JP4132252B2

JP4132252B2 - High-strength cold-rolled steel sheet that is resistant to corrosion and has excellent geomagnetic shielding properties, and a method for producing the same

Info

Publication number: JP4132252B2
Application number: JP20513098A
Authority: JP
Inventors: 暁田中; 康治佐久間; 優二郎宮内
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1998-07-21
Filing date: 1998-07-21
Publication date: 2008-08-13
Anticipated expiration: 2018-07-21
Also published as: JP2000038644A

Description

【０００１】
【発明の属する技術分野】
本発明は、地磁気シールド性に優れ、冷延ままで腐食しにくい冷延鋼板とその製造方法に関するものである。
【０００２】
【従来の技術】
薄鋼板が使用される家庭電気製品、自動車、家具、建築用途に必要な特性としては、強度、防錆性、加工性等があげられるが、たとえばＴＶブラウン管用材料などでは、電子ビームが地磁気により偏向しないようにすることが重要となっており、地磁気シールド性が求められている。地磁気シールド性とは、地磁気に相当する直流地場における磁化力、０．３エールステッド前後での初透磁率μ_0.3で表され、大きい方が優れている。またこうした材料でも他の家電製品や自動車等に見られるように軽量化のために高強度化についても要請されている。
【０００３】
高強度でかつ地磁気シールド性を良好とすることは、ＪＩＳＣ２５５２に規定するような無方向性電磁鋼板を用いることで容易に実現できるが、無方向性電磁鋼板は、製造設備と製造方法が限定され、プレス用薄鋼板のような様々な板厚のものを製造することが出来ず高コストである。また、前記ＴＶブラウン管用材料では電磁鋼板のように高磁場での特性を必要とせず、これを用いることは過剰品質となる。
【０００４】
本発明者らはこれに鑑み、特願平９−３０２６３０号や特願平９−３０２６３１号に示すように、降伏強度が２５０〜３００ＭＰａ以上の高強度でかつ、初透磁率μ_0.3が地磁気シールド性を満たすレベル、すなわちμ_0.3が５００程度となる鋼板を製造する技術を開発してきた。しかし、ＳｉやＭｎ等の酸化しやすい元素を多く含むため、冷延ままにおける耐錆性に課題があり、地磁気シールド性と耐錆性を両立し得る高強度冷延鋼板がないというのが現状であった。
【０００５】
【発明が解決しようとする課題】
本発明の目的は上述のような問題を解決し、腐食しにくく、地磁気シールド性に優れた高強度冷延鋼板とその製造方法を提供するものである。
【０００６】
【課題を解決するための手段】
本発明では、高強度化のためＳｉ，Ｍｎ，Ｐ等の添加を基本としており、さらに磁気特性の向上と腐食しにくい冷延鋼板であることを両立させる必要があることから、冷延鋼板の表面についてＥＳＣＡ、ＡＥＳ等の表面分析手段を用いて鋭意研究を重ねた。その結果、鋼板表面にＳｉ酸化膜がない場合や反対に厚過ぎる場合に点錆が発生し易いということを知見した。このメカニズムの詳細は明らかでないが恐らくＳｉ酸化膜は耐錆性があり、厚過ぎる場合はＳｉ酸化膜が脆いために調質圧延や取り扱い時に割れが入り、これが錆の起点となると考えられる。
【０００７】
これは、スケールキズの発生と、ある程度の相関が見られたことから、スケールキズが発生しないような手段をとることが有効であると考え、Ｎｉ等の元素を抑制したり、圧延ロール粗さの制限やデスケーリングの強化を行った。しかし、このような手段を講じても耐錆性の向上があまり見られなかった。そこで、鋼板表面の元素の濃化は、焼鈍温度、雰囲気、冷却方法に影響されることも考えられるため、熱延条件に加えて、焼鈍条件についても検討を行った。そして、再結晶焼鈍の雰囲気や温度をある範囲としたところＳｉの酸化膜厚が１．０〜７．５ｎｍとなり、耐錆性に向上が見られることを突き止めた。
【０００８】
本発明は、これらの新知見に基づいてさらに構成分や製造条件を鋭意検討した結果、磁化力０．３エールステッドでの初透磁率が５００以上の地磁気シールド性に優れ、腐食しにくい高強度冷延鋼板を得ることを可能としたものであり、その要旨は下記の通りである。
（１）Ｃ：０．０００３〜０．００７％、Ｓｉ：０．５〜２．５％、Ｍｎ：０．０５〜２．０％、Ｐ：０．００５〜０．１％、Ｓ：０．０００１〜０．０５％、Ａｌ：０．０００１〜０．１％、Ｎ：０．０００５〜０．００５％、Ｎｉ：０．０５％以下を含有し、残部Ｆｅ及び不可避的不純物から成り、表層Ｓｉ酸化膜厚が１．０〜７．５ｎｍであることを特徴とする、腐食しにくく地磁気シールド性に優れた高強度冷延鋼板。
【０００９】
（２）請求項１記載の鋼にさらにＢ：０．０００１〜０．００３０％を含有し、残部Ｆｅ及び不可避的不純物から成り、表層Ｓｉ酸化膜厚が１．０〜７．５ｎｍであることを特徴とする、腐食しにくく地磁気シールド性に優れた高強度冷延鋼板。
（３）請求項１又は請求項２記載の鋼スラブを１２００℃以下で加熱後、ロール中心線粗さを２μｍ以下とした仕上圧延ロールを使用し、巻取温度を６００〜７５０℃以下とし、酸洗後連続焼鈍するに当たり、水素濃度を１．０％以上、残部窒素ガス及び不可避的ガスからなる雰囲気で均熱温度を７５０〜９００℃、露点１０℃以下とし、表層Ｓｉ酸化膜厚が１．０〜７．５ｎｍであることを特徴とする、腐食しにくく地磁気シールド性に優れた高強度冷延鋼板の製造方法にある。
【００１０】
【発明の実施の形態】
以下、本発明を詳細に説明する。まず、鋼成分について説明する。
Ｃは鋼を強化する重要な元素であり、単に高強度化するには添加量を増やせばよいが、添加量が増えると炭化物の析出により磁化を阻害し地磁気シールド性を劣化させるため極低炭素鋼とすべきであり、上限を０．００７％とする。一方０．０００３％未満では製鋼工程における脱炭時間がかかりすぎるため、これを下限とした。
【００１１】
Ｓｉは、固溶強化元素であるとともに初透磁率への悪影響がＰ、Ｍｎより小さく、重要な元素であり、０．５％以上の添加が必要である。そして２５０〜３００ＭＰａ以上の降伏強度を得るには、０．６％以上の添加が望ましい。しかし２．０％を超えて添加すると鋼が脆化するためこれを上限とした。
Ｍｎも固溶して鋼を強化し、また、Ｓによる鋼の脆化を防止するため、０．０５％以上の添加が必要である。しかし、ＰやＳｉに比べ強化代が小さいため高強度を得るためには多量の添加を必要とするが、２．０％を越えて添加すると鋼が脆化するため上限を２．０％とした。
【００１２】
Ｐも鋼を強化し、強化代はＭｎ、Ｓｉよりも大きいことから有力な元素であるが、結晶粒界に析出しやすく、結晶粒を微細化するため地磁気シールド性への悪影響が大きい。また、多量の添加は鋼を脆化させるため上限を０．１％とした。一方０．００５％未満とするには脱Ｐコストがかかりすぎるため、これを下限とした。
Ｓは、鋼を脆化し、またＭｎＳとしてＳを固定し脆化を防止した場合でも、地磁気シールド性を劣化させるため、０．０５％を上限とする。一方、０．０００１％未満とすることは工業的に困難であるため、０．０００１〜０．０５％とした。
【００１３】
Ａｌは、ＡｌＮとして微細析出すると地磁気シールド性が極端に劣化するため、なるべく少ない方が望ましいがＮ時効による機械特性の劣化を防止する役割も持っており上限を０．１％とする。一方、Ａｌは脱酸剤として製鋼工程にて添加されるため０．０００１％以下とすることは困難であるため、これを下限とし、０．０００１〜０．１％とした。
Ｎは、ＡｌＮとして微細析出することで地磁気シールド性を劣化させるため上限を０．００５％とし、一方０．０００５％未満とするには高コストとなるので、０．０００５〜０．００５％とした。
【００１４】
Ｎｉは、本発明にとって重要な元素であり、この添加は、局部的濃化を通じて地鉄・スケール界面の粗さを粗くし、スケールキズを発生しやすくするとともに、Ｓｉの酸化膜が局部的に濃化し、耐錆性を劣化するため、上限を０．０５％とする。望ましくは０．０２％以下である。
Ｂは、鋼中でＢＮを形成し、地磁気シールド性に悪影響を与えるＡｌＮの微細析出を抑制するために必要に応じて添加される。０．０００１％未満ではその効果がなく、０．０３０％を越えるとＢＮの析出が地磁気シールド性に悪影響を及ぼすため０．０００１〜０．００３０％とした。
【００１５】
次に製造方法について述べる。鋼の鋳造から熱間圧延に至るまでの工程については特に限定はないが、熱延工程での加熱温度は、１２００℃を越えるとＳｉ起因のスケールによって表面性状が劣化するとともに、鋼中のＭｎＳが再固溶し、熱間圧延工程中に微細析出し粒成長を抑制し、地磁気シールド性が劣化するため１２００℃以下とする。
熱間圧延工程は冷延・焼鈍後の表面の状態に影響を与えるが、仕上げ工程までは通常の方法で構わない。仕上圧延に用いるロールの中心線粗さは２μｍ以上とすることで、均一にスケールが剥離されスケールキズの発生とＳｉ酸化膜厚の不均一さを回避することが出来る。
【００１６】
巻取温度は、冷延・焼鈍後の粒径を通して磁気特性に影響し、地磁気シールド性の向上と冷延焼鈍後のＳｉ酸化膜の成長のためには、なるべく高温であることが望ましく、少なくとも６００℃以上とすべきであり、好ましくは６５０℃以上とする。一方、高温にしすぎると酸洗ラインでの脱スケール時間が長くなり、またＳｉ酸化膜厚が厚くなりすぎて耐錆性も劣化するため上限を７５０℃とする。
【００１７】
酸洗、冷延後の焼鈍工程は本発明にとり重要である。焼鈍均熱温度は、フェライト粒径に影響し、粒径が大きい程地磁気シールド性が良いことからなるべく高温であることが望ましく、少なくとも７５０℃以上であることが必要である。しかし、高温にし過ぎるとα鉄からγ鉄への変態が生じ、冷却時のフェライト粒径が小さくなり地磁気シールド性が劣化するとともに、Ｓｉ酸化膜が厚くなり過ぎて耐錆性が劣化するので上限を９００℃とする。焼鈍雰囲気は、水素濃度を１％以上、残部窒素ガス及び不可避的ガスからなる雰囲気とする。この理由は水素濃度が１％未満であるとＳｉの選択酸化が高温中で起こるとともに拡散が伴い、Ｓｉ酸化膜が局所的に濃化し、膜厚が大きくなり過ぎて錆の原因となるためである。また、焼鈍露点も同様の理由で１０℃以下とする。
【００１８】
焼鈍後の調質圧延については特に規定しないが、地磁気シールド性にとっては、なるべく小さくすることが有効であり、０．５％以下とすることが望ましい。以上のようにすることで焼鈍後の鋼板表面のＳｉの濃化層の厚さが１．０〜７．５ｎｍとなり、腐食しにくく、地磁気シールド性に優れた冷延鋼板となる。次に本発明を実施例にて説明する。
【００１９】
【実施例】
表１に示す組成からなるスラブを溶製し、表２に示すように、抽出温度１１２１〜１２５３℃で抽出後、ロール中心線粗さを０．８〜２．４μｍとした仕上げロールを用いて仕上温度８８３〜９４８℃、巻取温度５９０〜８１０℃とした熱間圧延を行った。その後冷延率６５〜８０％の冷間圧延を行い、連続焼鈍ラインにおいて、水素濃度０．５〜２．０％で露点−２０〜＋２０℃の焼鈍雰囲気において６８３〜９０２℃で均熱後、０．３％の調質圧延を行った。このようにして製造された鋼帯から圧延方向に平行に試片を切り出し、表面をＸ線光電子分光法により分析し、Ｓｉ酸化膜の厚みを計測した。磁気特性は、鋼帯から切り出した３０ｍｍ×３００ｍｍの試験片を組み合わせてＪＩＳＣ２５５０の方法にて直流エプスタイン法にて磁化力０．３エールステッドでの初透磁率を求めた。また圧延方向に平行にＪＩＳ５号引張試験片を切り出し、引張試験を行った。
【００２０】
【表１】

【００２１】
【表２】

【００２２】
耐錆性は、防錆油を各１ｇ／ｍ² 塗油したものを５ヶ月放置した後の発錆状況を目視で評価した。Ｎｏ１〜１２は本発明例であり、耐錆性に優れ初透磁率も大きく地磁気シールド性に優れている。一方、Ｎｏ１３〜２９は比較例であり、Ｎｏ１３は抽出温度が高過ぎるため地磁気シールド性が悪い。Ｎｏ１５と１９は仕上ロール中心線粗さが大き過ぎるため、耐錆性が悪い。Ｎｏ１６は巻取温度が高過ぎるため、表層Ｓｉ酸化膜が厚く生成するが脆いために調質圧延中やその後の取り扱い時に割れが生じ錆発生の起点となる。
【００２３】
Ｎｏ１７は巻取温度が低すぎるためＳｉ酸化膜が生成せず耐錆性に劣るとともに初透磁率も小さい。Ｎｏ２２は焼鈍均熱温度が高くＳｉ酸化膜が厚過ぎるため錆発生しやすい。Ｎｏ２６は焼鈍温度が低すぎるため初透磁率が低く、かつＳｉ酸化膜厚みが小さ過ぎて耐錆性が悪い。Ｎｏ１４と２３は焼鈍雰囲気の水素濃度が低すぎるためＳｉ酸化膜が厚くなり過ぎて耐錆性が悪い。Ｎｏ１８は露点が高過ぎてＳｉ酸化膜が厚くなり過ぎて内部酸化状態となっており耐錆性が悪い。Ｎｏ２４から２９は鋼成分が本発明から外れており耐錆性又は地磁気シールド性が悪い。
【００２４】
【発明の効果】
以上述べてきたように、本発明によれば地磁気レベルの直流磁場における初透磁率が大きく、冷延ままにおける耐錆性に優れた高強度冷延鋼板とその製造方法を提供出来る。もちろん本発明鋼は、電気亜鉛めっきや溶融亜鉛めっき等のめっき用原板としても使用可能である。またプレス加工用の薄鋼板の製造に一般的に用いられている連続焼鈍設備を用いて容易に製造できるためＴＶブラウン管用の防爆バンドや各種磁気シールド材に用いることが出来るばかりか、家庭電気製品や自動車、家具、建築等の広い用途に適用できるため、産業上に与える効果は極めて大きい。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a cold-rolled steel sheet that is excellent in geomagnetic shielding properties and hardly corrodes as it is cold-rolled, and a method for producing the same.
[0002]
[Prior art]
Properties required for household electrical appliances, automobiles, furniture, and building applications in which thin steel plates are used include strength, rust resistance, and workability. For example, in TV Braun tube materials, the electron beam is generated by geomagnetism. It is important not to be deflected, and geomagnetic shielding is required. The geomagnetic shielding properties, magnetizing force in the DC local corresponding to geomagnetism, expressed in initial permeability mu _0.3 between before and after the 0.3 oersteds, it is better larger. In addition, these materials are also required to have high strength for weight reduction as seen in other home appliances and automobiles.
[0003]
High strength and good geomagnetic shielding properties can be easily realized by using a non-oriented electrical steel sheet as defined in JIS C2552, but the non-oriented electrical steel sheet has limited manufacturing equipment and manufacturing method. In addition, various sheet thicknesses such as a thin steel sheet for press cannot be manufactured, and the cost is high. Further, the TV cathode ray tube material does not require the characteristics in a high magnetic field unlike the electromagnetic steel plate, and the use of this material becomes excessive quality.
[0004]
In view of this, the present inventors, as shown in Japanese Patent Application Nos. 9-302630 and 9-302631, have a high yield strength of 250 to 300 MPa or more and an initial magnetic permeability μ _0.3 of a geomagnetic shield. Has developed a technology for producing a steel sheet having a level satisfying the property, that is, μ _0.3 is about 500. However, since it contains many oxidizable elements such as Si and Mn, there is a problem in rust resistance in cold rolling, and there is no high-strength cold-rolled steel sheet that can achieve both geomagnetic shielding and rust resistance. Met.
[0005]
[Problems to be solved by the invention]
The object of the present invention is to solve the above-mentioned problems, to provide a high-strength cold-rolled steel sheet that is difficult to corrode and has excellent geomagnetic shielding properties, and a method for producing the same.
[0006]
[Means for Solving the Problems]
In the present invention, Si, Mn, P and the like are basically added for high strength, and further, it is necessary to satisfy both the improvement of magnetic properties and the cold-rolled steel plate which is not easily corroded. The surface was intensively researched using surface analysis means such as ESCA and AES. As a result, it was found that spot rust is likely to occur when there is no Si oxide film on the surface of the steel sheet or when it is too thick. Although the details of this mechanism are not clear, it is probable that the Si oxide film is resistant to rust, and if it is too thick, the Si oxide film is brittle and cracks occur during temper rolling and handling, which is considered to be the starting point of rust.
[0007]
Since a certain degree of correlation was observed with the occurrence of scale scratches, it was considered effective to take measures not to generate scale scratches, and elements such as Ni were suppressed, or the rolling roll roughness Restrictions and descaling enhancements were made. However, even if such measures were taken, the improvement in rust resistance was not so much seen. Therefore, the concentration of elements on the surface of the steel sheet may be affected by the annealing temperature, atmosphere, and cooling method, so the annealing conditions were examined in addition to the hot rolling conditions. And when the atmosphere and temperature of recrystallization annealing were made into a certain range, the oxide film thickness of Si became 1.0-7.5 nm, and it discovered that improvement was seen in rust resistance.
[0008]
Based on these new findings, the present invention has further intensively studied the components and manufacturing conditions. As a result, it has excellent geomagnetic shielding properties with an initial magnetic permeability of 500 or more at a magnetizing force of 0.3 aersted and is highly resistant to corrosion. It is possible to obtain a cold-rolled steel sheet, the summary of which is as follows.
(1) C: 0.0003 to 0.007%, Si: 0.5 to 2.5%, Mn: 0.05 to 2.0%, P: 0.005 to 0.1%, S: 0 .0001-0.05%, Al: 0.0001-0.1%, N: 0.0005-0.005%, Ni: 0.05% or less, comprising the balance Fe and inevitable impurities, A high-strength cold-rolled steel sheet that is resistant to corrosion and excellent in geomagnetic shielding properties, characterized in that the surface Si oxide film thickness is 1.0 to 7.5 nm.
[0009]
(2) The steel of claim 1 further contains B: 0.0001 to 0.0030%, consists of the balance Fe and inevitable impurities, and the surface Si oxide film thickness is 1.0 to 7.5 nm. A high-strength cold-rolled steel sheet that is resistant to corrosion and has excellent geomagnetic shielding properties.
(3) After heating the steel slab according to claim 1 or claim 2 at 1200 ° C. or less, a finish rolling roll having a roll centerline roughness of 2 μm or less is used, and the coiling temperature is 600 to 750 ° C. or less. In continuous annealing after pickling, the hydrogen concentration is 1.0% or more, the soaking temperature is 750 to 900 ° C., the dew point is 10 ° C. or less in an atmosphere consisting of the remaining nitrogen gas and unavoidable gas, and the surface Si oxide film thickness is 1. The present invention is a method for producing a high-strength cold-rolled steel sheet that is resistant to corrosion and has excellent geomagnetic shielding properties, characterized by being from 0.0 to 7.5 nm.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail. First, steel components will be described.
C is an important element for strengthening steel. To increase the strength simply, the addition amount may be increased. However, if the addition amount is increased, the magnetization is inhibited by the precipitation of carbides, and the geomagnetic shielding properties are deteriorated. The upper limit should be 0.007%. On the other hand, if it is less than 0.0003%, it takes too much decarburization time in the steel making process, so this was made the lower limit.
[0011]
Si is a solid solution strengthening element and has an adverse effect on the initial permeability smaller than that of P and Mn. It is an important element and needs to be added in an amount of 0.5% or more. In order to obtain a yield strength of 250 to 300 MPa or more, addition of 0.6% or more is desirable. However, if added over 2.0%, the steel becomes brittle, so this was made the upper limit.
Addition of 0.05% or more is necessary in order to strengthen the steel by solid solution of Mn and to prevent embrittlement of the steel by S. However, since the strengthening allowance is small compared to P and Si, a large amount of addition is required to obtain high strength, but if added over 2.0%, the steel becomes brittle, so the upper limit is 2.0%. did.
[0012]
P also strengthens the steel and is a powerful element because the strengthening allowance is larger than Mn and Si. However, P is likely to precipitate at the grain boundaries, and the crystal grains are miniaturized, so the adverse effect on the geomagnetic shielding properties is great. Moreover, since the addition of a large amount embrittles the steel, the upper limit was made 0.1%. On the other hand, to make it less than 0.005%, the P removal cost is too high, so this was made the lower limit.
Even if S is embrittled and steel is fixed as MnS to prevent embrittlement, the upper limit is set to 0.05%. On the other hand, since it is industrially difficult to make it less than 0.0001%, it was made 0.0001-0.05%.
[0013]
When Al is finely deposited as AlN, the geomagnetic shielding properties are extremely deteriorated. Therefore, it is desirable that Al be as small as possible. However, Al also has a role of preventing deterioration of mechanical properties due to N aging, and the upper limit is set to 0.1%. On the other hand, since Al is added as a deoxidizer in the steelmaking process, it is difficult to make it 0.0001% or less, so this was made the lower limit and made 0.0001 to 0.1%.
N is finely precipitated as AlN to degrade the geomagnetic shielding properties, so the upper limit is made 0.005%, while on the other hand, it is expensive to make it less than 0.0005%, so 0.0005 to 0.005% did.
[0014]
Ni is an important element for the present invention, and this addition increases the roughness of the iron-steel interface through local concentration, thereby facilitating the generation of scale flaws, and the oxide film of Si is locally localized. In order to thicken and deteriorate rust resistance, the upper limit is made 0.05%. Desirably, it is 0.02% or less.
B is added as necessary to suppress the fine precipitation of AlN which forms BN in steel and adversely affects geomagnetic shielding properties. If the content is less than 0.0001%, the effect is not obtained. If the content exceeds 0.030%, the precipitation of BN adversely affects the geomagnetic shielding property, so the content is set to 0.0001 to 0.0030%.
[0015]
Next, a manufacturing method will be described. The process from casting of steel to hot rolling is not particularly limited, but when the heating temperature in the hot rolling process exceeds 1200 ° C., the surface properties deteriorate due to the Si-derived scale, and MnS in the steel. Is re-dissolved and finely precipitated during the hot rolling process to suppress grain growth and deteriorate the geomagnetic shielding properties, so that the temperature is 1200 ° C. or lower.
The hot rolling process affects the state of the surface after cold rolling and annealing, but a normal method may be used until the finishing process. By setting the center line roughness of the roll used for finish rolling to 2 μm or more, the scale is uniformly peeled off, and generation of scale scratches and non-uniformity of the Si oxide film thickness can be avoided.
[0016]
The coiling temperature affects the magnetic properties through the grain size after cold rolling / annealing, and is preferably as high as possible in order to improve the geomagnetic shielding properties and grow the Si oxide film after cold rolling annealing, It should be 600 ° C. or higher, preferably 650 ° C. or higher. On the other hand, if the temperature is too high, the descaling time in the pickling line becomes long, the Si oxide film thickness becomes too thick, and the rust resistance deteriorates, so the upper limit is set to 750 ° C.
[0017]
The annealing process after pickling and cold rolling is important for the present invention. The annealing soaking temperature affects the ferrite grain size, and the larger the grain size, the better the geomagnetic shielding properties. Therefore, it is desirable that the annealing temperature is as high as possible, and at least 750 ° C. or more is required. However, if the temperature is too high, transformation from α iron to γ iron occurs, the ferrite grain size during cooling decreases and the geomagnetic shielding performance deteriorates, and the Si oxide film becomes too thick and the rust resistance deteriorates. Is 900 ° C. The annealing atmosphere is an atmosphere composed of a hydrogen concentration of 1% or more and the balance nitrogen gas and inevitable gas. The reason for this is that if the hydrogen concentration is less than 1%, selective oxidation of Si occurs at a high temperature and diffusion occurs, the Si oxide film is locally concentrated, and the film thickness becomes too large, causing rust. is there. Also, the annealing dew point is set to 10 ° C. or less for the same reason.
[0018]
The temper rolling after annealing is not particularly defined, but it is effective to make it as small as possible for the geomagnetic shielding property, and it is desirable to make it 0.5% or less. By doing as mentioned above, the thickness of the Si concentrated layer on the surface of the steel sheet after annealing becomes 1.0 to 7.5 nm, and it becomes a cold-rolled steel sheet that is not easily corroded and has excellent geomagnetic shielding properties. Next, the present invention will be described with reference to examples.
[0019]
【Example】
A slab having the composition shown in Table 1 was melted and, as shown in Table 2, after extraction at an extraction temperature of 1121 to 1253 ° C., a finish roll having a roll centerline roughness of 0.8 to 2.4 μm was used. Hot rolling was performed at a finishing temperature of 883 to 948 ° C and a winding temperature of 590 to 810 ° C. Thereafter, cold rolling with a cold rolling rate of 65 to 80% is performed, and in a continuous annealing line, after soaking at 683 to 902 ° C. in an annealing atmosphere with a hydrogen concentration of 0.5 to 2.0% and a dew point of −20 to + 20 ° C., 0.3% temper rolling was performed. A specimen was cut out from the steel strip thus produced in parallel with the rolling direction, the surface was analyzed by X-ray photoelectron spectroscopy, and the thickness of the Si oxide film was measured. Magnetic properties were obtained by combining the test pieces of 30 mm × 300 mm cut out from the steel strip and determining the initial permeability at a magnetizing force of 0.3 Eersted by the DC Epstein method according to the method of JISC2550. Further, a JIS No. 5 tensile test piece was cut out in parallel with the rolling direction, and a tensile test was performed.
[0020]
[Table 1]

[0021]
[Table 2]

[0022]
The rust resistance was evaluated by visual observation of the rusting state after leaving rust preventive oil applied at 1 g / m ² for 5 months. Nos. 1 to 12 are examples of the present invention, excellent in rust resistance and large in initial permeability, and excellent in geomagnetic shielding properties. On the other hand, Nos. 13 to 29 are comparative examples, and No. 13 has poor geomagnetic shielding properties because the extraction temperature is too high. Nos. 15 and 19 have poor rust resistance because the finish roll center line roughness is too large. No. 16 has a coiling temperature that is too high, so that the surface Si oxide film is formed thick. However, since it is brittle, cracking occurs during temper rolling and subsequent handling, and becomes the starting point for rust generation.
[0023]
In No17, since the coiling temperature is too low, a Si oxide film is not formed, and the rust resistance is inferior and the initial permeability is small. No. 22 has a high annealing soaking temperature and the Si oxide film is too thick, and rust is likely to occur. No. 26 has a low initial permeability because the annealing temperature is too low, and the thickness of the Si oxide film is too small, resulting in poor rust resistance. In Nos. 14 and 23, since the hydrogen concentration in the annealing atmosphere is too low, the Si oxide film becomes too thick and the rust resistance is poor. In No. 18, the dew point is too high, the Si oxide film becomes too thick and is in an internal oxidation state, and the rust resistance is poor. In Nos. 24 to 29, the steel components are out of the present invention, and rust resistance or geomagnetic shielding properties are poor.
[0024]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a high-strength cold-rolled steel sheet having a large initial permeability in a DC magnetic field at a geomagnetic level and excellent in rust resistance as it is cold-rolled, and a method for producing the same. Of course, the steel of the present invention can also be used as an original plate for plating such as electrogalvanizing and hot dip galvanizing. In addition, it can be easily manufactured using continuous annealing equipment generally used for manufacturing thin steel sheets for press work, so it can be used for explosion-proof bands for TV CRTs and various magnetic shield materials. Since it can be applied to a wide range of applications such as automobiles, furniture, and architecture, the effect on the industry is extremely large.

Claims

% By weight (hereinafter the same for steel components)
C: 0.0003 to 0.007%,
Si: 0.5 to 2.5%
Mn: 0.05 to 2.0%,
P: 0.005-0.1%,
S: 0.0001 to 0.05%,
Al: 0.0001 to 0.1%,
N: 0.0005 to 0.005%,
Ni: 0.05% or less,
A high-strength cold-rolled steel sheet which is composed of the remaining Fe and inevitable impurities and has a surface Si oxide film thickness of 1.0 to 7.5 nm, which is resistant to corrosion and excellent in geomagnetic shielding properties.

The steel according to claim 1 further contains B: 0.0001 to 0.0030%, consists of the balance Fe and inevitable impurities, and has a surface Si oxide film thickness of 1.0 to 7.5 nm. A high-strength cold-rolled steel sheet that resists corrosion and has excellent geomagnetic shielding properties.

After heating the steel slab according to claim 1 or 2 at 1200 ° C. or less, a finish rolling roll having a roll center line roughness of 2 μm or less is used, the coiling temperature is set to 600 to 750 ° C. or less, and after pickling In continuous annealing, the surface Si oxide film thickness obtained by setting the soaking temperature to 750 to 900 ° C. and the dew point of 10 ° C. or less in an atmosphere composed of hydrogen concentration of 1.0% or more, the balance nitrogen gas and inevitable gas. A method for producing a high-strength cold-rolled steel sheet that is resistant to corrosion and excellent in geomagnetic shielding properties, characterized by being 1.0 to 7.5 nm.