JP2010132997A - Method for controlling atmosphere in heat treating furnace - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for controlling atmosphere in a heat treating furnace by which an index value of carbon potential in furnace atmospheric gas can be highly accurately maintained at a reference value without generating hunching caused by CO<SB>2</SB>concentration. <P>SOLUTION: The method for controlling the atmosphere is provided by which the index value of the carbon potential in the furnace is controlled to the reference value by adjusting at least endothermic converted gas among the endothermic converted gas and nitrogen gas supplied into the furnace. In the method, when the CO<SB>2</SB>concentration in the furnace is decreased to a prescribed set value SV or lower, the air is fed into the furnace, and on the basis of a total control output S obtained by adding a feed-forward control output according to feeding amount of the endothermic converted gas and the N<SB>2</SB>gas into the furnace to a feed-back control output according to the deviation between the set value SV and an detected value of the CO<SB>2</SB>concentration, the feeding amount of the air is adjusted and thereby the CO<SB>2</SB>concentration is controlled using the set value SV as the target value, and thereafter, when the CO<SB>2</SB>concentration is increased to a value larger than the set value SV by the prescribed value or more, the feeding of the air is stopped to maintain the CO<SB>2</SB>concentration at the set value SV or higher. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for controlling the atmosphere in a furnace during heat treatment of a steel material in a batch-type heat treatment furnace.

Conventionally, in a batch-type heat treatment furnace, the composition of the atmospheric gas in the furnace at the time of heat treatment of the steel material is generally managed by the index value PF of the carbon potential according to the heat treatment content such as carburization. This index value PF is a value determined by the ratio of the CO ₂ concentration (%) in the furnace atmosphere gas to the square of the CO concentration (%) as shown in the following equation (1).
PF = (CO) ² / CO ₂ (1)

When the heat treatment furnace is operated, the measured value of the PF value calculated based on the CO ₂ concentration in the atmospheric gas in the furnace and the detected value of the CO concentration matches the reference value of the PF value predetermined according to the heat treatment content. Thus, the supply amount of the carbon potential adjusting gas such as the endothermic metamorphic gas or N ₂ gas fed into the furnace is adjusted. However, as is clear from the above equation, when the CO ₂ concentration in the furnace is decreased, the measured value of the index value PF is greatly increased and the control becomes hypersensitive, which causes a hunting phenomenon in the PF control. Therefore, as a solution to this problem, when the CO ₂ concentration is lowered, hunting of the PF value is prevented by adjusting the gas supply amount using the control gain that is lowered corresponding to the lowered CO ₂ concentration. A method is described in US Pat.
Japanese Patent Publication No. 7-5958

However, even with the control method described in Patent Document 1, if the CO ₂ concentration in the furnace is reduced to a slight value (for example, 0.2%), it is possible to sufficiently avoid a large fluctuation in the measured value of the PF value. In addition to the resulting hunting, there is a limit to the detection accuracy of the CO ₂ concentration detector (CO ₂ analyzer), so the calculation accuracy of the PF value becomes inaccurate, and a steel material in the desired heat treatment state is obtained. It had the problem that it was not possible.

The present invention has been made in view of the above points, and is a heat treatment furnace capable of accurately maintaining the index value of the carbon potential of the atmosphere gas in the furnace at a reference value without causing hunting due to CO ₂ concentration. An object is to provide an atmosphere control method.

In order to achieve the above object, an atmosphere control method for a heat treatment furnace according to the present invention provides an index value of a carbon potential in a heat treatment furnace, wherein at least an endothermic type modified gas amount of an endothermic type transformed gas and nitrogen gas supplied into the furnace. In the atmosphere control method of the heat treatment furnace that controls to a reference value by adjusting the air, when the CO ₂ concentration in the furnace falls below a predetermined set value, air is fed into the furnace, and the set value and the CO ₂ Based on a total control output obtained by adding a feedforward control output corresponding to the amount of feed of the endothermic modified gas and N ₂ gas into the furnace to a feedback control output corresponding to the deviation of the detected concentration value, the air the feed of CO ₂ concentration by controlling the setting value as a target value by adjusting the Iriryou, that thereafter the CO ₂ concentration to stop the delivery of the air when it becomes larger than a predetermined value than the set value Ri, and maintains the CO ₂ concentration than the set value.

With this configuration, the first by oxygen in the air CO ₂ concentration in the furnace is fed into the furnace when drops below a predetermined set value, to oxidize the CO gas in the furnace CO ₂ Gas can be generated, and the CO ₂ concentration can be prevented from decreasing below the set value. And in addition to the feedback control of the CO ₂ concentration, by adjusting the feeding of air Iriryou by the feed-forward control in accordance with the input amount fed into the furnace of the endothermic type converted gas and N ₂ gas, CO ₂ concentration accurately well is maintained at the set value, since the set value of CO ₂ concentration in addition to can be selected in the range corresponding to the detection accuracy of the CO ₂ concentration detector, also becomes correct operation accuracy of the PF value, the carbon potential The index value can be accurately maintained at the reference value without causing hunting. When the CO ₂ concentration exceeds the set value by a predetermined value or more, by stopping the air supply to the furnace, no endothermic gas or N ₂ gas due to excessive air supply is required. Therefore, it is possible to avoid the inflow control.

As described above, according to the present invention, the index value of the carbon potential of the furnace atmosphere gas can be accurately maintained at the reference value without causing hunting due to the CO ₂ concentration, and the desired heat treatment state can be maintained. Steel material can be obtained.

Hereinafter, an embodiment of the present invention will be described with reference to an example shown in FIGS. In FIG. 1, reference numeral 1 denotes a batch type heat treatment furnace, which has a well-known furnace heating means and heats a workpiece W made of steel in a predetermined furnace atmosphere gas, and is connected to the furnace body. The gas feed pipe ₂ is connected with an endothermic modified gas supply pipe 3, an N ₂ gas supply pipe 4, 5 and an air supply pipe 6, and an exhaust damper 8 is connected to the gas delivery pipe 7 connected to the furnace body. Is connected.

The N ₂ gas supply pipe 5 supplies a constant amount Q ₀ of N ₂ gas into the furnace through the on-off valve 10 for purging the furnace and maintaining the furnace pressure. On the other hand, the endothermic modified gas supply pipe 3 is provided with a flow rate adjustment valve 11, and the N ₂ gas supply pipe 4 is provided with a flow rate adjustment valve 12.

During furnace atmosphere control, it receives the measurement signal of the CO concentration in the furnace measurement (%) and CO ₂ concentration in the furnace as measured by CO ₂ analyzer 14 (%) by CO analyzer 13, the furnace PF The calculation / instruction controller 15 calculates the index value (PF) of the carbon potential shown in the above formula (1) and selects it in advance according to the furnace temperature in the heat treatment furnace 1 and the heat treatment contents / process of the workpiece W. The control output MV ₀ for controlling each flow rate control valve opening is generated so that the calculated actual PF value follows the reference value of the index value of the carbon potential (PF setting pattern shown in FIG. 2). The control outputs are input to the flow rate control valves 11 and 12 by the control valve output converters 16 and 17 as the endothermic modified gas control valve opening control output MV ₁ and the N ₂ gas control valve opening control output MV ₂ , respectively. Both of these By controlling the flow rate of the endothermic type converted gas and N ₂ gas is supplied into the heat treatment furnace 1 via a regulating valve, PF control of furnace atmosphere is performed.

On the other hand, an air flow rate adjusting valve 21 is provided in the middle of the air supply pipe 6 connected to an air supply source such as a blower (not shown), and the opening degree of the air flow rate adjusting valve 21 is determined as a CO ₂ control period determining factor. 22, the CO ₂ indicating controller 23 for feedback control, and the CO ₂ control means 20 including the feedforward amount calculator 24, thereby adjusting the CO ₂ concentration in the furnace to a set value SV (for example, 0.5 %) Or more.

The control operation will be described below with reference to FIGS. 1 and 2. When the workpiece W is heat-treated in the heat treatment furnace 1, the opening control of the flow rate control valves 11 and 12 by the in-furnace PF calculation / instruction controller 15 is performed. in the PF control period for controlling the PF value of the furnace PF setting pattern (reference value) shown in FIG. 2, when the CO ₂ concentration in the furnace vary according to the "behavior of CO _2" curve in Figure 2, CO ₂ When the detected value of the CO ₂ concentration in the furnace by the analyzer 14 falls below a predetermined set value SV (for example, 0.5%), the CO ₂ control period determining element 22 that incorporates this set value SV as a comparison value. Outputs a CO ₂ control start command to the switch 25 connected to the output unit of the CO ₂ indicating controller 23 and the feedforward amount calculator 24, respectively.

CO ₂ instruction adjusting meter 23 outputs and the set value SV, the feedback control output Sb corresponding to the deviation of the detected value of CO ₂ concentration by the CO ₂ analyzer 14. On the other hand, in the feedforward quantity calculator 24, endothermic type converted gas flow rate value Q _{1 (industrial} by flow rate conversion element 26 receives the endothermic type converted gas control valve position control output MV ₁ input to the flow rate adjusting valve 11 The N ₂ gas control valve opening control output MV ₂ that is also input to the flow rate control valve 12 and converted to the N ₂ gas flow rate value Q ₂ by the flow rate conversion element 27. The amount of change ΔQ of the total gas supply amount Qt obtained by adding the N ₂ gas constant flow rate value Q ₀ of the N ₂ gas supply pipe 5 for supplying the constant flow rate N ₂ gas, and the FF gain Kf selected in advance by simulation or the like Based on the above, the feedforward control output Sf _n is calculated and output by the following equation (2) at a predetermined frequency (for example, every second) by the FF output conversion and gain element 28, and this is fed to the feed The opening degree of the air flow rate adjustment valve 21 is adjusted by the total control output S obtained by adding to the back control output Sb.
Sf _n = Kf × ΔQ / (Q _R + Q _N + Q ₀ ) + Sf _{n−1
= Kf × (Qt n -Qt n} -1) / (Q R + Q N + Q 0) + Sf n-1 ...... (2)
Sf _n = current FF control output amount Sf _n-1 = previous FF control output amount Qt _n = current total air supply gas amount Qt _n-1 = previous total air supply gas amount Q _R = flow rate control valve 11 Endothermic modified gas flow rate at the time of maximum opening (MV ₁ = 100%) Q _N = N ₂ gas flow rate at the time of maximum opening of the flow control valve 12 (MV ₂ = 100%)

By sending air into the heat treatment furnace 1 through the air supply pipe 6, the CO gas in the furnace is oxidized and CO ₂ gas is generated, and the CO ₂ concentration is prevented from lowering below the set value SV. . Then, the feedback control by the CO ₂ control means 20 and the feed-in control according to the feed amount of the endothermic modified gas and N ₂ gas having a strong correlation between the feed amount of air and the feed amount of air into the furnace. By adjusting the amount, the CO ₂ concentration in the furnace is accurately maintained at the SV value as shown in FIG. 2, and it is avoided that the CO ₂ concentration becomes a small value and the PF value fluctuates greatly. By selecting the set value SV within an allowable range on the detection accuracy of the CO ₂ analyzer 14, the calculation accuracy of the PF value becomes accurate. As a result, the PF value in the furnace can be accurately maintained at the reference value without causing hunting due to the CO ₂ concentration, and the workpiece W in a desired heat treatment state can be obtained.

Then, CO ₂ concentration detected by the CO ₂ analyzer 14 is raised from the SV value, when it becomes a predetermined value alpha (e.g. 0.2%) or more larger than the set value SV as shown in Figure 2, CO ₂ controller to exit the CO ₂ concentration control by 20 to stop the delivery of air into the furnace by the air supply pipe 6. This avoids unnecessary control of the amount of incoming heat, such as the endothermic modified gas or N ₂ gas, due to excessive air supply.

The present invention is not limited to the above example. For example, when the PF control of the furnace atmosphere is completed within the above-described CO ₂ control period, the CO ₂ control is also terminated at the end of the PF control and the air supply is performed. The entrance shall be stopped. The above example controls the flow rate of both the endothermic modified gas and N ₂ gas during PF control. N ₂ gas controls only the flow rate of the endothermic modified gas as a constant flow rate. The present invention can also be applied to an atmosphere control method for a heat treatment furnace that performs PF control.

It is a control system diagram of a heat treatment furnace showing an example of an embodiment of the present invention. FIG. ₂ is a diagram showing the relationship between the elapse of time in the heat treatment process by the heat treatment furnace of FIG.

Explanation of symbols

1 ... heat treatment furnace 3 ... endothermic type converted gas supply pipe, 4 ... N ₂ gas feed pipe, 5 ... N ₂ gas feed pipe, 6 ... air supply pipe, 11 ... flow control valve, 12 ... flow control valve, 13 ... CO analyzer, 14 ... CO ₂ analyzer, 15 ... In-furnace PF calculation / indication controller, 20 ... CO ₂ control means, 21 ... Air flow control valve, 22 ... CO ₂ control period determining element, 23 ... CO ₂ indication Controller, 24 ... feed forward amount calculator, W ... processed object.

Claims (1)

In the atmosphere control method of the heat treatment furnace, the index value of the carbon potential in the heat treatment furnace is controlled to a reference value by adjusting at least the amount of the endothermic modification gas and nitrogen gas supplied into the furnace.
When the CO ₂ concentration in the furnace falls below a predetermined set value, air is sent into the furnace,
Total control output obtained by adding a feedforward control output corresponding to the amount of the endothermic modified gas and N ₂ gas fed into the furnace to a feedback control output corresponding to a deviation between the set value and a detected value of the CO ₂ concentration Based on the control, the CO ₂ concentration is controlled with the set value as a target value by adjusting the amount of air fed,
Thereafter, when the CO ₂ concentration becomes larger than the set value by a predetermined value or more, the atmosphere control method for the heat treatment furnace is characterized in that the CO ₂ concentration is maintained at the set value or more by stopping the air supply. .

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