Methods for determining the end point of the decarburization process, equipment for determining the end point of the decarburization process, operating methods for the vacuum degassing process, and methods for producing melted steel.

Abstract

The invention proposes a method for determining the endpoint of a decarbonization process, comprising steps for estimating the carbon concentration in molten steel (S3, S4) and steps for determining the endpoint of the decarbonization process (S5). The step for estimating the carbon concentration in molten steel divides the decarbonization process into several stages and estimates the carbon concentration using different models for estimating the carbon concentration in molten steel for each stage. The model for estimating the carbon concentration in molten steel at the final stage expresses the decarbonization reaction rate as a differential equation with a term proportional to the product of the linear expression of the carbon concentration in a portion of molten steel placed in a depressurized environment Cv and the decarbonization reactivity coefficient ak. The decarbonization reactivity coefficient ak is calculated as a linear expression of the logarithm of the pressure P in the vacuum vessel.

Description

Decarburization treatment completion determination method, decarburization treatment completion determination device, vacuum degassing treatment operation method, and molten steel manufacturing method

[0001] The present disclosure relates to a method for determining the completion of decarburization treatment, a device for determining the completion of decarburization treatment, a method for operating vacuum degassing treatment, and a method for producing molten steel.

[0002] In the steelmaking process, the molten steel composition is adjusted by removing impurities such as carbon from the molten iron and adding useful alloying elements. Regarding carbon in particular, decarburization is promoted by placing the molten steel in a vacuum environment using vacuum degassing equipment, making it possible to produce ultra-low carbon steel with a carbon concentration of less than 10 ppm in the molten steel.

[0003] In the vacuum degassing process, the carbon concentration in the molten steel is not measured directly, but is only indirectly estimated from the concentrations of carbon monoxide and carbon dioxide in the exhaust gas. In the production of ultra-low carbon steel, operators tend to perform the decarburization process for an excessively long time due to concerns about the carbon concentration not being within specifications.

[0004] In order to solve the problem of excessive decarburization treatment resulting in a long treatment time, it is effective to estimate the carbon concentration in the molten steel with high accuracy during treatment. Several decarburization reaction models have been proposed that physically consider the details of the decarburization reaction in vacuum degassing treatment (for example, Non-Patent Documents 1 and 2).

[0005] On the other hand, decarburization reaction models based on physical considerations have the problem that they contain model parameters whose true values ​​are difficult to determine. To address this problem, several methods have been proposed that attempt to resolve it by determining model parameters using exhaust gas measurement values ​​(e.g., Patent Documents 1 and 2).

[0006] JP 2005-330512 A JP 2021-152191 A

[0007] Shinya Kitamura and three others, "Decarburization Reaction Model in Vacuum Degassing Furnace," Tetsu-to-Hagané, Vol. 80 (1994) No. 3, pp. 213-218; Yoshihiko Higuchi and two others, "Effects of [C], [O], and Vacuum Level on RH Vacuum Decarburization," Tetsu-to-Hagané, Vol. 84 (1998) No. 10, pp. 709-714

[0008] As in the techniques of Patent Documents 1 and 2, the measured values ​​of the flow rate and concentration of the exhaust gas emitted from the vacuum degassing equipment during processing contain information about the progress of decarburization. Therefore, it is believed that using the measured values ​​of the exhaust gas enables highly accurate estimation of the carbon concentration. Furthermore, if highly accurate carbon concentration estimation is possible, it is possible to target a carbon concentration near the upper limit of the target range, thereby shortening the time required for decarburization processing of ultra-low carbon steel, etc. For example, Tables 3 and 4 of Patent Document 1 show an example in which the processing time for ultra-low carbon steel was shortened by several minutes by improving the standard deviation of the carbon concentration by several ppm.

[0009] However, the use of exhaust gas measurement values ​​poses two challenges. First, there is generally a time lag between the generation and measurement of exhaust gas, which means that even if the carbon concentration in molten steel is estimated with high accuracy, it takes a long time to process. Second, instantaneous values ​​in time-series exhaust gas measurement values ​​have large variations. If instantaneous values ​​are used to determine model parameters, the variations are also reflected in the model parameters, resulting in a decrease in the accuracy of the carbon concentration estimation. In particular, the allowable error in the estimated carbon concentration in molten steel at the end of the decarburization process (the final stage of the decarburization process) is approximately several ppm. Therefore, the variations in the time-series exhaust gas measurement values ​​can lead to unacceptable errors. Therefore, a method different from that for determining model parameters of a decarburization reaction model using time-series exhaust gas measurement values ​​is needed.

[0010] In view of the above circumstances, an object of the present disclosure is to provide a method and an apparatus for determining the completion of decarburization treatment, which estimate the carbon concentration in molten steel at the final stage with high accuracy and without time delay, and terminate the decarburization treatment at an appropriate timing. Another object of the present disclosure is to provide a method for operating a vacuum degassing treatment and a method for producing molten steel, which estimate the carbon concentration in molten steel at the final stage with high accuracy and without time delay, and terminate the decarburization treatment at an appropriate timing.

[0011] (1) A method for determining the completion of decarburization treatment according to one embodiment of the present disclosure is a method for determining the completion of decarburization treatment in a vacuum degassing treatment in which molten steel is decarburized by placing the molten steel in a reduced pressure environment, the method including: a step of estimating a carbon concentration in molten steel for estimating a carbon concentration of the molten steel; and a step of determining the completion of the decarburization treatment when the estimated carbon concentration is equal to or less than a target value, wherein the step of estimating the carbon concentration in molten steel divides the decarburization treatment into a plurality of stages, and estimates the carbon concentration using a different estimation model for the carbon concentration in molten steel in each stage, and the estimation model for the carbon concentration in molten steel in the final stage expresses a decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression for the carbon concentration Cv in molten steel of a portion of the molten steel placed in a reduced pressure environment and a decarburization reaction capacity coefficient a, and the decarburization reaction capacity coefficient a is determined as a linear expression for the logarithm of the pressure P in a vacuum vessel.

[0012] (2) As one embodiment of the present disclosure, in (1), the pressure P in the vacuum chamber is used as a criterion for dividing the final stage of the decarburization treatment from a stage before the final stage.

[0013] (3) A decarburization treatment completion determination device according to an embodiment of the present disclosure is a decarburization treatment completion determination device that determines the completion of decarburization treatment in a vacuum degassing treatment in which molten steel is decarburized by placing the molten steel in a reduced pressure environment, and includes: a molten steel carbon concentration estimation unit that estimates a carbon concentration of the molten steel; and a decarburization treatment completion determination unit that determines the completion of the decarburization treatment when the estimated carbon concentration is equal to or less than a target value, wherein the molten steel carbon concentration estimation unit divides the decarburization treatment into a plurality of stages and estimates the carbon concentration using a different molten steel carbon concentration estimation model for each stage, and the molten steel carbon concentration estimation model for the final stage expresses a decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression of the molten steel carbon concentration Cv of a portion of the molten steel that is placed in a reduced pressure environment and a decarburization reaction capacity coefficient ak, and the decarburization reaction capacity coefficient ak is determined as a linear expression of the logarithm of the pressure P in a vacuum tank.

[0014] (4) As one embodiment of the present disclosure, in (3), the pressure P in the vacuum chamber is used as a criterion for dividing the final stage of the decarburization treatment from a stage before the final stage.

[0015] (5) A method of operating a vacuum degassing treatment according to an embodiment of the present disclosure includes: subjecting the molten steel to a vacuum degassing treatment using the method of determining completion of decarburization treatment according to (1) or (2), thereby producing refined molten steel.

[0016] (6) A method for producing molten steel according to an embodiment of the present disclosure produces refined molten steel by subjecting the molten steel to a vacuum degassing treatment using the method for determining completion of decarburization treatment according to (1) or (2).

[0017] According to the present disclosure, it is possible to provide a method and an apparatus for determining the completion of decarburization treatment, which estimate the carbon concentration in molten steel at the final stage with high accuracy and without time delay, and terminate the decarburization treatment at an appropriate timing. Also, according to the present disclosure, it is possible to provide an operating method of vacuum degassing treatment, and a method for producing molten steel, which estimate the carbon concentration in molten steel at the final stage with high accuracy and without time delay, and terminate the decarburization treatment at an appropriate timing.

[0018] Fig. 1 is a block diagram showing the configuration of a decarburization treatment completion determination device according to an embodiment of the present disclosure. Fig. 2 is a flowchart showing the process flow of a decarburization treatment completion determination method according to an embodiment of the present disclosure. Fig. 3 is a graph showing the relationship between the pressure in a vacuum chamber and the decarburization rate in a vacuum degassing treatment of one charge.

[0019] Hereinafter, with reference to the drawings, a method for determining the completion of decarburization treatment, a device for determining the completion of decarburization treatment 20 (see FIG. 1 ), a method for operating a vacuum degassing treatment, and a method for producing molten steel according to one embodiment of the present disclosure will be described. In this embodiment, the vacuum degassing equipment 100 (see FIG. 1 ) will be described as an RH vacuum degassing equipment, but the present invention is not limited to an RH vacuum degassing equipment. For example, the method described below can also be implemented in an equipment (apparatus) having only a vacuum vessel 101 (see FIG. 1 ) and a single submerged pipe 103 (see FIG. 1 ) that is immersed in a ladle 102 (see FIG. 1 ) and draws molten steel into the vacuum vessel 101. Furthermore, the method described below can also be implemented in an equipment (apparatus) that does not have a vacuum vessel 101 and creates a vacuum on the surface of the molten steel in the ladle 102.

[0020] (Configuration) FIG. 1 is a schematic diagram showing the configuration of a decarburization treatment completion determination device 20 and a vacuum degassing facility 100 according to this embodiment. In this embodiment, the vacuum degassing facility 100 performs vacuum degassing by placing molten steel in a reduced-pressure environment. The decarburization treatment completion determination device 20 is a device that estimates the internal state of the vacuum degassing facility 100 and determines the timing of the end of the decarburization treatment while the vacuum degassing facility 100 is performing the vacuum degassing treatment. In other words, the decarburization treatment completion determination device 20 determines the end of the decarburization treatment in a vacuum degassing treatment, which at least performs decarburization by placing molten steel in a reduced-pressure environment. In the vacuum degassing treatment, impurities are removed from the molten iron, and the process of removing carbon from the impurities is called decarburization treatment. In this embodiment, the decarburization treatment completion determination device 20 executes a decarburization treatment completion determination method (described later) to determine the timing of the end of the decarburization treatment, thereby operating the vacuum degassing facility 100. In this embodiment, the vacuum degassing facility 100 constitutes part of a molten steel manufacturing facility. A molten steel manufacturing method is performed in the molten steel manufacturing facility. The method for producing molten steel includes refining molten steel in a vacuum degassing facility 100 to produce refined molten steel.

[0021] The RH vacuum degassing equipment 100 includes a vacuum chamber 101 and a ladle 102, which are connected by two immersion pipes 103. The vacuum chamber 101 is connected to an exhaust duct 104, through which gas inside the vacuum chamber 101 is evacuated to reduce the pressure inside the vacuum chamber 101 and to draw up the molten steel in the ladle 102. An inert gas is then blown into one end of the immersion pipe 103 through a pipe 105, causing the molten steel to circulate between the vacuum chamber 101 and the ladle 102. Oxygen can be blown into the molten steel from an injection lance 106 installed in the vacuum chamber 101. The vacuum chamber 101 is an example of a vacuum zone of the vacuum degassing equipment 100, i.e., a zone that is depressurized to create a vacuum.

[0022] A vacuum gauge 107 is installed inside the exhaust duct 104. The vacuum gauge 107 measures the pressure inside the vacuum chamber 101. Generally, the vacuum degassing equipment 100 is designed to maximize the conductance between the exhaust device (not shown) and the surface of the molten steel so that a lower pressure can be reached more quickly. Therefore, although the vacuum gauge 107 is not installed at the same location as the surface of the molten steel, the measurement value of the vacuum gauge 107 may be considered to be equal to the pressure at the surface of the molten steel at the time of measurement.

[0023] A vacuum degassing treatment control system in which the decarburization treatment completion determination device 20 is used comprises a control device 10 and the decarburization treatment completion determination device 20 as its main components. The control device 10 controls the overall operation of the vacuum degassing equipment 100. The control device 10 is configured with an information processing device such as a computer. The control device 10 controls operational manipulated variables, including the exhaust volume of the exhaust system, the reflux inert gas flow rate, and the blown oxygen flow rate, so that the component concentrations and temperature of the molten steel fall within target ranges after the vacuum degassing treatment from actual values ​​before the vacuum degassing treatment. The control device 10 also collects operational performance data, including the pressure in the vacuum chamber 101, the reflux inert gas flow rate, and the blown oxygen flow rate, and outputs the data to the decarburization treatment completion determination device 20.

[0024] As shown in FIG. 1 , the decarburization treatment completion determination device 20 includes an operation information input unit 21 , a molten steel carbon concentration estimation unit 22 , and a decarburization treatment completion determination unit 23 .

[0025] The operation information input unit 21 receives input information such as molten steel information before the start of decarburization treatment and operation results during decarburization treatment.

[0026] The molten steel carbon concentration estimation unit 22 estimates the carbon concentration in molten steel (i.e., the carbon concentration of molten steel) based on the input information acquired by the operation information input unit 21. The carbon concentration in molten steel is estimated by calculation using a model. Hereinafter, the model used to estimate the carbon concentration in molten steel will be referred to as the molten steel carbon concentration estimation model. The molten steel carbon concentration estimation unit 22 divides the decarburization treatment into a plurality of stages according to the progress, and estimates the carbon concentration using (selectively uses) a different molten steel carbon concentration estimation model at each stage.

[0027] The decarburization treatment completion determination unit 23 determines the completion of the decarburization treatment based on the carbon concentration in molten steel estimated (calculated) by the molten steel carbon concentration estimation unit 22. In the following, the estimated value of the carbon concentration in molten steel may be referred to as the "estimated value of the carbon concentration in molten steel" particularly to distinguish it from a measured value. The decarburization treatment completion determination unit 23 also outputs the determination result to the control device 10. The control device 10 may control manipulated variables related to the operation based on the determination result obtained from the decarburization treatment completion determination unit 23.

[0028] The decarburization treatment completion determination device 20 is configured by an information processing device such as a computer. The decarburization treatment completion determination device 20 may function as an operation information input unit 21, a molten steel carbon concentration estimation unit 22, and a decarburization treatment completion determination unit 23 by an arithmetic processing unit in the information processing device executing a computer program. The arithmetic processing unit in the information processing device is, for example, a CPU (Central Processing Unit).

[0029] The decarburization treatment completion determination device 20 can estimate the carbon concentration in molten steel at the final stage of the decarburization treatment with high accuracy and without time delay by executing the decarburization treatment completion determination process described below. By estimating the carbon concentration in molten steel with high accuracy, it is possible to avoid performing the decarburization treatment for an excessively long time due to concerns that the carbon concentration may not meet the specifications, and as a result, it is possible to shorten the decarburization treatment time.

[0030] (Decarburization Treatment Completion Determination Process) Hereinafter, the flow of the process (decarburization treatment completion determination process) of the method for determining the completion of decarburization treatment according to one embodiment of the present disclosure will be described with reference to Figure 2. The decarburization treatment completion determination process will be described below on the assumption that the decarburization treatment is divided into two stages, but the decarburization treatment completion determination process can also be performed in a similar manner even if the decarburization treatment is divided into three or more stages. The flowchart shown in Figure 2 starts, for example, when an operator inputs a command to execute the decarburization treatment, and the process of step S1 is performed.

[0031] In the process of step S1, the operation information input unit 21 acquires molten steel information before the start of decarburization treatment. The molten steel information may include, for example, the weight of the molten steel and analysis results obtained by component analysis. This completes the process of step S1, and the process of determining the completion of decarburization treatment proceeds to the process of step S2.

[0032] In the processing of step S2, the operation information input unit 21 acquires the operation results during the decarburization treatment. The operation results include items necessary for calculations in the molten steel carbon concentration estimation unit 22. Information such as the pressure inside the vacuum chamber 101 during the decarburization treatment, the reflux inert gas flow rate, the oxygen flow rate from the injection lance 106 (injected oxygen flow rate), and information about the auxiliary materials charged during the execution of the decarburization treatment may be input to the operation information input unit 21. Specific examples of the information about the auxiliary materials include the type and amount of the auxiliary materials charged. This completes the processing of step S2, and the decarburization treatment completion determination processing proceeds to the processing of step S3.

[0033] In the process of step S3, the stage of the decarburization treatment is determined based on the operational information and the like acquired in the processes up to step S2. As the criterion for dividing the decarburization treatment into the final stage and the stages before the final stage, it is preferable to use the pressure inside the vacuum vessel 101, taking into consideration the properties of the carbon concentration estimation model in molten steel, which will be described later. Here, when the decarburization treatment is divided into three or more stages, the criterion for the stages other than the final stage may be appropriately selected so that the carbon concentration in the molten steel becomes higher. For example, the estimated value of the carbon concentration in the molten steel, the treatment time, whether or not oxygen is blown in, or a combination of these may be used as the judgment criteria. This completes the process of step S3, and the process for determining the completion of the decarburization treatment proceeds to the process of step S4.

[0034] In the process of step S4, the carbon concentration in molten steel is estimated by the molten steel carbon concentration estimation unit 22. The calculation for estimating the carbon concentration in molten steel is performed using a molten steel carbon concentration estimation model corresponding to the stage of the decarburization treatment determined in the process of step S3.

[0035] In the vacuum degassing treatment using the RH vacuum degassing equipment, if it is assumed that the molten steel in the ladle 102 and the vacuum vessel 101 is completely mixed, the estimation model for the carbon concentration in the molten steel can be described by the differential equations of the following formulas (1) and (2).

[0036]

[0037] where w is the mass of molten steel [kg], C is the carbon concentration in the molten steel [ppm], Q is the molten steel reflux rate [kg / s], and ρ is the molten steel density [kg / m ３ ]. ak is the decarburization reaction capacity coefficient [m ３ / s]. C Ｅ is the equilibrium value [ppm] of the carbon concentration in the molten steel in the vacuum vessel 101. The subscript L indicates a physical quantity of the molten steel in the ladle 102. The subscript V indicates a physical quantity of the molten steel in the vacuum vessel 101. For example, C Ｖ indicates the carbon concentration [ppm] of the molten steel in the vacuum vessel 101. The mass w of the molten steel in the vacuum vessel 101 Ｌ is calculated from the balance of gravity acting on the molten steel in the vacuum vessel 101 and the pressure difference between the atmospheric pressure and the pressure inside the vacuum vessel 101. Ｖ is calculated as the total mass of molten steel minus the mass of molten steel in the vacuum vessel 101. The molten steel reflux rate Q can be calculated from a known formula based on operational results (see, for example, formula (5) in Non-Patent Document 2).

[0038] The first terms of the formulas (1) and (2) correspond to the molten steel reflux in the vacuum vessel 101 and the ladle 102. The amount of carbon removed from the molten steel per unit time is equal to the second term of the formula (2). Here, the flow rate of the exhaust gas during the vacuum degassing treatment, the CO concentration in the exhaust gas, and the CO ２ If the actual value of the concentration has been measured, the decarburization rate based on the actual value during the decarburization treatment can be calculated using the following equations (3) to (5).

[0039]

[0040] Here, q Ｃ (t) is the decarburization rate [kg / s] at time t. Ｃ，ＯＧ (t) is the amount of carbon in the exhaust gas per unit time [kg / s] at time t. α is a correction coefficient for matching the cumulative amount of carbon in the exhaust gas throughout the entire decarburization process with the actual reduction in the carbon concentration in the molten steel. α is a dimensionless quantity. Ｃ is the molar mass of carbon [g / mol]. ｏｆｆ (t) is the volumetric flow rate of exhaust gas at time t [Nm ３ / s]. ＣＯ (t) is the CO concentration [vol%] in the exhaust gas at time t. ＣＯ２ (t) is the CO in the exhaust gas at time t ２ Concentration [vol%]. ｏｆｆ is the delay time [s] of the exhaust gas flow rate measurement. ＣＯ is the delay time [s] of measuring the CO concentration in the exhaust gas. ＣＯ２ is the CO in the exhaust gas ２ Delay time [s] for concentration measurement. Ｃ is the actual decarburization amount [kg] calculated from the measured carbon concentration in the molten steel before and after the vacuum degassing treatment. ０ is the end time [s] of the decarburization process when the start time of the decarburization process is set to 0.

[0041] The error in the exhaust gas measurement value can be treated as constant during one vacuum degassing process, so α can be treated as a constant that does not depend on time.

[0042] As shown in FIG. 3, the inventors have found that the decarburization rate q ＣIt was found that there is a positive linear relationship between the logarithm of the pressure P and qC. As discussed below, it is believed that in the region where the pressure P in the vacuum chamber 101 is low, the higher the decarburization rate qC, the higher the pressure P. The pressure P in the vacuum chamber 101 is determined by the balance between the exhaust rate from the vacuum chamber 101 and the supply rate of gas to the vacuum chamber 101. The exhaust rate can be treated as a roughly constant value that corresponds to the exhaust capacity of the vacuum degassing equipment 100. On the other hand, the supply rate of gases such as CO gas to the vacuum chamber 101 increases as the carbon concentration in the molten steel increases. Therefore, the higher the decarburization rate, the higher the pressure P in the vacuum chamber 101. Furthermore, considering this relationship, it can be said that the pressure P in the vacuum chamber 101 is a physical quantity that reflects the decarburization performance.

[0043] Based on the above considerations, when the pressure P inside the vacuum chamber 101 is low, i.e., in the final stage of the decarburization treatment, the decarburization reaction capacity coefficient a can be calculated by the following formula (6): In other words, the decarburization reaction capacity coefficient a can be calculated as a linear expression of the logarithm of the pressure P inside the vacuum chamber.

[0044]

[0045] Here, β0 and β1 are constants that can be determined by fitting from past operational results, for example. In other words, the decarburization reaction capacity coefficient ak can be calculated based on the decarburization results, regardless of the exhaust gas measurement values.

[0046] As shown in FIG. 3, in a region where the pressure P in the vacuum chamber 101 is relatively high, the decarburization rate q Ｃ There is no linear relationship between the logarithm of pressure P and the pressure V. This can be explained as follows.

[0047] According to Non-Patent Documents 1 and 2, the decarburization reaction in vacuum degassing can be roughly divided into three types: reflux inert gas bubbles, inside the molten steel, and on the surface of the molten steel. Of these, the decarburization rate in reflux inert gas bubbles is low. Also, decarburization inside the molten steel requires bubble generation pressure to generate CO gas bubbles. Therefore, it is considered that decarburization on the surface of the molten steel is dominant at the end of the decarburization process (the final stage of the decarburization process). Therefore, the decarburization rate q ＣIt can be interpreted that the linear relationship between the logarithm of the pressure P and the carbon concentration in the molten steel is valid for the decarburization rate at the surface of the molten steel. In a region where the carbon concentration in the molten steel is relatively high and the pressure P in the vacuum vessel 101 is also relatively high, the contribution of decarburization inside the molten steel increases, and the decarburization rate q Ｃ It is considered that the linear relationship between the logarithm of the pressure P and the logarithm of the pressure P does not hold. With reference to Non-Patent Documents 1 and 2, it can be considered that the linear relationship does not hold at least in the region where the carbon concentration in the molten steel exceeds 100 ppm.

[0048] Although Equation (6) cannot be used in the region where the pressure P in the vacuum vessel 101 is relatively high, it is possible to estimate the carbon concentration in the molten steel using the known techniques described in Non-Patent Document 1 or Non-Patent Document 2. Furthermore, in the region where the pressure P in the vacuum vessel 101 is relatively high, exhaust gas measurement values ​​may be used to improve the accuracy of estimating the carbon concentration in the molten steel. Here, in the region where the pressure P in the vacuum vessel 101 is low, including at least the final stage of the decarburization treatment, the model for estimating the carbon concentration in the molten steel can be described by the differential equations of Equation (1) and Equation (2) above. That is, the model for estimating the carbon concentration in the molten steel at least in the final stage expresses the decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression for the carbon concentration Cv in the molten steel in the portion of the molten steel that is placed in a reduced-pressure environment and the decarburization reaction capacity coefficient ak. Furthermore, Equation (6) can be used for the decarburization reaction capacity coefficient ak. In decarburization, a delay in measurement time does not affect the decarburization time unless it is in the final stage, but in the final stage, a measurement delay causes an estimation error larger than the allowable estimation error (about several ppm), thereby directly affecting the decarburization time. Furthermore, the instantaneous values ​​of the exhaust gas measurement values ​​vary. The method for determining the completion of decarburization according to this embodiment uses the molten steel carbon concentration estimation model expressed as the above-mentioned differential equation in the final stage to estimate the carbon concentration without using the exhaust gas measurement values, thereby making it possible to calculate the carbon concentration in molten steel without being affected by a measurement time delay.

[0049] When the carbon concentration in the molten steel is calculated by the above method, the process of step S4 is completed, and the process of determining the completion of the decarburization process proceeds to the process of step S5. Here, steps S3 and S4 correspond to the step of estimating the carbon concentration in the molten steel.

[0050] In the processing of step S5, the decarburization treatment completion determination unit 23 determines whether the carbon concentration in molten steel estimated in step S4 has become equal to or less than a predetermined target value. If the estimated value of the carbon concentration in molten steel is higher than the target value (No in step S5), the decarburization treatment completion determination processing returns to the processing of step S2, and the processing from step S3 onwards is executed again using the newly input operational results. If the estimated value of the carbon concentration in molten steel is equal to or less than the target value (Yes in step S5), the decarburization treatment completion determination unit 23 determines that the decarburization treatment has ended, and the decarburization treatment ends. Here, step S5 corresponds to the decarburization treatment completion determination step.

[0051] The method of this embodiment can be implemented in an RH vacuum degassing facility. ２ The carbon concentration is generally measured using an infrared gas analyzer. An infrared gas analyzer is a device that measures the concentration of a gas based on the amount of infrared radiation absorbed by the target gas based on the absorption of infrared radiation of a specific wavelength. The delay in the exhaust gas measurement value, which is the sum of the delay due to the response of the infrared gas analyzer and the delay due to propagation through the piping, is, for example, 30 seconds to 1 minute. As described with reference to Patent Document 1, if highly accurate carbon concentration estimation is possible, it is possible to target a carbon concentration close to the upper limit of the target range, thereby shortening the time required for decarburization treatment of ultra-low carbon steel, etc. The method of this embodiment provides high accuracy in carbon concentration estimation, and as a result, the processing time for ultra-low carbon steel can be shortened by several minutes. Furthermore, refined molten steel may be produced by vacuum degassing molten steel using the decarburization treatment completion determination method according to this embodiment as a vacuum degassing treatment operating method or a molten steel manufacturing method.

[0052] As described above, the decarburization treatment completion determination method, decarburization treatment completion determination device 20, vacuum degassing treatment operation method, and molten steel manufacturing method can estimate the carbon concentration in molten steel that reflects the decarburization performance, at least for the final stage of decarburization, without relying on exhaust gas measurement values. Therefore, it is possible to estimate the carbon concentration in molten steel in the final stage with high accuracy and without time delay. Furthermore, it is possible to determine the completion of decarburization treatment at an appropriate time based on a highly accurate estimation, and it is possible to avoid performing decarburization treatment for an excessively long time due to concerns about the carbon concentration not exceeding the specification, thereby shortening the decarburization treatment time.

[0053] EXAMPLES The effects of the present disclosure will be specifically described below based on examples, but the present disclosure is not limited to the contents of the examples.

[0054] In this example, decarburization was carried out using RH vacuum degassing equipment to produce ultra-low carbon molten steel with a carbon concentration of 25 ppm as the upper limit of the standard. Before the start of the vacuum degassing treatment and after the end of the vacuum degassing treatment, portions of the molten steel were taken as samples, and the carbon concentrations in the molten steel were measured. In addition, the exhaust gas flow rate, CO concentration in the exhaust gas, and CO ２ The change in concentration over time was measured. The conventional method is a method in which the completion of decarburization treatment is determined based on the operator's experience. Here, the number of analytical data for the inventive method and the comparative method is 20 each.

[0055] Table 1 compares the results of estimating the carbon concentration in molten steel between the inventive method and the conventional method. The inventive method is the technique of the above-mentioned embodiment. According to the example of FIG. 3, in the inventive method, the decarburization process was divided into two stages at the time when the pressure P in the vacuum chamber 101 fell below 4 torr for the first time during the decarburization process. Furthermore, in the first stage (a stage before the final stage) of the conventional method and the inventive method, the carbon concentration in molten steel was estimated using the decarburization reaction model described in Non-Patent Document 1.

[0056]

[0057] The first row of Table 1 shows the standard deviation of the estimation error of the carbon concentration in molten steel at the end of the vacuum degassing process. It can be seen that the inventive method can estimate the carbon concentration in molten steel more accurately than the comparative method. The present disclosure aims to prevent excessive decarburization during vacuum degassing and shorten the processing time by estimating the carbon concentration in molten steel with high accuracy. The second and third rows of Table 1 show the effect of preventing excessive decarburization and the shortened decarburization time when the carbon concentration in molten steel is estimated using the model of the inventive method instead of the comparative method in this embodiment. When determining the end of decarburization based on the estimated carbon concentration in molten steel, the target value of the carbon concentration in molten steel referenced in step S5 needs to be set so as not to exceed the upper limit of the carbon concentration specification, taking into account the error of the carbon concentration estimation model. Specifically, the target value can be set by subtracting three times the standard error standard deviation of the carbon concentration estimation model from the upper limit of the carbon concentration specification. When setting the target value in this manner, the target value for determining the end of decarburization can be set to the amount of suppression of excessive decarburization by using the model of the example of the invention in comparison with the comparative example, as shown in the second row of Table 1. In this example, the effect of shortening the decarburization time by increasing the target value is as shown in the third row of Table 1.

[0058] Although embodiments of the present disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art would easily be able to make various modifications or alterations based on the present disclosure. Therefore, it should be noted that these modifications and alterations are included within the scope of the present disclosure. For example, functions included in each component or step can be rearranged so as not to cause logical inconsistencies, and multiple components or steps can be combined or divided into one. The embodiments of the present disclosure can also be realized as a program executed by a processor included in an apparatus and a storage medium on which the program is recorded. It should be understood that these are also included within the scope of the present disclosure.

[0059] REFERENCE SIGNS LIST 10 Control device 20 Decarburization treatment completion determination device 21 Operation information input unit 22 Molten steel carbon concentration estimation unit 23 Decarburization treatment completion determination unit 100 Vacuum degassing equipment 101 Vacuum tank 102 Ladle 103 Immersion pipe 104 Exhaust duct 105 Piping 106 Blowing lance 107 Vacuum gauge

Claims

1. A method for determining the completion of a decarburization process in a vacuum degassing process in which molten steel is placed in a reduced pressure environment to decarburize it, comprising: a step of estimating a carbon concentration in molten steel for estimating a carbon concentration of the molten steel; and a step of determining the completion of the decarburization process when the estimated carbon concentration is equal to or lower than a target value, wherein the step of estimating the carbon concentration in molten steel divides the decarburization process into a plurality of stages, and estimates the carbon concentration using a different estimation model for the carbon concentration in molten steel in each stage, and the estimation model for the carbon concentration in molten steel in the final stage expresses the decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression for the carbon concentration Cv in the molten steel of a portion of the molten steel placed in a reduced pressure environment and a decarburization reaction capacity coefficient ak, and the decarburization reaction capacity coefficient ak is calculated as a linear expression for the logarithm of the pressure P in a vacuum tank.

2. A method for determining the completion of decarburization treatment according to claim 1, wherein the pressure P within the vacuum chamber is used as a criterion for dividing the decarburization treatment into a final stage and a stage prior to the final stage.

3. A decarburization treatment completion determination device that determines the completion of decarburization treatment in a vacuum degassing process in which molten steel is placed in a reduced pressure environment to perform decarburization, comprising: a molten steel carbon concentration estimation unit that estimates a carbon concentration of the molten steel; and a decarburization treatment completion determination unit that determines the completion of the decarburization treatment when the estimated carbon concentration is equal to or lower than a target value, wherein the molten steel carbon concentration estimation unit divides the decarburization treatment into a plurality of stages and estimates the carbon concentration using a different molten steel carbon concentration estimation model for each stage, and the molten steel carbon concentration estimation model for the final stage expresses the decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression for the molten steel carbon concentration Cv of a portion of the molten steel placed in a reduced pressure environment and a decarburization reaction capacity coefficient ak, and the decarburization reaction capacity coefficient ak is calculated as a linear expression for the logarithm of the pressure P in a vacuum tank.

4. A decarburization treatment completion determination device as set forth in claim 3, wherein the pressure P within the vacuum chamber is used as a criterion for dividing the decarburization treatment into a final stage and a stage prior to the final stage.

5. A method of operating a vacuum degassing process, comprising the steps of: subjecting molten steel to a vacuum degassing process using the method for determining the completion of decarburization process according to claim 1 or 2, thereby producing refined molten steel.

6. A method for producing molten steel, comprising the steps of: subjecting the molten steel to a vacuum degassing process using the method for determining the completion of decarburization process according to claim 1 or 2, thereby producing refined molten steel.

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