JP2897639B2 - Refining method for extremely low sulfur steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining ultra-low sulfur steel having excellent cleanliness, and more particularly to a method for refining ultra-low sulfur steel by vacuum desulfurization treatment combined with vacuum heat treatment.

[0002]

2. Description of the Related Art In general, ultra-low sulfur steel is melted by a method in which gas is stirred under a condition in which a desulfurization flux is placed on molten steel in a ladle under atmospheric pressure, or powder for desulfurization is stirred while gas is stirred. A method of injecting into molten steel has been used. In this case, a ladle lid may be used to avoid air oxidation of the molten steel or an increase in the nitrogen concentration due to contact with air in the atmosphere.

According to these desulfurization treatments, gas stirring or powder blowing is performed under atmospheric pressure, so that stirring power is not sufficient, so that the desulfurization rate cannot be made too high. Therefore, when trying to reduce the sulfur concentration in the molten steel to an extremely low sulfur region such as 10 ppm or less,
The processing time is long, during which the temperature of the molten steel drops. Then, the reaction rate is further reduced due to the low temperature of the molten steel, and the repetition of the reaction does not eventually achieve the desired degree of desulfurization.

As described above, in the above-mentioned conventional desulfurization method,
Since the temperature drop during desulfurization is large, it is necessary to raise the tapping temperature prior to the desulfurization treatment to achieve extremely low sulfurization. However, if the tapping temperature is raised too much, the erosion of the converter refractory increases, and the life of the converter furnace is shortened. Further, in order to increase the tapping temperature, it is necessary to reduce the amount of scrap, which is a cold material, in the converter, or to perform a blow-down operation in the converter blowing. These respectively lead to an increase in the hot metal ratio and a decrease in the yield of iron, resulting in a decrease in productivity and an increase in cost. Therefore, it is necessary to raise the temperature of the molten steel by ladle refining in order not to excessively raise the converter tapping temperature.

As a method for heating molten steel under atmospheric pressure, an inert gas is blown from the bottom of a ladle to stir the molten steel while adding an oxidizing agent to an immersion tube immersed in the molten steel while blowing oxygen gas. A method of heating molten steel has been proposed. As a method for heating molten steel under vacuum, a method has been proposed in which oxygen gas is introduced into molten steel in a vacuum tank and an inert gas is blown from the bottom of the vacuum tank to stir the molten steel.

For example, in JP-A-63-266017,
"Immerse a large immersion tube in the molten steel in the ladle, heat the molten steel by supplying a heating agent and oxygen gas, and before and after this heating, add a slag-making agent and perform powder injection to perform ladle injection. Method for Performing Heat Refining of Molten Steel "has been proposed.

[0007] Japanese Patent Application Laid-Open No. 4-253213 discloses a method characterized in that a single-leg immersion pipe is immersed in molten steel in a ladle, the inside of the immersion pipe is evacuated, and oxygen gas is supplied while blowing a stirring gas. Method of heating molten steel in ladle "has been proposed.

On the other hand, as a desulfurization method using a ladle, for example, Japanese Unexamined Patent Publication No. 1-100216 discloses a method in which a dip tube is immersed in molten steel in a ladle and a heating agent is added to the dip tube from above. While heating with blowing acid, in the lower surface area of the immersion tube projection,
And a ladle refining method for molten steel characterized by blowing a desulfurizing agent below a flash point forming region associated with the blowing acid. This has the advantage that the heating of molten steel and desulfurization are performed simultaneously, and the total processing time can be shortened.

JP-A-1-92314 proposes a "method of immersing a single-leg immersion tube in molten steel in a ladle, evacuating the immersion tube and desulfurizing while blowing a stirring gas". A method has also been proposed in which the entire ladle containing molten steel is taken into a vacuum chamber, and the vacuum chamber is evacuated and depressurized, and the ultra-low sulfur steel is refined by gas agitation or gas agitation together with a desulfurizing agent injection.

[0010]

However, the above-mentioned conventional method has the following problems. In Japanese Patent Application Laid-Open No. 63-266017, as shown in the example, since the molten steel is heated by supplying oxygen gas under atmospheric pressure, a large amount of lower oxides is generated, which is a disadvantageous condition for desulfurization. .

Japanese Patent Application Laid-Open No. Hei 4-253213 discloses heating of molten steel under vacuum, and the utilization efficiency of the exothermic reactant is extremely high and the formation of lower oxides is suppressed. It is aimed at heating and it is disclosed that the cleanliness is improved, but nothing is mentioned about melting ultra-low sulfur steel.

On the other hand, in JP-A-1-100216, since a desulfurizing agent is blown below a hot spot forming region in which blowing acid is heated, temporary desulfurization reaction of a transient occurs during a process in which the desulfurizing agent rises in molten steel. However, this desulfurization agent reaches the hot spot formation area on the molten steel surface, and then resulfurizes at the hot flash point, which is a peroxide state, resulting in extremely low overall desulfurization efficiency. A big problem arises.

Japanese Patent Application Laid-Open No. 1-92314 discloses that the desulfurization rate is improved as compared with the case where the same treatment is performed under the atmospheric pressure. For example, when extremely low sulfur steel having a sulfur concentration of 5 ppm or less is melted, Even in such a process with high desulfurization efficiency,
The processing time increases, and the temperature drop during the processing increases. In the case of temperature compensation, the above-mentioned problems occur in the same way in the case of ladle molten steel heating by raising the converter output temperature or injecting oxygen gas under atmospheric pressure.

As described above, some ladle refining methods for ultra-low sulfur steel have been proposed in the prior art, but none of them has simultaneously satisfied ultra-low sulfuration and cleanliness.
More economical refining processes for ultra-low sulfur clean steel with ≤10 ppm and cleanliness index less than 1.0 have not been developed yet. The “cleanness index” is a numerical value obtained by microscopically examining a sample after rolling and using the number of large inclusions per unit area as an index.

Here, an object of the present invention is to provide an efficient and inexpensive refining method of ultra-low sulfur steel having excellent cleanliness. More specifically, an object of the present invention is to refining ultra-low sulfur steel having a cleanliness index of 1.0 or less and S ≦ 10 ppm, which can be performed more efficiently and inexpensively by performing desulfurization treatment under vacuum more effectively. Is to provide a way.

[0016]

Means for Solving the Problems The present inventors have conducted various studies in order to solve the above problems, and have obtained the following findings to complete the present invention. Considering the furnace economy, the ratio D of the inner diameter of the immersion tube to the inner diameter of the ladle is D
It ₁ / D ₂ to be from 0.5 to 0.8 is advantageous. Under such a premise, it is advantageous to perform desulfurization after heating. When combining the temperature rise of molten steel with desulfurization, if the temperature is raised under atmospheric pressure, the desulfurization efficiency is reduced and the cleanliness of the steel is deteriorated, but the molten steel heated under vacuum is also subjected to vacuum. When desulfurization treatment is performed, unexpectedly, S ≦ 10 ppm, particularly S ≦ 5 ppm, can be achieved in a short time, and cleanliness can be secured. Continue slag reforming by continuing molten steel bubbling after desulfurization to minimize resulfurization.

Here, the gist of the present invention is that a one-legged cylindrical immersion pipe is immersed in molten steel accommodated in a ladle, and the interior of the immersion pipe is evacuated to suck the molten steel into the immersion pipe. in the state in refining method very low硫鋼blowing inert gas from ladle bottom of under the projection surface of the dip tube, the ratio D ₁ / D ₂ of the dip tube inner diameter D ₁ and the ladle inner diameter D ₂ of 0.5 0.8 or more
Determine the inner diameter of the immersion pipe so as to have the following values, while adding Al to the molten steel sucked into the immersion pipe in a lump or divided or continuous, and by heating the molten steel by supplying an oxidizing gas to the molten steel And then subjecting the molten steel to desulfurizing and refining, and then returning the pressure in the dip tube to atmospheric pressure, performing a gas agitation treatment with the dip depth of the dip tube within 0.5 m, and performing gas agitation treatment. .

According to a preferred embodiment of the present invention, after the desulfurization refining, the pressure in the immersion pipe is restored to atmospheric pressure, and then the metal Ca-containing substance is blown upward, injected, added by a wire-shaped material or molten steel. You may make it supply to molten steel by means, such as pushing from the upper part.

The supply of the oxidizing gas and Al to the molten steel may be carried out from an upper blowing lance, an injection lance, or an immersion tuyere or an upper blowing tuyere provided inside the immersion tube, and there is no particular limitation.

Similarly, during the desulfurization treatment, the powder for desulfurization is
The addition may be made from an upper blowing lance or injection lance installed in the immersion tube, or from the immersion tuyere inside the immersion tube, or from the upper blowing tuyere, and there is no particular limitation.

According to another preferred embodiment of the present invention, the hydrogen concentration in the molten steel may be reduced by performing a vacuum refining process under a condition of 100 Torr or less in the immersion tube after heating the molten steel. According to still another preferred embodiment of the present invention, the hydrogen concentration in the molten steel may be reduced by subjecting the interior of the immersion pipe to vacuum refining treatment at 100 Torr or less after the desulfurization of the molten steel. According to still another preferred embodiment of the present invention, the gas agitation treatment may be performed before the heat of the molten steel is heated, by setting the inside of the immersion pipe to the atmospheric pressure and setting the immersion depth of the immersion pipe to 0.5 m or less.

[0022]

Next, the operation of the present invention will be briefly described with reference to the accompanying drawings, and then the reason for limiting the respective processing conditions in the present invention as described above will be described in detail together with its operation. In FIG. 1 of the accompanying drawings, the molten steel 12 that has been tapped from the converter (not shown) to the ladle 10 is usually at about 1620 to 1670 ° C., and the immersion pipe 14 is immersed therein. In the illustrated example, the immersion tube 14 is provided with an oxidizing gas upper blowing lance 16, an alloy charging port 18, and an exhaust port 20 appropriately connected to a vacuum device (not shown).

A blowing lance 22 is immersed in the molten steel 12 in the ladle in the projection plane of the immersion pipe 14, and an inert gas is blown into the molten steel. Alternatively, a porous plug 24 may be provided at the bottom of the ladle.

First, according to the present invention, molten steel contained in a ladle 10
12 is immersed in a cylindrical immersion tube 14 consisting of one foot.
This is a method in which the inside of 14 is evacuated and an inert gas is blown from the lower part of the ladle below the projection plane of the immersion tube via the blowing lance 22 or the porous plug 24. The blowing of the inert gas such as Ar gas is preferably continued from the start to the end of the ladle refining, and all the vacuum treatments in the present invention are performed under the blowing of the inert gas.

At this time, the inner diameter D _{1 of} the immersion tube and the inner diameter D _{2 of the} ladle
The reason why the inner diameter of the immersion tube is determined so that the ratio D ₁ / D ₂ to the value of 0.5 to 0.8 is as follows. The larger the area in the immersion tube where the desulfurization reaction occurs, the larger the reaction interface area in the immersion tube and the higher the desulfurization reaction rate. However,
If the inner diameter of the immersion tube is made larger than necessary, the erosion speed of the immersion tube increases, and problems such as an increase in the frequency of repair of refractories and a decrease in the life of the immersion tube also occur.

The ratio D ₁ / D ₂ between the inner diameter of the immersion tube and the inner diameter of the ladle is
The relationship between (A) desulfurization rate and (B) submerged pipe repair frequency index was investigated using molten steel stored in a 250-ton ladle. as a result,
The (A) (B) selecting the ratio D ₁ / D ₂ of the dip tube inner diameter D ₁ and the inner diameter D ₂ of the ladle as the optimum range for the same time satisfactory level two is 0.5 to 0.8 of It was done.
Preferably it is 0.6 to 0.75.

Next, first, Al is added to molten steel in a lump, dividedly or continuously under vacuum, and at the same time, an oxidizing gas, that is, an oxygen gas or an oxygen-containing gas is usually added to the inside of a top-blowing lance, injection or immersion tube. After the molten steel is heated by supplying it to the molten steel from the installed immersion tuyere or the upper blowing tuyere, the molten steel is desulfurized and refined under vacuum. In the example shown in the figure, a tuyere is provided above the oxidizing gas.

At this time, the input amounts of Al and the oxidizing gas are not limited. However, since the purpose is to compensate for the temperature drop during the subsequent desulfurization, the molten steel temperature is usually set to 1620 to 1700.
What is necessary is just to throw in the quantity necessary and sufficient to raise to about ° C. Note that Al is preferably added in powder form.

In the present invention, since the heating is performed under vacuum,
Compared with conventional heating under atmospheric pressure, lower oxides (FeO, MnO
) Is suppressed. If the desulfurization treatment is performed after the heating of the molten steel, inclusions suspended in the molten steel such as alumina generated by the reaction between Al and oxygen during the heating of the molten steel are adsorbed and separated by a desulfurizing agent or desulfurization slag used for the desulfurization treatment. It is possible to separate and remove from the molten steel. However, when the desulfurization treatment is performed first and then the molten steel is heated, some of the alumina-based inclusions generated during the heating are adsorbed by the slag and can be separated and removed, but most of them remain in the molten steel. However, the cleanliness of the steel is deteriorated. Therefore, the desulfurization treatment is performed after the heating of the molten steel. Also,
As a result, even when the desulfurization treatment time is very long, such as in the case of melting extremely low sulfur steel, desulfurization treatment can be performed without increasing the converter tapping temperature.

The desulfurizing agent used in the present invention is CaO ₂ -CaF
System, CaO-Al ₂ O _{3 system, CaO-Al 2 O 3 -CaF} 2 system and the like, those commonly available. Thus, in the present invention, desulfurization up to S ≦ 10 ppm becomes possible, and the molten steel temperature at the end of desulfurization is 1590 to 1620.
° C.

As described in Japanese Patent Application Laid-Open No. Hei 4-253213, the formation of a lower oxide during heating at a pressure higher than the atmospheric pressure is suppressed, and the cleanliness of the steel can be ensured. In addition, the subsequent desulfurization treatment is advantageous.

Further, the desulfurization treatment of molten steel under vacuum is because the desulfurization efficiency is more improved than under atmospheric pressure, particularly in the point that the stirring power is increased, as shown in JP-A-1-92314. After desulfurization, Fukuoshi the interior of the dip tube 14 to the atmospheric pressure, an inert gas blown from the blowing lance 22 or a porous slug 24 in immersion depth D ₃ of the following 0.5 m state of the atmospheric pressure addition dip tube 14 Gas stirring process is continued. The reason is as follows.

In order to accelerate the desulfurization reaction, it is necessary to reduce the oxygen potential at the interface of the slag metal, but it is necessary to reduce the lower oxides in the slag. In addition, the lower oxides in the slag cause reoxidation of the molten steel, which lowers the cleanliness of the molten steel.

When the desulfurization treatment is performed in a state where the inside of the immersion tube is kept under vacuum, the slag inside the immersion tube is well stirred, but the slag outside the immersion tube is insufficiently stirred. Further, even when the inside of the immersion tube is treated at atmospheric pressure, if the immersion depth of the immersion tube is large, the agitation of the slag outside the immersion tube also becomes insufficient.

Therefore, the present inventors investigated the influence of the immersion depth of the immersion tube on the lower oxide concentration in the slag by setting the inside of the immersion tube to atmospheric pressure. As a result, when the immersion depth was more than 0.5 m, there was a difference in the lower oxide concentration in the slag inside and outside the immersion tube, and when the immersion depth was less than 0.5 m, the difference in the concentration was significantly small. This is because the upward flow of the molten steel caused by the injection of the inert gas becomes a horizontal flow toward the inner wall of the immersion pipe at the bath surface of the immersion pipe, and when the immersion depth of the immersion pipe exceeds 0.5 m, the flow of the molten steel falls completely at the inner wall of the immersion pipe. When the immersion depth is less than 0.5 m, the horizontal molten steel flow does not change to a complete downward flow, but forms a molten steel flow outside the immersion pipe, thus enabling the slag to be stirred. It is because it becomes.

According to a preferred embodiment of the present invention, the pressure in the immersion tube is restored, and the molten steel is subjected to, for example, Ca-Si, Fe-Ca, Fe-Ca-Ni,
The metal Ca-containing substance such as Ca-Al may be supplied by means of upward blowing, injection, addition of wire or pushing from the top of molten steel, but this is the present invention which can be processed under vacuum and atmospheric pressure by the same apparatus. The purpose of this refining method is to melt HIC-resistant steel.

As a result, even in a HIC-resistant steel which requires the addition of metallic Ca for controlling the form of inclusions, the yield of Ca is not reduced as in the case of simple RH degassing, that is, the evaporation loss is reduced, and The process can complete the process.

During the vacuum desulfurization treatment, the desulfurizing agent powder is added from the upper blowing lance or the injection lance installed in the immersion tube or the immersion tuyere or the upper wing tuyere inside the immersion tube because of the desulfurization rate during the desulfurization treatment, the desulfurization This is to improve the rate.

When the desulfurization treatment is performed at a degree of vacuum of 100 Torr or less, the gas is blown under vacuum, so that the higher the pressure in the immersion tube (the worse the degree of vacuum), the weaker the agitation power of the blown gas. In addition, since the equilibrium hydrogen concentration increases, the dehydrogenation rate significantly decreases, which is disadvantageous for dehydrogenation. Therefore, in the present invention, the dehydrogenation treatment is performed at a degree of vacuum of 100 Torr or less.

The treatment at a degree of vacuum of 100 Torr or less after the desulfurization treatment may not be able to obtain a sufficient concentration of hydrogen in the molten steel due to the dehydrogenation reaction that occurs simultaneously during the desulfurization treatment under vacuum. This is because it is necessary to perform a dehydrogenation treatment. The reason why the degree of vacuum is set to 100 Torr or less is the same as above.

In a modified example of the present invention, the gas agitation treatment may be performed before the heat treatment by heating the inside of the immersion tube at atmospheric pressure and the immersion depth of the immersion tube within 0.5 m. If the lower oxide in the slag before the treatment is very high, the subsequent treatment alone may not be enough to completely modify the ladder outside ladle slag, and this is for the purpose. Next, specific working effects of the present invention will be described with reference to examples.

[0042]

【Example】

(Example 1) In this example, the device shown in FIG.
The single-footed immersion tube is immersed in molten steel at 1620 ° C stored in a ladle, and 3 Nm ³ / min of argon gas is blown from the bottom blown porous plug of the ladle while the interior of the immersion tube is evacuated to 100 Torr or less.
The gas was blown to stir the molten steel. While the gas was being stirred in this manner, the molten steel was heat-treated to 1670 ° C. under vacuum, and then desulfurized under vacuum of 100 Torr or less.

In this example, the sulfur concentration in the molten steel before the treatment was 20
The hydrogen concentration in the molten steel before the treatment was 5 to 15 ppm. After desulfurization, gas agitation was performed with the immersion tube having an immersion depth of 0.5 m or less. In this example, the heat of the molten steel was increased by adding Al powder to the molten steel from the alloy inlet, and supplying oxygen gas as oxidizing gas to the molten steel from the oxidizing gas upper blowing lance.

Before heating the molten steel, Al is added in an amount of 160 to 80 in accordance with the heat amount.
0 kg was added, the oxygen gas flow rate was 50 Nm ³ / min, and oxygen gas was supplied at each charge for 2 to 10 minutes. As a result, the heating rate was about 8 ° C./min for each charge.

The inner diameter of the ladle was kept constant at 4 m, and the low hydrogen extremely low sulfur steel was melted under the conditions of 1.5, 1.8, 2, 2.5, 3, and 3.5 m. Also, as desulfurization flux
10 kg / T of a desulfurizing agent composed of 85% by weight of CaO and 15% by weight of CaF ₂ was added from the alloy inlet of the dip tube after heating. Table 1 shows the results
Are shown together.

As shown in Table 1, the desulfurization time under vacuum of 100 Torr or less is 13 to 15 minutes, and it is found that desulfurization conditions of D ₁ / D ₂ of 0.5 or more are desirable. Although showing a dehydrogenation behavior during the vacuum desulfurization in Table 2, the D ₁ / D ₂
It turns out that 0.5 or more is desirable as a dehydrogenation condition.

The life of the immersion tube and the frequency of repair of the immersion tube are represented by D ₁ /
Table 3 shows values indexed based on the case of D ₂ = 0.375. From Table 3, it can be seen that when D ₁ / D ₂ exceeds 0.8, the life and repair frequency of the immersion tube are significantly increased.

(Example 2) After gas stirring after completion of desulfurization in Example 1, 0.8 kg of Ca-Si was charged per 1 ton of molten steel, and Ca treatment was performed. As a result, the obtained steel material has the following NA
The CE condition was satisfied.

NACE conditions : Solution: 5% NaCl + 0.8% CH ₃ COOH Temperature: 24 ± 2.8 ° C. pH: max 4.5 hours: 96 hours H ₂ S concentration: H ₂ S saturation 流量 Flow rate: 100-200 cc / min.

Comparative Example 1 In this example, Example 1 was repeated under the condition that D ₁ / D ₂ = 0.625, which is almost the same as No. 4 in Table 1.
Heating and desulfurization operations were performed simultaneously. The results are summarized in Table 4.

(Comparative Example 2) In this example, Example 1 was repeated under the condition of D ₁ / D ₂ = 0.625, almost the same as No. 4 in Table 1.
The heating was performed under atmospheric pressure, and then the pressure was reduced to perform desulfurization under a high vacuum of 100 Torr or less. The results are also summarized in Table 4. For reference, the results are also shown for an example in which only the temperature was raised in vacuum.

(Comparative Example 3) In this example, Example 1 was repeated under the condition of D ₁ / D ₂ = 0.625, almost the same as No. 4 in Table 1.
Gas stirring under atmospheric pressure after desulfurization was omitted. The results are summarized in Table 4.

(Comparative Example 4) In this example, Example 1 was repeated under the condition of D ₁ / D ₂ = 0.625 which is almost the same as that of No. 4 in Table 1.
Desulfurization was performed prior to heating. The results are summarized in Table 4.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

[Table 3]

[0057]

[Table 4]

[0058]

As described above, by using the method for refining ultra-low sulfur steel according to the present invention, the desulfurization rate under vacuum is stabilized at a high level, and the lower oxide concentration in the entire ladle slag is reduced. As a result, the sulfur concentration level obtained from the extremely low sulfur steel was reduced, and at the same time, the wear of the refractory was suppressed. Further, by controlling the degree of vacuum during the vacuum processing, low hydrogen steel can be refined at the same time.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic explanatory view showing a setting state of a ladle and a dip tube for implementing the present invention.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C21C 7/00 C21C 7/10

Claims (2)

(57) [Claims] A immersion pipe having a single foot is immersed in molten steel accommodated in a ladle, and the interior of the immersion pipe is evacuated to a vacuum and the molten steel is sucked into the immersion pipe. In the refining method of ultra-low sulfur steel, which is performed by blowing an inert gas from a lower part of the pot and performing a gas stirring process,
_The inner diameter of the immersion pipe is determined so that the ratio D ₁ / D _{2 of 1} to the ladle inner diameter D ₂ becomes a value of 0.5 or more and 0.8 or less, and Al is added to the molten steel sucked into the immersion pipe all at once or divided or continuously added. At the same time, the molten steel is heated by supplying an oxidizing gas to the molten steel, then the molten steel is desulfurized and refined, and then the inside of the dip tube is restored to the atmospheric pressure, and the dip depth of the dip tube is reduced.
A method for refining ultra-low sulfur steel, wherein the gas agitation treatment is continued by blowing the inert gas from the lower part of the ladle within 0.5 m. 2. A method for refining ultra-low sulfur steel, wherein after the desulfurization refining according to claim 1, the pressure inside the immersion pipe is restored to the atmospheric pressure, and then the metal Ca-containing substance is supplied to the molten steel.