Decarburization treatment end determination method, decarburization treatment end determination device, vacuum degassing treatment operation method, and molten steel production method

Abstract

A decarburization treatment end determination method includes a molten steel carbon concentration estimation step (S3, S4), and a decarburization treatment end determination step (S5). The molten steel carbon concentration estimation step 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. The molten steel carbon concentration estimation model of a final stage expresses a decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression of a carbon concentration in a portion of the molten steel placed in the reduced pressure environment Cv and a decarburization reaction capacity coefficient ak. The decarburization reaction capacity coefficient ak is calculated as a linear expression of the logarithm of a pressure P in a vacuum vessel.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a decarburization treatment end determination method, a decarburization treatment end determination device, a vacuum degassing treatment operation method, and a method of producing molten steel.BACKGROUND

[0002] In the steelmaking process, impurities such as carbon are removed from molten pig iron, and useful alloy components are added to adjust molten steel composition. With regard to carbon in particular, decarburization can be promoted by placing the molten steel in a vacuum environment using a vacuum degassing facility, making it possible to produce ultra-low carbon steel having a carbon concentration in the molten steel below 10 ppm.

[0003] Here, in the vacuum degassing treatment, the carbon concentration in the molten steel is not measured directly, but is only indirectly estimated from concentrations of carbon monoxide and carbon dioxide in exhaust gas. In the production of ultra-low carbon steel, operators tend to run the decarburization treatment too long due to concern about carbon concentration being out of specification.

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

[0005] On the other hand, decarburization reaction models based on physical considerations have the problem of containing model parameters whose true values are difficult to obtain. To address this problem, methods have been proposed that attempt to solve the problem by determining model parameters using exhaust gas measurement values (for example, see Patent Literature (PTL) 1 and 2).CITATION LISTPatent Literature

[0006] PTL 1: JP 2005-330512 A PTL 2: JP 2021-152191 A Non-Patent Literature

[0007] NPL 1: Shinya Kitamura and three others, "Decarburization Model for Vacuum Degasser", Tetsu-to-Hagane, Vol. 80 (1994) No. 3, pp. 213-218 NPL 2: Yoshihiko Higuchi and two others, "Effects of [C], [O] and Pressure on RH Vacuum Decarburization", Tetsu-to-Hagane, Vol. 84 (1998) No. 10, pp. 709-714 SUMMARY(Technical Problem)

[0008] As in the techniques of PTL 1 and PTL 2, measurement values of flow rate and concentration of exhaust gas discharged from a vacuum degassing facility during treatment contain information regarding the progress of decarburization. Accordingly, it may be considered that using exhaust gas measurement values enables high precision estimation of carbon concentration. Further, when high precision carbon concentration estimation is possible, targeting a carbon concentration close to an upper limit of a target range becomes possible, thereby shortening a time required for decarburization treatment of ultra-low carbon steel or the like. For example, Tables 3 and 4 of PTL 1 indicate an example in which the standard deviation of the carbon concentration was improved by some ppm, and the treatment time for ultra-low carbon steel was thereby shortened by some minutes.

[0009] However, the use of exhaust gas measurement values presents two problems. First, there is generally a time delay between the generation of exhaust gas and measurement, and even when the carbon concentration in the molten steel is estimated with high precision, the process takes time. Second, the instantaneous values in a time-series of exhaust gas measurement values exhibit large fluctuations. When such instantaneous values are used to determine model parameters, the fluctuations are reflected in the model parameters, leading to a decrease in carbon concentration estimation precision. In particular, in an end period of decarburization treatment (a final stage of the decarburization treatment) an allowable error in the estimated carbon concentration in the molten steel is some ppm. Therefore, unacceptable errors may occur due to the influence of fluctuations in the time-series of exhaust gas measurement values. Accordingly, a method different from conventional approaches for determining model parameters of a decarburization reaction model that uses a time-series of exhaust gas measurement values is required.

[0010] The present disclosure is made in consideration of the above circumstances, and aims to provide a decarburization treatment end determination method and a decarburization treatment end determination device that can be used to estimate the carbon concentration in molten steel in a final stage with high precision and without time delay, and end the decarburization treatment at an appropriate timing. Further, the present disclosure aims to provide a vacuum degassing treatment operation method and a method of producing molten steel that can be used to estimate the carbon concentration in molten steel in a final stage with high precision and without time delay, and end the decarburization treatment at an appropriate timing.(Solution to Problem)

[0011] (1) A decarburization treatment end determination method according to an embodiment of the present disclosure is a decarburization treatment end determination method that determines the end of decarburization treatment in a vacuum degassing treatment in which molten steel is decarburized by being placed in a reduced pressure environment, comprising: a molten steel carbon concentration estimation step of estimating a carbon concentration in the molten steel; and a decarburization treatment end determination step of determining the end of the decarburization treatment when the estimated carbon concentration is a target value or less, wherein the molten steel carbon concentration estimation step estimates the carbon concentration by dividing the decarburization treatment into a plurality of stages and using a different molten steel carbon concentration estimation model for each stage, the molten steel carbon concentration estimation model of a final stage expresses a decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression of a carbon concentration in a portion of the molten steel placed in the reduced pressure environment Cv and a decarburization reaction capacity coefficient ak, and the decarburization reaction capacity coefficient ak is calculated as a linear expression of the logarithm of a pressure P in a vacuum vessel. (2) As an embodiment of the present disclosure, (1), wherein the pressure P in the vacuum vessel is used as a criterion for dividing the decarburization treatment into the final stage and the stage before the final stage. (3) A decarburization treatment end determination device according to an embodiment of the present disclosure is a decarburization treatment end determination device configured to determine the end of decarburization treatment in a vacuum degassing treatment in which molten steel is decarburized by being placed in a reduced pressure environment, comprising: a molten steel carbon concentration estimation unit configured to estimate a carbon concentration in the molten steel; and a decarburization treatment end determination unit configured to determine the end of the decarburization treatment when the estimated carbon concentration is a target value or less, wherein the molten steel carbon concentration estimation unit estimates the carbon concentration by dividing the decarburization treatment into a plurality of stages and using a different molten steel carbon concentration estimation model for each stage, the molten steel carbon concentration estimation model of a final stage expresses a decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression of a carbon concentration in a portion of the molten steel placed in the reduced pressure environment Cv and a decarburization reaction capacity coefficient ak, and the decarburization reaction capacity coefficient ak is calculated as a linear expression of the logarithm of a pressure P in a vacuum vessel. (4) As an embodiment of the present disclosure, (3), wherein the pressure P in the vacuum vessel is used as a criterion for dividing the decarburization treatment into the final stage and the stage before the final stage. (5) A vacuum degassing treatment operation method according to an embodiment of the present disclosure uses the decarburization treatment end determination method of (1) or (2) in subjecting the molten steel to a vacuum degassing treatment, thereby producing refined molten steel. (6) A method of producing molten steel according to an embodiment of the present disclosure uses the decarburization treatment end determination method of (1) or (2) in subjecting the molten steel to a vacuum degassing treatment, thereby producing refined molten steel. (Advantageous Effect)

[0012] According to the present disclosure, it is possible to provide a decarburization treatment end determination method and a decarburization treatment end determination device that estimate the carbon concentration in molten steel in a final stage with high precision and without time delay, and end the decarburization treatment at an appropriate timing. Further, according to the present disclosure, it is possible to provide a vacuum degassing treatment operation method and a method of producing molten steel that estimate the carbon concentration in the molten steel in a final stage with high precision and without time delay, and end the decarburization treatment at an appropriate timing.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In the accompanying drawings: FIG. 1 is a block diagram illustrating a configuration of a decarburization treatment end determination device according to an embodiment of the present disclosure; FIG. 2 is a flowchart illustrating a processing flow of a decarburization treatment end determination method according to an embodiment of the present disclosure; and FIG. 3 is a graph illustrating a relationship between a pressure in a vacuum vessel and a decarburization rate in one charge of vacuum degassing treatment. DETAILED DESCRIPTION

[0014] Hereinafter, a decarburization treatment end determination method, a decarburization treatment end determination device 20 (see FIG. 1), a vacuum degassing treatment operation method, and a method of producing molten steel according to an embodiment of the present disclosure are described with reference to the drawings. According to the present embodiment, a vacuum degassing facility 100 (see FIG. 1) is described as an RH vacuum degasser, but is not limited to an RH vacuum degasser. For example, the method described below can also be implemented in a facility (apparatus) having only one snorkel 103 (see FIG. 1) that is immersed in a vacuum vessel 101 (see FIG. 1) and a ladle 102 (see FIG. 1) and that draws the molten steel into the vacuum vessel 101. Further, the method described below may be carried out in a facility (apparatus) that does not include the vacuum vessel 101 and creates a vacuum on a surface of the molten steel in the ladle 102.(Configuration)

[0015] FIG. 1 is a schematic diagram illustrating a configuration of the decarburization treatment end determination device 20 and the vacuum degassing facility 100 according to the present embodiment. According to the present embodiment, the vacuum degassing facility 100 carries out the vacuum degassing treatment by placing the molten steel in a reduced pressure environment. The decarburization treatment end determination device 20 is a device that estimates an internal state of the vacuum degassing facility 100 while the vacuum degassing treatment is being carried out in the vacuum degassing facility 100 and determines an end timing of the decarburization treatment. In other words, the decarburization treatment end determination device 20 determines the end of the decarburization treatment in the vacuum degassing treatment in which at least decarburization is carried out by placing the molten steel in a reduced pressure environment. Vacuum degassing treatment removes impurities from molten pig iron, and decarburization treatment is a process of removing carbon among the impurities. According to the present embodiment, the decarburization treatment end determination device 20 carries out the decarburization treatment end determination method described below to determine the end timing of the decarburization treatment, and the vacuum degassing facility 100 is operated accordingly. Further, according to the present embodiment, the vacuum degassing facility 100 constitutes a portion of a molten steel production facility. The method of producing molten steel is carried out in the molten steel production facility. The method of producing molten steel includes refining molten steel in the vacuum degassing facility 100 to produce refined molten steel.

[0016] The RH vacuum degasser 100 includes the vacuum vessel 101 and the ladle 102 that are connected by two snorkels 103. The vacuum vessel 101 is connected to an exhaust duct 104 through which gas inside the vacuum vessel 101 is exhausted to reduce pressure in the vacuum vessel 101 and suck up the molten steel in the ladle 102. Then, by blowing in an inert gas through a pipe or tube 105 from one of the snorkels 103, the molten steel is circulated between the vacuum vessel 101 and the ladle 102. Oxygen can be supplied to the molten steel by blowing oxygen from a blowing lance 106 installed in the vacuum vessel 101. The vacuum vessel 101 is an example of a vacuum region of the vacuum degassing facility 100, that is, a region that is depressurized to create a vacuum.

[0017] A vacuum gauge 107 is installed inside the exhaust duct 104. The vacuum gauge 107 measures pressure inside the vacuum vessel 101. Typically, the vacuum degassing facility 100 is designed to provide as much conductance as possible between an exhauster (not illustrated) and a molten steel surface so that lower pressures can be reached more quickly. Accordingly, although the vacuum gauge 107 is not installed at the same location as the molten steel surface, a measurement value of the vacuum gauge 107 may be treated as being equal to a pressure at the molten steel surface at the time of measurement.

[0018] A vacuum degassing treatment control system in which the decarburization treatment end determination device 20 is used includes a control device 10 and the decarburization treatment end determination device 20 as main components. The control device 10 controls overall operation of the vacuum degassing facility 100. The control device 10 comprises an information processing device such as a computer. The control device 10 controls operation variables related to operation, including an exhaust amount of the exhaust equipment, the flow rate of the circulating inert gas, and the blown oxygen flow rate, so that, from actual values before the vacuum degassing treatment, component concentration and temperature of the molten steel fall within a target range after the vacuum degassing treatment. Further, the control device 10 collects data of operation results including pressure inside the vacuum vessel 101, the flow rate of the circulating inert gas, and the blown oxygen flow rate, and outputs to the decarburization treatment end determination device 20.

[0019] As illustrated in FIG. 1, the decarburization treatment end determination device 20 includes an operation information input interface 21, a molten steel carbon concentration estimation unit 22, and a decarburization treatment end determination unit 23.

[0020] The operation information input interface 21 receives input information such as molten steel information before the start of decarburization treatment and operation results during decarburization treatment.

[0021] The molten steel carbon concentration estimation unit 22 estimates the carbon concentration in the molten steel (that is, the molten steel carbon concentration) based on the input information acquired by the operation information input interface 21. The carbon concentration in the molten steel is estimated by calculation using a model. Hereinafter, a model used to estimate the carbon concentration in the molten steel is referred to as a 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 degree of progress, and estimates the carbon concentration using (selectively using) different molten steel carbon concentration estimation models at each stage.

[0022] The decarburization treatment end determination unit 23 determines the end of the decarburization treatment based on the carbon concentration in the molten steel estimated (calculated) by the molten steel carbon concentration estimation unit 22. Hereinafter, the value of carbon concentration in the molten steel as estimated may be referred to as an "estimated value of carbon concentration in the molten steel" particularly to distinguish the value from a measured value. Further, the decarburization treatment end determination unit 23 outputs a determination result to the control device 10. The control device 10 may control operation variables related to the operation based on the determination result obtained from the decarburization treatment end determination unit 23.

[0023] The decarburization treatment end determination device 20 comprises an information processing device such as a computer. The decarburization treatment end determination device 20 may function as the operation information input interface 21, the molten steel carbon concentration estimation unit 22, and the decarburization treatment end determination unit 23 by an arithmetic processor in the information processing device executing a computer program. The arithmetic processor in the information processing device is, for example, a central processing unit (CPU).

[0024] The decarburization treatment end determination device 20 can estimate the carbon concentration in the molten steel in the final stage of the decarburization treatment with high precision and without time delay by carrying out the decarburization treatment end determination process described below. By estimating the carbon concentration in the molten steel with high precision, it is possible to avoid carrying out an excessively long decarburization treatment due to concern about the carbon concentration being out of specification, and as a result, it is possible to shorten the decarburization treatment time.(Decarburization treatment end determination process)

[0025] Hereinafter, with reference to FIG. 2, a processing flow of the decarburization treatment end determination method (decarburization treatment end determination process) according to an embodiment of the present disclosure is described. The following description of the decarburization treatment end determination process assumes that the decarburization treatment is divided into two stages, but the decarburization treatment end determination process may be carried out in the same way even when the decarburization treatment is divided into three or more stages. The flowchart illustrated in FIG. 2 starts, for example, when an operator inputs a command to execute the decarburization treatment, and processing of step S1 is carried out.

[0026] In the processing of step S1, the operation information input interface 21 acquires molten steel information before the start of the 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 processing of step S1, and the decarburization treatment end determination process proceeds to step S2.

[0027] In the processing of step S2, the operation information input interface 21 acquires operation results during the decarburization treatment. The operation results include values necessary for calculation in the molten steel carbon concentration estimation unit 22. Information such as pressure inside the vacuum vessel 101 during the decarburization treatment, the flow rate of the circulating inert gas, the oxygen flow rate from the blowing lance 106 (blown oxygen flow rate), and information regarding auxiliary material added during the decarburization treatment may be input to the operation information input interface 21. The information regarding auxiliary material is, for example, type and input amount of auxiliary material. This completes the processing of step S2, and the decarburization treatment end determination process proceeds to step S3.

[0028] In the processing of step S3, a stage of the decarburization treatment is determined based on the operation information and the like acquired in the processing up to step S2. As a criterion for dividing the decarburization treatment into the final stage and the stage before the final stage, taking into consideration properties of the molten steel carbon concentration estimation model, described later, it is preferable to use a pressure inside the vacuum vessel 101. Here, when the decarburization treatment is divided into three or more stages, determination criteria for stages other than the final stage may be appropriately selected so that the carbon concentration in the molten steel is higher. For example, the estimated value of carbon concentration in the molten steel, the treatment time, whether or not oxygen is blown, or a combination of these can be used as determination criteria. This completes the processing of step S3, and the decarburization treatment end determination process proceeds to step S4.

[0029] In the processing of step S4, the molten steel carbon concentration estimation unit 22 estimates the carbon concentration in the molten steel. Calculation for estimating the carbon concentration in the molten steel is carried out using a molten steel carbon concentration estimation model according to the stage of the decarburization treatment determined in the processing of step S3.

[0030] In the vacuum degassing treatment using the RH vacuum degasser, when assuming that the molten steel in the ladle 102 and the vacuum vessel 101 is completely mixed, the molten steel carbon concentration estimation model can be described by the differential equations of the following Expression (1) and Expression (2). w L dC L dt = Q C V − C L Expression 1 w V dC V dt = Q C L − C V − ρ ⋅ ak C V − C E Expression 2

[0031] Here, w is the molten steel mass in kg. C is the carbon concentration in the molten steel in ppm. Q is the molten steel circulation rate in kg / s. ρ is the molten steel density in kg / m 3< . ak is the decarburization reaction capacity coefficient in m 3< / s. C E is the equilibrium value, in ppm, of the carbon concentration in the molten steel in the vacuum vessel 101. Further, 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, in ppm, in the molten steel in the vacuum vessel 101. The molten steel mass w L in the vacuum vessel 101 is calculated from the balance between gravity acting on the molten steel in the vacuum vessel 101 and the differential pressure between the atmospheric pressure and the pressure inside the vacuum vessel 101. The molten steel mass w V in the ladle 102 is calculated as the total molten steel mass minus the molten steel mass in the vacuum vessel 101. The molten steel circulation rate Q can be calculated from a known formula based on operation results (see, for example, expression (5) in NPL 2).

[0032] The first terms in Expression (1) and Expression (2) correspond to the molten steel circulation in the vacuum vessel 101 and the ladle 102. The amount of carbon removed from molten steel per unit time is equal to the second term of Expression (2). Here, when the actual values of the exhaust gas flow rate, CO concentration in the exhaust gas, and CO 2 concentration in the exhaust gas during the vacuum degassing treatment are measured, the decarburization rate based on the actual results during the decarburization treatment can be calculated using the following Expressions (3) to (5). q C t = αq C , OG t Expression 3 q C , OG t = αm C v off t + t off 22.4 ⋅ r CO t + t CO + r CO 2 t + t CO 2 100 Expression 4 α = Q C ∫ 0 t 0 q C , OG t dt Expression 5

[0033] Here, q C (t) is the decarburization rate, in kg / s, at time t. q C,OG (t) is the amount of carbon in the exhaust gas per unit time, in kg / s, at time t. α is a correction coefficient for making the cumulative amount of carbon in the exhaust gas during the entire decarburization treatment coincide with the actual reduction in the carbon concentration in the molten steel. α is a dimensionless quantity. m C is the molar mass of carbon in g / mol. V off (t) is the volumetric flow rate of the exhaust gas, in Nm 3< / s, at time t. r CO (t) is the CO concentration in the exhaust gas, in vol%, at time t. r CO2 (t) is the CO 2 concentration in the exhaust gas, in vol%, at time t. t off is the delay time, in s, of exhaust gas flow rate measurement. t CO is the delay time, in s, in measuring the CO concentration in the exhaust gas. t CO2 is the delay time, in s, in measuring the CO 2 concentration in the exhaust gas. Q C is the actual decarburization amount, in kg, calculated from the measured values of the carbon concentration in the molten steel before and after the vacuum degassing treatment. t 0 is the decarburization treatment end time, in s, when the decarburization treatment start time is set to 0.

[0034] The error in the exhaust gas measurement values can be treated as constant during one vacuum degassing treatment. Therefore, α may be a constant that does not depend on time.

[0035] The inventors have found that, as illustrated in FIG. 3, in a range where the pressure P in the vacuum vessel 101 is low (a range of 4 Torr or less in the example of FIG. 3), there is a positive linear relationship between the decarburization rate q C and the logarithm of the pressure P. As described below, it may be considered that in a range where the pressure P in the vacuum vessel 101 is low, the higher the decarburization rate q C , the higher the pressure P. The pressure P inside the vacuum vessel 101 is determined by the balance between the exhaust rate from the vacuum vessel 101 and the gas supply rate into the vacuum vessel 101. The exhaust rate can be treated as a roughly constant value according to the exhaust capacity of the vacuum degassing facility 100. On the other hand, the supply rate of gas such as CO gas into the vacuum vessel 101 increases with higher carbon concentration in the molten steel. Therefore, the higher the decarburization rate, the higher the pressure P in the vacuum vessel 101 becomes. Further, taking such a relationship into consideration, it can be said that the pressure P inside the vacuum vessel 101 is a physical quantity that reflects actual decarburization.

[0036] Based on the above considerations, when the pressure P inside the vacuum vessel 101 is low, that is, in the final stage of the decarburization treatment, the decarburization reaction capacity coefficient ak can be calculated by the following Expression (6). That is, the decarburization reaction capacity coefficient ak can be calculated as a linear expression of the logarithm of the pressure P in the vacuum vessel. [Math. 3] ak = β 0 + β 1 log P

[0037] Here, β 0 and β 1 are constants that are determined by fitting based on past operation results, for example. That is, the decarburization reaction capacity coefficient ak can be calculated based on actual decarburization, regardless of the exhaust gas measurement values.

[0038] As illustrated in FIG. 3, in a range where the pressure P in the vacuum vessel 101 is relatively high, no linear relationship is observed between the decarburization rate q C and the logarithm of the pressure P. This can be considered as follows.

[0039] According to NPL 1 and NPL 2, decarburization reactions in vacuum degassing treatment can be broadly divided into three types: in circulating inert gas bubbles, inside the molten steel, and at the molten steel surface. Among these, the decarburization rate in circulating inert gas bubbles is low. Further, decarburization inside the molten steel requires a bubble formation pressure to generate CO gas bubbles. Therefore, it is considered that decarburization at the molten steel surface is dominant in an end period of decarburization treatment (the final stage of the decarburization treatment). Accordingly, it can be interpreted that the linear relationship between the decarburization rate q C and the logarithm of the pressure P holds true for the decarburization rate at the molten steel surface. In a range where the carbon concentration in the molten steel is relatively high and the pressure P in the vacuum vessel 101 is also relatively high, it is considered that the contribution of decarburization inside the molten steel becomes large, and the linear relationship between the decarburization rate q C and the logarithm of the pressure P no longer holds. With reference to NPL 1 and NPL 2, it can be considered that the linear relationship described above does not hold true at least in a range where the carbon concentration in the molten steel exceeds 100 ppm.

[0040] In a range where the pressure P in the vacuum vessel 101 is relatively high, Expression (6) cannot be used. However, it is possible to estimate the carbon concentration in the molten steel by using a known technique described in NPL 1 or NPL 2. Further, in the range where the pressure P in the vacuum vessel 101 is relatively high, the exhaust gas measurement values may be used to improve the precision of estimating the carbon concentration in the molten steel. Here, in the range where the pressure P in the vacuum vessel 101 is low, including at least the final stage of the decarburization treatment, the molten steel carbon concentration estimation model can be described by the differential equations of Expression (1) and Expression (2). That is, the molten steel carbon concentration estimation model for at least the final stage expresses the decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression of a carbon concentration in a portion of the molten steel placed in the reduced pressure environment Cv and the decarburization reaction capacity coefficient ak. Further, Expression (6) can be used for the decarburization reaction capacity coefficient ak. In the decarburization treatment, when the measurement time is delayed, the decarburization treatment time is not affected unless in the final stage. However, in the final stage, the measurement delay causes an estimation error that is larger than the allowable estimation error (some ppm), which has a direct impact. Further, instantaneous values of the exhaust gas measurement values fluctuate. The decarburization treatment end determination method according to the present embodiment uses the molten steel carbon concentration estimation model expressed as the differential equation described above in the final stage to estimate the carbon concentration without using exhaust gas measurement values, thereby making it possible to calculate the carbon concentration in the molten steel without being affected by a measurement time delay.

[0041] When the carbon concentration in the molten steel is calculated by the above method, the processing of step S4 is completed, and the decarburization treatment end determination process proceeds to step S5. Here, step S3 and step S4 correspond to the molten steel carbon concentration estimation step.

[0042] In the processing of step S5, the decarburization treatment end determination unit 23 determines whether the carbon concentration in the molten steel estimated in step S4 has become a predetermined target value or less. When the estimated value of carbon concentration in the molten steel is higher than the target value (No in step S5), the decarburization treatment end determination process returns to step S2, and the processing from step S3 onwards is executed again using newly input operation results. When the estimated value of the carbon concentration in the molten steel is the target value or less (Yes in step S5), the decarburization treatment end determination unit 23 determines the end of the decarburization treatment, and the decarburization treatment ends. Here, step S5 corresponds to the decarburization treatment end determination step.

[0043] The method according to the present embodiment can be implemented in an RH vacuum degasser. Here, the concentrations of CO and CO 2 in the exhaust gas are typically measured by an infrared gas analyzer. The infrared gas analyzer is a device that measures the concentration of a gas to be measured based on the amount of infrared absorption at a specific wavelength absorbed by the gas. The delay in the exhaust gas measurement values, which is the sum of delay due to response of the infrared gas analyzer and delay due to propagation through piping, is, for example, 30 seconds to one minute. As explained with reference to PTL 1, when high precision carbon concentration estimation is possible, it is possible to aim for 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, and the like. The method according to the present embodiment has high precision in carbon concentration estimation, and as a result, the processing time for ultra-low carbon steel can be reduced by some minutes. Further, as a vacuum degassing treatment operation method or a method of producing molten steel, refined molten steel may be produced by subjecting the molten steel to a vacuum degassing treatment using the decarburization treatment end determination method according to the present embodiment.

[0044] As described above, the decarburization treatment end determination method, the decarburization treatment end determination device 20, the vacuum degassing treatment operation method, and the method of producing molten steel can estimate the carbon concentration in molten steel that reflects actual decarburization, at least for the final stage of decarburization treatment, without relying on exhaust gas measurement values. Therefore, it is possible to estimate the carbon concentration in the molten steel in the final stage with high precision and without time delay. Further, it is possible to determine the end of the decarburization treatment at an appropriate time based on high precision estimation, which makes it possible to avoid carrying out the decarburization treatment for an excessively long time due to concerns about the carbon concentration being out of specification, thereby shortening the decarburization treatment time.EXAMPLES

[0045] Advantageous effects of the present disclosure are described in detail below based on examples, but the present disclosure is not limited to the content of the examples.

[0046] In the examples, decarburization treatment was carried out using an RH vacuum degasser to produce ultra-low carbon molten steel having a upper specification limit of carbon concentration of 25 ppm. Before the start of the vacuum degassing treatment and after the end of the vacuum degassing treatment, a portion of the molten steel was taken as a sample, and the carbon concentration in the molten steel of the sample was measured. Further, the exhaust gas flow rate, the CO concentration in the exhaust gas, and the CO 2 concentration changes over time in the exhaust gas during the vacuum degassing treatment were measured. According to a conventional method, the end of decarburization treatment is determined based on the experience of the operator. Here, the number of analysis data points for the example method and the comparative method was 20 each.

[0047] Table 1 compares results of estimating the carbon concentration in the molten steel between the example method (Example method) and the conventional method (Comparative method). The example method is the method according to the embodiment described above. In the example of FIG. 3, according to the example method, the decarburization treatment was divided into two stages at the time when the pressure P in the vacuum vessel 101 fell below 4 torr for the first time during the decarburization treatment. Further, in the first stage (stage before the final stage) of the conventional method and the example method, the carbon concentration in the molten steel was estimated using the decarburization reaction model described in NPL 1.[Table 1]

[0048] (Table 1)Comparative methodExample methodStandard deviation of carbon concentration estimate at end of vacuum degassing treatment2.98 ppm2.72 ppmExcess decarburization suppression amount compared to comparative method-0.78 ppmDecarburization treatment time reduction compared to comparative method-0.8 min

[0049] The first row of Table 1 indicates the standard deviation of the estimation error of the carbon concentration in the molten steel at the end of the vacuum degassing treatment. It can be seen that the example method can estimate the carbon concentration in the molten steel more precisely than the comparative method. An aim of the present disclosure is to prevent excessive decarburization treatment in vacuum degassing treatment and shorten the treatment time by estimating the carbon concentration in molten steel with high precision. The second and third lines of Table 1 indicate the effect of suppressing excessive decarburization and the shortened time for decarburization treatment when the carbon concentration in the molten steel is estimated using the model of the example method instead of the comparative method. When determining the end of the decarburization treatment based on the estimated value of the carbon concentration in the molten steel, the target value of the carbon concentration in the molten steel referenced in the processing of step S5 needs to be set so as not to exceed the upper specification limit of the carbon concentration, taking into account the error of the carbon concentration estimation model. Specifically, the target value may be set to a value obtained by subtracting three times the standard deviation of the error of the carbon concentration estimation model from the upper specification limit of the carbon concentration. When setting the target value in this way, when the model of the example is used in comparison with the comparative example, as indicated in the second row of Table 1, the target value for determining the end of decarburization treatment can be set to an excess decarburization suppression amount. In this example, the effect of shortening the decarburization treatment time by increasing the target value is as indicated in the third row of Table 1.

[0050] Although an embodiment of the present disclosure has been described based on the drawings and examples, it should be noted that a person skilled in the art may make variations and modifications based on the present disclosure. Therefore, it should be noted that such variations and modifications are included within the scope of the present disclosure. For example, functions and the like included in each component and step may be rearranged, and a plurality of components and steps may be combined into one or divided, as long as no logical inconsistency results. An embodiment according to the present disclosure may be realized as a program executed by a processor provided to a device and as a storage medium on which the program is stored. The scope of the present disclosure should be understood to include these examples.REFERENCE SIGNS LIST

[0051] 10control device 20decarburization treatment end determination device 21operation information input interface 22molten steel carbon concentration estimation unit 23decarburization treatment end determination unit 100vacuum degassing facility 101vacuum vessel 102ladle 103snorkel 104exhaust duct 105pipe 106blowing lance 107vacuum gauge

Examples

examples

[0045]Advantageous effects of the present disclosure are described in detail below based on examples, but the present disclosure is not limited to the content of the examples.

[0046]In the examples, decarburization treatment was carried out using an RH vacuum degasser to produce ultra-low carbon molten steel having a upper specification limit of carbon concentration of 25 ppm. Before the start of the vacuum degassing treatment and after the end of the vacuum degassing treatment, a portion of the molten steel was taken as a sample, and the carbon concentration in the molten steel of the sample was measured. Further, the exhaust gas flow rate, the CO concentration in the exhaust gas, and the CO 2 concentration changes over time in the exhaust gas during the vacuum degassing treatment were measured. According to a conventional method, the end of decarburization treatment is determined based on the experience of the operator. Here, the number of analysis data points for the example meth...

TECHNICAL FIELD

[0001] The present disclosure relates to a decarburization treatment end determination method, a decarburization treatment end determination device, a vacuum degassing treatment operation method, and a method of producing molten steel.BACKGROUND

[0002] In the steelmaking process, impurities such as carbon are removed from molten pig iron, and useful alloy components are added to adjust molten steel composition. With regard to carbon in particular, decarburization can be promoted by placing the molten steel in a vacuum environment using a vacuum degassing facility, making it possible to produce ultra-low carbon steel having a carbon concentration in the molten steel below 10 ppm.

[0003] Here, in the vacuum degassing treatment, the carbon concentration in the molten steel is not measured directly, but is only indirectly estimated from concentrations of carbon monoxide and carbon dioxide in exhaust gas. In the production of ultra-low carbon steel, operators tend to run the decarburization treatment too long due to concern about carbon concentration being out of specification.

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

[0005] On the other hand, decarburization reaction models based on physical considerations have the problem of containing model parameters whose true values are difficult to obtain. To address this problem, methods have been proposed that attempt to solve the problem by determining model parameters using exhaust gas measurement values (for example, see Patent Literature (PTL) 1 and 2).CITATION LISTPatent Literature

[0006] PTL 1: JP 2005-330512 A PTL 2: JP 2021-152191 A Non-Patent Literature

[0007] NPL 1: Shinya Kitamura and three others, "Decarburization Model for Vacuum Degasser", Tetsu-to-Hagane, Vol. 80 (1994) No. 3, pp. 213-218 NPL 2: Yoshihiko Higuchi and two others, "Effects of [C], [O] and Pressure on RH Vacuum Decarburization", Tetsu-to-Hagane, Vol. 84 (1998) No. 10, pp. 709-714 SUMMARY(Technical Problem)

[0008] As in the techniques of PTL 1 and PTL 2, measurement values of flow rate and concentration of exhaust gas discharged from a vacuum degassing facility during treatment contain information regarding the progress of decarburization. Accordingly, it may be considered that using exhaust gas measurement values enables high precision estimation of carbon concentration. Further, when high precision carbon concentration estimation is possible, targeting a carbon concentration close to an upper limit of a target range becomes possible, thereby shortening a time required for decarburization treatment of ultra-low carbon steel or the like. For example, Tables 3 and 4 of PTL 1 indicate an example in which the standard deviation of the carbon concentration was improved by some ppm, and the treatment time for ultra-low carbon steel was thereby shortened by some minutes.

[0009] However, the use of exhaust gas measurement values presents two problems. First, there is generally a time delay between the generation of exhaust gas and measurement, and even when the carbon concentration in the molten steel is estimated with high precision, the process takes time. Second, the instantaneous values in a time-series of exhaust gas measurement values exhibit large fluctuations. When such instantaneous values are used to determine model parameters, the fluctuations are reflected in the model parameters, leading to a decrease in carbon concentration estimation precision. In particular, in an end period of decarburization treatment (a final stage of the decarburization treatment) an allowable error in the estimated carbon concentration in the molten steel is some ppm. Therefore, unacceptable errors may occur due to the influence of fluctuations in the time-series of exhaust gas measurement values. Accordingly, a method different from conventional approaches for determining model parameters of a decarburization reaction model that uses a time-series of exhaust gas measurement values is required.

[0010] The present disclosure is made in consideration of the above circumstances, and aims to provide a decarburization treatment end determination method and a decarburization treatment end determination device that can be used to estimate the carbon concentration in molten steel in a final stage with high precision and without time delay, and end the decarburization treatment at an appropriate timing. Further, the present disclosure aims to provide a vacuum degassing treatment operation method and a method of producing molten steel that can be used to estimate the carbon concentration in molten steel in a final stage with high precision and without time delay, and end the decarburization treatment at an appropriate timing.(Solution to Problem)

[0011] (1) A decarburization treatment end determination method according to an embodiment of the present disclosure is a decarburization treatment end determination method that determines the end of decarburization treatment in a vacuum degassing treatment in which molten steel is decarburized by being placed in a reduced pressure environment, comprising: a molten steel carbon concentration estimation step of estimating a carbon concentration in the molten steel; and a decarburization treatment end determination step of determining the end of the decarburization treatment when the estimated carbon concentration is a target value or less, wherein the molten steel carbon concentration estimation step estimates the carbon concentration by dividing the decarburization treatment into a plurality of stages and using a different molten steel carbon concentration estimation model for each stage, the molten steel carbon concentration estimation model of a final stage expresses a decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression of a carbon concentration in a portion of the molten steel placed in the reduced pressure environment Cv and a decarburization reaction capacity coefficient ak, and the decarburization reaction capacity coefficient ak is calculated as a linear expression of the logarithm of a pressure P in a vacuum vessel. (2) As an embodiment of the present disclosure, (1), wherein the pressure P in the vacuum vessel is used as a criterion for dividing the decarburization treatment into the final stage and the stage before the final stage. (3) A decarburization treatment end determination device according to an embodiment of the present disclosure is a decarburization treatment end determination device configured to determine the end of decarburization treatment in a vacuum degassing treatment in which molten steel is decarburized by being placed in a reduced pressure environment, comprising: a molten steel carbon concentration estimation unit configured to estimate a carbon concentration in the molten steel; and a decarburization treatment end determination unit configured to determine the end of the decarburization treatment when the estimated carbon concentration is a target value or less, wherein the molten steel carbon concentration estimation unit estimates the carbon concentration by dividing the decarburization treatment into a plurality of stages and using a different molten steel carbon concentration estimation model for each stage, the molten steel carbon concentration estimation model of a final stage expresses a decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression of a carbon concentration in a portion of the molten steel placed in the reduced pressure environment Cv and a decarburization reaction capacity coefficient ak, and the decarburization reaction capacity coefficient ak is calculated as a linear expression of the logarithm of a pressure P in a vacuum vessel. (4) As an embodiment of the present disclosure, (3), wherein the pressure P in the vacuum vessel is used as a criterion for dividing the decarburization treatment into the final stage and the stage before the final stage. (5) A vacuum degassing treatment operation method according to an embodiment of the present disclosure uses the decarburization treatment end determination method of (1) or (2) in subjecting the molten steel to a vacuum degassing treatment, thereby producing refined molten steel. (6) A method of producing molten steel according to an embodiment of the present disclosure uses the decarburization treatment end determination method of (1) or (2) in subjecting the molten steel to a vacuum degassing treatment, thereby producing refined molten steel. (Advantageous Effect)

[0012] According to the present disclosure, it is possible to provide a decarburization treatment end determination method and a decarburization treatment end determination device that estimate the carbon concentration in molten steel in a final stage with high precision and without time delay, and end the decarburization treatment at an appropriate timing. Further, according to the present disclosure, it is possible to provide a vacuum degassing treatment operation method and a method of producing molten steel that estimate the carbon concentration in the molten steel in a final stage with high precision and without time delay, and end the decarburization treatment at an appropriate timing.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In the accompanying drawings: FIG. 1 is a block diagram illustrating a configuration of a decarburization treatment end determination device according to an embodiment of the present disclosure; FIG. 2 is a flowchart illustrating a processing flow of a decarburization treatment end determination method according to an embodiment of the present disclosure; and FIG. 3 is a graph illustrating a relationship between a pressure in a vacuum vessel and a decarburization rate in one charge of vacuum degassing treatment. DETAILED DESCRIPTION

[0014] Hereinafter, a decarburization treatment end determination method, a decarburization treatment end determination device 20 (see FIG. 1), a vacuum degassing treatment operation method, and a method of producing molten steel according to an embodiment of the present disclosure are described with reference to the drawings. According to the present embodiment, a vacuum degassing facility 100 (see FIG. 1) is described as an RH vacuum degasser, but is not limited to an RH vacuum degasser. For example, the method described below can also be implemented in a facility (apparatus) having only one snorkel 103 (see FIG. 1) that is immersed in a vacuum vessel 101 (see FIG. 1) and a ladle 102 (see FIG. 1) and that draws the molten steel into the vacuum vessel 101. Further, the method described below may be carried out in a facility (apparatus) that does not include the vacuum vessel 101 and creates a vacuum on a surface of the molten steel in the ladle 102.(Configuration)

[0015] FIG. 1 is a schematic diagram illustrating a configuration of the decarburization treatment end determination device 20 and the vacuum degassing facility 100 according to the present embodiment. According to the present embodiment, the vacuum degassing facility 100 carries out the vacuum degassing treatment by placing the molten steel in a reduced pressure environment. The decarburization treatment end determination device 20 is a device that estimates an internal state of the vacuum degassing facility 100 while the vacuum degassing treatment is being carried out in the vacuum degassing facility 100 and determines an end timing of the decarburization treatment. In other words, the decarburization treatment end determination device 20 determines the end of the decarburization treatment in the vacuum degassing treatment in which at least decarburization is carried out by placing the molten steel in a reduced pressure environment. Vacuum degassing treatment removes impurities from molten pig iron, and decarburization treatment is a process of removing carbon among the impurities. According to the present embodiment, the decarburization treatment end determination device 20 carries out the decarburization treatment end determination method described below to determine the end timing of the decarburization treatment, and the vacuum degassing facility 100 is operated accordingly. Further, according to the present embodiment, the vacuum degassing facility 100 constitutes a portion of a molten steel production facility. The method of producing molten steel is carried out in the molten steel production facility. The method of producing molten steel includes refining molten steel in the vacuum degassing facility 100 to produce refined molten steel.

[0016] The RH vacuum degasser 100 includes the vacuum vessel 101 and the ladle 102 that are connected by two snorkels 103. The vacuum vessel 101 is connected to an exhaust duct 104 through which gas inside the vacuum vessel 101 is exhausted to reduce pressure in the vacuum vessel 101 and suck up the molten steel in the ladle 102. Then, by blowing in an inert gas through a pipe or tube 105 from one of the snorkels 103, the molten steel is circulated between the vacuum vessel 101 and the ladle 102. Oxygen can be supplied to the molten steel by blowing oxygen from a blowing lance 106 installed in the vacuum vessel 101. The vacuum vessel 101 is an example of a vacuum region of the vacuum degassing facility 100, that is, a region that is depressurized to create a vacuum.

[0017] A vacuum gauge 107 is installed inside the exhaust duct 104. The vacuum gauge 107 measures pressure inside the vacuum vessel 101. Typically, the vacuum degassing facility 100 is designed to provide as much conductance as possible between an exhauster (not illustrated) and a molten steel surface so that lower pressures can be reached more quickly. Accordingly, although the vacuum gauge 107 is not installed at the same location as the molten steel surface, a measurement value of the vacuum gauge 107 may be treated as being equal to a pressure at the molten steel surface at the time of measurement.

[0018] A vacuum degassing treatment control system in which the decarburization treatment end determination device 20 is used includes a control device 10 and the decarburization treatment end determination device 20 as main components. The control device 10 controls overall operation of the vacuum degassing facility 100. The control device 10 comprises an information processing device such as a computer. The control device 10 controls operation variables related to operation, including an exhaust amount of the exhaust equipment, the flow rate of the circulating inert gas, and the blown oxygen flow rate, so that, from actual values before the vacuum degassing treatment, component concentration and temperature of the molten steel fall within a target range after the vacuum degassing treatment. Further, the control device 10 collects data of operation results including pressure inside the vacuum vessel 101, the flow rate of the circulating inert gas, and the blown oxygen flow rate, and outputs to the decarburization treatment end determination device 20.

[0019] As illustrated in FIG. 1, the decarburization treatment end determination device 20 includes an operation information input interface 21, a molten steel carbon concentration estimation unit 22, and a decarburization treatment end determination unit 23.

[0020] The operation information input interface 21 receives input information such as molten steel information before the start of decarburization treatment and operation results during decarburization treatment.

[0021] The molten steel carbon concentration estimation unit 22 estimates the carbon concentration in the molten steel (that is, the molten steel carbon concentration) based on the input information acquired by the operation information input interface 21. The carbon concentration in the molten steel is estimated by calculation using a model. Hereinafter, a model used to estimate the carbon concentration in the molten steel is referred to as a 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 degree of progress, and estimates the carbon concentration using (selectively using) different molten steel carbon concentration estimation models at each stage.

[0022] The decarburization treatment end determination unit 23 determines the end of the decarburization treatment based on the carbon concentration in the molten steel estimated (calculated) by the molten steel carbon concentration estimation unit 22. Hereinafter, the value of carbon concentration in the molten steel as estimated may be referred to as an "estimated value of carbon concentration in the molten steel" particularly to distinguish the value from a measured value. Further, the decarburization treatment end determination unit 23 outputs a determination result to the control device 10. The control device 10 may control operation variables related to the operation based on the determination result obtained from the decarburization treatment end determination unit 23.

[0023] The decarburization treatment end determination device 20 comprises an information processing device such as a computer. The decarburization treatment end determination device 20 may function as the operation information input interface 21, the molten steel carbon concentration estimation unit 22, and the decarburization treatment end determination unit 23 by an arithmetic processor in the information processing device executing a computer program. The arithmetic processor in the information processing device is, for example, a central processing unit (CPU).

[0024] The decarburization treatment end determination device 20 can estimate the carbon concentration in the molten steel in the final stage of the decarburization treatment with high precision and without time delay by carrying out the decarburization treatment end determination process described below. By estimating the carbon concentration in the molten steel with high precision, it is possible to avoid carrying out an excessively long decarburization treatment due to concern about the carbon concentration being out of specification, and as a result, it is possible to shorten the decarburization treatment time.(Decarburization treatment end determination process)

[0025] Hereinafter, with reference to FIG. 2, a processing flow of the decarburization treatment end determination method (decarburization treatment end determination process) according to an embodiment of the present disclosure is described. The following description of the decarburization treatment end determination process assumes that the decarburization treatment is divided into two stages, but the decarburization treatment end determination process may be carried out in the same way even when the decarburization treatment is divided into three or more stages. The flowchart illustrated in FIG. 2 starts, for example, when an operator inputs a command to execute the decarburization treatment, and processing of step S1 is carried out.

[0026] In the processing of step S1, the operation information input interface 21 acquires molten steel information before the start of the 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 processing of step S1, and the decarburization treatment end determination process proceeds to step S2.

[0027] In the processing of step S2, the operation information input interface 21 acquires operation results during the decarburization treatment. The operation results include values necessary for calculation in the molten steel carbon concentration estimation unit 22. Information such as pressure inside the vacuum vessel 101 during the decarburization treatment, the flow rate of the circulating inert gas, the oxygen flow rate from the blowing lance 106 (blown oxygen flow rate), and information regarding auxiliary material added during the decarburization treatment may be input to the operation information input interface 21. The information regarding auxiliary material is, for example, type and input amount of auxiliary material. This completes the processing of step S2, and the decarburization treatment end determination process proceeds to step S3.

[0028] In the processing of step S3, a stage of the decarburization treatment is determined based on the operation information and the like acquired in the processing up to step S2. As a criterion for dividing the decarburization treatment into the final stage and the stage before the final stage, taking into consideration properties of the molten steel carbon concentration estimation model, described later, it is preferable to use a pressure inside the vacuum vessel 101. Here, when the decarburization treatment is divided into three or more stages, determination criteria for stages other than the final stage may be appropriately selected so that the carbon concentration in the molten steel is higher. For example, the estimated value of carbon concentration in the molten steel, the treatment time, whether or not oxygen is blown, or a combination of these can be used as determination criteria. This completes the processing of step S3, and the decarburization treatment end determination process proceeds to step S4.

[0029] In the processing of step S4, the molten steel carbon concentration estimation unit 22 estimates the carbon concentration in the molten steel. Calculation for estimating the carbon concentration in the molten steel is carried out using a molten steel carbon concentration estimation model according to the stage of the decarburization treatment determined in the processing of step S3.

[0030] In the vacuum degassing treatment using the RH vacuum degasser, when assuming that the molten steel in the ladle 102 and the vacuum vessel 101 is completely mixed, the molten steel carbon concentration estimation model can be described by the differential equations of the following Expression (1) and Expression (2). w L dC L dt = Q C V − C L Expression 1 w V dC V dt = Q C L − C V − ρ ⋅ ak C V − C E Expression 2

[0031] Here, w is the molten steel mass in kg. C is the carbon concentration in the molten steel in ppm. Q is the molten steel circulation rate in kg / s. ρ is the molten steel density in kg / m 3< . ak is the decarburization reaction capacity coefficient in m 3< / s. C E is the equilibrium value, in ppm, of the carbon concentration in the molten steel in the vacuum vessel 101. Further, 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, in ppm, in the molten steel in the vacuum vessel 101. The molten steel mass w L in the vacuum vessel 101 is calculated from the balance between gravity acting on the molten steel in the vacuum vessel 101 and the differential pressure between the atmospheric pressure and the pressure inside the vacuum vessel 101. The molten steel mass w V in the ladle 102 is calculated as the total molten steel mass minus the molten steel mass in the vacuum vessel 101. The molten steel circulation rate Q can be calculated from a known formula based on operation results (see, for example, expression (5) in NPL 2).

[0032] The first terms in Expression (1) and Expression (2) correspond to the molten steel circulation in the vacuum vessel 101 and the ladle 102. The amount of carbon removed from molten steel per unit time is equal to the second term of Expression (2). Here, when the actual values of the exhaust gas flow rate, CO concentration in the exhaust gas, and CO 2 concentration in the exhaust gas during the vacuum degassing treatment are measured, the decarburization rate based on the actual results during the decarburization treatment can be calculated using the following Expressions (3) to (5). q C t = αq C , OG t Expression 3 q C , OG t = αm C v off t + t off 22.4 ⋅ r CO t + t CO + r CO 2 t + t CO 2 100 Expression 4 α = Q C ∫ 0 t 0 q C , OG t dt Expression 5

[0033] Here, q C (t) is the decarburization rate, in kg / s, at time t. q C,OG (t) is the amount of carbon in the exhaust gas per unit time, in kg / s, at time t. α is a correction coefficient for making the cumulative amount of carbon in the exhaust gas during the entire decarburization treatment coincide with the actual reduction in the carbon concentration in the molten steel. α is a dimensionless quantity. m C is the molar mass of carbon in g / mol. V off (t) is the volumetric flow rate of the exhaust gas, in Nm 3< / s, at time t. r CO (t) is the CO concentration in the exhaust gas, in vol%, at time t. r CO2 (t) is the CO 2 concentration in the exhaust gas, in vol%, at time t. t off is the delay time, in s, of exhaust gas flow rate measurement. t CO is the delay time, in s, in measuring the CO concentration in the exhaust gas. t CO2 is the delay time, in s, in measuring the CO 2 concentration in the exhaust gas. Q C is the actual decarburization amount, in kg, calculated from the measured values of the carbon concentration in the molten steel before and after the vacuum degassing treatment. t 0 is the decarburization treatment end time, in s, when the decarburization treatment start time is set to 0.

[0034] The error in the exhaust gas measurement values can be treated as constant during one vacuum degassing treatment. Therefore, α may be a constant that does not depend on time.

[0035] The inventors have found that, as illustrated in FIG. 3, in a range where the pressure P in the vacuum vessel 101 is low (a range of 4 Torr or less in the example of FIG. 3), there is a positive linear relationship between the decarburization rate q C and the logarithm of the pressure P. As described below, it may be considered that in a range where the pressure P in the vacuum vessel 101 is low, the higher the decarburization rate q C , the higher the pressure P. The pressure P inside the vacuum vessel 101 is determined by the balance between the exhaust rate from the vacuum vessel 101 and the gas supply rate into the vacuum vessel 101. The exhaust rate can be treated as a roughly constant value according to the exhaust capacity of the vacuum degassing facility 100. On the other hand, the supply rate of gas such as CO gas into the vacuum vessel 101 increases with higher carbon concentration in the molten steel. Therefore, the higher the decarburization rate, the higher the pressure P in the vacuum vessel 101 becomes. Further, taking such a relationship into consideration, it can be said that the pressure P inside the vacuum vessel 101 is a physical quantity that reflects actual decarburization.

[0036] Based on the above considerations, when the pressure P inside the vacuum vessel 101 is low, that is, in the final stage of the decarburization treatment, the decarburization reaction capacity coefficient ak can be calculated by the following Expression (6). That is, the decarburization reaction capacity coefficient ak can be calculated as a linear expression of the logarithm of the pressure P in the vacuum vessel. [Math. 3] ak = β 0 + β 1 log P

[0037] Here, β 0 and β 1 are constants that are determined by fitting based on past operation results, for example. That is, the decarburization reaction capacity coefficient ak can be calculated based on actual decarburization, regardless of the exhaust gas measurement values.

[0038] As illustrated in FIG. 3, in a range where the pressure P in the vacuum vessel 101 is relatively high, no linear relationship is observed between the decarburization rate q C and the logarithm of the pressure P. This can be considered as follows.

[0039] According to NPL 1 and NPL 2, decarburization reactions in vacuum degassing treatment can be broadly divided into three types: in circulating inert gas bubbles, inside the molten steel, and at the molten steel surface. Among these, the decarburization rate in circulating inert gas bubbles is low. Further, decarburization inside the molten steel requires a bubble formation pressure to generate CO gas bubbles. Therefore, it is considered that decarburization at the molten steel surface is dominant in an end period of decarburization treatment (the final stage of the decarburization treatment). Accordingly, it can be interpreted that the linear relationship between the decarburization rate q C and the logarithm of the pressure P holds true for the decarburization rate at the molten steel surface. In a range where the carbon concentration in the molten steel is relatively high and the pressure P in the vacuum vessel 101 is also relatively high, it is considered that the contribution of decarburization inside the molten steel becomes large, and the linear relationship between the decarburization rate q C and the logarithm of the pressure P no longer holds. With reference to NPL 1 and NPL 2, it can be considered that the linear relationship described above does not hold true at least in a range where the carbon concentration in the molten steel exceeds 100 ppm.

[0040] In a range where the pressure P in the vacuum vessel 101 is relatively high, Expression (6) cannot be used. However, it is possible to estimate the carbon concentration in the molten steel by using a known technique described in NPL 1 or NPL 2. Further, in the range where the pressure P in the vacuum vessel 101 is relatively high, the exhaust gas measurement values may be used to improve the precision of estimating the carbon concentration in the molten steel. Here, in the range where the pressure P in the vacuum vessel 101 is low, including at least the final stage of the decarburization treatment, the molten steel carbon concentration estimation model can be described by the differential equations of Expression (1) and Expression (2). That is, the molten steel carbon concentration estimation model for at least the final stage expresses the decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression of a carbon concentration in a portion of the molten steel placed in the reduced pressure environment Cv and the decarburization reaction capacity coefficient ak. Further, Expression (6) can be used for the decarburization reaction capacity coefficient ak. In the decarburization treatment, when the measurement time is delayed, the decarburization treatment time is not affected unless in the final stage. However, in the final stage, the measurement delay causes an estimation error that is larger than the allowable estimation error (some ppm), which has a direct impact. Further, instantaneous values of the exhaust gas measurement values fluctuate. The decarburization treatment end determination method according to the present embodiment uses the molten steel carbon concentration estimation model expressed as the differential equation described above in the final stage to estimate the carbon concentration without using exhaust gas measurement values, thereby making it possible to calculate the carbon concentration in the molten steel without being affected by a measurement time delay.

[0041] When the carbon concentration in the molten steel is calculated by the above method, the processing of step S4 is completed, and the decarburization treatment end determination process proceeds to step S5. Here, step S3 and step S4 correspond to the molten steel carbon concentration estimation step.

[0042] In the processing of step S5, the decarburization treatment end determination unit 23 determines whether the carbon concentration in the molten steel estimated in step S4 has become a predetermined target value or less. When the estimated value of carbon concentration in the molten steel is higher than the target value (No in step S5), the decarburization treatment end determination process returns to step S2, and the processing from step S3 onwards is executed again using newly input operation results. When the estimated value of the carbon concentration in the molten steel is the target value or less (Yes in step S5), the decarburization treatment end determination unit 23 determines the end of the decarburization treatment, and the decarburization treatment ends. Here, step S5 corresponds to the decarburization treatment end determination step.

[0043] The method according to the present embodiment can be implemented in an RH vacuum degasser. Here, the concentrations of CO and CO 2 in the exhaust gas are typically measured by an infrared gas analyzer. The infrared gas analyzer is a device that measures the concentration of a gas to be measured based on the amount of infrared absorption at a specific wavelength absorbed by the gas. The delay in the exhaust gas measurement values, which is the sum of delay due to response of the infrared gas analyzer and delay due to propagation through piping, is, for example, 30 seconds to one minute. As explained with reference to PTL 1, when high precision carbon concentration estimation is possible, it is possible to aim for 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, and the like. The method according to the present embodiment has high precision in carbon concentration estimation, and as a result, the processing time for ultra-low carbon steel can be reduced by some minutes. Further, as a vacuum degassing treatment operation method or a method of producing molten steel, refined molten steel may be produced by subjecting the molten steel to a vacuum degassing treatment using the decarburization treatment end determination method according to the present embodiment.

[0044] As described above, the decarburization treatment end determination method, the decarburization treatment end determination device 20, the vacuum degassing treatment operation method, and the method of producing molten steel can estimate the carbon concentration in molten steel that reflects actual decarburization, at least for the final stage of decarburization treatment, without relying on exhaust gas measurement values. Therefore, it is possible to estimate the carbon concentration in the molten steel in the final stage with high precision and without time delay. Further, it is possible to determine the end of the decarburization treatment at an appropriate time based on high precision estimation, which makes it possible to avoid carrying out the decarburization treatment for an excessively long time due to concerns about the carbon concentration being out of specification, thereby shortening the decarburization treatment time.EXAMPLES

[0045] Advantageous effects of the present disclosure are described in detail below based on examples, but the present disclosure is not limited to the content of the examples.

[0046] In the examples, decarburization treatment was carried out using an RH vacuum degasser to produce ultra-low carbon molten steel having a upper specification limit of carbon concentration of 25 ppm. Before the start of the vacuum degassing treatment and after the end of the vacuum degassing treatment, a portion of the molten steel was taken as a sample, and the carbon concentration in the molten steel of the sample was measured. Further, the exhaust gas flow rate, the CO concentration in the exhaust gas, and the CO 2 concentration changes over time in the exhaust gas during the vacuum degassing treatment were measured. According to a conventional method, the end of decarburization treatment is determined based on the experience of the operator. Here, the number of analysis data points for the example method and the comparative method was 20 each.

[0047] Table 1 compares results of estimating the carbon concentration in the molten steel between the example method (Example method) and the conventional method (Comparative method). The example method is the method according to the embodiment described above. In the example of FIG. 3, according to the example method, the decarburization treatment was divided into two stages at the time when the pressure P in the vacuum vessel 101 fell below 4 torr for the first time during the decarburization treatment. Further, in the first stage (stage before the final stage) of the conventional method and the example method, the carbon concentration in the molten steel was estimated using the decarburization reaction model described in NPL 1.[Table 1]

[0048] (Table 1)Comparative methodExample methodStandard deviation of carbon concentration estimate at end of vacuum degassing treatment2.98 ppm2.72 ppmExcess decarburization suppression amount compared to comparative method-0.78 ppmDecarburization treatment time reduction compared to comparative method-0.8 min

[0049] The first row of Table 1 indicates the standard deviation of the estimation error of the carbon concentration in the molten steel at the end of the vacuum degassing treatment. It can be seen that the example method can estimate the carbon concentration in the molten steel more precisely than the comparative method. An aim of the present disclosure is to prevent excessive decarburization treatment in vacuum degassing treatment and shorten the treatment time by estimating the carbon concentration in molten steel with high precision. The second and third lines of Table 1 indicate the effect of suppressing excessive decarburization and the shortened time for decarburization treatment when the carbon concentration in the molten steel is estimated using the model of the example method instead of the comparative method. When determining the end of the decarburization treatment based on the estimated value of the carbon concentration in the molten steel, the target value of the carbon concentration in the molten steel referenced in the processing of step S5 needs to be set so as not to exceed the upper specification limit of the carbon concentration, taking into account the error of the carbon concentration estimation model. Specifically, the target value may be set to a value obtained by subtracting three times the standard deviation of the error of the carbon concentration estimation model from the upper specification limit of the carbon concentration. When setting the target value in this way, when the model of the example is used in comparison with the comparative example, as indicated in the second row of Table 1, the target value for determining the end of decarburization treatment can be set to an excess decarburization suppression amount. In this example, the effect of shortening the decarburization treatment time by increasing the target value is as indicated in the third row of Table 1.

[0050] Although an embodiment of the present disclosure has been described based on the drawings and examples, it should be noted that a person skilled in the art may make variations and modifications based on the present disclosure. Therefore, it should be noted that such variations and modifications are included within the scope of the present disclosure. For example, functions and the like included in each component and step may be rearranged, and a plurality of components and steps may be combined into one or divided, as long as no logical inconsistency results. An embodiment according to the present disclosure may be realized as a program executed by a processor provided to a device and as a storage medium on which the program is stored. The scope of the present disclosure should be understood to include these examples.REFERENCE SIGNS LIST

[0051] 10control device 20decarburization treatment end determination device 21operation information input interface 22molten steel carbon concentration estimation unit 23decarburization treatment end determination unit 100vacuum degassing facility 101vacuum vessel 102ladle 103snorkel 104exhaust duct 105pipe 106blowing lance 107vacuum gauge

Examples

examples

[0045]Advantageous effects of the present disclosure are described in detail below based on examples, but the present disclosure is not limited to the content of the examples.

[0046]In the examples, decarburization treatment was carried out using an RH vacuum degasser to produce ultra-low carbon molten steel having a upper specification limit of carbon concentration of 25 ppm. Before the start of the vacuum degassing treatment and after the end of the vacuum degassing treatment, a portion of the molten steel was taken as a sample, and the carbon concentration in the molten steel of the sample was measured. Further, the exhaust gas flow rate, the CO concentration in the exhaust gas, and the CO 2 concentration changes over time in the exhaust gas during the vacuum degassing treatment were measured. According to a conventional method, the end of decarburization treatment is determined based on the experience of the operator. Here, the number of analysis data points for the example meth...

Claims

1. A decarburization treatment end determination method that determines the end of decarburization treatment in a vacuum degassing treatment in which molten steel is decarburized by being placed in a reduced pressure environment, comprising: a molten steel carbon concentration estimation step of estimating a carbon concentration in the molten steel; and a decarburization treatment end determination step of determining the end of the decarburization treatment when the estimated carbon concentration is a target value or less, wherein the molten steel carbon concentration estimation step estimates the carbon concentration by dividing the decarburization treatment into a plurality of stages and using a different molten steel carbon concentration estimation model for each stage, the molten steel carbon concentration estimation model of a final stage expresses a decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression of a carbon concentration in a portion of the molten steel placed in the reduced pressure environment Cv and a decarburization reaction capacity coefficient ak, and the decarburization reaction capacity coefficient ak is calculated as a linear expression of the logarithm of a pressure P in a vacuum vessel.

2. The decarburization treatment end determination method according to claim 1, wherein the pressure P in the vacuum vessel is used as a criterion for dividing the decarburization treatment into the final stage and the stage before the final stage.

3. A decarburization treatment end determination device configured to determine the end of decarburization treatment in a vacuum degassing treatment in which molten steel is decarburized by being placed in a reduced pressure environment, comprising: a molten steel carbon concentration estimation unit configured to estimate a carbon concentration in the molten steel; and a decarburization treatment end determination unit configured to determine the end of the decarburization treatment when the estimated carbon concentration is a target value or less, wherein the molten steel carbon concentration estimation unit estimates the carbon concentration by dividing the decarburization treatment into a plurality of stages and using a different molten steel carbon concentration estimation model for each stage, the molten steel carbon concentration estimation model of a final stage expresses a decarburization reaction rate as a differential equation having a term proportional to the product of a linear expression of a carbon concentration in a portion of the molten steel placed in the reduced pressure environment Cv and a decarburization reaction capacity coefficient ak, and the decarburization reaction capacity coefficient ak is calculated as a linear expression of the logarithm of a pressure P in a vacuum vessel.

4. The decarburization treatment end determination device according to claim 3, wherein the pressure P in the vacuum vessel is used as a criterion for dividing the decarburization treatment into the final stage and the stage before the final stage.

5. A vacuum degassing treatment operation method, comprising using the decarburization treatment end determination method of claim 1 or 2 in subjecting the molten steel to a vacuum degassing treatment, and thereby producing refined molten steel.

6. A method of producing molten steel, the method comprising using the decarburization treatment end determination method of claim 1 or 2 in subjecting the molten steel to a vacuum degassing treatment, and thereby producing refined molten steel.

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