JP2004100019A - Cu PRECIPITATION HARDENING TYPE STEEL MEMBER FOR WARM PRESS FORMING AND WARM PRESS FORMING METHOD THEREFOR - Google Patents
Cu PRECIPITATION HARDENING TYPE STEEL MEMBER FOR WARM PRESS FORMING AND WARM PRESS FORMING METHOD THEREFOR Download PDFInfo
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Abstract
Description
【0001】
【発明の属する技術分野】
本発明は、短時間で析出硬化することを特徴としたCu析出硬化型鋼材の温間プレス成形用Cu析出硬化型鋼材及びその温間プレス成形方法に関するものであり、プレス成形して作られる自動車用や電機製品用の内外板パネル・構造用部品、容器・缶用材料に好適な、引張強度300MPaから800MPa程度の熱延鋼材、冷延鋼材に適用可能である。また高強度でありかつ優れた疲労特性、溶接部靭性、耐食性、耐デント性を有することから、上述の他に建築物の構造部材としての適用が可能である。
【0002】
【従来の技術】
地球環境保護の観点から自動車が排出するCO2 軽減が重要な課題となっている。CO2 軽減のためには車体重量の軽減が有効であり、そのために鋼材高強度化のニーズが高まっている。ところが、一般的に材料の高強度化は形状凍結性の低下や成形時の割れといったプレス成形性の劣化を伴うことが知られており、加工性を低下させずに高強度化する方法が強く望まれていた。
【0003】
このような要望に対し、成形加工性を確保した上で高強度化を達成する技術として、成形加工時にはCuを固溶状態にさせて鋼板を軟質に保ち、室温での成形加工後に450〜700℃程度に昇温をしてCuの析出硬化現象により部品強度を増加させる方法や、成形加工自体をCuの析出硬化が起こる温間で行う方法が提案されている。
【0004】
しかしながら、鋼中におけるCuの析出は、下記の非特許文献1に示されているように、ある温度と時間の組み合わせのときに最大の強化量が得られることが知られている。例えば550℃では30分〜1時間程度の等温保持が必要であり、これ以下の温度ではさらに長時間の時効析出処理が必要である。このため、現実的にはこれらの方法ではCuの時効析出処理に長時間を要し、そのため部品の生産性が低く、その結果として製造コスト的にも高いという難点があり、現実的にCu析出強化を利用した方法はほとんど用いられてこなかった。
【0005】
【非特許文献1】
Acta Metallurgica、第20巻(1972)、971頁
【0006】
しかしながら、条件によっては降伏強度変化量で300MPa以上もの析出硬化量を期待できるCu含有鋼は、単に強度上昇だけでなく、疲労特性や溶接部靭性の向上、鋼材の耐食性の向上、耐デント性の顕著な向上が期待できるため、生産性の問題が解決されれば、その適用部材が拡がることが期待されている。このためCuの析出を促進させて短時間で硬化するプレス成形技術の開発が強く望まれていた。
【0007】
【発明が解決しようとする課題】
本発明は、短時間で製造が可能な、含Cu時効析出型鋼材及びそのプレス成形方法を提供することを目的とする。
【0008】
【課題を解決するための手段】
本発明者らは、上記の目的を達成すべく、鋭意実験と検討を重ねた結果、低歪み速度でプレス成形を行う、あるいは更に、MnあるいはCrを適正量添加した素材を用いることで、従来の技術で問題となっていた時効処理時間を大幅に短縮できることを見出した。
【0009】
すなわち、本発明の要旨は以下の通りである。
第1の発明は、質量%で、
C :0.0005〜0.2%、 Si:0.001〜2.0%、
P :0.001〜0.2%、 S :0.1%以下、
Al:0.002〜0.2%、 N :0.0005〜0.1%、
Cu:0.7〜2.0%
を含み、残部がFeおよび不可避不純物からなり、添加したCuの70%以上が固溶状態であることを特徴とする温間プレス成形用Cu析出硬化型鋼材である。
【0010】
第2の発明は、前記組成に加えて鋼材が更に質量%で、
Mn:0.1〜3.0%、 Cr:0.1〜3.0%
のうち1種または2種を含み、さらに(Mn+Cr)/Cuが0.2以上であることを特徴とする。
第3の発明は、前記第1又は第2の発明組成に加えて、鋼材が更に質量%で、
Ni:0.1〜2.0%、 Mo:0.01〜1.0%、
Nb:0.003〜0.1%、 Ti:0.003〜0.1%、
V :0.003〜0.1%、 B :0.0003〜0.1%
のうち、1種または2種以上を含むことを特徴とする。
【0011】
第4の発明は、前記第1〜第3の発明の何れか1項に記載の鋼材を、450〜700℃の温度範囲内に加熱し、当該温度範囲到達後2分以内に、1×10−5〜5×10−2s−1の歪速度の条件でプレス成形を行った後、450〜700℃で保持することを特徴とする温間プレス成形方法である。
【0012】
なお、本発明により達成されるCu粒子の450〜700℃における時効析出処理工程の最高硬さ到達までの時間短縮は、Mn+Cr量が0.1%未満の時に比べてMn+Cr量を0.1%以上含有し、(Mn+Cr)/Cuが0.2以上を満足することにより20%以上、あるいは450〜700℃の温度範囲で歪速度1s−1の温間加工を行った時に比べて本発明のプレス成形条件を満足することにより、20%以上の短縮効果が得られる条件である。
プレス成形方法としては、深絞り変形、張り出し変形、伸びフランジ変形等の立体形状が形成される際に材料移動が伴う成形法である必要があり、型を使った押出成形法の他に液圧を使った成形法も適用可能である。
【0013】
【発明の実施の形態】
本発明者らは、Cu粒子の時効析出速度に影響する因子を明らかにするために、基礎的観点に立ち返り数多くの実験と検討を重ねた。その結果、Cu粒子の析出が起こる温度域でかつその温度域に入ってから短時間の間に連続的あるいは断続的に鋼材中に空孔を導入すると、Cu粒子の析出が促進されるという全く新しい事実を見出した。
プレス成形中には変形により過剰空孔が導入されることが予測できたため、以上の知見を元にプレス成形の条件について検討を重ねた結果、図1に示すようにCuが析出する温度域において、析出温度範囲到達後の早い段階で小さい歪み速度で成形加工した後、450〜700℃で保持することにより、従来技術の大きな歪み速度で成形加工をして保持する場合より急速に析出硬化することを見出した。
【0014】
さらに本発明者らはCuの析出に及ぼす第3元素の影響について調査を行い、Cuの添加量に対し適正量のMnあるいはCrを添加することがCu粒子の時効硬化促進に効果的であることを見出した。
発明者らはその促進原因について解析を行った結果、Cu粒子の周囲にMnあるいはCrが高濃度に偏析することにより、Cu粒子成長時にCu粒子周囲に形成される歪み場が緩和される結果、Cu粒子成長の駆動力が見かけ上大きくなり、その結果、Cu粒子の析出促進が達成されるという全く新しいメカニズムにもとづくことを見出した。さらにCuの添加量に対し適正量のMnあるいはCrを添加した鋼材を、前記したような条件で温間プレス成形を行うことにより、その析出促進効果が相乗的に増大することを見出し、本発明に至った。
【0015】
以下に、本発明について詳細に説明する。
まず成分の限定理由について説明する。成分含有量は質量%である。
C:Cは鋼の組織制御に必須の添加元素であり、0.0005%以上添加するものとする。しかし、0.2%を超えると、組織がマルテンサイトのような高転位密度の組織になり、温間圧延時に導入された空孔がこの転位に吸収されてしまい、Cu粒子の成長促進に効かなくなる。このためその上限を0.2%に限定した。
【0016】
Si:SiはCと同様に、母相組織を制御するのに必須の元素であり、また脱酸元素としても必要であるので、0.001%以上添加するものとする。しかし、2.0%を超えると熱延時の脱スケール性の悪化やコスト高を招く。従ってSi含有量は2.0%以下の範囲に制限した。
【0017】
P:Pは鋼中の転位密度を制御するために用いられる元素であるので、0.001%以上含有するものとする。0.2%を超えると加工割れを起こすので、P含有量の範囲を0.2%以下とした。
【0018】
S:Sは不純物であり、多量に含有すると熱間加工割れを起こすので、0.1%以下とした。
【0019】
Al:Alは脱酸元素として用いる元素であるので、0.002%以上含有するものとする。しかしAlの添加量が0.2%を超えると、AlNの析出量が増加しCuの添加効果が失われるため、Al含有量の適正添加範囲を0.2%以下とした。
【0020】
N:Nは窒化物として、主にオーステナイト域の結晶粒径制御に用いられるので、0.0005%以上含有するものとする。Nが0.1%を超えると、フェライト粒内に多量の炭窒化物が析出しCuの添加効果が失われ、さらにCの場合と同様に高転位密度の組織となるため、N含有量の範囲を0.1%以下とした。
【0021】
Cu:Cuは本発明における不可欠な構成元素である。しかし、0.7%未満であるとCuの析出硬化が十分に発現せず、また2.0%を超えるとCuの熱間脆性による鋼板の表面割れが顕著になるために、Cu含有量の範囲を0.7〜2.0%の範囲に制限した。ただし、添加量の下限については、Cu析出粒子の体積分率をより多くするという観点から1.0%以上の添加が望ましい。
なお、より軟質な状態で温間プレス成形を開始することが望ましいので、成形前のCuは固溶状態であることがより望ましい。Cuは炭素当量を上げない元素でもあるので溶接性の向上に有効であり、疲労特性、耐食性の向上にも効果があるという付加的効果がある。
【0022】
固溶Cu:Cuは温間成形中に析出することにより硬化に寄与する。しかしながら、含有するCu量の内、温間成形前にCu粒子として既に析出してしまっているもの、あるいはCuS等の硫化物として存在しているものは、温間成形中の時効硬化に寄与しない。従って、固溶状態のCu量が含有Cu量の70%以上であるものとし、さらに90%以上であることが好ましい。
なお、Cuの固溶量は、3次元アトムプローブ電界イオン顕微鏡を用いて10000nm3 以上の体積のマトリックス分析を行った際に、Cuのクラスタリングあるいは析出が起こっている部分を除いた領域の平均Cu濃度を測定することによって求められる。
【0023】
成形前の素材において添加したCuの70%以上を固溶状態にする方法としては、鋼材の製造時において、950℃以上の温度でCuの溶体化処理を行った後、熱間圧延・冷却プロセスにおいてCuの析出温度域である700〜500℃の温度範囲を平均冷却速度10℃/s以上で冷却する方法が好適である。
【0024】
Mn:MnはCu粒子の析出を促進させるために有効であり、またAr3 変態点を低下させることで母相組織を制御するのに好ましい元素である。しかしながら0.1%未満であると十分なCu粒子析出促進効果が得られず、3.0%を超えると鋼材の熱間変形抵抗が増大する傾向になり、また溶接性も悪化する。このためMn含有量は0.1〜3.0%の範囲に制限した。
【0025】
Cr:CrはMnと同様に、Cu粒子の析出を促進させるために有効である。しかしながら0.1%未満であると十分なCu粒子析出促進効果が得られず、3.0%を超えると鋼材の熱間変形抵抗が増大する傾向になり、また溶接性も悪化する。このためCr含有量は0.1〜3.0%の範囲に制限した。
【0026】
(Mn+Cr)/Cu比:CrとMnはいずれもCuと同時に添加することでCu粒子の析出を促進させる効果があるが、Cuの添加量に対して0.2未満であるとMnあるいはCrがCu粒子周囲に十分に偏析せず、その結果20%以上の時効硬化時間短縮量が得られないため、その範囲を0.2以上に制限した。
【0027】
Ni:NiはCu添加に起因する熱間脆性の抑制と母相組織の制御に用いられる。一般的に、添加Cu量と等量のNiを添加するとCuによる顕著な熱間割れを抑制できる。0.1%未満であるとCu起因の熱間割れを抑制できず、また2.0%を超えるとコスト高を招く。従って、その適正添加範囲を0.1〜2.0%に限定した。
【0028】
Mo:Moは炭窒化物として再加熱時のオーステナイト粒径を制御する元素として0.01%以上必要である。しかしながら、1.0%を超えると、多量の炭窒化物の析出によりCu析出物の効果が失われる。従って、その適正添加範囲を0.01〜1.0%に限定した。
【0029】
Nb:Nbは炭窒化物として再加熱時のオーステナイト粒径を制御する元素として0.003%以上必要である。しかしNbの添加量が0.1%を超えるとフェライト中の炭窒化物量が増え、Cuの添加効果が失われるため、Nb含有量の適正添加範囲を0.003〜0.1%とした。
【0030】
Ti:Tiは脱酸元素として、また炭窒化物として再加熱時のオーステナイト粒径を制御する元素として0.003%以上必要である。しかしTiの添加量が0.1%を超えるとCuの添加効果が失われるため、Ti含有量の適正添加範囲を0.003〜0.1%以下とした。
【0031】
V:Vは炭窒化物として再加熱時のオーステナイト粒径を制御する元素として0.003%以上必要である。しかしVの添加量が0.1%を超えるとフェライト中の炭窒化物量が増え、Cuの添加効果が失われるため、V含有量の適正添加範囲を0.003〜0.1%とした。
【0032】
B:Bは母相組織を制御するために用いられる元素であるので、0.0003%以上必要である。0.1%を超えると粒界に炭ホウ化物、ホウ窒化物が析出し延性の悪化を引き起こす。従って、その適正添加範囲を0.0003〜0.1%に限定した。
【0033】
平均結晶粒径については、平均フェライト粒径が3μmより小さいと、Cu粒子の大部分が粒界に析出してしまい十分な時効硬化が得られず、さらに温間成形により導入された空孔が粒界に拡散してしまい時効の促進効果も得られないので、平均フェライト粒径は3μm以上であることが好ましく、10μm以上であることがより好ましい。平均フェライト粒径の上限は特に限定することなく本発明の効果を得ることができる。
【0034】
次に、プレス成形方法の限定理由について説明する。
温間プレス成形温度が450℃未満であると、Cuの析出に長時間を要し製造コスト面で不利であり、また700℃を超えるとフェライト中で析出するCu粒子の量が少なくなり、Cu鋼特有の大きな析出硬化量を期待できない。また450〜700℃の温度範囲に入ったあと2分を超えてからプレス成形を行うか、あるいは5×10−5s−1を超える歪速度では時効時間短縮効果は小さい。
従って、温間プレス成形の温度を450℃〜700℃の温度範囲内に制限し、さらに成形タイミング、成形の歪み速度を、それぞれ当該温度範囲到達後2分以内、5×10−5s−1以下に制限した。
なお、成形加工開始のタイミングについては、より効果的にCu粒子の析出を促進するためには、当該温度範囲到達後10秒以内に開始し、1×10−3s−1以下の遅い歪み速度ですることがより好ましい。
【0035】
その後、Cu析出を促進させるために、450℃〜700℃で保持を行う。保持時間は特に限定しないが、生産性の観点から、600秒以下とすることが好ましい。また、析出強化量確保の点から5秒以上とすることが好ましい。
【0036】
なお、本発明に係る成形方法は、Mn若しくはCrを適正量添加せず、450〜700℃に到達後2分以内に低歪速度でプレス成形しない従来方法、又はMn若しくはCrを適正量添加しても同じ温度時間条件において高歪み速度でプレス成形加工を行う従来方法に比して時効硬化時間短縮効果が得られることを特徴としているが、実製造現場では1つの成形に費やされる時間の絶対値が短ければ短いほど良い。
【0037】
このように生産性の観点からは、630℃以上で本発明に示す組成の鋼材を所定の条件で成形加工することが望ましい。またMn,Crを含有する鋼材を用い、本発明のプレス成形条件で成形加工することにより、相乗的な時間短縮効果が得られ、具体的には従来素材を1s−1の歪み速度で成形したときに比べて、同一のCu析出硬化量が得られるまでの時間を40%以上短縮することができる。
【0038】
【実施例】
次にこの発明を実施例により詳細に説明する。
表1に示す成分の鋼材A〜Gを、表2に示す条件で温間引張試験を行うことにより、鋼材を温間成形した時の強度変化のシミュレート実験を行った。
鋼材はそれぞれの成分からなる鋼片を1100〜1270℃に加熱して、1100〜900℃で熱間圧延を開始し、950〜700℃で仕上げ圧延を終了した後、700〜500℃の温度範囲を10〜100℃/sで冷却し、一部についてはさらに200〜500℃で巻き取ることにより製造した。
【0039】
なお、実際の成形加工においては、予歪量やその変形モード(引張か圧縮か、あるいは1軸引張か2軸引張か等)は被成形部材の場所ごとに異なるが、本発明者らが様々な成形加工形式と歪み時効硬化挙動を詳細に検討した結果、単純な1軸の引張歪みで上記の変形部の特性が良く代表できることを突き止めており、本発明ではこの知見を基に1軸の引張歪を温間で印加してその硬化挙動を調査する手法を用いた。
【0040】
試験片は1mm厚の板状高温引張試験片を用い、はじめに所定の加工温度まで昇温し、所定の時間保持した後、表2に記載の歪み速度、歪み量で引張歪みを加えた後、一旦除荷し、さらにいくつかの異なった時間で加工温度と同じ温度で等温保持を行った後室温まで冷却し、この温度にて再引張試験を行うことで時効後の試験片の引張強度を測定し、この引張強度を図1に示すように時効時間に対してプロットした。次いで、この各プロットから硬化量が最大になる時間を求め、この値を指標として比較材に比べた際のCu析出硬化時間の短縮効果を定量的に算出した。
【0041】
試料No.1〜20は全て本発明の範囲を満たす鋼材の例であるが、試料No.1、6、9、12、15、17は引張歪を付加せずに温間で等温保持のみ行った試料である。
Mn,Crを添加せず、温間加工も実施していないNo.1はNo.1〜20の中で硬化量が最大になるまでの時間が最も長い。
Mn,Crを添加していないNo.1に対して、Mn、Crを添加したNo.6、9は硬度が最大値になるまでの時間がNo.1に比べて43%短くなっている。
同様に、Mn,Crを添加していないNo.15に対して、Mn、Crの両方を適正量添加したNo.17は硬度が最大値になるまでの時間がNo.15に比べて25%短くなっている。
【0042】
試料No.2〜5、7、8、10、11、13、14、16、18〜20は、Cu析出挙動に及ぼす温間加工条件を調べたものである。試料No.2〜5を比較すると、歪み速度あるいは適正温度到達後から加工開始までの時間が適正範囲外であるNo.3、4は無加工のNo.1に比しても10%以下の時効短縮効果しか得られず、一方、歪み速度あるいは適正温度到達後から加工開始までの時間が適正範囲にあるNo.2と5はNo.1、3、4に比して、大きな時効時間短縮効果が得られている。
【0043】
No.8、14、19は歪み速度あるいは適正温度到達後から加工開始までの時間が適正範囲外であるために、顕著な時効時間短縮効果が得られなかった例である。
No.7、10は、Mn又はCr、温間引張条件の双方が本発明範囲内にある例であり、無加工、Mn、Cr添加無しのNo.1に比べてそれぞれ76%、70%の極めて大きな時効時間短縮効果が得られている。
【0044】
No.8、11は、加工条件が発明範囲外にあるものである。Mn又はCr、温間引張条件の双方が適正範囲内にあるNo.7、10と比べるとその時間短縮効果は小さいが、Mn又はCrを含有しないNo.1に比べると大きな時間短縮効果が得られている。
No.19、20も、Mn及びCrを含有するが、加工条件が発明範囲外にあるものである。加工条件が適正範囲内にあるNo.18に比べるとその時間短縮効果は小さいが、Mn又はCrを含有しないNo.15に比べると大きな時間短縮効果が得られている。
試料No.21は本発明の成分を外れる鋼材であり、加工条件が本発明の範囲を満たしても顕著な析出硬化は見られず、従って時間短縮効果も得られなかった。
【0045】
【表1】
【0046】
【表2】
【0047】
【表3】
【0048】
【発明の効果】
本発明は、自動車用や電機製品用の内外板パネル・構造用部品に好適な含Cu鋼材の温間プレス成形プロセスにかかる時間を従来に比して最大で1/5程度に著しく短縮させる効果があり、産業上の効果は極めて高い。またMn又はCrを含有し、かつ本発明の成形方法の組み合わせにより製造される部材は高強度でありかつ優れた疲労特性、溶接部靭性、耐食性、耐デント性を有することから、自動車用や電機製品用だけでなく、建築物の構造部材としても適用が可能であるという波及的効果を有する。
【図面の簡単な説明】
【図1】Fe−1.5Cu合金の時効曲線であり、530℃の時効初期に1×10−3s−1の歪み速度で0.1の歪み量で温間引張をすることによってCu粒子の析出が促進されて、硬度ピークまでの時間が1×100 s−1の歪み速度の場合に比べて約1/2になった例を示す図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a Cu precipitation hardening type steel material for warm press forming a Cu precipitation hardening type steel material characterized by precipitation hardening in a short time, and a warm press forming method thereof, and an automobile formed by press forming. It can be applied to hot-rolled steel materials and cold-rolled steel materials having a tensile strength of about 300 MPa to 800 MPa, which are suitable for inner / outer panel panels / structural parts for containers and electric appliances, and materials for containers / cans. Further, since it has high strength and excellent fatigue characteristics, weld toughness, corrosion resistance, and dent resistance, it can be applied as a structural member of a building in addition to the above.
[0002]
[Prior art]
CO 2 mitigation for discharging the automobile from the viewpoint of global environmental protection has become an important issue. To reduce CO 2 , it is effective to reduce the weight of the vehicle body. Therefore, there is a growing need for higher strength steel materials. However, it is generally known that increasing the strength of a material is accompanied by deterioration of press formability such as a decrease in shape freezing property and cracking during molding, and a method of increasing strength without reducing workability is strongly used. Was desired.
[0003]
In response to such demands, as a technique for achieving high strength while ensuring formability, as a technique for forming a solid solution of Cu during forming, the steel sheet is kept soft, and after forming at room temperature, 450 to 700 A method has been proposed in which the strength of the component is increased by increasing the temperature to about ° C. by the precipitation hardening phenomenon of Cu, or a method in which the forming process itself is performed at a temperature at which the precipitation hardening of Cu occurs.
[0004]
However, as shown in Non-Patent Document 1 below, it is known that the maximum amount of strengthening can be obtained at a certain combination of temperature and time, as shown in Non-Patent Document 1 below. For example, at 550 ° C., isothermal holding for about 30 minutes to 1 hour is required, and at temperatures lower than this, aging precipitation treatment for a longer time is required. For this reason, in practice, these methods require a long time for the aging precipitation treatment of Cu, so that the productivity of parts is low, and as a result, the production cost is high. Reinforcement methods have rarely been used.
[0005]
[Non-patent document 1]
Acta Metallurgica, Vol. 20 (1972), p. 971.
However, depending on conditions, Cu-containing steel, which can be expected to have a precipitation hardening amount of 300 MPa or more in yield strength change amount, not only increases strength but also improves fatigue characteristics and weld toughness, improves corrosion resistance of steel materials, and improves dent resistance. Since remarkable improvement can be expected, if the problem of productivity is solved, it is expected that the applicable members will be expanded. For this reason, there has been a strong demand for the development of a press molding technique for accelerating the precipitation of Cu and hardening in a short time.
[0007]
[Problems to be solved by the invention]
An object of the present invention is to provide a Cu-containing aging-precipitation-type steel material that can be manufactured in a short time and a press forming method thereof.
[0008]
[Means for Solving the Problems]
The present inventors have conducted intensive experiments and studies in order to achieve the above object, and as a result, by performing press forming at a low strain rate, or by using a material further added with an appropriate amount of Mn or Cr, It has been found that the aging treatment time, which has been a problem with the technology, can be greatly reduced.
[0009]
That is, the gist of the present invention is as follows.
The first invention is, in mass%,
C: 0.0005 to 0.2%, Si: 0.001 to 2.0%,
P: 0.001 to 0.2%, S: 0.1% or less,
Al: 0.002 to 0.2%, N: 0.0005 to 0.1%,
Cu: 0.7 to 2.0%
And a balance consisting of Fe and unavoidable impurities, wherein 70% or more of the added Cu is in a solid solution state, and is a Cu precipitation hardening steel material for warm press forming.
[0010]
According to a second aspect of the present invention, in addition to the composition, the steel material further includes
Mn: 0.1-3.0%, Cr: 0.1-3.0%
And (Mn + Cr) / Cu is 0.2 or more.
According to a third invention, in addition to the first or second invention composition, the steel material further includes
Ni: 0.1 to 2.0%, Mo: 0.01 to 1.0%,
Nb: 0.003 to 0.1%, Ti: 0.003 to 0.1%,
V: 0.003 to 0.1%, B: 0.0003 to 0.1%
Among them, one or more kinds are included.
[0011]
A fourth invention heats the steel material according to any one of the first to third inventions in a temperature range of 450 to 700 ° C., and within 2 minutes after reaching the temperature range, 1 × 10 4 This is a warm press forming method characterized in that after performing press forming under a condition of a strain rate of −5 to 5 × 10 −2 s −1 , the temperature is maintained at 450 to 700 ° C.
[0012]
The shortening of the time required for the aging precipitation treatment step at 450 to 700 ° C. of the Cu particles achieved by the present invention to reach the maximum hardness is achieved by reducing the amount of Mn + Cr by 0.1% as compared with the case where the amount of Mn + Cr is less than 0.1%. More than 20% when (Mn + Cr) / Cu satisfies 0.2 or more, or when compared with the case where warm working was performed at a strain rate of 1 s -1 in a temperature range of 450 to 700 ° C. By satisfying the press molding conditions, it is a condition under which a shortening effect of 20% or more can be obtained.
The press molding method must be a molding method that involves the movement of materials when a three-dimensional shape such as deep drawing deformation, overhang deformation, stretch flange deformation, etc. is formed. A molding method using is also applicable.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors went back to the basic viewpoint and repeated many experiments and studies in order to clarify factors affecting the aging precipitation rate of Cu particles. As a result, when vacancies are introduced into the steel material continuously or intermittently within a temperature range where precipitation of Cu particles occurs and within a short time after entering the temperature range, precipitation of Cu particles is promoted. I found a new fact.
Since it was predicted that excessive vacancies were introduced due to deformation during press molding, based on the above findings, repeated examinations were conducted on the conditions of press molding. As a result, as shown in FIG. After forming at a low strain rate at an early stage after reaching the precipitation temperature range, by holding at 450 to 700 ° C., precipitation hardens more rapidly than when forming and holding at a high strain rate in the prior art. I found that.
[0014]
Furthermore, the present inventors investigated the effect of the third element on the precipitation of Cu, and found that adding an appropriate amount of Mn or Cr to the added amount of Cu is effective in promoting age hardening of Cu particles. Was found.
The present inventors have analyzed the cause of the promotion, and as a result of segregation of Mn or Cr at a high concentration around the Cu particles, the strain field formed around the Cu particles during the growth of the Cu particles is relaxed. It has been found that the driving force for the growth of Cu particles is apparently large, and as a result, it is based on a completely new mechanism of promoting the precipitation of Cu particles. Further, the present inventors have found that a steel material to which an appropriate amount of Mn or Cr is added with respect to the added amount of Cu is subjected to warm press forming under the above-described conditions, whereby the effect of accelerating the precipitation is synergistically increased. Reached.
[0015]
Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the components will be described. The component content is% by mass.
C: C is an additive element indispensable for controlling the structure of steel, and should be added at 0.0005% or more. However, if it exceeds 0.2%, the structure becomes a structure having a high dislocation density such as martensite, and the vacancies introduced during warm rolling are absorbed by these dislocations, which is not effective in promoting the growth of Cu particles. Disappears. Therefore, the upper limit is limited to 0.2%.
[0016]
Si: Like C, Si is an essential element for controlling the matrix structure, and is also required as a deoxidizing element. Therefore, Si is added in an amount of 0.001% or more. However, if it exceeds 2.0%, the descaling property at the time of hot rolling is deteriorated and the cost is increased. Therefore, the Si content was limited to a range of 2.0% or less.
[0017]
P: Since P is an element used to control the dislocation density in steel, it should be contained at 0.001% or more. If it exceeds 0.2%, a work crack occurs, so the range of the P content is set to 0.2% or less.
[0018]
S: S is an impurity, and if contained in a large amount, causes hot working cracking.
[0019]
Al: Since Al is an element used as a deoxidizing element, it should be contained at 0.002% or more. However, when the addition amount of Al exceeds 0.2%, the precipitation amount of AlN increases and the effect of adding Cu is lost, so the appropriate addition range of the Al content is set to 0.2% or less.
[0020]
N: Since N is mainly used for controlling the crystal grain size in the austenite region as a nitride, it should be contained at 0.0005% or more. If N exceeds 0.1%, a large amount of carbonitride precipitates in the ferrite grains, losing the effect of adding Cu, and further has a high dislocation density structure as in the case of C. The range was set to 0.1% or less.
[0021]
Cu: Cu is an essential constituent element in the present invention. However, if it is less than 0.7%, the precipitation hardening of Cu is not sufficiently exhibited, and if it exceeds 2.0%, the surface cracks of the steel sheet due to the hot embrittlement of Cu become remarkable, so that the Cu content is lowered. The range was restricted to the range of 0.7-2.0%. However, the lower limit of the addition amount is desirably 1.0% or more from the viewpoint of increasing the volume fraction of Cu precipitated particles.
Since it is desirable to start warm press forming in a softer state, it is more preferable that Cu before forming is in a solid solution state. Cu is an element that does not increase the carbon equivalent, and thus is effective in improving weldability, and has the additional effect of improving fatigue characteristics and corrosion resistance.
[0022]
Solid solution Cu: Cu contributes to hardening by precipitating during warm forming. However, of the contained Cu amounts, those that have already precipitated as Cu particles before warm forming or those that exist as sulfides such as CuS do not contribute to age hardening during warm forming. . Therefore, the amount of Cu in the solid solution state should be 70% or more of the contained Cu amount, and more preferably 90% or more.
In addition, when the solid solution amount of Cu is determined by performing a matrix analysis of 10,000 nm 3 or more using a three-dimensional atom probe field ion microscope, the average Cu in a region excluding a portion where clustering or precipitation of Cu occurs is performed. It is determined by measuring the concentration.
[0023]
As a method for converting 70% or more of Cu added to a material before forming into a solid solution state, a solution treatment of Cu is performed at a temperature of 950 ° C. or more during the production of a steel material, followed by a hot rolling / cooling process. In this method, a method of cooling a temperature range of 700 to 500 ° C., which is a temperature range for depositing Cu, at an average cooling rate of 10 ° C./s or more is preferable.
[0024]
Mn: Mn is effective for accelerating the precipitation of Cu particles, and is a preferable element for controlling the matrix structure by lowering the Ar3 transformation point. However, if it is less than 0.1%, a sufficient effect of accelerating the precipitation of Cu particles cannot be obtained, and if it exceeds 3.0%, the hot deformation resistance of the steel material tends to increase, and the weldability also deteriorates. For this reason, the Mn content is limited to the range of 0.1 to 3.0%.
[0025]
Cr: Like Cr, Cr is effective for promoting the precipitation of Cu particles. However, if it is less than 0.1%, a sufficient effect of accelerating the precipitation of Cu particles cannot be obtained, and if it exceeds 3.0%, the hot deformation resistance of the steel material tends to increase, and the weldability also deteriorates. For this reason, the Cr content was limited to the range of 0.1 to 3.0%.
[0026]
(Mn + Cr) / Cu ratio: When both Cr and Mn are added simultaneously with Cu, they have an effect of accelerating the precipitation of Cu particles. Since the segregation does not sufficiently occur around the Cu particles and as a result, the age hardening time reduction amount of 20% or more cannot be obtained, the range is limited to 0.2 or more.
[0027]
Ni: Ni is used for suppressing hot embrittlement and controlling the matrix structure due to the addition of Cu. Generally, when Ni is added in an amount equal to the amount of added Cu, remarkable hot cracking due to Cu can be suppressed. If it is less than 0.1%, hot cracking caused by Cu cannot be suppressed, and if it exceeds 2.0%, cost increases. Therefore, the appropriate addition range was limited to 0.1 to 2.0%.
[0028]
Mo: Mo needs to be 0.01% or more as an element for controlling the austenite grain size at the time of reheating as a carbonitride. However, if it exceeds 1.0%, the effect of the Cu precipitate is lost due to the precipitation of a large amount of carbonitride. Therefore, the appropriate addition range was limited to 0.01 to 1.0%.
[0029]
Nb: Nb is required to be at least 0.003% as an element for controlling the austenite grain size during reheating as a carbonitride. However, if the addition amount of Nb exceeds 0.1%, the amount of carbonitride in ferrite increases, and the effect of adding Cu is lost. Therefore, the appropriate addition range of the Nb content is set to 0.003 to 0.1%.
[0030]
Ti: Ti needs to be 0.003% or more as an element for controlling the austenite particle size at the time of reheating as a deoxidizing element and as a carbonitride. However, if the added amount of Ti exceeds 0.1%, the effect of adding Cu is lost, so the proper addition range of the Ti content is set to 0.003 to 0.1% or less.
[0031]
V: V needs to be 0.003% or more as an element for controlling the austenite grain size at the time of reheating as a carbonitride. However, if the added amount of V exceeds 0.1%, the amount of carbonitride in the ferrite increases and the effect of adding Cu is lost. Therefore, the appropriate addition range of the V content is set to 0.003 to 0.1%.
[0032]
B: Since B is an element used to control the matrix structure, 0.0003% or more is required. If it exceeds 0.1%, carbon borides and boron nitrides precipitate at the grain boundaries, causing deterioration in ductility. Therefore, the appropriate addition range was limited to 0.0003 to 0.1%.
[0033]
Regarding the average crystal grain size, if the average ferrite grain size is smaller than 3 μm, most of the Cu particles are precipitated at the grain boundaries, and sufficient age hardening cannot be obtained. The average ferrite particle size is preferably 3 μm or more, more preferably 10 μm or more, because the average ferrite particle diameter is diffused to the grain boundaries and the effect of promoting aging cannot be obtained. The upper limit of the average ferrite grain size is not particularly limited, and the effects of the present invention can be obtained.
[0034]
Next, the reasons for the limitation of the press molding method will be described.
If the warm press forming temperature is lower than 450 ° C., it takes a long time to precipitate Cu, which is disadvantageous in terms of manufacturing cost. If the temperature exceeds 700 ° C., the amount of Cu particles precipitated in ferrite decreases, and Cu A large precipitation hardening characteristic of steel cannot be expected. Pressing is performed after 2 minutes after entering the temperature range of 450 to 700 ° C., or the effect of shortening the aging time is small at a strain rate exceeding 5 × 10 −5 s −1 .
Therefore, the temperature of the warm press forming is limited to a temperature range of 450 ° C. to 700 ° C., and the forming timing and the forming strain rate are each set within 5 minutes after reaching the temperature range, 5 × 10 −5 s −1. Limited to:
In order to more effectively promote the precipitation of Cu particles, the timing of the start of the forming process starts within 10 seconds after reaching the temperature range, and starts at a low strain rate of 1 × 10 −3 s −1 or less. Is more preferable.
[0035]
Thereafter, in order to promote Cu deposition, the temperature is held at 450 ° C to 700 ° C. The holding time is not particularly limited, but is preferably 600 seconds or less from the viewpoint of productivity. Further, it is preferable to set it to 5 seconds or more from the viewpoint of securing the precipitation strengthening amount.
[0036]
In addition, the molding method according to the present invention does not add an appropriate amount of Mn or Cr, a conventional method in which press molding is not performed at a low strain rate within 2 minutes after reaching 450 to 700 ° C., or an appropriate amount of Mn or Cr is added. However, it is characterized in that the effect of shortening the age hardening time can be obtained as compared with the conventional method of performing press forming at a high strain rate under the same temperature and time conditions. The shorter the value, the better.
[0037]
Thus, from the viewpoint of productivity, it is desirable to form a steel material having the composition shown in the present invention at 630 ° C. or higher under predetermined conditions. By using a steel material containing Mn and Cr and forming under the press forming conditions of the present invention, a synergistic time reduction effect can be obtained. Specifically, the conventional material was formed at a strain rate of 1 s -1 . The time required for obtaining the same Cu precipitation hardening amount can be shortened by 40% or more as compared with the case where it is sometimes.
[0038]
【Example】
Next, the present invention will be described in detail with reference to examples.
By performing a warm tensile test on the steel materials A to G having the components shown in Table 1 under the conditions shown in Table 2, a simulation experiment of a change in strength when the steel material was warm formed was performed.
The steel material is obtained by heating a slab composed of the respective components to 1100 to 1270 ° C, starting hot rolling at 1100 to 900 ° C, and finishing finish rolling at 950 to 700 ° C, and then a temperature range of 700 to 500 ° C. Was cooled at 10 to 100 ° C./s, and a portion was further wound at 200 to 500 ° C.
[0039]
In the actual forming process, the amount of pre-strain and its deformation mode (tensile or compressive, uniaxial or biaxial tension, etc.) differ depending on the location of the molded member, but the present inventors have As a result of a detailed study of the various forming processes and strain age hardening behavior, it has been found that the characteristics of the deformed portion can be well represented by a simple uniaxial tensile strain. A method of applying a tensile strain in a warm state and investigating its hardening behavior was used.
[0040]
The test piece was a plate-shaped high-temperature tensile test piece having a thickness of 1 mm. First, the temperature was raised to a predetermined processing temperature, and after holding for a predetermined time, tensile strain was applied at the strain rate and strain amount shown in Table 2, Once unloaded, further maintained at the same temperature as the processing temperature for several different times, cooled to room temperature, and re-tensile test at this temperature to increase the tensile strength of the aged specimen. The tensile strength was measured and plotted against the aging time as shown in FIG. Next, the time at which the hardening amount was maximized was determined from each plot, and using this value as an index, the effect of shortening the Cu precipitation hardening time as compared with the comparative material was quantitatively calculated.
[0041]
Sample No. Sample Nos. 1 to 20 are all steel materials satisfying the scope of the present invention. Samples 1, 6, 9, 12, 15, and 17 are samples that have been subjected to only isothermal holding at warm without adding tensile strain.
No. Mn and Cr were not added and warm working was not performed. No. 1 is No. It takes the longest time to reach the maximum curing amount among 1 to 20.
Nos. Mn and Cr were not added. No. 1 to which Mn and Cr were added. In Nos. 6 and 9, the time required for the hardness to reach the maximum value was No. 6; 43% shorter than 1.
Similarly, the sample No. to which Mn and Cr were not added. No. 15 in which both Mn and Cr were added in appropriate amounts. No. 17 is the time required for the hardness to reach the maximum value. It is 25% shorter than 15.
[0042]
Sample No. 2 to 5, 7, 8, 10, 11, 13, 14, 16, and 18 to 20 were obtained by examining warm working conditions affecting the Cu deposition behavior. Sample No. Comparing No. 2 to No. 5, the time from the arrival of the strain rate or the appropriate temperature to the start of machining is out of the appropriate range. Nos. 3 and 4 are unprocessed Nos. In comparison with No. 1, only an aging reduction effect of 10% or less was obtained. Nos. 2 and 5 are Nos. Compared with 1, 3, and 4, a large aging time shortening effect is obtained.
[0043]
No. 8, 14, and 19 are examples in which a remarkable aging time shortening effect was not obtained because the time from the arrival of the strain rate or the appropriate temperature to the start of processing was outside the appropriate range.
No. Nos. 7 and 10 are examples in which both Mn or Cr and warm tensile conditions are within the scope of the present invention, and No. The aging time shortening effects of 76% and 70%, respectively, are significantly greater than those of No. 1.
[0044]
No. Nos. 8 and 11 are processing conditions outside the scope of the invention. No. 3 in which both Mn or Cr and the warm tensile conditions are within the appropriate ranges. Although the time shortening effect is small as compared with Nos. 7 and 10, No. 7 containing no Mn or Cr was used. Compared with 1, a large time reduction effect is obtained.
No. 19 and 20 also contain Mn and Cr, but the processing conditions are outside the scope of the invention. No. whose processing conditions are within the proper range. The time shortening effect is smaller than that of No. 18, but No. 18 containing no Mn or Cr. Compared with No. 15, a large time reduction effect is obtained.
Sample No. Reference numeral 21 denotes a steel material deviating from the components of the present invention. No remarkable precipitation hardening was observed even when the processing conditions satisfied the range of the present invention, and therefore, no time shortening effect was obtained.
[0045]
[Table 1]
[0046]
[Table 2]
[0047]
[Table 3]
[0048]
【The invention's effect】
The present invention has the effect of significantly reducing the time required for the warm press forming process of a Cu-containing steel material suitable for inner and outer panel panels and structural parts for automobiles and electric appliances to at most about 1/5 as compared with the related art. The industrial effect is extremely high. A member containing Mn or Cr and manufactured by the combination of the molding method of the present invention has high strength and excellent fatigue characteristics, weld toughness, corrosion resistance, and dent resistance. It has a ripple effect that it can be applied not only to products but also to structural members of buildings.
[Brief description of the drawings]
FIG. 1 is an aging curve of an Fe-1.5Cu alloy, in which Cu particles are obtained by performing a warm tension at a strain rate of 1 × 10 −3 s −1 and a strain amount of 0.1 in an early stage of aging at 530 ° C. FIG. 6 is a diagram showing an example in which precipitation of GaN is promoted, and the time until a hardness peak is reduced to about 比 べ as compared with the case of a strain rate of 1 × 10 0 s −1 .
Claims (4)
C :0.0005〜0.2%、
Si:0.001〜2.0%、
P :0.001〜0.2%、
S :0.1%以下、
Al:0.002〜0.2%、
N :0.0005〜0.1%、
Cu:0.7〜2.0%
を含み、残部がFeおよび不可避不純物からなり、Cuの70%以上が固溶状態であることを特徴とする温間プレス成形用Cu析出硬化型鋼材。In mass%,
C: 0.0005 to 0.2%,
Si: 0.001 to 2.0%,
P: 0.001 to 0.2%,
S: 0.1% or less,
Al: 0.002 to 0.2%,
N: 0.0005 to 0.1%,
Cu: 0.7 to 2.0%
And a balance consisting of Fe and unavoidable impurities, and 70% or more of Cu is in a solid solution state.
Mn:0.1〜3.0%、
Cr:0.1〜3.0%
のうち1種または2種を含み、さらに(Mn+Cr)/Cuが0.2以上であることを特徴とする請求項1記載の温間プレス成形用Cu析出硬化型鋼材。In addition to the above composition, by mass%,
Mn: 0.1-3.0%,
Cr: 0.1-3.0%
The Cu precipitation hardening steel material for warm press forming according to claim 1, wherein one or two of the above are included, and (Mn + Cr) / Cu is 0.2 or more.
Ni:0.1〜2.0%、
Mo:0.01〜1.0%、
Nb:0.003〜0.1%、
Ti:0.003〜0.1%、
V :0.003〜0.1%、
B :0.0003〜0.1%
のうち、1種または2種以上を含むことを特徴とする請求項1又は2記載の温間プレス成形用Cu析出硬化型鋼材。In addition to the above composition, by mass%,
Ni: 0.1 to 2.0%,
Mo: 0.01 to 1.0%,
Nb: 0.003 to 0.1%,
Ti: 0.003 to 0.1%,
V: 0.003-0.1%,
B: 0.0003-0.1%
The Cu precipitation hardening type steel material for warm press forming according to claim 1, wherein one or more types are included.
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Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2008075145A (en) * | 2006-09-22 | 2008-04-03 | Kobe Steel Ltd | High-strength steel material excellent in fatigue characteristic, and producing method therefor |
JP2010188393A (en) * | 2009-02-19 | 2010-09-02 | Sumitomo Metal Ind Ltd | Method for press-forming steel sheet |
-
2002
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Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2008075145A (en) * | 2006-09-22 | 2008-04-03 | Kobe Steel Ltd | High-strength steel material excellent in fatigue characteristic, and producing method therefor |
JP4671238B2 (en) * | 2006-09-22 | 2011-04-13 | 株式会社神戸製鋼所 | High-strength steel material with excellent fatigue characteristics and method for producing the same |
JP2010188393A (en) * | 2009-02-19 | 2010-09-02 | Sumitomo Metal Ind Ltd | Method for press-forming steel sheet |
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