas was blown as a carrier gas. The degree of vacuum in the tank was set to a high vacuum of 0.5 to 2 torr from the start of the addition of the desulfurization flux to the end of the addition of the above-mentioned flux 7, and the desulfurization treatment and the dehydrogenation treatment were simultaneously performed to shorten the treatment time.

Table 2 shows the results of Examples according to the present invention and Comparative Examples. In the examples, eight types of flux-reducing flux compositions were applied by the above-mentioned two addition methods. The determination of the effect of preventing re-sulfurization according to the present invention is carried out by analyzing the S value in three molten steels before desulfurization treatment (at the end of decarburization), after desulfurization treatment, and in the mold during continuous casting (points of raw steel composition analysis). It evaluated by comparing. The desulfurization effect is a desulfurization rate (definition formula = [1- (S in mold) calculated from change in S value before desulfurization treatment and S value in mold].
Value) / (S value before desulfurization treatment)] × 100%).

In the comparative examples (six examples) in which the vulcanization preventing flux 7 was not added, the S value could be reduced to 5 to 6 ppm after the desulfurization treatment, but the S value in the mold was 10 to 1 in all cases.
It has increased to 2ppm, and has been re-sulphurized. For this reason,
The desulfurization rate is low in the range of 60 to 67%. On the other hand, in the examples (8 examples) in which the flux 7 for preventing sulfur reduction was added, the S value in the mold remained at 5 to 6 ppm, which was the same as the S value after desulfurization treatment, or increased by 1 ppm or less. And prevents vulcanization. Therefore, 79-83%
A high desulfurization rate in the range of was achieved.

Under the desulfurization conditions in which the viscosity of the ladle slag is low and the molten steel level in the ladle is large, the flux layer 9 for preventing desulfurization breaks, the ladle slag and molten steel come into contact with each other, and Sulfur occurs. In the present invention, by adding from both the molten steel that recirculates and the molten steel in the ladle, the thickness of the vulcanization preventing flux layer 9 can be secured and vulcanization can be prevented without extending the addition time.

When the flux 7 for preventing re-sulfurization is added only from the molten steel which recirculates, it is evenly distributed over the entire ladle depending on the desulfurization conditions such as the amount of Ar gas blown for recirculation, the diameter of the recirculation pipe and the pot capacity. In some cases, the barrier layer composed of the thick flux layer 9 is not formed. In this case, when the thickness of the flux layer 9 is thin, the effect of the blocking layer does not work and re-sulfurization occurs. As a countermeasure against this, by adding the molten steel in the ladle aiming at a thin portion, the lack of thickness was eliminated and vulcanization could be prevented.

Further, when the vulcanization preventing flux 7 is added to the circulating molten steel, the above-mentioned flux 7 may adhere and deposit on the refractory surface of the vacuum chamber or the reflux pipe, which may impair the function of the RH. is there. As a measure against this, by blowing into the molten steel in the ladle using a plurality of lances without passing through the RH, the above-mentioned adhesion and accumulation were eliminated and vulcanization could be prevented. In this case, in order to form a uniform vulcanization preventing flux layer 9 in the entire ladle, it is desirable to immerse and blow a plurality of lances into the molten steel from a plurality of locations.

In the ladle refining desulfurization technique, after the degassing treatment in the previous step, the ladle slag may have a high viscosity and may not be completely discharged. In this case, even if a predetermined amount of the reforming flux is added, the (FeO + MnO) content exceeds 2%, and therefore, desulfurization occurs after desulfurization. As a countermeasure against this problem, after the desulfurizing agent is blown in, the immersion lance containing the desulfurizing agent is used to blow the powdery re-sulfurization preventing flux 7 together with the carrier gas into the molten steel in the ladle. By forming the flux layer 9 below the slag, it was possible to prevent re-sulfurization.

In the present invention, four kinds of oxides of magnesia, alumina, calcia and zirconia were selected. The melting point of the oxide simple substance and the flux for preventing the re-sulfurization is not higher than the desulfurization treatment temperature, the standard free energy of formation is low and stable, and CaS (desulfurization reaction product) and solute elements in molten steel do not react, _{CaO-SiO 2 -Al 2 O 3} -M
Oxides other than the above four types may be selected as long as they satisfy the property that neither the gO-based ladle slag melts.

[0046]

EFFECTS OF THE INVENTION According to the present invention, by forming a flux layer for preventing vulcanization in the lower part of the slag in the ladle during the RH treatment, it is possible to stably prevent vulcanization without modifying the ladle slag. As a result, it becomes possible to produce ultra low carbon steel in RH.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a (FeO + MnO) content in a ladle slag and a desulfurization rate.

FIG. 2 is a diagram showing an embodiment of the present invention in which a vulcanization preventing flux is sprayed onto the steel bath surface in the tank to form a vulcanization preventing flux layer under the slag in the ladle to prevent vulcanization. .

[Explanation of symbols]

 1 RH vacuum tank 2a Ascending pipe (immersion pipe) 2b Downcomer pipe (immersion pipe) 3 Ladle 5 Top blowing lance 7 Flux for preventing vulcanization 8 Ladle slag 9 Flux layer for preventing vulcanization

Claims (2)

[Claims] 1. A desulfurization treatment method for desulfurization using RH degassing equipment, wherein magnesia, alumina, calcia and zirconia are added to molten steel which is refluxed after addition of a desulfurizing agent or molten steel in a ladle, or both. Among them, a method for desulfurizing molten steel, characterized by adding a powdery flux having one or more kinds of melting points and having a melting point of desulfurization treatment temperature or higher to form the flux layer below the slag layer in the ladle. 2. A desulfurization treatment method of desulfurizing using a ladle refining facility, wherein the powdery flux is added to the molten steel in the ladle after adding a desulfurizing agent. 1. Molten steel desulfurization method according to 1.