Decarburization treatment end determination method, decarburization treatment end determination device, operation method of vacuum degassing treatment, and method for producing molten steel

Abstract

The decarburization process end determination method includes a molten steel carbon concentration estimation step (S3, S4) for estimating the carbon concentration of the molten steel, and a decarburization process end determination step (S5) for determining the end of the decarburization process when the estimated carbon concentration is less than or equal to the target value. The molten steel carbon concentration estimation step divides the decarburization process into a plurality of stages, and uses different molten steel carbon concentration estimation models at each stage to estimate the carbon concentration. The molten steel carbon concentration estimation model in the final stage is expressed as a differential equation having a term proportional to the product of the decarburization reaction rate and the product of the molten steel carbon concentration Cv in the part of the molten steel placed under a reduced pressure environment in the molten steel, and the decarburization reaction volume coefficient ak. The decarburization reaction volume coefficient ak is obtained as a linear expression of the logarithm of the pressure P in the vacuum chamber.

Description

Technical Field

[0001] The present disclosure relates to a method for determining the end of decarburization treatment, an apparatus for determining the end of decarburization treatment, an operation method of vacuum degassing treatment, and a method for producing molten steel.

Background Art

[0002] In the steelmaking process, the components of molten steel are adjusted by removing impurities in hot metal such as carbon and adding useful alloy components. Particularly with respect to carbon, decarburization is promoted by placing molten steel in a vacuum environment using a vacuum degassing facility, and it is possible to produce extra-low carbon steel in which the carbon concentration in the molten steel is less than 10 ppm.

[0003] Here, in the vacuum degassing treatment, the carbon concentration in the molten steel is not directly measured but is only indirectly estimated from the concentrations of carbon monoxide and carbon dioxide in the exhaust gas. In the production of extra-low carbon steel, operators tend to perform decarburization treatment for an overly long time due to concerns about out-of-specification carbon concentrations.

[0004] In order to solve the problem of prolonged treatment time due to excessive decarburization treatment, it is effective to accurately estimate the carbon concentration in the molten steel during treatment. A plurality of decarburization reaction models that physically consider the details of the decarburization reaction in vacuum degassing treatment have been proposed (for example, Non-Patent Document 1 and Non-Patent Document 2).

[0005] On the other hand, the decarburization reaction model based on physical considerations has a problem that it includes model parameters that are difficult to obtain the true values. For this problem, a plurality of methods have been proposed to attempt to solve it by determining model parameters using exhaust gas measurement values (for example, Patent Document 1 and Patent Document 2).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Non-Patent Document

[0007]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] The measured values of the flow rate and concentration of the exhaust gas discharged from the vacuum degassing equipment during processing, like the technologies of Patent Document 1 and Patent Document 2, contain information regarding the progress of decarburization. Therefore, it is considered that using the exhaust gas measurement values enables highly accurate carbon concentration estimation. Also, if highly accurate carbon concentration estimation is possible, since it is possible to aim near the upper limit of the target range of the carbon concentration, the time required for decarburization treatment for extra-low carbon steel and the like can be shortened. For example, Tables 3 and 4 of Patent Document 1 show examples where the processing time of extra-low carbon steel was shortened by several minutes due to an improvement in the standard deviation of the carbon concentration by several ppm.

[0009] However, the use of exhaust gas measurement values has two problems. First, generally, there is a time lag from the generation of exhaust gas to the measurement, and even if the carbon concentration in the molten steel is estimated with high precision, it takes time for the process. Second, in the time-series exhaust gas measurement values, the instantaneous values have large variations. If the instantaneous values are used to determine the model parameters, the variations will also be reflected in the model parameters, leading to a decrease in the accuracy of carbon concentration estimation. In particular, at the end stage of the decarburization process (the final stage of the decarburization process), the allowable error in the estimated value of the carbon concentration in the molten steel is about several ppm. Therefore, unacceptable errors may occur due to the variations in the time-series exhaust gas measurement values. Accordingly, a method different from the determination of the model parameters of the decarburization reaction model using the time-series exhaust gas measurement values is required.

[0010] In view of such circumstances, an object of the present disclosure is to provide a decarburization process end determination method and a decarburization process end determination device that accurately estimate the carbon concentration in the molten steel at the final stage without time lag and terminate the decarburization process at an appropriate timing. Another object of the present disclosure is to provide an operation method of a vacuum degassing process and a method for manufacturing molten steel that accurately estimate the carbon concentration in the molten steel at the final stage without time lag and terminate the decarburization process at an appropriate timing.

Means for Solving the Problems

[0011] (1) The decarburization process end determination method according to an embodiment of the present disclosure is a decarburization process end determination method for determining the end of a decarburization process in a vacuum degassing process in which decarburization is performed by placing molten steel in a reduced-pressure environment, including a step of estimating the carbon concentration in the molten steel for estimating the carbon concentration of the molten steel, and a decarburization process end determination step of determining the end 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 the molten steel divides the decarburization process into a plurality of stages, and estimates the carbon concentration using different carbon concentration estimation models for each stage. The carbon concentration estimation model in the molten steel at the final stage is expressed as a differential equation having a term proportional to the product of the decarburization reaction rate and the primary expression of the carbon concentration Cv in the molten steel in the portion of the molten steel placed in a reduced-pressure environment, The decarburization reaction volume coefficient ak is obtained as a primary expression of the logarithm of the pressure P in the vacuum chamber.

[0012] (2) As an 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 and the stage before the final stage.

[0013] (3) The decarburization treatment end determination device according to an embodiment of the present disclosure is A decarburization treatment end determination device for determining the end of decarburization treatment in a vacuum degassing treatment for performing decarburization by placing molten steel in a reduced-pressure environment, A molten steel carbon concentration estimation unit for estimating the carbon concentration of the molten steel, A decarburization treatment end determination unit for determining the end of the decarburization treatment when the estimated carbon concentration is equal to or less than a target value, and The molten steel carbon concentration estimation unit divides the decarburization treatment into a plurality of stages, and estimates the carbon concentration using different molten steel carbon concentration estimation models at each stage, The carbon concentration estimation model of the molten steel in the final stage is expressed as a differential equation having a term proportional to the product of the decarburization reaction rate and the primary expression of the carbon concentration Cv in the molten steel in the portion of the molten steel placed in a reduced-pressure environment, The decarburization reaction volume coefficient ak is obtained as a primary expression of the logarithm of the pressure P in the vacuum chamber.

[0014] (4) As an 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 and the stage before the final stage.

[0015] (5) The operation method of the vacuum degassing treatment according to an embodiment of the present disclosure is Using the decarburization treatment end determination method of (1) or (2), the molten steel is subjected to vacuum degassing treatment to produce the refined molten steel.

[0016] (6) The method for producing molten steel according to an embodiment of the present disclosure is Using the decarburization treatment end determination method of (1) or (2), the molten steel is subjected to vacuum degassing treatment to produce the refined molten steel.

Effects of the Invention

[0017] According to the present disclosure, it is possible to provide a decarburization treatment end determination method and a decarburization treatment end determination device that accurately estimate the carbon concentration in molten steel at the final stage without time delay and end the decarburization treatment at an appropriate timing. Further, according to the present disclosure, it is possible to provide an operation method for vacuum degassing treatment and a method for producing molten steel that accurately estimate the carbon concentration in molten steel at the final stage without time delay and end the decarburization treatment at an appropriate timing.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0019] Hereinafter, with reference to the drawings, a decarburization process end determination method, a decarburization process end determination device 20 (see FIG. 1), an operation method of vacuum degassing treatment, and a method for manufacturing molten steel according to an embodiment of the present disclosure will be described. In this embodiment, the vacuum degassing facility 100 (see FIG. 1) will be described as an RH vacuum degassing facility, but it is not limited to the RH vacuum degassing facility. For example, a facility (device) having only one immersion tube 103 (see FIG. 1) that immerses in the vacuum vessel 101 (see FIG. 1) and the ladle 102 (see FIG. 1) to suck the molten steel into the vacuum vessel 101 can also implement the method described below. Further, for example, a facility (device) that does not have a vacuum vessel 101 and makes the surface of the molten steel in the ladle 102 in a vacuum state can also implement the method described below.

[0020] (Configuration) FIG. 1 is a schematic diagram showing the configuration of the decarburization process end determination device 20 and the vacuum degassing facility 100 according to this embodiment. In this embodiment, the vacuum degassing facility 100 performs vacuum degassing treatment by placing molten steel in a reduced-pressure environment. The decarburization process end determination device 20 is a device that estimates the internal state of the vacuum degassing facility 100 and determines the end timing of the decarburization process during the execution of the vacuum degassing treatment in the vacuum degassing facility 100. In other words, the decarburization process end determination device 20 determines the end of the decarburization process in the vacuum degassing treatment that performs at least decarburization by placing molten steel in a reduced-pressure environment. In the vacuum degassing treatment, impurities in the hot metal are removed, and the treatment of removing carbon among the impurities is the decarburization process. In this embodiment, the vacuum degassing facility 100 is operated by the decarburization process end determination device 20 executing the decarburization process end determination method described later to determine the end timing of the decarburization process. Further, in this embodiment, the vacuum degassing facility 100 constitutes a part of the molten steel manufacturing facility. A method for manufacturing molten steel is executed in the molten steel manufacturing facility. The method for manufacturing molten steel includes refining molten steel in the vacuum degassing facility 100 to produce refined molten steel.

[0021] The RH vacuum degassing facility 100 includes a vacuum chamber 101 and a ladle 102, which are connected by two immersion tubes 103. The vacuum chamber 101 is connected to an exhaust duct 104, and the gas inside the vacuum chamber 101 is exhausted through this to reduce the pressure in the vacuum chamber 101 and suck up the molten steel in the ladle 102. Then, by blowing an inert gas through a pipe 105 from one side of the immersion tube 103, the molten steel refluxes between the vacuum chamber 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 chamber 101. The vacuum chamber 101 is an example of the evacuation area of the RH vacuum degassing facility 100, that is, the area to be evacuated to a vacuum state.

[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 RH vacuum degassing facility 100 is designed so that the conductance between the exhaust device (not shown) and the molten steel surface can be as large as possible to reach a lower pressure more quickly. Therefore, although the vacuum gauge 107 is not installed at the same location as the molten steel surface, the measured value of the vacuum gauge 107 can be treated as equal to the pressure at the molten steel surface at the time of measurement.

[0023] The RH vacuum degassing process control system in which the decarburization process end determination device 20 is used includes a control device 10 and the decarburization process end determination device 20 as main components. The control device 10 controls the overall operation of the RH vacuum degassing facility 100. The control device 10 is composed of an information processing device such as a computer. The control device 10 controls the operation amounts related to the operation including the exhaust amount of the exhaust facility, the flow rate of the reflux inert gas, and the flow rate of the blown-in oxygen so that the component concentration and temperature of the molten steel are within the target range after the RH vacuum degassing process from the actual values before the RH vacuum degassing process. Also, the control device 10 collects data on the operation results including the pressure inside the vacuum chamber 101, the flow rate of the reflux inert gas, and the flow rate of the blown-in oxygen, and outputs it to the decarburization process end determination device 20.

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

[0025] In the operation information input unit 21, the molten steel information before the start of the decarburization treatment and the operation results during the decarburization treatment are input as input information.

[0026] Based on the input information acquired by the operation information input unit 21, the molten steel carbon concentration estimation unit 22 estimates the molten steel carbon concentration (i.e., the carbon concentration of the molten steel). The molten steel carbon concentration is estimated by calculation using a model. Hereinafter, the model used for estimating the molten steel carbon concentration is 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 degree, and estimates the carbon concentration using different molten steel carbon concentration estimation models (using them properly) at each stage.

[0027] Based on the molten steel carbon concentration estimated (calculated) by the molten steel carbon concentration estimation unit 22, the decarburization treatment end determination unit 23 determines the end of the decarburization treatment. Hereinafter, in order to distinguish it from the measured value in particular, the value of the estimated molten steel carbon concentration may be described as the "estimated value of the molten steel carbon concentration". Further, the decarburization treatment end determination unit 23 outputs the determination result to the control device 10. The control device 10 may control the operation operation amount based on the determination result obtained from the decarburization treatment end determination unit 23.

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

[0029] The decarburization process end determination device 20 can accurately estimate the carbon concentration in the molten steel at the final stage of the decarburization process without time delay by executing the decarburization process end determination process described below. By accurately estimating the carbon concentration in the molten steel, it is possible to avoid performing the decarburization process for an excessive length of time due to concerns about out-of-specification carbon concentrations, and as a result, it becomes possible to shorten the time of the decarburization process.

[0030] (Decarburization process end determination process) Hereinafter, with reference to FIG. 2, the flow of the process (decarburization process end determination process) of the decarburization process end determination method according to an embodiment of the present disclosure will be described. Hereinafter, the decarburization process end determination process will be described on the premise that the decarburization process is divided into two stages, but it is also possible to perform the decarburization process end determination process in the same manner even if it is divided into three or more stages. The flowchart shown in FIG. 2 starts, for example, at the timing when an execution command for the decarburization process is input by an operator, and the process of step S1 is performed.

[0031] In the process of step S1, the operation information input unit 21 acquires the molten steel information before the start of the decarburization process. The molten steel information may include, for example, the weight of the molten steel and the analysis results obtained by component analysis. Thereby, the process of step S1 is completed, and the decarburization process end determination process proceeds to the process of step S2.

[0032] In the process of step S2, the operation information input unit 21 acquires the operation results during the decarburization process. The operation results include items necessary for the calculation in the molten steel carbon concentration estimation unit 22. Information such as the pressure in the vacuum chamber 101, the flow rate of the reflux inert gas, and the oxygen flow rate (injected oxygen flow rate) from the injection lance 106 during the decarburization process, and information regarding the auxiliary raw materials input during the execution of the decarburization process may be input to the operation information input unit 21. The information regarding the auxiliary raw materials is, as a specific example, the type and input amount of the auxiliary raw materials. Thereby, the process of step S2 is completed, and the decarburization process end determination process proceeds to the process of step S3.

[0033] In the process of step S3, the stage of the decarburization treatment is determined based on the operation information obtained in the processes up to step S2. Regarding the criterion for dividing the final stage of the decarburization treatment and the stage before the final stage, it is preferable to use the pressure in the vacuum chamber 101 in consideration of the properties of the molten steel carbon concentration estimation model described later. Here, when the decarburization treatment is divided into three or more stages, the determination criteria for the stages other than the final stage may be appropriately selected so that the carbon concentration in the molten steel increases. For example, the estimated value of the carbon concentration in the molten steel, the treatment time, the presence or absence of oxygen blowing, or a combination thereof can be used as the determination criteria. Thereby, the process of step S3 is completed, and the decarburization treatment end determination process proceeds to the process of step S4.

[0034] In the process of step S4, the molten steel carbon concentration estimation unit 22 estimates the carbon concentration in the molten steel. The calculation for estimating the carbon concentration in the 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 by the RH vacuum degassing facility, assuming that the molten steel in the ladle 102 and the vacuum chamber 101 is completely mixed, the molten steel carbon concentration estimation model can be described by the differential equations of the following formulas (1) and (2).

[0036]

Number

[0037] Here, w is the mass of the molten steel [kg]. C is the carbon concentration in the molten steel [ppm]. Q is the molten steel reflux rate [kg / s]. 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 of the carbon concentration in the molten steel in the vacuum chamber 101 [ppm]. Also, the subscript L indicates that it is a physical quantity of the molten steel in the ladle 102. The subscript V indicates that it is a physical quantity of the molten steel in the vacuum chamber 101. For example, C V indicates the carbon concentration in the molten steel in the vacuum chamber 101 [ppm]. The mass of the molten steel w in the vacuum chamber 101 LIt is calculated from the balance between the gravity acting on the molten steel in the vacuum chamber 101 and the differential pressure between the atmospheric pressure and the pressure in the vacuum chamber 101. The mass of the molten steel w in the ladle 102 V is calculated as the value obtained by subtracting the mass of the molten steel in the vacuum chamber 101 from the total mass of the molten steel. The molten steel reflux rate Q can be calculated from a known formula based on the operating results (see, for example, formula (5) in Non-Patent Document 2).

[0038] The first terms of formula (1) and formula (2) correspond to the molten steel reflux between the vacuum chamber 101 and the ladle 102. The amount of carbon removed from the molten steel per unit time is equal to the second term of formula (2). Here, if the actual values of the exhaust gas flow rate, the CO concentration in the exhaust gas, and the CO2 concentration in the exhaust gas during the vacuum degassing process are measured, the decarburization rate based on the actual results during the decarburization process can be calculated by the following formulas (3) to (5).

[0039]

Equation

[0040] Here, q C (t) is the decarburization rate [kg / s] at time t. q 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 during the entire decarburization process with the reduction result of the carbon concentration in the molten steel. α is a dimensionless quantity. m C is the molar mass of carbon [g / mol]. V off (t) is the volume flow rate of the exhaust gas [Nm 3 / s] at time t. r CO (t) is the CO concentration [vol%] in the exhaust gas at time t. r CO2 (t) is the CO2 concentration [vol%] in the exhaust gas at time t. t off is the delay time [s] for exhaust gas flow rate measurement. t CO is the delay time [s] for CO concentration measurement in the exhaust gas. t CO2 is the delay time [s] for CO2 concentration measurement in the exhaust gas. Q Cis the actual decarburization amount [kg] calculated from the measured values of the carbon concentration in the molten steel before and after the vacuum degassing treatment. t0 is the end time [s] of the decarburization treatment when the start time of the decarburization treatment is set to 0.

[0041] The error in the exhaust gas measurement value can be treated as constant during one vacuum degassing treatment. Therefore, α can be regarded as a constant independent of time.

[0042] As shown in FIG. 3, the inventors have found that in the region where the pressure P in the vacuum chamber 101 is low (the region below 4 Torr in the example of FIG. 3), the decarburization rate q C has a positive linear relationship with the logarithm of the pressure P. As described in the following discussion, in the region where the pressure P in the vacuum chamber 101 is low, it is considered that 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 generally 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 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. Considering such a relationship, it can be said that the pressure P in the vacuum chamber 101 is a physical quantity that reflects the decarburization result.

[0043] Based on the above discussion, in the state where the pressure P in the vacuum chamber 101 is low, that is, at the final stage of the decarburization treatment, the decarburization reaction volume coefficient ak can be obtained by the following formula (6). That is, the decarburization reaction volume coefficient ak is obtained as a linear expression of the logarithm of the pressure P in the vacuum chamber.

[0044]

Equation

[0045] Here, β0 and β1 are constants, and are obtained by fitting, for example, from past operation results. That is, the decarburization reaction volume coefficient ak can be calculated based on the decarburization result regardless of the exhaust gas measurement value.

[0046] Here, 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 and the logarithmic relationship with the pressure P is not recognized. This can be considered as follows.

[0047] According to Non-Patent Document 1 and Non-Patent Document 2, the decarburization reaction in the vacuum degassing process can be roughly classified into three types: reflux inert gas bubbles, the inside of the molten steel, and the surface of the molten steel. Among these, the decarburization by reflux inert gas bubbles has a low decarburization rate. Also, for decarburization inside the molten steel, a bubble generation pressure for generating CO gas bubbles is required. Therefore, it is considered that decarburization on the surface of the molten steel is dominant at the end stage of the decarburization process (the final stage of the decarburization process). Therefore, the linear relationship between the decarburization rate q C and the logarithm of the pressure P can be interpreted as holding for the decarburization rate on 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 chamber 101 is also relatively high, the contribution of decarburization inside the molten steel becomes large, and it is considered that the linear relationship between the decarburization rate q C and the logarithm of the pressure P no longer holds. Referring to Non-Patent Document 1 and Non-Patent Document 2, it is considered that the above linear relationship does not hold at least in a region where the carbon concentration in the molten steel exceeds 100 ppm.

[0048] In a region where the pressure P in the vacuum chamber 101 is relatively high, Equation (6) cannot be used, but 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. Also, in a region where the pressure P in the vacuum chamber 101 is relatively high, the exhaust gas measurement value may be used for the purpose of improving the estimation accuracy of the carbon concentration in the molten steel. Here, in a region where the pressure P in the vacuum chamber 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 the above Equations (1) and (2). That is, at least the molten steel carbon concentration estimation model in the final stage is expressed as a differential equation having a term in which the decarburization reaction rate is proportional to the product of a linear expression of the carbon concentration Cv in the molten steel in the portion of the molten steel placed in the reduced-pressure environment and the decarburization reaction volume coefficient ak. Also, Equation (6) can be used for the decarburization reaction volume coefficient ak. In the decarburization treatment, if it is not the final stage, even if the measurement time is delayed, it does not affect the decarburization treatment time. However, in the final stage, the measurement delay causes an estimation error larger than the allowable estimation error (about several ppm), having a direct impact. Also, the instantaneous value of the exhaust gas measurement value has variations. The decarburization treatment end determination method according to the present embodiment uses the molten steel carbon concentration estimation model expressed as the above differential equation in the final stage, and estimates the carbon concentration without using the exhaust gas measurement value, thereby calculating the carbon concentration in the molten steel without being affected by the 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 decarburization treatment end determination process proceeds to the process of Step S5. Here, Step S3 and Step S4 correspond to the molten steel carbon concentration estimation steps.

[0050] In the process of step S5, the decarburization process end determination unit 23 determines whether the carbon concentration in the molten steel estimated in step S4 is equal to or lower than a predetermined target value. When the estimated value of the carbon concentration in the molten steel is higher than the target value (No in step S5), the decarburization process end determination process returns to the process of step S2, and the processes after step S3 are executed again using the newly input operation results. When the estimated value of the carbon concentration in the molten steel is equal to or lower than the target value (Yes in step S5), the decarburization process end determination unit 23 determines the end of the decarburization process, and the decarburization process ends. Here, step S5 corresponds to the decarburization process end determination step.

[0051] The method of this embodiment can be executed in an RH vacuum degassing facility. Here, the concentrations of CO and CO2 in the exhaust gas are generally measured by an infrared gas analyzer. An infrared gas analyzer is a device that measures the concentration of a gas based on the absorption amount of infrared rays with a specific wavelength absorbed by the gas to be measured. The delay in the exhaust gas measurement value is, for example, 30 seconds to 1 minute when the delay due to the response of the infrared gas analyzer and the delay due to the propagation in the piping are added together. Here, as described with reference to Patent Document 1, if high-precision carbon concentration estimation is possible, it is possible to aim near the upper limit of the target range of the carbon concentration, so that the time required for the decarburization process for extra-low carbon steel and the like can be shortened. In the method of this embodiment, the accuracy of carbon concentration estimation is high, and as a result, the processing time for extra-low carbon steel can be shortened by about several minutes. Further, as an operation method of the vacuum degassing process or a method for manufacturing molten steel, refined molten steel may be manufactured by subjecting the molten steel to a vacuum degassing process using the decarburization process end determination method according to this embodiment.

[0052] As described above, the decarburization process end determination method, the decarburization process end determination apparatus 20, the operation method of the vacuum degassing process, and the molten steel manufacturing method can estimate the carbon concentration in the molten steel that reflects the decarburization results regardless of the exhaust gas measurement values for at least the final stage of the decarburization process. Therefore, it is possible to accurately estimate the carbon concentration in the molten steel at the final stage without time delay. In addition, based on the accurate estimation, it becomes possible to determine the end of the decarburization process at an appropriate timing, and it is possible to avoid performing the decarburization process for an excessively long time due to concerns about out-of-specification carbon concentration, and as a result, it becomes possible to shorten the time of the decarburization process.

[0053] (Example) Hereinafter, the effects of the present disclosure will be specifically described based on examples, but the present disclosure is not limited to the contents of the examples.

[0054] As an example of this embodiment, decarburization treatment was performed using an RH vacuum degassing facility, and ultra-low carbon molten steel with a carbon concentration specification upper limit of 25 ppm was produced. A part of the molten steel was sampled before the start of the vacuum degassing treatment and after the end of the vacuum degassing treatment, and the carbon concentration in the molten steel of this sample was actually measured. In addition, the time changes in the exhaust gas flow rate, the CO concentration in the exhaust gas, and the CO2 concentration in the exhaust gas during the vacuum degassing treatment were measured. The conventional method is a method of determining the end of the decarburization process based on the experience of the operator. Here, the number of analysis data for the inventive method and the comparative method is 20 each.

[0055] Table 1 compares the estimated results of the carbon concentration in the molten steel between the inventive method and the conventional method. The inventive method is the method of the above-described 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 first dropped below 4 torr during the decarburization process. In addition, in the first stage (the stage before the final stage) of the conventional method and the inventive method, the carbon concentration in the molten steel was estimated using the decarburization reaction model described in Non-Patent Document 1.

[0056]

Table 1

[0057] The first row of Table 1 shows the standard deviation of the estimation error of the carbon concentration in the molten steel at the end of the vacuum degassing process. It can be seen that the inventive method can estimate the carbon concentration in the molten steel more accurately than the comparative method. The present disclosure aims to suppress excessive decarburization treatment in the vacuum degassing process and shorten the treatment time by accurately estimating the carbon concentration in the molten steel. The second and third rows of Table 1 show the excessive decarburization suppression effect and the decarburization treatment shortening time when the carbon concentration in the molten steel is estimated by the model of the inventive method instead of the comparative method in this example. 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 referred to in the process of step S5 needs to be set so as not to exceed the upper limit of the carbon concentration standard considering the error of the carbon concentration estimation model. Specifically, a value obtained by subtracting three times the standard deviation of the error of the carbon concentration estimation model from the upper limit of the carbon concentration standard may be set as the target value. When setting the target value in this way, as shown in the second row of Table 1, if the model of the inventive example is used for the comparative example, the target value for determining the end of the decarburization treatment can be set to the excessive decarburization suppression amount. In this example, the effect of shortening the decarburization treatment time due to the increase in the target value is as shown in the third row of Table 1.

[0058] Regarding the embodiments according to the present disclosure, although they have been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various modifications or corrections based on the present disclosure. Therefore, it should be noted that these modifications or corrections are included in the scope of the present disclosure. For example, the functions included in each component or each step, etc., can be rearranged so as not to be logically contradictory, and it is possible to combine or divide a plurality of components or steps, etc. into one. The embodiments according to the present disclosure can also be realized as a program executed by a processor provided in the device and a storage medium recording the program. It should be understood that these are also included in the scope of the present disclosure.

Explanation of Reference Signs

[0059] 10 Control device 20 Decarburization treatment end determination device 21 Operating information input unit 22 Molten steel carbon concentration estimation unit 23 Decarburization process end determination unit 100 Vacuum degassing facility 101 Vacuum chamber 102 Ladle 103 Submerged lance 104 Exhaust duct 105 Pipe 106 Injection lance 107 Vacuum gauge

Claims

1. A method for determining the end of decarburization treatment in a vacuum degassing treatment for decarburizing molten steel by placing the molten steel in a reduced-pressure environment, comprising: a step of estimating the carbon concentration in the molten steel; and a step of determining the end of the decarburization treatment when the estimated carbon concentration is equal to or lower than a target value. In the step of estimating the carbon concentration in the molten steel, the decarburization treatment is divided into a plurality of stages, and the carbon concentration is estimated using different carbon concentration estimation models for the molten steel at each stage. The carbon concentration estimation model for the final stage is expressed as a differential equation having a term proportional to the product of the decarburization reaction rate and the decarburization reaction volume coefficient ak, where the decarburization reaction rate is a linear expression of the carbon concentration Cv in the portion of the molten steel placed in the reduced-pressure environment among the molten steel. The decarburization reaction volume coefficient ak is obtained as a linear expression of the logarithm of the pressure P in the vacuum chamber. A method for determining the end of decarburization treatment.

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

3. A device for determining the end of decarburization treatment in a vacuum degassing treatment for decarburizing molten steel by placing the molten steel in a reduced-pressure environment, comprising: a unit for estimating the carbon concentration in the molten steel; and a unit for determining the end of the decarburization treatment when the estimated carbon concentration is equal to or lower than a target value. The unit for estimating the carbon concentration in the molten steel divides the decarburization treatment into a plurality of stages, and estimates the carbon concentration using different carbon concentration estimation models for the molten steel at each stage. The carbon concentration estimation model for the final stage is expressed as a differential equation having a term proportional to the product of the decarburization reaction rate and the decarburization reaction volume coefficient ak, where the decarburization reaction rate is a linear expression of the carbon concentration Cv in the portion of the molten steel placed in the reduced-pressure environment among the molten steel. The decarburization reaction volume coefficient ak is obtained as a linear expression of the logarithm of the pressure P in the vacuum chamber. A device for determining the end of decarburization treatment.

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

5. An operation method of vacuum degassing treatment for producing the refined molten steel by subjecting the molten steel to vacuum degassing treatment using the method for determining the end of decarburization treatment according to claim 1 or 2.

6. A method for producing molten steel, which uses the decarburization treatment end determination method according to claim 1 or 2 to subject the molten steel to vacuum degassing treatment to produce the refined molten steel.

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