JP5915569B2 - Method for producing hot dip galvanized steel sheet and continuous hot dip galvanizing apparatus - Google Patents

Method for producing hot dip galvanized steel sheet and continuous hot dip galvanizing apparatus Download PDF

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JP5915569B2
JP5915569B2 JP2013040207A JP2013040207A JP5915569B2 JP 5915569 B2 JP5915569 B2 JP 5915569B2 JP 2013040207 A JP2013040207 A JP 2013040207A JP 2013040207 A JP2013040207 A JP 2013040207A JP 5915569 B2 JP5915569 B2 JP 5915569B2
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steel sheet
air
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steel plate
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JP2014169465A (en
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玄太郎 武田
玄太郎 武田
高橋　秀行
秀行 高橋
田中　稔
稔 田中
鈴木　克一
克一 鈴木
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JFE Steel Corp
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Priority to PCT/JP2014/001022 priority patent/WO2014132638A1/en
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23D—BURNERS
    • F23D91/00—Burners specially adapted for specific applications, not otherwise provided for
    • F23D91/02—Burners specially adapted for specific applications, not otherwise provided for for use in particular heating operations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/54—Furnaces for treating strips or wire
    • C21D9/56—Continuous furnaces for strip or wire
    • C21D9/561—Continuous furnaces for strip or wire with a controlled atmosphere or vacuum
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/54—Furnaces for treating strips or wire
    • C21D9/56—Continuous furnaces for strip or wire
    • C21D9/562—Details
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23D—BURNERS
    • F23D14/00—Burners for combustion of a gas, e.g. of a gas stored under pressure as a liquid
    • F23D14/46—Details, e.g. noise reduction means
    • F23D14/68—Treating the combustion air or gas, e.g. by filtering, or moistening
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23D—BURNERS
    • F23D23/00—Assemblies of two or more burners
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23N—REGULATING OR CONTROLLING COMBUSTION
    • F23N1/00—Regulating fuel supply
    • F23N1/02—Regulating fuel supply conjointly with air supply
    • F23N1/022—Regulating fuel supply conjointly with air supply using electronic means
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23N—REGULATING OR CONTROLLING COMBUSTION
    • F23N2225/00—Measuring
    • F23N2225/26—Measuring humidity
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Description

本発明は、焼鈍炉に直火型加熱炉を備えた溶融亜鉛めっき鋼板の製造方法および連続溶融亜鉛めっき装置に関するものである。   The present invention relates to a method for producing a hot-dip galvanized steel sheet having a direct-fired heating furnace in an annealing furnace and a continuous hot-dip galvanizing apparatus.

近年、自動車、家電、建材等の分野において、構造物の軽量化等に利用可能な高張力鋼板（ハイテン鋼材）の需要が高まっている。ハイテン鋼材としては、例えば、鋼中にＳｉを含有することにより穴広げ性の良好な鋼板や、また、ＳｉやＡｌを含有することにより残留γが形成しやすく延性の良好な鋼板を得られることがわかっている。   In recent years, in fields such as automobiles, home appliances, and building materials, there is an increasing demand for high-tensile steel sheets (high-tensile steel materials) that can be used for reducing the weight of structures. As high-tensile steel materials, for example, steel sheets with good hole expansibility by containing Si in the steel, and steel sheets with good ductility can easily be formed by containing Si and Al, and residual γ is easily formed. I know.

しかし、Ｓｉを多量に含有する高強度鋼板を母材とする溶融亜鉛めっき鋼板及び合金化溶融亜鉛めっき鋼板を製造する場合、以下の問題がある。溶融亜鉛めっき鋼板は非酸化性雰囲気中あるいは還元雰囲気中で６００〜９００℃程度の温度で加熱焼鈍を行った後に、溶融亜鉛めっき処理を行う。しかし、鋼中のＳｉは易酸化性元素であり、一般的に用いられる非酸化性雰囲気中あるいは還元雰囲気中でも選択酸化されて、表面に濃化し酸化物を形成する。この酸化物は、めっき処理時の溶融亜鉛との濡れ性を低下させて不めっきを生じさせるので、鋼中Ｓｉ濃度の増加と共に濡れ性が急激に低下し不めっきが多発する。また、不めっきに至らなかった場合でも、めっき密着性に劣るという問題がある。さらに、鋼中のＳｉが選択酸化されて表面に濃化すると、溶融亜鉛めっき後の合金化過程において著しい合金化遅延が生じる。その結果、生産性を著しく阻害する。生産性を確保するために過剰に高温で合金化処理しようとすると、耐パウダリング性の劣化を招くという問題もあり、高い生産性と良好な耐パウダリング性を両立させることは困難である。   However, when producing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet that use a high-strength steel sheet containing a large amount of Si as a base material, there are the following problems. The hot dip galvanized steel sheet is subjected to hot dip galvanizing treatment after heat annealing at a temperature of about 600 to 900 ° C. in a non-oxidizing atmosphere or a reducing atmosphere. However, Si in steel is an easily oxidizable element, and is selectively oxidized in a generally used non-oxidizing atmosphere or reducing atmosphere to concentrate on the surface to form an oxide. This oxide lowers the wettability with molten zinc during the plating process and causes non-plating. Therefore, the wettability rapidly decreases as the Si concentration in the steel increases, and non-plating occurs frequently. In addition, even when non-plating does not occur, there is a problem that the plating adhesion is poor. Further, when Si in the steel is selectively oxidized and concentrated on the surface, a significant alloying delay occurs in the alloying process after hot dip galvanizing. As a result, productivity is significantly inhibited. If an alloying treatment is attempted at an excessively high temperature in order to ensure productivity, there is a problem that the powdering resistance is deteriorated, and it is difficult to achieve both high productivity and good powdering resistance.

このような問題に対して、例えば、特許文献１および２には、直火型加熱炉（ＤＦＦ）あるいは無酸化炉（ＮＯＦ）を用いて、鋼板表面を一旦酸化させた後、還元帯で還元することでＳｉを内部酸化させ、Ｓｉ表面濃化を抑制し、溶融亜鉛めっき濡れ性および密着性を向上させる方法が開示されている。   For such problems, for example, Patent Documents 1 and 2 disclose that a steel sheet surface is once oxidized using a direct-fired heating furnace (DFF) or a non-oxidizing furnace (NOF) and then reduced in a reduction zone. Thus, a method is disclosed in which Si is internally oxidized, Si surface concentration is suppressed, and wettability and adhesion of hot dip galvanizing are improved.

特開２０１０−２０２９５９号公報JP 2010-202959 A 特開２０１１−１１７０６９号公報JP 2011-1117069 A

しかし、特許文献１および２に記載の方法では、所望のＦｅ酸化量を確保するために、直火型加熱炉出口温度を７００℃程度（少なくとも６５０℃以上）の高温にすることが必須条件となっている。また、通常、直火型加熱炉能力には制約があり、生産性の指標ＳＴ（＝ライン速度［ｍ／ｍｉｎ］×板厚［ｍｍ］）を用いると、例えば、直火型加熱炉出口温度５５０℃であれば、ＳＴ＝２５０、６５０℃であればＳＴ＝１４０、７００℃であればＳＴ＝１２０などと表すことができる。このような直火型加熱炉では、１．６ｍｍの鋼板の場合、通常は最大１００ｍ／ｍｉｎまでライン速度を上げられるのに対し、Ｓｉ添加鋼では６２．５〜７５ｍ／ｍｉｎ程度のライン速度に留まることになり、生産性が著しく低下することになる。予め、直火型加熱炉の炉長を長くする方法もあるが、建設済みの設備の場合、延長は困難であるし、新設であっても、本来コンパクトなスペースで高い昇温速度が得られるのが特徴の加熱炉であるので、炉長増加は非常にコストがかかる。また、特許文献２に記載の方法には、最適な雰囲気含有水蒸気分圧等が示されている。具体的には、加熱炉内燃焼後雰囲気の水分量１〜５０％が好適範囲とされているが、その限定根拠は明確ではないし、その制御方法も示されていない。例えば、投入する空気の露点が、通常大気雰囲気で露点−１０℃〜３０℃（水分量０．２５７〜４．５３％）で変動したとすると、コークスガス燃焼後の炉内水分量は２０〜２４％程度である。しかし、実際の操業においては気温、天候によって投入ガスの湿分は変更し、Ｆｅ酸化量が変化するので、Ｆｅ酸化量の制御が難しく、最適な直火型加熱炉出口温度は刻々と変化するのが実態である。   However, in the methods described in Patent Documents 1 and 2, in order to secure a desired amount of Fe oxidation, it is an essential condition that the outlet temperature of the direct flame furnace is about 700 ° C. (at least 650 ° C. or higher). It has become. In addition, there is usually a limitation on the direct-fired heating furnace capacity, and when the productivity index ST (= line speed [m / min] × plate thickness [mm]) is used, for example, the outlet temperature of the direct-fired heating furnace If it is 550 ° C., ST = 250, if it is 650 ° C., ST = 140, if it is 700 ° C., it can be expressed as ST = 120. In such a direct-fired heating furnace, in the case of a 1.6 mm steel plate, the line speed is usually increased up to a maximum of 100 m / min, whereas in a Si-added steel, the line speed is about 62.5 to 75 m / min. Productivity will be significantly reduced. There is a method to increase the length of the direct-fired heating furnace in advance, but it is difficult to extend in the case of already constructed equipment, and even if it is newly installed, a high heating rate can be obtained in an originally compact space Since this is a characteristic heating furnace, increasing the furnace length is very costly. Further, the method described in Patent Document 2 shows an optimum atmosphere-containing water vapor partial pressure and the like. Specifically, the moisture content in the atmosphere after combustion in the heating furnace is 1 to 50%, but the reason for limitation is not clear and the control method is not shown. For example, assuming that the dew point of the input air fluctuates at a dew point of −10 ° C. to 30 ° C. (water content of 0.257 to 4.53%) in a normal air atmosphere, the water content in the furnace after coke gas combustion is 20 to 20%. It is about 24%. However, in actual operation, the moisture content of the input gas changes depending on the temperature and weather, and the amount of Fe oxidation changes. Therefore, it is difficult to control the amount of Fe oxidation, and the optimum direct-fired heating furnace outlet temperature changes every moment. Is the actual situation.

本発明は、かかる事情に鑑み、Ｓｉ添加鋼であってもめっき外観に優れる溶融亜鉛めっき鋼板を得ることができ、生産性の高い溶融亜鉛めっき鋼板の製造方法および連続溶融亜鉛めっき装置を提供することを目的とする。   In view of such circumstances, the present invention can provide a hot-dip galvanized steel sheet that is excellent in plating appearance even with Si-added steel, and provides a high-productivity hot-dip galvanized steel sheet manufacturing method and continuous hot-dip galvanizing apparatus. For the purpose.

上記課題を解決するための本発明の要旨は、以下のとおりである。
［１］鋼板面にバーナーを対向配置した直火型加熱炉を備えた連続溶融亜鉛めっき装置を用いて溶融亜鉛めっき鋼板を製造する際に、前記バーナーに投入するガスの露点を調整することを特徴とする溶融亜鉛めっき鋼板の製造方法。
［２］［１］に記載の溶融亜鉛めっき鋼板の製造方法において、投入するガスの露点を４０℃〜８０℃とすることを特徴とする溶融亜鉛めっき鋼板の製造方法。
［３］［１］または［２］に記載の溶融亜鉛めっき鋼板の製造方法において、投入するガスは燃料ガスと空気であって、前記空気の露点を調整することを特徴とする溶融亜鉛めっき鋼板の製造方法。
［４］［２］または［３］のいずれかに記載の溶融亜鉛めっき鋼板の製造方法において、鋼板長手方向に燃焼率および空気比を独立に制御可能な複数のバーナー群を用いて、鋼板移動方向最下流のバーナー群以外のバーナー群の燃料ガスおよびまたは空気の露点を４０〜８０℃とし、かつ、空気比１．０以上１．５以下の燃焼を行ない、鋼板移動方向最下流のバーナー群は空気比０．５以上０．９５以下の燃焼を行うことを特徴とする溶融亜鉛めっき鋼板の製造方法。
［５］鋼板面に対向してバーナーを複数配置した直火型加熱炉を備えた連続溶融亜鉛めっき装置において、前記バーナーを、鋼板長手方向に燃焼率及び空気比を独立に制御可能な複数のバーナー群に分け、鋼板移動方向最下流のバーナー群以外のバーナー群のバーナーには、露点４０〜８０℃の範囲で任意に調湿された空気を投入する調湿装置を備え、かつ鋼板移動方向最下流のバーナー群以外のバーナーは空気比１．０以上１．５未満での燃焼又は燃焼停止の選択が自在であり、鋼板移動方向最下流のバーナー群のバーナーは空気比０．５以上０．９５以下の燃焼が可能であることを特徴とする連続溶融亜鉛めっき装置。 The gist of the present invention for solving the above problems is as follows.
[1] When producing a hot dip galvanized steel sheet using a continuous hot dip galvanizing apparatus equipped with a direct-fired heating furnace having a burner opposed to the steel sheet surface, adjusting the dew point of the gas introduced into the burner A method for producing a hot-dip galvanized steel sheet.
[2] A method for producing a hot-dip galvanized steel sheet according to [1], wherein the dew point of the input gas is 40 ° C to 80 ° C.
[3] In the method for producing a hot-dip galvanized steel sheet according to [1] or [2], the gases to be input are fuel gas and air, and the dew point of the air is adjusted. Manufacturing method.
[4] In the method for producing a hot dip galvanized steel sheet according to any one of [2] and [3], the steel sheet is moved by using a plurality of burner groups capable of independently controlling the combustion rate and the air ratio in the longitudinal direction of the steel sheet. The dew point of the fuel group and / or air of the burner group other than the burner group at the most downstream in the direction is set to 40 to 80 ° C. and combustion is performed at an air ratio of 1.0 to 1.5, and the burner group at the most downstream in the steel plate moving direction Is a method for producing a hot-dip galvanized steel sheet, wherein combustion is performed at an air ratio of 0.5 to 0.95.
[5] In a continuous hot dip galvanizing apparatus provided with a direct-fired heating furnace in which a plurality of burners are arranged opposite to the steel plate surface, the burner can be controlled by a plurality of burn rates and air ratios that can be controlled independently in the longitudinal direction of the steel plate. Divided into burner groups, the burners of the burner groups other than the most downstream burner group in the steel plate moving direction are equipped with a humidity control device for introducing air conditioned at a dew point of 40 to 80 ° C., and the steel plate moving direction Burners other than the most downstream burner group can freely select combustion or combustion stop at an air ratio of 1.0 or more and less than 1.5. A continuous hot dip galvanizing apparatus characterized by being capable of burning below 95.

本発明によれば、焼鈍炉に直火型加熱炉を備えた連続溶融亜鉛めっき装置を用いて溶融亜鉛めっき鋼板を製造するに際し、Ｓｉを０．１％以上含む鋼であっても、美麗な表面外観を有する溶融亜鉛めっき鋼板を、生産性の低下もなく安定して製造することができる。また、気温や天候等の外乱に影響されずに、非常に安定して溶融亜鉛めっき鋼板を製造することができる。   According to the present invention, when a hot dip galvanized steel sheet is produced using a continuous hot dip galvanizing apparatus equipped with a direct-fired heating furnace in an annealing furnace, even a steel containing 0.1% or more of Si is beautiful. A hot-dip galvanized steel sheet having a surface appearance can be stably produced without a decrease in productivity. Moreover, a hot-dip galvanized steel sheet can be manufactured very stably without being influenced by disturbances such as temperature and weather.

本発明の連続溶融亜鉛めっき装置に配置される直火型加熱炉の一実施形態を示し、（ａ）は直火型加熱炉の縦断面図、（ｂ）は直火型加熱炉壁面に複数配置した直火バーナーのバーナー群を示す正面図である。1 shows an embodiment of a direct-fired heating furnace disposed in the continuous hot-dip galvanizing apparatus of the present invention, (a) is a longitudinal sectional view of the direct-fired heating furnace, and (b) is a plurality of direct-fired heating furnace walls. It is a front view which shows the burner group of the arrange | positioned direct fire burner. バーナーに投入する空気（Ａｉｒ）の露点とＤＦＦ内のＨ２Ｏガス量との関係を調べた結果を示す図である。Is a diagram showing the results of examining the relationship between the H 2 O gas quantity in the dew point and DFF air (Air) to be introduced into the burner. バーナーに投入する空気（Ａｉｒ）の露点とＳｉ添加鋼のＦｅ酸化量との関係を調べた結果を示す図である。It is a figure which shows the result of having investigated the relationship between the dew point of the air (Air) thrown into a burner, and the amount of Fe oxidation of Si addition steel. バーナーに投入する空気（Ａｉｒ）およびコークスガス（ＣＯＧ）の流れを示す図である。It is a figure which shows the flow of the air (Air) thrown into a burner, and coke gas (COG). 本発明の調湿装置を表す模式図である。It is a schematic diagram showing the humidity control apparatus of this invention. 鋼板の走行距離と板温および酸化量との関係を示す図であり、（ａ）は発明例（条件３）、（ｂ）は比較例（条件７）の結果を示す図である。It is a figure which shows the relationship between the travel distance of a steel plate, plate | board temperature, and oxidation amount, (a) is a figure which shows the result of an invention example (condition 3), (b) is a comparative example (condition 7).

以下に、本発明の実施形態について、図１〜図６に基づき具体的に説明する。   Embodiments of the present invention will be specifically described below with reference to FIGS.

図１は、本発明の実施の形態に係る連続溶融亜鉛めっき装置の焼鈍炉に配置される直火型加熱炉の要部を示す。図１において、（ａ）は直火型加熱炉の縦断面図、（ｂ）は直火型加熱炉壁面に複数配置した直火バーナーのバーナー群を示す正面図である。図１において、１は直火型加熱炉、２はバーナー（直火バーナー）、３は鋼板である。   FIG. 1 shows a main part of a direct-fired heating furnace disposed in an annealing furnace of a continuous hot-dip galvanizing apparatus according to an embodiment of the present invention. In FIG. 1, (a) is a longitudinal sectional view of a direct-fired heating furnace, and (b) is a front view showing a plurality of burners of direct-fired burners arranged on the wall of the direct-fired heating furnace. In FIG. 1, 1 is a direct-fired heating furnace, 2 is a burner (direct-fired burner), and 3 is a steel plate.

なお、直火型加熱炉１の下流には、ラジアントチューブ（ＲＴ）炉、冷却炉、溶融めっき設備、合金化処理設備等が配置される（図示なし。）。ＲＴ炉、冷却炉、溶融めっき設備、合金化処理設備等は特に限定されず、通常採用されるものでよい。直火型加熱炉の上流に予熱炉が配置されることもある。   A radiant tube (RT) furnace, a cooling furnace, a hot dipping equipment, an alloying equipment, etc. are arranged downstream of the direct-fired heating furnace 1 (not shown). The RT furnace, cooling furnace, hot dipping equipment, alloying equipment, etc. are not particularly limited and may be those usually employed. A preheating furnace may be arranged upstream of the direct-fired heating furnace.

バーナー２は、鋼板面に対向して複数配置する。本実施形態では、鋼板面に対向して複数配置したバーナー２は、鋼板長手方向に４つのバーナー群（グループ）、１Ｚ〜４Ｚに分かれる。バーナー群１Ｚ〜３Ｚは、バーナー群毎に燃焼率及び空気比を独立に制御可能である。バーナー群１Ｚ〜３Ｚのバーナーは、燃焼率が予め定めた閾値以上の燃焼率となる条件で燃焼する。   A plurality of burners 2 are arranged to face the steel plate surface. In the present embodiment, the plurality of burners 2 arranged facing the steel plate surface are divided into four burner groups (groups) 1Z to 4Z in the longitudinal direction of the steel plate. The burner groups 1Z to 3Z can independently control the combustion rate and the air ratio for each burner group. The burners in the burner groups 1Z to 3Z burn under the condition that the combustion rate becomes a combustion rate equal to or higher than a predetermined threshold.

燃焼率は、最大燃焼負荷時のバーナーの燃料ガス量で、実際にバーナーに導入した燃料ガス量を割った値である。バーナーを最大燃焼負荷で燃焼したときが燃焼率１００％である。バーナーは、燃焼負荷が低くなると安定した燃焼状態が得られなくなる。燃焼率の予め定めた閾値は、最大燃焼負荷時の燃料ガス量に対する、安定した燃焼状態を確保できる燃焼負荷の下限における燃料ガス量の割合である。燃焼率の閾値は、バーナーの構造等によって幾分異なるが、燃焼試験を行うこと等で容易に決定できる。通常、閾値は３０％程度となる。   The combustion rate is a value obtained by dividing the amount of fuel gas actually introduced into the burner by the amount of fuel gas of the burner at the maximum combustion load. When the burner is burned at the maximum combustion load, the burning rate is 100%. The burner cannot obtain a stable combustion state when the combustion load becomes low. The predetermined threshold value of the combustion rate is the ratio of the fuel gas amount at the lower limit of the combustion load that can ensure a stable combustion state with respect to the fuel gas amount at the maximum combustion load. The threshold value of the combustion rate varies somewhat depending on the structure of the burner, but can be easily determined by performing a combustion test or the like. Usually, the threshold is about 30%.

燃料ガスには、コークス炉で発生する副生ガス（コークスガス）が用いられることが多い。コークスガスの組成としては、Ｈ２：５０〜６０ｖｏｌ％、ＣＨ４：２５〜３０ｖｏｌ％、ＣＯ：５〜１０ｖｏｌ％、ＣＯ２：２〜４ｖｏｌ％、Ｎ２：４〜８ｖｏｌ％程度である。例えば、表１のコークスガス成分のとき、空気比１で燃焼後の排ガス成分は、Ｈ２Ｏ：２２ｖｏｌ％、ＣＯ２：８ｖｏｌ％程度となる。なお、空気比は、燃料ガスを完全燃焼するために必要な空気量で、実際のバーナーに導入した空気量を割った値である。 By-product gas (coke gas) generated in a coke oven is often used as the fuel gas. The composition of coke gas, H 2: 50~60vol%, CH 4: 25~30vol%, CO: 5~10vol%, CO 2: 2~4vol%, N 2: is about 4~8vol%. For example, in the case of the coke gas components in Table 1, the exhaust gas components after combustion at an air ratio of 1 are about H 2 O: 22 vol% and CO 2 : 8 vol%. The air ratio is a value obtained by dividing the amount of air introduced into the actual burner by the amount of air necessary for complete combustion of the fuel gas.

Figure 0005915569
Figure 0005915569

空気比１以上では、燃焼しない余剰酸素が存在することにより、鋼板表面の酸化が促進される。このような空気比制御により、直火型加熱炉（以下、ＤＦＦと称することもある。）前段で酸化膜を生成し、ＤＦＦ後段で還元させることで、Ｓｉの表面濃化を抑制し、めっき濡れ性を向上させることが可能になる。しかしながら、通常の露点範囲（０〜２５℃程度）のコークスガスや空気等のガス成分ではＤＦＦ出口での鋼板温度は少なくとも６５０℃以上にしなければ、十分な酸化膜を得られないことがわかっている。発明者らは、加熱負荷の少ない状態で酸化膜制御が可能になる方法について鋭意検討を行った。その結果、バーナーに投入するガスの露点を予め調整することにより、ＤＦＦ内のＨ２Ｏガス量が増加し、Ｈ２Ｏ分子と鋼板表面との接触頻度が増加するため、鋼板表面の酸化が促進されることを見出した。 When the air ratio is 1 or more, oxidation of the steel sheet surface is promoted by the presence of surplus oxygen that does not burn. By controlling the air ratio, an oxide film is generated at the front stage of the direct-fired heating furnace (hereinafter also referred to as DFF) and reduced at the rear stage of the DFF, thereby suppressing the surface concentration of Si and plating. It becomes possible to improve wettability. However, it has been found that with a gas component such as coke gas or air in the normal dew point range (about 0 to 25 ° C.), a sufficient oxide film cannot be obtained unless the steel plate temperature at the DFF outlet is at least 650 ° C. or higher. Yes. The inventors diligently studied a method capable of controlling the oxide film with a small heating load. As a result, by adjusting in advance the dew point of the gas introduced into the burner, the amount of H 2 O gas in the DFF increases and the contact frequency between the H 2 O molecules and the steel plate surface increases, so that the oxidation of the steel plate surface is reduced. I found that it was promoted.

図２は、バーナーに投入する空気の露点（投入Ａｉｒ露点）とＤＦＦ内のＨ２Ｏガス量との関係を示す図である。図２のＨ２Ｏガス量は、表１におけるＣガスと、加湿されたＡｉｒとを空気比１．１５で燃焼させた後の排ガス中のＨ２Ｏガス量（理論値）である。図２より、投入する空気の露点を予め調整することにより、ＤＦＦ内のＨ２Ｏガス量が増加することがわかる。したがって、投入するガスの露点を調整することにより、ＤＦＦ内のＨ２Ｏガス量が増加し、鋼板表面の酸化が促進される。 FIG. 2 is a diagram showing the relationship between the dew point of the air introduced into the burner (input Air dew point) and the amount of H 2 O gas in the DFF. The amount of H 2 O gas in FIG. 2 is the amount of H 2 O gas (theoretical value) in the exhaust gas after burning C gas in Table 1 and humidified Air at an air ratio of 1.15. FIG. 2 shows that the amount of H 2 O gas in the DFF increases by adjusting the dew point of the input air in advance. Therefore, by adjusting the dew point of the input gas, the amount of H 2 O gas in the DFF increases, and the oxidation of the steel sheet surface is promoted.

燃焼ガス、空気それぞれのガス露点を調整することにより、ＤＦＦ内のＨ２Ｏガス量を調整することができる。なお、コークスガスの体積１に対して、完全燃焼に必要な空気量は通常４〜５倍の体積である。図２のように、体積の大きい空気の露点のみを調整して、ＤＦＦ内のＨ２Ｏ量を制御することもできる。 The amount of H 2 O gas in the DFF can be adjusted by adjusting the gas dew points of the combustion gas and air. The amount of air required for complete combustion is usually 4 to 5 times the volume of coke gas volume 1. As shown in FIG. 2, the amount of H 2 O in the DFF can also be controlled by adjusting only the dew point of air having a large volume.

本発明において、投入するガスの露点は、４０〜８０℃であることが好ましい。本発明者らは、Ｃ：０．１２％、Ｓｉ：２．０％、Ｍｎ：１．０％、Ａｌ：０．０３％、Ｓ：０．００５％。Ｐ：０．０１％の成分組成を有し残部をＦｅおよび不可避的不純物とする鋼板を用いて、５００℃まで加熱するオフライン酸化実験を行った。結果を図３に示す。図３より、投入する空気（Ａｉｒ）の露点が４０℃以上になると、鋼板表面に生成したＦｅ酸化量は、亜鉛めっき付着に必要な酸化量である２００ｍｇ／ｍ２以上となることがわかった。その他の合金成分鋼であっても同様の傾向が確認できた。一方、露点８０℃を越えると、水分が多すぎてバーナー燃焼性が悪化することも判明した。以上の理由から、本発明において、投入するガスの露点は、４０〜８０℃であることが好ましい。 In the present invention, the dew point of the input gas is preferably 40 to 80 ° C. The inventors have C: 0.12%, Si: 2.0%, Mn: 1.0%, Al: 0.03%, S: 0.005%. P: An off-line oxidation experiment was carried out by heating to 500 ° C. using a steel sheet having a component composition of 0.01% and the balance being Fe and inevitable impurities. The results are shown in FIG. From FIG. 3, it was found that when the dew point of the input air (Air) was 40 ° C. or more, the amount of Fe oxidation generated on the steel sheet surface was 200 mg / m 2 or more, which is the amount of oxidation necessary for galvanizing adhesion. . The same tendency could be confirmed with other alloy component steels. On the other hand, it has also been found that when the dew point exceeds 80 ° C., there is too much moisture and burner combustibility deteriorates. For the above reasons, in the present invention, the dew point of the input gas is preferably 40 to 80 ° C.

バーナー群１Ｚ〜３Ｚは、投入する空気あるいは燃焼ガスを適宜調湿可能な調湿装置を備えており、調湿装置により所望の露点に制御することができる。図４に示すように、調湿装置４は、例えば、空気（Ａｉｒ）を直火バーナー２に投入する手前に設置すればよく、調湿装置を通過したＡｉｒは、直火バーナーへ投入される。また、コークスガス（ＣＯＧ）は、別のラインから直火バーナーへ投入される。燃焼ガスを調湿する際は、ＣＯＧを直火バーナー２に投入する手前に調湿装置４を設ければよい。   The burner groups 1Z to 3Z are provided with a humidity control device that can appropriately adjust the humidity of the input air or combustion gas, and can be controlled to a desired dew point by the humidity control device. As shown in FIG. 4, the humidity control apparatus 4 may be installed, for example, before the air (Air) is input to the direct fire burner 2, and the Air that has passed through the humidity control apparatus is input to the direct fire burner. . Moreover, coke gas (COG) is thrown into a direct-fired burner from another line. When the humidity of the combustion gas is adjusted, the humidity control device 4 may be provided before the COG is put into the direct fire burner 2.

調湿装置としては、特に限定されないが、中空糸膜フィルターを用いることが望ましい。中空糸膜とは、水分子との親和力を有するイオン交換膜の一種である。中空糸膜の内側と外側に水分濃度差が生じると、その濃度差を均等にしようとする力が発生し、水分はその力をドライビングフォースとして低い水分濃度の方へ膜を透過し移動する。図５に示すように、加湿用として使用する場合は、中空糸膜フィルター５の近くに恒温水槽６を設け、所定の温度に調整された純水を恒温水槽６から中空糸膜フィルター５に投入することで、露点を水温と同じ温度にすることができるため、露点を精密に制御することができる。また、除湿用として使用する場合は、上記の純水の代わりにパージエアを投入し、パージエア流量あるいはパージエア圧力を調整することで露点を精密に制御することができる。   The humidity control device is not particularly limited, but it is desirable to use a hollow fiber membrane filter. A hollow fiber membrane is a kind of ion exchange membrane having an affinity for water molecules. When a difference in moisture concentration occurs between the inside and outside of the hollow fiber membrane, a force is generated to make the concentration difference uniform, and the moisture permeates through the membrane toward a lower moisture concentration using the force as a driving force. As shown in FIG. 5, when used for humidification, a constant temperature water tank 6 is provided near the hollow fiber membrane filter 5, and pure water adjusted to a predetermined temperature is supplied from the constant temperature water tank 6 to the hollow fiber membrane filter 5. By doing so, since the dew point can be set to the same temperature as the water temperature, the dew point can be precisely controlled. When used for dehumidification, the dew point can be precisely controlled by supplying purge air in place of the pure water and adjusting the purge air flow rate or purge air pressure.

調湿装置は、各バーナー群毎に各１個あるいは複数個設置し、個別に調整できるようにすることが望ましい。調湿するガスは調湿装置前後で所定露点以上に加熱するか、調湿装置出側からバーナーまで温度低下しないよう保温することが望ましい。調質装置を個別に調整することにより、単純にガス温度を所定露点以上に上げた際に生じ得る配管内での結露を防ぐことができる。その結果、調質装置により所望の露点に制御することができる。   It is desirable to install one or more humidity control devices for each burner group so that they can be individually adjusted. It is desirable that the gas to be conditioned be heated to a predetermined dew point or higher before and after the humidity control device, or to keep the temperature from dropping from the humidity control device to the burner. By individually adjusting the tempering device, it is possible to prevent dew condensation in the piping that may occur when the gas temperature is simply raised above a predetermined dew point. As a result, the tempering device can control the desired dew point.

バーナー群１Ｚ〜３Ｚは、各バーナー群毎に、燃焼又は燃焼停止の選択が自在である。燃焼するときは、燃焼率を予め定めた設定値以上とし、かつ空気比が１．０以上１．５未満（余剰空気あり）で燃焼することが好ましい。   The burner groups 1Z to 3Z can freely select combustion or combustion stop for each burner group. When burning, it is preferable that the combustion rate is set to a predetermined value or more and the air ratio is 1.0 or more and less than 1.5 (with surplus air).

バーナー群４Ｚのバーナーは空気比０．５以上０．９５以下の燃焼が可能で、燃焼率の制御も可能である。バーナー群４Ｚにおいて、バーナーを空気比０．５以上０．９５以下で燃焼することで、鋼板表面に生成しているＦｅ酸化物を還元し、表層に還元Ｆｅを生成させることができる。直火型加熱炉を出た鋼板がＲＴ炉内のロールに接触したときに鋼板表層部に還元Ｆｅが存在していることで、ロールへの酸化物の付着が防止され、酸化物付着に起因する欠陥（ピックアップ）を防止できる。   The burners in the burner group 4Z can burn with an air ratio of 0.5 to 0.95, and the combustion rate can also be controlled. In the burner group 4Z, by burning the burner at an air ratio of 0.5 or more and 0.95 or less, the Fe oxide generated on the surface of the steel sheet can be reduced, and reduced Fe can be generated on the surface layer. Due to the presence of reduced Fe on the steel sheet surface layer when the steel sheet exiting the direct-fired heating furnace comes into contact with the roll in the RT furnace, the adhesion of oxide to the roll is prevented, resulting in the oxide adhesion To prevent defects (pickup).

良好なめっき性を得るには、最適な酸化量を確保する必要があり、実操業においては、鋼成分、鋼板サイズ、ライン速度に応じてＤＦＦ出側鋼板温度とガス露点を調整する必要がある。発明者らは鋭意検討した結果、下記式（１）でＦｅ酸化量を予測できることを見出した。   In order to obtain good plating properties, it is necessary to secure an optimum amount of oxidation. In actual operation, it is necessary to adjust the DFF outlet steel plate temperature and gas dew point according to the steel composition, steel plate size, and line speed. . As a result of intensive studies, the inventors found that the amount of Fe oxidation can be predicted by the following formula (1).

Figure 0005915569
Figure 0005915569

なお、式（１）において、Ｏ：Ｆｅ酸化量［ｇ／ｍ２］、ＰＨ２Ｏ：燃焼後ガス水蒸気分圧（ガス成分から理論計算）、Ｑ：活性化エネルギー［ｋＪ／ｍｏｌ］（鋼種毎に決定）、ＴＮ：直火型加熱炉Ｎ群出鋼板温度［Ｋ］、ｔＮ：直火型加熱炉Ｎ群滞在時間［ｓｅｃ］、Ｃ：調整係数（空気比設定により変化）である。 In formula (1), O: Fe oxidation amount [g / m 2 ], P H2O : Gas water vapor partial pressure after combustion (theoretical calculation from gas components), Q: Activation energy [kJ / mol] (each steel type) ): T N : Direct-fired heating furnace N group out-rolling steel plate temperature [K], t N : Direct-fired heating furnace N group staying time [sec], C: Adjustment coefficient (changes depending on air ratio setting) .

焼鈍炉にＤＦＦ（直火型加熱炉）を備えるＣＧＬにおいて、図１に示すように加熱用バーナーを４つの群（＃１〜＃４）に分割したＤＦＦを用い、鋼板移動方向上流側の３つの群（＃１〜＃３）は酸化ゾーン、最終ゾーン（＃４）は還元ゾーンとし、酸化ゾーンはゾーン毎に空気比、燃焼率、投入する空気の露点（以下、Ａｉｒ露点と称す。）を個別に制御するようにして試験を行った。なお、各ゾーンの長さは４ｍである。調湿装置としては、酸化ゾーン（＃１〜＃３）のバーナーに投入する空気を、１ゾーンあたり４系統に分岐し、それぞれの系統に中空糸膜フィルターを設置した。恒温水槽はゾーン毎１台設置し、４台の中空糸膜フィルターに温度調整した純水を送水した。   In CGL equipped with DFF (direct-fired heating furnace) in the annealing furnace, as shown in FIG. 1, the DFF in which the heating burner is divided into four groups (# 1 to # 4) is used. One group (# 1 to # 3) is an oxidation zone, and the last zone (# 4) is a reduction zone. The oxidation zone is the air ratio, the combustion rate, and the dew point of the input air (hereinafter referred to as Air dew point). The test was carried out in such a way as to control each individually. The length of each zone is 4 m. As the humidity controller, the air to be introduced into the burners in the oxidation zone (# 1 to # 3) was branched into 4 systems per zone, and a hollow fiber membrane filter was installed in each system. One thermostatic water tank was installed for each zone, and pure water whose temperature was adjusted was fed to four hollow fiber membrane filters.

試験に用いた鋼板の成分組成を表２に示す。   Table 2 shows the component composition of the steel sheet used in the test.

Figure 0005915569
Figure 0005915569

その他の製造条件は表３に示す。なお、焼鈍温度は８３０℃、めっき浴温は４６０℃、めっき浴中Ａｌ濃度０．１３０％、付着量はガスワイピングにより片面当り４５ｇ／ｍ２に調節した。また、溶融亜鉛めっきを施した後に合金化温度５３０℃で合金化処理を行った。 Other manufacturing conditions are shown in Table 3. The annealing temperature was 830 ° C., the plating bath temperature was 460 ° C., the Al concentration in the plating bath was 0.130%, and the adhesion amount was adjusted to 45 g / m 2 per side by gas wiping. Further, after hot dip galvanization, alloying was performed at an alloying temperature of 530 ° C.

めっき外観の評価は、光学式の表面欠陥計による検査（φ０．５以上の不めっき欠陥や過酸化性欠陥を検出）および目視による合金化ムラ判定を行い、全ての項目が合格で○、一つでも不合格があれば×とした。また、表３中の夏季は気温３０℃、冬季は気温０℃とした。   Evaluation of plating appearance is performed by inspection with an optical surface defect meter (detecting non-plating defects or peroxide defects of φ0.5 or more) and visual judgment of alloying unevenness. If there was any failure, it was marked as x. In Table 3, the summer temperature was 30 ° C. and the winter temperature was 0 ° C.

Figure 0005915569
Figure 0005915569

また、鋼板の走行距離と板温およびＦｅ酸化量との関係について調べた。式（１）に基づいて計算されたＦｅ酸化量のうち、発明例（条件３）および比較例（条件７）の計算結果を図６に示す。なお、対象の鋼成分のときの活性化エネルギーは２２４０５Ｊ／ｍｏｌとした。また、調整係数Ｃは１．４４とした。   Further, the relationship between the travel distance of the steel plate, the plate temperature, and the amount of Fe oxidation was examined. FIG. 6 shows the calculation results of the invention example (condition 3) and the comparative example (condition 7) among the Fe oxidation amounts calculated based on the formula (1). The activation energy for the target steel component was 22405 J / mol. The adjustment coefficient C was 1.44.

試験に用いた鋼板の場合、亜鉛めっき処理に必要なＦｅ酸化量の目標範囲は２００〜６００ｍｇ／ｍ２である。本発明例である条件１〜４では、投入する空気の露点を適宜上昇させることにより、季節や板厚やライン速度にかかわらず、十分なＦｅ酸化量を確保できるため（図５（ａ））、めっき外観が良好であり、かつＳＴが低下することなく生産性を維持することができた。 In the case of the steel plate used for the test, the target range of the amount of Fe oxidation necessary for the galvanizing treatment is 200 to 600 mg / m 2 . In conditions 1 to 4 which are examples of the present invention, a sufficient amount of Fe oxidation can be ensured by appropriately increasing the dew point of the input air regardless of the season, the plate thickness, and the line speed (FIG. 5A). The plating appearance was good and the productivity could be maintained without lowering ST.

一方、板厚１．２ｍｍの鋼板を製造する場合、ライン速度１２０ｍｐｍ（条件５、９）では、ＤＦＦ出側温度が７５５℃と高いため、必要Ｆｅ酸化量を確保できるが、ライン速度１６０ｍｐｍ（条件６、１０）では、ＤＦＦ出側平均温度が低いために不めっきが発生し、外観が×となった。板厚１．８ｍｍの鋼板を製造する場合、ライン速度１２０ｍｐｍでは、夏季（条件１１）と冬季（条件７）とでは同じＤＦＦ出側平均温度であっても、冬季の場合では酸化不足となりめっき外観が×となった。また、ライン速度１６０ｍｐｍ（条件８、１２）の場合もめっき外観が×となった。したがって、板厚１．８ｍｍの鋼板を製造する場合、必要酸化量を確保するには速度を低下せざるを得ないために、生産効率が著しく低下する。   On the other hand, when manufacturing a steel plate having a thickness of 1.2 mm, the required Fe oxidation amount can be secured at a line speed of 120 mpm (conditions 5 and 9) because the DFF exit side temperature is as high as 755 ° C., but the line speed of 160 mpm (conditions 6 and 10), the average temperature on the DFF exit side was low, resulting in non-plating, and the appearance was x. When manufacturing a steel plate with a thickness of 1.8 mm, at a line speed of 120 mpm, even in the summer (condition 11) and winter (condition 7), the same average DFF outlet side temperature, the oxidation is insufficient in the winter and the plating appearance Became x. Also, the plating appearance was x when the line speed was 160 mpm (conditions 8 and 12). Therefore, when a steel plate having a thickness of 1.8 mm is manufactured, the production efficiency is remarkably lowered because the speed must be reduced in order to secure the necessary amount of oxidation.

１ 直火型加熱炉
２ バーナー（直火バーナー）
３ 鋼板
４ 調湿装置
５ 中空糸膜フィルター
６ 恒温水槽
１Ｚ〜４Ｚ バーナー群 1 Direct-fired furnace 2 Burner (direct fire burner)
3 Steel Plate 4 Humidity Control Device 5 Hollow Fiber Membrane Filter 6 Constant Temperature Water Tank 1Z-4Z Burner Group

Claims (2)

鋼板面にバーナーを対向配置した直火型加熱炉を備えた連続溶融亜鉛めっき装置を用いて溶融亜鉛めっき鋼板を製造する際に、鋼板長手方向に燃焼率および空気比を独立に制御可能な複数のバーナー群を用いて、鋼板移動方向最下流のバーナー群以外のバーナー群の燃料ガスおよびまたは空気の露点を４０〜８０℃とし、かつ、空気比１．０以上１．５以下の燃焼を行ない、鋼板移動方向最下流のバーナー群は空気比０．５以上０．９５以下の燃焼を行うことを特徴とする溶融亜鉛めっき鋼板の製造方法。 When manufacturing a hot-dip galvanized steel sheet using a continuous hot-dip galvanizing apparatus equipped with a direct-fired heating furnace with a burner facing the steel sheet surface, a plurality of burn rates and air ratios can be controlled independently in the longitudinal direction of the steel sheet The fuel gas and / or air dew point of the burner group other than the burner group at the most downstream in the direction of movement of the steel sheet is set to 40 to 80 ° C., and combustion is performed at an air ratio of 1.0 to 1.5. The method for producing a hot-dip galvanized steel sheet, characterized in that the burner group at the downstream end in the steel sheet moving direction performs combustion at an air ratio of 0.5 to 0.95 . 鋼板面に対向してバーナーを複数配置した直火型加熱炉を備えた連続溶融亜鉛めっき装置において、前記バーナーを、鋼板長手方向に燃焼率及び空気比を独立に制御可能な複数のバーナー群に分け、鋼板移動方向最下流のバーナー群以外のバーナー群のバーナーには、露点４０〜８０℃の範囲で任意に調湿された空気を投入する調湿装置を備え、かつ鋼板移動方向最下流のバーナー群以外のバーナーは空気比１．０以上１．５未満での燃焼又は燃焼停止の選択が自在であり、鋼板移動方向最下流のバーナー群のバーナーは空気比０．５以上０．９５以下の燃焼が可能であることを特徴とする連続溶融亜鉛めっき装置。   In a continuous hot dip galvanizing apparatus equipped with a direct-fired heating furnace in which a plurality of burners are arranged facing the steel plate surface, the burner is divided into a plurality of burner groups capable of independently controlling the combustion rate and the air ratio in the longitudinal direction of the steel plate. The burners in the burner group other than the burner group at the most downstream side in the steel plate moving direction are equipped with a humidity control device for introducing air that is arbitrarily conditioned at a dew point in the range of 40 to 80 ° C., and at the most downstream in the steel plate moving direction Burners other than the burner group can freely select combustion or stop combustion at an air ratio of 1.0 to less than 1.5, and the burner of the burner group at the most downstream side in the steel plate moving direction has an air ratio of 0.5 to 0.95. A continuous hot dip galvanizing apparatus characterized by being capable of burning.
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