JP7139878B2 - 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 slag layer is formed on the surface of molten steel in a ladle, an electrode is immersed in the slag layer and energized, and gas is blown into the molten steel from a gas blowing plug placed at the bottom of the ladle. In the molten steel ladle refining method,
The number of the electrodes immersed in the slag layer is three ,
two said gas injection plugs are placed at the bottom of said ladle,
The ladle inner diameter Ds [m] on the surface of the molten steel is 2.5 to 4.7 m, and when the surface of the molten steel is viewed from above the ladle, the three electrodes on the surface of the molten steel When the diameter 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 D [m], the electrode circumscribes The ratio D _S /D of the ladle inner diameter D _s to the circle diameter D satisfies the following formula (1),
When the inner bottom surface of the ladle is viewed from above the ladle,
Each of the centers of the two gas injection plugs is located outside the circumscribed circle of the electrode projected on the inner bottom surface of the ladle,
Let the center of the electrode circumscribed circle projected on the inner bottom surface of the ladle be the point O, and the center of the first gas injection plug of the two gas injection plugs at the inner bottom surface of the ladle A straight line OA passing through points O and A when the center of the second gas blowing plug of the two gas blowing plugs on the inner bottom surface of the ladle is set to point B, and , the angle θ formed by the straight line OB passing through the point O and the point B is 90 degrees or more and 180 degrees or less,
When the radius of the inner bottom surface of the ladle is R, at the inner bottom surface of the ladle, the distance between the center of each of the two gas injection plugs and the inner wall surface of the ladle is 0.1R and
The center of the electrode circumscribed circle on the surface of the molten steel is located within a region of 0.1 × R from the center position of the ladle on the surface of the molten steel,
The flow rate _PA of the gas blown into the molten steel from the first gas blowing plug is 0.3 NL/min/t or more and 4.5 NL/min/t or less,
The flow rate _PB of the gas injected into the molten steel from the second gas injection plug is 0.3 NL/min/t or more and 4.5 NL/min/t or less,
The ratio P _A /P _B of the flow rate P _A to the flow rate P _B is a molten steel ladle refining method that satisfies the following formula (2) (however, the bottom inner diameter of the ladle is 3968 mm, and the circle passing through the center of the electrode except when the diameter is 1300 mm) .
1.8≦Ds/D≦3.5 Formula (1)
2.00/3.20 ≤ P _A /P _B ≤ 3.20/2.00 Formula (2)
[2] The ladle refining method for molten steel according to [1], wherein the thickness of the slag layer is 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 sectional drawing which showed typically the horizontal cross section in the molten-steel height H of the ladle refining equipment which concerns on 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. 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. Also, the ratios and dimensions of each component in the drawings do not represent the actual ratios and dimensions of each component.

(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 12. FIG. FIG.1 and FIG.2 is sectional drawing which showed typically the cross section at the time of cutting the ladle refining equipment which concerns on this embodiment in the depth direction of a ladle. FIG. 3 is a cross-sectional view schematically showing a horizontal cross section at the molten steel height H of the ladle refining equipment according to the present embodiment. 4 to 12 are explanatory diagrams for explaining the ladle refining method according to the present embodiment.

<About ladle refining equipment>
First, with reference to FIGS. 1 and 2, 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 addition, below, the coordinate system shown in FIG.1 and FIG.2 shall be used for description for convenience.

The ladle refining equipment 1 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.

Also, two porous plugs 20 as an example of gas injection plugs are provided at the bottom of the ladle 10 . Such a porous plug 20 is used as a gas discharge port for blowing a predetermined inert gas into the molten steel 30 held inside the ladle 10 to stir the molten steel 30 . 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 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 completed, or may have other structures. may be provided.

Molten steel 30 is held inside the ladle 10, and a slag layer 40 containing CaO, SiO ₂ , Al ₂ O ₃ , FeO, etc. is formed on the surface of the molten steel 30 (surface on the z-axis positive direction side). It exists in a floating state. Also, the slag layer 40 may contain various fluxes (slag-forming agents) added in the ladle refining process. Such a slag layer 40 is sometimes called a flux layer.

Here, in the present embodiment, as schematically shown in FIG. 1, the position of the bottom surface E of the ladle 10 is conveniently the position of the origin (z=0) in the z-axis direction. In addition, as schematically shown in FIG. 1, the height H of the molten steel 30 is the position of the surface of the molten steel 30 after the molten steel 30 and the slag layer 40 are tapped inside the ladle 10 and allowed to stand still. and

In addition, the radius of the ladle 10 at the position of the bottom E (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 40 is d [mm].

When applying the ladle refining method according to the present embodiment to the molten steel 30 held in the ladle 10 , two or three electrodes 50 are immersed inside the slag layer 40 . In this embodiment, three electrodes 50 are immersed inside the slag layer 40, but the number of electrodes 50 may be two.

The electrode 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) are used. It is convenient to use. Also, the shape and size of the electrodes are not particularly limited, and various known electrodes can be appropriately used.

However, it is preferable that the electrode 50 is immersed in the slag layer 40 to a depth such that the electrode 50 does not come into contact with the molten steel 30 . In particular, when a carbon electrode is used as the electrode 50, if the electrode 50 comes into contact with the molten steel 30, the heat of the molten steel 30 may melt the carbon electrode. When the carbon electrode melts, the molten steel 30 may be mixed with the dissolved carbon and the carbon content of the molten steel 30 may change. Further, even when electrodes made of other materials are used, if the electrodes come into contact with the molten steel 30, there is a possibility that the electrodes will melt. Therefore, by immersing the electrodes to such a depth that they do not come into contact with the molten steel 30, it is possible to prevent impurities from entering the molten steel 30.

By applying a predetermined power to the electrode 50 immersed in the slag layer 40, arc plasma is generated between the tip of the electrode 50 and the molten steel 30, and the generated arc plasma is generated through the molten steel 30. is energized. 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 30 and the slag layer 40 .

In addition to the above-described energization, 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, as shown in FIG. , inert gas bubbles 60 are generated in the molten steel 30 . A bubble rising region 61 where the bubbles 60 rise changes depending on the flow rate of the inert gas from the porous plug 20 . These bubbles 60 cause a flow in the molten steel 30, and, for example, the molten slag moves on the surface of the molten steel along with the flow of the molten steel as indicated by the arrows in FIG. 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 30 . As a result, contact between the nitrogen gas present in the atmosphere inside the ladle 10 and the molten steel 30 can be cut off. As a result, the reaction rate of the nitriding reaction in the molten steel 30 can be reduced more reliably.

As described above, the arc plasma generated between the electrode 50 and the molten steel 30 is used to heat and melt the slag. is preferably immersed in the slag layer 40 to a depth that allows it to reach the molten steel 30 .

<Details of the ladle refining method>
Next, the ladle refining method according to the present embodiment will be described in detail with reference to FIGS. 3 to 12. FIG.

In the ladle refining method according to this embodiment, two or three electrodes 50 are immersed in the slag layer 40 existing in the ladle 10, as shown in FIGS. In addition, below, the case where the three electrodes 50 are immersed with respect to the slag layer 40 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. This electrode circumscribed circle C is two or three lines on the surface of the molten steel 30 when viewed from the surface of the molten steel 30 (z = H plane) from above the ladle 10 (positive direction in the z-axis direction). It is a circle that circumscribes the position of the electrode 50 or the projection position of the two or three electrodes 50 onto the surface of the molten steel 30 and has the smallest diameter.

In the ladle refining method according to the present embodiment, the center of the electrode circumscribed circle C on the surface of the molten steel 30 is the ladle 10 on the surface of the molten steel 30 when the radius of the bottom surface E of the ladle is R [m]. It is preferably positioned within a region from the center position of to 0.1×R. The electrodes are usually arranged so that the positional relationship between the electrodes and the ladle satisfies this requirement. By positioning the center of the electrode circumscribed circle C within the above region, the slag layer 40 present in the ladle 10 can be heated more evenly while suppressing uneven heat transfer. Become.

Further, the positions of the two or three electrodes 50 on the electrode circumscribed circle C are not particularly limited, but it is preferable that they are arranged as evenly as possible with respect to the center of the electrode circumscribed circle C. preferable.

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 that should be satisfied between the minimum diameter D of the electrode circumscribed circle C and the inner diameter Ds of the ladle 10 on the surface of the molten steel 30 (at z=H). Focusing on the ladle 10 having various inner diameters Ds (Ds: 2.5 to 4.7 m) within the inner diameter range, the input power and energization time to the electrode 50 per unit amount of molten steel are In consideration of the pot refining process, we made a thorough study with 0.5 to 2.0 kW/t and 3 to 60 minutes, respectively. As a result, when the ratio (Ds/D) of the diameter D of the electrode circumscribing circle 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 We have found that it is possible to efficiently heat and melt the slag while transferring the radiant heat from the electrode to the ladle wall surface.

1.8 ≤ Ds/D ≤ 3.5 Expression (101)

When power is applied to the electrode 50 immersed in the slag layer 40, the slag and molten steel in the region inside the electrode circumscribed circle C are heated. When the ratio (Ds/D) satisfies the above formula (101), the size of the region (heating region) where the slag and molten steel are heated becomes an appropriate size with respect to the inner diameter Ds of the ladle 10. , the slag is efficiently heated and melted. In addition, the radiant heat from the two or three electrodes 50 reliably propagates to the inner wall of the ladle 10, heats the slag in the area near the inner wall of the ladle 10, and unmelted slag (slag mass ) 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.

Further, by discharging the inert gas from the porous plug 20 described later, for example, a molten steel flow as schematically shown by the arrow in FIG. to flow. The slag melting promotion effect and the molten slag flow associated with the optimization of the ratio (Ds/D) as described above function synergistically so that the surface of the molten steel 30 is coated with the slag layer 40. Become.

Here, when the ratio (Ds/D) is less than 1.8, as schematically shown in FIG. Therefore, the diameter D of the electrode circumscribed circle C1 becomes relatively too large. As a result, the range of the heating area is widened, and the temperature deviation in heating the slag between the electrodes 50 becomes remarkable. As schematically shown in FIG. 5, the slag in the area between the electrodes A relatively low-temperature low-temperature slag 42A is formed, and unmelted slag (slag mass) is generated in the low-temperature slag 42A.

Also, when the ratio (Ds/D) exceeds 3.5, as schematically shown in FIG. The diameter D of the electrode circumscribed circle C2 becomes relatively too small. As a result, the range of the heating region is narrowed, the amount of slag heated by electric heating is reduced, and the entire slag cannot be heated appropriately. In addition, since the distance between the electrode 50 and the inner wall surface of the ladle 10 increases, the radiant heat from the electrode 50 is less likely to propagate to the inner wall surface of the ladle 10, and the temperature near the inner wall surface of the ladle 10 decreases. Then, the slag positioned near the inner wall becomes the low-temperature slag 42B, and unmelted slag (slag mass) is generated in the low-temperature slag 42B.

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 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 or more. and 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 10 on the surface of the molten steel 30 is obtained by actually measuring the inner diameter of the ladle 10 after the molten steel 30 and the slag layer 40 are tapped inside the ladle 10 and allowed to stand still. , can be specified. Also, the diameter and center of the electrode circumscribed circle C can be specified from the geometric arrangement of the electrodes 50 .

[Center position of porous plug 20]
In the ladle refining method according to this embodiment, a plurality of porous plugs 20 are provided at the bottom of the ladle 10 . In the ladle refining method according to the present embodiment, as schematically shown in FIG. It is assumed that the center A of the first porous plug 20A and the center B of the second porous plug 20B are positioned outside the circumscribed circle C of the electrode. The center O of the electrode circumscribed circle C projected onto the bottom surface E of the ladle 10 from the vertical direction and the center A of the first porous plug 20A shown in FIG. Let r _A /(D/2) be the ratio of the distances OA (r _A ). Similarly, for example, the ratio of the distance OB (r _B ) between the center O and the center _B of the second porous plug 20B shown in FIG. (D/2). When both r _A /(D/2) and r _B /(D/2) are greater than 1, both the center of the first porous plug 20A and the center of the second porous plug 20B are outside the electrode circumscribed circle. means that it is located in

When the above-described first porous plug 20A and second porous plug 20B are arranged at the above-described positions, when viewed from above the ladle 10, a straight line OA passing through the center O and the center A is the bottom surface E of the ladle 10 and a straight line OB passing through the center O and the center B, the first porous plug 20A and the second porous plug 20B must be positioned so that the angle θ between them is 90 degrees or more and 180 degrees or less. In other words, the first porous plug 20A and the second porous plug 20B are located on different sides of the center A and the center B when the bottom surface E of the ladle 10 is divided by a straight line passing through the center O and perpendicular to the straight line OA. are arranged to exist in the By positioning the first porous plug 20A and the second porous plug 20B so that the angle θ is 90 degrees or more and 180 degrees or less, for example, as shown in FIG. The molten steel formed by the inert gas and the molten steel formed by the inert gas blown from the second porous plug 20B flow toward the electrode 50, and the molten steel melts in the electrode heating region near the electrode 50 and in the vicinity thereof. The currents will collide. Since the flux is melted in the region where this molten steel flow collides, the molten slag is carried by the molten steel flow without staying. In addition to the downward flow of the molten steel in the ladle, the molten steel flow also occurs in a direction generally perpendicular to the traveling direction of the molten steel flow before collision (carry-out flow). The outflow, as schematically shown in FIG. 4, conveys the hot slag heated by the electrodes in a direction perpendicular to the line connecting the two gas injection plugs. Also, the carry-out flow collides with the inner wall of the ladle to create a flow along the inner wall of the ladle. Therefore, it becomes possible to supply the hot slag melted in the vicinity of the electrode to the entire surface of the molten steel. As a result, the slag is transported over the surface of the molten steel, and the surface of the molten steel is coated with the slag more quickly.

When the angle θ formed by the straight lines OA and OB is less than 90 degrees, the distance between the first porous plug 20A and the second porous plug 20B becomes short, and the molten steel flow around these porous plugs 20 becomes strong. As a result, as schematically shown in FIG. 8, the slag is biased to the side opposite to the position where the first porous plug 20A and the second porous plug 20B are arranged, and the electrode heating region near the electrode 50 Slag cannot be stably supplied. As a result, a molten steel exposed surface is produced in which the molten steel 30 is exposed without being covered with molten slag. As a result, the molten steel 30 and nitrogen in the atmosphere come into contact with each other on the exposed surface of the molten steel.

Further, when the radius of the bottom surface E of the ladle 10 is R, the distance between the center of the plurality of porous plugs 20 and the inner wall surface of the ladle 10 at the bottom surface E of the ladle 10 is 0.1R or more. For example, if the ratio of the distance (r _A ) between the center O and the center A of the first porous plug 20A at the bottom E of the ladle 10 to the radius R of the bottom E of the ladle is r _A /R, then r _A When /R is 0.9 or less, it means that the distance between the center A of the first porous plug 20A and the inner surface of the inner bottom surface of the ladle is 0.1R or more. Similarly for the second porous plug 20B, for example, the distance (r _B ) between the center O and the center B of the second porous plug 20B on the bottom surface E of the ladle 10 with respect to the radius R of the bottom surface E of the ladle Assuming that the ratio is r _B /R, when r _B /R is 0.9 or less, the distance between the center B of the second porous plug 20B and the inner surface of the inner bottom surface of the ladle is 0.1R. means greater than or equal to

When the distance between the center of the porous plug 20 and the inner surface of the ladle 10 at the bottom E of the ladle 10 is less than 0.1R, the inert gas bubbles discharged from the porous plug 20 float. Since it is disturbed by the inner wall of the pot 10, the flow of the molten steel 30 is disturbed. As a result, for example, unmelted slag in the low-temperature slag 42C moves on the surface of the molten steel 30, as schematically shown in FIG. When the unmelted slag moves, for example, a crack occurs in the unmelted slag mass, creating a gap. Resulting in.

In addition, for example, when a structure such as a hot water contact block that directly receives the tapping flow from the converter is provided at the bottom of the ladle, the porous plug is preferably provided so as to avoid the structure. For example, in the vicinity of the center of the bottom of the ladle 10, there is a case where a hot water contact block is arranged for receiving the stream of tapped steel from the converter. Therefore, the first porous plug 20A and the second porous plug 20B are arranged such that the central axis Ca and the central axis Cb are arranged such that the distance from the center O of the electrode circumscribed circle C is more than D/2×1.0 and D/2×1.1. If arranged so as to be located in the area below the hot water, the first porous plug 20A and the second porous plug 20B may overlap the position of the hot water contact block. Therefore, for example, the first porous plug 20A and the second porous plug 20B are the distance between the central axis Ca of the first porous plug 20A and the circumscribed circle center O, and the distance between the central axis Cb of the second porous plug 20B and the circumscribed circle center O are preferably arranged at positions where the distance between them is D/2×1.1 or more.

In this embodiment, the case of using two porous plugs has been described, but the number of porous plugs is not limited to two. Three or more porous plugs may be provided. For example, when three porous plugs are arranged, each porous plug is preferably arranged outside the circumscribed circle of the electrode and rotationally symmetrical with respect to the center of the ladle.

Here, as shown in FIG. It can be specified geometrically from the installation position and installation angle of the porous plug 20 on the bottom surface and the height H of the molten steel 30 .

[Flow rate of stirring gas]
In the ladle refining method according to the present embodiment, the flow rate P 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 ] as a unit. At this time, the flow rate P _A of the inert gas discharged from the first porous plug 20A and the flow rate P _B of the inert gas discharged from the second porous plug 20B are 0.3 NL/min/t or more and 4.5 NL. /min/t or less. The flow rate ratio P _A /P _B , which is the ratio of the flow rate P _A of the inert gas discharged from the first porous plug 20A to the flow rate P _B of the inert gas discharged from the second porous plug 20B, is given below. (102) must be satisfied.

0.61≦P _A /P _B ≦1.66 Expression (102)

When the flow rate P _A and the flow rate P _B are within the above ranges, the slag inside the electrode circumscribed circle C and The replacement of the slag outside the electrode circumscribing circle C is promoted, the slag heated inside the electrode circumscribing circle C spreads over the entire surface of the molten steel, and the relatively low-temperature slag existing outside the electrode circumscribing circle C C is transported inside. This allows a suitable molten steel 30 flow to occur throughout the ladle 10 . As a result, the entire surface of the molten steel 30 can be covered with molten slag generated by electric heating, and contact between the molten steel 30 and nitrogen gas in the atmosphere can be more reliably suppressed. Thereby, the occurrence of nitrogen absorption reaction in the molten steel 30 can be suppressed more reliably.

When the flow rate P _A of the inert gas discharged from the first porous plug 20A or the flow rate P _B of the inert gas discharged from the second porous plug 20B is less than 0.3 NL/min/t, As schematically shown in FIG. 10, the flow rate of the inert gas discharged into the molten steel 30 is too small to form a flow that conveys the molten slag, and the surface of the molten steel 30 is unmelted. of slag remains. As a result, there remains a portion where the molten steel 30 and the nitrogen gas in the atmosphere can come into contact with each other, and the nitriding reaction of the molten steel 30 progresses. The flow rate P _A and the flow rate P _B are 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 P _A of the inert gas discharged from the first porous plug 20A or the flow rate P _B of the inert gas discharged from the second porous plug 20B exceeds 4.5 NL/min/t , as schematically shown in FIG. 11, the bubble rising region 61 expands, the inert gas bubbles break on the surface of the molten steel 30, and the molten steel 30 is exposed to the atmosphere. As a result, nitrogen gas in the atmosphere is involved in the molten steel 30 . As a result, the occurrence of nitrogen absorption reaction in the molten steel 30 cannot be suppressed. The flow rate P _A and the flow rate P _B are preferably 3.0 NL/min/t or less, more preferably 2.5 NL/min/t or less.

Further, when the flow rate ratio P _A /P _B satisfies the formula (102), as described above, the molten steel flow formed by the inert gas blown from the first porous plug 20A and the second porous plug 20B The molten steel stream formed by the inert gas blown in from the electrode 50 collides with it in the vicinity of the electrode 50 . The flux is melted in the region where this molten steel flow collides, and the molten slag is carried by the molten steel flow without staying. In addition, the carry-out flow generated by the collision of the molten steel flow formed by the inert gas blown from the first porous plug 20A and the molten steel flow formed by the inert gas blown from the second porous plug 20B causes the electrode It becomes possible to supply hot slag melted in the vicinity to the entire molten steel surface. The flow ratio P _A /P _B is preferably 0.7 or more, more preferably 0.75 or more. Also, the flow rate ratio P _A /P _B is preferably 1.42 or less, more preferably 1.33 or less.

When P _A /P _B is less than 0.61, for example, the flow rate of the inert gas blown from the first porous plug 20A is relatively small with respect to the flow rate of the inert gas blown from the second porous plug 20B. . Therefore, the molten steel flow from the first porous plug 20A toward the electrode 50 becomes weaker than the molten steel flow from the second porous plug 20B toward the electrode 50, so that the two molten steel flows collide in the electrode heating region near the electrode 50. Instead, both molten steel streams collide outside the electrode heating area. As a result, as schematically shown in FIG. 12, the slag layer 40 is biased toward the bubble rising region 61G blown from the first porous plug 20A, and the slag is transported inside the electrode circumscribed circle C, which has a higher temperature. and unmelted slag remains.

When P _A /P _B exceeds 1.67, contrary to the case where P _A /P _B is less than 0.61, for example, the flow rate of the inert gas blown from the second porous plug 20B is the highest. It is relatively small compared to the flow rate of the inert gas blown from the one-porous plug 20A. Therefore, the molten steel flow from the second porous plug 20B toward the electrode 50 becomes weaker than the molten steel flow from the first porous plug 20A toward the electrode 50, so that the two molten steel flows collide in the electrode heating region near the electrode 50. Instead, both molten steel streams collide outside the electrode heating area. As a result, the slag layer 40 is biased toward the second porous plug 20B, and the slag is no longer transported inside the electrode circumscribing circle C where the temperature is higher, leaving unmelted slag.

Note that there is a predetermined distance between the height of the molten steel 30 (height H in FIG. 1), the size (area) of the inert gas bubbles 60 on the surface of the molten steel 30, and the magnitude of the impact force when the bubbles break. relationship is established. That is, as the height H of the molten steel 30 increases, the size of the inert gas bubbles 60 on the surface of the molten steel 30 increases, but the impact force when the bubbles break becomes relatively small. is smaller, the size of the inert gas bubble 60 on the surface of the molten steel 30 is smaller, but the impact force when the bubble is broken is relatively larger. However, if the flow rate P _A of the inert gas discharged from the first porous plug 20A or the flow rate P _B of the inert gas discharged from the second porous plug 20B is set to 0.3 NL/min/t or more and 4.5 NL/min. / t or less, at the height H of the molten steel 30 in general operation, the molten steel 30 does not cause bubble breakage to the extent that the molten steel 30 contacts the atmosphere, and an appropriate flow of the molten steel 30 is generated. be able to.

Here, the flow rate of the inert gas can be 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. is possible.

In addition, the inert gas blowing time (which can also be regarded as the stirring time of the molten steel 30) is not particularly limited, but is preferably set to 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 30 can be more reliably covered with the molten slag.

[Thickness of slag layer 40]
In the ladle refining method according to the present embodiment, the thickness d of the slag layer 40 schematically shown in FIG. 1 is preferably 100 mm or more. Here, the thickness d of the slag layer 40 is obtained by tapping the molten steel 30 and the slag layer 40 inside the ladle 10 and standing still, adding flux as necessary, and then energizing and blowing an inert gas. Let it be the thickness of the slag layer 40 before.

By setting the thickness d of the slag layer 40 to 100 mm or more, the exposure of the molten steel 30 to the atmosphere can be more reliably suppressed, and the absorption and nitriding reaction in the molten steel 30 can be more reliably prevented. The upper limit of the thickness d of the slag layer 40 is not particularly specified. However, the thickness d of the slag layer 40 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 in the vicinity of the electrode is suppressed from re-solidifying, 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.

Subsequently, the ladle was transferred to a treatment position where electric heating 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 the ladle was moved to the treatment position where the electric heating treatment was performed, a container cover was attached to the ladle, and three electrodes for electric current having a diameter of 320 mm were lowered into the slag layer on the surface of the molten steel. Three electrodes, the first porous plug and the second porous plug were arranged under the conditions shown in Table 1. Table 1 shows the distance (r _A ) between the center O of the electrode circumscribed circle projected on the bottom surface of the ladle and the center A of the first porous plug at the bottom surface of the ladle with respect to the radius R of the inner bottom surface of the ladle. , the ratio r _A /R was shown. When r _A /R is 0.9 or less, it means that the distance between the center A of the first porous plug and the inner surface of the inner bottom surface of the ladle is 0.1R or more. Table 1 also shows the ratio r _A /(D/2) of the distance OA (r _A ) to the radius (D/2) of the circumscribed circle C of the electrode. When r _A /(D/2) is more than 1, it means that the center of the horizontal cross section of the first porous plug is located outside the electrode circumscribed circle. Similarly, for the second porous plug, Table 1 shows the ratio of the distance (r _B ) between the center O and the center _B of the second porous plug at the bottom surface of the ladle to the radius R of the inner bottom surface of the ladle. /R and the ratio r _B /(D/2) of the distance OB (r _B ) to the radius (D/2) of the circumscribed circle of the electrode.

In addition, under the conditions shown in Table 1, heat treatment by energization was started while introducing an inert gas (Ar) from the first porous plug and the second porous plug at the bottom of the ladle and 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 5 minutes. In addition, for comparison, a case where no energization was performed was also conducted. Note that the radius of the ladle at the ladle bottom is R [m] is 1.2, and the center of the electrode circumscribed circle C on the surface of the molten steel 30 is 0 to 0 from the center position of the ladle 10 on the surface of the molten steel 30. .1×R[m].

Samples were collected before and after energization, and the N concentration after energization and stirring was evaluated. In this evaluation, Test No. shown below. The increment (nitrogen absorption amount) of nitrogen concentration (% by mass) before and after energization of No. 18 was set to 1.0, and other conditions were indexed. 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 that of A, but practical)
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, test no. 1 to No. 10 (No. 3 is a reference example) was evaluated as "B". In particular, Test No. in which the thickness d of the slag layer is 100 mm or more. 13 to No. 16 (No. 13 to 16 are reference examples) , the evaluation of the amount of nitrogen absorption is "A", and the test No. 1 to No. The amount of nitrogen absorption was lower than in the case of No. 10. Moreover, test No. in which Ds/D is within the preferable range. 11, and test no. In No. 12, the evaluation of the amount of nitrogen absorption was "A" even though the slag thickness was 50 mm.

On the other hand, test no. 17 to No. No. 29 was rated as "C" in the evaluation of the amount of nitrogen absorption. Test no. 17 to No. This is the reason why the amount of nitrogen absorption was high under each condition of No. 29.

Test no. Comparative Example No. 17 has a small D _S /D and a large area to be electrically heated. Therefore, as schematically shown in FIG. Part occurred and slag clumps remained.

Test no. Comparative Example No. 18 has a large D _S /D and a small area to be electrically heated. Therefore, the proportion of slag existing in the electrically heated area is reduced, and the amount of slag to be heated is reduced. As schematically shown in , the molten slag did not reach the vicinity of the ladle wall.

Test no. 19 comparative examples and test no. In Comparative Example No. 21, the inert gas flow rate from the first porous plug or the inert gas flow rate from the second porous plug was too small, so a flow for conveying the molten slag could not be formed, and It was not possible to melt the entire slag present.

Test no. 20 comparative examples and test no. In Comparative Example No. 22, the flow rate of the inert gas from the first porous plug or the flow rate of the inert gas from the second porous plug was excessively large, so that the stirring gas bubbles were generated as schematically shown in FIG. Bubbles broke on the surface of the molten steel, nitrogen was caught in the molten steel, and the nitriding reaction could not be suppressed.

Test no. 23 and test no. In No. 24, since P _A /P _B is less than 0.61 or more than 1.66, as schematically shown in FIG. There was a tendency for the slag to be transported to the heated area to be delayed, and unmelted slag was present, making it impossible to suppress the absorption of nitrogen.

Test no. 25 and test no. 26, since the first porous plug or the second porous plug is arranged near the ladle wall surface, when the inert gas bubbles float in the molten steel, the bubbles come into contact with the ladle wall surface, and the floating behavior of the bubbles is Due to the instability, the flow of molten steel also became unstable, and as a result, as schematically shown in FIG.

Test no. Comparative Example No. 27 has the first porous plug and the second porous plug so that the angle ? It is an example in which a plug is installed. The first porous plug and the second porous plug are on the same side of the ladle (in other words, the plane of the ladle containing the center O, center A and center B is divided by a straight line passing through the center O and perpendicular to the straight line OA. Since the center A and the center B were placed on the same side when the center A and the center B were on the same side), there was a deviation in the conveying direction of the molten slag. A region remained uncovered by the molten slag.

Test no. 28 comparative examples and test no. In Comparative Example No. 29, slag melting did not progress and slag clumps 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.

1 ladle refining device 10 ladle 20, 20A, 20B porous plug 30 molten steel 40 slag layer 50 electrode 41, 41A, 41B, 41C, 41D high temperature slag 42, 42A, 42B, 42C, 42D low temperature slag 60 bubbles 61, 61A, 61B, 61C, 61D, 61E, 61F bubble rising area C, C1, C2 electrode circumscribed circle

Claims (2)

A slag layer is formed on the surface of molten steel in a ladle, an electrode is immersed in the slag layer and energized, and gas is blown into the molten steel from a gas blowing plug placed at the bottom of the ladle. In the pot refining method,
The number of the electrodes immersed in the slag layer is three ,
two said gas injection plugs are placed at the bottom of said ladle,
The ladle inner diameter Ds [m] on the surface of the molten steel is 2.5 to 4.7 m, and when the surface of the molten steel is viewed from above the ladle, the three electrodes on the surface of the molten steel When the diameter 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 D [m], the electrode circumscribes The ratio D _S /D of the ladle inner diameter D _s to the circle diameter D satisfies the following formula (1),
When the inner bottom surface of the ladle is viewed from above the ladle,
Each of the centers of the two gas injection plugs is located outside the circumscribed circle of the electrode projected on the inner bottom surface of the ladle,
Let the center of the electrode circumscribed circle projected on the inner bottom surface of the ladle be the point O, and the center of the first gas injection plug of the two gas injection plugs at the inner bottom surface of the ladle A straight line OA passing through points O and A when the center of the second gas blowing plug of the two gas blowing plugs on the inner bottom surface of the ladle is set to point B, and , the angle θ formed by the straight line OB passing through the point O and the point B is 90 degrees or more and 180 degrees or less,
When the radius of the inner bottom surface of the ladle is R, at the inner bottom surface of the ladle, the distance between the center of each of the two gas injection plugs and the inner wall surface of the ladle is 0.1R and
The center of the electrode circumscribed circle on the surface of the molten steel is located within a region of 0.1 × R from the center position of the ladle on the surface of the molten steel,
The flow rate _PA of the gas blown into the molten steel from the first gas blowing plug is 0.3 NL/min/t or more and 4.5 NL/min/t or less,
The flow rate _PB of the gas injected into the molten steel from the second gas injection plug is 0.3 NL/min/t or more and 4.5 NL/min/t or less,
The ratio P _A /P _B of the flow rate P _A to the flow rate P _B is a molten steel ladle refining method that satisfies the following formula (2) (however, the bottom inner diameter of the ladle is 3968 mm, and the circle passing through the center of the electrode except when the diameter is 1300 mm) .
1.8≦Ds/D≦3.5 Formula (1)
2.00/3.20 ≤ P _A /P _B ≤ 3.20/2.00 Formula (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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