JP3592490B2 - High ductility and high strength steel sheet with excellent low temperature toughness - Google Patents
High ductility and high strength steel sheet with excellent low temperature toughness Download PDFInfo
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Description
【０００１】
【発明が属する技術分野】
本発明は高延性高強度鋼板であり、特に低温靱性に優れ、主として自動車用の内板、足回り部品などの強度部材をはじめ、衝突安全性に必要な耐衝撃用部材などに用いられ、さらには高い強度と加工性が要求される建築用や機械構造用鋼板としても用いられる。
【０００２】
【従来の技術】
自動車用鋼板の軽量化と衝突時の安全性確保を背景として、鋼板の高強度化の要請は強い。さらにまた、高強度化とともにプレス成形性についての要求も高く、強度と加工性を両立させる鋼板が必要となっている。
【０００３】
高強度鋼の延性を高める方法としては、特開昭５５−１４５１２１号公報などに記載されているように、残留オーステナイト（残留γと記載する）の変形誘起塑性を利用する方法がある。さらに特開平４−２２８５３８号公報などには、フェライトの体積率と粒径などの組織を制御し、この残留γの変形誘起塑性を向上させる工夫が示されている。また、特開平６−２６４１８３号公報などには、残留γ鋼板の欠点である穴拡げ特性や低温靱性などの劣化を防止するため、Ｓｉ添加量を抑えてＡｌを添加することが記載され、特開平５−１７１３４５号公報などには、残留γを微細化することにより穴拡げ性を向上させることが提案されている。
【０００４】
【発明が解決しようとする課題】
これらの残留γ鋼板により、強度、延性の向上、さらには局部延性ともいえる穴拡げ性の向上が図られたものの、低温靱性の改善が不十分であり、低温靱性に優れた高延性高強度鋼板が強く求められている。
【０００５】
【課題を解決するための手段】
従来の残留γ鋼板は、ポリゴナル・フェライトを主体とした組織を有しており、延性を向上させる残留γはフェライト・マトリックス内やベイナイト組織の境界にブロック状に存在している。この形態の残留γは鋼板全体に加工が加わった場合に、マトリックスとの塑性様式が異なるためより大きな歪みが発生し、オーステナイト（γ）からマルテンサイトに変態し易く、このため歪み誘起変態の程度が大きくなり、延性向上の効果をもたらすものと考えられる。その一方で、この残留γ周辺ではマトリックスと異なる変形のために不均一変形が生じ易く、局部延性などは劣化し、さらに低温での変形や衝撃については、この悪影響はさらに顕著になり低温靱性が低下するものと推察される。
【０００６】
本発明者らは、かかる残留γ鋼板の変形メカニズムに鑑み、組織を従来のポリゴナル・フェライトや第２相組織より微細な下部組織（微細炭化物、ベイニテイック・フェライトなど）を有するベイナイト組織を主体として構成し、ベイナイト粒の内部に残留γを生成させることにより、ベイナイトを構成するベイニテイック・フェライトや微細炭化物と残留γとの塑性変形能の違いを残留γの歪み誘起変態によって吸収緩和させ、変形時のベイナイト粒内のボイドの生成をおさえて、ベイナイト粒そのものの延性を向上させることにより、延性とともに低温靱性の向上を図ることに成功したものである。
【０００７】
すなわち、本発明の鋼板は、重量％で、Ｃ ：０．１０〜０．２２％、Ｓｉ：０．５〜２．０％、Ｍｎ：１．０〜１．８１％、Ａｌ：１．０％以下を含み、残部Ｆｅ及び不可避的不純物からなり、面積％で、ベイナイトが６０％以上好ましくは７０％以上、残留γが１〜２０％、残部実質的にフェライトからなる組織を有し、残留γがベイナイト粒内に存在することを特徴とする。
【０００８】
ここで、組織限定理由について説明すると、ベイナイト量が６０％未満ではポリゴナル・フェライト組織が多くなるため、微小領域での変形が不均一となったり、ベイナイト粒外すなわちフェライト粒内やベイナイトとフェライトの粒界に生成する残留γが多くなり、低温靱性が劣化するようになる。また、残留γが１％未満ではベイナイト粒内の残留γも過少となり、ベイナイトの延性向上作用が不足するようになる。一方、残留γが２０％を越えると、ベイナイト粒内の残留γによりベイナイトの延性が向上するものの、ベイナイト粒外に生成する残留γも多くなるため低温靱性が劣化するようになる。
【０００９】
なお、残留γがベイナイト粒内に存在するようにするには、熱延鋼板では仕上圧延後に４００〜５００℃までを急冷し、冷延鋼板では冷間圧延後の再結晶焼鈍後に４００〜５００℃までを急冷すればよく、ベイナイト量が６０％以上、残留γ量が２０％以下であれば、生成した残留γの大部分はベイナイト粒内に存在するものと考えられる。
【００１０】
ベイナイト粒内の残留γの存在形態は、ベイナイト粒内に在る限り、ベイナイト組織中のベイニテイック・フェライトのラス間に生成していてもよく、また微細炭化物の間やその周囲に生成していてもよく、その形状も塊状やフィルム状であってもよい。
【００１１】
本発明の鋼板は、上記ベイナイトを主体とする特定の組織を有しており、その鋼成分（重量％）を以下に説明する。
【００１２】
Ｃ：０．１０〜０．２２％
Ｃはオーステナイト安定化元素であり、Ａｒ３点以上の温度ではγに固溶しているが、熱延仕上げ以後の冷却や巻取中でもγの分解を抑える作用がある。また、ベイナイト中に残存した残留γに濃化することによって化学的に安定化したγが１％〜２０％残存するようになり、変形誘起塑性により成形性を良好にする。Ｃ量が０．１０％未満ではγは安定化せず、熱延以後のプロセスにおいて分解するようになる。一方、０．２５％を超えて添加した場合には、溶接性を劣化させるばかりでなく、低温靱性も急激に劣化する。このため、下限を０．１０％、上限を０．２２％とする。
【００１３】
Ｓｉ：０．５〜２．０％
Ｓｉは鋼をオーステナイトから冷却した場合にセメンタイト等の析出を抑え、冷却中の変態を抑える作用がある。このために熱延以後の冷却や巻取中にγを残存しやすくし、さらにこの残留γへのＣ濃化を促進する。Ｓｉ量が０．５％未満では十分な残留γが残らず、また２．０％を超えて添加する場合には、変態の抑制効果が高くなるためベイナイト主体の組織を生じにくい。このため、下限を０．５％、上限を２．０％とする。
【００１４】
Ｍｎ：１．０〜１．８１％
Ｍｎはγ形成元素であり、γがパーライトに分解するのを抑える。このため、残留γへのＣ濃化を促進し、さらにベイナイト組織の生成に有効である。この効果を得るためには、１．０％以上必要である。また、３．０％を超えて添加する場合には、バンド組織が著しくなるため、割れや欠陥による加工不良が発生するようになる。加工不良を十分に防止するため、上限を１．８１とする。
【００１５】
Ａｌ：１．０％以下
ＡｌはＳｉと同様にセメンタイト等の析出を抑え、未変態γへのＣ濃縮を促進する作用があり、残留γ生成のためのもう一つの重要な元素である。しかし、過剰に添加した場合には、鋼中に酸化物系介在物が生成し易くなり、加工性を劣化させる。このために、上限を１．０％とする。
【００１６】
鋼成分としては以上の成分を必須成分とし、残部Ｆｅ及び不可避的不純物で形成される。不純物であるＰおよびＳについては以下の理由により含有量を抑えることが望ましい。
【００１７】
Ｐは高強度化に有効で必要に応じて添加され、しかも残留γ量に対してとりわけ悪影響を及ぼす元素ではない。しかし、過剰に添加すると、低温靱性を劣化させるとともに、スポット溶接性を劣化させる。好ましくは０．０１％以下に止めるのがよい。
【００１８】
Ｓは鋼中に硫化物系介在物を形成する元素であり、過剰に添加すると加工性を劣化させるので、好ましくは０．０１％以下に止めるのがよい。
【００２１】
本発明鋼板の製造方法としては、熱延鋼板の場合、仕上温度を８８１〜８８５℃として熱間圧延を終了し、仕上圧延後から巻取までを４１〜８２℃／ｓで急冷し、４００〜５００℃で巻取る。また、冷延鋼板の場合、常法で熱間圧延および冷間圧延を行い、再結晶焼鈍後、４００〜５００℃まで急冷し、同温度で２０〜４０分程度保持後、空冷する。そのほか、合金化溶融Ｚｎめっき鋼板としても製造可能であり、めっき条件は特に限定されない。
【００２２】
【実施例】
表１に示す化学成分の鋼を試験溶解炉にて溶製し、実験室にて１１００℃に加熱後、８００℃以上９００℃以下の仕上温度にて熱間圧延を終了し、およそ１０℃／ｓ〜８０℃／ｓまでの冷却速度にて４００〜５５０℃の温度域まで冷却し、この温度に３０〜８０分保持の巻取処理を行って熱延鋼板を作製した。
【００２３】
また、表１の鋼種Ｎｏ． ３および鋼種Ｎｏ． ４については実験室にて通常の熱延後（加熱温度：１２００℃、仕上温度：８５０℃、巻取温度：５５０℃）、８０％の冷間圧延を施し、９２０〜９３０℃にて焼純加熱後、５０℃／ｓの急冷により、４５０℃（過時効温度）まで冷却し、この温度で３０分保持後、空冷した冷延鋼板も作製した。
【００２４】
得られた熱延鋼板、冷延鋼板につき、板厚ｔのｔ／４部における組織をレペラー腐食にて腐食し、ベイナイト粒内の残留γ組織の有無とベイナイトの面積％（残部実質的にフェライト）を観察した。また、Ｘ線測定により全体の残留γ量を測定した。これらの結果を製造条件と共に表２に示す。
【００２５】
【表１】
【表２】
表２において、試料Ｎｏ． １〜７は基本成分の鋼Ｎｏ． ３を用い、またＮｏ． １０〜１７は鋼Ｎｏ． ２や鋼Ｎｏ． ４を用いて熱延の冷却速度や巻取温度を変化させたものである。また、試料Ｎｏ． ２６や試料Ｎｏ． ２７は鋼Ｎｏ． ８および鋼Ｎｏ． ９を用いた例で、Ｐ量やＡｌ量が多い場合である。試料Ｎｏ． ２８や試料Ｎｏ． ２９は好適成分を有するものの、残留γがベイナイト中ではなくフェライト中や粒界に生成する例である。試料Ｎｏ． ３２と試料Ｎｏ． ３３は冷延鋼板の例を示す。
【００２８】
それぞれの鋼板からＪＩＳ５号試験片を採取して引張試験によりＴＳ（引張強さ）及びＥｌ（伸び）を求めた。また、同様に鋼板からシャルピー試験片を採取して、延性破面率５０％の ＶＴｒＳ（遷移温度）と ＶＥ−２０ （−２０℃における吸収エネルギー）を求めた。さらに、６０ｍｍＷ×６０ｍｍＬの穴拡げ試験片を採取し、中央にパンチ穴（初期穴）を開けて頂角６０℃の円錐ポンチにてパンチ穴を押し拡げて、下記式によってλ値を求めた。これらの値を表３に示す。
λ値（％）＝１００×（穴拡げ後の穴径−初期穴径）／初期穴径
【００２９】
【表３】
表２および表３の内、残留γ（γＲ ）が０％および２５％超のもの（試料Ｎｏ． ６〜９，２２〜２５，３０及び３１）およびＢ（ベイナイト）量が６０％以上でもベイナイト粒内にγＲ が存在しないもの（試料Ｎｏ． ２８，２９）、すなわち明らかに発明範囲外の試料を除く他の試料について、Ｂ量の区分に従って、γＲ と ＶＴｒＳとの関係を整理したものを図１に、残留γ（γＲ ）とＴＳ×Ｅｌとの関係を整理したものを図２に示す。
【００３１】
図１より、ベイナイト量が６０％未満では、残留γが多く生成しても遷移温度にあまり変化はないが、ベイナイト量が６０％以上かつ残留γ量が２０％以下では遷移温度が低くなり、低温靱性が良好であることがわかる。一方、ＴＳ×Ｅｌ値は、図２に示すとおり、残留γ量が１％以上であれば組織の違いによらず良好であることが認められる。なお、残留γ量が０％のもの（試料Ｎｏ． ６〜９、２２〜２５、３０及び３１）では、ＴＳ×Ｅｌ値は１５０００（Ｎ／ｍｍ２ ・％）程度であり、延性の劣化が著しい。
【００３２】
【発明の効果】
本発明の鋼板によれば、特定の鋼組成を基に、組織をベイナイトを主体とし、残留γ量を１〜２０％とし、残留γがベイナイト粒内に存在するので、微小な領域においても組織の均一な変形が可能となり、高延性、高強度を有し、しかも優れた低温靱性を兼備することができる。
【図面の簡単な説明】
【図１】実施例におけるＢ（ベイナイト）量の区分に従って、γＲ （残留オーステナイト）と ＶＴｒＳ（遷移温度）との関係を整理したグラフを示す。
【図２】実施例におけるＢ量の区分に従って、γＲ とＴＳ×Ｅｌとの関係を整理したグラフを示す。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention is a high-ductility high-strength steel sheet, particularly excellent in low-temperature toughness, mainly used for strength members such as inner plates for automobiles, undercarriage parts, impact-resistant members necessary for collision safety, and the like. Is also used as a steel sheet for construction and mechanical structures requiring high strength and workability.
[0002]
[Prior art]
With the background of reducing the weight of automotive steel sheets and ensuring safety during collisions, there is a strong demand for higher strength steel sheets. Furthermore, there is a high demand for press formability as well as high strength, and a steel sheet that achieves both strength and workability is required.
[0003]
As a method for increasing the ductility of a high-strength steel, there is a method utilizing the deformation-induced plasticity of retained austenite (described as residual γ) as described in Japanese Patent Application Laid-Open No. 55-145121. Further, Japanese Patent Application Laid-Open No. 4-228538 and the like show a device for controlling the structure such as the volume ratio and the grain size of ferrite and improving the deformation-induced plasticity of the residual γ. In addition, Japanese Patent Application Laid-Open No. 6-264183 and the like disclose that in order to prevent deterioration of hole expansion characteristics and low-temperature toughness, which are defects of a residual γ steel plate, Al is added while suppressing the amount of added Si. Japanese Patent Application Laid-Open No. 5-171345 proposes improving the hole expandability by reducing the residual γ.
[0004]
[Problems to be solved by the invention]
These residual γ steel sheets have improved strength, ductility, and even hole expandability, which can be called local ductility, but have insufficient improvement in low-temperature toughness and high-ductility, high-strength steel sheets with excellent low-temperature toughness. Is strongly required.
[0005]
[Means for Solving the Problems]
The conventional residual γ steel sheet has a structure mainly composed of polygonal ferrite, and the residual γ for improving ductility exists in a block shape in the ferrite matrix or at the boundary of the bainite structure. In the form of residual γ, when the entire steel sheet is worked, a larger strain is generated due to a difference in plasticity with the matrix, and the transformation is easy to transform from austenite (γ) to martensite. Is increased, and the effect of improving ductility is considered to be obtained. On the other hand, non-uniform deformation is likely to occur around the residual γ due to deformation different from the matrix, local ductility and the like deteriorate, and furthermore, regarding low-temperature deformation and impact, this adverse effect becomes even more pronounced and low-temperature toughness decreases. It is inferred that it will decrease.
[0006]
In view of the deformation mechanism of the residual γ steel sheet, the present inventors mainly constituted a bainite structure having a substructure (fine carbide, bainitic ferrite, etc.) finer than the conventional polygonal ferrite or the second phase structure. Then, by generating residual γ inside the bainite grains, the difference in plastic deformability between the residual γ and bainitic ferrite or fine carbide constituting bainite is absorbed and relaxed by strain-induced transformation of the residual γ, By suppressing the formation of voids in the bainite grains and improving the ductility of the bainite grains themselves, they succeeded in improving ductility and low-temperature toughness.
[0007]
That is, in the steel sheet of the present invention, C: 0.10 to 0.22%, Si: 0.5 to 2.0%, Mn: 1.0 to 1.81%, Al: 1.0% by weight. %, And the balance is composed of Fe and unavoidable impurities, and has an area% of bainite of 60% or more, preferably 70% or more, a residual γ of 1 to 20%, and a balance substantially composed of ferrite. γ is present in bainite grains.
[0008]
Here, the reasons for the structure limitation will be described. If the bainite amount is less than 60%, the polygonal ferrite structure increases, so that the deformation in the minute region becomes nonuniform or the bainite grains are not uniform, ie, inside the ferrite grains or between bainite and ferrite. The residual γ generated at the grain boundary increases, and the low-temperature toughness deteriorates. If the residual γ is less than 1%, the residual γ in the bainite grains will be too small, and the effect of improving the ductility of bainite will be insufficient. On the other hand, if the residual γ exceeds 20%, the ductility of the bainite is improved by the residual γ in the bainite grains, but the residual γ generated outside the bainite grains also increases, so that the low-temperature toughness deteriorates.
[0009]
In order to make residual γ exist in the bainite grains, the hot-rolled steel sheet is rapidly cooled to 400 to 500 ° C. after finish rolling, and the cold-rolled steel sheet is 400 to 500 ° C. after recrystallization annealing after cold rolling. If the amount of bainite is 60% or more and the amount of residual γ is 20% or less, most of the generated residual γ is considered to be present in the bainite grains.
[0010]
The existence form of the residual γ in the bainite grains may be generated between the laths of bainitic ferrite in the bainite structure, as long as they are present in the bainite grains, and may be formed between and around the fine carbides. And the shape may be a lump or a film.
[0011]
The steel sheet of the present invention has a specific structure mainly composed of bainite, and its steel component (% by weight) will be described below.
[0012]
C: 0.10 to 0.22 %
C is an austenite stabilizing element, and forms a solid solution with γ at a temperature of Ar 3 points or more, but has an effect of suppressing the decomposition of γ even during cooling or winding after hot rolling. Further, by concentrating to the residual γ remaining in the bainite, 1% to 20% of chemically stabilized γ remains, and the formability is improved by deformation-induced plasticity. If the C content is less than 0.10%, γ will not be stabilized and will be decomposed in the process after hot rolling. On the other hand, if it exceeds 0.25%, not only does the weldability deteriorate, but also the low-temperature toughness rapidly deteriorates. Therefore, the lower limit is 0.10% and the upper limit is 0.22% .
[0013]
Si: 0.5 to 2.0%
Si has an effect of suppressing precipitation of cementite and the like when the steel is cooled from austenite and suppressing transformation during cooling. For this reason, γ is likely to remain during cooling or winding after hot rolling, and furthermore, the concentration of C into the residual γ is promoted. If the amount of Si is less than 0.5%, sufficient residual γ does not remain. If the amount of Si exceeds 2.0%, the effect of suppressing transformation is increased, so that a bainite-based structure is not easily formed. Therefore, the lower limit is set to 0.5% and the upper limit is set to 2.0%.
[0014]
Mn: 1.0 to 1.81 %
Mn is a γ-forming element and suppresses γ from being decomposed into pearlite. Therefore, it promotes the concentration of C into the residual γ, and is effective for generating a bainite structure. To obtain this effect, 1.0% or more is required. In addition, when the content exceeds 3.0%, the band structure becomes remarkable, so that processing failure due to cracks or defects occurs. In order to sufficiently prevent processing defects, the upper limit is set to 1.81.
[0015]
Al: 1.0% or less Al has the effect of suppressing the precipitation of cementite and the like and promoting the enrichment of C into untransformed γ like Si, and is another important element for the generation of residual γ. However, when added excessively, oxide inclusions are liable to be formed in the steel, thereby deteriorating the workability. For this reason, the upper limit is set to 1.0%.
[0016]
As a steel component, the above components are essential components, and the balance is formed by Fe and unavoidable impurities. It is desirable to reduce the contents of P and S, which are impurities, for the following reasons.
[0017]
P is effective for increasing the strength and is added as needed, and is not an element that particularly adversely affects the residual γ content. However, excessive addition deteriorates low-temperature toughness and deteriorates spot weldability. Preferably, the content is limited to 0.01% or less.
[0018]
S is an element that forms sulfide-based inclusions in steel. If added excessively, the workability is deteriorated. Therefore, the content of S is preferably set to 0.01% or less.
[0021]
As a manufacturing method of the steel sheet of the present invention, in the case of a hot-rolled steel sheet, the finishing temperature is set to 881 to 885 ° C., hot rolling is completed, and from finish rolling to winding is rapidly cooled at 41 to 82 ° C./s , Wind at 500 ° C. In the case of a cold-rolled steel sheet, hot rolling and cold rolling are performed in a usual manner, and after recrystallization annealing, the steel sheet is rapidly cooled to 400 to 500 ° C., kept at the same temperature for about 20 to 40 minutes, and then air-cooled. In addition, it can be manufactured as an alloyed hot-dip galvanized steel sheet, and the plating conditions are not particularly limited.
[0022]
【Example】
Steel having the chemical composition shown in Table 1 was melted in a test melting furnace, heated to 1100 ° C. in a laboratory, and then hot-rolled at a finishing temperature of 800 ° C. to 900 ° C. The steel sheet was cooled to a temperature range of 400 to 550 ° C. at a cooling rate of s to 80 ° C./s, and a winding treatment was performed at this temperature for 30 to 80 minutes to produce a hot-rolled steel sheet.
[0023]
Further, in Table 1, steel type No. 3 and steel type No. As for No. 4, after normal hot rolling in a laboratory (heating temperature: 1200 ° C., finishing temperature: 850 ° C., winding temperature: 550 ° C.), cold rolling of 80% was performed, and ingot at 920 to 930 ° C. After the heating, the steel sheet was cooled to 450 ° C. (overage temperature) by rapid cooling at 50 ° C./s. After maintaining at this temperature for 30 minutes, a cold-rolled steel sheet cooled by air was also produced.
[0024]
With respect to the obtained hot-rolled steel sheet and cold-rolled steel sheet, the structure at t / 4 part of the sheet thickness t is corroded by repeller corrosion, the presence or absence of residual γ structure in the bainite grains, and the area% of bainite (substantially ferrite ) Was observed. Further, the total amount of residual γ was measured by X-ray measurement. Table 2 shows these results together with the production conditions.
[0025]
[Table 1]
[Table 2]
In Table 2, the sample No. Steel Nos. 1 to 7 are basic components. No. 3 and No. 3 Steel Nos. 10 to 17 are steel Nos. 2 and steel No. 4, the cooling rate of hot rolling and the winding temperature were changed. Further, the sample No. 26 and sample no. No. 27 is steel No. 8 and steel no. 9 is an example using a large amount of P or Al. Sample No. 28 and sample no. 29 is an example in which, although having a suitable component, residual γ is formed not in bainite but in ferrite or at grain boundaries. Sample No. 32 and sample no. 33 shows the example of a cold rolled steel plate.
[0028]
A JIS No. 5 test piece was sampled from each steel sheet, and TS (tensile strength) and El (elongation) were determined by a tensile test. Similarly, a Charpy test piece was sampled from a steel sheet, and V Trs (transition temperature) and V E- 20 (absorbed energy at −20 ° C.) at a ductile fracture ratio of 50% were determined. Further, a hole-expanded test piece of 60 mmW × 60 mmL was sampled, a punched hole (initial hole) was opened in the center, and the punched hole was pushed and expanded with a conical punch having a vertex angle of 60 ° C., and the λ value was determined by the following equation. Table 3 shows these values.
λ value (%) = 100 × (hole diameter after hole expansion−initial hole diameter) / initial hole diameter
[Table 3]
Of Tables 2 and 3, those having a residual γ (γ R ) of 0% or more than 25% (Sample Nos. 6 to 9, 22 to 25, 30, and 31) and B (bainite) content of 60% or more The relationship between γ R and V T rS was determined according to the amount of B for the samples in which γ R was not present in the bainite grains (Sample Nos. 28 and 29), that is, other samples except those clearly outside the scope of the invention. FIG. 1 shows the arrangement and FIG. 2 shows the arrangement of the residual γ (γ R ) and TS × El.
[0031]
According to FIG. 1, when the amount of bainite is less than 60%, the transition temperature does not change much even if a large amount of residual γ is generated. However, when the amount of bainite is 60% or more and the amount of residual γ is 20% or less, the transition temperature becomes low. It turns out that low temperature toughness is good. On the other hand, as shown in FIG. 2, it is recognized that the TS × El value is good irrespective of the difference in the structure when the residual γ amount is 1% or more. In the case where the amount of residual γ is 0% (Sample Nos. 6 to 9, 22 to 25, 30, and 31), the TS × El value is about 15000 (N / mm 2 ·%), and the ductility is deteriorated. Remarkable.
[0032]
【The invention's effect】
According to the steel sheet of the present invention, based on a specific steel composition, the structure is mainly bainite, the amount of residual γ is 1 to 20%, and the residual γ is present in the bainite grains. Can be uniformly deformed, have high ductility and high strength, and also have excellent low-temperature toughness.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between γ R (retained austenite) and V T rS (transition temperature) according to the B (bainite) content in Examples.
According the amount of B segment in Figure 2 Example shows a graph organizing the relationship between gamma R and TS × El.
Claims (1)
Ｃ ：０．１０〜０．２２％、
Ｓｉ：０．５〜２．０％、
Ｍｎ：１．０〜１．８１％、
Ａｌ：１．０％以下
を含み、残部Ｆｅ及び不可避的不純物からなり、面積％で、ベイナイトが６０％以上、残留γが１〜２０％、残部実質的にフェライトからなる組織を有し、残留γがベイナイト粒内に存在することを特徴とする低温靱性に優れた高延性高強度鋼板。In weight percent,
C: 0.10 to 0.22%,
Si: 0.5 to 2.0%,
Mn: 1.0 to 1.81%,
Al: contains 1.0% or less, the balance consists of Fe and inevitable impurities, and has an area% of bainite of 60% or more, a residual γ of 1 to 20%, and a balance substantially consisting of ferrite. A high-ductility, high-strength steel sheet excellent in low-temperature toughness, characterized in that γ exists in bainite grains.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP19316597A JP3592490B2 (en) | 1997-07-02 | 1997-07-02 | High ductility and high strength steel sheet with excellent low temperature toughness |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP19316597A JP3592490B2 (en) | 1997-07-02 | 1997-07-02 | High ductility and high strength steel sheet with excellent low temperature toughness |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH1121653A JPH1121653A (en) | 1999-01-26 |
| JP3592490B2 true JP3592490B2 (en) | 2004-11-24 |
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| JP4806887B2 (en) * | 2002-06-18 | 2011-11-02 | Ｊｆｅスチール株式会社 | Steel material excellent in fatigue crack propagation characteristics and method for producing the same |
| JP3764411B2 (en) | 2002-08-20 | 2006-04-05 | 株式会社神戸製鋼所 | Composite steel sheet with excellent bake hardenability |
| ES2755414T3 (en) | 2011-07-29 | 2020-04-22 | Nippon Steel Corp | High strength steel sheet excellent in impact resistance and manufacturing method thereof, and high strength galvanized steel sheet and manufacturing method thereof |
| JP5728115B1 (en) * | 2013-09-27 | 2015-06-03 | 株式会社神戸製鋼所 | High strength steel sheet excellent in ductility and low temperature toughness, and method for producing the same |
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