Molten steel production method

Abstract

There is provided a molten steel production method which can control the Al concentration of molten steel to a target content of 0.01 mass % or more, without significantly increasing processing cost, when deoxidation refining is performed using a vacuum degassing apparatus. The method for producing molten steel having an Al concentration of 0.01 mass % or more by adding Al to molten steel at least twice in a vacuum degassing apparatus, includes: predicting an Al yield from a change in the temperature of a slag before and after a first addition of Al; and using the predicted Al yield to determine an amount of Al to be added in at least one subsequent addition of Al.

Description

Technical Field

[0001] The present invention relates to a molten steel production method for producing deoxidized molten steel using a vacuum degassing apparatus.Background Art

[0002] In a vacuum degassing of non-deoxidized (undeoxidized) or partially deoxidized molten steel using a vacuum degassing apparatus such as RH or DH, the molten steel is generally deoxidized and refined by adding metallic Al to the molten steel in a vacuum atmosphere during the treatment. The yield of metallic Al upon the deoxidization varies greatly depending on the operating conditions, and the variation causes off-specification and an increase in Al-alloying cost. Some techniques have been reported which are directed to reducing such variation in the yield of Al.

[0003] For example, Patent Literature 1 discloses a technique which involves adding, in a ladle (steel ladle), a deoxidizer to a slag on molten steel which has been tapped from a converter, thereby suppressing the oxidation of Al by the slag and controlling the Al concentration of the molten steel. Patent Literature 2 discloses a technique which involves determining the degree of oxidation of a slag by measuring the concentrations of T. Fe and MnO in the slag using an oxygen probe or the like before vacuum refining, and determining an amount of Al to be added according to the measured degree of oxidation of the slag.Citation ListPatent Literature

[0004] PTL 1: Japanese Unexamined Patent Application Publication No. 63-183117 PTL 2: Japanese Unexamined Patent Application Publication No. 2006-283089 Non-Patent Literature

[0005] NPL 1: Thermodynamic Data for Steelmaking, edited by Mitsutaka Hino and Kimihisa Ito, Tohoku University Press, 2010Summary of InventionTechnical Problem

[0006] In the method disclosed in Patent Literature 1 in which a deoxidizer is added onto a slag, oxides in the slag are not completely reduced by the addition of the deoxidizer. Therefore, some Al reacts with the slag, resulting in an increased variation in the Al concentration of molten steel. When oxygen-blowing decarburization (under vacuum) is performed by vacuum degassing on non-deoxidized molten steel which has been tapped from a converter, as in processing of ultra-low carbon steel, FeO is formed by blowing of oxygen after the addition of a deoxidizer onto a slag, and the FeO reacts with Al, resulting in a further increased variation in the Al concentration of molten steel.

[0007] In the method of Patent Literature 2, the concentrations of T. Fe and MnO in a slag are measured using an oxygen probe. An oxygen probe specifically for slag is required in order to quickly measure the oxidation degree of a slag in a steelmaking process. An oxygen probe specifically for slag is expensive, leading to a significantly increased processing cost.

[0008] The present invention has been made in view of the above situation. It is therefore an object of the present invention to provide a molten steel production method which can control the Al concentration of molten steel to a target content of 0.01 mass % or more, without significantly increasing processing cost, when deoxidation refining is performed using a vacuum degassing apparatus.Solution to Problem

[0009] In order to solve the above problems, the present invention has the following features. [1] A method for producing molten steel having an Al concentration of 0.01 mass % or more by adding Al to molten steel at least twice in a vacuum degassing apparatus, the method including: predicting an Al yield from a change in the temperature of a slag before and after a first addition of Al; and using the predicted Al yield to determine an amount of Al to be added in at least one subsequent addition of Al. [2] The method for producing molten steel according to [1], wherein an evaporation fraction of Al is determined from a change in the heat content of the molten steel before and after the first addition of Al, and an Al yield is predicted from the change in the temperature of the slag and from the evaporation fraction. [3] The method for producing molten steel according to [2], wherein the Al yield is predicted using the following formula (1), and the amount of Al to be added in at least one subsequent addition of Al is determined using the following formula (2): [Math. 1] η = 1 − α × Δ T / d − 0.01 a o f o × W steel W Al _ 1 st − β [Math. 2] W Al _ 2 nd = 0.01 × Al Target − Al 0 × W steel − η × W Al _ 1 st where η is the Al yield (-), ΔT is the change in the temperature (°C) of the slag, d is the thickness (mm) of the slag, α is a correction factor (-), β is the evaporation fraction (-), a 0 is the oxygen concentration (mass %) of the molten steel before the first addition of Al, f 0 is the activity coefficient (-) of oxygen in the molten steel, W steel is the weight (kg) of the molten steel, W Al_1st is the amount (kg) of the first-added Al, W Al_2nd is the amount (kg) of Al to be added in at least one subsequent addition of Al, [Al] Target is a target Al concentration (mass %) of the molten steel, and [Al] 0 is the Al concentration (mass %) of the molten steel before the first addition of Al. [4] The method for producing molten steel according to [2] or [3], wherein the evaporation fraction is determined using the following formulae (3) to (6): [Math. 3] Δ Q = Δ Q exo + Δ Q sens [Math. 4] Δ Q exo = 10 × a o f o × 54 48 × 6464 [Math. 5] Δ Q sens = 517 × W Al _ 1 st 0.001 × W steel [Math. 6] β = X × ln Δ Q + Y where ΔQ is a change in the heat content (kcal / t-steel) of the molten steel due to the first-added Al, ΔQ exo is a change in the heat of reaction (kcal / t-steel) of the molten steel due to the first-added Al, ΔQ sens is a change in the sensible heat (kcal / t-steel) of the molten steel due to the first-added Al, a 0 is the oxygen concentration (mass %) of the molten steel before the first addition of Al, f 0 is the activity coefficient of oxygen in the molten steel (-), β is the evaporation fraction (-), and X and Y are constants. Advantageous Effects of Invention

[0010] According to the present invention, molten steel whose Al concentration is controlled to a target content of 0.01 mass % or more can be produced without significantly increasing processing cost.Brief Description of Drawings

[0011] [FIG. 1] FIG. 1 is a schematic cross-sectional view of an RH vacuum degassing apparatus 10. [FIG. 2] FIG. 2 is a graph showing the relationship between the concentration of soluble Al (Sol.Al) and the number of heats after the completion of an RH vacuum vacuum degassing. Description of Embodiments

[0012] Embodiment of the present invention will now be described with reference to the drawings. Vacuum degassing facilities, which can perform the molten steel production method according to the present invention, include an RH vacuum degassing apparatus, a DH vacuum degassing apparatus, and a REDA vacuum degassing apparatus. An RH vacuum degassing apparatus, which is most typical among these apparatuses, is used to describe embodiments of the molten steel production method according to the present invention.

[0013] FIG. 1 is a schematic cross-sectional view of an RH vacuum degassing apparatus 10. In FIG. 1, reference sign 12 denotes a ladle, reference sign 14 denotes molten steel, and reference sign 16 denotes a slag. Reference sign 18 denotes a vacuum vessel. The vacuum vessel 18 is composed of an upper chamber 20 and a lower chamber 22. Reference sign 24 denotes an up leg snorkel (immersion tube), and reference sign 26 denotes a down leg snorkel (immersion tube). Reference sign 28 denotes a circulation gas injection pipe, reference sign 30 denotes a duct, reference sign 32 denotes a feed port, and reference sign 34 denotes a top lance (oxygen lance). The top lance 34 is a device for blowing and adding oxygen gas and a flux to the molten steel in the vacuum vessel, is installed at the top of the vacuum vessel 18, and is vertically movable within the vacuum vessel 18. Reference sign 36 denotes a thermal camera. The thermal camera 36 is used to measure the temperature of the slag.

[0014] In the RH vacuum degassing apparatus 10, the ladle 12 containing the molten steel 14 that has undergone dephosphorization and / or decarburization is raised by a lifting device (not shown) to immerse the up leg snorkel 24 and the down leg snorkel 26 in the molten steel 14 in the ladle 12. The vacuum vessel 18 is then evacuated by an exhaust device (not shown) connected to the duct 30 to reduce the pressure in the vacuum vessel 18, while a circulation gas is blown into the up leg snorkel 24 from the circulation gas injection pipe 28. As the pressure in the vacuum vessel 18 decreases, the molten steel 14 in the ladle 12 ascends in proportion to the difference between the atmospheric pressure and the pressure (vacuum level) in the vacuum vessel, and flows into the vacuum vessel 18. At the same time, due to the gas lift effect of the circulation gas, which has been blown from the circulation gas injection pipe 10, the molten steel 14, together with the circulation gas, ascends in the up leg snorkel 24 and flows into the vacuum vessel 18. Thereafter, the molten steel returns to the ladle 12 via the down leg snorkel 26, forming a so-called circulation, while vacuum degassing refining of the molten steel is in progress. The molten steel 14 is exposed to a reduced-pressure atmosphere in the vacuum vessel 18, and gas components in the molten steel 14 move into the atmosphere in the vacuum vessel 18, whereby degassing reactions of the molten steel 14 proceed.

[0015] When a deoxidation treatment is performed on the non-deoxidized or partially deoxidized molten steel in vacuum degassing refining of the molten steel, an alloy that reacts with oxygen to form an oxide is added as a deoxidizer to the molten steel 14 in the vacuum vessel 18 from the feed port 32. Because of the strong deoxidizing power, metallic Al or an Al-containing alloy is generally used as a deoxidizer. In the molten steel production method of this embodiment, metallic Al or an Al-containing alloy, hereinafter referred to collectively as Al, is used as a deoxidizer.

[0016] When Al, added as a deoxidizer, reacts with an oxide in the slag 16, the slag 16 is heated by the heat of reaction of Al. The present inventors, on the assumption that the temperature of the slag will increase according to the amount of reaction of Al, measured the temperature of the slag before and after the addition of Al during a vacuum vacuum degassing using a thermal camera. As a result, it was found that there is a good correlation between the change in the temperature of the slag 16 and the amount of reaction of Al. The inventors then found that the Al concentration of the molten steel 14 after the deoxidation treatment can be controlled with high accuracy by predicting an Al yield, taking account of the amount of Al consumed by the slag 16, from a change in the temperature of the slag 16 before and after a first addition, of a total of two or more additions, of Al to the molten steel 14, and using the Al yield to determine an amount of Al to be added to the molten steel 14 in at least one subsequent addition of Al.

[0017] Thus, in the molten steel production method of this embodiment, in performing deoxidation of molten steel in the RH vacuum degassing apparatus 10, Al as a deoxidizer is added at least twice to produce molten steel having an Al concentration of 0.01 mass % or more. First, an approximate Al yield is estimated by referring to the oxygen concentration of the molten steel 14 based on past operational results, and an amount of Al to be added in a first addition of Al is determined so that the oxygen concentration of the molten steel 14 after the addition of Al will be less than 0.0002 mass %, and the determined amount of Al is added to the molten steel 14. In advance of the addition of Al, the thermal camera 36 is installed at a position where the slag 16 in the ladle 12 can be measured from above, and the slag temperature is continuously measured using the thermal camera 36. The thermal camera 36 is an example of a radiation thermometer.

[0018] Upon the first addition of Al, oxides such as FeO in the slag 16 react with Al, causing an increase in the slag temperature. Using the change ΔT in the slag temperature, obtained by subtracting the slag temperature before the first addition of Al from the slag temperature after the first addition of Al, the yield of the first-added Al can be predicted by the following formula (1). The slag temperature is the average temperature of a given area measured by the thermal camera 36. The slag temperature after the addition of Al refers to the slag temperature after the increase in the slag temperature due to the addition of Al has ceased. [Math. 7] η = 1 − α × Δ T / d − 0.01 a o f o × W steel W Al _ 1 st − β

[0019] In the above formula (1), η is the Al yield (-). ΔT is the change in the temperature (°C) of the slag 16. d is the thickness (mm) of the slag 16. α is a correction factor (-). β is the evaporation fraction (-) of the first-added Al. a 0 is the oxygen concentration (mass %) of the molten steel before the first addition of Al. f 0 is the activity coefficient (-) of oxygen in the molten steel 14. W steel is the weight (kg) of the molten steel 14. W Al_1st is the amount (kg) of the first-added Al. The unit (-) means dimensionless. The correction factor is determined so that the Al yield, determined from the actual amount of Al added and the analytical value of Al (average values in the past about three months to one year) in actual operation, matches the Al yield determined by formula (1). The activity coefficient of oxygen in the molten steel 14 is determined using the following formula (7). [Math. 8] logf o = ∑ i e o i % i

[0020] In the above formula (7), f 0 is the activity coefficient (-) of oxygen in the molten steel 14. e i< 0 is the coefficient (-) of an interaction of a component i in the molten steel 14 with oxygen. (%i) is the concentration (mass %) of the component i in the molten steel 14. A value described in Non-Patent Literature 1 is used for the interaction parameter (first-order) e i< 0 .

[0021] The slag thickness in the above formula (1) can be determined by measuring the level of the surface of the molten steel using a molten steel level gauge, or by immersing a metal rod in the molten steel 14 in the ladle 12, and measuring the length of a portion melted by the slag 16. The oxygen concentration of the molten steel 14 may be one measured before deoxidation and after dephosphorization and decarburization. The amount of Al to be added in at least one subsequent addition of Al is determined using the Al yield, determined by the above formula (1), and the following formula (2). When an Al-containing alloy is used as Al, the amount of the Al-containing alloy to be added may be determined based on the amount of metallic Al to be added, which is determined by multiplying the amount of the Al-containing alloy to be added by the pure Al content of the alloy. [Math. 9] W Al _ 2 nd = 0.01 × Al Target − Al 0 × W steel − η × W Al _ 1 st

[0022] In the above formula (2), W Al_2nd is the amount (kg) of Al to be added in at least one subsequent addition addition(s) of Al. [Al] Target is a target Al concentration (mass %) of the molten steel 14. [Al] 0 is the Al concentration (mass %) of the molten steel 14 before the first addition of Al. W steel is the weight (kg) of the molten steel 14. η is the Al yield (-). W Al_1st is the amount (kg) of the first-added Al.

[0023] In this manner, a determined amount of Al is added in at least one subsequent addition of Al. By the phrase "in at least one subsequent addition of Al" is meant that all the determined amount of Al may be added at a time in the second addition of Al, or may be added in a divided manner to the molten steel 14, for example, in the second and third additions of Al, or in the second to fourth additions of Al.

[0024] By thus determining an amount of Al to be added in at least one subsequent addition of Al, the amount of Al can be determined, without using an oxygen probe specifically for slag, based on an Al yield taking account of the consumption of Al by the slag 16. By adding the determined amount of Al to the molten steel 14, variation in the amount of Al due to the slag 16 can be reduced. This makes it possible to produce molten steel, whose Al concentration is controlled with high accuracy to a target content of 0.01 mass % or more, without significantly increasing processing cost.

[0025] β in the above formula (1) is the evaporation fraction of the first-added Al. While the evaporation fraction of the first-added Al may be a predetermined amount based on past operational results, it is preferably determined from a change in the heat content of the molten steel 14 before and after the first addition of Al. In particular, it is preferred to determine a change ΔQ in the heat content of the molten steel 14 before and after the first addition of Al using the following formulae (3) to (5), and determine the evaporation fraction of the first-added Al using the ΔQ and the following formula (6). [Math. 10] Δ Q = Δ Q exo + Δ Q sens [Math. 11] Δ Q exo = 10 × a o f o × 54 48 × 6464 [Math. 12] Δ Q sens = 517 × W Al _ 1 st 0.001 × W steel [Math. 13] β = X × ln Δ Q + Y

[0026] In the above formulae (3) to (6), ΔQ is a change in the heat content (kcal / t-steel) of the molten steel 14 due to the first-added Al. ΔQ exo is a change in the heat of reaction (kcal / t-steel) of the molten steel 14 due to the first-added Al. ΔQ sens is a change in the sensible heat (kcal / t-steel) of the molten steel 14 due to the first-added Al. a 0 is the oxygen concentration (mass %) of the molten steel before the first addition of Al. f 0 is the activity coefficient (-) of oxygen in the molten steel. W Al_1st is the amount (kg) of the first-added Al. W steel is the weight (kg) of the molten steel 14. β is the evaporation fraction (-) of the first-added Al. X and Y are constants. f 0 can be determined using the above formula (7). The constants X and Y may be determined from laboratory experiments, or may be determined by fitting between calculated values and actual evaporation amounts of Al in actual operation.

[0027] By thus determining the evaporation fraction of Al from a change in the heat content of the molten steel 14 before and after the first addition of Al, the evaporation fraction of Al can be determined with high accuracy. The amount of Al to be added in at least one subsequent addition of Al is determined using the evaporation fraction and the above formulae (1) and (2). By adding the thus-determined amount of Al to the molten steel 14, it becomes possible to produce molten steel whose Al concentration is controlled to a target content of 0.01 mass % or more with higher accuracy.EXAMPLES

[0028] A description will now be given of examples in which an experiment was performed in the following manner. 300 tons of molten steel, which had been produced by decarburizing and refining molten pig iron in a converter, was tapped from the converter to a ladle, where the molten steel was subjected to vacuum degassing refining using the RH vacuum degassing apparatus 10 shown in FIG. 1. The steel grade tested was ultra-low carbon steel having a carbon concentration of up to 25 ppm and a target Al concentration of 0.04 mass %. The molten steel used in the experiment had the following chemical composition: C: 0.04 to 0.06 mass %, Si: 0.15 to 0.25 mass %, Mn: 1.2 to 1.4 mass %, P: 0.02 mass % or less, and S: 0.003 mass % or less. The temperature of the molten steel before deoxidation was 1580 to 1630°C, and the oxygen concentration of the molten steel before deoxidation was 200 to 600 ppm. The vacuum level in the RH vacuum degassing apparatus was set to 2 torr, and the flow rate of the circulation gas was set to 2500 NL / min. Metallic Al was used as a deoxidizer.

[0029] After decarburization had progressed sufficiently, the oxygen concentration a 0 of the molten steel was measured using an oxygen measuring probe, and an amount of Al to be added in the first addition of Al was determined in the range of 200 to 600 kg from the value of a 0 and the amount of the molten steel, and then Al was added to the molten steel. The experiment was conducted on charge groups A, B, and C of 100 charges each, and an amount of Al to be added in the second addition of Al was determined for each charge group in the following manner:

[0030] For charge group A (Comparative Example 1), the oxygen concentration of the molten steel was measured with the oxygen measuring probe, and the Al concentration of the molten steel was estimated from the Al-O equilibrium and, based on the estimated Al concentration, an amount of Al to be added in the second addition of Al was determined. For charge group B (Inventive Example 1), an amount of Al to be added in the second addition of Al was determined using the above formulae (1) and (2). In the case of charge group B, a constant, determined from an average value in past operation, was used as the evaporation fraction of the first-added Al. For charge group C (Inventive Example 2), an amount of Al to be added in the second addition of Al was determined using the above formulae (1) and (2). In the case of charge group C, a value determined by the above formulae (3) to (6) was used as the evaporation fraction of the first-added Al.

[0031] In each charge group, after completion of the RH vacuum vacuum degassing, metal samples were taken from the molten steel to measure the soluble Al concentrations of the molten steel.

[0032] FIG. 2 is a graph showing the relationship between the concentration of soluble Al and the number of heats after the completion of the RH vacuum degassing. In FIG. 2, the abscissa axis represents the soluble Al concentration (mass %), and the ordinate axis represents the number of heats (ch). As shown in FIG. 2, variation in the soluble Al concentration from the target content of 0.04 mass % was smaller in charge groups B and C, which are inventive examples, than in charge group A which is a comparative example. The standard deviation of the soluble Al concentrations of the molten steel after completion of the RH vacuum degassing in each charge group is shown in Table 1 below. [Table 1]ClassificationStandard deviation σ (%)Inventive Example 1Charge group B0.0027Inventive Example 2Charge group C0.0009Comparative Example 1Charge group A0.0114

[0033] As shown in Table 1, the standard deviations of the soluble Al concentrations of the molten steel after completion of the RH vacuum degassing in Inventive Examples 1 and 2 were smaller than that in Comparative Example 1. This result demonstrates that the Al concentration of molten steel can be controlled to a target content of 0.01 mass % or more (0.04 mass %) with high accuracy by predicting an Al yield from a change in the temperature of a slag before and after the first addition of Al, and using the predicted Al yield to determine the amount of Al to be added in the second addition of Al.

[0034] The standard deviation of the soluble Al concentrations of the molten steel after completion of the RH vacuum degassing in Inventive Example 2 was smaller than that in Inventive Example 1. This result demonstrates that the Al concentration of molten steel can be controlled to a target content of 0.01 mass % or more (0.04 mass %) with higher accuracy by determining the evaporation fraction of the first-added Al in the above formula (1) from a change in the heat content of the molten steel before and after the first addition of Al.Reference Signs List

[0035] 10RH vacuum degassing apparatus 12ladle 14molten steel 16slag 18vacuum vessel 20upper chamber 22lower chamber 24up leg snorkel 26down leg snorkel 28circulation gas injection pipe 30duct 32feed port 34top lance 36thermal camera

Claims

1. A method for producing molten steel having an Al concentration of 0.01 mass % or more by adding Al to molten steel at least twice in a vacuum degassing apparatus, the method comprising: predicting an Al yield from a change in the temperature of a slag before and after a first addition of Al; and using the predicted Al yield to determine an amount of Al to be added in at least one subsequent addition of Al.

2. The method for producing molten steel according to claim 1, wherein an evaporation fraction of Al is determined from a change in the heat content of the molten steel before and after the first addition of Al, and an Al yield is predicted from the change in the temperature of the slag and from the evaporation fraction.

3. The method for producing molten steel according to claim 2, wherein the Al yield is predicted using the following formula (1), and the amount of Al to be added in at least one subsequent addition of Al is determined using the following formula (2): [Math. 1] η = 1 − α × ΔT / d − 0.01 a o f o × W steel W Al _ 1 st − β [Math. 2] W Al _ 2 nd = 0.01 × Al Target − Al 0 × W steel − η × W Al _ 1 st where η is the Al yield (-), ΔT is the change in the temperature (°C) of the slag, d is the thickness (mm) of the slag, α is a correction factor (-), β is the evaporation fraction (-), a0 is the oxygen concentration (mass %) of the molten steel before the first addition of Al, f0 is the activity coefficient (-) of oxygen in the molten steel, Wsteel is the weight (kg) of the molten steel, WAl_1st is the amount (kg) of the first-added Al, WAl_2nd is the amount (kg) of Al to be added in at least one subsequent addition of Al, [Al]Target is a target Al concentration (mass %) of the molten steel, and [Al]0 is the Al concentration (mass %) of the molten steel before the first addition of Al.

4. The method for producing molten steel according to claim 2 or 3, wherein the evaporation fraction is determined using the following formulae (3) to (6): [Math. 3] Δ Q = Δ Q exo + Δ Q sens [Math. 4] Δ Q exo = 10 × a o f o × 54 48 × 6464 [Math. 5] Δ Q sens = 517 × W Al _ 1 st 0.001 × W steel [Math. 6] β = X × ln Δ Q + Y where ΔQ is a change in the heat content (kcal / t-steel) of the molten steel due to the first-added Al, ΔQexo is a change in the heat of reaction (kcal / t-steel) of the molten steel due to the first-added Al, ΔQsens is a change in the sensible heat (kcal / t-steel) of the molten steel due to the first-added Al, a0 is the oxygen concentration (mass %) of the molten steel before the first addition of Al, f0 is the activity coefficient of oxygen in the molten steel (-), WAl_1st is the amount (kg) of the first-added Al, Wsteel is the weight (kg) of the molten steel, β is the evaporation fraction (-), and X and Y are constants.

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