Method for operating LF apparatus

Abstract

To provide a method for operating an LF apparatus with which an increase of hydrogen concentration in molten steel in LF operation can effectively be restrained.SOLUTION: In a method for operating an LF apparatus, an electrode 12 is inserted into slag on molten steel in a molten steel ladle 11 to perform an electric treatment. Before starting the electric treatment, a CaO source containing calcium aluminate and having a water content of 2 mass% or less is charged into the molten steel in the molten steel ladle 11, and a deoxidizing material is charged into the molten steel into which the CaO source has been charged. At least from the charging of the CaO source to the start of the electric treatment, the molten steel in the molten steel ladle 11 is stirred with a bottom-blown gas at 30 W of less of the stirring power density per ton of the molten steel.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for operating an LF (Ladle Furnace) that heat-treats molten steel by arc discharge. [Background technology]

[0002] In an LF device, an electrode is inserted into the slag above the molten steel in a molten steel ladle (also called a ladle), and an arc is generated between the electrode and the molten steel (metal), heating the molten steel. In this operation using an LF device (hereinafter also referred to as LF operation), a CaO source (auxiliary raw material) is added to ensure the slag thickness necessary for current application and to improve the cleanliness of the molten steel, but the water contained in the CaO source raises the problem of increasing the hydrogen concentration in the molten steel. If the hydrogen concentration in the molten steel increases in this way, the possibility of breakouts and cracking of the cast piece increases during the subsequent continuous casting.

[0003] For this reason, it is necessary to reduce the increased hydrogen concentration in the molten steel by vacuum degassing, but in this case, there is a problem that the vacuum degassing process increases manufacturing costs. On the other hand, the following methods have been proposed as methods for reducing the hydrogen concentration in molten steel.

[0004] For example, Patent Document 1 describes that, in order to promote dehydrogenation of molten steel, the oxidizer powder or refining powder used in a reduced pressure vessel is dried in advance to reduce the moisture content to 0.05% by weight or less, thereby reducing the hydrogen concentration reached in the molten steel after the vacuum degassing process.

[0005] Patent Document 2 describes a method for producing a forging steel ingot that has excellent fatigue properties and hydrogen cracking resistance, in which molten steel that has undergone a first secondary refining treatment (LF) is degassed, and then this molten steel is subjected to a second secondary refining treatment (LF), in which the molten steel is stirred at a low stirring power density to prevent new slag entrainment, thereby suppressing an increase in the hydrogen concentration in the molten steel and preventing reoxidation of the molten steel components. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 63-203718 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-266768 Summary of the Invention [Problem to be solved by the invention]

[0007] However, although the method described in Patent Document 1 is effective in reducing the amount of hydrogen supplied to molten steel, it is difficult to reduce the hydrogen concentration in the molten steel thereafter, and it does not go so far as to eliminate the need for vacuum degassing treatment.

[0008] Furthermore, in the method described in Patent Document 2, no measures are taken to prevent an increase in the hydrogen concentration in the molten steel during the first secondary refining treatment (LF), and hydrogen is removed by a subsequent degassing treatment. In other words, in order to reduce the hydrogen concentration in the molten steel that has increased during LF operation, a degassing treatment is required, which poses a problem of increased manufacturing costs.

[0009] An object of the present invention is to provide a method for operating an LF apparatus that can effectively suppress an increase in hydrogen concentration in molten steel during LF operation. [Means for solving the problem]

[0010] The present invention is as follows. A method for operating an LF device in which an electrode is inserted into slag above molten steel in a molten steel ladle and an electric current is applied to the slag, before the start of the current application treatment, a CaO source containing calcium aluminate and having a water content of 2 mass% or less is charged into the molten steel in the molten steel ladle, and a deoxidizer is charged into the molten steel into which the CaO source has been charged; At least from the time when the CaO source is charged until the start of the current application treatment, the molten steel in the molten steel ladle is stirred with bottom blown gas at a stirring power density of 30 W or less per ton of molten steel. A method for operating an LF device, characterized by: [Effects of the Invention]

[0011] According to the method for operating an LF apparatus of the present invention, it is possible to effectively suppress an increase in the hydrogen concentration in molten steel during LF operation. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is an explanatory diagram showing an overview of an LF apparatus to which a method for operating an LF apparatus according to the present invention is applied; DETAILED DESCRIPTION OF THE INVENTION

[0013] The method for operating an LF apparatus of the present invention is a method for operating an LF apparatus in which an electrode is inserted into slag above molten steel in a molten steel ladle and an electric current treatment is performed, before the start of the current application treatment, a CaO source containing calcium aluminate and having a water content of 2 mass% or less is charged into the molten steel in the molten steel ladle, and a deoxidizer is charged into the molten steel into which the CaO source has been charged; At least from the time when the CaO source is charged until the start of the current application treatment, the molten steel in the molten steel ladle is stirred with bottom blown gas at a stirring power density of 30 W or less per ton of molten steel. It is characterized by: Here, the stirring power density of bottom-blown gas stirring is also expressed as XW per ton of molten steel, or X(W / ton of molten steel). The significance of the features of the present invention will be explained in detail below.

[0014] 1) Regarding the point that an electrode is inserted into the slag above the molten steel in the ladle to conduct the current As shown in Figure 1, an LF device 10 applies current to slag formed on molten steel in a molten steel ladle 11 by inserting an electrode 12 from above the slag, generating an arc between the electrode 12 and the molten steel to heat the molten steel. During the current application, an inert gas (Ar gas) is blown in through a porous plug (porous refractory material) 13 installed in the bottom of the ladle, and the molten steel is stirred by this bottom-blown gas. From the viewpoint of promoting slag formation, the stirring power density per ton of molten steel by the bottom-blown gas when the CaO source is added is usually 35 W or more, and preferably 40 W or more.

[0015] 1 shows the state before the start of the current application process, so there is flux on the molten steel, which will later become slag. A lid 14 is placed on the molten steel ladle 11, creating an inert atmosphere inside the molten steel ladle 11 (above the flux (slag)).

[0016] 2) Before the start of the current treatment, a CaO source containing calcium aluminate and having a water content of 2 mass% or less is added to the molten steel in the molten steel ladle, and a deoxidizer is added to the molten steel into which the CaO source has been added. In general, in LF operation, CaO sources (auxiliary raw materials) such as ingot-making materials and flux are added to the ladle to ensure the slag thickness required for current treatment and to improve the cleanliness of the molten steel.

[0017] As shown in equation (1), quicklime in the CaO source reacts with water to produce slaked lime, which, when added to deoxidized molten steel, reacts with Al in the molten steel, increasing the hydrogen concentration. Specifically, 6H on the right side of equation (2) is thought to be the main cause of the increase in the hydrogen concentration in molten steel.

[0018] CaO + H2O → Ca(OH)2···(1) 3Ca(OH)2+ 2Al → 3CaO + Al2O3+ 6H ···(2)

[0019] In the above formula (2), reducing the amount of Ca(OH)2, which serves as the hydrogen source, is effective in suppressing an increase in the hydrogen concentration in molten steel. For this reason, the water content of the CaO source added before the start of the current treatment is set to 2 mass% or less, preferably 1.5 mass% or less. Here, the water content of the CaO source refers to the total of the amount of adhered water, the amount of water of crystallization, and the amount of slaked lime (Ca(OH)2) converted into HO. As a method for reducing the amount of adhering water and the amount of crystallization water in the CaO source, known methods such as drying treatment using a furnace such as a kiln, air cooling treatment, etc. can be used.

[0020] Furthermore, the inclusion of calcium aluminate in the CaO source is an essential feature of the present invention, in that the deoxidizer is added after the CaO source. That is, if the CaO source is added before the deoxidizer, the high melting point of the CaO source will cause the flux to solidify (solidify CaO), and even if the deoxidizer is subsequently added, this will result in a problem in which the deoxidizing effect cannot be expected. However, by including calcium aluminate in the CaO source, the melting point of the flux is lowered, which suppresses the solidification of the flux, allowing the deoxidizing effect of the deoxidizer to be fully exerted.

[0021] Here, calcium aluminate is a complex oxide with the molecular formula CaO·Al2O3, and has a low melting point. When calcium aluminate is added to a flux that mainly contains CaO, the flux tends to turn into slag. As a CaO source containing calcium aluminate, a mixture of calcium aluminate and quicklime, such as ingot slag (slag in the molten steel ladle after continuous casting), can be suitably used.

[0022] Calcium aluminate is preferably contained in the CaO source so that the basicity, "(CaO mass %) / (Al2O3 mass %)," which is an index of the mass ratio of CaO to Al2O3 in the CaO source, is 1.3 or less, preferably 1.1 or less. In addition, since an excessive amount of Al2O3 may result in poor slag formation, the basicity is preferably 0.7 or more, preferably 0.8 or more.

[0023] Next, the introduction of a deoxidizer into the molten steel into which the CaO source has been introduced will be described. When a CaO source (containing Ca(OH)2) is added to molten steel in a state where a deoxidizer has been added, the reaction of the above-mentioned formula (2) proceeds to the right, resulting in an increase in the hydrogen concentration. For this reason, by adding a CaO source to molten steel before adding a deoxidizer, it is possible to suppress the progress of the hydrogen absorption reaction shown in the above-mentioned formula (2). Furthermore, in the absence of a deoxidizer, H2O is removed from the CaO source according to the following formula (3), reducing Ca(OH)2, which is thought to further suppress the progress of formula (2).

[0024] Ca(OH)2→ CaO + H2O ···(3)

[0025] Here, it is preferable to use a deoxidizer containing metallic aluminum (hereinafter referred to as metallic Al), such as aluminum dross. Note that the deoxidizer reacts in the molten steel ladle with the reaction product (slaked lime) produced by the reaction of quicklime with water as shown in the above formula (1), thereby producing Al2O3 and atomic hydrogen as shown in the above formula (2), which causes hydrogen pickup in the molten steel.

[0026] As described above, by adopting the configuration of 2), it is possible to suppress an increase in the hydrogen concentration to a level that allows molten steel to be cast without any hydrogen-induced troubles in the subsequent continuous casting.

[0027] 3) At least from the time of CaO source addition until the start of current treatment, the molten steel in the ladle should be stirred with bottom blown gas at a stirring power density of 30 W or less per ton of molten steel. After the deoxidizer is added, the Ca(OH)2 inevitably present in the slag and the deoxidizer (e.g., metallic Al) may cause the reaction of formula (2) described above. If the molten steel is strongly stirred with bottom-blown gas in this state, the reaction interface area between the flux (containing Ca(OH)2) and the molten steel (containing the added metallic Al) increases, and the reaction of formula (2) described above progresses. In the present invention, weak stirring is performed with a stirring power density of 30 W / ton of molten steel or less between the time when the deoxidizer is added and the time when current is applied is started, thereby suppressing the progress of the reaction of formula (2).

[0028] Furthermore, by carrying out weak stirring of the molten steel with a stirring power density of 30 (W / ton of molten steel) or less during the period from the addition of the CaO source to the addition of the deoxidizer, it is possible to suppress the entrainment of slag into the molten steel and the shaking of the molten steel surface, and the evaporation reaction shown in the above-mentioned formula (3) can be advanced by the heat of the molten steel, thereby suppressing the occurrence of the reaction shown in the above-mentioned formula (2) after the addition of the deoxidizer.

[0029] The stirring power density per ton of molten steel is preferably 25 W or less, and more preferably 20 W or less.

[0030] Here, the method for calculating the stirring power density will be described below. The stirring power density can be calculated, for example, based on the following formula described in JP 2013-023739 A: ε M is the stirring power density per ton of molten steel due to bottom-injected gas stirring (W / ton of molten steel), and Q is the injected gas flow rate (Nm 3 / sec), T1 is the molten steel temperature (K), T n is the injection gas temperature (K), P2 is the ambient pressure (N / m 2 ), W is the mass of molten steel (ton), ρ is the density of molten steel (ton / m 3 ), and h is the injection depth of the injected gas (m).

[0031] ε M ={(371×Q×T1) / W}×[ln{1+(9.8×ρ×h) / P2}+{1-T n / T1}]

[0032] As described above, in the LF equipment 10, the CaO source and the deoxidizer are sequentially added to the molten steel in the molten steel ladle 11 while bottom-blowing gas agitation is being performed on the molten steel, and then the electrode 12 is inserted into the slag above the molten steel, and the current application treatment is started. [Example]

[0033] Next, examples carried out to confirm the effects of the present invention will be described. In normal LF operation, deoxidizers and flux are sequentially added to a ladle containing 320 to 350 tons of crude molten steel, and the slag is stirred by bottom blowing to promote slag formation. Once a sufficient slag thickness is achieved, the slag is subjected to electrical treatment. Therefore, this experiment was conducted based on the above-mentioned normal LF operation.

[0034] 1. Experimental Conditions 320 to 350 tons of molten steel was stored in a steel ladle, and during the slag formation process before the start of electric current treatment, the basicity of the CaO source ((CaO mass%) / (Al2O3 mass%)), the moisture content of the CaO source (mass% in terms of HO), the timing of adding the deoxidizer, and the conditions of bottom-blown gas stirring were changed, and the hydrogen concentration in the molten steel before and after LF operation, specifically, after the molten steel ladle arrived at the LF equipment and immediately before adding the CaO source, and after the end of electric current treatment, was investigated.

[0035] The amount of deoxidizer added was kept constant at 9.0 kg / ton of molten steel. The stirring power density was set at 40 W / ton of molten steel immediately after the molten steel ladle arrived at the LF equipment, and remained constant at 40 W / ton of molten steel after the start of the current treatment. The hydrogen concentration in the molten steel was compared with the hydrogen concentration in the molten steel of the conventional example shown in Table 1, which will be described later, and evaluated according to the following criteria.

[0036] ×: Worsening to 5% improvement (suppression of increase in hydrogen concentration) ○: Improvement of over 5% (suppression of increase in hydrogen concentration) ◎: Improvement of over 10% (suppression of increase in hydrogen concentration)

[0037] The above experimental conditions and evaluation results are shown in Table 1.

[0038] [Table 1]

[0039] The results in Table 1 reveal the following: From Inventive Example 1 and Comparative Example 2, it was found that a good effect of reducing the hydrogen concentration was obtained by reducing the water content of the CaO source charged into molten steel. From Inventive Example 1 and Comparative Example 3, it was found that a good effect of reducing the hydrogen concentration was obtained by adding the deoxidizer after adding the CaO source. Inventive Example 1 and Comparative Examples 4 and 5 show that lowering the stirring power density by bottom blowing when the CaO source is added is effective in reducing the hydrogen concentration in the molten steel. From Comparative Example 4, it was found that lowering the stirring power density when adding the deoxidizer can suppress the increase in hydrogen concentration, but the effect was small, with an improvement of less than 5%. In other words, although weak stirring is necessary when adding the deoxidizer, the results show that lowering the stirring power density when adding the CaO source is more effective in suppressing the increase in hydrogen concentration.

[0040] That is, as in Example 1 of the invention, before the start of the current application treatment, the molten steel in the molten steel ladle 11 was subjected to weak bottom-blown gas stirring at 30 W / ton of molten steel while a CaO source containing calcium aluminate and having a water content of 2 mass% was added to the molten steel, and then a deoxidizer was added to the molten steel into which the CaO source had been added.This allowed the deoxidizer to be added to the molten steel without the CaO source solidifying, and a good evaluation result (◯) was obtained. Furthermore, similar to Inventive Example 1, Inventive Examples 2 and 3, in which the basicity of the CaO source was changed, also obtained good evaluation results (◯). In particular, in Example 4, the bottom gas mixing was weaker than in Examples 1 to 3, at 10 W / ton of molten steel, which further reduced the entrainment of slag into the molten steel and the fluctuation of the molten steel surface, and thus the most advantageous evaluation results were obtained.

[0041] From the above, it was confirmed that the increase in hydrogen concentration in molten steel during LF operation can be suppressed compared to conventional technologies, making it possible to omit or shorten the vacuum degassing treatment that was previously essential after LF operation.

[0042] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the configurations described in the above embodiments and includes other embodiments and modifications that are conceivable within the scope of the claims. For example, a method for operating an LF device of the present invention that combines some or all of the above embodiments and modifications is also within the scope of the present invention. [Industrial Applicability]

[0043] INDUSTRIAL APPLICABILITY The present invention is industrially useful because it can provide a method for operating an LF apparatus that can effectively suppress an increase in hydrogen concentration in molten steel during LF operation. [Explanation of symbols]

[0044] 10: LF device, 11: molten steel ladle, 12: electrode, 13: porous plug, 14: lid

Claims

[Claim 1] A method for operating an LF device in which an electrode is inserted into slag above molten steel in a molten steel ladle and an electric current is applied, comprising the steps of: before the start of the current application treatment, a CaO source containing calcium aluminate and having a water content of 2 mass% or less is charged into the molten steel in the molten steel ladle, and a deoxidizer is charged into the molten steel into which the CaO source has been charged; At least from the time when the CaO source is charged until the start of the current application treatment, the molten steel in the molten steel ladle is stirred with bottom blown gas at a stirring power density of 30 W or less per ton of molten steel. A method for operating an LF device.

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