JP6673051B2 - Desulfurization method of molten steel - Google Patents

Description

  The present invention relates to a method for desulfurizing molten steel in which a flux is supplied to the upper layer of molten steel in a ladle to desulfurize the molten steel.

  Molten steel melted by blowing acid decarburization in a converter (primary refining) is subjected to secondary refining according to the application. In the secondary refining, a component addition treatment, a further decarburization treatment, a desulfurization treatment for removing sulfur (hereinafter, also referred to as “S”) as an impurity, etc. are performed according to the product components. In the desulfurization treatment of molten steel, for example, a desulfurization flux (hereinafter, simply referred to as “flux”) is arranged on the upper layer of molten steel in a ladle, and a current-carrying electrode is immersed in the flux to stir the molten steel while energizing. Done in Thereby, desulfurization of molten steel can be advanced. Depending on the product (application), it is necessary to reduce the S content below the target value, so desulfurization is important, and there is a need for more efficient desulfurization. The target value of the S content is set, for example, within a range of 15 ppm or more and 100 ppm or less.

  For example, Patent Literature 1 discloses an electric heating type refining ladle that aims to shorten the desulfurization time by imparting appropriate mixing and stirring to molten steel, and to set the position of the ladle bottom of the stirring gas injection plug and the electrode and the like. A technique for defining the relative positions of the two has been disclosed. In addition, Patent Literature 2 has an object to shorten the desulfurization time by strengthening the mixing and stirring power of molten steel while minimizing turbulence of the molten steel surface layer, and to improve the ladle shape (relation between steel bath height and inner diameter). ) Is disclosed.

JP-A-2001-040411 JP 2008-173675 A

  However, in the methods described in Patent Documents 1 and 2, although the desulfurization time can be reduced, the desulfurization effect may be reduced depending on the amount of the flux used, and the flux added to the molten steel is not sufficiently utilized. There was a case.

  Therefore, the present invention has been made in view of the above-described problems, and an object of the present invention is to provide a novel and novel method that can efficiently utilize the added flux without extending the desulfurization time. An object of the present invention is to provide an improved method for desulfurizing molten steel.

In order to solve the above-described problems, according to an aspect of the present invention, a desulfurization treatment of molten steel is performed by arranging a flux containing CaO on top of molten steel in a ladle, immersing an electrode in the flux and energizing the flux. (Except for the case where the pressure is reduced by immersing the cylindrical immersion pipe in the molten steel ) , the desulfurization method of the molten steel by ladle refining, wherein the thickness of the flux disposed on the molten steel in the ladle is 100 mm. 200 mm or less (excluding 200 mm or more), and CaO (mass%) / Al ₂ which is the ratio of the content (mass%) of CaO contained in the flux to the content (mass%) of Al ₂ O _3. O ₃ (mass%) is set to 1.5 or more and 2.5 or less, and the CaO amount (kg) added at the time of desulfurization treatment is 5 (kg / t) as a value per unit amount (t) of molten steel to be desulfurized. above 15 and (kg / t) or less, de Stirring power density of the stirring gas for stirring the molten steel during processing is less 30 W / t or 80W / t, the desulfurization method of the molten steel is provided.

  As described above, according to the present invention, it is possible to efficiently utilize the added flux without lengthening the desulfurization time.

It is a graph showing one relationship between the amount of CaO added in the electric heating type molten steel desulfurization treatment and the desulfurization result. It is explanatory drawing which shows the outline of the smelting equipment which performs an electric heating type molten steel desulfurization process. FIG. 3 is a partial plan view of a ladle for explaining an arrangement of electrodes and gas blowing plugs in plan view.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant description is omitted.

<1. Overview>
First, an outline of a method for desulfurizing molten steel according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a graph showing one relationship between the amount of CaO added in the electric heating type molten steel desulfurization treatment and the desulfurization result. In FIG. 1, for two cases where the thickness of the flux disposed on the molten steel is 90 mm and 150 mm, the amount of CaO contained in the added flux is changed, and the S Was determined. In this study, the initial content of S in molten steel was 100 ppm, and the content of S in molten steel was examined 35 minutes after the start of desulfurization treatment. The stirring power density of the molten steel during the desulfurization treatment was 60 W / t.

  Since the desulfurization treatment using CaO is performed by the reaction between CaO and S, the amount of CaO added to the molten steel is important. Theoretically, as shown in FIG. 1, the larger the basic unit of CaO added, the larger the amount of S removed from the molten steel after a predetermined time has elapsed. However, in practice, it was found that as shown in FIG. 1, the tendency was different from the theoretical value. That is, although the amount of S in the molten steel decreases to a certain degree when the added CaO unit increases, the amount of S in the molten steel is not reduced when the added CaO unit increases beyond a certain value. Further, for example, as in the case of the actual value in the case where the thickness of the flux is 90 mm, there are few sections that match the theoretical value, and the interval may deviate greatly from the theoretical value.

  Therefore, the inventors of the present application focused on the convection of the flux in the flux layer, and conceived of a desulfurization method for molten steel in which convection is generated in the entire flux layer to promote desulfurization and to increase the CaO basic unit added in the desulfurization treatment. According to such a method, for example, as in the case where the thickness of the flux in FIG. 1 is 150 mm, the amount of S in the molten steel after the desulfurization treatment substantially coincides with the theoretical value in a section in which the amount of S tends to decrease. The added unit of CaO is improved. Hereinafter, the method for desulfurizing molten steel according to the present embodiment will be described in detail.

<2. Desulfurization of molten steel>
[2-1. Refining equipment]
First, an outline of a refining facility to which the method for desulfurizing molten steel according to the present embodiment is applied will be described with reference to FIG. FIG. 2 is an explanatory view showing an outline of a smelting facility for performing an electric heating type molten steel desulfurization treatment.

  The method for desulfurizing molten steel according to the present embodiment is performed in a refining facility that performs a molten steel desulfurization treatment of an electric heating type. As shown in FIG. 2, the refining equipment includes a ladle 10 for accommodating molten steel 5, an electrode 20 that is energized while being immersed in a flux 7 disposed on the molten steel 5, and a molten steel in the ladle 10. 5 is provided with a gas blowing plug 30 for blowing a stirring gas 3 for stirring the molten steel 5. The electrode 20 includes, for example, three rod-shaped heating electrodes 21, 23, and 25 for energization, and is provided to penetrate the upper lid 12 of the ladle 10. The gas blowing plug 30 is provided at the bottom of the ladle 10.

  When the flux 7 is disposed above the molten steel 5 accommodated in the ladle 10, the upper lid 12 is disposed so as to cover the opening of the ladle 10, and the molten steel 5 and the flux 7 are placed in a non-oxidizing atmosphere. The tips of the current-carrying heating electrodes 21, 23, 25 provided through the upper lid 12 are immersed in the flux 7. Then, energization of the heating electrodes 21, 23, and 25 and the injection of the stirring gas 3 from the gas injection plug 30 are started, and the desulfurization treatment of the molten steel 5 is started. During the desulfurization treatment, CaO (flux containing CaO) is further added to the molten steel 5.

[2-2. Desulfurization conditions]
In the present embodiment, the desulfurization treatment of molten steel is performed under the following conditions. Thereby, the convection of the whole flux layer can be promoted, and CaO added during the desulfurization treatment can be utilized for desulfurization without waste.

(A) Flux Thickness First, the thickness of the flux disposed above the molten steel in the ladle is set to 100 mm or more and 200 mm or less. Flux thickness is difficult to measure during desulfurization. For this reason, the thickness of the flux specified here is the thickness of the flux after the desulfurization treatment, in which the stirring of the molten steel is stopped and the molten steel is allowed to stand. The thickness of the flux can be obtained, for example, by inserting a steel rod into a ladle and then measuring the thickness of the flux adhered to the steel rod withdrawn from the ladle. In addition, there is no substantial change in the thickness of the flux even if the desulfurization treatment is performed.

  When the thickness of the flux is less than 100 mm, convection hardly occurs in the flux layer, and the progress of desulfurization becomes slow. Therefore, the thickness of the flux needs to be 100 mm or more. On the other hand, if the thickness of the flux is more than 200 mm, unslagged (unmelted) portions become noticeable on the surface of the flux layer during the desulfurization treatment. Even if the slag is formed, there is a concern that the viscosity may be increased due to a decrease in temperature, and it is considered that flux convection of the entire flux layer is unlikely to occur. As a result, the CaO basic unit deteriorates. Therefore, the thickness of the flux needs to be 200 mm or less. In addition, in order to suppress unslagging or high viscosity of the surface layer of the flux layer, the thickness of the flux is desirably 150 mm or less.

(B) CaO (% by mass) / Al ₂ O ₃ (% by mass)
CaO (mass%) / Al ₂ O ₃ (mass%), which is the ratio of the content (mass%) of CaO contained in the flux and the content (mass%) of Al ₂ O ₃ , is 1.5 or more and 2 .5 or less. Since desulfurization using CaO as in this embodiment is caused by the reaction between CaO and S, the amount of CaO contained in the flux is important. In the present embodiment, by setting CaO (mass%) / Al ₂ O ₃ (mass%) to be 1.5 or more and 2.5 or less, convection occurs in the entire flux layer having the thickness of the flux, and desulfurization occurs. Processing can be accelerated. More preferably, CaO (mass%) / Al ₂ O ₃ (mass%) is set to 1.8 or more and 2.4 or less.

When the ratio of CaO (mass%) / Al ₂ O ₃ (mass%) is less than 1.5, the relative concentration of CaO with respect to Al ₂ O ₃ is too low, so that the outflow rate is significantly reduced. For this reason, in order to complete the desulfurization treatment within a certain time, it is necessary to add a large amount of flux, and as a result, the flux layer becomes thick, and the added CaO basic unit deteriorates. On the other hand, when CaO (mass%) / Al ₂ O ₃ (mass%) is more than 2.5, the dissolution rate of the flux decreases, and effective convection of the flux layer cannot be secured. In addition, an increase in the melting temperature of the flux may cause unslagging. As described above, the ratio (CaO (mass%) / Al ₂ O ₃ (mass%)) of the content (mass%) of CaO contained in the flux to the content (mass%) of Al ₂ O ₃ is 1. 5 or more and 2.5 or less.

(C) Added CaO Amount The CaO amount (kg) added at the time of desulfurization treatment is set to 5 (kg / t) or more and 15 (kg / t) or less per unit amount (t) of molten steel to be treated. As a source of CaO to be added, for example, quicklime can be used. Quick lime often contains 90% by mass or more of CaO, and approximately 100% by mass. Further, as another CaO supply source, a material containing CaO such as calcium aluminate, lightly-burned dolomite, and ingot slag may be used. When these are used, the amount of CaO to be added may be calculated using the mass ratio of CaO contained.

  When the amount of CaO added during the desulfurization treatment is less than 5 (kg / t) per unit amount of molten steel (t), the amount of CaO in the molten steel after desulfurization treatment is small because the amount of CaO is smaller than the amount of molten steel to be desulfurized. Does not reduce. Therefore, the amount of CaO to be added is preferably 5.0 (kg / t) or more. On the other hand, when the amount of CaO added during the desulfurization treatment exceeds 15.0 (kg / t) per unit amount (t) of molten steel, the thickness of the flux above the molten steel satisfies the above, and the commonly used ladle dimensions (molten steel) (Depth, ladle inner diameter), the CaO concentration in the flux increases, and the viscosity of the flux deteriorates. Due to the deterioration of the viscosity of the flux (defective slagging in a remarkable case), the convection of the flux layer hardly occurs, and the progress of desulfurization is slowed. Therefore, the amount of CaO to be added is preferably 15.0 (kg / t) or less.

(D) Stirring Power Density of Stirring Gas The stirring power density of the stirring gas for stirring the molten steel during the desulfurization treatment is set to 20 W / t or more and 90 W / t or less.

  It is important for the desulfurization treatment to blow a stirring gas such as an inert gas or a nitrogen gas from a ladle bottom from a gas blowing plug to stir the molten steel. However, the stirring of the molten steel needs to be appropriately performed according to the amount of the molten steel and the shape of the ladle. If the stirring of the molten steel is too strong, the thickness of the flux at the site where the electrode is immersed becomes extremely thin, and the effect of increasing the temperature of the convective flux decreases. On the other hand, if the stirring of the molten steel is too weak, the convection of the flux layer generated by the shear force of the molten steel flow becomes weak, and efficient desulfurization is not performed. Therefore, in this embodiment, the stirring power density of the stirring gas is specified to be 20 (W / t) or more and 90 (W / t) or less. More preferably, the stirring power density of the stirring gas is 30 (W / t) or more and 80 (W / t) or less.

The stirring power density can be calculated based on, for example, the following equation (1) described in JP2013-023739A. Here, ε _M is the stirring power density (W / t), Q is the stirring gas flow rate (Nm ³ / sec), T ₁ is the molten steel temperature (K), T _n is the stirring gas temperature (K), and P ₂ is the atmosphere. Pressure (Pa), W is the mass (t) of molten steel, ρ is the specific gravity (t / m ³ ), and h is the stirring gas blowing depth (m).

(E) Position of Electrode and Plug When the thickness of the flux and the stirring power density of the stirring gas are set as described above, if the ladle is viewed from above and the electrode position is too close to the plug position, the flux at the electrode position may be too small. Since the thickness becomes thin, convection of the heated flux cannot be promoted, and promotion of desulfurization becomes difficult. Therefore, when the ladle is viewed in plan, the electrode position and the plug position need to be separated from each other at an appropriate distance, not at least at the same position.

  Specifically, as shown in FIG. 3, the ladle 10 is viewed in a plan view, and the diameter of the electrode 20 (here, any one of the heating electrodes 21, 23, and 25 for energization is indicated) (hereinafter, referred to as ““ The diameter of the gas blowing plug 30 (hereinafter, also referred to as “plug diameter”) is D, and the electrode diameter is also referred to as D. At this time, the positions of the electrode 20 and the gas blowing plug 30 are determined so that the distance L between the center of the electrode 20 and the center of the gas blowing plug 30 satisfies the following expression (2).

  L ≧ 1.7 × (D / 2) + (d / 2) (2)

  When the cross section of the electrode 20 is not a circle, such as a polygon, for example, a diameter of a circle having an equivalent cross-sectional area may be used as the electrode diameter D. A good center of gravity may be used. The diameter of the plug may be, for example, the diameter of a circle circumscribing the hole of the gas blowing plug 30. At this time, the center of the gas blowing plug 30 may be the center of the circumscribed circle.

<3. Summary>
The method for desulfurizing molten steel according to one embodiment of the present invention has been described above. According to the present embodiment, when the flux containing CaO is arranged on the molten steel in the ladle, and the electrode is immersed in the flux and the current is applied to perform the desulfurization treatment of the molten steel, the thickness of the flux is 100 mm or more. 200 mm or less, the ratio of the content (% by mass) of CaO contained in the flux to the content (% by mass) of Al ₂ O ₃ is 1.5 or more and 2.5 or less, and the amount of CaO added at the time of desulfurization treatment (Kg) is set to 5 (kg / t) or more and 15 (kg / t) or less per unit amount (t) of molten steel to be treated.

  Thereby, the convection of the flux generated in the flux layer is promoted as a whole, and S in the molten steel is efficiently reacted with CaO in the flux and distributed in the flux, and as a result, the S content in the molten steel is reduced. it can. Thus, the added CaO can be effectively used for desulfurization. By improving the desulfurization capacity, CaO that does not contribute to desulfurization can be reduced, so that the amount of CaO used can be reduced. In addition, the desulfurization method of molten steel according to the present embodiment can increase the thickness of the flux layer at the electrode position where the flux temperature is highest, so that the flux thickness becomes too thin and the convection of the flux is promoted. And desulfurization is not promoted.

  Hereinafter, the results of verifying the effectiveness of the method for desulfurizing molten steel by ladle refining of the present invention will be described. In the present embodiment, the electric heating type molten steel desulfurization treatment was performed using the refining equipment shown in FIG. A 50-70 t crude molten steel was placed in a ladle as a target for desulfurization treatment, and a flux was injected into the upper part of the crude molten steel. Thereafter, the electrodes were immersed in a flux to start energization, and a bottom-blowing stirrer for blowing a stirring gas from a gas blowing plug at the bottom of the ladle was started to start desulfurization treatment. The desulfurization treatment was performed for 35 minutes. The electrodes and the gas blowing plugs of the refining facility are arranged such that the distance L between the electrode center and the plug center of the gas blowing plug satisfies the above expression (2).

  For Examples and Comparative Examples shown in Table 1 below, molten steel was desulfurized under the above desulfurization conditions, and based on the desulfurization amount per unit amount of CaO (ppm / CaO amount kg), whether or not the amount of CaO used was reduced. Was evaluated. Based on Comparative Example 2, the evaluation was ◎ when the desulfurization amount increased by 12% or more, ○ when the desulfurization amount was 5% or more and less than 12%, and the desulfurization amount was equivalent to Comparative Example 2 (± (Less than 5%) or worsened, it was evaluated as x.

As shown in Table 1, in Example 1 and Reference Example 2, CaO (mass%), which is a ratio of the content (mass%) of CaO contained in the flux to the content (mass%) of Al ₂ O _3. / Al ₂ O ₃ (mass%): 2.3, CaO amount (kg) per unit amount (t) of molten steel added during desulfurization treatment: 8.0 (kg / t), stirring power density: 63 W / t , A desulfurization treatment of molten steel was performed. In Example 1, when the thickness of the flux was 100 mm, the desulfurization amount per CaO unit amount was good. Further, in Reference Example 2, when the thickness of the flux was 200 mm, the desulfurization amount per CaO unit amount was inferior to that of Example 1, but was improved from Comparative Example 2 which was the evaluation standard.

In Examples 3 to 6, the flux thickness was 150 mm, the CaO amount (kg) per molten steel unit amount (t) added during desulfurization treatment was 8.0 (kg / t), and the stirring power density was 63 W / t. , A desulfurization treatment of molten steel was performed. The values of CaO (% by mass) / Al ₂ O ₃ (% by mass) were different values in Examples 3 to 6. From Table 1, the desulfurization amount per CaO unit amount was evaluated in Examples 3 and 6 where CaO (mass%) / Al ₂ O ₃ (mass%) was 1.5 and 2.5. In Comparative Example 2 which was improved as compared with Comparative Example 2, and in Example 4 in which CaO (mass%) / Al ₂ O ₃ (mass%) was 1.8 and in Example 5 in which 2.4 was 2.4, Example 3 was used. , 6 were further improved.

In Examples 7 and 8, molten steel was desulfurized with a flux thickness of 150 mm, CaO (mass%) / Al ₂ O ₃ (mass%) of 2.3, and a stirring power density of 63 W / t. In Example 7, the CaO amount (kg) per unit amount (t) of molten steel added at the time of desulfurization treatment was 5.0 (kg / t), and in Example 8, it was 15.0 (kg / t). And the amount of desulfurization per unit amount of CaO was good.

In Reference Examples 9 and 10 and Examples 11 and 12, the flux thickness was 150 mm, CaO (% by mass) / Al ₂ O ₃ (% by mass) was 2.3, and the unit amount of molten steel added during desulfurization treatment (t The desulfurization treatment of the molten steel was performed with the CaO amount (kg) per 8.0) being 8.0 (kg / t). The stirring power density was different in Reference Examples 9 and 10 and Examples 11 and 12. From Table 1, in Reference Example 9 in which the stirring power density was 20 W / t, and in Reference Example 10 in which the stirring power density was 90 W / t, the stirring power density was 30 W / t, which was improved over Comparative Example 2 which was the evaluation criterion. In Example 11 and Example 12 in which the power consumption was 80 W / t, the results were further improved as compared with Reference Examples 9 and 10.

On the other hand, Comparative Example 1 is a case where the thickness of the flux is further increased than that of Reference Example 2, and Comparative Example 2 is a case where the thickness of the flux is further reduced than that of Example 1. In each case, no improvement in the desulfurization amount per CaO unit amount was found. In Comparative Example 1, the flux in the surface layer was in an unslagged state or at a low temperature and became highly viscous, so that convection of the entire flux layer did not occur. Was difficult to happen.

Comparative Example 3 is a case where the value of CaO (% by mass) / Al ₂ O ₃ (% by mass) is smaller than that of Example 3, and Comparative Example 4 is a case where CaO (% by mass) / Al _This is the case where the value of ₂ O ₃ (% by mass) is increased. In each case, no improvement in the desulfurization amount per CaO unit amount was found. It is considered that in Comparative Example 3, the relative concentration of CaO was too low, and the desulfurization rate was remarkably reduced. In Example 4, the fluidity of the flux was poor, and effective convection of the flux layer could not be secured.

  Comparative Example 5 is a case where the amount of CaO per unit amount of molten steel (t) added at the time of desulfurization treatment is smaller than that of Example 7, and Comparative Example 6 is a case where molten steel added at the time of desulfurization treatment more than that of Example 8. This is a case where the amount of CaO per unit amount (t) is increased. In each case, no improvement in the desulfurization amount per CaO unit amount was found. In Comparative Example 5, the S content of the molten steel after the desulfurization treatment was large, and the CaO amount was smaller than the amount of the molten steel to be desulfurized. Therefore, in Comparative Example 6, the CaO concentration in the flux was increased, and the viscosity of the flux was deteriorated. It is considered that the convection of the flux layer did not easily occur.

Comparative Example 7 is a case where the stirring power density of the stirring gas is further reduced than in Reference Example 9, and Comparative Example 8 is a case where the stirring power density of the stirring gas is further increased than in Reference Example 10. In each case, no improvement in the desulfurization amount per CaO unit amount was found. In Comparative Example 7, the stirring of the molten steel was too weak, the convection of the flux layer generated by the shear force of the molten steel flow was weak, and efficient desulfurization was not performed. In Comparative Example 8, the stirring of the molten steel was too strong, It is considered that the thickness of the flux at the electrode immersion site became extremely thin, and the effect of increasing the temperature of the convective flux was reduced.

  As described above, the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that those skilled in the art to which the present invention pertains can conceive various changes or modifications within the scope of the technical idea described in the claims. It is understood that these also belong to the technical scope of the present invention.

Reference Signs List 3 Stirring gas 5 Molten steel 7 Flux 10 Ladle 12 Top lid 20 Electrode 21, 23, 25 Heating electrode for electricity supply 30 Gas blowing plug

Claims (1)

A flux containing CaO is placed above the molten steel in the ladle, and the electrode is immersed in the flux and energized to conduct desulfurization of the molten steel (however, the cylindrical immersion pipe is immersed in the molten steel to reduce the pressure. ) , A method of desulfurizing molten steel by ladle refining,
The thickness of the flux disposed above the molten steel in the ladle is 100 mm or more and 200 mm or less (however, excluding 200 mm or more) ,
CaO (mass%) / Al ₂ O ₃ (mass%), which is the ratio of the CaO content (mass%) and Al ₂ O ₃ content (mass%) contained in the flux, is 1.5 or more and 2 .5 or less,
The CaO amount (kg) added at the time of the desulfurization treatment is 5 (kg / t) or more and 15 (kg / t) or less per unit amount (t) of molten steel to be desulfurized ,
A method for desulfurizing molten steel, wherein a stirring power density of a stirring gas for stirring the molten steel during the desulfurization treatment is set to 30 W / t or more and 80 W / t or less .

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