JP3840901B2 - Cold-rolled steel sheet, plated steel sheet, and method for producing cold-rolled steel sheet having excellent strength increasing ability by heat treatment after forming - Google Patents

Description

【００１７】
かくして得られた冷延鋼板から、JIS ５号引張試験片を採取し、歪み速度：0.02ｓ^-1の条件で通常の引張試験機を用いて引張強さ(TS₀) を測定した。また、これらの冷延鋼板に５％の引張歪みを付与し、 170℃, 20分の熱処理（塗装焼付け処理相当）を施した後、引張試験片を採取し、同様の引張試験を施して引張強度(TS₁) を求めた。
成形後の熱処理による強度上昇代ΔTS（＝TS₁ −TS₀ ）を求め、〔析出Mn％〕および〔析出Si％〕との関係について調査した結果を、図１に示す。
図１に示したとおり、〔析出Mn％〕×〔析出Si％〕の値が 0.00010以下の場合に、ΔTSが60 MPa以上となることが判明した。なお、ΔTSが60 MPa以上となった場合は、全て0.0015mass％以上の固溶Ｎ量が確保されていた。
【００１８】
次に、本発明の構成要件とその作用について説明する。
まず、本発明において鋼板の成分組成を前記の範囲に限定した理由について説明する。なお、以下に示す成分組成の％表示は「質量％」である。
Ｃ：0.15％以下
Ｃは、鋼の強度を増加させる元素であり、強度確保の観点からは0.01％以上を添加することが好ましい。一方、0.15％を超えて含有すると溶接性が低下するため、0.15％以下とした。なお、鋼板の深絞り性を優先させる場合には、Ｃはできるだけ少量である方がプレス成形性には有利である。また、フェライト相に対するＣの固溶限はＮよりもかなり低いため、熱間圧延以降の冷延工程において再溶解が進行して結晶粒内の固溶Ｃが増加するため、耐常温時効性を低下させ易い。従って、耐常温時効性を重視する場合にはＣ量は0.01％以下とすることが望ましい。一方、過度にＣを低減させることは製鋼コストの増大を招くので、下限は0.0005％程度とすることが望ましい。
【００１９】
Si：0.005 〜1.0 ％
Siは、伸びの低下を抑制し、また強度を向上させる有用な成分であるが、含有量が 0.005％に満たないとその添加効果に乏しく、一方 1.0％を超えると表面性状が悪化するだけでなく延性の低下を招くので、Siは 0.005〜1.0 ％の範囲に限定した。より好ましくは 0.005〜0.75％の範囲である。なお、SiおよびMn含有量は、冷延鋼板の〔析出Mn％〕および〔析出Si％〕に影響を及ぼすため、これらの含有量に応じて熱延条件や焼鈍条件を適正化する必要がある。これは、Mn，SiおよびＮを主成分とする窒化物が形成し、固溶Ｎが変動するためである。
【００２０】
Mn：0.01〜3.0 ％
Mnは、鋼の強化成分として有効に作用するが、含有量が 3.0％を超えると表面性状の悪化のみならず延性の低下を招くので 3.0％以下に限定した。より好ましくは 2.0％以下である。一方、Mnは、MnＳを形成しＳによる脆化を抑制する作用があるが、含有量が0.01％未満では十分な効果が得られない。
なお、Mnは、Siと同様に、含有量に応じて熱延条件や焼鈍条件を適正化する必要がある。
【００２１】
Al：0.005 〜0.02％
Alは、脱酸剤として、また炭窒化物形成成分の歩留り向上のために添加されるが、添加量が 0.005％未満では十分な効果がなく、一方0.02％を超えると鋼中に添加が必要なＮ量が増大するため、製鋼時のＮの歩留り的中が困難となる。
【００２２】
Ｎ：0.006 〜0.020 ％
Ｎが 0.006％未満では十分な成形後強度上昇熱処理能が得られず、一方 0.020％を超えてＮを含有させると、製鋼時にブローホールが発生しプレス成形性が低下するので、Ｎは 0.006〜0.02％の範囲に限定した。好ましくは 0.008〜0.019％の範囲である。
【００２３】
Ｎ(%)/Al(%) ≧0.3
上述したとおり、Ｎは、本発明において成形後強度上昇能を鋼板に付与する重要な役割を果たす。そのためには、Ｎ含有量とAl含有量の比Ｎ(%)/Al(%) を 0.3以上とする必要がある。というのは、製造条件の変動の如何にかかわらず、安定して0.0015％以上の固溶Ｎを残留させるために、Ｎを強力に固定する元素であるAlとの組み合わせについて広範囲にわたって検討した結果、最終製品での固溶Ｎを安定して0.0015％以上とするためには、Ｎ(%)/Al(%) ≧0.3 とする必要があることが判明したからである。
さらに、MnおよびSi含有量の積〔Mn％〕×〔Si％〕が増加すると、最終製品での析出Ｎ量が増加するため、鋼成分としてMnおよびSi含有量に応じて、製造条件を制御することが好ましい。
【００２４】
固溶Ｎ：0.0015％以上
成形後の熱処理温度：120 〜200 ℃の温度範囲で十分な強度上昇能を確保するには、例えば５％の予歪で 170℃時効処理後に 60 MPa 以上のΔTSを確保するためには、0.0015％以上の固溶Ｎを必要とする。
従って、本発明では、Ｎ量が上記の 0.006〜0.020 ％を満足する範囲において、さらに固溶Ｎを0.0015％以上の範囲で含有させるものとした。
【００２５】
Ｐ：0.002 〜0.10％
Ｐは、固溶強化成分として鋼の強化に有効に寄与するが、0.10％を超えて添加すると、(FeNb)_X Ｐなどの燐化物が形成されるため深絞り性が低下するので、Ｐ量は0.10％以下に制限した。好ましくは0.08％以下である。一方、Ｐは、現状の製鋼段階で 0.002％未満とするにはコストが嵩むため、下限は現状の不純物レベルの下限である 0.002％とした。
【００２６】
〔析出Mn％〕×〔析出Si％〕≦ 0.00010
また、本発明では、析出Mn量と析出Si量を極力低減することが重要で、前掲図１に示したとおり、析出Mn％と析出Si％の積を 0.00010以下とすることにより、0.0015％以上の固溶Ｎが確保されて、成形後の熱処理による強度上昇代ΔTSを60MPa 以上とすることができるので、本発明ではこれらの積〔析出Mn％〕×〔析出Si％〕を 0.00010以下に限定した。
【００２７】
以上、必須成分について説明したが、本発明では、その他にも以下の元素を適宜含有させることができる。
Crおよび／またはMo：0.05〜2.0 ％
CrおよびMoはいずれも、鋼の焼入れ性を向上させ、マルテンサイト相の生成を促進させる効果がある。しかしながら、これらの含有量が0.05％未満ではその添加効果に乏しく、一方 2.0％を超えると成形性、めっき性およびスポット溶接性の劣化を招く。従って、これらは単独使用または併用いずれの場合においても、0.05〜2.0 ％の範囲で含有させるものとした。
【００２８】
Ni：0.1 〜1.5 、Cu：0.1 〜1.5 ％
Ni, Cuは、固溶強化により鋼の強度を増加させる元素であり、また焼鈍後の冷却過程でオーステナイトを安定化し、２相組織および低温変態相を形成し易くする効果がある。かような効果は、Ni, Cuとも含有量が 0.1％以上で認められるものの、1.5 ％を超えて添加すると成形性、めっき性、溶接性を低下させるので、NiおよびCuはそれぞれ 0.1〜1.5 ％の範囲に限定した。
そして、上記したCrおよび／またはMo、NiおよびCuのうちいずれか一種または二種以上を含有させることにより、第２相としてマルテンサイトを効果的に生成させることができる。
【００２９】
Nb，TiおよびＶのうちから選んだ１種または２種以上合計：0.3 ％以下
Nb，TiおよびＶはそれぞれ、炭窒化物形成元素であり、熱延組織および冷延再結晶焼鈍組織を微細化させる効果がある。この効果は、0.001 ％以上で認められるが、0.3 ％を超えると炭窒化物形成量が増大し、成形後の熱処理による引張強度上昇量の低下を招くので、これらの元素は合計で 0.3％以下の範囲に限定した。
【００３０】
Ｂ：0.0003〜0.0015％
Ｂは、NbやTi, Ｖと複合添加することにより、熱延組織および冷延再結晶組織の微細化に寄与し、かつ耐二次加工脆性を改善する働きがある。しかしながら、Ｂ量が0.0003％未満では十分な微細化効果が得られず、一方0.0015％を超えるとＢＮ析出量が増大し、またスラブ加熱段階での溶体化に支障をきたすので、Ｂは0.0003〜0.0015％の範囲で含有させるものとした。特に好ましくは0.0007〜0.0012％の範囲である。
【００３１】
Ｓ：0.01％以下
その他、不純物中特にＳが多量に含有されると、介在物量が増大して延性の低下を招くので、Ｓの混入は極力避けることが望ましいが、0.01％までなら許容される。
【００３２】
上記した成分組成範囲において、請求項１の成分組成に調整した場合には、鋼組織はフェライトまたはフェライト主体の組織となる。ここに、フェライト以外の相としては、パーライト、ベイナイト、残留オーステナイトおよびマルテンサイト等が挙げられる。
なお、ここでフェライト主体の組織とは、フェライト相を体積分率で60％以上含有する組織を意味する。また、本発明でいうフェライトは、通常の意味のフェライト（ポリゴナルフェライト）だけではなく、炭化物を含まないベイニティックフェライトやアシキュラーフェライトを含むものとする。
【００３３】
また、請求項２のように、鋼中にさらに、CrやMo, Ni, Cuを添加すると、フェライトを主体とし、第２相としてマルテンサイトを体積率で３％以上（３ vol％以上）生成させることができ、このマルテンサイトは３ vol％以上でとりわけ良好に延性を向上させる効果があるが、40 vol％を超えると強度が高くなりすぎ、十分な延性を確保できないので、第２相としてマルテンサイトを生成させる場合、その生成量は40 vol％以下とすることが好ましい。なお、この場合に少量の低温変態相（ベイナイト）が生成する場合がある。このような変態組織鋼にすることによって、延性を大幅に高めることができる。
【００３４】
次に、本発明に従う製造方法について説明する。
上記の好適成分組成に調整した鋼を、転炉等の通常公知の溶製方法で溶製し、造塊法あるいは連続鋳造法で凝固させ、鋼素材とする。連続鋳造スラブは、鋳造ままで熱延工程へ直送してもよいし、一旦冷却後に再加熱して熱延工程に供給してもよい。
これら鋼素材を、加熱、均熱したのち、熱間圧延により熱延板とする。本発明では、熱間圧延の加熱温度は特に規定するものではないが、Ｎを溶体化するためには加熱温度は1150℃以上とするのがよい。なお、溶体化のより一層の向上のためには、1200℃以上とするのが好ましい。しかしながら、加熱温度が1300℃を超えると溶体化の改善効果は飽和し、逆に結晶粒の粗大化に伴う加工性の低下を招く。
【００３５】
また、熱間圧延の全圧下率は70％以上とすることが好ましい。というのは、70％未満では熱延板の結晶粒微細化が不十分となるからである。さらに、熱間圧延仕上温度は、Ａr₃変態点以上のγ域あるいはＡr₃変態点以下のα域のいずれでもよいが、特に好ましく 960〜650 ℃の温度範囲である。というのは、熱間圧延仕上温度が 960℃超えると熱延板の結晶粒が粗大化して、冷延・焼鈍後の加工性が低下し、一方 650℃未満では変形抵抗が増加するため熱延負荷の増大を招き、圧延が困難になるからである。
【００３６】
熱間仕上圧延終了後は直ちに冷却することが、粒成長を防止し、かつ冷却過程でのAlＮの析出およびMnSiN₂あるいはMnSiＮといったMn, Siの析出を抑制する上で有利であり、かくして結晶粒の微細化を図ることができる。
【００３７】
ついで、熱延板はコイル状に巻取られる。熱延板の巻取り温度は高温ほど炭化物の粗大化には有利であるが、800 ℃を超えると熱延板表面に形成されるスケールが厚くなってスケール除去作業の負荷が増大するだけでなく、窒化物形成が進行しコイル長手方向の固溶Ｎ量の変動を招く。また、巻取り温度が 200℃未満では、巻取り作業が困難になる。
従って、これらの観点からは、巻取り温度は 200℃以上、 800℃以下とすることが好ましい。
【００３８】
前掲表１に示した成分組成になるシートバーを、1270℃で均一加熱したのち、仕上温度が 920℃となるように３パス圧延を行い、熱延終了後、直ちに急冷し、コイル巻取り温度を 750, 610, 480, 350 ℃と変化させて１時間保持した。
得られた４mm厚の熱延板に、圧下率：75％の冷間圧延を施したのち、820 ℃で40秒間の再結晶焼鈍を施し、さらに圧下率：１％のスキンパス圧延を施した。
かくして得られた冷延鋼板から、JIS ５号引張試験片を採取し、歪み速度：0.02ｓ^-1の条件で通常の引張試験機を用いて、引張強さ(TS₀) を測定した。また、これらの冷延鋼板に５％の引張歪みを付与し、 170℃, 20分の熱処理（塗装焼付け処理相当）を施したのち、引張試験片を採取し、同様の引張試験を施して引張強度(TS₁) を求めた。
【００３９】
成形後の熱処理による強度上昇代ΔTS（＝TS₁ −TS₀ ）を求め、〔Mn％〕および〔Si％〕との関係について調査した結果を、図２に示す。図中の○内の数値がΔTSである。
同図から明らかなように、MnとSiの含有量の積、すなわち〔Mn％〕×〔Si％〕が 1.0以下の場合にはCT≦700 ℃とし、一方〔Mn％〕×〔Si％〕が 1.0より大きい場合には、CT≦ 300＋ 400／（〔Mn％〕×〔Si％〕）とすることにより、安定して0.0015％以上の固溶Ｎ量を確保でき、ΔTS≧60 MPaを達成することができた。
【００４０】
ついで、熱延板を酸洗したのち、冷間圧延を施す。この冷間圧延における圧下率：60〜95％とすることが好ましい。というのは、冷延圧下率が60％未満では、再結晶時のストアードエネルギーが小さく、一方95％を超えると圧延負荷が増大するからである。
【００４１】
冷間圧延を施された冷延鋼板は、ついで再結晶焼鈍に供される。再結晶焼鈍条件は 650℃以上、５秒以上とすることが好ましい。というのは、焼鈍温度および時間がそれぞれ 650℃未満、５秒未満では再結晶が完了せず、そのため加工性が低下するからである。加工性をより向上させるためには 800℃以上で５秒以上とすることが望ましい。なお、焼鈍温度の上限は 950℃とすることが好ましい。というのは、焼鈍温度が 950℃を超えると、炭化物の再溶解が進行し固溶Ｃが過度に増加するため、遅時効性が劣化するからである。
なお、再結晶焼鈍は、連続焼鈍ラインまたは連続めっきラインで行うことが好ましい。
また、焼鈍雰囲気の主なガス種は、H₂とN₂であり、H₂とN₂の混合ガスとは３〜９％のH₂を含むN₂ガスとすることが好ましい。
【００４２】
さらに、連続焼鈍における再結晶焼鈍後の冷却は、組織の微細化、固溶Ｎ量の確保の観点から、本発明では、焼鈍後少なくとも10℃/s以上で冷却することが好ましい。より好ましくは20℃/s以上である。なお、冷却速度が 300℃/sを超えると、鋼板の幅方向での材質の均一性の低下などの不具合が発生するので、冷却速度は 300℃/s以下とすることが好ましい。
【００４３】
前掲表１に示した鋼Ａ〜Ｅのシートバーを、1270℃に均一加熱したのち、仕上温度が 920℃となるように３パス圧延を行い、圧延終了後直ちに急冷し、コイル巻取り温度を 450℃とし１時間保持した。得られた４mm厚の熱延板を圧下率：75％の冷間圧延を施したのち、種々の温度で40秒間の再結晶焼鈍を施し、さらに圧下率：１％のスキンパス圧延を施した。
かくして得られた冷延鋼板から、JIS ５号引張試験片を採取し、歪み速度：0.02ｓ^-1の条件で通常の引張試験機を用いて、引張強さ(TS₀) を測定した。また、これらの冷延鋼板に５％の引張歪みを付与し、 170℃, 20分の熱処理（塗装焼付け処理相当）を施したのち、引張試験片を採取し、同様の引張試験を施して引張強度(TS₁) を求めた。
【００４４】
成形後の熱処理による強度上昇代（ΔTS＝TS₁ −TS₀ ）を求め、〔Mn％〕および〔Si％〕との関係について調査した結果を、図３に示す。図中の○内の数字がΔTSである。
同図から明らかなように、〔Mn％〕×〔Si％〕が1.0 以下の場合には、焼鈍温度が 650〜950 ℃の範囲でΔTS≧60 MPaを達成できる。一方、〔Mn％〕×〔Si％〕が 1.0よりも大きい場合には、 950−300/（〔Mn％〕×〔Si％〕）≦焼鈍温度（℃）≦950 の範囲においてΔTS≧60 MPaを達成することができた。
【００４５】
なお、上記の再結晶焼鈍後、鋼板には、形状矯正、表面粗さ調整のため、10％以下の調質圧延を行ってもよい。
本発明では、上記のようにして得られた冷延鋼板の表面に、電気めっきまたは溶融めっきを施しても何ら問題はない。これらのめっき鋼板も、冷延鋼板と同程度のTS、BH量およびΔTS量を示す。
また、めっきの種類としては、電気亜鉛めっき、溶融亜鉛めっき、合金化溶融亜鉛めっき、電気錫めっき、電気クロムめっきおよび電気ニッケルめっきなどいずれもが有利に適合する。
【００４６】
なお、再結晶焼鈍を経た冷延鋼板に溶融亜鉛めっき処理を施して鋼板表面に溶融亜鉛めっき層を被成する場合、めっき処理は、通常、溶融亜鉛めっきラインで行われる条件と同様に、板温が 450〜550 ℃の温度範囲で溶融亜鉛めっきを施すことが好ましい。亜鉛浴は、Alを0.10〜0.15％含有するZn浴とすることが好ましい。かかるめっき処理後、必要に応じて目付量調整のためのワイピングを行っても良いのはいうまでもない。
また、再結晶焼鈍工程を経た冷延鋼板を連続めっき焼鈍ラインに通板し、再度焼鈍後、めっきすることも可能である。ただし、この場合も焼鈍温度は前述したように〔Mn％〕と〔Si％〕で規制される条件を満足させる必要がある。
さらに、焼鈍後は、 550℃までを10℃/s以上、 300℃/s以下の速度で冷却することが好ましい。
【００４７】
また、上記の溶融亜鉛めっき処理後、めっき層を合金化する合金化処理を施してもよい。合金化処理における加熱温度は 450℃〜Ａc₁変態点程度とすることが好ましい。というのは、加熱温度が 450℃に満たないと合金化の進行が遅くて生産性の低下を招き、一方Ａc₁変態点を超えるとめっき層の合金化が進行しすぎてめっき層が脆化するからである。
なお、めっき処理後、鋼板は冷却されるが、その工程中 300℃までの温度域については５℃/s以上の速度で冷却することが好ましい。
また、上記合金化溶融亜鉛めっき鋼板などとしたのち、加工性の向上や加工後の外観のために調質圧延を施した鋼板（ダル仕上鋼板、ブライト仕上鋼板、表面に特定の形状パタンを形成した鋼板）、さらには表面に防錆油、潤滑油などの油膜層を有する鋼板など、通常、薄鋼板として採用する表面処理を施した鋼板については、いずれも本発明を適用することができ、その効果を十分に享受することができる。
【００４８】
その後、加工成形、例えば絞り加工などのプレス加工が施される。このプレス加工を行うに際しては、鋼板に適当な量の転位を付与する必要がある。熱処理後に強度を上昇させるには、強度や硬度が必要とされる部位に、少なくとも２％の塑性相当歪みが付与することが必要である。歪み量が少なすぎる場合には、成形後熱処理を施しても十分な強度上昇が発現しない。好適には５％以上の塑性相当歪みを付与することが好ましく、この場合ΔTS≧60 MPaを確保することができる。
【００４９】
プレス成形後、低温での熱処理を施す。この際、熱処理温度は従来塗装焼付け処理で行われている 120〜200 ℃程度でよい。熱処理温度が 120℃未満では成形後強度上昇熱処理能が塑性相当歪みが低い場合に十分に得られない。一方、200℃を超える加熱処理は成形後強度上昇熱処理能は満足するものの、格別な加熱装置が必要となる場合がある。なお、加熱方法としては、熱風加熱、赤外炉加熱、温浴熱処理、通電加熱、高周波加熱などの方法が適用でき、特に規定されない。また、強度を上昇させたい部分のみを選択的に加熱する場合でもよい。なお 250℃を超える熱処理は表面性状を損なう懸念がある。
【００５０】
【実施例】
実施例１
表２に示す成分組成になる鋼スラブを、表３に示す熱延条件で熱延板とした。ついで、これらの熱延板に冷間圧延を施して冷延板としたのち、連続焼鈍ラインにて再結晶焼鈍を施し、さらに圧下率：1.0 ％の調質圧延を施した。
かくして得られた製品板の組織、機械的特性および予変形−塗装焼付け後特性について調べた結果を表４に示す。
【００５１】
なお、引張特性は、製品板からJIS ５号試験片を採取して実施した。
また、固溶Ｎ量および〔析出Mn％〕〔析出Si％〕に次のようにして求めた。
固溶Ｎ量、析出Mn量および析出Si量は、定電位電解法を用いた電解抽出分析法により求めるのが有効であり、抽出分析に用いる地鉄を溶解する方法としては、酸分解法、ハロゲン法および電解法がある。この中で電解法は、炭化物や窒化物などの極めて不安定な析出物を分解させることなく安定して抽出し、地鉄のみを溶解することができる。
本発明では、上記の方法で求めた電解抽出物中のMn，Si量を測定し、〔析出Mn％〕、〔析出Si％〕とした。
また、固溶Ｎ量は、上記のようにして電解抽出した電解抽出物中のＮを析出Ｎとし、鋼中の全Ｎ量から析出Ｎ量を差し引いて求めた。
【００５２】
さらに、その他の特性は次のようにして求めた。
・歪時効硬化特性
各製品板からJIS ５号試験片を圧延方向に採取し、予変形として５％の引張予歪みを与えて、ついで 170℃, 20分の塗装焼付け処理相当の熱処理を施した後、歪速度：0.02ｓ^-1の条件で引張試験を実施し、予変形を行い、さらに塗装焼付け処理を施した後の引張強さTS_BHを求め、ΔTS＝TS_BH−TSを求めた。TSは製品板の引張強さである。
・組織
各鋼板から試験片を採取し、圧延方向に直交する断面（Ｃ断面）について、光学顕微鏡あるいは走査型電子顕微鏡を用いて微視組織を撮像し、画像解析装置を用いて組織の種類、フェライトの組織分率を求めた。
【００５３】
・耐衝撃特性
各製品板から衝撃試験片を圧延方向に採取して、「Journa1 of Society of Materials Science Japan. 10 (1998). P.1058」に記載された高速引張試験方法に準拠して、歪み速度：2000ｓ^-1で高速引張試験を実施し、応力−歪み曲線を測定した。得られた応力−歪み曲線を用いて、応力を歪み：０〜30％の範囲で積分して、吸収エネルギ−Ｅを求めた。また、予変形として５％の引張変形を与え、ついで 170℃_, 20分の塗装焼付け相当熱処理を施したのち、同様な衝撃試験を実施し、吸収エネルギ−Ｅ_BHを求め、予変形−塗装焼付け処理による耐衝撃特性の向上代Ｅ_BH／Ｅを評価した。
【００５４】
【表２】

【００５５】
【表３】

【００５６】
【表４】

【００５７】
表４から明らかなように、本発明の要件を満足する製品板はいずれも、比較例に比べて、成形後熱処理による引張強度の上昇が高い。
【００５８】
実施例２
表５に示す成分組成になる鋼スラブを、表６に示す熱延条件で熱延板とした。ついで、これらの熱延板を、表６に示す圧下率で冷間圧延したのち、 840℃, 20秒の再結晶焼鈍を施した。なお、この 840℃という温度は、鋼種Ｘ，Ｙ，Ｚともに、MnおよびSi含有量で規制される焼鈍温度下限値よりも高い温度である。
ついで、鋼板表面の酸化層を酸洗または機械的に除去したのち、表６に示す連続めっき焼鈍条件で溶融亜鉛めっきまたは合金化溶融亜鉛めっきを施した。この時、合金化処理温度は全てＡc₁変態点以下とした。その後、圧下率：0.8 〜1.2％の調質圧延を行った。
かくして得られた製品板の組織、機械的特性および予変形−塗装焼付け後特性について調べた結果を表７に示す。
【００５９】
【表５】

【００６０】
【表６】

【００６１】
【表７】

【００６２】
表７に示したとおり、本発明の要件を満足するめっき鋼板はいずれも、冷延鋼板で検討した場合と同様に、成形後熱処理による引張強度の上昇が比較例に比べて高い。
【００６３】
【発明の効果】
かくして、本発明によれば、プレス成形時に優れた加工性を維持しつつ、プレス成形−熱処理によって引張強度が大幅に向上する冷延鋼板、さらには合金化溶融亜鉛めっき鋼板等のめっき鋼板を、工業的に安定して製造することができる。
【図面の簡単な説明】
【図１】 成形後の熱処理による強度上昇代ΔTS（＝TS₁ −TS₀ ）に及ぼす〔析出Mn％〕×〔析出Si％〕の影響を示した図である。
【図２】 成形後の熱処理による強度上昇代ΔTS（＝TS₁ −TS₀ ）に及ぼす〔Mn％〕×〔Si％〕と巻取温度の影響を示した図である。
【図３】 成形後の熱処理による強度上昇代（ΔTS＝TS₁ −TS₀ ）に及ぼす〔Mn％〕×〔Si％〕と焼鈍温度の影響を示した図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is a steel plate suitable for application to a location where structural strength, particularly strength and / or rigidity is required after molding, such as construction members, machine structural parts, and automotive structural parts, The present invention relates to a cold-rolled steel sheet, a plated steel sheet, and a method for producing a cold-rolled steel sheet, which are suitable as a raw material steel sheet for a molded body to be heat-treated after press forming or the like, and have excellent strength increasing ability by heat treatment after forming.
[0002]
[Prior art]
In the production of a press-formed body of a thin steel plate, there is a method of applying paint baking at less than 200 ° C as a method of increasing the strength of the part by making it soft and easy to press before press forming and hardening after press forming, A BH steel sheet has been developed as such a steel sheet for paint baking.
[0003]
For example, in Japanese Patent Laid-Open Nos. 55-141526 and 55-141555, Nb is added according to the C, N, and Al contents in steel, and the atomic ratio is Nb / (solid solution C + solid The method of adjusting the solid solution C and the solid solution N in the steel sheet by controlling the cooling rate after annealing while limiting the solution N) within a specific range is disclosed in Japanese Patent Publication No. 61-45689. A method for improving the bake hardenability by the combined addition of Ti and Nb is disclosed.
However, since the above steel plates place importance on deep drawability, the strength of the raw steel plates is low and is not necessarily sufficient as a structural material.
Japanese Patent Application Laid-Open No. 5-25549 discloses a method for improving bake hardenability by adding W, Cr, or Mo alone or in combination to steel.
[0004]
However, the increase in strength due to bake hardening does not increase the tensile strength (tensile strength) only by increasing the yield strength of the material using the solid solution C or solid solution N contained in the steel sheet. Absent.
For this reason, it has only the effect of increasing the deformation start stress of the member or part, and the effect of increasing the stress required for deformation (hereinafter referred to as the deformation strength characteristic) over the entire deformation region from the start of deformation of the member or part to the end of the deformation is not necessarily sufficient is not.
[0005]
On the other hand, as a curing method other than the coating baking of the press-molded body, there is a method of performing a nitriding treatment after the press molding.
For example, Japanese Patent Laid-Open No. 2-80539 discloses a method in which a nitride forming element such as Cr, Al, V or the like is contained in steel so that the strength is increased by nitriding, and Japanese Patent Laid-Open No. 3-122255 discloses. A method of increasing the hardness of a member by precipitation hardening of Cu using the heat of nitriding is disclosed.
However, since the heating temperature is higher than 450 ° C. in these methods, the use of a normal galvanized steel sheet with improved corrosion resistance causes the plating layer to evaporate and a corrosion-resistant product cannot be obtained. was there.
[0006]
As another cold-rolled steel sheet whose tensile strength increases after forming, for example, JP-A-10-310847 discloses an alloyed hot-dip galvanized steel sheet whose tensile strength increases by 60 MPa or more in a heat treatment temperature range of 200 to 450 ° C. Has been.
This steel sheet contains, by mass percentage, C: 0.01 to 0.08%, Mn: 0.01 to 3.0%, and a total of 0.05 to 3.0% of one or more of W, Cr and Mo, and is necessary. Depending on the composition of Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, or a composition containing one or more, and the microstructure of the steel is mainly composed of ferrite or ferrite It is.
However, this technique is to form fine carbides in the steel sheet by heat treatment after forming, and effectively increase the amount of strain by increasing the dislocations effectively against the strain imparted during pressing. Heat treatment needs to be performed in a temperature range of 450 ° C., and the heat treatment temperature required is higher than a general baking hardening temperature. The paint baking temperature normally used in automobile production lines is around 170 ° C, and heat treatment conditions reaching 450 ° C are difficult to implement.
For this reason, development of a cold-rolled steel sheet having a hardening ability of 60 MPa or more at a lower temperature has been desired.
[0007]
[Problems to be solved by the invention]
The present invention advantageously responds to the above-mentioned demands, and is easy to work such as press forming with a relatively soft high-strength steel having a strength level of 340 to 700 MPa class before work forming. Short-term heat treatment at a relatively low temperature for the purpose of increasing strength after molding can effectively increase tensile strength and hardness, and improve the deformation strength characteristics and rigidity of members and parts. It aims at proposing the cold-rolled steel plate and the plated steel plate which were excellent in the strength raising ability by heat processing with the advantageous manufacturing method of a cold-rolled steel plate.
[0008]
[Means for Solving the Problems]
Now, the inventors are able to increase the tensile strength even when processing is performed in a low temperature range where the heat treatment temperature after forming is 120 to 200 ° C., particularly when a strain of 5% or more is applied, cold-rolled steel sheet. As a result of intensive studies on means that can increase the tensile strength of the steel by 60 MPa or more, the following findings were obtained.
(1) In order to increase the tensile strength after heat treatment, it is necessary to introduce new dislocations by molding. Due to the interaction between dislocations introduced by forming and interstitial elements or precipitates, it is necessary that the dislocations introduced by pre-deformation do not move even when the upper yield point is reached.
(2) In order to obtain the above interaction by forming carbides, nitrides or carbonitrides such as W, Cr, Mo, Ti, Nb, Al, etc., the heat treatment temperature after molding should be 200 ° C or higher. Need to increase. Therefore, active utilization of interstitial elements is advantageous in reducing the heat treatment temperature after molding.
(3) Among the interstitial elements, solute N is more likely to interact with dislocations introduced by molding, even if the heat treatment temperature after molding is lower than solute C, even if the upper yield point is reached. Dislocations introduced by pre-deformation are difficult to move.
(4) Control of nitride formation is particularly important when the tensile strength is increased by the heat treatment after forming due to the interaction between the solute N and the dislocations. Mn and Si added for the purpose of strengthening the solid solution and strengthening the transformation structure of the steel plate have a great influence on the solid solution N of the cold-rolled steel plate.
That is, in order to secure solid solution N in order to obtain a sufficient strength increasing ability, it was determined that it is important to keep the product of precipitated Mn% and precipitated Si% of a cold-rolled steel sheet within a predetermined range. It is.
The present invention is based on the above findings.
[0009]
  That is, the gist configuration of the present invention is as follows.
1. In mass percentage,
    C: 0.15% or less,
    Si: 0.005 to 1.0%,
    Mn: 0.01 to 3.0%,
    Al: 0.005 to 0.02%,
    N: 0.006 to 0.020% and
    P: 0.002 to 0.10%
In a range that satisfies N (%) / Al (%) ≧ 0.3, the balance being the composition of Fe and inevitable impurities,Mn When Si Product of the content of [ Mn %] × [ Si %〕But 1.0 Less than,The product of precipitated Mn% and precipitated Si% in steel is 0.00010 or less andMeltingA cold-rolled steel sheet containing 0.0015% or more of N, and further having a steel structure of ferrite or a ferrite-based structure, which is excellent in strength increasing ability by heat treatment after forming.
2. In mass percentage,
    C: 0.15 %Less than,
    Si : 0.005 ~ 1.0 %,
    Mn : 0.01 ~ 3.0 %,
    Al : 0.005 ~ 0.02 %,
    N: 0.006 ~ 0.020 %and
    P: 0.002 ~ 0.10 %
N (%) / Al (%) ≧ 0.3 In the range that satisfies Fe And the composition of inevitable impurities, and Mn When Si Product of the content of [ Mn %] × [ Si %〕But 1.0 Larger, precipitation in steel Mn % And precipitation Si % Product 0.00010 Below, and solute N 0.0015 A cold-rolled steel sheet excellent in strength increasing ability by heat treatment after forming, wherein the steel structure is ferrite or a structure mainly composed of ferrite.
[0010]
3. 1 or 2 aboveIn steel, the percentage by mass
    Cr and / or Mo: 0.05 to 2.0%,
    Ni: 0.1-1.5% and
    Cu: 0.1-1.5%
One or two or more selected from among the above, and the steel structure is a composite structure mainly containing ferrite and containing martensite as a second phase in a volume fraction of 3 to 40%. Cold-rolled steel sheet with excellent strength increasing ability by heat treatment after forming.
[0011]
4). 1, 2 or 3 aboveIn steel, the percentage by mass
    One or more selected from Nb, Ti and V Total: 0.3% or less
A cold-rolled steel sheet excellent in strength increasing ability by heat treatment after forming, characterized in that it has a composition containing
[0012]
5. Above 4In steel, the percentage by mass
    B: 0.0003-0.0015%
A cold-rolled steel sheet excellent in strength increasing ability by heat treatment after forming, characterized in that it has a composition containing
[0013]
6). 1-5 aboveA plated steel sheet excellent in strength increasing ability by heat treatment after forming, wherein an electroplated layer or a hot-dip plated layer is formed on the surface of the cold rolled steel sheet according to any one of the above.
[0014]
7. In mass percentage,
    C: 0.15% or less,
    Si: 0.005 to 1.0%,
    Mn: 0.01 to 3.0%,
    Al: 0.005 to 0.02%,
    N: 0.006 to 0.020% and
    P: 0.002 to 0.10%
The,N (%) / Al (%) ≧ 0.3, [ Mn %] × [ Si %] ≦ 1.0When a steel slab having a composition that satisfies the above requirements is hot-rolled and then wound after hot rolling,rollTaking temperature ≦ 700 ℃, OneAfter pickling, after cold rolling, in the recrystallization annealing processGrilledThe blunt temperature is 650 ~ 950 ℃DoThe manufacturing method of the cold-rolled steel plate excellent in the strength raise ability by the heat processing after shaping | molding characterized by the above-mentioned.
8). In mass percentage,
    C: 0.15 %Less than,
    Si : 0.005 ~ 1.0 %,
    Mn : 0.01 ~ 3.0 %,
    Al : 0.005 ~ 0.02 %,
    N: 0.006 ~ 0.020 %and
    P: 0.002 ~ 0.10 %
N (%) / Al (%) ≧ 0.3 , [ Mn %] × [ Si %]> 1.0 When the steel slab having a composition containing in a range satisfying the above is hot-rolled and then wound after hot rolling, the winding temperature ≦ 300 + 400 / ([ Mn %] × [ Si %]), And after pickling and cold rolling, the annealing temperature in the recrystallization annealing step is expressed by the following equation:
    950 − 300 / ([ Mn %] × [ Si %]) ≦ Annealing temperature (° C) ≦ 950
The manufacturing method of the cold-rolled steel plate excellent in the strength raising ability by the heat processing after shaping | molding characterized by setting it as the temperature which satisfy | fills.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
  First, the basics of the present inventionWhenExplain the research results.
  After the sheet bar having the composition shown in Table 1 is uniformly heated to 1250 ° C, it is subjected to three-pass rolling so that the finishing temperature is 920 ° C, immediately cooled immediately after the end of rolling, and the coil winding temperature is changed. Hold for 1 hour. Next, the obtained hot-rolled sheet having a thickness of 4 mm was subjected to cold rolling at a reduction ratio of 75%, and then subjected to recrystallization annealing at various temperatures of 700 to 800 ° C. for 40 seconds, and the reduction ratio: 1% skin pass rolling was applied.
[0016]
[Table 1]

[0017]
A JIS No. 5 tensile specimen was collected from the cold-rolled steel sheet thus obtained, and the strain rate was 0.02 s.^-1Tensile strength (TS₀) Was measured. In addition, 5% tensile strain was applied to these cold-rolled steel sheets, and after heat treatment at 170 ° C for 20 minutes (equivalent to paint baking treatment), tensile specimens were collected and subjected to the same tensile test for tension. Strength (TS₁)
Increase in strength due to heat treatment after molding ΔTS (= TS₁ −TS₀ ), And the results of investigating the relationship with [precipitation Mn%] and [precipitation Si%] are shown in FIG.
As shown in FIG. 1, it was found that ΔTS was 60 MPa or more when the value of [Precipitation Mn%] × [Precipitation Si%] was 0.00010 or less. When ΔTS was 60 MPa or more, a solid solution N amount of 0.0015 mass% or more was ensured in all cases.
[0018]
Next, the configuration requirements and the operation of the present invention will be described.
First, the reason why the component composition of the steel sheet is limited to the above range in the present invention will be described. In addition, the% display of the component composition shown below is "mass%".
C: 0.15% or less
C is an element that increases the strength of steel, and it is preferable to add 0.01% or more from the viewpoint of securing the strength. On the other hand, if the content exceeds 0.15%, the weldability deteriorates, so the content was made 0.15% or less. In addition, when giving priority to the deep drawability of the steel sheet, it is advantageous for press formability that C is as small as possible. In addition, since the solid solubility limit of C in the ferrite phase is considerably lower than N, remelting proceeds in the cold rolling process after hot rolling to increase the solid solution C in the crystal grains. Easy to lower. Therefore, when importance is attached to room temperature aging resistance, the C content is desirably 0.01% or less. On the other hand, excessively reducing C causes an increase in steelmaking costs, so the lower limit is preferably about 0.0005%.
[0019]
Si: 0.005 to 1.0%
Si is a useful component that suppresses the decrease in elongation and improves strength. However, if the content is less than 0.005%, its additive effect is poor, while if it exceeds 1.0%, surface properties only deteriorate. Therefore, Si was limited to the range of 0.005 to 1.0%. More preferably, it is 0.005 to 0.75% of range. In addition, since the Si and Mn contents affect the [precipitated Mn%] and [precipitated Si%] of the cold-rolled steel sheet, it is necessary to optimize the hot-rolling conditions and annealing conditions according to these contents. . This is because a nitride mainly composed of Mn, Si and N is formed, and the solid solution N varies.
[0020]
Mn: 0.01-3.0%
Mn effectively acts as a strengthening component of steel, but when the content exceeds 3.0%, not only the surface properties are deteriorated but also the ductility is lowered, so the content is limited to 3.0% or less. More preferably, it is 2.0% or less. On the other hand, Mn forms MnS and suppresses embrittlement due to S, but if the content is less than 0.01%, a sufficient effect cannot be obtained.
In addition, like Mn, it is necessary to optimize hot rolling conditions and annealing conditions according to the content of Mn.
[0021]
Al: 0.005 to 0.02%
Al is added as a deoxidizer and to improve the yield of carbonitride-forming components, but if the added amount is less than 0.005%, there is no sufficient effect, while if it exceeds 0.02%, it must be added to the steel. As the amount of N increases, it becomes difficult to keep the yield of N during steel making.
[0022]
N: 0.006 to 0.020%
If N is less than 0.006%, sufficient post-molding strength-increasing heat treatment ability cannot be obtained. On the other hand, if N exceeds 0.020%, blow holes are generated during steel making and press formability is reduced. Limited to 0.02% range. Preferably it is 0.008 to 0.019% of range.
[0023]
N (%) / Al (%) ≧ 0.3
As described above, N plays an important role in imparting strength to the steel sheet after forming in the present invention. For this purpose, the ratio N (%) / Al (%) of N content to Al content needs to be 0.3 or more. This is because, as a result of extensive studies on the combination with Al, which is an element that strongly fixes N, in order to stably retain solute N of 0.0015% or more regardless of fluctuations in production conditions, This is because it has been found that it is necessary to satisfy N (%) / Al (%) ≧ 0.3 in order to make the solid solution N in the final product stably 0.0015% or more.
Furthermore, if the product of Mn and Si contents [Mn%] x [Si%] increases, the amount of precipitated N in the final product increases, so the production conditions are controlled according to the Mn and Si contents as steel components. It is preferable to do.
[0024]
Solid solution N: 0.0015% or more
Heat treatment temperature after forming: In order to ensure sufficient strength increasing ability in the temperature range of 120-200 ° C, for example, to ensure ΔTS of 60 MPa or more after 170 ° C aging treatment with 5% pre-strain, 0.0015 % Of solid solution N is required.
Therefore, in the present invention, in the range where the N amount satisfies the above 0.006 to 0.020%, the solid solution N is further contained in the range of 0.0015% or more.
[0025]
P: 0.002 to 0.10%
P contributes effectively to strengthening steel as a solid solution strengthening component, but if added over 0.10%, (FeNb)_X Since deep drawability deteriorates because phosphides such as P are formed, the P content is limited to 0.10% or less. Preferably it is 0.08% or less. On the other hand, since it is expensive to make P less than 0.002% at the present steelmaking stage, the lower limit is set to 0.002%, which is the lower limit of the current impurity level.
[0026]
[Precipitation Mn%] x [Precipitation Si%] ≤ 0.00010
In the present invention, it is important to reduce the amount of precipitated Mn and the amount of precipitated Si as much as possible. As shown in FIG. 1, the product of precipitated Mn% and precipitated Si% is set to 0.00010 or less, so that 0.0015% or more. In this invention, the product [precipitation Mn%] × [precipitation Si%] is limited to 0.00010 or less because the solid solution N can be secured and the strength increase allowance ΔTS by heat treatment after molding can be 60 MPa or more. did.
[0027]
As described above, the essential components have been described. However, in the present invention, the following elements can be appropriately contained.
Cr and / or Mo: 0.05 to 2.0%
Both Cr and Mo have the effect of improving the hardenability of the steel and promoting the formation of the martensite phase. However, if the content is less than 0.05%, the effect of addition is poor, while if it exceeds 2.0%, the formability, plating property and spot weldability are deteriorated. Therefore, they are contained in the range of 0.05 to 2.0% in either case of single use or combined use.
[0028]
Ni: 0.1-1.5, Cu: 0.1-1.5%
Ni and Cu are elements that increase the strength of the steel by solid solution strengthening, and also have the effect of stabilizing austenite in the cooling process after annealing and facilitating the formation of a two-phase structure and a low-temperature transformation phase. Such effects are recognized when the content of Ni and Cu is 0.1% or more, but if added over 1.5%, the formability, plating property and weldability deteriorate, so Ni and Cu are 0.1 to 1.5% respectively. It was limited to the range.
Then, by including any one or more of Cr and / or Mo, Ni and Cu described above, martensite can be effectively generated as the second phase.
[0029]
One or more selected from Nb, Ti and V Total: 0.3% or less
Nb, Ti and V are carbonitride forming elements, respectively, and have the effect of refining the hot rolled structure and the cold rolled recrystallization annealed structure. This effect is recognized at 0.001% or more. However, if it exceeds 0.3%, the amount of carbonitride formation increases and the amount of increase in tensile strength due to heat treatment after molding is reduced, so these elements total 0.3% or less. It was limited to the range.
[0030]
B: 0.0003-0.0015%
B, when added in combination with Nb, Ti, or V, contributes to the refinement of the hot-rolled structure and the cold-rolled recrystallized structure, and has the function of improving the secondary work brittleness resistance. However, if the amount of B is less than 0.0003%, a sufficient effect of refining cannot be obtained. On the other hand, if the amount exceeds 0.0015%, the amount of BN precipitation increases, and the solution formation in the slab heating stage is hindered. It was made to contain in 0.0015% of range. Especially preferably, it is 0.0007 to 0.0012% of range.
[0031]
S: 0.01% or less
In addition, if impurities are contained in a particularly large amount, the amount of inclusions is increased and ductility is reduced. Therefore, it is desirable to avoid the incorporation of S as much as possible, but it is acceptable up to 0.01%.
[0032]
In the above-described component composition range, when adjusted to the component composition of claim 1, the steel structure becomes ferrite or a ferrite-based structure. Here, examples of phases other than ferrite include pearlite, bainite, retained austenite, and martensite.
Here, the structure mainly composed of ferrite means a structure containing a ferrite phase in a volume fraction of 60% or more. In addition, the ferrite referred to in the present invention includes not only the ordinary meaning of ferrite (polygonal ferrite) but also bainitic ferrite and acicular ferrite that do not contain carbides.
[0033]
In addition, as in claim 2, when Cr, Mo, Ni, Cu is further added to the steel, ferrite is the main component and martensite as the second phase is produced in a volume ratio of 3% or more (3 vol% or more). This martensite has the effect of improving ductility particularly well at 3 vol% or more, but if it exceeds 40 vol%, the strength becomes too high and sufficient ductility cannot be secured, so as a second phase When martensite is produced, the production amount is preferably 40 vol% or less. In this case, a small amount of low-temperature transformation phase (bainite) may be generated. By using such a transformation structure steel, the ductility can be greatly increased.
[0034]
Next, the manufacturing method according to the present invention will be described.
The steel adjusted to the above preferred component composition is melted by a generally known melting method such as a converter and solidified by an ingot-making method or a continuous casting method to obtain a steel material. The continuous cast slab may be sent directly to the hot rolling process as cast, or may be reheated after cooling and supplied to the hot rolling process.
These steel materials are heated and soaked, and then hot rolled into hot rolled sheets. In the present invention, the heating temperature for hot rolling is not particularly specified, but the heating temperature is preferably 1150 ° C. or higher in order to form a solution of N. In order to further improve the solution treatment, the temperature is preferably 1200 ° C. or higher. However, when the heating temperature exceeds 1300 ° C., the effect of improving the solution is saturated, and conversely, the workability is reduced due to the coarsening of the crystal grains.
[0035]
The total rolling reduction of hot rolling is preferably 70% or more. This is because if it is less than 70%, the crystal grain refinement of the hot-rolled sheet is insufficient. Furthermore, the hot rolling finishing temperature is Ar_ThreeΓ region above the transformation point or Ar_ThreeAlthough it may be any α region below the transformation point, it is particularly preferably a temperature range of 960 to 650 ° C. This is because when the hot rolling finish temperature exceeds 960 ° C, the crystal grain of the hot-rolled sheet becomes coarse and the workability after cold rolling / annealing deteriorates. This is because the load increases and rolling becomes difficult.
[0036]
Immediate cooling after finishing hot finish rolling prevents grain growth and precipitates AlN and MnSiN during the cooling process.₂Alternatively, it is advantageous in suppressing the precipitation of Mn and Si such as MnSiN, and thus the crystal grains can be made finer.
[0037]
Next, the hot rolled sheet is wound into a coil shape. The higher the hot-rolled coil winding temperature, the more advantageous is the coarsening of the carbides. However, if the temperature exceeds 800 ° C, the scale formed on the surface of the hot-rolled sheet becomes thicker, which not only increases the load of descaling work. Nitride formation proceeds, causing fluctuations in the amount of solute N in the longitudinal direction of the coil. Also, if the winding temperature is less than 200 ° C, the winding operation becomes difficult.
Therefore, from these viewpoints, the winding temperature is preferably 200 ° C. or higher and 800 ° C. or lower.
[0038]
The sheet bar having the composition shown in Table 1 above is heated uniformly at 1270 ° C, then rolled in 3 passes so that the finishing temperature is 920 ° C, and immediately after the hot rolling is completed, it is immediately cooled to the coil winding temperature. Was changed to 750, 610, 480, 350 ° C. and held for 1 hour.
The obtained hot-rolled sheet having a thickness of 4 mm was subjected to cold rolling with a reduction ratio of 75%, then subjected to recrystallization annealing at 820 ° C. for 40 seconds, and further subjected to skin pass rolling with a reduction ratio of 1%.
A JIS No. 5 tensile specimen was collected from the cold-rolled steel sheet thus obtained, and the strain rate was 0.02 s.^-1The tensile strength (TS₀) Was measured. In addition, 5% tensile strain was applied to these cold-rolled steel sheets, and after heat treatment at 170 ° C for 20 minutes (equivalent to paint baking), tensile test specimens were collected and subjected to the same tensile test for tension. Strength (TS₁)
[0039]
Increase in strength due to heat treatment after molding ΔTS (= TS₁ −TS₀ FIG. 2 shows the results of investigation on the relationship between [Mn%] and [Si%]. The numerical value in circles in the figure is ΔTS.
As is apparent from the figure, when the product of the contents of Mn and Si, that is, [Mn%] × [Si%] is 1.0 or less, CT ≦ 700 ° C., while [Mn%] × [Si%] If is greater than 1.0, CT ≦ 300 + 400 / ([Mn%] × [Si%]) ensures a stable solid solution N content of 0.0015% or more and achieves ΔTS ≧ 60 MPa. We were able to.
[0040]
Next, the hot-rolled sheet is pickled and then cold-rolled. The rolling reduction in this cold rolling is preferably 60 to 95%. This is because if the cold rolling reduction is less than 60%, the stored energy during recrystallization is small, while if it exceeds 95%, the rolling load increases.
[0041]
The cold-rolled steel sheet that has been cold-rolled is then subjected to recrystallization annealing. The recrystallization annealing condition is preferably 650 ° C. or more and 5 seconds or more. This is because if the annealing temperature and time are less than 650 ° C. and less than 5 seconds, respectively, recrystallization is not completed, so that workability is lowered. In order to further improve the workability, it is desirable to set the temperature at 800 ° C. or higher for 5 seconds or longer. The upper limit of the annealing temperature is preferably 950 ° C. This is because when the annealing temperature exceeds 950 ° C., the re-dissolution of the carbide proceeds and the solid solution C increases excessively, so that the delayed aging property deteriorates.
In addition, it is preferable to perform recrystallization annealing in a continuous annealing line or a continuous plating line.
The main gas type in the annealing atmosphere is H₂And N₂And H₂And N₂3-9% H is a mixed gas of₂N including₂It is preferable to use gas.
[0042]
Furthermore, in the present invention, cooling after recrystallization annealing in continuous annealing is preferably performed at least 10 ° C./s or more after annealing from the viewpoint of refining the structure and securing the amount of solute N. More preferably, it is 20 ° C./s or more. If the cooling rate exceeds 300 ° C./s, problems such as a decrease in material uniformity in the width direction of the steel sheet occur. Therefore, the cooling rate is preferably 300 ° C./s or less.
[0043]
The steel bars A to E shown in Table 1 above are uniformly heated to 1270 ° C and then rolled in 3 passes so that the finishing temperature is 920 ° C. The temperature was kept at 450 ° C. for 1 hour. The obtained hot-rolled sheet having a thickness of 4 mm was subjected to cold rolling at a reduction ratio of 75%, then subjected to recrystallization annealing at various temperatures for 40 seconds, and further subjected to skin pass rolling at a reduction ratio of 1%.
A JIS No. 5 tensile specimen was collected from the cold-rolled steel sheet thus obtained, and the strain rate was 0.02 s.^-1The tensile strength (TS₀) Was measured. In addition, 5% tensile strain was applied to these cold-rolled steel sheets, and after heat treatment at 170 ° C for 20 minutes (equivalent to paint baking), tensile test specimens were collected and subjected to the same tensile test for tension. Strength (TS₁)
[0044]
Strength increase due to heat treatment after molding (ΔTS = TS₁ −TS₀ ), And the results of investigation on the relationship between [Mn%] and [Si%] are shown in FIG. The number in circles in the figure is ΔTS.
As is apparent from the figure, when [Mn%] × [Si%] is 1.0 or less, ΔTS ≧ 60 MPa can be achieved in the annealing temperature range of 650 to 950 ° C. On the other hand, if [Mn%] x [Si%] is greater than 1.0, ΔTS≥60 MPa in the range of 950-300 / ([Mn%] x [Si%]) ≤ annealing temperature (° C) ≤ 950 Could be achieved.
[0045]
After the recrystallization annealing, the steel sheet may be subjected to temper rolling of 10% or less for shape correction and surface roughness adjustment.
In the present invention, there is no problem even if the surface of the cold rolled steel sheet obtained as described above is subjected to electroplating or hot dipping. These plated steel sheets also show the same TS, BH amount and ΔTS amount as cold-rolled steel sheets.
As the type of plating, any of electrogalvanizing, hot dip galvanizing, alloying hot dip galvanizing, electrotin plating, electrochromic plating and electronickel plating is advantageously suitable.
[0046]
In addition, when a hot-dip galvanizing process is performed on a cold-rolled steel sheet that has undergone recrystallization annealing to form a hot-dip galvanized layer on the surface of the steel sheet, the plating process is usually performed in the same manner as the conditions performed in the hot-dip galvanizing line. It is preferable to perform hot dip galvanization in a temperature range of 450 to 550 ° C. The zinc bath is preferably a Zn bath containing 0.10 to 0.15% Al. It goes without saying that wiping for adjusting the basis weight may be performed as necessary after the plating treatment.
Moreover, it is also possible to plate the cold-rolled steel sheet that has undergone the recrystallization annealing process through a continuous plating annealing line, and after annealing again. However, also in this case, the annealing temperature needs to satisfy the conditions regulated by [Mn%] and [Si%] as described above.
Furthermore, after annealing, it is preferable to cool to 550 ° C. at a rate of 10 ° C./s or more and 300 ° C./s or less.
[0047]
Moreover, you may perform the alloying process which alloyes a plating layer after said hot dip galvanization process. Heating temperature in alloying process is 450 ℃ ~ Ac₁It is preferable to set the transformation point. This is because if the heating temperature is less than 450 ° C., the progress of alloying is slow, which leads to a decrease in productivity.₁This is because when the transformation point is exceeded, alloying of the plating layer proceeds excessively and the plating layer becomes brittle.
In addition, although a steel plate is cooled after a plating process, it is preferable to cool at a speed | rate of 5 degree-C / s or more about the temperature range to 300 degreeC in the process.
In addition, the above alloyed hot-dip galvanized steel sheets are used, and steel sheets that have undergone temper rolling to improve workability and appearance after processing (dull-finished steel sheets, bright-finished steel sheets, and form specific shapes on the surface) Steel sheet), and furthermore, a steel sheet having an oil film layer such as a rust-preventing oil or a lubricating oil on the surface, such as a steel sheet that has been subjected to a surface treatment usually employed as a thin steel sheet, the present invention can be applied to any of them, The effect can be fully enjoyed.
[0048]
Thereafter, press forming such as work forming, for example, drawing is performed. When performing this press work, it is necessary to give an appropriate amount of dislocations to the steel sheet. In order to increase the strength after the heat treatment, it is necessary to impart a plastic equivalent strain of at least 2% to the site where the strength and hardness are required. When the amount of strain is too small, a sufficient increase in strength is not exhibited even after heat treatment after molding. Preferably, a plastic equivalent strain of 5% or more is preferably applied. In this case, ΔTS ≧ 60 MPa can be secured.
[0049]
After press molding, heat treatment is performed at a low temperature. At this time, the heat treatment temperature may be about 120 to 200 ° C., which is conventionally performed in the paint baking process. If the heat treatment temperature is less than 120 ° C, the post-molding strength increasing heat treatment ability cannot be obtained sufficiently when the plastic equivalent strain is low. On the other hand, although heat treatment exceeding 200 ° C. satisfies the post-molding strength increasing heat treatment ability, a special heating device may be required. As a heating method, methods such as hot air heating, infrared furnace heating, warm bath heat treatment, energization heating, and high frequency heating can be applied and are not particularly defined. Further, it may be possible to selectively heat only the portion whose strength is to be increased. There is a concern that heat treatment above 250 ° C may damage the surface properties.
[0050]
【Example】
Example 1
The steel slab having the component composition shown in Table 2 was used as a hot rolled sheet under the hot rolling conditions shown in Table 3. Subsequently, these hot-rolled sheets were cold-rolled to form cold-rolled sheets, and then subjected to recrystallization annealing in a continuous annealing line, and further subjected to temper rolling at a rolling reduction of 1.0%.
Table 4 shows the results of examining the structure, mechanical properties, and predeformation-post-baking properties of the product plate thus obtained.
[0051]
The tensile properties were obtained by collecting JIS No. 5 test pieces from the product plates.
Moreover, it calculated | required as follows in the amount of solute N and [precipitation Mn%] [precipitation Si%].
The amount of solid solution N, the amount of precipitated Mn and the amount of precipitated Si are effective to be determined by electrolytic extraction analysis using a constant potential electrolysis method. There are halogen methods and electrolytic methods. Among them, the electrolytic method can stably extract very unstable precipitates such as carbides and nitrides without decomposing them, and can dissolve only the iron.
In the present invention, the amounts of Mn and Si in the electrolytic extract obtained by the above method were measured and determined as [Precipitated Mn%] and [Deposited Si%].
Further, the amount of solid solution N was determined by subtracting the amount of precipitated N from the total amount of N in the steel, with N in the electrolytic extract electrolytically extracted as described above defined as precipitated N.
[0052]
Further, other characteristics were obtained as follows.
・ Strain age hardening characteristics
JIS No. 5 test specimens were taken from each product plate in the rolling direction, pre-deformed with a 5% tensile pre-strain, and then subjected to a heat treatment equivalent to 170 ° C for 20 minutes, followed by a strain rate: 0.02s^-1Tensile strength TS after conducting a tensile test under the conditions of_BHΔTS = TS_BH-Asked for TS. TS is the tensile strength of the product plate.
・ Organization
Test specimens are collected from each steel plate, and a microscopic structure is imaged using an optical microscope or a scanning electron microscope with respect to a cross section (C cross section) orthogonal to the rolling direction. The tissue fraction was determined.
[0053]
・ Shock resistance
An impact test piece was taken from each product plate in the rolling direction, and in accordance with the high-speed tensile test method described in “Journa1 of Society of Materials Science Japan. 10 (1998). P.1058”, the strain rate was 2000 s.^-1A high-speed tensile test was carried out and a stress-strain curve was measured. Using the obtained stress-strain curve, the stress was integrated in the range of strain: 0 to 30%, and the absorbed energy E was determined. Also, 5% tensile deformation was given as pre-deformation, and then 170 ° C_, After a heat treatment equivalent to 20 minutes of paint baking, the same impact test was conducted and the absorbed energy E_BHTo improve impact resistance by pre-deformation-paint baking process E_BH/ E was evaluated.
[0054]
[Table 2]

[0055]
[Table 3]

[0056]
[Table 4]

[0057]
As is clear from Table 4, any product plate that satisfies the requirements of the present invention has a higher increase in tensile strength due to post-molding heat treatment than the comparative example.
[0058]
Example 2
The steel slab having the component composition shown in Table 5 was used as a hot rolled sheet under the hot rolling conditions shown in Table 6. Next, these hot-rolled sheets were cold-rolled at a reduction rate shown in Table 6 and then subjected to recrystallization annealing at 840 ° C. for 20 seconds. This temperature of 840 ° C. is a temperature higher than the lower limit of the annealing temperature regulated by the Mn and Si contents for all of the steel types X, Y, and Z.
Next, after the oxide layer on the steel sheet surface was pickled or mechanically removed, hot dip galvanizing or alloying hot dip galvanizing was performed under the continuous plating annealing conditions shown in Table 6. At this time, all alloying temperatures are Ac.₁Below the transformation point. Thereafter, temper rolling was performed at a rolling reduction of 0.8 to 1.2%.
Table 7 shows the results of examining the structure, mechanical properties and pre-deformation-post-baking properties of the product plate thus obtained.
[0059]
[Table 5]

[0060]
[Table 6]

[0061]
[Table 7]

[0062]
As shown in Table 7, all the plated steel sheets that satisfy the requirements of the present invention have a higher increase in tensile strength due to post-forming heat treatment than in the comparative example, as in the case of studying cold-rolled steel sheets.
[0063]
【The invention's effect】
Thus, according to the present invention, while maintaining excellent workability at the time of press forming, cold-rolled steel sheets whose tensile strength is greatly improved by press forming-heat treatment, and further, plated steel sheets such as alloyed hot-dip galvanized steel sheets, It can be manufactured industrially stably.
[Brief description of the drawings]
[Fig. 1] Increase in strength due to heat treatment after molding ΔTS (= TS₁ −TS₀ It is the figure which showed the influence of [Precipitated Mn%] x [Precipitated Si%].
[Fig. 2] ΔTS (= TS for increasing strength due to heat treatment after molding₁ −TS₀ It is the figure which showed the influence of [Mn%] x [Si%] and coiling temperature which affect).
[Fig. 3] Strength increase due to heat treatment after molding (ΔTS = TS₁ −TS₀ It is the figure which showed the influence of [Mn%] x [Si%] and annealing temperature which affect).

Claims (8)

In mass percentage,
C: 0.15% or less,
Si: 0.005 to 1.0%,
Mn: 0.01 to 3.0%,
Al: 0.005 to 0.02%,
N: 0.006 to 0.020% and P: 0.002 to 0.10%
In the range satisfying N (%) / Al (%) ≧ 0.3, the balance being the composition of Fe and inevitable impurities, and the product of the contents of Mn and Si [ Mn %] × [ Si %] but 1.0 or less, with precipitation Mn% and precipitation Si% of the product in the steel 0.00010 or less, and a solid solution N containing 0.0015% or more, and wherein the further steel structure is a tissue of ferrite or ferrite-based, Cold-rolled steel sheet with excellent strength increasing ability by heat treatment after forming. In mass percentage,
C: 0.15 %Less than,
Si : 0.005 ~ 1.0 %,
Mn : 0.01 ~ 3.0 %,
Al : 0.005 ~ 0.02 %,
N: 0.006 ~ 0.020 %and
P: 0.002 ~ 0.10 %
N (%) / Al (%) ≧ 0.3 In the range that satisfies Fe And the composition of inevitable impurities, and Mn When Si Product of the content of [ Mn %] × [ Si %〕But  1.0 Larger, precipitation in steel Mn % And precipitation Si % Product  0.00010 Below, and solute N 0.0015 A cold-rolled steel sheet excellent in strength increasing ability by heat treatment after forming, wherein the steel structure is ferrite or a structure mainly composed of ferrite. The steel according to claim 1 or 2, wherein the steel is in percentage by mass,
Cr and / or Mo: 0.05 to 2.0%,
Ni: 0.1-1.5% and
Cu: 0.1-1.5%
One or two or more selected from among the above, and the steel structure is a composite structure mainly containing ferrite and containing martensite as a second phase in a volume fraction of 3 to 40%. Cold-rolled steel sheet with excellent strength increasing ability by heat treatment after forming. The steel according to claim 1, 2 or 3, wherein the steel is in percentage by mass,
A cold-rolled steel sheet excellent in strength increasing ability by heat treatment after forming, characterized in that the composition contains one or more selected from Nb, Ti and V and a total of 0.3% or less. In Claim 4 , steel is a mass percentage, and also B: 0.0003-0.0015%
A cold-rolled steel sheet excellent in strength increasing ability by heat treatment after forming, characterized in that it has a composition containing A plated steel sheet excellent in strength increasing ability by heat treatment after forming, wherein an electroplated layer or a hot-dip plated layer is formed on the surface of the cold-rolled steel sheet according to any one of claims 1 to 5 . In mass percentage,
C: 0.15% or less,
Si: 0.005 to 1.0%,
Mn: 0.01 to 3.0%,
Al: 0.005 to 0.02%,
N: 0.006 to 0.020% and P: 0.002 to 0.10%
, A steel slab having a composition containing N (%) / Al (%) ≧ 0.3 and [ Mn %] × [ Si %] ≦ 1.0 is hot-rolled and then wound after hot rolling intensity upon, the coiling temperature ≦ 700 ° C., one Ide after pickling, by after cold rolling, characterized by a 650 to 950 ° C. baked blunt temperature Te recrystallization annealing step smell, heat treatment after the molding A method for producing a cold-rolled steel sheet having excellent climbing ability. In mass percentage,
C: 0.15 %Less than,
Si : 0.005 ~ 1.0 %,
Mn : 0.01 ~ 3.0 %,
Al : 0.005 ~ 0.02 %,
N: 0.006 ~ 0.020 %and
P: 0.002 ~ 0.10 %
N (%) / Al (%) ≧ 0.3 , [ Mn %] × [ Si %]> 1.0 When the steel slab having a composition containing in a range satisfying the above is hot-rolled and then wound after hot rolling, the winding temperature ≦  300 +  400 / ([ Mn %] × [ Si %]), And after pickling and cold rolling, the annealing temperature in the recrystallization annealing step is expressed by the following equation:
 950 − 300 / ([ Mn %] × [ Si %]) ≦ Annealing temperature (° C) ≦  950
The manufacturing method of the cold-rolled steel plate excellent in the strength raising ability by the heat processing after shaping | molding characterized by setting it as the temperature which satisfy | fills.

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