JP2017210671A - Desulfurization method of molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for effectively using a desulfurization agent containing practically no F in a method for desulfurization by spraying a desulfurization agent to a molten steel surface 11 in a vacuum container from a desulfurization agent spraying nozzle 5 in a vacuum degasser.SOLUTION: As a vacuum degasser, a device 6 for blowing a gas for stirring from a molten steel ladle bottom is used and pressure in a vacuum tank is set at 1300 Pa to 67000 Pa. By setting the pressure in the vacuum tank at 1300 Pa or more, impact of particles of the desulfurization agents is hardly generated. By using the device 6 for blowing a gas for stirring from the molten steel ladle bottom, reaction efficiency can be secured even when using CaO-based desulfurization agent containing practically no F is used.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for desulfurizing molten steel using a vacuum degassing apparatus.

  When melting low-sulfur steel with a sulfur content of several tens of ppm (hereinafter referred to as “S”), it is sufficient to decarburize it in a converter after desulfurizing with hot metal pretreatment. Since the sulfur level is not reached, a molten steel desulfurization process is required.

  As a molten steel desulfurization process, a method of removing S by slag refining while heating the molten steel (so-called LF method) has been put into practical use, but the LF process as a secondary refining process is indispensable. Has led to an increase.

  Therefore, establishment of a desulfurization process in a vacuum degassing apparatus that is widely used as a secondary refining process is required. As a desulfurization process in a vacuum degassing apparatus, a method of spraying a powdery desulfurization agent from above on a molten steel surface in a vacuum tank is generally used. As the desulfurizing agent, CaO-based flux is generally used. The desulfurization agent sprayed on the surface of the molten steel penetrates into the molten steel, and a desulfurization reaction occurs with the molten steel. Since the specific gravity of CaO-based flux is smaller than that of molten steel and eventually floats and separates on the surface of the molten steel due to buoyancy, it is important how the desulfurization reaction proceeds until it floats and separates.

Conventionally, when a CaO-based flux is used, CaF ₂ (fluorite) has been used as a solvent medium. When CaF ₂ is added to the CaO-based flux, a liquid phase having a high desulfurization ability can be formed. By forming a liquid phase in the desulfurizing agent, the movement of S inside the desulfurizing agent is facilitated, and the desulfurization reaction is rapidly advanced. Therefore, even if the time until the desulfurization agent floats and separates on the surface of the molten steel is short, the desulfurization reaction can proceed sufficiently.

  When desulfurization treatment is performed with a flux containing fluorine, fluorine remains in the slag after treatment. Therefore, in consideration of the influence of fluorine in the slag on the environment, a molten steel desulfurization method that does not use a fluorine source is desired recently.

  When performing molten steel desulfurization using a CaO-based flux that does not substantially contain F, the desulfurizing agent is mainly a solid phase at the molten steel processing temperature, and S moves inside the desulfurizing agent by diffusion in the solid phase. The movement of S due to diffusion in the solid phase is extremely slow compared to the movement in the liquid phase, and it takes time for the desulfurization reaction with the molten steel.

  Therefore, in the molten steel desulfurization using CaO-based flux that does not substantially contain F, the desulfurizing agent floats and separates on the surface of the molten steel before sufficiently absorbing S, and thus there is a problem that the utilization efficiency of the desulfurizing agent decreases. .

  Patent Document 1 defines the relationship between the Al concentration and the powder supply rate for preventing aggregation of the desulfurization agent when a CaO-based desulfurization agent substantially free of F is used in the vacuum degassing apparatus. By preventing agglomeration of the desulfurizing agent in the molten steel, a reaction interface area between the desulfurizing agent and the molten steel can be secured, and the desulfurization reaction can be advanced. However, since the Al concentration changes with time during the desulfurization treatment, it is difficult to control the concentration within an appropriate range, and as a result, the desulfurization rate may decrease.

  Patent Document 2 states that the penetration rate of the desulfurizing agent into the molten steel is improved by lowering the lance height and making it hard blow. At this time, the desulfurizing agent contains 1% by weight or more of fluoride.

JP 2007-270178 A Japanese Patent Laid-Open No. 6-322431

  The inventors tried a desulfurization treatment by the method of Patent Document 2 in a CaO-based desulfurization agent substantially free of F. However, in the method of Patent Document 2, although the powder penetrates into the molten steel, the powders entering the molten steel from the molten steel easily collide with each other, forming a laminate mainly composed of the collided powder, and desulfurizing. It was found that a desulfurizing agent (powder) that does not contribute to the generation of the desulfurizing agent is generated, and the desulfurization rate decreases.

  The present invention relates to a method for desulfurizing molten steel that efficiently uses a desulfurization agent that does not substantially contain F in a desulfurization method in which a desulfurization agent is sprayed onto a surface of molten steel in a vacuum vessel from a desulfurization agent spray nozzle in a vacuum degassing apparatus The purpose is to provide.

That is, the gist of the present invention is as follows.
(1) When performing desulfurization treatment of molten steel in a vacuum degassing apparatus, a desulfurizing agent is sprayed onto the surface of the molten steel in a vacuum vessel from a desulfurizing agent spray nozzle, and the CaO-based desulfurizing agent substantially not containing F as the desulfurizing agent. In this method, as a vacuum degassing device, a device for blowing a stirring gas from the bottom of the molten steel pan is used, and the pressure in the vacuum chamber is 1300 Pa or more and 67000 Pa or less, a method for desulfurizing molten steel .
(2) The method for desulfurizing molten steel according to (1) above, wherein the total of the FeO component and the MnO component of the slag existing on the molten steel surface in the molten steel pan before desulfurization is 10% by mass or less.

  According to the present invention, a CaO-based desulfurization agent that substantially does not contain F can be supplied uniformly in the molten steel as compared with the prior art, and a reduction in reaction efficiency due to the laminated coalescence can be prevented.

  By utilizing the circulating flow in the molten steel, the reaction time in the molten steel is ensured, and even a F-less flux having a high solid phase ratio can be sufficiently reacted.

It is a figure which shows the desulfurization method of the molten steel performed using a vacuum degassing apparatus, (A) is the example which used the large diameter dip tube system, (B) is the example which used the RH system. It is a figure which shows the gas jet sprayed on the molten steel surface in a vacuum chamber, (A) is a case where the pressure in a vacuum chamber is high, (B) is a case where the pressure in a vacuum chamber is low.

  In the molten steel desulfurization method of the present invention, the molten steel is desulfurized using a vacuum degassing apparatus. There are various types of vacuum degassing apparatuses for molten steel. When degassing the molten steel, a method in which the entire molten steel pan is accommodated in a vacuum vessel and the vacuum vessel is evacuated (for example, VOD (Vacuum Oxygen Decarburization) method, VAD (Vacuum Arc Degassing) method, etc.), or vacuum vessel There is a method (for example, a large-diameter dip tube system, RH vacuum degassing apparatus, etc.) in which the lower opening is immersed in the molten steel in the molten steel pan to evacuate the vacuum vessel. As shown in FIG. 1 (A), the large-diameter dip tube type vacuum degassing device 21 is provided with one large-diameter dip tube 3 at the bottom of the vacuum vessel 2, and this large-diameter dip tube 3 is used as a molten steel pan 1. The molten steel 10 is immersed in the molten steel 10, the vacuum vessel 2 is depressurized to suck the molten steel into the vacuum vessel, and the gas is blown from the apparatus 6 for blowing the stirring gas at the bottom of the molten steel pan. In this method, the molten steel is stirred and mixed by forming a stirring flow between the molten steel and the molten steel in the vacuum vessel.

  Here, the vacuum degassing apparatus is classified according to the difference in the blowing position of the inert gas for stirring and mixing the molten steel. For the vacuum degassing device, there is a method of supplying stirring gas from the bottom of the molten steel pan (VOD method, large dip tube method, etc.) and an RH vacuum that supplies stirring and mixing gas from the middle of the dip tube immersed from above the molten steel. There is a degasser.

In the case of the RH type vacuum degassing apparatus 22, as shown in FIG. 1B, two dip pipes are provided at the bottom of the vacuum vessel 2, one of which is the riser pipe 7 and the other is the downfall pipe 8. Gas is blown into the molten steel from the gas blowing port 9 at the intermediate side wall of the rising pipe 7, the molten steel rises in the rising pipe 7 due to the buoyancy of the gas, and the molten steel descends from the downcomer pipe 8 through the vacuum vessel 2. And discharged into the molten steel pan 1. The flow of molten steel from the downcomer 8 toward the molten steel pan 1 is strong, and the flow collides with the bottom of the molten steel pan 1, and then the flow is dispersed and weakened. The powder that has entered the molten steel in the vacuum vessel rides on the molten steel flow from the downcomer 8 and is dispersed at the bottom of the molten steel pan 1, and then slowly rises mainly along the side wall of the molten steel pan 1. In addition to reaching the vicinity of the molten steel surface 12 in the molten steel pan, the molten steel flow reaches a weak portion. The force acting on the powder in the weak part of the molten steel flow is more influenced by the buoyancy than the viscous force received from the molten steel, and is likely to float and separate. The powder that floats and separates in the vicinity of the molten steel surface 12 of the molten steel pan is taken into the slag 13 existing on the surface of the molten steel pan, and thereafter the contribution to desulfurization is greatly reduced. A CaO-based flux containing CaF ₂ is not a problem because the reaction rate is high and desulfurization proceeds sufficiently even in a short time. On the other hand, when a CaO-based desulfurization agent that does not substantially contain F is used, since the movement of S in the desulfurization agent is due to diffusion in the solid phase, it takes time for the desulfurization reaction. The reaction is difficult to proceed sufficiently in time. Therefore, in the RH system, when a CaO-based desulfurizing agent that does not substantially contain F is used, the reduction in reaction efficiency is significant.

  In this invention, the apparatus which blows in the gas for stirring from a molten steel pan bottom part is used as a vacuum degassing apparatus. Large diameter dip tube method, VOD method, VAD method, etc. are vacuum degassing devices that fall into this category. It has been found that the above-mentioned problem that becomes a problem in the RH method can be solved by using a device 6 that blows a stirring gas from the bottom of the molten steel pan like the VOD method or the large dip tube method. The apparatus 6 for blowing the stirring gas at the bottom of the molten steel pan is arranged so that the bubbles blown into the molten steel from the apparatus 6 for blowing the stirring gas rise and reach the molten steel surface 11 in the vacuum vessel. (See FIG. 1A).

  When the apparatus 6 for blowing the stirring gas from the bottom of the molten steel pan as described above is used, the molten steel rises along the bubble rising flow from the apparatus 6 for blowing the stirring gas toward the molten steel surface 11 in the vacuum vessel. A stream 15 is formed. The ascending flow 15 of the molten steel that has reached the molten steel surface 11 in the vacuum vessel flows radially from the reaching point of the ascending flow on the molten steel surface 11 in the vacuum vessel, reaches the side wall of the vacuum vessel 2, and turns downward from that position. The downward flow 16 forms a downward flow that continues in the molten steel pan to reach the bottom of the molten steel pan. That is, in these apparatuses, a large circulating flow is generated in the molten steel. The powder sprayed from the desulfurizing agent spray nozzle 5 onto the molten steel surface 11 in the vacuum vessel and invaded into the molten steel descends in the molten steel pan along with the descending flow 16, and rises due to gas bubbles at the bottom of the molten steel pan. 15 is circulated to the molten steel surface 11 in the vacuum vessel. The molten steel is vigorously mixed on the molten steel surface 11 in the vacuum vessel due to the expansion of bubbles, and the floated powder is taken into the molten steel without staying on the molten steel surface, and descends in the molten steel pan together with the descending flow 16 of the molten steel. Thus, the reaction time can be secured by the powder remaining in the circulating flow of molten steel. Therefore, even when a CaO-based desulfurization agent that does not substantially contain F is used, although the movement of S in the desulfurization agent is diffusion in the solid phase and it takes time for the desulfurization reaction, sufficient time necessary for the desulfurization reaction Because it stays in the molten steel, the reaction efficiency can be secured.

  The fact that F is not substantially contained means that the elution of fluorine (F) from the slag after desulfurization refining is not recognizable, and according to the knowledge of the present inventors, the composition of slag after refining. The case where F is 1% by mass or less. More preferably, F is 0.5% by mass or less.

In the present invention, the CaO-based desulfurizing agent refers to a desulfurizing agent containing 50% by mass or more of CaO. The CaO source may be CaCO ₃ or Ca (OH) ₂ in addition to CaO, and the CaO content can be calculated from the mass when completely pyrolyzed into CaO. Components other than CaO in the desulfurization agent are not particularly limited. It is more preferable to include a component having desulfurization ability by itself such as Mg.

  In desulfurization of molten steel using a vacuum degassing apparatus, a desulfurizing agent blowing lance 4 is inserted into the vacuum vessel 2 facing downward. A desulfurizing agent blowing nozzle 5 is disposed at the tip of the desulfurizing agent blowing lance 4. A desulfurizing agent is sprayed from the desulfurizing agent spray nozzle 5 onto the molten steel surface 11 in the vacuum vessel. The present invention is characterized in that the pressure in the vacuum chamber is 1300 Pa or more and 67000 Pa or less when spraying the desulfurizing agent. Details will be described below.

  The desulfurizing agent powder is conveyed in the desulfurizing agent blowing lance 4 by the carrier gas, and is ejected as a gas jet 14 from the desulfurizing agent blowing nozzle 5 at the tip of the desulfurizing agent blowing lance 4 toward the molten steel surface 11 in the vacuum vessel. At this time, the gas rapidly expands due to the pressure difference between the inside and outside of the nozzle, and forms a jet core in the vicinity of the lance. The gas jet 14 blown vertically downward from the nozzle advances toward the molten steel surface and spreads horizontally in the wake behind the jet core. At this time, the powder is also dispersed in the horizontal direction.

  In FIG. 2, the gas jet 14 sprayed on the molten steel surface 11 in a vacuum chamber is shown. 2A shows a case where the pressure inside the vacuum chamber is as high as 1300 Pa or higher, and FIG. 2B shows a case where the pressure inside the vacuum chamber is low as less than 1300 Pa. When the pressure in the vacuum chamber is low, the straightness of the gas jet 14 including the powder is high, so that the spread of the powder is small as shown in FIG. Will be supplied. The powder supplied locally increases the probability that particles collide with each other. When a CaO-based desulfurization agent that does not substantially contain F is used, when particles collide with each other, they are laminated on the molten steel surface and united. When the laminated coalescence occurs, the specific surface area (ratio of the surface area to the volume) of the powder becomes small. As a result, the reaction rate is lower than when the particles are present alone.

  On the other hand, it discovered that the said subject could be solved by making the pressure in a vacuum chamber high. That is, if the pressure in the vacuum chamber is high, as shown in FIG. 2 (A), the straightness of the gas jet 14 containing powder decreases, and the powder reaches the molten steel surface in a dispersed state. It becomes difficult to cause a collision. Therefore, even when a CaO-based desulfurization agent that does not substantially contain F is used, the reaction efficiency can be ensured.

  In order to acquire the said effect, it is good to set the pressure in a vacuum chamber to 1300 Pa (9.8 Torr) or more according to the knowledge of the present inventors. If it is less than 1300 Pa, as described above, the straightness of the gas jet 14 is high, so that the spread of the powder is small and the desulfurization reaction efficiency is lowered.

  On the other hand, when the pressure in the vacuum chamber is excessively increased, a phenomenon occurs in which the desulfurization reaction efficiency decreases. This is because the stirring force by the bottom blowing stirring gas is reduced. This tendency begins to appear when the pressure in the vacuum chamber is 13332 Pa (100 Torr) or more. According to the knowledge of the present inventors, the effect of the present invention can be obtained if the pressure in the vacuum chamber is 67000 Pa (503 Torr) or less.

  As described above, when performing the desulfurization treatment of the molten steel in the vacuum degassing apparatus, the desulfurizing agent is sprayed from the desulfurizing agent spray nozzle 5 to the molten steel surface 11 in the vacuum vessel, and CaO substantially free of F as the desulfurizing agent. In the method using the system desulfurization agent, when the pressure in the vacuum chamber is 1300 Pa (9.8 Torr) or more, collision between particles of the desulfurization agent is less likely to occur. Moreover, even when the CaO type | system | group desulfurization agent which does not contain F substantially is used by using the apparatus 6 which blows in the gas for stirring from a molten steel pan bottom as a vacuum degassing apparatus, reaction efficiency is securable.

  In this invention, it is preferable in the sum total of the FeO component and MnO component of the slag 13 which exists in the molten steel surface 12 in the molten steel pan before desulfurization being 10 mass% or less. This will be described in detail below.

  As described above, in the present invention that uses the apparatus 6 that blows the stirring gas from the bottom of the molten steel pan, the desulfurizing agent particles in the circulating molten steel flow circulate in the molten steel and continue to contribute to desulfurization. It deviates from the circulating molten steel flow with the probability of, rises to the molten steel pan bath surface, and is taken into the slag 13 existing on the molten steel pan bath surface. The slag 13 on the surface of the molten steel pan is mainly composed of oxidized refining slag that has flowed out of the converter when steel is discharged from the converter, and has a high content of FeO component and MnO component and is a so-called highly oxidizable slag. . Therefore, the desulfurization product that constitutes the desulfurization agent particles that have floated to the molten steel pan bath surface may be oxidized by the slag on the bath surface and resulfurized into the molten steel.

  Therefore, in the present invention, the slag present on the surface of the molten steel before desulfurization preferably has a total of FeO component and MnO component of 10% by mass or less (preferably when the oxidation degree of slag is low). As a method of reducing the concentration of the FeO component and MnO component in the slag, for example, a method of reducing by adding a strong deoxidizing element such as Al, a method of adding a CaO source to the slag and diluting the slag, etc. Can do. In actual operation, the amount of FeO component (mass%) is T.I. Based on the analytical value of the amount of Fe (mass%), T.W. The FeO content may be calculated assuming that all of Fe is FeO.

  A verification test performed to verify the effect of the present invention will be described.

  For the verification test, molten steel of 300 to 350 ton / charge, low carbon aluminum killed steel (S concentration before desulfurization was 30 ppm to 40 ppm) was used. The inner diameter of the molten steel pan is 5 m. The vacuum degassing apparatus was a large-diameter dip tube method (see FIG. 1A) (inner diameter of the large-diameter dip tube 3: 2 m) and an RH method (see FIG. 1B). In the present invention, it is assumed that the inner diameter of the molten steel pan is 3 to 6 m and the inner diameter of the large-diameter dip tube is 1.5 to 2.5 m.

Here, the vertical distance from the surface of the molten steel in the vacuum chamber to the tip of the nozzle is referred to as the lance height. In the case of the large-diameter dip tube method and the RH method, the height of the molten steel surface 11 in the vacuum chamber changes according to the pressure in the vacuum chamber. The difference ΔP (mm iron column) between the pressure in the vacuum chamber and the atmospheric pressure is the height J (mm) of the molten steel surface 11 in the vacuum chamber where the position of the molten steel surface 12 in the molten steel pan outside the vacuum chamber is zero. Equivalent. When ΔP is displayed in the pressure unit Pa that is commonly used, ΔP = ρgJ / 1000. However, (rho) is the density (kg / m < ³ >) of molten steel, g is a gravitational acceleration (m / s < ² >). The turbulence on the surface of the molten steel due to gas stirring was ignored, and it was calculated assuming that it was a stationary surface. Moreover, the height J can be calculated | required more correctly if the slag 13 which exists in the molten steel surface in a molten steel pan is considered.

  In this verification test, desulfurization treatment was performed with a lance height of 1 to 2 m. In the present invention, operation at a lance height of 1 m or more is assumed. The larger the lance height, the better because the powder is easy to spread and advantageous for desulfurization. This effect is remarkable at 2 m or more, and becomes more remarkable at 2.6 m or more. The lance height is usually about 6 m at most.

  In the level using the large-diameter dip tube system, the stirring gas was supplied from the stirring gas blowing plug (device 6 for blowing the stirring gas) provided at the bottom of the pan. In the level using the RH method, the stirring gas was supplied from the gas inlet 9 of one dip tube (rising pipe 7) out of the two dip tubes provided in the vacuum chamber. At any level, Ar gas was used as the stirring gas.

  The desulfurizing agent is jetted together with the carrier gas from the desulfurizing agent blowing nozzle 5 at the tip of the desulfurizing agent blowing lance 4 toward the molten steel surface 11 in the vacuum vessel. Ar gas was used as a carrier gas for spraying the CaO-based desulfurizing agent, and an amount of 5 kg / ton-steel was sprayed on the molten steel at 175 kg / min. As the desulfurizing agent spray nozzle 5, a Laval nozzle having a nozzle lower end opening diameter (diameter) of 120 mm was used. In this invention, the thing of 75-150 mm is assumed.

As a component of the CaO-based desulfurizing agent, 80% by mass CaO + 20% by mass Al ₂ O ₃ was used for Examples 1 to 5 and Comparative Examples 1 to 3. In the level using the prior art, 80 mass% CaO + 20 mass% CaF ₂ was used. In the present invention, the supply amount of CaO-based desulfurization agent per unit time is assumed to be 50 to 300 (kg / min).

The test results were evaluated as follows.
The S concentration of 30 ppm to 40 ppm before desulfurization was changed to an ultimate S concentration of 5 ppm to 13 ppm by desulfurization treatment, and the time from the start of the desulfurization treatment to the end of the desulfurization treatment was measured. Next, assuming that the change in S concentration is a linear function of time, it was converted to the time required to decrease from 30 ppm to 15 ppm using this function, and was defined as “desulfurization treatment time”. Then, based on the desulfurization treatment time in Comparative Example 1, an improvement of more than 10% was evaluated as ◎, an improvement of 5% or more and 10% or less was evaluated as ◯, a deterioration or an improvement of less than 5% was evaluated as ×. .

The results of the verification test are shown in Table 1.
In comparison with the case of using a CaO-based desulfurization agent that does not contain F, the desulfurization treatment time is shorter in the large-diameter dip tube method (Example 1) than in the RH method (Comparative Example 3). The large-diameter dip tube system uses a device 6 that blows a gas for stirring from the bottom of the molten steel pan, so the desulfurization agent that has been blown in the molten steel stays in the molten steel for a long time, and therefore contains F. It is presumed that sufficient desulfurization was performed even though the desulfurization agent had a long desulfurization time.

Further, when compared with the same large-diameter dip tube system, the pressure in the vacuum chamber is 1300 Pa (9.8 Torr) or more (Example 1), and the desulfurization treatment time is shorter than 400 Pa (Comparative Example 1). Further, the pressure in the vacuum chamber is 67000 Pa (503 Torr) or less (Example 4), and the desulfurization time is shortened compared to 90000 Pa (Comparative Example 2). When the pressure in the vacuum chamber is 13000 Pa (98 Torr) or less (Example 5), the effect of shortening the desulfurization treatment time is further obtained than 67000 Pa (Example 4).
The higher the lance height, the shorter the desulfurization processing time (Example 1, Example 3).

  As the ratio of FeO component + MnO component of the slag decreases, the desulfurization treatment time becomes shorter (Example 1, Example 2).

DESCRIPTION OF SYMBOLS 1 Molten steel pan 2 Vacuum vessel 3 Large diameter immersion pipe 4 Desulfurizing agent blowing lance 5 Desulfurizing agent blowing nozzle 6 Agitating gas blowing device 7 Rising pipe 8 Lowering pipe 9 Gas blowing port 10 Molten steel 11 Molten steel surface 12 in the vacuum vessel Molten steel surface in molten steel pan 13 Slag 14 Gas jet 15 Upflow 16 Downflow 21 Large diameter dip tube type vacuum degassing device 22 RH type vacuum degassing device

Claims (2)

When performing desulfurization treatment of molten steel in a vacuum degassing apparatus, a desulfurizing agent is sprayed onto the surface of the molten steel in a vacuum vessel from a desulfurizing agent spray nozzle, and a CaO-based desulfurizing agent substantially free of F is used as the desulfurizing agent. In the method
As a vacuum degassing device, while using a device that blows the stirring gas from the bottom of the molten steel pan,
The method for desulfurizing molten steel, wherein the pressure in the vacuum chamber is 1300 Pa or more and 67000 Pa or less.   The desulfurization method for molten steel according to claim 1, wherein the total of the FeO component and the MnO component of the slag existing on the molten steel surface in the molten steel pan before desulfurization is 10% by mass or less.

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