JP4288022B2

JP4288022B2 - Unidirectional silicon steel sheet and manufacturing method thereof

Info

Publication number: JP4288022B2
Application number: JP2001174669A
Authority: JP
Inventors: 元一重里; 浩康藤井; 健一村上; 義行牛神; 修一中村; 昌章杉山
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2001-06-08
Filing date: 2001-06-08
Publication date: 2009-07-01
Anticipated expiration: 2021-06-08
Also published as: JP2002363763A

Description

【０００１】
【発明の属する技術分野】
本発明はフォルステライト（Ｍｇ₂ＳｉＯ₄）等で構成される無機鉱物質皮膜の生成を意図的に防止して製造したり、あるいは研削や酸洗等の手段によって除去したり、さらには鏡面光沢を呈するまで表面を平坦化させたりして調製した仕上げ焼鈍済みの一方向性珪素鋼板に対し、張力付与性の絶縁性皮膜を形成させた一方向性珪素鋼板とその製造方法に関するものである。
【０００２】
【従来の技術】
一方向性珪素鋼板は磁気鉄芯材料として多用されており、特にエネルギ−ロスを少なくするために鉄損の少ない材料が求められている。鉄損の低減には鋼板に張力を付与することが有効であることから、鋼板に比べ熱膨張係数の小さい材質からなる皮膜を高温で形成することによって鋼板に張力を付与し、鉄損低減が図られてきた。仕上げ焼鈍工程で鋼板表面の酸化物と焼鈍分離剤とが反応して生成するフォルステライト系皮膜は、鋼板に張力を与えることができ、皮膜密着性も優れている。
【０００３】
一方、特開昭４８−３９３３８号公報で開示されたコロイド状シリカとリン酸塩を主体とするコ−ティング液を鋼板表面に塗布し、焼き付けることによって絶縁皮膜を形成する方法は、鋼板に対する張力付与の効果が大きく、鉄損低減に有効である。
そこで、仕上げ焼鈍工程で生じたフォルステライト系皮膜を残した上でリン酸塩を主体とする絶縁皮膜を形成することが一般的な一方向性珪素鋼板の製造方法となっている。
【０００４】
近年、フォルステライト系皮膜と地鉄の乱れた界面構造が、皮膜張力による鉄損改善効果をある程度減少させていることが明らかになってきた。そこで、例えば、特開昭４９−９６９２０号公報に開示されている如く、仕上げ焼鈍工程で生ずるフォルステライト系皮膜を除去したり、更に鏡面化仕上げを行った後、改めて張力皮膜を形成させることにより、更なる鉄損低減を試みる技術が開発された。
【０００５】
しかしながら、上記絶縁皮膜はフォルステライトを主体とする皮膜の上に形成した場合はかなりの密着性が得られるものの、フォルステライト系皮膜を除去したり、あるいは仕上げ焼鈍工程で意図的にフォルステライト形成を行わなかったものに対しては皮膜密着性が十分ではない。フォルステライト系皮膜の除去を行った場合はコ−ティング液を塗布して形成させる張力付与型絶縁皮膜のみで所要の皮膜張力を確保する必要があり、必然的に厚膜化しなければならず、より一層の密着性が必要である。したがって、従来の皮膜形成法では鏡面化の効果を十分に引き出すほどの皮膜張力を達成し、かつ皮膜密着性をも確保する事は困難であり、十分な鉄損低減が図られていなかった。そこで、張力付与性絶縁皮膜の密着性を確保するための技術として、張力付与性絶縁皮膜の形成に先立ち、仕上げ焼鈍済みの一方向性珪素鋼板の表面に酸化膜を形成させる方法が、例えば、特開昭６０−１３１９７６号公報、特開平６−１８４７６２号公報、特開平７−２７８８３３号公報、特開平８−１９１０１０号公報、特開平９−０７８２５２号公報、において開示された。
【０００６】
特開昭６０−１３１９７６号公報は鏡面化した仕上げ焼鈍済みの一方向性珪素鋼板の鋼板表面付近を内部酸化させる方法で、この内部酸化層によって張力皮膜の密着性を向上させ、内部酸化、即ち鏡面度減退で生じる鉄損劣化を皮膜密着性向上によってもたらされる付与張力の増大で補おうする方法である。
特開平６−１８４７６２号公報は鏡面化ないしはそれに近い状態に調製した仕上げ焼鈍済みの一方向性珪素鋼板に対し、温度ごとに特定の雰囲気で焼鈍を施す事により鋼板表面に外部酸化型の酸化膜を形成し、この酸化膜でもって張力付与性絶縁皮膜の皮膜と鋼板との皮膜密着性を確保する方法である。
【０００７】
特開平７−２７８８３３号公報は張力付与性の絶縁皮膜が結晶質である場合において、無機鉱物質皮膜のない仕上げ焼鈍済みの一方向性珪素鋼板の表面に非晶質の酸化物の下地皮膜を形成させることで、結晶質の張力付与性絶縁皮膜が形成される際に起こる鋼板酸化、即ち、鏡面度減退を防止する技術である。
特開平８−１９１０１０号公報は非金属物質を除去した仕上げ焼鈍済みの一方向性珪素鋼板の表面に結晶性のファイヤライトを形成させることでファイヤライト結晶による張力付与効果と張力付与性の絶縁皮膜との密着性向上効果により鉄損低減を図る方法である。
【０００８】
特開平９−０７８２５２号公報は無機鉱物質皮膜のない仕上げ焼鈍済みの一方向性珪素鋼板の表面に形成させる下地シリカ層の量を１００ｍｇ／ｍ²以下にすることで張力皮膜の密着性確保だけでなく、良好な鉄損値をも実現しようとする方法である。
【０００９】
【発明が解決しようとする課題】
上述の技術を適用し、無機鉱物質のない一方向性珪素鋼板の表面に酸化膜を形成させることで、皮膜密着性改善や鉄損値低減の効果はそれなりに認められる。しかしながら、張力付与性絶縁皮膜の皮膜密着性が必ずしも完全ではなかった。
【００１０】
【課題を解決するための手段】
本発明は上述の問題点を解決し、無機鉱物質皮膜のない仕上げ焼鈍済みの一方向性珪素鋼板に対し、十分な皮膜密着性を得ることができるよう張力付与型の絶縁性皮膜を形成させた一方向性珪素鋼板およびその製造方法である。本発明の要旨は次の通りである。
【００１１】
（１）仕上げ焼鈍皮膜が実質的に存在しない鋼板表面に張力付与性の絶縁皮膜を有する一方向性珪素鋼板であって、張力付与性絶縁皮膜と鋼板との界面に、膜厚が４０ｎｍ以上５００ｎｍ以下、空洞が断面面積率にして３０％以下であるシリカ主体の外部酸化型酸化膜を有することを特徴とする一方向性珪素鋼板。
【００１２】
（２）フォルステライトの無機鉱物質皮膜の生成を意図的に防止して製造した仕上げ焼鈍済み一方向性珪素鋼板に対し、張力付与性絶縁皮膜と鋼板との密着性を確保するため、張力付与性絶縁皮膜の形成に先立ち、シリカ主体の外部酸化型酸化膜を形成させる方法において、外部酸化型酸化膜を形成するための熱処理を１０００℃以上の温度で行い、外部酸化型酸化膜の形成温度から２００℃までの温度域の冷却速度を１００℃／秒以下とすることを特徴とする一方向性珪素鋼板の製造方法。
【００１３】
（３）リン酸塩とコロイド状シリカを主体とする塗布液を焼き付けて生成させた張力付与性絶縁皮膜を有することを特徴とする前記（１）記載の一方向性珪素鋼板。
（４）アルミナゾルとホウ酸を主体とする塗布液を焼き付けて生成させた張力付与性絶縁皮膜を有することを特徴とする前記（１）記載の一方向性珪素鋼板。
【００１４】
【発明の実施の形態】
以下、発明の詳細について説明する。
発明者らは、皮膜密着性が必ずしも完全ではない原因として外部酸化型酸化膜を形成させる条件、特に、冷却速度に問題があり、冷却速度によって外部酸化型酸化膜の構造に差異が生じ、そのため張力付与性の絶縁皮膜の密着性が変動するのではないかと推測した。そこで、次に述べるような実験を行ない、皮膜密着性に対する冷却速度と外部酸化型酸化膜構造との関係を調べた。なお、ここでいう外部酸化膜とは、低酸素分圧下で生成する酸化膜であって、合金元素（主にＳｉ）が鋼板表層まで拡散した後に皮膜状になる酸化膜である。
【００１５】
実験用素材として、板厚０．２２５ｍｍの脱炭焼鈍板に対し、アルミナを主体とする焼鈍分離剤を塗布して仕上げ焼鈍を行ない、二次再結晶させ、鏡面光沢を有する一方向性珪素鋼板を準備した。この鋼板に対し、窒素２５％、水素７５％、露点−５℃の雰囲気において均熱時間１０秒で、かつ、種々の温度と冷却速度の条件で熱処理を施し、外部酸化型酸化膜を形成させた。ついで、張力付与性の絶縁皮膜を形成するため、リン酸塩、クロム酸、コロイダルシリカを主体とする塗布液を塗布し、窒素雰囲気中で８３５℃で３０秒間焼き付けた。このようにして作製した鋼板の皮膜密着性を調べた。
【００１６】
皮膜密着性は直径２０ｍｍの円筒に試料を巻き付けた時、鋼板から剥離せず、鋼板と皮膜が密着したままであった部分の面積率（以後、皮膜残存面積率と称する）で評価した。密着性が不良で皮膜が完全に剥離した場合は０％、皮膜密着性が良好で皮膜が全く剥離しなかった場合を１００％と判定した。評価は皮膜残存面積率が９０％以下の場合を×、９５％のものを○、１００％のものを◎とした。
【００１７】
また、外部酸化型酸化膜を含む張力付与性絶縁皮膜と鋼板との界面構造を調べるため、集束イオンビ−ム法（以下、ＦＩＢ法と称する）によって試料を作製し、透過型電子顕微鏡（以下、ＴＥＭと称する）で断面構造を観察した。
断面観察の結果、外部酸化型酸化膜の中に部分的に空洞が観察された。ＴＥＭ写真から断面の空洞面積率を算出した。
【００１８】
このようにして調べた結果を表１にまとめた。なお、図１に、断面観察結果の一例として、試料番号４０の試料の断面ＴＥＭ観察像を示した。ただし、試料番号４０の試料は、張力付与性絶縁皮膜の密着性が非常に悪く、張力皮膜を塗布した状態では断面ＴＥＭ観察が困難であったため、張力付与性絶縁皮膜を塗布する前の鋼板断面を観察した。外部酸化型酸化膜中に観察された空洞の断面積率は４０％であった。
【００１９】
【表１】

【００２０】
表１から、張力付与性絶縁皮膜の密着性を確保できる条件を求めると次のようになる。
まず、空洞面積率に関わらず、外部酸化型酸化膜の膜厚が２ｎｍ未満の試料番号１から試料番号４の熱処理温度（外部酸化型酸化膜の形成温度）５００℃の条件では、皮膜密着性が確保できない。一方、外部酸化型酸化膜の膜厚が２ｎｍ以上の試料番号５から試料番号４０の熱処理温度が６００℃から１１５０℃の条件においては、概ね、皮膜密着性が確保できるようになる。特に、試料番号２６から試料番号４０の外部酸化型酸化膜の膜厚が４０ｎｍ以上の熱処理温度が１０００℃以上の条件では特に皮膜密着性が良好である。但し、冷却速度が５℃／秒以上１００℃／秒以下の条件で、外部酸化型酸化膜中の空洞面積率が３０％以下の条件では、皮膜密着性が良好であるが、冷却速度が２００℃／秒で空洞面積率が３０％よりも大きい条件では外部酸化型酸化膜の膜厚が厚くとも、皮膜密着性が必ずしも完全とは言えず、皮膜残存面積率で９０％となった。
【００２１】
表１から、張力付与性絶縁皮膜の皮膜密着性確保するためには外部酸化型酸化膜の膜厚が２ｎｍ以上で、かつ外部酸化型酸化膜における空洞面積率が３０％以下であることが必須であり、こうした外部酸化型酸化膜を形成させるためには外部酸化型酸化膜を形成するための熱処理を６００℃以上、特に好ましくは１０００℃以上の温度で行ない、かつ、その時の冷却速度を５℃／秒以上１００℃／秒以下にする必要があることがわかる。
【００２２】
このように、皮膜密着性について外部酸化型酸化膜の膜厚と空洞面積率が大きく影響していることについて、本発明者らはその機構を次のように考えている。
まず、外部酸化型酸化膜の膜厚依存性について述べる。
鋼板と張力付与性絶縁皮膜との密着性は、両者の界面に形成させた外部酸化型酸化膜によって決まる。一般に外部酸化型酸化膜は金属原子が鋼中から表面に拡散し、表面で酸化性ガスと反応することで成長すると言われている。そのため、酸化膜の成長速度は金属原子の拡散速度によって決まる。金属中の原子の拡散は熱エネルギ−によって高められる。したがって、温度が高いほど原子の拡散が促進され、外部酸化型酸化膜はより成長する。こうした機構のため熱処理温度が５００℃と低い条件では外部酸化型の酸化膜の成長が十分発達せず、皮膜密着性が劣る。一方、熱処理温度が６００℃以上では十分な外部酸化型酸化膜が成長するので皮膜密着性は良好で、さらに１０００℃以上では更に酸化膜が成長し易くなるので皮膜密着性が極めて良好となるものと考えられる。
【００２３】
こうした推測が妥当であることが透過型電子顕微鏡を使った外部酸化型酸化膜の膜厚測定の結果からわかる。即ち、膜厚が１ｎｍで、外部酸化型酸化膜の成長が十分でない、熱処理温度５００℃の条件では張力付与型絶縁皮膜の密着性が不良であるのに対し、膜厚２ｎｍ以上で、外部酸化型酸化膜が成長した、熱処理温度６００℃以上の条件では皮膜密着性は良好である。
【００２４】
次に張力付与性絶縁皮膜の密着性と外部酸化型酸化膜の空洞面積率の関係について述べる。
外部酸化型酸化膜中に空洞が形成される反応機構についてはその詳細は未だ不明であるが、本発明者らは、外部酸化型酸化膜が形成される時に酸化膜と鋼板との界面付近に蓄積された格子欠陥などが、外部酸化膜中に集積するため、空洞が生成するのではないかと推測している。この空洞形成と冷却速度との関係については冷却が緩やかに行なわれた場合、こうした欠陥は系外に取り除かれるが、急速に冷却が行なわれた場合、欠陥が系外に取り除かれるのに十分な時間がないため、欠陥が外部酸化型酸化膜中に集積し、空洞を形成するのではないかと推測している。
【００２５】
皮膜張力は張力付与性絶縁皮膜と鋼板との熱膨張係数の差によってもたらされる。この時、張力付与性絶縁皮膜と鋼板との界面には多大な応力が発生する。この応力に耐え、鋼板と張力付与性絶縁皮膜の密着性を確保するのが外部酸化型酸化膜である。本発明者らは、こうした応力耐性の必要な外部酸化型酸化膜に空洞が少ないと応力に耐えうるが、空洞が多い、即ち、空洞断面面積率が高いと応力に耐えることができず、破壊されてしまうのではないかと考えている。
【００２６】
また、図１に示したように、外部酸化型酸化膜中にはシリカ（シリコン酸化物）以外に、アルミニウム、マンガン、クロム、鉄などの酸化物が存在したが、今回の結果では、鋼板と張力付与性絶縁皮膜との密着性はこれらの酸化物の有無に依らず、外部酸化型酸化膜中の空洞面積率で一義的に決まっていた。
【００２７】
【実施例】
〔実施例１〕
板厚０．２２５ｍｍ、Ｓｉ濃度３．３５％の一方向性珪素鋼板製造用の冷延板に脱炭焼鈍を施し、表面にマグネシアと塩化ビスマスを主体とする焼鈍分離剤の水スラリ−を塗布し、乾燥した。ついで乾燥水素雰囲気中、１２００℃、２０時間の仕上げ焼鈍を行ない、表面に無機鉱物質の殆どない二次再結晶の完了した一方向性珪素鋼板を得た。この鋼板に対し、窒素２５％、水素７５％、露点−２０℃の雰囲気中、温度１１５０℃で熱処理を行なう事でシリカを主体とする外部酸化型酸化膜を形成させた。この時、冷却速度を１０℃／秒（実施例）と２００℃／秒（比較例）の２条件で行なった。こうして調製した鋼板に対し、濃度５０％のリン酸マグネシム水溶液５０ｍｌ、濃度２０％のコロイダルシリカ水分散液１００ｍｌ、無水クロム酸５ｇからなる混合液を塗布し、８５０℃で３０秒間焼き付け、張力付与性の絶縁皮膜を形成させた。こうして調製した絶縁皮膜付き一方向性珪素鋼板について、直径２０ｍｍの円筒に試料を巻き付けた時の皮膜残存面積率で絶縁皮膜の密着性を評価した。その結果を表２に示す。
【００２８】
【表２】

【００２９】
表２から、冷却速度２００℃／秒、空洞面積率４０％で皮膜残存面積率９０％である比較例に比べ、冷却速度１０℃／秒，空洞面積率１５％で皮膜残存面積率１００％である実施例の方が皮膜密着性が良好で優れている。
【００３３】
〔実施例４〕
板厚０．２３ｍｍ、Ｓｉ濃度３．３０％の一方向性珪素鋼板製造用の冷延板に脱炭焼鈍を施し、表面にマグネシアを主体とする焼鈍分離剤の水スラリ−を塗布し、乾燥した後、乾燥水素雰囲気中、１２００℃、２０時間の仕上げ焼鈍を行なった。こうして調製した二次再結晶の完了した一方向性珪素鋼板の表面にはフォルステライトを主体とする皮膜が生成している。ついで、ふっ化アンモニムと硫酸の混合溶液中で酸洗し、表面皮膜を溶解除去した後、ふっ酸と過酸化水素水の混合溶液中で化学研磨し、鋼板表面に無機鉱物質がなく、かつ鏡面光沢をもつ鋼板を得た。この鋼板に対し、窒素２５％、水素７５％、露点０℃の雰囲気中、温度１０５０℃で熱処理を行なうことで外部酸化型酸化膜を形成させた。この時、冷却速度を１００℃／秒（実施例）と２００℃／秒（比較例）の２条件で行なった。こうして調製した鋼板に対し、１０％濃度のコロイダルアルミナ水分散液１００ｍｌ、不定形アルミナ粉末１０ｇ、ホウ酸５ｇ、水２００ｍｌからなる混合液を塗布し、８５０℃で３０秒間焼き付け、張力付与性の絶縁皮膜を形成させた。こうして調製した絶縁皮膜付き一方向性珪素鋼板について、直径２０ｍｍの円筒に試料を巻き付けた時の皮膜残存面積率で皮膜密着性を評価した。その結果を表５に示す。
【００３４】
【表５】

【００３５】
表５から、冷却速度２５０℃／秒、空洞面積率３５％で皮膜残存面積率９０％である比較例に比べ冷却速度１００℃／秒、空洞面積率１０％で皮膜残存面積率１００％である実施例のほうが皮膜密着性が良好で優れている。
【００３６】
【発明の効果】
本発明により仕上げ焼鈍皮膜が実質的に存在しない鋼板表面に、皮膜密着性の良好な絶縁皮膜を有する一方向性珪素鋼板を提供することができる。
【図面の簡単な説明】
【図１】表１中の試料番号４０の試料の断面ＴＥＭ観察像。（符号Ｗは、ＴＥＭ試料作成時の最表面保護用のＷ蒸着膜である。また、Ｆｅ−３％Ｓｉ鋼のマトリックス中の白い斑点模様はＴＥＭ試料作成時に生成した転位ループである。）[0001]
BACKGROUND OF THE INVENTION
The present invention is produced by intentionally preventing the formation of an inorganic mineral film composed of forsterite (Mg ₂ SiO ₄ ) or the like, or is removed by means such as grinding or pickling, and further has a specular gloss. The present invention relates to a unidirectional silicon steel sheet in which a tension-imparting insulating film is formed on a unidirectional silicon steel sheet that has been subjected to finish annealing and that has been prepared by flattening the surface until it exhibits, and a method for producing the same.
[0002]
[Prior art]
Unidirectional silicon steel sheets are frequently used as magnetic iron core materials, and materials with low iron loss are particularly required to reduce energy loss. Since it is effective to apply tension to the steel sheet to reduce iron loss, it is possible to reduce the iron loss by applying tension to the steel sheet by forming a coating made of a material having a smaller thermal expansion coefficient than that of the steel sheet at a high temperature. It has been planned. The forsterite-based film produced by the reaction of the oxide on the surface of the steel sheet and the annealing separator in the final annealing step can give tension to the steel sheet and has excellent film adhesion.
[0003]
On the other hand, the method of forming an insulating film by applying a coating liquid mainly composed of colloidal silica and phosphate disclosed in JP-A-48-39338 on the surface of a steel sheet and baking the coating liquid, The effect of imparting is great and effective in reducing iron loss.
Therefore, it is a general method for producing a unidirectional silicon steel sheet to leave the forsterite-based film produced in the finish annealing step and form an insulating film mainly composed of phosphate.
[0004]
In recent years, it has become clear that the disordered interface structure between the forsterite film and the ground iron reduces the iron loss improvement effect due to the film tension to some extent. Therefore, for example, as disclosed in Japanese Patent Laid-Open No. 49-96920, by removing the forsterite-based film generated in the finish annealing process or performing a mirror finish, a tension film is formed again. A technology to further reduce iron loss has been developed.
[0005]
However, when the above insulating film is formed on a film mainly composed of forsterite, considerable adhesion can be obtained. However, the forsterite film is removed or the forsterite is intentionally formed in the final annealing process. Film adhesion is not sufficient for those not performed. When the forsterite film is removed, it is necessary to secure the required film tension only with the tension-imparting type insulating film formed by applying a coating solution, and inevitably the film must be thickened. Further adhesion is required. Therefore, it has been difficult to achieve a film tension sufficient to bring out the effect of mirror finish and to secure the film adhesion with the conventional film forming method, and the iron loss has not been sufficiently reduced. Therefore, as a technique for ensuring the adhesion of the tension-imparting insulating film, prior to the formation of the tension-imparting insulating film, a method of forming an oxide film on the surface of the finished unidirectional silicon steel sheet, for example, This is disclosed in JP-A-60-131976, JP-A-6-184762, JP-A-7-278833, JP-A-8-191010, and JP-A-9-078252.
[0006]
JP-A-60-131976 discloses a method of internally oxidizing the vicinity of a steel plate surface of a mirror-finished annealed unidirectional silicon steel sheet. This internal oxide layer improves the adhesion of the tension film, and thus internal oxidation, This is a method of compensating for the iron loss deterioration caused by the decrease in specularity by increasing the applied tension brought about by the improvement of the film adhesion.
Japanese Patent Laid-Open No. 6-184762 discloses an external oxide type oxide film on the surface of a steel sheet by annealing it in a specific atmosphere at every temperature to a finish-finished unidirectional silicon steel sheet prepared in a mirror-like or close state. This is a method of securing the film adhesion between the tension-providing insulating film and the steel sheet with this oxide film.
[0007]
JP-A-7-278833 discloses an amorphous oxide undercoat on the surface of a finish annealed unidirectional silicon steel plate without an inorganic mineral coating when the tension-providing insulating coating is crystalline. This is a technique for preventing steel plate oxidation that occurs when a crystalline tension-imparting insulating film is formed, that is, reduction in specularity.
Japanese Unexamined Patent Publication No. Hei 8-191010 discloses a tension imparting effect and a tension imparting insulating film by forming a crystalline firelite on the surface of a finish annealed unidirectional silicon steel plate from which nonmetallic substances have been removed. This is a method for reducing iron loss by improving the adhesion.
[0008]
Japanese Patent Laid-Open No. 9-078252 only ensures the adhesion of the tension coating by setting the amount of the underlying silica layer to be formed on the surface of the unidirectional silicon steel plate that has been annealed without an inorganic mineral coating to 100 mg / m ² or less. In addition, it is a method of trying to realize a good iron loss value.
[0009]
[Problems to be solved by the invention]
By applying the above-described technique and forming an oxide film on the surface of a unidirectional silicon steel sheet having no inorganic mineral substance, the effects of improving the film adhesion and reducing the iron loss value are recognized as such. However, the film adhesion of the tension-imparting insulating film is not always perfect.
[0010]
[Means for Solving the Problems]
The present invention solves the above-mentioned problems and forms a tension-imparting type insulating film so that sufficient film adhesion can be obtained with respect to a unidirectional silicon steel sheet that has been annealed without an inorganic mineral film. Another unidirectional silicon steel sheet and a method for producing the same. The gist of the present invention is as follows.
[0011]
(1) A unidirectional silicon steel sheet having a tension-imparting insulating film on the surface of a steel sheet substantially free of a finish annealing film, and having a film thickness of 40 nm or more and 500 nm at the interface between the tension-imparting insulating film and the steel sheet. A unidirectional silicon steel sheet having a silica-based external oxide type oxide film having a cavity whose cross-sectional area ratio is 30% or less.
[0012]
(2) Applying tension to ensure the adhesion between the tension-imparting insulating coating and the steel sheet for the unidirectional silicon steel sheet that has been annealed by intentionally preventing the formation of inorganic mineral film of forsterite. In the method of forming a silica-based external oxide film prior to the formation of the conductive insulating film, the heat treatment for forming the external oxide film is performed at a temperature of 1000 ° C. or more, and the external oxide film formation temperature A cooling method in a temperature range from 1 to 200 ° C. is set to 100 ° C./second or less, and the method for producing a unidirectional silicon steel sheet is characterized in that:
[0013]
(3) The unidirectional silicon steel sheet according to (1) above, which has a tension-imparting insulating film formed by baking a coating solution mainly composed of phosphate and colloidal silica.
(4) The unidirectional silicon steel sheet according to (1) above, which has a tension-imparting insulating film formed by baking a coating solution mainly composed of alumina sol and boric acid.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, details of the invention will be described.
The inventors have a problem in the conditions for forming the external oxide oxide film, particularly the cooling rate, as the cause of the film adhesion not necessarily being perfect, and the structure of the external oxide oxide film varies depending on the cooling rate. It was speculated that the adhesion of the tension-imparting insulating film might fluctuate. Therefore, the following experiment was conducted to investigate the relationship between the cooling rate for the film adhesion and the external oxide type oxide film structure. The external oxide film referred to here is an oxide film formed under a low oxygen partial pressure, and is an oxide film that forms a film after the alloy element (mainly Si) diffuses to the steel sheet surface layer.
[0015]
As an experimental material, a 0.25-mm thick decarburized annealed plate is coated with an annealing separator mainly composed of alumina, subjected to finish annealing, secondary recrystallized, and a unidirectional silicon steel plate having a specular gloss. Prepared. This steel sheet was heat-treated in an atmosphere of 25% nitrogen, 75% hydrogen, and dew point of -5 ° C. for a soaking time of 10 seconds and under various temperature and cooling rate conditions to form an external oxide film. It was. Then, in order to form a tension-providing insulating film, a coating solution mainly composed of phosphate, chromic acid, and colloidal silica was applied and baked at 835 ° C. for 30 seconds in a nitrogen atmosphere. The film adhesion of the steel sheet thus prepared was examined.
[0016]
The film adhesion was evaluated by the area ratio (hereinafter referred to as the film remaining area ratio) of the part where the steel sheet and the film remained in close contact with each other when the sample was wound around a cylinder having a diameter of 20 mm. When the adhesion was poor and the film was completely peeled off, it was judged as 0%, and when the film adhesion was good and the film was not peeled off at all, it was judged as 100%. In the evaluation, the case where the film remaining area ratio was 90% or less was evaluated as x, the film was 95%, and the film was 100%.
[0017]
In addition, in order to investigate the interface structure between a tension-imparting insulating film including an external oxide type oxide film and a steel sheet, a sample was prepared by a focused ion beam method (hereinafter referred to as FIB method), and a transmission electron microscope (hereinafter referred to as “FIELD”). The cross-sectional structure was observed by TEM).
As a result of cross-sectional observation, cavities were partially observed in the external oxide oxide film. The cavity area ratio of the cross section was calculated from the TEM photograph.
[0018]
The results thus examined are summarized in Table 1. In addition, in FIG. 1, the cross-sectional TEM observation image of the sample of the sample number 40 was shown as an example of a cross-section observation result. However, the sample of Sample No. 40 had very poor adhesion to the tension-imparting insulating film, and it was difficult to observe the cross-section TEM when the tension film was applied. Was observed. The cross-sectional area ratio of the cavity observed in the external oxide film was 40%.
[0019]
[Table 1]

[0020]
From Table 1, the conditions for ensuring the adhesion of the tension-imparting insulating film are as follows.
First, regardless of the cavity area ratio, the film adhesion is obtained under the conditions of the heat treatment temperature (formation temperature of the external oxide oxide film) of 500 ° C. of Sample No. 1 to Sample No. 4 where the thickness of the external oxide oxide film is less than 2 nm. Cannot be secured. On the other hand, under the conditions where the heat treatment temperature of Sample No. 5 to Sample No. 40 in which the thickness of the external oxide oxide film is 2 nm or more is 600 ° C. to 1150 ° C., the film adhesion can generally be ensured. In particular, the film adhesion is particularly good when the heat treatment temperature of the outer oxide type oxide film of Sample No. 26 to Sample No. 40 is 40 nm or more and the heat treatment temperature is 1000 ° C. or more. However, when the cooling rate is 5 ° C./second or more and 100 ° C./second or less and the cavity area ratio in the external oxide film is 30% or less, the film adhesion is good, but the cooling rate is 200 Under conditions where the cavity area ratio was greater than 30% at ° C./second, the film adhesion was not necessarily perfect even when the external oxide type oxide film was thick, and the film remaining area ratio was 90%.
[0021]
From Table 1, it is essential that the film thickness of the external oxide film is 2 nm or more and the cavity area ratio in the external oxide film is 30% or less in order to ensure the film adhesion of the tension-imparting insulating film. In order to form such an external oxide film, heat treatment for forming the external oxide film is performed at a temperature of 600 ° C. or higher, particularly preferably 1000 ° C. or higher, and the cooling rate at that time is 5 It can be seen that it is necessary to set the temperature to 100 ° C./second or less.
[0022]
As described above, the present inventors consider the mechanism of the film adhesion as follows, as the film thickness and the cavity area ratio of the external oxide oxide film have a great influence.
First, the film thickness dependence of the external oxide type oxide film will be described.
Adhesion between the steel sheet and the tension-imparting insulating film is determined by an external oxidation type oxide film formed at the interface between the two. In general, it is said that an external oxide oxide film grows when metal atoms diffuse from the steel to the surface and react with an oxidizing gas on the surface. Therefore, the growth rate of the oxide film is determined by the diffusion rate of metal atoms. The diffusion of atoms in the metal is enhanced by thermal energy. Therefore, the higher the temperature, the more the atom diffusion is promoted and the outer oxide oxide film grows more. Due to such a mechanism, the growth of the external oxidation type oxide film is not sufficiently developed under the condition where the heat treatment temperature is as low as 500 ° C., and the film adhesion is poor. On the other hand, when the heat treatment temperature is 600 ° C. or higher, a sufficient external oxide type oxide film grows, so that the film adhesion is good, and when the heat treatment temperature is 1000 ° C. or higher, the oxide film is more easily grown, so the film adhesion is extremely good. it is conceivable that.
[0023]
It can be seen from the results of the measurement of the thickness of the external oxide film using a transmission electron microscope that such an assumption is valid. That is, the film thickness is 1 nm, the growth of the external oxide type oxide film is not sufficient, and the adhesion of the tension-imparting type insulating film is poor on the condition of the heat treatment temperature of 500 ° C. The film adhesion is good under the condition where the mold oxide film is grown and the heat treatment temperature is 600 ° C. or higher.
[0024]
Next, the relationship between the adhesion of the tension-imparting insulating film and the cavity area ratio of the external oxide oxide film will be described.
Although the details of the reaction mechanism for forming cavities in the external oxide film are still unknown, the present inventors have found that the external oxide oxide film is formed near the interface between the oxide film and the steel plate when the external oxide film is formed. Since accumulated lattice defects and the like are accumulated in the external oxide film, it is speculated that cavities may be generated. Regarding the relationship between the cavity formation and the cooling rate, when the cooling is performed slowly, these defects are removed from the system. However, when the cooling is performed rapidly, the defects are sufficiently removed from the system. Since there is no time, it is assumed that defects accumulate in the external oxide film and form cavities.
[0025]
The film tension is caused by the difference in thermal expansion coefficient between the tension-imparting insulating film and the steel sheet. At this time, a great amount of stress is generated at the interface between the tension-imparting insulating film and the steel plate. It is the external oxide type oxide film that can withstand this stress and ensure the adhesion between the steel sheet and the tension-imparting insulating film. The present inventors can withstand stress when there are few cavities in the external oxide oxide film that requires stress resistance, but there are many cavities, that is, when the cavity cross-sectional area ratio is high, the stress cannot be withstood. I think that will be done.
[0026]
Moreover, as shown in FIG. 1, in the external oxide film, oxides such as aluminum, manganese, chromium, and iron were present in addition to silica (silicon oxide). The adhesion to the tension-imparting insulating film was uniquely determined by the void area ratio in the external oxide film regardless of the presence or absence of these oxides.
[0027]
【Example】
[Example 1]
Decarburized and annealed cold-rolled sheet for unidirectional silicon steel sheet with 0.225mm thickness and Si concentration of 3.35%, and water slurry of annealing separator mainly composed of magnesia and bismuth chloride is applied on the surface. And dried. Next, finish annealing was performed at 1200 ° C. for 20 hours in a dry hydrogen atmosphere, and a unidirectional silicon steel sheet having a secondary recrystallization almost free of inorganic minerals on the surface was obtained. This steel plate was heat-treated at a temperature of 1150 ° C. in an atmosphere of 25% nitrogen, 75% hydrogen, and a dew point of −20 ° C. to form an external oxide type oxide film mainly composed of silica. At this time, the cooling rate was 10 ° C./second (Example) and 200 ° C./second (Comparative Example). The steel plate thus prepared was coated with a mixed solution consisting of 50 ml of a 50% magnesium phosphate aqueous solution, 100 ml of a 20% colloidal silica aqueous dispersion, and 5 g of anhydrous chromic acid, and baked at 850 ° C. for 30 seconds to impart tension. An insulating film was formed. About the unidirectional silicon steel plate with an insulating film prepared in this way, the adhesiveness of the insulating film was evaluated by the film remaining area ratio when the sample was wound around a cylinder having a diameter of 20 mm. The results are shown in Table 2.
[0028]
[Table 2]

[0029]
From Table 2, compared to the comparative example in which the cooling rate is 200 ° C./second, the cavity area rate is 40%, and the remaining film area rate is 90%, the cooling rate is 10 ° C./second, the cavity area ratio is 15%, and the remaining film area rate is 100%. Some examples have better film adhesion and are superior.
[0033]
Example 4
Decarburized and annealed cold-rolled sheet for unidirectional silicon steel sheet production with a thickness of 0.23mm and Si concentration of 3.30%, coated with a water slurry of an annealing separator mainly composed of magnesia on the surface, and dried Then, finish annealing was performed in a dry hydrogen atmosphere at 1200 ° C. for 20 hours. A film mainly composed of forsterite is formed on the surface of the unidirectional silicon steel sheet that has been subjected to secondary recrystallization thus prepared. Next, pickling in a mixed solution of ammonium fluoride and sulfuric acid, dissolving and removing the surface film, and then chemically polishing in a mixed solution of hydrofluoric acid and hydrogen peroxide solution, there is no inorganic mineral on the steel sheet surface, and A steel sheet with a specular gloss was obtained. This steel plate was heat-treated at a temperature of 1050 ° C. in an atmosphere of 25% nitrogen, 75% hydrogen and 0 ° C. dew point to form an external oxide type oxide film. At this time, the cooling rate was 100 ° C./second (Example) and 200 ° C./second (Comparative Example). To the steel sheet thus prepared, a mixed liquid composed of 100 ml of a 10% colloidal alumina aqueous dispersion, 10 g of amorphous alumina powder, 5 g of boric acid, and 200 ml of water was applied and baked at 850 ° C. for 30 seconds to provide tension-providing insulation. A film was formed. About the unidirectional silicon steel plate with an insulating film prepared in this way, the film adhesion was evaluated by the film remaining area ratio when the sample was wound around a cylinder with a diameter of 20 mm. The results are shown in Table 5.
[0034]
[Table 5]

[0035]
From Table 5, a cooling rate of 100 ° C./second, a cavity area ratio of 10%, and a remaining film area ratio of 100% are compared with the comparative example in which the cooling rate is 250 ° C./second, the cavity area ratio is 35%, and the remaining film area ratio is 90%. The example has better film adhesion and is superior.
[0036]
【The invention's effect】
According to the present invention, it is possible to provide a unidirectional silicon steel sheet having an insulating film with good film adhesion on a steel sheet surface substantially free of a finish annealing film.
[Brief description of the drawings]
1 is a cross-sectional TEM observation image of a sample of Sample No. 40 in Table 1. FIG. (W is a W vapor-deposited film for protecting the outermost surface at the time of TEM sample preparation. The white spot pattern in the matrix of Fe-3% Si steel is a dislocation loop generated at the time of TEM sample preparation.)

Claims

A unidirectional silicon steel sheet having a tension-imparting insulating film on the surface of a steel sheet substantially free of a finish annealing film, wherein the film thickness is 40 nm or more and 500 nm or less at the interface between the tension-imparting insulating film and the steel sheet. A unidirectional silicon steel sheet characterized by having a silica-based external oxidation type oxide film whose cross-sectional area ratio is 30% or less.

In order to ensure the adhesion between the tension-imparting insulation film and the steel sheet, which is produced by intentionally preventing the formation of forsterite inorganic mineral film, In the method of forming an external oxide type oxide film mainly composed of silica prior to the formation of the heat treatment, a heat treatment for forming the external oxide type oxide film is performed at a temperature of 1000 ° C. or more, and the temperature from the formation temperature of the external oxide type oxide film to 200 ° C. The manufacturing method of the unidirectional silicon steel plate characterized by making the cooling rate of the temperature range to 100 degrees C / sec or less.

The unidirectional silicon steel sheet according to claim 1, further comprising a tension-imparting insulating film formed by baking a coating solution mainly composed of phosphate and colloidal silica.

The unidirectional silicon steel sheet according to claim 1, further comprising a tension-imparting insulating film formed by baking a coating liquid mainly composed of alumina sol and boric acid.