JP4286544B2 - Method and apparatus for forced convection cooling of steel strip in continuous heat treatment equipment - Google Patents

Description

【００１５】
なお、１３は入側板温計、８はガス温度検出器である。また、ガス露点計１０及び水素濃度計１１の順は入れ替わっても特に問題ない。
水素ガス自体は熱伝導率が良く、冷却効果に優れているため、従来のように水素ガス濃度を高くすれば、勿論冷却効果は高くなるが、本発明による方法においては、鋼帯１に噴射する冷却ガスの水素ガス濃度を３０〜７５％の範囲に設定している。ここで、水素濃度が３０％よりも小さいと水又は水蒸気を添加していない従来の水素濃度７０％の冷却ガスよりも冷却効果が下回ってしまう。
【００１６】
又、水素濃度が７５％よりも大きいと、冷却効果は大きくなるが水素使用量が増大し、冷却ガスコスト原単位が悪くなってしまうためである。本発明では、このように冷却ガスの水素ガス濃度を３０〜７５％の範囲に抑えながらも水素ガス分圧Ｐ_H2と水蒸気分圧Ｐ_H2Oの分圧比を一定範囲の関係に保つように冷却ガスに水分又は水蒸気分を含有させることによって鋼帯１を酸化させることなく、高い冷却効果を得ることができる。
即ち、本願発明は種々の検討を重ねた結果、冷却ガスの水素濃度が３０〜７５％であっても、水蒸気分圧Ｐ_H2Oと水素分圧Ｐ_H2の分圧比を（１）式に示すような範囲に保つことで従来の冷却ガスの水素濃度７０〜１００％と同等の冷却効果を得ることができることがわかった。
【００１７】
図２は、本発明の適用領域を鋼帯温度Ｔと（水蒸気分圧Ｐ_H2Oと水素分圧Ｐ_H2の比Ｐ_H2O/ Ｐ_H2）との関係から示した図である。この図２に示すように、冷却ガス中の水蒸気分圧比が高くなると、図中の×印で示されているように鋼帯表面に水蒸気による酸化膜が生じてしまい、酸洗などによる酸化膜除去が必要となるが、本発明によれば、図中点線範囲で示される領域、即ち、水蒸気分圧Ｐ_H2Oを（１）式に示すように水素分圧Ｐ_H2と鋼帯温度Ｔとの関係に保つことで鋼帯の酸化の発生がなく、また極めて高い冷却効果を得ることができる。
【００１８】
（１）式において、Ｐ_H2O ／Ｐ_H2の分圧比の上限値は、鋼帯の表面を酸化させないように設定されており本発明の適用領域から導きだした回帰式である。この上限値は、［０．５６・ｌｎ（Ｔ）−３．２６］若しくは［０．０７］のうち、いずれか最大となる方の値を適用する。即ち、図２からもわかるように、鋼板温度Ｔが低温域にある場合は、表面酸化が実質上問題にならないため、（１）式におけるＰ_H2O ／Ｐ_H2の分圧比の上限値は回帰式で求められる値と［０．０７］のいずれか最大となる方を適用することにより、低温域においても酸化膜の発生を防止しつつ高い冷却効果を得ることができる。
【００１９】
また、下限値は、冷却効果が従来の冷却装置に比べて改善できる範囲となるように０．００１に設定した。ここで、矢印（←）は鋼帯の冷却領域を示し、矢印の右側の○印は冷却開始温度、矢印の左側の○印は冷却終了温度を示している。また、例えば板厚が０．８ｍｍの鋼帯を走行速度が毎分２５０ｍで７００℃から４００℃へ急冷する場合、図１に示す強制対流式冷却装置Ａを３ユニット連結させて冷却することが行われる。第一ユニットでは、鋼帯を７００℃から約５９５℃まで冷却され、引き続き第二ユニットで約４９０℃まで冷却され、第三ユニットでは４００℃まで最終冷却される。このとき、水蒸気分圧Ｐ_H2Oと水素分圧Ｐ_H2の比は、（１）式に基づいて第一ユニットで０．３１、第二ユニットで０．２０、第三ユニットで０．０９以下になるようにそれぞれ水蒸気吹き込み量を調整することによって制御される。
【００２０】
ここで、鋼帯温度は各ユニット出口の鋼帯温度が用いられる。水素濃度は、従来のガスジェット冷却に比べて冷却効果の改善が見込まれるよう３０％以上とし、本発明によれば水蒸気による冷却効果により水素７５％であっても従来の１００％水素冷却と同等の冷却効果が得られることから、水素濃度の上限を７５％とした。これにより必要以上の水蒸気分を含有する必要がなくなるので、ガス循環ダクト系内で水蒸気の一部が不必要に結露することもなくなる。
【００２１】
なお、本発明の冷却ガスは窒素ガス又は窒素ガスと低濃度の水素ガスの混合ガスに比べて、同じ噴出速度の場合には、鋼帯１への衝突力が小さくなり、従って、鋼帯のバタツキやツイスト現象が軽減される。
本発明を実施するにあたっては、前記実公昭６３−２４１１７号公報に示されている如く、ノズル群の先端と鋼帯との間の距離を７０ｍｍ以下とし、さらに、ノズル群の開口面積比率を２〜４％とし、且つ、それぞれのノズル径をノズル群の先端と鋼帯との間の距離の１／５より小さくすることによって、最も冷却効果を上げることができる。
【００２２】
本発明は、鋼帯の連続焼鈍設備における一次冷却帯において従来ガスジェットクーラー方式、気水冷却方式或いはロール冷却方式またはこれらの組み合わせ方式などが採用されていた箇所にこれらの冷却方式に代わる冷却方法として適用することができ、鋼種に応じて鋼帯を２５０℃〜４５０℃の温度まで急冷する際に用いることができる。その際、冷却チャンバーは複数に分割し、鋼帯の温度降下に応じて本発明を適用して水蒸気分圧を調整することができる。また、本発明は連続式溶融亜鉛メッキラインの焼鈍炉やステンレス焼鈍炉、電磁鋼板の熱処理炉等における急冷装置などにも適用することができる。
【００２３】
【発明の効果】
以上述べたように、本発明によれば、冷却ガスに水分又は水蒸気分を含むものの鋼帯を酸化させることがなく、実質非酸化状態で、高価な水素ガス使用量を大きくすることなく高い冷却効果を得ることが可能となる。従って、気水冷却装置や水浸漬冷却装置において必要である冷却後の酸洗等の後処理が不要となる。また、窒素ガスや窒素ガスと低濃度の水素ガスの混合ガスを使用する場合に比較して、同じ噴出速度では鋼帯の噴流衝突に起因するバタツキやツイスト現象を抑えることができるので、ノズル先端と鋼帯との間の距離を従来よりさらに小さく設定することが可能になり、より高い冷却効果を得ることができるようになる。
【００２４】
また、鋼帯を酸化させない範囲で冷却ガスに水分又は水蒸気分を含有させることにより、従来よりも低い水素ガス濃度で高い冷却効果を得ることができ、その結果水素ガス使用量を抑制することが可能であり冷却ガスコスト原単位を悪くすることもない。また、本発明は鋼帯を酸化させることなく高い冷却速度を得ることができるので、冷却後に引き続いて亜鉛メッキを施すような連続式溶融亜鉛めっき設備の冷却帯へ適用することが可能である。
【図面の簡単な説明】
【図１】本発明の強制対流式冷却装置の実施例を示す図である。
【図２】本発明の適用領域を鋼帯温度と（水蒸気分圧と水素分圧の比）との関係から示した図である。
【符号の説明】
１ 鋼帯
２ ガスノズル群
３ 冷却ガスボックス
４ 入口シールロール
５ 出口シールロール
６ ガス冷却器
７ 循環ブロワー
８ ガス温度検出器
９ ミキシングチャンバー
１０ ガス露点計
１１ 水素濃度計
１２ 冷却チャンバー
１３ 入側板温計
１４ 出側板温計
１５ 演算器
１６ 調節弁
１７ 循環経路
１８ 水又は水蒸気の調整供給手段
Ａ 強制対流式冷却装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a forced convection cooling method and apparatus for a steel strip in a continuous heat treatment facility for continuously heat treating a steel strip such as a continuous annealing facility for a steel strip or a continuous hot dip galvanizing line.
[0002]
[Prior art]
Conventionally, for example, in order to increase the primary cooling rate in a continuous annealing facility for steel strips, an air-water cooling device or a water immersion cooling device has been used. However, in these cooling methods, a thin oxide film is generated on the surface of the steel strip. Therefore, it is necessary to remove the oxide film by pickling after annealing. Therefore, in steel strip continuous annealing equipment and continuous hot dip galvanizing line, forced cooling of the steel strip is a forced convection system in which non-oxidizing gas such as nitrogen gas, hydrogen gas, or mixed gas is blown from a plurality of nozzle groups. A cooling device is used.
[0003]
For example, in Japanese Utility Model Publication No. 63-24117 (Patent Document 1), in this forced convection type cooling device, the distance between the tip of the nozzle group and the steel strip is set to 70 mm or less, and the opening area ratio of the nozzle group is set as follows. High cooling capacity and uniform cooling in the width direction of the steel strip can be obtained by setting each nozzle diameter to less than 1/5 of the distance between the tip of the nozzle group and the steel strip. It has been shown that you can.
In Japanese Patent Application No. 5-136252 (Patent Document 2), a plurality of nozzle groups forming the nozzle groups are formed as nozzles each having a protruding tip opening, and a hydrogen gas concentration of 70 to 90% from each nozzle. It has been shown that a higher cooling effect can be obtained by spraying a non-oxidizing gas having the following.
[0004]
[Cited document]
(1) Patent Document 1 (Japanese Utility Model Publication No. 63-24117)
(2) Patent Document 2 (Japanese Patent Application No. 5-136252)
[0005]
[Problems to be solved by the invention]
However, in these conventional forced convection cooling devices, it is necessary to increase the ejection speed of the non-oxidizing gas from the nozzle when obtaining a high cooling effect, and when a high flow rate of cooling gas is injected into the steel strip. There was a fear of fluttering and twisting of the steel strip. Further, even in a circulation type forced convection type cooling device that circulates and uses most of the cooling gas, a non-oxidizing gas may leak from a steel strip inlet or outlet sealing device to adjacent zones before and after the cooling device. Therefore, when using a non-oxidizing gas containing a high concentration of hydrogen gas with a small gas specific gravity in order to suppress fluttering and twisting phenomenon of the steel strip, the amount of hydrogen used increases and the cooling gas cost basic unit deteriorates. There was a problem. Accordingly, the present invention provides a cooling method that can obtain a higher cooling effect than before without oxidizing the steel strip, reduce the amount of hydrogen used, and does not cause a flutter or twist phenomenon of the steel strip. It is intended to propose.
[0006]
The present invention is a forced convection cooling method for a steel strip in a continuous heat treatment facility having a circulation path for circulating and using cooling gas injected into the steel band, and having a gas cooler and a circulation blower in the circulation path. And adjusting the water or water vapor content in the cooling gas in the circulation path, and providing water or water vapor adjusting and supplying means for enhancing the cooling effect of the cooling gas. Hydrogen gas concentration is 30-75%,
[0007]
Partial pressure ratio of water vapor partial pressure P _{H2 O} and the hydrogen partial pressure P _H2 is characterized by being adjusted to the range of the following formula (1).
0.001 ≦ P _H2O / P _H2 ≦ [0.56 · ln (T) −3.26]
Or [0.07] any maximum value (1) where P _H2O : water vapor partial pressure
P _H2 : Hydrogen partial pressure
T: Steel strip temperature (° C)
[0008]
Further, the forced convection cooling device is divided into a plurality of portions, and the water vapor blowing amount is adjusted according to the temperature drop of the steel strip using the steel strip temperature at each outlet.
[0009]
Furthermore, a forced convection cooling device for a steel strip in a continuous heat treatment facility having a circulation path for circulating and using cooling gas injected into the steel band, and having a gas cooler and a circulation blower in the circulation path, In the circulation path, adjusting the water or water vapor content in the cooling gas according to the hydrogen concentration of the gas and the steel strip temperature, as water or water vapor adjustment supply means for enhancing the cooling effect of the cooling gas, wherein an installation moisture or mixing chamber of the steam partial to the downstream side of the circulation blower in the circulation path, the hygrometer and the hydrogen concentration meter installed downstream of the mixing chamber, the dew point data and the hydrogen concentration meter from the dew Of the water vapor partial pressure P _H2O and the hydrogen partial pressure P _{H2 in} the cooling gas based on the hydrogen concentration data from and the plate temperature data from the outlet side thermometer installed on the steel strip outlet side of the forced convection cooling device Consists of an arithmetic unit that calculates moisture or water vapor so that the partial pressure ratio falls within the range of the following formula (1) , and a control valve that adjusts the moisture or water vapor supplied to the mixing chamber in accordance with instructions from the arithmetic unit It is characterized by that.
0.001 ≦ P _H2O / P _H2 ≦ [0.56 · ln (T) −3.26]
Or [0.07], whichever is the maximum value (1)
However, P _H2O : Water vapor partial pressure
P _H2 : Hydrogen partial pressure
T: Steel strip temperature (° C)
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail below with reference to the drawings.
FIG. 1 is a diagram showing an embodiment of the forced convection cooling device of the present invention. As shown in this figure, the steel strip 1 that has been subjected to the heat treatment through the heating step and the soaking step is then cooled in the quenching step. For example, a forced convection cooling device A as shown in FIG. 1 is installed in the rapid cooling process, and the steel strip 1 enters the cooling chamber 12 through the upper seal roll 4 and faces the front and back of the steel strip 1. After being rapidly cooled by the non-oxidizing cooling gas sprayed from the nozzle group 2 provided in this way, it passes through the lower seal roll 5 and is sent to the next process.
[0011]
The forced convection cooling device A has a circulation path 17 for circulating and using cooling gas, and the cooling gas injected into the steel strip 1 is cooled to a predetermined temperature by a gas cooler 6 installed in the circulation path 17. After that, the pressure is raised by the circulation blower 7 and used in circulation. Here, in the present invention, on the downstream side of the circulation blower 7, water or water vapor supply means for adjusting the water or water vapor content in the cooling gas according to the hydrogen concentration of the cooling gas and the steel strip temperature. 18 is provided.
[0012]
Specifically, a mixing chamber 9, a gas dew point meter 10, and a hydrogen concentration meter 11 are installed on the downstream side of the circulation blower 7 in the circulation path 17, and the cooling gas is introduced into the cooling box 3 in the cooling chamber 12. Before starting, the content of moisture or water vapor is adjusted in the mixing chamber 9. This adjustment of moisture or water vapor content is accomplished by measuring the water vapor partial pressure P _H2O obtained by measuring the dew point of the cooling gas with the gas dew point meter 10, and the hydrogen partial pressure P obtained from the hydrogen concentration of the cooling gas measured by the hydrogen concentration meter 11. _This is done by adjusting the amount of water vapor or moisture blown into the mixing chamber 9 by the control valve 16 in accordance with the _H2 and the steel plate temperature T.
[0013]
Calculator 15, the dew point data and the hydrogen concentration meter 11 hydrogen concentration data Contact and forced convection cooling device out installed at the exit side of the steel strip 1 in the A side plate temperature gauge 14 from the from the gas dew 10 based on the sheet temperature data, the water vapor partial pressure P _{H2 O} and partial pressure ratio of hydrogen partial pressure P _H2 in the cooling gas calculates the required moisture or steam partial such that the following ranges (1), regulating valve 16 performs feedback and feedforward control of moisture or water vapor content in the cooling gas.
[0014]
[Equation 3]

[0015]
Reference numeral 13 denotes an inlet side thermometer, and 8 denotes a gas temperature detector. Further, there is no particular problem even if the order of the gas dew point meter 10 and the hydrogen concentration meter 11 is changed.
Since hydrogen gas itself has good thermal conductivity and excellent cooling effect, if the hydrogen gas concentration is increased as in the prior art, the cooling effect is naturally enhanced. However, in the method according to the present invention, the hydrogen gas is injected into the steel strip 1. The hydrogen gas concentration of the cooling gas to be set is set in the range of 30 to 75%. Here, if the hydrogen concentration is smaller than 30%, the cooling effect is lower than that of the conventional cooling gas having a hydrogen concentration of 70% in which water or water vapor is not added.
[0016]
On the other hand, if the hydrogen concentration is higher than 75%, the cooling effect increases, but the amount of hydrogen used increases, and the cooling gas cost unit becomes worse. In the present invention, the cooling gas is maintained so that the partial pressure ratio between the hydrogen gas partial pressure P _H2 and the water vapor partial pressure P _H2O is maintained within a certain range while the hydrogen gas concentration of the cooling gas is suppressed to a range of 30 to 75%. A high cooling effect can be obtained without causing the steel strip 1 to be oxidized by containing water or water vapor.
That is, as a result of various investigations in the present invention, even if the hydrogen concentration of the cooling gas is 30 to 75%, the partial pressure ratio of the water vapor partial pressure P _H2O and the hydrogen partial pressure P _H2 is expressed by the equation (1). It was found that a cooling effect equivalent to the hydrogen concentration of 70 to 100% of the conventional cooling gas can be obtained by keeping the temperature within this range.
[0017]
Figure 2 is a diagram of the application area shown the relationship between the steel strip temperature T (the ratio P _H2O / P _H2 of a steam partial pressure P _{H2 O} and the hydrogen partial pressure P _H2) of the present invention. As shown in FIG. 2, when the water vapor partial pressure ratio in the cooling gas is increased, an oxide film due to water vapor is generated on the surface of the steel strip as indicated by x in the figure, and the oxide film due to pickling or the like. Although removal is required, according to the present invention, the region indicated by the dotted line in the figure, that is, the water vapor partial pressure P _H2O is represented by the hydrogen partial pressure P _H2 and the steel strip temperature T as shown in equation (1). By maintaining the relationship, there is no occurrence of oxidation of the steel strip, and an extremely high cooling effect can be obtained.
[0018]
In the equation (1), the upper limit value of the partial pressure ratio of P _H2O / P _H2 is set so as not to oxidize the surface of the steel strip, and is a regression equation derived from the application area of the present invention. As the upper limit value, the maximum value of [0.56 · ln (T) -3.26] or [0.07] is applied. That is, as can be seen from FIG. 2, when the steel plate temperature T is in a low temperature range, surface oxidation is not a problem. Therefore, the upper limit value of the partial pressure ratio of P _H2O / P _H2 in equation (1) is a regression equation. By applying the maximum value of [0.07] and the value obtained in (1), a high cooling effect can be obtained while preventing the generation of an oxide film even in a low temperature region.
[0019]
The lower limit value was set to 0.001 so that the cooling effect could be improved as compared with the conventional cooling device. Here, the arrow (←) indicates the cooling region of the steel strip, the ○ mark on the right side of the arrow indicates the cooling start temperature, and the ○ mark on the left side of the arrow indicates the cooling end temperature. For example, when a steel strip having a thickness of 0.8 mm is rapidly cooled from 700 ° C. to 400 ° C. at a traveling speed of 250 m / min, three units of the forced convection cooling device A shown in FIG. Done. In the first unit, the steel strip is cooled from 700 ° C. to about 595 ° C., subsequently cooled to about 490 ° C. in the second unit, and finally cooled to 400 ° C. in the third unit. At this time, the ratio of the water vapor partial pressure P _H2O to the hydrogen partial pressure P _H2 is 0.31 for the first unit, 0.20 for the second unit, and 0.09 or less for the third unit based on the equation (1). It is controlled by adjusting the amount of water vapor blown respectively.
[0020]
Here, the steel strip temperature at the outlet of each unit is used. The hydrogen concentration is 30% or more so that the cooling effect is expected to be improved compared to the conventional gas jet cooling, and according to the present invention, even if the hydrogen is 75% due to the cooling effect by water vapor, it is equivalent to the conventional 100% hydrogen cooling. Therefore, the upper limit of the hydrogen concentration was set to 75%. As a result, it is not necessary to contain an excessive amount of water vapor, so that part of the water vapor is not unnecessarily condensed in the gas circulation duct system.
[0021]
Note that the cooling gas of the present invention has a smaller impact force on the steel strip 1 at the same ejection speed as compared with nitrogen gas or a mixed gas of nitrogen gas and low-concentration hydrogen gas. Fluttering and twisting phenomena are reduced.
In carrying out the present invention, as shown in the Japanese Utility Model Publication No. 63-24117, the distance between the tip of the nozzle group and the steel strip is 70 mm or less, and the opening area ratio of the nozzle group is 2 The cooling effect can be maximized by setting the nozzle diameter to ˜4% and making each nozzle diameter smaller than 1/5 of the distance between the tip of the nozzle group and the steel strip.
[0022]
The present invention is a cooling method that replaces these cooling methods in places where a conventional gas jet cooler method, air-water cooling method, roll cooling method, or a combination method thereof has been adopted in a primary cooling zone in a continuous annealing facility for steel strips. And can be used when rapidly cooling a steel strip to a temperature of 250 ° C. to 450 ° C. depending on the steel type. At that time, the cooling chamber can be divided into a plurality of parts, and the water vapor partial pressure can be adjusted by applying the present invention in accordance with the temperature drop of the steel strip. The present invention can also be applied to a rapid cooling apparatus in an annealing furnace of a continuous hot dip galvanizing line, a stainless steel annealing furnace, a heat treatment furnace of an electromagnetic steel sheet, or the like.
[0023]
【The invention's effect】
As described above, according to the present invention, the cooling gas containing moisture or water vapor is not oxidized, and the steel strip is substantially non-oxidized and highly cooled without increasing the amount of expensive hydrogen gas used. An effect can be obtained. Accordingly, post-treatment such as pickling after cooling, which is necessary in the air-water cooling device and the water immersion cooling device, is not necessary. Compared to using nitrogen gas or a mixed gas of nitrogen gas and low-concentration hydrogen gas, the flutter and twist phenomenon caused by jet collision of the steel strip can be suppressed at the same jet velocity, so the nozzle tip It is possible to set the distance between the steel strip and the steel strip even smaller than before, and a higher cooling effect can be obtained.
[0024]
In addition, by containing moisture or water vapor in the cooling gas within a range that does not oxidize the steel strip, it is possible to obtain a higher cooling effect at a lower hydrogen gas concentration than the conventional one, and as a result, the amount of hydrogen gas used can be suppressed. This is possible and does not deteriorate the cooling gas cost unit. Moreover, since this invention can obtain a high cooling rate, without oxidizing a steel strip, it can be applied to the cooling zone of a continuous hot dip galvanization equipment which performs galvanization after cooling.
[Brief description of the drawings]
FIG. 1 is a diagram showing an embodiment of a forced convection cooling device according to the present invention.
FIG. 2 is a diagram showing the application range of the present invention from the relationship between steel strip temperature and (ratio of water vapor partial pressure to hydrogen partial pressure).
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Steel strip 2 Gas nozzle group 3 Cooling gas box 4 Inlet seal roll 5 Outlet seal roll 6 Gas cooler 7 Circulating blower 8 Gas temperature detector 9 Mixing chamber 10 Gas dew point meter 11 Hydrogen concentration meter 12 Cooling chamber 13 Inlet side thermometer 14 Outlet plate thermometer 15 Calculator 16 Control valve 17 Circulation path 18 Water or steam adjustment supply means A Forced convection cooling device

Claims (3)

It has a circulation path for recycling the cooled gas injected into the steel strip, a forced convection cooling method of the steel strip in a continuous heat treatment installation comprising a circulating blower and a gas cooler in the circulation path, the circulation Adjusting the water or water vapor content in the cooling gas in the path, providing water or water vapor adjustment supply means for enhancing the cooling effect of the cooling gas, the hydrogen gas concentration of the cooling gas injected into the steel strip is Forced convection of a steel strip in a continuous heat treatment facility characterized in that the partial pressure ratio of water vapor partial pressure P _H2O and hydrogen partial pressure P _H2 is adjusted to be in the range of the following formula (1) at 30 to 75% Cooling method.
0.001 ≦ P _H2O / P _H2 ≦ [0.56 · ln (T) −3.26]
Or [0.07] either maximum value (1) equation where P _H2O : partial pressure of water vapor P _H2 : partial pressure of hydrogen T: steel strip temperature (° C) 2. The continuous heat treatment equipment according to claim 1, wherein the forced convection cooling device is divided into a plurality of sections, and the water vapor blowing amount is adjusted according to the temperature drop of the steel strip by using the steel strip temperature at each outlet. Forced convection cooling method for steel strips in Japan . A forced convection cooling device for a steel strip in a continuous heat treatment facility having a circulation path for circulatingly using cooling gas injected into the steel band, and having a gas cooler and a circulation blower in the circulation path, wherein the circulation In the path, the content of water or water vapor in the cooling gas is adjusted according to the hydrogen concentration of the gas and the steel strip temperature, and the circulation as water or water vapor adjustment supply means for enhancing the cooling effect of the cooling gas A mixing chamber for moisture or water vapor installed downstream of the circulation blower in the path, a dew point meter and a hydrogen concentration meter installed downstream of the mixing chamber, dew point data from the dew point meter and a hydrogen concentration meter the hydrogen concentration data and on the basis of the sheet temperature data from the installation the delivery side temperature gauge steel home use side of the forced convection cooling device, the water vapor partial pressure P _{H2 O} and partial pressure ratio of hydrogen partial pressure P _H2 in the cooling gas Is composed of a computing unit that calculates the moisture or water vapor content such that is within the range of the following equation (1) , and a control valve that adjusts the moisture or water vapor content supplied to the mixing chamber in accordance with instructions from the computing device. A forced convection cooling device for steel strip in a continuous heat treatment facility.
0.001 ≦ P _H2O / P _H2 ≦ [0.56 · ln (T) −3.26]
Or [0.07], whichever is the maximum value (1)
However, P _H2O : Water vapor partial pressure
P _H2 : Hydrogen partial pressure
T: Steel strip temperature (° C)

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