JPH06306445A - Method for decarburizing and desulfurizing molten steel - Google Patents

Abstract

PURPOSE:To promote a vacuum-decarburizing and desulfurizing method for steel by a simplified process. CONSTITUTION:(1) In the method for melting the steel having <=300ppm C and <=20ppm S by using a vacuum refining apparatus providing a device for top-blowing refining gas and refining powder, metal oxide powder is blown on the molten steel surface in a vacuum vessel in the apparatus by using inert gas, gaseous oxygen or the mixed gas thereof as carrier gas to execute the carburizing treatment. Successively, by adding deoxidizing agent, the molten steel is deoxidized and also, after adjusting to >=0.1wt.% sol.Al,, the desulfurizing agent powder is blown on the molten steel surface in the same vacuum vessel by using the inert gas as the carrier gas to execute the desulfurizing treatment. (2) In the case of being the steel containing >=0.3% Mn, the powder containing Mn oxide is blown on the molten steel surface to execute the decarburizing treatment and successively, by using the above method, to execute the desulfurizing treatment. By this method and using the same vacuum vessel and the same top blowing, the steel into the extra low carbon and the extra low sulfur can efficiently be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for vacuum refining steel, and more particularly to a method for reducing carbon, reducing sulfur and reducing nitrogen.

[0002]

2. Description of the Related Art In recent years, as the grade of steel becomes higher, there are cases where ultra-low carbon, ultra-low sulfur and ultra-nitrogen are required at the same time depending on the type of steel. For example, in electromagnetic steel plate materials and the like, it is essential to meet these requirements in order to reduce iron loss and improve magnetic properties.

FIG. 5 is a diagram showing the current state of the decarburizing, desulfurizing and denitrifying process of molten steel using the ordinary VOD method. The desulfurization treatment is performed by deoxidizing molten steel produced in a converter or an electric furnace with a deoxidizing agent such as Al or Si, and then blowing the desulfurizing agent into the molten steel in the ladle (LT treatment). Since this treatment is performed under atmospheric pressure, nitrogen is picked up in the molten steel. Therefore,
When a low nitrogen content is required, this treatment is usually performed before VOD refining (decarburization and denitrification treatment using vacuum). Therefore, in the current treatment process shown in FIG. 5, oxidation refining → reduction refining (desulfurization treatment) → oxidative refining (decarburization denitrification treatment) → reduction refining (deoxidation treatment) is performed, which is low in temperature and time. It is efficient.

The decarburization treatment in this VOD refining is usually
It is carried out by blowing oxygen gas onto the surface of the molten steel from a top blowing lance. However, in such an ordinary treatment, the decarburization reaction is stagnated when [C] is in the low carbon region of 20 to 50 ppm or less, and a long treatment is required to obtain an ultra low carbon steel. In addition, when the decarburization treatment by blowing oxygen gas over a long period of time is performed, when a metal such as Mn that easily evaporates is contained,
The evaporation loss of these alloys becomes a problem.

In the low carbon region as described above, the decarburization reaction is the rate-determination of carbon transfer to the reaction interface, so it is important to increase the reaction interfacial area in order to accelerate the reaction. Conventionally, as a method of increasing the reaction interfacial area, a method of strongly stirring molten steel by increasing the flow rate of Ar gas for stirring has been carried out, but the degree of vacuum is deteriorated with the increase of the gas flow rate, and the gas blowing tuyere There are problems such as melting and splashing.

In order to solve such a problem, Japanese Patent Publication No. 1-25370 discloses that a powder of a metal oxide is sprayed on the surface of molten steel to supply oxygen necessary for a decarburization reaction and A vacuum treatment method for promoting decarburization by using the body as a reaction nucleus is disclosed. However, this method is a decarburization-only treatment method, and the treatment process cannot be simplified so that decarburization and desulfurization can be performed continuously. That is, also in the case of melting ultra-low carbon, ultra-low sulfur and ultra-nitrogen steel by this method, similar to the process of FIG. 5, oxidation refining → reduction refining (desulfurization treatment) → oxidative refining (decarburization) There is no choice but to go through the process of denitrification). Similarly, there remains the problem of nitrogen pickup in the desulfurization process performed under atmospheric pressure.

[0007]

The present invention has been made to solve the above problems, and the object of the present invention is to:
An object of the present invention is to provide a vacuum decarburization desulfurization method capable of melting ultra-low carbon, ultra-low sulfur and ultra-nitrogen steel in a short time by a simplified process.

[0008]

Means for Solving the Problems The gist of the present invention is as follows (1)
And method (2).

(1) A method for smelting steel having C: 30 ppm or less and S: 20 ppm or less by using a vacuum refining device equipped with a device for upwardly blowing a refining gas and a refining powder, Deoxidize the molten steel by adding a deoxidizing agent to the surface of the molten steel by spraying the metal oxide powder as a carrier gas with the metal oxide powder in the vacuum tank of the equipment as a carrier gas. Sol.Al: 0.1 Wt% or more, and then desulfurization treatment is performed by spraying desulfurizing agent powder on the surface of molten steel with an inert gas as a carrier gas in the same vacuum chamber. Decarburization desulfurization method.

(2) When the steel is a steel containing Mn: 0.3% or more, decarburization treatment is performed by spraying powder containing Mn oxide onto the surface of molten steel.
The method for decarburizing and desulfurizing molten steel according to (1).

In the method of the present invention, the oxidizer powder is blown up in the vacuum chamber for decarburization, the molten steel is then deoxidized, and then the desulfurizing agent is added in the same vacuum chamber using the same blowing device. Continuous blowing is performed to prevent nitrogen pickup and simplify the treatment process.

The oxidizer powder is made fine, and oxygen required for decarburization is supplied, and the oxidizer powder acts as a CO gas generating nucleus to promote the decarburization reaction. Even during desulfurization, the desulfurization agent powder is blown upward to increase the reaction interfacial area with the molten steel and contribute to the promotion of the desulfurization reaction.

[0013]

1 and 2 are views showing a processing process according to the method of the present invention.

In the process of FIG. 1, elementary decarburization is carried out in a converter,
High carbon ferromanganese (hereinafter referred to as HcFeMn) was added to the VO to obtain undeoxidized molten steel in which only the Mn content was adjusted.
Blow oxide powder in the furnace D to decarburize to the target low carbon content, deoxidize Al immediately after decarburization to adjust the Al content in the steel to a predetermined value, and then continue to desulfurization agent powder After being subjected to desulfurization treatment by blowing in, the necessary components and temperature are adjusted and subjected to a continuous casting process.

The process of FIG. 2 is similar to that of FIG.
Instead of HcFeMn, low carbon ferromanganese (hereinafter,
LcFeMn), after deoxidation of Al, Si and metal (metal) Mn,
This is an example of adding each. In the method of the present invention, the target molten steel may be electric furnace steel or the like.

FIG. 3 is a view showing a vertical section of a VOD furnace to which the method of the present invention is applied. A ladle 2 containing molten steel 5 such as the converter steel or the electric furnace steel is left standing in a vacuum tank 1 capable of being evacuated. The powder and carrier gas upper blowing lance 4 is inserted from the upper part of the vacuum chamber 1, and the powder such as oxide or desulfurizing agent and the carrier gas 5 are blown onto the surface of the molten steel 6. The powder finally becomes slag 7 and molten steel 6
Levitate on the surface of (the slag on this surface is also called top slag). At this time, in order to stir the molten steel 6 and promote contact with the slag 7 on the surface, from the porous plug 3 provided at the bottom of the ladle 2, an inert material of a type that does not adversely affect the target steel composition. Gas is blown.

As described above, in the method of the present invention, the oxidizer powder is sprayed in the vacuum chamber to complete the decarburization treatment, the molten steel is immediately deoxidized, and the oxidizer powder is continuously deoxidized in the same vacuum chamber. Since the desulfurizing agent powder is continuously blown up by the method used to blow the body, it is possible to simplify the treatment process and shorten the treatment time, and lower the temperature of molten steel. Can also be reduced. Further, the desulfurization treatment is also carried out under vacuum, and nitrogen pickup from the atmosphere can be prevented, which is advantageous for the melting of low-nitrogen steel.

That is, by spraying a fine oxidizer powder during decarburization, oxygen necessary for decarburization is supplied, and the oxidizer powder becomes a nucleus for producing CO gas, so that the decarburization reaction is remarkably accelerated. However, it is possible to easily obtain an extremely low coal level of [C] of 30 ppm or less.

Further, by applying this method of spraying powder on the surface of molten steel in a vacuum chamber to desulfurization treatment, decarburization and desulfurization can be continuously performed, and the treatment process is decarburized (oxidative refining). It is possible to simplify so that only the reduction refining such as deoxidation and desulfurization is performed after the completion of the above.

By spraying the desulfurizing agent powder, the reaction interfacial area with the molten steel is remarkably increased and the desulfurization reaction is promoted. Therefore, it is possible to obtain a very low sulfur steel having [S] of 20 ppm or less. Further, since the desulfurization process is performed in the vacuum tank, the problem of nitrogen pickup is eliminated.

However, when performing desulfurization treatment after decarburization treatment,
The desulfurization reaction may be hindered by the influence of the oxidizing slag generated during decarburization. Therefore, it is necessary to sufficiently deoxidize the molten steel and the slag after the decarburization treatment.

Next, the reasons for limiting the decarburization and desulfurization treatment conditions as described above will be explained.

[1] Decarburization The metal oxide powder is used as the oxidizing agent for the decarburization treatment.
The reason is that the decomposing reaction is remarkably promoted because the oxidant powder serves as a CO gas generating nucleus while being melted and decomposed to serve as an oxygen supply source. Further, the temperature of the reaction interface is lowered as compared with the case of oxygen gas to reduce the loss of useful metal components due to evaporation. The particle size range of the oxidizer powder having such an action is preferably as fine as 10 to 500 μm.

Iron ore powder is generally used as the oxidizing agent, but Mn ore, Cr ore, or a mixture thereof may be used depending on the composition of the product steel. Examples of the powder containing Mn oxide include Mn ore and MnO ₂ .

In particular, when decarburizing a steel type having a Mn content of 0.2% or more by using iron ore powder, Mn, which easily oxidizes and evaporates, has a large loss due to oxidation and evaporation, and is expensive metal (metal) after the treatment. ) Mn must be added to adjust the composition. In such a case, by using, for example, Mn ore powder as the oxidizer, the MnO 2 concentration at the reaction interface becomes high, and Mn in the molten steel becomes difficult to oxidize. Therefore, decarburization can be preferentially promoted without lowering [Mn] in the molten steel.

As the carrier gas, an inert gas, an oxygen gas or a mixed gas thereof is used. However, when trying to obtain low-nitrogen steel or ultra-low-nitrogen steel, it is natural that N ₂ gas is not desirable.

In the early stage of decarburization where the decarburization reaction is active, oxygen gas or a mixed gas of oxygen gas and an inert gas is used, and the decarburization reaction is delayed. In each region, it is desirable to use an inert gas as a carrier gas. In this area where decarburization is stagnant,
Since the decarburization reaction is the rate-determining movement of carbon to the reaction interface, the decarburization is not promoted even if oxygen is excessively supplied, and only the loss of metal components such as Mn that easily oxidize or vaporize increases.

[2] Desulfurization In the desulfurization treatment, the molten steel after the decarburization treatment and the oxides produced in the oxidizing and refining atmosphere during the decarburization treatment are sufficiently deoxidized with Al, and then the desulfurization agent powder is used. It is performed by spraying an inert gas as a carrier gas onto the surface of the molten steel.

The desulfurizing agent powder sprayed on the surface of the molten steel penetrates into the molten steel and continues the desulfurization reaction while floating. VO
When the D furnace is used, the slag (top slag) on the surface of the molten steel and the molten steel are agitated by the bottom blowing gas, so that the desulfurizing agent after the floating also contributes to the reaction.

In order to effectively carry out the desulfurization reaction with this top slag, it is essential to reduce the oxygen concentration in the molten steel and the top slag. When the oxygen concentration in the molten steel is high, the desulfurization reaction does not proceed. Therefore, as the carrier gas, only an inert gas is used.

Further, when the oxygen concentration in the top slag is high, the desulfurization ability of the slag is low, and therefore, the reaction of returning S in the desulfurizing agent that has completed desulfurization in the steel and has floated to the molten steel. (Resulfurization) occurs. Therefore, the apparent desulfurization effect is reduced.

In order to reduce the oxygen concentration in the molten steel and the top slag, a deoxidizing agent such as Al is added after the decarburizing treatment,
It is necessary to completely deoxidize the molten steel by refluxing it with sufficient stirring. For this reason, it was decided to use Al, which has a strong deoxidizing power. In order to completely deoxidize, the Al content in the molten steel before desulfurization should be sol.
l must be kept at 0.1 wt% or more.

In general, in the slag-metal reaction, the better the slag slag formation property, the higher the reaction efficiency. That is, the desulfurizing agent used here is more effective if it has a low melting point and a good slag forming property. Therefore, the Ca normally used for desulfurization
In addition to O (quick lime), it is also effective to use CaO mixed with CaF ₂ to lower its melting point. The range of the particle size of the desulfurizing agent is preferably as fine as about 10 to 50 μm in order to increase the reaction interfacial area with the molten steel and accelerate the desulfurization reaction.

It is desirable that the lance used for the upper blowing can be cooled by using cooling water or the like. When an uncooled lance is used, problems such as melting damage of the lance and inability to move the lance up and down due to metal adhesion may occur, making it impossible to accurately maintain the distance between the lance and the molten steel surface. is there. Even if the lance main body is sound, if the nozzle portion at the tip of the lance is melted, the gas flow velocity changes and the powder blowing direction also changes, so that the predetermined blowing condition cannot be maintained.

A desirable range of distance between the tip of the lance and the surface of the molten steel is 0.5 to 1.0 m in the VOD furnace. The shape of the ejection hole of the lance is preferably Laval, straight or the like.

[0036]

EXAMPLE An explanation will be given based on an example carried out by using a 50 ton scale VOD furnace shown in FIG.

Blowing was completed in a converter, HcFeMn or LcFeMn was added in an amount corresponding to a predetermined [Mn] level, and then molten steel was tapped in an undeoxidized state.

The basic process conditions for a VOD furnace are as follows.

The molten steel in the ladle (temperature: 1640 to 1680 ° C) was placed in the VOD furnace together with the ladle, and the furnace (in the vacuum chamber) was heated to 1 to 2T.
Evacuate to orr, blow Ar gas for stirring molten steel at 180 Nl / min from the porous plug at the bottom of the ladle, and after the degree of vacuum has stabilized, apply a water cooling lance from the top of the vacuum tank to the molten steel surface.
It was lowered to a position of 00 mm, and iron ore powder or Mn ore powder was sprayed from a nozzle having a hole diameter of φ25 mm at the tip of the lance, using Ar gas as a carrier gas. The carrier gas flow rate was kept constant at 5 Nl / min, and the supply rate of the iron ore powder or Mn ore powder was changed in the range of 10 to 40 kg / min according to the decarburization rate.

The iron ore powder is T.I. Fe is 63Wt% or more and its grain size is 100mesh under, Mn ore powder has Mn content of 49-
A 50 Wt% and S content of 0.006 Wt% or less and a particle size of 100 mesh under were used.

After completion of decarburization, metal Al was added and stirring with bottom-blown Ar gas was continued to deoxidize the molten steel to a predetermined level.
The Al content was adjusted.

After the completion of deoxidation, iron ore powder or Mn
The desulfurizing agent powder was sprayed under the same carrier gas condition from the same lance as the spraying of the ore powder. The desulfurizing agent used is
The composition is 60Wt% CaO −40Wt% CaF ₂ , and its particle size is 100mesh.
I used the under one. The desulfurizing agent was supplied at a rate of 50 kg / min and was blown for 6 to 8 minutes.

[Test 1] The molten steel after being discharged from the converter was treated according to the process shown in FIG. At this time, the oxidizer powder is only the above iron ore. At that time, sol. Due to Al deoxidation after decarburization.
The effect of sol.Al content in molten steel on desulfurization was investigated by changing the Al content level from 0.03 to 0.15wt%.

FIG. 4 is a graph showing the relationship between the sol.Al content in molten steel and the desulfurization rate. As is clear from FIG. 4, the desulfurization rate improves as the sol.Al content in the molten steel increases. Since [S] is about 20 to 30 ppm after the steel is taken out of the converter during melting of extremely low sulfur steel, the desulfurization rate is 80 to 80% to obtain extremely low sulfur steel with [S] of several ppm by desulfurization treatment. 85% or more is required. Therefore, it is understood that the sol.Al content in the molten steel must be maintained at 0.1 Wt% or more.

[Test 2] In the same manner as in Test 1 above, Mn
Molten steel with a content of about 0.3% was decarburized and desulfurized according to the process shown in Fig. 1, and the sol.Al content after desulfurization was 0.49Wt.
% And 1.14 Wt% are maintained, Table 1 and Table 2 show examples of component behavior by VOD furnace treatment.

[0046]

[Table 1]

[0047]

[Table 2]

As is clear from Tables 1 and 2, the decarburization reaction is promoted by the iron ore powder top-spraying and [C] is 5 to 6 ppm.
Of ultra low carbon steel was obtained. However, 0.1% of Mn oxidation loss due to decarburization is observed. This oxidation loss does not have a significant effect, but in this case, the Mn content of the product was steel that was strictly regulated to about 0.30%,
It was necessary to add Mn to adjust the Mn content after desulfurization. In addition, denitrification reaction also occurs with vacuum decarburization.

After the desulfurization treatment, 2 due to the influence of C in the desulfurizing agent.
Although there was a C pickup of ppm, [C] kept below 10 ppm. The desulfurization rate reached 85%, and the ultimate [S] achieved an extremely low sulfurization of 4 ppm.

[Test 3] The Mn content after tapping the converter was about 1.2.
The molten steel adjusted to the% level was decarburized and desulfurized according to the process of FIG. At this time, the oxidant powder is also the iron ore described above. The sol.Al content after this desulfurization treatment is 0.51 Wt%
Tables 3 and 4 show examples of the component behavior by the VOD furnace treatment when it was maintained at 1.19 Wt%.

[0051]

[Table 3]

[0052]

[Table 4]

As is clear from Tables 3 and 4, about 0.7% of Mn is lost by oxidation due to the iron ore powder top-spraying. Therefore, after VOD treatment, it is necessary to adjust the components by adding metal Mn, which is more expensive than ferromanganese. The other component behaviors of C, S and N are the same as in Test 2.

[Test 4] The Mn content after tapping the converter was about 1.0.
The molten steel adjusted to the% level was decarburized and desulfurized according to the process of FIG. The powder containing Mn oxide at this time is only Mn ore. The sol.Al content after this desulfurization is 0.45Wt%
Tables 5 and 6 show examples of the component behavior by the VOD treatment in the case of being maintained at 1.16 Wt%.

[0055]

[Table 5]

[0056]

[Table 6]

As is clear from Tables 5 and 6, the oxidation loss of Mn in the molten steel was reduced as compared with Test 3 by the Mn ore powder top spraying. In addition, in the analysis results of the top slag and molten steel after decarburization treatment and deoxidation treatment, it was supplied by top blowing.
About 50% of Mn content in Mn ore powder is present in the slag in the form of MnO after decarburization treatment, but it is reduced by deoxidation before desulfurization treatment or adjustment of sol.Al content. , Most of them returned to molten steel and [Mn] increased by about 0.3%. Therefore, it is not necessary to add the metal Mn after the VOD treatment. The behaviors of the other components are the same as those in the tests 2 and 3.

[Test 5] The Mn content after tapping the converter was about 0.3.
The molten steel adjusted to the% level was decarburized and desulfurized according to the process of FIG. The decarburizing oxidizer powder at this time is only the iron ore described above, but only carrier gas from Ar50 to Ar50
% + O ₂ 50% mixed gas was changed. Other decarburization desulfurization conditions are the same as the basic conditions described above. Sol after this desulfurization treatment.
V when Al content is maintained at 0.49Wt% and 1.14Wt%
Tables 7 and 8 show examples of the component behavior by the OD treatment.

[0059]

[Table 7]

[0060]

[Table 8]

As is apparent from Tables 7 and 8, since the above-mentioned mixed gas was used as the carrier gas, compared with Test 1.
The evaporation loss of Mn increases and the arrival [Mn] is low.
However, the value is higher than that of the comparative example using the conventional process described later. The behavior of the other components is almost the same as in Test 1.

[Comparative Example] The Mn content after tapping the converter was about 0.3.
% Of the molten steel adjusted to LT level and VO
Processed according to the process using D. The illustrated process is basically the same as the conventional discontinuous processing method shown in FIG. That is, desulfurization was performed in the atmosphere by LT treatment, and decarburization and deoxidation were performed in a vacuum by a VOD furnace.

The desulfurization treatment was performed by using Ar as a carrier gas and injecting the same desulfurizing agent powder as in the above example into the molten steel at a supply rate of 50 kg / min. The decarburization treatment was performed by using only a top blowing lance (nozzle diameter 25 mmφ, distance from molten steel surface 1 m) and blowing oxygen gas at a supply rate of 0.9 Nm ³ / min · ton. This LT processing-V
Tables 9 and 10 show examples of component behavior by OD treatment.

[0064]

[Table 9]

[0065]

[Table 10]

As is clear from Tables 9 and 10, in the decarburization treatment in the VOD furnace, the decarburization reaction stagnates in the low carbon region, and therefore the arrival [C] is 13 to 16 ppm, and the [C] of 10 ppm or less is reached. ] Was not obtained. Also, to blow in oxygen gas
The oxidation loss of Mn is large, and [Mn] decreases from 0.32% to 0.15%. Therefore, it is necessary to add metal Mn after decarburization to adjust the components.

The desulfurization effect is almost the same level as that of the method of the present invention, but since it is desulfurization treatment in the air atmosphere, N pickup from the air is large, and [N] after desulfurization is 38 to 42 pp.
It has a high value of m. Therefore, the arrival [N] after decarburization is also high at 15 to 17 ppm.

[0068]

EFFECTS OF THE INVENTION According to the method of the present invention, powder such as a metal oxide is blown in the same vacuum chamber using an upper blowing lance to decarburize, and then deoxidation is performed to reduce the Al content in molten steel. By appropriately maintaining the temperature and then continuously spraying the desulfurizing agent powder onto the surface for continuous treatment, it is possible to efficiently perform extremely low carbonization and extremely low sulfurization of steel. Since it is a vacuum continuous processing method, there is no nitrogen pickup.

[Brief description of drawings]

1 illustrates an example of a process using the method of the present invention.

FIG. 2 illustrates another example of a process that uses the method of the present invention.

FIG. 3 is a vertical sectional view of a VOD furnace for carrying out the method of the present invention.

FIG. 4 is a diagram showing the effect of the sol.Al content in molten steel on the desulfurization rate.

FIG. 5 is a diagram showing an example of a conventional desulfurization decarburization process.

FIG. 6 is a diagram showing another example of a conventional desulfurization decarburization process.

[Explanation of symbols]

1: vacuum tank, 2: ladle, 3: porous plug,
4: Powder and carrier gas top blowing lance, 5: Powder and carrier gas, 6: Molten steel, 7: Slag

Claims (2)

[Claims] 1. A vacuum refining device equipped with a device for blowing a refining gas and a refining powder onto the surface of the powder, C: 30 ppm or less,
S: A method for melting steel of 20 ppm or less, which is carried out by spraying the metal oxide powder on the surface of the molten steel as an inert gas, oxygen gas or a mixed gas thereof as a carrier gas in the vacuum chamber of the above apparatus. After performing charcoal treatment and then adding a deoxidizer to deoxidize the molten steel and adjust the sol.Al: 0.1 Wt% or more, desulfurization agent powder is further melted in the same vacuum chamber using an inert gas as a carrier gas. A method for decarburizing desulfurization of molten steel, characterized by performing desulfurization treatment by spraying on the surface. 2. When the steel is a steel containing Mn: 0.3% or more, decarburization treatment is performed by spraying powder containing Mn oxide onto the surface of the molten steel.
A method for decarburizing and desulfurizing molten steel as described.