JP5573265B2 - High strength thick steel plate excellent in ductility with a tensile strength of 590 MPa or more and method for producing the same - Google Patents
High strength thick steel plate excellent in ductility with a tensile strength of 590 MPa or more and method for producing the same Download PDFInfo
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本発明は、橋梁、建築、造船、海洋構造物、貯蔵タンク、圧力容器、ラインパイプ等に供して好適な引張強度５９０ＭＰａ以上で板厚１２ｍｍ以上の高強度厚鋼板およびその製造方法に係り、特に、母材の低温靭性と延性に優れるものに関する。 The present invention relates to a high-strength steel plate having a tensile strength of 590 MPa or more and a plate thickness of 12 mm or more suitable for use in bridges, buildings, shipbuilding, offshore structures, storage tanks, pressure vessels, line pipes, etc. , Relating to a base material having excellent low-temperature toughness and ductility.
橋梁、建築、貯蔵タンク、圧力容器、ラインパイプなどの鉄鋼構造物に用いられる鋼板は、強度が高いことは勿論、安全性の観点から高い靭性や延性が要求される。例えば、建築用鋼材の場合、低降伏比でかつ一様伸びを大きくして塑性変形能を高めて、耐震性を確保している。 Steel sheets used for steel structures such as bridges, buildings, storage tanks, pressure vessels, and line pipes are required to have high toughness and ductility from the viewpoint of safety as well as high strength. For example, in the case of steel for construction, the low yield ratio and uniform elongation are increased to enhance the plastic deformability and ensure the earthquake resistance.
一方で、ラインパイプなどでは、全伸び（一様伸び＋局部伸び）が大きいことが要求されている。全伸びが大きいことは外部からの応力により変形が始まってから破壊するまでに変形する量が大きいことを意味し、鋼材の破壊に対する安全性の指標となっている。 On the other hand, a line pipe or the like is required to have a large total elongation (uniform elongation + local elongation). A large total elongation means that the amount of deformation from the start of deformation to the failure due to external stress is large, and is an indicator of safety against the destruction of steel materials.
引張試験片の標点距離が長いほど、全伸びに占める一様伸びの比率は大きくなるが、一般的に使用されている引張試験片では、局部伸びの割合も４０〜５０％程度のことが多く、一様伸びと局部伸びの両者を共に大きくして全伸びを大きくすることが望ましい。 The longer the gauge distance of a tensile test piece, the larger the ratio of uniform elongation to the total elongation. However, in a generally used tensile test piece, the ratio of local elongation is about 40 to 50%. In many cases, it is desirable to increase both the uniform elongation and the local elongation to increase the total elongation.
一般に、鋼材が高強度化するにしたがって、一様伸び、局部伸びともに低下し、特に引張強度が５９０ＭＰａを超えるとその傾向が顕著になるため、ミクロ組織や成分組成の調整による優れた全伸びを備えた引張強度５９０ＭＰａ級鋼板が種々提案されている。 In general, as steel materials increase in strength, both uniform elongation and local elongation decrease. Especially when the tensile strength exceeds 590 MPa, the tendency becomes remarkable. Various provided tensile strength 590 MPa grade steel sheets have been proposed.
特許文献１は、一様伸びの優れた高強度鋼板の製造方法に関し、オーステナイトの再結晶温度域で圧延終了後、２相域での冷却を制御することにより成長したフェライト＋マルテンサイト組織とし、降伏比の低下により、一様伸びを向上させている。 Patent Document 1 relates to a method for producing a high-strength steel sheet having excellent uniform elongation, and has a ferrite + martensite structure grown by controlling cooling in a two-phase region after rolling in the recrystallization temperature region of austenite, Uniform elongation is improved by lowering the yield ratio.
特許文献２は、残留オーステナイトを生成させ全伸びを向上させた厚鋼板およびその製造方法に関し、薄鋼板における、残留オーステナイトのＴＲＩＰ効果が厚鋼板で得られるように残留オーステナイト量と残留オーステナイト中のＣ量を調整することを特徴とする。 Patent Document 2 relates to a thick steel plate in which residual austenite is generated and the total elongation is improved, and a method for producing the same. It is characterized by adjusting the amount.
また、特許文献３は、一様伸びに優れた高強度鋼に関し、Ｃｕ析出を利用することにより、伸びが向上することが報告されている。強化粒子自体の塑性変形能が高く、周囲のＦｅより軟質な強化粒子として、９Ｒ構造を有するＣｕ粒子と、ｂｃｃ構造を有するＣｕ粒子またはｆｃｃ構造を有するＣｕ粒子のいずれかを複合的に分散させることにより、ミクロ的な不均一変形が抑制されるためと考えられている。 Moreover, patent document 3 is related to the high strength steel excellent in uniform elongation, and it is reported that elongation improves by utilizing Cu precipitation. The reinforcing particles themselves have high plastic deformability and are softer than the surrounding Fe, and as a reinforcing particle softer, either Cu particles having 9R structure and Cu particles having bcc structure or Cu particles having fcc structure are dispersed in a composite manner. Therefore, it is considered that microscopic nonuniform deformation is suppressed.
しかしながら、特許文献１に記載された技術では、フェライト粒が粗大化するために、低温靭性は良好とは言えず、また、ミクロ組織が不均一であることから、局部伸びが著しく低下する恐れがある。 However, in the technique described in Patent Document 1, since ferrite grains are coarsened, the low-temperature toughness cannot be said to be good, and since the microstructure is non-uniform, local elongation may be significantly reduced. is there.
特許文献２に記載された技術では、合金成分コストが高くなることや、溶接性が著しく劣化することが問題である。 In the technique described in Patent Document 2, there are problems that the cost of the alloy components becomes high and the weldability is remarkably deteriorated.
特許文献３に記載された技術では、概ね１％以上のＣｕの多量添加が不可欠であり、圧延中の鋼板表面割れ等、表面性状が劣化するため、実用鋼としての実現可能性が低い。 In the technique described in Patent Document 3, it is indispensable to add a large amount of Cu of approximately 1% or more, and the surface properties such as surface cracking of the steel sheet during rolling deteriorate, so that the feasibility as practical steel is low.
そこで、本発明は、経済性に優れ、優れた低温靭性と延性を兼備した引張強度５９０ＭＰａ以上の高強度厚鋼板およびその製造方法を提供することを目的とする。 Therefore, an object of the present invention is to provide a high-strength thick steel plate having a tensile strength of 590 MPa or more, which is excellent in economic efficiency and has both excellent low-temperature toughness and ductility, and a method for producing the same.
本発明者らは、厚鋼板を対象として優れた低温靭性と５９０ＭＰａ以上の引張強度の確保を前提に、全伸びを決定する各種要因に関して鋭意研究を行い、以下の知見を得た。 The present inventors diligently researched various factors that determine the total elongation on the premise of securing excellent low temperature toughness and tensile strength of 590 MPa or more for thick steel plates, and obtained the following knowledge.
１．５９０ＭＰａ以上の引張強度と優れた低温靭性を確保しつつ、一様伸びを向上するためには、軟質相であるポリゴナルフェライト相と硬質相である上部ベイナイト相の混合組織とし、各相の面積分率を制御することが重要である。軟質相として加工フェライトが、また硬質相として下部ベイナイトやマルテンサイトが混入した場合には、伸びに対して不利である。
２．ポリゴナルフェライト相と上部ベイナイト相の混合組織とすることにより一様伸びを向上させ、さらに局部伸びを向上させるため、各相の存在形態を厳格に制御することが必要である。すなわち、ポリゴナルフェライト相は、平均結晶粒径を１８μｍ以下かつ結晶粒径の標準偏差を８μｍ以下に制御する。
In order to improve the uniform elongation while ensuring a tensile strength of 1.590 MPa or more and excellent low temperature toughness, a mixed structure of a polygonal ferrite phase that is a soft phase and an upper bainite phase that is a hard phase is used. It is important to control the area fraction. When processed ferrite is mixed as a soft phase and lower bainite or martensite is mixed as a hard phase, it is disadvantageous for elongation.
2. In order to improve the uniform elongation by making the mixed structure of the polygonal ferrite phase and the upper bainite phase and further improve the local elongation, it is necessary to strictly control the existence form of each phase. That is, the polygonal ferrite phase controls the average crystal grain size to 18 μm or less and the standard deviation of crystal grain size to 8 μm or less.
均一で細粒の結晶粒主体の組織として、粗大な結晶粒を抑制することによって、引張試験時における、粗大結晶粒近傍の結晶粒界への応力集中が低減し、早期のマイクロボイドの発生が抑制されて、局部伸びが向上する。 Suppressing coarse crystal grains as a uniform and fine-grained grain structure reduces stress concentration at the grain boundaries in the vicinity of coarse crystal grains during tensile testing, resulting in early microvoids. It is suppressed and local elongation is improved.
上部ベイナイト相は、組織中に含まれる島状マルテンサイトの面積分率を５％以下とする。極度に硬い脆化組織である島状マルテンサイトと母相の界面は、引張試験時に応力が集中しやすく、マイクロボイドの発生が助長され、局部伸びに不利である。
３．上記混合組織の達成には、鋼板組成だけでなく、熱間圧延および加速冷却、さらには再加熱焼戻し条件を厳格に管理することが重要である。
The upper bainite phase has an area fraction of island martensite contained in the structure of 5% or less. The interface between the island-like martensite, which is an extremely hard embrittled structure, and the matrix phase tends to concentrate stress during the tensile test, and the generation of microvoids is promoted, which is disadvantageous for local elongation.
3. In order to achieve the above mixed structure, it is important to strictly manage not only the steel sheet composition but also hot rolling and accelerated cooling, and further, reheating and tempering conditions.
本発明は、得られた知見に、さらに検討を加えてなされたもので、すなわち、本発明は、
１．質量％で、
Ｃ：０．０３〜０．２０％
Ｓｉ：０．０５〜０．６０％
Ｍｎ：０．３〜２．０％
Ｐ：０．０１５％以下
Ｓ：０．００３％以下
Ａｌ：０．０７％以下
Ｎ：０．０１％以下
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と、
ポリゴナルフェライト相と上部ベイナイト相の混合組織であって、ポリゴナルフェライト相の面積分率が１０〜４５％、平均結晶粒径が１８μｍ以下、結晶粒径の標準偏差が８μｍ以下で、上部ベイナイト相中の島状マルテンサイトの面積分率が５％以下となるミクロ組織を備えたことを特徴とする引張強度５９０ＭＰａ以上の延靭性に優れた高強度厚鋼板。
２．鋼組成に、質量％で、更に、
Ｃｕ：１．５％以下
Ｎｉ：２．０％以下
Ｃｒ：１．０％以下
Ｍｏ：１．０％以下
Ｎｂ：０．１％以下
Ｖ：０．１％以下
Ｔｉ：０．０３％以下
Ｂ：０．００５％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記載の引張強度５９０ＭＰａ以上の延靭性に優れた高強度厚鋼板。
３．鋼組成に、質量％で、更に、
ＲＥＭ：０．０２％以下
Ｃａ：０．００５％以下
Ｍｇ：０．００５％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または２記載の引張強度５９０ＭＰａ以上の延靭性に優れた高強度厚鋼板。
４．１乃至３のいずれか一つに記載した鋼組成からなる鋳片または鋼片を、１０００〜１２５０℃に再加熱後、９００℃以下での累積圧下率が３０％以上で、かつＡｒ３〜Ａｒ３＋８０℃の温度で終了する熱間圧延を行い、Ａｒ３−６０℃〜Ａｒ３−１０℃の温度まで放冷をした後、３〜１００℃／ｓで４００〜６００℃まで加速冷却を行うことを特徴とする引張強度５９０ＭＰａ以上の延靭性に優れた高強度厚鋼板の製造方法。
５．加速冷却後、更に、４００〜６５０℃で焼戻すことを特徴とする４記載の引張強度５９０ＭＰａ以上の延靭性に優れた高強度厚鋼板の製造方法。
The present invention has been made by further studying the obtained knowledge, that is, the present invention
1. % By mass
C: 0.03-0.20%
Si: 0.05-0.60%
Mn: 0.3 to 2.0%
P: 0.015% or less S: 0.003% or less Al: 0.07% or less N: 0.01% or less, the steel composition comprising the balance of Fe and inevitable impurities,
A mixed structure of a polygonal ferrite phase and an upper bainite phase, wherein the area fraction of the polygonal ferrite phase is 10 to 45%, the average crystal grain size is 18 μm or less, and the standard deviation of the crystal grain size is 8 μm or less. A high-strength thick steel plate excellent in ductility having a tensile strength of 590 MPa or more, comprising a microstructure in which the area fraction of island-like martensite in the phase is 5% or less.
2. In steel composition by mass%,
Cu: 1.5% or less Ni: 2.0% or less Cr: 1.0% or less Mo: 1.0% or less Nb: 0.1% or less V: 0.1% or less Ti: 0.03% or less B A high strength thick steel plate excellent in ductility with a tensile strength of 590 MPa or more according to 1, characterized by containing one or more of 0.005% or less.
3. In steel composition by mass%,
REM: 0.02% or less Ca: 0.005% or less Mg: 0.005% or less 1 type or 2 types or more characterized by excellent tensile toughness of 590 MPa or more according to 1 or 2 High strength thick steel plate.
4. After reheating the slab or slab comprising the steel composition described in any one of 4.1 to 3 to 1000 to 1250 ° C., the cumulative rolling reduction at 900 ° C. or less is 30% or more, and Ar 3 After performing hot rolling finished at a temperature of ˜Ar 3 + 80 ° C. and allowing to cool to a temperature of Ar 3 -60 ° C. to Ar 3 -10 ° C., accelerated cooling to 400 to 600 ° C. at 3 to 100 ° C./s A method for producing a high-strength thick steel plate having excellent tensile toughness with a tensile strength of 590 MPa or more.
5. 5. The method for producing a high-strength thick steel plate excellent in ductility with a tensile strength of 590 MPa or more according to 4, wherein the steel is further tempered at 400 to 650 ° C. after accelerated cooling.
本発明によれば、引張強度５９０ＭＰａ以上で、低温靭性および全伸びに優れた、板厚１２ｍｍ以上の厚鋼板が得られ、鋼構造物の大型化、鋼構造物の耐震性および安全性の向上に大きく寄与し、産業上格段の効果を奏する。 According to the present invention, a steel plate having a tensile strength of 590 MPa or more, excellent low-temperature toughness and total elongation, and a plate thickness of 12 mm or more can be obtained, increasing the size of the steel structure, and improving the earthquake resistance and safety of the steel structure. It contributes greatly to the industry and has a remarkable industrial effect.
本発明では成分組成とミクロ組織を規定する。成分組成の説明において％は質量％、ミクロ組織の説明において％は面積％とする。
［成分組成］
Ｃ：０．０３〜０．２０％
Ｃは、鋼の強度を増加させ、構造用鋼材として必要な強度を確保するために必要な元素でその効果を得るため、０．０３％以上の含有を必要とする。一方、０．２０％を超える含有は、低温靭性を顕著に劣化させるとともに、上部ベイナイト中の島状マルテンサイトの面積分率が上昇し、局部伸びの低下により全伸びを低下させるため、０．０３〜０．２０％の範囲に限定する。好ましくは、０．０４〜０．１８％である。
In the present invention, the component composition and the microstructure are defined. In the description of the component composition,% means mass%, and in the description of the microstructure,% means area%.
[Ingredient composition]
C: 0.03-0.20%
C increases the strength of the steel and needs to be contained in an amount of 0.03% or more in order to obtain the effect with an element necessary for ensuring the strength required as a structural steel material. On the other hand, the content exceeding 0.20% significantly deteriorates the low temperature toughness and increases the area fraction of island martensite in the upper bainite, and decreases the total elongation due to the decrease in local elongation. The range is limited to 0.20%. Preferably, it is 0.04 to 0.18%.
Ｓｉ：０．０５〜０．６０％
Ｓｉは、脱酸材として作用し、製鋼上、少なくとも０．０５％必要である。一方、０．６０％を超えて含有すると、母材の靭性、溶接部の低温割れ性が顕著に劣化するだけでなく、島状マルテンサイトの生成を助長して、局部伸びの低下により全伸びを低下させるため、０．０５〜０．６０％の範囲に限定する。好ましくは、０．１０〜０．５５％である。
Si: 0.05-0.60%
Si acts as a deoxidizer and needs to be at least 0.05% for steelmaking. On the other hand, if the content exceeds 0.60%, not only the toughness of the base metal and the low temperature cracking property of the welded portion are remarkably deteriorated, but also the formation of island martensite is promoted, and the total elongation is reduced due to the decrease in local elongation. In order to reduce the content, it is limited to the range of 0.05 to 0.60%. Preferably, it is 0.10 to 0.55%.
Ｍｎ：０．３〜２．０％
Ｍｎは、鋼の焼入れ性を増加させる効果を有し、母材の強度を確保するために０．３％以上は必要である。一方、２．０％を超えて含有すると、母材の靭性、延性および溶接性が著しく劣化するため、０．３〜２．０％の範囲に限定する。好ましくは、０．４〜１．９％である。
Mn: 0.3 to 2.0%
Mn has the effect of increasing the hardenability of steel, and 0.3% or more is necessary to ensure the strength of the base material. On the other hand, if the content exceeds 2.0%, the toughness, ductility and weldability of the base material are significantly deteriorated, so the content is limited to the range of 0.3 to 2.0%. Preferably, it is 0.4 to 1.9%.
Ｐ：０．０１５％以下、Ｓ：０．００３％以下
Ｐは鋼の強度を増加させ靭性を劣化させるとともに、島状マルテンサイトの生成を助長して、局部伸びの低下により全伸びを低下させるため、０．０１５％を上限とする。可能なかぎり低減することが望ましいが、過度の低減は精錬コストを高騰させ経済的に不利となるため、０．００１％以上とすることが望ましい。
P: 0.015% or less, S: 0.003% or less P increases the strength of steel and deteriorates toughness, promotes the formation of island martensite, and reduces the total elongation due to a decrease in local elongation. Therefore, the upper limit is 0.015%. Although it is desirable to reduce it as much as possible, excessive reduction raises the refining cost and is economically disadvantageous, so 0.001% or more is desirable.
Ｓは母材の低温靭性や延性を劣化させるため、０．００３％を上限として可能なかぎり低減することが望ましい。 Since S deteriorates the low-temperature toughness and ductility of the base material, it is desirable to reduce 0.003% as much as possible.
Ａｌ：０．０７％以下
Ａｌは、脱酸剤として作用し、鋼材の溶鋼脱酸プロセスに於いて、もっとも汎用的に使われる。また、鋼中のＮをＡｌＮとして固定し、母材および溶接部の靭性向上に寄与する。一方、０．０７％を超えて含有すると、母材の靭性が低下するとともに、溶接時に溶接金属部に混入して、溶接金属の靭性を劣化させるため、０．０７％以下に限定する。
Al: 0.07% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel. Further, N in the steel is fixed as AlN, which contributes to improvement of the toughness of the base material and the welded portion. On the other hand, if the content exceeds 0.07%, the toughness of the base metal is lowered and mixed into the weld metal part during welding to deteriorate the toughness of the weld metal, so the content is limited to 0.07% or less.
Ｎ：０．０１％以下
Ｎは不可避的不純物として鋼中に含まれ、０．０１％を超えて含有すると、母材および溶接部靭性が著しく低下するため、０．０１％以下に限定する。
N: 0.01% or less N is contained in steel as an unavoidable impurity, and if it exceeds 0.01%, the toughness of the base metal and the welded portion is remarkably reduced, so it is limited to 0.01% or less.
以上が、本発明の基本成分組成であるが、更に、特性を向上させるため、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｎｂ、Ｖ、Ｔｉ、Ｂ、ＲＥＭ、Ｃａ、Ｍｇの１種または２種以上を含有することができる。 The above is the basic component composition of the present invention, but in order to further improve the characteristics, one or more of Cu, Ni, Cr, Mo, Nb, V, Ti, B, REM, Ca, Mg are added. Can be contained.
Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｎｂ、Ｖ、Ｔｉ、Ｂは、いずれも鋼の強度向上に寄与する元素であり、所望する強度に応じて適宜含有できる。
Ｃｕ：１．５％以下
Ｃｕを添加する場合は、０．１％以上とすることが好ましいが、１．５％を超えると熱間脆性を生じて鋼板の表面性状を劣化させるため、１．５％以下とする。
Cu, Ni, Cr, Mo, Nb, V, Ti, and B are all elements that contribute to improving the strength of steel, and can be appropriately contained depending on the desired strength.
Cu: 1.5% or less When Cu is added, the content is preferably 0.1% or more. However, if it exceeds 1.5%, hot brittleness is caused to deteriorate the surface properties of the steel sheet. 5% or less.
Ｎｉ：２．０％以下
Ｎｉを添加する場合は、０．１％以上とすることが好ましいが、２．０％を超えると効果が飽和し、経済的に不利になるため、２．０％以下とする。
Ni: 2.0% or less When adding Ni, it is preferable to make it 0.1% or more, but if it exceeds 2.0%, the effect is saturated and economically disadvantageous, so 2.0% The following.
Ｃｒ：１．０％以下
Ｃｒを添加する場合は、０．０５％以上とすることが好ましいが、１．０％を超えて含有すると、母材靭性、延性および溶接性が著しく劣化するため、１．０％以下とする。
Cr: 1.0% or less When adding Cr, it is preferable to be 0.05% or more, but if it exceeds 1.0%, the base material toughness, ductility and weldability deteriorate significantly, 1.0% or less.
Ｍｏ：１．０％以下
Ｍｏを添加する場合は、０．０５％以上とすることが好ましいが、１．０％を超えると、母材靭性、延性および耐溶接割れ性に悪影響を及ぼすため、１．０％以下とする。
Mo: 1.0% or less When adding Mo, 0.05% or more is preferable, but if it exceeds 1.0%, the base material toughness, ductility and weld crack resistance are adversely affected. 1.0% or less.
Ｎｂ：０．１％以下
Ｎｂを添加する場合は、０．００５％以上とすることが好ましいが、０．１％を超えると、母材靭性および延性を劣化させるため、０．１％以下とする。
Nb: 0.1% or less When Nb is added, the content is preferably 0.005% or more. However, if it exceeds 0.1%, the base material toughness and ductility are deteriorated. To do.
Ｖ：０．１％以下
Ｖを添加する場合は、０．０１％以上とすることが好ましいが、０．１％を超えると、母材靭性および延性を劣化させるため、０．１％以下とする。
V: 0.1% or less When adding V, it is preferably 0.01% or more. However, if it exceeds 0.1%, the base material toughness and ductility are deteriorated. To do.
Ｔｉ：０．０３％以下
Ｔｉは、Ｎとの親和力が強く凝固時にＴｉＮとして析出し、溶接熱影響部でのオーステナイト粒の粗大化を抑制して高靭化に寄与する添加元素である。一方、０．０３％を超えて添加するとＴｉＮ粒子が粗大化して、母材および溶接部靭性を劣化させるため、０．０３％以下とする。
Ti: 0.03% or less Ti is an additive element that has a strong affinity for N and precipitates as TiN during solidification and contributes to high toughness by suppressing coarsening of austenite grains in the weld heat affected zone. On the other hand, if added over 0.03%, TiN particles become coarse and deteriorate the base material and weld toughness, so the content is made 0.03% or less.
Ｂ：０．００５％以下
Ｂは、焼入れ性の向上を介して、鋼の強度を増加させる作用を有するので添加する場合は０．０００５％以上とすることが好ましい。一方、０．００５％を超える含有は焼入れ性を著しく増加させ、母材の靭性、延性の劣化をもたらすため、０．００５％以下とする。
B: 0.005% or less B has an effect of increasing the strength of the steel through the improvement of hardenability, so when added, it is preferably made 0.0005% or more. On the other hand, if the content exceeds 0.005%, the hardenability is remarkably increased and the toughness and ductility of the base material are deteriorated, so the content is made 0.005% or less.
ＲＥＭ、ＣａおよびＭｇは、いずれも靭性向上に寄与し、所望する特性に応じて選択して添加する。 REM, Ca, and Mg all contribute to the improvement of toughness, and are selected and added according to desired characteristics.
ＲＥＭ：０．０２％以下
ＲＥＭを添加する場合は、０．００２％以上とすることが好ましいが、０．０２％を超えても効果が飽和するため、０．０２％を上限とする。
REM: 0.02% or less When REM is added, the content is preferably 0.002% or more. However, if the content exceeds 0.02%, the effect is saturated, so 0.02% is made the upper limit.
Ｃａ：０．００５％以下
Ｃａを添加する場合は、０．０００５％以上とすることが好ましいが、０．００５％を超えても効果が飽和するため、０．００５％を上限とする。
Ca: 0.005% or less When Ca is added, the content is preferably 0.0005% or more, but even if it exceeds 0.005%, the effect is saturated, so 0.005% is made the upper limit.
Ｍｇ：０．００５％以下
Ｍｇを添加する場合は、０．００１％以上とすることが好ましいが、０．００５％を超えても効果が飽和するため、０．００５％を上限とする。
Mg: 0.005% or less When adding Mg, the content is preferably 0.001% or more. However, if the content exceeds 0.005%, the effect is saturated, so 0.005% is made the upper limit.
上記成分以外の残部は、Ｆｅおよび不可避的不純物である。 The balance other than the above components is Fe and inevitable impurities.
［ミクロ組織］
本発明では、引張強度５９０ＭＰａ以上と優れた低温靭性を両立させ、さらに高い一様伸びを確保するため、ミクロ組織を軟質相のポリゴナルフェライト相と硬質相の上部ベイナイト相の混合組織（複相組織）とする。
[Microstructure]
In the present invention, in order to achieve both a tensile strength of 590 MPa or more and excellent low temperature toughness, and to secure a high uniform elongation, the microstructure is a mixed structure of a polygonal ferrite phase of a soft phase and an upper bainite phase of a hard phase (double phase). Organization).
また、局部伸びを向上させるため、ポリゴナルフェライト相の結晶粒径と上部ベイナイト相中の島状マルテンサイト量を規定する。 Further, in order to improve the local elongation, the crystal grain size of the polygonal ferrite phase and the amount of island martensite in the upper bainite phase are defined.
ポリゴナルフェライト相の面積分率が１０％未満では、一様伸びを上昇する効果が得られない。一方、面積分率が４５％を超えると引張強度が低下し、引張強度５９０ＭＰａ以上を満足できない。このため、ポリゴナルフェライト相の面積分率は１０〜４５％とする。 If the area fraction of the polygonal ferrite phase is less than 10%, the effect of increasing uniform elongation cannot be obtained. On the other hand, if the area fraction exceeds 45%, the tensile strength decreases and the tensile strength of 590 MPa or more cannot be satisfied. For this reason, the area fraction of the polygonal ferrite phase is 10 to 45%.
ポリゴナルフェライト相の平均結晶粒径が１８μｍより大きければ、低温靭性が劣化するだけでなく、引張試験時にはマイクロボイドの発生を助長し、局部伸びが低下する。このためポリゴナルフェライト相の平均結晶粒径は１８μｍ以下とする。 If the average crystal grain size of the polygonal ferrite phase is larger than 18 μm, not only the low temperature toughness is deteriorated, but also microvoids are promoted during the tensile test, and the local elongation is lowered. Therefore, the average crystal grain size of the polygonal ferrite phase is set to 18 μm or less.
また、ポリゴナルフェライト相の結晶粒径の標準偏差が８μｍより大きければ、粗大粒が存在することになり、引張試験時に粗大結晶粒近傍の結晶粒界に歪が集中して、マイクロボイドの発生を助長し、局部伸びが低下する。このため、ポリゴナルフェライト相の結晶粒径の標準偏差は８μｍ以下とする。 In addition, if the standard deviation of the grain size of the polygonal ferrite phase is larger than 8 μm, coarse grains will exist, and strain will concentrate at the grain boundaries near the coarse grains during the tensile test, generating microvoids. And local growth is reduced. For this reason, the standard deviation of the crystal grain size of the polygonal ferrite phase is 8 μm or less.
上部ベイナイト相中の島状マルテンサイトの面積分率が５％を超えると、引張試験時に島状マルテンサイトと母相の界面に歪が集中することになり、マイクロボイドの発生を助長し、局部伸びが低下する。このため、上部ベイナイト相中の島状マルテンサイトの面積分率は５％以下とする。 If the area fraction of island martensite in the upper bainite phase exceeds 5%, strain concentrates on the interface between the island martensite and the parent phase during the tensile test, which promotes the generation of microvoids and increases the local elongation. descend. For this reason, the area fraction of island martensite in the upper bainite phase is set to 5% or less.
混合組織は、本発明の作用効果を損なわない程度に他の組織を少量含むものであってもよい。軟質相として、加工フェライトが混在すると一様伸びが低下するが面積分率で５％以下の場合には影響が無視できる。硬質相として、パーライトが混在すると強度低下を招き、下部ベイナイトやマルテンサイトが混在すると局部伸びを低下させるが、パーライト、下部ベイナイトおよびマルテンサイト等を合計した硬質第２相組織の面積分率が５％以下の場合には影響が無視できる。 The mixed tissue may contain a small amount of other tissues to the extent that the effects of the present invention are not impaired. When processed ferrite is mixed as a soft phase, the uniform elongation decreases, but the influence can be ignored when the area fraction is 5% or less. When the pearlite is mixed as the hard phase, the strength is reduced, and when the lower bainite and martensite are mixed, the local elongation is decreased. However, the area fraction of the hard second phase structure including the pearlite, the lower bainite, and martensite is 5 The effect is negligible when it is less than%.
以上の説明において、面積分率とは鋼板断面のミクロ組織から測定される領域中の平均の面積分率を示す。ポリゴナルフェライト相の同定は、例えば、鋼板の圧延方向と平行な断面について、ミクロ組織観察用サンプルを採取し、ナイタール腐食の後、倍率４００倍の光学顕微鏡で観察される組織を撮影し、画像解析装置を用いて行う。 In the above description, the area fraction indicates the average area fraction in the region measured from the microstructure of the steel sheet cross section. For example, the polygonal ferrite phase is identified by taking a microstructure observation sample for a cross section parallel to the rolling direction of the steel sheet, photographing the structure observed with an optical microscope at a magnification of 400 times after the nital corrosion, Use an analysis device.
島状マルテンサイトの面積分率は、試料にレペラ腐食（ＪＯＵＲＮＡＬ ＯＦ ＭＥＴＡＬＳ、Ｍａｒｃｈ、１９８０、ｐ．３８−３９）を実施して倍率１０００倍の光学顕微鏡で観察される組織を撮影し、画像解析装置を用いて求めた。 The area fraction of island-like martensite was measured by imaging the structure observed with an optical microscope at a magnification of 1000 times after performing a repeller corrosion (JOURNAL OF METALS, March, 1980, p.38-39) on the sample. It was determined using an apparatus.
本発明鋼の好ましい製造条件は以下の様である。説明において、温度に関する「℃」表示は、板厚の１／２における温度を意味するものとする。 Preferred production conditions for the steel of the present invention are as follows. In the description, the “° C.” display relating to the temperature means a temperature at half the plate thickness.
鋼素材加熱温度：１０００℃〜１２５０℃
上述した組成の鋳片または鋼片の鋼素材を転炉、電気炉、真空溶解炉等、通常公知の方法による溶鋼から製造し、１０００℃〜１２５０℃に再加熱する。
Steel material heating temperature: 1000 ° C-1250 ° C
A steel material of a slab or steel slab having the above-described composition is produced from molten steel by a generally known method such as a converter, electric furnace, vacuum melting furnace, etc., and reheated to 1000 ° C to 1250 ° C.
再加熱温度が１０００℃未満では、熱間圧延での変形抵抗が高く、１パス当たりの圧下量が大きく取れず、圧延パス数が増加し、圧延能率の低下を招くとともに、鋼素材（スラブ）中の鋳造欠陥を圧着することができない場合が生じる。 If the reheating temperature is less than 1000 ° C., the deformation resistance in hot rolling is high, the amount of rolling per pass cannot be increased, the number of rolling passes increases, the rolling efficiency decreases, and the steel material (slab) There are cases where the casting defects inside cannot be crimped.
一方、再加熱温度が１２５０℃を超えると、加熱時のスケールによって表面疵が生じやすく、圧延後の手入れ負荷が増大する。また、結晶粒が粗大化して所望のミクロ組織が得られず、局部伸びが低下して全伸びが低下するため、１０００〜１２５０℃の範囲とする。 On the other hand, when the reheating temperature exceeds 1250 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. Moreover, since a crystal grain coarsens and a desired micro structure is not obtained and local elongation falls and total elongation falls, it is set as the range of 1000-1250 degreeC.
熱間圧延：９００℃以下での累積圧下率が３０％以上、圧延終了温度：Ａｒ３〜Ａｒ３＋８０℃
９００℃以下での累積圧下率が３０％未満では、靭性が劣化するとともに、オーステナイト粒への加工歪の導入が不足し、最終組織のポリゴナルフェライト粒の平均結晶粒径が大きくなるとともに、粗大粒が混在して、局部伸びの低下により全伸びが低下する。
Hot rolling: Cumulative rolling reduction at 900 ° C. or lower is 30% or more, Rolling end temperature: Ar 3 to Ar 3 + 80 ° C.
If the cumulative rolling reduction at 900 ° C. or less is less than 30%, the toughness is deteriorated, the introduction of processing strain into the austenite grains is insufficient, the average crystal grain size of the polygonal ferrite grains in the final structure is increased, and the coarseness is increased. Grains are mixed, and the total elongation is reduced due to a decrease in local elongation.
圧延終了温度がＡｒ３＋８０℃より高い場合には、オーステナイト粒への加工歪の導入が不足し、最終組織のポリゴナルフェライト粒の平均結晶粒径が大きくなるだけでなく、粗大粒が混在して、局部伸びの低下により全伸びが低下する。 When the rolling end temperature is higher than Ar 3 + 80 ° C., the introduction of processing strain into the austenite grains is insufficient, and not only the average crystal grain size of the polygonal ferrite grains in the final structure becomes large but also coarse grains are mixed. Thus, the total elongation decreases due to the decrease in local elongation.
一方、圧延終了温度がＡｒ３未満の場合、初析フェライトを圧延するので、転位を含む加工フェライトが生成して、一様伸びの低下により全伸びが低下するので、圧延終了温度はＡｒ３〜Ａｒ３＋８０℃とする。 On the other hand, if the rolling end temperature is less than Ar 3, since rolling the pro-eutectoid ferrite generates the deformed ferrite containing dislocations, because the total elongation is decreased by a decrease in uniform elongation, the finish rolling temperature is Ar 3 ~ Ar 3 + 80 ° C.
板厚が７０ｍｍを超える極厚鋼板の場合には、ザク圧着のために１パスあたりの圧下率が１５％以上となる圧延パスを少なくとも１パス以上確保することが望ましい。 In the case of an extremely thick steel plate having a plate thickness exceeding 70 mm, it is desirable to secure at least one rolling pass at which the rolling reduction per pass is 15% or more for the Zaku pressure bonding.
熱間圧延後の冷却：熱間圧延後、Ａｒ３−６０℃〜Ａｒ３−１０℃の温度まで放冷をした後、冷却速度３〜１００℃／ｓで４００〜６００℃まで加速冷却
熱間圧延後の放冷をＡｒ３−１０℃よりも高い温度で終了して加速冷却を開始すると、ポリゴナルフェライトが得られず、一様伸びの低下により全伸びが低下する。
Cooling after hot rolling: After hot rolling, after cooling to a temperature of Ar 3 -60 ° C to Ar 3 -10 ° C, accelerated cooling to 400 to 600 ° C at a cooling rate of 3 to 100 ° C / s When cooling after rolling is finished at a temperature higher than Ar 3 -10 ° C. and accelerated cooling is started, polygonal ferrite cannot be obtained, and the total elongation is lowered due to a decrease in uniform elongation.
一方、Ａｒ３−６０℃未満まで放冷した場合には、ポリゴナルフェライト量が過剰になるとともに、硬質第２相として粗大なパーライトが混在するようになり、引張強度が低下する。このため、熱間圧延後の放冷は、Ａｒ３−６０℃〜Ａｒ３−１０℃の範囲とする。 On the other hand, when it is allowed to cool to less than Ar 3 −60 ° C., the amount of polygonal ferrite becomes excessive, and coarse pearlite is mixed as the hard second phase, resulting in a decrease in tensile strength. Therefore, cooling after hot rolling is in the range of Ar 3 -60 ℃ ~Ar 3 -10 ℃ .
放冷後の加速冷却の冷却速度が３℃／ｓ未満では、ポリゴナルフェライト量が過剰になるとともに、硬質第２相として粗大なパーライトが混在し、引張強度が低下する。 When the cooling rate of accelerated cooling after standing to cool is less than 3 ° C./s, the amount of polygonal ferrite becomes excessive, coarse pearlite is mixed as the hard second phase, and the tensile strength is lowered.
一方、冷却速度が１００℃／ｓを超えると、鋼板内の各位置における温度制御が困難となり、引張特性などの材質上のばらつきが生じる。 On the other hand, when the cooling rate exceeds 100 ° C./s, it becomes difficult to control the temperature at each position in the steel sheet, resulting in variations in materials such as tensile properties.
加速冷却の停止温度が６００℃よりも高いと、ポリゴナルフェライト量が過剰になるとともに、硬質第２相として粗大なパーライトが混在し、引張強度が低下する。一方、４００℃未満になると、硬質第２相に下部ベイナイトやマルテンサイトが混入し、局部伸びの低下を介して全伸びが低下する。以上より、加速冷却は冷却速度：３〜１００℃／ｓで冷却停止温度：４００〜６００℃とする。加速冷却の停止後は、室温まで放冷する。 When the accelerated cooling stop temperature is higher than 600 ° C., the amount of polygonal ferrite becomes excessive, coarse pearlite is mixed as the hard second phase, and the tensile strength is lowered. On the other hand, if it becomes less than 400 degreeC, a lower bainite and a martensite will mix in a hard 2nd phase, and total elongation will fall through the fall of local elongation. From the above, accelerated cooling is performed at a cooling rate of 3 to 100 ° C./s and a cooling stop temperature of 400 to 600 ° C. After accelerating cooling stops, it is allowed to cool to room temperature.
なお、Ａｒ３（℃）は例えば、下式で求めることができる。
Ａｒ３＝９１０−３１０Ｃ−８０Ｍｎ−２０Ｃｕ−１５Ｃｒ−５５Ｎｉ−８０Ｍｏ
（元素記号は鋼材中の各元素の含有量（質量％）を表す）
本発明では、加速冷却後、焼戻し処理を施しても良い。焼戻し温度は、母材の靭性および延性を向上させるため４００℃以上とし、一方、６５０℃を超えると母材強度が大幅に低下するため、４００〜６５０℃で行うことが望ましい。保持時間は母材強度が低下しないように板厚によって適宜調整する。板厚１０〜１００ｍｍの場合、１ｈｒを超えると、母材強度が大幅に低下するので１ｈｒ以内とするが、熱処理炉内の均熱状態が良ければ、短時間の保持でもかまわない。
Ar 3 (° C.) can be obtained by the following equation, for example.
Ar 3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
(The element symbol represents the content (% by mass) of each element in the steel)
In the present invention, tempering treatment may be performed after accelerated cooling. The tempering temperature is set to 400 ° C. or higher in order to improve the toughness and ductility of the base material. On the other hand, if it exceeds 650 ° C., the strength of the base material is greatly reduced. The holding time is appropriately adjusted depending on the plate thickness so that the strength of the base material does not decrease. In the case of a plate thickness of 10 to 100 mm, if it exceeds 1 hr, the strength of the base material is significantly reduced, so that it is within 1 hr. However, if the soaking state in the heat treatment furnace is good, it may be held for a short time.
転炉−取鍋精錬−連続鋳造法で、種々の成分組成の鋼スラブを、熱間圧延により板厚１２ｍｍ以上の鋼板とした後、加速冷却し、一部の鋼板には、更に焼戻し処理を施した。表１に供試鋼の成分組成を示す。 After converting steel slabs of various component compositions into steel plates with a thickness of 12 mm or more by hot rolling by converter-ladder refining-continuous casting method, accelerated cooling is performed, and some steel plates are further tempered. gave. Table 1 shows the component composition of the test steel.
得られた鋼板の板厚１／２位置で圧延方向と直角な方向から、ＪＩＳ４号引張試験片を採取し、ＪＩＳＺ２２４１（１９９８年）の既定に準拠して引張試験を実施し、引張特性を調査した。 JIS No. 4 tensile test specimens were sampled from the direction perpendicular to the rolling direction at the plate thickness 1/2 position of the obtained steel sheet, and tensile tests were conducted in accordance with JISZ2241 (1998) defaults to investigate the tensile properties. did.
各鋼板の板厚１／２位置で圧延方向と直角な方向から、ＪＩＳＺ２２０２（１９９８年）の規定に準拠してＶノッチ試験片を採取し、ＪＩＳＺ２２４２（１９９８年）の規定に準拠してシャルピー衝撃試験を実施し、延性−脆性破面遷移温度（ｖＴｒｓ）を求め、母材靭性を評価した。ミクロ組織観察用の試験片を、板厚１／２位置から採取して、先に述べた方法で、ミクロ組織を観察した。 V-notch test specimens were collected from the direction perpendicular to the rolling direction at a thickness 1/2 position of each steel plate in accordance with JISZ2202 (1998) and Charpy impact in accordance with JISZ2242 (1998). A test was conducted to determine the ductile-brittle fracture surface transition temperature (vTrs), and the base material toughness was evaluated. A specimen for microstructural observation was taken from a position of 1/2 the plate thickness, and the microstructure was observed by the method described above.
表２に製造条件（熱間圧延、加速冷却）およびミクロ組織観察結果を、表３に引張特性とシャルピー衝撃試験結果を示す。表２において、組織形態の欄のＦはポリゴナルフェライト相、ＵＢは上部ベイナイト相、ＬＢは下部ベイナイト相、Ｐはパーライト相をそれぞれ示している。なお、表２、３の鋼Ｎｏ．は共通とする。 Table 2 shows manufacturing conditions (hot rolling, accelerated cooling) and microstructure observation results, and Table 3 shows tensile properties and Charpy impact test results. In Table 2, F in the column of structure form indicates a polygonal ferrite phase, UB indicates an upper bainite phase, LB indicates a lower bainite phase, and P indicates a pearlite phase. In Tables 2 and 3, the steel No. Are common.
発明例（鋼Ｎｏ．１−１、１−２、２、３−１、３−２、４−１、５、６−１、６−２、７、８）は、いずれも引張強度５９０ＭＰａ以上の高強度、ｖＴｒｓ＜−８０℃の高靭性、且つ全伸び３２％以上の高延性の母材特性を有することが確認された。
一方、比較例（鋼Ｎｏ．１−３〜１−７、３−３〜３−５、４−２、６−３〜６−６、９〜１３）は、強度、靭性、延性のいずれか、あるいは複数の特性が目標値を満足しない。
Inventive examples (steel Nos. 1-1, 1-2, 2, 3-1, 3-2, 4-1, 5, 6-1, 6-2, 7, 8) all have a tensile strength of 590 MPa or more. Of high strength, vTrs <−80 ° C., and high ductility base material characteristics with a total elongation of 32% or more.
On the other hand, Comparative Examples (Steel Nos. 1-3 to 1-7, 3-3 to 3-5, 4-2, 6-3 to 6-6, 9 to 13) are either strength, toughness, or ductility. Or a plurality of characteristics do not satisfy the target value.
Claims (5)
Ｃ：０．０３〜０．２０％
Ｓｉ：０．０５〜０．６０％
Ｍｎ：０．３〜２．０％
Ｐ：０．０１５％以下
Ｓ：０．００３％以下
Ａｌ：０．０７％以下
Ｎ：０．０１％以下
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と、
ポリゴナルフェライト相と上部ベイナイト相の混合組織であって，ポリゴナルフェライト相の面積分率が１０〜４５％、平均結晶粒径が１８μｍ以下，結晶粒径の標準偏差が８μｍ以下で、上部ベイナイト相中の島状マルテンサイトの面積分率が５％以下となるミクロ組織を備えたことを特徴とする引張強度５９０ＭＰａ以上の延靭性に優れた高強度厚鋼板。 % By mass
C: 0.03-0.20%
Si: 0.05-0.60%
Mn: 0.3 to 2.0%
P: 0.015% or less S: 0.003% or less Al: 0.07% or less N: 0.01% or less, the steel composition comprising the balance of Fe and inevitable impurities,
A mixed structure of a polygonal ferrite phase and an upper bainite phase, the area fraction of the polygonal ferrite phase is 10 to 45%, the average crystal grain size is 18 μm or less, and the standard deviation of the crystal grain size is 8 μm or less. A high-strength thick steel plate excellent in ductility having a tensile strength of 590 MPa or more, comprising a microstructure in which the area fraction of island-like martensite in the phase is 5% or less.
Ｃｕ：１．５％以下
Ｎｉ：２．０％以下
Ｃｒ：１．０％以下
Ｍｏ：１．０％以下
Ｎｂ：０．１％以下
Ｖ：０．１％以下
Ｔｉ：０．０３％以下
Ｂ：０．００５％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記載の引張強度５９０ＭＰａ以上の延靭性に優れた高強度厚鋼板。 In steel composition by mass%,
Cu: 1.5% or less Ni: 2.0% or less Cr: 1.0% or less Mo: 1.0% or less Nb: 0.1% or less V: 0.1% or less Ti: 0.03% or less B The high-strength thick steel plate excellent in ductility having a tensile strength of 590 MPa or more according to claim 1, comprising one or more of 0.005% or less.
ＲＥＭ：０．０２％以下
Ｃａ：０．００５％以下
Ｍｇ：０．００５％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または２記載の引張強度５９０ＭＰａ以上の延靭性に優れた高強度厚鋼板。 In steel composition by mass%,
3. REM: 0.02% or less, Ca: 0.005% or less, Mg: 0.005% or less, containing 1 type or 2 types or more, Tensile strength with a tensile strength of 590 MPa or more according to claim 1 or 2 Excellent high-strength thick steel plate.
Ａｒ ３ ＝９１０−３１０Ｃ−８０Ｍｎ−２０Ｃｕ−１５Ｃｒ−５５Ｎｉ−８０Ｍｏ
（元素記号は鋼材中の各元素の含有量（質量％）を表す。） A slab or steel slab comprising the steel composition according to any one of claims 1 to 3 is reheated to 1000 to 1250 ° C, and the cumulative reduction at 900 ° C or lower is 30% or more, and Ar 3 After performing hot rolling that ends at a temperature of ˜Ar 3 + 80 ° C. and allowing to cool to a temperature of Ar 3 −60 ° C. to Ar 3 −10 ° C. , cooling is stopped at 420 to 600 ° C. at 3 to 100 ° C./s. A mixed structure of a polygonal ferrite phase and an upper bainite phase characterized by accelerated cooling to a temperature, wherein the area fraction of the polygonal ferrite phase is 10 to 45%, the average crystal grain size is 18 μm or less, and the crystal grains A method for producing a high-strength thick steel sheet having a tensile strength of 590 MPa or more and having a microstructure in which the standard deviation of diameter is 8 μm or less and the area fraction of island martensite in the upper bainite phase is 5% or less . However, Ar 3 is obtained from the following formula.
Ar 3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
(The element symbol represents the content (mass%) of each element in the steel.)
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