JP4619404B2 - Hot-rolled steel sheet manufacturing method - Google Patents

Hot-rolled steel sheet manufacturing method Download PDF

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JP4619404B2
JP4619404B2 JP2007511220A JP2007511220A JP4619404B2 JP 4619404 B2 JP4619404 B2 JP 4619404B2 JP 2007511220 A JP2007511220 A JP 2007511220A JP 2007511220 A JP2007511220 A JP 2007511220A JP 4619404 B2 JP4619404 B2 JP 4619404B2
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steel sheet
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誠 勝部
昌幸 三宅
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Nippon Steel Corp
Nippon Steel Engineering Co Ltd
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • 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
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • 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
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • 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
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/003—Apparatus
    • C23C2/0038—Apparatus characterised by the pre-treatment chambers located immediately upstream of the bath or occurring locally before the dipping process
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    • 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
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    • 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
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    • 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
    • C23C2/0222—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating in a reactive atmosphere, e.g. oxidising or reducing atmosphere
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • 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
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    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment

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Description

本発明は，薄スラブ連続鋳造法で製造された熱延鋼板を溶融めっきする溶融めっき熱延鋼板の製造方法に関する。   The present invention relates to a method for producing a hot-rolled hot-rolled steel sheet, which hot-rolls a hot-rolled steel sheet produced by a thin slab continuous casting method.

近年，省エネルギー化及びコスト削減の必要性から，特開平２−１９７３５８号公報に記載されるような薄スラブ連続鋳造法（Ｔｈｉｎ Ｓｌａｂ Ｃａｓｔｉｎｇ Ｐｒｏｃｅｓｓ）を用いた鋼板製造技術が世界的に脚光を浴びるようになってきた。この薄スラブ連続鋳造法は，鋼板が連続鋳造工程から圧延工程に直送される点に特徴がある。このため，連続鋳造工程と圧延工程との間に，鋼片の冷却，欠陥検査，欠陥除去，及び加熱等の多数の工程を必要とする従来式の連続鋳造機に比べ，非常にエネルギー効率がよく，設備費用も低く抑えることができる。さらに，この薄スラブ連続鋳造機を，スクラップを原料にする電炉と共に利用可能であることも注目を集めることになった大きな要因である。
しかし，薄スラブ連続鋳造法で製造した鋼板は，従来の連続鋳造機で製造した鋼板よりも表面品質の作り込みが難しいという問題点がある。そのため，最近まで薄スラブ連続鋳造法は，広く普及していなかった。また，薄スラブ連続鋳造法で製造された熱延鋼板に対する情報も非常に少なく，この熱延鋼板に対して溶融亜鉛めっきをする際には，従来の連続鋳造機による熱延鋼板に対して用いる方法をそのまま適用していた。
熱延鋼板を溶融亜鉛めっきする方法としては，一般に「無酸化炉方式」が用いられる。この方法では，熱延鋼板を連続的に無酸化炉，還元炉（焼鈍炉），及び冷却炉を通過させて，高温加熱して酸化・還元処理を行う。このように，無酸化炉内で酸化処理してから還元炉内で還元処理することによって，熱延鋼板表面にＦｅ層を形成することができる。鋼板表面のＦｅＯ等の酸化膜は溶融めっきを付着し難くするので，鋼板の表面からこれを除去することによって溶融めっきに対するめっき濡れ性を向上させる効果がある。
上記のような従来の溶融メッキ設備は，冷延鋼板を通板することを主な目的で設計されているので，加熱帯での昇温速度はほぼ１０℃／ｓ〜２０℃／ｓの範囲であった。更に，この溶融メッキ設備を使用して熱延鋼板にメッキ処理を行う場合には，一般的な鋼においては再結晶焼鈍を行う必要は無いので，焼鈍時の最高温度は６４０℃〜６６０℃程度に調整するのが普通であった。
なお，他の方法として「どぶ漬けめっき法（フラックス法）」等も知られている。この方法では，鋼板表面に塩化亜鉛，塩化アンモニウム等のフラックスを塗布し，鋼板表面を活性化させて溶融めっきに対する濡れ性を向上させる。しかしながら，この方法は，連続的な製造が難しいことやめっき密着性の点で溶融めっき鋼板製造にはあまり一般的ではない。
薄スラブ連続鋳造法を用いて製造した熱延鋼板に，上述した「無酸化炉方式のめっき設備」を用いた溶融めっき鋼板製造方法で溶融亜鉛めっきを行うと，溶融亜鉛めっき圧延鋼板の表面に不めっきが発生する。これは，薄スラブ連続鋳造法に特有のＣａ添加に一因するものと考えられる。
薄スラブ連続鋳造機は，従来の連続鋳造機に比べて鋳型の幅が非常に狭く，注入ノズルも特殊構造をしているので，アルミナによるノズル詰まりが生じやすい。そこで，これを防止するために，薄スラブ連続鋳造機では，取鍋内にＣａを添加してアルミナの融点を下げている。
薄スラブ連続鋳造法では，鋳造された５０ｍｍから８０ｍｍ程度のスラブを高温に保ったまま圧延工程に直送して圧延する。この熱間圧延機は，従来の熱間圧延工程の仕上げ圧延機に相当する熱間圧延機であり，１．２ｍｍから４ｍｍ程度の厚みまで圧延して，熱延鋼板を製造する。この場合に，薄スラブを保温するために，滞在時間が長いトンネル炉を用いるので，圧延前のスラブ表面に生成するスケールの量が多い。
前述したように添加して薄スラブ内に残存したＣａは上記のスケール内で酸化して，ＣａＯの形態で留まる。その結果，このＣａ添加によって生成された酸化物ＣａＯが，メッキ工程での無酸化炉内で酸化する際に，熱延鋼板表面の酸化膜にむらやピットを生じさせて，溶融亜鉛めっきとのめっき濡れ性を部分的に劣化させ，めっき不良が生じていると考えられる。
また，薄スラブ連続鋳造法を用いて製造した熱延鋼板は，従来の連続鋳造機に比べてスマットの量が多くなることが観察されている。これは，薄スラブ連続鋳造法では，鋳造された鋼板を高温に保ったまま圧延工程に直送して圧延するので，Ｆｅ３Ｃ及びＣが鋼板表面に分離したまま残存しやすいためである。熱延鋼板の表面にこれらのＦｅ３Ｃ等が多く残存すると，無酸化炉内で酸化をする際に，Ｃが酸素と反応して，Ｆｅの酸化膜の生成が部分的に遅れて，酸化膜にむらやピットが生成される。これらのむらやピットも，亜鉛とのめっき濡れ性を低下させてめっき不良を生じさせると考えられる。
更に，薄スラブ連続鋳造法を用いて製造した熱延鋼板を，従来の溶融めっきラインで製造すると腰折れが発生することが判った。特に２ｍｍ以上の厚みの熱延鋼板で顕著に腰折れが発生した。この理由は，従来の溶融めっきラインで製造すると，加熱，焼鈍段階で必要以上に降伏点が下がる為に，特に２ｍｍ以上の板厚の熱延鋼板を通板すると，メッキ後の通板ラインで腰折れが生じる為である。
腰折れを防止するため，従来，メッキ後に鋼板を加熱して降伏点を調整する技術や，メッキ後の通板ラインのロール径を大きくして曲げ歪量を小さくする技術が提案されているが，前者の技術は操業が煩雑になる。後者の技術は，ロールプロフィルなどを精度良く加工して径の大きいロールを製造することは，高度な技術と加工設備を要するので，その結果としてロールの製造費用が従来よりも，かなり高くなる。In recent years, steel plate manufacturing technology using a thin slab casting process (Thin Slab Casting Process) as described in Japanese Patent Application Laid-Open No. 2-197358 has attracted worldwide attention due to the necessity of energy saving and cost reduction. It has become. This thin slab continuous casting method is characterized in that the steel sheet is directly sent from the continuous casting process to the rolling process. Therefore, it is much more energy efficient than conventional continuous casting machines that require many processes such as billet cooling, defect inspection, defect removal, and heating between the continuous casting and rolling processes. Well, equipment costs can be kept low. Furthermore, the fact that this thin slab continuous casting machine can be used with electric furnaces made from scrap is also a major factor that has attracted attention.
However, steel plates manufactured by the thin slab continuous casting method have a problem that it is more difficult to make surface quality than steel plates manufactured by conventional continuous casting machines. Therefore, until recently, thin slab continuous casting has not been widely used. In addition, there is very little information on hot-rolled steel sheets manufactured by the thin slab continuous casting method. When hot-dip galvanizing is performed on these hot-rolled steel sheets, it is used for hot-rolled steel sheets by conventional continuous casting machines. The method was applied as it was.
As a method of hot-dip galvanizing hot-rolled steel sheets, the “non-oxidation furnace method” is generally used. In this method, the hot-rolled steel sheet is continuously passed through a non-oxidation furnace, a reduction furnace (annealing furnace), and a cooling furnace, and heated and heated to perform oxidation / reduction treatment. Thus, an Fe layer can be formed on the surface of a hot-rolled steel sheet by oxidizing in a non-oxidizing furnace and then reducing in a reducing furnace. Since an oxide film such as FeO on the surface of the steel sheet makes it difficult to adhere hot-dip plating, removing this from the surface of the steel sheet has the effect of improving the plating wettability against hot-dip plating.
Since the conventional hot dipping equipment as described above is designed mainly for passing cold-rolled steel sheets, the heating rate in the heating zone is in the range of about 10 ° C / s to 20 ° C / s. Met. Furthermore, when the hot-rolled steel sheet is plated using this hot dipping equipment, it is not necessary to perform recrystallization annealing in general steel, so the maximum temperature during annealing is about 640 ° C to 660 ° C. It was normal to adjust.
Other methods such as the “dough pickling plating method (flux method)” are also known. In this method, flux such as zinc chloride or ammonium chloride is applied to the surface of the steel sheet, and the surface of the steel sheet is activated to improve the wettability with respect to hot dipping. However, this method is not very common in hot-dip galvanized steel sheet manufacturing because of its difficulty in continuous manufacturing and plating adhesion.
When hot-dip galvanizing is performed on a hot-rolled steel sheet manufactured using the thin slab continuous casting method by the hot-dip galvanized steel sheet manufacturing method using the “non-oxidation furnace type plating equipment” described above, Unplating occurs. This is considered to be due to Ca addition peculiar to the thin slab continuous casting method.
The thin slab continuous caster has a very narrow mold width compared to the conventional continuous caster and the injection nozzle has a special structure, so nozzle clogging with alumina tends to occur. Therefore, in order to prevent this, in the thin slab continuous casting machine, Ca is added to the ladle to lower the melting point of alumina.
In the thin slab continuous casting method, the cast slab of about 50 mm to 80 mm is directly sent to the rolling process and kept rolling at a high temperature. This hot rolling mill is a hot rolling mill corresponding to a finish rolling mill in a conventional hot rolling process, and rolls to a thickness of about 1.2 mm to 4 mm to produce a hot rolled steel sheet. In this case, a tunnel furnace with a long residence time is used to keep the thin slab warm, so a large amount of scale is generated on the surface of the slab before rolling.
As described above, Ca added and remaining in the thin slab is oxidized in the scale and remains in the form of CaO. As a result, when the oxide CaO produced by the addition of Ca is oxidized in a non-oxidizing furnace in the plating process, unevenness and pits are generated in the oxide film on the surface of the hot-rolled steel sheet, resulting in It is thought that plating wettability has been partially degraded, resulting in poor plating.
In addition, it has been observed that hot-rolled steel sheets manufactured using a thin slab continuous casting method have a larger amount of smut than conventional continuous casting machines. This is because in the thin slab continuous casting method, the cast steel sheet is directly fed to the rolling process while being kept at a high temperature and rolled, so that Fe 3 C and C are likely to remain separated on the steel sheet surface. If a large amount of Fe 3 C or the like remains on the surface of the hot-rolled steel sheet, when it oxidizes in a non-oxidizing furnace, C reacts with oxygen, and the formation of the Fe oxide film is partially delayed, causing oxidation. Unevenness and pits are generated in the film. These unevenness and pits are considered to cause poor plating by reducing the plating wettability with zinc.
Furthermore, it has been found that when a hot-rolled steel sheet manufactured using a thin slab continuous casting method is manufactured on a conventional hot dipping line, hip folding occurs. In particular, hip folding occurred significantly in a hot-rolled steel sheet having a thickness of 2 mm or more. The reason for this is that when the conventional hot dipping line is used, the yield point drops more than necessary at the heating and annealing stages. Especially when hot-rolled steel sheets with a thickness of 2 mm or more are passed, This is because hip breakage occurs.
In order to prevent hip breakage, conventionally, technologies have been proposed to adjust the yield point by heating the steel sheet after plating, and to reduce the amount of bending strain by increasing the roll diameter of the through-plate line after plating. The former technique is complicated to operate. In the latter technique, manufacturing a roll having a large diameter by accurately processing a roll profile or the like requires advanced techniques and processing equipment, and as a result, the manufacturing cost of the roll is considerably higher than before.

本発明は，上記課題に鑑みてなされたものであり，薄スラブ連続鋳造法で製造した熱延鋼板を溶融めっきする際に，特に，めっき表面に発生する不めっきを防止するための手段を提供するものである。
上記課題を解決するために，本発明によれば，質量％で，Ｃ：０．０３％以上，Ｓｉ：０．０２％以上，Ｍｎ：０．１５％以上，Ｃａ：０．００１％以上を含有する鋼を薄スラブ連続鋳造法で鋳造及び熱間圧延して製造した鋼板を，最高到達鋼板温度が５５０℃以上６５０℃未満であって，昇温速度が２５℃／秒以上で１５秒以上の間加熱して酸化処理し，最高到達鋼板温度が７００℃以上７６０℃以下であって，鋼板温度が５７０℃以上である時間が、２５秒以上４５秒以下であるように加熱して還元処理し，その後，溶融めっきすることによりなる溶融めっき熱延鋼板の製造方法が提供される。
なお，上記の溶融めっき熱延鋼板の製造方法において，溶融めっきを溶融亜鉛めっきにしてもよい。
また，本発明によれば，薄スラブ連続鋳造法で鋳造及び熱間圧延して製造した鋼板を溶融めっきする溶融めっき熱延鋼板の製造設備であって，酸化に供する炉と還元に供する炉とを有し，前記酸化に供する炉と前記還元に供する炉との，前記鋼板の搬送方向に沿った長さの比は，０．５以上０．９以下であることを特徴とする，溶融めっき熱延鋼板の製造設備が提供される。
なお，上記溶融めっき熱延鋼板の製造設備において，前記鋼板が前記酸化に供する炉を通過する時間を１５秒以上２５秒以下にしてもよい。
本発明によれば，薄スラブ連続鋳造法で製造した熱延鋼板を溶融めっきする際に，めっき表面に発生する不めっきを防止することが可能となる。また，腰折れを発生することなく溶融めっきすることも可能となる。The present invention has been made in view of the above problems, and provides a means for preventing non-plating generated on the plating surface particularly when hot-rolling a hot-rolled steel sheet produced by a thin slab continuous casting method. To do.
In order to solve the above-mentioned problems, according to the present invention, by mass%, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, Ca: 0.001% or more. A steel plate produced by casting and hot rolling the contained steel by a thin slab continuous casting method, the maximum attained steel plate temperature is 550 ° C. or more and less than 650 ° C., and the heating rate is 25 ° C./second or more and 15 seconds or more. During the heating, oxidation treatment is performed, and the maximum reached steel sheet temperature is 700 ° C. or higher and 760 ° C. or lower, and the time that the steel plate temperature is 570 ° C. or higher is reduced to 25 seconds or longer and 45 seconds or shorter. Then, a method for producing a hot-dip hot-rolled steel sheet by hot-dip plating is provided.
In the above hot-rolled hot-rolled steel sheet manufacturing method, hot-dip galvanizing may be used as hot-dip plating.
Further, according to the present invention, there is provided a hot-rolled hot-rolled steel plate manufacturing facility for hot-plating a steel plate manufactured by casting and hot rolling by a thin slab continuous casting method, a furnace for oxidation and a furnace for reduction. The ratio of the length along the conveying direction of the steel sheet between the furnace for oxidation and the furnace for reduction is 0.5 or more and 0.9 or less. A hot-rolled steel sheet manufacturing facility is provided.
In the hot-rolled hot-rolled steel sheet manufacturing facility, the time for the steel sheet to pass through the furnace for oxidation may be 15 seconds or more and 25 seconds or less.
ADVANTAGE OF THE INVENTION According to this invention, when hot-rolling the hot-rolled steel plate manufactured by the thin slab continuous casting method, it becomes possible to prevent the non-plating which generate | occur | produces on the plating surface. It is also possible to perform hot dip plating without causing hip breakage.

図１は，本発明に係る好適な溶融亜鉛めっき熱延鋼板製造設備の構成図である。
図２は，本発明に係る好適な溶融亜鉛めっき熱延鋼板製造設備の無酸化炉及び焼鈍炉での温度変化を説明した図である。
図３は，薄スラブ連続鋳造法により製造された熱延鋼板を，酸化する前後の図である。（ａ）は酸化する前の熱延鋼板を示し，（ｂ）は本発明によって酸化された後の熱延鋼板を示し，（ｃ）は従来技術によって酸化された後の熱延鋼板を示す。
図４は，無酸化炉で酸化された熱延鋼板を，還元する前後の図である。（ｄ）は還元する前の熱延鋼板を示し，（ｅ）は過不足なく還元された熱延鋼板を，（ｆ）は還元が不十分な熱延鋼板を，（ｇ）は還元が過剰な熱延鋼板をそれぞれ示す。
図５は，溶融めっき設備前面の洗浄装置の構成図である。FIG. 1 is a block diagram of a suitable hot-dip galvanized hot-rolled steel sheet manufacturing facility according to the present invention.
FIG. 2 is a diagram illustrating temperature changes in a non-oxidation furnace and an annealing furnace of a suitable hot-dip galvanized hot-rolled steel sheet manufacturing facility according to the present invention.
FIG. 3 is a view before and after oxidizing a hot-rolled steel sheet manufactured by a thin slab continuous casting method. (A) shows the hot-rolled steel sheet before oxidation, (b) shows the hot-rolled steel sheet after being oxidized according to the present invention, and (c) shows the hot-rolled steel sheet after being oxidized by the prior art.
FIG. 4 is a view before and after reducing the hot-rolled steel sheet oxidized in a non-oxidizing furnace. (D) shows the hot-rolled steel plate before reduction, (e) shows the hot-rolled steel plate reduced without excess, (f) shows the hot-rolled steel plate with insufficient reduction, and (g) shows excessive reduction. Each hot-rolled steel sheet is shown.
FIG. 5 is a configuration diagram of a cleaning device for the front surface of the hot dipping facility.

以下，図面を参照しながら，本発明の好適な実施形態について説明をする。なお，本明細書及び図面において，実質的に同一の機能構成を有する要素については，同一の符号を付した。
本発明では，溶融亜鉛めっき熱延鋼板製造方法で製造する溶融めっき鋼板として，ＪＩＳ Ｇ ３３０２に定める溶融亜鉛めっき鋼板ＳＧＨＣ，ＳＧＨ３４０，ＳＧＨ４００，ＳＧＨ４４０，ＳＧＨ５４０等を対象とし，質量％で，Ｃ：０．０３％以上，Ｓｉ：０．０２％以上，Ｍｎ：０．１５％以上，Ｃａ：０．００１％以上を含有する鋼を薄スラブ連続鋳造法で鋳造・圧延して製造した鋼板を用いる。
Ｃａは，０．００１％未満では，ノズル詰まりが防止できないことがあるので，それ以上含有される。Ｃａの添加は通常，製鋼工程で，脱酸した後の溶鋼中にＣａＡｌやＣａＳｉもしくはＦｅＣａ，メタリックＣａを添加することにより行われる。
図１は，本発明に係る好適な溶融亜鉛めっき熱延鋼板製造設備１の構成図である。この溶融亜鉛めっき熱延鋼板製造設備は，溶融亜鉛めっき工程ラインの始点である送出リール１０，終点である巻取リール１１，それらのリール１０，１１間に配設された予熱炉（図示せず），無酸化炉１２，還元帯１３と冷却帯１４とを含む焼鈍炉１５，溶融亜鉛めっき槽１６，ワイピング装置１７，及び冷却炉１８で構成される。
送出リール１０は，質量％で，Ｃ：０．０３％以上，Ｓｉ：０．０２％以上，Ｍｎ：０．１５％以上，Ｃａ：０．００１％以上を含有する鋼を，薄スラブ連続鋳造法で鋳造後，温度を下げずにそのまま圧延して製造した熱延鋼板が巻かれたリールである。
無酸化炉１２は，送出リールから送出された熱延鋼板を酸化するための，鋼板の搬送方向の長さが例えば１５ｍ以上２５ｍ以下の炉である。この実施の形態の場合，通板速度が１２０ｍ／分であるので，無酸化炉１２内での熱延鋼板の酸化時間は７秒以上１２秒以下になる。無酸化炉１２内の燃料空気比は，０．９以上０．９８以下程度に設定されている。また，無酸化炉１２に予熱炉を加えた搬送方向の長さは，例えば３０ｍ以上５０ｍ以下に設定されている。無酸化炉１２と予熱炉における全体酸化時間（通過時間）は，１５秒以上２５秒以下になる。
無酸化炉１２に連続的に配設された焼鈍炉１５は，酸化された熱延鋼板を還元するための還元帯１３と，その後に熱延鋼板を冷却するための冷却帯１４とから構成され，その搬送方向の長さが例えば７０ｍ以上１００ｍ以下の炉である。この実施の形態の場合，通板速度が１２０ｍ／分であるので，焼鈍炉１５内での熱延鋼板の還元時間は，例えば，比較的還元が早い５７０℃以上の領域で２５秒以上４５秒以下になる。また，Ｈ２及びＮ２等を焼鈍炉１５内の雰囲気とする。なお，還元が主に行われる還元帯１３は，還元炉及び均熱炉，若しくは還元炉のみで構成され，その搬送方向の長さは，例えば５０ｍ以上７０ｍ以下に設定されている。
溶融亜鉛めっき槽１６は，熱延鋼板を浸漬させて溶融めっきを付着させるための槽である。ワイピング装置１７は，熱延鋼板に付着した過剰の溶融金属を気体により払拭する装置である。冷却炉１８は，その後に熱延鋼板を冷却するための炉である。
次に，上述の溶融亜鉛めっき熱延鋼板製造設備１を用いた溶融亜鉛めっき熱延鋼板製造方法を図２〜図４を用いて説明する。
図２は，熱延鋼板が溶融亜鉛めっき熱延鋼板製造設備１の無酸化炉１２，還元帯１３，及び冷却帯１４を通過する際の鋼板表面の温度変化を示した図である。図２において，熱延鋼板が無酸化炉１２に進入する温度点がＯ，無酸化炉１２から退出する温度点がＰ，還元帯１３の還元炉に進入する温度点がＱ，還元帯１３の還元炉から退出して還元帯１３の均熱炉に侵入する温度点がＳ，還元帯１３の均熱炉から退出して冷却帯１４に進入する温度点がＴ，そして，冷却帯１４から退出する温度点がＶである。
まず，薄スラブ連続鋳造法で製造された熱延鋼板が，送出リール１０から送出されて，ライン上を進行し，予熱炉を経て無酸化炉１２内に入る。
無酸化炉１２内に入った熱延鋼板は，図２の区間Ｉに示すように，最高到達鋼板温度が５５０℃以上６００℃未満になるように，昇温速度２５℃／秒以上で１５秒以上２５秒以下の間，加熱されて，熱延鋼板の表面が酸化処理される。ここで酸化処理される時間とは，予熱帯と無酸化炉の通過時間である。
この酸化処理の前後の熱延鋼板表面を図３に示す。図３（ａ）は，酸化する前の熱延鋼板を示し，図３（ｂ）は本発明によって酸化された後の熱延鋼板を示し，そして，図３（ｃ）は，従来技術によって酸化された後の熱延鋼板を示す。
図２の区間Ｉでの昇温速度を，上述した従来の昇温速度よりも速い２５℃／秒以上に設定したことにより，不めっきの発生を防止する効果が得られる。これに対して，区間Ｉでの昇温速度を２５℃／秒未満にすると，Ｃａ添加によって生成された酸化物ＣａＯ及びカルシウム−アルミネート，並びにスマットのＦｅ３Ｃ等が原因で不めっきが発生してしまう。昇温速度を２５℃／秒以上に設定したことで不めっきが防止される理由を以下に説明する。
図３（ａ）に示すように，熱延鋼板表面のＦｅ酸化膜は，Ｆｅ層のＦｅ原子が表層に移動し，酸素と反応して生成される。また，Ｆｅ酸化膜が生成される際には，鋼板内に存在するＳｉ及びＭｎもＦｅ同様に酸化されるので，Ｆｅ酸化膜の下にはＳｉＯ２及びＭｎＯ等の２次酸化膜が生成される。ここで，Ｆｅ酸化膜が生成される際に，図３（ａ）に示すＣａＯやＦｅ３Ｃ等が鋼板表面に付着していると，Ｆｅ酸化膜の生成が阻害され，図３（ｃ）に示すピット１９を形成してしまう。Ｆｅ３Ｃの場合は，Ｃに分解されて酸素と反応し，図３（ｃ）に示すように，Ｆｅ酸化膜の生成を阻害する。上述のように，ピット１９が形成されると，図３（ｃ）に示すように，ＳｉＯ２及びＭｎＯ等の２次酸化膜が表面に出てしまう。これらＳｉＯ２及びＭｎＯ等の２次酸化膜は，溶融亜鉛めっきとの濡れ性を劣化させるので，溶融亜鉛めっきをする際に不めっきを発生させてしまう。
そこで，本願では，昇温速度を２５℃／秒以上と高い値に設定し，Ｆｅ酸化膜の生成速度を大きくした。
加熱温度が高くなると，酸化膜の生成は促進されるので，加熱速度が大きいほど，酸化膜の生成速度が大きくなる。酸化膜の生成は主にＦｅの表面への移動で起きるので，酸化膜の生成速度が大きいと，結果的にＣａＯやＦｅ３Ｃ等を鋼板表面に押し出すことになり，ＣａＯやＦｅ３Ｃ等でたとえピットが生成されても底部にもＦｅ酸化膜が形成されることになる。
この作用は，加熱時には，鋼板表面の酸素濃度が高いので，鋼板の極表面にはＦｅ２Ｏ３（ヘマタイト）が形成されていると推定される。Ｆｅ２Ｏ３の生成は，酸素が鋼板の内側に拡散することで進行すると言われている。このことから，結果的にＣａＯやＦｅ３Ｃ等を鋼板表面に押し出されていると考えられる。
表面のＦｅ酸化膜の内部の酸素濃度は，表層から内部になるほど少なくなるので，Ｆｅ２Ｏ３の下側には，５７０℃以下ではＦｅ３Ｏ４（マグネタイト）が生成して，５７０℃以上ではＦｅＯ（ウスタイト）が生成する。これらのＦｅ３Ｏ４やＦｅＯはＦｅイオンの外方拡散により成長する。そこで，５７０℃以上では，前記の鋼板の極表層にはＦｅ２Ｏ３が生成し，その下にはＦｅ３Ｏ４，その下にはＦｅＯが生成する。５７０℃未満では極表層にはＦｅ２Ｏ３が，その下にはＦｅ３Ｏ４が生成する。
これらの，ＦｅＯやＦｅ２Ｏ３の下には，鋼中のＳｉやＭｎ濃度が高い場合には，ＳｉもしくはＭｎの酸化物もしくはＳｉとＭｎの複合酸化物からなる２次酸化膜が生成する。
もしも，ＣａＯやＦｅ３Ｃ等が鋼板表面に付着していて，表面に押し出されない状態が発生すると，ＣａＯやＦｅ３Ｃ等により，表層からの酸素の供給が遮断されるので，ＣａＯやＦｅ３Ｃ等の下には，直接，ＳｉもしくはＭｎの酸化物もしくはＳｉとＭｎの複合酸化物からなる２次酸化膜が生成する。この場合には，引き続く還元処理の過程で，表面のＣａＯやＦｅ３Ｃ等が落ちると，ＳｉもしくはＭｎの酸化物もしくはＳｉとＭｎの複合酸化物が表面に露出した状態のピットが生成されて，その結果，メッキ後に不メッキが検出される。
しかし，前述したように，昇温速度を２５℃／秒以上と高い値に設定した場合には，鋼板表面に付着したＣａＯやＦｅ３Ｃ等が，表面に押し出されるので，押し出された後のピットの酸素濃度が高くなり，この部分にＦｅ３Ｏ４やＦｅＯが生成するので，ＳｉもしくはＭｎの酸化物もしくはＳｉとＭｎの複合酸化物が表面に露出した状態になることは無い。
これにより，ＣａＯやＦｅ３Ｃの阻害を受けてＦｅ酸化膜に，図３（ｂ）に示すピット１９が形成されても，このピット１９の底部にもＦｅ酸化膜が形成される。従って，ＳｉＯ２及びＭｎＯ等の２次酸化膜がＦｅ酸化膜で覆われて鋼板表面に出てこない。
すなわち，昇温過程を終了した時の鋼板表面の性状は，図３（ｂ）に示す様に，内側から，Ｆｅ（鋼板），ＳｉもしくはＭｎの酸化物もしくはＳｉとＭｎの複合酸化物からなる２次酸化膜，その上に，Ｆｅ３Ｏ４とＦｅＯもしくはＦｅＯからなる酸化膜，表面にＣａＯ，Ｆｅ３Ｃが存在して，ＣａＯ，Ｆｅ３Ｃの下にピットは有るが，ＦｅＯ層が存在している形態になっている。
これに対して，昇温速度を２５℃／秒未満に設定すると，ＣａＯやＦｅ３Ｃ等が表面に押し出されにくいので，図３（ｃ）のように，ＳｉもしくはＭｎの酸化物もしくはＳｉとＭｎの複合酸化物からなる２次酸化膜が表面に出てしまう。
なお，Ｆｅ（鋼板）上のＳｉもしくはＭｎの酸化物もしくはＳｉとＭｎの複合酸化物からなる２次酸化膜を，図３（ｂ），（ｃ）では，”ＳｉＯ２，ＭｎＯ”と簡略化して記載した。
また，無酸化炉内での最高到達鋼板温度を５５０℃以上に設定したことにより，酸化層が均一に生成されて，酸化膜表層部分に存在するＣａＯやＦｅ３Ｃ等の除去が容易になる効果が得られる。この効果は，最高到達鋼板温度を５５０℃未満にすると得られない。
さらに，無酸化炉内での最高到達鋼板温度を６００℃未満に設定したことにより，酸化膜の過剰の生成が防止される。無酸化炉内での最高到達鋼板温度を６００℃以上にすると，酸化膜が過剰に生成され，後の還元処理で酸化膜が残存してしまう。
この場合に，昇温速度を２５℃／秒以上に保持する時間は１５秒以上とする。１５秒未満では，充分な酸化膜厚みが有られないで，その結果，ＳｉもしくはＭｎの酸化物もしくはＳｉとＭｎの複合酸化物からなる２次酸化膜がＦｅＯ膜で覆われる事無く表面に露出してしまう。
次に，図２の区間ＩＩに示すように，酸化された熱延鋼板は，ライン上を進行し，焼鈍炉１５内の還元帯１３に入る。焼鈍炉１５内では，まず，還元帯１３で，最高到達鋼板温度が７００℃以上７６０℃以下になるように，加熱されて，その後，冷却帯１４に進められて冷却される。熱延鋼板は，焼鈍炉内の還元帯１３及び冷却帯１４において，鋼板温度を５７０℃以上に保った状態で２５秒以上４５秒以下の間，還元処理される。即ち，図２において，鋼板温度が５７０℃である温度点Ｒから温度点Ｕまでの時間が，２５秒以上４５秒以下に設定される。
ここで，５７０℃以上の温度の領域に還元処理の温度を限った理由は次の通りである。すなわち，５７０℃以上ではＦｅＯがＦｅ酸化物の主体になり還元されるのに対し，５７０℃未満では，Ｆｅ３Ｏ４がＦｅ酸化物の主体になり還元される。ＦｅＯはＦｅ３Ｏ４に比べて，処理温度も高いこともあり還元されやすい。したがって，ＦｅＯを還元処理する方が，Ｆｅ３Ｏ４を還元処理するよりも制御しやすいためである。
上記の還元処理の前後の熱延鋼板表面を図４に示す。還元処理する前の熱延鋼板が（ｄ），過不足なく還元処理された熱延鋼板が（ｅ），還元処理が不十分な熱延鋼板が（ｆ），及び還元処理が過剰な熱延鋼板が（ｇ）である。なお，図４において，図３に示したＣａＯ及びＦｅ３Ｃが図示されていないが，これは，これらのＣａＯ及びＦｅ３Ｃが，焼鈍炉１３等を通過する際に還元雰囲気Ｈ２及びＮ２等の流れにより鋼板表面から吹き飛ばされてしまうためである。
なお，Ｆｅ（鋼板）上に形成されるＳｉもしくはＭｎの酸化物もしくはＳｉとＭｎの複合酸化物からなる２次酸化膜を，図４でも”ＳｉＯ２，ＭｎＯ”と簡略化して記載した。
その結果，図３（ｂ）の形態の酸化膜が適度に還元されて図４（ｅ）の様に，内側から，Ｆｅ（鋼板），ＳｉもしくはＭｎの酸化物もしくはＳｉとＭｎの複合酸化物からなる２次酸化膜，その上にＦｅからなる膜が存在して，表面にＣａＯ，Ｆｅ３Ｃが存在していたピットは残存するが，その底にはＦｅ層が存在している形態になる。
熱延鋼板を，最高到達鋼板温度が７００℃以上７６０℃以下になるように，鋼板温度を５７０℃以上に保った状態で２５秒以上４５秒以下の間，還元処理することにより，図４（ｄ）に示す熱延鋼板の表面が焼鈍炉１５内で過不足なく還元される。
即ち，図４（ｅ）に示すように，無酸化膜で生成されたＦｅ酸化膜が，還元処理されて全てＦｅ層になる。また，このＦｅ層は，酸化処理及び還元処理等で生成されるＳｉＯ２及びＭｎＯ等の２次酸化膜も完全に覆っている。溶融亜鉛めっきとのめっき濡れ性を劣化させるＳｉＯ２及びＭｎＯ等の２次酸化膜が，完全に覆われているので，めっき濡れ性が非常に良好となり，不めっきは発生しない。
これに対して，最高到達鋼板温度が７００℃未満である場合又は鋼板温度を５７０℃以上に保つ時間が２５秒未満である場合には，焼鈍炉１５内での還元が不十分になり，図４（ｆ）に示すように，Ｆｅ酸化膜が残存してしまう。従って，このＦｅ酸化膜が，溶融めっきに対するめっき濡れ性を劣化させるので不めっきが発生してしまう。
また，最高到達鋼板温度が７６０℃を超過する場合又は鋼板温度を５７０℃以上に保つ時間が４５秒を超過する場合には，焼鈍炉１５内での還元が過剰になる。この場合，図４（ｇ）に示すように，Ｆｅ酸化膜は，十分に還元処理されてＦｅ層が形成される。しかしながら，Ｓｉ及びＭｎはＦｅよりも酸化力が強いので，Ｆｅ酸化膜が焼鈍炉１５で還元される際にもＳｉＯ２及びＭｎＯの二次酸化層は過剰に成長し，鋼板表面に出てしまう。前述したように，ＳｉＯ２及びＭｎＯは，鋼板のめっき濡れ性を劣化させるので，不めっきが生じてしまう。
次に，還元された熱延鋼板は，ライン上を焼鈍炉１５から所定温度に加熱された溶融亜鉛めっき槽１６に進行し，浸漬されて溶融亜鉛めっきが付着される。
次に，溶融亜鉛めっきの付着した熱延鋼板は，ライン上を進行し，熱延鋼板上の溶融亜鉛めっき付着量がワイピング装置１７で所定量に調整される。
次に，熱延鋼板は，ライン上を進行し，冷却炉１８内で冷却される。
以上の実施の形態では，無酸化炉１２内に入った熱延鋼板を，最高到達鋼板温度が５５０℃以上６００℃未満になるように，昇温速度２５℃／秒以上で１５秒以上２５秒以下の間，加熱酸化処理したので，Ｆｅ酸化膜が生成される際に，Ｆｅ３Ｃ等のスマット及びＣａ系酸化物によってピット１９が生じても，このピット１９の底部がＦｅ酸化膜によって覆われる。
また，以上の実施の形態では，酸化された熱延鋼板を，最高到達鋼板温度が７００℃以上７６０℃以下になるように，鋼板温度を５７０℃以上に保った状態を２５秒以上４５秒以下の間，加熱して還元処理したので，熱延鋼板表面のＦｅ酸化膜が過不足なく還元される。さらに，ＳｉＯ２及びＭｎＯの二次酸化層も表面に出てこない。従って，不めっきの発生が防止される。
また，以上の実施の形態では，酸化に供する炉（予熱炉及び無酸化炉１２）の搬送方向の長さを３０ｍ以上５０ｍ以下に設定し，還元に供する炉（還元帯１３）の搬送方向の長さを５０ｍ以上７０ｍ以下に設定した。実験により，酸化に供する炉と還元に供する炉との，熱延鋼板の搬送方向に沿った長さの比が，０．５以上０．９以下の場合に，良好なめっき状態が得られることが判明した。本実施の形態において，酸化に供する炉と還元に供する炉との搬送方向に沿った長さの比を，０．５以上０．９以下になるように設定することで，不めっきの発生を防止できる。また，酸化に供する炉と還元に供する炉が，過不足の無い適正な長さに設定されるので，設備コストの投資が適正化される。
以上，添付図面を参照しながら本発明の好適な実施形態について説明したが，本発明は係る例に限定されない。当業者であれば，特許請求の範囲に記載された技術的思想の範疇内において，各種の変更例又は修正例に想到し得ることは明らかであり，それらについても当然に本発明の技術的範囲に属するものと了解される。
また，上述した実施形態においては，熱延鋼板が送出リールから送出されているが，薄スラブ連続鋳造法を行うラインに直接接続されてもよい。
また，上述した実施形態においては，熱延鋼板が送出リールから無酸化炉に送出されるが，無酸化炉に送出される前に酸洗及び表面のスクラブ処理等の処理が行われてもよい。
また，上述した実施形態においては，熱延鋼板が送出リールから無酸化炉内に送出されて酸化されているが，酸化の前に酸洗及び表面のスクラブ処理等の処理のための装置を設けてもよい。
また，上述した実施形態においては，還元帯及び冷却帯を含む焼鈍炉を用いているが，還元炉及び冷却炉等のように別個の炉を用いてもよい。
また，上述した実施形態においては，溶融めっきとして溶融亜鉛めっきを用いているが，亜鉛以外にアルミニウム，鉛，錫等を用いてもよい。
また，上述した実施形態においては，本発明は，熱延鋼板において特に効果を発揮する。この理由は，熱延鋼板の表面の方が冷延鋼板の表面よりも，粒界が粗く，表面積が大きく，酸化及び還元が容易であり，酸化層の成長速度が大きいためと推定される。
ここで，冷延鋼板の溶融亜鉛めっき条件における酸化量及び還元量と比較するために，本発明の酸化，還元条件下で良好なめっき状態が得られた熱延鋼板に対して，従来的な冷延鋼板の酸化量，還元量を推定する式を適用して，熱延鋼板の酸化量と還元量を算出する。
冷延鋼板の酸化量を推定する式は，予熱炉及び無酸化炉内に滞在する時間と鋼板の到達温度との２変数から酸化量を推定する。冷延鋼板の還元量を推定する式は，還元処理を行う炉内に滞在する時間と鋼板の到達温度との２変数から還元量を推定する。この還元量を推定する際には，還元炉の温度が５７０℃以上の場合の還元量と，５７０℃未満の場合の還元量とを別個に算出し，両者の和を還元量と推定する。これらの酸化量及び還元量を推定する式の具体的な形は示さないが，実験から導出可能である。
薄スラブ鋳造機で得られた鋳片を熱延して得られた熱延鋼板を，本発明で規定する好適な酸化，還元の条件下で酸化，還元し，その際の酸化量，還元量の値を上記酸化量，還元量を推定する式より求めた。その結果，酸化量は０．１２〜０．２ｍｇ／ｍ２程度になり，還元量は０．２〜０．３５ｍｇ／ｍ２程度になった。これらの値は，同じ式より得られる冷延板の酸化量０．１〜０．８ｍｇ／ｍ２，還元量０．４５〜１ｍｇ／ｍ２に比べて小さくなっている。
上記結果から，冷延鋼板の場合よりも酸化速度及び還元速度が速いので熱延鋼板を溶融亜鉛めっきする際の好適な酸化量及び還元量の計算値が冷延鋼板の場合のそれらの値よりも小さい値が得られていると推定できる。
本発明を熱延鋼板の溶融亜鉛めっきに適用することにより，冷延鋼板に適用する場合に比べて，酸化時間，還元時間を短縮できる。また，酸化や還元を行う炉の長さを短くすることができ，溶融亜鉛めっき設備を小型化できる。
ところで，本発明の溶融メッキ設備の前面には，図５に示すように，アルカリスプレータンク２０，アルカリスクラバタンク２１，温水リンスタンク２２，ホットエアドライヤ２３よりなる電解洗浄を用いないアルカリ洗浄装置とナイロンブラシ２４によるアルカリスクラバを設置している。一般的に用いられている電解洗浄を用いない理由は，薄スラブ連続鋳造機と，これに直結した熱間圧延機で熱延鋼板を製造する場合には，熱間圧延した後に，鋼板表面を酸洗して，防錆剤を塗布するが，酸洗してから溶融メッキを行うまでの時間が２日以内程度であり短い為に，防錆剤の塗布量が通常より少なくても良いためである。
しかし，酸洗後の鋼板表面には，通常よりも少ない量の防錆剤やＦｅ３Ｃ等が残存しているので，電解洗浄を用いないアルカリ洗浄装置を用いて表面に付着している防錆剤やＦｅ３Ｃ等を洗浄した後にナイロンブラシによるアルカリスクラバを行い，防錆剤やＦｅ３Ｃ等を除去する。
この洗浄により，通常は，加熱炉で燃焼除去している防錆剤は除去させるので，加熱炉では，鋼板表面の酸化に安定して雰囲気中の酸素が使用される。したがって，酸化膜の生成量が安定するので，安定した不メッキの防止には好ましい条件になる。
なお，薄スラブ鋳造機で得られた鋳片を熱延して得られた熱延鋼板を対象とした時の酸化量と還元量の適正の比は，実験により０．４〜０．５５程度であることが判明した。これに対し，従来の冷延鋼板の場合には，０．２〜１．２程度と値にばらつきがあった。
さらに、本発明のような酸化工程，還元工程を用いると，薄スラブ連続鋳造機で製造したスラブを直接熱延して製造した熱延鋼板の厚みが２ｍｍ以上であっても，メッキ後の工程で，通常の１５００ｍｍの径の搬送ロールを用いても腰折れが発生しないことを確認した。
この理由は，酸化工程での昇温速度を２５℃／ｓとしたことと，還元時間を従来の冷延鋼板の還元工程よりも短くしたことで，鋼板の降伏点が高くなり，降伏伸びを生じる歪以下で通板出来る様になった為に，腰折れを発生することが無くなったためと推定される。
なお，現状の技術での，通常の通板速度は９０ｍｐｍ〜１８０ｍｐｍであるので，本発明を適用して，この速度範囲を有する溶融めっき設備を新設または改造できる。溶融めっき設備の通板速度の上限は，現状の技術では１８０ｍｐｍ程度である。しかし，もしも，更に通板速度が大きい溶融めっき設備が出来ても，本技術は適用できる。また，通板速度の下限は，本発明の条件を実現できれば，いくらでも良い。
溶融亜鉛めっき設備の中には，炉の経済的トン／ｈｒ制限を行っている場合があり，この様な場合には，板厚が厚くなると通板速度を下げるので，酸化炉を通過する時間が長くなり，その結果，平均の昇温速度は小さくなる。この場合には，昇温工程の一部が，本発明の昇温速度を満足するように操業しても良い。Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, in this specification and drawing, the same code | symbol was attached | subjected about the element which has substantially the same function structure.
In the present invention, hot dip galvanized steel sheets manufactured by the hot dip galvanized hot-rolled steel sheet manufacturing method are intended for hot dip galvanized steel sheets SGHC, SGH340, SGH400, SGH440, SGH540, and the like as defined in JIS G 3302, and in mass%, C: 0 A steel plate produced by casting and rolling a steel containing 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, and Ca: 0.001% or more by a thin slab continuous casting method is used.
If Ca is less than 0.001%, nozzle clogging may not be prevented, so it is contained in excess. The addition of Ca is usually performed by adding CaAl, CaSi, FeCa, or metallic Ca into the molten steel after deoxidation in the steelmaking process.
FIG. 1 is a configuration diagram of a suitable hot-dip galvanized hot-rolled steel sheet manufacturing facility 1 according to the present invention. This hot-dip galvanized hot-rolled steel sheet manufacturing facility includes a delivery reel 10 that is a starting point of a hot-dip galvanizing process line, a take-up reel 11 that is an end point, and a preheating furnace (not shown) disposed between the reels 10 and 11. ), An non-oxidizing furnace 12, an annealing furnace 15 including a reduction zone 13 and a cooling zone 14, a hot dip galvanizing tank 16, a wiping device 17, and a cooling furnace 18.
The delivery reel 10 is a thin slab continuous casting of steel containing, by mass%, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, Ca: 0.001% or more. This is a reel on which hot-rolled steel sheets are rolled after being cast by the method and rolled as it is without lowering the temperature.
The non-oxidizing furnace 12 is a furnace having a length in the conveyance direction of the steel plate of, for example, 15 m or more and 25 m or less for oxidizing the hot rolled steel plate sent from the sending reel. In this embodiment, since the sheet passing speed is 120 m / min, the oxidation time of the hot-rolled steel sheet in the non-oxidizing furnace 12 is 7 seconds or more and 12 seconds or less. The fuel air ratio in the non-oxidizing furnace 12 is set to about 0.9 or more and 0.98 or less. Moreover, the length of the conveyance direction which added the preheating furnace to the non-oxidizing furnace 12 is set to 30 m or more and 50 m or less, for example. The total oxidation time (passage time) in the non-oxidizing furnace 12 and the preheating furnace is 15 seconds or more and 25 seconds or less.
An annealing furnace 15 continuously disposed in the non-oxidizing furnace 12 includes a reduction zone 13 for reducing the oxidized hot-rolled steel sheet, and a cooling zone 14 for cooling the hot-rolled steel sheet thereafter. The furnace has a length in the transport direction of, for example, 70 m to 100 m. In the case of this embodiment, since the sheet passing speed is 120 m / min, the reduction time of the hot rolled steel sheet in the annealing furnace 15 is, for example, 25 seconds or more and 45 seconds in a region where the reduction is relatively fast at 570 ° C. or higher. It becomes the following. Further, H 2 and N 2 or the like are used as the atmosphere in the annealing furnace 15. Note that the reduction zone 13 in which reduction is mainly performed includes only a reduction furnace and a soaking furnace, or a reduction furnace, and the length in the transport direction is set to, for example, 50 m or more and 70 m or less.
The hot dip galvanizing tank 16 is a tank for immersing the hot-rolled steel sheet to adhere the hot dip plating. The wiping device 17 is a device that wipes excess molten metal adhering to the hot-rolled steel sheet with gas. The cooling furnace 18 is a furnace for subsequently cooling the hot-rolled steel sheet.
Next, a hot-dip galvanized hot-rolled steel sheet manufacturing method using the hot-dip galvanized hot-rolled steel sheet manufacturing facility 1 will be described with reference to FIGS.
FIG. 2 is a view showing a temperature change of the steel sheet surface when the hot-rolled steel sheet passes through the non-oxidizing furnace 12, the reduction zone 13, and the cooling zone 14 of the hot-dip galvanized hot-rolled steel plate production facility 1. In FIG. 2, the temperature point at which the hot-rolled steel sheet enters the non-oxidation furnace 12 is O, the temperature point at which the hot-rolled steel sheet enters the non-oxidation furnace 12 is P, the temperature point at which the hot-rolled steel sheet enters the reduction furnace in the reduction zone 13 is Q, The temperature point exiting from the reduction furnace and entering the soaking furnace in the reduction zone 13 is S, the temperature point exiting from the soaking furnace in the reduction zone 13 and enters the cooling zone 14 is T, and exits from the cooling zone 14 The temperature point to perform is V.
First, a hot-rolled steel sheet manufactured by a thin slab continuous casting method is sent out from the delivery reel 10, proceeds on the line, and enters the non-oxidizing furnace 12 through a preheating furnace.
As shown in section I of FIG. 2, the hot-rolled steel sheet entering the non-oxidizing furnace 12 is 15 seconds at a temperature increase rate of 25 ° C./second or more so that the maximum reached steel plate temperature is 550 ° C. or more and less than 600 ° C. The surface of the hot-rolled steel sheet is oxidized for heating for 25 seconds or less. Here, the oxidation time is the passage time between the pre-tropical zone and the non-oxidizing furnace.
The hot rolled steel sheet surface before and after this oxidation treatment is shown in FIG. FIG. 3 (a) shows a hot-rolled steel sheet before oxidation, FIG. 3 (b) shows a hot-rolled steel sheet after being oxidized according to the present invention, and FIG. The hot-rolled steel plate after being done is shown.
The effect of preventing the occurrence of non-plating can be obtained by setting the temperature rising rate in section I in FIG. 2 to 25 ° C./second or higher, which is faster than the conventional temperature rising rate described above. On the other hand, when the temperature increase rate in section I is less than 25 ° C./second, non-plating occurs due to oxides CaO and calcium-aluminate produced by the addition of Ca and Fe 3 C of smut. Resulting in. The reason why non-plating is prevented by setting the heating rate to 25 ° C./second or more will be described below.
As shown in FIG. 3A, the Fe oxide film on the surface of the hot-rolled steel sheet is generated by the Fe atoms in the Fe layer moving to the surface layer and reacting with oxygen. In addition, when the Fe oxide film is generated, Si and Mn existing in the steel plate are oxidized in the same manner as Fe, so that secondary oxide films such as SiO 2 and MnO are generated under the Fe oxide film. The Here, when the Fe oxide film is formed, if CaO, Fe 3 C, or the like shown in FIG. 3A adheres to the surface of the steel sheet, the formation of the Fe oxide film is inhibited, and FIG. The pit 19 shown in FIG. In the case of Fe 3 C, it is decomposed into C and reacts with oxygen, thereby inhibiting the formation of an Fe oxide film as shown in FIG. As described above, when the pits 19 are formed, secondary oxide films such as SiO 2 and MnO appear on the surface as shown in FIG. Since these secondary oxide films such as SiO 2 and MnO deteriorate the wettability with hot dip galvanizing, non-plating occurs during hot dip galvanizing.
Therefore, in this application, the rate of temperature increase was set to a high value of 25 ° C./second or more, and the rate of formation of the Fe oxide film was increased.
As the heating temperature increases, the generation of the oxide film is promoted, so that the higher the heating rate, the higher the generation rate of the oxide film. Since the formation of the oxide film mainly occurs due to the movement of Fe to the surface, if the generation rate of the oxide film is large, CaO, Fe 3 C, etc. are pushed out to the surface of the steel plate as a result, CaO, Fe 3 C, etc. Even if pits are generated, an Fe oxide film is also formed at the bottom.
This effect is presumed that Fe 2 O 3 (hematite) is formed on the extreme surface of the steel sheet because the oxygen concentration on the steel sheet surface is high during heating. The production of Fe 2 O 3 is said to proceed as oxygen diffuses inside the steel sheet. Therefore, it is consequently considered that extruded a CaO and Fe 3 C, etc. on the surface of the steel sheet.
Since the oxygen concentration inside the Fe oxide film on the surface decreases as it goes from the surface layer to the inside, Fe 3 O 4 (magnetite) is formed below Fe 2 O 3 at 570 ° C. or lower, and 570 ° C. or higher. Then, FeO (wustite) is generated. These Fe 3 O 4 and FeO grow by outward diffusion of Fe ions. Therefore, at 570 ° C. or higher, Fe 2 O 3 is generated in the extreme surface layer of the steel sheet, Fe 3 O 4 is formed below it, and FeO is formed below it. Below 570 ° C., Fe 2 O 3 is formed on the extreme surface layer, and Fe 3 O 4 is formed below it.
Under these FeO and Fe 2 O 3 , when the concentration of Si or Mn in the steel is high, a secondary oxide film made of an oxide of Si or Mn or a complex oxide of Si and Mn is formed. .
If CaO, Fe 3 C, etc. are attached to the surface of the steel sheet and are not pushed onto the surface, the supply of oxygen from the surface layer is blocked by CaO, Fe 3 C, etc. A secondary oxide film made of an oxide of Si or Mn or a composite oxide of Si and Mn is directly formed under 3 C or the like. In this case, when CaO, Fe 3 C, etc. on the surface drop during the subsequent reduction treatment, pits with Si or Mn oxide or Si and Mn composite oxide exposed on the surface are generated. As a result, non-plating is detected after plating.
However, as described above, when the temperature rising rate is set to a high value of 25 ° C./second or more, CaO, Fe 3 C, etc. adhering to the steel sheet surface are pushed out to the surface. Since the oxygen concentration of the pit is increased and Fe 3 O 4 or FeO is generated in this portion, the oxide of Si or Mn or the complex oxide of Si and Mn is not exposed on the surface.
Thereby, even if the pit 19 shown in FIG. 3B is formed in the Fe oxide film due to the inhibition of CaO or Fe 3 C, the Fe oxide film is also formed at the bottom of the pit 19. Therefore, the secondary oxide film such as SiO 2 and MnO is covered with the Fe oxide film and does not come out on the steel plate surface.
That is, as shown in FIG. 3B, the properties of the steel plate surface when the temperature raising process is finished are composed of Fe (steel plate), Si or Mn oxide, or Si and Mn composite oxide from the inside. secondary oxide film, thereon, Fe 3 O 4 and FeO or consisting FeO oxide film, the surface of CaO, and Fe 3 C is present, CaO, but below the Fe 3 C pit there, FeO layer It is in a form that exists.
On the other hand, when the rate of temperature rise is set to less than 25 ° C./sec, CaO, Fe 3 C, etc. are not easily pushed out to the surface. Therefore, as shown in FIG. A secondary oxide film made of a complex oxide of Mn appears on the surface.
A secondary oxide film made of an oxide of Si or Mn or a composite oxide of Si and Mn on Fe (steel plate) is simplified as “SiO 2 , MnO” in FIGS. 3B and 3C. It was described.
Moreover, by setting the maximum steel plate temperature in the non-oxidizing furnace to 550 ° C. or more, an oxide layer is uniformly generated, and it becomes easy to remove CaO, Fe 3 C, etc. existing in the oxide film surface layer portion. An effect is obtained. This effect cannot be obtained if the maximum steel sheet temperature is less than 550 ° C.
Furthermore, by setting the maximum steel sheet temperature in the non-oxidizing furnace to be less than 600 ° C., excessive generation of the oxide film can be prevented. When the maximum steel sheet temperature in the non-oxidizing furnace is set to 600 ° C. or more, an oxide film is excessively generated, and the oxide film remains in the subsequent reduction treatment.
In this case, the time for keeping the temperature rising rate at 25 ° C./second or more is 15 seconds or more. If the time is less than 15 seconds, the oxide film does not have a sufficient thickness. As a result, the secondary oxide film made of an oxide of Si or Mn or a composite oxide of Si and Mn is exposed on the surface without being covered with the FeO film. Resulting in.
Next, as shown in section II of FIG. 2, the oxidized hot-rolled steel sheet proceeds on the line and enters the reduction zone 13 in the annealing furnace 15. In the annealing furnace 15, first, heating is performed in the reduction zone 13 so that the highest steel sheet temperature is 700 ° C. or higher and 760 ° C. or lower, and then the cooling zone 14 is advanced to be cooled. The hot-rolled steel sheet is reduced in the reduction zone 13 and the cooling zone 14 in the annealing furnace for 25 seconds to 45 seconds with the steel plate temperature kept at 570 ° C. or higher. That is, in FIG. 2, the time from the temperature point R to the temperature point U where the steel plate temperature is 570 ° C. is set to 25 seconds or more and 45 seconds or less.
Here, the reason why the temperature of the reduction treatment is limited to a temperature range of 570 ° C. or higher is as follows. That is, at 570 ° C. or higher, FeO becomes the main component of Fe oxide and is reduced, whereas at less than 570 ° C., Fe 3 O 4 becomes the main component of Fe oxide and reduced. FeO is easy to be reduced because its processing temperature is higher than that of Fe 3 O 4 . Therefore, the reduction treatment of FeO is easier to control than the reduction treatment of Fe 3 O 4 .
The hot-rolled steel sheet surface before and after the reduction treatment is shown in FIG. The hot-rolled steel sheet before reduction treatment is (d), the hot-rolled steel sheet that has been reduced without excess (d), the hot-rolled steel sheet with insufficient reduction treatment (f), and the hot-rolled steel with excessive reduction treatment. The steel sheet is (g). In FIG. 4, CaO and Fe 3 C shown in FIG. 3 are not shown, but this is because when these CaO and Fe 3 C pass through the annealing furnace 13 and the like, the reducing atmospheres H 2 and N 2 It is because it will be blown off from the steel plate surface by the flow of 2 grades.
Note that the secondary oxide film made of an oxide of Si or Mn or a composite oxide of Si and Mn formed on Fe (steel plate) is also simplified as “SiO 2 , MnO” in FIG.
As a result, the oxide film in the form of FIG. 3 (b) is moderately reduced, and as shown in FIG. 4 (e), from the inside, Fe (steel plate), Si or Mn oxide, or Si and Mn composite oxide. A secondary oxide film made of Fe, and a film made of Fe on it, and pits where CaO and Fe 3 C existed on the surface remain, but an Fe layer exists on the bottom. Become.
By reducing the hot-rolled steel sheet for 25 seconds or more and 45 seconds or less while maintaining the steel sheet temperature at 570 ° C. or higher so that the highest reached steel sheet temperature is 700 ° C. or higher and 760 ° C. or lower, FIG. The surface of the hot-rolled steel sheet shown in d) is reduced without excess or deficiency in the annealing furnace 15.
That is, as shown in FIG. 4E, the Fe oxide film generated by the non-oxidized film is all reduced to become an Fe layer. The Fe layer also completely covers secondary oxide films such as SiO 2 and MnO produced by oxidation treatment and reduction treatment. Since the secondary oxide film such as SiO 2 and MnO which deteriorates the plating wettability with the hot dip galvanizing is completely covered, the plating wettability is very good and no plating is generated.
On the other hand, when the maximum steel plate temperature is less than 700 ° C. or when the time for keeping the steel plate temperature at 570 ° C. or more is less than 25 seconds, the reduction in the annealing furnace 15 becomes insufficient. As shown in 4 (f), the Fe oxide film remains. Therefore, this Fe oxide film deteriorates the plating wettability with respect to the hot dipping so that non-plating occurs.
Further, when the maximum steel plate temperature exceeds 760 ° C. or when the time for keeping the steel plate temperature at 570 ° C. or more exceeds 45 seconds, the reduction in the annealing furnace 15 becomes excessive. In this case, as shown in FIG. 4G, the Fe oxide film is sufficiently reduced to form an Fe layer. However, since Si and Mn have stronger oxidizing power than Fe, even when the Fe oxide film is reduced in the annealing furnace 15, the secondary oxide layer of SiO 2 and MnO grows excessively and appears on the steel plate surface. . As described above, since SiO 2 and MnO deteriorate the plating wettability of the steel sheet, non-plating occurs.
Next, the reduced hot-rolled steel sheet travels on the line from the annealing furnace 15 to the hot dip galvanizing tank 16 heated to a predetermined temperature, and is immersed in the hot dip galvanizing.
Next, the hot-rolled steel sheet to which the hot-dip galvanizing is adhered proceeds on the line, and the amount of hot-dip galvanized adhesion on the hot-rolled steel sheet is adjusted to a predetermined amount by the wiping device 17.
Next, the hot-rolled steel sheet proceeds on the line and is cooled in the cooling furnace 18.
In the above embodiment, the hot-rolled steel sheet that has entered the non-oxidizing furnace 12 is 15 seconds or more and 25 seconds at a temperature increase rate of 25 ° C./second or more so that the maximum attained steel sheet temperature is 550 ° C. or more and less than 600 ° C. Since the heat oxidation treatment was performed during the following, even when the Fe oxide film was formed, the bottom of the pit 19 was covered with the Fe oxide film even if the pit 19 was formed by the smut such as Fe 3 C and the Ca-based oxide. Is called.
Moreover, in the above embodiment, the state in which the steel plate temperature is maintained at 570 ° C. or higher is set to 25 seconds or more and 45 seconds or less so that the highest reached steel plate temperature is 700 ° C. or higher and 760 ° C. or lower. In the meantime, since the reduction treatment was performed by heating, the Fe oxide film on the surface of the hot rolled steel sheet was reduced without excess or deficiency. Further, the secondary oxide layer of SiO 2 and MnO does not come out on the surface. Therefore, the occurrence of non-plating is prevented.
Moreover, in the above embodiment, the length of the transport direction of the furnace (preheating furnace and non-oxidation furnace 12) used for oxidation is set to 30 m or more and 50 m or less, and the length of the transport direction of the furnace (reduction zone 13) used for reduction is set. The length was set to 50 m or more and 70 m or less. According to the experiment, when the ratio of the length along the conveying direction of the hot-rolled steel sheet between the furnace used for oxidation and the furnace used for reduction is 0.5 or more and 0.9 or less, a good plating state is obtained. There was found. In this embodiment, by setting the ratio of the length along the conveying direction of the furnace for oxidation and the furnace for reduction to be 0.5 or more and 0.9 or less, the occurrence of non-plating can be prevented. Can be prevented. In addition, the furnace used for oxidation and the furnace used for reduction are set to appropriate lengths with no excess or deficiency, so investment in equipment costs is optimized.
As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, this invention is not limited to the example which concerns. It is obvious for those skilled in the art that various changes or modifications can be conceived within the scope of the technical idea described in the claims. It is understood that it belongs to.
In the above-described embodiment, the hot-rolled steel sheet is fed from the feed reel, but may be directly connected to a line for performing the thin slab continuous casting method.
In the above-described embodiment, the hot-rolled steel sheet is sent from the delivery reel to the non-oxidizing furnace, but before being sent to the non-oxidizing furnace, processing such as pickling and surface scrubbing may be performed. .
In the above-described embodiment, the hot-rolled steel sheet is fed from the feed reel into the non-oxidizing furnace and oxidized. However, before oxidation, an apparatus for pickling and surface scrubbing is provided. May be.
In the embodiment described above, an annealing furnace including a reduction zone and a cooling zone is used. However, separate furnaces such as a reduction furnace and a cooling furnace may be used.
In the embodiment described above, hot dip galvanizing is used as hot dip plating, but aluminum, lead, tin, or the like may be used in addition to zinc.
Moreover, in embodiment mentioned above, this invention exhibits an effect especially in a hot-rolled steel plate. This is presumably because the surface of the hot-rolled steel sheet is rougher than the surface of the cold-rolled steel sheet, the grain boundary is rough, the surface area is large, oxidation and reduction are easy, and the growth rate of the oxide layer is high.
Here, in order to compare the oxidation amount and reduction amount of hot-rolled steel sheet under hot dip galvanizing conditions, the conventional hot-rolled steel sheet with good plating condition obtained under the oxidation and reduction conditions of the present invention is compared with the conventional method. Applying equations to estimate the amount of oxidation and reduction of cold-rolled steel sheets, the amount of oxidation and reduction of hot-rolled steel sheets is calculated.
The equation for estimating the amount of oxidation of a cold-rolled steel sheet estimates the amount of oxidation from two variables: the time spent in the preheating furnace and non-oxidation furnace and the temperature reached by the steel sheet. The formula for estimating the amount of reduction of the cold-rolled steel sheet estimates the amount of reduction from two variables: the time spent in the furnace where the reduction treatment is performed and the temperature reached by the steel sheet. When estimating the reduction amount, the reduction amount when the temperature of the reduction furnace is 570 ° C. or higher and the reduction amount when the temperature is lower than 570 ° C. are calculated separately, and the sum of both is estimated as the reduction amount. Although the specific form of the equation for estimating the oxidation amount and reduction amount is not shown, it can be derived from experiments.
A hot-rolled steel sheet obtained by hot-rolling a slab obtained by a thin slab caster is oxidized and reduced under the suitable oxidation and reduction conditions specified in the present invention. Was obtained from the above formula for estimating the amount of oxidation and reduction. As a result, the oxidation amount was about 0.12 to 0.2 mg / m 2 and the reduction amount was about 0.2 to 0.35 mg / m 2 . These values are smaller than the oxidation amount 0.1 to 0.8 mg / m 2 and the reduction amount 0.45 to 1 mg / m 2 of the cold-rolled sheet obtained from the same formula.
From the above results, since the oxidation rate and reduction rate are faster than in the case of cold-rolled steel sheets, the calculated values of the preferred oxidation amount and reduction amount when hot-dip galvanizing hot-rolled steel sheets are higher than those in the case of cold-rolled steel sheets. It can be estimated that a small value is obtained.
By applying the present invention to hot dip galvanizing of hot-rolled steel sheets, the oxidation time and reduction time can be shortened compared to the case of applying to cold-rolled steel sheets. In addition, the length of the furnace for oxidation and reduction can be shortened, and the hot dip galvanizing equipment can be downsized.
By the way, on the front surface of the hot dipping apparatus of the present invention, as shown in FIG. 5, an alkaline cleaning apparatus and nylon made of an alkaline spray tank 20, an alkaline scrubber tank 21, a hot water rinse tank 22, and a hot air dryer 23 are used. An alkali scrubber with a brush 24 is installed. The reason for not using electrolytic cleaning, which is generally used, is that when a hot-rolled steel sheet is manufactured by a thin slab continuous casting machine and a hot rolling machine directly connected to this, the surface of the steel sheet is removed after hot rolling. Pickling and applying a rust preventive, but since the time from pickling to hot dipping is about 2 days or less, the amount of rust preventive applied may be less than usual. It is.
However, since a smaller amount of rust preventive and Fe 3 C remain than usual on the steel plate surface after pickling, it is necessary to prevent the surface from adhering to the surface using an alkaline cleaning device that does not use electrolytic cleaning. alkali scrubber with a nylon brush after washing the Sabizai and Fe 3 C, etc., to remove rust and Fe 3 C, and the like.
This cleaning usually removes the rust preventive that has been burned and removed in the heating furnace, so in the heating furnace, oxygen in the atmosphere is used stably for oxidation of the steel sheet surface. Therefore, the amount of oxide film produced is stable, which is a preferable condition for preventing stable unplating.
The appropriate ratio of the amount of oxidation and the amount of reduction when a hot-rolled steel sheet obtained by hot rolling a slab obtained by a thin slab caster is about 0.4 to 0.55 by experiment. It turned out to be. On the other hand, in the case of the conventional cold-rolled steel sheet, the values varied from about 0.2 to 1.2.
Furthermore, when an oxidation process and a reduction process as in the present invention are used, even if the thickness of a hot-rolled steel sheet produced by directly hot-rolling a slab produced by a thin slab continuous casting machine is 2 mm or more, a process after plating is performed. Thus, it was confirmed that the waist break did not occur even when a normal conveyance roll having a diameter of 1500 mm was used.
The reason for this is that the rate of temperature increase in the oxidation process is 25 ° C / s and that the reduction time is shorter than that of the conventional cold-rolled steel sheet, which increases the yield point of the steel sheet and increases the yield elongation. It is presumed that the waist break was not generated because it was possible to pass the plate below the generated strain.
In addition, since the normal sheet-feeding speed in the current technology is 90 mpm to 180 mpm, the present invention can be applied to newly install or remodel a hot dipping equipment having this speed range. The upper limit of the sheet feeding speed of the hot dipping equipment is about 180 mpm in the current technology. However, this technology can be applied even if a hot dipping equipment with a higher sheet feeding speed is available. Further, the lower limit of the sheet passing speed is not limited as long as the conditions of the present invention can be realized.
Some hot dip galvanizing equipment may limit the furnace's economic ton / hr. In such a case, the plate passing speed decreases as the plate thickness increases, so the time required to pass through the oxidation furnace. As a result, the average heating rate decreases. In this case, a part of the temperature raising step may be operated so as to satisfy the temperature raising rate of the present invention.

薄スラブ連続鋳造法を用いて製造した４種類の熱延鋼板Ａ，Ｂ，Ｃ，及びＤの各成分を質量％で表したものを表１に示す。

Figure 0004619404
本発明に係る溶融亜鉛めっき熱延鋼板製造の方法を用いて，これら４種の熱延鋼板から溶融亜鉛めっき熱延鋼板を製造した際の，種々の条件及びその結果を表２に示す。溶融亜鉛めっき熱延鋼板の製造は，４種の熱延鋼板を予熱炉，無酸化炉，還元炉，均熱炉，及び冷却炉内を通過させて，酸化処理，還元処理，及び冷却処理し，その後，溶融亜鉛めっきして行った。
溶融亜鉛めっきの付着量は８０〜１２０ｇ／ｍ２（片面）の範囲であった。
Figure 0004619404
 表２に示すように，データ番号１〜４は，本発明で規定する条件を全て満たす実施例であり，製造された溶融亜鉛めっき熱延鋼板の表面は，非常に良好なめっき状態になっている。
一方，表２に示すデータ番号５〜９は，本発明で規定する条件のいずれかを満たさない比較例であり，製造された溶融亜鉛めっき熱延鋼板の表面は，不めっき又はスケール残り等のめっき不良状態になっている。Table 1 shows the contents of each of the four types of hot-rolled steel sheets A, B, C, and D manufactured using the thin slab continuous casting method expressed in mass%.
Figure 0004619404
 Table 2 shows various conditions and results when the hot-dip galvanized hot-rolled steel sheet was produced from these four types of hot-rolled steel sheets using the hot-dip galvanized hot-rolled steel sheet manufacturing method according to the present invention. Hot-rolled galvanized hot-rolled steel sheets are manufactured by passing four types of hot-rolled steel sheets through the preheating furnace, non-oxidation furnace, reduction furnace, soaking furnace, and cooling furnace, followed by oxidation treatment, reduction treatment, and cooling treatment. Then, hot dip galvanizing was performed.
The adhesion amount of hot dip galvanization was in the range of 80 to 120 g / m 2 (single side).
Figure 0004619404
 As shown in Table 2, data Nos. 1 to 4 are examples that satisfy all of the conditions specified in the present invention, and the surface of the hot-dip galvanized hot-rolled steel sheet produced is in a very good plating state. Yes.
On the other hand, data numbers 5 to 9 shown in Table 2 are comparative examples that do not satisfy any of the conditions specified in the present invention, and the surface of the hot-dip galvanized hot-rolled steel sheet produced is not plated or has a scale residue. The plating is in poor condition.

薄スラブ連続鋳造法を用いて製造した２種類の熱延鋼板Ａ及びＢの各成分を質量％で表したものを表３に示す。

Figure 0004619404
本発明に係る溶融亜鉛めっき熱延鋼板製造の方法を用いて，これら２種の熱延鋼板から溶融亜鉛めっき熱延鋼板を製造した際の種々の条件及びその結果を表４に示す。溶融亜鉛めっき熱延鋼板の製造は，２種の熱延鋼板を予熱炉及び無酸化炉で酸化処理し，還元帯（還元炉及び均熱炉）で還元処理し，その後，溶融亜鉛めっきして行った。なお，この実験においては，予熱炉及び無酸化炉が酸化に供する炉に相当し，還元帯が還元に供する炉に相当する。
Figure 0004619404
 表４に示すデータ番号３及び４は，予熱炉の長さを１７ｍ，無酸化炉の長さを２１ｍに固定し，冷却条件を変化させ，還元帯の長さが擬似的に４１ｍと７８ｍになるように調整した。還元時間は，１２０ｍ／分の通板速度から，算出した値である。
表４に示すように，データ番号１及び２は，予熱炉及び無酸化炉の合計の長さと還元帯の長さとの比が，本発明で規定する０．５以上０．９以下の範囲内にある条件を満たす実施例であり，製造された溶融亜鉛めっき熱延鋼板の表面は，非常に良好なめっき状態になっている。
一方，表４に示すデータ番号３及び４は，予熱炉及び無酸化炉の合計の長さと還元帯の長さとの比が，本発明で規定する０．５以上０．９以下の範囲から外れている比較例であり，製造された溶融亜鉛めっき熱延鋼板の表面は，不めっき等のめっき不良状態になっている。
なお，本発明は，上記の実施例に示した通板速度範囲で実施している。この場合に，通板速度の上限は，現状の技術では１８０ｍｐｍ程度である。しかし，もしも，更に通板速度が大きい溶融めっき設備が出来ても，本技術は適用できる。
また，通板速度の下限は，本発明の条件を実現できれば，いくらでも良い。現状の技術での，通常の通板速度は９０ｍｐｍ〜１８０ｍｐｍであるので，溶融亜鉛めっき設備の中には，炉の経済的トン／ｈｒ制限を行っている場合があり，この様な場合には，板厚が厚くなると通板速度を下げるので，酸化炉を通過する時間が長くなり，その結果，昇温速度は小さくなる。この場合には，昇温工程の一部が，本発明の昇温速度を満足するように操業しても良い。Table 3 shows the components of the two types of hot-rolled steel sheets A and B manufactured by using the thin slab continuous casting method, expressed in mass%.
Figure 0004619404
 Table 4 shows various conditions and results when a hot-dip galvanized hot-rolled steel sheet was manufactured from these two types of hot-rolled steel sheets using the hot-dip galvanized hot-rolled steel sheet manufacturing method according to the present invention. Hot-dip galvanized hot-rolled steel sheets are manufactured by oxidizing two types of hot-rolled steel sheets in a preheating furnace and non-oxidizing furnace, reducing them in a reduction zone (reduction furnace and soaking furnace), and then hot-dip galvanizing. went. In this experiment, the preheating furnace and the non-oxidizing furnace correspond to the furnace used for oxidation, and the reduction zone corresponds to the furnace used for reduction.
Figure 0004619404
 Data Nos. 3 and 4 shown in Table 4 indicate that the length of the preheating furnace is fixed at 17 m, the length of the non-oxidizing furnace is fixed at 21 m, the cooling conditions are changed, and the length of the reduction zone is set to 41 m and 78 m in a pseudo manner. It adjusted so that it might become. The reduction time is a value calculated from a plate speed of 120 m / min.
As shown in Table 4, data numbers 1 and 2 indicate that the ratio of the total length of the preheating furnace and the non-oxidation furnace to the length of the reduction zone is within the range of 0.5 to 0.9 specified in the present invention. The surface of the manufactured hot-dip galvanized hot-rolled steel sheet is in a very good plating state.
On the other hand, data numbers 3 and 4 shown in Table 4 indicate that the ratio of the total length of the preheating furnace and the non-oxidation furnace to the length of the reduction zone is outside the range of 0.5 to 0.9 specified in the present invention. The surface of the manufactured hot-dip galvanized hot-rolled steel sheet is in a poor plating state such as non-plating.
In addition, this invention is implemented in the plate | board speed range shown in said Example. In this case, the upper limit of the sheet passing speed is about 180 mpm in the current technology. However, this technology can be applied even if a hot dipping equipment with a higher sheet feeding speed is available.
Further, the lower limit of the sheet passing speed is not limited as long as the conditions of the present invention can be realized. In the current technology, the normal sheeting speed is 90 mpm to 180 mpm, so some hot dip galvanizing equipment may limit the economic ton / hr of the furnace. As the plate thickness increases, the plate passing speed decreases, so the time for passing through the oxidation furnace increases, and as a result, the heating rate decreases. In this case, a part of the temperature raising step may be operated so as to satisfy the temperature raising rate of the present invention.

本発明によれば，薄スラブ連続鋳造法で製造した熱延鋼板を溶融亜鉛めっきする場合において，めっき表面に発生する不めっきを防止する際に有効である。   According to the present invention, when hot-rolled steel sheets manufactured by a thin slab continuous casting method are hot dip galvanized, they are effective in preventing non-plating generated on the plating surface.

Claims (4)

質量％で，Ｃ：０．０３％以上，Ｓｉ：０．０２％以上，Ｍｎ：０．１５％以上，Ｃａ：０．００１％以上を含有する鋼を薄スラブ連続鋳造法で鋳造及び熱間圧延して製造した鋼板を，
最高到達鋼板温度が５５０℃以上６５０℃未満であって，昇温速度が２５℃／秒以上で１５秒以上の間加熱して酸化処理し，
最高到達鋼板温度が７００℃以上７６０℃以下であって，鋼板温度が５７０℃以上である時間が、２５秒以上４５秒以下であるように加熱して還元処理し，
その後，溶融めっきすることを特徴とする，溶融めっき熱延鋼板の製造方法。Casting and hot casting steel containing, by mass, C: 0.03% or more, Si: 0.02% or more, Mn: 0.15% or more, Ca: 0.001% or more by thin slab continuous casting method Rolled steel sheet
The highest steel sheet temperature is 550 ° C. or more and less than 650 ° C., and the heating rate is 25 ° C./second or more and heated for 15 seconds or more to oxidize,
The maximum steel plate temperature is 700 ° C. or higher and 760 ° C. or lower, and the time that the steel plate temperature is 570 ° C. or higher is reduced by heating so that it is 25 seconds or longer and 45 seconds or shorter,
Then, hot-rolled steel sheet manufacturing method, characterized by hot-dip plating. 前記溶融めっきは，溶融亜鉛めっきであることを特徴とする，請求の範囲１に記載の溶融めっき熱延鋼板の製造方法。The method for producing a hot-rolled hot-rolled steel sheet according to claim 1, wherein the hot-dip plating is hot-dip galvanizing. 薄スラブ連続鋳造法で鋳造及び熱間圧延して製造した鋼板を溶融めっきする溶融めっき熱延鋼板の製造設備であって，
酸化に供する炉と還元に供する炉とを有し，
前記酸化に供する炉と前記還元に供する炉との，前記鋼板の搬送方向に沿った長さの比は，０．５以上０．９以下であることを特徴とする，溶融めっき熱延鋼板の製造設備。A hot-rolled hot-rolled steel plate manufacturing facility that hot-plates steel plates produced by hot slab casting and hot rolling.
A furnace for oxidation and a furnace for reduction;
The ratio of the length along the conveying direction of the steel sheet between the furnace used for oxidation and the furnace used for reduction is 0.5 or more and 0.9 or less. production equipment. 前記鋼板が前記酸化に供する炉を通過する時間は，１５秒以上２５秒以下であることを特徴とする，請求の範囲３に記載の溶融めっき熱延鋼板の製造設備。The hot-rolled hot-rolled steel sheet manufacturing equipment according to claim 3, wherein the time for the steel sheet to pass through the furnace for oxidation is 15 seconds or more and 25 seconds or less.
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