State Estimation Method for Vacuum Degassing Treatment, Operation Method, Molten Steel Production Method, and State Estimation Device for Vacuum Degassing Treatment

The present disclosure relates to a state estimation method for vacuum degassing treatment, an operation method, a molten steel production method, and a state estimation device for vacuum degassing treatment.

When casting molten steel by continuous casting, it is necessary to keep the oxygen concentration in the molten steel at a very low value. In a typical steelmaking process, the oxygen concentration in the molten steel is decreased by adding a deoxidizer to the molten steel. When performing secondary refining using vacuum degassing treatment after converter treatment, on the other hand, oxygen is blown into the molten steel in order to increase the oxygen concentration in the molten steel in some cases. Oxygen blowing may be intended, for example, to promote decarburization for steel types with a low target carbon concentration in the molten steel. By increasing the oxygen concentration in the molten steel, the reaction in which carbon in the molten steel reacts with oxygen in the molten steel to generate CO gas can be balanced with a lower carbon concentration in the molten steel. Oxygen blowing may be intended, for example, to adjust the temperature of the molten steel by utilizing the heat generated as a result of the reaction with the component elements in the molten steel. Here, the components in the molten steel that are reacted are not limited to components present in the molten steel at the time of oxygen blowing but also include components added after oxygen blowing. Especially in the case where a deoxidizer is added after oxygen blowing, the increase in the temperature of the molten steel depends on the oxygen concentration in the molten steel. Hence, in order to control the components and temperature of the molten steel to the desired values by performing secondary refining, it is necessary to control the oxygen concentration in the molten steel to the desired value.

Here, oxygen blown in by oxygen blowing (hereafter referred to as “blown oxygen”) does not fully dissolve in the molten steel and contribute to increasing the oxygen concentration in the molten steel. Part of the blown oxygen may react with CO gas in the gas phase to form COgas. Part of the blown oxygen may be exhausted to the outside of the system as Ogas. Therefore, in order to increase the oxygen concentration in the molten steel to the desired value by oxygen blowing, the proportion of blown oxygen dissolved in the molten steel needs to be estimated accurately.

Since blown oxygen either dissolves in the molten steel or is discharged from the exhaust system, determining the amount of oxygen discharged from the exhaust system using an exhaust gas measurement device makes it possible to estimate the dissolution rate of blown oxygen in the molten steel (i.e. the proportion of blown oxygen dissolved in the molten steel). However, the oxygen exhausted from the vacuum degassing line contains oxygen other than the oxygen blown in by oxygen blowing during vacuum degassing treatment, and such other oxygen needs to be subtracted. Examples of such oxygen supply sources include the molten steel to be treated, air present in the vacuumized region before the start of the treatment, and air entering the vacuumized region from an insufficiently sealed part of the vacuum exhaust system during the treatment.

The amount of oxygen supplied from air can be determined by calculating the amount of Ngas contained in the exhaust gas. In the refining process, approximately the whole flow rate of the exhaust gas is occupied by components such as CO gas, COgas, Ogas, inert gas blown in to stir the molten steel, and Ngas. The flow rates of CO gas, COgas, and Ogas are measured by an exhaust gas measurement device. The flow rate of inert gas is a manipulated variable for operation. Hence, the remainder obtained by subtracting the flow rates of these gases from the measured flow rate of the exhaust gas can be estimated to be the flow rate of Ngas.

For example, in JP 2019-183227 A (PTL 1), the amount of air entering an exhaust system during operation is estimated by the foregoing calculation method for the purpose of deriving converter parameters. In JP 6583594 B1 (PTL 2), estimation that takes into account the amount of entrained air is described as an embodiment of estimating the composition of molten metal in a refining process, and the amount of entrained air is estimated by the foregoing calculation method.

The techniques described in PTL 1 and PTL 2 are both intended for a converter process. In the case of a converter, there is a large gap between the converter and the skirt and air enters from this gap, so that the flow rate of incoming air changes greatly with time. In a vacuum degassing line, on the other hand, the air pressure is kept very low in the exhaust system, and the flow rate of air entering from an insufficiently sealed part is considered to change little with time. Accordingly, the estimation methods proposed in PTL 1 and PTL 2 cannot be directly applied to vacuum degassing treatment.

Moreover, the ultimate purpose in PTL 1 is to derive converter parameters, and the ultimate purpose in PTL 2 is to estimate the carbon concentration in molten metal and the FeO concentration in slag. Thus, no method has been proposed that, for example, estimates the proportion of blown oxygen dissolved in molten steel and the oxygen concentration in molten steel in vacuum degassing treatment.

It could therefore be helpful to provide a state estimation method for vacuum degassing treatment, an operation method, a molten steel production method, and a state estimation device for vacuum degassing treatment that are capable of highly accurate state estimation in vacuum degassing treatment.

With the presently disclosed techniques, the sources of the gases that constitute the exhaust gas in vacuum degassing treatment are classified into a plurality of sources including blown oxygen and air entering the vacuumized region in the vacuum degassing line before the start of the vacuum degassing treatment or during the treatment, and the constituent ratio of the classified plurality of sources is estimated. It is thus possible to provide a state estimation method for vacuum degassing treatment, an operation method, a molten steel production method, and a state estimation device for vacuum degassing treatment that are capable of highly accurate state estimation in vacuum degassing treatment.

A state estimation device and state estimation method for vacuum degassing treatment according to an embodiment of the present disclosure will be described below, with reference to the drawings. Although this embodiment describes an example in which the vacuum degassing treatment is RH vacuum degassing treatment performed using an RH vacuum degassing line, the vacuum degassing treatment is not limited to RH vacuum degassing treatment. The below-described state estimation method can also be applied, for example, to vacuum degassing treatment performed using a line that includes a vacuum vessel and only one immersion tube that is immersed in a ladle and sucks up molten steel into the vacuum vessel or a line (apparatus) that includes no vacuum vessel and creates a vacuum state on the surface of molten steel in a ladle.

is a schematic diagram illustrating the structure of a state estimation deviceand a vacuum degassing lineaccording to this embodiment. The state estimation deviceestimates, for example, the internal state of the vacuum degassing lineduring vacuum degassing treatment in the vacuum degassing line. In this embodiment, the vacuum degassing lineis operated by the state estimation deviceexecuting the below-described state estimation method for vacuum degassing treatment. That is, state estimation for vacuum degassing treatment is executed as an operation method for the vacuum degassing line. In this embodiment, the vacuum degassing lineforms part of a molten steel production line. A molten steel production method is executed in the molten steel production line. The molten steel production method includes refining molten steel in the vacuum degassing lineto produce refined molten steel.

The RH vacuum degassing lineincludes a vacuum vesseland a ladlethat are connected to each other by two immersion tubes. The vacuum vesselis connected to an exhaust duct. The gas inside the vacuum vesselis exhausted through the exhaust duct to reduce the pressure in the vacuum vesseland suck up the molten steel in the ladle. Then, inert gas is blown in through a pipefrom one of the immersion tubesto circulate the molten steel between the vacuum vesseland the ladle. Oxygen is blown in from a blowing lanceinstalled in the vacuum vessel. Thus, oxygen can be supplied to the molten steel. The vacuum vesselis an example of a vacuumized region, i.e. a region depressurized to create a vacuum, in the vacuum degassing line. The vacuumized region in the vacuum degassing linealso includes the exhaust ductconnected to the vacuum vessel.

An exhaust gas flowmeterand an exhaust gas component concentration meterare installed inside the exhaust duct. The exhaust gas flowmetermeasures the flow rate of the exhaust gas. The exhaust gas component concentration metermeasures the concentrations of the components in the exhaust gas, including CO gas, COgas, and Ogas.

A vacuum degassing treatment control system to which the state estimation devicefor vacuum degassing treatment is applied includes a control deviceand the state estimation devicefor vacuum degassing treatment as main components. The control deviceis composed of an information processing device such as a computer, and controls operation-related manipulated variables, such as the exhaust volume of the exhaust line, the flow rate of inert gas for circulation, and the flow rate of blown oxygen, so that the component concentrations and temperature of the molten steel will fall within the target ranges after vacuum degassing treatment from the track records before the vacuum degassing treatment. The control devicealso collects operation track record data such as the degree of vacuum in the vacuum vessel, the flow rate of inert gas for circulation, the flow rate of blown oxygen, the flow rate of exhaust gas, and the component concentrations of exhaust gas, and outputs the data to the state estimation device.

As illustrated in, the state estimation deviceincludes an operation information input unit, a calculation unit, and an output unit. The calculation unit is a functional unit that executes calculations to estimate the state of vacuum degassing treatment. In this embodiment, the calculation unit includes an exhaust gas classification calculation unit, a blown oxygen dissolution rate calculation unit, and an in-molten steel oxygen concentration increase calculation unit.

The operation information input unitreceives input of operation track records related to manipulated variables during vacuum degassing treatment and time-series exhaust gas measured values including the flow rate of exhaust gas discharged from the vacuum degassing linethat performs the vacuum degassing treatment and the component concentrations of CO gas, COgas, and Ogas contained in the exhaust gas, as input information.

The exhaust gas classification calculation unitclassifies the sources of the gases that constitute the exhaust gas discharged from the vacuum degassing lineinto a plurality of sources and estimates the constituent ratio of the classified plurality of sources, based on the input information acquired by the operation information input unit. The plurality of sources include blown oxygen and air entering the vacuumized region in the vacuum degassing linebefore the start of the vacuum degassing treatment or during the treatment. The exhaust gas classification calculation unitmay estimate the proportion of Ngas in the exhaust gas from the input information, and calculate the proportion of the air entering the vacuumized region based on the Ngas in the exhaust gas.

The blown oxygen dissolution rate calculation unitestimates the dissolution rate of the blown oxygen in the molten steel based on the constituent ratio of the plurality of sources estimated by the exhaust gas classification calculation unit.

The in-molten steel oxygen concentration increase calculation unitestimates the increase of the oxygen concentration in the molten steel based on the dissolution rate estimated by the blown oxygen dissolution rate calculation unit.

The output unitoutputs the results of the calculations performed by the calculation unit to estimate the state of the vacuum degassing treatment, to the control device. The control devicemay control the operation-related manipulated variables based on the calculation results obtained from the output section.

The state estimation devicefor vacuum degassing treatment is composed of an information processing device such as a computer. The state estimation devicefor vacuum degassing treatment functions as the operation information input unit, the exhaust gas classification calculation unit, the blown oxygen dissolution rate calculation unit, the in-molten steel oxygen concentration increase calculation unit, and the output unitby a processor such as a central processing unit (CPU) in the information processing device executing a computer program.

The state estimation devicefor vacuum degassing treatment having the foregoing structure performs the below-described state estimation process for vacuum degassing treatment to classify the gases that constitute the exhaust gas and estimate the constituent ratio. For a charge in which oxygen blowing is performed during treatment, the dissolution rate of the blown oxygen in the molten steel is estimated from the estimated exhaust gas constituent ratio. The estimation result can then be used to estimate the increase of the oxygen concentration in the molten steel due to the blown oxygen with high accuracy. The operation of the state estimation devicefor vacuum degassing treatment will be described below, with reference to the flowchart in. The following description assumes that oxygen blowing is performed during vacuum degassing treatment.

is a flowchart illustrating the state estimation process for vacuum degassing treatment according to an embodiment of the present disclosure. When an instruction to execute vacuum degassing treatment is input, the flowchart instarts and the state estimation process proceeds to step S.

In step S, the operation information input unitacquires 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 the measurement and analysis results obtained by component analysis. This completes step S, and the state estimation process proceeds to step S.

In step S, the operation information input unitacquires operation track records related to manipulated variables during vacuum degassing treatment. Items necessary for the calculations in the exhaust gas classification calculation unit, blown oxygen dissolution rate calculation unit, and in-molten steel oxygen concentration increase calculation unitare acquired as the operation track records. For example, the operation information input unitacquires the degree of vacuum in the vacuum vessel, the flow rate of inert gas for circulation, and the flow rate of blown oxygen as the operation track records. In this embodiment, the operation information input unitalso acquires, as input information, time-series exhaust gas measured values including the flow rate of exhaust gas and the component concentrations of CO gas, COgas, and Ogas contained in the exhaust gas, together with the operation track records. This completes step S, and the state estimation process proceeds to step S. Steps Sand Scorrespond to the input step.

In step S, the exhaust gas classification calculation unitclassifies the gases that constitute the exhaust gas discharged during the vacuum degassing treatment and estimates the constituent ratio.

In RH vacuum degassing treatment, the supply sources of exhaust gas are classified into the following five types: impurity components contained in molten steel and removed as gas by depressurization, inert gas for circulation, air present in the vacuum vesselbefore the start of vacuum degassing treatment, leakage air entering the vacuumized region (the vacuum vesseland the exhaust duct) during the vacuum degassing treatment, and blown oxygen.

The main impurity components contained in the molten steel are hydrogen, nitrogen, and carbon. In most steel types, the amount of impurity components other than carbon is small enough to be negligible. Since carbon is removed from the molten steel as CO gas, its discharge amount can be obtained by exhaust gas measurement. The inert gas for circulation is an operation-related manipulated variable, and therefore its amount can be obtained.

The air present in the vacuum vesselbefore the start of vacuum degassing treatment and the leakage air entering the vacuumized region during the treatment can be distinguished by calculating the amount of Ncontained in the exhaust gas. Approximately the whole flow rate of the exhaust gas is occupied by CO gas, COgas, Ogas, inert gas blown in to stir the molten steel, and Ngas as components of the exhaust gas. The exhaust volumes of components other than Ngas can be calculated from exhaust gas measurement results or operation control track records. Assuming an unknown component of the exhaust gas as Ngas, its amount can be calculated using the following formula (1).

Here, fis the Nflow rate [Nm/h] in the exhaust gas. fis the flow rate of the exhaust gas [Nm/h]. ris the CO concentration [vol %] in the exhaust gas. ris the COconcentration [vol %] in the exhaust gas. ris the Oconcentration [vol %] in the exhaust gas. fis the flow rate of Ar gas blown in for circulation [Nm/h]. In the case where the measurement results of the flow rate and component concentrations of the exhaust gas contain known errors, it is preferable that the exhaust gas classification calculation unitremoves or reduces the known errors before performing the calculation of formula (1). The known errors are assumed to be, for example, errors such as offsets contained in measured values. If the time taken for the inert gas for circulation to reach the exhaust gas flowmeter is known, it is desirable to use freflecting this time delay in the calculation of formula (1).

In the case where the Nflow rate in the exhaust gas is calculated by formula (1), the air flow rate f[Nm/h] in the exhaust gas can be calculated from the nitrogen abundance ratio in air according to the following formula (2).

Based on track record data of vacuum degassing treatment, I found that, when the vacuum vesselis evacuated to near the ultimate vacuum (target degree of vacuum) in vacuum degassing treatment, the flow rate of air in the exhaust gas calculated by the foregoing formula (2) is approximately constant. Since the degree of vacuum changes little near the ultimate vacuum, the entire amount of air in the exhaust gas can be assumed to derive from leakage during the treatment. Moreover, the flow rate of leakage air is constant if the air pressure in the vacuum vesselis sufficiently low. It is therefore reasonable that the flow rate of air in the exhaust gas is approximately constant.

As described above, the constituent ratio of the four supply sources other than blown oxygen from among the five supply sources constituting the exhaust gas can be quantitatively estimated. The remainder (balance) can then be estimated as exhaust gas that derives from blown oxygen. The constituent ratio of all of the plurality of sources is thus estimated. This completes step S, and the state estimation process proceeds to step S.

In step S, whether the time elapsed from the oxygen blowing end time to the time at which the exhaust gas constituent ratio estimation is performed is longer than or equal to a predetermined reference time T is determined. If the elapsed time after the oxygen blowing is shorter than the reference time, the state estimation process returns to step Sto repeat from step Sonward. If the elapsed time is longer than the reference time, the state estimation process proceeds to step S. The reason for performing such a conditional branching process will be explained in the description of step S.

In step S, the blown oxygen dissolution rate calculation unitestimates the proportion of blown oxygen dissolved in the molten steel.

First, the amount of oxygen in the exhaust gas can be calculated using the following formula (3).

Here, the amount of oxygen in the exhaust gas is evaluated in terms of Ovolumetric flow rate. fis the Oflow rate [Nm/h] in the exhaust gas. In the following, the amount of oxygen in the exhaust gas is equally evaluated in terms of Ovolumetric flow rate.

Of the amount of oxygen in the exhaust gas, the flow rate f[Nm/h] of oxygen derived from decarburization can be calculated using the following formula (4) because it is all supplied to the vacuum vesselas CO.

Of the amount of oxygen in the exhaust gas, the flow rate f[Nm/h] of oxygen derived from incoming air can be calculated from the oxygen abundance ratio in air using the following formula (5).

The only other supply source of oxygen to the exhaust gas is blown oxygen. Hence, of the amount of oxygen in the exhaust gas, the flow rate f[Nm/h] of oxygen derived from oxygen blowing can be calculated using the following formula (6).

Based on track record data of vacuum degassing treatment, I obtained three findings about the temporal changes of f. Firstly, there is a time delay from the start of oxygen blowing until fincreases. Secondly, after the end of oxygen blowing, frapidly converges to 0 after a certain time delay. Thirdly, the time from when the blown oxygen not dissolved in the molten steel is blown into the molten steel to when it is observed by the exhaust gas measurement device cannot be considered constant. The third finding means that the temporal change pattern of fcannot be regarded as the same as the oxygen blowing pattern and it is difficult to continuously estimate the dissolution rate of blown oxygen. The difference in pattern is considered to be because, while part of blown oxygen reacts with CO gas released into the vacuum vesselto form COgas, the time from when oxygen is blown in to when it reaches the exhaust gas measurement device varies depending on the time required for this reaction.