JP4042897B2 - Steel plate for CRT frame - Google Patents

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／ｍｍ²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／ｍｍ²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／ｍｍ²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／ｍｍ²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／ｍｍ²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／ｍｍ²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／ｍｍ²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／ｍｍ²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 application fields]
The present invention relates to a CRT frame excellent in high-temperature creep resistance during blackening heat treatment or baking in a CRT assembly process, and more particularly to a steel plate for CRT frames in the left and right vertical direction and a manufacturing method thereof.
[0002]
[Prior art and problems]
With the flattening of TV cathode ray tubes, etc., tension was applied in the vertical direction to the mask surface with fine holes opened at regular intervals by cold etching on a cold rolled steel plate with a thickness of 0.12 to 0.24 mm. The method of fixing to the frame in the state is increasing.
In the assembling process of the cathode ray tube of this method, a steel plate or a bar material used as the material of the cathode ray tube frame is formed, and a total of four cathode ray tube frames are assembled by welding, two in the horizontal direction and two in the vertical direction. In order to prevent halation or prevent rust, blackening heat treatment is performed at a temperature of 450 to 650 ° C. for 10 to 30 minutes. Similarly, the mask subjected to the blackening heat treatment is welded and fixed to the horizontal cathode ray tube frame in a state where a predetermined tension is maintained on the mask surface. Thereafter, strain relief annealing is performed at a temperature of 450 to 650 ° C. called baking treatment for 10 to 60 minutes.
As another process, blackening heat treatment may be performed after forming, assembling, baking, and attaching the mask to the cathode ray tube frame.
[0003]
In the former process, the heat treatment is performed in a state where the mask is stretched during the baking process, and in the latter process, during the blackening heat treatment. In any process, the stress is applied to the four CRT frames at 400 to 700 ° C. Due to the exposure to temperature, the mask and frame members are likely to be deformed by the creep phenomenon. For this reason, after these heat treatments, the tension on the mask surface is reduced. When the reduction in the mask tension on the mask surface is large, the mask surface becomes sensitive to the occurrence of distortion or vibration on the mask surface. As a result, the performance of the cathode ray tube such as color misregistration is reduced.
[0004]
In order to prevent a decrease in the tension, Japanese Patent Laid-Open No. 2001-316766 discloses Mo, in which a metal structure is arranged with a pearlite or bainite phase of 10% by volume or more with respect to the ferrite phase, and further, Mo, which increases high temperature strength to iron, The addition of V, Cr, etc. is disclosed. However, if the thermal expansion coefficient of the CRT frame in the vertical direction is equal to or greater than the thermal expansion coefficient of the cold-rolled steel sheet that is the mask material, the amount of creep during the heat treatment with the above-described mask stretched increases. Decrease in rack tension becomes large. For this reason, the vertical frame is desired to have a smaller thermal expansion coefficient than that of the cold-rolled steel sheet, but the frame material proposed in Japanese Patent Application Laid-Open No. 2001-316766 has the largest Cr content that reduces the thermal expansion coefficient of iron. However, it is 2%, and even if other components are taken into consideration, the coefficient of thermal expansion is almost the same as that of a cold-rolled steel sheet mask and is not suitable for a vertical cathode ray tube frame.
[0005]
In order to reduce the coefficient of thermal expansion, Japanese Patent Application Laid-Open No. 2001-181801 proposes a ferritic stainless steel containing 10.5% or more of Cr. In order to increase the high temperature strength, P, Mo, V, and W are further added to this stainless steel.
Japanese Patent Laid-Open No. 2001-234293 discloses that Cr is added at 8% or more and the hot-rolled steel sheet is rolled as it is or after soft annealing (box annealing) at a reduction rate of 5 to 20%. It is disclosed that the yield stress at room temperature is 400 MPa or more and the amount of creep during heat treatment in a state where the mask is stretched is reduced.
[0006]
The cathode ray tube with the mask stretched (hereinafter referred to as the “stretching method”) was forced to have a thick frame to withstand the mask tension, and was pressed into a spherical shape. There is a drawback that it is heavier than a method in which the mask is welded to the frame and fixed (hereinafter referred to as “press molding method”). With the techniques proposed in the above Japanese Patent Laid-Open Nos. 2001-316766, 2001-181801, and 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 3 mm even if it is thin, and typically a stainless steel plate thickness of 4 to 6 mm is exemplified, which is thicker and heavier than the frame thickness of the press forming method. When the strength of the CRT frame material is further increased, the amount of creep is reduced, but the bending workability is lowered and cracking is likely to occur during press working.
[0007]
As described above, there has not yet been proposed a CRT frame steel sheet that can meet both the needs of CRT frame weight reduction and the need to increase the mask tension to improve the performance of the CRT itself. Currently.
The present invention has been devised to solve such problems, has good press workability, can reduce the amount of creep during heat treatment in a stretched state, and The purpose of the present invention is to provide a CRT frame steel sheet that can apply a frame tension equal to or greater than that of the CRT and can significantly reduce the weight of the CRT frame.
[0008]
[Means for Solving the Problems]
In order to achieve the object, the CRT frame steel sheet of the present invention is, in mass%, C : 0.03-0.08 %, Si: 0.2-1.0%, Mn: 0.1-1. 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: 0.0. Hot-rolling a continuous cast slab having a steel composition comprising 02% or less , the balance being Fe and inevitable impurities, and after the heat treatment of the obtained hot-rolled steel strip at 750 to 850 ° C. for 1 hour or more, After cold-rolling and subjecting the obtained cold-rolled steel strip to continuous annealing at 750 to 850 ° C., the average cooling rate to 200 ° C. is cooled at a rate of 40 ° C./min or more, and the rolling rate is 22 to 32 % In the processed ferrite structure in which C is dissolved, and has a maximum grain size of 3 μm. m has a metal structure in which carbides are dispersed, and 0.2% proof stress is 650 to 870 N / mm ² , and an average coefficient of thermal expansion of 30 to 650 ° C. is 12.0 × 10 ⁻⁶ / ° C. or less. It is characterized by that .
[0009]
[Action]
When using a steel plate as a cathode ray tube frame material, the thermal expansion coefficient of the frame material needs to be smaller than the average thermal expansion coefficient of the cold-rolled steel sheet mask material.
During the heat treatment with the mask stretched, if the thermal expansion amount of the vertical frame is larger than that of the cold-rolled steel sheet mask, the tension of both the mask and the frame increases. The average thermal expansion coefficient was limited to 12.0 × 10 ⁻⁶ / ° C. or less, which is smaller than the thermal expansion coefficient 13.0 × 10 ⁻⁶ / ° C. of the cold-rolled steel sheet mask.
For this reason, the present inventors have processed a cathode ray tube frame in an Fe—Cr alloy containing 10.0% or more of Cr in iron in order to set the thermal expansion coefficient to 12.0 × 10 ⁻⁶ / ° C. or less. The workability suitable for the heat treatment and the creep resistance during heat treatment were investigated.
[0010]
In general, mechanical properties including workability of steel depend on the behavior of contained C and carbide. Therefore, steels having the component compositions shown in Table 1 with different carbides were produced in the laboratory. That is, a 200 mmt continuous cast slab having the components shown in Table 1 was hot-rolled, and samples were taken from the obtained 5 mm and 3 mm hot rolled 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 5 mmt sample was descaled after soft annealing in a laboratory. The temperature of this softening annealing did not enter the austenite region, and was heated for 1 hour or more in the temperature range of 750 to 850 ° C. for the purpose of causing recrystallization and softening, and then cooled in the furnace. In this experiment, after heating at 820 ° C. for 7 hours, it was gradually cooled to an average cooling rate of 2 ° C./min up to 200 ° C. Next, it is cold-rolled to a thickness of 3 mm (hereinafter referred to as intermediate rolling), annealed at 820 ° C. × 0 second, and then cooled at an average cooling rate of 40 ° C./min up to 200 ° C. (hereinafter, this annealing is finished) After annealing, cold rolling (hereinafter referred to as finish rolling) was performed at a rolling rate of 28% up to 2.2 mm.
[0011]

[0012]
Under another condition (hereinafter referred to as condition (2)), a 3 mmt sample was subjected to soft annealing (820 ° C. × 7 hours, slow cooling) and then descaled, and 28% as in condition (1). Cold rolled to 2 mm.
A 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. In the bending test, the bending axis was parallel to the rolling direction (C direction bending), and a 90 ° bending test was performed with a V block with the tip R changed, and the tip R where the crack occurred was investigated.
Assuming blackening or distortion removal, after heat treatment at 600 ° C. for 15 minutes (hereinafter referred to as heat treatment), 0.2% yield strength of each sample was measured, and 0.2% yield strength before and after this heat treatment was measured. The difference was calculated.
For creep property evaluation, each plate was processed into a tensile test piece with a width of 12.5 mm and a distance between marks of 50 mm, assuming a blackening or strain relief annealing process with the mask stretched. After holding at 450 ° C. for 1 hour under a stress of 300 N / mm ² , the creep strain was measured.
[0013]
The results are shown in Table 2.
(1) The bending workability as finished rolling is superior to the condition (2) in the sample fabricated under the condition (1) when compared at the same rolling rate.
(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 creep distortion of the frame is smaller in the sample of condition (1) than in the sample of condition (2).
[0014]
Apart from the above characteristic evaluation, the maximum size of carbides in the steel was measured.
Using the SPEED method, which is a controlled potential electrolytic etching method using a non-aqueous solvent-based electrolytic solution, the steel plate surface is dissolved to expose carbides, and 10 fields of view are observed at 15000 times using a scanning electron microscope. The longest diameter of the largest carbide was measured to obtain the largest size of the carbide.
The results are also shown in Table 2.
[0015]

[0016]
The cause of the difference between the condition (1) and the condition (2) was considered as follows from the existence form of contained C and carbide.
First, regarding (1), in the hot-rolled sheet, carbides are generated along the segregation generated during solidification and take a linear form. In the case of condition (2) without intermediate annealing and finish annealing, carbide diffusion of 3 μm or more long in the rolling direction was recognized because of insufficient diffusion of carbide. On the other hand, the condition (1) subjected to intermediate cold rolling and finish annealing promotes the fragmentation of the carbide, and the maximum carbide is in the form of a granular material that is smallly divided to 3 μm or less.
In general, at the time of bending, the starting point of cracking is carbide, and the finer and granular the form of the carbide is, the less likely to be the starting point of cracking and the better the bending workability. Therefore, even if the condition (1) is the same finish rolling rate as the condition (2), the condition (1) is finer and more granular than the condition (2), so the condition (1) It is inferred that the bending workability was better.
[0017]
Next, with respect to (2), the condition (1) has a smaller decrease in 0.2% proof stress after heat treatment than the condition (2). Under the condition (1), a part of C dissolves during the final annealing. This is presumably because 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 due to strain aging can be seen from the fact that the state of the 0.2% proof stress before and after the heat treatment changes due to the difference in the cooling rate during the final annealing. That is, as shown in FIG. 1, due to the difference in the average cooling rate from the annealing temperature to 200 ° C., the change state of 0.2% proof stress after the heat treatment changes, and when the average cooling rate is 40 ° C./min or more, 0 .2% decrease in physical strength is small. When the cooling rate after annealing is high, precipitation of solute C does not occur, and solute C contributes to subsequent strain aging and suppresses a decrease in 0.2% yield strength. On the other hand, if the cooling rate after annealing is slow, the C that has been dissolved is almost precipitated as carbides. Therefore, strain aging due to the solid solution C cannot be expected even if work hardening is performed. .2% yield strength lowering suppression effect does not occur. For this reason, the condition (2) has a large 0.2% yield strength drop during 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.
When the relationship between 0.2% proof stress after heat treatment and creep strain was separately investigated using the samples in Table 1, as shown in FIG. 2, the 0.2% proof stress after heat treatment was 540 N / mm. ^When it became ² or less, it increased rapidly.
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 greatly reduced by the heat treatment, This is because the 0.2% proof stress was less than 540 N / mm ² .
[0019]
Here, the work ferrite structure is a structure obtained by work hardening of a structure that is a ferrite phase, and is obtained by cold rolling a steel strip having an annealed ferrite structure. The processed ferrite structure obtained by cold rolling at an appropriate rolling rate can improve 0.2% proof stress while maintaining good bendability.
As a result of examining the appropriate finish rolling rate of the steel strip, if the rolling rate 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 rate is less than 22%, a 0.2% proof stress of 650 N / mm ² or more cannot be obtained, and if the rolling rate exceeds 32%, the bendability is significantly lowered.
When martensite is present in the processed ferrite, the 0.2% proof stress is increased. However, when the volume ratio of martensite exceeds 5%, the bending workability is remarkably deteriorated. A ferrite structure was used. The processed ferrite structure of the present invention allows mixing of 5% by volume or less of martensite.
[0020]
Below, the alloy component, content, etc. which are contained in the steel plate for cathode-ray tube frames of the present invention are explained.
C:.. 0 03~0 08 mass%
C is contained for the purpose of solid solution strengthening and precipitation hardening. In order to make it act effectively, 0.03% or more is necessary at least. However, if the content exceeds 0.08%, coarse carbides are easily generated, and bending workability is lowered.
Si:.. 0 2~1 0 wt%
Si is an alloy component added as a deoxidizer in the steelmaking stage, and is effective in improving the strength of the steel material. The effect of addition becomes remarkable with 0.2 mass% or more of Si. It is also effective for generating ferrite. However, if the content exceeds 1.0%, the bending workability decreases.
[0021]
Mn:. 0 1~1.0 mass%
Mn is an alloy component added as a deoxidizer in the steelmaking stage, and is effective in improving the strength of steel materials. However, since it is an austenite forming element, if it is contained in a large amount, it becomes easy to form martensite, so the upper limit was made 1.0%.
P:. 0 04 mass% or less P is an effective ingredient in solid solution strengthening, it decreases cold workability when the content exceeds 0.04%.
[0022]
S:. 0 03 wt% or less S is detrimental element to produce such adverse MnS-based inclusions in workability, the content is preferably as small. If it exceeds 0.03%, the hot workability is particularly deteriorated.
N: 0.04% by mass or less N is effective for solid solution strengthening in the same manner as C, but is an element that easily generates austenite, so the upper limit was made 0.04%.
[0023]
Cr: 10.0-18.0 mass%
In order to improve the corrosion resistance and make the average coefficient of thermal expansion from 30 to 650 ° C. 12.0 × 10 ⁻⁶ / ° C. or less, at least 10.0% or more is necessary. However, if it exceeds 18.0%, a black oxide scale is hardly generated during the blackening treatment.
Ti: 0.05% by mass or less Ti has an extremely strong affinity for C. Therefore, when Ti is contained, the C solid solution amount after finish annealing decreases. Therefore, mixing of Ti should be avoided as much as possible, and at most 0.05% or less, preferably 0.01% or less.
Nb: 0.02 mass% or less Nb has a very strong affinity for C as well as Ti . Therefore , when Nb is contained, the amount of C solid solution after finish annealing decreases. Therefore, Nb contamination should be avoided as much as possible, and it should be 0.02% or less, preferably 0.01% or less at most.
[0024]
The soft annealing conditions after hot rolling, the finish annealing conditions and the cooling conditions after annealing are set as described in the description of the preliminary experiment. To get. If this condition is not satisfied, the C solid solution state and the carbide dispersion state are insufficient and desired physical properties cannot be obtained, as shown in a comparative example described later. Further, the rolling rate during finish rolling greatly affects the mechanical properties and workability of the steel sheet.
The reason for setting these conditions will be described below.
Annealing conditions of hot-rolled steel strip: 750 to 850 ° C. × 1 hour or more and less than 750 ° C., the martensite structure generated during hot rolling cannot be recrystallized. Sometimes a martensite phase occurs. In the annealing of a hot-rolled steel strip, it is necessary to heat it by box-type annealing for 1 hour or longer and gradually cool it to make it as soft as possible.
[0025]
Finish annealing conditions: 750 to 850 ° C. × continuous annealing In finish annealing before 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. In this annealing, recrystallization of the processed structure and a part of C are dissolved, so that heating for a short time (soaking 0 to 10 minutes) is sufficient, and continuous annealing can be applied.
Cooling conditions and finish rolling rate after annealing In order to suppress precipitation of solute C during cooling after annealing, it is necessary to regulate the cooling rate after annealing, and as described above, 200 ° C after annealing. The cooling rate is required to be 40 ° C./min or more.
Further, in the ferritic steel of the present invention, as described above, if the rolling rate is less than 22%, the 0.2% proof stress cannot be obtained 650 N / mm ² or more, and if it exceeds 32%, the bendability is increased. It drops significantly. For this reason, the rolling rate at the time of finish rolling needs to be 22 to 32%.
[0026]
【Example】
Steel materials having the components shown in Table 3 were melted and slabs were produced by continuous casting. The slab was heated to 1200 ° C. and then hot-rolled to obtain a 5 mm thick hot rolled coil. Next, after heating in a box-type annealing furnace at 820 ° C. for 7 hours, cooling in the furnace, pickling, intermediate annealing to 2.6 to 2.8 mm pressure, and finishing and annealing in the range of 740 to 910 ° C. Finish rolling was performed at a rolling rate within a range of 11 to 36%, and a cold-rolled coil having a plate thickness of 2.0 mm was produced. The average cooling rate up to 200 ° C. after finish annealing was about 90 ° C./min.
In order to confirm the effects and effects of intermediate cold rolling and finish annealing, as a comparative example, a hot rolled coil having a thickness of 2.8 mm was manufactured by hot rolling using a slab of component C in Table 3, and a box annealing furnace Was heated at 820 ° C. for 7 hours and cooled in the furnace, pickled, and then finish-rolled directly at a rolling rate of 28% to produce a cold-rolled coil having a thickness of 2.0 mm.
In any of the examples, the average cooling rate up to 200 ° C. after annealing in the box-type annealing furnace was 2 ° C./min or less.
By the way, in the steel used for the present Example, since Cr content is all 10.0% or more, all the average thermal expansion coefficients of 30-650 degreeC were 12.0 * 10 < ^-6 > / degrees C or less.
[0027]

[0028]
A sample was taken from the obtained cold-rolled coil, 0.2% proof stress in the C direction (direction perpendicular to the rolling direction) was measured, and the structure was observed and the maximum size of the carbide was measured. The maximum size of the carbide was observed and measured with a scanning electron microscope by exposing the carbide by the same SPEED method as in the preliminary experiment described above.
By the way, the cathode ray tube frame is manufactured from a cold-rolled coil by pressing. At that time, the material is subjected to L direction bending and C direction bending whose bending axis is perpendicular to the rolling direction. Since the bending in the C direction is strict as the bending process, and the direction in which cracking is a concern during the bending process is the C direction bending, the bending test in the C direction was performed.
The bendability was evaluated by performing a 90 ° bend test using a V block with a tip of R1.0 mm in order to match the bending inside R1.0 mm at the time of bending of the frame.
[0029]
Then, after heat treatment at 600 ° C. for 15 minutes assuming blackening or strain relief annealing, each sample was subjected to a creep test. In the creep test, each plate was processed into a tensile test piece having a width of 12.5 mm and a distance between marks of 50 mm, and assuming a blackening or strain relief annealing process with the mask stretched, a load stress of 300 N / After holding at 450 ° C. for 1 hour under mm ² , the creep strain was measured. At that time, in order to obtain a level at which the actual decrease in mask tension does not become a problem, the creep strain needs to be 0.05% or less.
[0030]
The evaluation results are shown in Table 4.
Test No. according to the invention. About 1-9, the crack test did not generate | occur | produce in a bending test and the creep distortion was also 0.05% or less.
In contrast, test no. No. 10 uses steel type E with a low C content, so 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 of 0 Greatly exceeded below 05%. Test No. On the contrary, No. 11 uses steel F with a high C content, so that the carbide becomes coarse and cracks occur in the bending test. Test No. Nos. 12 and 13 use steel types G and H containing a large amount of Ti and Nb, which have a strong affinity with C. Therefore, there is almost no solid solution of C during the final annealing, and as a result, it is improved by work hardening. The 2% proof stress was greatly reduced by the subsequent heat treatment, and although not shown in Table 4, it was below 540 N / mm ² , and the creep strain exceeded the target of 0.05% or less.
[0031]
Test No. In No. 14, the finish annealing temperature was as high as 910 ° C., so some martensite was generated and cracks occurred in the work bending test. Test No. On the other hand, No. 15 was not sufficiently recovered 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 rate was too low and the 0.2% proof stress was low, and as a result, the creep strain greatly exceeded the target of 0.05% or less. Test No. No. 18 was soft-annealed without intermediate annealing and finish annealing, and then finish-rolled, so that carbide was not finely dispersed to 3 μm or less, and C was hardly dissolved. The distortion also exceeded the target of 0.05% or less.
[0032]

[0033]
【The invention's effect】
As described above, the CRT frame steel sheet according to the present invention has a ferrite structure in which carbides are finely dispersed, and therefore has good press bendability with respect to processing into a frame shape. Moreover, since C is dissolved in the ferrite phase, it is excellent in work hardening and creep resistance. For this reason, even if blackening or distortion removal annealing is performed in a state where the mask is stretched, a high-quality CRT that does not deform and does not cause color misregistration or image disturbance can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining the difference in 0.2% proof stress before and after heat treatment depending on the cooling rate after annealing. FIG. 2 is a diagram for explaining the relationship between 0.2% proof stress after heat treatment and creep strain.

Claims (1)

In mass%, C : 0.03-0.08 %, Si: 0.2-1.0%, Mn: 0.1-1.0%, P: 0.04% or less, S: 0.03 %: N: 0.04% or less, Cr: 10.0-18.0%, Ti: 0.05% or less, Nb: 0.02% or less, with the balance being Fe and inevitable impurities A continuous cast slab having a steel composition is hot-rolled, and the obtained hot-rolled steel strip is subjected to heat treatment at 750 to 850 ° C. for 1 hour or more, followed by cold rolling, and the obtained cold-rolled steel strip is obtained from 750 to 850. A steel sheet for a CRT frame obtained by performing continuous annealing at ℃, cooling at an average cooling rate of up to 200 ℃ at a rate of 40 ℃ / min or more, and performing cold rolling at a rolling rate of 22 to 32%. And
An average thermal expansion coefficient having a metal structure in which carbide having a maximum particle size of 3 μm or less is dispersed in a processed ferrite structure in which C is dissolved, and 0.2% proof stress is 650 to 870 N / mm ² and 30 to 650 ° C. Is a steel plate for a cathode ray tube frame, characterized by being 12.0 × 10 ⁻⁶ / ° C. or less.

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