JP5966598B2 - High yield ratio high strength cold-rolled steel sheet excellent in workability and method for producing the same - Google Patents
High yield ratio high strength cold-rolled steel sheet excellent in workability and method for producing the same Download PDFInfo
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Description
本発明は、自動車の骨格構造部品に用いられる高強度冷延鋼板、特に、引張強度TSが980MPa以上の高降伏比高強度冷延鋼板およびその製造方法に関する。 The present invention relates to a high-strength cold-rolled steel sheet used for a skeleton structure part of an automobile, in particular, a high-yield ratio high-strength cold-rolled steel sheet having a tensile strength TS of 980 MPa or more and a method for producing the same.
近年、より一層の自動車車体軽量化による燃費向上や衝突安全性確保のため、TSが980MPa級の高強度鋼板が自動車の複雑な形状を有する骨格構造部品に採用される機会が多くなっている。この骨格構造部品はプレス加工により製造されるため、用いられる鋼板には、高強度のみならず、全伸びEl、伸びフランジ性、曲げ性などの延性にも優れていることが必要である。また、衝突安全性の観点からは、降伏強度YSの高い、すなわち降伏比YR[=(YS/TS)×100(%)]が65%以上の鋼板が求められている。 In recent years, high-strength steel sheets with a TS of 980 MPa class have been increasingly used for skeletal structural parts with complex shapes of automobiles in order to further improve fuel economy and ensure collision safety by further reducing the weight of automobile bodies. Since this skeletal structure component is manufactured by press working, it is necessary that the steel sheet to be used has not only high strength but also excellent ductility such as total elongation El, stretch flangeability, and bendability. Further, from the viewpoint of collision safety, a steel sheet having a high yield strength YS, that is, a yield ratio YR [= (YS / TS) × 100 (%)] of 65% or more is required.
このような状況下で、これまでに、TSが980MPa以上の加工性に優れた高強度冷延鋼板がいくつか提案されている。 Under such circumstances, several high-strength cold-rolled steel sheets excellent in workability with TS of 980 MPa or more have been proposed so far.
例えば、特許文献1には、mass%で、C:0.06〜0.6%、Si+Al:0.5〜3%、Mn:0.5〜3%、P:0.15%以下、S:0.02%以下を含有するとともに、焼戻マルテンサイト相:全組織に対して面積率で15%以上、フェライト相:全組織に対して面積率で5〜60%、残留オーステナイト相:全組織に対して体積率で5%以上、さらにベイナイト相および／またはマルテンサイト相を含有してもよい組織を有し、かつ、前記残留オーステナイト相のうち、2%歪を加えることによりマルテンサイト相へ変態する残留オーステナイト相の割合が20〜50%であることを特徴とする加工性および形状凍結性に優れた高強度鋼板が開示されている。 For example, Patent Document 1 includes mass%, C: 0.06 to 0.6%, Si + Al: 0.5 to 3%, Mn: 0.5 to 3%, P: 0.15% or less, and S: 0.02% or less. Tempered martensite phase: 15% or more in area ratio relative to the entire structure, ferrite phase: 5 to 60% in area ratio relative to the entire structure, residual austenite phase: 5% or more in volume ratio relative to the entire structure Further, the ratio of the residual austenite phase that has a structure that may contain a bainite phase and / or a martensite phase and transforms into a martensite phase by applying 2% strain among the residual austenite phase is 20 A high-strength steel sheet excellent in workability and shape freezing property, characterized by being ˜50%, is disclosed.
特許文献2には、mass%で、C:0.06超〜0.24%、Si:0.3%以下、Mn:0.5〜2.0%、P:0.06%以下、S:0.005%以下、Al:0.06%以下、N:0.006%以下、Mo:0.05〜0.5%、Ti:0.03〜0.2%、V:0.15超〜1.2%を含み、残部がFeおよび不可避的不純物からなり、C、Ti、Mo、V含有量が、0.8≦(C/12)/{(Ti/48)+(Mo/96)+(V/51)}≦1.5を満足する成分組成を有し、フェライト相が面積比率で95%以上であり、平均粒径10nm未満のTi、MoおよびVを含む炭化物が分散析出するとともに、該Ti、MoおよびVを含む炭化物は、原子%で表されるTi、Mo、Vが、V/(Ti+Mo+V)≧0.3を満たす平均組成を有することを特徴とするTSが980MPa以上の高降伏比高強度冷延鋼板が開示されている。 Patent Document 2 includes mass%, C: more than 0.06 to 0.24%, Si: 0.3% or less, Mn: 0.5 to 2.0%, P: 0.06% or less, S: 0.005% or less, Al: 0.06% or less, N : 0.006% or less, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.2%, V: more than 0.15 to 1.2%, the balance consists of Fe and inevitable impurities, and the contents of C, Ti, Mo, V are 0.8 ≦ (C / 12) / {(Ti / 48) + (Mo / 96) + (V / 51)} ≦ 1.5, the ferrite phase is 95% or more by area ratio, Carbides containing Ti, Mo, and V having an average particle size of less than 10 nm are dispersed and precipitated, and the carbides containing Ti, Mo, and V are Ti / Mo / V expressed in atomic% as V / (Ti + Mo A high yield ratio high strength cold-rolled steel sheet having a TS of 980 MPa or more, characterized by having an average composition satisfying + V) ≧ 0.3 is disclosed.
特許文献3には、mass%で、C:0.17〜0.73%、Si:3.0%以下、Mn:0.5〜3.0%、P:0.1%以下、S:0.07%以下、Al:3.0%以下およびN:0.010%以下を含有し、かつSi+Al:0.7%以上を満足し、残部がFeおよび不可避的不純物からなり、鋼板組織として、マルテンサイト相の鋼板組織全体に対する面積率が10〜90%、残留オーステナイト相の量が5〜50%、上部ベイナイト相中のベイニティックフェライト相の鋼板組織全体に対する面積率が5%以上であり、前記マルテンサイト相のうち25%以上が焼戻マルテンサイト相であり、前記マルテンサイト相の鋼板組織全体に対する面積率、前記残留オーステナイト相の量および前記上部ベイナイト相中のベイニティックフェライト相の鋼板組織全体に対する面積率の合計が65%以上、ポリゴナルフェライト相の鋼板組織全体に対する面積率が10%以下を満足し、かつ前記残留オーステナイト相中の平均C量が0.70%以上であって、TSが980MPa以上であることを特徴とする高強度鋼板が開示されている。 Patent Document 3 includes mass%, C: 0.17 to 0.73%, Si: 3.0% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, and N: Containing 0.010% or less, and satisfying Si + Al: 0.7% or more, the balance is composed of Fe and inevitable impurities, and the steel sheet structure is 10% to 90% in area ratio with respect to the entire steel structure of the martensite phase. The amount of austenite phase is 5 to 50%, the area ratio of the bainitic ferrite phase in the upper bainite phase to the entire steel sheet structure is 5% or more, and 25% or more of the martensite phase is tempered martensite phase. Yes, the total area ratio of the martensite phase to the entire steel sheet structure, the amount of the retained austenite phase and the area ratio of the bainitic ferrite phase in the upper bainite phase to the entire steel sheet structure is 65% or more, the polygonal ferrite phase Satisfies 10% or less of the area ratio for the entire steel sheet structure In addition, there is disclosed a high-strength steel sheet characterized in that the average C content in the retained austenite phase is 0.70% or more and TS is 980 MPa or more.
特許文献4には、mass%で、C:0.05〜0.3%、Si:0.01〜2.5%、Mn:0.5〜3.5%、P:0.003〜0.100%、S:0.02%以下、Al:0.010〜1.5%を含有し、Si+Al:0.5〜3.0%であり、残部がFeおよび不可避的不純物からなる成分組成を有し、面積率でフェライト相を20%以上、焼戻マルテンサイト相を10〜60%、マルテンサイト相を0〜10%を含み、体積率で残留オーステナイト相を3〜10%含み、焼戻マルテンサイト相のビッカース硬度(m)とフェライト相のビッカース硬度(f)の比(m)/(f)が3.0以下である金属組織を有することを特徴とする加工性に優れた高強度鋼板が開示されている。 In Patent Document 4, in mass%, C: 0.05 to 0.3%, Si: 0.01 to 2.5%, Mn: 0.5 to 3.5%, P: 0.003 to 0.100%, S: 0.02% or less, Al: 0.010 to 1.5% Si + Al: 0.5-3.0%, the balance is composed of Fe and unavoidable impurities, the area ratio is 20% or more ferrite phase, tempered martensite phase 10-60% , Containing 0 to 10% of martensite phase, 3 to 10% of retained austenite phase by volume, ratio of Vickers hardness (m) of tempered martensite phase to Vickers hardness (f) of ferrite phase (m) A high-strength steel sheet excellent in workability characterized by having a metal structure with / (f) being 3.0 or less is disclosed.
特許文献5には、mass%で、C:0.05〜0.35%、Si:0.05〜2.0%、Mn:0.8〜3.0%、P:0.0010〜0.1%、S:0.0005〜0.05%、N:0.0010〜0.010%、Al:0.01〜2.0%を含有し、残部がFeおよび不可避的不純物からなる鋼組成を持ち、金属組織はフェライト相またはベイナイト相または焼戻マルテンサイト相を主体とし、残留オーステナイト相を3〜30%含む鋼板において、前記オーステナイト相がフェライト相、ベイナイト相およびマルテンサイト相と接する相界面において、前記オーステナイト相の中心濃度Cgcとオーステナイト粒の粒界の濃度CgbがCgb/Cgc > 1.3を満たす範囲にあるオーステナイト粒が50%以上あることを特徴とする伸びと穴拡げ性に優れた高強度薄鋼板が開示されている。 In Patent Document 5, mass%, C: 0.05 to 0.35%, Si: 0.05 to 2.0%, Mn: 0.8 to 3.0%, P: 0.0010 to 0.1%, S: 0.0005 to 0.05%, N: 0.0010 to 0.010 %, Al: 0.01 to 2.0%, the balance is a steel composition consisting of Fe and inevitable impurities, the metal structure is mainly ferrite phase, bainite phase or tempered martensite phase, residual austenite phase 3 to In the steel sheet containing 30%, at the phase interface where the austenite phase is in contact with the ferrite phase, bainite phase, and martensite phase, the central concentration Cgc of the austenite phase and the concentration Cgb of the austenite grain boundary satisfy Cgb / Cgc> 1.3 There is disclosed a high-strength thin steel sheet excellent in elongation and hole expansibility characterized by having 50% or more of austenite grains.
しかしながら、特許文献1に記載の高強度鋼板では、降伏比が50%以下で極端に低く、衝突安全性の観点から問題がある。特許文献2に記載の高降伏比高強度冷延鋼板では、高価な元素であるTi、Mo、Vを用いているためコスト高であるのみならず、Elが高々19%程度と低い。また、特許文献3〜5に記載の高強度鋼板では、980MPa以上のTSを得ようとすると、Elが高々27%程度で十分とはいえない。 However, the high-strength steel sheet described in Patent Document 1 has a yield ratio of 50% or less and extremely low, which is problematic from the viewpoint of collision safety. The high yield ratio high strength cold-rolled steel sheet described in Patent Document 2 uses expensive elements such as Ti, Mo, and V, so that the cost is high and El is as low as about 19% at most. In addition, in the high-strength steel sheets described in Patent Documents 3 to 5, when trying to obtain a TS of 980 MPa or more, El of at most about 27% is not sufficient.
本発明は、安価で、980MPa以上のTS、28%以上のElを有し、かつYRが65%以上の加工性に優れる高降伏比高強度冷延鋼板およびその製造方法を提供することを目的とする。 An object of the present invention is to provide a high yield ratio high strength cold-rolled steel sheet that is inexpensive, has a TS of 980 MPa or more, an El of 28% or more, and excellent YR of 65% or more, and a method for producing the same. And
本発明者らは、上記の目的とする高降伏比高強度冷延鋼板の製造方法について検討したところ、高価な元素であるTi、Mo、Vなどを用いずに、フェライト相、ベイナイト相、焼戻マルテンサイト相の面積比率および残留オーステナイト相の量を制御するとともに、長軸が3μm以上の粗大な焼戻マルテンサイト相の個数を制限することが効果的であることを見出した。 The inventors of the present invention have studied the manufacturing method of the high yield ratio high strength cold-rolled steel sheet for the above purpose, and without using expensive elements such as Ti, Mo, V, etc., ferrite phase, bainite phase, sintered It was found that it is effective to control the area ratio of the returned martensite phase and the amount of retained austenite phase, and to limit the number of coarse tempered martensite phases having a major axis of 3 μm or more.
本発明は、このような知見に基づいてなされたものであり、mass%で、C:0.15〜0.25%、Si:1.0〜2.0%、Mn:1.8〜2.8%、P:0.020%以下、S:0.0040%以下、Al:0.005〜0.08%、N:0.008%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成を有し、フェライト相とベイナイト相の合計の組織全体に対する面積比率が40〜70%、焼戻マルテンサイト相の組織全体に対する面積比率が20〜40%で、残留オーステナイト相の量が2〜20%であり、かつ長軸が3μm以上の焼戻マルテンサイト相の個数が2.0×104個/mm2以下であることを特徴とする加工性に優れる高降伏比高強度冷延鋼板を提供する。 The present invention has been made based on such findings, in mass%, C: 0.15-0.25%, Si: 1.0-2.0%, Mn: 1.8-2.8%, P: 0.020% or less, S: 0.0040% or less, Al: 0.005 to 0.08%, N: 0.008% or less, the remainder has a component composition consisting of Fe and inevitable impurities, the area ratio of the total of the ferrite phase and bainite phase to the entire structure is 40 ~ 70%, the area ratio of the tempered martensite phase to the whole structure is 20-40%, the amount of retained austenite phase is 2-20%, and the number of tempered martensite phases with a major axis of 3 μm or more is Provided is a high yield ratio high strength cold-rolled steel sheet excellent in workability characterized by being 2.0 × 10 4 pieces / mm 2 or less.
本発明の高降伏比高強度冷延鋼板は、上記の化学組成を有する鋼スラブに、順に、熱間圧延、酸洗、冷間圧延を施した後、850〜950℃の温度域に加熱後冷却する第一の熱処理を行い、次いで750〜850℃の温度域に加熱後冷却中の300〜500℃の温度域で過時効処理する第二の熱処理を行い、次いで100〜300℃の温度域に加熱する第三の熱処理を行う方法により製造できる。 The high yield ratio high strength cold-rolled steel sheet of the present invention is subjected to hot rolling, pickling and cold rolling in this order on a steel slab having the above chemical composition, and then heated to a temperature range of 850 to 950 ° C. Perform a first heat treatment to cool, then perform a second heat treatment to overheat in the temperature range of 300 to 500 ° C during cooling after heating to the temperature range of 750 to 850 ° C, then the temperature range of 100 to 300 ° C It can manufacture by the method of performing the 3rd heat processing which heats.
本発明により、安価で、980MPa以上のTS、28%以上のElを有し、かつYRが65%以上の加工性に優れる高降伏比高強度冷延鋼板を製造できるようになった。本発明の高降伏比高強度冷延鋼板は、自動車の骨格構造部品のみならず、建築や家電分野など厳しい寸法精度と加工性が必要とされる用途にも好適である。 According to the present invention, it is possible to produce a high yield ratio high strength cold-rolled steel sheet that is inexpensive, has TS of 980 MPa or more, El of 28% or more, and has excellent workability with YR of 65% or more. The high yield ratio high strength cold-rolled steel sheet according to the present invention is suitable not only for automobile frame structure parts but also for applications that require strict dimensional accuracy and workability, such as in the field of architecture and home appliances.
以下に、本発明の詳細を説明する。 Details of the present invention will be described below.
1) 化学組成
以下、成分元素の含有量の単位である%は、mass%を意味するものとする。
1) Chemical composition Hereinafter,%, which is a unit of content of component elements, means mass%.
C:0.15〜0.25%
Cはオーステナイト安定化元素であり、オーステナイト相からの変態相であるベイナイト相やマルテンサイト相を生成させて高強度化に寄与する。また、焼戻マルテンサイト相の量や硬さに影響し、伸びフランジ性や曲げ性に影響を及ぼす。C量が0.15%未満ではフェライト相が過剰に生成し、980MPa以上のTSを確保することが困難になる。一方、C量が0.25%を超えると焼戻マルテンサイト相が過度に硬質化し、El、伸びフランジ性、曲げ性が低下したり、スポット溶接性が著しく劣化する。したがって、C量は0.15〜0.25%以下とする。
C: 0.15-0.25%
C is an austenite stabilizing element and contributes to high strength by generating a bainite phase and a martensite phase which are transformation phases from the austenite phase. It also affects the amount and hardness of the tempered martensite phase and affects stretch flangeability and bendability. If the C content is less than 0.15%, the ferrite phase is excessively generated, and it becomes difficult to secure TS of 980 MPa or more. On the other hand, if the C content exceeds 0.25%, the tempered martensite phase becomes excessively hard, and El, stretch flangeability and bendability are lowered, and spot weldability is remarkably deteriorated. Therefore, the C content is 0.15 to 0.25% or less.
Si:1.0〜2.0%
Siはフェライト相の固溶強化を通じ、硬質な焼戻マルテンサイト相との硬度差を低減し、伸びフランジ性や曲げ性の向上に寄与する。また、セメンタイト相の生成を遅延させ、残留オーステナイト相の生成を促進し、Elの向上に寄与する。Si量が1.0%未満ではこのような効果が得られず、2.0％を超えると鋼板が脆くなる。したがって、Si量1.0〜2.0%とする。
Si: 1.0-2.0%
Si reduces the hardness difference from the hard tempered martensite phase through solid solution strengthening of the ferrite phase and contributes to the improvement of stretch flangeability and bendability. It also delays the formation of cementite phase, promotes the formation of retained austenite phase, and contributes to the improvement of El. If the Si content is less than 1.0%, such an effect cannot be obtained, and if it exceeds 2.0%, the steel sheet becomes brittle. Therefore, the Si amount is set to 1.0 to 2.0%.
Mn:1.8〜2.8%
Mnはオーステナイト安定化元素であり、焼鈍後の冷却過程で炭化物の析出を抑制し、ベイナイト相やマルテンサイト相を生成させて高強度化に寄与する。こうした効果を得るには、Mn量を1.8%以上にする必要がある。一方、Mn量が2.8%を超えるとMnの偏析に起因したバンド状組織が形成され、鋼板の均一な変形を阻害して、伸びフランジ性や曲げ性を低下させる。したがって、Mn量は1.8〜2.8%とする。
Mn: 1.8-2.8%
Mn is an austenite stabilizing element, which suppresses the precipitation of carbides in the cooling process after annealing and generates a bainite phase and a martensite phase, thereby contributing to high strength. In order to obtain such an effect, the Mn content needs to be 1.8% or more. On the other hand, if the amount of Mn exceeds 2.8%, a band-like structure resulting from segregation of Mn is formed, hindering uniform deformation of the steel sheet and reducing stretch flangeability and bendability. Therefore, the Mn content is 1.8 to 2.8%.
P:0.020%以下
P量が0.020%を超えると偏析して伸びフランジ性や曲げ性を低下させるのみならず、スポット溶接性に悪影響を及ぼす。したがって、P量は0.020%以下とする。なお、P量は極力低減することが好ましいが、過度に低減すると製鋼工程での生産能率が低下し、コスト増を招くので、その下限は0.001%とすることが好ましい。
P: 0.020% or less
If the amount of P exceeds 0.020%, it segregates to reduce stretch flangeability and bendability, and also adversely affects spot weldability. Therefore, the P content is 0.020% or less. The amount of P is preferably reduced as much as possible. However, if the amount is excessively reduced, the production efficiency in the steel making process is lowered and the cost is increased, so the lower limit is preferably made 0.001%.
S:0.0040%以下
S量が0.0040%を超えるとMnSなどの硫化物系介在物が形成され、変形時に割れの起点となって伸びフランジ性、および曲げ性を低下させる。したがって、S量は0.0040%以下とする。なお、S量は極力低減することが好ましいが、過度に低減すると製鋼工程での生産能率が低下し、コスト増を招くので、その下限は0.0001%とすることが好ましい。
S: 0.0040% or less
If the amount of S exceeds 0.0040%, sulfide inclusions such as MnS are formed, which becomes a starting point of cracking during deformation and deteriorates stretch flangeability and bendability. Therefore, the S content is 0.0040% or less. The amount of S is preferably reduced as much as possible. However, if the amount is excessively reduced, the production efficiency in the steel making process is lowered and the cost is increased, so the lower limit is preferably made 0.0001%.
Al:0.005〜0.08%
Alは鋼の脱酸に必要であるため、その量は0.005%以上にする必要がある。しかし、Al量が0.08%を超えるとアルミナなどの鋼板表層部に存在する介在物が増加して曲げ性が低下する。したがって、Al量は0.005〜0.08%とする。
Al: 0.005-0.08%
Since Al is necessary for deoxidation of steel, its amount needs to be 0.005% or more. However, when the Al content exceeds 0.08%, inclusions present in the surface layer of the steel plate such as alumina increase and the bendability decreases. Therefore, the Al content is 0.005 to 0.08%.
N:0.008%以下
N量が0.008％を超えると粗大な窒化物が生成し、伸びフランジ性や曲げ性が低下する。したがって、N量は0.008%以下とする。なお、N量は極力低減することが好ましいが、過度に低減すると製鋼工程での生産能率が低下し、コスト増を招くので、その下限は0.0001%とすることが好ましい。
N: 0.008% or less
If the N content exceeds 0.008%, coarse nitrides are formed, and stretch flangeability and bendability are deteriorated. Therefore, the N content is 0.008% or less. The N amount is preferably reduced as much as possible. However, if the amount is excessively reduced, the production efficiency in the steel making process is lowered and the cost is increased, so the lower limit is preferably made 0.0001%.
残部はFeおよび不可避的不純物である。 The balance is Fe and inevitable impurities.
2) 組織
フェライト相とベイナイト相の合計の組織全体に対する面積比率:40〜70%
フェライト相とベイナイト相は焼戻マルテンサイト相より軟質であり、El、伸びフランジ性および曲げ性の向上に寄与する。
2) Structure Area ratio of the total of ferrite phase and bainite phase to the entire structure: 40-70%
The ferrite phase and bainite phase are softer than the tempered martensite phase and contribute to the improvement of El, stretch flangeability and bendability.
目的とするEl、伸びフランジ性および曲げ性を得るには、フェライト相とベイナイト相の合計の組織全体に対する面積比率を40%以上にする必要がある。面積比率が40%に満たない場合、硬質な焼戻マルテンサイト相の面積比率が増加し、過度に高強度化し、目的とするEl、伸びフランジ性および曲げ性の確保が困難となる。一方、面積比率が70％を超えると、980MPa以上のTSの確保が困難となる。したがって、フェライト相とベイナイト相の合計の組織全体に対する面積比率は40〜70%とする。 In order to obtain the target El, stretch flangeability and bendability, the area ratio of the total of the ferrite phase and the bainite phase to the entire structure needs to be 40% or more. When the area ratio is less than 40%, the area ratio of the hard tempered martensite phase is increased, the strength is excessively increased, and it is difficult to ensure the target El, stretch flangeability and bendability. On the other hand, if the area ratio exceeds 70%, it is difficult to secure TS of 980 MPa or more. Therefore, the area ratio of the total of the ferrite phase and the bainite phase to the entire structure is 40 to 70%.
焼戻マルテンサイト相の組織全体に対する面積比率:20〜40%
焼鈍後の冷却時に生成されるマルテンサイト相は過度に硬質であり、El、伸びフランジ性、曲げ性を低下させる。そのため、本発明では、100〜300℃の温度域で第三の熱処理を行い、焼戻マルテンサイト相により高強度化を図っている。980MPa以上のTSを確保するには、焼戻マルテンサイト相の組織全体に対する面積比率を20%以上にする必要がある。しかし、面積比率が40%を超えると過度に高強度化し、El、伸びフランジ性、曲げ性の低下を招く。したがって、焼戻マルテンサイト相の組織全体に対する面積比率は20〜40%とする。
Area ratio of tempered martensite phase to the entire structure: 20-40%
The martensite phase produced during cooling after annealing is excessively hard and reduces El, stretch flangeability and bendability. Therefore, in the present invention, the third heat treatment is performed in the temperature range of 100 to 300 ° C., and the strength is increased by the tempered martensite phase. In order to secure TS of 980 MPa or more, the area ratio of the tempered martensite phase to the entire structure needs to be 20% or more. However, if the area ratio exceeds 40%, the strength is excessively increased, and the El, stretch flangeability, and bendability are reduced. Therefore, the area ratio of the tempered martensite phase to the entire structure is 20 to 40%.
残留オーステナイト相の量:2〜20%
残留オーステナイト相はElを向上させる効果があり、高El化するには2％以上の残留オーステナイト相を含有させることが必要である。しかし、残留オーステナイト相はC濃度が高く硬質であり、また歪を受けてマルテンサイト変態した部分が硬質化し、鋼板の均一な変形を阻害し、伸びフランジ性や曲げ性を低下させるため、その量は20％以下にする必要がある。したがって、残留オーステナイト量は2〜20％とする。
Amount of residual austenite phase: 2-20%
The residual austenite phase has the effect of improving El, and in order to achieve high El, it is necessary to contain 2% or more of the residual austenite phase. However, the retained austenite phase has a high C concentration and is hard, and the part that has undergone martensite transformation due to strain becomes hard, obstructing uniform deformation of the steel sheet and reducing stretch flangeability and bendability. Must be 20% or less. Therefore, the amount of retained austenite is 2 to 20%.
長軸が3μm以上の焼戻マルテンサイト相の個数:2.0×104個/mm2以下
フェライト相およびベイナイト相より硬質な焼戻マルテンサイト相が微細かつ均一に存在すると、加工時に鋼板は均一な変形が可能であり、優れた伸びフランジ性や曲げ性が得られる。しかし、長軸が3μm以上の粗大な焼戻マルテンサイト相の個数が2.0×104個/mm2を超えると均一な変形を阻害し、伸びフランジ性や曲げ性を低下させる。したがって、長軸が3μm以上の焼戻マルテンサイト相の個数は2.0×104個/mm2以下とする。
Number of tempered martensite phases with a major axis of 3 μm or more: 2.0 × 10 4 pieces / mm 2 or less If the tempered martensite phase harder than the ferrite phase and bainite phase is finely and evenly present, the steel plate is uniform during processing. Deformation is possible, and excellent stretch flangeability and bendability are obtained. However, if the number of coarse tempered martensite phases with a major axis of 3 μm or more exceeds 2.0 × 10 4 / mm 2 , uniform deformation is inhibited and stretch flangeability and bendability are deteriorated. Therefore, the number of tempered martensite phases with a major axis of 3 μm or more is 2.0 × 10 4 pieces / mm 2 or less.
3) 製造方法
本発明の高降伏比高強度冷延鋼板は、上記組成を有する鋼スラブを、通常行われている条件で、加熱後、熱間圧延し、次いで酸洗後、冷間圧延して冷延板とし、850〜950℃の温度域に加熱後冷却する第一の熱処理を行い、次いで750〜850℃の温度域に加熱後冷却中の300〜500℃の温度域で過時効処理する第二の熱処理を行い、次いで100〜300℃の温度域に加熱する第三の熱処理を行って製造される。
3) Manufacturing method The high yield ratio high strength cold-rolled steel sheet of the present invention is a steel slab having the above composition, heated under normal conditions, hot-rolled, then pickled and cold-rolled. Cold-rolled sheet, first heat treatment to cool after heating to 850-950 ° C, then over-aging at 300-500 ° C during cooling to 750-850 ° C after cooling The second heat treatment is performed, and then the third heat treatment is performed by heating to a temperature range of 100 to 300 ° C.
第一の熱処理：850〜950℃の温度域に加熱後冷却
冷間圧延後の冷延板には、まず、第一の熱処理（焼鈍）が行われる。このとき、第一の熱処理の温度、すなわち焼鈍温度が850℃より低い場合は、焼鈍中の鋼中のC、Si、Mn、P元素の拡散が不十分であり、熱間圧延後に生成していたパーライト相、ベイナイト相、マルテンサイト相の影響を受けて、焼鈍後の鋼板において、C、Si、Mn、P濃度の不均一な組織分布が形成される。この不均一なC、Si、Mn、P濃度の不均一な組織分布は、その後の第二の熱処理および第三の熱処理においても解消されず、最終的に得られる組織が不均一組織となり、硬さや長軸の異なる焼戻マルテンサイト相が存在して、伸びフランジ性や曲げ性の低下を招く。一方、焼鈍温度が950℃を超えてオーステナイト単相の温度域まで加熱すると、C、Si、Mn、P濃度は均一になるが、オーステナイト粒径が過度に粗大化するため、最終的に得られる焼戻マルテンサイト相も粗大化し、伸びフランジ性や曲げ性の低下を招く。したがって、焼鈍温度は850〜950℃とする。
First heat treatment: cooling after heating to a temperature range of 850 to 950 ° C. First, a first heat treatment (annealing) is performed on the cold-rolled sheet after cold rolling. At this time, when the temperature of the first heat treatment, that is, the annealing temperature is lower than 850 ° C., the diffusion of C, Si, Mn, and P elements in the steel during annealing is insufficient, and is generated after hot rolling. Under the influence of the pearlite phase, bainite phase, and martensite phase, a non-uniform structure distribution of C, Si, Mn, and P concentrations is formed in the annealed steel sheet. This non-uniform structure distribution of C, Si, Mn, P concentration is not eliminated in the subsequent second heat treatment and third heat treatment, and the finally obtained structure becomes a non-uniform structure. Tempered martensite phases with different sheaths and long axes are present, leading to a reduction in stretch flangeability and bendability. On the other hand, when the annealing temperature exceeds 950 ° C and it is heated to the temperature range of the austenite single phase, the C, Si, Mn, P concentration becomes uniform, but the austenite grain size becomes excessively coarse, so it is finally obtained The tempered martensite phase is also coarsened, leading to a reduction in stretch flangeability and bendability. Therefore, the annealing temperature is 850 to 950 ° C.
焼鈍後の冷却速度は、特に規定しないが、次に行う第二の熱処理後にフェライト相、ベイナイト相、マルテンサイト相、残留オーステナイト相をバランス良く得るために、10〜80℃/秒とすることが好ましい。また、冷却手段は、ガス冷却が好ましいが、特に規定する必要はなく、空冷、ミスト冷却、ロール冷却、水冷など、従来行われている冷却手段を用いて組み合わせて行うことが可能である。また、特に限定するものではないが、高生産性、冷却速度の制御のため、第一の熱処理は、連続焼鈍炉にて行うことが好ましい。 The cooling rate after annealing is not particularly specified, but in order to obtain a well-balanced ferrite phase, bainite phase, martensite phase, and retained austenite phase after the second heat treatment to be performed next, it may be set to 10 to 80 ° C./second. preferable. The cooling means is preferably gas cooling, but there is no particular need to define it, and it is possible to use a combination of conventionally used cooling means such as air cooling, mist cooling, roll cooling, and water cooling. Although not particularly limited, the first heat treatment is preferably performed in a continuous annealing furnace in order to control high productivity and cooling rate.
第二の熱処理：750〜850℃の温度域に加熱後冷却中の300〜500℃の温度域で過時効処理
第一の熱処理後（焼鈍後）の鋼板に、第二の熱処理を施す。このとき、第二の熱処理温度が750℃より低い場合は、熱処理中のフェライト相の面積比率が過度に多くなり、980MPa以上のTSの確保が困難となる。一方、第二の熱処理温度が850℃を超えると熱処理中のオーステナイト相の面積比率が増加し、冷却保持後の鋼板のフェライト相の面積比率が少なく、マルテンサイト相の面積比率が大きくなり、最終的に焼戻マルテンサイト相の面積比率が40%を超え、28%以上のElの確保が困難となる。したがって、第二の熱処理温度は750〜850℃、より好ましくは800〜850℃とする。
Second heat treatment: Heat treatment in the temperature range of 750 to 850 ° C. and overaging in the temperature range of 300 to 500 ° C. during cooling. The second heat treatment is performed on the steel plate after the first heat treatment (after annealing). At this time, when the second heat treatment temperature is lower than 750 ° C., the area ratio of the ferrite phase during the heat treatment becomes excessively large, and it becomes difficult to secure TS of 980 MPa or more. On the other hand, when the second heat treatment temperature exceeds 850 ° C., the area ratio of the austenite phase during the heat treatment increases, the area ratio of the ferrite phase of the steel sheet after cooling is small, the area ratio of the martensite phase is large, and the final In particular, the area ratio of the tempered martensite phase exceeds 40%, and it is difficult to secure El of 28% or more. Therefore, the second heat treatment temperature is 750 to 850 ° C, more preferably 800 to 850 ° C.
上記750〜850℃で熱処理した後、冷却中に300〜500℃で冷却を停止し該温度域に保持する過時効処理を行う。冷却停止温度が300℃未満の場合、残留オーステナイトの生成が抑制され、過度にマルテンサイト相が生成するため、最終的に得られる鋼板の強度が高くなりすぎ、Elの確保が困難となる。一方、500℃を超える場合、ベイナイト変態は遅延し、残留オーステナイト相の生成は抑制され、最終的に得られる鋼板において、優れたElを得ることが困難となる。フェライト相、ベイナイト相を主体とし、マルテンサイト相および残留オーステナイト相の存在比率を制御し、TS980MPa級以上の強度を確保するとともにEl、伸びフランジ性、および曲げ性をバランス良く得るに、冷却停止温度（過時効温度）は300〜500℃の範囲とする。 After the heat treatment at 750 to 850 ° C., an overaging treatment is performed in which the cooling is stopped at 300 to 500 ° C. and kept in the temperature range during the cooling. When the cooling stop temperature is less than 300 ° C., the generation of retained austenite is suppressed and the martensite phase is excessively generated, so that the strength of the finally obtained steel sheet becomes too high, and it becomes difficult to ensure El. On the other hand, when the temperature exceeds 500 ° C., the bainite transformation is delayed, the formation of the retained austenite phase is suppressed, and it becomes difficult to obtain excellent El in the steel sheet finally obtained. Mainly ferrite phase and bainite phase, control the abundance ratio of martensite phase and residual austenite phase to ensure the strength of TS980MPa class or higher and to obtain a good balance of El, stretch flangeability and bendability. (Overaging temperature) is in the range of 300 to 500 ° C.
冷却停止後、オーステナイト相へのC濃化を進行させ、所望の残留オーステナイト量を得るため、該温度域に保持する過時効処理を行う。保持時間（過時効時間）は、100秒〜1000秒とすることが好ましい。保持時間が100秒未満では、所望の残留オーステナイト量を得ることが困難であり、1000秒を超えると、生産性が低下する。 After the cooling is stopped, C concentration to the austenite phase is advanced, and in order to obtain a desired amount of retained austenite, an overaging treatment is performed while maintaining the temperature range. The holding time (overaging time) is preferably 100 seconds to 1000 seconds. If the holding time is less than 100 seconds, it is difficult to obtain a desired amount of retained austenite, and if it exceeds 1000 seconds, the productivity decreases.
上記時間保持後は、特に限定するものではないが、空冷、ガス冷却あるいは水冷などの常法により常温まで冷却すればよい。また、特に限定するものではないが、第二の熱処理は、高生産性、高精度ヒートサイクル制御のため、連続焼鈍炉にて行うことが好ましい。 Although there is no particular limitation after the above-mentioned time holding, it may be cooled to room temperature by an ordinary method such as air cooling, gas cooling or water cooling. Further, although not particularly limited, the second heat treatment is preferably performed in a continuous annealing furnace for high productivity and high-accuracy heat cycle control.
第三の熱処理：100〜300℃の温度域に加熱
第二の熱処理後の鋼板には、第三の熱処理（再加熱熱処理）を施す。このとき、再加熱処理温度が100℃より低い場合は、マルテンサイト相の焼戻軟質化が不十分となり過度に硬質化し、伸びフランジ性および曲げ性が低下する。また、残留応力も除去されず、YRが65%未満と低くなる。一方、再加熱処理温度が300℃を超えるとマルテンサイト相が過度に焼戻軟化し、980MPa以上のTSを確保することが困難になるとともに、第二の熱処理により得られた残留オーステナイト相が分解し、最終的に所望の残留オーステナイト相が得られず、28%以上のElが得られなくなる。したがって、再加熱処理温度は100℃〜300℃の範囲とする。
Third heat treatment: Heating to a temperature range of 100 to 300 ° C. A third heat treatment (reheating heat treatment) is applied to the steel plate after the second heat treatment. At this time, when the reheating treatment temperature is lower than 100 ° C., the temper softening of the martensite phase becomes insufficient and becomes excessively hard, and stretch flangeability and bendability are deteriorated. Also, residual stress is not removed, and YR is as low as less than 65%. On the other hand, if the reheating temperature exceeds 300 ° C, the martensite phase becomes excessively tempered and softened, making it difficult to secure TS of 980 MPa or more, and the residual austenite phase obtained by the second heat treatment decomposes. As a result, the desired retained austenite phase cannot be obtained, and 28% or more of El cannot be obtained. Therefore, the reheating treatment temperature is in the range of 100 ° C to 300 ° C.
処理時間は、マルテンサイト相を十分焼戻し、焼戻マルテンサイト相とし、65%以上のYRを得るためには1〜100分とすることが好ましい。 The treatment time is preferably 1 to 100 minutes in order to sufficiently temper the martensite phase to obtain a tempered martensite phase and obtain a YR of 65% or more.
なお、再加熱処理後の冷却は、特に限定するものではないが、空冷あるいはガス冷却などの常法により常温まで冷却する。 In addition, although the cooling after a reheating process is not specifically limited, it cools to normal temperature by conventional methods, such as air cooling or gas cooling.
また、再加熱処理の方法は上記条件を満たす方法であれば特に規定する必要はなく、箱焼鈍炉にて行ってもよいし、連続焼鈍炉にて行ってもよい。 The reheating treatment method is not particularly limited as long as it satisfies the above conditions, and may be performed in a box annealing furnace or a continuous annealing furnace.
上記のように、本発明では、通常行われている条件で、鋼スラブの製造から冷間圧延までは行われる。このとき、鋼スラブは、連続鋳造法や造塊法で製造できるが、偏析を軽減するために、連続鋳造法で製造することが好ましい。熱間圧延時の加熱温度は1100℃以上にすることが好ましい。スケール生成の抑制、燃料原単位の低減の観点からその上限温度は1300℃にすることが好ましい。熱間圧延はフェライト相やパーライト相などの層状組織を回避すべく、850℃以上の仕上温度で圧延を終了することが好ましい。スケール生成の抑制、結晶粒粗大化の抑制による組織の微細均一化の観点から仕上温度の上限は950℃にすることが好ましい。熱間圧延後の巻取温度は、冷間圧延性、表面性状の観点から450〜600℃にすることが好ましい。巻取り後の鋼板には、酸洗、冷間圧延を経て、上記の条件で第一の熱処理（焼鈍）、第二の熱処理および第三の熱処理（再加熱処理）が行われる。 As mentioned above, in this invention, it is performed from manufacture of a steel slab to cold rolling on the conditions normally performed. At this time, the steel slab can be manufactured by a continuous casting method or an ingot-making method, but is preferably manufactured by a continuous casting method in order to reduce segregation. The heating temperature during hot rolling is preferably 1100 ° C. or higher. The upper limit temperature is preferably set to 1300 ° C. from the viewpoint of suppressing scale generation and reducing the fuel consumption rate. In the hot rolling, it is preferable to finish the rolling at a finishing temperature of 850 ° C. or higher so as to avoid a layered structure such as a ferrite phase and a pearlite phase. The upper limit of the finishing temperature is preferably 950 ° C. from the viewpoint of finer and uniform structure by suppressing scale formation and coarsening of crystal grains. The coiling temperature after hot rolling is preferably 450 to 600 ° C. from the viewpoints of cold rollability and surface properties. The steel sheet after winding is subjected to pickling and cold rolling, and the first heat treatment (annealing), the second heat treatment, and the third heat treatment (reheating treatment) are performed under the above conditions.
表1に示す成分組成を有する鋼を溶製してスラブとし、1200℃に加熱後、仕上温度900℃で熱間圧延し、圧延後平均冷却速度50℃/秒で冷却し、巻取温度450℃で巻取り、引き続き塩酸酸洗し、冷間圧延後、表2に示す条件で焼鈍、第一の熱処理、第二の熱処理、第三の熱処理を行い、板厚1.6mmの冷延鋼板No.1〜14を製造した。なお、第一の熱処理、第二の熱処理は連続焼鈍炉にて行い、第三の熱処理は連続焼鈍炉または箱焼鈍炉にて行った。そして、得られた冷延鋼板について、下記の方法でフェライト相とベイナイト相の合計の面積比率、焼戻マルテンサイト相の面積比率、残留オーステナイト相の量および長軸が3μm以上の焼戻マルテンサイト相の個数を求めた。 Steel having the composition shown in Table 1 is melted to form a slab, heated to 1200 ° C, hot-rolled at a finishing temperature of 900 ° C, cooled at an average cooling rate of 50 ° C / second after rolling, and a coiling temperature of 450 Winding at 0 ° C, followed by hydrochloric acid pickling, cold rolling, annealing, first heat treatment, second heat treatment, third heat treatment under the conditions shown in Table 2, cold rolled steel plate No. 1.6mm thick .1-14 were produced. The first heat treatment and the second heat treatment were performed in a continuous annealing furnace, and the third heat treatment was performed in a continuous annealing furnace or a box annealing furnace. And about the obtained cold-rolled steel sheet, the total area ratio of the ferrite phase and the bainite phase, the area ratio of the tempered martensite phase, the amount of residual austenite phase, and the tempered martensite whose major axis is 3 μm or more by the following method The number of phases was determined.
ここで、フェライト相とベイナイト相の合計の組織全体に対する面積比率や焼戻マルテンサイト相の組織全体に対する面積比率は、以下のようにして求めた。すなわち、鋼板の圧延方向に沿った板厚断面の板厚1/4の位置を研磨し、3 mass %ピクラールと3 mass %ピロ亜硫酸ソーダの混合液でエッチング後、光学顕微鏡を用いて倍率1000倍で5視野観察し、画像解析により100μmm×100μmm四方の領域内に存在する各相の平均の占有面積率を求めた。ここで、3 mass %ピクラールと3 mass %ピロ亜硫酸ソーダの混合液でエッチングして黒色となった領域をフェライト相とベイナイト相とし、それ以外の領域を焼戻マルテンサイト相と残留オーステナイト相の合計面積比率とした。 Here, the area ratio of the total of the ferrite phase and the bainite phase to the entire structure and the area ratio of the tempered martensite phase to the entire structure were determined as follows. That is, the position of the thickness 1/4 of the thickness cross section along the rolling direction of the steel plate is polished, etched with a mixed solution of 3 mass% picral and 3 mass% sodium pyrosulfite, and then magnified 1000 times using an optical microscope. 5 fields of view were observed, and the average occupied area ratio of each phase existing in a 100 μm × 100 μm square region was determined by image analysis. Here, the blackened areas etched with a mixture of 3 mass% picral and 3 mass% sodium pyrosulfite are the ferrite phase and bainite phase, and the other areas are the sum of the tempered martensite phase and the retained austenite phase. The area ratio was used.
残留オーステナイト相の量はMoのKα線を用いたX線回折法により、以下のようにして求めた。すなわち、鋼板の板厚1/4付近の圧延面に平行な面において、オーステナイト相の(211)および(220)面とフェライト相の(200)および(220)面のピーク強度比から残留オーステナイト相の体積率を算出し残留オーステナイト相の量とした。焼戻マルテンサイト相の面積比率は組織観察より求めた焼戻マルテンサイト相と残留オーステナイト相の合計面積比率とX線回折より求めた残留オーステナイト量の差分として求めた。3回目の熱処理温度が100℃未満の場合、SEM観察を行い、平滑な表面の場合、焼戻マルテンサイト相ではなく、マルテンサイト相と判定した。 The amount of residual austenite phase was determined by the X-ray diffraction method using Mo Kα rays as follows. That is, in the plane parallel to the rolling surface near the thickness of the steel sheet 1/4, the residual austenite phase is determined from the peak intensity ratio of the (211) and (220) surfaces of the austenite phase and the (200) and (220) surfaces of the ferrite phase. Was calculated as the amount of residual austenite phase. The area ratio of the tempered martensite phase was determined as the difference between the total area ratio of the tempered martensite phase and the retained austenite phase obtained from the structure observation and the amount of retained austenite determined from the X-ray diffraction. When the third heat treatment temperature was less than 100 ° C., SEM observation was performed, and in the case of a smooth surface, it was determined that the martensite phase was not a tempered martensite phase.
長軸が3μm以上の焼戻マルテンサイト相は、上記のように、鋼板の圧延方向に沿った板厚断面の板厚1/4の位置を研磨し、3 mass%ピクラールと3 mass%ピロ亜硫酸ソーダの混合液でエッチング後、光学顕微鏡で倍率1000倍で観察し、画像解析により求めた50μm×50μm四方の領域内に存在する焼戻マルテンサイト相の結晶粒について、直径3μmの円を重ねあわせ、結晶が円内（円ふくまない）に納まり、円より小さい場合、長軸3μm未満の焼戻マルテンサイトとし、円を含むまたは円内に納まらない場合、長軸が3μm以上の焼戻マルテンサイト相とした。また、長軸が3μm以上の焼戻マルテンサイト相の個数は、長軸が3μm以上の焼戻マルテンサイト相の個数を求め、単位面積(1mm2)当たりに換算した値である。 As described above, the tempered martensite phase with a major axis of 3 μm or more is polished at a thickness of 1/4 of the thickness cross section along the rolling direction of the steel sheet, and 3 mass% picral and 3 mass% pyrosulfite. After etching with a mixed solution of soda, observe with an optical microscope at 1000 times magnification, and superimpose circles with a diameter of 3 μm on the tempered martensite phase grains present in the 50 μm x 50 μm square area obtained by image analysis If the crystal fits in a circle (the circle does not have a full circle) and is smaller than the circle, the tempered martensite with a major axis of less than 3 μm is used. Phased. The number of tempered martensite phases with a major axis of 3 μm or more is a value obtained by calculating the number of tempered martensite phases with a major axis of 3 μm or more and converting per unit area (1 mm 2 ).
また、以下の方法で引張特性、伸びフランジ性、曲げ性を調査し、材料特性を評価した。 In addition, the tensile properties, stretch flangeability and bendability were investigated by the following methods to evaluate the material properties.
引張特性：圧延方向と90°の方向を長手方向(引張方向)とするJISZ2201に準拠した5号試験片を用い、JISZ2241に準拠した引張試験を行った。そして、YS、TS、YR、El、TS×Elを求めた。 Tensile properties: No. 5 test piece conforming to JISZ2201 with the rolling direction and 90 ° direction as the longitudinal direction (tensile direction) was used, and a tensile test based on JISZ2241 was performed. YS, TS, YR, El, and TS × El were obtained.
伸びフランジ性：日本鉄鋼連盟規格JFST1001に基づき、直径d0=10mmの穴を打抜き、60°の円錐ポンチを上昇させ穴拡げ試験を行い、亀裂が板厚貫通したところでポンチ上昇を止め、亀裂貫通後の打抜き穴径dを測定し、穴拡げ率λ(%)=[(d-d0)/d0]×100を算出した。試験は同一の鋼板について3回実施し、平均のλとTS×λを求めた。 Stretch flangeability: Based on the Japan Iron and Steel Federation standard JFST1001, punch holes with a diameter of d 0 = 10 mm, raise a 60 ° conical punch, and conduct a hole expansion test. The subsequent punched hole diameter d was measured, and the hole expansion ratio λ (%) = [(dd 0 ) / d 0 ] × 100 was calculated. The test was performed three times on the same steel sheet, and average λ and TS × λ were obtained.
曲げ性：下記のように、JISZ2204に準拠した3号試験片を用い、JISZ2248に準拠した曲げ角度90°のVブロック法による曲げ試験を行った。なお、同一番号の鋼板について３枚の曲げ試験片を用意し、各々の試験片について曲げ試験を行い、曲げ先端の割れが３枚とも観察されない場合を割れ無し、１枚でも観察された場合を割れ有りとして評価した。曲げ評価方法の詳細は以下のとおりである。すなわち、曲げ部の稜線と圧延方向が平行になるように40mm×150mm(サンプルの長手が圧延直角方向)のサンプルを採取し、先端曲げR=2.0mmの金型を用いて、下死点での決め押し荷重3トン（3000kgf）で90°V曲げの試験を行い、曲げ先端における割れの有無を目視判定した。 Bendability: Using a No. 3 test piece conforming to JISZ2204 as described below, a bending test was conducted by the V-block method with a bending angle of 90 ° conforming to JISZ2248. In addition, three bending test pieces are prepared for the steel plates with the same number, a bending test is performed for each test piece, and when no cracks at the bending tip are observed, no cracks are observed, and even when one is observed. Evaluated as having cracks. Details of the bending evaluation method are as follows. That is, a sample of 40 mm x 150 mm (the length of the sample is perpendicular to the rolling direction) is taken so that the ridge line of the bent part and the rolling direction are parallel, and a die with a tip bending R = 2.0 mm is used at the bottom dead center. A 90 ° V-bending test was performed at a determined pushing load of 3 tons (3000 kgf), and the presence or absence of cracks at the bending tip was visually judged.
結果を表3に示す。 The results are shown in Table 3.
本発明例は、980MPa以上のTS、28%以上のEl、65%以上のYRを有し、かつTS×λが20000MPa・%以上で伸びフランジ性に優れるとともに、曲げ先端の割れも無く曲げ性にも優れていることがわかる。 Example of the present invention has TS of 980 MPa or more, El of 28% or more, YR of 65% or more, and TS × λ is 20000 MPa ·% or more and has excellent stretch flangeability and bendability without cracking at the bending tip. It turns out that it is also excellent.
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