JPH0717942B2 - Atmosphere control method in continuous annealing furnace - Google Patents

Description

TECHNICAL FIELD The present invention relates to a furnace atmosphere control method in a continuous annealing furnace.

[Conventional technology]

As an annealing furnace used for annealing a steel tube, there is an indirect heating type continuous furnace annealing using radiant heat from a radiant tube. In this annealing furnace, both ends of the furnace body are normally opened, and atmospheric gas is continuously supplied into the furnace so that the inside of the furnace is maintained in a predetermined atmosphere.

By the way, when a steel pipe used for annealing is manufactured hot, a decarburized layer is often formed in the surface layer portion. Such a steel pipe can remove the decarburized layer by making the atmosphere a carburizing atmosphere during annealing.

RX gas is used as the carburizing atmosphere gas.
Japanese Patent No. 156707 discloses a furnace in which both ends of the furnace body are closed,
An in-furnace atmosphere control method is disclosed in which an atmosphere gas obtained by diluting RX gas with N ₂ gas is supplied into the furnace. in this way,
The amount of atmospheric gas N ₂ introduced into the heating zone is increased to suppress the generation of CO ₂ , and a forced flow of atmospheric gas is formed from the furnace outlet side to the furnace inlet side to remove the CO ₂ generated in the heating zone. Discharge sequentially.

[Problems to be Solved by the Invention]

However, in such a furnace atmosphere control method, the decarburized layer formed in the surface layer portion of the steel pipe during the manufacturing process is not sufficiently removed, and in some cases the structure may deteriorate.
This is because this method creates a carburizing atmosphere throughout the furnace.

That is, when the entire furnace is maintained in a carburizing atmosphere, C, which should contribute to carburizing, adheres to the surface of the steel pipe as soot in the first half of the heating zone and the latter half of the cooling zone where the atmospheric temperature is low. The carburization of the surface layer does not proceed and the RX gas is wasted. On the contrary, in the vicinity of the soaking zone where the ambient temperature is high, there is a risk that C may deteriorate the structure of the surface layer.

For this reason, in order to carburize a steel pipe with a decarburized layer formed on the surface layer without any problem during the annealing process, the ambient temperature is neither high nor low, and the intermediate temperature of the cooling zone is from the middle to the first half. Create a strong carburizing atmosphere in the part
Equilibrium or weak decarburization atmosphere should be used in the latter half of the soaking zone where the ambient temperature is high and the heating zone and the cooling zone where the ambient temperature is low.

An object of the present invention is to provide a method for controlling the atmosphere in a continuous annealing furnace in a continuous annealing furnace in which only the intermediate temperature portion in the longitudinal direction of the furnace has a carburizing atmosphere in response to such a request.

[Means for Solving the Problems]

As a method of giving a predetermined atmosphere distribution in the longitudinal direction in the continuous annealing furnace, a method of using a mixed gas of RX gas and NX gas and changing the mixing ratio in each zone in the longitudinal direction of the furnace can be considered. With this method, the densest mixture of RX gas is supplied to the zone in the middle of the cooling zone, and RX becomes more distant from that zone.
If the gas mixing ratio is lowered, an ideal atmosphere distribution should be obtained at the beginning of annealing. However, in the zone where the carburization reaction proceeds, CO ₂ is generated along with the reaction, and the carburizing ability of the atmospheric gas decreases as time elapses from the start of annealing. Therefore, if it is attempted to maintain a predetermined atmosphere distribution even after the annealing start period, the supply amount of the atmosphere gas is remarkably increased, and the gas unit consumption is lowered.

A first method for controlling an atmosphere in a furnace of the present invention is to set a carburizing atmosphere at an intermediate temperature portion in a longitudinal direction of the furnace while reducing the atmosphere gas. In an indirect heating type continuous annealing furnace in which atmospheric gas is independently supplied to each zone in the direction, NX gas is used as the basic atmospheric gas, and at least one that belongs to the cooling zone and has an atmospheric temperature that allows carburization In the zone
The point is that a mixed gas of NX gas and RX gas is supplied, and the supply amount of atmospheric gas in the zone is made larger than the supply amount of atmospheric gas in other zones.

A second furnace atmosphere control method according to the present invention is the first method, in which the O ₂ concentration and CO concentration in the furnace atmosphere are measured for each zone in the furnace longitudinal direction, and each zone is calculated from the measurement results. It is characterized in that the mixing ratio and the supply amount of the mixed gas are controlled so that each carbon pontitial has a predetermined value set in advance in each zone.

[Action]

In the first method of the present invention, RX gas is provided in at least one zone belonging to the cooling zone and having an ambient temperature at which the carburizable temperature is reached.
By supplying a mixed gas with NX gas, the intermediate temperature part can be made into a carburizing atmosphere. The intermediate temperature portion is a portion excluding a portion having a high temperature (soaking zone) and a portion having a low temperature (the latter half of the heating zone and the cooling zone). By supplying the mixed gas to the carburizable temperature portion of the cooling zone, the intermediate temperature portion becomes a carburizing atmosphere because there is substantially no high temperature portion in the cooling zone, and it seems that the latter half of the cooling zone. This is because carburization does not proceed in such a low temperature portion, so that the temperature portion where carburization is possible in the cooling zone is nothing but the intermediate temperature portion. Moreover,
Since the gas supply amount in the zone is made larger than the gas supply amounts in the other zones, a gas flow from the zone to both the furnace inlet and the furnace outlet is formed. as a result,
The CO ₂ generated by the carburization reaction goes to the furnace inlet side and the furnace outlet side, and the predetermined carburizing atmosphere is maintained near the zone where the gas flow branches, and the atmosphere becomes more distant from the zone. The carburizing ability of gas gradually decreases.
Thus, an ideal atmosphere distribution in which only the intermediate temperature portion in the longitudinal direction of the furnace becomes the carburizing atmosphere, and the carburizing ability decreases with distance from the portion, is obtained with a relatively small amount of gas supply.

In the second method of the present invention, the carbon potential is controlled to a predetermined distribution in the longitudinal direction in the carbon potential furnace, which serves as an index of carburizing ability.

〔Example〕

 Examples of the present invention will be described below.

A continuous annealing furnace with a total length of 60 m, a sectional view of 5 m ²² , divided into 9 zones in the longitudinal direction of the furnace, with both ends open. When the steel pipe was annealed at a transfer speed of 1 m / H in the furnace, a mixed gas of NX gas and RX gas was supplied to the No. 5, 6, and 7 zones, and only NX gas was supplied to the other zones.

The RX gas ratio in the mixed gas was 20% by volume in the No. 5 zone, 30% by volume in the No. 6 zone, and 50% by volume in the No. 7 zone, as shown in FIG. The total amount of gas supplied was 300 N ³ / H, which was distributed to each zone at the ratio shown in FIG. 1 (b).

That is, the supply amount was concentrated in the No. 5 to 7 zones where the mixed gas was supplied, and the maximum amount of the ambient gas was supplied to the No. 7 zone in which the RX gas ratio was the maximum. As a result, the furnace gas flowed from the No. 7 zone to both the furnace inlet and the furnace outlet. The heat pattern at this time is shown in FIG. 2, and the carbon potential distribution is shown in FIG.

As can be seen from FIGS. 2 and 3, the atmosphere in the furnace is
The No. 7 zone located in the middle of the cooling zone shows the strongest carburizing ability, the carburizing ability decreases as it moves away from the No. 7 zone, and the equilibrium or weak decarburizing atmosphere is established in the No. 1 to 5 zones. It was As a result, the decarburized layer that had been formed to an average depth of 0.3 mm before annealing was significantly reduced to an average depth of 0.1 mm or less after annealing. Further, it was also confirmed that the adverse effect of C on the surface layer structure was not observed, while the RX gas was effectively used without soot.

FIG. 4 is a block diagram of a control system suitable for carrying out the second method of the present invention.

RX gas is supplied to each zone of the furnace body 1 through the control valve 2 and NX gas is supplied through the control valve 3. Further, the CO concentration and the O ₂ concentration of each zone atmosphere are measured by the measuring instrument 4 and the measuring instrument 5, respectively. The instrument 4 for measuring the CO concentration is, for example, an infrared CO analyzer, and the instrument 5 for measuring the O ₂ concentration is, for example, a trace oxygen concentration meter using a zirconia solid electrolyte.

The measured CO concentration and O ₂ concentration are input to the atmospheric gas supply control device 6, where the carbon potential Pc is calculated by the following equation.

Pco: CO concentration Po ₂ : Trace oxygen concentration K ₁ : Equilibrium constant K ₂ : Correction coefficient The carbon potential Pc calculated value is compared with the target value in the relevant zone, and the supply amount required to match the calculated value with the target value And control valves 2, 3
Control the opening of. Do this for all zones,
Further, by repeating during the annealing operation, a furnace atmosphere distribution suitable for carburizing in which only a part in the furnace longitudinal direction is a carburizing atmosphere can be obtained with high accuracy throughout the operation.

〔The invention's effect〕

As is clear from the above description, the first furnace atmosphere control method of the present invention achieves effective use of C by reducing the gas consumption rate by making only the intermediate temperature portion in the furnace longitudinal direction a carburizing atmosphere. Can be reduced. Further, it prevents the adverse effect of C on the structure and guarantees excellent annealing quality.

The second furnace atmosphere control method of the present invention enables highly accurate atmosphere control throughout the entire annealing operation by controlling the carbon Pontial distribution in the furnace longitudinal direction with high accuracy.

[Brief description of drawings]

FIGS. 1 (a) and 1 (b) are charts showing the RX gas ratio and gas supply amount for each zone in the embodiment of the first method of the present invention, and FIGS. 2 and 3 are the same heat pattern and carbon potential. FIG. 4 is a block diagram of a control system suitable for carrying out the second method of the present invention. 1: Furnace body, 2, 3: Control valve, 4,5: Instrument, 6: Atmosphere gas supply control device.

Claims (2)

[Claims] 1. In an indirect heating type continuous annealing furnace in which both ends of a furnace are opened and an atmospheric gas is independently supplied to each zone in the longitudinal direction of the furnace, NX gas is used as a basic atmospheric gas and cooling is performed. A mixed gas of NX gas and RX gas is supplied to at least one zone that belongs to the zone and the atmospheric temperature is a temperature at which carburization is possible, and the amount of atmospheric gas supplied in that zone is set larger than the amount of atmospheric gas supplied in other zones. A method for controlling an in-furnace atmosphere in a continuous annealing furnace, wherein a carburizing atmosphere is provided only in an intermediate temperature portion in a longitudinal direction of the in-furnace. 2. The furnace atmosphere control method for a continuous annealing furnace according to claim 1, wherein the O ₂ concentration and the CO concentration in the furnace atmosphere are measured for each zone in the furnace longitudinal direction and calculated from the measurement results. A method for controlling an atmosphere in a furnace, characterized in that the mixing ratio and the supply amount of the mixed gas are controlled so that the carbon potential of each zone becomes a predetermined value set in advance in each zone.