TW201829790A - Desulfurization treatment method for molten steel, and desulfurization agent - Google Patents

Abstract

The desulfurization treatment method for molten steel according to the present invention is for reducing the sulfur concentration of molten steel, by adding a desulfurization agent containing quicklime to a ladle containing molten steel, and stirring the molten steel in the ladle, and is characterized in that the desulfurization agent used is one which contains quicklime, and in that the sum of the volumes of pores having a pore diameter falling within the range of 0.5-10 [mu]m or less is 0.1 mL/g or more. This allows the desulfurization treatment to be efficiently performed without use of CaF2 or pre-melt flux.

Description

Desulfurization treatment method for molten iron and desulfurizing agent

The invention relates to a method for desulfurizing a molten steel and a desulfurizing agent.

In recent years, due to the increase in material characteristics accompanying the increase in the value-added steel or the expansion of the use of steel materials, the demand for high-purity steel manufacturing has increased, especially as an element that reduces the toughness of steel materials. Very low sulfur steels with low sulfur content have become more demanding. In the melting process of iron and steel materials, there are desulfurization treatment in the molten iron stage and desulfurization treatment in the molten steel stage. Generally, steel materials are melted only by using the desulfurization treatment in the molten iron stage. However, in the process of melting ultra-low sulfur steels such as high-grade electromagnetic steel plates and pipeline steels, the desulfurization treatment in the molten iron stage is not sufficient. In addition to the desulfurization treatment in the molten iron stage, a molten steel is further required. Desulfurization treatment in the stage.

Generally, the desulfurization treatment in the molten steel stage is carried out by a ladle refining (ASEA-SKF) method in which molten steel has an arc heating mechanism or a stirring mechanism, and is then blown into a powder such as a flux or alloy powder in the molten steel. , Vacuum arc heating degassing (VAD) method, submerged arc ladle furnace (ladle furnace, LF) method and other ladle refining methods. The ladle refining method is performed by adding a desulfurizing agent to a ladle containing molten steel melted by decarburization refining in a converter, and stirring and mixing the molten steel and the desulfurizing agent. Or arc heating, thereby slagging the desulfurizing agent, a slag-metal reaction occurs between the slag formed by the slagging of the desulfurizing agent and the molten steel, and the sulfur component in the molten steel is transferred to the molten metal. In the slag.

Here, as the desulfurizing agent, there can be used Al ₂ O ₃ (alumina), CaF ₂ (fluorite), etc., which contains CaO (quick lime) as the main component and has the purpose of lowering the melting point of the desulfurizing agent. Desulfurizer. In order to have an efficient desulfurization reaction by a desulfurization treatment method using a ladle refining method, it is important to slag the added desulfurizing agent as soon as possible, and to increase the stirring strength to slag the desulfurizing agent. The contact area between the formed slag and the metal is increased. The desulfurizing agent is usually placed above the molten steel in the ladle and added. Whether it is slagging the desulfurizing agent by electric arc heating after the addition, or after the addition, the desulfurizing agent is stirred and mixed with the molten steel. Slagging takes a long time.

Therefore, in order to promote the slagging of the desulfurizing agent, Patent Document 1 discloses a method of adding a flux as a mixture of quicklime, alumina, and fluorite, and then performing a bubbling treatment to desulfurize the The slag composition was set to CaO / Al ₂ O ₃ ≧ 1.5 and CaF ₂ ≧ 5 mass% to desulfurize the molten steel. In addition, Patent Document 2 discloses a method of using CaO-Al ₂ O ₃ pre-flux flux (made by mixing and dissolving in advance) or CaO-Al ₂ O in order to promote slagging of the desulfurizing agent. _{The 3} -CaF ₂ pre-flux is used as a desulfurizing agent. On the other hand, as for a method for enhancing the stirring of molten steel, as a method of increasing the stirring intensity without increasing the flow rate of the stirring gas, Patent Documents 3, 4, and 5 disclose that a flux is mixed into the stirring gas and blown. Into the method. [Prior Art Literature] [Patent Literature]

Patent Literature 1: Japanese Patent Laid-Open No. 8-260025 Patent Literature 2: Japanese Patent Laid-Open No. 9-217110 Patent Literature 3: Japanese Patent Laid-Open No. 61-91318 Patent Literature 4: Japanese Patent Laid-Open No. 61 -281809 Patent Document 5: Japanese Patent Laid-Open No. 2000-234119

[Problems to be Solved by the Invention] However, according to the method described in Patent Document 1, when a desulfurizing agent containing CaF ₂ is used, there is a problem that a cask is formed by CaF ₂ in the generated slag. The refractory material is severely melted and the life of the ladle is greatly shortened. In addition, according to the method described in Patent Document 2, the pre-flux type flux is very expensive, and there is a problem that the processing cost increases. In addition, in the desulfurizing agent containing CaF ₂ , the above-mentioned problems also occur in the same manner.

On the other hand, in the methods described in Patent Literature 3, Patent Literature 4, and Patent Literature 5, the amount of flux to be injected is limited with respect to the flow rate of the injection gas (the solid-gas ratio is limited to 5 kg / kg to 30 kg / kg ), There is a limit to the stirring power that can be increased. In addition, when the flow rate of the stirring gas is increased, problems such as the sloshing (shaking) of the molten steel level in the ladle are intense, splashing occurs, and the raw metal adheres to the cover, or between the electrode and the molten steel. An arc is unstable due to a short circuit or the like, and arc heating becomes difficult.

The present invention has been made in view of the problems described above, and an object thereof is to provide a desulfurization treatment method and a desulfurization agent for a molten steel that can efficiently perform desulfurization treatment without using CaF ₂ or a pre-flux. [Means for solving problems]

The method for desulfurizing the molten steel of the present invention is a method of reducing the sulfur concentration in the molten steel by adding a desulfurizing agent containing quicklime to the ladle containing the molten steel and stirring the molten steel in the ladle. In addition, the method for desulfurizing a molten steel is characterized by using a volume of pores having a pore diameter within a range of 0.5 μm to 10 μm or less and a desulfurizing agent of quicklime of 0.1 mL / g or more. Mentioned desulfurizing agent.

The method for desulfurizing a molten steel according to the present invention is characterized in that, in the above invention, the quicklime contains 90% or more of particles having a particle diameter in a range of 1 mm to 30 mm.

The desulfurizing agent of the present invention is characterized by including a volume of pores having a pore diameter in a range of 0.5 μm to 10 μm or less, and quicklime of 0.1 mL / g or more. Particles in the range of 1 mm to 30 mm.

The method for desulfurizing a molten steel of the present invention is characterized in that, in the invention, the molten steel is stirred so as to satisfy a condition of a stirring power density represented by the following formula (1). Further, in this specification, the term "Nm ^3" means pressure 101325 Pa, the volume of gas at standard conditions of a temperature of 273.15 K.

[Number 1] ε: stirring power density of molten steel using gas (W / t), Q: gas flow rate (Nm ³ / min), W: amount of molten steel (t), T ₁ : temperature of molten steel (° C), T _g : Gas temperature (℃), h: bath depth (m), P: ambient pressure (Pa)

The method for desulfurizing a molten steel according to the present invention is characterized in that in the invention, the amount of aluminum put into the molten steel within 10 minutes after the molten steel spins from the start of the converter to the start of the desulfurizing treatment satisfies the following conditions: Formula (2).

[Number 2] [sol.Al] ₁ : Upper limit of Al concentration specification (mass%) of the steel to be melted [sol.Al] ₂ : Al concentration (mass%) in molten steel after converter tapping W _Al : Desulfurization in ladle Amount of Al input (kg / t) within 10 minutes of the start of processing

The method for desulfurizing a molten steel according to the present invention is characterized in that, in the invention, Ar gas is blown into the ladle so that the oxygen concentration in the ladle becomes 15% or less. [Effect of the invention]

According to the desulfurization treatment method and desulfurizing agent for molten steel of the present invention, the desulfurization treatment can be performed efficiently without using CaF ₂ or a pre-flux type flux.

In order to solve the above-mentioned problems, the inventors of the present invention have focused on the particle size and pore size of the lime, or the composition of the molten steel, and have repeatedly studied. More specifically, the inventors of the present invention have repeatedly performed various tests and studies for the purpose of using a CaO-containing substance as a main constituent of a desulfurizing agent and using a ladle refining method. Desulfurization process When melting low-sulfur steel with a sulfur concentration of 0.0030% by mass or less, even if CaF _{2 is} not used as a part of the desulfurizing agent, it can also be used as a desulfurizing agent even if the desulfurizing agent is not a pre-fluxing flux. The flux added with the flux quickly slags, and the desulfurization treatment is performed efficiently.

As a result, the inventors of the present invention discovered that in order to promote the slagging of the flux added as a desulfurizing agent, it is important that the temperature of the molten steel, the sol.Al concentration, the particle size of the lime, and the fineness of the lime when the flux is added Aperture. Among them, the temperature of the molten steel is determined by the temperature of the molten steel at the time of tapping from the converter, and excessively increasing the temperature of the molten steel at the time of tapping will increase the melting loss of the refractory of the converter and increase the processing cost instead of The best policy.

Therefore, the inventors of the present invention have found that by using the pore volume of pores having a pore diameter in the range of 0.5 μm to 10 μm of lime and the quicklime of 0.1 mL / g or more as the main components The present invention was conceived because a powdery desulfurizing agent can efficiently perform desulfurization treatment. The pore size distribution of quicklime was measured by the method shown below.

First, as a pretreatment, quicklime was subjected to constant temperature drying at 120 ° C for 4 hours. Then, using Autopore IV9520 manufactured by Micromerities, the pore diameter of the dried quicklime was determined by the mercury intrusion method to be within a range of about 0.0036 μm to 200 μm. Pore distribution and the cumulative pore volume curve was calculated. Furthermore, based on the calculated cumulative pore volume curve, the volume sum of pores having a pore diameter in the range of 0.5 μm to 10 μm was obtained.

The pore diameter was calculated using a Washburn formula shown in the following formula (3). In equation (3), P is the pressure, D is the diameter of the pores, σ is the surface tension of mercury (= 480 dynes / cm), and θ is the contact angle between mercury and the sample (= 140 Degrees (degrees)).

[Number 3]

The molten iron discharged from the blast furnace is received in a molten iron transfer container such as a molten iron pot or a torpedo car, and transferred to a converter for decarburization refining in the next step. Usually, the molten iron is subjected to a hot metal pretreatment such as a desulfurization treatment or a dephosphorization treatment during the transportation. The present invention is a technique for manufacturing a low-sulfur steel, so a desulfurization treatment is performed. In addition, even in the case where the desulfurization treatment is not required in terms of the component specifications of the low-sulfur steel, the dephosphorization treatment is performed in order to prevent the rephosphorization from the converter slag during the desulfurization treatment after the converter tapping.

Next, the molten iron subjected to desulfurization treatment and dephosphorization treatment was subjected to decarburization refining in a converter, and the obtained molten steel was tapped in a ladle. Regarding the decarburization refining in the converter, since molten iron has been subjected to desulfurization treatment and dephosphorization treatment, a small amount of quick lime (CaO) and a small amount of dolomite (MgCO ₃ -CaCO ₃ ) or calcined dolomite (MgO- CaO) is used as a flux to form slag in the furnace (hereinafter referred to as "converter slag"). The converter slag plays the role of promoting the dephosphorization reaction of the molten iron, but since the molten iron has been dephosphorized, the main role is to prevent the generation of iron splash during the blowing process and suppress the refractory of the converter lining refractory. Melting loss.

At the end of tapping, the converter slag is mixed into the molten steel and flows out into the ladle. Therefore, in order to prevent this, measures to prevent slag outflow are usually implemented. Even if countermeasures against slag outflow are implemented, it is difficult to completely prevent the outflow of converter slag. A certain amount of converter slag in the ladle is mixed into the molten steel and flows out. After tapping, the converter slag mixed into the molten steel can also be removed from the ladle, but the SiO ₂ component in the converter slag contributes to the slag formation of the CaO-containing substance added later as a desulfurizing agent. , So it can not be removed.

In order to form CaO-MgO-Al ₂ O ₃ -SiO ₂ based slag for desulfurization in a ladle, CaO-containing materials, MgO-containing materials, and Al ₂ O-containing materials are added to the ladle as fluxes. ₃ substances and substances containing SiO ₂ . Among them, as described above, since MgO has a lower desulfurization ability than CaO, it is not necessary to add a MgO-containing substance. In addition, in order to deoxidize molten steel and reduce slag (reduction of Fe oxide and Mn oxide in the slag), metal Al is added to the ladle.

These substances can also be added using equipment that performs the post-desulfurization process by any of the ASEA-SKF method, the VAD method, and the LF method. However, it is preferable from the viewpoint of promoting the slagging of CaO. It is added to the ladle when the steel is spun out of the rotary kiln or just after tapping. The quicklime added immediately after tapping is preferably a pore with a pore diameter within the range of 0.5 μm to 10 μm in the pores of the quicklime, the sum of the pore volume is 0.1 mL / g or more, and 90% or more of the pores are included. Particles with a diameter in the range of 1 mm to 30 mm.

Regarding the addition amounts of the CaO-containing substance, MgO-containing substance, metal Al, Al ₂ O ₃ -containing substance, and SiO ₂ -containing substance, considering the quality and composition of the converter slag flowing out of the ladle, the converter melt is included. The composition of the slag generated in the ladle after the slag containing the added flux becomes SiO ₂ content = 5 mass% to 15 mass% and [(mass% CaO) + (mass% MgO)] / The method of (mass% Al ₂ O ₃ ) = 1.5 to 3.0, preferably [(mass% CaO) + (mass% MgO)] / (mass% Al ₂ O ₃ ) = 1.8 to 2.5 In the method, the respective addition amounts of the CaO-containing substance, MgO-containing substance, metal Al, Al ₂ O ₃ -containing substance, and SiO ₂ -containing substance are specified.

In this case, it is more preferable to specify the respective addition amounts so that (mass% MgO) / (mass% CaO) of the generated slag becomes 0.10 or less. These substances are added to the ladle in a predetermined amount. Regarding the metal Al, not all of the added amount becomes Al ₂ O ₃ , and it dissolves in the molten steel and remains. Therefore, the ratio of the amount of dissolved Al dissolved in the molten steel to the amount of Al ₂ O ₃ in the molten slag is determined by experiments in advance, and the amount of metal Al added is set based on the ratio. No CaF _{2 was} added.

In addition, in the present invention, "the composition of the molten slag in the ladle after the desulfurization treatment is adjusted to a composition that does not substantially contain CaF ₂ " means that a fluoride such as CaF ₂ is not used as a slag accelerator for CaO. The composition of the slag after the desulfurization treatment is also defined, even if the fluorine that is unavoidably mixed into the CaO-containing substance or the Al ₂ O ₃ -containing substance used in the slag after the desulfurization treatment is also defined. It is a slag which is substantially free of CaF ₂ .

As the CaO-containing substance to be added, quicklime (CaO), limestone (CaCO ₃ ), slaked lime (Ca (OH) ₂ ), dolomite (MgCO ₃ -CaCO ₃ ), calcined dolomite (MgO-CaO), etc. are used as MgO-containing substances include magnesia (MgO), dolomite (MgCO ₃ -CaCO ₃ ), and calcined dolomite (MgO-CaO).

Regarding the particle size of the lime, from the viewpoint of reaction efficiency and addition yield, the average particle size is preferably in the range of 1 mm to 30 mm. From the viewpoint of reducing the amount of inhalation in the exhaust system, it is desirable that the amount of fine powder is small, and preferably, the amount of lime having an average particle diameter of 30 mm or more is small. The measurement method of the average particle diameter is as follows. 1 kg of desulfurizing agent, below 500 μm, 500 μm ～ 1 mm, 1 mm ～ 5 mm, 5 mm ～ 10 mm, 10 mm ～ 15 mm, 15 mm ～ 20 mm, 20 mm ～ 25 mm, The sieve was sieved in 9 stages of 25 mm to 30 mm and 30 mm or more, and the average particle diameter was calculated by a weight ratio, and was calculated by the following formula (4).

[Number 4] D _a : average particle diameter (mm) d _i : average particle diameter (mesh center value) in each particle size range (mm) w _i : weight of slag on each sieve (kg)

As the Al ₂ O ₃ -containing substance, aluminum scum (containing 20% to 70% by mass of metal Al, the main component of the remaining portion is Al ₂ O ₃ ), alumina (Al ₂ O ₃ · 2H ₂ O), Alumina (Al ₂ O ₃ ) and the like are fired. Aluminum scum has also become a substitute for the metal Al. As the SiO ₂ -containing substance, silica sand (SiO ₂ ), wollastonite (CaO-SiO ₂ ), and the like are used. In this case, when the mass of the converter slag flowing out into the ladle is large, it is not necessary to add a SiO ₂ -containing substance. In addition, regarding the MgO-containing substance, if the MgO-containing substance is not added, the slag composition can also be in the range of [(mass% CaO) + (mass% MgO)] / (mass% Al ₂ O ₃ ) = 1.5 to 3.0, It is preferably in the range of 1.8 to 2.5, but may not be added.

Then, the ladle containing the molten steel is transported to a facility that performs a desulfurization treatment by any one of the ASEA-SKF method, the VAD method, and the LF method, and the molten steel is desulfurized. In the present invention, a case where a desulfurization treatment is performed using an LF device is described as an example. FIG. 1 is a schematic side view of an LF device used in carrying out the present invention. In Figure 1, symbol 1 indicates LF equipment, symbol 2 indicates a pouring bucket, symbol 3 indicates a lifting lid, symbol 4 indicates an electrode for arc heating, symbols 5 and 6 indicate impregnated spears, and symbols 7 and 8 indicate bottom-blown porous bricks. Reference numeral 9 indicates molten steel, reference numeral 10 indicates slag, reference numeral 11 indicates raw material input to the chute, and reference numeral 12 indicates an Ar gas introduction pipe.

In the LF equipment 1, the ladle 2 containing the molten steel 9 loaded on a mobile trolley (not shown) is arranged at a predetermined position directly below the lid 3, and the lid 3 is lowered to closely contact the ladle 2. The upper end portion, in this state, supplies Ar gas from the Ar gas introduction pipe 12 and sets the space surrounded by the ladle 2 and the lid 3 to an Ar gas environment. The Ar gas is preferably blown from a pipe installed around the furnace lid so that the oxygen concentration in the ladle 2 becomes 15% or less. By reducing the oxygen concentration in the ladle 2, the amount of Al lost due to the reaction with oxygen in the air during the LF treatment can be reduced. 2 blown from the ladle Ar gas flow rate is preferably set to a value πL ² / 4Q flow becomes within a range of 50 ~ 150 (m / min) is, more preferably be 70 ~ 100 (m / min) of Traffic within range. Here, L is the diameter (m) of the ladle, and Q is the Ar gas flow rate (Nm ³ / min). The reason is that if the flow rate of the Ar gas is small, the oxygen concentration does not sufficiently decrease, whereas if the flow rate of the Ar gas is too large, the temperature of the molten steel decreases.

When CaO-containing material, MgO-containing material, metal Al, Al ₂ O ₃ -containing material, and SiO ₂ -containing material are not added to the ladle 2 in advance, and when the amount of these materials is insufficient, in this state The flux of these substances and metal Al are put into the ladle 2 through the raw material input chute 11. The metal Al is preferably added within 10 minutes from the start so as to satisfy the following formula (5). That is, in terms of promoting the desulfurization treatment, it is preferable to increase the Al concentration in the molten steel by adding metal Al in accordance with the Al concentration after the tapping of the converter.

[Number 5] [sol.Al] ₁ : Upper limit of Al concentration specification (mass%) of the steel to be melted [sol.Al] ₂ : Al concentration (mass%) in molten steel after converter tapping W _Al : Desulfurization in ladle Amount of Al input (kg / t) within 10 minutes of the start of processing

Then, if necessary, an electric current is applied to the electrode 4 to generate an arc, and the molten steel 9 is heated. At the same time, the added flux is slagged, and then the immersion lance 5 or the immersion lance 6 is immersed in the molten steel 9. Ar gas, which is a stirring gas, is blown into the molten steel 9 from at least one of the impregnated lance 5, the impregnated lance 6, or the bottom-blown porous brick 7, and the bottom-blown porous brick 8, and the molten steel 9 is stirred. By stirring the molten steel 9, the flux is mixed with the molten steel 9 to slag the flux, thereby generating slag 10.

The generated slag 10 is stirred and mixed with the molten steel 9 by the stirring of the molten steel 9, a slag-metal reaction occurs between the molten steel 9 and the molten slag 10, and a sulfur component in the molten steel 9 is generated. Desulfurization reaction transferred in slag. In this case, from the viewpoint of promoting the desulfurization reaction, as described above, any one or two or more of the Ca alloy powder, the metal Mg powder, and the Mg alloy powder are preferably self-impregnated with the Ar gas. 5, The impregnation spear 6 is blown into the molten steel 9 or the self-impregnation spear 5 and impregnation spear 6 are blown into the molten steel 9 at the same time as the desulfurization treatment. The gas for stirring of the porous brick 8 is blown in.

As the Ca alloy powder, Ca-Si alloy powder or Ca-Al alloy powder is used, and as the Mg alloy powder, Mg-Al-Zn alloy powder or Mg-Si-Fe alloy powder is used. The particle diameters of these metal powders are not particularly limited as long as they can be added by blowing. From the viewpoint of ensuring the reaction interfacial area, the maximum particle diameter is preferably 1 mm or less. When the sulfur concentration of the molten steel 9 becomes 0.0010 mass% or less, the blowing of Ar gas into the molten steel 9 is stopped, and the desulfurization process is terminated. When the temperature of the molten steel 9 is lower than the target temperature at the time when the desulfurization process is completed, arc heating is performed, and when the composition of the molten steel 9 is not within the target range, the composition is adjusted through the raw material input chute 11 Use alloy iron or metal. After the desulfurization treatment is completed, if necessary, degassing refining is performed by using a RH vacuum degassing device, etc., and then a continuous casting machine is used to cast into a slag slab.

As described above, according to the present invention, in the desulfurization treatment of molten steel 9 using a ladle refining method using a CaO-containing substance as a main constituent of the desulfurization agent, the slag after the desulfurization treatment is composed The content of SiO ₂ is adjusted in a range of 5% to 15% by mass. Therefore, SiO ₂ functions as a slagging accelerator for CaO to promote the slagging of CaO. In addition, the composition of the slag after the desulfurization treatment is changed to [(Mass% CaO) + (mass% MgO)] / (mass% Al ₂ O ₃ ) is adjusted so that it falls within a range of 1.5 to 3.0. Therefore, for the slag 10, a high desulfurization ability can be ensured, and as a result, The use of CaF ₂ as a part of the desulfurizing agent enables efficient desulfurization treatment of the molten steel 9 even if the desulfurizing agent is not a pre-melt flux. In addition, the description is an example of implementing the present invention using an LF device, and the present invention can also be applied as a reference in an ASEA-SKF device and a VAD device.

[Example 1] After the de-siliconization treatment, desulfurization treatment, and dephosphorization treatment were performed on the molten iron discharged from the blast furnace, the molten iron was charged into a converter to perform decarburization refining to obtain a carbon concentration of 0.05 mass. About 250 tons of molten steel having a sulfur concentration in a range of 0.0041 to 0.0043% by mass and a phosphorus concentration in a range of 0.004 to 0.010% by mass in a range of% to 0.09% by mass. After tapping, the converter slag flowing out to the ladle is not slowly slagged, and the ladle to which the metal Al, quicklime, light-burned dolomite and aluminum scum are added is transferred to the LF equipment shown in FIG. 1. The electrode was immersed in the slag, while arc heating was carried out, while Ar gas of 2000 NL / min was blown into the molten steel from the immersion spear, the molten steel was stirred, and the desulfurization treatment was performed for about 30 minutes so that the sulfur concentration was 0.0024. % Or less is the target for desulfurization.

Table 1 shown below shows the sulfur concentration (chemical analysis value) and the desulfurization rate in the molten steel before and after the desulfurization treatment in each desulfurization test. In addition, in the remarks column of Table 1, the test in the range of this invention is shown as "the example of this invention", and the other is shown as a "comparative example." The desulfurization rate is a value representing the difference between the sulfur concentration in the molten steel before and after the desulfurization treatment with respect to the sulfur concentration in the molten steel before the desulfurization treatment. In addition, the "sulfurization evaluation" "○" indicates that the sulfur concentration in the molten steel after desulfurization treatment is 0.0024% or less, and the "x" indicates that the sulfur concentration in the molten steel after desulfurization treatment exceeds 0.0024%.

[Table 1] (Table 1)

The test levels and results are shown in Table 1. The desulfurization rate of the pore volume in the pore diameter range of 0.5 μm to 10 μm and the inappropriate comparative examples (test numbers 1 to 3) compared with the examples of the present invention (test numbers 4 to 15) low. In addition, in the example of the present invention, when the average particle diameter of the quicklime is within a range of 1 mm to 30 mm, slagging is promoted and the desulfurization rate of the molten steel is also high.

[Example 2] After the desilication treatment, desulfurization treatment, and dephosphorization treatment were performed on the molten iron discharged from the blast furnace, the molten iron was charged into a converter to perform decarburization refining to obtain a carbon concentration of 0.05 mass. About 250 t of molten steel having a sulfur concentration in a range of 0.0041 to 0.0043% by mass and a phosphorus concentration in a range of 0.004 to 0.010% by mass in a range of% to 0.09% by mass. After tapping, the converter slag flowing out to the ladle is not slowly slagged, and the ladle to which the metal Al, quicklime, light-burned dolomite and aluminum scum are added is transferred to the LF equipment shown in FIG. 1. The electrode was immersed in the slag, and while arc heating was performed, an Ar gas of 500 NL / min to 2000 NL / min was blown into the molten steel from the immersion spear, and the molten steel was stirred for about 30 minutes for desulfurization treatment. Desulfurization treatment is carried out with a sulfur concentration of 0.0024% or less as a target.

Table 2 shown below shows the sulfur concentration (chemical analysis value) and the desulfurization rate in the molten steel before and after the desulfurization treatment in each desulfurization test. It should be noted that the desulfurization evaluation "○" indicates that the sulfur concentration in the molten steel after the desulfurization treatment was 0.0024% or less.

[Table 2] (Table 2)

The test levels and results are shown in Table 2. It was confirmed that the slagging rate and the desulfurization rate increased 5 minutes after the start of the LF treatment with the increase in the stirring power. In addition, it was confirmed that a high slagging rate and a desulfurization rate can be obtained by satisfying the formula (6) shown below.

[Number 6] ε: stirring power density of molten steel using gas (W / t), Q: gas flow rate (Nm ³ / min), W: amount of molten steel (t), T ₁ : temperature of molten steel (° C), T _g : Gas temperature (℃), h: bath depth (m), P: ambient pressure (Pa)

[Example 3] Fig. 2 is a graph showing the slagging rate of the examples of the present invention and the comparative examples. The volume of the pores with a pore diameter in the range of 0.5 μm to 10 μm and the quicklime with a particle diameter of 0.2 mL / g and a particle diameter of 20 mm or less are taken as examples of the present invention. The sum of the volume of pores in the range was 0.03 mL / g, and quicklime having a particle diameter of 20 mm or less was used as a comparative example. As shown in FIG. 2, it was confirmed that in the examples of the present invention, slagging can be promoted even when the stirring power density (135 W / t) is the same as that of the comparative example.

[Example 4] After the de-siliconization treatment, desulfurization treatment, and dephosphorization treatment were performed on the molten iron discharged from the blast furnace, the molten iron was charged into a converter to perform decarburization refining to obtain a carbon concentration of 0.05 mass. Approximately 250 t of molten steel having a sulfur concentration in a range of 0.0041 to 0.0044 mass% and a phosphorus concentration in a range of 0.004 to 0.010 mass% in a range of% to 0.09 mass%. After tapping, the converter slag flowing out to the ladle is not slowly slagged, and the ladle to which the metal Al, quicklime, light-burned dolomite and aluminum scum are added is transferred to the LF equipment shown in FIG. 1. In the LF treatment, quicklime having a volume of pores having a pore diameter in a range of 0.5 μm to 10 μm and a particle diameter of 0.2 mL / g and a particle diameter of 20 mm or less was used.

Table 3 shown below shows the sulfur concentration (chemical analysis value) and the desulfurization rate in the molten steel before and after the desulfurization treatment in each desulfurization test. Here, [sol.Al] ₁ is the upper limit value (mass%) of the Al concentration specification of the steel to be melted, and [sol.Al] ₂ is the Al concentration (mass%) in the molten steel after the converter is tapped. It should be noted that the desulfurization evaluation "○" indicates that the sulfur concentration in the molten steel after the desulfurization treatment was 0.0024% or less.

[Table 3] (Table 3)

As shown in Table 3, when the amount of Al input within 10 minutes of the start of the LF process is at a level within the range of the formula (5), the [sol.Al] ₃ value at the end of the LF process is the specification range. Within, the desulfurization rate is also high. On the other hand, when the amount of Al input within 10 minutes from the start of the LF process is higher than the range shown in the above formula (5), the [sol.Al] ₃ value at the end of the LF process is higher than the specification The upper limit value causes the need for de-Al treatment using RH in the next step, and the processing time of RH is prolonged.

[Example 5] After the desilication treatment, desulfurization treatment, and dephosphorization treatment were performed on the molten iron discharged from the blast furnace, the molten iron was charged into a converter to perform decarburization refining to obtain a carbon concentration of 0.05% by mass to In the range of 0.09 mass%, the sulfur concentration is within the range of 0.0041 mass% to 0.0044 mass%, and the phosphorus concentration is in the range of 0.004 mass% to 0.010 mass%. After tapping, the converter slag flowing out to the ladle is not slowly slagged, and the ladle to which the metal Al, quicklime, light-burned dolomite and aluminum scum are added is transferred to the LF equipment shown in FIG. 1. In the LF treatment, a volume of pores having a pore diameter in the range of 0.5 μm to 10 μm and quicklime having a particle size of 0.2 mL / g and a particle diameter of 20 mm or less were used within 10 minutes from the start of the LF treatment to satisfy all requirements. The metal Al is added in the manner of formula (5).

Table 4 shown below shows the sulfur concentration (chemical analysis value) and the desulfurization rate in the molten steel before and after the desulfurization treatment in each desulfurization test. It should be noted that the desulfurization evaluation "○" indicates that the sulfur concentration in the molten steel after the desulfurization treatment was 0.0024% or less.

[Table 4] (Table 4)

As shown in Table 4, it was confirmed that when the oxygen concentration in the ladle was 15% or less (Test No. 37 to Test No. 39), the Al loss was reduced during processing. It should be noted that the Al loss (involved air) during the process is calculated using the following formula (7).

[Number 7] [sol.Al] ₂ : Al concentration (mass%) in molten steel after converter tapping, [sol.Al] ₃ : Al concentration (mass%) in molten steel after ladle desulfurization treatment, W _{Al- all} : Al amount (kg / t) input during desulfurization of ladle, X _Al-loss- De _{- S} : Al amount lost due to desulfurization reaction (3CaO + 3S + 2Al → 3CaS + Al ₂ O ₃ ) (Kg / t), X _Al-loss-slag : the amount of Al (kg / t) lost in the slag due to reduction (6MnO + 4Al → 2Al ₂ O ₃ + 6Mn, etc.), X _Al-loss : Amount of Al lost due to air entrainment (kg / t) [Industrial availability]

According to the present invention, it is possible to provide a desulfurization treatment method and a desulfurization agent for a molten steel that can efficiently perform desulfurization treatment without using CaF ₂ or a pre-flux.

1‧‧‧LF equipment

2‧‧‧ pouring bucket

3‧‧‧ cover

4‧‧‧ electrode

5, 6, ‧ ‧ impregnated spear

7, 8‧‧‧ bottom blow porous brick

9‧‧‧ molten steel

10‧‧‧ slag

11‧‧‧ Raw materials are put into the chute

12‧‧‧Ar gas introduction tube

FIG. 1 is a schematic side view of an LF device used in carrying out the present invention. FIG. 2 is a graph showing the slagging rates of Examples of the present invention and Comparative Examples.

Claims (6)

A method for desulfurizing a molten steel, which reduces the sulfur concentration in the molten steel by adding a desulfurizing agent containing quicklime to the molten steel containing the molten steel and stirring the molten steel in the molten steel bucket. The method for desulfurizing a molten steel is characterized by using a volume of pores having a pore diameter in a range of 0.5 μm to 10 μm or less and a desulfurizing agent of quick lime of 0.1 mL / g or more. The desulfurizing agent. The method for desulfurizing a molten steel according to item 1 of the scope of the patent application, wherein the quicklime contains more than 90% of particles having a particle diameter in a range of 1 mm to 30 mm. A desulfurizing agent characterized by comprising a volume of pores having a pore diameter in a range of 0.5 μm to 10 μm or less, and quicklime of 0.1 mL / g or more, the quicklime containing more than 90% of the particle diameters in the range of 1 Particles in the range of mm to 30 mm. The method for desulfurizing a molten steel according to item 1 or 2 of the scope of application for a patent, wherein the molten steel is stirred in such a manner as to satisfy a condition of a stirring power density shown by the following formula (1); ε: stirring power density of molten steel using gas (W / t), Q: gas flow rate (Nm ³ / min), W: amount of molten steel (t), T ₁ : temperature of molten steel (° C), T _g : Gas temperature (° C), h: bath depth (m), P: ambient pressure (Pa). The method for desulfurizing a molten steel as described in any one of the items 1, 2, and 4 in the scope of the patent application, wherein the molten steel is within 10 minutes from the start of the steelmaking of the converter to the beginning of the desulfurizing treatment. The amount of aluminum put into the molten steel satisfies the following formula (2); [sol.Al] ₁ : Upper limit of Al concentration specification (mass%) of the steel to be melted [sol.Al] ₂ : Al concentration (mass%) in molten steel after converter tapping W _Al : Desulfurization in ladle The amount of Al (kg / t) charged within 10 minutes of the start of the treatment. The method for desulfurizing a molten steel according to any one of claims 1, 2, 4, and 5, wherein the oxygen concentration in the ladle becomes 15% or less Ar gas was blown into the ladle.