JP2005024294A - Magnetic flaw detection method of ferromagnetic body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic flaw detection method of a ferromagnetic body, for setting the magnetization level and magnetization conditions of a specimen in flaw detection with a simple and inexpensive method, even if a steel type and a plate thickness change, and to provide a magnetic flaw detection method of a ferromagnetic body for accurately setting the magnetization level. <P>SOLUTION: In the magnetic flaw detection method of the ferromagnetic body, magnetization conditions are adjusted so that a magnetic field near the surface of the flaw detection region of a specimen ferromagnetic body becomes a preset magnetic field value (H0), and the magnetization level in flaw detection is set in a method for performing flaw detection by magnetizing the ferromagnetic body. In this manner, an appropriate magnetization level according to the magnetization level of a measurement object can be determined using a simple method, and the flaw detection time can be reduced significantly. Furthermore, flaw detection having high precision can be made. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００２１】
図２の関係を用いることで、鋼板の磁場の値から鋼板内部の磁束密度の値を予測可能である。また、鋼板内部の磁束密度の設定値（Ｂ０）に対応する磁場の設定値（Ｈ０）を決定できる。また、鋼板の磁場を測定し、測定値が前述の設定値（Ｈ０）になるように調整することで、鋼板内部の磁束密度を前述の磁束密度の設定値（Ｂ０）に設定することができる。
【００２２】
鋼板内部の磁束密度を簡易に測定することは困難であるが、鋼板表面近傍の磁場の測定は容易である。また、この２つの媒質の境界面付近では、境界面に電荷や電流がない場合はその境界面に対する電場の接線成分が等しい事が知られていることから、表面近傍の表面に沿った方向の磁場の値は鋼板内の表面に沿った磁場と等しいと考えてよい。また鋼板の磁場の調整は磁化条件を調整することで容易である。従って、ある鋼板について、エプスタイン試験等の公知の方法で、鋼板の磁場（Ｈ）と内部磁束（Ｂ）の関係を、あらかじめ求めておくと、この結果を用いることで、鋼種、板厚の異なる鋼板に対して、探傷時の鋼板内部の磁束密度の設定値（Ｂ０）と、それに対応する磁場の設定値（Ｈ０）を決定できる。そして、探傷時に、鋼板表面近傍の表面に沿った方向の磁場（Ｈ）を測定し、その値が前述の磁場の設定値（Ｈ０）となるように磁化条件を調整することで、探傷時の鋼板内部の磁束密度を予め測定した磁場の方向と同一の方向成分を所定の磁束密度の設定値（Ｂ０）に設定できる。
【００２３】
前述のように探傷時の鋼板内部の磁束密度を設定することで、探傷時の鋼板内部の磁束密度の設定を簡易化でき、また鋼板内部の磁束密度を探傷に最適な磁束密度に設定できる。
【００２４】
探傷する鋼板の鋼板内部の磁束密度の設定をより簡易に行うために、既知の鋼板の磁化特性のデータを利用することもできる。既知の鋼板の磁化特性のデータは、公知の技術文献に基づくものであってもよく、非公知の技術資料、試験、操業のデータ情報に基づくものであっても良い。既知の鋼板の磁化特性のデータは、探傷する鋼板と鋼種、板厚が同一の鋼板のデータだけでなく、鋼種、板厚の異なる鋼板であっても、例えば、図２に示されるにように高磁場領域でＢとＨの関係が一致する鋼板であれば、該鋼板のデータを利用できる。探傷する鋼板の鋼板内部の磁束密度の設定にあたって、参照できるデータ量が増大することで、鋼板内部の磁束密度の設定が簡易になるだけでなく、鋼板内部の磁束密度を探傷に適した磁束密度に設定しやすくなる。
【００２５】
本発明は、前述の知見及び考えに基づきなされたものである。
【００２６】
図３は、磁気探傷のフローを説明する図である。本発明では、予め探傷時の磁化レベルを決定する（磁化レベル決定ステップ）。次いで、探傷時に、被検体表面近傍の磁場を測定し、測定値があらかじめ設定した磁場の値（Ｈ０）になるように、磁化条件を調整して、磁化レベルを設定し（磁化レベル設定ステップ）、被検体を磁気探傷する（探傷ステップ）。このようにすることで、探傷時の磁化レベルの設定が簡易になる（第１ないし第３の手段）。
【００２７】
磁化レベル決定ステップでは、欠陥信号のＳ／Ｎが最大になる磁化レベルに対応する磁場の値（Ｈ０）または磁気飽和領域となる磁場の値（Ｈ０）を決定する。探傷時に、磁場の測定値が前記で決定した磁場の値（Ｈ０）なるように磁化条件を調整して被検体を探傷する。このようにすることで、探傷時に磁化レベルの設定を簡易に行うことができるだけでなく、探傷時の磁化レベルを欠陥探傷に適した条件に確実に設定できるようになる（第４および第５の手段）。
【００２８】
以下、本発明について詳しく説明する。
【００２９】
本発明では、強磁性体である被検体を磁化させて探傷するにあたって、探傷時の被検体の磁化レベルを次のように設定する。
【００３０】
被検体の探傷時の磁場の設定値（Ｈ０）は、既知の強磁性体の磁化特性（強磁性体の表面近傍の磁場と強磁性体内部の磁束密度の関係）に基いて、あらかじめ決定する。被検体が鋼板の場合を例に挙げて説明する。
【００３１】
既知の鋼板の磁化特性のデータは、公知文献、非公知の技術資料、試験、操業データ等の何れに基づくものでもよい。前記データは、探傷技術を対象とするものでも、対象としないものでもよい。既知の鋼板の鋼種、サイズは特に限定されないが、被検体と鋼種、板厚が同一である方が好ましい。鋼種、板厚が同一でない場合、被検体と磁気特性（強磁性体の表面近傍の磁場と強磁性体内部の磁束密度の関係）が同等又は同等と推定されるものが好ましい。磁化特性中に、磁場のデータが示されている場合、その値を探傷時の磁場の設定値（Ｈ０）としてもよい。磁場の値が示されていなくても、磁化特性のデータから磁場の値の算出または推定が可能であれば、その値を探傷時の磁場の設定値（Ｈ０）としてもよい。
【００３２】
また、前記データに基き、さらに探傷時に欠陥信号レベルが高くなる領域、又は磁気飽和領域になる磁場領域を推定し、探傷時の磁場の設定値（Ｈ０）を、前記磁気飽和領域の磁場の値に決定することがより好ましい。ここで、被検体が鋼板である場合、磁気飽和領域は、磁場の値（Ｈ０）が４００００Ａ／ｍ以上、または比微分透磁率が４以下を例示できる。
【００３３】
次に、図７（後記）の装置を用いて、探傷時に、磁気センサの直下の被検体の探傷位置に対応する極力被検体表面に近い場所で磁場（Ｈ）を測定し、測定値が所定の設定値（Ｈ０）になるように磁化条件、例えば磁化電流を調整する。このように磁化条件を調整することで、探傷時の磁化レベルを設定する。
【００３４】
測定される外部磁場（Ｈ）は、磁場測定部が被検体から離れるとその値が変わり、磁場測定方向が変わるとその値が変わってくる。そのため、被検体の探傷する位置に対応する被検体表面に接する空間における磁化方向に平行な磁場を測定する。
【００３５】
本手段では、被検体の探傷する位置に対応する被検体表面に接する空間における磁化方向に平行な磁場を測定し、磁化条件を調整してその磁場の値があらかじめ設定した磁場の値（Ｈ０）になるように磁化条件を調整することで、被検体の磁化レベルを簡易に設定できる。
【００３６】
ここで、磁化条件とは、例えば磁化器の磁化電流、磁化器と測定対象の距離、磁化器ヨークの間隔、形状、材質、磁化コイルの巻き数、などの被検体の磁化レベルを変えることのできる条件の事である。
【００３７】
また、磁化レベルとは、磁化器によって被検体がどの程度磁化されたかの程度のことで、磁化の強度ともいう。具体的には、たとえば被検体内部の磁束密度の値で示される。
【００３８】
前述のように、第２の手段では、探傷の都度、被検体の探傷する位置に対応する被検体表面に接する空間における磁化方向に平行な磁場（Ｈ）を測定し、その値があらかじめ決定した所定の設定値（Ｈ０）になるように磁化条件を調整することで磁化レベルを設定した。
【００３９】
第３の手段では、あらかじめ、被検体の探傷する位置に対応する被検体表面に接する空間における磁化方向に平行な磁場（Ｈ）と磁化条件との関係を求める。そして、前記で求めた磁場（Ｈ）と磁化条件との関係を用いて、探傷時に、磁化条件を、磁場の値があらかじめ設定した所定の設定値（Ｈ０）となる条件に調整することで、探傷時の磁化レベルと設定する。ここで、磁場の設定値（Ｈ０）は、第２の手段と同様にして決定される。
【００４０】
被検体が鋼板で、磁化条件の内、磁化電流を調整する場合を例に挙げて説明する。図７に示す磁化装置を用いて、あらかじめ、鋼板サイズ毎に磁化電流（Ｉ）を種々変えて、被検体の探傷する位置に対応する被検体表面に接する空間における磁化方向に平行な磁場（Ｈ）を測定し、磁化電流（Ｉ）と前記磁場（Ｈ）との関係を求める。磁化電流（Ｉ）と前記磁場（Ｈ）との関係を示す特性図の一例を図４に示す。図４は、後記実施例に記載される厚さ１．８ｍｍの熱延鋼板の例である。
【００４１】
次に、図４に示される磁化電流（Ｉ）と磁場（Ｈ）との関係を用いて、実際の探傷時に、磁化電流（Ｉ）を、磁場の設定値（Ｈ０）に対応する磁化電流（Ｉ０）に調整する。本手段では、探傷時に、磁化電流（Ｉ）を前記電流値（Ｉ０）に調整するだけで、磁場（Ｈ）を測定しなくてもよいので、被検体の磁化レベルの設定がより簡易になる。図４に示される磁化電流（Ｉ）と前記磁場（Ｈ）との関係の実験式を求めておき、これに基づいて磁化電流（Ｉ）を調整してもよい。例えば、図４に示される例では、磁化電流（Ｉ：ｍＡ）と磁場（Ｈ：Ａ／ｍ）との関係は、Ｈ＝６８＊Ｉ−１３６８０で表される。
【００４２】
第４の手段では、第１段階の磁化レベル決定ステップは、被検体と磁気的に同等な強磁性体サンプルに設けられた標準欠陥を探傷して欠陥信号レベルを測定し、同時に前記強磁性体サンプルの探傷する位置に対応する強磁性体サンプル表面に接する空間における磁化方向に平行な磁場を測定して、前記欠陥信号レベルが最大になるときの磁場の値（Ｈ０）を決定する。なお、標準欠陥は、自然欠陥、人工欠陥何れであってもよい。
【００４３】
ここで、被検体と磁気的に同等は、処女磁化したときに得られる励磁磁場（Ｈ）と内部磁束密度（Ｂ）の関係を示すＢ−Ｈカーブが、高磁場領域において、被検体と強磁性体とで、ほぼ一致することである。熱延鋼板を例に挙げると、５０００Ａ／ｍ以上の磁場領域、より厳密には４００００Ａ／ｍ以上の磁束密度の飽和領域において、さらには交流磁気特性についても、略同一のＢ−Ｈカーブが得られるものである。前述のように、Ｃ含有量が０．０５質量％以下の低炭素熱延鋼板（Ｃ含有量が０．０１質量％以下の極低炭素熱延鋼板を含む。以下、同じ。）は、成分、板厚が変わっても、図２に示すような同一のＢ−Ｈカーブが得られるので、Ｃ含有量が前記範囲内にある低炭素熱延鋼板は、磁気的に同等であると判断される。
【００４４】
従って、例えば、被検体が、Ｃ含有量が０．０５質量％以下の低炭素熱延鋼板である場合、強磁性体サンプルとして、前述のＣ含有量が０．０５質量％以下の低炭素熱延鋼板の中から、適宜板厚の１の鋼板を選び、該鋼板に標準欠陥となる人工欠陥を作製する。前述の鋼板の中から自然欠陥のある鋼板を選んでも良い。
【００４５】
次に、前記標準欠陥を探傷して欠陥信号レベルを測定し、同時に前記強磁性体サンプルの探傷する位置に対応する強磁性体サンプル表面に接する空間における磁化方向に平行な磁場を測定して、前記欠陥信号レベルが最大になるときの磁場の値（Ｈ０）を決定する。
【００４６】
磁気探傷の行われる被検体種が多岐である場合、あらかじめ、種々の被検体について、前述のＢ−Ｈカーブを調査し、Ｂ−Ｈカーブが一致するもの同士をグループ化しておくことが好ましい。
【００４７】
第２段階の磁化レベル設定ステップは、実際の探傷時に、被検体を探傷するときの磁場の値が前記第１段階で決定した磁場の値（Ｈ０）となるように、前述の第２又は第３の手段に記載した方法で、磁化条件を調整する。
【００４８】
すなわち、第２の手段に記載した方法では、図７の装置を用い、被検体を探傷するときに、被検体の探傷する位置に対応する被検体表面に接する空間における磁化方向に平行な磁場（Ｈ）を測定し、その磁場の値（Ｈ）が前記決定した磁場の値（Ｈ０）になるように、磁化条件、例えば磁化電流を調整する。
【００４９】
また、第３の手段に記載した方法では、あらかじめ、図７の装置を用い、被検体の探傷する位置に対応する被検体表面に接する空間における磁化方向に平行な磁場と磁化条件との関係を求める。例えば、磁化電流を調整する場合、あらかじめ、被検体について、図４に示すような磁化電流（Ｉ）と前記磁場（Ｈ）との関係を求める。この関係を用いて、第１段階で決定した磁場の値（Ｈ０）になる磁化電流の設定値（Ｉ０）を求め、探傷時の磁化電流を前記た電流値（Ｉ０）に調整する。
【００５０】
第３段階の探傷ステップは、前記で設定した磁化条件（磁化電流（Ｉ０））で被検体を磁気探傷する。
【００５１】
標準欠陥による探傷を行い、欠陥信号のＳ／Ｎが最大となるような磁化レベルを、基準となる磁化レベルとして決定する点は、従来技術と同様である。
【００５２】
従来技術では、鋼種、板厚が異なる被検体を探傷する場合、鋼種、板厚毎に、標準欠陥による探傷を行い、基準となる磁化レベルを設定していた。そのため、磁化レベルの設定が煩雑であった。
【００５３】
これに対して、本手段では、既知に強磁性体の磁化特性のデータに基いて決定した磁場の値（Ｈ０）になるように磁化条件を調整して探傷時の磁化レベルを設定し、又は、磁気的に同等な被検体にあっては、複数の標準欠陥がある強磁性体サンプルのうち、代表的なある１の強磁性体サンプルについて標準欠陥の探傷を行い、欠陥信号レベルが最大になる磁場の値（Ｈ０）を決定するだけで、これと磁気的に同等な被検体については、強磁性体サンプルと鋼種、板厚が異なっていても、前述の磁場の値（Ｈ０）に基いて磁化条件を調整して探傷時の磁化レベルを設定する。そのため、探傷時の磁化レベル設定の煩雑さが解消される。
【００５４】
第５の手段では、第１段階の磁化レベル決定ステップは、あらかじめ、被検体と磁気的に同等な強磁性体サンプルに外部より磁場を与え、該強磁性体を処女磁化したときの外部より与えられる磁場（Ｈ）と強磁性体内部の磁束密度（Ｂ）の関係を調査して、外部より与えられる磁場（Ｈ）と内部の磁束密度（Ｂ）の関係（Ｂ−Ｈカーブ）を求める。その関係を用いて、被検体探傷時の被検体内部の磁束密度の設定値（Ｂ０）、前記被検体内部の磁束密度の設定値（Ｂ０）に対応する磁場の値（Ｈ０）を決定する。
【００５５】
例えば、被検体が、Ｃ含有量が０．０５質量％以下の低炭素熱延鋼板である場合、強磁性体サンプルとして、前述の鋼板の中から１の鋼板を選ぶ。この強磁性体サンプルについて、処女磁化したときの励磁磁場（Ｈ）と鋼板の内部磁束密度（Ｂ）の関係を調査し、図２に示すようなＢ−Ｈカーブを求める。次に、このＢ−Ｈカーブを用いて、被検体を探傷する時の被検体内部の磁束密度の設定値（Ｂ０）、及び前記被検体内部の磁束密度の設定値（Ｂ０）に対応する磁場の値（Ｈ０）を決定する。
【００５６】
第２段階の磁化レベル設定ステップは、実際の探傷時に、前述の第２又は第３の手段に記載した方法で、被検体を探傷するときの磁場の値が前記第１段階で決定した磁場の値（Ｈ０）となるように磁化条件を調整する。具体的には、前記第４の手段の第２段階の磁化レベル設定ステップと同様に行う。
【００５７】
第３段階の探傷ステップは、前記で設定した磁化条件で被検体を磁気探傷する。
【００５８】
本手段では、磁場の値が、被検体と磁気的に同等な強磁性体サンプルのＢ−Ｈカーブに基いて、あらかじめ設定した被検体探傷時の磁束密度の値（Ｂ０）となる被検体の磁場の値（Ｈ０）となるように磁化条件を調整して探傷時の磁化レベルを設定するので、探傷時の磁化レベルの設定の煩雑さを解消できる。また、探傷時の被検体内部の磁束密度を被検体の探傷に適した磁束密度に確実に設定できる。
【００５９】
漏洩する磁束を測定する探傷方式では、被検体を磁気飽和させる、すなわち、探傷時の磁場の設定値（Ｈ０）を磁気飽和領域の値に設定する方が、より強い欠陥信号が出る。磁気飽和の指標として比微分透磁率が用いられている。ここで、比微分透磁率は、真空の透磁率μ_０を磁場（Ｈ）に掛けたものと、磁束密度をプロットしたＢ−μ_０Ｈカーブ（Ｂ−Ｈカーブの横軸にμ_０を掛けたもの）の傾きに相当する。
【００６０】
強磁性体内部に形成される磁束と磁場の関係は、Ｂ＝μ_０Ｈ＋Ｊであることが知られている。ここで、Ｂは磁束密度、Ｈは磁場、Ｊは強磁性体内に外部磁場により励磁される磁化である。強磁性体に外部より徐々に強い磁場を掛けていくと、はじめは、強磁性体内部に磁化が形成されることで磁束は急激に増加する。さらに磁場を掛けていくと、強磁性体は磁気飽和に近づき、やがて磁化がそれ以上増加しない磁気飽和に達する。磁気飽和状態に達すると、磁場の増加に対して磁束密度の増加は、磁場の増加分のみとなるため比微分透磁率は１となる。そのため、比微分透磁率が１に近づくと強磁性体が磁気飽和状態に近づいていると判断できる。
【００６１】
図５は、図２に示したＢ−Ｈカーブに対応する、磁場（Ｈ）と比微分透磁率の関係を示す図で、横軸は磁場（Ｈ）、縦軸は比微分透磁率である。図５から、磁場（Ｈ）が４００００Ａ／ｍ以上では、比微分透磁率が４以下であり、磁束密度が実質的に飽和して来ていることがわかる。このことから、磁場の設定値（Ｈ０）を４００００Ａ／ｍ以上、比微分透磁率を４以下とすることが好ましい。
【００６２】
探傷時に、磁場の設定値（Ｈ０）又は比微分透磁率を前記で規定される値に設定することで、内部欠陥の漏洩磁束探傷に適した磁気探傷が可能となる。磁場の設定値（Ｈ０）が４５０００Ａ／ｍ以上では、比微分透磁率が２以下となり、さらに磁気飽和に近い条件で探傷できるのでより好ましい。なお、比微分透磁率が１となる磁場の値を設定すると、完全な磁気飽和条件での探傷が可能となる。しかし、磁気飽和に近い条件で探傷すると、完全に消磁されにくくなる。従って、求められる欠陥探傷レベルや消磁を考慮して、前記で規定される条件の範囲内で、適宜条件を設定して探傷を行うのがよい。
【００６３】
次に、本発明の実施の形態に係る強磁性体の探傷方法の一例について、図６及び図７を参照して具体的に説明する。
【００６４】
図６は、漏洩磁束探傷装置を備える鋼板の磁気探傷装置の概略図である。図６において、１は鋼板、２ａ、２ｂは搬送ローラ、３は漏洩磁束探傷装置、４は内部欠陥である。鋼板１の搬送路に沿って漏洩磁束探傷装置３が設置されている。前記漏洩磁束探傷装置３は、磁気センサ６、磁化器５、信号処理装置７、磁化器電源８を備える。磁気センサ６と鋼板１との距離（リフトオフ）はＬである。
【００６５】
図７は、鋼板表面に接する空間における磁化方向に平行な磁場（Ｈ）の測定に使用される磁化装置の斜視図である。図７において、９はガウスメータであり、それ以外の符号は図６で説明した符号と同じである。
【００６６】
本発明では、磁化レベル決定ステップで、鋼板１を探傷する前に、あらかじめ、鋼板を処女磁化したときの鋼板内部の磁束密度（Ｂ）と外部磁界（Ｈ）を公知の方法（例えばエプスタイン試験）で測定し、Ｂ−Ｈカーブからは比微分透磁率は不明であるので、「Ｂ−Ｈカーブを用いて、さらに磁場（Ｈ）と比微分透磁率の関係を示す図を求め、比微分透磁率が４以下である磁場の設定値（Ｈ０）を決定する。
【００６７】
次に、磁化レベル設定ステップで、探傷前に、あらかじめ、図７に示す装置を用いて、磁化器電源８より磁化器５に流す磁化電流（Ｉ）を変えながら鋼板１の探傷する位置に対応する鋼板表面に垂直に当てたガウスメータ９で、鋼板表面近傍の鋼板表面の磁化方向に沿った向きの磁場（Ｈ）を測定し、磁場（Ｈ）が前記で決定した磁場（Ｈ０）となる磁化電流（Ｉ０）を決定する。次いで、図６の探傷装置における探傷時の磁化電流（Ｉ）の設定値を前記で決定した磁化電流（Ｉ０）に設定する。ここで、探傷する位置に対応する点としては、図６の示された磁気センサ６から鋼板表面への垂線をおろし、鋼板表面と交わる点を用いた。また、ガウスメータは磁場を安定して測定するために被検体である鋼板表面に垂直にあて、ガウスメータのセンサ部のサイズは２〜３ｍｍ程度で、鋼板表面から２〜３ｍｍ程度の高さまでの磁場を測定するのが好ましい。
【００６８】
次に探傷ステップについて説明する。
【００６９】
図６の磁化装置を用いて、鋼板１を搬送ローラ２ａ、２ｂで搬送しながら、磁化器電源８により磁化器５に前記で設定した電流値（Ｉ０）の電流を供給して鋼板１を磁化する。磁気センサー６は鋼板表面へ漏洩する磁束を検出し、その信号は信号処理装置７へ出力される。内部欠陥４などの欠陥が磁気センサー６位置を通過すると漏洩する磁束が大きくなり、磁気センサー６から信号処理装置７への出力が大きくなる。磁気センサ６の出力は信号処理装置７で処理され、欠陥判定が行われる。信号処理装置７では、磁気センサー６で検出される信号に基き、常法で処理される。
【００７０】
前述の探傷方法では、磁化レベル設定ステップで、あらかじめ、磁化電流（Ｉ０）を決定し、探傷時に、磁化電流が前記で決定した磁化電流（Ｉ０）になるように磁化電流を設定したが、漏洩磁束探傷装置３に磁気センサー６とは別に、さらにガウスメータを付設し、ガウスメータで検出する磁場が前記Ｈ０になるように磁化電流を調整しながら、探傷してもよい。
【００７１】
前述の説明では、磁化電流の値を変えることで磁化条件を調整したが、磁化器と測定対象の距離、磁化器ヨークの間隔、形状、材質、磁化コイルの巻き数を変えることで磁化条件を調整してもかまわない。
【００７２】
前述の説明では、強磁性体を磁化する磁化器と反対側の強磁性体に接する空間の磁化方向と同じ方向の磁化を測定したが、磁化器と同一側の強磁性体に接する空間の磁化方向と同じ方向の磁化を測定しても構わない。また、磁化方向と同じ方向の磁化との対応をあらかじめ測定しておけば、磁化方向と異なる方向の磁化を測定する方式でも構わない。
【００７３】
前述の説明では、主に漏洩磁束を探傷する方式について説明したが、直流磁場ないしは交流磁場を用いて強磁性体を磁化し磁気飽和状態で、渦流探傷を行うような磁気探傷方式にも適用可能である。
【００７４】
【実施例】
図６及び図７に示した装置を用いて、板厚１．８ｍｍの低炭素熱延鋼板（Ｃ含有量０．０４質量％）の内部欠陥を漏洩磁束探傷によって探傷した例を示す。図６にて、リフトオフＬ＝０．５ｍｍ、鋼板１の搬送速度は１ｍ／ｓｅｃである。探傷に供した鋼板１のＢ−Ｈカーブは、図２で示したＢ−Ｈカーブと一致したので、鋼板１のＢ−Ｈカーブは、図２に示したＢ−Ｈカーブを用いた。
【００７５】
次に、あらかじめ、図７の磁化装置を用いて、磁化器電源８から磁化器５に流す磁化電流を変えながら鋼板１の表面に垂直に当てたガウスメータ９で、鋼板１表面の磁化方向に沿った向きの磁場（Ｈ）を測定し、磁化電流（Ｉ）と前記磁場（Ｈ）の関係を調査した。調査結果を図４に示す。
【００７６】
次いで、図６に示した磁気探傷装置を用いて、磁化器電源８から磁化器５に流す磁化電流（Ｉ）を変えることで、磁場Ｈ（すなわち磁束密度Ｂ）を変化させながら、内部欠陥を複数探傷したもののうち代表的な１例を図８に示す。
【００７７】
図８で、横軸は比微分透磁率、縦軸は欠陥信号のＳ／Ｎである。本発明者らは、正確な欠陥探傷を行う観点からは、欠陥信号のＳ／Ｎが２．５以上であることが好ましく、３．０以上であることがより好ましいことを経験的に知得している。図８から、比微分透磁率が４以下でノイズと欠陥信号の比であるＳ／Ｎが２．５以上となりノイズに対して欠陥信号を判別可能となり、欠陥探傷可能である。比微分透磁率が２以下となる磁化レベルを選べば、Ｓ／Ｎが３．０以上となり、さらに正確な欠陥探傷が可能であることがわかる。
【００７８】
【発明の効果】
本発明により、直流漏洩磁束探傷方法、交流漏洩磁束探傷方法、直流磁紛探傷方法、交流磁紛探傷方法などの、強磁性体を磁化させる磁気探傷方法において、簡易な方式で、測定対象の磁化レベルに応じた適切な磁化レベルを決定する事が可能となり、大幅に探傷時間を短縮する事が可能となる。さらに精度のよい探傷が可能になる。
【図面の簡単な説明】
【図１】外部磁場（Ｈ）と被検体内部の磁束密度（Ｂ）の一般的なヒステリシス曲線（Ｂ−Ｈカーブ）を示す図である。
【図２】鋼種、板厚の異なる３種類の熱延鋼板を処女磁化したときの鋼板の励磁磁場（Ｈ）と鋼板の内部磁束密度（Ｂ）の関係を示す図である。
【図３】磁気探傷のフローを説明する図である。
【図４】磁化電流（Ｉ）と前記磁場（Ｈ）との関係の一例を示す図である。
【図５】図２に示した鋼板のＢ−Ｈカーブに対応する、磁場（Ｈ）と比微分透磁率の関係を示す図である。
【図６】漏洩磁束探傷装置を備える鋼板の磁気探傷装置の概略図である。
【図７】鋼板表面に接する空間における磁化方向に平行な磁場（Ｈ）の測定に使用される磁化装置の斜視図である。
【図８】鋼板の比微分透磁率と内部欠陥の信号のＳ／Ｎの関係を示す図である。
【符号の説明】
１ 鋼板
２ａ、２ｂ 搬送ローラ
３ 漏洩磁束探傷装置
４ 内部欠陥
５ 磁化器
６ 磁気センサ
７ 信号処理装置
８ 磁化器電源
９ ガウスメータ[0001]
BACKGROUND OF THE INVENTION
The present invention belongs to a magnetic flaw detection method in which a ferromagnetic material is magnetized and flawed, such as a DC leakage magnetic flux inspection, an AC leakage magnetic flux inspection, a DC magnetic particle inspection, and an AC magnetic particle inspection.
[0002]
[Prior art]
A magnetic flaw detection method for a ferromagnetic material will be described by taking as an example a magnetic flaw detection of steel that is most commonly used as a ferromagnetic material. In magnetic flaw detection methods such as DC leakage magnetic flux inspection, AC leakage magnetic flux inspection, DC magnetic particle inspection, and AC magnetic particle inspection, it is necessary to determine the magnetization level of the ferromagnetic object at the time of flaw detection. As a method of determining the magnetization level of a ferromagnetic specimen at the time of flaw detection, a standard defect is flaw-detected by changing the magnetization conditions in advance, and the magnetization level at which the S / N of the defect signal is maximized is determined. A method of performing flaw detection under a magnetization condition that achieves a level is common.
[0003]
In this method, since the standard defect signal is used as a reference, if the natural defect is a standard defect, the same defect cannot be produced, and therefore there is a problem that the conditions are different for each steel type. In addition, if an artificial defect is a standard defect, the same defect can be created with good reproducibility. However, a natural defect that is actually inspected produces a defect signal that is different from that of an artificial defect, so it is difficult to accurately detect natural defects. There are problems such as becoming.
[0004]
Also, with this method, when testing specimens of different steel types and plate thicknesses, the magnetization level is determined by the standard defect of a sample with the same specimen and steel type and plate thickness each time, and the magnetization conditions are set accordingly. It was necessary to rework. Therefore, the determination of the magnetization level at the time of flaw detection and the setting method of the magnetization conditions are complicated.
[0005]
In addition, in the conventional method of determining the magnetization level from the standard defect signal, it is impossible to confirm the magnetization state of the measurement object, so it is possible to grasp the level of the magnetization state of the subject at the time of flaw detection. There is a problem that it is not made.
[0006]
In particular, in the leakage magnetic flux inspection method and the magnetic particle inspection method, since the defect signal due to the discontinuous part in the ferromagnetic body leaks to the surface of the ferromagnetic body, the defect signal Does not leak to the surface of the object, and the defect cannot be detected. Therefore, in these flaw detection methods, it is particularly important that the magnetization state of the subject can be accurately grasped at the time of flaw detection.
[0007]
In general, the magnetic flux density inside the subject is affected by the magnetic field applied from the outside and the permeability (relative permeability) of the material (subject). As is well known, the external magnetic field (H) and the magnetic flux density (B) inside the subject form a hysteresis curve (BH curve) as shown in FIG. 1, and are strongly influenced by the magnetization history. . For this reason, since the internal magnetic flux density (B) cannot generally be determined from the value of the external magnetic field (H), the magnetization state of the subject at the time of flaw detection cannot be easily grasped.
[0008]
In order to measure the relationship between the magnetic flux density (B) inside the ferromagnetic material and the external magnetic field (H), for example, it is necessary to perform measurement by a method such as an Epstein test. However, a dedicated device is required to evaluate these magnetic characteristics. When actually performing leakage magnetic flux inspection on-line, it is necessary to evaluate the magnetic field characteristics for each coil. In order to do this, (1) a sample is cut out from the coil and evaluated by an off-line apparatus, and (2) an on-line dedicated magnetic field characteristic evaluation apparatus is provided and automatically evaluated.
[0009]
However, in the method (1), a sample is cut out from each coil and transported to an off-line device. After evaluating the magnetic characteristics, the procedure for performing on-line leakage magnetic flux flaw detection is performed based on the result. It takes. In the method (2), the magnetization conditions of the leakage magnetic flux flaw detector can be determined based on the magnetic characteristics automatically measured by the online device, but there is a problem that the device itself becomes complicated and expensive. Therefore, it is not realistic to evaluate the magnetic characteristics for each coil one by one.
[0010]
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and magnetic inspection of a ferromagnetic material that can set the magnetization level and magnetization conditions of a subject at the time of flaw detection by a simple and inexpensive method even if the steel type and plate thickness change. The purpose is to provide a method. It is another object of the present invention to provide a magnetic flaw detection method for a ferromagnetic material that can accurately set the magnetization level of a subject.
[0011]
[Means for Solving the Problems]
Means of the present invention for solving the above-mentioned problems are as follows.
[0012]
The first means adjusts the magnetization condition so that the magnetic field in the vicinity of the surface of the flaw detection area of the ferromagnetic specimen becomes a preset magnetic field value (H0) in the method of flaw detection by magnetizing a ferromagnetic material. A magnetic flaw detection method for a ferromagnetic material, characterized in that the magnetization level at the time of flaw detection is set.
[0013]
The second means is a method in which a flaw is detected by magnetizing a ferromagnetic material, and a magnetic field parallel to the magnetization direction in the vicinity of the surface of the flaw detection area of the ferromagnetic object is measured at the time of flaw detection. This is a magnetic flaw detection method for a ferromagnetic material, characterized in that the magnetization level at the time of flaw detection is set by adjusting the magnetization conditions so as to be a value (H0).
[0014]
According to a third method, in a method for flaw detection by magnetizing a ferromagnetic material, a relationship between a magnetic field parallel to the magnetization direction in the vicinity of the surface of the flaw detection region of the ferromagnetic object and the magnetization condition is obtained in advance, and the relationship is used. Thus, the magnetic flaw detection method for a ferromagnetic material is characterized in that the magnetization condition is adjusted to set the magnetization level so that the magnetic field value becomes a preset magnetic field value (H0).
[0015]
A fourth means is a method of flaw detection by magnetizing a ferromagnetic material, and a magnetic field that maximizes the standard defect signal level when a standard defect of a ferromagnetic sample magnetically equivalent to a ferromagnetic specimen is detected. A ferromagnetic material characterized by flaw detection of the ferromagnetic specimen by adjusting the magnetization condition by any one of the first to third methods to set the magnetization level so that the value becomes (H0). This is a magnetic flaw detection method.
[0016]
The fifth means is a method of flaw detection by magnetizing a ferromagnetic material, and a magnetic field (H) applied from the outside and a ferromagnetic material sample obtained in advance with a ferromagnetic sample magnetically equivalent to the ferromagnetic object. From the relationship with the internal magnetic flux density (B), the magnetic field value (H0) of the subject at which the magnetic flux density at the time of the subject flaw detection becomes a predetermined value (B0) is set to any one of the first to third means. In this method, the magnetic condition is adjusted, and the subject is flaw-detected.
[0017]
A sixth means is the magnetic flaw detection method for a ferromagnetic material according to any one of the first to fifth means, wherein the set value (H0) of the magnetic field is set to a value in a magnetic saturation region. .
[0018]
DETAILED DESCRIPTION OF THE INVENTION
In general magnetic flaw detection, a magnetic saturation region in which a subject is strongly magnetized and can be placed in a stable state is often used. In general, in the case of a material having similar components, the relationship between B and H in the magnetic saturation region is expected to be almost constant regardless of the material change. Actually, the present inventors conducted various investigations on the magnetization characteristics of a low carbon steel sheet having a C content of 0.05% by mass or less and an ultra low carbon steel sheet having a C content of 0.01% by mass or less. As a result, when the steel plate is virginally magnetized, in the high magnetic field region, the BH curve of the steel plate showing the relationship between the excitation magnetic field (H) of the steel plate and the internal magnetic flux density (B) of the steel plate is the steel type and thickness. It was newly found that almost the same BH curve can be obtained even if different.
[0019]
That is, FIG. 2 shows the relationship between the excitation magnetic field (H) of the steel sheet and the internal magnetic flux density (B) of the steel sheet when virginally magnetized three types of hot-rolled steel sheets with different steel types and thicknesses shown in Table 1. The horizontal axis represents the external excitation magnetic field (H), and the vertical axis represents the internal magnetic flux density (B) of the steel sheet. In FIG. 2, only the BH curve in the high magnetic field region of 5000 A / m or more in the first quadrant of the BH curve is shown. In a high magnetic field region of 5000 A / m or more, the BH curves of three types of steel plates having different steel types and plate thicknesses coincide with each other, and are represented by one BH curve as shown in FIG.
[0020]
[Table 1]

[0021]
By using the relationship of FIG. 2, it is possible to predict the value of the magnetic flux density inside the steel plate from the value of the magnetic field of the steel plate. Further, the magnetic field setting value (H0) corresponding to the magnetic flux density setting value (B0) inside the steel plate can be determined. Further, by measuring the magnetic field of the steel sheet and adjusting the measured value to be the aforementioned set value (H0), the magnetic flux density inside the steel sheet can be set to the aforementioned set value (B0) of the magnetic flux density. .
[0022]
Although it is difficult to easily measure the magnetic flux density inside the steel sheet, it is easy to measure the magnetic field in the vicinity of the steel sheet surface. Also, in the vicinity of the boundary surface between these two media, it is known that the tangential component of the electric field to the boundary surface is equal when there is no charge or current on the boundary surface. It can be considered that the value of the magnetic field is equal to the magnetic field along the surface in the steel sheet. The magnetic field of the steel plate can be easily adjusted by adjusting the magnetization conditions. Accordingly, when a relationship between the magnetic field (H) of the steel plate and the internal magnetic flux (B) is obtained in advance for a certain steel plate by a known method such as an Epstein test, the steel type and the plate thickness are different by using this result. For the steel plate, a set value (B0) of magnetic flux density inside the steel plate at the time of flaw detection and a set value (H0) of the magnetic field corresponding to the set value can be determined. And at the time of flaw detection, the magnetic field (H) in the direction along the surface in the vicinity of the steel sheet surface is measured, and the magnetization condition is adjusted so that the value becomes the above-mentioned magnetic field setting value (H0). The same direction component as the direction of the magnetic field in which the magnetic flux density inside the steel plate is measured in advance can be set to a predetermined magnetic flux density setting value (B0).
[0023]
By setting the magnetic flux density inside the steel plate at the time of flaw detection as described above, setting of the magnetic flux density inside the steel plate at the time of flaw detection can be simplified, and the magnetic flux density inside the steel plate can be set to the optimum magnetic flux density for flaw detection.
[0024]
In order to more easily set the magnetic flux density inside the steel plate of the steel plate to be inspected, it is possible to use data on the magnetization characteristics of the known steel plate. The data on the magnetization characteristics of known steel sheets may be based on known technical literature, or may be based on non-known technical data, test, and operation data information. As shown in FIG. 2, for example, as shown in FIG. 2, the data on the magnetic properties of known steel sheets include not only data on steel sheets having the same steel type and thickness but also steel sheets having different steel types and thicknesses. As long as the relationship between B and H is the same in the high magnetic field region, the data of the steel plate can be used. In setting the magnetic flux density inside the steel plate of the steel plate to be inspected, the amount of data that can be referred to increases, which not only simplifies the setting of the magnetic flux density inside the steel plate, but also makes the magnetic flux density inside the steel plate suitable for flaw detection. Easy to set.
[0025]
The present invention has been made on the basis of the aforementioned knowledge and idea.
[0026]
FIG. 3 is a diagram for explaining the flow of magnetic flaw detection. In the present invention, the magnetization level at the time of flaw detection is determined in advance (magnetization level determination step). Next, at the time of flaw detection, the magnetic field in the vicinity of the surface of the subject is measured, the magnetization condition is adjusted so that the measured value becomes a preset magnetic field value (H0), and the magnetization level is set (magnetization level setting step). Then, the object is magnetically flawed (flaw detection step). By doing in this way, the setting of the magnetization level at the time of a flaw detection becomes easy (1st thru | or 3rd means).
[0027]
In the magnetization level determination step, the magnetic field value (H0) corresponding to the magnetization level at which the S / N of the defect signal is maximized or the magnetic field value (H0) corresponding to the magnetic saturation region is determined. At the time of flaw detection, the specimen is flawed by adjusting the magnetization conditions so that the measured value of the magnetic field becomes the magnetic field value (H0) determined above. This makes it possible not only to easily set the magnetization level during flaw detection, but also to reliably set the magnetization level during flaw detection to a condition suitable for defect flaw detection (fourth and fifth). means).
[0028]
The present invention will be described in detail below.
[0029]
In the present invention, when magnetizing a subject that is a ferromagnetic material and performing flaw detection, the magnetization level of the subject at the time of flaw detection is set as follows.
[0030]
The set value (H0) of the magnetic field at the time of flaw detection of the subject is determined in advance based on the magnetization characteristics of the known ferromagnetic material (the relationship between the magnetic field in the vicinity of the surface of the ferromagnetic material and the magnetic flux density inside the ferromagnetic material). . The case where the subject is a steel plate will be described as an example.
[0031]
The data of the known magnetic properties of the steel sheet may be based on any of publicly known literature, non-known technical data, tests, operation data, and the like. The data may or may not be for flaw detection technology. The steel type and size of the known steel plate are not particularly limited, but it is preferable that the specimen, the steel type, and the plate thickness are the same. When the steel type and the plate thickness are not the same, it is preferable that the specimen and the magnetic properties (relationship between the magnetic field in the vicinity of the surface of the ferromagnetic material and the magnetic flux density inside the ferromagnetic material) are estimated to be equivalent or equivalent. When magnetic field data is indicated in the magnetization characteristics, the value may be set as the magnetic field setting value (H0) at the time of flaw detection. If the value of the magnetic field can be calculated or estimated from the magnetization characteristic data even if the value of the magnetic field is not shown, the value may be set as the setting value (H0) of the magnetic field at the time of flaw detection.
[0032]
Further, based on the data, a region where the defect signal level becomes higher at the time of flaw detection or a magnetic field region that becomes a magnetic saturation region is estimated, and the setting value (H0) of the magnetic field at the time of flaw detection is set to the value of the magnetic field in the magnetic saturation region. More preferably, it is determined. Here, when the subject is a steel plate, the magnetic saturation region can be exemplified by a magnetic field value (H0) of 40000 A / m or more or a relative differential permeability of 4 or less.
[0033]
Next, using the apparatus shown in FIG. 7 (described later), the magnetic field (H) is measured as close as possible to the surface of the subject corresponding to the flaw detection position of the subject directly below the magnetic sensor during flaw detection, and the measured value is predetermined. The magnetizing condition, for example, the magnetizing current is adjusted so that the set value (H0) is obtained. Thus, the magnetization level at the time of flaw detection is set by adjusting the magnetization conditions.
[0034]
The value of the external magnetic field (H) to be measured changes when the magnetic field measurement unit moves away from the subject, and changes when the magnetic field measurement direction changes. Therefore, a magnetic field parallel to the magnetization direction in a space in contact with the subject surface corresponding to the position where the subject is inspected is measured.
[0035]
In this means, a magnetic field parallel to the magnetization direction in the space in contact with the subject surface corresponding to the position where the subject is inspected is measured, the magnetization condition is adjusted, and the magnetic field value is set in advance (H0). The magnetization level of the subject can be easily set by adjusting the magnetization conditions so that
[0036]
Here, the magnetizing conditions include, for example, changing the magnetization level of the object such as the magnetizing current of the magnetizer, the distance between the magnetizer and the object to be measured, the gap between the magnetizer yoke, the shape, the material, and the number of turns of the magnetizing coil. It is a condition that can be done.
[0037]
Further, the magnetization level is the degree to which the subject is magnetized by the magnetizer, and is also referred to as the intensity of magnetization. Specifically, for example, it is indicated by a value of magnetic flux density inside the subject.
[0038]
As described above, the second means measures the magnetic field (H) parallel to the magnetization direction in the space in contact with the surface of the subject corresponding to the position where the subject is inspected for each flaw detection, and the value is determined in advance. The magnetization level was set by adjusting the magnetization condition so as to be a predetermined set value (H0).
[0039]
In the third means, the relationship between the magnetization condition and the magnetic field (H) parallel to the magnetization direction in the space in contact with the subject surface corresponding to the position where the subject is inspected is obtained in advance. Then, by using the relationship between the magnetic field (H) obtained above and the magnetization condition, at the time of flaw detection, the magnetization condition is adjusted to a condition where the value of the magnetic field becomes a predetermined setting value (H0) set in advance. Set the magnetization level at the time of flaw detection. Here, the set value (H0) of the magnetic field is determined in the same manner as the second means.
[0040]
The case where the object is a steel plate and the magnetization current is adjusted in the magnetization conditions will be described as an example. Using the magnetizing apparatus shown in FIG. 7, the magnetic current (I) is changed for each steel plate size in advance, and the magnetic field (H parallel to the magnetization direction in the space in contact with the subject surface corresponding to the position where the subject is flawed is detected. ) To determine the relationship between the magnetizing current (I) and the magnetic field (H). An example of a characteristic diagram showing the relationship between the magnetizing current (I) and the magnetic field (H) is shown in FIG. FIG. 4 is an example of a hot-rolled steel sheet having a thickness of 1.8 mm described in Examples described later.
[0041]
Next, using the relationship between the magnetizing current (I) and the magnetic field (H) shown in FIG. 4, the magnetizing current (I) is converted into a magnetizing current (I) corresponding to the set value (H0) of the magnetic field during actual flaw detection. I0). In this means, it is not necessary to measure the magnetic field (H) only by adjusting the magnetizing current (I) to the current value (I0) at the time of flaw detection, so the setting of the magnetization level of the subject becomes easier. . An empirical formula of the relationship between the magnetization current (I) and the magnetic field (H) shown in FIG. 4 may be obtained and the magnetization current (I) may be adjusted based on this. For example, in the example shown in FIG. 4, the relationship between the magnetization current (I: mA) and the magnetic field (H: A / m) is represented by H = 68 * I-13680.
[0042]
In the fourth means, in the first step of determining the magnetization level, the defect signal level is measured by detecting a standard defect provided in a ferromagnetic sample magnetically equivalent to the subject, and at the same time, the ferromagnetic material A magnetic field parallel to the magnetization direction in a space in contact with the ferromagnetic sample surface corresponding to the position at which the sample is inspected is measured to determine a magnetic field value (H0) when the defect signal level is maximized. The standard defect may be a natural defect or an artificial defect.
[0043]
Here, the BH curve indicating the relationship between the excitation magnetic field (H) and the internal magnetic flux density (B) obtained when virgin magnetization is magnetically equivalent to the subject is strong in the high magnetic field region. It is almost the same with the magnetic material. Taking a hot-rolled steel sheet as an example, in the magnetic field region of 5000 A / m or more, more strictly in the saturation region of the magnetic flux density of 40000 A / m or more, substantially the same BH curve is obtained for the AC magnetic characteristics. It is what As described above, a low carbon hot-rolled steel sheet having a C content of 0.05% by mass or less (including an extremely low carbon hot-rolled steel sheet having a C content of 0.01% by mass or less. The same shall apply hereinafter) is a component. Even if the plate thickness changes, the same BH curve as shown in FIG. 2 can be obtained, so that the low carbon hot-rolled steel plate having the C content within the above range is judged to be magnetically equivalent. The
[0044]
Therefore, for example, when the specimen is a low carbon hot-rolled steel sheet having a C content of 0.05% by mass or less, the above-mentioned C content is 0.05% by mass or less. A steel sheet having a thickness of 1 is appropriately selected from the rolled steel sheets, and an artificial defect serving as a standard defect is produced in the steel sheet. You may choose the steel plate with a natural defect from the above-mentioned steel plates.
[0045]
Next, the standard defect is detected to measure the defect signal level, and at the same time, the magnetic field parallel to the magnetization direction in the space in contact with the ferromagnetic sample surface corresponding to the position to be detected of the ferromagnetic sample is measured, A magnetic field value (H0) when the defect signal level is maximized is determined.
[0046]
In the case where there are a wide variety of specimen types to be subjected to magnetic flaw detection, it is preferable to investigate the BH curves described above for various specimens in advance and to group the BH curves that coincide with each other.
[0047]
In the second stage magnetization level setting step, during the actual flaw detection, the above-described second or second magnetic field value is set so that the magnetic field value when the subject is flawed becomes the magnetic field value (H0) determined in the first stage. The magnetization condition is adjusted by the method described in the third means.
[0048]
That is, in the method described in the second means, when using the apparatus of FIG. 7 to detect a subject, a magnetic field (parallel to the magnetization direction in a space in contact with the subject surface corresponding to the position where the subject is inspected) ( H) is measured, and the magnetization condition, for example, the magnetization current is adjusted so that the magnetic field value (H) becomes the determined magnetic field value (H0).
[0049]
Further, in the method described in the third means, the relationship between the magnetic field parallel to the magnetization direction and the magnetization condition in the space in contact with the surface of the subject corresponding to the position where the subject is inspected is previously determined using the apparatus of FIG. Ask. For example, when adjusting the magnetizing current, the relationship between the magnetizing current (I) and the magnetic field (H) as shown in FIG. Using this relationship, a setting value (I0) of the magnetizing current that becomes the magnetic field value (H0) determined in the first stage is obtained, and the magnetizing current at the time of the flaw detection is adjusted to the current value (I0).
[0050]
In the third stage flaw detection step, the subject is magnetically flawed under the magnetization conditions (magnetization current (I0)) set as described above.
[0051]
Similar to the prior art, flaw detection is performed using a standard defect, and a magnetization level that maximizes the S / N of the defect signal is determined as a reference magnetization level.
[0052]
In the prior art, when a specimen having a different steel type and plate thickness is to be detected, a standard defect level is set for each steel type and plate thickness, and a reference magnetization level is set. Therefore, the setting of the magnetization level is complicated.
[0053]
On the other hand, in this means, the magnetization level is adjusted so that the value (H0) of the magnetic field determined based on the magnetization characteristic data of the ferromagnetic material is known, and the magnetization level at the time of flaw detection is set, or In the case of a magnetically equivalent specimen, a standard defect is detected for one representative ferromagnetic sample among a plurality of ferromagnetic samples having standard defects, and the defect signal level is maximized. For a subject that is magnetically equivalent to this, even if the ferromagnetic sample is different from the steel type and plate thickness, it is based on the magnetic field value (H0) described above. And adjusting the magnetization condition to set the magnetization level at the time of flaw detection. This eliminates the complexity of setting the magnetization level during flaw detection.
[0054]
In the fifth means, in the first step of determining the magnetization level, a magnetic field is applied in advance to a ferromagnetic sample that is magnetically equivalent to the subject, and is applied from the outside when the ferromagnetic material is virginally magnetized. The relationship between the applied magnetic field (H) and the magnetic flux density (B) inside the ferromagnetic material is investigated, and the relationship (BH curve) between the magnetic field (H) applied from the outside and the internal magnetic flux density (B) is obtained. Using this relationship, a setting value (B0) of the magnetic flux density inside the subject at the time of subject inspection and a magnetic field value (H0) corresponding to the setting value (B0) of the magnetic flux density inside the subject are determined.
[0055]
For example, when the specimen is a low carbon hot-rolled steel sheet having a C content of 0.05% by mass or less, one steel sheet is selected from the aforementioned steel sheets as the ferromagnetic sample. For this ferromagnetic sample, the relationship between the excitation magnetic field (H) when virginally magnetized and the internal magnetic flux density (B) of the steel sheet is investigated, and a BH curve as shown in FIG. 2 is obtained. Next, using this BH curve, the magnetic flux density setting value (B0) inside the subject when the subject is flawed, and the magnetic field corresponding to the magnetic flux density setting value (B0) inside the subject. The value of (H0) is determined.
[0056]
In the second stage magnetization level setting step, at the time of actual flaw detection, the value of the magnetic field at the time of flaw detection is determined by the method described in the second or third means. The magnetization conditions are adjusted so that the value (H0) is obtained. Specifically, it is performed in the same manner as the second stage magnetization level setting step of the fourth means.
[0057]
In the third stage flaw detection step, the subject is magnetically flawed under the magnetization conditions set above.
[0058]
In this means, the magnetic field value of the subject is a preset magnetic flux density value (B0) at the time of subject inspection based on the BH curve of a ferromagnetic sample that is magnetically equivalent to the subject. Since the magnetization level at the time of flaw detection is set by adjusting the magnetization condition so that the magnetic field value (H0) is obtained, the complexity of setting the magnetization level at the time of flaw detection can be eliminated. In addition, the magnetic flux density inside the subject at the time of flaw detection can be reliably set to the magnetic flux density suitable for the flaw detection of the subject.
[0059]
In the flaw detection method for measuring the leaked magnetic flux, a stronger defect signal is produced when the subject is magnetically saturated, that is, when the set value (H0) of the magnetic field at the time of flaw detection is set to a value in the magnetic saturation region. The relative differential permeability is used as an index of magnetic saturation. Where the relative differential permeability is the vacuum permeability μ ₀ Multiplied by the magnetic field (H) and B-μ plotting the magnetic flux density ₀ H curve (μ on the horizontal axis of BH curve ₀ Is equivalent to the slope of
[0060]
The relationship between the magnetic flux formed inside the ferromagnetic material and the magnetic field is B = μ ₀ It is known to be H + J. Here, B is a magnetic flux density, H is a magnetic field, and J is a magnetization excited by an external magnetic field in the ferromagnetic body. When a strong magnetic field is gradually applied to the ferromagnet from the outside, the magnetic flux suddenly increases at first because magnetization is formed inside the ferromagnet. When a magnetic field is further applied, the ferromagnetic material approaches magnetic saturation and eventually reaches magnetic saturation where magnetization does not increase any more. When the magnetic saturation state is reached, the relative differential permeability becomes 1 because the increase of the magnetic flux density is only the increase of the magnetic field with respect to the increase of the magnetic field. For this reason, when the relative differential permeability approaches 1, it can be determined that the ferromagnetic body is approaching the magnetic saturation state.
[0061]
FIG. 5 is a diagram showing the relationship between the magnetic field (H) and the relative differential permeability corresponding to the BH curve shown in FIG. 2, where the horizontal axis is the magnetic field (H) and the vertical axis is the relative differential permeability. . FIG. 5 shows that when the magnetic field (H) is 40000 A / m or more, the relative differential permeability is 4 or less, and the magnetic flux density is substantially saturated. Therefore, it is preferable to set the magnetic field setting value (H0) to 40000 A / m or more and the relative differential permeability to 4 or less.
[0062]
At the time of flaw detection, magnetic flaw detection suitable for leakage magnetic flaw detection of internal defects can be performed by setting the magnetic field setting value (H0) or relative differential permeability to the value specified above. When the set value (H0) of the magnetic field is 45000 A / m or more, the relative differential permeability is 2 or less, and it is more preferable because flaw detection can be performed under conditions close to magnetic saturation. If the value of the magnetic field at which the relative differential permeability is 1 is set, flaw detection can be performed under complete magnetic saturation conditions. However, when flaw detection is performed under conditions close to magnetic saturation, it becomes difficult to completely demagnetize. Accordingly, in consideration of the required defect inspection level and demagnetization, it is preferable to perform the inspection by appropriately setting conditions within the range of the conditions defined above.
[0063]
Next, an example of the ferromagnetic flaw detection method according to the embodiment of the present invention will be specifically described with reference to FIGS.
[0064]
FIG. 6 is a schematic view of a steel plate magnetic flaw detector provided with a leakage magnetic flux flaw detector. In FIG. 6, 1 is a steel plate, 2a and 2b are conveyance rollers, 3 is a leakage magnetic flux flaw detector, and 4 is an internal defect. A leakage magnetic flux flaw detector 3 is installed along the conveyance path of the steel plate 1. The leakage magnetic flux flaw detector 3 includes a magnetic sensor 6, a magnetizer 5, a signal processing device 7, and a magnetizer power supply 8. The distance (lift-off) between the magnetic sensor 6 and the steel plate 1 is L.
[0065]
FIG. 7 is a perspective view of a magnetization apparatus used for measuring a magnetic field (H) parallel to the magnetization direction in a space in contact with the steel plate surface. In FIG. 7, 9 is a gauss meter, and the other reference numerals are the same as those described in FIG.
[0066]
In the present invention, the magnetic flux density (B) and the external magnetic field (H) inside the steel plate when the steel plate is virginally magnetized in advance before the steel plate 1 is flawed in the magnetization level determination step are known methods (for example, Epstein test). Since the differential differential permeability is unknown from the BH curve, “a graph showing the relationship between the magnetic field (H) and the relative differential permeability is obtained using the BH curve, and the differential differential permeability is obtained. A set value (H0) of a magnetic field having a magnetic susceptibility of 4 or less is determined.
[0067]
Next, in the magnetization level setting step, before the flaw detection, the apparatus shown in FIG. 7 is used in advance to correspond to the position at which the steel plate 1 is flawed while changing the magnetization current (I) flowing from the magnetizer power supply 8 to the magnetizer 5. The magnetic field (H) in the direction along the magnetization direction of the steel sheet surface in the vicinity of the steel sheet surface is measured with a gauss meter 9 applied perpendicularly to the surface of the steel sheet, and the magnetic field (H) becomes the magnetic field (H0) determined above. The current (I0) is determined. Next, the set value of the magnetizing current (I) at the time of flaw detection in the flaw detecting apparatus of FIG. 6 is set to the magnetizing current (I0) determined as described above. Here, as a point corresponding to the flaw detection position, a perpendicular line from the magnetic sensor 6 shown in FIG. 6 to the steel plate surface was taken and a point crossing the steel plate surface was used. In addition, the Gauss meter is perpendicular to the surface of the steel plate, which is the subject, in order to stably measure the magnetic field, and the size of the sensor portion of the Gauss meter is about 2 to 3 mm, and the magnetic field from the steel plate surface to a height of about 2 to 3 mm. It is preferable to measure.
[0068]
Next, the flaw detection step will be described.
[0069]
Using the magnetizing apparatus shown in FIG. 6, while the steel plate 1 is being transported by the transport rollers 2a and 2b, the magnetizer 5 is supplied with the current of the current value (I0) set to the magnetizer 5 to magnetize the steel plate 1. To do. The magnetic sensor 6 detects the magnetic flux leaking to the steel plate surface, and the signal is output to the signal processing device 7. When a defect such as the internal defect 4 passes through the position of the magnetic sensor 6, the magnetic flux that leaks increases, and the output from the magnetic sensor 6 to the signal processing device 7 increases. The output of the magnetic sensor 6 is processed by the signal processing device 7, and defect determination is performed. In the signal processing device 7, processing is performed in a conventional manner based on the signal detected by the magnetic sensor 6.
[0070]
In the flaw detection method described above, the magnetization current (I0) is determined in advance in the magnetization level setting step, and the magnetization current is set so that the magnetization current becomes the magnetization current (I0) determined above during the flaw detection. In addition to the magnetic sensor 6, the magnetic flux flaw detector 3 may be further provided with a gauss meter, and the flaw detection may be performed while adjusting the magnetization current so that the magnetic field detected by the gauss meter becomes H0.
[0071]
In the above description, the magnetization condition is adjusted by changing the value of the magnetizing current, but the magnetization condition is changed by changing the distance between the magnetizer and the object to be measured, the gap between the magnetizer yoke, the shape, the material, and the number of turns of the magnetizing coil. You can adjust it.
[0072]
In the above description, the magnetization in the same direction as the magnetization direction of the space in contact with the ferromagnet on the opposite side of the magnetizer that magnetizes the ferromagnet was measured. The magnetization in the same direction as the direction may be measured. Further, if the correspondence between the magnetization direction and the magnetization in the same direction is measured in advance, a method of measuring magnetization in a direction different from the magnetization direction may be used.
[0073]
In the above description, the method of flaw detection is mainly explained, but it can also be applied to a magnetic flaw detection method in which a ferromagnet is magnetized using a DC magnetic field or an AC magnetic field to conduct eddy current flaw detection in a magnetic saturation state. It is.
[0074]
【Example】
An example in which an internal defect of a low carbon hot-rolled steel sheet having a thickness of 1.8 mm (C content: 0.04 mass%) is detected by leakage magnetic flux inspection using the apparatus shown in FIGS. In FIG. 6, the lift-off L = 0.5 mm, and the conveying speed of the steel plate 1 is 1 m / sec. Since the BH curve of the steel plate 1 subjected to flaw detection coincided with the BH curve shown in FIG. 2, the BH curve shown in FIG. 2 was used as the BH curve of the steel plate 1.
[0075]
Next, using the magnetizing apparatus shown in FIG. 7, along the magnetization direction of the surface of the steel plate 1 with a gauss meter 9 applied perpendicularly to the surface of the steel plate 1 while changing the magnetization current flowing from the magnetizer power supply 8 to the magnetizer 5. The magnetic field (H) in the opposite direction was measured, and the relationship between the magnetization current (I) and the magnetic field (H) was investigated. The survey results are shown in FIG.
[0076]
Next, by using the magnetic flaw detector shown in FIG. 6, the internal current is changed while changing the magnetic field H (that is, the magnetic flux density B) by changing the magnetization current (I) flowing from the magnetizer power supply 8 to the magnetizer 5. A representative example of a plurality of flaws is shown in FIG.
[0077]
In FIG. 8, the horizontal axis represents the relative differential permeability, and the vertical axis represents the S / N of the defect signal. The present inventors have empirically learned from the viewpoint of accurate defect inspection that the S / N of the defect signal is preferably 2.5 or more, and more preferably 3.0 or more. is doing. From FIG. 8, the relative differential permeability is 4 or less, and the S / N, which is the ratio of the noise and the defect signal, is 2.5 or more, so that the defect signal can be discriminated with respect to the noise and the defect can be detected. It can be seen that if a magnetization level at which the relative differential permeability is 2 or less is selected, the S / N is 3.0 or more, and a more accurate defect inspection is possible.
[0078]
【The invention's effect】
According to the present invention, in a magnetic flaw detection method for magnetizing a ferromagnetic material, such as a DC leakage magnetic flux flaw detection method, an AC leakage magnetic flux flaw detection method, a DC magnetic flaw flaw detection method, and an AC magnetic flaw flaw detection method, It is possible to determine an appropriate magnetization level according to the level, and to greatly reduce the flaw detection time. In addition, accurate flaw detection is possible.
[Brief description of the drawings]
FIG. 1 is a diagram showing a general hysteresis curve (BH curve) of an external magnetic field (H) and a magnetic flux density (B) inside a subject.
FIG. 2 is a diagram showing a relationship between an excitation magnetic field (H) of a steel plate and an internal magnetic flux density (B) of the steel plate when three kinds of hot-rolled steel plates having different steel types and thicknesses are virginally magnetized.
FIG. 3 is a diagram illustrating a flow of magnetic flaw detection.
FIG. 4 is a diagram illustrating an example of a relationship between a magnetizing current (I) and the magnetic field (H).
5 is a diagram showing the relationship between the magnetic field (H) and the relative differential permeability corresponding to the BH curve of the steel sheet shown in FIG.
FIG. 6 is a schematic view of a steel plate magnetic flaw detector provided with a leakage magnetic flux flaw detector.
FIG. 7 is a perspective view of a magnetization apparatus used for measuring a magnetic field (H) parallel to the magnetization direction in a space in contact with the steel plate surface.
FIG. 8 is a diagram showing the relationship between the relative differential permeability of a steel sheet and the S / N of an internal defect signal.
[Explanation of symbols]
1 Steel plate
2a, 2b Transport roller
3 Leakage magnetic flux inspection equipment
4 Internal defects
5 Magnetizer
6 Magnetic sensor
7 Signal processor
8 Magnetizer power supply
9 Gauss meter

Claims (6)

In the method of flaw detection by magnetizing a ferromagnetic material, the magnetization condition is adjusted so that the magnetic field in the vicinity of the surface of the flaw detection area of the ferromagnetic object becomes a preset magnetic field value (H0). A magnetic flaw detection method for a ferromagnetic material, characterized in that In the method of flaw detection by magnetizing a ferromagnetic material, a magnetic field parallel to the magnetization direction in the vicinity of the surface of the flaw detection area of the ferromagnetic object is measured during flaw detection, and the measured value becomes a preset magnetic field value (H0). A magnetic flaw detection method for a ferromagnetic material, characterized in that the magnetization condition is adjusted to set the magnetization level at the time of flaw detection. In the method of flaw detection by magnetizing a ferromagnetic material, the relationship between the magnetic field parallel to the magnetization direction in the vicinity of the surface of the flaw detection area of the ferromagnetic object and the magnetization condition is obtained in advance, and the value of the magnetic field is calculated using the relationship. A magnetic flaw detection method for a ferromagnetic material, characterized by adjusting a magnetization condition so as to be a preset magnetic field value (H0) and setting a magnetization level at the time of flaw detection. In the method of flaw detection by magnetizing a ferromagnetic material, the magnetic field value (H0) that maximizes the standard defect signal level when a standard defect of a ferromagnetic sample that is magnetically equivalent to the ferromagnetic specimen is detected. A magnetic flaw detection of a ferromagnetic material, characterized in that the flaw detection is performed on the ferromagnetic object by adjusting the magnetization conditions by the method according to any one of claims 1 to 3 to set a magnetization level. Method. In the method of flaw detection by magnetizing a ferromagnetic material, the magnetic field (H) given from the outside and the magnetic flux density (B) inside the ferromagnetic sample, which are previously obtained from a ferromagnetic sample that is magnetically equivalent to the ferromagnetic specimen. )), The magnetic field value (H0) of the subject at which the magnetic flux density at the time of the subject flaw detection is a predetermined value (B0) is determined by the method according to any one of claims 1 to 3. A magnetic flaw detection method for a ferromagnetic material, characterized by adjusting a magnetization condition and flaw-detecting a subject. 6. The magnetic flaw detection method for a ferromagnetic material according to claim 1, wherein the set value (H0) of the magnetic field is set to a value in a magnetic saturation region.

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