JP2005307246A - High-tensile-strength steel with composite structure of fine crystal grain - Google Patents

High-tensile-strength steel with composite structure of fine crystal grain Download PDF

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JP2005307246A
JP2005307246A JP2004123325A JP2004123325A JP2005307246A JP 2005307246 A JP2005307246 A JP 2005307246A JP 2004123325 A JP2004123325 A JP 2004123325A JP 2004123325 A JP2004123325 A JP 2004123325A JP 2005307246 A JP2005307246 A JP 2005307246A
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steel
fine
strength
strength steel
crystal grain
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JP4408386B2 (en
Inventor
Masaaki Fujioka
政昭 藤岡
Shuji Aihara
周二 粟飯原
Mitsuru Sato
満 佐藤
Hiroshi Yaguchi
浩 家口
Takehiro Tsuchida
武広 土田
Masayuki Wakita
昌幸 脇田
Yasunobu Nagataki
康伸 長滝
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JFE Steel Corp
Kobe Steel Ltd
Nippon Steel Corp
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JFE Steel Corp
Kobe Steel Ltd
Nippon Steel Corp
Sumitomo Metal Industries Ltd
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-tensile-strength steel with a composite structure, which has such high strength and ductility as the product of yield strength×total elongation exceeds 14,000 MPa×%, and consists of fine crystal grains. <P>SOLUTION: The high-tensile-strength steel has a composition comprising, by mass%, 0.001-0.75% C, 0.01-0.5% Si, 0.1-5.0% Mn and the balance Fe with unavoidable impurities; and has a metallographic mixed structure consisting of 50-95 % ferrite by vol.% and the balance being martensite or tempered martensite, and consisting of grains with an average diameter of 2 μm or smaller. The high-tensile-strength steel further includes 0.1-3.0% Cu by mass%, and 0.1-3.0% metallic Cu-based precipitates by vol.%, with an average diameter of 5 nm or larger in the metallographic structure. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

本発明は、熱間圧延によって製造される鋼製品(薄鋼板、厚鋼板、線材、形鋼、棒鋼、鋼管など)において、その基本特性たる強度、靭性、延性に優れた、結晶粒の微細な複合組織高張力鋼に関するものである。   The present invention is a steel product (thin steel plate, thick steel plate, wire rod, shape steel, bar steel, steel pipe, etc.) manufactured by hot rolling, which has excellent basic properties such as strength, toughness, ductility, and fine crystal grains. It relates to a high-strength steel with a composite structure.

近年、鋼製品の軽量化、鋼構造物の使用条件の過酷化にともない、より強靭で安全性の高い鋼の開発が求められている。
この様な要求に対し、従来、鋼板の製造方法を改善して、金属組織の微細化(結晶粒の細粒化)を図り、鋼の強度、靭性を改善するための圧延方法が開発されてきた。この様な方法の例としては、いわゆる制御圧延法が上げられ、加速冷却法と組み合わせた製造法として、特許文献1や特許文献2などに開示されている。
また、より結晶粒を微細化する方法として、制御圧延法に比較してさらに低温かつ大歪で行うなどの方法が提案され、結晶粒を1μm以下に顕著に微細化できることが、特許文献3、特許文献4などに開示されている。
特開昭63−223124号公報 特開昭63−128117号公報 特開平11−032348号公報 特開2000−104115号公報
In recent years, with the reduction in weight of steel products and the harsh use conditions of steel structures, the development of stronger and safer steel has been demanded.
In response to such demands, a rolling method has been developed to improve the strength and toughness of steel by improving the steel sheet manufacturing method to refine the metal structure (crystal grains). It was. An example of such a method is a so-called controlled rolling method, which is disclosed in Patent Document 1 and Patent Document 2 as a manufacturing method combined with an accelerated cooling method.
Further, as a method for further refinement of crystal grains, a method such as performing at a lower temperature and larger strain compared to the controlled rolling method has been proposed, and it is possible to remarkably refine crystal grains to 1 μm or less. It is disclosed in Patent Document 4 and the like.
JP 63-223124 A JP 63-128117 A JP-A-11-032348 JP 2000-104115 A

しかし、上記の特許文献1、2に記載の発明で得られるフェライトの粒径は小さいといってもせいぜい5μm程度である。
また、上記特許文献3、4に記載の発明は、結晶粒の微細化のみに着目したものであった。従って、非常に微細な結晶粒を有する鋼という新しいコンセプトに対して、機械的特性の向上を目的とし、最も望ましい金属組織状態が実現できていない。
However, the particle size of the ferrite obtained by the inventions described in Patent Documents 1 and 2 is about 5 μm at most.
Further, the inventions described in Patent Documents 3 and 4 focus on only the refinement of crystal grains. Therefore, for the new concept of steel having very fine crystal grains, the most desirable metallographic state has not been realized for the purpose of improving mechanical properties.

そこで、超細粒鋼に最も適当な混合組織状態や析出物などの金属組織状態を解明し、これを実現することが求められている。
本発明は、特性向上の観点から最も適当な混合組織状態や析出物などの金属組織状態を有する、結晶粒の微細な複合組織高張力鋼を提供することを目的とするものである。
Therefore, it is required to clarify the mixed structure state and the metal structure state such as precipitates most suitable for ultrafine-grained steel and to realize this.
An object of the present invention is to provide a high-strength steel with a fine-grained composite structure having a most appropriate mixed structure state and metal structure state such as precipitates from the viewpoint of improving characteristics.

本発明者らは、上記の課題を解決するために、超細粒鋼における細粒組織の形成機構や特性について吟味し、以下の発明を創案した。
(1) 質量%で、
C :0.001〜0.75%、 Si:0.01〜0.5%、
Mn:0.1〜5.0%
を満たす成分を含有し、残部がFeおよび不可避的不純物からなる鋼で、その金属組織が体積%で50〜95%のフェライトと残部がマルテンサイトまたは焼き戻しマルテンサイトの混合組織であって、各々の粒径が平均直径で2μm以下であることを特徴とする、結晶粒の微細な複合組織高張力鋼。
(2) さらに質量%で、 Cu:0.1〜3.0%
を含有し、前記金属組織中に平均直径で5nm以上のCuの金属系析出物を体積%で、0.1〜3.0%含有することを特徴とする、上記(1)に記載の結晶粒の微細な複合組織高張力鋼。
(3) さらに質量%で、
Ti:0.001〜0.3%、 Nb:0.001〜0.3%、
V :0.001〜0.3%
のいずれか1種または2種以上を含有することを特徴とする、上記(1)または(2)に記載の結晶粒の微細な複合組織高張力鋼。
(4) さらに質量%で、
Ni:0.01〜5.0%、 Cr:0.01〜3.0%、
Mo:0.01〜1.0%、 B :0.0001〜0.003%
のいずれか1種または2種以上を含有することを特徴とする、上記(1)ないし(3)のいずれか1項に記載の結晶粒の微細な複合組織高張力鋼。
(5) さらに質量%で、
REM:0.001〜0.10%、Ca:0.0003〜0.0030%
のいずれか1種または2種を含有することを特徴とする、上記(1)ないし(4)のいずれか1項に記載の結晶粒の微細な複合組織高張力鋼。
(6) さらに質量%で、
N :0.001〜0.1%を含有するとともに、
Al:0.001〜0.10%、 Zr:0.001〜0.30%、
Ta:0.001〜0.30%、 Hf:0.001〜0.30%
のいずれか1種または2種以上を含有することを特徴とする、上記(1)ないし(5)のいずれか1項に記載の結晶粒の微細な複合組織高張力鋼。
In order to solve the above-mentioned problems, the present inventors examined the formation mechanism and characteristics of a fine-grained structure in ultrafine-grained steel, and created the following invention.
(1) In mass%,
C: 0.001 to 0.75%, Si: 0.01 to 0.5%,
Mn: 0.1 to 5.0%
And the balance is a steel composed of Fe and inevitable impurities, the metal structure of which is 50% to 95% by volume of ferrite and the balance is a mixed structure of martensite or tempered martensite, A high-strength steel with a fine-grained composite structure, wherein the average particle diameter is 2 μm or less.
(2) Further, by mass%, Cu: 0.1 to 3.0%
The crystal according to (1) above, wherein the metal structure contains 0.1 to 3.0% by volume of a metal-based precipitate of Cu having an average diameter of 5 nm or more in the metal structure. High-strength steel with a fine-grained complex structure.
(3) Furthermore, in mass%,
Ti: 0.001 to 0.3%, Nb: 0.001 to 0.3%,
V: 0.001 to 0.3%
The high-strength steel with a fine crystal grain structure according to the above (1) or (2), characterized by containing any one or more of the above.
(4) Furthermore, in mass%,
Ni: 0.01-5.0%, Cr: 0.01-3.0%,
Mo: 0.01 to 1.0%, B: 0.0001 to 0.003%
The high-strength steel with a fine crystal grain structure according to any one of the above (1) to (3), characterized by containing any one or more of the above.
(5) Furthermore, in mass%,
REM: 0.001-0.10%, Ca: 0.0003-0.0030%
The high-strength steel with a fine crystal grain structure according to any one of the above (1) to (4), which contains any one or two of the above.
(6) Furthermore, in mass%,
N: containing 0.001 to 0.1%,
Al: 0.001 to 0.10%, Zr: 0.001 to 0.30%,
Ta: 0.001 to 0.30%, Hf: 0.001 to 0.30%
The high-strength steel with a fine grain structure according to any one of the above (1) to (5), characterized by containing any one or more of the above.

本発明によれば、降伏強度×全伸びの値が14000MPa・%を超えるような、強度が高く、延性に優れた、結晶粒の微細な複合組織高張力鋼を得ることができる。その結果、近年の鋼製品の軽量化、鋼構造物の使用条件の過酷化にともない、より強靭で安全性の高い鋼の需用にも有利に応えることができるため、産業上の効果は計り知れない。   According to the present invention, it is possible to obtain a high-strength steel with a high-strength, excellent ductility and fine-grained composite structure in which the yield strength × total elongation value exceeds 14000 MPa ·%. As a result, it is possible to respond to the demand for stronger and safer steel as the steel products become lighter and the use conditions of steel structures become severer in recent years. I don't know.

以下、本発明について説明する。
まず、細粒鋼の機械的特性について考察してみると、以下のようになるものと考えられる。
結晶粒径が微細化すると降伏強度(YS)が増加する。一方、引張強度(TS)も増加するが、その増加の程度は降伏強度の増加に比較して小さい。このため、結晶粒径が1μm程度以下にまで顕著に微細化すると、降伏強度と引張強度がほぼ同等となってしまい塑性不安定性が発現するために、加工硬化することなく破断に至る。
このような細粒鋼の引張特性における均一伸びを改善するためには、比較的に大きな硬質相を細粒組織内に分散させ、降伏強度をあまり上昇させることなく、引張強度を上げて、細粒組織においても十分な加工硬化を現出させてやることが必要である。
The present invention will be described below.
First, the mechanical properties of fine-grained steel are considered as follows.
Yield strength (YS) increases as the crystal grain size becomes finer. On the other hand, the tensile strength (TS) also increases, but the degree of increase is small compared to the increase in yield strength. For this reason, when the crystal grain size is remarkably refined to about 1 μm or less, the yield strength and the tensile strength become substantially equal, and plastic instability is exhibited, so that breakage occurs without work hardening.
In order to improve the uniform elongation in the tensile properties of such fine-grained steel, a relatively large hard phase is dispersed in the fine-grained structure, and the tensile strength is increased without significantly increasing the yield strength. It is necessary to show sufficient work hardening even in the grain structure.

しかしながら、このような硬質相の分散は加工硬化を増加させることによって、均一伸びを増加させることが予想されるが、硬質相周囲に歪みが集中するために局部延性を劣化させ、局部伸びを減少させる可能性がある。従って、局部延性を劣化させずに引張強度を上昇させて、均一伸びを現出させトータルな伸び特性を改善する金属組織状態を実現することが望まれる。
そこで本発明者らは、上記のような結晶粒が1μm程度以下に微細化した細粒鋼の組織状態と引張特性の関連を吟味し、強度を低下させることなく延性を良好とする、結晶粒の微細な複合組織高張力鋼を創案した。
However, the dispersion of the hard phase is expected to increase the uniform elongation by increasing the work hardening. However, since the strain concentrates around the hard phase, the local ductility is deteriorated and the local elongation is reduced. There is a possibility to make it. Therefore, it is desired to realize a metallographic state that increases the tensile strength without deteriorating the local ductility, reveals uniform elongation, and improves the total elongation characteristics.
Therefore, the present inventors examined the relationship between the microstructure of the fine-grained steel in which the crystal grains were refined to about 1 μm or less and the tensile properties, and improved the ductility without reducing the strength. Of high-strength steel with a fine composite structure.

本発明の根幹となる技術的思想の要点は以下の通りである。
(1)フェライトの微細化
フェライトは、変態や再結晶などの現象を極限的に利用することによって、その結晶粒径を直径で1μm以下に微細化できる。このとき、結晶粒径を1μm以下まで微細化することによって得られる降伏強度および引張強度は概ね700MPa以上であり、結晶粒径2μm以下では500MPa以上である。一方、同一の成分系での従来鋼の粒径の下限は5μm程度であり、その場合の引張強度は350MPa程度であることを考慮すれば、細粒化による強化を十分に享受するためには、2μm以下の結晶粒径が必要である。
The main points of the technical idea that forms the basis of the present invention are as follows.
(1) Refinement of ferrite Ferrite can have its crystal grain size reduced to 1 μm or less in diameter by making extreme use of phenomena such as transformation and recrystallization. At this time, the yield strength and the tensile strength obtained by refining the crystal grain size to 1 μm or less are approximately 700 MPa or more, and when the crystal grain size is 2 μm or less, it is 500 MPa or more. On the other hand, considering that the lower limit of the grain size of the conventional steel in the same component system is about 5 μm, and the tensile strength in that case is about 350 MPa, in order to fully enjoy the strengthening by fine graining A crystal grain size of 2 μm or less is required.

(2)硬質相の存在
次に、上記したような細粒組織中に分散させるべき硬質相は、母相の微細フェライト組織との硬度差が大きい程良い。このような観点から実際の実験結果を見ると、硬質相はマルテンサイトおよび焼き戻しマルテンサイトが好ましいことが判った。
硬質相としては、当然、母相であるフェライトの硬度(ビッカース硬さ200程度)以上であることを考えると、ビッカース硬さ220程度であるパーライトやそれと同等の硬さであるベイナイトは、硬質相としてあまり相応しくないことが判る。このような硬質相の存在により、細粒組織鋼においては、降伏強度の増加に比較して引張強度が顕著に増加し、均一伸びの改善が認められた。
このような硬質相による均一伸びの改善は、硬質相の硬さ及び母相中での存在量が支配的であり、硬質相の結晶粒径にはあまり依存しないことが判った。また、このとき硬質相の量は概ね体積分率で5%以上50%以下の範囲で良好な均一伸びの改善が認められた。
(2) Presence of a hard phase Next, the hard phase which should be disperse | distributed in the above-mentioned fine grain structure is so good that a hardness difference with the fine ferrite structure of a mother phase is large. From the actual experimental results from this point of view, it was found that the hard phase is preferably martensite and tempered martensite.
Naturally, considering that it is higher than the hardness of ferrite (the Vickers hardness of about 200) as the parent phase, pearlite having a Vickers hardness of about 220 or bainite having a hardness equivalent to that is a hard phase. It turns out that it is not so suitable. Due to the presence of such a hard phase, in the fine-grained steel, the tensile strength was remarkably increased as compared with the increase in yield strength, and an improvement in uniform elongation was observed.
It has been found that the improvement of uniform elongation by such a hard phase is dominated by the hardness of the hard phase and the abundance in the matrix phase, and does not depend much on the crystal grain size of the hard phase. At this time, a good improvement in uniform elongation was observed when the amount of the hard phase was approximately in the range of 5% to 50% in terms of volume fraction.

(3)硬質相の分散状態
次に、硬質相の分散状態については、局部延性の観点から調査した結果、硬質相の結晶粒径が小さいほど局部延性の低下が抑制されることが判った。さらに、このような硬質相による局部延性の劣化は、硬質相の分散間隔が母相の結晶粒径以上になる場合に顕著に現れることが判明した。これは、延性破壊の起点が結晶粒界から硬質相−母相界面に変化するためと考えられる。超微細粒鋼の場合には1μm以下の分散状態が好ましいものと考えられ、このような観点から、顕著に局部延性の劣化を抑制可能な硬質相のサイズとして、母相の結晶粒径と同程度以下の2μm以下が好ましいことが判った。
(3) Hard Phase Dispersion State Next, the hard phase dispersion state was investigated from the viewpoint of local ductility. As a result, it was found that the smaller the hard phase crystal grain size, the lower the local ductility reduction. Further, it has been found that such deterioration of local ductility due to the hard phase appears prominently when the dispersion interval of the hard phase is larger than the crystal grain size of the parent phase. This is presumably because the starting point of ductile fracture changes from the grain boundary to the hard phase-matrix interface. In the case of ultrafine-grained steel, a dispersion state of 1 μm or less is considered preferable. From this viewpoint, the size of the hard phase that can remarkably suppress the deterioration of local ductility is the same as the crystal grain size of the parent phase. It was found that 2 μm or less, which is less than or equal to about, is preferable.

しかし、このような硬質相の分散状態は硬質相の体積分率との関係で決定されるべきであるが、均一伸び改善の目的から、硬質相の体積分率は5%以上50%以下の範囲が好ましいことを先に述べた。このような体積分率の範囲においては、硬質相同士が接触する程稠密に分布しないので、硬質相の直径を概ね2μm以下に微細化しておけば、細粒鋼の局部延性を劣化させることがなく、良好なトータル伸び(全伸び)を確保するに至ること判ったのである。   However, such a hard phase dispersion state should be determined in relation to the volume fraction of the hard phase. For the purpose of improving uniform elongation, the volume fraction of the hard phase is 5% or more and 50% or less. I mentioned earlier that the range is preferred. In such a volume fraction range, the hard phases are not distributed so densely as to come into contact with each other. Therefore, if the diameter of the hard phase is reduced to approximately 2 μm or less, the local ductility of the fine-grained steel may be deteriorated. However, it was found that good total elongation (total elongation) was secured.

(4)Cu析出物による微細化と強化
次に、上記したような微細硬質相を分散した細粒組織鋼において、Cuを添加することは特別に有効な延性を損なわない強化方法であることが判った。その析出相を利用することによって、母相である微細組織の結晶粒をより微細化し、細粒化強化を促進するとともに、析出相自身による析出強化が得られる。
このような強化では延性をあまり損なわない点について考察すると、Cu添加による強化は、上に述べたように母相の細粒化強化とCuの析出相による析出強化の2者であり、これらはいずれも延性の劣化の抑制に有効であると考えられるからである。
(4) Refinement and strengthening by Cu precipitates Next, in the fine-grained steel in which the fine hard phase is dispersed as described above, adding Cu is a strengthening method that does not impair the particularly effective ductility. understood. By utilizing the precipitated phase, the crystal grains of the microstructure that is the parent phase are further refined, and the strengthening of the fine grain is promoted, and the precipitation strengthening by the precipitated phase itself is obtained.
Considering the point that ductility does not deteriorate much with such strengthening, the strengthening by Cu addition is the two of the strengthening by refining the parent phase and the precipitation strengthening by the precipitated phase of Cu as described above. This is because both are considered to be effective in suppressing the deterioration of ductility.

まず、結晶粒の細粒化は、他の強化機構に比較して局部延性をあまり劣化させないと考えられていることによる。
また、後者はCuの析出相が金属Cuであるという特殊性に基づいている。これは、以下のように考えられる。
Cuの析出相は、鋼中の析出相としては他にはない母相より柔らかい面心立方構造の金属Cuである。従って、加工初期の歪み量が小さい場合には、Cu析出相の結晶構造が母相の鉄(体心立方構造)とは異なるので、母相中を移動してきた転位はCu析出相中に進入せず移動を抑制するので、転位のピン止めによって析出強化が得られる。
First, crystal grain refinement is due to the fact that local ductility is considered not to deteriorate much compared to other strengthening mechanisms.
The latter is based on the special feature that the precipitated phase of Cu is metallic Cu. This is considered as follows.
The precipitation phase of Cu is a metal Cu having a face-centered cubic structure that is softer than a parent phase that is not found as a precipitation phase in steel. Therefore, when the strain amount at the initial stage of processing is small, the crystal structure of the Cu precipitation phase is different from that of the parent phase iron (body-centered cubic structure), so the dislocations that have moved in the parent phase enter the Cu precipitation phase. Therefore, precipitation strengthening can be obtained by dislocation pinning.

しかし、加工が進み歪み量が増加すると析出相周囲に蓄積した転位が増加し、析出相は強い応力を受けることとなる。この様な状況では、通常の硬質な析出物が全く変形せず、応力が増加し続け、やがて破壊応力に達して亀裂が発生するのに対して、Cu析出相の場合には、その前に変形が始まり、析出相周囲に亀裂が発生することを抑制するものと考えられる。   However, as processing proceeds and the amount of strain increases, dislocations accumulated around the precipitate phase increase, and the precipitate phase is subjected to strong stress. In such a situation, the normal hard precipitate is not deformed at all, the stress continues to increase, and eventually the fracture stress is reached and a crack is generated. It is considered that the deformation starts and the occurrence of cracks around the precipitated phase is suppressed.

しかし、このような微細Cu析出物による強化は、引張強度の増加に比較して降伏強度の増加の程度が大きく、超微細粒鋼の場合には、そもそも微細化強化により顕著に降伏強度が上昇し、引張試験における加工硬化が小さく均一伸びが小さい傾向にあるので、この降伏強度の上昇を極端に大きくすることは延性改善の観点から回避しなければならない。 このためには、Cu析出物の分散間隔をある値以上に確保する必要があり、その条件を実験的に検討した結果、析出Cuの体積分率として、3%未満、析出物サイズとして5nm以上のサイズであることが必要であることが判明した。   However, the strengthening by such fine Cu precipitates has a large degree of increase in yield strength compared to the increase in tensile strength. In the case of ultra-fine grain steel, the yield strength is significantly increased by refinement strengthening in the first place. However, since the work hardening in the tensile test is small and the uniform elongation tends to be small, it is necessary to avoid increasing the yield strength extremely from the viewpoint of improving ductility. For this purpose, it is necessary to secure a dispersion interval of Cu precipitates to a certain value or more. As a result of experimentally examining the conditions, the volume fraction of precipitated Cu is less than 3%, and the precipitate size is 5 nm or more. It was found that it was necessary to be of the size.

Cu析出物の利用は、顕著な析出強化を得ると共に、細粒鋼の結晶粒径を一層微細にする。この析出を上記範囲で適度に制御することにより、細粒鋼をさらに高強度としながら、弱点である延性の劣化を有利に回避できることが判ったわけである。
上記したような発見に基づき、本発明の結晶粒の微細な複合組織高張力鋼の満たすべき成分、組織条件を明確にした。
The use of Cu precipitates provides remarkable precipitation strengthening and further refines the crystal grain size of fine-grained steel. It has been found that by appropriately controlling the precipitation within the above range, it is possible to advantageously avoid the deterioration of ductility, which is a weak point, while further increasing the strength of the fine-grained steel.
Based on the findings as described above, the components and structure conditions to be satisfied of the high-strength steel with a fine grain structure of the present invention were clarified.

まず、各成分、組織状態の限定の理由について以下に述べる。
Cは、鋼の強化を行うのに有効な元素であり、0.001%未満では強度を得るための量が十分でない。一方、その含有量が0.75%を超えると溶接性を顕著に劣化させる。そこでC含有量は0.001〜0.75%とする。
First, the reasons for limiting each component and the tissue state will be described below.
C is an element effective for strengthening steel, and if less than 0.001%, the amount for obtaining strength is not sufficient. On the other hand, if the content exceeds 0.75%, the weldability is remarkably deteriorated. Therefore, the C content is set to 0.001 to 0.75%.

Siは、脱酸元素として、また鋼の強化元素として有効であるが、0.01%未満の含有量ではその効果がない。一方、0.5%を超えると鋼の表面性状を損なう。そこでSi含有量は0.01〜0.5%とする。   Si is effective as a deoxidizing element and as a strengthening element for steel, but if the content is less than 0.01%, there is no effect. On the other hand, if it exceeds 0.5%, the surface properties of the steel are impaired. Therefore, the Si content is set to 0.01 to 0.5%.

Mnは、鋼の強化および焼き入れ性を向上させ圧延前の組織を適正に導くのに有効な元素であり、0.1%未満では十分な効果が得られない。一方、その含有量が5.0%を超えると鋼の加工性を劣化させる。そこでMn含有量は0.1〜5.0%とする。   Mn is an element effective for improving the strengthening and hardenability of steel and appropriately leading the structure before rolling, and if it is less than 0.1%, a sufficient effect cannot be obtained. On the other hand, if the content exceeds 5.0%, the workability of the steel is deteriorated. Therefore, the Mn content is set to 0.1 to 5.0%.

Cuは、上記説明したように、鋼中において金属Cu(FCC構造)の析出相を形成し、微細組織の形成の促進や延性の劣化を抑制した析出強化が得られるため、必要に応じて添加する。このような効果は、その含有量が0.1%未満では得られない。一方、その含有量が3.0%を超えると鋼の熱間加工性などを劣化させる。そこでCu含有量は0.1〜3.0%とする。   As described above, Cu forms a precipitation phase of metal Cu (FCC structure) in steel, and can enhance precipitation and suppress the deterioration of ductility by promoting the formation of microstructure and added as necessary. To do. Such an effect cannot be obtained if the content is less than 0.1%. On the other hand, when the content exceeds 3.0%, the hot workability of steel is deteriorated. Therefore, the Cu content is set to 0.1 to 3.0%.

Ti,NbおよびVは、結晶粒の微細化と析出強化の面で有効に機能するので靭性を劣化させない範囲で使用しても良い。このような観点からその添加量の上限をそれぞれ0.3%とする。また、その添加量の下限をそれぞれ0.001%とするのは、これ未満では効果がないからである。そこで、Ti,NbおよびVの添加量は、それぞれ0.001〜0.3%とする。   Ti, Nb, and V function effectively in terms of crystal grain refinement and precipitation strengthening, and may be used within a range that does not deteriorate toughness. From such a viewpoint, the upper limit of the addition amount is set to 0.3%. Moreover, the reason why the lower limit of the addition amount is 0.001% is that if it is less than this, there is no effect. Therefore, the addition amounts of Ti, Nb, and V are 0.001 to 0.3%, respectively.

Ni,Cr,Mo,Bは、いずれも鋼の焼入れ性を向上させる元素であり、本発明の場合、その添加により鋼の強度を高めることができる。しかし、過度の添加は鋼の靭性および溶接性を損なうため、Ni:0.01〜5.0%、Cr:0.01〜3.0%、Mo:0.01〜1.0%、B:0.0001〜0.003%に限定する。Ni,Cr,Moのそれぞれの下限を0.01%、Bの下限を0.0001%としたのは、これ未満では効果がないからである。   Ni, Cr, Mo, and B are all elements that improve the hardenability of the steel, and in the case of the present invention, the strength of the steel can be increased by addition thereof. However, excessive addition impairs the toughness and weldability of steel, so Ni: 0.01 to 5.0%, Cr: 0.01 to 3.0%, Mo: 0.01 to 1.0%, B : Limited to 0.0001 to 0.003%. The reason why the lower limit of Ni, Cr and Mo is 0.01% and the lower limit of B is 0.0001% is that if it is less than this, there is no effect.

REM,CaはSの無害化に有効であるが、添加量が少ないとその効果が無く、また過度の添加は靭性を損なうため、REMについては0.001〜0.10%、Caについては0.0003〜0.0030%に限定する。   REM and Ca are effective for detoxification of S. However, if the addition amount is small, the effect is not obtained, and excessive addition impairs toughness, so 0.001 to 0.10% for REM and 0 for Ca. Limited to .0003-0.0030%.

Al,Zr,Ta,Hfは脱酸元素あるいは炭窒化物形成元素として添加されるが、それぞれ0.001%未満の含有量ではその効果がなく、Alについては0.1%、Zr,Ta,Hfについては0.3%を超えると、鋼の靱性や表面性状を劣化させる。そこで、Alについては0.001〜0.10%、Zr,Ta,Hfについてはそれぞれ0.001〜0.30%とする。   Al, Zr, Ta, and Hf are added as deoxidizing elements or carbonitride-forming elements. However, when the content is less than 0.001%, there is no effect. For Al, 0.1%, Zr, Ta, If Hf exceeds 0.3%, the toughness and surface properties of the steel deteriorate. Therefore, 0.001 to 0.10% for Al and 0.001 to 0.30% for Zr, Ta, and Hf, respectively.

Nは、TiおよびAl,Zr,Ta,Hfと窒化物を形成し、オーステナイトの細粒化及びフェライトの再結晶粒の微細化に有効に作用するため靭性を劣化させない範囲で添加する。このような観点からその上限を0.1%、下限を0.001%とする。
その他、不可避的不純物であるP,Sの含有量は、延性確保の観点から、それぞれ0.02%以下、0.008%以下が好ましい。
N forms nitrides with Ti and Al, Zr, Ta, and Hf, and effectively adds to the austenite and the recrystallized grains of ferrite, so N is added in a range that does not deteriorate the toughness. From such a viewpoint, the upper limit is 0.1% and the lower limit is 0.001%.
In addition, the contents of P and S, which are inevitable impurities, are preferably 0.02% or less and 0.008% or less, respectively, from the viewpoint of ensuring ductility.

次に、本発明の細粒鋼の満たすべき組織条件の限定の理由について述べる。
本発明の要点は、以下の点にある。
(1)2μm以下の平均結晶粒径のフェライトを50〜95%含有する。
(2)上記の残部5〜50%は平均結晶粒径2μm以下のマルテンサイトもしくは焼き戻 しマルテンサイトの分散組織とする。
(3)さらに、好ましくは、体積分率で0.1%以上3.0%以下のCuの金属析出物を 平均粒径で5nm以上のサイズで分散した組織とする。
Next, the reason for limiting the structure conditions to be satisfied in the fine-grained steel of the present invention will be described.
The main points of the present invention are as follows.
(1) 50 to 95% of ferrite having an average grain size of 2 μm or less is contained.
(2) The remaining 5 to 50% is made a martensite or tempered martensite dispersed structure having an average crystal grain size of 2 μm or less.
(3) Further, preferably, a structure in which Cu metal precipitates having a volume fraction of 0.1% or more and 3.0% or less are dispersed with an average particle size of 5 nm or more is used.

上記の限定の理由は、まず、フェライトの結晶粒径が2μm超では鋼の強度が十分とならないからである。結晶粒径を1μm以下まで微細化することによって得られる降伏強度および引張強度は概ね700MPa以上であり、結晶粒径2μm以下では500MPa以上であった。一方、同一の成分系で従来鋼の粒径の下限が5μm程度である場合の引張強度が350MPa程度であることを考慮すれば、細粒化による強化を十分に享受するためには、2μm以下の結晶粒径が必要なのである。   The reason for the above limitation is that the strength of the steel is not sufficient if the crystal grain size of ferrite exceeds 2 μm. The yield strength and tensile strength obtained by refining the crystal grain size to 1 μm or less were generally 700 MPa or more, and 500 MPa or more when the crystal grain size was 2 μm or less. On the other hand, considering that the tensile strength is about 350 MPa when the lower limit of the grain size of the conventional steel is about 5 μm in the same component system, 2 μm or less in order to fully enjoy the strengthening by fine graining Is required.

次に、微細なフェライト以外の金属組織の残部をマルテンサイトもしくは焼き戻しマルテンサイトとするのは、以下のような理由による。
一般に、細粒鋼では結晶粒微細化による降伏強度の上昇が顕著で、降伏強度と引張強度が同等となり、加工硬化が起こらないために引張試験における均一伸びが極めて小さいという欠点がある。これを打開するためには、細粒組織中に硬質の金属組織を分散させることが必要である。このような硬質相は、母相の微細フェライト組織との硬度差が大きい程良い。
Next, the remainder of the metal structure other than fine ferrite is martensite or tempered martensite for the following reason.
In general, fine-grained steel has a significant increase in yield strength due to crystal grain refinement, yield strength and tensile strength are equal, and there is a disadvantage that uniform elongation in a tensile test is extremely small because work hardening does not occur. In order to overcome this, it is necessary to disperse the hard metal structure in the fine grain structure. Such a hard phase is better as the hardness difference from the fine ferrite structure of the parent phase is larger.

マルテンサイトは、含有する炭素量によって400〜800程度のビッカース硬さを示す。焼き戻しマルテンサイトも同様に、ビッカース硬さ250〜500程度の硬質相である。母相であるフェライトのビッカース硬さが200程度であることを考えると、ビッカース硬さ220程度であるパーライトやそれと同等の硬さであるベイナイトは、硬質相としてあまり相応しくない。そこで硬質相としては、マルテンサイトもしくは焼き戻しマルテンサイトに限定した。   Martensite exhibits a Vickers hardness of about 400 to 800 depending on the amount of carbon contained. Similarly, tempered martensite is a hard phase having a Vickers hardness of about 250 to 500. Considering that the Vickers hardness of ferrite as a parent phase is about 200, pearlite having a Vickers hardness of about 220 and bainite having a hardness equivalent thereto are not very suitable as a hard phase. Therefore, the hard phase was limited to martensite or tempered martensite.

また、硬質相の体積率を5〜50%(フェライト相50〜95%)としたのは、硬質相の体積率が5%未満では、硬質相による引張強度上昇の効果が少なく均一伸びを改善できないからであり、50%以下と限定するのは、50%を超えると硬質相が互いに極めて隣接するか、連結するようになり、鋼の降伏強度が顕著に上昇し、同様に均一伸びを改善できなくなるからである。   The volume ratio of the hard phase is 5 to 50% (ferrite phase 50 to 95%). If the volume ratio of the hard phase is less than 5%, the effect of increasing the tensile strength due to the hard phase is small and the uniform elongation is improved. This is because it is not possible to limit it to 50% or less. If it exceeds 50%, the hard phases become very adjacent to each other or become connected, and the yield strength of the steel is remarkably increased, and the uniform elongation is similarly improved. Because it becomes impossible.

このような硬質相の効果は硬質相のサイズにあまり依存しないが、硬質相分率一定のもとで硬質相の平均結晶粒径が大きくなると、硬質相の分散間隔がフェライトの結晶流サイズに比較して十分大きくなり、延性破壊の起点となるため、局部延性ひいては全伸びを低下させる。そこで硬質相の平均結晶サイズは、硬質相の分散間隔がフェライト結晶粒と同等となる2μm以下と限定した。   The effect of such a hard phase does not depend much on the size of the hard phase, but when the average grain size of the hard phase increases with a constant hard phase fraction, the dispersion interval of the hard phase depends on the crystal flow size of the ferrite. Since it becomes sufficiently large in comparison and becomes a starting point of ductile fracture, the local ductility and thus the total elongation is lowered. Therefore, the average crystal size of the hard phase is limited to 2 μm or less so that the dispersion interval of the hard phase is equivalent to the ferrite crystal grains.

次に、Cu析出物の状態について述べる。
Cuの析出相を利用することによって、母相であるフェライト微細組織の結晶粒をより微細化し、細粒化強化を促進するとともに、析出相自身による析出強化が得られる。なお、このような強化では延性をあまり損なわないことを先に述べた。しかし、このような微細Cu析出物による強化は、引張強度の増加に比較して降伏強度の増加の程度が大きく、超微細粒鋼の場合には、そもそも微細化強化により顕著に降伏強度が上昇し、引張試験における加工硬化が小さく均一伸びが小さい傾向にあるので、この降伏強度の上昇を極端に大きくすることは延性改善の観点から回避しなければならない。
Next, the state of the Cu precipitate will be described.
By utilizing the precipitated phase of Cu, the crystal grains of the ferrite microstructure, which is the parent phase, are further refined and the strengthening of the refinement is promoted, and the precipitation strengthening by the precipitated phase itself is obtained. In addition, it was mentioned earlier that such strengthening does not significantly impair the ductility. However, the strengthening by such fine Cu precipitates has a large degree of increase in yield strength compared to the increase in tensile strength. In the case of ultra-fine grain steel, the yield strength is significantly increased by refinement strengthening in the first place. However, since the work hardening in the tensile test is small and the uniform elongation tends to be small, it is necessary to avoid increasing the yield strength extremely from the viewpoint of improving ductility.

このためには、Cu析出物の分散間隔をある値以上に確保する必要があり、その条件を実験的に検討した結果、析出Cuの上限体積分率として3.0%以下とした。3.0%を超える体積分率では、降伏強度の増加が過度となり、鋼の均一伸びを著しく低下させる。 また、Cu析出物の体積分率下限を0.1%としたのは、これ未満では効果がないからである。また同様に、実験的にCu析出物サイズが5nm未満となると、降伏強度の上昇が過度に大きくなり、延性が劣化した。このことから、Cu析出物のサイズについては5nm以上と限定する。なお、Cu析出物サイズの上限については、細粒化効果および強化の効果が観られるのは概ね100nm程度であったが、その他の効果が失われることがなかったので、上限は規定しない。   For this purpose, it is necessary to ensure the dispersion interval of the Cu precipitates to a certain value or more. As a result of experimentally examining the conditions, the upper limit volume fraction of precipitated Cu was set to 3.0% or less. When the volume fraction exceeds 3.0%, the yield strength increases excessively, and the uniform elongation of the steel is significantly reduced. The reason why the lower limit of the volume fraction of the Cu precipitate is set to 0.1% is that if it is less than this, there is no effect. Similarly, when the Cu precipitate size was experimentally less than 5 nm, the yield strength increased excessively and the ductility deteriorated. For this reason, the size of the Cu precipitate is limited to 5 nm or more. As for the upper limit of the Cu precipitate size, the effect of refining and strengthening was observed at about 100 nm, but the other effects were not lost, so the upper limit is not specified.

次に、実施例によって本発明の有効性を示す。
表1は、実施例の鋼の成分を示すものである。なお、表中で下線をつけて示した番号の鋼は比較鋼であることを示しており、本発明に一致しない項目に下線をつけて示してある。
次に表2〜4には、このような成分の鋼を用い種々の製造条件で製造した本発明鋼及び比較鋼について得られた金属組織状態(各組織体積分率と結晶粒径、金属Cu析出物体積率と粒径)、降伏強度(YS)、引張強度(TS)、降伏比、均一伸び、全伸びを示す。 また、本発明の狙いは強度(降伏強度)が高く、延性に優れた鋼とすることであり、この指標として降伏強度(MPa)×全伸び(%)の値を示した。概ね、これらの積が14000MPa・%を超えるものを優れた特性を有するものと考えられる。
Next, the effectiveness of the present invention will be shown by examples.
Table 1 shows the components of the steels of the examples. In addition, the steel of the number shown with the underline in the table | surface has shown that it is a comparative steel, and the item which does not correspond to this invention is shown with the underline.
Next, Tables 2 to 4 show the metallographic state (each structure volume fraction and crystal grain size, metal Cu obtained for the steels of the present invention and comparative steels produced under various production conditions using steels having such components. Precipitate volume fraction and particle size), yield strength (YS), tensile strength (TS), yield ratio, uniform elongation, total elongation. The aim of the present invention is to make the steel high in strength (yield strength) and excellent in ductility, and this index shows the value of yield strength (MPa) × total elongation (%). In general, those having a product exceeding 14000 MPa ·% are considered to have excellent characteristics.

なお、今回示した実施例はいずれも板厚が3〜10mmの鋼材であり、その製造に当たっては、フェライトの再結晶を用いた加工熱処理で作成した。
代表的製造方法は、板厚100mmの素材を1400℃に加熱し、一度、室温まで水冷する。次にこの素材を700℃に加熱し、圧延機を用いた3〜8パスの連続する圧延によって板厚3〜10mmに圧延する。圧延終了後は直ちに室温まで水冷した。この際、フェライト結晶粒径は圧延温度を制御することによって実施する。
In addition, all the examples shown this time are steel materials having a plate thickness of 3 to 10 mm, and in the production thereof, they were prepared by a thermomechanical treatment using recrystallization of ferrite.
In a typical manufacturing method, a material having a thickness of 100 mm is heated to 1400 ° C. and once water-cooled to room temperature. Next, this material is heated to 700 ° C. and rolled to a thickness of 3 to 10 mm by continuous rolling of 3 to 8 passes using a rolling mill. Immediately after the completion of rolling, the product was cooled to room temperature. At this time, the ferrite crystal grain size is carried out by controlling the rolling temperature.

また、マルテンサイト分率と結晶粒径は、上記圧延によって製造した鋼板を700〜850℃のフェライト−オーステナイト2相域の種々の温度に加熱し、水冷する一連の熱処理の中で、その熱処理温度および保持時間を制御することによって作り分けた。Cu析出物のサイズについても同様に熱処理によって作り分けた。また、場合によっては圧延の温度あるいは圧延に先立って行う再加熱温度の制御だけによって製造したものもある。   The martensite fraction and the crystal grain size are determined by the heat treatment temperature in a series of heat treatments in which the steel sheet produced by rolling is heated to various temperatures in the two-phase region of ferrite to austenite at 700 to 850 ° C. and water cooled. And made by controlling the holding time. Similarly, the size of the Cu precipitate was prepared by heat treatment. Also, in some cases, it is manufactured only by controlling the rolling temperature or the reheating temperature performed prior to rolling.

表2〜4によれば、いずれの鋼の場合も本発明法の要件を満たす鋼は強度、延性に優れ、いずれも降伏強度×全伸びの値が14000MPa・%以上となっており、良好な特性を有する。これに対し、本発明の要件を満たさない鋼では、十分な降伏強度が得られないか、均一伸びもしくは局部延性の劣化に基づく全伸びの低下によって、降伏強度×全伸びの値は低い。
以上のことより、本発明の結晶粒の微細な混合組織鋼は、強度、延性に優れていることが明らかであり、本発明は有効であることが判る。
According to Tables 2 to 4, the steel satisfying the requirements of the method of the present invention is excellent in strength and ductility in any steel, and the yield strength × total elongation value is 14000 MPa ·% or more in both cases. Has characteristics. On the other hand, in steels that do not satisfy the requirements of the present invention, the yield strength × total elongation value is low because sufficient yield strength cannot be obtained, or due to a decrease in total elongation due to uniform elongation or degradation of local ductility.
From the above, it is clear that the mixed grain steel with fine crystal grains of the present invention is excellent in strength and ductility, and it is understood that the present invention is effective.

Claims (6)

質量%で、
C :0.001〜0.75%、
Si:0.01〜0.5%、
Mn:0.1〜5.0%
を満たす成分を含有し、残部がFeおよび不可避的不純物からなる鋼で、その金属組織が体積%で50〜95%のフェライトと残部がマルテンサイトまたは焼き戻しマルテンサイトの混合組織であって、各々の粒径が平均直径で2μm以下であることを特徴とする、結晶粒の微細な複合組織高張力鋼。
% By mass
C: 0.001 to 0.75%,
Si: 0.01 to 0.5%,
Mn: 0.1 to 5.0%
And the balance is a steel composed of Fe and inevitable impurities, the metal structure of which is 50% to 95% by volume of ferrite and the balance is a mixed structure of martensite or tempered martensite, A high-strength steel with a fine-grained composite structure, wherein the average particle diameter is 2 μm or less.
さらに質量%で、
Cu:0.1〜3.0%
を含有し、前記金属組織中に平均直径で5nm以上のCuの金属系析出物を体積%で、0.1〜3.0%含有することを特徴とする、請求項1に記載の結晶粒の微細な複合組織高張力鋼。
In addition,
Cu: 0.1 to 3.0%
The crystal grain according to claim 1, wherein the metal structure contains 0.1 to 3.0% by volume of a metallic precipitate of Cu having an average diameter of 5 nm or more in the metal structure. High-strength steel with a fine composite structure.
さらに質量%で、
Ti:0.001〜0.3%、
Nb:0.001〜0.3%、
V :0.001〜0.3%
のいずれか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に記載の結晶粒の微細な複合組織高張力鋼。
In addition,
Ti: 0.001 to 0.3%,
Nb: 0.001 to 0.3%,
V: 0.001 to 0.3%
The high-strength steel with a fine grained composite structure according to claim 1, wherein the high-strength steel has a fine grain structure.
さらに質量%で、
Ni:0.01〜5.0%、
Cr:0.01〜3.0%、
Mo:0.01〜1.0%、
B :0.0001〜0.003%
のいずれか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ないし3のいずれか1項に記載の結晶粒の微細な複合組織高張力鋼。
In addition,
Ni: 0.01 to 5.0%,
Cr: 0.01 to 3.0%,
Mo: 0.01 to 1.0%,
B: 0.0001 to 0.003%
4. The high-strength steel having a fine grain structure according to claim 1, wherein the high-strength steel has a fine crystal grain. 5.
さらに質量%で、
REM:0.001〜0.10%、
Ca :0.0003〜0.0030%
のいずれか1種または2種を含有することを特徴とする、請求項1ないし4のいずれか1項に記載の結晶粒の微細な複合組織高張力鋼。
In addition,
REM: 0.001 to 0.10%,
Ca: 0.0003 to 0.0030%
5. The high-strength steel with a fine grain structure according to any one of claims 1 to 4, wherein the high-strength steel has a fine grain structure.
さらに質量%で、
N :0.001〜0.1%
を含有するとともに、
Al:0.001〜0.10%、
Zr:0.001〜0.30%、
Ta:0.001〜0.30%、
Hf:0.001〜0.30%
のいずれか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ないし5のいずれか1項に記載の結晶粒の微細な複合組織高張力鋼。
In addition,
N: 0.001 to 0.1%
And containing
Al: 0.001 to 0.10%,
Zr: 0.001 to 0.30%,
Ta: 0.001 to 0.30%,
Hf: 0.001 to 0.30%
6. The high-strength steel having a fine grain structure according to claim 1, wherein the high-strength steel has a fine crystal grain.
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