JP7568919B2 - Refining equipment and method for producing low-nitrogen steel - Google Patents

Description

The present invention relates to a refining apparatus and a method for producing low-nitrogen steel.

Steel refining methods are classified into several methods depending on whether or not heating and vacuum equipment is used. LF refining is a refining method in which molten steel is heated with electrodes and refined under atmospheric pressure.

In LF refining, suppressing nitrogen absorption from the outside air is an important issue. JP 01-208413 A discloses a method for producing low-nitrogen steel by arc-heated ladle refining. The publication discloses that a slag former is added and the molten steel is stirred to melt at least a part of the slag former using the heat of the molten steel, and then the electrode part of an electric arc rod is inserted into the molten slag former to perform arc heating. This method aims to suppress the generation of nitrogen plasma by open arcs by melting the slag former in advance and immersing the electrode in the molten slag, thereby suppressing the absorption of nitrogen into the molten steel.

JP 2008-266759 A discloses a method for preventing nitrogen pickup in ladle refining using an electrode heating device. The publication describes controlling the amount of suction of dust-containing gas generated inside the ladle so that the dust concentration leaking from the ladle mouth to the surrounding area falls within a predetermined range.

Although not related to LF refining, Japanese Patent No. 3804143 discloses a method of controlling the atmosphere when stirring molten pig iron for stainless steel in a ladle as a technology for refining molten steel under atmospheric pressure. The publication describes supplying a sealing gas into the ladle space in addition to the stirring inert gas to control the atmosphere during stirring in the ladle.

As another technique for refining molten steel under atmospheric pressure, Japanese Patent Publication Nos. 5,082,417, 5,505,432, 5,979,029, and 6,645,374 disclose a method for producing low-nitrogen steel. These publications describe the sequential implementation of the following steps: (1) adding CaO-based flux to molten steel in a ladle under atmospheric pressure; (2) installing a ladle lid and injecting stirring gas into the molten steel in the ladle to stir the steel while preventing air from entering the inside of the lid, while also supplying oxidizing gas to the molten steel and mixing the oxides formed with the CaO-based flux to form a cover slag; and (3) stopping the supply of oxidizing gas and injecting stirring gas into the molten steel in the ladle to desulfurize and remove inclusions.

As a technique for performing refining after evacuating the inside of a ladle, Japanese Patent Application Laid-Open No. 2006-111950 discloses a vacuum dehydrogenation method for molten steel for steel forgings. This publication describes that the ladle is covered with a lid, the distance from the nozzle of an Ar blowing lance provided on the lid to the slag surface is set to 800 to 3000 mm, Ar is blown from the nozzle onto the slag surface at a flow rate of 15 to 200 ^Nm3 /hr, and Ar is blown from a bottom blowing tuyeres provided at the bottom of the ladle at a flow rate of 1500 to 12500 NL/hr.

Japanese Unexamined Patent Publication No. 1-208413 JP 2008-266759 A Patent No. 3804143 Patent No. 5082417 Patent No. 5505432 Patent No. 5979029 Patent No. 6645374 JP 2006-111950 A

In recent years, in addition to desulfurization and deoxidization, there has been a demand for a high level of nitrogen reduction in LF refining. To achieve this, it is preferable to be able to suppress nitrogen absorption without reducing desulfurization performance, etc. However, there have not been many reports on the suppression of nitrogen absorption in LF refining.

The object of the present invention is to provide a refining apparatus capable of suppressing nitrogen absorption in LF refining without significantly changing the operating conditions that affect desulfurization performance, etc. Another object of the present invention is to provide a method for producing low-nitrogen steel capable of suppressing nitrogen absorption in LF refining without significantly changing the operating conditions that affect desulfurization performance, etc.

The refining apparatus according to one embodiment of the present invention is a refining apparatus for LF refining that heats molten steel and slag with electrodes, and includes a ladle that contains the molten steel and the slag, a lid that is arranged to cover the opening of the ladle, and a bottom blowing gas introduction member that is provided at the bottom of the ladle and that blows gas from the bottom into the molten steel. The lid has a purge gas introduction port for blowing gas into the slag, and the purge gas introduction port is arranged between the bottom blowing gas introduction member and the furnace wall of the ladle that is located behind the bottom blowing gas introduction member when viewed from above.

The method for producing low-nitrogen steel according to one embodiment of the present invention is a method for producing low-nitrogen steel using the above-mentioned refining apparatus, in which an inert gas is supplied to the slag from the purge gas inlet.

According to the present invention, it is possible to suppress nitrogen absorption without significantly changing the operating conditions that affect desulfurization performance, etc.

FIG. 1 is a cross-sectional view showing a schematic configuration of a refining apparatus according to an embodiment of the present invention. FIG. 2 is a diagram showing a schematic diagram of the positional relationship of the furnace wall of the ladle, the porous plug, the purge gas inlet, and the plume eye in a plan view. FIG. 3 is a graph showing the relationship between the Ar purge position and the amount of nitrogen absorption. FIG. 4 is a graph showing the relationship between the Ar flow rate and the amount of nitrogen absorption.

The inventors have investigated methods to suppress nitrogen absorption from the outside air during LF refining and have obtained the following findings.

During refining, the surface of molten steel is usually covered with slag and is isolated from the outside air. However, stirring gas is blown into the molten steel from the bottom of the ladle during refining, and when this stirring gas leaves the molten steel, it may push aside the nearby slag, forming an exposed surface of the molten steel. This exposed surface of the molten steel is called the plume eye, and when outside air comes into contact with this plume eye, nitrogen absorption may occur.

Methods for suppressing the occurrence of plume eyes include adjusting the slag thickness and controlling the flow rate of the stirring gas. However, due to issues such as slag adhesion to the furnace walls, it may not be possible to ensure sufficient slag thickness. Also, in order to ensure the desulfurization speed, it may not be possible to reduce the flow rate of the stirring gas. In order to be able to suppress nitrogen absorption even when these operating conditions fluctuate, it is preferable to be able to suppress nitrogen absorption even in situations where plume eyes have occurred.

The inventors performed computational fluid analysis to calculate the gas flows inside and outside the refining equipment used in LF refining. As a result, they found that outside air flows in through the gap between the ladle and the lid, and gas inside the lid is exhausted through the gap between the electrode and the lid. This unidirectional gas flow is thought to be generated by an updraft caused by the chimney effect. The chimney effect is a phenomenon in which high-temperature, low-density air rises while low-temperature, high-density air from the outside is drawn into the chimney through the air intake at the bottom of the chimney.

In other words, in the refining equipment used for LF refining, a gas flow occurs from the gap between the ladle and the lid, located at the outermost periphery of the equipment, toward the gap between the electrode and the lid, located near the center of the equipment. The flow rate of the invading outside air is estimated to be more than 10 times the realistic flow rate of purge gas, even if the dust collection flow rate is set to zero, making it difficult to replace the entire inside of the lid with inert gas. In addition, the effect of reducing the dust collection flow rate is thought to be limited.

After extensive investigation, the inventors discovered that by providing a purge gas inlet between the plume eye and the furnace wall in a plan view and spraying an inert gas onto the slag surface, it is possible to prevent outside air entering from the furnace wall side from flowing toward the plume eye, thereby suppressing contact between the outside air and the plume eye.

The present invention has been completed based on the above findings. Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are given the same reference numerals and their description will not be repeated. The dimensional ratios between the components shown in each drawing do not necessarily represent the actual dimensional ratios.

[Refining Equipment]
1 is a cross-sectional view showing a schematic configuration of a refining apparatus 1 according to an embodiment of the present invention. The refining apparatus 1 is an apparatus for LF refining in which molten steel and slag are heated by electrodes.

The refining apparatus 1 comprises a ladle 11, a lid 12, an electrode 13, and a porous plug 14, which is a bottom blowing gas introduction member. The lid 12 has a purge gas introduction port 12a. For convenience, in FIG. 1, the electrode 13, the porous plug 14, and the purge gas introduction port 12a are illustrated as being in the same cross section, but FIG. 1 is merely a schematic diagram and does not show the exact positional relationship of these members. The electrode 13, the porous plug 14, and the purge gas introduction port 12a do not have to be in the same cross section.

The ladle 11 contains molten steel 21 and slag 22.

The lid 12 is placed on top of the ladle 11 so as to cover the opening of the ladle 11. The lid 12 has a purge gas inlet 12a and a hole for inserting the electrode 13.

The electrode 13 heats the molten steel 21. The electrode 13 is, for example, an artificial graphite electrode, and heats the molten steel 21 and slag 22 by arc heating. The electrode 13 is usually placed away from the furnace wall 11a of the ladle 11 to reduce wear on the refractory of the ladle 11. In other words, the electrode 13 is usually placed near the center of the ladle 11 when viewed from above.

In this specification, "plan view" means a view parallel to the vertical direction from above. The positional relationship between each component in plan view corresponds to the positional relationship when each component is projected onto the cut surface of ladle 11 if it is cut horizontally at line A-A' in Figure 1.

The porous plug 14 is provided at the bottom of the ladle 11. The porous plug 14 blows gas into the molten steel 21 from the bottom to stir the molten steel 21. It is preferable that the porous plug 14 is positioned away from the electrode 13 in a plan view. This is because, when gas blown from the porous plug 14 is supplied to the electrode 13, the voltage fluctuates, causing wear on the electrode 13. As described above, the electrode 13 is usually positioned near the center of the ladle 11 in a plan view. Therefore, it is preferable that the porous plug 14 is positioned away from the center of the ladle 11 in a plan view.

The purge gas inlet 12a blows gas onto the slag 22. More precisely, the purge gas inlet 12a is disposed between the porous plug 14 and the furnace wall 11a of the ladle 11, which is located further back than the porous plug 14 when viewed from the center of the ladle 11, in a plan view. A more preferable positioning of the purge gas inlet 12a will be described later.

[Method of manufacturing low-nitrogen steel]
Next, an example of a method for producing low nitrogen steel using the refining apparatus 1 will be described. First, molten steel 21 and auxiliary materials are charged into the ladle 11. Slag 22 is formed by melting the auxiliary materials. The molten steel 21 and slag 22 are heated by the electrode 13. Inert gas is bottom-injected from the porous plug 14 to stir the molten steel 21 and slag 22. Ar gas is usually used as the inert gas. This causes desulfurization and deoxidization of the molten steel 21. If necessary, alloy elements may be added to adjust the components.

During refining, the surface of the molten steel 21 is usually covered with slag 22 and is isolated from the outside air. However, when the gas blown from the bottom of the porous plug 14 leaves the molten steel 21, the nearby slag 22 may be pushed aside, forming a plume eye 21a, which is an exposed surface of the molten steel. The plume eye 21a is formed at approximately the same position as the porous plug 14 in a plan view. When the outside air comes into contact with the plume eye 21a, nitrogen absorption may occur.

In this embodiment, an inert gas is sprayed onto the slag 22 from a purge gas inlet 12a formed in the lid 12. Ar gas is usually used as the inert gas.

In the refining apparatus 1, as shown by the open arrows in FIG. 1, outside air flows in through the gap between the ladle 11 and the lid 12 and is exhausted through the gap between the lid 12 and the electrode 13. By spraying inert gas from the purge gas inlet 12a, the outside air flowing in through the gap between the ladle 11 and the lid 12 is prevented from heading toward the plume eye 21a, and contact between the outside air and the plume eye 21a can be suppressed. This makes it possible to suppress nitrogen absorption even in a situation where the plume eye 21a is present, and makes it possible to produce steel with a low nitrogen content.

Figure 2 is a diagram showing the positional relationship of the furnace wall 11a of the ladle 11, the porous plug 14, the purge gas inlet 12a, and the plume eye 21a in a plan view. As described above, the purge gas inlet 12a is located between the porous plug 14 and the furnace wall 11a of the ladle 11, which is located further back than the porous plug 14 when viewed from the center c0 of the ladle 11, in a plan view.

The center c0 of the ladle 11, the porous plug 14, and the purge gas inlet 12a do not necessarily have to be on the same line in a plan view. The purge gas inlet 12a may be located in any position that can prevent the outside air flowing in from the furnace wall 11a side from heading toward the plume eye 21a. Depending on the distance from the center c0 of the ladle 11 to the center of the porous plug 14 and the size of the plume eye 21a, the angle θ between the line connecting the center c0 of the ladle 11 and the center of the porous plug 14 and the line connecting the center c0 of the ladle 11 and the center of the purge gas inlet 12a in a plan view is preferably 20° or less, more preferably 10° or less, and even more preferably 5° or less.

In terms of distance from the center c0 of the ladle 11, the purge gas inlet 12a is positioned such that r1 < r2 < r3, where r1 is the distance from the center c0 of the ladle 11 to the center of the porous plug 14 in a plan view, r2 is the distance from the center c0 of the ladle 11 to the center of the purge gas inlet 12a, and r3 is the distance from the center c0 of the ladle 11 to the furnace wall 11a.

If the purge gas inlet 12a is located directly above the porous plug 14, the gas blown from the purge gas inlet 12a may draw in the surrounding outside air and come into contact with the plume eye 21a, which may result in insufficient suction suppression effect. Therefore, the purge gas inlet 12a is located closer to the furnace wall 11a than the center of the porous plug 14 in a plan view. It is preferable to install the purge gas inlet 12a closer to the furnace wall than the outer periphery of the plume eye 21a. When the plume eye 21a is circular in a plan view, its diameter is, for example, about 300 to 400 mm, depending on the operating conditions. Therefore, the size of r2 is preferably r1 + 150 mm or more, more preferably r1 + 200 mm or more, even more preferably r1 + 300 mm or more, and even more preferably r1 + 500 mm or more.

If the flow rate of the inert gas (hereinafter simply referred to as "purge gas") supplied from the purge gas inlet 12a is too small, it may not be effective enough to block outside air, and if it is too large, it may draw in outside air and contaminate the plume eye 21a.

The flow rate of the purge gas is preferably determined so that the average flow velocity _vAr of the purge gas on the surface of the slag 22 satisfies the following formula (1).

V _air is the average flow velocity of the outside air flowing in through the gap between the ladle 11 and the lid 12, and can be calculated from the following equations (2) and (3).

Here, Q _air is the supply air flow rate, A ₁ is the opening area between the ladle 11 and the lid 12, A ₂ is the opening area between the lid 12 and the electrode 13, C is the flow coefficient (=0.7), g is the gravitational acceleration, H ₂ is the height of the lid 12, T _i is the average temperature inside the furnace, and T ₀ is the outside air temperature.

When the ladle 11, the electrode 13, and the electrode holes are each circular in a plan view, _A1 and _A2 can also be calculated from the following equations (4) and (5), respectively.

Here, _Di is the inside diameter of the ladle 11, _H1 is the vertical clearance between the ladle 11 and the lid 12, _D2 is the diameter of the electrode hole in the lid 12, _Dc is the outside diameter of the electrode, and n is the number of electrodes.

_vAr can be calculated from the following equations (6) and (7).

Here, v _Ar ' is the central flow velocity of the purge gas on the surface of the slag 22, v ₀ is the outlet flow velocity of the purge gas at the purge gas inlet 12a, d ₀ is the inner diameter of the purge gas inlet 12a, and x is the distance from the outlet of the purge gas inlet 12a to the surface of the slag 22.

The relationship between the flow rates Q _Ar and v _Ar of the purge gas is given by the following formula (8) where v ₀ =4Q _Ar /(π·d ₀ ² ).

In summary, it is preferable that the flow rate Q _Ar of the purge gas satisfies the following formula (9).

When the ladle 11, the electrode 13, and the electrode holes are each circular in a plan view, _Q0 can also be expressed as follows using equations (4) and (5).

The above describes a refining apparatus and a method for producing low-nitrogen steel according to one embodiment of the present invention. The refining apparatus 1 according to this embodiment has a purge gas inlet 12a on the lid 12. In a plan view, the purge gas inlet 12a is disposed between the porous plug 14 and the furnace wall 11a of the ladle 11, which is located on the rear side of the porous plug 14 when viewed from the center of the ladle 11. According to this configuration, by blowing inert gas from the purge gas inlet 12a onto the slag 22, it is possible to prevent the outside air flowing in from the furnace wall 11a side from coming into contact with the plume eye 21a, and to suppress nitrogen absorption of the molten steel 21. Therefore, even in a situation where the plume eye 21a exists, nitrogen absorption can be suppressed, and steel with a low nitrogen content can be produced. From another perspective, nitrogen absorption can be suppressed without significantly changing the operating conditions that affect desulfurization performance, etc.

In the above embodiment, a case has been described in which the bottom blowing gas introduction member for bottom blowing gas into the molten steel 21 is a porous plug, but the bottom blowing gas introduction member does not have to be a porous plug and can be any member capable of bottom blowing gas into the molten steel 21.

In the above embodiment, the ladle 11 is provided with only one porous plug 14 (bottom blowing gas introduction member), but the ladle 11 may be provided with multiple porous plugs. In this case, if a purge gas inlet is provided between the porous plug and the furnace wall 11a in a plan view for at least one of the multiple porous plugs, contact between the plume eye formed by the porous plug and the outside air can be suppressed, so that a certain degree of nitrogen absorption suppression effect can be obtained. However, when there are multiple porous plugs, it is more preferable that a purge gas inlet is provided between the porous plug and the furnace wall 11a in a plan view for each of the multiple porous plugs. Also, multiple purge gas inlets may be provided for one porous plug.

In addition to the purge gas inlet 12a, the refining apparatus 1 may be provided with another purge gas inlet at any location.

In Figures 1 and 2, the refining device 1 is illustrated as being of a three-phase AC type having three electrodes 13, but the refining device 1 may also be of a DC type.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to these examples.

A 100-ton scale ladle-type refining equipment with arc heating was used to carry out a nitrogen absorption suppression test. Specifically, LF refining was carried out using molten steel with an N content of 0.0020-0.0030 mass% as the raw material under conditions such that the S content after refining was 0.002-0.003 mass% and the Sol. Al content was 0.05-0.06 mass%, and the difference in nitrogen content before and after refining was calculated as the amount of nitrogen absorption (ppm by mass).

After producing 10 kg/t of slag from the auxiliary materials, the alloy components were adjusted. The ladle had an inner diameter of 2720 mm, and the porous plug was placed at a distance of 640 mm from the center of the ladle. Ar gas was supplied from this porous plug at 100 NL/min to stir the molten steel and slag. The molten steel was heated from 1530°C to 1590°C by arc heating from the AC electrodes. The treatment time was 40 minutes.

The purge gas inlets were installed at different distances from the center of the ladle, in the same direction as the porous plug when viewed from the center of the ladle (θ = 0° in Figure 2) in a plan view. Argon gas was supplied vertically downward from these purge gas inlets at rates of 0 to 750 NL/min.

In this test, the average flow velocity of the outside air v _air was 0.3 m/s and Q ₀ was 160 NL/min.

Ten tests were performed under each condition, and the amount of nitrogen absorption was evaluated as the average value. The results are shown in Table 1. When the amount of nitrogen absorption was reduced by 10% or more compared to the condition without Ar purging (Comparative Example 1), it was evaluated that there was an effect of suppressing nitrogen absorption.

The relationship between the Ar purge position and the amount of nitrogen absorbed, and the relationship between the Ar flow rate and the amount of nitrogen absorbed are shown in Figures 3 and 4, respectively.

In Examples 1 to 3, a purge gas inlet was placed between the porous plug and the furnace wall of the ladle, which is located behind the porous plug when viewed from the center of the ladle in a plan view. As a result, a reduction in the amount of nitrogen absorption was observed compared to Comparative Examples 1 to 3. This is thought to be due to the effect of blocking the outside air with the purge gas.

In Comparative Examples 2 and 3, a purge gas inlet was placed between the porous plug and the center of the ladle in a plan view. As a result, no reduction in the amount of nitrogen absorption was observed compared to the condition without Ar purging (Comparative Example 1).

In Examples 3 to 7, tests were conducted with different Ar flow rates. As a result, Example 3 showed the greatest reduction in the amount of suctioned nitrogen. While Ar blocks outside air, the plume eye is contaminated by the Ar purge jet that draws in outside air, so it is believed that there is an optimal range for the Ar flow rate.

The above describes an embodiment of the present invention. The above-mentioned embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-mentioned embodiment, and it is possible to carry out the above-mentioned embodiment by appropriately modifying it within the scope of the gist of the present invention.

Reference Signs List 1 Refining apparatus 11 Ladle 11a Furnace wall 12 Lid 12a Purge gas inlet 13 Electrode 14 Porous plug (bottom blowing gas introduction member)
21 molten steel 21a plume eye 22 slag

Claims (3)

A refining apparatus for LF refining in which molten steel and slag are heated by electrodes,
a ladle for containing the molten steel and the slag;
A lid disposed to cover the opening of the ladle;
a bottom blowing gas introducing member provided at a bottom of the ladle for bottom blowing gas into the molten steel,
The lid has a purge gas inlet for blowing gas onto the slag;
the bottom blowing gas introducing member is disposed at a position different from the center of the ladle in a plan view,
The purge gas inlet is disposed between the bottom blowing gas introducing member and a furnace wall of the ladle located on the rear side of the bottom blowing gas introducing member when viewed from a plane ,
A refining apparatus, wherein, in a plan view, an angle formed between a line connecting the center of the ladle and the center of the bottom blowing gas introduction member and a line connecting the center of the ladle and the center of the purge gas introduction port is 20° or less . A method for producing low nitrogen steel using the refining apparatus according to claim 1,
A method for producing low nitrogen steel, comprising: supplying an inert gas to the slag through the purge gas inlet. A method for producing a low nitrogen steel according to claim 2,
The method for producing low nitrogen steel, wherein the flow rate Q _Ar of the inert gas satisfies the following formula (1):

Here, C is the flow coefficient (=0.7), _Di is the inner diameter of the ladle, _H1 is the vertical clearance between the ladle and the lid, _D2 is the diameter of the hole for the electrode in the lid, _Dc is the outer diameter of the electrode, n is the number of electrodes, g is the gravitational acceleration, _H2 is the height of the lid, _Ti is the average temperature in the furnace, _T0 is the outside air temperature, _d0 is the inner diameter of the purge gas inlet, and x is the distance from the outlet of the purge gas inlet to the slag surface.

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