JP7139877B2 - Ladle refining method for molten steel - Google Patents

Description

The present invention relates to a ladle refining method for molten steel.

During the production of steel materials, the molten steel decarburized in the converter is subjected to secondary refining depending on the application. In such secondary refining, alloy addition, temperature elevation, reduction, and removal of impurity elements are performed according to the specifications of the product to be manufactured.

In one of the secondary refining methods as described above, while an energizing electrode is immersed in the slag layer existing on the surface of the molten steel and electrically heated, inert gas is introduced into the molten steel through a porous plug provided at the bottom of the ladle. There is a method of stirring molten steel by blowing gas. In such a secondary refining method involving electric heating, a refining reaction occurs between the molten steel and the slag by stirring with an inert gas.

In the secondary refining method involving such electric heating, for example, as disclosed in Patent Document 1 below, while paying attention not to melt the refractory provided on the inner wall surface of the ladle by electric heating, Electrodes are arranged for energization.

Here, in the secondary refining method involving electric heating, when the nitrogen gas in the atmosphere comes into contact with the molten steel, a nitrogen absorption reaction (hereinafter referred to as "nitrogen absorption reaction") occurs in the molten steel, and the nitrogen concentration in the molten steel increases. it rises. Therefore, conventionally, various techniques have been proposed for preventing absorption and nitriding reactions.

For example, in Patent Document 2 below, a gas flow path that opens at the bottom of an electrode is formed, and an inert gas is discharged through the gas flow path to create an inert gas atmosphere in the upper part of the molten steel surface. Techniques are disclosed.

In addition, Patent Document 3 below discloses a technique in which carbon dioxide is supplied into the ladle lid to make the gas phase in contact with the molten steel a carbon dioxide gas atmosphere, and the gas atmosphere above the surface of the molten steel has a low nitrogen concentration. ing.

Further, in Patent Document 4 below, a slag-forming agent is added and molten steel is stirred to melt at least part of the slag-forming agent by the heat of the molten steel. A low-nitrogenization technique using heating is disclosed.

JP 2010-17756 A JP-A-61-276684 JP-A-3-104814 JP-A-1-208413

However, the techniques disclosed in Patent Documents 2 and 3 are techniques for reducing nitrogen in the atmosphere above the molten steel surface, and nitrogen absorption may progress until the atmosphere is reduced in nitrogen. Further, in the technique disclosed in Patent Document 4, the energization time required for melting the slag-forming agent may become long, and the adsorption/nitrogenation reaction may rather proceed.

As described above, the techniques disclosed in Patent Documents 2 to 4 still have room for improvement from the viewpoint of more reliable suppression of nitrogen absorption reaction.

Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to more reliably suppress the occurrence of nitrogen absorption reaction in ladle refining accompanied by electric heating. , to provide a ladle refining method for molten steel.

The present inventors have made intensive studies to solve the above problems, and found that the absorption of nitrogen is caused by (a) the exposed surface of the molten steel resulting from the bursting of bubbles of the stirring gas on the surface of the molten steel, and (b) the unmelted steel surface. and (c) exposed molten steel surfaces that are not locally covered due to uneven distribution of molten slag. Based on this knowledge, it was conceived that the occurrence of nitriding reaction can be more reliably suppressed by covering the entire surface of the molten steel with molten slag and preventing the bubbles of the stirring gas from breaking on the surface of the molten steel. I got The present inventors have completed the present invention as a result of further studies based on this idea.
The gist of the present invention, which was completed based on the results of these studies, is as follows.

[1] A ladle refining method for molten steel in which a slag layer is formed on the surface of the molten steel in the ladle, and electrodes are immersed in the slag layer and energized , in which three electrodes are immersed in the slag layer. , A gas blowing plug for blowing a stirring gas for stirring the molten steel held in the ladle is provided on the bottom surface of the ladle, and the inner diameter of the ladle on the surface of the molten steel is Ds [m] , the ladle inner diameter Ds is 2.5 to 4.7 m, and when the molten steel surface is viewed from above the ladle, the three electrodes on the molten steel surface are The center of the electrode circumscribing circle, which is a circle that circumscribes the position or the projection position of the three electrodes onto the molten steel surface and has the smallest diameter, is the center of the electrode circumscribing circle on the molten steel surface. When the radius of the ladle at the bottom of the ladle is R [m], it is located within a region from the center position of the ladle on the surface of the molten steel to 0.1 × R, and the electrode circumscribed circle When the diameter of the electrode is D [m], the ratio of the diameter of the electrode circumscribing circle to the inner diameter of the ladle (Ds/D) satisfies the following formula (1), and the gas is blown on the surface of the molten steel When the central axis of the plug passes through the electrode circumscribed circle or inside the electrode circumscribed circle, and the flow rate of the stirring gas is Q [NL/min/t], the flow rate of the stirring gas is is a ladle refining method for molten steel that satisfies the following formula (2).
1.8 ≤ Ds/D ≤ 3.5 Expression (1)
0.3 ≤ Q ≤ 4.5 Expression (2)
[2] The ladle refining method for molten steel according to [1], wherein the slag layer has a thickness of 100 mm or more.

As described above, according to the present invention, it is possible to more reliably suppress the occurrence of nitrogen absorption reaction in ladle refining accompanied by electric heating.

It is a sectional view showing typically a section at the time of cutting ladle refining equipment concerning an embodiment of the present invention in the depth direction of a ladle. It is a sectional view showing typically a section at the time of cutting ladle refining equipment concerning the embodiment in the depth direction of a ladle. It is a sectional view showing typically a section at the time of horizontally cutting the ladle refining equipment concerning the same embodiment at the position of molten steel height H. It is an explanatory view for explaining the ladle refining method according to the same embodiment. It is an explanatory view for explaining the ladle refining method according to the same embodiment. It is an explanatory view for explaining the ladle refining method according to the same embodiment. It is an explanatory view for explaining the ladle refining method according to the same embodiment. It is an explanatory view for explaining the ladle refining method according to the same embodiment. It is an explanatory view for explaining the ladle refining method according to the same embodiment. It is an explanatory view for explaining the ladle refining method according to the same embodiment.

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the present specification and drawings, constituent elements having substantially the same functional configuration are denoted by the same reference numerals, thereby omitting redundant description.

(About ladle refining method of molten steel)
Hereinafter, a ladle refining method for molten steel according to an embodiment of the present invention will be described in detail with reference to FIGS. 1 to 8. FIG.
1 and 2A are cross-sectional views schematically showing a cross section of the ladle refining equipment according to the present embodiment when cut in the depth direction of the ladle. FIG. 2B is a cross-sectional view schematically showing a cross-section of the ladle refining equipment according to the present embodiment when cut horizontally at the position of the molten steel height H. FIG. 3A to 8 are explanatory diagrams for explaining the ladle refining method according to the present embodiment.

<About ladle refining equipment>
First, with reference to FIGS. 1 to 2B, the ladle refining equipment used in the ladle refining method for molten steel according to the present embodiment (hereinafter also simply referred to as “the ladle refining method”) will be described. In the following description, the coordinate system shown in FIGS. 1 to 2B will be used for convenience.

The ladle refining equipment used in the ladle refining method according to the present embodiment has at least a ladle 10 with a predetermined capacity, as schematically shown in FIG. The size (capacity) of the ladle 10 is not particularly limited, and various known ladles can be used.

In addition, one porous plug 20 as an example of a gas blowing plug is provided on the bottom surface of the ladle 10 . The porous plug 20 is used as a gas outlet for stirring the molten steel 11 by blowing a predetermined inert gas into the molten steel held inside the ladle 10 . Various known porous plugs can be used as the porous plug 20 as long as the gas flow rate described in detail below can be achieved.

In addition, in this embodiment, for example, as shown in FIG. 1, the shape of the ladle 10 is schematically illustrated, but the detailed structure of the ladle is not particularly limited. For example, the ladle 10 used in the ladle refining method according to the present embodiment may have a molten steel outlet for taking out the molten steel after the secondary refining is finished, or may have other structures. may be provided.

Molten steel 11 is held inside the ladle 10, and a slag layer 13 containing CaO, SiO ₂ , Al ₂ O ₃ , FeO, etc. is formed on the surface of the molten steel 11 (the surface on the positive z-axis side) exists in a floating state. Also, the slag layer 13 may contain various fluxes (slag-forming agents) added in the ladle refining process. Such a slag layer 13 is sometimes called a flux layer.

Here, in the present embodiment, as schematically shown in FIG. 1, the position of the bottom surface of the ladle 10 is conveniently the position of the origin (z=0) in the z-axis direction. Further, the height H of the molten steel 11, as schematically shown in FIG. and

In addition, the radius of the ladle 10 at the position of the bottom (z = 0) of the ladle 10 to be used is represented as R [m], and the inner diameter of the ladle 10 at the molten steel surface (z = H) is Ds [m ], and the thickness of the slag layer 13 is represented as d [mm].

When applying the ladle refining method according to the present embodiment to the molten steel 11 held in the ladle 10 , two or three electrodes are immersed inside the slag layer 13 . FIGS. 2A and 2B illustrate the case where three electrodes 30A, 30B, and 30C (hereinafter collectively referred to as “electrodes 30”) are immersed inside the slag layer 13, 2A and 2B, the number of electrodes may be two.

The electrode 30 used in the ladle refining method according to the present embodiment is not particularly limited, and electrodes using various known materials can be used, but carbon electrodes (carbon electrodes) It is convenient to use Also, the shape and size of the electrode 30 are not particularly limited, and various known electrodes can be appropriately used.

However, it is preferable that the electrode 30 is immersed in the slag layer 13 to a depth that does not contact the molten steel 11 . In particular, when a carbon electrode is used as the electrode 30, if the electrode 30 comes into contact with the molten steel 11, the heat of the molten steel 11 may melt the carbon electrode. When the carbon electrode melts, the molten steel 11 may be mixed with the dissolved carbon and the carbon content of the molten steel 11 may change. Moreover, even when electrodes made of other materials are used, if the electrodes come into contact with the molten steel 11, there is a possibility that the electrodes will melt. Therefore, by immersing the electrode 30 to such a depth that it does not come into contact with the molten steel 11 , it is possible to prevent impurities from entering the molten steel 11 .

A power supply is installed as shown in FIG. 2A for the electrode 30 immersed in the slag layer 13, and a predetermined power is applied to the electrode 30, so that a Arc plasma is generated, and the generated arc plasma is energized through the molten steel 11 . The slag is heated and melted by the heat generated by the arc plasma and energization, and various refining reactions proceed between the molten steel 11 and the slag layer 13 .

In addition to the energization as described above, a flow is generated in the molten steel 11 by discharging an inert gas such as argon from the porous plug 20 provided at the bottom of the ladle at a flow rate as described in detail below. , the molten slag moves on the surface of the molten steel on the flow of the molten steel. As a result, it is possible to promote the melting of the unmelted slag while preventing gas bubbles from breaking on the surface of the molten steel, and the molten slag covers the entire surface of the molten steel 11 . As a result, contact between the nitrogen gas present in the atmosphere inside the ladle 10 and the molten steel 11 can be cut off. As a result, the reaction rate of the nitriding reaction in the molten steel 11 can be reduced more reliably.

As described above, the arc plasma generated between the electrode 30 and the molten steel 11 is used to heat and melt the slag. is preferably immersed in the slag layer 13 to a depth that can reach the molten steel 11 .

<Details of the ladle refining method>
Next, the ladle refining method according to this embodiment will be described in detail with reference to FIGS. 2A to 8. FIG.

In the ladle refining method according to the present embodiment, two or three electrodes 30 are immersed in the slag layer 13 existing in the ladle 10, as shown in FIGS. 2A and 2B. In addition, below, the case where three electrodes 30A, 30B, and 30C are immersed with respect to the slag layer 13 shall be mentioned as an example, and shall be demonstrated.

[Ratio between ladle inner diameter and electrode circumscribed circle diameter]
In the ladle refining method according to the present embodiment, for example, as schematically shown in FIG. 2B, attention is paid to the electrode circumscribed circle 35 defined based on the immersion positions of the two or three electrodes 30 . This electrode circumscribing circle 35 is two or three lines on the surface of the molten steel 11 when viewed from the surface of the molten steel 11 (z=H plane) from above the ladle 10 (positive side in the z-axis direction). It is a circle that circumscribes the position of the electrode 30 or the projection position of the two or three electrodes 30 onto the surface of the molten steel 11 and has the smallest diameter.

In the ladle refining method according to the present embodiment, the center of the electrode circumscribed circle 35 on the surface of the molten steel 11 is the ladle on the surface of the molten steel 11 when the radius of the ladle at the bottom of the ladle is R [m]. 10 (point A in FIG. 2B) to 0.1×R, and the positional relationship between the electrode and the ladle is usually set based on this requirement. By positioning the center of the electrode circumscribed circle 35 within the above region, the slag layer 13 present in the ladle 10 can be heated more evenly while suppressing uneven heat transfer. Become.

The positions of the electrodes 30A, 30B, and 30C within the electrode circumscribing circle 35 are not particularly limited, but they are preferably arranged as evenly as possible with respect to the center of the electrode circumscribing circle 35. FIG.

Here, the temperature of the slag in the ladle decreases as it approaches the inner wall surface of the ladle, and unmelted slag tends to remain. Therefore, based on the knowledge mentioned above, the present inventors have found that in order to efficiently heat and melt the slag, the melting of the slag is promoted by electric heating, and the ladle wall surface is heated by the radiant heat from the electrode. By doing so, it has been found that it is important to heat the entire surface of the molten steel. In order to efficiently heat and melt the slag in the electrode heating area, which is the area where the slag is heated by the electrode, and to transmit the radiant heat from the electrode to the ladle wall surface, the inner diameter of the ladle and the immersion position of the electrode , is important to set appropriately. Therefore, the present inventors have investigated the relationship between the minimum diameter D of the electrode circumscribing circle 35 and the inner diameter Ds of the ladle 10 on the surface of the molten steel 11 (at the position of z=H). We focused on ladles 10 having various inner diameters Ds (Ds: 2.5 to 4.7 m) within the inner diameter range. In addition, the input power and energization time to the electrode 30 per unit amount of molten steel were set to 0.5 to 2.0 kW / t and 3 to 60 minutes, respectively, considering a general ladle refining process. I have done a thorough review on the above. As a result, when the ratio (Ds/D) of the diameter of the electrode circumscribing circle D to the inner diameter Ds of the ladle satisfies the following formula (101), in the electrode heating region, which is the region where the slag is heated by the electrode It has been found that the slag can be efficiently heated and melted, and the radiant heat from the electrode can be transferred to the ladle wall surface.

1.8 ≤ Ds/D ≤ 3.5 Expression (101)

When the electrodes 30A, 30B, and 30C immersed in the slag layer 13 are powered, the area inside the electrode circumscribed circle 35 becomes the electrode heating area R1, as schematically shown in FIG. 3A. When the ratio (Ds/D) satisfies the above formula (101), the size of the electrode heating region R1 becomes an appropriate size with respect to the inner diameter Ds of the ladle 10, and the inside of the electrode heating region R1 The slag existing in is efficiently heated and melted. In addition, the radiant heat from the electrodes 30A, 30B, and 30C is reliably propagated to the inner wall of the ladle 10, and the slag in the region near the inner wall of the ladle 10 (region R2 in FIG. 3A) is heated. Remaining molten slag (slag lumps) can be suppressed. As a result, the refining reaction occurs more easily between the molten steel and the slag, and various refining reaction speeds can be improved.

In addition, by discharging the inert gas from the porous plug 20, a molten steel flow FL is generated, for example, as schematically shown in FIG. do. The slag melting promotion effect and the molten slag flow associated with the optimization of the ratio (Ds/D) as described above function synergistically, and for example, as schematically shown in FIG. The surface becomes covered with molten slag 15 .

Here, when the ratio (Ds/D) exceeds 3.5, as schematically shown in FIG. , the diameter D of the electrode circumscribed circle 35 becomes relatively too small. As a result, the range of the electrode heating region R1 is narrowed, and the proportion of slag existing within the electrode heating region R1 is reduced. As a result, the amount of slag heated by electrical heating is reduced, and the entire slag cannot be heated appropriately. In addition, since the distance between the electrode 30 and the inner wall surface of the ladle increases, the radiant heat from the electrode is less likely to propagate to the inner wall surface of the ladle, and the temperature of the slag in the region R3 near the inner wall surface of the ladle increases. As a result, unmelted slag (slag mass) is generated.

Also, when the ratio (Ds/D) is less than 1.8, as schematically shown in FIG. , the diameter D of the electrode circumscribed circle 35 becomes relatively too large. As a result, the range of the electrode heating region R1 is widened, and the temperature deviation in heating the slag between the electrodes 30 becomes conspicuous. Unmelted slag (slag lumps) may remain in the region R4 schematically shown in FIG.

In addition, since the part where the slag has become low temperature and the part where unmelted slag remains are more black than the slag in the molten state, it is easy to visually check whether or not such a part exists. can be judged.

In order to more reliably realize efficient slag heating in the electrode heating region and suppression of unmelted slag in the region near the inner wall surface of the ladle, the ratio (Ds / D) is 2.0. It is preferably 2.1 or more, more preferably 2.1 or more. In addition, in order to more reliably realize efficient slag heating in the electrode heating region and suppression of unmelted slag in the region near the inner wall surface of the ladle, the ratio (Ds / D) is 3 0.2 or less, and more preferably 3.0 or less.

Here, the inner diameter of the ladle on the surface of the molten steel 11 is determined by actually measuring the inner diameter of the ladle 10 after the molten steel 11 and the slag layer 13 are tapped inside the ladle 10 and allowed to stand still. can do. Also, the diameter of the electrode circumscribed circle 35 can be determined from the geometric arrangement of the electrodes 30 .

[Position of central axis of porous plug 20]
In the ladle refining method according to the present embodiment, the central axis of the porous plug 20 on the surface of the molten steel 11 (at z=H) is on the electrode circumscribing circle 35 or inside the electrode circumscribing circle 35 (in other words, within the electrode heating area). By satisfying these conditions, the distance between the position of the central axis of the porous plug 20 on the surface of the molten steel 11 and the center of the electrode circumscribing circle 35 becomes a value equal to or less than the radius of the electrode circumscribing circle 35 .

When the center axis of the porous plug 20 is positioned only outside the electrode circumscribed circle 35 on the surface of the molten steel 11, the direction in which the molten slag 15 is conveyed is limited to a specific direction, as schematically shown in FIG. As a result, a region where the molten slag 15 does not cover the surface of the molten steel 11 is generated. As a result, as schematically shown in FIG. 6, a surface where the molten steel 11 is exposed is generated, and the molten steel 11 comes into contact with nitrogen in the atmosphere.

The position of the central axis of the porous plug 20 inside the electrode circumscribing circle 35 is not particularly limited. , or may be positioned near the circumference of the electrode circumscribed circle 35 . When the position of the central axis of the porous plug 20 is at or near the center of the electrode circumscribed circle 35, it is possible to prevent the absorption of nitrogen more efficiently. If it is on the side closer to the circumference of the electrode circumscribing circle 35, it becomes possible to stir the molten steel 11 more efficiently.

Here, the position of the central axis of the porous plug 20 on the surface of the molten steel 11 is geometrically specified from the installation position and installation angle of the porous plug 20 on the bottom surface of the ladle 10 and the height H of the molten steel 11. It is possible to

[Flow rate of stirring gas]
In the ladle refining method according to the present embodiment, the flow rate Q of the inert gas as an example of the stirring gas discharged from the porous plug 20 is set to normal liters per minute per ton of molten steel [NL/min/t ], the flow rate Q needs to be 0.3 NL/min/t or more and 4.5 NL/min/t or less. By setting the flow rate Q of the inert gas within the above range, the molten steel 11 does not break on the surface of the molten steel 11 (at z = H), and the molten steel 11 flows properly into the ladle 10. can be generated throughout the As a result, the entire surface of the molten steel 11 can be covered with molten slag generated by electric heating, and contact between the molten steel 11 and nitrogen gas in the atmosphere can be more reliably suppressed. Thereby, the occurrence of nitrogen absorption reaction in the molten steel 11 can be suppressed more reliably.

When the flow rate Q of the inert gas is less than 0.3 NL/min/t, the flow rate of the inert gas discharged into the molten steel 11 is too small, as schematically shown in FIG. It is not possible to create a flow that carries molten slag. As a result, a portion where the molten steel 11 and the nitrogen gas in the atmosphere can come into contact remains, and the absorption of nitrogen gas in the molten steel 11 progresses. The inert gas flow rate Q is preferably 1.0 NL/min/t or more, more preferably 1.4 NL/min/t or more.

On the other hand, when the flow rate Q of the inert gas exceeds 4.5 NL/min/t, as schematically shown in FIG. As a result of the exposure of 11 to the atmosphere, nitrogen gas in the atmosphere is involved in the molten steel 11 . As a result, the occurrence of nitrogen absorption reaction in the molten steel 11 cannot be suppressed. The inert gas flow rate Q is preferably 3.0 NL/min/t or less, more preferably 2.5 NL/min/t or less.

It should be noted that the height of the molten steel 11 (height H in FIG. 1), the size (area) of the inert gas bubbles on the surface of the molten steel 11, and the magnitude of the impact force when the bubbles break, must be within a predetermined range. A relationship is established. That is, as the height H of the molten steel 11 increases, the size of the inert gas bubbles on the surface of the molten steel 11 increases, but the impact force when the bubbles break becomes relatively small, and the height H of the molten steel 11 increases. The lower the temperature, the smaller the size of the inert gas bubbles on the surface of the molten steel 11, but the greater the impact force when the bubbles break. However, by setting the flow rate Q of the inert gas within the range of 0.3 NL/min/t or more and 4.5 NL/min/t or less, the molten steel 11 at the height H of the molten steel 11 in general operation An appropriate flow of the molten steel 11 can be generated without causing bubble breakage to the extent that it comes into contact with.

Here, the flow rate Q of the inert gas is controlled to a desired value by controlling the opening and closing of various valve elements such as valves installed in the piping that supplies the inert gas to the porous plug 20. It is possible.

In addition, the inert gas blowing time (which can also be regarded as the stirring time of the molten steel 11) is not particularly limited, but is preferably 2 minutes or more and 60 minutes or less, for example. By setting the blowing time of the inert gas within the above range, the entire surface of the molten steel 11 can be more reliably covered with the molten slag.

[Thickness of slag layer 13]
In the ladle refining method according to the present embodiment, the thickness d of the slag layer 13 schematically shown in FIG. 1 is preferably 100 mm or more. Here, the thickness d of the slag layer 13 is measured after the molten steel 11 and the slag layer 13 are tapped inside the ladle 10 and allowed to stand still, and flux is added as necessary, before energization and inert gas blowing. is the thickness of the slag layer 13 in

By setting the thickness d of the slag layer 13 to 100 mm or more, the exposure of the molten steel 11 to the atmosphere can be more reliably suppressed, and the occurrence of nitrogen absorption reaction in the molten steel 11 can be more reliably prevented. The upper limit of the thickness d of the slag layer 13 is not particularly specified. However, the thickness d of the slag layer 13 is preferably 250 mm or less from the viewpoint of ensuring ease of operation such as suppression of slag foaming.

As described above, according to the ladle refining method for molten steel according to the present embodiment, the slag that is heated and melted directly under the electrode is suppressed from resolidifying, and the surface flow of the molten steel due to bottom-blown stirring causes It covers the entire surface of the molten steel and blocks the reaction between the molten steel and nitrogen gas in the atmosphere mixed in the lid. As a result, in the ladle refining method for molten steel according to the present embodiment, the speed of the nitrogen absorption reaction can be reduced, and in the ladle refining involving electric heating, the occurrence of the absorption nitrogen reaction can be more reliably suppressed. It becomes possible.

The ladle refining method for molten steel according to the present embodiment has been described above in detail.

Hereinafter, the method for ladle refining of molten steel according to the present invention will be specifically described while showing examples of the present invention and comparative examples. The examples of the present invention shown below are merely examples of the ladle refining method for molten steel according to the present invention, and the ladle refining method for molten steel according to the present invention is not limited to the examples shown below.

First, 80 to 90 tons of molten steel decarburized in a converter was tapped into a ladle. At this time, about 500 kg of converter slag composed of CaO, SiO ₂ , Al ₂ O ₃ , FeO, etc. flowed out. During tapping, an alloy such as Al as a deoxidizing element and a slag forming agent (flux) mainly composed of CaO were added so that the slag thickness d would be 50 to 250 mm. The inner diameter (D _S ) of the ladle at the surface of the molten steel was 2.8 m.

The ladle was transferred to the treatment position where the electric heat treatment was performed. The Al concentration in molten steel at the start of electric heating is 0.03 to 0.10% by mass, the S concentration is 0.0020 to 0.0050% by mass, and the N concentration is 0.0020 to 0.005%. 0024% by mass.

After transferring the ladle to the treatment position where the energization heat treatment is performed, the container lid is attached to the ladle, and the three electrodes for energization with a diameter of 320 mm are placed in a positional relationship as schematically shown in FIG. 2B. It descended into the slag layer on the molten steel surface so as to become. An inert gas (Ar) was introduced from the porous plug at the bottom of the ladle, and heat treatment by energization was started while stirring. Here, the molten steel height (bath depth) H was about 2.0 m. Here, in this heat treatment, the input electric power per unit amount of molten steel was set to 1.0 to 1.1 kW/t, and the energization time and gas stirring time were set to 20 minutes. In addition, for comparison, a case where no energization was performed was also conducted.

In this embodiment, the radius R [m] of the ladle at the bottom of the ladle is 1.2, and the center of the electrode circumscribed circle 25 on the surface of the molten steel 11 is the radius of the ladle 10 on the surface of the molten steel 11. It is located within a region of 0 to 0.1×R [m] from the center position (point A in FIG. 2B).

Samples were collected before and after energization, and the N concentration after energization and stirring was evaluated. In this evaluation, No. shown below. Other conditions were indexed with the increment (nitrogen absorption amount) of nitrogen concentration (% by mass) before and after energization in the 11 comparative examples being 1.0. The index evaluation criteria are as follows.
A: Index less than 0.72 B: Index 0.72 or more and less than 0.94 (N concentration is higher than A evaluation but practical range)
C: index of 0.94 or more

The results obtained are shown in Table 1 below. Note that the underlined parameters in the table are out of the scope of the present invention.

As shown in Table 1, no. 1 to No. 4 was rated as "B". In particular, Test No. in which the thickness d of the slag layer is 100 mm or more. 5, No. No. 6, D _S /D is within the range of 2.0 to 3.2. 7, No. No. 8, and No. 8 with a stirring gas flow rate in the range of 1.0 to 3.0 NL/min/t. 9, No. No. 10 has an evaluation of the amount of nitrogen absorption of "A". 1 to No. The amount of nitrogen absorption was lower than in the case of 4.

On the other hand, No. 1, whose test conditions were outside the scope of the present invention. 11 to No. No. 16 had a nitrogen absorption rating of "C". The following are No. 11 to No. This is the reason why the amount of nitrogen absorption was high under each of the 16 conditions.

No. Comparative Example No. 11 has a small D _S /D and a large area to be electrically heated (electrode heating area R1). Therefore, as schematically shown in FIG. As a result, an unmelted portion was generated in the slag, and slag lumps remained.

No. Comparative Example No. 12 has a large D _S /D and a small area (electrode heating area R1) to be electrically heated. Therefore, as schematically shown in FIG. decreased, the amount of slag heated decreased, and the molten slag did not reach the vicinity of the ladle wall surface.

No. In Comparative Example 13, as schematically shown in FIG. 6, since the porous plug was arranged outside the circumscribed circle of the electrode, the direction in which the molten slag was conveyed was limited to one direction. An uncovered area was created.

No. In Comparative Example No. 14, since the stirring gas flow rate was low, as schematically shown in FIG. 7, it was not possible to form a flow that conveys the molten slag, and the entire slag existing on the surface of the molten steel could not be melted.

No. In Comparative Example No. 15, since the flow rate of the stirring gas was large, as schematically shown in FIG. 8, the stirring gas bubbles burst on the surface of the molten steel, nitrogen was caught in the molten steel, and the absorption of nitrogen could not be suppressed. rice field.

No. In Comparative Example No. 16, slag melting did not progress and slag lumps remained because no electrical heating was performed.

The above examples are the results of examination using a ladle with a ladle inner diameter Ds of 2.8 m. Based on this knowledge, even when using a ladle with an inner diameter Ds of 2.5 to 3.0 m, it is considered that the same tendency as in this example can be obtained.

Also, when the inner diameter Ds of the ladle was 4.7 m (the amount of molten steel was increased to 350 tons as the inner diameter of the ladle was increased), the same tendency as in this example was obtained. Therefore, the present invention is considered to be effective at least when the inner diameter Ds of the ladle is in the range of 2.5 to 4.7 m.

Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention belongs can conceive of various modifications or modifications within the scope of the technical idea described in the claims. It is understood that these also naturally belong to the technical scope of the present invention.

10 ladle
REFERENCE SIGNS LIST 11 molten steel 13 slag layer 15 molten slag 20 porous plug 30 electrode 35 electrode circumscribed circle

Claims (2)

In a ladle refining method for molten steel in which a slag layer is formed on the surface of the molten steel in the ladle, and an electrode is immersed in the slag layer and energized,
The electrodes immersed in the slag layer are three ,
A gas blowing plug for blowing a stirring gas for stirring the molten steel held in the ladle is provided on the bottom surface of the ladle,
When the ladle inner diameter on the molten steel surface is Ds [m], the ladle inner diameter Ds is 2.5 to 4.7 m,
When the molten steel surface is viewed from above the ladle, circumscribe the positions of the three electrodes on the surface of the molten steel or the projected positions of the three electrodes on the surface of the molten steel, In addition, regarding the electrode circumscribing circle, which is the circle with the smallest diameter , the center of the electrode circumscribing circle on the surface of the molten steel is the radius of the ladle at the bottom of the ladle R [m], the molten steel Located within a region from the center position of the ladle on the surface to 0.1 × R,
When the diameter of the electrode circumscribed circle is D [m], the ratio of the diameter of the electrode circumscribed circle to the inner diameter of the ladle (Ds / D) satisfies the following formula (1),
on the surface of the molten steel, the central axis of the gas injection plug passes on the electrode circumscribed circle or inside the electrode circumscribed circle,
A ladle refining method for molten steel, wherein the flow rate of the stirring gas satisfies the following formula (2), where Q [NL/min/t] is the flow rate of the stirring gas.

1.8 ≤ Ds/D ≤ 3.5 Expression (1)
0.3 ≤ Q ≤ 4.5 Expression (2)
The ladle refining method of molten steel according to claim 1, wherein the thickness of the slag layer is 100 mm or more.

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