JP3828466B2 - Steel sheet with excellent bending properties - Google Patents
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- JP3828466B2 JP3828466B2 JP2002219662A JP2002219662A JP3828466B2 JP 3828466 B2 JP3828466 B2 JP 3828466B2 JP 2002219662 A JP2002219662 A JP 2002219662A JP 2002219662 A JP2002219662 A JP 2002219662A JP 3828466 B2 JP3828466 B2 JP 3828466B2
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0273—Final recrystallisation annealing
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
- C21D1/20—Isothermal quenching, e.g. bainitic hardening
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/003—Cementite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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Description
【０００１】
【発明の属する技術分野】
本発明は、曲げ特性、特に密着曲げ加工性（極限変形能）に優れた高強度鋼板に関し、殊に６００〜１４００ＭＰａ級の高強度および超高強度域において、優れた曲げ特性を発揮する高強度鋼板に関するものである。
【０００２】
【従来の技術】
自動車や産業用機械等にプレス成形して使用される鋼板は、優れた強度と延性を兼ね備えていることが要求され、この様な要求特性は近年、益々、高まっている。
【０００３】
従来より、強度と延性の両立を図った鋼板として、フェライト素地中に主としてマルテンサイトからなる低温変態組織を含むフェライト・マルテンサイトの複合組織鋼板［デュアルフェイズ（ＤＰ）鋼板］が知られている（例えば、特開昭５５−１２２８２０号等）。上記鋼板は、延性が良好なだけでなく、マルテンサイト生成域に導入された多量の自由転位のために降伏伸びが現れず、降伏応力が低くなる為、加工時の形状凍結特性が良好である。上記組織に制御することにより、引張強度（ＴＳ）が高く、伸び（Ｅｌ）特性にも優れた鋼板が得られている。
【０００４】
一方、組織中に残留オーステナイト（以下、「残量γ」と記すことがある）を生成させ、加工変形中に残留γが誘起変態（歪み誘起変態：ＴＲＩＰ）して延性を向上させる残留γ鋼板も知られている。例えば特開昭６０−４３４２５号には、複合組織鋼板としての組織を、体積分率で１０％以上のフェライトと１０％以上の残留γを有し、残部がベイナイトまたはマルテンサイト若しくはそれらの混合組織に制御することにより、高強度で、且つ極めて延性に優れた鋼板が開示されている。上記組織とすることにより、残留γの加工誘起変態効果に加えて、軟質のフェライトによる高延性が発揮される結果、延性はフェライトおよび残留γによって、強度はベイナイトまたはマルテンサイトによって確保される旨記載されている。
【０００５】
上記いずれの鋼板においても、伸び特性（特に均一伸び）に優れており、とりわけＴＲＩＰ鋼板では残留γを含むことによって非常に伸びが高く、成形性（張出し性や絞り成形性）が良好であるという特徴を有するものである。しかしながらこれらの鋼板では、いずれも局部変形特性（曲げ加工性や穴拡げ性）や極限変形特性（密着曲げ加工性）等が固溶強化鋼と比べて一般的に悪いということが知られている。特に、自動車用鋼板の様にプレス成形して使用される鋼板においては、曲げ特性（曲げ加工性および密着曲げ加工性）が良好であることは必要不可欠な要件であるが、これまで開発されている鋼板では曲げ特性、特に密着曲げ加工性が良好であるものは実現されていないのが実状である。
【０００６】
【発明が解決しようとする課題】
本発明は上記事情に着目してなされたものであり、その目的は、６００〜１４００ＭＰａ級の高強度および超高強度域において、優れた曲げ特性（曲げ加工性および密着曲げ加工性）を発揮し、自動車用鋼板として最適な高強度鋼板を提供することにある。
【０００７】
【課題を解決するための手段】
上記課題を解決し得た本発明の鋼板とは、残留オーステナイト：５〜３０面積％およびフェライト：５０面積％以上を含有する組織を主体とすると共に、残留オーステナイトとフェライトの界面に存在する炭化物の個数が２０００（μｍ）2当たり４０個以下である点に要旨を有するものである。
【０００８】
本発明の鋼板における基本成分としては、Ｃ：０．０６〜０．２５質量％未満、Ｓｉ＋Ａｌ：０．５〜３質量％、Ｍｎ：０．５〜３質量％、Ｐ：０．１５質量％以下（０質量％を含まない）、Ｓ：０．０２質量％以下（０質量％を含まない）を夫々含有し、残部鉄および不可避的不純物であるものが挙げられる。
【０００９】
また、本発明の鋼板には上記基本成分の他、必要によって（ａ）Ｍｏ：１質量％以下（０質量％を含まない），Ｎｉ：０．５質量％以下（０質量％を含まない）およびＣｕ：０．５質量％以下（０質量％を含まない）の少なくとも１種、（ｂ）Ｃａ：０．００３質量％以下（０質量％を含まない）等を含有させることも有効であり、含有させる成分に応じて鋼板の特性が更に改善される。
【００１０】
【発明の実施の形態】
本発明者らは、良好な曲げ特性を発揮する鋼板を実現するべく様々な角度から検討してきた。これまで開発されてきたＤＰ鋼板やＴＲＩＰ鋼板では、母相と第２相の強度比が高くなっており、母相と第２相の界面（母相側）に変形が集中することによって、曲げ特性が悪くなると考えられる。
【００１１】
そして本発明者らが更に検討したところによれば、ＴＲＩＰ鋼板における極限変形特性（密着曲げ加工性）には、母相と第２相の強度比ばかりでなく、塑性変形しない（第２相よりも硬い）炭化物（セメンタイト）の存在が影響していることを突き止めた。この炭化物は残留γとフェライトの界面に存在するのであるが、こうした炭化物を極力低減することによって、優れた曲げ特性を発揮する鋼板（ＴＲＩＰ鋼板）が実現できることを見出し、本発明を完成した。
【００１２】
上記の如く本発明の鋼板は、炭化物（即ち残留オーステナイトとフェライトの界面に存在する炭化物）の個数をできるだけ低減したものであるが、具体的にはこの炭化物の個数は２０００（μｍ）2当たり４０個以下とする必要がある。この炭化物の個数が４０個を超えると、曲げ特性（特に密着曲げ加工性）が劣化することになる。尚、この炭化物は、より好ましくは３０個以下に低減するのが良い。
【００１３】
本発明の鋼板は、その組織も適切に制御する必要がある。即ち、本発明の鋼板では、高い強度と良好な伸びを確保する観点から、その組織を残留γ：５〜３０面積％およびフェライト：５０面積％以上を含む組織を主体とするものであるが、こうした組織を構成する各相の範囲限定理由は次の通りである。
【００１４】
残留γ：５〜３０面積％
残留γは全伸びの向上に有用であり、この様な作用を有効に発揮させる為には、５面積％（好ましくは８面積％以上）存在することが必要である。一方、３０面積％を超えて存在すると、逆に曲げ加工性が劣化するので、その上限を３０面積％に定めた。より好ましくは２０面積％以下である。
【００１５】
フェライト：５０面積％以上
本発明におけるフェライトは、鋼板における良好な延性を確保するた為に、その面積率（占積率）は少なくとも５０面積％以上とする必要がある。
【００１６】
本発明の鋼板では、上記の残留γとフェライトを含む組織を主体（例えば、７０面積％以上）とするものであるが、本発明の作用を損なわない範囲で、他の異種組織として、ベイナイトやマルテンサイトを一部有していても良い。これらの組織は本発明の製造過程で必然的に残存し得るものであるが、マルテンサイトは少なければ少ない程、好ましい。
【００１７】
従来のＴＲＩＰ鋼板の熱処理（熱間圧延および冷間圧延後の熱処理）においては、Ａ1点以上Ａ3点以下の温度で６０〜１８０秒程度加熱保持した後、１０℃／ｓ以上の平均冷却速度でベイナイト変態域（例えば４００±５０℃程度）まで冷却し、その温度域で長時間（例えば３００秒程度）保持することによって、γ相へのＣ濃化を促進してγ相を安定化させ、所定量の残留γを確保するようにしている。しかしながら、こうした方法では残留γ相の内部と外部のＣ濃度勾配が大きくなってセメンタイト（炭化物）が生成し易くなって、曲げ特性が劣化することになる。
【００１８】
炭化物の形態を制御して本発明鋼板を製造するには、例えば次の方法を実施すればよい。即ち、上記のような熱処理に先立ち、フェライト変態域（例えば７００±３０℃程度）で所定時間保持することによって（即ち、２段階の熱処理を行うことによって）、残留γ相の内部と外部のＣ濃度勾配を小さくし、残留γとフェライトの界面での炭化物の生成を抑制することが有効である。但し、上記のフェライト変態域はパーライト変態域と重なることになるので、その温度で余り長時間加熱保持するとパーライト組織が析出することによる特性劣化が生じることになる。従って、この温度での保持時間は上記効果が生じる範囲内で適切に調整する必要があり、好ましくは１０〜３０秒程度である。またフェライト変態域では、γ相へのＣ濃化が迅速に進行するのであまり長時間保持する必要はなので、こうした熱処理は熱間圧延に引き続いて行う焼鈍工程（連続焼鈍工程）の一環として行うことができる。
【００１９】
尚、こうした熱処理に先立って行う熱延工程および冷延工程は、特に限定されず、通常実施されている条件を適宜選択して採用することができる。また、各熱処理温度からの冷却速度は、適宜調整すれば良いが、例えばフェライト変態域で所定時間保持した後、ベイナイト変態域まで冷却する際の平均冷却速度はセメンタイトの生成を防止するという観点から、１０℃／ｓ以上であることが好ましい。
【００２０】
本発明の残留γ鋼板は、上記のように組織および炭化物の存在個数を適切に制御することによって、本発明の目的を達成することができ、鋼板の化学成分組成については特に限定するものではないが、Ｃ，Ｓｉ，Ａｌ，Ｍｎ，Ｐ，Ｓ等の基本成分については下記の様に成分範囲を制御することが好ましい。
【００２１】
Ｃ：０．０６〜０．２５質量％未満
Ｃは、高強度を確保し、且つ、残留γを確保するために必須の元素である。詳細には、γ相中に充分なＣ量を含み、室温でも所望のγ相を残留させる為に重要な元素である。こうした効果を発揮させるためには、０．０６質量％以上含有させることが好ましいが、０．２５質量％以上含有させると溶接性が劣化する。
【００２２】
Ｓｉ＋Ａｌ：０．５〜３質量％
ＳｉおよびＡｌは、残留γが分解して炭化物が生成するのを有効に抑える元素である。特にＳｉは、固溶強化元素としても有用である。この様な作用を有効に発揮させる為には、ＳｉおよびＡｌを合計で０．５実量％以上含有させることが好ましい。好ましくは０．７質量％以上、より好ましくは質量１％以上とするのが良い。但し、上記元素を合計で、３質量％を超えて含有させても上記効果は飽和してしまい、経済的に無駄である他、多量に添加すると、熱間脆性を起こす為、その上限を３質量％とする。より好ましくは２．５実量％以下、更に好ましくは２実量％以下とするのが良い。
【００２３】
Ｍｎ：０．５〜３質量％
Ｍｎは、γを安定化し、所望の残留γを得る為に必要な元素である。この様な作用を有効に発揮させる為には、０．５質量％以上含有させるのが良い。より好ましくは０．７質量％以上、更に好ましくは１質量％以上とするのが良い。但し、３質量％を超えて含有させるとと、鋳片割れが生じる等の悪影響が見られる。より好ましくは２．５質量％以下、更に好ましくは２質量％以下とするのが良い。
【００２４】
Ｐ：０．１５質量％以下（０質量％を含まない）
Ｐは、所望の残留γを確保するのに有効な元素である。この様な作用を有効に発揮させる為には、０．０３質量％以上（より好ましくは０．０５実量％以上）含有することが推奨される。但し、０．１５質量％を超えて含有させると二次加工性が劣化する。より好ましくは０．１質量％以下とするのが良い。
【００２５】
Ｓ：０．０２質量％以下（０質量％を含まない）
Ｓは、ＭｎＳ等の硫化物系介在物を形成し、割れの起点となって加工性を劣化させる元素であるので、できるだけ抑制するのが良く、こうした観点から０．０２質量％以下とするのが良い。より好ましくは０．０１５質量％以下である。
【００２６】
本発明の鋼板には、上記基本成分の他必要によってＭｏ，Ｎｉ，Ｃｕ，Ｃａおよび希土類元素等を含有することも有効であり、含有される元素の種類に応じて鋼板の特性が更に改善される。必要によって含有される各元素の範囲限定理由は下記の通りである。
【００２７】
Ｍｏ：１質量％以下（０質量％を含まない），Ｎｉ：０．５質量％以下（０質量％を含まない）およびＣｕ：０．５質量％以下（０質量％を含まない）の少なくとも一種
これらの元素は、鋼の強化元素として有用であると共に、残留γの安定化や所定量の確保に有効な元素である。この様な作用を有効に発揮させる為には、Ｍｏ：０．０５質量％以上（より好ましくは０．１質量％以上）、Ｎｉ：０．０５質量％以上（より好ましくは０．１質量％以上）、Ｃｕ：０．０５質量％以上（より好ましくは０．１質量％以上）、Ｃｒ：０．０５質量％以上（より好ましくは０．１質量％以上）を、夫々含有させることが推奨される。但し、Ｍｏは１質量％、ＮｉおよびＣｕは０．５質量％を超えて含有させても上記効果が飽和してしまい、経済的に無駄である。より好ましくはＭｏ：０．８質量％以下、Ｎｉ：０．４質量％以下、Ｃｕ：０．４％質量以下、Ｃｒ：０．８％質量以下である。
【００２８】
Ｃａ：０．００３質量％以下（０質量％を含まない）
Ｃａは、鋼中硫化物の形態を制御し、加工性向上に有効な元素である。ここで、本発明に、こうした効果を発揮させる為には、Ｃａは０．０００３質量％以上（より好ましくは０．０００５質量％以上）含有させることが推奨される。但し、０．００３質量％を超えて添加しても上記効果が飽和してしまい、経済的に無駄である。より好ましくは０．００２５質量％以下である。
【００２９】
本発明の鋼板においては、上記成分の他（残部）基本的には鉄からなるものであるが、これら以外にも微量成分（例えば、Ｔｉ，Ｎｂ，Ｖ等）を含み得るものであり、こうした成分を含むものも本発明の技術的範囲に含まれるものである。また本発明の鋼板には、不可避的に不純物（例えば、Ｚｒ，Ｂ等）が含まれることになるが、それらは本発明の効果を損なわない限度（０．００１質量％以下）で許容される。
【００３０】
以下本発明を実施例によって更に具体的に示すが、下記実施例は本発明を制限するものではなく、前・後記の趣旨を逸脱しない範囲で変更実施することは全て本発明の技術的範囲に含まれるものである。
【００３１】
【実施例】
［実施例１］
下記表１に示す化学成分組成からなる供試鋼を真空溶製してスラブとした後に、熱間圧延および連続焼鈍を行い、板厚：１．２ｍｍの鋼板を得た。具体的には、１３００℃に加熱したスラブを、Ａｒ３変態点以上の温度である約９００℃で熱間圧延を終了し（仕上温度：ＦＤＴ）、約４５０℃で巻き取り処理した。こうして得られた熱延鋼板（厚さ：２〜３ｍｍ）を冷間圧延により板厚：１．２ｍｍに圧延した後、下記各種パターンの熱処理（連続焼鈍）を施して鋼板とした。
【００３２】
（熱処理パターン）
Ｎｏ．１〜１０
８５０℃（Ａ1変態点以上Ａ3点以下の温度）で１２０秒加熱保持（焼鈍）した後、平均冷却速度：５℃／ｓで７００℃まで冷却して１５秒保持し、次いで平均冷却速度１５℃／ｓで４２０℃まで冷却して１５秒保持（オーステンパ処理）し、空冷（平均冷却速度５℃／ｓ）で室温まで冷却した。
【００３３】
Ｎｏ．１１
８５０℃（Ａ1変態点以上Ａ3点以下の温度）で１２０秒加熱保持（焼鈍）した後、平均冷却速度：５℃／ｓで７００℃まで冷却して６０秒保持し、次いで平均冷却速度１５℃／ｓで４２０℃まで冷却して１５秒保持（オーステンパ処理）し、空冷（平均冷却速度５℃／ｓ）で室温まで冷却した。
【００３４】
Ｎｏ．１２
８５０℃（Ａ1変態点以上Ａ3点以下の温度）で１２０秒加熱保持（焼鈍）した後、平均冷却速度：１５℃／ｓで４２０℃まで冷却して１５秒保持（オーステンパ処理）し、空冷（平均冷却速度５℃／ｓ）で室温まで冷却した。
【００３５】
Ｎｏ．１３
８５０℃（Ａ1変態点以上Ａ3点以下の温度）で１２０秒加熱保持（焼鈍）した後、平均冷却速度：１５℃／ｓで４２０℃まで冷却して２００秒保持（オーステンパ処理）した後、空冷（平均冷却速度５℃／ｓ）で室温まで冷却した。
【００３６】
【表１】
この様にして得られた鋼板について、２０００（μｍ）2当たりの炭化物の個数、引張強度（ＴＳ）、伸び［全伸び（ＥＩ）］、各組織の面積率（占積率）および曲げ特性（完全密着曲げ加工性Ｒ０および曲げ加工性Ｒ１）を、下記要領で夫々測定した。これらの結果を、下記表２に示す。
【００３８】
［炭化物の個数］
（５％過塩素酸＋酢酸）溶液で電解研磨（６０Ｖ−０．５Ａ）した後、１０％アセチルアセトン＋９０％メタノール溶液に１ｇのテトラメチルアンモニウムクロライドを加えた溶液でエッチング（２Ｖ−２０ｍＡ，２ｍｉｎ）し、カーボン蒸着剥離して作製した抽出レプリカを、倍率７５００倍の透過型電子顕微鏡（ＴＥＭ）で任意の３箇所を写真撮影し（１箇所当たり４０μｍ×１７μｍ）、その合計［２０００（μｍ）2当たり］で残留γとフェライトの界面に存在する炭化物個数を計算した。
【００３９】
［引張強度（ＴＳ）、伸び（Ｅｌ）］
引張試験はＪＩＳ５号試験片を用い引張試験を行い、引張強度（ＴＳ）および伸び（Ｅｌ）を測定した。
【００４０】
［各組織の面積率］
上記鋼板中の組織を、レペラー腐食による光学顕微鏡観察および透過型電子顕微鏡（ＴＥＭ）観察を行うと共に、写真による面積率、更に残留γについては、Ｘ線測定によって面積率を測定した（ISIJ Int.Vol.33.(1933),No.7,P.776）。
【００４１】
［曲げ特性］
上記鋼板から試験片（幅：４０ｍｍ×長さ：１００ｍｍ×厚さ：１．２ｍｍ）を切り出し、完全密着曲げＲ０と１ｍｍの鋼板を挟んでの曲げＲ１（いずれも１８０°曲げ）を行い、割れの有無（有：「×」、無：「○」）によって曲げ特性を評価した。
【００４２】
【表２】
これらの結果より、以下の様に考察することができる。まず、Ｎｏ．２〜５、７〜１０はいずれも、本発明で規定する要件を満足しているので、曲げ特性の良好な高強度鋼板が得られている。参考までに、図１に、本発明鋼板（Ｎｏ．４）のＴＥＭ写真（倍率：７５００倍）を示す。この写真より、本発明鋼板は、残留γとフェライトの界面に存在する炭化物が少なくなっていることが分かる。
【００４４】
これに対し、Ｎｏ．１，６，１１〜１３のものは、本発明で規定する要件（または好ましい要件）のいずれかを満足しない例であり、夫々以下の不具合を有している。まず、Ｎｏ．１はＣ量が少ない例であり、強度が低くなっている。またＮｏ．６のものでは、Ｍｎ量および（Ｓｉ＋Ａｌ）の合計量が少ない例であり、所望の残留γが得られず、またパーライト組織が生じているので、強度およびＥｌが低くなっていると共に、曲げ特性も劣化している。
【００４５】
一方、Ｎｏ．１１のものでは、熱処理の際に７００℃での保持時間が長くなり、パーライト組織が多くなって所定の残留γが得られない為、Ｅｌが低くなっていると共に、曲げ特性も劣化している。また、Ｎｏ．１２のものでは７００℃における保持時間が短い（７００℃で保持していない）ので、炭化物の個数が多くなって、曲げ特性が劣化している。更に、Ｎｏ．１３のものでは、７００℃で保持せずしかも４００℃における保持時間が長いので、Ｃ濃度の高い安定な残留γが生成することによって曲げ加工性は良好であるが、炭化物の個数が多くなって特に密着曲げ加工性Ｒ０が劣化している。参考までに、図２に、Ｎｏ．１３で得られた鋼板のＴＥＭ写真（倍率：７５００倍）を示す。この写真より、従来の鋼板では、残留γとフェライトの界面に存在する炭化物が多いことが分かる。
【００４６】
【発明の効果】
本発明は上記の様に構成されているので、６００〜１４００ＭＰａ級の高強度および超高強度域において、優れた曲げ特性を発揮する高強度鋼板が実現でき、こうした鋼板は自動車用鋼板として最適である。
【図面の簡単な説明】
【図１】本発明鋼板（Ｎｏ．４）のＴＥＭ写真である。
【図２】従来鋼板（Ｎｏ．１３）のＴＥＭ写真である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel sheet having excellent bending characteristics, particularly close-contact bending workability (extreme deformability), and particularly high strength exhibiting excellent bending characteristics in a high strength and ultrahigh strength range of 600 to 1400 MPa class. It relates to steel plates.
[0002]
[Prior art]
Steel sheets used for press forming in automobiles, industrial machines and the like are required to have both excellent strength and ductility, and such required characteristics have been increasingly increased in recent years.
[0003]
Conventionally, a ferrite-martensitic composite steel sheet [dual phase (DP) steel sheet] including a low-temperature transformation structure mainly composed of martensite in a ferrite base is known as a steel sheet that achieves both strength and ductility ( For example, Japanese Patent Laid-Open No. 55-122820). The steel sheet not only has good ductility, but also yield elongation does not appear due to a large amount of free dislocations introduced into the martensite formation region, yield stress is low, and shape freezing characteristics during processing are good. . By controlling to the above structure, a steel sheet having high tensile strength (TS) and excellent elongation (El) characteristics is obtained.
[0004]
On the other hand, residual austenite (hereinafter sometimes referred to as “residual amount γ”) is generated in the structure, and residual γ is induced transformation (strain-induced transformation: TRIP) during work deformation to improve ductility. Is also known. For example, in Japanese Patent Laid-Open No. 60-43425, a structure as a composite structure steel sheet has a volume fraction of ferrite of 10% or more and a residual γ of 10% or more, and the balance is bainite, martensite or a mixed structure thereof. A steel sheet having a high strength and extremely excellent ductility is disclosed by controlling to. As a result of the above structure, in addition to the processing-induced transformation effect of residual γ, high ductility due to soft ferrite is exhibited, and as a result, ductility is ensured by ferrite and residual γ, and strength is secured by bainite or martensite. Has been.
[0005]
In any of the above steel plates, the elongation properties (particularly uniform elongation) are excellent, and in particular, the TRIP steel plate has a very high elongation by containing residual γ, and the formability (extrusion property and drawability) is good. It has characteristics. However, it is known that these steel plates generally have poor local deformation characteristics (bending workability and hole expandability), extreme deformation characteristics (adhesion bending workability), etc. compared to solid solution strengthened steel. . In particular, in steel sheets used for press forming, such as automotive steel sheets, good bending characteristics (bending workability and adhesion bending workability) are indispensable requirements. In fact, the steel sheets that have good bending characteristics, particularly good adhesion bending workability, have not been realized.
[0006]
[Problems to be solved by the invention]
The present invention has been made paying attention to the above circumstances, and its purpose is to exhibit excellent bending characteristics (bending workability and adhesion bending workability) in a high strength and ultrahigh strength range of 600 to 1400 MPa class. An object of the present invention is to provide an optimum high-strength steel plate as a steel plate for automobiles.
[0007]
[Means for Solving the Problems]
The steel sheet of the present invention that has solved the above problems is mainly composed of a structure containing residual austenite: 5 to 30 area% and ferrite: 50 area% or more, and is a carbide present at the interface between residual austenite and ferrite. The gist is that the number is 40 or less per 2000 (μm) 2 .
[0008]
As basic components in the steel sheet of the present invention, C: 0.06 to less than 0.25% by mass, Si + Al: 0.5 to 3% by mass, Mn: 0.5 to 3% by mass, P: 0.15% by mass The following are included (excluding 0% by mass) and S: 0.02% by mass or less (not including 0% by mass), respectively, and the remainder is iron and inevitable impurities.
[0009]
In addition to the basic components described above, the steel plate of the present invention, if necessary, (a) Mo: 1% by mass or less (not including 0% by mass), Ni: 0.5% by mass or less (not including 0% by mass) And Cu: 0.5% by mass or less (not including 0% by mass), (b) Ca: 0.003% by mass or less (not including 0% by mass), etc. are also effective. The properties of the steel sheet are further improved depending on the components to be contained.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have studied from various angles in order to realize a steel sheet that exhibits good bending characteristics. In DP steel plates and TRIP steel plates that have been developed so far, the strength ratio between the parent phase and the second phase is high, and the deformation concentrates on the interface between the parent phase and the second phase (the parent phase side), so that bending occurs. It is thought that the characteristics deteriorate.
[0011]
According to further investigation by the present inventors, not only the strength ratio of the parent phase and the second phase but also the plastic deformation (from the second phase) is not limited to the ultimate deformation characteristics (adhesion bending workability) of the TRIP steel sheet. (Hard) was found to be affected by the presence of carbide (cementite). This carbide is present at the interface between residual γ and ferrite, but by reducing such carbide as much as possible, it has been found that a steel plate (TRIP steel plate) exhibiting excellent bending characteristics can be realized, and the present invention has been completed.
[0012]
As described above, the steel sheet of the present invention has a reduced number of carbides (that is, carbides present at the interface between retained austenite and ferrite) as much as possible. Specifically, the number of carbides is 40 per 2000 (μm) 2. Must be less than If the number of carbides exceeds 40, bending characteristics (particularly adhesion bending workability) will deteriorate. This carbide is preferably reduced to 30 or less.
[0013]
The steel sheet of the present invention needs to be appropriately controlled in its structure. That is, in the steel sheet of the present invention, from the viewpoint of ensuring high strength and good elongation, the structure is mainly composed of a structure containing residual γ: 5 to 30 area% and ferrite: 50 area% or more. The reasons for limiting the range of each phase constituting such a structure are as follows.
[0014]
Residual γ: 5-30 area%
Residual γ is useful for improving the total elongation, and it is necessary that 5% by area (preferably 8% by area or more) be present in order to effectively exhibit such an action. On the other hand, if it exceeds 30 area%, the bending workability deteriorates conversely, so the upper limit was set to 30 area%. More preferably, it is 20 area% or less.
[0015]
Ferrite: 50 area% or more The ferrite in the present invention needs to have an area ratio (space factor) of at least 50 area% or more in order to ensure good ductility in the steel sheet.
[0016]
In the steel sheet of the present invention, the structure containing the above residual γ and ferrite is the main component (for example, 70 area% or more), but as long as the other different structures are not impaired, bainite, You may have some martensite. Although these structures can inevitably remain in the production process of the present invention, the less martensite, the better.
[0017]
In the conventional heat treatment of TRIP steel sheet (heat treatment after hot rolling and cold rolling), an average cooling of 10 ° C./s or more is performed after heating and holding at a temperature of A 1 point or more and A 3 point or less for about 60 to 180 seconds. By cooling to the bainite transformation region (eg, about 400 ± 50 ° C) at a high speed and maintaining the temperature range for a long time (eg, about 300 seconds), C concentration to the γ phase is promoted to stabilize the γ phase. And a predetermined amount of residual γ is secured. However, in such a method, the C concentration gradient inside and outside the residual γ phase becomes large and cementite (carbide) is easily generated, and the bending characteristics are deteriorated.
[0018]
In order to manufacture the steel sheet of the present invention by controlling the form of carbide, for example, the following method may be carried out. That is, prior to the heat treatment as described above, by holding the ferrite transformation region (for example, about 700 ± 30 ° C.) for a predetermined time (that is, by performing a two-step heat treatment), the residual γ-phase inside and outside C It is effective to reduce the concentration gradient and suppress the formation of carbides at the interface between residual γ and ferrite. However, since the ferrite transformation region is overlapped with the pearlite transformation region, if it is heated and held at that temperature for an excessively long time, characteristic deterioration due to precipitation of the pearlite structure occurs. Therefore, the holding time at this temperature needs to be appropriately adjusted within the range where the above-described effect occurs, and is preferably about 10 to 30 seconds. Also, in the ferrite transformation region, C concentration into the γ phase proceeds rapidly, so it is necessary to keep it for a long time, so such heat treatment should be performed as part of the annealing process (continuous annealing process) that follows hot rolling. Can do.
[0019]
In addition, the hot rolling process and the cold rolling process performed prior to the heat treatment are not particularly limited, and the conditions that are usually performed can be appropriately selected and employed. In addition, the cooling rate from each heat treatment temperature may be adjusted as appropriate.For example, the average cooling rate when cooling to the bainite transformation region after holding for a predetermined time in the ferrite transformation region prevents the formation of cementite. It is preferable that it is 10 degrees C / s or more.
[0020]
The residual γ steel sheet of the present invention can achieve the object of the present invention by appropriately controlling the structure and the number of carbides as described above, and the chemical composition of the steel sheet is not particularly limited. However, for basic components such as C, Si, Al, Mn, P, and S, it is preferable to control the component ranges as described below.
[0021]
C: Less than 0.06 to 0.25% by mass C is an essential element for securing high strength and securing residual γ. Specifically, it is an important element for containing a sufficient amount of C in the γ phase and for leaving the desired γ phase even at room temperature. In order to exhibit such an effect, it is preferable to contain 0.06 mass% or more, but if it contains 0.25 mass% or more, weldability deteriorates.
[0022]
Si + Al: 0.5-3 mass%
Si and Al are elements that effectively suppress the decomposition of residual γ and the formation of carbides. In particular, Si is useful as a solid solution strengthening element. In order to effectively exhibit such an action, it is preferable to contain 0.5% by mass or more of Si and Al in total. The content is preferably 0.7% by mass or more, more preferably 1% by mass or more. However, even if the total amount of the above elements exceeds 3% by mass, the above effect is saturated, which is economically wasteful, and when added in a large amount causes hot brittleness. Mass%. More preferably, it is 2.5% or less by actual amount, and further preferably 2% or less by actual amount.
[0023]
Mn: 0.5-3 mass%
Mn is an element necessary for stabilizing γ and obtaining a desired residual γ. In order to exhibit such an action effectively, it is preferable to contain 0.5% by mass or more. More preferably, it is 0.7 mass% or more, More preferably, it is good to set it as 1 mass% or more. However, if the content exceeds 3% by mass, adverse effects such as slab cracking are observed. More preferably, it is 2.5 mass% or less, More preferably, it is good to set it as 2 mass% or less.
[0024]
P: 0.15% by mass or less (excluding 0% by mass)
P is an element effective for securing a desired residual γ. In order to effectively exhibit such an action, it is recommended to contain 0.03% by mass or more (more preferably 0.05% by mass or more). However, if it exceeds 0.15 mass%, the secondary workability deteriorates. More preferably, it is good to set it as 0.1 mass% or less.
[0025]
S: 0.02% by mass or less (excluding 0% by mass)
S is an element that forms sulfide inclusions such as MnS and becomes a starting point of cracks and deteriorates workability. Therefore, S should be suppressed as much as possible, and from this point of view, it should be 0.02% by mass or less. Is good. More preferably, it is 0.015 mass% or less.
[0026]
It is also effective for the steel sheet of the present invention to contain Mo, Ni, Cu, Ca and rare earth elements as necessary in addition to the basic components described above, and the characteristics of the steel sheet are further improved according to the type of element contained. The The reasons for limiting the range of each element contained as necessary are as follows.
[0027]
Mo: 1% by mass or less (not including 0% by mass), Ni: 0.5% by mass or less (not including 0% by mass) and Cu: 0.5% by mass or less (not including 0% by mass) Type 1 These elements are useful elements for strengthening steel and are effective in stabilizing residual γ and securing a predetermined amount. In order to effectively exhibit such an action, Mo: 0.05% by mass or more (more preferably 0.1% by mass or more), Ni: 0.05% by mass or more (more preferably 0.1% by mass) Above), Cu: 0.05% by mass or more (more preferably 0.1% by mass or more), and Cr: 0.05% by mass or more (more preferably 0.1% by mass or more) are recommended. Is done. However, even if Mo is contained in an amount of more than 1% by mass and Ni and Cu are contained in an amount exceeding 0.5% by mass, the above effect is saturated, which is economically wasteful. More preferably, Mo: 0.8% by mass or less, Ni: 0.4% by mass or less, Cu: 0.4% by mass or less, and Cr: 0.8% by mass or less.
[0028]
Ca: 0.003 mass% or less (excluding 0 mass%)
Ca is an element that controls the form of sulfide in steel and is effective in improving workability. Here, in order to exert such effects in the present invention, it is recommended that Ca is contained in an amount of 0.0003 mass% or more (more preferably 0.0005 mass% or more). However, even if added over 0.003% by mass, the above effect is saturated, which is economically useless. More preferably, it is 0.0025 mass% or less.
[0029]
In the steel sheet of the present invention, in addition to the above components (remainder) basically consists of iron, but in addition to these, trace components (for example, Ti, Nb, V, etc.) may be included. Those containing components are also included in the technical scope of the present invention. In addition, the steel sheet of the present invention inevitably contains impurities (for example, Zr, B, etc.), but these are allowed as long as the effects of the present invention are not impaired (0.001% by mass or less). .
[0030]
Hereinafter, the present invention will be described more specifically by way of examples. However, the following examples are not intended to limit the present invention, and all modifications may be made within the technical scope of the present invention without departing from the spirit described above and below. It is included.
[0031]
【Example】
[Example 1]
A test steel having the chemical composition shown in Table 1 below was vacuum-melted into a slab, followed by hot rolling and continuous annealing to obtain a steel sheet having a thickness of 1.2 mm. Specifically, the slab heated to 1300 ° C. was hot-rolled at a temperature equal to or higher than the Ar 3 transformation point at about 900 ° C. (finishing temperature: FDT) and wound at about 450 ° C. The hot-rolled steel sheet (thickness: 2 to 3 mm) thus obtained was rolled to a thickness of 1.2 mm by cold rolling, and then subjected to heat treatment (continuous annealing) in the following various patterns to obtain a steel sheet.
[0032]
(Heat treatment pattern)
No. 1-10
After heating and holding (annealing) at 850 ° C. (temperature from A 1 transformation point to A 3 point) for 120 seconds, cooling to 700 ° C. at 5 ° C./s and holding for 15 seconds, then average cooling rate It cooled to 420 degreeC at 15 degreeC / s, hold | maintained for 15 seconds (austempering process), and cooled to room temperature by air cooling (average cooling rate 5 degreeC / s).
[0033]
No. 11
After heating and holding (annealing) at 850 ° C. (temperature of A 1 transformation point to A 3 point) for 120 seconds, average cooling rate: 5 ° C./s, cooling to 700 ° C. and holding for 60 seconds, then average cooling rate It cooled to 420 degreeC at 15 degreeC / s, hold | maintained for 15 seconds (austempering process), and cooled to room temperature by air cooling (average cooling rate 5 degreeC / s).
[0034]
No. 12
After heating and holding (annealing) at 850 ° C. (temperature of A 1 transformation point to A 3 point), cooling to 420 ° C. at an average cooling rate of 15 ° C./s and holding for 15 seconds (austempering) It cooled to room temperature by air cooling (average cooling rate 5 degrees C / s).
[0035]
No. 13
After heating and holding (annealing) at 850 ° C. (temperature of A 1 transformation point to A 3 point), after cooling to 420 ° C. at an average cooling rate of 15 ° C./s and holding for 200 seconds (austempering) And cooled to room temperature by air cooling (average cooling rate 5 ° C./s).
[0036]
[Table 1]
About the steel plate thus obtained, the number of carbides per 2000 (μm) 2 , tensile strength (TS), elongation [total elongation (EI)], area ratio (space factor) of each structure and bending characteristics ( The complete adhesion bending workability R0 and bending workability R1) were measured in the following manner. These results are shown in Table 2 below.
[0038]
[Number of carbides]
After electropolishing with (5% perchloric acid + acetic acid) solution (60V-0.5A), etching with a solution of 1% tetramethylammonium chloride in 10% acetylacetone + 90% methanol solution (2V-20mA, 2min) Then, the extraction replica prepared by carbon vapor deposition was photographed at three arbitrary positions with a transmission electron microscope (TEM) at a magnification of 7500 times (40 μm × 17 μm per position), and the total [2000 (μm) 2 The number of carbides present at the interface between residual γ and ferrite was calculated.
[0039]
[Tensile strength (TS), elongation (El)]
The tensile test was conducted using a JIS No. 5 test piece, and the tensile strength (TS) and elongation (El) were measured.
[0040]
[Area ratio of each organization]
The structure in the steel sheet was subjected to optical microscope observation and transmission electron microscope (TEM) observation by repeller corrosion, and the area ratio by photographs and the remaining γ were measured by X-ray measurement (ISIJ Int. Vol.33. (1933), No.7, P.776).
[0041]
[Bending characteristics]
A test piece (width: 40 mm × length: 100 mm × thickness: 1.2 mm) is cut out from the above steel plate, and a completely intimate bend R0 and a bend R1 with a 1 mm steel plate sandwiched between them (both 180 ° bends) and cracked. The bending characteristics were evaluated based on the presence or absence (existence: “x”, absence: “◯”).
[0042]
[Table 2]
From these results, it can be considered as follows. First, no. Since Nos. 2 to 5 and 7 to 10 satisfy the requirements defined in the present invention, a high-strength steel sheet having good bending characteristics is obtained. For reference, a TEM photograph (magnification: 7500 times) of the steel sheet of the present invention (No. 4) is shown in FIG. From this photograph, it can be seen that the steel sheet of the present invention has less carbides present at the interface between residual γ and ferrite.
[0044]
In contrast, no. Those of 1, 6, 11 to 13 are examples that do not satisfy any of the requirements (or preferable requirements) defined in the present invention, and each has the following problems. First, no. 1 is an example in which the amount of C is small, and the strength is low. No. No. 6 is an example in which the total amount of Mn and (Si + Al) is small, the desired residual γ cannot be obtained, and a pearlite structure is formed, so that the strength and El are lowered, and the bending characteristics Has also deteriorated.
[0045]
On the other hand, no. In the case of No. 11, since the holding time at 700 ° C. becomes longer during the heat treatment, the pearlite structure increases and the predetermined residual γ cannot be obtained, the El is lowered and the bending characteristics are also deteriorated. . No. In the case of No. 12, since the holding time at 700 ° C. is short (not held at 700 ° C.), the number of carbides increases, and the bending characteristics deteriorate. Furthermore, no. No. 13 is not held at 700 ° C. and has a long holding time at 400 ° C., so that a stable residual γ having a high C concentration is produced, so that bending workability is good, but the number of carbides increases. In particular, the close contact bending workability R0 is deteriorated. For reference, FIG. 13 shows a TEM photograph (magnification: 7500 times) of the steel plate obtained in FIG. From this photograph, it can be seen that in the conventional steel sheet, there are many carbides present at the interface between residual γ and ferrite.
[0046]
【The invention's effect】
Since the present invention is configured as described above, it is possible to realize a high-strength steel sheet exhibiting excellent bending characteristics in a high strength and ultrahigh strength range of 600 to 1400 MPa class, and such a steel sheet is optimal as a steel sheet for automobiles. is there.
[Brief description of the drawings]
FIG. 1 is a TEM photograph of a steel plate (No. 4) of the present invention.
FIG. 2 is a TEM photograph of a conventional steel plate (No. 13).
Claims (4)
Priority Applications (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002219662A JP3828466B2 (en) | 2002-07-29 | 2002-07-29 | Steel sheet with excellent bending properties |
| EP03254606A EP1389639B1 (en) | 2002-07-29 | 2003-07-24 | Steel sheet with excellent bendability |
| US10/626,612 US20040159373A1 (en) | 2002-07-29 | 2003-07-25 | Steel sheet with excellent bendability |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002219662A JP3828466B2 (en) | 2002-07-29 | 2002-07-29 | Steel sheet with excellent bending properties |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JP2004059996A JP2004059996A (en) | 2004-02-26 |
| JP3828466B2 true JP3828466B2 (en) | 2006-10-04 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP2002219662A Expired - Fee Related JP3828466B2 (en) | 2002-07-29 | 2002-07-29 | Steel sheet with excellent bending properties |
Country Status (3)
| Country | Link |
|---|---|
| US (1) | US20040159373A1 (en) |
| EP (1) | EP1389639B1 (en) |
| JP (1) | JP3828466B2 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1431406A1 (en) * | 2002-12-20 | 2004-06-23 | Sidmar N.V. | A steel composition for the production of cold rolled multiphase steel products |
| US7314532B2 (en) * | 2003-03-26 | 2008-01-01 | Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) | High-strength forged parts having high reduction of area and method for producing same |
| EP1512760B1 (en) * | 2003-08-29 | 2011-09-28 | Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) | High tensile strength steel sheet excellent in processibility and process for manufacturing the same |
| US20050150580A1 (en) * | 2004-01-09 | 2005-07-14 | Kabushiki Kaisha Kobe Seiko Sho(Kobe Steel, Ltd.) | Ultra-high strength steel sheet having excellent hydrogen embrittlement resistance, and method for manufacturing the same |
| EP1559798B1 (en) * | 2004-01-28 | 2016-11-02 | Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) | High strength and low yield ratio cold rolled steel sheet and method of manufacturing the same |
| DE602005013442D1 (en) * | 2004-04-22 | 2009-05-07 | Kobe Steel Ltd | High-strength and cold-rolled steel sheet with excellent ductility and clad steel sheet |
| JP4288364B2 (en) * | 2004-12-21 | 2009-07-01 | 株式会社神戸製鋼所 | Composite structure cold-rolled steel sheet with excellent elongation and stretch flangeability |
| JP4716358B2 (en) * | 2005-03-30 | 2011-07-06 | 株式会社神戸製鋼所 | High-strength cold-rolled steel sheet and plated steel sheet with excellent balance between strength and workability |
| JP4716359B2 (en) * | 2005-03-30 | 2011-07-06 | 株式会社神戸製鋼所 | High strength cold-rolled steel sheet excellent in uniform elongation and method for producing the same |
| EP1767659A1 (en) * | 2005-09-21 | 2007-03-28 | ARCELOR France | Method of manufacturing multi phase microstructured steel piece |
| KR100985298B1 (en) | 2008-05-27 | 2010-10-04 | 주식회사 포스코 | Low Density Gravity and High Strength Hot Rolled Steel, Cold Rolled Steel and Galvanized Steel with Excellent Ridging Resistibility and Manufacturing Method Thereof |
| JP5867436B2 (en) * | 2013-03-28 | 2016-02-24 | Ｊｆｅスチール株式会社 | High strength galvannealed steel sheet and method for producing the same |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2952624B2 (en) * | 1991-05-30 | 1999-09-27 | 新日本製鐵株式会社 | High yield ratio type hot rolled high strength steel sheet excellent in formability and spot weldability and its manufacturing method and high yield ratio type hot rolled high strength steel sheet excellent in formability and its manufacturing method |
| TW363082B (en) * | 1994-04-26 | 1999-07-01 | Nippon Steel Corp | Steel sheet having high strength and being suited to deep drawing and process for producing the same |
| CA2278841C (en) * | 1997-01-29 | 2007-05-01 | Nippon Steel Corporation | High strength steels having excellent formability and high impact energy absorption properties, and a method for producing the same |
| JP3320013B2 (en) * | 1997-05-29 | 2002-09-03 | 川崎製鉄株式会社 | High strength and high workability hot rolled steel sheet with excellent impact resistance |
| JP3320014B2 (en) * | 1997-06-16 | 2002-09-03 | 川崎製鉄株式会社 | High strength, high workability cold rolled steel sheet with excellent impact resistance |
| JP3619357B2 (en) * | 1997-12-26 | 2005-02-09 | 新日本製鐵株式会社 | High strength steel sheet having high dynamic deformation resistance and manufacturing method thereof |
| JP3172505B2 (en) * | 1998-03-12 | 2001-06-04 | 株式会社神戸製鋼所 | High strength hot rolled steel sheet with excellent formability |
| JPH11323489A (en) * | 1998-05-13 | 1999-11-26 | Nippon Steel Corp | High strength cold rolled steel sheet having superior workability and excellent in shape fixability and its production |
| JP2000256789A (en) * | 1999-03-10 | 2000-09-19 | Kobe Steel Ltd | Cold-rolled steel sheet excellent in workability and spot weldability and pre-galvannealed steel sheet and production thereof |
| JP2000313936A (en) * | 1999-04-27 | 2000-11-14 | Kobe Steel Ltd | Galvannealed steel sheet excellent in ductility and production thereof |
| FR2801061B1 (en) * | 1999-11-12 | 2001-12-14 | Lorraine Laminage | PROCESS FOR PRODUCING A VERY HIGH STRENGTH HOT LAMINATED SHEET METAL FOR USE IN FORMING AND IN PARTICULAR FOR STAMPING |
| JP3750789B2 (en) * | 1999-11-19 | 2006-03-01 | 株式会社神戸製鋼所 | Hot-dip galvanized steel sheet having excellent ductility and method for producing the same |
| JP2001152287A (en) * | 1999-11-26 | 2001-06-05 | Kobe Steel Ltd | High strength cold rolled steel sheet excellent in spot weldability |
| EP1201780B1 (en) * | 2000-04-21 | 2005-03-23 | Nippon Steel Corporation | Steel plate having excellent burring workability together with high fatigue strength, and method for producing the same |
| CA2387322C (en) * | 2001-06-06 | 2008-09-30 | Kawasaki Steel Corporation | High-ductility steel sheet excellent in press formability and strain age hardenability, and method for manufacturing the same |
-
2002
- 2002-07-29 JP JP2002219662A patent/JP3828466B2/en not_active Expired - Fee Related
-
2003
- 2003-07-24 EP EP03254606A patent/EP1389639B1/en not_active Expired - Fee Related
- 2003-07-25 US US10/626,612 patent/US20040159373A1/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| US20040159373A1 (en) | 2004-08-19 |
| EP1389639A2 (en) | 2004-02-18 |
| EP1389639A3 (en) | 2005-06-08 |
| EP1389639B1 (en) | 2011-01-05 |
| JP2004059996A (en) | 2004-02-26 |
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