KR20240008923A - Molten steel refining method - Google Patents

Abstract

In the steelmaking process, when hydrogen plasma is applied to molten steel in a ladle, the refining reactions of deoxidation, denitrification, and desulfurization proceed quickly, and high purity molten steel with few impurities is efficiently produced. A method of refining molten steel, in which a gas for stirring is blown into the molten steel 9 contained in a ladle to stir the molten steel, the ladle is installed above the molten steel in the ladle on the surface of the molten steel in a fluid state due to the stirring process. A plasma treatment is performed in which hydrogen gas or an inert gas containing hydrogen gas is irradiated from the plasma generators 11 and 12 as a plasma gas under conditions that satisfy the equation (1) below, thereby removing oxygen and nitrogen contained in the molten steel. , the content of one or two or more elements selected from sulfur is reduced. (1) In the formula, G _P is the flow rate of the plasma gas (Nm ³ /min), (H ₂ ) is the hydrogen gas concentration (volume %) in the plasma gas, and Q is the molten steel circulation flow rate (ton/min).
(G _P × (H ₂ )/Q) ≥ 0.05 … (One)

Description

Molten steel refining method

The present invention relates to a refining method for producing molten steel with a low content of oxygen, nitrogen, and sulfur, which are impurity elements, and specifically, to irradiate hydrogen gas or an inert gas containing hydrogen gas as a plasma gas to the surface of molten steel in a ladle. Thus, it relates to a method of refining molten steel, in which impurities are removed using plasma gas.

It is generally known that non-metallic inclusions in steel materials have a negative effect on material properties and quality. Additionally, oxide-based non-metallic inclusions cause blockage of the submerged nozzle during continuous casting, resulting in lower productivity due to a decrease in casting speed and, in the worst case, forcing casting to be stopped. Non-metallic inclusions include oxide-based deoxidation products generated during deoxidation of molten steel, and sulfides and nitrides of alloy elements in steel. In order to reduce the amount of these non-metallic inclusions (hereinafter also simply referred to as “inclusions”), it is important to reduce oxygen, nitrogen, and sulfur in molten steel as much as possible, and various measures have been taken in the past.

Regarding the oxygen in the molten steel, the dissolved oxygen in the molten steel is fixed as Al ₂ O ₃ or SiO ₂ by adding a deoxidizing agent such as aluminum (Al) or silicon (Si). The generated oxide-based inclusions are removed by flotation by using the difference in specific gravity from the molten steel by gas stirring treatment of the molten steel or reflux treatment in an RH vacuum degassing device. However, under the current situation, it is impossible to separate and remove all oxide-based inclusions, and remaining oxide-based inclusions in the molten steel cannot be avoided.

Nitrogen in molten steel is reduced by vacuum treatment in a vacuum degassing facility. However, nitrogen in molten steel is influenced by oxygen and sulfur, which are surface active elements, and it is difficult to avoid adsorption due to air incorporation from outside the vacuum system, so the current situation is that a stable low concentration nitrogen level cannot be achieved.

Sulfur in molten steel is being reduced by adding a CaO-based flux or a CaO-Al ₂ O _3- based flux (addition of a desulfurizing agent). For example, in ladle refining in a ladle refining furnace, argon gas is blown into the molten steel from the bottom of the ladle and stirred to promote the reaction between the molten steel in the ladle and the CaO-Al ₂ O ₃ -based flux, and the flux By moving sulfur to the slag side, we are trying to reduce sulfur in molten steel. However, in the treatment in such a ladle refining furnace, since arc heating is performed using a graphite electrode, carbon infiltration into the molten steel occurs, making it difficult to apply to steel types such as ultra-low carbon steel.

Additionally, in the RH vacuum degassing device, there is a method of performing desulfurization by adding a CaO-based flux or a CaO-Al ₂ O _3- based flux to the molten steel refluxing in the vacuum tank. In addition, there is also a method of performing desulfurization by projecting (blotting) CaO-based flux or CaO-Al ₂ O ₃ -based flux onto the molten steel refluxing in the vacuum tank from the top blowing lance using an inert gas such as argon gas as a conveying gas. However, in these methods, the reaction time between the molten steel and the flux is not sufficient, and it is difficult to efficiently obtain molten steel with a low sulfur concentration.

However, the use of hydrogen plasma is known as a refining technology to reduce impurities in metal. Since the temperature in the plasma reaches thousands of degrees or more, the hydrogen gas in the plasma gas is in an atomic or ionic state and is in a very active state. By irradiating this to the molten steel surface, an excellent refining effect that cannot be achieved by ordinary hydrogen gas irradiation alone can be expected. That is, oxygen, nitrogen, and sulfur in the molten steel can be quickly removed through reactions of formulas (6) to (8) shown below.

2H + [O] = H ₂ O... … (6)

xH + [N] = NH _x ... … (7)

yH + [S] = H _y S... … (8)

Here, [O] represents oxygen in the molten steel, [N] represents nitrogen in the molten steel, and [S] represents sulfur in the molten steel.

In addition to the fact that oxygen, nitrogen _, and sulfur in molten steel can be removed to the outside of the system as gases as H ₂ O, _NH A river is obtained.

As a technology for refining high-purity metal using such hydrogen plasma, Patent Document 1 describes the hydrogen concentration in the plasma gas and the concentration in the furnace to reduce oxygen, nitrogen, or carbon in the metal when melting metal using hydrogen plasma. Preferred ranges of pressure are disclosed.

Japanese Patent Publication No. 4305792

However, there are the following problems in applying the technology of Patent Document 1 to an industrial-scale steelmaking process.

In the example described in Patent Document 1, the refining effect when metal weighing from tens of g to tens of kg is processed in a plasma melting furnace is described. However, in an industrial-scale steelmaking process, it is necessary to process molten steel exceeding 100 tons, and it is difficult to irradiate the entire molten steel with plasma gas. Therefore, there are concerns that a rapid impurity removal effect cannot be obtained with the technology disclosed in Patent Document 1. In order to obtain a rapid impurity removal effect, it is important to perform hydrogen plasma treatment efficiently by appropriate not only the plasma conditions but also the flow conditions on the molten steel side.

Additionally, in the technology disclosed in Patent Document 1, the amount of metal to which hydrogen plasma is applied or the relationship between the amount of metal and the plasma gas flow rate is not specified. Therefore, even if the plasma gas composition and atmospheric pressure are appropriately controlled, a case is assumed in which a sufficient impurity reduction effect cannot be obtained due to insufficient plasma gas flow rate or hydrogen amount relative to the amount of metal. Furthermore, Patent Document 1 is not a technology for applying hydrogen plasma to already molten iron, but also has the role of heating and melting the target metal by plasma. Therefore, there is concern that the same expected effect may not be obtained even if the disclosed plasma gas conditions are applied to already molten steel, such as in the steelmaking process.

The present invention was made in consideration of the above circumstances, and its purpose is to rapidly proceed with the refining reactions of deoxidation, denitrification, and desulfurization when hydrogen plasma is applied to the molten steel in the ladle in the steelmaking process, thereby removing impurities. The purpose of this article is to provide a method for refining molten steel that efficiently melts high-purity molten steel.

The gist of the present invention for solving the above problems is as follows.

[1] In the process of performing a gas stirring treatment in which a stirring gas is blown into the molten steel contained in a ladle to stir the molten steel in the ladle,

From the plasma generator installed above the molten steel in the ladle, hydrogen gas or an inert gas containing hydrogen gas is applied to the surface of the molten steel in the ladle in a fluid state by the gas stirring treatment as a plasma gas as shown in (1). ) A method of refining molten steel in which plasma treatment is performed under conditions that satisfy the formula, and the plasma treatment reduces the content of one or two or more elements selected from oxygen, nitrogen, and sulfur contained in the molten steel.

Here, G _P is the flow rate of the plasma gas (Nm ³ /min), (H ₂ ) is the hydrogen gas concentration (volume %) in the plasma gas, and Q is the molten steel circulation flow rate of the molten steel in the ladle (ton/min).

[2] The molten steel refining method according to [1] above, wherein the molten steel circulation flow rate of the molten steel in the ladle is calculated using the following equations (2), (3), and (4).

Here, Q is the molten steel circulation flow rate of the molten steel in the ladle (ton/min), W _m is the mass of molten steel in the ladle (ton), t _c is the molten steel circulation time of the molten steel in the ladle (min), and D is the ladle The average diameter of the molten steel bath in the ladle (m), H is the depth of the molten steel bath in the ladle (m), ε is the stirring power (W/ton), G _B is the total stirring gas injection flow rate for the molten steel in the ladle (Nm) ³ /min), T _L is the molten steel temperature (K) in the ladle, and P ₀ is the atmospheric pressure (torr) in the plasma irradiation area.

[3] The gas stirring process is performed by installing a gas inlet at one point or at two or more points at the bottom of the ladle, and blowing a gas for stirring into the molten steel in the ladle from the gas inlet, The molten steel refining method according to [1] or [2] above, wherein the stirring power (ε) calculated by equation (4) is 25 W/ton or more.

[4] The plasma gas is irradiated in a range within the plasma irradiation range radius (r) calculated by the equation (5) below, centered just above the molten steel surface vertically above at least one of the gas inlet points. The method for refining molten steel according to [3] above.

Here, r is the radius of the plasma irradiation range (m), and g _B is the injection flow rate of the stirring gas (Nm ³ /min/one gas injection point).

[5] The method for refining molten steel according to any one of [1] to [4] above, wherein the atmospheric pressure when irradiating plasma gas to the molten steel in the ladle is 150 torr or less.

[6] The slag floating on the surface of the molten steel contained in the ladle is the molten steel according to any one of [1] to [5] above, wherein the total concentration of iron oxide and manganese oxide is 5% by mass or less. Refining method.

[7] The method of refining molten steel according to any one of [1] to [6] above, wherein the content of the three elements of oxygen, nitrogen, and sulfur contained in the molten steel is simultaneously reduced by the plasma treatment.

According to the present invention, hydrogen plasma treatment can be appropriately performed on molten steel contained in a ladle, and as a result, high-purity steel with few impurities can be quickly melted, resulting in an industrially beneficial effect.

1 is a schematic longitudinal cross-sectional view showing an example of a typical ladle refining furnace.

Hereinafter, the present invention will be described in detail.

The method for refining molten steel according to the present invention is a process of performing a gas stirring treatment in which a stirring gas is blown into the molten steel contained in a ladle to agitate the molten steel in the ladle, wherein the ladle is in a fluid state by the gas stirring treatment. The surface of the molten steel is irradiated as plasma gas with plasmaized hydrogen gas or a mixture of plasmaized hydrogen gas and an inert gas from a plasma generator installed above the molten steel in the ladle, and irradiated with this plasma gas. This is a solvent method that removes one or two or more elements selected from oxygen, nitrogen, and sulfur in molten steel to reduce their content. In this specification, irradiating hydrogen gas or an inert gas containing hydrogen gas as a plasma gas to the molten steel surface is referred to as “plasma treatment” or “hydrogen plasma treatment.”

A refining facility capable of implementing the present invention is a secondary refining furnace capable of agitating the molten steel by blowing a stirring gas into the molten steel in a ladle, for example, a ladle furnace (LF: Ladle Furnace) and a VOD furnace ( There are Vacuum OxygenDecarburization Furnace), VAD Furnace (Vacuum Arc Decarburization Furnace), REDA (Revolutionary Degassing Activator) vacuum degassing device, etc.

In Fig. 1, an example of a typical ladle refining furnace is shown in a schematic longitudinal cross-sectional view. In Figure 1, symbol 1 is a ladle refining furnace, 2 is a ladle, 3 is an upper cover, 4 is a graphite electrode, 5 is an iron shell, 6 is a lining refractory material, 7 is a permanent refractory material, 8, 8a is a bottom odor plug, 9 is molten steel, 10 is slag, 11 and 12 are plasma torches, and 13 is gas bubbles for stirring. The ladle (2), which accommodates the molten steel (9), has an outer shell with a steel shell (5), and refractories are constructed on the inside of the steel shell (5) in the following order: permanent refractory material (7) and lining refractory material (6). , at least part of the lining refractory material 6 (mainly the slag line) is constructed with MgO-based refractory material. Additionally, at the bottom of the ladle 2, bottom blowing plugs 8 and 8a are provided as gas inlet portions for injecting gas for stirring such as rare gas. The plasma torches 11 and 12 are devices that constitute a part of the plasma generating device, and are devices that perform hydrogen plasma treatment by irradiating plasma gas from their tip to the surface of the molten steel 9 in the ladle, and have an upper cover ( 3), and is capable of moving up and down within the space surrounded by the ladle (2) and the upper cover (3). In FIG. 1, two plasma torches are installed, but there may be one contribution of plasma torches, and there may be three or more plasma torches. Also, in Fig. 1, bottom odor plugs are installed at two points, but the bottom odor plugs may be at one point or at three or more points.

The ladle refining furnace 1 blows a stirring gas such as argon gas into the molten steel 9 in the ladle from the bottom blowing plugs 8 and 8a, and while stirring the molten steel 9, a refining flux and This is a facility for adding alloy materials. The ladle refining furnace 1 is also a facility that performs electric heating using the graphite electrode 4 to adjust the components and temperature of the molten steel 9 to target values. In addition, in equipment that performs electric heating with an alternating current power supply, three graphite electrodes 4 are often installed, but in Figure 1, description of two of the three graphite electrodes 4 is omitted, and the graphite electrode ( 4) This is a single drawing. In addition, in the ladle refining furnace 1, the added refining flux is melted to form slag 10 of the desired composition, and the reaction between this slag 10 and the molten steel 9 forms the form of inclusions in the molten steel. Control and desulfurization reaction of molten steel are carried out. The reason why the lining refractory material 6 of the slag line of the ladle 2 is made of MgO-based refractory material is that the MgO-based refractory material has high corrosion resistance against the slag 10. In the ladle refining furnace (1), the pressure of the atmosphere in the space surrounded by the ladle (2) and the upper cover (3) is equivalent to atmospheric pressure. In short, in the ladle refining furnace 1, refining under reduced pressure cannot be performed.

The VOD furnace (not shown) and the VAD furnace (not shown) have a vacuum chamber connected to an exhaust device, and a ladle 2 containing the molten steel 9 is placed inside the vacuum chamber. Then, the inside of the vacuum chamber is depressurized and the rare gas or non-oxidizing gas for stirring is blown in from the bottom blowing plugs 8 and 8a disposed at the bottom of the ladle 2. In this way, while stirring the molten steel 9 in the ladle, it is an equipment for refining by spraying a refining agent such as a desulfurization agent along with oxygen gas or conveying gas from a top blowing lance installed through a vacuum chamber to the molten steel 9 in the ladle. The VAD furnace, like the ladle refining furnace 1, is equipped with a graphite electrode for heating the molten steel 9. In VOD furnaces and VAD furnaces, refining is usually carried out under reduced pressure. The present invention can be practiced by installing a plasma torch on the upper part of the vacuum chamber and penetrating the vacuum chamber.

The REDA vacuum degassing device (not shown) is a degassing furnace that combines bottom blowing agitation of the molten steel in the ladle using a stirring gas and a large-diameter immersion tank with a depressurized interior in which the tip is immersed in the molten steel in the ladle. This is a device that elevates molten steel into a large-diameter immersion tank and refines it while stirring. In the REDA vacuum degassing device, refining is usually carried out under reduced pressure. The present invention can be implemented by installing a plasma torch on the upper part of the large-diameter immersion tank through the large-diameter immersion tank.

In the molten steel refining method according to the present embodiment, the molten steel is stirred by a stirring gas such as argon gas during refining of the molten steel 9 in a secondary refining furnace such as a ladle refining furnace 1 or a VOD furnace. Hydrogen gas or an inert gas containing hydrogen gas is irradiated as plasma gas from the plasma torches 11 and 12 on the surface of (9). Since the temperature in the plasma reaches thousands of degrees or more, the hydrogen gas in the plasma gas is in an atomic or ionic state and is in a very active state. By irradiating active hydrogen in the atomic or ionic state to the surface of the molten steel, reactions of the formulas (6), (7), and (8) shown below are formed, allowing rapid removal of oxygen, nitrogen, and sulfur in the molten steel. You can.

2H + [O] = H ₂ O... … (6)

xH + [N] = NH _x ... … (7)

yH + [S] = H _y S... … (8)

In the formulas (6), (7), and (8), [O] represents oxygen in the molten steel, [N] represents nitrogen in the molten steel, and [S] represents sulfur in the molten steel.

In Fig. 1, the stirring gas is blown from the bottom blowing plugs 8 and 8a, but an injection lance (not shown) is immersed in the molten steel 9, and the stirring gas is blown into the molten steel 9 from the tip of the injection lance. You can do it. As the gas for stirring, argon, which is an inert gas, or hydrogen gas or propane, which is a reducing gas, can be used. In hydrogen plasma treatment, when the purpose is not denitrification reaction, nitrogen gas may be used as a stirring gas. Additionally, inert gas or nitrogen gas can be mixed and used, or can be switched appropriately during hydrogen plasma treatment.

There are various methods for generating plasma, but as shown in FIG. 1, a method of generating plasma using plasma torches 11 and 12 is common. The plasma torches 11 and 12 are one of devices that generate arc plasma stably and with good control in a form suitable for various applications by mainly using direct current and by the action of an air current or water cooling nozzle.

There are two types of plasma torches using the above-described direct current power supply: non-transitional type and transitional type. In the non-transition type plasma torch, there is no need to form an electrode on the molten steel side, so there are fewer equipment restrictions and installation costs are low. From this point of view, a non-transition type plasma torch using direct current arc discharge is used. It is desirable to do so.

Additionally, the plasma generating device can be installed above the molten steel 9, and the method is not particularly limited as long as it can stably supply hydrogen plasma to the surface of the molten steel. For example, when performing hydrogen plasma treatment in the ladle refining furnace 1, hydrogen gas or an inert gas containing hydrogen is supplied to the arc generated from the graphite electrode 4, and hydrogen gas or an inert gas containing hydrogen gas is supplied to the arc generated from the graphite electrode 4. A method of converting the inert gas into plasma may be used. In addition, for a process that does not have a heating electrode, such as a VOD furnace, an electrode for generating an alternating current arc is formed above the molten steel 9, and hydrogen gas or an inert gas containing hydrogen gas is supplied between these electrodes. By doing so, hydrogen plasma treatment can be performed.

As the plasma gas, hydrogen gas or a mixed gas containing hydrogen gas and an inert gas is used. The reason for using hydrogen gas is that impurities in molten steel can be directly removed by turning hydrogen gas into plasma, as described above. In order to obtain a rapid impurity removal effect, it is preferable to mix 0.5 volume% or more of hydrogen gas in the plasma gas. Since the higher the hydrogen gas concentration in the hydrogen plasma gas, the impurity removal effect increases, there is no particular upper limit to the hydrogen gas concentration in the hydrogen plasma gas. As the inert gas, argon gas or helium gas can be used.

In order to quickly reduce impurities such as oxygen, nitrogen, and sulfur in molten steel, the three elements of the flow rate of the plasma gas, the hydrogen gas concentration in the plasma gas, and the circulation flow rate of the molten steel in the ladle must be controlled to an appropriate range. There is a need.

In other words, in order to achieve a rapid impurity removal effect, it is necessary not only to increase the hydrogen gas concentration in the plasma gas, but also to supply an appropriate amount of hydrogen gas for the molten steel circulation flow rate circulating in the molten steel bath in the ladle by stirring the bottom gas. . Specifically, as shown in equation (1) below, the flow rate of the plasma gas (G _P ), the hydrogen concentration in the plasma gas (H ₂ ), and the molten steel circulation flow rate (Q) of the molten steel in the ladle. The element must be a condition that satisfies the relationship in equation (1) below. Also, the relationship between the three elements (G _P × (H ₂ )/Q) is preferably 0.1 or more, and more preferably 0.5 or more. On the other hand, when (G _P × (H ₂ )/Q) becomes greater than 3.0, a large output is required to dissociate or ionize hydrogen in the plasma gas. Furthermore, since the accompanying wear and tear of the plasma torch begins to become significant, it is more preferable to set (G _P × (H ₂ )/Q) to 3.0 or less.

(1) In the equation, G _P is the flow rate of the plasma gas (Nm ³ /min), (H ₂ ) is the hydrogen gas concentration (volume %) in the plasma gas, and Q is the molten steel circulation flow rate of the molten steel in the ladle. (ton/min). In addition, “Nm ³ /min” of the flow rate of the plasma gas is a unit representing the volumetric flow rate of the plasma gas, and “Nm ³ ” means the volume of the plasma gas in a standard state. In this specification, the standard state of plasma gas is 0°C and 1 atm (101325 Pa).

The molten steel circulation flow rate (Q) of the molten steel in the ladle is influenced by the mass of molten steel in the ladle and the stirring power caused by the bottom blowing gas. Therefore, for each of these conditions, the molten steel circulation time of the molten steel in the ladle is measured in the ladle 2 of the actual machine, and the molten steel mass in the ladle is divided by the measured molten steel circulation time, The molten steel circulation flow rate (Q) of the molten steel 9 in the ladle can be determined.

Here, tracer elements (e.g., copper, nickel, etc.) are added to the molten steel 9 in the ladle, and the variation in tracer element concentration of the sample for component analysis taken in time series from within the ladle is within ±5%. If the time required to achieve this is considered the uniform mixing time, the molten steel circulation time is about 1/3 of the uniform mixing time, so the time that is 1/3 of the obtained uniform mixing time can be used as the molten steel circulation time. .

In addition, the molten steel circulation time for the molten steel in the ladle can be obtained by an empirical regression equation expressed in equation (3) below, and the stirring power (ε) in equation (3) below is: It is well known that it can be obtained by an empirical regression equation expressed in equation (4) below. Therefore, it is desirable to determine the molten steel circulation flow rate (Q) of the molten steel in the ladle using the following equations (2), (3), and (4).

In equations (2) to (4), Q is the molten steel circulation flow rate of the molten steel in the ladle (ton/min), W _m is the mass of molten steel in the ladle (ton), and t _c is the molten steel of the molten steel in the ladle. Circulation time (min), D is the average diameter of the molten steel bath in the ladle (m), H is the depth of the molten steel bath in the ladle (m), ε is the stirring power (W/ton), G _B is the molten steel in the ladle is the total stirring gas blowing flow rate (Nm ³ /min), T _L is the molten steel temperature in the ladle (K), and P ₀ is the atmospheric pressure (torr) in the plasma irradiation area. Also, regarding the total agitation gas injection flow rate ( _GB ) to the molten steel in the ladle, “Nm ³ ” means the total agitation gas volume in the standard state, 0°C, 1 atm (101325 Pa) ) is set to the standard state. Also, “torr” is a pressure unit, and 1 torr is 133.32 Pa. Additionally, the side wall of the ladle 2 may be inclined to widen upward. Therefore, the average diameter of the molten steel bath in the ladle is the average value of the diameter of the lower end and the upper end of the molten steel bath in the ladle. am.

The molten steel 9 accommodated in the ladle and before hydrogen plasma treatment is tapped into the ladle 2 from a converter or electric furnace, and subjected to vacuum degassing in a vacuum degassing facility such as, for example, an RH vacuum degassing device. It may go through a degassing refining process and be returned to a gas stirring treatment process in which the molten steel in the ladle is stirred by blowing a stirring gas.

The molten steel 9 before the hydrogen plasma treatment may be in a non-deoxidized state, but before the hydrogen plasma treatment, a reducing gas such as hydrogen gas or propane is supplied to the molten steel 9 to preliminarily deoxidize the molten steel 9. You can do it. By preliminary deoxidation with a reducing gas before plasma treatment, plasma treatment can be started from a state in which the oxygen concentration in the molten steel has decreased to a certain extent, so the load of the reaction according to equation (6) above is reduced, and the plasma treatment time is reduced. It can be shortened.

In addition, when the removal of nitrogen and sulfur in the molten steel is mainly performed, the oxygen concentration in the molten steel may be reduced in advance by adding a deoxidizing agent such as aluminum or silicon to deoxidize the molten steel 9 before the plasma treatment. In this case, since the oxygen concentration in the molten steel is already low, the deoxidation effect by plasma treatment is limited. Oxygen in molten steel functions as an interface active element, and by reducing the oxygen concentration in molten steel through deoxidation, the escape of nitrogen gas, hydrogen nitride, and hydrogen sulfide from the surface of molten steel into the gas phase (atmosphere within the ladle) can be prevented. . However, by lowering the oxygen concentration in the molten steel to a low level through deoxidation treatment with aluminum or silicon, the effect of quickly removing oxygen, nitrogen, sulfur, etc. in the molten steel by hydrogen plasma can be obtained. In addition, by deoxidizing the molten steel 9, not only the direct sulfur removal effect by hydrogen plasma treatment but also acceleration of the desulfurization reaction by the molten steel-slag reaction can be expected.

It is more preferable that the plasma output (E) satisfies the equation (9) below. In order to dissociate hydrogen gas into an atomic state at a high rate, a certain level of output is required, but the required output varies depending on the flow rate of the introduced plasma gas or the hydrogen concentration in the plasma gas. As a result of examining them, it was found that the plasma output only needs to satisfy the relationship in equation (9). If the output is increased, the impurity removal effect becomes more noticeable because not only the dissociation into hydrogen atoms but also the rate of ionization into hydrogen ions increases. On the other hand, since the power cost increases with the increase in output, the plasma output can be selected according to the balance between required quality and cost.

E ≥ G _P × (1.5 × (H ₂ ) + 11.5) … (9)

(9) In the equation, E is the plasma output (kW).

During plasma treatment, impurities can be reduced more efficiently by providing a certain level of flow to the molten steel 9 in the ladle. That is, since plasma irradiation is performed on a relatively localized area of the molten steel surface, the impurity concentration of the entire molten steel in the ladle can be quickly reduced by continuously sending new molten steel 9 to the plasma irradiation section by stirring the molten steel.

It can be seen that the flow on the surface of molten steel is correlated with the stirring power (ε) expressed in equation (4) above, and as the stirring power (ε) increases, the flow on the surface of molten steel increases. Therefore, from the viewpoint of improving the efficiency of plasma treatment, it is preferable that the stirring power (ε) is set to 25 W/ton or more. If the stirring power (ε) is less than 25 W/ton, the circulation and mixing of the molten steel surface, which is the plasma irradiation area, and the bulk molten steel become stagnant, and a rapid impurity reduction effect is not obtained. There is no particular upper limit to the stirring power (ε), but if the stirring power (ε) is too large, gas blowing or molten steel scattering increases, so it is preferably set to 150 W/ton or less.

Additionally, there is a preferable range for the position at which plasma is irradiated. That is, it is preferable to irradiate hydrogen plasma at or near the position where the stirring gas blown in from the bottom blowing plugs 8 and 8a becomes the stirring gas bubble 13 and floats on the molten steel surface. In Fig. 1, a plasma torch 11 is installed vertically above the bottom odor plug 8, and a plasma torch 12 is installed vertically above the bottom odor plug 8a. In short, in the present invention, the plasma irradiation range within the radius (r) calculated by the equation (5) below, centered just above the molten steel surface vertically above at least one point among the gas injection portions (bottom blowing plug). In the range, it is desirable to irradiate plasma gas.

(5) In the equation, r is the radius of the plasma irradiation range (m), and g _B is the injection flow rate of the stirring gas (Nm ³ /min/one gas injection point). Also, regarding the gas blowing flow rate (g _B ) for stirring, “Nm ³ ” means the volume of gas for stirring in a standard state, and 0° C. and 1 atm (101325 Pa) are taken as the standard state.

The area within the plasma irradiation range radius (r) is the area where the flow is the fastest (violent) on the surface of the molten steel in the ladle, and by irradiating plasma there, the reaction between the hydrogen plasma and the molten steel in the ladle progresses quickly. In addition, in the area corresponding to the radius r of the plasma irradiation range, the molten steel 9 rising together with the gas bubbles 13 for stirring pushes out the slag 10 and the molten steel surface is exposed, or the slag thickness is exposed. Since it is a relatively thin area, plasma treatment can be performed on the molten steel surface without being hindered by the slag 10.

On the other hand, when the plasma is irradiated outside the area of the plasma irradiation range radius (r), since the molten steel flow is in a slow range, the extruded slag 10 is deposited and the slag thickness becomes large, and the plasma extends to the molten steel 9. There is a risk that it may not be reached.

When plasma irradiation is performed on the surface of molten steel in a ladle, the atmosphere is preferably under reduced pressure, specifically 150 torr or less. By irradiating plasma gas under reduced pressure of 150 torr or less, it can be expected to increase the flow rate of the plasma jet and further promote the dissociation of hydrogen gas molecules into atoms and ions, so the effect of reducing impurities in molten steel is greater than plasma treatment under atmospheric pressure. It gets bigger. When the atmospheric pressure is higher than 150 torr, the above effect is small, so the effect of reduced pressure is not obtained.

A specific example for performing hydrogen plasma treatment under reduced pressure is shown below. For example, as in a VOD furnace, the ladle (2) containing the molten steel 9 is placed in a vacuum chamber, and the surface of the molten steel in the ladle is irradiated with hydrogen plasma from a plasma generator installed at the top of the vacuum chamber. . Alternatively, like the REDA vacuum degassing device, the large-diameter immersion tank is immersed in the molten steel in the ladle, the inside of the large-diameter immersion tank is evacuated to create a reduced-pressure atmosphere, and then the large-diameter immersion tank is pumped into the large-diameter immersion tank from the plasma generator provided on the upper part of the large-diameter immersion tank. Plasma irradiation is performed on the surface of the sucked molten steel. These are examples of processing under reduced pressure and are not limited to these methods. Additionally, the atmospheric pressure when performing plasma irradiation is more preferably 100 torr or less, and even more preferably 50 torr or less.

Among the components of the slag 10 floating on the surface of the molten steel 9 in the ladle, especially iron oxide and manganese oxide in the slag can serve as an oxygen source for the molten steel 9. For this reason, the total of the iron oxide concentration and the manganese oxide concentration of the slag 10 is preferably 5 mass% or less, more preferably 3 mass% or less, and even more preferably 1 mass% or less. If the total concentration of the iron oxide concentration and the manganese oxide concentration is higher than 5% by mass, oxygen is supplied from the slag 10 to the molten steel 9 simultaneously during the plasma treatment, and the impurity reduction effect cannot be sufficiently obtained.

As a method of reducing iron oxide and manganese oxide in the slag 10, before plasma treatment, metal aluminum or aluminum dross is added to the slag 10 floating on the molten steel, and iron oxide or manganese oxide is converted into aluminum. It is effective to carry out the reduction of . Additionally, it is also effective to remove the slag 10 from the ladle 2 and then add an additive into the ladle to create new slag containing less iron oxide or manganese oxide. Additionally, reduction treatment of iron oxide and manganese oxide in the slag can also be performed by irradiating the slag 10 with hydrogen plasma gas.

After hydrogen plasma treatment, the timing of addition of a deoxidizing agent such as aluminum or silicon is not particularly limited. For example, after stopping the hydrogen plasma, oxygen is supplied to the molten steel 9 from the atmosphere, the slag 10, or the ladle refractory material, and the oxygen concentration in the molten steel increases, so after the hydrogen plasma treatment, the molten steel 9 It is desirable to quickly add a deoxidizer such as aluminum or silicon and maintain the oxygen concentration in the molten steel reduced by hydrogen plasma treatment at a low level. When it is necessary to adjust the molten steel composition in addition to the deoxidizing agent such as aluminum or silicon, a predetermined ferroalloy or pure metal is added to the molten steel 9 in the ladle after hydrogen plasma treatment.

Additionally, since the hydrogen concentration in the molten steel increases to several mass ppm or more due to the hydrogen plasma treatment, it is preferable to perform dehydrogenation treatment for 5 minutes or more under reduced pressure of 10 torr or less after the hydrogen plasma treatment. For example, when plasma treatment is performed in a ladle refining furnace (1), refining is provided in a vacuum degassing facility such as an RH vacuum degassing device in the post-process, and dehydrogenation treatment is performed in a vacuum degassing facility. do. When plasma treatment is performed in a VOD furnace or REDA vacuum degassing device having a degassing function, dehydrogenation treatment is continuously performed after the plasma treatment.

By the refining method using hydrogen plasma treatment as described above, oxygen, nitrogen, and sulfur in molten steel can be quickly reduced to 30 mass ppm or less each.

Example

In a practical machine with a molten steel volume of 200 tons or more and 350 tons or less per charge, a ladle refining furnace (LF) was used and a test was conducted in which molten steel tapped from the converter was subjected to hydrogen plasma treatment under atmospheric pressure (this invention) Honors 1 to 9, Comparative Examples 1 to 3). In addition, in a test to evaluate the influence of atmosphere under reduced pressure, a VOD furnace is used in an actual machine with a molten steel volume of 200 tons to 350 tons per charge, and hydrogen plasma treatment is performed under reduced pressure on the molten steel tapped from the converter. Tests were conducted (invention examples 10 to 19, comparative examples 4 and 5).

In the ladle refining furnace, a non-transition type plasma torch using direct current arc discharge was installed on the upper part of the furnace cover. Additionally, in the VOD furnace, a non-transition type plasma torch by direct current arc discharge is installed at the upper part of the vacuum chamber, and the plasma gas flow rate and the hydrogen gas concentration in the plasma gas are changed from these plasma torches to change the molten steel in the ladle. Hydrogen plasma was irradiated on the surface. Additionally, the operating conditions and molten steel composition (oxygen concentration, nitrogen concentration, sulfur concentration, etc.) in the ladle refining furnace and VOD furnace were changed. In addition, hydrogen plasma treatment in the ladle refining furnace was performed with arc heating by the graphite electrode stopped.

Before and after the hydrogen plasma treatment, samples for component analysis were collected from the molten steel in the ladle, and the oxygen concentration, nitrogen concentration, and sulfur concentration in the molten steel were analyzed to confirm the effect of the plasma treatment. The plasma treatment time was standardized to 15 minutes. Additionally, deoxidizing agents such as aluminum were not added during the period from steel tapping in the converter to plasma treatment. The iron oxide concentration and manganese oxide concentration of the slag in the ladle were adjusted by adding aluminum dross to the slag in the ladle before starting treatment in the ladle refining furnace or VOD furnace.

Table 1 shows the test conditions for each test, and Table 2 shows the evaluation results.

In the present invention example, by performing hydrogen plasma treatment for 15 minutes, the oxygen concentration, nitrogen concentration, and sulfur concentration in the molten steel were simultaneously and quickly reduced to 30 mass ppm or less. The removal rates of each element from before the start of the plasma treatment to after the end were 94% or more for oxygen in the molten steel, 33% or more for nitrogen in the molten steel, and 20% or more for sulfur in the molten steel.

On the other hand, in the comparative example that does not satisfy the conditions of the present invention, the reduction of oxygen, nitrogen, and sulfur in the molten steel is insufficient even after the hydrogen plasma treatment, and the removal rate of each element from before the start of the plasma treatment to after the end is lower than the oxygen in the molten steel. was 90% or less, nitrogen in molten steel was 19% or less, and sulfur in molten steel was 15% or less, all of which were low.

1: Ladle refining furnace
2: Ladle
3: upper cover
4: graphite electrode
5: iron skin
6: Lining refractory material
7: Permanent refractory material
8: Low odor plug
9: molten steel
10: Slag
11: Plasma Torch
12: Plasma Torch
13: Gas bubble for stirring

Claims (7)

In the process of performing a gas stirring treatment in which a stirring gas is blown into the molten steel contained in the ladle to stir the molten steel in the ladle,
From the plasma generator installed above the molten steel in the ladle, hydrogen gas or an inert gas containing hydrogen gas is applied to the surface of the molten steel in the ladle in a fluid state by the gas stirring treatment as a plasma gas as shown in (1). ) A method of refining molten steel in which plasma treatment is performed under conditions that satisfy the formula, and the plasma treatment reduces the content of one or two or more elements selected from oxygen, nitrogen, and sulfur contained in the molten steel.

Here, G _P is the flow rate of the plasma gas (Nm ³ /min), (H ₂ ) is the hydrogen gas concentration (volume %) in the plasma gas, and Q is the molten steel circulation flow rate of the molten steel in the ladle (ton/min). According to claim 1,
A method for refining molten steel, wherein the molten steel circulation flow rate of the molten steel in the ladle is calculated using the following equations (2), (3), and (4).

Here, Q is the molten steel circulation flow rate of the molten steel in the ladle (ton/min), W _m is the mass of molten steel in the ladle (ton), t _c is the molten steel circulation time of the molten steel in the ladle (min), and D is the ladle The average diameter of the molten steel bath in the ladle (m), H is the depth of the molten steel bath in the ladle (m), ε is the stirring power (W/ton), G _B is the total stirring gas injection flow rate for the molten steel in the ladle (Nm) ³ /min), T _L is the molten steel temperature (K) in the ladle, and P ₀ is the atmospheric pressure (torr) in the plasma irradiation area. The method of claim 1 or 2,
The gas stirring process is performed by installing a gas inlet at one point or at two or more points at the bottom of the ladle, and blowing a gas for stirring into the molten steel in the ladle from the gas inlet, and at that time ( 4) A method of refining molten steel in which the stirring power (ε) calculated by the formula is 25 W/ton or more. According to claim 3,
The molten steel is irradiated with the plasma gas in a range within the radius (r) of the plasma irradiation range calculated by the equation (5) below, centered just above the surface of the molten steel vertically above at least one of the gas inlet points. refining method.

Here, r is the radius of the plasma irradiation range (m), and g _B is the injection flow rate of the stirring gas (Nm ³ /min/one gas injection point). The method according to any one of claims 1 to 4,
A method of refining molten steel, wherein the atmospheric pressure when irradiating plasma gas to the molten steel in the ladle is 150 torr or less. The method according to any one of claims 1 to 5,
A method of refining molten steel, wherein the slag floating on the surface of molten steel contained in a ladle has a total concentration of iron oxide and manganese oxide of 5% by mass or less. The method according to any one of claims 1 to 6,
A method of refining molten steel in which the content of the three elements of oxygen, nitrogen, and sulfur contained in the molten steel is simultaneously reduced by the plasma treatment.

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