JP4276482B2 - High-strength hot-rolled steel sheet with excellent ultimate deformability and shape freezing property and its manufacturing method - Google Patents
High-strength hot-rolled steel sheet with excellent ultimate deformability and shape freezing property and its manufacturing method Download PDFInfo
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- JP4276482B2 JP4276482B2 JP2003182675A JP2003182675A JP4276482B2 JP 4276482 B2 JP4276482 B2 JP 4276482B2 JP 2003182675 A JP2003182675 A JP 2003182675A JP 2003182675 A JP2003182675 A JP 2003182675A JP 4276482 B2 JP4276482 B2 JP 4276482B2
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- 229910000831 Steel Inorganic materials 0.000 title claims description 115
- 239000010959 steel Substances 0.000 title claims description 115
- 238000007710 freezing Methods 0.000 title claims description 64
- 230000008014 freezing Effects 0.000 title claims description 62
- 238000004519 manufacturing process Methods 0.000 title claims description 16
- 238000005098 hot rolling Methods 0.000 claims description 33
- 238000005096 rolling process Methods 0.000 claims description 29
- 230000009467 reduction Effects 0.000 claims description 12
- 239000013078 crystal Substances 0.000 claims description 11
- 239000000203 mixture Substances 0.000 claims description 9
- 238000001816 cooling Methods 0.000 claims description 8
- 239000000126 substance Substances 0.000 claims description 7
- 229910000859 α-Fe Inorganic materials 0.000 claims description 7
- 230000032683 aging Effects 0.000 claims description 6
- 229910001563 bainite Inorganic materials 0.000 claims description 6
- 229910001567 cementite Inorganic materials 0.000 claims description 6
- 229910052758 niobium Inorganic materials 0.000 claims description 4
- 229910052719 titanium Inorganic materials 0.000 claims description 4
- 239000012535 impurity Substances 0.000 claims description 3
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 3
- 238000004804 winding Methods 0.000 claims description 2
- 238000005452 bending Methods 0.000 description 16
- 238000012545 processing Methods 0.000 description 15
- 230000000694 effects Effects 0.000 description 14
- 238000000034 method Methods 0.000 description 11
- 239000000463 material Substances 0.000 description 7
- 229910001566 austenite Inorganic materials 0.000 description 5
- 238000009864 tensile test Methods 0.000 description 5
- 230000037303 wrinkles Effects 0.000 description 5
- 230000007423 decrease Effects 0.000 description 4
- 238000005259 measurement Methods 0.000 description 4
- 238000003303 reheating Methods 0.000 description 4
- 238000011156 evaluation Methods 0.000 description 3
- 238000007747 plating Methods 0.000 description 3
- 238000005498 polishing Methods 0.000 description 3
- 230000008569 process Effects 0.000 description 3
- 238000005728 strengthening Methods 0.000 description 3
- 230000035882 stress Effects 0.000 description 3
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- 238000002441 X-ray diffraction Methods 0.000 description 2
- 238000005336 cracking Methods 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 238000010438 heat treatment Methods 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- 238000002347 injection Methods 0.000 description 2
- 239000007924 injection Substances 0.000 description 2
- 229910052742 iron Inorganic materials 0.000 description 2
- 229910000734 martensite Inorganic materials 0.000 description 2
- 150000001247 metal acetylides Chemical class 0.000 description 2
- 229910052761 rare earth metal Inorganic materials 0.000 description 2
- 230000009466 transformation Effects 0.000 description 2
- 229910052720 vanadium Inorganic materials 0.000 description 2
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 1
- 229910001122 Mischmetal Inorganic materials 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 230000003679 aging effect Effects 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 229910002092 carbon dioxide Inorganic materials 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 238000005097 cold rolling Methods 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000005520 cutting process Methods 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 230000018109 developmental process Effects 0.000 description 1
- 238000007598 dipping method Methods 0.000 description 1
- 238000009713 electroplating Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 239000010419 fine particle Substances 0.000 description 1
- 230000014509 gene expression Effects 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- -1 iron carbides Chemical class 0.000 description 1
- 229910052747 lanthanoid Inorganic materials 0.000 description 1
- 150000002602 lanthanoids Chemical class 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 230000037311 normal skin Effects 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 238000005070 sampling Methods 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 229910052712 strontium Inorganic materials 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
- 229910052718 tin Inorganic materials 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
- 238000007740 vapor deposition Methods 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 239000013585 weight reducing agent Substances 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
Landscapes
- Heat Treatment Of Sheet Steel (AREA)
- Metal Rolling (AREA)
- Heat Treatment Of Steel (AREA)
Description
【0001】
【発明の属する技術分野】
本発明は、自動車部材等に使用され、効率よく自動車部材の軽量化を達成することのできる極限変形能と形状凍結性に優れた高強度熱延鋼板と、その製造方法に関するものである。
【0002】
【従来の技術】
自動車からの炭酸ガスの排出量を抑えるために、高強度鋼板を使用して自動車車体の軽量化が進められている。また、搭乗者の安全性確保のためにも、自動車車体には軟鋼板の他に高強度鋼板が多く使用されるようになってきている。さらに自動車車体の軽量化を今後進めていくために、従来以上に、高強度鋼板の使用強度レベルを高めたいという新たな要請が非常に高まりつつある。
【0003】
しかしながら、高強度鋼板に曲げ変形を加えると、加工後の形状は、その高強度ゆえに、加工冶具の形状から離れて、加工前の形状にもどりやすくなる。加工を与えても、加工後の形状が元の形状の方向にもどろうとする現象はスプリング・バックと呼ばれている。このスプリング・バックが発生すると、狙いとする加工部品の形状が得られない。
【0004】
したがって、従来の自動車の車体では、主として440MPa以下の高強度鋼板に限って使用されてきた。自動車車体にとっては、490MPa以上の高強度鋼板を使用して車体の軽量化を進めていく必要があるにもかかわらず、スプリング・バックが少なく、かつ、形状凍結性の良い高強度鋼板が存在しないのが実状である。
【0005】
一方、高強度鋼板を自動車用部品等へ加工する際には、形状凍結性以外にも様々な特性が要求される。特に、伸びフランジ加工やバーリング加工の際に要求される極限変形能は重要な特性であり、該特性と形状凍結性が両立することで、自動車車体への高強度鋼板の適用範囲が、一層広範なものとなる。
【0006】
本発明者らは、これまでも、特許文献1などで結晶方位とr値を規定した形状凍結性に優れた鋼板およびその製造方法について開示してきた。
【0007】
今回、さらに鋭意検討を重ねた結果、形状凍結性と加工性が両立するためには、さらなる集合組織制御、および、延性の異方性制御が極めて重要であることを新たに見出した。そして、これらを満足するための最適造条件を、新たに見出した。
【0008】
【特許文献1】
WO00/06791号国際出願公報
【0009】
【発明が解決しようとする課題】
曲げ加工を施す自動車用部材に適用する鋼板の強度を増すと、鋼板強度の上昇にしたがってスプリング・バックの量が増大し、形状不良が発生し、高強度鋼板の適用が制限されているのが現状である。
【0010】
また、良好なプレス成形性と高い衝撃エネルギー吸収能は、高強度鋼板が自動車部品等に適用されるためには欠くことのできない特性である。
【0011】
本発明は、この問題を抜本的に解決して、良好な形状凍結性と良好な極限変形能を有する高強度熱延鋼板、および、その製造方法を提供するものである。
【0012】
【課題を解決するための手段】
従来の知見によれば、スプリング・バックを抑えるための方策としては、鋼板の降伏点を低くすることが、とりあえず重要であると考えられていた。そして、降伏点を低くするためには、引張強さの低い鋼板を使用せざるをえなかった。
【0013】
しかし、これだけでは、鋼板の曲げ加工性を向上させ、スプリング・バック量を低く抑えるための根本的な解決にはならない。
【0014】
そこで、本発明者らは、曲げ加工性を向上させてスプリング・バックの発生を根本的に解決するために、新たに鋼板の集合組織の曲げ加工性への影響に着目して、その作用効果を詳細に調査、研究した。
【0015】
そして、その結果、曲げ加工性に優れた鋼板を見いだした。即ち、本発明者らは、{100}<011>〜{223}<110>方位群、その中でも、特に、{100}<011>方位、さらに、{554}<225>、{111}<112>、{111}<110>の各方位のX線ランダム強度比を制御すること、さらには、圧延方向のr値および圧延方向と直角方向のr値のうち少なくとも1つをできるだけ低い値にすること、および、局部伸びの異方性を2%以上にすることで、曲げ加工性が飛躍的に向上することを明らかにした。
【0016】
しかし、局部伸びの異方性が大きくなると伸びフランジ性が劣化することが予想され、形状凍結性と加工性の両立が困難となる。そこで、本発明者らは、鋭意研究の結果、上記集合組織制御と炭化物制御を同時に成立させることによって、形状凍結性と極限変形能を同時に高めることができることを明らかにした。
【0017】
また、種々の部品を成形するためのブランク採取方向を限定しないことは、鋼材の歩留まり向上に大きく貢献するが、このためには、延性の異方性、とりわけ、均一伸びの異方性を小さくすることが重要な意味を持つ。
【0018】
本発明者らは、実験によって、鋼板の仕上熱間圧延の開始温度と終了温度を制御することによって、{100}<011>方位を主方位として発達せしめ、それによって、上記形状凍結性と加工性を確保しつつ、均一伸びの異方性を小さくすることが可能であることを見出した。
【0019】
本発明は、前述の知見に基づいて構成されており、その主旨とするところは、以下のとおりである。
【0020】
(1) 質量%で、
C:0.01%以上、0.2%以下、
Si:0.001%以上、2.5%以下、
Mn:0.01%以上、2.5%以下、
P:0.2%以下、
S:0.03%以下、
Al:0.01%以上、2.0%以下、
N:0.01%以下、
O:0.01%以下
含み、残部がFeおよび不可避的不純物からなり、ミクロ組織がフェライトもしくはベイナイトを体積分率最大の相とし、少なくとも1/2板厚における板面の、
(1) {100}<011>〜{223}<110>方位群のX線ランダム強度比の平
均値が2.5以上、
(2) {554}<225>、{111}<112>および{111}<110>の3
つの結晶方位のX線ランダム強度比の平均値が3.5以下、
(3) {100}<011>X線ランダム強度比が{211}<011>X線ランダム
強度比以上、および、
(4) {100}<011>X線ランダム強度比が2.5以上
の全てを満足し、かつ、圧延方向のr値および圧延方向と直角方向のr値のうち少なくとも1つが0.7以下であり、さらに、均一伸びの異方性△uElが4%以下、局部伸びの異方性△LElが2%以上で、かつ、△uElが△LEl以下であることを特徴とする極限変形能と形状凍結性に優れた高強度熱延鋼板。
【0021】
であり、圧延方向と平行(L方向)、垂直(C方向)、および、45°方向の均一伸びを、それぞれ、uEl(L)、uEl(C)、および、uEl(45°)とし、圧延方向と平行(L方向)、垂直(C方向)、および、45°方向の局部伸びを、それぞれ、LEl(L)、LEl(C)、および、LEl(45°)とする。
【0022】
(2) さらに、直径0.2μm以上の鉄炭化物の占積率が0.3%以下であることを特徴とする(1)記載の極限変形能と形状凍結性に優れた高強度熱延鋼板。
【0023】
(3) さらに、時効指数AIが8MPa以上であることを特徴とする(1)または(2)記載の極限変形能と形状凍結性に優れた高強度熱延鋼板。
【0025】
(4) さらに、質量%で、Nb、Ti、Vの1種または2種以上を合計で0.001%以上0.8%以下含むことを特徴とする(1)〜(3)のいずれかに記載の極限変形能と形状凍結性に優れた高強度熱延鋼板。
【0026】
(5) さらに、質量%で、Bを0.01%以下含むことを特徴とする(1)〜(4)のいずれかに記載の極限変形能と形状凍結性に優れた高強度熱延鋼板。
【0027】
(6) さらに、質量%で、
Mo:1%以下、
Cr:1%以下、
Cu:2%以下、
Ni:1%以下、
Sn:0.2%以下、
Co:2%以下
の1種または2種以上を含有することを特徴とする(1)〜(5)のいずれかに記載の極限変形能と形状凍結性に優れた高強度熱延鋼板。
【0028】
(7) 質量%で、
Ca:0.0005〜0.005%、
Rem:0.001〜0.05%、
Mg:0.0001〜0.05%、
Ta:0.0001〜0.05%
の1種または2種以上を含むことを特徴とする(1)〜(6)のいずれかに記載の極限変形能と形状凍結性に優れた高強度熱延鋼板。
【0029】
(8) (1)〜(7)のいずれかに記載の極限変形能と形状凍結性に優れた高強度熱延鋼板にめっきを施したことを特徴とする極限変形能と形状凍結性に優れた高強度熱延鋼板。
【0030】
(9) (1)〜(8)のいずれかに記載の極限変形能と形状凍結性に優れた高強度熱延鋼板を製造するにあたり、(1)、(4)〜(7)のいずれかに記載の成分組成を有する鋳造スラブを、鋳造ままもしくは一旦冷却した後に1000〜1300℃の範囲に再度加熱し、熱間圧延をする際、Ar3℃〜(Ar3+150)℃の温度範囲における圧下率の合計が25%以上となるように制御し、仕上熱延開始温度TFSと仕上熱延完了温度TFE(℃)が下記(1)〜(4)式を全て同時に満足するように熱間圧延を終了し、熱間圧延後冷却して(5)式に示す鋼の化学成分で決まる臨界温度To(℃)以下で、かつ、700℃以下400℃以上の温度で巻き取ることを特徴とする極限変形能と形状凍結性に優れた高強度熱延鋼板の製造方法。
【0031】
TFE≧Ar3(℃) (1)
TFE≧800℃ (2)
TFS≦1100℃ (3)
20℃≦(TFS−TFE)≦120℃ (4)
To=−650.4×{C%/(1.82×C%−0.001)}+B (5)
ここで、Bは質量%で表現した鋼の成分より求まる。
【0032】
B=−50.6×Mneq+894.3
Mneq=Mn%+0.24×Ni%+0.13×Si%+0.38×Mo%
+0.55×Cr%+0.16×Cu%−0.50×Al%−0.45×Co%
+0.90×V%
ただし、
Ar3=901−325×C%+33×Si%+287×P%+40×Al%
−92×(Mn%+Mo%+Cu%)−46×(Cr%+Ni%)
(10) さらに、Ar3〜(Ar3+150)℃の温度範囲における熱間圧延の内少なくとも1パス以上において摩擦係数が0.2以下となるように制御することを特徴とする(9)記載の極限変形能と形状凍結性に優れた高強度熱延鋼板の製造方法。
【0033】
(11) (9)または(10)記載の極限変形能と形状凍結性に優れた高強度熱延鋼板の製造方法で製造された高強度熱延鋼板に、0.1%以上5%以下のスキンパス圧延を施すことを特徴とする極限変形能と形状凍結性に優れた高強度熱延鋼板の製造方法。
【0034】
【発明の実施の形態】
以下に、本発明の内容を詳細に説明する。
【0035】
1/2板厚における板面の{100}<011>〜{223}<110>方位群のX線ランダム強度比の平均値:
板厚中心位置での板面のX線回折を行い、ランダム試料に対する各方位の強度比を求めたとき、{100}<011>〜{223}<110>方位群の平均値は2.5以上でなくてはならない。これが2.5未満では、形状凍結性が劣悪となる。
【0036】
この方位群に含まれる主な方位は、{100}<011>、{116}<110>、{114}<110>、{113}<110>、{112}<110>、{335}<110>、および、{223}<110>である。
【0037】
これら各方位のX線ランダム強度比は、{110}極点図に基づきベクトル法により計算した3次元集合組織や、{110}、{100}、{211}、および、{310}の極点図のうち、複数の極点図(好ましくは3つ以上)を用いて級数展開法で計算した3次元集合組織から求めればよい。
【0038】
例えば、後者の方法における上記各結晶方位のX線ランダム強度比には、3次元集合組織のφ2=45゜断面における(001)[1−10]、(116)[1−10]、(114)[1−10]、(113)[1−10]、(112)[1−10]、(335)[1−10]、(223)[1−10]の強度をそのまま用いればよい。
【0039】
{100}<011>〜{223}<110>方位群の平均値とは、上記の各方位の相加平均である。上記の全ての方位の強度を得ることができない場合には、{100}<011>、{116}<110>、{114}<110>、{112}<110>、および、{223}<110>の方位の相加平均で代替してもよい。
【0040】
さらに、望ましくは、{100}<011>〜{223}<110>方位群のX線ランダム強度比の平均値が4.0以上である。
【0041】
1/2板厚における板面の{554}<225>、{111}<112>および{111}<110>の3つの結晶方位のX線ランダム強度比の平均値:
1/2板厚における板面の{554}<225>、{111}<112>および{111}<110>の3つの結晶方位のX線ランダム強度比の平均値は、3.5以下でなくてはならない。これが3.5超であると、{100}<011>〜{223}<110>方位群の強度が適正であっても、良好な形状凍結性を得ることが困難となる。
【0042】
{554}<225>、{111}<112>および{111}<110>のX線ランダム強度比も、上記の方法に従って計算した3次元集合組織から求めればよい。
【0043】
さらに、望ましくは、{554}<225>、{111}<112>および{111}<110>のX線ランダム強度比の相加平均値が、2.5未満である。
【0044】
1/2板厚における板面の{100}<011>および{211}<011>X線ランダム強度比:
1/2板厚における板面の{100}<011>X線ランダム強度比は、{112}<011>X線ランダム強度比以上でなければならない。{211}<011>方位のX線ランダム強度比が{100}<011>X線ランダム強度比より大きくなると、均一伸びの異方性が大きくなり、加工性が劣化する。
【0045】
また、{100}<011>X線ランダム強度比は、2.5以上でなければならない。これが2.5未満になると、良好な形状凍結性を得ることができない。
【0046】
なお、ここで述べる{100}<011>および{211}<011>は、それぞれ、同様の効果を有する方位の範囲として、圧延方向に対して直角な方向(Transverse direction)を回転軸として、±12°を許容する。さらに、望ましくは、±6°とする。
【0047】
以上述べた結晶方位のX線強度が、曲げ加工時の形状凍結性や伸びの異方性に対して重要であることの理由は、必ずしも明らかではないが、曲げ変形時の結晶のすべり挙動と関係があるものと推測される。
【0048】
X線回折に供する試料は、機械研磨などによって鋼板を所定の板厚まで減厚し、次いで、化学研磨や電解研磨などによって歪みを除去すると同時に、板厚1/2面が測定面となるように作製する。
【0049】
鋼板の板厚中心層に偏析帯や欠陥などが存在し、測定上不都合が生ずる場合には、板厚の3/8〜5/8の範囲で適当な面が測定面となるように、上述の方法に従って試料を調整して測定すればよい。
【0050】
当然のことであるが、上述のX線強度の限定が、板厚1/2近傍だけでなく、なるべく多くの厚み(特に最表層〜板厚の1/4)について満たされることで、より一層、形状凍結性が良好になる。
【0051】
なお、{hkl}<uvw>で表される結晶方位とは、板面の法線方向が<hkl>に平行で、圧延方向が<uvw>と平行であることを示している。
【0052】
圧延方向のr値(rL)および圧延方向と直角方向のr値(rC):
本発明において重要な要件である。すなわち、本発明者らが鋭意検討した結果、上述した種々の結晶方位のX線強度が適正であっても、必ずしも良好な形状凍結性が得られないことが判明した。
【0053】
上記のX線強度と同時に、rLおよびrCのうち少なくとも1つが0.7以下であることが必須である。より好ましくは0.55以下である。
【0054】
rLおよびrCの下限は特に定めることなく、本発明の効果を得ることができるが、r値はJIS5号引張試験片を用いた引張試験により評価する。引張歪みは通常15%であるが、均一伸びが15%を下回る場合には、均一伸びの範囲で、できるだけ15%に近い歪みで評価すればよい。
【0055】
なお、曲げ加工を施す方向は、加工部品によって異なるので、特に限定するものではないが、r値が小さい方向に対して垂直もしくは垂直に近い方向に折り曲げる加工を主とすることが好ましい。
【0056】
ところで、一般に、集合組織とr値とは相関があることが知られているが、本発明においては、既述の結晶方位のX線強度比に関する限定と、r値に関する限定とは互いに同義ではなく、両方の限定が同時に満たされなくては、良好な形状凍結性を得ることはできない。
【0057】
延性の異方性:
鋼板をプレス成形する場合には、鋼板の均一伸び、すなわち、n値が重要な意味を持つ。特に、張りだし成形が主となる高強度鋼板においては、この均一伸び(n値)が異方性を持つ場合には、部品によってブランク切りだし方向を注意深く選定することが必要となり、生産性の低下や鋼板歩留まりの低下を招く。また、場合によっては、所望の形状に成形できない場合も生じる。
【0058】
400MPa程度以上の引張り強度(引張り試験で得られる最大強度)を持つ鋼においては、この均一伸びの異方性△uElが4%以下であれば、方向によらない良好な成形性を示すことが判明した。
【0059】
特に厳しい加工性が要求される場合には、異方性△uElが3%以下であることが望ましい。均一伸びの異方性△uElの下限は特に限定しないが、加工性の観点からは0%にすることが最も好ましい。
【0060】
また、局部伸びの異方性△LElが2%未満になると、形状凍結性が劣化することから、△LElの下限は2%とする。△LElの上限は特に設定しないが、△LElが大きくなりすぎると成形性が低下することから、12%以下とすることが望ましい。
【0061】
ただし、上記の条件を満足しても、△uEl>△LElとなる場合には、良好な成形性と形状凍結性が両立しなかったので、△uElは△LEl以下とした。
【0062】
なお、均一伸びと局部伸びの異方性は、圧延方向と平行(L方向)、垂直(C方向)、および、45°方向の伸び(均一伸びuEl、局部伸びLEl)を用いて、
と定義される。
【0063】
ミクロ組織:
実際の自動車部品においては、1つの部品の中で、上記のような曲げ加工に起因する形状凍結性が問題になるだけではなく、同一部品の他の部位においては、伸びフランジ加工やバーリング加工等の加工を受ける場合が少なくない。
【0064】
したがって、上述の集合組織を制御した曲げ加工時の形状凍結性の向上とともに、鋼板の極限変形能も向上させる必要がある。
【0065】
この観点から、金属組織は、高い穴拡げ性を有するフェライトもしくはベイナイト相を体積分率最大の相とする。ただし、集合組織の観点からは、低温で変態生成するベイナイト相の方が集合組織の発達が強いことから、ベイナイトを主相とする方が好ましい。
【0066】
なお、ここで述べるベイナイトは、ミクロ組織中に鉄炭化物粒子を含んでも含まなくてもよい。また、変態後に加工を受け、内部の転位密度が非常に高くなったフェライト(加工フェライト)は、延性が著しく劣化し、部品加工には適さないことから、本発明に規定するフェライトとは区別する。
【0067】
さらに、伸びフランジ性を著しく劣化させる直径0.2μm以上の鉄炭化物の占積率は0.3%以下に限定することが好ましい。鉄炭化物の占積率は、倍率500倍以上の光学顕微鏡観察写真において、画像処理によって鉄炭化物の面積率を求めて代替してもよい。また、写真上に描いたn個の格子点のうち0.2μm以上の鉄炭化物が占める格子点の数mを求め、m/nを占積率としてもよい。
【0068】
時効指数AI(Aging Index ):
鋼板の時効性を示す指数であるAIは、8MPa以上とすることが好ましい。AIが8MPa未満になると、形状凍結性が低下するので、8MPaを下限とする。AIが低下すると形状凍結性が劣化する要因は明らかでないが、AIは鋼材中の可動転位密度と相関があることから、この可動転位密度の違いが変形になんらかの影響を及ぼしていると考えられる。
【0069】
AIの上限は特に規定しないが、AIが100MPa超になると、ストレッチャーストレインが発生し、鋼板の外観を著しく損ねるおそれがあることから、AIは100MPa以下とすることが望ましい。
【0070】
なお、時効指数の測定には、L方向またはC方向のJIS5号引張試験片を用い、予歪み10%与えた際の変形応力と、その後一旦除荷し、100℃で一時間の時効を行った後、再度引張試験を行った際の降伏応力(降伏伸びが発生する場合には下降伏応力)との差を時効指数AIとする。
【0071】
以下に、本発明の好ましい化学成分について述べる(単位は質量%である)。
【0072】
C:
Cの下限を0.01%としたのは、Cが0.01%未満では高い加工性を維持したまま鋼板の強度を確保することが困難なためである。一方、0.2%超になると、極限変形能を低下させるオーステナイト相やマルテンサイト相、粗大炭化物ができやすくなるうえ、溶接性も低下するので、上限を0.2%とする。
【0073】
Si:
鋼板の機械的強度を高めるのに有効な元素であるが、2.5%超となると加工性が劣化したり、表面疵が発生したりするので、2.5%を上限とする。一方、実用鋼で、Siを0.001%未満とするのは困難であるので、0.001%を下限とする。
【0074】
Mn:
鋼板の機械的強度を高めるのに有効な元素であるが、2.5%超となると加工性が劣化するので、2.5%を上限とする。一方、実用鋼で、Mnを0.01%未満とするのは困難であるので、0.01%を下限とする。
【0075】
また、Mn以外に、Sによる熱間割れの発生を抑制するTiなどの元素が十分に添加されない場合には、Mnを、質量%でMn/S≧20となる量添加することが望ましい。
【0076】
P、S:
それぞれ、0.2%以下、および、0.03%以下とする。これは、加工性の劣化や熱間圧延または冷間圧延時の割れを防ぐためである。
【0077】
Al:
脱酸のために0.01%以上添加する。しかし、多すぎると加工性が低下したり、表面性状が劣悪となるため、上限を2.0%とする。
【0078】
N、O:
不純物であり、加工性を悪くさせないように、それぞれ、0.01%以下、および、0.01%以下とする。
【0079】
Ti、Nb、V:
析出強化、組織制御、細粒強化などの機構を通じて材質を改善する元素であるので、必要に応じて、1種または2種以上を、合計で0.001%以上添加することが望ましい。
【0080】
しかし、過度に添加しても格段の効果はなく、むしろ加工性や表面性状を劣化させるので、1種または2種以上の合計で0.8%を上限とする。
【0081】
B:
粒界の強化や鋼材の高強度化に有効であるが、その添加量が0.01%を超えると、その効果が飽和するばかりでなく、必要以上に鋼板強度を上昇させ、部品への加工性も低下させるので、上限を0.01%とした。ただし、Bの添加効果を得るためには、0.0002%以上添加することが好ましい。
【0082】
Mo、Cr、Cu、Ni、Sn、Co:
機械的強度を高めたり材質を改善する効果があるので、必要に応じて、各成分とも0.001%以上を添加することが望ましい。しかし、過度の添加は、逆に、加工性を劣化させるので、上限を、それぞれ、1%、1%、2%、1%、0.2%、2%とする。
【0083】
Ca、希土類元素(Rem):
介在物制御に有効な元素で、適量添加は熱間加工性を向上させるが、過剰の添加は、逆に、熱間脆化を助長させるので、必要に応じて、それぞれ、Ca:0.0005〜0.005%、Rem:0.001〜0.05%とした。ここで、希土類元素とは、Y,Srおよびランタノイド系の元素を指し、工業的には、これらの混合物である。
【0084】
また、Mgを0.0001%以上0.05%以下、Taを0.0001%以上0.05%以下添加することも、等価な効果を発現する。
【0085】
ここで、全ての場合に、下限値は介在物制御効果の発現する最低添加量を示し、最大値以上では、逆に、介在物が大きく成長しすぎることによって、伸びフランジ性等の極限変形能を低下させる。ミッシュメタル(混合物)として添加することが、コスト的に有利である。
【0086】
以下に、本発明の製造方法について述べる。
【0087】
スラブ再加熱温度:
所定の成分に調整された鋼は、鋳造後直接もしくは一旦Ar3変態温度以下まで冷却された後に再加熱され、その後に熱間圧延される。
【0088】
この時の再加熱温度が1000℃未満の場合には、所定の仕上熱延完了温度を確保することが難しくなるので、1000℃を下限とした。また、再加熱温度が1300℃を超える場合には、加熱時のスケール生成による歩留まり劣化を招くと同時に、製造コストの上昇も招くことから、1300℃を再加熱温度の上限値とした。
【0089】
加熱後の鋼片は、熱間圧延の途中で局部的にもしくは全体を加熱されても、本発明の特性に何ら影響を及ぼさない。
【0090】
熱間圧延条件:
熱間圧延およびその後の冷却によって、所定のミクロ組織と集合組織に制御される。最終的に得られる鋼板の集合組織は、熱間圧延の温度領域によって大きく変化する。熱延完了温度TFEがAr3℃未満になった場合には、均一伸びの異方性△uElが4%超となり、成形性を著しく劣化させるために、
TFE≧Ar3(℃) (1)
とした。
【0091】
TFEは、熱延の最終圧延を施すスタンドの後方で測定されるのが一般的であるが、必要な場合には、計算によって得られる温度を用いてもよい。
【0092】
また、熱延完了温度の上限は特に限定しないが、(Ar3+180℃)超の場合には、鋼板の表面に生成する酸化物層により表面品位が低下するので、(Ar3+180℃)以下であることが望ましい。
【0093】
より厳格な表面品位が求められる場合には、TFEを(Ar3+150℃)以下にすることが望ましい。
【0094】
ただし、鋼板の化学成分によらず、TFEが800℃未満になった場合には、熱延時の圧延荷重が高くなりすぎると同時に、鋼板の延性異方性が大きくなることから、
Ar3≧800℃ (2)
とした。
【0095】
また、仕上熱延開始温度TFSが1100℃超の場合には、鋼板表面品位が著しく低下することから、
TFS≦1100℃ (3)
とした。
【0096】
また、TFSとTFEの差が120℃超の場合には、集合組織の発達が十分でなく、良好な形状凍結性と低い異方性が両立せず、また、この差を20℃未満にすることは、操業上困難であることから、
20℃≦(TFS−TFE)≦120℃ (4)
とした。
【0097】
また、熱間圧延において、Ar3℃〜(Ar3+150)℃の温度範囲における圧下率は、最終的な鋼板の集合組織形成に大きな影響を及ぼし、この温度範囲での圧延率が25%未満の場合には、集合組織の発達が十分でなく、最終的に得られる鋼板が良好な形状凍結性を示さないので、Ar3℃〜(Ar3+150)℃の温度範囲における圧下率の下限を25%とした。
【0098】
この圧下率が高いほど、所望の集合組織が発達するから、圧下率は、50%以上であることが好ましく、また、75%以上であれば、さらに好ましい。
【0099】
圧下率の上限は特に定めないが、99%以上圧下することは、装置への負荷が大きく、また、特段の効果も得られないので、99%未満とすることが好ましい。
【0100】
ただし、
Ar3=901−325×C%+33×Si%+287×P%+40×Al%−92×(Mn%+Mo%+Cu%)−46×(Cr%+Ni%)
とする。
【0101】
この温度範囲での熱間圧延を、通常の条件で行っても、最終的な鋼板の形状凍結性は高いが、この温度範囲で行われる熱間圧延の少なくとも1パス以上において、その摩擦係数が0.2以下となるように制御した場合には、さらに、最終的な鋼板の形状凍結性が高くなる。
【0102】
また、仕上熱延に先立って、スケール除去を目的とした加工や高圧水噴射、微粒子噴射等を行うことは、最終鋼板の表面品位を高める効果があり、好ましい。
【0103】
熱間圧延後の冷却は、巻取り温度を制御することが最も重要であるが、平均の冷却速度が15℃/秒以上であることが好ましい。冷却は熱間圧延後、速やかに開始されることが望ましい。また、冷却の途中に空冷を設けることも、最終的な鋼板の特性を劣化させない。
【0104】
このようにして形成されたオーステナイトの集合組織を、最終的な熱延鋼板に受け継がせるためには、(5)式に示す臨界温度To(℃)以下で巻き取る必要がある。したがって、鋼の成分で決まるToを巻取り温度の上限とした。
【0105】
このTo温度は、オーステナイトとオーステナイトと同一成分のフェライトが同一の自由エネルギーを持つ温度として熱力学的に定義され、C以外の成分の影響も考慮して、(5)式を用いて簡易的に計算することができる。
【0106】
To温度に及ぼすとして本発明に規定された成功以外の成分の影響はそれほど大きくないので、ここでは無視した。
【0107】
冷却が鋼材の化学成分で決まる温度To以上で完了し、そのまま巻取り処理が行われた場合には、上記の熱間圧延条件が満足されていた場合でも、最終的に得られる鋼板で所望の集合組織が十分に発達せず、鋼板の形状凍結性が高くならない。
【0108】
To=−650.4×{C%/(1.82×C%−0.001)}+B (5)
ここで、Bは質量%で表現した鋼の成分より求まる。
【0109】
B=−50.6×Mneq+894.3
Mneq=Mn%+0.24×Ni%+0.13×Si%+0.38×Mo%+0.55×Cr%+0.16×Cu%−0.50×Al%−0.45×Co%+0.90×V%
また、巻取り温度が700℃超になると、コイル全長での巻取り温度の確保が困難になり、材質バラツキの原因になるうえ、Ti、Nb、および/または、Vの炭化物形成元素が含有されている場合には、これらの炭化物が粒界で粗大化し、極限変形能が著しく損なわれる。したがって、700℃を巻取り温度の上限値とした。
【0110】
一方、巻取り温度が400℃未満となると、鋼板中にオーステナイト相やマルテンサイト相が多量に生成され極限変形能が低下するので、400℃を巻取り温度の下限値とした。
【0111】
スキンパス圧延:
以上の方法で製造された本発明鋼に、出荷前にスキンパス圧延を施すことは、鋼板の形状を良好にする。この時、スキンパス圧下率が0.1%未満では、この効果が小さいことから、0.1%をスキンパス圧下率の下限とした。
【0112】
また、圧下率5%超のスキンパス圧延を行うためには、通常のスキンパス圧延機の改造が必要となり、経済的なデメリットが生じるとともに、加工性が著しく劣化するので、5%をスキンパス圧下率の上限とした。
【0113】
めっき:
めっきの種類や方法は特に限定されるものではなく、電気めっき、溶融めっき、蒸着めっき等のいずれでも、本発明の効果を得ることができる。
【0114】
本発明の鋼板は、曲げ加工だけではなく、曲げ、張り出し、絞り等、曲げ加工を主体とする複合成形にも適用できる。
【0115】
【実施例】
(実施例)
表1に示すA〜Kの鋼材を1100℃から1270℃に加熱し、表2中に示した熱延条件で熱延し、2.5mm厚の熱延鋼板とした。この熱延鋼板に対する各種評価の結果を、表3および表4に示す。
【0116】
【表1】
【0117】
【表2】
【0118】
【表3】
【0119】
【表4】
【0120】
形状凍結性の評価は、270mm長さ×50mm幅×板厚の短冊状のサンプルを用い、パンチ幅78mm、パンチ肩R5mm、ダイ肩R5mmにて、種々のしわ押さえ圧でハット型に成形した後、壁部の反り量を曲率半径ρ(mm)として測定し、その逆数1000/ρによって行った。1000/ρが小さいほど、形状凍結性は良好である。
【0121】
一般に、鋼板の強度が上昇すると形状凍結性が劣化することが知られている。本発明者らが実際の部品成形を行った結果から、上記方法によって測定したしわ押さえ圧70kNでの1000/ρが、0(mm−1)以上で、かつ、鋼板の引張り強度TS[MPa]に対して(0.012×TS−4.5)(mm−1)以下となる場合には、際だって形状凍結性が良好となる。
【0122】
それ故、0≦1000/ρ≦(0.012×TS−4.5)を、良好な形状凍結性の条件として評価した。
【0123】
ここで、しわ押さえ圧を増加すると、1000/ρは減少する傾向にある。しかしながら、どのようなしわ押さえ圧を選択しても、鋼板の形状凍結性の優位性の順位は変化しない。したがって、しわ押さえ圧70kNでの評価は、鋼板の形状凍結性をよく代表している。
【0124】
穴拡げ性は、1辺100mmの試験片の中央に径10mmの打ち抜き穴を加工し、その初期穴を頂角60°の円錐ポンチにて押し広げ、割れが鋼板を貫通した時点での穴径d(mm)の初期穴径10mmに対する穴広げ率λ(次式)で評価した。
【0125】
λ={(d−10)/10}×100(%)
穴広げ率も、一般的に、鋼板の強度が上昇すると劣化する。
【0126】
そこで、(穴広げ率λ[%])/(鋼板の引張り強度TS[MPa])を穴広げ性の指標とし、その値が0.15以上のものを穴広げ性良好として評価した。
【0127】
r値、延性の異方性、AIは、JIS5号引張り試験片を用いて測定した。また、X線の測定は、鋼板の代表値として板厚の7/16厚の位置で板面に平行なサンプルを調製して実施した。
【0128】
表2において、No.5〜11、および、No.13,No15は、いずれも、熱延条件が本発明の範囲から外れていることから、延性の異方性が大きく、一部では形状凍結性も十分でなく、伸びフランジ性も不十分であり、結果として、形状凍結性と低異方性および極限変形能を兼備した高強度鋼板になっていない。
【0129】
No.21は、成分、熱延条件ともが本発明範囲にないことから、形状凍結性、極限変形能ともに満足できていない。
【0130】
本発明範囲内の化学成分の鋼を本発明範囲内の熱延条件によって製造した場合には、良好な延性異方性、極限変形能とともに、良好な形状凍結性が得られていることがわかる。
【0131】
【発明の効果】
本発明によって、スプリング・バック量が少なく、形状凍結性に優れると同時に異方性が少ない良好なプレス成形性を有する薄鋼板が提供できるようになり、従来は、形状不良の問題から高強度鋼板の適用が難しかった部品にも、高強度鋼板が使用できるようになると同時に、効率的に自動車の安全性と車体の軽量化を両立することが可能となり、CO2排出削減等の環境・社会からの要請に応える自動車製造に大きく貢献することができる。
【0132】
したがって、本発明は、工業的に極めて高い価値のある発明である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength hot-rolled steel sheet excellent in ultimate deformability and shape freezing property that can be used in automobile members and the like and can efficiently reduce the weight of automobile members, and a method for producing the same.
[0002]
[Prior art]
In order to reduce carbon dioxide emissions from automobiles, the weight of automobile bodies is being reduced using high-strength steel sheets. In addition, in order to ensure the safety of passengers, high strength steel plates are often used in automobile bodies in addition to mild steel plates. Furthermore, in order to further reduce the weight of automobile bodies in the future, new demands to increase the use strength level of high-strength steel sheets are increasing much more than ever.
[0003]
However, when bending deformation is applied to a high-strength steel plate, the shape after processing is easily separated from the shape of the processing jig and returned to the shape before processing because of its high strength. Even if processing is applied, the phenomenon in which the shape after processing tries to return to the original shape direction is called spring back. When this spring back occurs, the shape of the target processed part cannot be obtained.
[0004]
Therefore, in a conventional automobile body, it has been mainly used only for high-strength steel sheets of 440 MPa or less. For automobile bodies, there is no high-strength steel sheet with less spring back and good shape freezing, despite the need to reduce the weight of the body using high-strength steel sheets of 490 MPa or higher. This is the actual situation.
[0005]
On the other hand, when a high-strength steel sheet is processed into an automotive part or the like, various characteristics are required in addition to the shape freezing property. In particular, the ultimate deformability required for stretch flange processing and burring processing is an important characteristic, and the compatibility of the characteristics and shape freezing properties makes it possible to further broaden the application range of high-strength steel sheets to automobile bodies. It will be something.
[0006]
The present inventors have so far disclosed a steel sheet excellent in shape freezing property and a manufacturing method thereof in which crystal orientation and r value are defined in Patent Document 1 and the like.
[0007]
As a result of further intensive studies, we have newly found that further texture control and ductility anisotropy control are extremely important in order to achieve both shape freezing and workability. And the optimum production conditions for satisfying these were newly found.
[0008]
[Patent Document 1]
WO00 / 066791 International Application Publication
[0009]
[Problems to be solved by the invention]
Increasing the strength of steel plates applied to automotive parts subjected to bending increases the amount of spring back as the strength of the steel plates increases, resulting in shape defects and limiting the application of high strength steel plates. Currently.
[0010]
In addition, good press formability and high impact energy absorption ability are indispensable characteristics for applying high-strength steel sheets to automobile parts and the like.
[0011]
The present invention drastically solves this problem and provides a high-strength hot-rolled steel sheet having a good shape freezing property and a good ultimate deformability, and a method for producing the same.
[0012]
[Means for Solving the Problems]
According to the conventional knowledge, as a measure for suppressing the spring back, it was considered to be important to lower the yield point of the steel plate for the time being. In order to lower the yield point, a steel plate having a low tensile strength has to be used.
[0013]
However, this alone is not the fundamental solution for improving the bending workability of the steel sheet and keeping the amount of spring back low.
[0014]
Therefore, in order to improve the bending workability and fundamentally solve the occurrence of springback, the present inventors have newly focused on the influence on the bending workability of the texture of the steel sheet, and its effects Were investigated and studied in detail.
[0015]
As a result, a steel sheet excellent in bending workability was found. That is, the inventors of the present invention have {100} <011> to {223} <110> orientation groups, and in particular, {100} <011> orientations, and {554} <225>, {111} < 112>, {111} <110> to control the X-ray random intensity ratio in each direction, and further, at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is set to the lowest possible value. It has been clarified that bending workability is remarkably improved by setting the anisotropy of local elongation to 2% or more.
[0016]
However, when the anisotropy of local elongation increases, the stretch flangeability is expected to deteriorate, making it difficult to achieve both shape freezing and workability. As a result of intensive studies, the present inventors have clarified that the shape freezing property and the ultimate deformability can be enhanced simultaneously by simultaneously establishing the texture control and carbide control.
[0017]
In addition, not restricting the blank sampling direction for forming various parts greatly contributes to the improvement of the yield of steel materials. For this purpose, the anisotropy of ductility, especially the anisotropy of uniform elongation, is reduced. It has an important meaning.
[0018]
The present inventors have experimentally developed the {100} <011> orientation as the main orientation by controlling the start and end temperatures of finish hot rolling of the steel sheet, whereby the shape freezeability and the workability are improved. It has been found that the anisotropy of uniform elongation can be reduced while securing the properties.
[0019]
The present invention is configured based on the above-described knowledge, and the main points thereof are as follows.
[0020]
(1)% By mass
C: 0.01% or more, 0.2% or less,
Si: 0.001% or more, 2.5% or less,
Mn: 0.01% or more, 2.5% or less,
P: 0.2% or less,
S: 0.03% or less,
Al: 0.01% or more, 2.0% or less,
N: 0.01% or less,
O: 0.01% or less
Containing, the balance consisting of Fe and inevitable impurities,The microstructure of ferrite or bainite is the phase with the largest volume fraction, and the plate surface at least 1/2 plate thickness,
(1) X-ray random intensity ratio of {100} <011> to {223} <110> orientation group
Average value is 2.5 or more,
(2) {554} <225>, {111} <112> and {111} <110>
The average value of the X-ray random intensity ratio of one crystal orientation is 3.5 or less,
(3) {100} <011> X-ray random intensity ratio is {211} <011> X-ray random
More than intensity ratio, and
(4) {100} <011> X-ray random intensity ratio is 2.5 or more
And at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.7 or less, and the anisotropy ΔuEl of uniform elongation is 4% or less, A high-strength hot-rolled steel sheet excellent in ultimate deformability and shape freezing property, characterized in that an elongation anisotropy ΔLE1 is 2% or more and ΔuEl is ΔLE1 or less.
[0021]
Uniform elongation in the direction parallel to the rolling direction (L direction), vertical (C direction), and 45 ° is uEl (L), uEl (C), and uEl (45 °), respectively. The local elongation in the direction parallel to the direction (L direction), vertical (C direction), and 45 ° direction is defined as LEl (L), LEl (C), and LEl (45 °), respectively.
[0022]
(2) Further, the space factor of iron carbide having a diameter of 0.2 μm or more is 0.3% or less, and the high strength hot-rolled steel sheet having excellent ultimate deformability and shape freezing property according to (1) .
[0023]
(3) The high-strength hot-rolled steel sheet excellent in ultimate deformability and shape freezing property according to (1) or (2), wherein the aging index AI is 8 MPa or more.
[0025]
(4(1) to (%), further comprising 0.001% or more and 0.8% or less of Nb, Ti, and V in total by mass.3A high-strength hot-rolled steel sheet excellent in ultimate deformability and shape freezing property.
[0026]
(5Furthermore, (1)-(characteristically) containing 0.01% or less of B by mass%4A high-strength hot-rolled steel sheet excellent in ultimate deformability and shape freezing property.
[0027]
(6In addition, in mass%,
Mo: 1% or less,
Cr: 1% or less,
Cu: 2% or less,
Ni: 1% or less,
Sn: 0.2% or less,
Co: 2% or less
(1) to (1) characterized by containing one or more of5A high-strength hot-rolled steel sheet excellent in ultimate deformability and shape freezing property.
[0028]
(7)% By mass
Ca: 0.0005 to 0.005%,
Rem: 0.001 to 0.05%,
Mg: 0.0001 to 0.05%,
Ta: 0.0001 to 0.05%
1 type or 2 types or more of (1)-(6A high-strength hot-rolled steel sheet excellent in ultimate deformability and shape freezing property.
[0029]
(8(1) to (7A high-strength hot-rolled steel sheet excellent in ultimate deformability and shape-freezing property, wherein the high-strength hot-rolled steel plate excellent in ultimate deformability and shape-freezing property is plated.
[0030]
(9(1) to (8In producing a high-strength hot-rolled steel sheet excellent in the ultimate deformability and shape freezing property described in any one of(1),(4) to (7) When the cast slab having the component composition described in any of the above is cast as it is or once cooled to 1000 to 1300 ° C. and hot-rolled, the temperature ranges from Ar 3 ° C. to (Ar 3 +150) ° C. The total rolling reduction is controlled so as to be 25% or more, and heat is applied so that finishing hot rolling start temperature TFS and finishing hot rolling completion temperature TFE (° C.) satisfy all the following expressions (1) to (4) at the same time. The hot rolling is finished, the steel is cooled after hot rolling, and wound at a temperature not more than the critical temperature To (° C) determined by the chemical composition of the steel shown in the formula (5) and not less than 700 ° C and not less than 400 ° C. A method for producing a high-strength hot-rolled steel sheet excellent in ultimate deformability and shape freezing property.
[0031]
TFE ≧ Ar3 (℃) (1)
TFE ≧ 800 ° C (2)
TFS ≦ 1100 ° C (3)
20 ° C. ≦ (TFS-TFE) ≦ 120 ° C. (4)
To = −650.4 × {C% / (1.82 × C% −0.001)} + B (5)
Here, B is obtained from the steel component expressed in mass%.
[0032]
B = −50.6 × Mneq + 894.3
Mneq = Mn% + 0.24 × Ni% + 0.13 × Si% + 0.38 × Mo%
+ 0.55 × Cr% + 0.16 × Cu% −0.50 × Al% −0.45 × Co%
+ 0.90 × V%
However,
Ar3 = 901-325 × C% + 33 × Si% + 287 × P% + 40 × Al%
−92 × (Mn% + Mo% + Cu%) − 46 × (Cr% + Ni%)
(10Further, the friction coefficient is controlled to be 0.2 or less in at least one pass of hot rolling in a temperature range of Ar3 to (Ar3 + 150) ° C. (9) A method for producing a high-strength hot-rolled steel sheet having excellent ultimate deformability and shape freezing properties.
[0033]
(11()9) Or (10The high-strength hot-rolled steel sheet produced by the method for producing a high-strength hot-rolled steel sheet having excellent ultimate deformability and shape freezing described in (1) is subjected to skin pass rolling of 0.1% to 5%. A method for producing high-strength hot-rolled steel sheets with excellent ultimate deformability and shape freezing properties.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
The contents of the present invention will be described in detail below.
[0035]
Average value of X-ray random intensity ratio of {100} <011> to {223} <110> orientation group on the plate surface at 1/2 plate thickness:
When the X-ray diffraction of the plate surface at the plate thickness center position was performed and the intensity ratio of each orientation with respect to the random sample was determined, the average value of {100} <011> to {223} <110> orientation group was 2.5. It must be above. If this is less than 2.5, the shape freezing property is poor.
[0036]
The main orientations included in this orientation group are {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335} < 110> and {223} <110>.
[0037]
The X-ray random intensity ratio in each of these directions is obtained from the three-dimensional texture calculated by the vector method based on the {110} pole figure, and the pole figures of {110}, {100}, {211}, and {310}. Of these, a three-dimensional texture calculated by a series expansion method using a plurality of pole figures (preferably three or more) may be used.
[0038]
For example, the X-ray random intensity ratio of each crystal orientation in the latter method includes (001) [1-10], (116) [1-10], (114 in the φ2 = 45 ° cross section of the three-dimensional texture. ) [1-10], (113) [1-10], (112) [1-10], (335) [1-10], (223) [1-10] may be used as they are.
[0039]
The average value of {100} <011> to {223} <110> orientation group is an arithmetic average of each of the above-mentioned orientations. If it is not possible to obtain the strengths of all the above directions, {100} <011>, {116} <110>, {114} <110>, {112} <110>, and {223} < An arithmetic average of 110> orientations may be substituted.
[0040]
Furthermore, desirably, the average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> orientation groups is 4.0 or more.
[0041]
Average value of X-ray random intensity ratio of three crystal orientations of {554} <225>, {111} <112> and {111} <110> on the plate surface at 1/2 plate thickness:
The average value of the X-ray random intensity ratio of the three crystal orientations of {554} <225>, {111} <112> and {111} <110> on the plate surface at 1/2 plate thickness is 3.5 or less. Must-have. If this exceeds 3.5, it will be difficult to obtain good shape freezing properties even if the strength of the {100} <011> to {223} <110> orientation groups is appropriate.
[0042]
The X-ray random intensity ratio of {554} <225>, {111} <112>, and {111} <110> may be obtained from the three-dimensional texture calculated according to the above method.
[0043]
Furthermore, desirably, the arithmetic average value of the X-ray random intensity ratios of {554} <225>, {111} <112> and {111} <110> is less than 2.5.
[0044]
{100} <011> and {211} <011> X-ray random intensity ratio of the plate surface at 1/2 plate thickness:
The {100} <011> X-ray random intensity ratio of the plate surface at 1/2 plate thickness must be greater than or equal to the {112} <011> X-ray random intensity ratio. When the X-ray random intensity ratio in the {211} <011> orientation is larger than the {100} <011> X-ray random intensity ratio, the anisotropy of uniform elongation increases and the workability deteriorates.
[0045]
Further, the {100} <011> X-ray random intensity ratio must be 2.5 or more. If this is less than 2.5, good shape freezing property cannot be obtained.
[0046]
In addition, {100} <011> and {211} <011> described here are respectively ± ranges having a direction perpendicular to the rolling direction (Transverse direction) as a rotation axis as a range of orientations having the same effect. Allow 12 °. Further, it is desirably ± 6 °.
[0047]
The reason why the X-ray intensity of the crystal orientation described above is important for the shape freezing property and elongation anisotropy at the time of bending is not necessarily clear, but the sliding behavior of the crystal at the time of bending deformation Presumed to be related.
[0048]
The sample to be subjected to X-ray diffraction is thinned to a predetermined plate thickness by mechanical polishing or the like, and then the distortion is removed by chemical polishing or electrolytic polishing, and at the same time, the 1/2 plate thickness becomes the measurement surface. To make.
[0049]
When there is a segregation zone or a defect in the thickness center layer of the steel sheet, which causes inconvenience in measurement, the above-described surface is set so that an appropriate surface becomes the measurement surface in the range of 3/8 to 5/8 of the plate thickness. The sample may be adjusted according to the above method.
[0050]
As a matter of course, the above-mentioned limitation of the X-ray intensity is satisfied not only in the vicinity of the plate thickness ½ but also as much as possible (especially, the outermost layer to ¼ of the plate thickness), thereby further increasing the thickness. The shape freezing property is improved.
[0051]
The crystal orientation represented by {hkl} <uvw> indicates that the normal direction of the plate surface is parallel to <hkl> and the rolling direction is parallel to <uvw>.
[0052]
R value (rL) in the rolling direction and r value (rC) in the direction perpendicular to the rolling direction:
This is an important requirement in the present invention. That is, as a result of intensive studies by the present inventors, it has been found that even if the X-ray intensities of the various crystal orientations described above are appropriate, good shape freezing properties cannot always be obtained.
[0053]
At the same time as the above X-ray intensity, it is essential that at least one of rL and rC is 0.7 or less. More preferably, it is 0.55 or less.
[0054]
The lower limit of rL and rC is not particularly defined, and the effect of the present invention can be obtained. The r value is evaluated by a tensile test using a JIS No. 5 tensile test piece. The tensile strain is usually 15%. However, when the uniform elongation is less than 15%, the strain may be evaluated as close to 15% as possible within the range of uniform elongation.
[0055]
The direction in which the bending process is performed differs depending on the processed part, and is not particularly limited. However, it is preferable that the bending process is mainly performed in a direction perpendicular to or near to the direction where the r value is small.
[0056]
Incidentally, it is generally known that there is a correlation between the texture and the r value. However, in the present invention, the above-described limitation on the X-ray intensity ratio of the crystal orientation and the limitation on the r value are synonymous with each other. No good shape freezing property can be obtained unless both limitations are satisfied at the same time.
[0057]
Ductile anisotropy:
In the case of press forming a steel plate, the uniform elongation of the steel plate, that is, the n value is important. Especially in the case of high-strength steel sheets that are mainly stretch-formed, if this uniform elongation (n value) has anisotropy, it is necessary to carefully select the blank cutting direction depending on the part. This leads to a decrease in the steel plate yield. In some cases, it may not be possible to mold into a desired shape.
[0058]
In a steel having a tensile strength of about 400 MPa or more (the maximum strength obtained by a tensile test), if this uniform elongation anisotropy ΔuE1 is 4% or less, good formability independent of the direction may be exhibited. found.
[0059]
When particularly severe workability is required, the anisotropy ΔuE1 is desirably 3% or less. The lower limit of the uniform elongation anisotropy ΔuEl is not particularly limited, but is most preferably 0% from the viewpoint of workability.
[0060]
Further, when the anisotropy ΔLE1 of local elongation is less than 2%, the shape freezeability deteriorates, so the lower limit of ΔLEl is set to 2%. The upper limit of ΔLEl is not particularly set, but if ΔLEl becomes too large, the moldability deteriorates, so it is desirable to make it 12% or less.
[0061]
However, even if the above conditions were satisfied, if ΔuE1> ΔLE1, the good moldability and the shape freezing property were not compatible, so ΔuE1 was set to ΔLE1 or less.
[0062]
The anisotropy of uniform elongation and local elongation is parallel to the rolling direction (L direction), perpendicular (C direction), and 45 ° direction elongation (uniform elongation uEl, local elongation LEl),
Is defined.
[0063]
Microstructure:
In an actual automobile part, not only the shape freezing property caused by bending as described above becomes a problem in one part, but in other parts of the same part, stretch flange processing, burring processing, etc. There are many cases of receiving processing.
[0064]
Therefore, it is necessary to improve the ultimate deformability of the steel sheet as well as improving the shape freezing property during bending by controlling the texture described above.
[0065]
From this viewpoint, the metal structure has a ferrite or bainite phase having a high hole expansibility as a phase with the maximum volume fraction. However, from the viewpoint of the texture, it is preferable to use bainite as the main phase because the bainite phase transformed and generated at a low temperature has a stronger texture development.
[0066]
Note that the bainite described here may or may not contain iron carbide particles in the microstructure. In addition, ferrite (processed ferrite) that has undergone processing after transformation and has a very high internal dislocation density is significantly different in ductility and is not suitable for parts processing. Therefore, it is distinguished from ferrite specified in the present invention. .
[0067]
Furthermore, it is preferable to limit the space factor of iron carbide having a diameter of 0.2 μm or more that significantly deteriorates stretch flangeability to 0.3% or less. The space factor of iron carbide may be replaced by obtaining the area ratio of iron carbide by image processing in an optical microscope observation photograph having a magnification of 500 times or more. Moreover, the number m of lattice points occupied by 0.2 μm or more of iron carbides among n lattice points drawn on the photograph may be obtained, and m / n may be used as the space factor.
[0068]
Aging Index (AI):
AI, which is an index indicating the aging property of the steel sheet, is preferably 8 MPa or more. When AI is less than 8 MPa, the shape freezing property decreases, so 8 MPa is set as the lower limit. The reason why the shape freezing property deteriorates when AI decreases is not clear, but since AI correlates with the movable dislocation density in the steel material, it is considered that the difference in the movable dislocation density has some influence on the deformation.
[0069]
The upper limit of AI is not particularly defined, but if AI exceeds 100 MPa, stretcher strain occurs and the appearance of the steel sheet may be significantly impaired. Therefore, AI is preferably set to 100 MPa or less.
[0070]
The aging index was measured using a JIS No. 5 tensile specimen in the L or C direction, and the deformation stress when 10% pre-strain was applied and then unloaded once and subjected to aging at 100 ° C for one hour. After that, the difference from the yield stress when the tensile test is performed again (the yield stress when yield elongation occurs) is defined as the aging index AI.
[0071]
Below, the preferable chemical component of this invention is described (a unit is the mass%).
[0072]
C:
The lower limit of C is set to 0.01% because if C is less than 0.01%, it is difficult to ensure the strength of the steel sheet while maintaining high workability. On the other hand, if it exceeds 0.2%, an austenite phase, a martensite phase, and coarse carbides that lower the ultimate deformability are easily formed, and weldability is also lowered. Therefore, the upper limit is made 0.2%.
[0073]
Si:
Although it is an element effective for increasing the mechanical strength of the steel sheet, if it exceeds 2.5%, workability deteriorates or surface flaws occur, so 2.5% is made the upper limit. On the other hand, in practical steel, it is difficult to make Si less than 0.001%, so 0.001% is made the lower limit.
[0074]
Mn:
Although it is an element effective for increasing the mechanical strength of the steel sheet, if it exceeds 2.5%, the workability deteriorates, so 2.5% is made the upper limit. On the other hand, in practical steel, it is difficult to make Mn less than 0.01%, so 0.01% is made the lower limit.
[0075]
In addition to Mn, when an element such as Ti that suppresses the occurrence of hot cracking due to S is not sufficiently added, it is desirable to add Mn in an amount such that Mn / S ≧ 20 by mass%.
[0076]
P, S:
Respectively, the content is 0.2% or less and 0.03% or less. This is to prevent workability deterioration and cracking during hot rolling or cold rolling.
[0077]
Al:
Add 0.01% or more for deoxidation. However, if the amount is too large, the workability deteriorates or the surface properties become poor, so the upper limit is made 2.0%.
[0078]
N, O:
It is an impurity, and is 0.01% or less and 0.01% or less, respectively, so as not to deteriorate the workability.
[0079]
Ti, Nb, V:
Since it is an element that improves the material through mechanisms such as precipitation strengthening, structure control, and fine grain strengthening, it is desirable to add one or two or more kinds in total in an amount of 0.001% or more as necessary.
[0080]
However, even if added excessively, there is no remarkable effect, but rather the workability and surface properties are deteriorated, so the upper limit is 0.8% in total of one or more.
[0081]
B:
It is effective for strengthening grain boundaries and increasing the strength of steel. However, if the added amount exceeds 0.01%, not only will the effect be saturated, but the steel sheet strength will be increased more than necessary, and processing into parts will be performed. Therefore, the upper limit was made 0.01%. However, in order to obtain the addition effect of B, it is preferable to add 0.0002% or more.
[0082]
Mo, Cr, Cu, Ni, Sn, Co:
Since there exists an effect which raises mechanical strength or improves a material, it is desirable to add 0.001% or more of each component as needed. However, excessive addition, on the contrary, deteriorates workability, so the upper limit is set to 1%, 1%, 2%, 1%, 0.2%, and 2%, respectively.
[0083]
Ca, rare earth element (Rem):
It is an element effective for inclusion control, and the addition of an appropriate amount improves hot workability, but excessive addition conversely promotes hot embrittlement. Therefore, if necessary, Ca: 0.0005, respectively. -0.005%, Rem: 0.001-0.05%. Here, the rare earth elements refer to Y, Sr and lanthanoid elements, and are industrially a mixture thereof.
[0084]
Further, adding Mg in the range of 0.0001% to 0.05% and adding Ta in the range of 0.0001% to 0.05% also exhibits an equivalent effect.
[0085]
Here, in all cases, the lower limit value indicates the minimum addition amount at which inclusion control effects are manifested. Above the maximum value, conversely, the inclusion grows too much, thereby causing ultimate deformability such as stretch flangeability. Reduce. Addition as a misch metal (mixture) is advantageous in terms of cost.
[0086]
Below, the manufacturing method of this invention is described.
[0087]
Slab reheating temperature:
The steel adjusted to a predetermined component is reheated directly after casting or once cooled to below the Ar3 transformation temperature and then hot-rolled.
[0088]
When the reheating temperature at this time is less than 1000 ° C., it becomes difficult to ensure a predetermined finish hot rolling completion temperature, so 1000 ° C. was set as the lower limit. Further, when the reheating temperature exceeds 1300 ° C., the yield is deteriorated due to scale generation during heating, and at the same time, the manufacturing cost is increased. Therefore, 1300 ° C. is set as the upper limit of the reheating temperature.
[0089]
Even if the steel slab after heating is locally or entirely heated during hot rolling, it does not affect the characteristics of the present invention.
[0090]
Hot rolling conditions:
It is controlled to a predetermined microstructure and texture by hot rolling and subsequent cooling. The texture of the steel sheet finally obtained varies greatly depending on the temperature range of hot rolling. When the hot rolling completion temperature TFE is less than Ar3 ° C., the uniform elongation anisotropy ΔuE1 exceeds 4%, and the formability is significantly deteriorated.
TFE ≧ Ar3 (℃) (1)
It was.
[0091]
The TFE is generally measured behind the stand where the hot rolling final rolling is performed, but if necessary, a temperature obtained by calculation may be used.
[0092]
The upper limit of the hot rolling completion temperature is not particularly limited, but when it exceeds (Ar 3 + 180 ° C.), the surface quality deteriorates due to the oxide layer generated on the surface of the steel sheet, so it may be (Ar 3 + 180 ° C.) or less. desirable.
[0093]
When stricter surface quality is required, it is desirable to set TFE to (Ar 3 + 150 ° C.) or less.
[0094]
However, regardless of the chemical composition of the steel sheet, when TFE is less than 800 ° C., the rolling load at the time of hot rolling becomes too high, and at the same time, the ductility anisotropy of the steel sheet increases,
Ar3 ≧ 800 ° C (2)
It was.
[0095]
In addition, when the finish hot rolling start temperature TFS is more than 1100 ° C., the steel sheet surface quality is significantly lowered.
TFS ≦ 1100 ° C (3)
It was.
[0096]
Further, when the difference between TFS and TFE exceeds 120 ° C., the texture is not sufficiently developed, and good shape freezing property and low anisotropy are not compatible, and this difference is set to less than 20 ° C. Because it is difficult to operate,
20 ° C. ≦ (TFS-TFE) ≦ 120 ° C. (4)
It was.
[0097]
In hot rolling, the rolling reduction in the temperature range of Ar3 ° C. to (Ar 3 +150) ° C. has a great influence on the formation of the texture of the final steel sheet, and the rolling reduction in this temperature range is less than 25%. Since the texture is not sufficiently developed and the finally obtained steel sheet does not show good shape freezing property, the lower limit of the rolling reduction in the temperature range of Ar3 ° C. to (Ar 3 +150) ° C. is set to 25%.
[0098]
The higher the rolling reduction, the more the desired texture develops. Therefore, the rolling reduction is preferably 50% or more, and more preferably 75% or more.
[0099]
Although the upper limit of the reduction ratio is not particularly defined, reducing 99% or more is preferably less than 99% because the load on the apparatus is large and a special effect cannot be obtained.
[0100]
However,
Ar3 = 901-325 × C% + 33 × Si% + 287 × P% + 40 × Al% −92 × (Mn% + Mo% + Cu%) − 46 × (Cr% + Ni%)
And
[0101]
Even if hot rolling in this temperature range is performed under normal conditions, the shape freezing property of the final steel sheet is high, but in at least one pass of hot rolling performed in this temperature range, the friction coefficient is When controlled to be 0.2 or less, the shape freezing property of the final steel plate is further increased.
[0102]
Further, prior to finish hot rolling, it is preferable to perform processing for removing the scale, high-pressure water injection, fine particle injection, and the like because of the effect of improving the surface quality of the final steel plate.
[0103]
In the cooling after hot rolling, it is most important to control the coiling temperature, but the average cooling rate is preferably 15 ° C./second or more. It is desirable that cooling be started immediately after hot rolling. Further, providing air cooling in the middle of cooling does not deteriorate the properties of the final steel plate.
[0104]
In order to transfer the austenite texture thus formed to the final hot-rolled steel sheet, it is necessary to wind it at a temperature equal to or lower than the critical temperature To (° C.) shown in the equation (5). Therefore, To determined by the steel component is set as the upper limit of the coiling temperature.
[0105]
This To temperature is thermodynamically defined as the temperature at which austenite and ferrite of the same component as austenite have the same free energy, and considering the influence of components other than C, it can be simplified using equation (5). Can be calculated.
[0106]
The influence of components other than the success defined in the present invention on the To temperature is not so great and was ignored here.
[0107]
When the cooling is completed at a temperature To or higher determined by the chemical composition of the steel material and the winding process is performed as it is, even if the above hot rolling conditions are satisfied, the desired steel sheet is finally obtained. The texture does not develop sufficiently and the shape freezing property of the steel sheet does not increase.
[0108]
To = −650.4 × {C% / (1.82 × C% −0.001)} + B (5)
Here, B is obtained from the steel component expressed in mass%.
[0109]
B = −50.6 × Mneq + 894.3
Mneq = Mn% + 0.24 × Ni% + 0.13 × Si% + 0.38 × Mo% + 0.55 × Cr% + 0.16 × Cu% −0.50 × Al% −0.45 × Co% + 0. 90 x V%
Further, when the coiling temperature exceeds 700 ° C., it becomes difficult to ensure the coiling temperature over the entire length of the coil, which causes variations in material and contains Ti, Nb, and / or V carbide forming elements. In such a case, these carbides are coarsened at the grain boundaries, and the ultimate deformability is significantly impaired. Therefore, 700 ° C. was set as the upper limit value of the coiling temperature.
[0110]
On the other hand, when the coiling temperature is less than 400 ° C., a large amount of austenite phase and martensite phase are generated in the steel sheet and the ultimate deformability is lowered. Therefore, 400 ° C. is set as the lower limit value of the coiling temperature.
[0111]
Skin pass rolling:
Applying skin pass rolling to the steel of the present invention manufactured by the above method before shipment makes the shape of the steel plate good. At this time, if the skin pass reduction ratio is less than 0.1%, this effect is small, so 0.1% was set as the lower limit of the skin pass reduction ratio.
[0112]
In addition, in order to perform skin pass rolling with a rolling reduction exceeding 5%, it is necessary to modify a normal skin pass rolling mill, resulting in economic demerits and significant deterioration in workability. The upper limit.
[0113]
Plating:
The type and method of plating are not particularly limited, and the effects of the present invention can be obtained by any of electroplating, hot dipping, vapor deposition plating and the like.
[0114]
The steel sheet of the present invention can be applied not only to bending work but also to composite forming mainly composed of bending work such as bending, overhanging and drawing.
[0115]
【Example】
(Example)
The steel materials A to K shown in Table 1 were heated from 1100 ° C. to 1270 ° C. and hot-rolled under the hot rolling conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 2.5 mm. Tables 3 and 4 show the results of various evaluations on this hot-rolled steel sheet.
[0116]
[Table 1]
[0117]
[Table 2]
[0118]
[Table 3]
[0119]
[Table 4]
[0120]
The shape freezing property was evaluated by using a strip-shaped sample of 270 mm long × 50 mm wide × plate thickness, and after forming into a hat shape with various wrinkle holding pressures with a punch width of 78 mm, a punch shoulder R5 mm, and a die shoulder R5 mm. The amount of warpage of the wall portion was measured as the radius of curvature ρ (mm), and the reciprocal thereof was 1000 / ρ. The smaller the 1000 / ρ, the better the shape freezing property.
[0121]
In general, it is known that the shape freezeability deteriorates when the strength of a steel plate increases. From the result of actual part molding by the present inventors, 1000 / ρ at a wrinkle pressure 70 kN measured by the above method is 0 (mm-1) Above, and (0.012 × TS-4.5) (mm for the tensile strength TS [MPa] of the steel sheet-1) The shape freezing property is remarkably good in the following cases.
[0122]
Therefore, 0 ≦ 1000 / ρ ≦ (0.012 × TS−4.5) was evaluated as a condition of good shape freezing property.
[0123]
Here, when the wrinkle pressure is increased, 1000 / ρ tends to decrease. However, no matter what wrinkle holding pressure is selected, the order of superiority of the shape freezing property of the steel sheet does not change. Therefore, the evaluation at the wrinkle holding pressure of 70 kN well represents the shape freezing property of the steel sheet.
[0124]
The hole expandability is the diameter of the hole when the punched hole with a diameter of 10 mm is machined in the center of a 100 mm side test piece, the initial hole is expanded with a conical punch with a vertex angle of 60 °, and the crack penetrates the steel plate. Evaluation was made by a hole expansion ratio λ (the following formula) with respect to an initial hole diameter of 10 mm of d (mm).
[0125]
λ = {(d−10) / 10} × 100 (%)
The hole expansion rate generally deteriorates as the strength of the steel plate increases.
[0126]
Therefore, (hole expanding rate λ [%]) / (tensile strength TS [MPa] of steel sheet) was used as an index of hole expanding property, and a value of 0.15 or more was evaluated as good hole expanding property.
[0127]
The r value, ductility anisotropy, and AI were measured using a JIS No. 5 tensile test piece. X-ray measurement was performed by preparing a sample parallel to the plate surface at a position of 7/16 of the plate thickness as a representative value of the steel plate.
[0128]
In Table 2, no. 5-11, and No. 13 and No. 15 both have high ductility anisotropy because the hot-rolling conditions are out of the scope of the present invention, and part of the shape is not sufficiently frozen and stretch flangeability is insufficient. As a result, it is not a high-strength steel sheet having both shape freezing property, low anisotropy and ultimate deformability.
[0129]
No. No component 21 and hot rolling conditions are not within the scope of the present invention, so that shape freezing property and ultimate deformability are not satisfied.
[0130]
When steel of chemical composition within the scope of the present invention is produced by hot rolling conditions within the scope of the present invention, it can be seen that good shape freezeability is obtained along with good ductility anisotropy and ultimate deformability. .
[0131]
【The invention's effect】
According to the present invention, it is possible to provide a thin steel plate having a good press formability with a small amount of spring back and excellent shape freezing property and at the same time low anisotropy. High-strength steel sheets can be used for parts that were difficult to apply, and at the same time, it is possible to achieve both the safety of automobiles and the weight reduction of vehicle bodies efficiently.2It can greatly contribute to automobile manufacturing that responds to environmental and social demands such as emission reduction.
[0132]
Therefore, the present invention is industrially extremely valuable.
Claims (11)
C:0.01%以上、0.2%以下、
Si:0.001%以上、2.5%以下、
Mn:0.01%以上、2.5%以下、
P:0.2%以下、
S:0.03%以下、
Al:0.01%以上、2%以下、
N:0.01%以下、
O:0.01%以下
含み、残部がFeおよび不可避的不純物からなり、ミクロ組織がフェライトもしくはベイナイトを体積分率最大の相とし、少なくとも1/2板厚における板面の、
(1) {100}<011>〜{223}<110>方位群のX線ランダム強度比の平 均値が2.5以上、
(2) {554}<225>、{111}<112>および{111}<110>の3 つの結晶方位のX線ランダム強度比の平均値が3.5以下、
(3) {100}<011>X線ランダム強度比が{211}<011>X線ランダム 強度比以上、および、
(4) {100}<011>X線ランダム強度比が2.5以上
の全てを満足し、かつ、圧延方向のr値および圧延方向と直角方向のr値のうち少なくとも1つが0.7以下であり、さらに、均一伸びの異方性△uElが4%以下、局部伸びの異方性△LElが2%以上で、かつ、△uElが△LEl以下であることを特徴とする極限変形能と形状凍結性に優れた高強度熱延鋼板。
ただし、△uEl={|uEl(L)−uEl(45°)|+|uEl(C)
−uEl(45°)|}/2
△LEl={|LEl(L)−LEl(45°)|+|LEl(C)
−LEl(45°)|}/2
であり、圧延方向と平行(L方向)、垂直(C方向)、および、45°方向の均一伸びを、それぞれ、uEl(L)、uEl(C)、および、uEl(45°)とし、圧延方向と平行(L方向)、垂直(C方向)、および、45°方向の局部伸びを、それぞれ、LEl(L)、LEl(C)、および、LEl(45°)とする。 % By mass
C: 0.01% or more, 0.2% or less,
Si: 0.001% or more, 2.5% or less,
Mn: 0.01% or more, 2.5% or less,
P: 0.2% or less,
S: 0.03% or less,
Al: 0.01% or more, 2% or less,
N: 0.01% or less,
O: 0.01% or less
Including the balance of Fe and inevitable impurities, the microstructure having ferrite or bainite as the volume fraction maximum phase, and at least 1/2 of the plate thickness,
(1) The average value of the X-ray random intensity ratio of {100} <011> to {223} <110> orientation group is 2.5 or more
(2) The average value of X-ray random intensity ratios of three crystal orientations of {554} <225>, {111} <112> and {111} <110> is 3.5 or less,
(3) {100} <011> X-ray random intensity ratio is {211} <011> X-ray random intensity ratio or more, and
(4) {100} <011> The X-ray random intensity ratio satisfies all of 2.5 or more, and at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.7 or less. Further, the ultimate deformability characterized in that the uniform elongation anisotropy ΔuE1 is 4% or less, the local elongation anisotropy ΔLE1 is 2% or more, and the ΔuE1 is ΔLE1 or less. High strength hot-rolled steel sheet with excellent shape freezing properties.
However, ΔuEl = {| uEl (L) −uEl (45 °) | + | uEl (C)
-UEl (45 °) |} / 2
ΔLEl = {| LEl (L) −LEl (45 °) | + | LEl (C)
-LEl (45 °) |} / 2
The uniform elongation in the direction parallel to the rolling direction (L direction), vertical (C direction), and 45 ° is uEl (L), uEl (C), and uEl (45 °), respectively. The local elongation in the direction parallel to the direction (L direction), vertical (C direction), and 45 ° direction is defined as LEl (L), LEl (C), and LEl (45 °), respectively.
Mo:1%以下、
Cr:1%以下、
Cu:2%以下、
Ni:1%以下、
Sn:0.2%以下、
Co:2%以下
の1種または2種以上を含有することを特徴とする請求項1〜5のいずれか1項に記載の極限変形能と形状凍結性に優れた高強度熱延鋼板。Furthermore, in mass%,
Mo: 1% or less,
Cr: 1% or less,
Cu: 2% or less,
Ni: 1% or less,
Sn: 0.2% or less,
The high-strength hot-rolled steel sheet having excellent ultimate deformability and shape freezing properties according to any one of claims 1 to 5 , wherein Co: 2% or less is contained.
Ca:0.0005〜0.005%、
Rem:0.001〜0.05%、
Mg:0.0001〜0.05%、
Ta:0.0001〜0.05%、
の1種または2種以上を含むことを特徴とする請求項1〜6のいずれか1項に記載の極限変形能と形状凍結性に優れた高強度熱延鋼板。Furthermore, in mass%,
Ca: 0.0005 to 0.005%,
Rem: 0.001 to 0.05%,
Mg: 0.0001 to 0.05%,
Ta: 0.0001 to 0.05%,
The high-strength hot-rolled steel sheet excellent in ultimate deformability and shape freezing property according to any one of claims 1 to 6 , comprising one or more of the following.
TFE≧Ar3(℃) (1)
TFE≧800℃ (2)
TFS≦1100℃ (3)
20℃≦(TFS−TFE)≦120℃ (4)
To=−650.4×{C%/(1.82×C%−0.001)}+B (5)
ここで、Bは質量%で表現した鋼の成分より求まる。
B=−50.6×Mneq+894.3
Mneq=Mn%+0.24×Ni%+0.13×Si%+0.38×Mo%
+0.55×Cr%+0.16×Cu%−0.50×Al%−0.45×Co%
+0.90×V%
ただし、
Ar3=901−325×C%+33×Si%+287×P%+40×Al%
−92×(Mn%+Mo%+Cu%)−46×(Cr%+Ni%)In producing the high-strength hot-rolled steel sheet having excellent ultimate deformability and shape freezing property according to any one of claims 1 to 8, the component composition according to any one of claims 1 and 4 to 7. When the cast slab having slab is cast as it is or once cooled to 1000 to 1300 ° C. and hot-rolled, the total rolling reduction in the temperature range of Ar 3 ° C. to (Ar 3 +150) ° C. is 25% or more. The hot rolling is finished so that the finishing hot rolling start temperature TFS and the finishing hot rolling completion temperature TFE satisfy all the following formulas (1) to (4) at the same time, and cooling is performed after hot rolling. (5) excellent in ultimate deformability and shape freezing property, characterized by winding at a temperature not more than the critical temperature To (° C.) determined by the chemical composition of the steel shown in the formula (5) and not higher than 700 ° C. and not lower than 400 ° C. Manufacturing method of high-strength hot-rolled steel sheet.
TFE ≧ Ar3 (℃) (1)
TFE ≧ 800 ° C (2)
TFS ≤ 1100 ° C (3)
20 ° C. ≦ (TFS-TFE) ≦ 120 ° C. (4)
To = −650.4 × {C% / (1.82 × C% −0.001)} + B (5)
Here, B is obtained from the steel component expressed in mass%.
B = −50.6 × Mneq + 894.3
Mneq = Mn% + 0.24 × Ni% + 0.13 × Si% + 0.38 × Mo%
+ 0.55 × Cr% + 0.16 × Cu% −0.50 × Al% −0.45 × Co%
+ 0.90 × V%
However,
Ar3 = 901-325 × C% + 33 × Si% + 287 × P% + 40 × Al%
−92 × (Mn% + Mo% + Cu%) − 46 × (Cr% + Ni%)
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---|---|---|---|---|
CN1072272C (en) * | 1997-01-29 | 2001-10-03 | 新日本制铁株式会社 | High-strength steel sheet highly resistant to dynamic deformation and excellent in workability and process for production thereof |
JP4060997B2 (en) * | 1999-08-27 | 2008-03-12 | 新日本製鐵株式会社 | High-strength cold-rolled steel sheet and high-strength galvanized cold-rolled steel sheet excellent in bendability and deep drawability and manufacturing method thereof |
EP1264910B1 (en) * | 2000-02-28 | 2008-05-21 | Nippon Steel Corporation | Steel pipe having excellent formability and method for production thereof |
JP3990554B2 (en) * | 2000-06-30 | 2007-10-17 | 新日本製鐵株式会社 | Steel sheet with excellent shape freezing property and method for producing the same |
JP3990553B2 (en) * | 2000-08-03 | 2007-10-17 | 新日本製鐵株式会社 | High stretch flangeability steel sheet with excellent shape freezing property and method for producing the same |
JP3814134B2 (en) * | 2000-09-21 | 2006-08-23 | 新日本製鐵株式会社 | High formability, high strength cold-rolled steel sheet excellent in shape freezing property and impact energy absorption ability during processing and its manufacturing method |
JP3927384B2 (en) * | 2001-02-23 | 2007-06-06 | 新日本製鐵株式会社 | Thin steel sheet for automobiles with excellent notch fatigue strength and method for producing the same |
JP2002317246A (en) * | 2001-04-19 | 2002-10-31 | Nippon Steel Corp | Automobile thin steel sheet having excellent notch fatigue resistance and burring workability and production method therefor |
TWI290177B (en) * | 2001-08-24 | 2007-11-21 | Nippon Steel Corp | A steel sheet excellent in workability and method for producing the same |
DE60224557D1 (en) * | 2001-10-04 | 2008-02-21 | Nippon Steel Corp | PULLABLE HIGH STRENGTH STEEL PLATE WITH OUTSTANDING FORMFIXING PROPERTY AND METHOD OF MANUFACTURING THEREOF |
-
2003
- 2003-06-26 JP JP2003182675A patent/JP4276482B2/en not_active Expired - Fee Related
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2004
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CN100374586C (en) | 2008-03-12 |
JP2005015854A (en) | 2005-01-20 |
CN1809646A (en) | 2006-07-26 |
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