KR20240008924A - Molten steel refining method - Google Patents

Abstract

In the RH vacuum degassing device in the steelmaking process, when hydrogen plasma is applied to molten steel, refining reactions such as deoxidation, denitrification, and desulfurization proceed quickly, and molten steel with few impurities is efficiently manufactured. The method of refining molten steel according to the present invention is a process of refining molten steel (3) contained in a ladle using an RH vacuum degassing device (1), and the molten steel is refluxed in the vacuum tank of the RH vacuum degassing device. The surface of is subjected to plasma treatment by irradiating hydrogen gas or an inert gas containing hydrogen gas as a plasma gas under conditions that satisfy the equation (1) below, and 1 selected from oxygen, nitrogen, and sulfur contained in the molten steel. Reduce the content of a species or two or more elements. (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 refluxing in the vacuum tank. Reflux amount (ton/min).
G _P × (H ₂ )/Q ≥ 0.1 … … (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. Specifically, the present invention relates to a refining method for producing hydrogen gas or an inert gas containing hydrogen gas in a vacuum chamber of an RH vacuum degassing device. It relates to a refining method in which molten steel is irradiated with 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 such treatment in 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 (5) to (7) shown below.

2H + [O] = H ₂ O... … (5)

xH + [N] = NH _x ... … (6)

yH + [S] = H _y S... … (7)

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 molten steel in an RH vacuum degassing device in the steelmaking process. The aim is to provide a refining method that efficiently produces high-purity molten steel with few impurities.

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

[1] In the process of refining molten steel contained in a ladle by refluxing it in the vacuum tank of the RH vacuum degassing device, plasma installed in the vacuum tank is applied to the surface of the molten steel refluxing in the vacuum tank of the RH vacuum degassing device. A plasma treatment is performed in which hydrogen gas or an inert gas containing hydrogen gas is irradiated from the generator under conditions that satisfy the equation (1) below as a plasma gas, and the oxygen, nitrogen, and oxygen contained in the molten steel are reduced by the plasma treatment. A method of refining molten steel that reduces the content of one or two or more elements selected from sulfur.

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 reflux amount of molten steel refluxing in the vacuum tank (ton/ min).

[2] The molten steel refining method according to [1] above, wherein the reflux amount of the molten steel flowing in the vacuum tank is calculated using the equation (2) below.

Here, Q is the reflux amount of molten steel refluxing in the vacuum tank (ton/min), G _C is the flow rate of reflux gas (Nm ³ /min), and D is the inner diameter of the immersion tube of the RH vacuum degassing device. (m), P ₀ is the pressure (torr) at the injection position of the reflux gas, and P is the pressure (torr) in the vacuum chamber.

[3] In the above [1] or the above [2], the surface flow rate of the molten steel refluxing in the vacuum chamber to which the plasma gas is irradiated satisfies the relationships of the following equations (3) and (4). Method of refining the described molten steel.

Here, V is the surface flow rate of the molten steel refluxing in the vacuum tank (m/min), G _P is the flow rate of the plasma gas (Nm ³ /min), π is the pi ratio, and L is the rising side immersion pipe and the falling side. Q is the distance between the centers of the side immersion pipes (m), Q is the reflux amount of molten steel in the vacuum tank (ton/min), ρ is the density of molten steel (kg/㎥), and H is the height of molten steel in the vacuum tank. (m) and d are the inner diameter (m) of the vacuum tank.

[4] 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 [3] above, wherein the total concentration of iron oxide and manganese oxide is 5% by mass or less. Refining method.

[5] The method of refining molten steel according to any one of [1] to [4] 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 being refined in an RH vacuum degassing device, and as a result, molten steel with few impurities can be quickly refined, resulting in an industrially beneficial effect.

1 is a schematic longitudinal cross-sectional view of an RH vacuum degassing device showing an example of a form in which hydrogen plasma processing is performed in the RH vacuum degassing device.

Hereinafter, the present invention will be described in detail.

The method of refining molten steel according to the present invention is a process of refining molten steel contained in a ladle by refluxing it in the vacuum chamber of the RH vacuum degassing device, and generating plasma from a plasma generator installed in the vacuum chamber of the RH vacuum degassing device. Hydrogen gas converted into plasma, or a mixed gas of hydrogen gas converted into plasma and an inert gas, is irradiated as a plasma gas to the surface of the molten steel refluxing in the vacuum chamber, and the irradiation of this plasma gas removes oxygen, nitrogen, and sulfur from the molten steel. One or two or more selected elements are removed 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.”

1 is a schematic longitudinal cross-sectional view of an RH vacuum degassing device showing an example of a form in which plasma processing is performed in the RH vacuum degassing device. In Figure 1, symbol 1 is RH vacuum degassing device, 2 is ladle, 3 is molten steel, 4 is slag, 5 is vacuum tank, 6 is upper tank, 7 is lower tank, 8 is rising side immersion pipe, 9 is a descending side immersion pipe, 10 is a reflux gas inlet pipe, 11 is a duct, 12 is a raw material inlet, and 13 is a plasma torch. The vacuum tank 5 is composed of an upper tank 6 and a lower tank 7. In addition, the plasma torch 13 is a device that constitutes a part of the plasma generating device and is a device that performs hydrogen plasma treatment by irradiating plasma gas from its tip to the surface of the molten steel 3 refluxing in the vacuum tank. The plasma torch 13 is installed to penetrate the upper part of the vacuum chamber 5, and can move up and down inside the vacuum chamber 5.

In the RH vacuum degassing device 1, the ladle 2 containing the molten steel 3 is raised by a lifting device (not shown), and the rising side immersion pipe 8 and the descending side immersion pipe 9 are raised. It is immersed in the molten steel (3) in the ladle. Then, the inside of the vacuum tank 5 is evacuated by an exhaust device (not shown) connected to the duct 11 to depressurize the inside of the vacuum tank 5, and the gas is raised from the reflux gas intake pipe 10. Reflux gas is blown into the side immersion pipe (8). When the inside of the vacuum tank 5 is depressurized, the molten steel 3 in the ladle rises in proportion to the difference between the atmospheric pressure and the pressure (vacuum degree) within the vacuum tank, and flows into the vacuum tank. In addition, the molten steel 3 in the ladle rises up the rising side immersion pipe 8 together with the reflux gas due to the gas lift effect caused by the reflux gas blown in from the reflux gas injection pipe 10, and enters the vacuum chamber. (5) flows into the interior of . Argon gas is generally used as a reflux gas.

The molten steel 3 flowing into the vacuum tank 5 due to the pressure difference and gas lift effect returns to the ladle 2 via the descending side immersion pipe 9. The flow of molten steel that flows into the vacuum tank 5 from the ladle 2 and then returns from the vacuum tank 5 to the ladle 2 is called “reflux.” In this way, the molten steel 3 forms a reflux, and the molten steel 3 is subjected to RH vacuum degassing refining.

In short, the molten steel 3 is exposed to an atmosphere under reduced pressure in a vacuum tank, so that gas components such as hydrogen and nitrogen in the molten steel change from an equilibrium relationship in a state in contact with the atmosphere to an equilibrium relationship in contact with an atmosphere under reduced pressure. It is fulfilled. As a result, hydrogen and nitrogen move from the molten steel 3 into the atmosphere in the vacuum tank, and the molten steel 3 is subjected to degassing treatment (dehydrogenation treatment and denitrification treatment). In addition, since the molten steel 3 is refluxed between the ladle 2 and the vacuum tank 5, that is, the molten steel 3 is stirred, if the molten steel 3 is deoxidized with aluminum or the like, the molten steel 3 Separation of the oxide-based inclusions suspended in the steel and generated by the deoxidation treatment from the molten steel 3 to the slag 4 is promoted.

In the molten steel refining method according to the present embodiment, after the molten steel 3 in the ladle starts to flow back into the vacuum tank 5, a plasma torch 13 is applied to the surface of the molten steel 3 flowing back into the vacuum tank. , hydrogen gas or an inert gas containing hydrogen gas is irradiated as plasma gas. 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 (5), (6), and (7) shown below are formed, and oxygen, nitrogen, and sulfur in the molten steel are more quickly removed. It can be removed.

2H + [O] = H ₂ O... … (5)

xH + [N] = NH _x ... … (6)

yH + [S] = H _y S... … (7)

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

There are various methods for generating plasma, but as shown in FIG. 1, a method of generating plasma using a plasma torch 13 is common. The plasma torch 13 is one of devices that mainly uses a direct current power source and generates arc plasma in a form suitable for various applications stably and with good controllability through 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 method of the plasma generating device is not particularly limited as long as it can be installed in the vacuum chamber of the RH vacuum degassing device 1 and can stably supply hydrogen plasma to the molten steel 3. For example, an electrode for generating an alternating current arc is formed in the vacuum chamber of the RH vacuum degassing device 1, hydrogen gas or an inert gas containing hydrogen is supplied between these electrodes, and hydrogen gas or an inert gas containing hydrogen is formed. A method of converting an inert gas into plasma may be used.

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 plasma gas, the impurity removal effect increases, there is no particular upper limit to the hydrogen gas concentration in the 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 factors of the flow rate of the plasma gas, the hydrogen gas concentration in the plasma gas, and the reflux amount of the molten steel inside the vacuum tank must be adjusted to an appropriate range. There is a need to control it.

In other words, in order to obtain a rapid impurity removal effect, it is important not only to increase the hydrogen gas concentration in the plasma gas but also to supply an appropriate amount of hydrogen gas for the amount of molten steel transferred to the vacuum tank 5 of the RH vacuum degassing device 1. need. 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 reflux amount (Q) of the molten steel refluxing in the vacuum tank. It is necessary that the three elements satisfy the relationship in equation (1) below. Also, preferably the relationship between the three elements (G _P × (H ₂ )/Q) is set to 0.5 or more, and more preferably is set to 1.0 or more. On the other hand, when (G _P × (H ₂ )/Q) becomes greater than 20.0, a large output is required to dissociate or ionize hydrogen in the plasma gas. Moreover, since the accompanying wear and tear of the plasma torch 13 begins to become significant, it is more preferable to set (G _P × (H ₂ )/Q) to 20.0 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 refluxing in the vacuum tank. Reflux amount (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 reflux amount (Q) of the molten steel 3 that refluxes in the vacuum tank is influenced by the flow rate of the reflux gas, the atmospheric pressure in the vacuum tank, and the cross-sectional area of the rising side immersion pipe. Therefore, for each of these conditions, the uniform mixing time is measured in the actual RH vacuum degassing device 1, and the amount of molten steel contained in the ladle is determined by the molten steel reflux time obtained from the measured uniform mixing time. By dividing, the reflux amount (Q) of the molten steel 3 can be obtained. Here, the uniform mixing time is calculated by adding a tracer element (e.g., copper, nickel, etc.) to the molten steel in the ladle immediately below the rising side immersion pipe or the molten steel in the vacuum tank, and taking time series from within the ladle. It can be obtained as the time required for the change in tracer element concentration of the sample for component analysis to be within ±5%. At this time, since the molten steel reflux time is about 1/3 of the uniform mixing time, the time that is 1/3 of the obtained uniform mixing time can be used as the molten steel reflux time.

In addition, it is well known that the reflux amount (Q) of the molten steel 3 that refluxes in the vacuum tank can be obtained by an empirical regression equation expressed in equation (2) below. Therefore, equation (2) below It is desirable to determine the reflux amount (Q) of the molten steel 3 refluxing in the vacuum tank using .

(2) In the formula, Q is the reflux amount of the molten steel refluxing in the vacuum tank (ton/min), G _C is the flow rate of the reflux gas (Nm ³ /min), and D is the RH vacuum degassing device. The inner diameter (m) of the immersion tube, P ₀ is the pressure (torr) at the injection position of the reflux gas, and P is the pressure (torr) in the vacuum tank. Additionally, “torr” is a pressure unit, and 1 torr is 133.32 Pa. Also, regarding the flow rate of the reflux gas, “Nm ³ ” means the volume of the reflux gas in a standard state, and 0° C. and 1 atm (101325 Pa) are taken as the standard state.

The molten steel 3 accommodated in the ladle and before hydrogen plasma treatment may be tapped from a converter or electric furnace into the ladle 2 and then conveyed to the RH vacuum degassing device 1. In addition, steel may be tapped from a converter or electric furnace into a ladle, undergo refining outside the furnace in a heating and stirring treatment facility (sometimes called a ladle furnace), etc., and then be returned to the RH vacuum degassing device (1). do.

The molten steel 3 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 3 to preliminarily deoxidize the molten steel 3. 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 (5) above is reduced, and the plasma treatment time is reduced. It can be shortened. The timing of performing preliminary deoxidation with a reducing gas may be before treatment in the RH vacuum degassing device or before plasma processing during refining in the RH vacuum degassing device.

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 3 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 can inhibit the escape of nitrogen gas, hydrogen nitride, and hydrogen sulfide from the surface of molten steel into the gas phase (atmosphere within the vacuum tank). 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.

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).

Additionally, the present inventors discovered that when performing hydrogen plasma treatment, impurities in molten steel can be reduced more efficiently by appropriately controlling the flow of molten steel in the vacuum tank.

From numerical calculations and water model experiments imitating the RH vacuum degassing device, the present inventors have found that the flow rate of the steel bath in the vacuum tank is not uniform, the flow along the inner wall of the vacuum tank 5 is fast, and the flow rate in the steel bath center of the vacuum tank 5 is fast. It was confirmed that the flow was slower than that on the wall side. On the other hand, since the location of hydrogen plasma irradiation is near the center of the steel bath of the vacuum tank 5, it was thought that appropriate flow of molten steel in the vacuum tank was important to increase the impurity removal efficiency during hydrogen plasma treatment.

Therefore, the relationship between plasma irradiation conditions and the surface flow rate of molten steel refluxing in the vacuum tank was evaluated. As a result, it was found that in order to quickly remove impurities in molten steel, by setting the surface flow rate (V) of molten steel in the vacuum tank to a range that satisfies the equation (3) below, impurity removal by hydrogen plasma proceeds efficiently. I was able to.

(3) In the formula, V is the surface flow rate of the molten steel refluxing in the vacuum tank (m/min), G _P is the flow rate of the plasma gas (Nm ³ /min), π is the pi, and L is the rise. It is the distance (m) between the centers of the side immersion pipe and the descending side immersion pipe.

That is, the area in the central part of the river bath in the above-described vacuum tank, where the flow speed is slower than the flow along the inner wall of the vacuum tank 5, is generally the distance between the centers of the rising side immersion pipe 8 and the descending side immersion pipe 9. It can be expressed using By increasing the surface flow velocity (V) of the molten steel in the vacuum tank relative to the linear flow velocity of the plasma gas blown into the center of the steel bath in the vacuum layer, new molten steel (3) can always be supplied to the hydrogen plasma irradiation section in the central part of the steel bath. , it is believed that impurities in molten steel can be quickly removed.

The surface flow velocity (V) of the molten steel refluxing in the vacuum tank can be obtained from the equation (4) below.

(4) In the equation, V is the surface flow rate of the molten steel refluxing in the vacuum tank (m/min), Q is the reflux amount of the molten steel refluxing in the vacuum tank (ton/min), and ρ is the reflux of the molten steel. Density (kg/m3), H is the molten steel height in the vacuum tank (m), and d is the inner diameter of the vacuum tank (m).

When the surface flow velocity (V) of the molten steel in the vacuum tank is smaller than the right side of equation (3), the supply and mixing of new molten steel to the hydrogen plasma irradiation section in the center of the steel bath does not proceed, resulting in the effect of removing impurities by hydrogen plasma. becomes smaller.

From the above, it is preferable that the molten steel flow in the vacuum tank, that is, the surface flow velocity (V) of the molten steel refluxing in the vacuum tank, is within the range of equation (3). In order to make the surface flow rate (V) of the molten steel refluxing in the vacuum tank within the range of equation (3), the reflux amount of molten steel refluxing in the vacuum tank (Q; ton/min) so as to satisfy the equation (8) below. ) and the flow rate of the plasma gas (G _P ; Nm ³ /min). Expression (8) is an expression obtained from expressions (3) and (4), and each variable is the same as expressions (3) and (4).

Among the components of the slag 4 floating on the surface of the molten steel 3 in the ladle, especially iron oxide and manganese oxide in the slag can serve as an oxygen source for the molten steel 3. For this reason, the total of the iron oxide concentration and the manganese oxide concentration of the slag 4 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 4 to the molten steel 3 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 4, before starting treatment in the RH vacuum degassing device 1, metal aluminum or aluminum dross is applied to the slag 4 floating on the molten steel. Additionally, it is effective to reduce iron oxide or manganese oxide with aluminum. Additionally, it is also effective to remove the slag 4 from the ladle 2 and then add an additive into the ladle to create new slag containing less iron oxide or manganese oxide.

After hydrogen plasma treatment, the timing of addition of a deoxidizer such as aluminum or silicon to the molten steel 3 is not particularly limited. For example, after the hydrogen plasma is stopped, oxygen is supplied to the molten steel 3 from the atmosphere, the slag 4, or the ladle refractory material, and the oxygen concentration in the molten steel increases. For this reason, after the hydrogen plasma treatment, it is advisable to quickly add a deoxidizing material such as aluminum or silicon to the molten steel 3 from the raw material inlet 12 to maintain the oxygen concentration in the molten steel reduced by the hydrogen plasma treatment at a low level. desirable. When it is necessary to adjust the molten steel composition in addition to the deoxidizing agent such as aluminum or silicon, after hydrogen plasma treatment, a predetermined ferroalloy or pure metal is added to the molten steel in the vacuum tank refluxing from the raw material inlet 12.

Additionally, since the hydrogen concentration in the molten steel increases to several mass ppm or more due to the hydrogen plasma treatment, the hydrogen plasma is not irradiated after the hydrogen plasma treatment, and the atmospheric pressure in the vacuum chamber is set to 10 torr or less. Then, it is preferable to continue to reflux the molten steel 3 into the vacuum tank 5 for 5 minutes or more under a reduced pressure of 10 torr or less to reduce the hydrogen concentration in the molten steel.

By the refining method using hydrogen plasma treatment as described above, oxygen, nitrogen, and sulfur in molten steel can be quickly reduced to 20 mass ppm or less each.

Example

In an actual machine with a molten steel volume of 200 tons or more and 350 tons or less per charge, a test was conducted in which the molten steel tapped from the converter was subjected to hydrogen plasma treatment using the RH vacuum degassing device shown in FIG. 1. From a non-transition type plasma torch by direct current arc discharge installed on the upper part of the vacuum chamber of the RH vacuum degassing device, hydrogen plasma is generated on the surface of the molten steel refluxing in the vacuum chamber by changing the plasma gas flow rate or the hydrogen concentration in the plasma gas. investigated. Additionally, the operating conditions and molten steel composition (oxygen concentration, nitrogen concentration, sulfur concentration, etc.) of the RH vacuum degassing device were changed.

In the RH vacuum degassing device, samples for component analysis are taken from the molten steel in the ladle before and after hydrogen plasma treatment, and the oxygen concentration, nitrogen concentration, and sulfur concentration in the molten steel are analyzed to confirm the effect of plasma treatment. did. The plasma treatment time was generally 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. In addition, the amount of molten steel being returned to the vacuum tank (Q) was calculated using equation (2). 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 RH vacuum degassing device.

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 20 mass ppm or less. The removal rates of each element from before the start of the plasma treatment to after the end were 95% or more for oxygen in the molten steel, 54% or more for nitrogen in the molten steel, and 21% or more for sulfur in the molten steel.

On the other hand, in the comparative example that did not satisfy the conditions of the present invention, the reduction of oxygen, nitrogen, and sulfur in the molten steel was insufficient even after the hydrogen plasma treatment, and the concentration after the hydrogen plasma treatment exceeded 20 mass ppm for any element. The removal rates of each element from before the start of the plasma treatment to after the end were low at 91% or less for oxygen in the molten steel, 22% or less for nitrogen in the molten steel, and 9% or less for sulfur in the molten steel.

1: RH vacuum degassing device
2: Ladle
3: molten steel
4: Slag
5: Vacuum tank
6: Upper bracket
7: lower part
8: Rising side immersion pipe
9: Downward side immersion pipe
10: Gas injection pipe for reflux
11: duct
12: Raw material input port
13: Plasma Torch

Claims (5)

In the process of refining molten steel contained in a ladle by refluxing it in the vacuum tank of the RH vacuum degassing device, a plasma generator installed in the vacuum tank is applied to the surface of the molten steel refluxing in the vacuum tank of the RH vacuum degassing device. , plasma treatment is performed by irradiating hydrogen gas or an inert gas containing hydrogen gas as a plasma gas under conditions that satisfy the equation (1) below, and the plasma treatment selects oxygen, nitrogen, and sulfur contained in the molten steel. A method of refining molten steel that reduces the content of one or two or more elements.

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 reflux amount of molten steel refluxing in the vacuum tank (ton/ min). According to claim 1,
A method of refining molten steel in which the reflux amount of molten steel flowing in the vacuum tank is calculated using the equation (2) below.

Here, Q is the reflux amount of molten steel refluxing in the vacuum tank (ton/min), G _C is the flow rate of reflux gas (Nm ³ /min), and D is the inner diameter of the immersion tube of the RH vacuum degassing device. (m), P ₀ is the pressure (torr) at the injection position of the reflux gas, and P is the pressure (torr) in the vacuum chamber. The method of claim 1 or 2,
A method for refining molten steel, wherein the surface flow rate of molten steel refluxing in the vacuum chamber to which the plasma gas is irradiated satisfies the relationships of equations (3) and (4) below.

Here, V is the surface flow rate of the molten steel refluxing in the vacuum tank (m/min), G _P is the flow rate of the plasma gas (Nm ³ /min), π is the pi ratio, and L is the rising side immersion pipe and the falling side. Q is the distance between the centers of the side immersion pipes (m), Q is the reflux amount of molten steel in the vacuum tank (ton/min), ρ is the density of molten steel (kg/㎥), and H is the height of molten steel in the vacuum tank. (m) and d are the inner diameter (m) of the vacuum tank. The method according to any one of claims 1 to 3,
A method for 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 4,
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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