JP3873638B2

JP3873638B2 - Hot-dip galvanized steel sheet and manufacturing method thereof

Info

Publication number: JP3873638B2
Application number: JP2001067278A
Authority: JP
Inventors: 総人北野; 康伸長滝; 康弘重田; 昭蒲
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2001-03-09
Filing date: 2001-03-09
Publication date: 2007-01-24
Anticipated expiration: 2021-03-09
Also published as: JP2002266032A

Description

【０００１】
【発明の属する技術分野】
本発明は溶融亜鉛めっき鋼板の製造方法に関する。
【０００２】
【従来の技術】
近年、自動車メーカーにおける地球環境保護に対する取り組みの一つとして、低燃比化を目的とした自動車車体の軽量化が実施されている。車体の軽量化の有効な手法として、自動車のメンバー、ロッカー、ピラーなどの各種構造部品の素材に高強度鋼板を適用し、素材板厚を薄肉化することが検討されてきた。また、最近では、自動車の衝突安全規制が高まる状況にあり、各種補強部材にも高強度鋼板の適用化が進められている。
【０００３】
高強度鋼板を実際の自動車部品に成形する場合、主として引張変形が付与される部位には、張り出し性や伸びフランジ性などのプレス成形性が要求されており、こうした要求に対し、従来から種々の高強度鋼板が開発されている。例えば、特開昭58-39770号公報には伸びフランジ性の優れた高強度溶融亜鉛めっき鋼板の製造方法が開示されている。この技術では、フェライト相を下地として、ベイナイト、マルテンサイトなどの硬質相を各々面積率で5〜50%、1〜20%と多く含有し、55〜60kgf/mm²級の強度と141〜152%と高い伸びフランジ性を有する鋼板が得られている。
【０００４】
一方、絞り成形などのプレス成形時に圧縮変形が付加される部位には、成形後の耐二次加工脆性（耐縦割れ性）が求められており、特開平6-57373号公報には、耐二次加工脆性に優れる高ｒ値高張力冷延鋼板を製造する技術が開示されている。この技術は、P添加の極低C-Ti-Nb-B系の鋼で、B量をSi,Mn,Pの重み付き合計量で定まる所定範囲内に調整した鋼を用いることにより、耐二次加工脆性の良好な367.5〜501.8MPa(37.5〜51.2kgf/mm2)の強度の鋼板が得られるというものである。
【０００５】
【発明が解決しようとする課題】
しかし、特開昭58-39770号公報の技術のように、ベイナイト、マルテンサイトなどの硬質相を多く含む鋼板を圧縮変形すると、フェライトとベイナイトまたはマルテンサイトの界面に応力が集中する。そのため、成形後の圧壊試験でフェライトと第二相の界面から縦割れ破壊を起こし易い。このため、この技術の鋼板は耐二次加工脆性には好ましいとは考えにくい。
【０００６】
一方、特開平6-57373号公報に開示された技術では、500MPa程度までの強度の耐二次加工脆性の良好な鋼板は得られるが、化学成分から見て590 MPa以上の強度を有する鋼板を安定して製造することは困難と思われる。
【０００７】
ロッカー、シートアウターなどの自動車の構造部品に使用される鋼板としては、耐二次加工脆性が良好であるとともに590 MPa以上の強度が要求されているが、鋼板の高強度化に伴ない、プレス成形時に結晶粒界への応力集中が大きくなるため、耐二次加工脆性にとってはより厳しい状況となる。従って、従来技術ではこれら両方の要求を同時に満足することができないという問題がある。
【０００８】
そこで、本発明では、以上の問題点を解決し、590MPa以上の引張強度を有し、プレス成形後の耐二次加工脆性に優れた溶融亜鉛めっき鋼板を安定して製造する技術を提供することを目的とする。
【０００９】
【課題を解決するための手段】
上記の課題は、次の発明により解決される。その発明は、化学成分が、mass%でC:0.050 〜 0.130、Si≦0.7、Mn:1.3〜2.5、P≦0.08、S≦0.01、sol.Al:0.01〜0.1、N≦0.005、Nb:0.007〜0.06を含有し、残部鉄および不可避的不純物からなる鋼を溶製して鋳造した後、熱間圧延を施して熱延鋼板とし、この熱延鋼板を、あるいはさらに冷間圧延を施して得られた冷延鋼板を、加熱した後冷却し、溶融亜鉛めっき工程（溶融亜鉛めっき後合金化処理を行う場合を含む）を施して溶融亜鉛めっき鋼板を製造する際に、前記加熱は鋼板を下記の不等式を満たす温度T(℃)に加熱し、加熱後の冷却過程から前記溶融亜鉛めっき工程において鋼板を600〜350℃の温度域に下記の不等式を満たす時間t(sec)保持することを特徴とする、引張強度が 590MPa 以上で耐二次加工脆性に優れる溶融亜鉛めっき鋼板の製造方法。
【００１０】
89×log(Nb+C/8)+900≦T＜900 (1)
t≦107-338×log(3×P+0.5) (2)
ここで、式中の元素記号はそれぞれのmass%を示す。
【００１１】
また、この発明の溶融亜鉛めっき鋼板の製造方法において、化学成分としてさらに、mass%でV:0.003〜0.08を含有することを特徴とする、引張強度が 590MPa 以上で耐二次加工脆性に優れる溶融亜鉛めっき鋼板の製造方法とすることもできる。
また、この発明の溶融亜鉛めっき鋼板の製造方法において、化学成分としてさらに、 mass% で Ti ≦ 0.05 、 Cr ≦ 0.5 のうちの 1 種または 2 種を含有することを特徴とする、引張強度が 590MPa 以上で耐二次加工脆性に優れる溶融亜鉛めっき鋼板の製造方法とすることもできる。
【００１２】
この発明は、耐二次加工脆性に優れた高強度溶融亜鉛めっき鋼板を得るために、鋭意検討を重ねた結果見出された知見に基づきなされた。それは、結晶粒界の強化に加え、鋼中にNb系炭化物を分散させた微細なフェライト組織とすることにより、鋼板の高強度化に伴なうプレス成形時の結晶粒界への応力集中に対して、特性の向上が図れるということである。
【００１３】
それに伴い、この発明では、Nb量とC量にて加熱温度を適正化し、さらに、加熱後の冷却過程から溶融亜鉛めっき工程においては、P量に応じて600〜350℃の温度域に滞在する時間、即ち保持時間を適正化することにより、結晶粒界を脆化させるPの偏析を抑制し、粒界強度を維持している。このようにして、590MPa以上の引張強度を有し、プレス成形後の耐二次加工脆性に優れた溶融亜鉛めっき鋼板を得るため、本発明は以下の構成要件を必須とする。以下に、本発明の鋼成分の添加理由、成分限定理由、組織形態および製造条件の限定理由について説明する。なお、以下の%はmass%を示す。
【００１４】
（１）鋼成分の範囲
C:0.050 〜 0.130%
Cは、組織の細粒化に寄与するNb系の炭化物を得るために重要な元素であり、また、鋼の強化に有効な元素であり、0.050%以上の添加量を要する。しかし、0.130%を超えてCを添加すると、延性が劣化し、プレス成形性に好ましくない。このため、C量は0.050 〜 0.130%の範囲内とする。
【００１５】
Si:≦0.7%
Siは、鋼の強化に有効な元素であり、適宜添加することができる。しかし、Si量が0.7%を超えると、溶融亜鉛めっきの密着性の劣化を招くばかりか、化成処理性にも好ましくない。よって、Si量は0.7%以下とする。
【００１６】
Mn:1.3〜2.5%
Mnは、鋼の強化に有効な元素であるが、添加量が1.3%未満では強化能が小さい。一方、Mnの添加量が2.5%を超えると、Mnの鋳造偏析に起因したバンド組織が顕在化し、このような不均一組織が形成されるとプレス成形性の劣化が懸念される。このため、Mn量は1.3〜2.5%の範囲内とする。
【００１７】
P:≦0.08%
Pは、鋼の強化に有効な元素であり、適宜添加することができる。しかし、Pは、結晶粒界に偏析して粒界を脆化させるため、プレス成形後の耐二次加工脆性に好ましくない。また、Pは溶融亜鉛めっき処理時の合金化むらを引き起こし、密着性を低下させる元素である。特にP量が0.08%を超えると、これらの悪影響が顕著となる。従って、Pの添加量を0.08%以下とする。
【００１８】
S:≦0.01%
Sは鋼中に過剰に存在すると、熱間圧延時の赤熱脆性が懸念される。また、結晶粒界に析出した硫化物は粒界強度を低下させ、プレス成形後の耐二次加工脆性に好ましくない。特にS量が0.01%を超えると、これらの悪影響が顕著となる。よって、S量を0.01%以下とする。
【００１９】
sol.Al:0.01〜0.1%
Alは鋼の脱酸とNを析出固定するためから、0.01%以上必要である。しかし、Alの添加量が0.1%を超えると、溶融亜鉛めっき処理後のめっき表面性状に好ましくない。このため、Al量は0.01〜0.1%の範囲内とする。
【００２０】
N:≦0.005%
Nは、AlまたはVで析出固定されるので多少含まれていてもよい。しかし、N量が0.005%を超えると、微細な窒化物が多く形成され、これにより焼鈍時のフェライトの再結晶は遅滞し、加工組織が残留し易くなる。このような鋼板組織はプレス成形性に好ましくないばかりか、成形後の耐二次加工脆性にも望ましくない。よって、N量は0.005%以下とする。
【００２１】
Nb:0.007〜0.06%
Nbは、フェライトの細粒化と鋼の強化に寄与するNb系炭化物を得るため、0.007%以上の添加を要する。しかし、添加量が0.06%を超えると、Nb系炭化物が過剰に形成され、これにより焼鈍時のフェライトの再結晶は遅滞し、鋼中に加工組織が残留し易くなる。この結果、鋼板の延性は低下し、プレス成形性の劣化を引き起こす。また、このように加工歪みの残留する鋼板では、プレス成形時に応力集中が起り、加工歪みの局在化を招き易くなるため、プレス成形後の耐二次加工脆性にも好ましくない。このため、Nb量は0.007〜0.06%の範囲内とする。
【００２２】
V:添加する場合0.003〜0.08%
Vは、鋼の強化とNを析出固定するために有効な元素であり、必要に応じて添加することができる。V系窒化物が多く形成されると鋼板組織の微細化に好ましく、この効果は、0.003%以上の添加により達成される。しかし、V量が0.08%を超えると、過剰なV系窒化物によって、フェライトの再結晶は遅滞し、加工組織が残留し易くなる。こうした鋼板組織では、プレス成形性や成形後の耐二次加工脆性の向上は望めない。従って、Vを添加する場合、その添加量は0.003〜0.08%の範囲内とする。
【００２３】
上記の鋼成分以外の化学成分については、過剰に添加しなければ、本発明の効果を損なうことはない。例えば、Tiは0.05%以下、Crは0.5%以下であれば、本発明の目的とする特性に悪影響を及ぼさない。
【００２４】
本発明の溶融亜鉛めっき鋼板は、優れた耐二次加工脆性を意図としており、上記(1)のように所定の成分を調整した鋼板であり、以下の方法にて製造することができる。
【００２５】
（２）鋼板の製造方法
上記（１）で述べた化学成分の鋼を溶製し、鋳造した後、熱間圧延を施す。鋼の溶製、溶製後の鋳造、熱間圧延の方法については特に限定はなく、特に組織が不均一でなければ良い。得られた熱延鋼板を酸洗し、必要に応じて冷間圧延した後、連続溶融亜鉛めっき処理を施す。その際、プレス成形後の耐二次加工脆性に好ましい微細なフェライト組織を安定して形成させ、更に、フェライト粒界の強度を維持するため、焼鈍（加熱）プロセスの適正化が必要である。また、均熱（加熱）後の冷却ないし溶融亜鉛めっき処理過程においては、フェライト粒界を脆化させるPの偏析を抑制するため、600〜350℃の温度域に滞在する時間、即ち保持時間を適正化することが必要である。
【００２６】
均熱（加熱）温度T(℃)：89×log(Nb+C/8)+900≦ T＜900
均熱（加熱）温度がこの温度範囲より低い場合には、焼鈍（加熱）後でも加工組織が残留し易く、成形後の縦割れ臨界温度Tcは-29℃以上と高くなり、耐二次加工脆性は劣化する。また、均熱（加熱）温度が900℃以上の場合には、粗大なフェライト粒が認められるため、やはりTcは-29℃以上と高くなり、耐二次加工脆性は劣化する。均熱温度T(℃)をこの温度範囲内の場合は、後述のように、微細な再結晶フェライト組織が安定して得られ、Tcは許容範囲の-30℃以下ないし良好とされる-61℃以下を達成できる。従って、均熱温度T(℃)を89×log(Nb+C/8)+900≦ T＜900を満たす温度とする。
【００２７】
600〜350℃の温度域の保持時間t(sec)：t≦107-338×log(3×P+0.5)
この温度域の保持時間tが107-338×log(3×P+0.5)を超える場合には、成形後の縦割れ臨界温度Tcは-29℃以上と高くなり、耐二次加工脆性は劣化する。この温度域の保持時間tが107-338×log(3×P+0.5)以下の場合には、Tcは許容範囲の-30℃以下ないし良好とされる-61℃以下を達成できる。よって、粒界へのPの偏析を抑制し、フェライトの粒界強度を維持するために、均熱（加熱）後の冷却過程以降の600〜350℃の温度域での保持時間を107-338×log(3×P+0.5)以下とする。600〜350℃の温度域を所定の時間保持しながら、鋼板に溶融亜鉛めっきを施し、また、必要に応じて合金化処理を実施しても良い。
【００２８】
以上の製造工程を経て、本発明の意図とする耐二次加工脆性に優れた溶融亜鉛めっき鋼板を製造することが出来る。また、このようにして得られた鋼板に、電気めっき、化成処理などの表面処理を施しても所望の鋼板特性を損なうことはない。
【００２９】
溶融亜鉛めっき鋼板の発明は、化学成分が上記の化学成分であり、フェライト粒径が10μm以下の組織を有し、強度が590MPa以上であり、耐二次加工脆性に優れていることを特徴とする溶融亜鉛めっき鋼板である。
【００３０】
この発明は、前述の製造方法の発明と同様、加熱温度を適正化し、さらに、加熱後の冷却過程から溶融亜鉛めっき工程においては、P量に応じて600〜350℃の温度域における保持時間を適正化することにより製造することができる。この溶融亜鉛めっき鋼板は、前述の化学成分に調整されているので、590MPa以上の引張強度を有することが可能である。また、フェライト粒径を10μm以下に微細化し、結晶粒界を脆化させるPの偏析を抑制して粒界強度を維持しているので、プレス成形後の耐二次加工脆性に優れた溶融亜鉛めっき鋼板とすることができる。
【００３１】
なお、この発明で、耐二次加工脆性に優れているというのは、例えば後述のように、絞り比1.6で成形したカップの縦割れ遷移温度が-30℃以下ということである。
【００３２】
【発明の実施の形態】
極低炭素IF鋼を用いた従来知見から明らかなように、耐二次加工脆性を向上させる方法として、結晶粒界を強化することが有効であることが知られている。しかし、鋼板の高強度化に伴ない、プレス成形時に結晶粒界への応力集中が大きくなるため、粒界強度を高めることのみでは、耐二次加工脆性の向上は難しい。そこで、結晶粒界の強化に加え、鋼板組織の細粒化に主眼を置き、耐二次加工脆性の向上を検討した所、鋼中にNb系炭化物を分散させた平均粒径10μm以下のフェライト組織を主体とする微細組織により、特性の向上が可能となった。
【００３３】
鋼板の組織制御は、主に熱間圧延後ないし冷間圧延後の焼鈍プロセスにて実施し、この場合、特に、組織の微細化に寄与するNb系炭化物を形成するNb量とC量にて均熱温度を適正化し、さらに、均熱後の冷却過程において、結晶粒界を脆化させるPの偏析を抑制し、粒界強度を維持するためから、P量に応じて600〜350℃の中間温度域での保持時間を適正化することが必須であることが明らかとなった。
【００３４】
そこで、発明の実施に当たっては、前述の化学成分の鋼を溶製し、鋳造した後、熱間圧延を施す。鋼の溶製は、転炉法、電気炉法のいずれの方法でも差し支えなく、また、溶製後の鋳造は、連続鋳造あるいは造塊のいずれの方法で実施しても問題はない。熱間圧延は鋳造後、直ちに開始しても良いし、また、一旦冷却し、加熱してから実施しても良い。
【００３５】
熱間圧延では、粗圧延後、仕上圧延を行ない、コイルに巻き取る。熱延板の表層部で伸長粒や粗大粒が発達すると、最終焼鈍後の板厚方向の組織が不均一となり、プレス成形性、成形後の耐二次加工脆性に好ましくない。このため、仕上圧延温度はAr₃点以上とし、また、巻取温度は750℃以下とした方が良い。
【００３６】
得られた熱延板を酸洗し、必要に応じて30%以上の圧下率で冷間圧延した後、連続溶融亜鉛めっき処理を施す。プレス成形後の耐二次加工脆性に好ましい平均粒径10μm以下の微細なフェライト組織を安定して形成させ、更に、フェライト粒界の強度を維持するため、焼鈍プロセスを適正化することが必要である。つまり、フェライトの細粒化に寄与するNb系炭化物を形成するNb量とC量にて均熱温度を適正化し、均熱後の冷却過程において、フェライト粒界を脆化させるPの偏析を抑制するため、P量に応じて600〜350℃の中間温度域での保持時間、即ちこの温度域に滞在する時間を適正化することが必要である。
【００３７】
具体的な数値を求めるため、本発明の化学成分範囲内の鋼板について、カップ成形後の縦割れ試験を行い、プレス成形後の耐二次加工脆性を調査した。用いた鋼板は、C:0.04〜0.10%、Si:0.01〜0.45%、Mn:1.5〜1.9%、P:0.01〜0.03%、S:0.001〜0.004%、sol.Al:0.02〜0.06%、N:0.0020〜0.0035%、Nb:0.01〜0.06%、V:0.02〜0.06%の化学成分の冷延板(板厚1.2mm)を種々の均熱温度で180sec保持し、その後600〜350℃の温度域の保持時間が150 secで室温まで冷却した焼鈍板である。
【００３８】
試験では、この焼鈍板より120mmφのブランクを採取し、絞り比1.6（75mmφ）でカップ成形し、カップ側壁を高さ30mmにトリムして縦割れ試験用の成形カップとした。縦割れ試験では、冷媒中でこの成形カップの内面を円錐台形の台座に押付け、カップ側壁を押し拡げる。その際、カップ側壁に縦割れが生じない最低温度（冷媒の温度T）を、縦割れ臨界温度Tcとする。以上の試験方法を図１に示す。
【００３９】
縦割れ試験結果を、Nb,C量と均熱温度で整理して図２に示す。なお、Nb,C量は対数目盛でプロットしてあり、縦割れ試験結果はNb+C/8により整理できることが分かる。図２に示すように、均熱温度T(℃)が図中の直線以上、即ち89×log(Nb+C/8)+900以上（Nb,Cはそれぞれのmass%を示す）の温度で、かつ900℃未満の場合には、縦割れ臨界温度Tcは-30〜-60℃(△:許容)ないし-61〜-90℃(○:良好)と低温の特性値が得られている。これは、微細な再結晶フェライト組織が安定して得られていることによる。
【００４０】
これに対して、図中の直線より下、即ち89×log(Nb+C/8)+900未満の場合には、焼鈍後に加工組織が残留し易くなるため、成形後の縦割れ臨界温度Tcは25〜-29℃(×)と高く、耐二次加工脆性は劣化する。また、均熱温度が900℃の場合には、粗大なフェライト粒が認められるため、Tcは25〜-29℃と高く、耐二次加工脆性は劣化(×)する。
【００４１】
また、600〜350℃の温度域の保持時間の影響を調べるため、前述と同様、プレス成形後の耐二次加工脆性を調査した。用いた鋼板は、C:0.05〜0.10%、Si:0.01〜0.30%、Mn:1.6〜2.0%、P:0.012〜0.10%、S:0.001〜0.003%、sol.Al:0.02〜0.06%、N:0.0015〜0.0035%、Nb:0.01〜0.035%、V:0.005〜0.04%の成分の冷延板(板厚1.2mm)を830℃で180sec均熱した後、その後600〜350℃の温度域の保持時間を変化させ、室温まで冷却した焼鈍板である。
【００４２】
縦割れ試験結果を、図３に示す。図に示すように、600〜350℃での保持時間t(sec)が、図中の曲線より上、即ち107-338×log(3×P+0.5)を超える場合（Pはmass%を示す）には、成形後の縦割れ臨界温度Tcは25〜-29℃と高く、耐二次加工脆性は劣化(×)している。これは、粒界へのPの偏析による特性劣化と考えられる。一方、600〜350℃での保持時間が107-338×log(3×P+0.5)以下の場合には、Tcは-30〜-60℃(△:許容)ないし-61〜-90℃(○:良好)と低温の特性値が得られている。
【００４３】
このように、耐二次加工脆性の劣化を防止するには、600〜350℃の温度域を所定の時間以内で、鋼板に溶融亜鉛めっきを施す必要がある。その際、必要に応じて合金化処理を実施しても良い。以上の製造工程を経て、本発明の意図とする耐二次加工脆性に優れた溶融亜鉛めっき鋼板を製造することが出来る。また、このようにして得られた鋼板に、電気めっき、化成処理などの表面処理を施しても所望の鋼板特性を損なうことはない。
【００４４】
【実施例】
以下に本発明の実施例を示す。
【００４５】
[実施例１]
表１に示す成分の鋼(鋼番1〜8:本発明鋼、鋼番9〜16:比較鋼)を実験室にて溶製後、鋳造し、板厚60mmのスラブを作製した。但し、鋼番8の鋼は後述のように電気炉にて出鋼した鋼を用いた。このスラブを板厚30mmまで分塊圧延した後、大気炉で1270℃×1hrの加熱処理を施し、熱間圧延に供した。仕上圧延は890℃で実施し、続いて、620℃×1hrの巻取相当の熱処理を行ない、板厚4mmの熱延板を作製した。次に、熱延板を酸洗し、板厚1.2mmまで冷間圧延した。
【００４６】
【表１】

【００４７】
この後、冷延板を820℃で150sec均熱し、平均速度10℃/sで600℃まで冷却した後、460℃の溶融亜鉛めっき浴中に浸漬し、550℃で合金化処理を施した。焼鈍中、600〜350℃の温度域の保持時間を130 secとした。この焼鈍板に伸長率1.0%の調質圧延を施し、引張試験、耐二次加工脆性、表面外観の評価を実施した。
【００４８】
引張試験はJIS Z 2241(日本工業規格)に準拠した方法にて実施し、590MPa以上の引張強度(TS)が得られる場合、特性良好(○)とし、590MPa未満のTSの場合には、強度不足(×)と評価した。耐二次加工脆性は、図1に示す方法にて評価し、縦割れ臨界温度(Tc)が-61〜-90℃の時、特性良好(○)とし、Tcが-30〜-60℃の時、許容レベル(△)、また、Tcが25〜-29℃の時、特性劣化(×)と判定した。
【００４９】
また、巾120mm、長さ2000mmの範囲で、めっき表面を目視にて評価し、不めっきや線状欠陥などの表面欠陥が認められた場合には、表面不良(×)と評価した。これらの引張強度、縦割れ臨界温度およびめっき表面性状の評価結果を表２に示す。
【００５０】
【表２】

【００５１】
本発明例No.1〜8（鋼番1〜8）はいずれも本発明成分範囲にあり、TSが600〜680MPa、Tcが-65〜-85℃の特性値を有しており、引張強度、耐二次加工脆性ともに良好な特性が得られている。また、表面性状はいずれも良好である。
【００５２】
一方、比較例No.9〜16（鋼番9〜16）はいずれも本発明成分範囲外にあり、引張強度、耐二次加工脆性およびめっき表面性状を満足しない。比較例No.9、12はTcが-80〜-90℃と低く良好な耐二次加工脆性を有しており、また、めっき表面も好ましいが、TSが490〜520MPaと低く、590MPa以上の引張強度が得られていない。比較例No.10、11、14はTSが610〜750MPaと高く、引張強度は良好で、いずれも好ましいめっき表面を有しているが、Tcは25〜-10℃と高く、耐二次加工脆性は劣化している。
【００５３】
比較例No.13、15はTSが630〜660MPaと高く、良好な強度が得られているが、Tcは15〜20℃と高く、耐二次加工脆性は劣化している。また、いずれの表面も不めっきが存在しており、表面性状は好ましくない。また、比較例No.16はTSが605MPaで良好な強度を有しており、また、Tcは-45℃と低く耐二次加工脆性は許容レベルであるが、表面には線状欠陥が存在しており、表面外観は好ましくない。
【００５４】
[実施例2]
表１の鋼番8の鋼(本発明鋼)を電気炉にて出鋼した後、造塊し、板厚200mmのスラブを2本製造した。このスラブを1270℃で1時間加熱した後、粗圧延を開始し、880℃で仕上圧延を実施し、板厚2.0mmの熱延コイルを製造した。なお、巻取温度は550℃とした。次に、この2本の熱延コイルをそれぞれ酸洗した後、各々異なる焼鈍サイクルにて連続溶融亜鉛めっきを施した。
【００５５】
この内、1本のコイルを用いて、コイルの長手方向で720〜910℃に均熱温度を変化させた後、冷却し、続いて、460℃の溶融亜鉛めっき浴中に浸漬した後、直ちに500℃まで昇温し、合金化処理を施し、室温まで冷却した。この間、均熱後、室温までの焼鈍過程において、600〜350℃の温度域での保持時間を150secとした。
【００５６】
また、残り1本のコイルを用いて、800〜815℃で均熱した後、冷却し、460℃の溶融亜鉛めっき浴中に浸漬した後、直ちに550℃まで昇温し、合金化処理を施し、室温まで冷却した。尚、このコイルでは、コイル長手方向で焼鈍中の600〜350℃の温度域での保持時間を100〜280secに変化させた。
【００５７】
また、めっき後、各々の焼鈍板には、伸長率1.0%の調質圧延を施した。それぞれのコイルで均熱温度、または600〜350℃の中間温度域での保持時間を変化させた位置から、焼鈍板を採取し、実施例1と同様の方法にて、引張試験、耐二次加工脆性、表面外観の評価を実施した。
【００５８】
引張強度、縦割れ臨界温度およびめっき表面性状の評価結果を表３に示す。
【００５９】
【表３】

【００６０】
鋼板No.21,22,27は、いずれも均熱温度が本発明範囲外にあり、表面性状は良好であるが、TSは560〜750MPa、Tcは-25〜10℃であり、耐二次加工脆性は望ましくない。
【００６１】
No.23〜26はいずれも均熱温度が本発明範囲にあり、TSは600〜670MPaと高く、また、Tcは-35〜-75℃の特性値を示しており、耐二次加工脆性は許容範囲ないし良好な特性が得られている。また、いずれも良好なめっき表面を有している。
【００６２】
No.28,29はいずれも600〜350℃の温度域での保持時間が本発明範囲にあり、TSは610〜615MPaと高く、また、Tcは-45〜-70℃と低い特性値を有しており、強度、耐二次加工脆性ともに良好である。更に、好ましいめっき表面性状を有している。
【００６３】
No.30〜32はいずれも600〜350℃の温度域での保持時間が本発明範囲外にあり、TSは605〜620 MPa と高く、めっき表面も好ましい外観を示しているが、Tcは5〜-20℃と高く、耐二次加工脆性は劣化している。
【００６４】
【発明の効果】
本発明によれば、鋼の化学成分を規定するとともに、Nb量とC量にて加熱温度を適正化し、さらに、加熱後の冷却過程から溶融亜鉛めっき工程において、P量に応じて600〜350℃の温度域における保持時間を適正化することにより、結晶粒界を脆化させるPの偏析を抑制し、粒界強度を維持している。このように鋼の化学成分と製造条件を規定することにより、590MPa以上の強度を有する耐二次加工脆性に優れた溶融亜鉛めっき鋼板を安定して製造することが可能であり、靭性が求められる自動車の構造部品等への適用できることから、自動車業界における利用価値は大きい。
【図面の簡単な説明】
【図１】鋼板の耐二次加工脆性の評価方法を示す図。
【図２】耐二次加工脆性におよぼす均熱温度およびNb+C/8の影響を示す図。
【図３】耐二次加工脆性におよぼす600〜350℃での保持時間およびPの影響を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot dip galvanized steel sheet.
[0002]
[Prior art]
In recent years, as one of the efforts to protect the global environment by automobile manufacturers, the weight reduction of automobile bodies for the purpose of lowering the fuel ratio has been implemented. As an effective method for reducing the weight of the vehicle body, it has been studied to apply a high-strength steel sheet to various structural parts such as automobile members, lockers, and pillars to reduce the thickness of the material. In recent years, regulations on collision safety of automobiles are increasing, and the application of high-strength steel sheets to various reinforcing members is being promoted.
[0003]
When forming high-strength steel sheets into actual automobile parts, press formability such as stretchability and stretch flangeability is required mainly for the parts to which tensile deformation is applied. High strength steel plates have been developed. For example, JP-A-58-39770 discloses a method for producing a high-strength hot-dip galvanized steel sheet having excellent stretch flangeability. In this technology, the ferrite phase is used as a base, and hard phases such as bainite and martensite are contained in a large amount of 5 to 50% and 1 to 20%, respectively, with a strength of 55 to 60 kgf / mm ^{2 and} a strength of 141 to 152. A steel plate having a stretch flangeability as high as% is obtained.
[0004]
On the other hand, the secondary work brittleness resistance (longitudinal crack resistance) after molding is required for a portion to which compression deformation is applied during press molding such as draw molding. JP-A-6-57373 discloses resistance to resistance. A technique for producing a high r-value high-tensile cold-rolled steel sheet having excellent secondary work brittleness is disclosed. This technology is a P-added ultra-low C-Ti-Nb-B steel with a B content adjusted within a predetermined range determined by the weighted total amount of Si, Mn, and P. A steel plate having a strength of 367.5 to 501.8 MPa (37.5 to 51.2 kgf / mm 2) having good next processing brittleness can be obtained.
[0005]
[Problems to be solved by the invention]
However, when a steel sheet containing a large amount of hard phase such as bainite and martensite is compressively deformed as in the technique of Japanese Patent Application Laid-Open No. 58-39770, stress concentrates on the interface between ferrite and bainite or martensite. Therefore, it is easy to cause a vertical crack failure from the interface between the ferrite and the second phase in the crush test after molding. For this reason, it is unlikely that steel sheets of this technology are preferable for secondary work brittleness resistance.
[0006]
On the other hand, with the technology disclosed in Japanese Patent Laid-Open No. 6-57373, a steel plate having a strength of secondary work brittleness having a strength of up to about 500 MPa can be obtained. It seems difficult to produce stably.
[0007]
Steel plates used for structural parts of automobiles such as rockers and seat outers are required to have good secondary work brittleness resistance and strength of 590 MPa or more. Since the concentration of stress on the crystal grain boundaries increases during molding, the situation becomes more severe for the resistance to secondary work embrittlement. Therefore, there is a problem that the prior art cannot satisfy both of these requirements at the same time.
[0008]
Therefore, the present invention provides a technique for solving the above problems and stably producing a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent in secondary work brittleness resistance after press forming. With the goal.
[0009]
[Means for Solving the Problems]
The above problems are solved by the following invention. In the invention, the chemical composition is mass% C: 0.050 to 0.130 , Si ≤ 0.7, Mn: 1.3 to 2.5, P ≤ 0.08, S ≤ 0.01, sol.Al: 0.01 to 0.1, N ≤ 0.005, Nb: 0.007 It is obtained by melting and casting a steel containing ˜0.06 and the balance iron and inevitable impurities , and then hot rolling to obtain a hot rolled steel sheet, which is obtained by further cold rolling. The obtained cold-rolled steel sheet is heated and then cooled, and when a hot-dip galvanized steel sheet is manufactured by performing a hot-dip galvanizing step (including a case where an alloying treatment is performed after hot-dip galvanizing), Is heated to a temperature T (° C.) satisfying the inequality , and the steel sheet is maintained in the temperature range of 600 to 350 ° C. in the hot dip galvanizing process from the cooling process after heating, and the time t (sec) satisfying the following inequality is maintained A method for producing a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent secondary work brittleness resistance .
[0010]
89 × log (Nb + C / 8) + 900 ≦ T <900 (1)
t ≦ 107-338 × log (3 × P + 0.5) (2)
Here, the element symbol in a formula shows each mass%.
[0011]
Further, in the method for producing a hot dip galvanized steel sheet according to the present invention, the chemical component further contains V: 0.003 to 0.08 in mass%, and the tensile strength is 590 MPa or more and the secondary work brittleness resistance is excellent. It can also be set as the manufacturing method of a galvanized steel plate.
In the method of manufacturing the galvanized steel sheet of the present invention, further as a chemical component, which is characterized by containing a Ti ≦ 0.05, 1 kind or two kinds of Cr ≦ 0.5 in mass%, the tensile strength is 590MPa It can also be set as the manufacturing method of the hot dip galvanized steel plate which is excellent in the secondary work brittleness resistance above.
[0012]
The present invention has been made based on the knowledge found as a result of extensive studies in order to obtain a high-strength hot-dip galvanized steel sheet having excellent secondary work brittleness resistance. In addition to strengthening the grain boundaries, it has a fine ferrite structure in which Nb carbide is dispersed in the steel, thereby concentrating stress on the grain boundaries during press forming as the strength of the steel sheet increases. On the other hand, the characteristics can be improved.
[0013]
Accordingly, in the present invention, the heating temperature is optimized by the amount of Nb and the amount of C, and furthermore, in the hot dip galvanizing process from the cooling process after heating, it stays in a temperature range of 600 to 350 ° C. according to the amount of P. By optimizing the time, that is, the holding time, segregation of P that causes embrittlement of the grain boundaries is suppressed, and the grain boundary strength is maintained. Thus, in order to obtain a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent in secondary work brittleness resistance after press forming, the present invention requires the following constituent elements. Below, the reason for the addition of the steel component of the present invention, the reason for limiting the component, the structure form and the reason for limiting the manufacturing conditions will be described. In addition, the following% shows mass%.
[0014]
(1) Range of steel components
C: 0.050 to 0.130 %
C is an important element for obtaining Nb-based carbides contributing to the refinement of the structure, and is an element effective for strengthening steel, and requires an addition amount of 0.050 % or more. However, if C exceeds 0.130 %, the ductility deteriorates, which is not preferable for press formability. For this reason, the C content is in the range of 0.050 to 0.130 %.
[0015]
Si: ≦ 0.7%
Si is an element effective for strengthening steel, and can be added as appropriate. However, when the Si content exceeds 0.7%, not only the adhesion of hot dip galvanizing is deteriorated, but also the chemical conversion treatment property is not preferable. Therefore, the Si content is 0.7% or less.
[0016]
Mn: 1.3-2.5%
Mn is an element effective for strengthening steel, but if the addition amount is less than 1.3%, the strengthening ability is small. On the other hand, if the amount of Mn added exceeds 2.5%, a band structure due to Mn casting segregation becomes obvious, and if such a non-uniform structure is formed, there is a concern about deterioration of press formability. For this reason, the amount of Mn is set within a range of 1.3 to 2.5%.
[0017]
P: ≦ 0.08%
P is an element effective for strengthening steel and can be appropriately added. However, since P segregates at the crystal grain boundaries and embrittles the grain boundaries, it is not preferable for secondary work embrittlement resistance after press molding. P is an element that causes uneven alloying during hot dip galvanizing and reduces adhesion. In particular, when the amount of P exceeds 0.08%, these adverse effects become significant. Therefore, the addition amount of P is set to 0.08% or less.
[0018]
S: ≦ 0.01%
If S is excessively present in the steel, there is a concern about red hot embrittlement during hot rolling. Further, sulfides precipitated at the crystal grain boundaries decrease the grain boundary strength, and are not preferable for secondary work embrittlement resistance after press molding. In particular, when the amount of S exceeds 0.01%, these adverse effects become significant. Therefore, the S content is 0.01% or less.
[0019]
sol.Al:0.01-0.1%
Al needs to be 0.01% or more because it deoxidizes the steel and precipitates and fixes N. However, if the addition amount of Al exceeds 0.1%, it is not preferable for the surface properties of the plating after the hot dip galvanizing treatment. For this reason, the amount of Al is within a range of 0.01 to 0.1%.
[0020]
N: ≦ 0.005%
Since N is precipitated and fixed with Al or V, it may be contained somewhat. However, if the N content exceeds 0.005%, a lot of fine nitrides are formed, and as a result, recrystallization of ferrite during annealing is delayed, and the processed structure tends to remain. Such a steel sheet structure is not preferable for press formability, but is also not preferable for secondary work embrittlement resistance after forming. Therefore, the N content is 0.005% or less.
[0021]
Nb: 0.007-0.06%
Nb needs to be added in an amount of 0.007% or more in order to obtain Nb-based carbides that contribute to ferrite refinement and steel strengthening. However, if the addition amount exceeds 0.06%, Nb-based carbides are excessively formed, which delays the recrystallization of ferrite during annealing, and tends to leave a work structure in the steel. As a result, the ductility of the steel sheet is lowered, causing deterioration of press formability. Further, in such a steel sheet in which processing strain remains, stress concentration occurs at the time of press forming, and localization of the processing strain is likely to occur, which is not preferable for secondary work embrittlement resistance after press forming. For this reason, Nb amount shall be in the range of 0.007 to 0.06%.
[0022]
V: 0.003 to 0.08% when added
V is an element effective for strengthening steel and precipitating and fixing N, and can be added as necessary. When a large amount of V-based nitride is formed, it is preferable for refinement of the steel sheet structure, and this effect is achieved by addition of 0.003% or more. However, if the amount of V exceeds 0.08%, the recrystallization of ferrite is delayed by excess V-based nitride, and the processed structure tends to remain. In such a steel sheet structure, improvement of press formability and resistance to secondary work brittleness after forming cannot be expected. Therefore, when V is added, the amount added is within the range of 0.003 to 0.08%.
[0023]
About chemical components other than said steel component, unless it adds excessively, the effect of this invention will not be impaired. For example, if Ti is 0.05% or less and Cr is 0.5% or less, the target characteristics of the present invention are not adversely affected .
[0024]
The hot dip galvanized steel sheet of the present invention is intended to have excellent secondary work brittleness resistance, and is a steel sheet in which predetermined components are adjusted as described in the above (1), and can be manufactured by the following method.
[0025]
(2) Steel plate production method The steel having the chemical composition described in (1) above is melted and cast, and then hot-rolled. There are no particular limitations on the methods of melting steel, casting after melting, and hot rolling, and it is sufficient that the structure is not particularly uneven. The obtained hot-rolled steel sheet is pickled, cold-rolled as necessary, and then subjected to continuous hot dip galvanizing. At that time, it is necessary to optimize the annealing (heating) process in order to stably form a fine ferrite structure preferable for secondary work brittleness resistance after press molding and to maintain the strength of ferrite grain boundaries. In addition, in the cooling or hot dip galvanizing process after soaking (heating), in order to suppress the segregation of P that embrittles the ferrite grain boundaries, the time to stay in the temperature range of 600 to 350 ° C., that is, the holding time is set. It is necessary to optimize.
[0026]
Soaking (heating) temperature T (℃): 89 × log (Nb + C / 8) + 900 ≦ T <900
If the soaking (heating) temperature is lower than this temperature range, the processed structure tends to remain even after annealing (heating), and the critical temperature Tc of the vertical crack after forming becomes -29 ° C or higher, which is secondary processing resistant. Brittleness deteriorates. Further, when the soaking (heating) temperature is 900 ° C. or higher, coarse ferrite grains are observed, so that Tc becomes as high as −29 ° C. or higher, and the secondary work brittleness resistance deteriorates. When the soaking temperature T (° C.) is within this temperature range, as will be described later, a fine recrystallized ferrite structure is stably obtained, and Tc is considered to be an acceptable range of −30 ° C. or lower to −61 A temperature below ℃ can be achieved. Therefore, the soaking temperature T (° C.) is set to a temperature satisfying 89 × log (Nb + C / 8) + 900 ≦ T <900.
[0027]
Holding time t (sec) in the temperature range of 600 to 350 ° C: t ≦ 107-338 × log (3 × P + 0.5)
When the holding time t in this temperature range exceeds 107-338 x log (3 x P + 0.5), the critical temperature Tc for longitudinal cracking after molding becomes as high as -29 ° C or more, and the secondary work brittleness resistance deteriorates. To do. When the holding time t in this temperature range is 107-338 × log (3 × P + 0.5) or less, Tc can achieve an allowable range of −30 ° C. or less to −61 ° C. or less. Therefore, in order to suppress the segregation of P to the grain boundaries and maintain the grain boundary strength of ferrite, the holding time in the temperature range of 600 to 350 ° C. after the cooling process after soaking (heating) is set to 107-338. × log (3 × P + 0.5) or less. While maintaining a temperature range of 600 to 350 ° C. for a predetermined time, hot dip galvanization may be performed on the steel sheet, and alloying treatment may be performed as necessary.
[0028]
Through the above manufacturing steps, a hot-dip galvanized steel sheet excellent in secondary work brittleness resistance intended by the present invention can be manufactured. Moreover, even if the steel plate thus obtained is subjected to a surface treatment such as electroplating or chemical conversion treatment, the desired steel plate characteristics are not impaired.
[0029]
The invention of the hot-dip galvanized steel sheet is characterized in that the chemical component is the above-described chemical component, the ferrite particle size has a structure of 10 μm or less, the strength is 590 MPa or more, and the secondary work brittleness resistance is excellent. It is a hot dip galvanized steel sheet.
[0030]
This invention optimizes the heating temperature in the same manner as the invention of the manufacturing method described above. Further, in the hot dip galvanizing process from the cooling process after heating, the holding time in the temperature range of 600 to 350 ° C. is set according to the amount of P. It can be manufactured by optimizing. Since this hot-dip galvanized steel sheet is adjusted to the above-described chemical components, it can have a tensile strength of 590 MPa or more. In addition, the ferrite grain size is refined to 10 μm or less, and the grain boundary strength is maintained by suppressing the segregation of P, which causes embrittlement of the crystal grain boundaries, so the molten zinc has excellent secondary work brittleness resistance after press molding. It can be set as a plated steel plate.
[0031]
In the present invention, the excellent resistance to secondary work brittleness means that, for example, as described later, the longitudinal crack transition temperature of a cup molded at a draw ratio of 1.6 is −30 ° C. or lower.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
As is apparent from conventional knowledge using ultra-low carbon IF steel, it is known that strengthening the grain boundaries is effective as a method for improving secondary work brittleness resistance. However, as the strength of the steel sheet increases, the stress concentration at the crystal grain boundaries increases during press forming, so that it is difficult to improve the secondary work brittleness resistance only by increasing the grain boundary strength. Therefore, in addition to strengthening the grain boundaries, we focused on refinement of the steel sheet structure and studied the improvement of secondary work brittleness resistance. Ferrite with an average grain size of 10 μm or less in which Nb carbide was dispersed in the steel. The fine structure made mainly of the organization has made it possible to improve the characteristics.
[0033]
The structure control of the steel sheet is mainly performed in the annealing process after hot rolling or after cold rolling. In this case, in particular, the Nb amount and C amount that form Nb-based carbides that contribute to the refinement of the structure. In order to maintain the grain boundary strength by optimizing the soaking temperature and further suppressing the segregation of P, which causes embrittlement of the crystal grain boundaries in the cooling process after soaking, It became clear that it was essential to optimize the holding time in the intermediate temperature range.
[0034]
Therefore, in carrying out the invention, the above-described chemical component steel is melted and cast, and then hot-rolled. Steel can be melted by either a converter method or an electric furnace method, and casting after melting can be carried out by either continuous casting or ingot forming. Hot rolling may be started immediately after casting, or may be performed after cooling and heating.
[0035]
In hot rolling, after rough rolling, finish rolling is performed and wound into a coil. If elongated grains and coarse grains develop in the surface layer portion of the hot-rolled sheet, the structure in the thickness direction after the final annealing becomes non-uniform, which is undesirable for press formability and secondary work brittleness resistance after forming. For this reason, the finish rolling temperature should be Ar ₃ or higher, and the winding temperature should be 750 ° C. or lower.
[0036]
The obtained hot-rolled sheet is pickled and, if necessary, cold-rolled at a rolling reduction of 30% or more, and then subjected to continuous hot dip galvanizing treatment. It is necessary to optimize the annealing process in order to stably form a fine ferrite structure with an average particle size of 10 μm or less, which is preferable for secondary work brittleness resistance after press molding, and to maintain the strength of ferrite grain boundaries. is there. In other words, the soaking temperature is optimized by the amount of Nb and C that form the Nb carbide that contributes to the finer ferrite, and the segregation of P, which causes embrittlement of the ferrite grain boundary, is suppressed in the cooling process after soaking. Therefore, it is necessary to optimize the holding time in the intermediate temperature range of 600 to 350 ° C. according to the amount of P, that is, the time for staying in this temperature range.
[0037]
In order to obtain specific numerical values, the steel plate within the chemical composition range of the present invention was subjected to a longitudinal cracking test after cup forming, and the secondary work brittleness resistance after press forming was investigated. The steel plates used were C: 0.04-0.10%, Si: 0.01-0.45%, Mn: 1.5-1.9%, P: 0.01-0.03%, S: 0.001-0.004%, sol.Al: 0.02-0.06%, N : Cold-rolled sheets (plate thickness 1.2mm) with chemical composition of 0.0020-0.0035%, Nb: 0.01-0.06%, V: 0.02-0.06% are held at various soaking temperatures for 180 seconds, and then a temperature of 600-350 ° C This is an annealed plate cooled to room temperature with a holding time of 150 sec.
[0038]
In the test, a blank of 120 mmφ was collected from this annealed plate, cup-shaped with a draw ratio of 1.6 (75 mmφ), and the cup side wall was trimmed to a height of 30 mm to form a molded cup for a vertical crack test. In the vertical cracking test, the inner surface of the molded cup is pressed against a truncated cone-shaped base in a refrigerant, and the side wall of the cup is expanded. At that time, the lowest temperature at which vertical cracks do not occur on the cup side wall (refrigerant temperature T) is defined as the critical temperature Tc for vertical cracks. The above test method is shown in FIG.
[0039]
Fig. 2 shows the results of longitudinal cracking tests organized by Nb, C content and soaking temperature. The Nb and C amounts are plotted on a logarithmic scale, and it can be seen that the longitudinal crack test results can be organized by Nb + C / 8. As shown in FIG. 2, the soaking temperature T (° C.) is higher than the straight line in the figure, that is, 89 × log (Nb + C / 8) +900 or higher (Nb and C indicate mass% of each). When the temperature is lower than 900 ° C., the critical temperature Tc of the longitudinal crack is -30 to -60 ° C. (Δ: acceptable) or −61 to −90 ° C. (◯: good), and low temperature characteristic values are obtained. This is because a fine recrystallized ferrite structure is stably obtained.
[0040]
On the other hand, if it is below the straight line in the figure, that is, less than 89 × log (Nb + C / 8) +900, the processed structure tends to remain after annealing, so the critical temperature Tc for vertical cracking after molding Is as high as 25 to -29 ° C (x), and the secondary work brittleness resistance deteriorates. In addition, when the soaking temperature is 900 ° C., coarse ferrite grains are observed, Tc is as high as 25 to −29 ° C., and the secondary work brittleness resistance is deteriorated (×).
[0041]
Further, in order to investigate the influence of the holding time in the temperature range of 600 to 350 ° C., the secondary work embrittlement resistance after press molding was investigated as described above. The steel plates used were C: 0.05 to 0.10%, Si: 0.01 to 0.30%, Mn: 1.6 to 2.0%, P: 0.012 to 0.10%, S: 0.001 to 0.003%, sol.Al: 0.02 to 0.06%, N : 0.0015 to 0.0035%, Nb: 0.01 to 0.035%, V: 0.005 to 0.04% of cold-rolled sheet (thickness 1.2mm) is soaked at 830 ° C for 180 seconds, then in the temperature range of 600 to 350 ° C It is an annealed plate that is cooled to room temperature by changing the holding time.
[0042]
The longitudinal crack test results are shown in FIG. As shown in the figure, when the holding time t (sec) at 600 to 350 ° C. is above the curve in the figure, that is, exceeds 107-338 × log (3 × P + 0.5) (P indicates mass%) ), The critical critical temperature Tc for vertical cracking after molding is as high as 25 to -29 ° C, and the secondary work brittleness resistance is deteriorated (x). This is considered to be characteristic deterioration due to segregation of P to the grain boundary. On the other hand, when the holding time at 600 to 350 ° C. is 107-338 × log (3 × P + 0.5) or less, Tc is −30 to −60 ° C. (△: acceptable) to −61 to −90 ° C. ○: Good and low temperature characteristic values are obtained.
[0043]
Thus, in order to prevent the deterioration of the secondary work brittleness resistance, it is necessary to perform hot dip galvanization on the steel sheet within a predetermined time in a temperature range of 600 to 350 ° C. At that time, an alloying treatment may be performed as necessary. Through the above manufacturing steps, a hot-dip galvanized steel sheet excellent in secondary work brittleness resistance intended by the present invention can be manufactured. Moreover, even if the steel plate thus obtained is subjected to a surface treatment such as electroplating or chemical conversion treatment, the desired steel plate characteristics are not impaired.
[0044]
【Example】
Examples of the present invention are shown below.
[0045]
[Example 1]
Steels having the components shown in Table 1 (steel numbers 1-8: steel of the present invention, steel numbers 9-16: comparative steel) were melted in the laboratory and then cast to produce a slab having a thickness of 60 mm. However, the steel of steel No. 8 was steel produced in an electric furnace as described later. This slab was subjected to ingot rolling to a plate thickness of 30 mm, and then subjected to heat treatment at 1270 ° C. × 1 hr in an atmospheric furnace and subjected to hot rolling. Finish rolling was performed at 890 ° C., followed by heat treatment equivalent to winding at 620 ° C. × 1 hr to produce a hot rolled sheet having a thickness of 4 mm. Next, the hot-rolled sheet was pickled and cold-rolled to a sheet thickness of 1.2 mm.
[0046]
[Table 1]

[0047]
Thereafter, the cold-rolled plate was soaked at 820 ° C. for 150 seconds, cooled to 600 ° C. at an average rate of 10 ° C./s, then immersed in a 460 ° C. hot dip galvanizing bath, and alloyed at 550 ° C. During annealing, the holding time in the temperature range of 600 to 350 ° C. was 130 sec. The annealed plate was subjected to temper rolling with an elongation of 1.0%, and the tensile test, secondary work brittleness resistance, and surface appearance were evaluated.
[0048]
The tensile test is carried out by a method compliant with JIS Z 2241 (Japanese Industrial Standard) .If a tensile strength (TS) of 590 MPa or more is obtained, the property is good (○) .In the case of a TS of less than 590 MPa, the strength is Rated as deficient (x). The secondary work brittleness resistance was evaluated by the method shown in FIG. 1, and when the critical temperature (Tc) for longitudinal cracking was -61 to -90 ° C, the characteristics were good (○), and the Tc was -30 to -60 ° C. When the tolerance level (Δ) and Tc was 25 to -29 ° C., it was judged that the characteristic was deteriorated (×).
[0049]
Further, the plating surface was visually evaluated in the range of 120 mm in width and 2000 mm in length, and when surface defects such as non-plating and linear defects were observed, it was evaluated as surface defect (x). Table 2 shows the evaluation results of the tensile strength, critical crack critical temperature, and plating surface properties.
[0050]
[Table 2]

[0051]
Invention Examples Nos. 1 to 8 (Steel Nos. 1 to 8) are all within the range of the present invention, TS has characteristic values of 600 to 680 MPa, Tc of −65 to −85 ° C., and tensile strength In addition, good characteristics are obtained in both secondary work brittleness resistance. Further, the surface properties are all good.
[0052]
On the other hand, Comparative Examples Nos. 9 to 16 (steel numbers 9 to 16) are all out of the range of the present invention and do not satisfy tensile strength, secondary work brittleness resistance and plating surface properties. Comparative Examples No. 9 and 12 have low Tc of −80 to −90 ° C. and good secondary work brittleness resistance, and the plated surface is also preferable, but TS is as low as 490 to 520 MPa, and 590 MPa or more. Tensile strength is not obtained. Comparative Examples Nos. 10, 11, and 14 have high TS of 610 to 750 MPa, good tensile strength, and all have preferable plating surfaces, but Tc is as high as 25 to -10 ° C., secondary processing resistance Brittleness has deteriorated.
[0053]
In Comparative Examples Nos. 13 and 15, TS is as high as 630 to 660 MPa and good strength is obtained, but Tc is as high as 15 to 20 ° C., and the secondary work brittleness resistance is deteriorated. Moreover, non-plating exists in any surface and the surface property is not preferable. Comparative Example No. 16 has good strength with TS of 605 MPa, and Tc is as low as −45 ° C., which is an acceptable level of secondary work brittleness resistance, but there are linear defects on the surface. The surface appearance is not preferable.
[0054]
[Example 2]
Steel No. 8 shown in Table 1 (present invention steel) was steeled in an electric furnace, and then ingoted to produce two slabs with a thickness of 200 mm. The slab was heated at 1270 ° C. for 1 hour, then rough rolling was started, and finish rolling was performed at 880 ° C. to produce a hot-rolled coil having a thickness of 2.0 mm. The winding temperature was 550 ° C. Next, the two hot-rolled coils were pickled, and then subjected to continuous hot dip galvanization in different annealing cycles.
[0055]
Of these, using one coil, changing the soaking temperature in the longitudinal direction of the coil to 720-910 ° C, cooling, and then dipping in a 460 ° C hot dip galvanizing bath immediately The temperature was raised to 500 ° C., alloyed, and cooled to room temperature. During this period, after the soaking, the holding time in the temperature range of 600 to 350 ° C. was set to 150 seconds in the annealing process to room temperature.
[0056]
In addition, using the remaining one coil, soaking at 800-815 ° C, cooling, immersing in a hot-dip galvanizing bath at 460 ° C, and immediately raising the temperature to 550 ° C for alloying treatment And cooled to room temperature. In this coil, the holding time in the temperature range of 600 to 350 ° C. during annealing in the longitudinal direction of the coil was changed to 100 to 280 seconds.
[0057]
In addition, after the plating, each annealed plate was subjected to temper rolling with an elongation rate of 1.0%. From the position where the soaking temperature was changed in each coil or the holding time in the intermediate temperature range of 600 to 350 ° C., an annealed plate was collected, and in the same manner as in Example 1, tensile test, secondary resistance Processing brittleness and surface appearance were evaluated.
[0058]
Table 3 shows the evaluation results of tensile strength, longitudinal crack critical temperature, and plating surface properties.
[0059]
[Table 3]

[0060]
Steel plates No. 21, 22, and 27 all have a soaking temperature outside the scope of the present invention, and surface properties are good, but TS is 560 to 750 MPa, Tc is -25 to 10 ° C., secondary resistance Processing brittleness is undesirable.
[0061]
Nos. 23 to 26 all have a soaking temperature within the range of the present invention, TS is as high as 600 to 670 MPa, and Tc shows a characteristic value of -35 to -75 ° C. An acceptable range or good characteristics are obtained. Moreover, all have a favorable plating surface.
[0062]
Nos. 28 and 29 both have a retention time in the temperature range of 600 to 350 ° C., TS is high as 610 to 615 MPa, and Tc has a low characteristic value as −45 to −70 ° C. Both strength and secondary work brittleness resistance are good. Furthermore, it has preferable plating surface properties.
[0063]
Each of Nos. 30 to 32 has a holding time in the temperature range of 600 to 350 ° C. that is outside the range of the present invention, TS is as high as 605 to 620 MPa, and the plated surface shows a preferable appearance, but Tc is 5 As high as -20 ° C, the secondary work brittleness resistance has deteriorated.
[0064]
【The invention's effect】
According to the present invention, the chemical composition of steel is specified, the heating temperature is optimized by the Nb amount and the C amount, and further, in the hot dip galvanizing process from the cooling process after heating, depending on the P amount, 600 to 350 By optimizing the holding time in the temperature range of ° C., the segregation of P that causes embrittlement of the grain boundaries is suppressed, and the grain boundary strength is maintained. By defining the chemical composition and production conditions of the steel in this way, it is possible to stably produce a hot-dip galvanized steel sheet having a secondary work brittleness resistance having a strength of 590 MPa or more, and toughness is required. Since it can be applied to structural parts of automobiles, the utility value in the automobile industry is great.
[Brief description of the drawings]
FIG. 1 is a view showing a method for evaluating secondary work brittleness resistance of a steel sheet.
FIG. 2 is a graph showing the effects of soaking temperature and Nb + C / 8 on secondary work brittleness resistance.
FIG. 3 is a graph showing the effect of holding time at 600 to 350 ° C. and P on secondary work brittleness resistance.

Claims

Chemical composition, C in mass%: 0.050 ~ 0.130, Si ≦ 0.7, Mn: 1.3~2.5, P ≦ 0.08, S ≦ 0.01, sol.Al: 0.01~0.1, N ≦ 0.005, Nb: containing from 0.007 to 0.06 Then, after melting and casting the steel composed of the remaining iron and inevitable impurities , it is hot-rolled to obtain a hot-rolled steel sheet, and this hot-rolled steel sheet or cold-rolled obtained by further cold-rolling When a steel sheet is heated and then cooled and subjected to a hot dip galvanizing step (including a case where alloying treatment is performed after hot dip galvanizing) to produce a hot dip galvanized steel sheet, the heating satisfies the following inequality. Heating to a temperature T (° C.), and maintaining the time t (sec) satisfying the following inequality in the temperature range of 600 to 350 ° C. in the hot dip galvanizing process from the cooling process after heating , A method for producing hot-dip galvanized steel sheets with strength of 590 MPa or more and excellent secondary work brittleness resistance .
89 × log (Nb + C / 8) + 900 ≦ T <900
t ≦ 107-338 × log (3 × P + 0.5)
Here, the element symbol in a formula shows each mass%.

The hot-dip galvanized steel sheet manufacturing method according to claim 1, wherein the chemical component further contains V: 0.003 to 0.08 in mass% and has a tensile strength of 590 MPa or more and excellent secondary work brittleness resistance. Manufacturing method of galvanized steel sheet.

In the manufacturing method of the hot-dip galvanized steel sheet according to claim 1 or 2, further, as a chemical component, mass% so Ti ≦ 0.05 , Cr ≦ 0.5 Out of 1 Seed or 2 It has a tensile strength characterized by containing seeds. 590MPa The manufacturing method of the hot dip galvanized steel sheet which is excellent in the secondary work brittleness resistance above.

A hot-dip galvanized steel sheet, characterized in that the chemical component is the chemical component according to claim 1, 2 or 3 , the tensile strength is 590 MPa or more, and the secondary work brittleness resistance is excellent.