TWI490340B - Vacuum refining method of molten steel - Google Patents

Description

Vacuum refining method for molten steel

The present invention relates to a vacuum refining method for molten steel, and more particularly to a method for melting low carbon high manganese steel and low sulfur steel in a vacuum degassing apparatus.

In recent years, the use of steel materials has gradually diversified, and the use in more severe environments has increased. Along with this, the requirements for the mechanical properties of the product are gradually increasing and become more severe. Under such circumstances, a low-carbon high-manganese steel having both high strength and high workability has been developed for the purpose of increasing the strength, weight, and cost of the structure (hereinafter also referred to as "low C high Mn steel". In addition, it is widely used in various applications such as steel sheets for steel pipes for transportation and steel plates for automobiles. Here, the low C high Mn steel refers to a steel having a C concentration of 0.05 mass% or less and a Mn concentration of 0.5 mass% or more.

In the steel making step, an inexpensive manganese source for adjusting the concentration of Mn in the molten steel is made of manganese ore (hereinafter also referred to as "Mn ore") and high carbon iron manganese to melt the above low C high Mn steel. At the time of decarburization and refining in the converter, the Mn ore is put into the converter for reduction, or the high carbon iron and manganese is added to the molten steel during the tapping of the converter to melt The Mn concentration in the steel is increased to a predetermined concentration (for example, refer to Patent Document 1).

However, when such inexpensive manganese sources are used, the concentration of C in the molten steel cannot be sufficiently reduced in the refining of the converter, or the C contained in the high carbon iron manganese is used in the molten steel after tapping. The C concentration rises. As a result, when there is a concern that the C concentration exceeds the allowable range of the low C high Mn steel, it is necessary to separately perform the treatment for removing C from the molten steel.

A method for efficiently removing C in a molten steel, which is known to have a vacuum degassing apparatus using an RH vacuum degassing apparatus, a vacuum decarburization method for a molten steel in an undeoxidized state, and a vacuum in a vacuum A method in which an oxygen source such as oxygen is blown (steamed) to a molten steel to perform decarburization. In the above vacuum decarburization, a method of using high carbon iron manganese as an inexpensive manganese source, for example, in Patent Document 2, a high carbon iron manganese is proposed in the initial stage of decarburization refining in a vacuum degassing apparatus. In addition, in Patent Document 3, in the case of melting extremely low carbon steel in a vacuum degassing furnace, it is proposed that high carbon ferromanganese is introduced between 20% and 20% of the decarburization treatment time. The method. However, when oxygen is added at the time of vacuum decarburization treatment of a molten steel containing a large amount of Mn, oxygen reacts not only with C in the molten steel but also with Mn, which not only causes oxidation loss of Mn but also lowers Mn yield. And it is difficult to control the Mn concentration in the molten steel with high precision.

Further, regarding the oxygen source in the decarburization treatment using the vacuum degassing apparatus or the decarburization promoting method, for example, Patent Document 4 proposes to introduce solid oxygen such as rust dust into a vacuum chamber, thereby suppressing Mn. a method of preferentially performing a decarburization reaction, and Patent Document 5 proposes a method In the vacuum degassing apparatus, a method of decarburizing by adding Mn ore to a molten steel which restricts the amount of C and temperature at the time of stopping the blowing of the converter, and Patent Document 6 and Patent Document 7 proposes When the steel after the tapping of the converter is decarburized, the MnO powder or the Mn ore powder is blown together with the carrier gas toward the surface of the molten steel in the vacuum chamber to perform the decarburization treatment, and further, in Patent Document 8 In the present invention, it is proposed that a Mn ore powder is blown together with a carrier gas toward a molten steel in a vacuum chamber of an RH vacuum degassing device through a blow hole provided in a side wall of the vacuum chamber to melt the steel by oxygen in the Mn ore. Decarburization and a method of increasing the Mn concentration.

On the other hand, there is an increasing demand for the high added value of steel materials and the improvement of material properties accompanying the expansion of use. One of the means required is high purity of steel, specifically very low vulcanization. The desulfurization of molten iron is generally carried out in the melt-milling stage and the molten steel stage, but in the extremely low-sulfur steel used in advanced electromagnetic steel sheets and conveying steel pipes, it is necessary to carry out desulfurization in the molten steel stage. As a method of refining the extremely low-sulfur steel, for example, a method of ejecting a desulfurizing agent into a molten steel in a steel ladle, a method of adding a desulfurizing agent to a molten steel, and stirring, etc., various methods have been proposed in the past. However, in these methods, since a new step is added between the steel tapping from the converter and the vacuum degassing treatment, the temperature of the molten steel is lowered, the manufacturing cost is increased, and the productivity is lowered.

In order to solve such problems, attempts have been made to concentrate the secondary refining step and simplify it by making the vacuum degassing apparatus have a desulfurization function. For example, using a desulfurization method of a vacuum degassing apparatus, an RH vacuum degassing apparatus having a top blowing type blowing tube is proposed, and is removed from the top blowing type blowing tube. The sulfur agent is blown (projected) together with the carrier gas to the molten steel surface in the vacuum chamber, thereby desulfurizing the molten steel (for example, refer to Patent Document 9).

However, for example, in a vacuum degassing apparatus, when an oxide powder such as solid oxygen or a desulfurizing agent such as Mn ore is added during the decarburization treatment, latent heat required for sensible heat or thermal decomposition of the oxide powder may occur. And the temperature of the molten steel is lowered. A method for compensating for a decrease in the temperature of the molten steel is a method for preliminarily increasing the temperature of the molten steel in the preceding step of vacuum degassing, and a method of adding metal Al to the molten steel and increasing the temperature of the molten steel by the heat of combustion Wait. However, the method of increasing the temperature of the molten steel in the preceding step causes the loss of the refractory in the preceding step to increase, resulting in an increase in cost. Further, in the method of adding metal Al to raise the temperature, there is a possibility that the purity of the molten steel is lowered due to the generated Al oxide, or the cost of the auxiliary material is increased.

Therefore, a method of adding solid oxygen while suppressing a decrease in the temperature of the molten steel has been proposed as a method of projecting onto the molten steel surface while heating by the flame of the burner provided at the tip end of the top-blowing blow pipe ( For example, refer to Patent Documents 10 and 11). Further, in the method of adding a desulfurizing agent, it is proposed to form a flame by blowing oxygen and a combustion gas together with a desulfurizing agent from the tip end of the top-blowing blow pipe, and melting the desulfurizing agent by the flame to melt it. A method of melting a molten steel surface (for example, refer to Patent Document 12).

Prior technical literature Patent literature

Patent Document 1: Japanese Patent Publication No. 04-088114

Patent Document 2: Japanese Patent Laid-Open No. 02-047215

Patent Document 3: Japanese Laid-Open Patent Publication No. 01-301815

Patent Document 4: Japanese Laid-Open Patent Publication No. SHO 58-073715

Patent Document 5: Japanese Laid-Open Patent Publication No. SHO63-293109

Patent Document 6: Japanese Laid-Open Patent Publication No. 05-239534

Patent Document 7: Japanese Laid-Open Patent Publication No. 05-239526

Patent Document 8: Japanese Laid-Open Patent Publication No. 01-092312

Patent Document 9: Japanese Laid-Open Patent Publication No. 05-311231

Patent Document 10: Japanese Laid-Open Patent Publication No. SHO 64-039314

Patent Document 11: Japanese Laid-Open Patent Publication No. 07-041827

Patent Document 12: Japanese Laid-Open Patent Publication No. 07-041826

However, the techniques disclosed in Patent Documents 10 and 11 for promoting decarburization or denitrification, dehydrogenation, and projection of oxide powders are optimum conditions for adding Mn ore as a source of Mn in a vacuum degassing apparatus, and No discussion. Similarly, the technique disclosed in Patent Document 12 in which the desulfurizing agent is heated by the flame of the burner does not have any discussion on the optimum conditions for adding the desulfurizing agent to the vacuum degassing apparatus.

The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide a vacuum degassing apparatus capable of suppressing a decrease in temperature of a molten steel or a loss of Mn when Mn ore is added as a source of Mn, and is effective. Melting method of low carbon high manganese steel which is decarburized In the vacuum degassing apparatus, it is possible to suppress the reduction in the temperature of the molten steel when the desulfurizing agent is added, and to efficiently carry out the low-sulfur melting method of the desulfurization treatment.

In order to solve the above-mentioned problems, the inventors of the present invention have focused on the reaction operation of C and Mn and the change operation of the molten steel temperature during decarburization treatment by a vacuum degassing apparatus. As a result, it has been found that the above problems can be solved by optimizing the addition conditions of the Mn ore added to the molten steel. Specifically, the combustion conditions of the burner provided at the tip end of the top-blowing blow pipe are controlled to an appropriate range, and the Mn ore is controlled. The steel is heated and reduced and added to the molten steel in the vacuum tank. Thus, the Mn ore can be added at a high yield without causing a decrease in the temperature of the molten steel, and the decarburization promoting effect can be enjoyed as well. For the desulfurizing agent, the flame is heated and melted by the flame of the burner provided at the front end of the top-blowing blow pipe, and is added to the molten steel in the vacuum tank, so that the temperature of the molten steel is not caused. The desulfurization treatment is carried out under reduced pressure, and in addition, it has been found that it is preferred to use a properly constructed blow pipe, thus completing the present invention.

That is, the present invention is a vacuum refining method for molten steel, which heats the oxide powder by a flame formed by a burner disposed at the front end of the top-blown blow pipe of the vacuum degassing apparatus, and is heated from the upper portion. A vacuum refining method of a molten steel which is blown out and added to a molten steel surface of a molten steel in a degassing tank, characterized in that a fuel and a combustion gas are supplied to the burner so as to satisfy a flame gas, and a flame is formed. ≦(G/F)/(G/F) _st ≦1.1

Where G: combustion gas supply rate (Nm ³ /min)

F: fuel supply speed (Nm ³ /min)

(G/F): oxygen fuel ratio (= combustion gas supply speed / fuel supply speed)

(G/F) _st : The stoichiometric chemical value of the oxygen-to-fuel ratio of the complete combustion of the fuel.

In the vacuum refining method of the molten steel of the present invention, the oxide powder is a Mn ore and/or a CaO-based desulfurizing agent.

Further, in the vacuum refining method of the molten steel according to the present invention, the Mn ore and/or the CaO system are blown out together with the carrier gas from the blow hole provided at the tip end of the center hole of the axial portion of the top-blowing blow pipe. The desulfurizing agent supplies fuel and combustion gas from a burner disposed at a tip end of a plurality of peripheral holes disposed around the above-mentioned blowing holes, ignites to form a flame, and heats the oxide powder by the flame.

Further, in the vacuum refining method of the molten steel according to the present invention, at least one of a hydrocarbon-based gaseous fuel, a hydrocarbon-based liquid fuel, and a carbon-based solid fuel is supplied as the fuel.

According to the present invention, in the vacuum degassing apparatus, the decrease in the temperature of the molten steel can be suppressed and the Mn ore can be added to the molten steel at a high Mn yield, and in addition, the decarburization speed can be increased, so that high production can be achieved. Slim and low cost to melt low carbon high manganese steel. Further, according to the present invention, in the vacuum degassing apparatus, the decrease in the temperature of the molten steel can be suppressed to add the desulfurizing agent to the molten steel, and In addition, the desulfurization efficiency can be improved, so that low-sulfur steel can be efficiently melted.

1‧‧‧Fused steel

2‧‧‧Steel drum

3‧‧‧Degassing Department

4‧‧‧vacuum tank

5, 6‧‧‧ immersion tube

7‧‧‧Exhaust port

8‧‧‧Accessory input (flow path)

9‧‧‧Top blown blowpipe

10‧‧‧Return gas supply piping

11‧‧‧Oxygen pathway or powder/carrier gas path

12‧‧‧Blow holes

13‧‧‧Internal water-cooled cylinder

14‧‧‧External water-cooled cylinder

15‧‧‧Fuel/combustion gas path

16‧‧‧ burner

17‧‧‧Guided burner

20‧‧‧Oxygen access

21‧‧‧ throat

22‧‧‧End of the Department

23‧‧‧fuel gas pathway

24‧‧‧fuel gas supply hole

25‧‧‧ powder/carrier gas path

26‧‧‧ powder/carrier gas blowing hole

Figure 1 is a schematic vertical cross-sectional view of the RH vacuum degasser.

Fig. 2 is a view for explaining the structure of a top-blowing blow pipe used in the present invention.

Fig. 3 is a view for explaining the configuration of a top blow type blower of the prior art.

Fig. 4 is a graph showing the relationship between the C concentration before the vacuum degassing treatment and the unstatic decarburization rate.

Fig. 5 is a graph showing the relationship between the C concentration and the Mn yield before the vacuum degassing treatment.

Figure 6 is a graph showing the effect of burner combustion conditions on Mn yield.

Figure 7 is a graph showing the effect of burner combustion conditions on the desulfurization rate.

First, the basic technical idea of the present invention and an experiment for proving the technical idea will be described.

The Mn ore is mainly composed of Mn oxides having different oxidation numbers such as MnO _{2 ,} Mn ₂ O ₃ , and MnO. When the Mn ore is used as a source of Mn or an oxygen source for promoting decarburization is added to the molten steel, the Mn oxide having a different oxidation number in the Mn ore may be considered as follows by C in the molten steel (1) )~(3); MnO ₂ +2C → Mn+2CO...(1)

Mn ₂ O ₃ +3C → 2Mn+3CO...(2)

MnO+C → Mn+CO...(3) is reduced.

From the above formulas (1) to (3), it is found that the higher the oxidation number of the Mn oxide, the more the amount of C required for reducing the Mn oxide. From this content, it can be known that when Mn oxide in the Mn ore is added to the molten steel as the Mn oxide having a low oxidation number, the amount of C required for reducing the Mn ore can be reduced even if C in the molten steel The concentration is low, and the Mn ore can be sufficiently reduced, so that the improvement of the Mn yield can be expected.

Therefore, the present inventors have considered that when the powdered Mn ore is added to the molten steel from the top-blown blow pipe disposed in the vacuum degassing apparatus, the burner provided at the front end of the top-blown blow pipe is controlled. The combustion condition of the fuel (hereinafter also referred to as "burning condition of the burner") heats the Mn ore while reducing the Mn oxide in the Mn ore and adding it.

In order to confirm the above effects, in the laboratory experiment in which molten steel is not used, the combustion conditions of the burners at the front end of various top-blowing blowpipes and the input method of Mn ore are changed, and the preparation for the top blowing of the steel drum container is added. experiment.

Specifically, in the preliminary experiment, the top-blowing type blower uses a Mn ore which can be blown out together with a carrier gas (Ar gas) from a blow hole provided at a tip end of a center hole of the shaft core portion, and Blowing fuel from a burner disposed at the front end of a plurality of peripheral holes disposed around the center hole A multi-tube blower that burns a gas to form a flame is used to heat the Mn ore and add it in a top blow. At this time, as shown in the first table, the supply rate of the fuel and the combustion gas is changed, and whether it is added by heating by a burner, and the temperature change of the Mn ore before and after the top-blown addition and the Mn ore are investigated. The change in the composition ratio of the Mn oxides having different oxidation numbers. In the preliminary experiment described above, Ar gas was used as the carrier gas, propane gas was used as the fuel, and pure oxygen was used as the combustion gas.

The results of the above preparatory experiments are collectively shown in Table 1. G shown in the first table is the supply rate of the combustion gas (Nm ³ /min), F is the supply rate of the fuel (Nm ³ /min), and (G/F) is the supply speed of the combustion gas relative to the fuel. The ratio of the supply speed, (G/F) _st is the stoichiometric chemical value of the oxygen-to-fuel ratio of the complete combustion of the fuel. Further, when the propane gas is used as the fuel and the combustion gas is pure oxygen, the (G/F) _st is 5, that is, the supply speed F with respect to the fuel is 1 Nm ³ /min, and the supply speed G of the combustion gas is 5 Nm ^{3 .} /min.

From the above-mentioned first table, it can be seen that in the case where No. 1 is heated by the flame of the burner at the tip end of the blower when the Mn ore is added together with the carrier gas, no Mn ore is observed before and after the introduction. The change was made, but in the conditions of No. 2 to 7 in which the Mn ore was heated by the flame of the burner, the temperature rise of the Mn ore was observed, and in the above Nos. 2 to 7, the combustion condition of the burner was (G/). F) / (G / F) _st is in the range of 0.4 to 1.1 No. 4 to 7, the ratio of MnO ₂ and Mn ₂ O ₃ in the Mn ore is decreased, and the ratio of MnO is increased. That is, the Mn oxide having a high oxidation number is reduced and is modified to a Mn oxide having a low oxidation number, however, in No. 8 in which (G/F)/(G/F) _{st is} lowered to 0.3. Since the flame itself was not formed, as in the case of No. 1, no change in the Mn ore was observed before and after the addition.

From the above results, it is also known that when the combustion condition of the burner formed at the tip end portion of the blow pipe is controlled to an appropriate range, that is, the combustion condition of the burner is not controlled to the oxygen excess side, but is controlled to the insufficient side. The flame formed is reduced to promote the reduction of the Mn oxide in the Mn ore. Therefore, even when the amount of C in the molten steel is small, the flame can be sufficiently reduced. Mn ore increases the Mn yield. In addition, when the Mn ore is heated by the flame of the burner, the temperature of the molten steel is lowered due to the temperature rise of the Mn ore. Furthermore, due to the addition of Mn, the oxygen in the Mn ore can be used. Promote decarburization reaction. Further, even if a desulfurizing agent is added from the tip end of the blow pipe, the same effect as described above can be expected.

The present invention has been developed based on the above-mentioned new technical ideas and findings.

Next, a vacuum degassing apparatus used for vacuum refining of the molten steel of the present invention will be described.

The vacuum degassing apparatus which can be used in the vacuum refining of the molten steel of the present invention is an RH vacuum degassing device, a DH vacuum degassing device, a VOD furnace, etc., and the most representative one among them is an RH vacuum degassing device. Therefore, the RH vacuum degassing device will be described as an example.

Figure 1 is a vertical cross-sectional view of a typical RH vacuum degassing device.

The RH vacuum degassing apparatus is composed of a steel drum 2 for accommodating the molten steel 1, and a degassing unit 3 for vacuum degassing (hereinafter also referred to as "degassing treatment"). The deaeration unit 3 is composed of a vacuum chamber 4 that introduces molten steel into a degassing process, and an exhaust device (not shown) connected thereto. The upper side surface of the vacuum chamber 4 is provided with an exhaust port 7 connected to the exhaust device, and an inlet port (flow path) 8 to which an auxiliary material such as an alloy raw material (component regulator) or a medium solvent is added.

Further, in the lower portion of the vacuum chamber 4, two immersion tubes 5 and 6 are disposed, one of which is a dip tube (5 in the first drawing), and a return gas for causing reflux in the molten steel 1 is connected. Piping that is blown into the dip tube 10. Then, during the degassing treatment, the above two dip tubes are immersed in the molten steel in the steel ladle, and the inside of the vacuum tank 4 is exhausted by an exhaust device not shown in the drawing, and the ladle 2 is The inside molten steel 1 is introduced into the vacuum chamber 4, and a return gas (inert gas such as Ar gas) is supplied into the immersion pipe 5 through the pipe 10 to rise in the immersion pipe 5 as air bubbles. As a result, the molten steel in the immersion tube 5 also rises together with the air bubbles and flows into the vacuum degassing tank, and after degassing treatment, it is lowered by the other immersion tube (6 in the first drawing), and causes a return. The molten steel in the steel drum is refluxed and degassed.

Further, a top-blowing blow pipe 9 is disposed in the upper portion of the vacuum chamber 4 so as to be inserted into the vacuum chamber 4 from above. The top-blowing blow pipe 9 is provided with an oxygen powder, an oxide powder such as Mn ore or a CaO-based desulfurizing agent, and a passage for transporting the carrier gas, and blows and blows the gas at the front end of the passage. a blowhole to the surface of the molten steel, a passage of fuel and combustion gas for burning the fuel, and a multi-tube blower for burning the fuel at the front end of the passage to form a burner of the flame.

The top-blowing blow pipe 9 is connected to a supply hopper not shown in the drawing of the auxiliary raw material, and is supplied with an oxide powder such as Mn ore or a CaO-based desulfurizing agent together with the carrier gas. The CaO-based desulfurizing agent is mainly used by mixing a CaO slagging accelerator such as fluorite (CaF ₂ ) or alumina (Al ₂ O ₃ ) of about 5 to 30 mass% with quicklime (CaO) or limestone ( CaCO ₃ ), hydrated lime (Ca(OH) ₂ ), dolomite (CaO-MgO), and the like. Further, the carrier gas is usually an inert gas such as Ar gas or nitrogen gas.

Further, the top-blowing blow pipe 9 is connected to a fuel supply pipe and a combustion gas supply pipe (not shown), and the fuel is a hydrocarbon-based gas fuel such as propane gas or natural gas, or a hydrocarbon-based liquid such as heavy oil or kerosene. At least one of a carbon-based solid fuel such as fuel, coke or coal, and an oxygen-containing gas such as oxygen, oxygen-enriched air or air can be supplied to the combustion gas. Further, since the top-blowing type blower 9 is water-cooled, a cooling water supply and drain pipe (not shown) for supplying and discharging the cooling water is also connected.

Here, a top-blowing type blower used for vacuum refining of the molten steel of the present invention will be described.

Fig. 2 is a view showing an example of a preferred top-blowing type blower used in the present invention, wherein (a) is a vertical sectional view and (b) is a lower view. The top-blowing type blowpipe is an oxide powder/carrier gas passage that supplies both an oxygen passage for supplying oxygen to the molten steel and a carrier gas for supplying the oxide powder and the oxide powder. (hereinafter also referred to simply as "oxygen passage" or "powder/carrier gas passage") 11; the shaft core portion is provided with a blow hole 12 provided at the front end of the passage, that is, at the tip end of the blow pipe. The inner water-cooling cylinder 13 of the "central hole"; the outer water-cooling cylinder 14 surrounding the inner water-cooling cylinder 13; and the fuel or combustion gas is supplied between the inner water-cooling cylinder 13 and the outer water-cooling cylinder 14 The passage 15 is constituted by a plurality of "surrounding holes" formed at the front end of the passage, that is, the burner 16 provided at the tip end of the blow pipe. The peripheral hole has a double pipe structure, and the fuel is circulated on the inner pipe side, and the combustion gas is circulated on the outer pipe side. However, the fuel passage and the combustion gas passage may be interchanged.

Blowing out from the blow hole 12 at the front end of the powder/carrier gas passage 11 The oxide powder or the like is heated by the flame of the burner 16 formed at the tip end of the blow pipe, or heated and reduced, or heated and melted, and blown to the molten steel surface in the vacuum chamber.

In Fig. 2, since one of the eight surrounding holes is used as the pilot burner 17 for igniting the blown fuel, the number of burners is seven. However, since the fuel supplied from the fuel gas passage of the burner 16 and the combustion gas (oxidizing gas) supplied from the combustion gas passage are close to each other (repeated), they can be instantaneously mixed and placed in the combustion front. Within the limited range, but because the ambient temperature in the vacuum chamber is high, even if there is no ignition device, combustion starts, and a flame is formed below the top-blowing blow pipe 9. Therefore, guiding the burner is usually not required, but can also be set.

Here, the positional relationship between the center hole of the tip end of the top-blowing blow pipe and the surrounding hole, that is, the positional relationship between the blow hole 12 and the burner 16, although it may be reversed, is configured to surround the oxidant by the flame of the burner. In the periphery of the jet flow of the powder, the oxide powder can be efficiently heated. Therefore, as shown in Fig. 2, it is preferable to arrange the blow hole in the axial core portion of the blow pipe, and to arrange the burner around the burner. .

Further, in the second drawing, the shape of the blow hole 12 provided at the tip end of the center hole of the top-blowing blow pipe is a Laval blow hole formed by a reduced cross section and two enlarged cone portions, but may be A blow hole for a straight shape. The narrowest portion of the Laval blowhole where the reduced portion is connected to the enlarged cone of the two cones is commonly referred to as the throat.

The top-blowing blow pipe of Fig. 2 configured as described above includes an oxygen passage, a powder/carrier gas passage, a fuel passage, and a combustion gas. Since the body passage is used, it is possible to perform preheating of the vacuum chamber, heating (heating) of the molten steel in the vacuum chamber, heating/removal of the deposit in the vacuum chamber, and blowing of the powder into the molten steel.

The above-described top-blowing blow pipe 9 used in the present invention is not limited to the above-described range. For example, a plurality of burners may be disposed around the top-blowing blow pipe to use the burner from the top-blowing blow pipe. The Mn ore blown in is heated. Further, a top-blowing blowpipe and a burner for adding Mn ore may be separately provided.

Next, a method of melting low carbon high manganese steel using the above-described RH vacuum degassing apparatus will be described.

First, the smelting and tapping from the blast furnace is carried to a holding vessel or a transport container such as a melt mill or a torpedo vehicle, and then transported to a steel making step for decarburization refining. In the middle of the transportation, in many cases, the fusion milling is subjected to a desulphurization or dephosphorization preparation process such as desulfurization or dephosphorization. In the present invention, even if the composition specification does not require the need for the fusion milling preparation, it is preferable to apply the melt milling. deal with. This is because in the converter, the Mn ore added as a manganese source is added. When the pre-melting preparation process is not performed, especially when the dephosphorization treatment is not performed, the dephosphorization must be carried out together with the decarburization during the blowing in the converter. Therefore, a CaO-based flux is added in a large amount to increase the amount of slag in the converter, and the amount of manganese distributed to the slag increases, resulting in a decrease in Mn yield.

In the subsequent steel making step, after melt-milling is installed in the converter, Mn ore is added as a manganese source, and a small amount of a medium solvent such as quicklime may be added as needed to blow off oxygen from the upper portion and/or blow off from the bottom portion. After the carbon is blown to form a molten steel of a predetermined composition, in the state of not deoxidizing The steel is kept to a molten steel holding container such as a steel drum. At this time, an inexpensive alloy iron-based manganese source such as high-carbon iron-manganese or the like can be added.

In the above steelmaking step, since an inexpensive manganese source such as Mn ore or high carbon iron manganese is used, the carbon concentration in the molten steel is inevitably increased, and at this time, it is preferable to suppress the C concentration in the molten steel after the Mn concentration is adjusted. 0.2mass% or less. When the C concentration exceeds 0.2 mass%, not only the decarburization treatment time in the vacuum degassing apparatus of the subsequent step is increased but the productivity is lowered, and the temperature of the molten steel is lowered in order to compensate for the prolonged decarburization treatment time. However, it is not preferable to increase the tapping temperature, thereby causing a decrease in the iron yield and an increase in the cost of the refractory due to an increase in the amount of refractory loss.

Then, the steel tapped from the converter is sent to a vacuum degassing device such as an RH vacuum degassing device, a DH vacuum degassing device, or a VOD furnace to perform degassing treatment such as decarburization treatment. The following is a description of a method of melting low carbon high manganese steel using the RH vacuum degassing apparatus shown in Fig. 1.

In the RH vacuum degassing apparatus shown in Fig. 1, the molten steel 1 in the undeoxidized state is subjected to vacuum decarburization treatment (hereinafter, this treatment is also referred to as "unstatic treatment"), and in the unstatic treatment, In the top-blowing blow pipe 9, Mn ore is blown from the upper portion. At this time, the Mn ore is heated and reduced by the flame of the burner formed at the front end portion of the top-blowing blow pipe 9, and is blown to the molten steel surface of the molten steel to be added. Specifically, the fuel and the combustion gas are supplied to the burner 16 at the tip end of the blow pipe through the fuel passage and the combustion gas passage provided in the peripheral hole of the top-blowing blow pipe 9, and are ignited and ignited in the burner. A flame is formed on it. Then, Mn is taken from the blow hole 12 at the front end of the blow pipe via the powder/carrier gas passage 11 of the center hole. The ore is blown out, and the blown Mn ore is heated and reduced by the flame of the burner described above, and is blown from the upper portion. When the Mn ore is initially added, it is preferred to form a flame on the burner in advance.

Here, the flame of the burner for heating and reducing the Mn ore to form the front end of the blow pipe, the fuel and the combustion gas must satisfy the following formula, 0.4 ≦(G/F)/(G/F) _st ≦ 1.1

Where G: combustion gas supply rate (Nm ³ /min)

F: fuel supply speed (Nm ³ /min)

(G/F): oxygen fuel ratio (= combustion gas supply speed / fuel supply speed)

(G/F) _st : the stoichiometric chemical value of the oxygen-to-fuel ratio of the complete combustion of the fuel

As described above, when ((G/F)/(G/F) _st ) exceeds 1.1, the oxidizing property of the flame becomes strong, and although the Mn ore is heated, the Mn oxide in the Mn ore is not reduced. On the other hand, when ((G/F)/(G/F) _st ) is less than 0.4, the flame itself is not formed, so the Mn ore cannot be heated. Preferably, ((G/F)/(G/F) _st ) is in the range of 0.4 or more and less than 1.1.

By adding Mn ore in this manner, it is possible to suppress a decrease in temperature (temperature loss) of the molten steel due to the addition of Mn ore. Further, since the Mn ore heated by the flame satisfying the above combustion conditions is added to the molten steel by reduction, the reduction reaction of the Mn ore can be promoted and the Mn yield can be improved, so that the addition amount of the Mn alloy can be reduced. Furthermore, the addition of Mn ore allows the oxygen in the Mn ore to function as a solid oxygen to promote the decarburization reaction, so that the untreated time can be shortened and the production can be improved. Sex.

In the above-described non-static treatment, after the Mn ore is heated and added, the oxygen may be blown off and blown to the molten steel through the oxygen gas passage 11 and the blow hole 12 at the tip end to promote decarburization or heating the molten steel. In the decarburization treatment or the temperature elevation treatment, since the burner is not used, it is preferable to allow an inert gas such as nitrogen gas or Ar gas to flow through the fuel passage and the combustion gas passage to prevent the burner from being caused by splashing or the like. Blocking.

After the predetermined time is not statically treated, after the C concentration in the molten steel reaches a specific value equal to or less than the component specification value, a strong deoxidizer such as Al is added to the molten steel 1 from the raw material input port 8 to reduce the molten steel. The oxygen concentration (deoxidation) was stored and ended without static treatment. When the temperature of the molten steel after the end of the static treatment is lower than the temperature required for the subsequent step such as the continuous casting step, Al may be further added to the molten steel from the raw material inlet port, and oxygen is blown from the top-blowing blow pipe ( The oxygen is supplied to the surface of the molten steel and the temperature of the molten steel is increased by burning Al.

Then, the molten steel 1 which has been deoxidized by adding a deoxidizing agent is added, and the molten steel is continuously reflowed for several minutes (this process is referred to as "complete static treatment"), and Al, Si, and the like may be used from the raw material input port 8 as necessary. A component adjusting agent (alloy component) of Mn, Ni, Cr, Cu, Nb, Ti, or the like is added to the molten steel 1, and after the molten steel component is adjusted to a predetermined composition range, the vacuum chamber 4 is returned to the atmospheric pressure, and the degassing treatment is terminated. .

Next, a method of melting a low-sulfur steel using the above-described RH vacuum degassing apparatus will be described. The iron is melted and milled from the blast furnace, blown in a converter, tapped and delivered to the RH vacuum degassing device, and is combined with the above low carbon high manganese steel. Since the melting method is the same, the description is omitted.

The molten steel which is transported to the RH vacuum degassing apparatus in an undeoxidized state can be blown by oxygen for a predetermined period of time, that is, through the oxygen gas passage 11 of the top-blowing blow pipe 9 and the blow hole 12 of the front end. Demelting is performed in the molten steel, and after the C concentration in the molten steel reaches a specific value below the component specification value, a strong deoxidizing agent such as Al is added to the molten steel 1 from the raw material input port 8 to reduce melting in the molten steel. Oxygen concentration (deoxidation) was stored and ended without static treatment.

When the temperature after the completion of the static treatment, that is, the temperature of the molten steel after deoxidation, is lower than the temperature required for the subsequent step such as the continuous casting step, Al may be further added to the molten steel from the raw material input port, from the above-mentioned top blowing type. The blow pipe blows oxygen (sends oxygen) to the surface of the molten steel and increases the temperature of the molten steel by burning Al. Further, in the same manner as the melting method of the low carbon high manganese steel described above, the molten steel 1 in the undeoxidized state is not subjected to static treatment, and the Mn ore can be blown from the top blowing type blower 9 from the upper portion.

Then, a CaO-based desulfurizing agent is sprayed from the top-blowing blow pipe 9 to the above-mentioned deoxidized molten steel, heated and melted by a flame formed in the burner 16, and blown to the molten steel surface of the molten steel to be added. And desulfurization treatment. Specifically, the fuel and the combustion gas are supplied to the burner 16 at the tip end of the blow pipe through the fuel passage and the combustion gas passage provided in the peripheral hole of the top-blowing blow pipe 9, and are ignited and ignited in the burner. A flame is formed thereon, and at the same time, the CaO-based desulfurizing agent is blown out from the blow hole 12 at the tip end of the blow pipe through the powder/carrier gas passage 11 of the center hole, and the CaO-based desulfurization blown by the flame of the burner is used. Agent heating and melting Melt, and add it from the top. When the addition of the CaO-based desulfurizing agent is started, it is preferred to form a flame on the burner in advance.

Here, the flame of the burner for heating and melting the above-mentioned desulfurizing agent to be formed at the front end of the blow pipe must satisfy the following formula, 0.4 ≦(G/F)/(G/F) _st ≦ 1.1

Where G: combustion gas supply rate (Nm ³ /min)

F: fuel supply speed (Nm ³ /min)

(G/F): oxygen fuel ratio (= combustion gas supply speed / fuel supply speed)

(G/F) _st : the stoichiometric chemical value of the oxygen-to-fuel ratio of the complete combustion of the fuel

When ((G/F)/(G/F) _st ) exceeds 1.1, the oxidizing property of the flame becomes strong, and the desulfurizing agent is heated, but the desulfurization reaction as a reduction reaction does not proceed. On the other hand, when ((G/F)/(G/F) _st ) is less than 0.4, the flame itself is not formed, so that the desulfurizing agent cannot be heated. Preferably, ((G/F)/(G/F) _st ) is in the range of 0.4 or more and less than 1.1.

By adding the desulfurizing agent in this manner, it is possible to suppress a decrease in temperature (temperature loss) of the molten steel due to the addition of the desulfurizing agent. Further, since the flame satisfying the above combustion conditions does not become a peroxidation environment, the desulfurization reaction as a reduction reaction can be promoted, and the desulfurization rate can be improved.

When the composition specification does not need to be decarburized in the RH vacuum degassing device, when the molten steel is tapped from the converter to the ladle, the metal Al may be added to the molten steel melt stream in the tapping steel to The molten steel is deoxidized. At this time, in the molten steel melt stream, in addition to Al, lime or lime-containing help may be added. Flux. Then, the steel is tapped to the molten steel of the steel drum, and the slag which adds the slag modifier such as Al to the molten steel is applied to iron oxide such as FeO in the slag or manganese such as MnO. After the oxide reduction slag is reformed, it is transported to the RH vacuum degassing device.

Then, the molten steel 1 which has been deoxidized by adding the above-mentioned deoxidizing agent is applied to the RH vacuum degassing apparatus, and the molten steel is refluxed to perform the degassing treatment, and then Al, depending on the necessity, the raw material input port 8 may be used. A component adjusting agent (alloy component) of Si, Mn, Ni, Cr, Cu, Nb, Ti, or the like is added to the molten steel 1, and after the molten steel component is adjusted to a predetermined composition range, the vacuum chamber 4 is returned to the atmospheric pressure, and the end is removed. Gas treatment.

In the above description, the method of melting low carbon high manganese steel and low sulfur steel using the RH vacuum degassing device is described, but even when other vacuum degassing devices such as a DH vacuum degassing device or a VOD furnace are used, Melting low carbon high manganese steel and low sulfur steel.

Example 1

After the desulphurization and desulfurization of the blast furnace tapping is subjected to the desulphurization and desulphurization preparation, it is refined in a 350 ton converter to form C: 0.03 to 0.09 mass%, Si: 0.05 mass% or less, and Mn: 0.1. ~0.85 mass%, P: 0.03 mass% or less, and S: 0.003 mass% or less. In the above converter, Mn ore is added as a manganese source to adjust the Mn concentration.

The molten steel after refining in the converter is tapped to the ladle under the condition of no deoxidation, and is transported to the RH vacuum degassing device with the top blowing type blowpipe, and is used in the state of no deoxidation. Vacuum decarburization Degassing treatment in static treatment. The concentration of O in the molten steel at the time of reaching the RH vacuum degassing device is in the range of 0.03 to 0.07 mass%.

In the above-described non-static treatment, the flow rate of the reflux gas (Ar gas) is 1500 NL/min, and the degree of vacuum of the vacuum chamber is set to 6.7 to 40 kPa (each condition is maintained constant), and is changed as shown in Table 2. The type of top blown blowpipe used, whether or not Mn ore is added, the method of addition, the combustion conditions of the burner at the end of the blow pipe ((G/F)/(G/F) _st ), and the presence or absence of oxygen supply.

The added Mn ore is used in a particle size of 5 to 20 mm and a manganese content of about 58 mass%. The addition rate of Mn ore is 100 kg/min, the addition time is 10 min, and the total addition amount is 1000 kg.

Further, the target component of the molten steel after the static treatment is C: 0.002 to 0.003 mass%, Mn: 0.5 to 1.2 mass%, and after the completion of the static treatment, when the Mn concentration is too low, metal manganese is added to adjust Mn concentration.

Further, when oxygen is insufficient in the case of no static treatment, decarburization is performed while blowing oxygen (steaming) from the blow hole at the tip end of the top-blowing blow pipe to the surface of the molten steel.

Further, the top-blowing type of the above-described RH vacuum degassing apparatus is a type of the blowing tube shown in Fig. 3 which is suitable for the second aspect of the present invention, and the type of the blowing tube shown in Fig. 3 which is similar to the blowing tube disclosed in Patent Document 10. In the blow pipe of FIG. 3, the fuel gas supply hole 24 connected from the fuel gas passage 23 is provided in the end portion wide portion 22 connected to the throat portion 21 from the oxygen passage 20 provided in the axial center portion of the blow pipe, and Further, the structure is configured to blow out Mn ore from the blowing hole 26 at the front end of the powder/carrier gas passage 25.

When the Mn ore is added, when the blow pipe shown in Fig. 2 is used, that is, the Mn ore and the carrier gas (Ar gas) are blown from the upper portion to the molten steel via the powder/carrier gas passage 11 and the blow hole 12. The molten steel surface is carried out. Further, when a flame is formed on a top-blowing lance tip of the burner system to 240Nm ³ / hr LNG as fuel is supplied, in addition, changing the supplied pure oxygen as the combustion gas in the range of 120 ~ 600Nm ³ / hr and Change the combustion conditions of the burner ((G/F)/(G/F) _st ). At this time, (G/F) _st is 2 (the supply speed F with respect to the fuel is 1 Nm ³ /min, and the supply speed G of the combustion gas is 2 Nm ³ /min). The formation time of the flame is 10 min (constant).

On the other hand, when the blow pipe shown in Fig. 3 is used, the combustion gas (oxygen) is blown through the oxygen passage 20, and the Mn ore and the carrier gas (Ar gas) are supplied via the powder/carrier gas passage 25. The Mn ore is added by blowing from the upper portion to the surface of the molten steel. Further, when the flame is formed at a front end of the top-blowing lance, based at 240Nm ³ / hr LNG as fuel is supplied to 470Nm ³ / hr of pure oxygen is supplied as a combustion gas. The flame formation time was set to 10 min.

In the second table, the Mn concentration of the molten steel component (C, Mn) before the degassing treatment (before the static treatment) and after the static treatment (before the concentration adjustment by the addition of the manganese metal) is also described. The Mn yield in the Mn ore added during the untreated treatment, the decarburization rate at the time of no static treatment, and the difference in the temperature of the molten steel before and after the static treatment. The decarburization rate described in the above Table 2 is the average decarburization rate at which the decarburization amount from the arrival of RH to the end of the non-static treatment is divided by the static time. In addition, the temperature difference of the molten steel, when the value is positive, indicates that the temperature of the molten steel rises, and when it is negative, the temperature of the molten steel decreases.

The following contents can be found from the second table.

First, No. 16 to 18 are the top-blown blowpipes of Fig. 2, and a flame is formed at the tip end of the blow pipe when it is not statically treated. However, in the comparative example in which no Mn ore is added, although the temperature of the molten steel rises, the decarburization speed is 0.0033~0.0036mass%/min. On the other hand, in the No. 1 to 15 in which Mn ore was added, the decarburization rate was 0.0040 to 0.0052 mass%/min, and it was found that decarburization was promoted by the addition of Mn ore. This can be considered as the function of the Mn oxide in the Mn ore to effectively act as a solid oxygen, and promote the decarburization reaction of the molten steel. In the comparative example of No. 16 to 18, oxygen required for decarburization was insufficient and oxygen was supplied, so that Mn loss occurred.

Next, an example of adding Mn ore is analyzed.

No. 13 to 15 are comparative examples in which the top blowing type blowing pipe of Fig. 2 is used and the Mn ore is added from the auxiliary material inlet port (8 in Fig. 1) to the vacuum chamber without heating, because the Mn ore is accompanied. The temperature loss caused by the sensible heat or decomposition heat (latent heat) caused by the addition, the temperature of the molten steel is lowered by more than 30 ° C, the decarburization speed is only about 0.004 mass% / min, and the Mn yield is It is only about 40%~50%.

In addition, No. 10 to 12 are comparative examples in which the top blowing type blowing pipe of Fig. 2 is used, but the Mn ore is not heated by the flame of the burner and is blown from the upper portion, and is the same as No. 13 to 15 described above. The temperature loss caused by the sensible heat or decomposition heat (latent heat) caused by the addition of Mn ore causes the temperature of the molten steel to be greatly lowered, and the Mn yield is also at a low level as in the above Nos. 13-15.

On the other hand, No. 19 is an invention example in which a top-blowing type blower of the prior art shown in FIG. 3 is used and the Mn ore is heated by a flame formed at the tip end of the blow pipe. In the example of the invention, the temperature of the molten steel rises by 10 ° C or more. This can be considered by adding Mn ore to reduce the temperature loss and improve the thermal efficiency. In addition, the Mn yield has also increased to nearly 80%. This can be considered by heating the Mn ore by a reducing flame and reducing the addition of the Mn ore.

In addition, No. 1 to 6 are inventive examples in which the top-blown blowpipe of Fig. 2 is used to heat the Mn ore while being blown from the upper portion by the flame of the burner, and the temperature of the molten steel after the untreated treatment is increased by 9 ° C. As described above, the decarburization rate is a high value of 0.048 mass%/min or more, and the Mn yield in the Mn ore is also 80% or more.

Here, in the invention example of No. 4 and the invention example of No. 19, although the combustion conditions ((G/F)/(G/F) _st ) of the burner are the same, in the invention example of No. 4, the fusion The rise in steel temperature, the decarburization rate, and the Mn yield are all preferred. The difference is that in the top-blowing type blower of Fig. 3 used in No. 19, the Mn ore and the combustion gas are mixed and blown at the tip end of the blow pipe, whereas the top blow of the second figure used in No. 4 is used. In the blow pipe, the Mn ore is blown from the blow hole at the tip end of the blow pipe, and the Mn ore is heated to surround the jet by the flame of the burner disposed around the blow hole, so that the blow pipe of FIG. 2 can be considered. It can efficiently heat and reduce Mn ore.

On the other hand, No. 7 is the same as the invention example of No. 1 to 6 except that the combustion condition ((G/F) / (G/F) _st ) of the burner is higher than the range of the present invention. In the comparative example, since the Mn ore cannot be reduced due to the non-reducing property of the flame, the temperature of the molten steel rises, but the Mn yield is also at a low level as in the above Nos. 13 to 15.

On the contrary, No. 8 and 9 are the same as the invention examples of Nos. 1 to 6 except that the combustion conditions ((G/F)/(G/F) _st ) of the burner are lower than the range of the present invention. In the comparative example, since the supplied oxygen is insufficient, the flame cannot be formed and the Mn ore cannot be heated, and the temperature loss due to sensible heat or latent heat caused by the addition of the Mn ore causes the temperature of the molten steel to decrease, and the Mn yield is lowered. It is also at a low level as in No. 13~15 above.

Here, Fig. 4 shows the relationship between the C concentration before the RH treatment and the decarburization rate in the comparative examples of Nos. 1 to 6 and No. 7 to 15, and Fig. 5 is shown in the above No. In the comparative examples of 1 to 6 and the comparative examples of Nos. 7 to 15, the relationship between the C concentration before RH treatment and the Mn yield. From these figures, it can be seen that when the C concentration before the RH treatment is the same level, the decarburization speed of the invention example is higher and the Mn yield is improved as compared with the comparative example. As described above, when the Mn ore is heated by the flame of the optimum combustion condition and is blown from the upper portion, the reduction of the Mn oxide is performed before the Mn ore reaches the molten steel, and the amount of C required for the reduction of the Mn ore is reduced. The result can be tested The amount is such that the Mn ore can be sufficiently reduced even if the C concentration before the RH treatment is low, and the oxygen in the Mn ore can sufficiently function as a solid oxygen. That is, in the comparative example, it is considered that when the C concentration before the RH treatment is low, the amount of C used for reducing the Mn ore is insufficient, and the Mn yield is insufficient.

Further, Fig. 6 shows a comparison example of Nos. 1 to 6 and Nos. 7 to 9 which are added by heating the Mn ore by the flame of the burner, ((G/F)/(G/F). ) _st ) and the relationship between Mn yield. From these figures, it can be seen that ((G/F)/(G/F) _st ) is in the range of 0.4 to 1.1, and the Mn yield is 80% or more, among which ((G/F)/ When (G/F) _st ) is 0.4 or more and less than 1.0, an extremely high value of Mn yield of 90% or more can be obtained.

From the above results, it is understood that a blower suitable for the present invention is used, and a flame is formed at a burner at the front end of the blow pipe under conditions suitable for the present invention, and the Mn ore is heated and reduced to be blown from the upper portion, whereby Not only the reduction in the temperature of the molten steel but also the addition of Mn at a high yield can be suppressed, and the decarburization speed can be increased, so that the melting of the low carbon high manganese steel can be carried out efficiently and at low cost.

Example 2

After the desulphurization and desulfurization of the blast furnace tapping is subjected to the desulphurization and desulphurization preparation, it is refined in a 350 ton converter to form C: 0.03 to 0.09 mass%, Si: 0.05 mass% or less, and Mn: 0.1. ~0.85mass%, P: 0.03mass% or less, S: 0.0037~0.0042mass% of the composition of the steel.

The molten steel after refining in the converter is tapped to the steel in the state without deoxidation The drum is transported to an RH vacuum degassing apparatus equipped with a top-blowing blower, and a degassing treatment accompanying the vacuum treatment for vacuum decarburization is applied without deoxidation. The concentration of O in the molten steel at the time of reaching the RH vacuum degassing device is in the range of 0.03 to 0.07 mass%.

In the above-described non-static treatment, the flow rate of the reflux gas (Ar gas) is 1500 NL/min, and the vacuum degree of the vacuum chamber is set to 6.7 to 40 kPa (each condition is maintained constant), and the blow hole from the top-blowing blow pipe is used. Oxygen is supplied to the surface of the molten steel while oxygen is supplied, and after the C concentration in the molten steel reaches a specific value equal to or less than the component specification value, Al is added to the molten steel to perform deoxidation, and the static treatment is terminated. Then, a CaO-based desulfurizing agent is added to the above molten steel to perform a desulfurization treatment. The above-mentioned desulfurizing agent is a CaO-Al ₂ O ₃ premelting flux having a particle size of 2 mm or less, the addition rate of the desulfurizing agent is 100 kg/min, the addition time is 10 minutes, and the total addition amount is 1000 kg.

At this time, as shown in the third table, the addition conditions of the desulfurizing agent (with or without heating of the burner) and the combustion conditions of the burner ((G/F)/(G/F) _st ) are changed. For the desulfurizing agent, the top blowing type blowing pipe shown in Fig. 2 is used, and the desulfurizing agent and the carrier gas (Ar gas) are passed through the center hole, that is, via the powder/carrier gas passage 11 and the blow hole 12. It is added together to the molten steel surface.

Further, when a flame is formed on a top-blowing lance tip of the burner system to 240Nm ³ / hr LNG as a fuel supply, and changing the supplied pure oxygen as the combustion gas in the range of 120 ~ 600Nm ³ / hr and change Burning conditions of the burner ((G/F)/(G/F) _st ). At this time, (G/F) _st is 2 (the supply speed F with respect to the fuel is 1 Nm ³ /min, and the supply speed G of the combustion gas is 2 Nm ³ /min).

In the third table, the S concentration in the molten steel before and after the degassing treatment, the desulfurization ratio obtained from the value, and the temperature difference of the molten steel before and after the projection of the desulfurizing agent are also described. The molten steel temperature difference, when the value is positive, indicates that the molten steel temperature rises, and when it is negative, it indicates that the molten steel temperature is lowered.

The following is available from Table 3.

No. 9 is a comparative example in which the top blowing type blowing pipe of Fig. 2 is used, but the desulfurizing agent is not heated by the flame of the burner to be blown from the upper portion, and the sensible heat is caused by the addition of the desulfurizing agent. The temperature of the molten steel is greatly reduced, and the desulfurization rate is also about 60%, which is at a low level.

On the other hand, Nos. 1 to 6 are inventions in which the top-blown blowpipe of Fig. 2 is used and the desulfurizing agent is heated by the flame of the burner to be blown from the upper portion. For example, there is almost no temperature loss due to the addition of a desulfurizing agent. This can be considered by adding a desulfurizing agent by heating, which can reduce the temperature loss and improve the thermal efficiency. In addition, the desulfurization rate can also be obtained by more than 78%. This can be considered because the flame of the burner has reductive property, so the desulfurization reaction of the molten steel is promoted.

On the other hand, No. 7 is the same as the invention example of No. 1 to 6 except that the combustion condition ((G/F) / (G/F) _st ) of the burner is higher than the range of the present invention. In the comparative example, although the temperature of the molten steel rises, the desulfurization rate is also about 60%, which is at a low level. This can be considered because the flame is non-reducing, and the desulfurization reaction of the molten steel as the reduction reaction cannot be performed.

On the contrary, No. 8 is the same as the invention examples of Nos. 1 to 6 except that the combustion condition ((G/F) / (G/F) _st ) of the burner is lower than the range of the present invention. For example, since the supplied oxygen is insufficient, a flame cannot be formed and the desulfurizing agent cannot be heated, so that the temperature of the molten steel is largely lowered due to temperature loss. However, since the desulfurizing agent is supplied as an unburned reducing gas, the desulfurization rate is as high as 88.1%.

Here, Fig. 7 shows a comparison example of Nos. 1 to 6 and Nos. 7 and 8 which are added by heating a desulfurizing agent by a flame of a burner, ((G/F)/(G) /F) _st ) Relationship with desulfurization rate. From this figure, it can be seen that when ((G/F)/(G/F) _st ) is 1.1 or less, a desulfurization rate of 78% or more can be obtained, among which, ((G/F)/(G/) F) When _st ) is 0.4 or more and less than 1.0, an extremely high value of about 90% of the desulfurization rate can be obtained. When ((G/F)/(G/F) _st ) is 0.3, although a high desulfurization rate can be obtained, under these conditions, the flame cannot be formed as described above, and the temperature of the molten steel is greatly lowered, so that it is not preferable. .

From the above results, it is understood that a blower suitable for the present invention is used, and a flame is formed at a burner at the tip end of the blow pipe under the conditions suitable for the present invention, and the desulfurizing agent is heated and melted to be blown from the upper portion, whereby The reduction in the temperature of the molten steel can be suppressed and the desulfurization rate can be increased, so that the melting of the low-sulfur steel can be efficiently performed.

Claims (5)

A vacuum refining method for molten steel, which heats an oxide powder by a flame formed by a burner disposed at a front end of a top-blown blow pipe of a vacuum degassing device, and is blown from the upper portion to be added to the degassing A vacuum refining method for molten steel on a molten steel surface of a molten steel in a tank, which is characterized in that a fuel and a combustion gas satisfy the following formula are supplied to the burner to form a flame, 0.4 ≦ (G/F) /(G/F) _st ≦1.1 where G: combustion gas supply rate (Nm ³ /min) F: fuel supply speed (Nm ³ /min) (G/F): oxygen fuel ratio (= combustion gas supply Speed/fuel supply speed) (G/F) _st : The stoichiometric chemical value of the oxygen-to-fuel ratio of the complete combustion of the fuel. The vacuum refining method for molten steel according to claim 1, wherein the oxide powder is a Mn ore and/or a CaO-based desulfurizing agent. The vacuum refining method of the molten steel according to claim 1 or 2, wherein the Mn ore is blown out together with the carrier gas from a blow hole provided at a front end of the center hole of the shaft core portion of the top blow type blow pipe And a CaO-based desulfurizing agent, and a fuel and a combustion gas are supplied from a burner disposed at a front end of a plurality of peripheral holes disposed around the blowing hole, ignited to form a flame, and the flame is heated by the flame Powder. The vacuum refining method of the molten steel according to the first or second aspect of the invention, wherein the hydrocarbon fuel, the hydrocarbon-based liquid fuel, and the carbon-based solid fuel are supplied as the fuel. The vacuum refining method of the molten steel according to the third aspect of the invention, wherein the hydrocarbon fuel, the hydrocarbon-based liquid fuel, and the carbon-based solid fuel are supplied as the fuel.