JP2004068033A - Steel plate for frame of cathode-ray tube, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel plate for a frame of a cathode-ray tube, which has an adequate press formability, reduces creeping quantity in heat treatment, and greatly reduces the weight of the frame of the cathode-ray tube. <P>SOLUTION: The steel plate has a composition including by mass%, 0.03-0.08% C, 10.0-18.0% Cr, 0.05% or less Ti and 0.02% or less Nb, has a metallographic structure in which carbides with sizes of 3 μm or less are dispersed in a wrought ferrite structure containing dissolved C, and further has a 0.2% yield stress of 650-870 N/mm<SP>2</SP>and a mean coefficient of thermal expansion in 30-650°C of 12.0×10<SP>-6</SP>/°C or less. The manufacturing method comprises hot-rolling a steel slab having the above composition, heat-treating a provided hot-rolled steel strip at 750-850°C for one hour or longer, cold-rolling it, continuously annealing the provided cold-rolled steel strip at 750-850°C, cooling it at a mean cooling rate of 40°C/minute or higher before reaching 200°C, and cold-rolling it at a rolling reduction of 22-32%. <P>COPYRIGHT: (C)2004,JPO

Description

【００１２】
もう一つの条件（以後、条件▲２▼と称す。）では、３ｍｍｔのサンプルをそのまま軟化焼鈍（８２０℃×７時間、徐冷）後脱スケールし、条件▲１▼と同じく２８％で２．２ｍｍまで冷間圧延した。
条件▲１▼および条件▲２▼で得られた２．２ｍｍｔの板の０．２％耐力を測定するとともに、曲げ試験を行った。曲げ試験は、曲げ軸が圧延方向に対して平行（Ｃ方向曲げ）となるようにし、先端Ｒを変えたＶブロックによる９０°曲げ試験を行い、割れが発生した先端Ｒを調査した。
黒化、あるいは歪み取りを想定して、６００℃×１５分の熱処理後（以後、熱処理と称す。）、それぞれのサンプルの０．２％耐力を測定し、この熱処理前後の０．２％耐力の差を求めた。
クリープ特性評価については、それぞれの板を幅１２．５ｍｍ，標点間距離が５０ｍｍの引張試験片に加工し、マスクを架張した状態での黒化あるいは歪み取り焼鈍工程を想定して、負荷応力３００Ｎ／ｍｍ^２の下で４５０℃×１時間保持後、クリープ歪みを測定した。
【００１３】
その結果を表２に示す。
（１）仕上げ圧延ままの曲げ加工性は、同一圧延率で比較すると条件▲１▼で造りこんだサンプルの方が条件▲２▼に比べて優れている。
（２）条件▲１▼のサンプルは、条件▲２▼のサンプルに比べ熱処理による０．２％耐力の低下は小さい。
（３）フレームのクリープ歪みは条件▲１▼のサンプルが条件▲２▼のサンプルに比べ小さい。
【００１４】
上記特性評価とは別に、鋼中の炭化物の最大大きさを測定した。
非水溶媒系電解液を用いた定電位電解エッチング法であるＳＰＥＥＤ法を用い、鋼板表面を溶解して炭化物を露出させ、走査型電子顕微鏡を用いて１５０００倍で１０視野観察し、その中での最大炭化物の長径を測定して炭化物の最大の大きさとした。
その結果を表２中に併せて示す。
【００１５】

【００１６】
条件▲１▼と条件▲２▼との上記の差異の原因について、含有Ｃおよび炭化物の存在形態から次のように考察した。
まず（１）について、熱延板では、炭化物は凝固時に生じた偏析に沿って生じており、線状の形態を呈する。中間焼鈍・仕上げ焼鈍なしの条件▲２▼の場合、炭化物の拡散が不十分なため、圧延方向に長い３μｍ以上の炭化物が認められた。これに対して、中間冷延・仕上げ焼鈍を施した条件▲１▼は炭化物の分断が促進され、最大炭化物は３μｍ以下に小さく分断された粒状の形態になっている。
一般に、曲げ加工の際、割れの起点は炭化物であり、炭化物の形態が微細で粒状であるほど割れの起点になり難く、曲げ加工性が良好となる。したがって、条件▲１▼が条件▲２▼と同一仕上げ圧延率であっても、条件▲１▼の方が条件▲２▼に比べ炭化物が微細で粒状になっているため、条件▲１▼の方が曲げ加工性が良好であったと推察される。
【００１７】
次に、（２）について条件▲１▼が条件▲２▼に比べ熱処理後の０．２％耐力の低下が小さかった理由は、条件▲１▼では、仕上げ焼鈍時にＣが一部固溶し、熱処理時に固溶Ｃに起因した歪み時効により０．２％耐力の低下が軽減されたためと推測される。０．２％耐力の低下軽減が歪み時効によることは、仕上げ焼鈍時の冷却速度の違いにより熱処理前後の０．２％耐力の低下の状況が変化することからも窺える。すなわち、図１に示すように、焼鈍温度から２００℃までの平均冷却速度の違いにより、熱処理後の０．２％耐力の変化状況が変わり、平均冷却速度が４０℃／分以上であると０．２％体力の低下は少ない。焼鈍後の冷却速度が速いと固溶Ｃの析出が起こらず、固溶Ｃはその後の歪み時効に寄与して０．２％耐力の低下を抑制している。これに対して、焼鈍後の冷却速度が遅いと固溶されていたＣはほとんど炭化物として析出してしまうために、加工硬化させても固溶Ｃによる歪み時効は期待できず、熱処理時の０．２％耐力の低下抑制効果は生じない。このため、条件▲２▼は熱処理時の０．２％耐力低下が大きい。
【００１８】
さらに、（３）について、条件▲１▼が条件▲２▼に比べてクリープ歪みが小さかった理由について考察した。
表１のサンプルを用いて、熱処理後の０．２％耐力とクリープ歪みの関係を別途調査したところ、図２にみられるように、クリープ歪みは熱処理後の０．２％耐力が５４０Ｎ／ｍｍ^２以下になると急激に増大していた。
したがって、熱処理前の条件▲１▼および条件▲２▼の０．２％耐力はほぼ同等であったが、条件▲２▼の場合、熱処理によって０．２％耐力が大きく低下し、熱処理後の０．２％耐力が５４０Ｎ／ｍｍ^２を下回ったためである。
【００１９】
ここで、加工フェライト組織とは、フェライト相である組織を加工硬化させた組織であり、焼鈍したフェライト組織の鋼帯を冷間圧延することにより得られる。適正な圧延率の冷間圧延によって得られた加工フェライト組織により、良好な曲げ性を維持しつつ０．２％耐力を向上させることが可能である。
鋼帯の適正仕上げ圧延率について検討した結果、圧延率２２〜３２％であれば６５０Ｎ／ｍｍ^２以上の０．２％耐力および良好な曲げ性が得られる。しかし，圧延率が２２％に満たないと６５０Ｎ／ｍｍ^２以上の０．２％耐力が得られず、また、圧延率が３２％を超えると曲げ性が著しく低下する。
なお、加工フェライト中にマルテンサイトを存在させると０．２％耐力は増加するが、マルテンサイトの体積率が５％を超えると曲げ加工性が著しく低下するため、本発明では、実質的に加工フェライト組織とした。本発明の加工フェライト組織は、５体積％以下のマルテンサイトの混在を許容するものである。
【００２０】
以下に、本発明ブラウン管フレーム用鋼板に含まれる合金成分，含有量等を説明する。
Ｃ：０ ． ０３〜０ ． ０８質量％
Ｃは固溶強化，析出硬化を目的として含有される。有効に作用させるためには最低でも０．０３％以上必要である。しかしながら０．０８％を超えて含有すると粗大な炭化物が生成しやすくなり、曲げ加工性が低下する。
Ｓｉ：０ ． ２〜１ ． ０質量％
Ｓｉは製鋼段階で脱酸剤として添加される合金成分であり，鋼材の強度向上にも有効で、０．２質量％以上のＳｉで添加効果が顕著になる。フェライトの生成にも有効である。しかし、１．０％を超えて含有すると曲げ加工性が低下する。
【００２１】
Ｍｎ：０ ． １〜１．０質量％
Ｍｎも製鋼段階で脱酸剤として添加される合金成分であり，鋼材の強度向上にも有効である。しかし、オーステナイト形成元素であるために多量に含有するとマルテンサイトを形成しやすくなるため、上限を１．０％とした。
Ｐ：０ ． ０４質量％以下
Ｐは固溶強化に有効な成分であるが、０．０４％を超えて含有させると冷間加工性が低下する。
【００２２】
Ｓ：０ ． ０３質量％以下
Ｓは、加工性に有害なＭｎＳ系介在物等の生成させる有害元素であり、含有量は少ないほど好ましい。０．０３％を超えると特に熱間加工性が低下する。
Ｎ：０．０４質量％以下
Ｃと同様に固溶強化に有効であるが、オーステナイトを生成しやすい元素であるため、上限を０．０４％とした。
【００２３】
Ｃｒ：１０．０〜１８．０質量％
耐食性向上と、３０〜６５０℃までの平均熱膨張係数を１２．０×１０^−６／℃以下にするためには、最低でも１０．０％以上必要である。しかし、１８．０％を超えると黒化処理時に黒色の酸化スケールが生じにくくなる。
Ｔｉ：０．０５質量％以下
ＴｉはＣとの親和力が非常に強いため、Ｔｉを含有すると仕上げ焼鈍後のＣ固溶量が減少する。したがって、Ｔｉの混入は極力避けるべきであり、多くても０．０５％以下に、好ましくは０．０１％以下にする必要がある。
Ｎｂ：０ ． ０２質量％以下
ＮｂもＴｉと同様にＣとの親和力が非常に強いため、Ｎｂを含有すると仕上げ焼鈍後のＣ固溶量が減少する。したがって、Ｎｂの混入についても極力避けるべきであり、多くても０．０２％以下に、好ましくは０．０１％以下にする必要がある。
【００２４】
熱延後の軟化焼鈍条件，仕上げ焼鈍条件および焼鈍後の冷却条件の設定は、前記予備実験の説明の項で記載したように、含有Ｃの固溶量を多くし、炭化物の微細分散状態を得るためのものである。この条件を外れると、後述の比較例で示すように、Ｃ固溶状態、炭化物分散状態が不十分で所望の物性が得られない。また、仕上げ圧延時の圧延率は、鋼板の機械的特性や加工性に大きく影響する。
それらの条件について設定理由を、以下に説明する。
熱延鋼帯の焼鈍条件：７５０〜８５０℃×１時間以上
７５０℃未満であると熱延時に生じたマルテンサイト組織を再結晶させることができず、８５０℃を超えるとγ相に入るため冷却時にマルテンサイト相が生じる。熱延鋼帯の焼鈍では箱型の焼鈍で１時間以上加熱し、できるだけ軟質にするため徐冷する必要がある。
【００２５】
仕上げ焼鈍条件：７５０〜８５０℃×連続焼鈍
仕上げ圧延前の仕上げ焼鈍では、加工硬化した冷延鋼帯をマルテンサイトの生成を抑制しつつ再結晶させ、かつＣの一部を固溶させるため、再結晶温度７５０℃以上、マルテンサイトが生成しない上限である８５０℃以下の温度範囲で加熱する必要がある。この焼鈍では、加工組織の再結晶とＣの一部を固溶させるためであるから短時間（均熱０〜１０分）の加熱でよく、連続焼鈍を適用することができる。
焼鈍後の冷却条件および仕上げ圧延率
焼鈍後の冷却時に固溶Ｃの析出を抑制するためには、焼鈍後の冷却速度を規制する必要があり、前述したように焼鈍後２００℃までの冷却速度を４０℃／分以上にする必要がある。
また、本発明のフェライト系鋼においては、前述のように圧延率が２２％に満たないと０．２％耐力が６５０Ｎ／ｍｍ^２以上が得られず、また、３２％を超えると曲げ性が著しく低下する。このため、仕上げ圧延時の圧延率は２２〜３２％にする必要がある。
【００２６】
【実施例】
表３に示す成分の鋼材を溶製し、連続鋳造によりスラブを製造した。このスラブを１２００℃に加熱した後、熱間圧延して５ｍｍ厚の熱延コイルを得た。次いで、箱型焼鈍炉で８２０℃×７時間加熱後炉冷し、酸洗後、２．６〜２．８ｍｍ圧まで中間焼鈍し、７４０〜９１０℃の範囲内で仕上げ連続焼鈍・酸洗後、１１〜３６％範囲内の圧延率で仕上げ圧延を実施し、板厚２．０ｍｍの冷延コイルを作製した。なお仕上げ焼鈍後の２００℃までの平均冷却速度は約９０℃／分であった。
中間冷延・仕上げ焼鈍の作用・効果を確認するために、比較例として、表３のＣ成分のスラブを用いて熱間圧延により２．８ｍｍ厚の熱延コイルを製造し、箱型焼鈍炉で８２０℃×７時間加熱・炉冷し、酸洗後、圧延率２８％で直接仕上げ圧延し、板厚２．０ｍｍの冷延コイルを作製した。
なお、いずれの例でも、箱型焼鈍炉での焼鈍後の２００℃までの平均冷却速度は、２℃／分以下であった。
ところで、本実施例に用いた鋼では、Ｃｒ含有量がすべて１０．０％以上であるので、３０〜６５０℃の平均熱膨張係数はすべて１２．０×１０^−６／℃以下であった。
【００２７】

【００２８】
得られた冷延コイルからサンプルを採取し、Ｃ方向（圧延方向と直角方向）の０．２％耐力を測定するとともに、組織の観察と炭化物の最大大きさを測定した。炭化物の最大大きさは前述の予備実験と同じＳＰＥＥＤ法で炭化物を露出させ、走査電子顕微鏡で観察・測定した。
ところで、ブラウン管フレームは冷延コイルからプレス加工により製造される。その際、素材は曲げ軸が圧延方向と直角なＬ方向曲げとＣ方向曲げを受ける。曲げ加工としてはＣ方向曲げが厳しく、曲げ加工時に割れが懸念される方向はＣ方向曲げであるから、Ｃ方向の曲げ試験を行った。
曲げ加工性の評価は、フレームの曲げ加工時の曲げ内Ｒ１．０ｍｍに合わせるため、先端Ｒ１．０ｍｍのＶブロックによる９０°曲げ試験を行い、割れの有無で良，不良を判定した。
【００２９】
次いで、黒化あるいは歪み取り焼鈍を想定して６００℃×１５分の熱処理後、それぞれのサンプルをクリープ試験に供した。クリープ試験は、それぞれの板を幅１２．５ｍｍ，標点間距離が５０ｍｍの引張試験片に加工し、マスクを架張した状態での黒化あるいは歪み取り焼鈍工程を想定して負荷応力３００Ｎ／ｍｍ^２の下で４５０℃×１時間保持後、クリープ歪みを測定した。その際、実際のマスク架張力の低下が問題にならないレベルとするためには、クリープ歪みを０．０５％以下とする必要がある。
【００３０】
評価結果を、表４に示す。
本発明にしたがった試験Ｎｏ．１〜９については、曲げ試験において割れ発生はなく、またクリープ歪みも０．０５％以下であった。
これに対して、試験Ｎｏ．１０は、Ｃ含有量が少ない鋼種Ｅを使用しているため、圧延時の加工硬化の上昇が小さく、また仕上げ焼鈍時の固溶強化も小さいため、クリープ歪みが０．１７％と目標の０．０５％以下を大きく上回った。試験Ｎｏ．１１は、逆にＣ含有量が多い鋼Ｆを使用しているため、炭化物が粗大になって曲げ加工試験で割れが生じた。試験Ｎｏ．１２，１３は、Ｃとの親和力が強いＴｉおよびＮｂを多く含有した鋼種Ｇ，Ｈを使用しているため、仕上げ焼鈍時にＣの固溶がほとんどなく、この結果、加工硬化により向上した０．２％耐力がその後の熱処理で大きく減少し、表４には示していないが５４０Ｎ／ｍｍ^２を下回ることとなって、クリープ歪みが目標の０．０５％以下を上回った。
【００３１】
試験Ｎｏ．１４は、仕上げ焼鈍温度が９１０℃と高かったため、一部マルテンサイトが生成し、加工曲げ試験で割れが生じた。試験Ｎｏ．１５は、逆に仕上げ焼鈍温度が低すぎたため、十分に回復せず、その結果、強度が高くなり過ぎて曲げ試験で割れが生じた。試験Ｎｏ．１６は、仕上げ圧延率が低すぎて０．２％耐力が低く、その結果、クリープ歪みが目標の０．０５％以下を大きく上回った。試験Ｎｏ．１８は、中間焼鈍，仕上げ焼鈍なしで軟化焼鈍後、仕上げ圧延したために、炭化物が３μｍ以下に微細に分散されず、またＣもほとんど固溶していなかったため、曲げ加工時に割れが生じ、またクリープ歪みも目標の０．０５％以下を上回っていた。
【００３２】

【００３３】
【発明の効果】
以上に説明したように、本発明に係るのブラウン管フレーム用鋼板は、炭化物が微細に分散したフェライト組織であるため、フレーム形状への加工に対して良好なプレス曲げ性を有する。しかも、フェライト相にＣを固溶させているので、加工硬化性と耐クリープ性にも優れている。このため、マスクを架張した状態で黒化あるいは歪み取り焼鈍を施しても変形することがなく、色ずれや画像の乱れがない高品質のブラウン管が得られる。
【図面の簡単な説明】
【図１】焼鈍後の冷却速度の違いにより、熱処理前後の０．２％耐力の差を説明する図
【図２】熱処理後の０．２％耐力とクリープ歪みの関係を説明する図[0001]
[Industrial applications]
The present invention relates to a cathode ray tube frame excellent in high-temperature creep resistance during blackening heat treatment or baking in a cathode ray tube assembling process, and more particularly to a steel plate for a right and left vertical cathode ray tube frame and a method for producing the same.
[0002]
[Prior art and problems]
Along with the flattening of TV cathode ray tubes and the like, a mask surface with fine holes formed at regular intervals by photo-etching on a cold-rolled steel plate having a thickness of 0.12 to 0.24 mm was tensioned in the vertical direction. The method of fixing to a frame in a state is increasing.
In the process of assembling the cathode ray tube of this method, a steel plate or a rod material for the cathode ray tube frame is formed, and a total of four cathode ray tube frames, two vertically upper and lower and two vertically left and right, are assembled by welding. A blackening heat treatment is performed at a temperature of 450 to 650 ° C. for 10 to 30 minutes for the purpose of preventing halation or preventing rust. Similarly, the mask subjected to the blackening heat treatment is welded and fixed to the horizontal cathode ray tube frame while maintaining a predetermined tension on the mask surface. Thereafter, a strain relief annealing is performed at a temperature of 450 to 650 ° C. called a baking treatment for 10 to 60 minutes.
As another process, a blackening heat treatment may be performed after forming, assembling, baking, and attaching a mask to a CRT frame.
[0003]
In the former process, baking treatment is performed, and in the latter process, during blackening heat treatment, heat treatment is performed with a mask stretched. Therefore, in any of the processes, a temperature of 400 to 700 ° C. is applied with stress applied to four CRT frames. As a result, the mask and the frame are likely to be deformed by the creep phenomenon. Therefore, after these heat treatments, the tension on the mask surface decreases. If the reduction in the tension on the mask surface is large, the mask surface becomes sensitive to the occurrence of distortion or vibration, and as a result, the performance of the CRT such as color shift is reduced.
[0004]
Japanese Patent Application Laid-Open No. 2001-316766 discloses a metal structure in which a pearlite or bainite phase of 10% by volume or more with respect to a ferrite phase is disposed in order to prevent a decrease in bridge tension. It discloses that V, Cr and the like are added. However, if the thermal expansion coefficient of the vertical cathode ray tube frame is equal to or greater than the thermal expansion coefficient of the cold-rolled steel sheet as the mask material, the creep amount during the heat treatment with the above-described mask stretched increases, The drop in the bridge tension increases. For this reason, it is desired that the vertical frame has a smaller coefficient of thermal expansion than the cold-rolled steel sheet. However, the frame material proposed in Japanese Patent Application Laid-Open No. 2001-316766 has a maximum Cr content that lowers the coefficient of thermal expansion of iron. However, the thermal expansion coefficient is almost the same as that of a cold rolled steel sheet mask even if other components are taken into consideration, and it is not suitable for a vertical CRT frame.
[0005]
In order to reduce the thermal expansion coefficient, Japanese Patent Application Laid-Open No. 2001-181801 proposes a ferritic stainless steel containing 10.5% or more of Cr. P, Mo, V, and W are further added to the stainless steel to increase the high-temperature strength.
Japanese Patent Application Laid-Open No. 2001-234293 discloses that a hot-rolled steel sheet is rolled at a rolling reduction of 5 to 20% by adding Cr at 8% or more as it is or after softening annealing (box-type annealing). It is disclosed that the yield stress at room temperature is set to 400 MPa or more to reduce the amount of creep during heat treatment in a state where a mask is stretched.
[0006]
The cathode ray tube of the type in which the mask is stretched (hereinafter referred to as the “stretching method”) is formed into a spherical shape by press forming the frame to withstand the tension of the mask. There is a drawback that the mask becomes heavier than a method in which the mask is welded to the frame and fixed (hereinafter, referred to as “press forming method”). According to the techniques proposed in JP-A-2001-316766, JP-A-2001-181801, and JP-A-2001-234293, the strength of the frame is increased, and the weight of the frame is reduced to some extent. However, the thickness of the frame is at least 3 mm at the minimum, and the thickness of the stainless steel plate is typically 4 to 6 mm, which is thicker and heavier than the frame thickness of the press molding method. When the strength of the cathode ray tube frame material is further increased, the creep amount is reduced, but the bending workability is reduced and cracks are likely to occur during press working.
[0007]
As described above, there has not yet been proposed a steel plate for a CRT frame that can meet both the need to reduce the weight of the CRT frame and the need to increase the tension of the mask to increase the performance of the CRT itself. It is the current situation.
The present invention has been devised to solve such a problem, has good press workability, and can reduce the amount of creep during heat treatment in a stretched state. An object of the present invention is to provide a steel plate for a cathode ray tube frame capable of applying a tension equal to or higher than that of the cathode ray tube and capable of greatly reducing the weight of the cathode ray tube frame.
[0008]
[Means for Solving the Problems]
In order to achieve the object, the steel plate for a cathode ray tube frame of the present invention contains C in an amount of 0.03 to 0.08% by mass, and carbide having a size of 3 μm or less is dispersed in a processed ferrite structure in which C is dissolved. It has a metal structure, and has a 0.2% proof stress of 650 to 870 N / mm ² and an average coefficient of thermal expansion of 30 to 650 ° C. of 12.0 × 10 ⁻⁶ / ° C. or less.
Such a steel sheet is, in mass%, C: 0.03 to 0.08%, Si: 0.2 to 1.0%, Mn: 0.1 to 1.0%, P: 0.04% or less. , S: 0.03% or less, N: 0.04% or less, Cr: 10.0 to 18.0%, Ti: 0.05% or less, Nb: 0.02% or less, the balance being substantially Hot rolled steel strip obtained by heat rolling at 750 to 850 ° C. for 1 hour or more, and then subjected to cold rolling, and the obtained cold rolled steel strip is heated to 750 to 850 ° C. , And then cooled at an average cooling rate up to 200 ° C at a rate of 40 ° C / min or more and cold-rolled at a rolling reduction of 22 to 32%.
[0009]
[Action]
When a steel sheet is used as a CRT frame material, it is necessary that the thermal expansion coefficient of the frame material be smaller than the average thermal expansion coefficient of the cold-rolled steel sheet mask material.
During the heat treatment with the mask stretched, when the vertical frame has a larger thermal expansion amount than the cold-rolled steel plate mask, the bridging tension of both the mask and the frame increases. Was limited to 12.0 × 10 ⁻⁶ / ° C. or less, which is smaller than the thermal expansion coefficient of the cold-rolled steel sheet mask of 13.0 × 10 ⁻⁶ / ° C.
For this reason, the inventors of the present invention have studied the processing of a Fe-Cr alloy in which iron contains 10.0% or more of Cr to reduce the thermal expansion coefficient to 12.0 × 10 ⁻⁶ / ° C. or less into a cathode ray tube frame. The workability and creep resistance during heat treatment were investigated.
[0010]
In general, the mechanical properties, including workability of steel, depend on the content of C and the behavior of carbides. Therefore, steels having the component compositions shown in Table 1 were produced in a laboratory with different forms of carbides. That is, a continuous casting slab of 200 mmt of the components shown in Table 1 was hot-rolled, and samples were obtained from the obtained hot-rolled 5 mm and 3 mm coils. A 2.2 mm plate was obtained from the collected sample under the following conditions.
Under one condition (hereinafter referred to as condition (1)), a sample of 5 mmt was descaled after softening annealing in a laboratory. The temperature of this softening annealing did not fall within the austenite range, and for the purpose of causing recrystallization and softening, heating was performed in a temperature range of 750 to 850 ° C. for 1 hour or more, and then the furnace was cooled. In this experiment, after heating at 820 ° C. for 7 hours, the sample was gradually cooled to 200 ° C. at an average cooling rate of 2 ° C./min. Next, it is cold-rolled to a thickness of 3 mm (hereinafter referred to as intermediate rolling), annealed at 820 ° C. × 0 second, and cooled at an average cooling rate of up to 200 ° C. at 40 ° C./min or more (hereinafter, this annealing is finished After descaling, cold rolling (hereinafter referred to as finish rolling) was performed at a rolling ratio of 28% to 2.2 mm.
[0011]

[0012]
Under another condition (hereinafter referred to as condition (2)), a sample of 3 mmt is softened and annealed (820 ° C. × 7 hours, gradually cooled) and then descaled. It was cold rolled to 2 mm.
The 0.2% proof stress of the 2.2 mmt plate obtained under the conditions (1) and (2) was measured and a bending test was performed. The bending test was performed so that the bending axis was parallel to the rolling direction (bending in the C direction), and a 90 ° bending test was performed using a V-block having a different tip R to investigate the tip R where cracks occurred.
Assuming blackening or distortion removal, after heat treatment at 600 ° C. for 15 minutes (hereinafter referred to as heat treatment), the 0.2% proof stress of each sample was measured, and the 0.2% proof stress before and after this heat treatment was measured. Was determined.
Regarding the creep characteristic evaluation, each plate was processed into a tensile test piece having a width of 12.5 mm and a distance between gauge points of 50 mm, and a blackening or strain relief annealing process with a mask stretched was performed. After holding at 450 ° C. × 1 hour under a stress of 300 N / mm ² , the creep strain was measured.
[0013]
Table 2 shows the results.
(1) As for the bending workability as it is in the finish rolling, the sample fabricated under the condition (1) is superior to the condition (2) when compared at the same rolling ratio.
(2) The sample under condition (1) has a smaller decrease in 0.2% proof stress due to heat treatment than the sample under condition (2).
(3) The sample under condition (1) has a smaller creep distortion of the frame than the sample under condition (2).
[0014]
Separately from the above property evaluation, the maximum size of carbide in steel was measured.
Using the SPEED method, which is a constant potential electrolytic etching method using a non-aqueous solvent-based electrolytic solution, dissolves the steel sheet surface to expose carbides, and observes 10 visual fields at 15000 × using a scanning electron microscope. Of the largest carbide was measured to determine the largest size of the carbide.
The results are also shown in Table 2.
[0015]

[0016]
The cause of the above difference between the condition (1) and the condition (2) was considered as follows from the content of C and the form of the carbide.
First, regarding (1), in the hot-rolled sheet, the carbide is generated along with the segregation generated at the time of solidification, and exhibits a linear form. Under the condition (2) without intermediate annealing and finish annealing, carbides with a length of 3 μm or more in the rolling direction were recognized due to insufficient diffusion of carbides. On the other hand, under the condition (1) in which the intermediate cold rolling and finish annealing are performed, the fragmentation of the carbide is promoted, and the largest carbide is in the form of a small fragment of 3 μm or less.
In general, at the time of bending, the starting point of cracks is carbide, and the finer and more granular the form of the carbide, the less likely the starting point of cracking, and the better bending workability. Therefore, even if the condition (1) has the same finish rolling reduction as the condition (2), the carbide is finer and more granular in the condition (1) than in the condition (2). It is presumed that the bending workability was better.
[0017]
Next, the reason for the condition (1) that the decrease in the 0.2% proof stress after the heat treatment was smaller in the condition (1) than in the condition (2) is that in the condition (1), C was partially dissolved during the finish annealing. It is presumed that the decrease in 0.2% proof stress was reduced by strain aging caused by solid solution C during heat treatment. The fact that the reduction in the 0.2% proof stress is reduced by strain aging is also indicated by the fact that the 0.2% proof stress before and after the heat treatment changes depending on the cooling rate during the finish annealing. That is, as shown in FIG. 1, the change in the 0.2% proof stress after the heat treatment changes due to the difference in the average cooling rate from the annealing temperature to 200 ° C., and 0% when the average cooling rate is 40 ° C./min or more. .2% The decrease in physical strength is small. If the cooling rate after annealing is high, precipitation of solid solution C does not occur, and solid solution C contributes to the subsequent strain aging and suppresses a decrease in 0.2% proof stress. On the other hand, if the cooling rate after annealing is low, C dissolved in the solid solution is almost precipitated as carbide, so that strain aging due to the solid solution C cannot be expected even when work-hardened, and 0% during the heat treatment. No effect of suppressing a decrease in .2% proof stress occurs. Therefore, the condition (2) has a large decrease in the proof stress of 0.2% during the heat treatment.
[0018]
Further, regarding (3), the reason why the creep strain was smaller in the condition (1) than in the condition (2) was considered.
The relationship between the 0.2% proof stress after heat treatment and the creep strain was separately investigated using the samples in Table 1. As shown in FIG. 2, the creep strain was found to be 0.2% proof stress after heat treatment at 540 N / mm. ^When it became ² or less, it rapidly increased.
Therefore, the 0.2% proof stress of the conditions (1) and (2) before the heat treatment was almost the same, but in the case of the condition (2), the 0.2% proof stress was significantly reduced by the heat treatment, and the This is because the 0.2% proof stress was lower than 540 N / mm ² .
[0019]
Here, the processed ferrite structure is a structure obtained by work hardening a structure that is a ferrite phase, and is obtained by cold rolling a steel strip having an annealed ferrite structure. With the processed ferrite structure obtained by cold rolling at an appropriate rolling reduction, it is possible to improve 0.2% proof stress while maintaining good bendability.
As a result of examining the appropriate finish rolling ratio of the steel strip, if the rolling ratio is 22 to 32%, a 0.2% proof stress of 650 N / mm ² or more and good bendability can be obtained. However, if the rolling reduction is less than 22%, a 0.2% proof stress of 650 N / mm ² or more cannot be obtained, and if the rolling reduction exceeds 32%, the bendability is significantly reduced.
The presence of martensite in the processed ferrite increases the 0.2% proof stress, but when the volume fraction of martensite exceeds 5%, the bending workability is significantly reduced. A ferrite structure was obtained. The processed ferrite structure of the present invention permits the mixing of 5% by volume or less of martensite.
[0020]
Hereinafter, alloy components, contents, and the like contained in the steel plate for a cathode ray tube frame of the present invention will be described.
C: 0 . 03-0 . 08 mass%
C is contained for the purpose of solid solution strengthening and precipitation hardening. In order to function effectively, at least 0.03% or more is required. However, when the content exceeds 0.08%, coarse carbides are easily generated, and the bending workability decreases.
Si: 0 . 2-1 . 0% by mass
Si is an alloy component added as a deoxidizing agent in the steelmaking stage, and is also effective for improving the strength of steel materials. The effect of adding Si at 0.2% by mass or more becomes significant. It is also effective in producing ferrite. However, when the content exceeds 1.0%, the bending workability decreases.
[0021]
Mn: 0 . 1 to 1.0% by mass
Mn is also an alloy component added as a deoxidizing agent in the steelmaking stage, and is also effective in improving the strength of steel. However, since it is an austenite-forming element, if contained in a large amount, it becomes easy to form martensite, so the upper limit was made 1.0%.
P: 0 . P of not more than 04% by mass is an effective component for solid solution strengthening. However, if it is contained in excess of 0.04%, the cold workability is reduced.
[0022]
S: 0 . S in an amount of not more than 03% by mass is a harmful element that forms MnS-based inclusions that are harmful to workability, and the smaller the content, the better. If it exceeds 0.03%, hot workability is particularly deteriorated.
N: 0.04% by mass or less Effective for solid solution strengthening like C, but is an element that easily generates austenite, so the upper limit was made 0.04%.
[0023]
Cr: 10.0 to 18.0% by mass
In order to improve the corrosion resistance and to make the average thermal expansion coefficient up to 30 to 650 ° C. 12.0 × 10 ⁻⁶ / ° C. or less, at least 10.0% or more is required. However, if it exceeds 18.0%, black oxide scale is less likely to be generated during the blackening treatment.
Ti: 0.05% by mass or less Ti has a very strong affinity for C. Therefore, when Ti is contained, the amount of C solid solution after finish annealing decreases. Therefore, the incorporation of Ti should be avoided as much as possible, and should be at most 0.05% or less, preferably 0.01% or less.
Nb: 0 . Since Nb of not more than 02% by mass also has a very strong affinity for C like Ti, if Nb is contained, the amount of C solid solution after finish annealing decreases. Therefore, the incorporation of Nb should be avoided as much as possible, and should be at most 0.02% or less, preferably 0.01% or less.
[0024]
As described in the description of the preliminary experiment, the softening conditions after the hot rolling, the conditions for the final annealing, and the cooling conditions after the annealing were set by increasing the solid solution amount of the contained C and reducing the fine dispersion state of the carbide. To gain. If the conditions are not satisfied, as shown in the comparative examples described below, the C solid solution state and the carbide dispersion state are insufficient, and desired physical properties cannot be obtained. Further, the rolling reduction during finish rolling greatly affects the mechanical properties and workability of the steel sheet.
The reasons for setting these conditions will be described below.
Annealing condition of hot-rolled steel strip: If it is 750-850 ° C. × 1 hour or more and less than 750 ° C., the martensite structure generated during hot rolling cannot be recrystallized. Sometimes a martensitic phase occurs. In the annealing of a hot-rolled steel strip, it is necessary to heat the steel by box-type annealing for 1 hour or more, and to gradually cool the steel so as to be as soft as possible.
[0025]
Finish annealing conditions: 750 to 850 ° C. × continuous annealing In the finish annealing before the finish rolling, the work-hardened cold-rolled steel strip is recrystallized while suppressing the formation of martensite, and a part of C is solidified. In order to dissolve, it is necessary to heat at a recrystallization temperature of 750 ° C. or higher and a temperature range of 850 ° C. or lower which is the upper limit at which martensite is not generated. Since this annealing is for recrystallization of the processed structure and for dissolving a part of C, heating for a short time (soaking is 0 to 10 minutes) is sufficient, and continuous annealing can be applied.
Cooling conditions after annealing and finish rolling rate In order to suppress precipitation of solid solution C during cooling after annealing, it is necessary to regulate the cooling rate after annealing, and as described above, 200 ° C after annealing. It is necessary to set the cooling rate to 40 ° C./min or more.
In the ferritic steel of the present invention, as described above, a 0.2% proof stress of 650 N / mm ² or more cannot be obtained unless the rolling reduction is less than 22%. It decreases significantly. For this reason, the rolling reduction at the time of finish rolling needs to be 22 to 32%.
[0026]
【Example】
A slab was produced by smelting steel materials having the components shown in Table 3 and performing continuous casting. This slab was heated to 1200 ° C. and then hot-rolled to obtain a hot-rolled coil having a thickness of 5 mm. Then, after heating at 820 ° C. for 7 hours in a box-type annealing furnace, furnace-cooling, pickling, intermediate annealing to a pressure of 2.6 to 2.8 mm, and finishing continuous annealing and pickling within a range of 740 to 910 ° C. Finish rolling was performed at a rolling rate within the range of 11 to 36% to produce a cold-rolled coil having a thickness of 2.0 mm. The average cooling rate to 200 ° C. after the finish annealing was about 90 ° C./min.
As a comparative example, a 2.8 mm-thick hot-rolled coil was manufactured by hot rolling using a slab of the C component shown in Table 3 to check the action and effect of the intermediate cold rolling and finish annealing. After heating at 820 ° C. for 7 hours and cooling in a furnace, pickling was performed and then finish rolling was performed directly at a rolling reduction of 28% to produce a cold-rolled coil having a sheet thickness of 2.0 mm.
In each case, the average cooling rate to 200 ° C. after annealing in the box-type annealing furnace was 2 ° C./min or less.
By the way, in the steel used in this example, since the Cr content was 10.0% or more, the average thermal expansion coefficients at 30 to 650 ° C were all 12.0 × 10 ⁻⁶ / ° C or less.
[0027]

[0028]
A sample was taken from the obtained cold-rolled coil, and the 0.2% proof stress in the C direction (the direction perpendicular to the rolling direction) was measured, and at the same time, the structure was observed and the maximum size of the carbide was measured. The maximum size of the carbide was measured by exposing the carbide by the same SPEED method as in the above-mentioned preliminary experiment, and observing and measuring it with a scanning electron microscope.
By the way, the cathode ray tube frame is manufactured from a cold rolled coil by press working. At this time, the material is subjected to L-direction bending and C-direction bending whose bending axes are perpendicular to the rolling direction. Since bending in the C direction is severe as the bending process, and the direction in which cracking is likely to occur during bending is the C direction bending, a bending test in the C direction was performed.
The bending workability was evaluated by performing a 90 ° bending test using a V-block having a tip R of 1.0 mm in order to match the inside radius R of 1.0 mm during the bending of the frame.
[0029]
Next, each sample was subjected to a creep test after a heat treatment at 600 ° C. for 15 minutes assuming blackening or strain relief annealing. In the creep test, each plate was processed into a tensile test piece having a width of 12.5 mm and a distance between gauge points of 50 mm, and a load stress of 300 N / After holding at 450 ° C. × 1 hour under mm ² , the creep strain was measured. At that time, in order to reduce the actual mask tension to a level that does not cause a problem, the creep strain needs to be 0.05% or less.
[0030]
Table 4 shows the evaluation results.
Test No. according to the present invention. For Nos. 1 to 9, no cracking occurred in the bending test, and the creep strain was 0.05% or less.
On the other hand, Test No. Sample No. 10 uses steel type E having a low C content, so that the increase in work hardening during rolling is small, and the solid solution strengthening during finish annealing is also small, so the creep strain is 0.17%, which is the target 0. 0.05% or less. Test No. In No. 11, on the contrary, since steel F having a large C content was used, carbides became coarse and cracks occurred in a bending test. Test No. Since Nos. 12 and 13 use steel types G and H containing a large amount of Ti and Nb having a strong affinity for C, there is almost no solid solution of C at the time of finish annealing, and as a result, 0.1 is improved by work hardening. The 2% proof stress was significantly reduced by the subsequent heat treatment, and although not shown in Table 4, it was lower than 540 N / mm ² , and the creep strain exceeded the target of 0.05% or less.
[0031]
Test No. In No. 14, since the finish annealing temperature was as high as 910 ° C., martensite was partially generated, and cracks occurred in the working bending test. Test No. Conversely, No. 15 did not recover sufficiently because the finish annealing temperature was too low, and as a result, the strength became too high and cracks occurred in the bending test. Test No. In No. 16, the finish rolling reduction was too low and the 0.2% proof stress was low, and as a result, the creep strain was much higher than the target of 0.05% or less. Test No. No. 18 was softened and annealed without intermediate annealing and finish annealing followed by finish rolling, so that carbides were not finely dispersed to 3 μm or less, and C was hardly dissolved, so cracks were generated during bending and creep occurred. The distortion was also below the target of 0.05% or less.
[0032]

[0033]
【The invention's effect】
As described above, the steel sheet for a CRT frame according to the present invention has a ferrite structure in which carbides are finely dispersed, and thus has good press-bendability for processing into a frame shape. In addition, since C is dissolved in the ferrite phase, work hardenability and creep resistance are excellent. For this reason, even if blackening or strain relief annealing is performed with the mask being stretched, a high-quality cathode ray tube which does not deform and has no color shift or image disturbance can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a difference in 0.2% proof stress before and after heat treatment due to a difference in cooling rate after annealing. FIG. 2 is a diagram illustrating a relationship between 0.2% proof stress after heat treatment and creep strain.

Claims (3)

C contains 0.03 to 0.08 mass%, has a metal structure in which carbide having a size of 3 μm or less is dispersed in a processed ferrite structure in which C is dissolved, and has a 0.2% proof stress of 650 to 650%. A steel plate for a cathode ray tube frame, wherein the average thermal expansion coefficient at 870 N / mm ² and 30 to 650 ° C. is 12.0 × 10 ⁻⁶ / ° C. or less. In mass%, C: 0.03 to 0.08%, Si: 0.2 to 1.0%, Mn: 0.1 to 1.0%, P: 0.04% or less, S: 0.03% %, N: 0.04% or less, Cr: 10.0 to 18.0%, Ti: 0.05% or less, Nb: 0.02% or less, the balance being substantially Fe The steel plate for a cathode ray tube frame according to claim 1, which is: The continuous cast slab having the steel composition according to claim 2 is hot-rolled, and the obtained hot-rolled steel strip is subjected to a heat treatment at 750 to 850 ° C. for 1 hour or more, and then subjected to cold rolling, and the obtained cold-rolled steel strip. After continuously annealing the steel strip at 750 to 850 ° C., the average cooling rate up to 200 ° C. is cooled at a rate of 40 ° C./min or more, and cold rolling is performed at a rolling reduction of 22 to 32%. Manufacturing method for CRT frame.

Images