JP4335982B2

JP4335982B2 - Finish annealing heating method of unidirectional silicon steel sheet with low primary film defect occurrence area ratio and high productivity

Info

Publication number: JP4335982B2
Application number: JP13652998A
Authority: JP
Inventors: 浩康藤井; 武木村; 収田中; 申也林; 郁雄宮本; 幸司山崎
Original assignee: Nittetsu Plant Designing Corp; Nippon Steel Corp
Current assignee: Nippon Steel Corp; Nippon Steel Plant Designing Corp
Priority date: 1998-05-19
Filing date: 1998-05-19
Publication date: 2009-09-30
Anticipated expiration: 2018-05-19
Also published as: JPH11323444A

Description

【０００１】
【発明の属する技術分野】
本発明は、珪素含有冷延鋼板に鋼中からの脱炭と一次再結晶とを兼ねる脱炭焼鈍を施した後、焼鈍分離剤を塗布し、コイル状に巻き取り、仕上げ焼鈍を施す一方向性珪素鋼板製造方法に関するものである。
【０００２】
【従来の技術】
一方向性珪素鋼板の一般的な製造方法としては、冷間圧延された珪素含有鋼板に再結晶と鋼中からの脱炭を兼ねる焼鈍（以下、脱炭焼鈍と称する）が施され、次いで、ＭｇＯを主体とする焼鈍分離剤を鋼板表面に塗布し乾燥した後、コイル状に巻き取る。次に、このコイル状鋼板を純水素中で２０時間前後焼鈍する（以下、仕上げ焼鈍と称する）。この仕上げ焼鈍工程において、脱炭焼鈍の際に鋼板表面に生成したＳｉＯ₂と、焼鈍分離剤として塗布されたＭｇＯの一部とが固相反応を起こし、フォルステライト（2(ＭｇＯ)・ＳｉＯ₂）主体の無機鉱物質の皮膜が生成する（以下、一次皮膜と称する）。次いで、この一次皮膜の上にリン酸塩、クロム酸、ならびにコロイダルシリカを主体とするコーティング液を塗布し、乾燥した後、８００℃前後の温度で焼き付け、一次皮膜の上に更に皮膜（以下、二次皮膜と称する）を形成させて製品とする。
【０００３】
このようにして製造された一方向性珪素鋼板は、剪断、打ち抜きなどによって所定の寸法に加工され、積層された後、固着され鉄心等の電圧変換用の電機部品となる。鉄心等の製品にされた際に、最も重要視される特性は電圧変換の際の熱エネルギー損失で、これが少ないことが望ましく、この熱エネルギー損失分のうち、珪素鋼板に由来する部分を鉄損である。鉄損は、鋼板に発生する渦電流に大きく影響を受け、この渦電流の発生を抑制するには鋼板表面の絶縁性を高めることが有効であることから、鋼板表面に２種類もの皮膜を形成して、その絶縁性を高めている。特に、鋼板側のいわば下地皮膜となるフォルステライト質の一次皮膜は上層の二次皮膜の密着性にも関っていることから、この一次皮膜を鋼板表面に均一に欠陥なく形成させることは極めて重要なことである。
【０００４】
従来、一次皮膜はグラス皮膜と呼称されてきた。その理由は、この皮膜形成法が開発された当初、Ｘ線源の出力が弱かったため、ディフラクトメーターを用いた回折法で回折線が得られなかったためと推測される。しかしながら、現在のＸ線回折装置による分析においては明瞭な回折線を示し、明らかに結晶相が認められる。そのため、グラス、即ち、非晶質という呼び名は適当でない。そこで、本発明では二次皮膜との区別も容易であることから一次皮膜という呼称を採用する。
【０００５】
【発明が解決しようとする課題】
良好な一次皮膜を形成するために種々様々の技術が提案されている。それらは、▲１▼鋼板側における皮膜形成原料である脱炭焼鈍酸化層（主体はＳｉＯ₂と２（ＦｅＯ）・ＳｉＯ₂）及びその生成条件に関するもの、▲２▼もう一方の原料である焼鈍分離剤ＭｇＯと添加物に関するもの、▲３▼仕上げ焼鈍条件に関するもの、に大別できる。
【０００６】
本発明者らは、従来技術を適用し、現場試験を繰り返し、それら仕上げ焼鈍済みコイルの全長、全巾における一次皮膜形成状況を詳細に解析した。その結果、これら従来技術をそのまま適用しても、必ずしも安定して良好な一次皮膜をコイルの全長・全巾にわたって形成できないという知見を得た。特に、仕上げ焼鈍コイル外縁部、即ち、板巾方向における端部やコイル最外周部分や最内周部分において、慢性的に一次皮膜欠陥が発生しているのである。こうした状況は、仕上げ焼鈍コイルを展開した状態を非常に長い長辺と短辺を持つ長方形に見立てた時、一次皮膜欠陥が、丁度額縁状に発生していることを意味する。一次皮膜欠陥の発生度合いは、仕上げ焼鈍コイルの全面積に対する一次皮膜欠陥発生面積率でもって評価される。解析の結果、この外縁部における欠陥発生が製品歩留りの向上を阻んでいることが判明した。
【０００７】
【課題を解決するための手段】
本発明は、以上のような課題を解決し、一方向性珪素鋼板の全面積にわたって一次皮膜欠陥を少なくし、生産性の高い仕上焼鈍加熱方法を提案するもので、その主旨は以下のとおりである。
（１）最終板厚に調製された珪素含有冷延鋼板に鋼中から脱炭と一次再結晶とを兼ねる脱炭焼鈍を施した後、焼鈍分離剤を塗布し、コイル状に巻き取り、仕上げ焼鈍を施す一方向性珪素鋼板の製造方法において、仕上げ焼鈍コイルを常温から６００℃までを鋼板自体を直接発熱させる方法で行い、６００℃超から保持温度までを間接加熱法により行うことを特徴とする一方向性珪素鋼板の仕上焼鈍加熱方法。
（２）前記鋼板自体を直接発熱させる方法が通電加熱法であることを特徴とする（１）記載の一方向性珪素鋼板の仕上焼鈍加熱方法。
（３）前記鋼板自体を直接発熱させる方法が誘導加熱法であることを特徴とする請求項１記載の一方向性珪素鋼板の仕上焼鈍加熱方法。
【０００８】
【発明の実施の形態】
以下本発明を詳細に説明する。
本発明者らは、皮膜欠陥の発生と板間雰囲気との間に何らかの関係があるのではないかと考え、現場仕上げ焼鈍コイルの板間雰囲気に着目し検討を重ねた。
まず、仕上げ焼鈍コイル板間で起こる脱水反応について述べる。一方向性珪素鋼板の一次皮膜は脱炭焼鈍板に対しＭｇＯを塗布し、乾燥した後、コイル状に巻き取り、仕上げ焼鈍を施すことによって形成される。この時、ＭｇＯは水スラリー状態に調製された上で、鋼板に塗布される。そのため、ＭｇＯの一部が水和反応を起こし、水酸化マグネシウム（Ｍｇ（ＯＨ）₂）に変化する。塗布されたＭｇＯスラリーは鋼板上で乾燥されるが、この時、全ての水酸化マグネシウムが完全に脱水してしまうわけではない。そのため、水酸化マグネシウムの形でコイル板間に持ち込まれた水分は、温度がおよそ３５０℃の時、コイル板間に水分を放出することになる。こうした水酸化マグネシウムの脱水は粒子表面から進行するが、粒子内部に孤立してＭｇ（ＯＨ）₂領域が残存した場合、完全に脱水反応が完結するためには水分がＨ₂Ｏ等の形で粒内を拡散しなければならず、多大の拡散エネルギーを必要とする。そのために１０００℃近くまで脱水反応が継続する。
【０００９】
次に、仕上げ焼鈍工程における加熱方式について述べる。仕上げ焼鈍においては一次皮膜形成のほかに二次再結晶も行われる。二次再結晶は鋼中に分散析出した窒化物の分解を調整して発現させる。窒化物の分解は雰囲気の窒素分圧に依存する。そのため、二次再結晶の起こる仕上げ焼鈍前半においては窒素分圧を制御した雰囲気で行われる。従って、仕上げ焼鈍工程における加熱方式は加熱効率の良い重油やガスの燃焼炎をそのままコイルに吹き付ける直接加熱方式ではない。即ち、仕上げ焼鈍コイルは、インナーカバーと呼ばれる耐熱性の容器に入れられ、その容器の外壁を燃焼炎で加熱し、インナーカバーからの輻射熱でもって仕上げ焼鈍コイルを加熱している。このように、雰囲気調整のため敢えて間接加熱方式を採用していることから加熱効率が極めて悪い。
【００１０】
次に焼鈍コイルの形態について述べる。一方向性珪素鋼板は形状、即ちその平坦性が製品特性に大きく影響する。そのため、最高到達温度１２００℃という仕上げ焼鈍においては、鋼板の熱変形や座屈を防止するため、コイルをいわゆるルーズ巻きにすることができない。つまり、タイト巻きのコイルでないと焼鈍できない。その結果、板間は非常に狭くなり、伝熱や通気性が極めて悪くなる。
【００１１】
こうしたタイト巻きコイルをインナーカバー内に設置し間接加熱する場合、入熱はコイル外縁部からなされる。そのため、コイル外縁部では比較的急速に高温に到達するが、板間間隔が非常に狭く、また、板間が熱伝導性の低いＭｇＯで構成されているため、コイル内部ではなかなか温度が上昇しない。そのため、コイル部位による温度偏差が非常に大きいという状況が発生する。
【００１２】
このように、コイル部位間の温度偏差が非常に大きく、かつ伝熱性や通気性が良好とは言えない仕上げ焼鈍コイルの板間において、前述の脱水反応が進行することになる。上述の昇温特性を踏まえた上で板間で起こる脱水反応を推定してみると次のようになる。焼鈍時間の経過とともにコイル外縁温度が高まり、次いでコイル内部まで伝熱し昇温が進む。その結果、コイル内部でも脱水反応が始まる。発生した水分は板巾方向エッジ側に向けて拡散し、エッジ部分を通過した後、コイル外に放散される。従って、仕上げ焼鈍コイルにおいては著しい部位間温度偏差、雰囲気流通性の悪さに加え、板間水分濃度の不均一性を生じることになる。そのため、コイル各部位の酸化履歴が非常に異なってくる。
【００１３】
本発明者らは、このようなコイル部位間の酸化履歴のばらつきが一次皮膜欠陥発生の根本的発生原因であると考えた。すなわち、仕上げ焼鈍コイルの外縁部は他の部位と比較し、予想以上に高い酸化性履歴を経るため皮膜欠陥が発生するのではないかと考えたのである。
仕上げ焼鈍においてコイルを加熱する際、コイル外縁部が先に温度が上昇する。一方、コイル内部においては伝熱が良好でないため、非常にゆっくりとした温度上昇になる。やがて、コイル内部で温度上昇が始まると、発生した水分は板巾方向に拡散し、外縁部に到達する。この時、既に外縁部は高温になっている。従って、外縁部は絶えずコイル内部から拡散してくる水分に曝されることになる。つまり、外縁部は非常に広い温度範囲で板間雰囲気中に水分が存在する、即ち、酸化性が高い履歴をとることになる。そして、この酸化性雰囲気こそがコイル外縁部の一次皮膜を破壊しているのである。
【００１４】
そこで、本発明者らは仕上げ焼鈍コイル、それ自体を発熱させることを発明した。この方式であれば、仕上げ焼鈍コイルの部位によらずコイル全体を均一に加熱できる。そのため、酸化履歴を一様にでき、外縁部の方が履歴酸化性が高いという状況が生まれない。また、インナーカバーを通した間接加熱でなく、コイル中で発熱するのは鋼板であるため、コイル全体を急速に加熱できる。そのため、生産性の大幅な向上も期待できる。
【００１５】
以上の効果を達成する手段としては、通電加熱法、誘導加熱法もしくはその他の手段がある。このような加熱法を用いて仕上げ焼鈍の全行程を行ってもよいが、その昇温過程、特にＭｇＯからの水分放出が多い、室温〜６００℃までを本発明の方法で行うことによりコイル全体を均一に加熱することができることに加え、コイル全体を急速に加熱することが可能となる。
【００１６】
〈実施例１〉
本発明者らは以上の考え方に基づいて、次のような実験を行った。まず、脱炭・一次再結晶済みの一方向性珪素鋼板製造用素材にＭｇＯを主体とする焼鈍分離剤を塗布し、乾燥した後コイル状に巻き取った。このようにして調製したコイルを３本用意し、それぞれ、（１）従来の間接加熱法、（２）通電加熱法、（３）誘導加熱法の３種類の方法で加熱し、仕上げ焼鈍を行った。結果を表１に示す。なお、実験番号（３）の誘導加熱法においては、６００℃超の温度域は通常の間接加熱法で加熱した。
【００１７】
【表１】

【００１８】
表１から実験番号（１）の間接加熱法の場合（比較例）は、一次皮膜欠陥発生率が２．５％であるのに対し、実験番号（２）の通電加熱法の場合（参考例）や実験番号（３）の誘導加熱法の場合（実施例）は一次皮膜欠陥発生率がそれぞれ、０．２％、０．３％と極めて低く良好である。
【００１９】
【実施例】
〈実施例２〉
Ｓｉ：３．２５％含有し、鋼中炭素濃度を１０ｐｐｍまで脱炭した板厚さ０．２２５ｍｍの一次再結晶済みの一方向性珪素鋼板用素材を３本用意し、これらに、それぞれＭｇＯを主体とした焼鈍分離剤を塗布し、乾燥した後コイル状に巻き取った。３本のコイルを、（１）従来の間接加熱法、（２）通電加熱法、（３）誘導加熱法、の３種類の方法で加熱し、仕上げ焼鈍を行った。焼鈍終了後、余剰のＭｇＯを水洗によって除去した後、三者の一次皮膜欠陥発生面積率を比較した。その結果を表２に示す。なお、実験番号（３）の誘導加熱法においては、７００℃超の温度域は通常の間接加熱法で加熱した。
【００２０】
【表２】

【００２１】
表２から実験番号（１）の間接加熱法の場合（比較例）は、一次皮膜欠陥発生率が２．４％であるのに対し、実験番号（２）の通電加熱法の場合（参考例）や実験番号（３）の誘導加熱法の場合（参考例）は一次皮膜欠陥発生率がそれぞれ、０．３％、０．２％と極めて低く良好である。
〈実施例３〉
Ｓｉ：３．２５％含有し、鋼中炭素濃度を１２ｐｐｍまで脱炭した板厚さ０．３０ｍｍの一次再結晶済みの一方向性珪素鋼板用素材を３本用意し、これらに、それぞれＭｇＯを主体とした焼鈍分離剤を塗布し、乾燥した後コイル状に巻き取った。３本のコイルを、（１）従来の間接加熱法、（２）通電加熱法、（３）誘導加熱法、の３種類の方法で加熱し、仕上げ焼鈍を行った。焼鈍終了後、余剰のＭｇＯを水洗によって除去した後、三者の一次皮膜欠陥発生面積率を比較した。その結果を表３に示す。なお、実験番号（３）の誘導加熱法においては、６５０℃超の温度域は通常の間接加熱法で加熱した。
【００２２】
【表３】

【００２３】
表３から実験番号（１）の間接加熱法の場合（比較例）は、一次皮膜欠陥発生率が２．５％であるのに対し、実験番号（２）の通電加熱法の場合（参考例）や実験番号（３）の誘導加熱法の場合（参考例）は一次皮膜欠陥発生率がそれぞれ、０．２％、０．２％と極めて低く良好である。
【００２４】
【発明の効果】
以上のように、大型仕上げ焼鈍コイルの昇温特性と板間雰囲気の特殊性を踏まえた上で、コイル全面積に占める皮膜欠陥発生面積率を最小化するという観点に立ち、従来、全く提案のなかったコイル自体の発熱を利用したコイル全体の均一加熱が可能となった。この加熱方法で製造した仕上げ焼鈍板上の一次皮膜は、コイル全長全巾にわたって良好で、皮膜欠陥面積率が従来法に比し極めて低く、製品歩留りの大巾向上に貢献できる。また、加熱効率が高いためエネルギーの有効の点でも優れた効果を発揮することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention is directed to a silicon-containing cold-rolled steel sheet that has been subjected to decarburization annealing that serves both as decarburization and primary recrystallization from within the steel, and then applied with an annealing separator, wound into a coil, and subjected to finish annealing. The present invention relates to a method for producing a porous silicon steel sheet.
[0002]
[Prior art]
As a general method for producing a unidirectional silicon steel sheet, cold-rolled silicon-containing steel sheet is subjected to annealing that combines recrystallization and decarburization from the steel (hereinafter referred to as decarburization annealing). An annealing separator mainly composed of MgO is applied to the steel sheet surface and dried, and then wound into a coil. Next, this coiled steel sheet is annealed in pure hydrogen for about 20 hours (hereinafter referred to as finish annealing). In this final annealing step, SiO ₂ produced on the surface of the steel plate during decarburization annealing and a part of MgO applied as an annealing separator cause a solid-phase reaction to produce forsterite (2 (MgO) · SiO ₂ ) A main inorganic mineral film is formed (hereinafter referred to as the primary film). Next, a coating liquid mainly composed of phosphate, chromic acid, and colloidal silica is applied onto the primary film, dried, and then baked at a temperature of about 800 ° C. (Referred to as secondary film) to form a product.
[0003]
The unidirectional silicon steel plate manufactured in this way is processed into a predetermined size by shearing, punching, and the like, laminated, and then fixed to become an electrical component for voltage conversion such as an iron core. When a product such as an iron core is used, the most important characteristic is thermal energy loss during voltage conversion, and it is desirable that this is small. Of this thermal energy loss, the portion derived from the silicon steel sheet is the iron loss. It is. Iron loss is greatly affected by the eddy current generated on the steel sheet, and it is effective to increase the insulation of the steel sheet surface to suppress the generation of this eddy current. And the insulation is improved. In particular, since the forsterite primary film, which is the base film on the steel sheet side, is also related to the adhesion of the upper secondary film, it is extremely difficult to form this primary film uniformly on the steel sheet surface without any defects. It is important.
[0004]
Conventionally, the primary film has been called a glass film. The reason for this is presumed that since the output of the X-ray source was weak when this film forming method was developed, diffraction lines could not be obtained by a diffraction method using a diffractometer. However, analysis by the current X-ray diffractometer shows clear diffraction lines, and crystal phases are clearly recognized. Therefore, the name glass, that is, amorphous, is not appropriate. Therefore, in the present invention, the term “primary film” is adopted because it is easy to distinguish from the secondary film.
[0005]
[Problems to be solved by the invention]
Various techniques have been proposed to form a good primary film. They are: (1) Decarburized annealing oxide layer (mainly SiO ₂ and 2 (FeO) · SiO ₂ ), which is a film forming raw material on the steel sheet side, and conditions for its formation, ( ₂ ) Annealing, which is the other raw material It can be broadly divided into those relating to the separating agent MgO and additives, and (3) those relating to finish annealing conditions.
[0006]
The present inventors applied the conventional technique, repeated field tests, and analyzed the details of the primary film formation in the overall length and the entire width of the finish annealed coils. As a result, it has been found that even if these conventional techniques are applied as they are, a stable and good primary film cannot always be formed over the entire length and width of the coil. In particular, primary film defects are chronically generated at the outer edge of the finish annealing coil, that is, at the end in the plate width direction, the outermost peripheral portion of the coil, and the innermost peripheral portion. Such a situation means that when the state of the finished annealing coil is regarded as a rectangle having very long and short sides, primary film defects are generated just like a frame. The degree of occurrence of primary film defects is evaluated by the ratio of the area of primary film defects to the total area of the finish annealing coil. As a result of analysis, it was found that the occurrence of defects at the outer edge hinders the improvement of product yield.
[0007]
[Means for Solving the Problems]
The present invention proposes a finish annealing heating method that solves the above-described problems, reduces primary film defects over the entire area of the unidirectional silicon steel sheet, and has high productivity, and the gist thereof is as follows. is there.
(1) After applying decarburization annealing that combines decarburization and primary recrystallization from steel to the silicon-containing cold-rolled steel sheet prepared to the final thickness, it is applied with an annealing separator, wound into a coil, and finished. In the manufacturing method of the unidirectional silicon steel sheet to which annealing is performed, the finish annealing coil is performed from a normal temperature to 600 ° C. by a method of directly heating the steel sheet itself, and from 600 ° C. to a holding temperature is performed by an indirect heating method. Finishing annealing heating method for unidirectional silicon steel sheet.
(2) The method of finish annealing and heating a unidirectional silicon steel sheet according to (1), wherein the method of directly heating the steel sheet itself is an energization heating method.
(3) The method of finish annealing and heating a unidirectional silicon steel sheet according to claim 1, wherein the method of directly heating the steel sheet itself is an induction heating method.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail below.
The present inventors considered that there is some relationship between the occurrence of film defects and the atmosphere between plates, and repeated investigations paying attention to the atmosphere between plates of the on-site finish annealing coil.
First, the dehydration reaction that occurs between finish-annealed coil plates will be described. The primary coating of the unidirectional silicon steel sheet is formed by applying MgO to a decarburized annealing plate, drying it, winding it into a coil shape, and subjecting it to finish annealing. At this time, MgO is prepared in a water slurry state and then applied to the steel sheet. Therefore, a part of MgO causes a hydration reaction and changes to magnesium hydroxide (Mg (OH) ₂ ). The applied MgO slurry is dried on the steel plate, but at this time, not all magnesium hydroxide is completely dehydrated. Therefore, the moisture brought in between the coil plates in the form of magnesium hydroxide releases moisture between the coil plates when the temperature is about 350 ° C. Such dehydration of magnesium hydroxide proceeds from the particle surface, but when the Mg (OH) ₂ region remains isolated inside the particle, the moisture is in the form of H ₂ O or the like in order to completely complete the dehydration reaction. It must diffuse within the grains and requires a large amount of diffusion energy. Therefore, the dehydration reaction continues up to about 1000 ° C.
[0009]
Next, a heating method in the finish annealing process will be described. In the final annealing, secondary recrystallization is performed in addition to primary film formation. Secondary recrystallization is caused by adjusting the decomposition of nitrides dispersed and precipitated in the steel. The decomposition of nitride depends on the nitrogen partial pressure of the atmosphere. Therefore, in the first half of finish annealing in which secondary recrystallization occurs, it is performed in an atmosphere in which the nitrogen partial pressure is controlled. Therefore, the heating method in the finish annealing process is not a direct heating method in which a combustion flame of heavy oil or gas with good heating efficiency is blown directly onto the coil. That is, the finish annealing coil is placed in a heat-resistant container called an inner cover, the outer wall of the container is heated by a combustion flame, and the finish annealing coil is heated by radiant heat from the inner cover. Thus, the heating efficiency is extremely poor because the indirect heating method is used to adjust the atmosphere.
[0010]
Next, the form of the annealing coil will be described. The shape of the unidirectional silicon steel plate, that is, its flatness greatly affects the product characteristics. Therefore, in finish annealing at a maximum temperature of 1200 ° C., the coil cannot be so-called loosely wound in order to prevent thermal deformation and buckling of the steel sheet. In other words, it cannot be annealed unless it is a tightly wound coil. As a result, the space between the plates becomes very narrow, and heat transfer and air permeability become extremely poor.
[0011]
When such a tightly wound coil is installed in the inner cover and heated indirectly, heat input is made from the outer edge of the coil. For this reason, the outer edge of the coil reaches a high temperature relatively quickly, but the interval between the plates is very narrow, and the interval between the plates is composed of MgO having low thermal conductivity, so the temperature does not increase easily inside the coil. . Therefore, a situation occurs in which the temperature deviation due to the coil part is very large.
[0012]
As described above, the above-described dehydration reaction proceeds between the plates of the finish-annealed coil where the temperature deviation between the coil parts is very large and the heat transfer property and the air permeability are not good. An estimation of the dehydration reaction that occurs between the plates based on the above temperature rise characteristics is as follows. As the annealing time elapses, the outer edge temperature of the coil increases, and then the heat is transferred to the inside of the coil to increase the temperature. As a result, the dehydration reaction also starts inside the coil. The generated moisture diffuses toward the edge in the plate width direction, passes through the edge portion, and is then diffused out of the coil. Accordingly, in the finish annealing coil, in addition to the remarkable temperature deviation between the parts and the poor air circulation, non-uniformity of the moisture concentration between the plates is caused. Therefore, the oxidation history of each part of the coil is very different.
[0013]
The present inventors considered that such a variation in oxidation history between coil sites is a fundamental cause of primary film defects. In other words, the outer edge portion of the finish annealed coil is thought to cause film defects because it undergoes an oxidation history higher than expected compared to other portions.
When the coil is heated in finish annealing, the temperature of the outer edge of the coil rises first. On the other hand, since heat transfer is not good inside the coil, the temperature rises very slowly. When the temperature starts to rise in the coil, the generated moisture diffuses in the plate width direction and reaches the outer edge. At this time, the outer edge has already become hot. Therefore, the outer edge is constantly exposed to moisture diffusing from the inside of the coil. That is, the outer edge portion has a history in which moisture exists in the inter-plate atmosphere in a very wide temperature range, that is, has a high oxidation property. It is this oxidizing atmosphere that destroys the primary coating on the outer edge of the coil.
[0014]
Therefore, the present inventors have invented that the finish annealing coil itself generates heat. If it is this system, the whole coil can be heated uniformly irrespective of the site | part of a finish annealing coil. For this reason, the oxidation history can be made uniform, and the situation where the outer edge portion has higher history oxidation property does not occur. Moreover, since it is a steel plate that generates heat in the coil, not indirectly through the inner cover, the entire coil can be rapidly heated. Therefore, significant improvement in productivity can be expected.
[0015]
As means for achieving the above effects, there are an electric heating method, an induction heating method and other means. The entire process of finish annealing may be performed using such a heating method, but the entire coil is obtained by performing the temperature raising process, particularly from room temperature to 600 ° C., in which water release from MgO is large, by the method of the present invention. In addition to being able to heat the coil uniformly, the entire coil can be rapidly heated.
[0016]
<Example 1>
The present inventors conducted the following experiment based on the above concept. First, an annealing separator mainly composed of MgO was applied to a raw material for producing a unidirectional silicon steel sheet that had been decarburized and primary recrystallized, dried, and then wound into a coil. Three coils prepared in this way are prepared, and each is heated by three methods: (1) conventional indirect heating method, (2) current heating method, and (3) induction heating method, and finish annealing is performed. It was. The results are shown in Table 1. In addition, in the induction heating method of experiment number (3) , the temperature range exceeding 600 ° C. was heated by a normal indirect heating method.
[0017]
[Table 1]

[0018]
In the case of the indirect heating method of experiment number (1) from Table 1 (comparative example), the primary film defect occurrence rate is 2.5%, whereas in the case of the electric heating method of experiment number (2) (reference example) ) And the experiment number (3) in the induction heating method (Example), the primary film defect occurrence rates are 0.2% and 0.3%, which are very low and good, respectively.
[0019]
【Example】
<Example 2 >
Si: 3.25% containing three unidirectional silicon steel materials for primary recrystallization that have been decarburized to a carbon concentration of 10 ppm in steel and 0.25 mm in thickness. The main annealing separator was applied, dried, and wound into a coil. The three coils were heated by three methods: (1) a conventional indirect heating method, (2) an electric heating method, and (3) an induction heating method, and finish annealing was performed. After the annealing was completed, excess MgO was removed by water washing, and then the three primary film defect occurrence area ratios were compared. The results are shown in Table 2. In addition, in the induction heating method of the experiment number (3) , the temperature range exceeding 700 ° C. was heated by a normal indirect heating method.
[0020]
[Table 2]

[0021]
From Table 2, in the case of the indirect heating method of experiment number (1) (comparative example), the primary film defect occurrence rate is 2.4%, whereas in the case of the heating method of experiment number (2) (reference example) ) And the experiment number (3) induction heating method (reference example), the primary film defect occurrence rates are very low, 0.3% and 0.2%, respectively.
<Example 3 >
Si: 3.25% containing three unidirectional silicon steel materials for primary recrystallization that have been decarburized to a carbon concentration of 12 ppm in steel, and prepared MgO, respectively. The main annealing separator was applied, dried, and wound into a coil. The three coils were heated by three methods: (1) a conventional indirect heating method, (2) an electric heating method, and (3) an induction heating method, and finish annealing was performed. After the annealing was completed, excess MgO was removed by water washing, and then the three primary film defect occurrence area ratios were compared. The results are shown in Table 3. In addition, in the induction heating method of the experiment number (3) , the temperature range exceeding 650 ° C. was heated by a normal indirect heating method.
[0022]
[Table 3]

[0023]
From Table 3, in the case of the indirect heating method of experiment number (1) (comparative example), the primary film defect occurrence rate is 2.5%, whereas in the case of the heating method of experiment number (2) (reference example) ) And the experiment number (3) induction heating method (reference example), the primary film defect occurrence rates are 0.2% and 0.2%, which are very low and good, respectively.
[0024]
【The invention's effect】
As described above, in consideration of the temperature rise characteristics of large finish annealing coils and the speciality of the inter-plate atmosphere, from the viewpoint of minimizing the film defect occurrence area ratio in the total coil area, there has been no proposal in the past. It was possible to heat the entire coil uniformly using the heat generated by the coil itself. The primary film on the finish-annealed plate produced by this heating method is good over the entire length of the coil, and the film defect area ratio is extremely lower than that of the conventional method, which can contribute to the improvement of the product yield. Moreover, since the heating efficiency is high, an excellent effect can be exhibited in terms of energy efficiency.

Claims

The silicon-containing cold-rolled steel sheet prepared to the final thickness is subjected to decarburization annealing that combines decarburization and primary recrystallization from the steel, and then an annealing separator is applied, wound into a coil, and subjected to finish annealing. In the method for producing a unidirectional silicon steel sheet, the finish annealing coil is performed by a method in which the steel sheet itself directly generates heat from room temperature to 600 ° C., and from a temperature exceeding 600 ° C. to a holding temperature by an indirect heating method. Finish annealing method for heat-resistant silicon steel sheet.

2. The method of finish annealing and heating a unidirectional silicon steel sheet according to claim 1, wherein the method of directly heating the steel sheet itself is an energization heating method.

2. The method of finish annealing and heating a unidirectional silicon steel sheet according to claim 1, wherein the method of directly heating the steel sheet itself is an induction heating method.