Control Device for Vacuum Degassing Line, Control Method for Vacuum Degassing Line, Operation Method, and Method of Producing Molten Steel

The present disclosure relates to a control device for a vacuum degassing line, a control method for a vacuum degassing line, an operation method, and a method of producing molten steel.

In steelmaking processes, components of molten steel are adjusted, by removing impurities, including carbon, in hot metal and adding useful alloy components. For carbon in particular, it is possible to accelerate decarburization by placing the molten steel under a vacuum environment using a vacuum degassing line, so as to produce ultra-low carbon steel with a carbon concentration in the molten steel of less than 10 ppm.

Here, in vacuum degassing processing, the carbon concentration in the molten steel is not directly measured but 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 perform excessively long decarburization processing because of concerns about out-of-standard carbon concentrations.

To solve the problem of prolonged processing time due to excessive decarburization processing, a highly accurate estimation of the carbon concentration in molten steel during the processing is effective, and various methods have been proposed. Methods of estimating the carbon concentration in molten steel can be broadly classified into two categories. One method is to physically study details of decarburization reaction in a vacuum degassing line and build a decarburization reaction model (for example, Non-Patent Literature [NPL] 1). The other method is to estimate the carbon concentration in molten steel, by calculating the amount of decarburization from a flow rate and a measured value (for example, measured values of concentrations of components) of exhaust gas discharged from a vacuum degassing line during processing. As a combination of both, a method of determining parameters of a decarburization reaction model from a measured value of exhaust gas and estimating the carbon concentration in molten steel using the decarburization reaction model with the determined parameters has also been proposed (for example, Patent Literature [PTL] 1 and Patent Literature [PTL] 2).

In another example, Patent Literature (PTL) 3 also describes a method of correcting an estimated value of carbon concentration in molten steel using the difference between a decarburization rate calculated from a decarburization reaction model based on the observer theory and a decarburization rate calculated from a measurement value of exhaust gas.

In a case in which a decarburization reaction model is built from physical studies, it is often difficult to determine model parameters when trying to represent details of decarburization reaction. For example, the decarburization reaction model proposed in NPL 1 introduces an additional pressure parameter to formulate CO bubble formation inside molten steel, but this value is determined from results of basic experiments. As is pointed out in NPL 2, there is no verification that the same additional pressure parameter value is safe to use in an actual vacuum degassing line. Besides, it is assumed that model parameters for vacuum degassing lines vary due to the different equipment shapes and operating conditions. For the above reasons, even in a case in which the decarburization reaction model proposed in NPL 1 is introduced, a highly accurate estimation of carbon concentration in molten steel cannot be achieved when the equipment shapes or operating conditions are different.

As described above, the technology of PTL 1 and PTL 2 determine parameters of a decarburization reaction model from a measured value of exhaust gas that reflects track records of decarburization, so that model parameters suitable for, for example, equipment shapes and operating conditions can be set. However, because errors in the measured value of exhaust gas are directly reflected in the model parameters, there is a need for a method of further improving the accuracy of estimated value of carbon concentration in molten steel.

As described above, the technology of PTL 3 corrects an estimated value of carbon concentration in molten steel based on the difference between a decarburization rate calculated from the decarburization reaction model and a decarburization rate calculated from a measured value of exhaust gas, but it assumes that the decarburization reaction model is accurate. Accordingly, because errors in the decarburization reaction model are reflected in an estimation result, there is a need for a method of further improving the accuracy of estimated value of carbon concentration in molten steel.

Thus, according to the conventional technology, where there can be errors in a decarburization reaction model and errors in a measured value of exhaust gas, calculation is performed based on the assumption that at least one of the above is accurate. Because the conventional technology estimates a carbon concentration in molten steel by ignoring either errors, the accuracy of estimating the carbon concentration in molten steel is inadequate.

It would be helpful to provide a control device for a vacuum degassing line, a control method for a vacuum degassing line, an operation method, and a method of producing molten steel by which a carbon concentration in molten steel is highly accurately estimated and decarburization processing is ended at an appropriate timing.

(1) A control device for a vacuum degassing line according to an embodiment of the present disclosure is

(2) As an embodiment of the present disclosure, in (1),

(3) As an embodiment of the present disclosure, in (2),

(4) As an embodiment of the present disclosure, in (2) or (3),

(5) A control method for a vacuum degassing line according to an embodiment of the present disclosure is

(6) As an embodiment of the present disclosure, in (5),

(7) As an embodiment of the present disclosure, in (6),

(8) As an embodiment of the present disclosure, in (6) or (7),

(9) An operation method according to an embodiment of the present disclosure includes

(10) A method of producing molten steel according to an embodiment of the present disclosure includes

According to the method of the present disclosure, errors contained in a decarburization reaction model, a measured value of exhaust gas, and a carbon content of in the exhaust gas calculated from these can be corrected at the same time. Accordingly, there are provided the control device for a vacuum degassing line, the control method for a vacuum degassing line, the operation method, and the method of producing molten steel by which an in-molten-steel carbon concentration can be highly accurately estimated, decarburization processing can be ended at an appropriate timing for carbon concentration standards, and decarburization processing time can be reduced.

Hereinafter, a control device and a control method for a vacuum degassing line according to an embodiment of the present disclosure will be described with reference to the drawings. In the present embodiment, the vacuum degassing line is described as an RH vacuum degassing line, but it is not limited to the RH vacuum degassing line. The control method described below can also be implemented for a line (equipment) that includes a vacuum vessel and a single immersion tube to be immersed in a ladle and sucked up into the molten steel vacuum vessel, or a line (equipment) that does not include a vacuum vessel and brings a molten steel surface in the ladle to a vacuum state.

is a block diagram illustrating a configuration of a control deviceaccording to an embodiment of the present disclosure. The control deviceis a control devicefor a vacuum degassing lineand controls operations of the vacuum degassing line. In the vacuum degassing line, decarburization processing is performed by placing at least molten steel under a reduced pressure environment. In the present embodiment, the vacuum degassing lineis operated by the control deviceexecuting a later-described control method for the vacuum degassing line. That is, the control of the vacuum degassing lineis executed as an operation method of the vacuum degassing line. Additionally, in the present embodiment, the vacuum degassing lineconstitutes part of a molten steel production facility. A method of producing molten steel is executed in the molten steel production facility, and the method of producing molten steel includes producing refined molten steel, by refining molten steel in the vacuum degassing line.

As illustrated in, the control deviceincludes an operation information input unit, a component calculation unit, a correction calculation unit, and a decarburization processing control unit.

The operation information input unitacquires information about operation using the vacuum degassing line. In the present embodiment, the operation information input unitreceives information regarding a weight and concentrations of components of molten steel before decarburization processing, track records of operation including measurement results of flow rate and concentrations of components of exhaust gas discharged from the vacuum degassing linewhen the decarburization processing is being executed, and information regarding auxiliary raw materials charged when the decarburization processing is being executed.

The component calculation unitestimates an in-molten-steel carbon concentration in the molten steel, based on operation information acquired by the operation information input unit. In the present embodiment, the component calculation unitestimates the in-molten-steel carbon concentration in the molten steel, based on the information regarding the weight and the concentrations of components of the molten steel before the decarburization processing and the track records of operation.

The correction calculation unitcalculates correction parameters to correct an estimated value of carbon content discharged from the vacuum degassing lineand the estimated in-molten-steel carbon concentration in the molten steel. In the present embodiment, the correction calculation unitcalculates the correction parameters to correct the estimated value of carbon content discharged from the vacuum degassing lineand the estimated in-molten-steel carbon concentration in the molten steel, based on the estimated in-molten-steel carbon concentration in the molten steel, the measurement results of flow rate and concentrations of components of exhaust gas, and a result of carbon balance calculation.

The decarburization processing control unitends the decarburization processing when the in-molten-steel carbon concentration that has been corrected by the correction parameter reaches a target value.

The control deviceis composed, for example, of an information processing device, such as a computer. The control devicemay be configured to function as the operation information input unit, the component calculation unit, the correction calculation unit, and the decarburization processing control unitby having an arithmetic processing unit, such as a CPU (Central Processing Unit), of the information processing device execute a program.

The vacuum degassing linemay be of any known configuration. As described above, an RH vacuum degassing line is used in the present embodiment. The RH vacuum degassing line includes, for example, a vacuum vessel and a ladle, with two immersion tubes connecting them. The vacuum vessel is connected to an exhaust duct, through which gas inside the vacuum vessel is discharged, so as to depressurize the vacuum vessel and suck up molten steel in the ladle. The molten steel is then circulated between the vacuum vessel and the ladle, by blowing inert gas through the immersion tubes from their one ends. Oxygen may also be blown from a blowing lance installed in the vacuum vessel for the purpose of accelerating the decarburization processing.

The control devicewith the above configuration highly accurately estimates an in-molten-steel carbon concentration, by executing decarburization control processing that will be described below. The highly accurate estimation can avoid excessively long decarburization processing due to concerns about out-of-standard carbon concentrations, resulting in a reduction in decarburization processing time. The flow of the decarburization control processing according to an embodiment of the present disclosure will be described below with reference to.

is a flowchart illustrating flow of decarburization control processing executed by the control device. The flowchart ofstarts at a time when instructions to execute the decarburization processing is input, and processing of Step Sis performed.

In the processing of Step S, the operation information input unitacquires a weight of molten steel measured before the start of decarburization processing and concentrations of components obtained by component analysis. Examples of the components for which the concentrations are to be measured include C, Si, Mn, P, S, Al, Cu, Nb, and Ti. If necessary for calculation in the composition calculation unit, the operation information input unitmay also acquire a measurement result of molten steel temperature. In the example of, the temperature is also acquired. This completes the processing of Step S, and the decarburization control processing proceeds to processing of Step S.

In the processing of Step S, the operation information input unitacquires track records of operation during the decarburization processing. The track records of operation are acquired for items necessary for calculation in the component calculation unitand the correction calculation unit. In the present embodiment, the operation information input unitacquires measurement results of flow rate and concentrations of components of exhaust gas discharged from the vacuum degassing lineas the track records of operation. In the present embodiment, the operation information input unitalso acquires information regarding auxiliary raw materials charged when the decarburization processing is being executed. Concrete examples of the information regarding auxiliary raw materials include types and charge amounts of the auxiliary raw materials. In addition, information, such as pressure of the vacuum vessel, flow rate of inert gas to be circulated, or oxygen flow rate from a top-blowing lance during the decarburization processing, may be input to the operation information input unit. In a case in which the processing of Step Sis executed after Step S, which will be described later, the operation information input unitmay also acquires estimated values of molten steel components, including an estimated value of in-molten-steel carbon concentration. This completes the processing of Step S, and the decarburization control processing proceeds to processing of Step Sand Step S. Here, Steps Sand Step Scorrespond to an input step.

In the processing of Step S, the component calculation unitcalculates (estimates) an in-molten-steel carbon concentration according to a predetermined decarburization reaction model. In the present embodiment, the composition calculation unitacquires input information, such as track records of operation, every predetermined period or continuously, and estimates the in-molten-steel carbon concentration in the molten steel every predetermined period or continuously. There are two requirements of the decarburization reaction model used by the component calculation unit. One is that it is capable of estimating an in-molten-steel carbon concentration every predetermined period or continuously, and the other is that the decarburization rate, namely, the rate of change in in-molten-steel carbon concentration, is expressed as a function of in-molten-steel carbon concentration in an area in which decarburization reaction occurs. The area in which the decarburization reaction occurs corresponds to the vacuum vessel in the RH vacuum degassing line. These two requirements are conditions that general decarburization reaction models obviously satisfy.

In the present embodiment, a decarburization reaction model according to the following Formula (1) and Formula (2) is used, while it is assumed that each of a molten steel concentration in the vacuum vessel and that in the ladle is in a fully mixed state during the decarburization processing in the RH vacuum degassing line.

Here, w [kg] is molten steel mass. C [ppm] is in-molten-steel carbon concentration. Q [kg/s] is molten steel circulation rate. ak [kg/s] is decarburization reaction capacity coefficient. C[ppm] is equilibrium value of in-molten-steel carbon concentration in the vacuum vessel. C[ppm] is carbon weight in charged auxiliary raw materials in terms of in-molten-steel carbon concentration. Formula (2) explicitly indicates that the decarburization reaction capacity coefficient depends on the carbon concentration in the molten steel in the vacuum vessel. The subscript L indicates that it is a physical quantity of molten steel in the ladle. The subscript V indicates that it is a physical quantity of molten steel in the vacuum vessel. For example, C[ppm] indicates in-molten-steel carbon concentration in the vacuum vessel. The subscript i is used to identify a specific decarburization reaction site. Concreate examples of the decarburization reaction site may include a molten steel surface and inert gas bubbles for circulation.

A carbon content discharged as exhaust gas is calculated by the second term of Formula (2). The amount of change in in-molten-steel carbon concentration per very short period of time can also be calculated from Formula (1) and Formula (2) and subtracted from the current in-molten-steel carbon concentration, to thereby calculate an in-molten-steel carbon concentration after the very short period of time. This completes the processing of Step S. Here, Step Scorresponds to a component calculation step.

In the processing of Step S, the correction calculation unitcalculates a carbon content in exhaust gas, based on the measurement results of flow rate and concentrations of components of the exhaust gas. Given that carbon discharged from molten steel takes the form of CO or CO, the carbon content in the exhaust gas per unit time is represented by the following Formula (3). A cumulative amount of carbon discharged from the start of the processing (time) to time t is also represented by the following Formula (4).

Here, q(t) [kg/s] is carbon content in exhaust gas per unit time at time t. m[g/mol] is molar mass of carbon. V(t) [Nm/s] is volumetric flow rate of the exhaust gas at time t. r(t) [vol %] is CO concentration in the exhaust gas at time t. r(t) [vol %] is COconcentration in the exhaust gas at time t. Q(t) [kg] is cumulative amount of carbon discharged from timeto time t.

Here, in a case in which measurement results of flow rate and concentrations of components of exhaust gas contain known errors, it is preferable for the correction calculation unitto remove or reduce the known errors before executing the calculation of Formula (3). For example, in a case in which a measured value of CO concentration and a measured value of COconcentration take a non-zero value even at times when no measurements are being made (in a case in which the zero points are shifted), the measured values minus the amount of zero point shift may be used in the calculation. This completes the processing of Step S. Once Steps Sand Step Sare completed, the decarburization control processing proceeds to processing of Step S. Here, the processing of Step Scan be executed independently from the processing of Step S, and Steps Sand Step Smay be executed in parallel, as in the present embodiment. However, the present disclosure is not limited to parallel processing, and Steps Sand Step Smay be executed in sequence, and which comes first (execution order) can also be determined without any limitations in this case.

Here, from the law of conservation of mass, the sum of a carbon content in molten steel and a cumulative amount of carbon discharged from the molten steel is equal to the sum of a carbon content in the molten steel before the decarburization processing and a carbon content contained in the auxiliary raw materials charged during the processing. In general, however, calculation using the carbon content in the molten steel based on the in-molten-steel carbon concentration estimated in Step Sand the cumulative amount of discharged carbon estimated in Step Sdo not satisfy the law of conservation of mass. In the present embodiment, the correction calculation unitdetermines the deviation from the law of conservation of mass as carbon balance calculation and sets a parameter to correct each error, by assuming that this deviation is due to errors in both the decarburization reaction model and the measured value of exhaust gas.

In the processing of Step S, the correction calculation unitdetermines correction parameters for calculation results in the processing of Step Sand Step Sso that the low of conservation of mass is satisfied. The correction value for in-molten-steel carbon concentration in the vacuum vessel, ΔC[ppm], is a correction parameter for the decarburization reaction model. The correction coefficient for carbon content in exhaust gas, α, is a correction parameter for a measured value of exhaust gas. With these correction parameters, the calculation results in the processing of Step Sand Step Sare corrected as follows.

First, the in-molten-steel carbon concentration in the vacuum vessel is corrected to C+ΔC, by adding the correction value for in-molten-steel carbon concentration in the vacuum vessel, ΔC. The carbon content in exhaust gas per unit time is corrected to αq(t), by multiplying the correction coefficient for carbon content in exhaust gas, α. The accumulated amount of discharged carbon is also corrected to αQ(t), by multiplying the correction coefficient for carbon content in exhaust gas, α. The correction coefficient for carbon content in exhaust gas, α, and the correction value for in-molten-steel carbon concentration in the vacuum vessel, ΔC, which are correction parameters, are determined as solutions to the optimization problem represented by the following Formula (5).

Here, Q[kg] is the sum of the carbon content in the molten steel before the decarburization processing and the carbon content contained in the auxiliary raw materials charged during the processing. Q[kg] is the carbon content in the molten steel. The difference between Qand Qincludes the amount of decrease in carbon content in the molten steel. Furthermore, taking the difference from αQ(t) corresponds to evaluating the difference between the amount of decrease and the carbon content in exhaust gas (accumulated amount of discharged carbon). deC(ΔC) [kg/s] is a decarburization rate calculated from the decarburization reaction model by the component calculation unit. αis a standard value of a based on track records of operation. σ, σ, σand σare weighting coefficients, which are set by a user, for example. Q(ΔC) is defined by Formula (6). Furthermore, deC (ΔC) is defined by Formula (7).