JP3945161B2

JP3945161B2 - Heat treatment method for thick steel plate

Info

Publication number: JP3945161B2
Application number: JP2000394582A
Authority: JP
Inventors: 晃夫藤林; 善通日野; 章多賀根; 正敏杉岡; 宏関根
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2000-12-26
Filing date: 2000-12-26
Publication date: 2007-07-18
Anticipated expiration: 2020-12-26
Also published as: JP2002194430A

Description

【０００１】
【発明の属する技術分野】
本発明は、厚鋼板の熱処理方法に関し、特に、厚鋼板を連続的に加熱して熱処理する方法に関する。
【０００２】
【従来の技術】
厚鋼板は一般に焼き入れ処理、その後の焼戻し処理によって高強度・高靭性を得るプロセスが熱処理方法として行われている。特に焼戻処理は燃焼をエネルギー源とした炉による熱処理が一般的であり、特開平９ー２５６０５３号公報のように、温度パターンを工夫して能率を上げることが行われていた。また、具体的な手段の記載がないものの、特開平４ー３５８０２２号公報のように、温度上昇速度を大きくとることで能率を上げる方法もあった。
【０００３】
一方、誘導加熱による加熱方法も開示されている。特開平９ー２２５５１７号公報では、熱延鋼板の製造プロセスにおいて、粗圧延された粗バーの仕上圧延機入側の温度が粗バーの長手方向に一様になるように、誘導加熱でバーを加熱する方法が開示されている。この中には、熱圧延鋼板を一定速度で通過させ、仕上圧延機入側温度が一定になるように加熱する方法が示されている。
【０００４】
【発明が解決しようとする課題】
しかしながら、ガス燃焼における加熱方式では（熱処理）炉による鋼材の熱処理において、ふく射や対流伝熱によって鋼板に熱が伝わるために、急速な加熱はできなかった。そのために加熱効率が悪く、例えば厚み２０ｍｍ程度の厚鋼板であっても１５分から４０分程の加熱時間が必要で、そのために、加熱装置は鋼板をすべて挿入するいわゆる加熱炉のような方式が一般的であった。
【０００５】
一方、誘導加熱方式は、鋼板の表面の温度を急速に上げることは比較的容易にできるが、連続的に昇熱させるためには、搬送速度をかなり落としてゆっくり通板させる必要があり、その場合、表面は直ぐに所望の加熱温度に到達するが、中心部はある時間遅れて昇熱する。従って、特に均熱性が要求される厚鋼板の熱処理では、厚鋼板の各部を所望の温度領域に入れるためには誘導加熱装置のコイル長を長くする、長手方向の投入パワーを変更する等の工夫が必要で、厚鋼板各部の均熱性を確保する制御は非常に難しかった。
【０００６】
本発明は、上記のような課題を解決するためになされたもので、誘導加熱により温度上昇速度を高めるとともに、特にソレノイド型の誘導加熱を適用することにより、表面を加熱した後に引き続いて、均熱性を上げるために内部まで十分に熱が拡散し、その間、鋼板の温度が冷えないように保熱する保熱炉を設けることで、均熱性を十分に確保することができる高能率の熱処理方法を提供することを目的とする。
【０００７】
【課題を解決するための手段】
本発明にかかる厚鋼板の熱処理方法は、次のように構成した。
（１）厚鋼板を熱処理する方法であって、ソレノイド型誘導加熱装置と同一ライン上であってソレノイド型誘導加熱装置の上流側と下流側に放射率の小さい断熱板を配置した保熱炉を設け、この保熱炉内に厚鋼板を往復通過させて厚鋼板を加熱するようにしたものである。
【０００８】
（２）上記（１）の厚鋼板の熱処理方法であって、ソレノイド型誘導加熱装置が２基以上同一ライン上に近接配置されている。
【０００９】
【発明の実施の形態】
図１、図２はそれぞれ本発明に係る厚鋼板の熱処理設備の実施の形態を示す模式図である。１は熱処理を行う厚鋼板、２は厚鋼板１を連続的に加熱するソレノイド型誘導加熱装置（以下、誘導加熱装置という）で、この誘導加熱装置２は、第１〜第３の３基の誘導加熱装置２ａ〜２ｃを近接させて直線的に並べてある。
【００１０】
３は厚鋼板１を均熱化するための保熱炉で、誘導加熱装置２の直近（下流側である後部（図１）、または上流側と下流側である前後部（図２））に設けられ、断熱板４により上下左右を覆ってその中に厚鋼板１を通過させ、厚鋼板１の均熱化を行う。この断熱板４は、それ自身が放射率の小さい、例えば金属光沢のステンレス薄板などであって、厚鋼板１からの放射熱を反射して返すような素材が望ましい。断熱板４の放射率は小さい程性能が優れているが、目安として放射率が０．５以下、望ましくは０．３以下が好ましい。さらに、この断熱板４は、即応して表面温度があがるように極力薄く、裏側には断熱材が裏打ちされたような構造のものが望ましい。
５は第１〜第３の誘導加熱装置２ａ〜２ｃの間、誘導加熱装置２ａ〜２ｃの前後および保熱炉３内に配設した搬送用のロールで、回転数制御が細かく制御可能としている。なお、６は温度計である。
【００１１】
上記のように構成した厚鋼板１の熱処理設備においては、被加熱物である厚鋼板１を誘導加熱装置２によって連続的に加熱した後に、保熱炉３内を通過させ（図１）、または誘導加熱装置２と保熱炉３内を往復通過させながら（図２）、そのあいだに表面に発生した熱を拡散させて、厚鋼板１の内部まで均熱させる。
上記の誘導加熱装置２によって加熱制御を行う手順を、以下の実施例１，２で詳述する。
【００１２】
［実施例１］
まず、図１を用いて、実施例１を説明する。本実施例は、被加熱材である厚鋼板１をまず誘導加熱装置２によって連続的に加熱し、その後、厚鋼板１の上下を断熱材４ではさんで保熱炉３内を通過させて均熱化を行うものである。
誘導加熱装置２は、間口が幅４７００ｍｍ、高さ２００ｍｍで、長さが約８０ｃｍの第１、第２、第３の誘導加熱装置２ａ，２ｂ，２ｃを３基近接させて直線的に並べており、各誘導加熱装置２ａ，２ｂ，２ｃの間には搬送用のロール５が設けられている。一方、誘導加熱装置２の後において、保熱炉３内では厚鋼板１の下面の各ロール５間に断熱板４を挿入し、厚鋼板１の上面には厚鋼板１を覆うように断熱板４を設置して、断熱板４に挟まれた空間に厚鋼板１を通過させる。この断熱板４の長さは８ｍである。
【００１３】
断熱板４は、厚み１ｍｍの表面が光沢を持つステンレス板（放射率は０．１５〜０．２）をセラミック性の断熱材で裏打ちしたものである。光沢のあるステンレス鋼板は、放射率が低く、すなわち熱放射を多く反射するので断熱効果が高く、また、厚みが薄いために、雰囲気温度に追従してすぐにステンレス温度が上昇するので、熱慣性が小さくて済み、熱損失が小さく済む。
【００１４】
このような熱処理装置において、幅４５００ｍｍ、長さ４８ｍ、厚み２０ｍｍの常温の厚鋼板１を、誘導加熱装置２を通した後に、続けて保熱炉３内を連続的に通過させて加熱および均熱化させ、焼鈍温度である６５０℃まで加熱した。このときの誘導加熱装置２の周波数は１０００Ｈｚ、投入パワーは前段から順に、１、０．５、０．２ＭＷ／ｍ²、搬送速度は２ｍｐｍとした。厚鋼板１にとりつけた熱伝対の出力より求めた厚鋼板１の表面と板厚方向中心部の測温結果より、保熱炉３を出た時点の厚鋼板１の表面と板厚方向中心部の温度差は０．１℃以下であって、ほぼ均熱されて焼きあがった。
【００１５】
誘導加熱はその性格上、被加熱物の表面に発熱が集中し、表面部が先行して昇熱し、その傾向は、投入パワーが大きいほど、周波数が高いほど、表面の発熱効果が大きくなる。急速に加熱するためにはパワーを大きくするが、その分、誘導加熱装置２を出た時点での均熱度は悪く、すなわち、厚鋼板１の表面と板厚方向の中心部の温度差が大きくなる。そこで、本発明は、第１の誘導加熱装置２ａから第２、第３の誘導加熱装置２ａ，２ｂの投入パワーを順次減少させている。しかしながら、投入パワーを下げれば、能率が落ちるので、必要な能率に応じて投入パワーを選択すればよい。
【００１６】
また、本実施例では、第１から第３までの誘導加熱装置２ａ〜２ｃの周波数を１０００Ｈｚ一定としたが、さらに厚鋼板１の表面と板厚方向の中心部の焼戻しのパターンを均一化させるためには、例えば第１〜第３の誘導加熱装置２ａ〜２ｃの周波数を高周波数から低周波数へ、例えば１０００Ｈｚから５０Ｈｚまで順次下がるように設定すると、１つの設備で誘導発熱の浸透深さを徐々に深くすることで、より均熱性が得られる。
【００１７】
なお、本実施例で用いた誘導加熱装置２の周波数は１０００Ｈｚで、常温の鋼を加熱するのに適しているといわれる周波数５０Ｈｚ程度に比べて高周波数となっているが、これは、加熱して鋼がキュリー点（７３０℃）を越えると比透磁率が小さくなるので、熱処理装置としてキュリー点を越える温度域でも加熱が行えるように１０００Ｈｚを選んだ。しかしながら、設備の熱処理温度がキュリー点を越えない場合には、低周波側の周波数を例えば５０Ｈｚ〜５００Ｈｚと適宜の周波数として選定すればよい。
また、この熱処理装置でさまざまな厚みの厚鋼板を熱処理する場合は、各板厚毎に、表面と板厚中央部の温度が所望とされる均熱度に応じて熟熱されるように、使用する誘導加熱装置２の投入パワーを調整すればよい。
【００１８】
［実施例２］
次に、図２を用いて、実施例２を説明する。本実施例は、誘導加熱装置２の前後に断熱板４で上下左右を覆った保熱炉３を設け、その中で厚鋼板１を往復通過させながら加熱・均熱を行うものである。この誘導加熱装置２の構成は、実施例１と同じ３基の誘導加熱装置２ａ〜２ｃを近接させて直線的に並べたもので、各誘導加熱装置２ａ〜２ｃの間には搬送用のロール５が設けられている。一方、保熱炉３の断熱板４は、それ自身が放射率の低い、例えば金属光沢のステンレス薄板などで、厚鋼板１からの放射熱を反射して返すような素材で構成され、即応して表面温度があがるように、極力薄く、裏側には断熱材が裏打ちされたような構造となっている。
【００１９】
誘導加熱装置２は、間口が幅４７００ｍｍ、高さ２００ｍｍで、長さが約８０ｃｍの第１、第２、第３の誘導加熱装置２ａ，２ｂ，２ｃを３基近接させて直線的に並べており、各誘導加熱装置２ａ，２ｂ，２ｃの間には搬送用のロール５が設けられている。一方、誘導加熱装置２の前後において、保熱炉３内では厚鋼板１の下面の各ロール５間に断熱板３を挿入し、厚鋼板１の上面には、厚鋼板１を覆うように断熱板４を設置して、断熱板４に挟まれた空間に厚鋼板１を通過させる。この断熱板４の長さは２０ｍである。
【００２０】
断熱板４は、厚み１ｍｍの表面が光沢を持つステンレス板をセラミック性の断熱材で裏打ちしたものである。これは、光沢のあるステンレス鋼板は放射率が低くすなわち熱放射を多く反射するので断熱効果が高く、また、厚みが薄いために、雰囲気温度に追従してすぐにステンレス温度が上昇するので、熱慣性が小さくて済み、熱損失が小さく済む。
【００２１】
このような熱処理装置において、幅４５００ｍｍ、長さ１８ｍ、厚み５０ｍｍの常温の厚鋼板１を、誘導加熱装置２と保熱炉３との中を往復通過させ、保熱炉３からはみださないようにオッシレートさせながら連続的に加熱および均熱化させ、焼鈍温度である６５０℃まで加熱した。このときの誘導加熱装置２の周波数は１０００Ｈｚ、投入パワーは前段から順に、１、０．５、０．２ＭＷ／ｍ²、搬送速度は２０ｍｐｍとしたが、厚鋼板１の昇温にともなって投入パワーを徐々に下げていった。厚鋼板１にとりつけた熱電対の出力より求めた厚鋼板１の表面と板厚方向の中心部の測温結果より、保熱板を出た時点の厚鋼板１の表面と板厚方向の中心部の温度差は２℃以内であって、ほぼ均熱されて焼きあがった。
【００２２】
［比較例］
比較例として、実施例１，２で説明した設備において、誘導加熱装置を１回通過させて加熱した場合（比較例１）と、往復通過させて加熱した場合（比較例２）の厚鋼板１の表面温度と板厚中心部の温度との温度差（均熱度）を比較した。
比較例１の加熱条件は、実施例１において保熱炉３がない場合に相当し、すなわち、誘導加熱装置２の周波数は１０００Ｈｚ、投入パワーは前段から順に、１、０．５、０．２ＭＷ／ｍ²、搬送速度は２ｍｐｍに、幅４５００ｍｍで、長さ４８ｍ、厚み２０ｍｍの常温の厚鋼板１を誘導加熱装置２を１回通過させた場合の、厚鋼板１の表面と板厚方向の中心部の温度差を計測したところ、厚鋼板１の表面と板厚方向の中心部の温度差が３０℃と大きく、均熱性が劣っていた。その結果、この条件では表面は焼戻効果が得られたが、中心部は焼戻効果がなく、均質な材料が得られなかった。これは誘導加熱装置２を通過した直後は、厚鋼板表面の温度が上がっていたが、熱が板厚中心まで拡散するまえに放熱によって表面温度が下がり始めるため、十分な焼戻し効果が得られなかったと考えられる。
【００２３】
比較例２の加熱条件は、実施例２で示す装置において、幅４５００ｍｍ、長さ１８ｍ、厚み５０ｍｍの常温の厚鋼板１を、誘導加熱装置２を往復通過させ、連続的に加熱し、焼鈍温度である６５０℃まで加熱した。このときの誘導加熱装置２の周波数は１０００Ｈｚ、投入パワーは前段から順に、当初１、０．５、０．２ＭＷ／ｍ²、搬送速度は２０ｍｐｍとしたが、厚鋼板１の温度の昇温にともなって投入パワーは徐々に下げていった。本比較例２は実施例２において誘導加熱装置２のみを用いて加熱した場合に相当する。その結果、厚鋼板１先端部の表面が６５０℃になった時点で厚鋼板１内の各部の温度を調べたところ、中心部は６２０℃、また、板中央部の板厚中心温度が板先端部と板後端部の板厚中心温度より３０℃高く、均熱性が劣っていた。これは往復通過させる際に、誘導加熱装置２を出たところは順次放熱によって温度が下がるために、厚鋼板長手方向で板中央部が温度が高くなる傾向があるためである。
【００２４】
【発明の効果】
以上の説明から明らかなように、本発明は、誘導加熱によって大きな温度上昇速度を与えることにより、従来よりも高い加熱速度を得ることができ、かつ、続けて保熱炉内で均熱させることで、厚鋼板各部の温度が同じになるように熟熱加熱が連続的に行われ、結果として、非常に効率のよい熟熱加熱が可能となった。従って、本発明によれば、誘導過熱による高い昇熱速度と、保熱炉による高い均熱性とが同時に得られるので、温度の均一化を図りながら、効率的な加熱を行うことができる。また、設備長も短くて済み、省スペース化を実現できる。勿論、高い均熱度を得られることで、熱処理後の厚鋼板各部の強度差が少なく、より均一な材質にすることができる。その結果、材質はずれによる格落ちがなく、製品歩留まりが高い。
【図面の簡単な説明】
【図１】本発明の実施例１における熱処理設備を示す模式図である。
【図２】本発明の実施例２における熱処理設備を示す模式図である。
【符号の説明】
１厚鋼板
２誘導加熱装置
２ａ第１の誘導加熱装置
２ｂ第２の誘導加熱装置
２ｃ第３の誘導加熱装置
３保熱炉
４断熱板
５搬送用のロール[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a heat treatment how a steel plate, in particular, relates to how to heat treatment by heating the steel plate continuously.
[0002]
[Prior art]
In general, a process for obtaining high strength and high toughness by quenching treatment and subsequent tempering treatment is performed as a heat treatment method for thick steel plates. In particular, tempering is generally performed by a furnace using combustion as an energy source, and as described in Japanese Patent Laid-Open No. 9-256053, the temperature pattern is devised to increase efficiency. Further, although there is no description of specific means, there has been a method of increasing efficiency by increasing the temperature rise rate as disclosed in Japanese Patent Laid-Open No. 4-358022.
[0003]
On the other hand, a heating method by induction heating is also disclosed. In Japanese Patent Laid-Open No. 9-225517, in the process of manufacturing a hot rolled steel sheet, a bar is formed by induction heating so that the temperature at the entrance to the finishing mill of the rough rolled rough bar is uniform in the longitudinal direction of the rough bar. A method of heating is disclosed. In this, a method is shown in which a hot-rolled steel sheet is passed at a constant speed and heated so that the temperature on the finishing mill entry side is constant.
[0004]
[Problems to be solved by the invention]
However, in the heating method in gas combustion (heat treatment), in the heat treatment of the steel material in the furnace, heat is transferred to the steel plate by radiation or convective heat transfer, so that rapid heating cannot be performed. Therefore, the heating efficiency is poor. For example, even a thick steel plate having a thickness of about 20 mm requires a heating time of about 15 to 40 minutes. For this reason, the heating device is generally a so-called heating furnace in which all steel plates are inserted. It was the target.
[0005]
On the other hand, the induction heating method can relatively easily raise the temperature of the surface of the steel sheet, but in order to raise the temperature continuously, it is necessary to slow down the conveying speed and pass it slowly. In this case, the surface immediately reaches the desired heating temperature, but the central portion heats up with a certain delay. Therefore, especially in the heat treatment of thick steel plates that require soaking, in order to put each part of the thick steel plates in the desired temperature range, the coil length of the induction heating device is lengthened, and the input power in the longitudinal direction is changed. Therefore, it is very difficult to control the temperature uniformity of each part of the thick steel plate.
[0006]
The present invention has been made to solve the above-described problems. In addition to increasing the rate of temperature increase by induction heating, in particular, by applying solenoid-type induction heating, the surface is heated and subsequently leveled. A high-efficiency heat treatment method that ensures sufficient soaking by providing a heat-retaining furnace that keeps the steel plate warm enough so that the heat is sufficiently diffused to increase the heat resistance. The purpose is to provide the law .
[0007]
[Means for Solving the Problems]
Heat treatment how a steel plate according to the present invention is constituted as follows.
( 1 ) A method of heat-treating a thick steel plate, comprising a heat-retaining furnace in which heat insulating plates having a low emissivity are arranged on the same line as the solenoid induction heating device and upstream and downstream of the solenoid induction heating device. The thick steel plate is reciprocally passed through the heat-retaining furnace to heat the thick steel plate.
[0008]
( 2 ) The method for heat treating a thick steel plate according to (1 ) above, wherein two or more solenoid induction heating devices are arranged close to each other on the same line.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 and FIG. 2 are schematic views each showing an embodiment of a heat treatment facility for thick steel plates according to the present invention. 1 is a thick steel plate for heat treatment, 2 is a solenoid type induction heating device (hereinafter referred to as induction heating device) for continuously heating the thick steel plate 1, and this induction heating device 2 includes first to third three units. The induction heating devices 2a to 2c are arranged close to each other in a straight line.
[0010]
3 is a heat-retaining furnace for soaking the thick steel plate 1 in the immediate vicinity of the induction heating device 2 (the rear part on the downstream side (FIG. 1) or the front and rear parts on the upstream side and the downstream side (FIG. 2)). The heat insulating plate 4 is provided to cover the upper, lower, left, and right sides, and the thick steel plate 1 is passed therethrough so that the thick steel plate 1 is soaked. The heat insulating plate 4 is preferably a material that itself has a low emissivity, such as a metallic thin stainless steel plate, and reflects and returns the radiant heat from the thick steel plate 1. The smaller the emissivity of the heat insulating plate 4 is, the better the performance is. However, as a guide, the emissivity is 0.5 or less, preferably 0.3 or less. Further, the heat insulating plate 4 is preferably as thin as possible so that the surface temperature rises immediately, and has a structure in which a heat insulating material is lined on the back side.
Reference numeral 5 denotes a transport roll disposed between the first to third induction heating devices 2a to 2c, before and after the induction heating devices 2a to 2c and in the heat insulation furnace 3, and the rotation speed control can be finely controlled. . Reference numeral 6 denotes a thermometer.
[0011]
In the heat treatment facility for the thick steel plate 1 configured as described above, the thick steel plate 1 as the object to be heated is continuously heated by the induction heating device 2 and then passed through the heat insulating furnace 3 (FIG. 1), or While reciprocatingly passing through the induction heating device 2 and the heat-retaining furnace 3 (FIG. 2), the heat generated on the surface is diffused between them and is soaked to the inside of the thick steel plate 1.
The procedure for performing the heating control by the induction heating device 2 will be described in detail in Examples 1 and 2 below.
[0012]
[Example 1]
First, Example 1 will be described with reference to FIG. In this embodiment, the thick steel plate 1 that is the material to be heated is first heated continuously by the induction heating device 2, and then, the upper and lower sides of the thick steel plate 1 are passed through the heat insulating furnace 3 with the heat insulating material 4 interposed therebetween. Heating is performed.
The induction heating device 2 is linearly arranged with three first, second, and third induction heating devices 2a, 2b, and 2c having a frontage of 4700 mm in width and a height of 200 mm and a length of about 80 cm in proximity to each other. A transport roll 5 is provided between the induction heating devices 2a, 2b, and 2c. On the other hand, after the induction heating device 2, the heat insulating plate 4 is inserted between the rolls 5 on the lower surface of the thick steel plate 1 in the heat insulating furnace 3, and the upper surface of the thick steel plate 1 is covered with the thick steel plate 1. 4 is installed, and the thick steel plate 1 is passed through a space sandwiched between the heat insulating plates 4. The length of the heat insulating plate 4 is 8 m.
[0013]
The heat insulating plate 4 is a stainless steel plate (emissivity 0.15 to 0.2) having a glossy surface with a thickness of 1 mm and is backed with a ceramic heat insulating material. Glossy stainless steel sheet has low emissivity, that is, it reflects a lot of heat radiation, so it has a high thermal insulation effect, and since its thickness is thin, the stainless steel temperature rises immediately following the ambient temperature, so thermal inertia Is small and heat loss is small.
[0014]
In such a heat treatment apparatus, after passing through the induction heating apparatus 2, the room-temperature steel plate 1 having a width of 4500 mm, a length of 48 m, and a thickness of 20 mm is continuously passed through the heat-retaining furnace 3 for heating and equalization. Heated to 650 ° C. which is an annealing temperature. At this time, the frequency of the induction heating device 2 was 1000 Hz, the input power was 1, 0.5, 0.2 MW / m ² and the conveyance speed was ² mpm in order from the previous stage. From the temperature measurement results of the surface of the thick steel plate 1 and the central portion in the thickness direction obtained from the output of the thermocouple attached to the thick steel plate 1, the surface of the thick steel plate 1 and the center in the thickness direction when leaving the heat-retaining furnace 3 The temperature difference of the part was 0.1 ° C. or less, and it was almost soaked and baked.
[0015]
Due to the nature of induction heating, heat generation concentrates on the surface of the object to be heated, and the surface portion heats up first, and the tendency is that the larger the input power, the higher the frequency, the greater the surface heat generation effect. In order to heat rapidly, the power is increased, but the heat equalization degree when the induction heating device 2 is exited is accordingly reduced, that is, the temperature difference between the surface of the thick steel plate 1 and the central portion in the thickness direction is large. Become. Therefore, in the present invention, the input power from the first induction heating device 2a to the second and third induction heating devices 2a and 2b is sequentially reduced. However, if the input power is lowered, the efficiency is lowered. Therefore, the input power may be selected according to the required efficiency.
[0016]
In this embodiment, the frequency of the first to third induction heating devices 2a to 2c is constant at 1000 Hz, but the surface of the thick steel plate 1 and the tempering pattern at the center in the thickness direction are further uniformized. For this purpose, for example, if the frequency of the first to third induction heating devices 2a to 2c is set so as to decrease sequentially from a high frequency to a low frequency, for example, from 1000 Hz to 50 Hz, the penetration depth of induction heat generation can be reduced with one facility. By increasing the depth gradually, more uniform temperature can be obtained.
[0017]
The frequency of the induction heating device 2 used in this example is 1000 Hz, which is higher than the frequency of about 50 Hz which is said to be suitable for heating steel at room temperature. Therefore, when the steel exceeds the Curie point (730 ° C.), the relative permeability becomes small. Therefore, 1000 Hz was selected as a heat treatment apparatus so that heating can be performed even in a temperature range exceeding the Curie point. However, if the heat treatment temperature of the equipment does not exceed the Curie point, the frequency on the low frequency side may be selected as an appropriate frequency, for example, 50 Hz to 500 Hz.
When heat-treating thick steel plates of various thicknesses with this heat treatment device, use so that the temperature of the surface and the center of the plate thickness is matured according to the desired degree of soaking for each plate thickness. What is necessary is just to adjust the input power of the induction heating apparatus 2.
[0018]
[Example 2]
Next, Example 2 will be described with reference to FIG. In this embodiment, a heat-retaining furnace 3 is provided before and after the induction heating device 2 so as to cover the upper, lower, left and right sides with a heat insulating plate 4, and heating and soaking are performed while reciprocating the thick steel plate 1 therein. The configuration of the induction heating device 2 is the same three induction heating devices 2a to 2c as in the first embodiment, which are arranged in a straight line, and a transport roll is provided between the induction heating devices 2a to 2c. 5 is provided. On the other hand, the heat insulating plate 4 of the heat insulating furnace 3 is made of a material having a low emissivity, for example, a metallic thin stainless steel plate, which reflects and returns the radiant heat from the thick steel plate 1 and responds quickly. In order to increase the surface temperature, the structure is as thin as possible and the back is covered with a heat insulating material.
[0019]
The induction heating device 2 is linearly arranged with three first, second, and third induction heating devices 2a, 2b, and 2c having a frontage of 4700 mm in width and a height of 200 mm and a length of about 80 cm in proximity to each other. A transport roll 5 is provided between the induction heating devices 2a, 2b, and 2c. On the other hand, before and after the induction heating device 2, the heat insulating plate 3 is inserted between the rolls 5 on the lower surface of the thick steel plate 1 in the heat insulating furnace 3, and the upper surface of the thick steel plate 1 is insulated so as to cover the thick steel plate 1. The plate 4 is installed and the thick steel plate 1 is passed through the space between the heat insulating plates 4. The length of the heat insulating plate 4 is 20 m.
[0020]
The heat insulating plate 4 is made of a stainless steel plate having a glossy surface with a thickness of 1 mm and backed with a ceramic heat insulating material. This is because the glossy stainless steel sheet has a low emissivity, that is, a high thermal insulation effect because it reflects a large amount of heat radiation, and since the thickness is thin, the stainless steel temperature rises immediately following the ambient temperature, so the heat Inertia is small and heat loss is small.
[0021]
In such a heat treatment apparatus, a room-temperature thick steel plate 1 having a width of 4500 mm, a length of 18 m, and a thickness of 50 mm is reciprocated through the induction heating apparatus 2 and the heat insulation furnace 3 so as to protrude from the heat insulation furnace 3. Heating and soaking were carried out continuously while oscillating so as to avoid oxidization, and heating was performed to an annealing temperature of 650 ° C. At this time, the frequency of the induction heating device 2 is 1000 Hz, the input power is 1, 0.5, 0.2 MW / m ² and the conveyance speed is 20 mpm in order from the previous stage. The power was gradually lowered. From the temperature measurement results of the surface of the thick steel plate 1 and the central portion in the thickness direction obtained from the output of the thermocouple attached to the thick steel plate 1, the surface of the thick steel plate 1 and the center in the thickness direction at the time of exiting the heat insulating plate The temperature difference of the part was within 2 ° C. and was almost soaked and baked.
[0022]
[Comparative example]
As a comparative example, in the facilities described in Examples 1 and 2, the thick steel plate 1 when heated by passing through an induction heating device once (Comparative Example 1) and when heated by reciprocating passage (Comparative Example 2). The temperature difference between the surface temperature and the temperature at the center of the plate thickness (degree of soaking) was compared.
The heating conditions of Comparative Example 1 correspond to the case where there is no heat insulation furnace 3 in Example 1, that is, the frequency of the induction heating device 2 is 1000 Hz, and the input power is 1, 0.5, 0.2 MW in order from the previous stage. / M ² , the conveying speed is ² mpm, the width of 4500 mm, the length of 48 m, and the thickness of 20 mm of room temperature thick steel plate 1 when the induction heating device 2 is passed once through the surface of the thick steel plate 1 and the thickness direction When the temperature difference of the center portion was measured, the temperature difference between the surface of the thick steel plate 1 and the center portion in the plate thickness direction was as large as 30 ° C., and the heat uniformity was inferior. As a result, the tempering effect was obtained on the surface under these conditions, but the central part had no tempering effect and a homogeneous material could not be obtained. This is because the surface temperature of the thick steel plate was increased immediately after passing through the induction heating device 2, but before the heat diffused to the center of the plate thickness, the surface temperature began to decrease due to heat dissipation, so a sufficient tempering effect could not be obtained. It is thought.
[0023]
The heating conditions of Comparative Example 2 were as follows. In the apparatus shown in Example 2, the normal temperature steel plate 1 having a width of 4500 mm, a length of 18 m, and a thickness of 50 mm was passed through the induction heating apparatus 2 and continuously heated, and the annealing temperature was It heated to 650 degreeC which is. At this time, the frequency of the induction heating device 2 is 1000 Hz, the input power is 1, 0.5, 0.2 MW / m ² at the beginning in order from the previous stage, and the conveyance speed is 20 mpm. At the same time, the input power gradually decreased. The present comparative example 2 corresponds to the case of heating using only the induction heating device 2 in the second embodiment. As a result, the temperature of each part in the thick steel plate 1 was examined when the surface of the thick steel plate 1 tip surface reached 650 ° C., the center portion was 620 ° C., and the plate thickness center temperature of the plate center portion was The temperature was 30 ° C. higher than the center thickness of the plate and the rear end of the plate, and soaking was inferior. This is because the temperature at the center of the plate tends to increase in the longitudinal direction of the thick steel plate because the temperature gradually decreases due to heat radiation when the induction heating device 2 exits the reciprocating passage.
[0024]
【The invention's effect】
As is clear from the above description, the present invention can obtain a higher heating rate than the conventional one by giving a large temperature rise rate by induction heating, and continuously soaks the heat in the heat-retaining furnace. Thus, mature heat heating was continuously performed so that the temperature of each part of the thick steel plate was the same, and as a result, very efficient mature heat heating became possible. Therefore, according to the present invention, a high heating rate by induction overheating and a high temperature uniformity by the heat-retaining furnace can be obtained at the same time, so that efficient heating can be performed while achieving uniform temperature. In addition, the equipment length is short and space saving can be realized. Of course, by obtaining a high degree of soaking, there is little difference in the strength of each part of the thick steel plate after the heat treatment, and a more uniform material can be obtained. As a result, there is no disqualification due to material slippage, and the product yield is high.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a heat treatment facility in Example 1 of the present invention.
FIG. 2 is a schematic diagram showing a heat treatment facility in Example 2 of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Thick steel plate 2 Induction heating apparatus 2a 1st induction heating apparatus 2b 2nd induction heating apparatus 2c 3rd induction heating apparatus 3 Thermal insulation furnace 4 Heat insulation board 5 Roll for conveyance

Claims

In the method of heat-treating a thick steel plate, a heat-retaining furnace is provided on the same line as the solenoid-type induction heating device, and heat insulation plates having low emissivity are arranged on the upstream side and the downstream side of the solenoid-type induction heating device. A method for heat treating a thick steel plate, wherein the thick steel plate is heated by reciprocating the thick steel plate through a heating furnace.

Heat treatment method for steel plate of claim 1 Symbol mounting, characterized in that the solenoid-type induction heating device is arranged close to the same two or more pressure lines.