JP2018162486A - Heating method for hot-dip zinc-coated steel sheet - Google Patents
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本発明は、溶融亜鉛めっき用鋼板の加熱方法に関する。 The present invention relates to a method for heating a hot-dip galvanized steel sheet.
近年、自動車の軽量化等を目的として高張力鋼（ハイテン鋼）の需要が高まっている。ハイテン鋼は、鋼中にＳｉ等の添加元素が加えられ、結晶粒を組織制御するヒートパターンで焼鈍されることで高い強度を得る。 In recent years, demand for high-tensile steel (high-tensile steel) is increasing for the purpose of reducing the weight of automobiles. High strength steel is obtained by adding an additive element such as Si to the steel and annealing it with a heat pattern that controls the structure of crystal grains, thereby obtaining high strength.
Ｓｉは易酸化性であるため、鉄にとって還元雰囲気の条件であっても酸化してしまう。Ｓｉの酸化物は、溶融亜鉛との密着性が悪いため、鋼板の表面に生成されると、めっき処理の際に亜鉛の付着を阻害することにより不めっきを発生させる。そこで、一度鋼板を酸化して鋼板表面に酸化鉄層を成長させた後、この鋼板を還元して鋼板表面の酸化鉄層を鉄に戻すことにより、鋼板表面のめっき性を確保する酸化還元法が知られている。 Since Si is easily oxidizable, iron is oxidized even under conditions of a reducing atmosphere. Since the oxide of Si has poor adhesion to molten zinc, when it is generated on the surface of a steel sheet, it causes non-plating by inhibiting the adhesion of zinc during the plating process. So, after oxidizing the steel plate once and growing the iron oxide layer on the steel plate surface, this steel plate is reduced to return the iron oxide layer on the steel plate surface back to iron, thereby ensuring the plateability of the steel plate surface. It has been known.
鋼板を酸化させる方法としては、例えば、２ゾーンに分割された酸化ゾーンを有する直火型加熱炉において、１段目の出力を最大とし、２段目の出力をゼロとすることにより、鋼板の高温時間を長くして鋼板の酸化量を増大させる方法が提案されている（特許文献１）。一方、鋼板の酸化を抑制する目的で、複数の燃焼ゾーンを有する直火型加熱炉の前段ゾーンにおいて温度が４００℃から６００℃になるまで鋼板を急速加熱する鋼板の加熱方法が提案されている（特許文献２）。 As a method for oxidizing a steel plate, for example, in a direct-fired heating furnace having an oxidation zone divided into two zones, the output of the first stage is maximized and the output of the second stage is set to zero. A method for increasing the oxidation amount of a steel sheet by extending the high temperature time has been proposed (Patent Document 1). On the other hand, for the purpose of suppressing oxidation of the steel sheet, a method for heating the steel sheet is proposed in which the steel sheet is rapidly heated until the temperature reaches 400 ° C. to 600 ° C. in the front zone of the direct-fired heating furnace having a plurality of combustion zones. (Patent Document 2).
鋼板表面における酸化鉄層の成長速度は、鋼板の板温が上昇すると大幅に増大する。また、直火型加熱炉はバーナーで鋼板を加熱するため、鋼板表面の温度がばらつきやすい。このため、鋼板が直火型加熱炉で高い温度まで一定速度で加熱されると、特に加熱終了温度付近において鋼板表面の温度が大きくばらつき、鋼板表面で成長する酸化鉄層の厚みにばらつきが発生しやすい。そして、酸化鉄層の厚みにばらつきが発生すると、酸化鉄層を還元した後にも鋼板表面に酸化鉄層が残存し、鋼板表面のめっき性が低下するおそれがある。 The growth rate of the iron oxide layer on the steel sheet surface increases significantly as the plate temperature of the steel sheet increases. Moreover, since a direct-fired heating furnace heats a steel plate with a burner, the temperature of the steel plate surface tends to vary. For this reason, when a steel sheet is heated at a constant rate to a high temperature in a direct-fired heating furnace, the temperature of the steel sheet surface varies greatly, especially near the heating end temperature, and the thickness of the iron oxide layer grown on the steel sheet surface varies. It's easy to do. And when a variation occurs in the thickness of the iron oxide layer, the iron oxide layer remains on the surface of the steel sheet even after the iron oxide layer is reduced, and the plateability of the steel sheet surface may be lowered.
本発明は、上述のような事情に基づいてなされたものであり、鋼板を直火で加熱する場合であっても、鋼板表面に均一な厚みの酸化鉄層を形成できる溶融亜鉛めっき用鋼板の加熱方法を提供することを目的とする。 The present invention has been made on the basis of the circumstances as described above, and is a hot-dip galvanized steel sheet capable of forming an iron oxide layer having a uniform thickness on the steel sheet surface even when the steel sheet is heated by direct fire. An object is to provide a heating method.
上記課題を解決するためになされた発明の溶融亜鉛めっき用鋼板の加熱方法は、複数の加熱ゾーンを有し、これらの加熱ゾーンを通じて帯状の鋼板を長手方向に搬送しつつバーナーで加熱する直火型加熱炉を用いた溶融亜鉛めっき用鋼板の加熱方法であって、上記複数の加熱ゾーンの前段において鋼板を昇温速度が秒速３０℃以上の条件で加熱する第一工程と、上記複数の加熱ゾーンの後段において上記鋼板を昇温速度が秒速５℃以下の条件で加熱する第二工程とを有し、上記第二工程における鋼板の加熱時間が上記第一工程における鋼板の加熱時間の１／４以上である。 The method for heating a hot-dip galvanized steel sheet of the invention made to solve the above-mentioned problems has a plurality of heating zones, and a direct fire is heated by a burner while conveying the strip-shaped steel sheet in the longitudinal direction through these heating zones. A hot-dip galvanizing steel sheet heating method using a mold heating furnace, wherein the first step of heating the steel sheet at a temperature rising rate of 30 ° C. or more in the preceding stage of the plurality of heating zones, and the plurality of heating A second step of heating the steel plate at a temperature rising rate of 5 ° C. or less at the latter stage of the zone, and the heating time of the steel plate in the second step is 1 / of the heating time of the steel plate in the first step. 4 or more.
当該溶融亜鉛めっき用鋼板の加熱方法は、複数の加熱ゾーンの前段において鋼板を急速に加熱し、後段において鋼板をゆっくり加熱する。このため、当該溶融亜鉛めっき用鋼板の加熱方法は、酸化鉄層の成長速度が遅い温度領域での加熱時間を短縮し、高温領域での鋼板表面の温度のばらつきを小さくできる。さらに、当該溶融亜鉛めっき用鋼板の加熱方法は、後段における鋼板の加熱時間が前段における鋼板の加熱時間の１／４以上に設定されているので、温度のばらつきが小さい鋼板表面に時間をかけて酸化鉄層を成長させることで、鋼板表面に均一な厚みの酸化鉄層を形成できる。 In the heating method of the hot dip galvanizing steel sheet, the steel sheet is rapidly heated in the former stage of the plurality of heating zones, and the steel sheet is slowly heated in the latter stage. For this reason, the heating method of the steel sheet for hot dip galvanization shortens the heating time in the temperature region where the growth rate of the iron oxide layer is slow, and can reduce the variation in the temperature of the steel sheet surface in the high temperature region. Furthermore, in the heating method of the hot dip galvanized steel sheet, since the heating time of the steel sheet in the subsequent stage is set to ¼ or more of the heating time of the steel sheet in the previous stage, it takes time to the steel sheet surface with small temperature variation. By growing the iron oxide layer, an iron oxide layer having a uniform thickness can be formed on the steel sheet surface.
上記第一工程における鋼板の到達温度が５００℃以上７００℃以下であるとよい。本発明者らは、一般的な鋼板の酸化速度が、５００℃以上で急速に増大することを知得している。当該溶融亜鉛めっき用鋼板の加熱方法は、複数の加熱ゾーンの前段における鋼板の到達温度を５００℃以上７００℃以下とすることで、後段における鋼板表面への酸化鉄層の成長速度を適切に調整できる。 The ultimate temperature of the steel sheet in the first step is preferably 500 ° C. or higher and 700 ° C. or lower. The present inventors have known that the oxidation rate of a general steel sheet increases rapidly at 500 ° C. or higher. The heating method for the hot-dip galvanized steel sheet is appropriately adjusted to the growth rate of the iron oxide layer on the steel sheet surface in the subsequent stage by setting the ultimate temperature of the steel sheet in the former stage of the plurality of heating zones to 500 ° C. or higher and 700 ° C. or lower. it can.
なお、昇温速度とは、鋼板の温度上昇量を加熱時間で割ったものを意味し、例えば昇温速度がマイナスで示される場合は、鋼板の温度が降下していることを意味する。また、加熱ゾーンにおける鋼板の昇温速度とは、１つの加熱ゾーン全体における鋼板の平均昇温速度を意味する。 Note that the rate of temperature increase means a value obtained by dividing the amount of temperature rise of the steel sheet by the heating time. For example, when the rate of temperature increase is shown as negative, it means that the temperature of the steel sheet is decreasing. Moreover, the heating rate of the steel plate in a heating zone means the average heating rate of the steel plate in the whole one heating zone.
本発明は、鋼板を直火で加熱する場合であっても、鋼板表面に均一な厚みの酸化鉄層を形成できる。 The present invention can form an iron oxide layer having a uniform thickness on the surface of the steel sheet even when the steel sheet is heated by direct fire.
以下、本発明の実施形態について詳説する。 Hereinafter, embodiments of the present invention will be described in detail.
当該溶融亜鉛めっき用鋼板の加熱方法は、連続溶融亜鉛めっき装置の焼鈍設備において適用される。連続溶融亜鉛めっき装置では、帯状の鋼板を長手方向に搬送しつつ連続焼鈍し、焼鈍後の鋼板に対して溶解亜鉛めっきを行う。連続溶融亜鉛めっき装置は、例えば、帯状の鋼板を搬送する順に、直火型加熱炉、還元炉、冷却炉、溶融亜鉛めっき設備、及び合金化炉を備える。還元炉、冷却炉、溶融亜鉛めっき設備及び合金化炉については、特に限定されず、公知のものを利用することができる。 The heating method of the steel sheet for hot dip galvanizing is applied in the annealing equipment of a continuous hot dip galvanizing apparatus. In the continuous hot dip galvanizing apparatus, a strip-shaped steel plate is continuously annealed while being conveyed in the longitudinal direction, and hot dip galvanizing is performed on the annealed steel plate. The continuous hot dip galvanizing apparatus includes, for example, a direct-fired heating furnace, a reduction furnace, a cooling furnace, a hot dip galvanizing facility, and an alloying furnace in the order of transporting a strip-shaped steel plate. The reduction furnace, cooling furnace, hot dip galvanizing equipment and alloying furnace are not particularly limited, and known ones can be used.
［直火型加熱炉］
当該溶融亜鉛めっき用鋼板の加熱方法は、複数の加熱ゾーンを有し、これらの加熱ゾーンを通じて帯状の鋼板を長手方向に搬送しつつバーナーで加熱する直火型加熱炉を用いて実施される。
[Direct-fired furnace]
The hot-dip galvanized steel sheet heating method is implemented using a direct-fired heating furnace that has a plurality of heating zones and heats the strip-shaped steel sheet with a burner while conveying the strip-shaped steel sheet in the longitudinal direction.
直火型加熱炉は、帯状の鋼板の搬送方向（長手方向）に沿って配設され、直火で鋼板を加熱する複数のバーナーを備えている。これらの複数のバーナーの空気比は１以上に調整されており、鋼板はバーナーで加熱される際に酸化する。また、直火型加熱炉は、複数のバーナーが区画された複数の加熱ゾーンを有しており、帯状の鋼板は、搬送されつつ各加熱ゾーンに属するバーナーにより順次加熱される。直火型加熱炉は、少なくとも前段及び後段を含む２以上の加熱ゾーンに区画されており、各加熱ゾーンに配設されるバーナーは、加熱ゾーン毎に燃焼量（加熱出力）の変更が可能であるように構成されている。また、前段の加熱ゾーンの出口は後段の加熱ゾーンの入口と直接接続されており、後段の加熱ゾーンの長さは、前段の加熱ゾーンの長さの１／４以上に構成されている。 The direct-fired heating furnace includes a plurality of burners that are disposed along the conveying direction (longitudinal direction) of the belt-shaped steel plate and heat the steel plate with direct fire. The air ratio of these plural burners is adjusted to 1 or more, and the steel plate is oxidized when heated by the burner. The direct-fired heating furnace has a plurality of heating zones in which a plurality of burners are partitioned, and the strip-shaped steel sheet is sequentially heated by the burners belonging to each heating zone while being conveyed. The direct-fired heating furnace is divided into two or more heating zones including at least a front stage and a rear stage, and the burner disposed in each heating zone can change the combustion amount (heating output) for each heating zone. It is configured to be. Further, the outlet of the preceding heating zone is directly connected to the inlet of the succeeding heating zone, and the length of the succeeding heating zone is configured to be ¼ or more of the length of the preceding heating zone.
［溶融亜鉛めっき用鋼板の加熱方法］
当該溶融亜鉛めっき用鋼板の加熱方法は、複数の加熱ゾーンの前段において鋼板を加熱する第一工程と、複数の加熱ゾーンの後段において鋼板を加熱する第二工程とを有する。
[Method of heating steel sheet for hot dip galvanizing]
The method for heating a hot-dip galvanized steel sheet includes a first step of heating the steel sheet in the preceding stage of the plurality of heating zones, and a second process of heating the steel sheet in the subsequent stage of the plurality of heating zones.
＜第一工程＞
第一工程では、複数の加熱ゾーンの前段において鋼板を昇温速度が秒速３０℃以上の条件で加熱する。この工程では、前段の加熱ゾーンの炉温が高く設定される又は前段の加熱ゾーンのバーナーの燃焼量が大きく設定されることで、鋼板が急速に加熱される。また、第一工程における鋼板の到達温度は５００℃以上７００℃以下に設定される。これにより、酸化鉄層の成長速度が遅い低温領域での加熱時間が短縮される。
<First step>
In the first step, the steel sheet is heated at a temperature rising rate of 30 ° C. or more in the preceding stage of the plurality of heating zones. In this step, the steel plate is rapidly heated by setting the furnace temperature in the preceding heating zone high or setting the combustion amount of the burner in the preceding heating zone high. Moreover, the ultimate temperature of the steel plate in the first step is set to 500 ° C. or more and 700 ° C. or less. This shortens the heating time in a low temperature region where the growth rate of the iron oxide layer is slow.
前段の加熱ゾーンにおける鋼板の昇温速度の下限は、秒速３０℃が好ましく、秒速３５℃がより好ましく、秒速４０℃がさらに好ましい。昇温速度が上記下限未満であると、低温領域での加熱時間を短縮できないおそれがある。また、前段の加熱ゾーンでの加熱時間を確保するために、前段の加熱ゾーンの長さを長くする必要性や鋼板の搬送速度を減少させる必要性が生じるおそれがある。一方、昇温速度の上限としては、秒速１００℃が好ましく、秒速８０℃がより好ましく、秒速６０℃がさらに好ましい。昇温速度が上記上限を超えると、加熱コストが増大するおそれがある。 The lower limit of the temperature increase rate of the steel sheet in the preceding heating zone is preferably 30 ° C. per second, more preferably 35 ° C. per second, and still more preferably 40 ° C. per second. If the rate of temperature increase is less than the lower limit, the heating time in the low temperature region may not be shortened. Moreover, in order to ensure the heating time in the heating zone of the front | former stage, there exists a possibility that the necessity of lengthening the length of the heating | heated stage of a front | former stage or the need to reduce the conveyance speed of a steel plate may arise. On the other hand, the upper limit of the rate of temperature rise is preferably 100 ° C per second, more preferably 80 ° C per second, and still more preferably 60 ° C per second. If the rate of temperature rise exceeds the upper limit, the heating cost may increase.
前段の加熱ゾーンにおける鋼板の到達温度の下限は、５００℃が好ましく、５５０℃がより好ましく、６００℃がさらに好ましい。到達温度が上記下限未満であると、前段の加熱ゾーンでの加熱が不足し、後段の加熱ゾーンにおいて酸化鉄層の成長速度を適切に調整できないおそれがある。一方、到達温度の上限としては、７００℃が好ましく、６９０℃がより好ましく、６８０℃がさらに好ましい。到達温度が上記上限を超えると、鋼板の温度が酸化鉄層の成長速度が非常に速い温度領域に到達してしまい、後段の加熱ゾーンにおいて酸化鉄層の成長速度を適切に調整できないおそれがある。 The lower limit of the reached temperature of the steel sheet in the preceding heating zone is preferably 500 ° C, more preferably 550 ° C, and even more preferably 600 ° C. When the ultimate temperature is less than the lower limit, heating in the preceding heating zone is insufficient, and the growth rate of the iron oxide layer may not be appropriately adjusted in the subsequent heating zone. On the other hand, the upper limit of the ultimate temperature is preferably 700 ° C, more preferably 690 ° C, and further preferably 680 ° C. If the reached temperature exceeds the above upper limit, the temperature of the steel sheet reaches a temperature region where the growth rate of the iron oxide layer is very fast, and there is a possibility that the growth rate of the iron oxide layer cannot be adjusted appropriately in the subsequent heating zone. .
前段の加熱ゾーンにおける鋼板の加熱時間の下限は、４秒が好ましく、５秒がより好ましく、６秒がさらに好ましい。加熱時間が上記下限未満であると、前段の加熱ゾーンでの加熱が不足するおそれや、加熱コストが増大するおそれがある。一方、加熱時間の上限としては、１６秒が好ましく、１３秒がより好ましく、１０秒がさらに好ましい。加熱時間が上記上限を超えると、前段の加熱ゾーンでの加熱時間を短縮できないおそれや、鋼板の温度が酸化鉄層の成長速度が非常に速い温度領域に到達してしまうおそれがある。 The lower limit of the heating time of the steel sheet in the preceding heating zone is preferably 4 seconds, more preferably 5 seconds, and even more preferably 6 seconds. When the heating time is less than the above lower limit, there is a fear that heating in the preceding heating zone may be insufficient, or the heating cost may increase. On the other hand, the upper limit of the heating time is preferably 16 seconds, more preferably 13 seconds, and even more preferably 10 seconds. If the heating time exceeds the above upper limit, the heating time in the preceding heating zone may not be shortened, or the temperature of the steel sheet may reach a temperature region where the growth rate of the iron oxide layer is very fast.
＜第二工程＞
第二工程では、複数の加熱ゾーンの後段において鋼板を昇温速度が秒速５℃以下の条件で加熱する。この工程では、後段の加熱ゾーンの炉温が低く設定される又は後段の加熱ゾーンのバーナーの燃焼量が小さく設定されることで、鋼板がゆっくり加熱される。特に限定されないが、例えば後段の加熱ゾーンのバーナーの燃焼量が前段の加熱ゾーンのバーナーの燃焼量の１／４より小さく設定される。また、第二工程における鋼板の到達温度は、特に限定されないが、例えば６００℃以上７００℃以下に設定される。これにより、鋼板表面の温度のばらつきを小さくできる。
<Second step>
In the second step, the steel sheet is heated under the condition that the heating rate is 5 ° C. or less per second after the plurality of heating zones. In this step, the steel plate is heated slowly by setting the furnace temperature in the subsequent heating zone low or setting the combustion amount of the burner in the subsequent heating zone small. Although not particularly limited, for example, the combustion amount of the burner in the subsequent heating zone is set to be smaller than ¼ of the combustion amount of the burner in the preceding heating zone. Moreover, although the ultimate temperature of the steel plate in a 2nd process is not specifically limited, For example, it sets to 600 degreeC or more and 700 degrees C or less. Thereby, the dispersion | variation in the temperature of the steel plate surface can be made small.
後段の加熱ゾーンの長さは、前段の加熱ゾーンの長さの１／４以上である。したがって、第二工程での鋼板の加熱時間は、第一工程での鋼板の加熱時間の１／４以上である。当該溶融亜鉛めっき用鋼板の加熱方法は、第二工程における鋼板の加熱時間が第一工程における鋼板の加熱時間の１／４以上であるので、温度のばらつきが小さい鋼板表面に時間をかけて酸化鉄層を成長させることで、鋼板表面に均一な厚みの酸化鉄層を形成できる。なお、特に限定されないが、第一工程での鋼板の加熱時間と第二工程での鋼板の加熱時間との合計は、例えば２０秒程度に設定される。 The length of the subsequent heating zone is ¼ or more of the length of the preceding heating zone. Therefore, the heating time of the steel plate in the second step is ¼ or more of the heating time of the steel plate in the first step. In the heating method of the hot dip galvanized steel sheet, since the heating time of the steel sheet in the second step is ¼ or more of the heating time of the steel sheet in the first step, the surface of the steel sheet with small temperature variation is oxidized over time. By growing the iron layer, an iron oxide layer having a uniform thickness can be formed on the surface of the steel sheet. Although not particularly limited, the total of the heating time of the steel plate in the first step and the heating time of the steel plate in the second step is set to about 20 seconds, for example.
後段の加熱ゾーンにおける鋼板の昇温速度の下限は、秒速−２０℃が好ましく、秒速−１５℃がより好ましく、秒速−１０℃がさらに好ましい。昇温速度が上記下限未満であると、直火型加熱炉での鋼板の加熱が不十分となり、次の還元炉における鋼板の加熱コストが増大するおそれがある。一方、昇温速度の上限としては、秒速１０℃が好ましく、秒速７℃がより好ましく、秒速５℃がさらに好ましい。昇温速度が上記上限を超えると、鋼板表面の温度のばらつきが大きくなり、鋼板表面に均一な厚みの酸化鉄層を形成できないおそれがある。 The lower limit of the heating rate of the steel sheet in the subsequent heating zone is preferably −20 ° C. per second, more preferably −15 ° C. per second, and further preferably −10 ° C. per second. If the rate of temperature rise is less than the above lower limit, the heating of the steel plate in the direct-fired heating furnace becomes insufficient, and the heating cost of the steel plate in the next reduction furnace may increase. On the other hand, the upper limit of the rate of temperature increase is preferably 10 ° C per second, more preferably 7 ° C per second, and further preferably 5 ° C per second. When the rate of temperature rise exceeds the above upper limit, the temperature variation on the steel sheet surface becomes large, and an iron oxide layer having a uniform thickness may not be formed on the steel sheet surface.
前段の加熱ゾーンの長さに対する後段の加熱ゾーンの長さの比率の下限は、０．２５が好ましく、０．５がより好ましく、１がさらに好ましい。一方、上記比率の上限としては、４が好ましく、３がより好ましく、２がさらに好ましい。上記比率が上記範囲外であると、前段の加熱ゾーンにおける鋼板の昇温速度を適切に設定することができないおそれがある。 The lower limit of the ratio of the length of the subsequent heating zone to the length of the preceding heating zone is preferably 0.25, more preferably 0.5, and even more preferably 1. On the other hand, the upper limit of the ratio is preferably 4, more preferably 3, and even more preferably 2. If the ratio is out of the above range, it may not be possible to appropriately set the temperature increase rate of the steel sheet in the preceding heating zone.
後段の加熱ゾーンにおける鋼板の加熱時間の下限は、４秒が好ましく、５秒がより好ましく、６秒がさらに好ましい。加熱時間が上記下限未満であると、鋼板表面の温度のばらつきが抑制されず、鋼板表面に均一な厚みの酸化鉄層を形成できないおそれがある。一方、加熱時間の上限としては、１６秒が好ましく、１３秒がより好ましく、１０秒がさらに好ましい。加熱時間が上記上限を超えると、前段の加熱ゾーンにおける鋼板の加熱時間を確保できないおそれがある。 The lower limit of the heating time of the steel sheet in the subsequent heating zone is preferably 4 seconds, more preferably 5 seconds, and even more preferably 6 seconds. If the heating time is less than the above lower limit, variation in temperature on the steel sheet surface is not suppressed, and an iron oxide layer having a uniform thickness may not be formed on the steel sheet surface. On the other hand, the upper limit of the heating time is preferably 16 seconds, more preferably 13 seconds, and even more preferably 10 seconds. If the heating time exceeds the above upper limit, the heating time of the steel sheet in the preceding heating zone may not be ensured.
後段の加熱ゾーンにおける鋼板の昇温速度は、後段の加熱ゾーンの長さ（加熱時間）とのバランスを考慮して決定される。例えば、前段の加熱ゾーンの長さに対する後段の加熱ゾーンの長さの比率が１である場合、後段の加熱ゾーンにおける鋼板の昇温速度は秒速１０℃以下とすればよい。また、例えば、前段の加熱ゾーンの長さに対する後段の加熱ゾーンの長さの比率が０．２５である場合、後段の加熱ゾーンにおける鋼板の昇温速度は秒速０℃以下とすればよい。 The temperature increase rate of the steel sheet in the subsequent heating zone is determined in consideration of the balance with the length (heating time) of the subsequent heating zone. For example, when the ratio of the length of the subsequent heating zone to the length of the preceding heating zone is 1, the heating rate of the steel sheet in the subsequent heating zone may be 10 ° C. or less per second. For example, when the ratio of the length of the subsequent heating zone to the length of the preceding heating zone is 0.25, the rate of temperature increase of the steel sheet in the subsequent heating zone may be 0 ° C. or less per second.
後段の加熱ゾーンにおいて加熱された鋼板は、還元炉に搬送される。後段の加熱ゾーンにおける鋼板の加熱が十分でない場合は、還元炉において鋼板が適宜加熱され、鋼板の板温が調整される。 The steel plate heated in the subsequent heating zone is conveyed to a reduction furnace. When the heating of the steel plate in the subsequent heating zone is not sufficient, the steel plate is appropriately heated in the reduction furnace, and the plate temperature of the steel plate is adjusted.
（利点）
当該溶融亜鉛めっき用鋼板の加熱方法は、前段の加熱ゾーンで５００℃以上７００℃以下まで鋼板を急速に加熱するので、酸化鉄層の成長速度が遅い低温領域での加熱時間を短縮し、酸化鉄層の成長速度を適切に調整可能な温度領域まで鋼板を昇温できる。また、当該溶融亜鉛めっき用鋼板の加熱方法は、前段の加熱ゾーンでの鋼板の加熱時間の１／４以上の長い時間をかけて後段の加熱ゾーンで鋼板をゆっくり加熱するので、鋼板表面の温度のばらつきを抑制できる。つまり、当該溶融亜鉛めっき用鋼板の加熱方法は、温度のばらつきが小さい鋼板表面に時間をかけて酸化鉄層を成長させることで、鋼板表面に均一な厚みの酸化鉄層を形成できる。さらに、当該溶融亜鉛めっき用鋼板の加熱方法は、従来よりも鋼板の温度が低い状態で鋼板の酸化を終了させるので、後段の加熱ゾーンの出口以降における鋼板表面の酸化物のロールピックを抑制できる。
(advantage)
In the heating method of the hot dip galvanizing steel sheet, since the steel sheet is rapidly heated to 500 ° C. or more and 700 ° C. or less in the preceding heating zone, the heating time in the low temperature region where the growth rate of the iron oxide layer is slow is shortened, and the oxidation The steel sheet can be heated to a temperature range in which the growth rate of the iron layer can be adjusted appropriately. In addition, the heating method of the steel sheet for hot dip galvanizing slowly heats the steel sheet in the subsequent heating zone over a time longer than 1/4 of the heating time of the steel sheet in the preceding heating zone. The variation of can be suppressed. That is, in the heating method of the steel sheet for hot dip galvanizing, an iron oxide layer having a uniform thickness can be formed on the surface of the steel sheet by growing the iron oxide layer on the steel sheet surface having a small temperature variation over time. Furthermore, since the heating method of the hot dip galvanized steel sheet terminates the oxidation of the steel sheet in a state where the temperature of the steel sheet is lower than before, it is possible to suppress roll picking of the oxide on the surface of the steel sheet after the outlet of the subsequent heating zone. .
＜その他の実施形態＞
本発明の溶融亜鉛めっき用鋼板の加熱方法は上記実施形態に限定されるものではない。
<Other embodiments>
The method for heating the hot-dip galvanized steel sheet of the present invention is not limited to the above embodiment.
上記実施形態では、直火型加熱炉と還元炉とが独立した連続溶融亜鉛めっき装置の焼鈍設備について説明したが、直火型加熱炉と還元炉とが１台の炉で構成されてもよい。また、焼鈍設備が、直火型加熱炉の前に鋼板を予め加熱する予熱炉を備えていてもよいし、予熱炉と直火型加熱炉とが１台の炉で構成されていてもよい。さらに、直火型加熱炉が有している複数の加熱ゾーンが、各々独立した複数の炉で構成されてもよい。 In the above embodiment, the annealing equipment of the continuous hot dip galvanizing apparatus in which the direct-fired heating furnace and the reduction furnace are independent has been described. However, the direct-fired heating furnace and the reduction furnace may be configured as a single furnace. . Moreover, the annealing equipment may be provided with a preheating furnace that preheats the steel plate before the direct-fired heating furnace, or the preheating furnace and the direct-fired heating furnace may be configured by one furnace. . Further, the plurality of heating zones included in the direct-fired heating furnace may be composed of a plurality of independent furnaces.
また、焼鈍設備が予熱炉を備えている場合、当該溶融亜鉛めっき用鋼板の加熱方法は、第一工程の前に、直火型加熱炉の排ガスによる対流伝熱を用いた予熱炉で約２５０℃まで鋼板を予め加熱する予熱工程をさらに有していてもよい。 Further, when the annealing equipment includes a preheating furnace, the hot dip galvanizing steel sheet is heated by a preheating furnace using convection heat transfer by exhaust gas from a direct heating furnace before the first step. You may further have a preheating process which preheats a steel plate to ° C.
以下、実施例によって本発明をさらに詳細に説明するが、本発明はこれらの実施例に限定されるものではない。 EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.
Ｃ（炭素）元素の含有率が０．１５質量％、Ｓｉ（ケイ素）元素の含有率が１．８質量％、及びＭｎ（マンガン）元素の含有率が２．０質量％の帯状の鋼板を用い、上述の直火型加熱炉のモデルについてシミュレーションを行って、様々な炉温条件での鋼板の板温及び酸化鉄層の厚み（酸化層厚）を算出した。なお、鋼板の板厚を１．２ｍｍ、鋼板の板幅を１０００ｍｍ、鋼板の搬送速度を秒速１ｍ、前段の加熱ゾーン及び後段の加熱ゾーンの長さの合計を２０ｍとしている。また、鋼板は予め加熱されているものとし、前段の加熱ゾーンの入口での鋼板の板温を２５０℃としている。 A strip-shaped steel sheet having a C (carbon) element content of 0.15% by mass, a Si (silicon) element content of 1.8% by mass, and a Mn (manganese) element content of 2.0% by mass. The simulation was performed on the above-mentioned direct-fired heating furnace model, and the plate temperature of the steel sheet and the thickness of the iron oxide layer (oxide layer thickness) were calculated under various furnace temperature conditions. In addition, the plate thickness of the steel plate is 1.2 mm, the plate width of the steel plate is 1000 mm, the conveyance speed of the steel plate is 1 m / s, and the total length of the heating zone in the front stage and the heating zone in the rear stage is 20 m. Further, the steel plate is preheated, and the plate temperature of the steel plate at the entrance of the preceding heating zone is 250 ° C.
酸化層厚は、鋼板表面の酸化速度の時間積分により与えられる。また、鋼板表面の酸化速度は、酸化層厚をｘとした場合に下記式（１）により与えられる。
ｄｘ／ｄｔ＝Ａ・ｅｘｐ（−Ｑ／ＲＴ）／２ｘ ・・・（１）
ここで、Ａは頻度因子、Ｑは活性化エネルギー（定数）、Ｒは気体定数、Ｔは鋼板の温度である。
The oxide layer thickness is given by the time integration of the oxidation rate on the steel sheet surface. Further, the oxidation rate of the steel sheet surface is given by the following formula (1) when the oxide layer thickness is x.
dx / dt = A · exp (−Q / RT) / 2x (1)
Here, A is the frequency factor, Q is the activation energy (constant), R is the gas constant, and T is the temperature of the steel sheet.
本発明者らは、実際に上述の組成の鋼板を加熱して鋼板の酸化層厚のばらつきについて事前調査を行った結果、後段の加熱ゾーンの出口において鋼板の酸化層の成長速度が秒速０．０１５μｍ以下であれば、酸化層厚のばらつきが抑制されることを知得している。そこで、酸化層厚の目標値を０．３μｍとし、鋼板の到達温度が６００℃以上となり、かつ後段の加熱ゾーンの出口において鋼板の酸化層の成長速度が秒速０．０１５μｍ以下となる加熱方法について検証した。 The inventors of the present invention actually heated the steel plate having the above composition and conducted a preliminary investigation on the variation in the thickness of the oxide layer of the steel plate. As a result, the growth rate of the oxide layer of the steel plate at the outlet of the subsequent heating zone was 0. It is known that if the thickness is 015 μm or less, variation in the oxide layer thickness is suppressed. Therefore, a heating method in which the target value of the oxide layer thickness is 0.3 μm, the reached temperature of the steel sheet is 600 ° C. or higher, and the growth rate of the oxidized layer of the steel sheet is 0.015 μm or less per second at the outlet of the subsequent heating zone. Verified.
図１には、前段の加熱ゾーンの炉温を１２５０℃とし、後段の加熱ゾーンの炉温を７５０℃とし、２つの加熱ゾーンを通じて鋼板が搬送されつつ加熱された際の鋼板の板温及び酸化層厚の算出結果が示されている。後段の燃焼量は、前段の燃焼量の約９倍である。また、図１のシミュレーションでは、前段の加熱ゾーンでの加熱時間を８秒とし、後段の加熱ゾーンでの加熱時間を１２秒としている。なお、酸化層厚を示す曲線の傾きが、酸化層の成長速度を示している。 FIG. 1 shows the plate temperature and oxidation of the steel plate when the furnace temperature in the preceding heating zone is 1250 ° C., the furnace temperature in the latter heating zone is 750 ° C., and the steel plate is heated while being conveyed through the two heating zones. The calculation result of the layer thickness is shown. The amount of combustion in the latter stage is about nine times the amount of combustion in the former stage. In the simulation of FIG. 1, the heating time in the preceding heating zone is 8 seconds, and the heating time in the subsequent heating zone is 12 seconds. The slope of the curve indicating the oxide layer thickness indicates the growth rate of the oxide layer.
図１に示す通り、鋼板は、前段の加熱ゾーンにおいて約６００℃まで急速に昇温し、後段の加熱ゾーンにおいて約６７０℃までゆっくり昇温する。この際、酸化層の成長速度は、前段の加熱ゾーンにおいて増大するが、後段の加熱ゾーンにおいてわずかに減少した後、略一定となる。図１のシミュレーションでは、後段の加熱ゾーンの出口での酸化層の成長速度が秒速０．０１５μｍ以下となった。したがって、このシミュレーションのように、前段において鋼板を急速に加熱し、後段において鋼板をゆっくり加熱すると、鋼板の酸化層厚のばらつきが抑制されるといえる。 As shown in FIG. 1, the steel sheet is rapidly heated to about 600 ° C. in the preceding heating zone and slowly heated to about 670 ° C. in the subsequent heating zone. At this time, the growth rate of the oxide layer increases in the preceding heating zone, but becomes substantially constant after slightly decreasing in the succeeding heating zone. In the simulation of FIG. 1, the growth rate of the oxide layer at the outlet of the subsequent heating zone was 0.015 μm or less per second. Therefore, it can be said that when the steel sheet is rapidly heated in the former stage and the steel sheet is slowly heated in the latter stage as in this simulation, variations in the oxide layer thickness of the steel sheet are suppressed.
図２には、従来の加熱方法で鋼板を加熱した場合の計算結果が示されている。図２の短い破線は、前段の加熱ゾーン及び後段の加熱ゾーンの炉温を１０８０℃で一定とした例であり、図２の長い破線は、前段の加熱ゾーン及び後段の加熱ゾーンにおけるバーナーの燃焼量を一定とした例である。炉温を一定とした例の後段の燃焼量は、前段の燃焼量の約７６％である。炉温を一定とした例では、鋼板は、約７６０℃まで略一定の速度で昇温し、酸化層の成長速度は加熱時間全体を通じて増大する。また、バーナーの燃焼量を一定とした例では、炉温が１０００℃から１１４０℃まで少しずつ上昇したものの、鋼板の昇温速度及び酸化層の成長速度については、炉温を一定とした例と同様の振る舞いであった。これら２つの例では、後段の加熱ゾーンの出口での酸化層の成長速度が秒速０．０１５μｍを超えた。したがって、これらのシミュレーションのように実際に鋼板を加熱すると、鋼板の酸化層厚のばらつきは抑制されないといえる。 FIG. 2 shows a calculation result when the steel sheet is heated by the conventional heating method. The short broken line in FIG. 2 is an example in which the furnace temperature in the preceding heating zone and the subsequent heating zone is constant at 1080 ° C., and the long broken line in FIG. 2 indicates burner combustion in the preceding heating zone and the subsequent heating zone. This is an example in which the amount is constant. The amount of combustion in the latter stage of the example in which the furnace temperature is constant is about 76% of the amount of combustion in the former stage. In an example in which the furnace temperature is constant, the steel sheet is heated to approximately 760 ° C. at a substantially constant rate, and the growth rate of the oxide layer increases throughout the heating time. Further, in the example in which the burner combustion amount is constant, the furnace temperature is gradually increased from 1000 ° C. to 1140 ° C., but the temperature increase rate of the steel sheet and the growth rate of the oxide layer are The behavior was similar. In these two examples, the growth rate of the oxide layer at the outlet of the subsequent heating zone exceeded 0.015 μm per second. Therefore, when the steel sheet is actually heated as in these simulations, it can be said that the variation in the oxide layer thickness of the steel sheet is not suppressed.
図３には、前段の加熱ゾーンの炉温を１３００℃とし、後段の加熱ゾーンでの加熱条件を異ならせた場合の鋼板の板温及び酸化層厚の算出結果が示されている。図３の実線は、後段の加熱ゾーンの炉温を２５０℃とし、後段の加熱ゾーンの加熱時間を１２秒とした例である。図３の長い破線は、後段の加熱ゾーンの炉温を７５０℃とし、後段の加熱ゾーンの加熱時間を１３秒とした例である。図３の短い破線は、後段の加熱ゾーンの炉温を８５０℃とし、後段の加熱ゾーンの加熱時間を１４秒とした例である。 FIG. 3 shows the calculation results of the plate temperature and oxide layer thickness of the steel sheet when the furnace temperature in the heating zone in the previous stage is 1300 ° C. and the heating conditions in the heating zone in the subsequent stage are varied. The solid line in FIG. 3 is an example in which the furnace temperature in the latter heating zone is 250 ° C. and the heating time in the latter heating zone is 12 seconds. The long broken line in FIG. 3 is an example in which the furnace temperature in the subsequent heating zone is set to 750 ° C. and the heating time in the subsequent heating zone is set to 13 seconds. The short broken line in FIG. 3 is an example in which the furnace temperature in the subsequent heating zone is set to 850 ° C. and the heating time in the subsequent heating zone is set to 14 seconds.
図３の実線のシミュレーションでは、前段の加熱ゾーンにおいて鋼板が６７０℃まで昇温し、後段の加熱ゾーンにおいて鋼板が６００℃まで降温する。また、このシミュレーションの酸化層の成長速度は、前段の加熱ゾーンにおいて増大するが後段の加熱ゾーンにおいて減少する。 In the solid line simulation of FIG. 3, the temperature of the steel plate is increased to 670 ° C. in the preceding heating zone, and the temperature of the steel plate is decreased to 600 ° C. in the subsequent heating zone. In addition, the growth rate of the oxide layer in this simulation increases in the preceding heating zone but decreases in the subsequent heating zone.
図３の長い破線のシミュレーションでは、前段の加熱ゾーンにおいて鋼板が６２０℃まで昇温し、後段の加熱ゾーンにおいて鋼板が６５０℃まで昇温する。また、このシミュレーションの酸化層の成長速度は、前段の加熱ゾーンにおいて増大するが後段の加熱ゾーンにおいてわずかに減少する。 In the long broken line simulation of FIG. 3, the steel plate is heated to 620 ° C. in the preceding heating zone, and the steel plate is heated to 650 ° C. in the subsequent heating zone. In addition, the growth rate of the oxide layer in this simulation increases in the preceding heating zone but slightly decreases in the subsequent heating zone.
図３の短い破線のシミュレーションでは、前段の加熱ゾーンにおいて鋼板が５８０℃まで昇温し、後段の加熱ゾーンにおいて鋼板が６９０℃まで昇温する。また、このシミュレーションの酸化層の成長速度は、前段の加熱ゾーンにおいて増大するが後段の加熱ゾーンにおいて略一定となる。 In the short broken line simulation of FIG. 3, the steel plate is heated to 580 ° C. in the preceding heating zone, and the steel plate is heated to 690 ° C. in the subsequent heating zone. In addition, the growth rate of the oxide layer in this simulation increases in the preceding heating zone but becomes substantially constant in the subsequent heating zone.
図３の実線のシミュレーション及び長い破線のシミュレーションでは、後段の加熱ゾーンの出口での酸化層の成長速度が秒速０．０１５μｍ以下となった。したがって、これらのシミュレーションのように実際に鋼板を加熱すると、鋼板の酸化層厚のばらつきが抑制されるといえる。 In the solid line simulation and the long broken line simulation of FIG. 3, the growth rate of the oxide layer at the outlet of the subsequent heating zone was 0.015 μm or less per second. Therefore, when the steel sheet is actually heated as in these simulations, it can be said that variations in the oxide layer thickness of the steel sheet are suppressed.
一方、図３の短い破線のシミュレーションでは、前段の加熱ゾーンにおける鋼板の到達温度が６００℃未満であり、後段の加熱ゾーンの出口での酸化層の成長速度が秒速０．０１５μｍを超えた。したがって、これらのシミュレーションのように実際に鋼板を加熱すると、鋼板の酸化層厚のばらつきは抑制されないといえる。 On the other hand, in the short broken line simulation of FIG. 3, the temperature reached by the steel sheet in the preceding heating zone was less than 600 ° C., and the growth rate of the oxide layer at the outlet of the succeeding heating zone exceeded 0.015 μm per second. Therefore, when the steel sheet is actually heated as in these simulations, it can be said that the variation in the oxide layer thickness of the steel sheet is not suppressed.
本発明の溶融亜鉛めっき用鋼板の加熱方法は、鋼板を直火で加熱する場合であっても、鋼板表面に均一な厚みの酸化鉄層を形成できる。このため、本発明の溶融亜鉛めっき用鋼板の加熱方法は、高強度溶融亜鉛めっき鋼板を製造する方法として利用できる。 The method for heating a hot-dip galvanized steel sheet of the present invention can form an iron oxide layer having a uniform thickness on the surface of the steel sheet even when the steel sheet is heated by direct fire. For this reason, the method for heating a hot-dip galvanized steel sheet according to the present invention can be used as a method for producing a high-strength hot-dip galvanized steel sheet.
Claims (2)
上記複数の加熱ゾーンの前段において鋼板を昇温速度が秒速３０℃以上の条件で加熱する第一工程と、
上記複数の加熱ゾーンの後段において上記鋼板を昇温速度が秒速５℃以下の条件で加熱する第二工程と
を有し、
上記第二工程における鋼板の加熱時間が上記第一工程における鋼板の加熱時間の１／４以上である溶融亜鉛めっき用鋼板の加熱方法。 A heating method for a hot-dip galvanized steel sheet using a direct-fired heating furnace having a plurality of heating zones and heating with a burner while conveying a strip-shaped steel sheet in the longitudinal direction through these heating zones,
A first step of heating the steel sheet at a temperature increase rate of 30 ° C. or more in the preceding stage of the plurality of heating zones;
A second step of heating the steel sheet at a temperature rising rate of 5 ° C. or less at a later stage of the plurality of heating zones,
A method for heating a hot-dip galvanized steel sheet, wherein the heating time of the steel sheet in the second step is ¼ or more of the heating time of the steel sheet in the first step.
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| JPH0230720A (en) * | 1988-07-16 | 1990-02-01 | Kobe Steel Ltd | Method for heating steel sheet |
| JP2008266778A (en) * | 2007-03-22 | 2008-11-06 | Jfe Steel Kk | High-strength hot dip zinc-plated steel sheet having excellent moldability, and method for production thereof |
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| JP2015151605A (en) * | 2014-02-18 | 2015-08-24 | Ｊｆｅスチール株式会社 | High strength hot-dip galvanized steel sheet and method for producing the same |
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|---|---|---|---|---|
| JPH0230720A (en) * | 1988-07-16 | 1990-02-01 | Kobe Steel Ltd | Method for heating steel sheet |
| JP2008266778A (en) * | 2007-03-22 | 2008-11-06 | Jfe Steel Kk | High-strength hot dip zinc-plated steel sheet having excellent moldability, and method for production thereof |
| WO2013125399A1 (en) * | 2012-02-22 | 2013-08-29 | 新日鐵住金株式会社 | Cold-rolled steel sheet and manufacturing method for same |
| WO2014188697A1 (en) * | 2013-05-21 | 2014-11-27 | Ｊｆｅスチール株式会社 | Method for manufacturing high-strength alloyed hot-dip galvanized steel plate |
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