JP4671238B2 - High-strength steel material with excellent fatigue characteristics and method for producing the same - Google Patents

Description

本発明は、７５０ＭＰａ以上の高強度を有し、しかも疲労特性に優れた高強度鋼材及びその製造方法に関する。   The present invention relates to a high-strength steel material having a high strength of 750 MPa or more and excellent fatigue properties, and a method for producing the same.

昨今、自動車用鋼板は自動車軽量化、衝突安全性能の向上の観点から高強度化が進められている。また、足回り部品等の素材として用いられる熱延鋼板は、部材として使用される際に繰り返し荷重を受けるため、一般に高い疲労特性が要求される。疲労強度は一般に鋼材の強度が高くなれば上がることが知られているが、強度が上昇するほど疲労強度は向上せず、疲労限度比（疲労強度／強度）は高強度になるに従って低下する。このため、高強度鋼板の疲労強度の向上は、低強度鋼板に比べて困難である。しかし、昨今の鋼板の高強度化に伴い、疲労特性に対する工業的要望は高い。   In recent years, steel sheets for automobiles have been increased in strength from the viewpoint of reducing the weight of automobiles and improving the safety of collisions. Moreover, since a hot-rolled steel sheet used as a material for undercarriage parts is repeatedly subjected to a load when used as a member, generally high fatigue characteristics are required. It is known that the fatigue strength generally increases as the strength of the steel increases, but the fatigue strength does not improve as the strength increases, and the fatigue limit ratio (fatigue strength / strength) decreases as the strength increases. For this reason, improvement of the fatigue strength of a high-strength steel plate is difficult compared with a low-strength steel plate. However, with the recent increase in strength of steel sheets, industrial demand for fatigue properties is high.

このような要望に対して、特許文献１（特開平１１−９２８５９号公報）にはフェライト結晶粒を超微細化することにより、また特許文献２（特開２００４−１４３５１８号公報）にはフェライト結晶粒の微細化に加えて粒内にＶＮを析出させることによって、疲労強度を向上させた高強度熱延鋼板が記載されている。さらに、非特許文献１（新日鉄技報、第３８１号、２００４年、ｐ４５−５０）には、Ｃｕ含有鋼におけるＣｕの存在形態（固溶、クラスター、析出）が疲労特性に与える影響を調査したところ、Ｃｕは析出状態よりも固溶状態あるいはクラスター状態の方が疲労特性が向上することが記載されている。前記「固溶」とは、十分な溶体化処理を前提として固体マトリックス中に合金元素が均一に分布して溶けた状態を指しており、また「クラスター」とは時効処理（Ｃｕを析出させる温度条件で保持する熱処理）過程においてＣｕが析出物として析出するに至る前の、Ｃｕ原子が複数集合した前駆体を指している。
特開平１１−９２８５９号公報 特開２００４−１４３５１８号公報 新日鉄技報、第３８１号、２００４年、ｐ４５−５０ In response to such a request, Patent Document 1 (Japanese Patent Laid-Open No. 11-92959) discloses a method for making ferrite grains ultrafine, and Patent Document 2 (Japanese Patent Laid-Open No. 2004-143518) discloses a ferrite crystal. A high-strength hot-rolled steel sheet having improved fatigue strength by precipitating VN in the grains in addition to grain refinement is described. Furthermore, Non-Patent Document 1 (Nippon Steel Technical Report, No. 381, 2004, p45-50) investigated the effect of the presence of Cu (solid solution, cluster, precipitation) on fatigue properties in Cu-containing steel. However, it is described that the fatigue characteristics of Cu are improved in a solid solution state or a cluster state than in a precipitated state. The “solid solution” refers to a state in which the alloy elements are uniformly distributed and dissolved in the solid matrix on the premise of sufficient solution treatment, and “cluster” is an aging treatment (temperature at which Cu is precipitated). It refers to a precursor in which a plurality of Cu atoms are collected before Cu is precipitated as a precipitate in the process of heat treatment held under conditions).
Japanese Patent Laid-Open No. 11-92859 JP 2004-143518 A Nippon Steel Technical Report, No. 381, 2004, p45-50

しかしながら、特許文献１、特許文献２の技術については、フェライト結晶粒を超微細化するための圧延条件および成分管理が煩雑であり、また大圧下を要するために通常の圧延設備では実施が困難である。特許文献２の技術は、さらに、特殊元素として高価なＶを必須成分として用いるために、材料コスト高を招来するという問題がある。   However, the techniques of Patent Document 1 and Patent Document 2 are complicated in rolling conditions and component management for ultra-fine ferrite crystal grains, and require a large reduction, so that it is difficult to carry out with ordinary rolling equipment. is there. The technique of Patent Document 2 further has a problem of incurring high material costs because expensive V as a special element is used as an essential component.

また、非特許文献１には、Ｃｕの固溶あるいはクラスターが疲労特性を向上させることが記載されているが、引張強度と疲労限度との関係示す図７（ｐ４７）によれば、疲労限度比が０．５８程度の場合、引張強度は５５０ＭＰａ程度と低強度レベルに止まっており、引張強度がより高くなれば疲労限度比もより低下することが明らかである。   Further, Non-Patent Document 1 describes that Cu solid solution or cluster improves fatigue characteristics. According to FIG. 7 (p47) showing the relationship between tensile strength and fatigue limit, the fatigue limit ratio is described. When the tensile strength is about 0.58, the tensile strength remains at a low strength level of about 550 MPa, and it is clear that the fatigue limit ratio is further decreased as the tensile strength is increased.

本発明はかかる問題に鑑みなされたもので、フェライト結晶粒の超微細化や特殊元素の添加を必須条件とすることなく、７５０ＭＰａ以上の高強度を有し、しかも疲労特性に優れた高強度鋼材およびその製造方法を提供することを目的とする。   The present invention has been made in view of such problems, and has high strength of 750 MPa or more and excellent fatigue properties without requiring ultrafine ferrite grains or addition of special elements. And it aims at providing the manufacturing method.

本発明者は、強化元素であるＣｕを含有する高強度鋼材に対し、基地中のＣｕの存在形態が疲労強度に及ぼす影響を詳細に調査したところ、非特許文献１に記載のとおり、ｆｃｃ構造のＣｕ析出物（メタリックＣｕ）が存在すると疲労特性が低下するが、Ｃｕ析出物を十分大きく成長させた後、Ｃｕの濃度分布が完全に均一にならないようにＣｕ析出物を再固溶し、基地中にＣｕ析出物に起因するＣｕ濃化域を分散させることにより、高強度下においても優れた疲労特性が得られることを知見し、本発明を完成するに至った。   The present inventor conducted a detailed investigation on the influence of the presence form of Cu in the matrix on the fatigue strength with respect to high-strength steel material containing Cu as a strengthening element. As described in Non-Patent Document 1, the fcc structure When the Cu precipitate (metallic Cu) is present, the fatigue characteristics are reduced, but after the Cu precipitate is grown sufficiently large, the Cu precipitate is re-dissolved so that the Cu concentration distribution is not completely uniform, It has been found that excellent fatigue properties can be obtained even under high strength by dispersing a Cu-enriched region caused by Cu precipitates in the base, and the present invention has been completed.

すなわち、本発明の高強度鋼材は、含有率が２．０〜１０mass％のＣｕを含み、引張強さが７５０ＭＰａ以上の鋼材であって、組織内にＣｕ濃度がＣｕ含有率の２倍以上、８０mass％以下であるＣｕ濃化域が均一に分散し、前記Ｃｕ濃化域の平均粒径が１０ｎｍ以上、５０ｎｍ以下とされたものである。鋼材の形態としては、熱延鋼板とすることができる。   That is, the high-strength steel material of the present invention is a steel material containing 2.0 to 10 mass% of Cu and having a tensile strength of 750 MPa or more, and the Cu concentration in the structure is twice or more the Cu content, The Cu concentrated region of 80 mass% or less is uniformly dispersed, and the average particle size of the Cu concentrated region is 10 nm or more and 50 nm or less. The form of the steel material can be a hot-rolled steel plate.

疲労破壊は、繰り返し過重の負荷過程における転位の挙動に大きく影響される。転位に対するＣｕ析出物の相互作用は過大であり、他方、固溶状態あるいはクラスター状態にあるＣｕ原子の相互作用は小さい。このため、これらの状態では十分な疲労特性が得られないと考えられる。本発明の高強度鋼材では、所定サイズのＣｕ濃化域の転位への相互作用は、Ｃｕ析出物ほど強過ぎず、また固溶あるいはクラスター状態の原子ほど弱過ぎず、適度な状態にある。このため、本発明の含Ｃｕ高強度鋼材では、７５０ＭＰａ以上の高強度でありながら、優れた疲労特性を備えることができる。   Fatigue fracture is greatly affected by the behavior of dislocations during repeated heavy loading processes. The interaction of Cu precipitates with dislocations is excessive, while the interaction of Cu atoms in a solid solution state or cluster state is small. For this reason, it is considered that sufficient fatigue characteristics cannot be obtained in these states. In the high-strength steel material of the present invention, the interaction with dislocations in the Cu-enriched region of a predetermined size is not too strong as Cu precipitates, and is not too weak as solid solution or cluster atoms, and is in an appropriate state. For this reason, the Cu-containing high-strength steel material of the present invention can have excellent fatigue properties while having a high strength of 750 MPa or more.

また、前記高強度鋼材において、その鋼成分は、mass％で、Ｃ：０．００５〜０．３０％、Ｓｉ：０．２０〜３．０％、Ｍｎ：０．５〜３．０％、Ｃｕ：２．０〜１０％を含み、残部Ｆｅ及び不純物からなる。さらに、Ｃｒ：０．１０〜２．０％、Ｍｏ：０．１０〜２．０％、Ｎｂ，Ｔｉ：それぞれ０．００２〜０．３％の元素のうち１種以上を含むことができる。このような鋼成分によれば、７５０ＭＰａ以上の引張強度を容易に確保することができる。 Further, in the high-strength steel, the steel Ingredient is in mass%, C: 0.005~0.30%, Si: 0.20~3.0%, Mn: 0.5~3.0% Cu: 2.0 to 10%, and the balance is Fe and impurities . Furthermore, C r: 0.10~2.0%, Mo : 0.10~2.0%, N b, T i: that each include one or more of 0.002 to 0.3% elemental Can do . According to such a steel component, a tensile strength of 750 MPa or more can be easily ensured.

上記高強度鋼材は、含有率が２．０mass％以上、１０mass％以下のＣｕを含む上記鋼成分を有し、Ｃｕが均一に固溶した鋼片を４００〜６００℃で５ｈｒ以上保持してＣｕ析出物を析出、成長させる時効処理を行い、時効処理した鋼片を８５０〜１０００℃で０．５〜５ｈｒ保持する加熱処理を行った後、その鋼片を熱間加工し、冷却することによって製造することができる。   The high-strength steel material has the steel component including Cu with a content rate of 2.0 mass% or more and 10 mass% or less, and holds a steel piece in which Cu is uniformly solid-solved at 400 to 600 ° C. for 5 hr or more. By performing an aging treatment for depositing and growing precipitates, and performing a heat treatment for holding the aged steel slab at 850 to 1000 ° C. for 0.5 to 5 hours, and then hot working and cooling the steel slab. Can be manufactured.

この製造方法によれば、Ｃｕを所定量含有し、均一に固溶した鋼片に対し、熱間加工前に所定の時効処理及び加熱処理を施すので、時効処理により鋼片中に析出、成長したＣｕ析出物（メタリックＣｕ）をＣｕ濃度が均一にならないように再固溶（拡散）させて、結晶粒内に所定サイズのＣｕ濃化域を分散形成することができる。このため、かかるＣｕ濃化域が組織中に分散形成された鋼片を熱間加工することにより、高強度でありながら疲労特性に優れた高強度鋼材を容易に得ることができる。前記熱間加工としては、熱間圧延、熱間鍛造等の各種の熱間塑性加工を適用することができる。   According to this manufacturing method, since a predetermined amount of aging treatment and heat treatment are performed before hot working on a steel piece containing a predetermined amount of Cu and uniformly solid solution, precipitation and growth in the steel piece by aging treatment By re-dissolving (diffusing) the deposited Cu precipitate (metallic Cu) so that the Cu concentration is not uniform, a Cu-enriched region of a predetermined size can be dispersedly formed in the crystal grains. For this reason, by hot-working a steel piece in which such a Cu-concentrated region is dispersedly formed in the structure, a high-strength steel material having high strength and excellent fatigue characteristics can be easily obtained. As the hot working, various hot plastic workings such as hot rolling and hot forging can be applied.

本発明の高強度鋼材によれば、７５０ＭＰａ以上の高強度を有しながら、組織中に所定のＣｕ濃化域が分散して存在するため、転位への相互作用が適度に得られ、高強度でありながら優れた疲労特性を得ることができる。また、本発明の製造方法によれば、析出成長させたＣｕ析出物が完全に再固溶しないように所定の加熱処理を熱間加工前に行うので、Ｃｕ濃化域が分散した高強度鋼材を容易に得ることができる。   According to the high-strength steel material of the present invention, while having a high strength of 750 MPa or more, a predetermined Cu-concentrated region is dispersed in the structure, so that an interaction with dislocations can be appropriately obtained, and a high strength However, excellent fatigue characteristics can be obtained. In addition, according to the manufacturing method of the present invention, since the predetermined heat treatment is performed before hot working so that the Cu precipitate that has been grown and grown does not completely re-dissolve, the high-strength steel material in which the Cu concentration region is dispersed. Can be easily obtained.

本発明の高強度鋼材は、７５０ＭＰａ以上の引張強度を有し、含有率が２．０〜１０mass％（以下、単に「％」と表示する場合がある。）のＣｕを必須成分として含有する。Ｃｕ以外の成分は引張強度が７５０ＭＰａ以上になるように適宜調整される。Ｃｕは強化元素であり、Ｃｕ濃化域を形成するために必須の成分である。２．０％未満では基地中に分散するＣｕ濃化域の個数が過少となり、高強度下における疲労特性の向上が不十分になる。一方、１０％を超えると前記Ｃｕ濃化域の元となるｆｃｃ構造のメタリックＣｕからなるＣｕ析出物をＣｕ濃化域に再固溶することが困難になり、組織中にＣｕ析出物が残存するようになり、疲労特性が却って低下する。このため、Ｃｕ含有率は２．０〜１０％、好ましくは３．０〜６．０％とする。   The high-strength steel material of the present invention contains Cu having a tensile strength of 750 MPa or more and a content rate of 2.0 to 10 mass% (hereinafter sometimes simply referred to as “%”) as an essential component. Components other than Cu are appropriately adjusted so that the tensile strength is 750 MPa or more. Cu is a strengthening element and is an essential component for forming a Cu enriched region. If it is less than 2.0%, the number of Cu-concentrated regions dispersed in the matrix becomes too small, and the improvement of fatigue properties under high strength becomes insufficient. On the other hand, if it exceeds 10%, it becomes difficult to re-dissolve the Cu precipitate made of metallic Cu having the fcc structure, which is the origin of the Cu concentrated region, in the Cu concentrated region, and the Cu precipitate remains in the structure. The fatigue properties are reduced. Therefore, the Cu content is set to 2.0 to 10%, preferably 3.0 to 6.0%.

本発明の高強度鋼材の組織上の特徴は、組織内にＣｕ濃度がＣｕ含有率（mass％）の２倍以上、８０mass％以下で、平均粒径が１０ｎｍ以上、５０ｎｍ以下であるＣｕ濃化域が均一に分散している点にある。基地組織としては、通常の高強度鋼材と同様、例えば、ベイナイト単相組織、フェライト及びマルテンサイト二相組織、これらの組織に１０面積％程度以下の残留オーステナイトを含む組織とすることができる。残留オーステナイト相を含む組織とすることにより、加工誘起変態効果を利用して高い伸びを得ることができる。   The structural features of the high-strength steel material of the present invention are that the Cu concentration in the structure is not less than twice the Cu content (mass%) and not more than 80 mass%, and the average particle size is not less than 10 nm and not more than 50 nm. This is because the area is uniformly distributed. As the base structure, for example, a bainite single-phase structure, a ferrite and martensite two-phase structure, and a structure containing residual austenite of about 10 area% or less can be used as in the case of a normal high-strength steel material. By making the structure including the retained austenite phase, high elongation can be obtained by utilizing the processing-induced transformation effect.

前記Ｃｕ濃化域につき、そのＣｕ濃度をＣｕ含有率の２倍以上、８０mass％以下とするのは、基地中に固溶するＣｕあるいはＣｕ析出物およびこれらに近いＣｕ原子集団をＣｕ濃化域から排除するためである。また、そのサイズを１０〜５０ｎｍとするは以下の理由による。１０ｎｍ未満では転位に及ぼす影響が軽微となり、クラスターに比して顕著な疲労特性改善効果が期待できない。一方、５０ｎｍ超とするには、その元になるＣｕ析出物を時効過程で大形に成長させる必要があるが、そのための処理時間が膨大になり、生産性に劣る。また過大なＣｕ濃化域では、その分散が疎らになって疲労特性が却って低下するようになる。このため、Ｃｕ濃化域のサイズは平均粒径で１０〜５０ｎｍとする。   Regarding the Cu concentration region, the Cu concentration is set to be not less than twice the Cu content and not more than 80 mass% because Cu or Cu precipitates dissolved in the matrix and a Cu atom group close to these are concentrated in the Cu concentration region. This is to eliminate it. The size is set to 10 to 50 nm for the following reason. If the thickness is less than 10 nm, the influence on dislocations is slight, and a remarkable effect of improving fatigue characteristics cannot be expected as compared with clusters. On the other hand, in order to make it exceed 50 nm, it is necessary to grow a large amount of the Cu precipitate as a base during the aging process, but the processing time for that is enormous and the productivity is poor. Further, in an excessively concentrated Cu region, the dispersion becomes sparse and the fatigue characteristics are lowered. For this reason, the size of a Cu concentration area shall be 10-50 nm by an average particle diameter.

前記Ｃｕ濃化域の平均粒径は、Ｃｕ濃度がＣｕ含有率の２倍以上、８０mass％以下の領域の面積に等しい面積の円（相当円）の直径の平均値を意味する。具体的には以下の要領で求める。ＴＥＭによって観察した１０万倍の組織写真上ではＣｕの濃化域と基地とは黒白のコントラストとして観察される。Ｃｕ濃化域に対応する黒点部をＥＤＸ分析し、所定濃度範囲のＣｕ濃化域を選別し、画像ソフトによりその相当円を求める。そして、任意の５視野における相当円の平均値を平均粒径とする。なお、非特許文献１に記載されているようなクラスターは、Ｃｕ析出物の前駆段階の原子の集合体であり、上記ＴＥＭによる観察手法では観察することはできない。   The average particle diameter of the Cu-enriched region means an average value of diameters of circles (equivalent circles) having an area equal to the area of a region where the Cu concentration is not less than twice the Cu content and not more than 80 mass%. Specifically, it is calculated as follows. On the 100,000-fold tissue photograph observed by TEM, the thickened area of Cu and the base are observed as black and white contrast. EDX analysis is performed on the black spot portion corresponding to the Cu enriched region, the Cu enriched region in a predetermined concentration range is selected, and an equivalent circle is obtained by image software. And let the average value of the equivalent circle in arbitrary 5 visual fields be an average particle diameter. Note that the cluster as described in Non-Patent Document 1 is an aggregate of atoms at the precursor stage of the Cu precipitate, and cannot be observed by the observation method using the TEM.

上記のとおり、本発明の高強度鋼材は、引張強度が７５０ＭＰａ以上あればよく、成分的にはＣｕ以外の元素は特に限定されず、また組織も適宜の低温変態相（ベイナイト、マルテンサイト）を含む組織とすることができるが、好ましい鋼成分（単位mass％）は以下のとおりである。なお、「〜」の記号は、記号の左右の数値をその範囲に含む。   As described above, the high-strength steel material of the present invention only needs to have a tensile strength of 750 MPa or more, and the elements other than Cu are not particularly limited in terms of composition, and the structure also has an appropriate low-temperature transformation phase (bainite, martensite). Although it can be set as the structure | tissue which contains, a preferable steel component (unit mass%) is as follows. In addition, the symbol of "-" includes the numerical value on either side of a symbol in the range.

Ｃ：０．００５〜０．３０％
Ｃは鋼の強度を向上させる有効な元素であり、０．００５％未満ではかかる作用が過小であり、一方０．３０％を超えると延性劣化や溶接性の劣化が著しくなる。このため、下限を０．００５％、上限を０．３０％とする。好ましくは、０．０１〜０．２０％とするのがよい。 C: 0.005 to 0.30%
C is an effective element for improving the strength of the steel, and if it is less than 0.005%, such an action is too small. Therefore, the lower limit is set to 0.005% and the upper limit is set to 0.30%. Preferably, the content is 0.01 to 0.20%.

Ｓｉ：０．２０〜３．０％
Ｓｉは固溶強化元素であり、延性を低下させることなく、強度を向上させるのに有効な元素である。０．２０％未満では強度延性バランスが低下し、一方３．０％を超えると製造工程でスケールを多量に発生し、生産性を阻害するようになる。このため、下限を０．２０％、上限を３．０％とする。好ましくは０．３〜２．０％とするのがよい。 Si: 0.20 to 3.0%
Si is a solid solution strengthening element and is an element effective for improving the strength without lowering the ductility. If it is less than 0.20%, the strength ductility balance is lowered. On the other hand, if it exceeds 3.0%, a large amount of scale is generated in the production process, and productivity is inhibited. For this reason, the lower limit is 0.20% and the upper limit is 3.0%. Preferably it is 0.3 to 2.0%.

Ｍｎ：０．５〜３．０％
Ｍｎは鋼の強度、靭性を向上させる作用を有する元素であり、０．５％未満では強度が過小となり、一方３．０％超になると強度が高くなり過ぎて延性が劣化するようになる。このため、下限を０．５％、上限を３．０％とする。好ましくは０．８〜２．５％とするのがよい。 Mn: 0.5 to 3.0%
Mn is an element that has the effect of improving the strength and toughness of steel. If it is less than 0.5%, the strength is too low. On the other hand, if it exceeds 3.0%, the strength becomes too high and the ductility deteriorates. Therefore, the lower limit is 0.5% and the upper limit is 3.0%. Preferably it is 0.8 to 2.5%.

本発明の高強度鋼材は、上記の基本成分のほか、残部不可避的不純物からなるが、鋼材の機械的性質を向上させるため、下記元素の内、１種以上の元素を単独で、あるいは複合して含有させることができる。Ｎｉ，Ｃｒ，Ｍｏは焼入れ性向上元素として、Ｂは粒界強化元素として、Ｖ，Ｎｂ，Ｔｉ，Ｚｒ，Ｈｆは析出強化元素として強度向上に寄与する。
Ｎｉ：０．１０〜１０％、Ｃｒ：０．１０〜２．０％、Ｍｏ：０．１０〜２．０％、Ｂ：０．０００５〜０．００５０％、Ｖ，Ｎｂ，Ｔｉ，Ｚｒ，Ｈｆ：それぞれ０．００２〜０．３％ The high-strength steel material of the present invention is composed of the inevitable impurities remaining in addition to the basic components described above, but in order to improve the mechanical properties of the steel material, one or more of the following elements are used alone or in combination. Can be contained. Ni, Cr, and Mo contribute to improving strength as hardenability improving elements, B as grain boundary strengthening elements, and V, Nb, Ti, Zr, and Hf as precipitation strengthening elements.
Ni: 0.10 to 10%, Cr: 0.10 to 2.0%, Mo: 0.10 to 2.0%, B: 0.0005 to 0.0050%, V, Nb, Ti, Zr, Hf: 0.002 to 0.3% respectively

次に、本発明の高強度鋼材の製造方法について説明する。本発明の製造方法の特徴は、熱間圧延や熱間鍛造などの熱間加工を行う前に、Ｃｕを均一に固溶させた鋼片を時効処理し、これによってＣｕ析出物を析出させ、そのＣｕ析出物に加熱処理を施して、Ｃｕ濃度がＣｕ含有率の２倍以上、８０％以下で、平均粒径が１０〜５０ｎｍのＣｕ濃化域を形成するところにある。   Next, the manufacturing method of the high strength steel material of this invention is demonstrated. A feature of the production method of the present invention is that, before performing hot working such as hot rolling or hot forging, aging treatment is performed on a steel piece in which Cu is uniformly dissolved, thereby precipitating Cu precipitates, The Cu precipitate is subjected to a heat treatment to form a Cu enriched region having a Cu concentration of 2 to 80% and a mean particle size of 10 to 50 nm.

このようなサイズのＣｕ濃化域を形成するには、その元になるＣｕ析出物も大径化しておく必要があり、析出したＣｕ析出物を十分成長させるために、時効温度４００〜６００℃にて５ｈｒ以上保持する時効処理を施す。保持時間の上限は特に制限はないが、生産性を考慮すると４８ｈｒ程度以下でよい。また、前記加熱処理は、一旦析出成長したＣｕ析出物が完全に均一に再固溶することなく、所定のＣｕ濃化域を形成するように、加熱温度を８５０〜１０００℃、保持時間を０．５〜５ｈｒ、好ましくは１〜４ｈｒ程度に設定する。   In order to form a Cu-enriched region having such a size, it is necessary to enlarge the diameter of the Cu precipitate as the source. In order to sufficiently grow the deposited Cu precipitate, an aging temperature of 400 to 600 ° C. Is subjected to an aging treatment for 5 hours or more. The upper limit of the holding time is not particularly limited, but may be about 48 hr or less in consideration of productivity. The heat treatment is performed at a heating temperature of 850 to 1000 ° C. and a holding time of 0 so that a Cu precipitate once grown and grown does not completely re-dissolve completely and forms a predetermined Cu concentration region. .About.5 to 5 hours, preferably about 1 to 4 hours.

時効処理を施す鋼片は、通常、鋳造片を熱間粗加工し、１１００〜１３００℃程度で２０〜４０ｈｒ加熱する均熱処理を施す。かかる均熱処理によりＣｕを均一に固溶した鋼片が得られる。また、所定のＣｕ濃化域を生成した鋼片に対して行われる熱間加工は、通常、鋼片を１０００〜１２００℃程度に加熱し、Ａr3点以上の温度にて熱間加工を終了し、所期の低温変態相を生成させるように、またパーライトが生成しないように１０℃／ｓ程度以上の冷却速度にて冷却する。熱間加工として、熱間圧延や熱間鍛造が用いられる。熱間圧延の場合、通常、炭化物の析出を抑制するため、４００〜６５０℃程度の温度で巻き取られる。
以下、具体的実施例を挙げて本発明をより具体的に説明するが、本発明はかかる実施例によって限定的に解釈されるものではない。 The steel piece subjected to the aging treatment is usually subjected to a soaking process in which a cast piece is subjected to hot roughing and heated at about 1100 to 1300 ° C. for 20 to 40 hours. A steel piece in which Cu is uniformly dissolved is obtained by the soaking process. Moreover, the hot work performed with respect to the steel slab which produced | generated the predetermined Cu concentration area | region normally heats a steel slab to about 1000-1200 degreeC, and complete | finishes hot processing at the temperature more than Ar3 point. Then, cooling is performed at a cooling rate of about 10 ° C./s or more so that the desired low-temperature transformation phase is generated and pearlite is not generated. As hot working, hot rolling or hot forging is used. In the case of hot rolling, it is usually wound at a temperature of about 400 to 650 ° C. in order to suppress the precipitation of carbides.
Hereinafter, the present invention will be described more specifically with reference to specific examples, but the present invention is not construed as being limited to the examples.

表１に記載した鋼を大気溶解炉で溶解し、鋳造して鋳造片を得た。この鋳造片を小型圧延機で分解圧延し、２５mm厚の厚板を得た。この厚板を１２００℃×２４ｈｒの均熱処理を行い、その後、表２に示す条件で時効処理を行い、さらに同表に示す条件でＣｕ析出物をＣｕ濃化域とする加熱処理を行った。その厚板から２００mm×１２０mmの平面スラブを切り出し、９５０℃に加熱し、熱間圧延を行った。熱間圧延は、圧下量３０〜６０％の多パス圧延とし、最終板厚は３mmとした。熱間圧延の仕上げ温度（最終圧延の温度）を８００℃として圧延を終了し、その後、パーライトの生成を回避してベイナイト単相組織が得られるように４００℃まで５０℃／ｓで冷却し、その後空冷した。なお、表１において、鋼種Ｃは比較鋼、鋼種Ｄは参考鋼、その他鋼種は発明鋼である。 The steel described in Table 1 was melted in an atmospheric melting furnace and cast to obtain a cast piece. This cast piece was disassembled and rolled with a small rolling mill to obtain a 25 mm thick plate. This thick plate was subjected to a soaking treatment of 1200 ° C. × 24 hr, and then an aging treatment was performed under the conditions shown in Table 2, and further a heat treatment was performed using the Cu precipitate as a Cu-concentrated region under the conditions shown in the same table. A 200 mm × 120 mm flat slab was cut out from the thick plate, heated to 950 ° C., and hot rolled. The hot rolling was multipass rolling with a reduction amount of 30 to 60%, and the final plate thickness was 3 mm. The hot rolling finish temperature (final rolling temperature) is 800 ° C. to finish the rolling, and then cooled to 400 ° C. at 50 ° C./s so as to obtain a bainite single phase structure while avoiding the formation of pearlite. Then it was air cooled. In Table 1, steel type C is comparative steel, steel type D is reference steel, and other steel types are invention steels.

このようにして製造された熱延鋼板の幅方向の中央部において、板厚の中央部からＴＥＭ観察用試料を採取し、既述の方法でＴＥＭ組織解析を行い、Ｃｕ濃化域の平均粒径を求めた。さらに各熱延鋼板を用いて、引張試験、疲労試験を行った。引張試験は鋼板表裏面を０．０５mm研削し、その後、ＪＩＳＺ２２０１記載の５号試験片に加工し、ＪＩＳＺ２２４１に従って実施した。また疲労試験は、鋼板表裏面を０．０５mm研削し、その後、ＪＩＳＺ２２７５記載の表面曲げ疲れ試験に従って、疲労限度を測定した。これらの測定結果を表２に併せて示す。なお、表２において、試料No. に「＊」を付したものは比較例、試料No. ４は参考例、その他は発明例である。 In the central part in the width direction of the hot-rolled steel sheet manufactured in this way, a sample for TEM observation is collected from the central part of the plate thickness, TEM structure analysis is performed by the method described above, and the average grain in the Cu concentrated region The diameter was determined. Furthermore, a tensile test and a fatigue test were performed using each hot-rolled steel sheet. The tensile test was carried out according to JISZ2241, by grinding the steel sheet front and back surfaces by 0.05 mm, then processing into No. 5 test piece described in JISZ2201. In the fatigue test, the front and back surfaces of the steel plate were ground by 0.05 mm, and then the fatigue limit was measured according to the surface bending fatigue test described in JISZ2275. These measurement results are also shown in Table 2. In Table 2, samples marked with “*” in the sample No. are comparative examples, sample No. 4 is a reference example, and others are invention examples.

表２より、試料No. １，２，５，６及び８の鋼板（発明例）は、時効処理、時効後の加熱処理の条件が適正であるので、Ｃｕ濃化域の平均粒径が１０ｎｍ以上となっており、７５０ＭＰａ以上の高強度でありながら、疲労限度比０．５５以上が得られており、疲労特性に優れている。 From Table 2 , since the steel plates (invention examples) of Sample Nos. 1, 2, 5, 6 and 8 have appropriate conditions for aging treatment and heat treatment after aging, the average particle size in the Cu-concentrated region is 10 nm. The fatigue limit ratio of 0 .. 0 while being high strength of 750 MPa or more. 55 or more is obtained, and is excellent in fatigue characteristics.

一方、試料No. ３は、その鋼種ＣのＣｕ量が１．５％と低いため、時効処理、その後の加熱処理の条件が適正であるにもかかわらず、強度が低く、疲労限度比も発明例より劣っている。また、試料No. ７は時効処理における保持時間が短いため、Ｃｕ濃化域のサイズが小さく、このため疲労限度比が発明例より劣っている。また、試料No. ９は時効後の加熱処理における保持時間が１５min と短いため、Ｃｕ析出物が完全にＣｕ濃化域にならず、析出物のまま残存したため、疲労限度比が０．５未満に劣化した。   On the other hand, sample No. 3 has a low Cu content of steel type C of 1.5%, so that the strength is low and the fatigue limit ratio is invented even though the conditions of aging treatment and subsequent heat treatment are appropriate. It is inferior to the example. Sample No. 7 has a short retention time in the aging treatment, so the size of the Cu enriched region is small, and therefore the fatigue limit ratio is inferior to that of the invention example. Sample No. 9 has a short retention time of 15 minutes in the heat treatment after aging, so that the Cu precipitate does not completely enter the Cu-concentrated region and remains as a precipitate, so the fatigue limit ratio is less than 0.5. Deteriorated.

Claims (5)

鋼成分がmass％で、
Ｃ：０．００５〜０．３０％、
Ｓｉ：０．２０〜３．０％、
Ｍｎ：０．５〜３．０％、
Ｃｕ：２．０〜１０％
を含み、残部Ｆｅ及び不純物からなり、引張強さが７５０ＭＰａ以上の鋼材であって、組織内にＣｕ濃度がＣｕ含有率の２倍以上、８０mass％以下であるＣｕ濃化域が均一に分散し、前記Ｃｕ濃化域の平均粒径が１０ｎｍ以上、５０ｎｍ以下である、疲労特性に優れる高強度鋼材。 Steel component is mass%,
C: 0.005 to 0.30%,
Si: 0.20 to 3.0%,
Mn: 0.5 to 3.0%
Cu: 2.0 to 10%
A Cu enriched region having a balance of Fe and impurities and having a tensile strength of 750 MPa or more and a Cu concentration of not less than twice the Cu content and not more than 80 mass% in the structure. A high-strength steel material having excellent fatigue characteristics, wherein the Cu-concentrated region has an average particle size of 10 nm or more and 50 nm or less. 鋼材が熱延鋼板である、請求項１に記載した高強度鋼材。   The high-strength steel material according to claim 1, wherein the steel material is a hot-rolled steel sheet. さらに、Ｃｒ：０．１０〜２．０％、Ｍｏ：０．１０〜２．０％、Ｎｂ，Ｔｉ：それぞれ０．００２〜０．３％の元素のうち１種以上を含む、請求項１又は２に記載した高強度鋼材。 Furthermore, C r: 0.10~2.0%, Mo : 0.10~2.0%, N b, T i: each comprise one or more of 0.002 to 0.3% of the Elements, The high-strength steel material according to claim 1 or 2 . 請求項１に記載した成分を有する鋼の鋳造片を熱間粗加工し、１１００〜１３００℃で２０〜４０ｈｒ加熱する均熱処理を施して、Ｃｕが均一に固溶した鋼片を４００〜６００℃で５ｈｒ以上保持してＣｕ析出物を析出、成長させる時効処理を行い、時効処理した鋼片を８５０〜１０００℃で０．５〜５ｈｒ保持する加熱処理を行った後、その鋼片を熱間加工し、冷却する、請求項１に記載した高強度鋼材の製造方法。 A steel slab having the components described in claim 1 is subjected to hot roughing and subjected to a soaking treatment at 1100 to 1300 ° C. for 20 to 40 hours to obtain a steel slab in which Cu is uniformly solid-solved at 400 to 600 ° C. At 5 minutes or more, after performing an aging treatment for precipitating and growing Cu precipitates, and performing a heat treatment for holding the aging-treated steel pieces at 850 to 1000 ° C. for 0.5 to 5 hours, The manufacturing method of the high strength steel materials of Claim 1 which process and cool. 前記鋼はさらに、Ｃｒ：０．１０〜２．０％、Ｍｏ：０．１０〜２．０％、Ｎｂ，Ｔｉ：それぞれ０．００２〜０．３％の元素のうち１種以上を含む、請求項４に記載した高強度鋼材の製造方法。The steel further includes one or more elements selected from Cr: 0.10 to 2.0%, Mo: 0.10 to 2.0%, Nb and Ti: 0.002 to 0.3%, A method for producing a high-strength steel material according to claim 4.