Method for refining molten steel

Abstract

In a RH vacuum degassing device of a steelmaking process, a refining reaction such as deoxidation, denitrification, desulfurization, and the like is rapidly performed when hydrogen plasma is given to molten steel, and molten steel with less impurities is efficiently produced. The present application is a molten steel refining method in which, in a process of refining molten steel (3) accommodated in a ladle using a RH vacuum degassing device (1), a surface of the molten steel circulating in a vacuum tank of the RH vacuum degassing device is subjected to plasma treatment in which hydrogen gas or non-active gas containing hydrogen gas is irradiated as plasma gas under a condition satisfying the following formula (1), and the content of one or more than two elements selected from oxygen, nitrogen, and sulfur contained in the molten steel is reduced. In formula (1), G P is a flow rate (Nm 3 / minute) of the plasma gas, (H2) is a hydrogen gas concentration (vol%) in the plasma gas, and Q is a circulation flow rate (tons / minute) of the molten steel circulating in the vacuum tank. G P × (H2) / Q ≥ 0.1 …… (1).

Description

Technical Field

[0001] This invention relates to a refining method for producing molten steel with low contents of oxygen, nitrogen, and sulfur, which are impurity elements. More specifically, it relates to a refining method in which hydrogen or an inactive gas containing hydrogen is irradiated as a plasma gas onto molten steel in a vacuum cell of an RH vacuum degassing apparatus. Background Technology

[0002] It is well known that non-metallic inclusions in steel materials have adverse effects on material properties and quality. Furthermore, oxide-based non-metallic inclusions cause clogging of immersion nozzles in continuous casting, leading to reduced productivity due to decreased casting speed, or in the worst case, forcing casting to be interrupted. Examples of non-metallic inclusions include oxide-based deoxidation products generated during steel deoxidation, sulfides of alloying elements in steel, and nitrides. To reduce the amount of these non-metallic inclusions (hereinafter referred to as "inclusions"), it is important to minimize the amount of oxygen, nitrogen, and sulfur in the molten steel; various efforts have been made in the past to this end.

[0003] To remove oxygen from molten steel, deoxidizers such as aluminum (Al) and silicon (Si) are added to fix the dissolved oxygen into Al₂O₃ and SiO₂, respectively. The resulting oxide inclusions are removed by flotation through methods such as gas agitation of the molten steel or circulation treatment in an RH vacuum degassing unit, utilizing the difference in specific gravity between the oxide inclusions and the molten steel. However, currently, it is impossible to separate and remove all oxide inclusions; they inevitably remain in the molten steel.

[0004] Nitrogen in molten steel is reduced through vacuum treatment in vacuum degassing equipment. However, in addition to being affected by oxygen and sulfur, which are surface-active elements, nitrogen in molten steel is also susceptible to nitrogen adsorption caused by atmospheric entrainment from outside the vacuum system, making it currently impossible to consistently achieve low nitrogen concentrations.

[0005] Sulfur in molten steel can be reduced by adding CaO-based fluxes or CaO-Al2O3-based fluxes (desulfurizing agents). For example, in ladle refining in a ladle 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 and the CaO-Al2O3-based flux, causing sulfur to move to the flux side (slag side) and reducing sulfur content in the molten steel. However, in such ladle refining furnace processes, the use of graphite electrodes for arc heating results in the dissolution of carbon into the molten steel, making it difficult to apply to steel grades such as very low carbon steel.

[0006] In addition, in RH vacuum degassing units, there are methods for desulfurization by adding CaO-based flux or CaO-Al2O3-based flux to the molten steel circulating in the vacuum tank. Furthermore, there are methods for desulfurization by injecting (blowing) CaO-based flux or CaO-Al2O3-based flux into the molten steel circulating in the vacuum tank from a top-blown lance using an inert gas such as argon. However, in these methods, the reaction time between the molten steel and the flux is insufficient, making it difficult to effectively obtain molten steel with a low sulfur concentration.

[0007] However, hydrogen plasma is known as a refining technique for reducing impurities in metals. Since the temperature within the plasma reaches thousands of degrees, the hydrogen gas in the plasma is in an atomic or ionic state, becoming highly reactive. By irradiating the surface of molten steel with this plasma, superior refining effects that cannot be achieved by ordinary hydrogen irradiation alone can be expected. That is, oxygen, nitrogen, and sulfur in molten steel can be rapidly removed through the reactions described in equations (5) to (7) below.

[0008] 2H+[O]=H2O……(5)

[0009] xH+[N]=NH x ……(6)

[0010] yH+[S]=H y S……(7)

[0011] Here, [O] represents oxygen in molten steel, [N] represents nitrogen in molten steel, and [S] represents sulfur in molten steel.

[0012] In addition to being able to convert oxygen, nitrogen, and sulfur in molten steel into H2O, NH4+, and NH4+ respectively... x and H y S is removed from the system as a gas. Since no inclusions are generated during oxygen removal (deoxidation), steel with high cleanliness can be obtained.

[0013] As a refining technique for high-purity metals utilizing such hydrogen plasma, Patent Document 1 discloses a preferred range of hydrogen concentration in the plasma gas and furnace pressure used to reduce oxygen, nitrogen, or carbon in the metal when melting metals using hydrogen plasma.

[0014] Existing technical documents

[0015] Patent documents

[0016] Patent Document 1: Japanese Patent No. 4305792 Summary of the Invention

[0017] However, the following problems exist when applying the technology of the aforementioned Patent Document 1 to industrial-scale steelmaking processes.

[0018] The embodiments described in Patent Document 1 illustrate the refining effect when treating metals at levels of tens of grams to tens of kilograms in a plasma melting furnace. However, in industrial-scale steelmaking processes, which require processing over 100 tons of molten steel, it is difficult to irradiate the entire molten steel with plasma gas. Therefore, the technology disclosed in Patent Document 1 may not achieve rapid impurity removal. To achieve rapid impurity removal, it is important not only to ensure appropriate plasma conditions but also to ensure appropriate flow conditions on the molten steel side, and to perform hydrogen plasma treatment efficiently.

[0019] Furthermore, Patent Document 1 does not specify the amount of metal in the object to which hydrogen plasma is applied, nor the relationship between the metal amount and the plasma gas flow rate. Therefore, it is assumed that even with proper control of the plasma gas composition and atmosphere pressure, insufficient plasma gas flow rate and hydrogen content relative to the metal amount will prevent the achievement of adequate impurity reduction. Moreover, Patent Document 1 does not involve applying hydrogen plasma to molten iron; it also utilizes plasma to heat and melt the target metal. Therefore, even if the disclosed plasma gas conditions are applied to molten steel, as in a steelmaking process, the desired effect may not be achieved.

[0020] The present invention was made in view of the above circumstances, and its object is to provide a refining method in which the refining reactions of deoxidation, denitrification and desulfurization when hydrogen plasma is introduced into molten steel are carried out rapidly in the RH vacuum degassing device of the steelmaking process, so as to efficiently produce high-purity molten steel with fewer impurities.

[0021] The main idea of ​​the present invention for solving the above problems is as follows.

[0022] [1] A method for refining molten steel, in the process of refining molten steel contained in a ladle by circulating it in the vacuum tank of an RH vacuum degassing device, wherein the surface of the molten steel circulating in the vacuum tank of the RH vacuum degassing device is subjected to plasma treatment, in which hydrogen or an inactive gas containing hydrogen is irradiated as plasma gas by a plasma generator provided in the vacuum tank under the conditions of the following formula (1), thereby reducing the content of one or more elements selected from oxygen, nitrogen and sulfur contained in the molten steel.

[0023]

[0024] Here, G P The flow rate of the plasma gas (Nm³) 3 / min), (H2) is the hydrogen concentration in the plasma gas (volume%), and Q is the circulation flow rate of the molten steel circulating in the vacuum tank (tons / min).

[0025] [2] According to the steel refining method described in [1] above, the circulation volume of the steel circulating in the vacuum tank is calculated using the following formula (2).

[0026]

[0027] Here, Q represents the circulation rate (tons / minute) of the molten steel circulating in the vacuum tank, and G... C The flow rate of the circulating gas (Nm³) 3 / minute), D is the inner diameter (m) of the submersible tube of the RH vacuum degassing device, P0 is the pressure (Tor) at the blowing position of the circulating gas, and P is the pressure (Tor) in the vacuum tank.

[0028] [3] According to the steel refining method described in [1] or [2] above, the surface flow velocity of the molten steel circulating in the vacuum tank irradiated by the plasma gas satisfies the relationship between the following equations (3) and (4).

[0029]

[0030]

[0031] Here, V is the surface velocity (m / min) of the molten steel circulating in the vacuum tank, and G... P The flow rate of the plasma gas (Nm³) 3 / minute), π is pi, L is the distance between the centers of the rising and descending submersible tubes (m), Q is the circulation flow rate of molten steel in the vacuum tank (tons / minute), and ρ is the density of molten steel (kg / m³). 3 H is the height of molten steel in the vacuum tank (m), and d is the inner diameter of the vacuum tank (m).

[0032] [4] The refining method of molten steel according to any one of [1] to [3] above, wherein the total concentration of iron oxides and manganese oxides in the slag floating on the surface of the molten steel contained in the ladle is 5% by mass or less.

[0033] [5] The refining method of molten steel according to any one of [1] to [4] above, wherein the contents of the three elements oxygen, nitrogen and sulfur contained in the molten steel are reduced simultaneously by the plasma treatment described above.

[0034] According to the present invention, molten steel refined in an RH vacuum degassing unit can be appropriately subjected to hydrogen plasma treatment, resulting in the rapid melting of molten steel with fewer impurities, which brings beneficial industrial effects. Attached Figure Description

[0035] Figure 1This is a longitudinal cross-sectional schematic diagram of an RH vacuum degassing apparatus, illustrating an example of a mode of hydrogen plasma treatment performed in an RH vacuum degassing apparatus. Detailed Implementation

[0036] The present invention will now be described in detail.

[0037] The steel refining method of the present invention involves a refining process in which molten steel contained in a ladle is circulated into the vacuum tank of an RH vacuum degassing device. In this process, plasma-generated hydrogen gas, or a mixture of plasma-generated hydrogen gas and an inert gas, is irradiated onto the surface of the molten steel circulating in the vacuum tank by a plasma generator located within the vacuum tank of the RH vacuum degassing device. This plasma irradiation removes one or more elements selected from oxygen, nitrogen, and sulfur from the molten steel, reducing their content. In this specification, the irradiation of the molten steel surface with hydrogen gas or an inert gas containing hydrogen gas is referred to as "plasma treatment" or "hydrogen plasma treatment."

[0038] Figure 1 This is a longitudinal cross-sectional schematic diagram of an RH vacuum degassing apparatus, illustrating an example of how plasma treatment is performed in an RH vacuum degassing apparatus. Figure 1 In the diagram, symbol 1 represents the RH vacuum degassing device, 2 represents the ladle, 3 represents molten steel, 4 represents slag, 5 represents the vacuum tank, 6 represents the upper tank, 7 represents the lower tank, 8 represents the rising side submersible pipe, 9 represents the descending side submersible pipe, 10 represents the circulating gas inlet pipe, 11 represents the pipeline, 12 represents the raw material inlet, and 13 represents the plasma torch. The vacuum tank 5 consists of the upper tank 6 and the lower tank 7. Furthermore, the plasma torch 13 is a device that forms part of the plasma generating apparatus. It is used to irradiate plasma gas onto the surface of the molten steel 3 circulating within the vacuum tank from its front end to perform hydrogen plasma treatment. The plasma torch 13 is installed through the upper part of the vacuum tank 5 and can move up and down inside the vacuum tank 5.

[0039] In the RH vacuum degassing apparatus 1, a ladle 2 containing molten steel 3 is raised using a lifting device (not shown), immersing the rising-side submersible tube 8 and the descending-side submersible tube 9 in the molten steel 3 within the ladle. Furthermore, an exhaust device (not shown) connected to pipe 11 depressurizes the interior of the vacuum tank 5 by venting the exhaust gas, and circulating gas is blown into the rising-side submersible tube 8 from the circulating gas inlet pipe 10. As the pressure inside the vacuum tank 5 decreases, the molten steel 3 in the ladle rises proportionally to the pressure difference (vacuum degree) between atmospheric pressure and the vacuum tank, flowing into the vacuum tank. Additionally, due to the air-lift effect of the circulating gas blown in from the circulating gas inlet pipe 10, the molten steel 3 in the ladle, together with the circulating gas, causes the rising-side submersible tube 8 to rise and flow into the vacuum tank 5. Argon is typically used as the circulating gas.

[0040] Molten steel 3, flowing into the vacuum tank 5 due to pressure difference and air lift effect, returns to the ladle 2 via the descending side submersible pipe 9. The flow of molten steel from the ladle 2 into the vacuum tank 5 and back to the ladle 2 is called "circulation". In this way, molten steel 3 forms circulation, and RH vacuum degassing and refining is performed on molten steel 3.

[0041] That is, by exposing the molten steel 3 to a reduced-pressure atmosphere within a vacuum tank, the gaseous components such as hydrogen and nitrogen in the molten steel shift from an equilibrium state of contact with the atmosphere to an equilibrium state of contact with the reduced-pressure atmosphere. As a result, hydrogen and nitrogen move from the molten steel 3 into the atmosphere within the vacuum tank, thus degassing the molten steel 3 (dehydrogenation and denitrification treatment). Furthermore, because the molten steel 3 circulates between the ladle 2 and the vacuum tank 5, i.e., the molten steel 3 is strongly agitated, this promotes the separation of oxide inclusions suspended in the molten steel from the molten steel 3 to the slag 4, even when the molten steel 3 is deoxidized by aluminum and other materials.

[0042] In the steel refining method of this embodiment, after the molten steel 3 in the ladle begins to circulate into the vacuum tank 5, hydrogen or an inactive gas containing hydrogen is irradiated onto the surface of the molten steel 3 circulating in the vacuum tank as plasma gas from the plasma torch 13. Since the temperature inside the plasma reaches several thousand degrees, the hydrogen in the plasma gas becomes atomic or ionic, becoming a highly reactive state. By irradiating the surface of the molten steel with active hydrogen in atomic or ionic state, reactions are formed as shown in equations (5), (6), and (7) below, which can more quickly remove oxygen, nitrogen, and sulfur from the molten steel.

[0043] 2H+[O]=H2O……(5)

[0044] xH+[N]=NH x ……(6)

[0045] yH+[S]=H y S……(7)

[0046] In equations (5), (6), and (7), [O] represents oxygen in molten steel, [N] represents nitrogen in molten steel, and [S] represents sulfur in molten steel.

[0047] There are various ways to generate plasma, but generally it is as follows: Figure 1 The diagram shows a method for generating plasma using a plasma torch 13. The plasma torch 13 is one of the devices that primarily uses a DC power supply and generates arc plasma stably and controllably in a manner suitable for various applications through the action of airflow, water-cooled nozzles, etc.

[0048] The plasma torches using DC power supplies mentioned above include non-transfer and transfer types. Using a non-transfer plasma torch eliminates the need for electrodes on the molten steel side, thus reducing equipment limitations. Furthermore, the installation cost is low. From this perspective, the non-transfer plasma torch utilizing DC arc discharge is preferred.

[0049] It should be noted that there are no particular restrictions on the form of the plasma generator, as long as it can be set in the vacuum chamber of the RH vacuum degassing device 1 and can stably supply hydrogen plasma to the molten steel 3. For example, it can be a method in which electrodes for generating an alternating electric arc are set in the vacuum chamber of the RH vacuum degassing device 1, and hydrogen or a hydrogen-containing inactive gas is supplied between the electrodes to plasmaize the hydrogen or hydrogen-containing inactive gas.

[0050] Hydrogen or a mixture of hydrogen and an inert gas is used as the plasma gas. The reason for using hydrogen is, as mentioned above, that impurities in molten steel can be directly removed by plasmaizing the hydrogen. To achieve rapid impurity removal, it is preferable to mix at least 0.5% by volume of hydrogen into the plasma gas. The higher the hydrogen concentration in the plasma gas, the greater the impurity removal effect; therefore, no upper limit is specifically set for the hydrogen concentration in the plasma gas. Argon or helium can be used as the inert gas.

[0051] To rapidly reduce impurities such as oxygen, nitrogen, and sulfur in molten steel, it is necessary to control three factors within appropriate ranges: the flow rate of plasma gas, the hydrogen concentration in plasma gas, and the circulation volume of molten steel circulating in the vacuum tank.

[0052] That is, in order to achieve rapid impurity removal, it is necessary not only to increase the hydrogen concentration in the plasma gas, but also to supply an appropriate amount of hydrogen to the molten steel fed into the vacuum tank 5 of the RH vacuum degassing device 1. Specifically, as shown in equation (1) below, the flow rate of the plasma gas (G P The three factors—hydrogen concentration (H2) in the plasma gas, and circulation volume (Q) of the molten steel circulating in the vacuum tank—must satisfy the relationship described in equation (1). Furthermore, it is preferable that the relationship between the three factors (G) is such that... P ×(H2) / Q) is 0.5 or more, more preferably 1.0 or more. On the other hand, if (G P If (G ×(H2) / Q) is greater than 20.0, a large output is required to dissociate or ionize the hydrogen in the plasma gas. Furthermore, the loss of the plasma torch 13 becomes significant, therefore it is more preferable to make (G ×(H2) / Q) greater than 20.0. P ×(H2) / Q) is below 20.0.

[0053]

[0054] (1) In the formula, GP The flow rate of the plasma gas (Nm³) 3 / min), (H2) is the hydrogen concentration in the plasma gas (volume %), and Q is the circulation flow rate of the molten steel circulating in the vacuum tank (tons / min). It should be noted that the plasma gas flow rate is expressed in Nm³. 3 " / minute" is a unit representing the volumetric flow rate of plasma gas, "Nm". 3 "" refers to the volume of plasma gas under standard conditions. In this specification, the standard conditions of plasma gas are set to 0°C and 1 atm (101325 Pa).

[0055] The circulation rate (Q) of molten steel 3 circulating in the vacuum tank is affected by the flow rate of the circulating gas, the atmospheric pressure in the vacuum tank, and the cross-sectional area of ​​the rising-side submersible tube. Therefore, for each of these conditions, the uniform mixing time is measured in the actual RH vacuum degassing device 1, and the circulation time of molten steel 3 can be obtained by dividing the amount of molten steel contained in the ladle by the molten steel circulation time obtained from the measured uniform mixing time. Here, the uniform mixing time can be calculated as the time required for the concentration of tracer elements (e.g., copper, nickel, etc.) of the molten steel in the ladle directly below the rising-side submersible tube or in the vacuum tank to vary within ±5% from the concentration of tracer elements in the composition analysis samples collected sequentially from the ladle. Since the molten steel circulation time is approximately 1 / 3 of the uniform mixing time, this 1 / 3 of the obtained uniform mixing time can be used as the molten steel circulation time.

[0056] In addition, it is well known that the circulation volume (Q) of the molten steel 3 circulating in the vacuum tank can be obtained by the empirical regression equation expressed by the following equation (2). Therefore, it is preferable to use the following equation (2) to obtain the circulation volume (Q) of the molten steel 3 circulating in the vacuum tank.

[0057]

[0058] (2) In the formula, Q is the circulation flow rate of molten steel in the vacuum tank (tons / minute), and G C The flow rate of the circulating gas (Nm³) 3 / minute), D is the inner diameter (m) of the submersible tube of the RH vacuum degassing device, P0 is the pressure (Tor) at the injection point of the circulating gas, and P is the pressure (Tor) in the vacuum chamber. It should be noted that "Tor" is a pressure unit, 1 Tor is 133.32 Pa. It should also be noted that the flow rate of the circulating gas is expressed in Nm³. 3 "" refers to the volume of circulating gas under standard conditions, with 0℃ and 1 atm (101325 Pa) set as the standard conditions.

[0059] Molten steel 3, contained in a ladle and before undergoing hydrogen plasma treatment, can be tapped from a converter or electric furnace to a ladle 2 and then transported to an RH vacuum degassing unit 1. Alternatively, it can be tapped from a converter or electric furnace to a ladle, undergo ladle refining in a heating and stirring treatment device (sometimes called a ladle furnace), and then transported to an RH vacuum degassing unit 1.

[0060] The molten steel 3 before hydrogen plasma treatment can be in an undeoxidized state, but it can also be pre-deoxidized by supplying reducing gases such as hydrogen or propane to the molten steel 3 before hydrogen plasma treatment. By using reducing gases for pre-deoxidation before plasma treatment, plasma treatment can begin when the oxygen concentration in the molten steel has been reduced to a certain extent, thus reducing the reaction load of the above equation (5) and shortening the plasma treatment time. The pre-deoxidation using reducing gases can be carried out before treatment in the RH vacuum degassing unit, or it can be carried out before plasma treatment during refining in the RH vacuum degassing unit.

[0061] Furthermore, when the primary focus is on removing nitrogen and sulfur from molten steel, deoxidizers such as aluminum and silicon can be added before plasma treatment to deoxidize the molten steel, thereby reducing the oxygen concentration in the steel beforehand. In this case, since the oxygen concentration in the molten steel is already low, the deoxidation effect of plasma treatment is limited. Oxygen in molten steel functions as a surface-active element, inhibiting nitrogen, hydrogen nitride, and hydrogen sulfide from molten steel surface into the gas phase (atmosphere inside the vacuum tank). However, by using aluminum and silicon for deoxidation treatment to maintain a low oxygen concentration in the molten steel, the effect of removing oxygen, nitrogen, and sulfur from the molten steel using hydrogen plasma can be quickly achieved.

[0062] The plasma output (E) preferably satisfies the following equation (9). To achieve a high proportion of hydrogen dissociation into atomic states, a certain 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. Research results show that the plasma output preferably satisfies the relationship in equation (9). When the output is increased, not only does the dissociation into hydrogen atoms increase, but the proportion of ionization into hydrogen ions also increases, thus making the impurity removal effect more significant. On the other hand, as the output increases, the power cost increases; therefore, the plasma output can be selected based on the desired balance between quality and cost.

[0063] E≥G P ×(1.5×(H2)+11.5)…(9)

[0064] (9) In the formula, E is the plasma output (kW).

[0065] Furthermore, the inventors have discovered that by ensuring proper flow of molten steel within the vacuum tank during hydrogen plasma treatment, impurities in the molten steel can be reduced more effectively.

[0066] The inventors confirmed through numerical calculations and water model experiments simulating an RH vacuum degassing device that the flow velocity of the steel bath within the vacuum tank is inconsistent, with faster flow along the inner wall of the vacuum tank 5 and slower flow in the central part of the steel bath compared to the wall side. Furthermore, since the hydrogen plasma irradiation location is near the central part of the steel bath in the vacuum tank 5, it is believed that maintaining appropriate steel flow within the vacuum tank is crucial for improving the impurity removal efficiency during hydrogen plasma treatment.

[0067] Therefore, the relationship between plasma irradiation conditions and the surface velocity of molten steel circulating in the vacuum tank was evaluated. The results showed that, in order to quickly remove impurities from the molten steel, the surface velocity (V) of the molten steel in the vacuum tank was kept within the range of the following equation (3), which effectively enabled the removal of impurities using hydrogen plasma.

[0068]

[0069] (3) In the formula, V is the surface velocity (m / min) of the molten steel circulating in the vacuum tank, and G P The flow rate of the plasma gas (Nm³) 3 / minute), π is the mathematical constant pi, and L is the distance (m) between the centers of the ascending and descending submersibles.

[0070] That is, the region in the central part of the steel bath within the vacuum tank, where the flow velocity is slower than that along the inner wall of the vacuum tank 5, can be roughly represented by the distance between the centers of the rising-side submersible tube 8 and the descending-side submersible tube 9. It is believed that by increasing the surface velocity (V) of the molten steel within the vacuum tank relative to the linear velocity of the plasma gas blown into the central part of the steel bath, fresh molten steel 3 can be continuously supplied to the hydrogen plasma irradiation section in the central part of the steel bath, thereby rapidly removing impurities from the molten steel.

[0071] The surface velocity (V) of molten steel circulating in the vacuum tank can be calculated by the following equation (4).

[0072]

[0073] (4) In the formula, V is the surface velocity of the molten steel circulating in the vacuum tank (m / min), Q is the circulation volume of the molten steel circulating in the vacuum tank (tons / min), and ρ is the density of the molten steel (kg / m³). 3 H is the height of molten steel in the vacuum tank (m), and d is the inner diameter of the vacuum tank (m).

[0074] When the surface flow rate (V) of the molten steel in the vacuum tank is less than the right side of equation (3), no new molten steel is supplied to and mixed with the hydrogen plasma irradiation section in the center of the steel bath, and the impurity removal effect of the hydrogen plasma becomes smaller.

[0075] In summary, the surface velocity (V) of the molten steel flowing in the vacuum tank, i.e., the molten steel circulating in the vacuum tank, is preferably within the range of equation (3). To ensure that the surface velocity (V) of the molten steel circulating in the vacuum tank is within the range of equation (3), the circulation rate (Q; tons / minute) of the molten steel circulating in the vacuum tank and the flow rate (G) of the plasma gas can be controlled in a manner that satisfies the following equation (8). P Nm 3 / minute). Equation (8) is derived from equations (3) and (4), and the variables are the same as in equations (3) and (4).

[0076]

[0077] The composition of the slag 4 floating on the surface of the molten steel 3 in the ladle, particularly the iron oxides and manganese oxides, can serve as a source of oxygen supply to the molten steel 3. Therefore, it is preferable that the combined concentration of iron oxides and manganese oxides in the slag 4 is 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less. When the combined concentration of iron oxides and manganese oxides exceeds 5% by mass, oxygen supply from the slag 4 to the molten steel 3 occurs simultaneously during plasma treatment, and the impurity reduction effect cannot be sufficiently obtained.

[0078] As a method to reduce iron oxides and manganese oxides in slag 4, adding metallic aluminum or aluminum slag to the slag 4 floating on the molten steel before the start of treatment in the RH vacuum degassing unit 1 to reduce iron oxides and manganese oxides with aluminum is effective. Alternatively, removing slag 4 from the ladle 2 and then adding a slagging agent to the ladle to regenerate slag with less iron oxide and manganese oxides is also effective.

[0079] The timing of adding deoxidizers such as aluminum and silicon to the molten steel 3 after hydrogen plasma treatment is not particularly limited. For example, after the hydrogen plasma treatment stops, oxygen is supplied to the molten steel 3 from the atmosphere, slag 4, or ladle refractory, causing the oxygen concentration in the molten steel to rise. Therefore, it is preferable to add deoxidizers such as aluminum and silicon to the molten steel 3 rapidly through the raw material inlet 12 after hydrogen plasma treatment to maintain the oxygen concentration in the molten steel reduced by the hydrogen plasma treatment at a low level. If it is necessary to adjust the composition of the molten steel in addition to deoxidizers such as aluminum and silicon, specified alloy iron or pure metals are added to the molten steel in the circulating vacuum tank through the raw material inlet 12 after hydrogen plasma treatment.

[0080] Furthermore, since the hydrogen concentration in the molten steel rises to above a few ppm due to hydrogen plasma treatment, the atmosphere pressure in the vacuum tank is kept below 10 Torr after hydrogen plasma treatment without further irradiation with hydrogen plasma. Then, preferably, the molten steel 3 is continuously circulated into the vacuum tank 5 for more than 5 minutes under a reduced pressure of below 10 Torr to reduce the hydrogen concentration in the molten steel.

[0081] By using the refining method described above, which employs hydrogen plasma treatment, the oxygen, nitrogen, and sulfur in molten steel can be rapidly reduced to below 20 ppm by mass.

[0082] Example

[0083] In actual machines with a single charge capacity of 200 to 350 tons of molten steel, the following methods are used: Figure 1 The RH vacuum degassing device shown was used to treat molten steel from the converter with hydrogen plasma, and experiments were conducted. Hydrogen plasma was irradiated onto the surface of the molten steel circulating within the vacuum tank by a non-transfer plasma torch utilizing DC arc discharge, located above the vacuum tank of the RH vacuum degassing device, with varying plasma gas flow rate and hydrogen concentration. Furthermore, the operating conditions of the RH vacuum degassing device and the composition of the molten steel (oxygen concentration, nitrogen concentration, sulfur concentration, etc.) were altered.

[0084] In the RH vacuum degassing unit, samples for compositional analysis were collected from the molten steel in the ladle before and after hydrogen plasma treatment to analyze the oxygen, nitrogen, and sulfur concentrations in the molten steel and confirm the effectiveness of the plasma treatment. The plasma treatment time was uniformly set at approximately 15 minutes. It should be noted that no deoxidizers such as aluminum were added from the time the steel was tapped from the converter until the plasma treatment. In addition, the circulation flow rate (Q) of the molten steel in the vacuum tank was calculated using equation (2). The iron oxide and manganese oxide concentrations in the slag in the ladle were adjusted by adding aluminum slag to the slag in the ladle before the start of the treatment in the RH vacuum degassing unit.

[0085] Table 1 shows the experimental conditions for each experiment, and Table 2 shows the evaluation results.

[0086]

[0087]

[0088] In this invention example, by performing hydrogen plasma treatment for 15 minutes, the oxygen, nitrogen, and sulfur concentrations in the molten steel are simultaneously and rapidly reduced to below 20 ppm by mass. Regarding the removal rates of each element from before to after the plasma treatment, the oxygen content in the molten steel is above 95%, the nitrogen content is above 54%, and the sulfur content is above 21%.

[0089] On the other hand, in the comparative example that did not meet the conditions of the present invention, even after hydrogen plasma treatment, the reduction of oxygen, nitrogen, and sulfur in the molten steel was insufficient, and the concentration of all elements after hydrogen plasma treatment exceeded 20 ppm by mass. The removal rates of each element from before to after plasma treatment were low, with oxygen in the molten steel below 91%, nitrogen below 22%, and sulfur below 9%.

[0090] Symbol Explanation

[0091] 1 RH vacuum degassing device

[0092] 2 Steel Ladle

[0093] 3 Molten Steel

[0094] 4. Slag

[0095] 5 Vacuum bath

[0096] 6. Upper groove

[0097] 7. Lower groove

[0098] 8 Ascending Side Submersible

[0099] 9. Descent side tube

[0100] 10. Circulation gas inlet pipe

[0101] 11 Pipelines

[0102] 12 Raw material input port

[0103] 13 Plasma Torch

Claims

1. A method for refining molten steel, wherein in a step of refining molten steel contained in a ladle by circulating it in a vacuum tank of an RH vacuum degassing apparatus, the surface of the molten steel circulating in the vacuum tank of the RH vacuum degassing apparatus is subjected to plasma treatment to reduce the content of one or more elements selected from oxygen, nitrogen, and sulfur in the molten steel, wherein the plasma treatment is performed by irradiating the molten steel with hydrogen or an inactive gas containing hydrogen as the plasma gas from a plasma generator installed in the vacuum tank under conditions satisfying the following formula (1). wherein G P Q is the flow rate of the plasma gas, in Nm / min 3 H2 is the hydrogen concentration in the plasma gas, in vol%, and Q is the circulation flow rate of the molten steel circulating in the vacuum tank, in tons / min.

2. The molten steel refining method according to claim 1, wherein The circulation volume of the molten steel circulating in the vacuum tank can be calculated using the following formula (2). wherein Q is the circulation flow rate of molten steel circulating in the vacuum tank, in tons / min, G C is the flow rate of the gas for circulation, in Nm 3 / min, D is the inner diameter of the submerged tube of the RH vacuum degassing device, in m, P0 is the pressure at the position of the gas for circulation being blown in, in torr, and P is the pressure in the vacuum tank, in torr.

3. The molten steel refining method according to claim 1, wherein The surface velocity of the molten steel circulating in the vacuum tank, which is irradiated by the plasma gas, satisfies the following relationship between equations (3) and (4). Where V is the surface velocity of the molten steel circulating in the vacuum tank, in m / min, and G... P The flow rate of the plasma gas is expressed in Nm³. 3 / minute, π is pi, L is the distance between the centers of the rising and descending submersible tubes in meters, Q is the circulation rate of molten steel in the vacuum tank in tons per minute, and ρ is the density of molten steel in kg / m³. 3 H is the height of molten steel in the vacuum tank, in meters (m), and d is the inner diameter of the vacuum tank, in meters (m).

4. The molten steel refining method according to claim 2, wherein The surface velocity of the molten steel circulating in the vacuum tank, which is irradiated by the plasma gas, satisfies the following relationship between equations (3) and (4). Where V is the surface velocity of the molten steel circulating in the vacuum tank, in m / min, and G... P The flow rate of the plasma gas is expressed in Nm³. 3 / minute, π is pi, L is the distance between the centers of the rising and descending submersible tubes in meters, Q is the circulation rate of molten steel in the vacuum tank in tons per minute, and ρ is the density of molten steel in kg / m³. 3 H is the height of molten steel in the vacuum tank, in meters (m), and d is the inner diameter of the vacuum tank, in meters (m).

5. The molten steel refining method according to any one of claims 1 to 4, wherein The total concentration of slag, iron oxides and manganese oxides floating on the surface of the molten steel contained in the ladle is less than 5% by mass.

6. The molten steel refining method according to any one of claims 1 to 4, wherein The plasma treatment reduces the content of oxygen, nitrogen, and sulfur in molten steel simultaneously.

7. The molten steel refining method according to claim 5, wherein The plasma treatment reduces the content of oxygen, nitrogen, and sulfur in molten steel simultaneously.

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