Steel production method

Abstract

To provide a steel production method by which slag stuck onto the inside of an immersion tube of an RH vacuum degassing facility is reduced.SOLUTION: A steel production method using an RH vacuum degassing facility includes: a flux charging step of charging a flux having a CaO content of 30 wt% or more and a mass ratio FeO / CaO of FeO to CaO of 1.2 to 2.0 into molten steel having an oxygen concentration of 300 ppm or more; and a deoxidizing step of charging a deoxidizer into the molten steel. A reflux speed of the molten steel from when the flux is charged to when the deoxidizer is charged is 1.15 m / s or more. A reflux time, which is a time from the charge of the flux to the charge of the deoxidizer, exceeds 2.5 minutes.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for manufacturing steel.

Background Art

[0002] In a general steel manufacturing process, secondary refining is performed to remove gas components such as oxygen and nitrogen and adjust the chemical composition. In secondary refining, an RH vacuum degassing facility having a vacuum chamber equipped with a double-leg dipping tube may be used. This RH vacuum degassing facility blows Ar gas from one tube into the molten steel in the ladle while reducing the pressure, pushes the molten steel up into the vacuum chamber, and extrudes the molten steel from the other tube back into the ladle, thereby circulating the molten steel.

[0003] In an operation using an RH vacuum degassing facility, in order to remove oxygen, Al, which is a deoxidizer, is added to the molten steel. As a result, a large amount of slag such as Al2O3 is generated, and there is a problem that it adheres to the dipping tube. The adhesion of such slag to the dipping tube increases each time the RH vacuum degassing facility is repeatedly used. For this reason, Patent Document 1 discloses a slag removal device attached to the outside of the dipping tube.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] On the one hand, as a matter of course, slag adheres not only to the outside of the tube but also to the inside of the tube. As a result, there is a problem that the inner diameter of the tube narrows. When the inner diameter of the tube narrows, the manufacturability decreases. In addition, in order to repair the locally melted part generated in the tube, it is necessary to insert a mandrel into the tube. However, when the inner diameter of the tube narrows, this operation becomes impossible, and the service life of the device is shortened. However, in Patent Document 1, sufficient consideration has not been given to the slag adhering to the inside of the tube. Therefore, in the production of steel, it is required to reduce the slag adhering to the inside of the immersion tube.

[0006] Based on the above, an object of the present invention is to reduce the slag adhering to the inside of the immersion tube of the RH vacuum degassing facility.

Means for Solving the Problems

[0007] The present invention has been made to solve the above problems, and the gist thereof is the following method for producing steel.

[0008] (1) A method for producing steel using an RH vacuum degassing facility, comprising: a flux input step of inputting a flux having a CaO content of 30 wt% or more and a mass ratio FeO / CaO of FeO to CaO of 1.2 to 2.0 into molten steel having an oxygen concentration of 300 ppm or more; a deoxidation step of inputting a deoxidizer into the molten steel, wherein the reflux speed of the molten steel from the input of the flux to the input of the deoxidizer is 1.15 m / s or more, and the reflux time, which is the time from the input of the flux to the input of the deoxidizer, is more than 2.5 minutes.

[0009] (2) A method for producing steel, wherein when producing steel through a plurality of consecutive charges, the production method according to (1) above is performed in at least one charge, comprising: the input amount FL (kg) of the flux input through the plurality of charges, A method for manufacturing steel, wherein the amount of Al2O3 generated through the plurality of charges, denoted as AlF (kg), satisfies the following formula (i). FL > 0.127×AlF + 240 ··· (i)

Advantages of the Invention

[0010] According to the present invention, a method for manufacturing steel that reduces slag adhering to the inside of the immersion tube of an RH vacuum degassing facility can be obtained.

Brief Description of the Drawings

[0011]

Figure 1

Embodiments for Carrying Out the Invention

[0012] The inventors of the present invention conducted research on a method for manufacturing steel that reduces slag adhering to the inside of the immersion tube of an RH vacuum degassing facility and obtained the following findings.

[0013] (a) During secondary refining, Al, which is a deoxidizer, is added for deoxidation. At this time, a large amount of Al2O3 is generated in the bath of the RH vacuum degassing facility. This Al2O3 adheres to the inner wall of the immersion tube. The inner wall of the immersion tube is made of refractory MgO, and this MgO reacts with the adhered Al2O3 to form MgAl2O4 having a spinel-type crystal structure (hereinafter simply referred to as "spinel"). Since this spinel is a compound with a high melting point, it is difficult to dissolve in the RH vacuum degassing facility, and accumulatively deposits inside the immersion tube during a series of secondary refining processes repeated, narrowing the inner diameter of the tube.

[0014] (b) Therefore, from a thermodynamic perspective, the inventors focused on lowering the melting point of the formed spinel. And it was clarified that it is effective to introduce a flux with a CaO content of 30 wt% or more and an FeO / CaO mass ratio of 1.2 to 2.0 into the molten steel. This is because the spinel accumulated in the immersion tube reacts with the flux having the above composition to form an oxide with a low melting point. The oxide with a low melting point formed by reacting with the flux melts at about 1600 °C, the operating temperature, and is taken into the molten steel. As a result, the oxide peels off from the inner wall of the narrowed immersion tube, and the inner diameter of the tube increases.

[0015] (c) In addition, since the flux having the above composition itself is easily melted at the operating temperature, it can be easily circulated with the molten steel in the equipment, so the reaction can be promoted. Similarly, in order to sufficiently circulate the flux, it is also effective to set the reflux speed of the molten steel (hereinafter, also simply referred to as "reflux speed") to 1.15 m / s or more and the reflux time of the molten steel (hereinafter, also referred to as "reflux time") to more than 2.5 minutes.

[0016] One embodiment of the present invention is made based on the above findings. Hereinafter, each requirement of the steel manufacturing method of this embodiment will be described in detail. In this application, "A to B" indicates A% or more and B% or less.

[0017] 1. Flux injection step In steel manufacturing, removing carbon and the like from pig iron produced in a converter or an electric furnace to produce molten steel is called primary refining. And the process following this primary refining is called secondary refining. In secondary refining, degassing treatment, adjustment of chemical composition, etc. are performed. When performing secondary refining, various devices may be used, but RH vacuum degassing equipment is common. And the steel manufacturing method of this embodiment is a steel manufacturing method using RH vacuum degassing equipment.

[0018] Figure 1 is a diagram schematically showing the structure of an RH vacuum degassing facility. As shown in Figure 1, the RH vacuum degassing facility 1 is a device having two immersion tubes 3 and a vacuum chamber 4 above a ladle 2. First, molten steel 5 is contained in the ladle 2, the pressure in the vacuum chamber 4 is reduced, and while blowing an inert gas 6 such as Ar gas from one immersion tube 3 (hereinafter also referred to as the "ascending immersion tube"), the molten steel is sucked into the vacuum chamber 4, and at the same time, the molten steel is returned from the other immersion tube 3 (hereinafter also referred to as the "descending immersion tube") to the ladle, and the molten steel is circulated throughout the facility. In this way, while circulating the molten steel, degassing and adjustment of the chemical composition are performed by adding a deoxidizer or the like to the molten steel.

[0019] In the method for manufacturing steel according to the present embodiment, a flux having a CaO content of 30 wt% or more and an FeO / CaO mass ratio of 1.2 to 2.0 is charged into molten steel having an oxygen concentration of 300 ppm or more. The step of charging such a flux is referred to as a flux charging step. Before charging the flux, the vacuum chamber in the RH vacuum degassing facility is depressurized.

[0020] The oxygen concentration in the molten steel before charging the flux is 300 ppm or more. If the oxygen concentration in the molten steel is less than 300 ppm, the components in the flux described later will be reduced, and it will be difficult for the melting point of the spinel to decrease. For this reason, the oxygen concentration in the molten steel before charging the flux is 300 ppm or more, preferably 400 ppm or more, and more preferably 600 ppm or more. The above oxygen concentration is the average oxygen concentration in the molten steel.

[0021] Under the manufacturing conditions of the present embodiment, by reacting the charged flux with the deposits on the immersion tube, the melting point of the deposits (spinel) adhering to the immersion tube is lowered. For this reason, it is not preferable to simply use a flux of only CaO (quicklime) having a high melting point, and it is preferable to use calcium ferrite containing FeO as the flux because the melting point can be lowered.

[0022] In order to promote the reaction between the flux and the adherent, it is necessary for the flux to dissolve efficiently in the molten steel and circulate within the facility. A flux composed only of CaO has a high melting point and remains in powder form even at a treatment temperature of about 1600°C during vacuum degassing. Therefore, it may not dissolve well in the molten steel. On the other hand, calcium ferrite melts in the above temperature range, so it can easily melt in the molten steel, circulate within the facility, and react with the spinel formed on the immersion tube.

[0023] Therefore, the flux to be introduced into the molten steel has a CaO content of 30 wt% or more and a mass ratio of FeO to CaO, FeO / CaO, of 1.2 to 2.0. Note that the composition of the flux only needs to satisfy the above range, and the flux does not have to consist only of calcium ferrite. A mixture of calcium ferrite and CaO is also acceptable.

[0024] Calcium ferrite contains CaO and Fe oxides. In this application, Fe compounds are converted to FeO, and the mass ratio (mass of FeO to mass of CaO) is calculated. In addition to the above CaO and Fe oxides, calcium ferrite may contain compounds such as SiO2, CaF2, and free lime (also referred to as "f.CaO").

[0025] Considering the CaO contained in calcium ferrite, the CaO content of the flux is 30 wt% or more. The upper limit of the CaO content is not particularly limited, but considering the CaO content in general calcium ferrite, the CaO content is preferably 50 wt% or less, and more preferably 40 wt% or less.

[0026] Also, from the perspective of effectively reducing the melting point of the spinel adhering to the immersion tube, the mass ratio of FeO to CaO, FeO / CaO, is 1.2 to 2.0. When the mass ratio of FeO to CaO, FeO / CaO, is less than 1.2 or more than 2.0, the melting point of the spinel adhering to the immersion tube cannot be reduced, and the adherent cannot be effectively melted.

[0027] 2. Deoxidation step The method for manufacturing steel according to this embodiment has a deoxidation step of adding a deoxidizing agent for deoxidation after the flux addition step. In the deoxidation step, the deoxidizing agent to be used is not particularly limited, but it is preferably ordinary Al as the deoxidizing agent. After the deoxidation step, the chemical composition is usually adjusted and the like.

[0028] 3. Reflux rate The rate of refluxing the molten steel from the time of adding the flux until the deoxidizing agent is added, that is, the reflux rate, is preferably faster. Specifically, the reflux rate of the molten steel is 1.15 m / s or more. Usually, in steel manufacturing, considering damage to the immersion tube, the load during operation, etc., the reflux rate of the molten steel is generally less than 1.00 m / s.

[0029] However, in the method for manufacturing steel according to this embodiment, if the reflux rate is less than 1.15 m / s, the molten flux is less likely to be taken into the molten steel, and the flux is less likely to circulate. Therefore, the reflux rate of the molten steel is 1.15 m / s or more, preferably 1.20 m / s or more, and more preferably 1.30 m / s or more.

[0030] Note that the upper limit of the reflux rate of the molten steel is not particularly limited, but from the viewpoint of equipment and the like, the reflux rate of the molten steel is preferably 1.40 m / s or less. Note that the reflux rate can be calculated based on the following formula (a) (also referred to as "Ono's formula").

[0031] Reflux rate (t / min) = 3.8×10 -3 ×Du 0.3 ×Dd 1.1 ×G 0.31 ×H 0.5 ···(a) However, each symbol in the above formula (a) is defined as follows. Du (cm): Inner diameter of the riser tube before refluxing the molten steel Dd (cm): Inner diameter of the downcomer tube before refluxing the molten steel G (NL / min): Flow rate of the added gas H (cm): Depth of the injected additive gas

[0032] The above-mentioned additive gas is the gas injected from the riser pipe to reflux the molten steel, for example, Ar (argon) gas or N2 (nitrogen) gas. Also, the depth of the injected additive gas can be calculated from the degree of vacuum, etc.

[0033] 4. Reflux time In the method for manufacturing steel of this embodiment, the time from when the flux is charged until the deoxidizer is charged, that is, the reflux time, is more than 2.5 minutes. If the reflux time is 2.5 minutes or less, the flux cannot sufficiently circulate in the equipment and stays in the tank. As a result, the flux and the spinel attached to the immersion tube do not sufficiently contact each other, and it becomes difficult for the melting point of the deposit to decrease. For this reason, the reflux time is more than 2.5 minutes, preferably 3.0 minutes or more, and more preferably 4.0 minutes or more. Note that the upper limit of the reflux time is not particularly limited, but usually, considering productivity, etc., the reflux time is preferably 6.0 minutes or less.

[0034] 5. Relationship between the amount of flux charged and the amount of Al2O3 Normally, in the steel manufacturing process, after a series of processes are completed in one ladle, the ladle is changed and the processes are continuously repeated in another ladle. Although steel can be manufactured by charging the flux for each treatment (1 charge) of one ladle, in this case, the flux is wasted. For this reason, it is desirable to manufacture steel so as to satisfy formula (i) for the entire plurality of consecutive charges.

[0035] That is, when manufacturing steel through a plurality of consecutive charges, it is preferable to satisfy the conditions described in items 1 to 4 above with at least one charge. At this time, it is preferable that FL (kg), which is the input amount of flux input through a plurality of charges, and AlF (kg), which is the amount of Al2O3 generated through a plurality of charges, satisfy the following formula (i). That is, the above FL (kg) is the total input amount of flux, and ALF (kg) is the total amount of Al2O3 generated, calculated as follows. When FL (kg) is greater than the right side value of formula (i), the input amount of flux is sufficient, and it becomes easier for the spinel adhering to the immersion tube to react with the flux. FL > 0.127 × AlF + 240 ··· (i)

[0036] In addition, the above AlF in this case can be calculated from, for example, the following formula (b). AlF (kg) = a × total acid injection volume (Nm 3 ) + b × average oxygen concentration in molten steel (ppm) × total treatment amount (t) + c × total treatment amount (t) ··· (b)

[0037] The coefficients a, b, and c in the above formula (b) are calculated by obtaining a regression curve based on past performance data. That is, in the past steel manufacturing, the relationship between the actually generated amount of Al2O3, the oxygen concentration in molten steel, and the treatment amount is sorted out to calculate a, b, and c. The total acid injection volume in the above formula (b) is the total value of the amount of oxygen blown into the RH vacuum degassing facility through all charges when a plurality of consecutive charges are performed. The total treatment amount is the total amount of molten steel processed by a plurality of consecutive charges.

[0038] 6. Other Conditions Regarding other steel manufacturing conditions, conventional methods may be followed.

[0039] Hereinafter, the manufacturing method of this embodiment will be described more specifically by way of examples, but this embodiment is not limited to these examples.

Examples

[0040] The immersion tube of the RH vacuum degassing facility was pre - set with slag adhering to its inner surface. Then, flux was introduced into the molten steel, and then Al, which is a deoxidizer, was introduced to investigate the reduction amount of the slag on the inner surface of the tube (the increase amount of the inner diameter of the tube).

[0041] Here, due to the constraints of the actual machine, measuring the increase amount for each charge would lead to a decrease in productivity. Therefore, 3 or 4 charges were regarded as one cycle, and the inner diameter of the tube was measured. In each cycle (No.1 - 5), for the first and second charges among the continuously processed charges, flux was introduced, the reflux speed and reflux time were adjusted, and then Al, which is a deoxidizer, was introduced. On the other hand, for the charges after the third charge, deoxidation was carried out without introducing flux.

[0042] For the charges after the third charge, since no flux was introduced, spinel may be generated and deposited on the inner surface of the tube. However, assuming that the reduction amount of the inner diameter of the tube is small, the increase amount of the inner diameter of the tube was evaluated.

[0043] Table 1 shows the details of the manufacturing conditions. In Table 1, CF represents calcium ferrite. The chemical composition of calcium ferrite is that the content of CaO is 30 - 40 wt%, the content of FeO is 50 - 60 wt%, the content of SiO2 is 5.0% or less, the content of CaF2 is 0.1%, and the content of f.CaO is 3.0% or less. In calculating ALF, a, b, and c in formula (b) were set as a = 2.37, b = 0.004771, and c = - 0.813 respectively.

[0044]

Table 1

[0045] As shown in Table 1, in the examples that satisfy the composition and manufacturing conditions of the flux of this embodiment, the spinel pre - formed on the immersion tube was dissolved, and the inner diameter of the tube increased. On the other hand, in the examples that did not satisfy the requirements of this embodiment, no increase in the inner diameter of the tube was observed.

Explanation of Symbols

[0046] 1. RH vacuum degassing equipment 2. Ladle 3. Immersion tube 4. Vacuum chamber 5. Molten steel 6. Inert gas

Claims

1. A method for manufacturing steel using an RH vacuum degassing facility, comprising: a flux charging step of charging a molten steel having an oxygen concentration of 300 ppm or more with a flux having a CaO content of 30 wt% or more and a mass ratio FeO / CaO of FeO to CaO of 1.2 to 2.0; a deoxidation step of charging a deoxidizer into the molten steel; wherein the reflux rate of the molten steel from the time of charging the flux until the time of charging the deoxidizer is 1.15 m / s or more; a method for manufacturing steel, wherein the reflux time, which is the time from the time of charging the flux until the time of charging the deoxidizer, is more than 2.5 minutes.

2. A method for manufacturing steel, wherein when manufacturing steel through a plurality of consecutive charges, the manufacturing method according to Claim 1 is performed in at least one charge, wherein FL (kg) is the amount of the flux charged through the plurality of charges; Al produced through the plurality of charges ２ O ３ The amount of AlF (kg) satisfies the following formula (i). A method for producing steel FL > 0.127 × AlF + 240... (i)

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