JP6623933B2 - Desulfurization method of molten steel - Google Patents

Description

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

  When smelting low-sulfur steel having a sulfur (hereinafter sometimes referred to as S) content of several tens of ppm, it is sufficiently low only to perform desulfurization in hot metal pretreatment and then decarburize in a converter or the like. Since the sulfur level is not reached, molten steel desulfurization is required.

  As a method for desulfurizing molten steel, a method of removing S by slag refining while heating molten steel (so-called LF method) has been put to practical use. However, an LF step as a secondary refining step is indispensable, which leads to an increase in production time and cost. Is increasing.

  Therefore, it is required to establish a desulfurization treatment using a vacuum degassing device that is widely used as a secondary refining process. As a desulfurization treatment in a vacuum degassing apparatus, a method of spraying a powdery desulfurizing agent from above onto a molten steel surface in a vacuum vessel is generally used. As a desulfurizing agent, a CaO-based flux is generally used. The desulfurizing agent sprayed on the molten steel surface enters the molten steel, and a desulfurization reaction occurs with the molten steel. Since the CaO-based flux has a lower specific gravity than molten steel and eventually floats and separates on the molten steel surface due to buoyancy, it is important how the desulfurization reaction proceeds before the floatation and separation.

Conventionally, CaF ₂ (fluorite) has been used as a solvent when a CaO-based flux is used. When CaF ₂ is added to a CaO-based flux, a liquid phase having a high desulfurization ability can be formed. Forming a liquid phase in the desulfurizing agent facilitates the movement of S inside the desulfurizing agent, and has the effect of promptly promoting the desulfurization reaction. Therefore, the desulfurization reaction was able to proceed sufficiently even if the time until the desulfurizing agent floated and separated on the molten steel surface was short.

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

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

  Therefore, in the desulfurization of molten steel with a CaO-based flux substantially containing no F, the desulfurizing agent floats and separates on the surface of the molten steel before the sulfur is sufficiently absorbed, so that there is a problem that the use efficiency of the desulfurizing agent is reduced. .

  Patent Literature 1 specifies the relationship between the Al concentration and the powder supply rate for preventing agglomeration of a desulfurizing agent when a CaO-based desulfurizing agent containing no F is practically used in a 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 proceed. However, since the Al concentration changes over time during the desulfurization treatment, it is difficult to control the Al 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 performing hard blowing. At this time, the desulfurizing agent contains 1% by weight or more of fluoride.

JP 2007-270178 A JP-A-6-322431

  The inventors attempted a desulfurization treatment by the method of Patent Document 2 using a CaO-based desulfurizing agent containing substantially no F. However, in the method of Patent Document 2, although the powder intrudes into the molten steel from the surface of the molten steel, the powders tend to collide with each other, so that a laminate mainly composed of the collided powder is formed, and the desulfurization is performed. It has been found that a desulfurizing agent (powder) that does not contribute to the generation is generated, and the desulfurization rate is reduced.

  The present invention provides a method of desulfurizing a molten steel in a vacuum degassing apparatus by spraying a desulfurizing agent from a desulfurizing agent spray nozzle onto a surface of molten steel in a vacuum vessel to desulfurize the molten steel. The purpose is to provide.

The gist of the present invention is as follows.
(1) When desulfurizing molten steel in a vacuum degassing apparatus, a desulfurizing agent is sprayed from a desulfurizing agent spray nozzle onto the surface of the molten steel in the vacuum vessel, and a CaO-based desulfurizing agent containing substantially no F as the desulfurizing agent In the method to be used, a device for blowing a stirring gas from the bottom of a molten steel pot is used as a vacuum degassing device, and a distance between the desulfurizing agent spray nozzle and the surface of molten steel in a vacuum vessel (hereinafter referred to as "lance height"). ) Is set to 2.6 m or more.
(2) The method for desulfurizing molten steel according to (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 pot before desulfurization is 10% by mass or less.

  According to the present invention, a CaO-based desulfurizing agent containing substantially no F in molten steel can be supplied more uniformly than in the prior art, and a reduction in reaction efficiency due to lamination can be prevented.

  By utilizing the circulating flow in the molten steel, the reaction time in the molten steel can be secured, and the F-less flux having a high solid fraction 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 an example using a large diameter immersion tube system, (B) is an example using an RH system. It is a figure which shows the gas jet from a desulfurizing agent spray nozzle, (A) is when the lance height H is low, (B) is when the lance height H is high.

  In the method for desulfurizing molten steel of the present invention, desulfurization of molten steel is performed using a vacuum degassing apparatus. There are various types of vacuum degassing apparatus for molten steel. When vacuum degassing molten steel, a method in which the entire molten steel pot is housed in a vacuum vessel and the inside of the vacuum vessel is evacuated (eg, VOD (Vacuum Oxygen Decarburization) method, VAD (Vacuum Arc Degassing) method, etc.), or a vacuum vessel Is immersed in molten steel in a molten steel pot to evacuate the inside of a vacuum vessel (for example, a large-diameter immersion pipe method, an RH vacuum degassing apparatus, etc.). As shown in FIG. 1A, one large-diameter immersion pipe 3 is provided at the lower part of a vacuum vessel 2 and the large-diameter immersion pipe 3 is Is immersed in molten steel 10 housed in a vacuum vessel, the pressure inside the vacuum vessel 2 is reduced, the molten steel is sucked up into the vacuum vessel, and gas is blown from the stirring gas blower 6 at the bottom of the molten steel pot. Is a method of stirring and mixing the molten steel by forming a stirring flow between the molten steel and the molten steel in the vacuum vessel. In the case of the RH-type vacuum degassing device 22, as shown in FIG. 1B, two immersion tubes are provided at the bottom of the vacuum vessel 2, one of which is an ascending tube 7 and the other is a descending tube 8. Gas is blown into the molten steel from the gas injection port 9 on the side wall of the riser pipe 7 at the intermediate position, and the molten steel rises in the riser pipe 7 due to the buoyancy of the gas. Is discharged into the molten steel ladle 1.

  In desulfurization of molten steel using a vacuum degassing device, a desulfurizing agent blowing lance 4 is inserted downward into the vacuum vessel 2. A desulfurizing agent spray 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 to the molten steel surface 11 in the vacuum vessel together with the gas jet 14.

  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. Vacuum degassing systems include a method of supplying a stirring gas from the bottom of a molten steel pot (VOD method, large immersion tube method, etc.) and an RH vacuum that supplies a stirring / mixing gas from the middle of an immersion pipe immersed from above the molten steel. There is a degassing device.

In the case of the RH method, two immersion tubes are provided at the bottom of the vacuum vessel, one is an ascending tube, and the other is a descending tube. Gas is blown into the molten steel from a side wall at an intermediate position of the riser, and the molten steel rises in the riser by the buoyancy of the gas, and the molten steel descends from the downcomer through the vacuum vessel and is discharged into the molten steel ladle. The flow of the molten steel from the downcomer to the ladle is strong, which collides with the bottom of the ladle, after which the flow is dispersed and weakened. The powder that has entered the molten steel in the vacuum vessel is dispersed at the bottom of the molten steel pot along with the flow of the molten steel from the downcomer pipe, and then slowly rises mainly along the side wall of the molten steel pot, As well as reaching the vicinity of the molten steel surface inside, it reaches the weak part of the molten steel flow. The force acting on the powder in the weak portion of the molten steel flow has a greater effect of buoyancy than the viscous force received from the molten steel, and the powder easily floats and separates. The powder floating and separating near the molten steel surface of the molten steel pot is taken into the slag existing on the surface in the molten steel pot, and the contribution to desulfurization is significantly reduced thereafter. A CaO-based flux containing CaF ₂ does not pose a problem because the reaction rate is large and desulfurization proceeds sufficiently even in a short time. On the other hand, when a CaO-based desulfurizing agent containing substantially no F is used, since the movement of S in the desulfurizing agent is caused by diffusion in the solid phase, a time is required for the desulfurization reaction, and the time required for the powder to float and separate. The reaction does not proceed sufficiently in time. For this reason, in the RH method, when a CaO-based desulfurizing agent containing substantially no F was used, the reaction efficiency was significantly reduced.

  In the present invention, a device for blowing a stirring gas from the bottom of a molten steel pot is used as a vacuum degassing device. The large-diameter immersion tube method, the VOD method, the VAD method and the like are vacuum degassing devices falling into this category. It has been found that the use of the device 6 that blows the stirring gas from the bottom of the molten steel pot, such as the VOD method or the large immersion tube method, can solve the above-mentioned problems that are problematic in the RH method. The device 6 for blowing the stirring gas at the bottom of the molten steel pot is arranged such that the bubbles blown into the molten steel from the device 6 for blowing the gas for stirring rise and reach the molten steel surface 11 in the vacuum vessel. (See FIG. 1).

  When using the device 6 for blowing the stirring gas from the bottom of the molten steel pot as described above, the molten steel rises along the bubble rising flow from the device 6 for blowing the gas for stirring to 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 in the vacuum vessel, reaches the side wall of the vacuum vessel, and starts descending from that position. The downward flow 16 forms a downward flow in the ladle that lasts all the way to the bottom of the ladle. That is, in these devices, a large circulating flow occurs 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 entering the molten steel descends in the molten steel pot accompanying the descending flow 16, and rises due to gas bubbles at the bottom of the molten steel pan. And circulates to the molten steel surface 11 in the vacuum vessel. On the molten steel surface 11 in the vacuum vessel, the molten steel vigorously mixes due to the expansion of bubbles, and the floating powder is taken into the molten steel without remaining on the molten steel surface, and descends in the molten steel pot together with the descending flow 16 of the molten steel. In this way, the reaction time can be ensured by keeping the powder in the circulating flow of the molten steel. Therefore, even when a CaO-based desulfurizing agent containing substantially no F is used, although the movement of S in the desulfurizing agent is diffusion in the solid phase and requires a long time for the desulfurization reaction, a sufficient time for the desulfurization reaction is required. Because the steel stays in the molten steel, the reaction efficiency can be secured.

  In addition, the fact that F is not substantially contained means that elution of fluorine (F) from slag after desulfurization refining is not remarkably recognized, and the present inventors have found that the slag composition after refining is not considered. Refers to 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. As the CaO source, CaCO ₃ or Ca (OH) ₂ may be used in addition to CaO, and the CaO content can be calculated from the mass when completely pyrolyzed to CaO. Components other than CaO in the desulfurizing agent are not particularly limited. It is more preferable to include a component having a desulfurizing ability by itself such as Mg.

  In the desulfurization of molten steel using a vacuum degassing device, a lance for blowing a desulfurizing agent into a vacuum vessel is inserted downward. As shown in FIG. 2, a desulfurizing agent spray nozzle 5 is arranged at the tip of the desulfurizing agent blowing lance 4. A desulfurizing agent is sprayed from the desulfurizing agent spray nozzle 5 to the molten steel surface 11 in the vacuum vessel. The present invention is characterized in that the distance (lance height H) between the desulfurizing agent spray nozzle 5 and the molten steel surface 11 in the vacuum vessel is set to 2.6 m or more. The details will be described below.

The desulfurizing agent powder is transported through the desulfurizing agent blowing lance 4 by the carrier gas, and is jetted 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. Here, the vertical distance from the molten steel surface 11 in the vacuum vessel to the tip of the desulfurizing agent spray nozzle 5 is referred to as a lance height H. In detail, the pressure inside the vacuum vessel is set to the pressure at which the desulfurizing agent is sprayed, and the vertical distance between the surface of the molten steel in the vacuum vessel and the tip of the nozzle in the condition where the stirring gas is not blown from the bottom of the molten steel pot is lance height. H.
When blowing the desulfurizing agent together with the carrier gas from the nozzle, the gas expands rapidly due to the pressure difference between the inside and outside of the nozzle. The gas jet 14 (including the desulfurizing agent) blown vertically downward from the nozzle has a jet core 17 (spread area approximately equal to the nozzle opening area) near the lance. After the lance height is about 1 m, a divergent portion 18 is formed which advances toward the molten steel surface and spreads in the horizontal direction. At this time, the powder is also dispersed in the horizontal direction.

  When the lance height H is low, as shown in FIG. 2A, the spread of the powder is small, and the powder is locally supplied to a part of the surface of the molten steel. In the locally supplied powder, the probability that the particles collide with each other increases. When a CaO-based desulfurizing agent containing substantially no F is used, when particles collide with each other, they are laminated on the surface of the molten steel and coalesce. When lamination and coalescence occur, the specific surface area (ratio of surface area to volume) of the powder decreases. As a result, the reaction rate is lower than when the particles exist alone.

  In contrast, it has been found that the above problem can be solved by increasing the lance height H. That is, if the lance height H is high, as shown in FIG. 2 (B), the powder reaches the molten steel surface 11 in the vacuum vessel in a dispersed state, so that the collision of particles hardly occurs. Therefore, even when a CaO-based desulfurizing agent containing substantially no F is used, the reaction efficiency can be ensured.

  In order to obtain the above-mentioned effects, the present inventors have found that the lance height is preferably set to 2.6 m or more. If it is less than 2.6 m, the efficiency of the desulfurization reaction will decrease. On the other hand, when the lance height was 2.6 m or more, the area of the carrier gas spraying surface became 6 times or more the opening diameter of the nozzle lower end, and it was estimated that lamination of the desulfurizing agent on the molten steel surface could be sufficiently prevented. The higher the lance height, the lower the probability of collision between the desulfurizing agent particles. Therefore, it is not necessary to specify the upper limit, but when the lance height exceeds 6 m, the effect is saturated.

  As described above, when performing desulfurization treatment of molten steel in the vacuum degassing apparatus, a desulfurizing agent is sprayed from the desulfurizing agent spray nozzle 5 onto the molten steel surface 11 in the vacuum vessel, and a CaO-based material substantially free of F as the desulfurizing agent is used. In the method using the desulfurizing agent, the distance (lance height H) between the desulfurizing agent spray nozzle 5 and the molten steel surface 11 in the vacuum vessel is set to 2.6 m or more, so that the particles of the desulfurizing agent hardly occur. Become. In addition, by using the apparatus 6 that blows the stirring gas from the bottom of the molten steel pot as a vacuum degassing apparatus, the reaction efficiency can be ensured even when a CaO-based desulfurizing agent containing substantially no F is used.

  In the present invention, the total of the FeO component and the MnO component of the slag 13 present on the molten steel surface 12 in the molten steel pot before desulfurization is preferably 10% by mass or less. The details will be described below.

  As described above, in the present invention using the device 6 for blowing the stirring gas from the bottom of the molten steel pot, the desulfurizing agent particles in the circulating molten steel flow circulate in the molten steel and continue to contribute to desulfurization. Deviates from the circulating molten steel flow with the probability of, rises to the molten steel ladle bath surface, and is taken into the slag 13 existing on the molten steel ladle bath surface. The slag 13 on the bath surface of the molten steel pot is mainly composed of oxidized and refined slag flowing out of the converter at the time of tapping from the converter, and has a high FeO component and MnO component content, and is a so-called highly oxidizable slag. . Therefore, the desulfurization product constituting the desulfurization agent particles floating on the bath surface of the molten steel pot may be oxidized by the slag 13 on the bath surface and resulfurized in the molten steel.

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

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

  In 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 inside diameter of the molten steel pot is 5 m. The vacuum degassing apparatus was a large-diameter immersion pipe system (see FIG. 1) (inner diameter of the large-diameter immersion pipe 3: 2 m) and an RH system, and the desulfurization treatment was performed by setting the pressure in the vacuum vessel to 400 Pa to 1200 Pa. In the present invention, it is assumed that a molten steel pot has an inner diameter of 3 to 6 m and a large-diameter immersion pipe has an inner diameter of 1.5 to 2.5 m.

  At the level using the large-diameter immersion tube method, the stirring gas was supplied from a stirring gas injection plug (device 6 for blowing the stirring gas) provided at the bottom of the pot. In the level using the RH method, a stirring gas was supplied from the gas injection port 9 of one of the two immersion tubes (rising tube 7) among the two immersion tubes provided in the vacuum vessel 2. Ar gas was used as a stirring gas at any level.

  The desulfurizing agent is jetted together with the carrier gas from the desulfurizing agent spray nozzle 5 at the tip of the desulfurizing agent injection 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 175 (kg / min) was sprayed on the molten steel. As the desulfurizing agent spray nozzle 5, a Laval nozzle having a nozzle lower end opening diameter (diameter) of 120 mm was used. In the present invention, a size of 75 to 150 mm is assumed.

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

Evaluation of the test results was performed as follows.
The S concentration of 30 ppm to 40 ppm before desulfurization was changed to the ultimate S concentration of 5 ppm to 13 ppm by desulfurization treatment, and the time from the start of desulfurization treatment to the end of desulfurization treatment was measured. Next, the change in S concentration was assumed to be a linear function of time, and the time required to decrease from 30 ppm to 15 ppm was converted to “desulfurization treatment time” using the function. Then, based on the desulfurization treatment time in Comparative Example 1, the case of improving more than 10% was evaluated as ◎, the case of improving 5% or more and 10% or less was evaluated as 、, and the case of deterioration or improvement of less than 5% was evaluated as ×. .
Table 1 shows the results of the verification test.

  In comparison with the case where a CaO-based desulfurizing agent containing no F is used, the desulfurization treatment time is shorter in the large diameter immersion tube method (Example 1, Comparative Example 1) than in the RH method (Comparative Example 2). The large-diameter immersion pipe system uses the device 6 that blows gas for stirring from the bottom of the molten steel pot, so that the injected desulfurization agent stays in the molten steel for a long time on the molten steel circulating flow. It is presumed that sufficient desulfurization was performed despite the fact that the desulfurization agent required a long time for desulfurization.

In addition, when compared with the same large-diameter immersion tube method, the desulfurization treatment time becomes shorter as the lance height H increases, and sufficient desulfurization performance is realized when the lance height H is 2.6 m (Example 1, Comparative Example 1). By increasing the lance height H, the blown powder reaches the surface of the molten steel in a dispersed state, so that collision of particles hardly occurs. Therefore, a CaO-based desulfurizing agent containing substantially no F was used. Even in this case, it is presumed that the reaction efficiency has been secured. In addition, the "prior art" in Table 1 used CaF _{2 in the} desulfurizing agent, and although the lance height H was as low as 1.5 m, the evaluation result was good.

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

DESCRIPTION OF SYMBOLS 1 Molten steel pot 2 Vacuum container 3 Large diameter immersion pipe 4 Desulfurizing agent blowing lance 5 Desulfurizing agent spray nozzle 6 Device for blowing gas for agitation 7 Ascending tube 8 Descending tube 9 Gas blowing port 10 Molten steel 11 Molten steel surface in vacuum container 12 Molten steel Molten steel surface 13 in slag 14 Slag 14 Gas jet 15 Upflow 16 Downflow 17 Jet core 18 Divergent portion 21 Large diameter immersion 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 method of spraying a desulfurizing agent onto the surface of molten steel in a vacuum vessel from a desulfurizing agent spray nozzle and using a CaO-based desulfurizing agent containing substantially no F as the desulfurizing agent At
As a vacuum degassing device, while using a device that blows gas for stirring from the bottom of the molten steel pot,
A method for desulfurizing molten steel, comprising: setting a distance (hereinafter, referred to as a "lance height") between the desulfurizing agent spray nozzle and the surface of molten steel in a vacuum vessel to be 2.6 m or more. 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 pot before desulfurization is 10% by mass or less.

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