Molten steel manufacturing method

Abstract

A method for producing molten steel is provided that, when performing deoxidation refining using a vacuum degassing device, can control the Al concentration in the molten steel to a target concentration of 0.01 mass % or more without significantly increasing processing costs. A method for producing molten steel in which Al is added to molten steel two or more times in a vacuum degassing apparatus to produce molten steel with an Al concentration of 0.01 mass% or more, in which the Al yield is predicted from the temperature change in the slag before and after the first Al addition, and the amount of Al to be added from the second time onwards is determined using the predicted Al yield.

Description

[Technical field]

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

[0002] When non-deoxidized or semi-deoxidized molten steel is degassed using a vacuum degassing device such as RH or DH, metallic Al is generally added to the molten steel in a vacuum atmosphere during the deoxidation process to deoxidize and refine the molten steel. However, the yield of metallic Al during deoxidation varies greatly depending on the operating conditions, and this variation is a factor in removing the component and increasing the cost of Al alloys. For this reason, several technologies have been reported to reduce the variation in Al yield.

[0003] For example, Patent Document 1 discloses a technique for controlling the Al concentration in molten steel by adding a deoxidizer to the slag above the molten steel in a ladle after steel is tapped from a converter, thereby suppressing the oxidation of Al by the slag. Patent Document 2 discloses a technique for determining the degree of oxidation of the slag by measuring the T.Fe and MnO concentrations in the slag using an oxygen probe or the like before vacuum refining, and determining the amount of Al to be added according to the measured value of the degree of oxidation of the slag. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-open No. 63-183117 [Patent Document 2] JP 2006-283089 A [Non-patent literature]

[0005] [Non-Patent Document 1] Thermodynamic datafor steelmaking edited by Mitsutaka Hino and Kimihisa Ito Tohoku UniversityPress, 2010 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the method disclosed in Patent Document 1, a deoxidizer is added onto the slag, but the oxides in the slag are not completely reduced by adding the deoxidizer. As a result, some Al reacts with the slag, resulting in a large variation in the Al concentration in the molten steel. When oxygen supply decarburization is performed by vacuum degassing after tapping of undeoxidized steel, such as in the treatment of ultra-low carbon steel, FeO is generated by oxygen supply after adding the deoxidizer onto the slag, and the generated FeO reacts with Al, resulting in a further large variation in the Al concentration in the molten steel.

[0007] In Patent Document 2, the concentrations of T.Fe and MnO in the slag are measured using an oxygen probe, but in order to quickly measure the oxidation degree of the slag in the steelmaking process, an oxygen probe for the slag is required. The oxygen probe for the slag is expensive, so the processing cost increases significantly.

[0008] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a method for producing molten steel that can control the Al concentration in molten steel to a target concentration of 0.01 mass% or more without significantly increasing the processing cost when deoxidation refining is performed using a vacuum degassing apparatus. [Means for solving the problem]

[0009] The means for solving the above problems are as follows. [1] A method for producing molten steel in which Al is added to molten steel two or more times in a vacuum degassing apparatus to produce molten steel having an Al concentration of 0.01 mass% or more, the method comprising predicting an Al yield from the temperature change of the slag before and after the first Al addition, and determining the amount of Al to be added from the second time onwards using the predicted Al yield. [2] The evaporation rate of Al is calculated from the change in heat quantity of the molten steel before and after the first addition of Al; The method for producing molten steel according to [1], wherein an Al yield is predicted from a temperature change and an evaporation rate of the slag. [3] A method for producing molten steel according to [2], comprising predicting the Al yield using the following formula (1) and determining the amount of Al to be added for the second or subsequent times using the following formula (2):

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[0010] According to the present invention, it is possible to produce molten steel in which the Al concentration is controlled to a target concentration of 0.01 mass % or more without significantly increasing the processing cost. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic cross-sectional view of an RH vacuum degassing apparatus 10. [Diagram 2] FIG. 2 is a graph showing the relationship between the Sol. Al concentration and the number of charges after the completion of the RH vacuum degassing treatment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Vacuum degassing equipment capable of carrying out the method for producing molten steel according to the present invention includes an RH vacuum degassing apparatus, a DH vacuum degassing apparatus, and a REDA vacuum degassing apparatus. An embodiment of the method for producing molten steel according to the present invention will be described using the RH vacuum degassing apparatus, which is the most representative of these.

[0013] FIG. 1 is a schematic cross-sectional view of an RH vacuum degassing apparatus 10. In FIG. 1, reference numeral 12 denotes a ladle, reference numeral 14 denotes molten steel, and reference numeral 16 denotes slag. Reference numeral 18 denotes a vacuum vessel, which is composed of an upper vessel 20 and a lower vessel 22. Reference numeral 24 denotes an ascending side immersion tube (ascending tube), and reference numeral 26 denotes a descending side immersion tube (descending tube). Reference numeral 28 denotes a carbonization gas inlet tube, reference numeral 30 denotes a duct, reference numeral 32 denotes a raw material inlet, and reference numeral 34 denotes a top blowing lance. The top blowing lance 34 is a device for adding oxygen gas and flux to the molten steel in the vacuum vessel by blowing them, and is installed at the top of the vacuum vessel 18 and can move up and down inside the vacuum vessel 18. Reference numeral 36 denotes a thermal camera, which is used to measure the slag temperature.

[0014] In the RH vacuum degassing apparatus 10, the ladle 12 containing the molten steel 14 that has been subjected to dephosphorization and decarburization is raised by a lifting device (not shown), and the up-side immersion tube 24 and the down-side immersion tube 26 are immersed in the molten steel 14 in the ladle 12. The inside of the vacuum chamber 18 is then evacuated by an exhaust device (not shown) connected to a duct 30 to reduce the pressure inside the vacuum chamber 18, and reflux gas is blown into the up-side immersion tube 24 from the carbonization gas blowing tube 28. When the pressure inside the vacuum chamber 18 is reduced, the molten steel 14 in the ladle 12 rises in proportion to the difference between the atmospheric pressure and the pressure (degree of vacuum) inside the vacuum chamber, and flows into the vacuum chamber 18. At the same time, due to the gas lift effect of the reflux gas blown from the reflux gas blowing tube 10, the molten steel 14 rises up the up-side immersion tube 24 together with the reflux gas and flows into the vacuum chamber 18. Thereafter, a flow returning to the ladle 12 via the downflow side immersion pipe 26, a so-called reflux flow, is formed, and vacuum degassing refining is performed. The molten steel 14 is exposed to an atmosphere under reduced pressure in the vacuum vessel 18, and the gas components in the molten steel 14 move to the atmosphere in the vacuum vessel 18, whereby the degassing reaction of the molten steel 14 progresses.

[0015] In vacuum degassing refining, when deoxidizing undeoxidized or semi-deoxidized molten steel, an alloy that reacts with oxygen to produce an oxide as a deoxidizer is added to the molten steel 14 in the vacuum tank 18 through the raw material inlet 32. Metallic Al or an alloy containing Al is generally used as the deoxidizer because of its strong deoxidizing power. In the method for producing molten steel according to this embodiment, metallic Al and an alloy containing Al are also used as the deoxidizer, and these metallic Al and alloy containing Al are collectively referred to as Al.

[0016] When Al added as a deoxidizer reacts with oxides in the slag 16, the slag 16 is heated by the heat of reaction of Al. The inventors assumed that the slag temperature rises according to the amount of reaction of Al, and measured the slag temperature before and after the addition of Al during the vacuum degassing process using a thermal camera. As a result, it was confirmed that there is a good correlation between the temperature change of the slag 16 and the amount of reaction of Al. Therefore, the inventors found that, assuming that Al is added to the molten steel 14 two or more times, the Al yield is predicted from the temperature change of the slag 16 before and after the first addition of Al, taking into account the amount of Al consumed by the slag 16, and the amount of Al to be added to the molten steel 14 from the second addition onward is determined using the Al yield, thereby enabling the Al concentration of the molten steel 14 after the deoxidation process to be controlled with high accuracy.

[0017] Therefore, in the method for producing molten steel according to the present embodiment, when deoxidizing molten steel in the RH vacuum degassing apparatus 10, Al, a deoxidizing agent, is added two or more times to produce molten steel with an Al concentration of 0.01 mass% or more. First, the approximate Al yield is estimated with reference to the oxygen concentration of the molten steel 14 from past operational results, and the amount of Al added the first time is determined and added to the molten steel 14 so that the oxygen concentration of the molten steel 14 after the Al addition is less than 0.0002 mass%. At this time, a 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] When the first Al is added, Al reacts with oxides such as FeO in the slag 16, causing the slag temperature to rise. If the slag temperature change ΔT is the slag temperature after the first Al addition minus the slag temperature before the Al addition, the yield of the first added Al can be predicted using the following formula (1). The slag temperature is the average temperature of a specified area measured by the thermal camera 36. The slag temperature after the addition of Al is the slag temperature after the rise in the slag temperature due to the addition of Al stops.

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[0021] The slag thickness in the above formula (1) can be obtained by measuring the height of the molten steel surface 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 the portion dissolved by the slag 16. The oxygen concentration of the molten steel 14 measured before deoxidation and after dephosphorization and decarburization may be used. The amount of Al added from the second time onwards is determined using the Al yield obtained from the above formula (1) and the following formula (2). When an Al-containing alloy is used as Al, the amount of Al-containing alloy to be added may be determined based on the amount of metallic Al to be added, which is obtained by multiplying the amount of Al-containing alloy to be added by the pure Al content of the alloy.

[0022]

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[0023] In this manner, the determined amount of Al is added in the second or subsequent times. The second or subsequent times may mean that the determined amount of Al is added in its entirety in the second addition, or may be added in two or more separate times, for example, by further dividing the determined amount into two or three separate times and adding it to the molten steel 14.

[0024] By determining the amount of Al to be added from the second time onwards in this manner, it is possible to determine the amount of Al to be added from the second time onwards based on the Al yield taking into consideration the amount of Al consumed by the slag 16, without using an oxygen probe for the slag. By adding the determined amount of Al to the molten steel 14, it is possible to reduce the variation in the amount of Al due to the slag 16, and it becomes possible to produce molten steel in which the Al concentration in the molten steel is controlled with high precision to a target concentration of 0.01 mass % or more, without significantly increasing the processing cost.

[0025] In the above formula (1), β is the evaporation rate of Al added the first time. The evaporation rate of Al added the first time may be a predetermined amount based on past operational results, but it is preferable to obtain the evaporation rate of Al from the change in heat quantity of molten steel 14 before and after the first addition of Al. Specifically, it is preferable to obtain the change in heat quantity ΔQ of molten steel 14 before and after the first addition of Al using the following formulas (3) to (5), and to obtain the evaporation rate of Al added the first time using ΔQ and the following formula (6).

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[0030] In this way, by determining the evaporation rate of Al from the change in the heat quantity of molten steel 14 before and after the first addition of Al, the evaporation rate of Al can be determined with high accuracy. The amount of Al to be added from the second time onwards is determined using the evaporation rate and the above formulas (1) and (2). By adding the determined amount of Al to molten steel 14, it becomes possible to produce molten steel in which the Al concentration in the molten steel is controlled with even higher accuracy to the target concentration of 0.01 mass% or more. EXAMPLES

[0031] Next, an embodiment will be described in which 300 tons of molten steel produced by decarburization and refining molten pig iron in a converter was tapped from the converter into a ladle, and the molten steel in the ladle was vacuum degassed and refined in the RH vacuum degassing apparatus 10 shown in FIG. 1. The target steel type was an ultra-low carbon steel type with an upper limit of carbon concentration of 25 ppm and a target Al concentration of 0.04 mass%. The composition of the molten steel used in the test was 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 degree of vacuum of the RH vacuum degassing apparatus was 2 torr, and the reflux gas flow rate was 2500 NL / min. Metallic Al was used as the deoxidizer.

[0032] After the decarburization process had progressed sufficiently, the oxygen concentration a0 of the molten steel was measured using an acid measuring probe, and the amount of Al to be added in the first increment was determined to be in the range of 200 to 600 kg based on the value of a0 and the amount of molten steel. The test was carried out in charge groups A, B, and C, each of which had 100 charges, and the amount of Al to be added in the second increment was determined for each charge group as follows:

[0033] In charge group A (comparative example), the oxygen concentration of the molten steel was measured with an acid measuring probe, and the Al concentration of the molten steel was estimated from the Al-O equilibrium, and the amount of Al to be added the second time was determined. In charge group B (invention example 1), the amount of Al to be added the second time was determined using the above formulas (1) and (2). However, in charge group B, a constant calculated from the average of past operations was used as the evaporation rate of Al added the first time. In charge group C (invention example 2), the amount of Al to be added the second time was determined using the above formulas (1) and (2). However, in charge group C, the value calculated from the above formulas (3) to (6) was used as the evaporation rate of Al added the first time.

[0034] For each charge group, metal samples were taken after the RH vacuum degassing treatment, and the sol. Al concentration of the molten steel was measured.

[0035] Fig. 2 is a graph showing the relationship between the Sol.Al concentration after completion of the RH vacuum degassing treatment and the number of charges. In Fig. 2, the horizontal axis is the Sol.Al concentration (mass%) and the vertical axis is the number of charges (ch). As shown in Fig. 2, in charge groups B and C, which are examples of the invention, the variation in the Sol.Al concentration was smaller with respect to the target concentration of 0.04 mass% than in charge group A, which is a comparative example. The standard deviation of the Sol.Al concentration of molten steel after completion of the RH vacuum degassing treatment for each charge group is shown in Table 1 below.

[0036] [Table 1]

[0037] As shown in Table 1, the standard deviation of the Sol. Al concentration in the molten steel after completion of the RH vacuum degassing treatment in Examples 1 and 2 was smaller than the corresponding standard deviation in Comparative Example 1. From this result, it was confirmed that the Al concentration in the molten steel can be controlled with high accuracy to the target concentration of 0.01 mass % or more (0.04 mass %) by predicting the Al yield from the temperature change of the slag before and after the first Al addition and determining the amount of Al added in the second time using the predicted Al yield.

[0038] The standard deviation of the Sol. Al concentration in the molten steel after completion of the RH vacuum degassing treatment in Example 2 was smaller than the standard deviation in Example 1. From this result, it was confirmed that the Al concentration in the molten steel can be controlled with even higher accuracy to the target concentration of 0.01 mass % or more (0.04 mass %) by determining the evaporation rate of the Al added the first time in the above formula (1) from the change in the heat quantity of the molten steel before and after the first addition of Al. [Explanation of symbols]

[0039] 10 RH vacuum degasser 12 Ladle 14 Molten Steel 16 Slug 18 Vacuum chamber 20 Upper tank 22 Lower tank 24 Upward side immersion tube 26 Downward dip tube 28 Gas inlet tube for carbonization 30 Duct 32 Raw material input port 34 Top blowing lance 36 Thermal Camera

Claims

1. A method for producing molten steel, comprising the steps of: adding Al to molten steel in a vacuum degassing apparatus two or more times to produce molten steel having an Al concentration of 0.01 mass % or more, using an Al yield estimated from the oxygen concentration of the molten steel before the addition of Al, determining an amount of Al to be added for the first time so that the oxygen concentration of the molten steel after the addition of Al becomes less than a predetermined oxygen concentration, and adding the amount of Al to the molten steel; A method for producing molten steel, comprising predicting an Al yield from a change in temperature of slag before and after a first addition of Al, and determining an amount of Al to be added from a second time onward using the predicted Al yield.

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

3. The Al yield is predicted using the following formula (1): The method for producing molten steel according to claim 2 , wherein the amount of Al added for the second or subsequent times is determined using the following formula (2): [0010] [0025] In the above formulas (1) and (2), η is the Al yield (-), ΔT is the temperature change of the slag (°C), d is the thickness of the slag (mm), α is a correction coefficient (-), β is the evaporation ratio (-), and a ０ is the oxygen concentration (mass%) of the molten steel before the first Al addition, and f ０ is the activity coefficient of oxygen in the molten steel (-), and W ｓｔｅｅｌ is the weight of the molten steel (kg), and W Ａｌ_１ｓｔ is the amount of Al added in the first time (kg), and W Ａｌ_２ｎｄ is the amount of Al added after the second time (kg), [Al] Ｔａｒｇｅｔ is the target Al concentration (mass%) of the molten steel, [Al] ０ is the Al concentration (mass%) of the molten steel before the first Al addition.

4. The method for producing molten steel according to claim 2 or 3, wherein the evaporation rate is calculated using the following formulas (3) to (6): [0030] [0045] [0050] [006] In the above formulas (3) to (6), ΔQ is the change in heat quantity of the molten steel due to the first addition of Al (kcal / t-steel), and ΔQ ｅｘｏ is the change in the reaction heat of the molten steel due to the first addition of Al (kcal / t-steel), and ΔQ ｓｅｎｓ is the change in sensible heat of the molten steel due to the first addition of Al (kcal / t-steel), ０ is the oxygen concentration (mass%) of the molten steel before the first Al addition, and f ０ is the activity coefficient of oxygen in the molten steel (-), and W Ａｌ_１ｓｔ is the amount of Al added in the first time (kg), and W ｓｔｅｅｌ is the weight of the molten steel (kg), β is the evaporation rate (-), and X and Y are constants.

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