JP2004211199A - High strength hot rolled steel sheet having excellent fatigue resistance and excellent strength-ductility balance, and method of producing the same - Google Patents

High strength hot rolled steel sheet having excellent fatigue resistance and excellent strength-ductility balance, and method of producing the same Download PDF

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JP2004211199A
JP2004211199A JP2003373670A JP2003373670A JP2004211199A JP 2004211199 A JP2004211199 A JP 2004211199A JP 2003373670 A JP2003373670 A JP 2003373670A JP 2003373670 A JP2003373670 A JP 2003373670A JP 2004211199 A JP2004211199 A JP 2004211199A
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steel sheet
rolled steel
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JP4367091B2 (en
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Yukihiro Matsubara
行宏 松原
Toshiki Hiruta
敏樹 蛭田
Masanori Kitahama
正法 北浜
Takeshi Hirabayashi
毅 平林
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JFE Steel Corp
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet which has an excellent strength-ductility balance, and excellent fatigue resistance. <P>SOLUTION: A steel stock having a composition comprising, by mass, 0.05 to 0.3% C, 0.01 to 2.5% Si and 0.5 to 3.0% Mn is heated, and is then successively subjected to: a hot rolling stage; a primary cooling stage where cooling is performed to a temperature range of 650 to 950°C at a cooling rate of 5 to 500°C/s; a strain impartation stage where repeated bending and rebending working by a leveller is performed and a strain of 0.05 to 2.0 is imparted to the outermost surface part of the hot rolled steel sheet; and a secondary cooling stage where cooling is performed to a temperature of ≤650°C at a cooling rate of 10 to 300°C. Thus, the high strength hot rolled steel sheet is obtained having a crystal grain size gradient structure in which the volume fraction of polygonal ferrite as the main phase is ≥70%, the volume fraction of a secondary phase other than that is ≥5%, and the average crystal grain size of the polygonal ferrite gradually decreases from the center of the sheet thickness toward the surface layer. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

本発明は、自動車部品、機械構造部品等の用途に好適な、強度−延性バランスに優れた高強度熱延鋼板に係り、とくに耐疲労特性の向上に関する。なお、本発明でいう「高強度」とは引張強さ:440MPa以上の熱延鋼板をいうものとする。また、「強度−延性バランスに優れた」とは、(引張強さTS)×(伸びEl)が20000MPa%以上である熱延鋼板をいうものとする。さらに、熱延鋼板は熱延鋼帯をも含むものとする。   TECHNICAL FIELD The present invention relates to a high-strength hot-rolled steel sheet excellent in strength-ductility balance and suitable for applications such as automobile parts and mechanical structure parts, and more particularly to improvement of fatigue resistance characteristics. In the present invention, “high strength” refers to a hot-rolled steel sheet having a tensile strength of 440 MPa or more. Further, “excellent in strength-ductility balance” refers to a hot-rolled steel sheet having (tensile strength TS) × (elongation El) of 20,000 MPa% or more. Further, the hot-rolled steel sheet includes a hot-rolled steel strip.

自動車部品、機械構造部品などに用いられる鋼板には、高い強度や優れた加工性、さらに優れた耐疲労特性を有することが要求されている。強度、加工性、耐疲労特性などの機械的特性を向上させるためには、鋼板の結晶粒を細かくすることが有効であることは従来から良く知られていることであり、鋼板の結晶粒微細化方法が種々検討されている。その代表的なものとして、例えば、特許文献1に記載された、いわゆる制御圧延法がある。   Steel sheets used for automobile parts, mechanical structural parts, and the like are required to have high strength, excellent workability, and excellent fatigue resistance. It is well known that it is effective to reduce the crystal grains of steel sheets in order to improve mechanical properties such as strength, workability, and fatigue resistance. Various methods have been studied. A typical example is a so-called controlled rolling method described in Patent Document 1.

制御圧延法の特徴は、オーステナイト再結晶温度域の高温側で圧延を開始し、動的再結晶、あるいは静的再結晶を利用して、オーステナイト( 以下、単にγともいう) 粒を微細化すること、および、γ域の低温側であるγ未再結晶温度域で再び圧延し、γ粒内に転位などの格子欠陥を導入し、そこを起点としγ→α変態を促進させること、の2点により、フェライト( 以下、単にαともいう) 粒の微細化を実現することにある。すなわち、γ→α変態時のα核生成サイトであるγ粒界を増加すること、及び、転位などの格子欠陥をより多量に導入することにより、α粒を数多く生成し、結晶粒の微細化を図ろうとするものである。   The feature of the controlled rolling method is that rolling starts on the high temperature side of the austenite recrystallization temperature range, and refines austenite (hereinafter simply referred to as γ) grains by using dynamic recrystallization or static recrystallization. And rolling again in the γ non-recrystallization temperature range, which is the lower temperature side of the γ range, to introduce lattice defects such as dislocations in the γ grains, and to promote the γ → α transformation starting therefrom. In view of the above, it is an object to realize finer ferrite (hereinafter simply referred to as α) grains. In other words, increasing the number of γ grain boundaries, which are α nucleation sites during the γ → α transformation, and introducing a larger amount of lattice defects such as dislocations, generate a large number of α grains and refine the crystal grains. It is to try to.

また、特許文献2には、鋳片を再加熱後の圧延開始前および/または圧延途中で水冷し、表層を(γ+α)2相域またはαの単相状態にしたのち、累積圧下率20%以上の圧延を行い、復熱によりAC3変態点以上まで上昇した後圧延を終了して、引き続きAr3変態点以上の温度域で繰返し曲げ加工により表層に所定量以上の歪を付与することにより表層の結晶粒を微細化して、脆性亀裂伝播停止特性を向上させる厚鋼板の製造方法が提案されている。
特開昭63-223124 号公報 特開平7-76726 号公報
Further, Patent Document 2 discloses that a slab is water-cooled before rolling and / or during rolling after reheating to bring a surface layer into a (γ + α) two-phase region or a single-phase state of α, and a cumulative rolling reduction of 20% The above-mentioned rolling is performed, the rolling is finished after the temperature rises to the AC3 transformation point or more by recuperation, and the surface layer is subjected to a predetermined amount of strain or more by repeatedly bending at a temperature range of the Ar3 transformation point or more, thereby continuously forming the surface layer. There has been proposed a method for manufacturing a thick steel plate in which crystal grains are refined to improve brittle crack propagation stopping characteristics.
JP-A-63-223124 JP 7-76726 A

γ粒の微細化や格子欠陥の多量導入のためには、できるだけ多くの歪を鋼板に付与することが必要となる。しかし、スラブ厚、シートバー厚や製品厚は決定されているため、圧延プロセスで付与できる歪量には限界がある。このため、一般の制御圧延法により得られる結晶粒は、平均結晶粒径で5μm 程度までの微細化が限界であると言われている。   In order to refine γ grains and introduce a large amount of lattice defects, it is necessary to impart as much strain as possible to the steel sheet. However, since the slab thickness, sheet bar thickness, and product thickness are determined, there is a limit to the amount of strain that can be applied in the rolling process. For this reason, it is said that the crystal grain obtained by the general controlled rolling method has a limit of miniaturization to an average crystal grain size of about 5 μm.

一般的に、結晶粒の微細化に伴う強度等の機械的特性の改善効果は、結晶粒径の平方根に逆比例する。したがって、一般の制御圧延法におけるような50〜5μm までの領域における結晶粒微細化では、機械的特性の改善効果は小さい。更なる機械的特性の向上のために、5μm 以下となるような結晶粒の更なる微細化が熱望されている。   Generally, the effect of improving mechanical properties such as strength associated with the refinement of crystal grains is inversely proportional to the square root of the crystal grain size. Therefore, the effect of improving the mechanical properties is small when the crystal grains are refined in the range of 50 to 5 μm as in a general controlled rolling method. In order to further improve the mechanical properties, further refinement of the crystal grains to have a size of 5 μm or less has been desired.

また、特許文献2に記載された技術は、厚鋼板を対象にし、表層のみを(γ+α)の2相またはα単相まで冷却した後、復熱により、再びγ単相とする必要がある。この技術を薄鋼板である熱延鋼板に適用する場合、鋼板が薄いため、内部からの復熱により、α→γ変態させることは非常に困難であり、実質的に不可能である。また、さらに特許文献2に記載された技術では、脆性亀裂伝播停止特性の改善を目的としており、耐疲労特性の改善についてはなんの配慮もなされていない。   Further, the technique described in Patent Literature 2 needs to cool a surface layer only to a two-phase (γ + α) or an α-single phase for a thick steel plate, and then reheat the steel sheet to a γ-single phase again. When this technique is applied to a hot-rolled steel sheet, which is a thin steel sheet, it is very difficult and practically impossible to transform α → γ by reheating from inside because the steel sheet is thin. Further, the technique described in Patent Document 2 aims at improving the brittle crack propagation arresting property, and does not consider the improvement of the fatigue resistance property at all.

本発明は、上記した従来技術の問題を有利に解決し、平均結晶粒径が細かく、かつ表層部の平均結晶粒径が中央部に比べ微細であり、高強度で強度−延性バランスに優れ、かつ耐疲労特性に優れた高強度熱延鋼板およびその製造方法を提供することを目的とする。本発明では、鋼板組成が同一でも従来の強度レベルに比べ1ランク上の強度を有する高強度熱延鋼板を提供することを目的とする。   The present invention advantageously solves the above-mentioned problems of the prior art, the average crystal grain size is fine, and the average crystal grain size of the surface layer portion is finer than that of the central portion, high strength and excellent in strength-ductility balance, Another object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in fatigue resistance and a method for producing the same. An object of the present invention is to provide a high-strength hot-rolled steel sheet having a strength one rank higher than a conventional strength level even if the composition of the steel sheet is the same.

本発明者らは、上記した課題を解決するために、強度・加工性とともに耐疲労特性を向上させる方策について、鋭意検討した。その結果、本発明者らは、強度、加工性とともに耐疲労特性を向上させるには、板厚平均結晶粒径が細かく、かつ表層部の平均結晶粒径が中央部に比べ微細な鋼板とすることが重要であることに想到した。具体的には、鋼板の平均結晶粒径を6μm 以下と細かくし、さらに表層部の平均結晶粒径を中心部のそれに比べ小さくし、表層部平均結晶粒径を板厚全体の平均結晶粒径の90%〜30%まで細かくすることが重要であることを知見した。   Means for Solving the Problems The present inventors have diligently studied measures for improving fatigue resistance as well as strength and workability in order to solve the above problems. As a result, the present inventors, in order to improve the fatigue resistance along with the strength and workability, the thickness average crystal grain size is fine, and the average crystal grain size of the surface layer is to be a steel sheet finer than the central part. Came to the point that it was important. Specifically, the average grain size of the steel sheet is reduced to 6 μm or less, the average grain size of the surface layer is made smaller than that of the central portion, and the average grain size of the surface layer is reduced to the average grain size of the entire sheet thickness. It has been found that it is important to make the fineness up to 90% to 30%.

本発明者らは、鋼板の耐疲労特性は、鋼板の平均結晶粒径よりも、表層近傍の結晶粒径に依存しており、仕上圧延後の鋼板を所定の温度域まで冷却し、ついで繰返し曲げ・曲げ戻し加工を施し、再び冷却することにより、鋼板表層の結晶粒が鋼板中心にくらべ細かくなり、耐疲労特性が顕著に向上することを見出した。   The present inventors have found that the fatigue resistance characteristics of a steel sheet depend on the crystal grain size near the surface layer rather than the average crystal grain size of the steel sheet, and the steel sheet after finish rolling is cooled to a predetermined temperature range, and then repeatedly. It has been found that by performing bending / unbending processing and cooling again, the crystal grains in the surface layer of the steel sheet become finer than in the center of the steel sheet, and the fatigue resistance characteristics are remarkably improved.

所定温度域に冷却し、鋼板に繰返し曲げ・曲げ戻し加工を施すと、鋼板の板厚を変更することなく、鋼板表層部に曲げ・曲げ戻し歪を蓄積させることができる。とくに、板厚中心から表層に向かうほど、蓄積される曲げ歪が大きくなる。そのため、その後のγ→α変態時に、最表層により多くのα粒の核を生成でき、その後冷却することにより、板厚平均結晶粒径を細かくできるとともに、とくに鋼板表層の結晶粒微細化が有効に図れる。これにより、高強度で強度−延性バランスに優れるとともに、耐疲労特性が顕著に向上した熱延鋼板とすることができることを知見した。   When the steel sheet is cooled to a predetermined temperature range and subjected to repeated bending and bending back processing, bending and bending back strain can be accumulated in the surface layer of the steel sheet without changing the thickness of the steel sheet. In particular, the accumulated bending strain increases from the center of the plate thickness toward the surface layer. Therefore, at the time of the subsequent γ → α transformation, more α grains can be generated in the outermost layer, and after cooling, the thickness average crystal grain size can be reduced, and the grain refinement of the surface layer of the steel sheet is particularly effective. Can be planned. This has revealed that a hot-rolled steel sheet having high strength, excellent strength-ductility balance, and markedly improved fatigue resistance can be obtained.

本発明は、このような知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨はつぎのとおりである。
(1)mass%で、C:0.05〜0.3%、Si:0.01〜2.5 %、Mn:0.5 〜3.0 %を含み、残部Feおよび不可避的不純物からなる組成と、主相としてポリゴナルフェライトを体積分率で70%以上、ポリゴナルフェライト以外の第二相を体積分率で5%以上含み、さらに、前記ポリゴナルフェライトの平均結晶粒径が板厚中心から表層に向かい漸次小さくなる結晶粒径傾斜組織を有することを特徴とする耐疲労特性に優れ、かつ強度−延性バランスに優れた高強度熱延鋼板。
(2)(1)において、前記強度−延性バランスが、21000MPa%以上であることを特徴とする高強度熱延鋼板。
(3)(1)または(2)において、前記ポリゴナルフェライトの表層における平均結晶粒径が1μm以下、前記ポリゴナルフェライトの板厚全域を対象とする板厚平均結晶粒径が1.8μm以下であることを特徴とする高強度熱延鋼板。
(4)(1)ないし(3)のいずれかにおいて、前記組成に加えてさらに、mass%で、Ti:0.005〜0.25%、Nb:0.005 〜0.2 %のうちから選ばれた1種または2種を含有することを特徴とする高強度熱延鋼板。
(5)(1)ないし(4)のいずれかにおいて、前記組成に加えてさらに、mass%で、Cu:0.2 %以下、Ni:0.2 %以下、Cr:0.2 %以下、Mo:0.2 %以下、B:0.2 %以下、V:0.2 %以下のうちから選ばれた1種または2種以上を、合計で1.0 %以下含有することを特徴とする高強度熱延鋼板。
(6)mass%で、C:0.05〜0.3%、Si:0.01〜2.5 %、Mn:0.5 〜3.0 %を含み、残部Feおよび不可避的不純物からなる組成の鋼素材を加熱し、 仕上圧延を含む熱間圧延を施す熱間圧延工程と、該仕上圧延後の熱延鋼板に5℃/s以上500 ℃/s以下の冷却速度で650 ℃以上950 ℃以下の温度域まで冷却する一次冷却工程と、前記温度域まで冷却された熱延鋼板にレベラによる繰返し曲げ・曲げ戻し加工を施し、熱延鋼板最表部に0.05以上2.0 以下の歪を付与する歪付与工程と、前記歪付与工程を経た熱延鋼板に10℃/s以上300 ℃/s以下の冷却速度で650 ℃以下の温度まで冷却する二次冷却工程と、を順次施すことを特徴とする、耐疲労特性に優れ、かつ強度−延性バランスに優れた高強度熱延鋼板の製造方法。
(7)(6)において、前記一次冷却工程における冷却を停止する温度域を800℃以下とし、前記歪付与工程における歪を1.0超えとすることを特徴とする高強度熱延鋼板の製造方法。
(8)(6)または(7)において、前記組成に加えてさらに、mass%で、Ti:0.005〜0.25%、Nb:0.005 〜0.2 %のうちから選ばれた1種または2種を含有することを特徴とする高強度熱延鋼板の製造方法。
(9)(6)ないし(8)のいずれかにおいて、前記組成に加えてさらに、mass%で、Cu:0.2 %以下、Ni:0.2 %以下、Cr:0.2 %以下、Mo:0.2 %以下、B:0.2 %以下、V:0.2 %以下のうちから選ばれた1種または2種以上を、合計で1.0 %以下含有することを特徴とする高強度熱延鋼板の製造方法。
The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.05-0.3%, Si: 0.01-2.5%, Mn: 0.5-3.0%, composition consisting of balance Fe and unavoidable impurities and polygonal ferrite as main phase by volume Grain size gradient containing 70% or more by volume fraction and 5% by volume or more of the second phase other than polygonal ferrite, and the average crystal grain size of the polygonal ferrite gradually decreases from the center of the plate thickness toward the surface layer. A high-strength hot-rolled steel sheet having an excellent fatigue resistance characteristic characterized by having a structure and an excellent balance between strength and ductility.
(2) The high-strength hot-rolled steel sheet according to (1), wherein the strength-ductility balance is 21000 MPa% or more.
(3) In (1) or (2), the average crystal grain size in the surface layer of the polygonal ferrite is 1 μm or less, and the average thickness of the polygonal ferrite is 1.8 μm or less in the entire thickness range. A high-strength hot-rolled steel sheet characterized by the following.
(4) In any one of (1) to (3), in addition to the above composition, one or two members selected from Ti: 0.005 to 0.25% and Nb: 0.005 to 0.2% by mass%. A high-strength hot-rolled steel sheet comprising:
(5) In any one of the constitutions (1) to (4), in addition to the above-mentioned composition, in mass%, Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, A high-strength hot-rolled steel sheet characterized in that one or more selected from B: 0.2% or less and V: 0.2% or less are contained in a total of 1.0% or less.
(6) Mass%: C: 0.05-0.3%, Si: 0.01-2.5%, Mn: 0.5-3.0%, steel material with composition consisting of balance Fe and unavoidable impurities is heated, including finish rolling A hot rolling step of performing hot rolling, and a primary cooling step of cooling the hot-rolled steel sheet after the finish rolling to a temperature range of 650 ° C to 950 ° C at a cooling rate of 5 ° C / s to 500 ° C / s. The hot-rolled steel sheet cooled to the temperature range is subjected to a repeated bending / bending process by a leveler, a strain applying step of applying a strain of 0.05 or more and 2.0 or less to the outermost part of the hot-rolled steel sheet, and the strain applying step is performed. A secondary cooling step of cooling the hot-rolled steel sheet to a temperature of 650 ° C. or less at a cooling rate of 10 ° C./s or more and 300 ° C./s or less, characterized by excellent fatigue resistance and strength. Manufacturing method of high strength hot rolled steel sheet with excellent ductility balance.
(7) The method for producing a high-strength hot-rolled steel sheet according to (6), wherein the temperature range in which the cooling in the primary cooling step is stopped is 800 ° C. or less, and the strain in the strain applying step is more than 1.0.
(8) In (6) or (7), in addition to the above composition, one or two selected from Ti: 0.005 to 0.25% and Nb: 0.005 to 0.2% by mass% are further contained. A method for producing a high-strength hot-rolled steel sheet.
(9) In any one of (6) to (8), in addition to the above-mentioned composition, further, in mass%, Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, A method for producing a high-strength hot-rolled steel sheet, comprising one or more selected from B: 0.2% or less and V: 0.2% or less in total of 1.0% or less.

本発明によれば、強度−延性バランスに優れるうえ、耐疲労特性にも優れた高強度熱延鋼板を容易にしかも安価に提供することができ、産業上格段の効果を奏する。本発明の高強度熱延鋼板は自動車部品用、機械構造部品用など、種々の用途に適した鋼板である。   According to the present invention, a high-strength hot-rolled steel sheet excellent in strength-ductility balance and excellent in fatigue resistance can be easily and inexpensively provided, and an industrially remarkable effect is achieved. The high-strength hot-rolled steel sheet of the present invention is a steel sheet suitable for various uses such as for automobile parts and mechanical structural parts.

本発明の熱延鋼板は、mass%で、C:0.05〜0.3%、Si:0.01〜2.5 %、Mn:0.5 〜3.0 %を含み、好ましくはS:0.02%以下、P:0.03%以下、Al:0.2 %以下、N:0.02%以下を含み、残部Feおよび不可避的不純物からなる組成を有する。   The hot-rolled steel sheet of the present invention contains, by mass%, C: 0.05 to 0.3%, Si: 0.01 to 2.5%, Mn: 0.5 to 3.0%, preferably S: 0.02% or less, P: 0.03% or less, and Al: 0.03% or less. : N: 0.02% or less, N: 0.02% or less, with the balance being Fe and unavoidable impurities.

まず、本発明の熱延鋼板の組成限定理由について説明する。なお、以下、組成におけるmass%は単に%と記す。   First, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention will be described. Hereinafter, mass% in the composition is simply described as%.

C:0.05〜0.3 %
Cは、鋼の固溶強化、および組織強化に有効な元素であり、このような効果を得るために本発明では0.05%以上の含有を必要とする。0.05%未満では、十分な強度が得られない。一方、0.3 %を超えると、延性が低下する。このため、本発明では、Cは0.05〜0.3 %に限定した。
C: 0.05-0.3%
C is an element effective for solid solution strengthening and structure strengthening of steel, and in order to obtain such an effect, the present invention needs to contain 0.05% or more. If it is less than 0.05%, sufficient strength cannot be obtained. On the other hand, if it exceeds 0.3%, the ductility decreases. Therefore, in the present invention, C is limited to 0.05 to 0.3%.

Si:0.01〜2.5 %
Siは、鋼の固溶強化に有効に作用する元素であり、本発明では0.01%以上の含有を必要とする。一方、2.5 %を超えて含有すると、スケールの剥離性が低下し、表面品質が悪化する。このため、Siは0.01〜2.5 %に限定した。なお、好ましくは0.1 〜1.5 %である。
Si: 0.01-2.5%
Si is an element effectively acting on solid solution strengthening of steel, and in the present invention, the content of 0.01% or more is required. On the other hand, if the content exceeds 2.5%, the releasability of the scale decreases and the surface quality deteriorates. For this reason, Si was limited to 0.01 to 2.5%. Incidentally, the content is preferably 0.1 to 1.5%.

Mn:0.5 〜3.0 %
Mnは、鋼板冷却時の変態点を低下させる作用を有する元素であり、本発明では0.5 %以上の含有を必要とする。一方、3.0 %を超えて含有すると、延性が低下する。このため、Mnは0.5 〜3.0 %の範囲に限定した。なお、強度−延性バランスを高くする観点から、好ましくは0.7〜2.5%である。
Mn: 0.5 to 3.0%
Mn is an element having an effect of lowering the transformation point at the time of cooling the steel sheet. In the present invention, Mn needs to be contained at 0.5% or more. On the other hand, if the content exceeds 3.0%, the ductility decreases. For this reason, Mn is limited to the range of 0.5 to 3.0%. From the viewpoint of increasing the strength-ductility balance, the content is preferably 0.7 to 2.5%.

S:0.02%以下
Sは、硫化物として存在し、鋼の清浄度を低下させるとともに、鋼の延性および耐疲労特性を低下させる。このため、Sは0.02%以下に限定することが望ましい。なお、好ましくは0.01%以下である。
S: 0.02% or less S exists as a sulfide, and lowers the cleanliness of the steel and decreases the ductility and fatigue resistance of the steel. Therefore, S is desirably limited to 0.02% or less. In addition, it is preferably 0.01% or less.

P:0.03%以下
Pは、固溶して鋼の強度を増加させる元素であるが、結晶粒界に偏析しやすく、多量の含有は鋼の靭性劣化を招く。このため、Pは0.03%以下に限定することが好ましい。なお、より好ましくは0.02%以下である。
P: 0.03% or less P is an element which increases the strength of steel by forming a solid solution, but is easily segregated at the crystal grain boundary, and a large amount of P causes deterioration of the toughness of the steel. For this reason, P is preferably limited to 0.03% or less. Note that the content is more preferably 0.02% or less.

Al:0.2 %以下
Alは、酸素との結合力が強く、通常脱酸剤として用いられる元素であるが、0.2 %を超えて含有すると、鋼板の延性が低下する。このため、Alは0.2 %以下に限定することが望ましい。なお、好ましくは0.05%以下である。
Al: 0.2% or less
Al is an element which has a strong bonding force with oxygen and is usually used as a deoxidizing agent. However, if it exceeds 0.2%, the ductility of the steel sheet decreases. Therefore, it is desirable to limit Al to 0.2% or less. In addition, it is preferably 0.05% or less.

N:0.02%以下
Nは、固溶して鋼の強度を増加させる元素であるが、多量に含有すると鋼板の成形性の低下を招く。このため、Nは0.02%以下に限定することが好ましい。
N: 0.02% or less N is an element that increases the strength of steel by forming a solid solution. However, when N is contained in a large amount, the formability of the steel sheet is reduced. For this reason, it is preferable to limit N to 0.02% or less.

本発明では、上記した基本組成に加えてさらに、Ti:0.005 〜0.25%、Nb:0.005 〜0.2 %のうちから選ばれた1種または2種を含有することができる。   In the present invention, in addition to the above-mentioned basic composition, one or two selected from 0.005 to 0.25% of Ti and 0.005 to 0.2% of Nb can be further contained.

Ti、Nbは、いずれも、炭化物あるいは窒化物、炭窒化物を形成し、結晶粒の粒成長を抑制し結晶粒の微細化に有効に作用するとともに、析出強化にも寄与し、鋼板の強度向上に有効に作用する元素であり、必要に応じ選択して含有できる。このような効果を得るためには、それぞれTi:0.005 %以上、Nb:0.005 %以上を含有することが望ましい。一方、Ti:0.25%、Nb:0.2%を超えて含有しても、効果が飽和し、含有量に見合う効果が期待できなくなる。このため、Ti:0.005 〜0.25%、Nb:0.005 〜0.2 %の範囲に限定することが好ましい。   Both Ti and Nb form carbides, nitrides, and carbonitrides, suppress grain growth and effectively act on grain refinement, and contribute to precipitation strengthening, and contribute to the strengthening of steel sheets. It is an element that effectively works for improvement, and can be selected and contained as needed. In order to obtain such effects, it is desirable to contain Ti: 0.005% or more and Nb: 0.005% or more, respectively. On the other hand, if the content exceeds 0.25% of Ti and 0.2% of Nb, the effect is saturated, and the effect corresponding to the content cannot be expected. For this reason, it is preferable to limit the content to 0.005 to 0.25% for Ti and 0.005 to 0.2% for Nb.

本発明では、上記した各組成に加えてさらに、Cu:0.2 %以下、Ni:0.2 %以下、Cr:0.2 %以下、Mo:0.2 %以下、B:0.2 %以下、V:0.2 %以下のうちから選ばれた1種または2種以上を、合計で1.0 %以下含有することができる。   In the present invention, in addition to the above-described compositions, Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, B: 0.2% or less, V: 0.2% or less One or more selected from the above may be contained in a total of 1.0% or less.

Cu、Ni、Cr、Mo、B、Vはいずれも、鋼の強度を増加させる元素であり、必要に応じ1種または2種以上を選択して含有できる。Cu:0.2 %、Ni:0.2 %、Cr:0.2 %、Mo:0.2 %、B:0.2 %、V:0.2 %をそれぞれ超えて含有すると、また、合計で1.0 %を超えて含有すると、加工性が低下する。このため、Cu:0.2 %以下、Ni:0.2 %以下、Cr:0.2 %以下、Mo:0.2 %以下、B:0.2 %以下、V:0.2 %以下のうちから選ばれた1種または2種以上を、合計で1.0 %以下に限定することが好ましい。   Cu, Ni, Cr, Mo, B, and V are all elements that increase the strength of steel, and one or two or more of them can be selected and contained as necessary. If the content exceeds Cu: 0.2%, Ni: 0.2%, Cr: 0.2%, Mo: 0.2%, B: 0.2%, and V: 0.2%, and if the total content exceeds 1.0%, the workability will increase. Decreases. Therefore, one or two or more selected from Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, B: 0.2% or less, V: 0.2% or less Is preferably limited to 1.0% or less in total.

なお、上記した成分以外の残部は、Feおよび不可避的不純物である。不可避的不純物としては、Ca:0.01%以下、O:0.01%以下等が許容できる。   The balance other than the above components is Fe and unavoidable impurities. As unavoidable impurities, Ca: 0.01% or less, O: 0.01% or less can be tolerated.

次に、本発明の熱延鋼板における金属組織について説明する。   Next, the metallographic structure of the hot-rolled steel sheet of the present invention will be described.

本発明の熱延鋼板は、上記した組成を有するとともに、ポリゴナルフェライトを主相として、ポリゴナルフェライト以外を第二相とし、ポリゴナルフェライトの平均結晶粒径が板厚中心から表層に向かい漸次小さくなる結晶粒径傾斜組織を有する。   The hot-rolled steel sheet of the present invention has the above-described composition, has polygonal ferrite as a main phase, and has a second phase other than polygonal ferrite, and the average crystal grain size of the polygonal ferrite gradually increases from the center of the sheet thickness toward the surface layer. It has a grain size gradient structure that becomes smaller.

主相であるポリゴナルフェライトは、体積分率で70%以上とする。ポリゴナルフェライトが70%未満では、延性(伸び)が大きく低下し強度−延性バランスが低下する。なお、ポリゴナルフェライトは強度確保の観点から体積分率で95%以下とすることが好ましい。本発明の熱延鋼板では、主相であるポリゴナルフェライトの粒径が、板厚中心から表層に向かい漸次小さくなる、結晶粒径傾斜組織を有する。表層部のポリゴナルフェライトが中央部に比べ細かい結晶粒径を有することにより、耐疲労特性が顕著に向上する。本発明の熱延鋼板では、板厚方向全域を対象とする板厚平均結晶粒径は、6μm 以下であり、表層部の平均結晶粒径が板厚平均結晶粒径の90〜30%であることが好ましい。   Polygonal ferrite, which is the main phase, has a volume fraction of 70% or more. If the polygonal ferrite content is less than 70%, the ductility (elongation) is greatly reduced, and the strength-ductility balance is reduced. The polygonal ferrite is preferably set to 95% or less in volume fraction from the viewpoint of securing strength. The hot-rolled steel sheet according to the present invention has a crystal grain size gradient structure in which the polygonal ferrite as the main phase gradually decreases in size from the center of the sheet thickness toward the surface layer. When the polygonal ferrite in the surface portion has a finer crystal grain size than the central portion, fatigue resistance is significantly improved. In the hot-rolled steel sheet of the present invention, the average crystal grain size in the entire thickness direction is 6 μm or less, and the average crystal grain size in the surface layer portion is 90 to 30% of the average crystal grain size in the sheet thickness. Is preferred.

表層部の平均結晶粒径が板厚平均結晶粒径に比べて90%を超えて大きくなると耐疲労特性の改善が顕著でなくなる。また、板厚平均結晶粒径が6μm を超えて大きくなると、結晶粒微細化による、強度向上効果が小さくなる。なお、本発明でいう表層部とは、 最表層から板厚方向(深さ方向)に200 μm までの領域をいうものとする。より好ましくは、板厚平均結晶粒径が1.8μm以下、表層部の平均結晶粒径が1μm以下である。   When the average crystal grain size of the surface layer portion exceeds 90% as compared with the average thickness grain size, the improvement in fatigue resistance characteristics is not remarkable. On the other hand, when the average thickness of the sheet exceeds 6 μm, the effect of improving the strength due to the refinement of the crystal grains decreases. The surface layer portion in the present invention refers to a region from the outermost layer to 200 μm in the thickness direction (depth direction). More preferably, the average crystal grain size in the plate thickness is 1.8 μm or less, and the average crystal grain size in the surface layer is 1 μm or less.

本発明で第二相とは、主相であるポリゴナルフェライト以外の組織をいうものとする。第二相は、体積分率で合計5%以上とする。第二相の体積分率が5%未満では、鋼板の強度が大きく低下し、強度−延性バランスが低下する。なお、第二相は延性確保の観点から体積分率で30%以下とすることが好ましい。第二相としては、パーライト、ベイナイト、マルテンサイト、残留オーステナイトのうちの1種または2種以上とすることが好ましい。とくに、強度向上を図る場合には、マルテンサイト量を多くすることが好ましく、延性向上を図る場合には、残留オーステナイト量を多くすることが好ましい。   In the present invention, the second phase means a structure other than the main phase, polygonal ferrite. The second phase has a total volume fraction of 5% or more. If the volume fraction of the second phase is less than 5%, the strength of the steel sheet is greatly reduced, and the strength-ductility balance is reduced. In addition, it is preferable to set the volume of the second phase to 30% or less from the viewpoint of ensuring ductility. As the second phase, one or more of pearlite, bainite, martensite, and retained austenite are preferably used. In particular, it is preferable to increase the amount of martensite when improving the strength, and it is preferable to increase the amount of retained austenite when improving the ductility.

つぎに、本発明の熱延鋼板の好ましい製造方法について説明する。   Next, a preferred method for producing the hot-rolled steel sheet of the present invention will be described.

上記した組成の溶鋼を、転炉、 電気炉等の公知の溶製方法で溶製し、連続鋳造法等の公知の鋳造方法でスラブ等の鋼素材とする。本発明では鋼素材の製造方法はとくに限定されない。通常公知の方法がいずれも好適である。   The molten steel having the composition described above is smelted by a known smelting method such as a converter or an electric furnace, and is made into a steel material such as a slab by a known casting method such as a continuous casting method. In the present invention, the method for producing the steel material is not particularly limited. In general, any known method is suitable.

ついで、鋼素材は、好ましくは1000℃以上、1200℃以下の温度に加熱され、 熱間圧延工程を経て、熱延鋼板とされる。この熱間圧延における加熱は、鋼素材が圧延可能温度以上である場合には、加熱することなく、 あるいはわずかに加熱する程度の直送圧延としてもよい。   Next, the steel material is preferably heated to a temperature of 1000 ° C. or more and 1200 ° C. or less, and is subjected to a hot rolling step to be a hot-rolled steel sheet. The heating in the hot rolling may be a direct-feed rolling without heating or slightly heating when the steel material is at a rolling temperature or higher.

加熱された鋼素材は、粗圧延によりシートバーとされ、ついで仕上圧延を施されて所定板厚の熱延鋼板とされる。なお、シートバーあるいは薄スラブを素材とする場合には、粗圧延を省略してもよいことはいうまでもない。   The heated steel material is converted into a sheet bar by rough rolling, and then subjected to finish rolling to obtain a hot-rolled steel sheet having a predetermined thickness. When a sheet bar or a thin slab is used as a material, it goes without saying that rough rolling may be omitted.

本発明の製造方法では、 熱間圧延条件はとくに限定されないが、鋼板の板厚平均結晶粒径を6μm 以下とするためには、粗圧延を、900 〜1150℃の温度域で累積圧下率70〜90%とし、さらに仕上圧延を、800 〜1050℃の温度域で累積圧下率70〜98%とし、仕上圧延機出側温度:800 〜1000℃とすることが好ましい。   In the production method of the present invention, the hot rolling conditions are not particularly limited. However, in order to reduce the thickness average crystal grain size of the steel sheet to 6 μm or less, rough rolling is performed at a cumulative rolling reduction of 70% in a temperature range of 900 to 1150 ° C. Preferably, the finish rolling is performed at a cumulative rolling reduction of 70 to 98% in a temperature range of 800 to 1050 ° C., and the exit temperature of the finishing mill is set to 800 to 1000 ° C.

仕上圧延を施された熱延鋼板は、仕上圧延終了後、直ちに、所定温度域に冷却される一次冷却工程を施される。一次冷却工程では、熱延鋼板は、5℃/s以上500 ℃/s以下の冷却速度で650 ℃以上950 ℃以下の温度域まで冷却される。   After the finish rolling, the hot-rolled steel sheet subjected to the finish rolling is immediately subjected to a primary cooling step of cooling to a predetermined temperature range. In the primary cooling step, the hot-rolled steel sheet is cooled to a temperature range of 650 ° C to 950 ° C at a cooling rate of 5 ° C / s to 500 ° C / s.

一次冷却工程における冷却速度が、5 ℃/s未満では冷却中にγ粒が粗大化し、結晶粒微細化が十分に図れない。一方、500 ℃/sを超える冷却速度は現在の冷却設備能力では実現し難いため、一次冷却工程における冷却速度を5〜500 ℃/sに限定することが好ましい。なお、冷却速度が大きいほど、仕上圧延後のγ粒の粒成長を抑制できるため、30℃/s以上とすることがより好ましい。   If the cooling rate in the primary cooling step is less than 5 ° C./s, the γ grains are coarsened during cooling, and the crystal grains cannot be sufficiently refined. On the other hand, since a cooling rate exceeding 500 ° C./s is difficult to achieve with the current cooling facility capacity, it is preferable to limit the cooling rate in the primary cooling step to 5 to 500 ° C./s. Note that, as the cooling rate increases, the grain growth of γ grains after finish rolling can be suppressed, so that the rate is preferably 30 ° C./s or more.

また、一次冷却工程における冷却停止温度が650 ℃未満ではγ→α変態が大部分で完了しており、その後の加工(繰返し曲げ・曲げ戻し加工)によりγ粒に歪を蓄積することができず、γ→α変態時の変態核を増加させることができなくなり、結晶粒の微細化が十分に図れなくなる。一方、一次冷却工程における冷却停止温度が950 ℃を超えて高くなると、その後の加工(繰返し曲げ・曲げ戻し加工)により付与された歪の回復が顕著に起こり、十分に歪を蓄積することができず、結晶粒の微細化が十分に図れなくなる。なお、冷却停止温度は、その後に付与した歪の回復が起こり難い温度域である、700 〜800 ℃とすることが好ましい。   Further, when the cooling stop temperature in the primary cooling step is lower than 650 ° C., the γ → α transformation is largely completed, and strain cannot be accumulated in the γ grains by subsequent processing (repeated bending / bending back processing). , The transformation nuclei during the γ → α transformation cannot be increased, and the crystal grains cannot be sufficiently refined. On the other hand, if the cooling stop temperature in the primary cooling step is higher than 950 ° C., the strain applied by the subsequent processing (repeated bending / bending-back processing) is remarkably recovered, and the strain can be sufficiently accumulated. Therefore, the crystal grains cannot be sufficiently refined. Note that the cooling stop temperature is preferably set to 700 to 800 ° C., which is a temperature range in which recovery of the strain applied thereafter hardly occurs.

一次冷却工程により所定温度域に冷却された熱延鋼板は、ついでレベラによる繰返し曲げ・曲げ戻し加工を施され、鋼板最表部に0.05以上2.0 以下の歪を付与する歪付与工程が施される。   The hot-rolled steel sheet cooled to the predetermined temperature range by the primary cooling step is then subjected to repeated bending and bending back processing by a leveler, and subjected to a strain imparting step of imparting a strain of 0.05 or more and 2.0 or less to the outermost part of the steel sheet. .

レベラによる繰返し曲げ・曲げ戻し加工により鋼板最表部に付与される歪εは、(1) 式により、近似的に表される。   The strain ε imparted to the outermost part of the steel sheet by the repeated bending and bending back processing by the leveler is approximately expressed by the equation (1).

ε=(N−2) ×(2t)δ/ L2 ……… (1)
ここで、ε:歪、t:板厚、δ:レベラ締め込み量、2L:レベラワークロール(以下、単にWR)中心軸間隔、N:レベラWR数
なお、レベラ締め込み量δは、上下のレベラWRで鋼板を挟んだ状態から、レベラWRを締め込んだ距離で定義される。図2に、レベラによる1回の曲げ加工における鋼板1の曲げ加工状況を示す。ここで、δはレベラ締め込み量、rはレベラワークロール半径、2Lはレベラワークロール中心軸間隔である。
ε = (N−2) × (2t) δ / L 2 (1)
Here, ε: strain, t: plate thickness, δ: leveler tightening amount, 2L: leveler work roll (hereinafter simply referred to as WR) center axis interval, N: number of leveler WRs. It is defined as the distance from the state where the steel plate is sandwiched by the leveler WR and the leveler WR is tightened. FIG. 2 shows a bending state of the steel sheet 1 in one bending operation by a leveler. Here, δ is the leveler tightening amount, r is the leveler work roll radius, and 2L is the leveler work roll center axis interval.

レベラでの曲げ加工により付与される鋼板最表部の歪εが0.05未満では、蓄積される歪量が少なく、結晶粒の微細化を図ることができない。一方、2.0 を超える歪の付与は現実的に困難である。例えば、(1)式からNを109 本以上に大きくすると歪εは2.0 を超えて大きくできる(なお、δ=19mm、2L=180 mm、t=4mmとした)が、設備長が9.9 m超と長くなりすぎ現実的でなくなる。また、図2からも明らかなように、ワークロールの配置上、レベラ締め込み量には限界があり、歪εを2.0 を超えて大きくすることは困難である。また、(1)式からレベラWR中心軸間隔2Lを5.5 mm未満に小さくすることにより(なお、δ=7mm、N=29、t=4mmとした)、歪εは2.0 を超えて大きくできるが、WR直径を極度に小さくせねばならないことになるので、WRのたわみが大きくなり、鋼板形状が悪化する。   If the strain ε at the outermost portion of the steel sheet provided by the bending process with the leveler is less than 0.05, the amount of accumulated strain is small, and it is not possible to refine the crystal grains. On the other hand, it is practically difficult to apply a strain exceeding 2.0. For example, from equation (1), if N is increased to 109 or more, the strain ε can be increased beyond 2.0 (in addition, δ = 19 mm, 2L = 180 mm, t = 4 mm), but the equipment length exceeds 9.9 m. Becomes too long to be realistic. Further, as is apparent from FIG. 2, the leveler tightening amount is limited due to the arrangement of the work rolls, and it is difficult to increase the strain ε beyond 2.0. In addition, by reducing the leveler WR center axis interval 2L to less than 5.5 mm from equation (1) (where δ = 7 mm, N = 29 and t = 4 mm), the strain ε can be increased beyond 2.0. Since the WR diameter must be extremely small, the deflection of the WR increases and the shape of the steel sheet deteriorates.

また、レベラによる繰返し曲げ・曲げ戻し加工では、板厚1/4 の位置で考えても、板厚最表層の半分の歪を付与でき、板厚の1/4 の位置およびその他の部位でも、付与される歪量に応じて結晶粒の微細化が図れる。すなわち、レベラによる繰返し曲げ・曲げ戻し加工による歪付与により、板厚中心から板厚最表部に向かって、ポリゴナルフェライトの粒径が漸次小さくなる結晶粒径傾斜組織を得ることができるのである。なお、鋼板最表部に付与される歪εは、結晶粒微細化の観点から0.2 以上、設備的条件からは1.0 以下とすることがより好ましい。   In addition, in the repeated bending and bending back processing using a leveler, even at the position of 1/4 of the plate thickness, a strain of half of the outermost layer of the plate thickness can be given, and at the position of 1/4 of the plate thickness and other parts, Crystal grains can be refined according to the amount of strain applied. That is, by the strain imparting by the repeated bending and bending back processing by the leveler, it is possible to obtain a crystal grain size gradient structure in which the grain size of polygonal ferrite gradually decreases from the sheet thickness center toward the sheet thickness outermost part. . The strain ε applied to the outermost part of the steel sheet is more preferably 0.2 or more from the viewpoint of grain refinement and 1.0 or less from the equipment condition.

なお、熱延鋼板の板厚平均結晶粒径を1.8μm以下、表層部の平均結晶粒径を1μm以下とするためには、一次冷却工程における冷却停止温度を800℃以下、歪付与工程において付与する歪を1.0超えとすることが好ましい。一次冷却工程における冷却停止温度が800℃超え、あるいは歪付与工程において付与する歪が1.0以下では、上記したような微細組織とすることができない。また、Ti:0.05〜0.15%、Nb:0.05〜0.15%含有する組成とすることにより、より微細な組織を得ることができる。   In order to reduce the average thickness of the hot-rolled steel sheet to 1.8 μm or less and the average crystal grain size of the surface layer to 1 μm or less, the cooling stop temperature in the primary cooling step is 800 ° C. or less, It is preferable to set the distortion to be more than 1.0. If the cooling stop temperature in the primary cooling step exceeds 800 ° C. or the strain applied in the strain applying step is 1.0 or less, the above-described fine structure cannot be obtained. Further, a finer structure can be obtained by using a composition containing Ti: 0.05 to 0.15% and Nb: 0.05 to 0.15%.

ついで、歪付与された熱延鋼板は、10〜300 ℃/sの冷却速度で650 ℃以下の温度まで冷却する二次冷却工程を施される。   Next, the strained hot-rolled steel sheet is subjected to a secondary cooling step of cooling to a temperature of 650 ° C. or lower at a cooling rate of 10 to 300 ° C./s.

二次冷却工程の冷却速度が、10℃/s未満では、変態後のα粒の粒成長を招き、結晶粒径が大きくなってしまう。また、化学成分、熱延条件にもよるが、冷却中にγ→α変態が大幅に進行し、第二相の生成量が少なくなる。いずれの場合も強度が低下する。一方、二次冷却工程の冷却速度が、300 ℃/sを超えて大きくなると、主相であるポリゴナルフェライトの生成量が少なくなり、延性が低下する。このため、二次冷却工程の冷却速度を10〜300 ℃/sの範囲に限定することが好ましい。なお、より好ましくは30℃/s以上である。   If the cooling rate in the secondary cooling step is less than 10 ° C./s, the α grains after transformation will grow, and the crystal grain size will increase. Further, depending on the chemical composition and the hot rolling conditions, the γ → α transformation greatly proceeds during cooling, and the amount of the second phase formed decreases. In either case, the strength decreases. On the other hand, when the cooling rate in the secondary cooling step exceeds 300 ° C./s, the amount of polygonal ferrite, which is the main phase, decreases, and the ductility decreases. For this reason, it is preferable to limit the cooling rate in the secondary cooling step to a range of 10 to 300 ° C./s. Note that the temperature is more preferably 30 ° C./s or more.

また、二次冷却工程における冷却停止温度を650 ℃を超える高い温度とすると、α粒が粗大化しやすく、板厚平均結晶粒径で6μm 以下という微細結晶粒を得ることができない。このため、二次冷却工程における冷却停止温度を650℃以下とした。しかし、化学成分、熱延条件にもよるが、二次冷却工程における冷却停止温度を600 ℃を超えて高い温度とすると、冷却停止後もγ→α変態が進行し、第二相の生成量が減少し強度の低下が問題となる場合がある。一方、300℃を下回ると熱延鋼板が著しく硬質化しコイル状に巻取りにくくなるという問題が生じる。このため、二次冷却停止工程における冷却停止温度は300℃以上600℃以下とするのがより好ましい。なお、二次冷却工程における冷却停止温度は、第二相の種類、 生成量を制御するうえで重要であり、化学成分、熱延条件、冷却条件にもよるが、強度−延性バランスをより確保するためには、パーライト、ベイナイト等の顕著に生成する温度領域に冷却するのが好ましい。この観点から二次冷却工程における冷却停止温度は450℃以上600℃以下とするのがさらに好ましい。   If the cooling stop temperature in the secondary cooling step is set to a high temperature exceeding 650 ° C., α grains are likely to be coarsened, and fine crystal grains having a thickness average grain size of 6 μm or less cannot be obtained. For this reason, the cooling stop temperature in the secondary cooling step was set to 650 ° C. or less. However, depending on the chemical composition and hot rolling conditions, if the cooling stop temperature in the secondary cooling step is set to a high temperature exceeding 600 ° C, the γ → α transformation proceeds even after the cooling stop, and the amount of the second phase generated And the strength may be reduced. On the other hand, when the temperature is lower than 300 ° C., a problem occurs that the hot-rolled steel sheet becomes extremely hard and becomes difficult to be wound into a coil. For this reason, the cooling stop temperature in the secondary cooling stop step is more preferably set to 300 ° C. or more and 600 ° C. or less. The cooling stop temperature in the secondary cooling step is important in controlling the type and amount of the second phase, and depends on the chemical composition, hot rolling conditions, and cooling conditions, but a better balance between strength and ductility is obtained. In order to achieve this, it is preferable to cool to a temperature range in which pearlite, bainite and the like are remarkably generated. From this viewpoint, the cooling stop temperature in the secondary cooling step is more preferably set to 450 ° C. or more and 600 ° C. or less.

二次冷却工程を経た熱延鋼板は、ついでコイル状に巻き取られ、製品とされる。   The hot-rolled steel sheet that has undergone the secondary cooling step is then wound into a coil to obtain a product.

本発明の熱延鋼板は、図1に示す熱間圧延ラインを使用して製造することが好ましい。鋼素材Sは、 図示しない加熱炉で加熱され、あるいは上流工程から直接熱間状態で直送され、粗圧延機列2によりシートバーSBとされたのち、仕上圧延機列3により仕上圧延されて所定寸法の熱延鋼板1とされる。仕上圧延機列3における3aはワークロール、3bはバックアップロールである。仕上圧延機列3の出側には、第一の冷却装置4と、その下流にレベラ5、および第二の冷却装置6がその順に配列され、さらに巻取装置7が設けられ、仕上圧延後の熱延鋼板1をコイル状に巻き取っている。このほか、以上述べた主要な設備間には、図示しない多数のテーブルローラが設置されており、圧延中の鋼素材Sを搬送する。   The hot-rolled steel sheet of the present invention is preferably manufactured using the hot rolling line shown in FIG. The steel material S is heated in a heating furnace (not shown), or directly fed in a hot state from an upstream process, turned into a sheet bar SB by a row of rough rolling mills 2, and then finish-rolled by a row of finishing mills 3. A hot-rolled steel sheet 1 having dimensions is set. Reference numeral 3a in the finishing mill train 3 denotes a work roll, and 3b denotes a backup roll. On the exit side of the finishing mill train 3, a first cooling device 4, a downstream leveler 5, and a second cooling device 6 are arranged in that order, and a winding device 7 is further provided. Hot-rolled steel sheet 1 is wound in a coil shape. In addition, a number of table rollers (not shown) are provided between the above-described main facilities, and transport the steel material S being rolled.

レベラ5は、千鳥状に配列された3本以上のワークロール5a、あるいはさらにワークロール5aをバックアップするバックアップロール5bを備えている。なお、このレベラのワークロール直径は300mm 以下とすることが、レベラによる繰返し曲げ・曲げ戻し加工により付与できる歪を大きくできることから好ましい。   The leveler 5 includes three or more work rolls 5a arranged in a staggered manner, or a backup roll 5b that further backs up the work rolls 5a. The level of the work roll of the leveler is preferably 300 mm or less, because the strain that can be imparted by the repeated bending / bending process by the leveler can be increased.

つぎに、本発明を実施例に基づきさらに詳細に説明する。   Next, the present invention will be described in more detail based on examples.

(実施例1)
表1に示す組成の鋼素材(スラブ厚:260mm )を用いた。これらスラブを1100℃に加熱し、次いで、加熱後のスラブに対し、1000〜1100℃の温度範囲にて7パスで累積圧下率85%の粗圧延を施してシートバーとし、ついで、950 〜1050℃の温度範囲にて7パスで累積圧下率90%、仕上圧延機出側温度を950 ℃とする仕上圧延を行う熱間圧延工程を施して、板厚4mm の熱延鋼板とした。なお、仕上圧延後の熱延鋼板をコイル状に巻き取ったものを以下、コイルと呼ぶ。
(Example 1)
A steel material (slab thickness: 260 mm) having the composition shown in Table 1 was used. These slabs are heated to 1100 ° C, and the heated slabs are subjected to rough rolling at a cumulative rolling reduction of 85% in 7 passes in a temperature range of 1000 to 1100 ° C to form sheet bars, and then to 950 to 1050 ° C. A hot-rolled steel sheet having a thickness of 4 mm was obtained by performing a finish rolling process in which the rolling was performed at a cumulative draft of 90% in seven passes in a temperature range of 70 ° C. and the exit temperature of the finishing mill at 950 ° C. In addition, what rolled the hot-rolled steel plate after finish rolling in the shape of a coil is hereafter called a coil.

熱間圧延工程における仕上圧延後、 直ちに冷却速度:30℃/secの条件で、熱延鋼板の前半部分(コイル前半部分)は450 ℃まで冷却し、巻き取り、 比較例とした。一方、熱延鋼板の後半部分(コイル前半部分)は、熱間圧延工程における仕上圧延後、直ちに冷却速度:30℃/secの条件で、冷却停止温度:850 ℃まで冷却する一次冷却工程を施し、ついでその温度で、鋼板最表部の歪が0.5 となるように、レベラによる繰返し曲げ・曲げ戻し加工を施す歪付与工程を経るようにした。歪付与工程を経た熱延鋼板には、ついで、再び冷却速度:30℃/secの条件で450 ℃まで冷却する二次冷却工程を経るようにし、ついで巻き取り、本発明例とした。なお、歪付与工程で使用したレベラは、レベラWR直径2r:170mm φ、レベラWR中心軸間隔2L:180mm、レベラWR数N:29本であり、レベラ締め込み量δ:19mmとした。   Immediately after the finish rolling in the hot rolling step, the first half (the first half of the coil) of the hot-rolled steel sheet was cooled to 450 ° C under the condition of a cooling rate of 30 ° C / sec, and was taken up as a comparative example. On the other hand, the second half of the hot-rolled steel sheet (first half of the coil) is subjected to a primary cooling step of cooling to a cooling stop temperature of 850 ° C. immediately after the finish rolling in the hot rolling step, at a cooling rate of 30 ° C./sec. Then, at that temperature, a strain applying step of repeatedly bending and bending back with a leveler was performed so that the strain at the outermost portion of the steel sheet was 0.5. The hot-rolled steel sheet after the strain imparting step was subjected to a secondary cooling step of cooling again to 450 ° C. under the condition of a cooling rate of 30 ° C./sec, and then wound up to obtain an example of the present invention. The levelers used in the strain applying step had a leveler WR diameter of 2r: 170 mm, a center axis interval of the leveler WR of 2 L: 180 mm, the number of leveler WRs N: 29, and a leveler tightening amount δ: 19 mm.

得られた熱延鋼板から試験片を採取して、組織調査、引張試験、疲労試験を実施した。
(1)組織調査
得られた熱延鋼板の板幅中央から試験片を採取し、C方向断面について、走査型電子顕微鏡を用いて、最表部から板厚方向に0.1mm 位置から0.2mm の間隔で3.9 mm位置まで各位置を中心とし、それぞれ板幅方向に100 μm (中心振分各50μm )板厚方向に75μm (中心振分各37.5μm )の視野を計20視野、倍率1000倍で観察し、それぞれについて画像解析装置を用いて、ポリゴナルフェライトの体積分率、第二相の体積分率、ポリゴナルフェライトの平均結晶粒径を測定し、それぞれ各視野内で平均値を求め、板厚方向の各中心位置における値とし、それらをさらに板厚方向で平均して、ポリゴナルフェライトの体積分率Vα(%)、第二相の体積分率V2(%)、ポリゴナルフェライトの板厚平均結晶粒径D2(μm)とした。なお、各視野内で面積率を測定し、体積分率に換算した。また、ポリゴナルフェライトの平均結晶粒径は、JIS G 0552の規定に準拠して、結晶粒の平均断面積を求め、それを円形と仮定し、結晶粒径に換算し、 平均結晶粒径とした。表層部を対象とした平均結晶粒径D1は、最表部から板厚方向に0.1 mmの位置を中心とし、板幅方向に100 μm (中心振分各50μm )板厚方向に75μm (中心振分各37.5μm )の視野を倍率1000倍で観察し、前述と同様にJIS G 0552の規定に準拠し、視野内で結晶粒の平均断面積を求め、円形と仮定し、それを結晶粒径に換算して求めた。
(2)引張試験
得られた熱延鋼板から、JIS Z 2201の規定に準拠してL方向(圧延長手方向)を試験片長手方向とするJIS 5号試験片を採取し、JIS Z 2241の規定に準拠して引張試験を実施し、引張強さTS、伸びElを測定した。
(3)疲労試験
得られた熱延鋼板から、JIS Z 2275の規定に準拠して、試験片長手方向を鋼板のL方向とする1号試験片を採取し、JIS Z 2275の規定に準拠して、両振り平面曲げ疲労試験を行い、疲労限FLを求めた。
A test piece was sampled from the obtained hot-rolled steel sheet and subjected to a structure examination, a tensile test, and a fatigue test.
(1) Structural investigation A test specimen was sampled from the center of the obtained hot-rolled steel sheet in the width direction, and the cross section in the C direction was measured using a scanning electron microscope. Centering on each position up to the 3.9 mm position at intervals, 100 fields in the plate width direction (50 μm each in the center distribution), 75 μm in the thickness direction (37.5 μm each in the center distribution), a total of 20 fields of view at a magnification of 1000 times. Observe, using an image analyzer for each, measure the volume fraction of polygonal ferrite, the volume fraction of the second phase, the average crystal grain size of the polygonal ferrite, to determine the average value in each field of view, The values at each center position in the plate thickness direction are taken, and they are further averaged in the plate thickness direction to obtain a volume fraction Vα (%) of polygonal ferrite, a volume fraction V2 (%) of the second phase, and a value of polygonal ferrite. The thickness average crystal grain size was defined as D2 (μm). In addition, the area ratio was measured in each visual field and converted into a volume fraction. In addition, the average crystal grain size of polygonal ferrite is determined in accordance with the provisions of JIS G 0552, by calculating the average cross-sectional area of the crystal grains, assuming that it is circular, and converting it to a crystal grain size. did. The average crystal grain size D1 for the surface layer portion is centered at a position of 0.1 mm in the thickness direction from the outermost portion and is 100 μm in the width direction (50 μm each for center distribution) and 75 μm in the thickness direction (center vibration). Observe the field of view of 37.5 μm) at a magnification of 1000 times and determine the average cross-sectional area of the crystal grains within the field of view in the same manner as described above in accordance with the provisions of JIS G 0552. Was calculated.
(2) Tensile test From the obtained hot-rolled steel sheet, a JIS No. 5 test piece having a test piece longitudinal direction in the L direction (rolling longitudinal direction) in accordance with the provisions of JIS Z 2201 was sampled, and JIS Z 2241 A tensile test was performed in accordance with the regulations, and a tensile strength TS and an elongation El were measured.
(3) Fatigue test From the obtained hot-rolled steel sheet, in accordance with the provisions of JIS Z 2275, take the No. 1 test piece whose longitudinal direction is the L direction of the steel sheet and comply with the provisions of JIS Z 2275. Then, a swing plane bending fatigue test was performed to determine a fatigue limit FL.

得られた結果を表2に示す。   Table 2 shows the obtained results.

Figure 2004211199
Figure 2004211199

Figure 2004211199
Figure 2004211199

Figure 2004211199
Figure 2004211199

なお、鋼No. Aについて、板厚方向の結晶粒径分布を図3に示す。レベラでの繰返し曲げ・曲げ戻し加工を施していない比較例(コイル前半部分)では、板厚方向でほぼ均一な粒径を有している。一方、レベラでの繰返し曲げ・曲げ戻し加工を施した本発明例(コイル後半部分)では、表層部を対象とした平均結晶粒径D1 の方が板厚方向全域を対象とした板厚平均結晶粒径D2 よりも小さく、板厚中心から板厚表層に向かい漸次粒径が小さくなる結晶粒径傾斜組織となっている。   FIG. 3 shows the grain size distribution of steel No. A in the plate thickness direction. In the comparative example (the first half of the coil) in which the repetitive bending / bending process with the leveler was not performed, the particle diameter was substantially uniform in the thickness direction. On the other hand, in the example of the present invention (the latter half of the coil) subjected to repeated bending / bending back processing with a leveler, the average crystal grain size D1 for the surface layer portion is larger than that for the entire region in the thickness direction. The grain size is smaller than the grain size D2, and the grain size is gradually reduced from the center of the sheet thickness toward the surface layer of the sheet thickness.

本発明例はいずれも、ポリゴナルフェライトを主相とし、平均結晶粒径にくらべ表層の結晶粒径が75%以下と細かくなる結晶粒径傾斜組織であり、440MPa以上の引張強さTSを有し、かつ強度−延性バランスTS×Elが20000MPa%以上と強度−延性バランスに優れ、かつ疲労限FLも高く、強度−延性バランスに優れ、かつ耐疲労特性に優れた高強度熱延鋼板となっている。一方、本発明の範囲を外れる比較例は、強度−延性バランスが低いか、疲労限FLが低く耐疲労特性が不足するか、あるいは引張強さが440MPa未満であり高強度が得られていないかして、強度−延性バランスに優れ、かつ耐疲労特性に優れた高強度熱延鋼板が得られていない。   In each of the examples of the present invention, the main phase is polygonal ferrite, and the crystal grain size of the surface layer is as fine as 75% or less as compared with the average crystal grain size, and the crystal grain size gradient structure has a tensile strength TS of 440 MPa or more. High strength hot-rolled steel sheet with excellent strength-ductility balance, high fatigue limit FL, excellent strength-ductility balance, and excellent fatigue resistance characteristics, with a strength-ductility balance TS × El of 20,000 MPa% or more. ing. On the other hand, the comparative examples out of the range of the present invention have a low strength-ductility balance, a low fatigue limit FL and insufficient fatigue resistance, or a tensile strength of less than 440 MPa and high strength is not obtained. As a result, a high-strength hot-rolled steel sheet having excellent strength-ductility balance and excellent fatigue resistance has not been obtained.

表2に示す結果について、引張強さと伸びの関係で図4に、疲労限と引張強さの関係で図5にそれぞれ示す。図4から、本発明例は比較例に比べ強度−延性バランスが優れていることがわかる。また図5から、同一引張強さで比較して、本発明例は高い疲労限を示しており、耐疲労特性に優れていることがわかる。本発明例であるレベラでの繰返し曲げ・曲げ戻し加工により結晶粒が微細化され、同一引張強さで比較すると、顕著な疲労限の上昇が認められる。
(実施例2)
表1に示す、鋼No. A〜Dの鋼素材(スラブ厚:260mm )を用いた。これらスラブを1100℃に加熱し、次いで、加熱後のスラブに対し1000〜1100℃の温度範囲にて7パスで累積圧下率85%の粗圧延を施してシートバーとし、ついで、950 〜1050℃の温度範囲にて7パスで累積圧下率90%、仕上圧延機出側温度を950 ℃とする仕上圧延を行う熱間圧延工程を施して、板厚4mm の熱延鋼板とした。熱間圧延工程における仕上圧延後、直ちに表3に示す条件の、一次冷却工程、歪付与工程、二次冷却工程を施し、巻き取った。なお、歪付与工程で使用したレベラは、レベラWR直径2r:100mm φ、レベラWR中心軸間隔2L:120mm、レベラWR数N:49本であり、レベラの締め込み量δを変化させることにより、歪付与量を調整した。また、一次冷却工程、 二次冷却工程における冷却速度の変更は、冷却水量密度と、熱間圧延ラインにおける鋼素材Sの搬送方向の、冷却水を噴射するゾーンの長さの変更とにより行った。
With respect to the results shown in Table 2, the relationship between the tensile strength and the elongation is shown in FIG. 4, and the relationship between the fatigue limit and the tensile strength is shown in FIG. From FIG. 4, it can be seen that the present invention example has a better strength-ductility balance than the comparative example. Also, from FIG. 5, it can be seen that, in comparison with the same tensile strength, the example of the present invention shows a high fatigue limit and is excellent in fatigue resistance characteristics. The crystal grain is refined by the repeated bending and bending back processing by the leveler which is an example of the present invention, and a remarkable increase in fatigue limit is recognized when compared at the same tensile strength.
(Example 2)
The steel materials (slab thickness: 260 mm) of steel Nos. A to D shown in Table 1 were used. These slabs are heated to 1100 ° C, and the heated slabs are subjected to rough rolling at a temperature range of 1000 to 1100 ° C in seven passes with a cumulative rolling reduction of 85% to form sheet bars, and then 950 to 1050 ° C. In the temperature range of 7 mm, a hot rolling process was performed in which finishing rolling was performed in 7 passes with a cumulative rolling reduction of 90% and the exit temperature of the finishing mill at 950 ° C. to obtain a hot-rolled steel sheet having a thickness of 4 mm. Immediately after the finish rolling in the hot rolling step, a primary cooling step, a strain imparting step, and a secondary cooling step under the conditions shown in Table 3 were performed and wound. The leveler used in the strain imparting process has a leveler WR diameter of 2r: 100 mm φ, a center axis interval of the leveler WR 2 L: 120 mm, and the number of levelers WR N: 49. By changing the tightening amount δ of the leveler, The amount of distortion was adjusted. Further, the change of the cooling rate in the primary cooling step and the secondary cooling step was performed by changing the cooling water amount density and the length of the zone for injecting the cooling water in the conveying direction of the steel material S in the hot rolling line. .

得られた熱延鋼板について、実施例1と同様に組織調査、引張試験、疲労試験を実施した。得られた結果を表3に併記した。   About the obtained hot-rolled steel sheet, the structure | tissue investigation, the tensile test, and the fatigue test were implemented like Example 1. FIG. The results obtained are shown in Table 3.

Figure 2004211199
Figure 2004211199

Figure 2004211199
Figure 2004211199

本発明例はいずれも、ポリゴナルフェライトを主相とし、表層部の平均結晶粒径D1 が板厚方向全域を対象とした板厚平均結晶粒径D2 にくらべ75%以下と細かくなる結晶粒径傾斜組織であり、440MPa以上の引張強さと、20000MPa%を超える高い強度−延性バランスと高い疲労限とを有する高強度熱延鋼板となっている。本発明の範囲を外れる比較例は、上記した組織、機械的特性のいずれかを満足することができない。   In any of the examples of the present invention, the main phase is polygonal ferrite, and the average grain size D1 in the surface layer portion is 75% or less as compared with the average thickness grain size D2 in the entire thickness direction. This is a high-strength hot-rolled steel sheet having an inclined structure, a tensile strength of 440 MPa or more, a high strength-ductility balance exceeding 20,000 MPa%, and a high fatigue limit. Comparative examples outside the scope of the present invention cannot satisfy any of the above-mentioned structures and mechanical properties.

本発明例では、Ti、Nbを含有しない場合でも板厚平均結晶粒径が2.0μm未満、表層部の平均結晶粒径が1.2μm以下とポリゴナルフェライトが微細化しており、本発明によれば、高価な合金元素を含有することなく、高価な合金元素を含有した場合と同等以上の優れた機械的特性を保持する熱延鋼板とすることができる。さらにTi、Nbを含有すれば、表層部の平均結晶粒径が1.0μm以下、板厚平均結晶粒径が1.8μm以下とすることができる。なお、本発明の高強度熱延鋼板は、リサイクル性にも優れた材料であるといえる。   In the present invention example, even if not containing Ti, Nb, the average crystal grain size of the plate thickness is less than 2.0μm, the average crystal grain size of the surface layer portion is 1.2μm or less and polygonal ferrite is fine, according to the present invention Thus, a hot-rolled steel sheet can be obtained which does not contain expensive alloy elements and retains excellent mechanical properties equal to or more than those of the case where expensive alloy elements are contained. Further, when Ti and Nb are contained, the average crystal grain size of the surface layer portion can be 1.0 μm or less, and the average crystal grain size of the plate thickness can be 1.8 μm or less. It can be said that the high-strength hot-rolled steel sheet of the present invention is a material excellent in recyclability.

本発明の実施に好適な熱間圧延ラインの一例を模式的に示す説明図である。It is explanatory drawing which shows typically an example of the hot rolling line suitable for implementation of this invention. レベラを用いた鋼板への歪付与の概略を模式的に示す説明図である。It is explanatory drawing which shows typically the outline of distortion imparting to the steel plate using a leveler. 実施例における板厚方向の粒径分布を示すグラフである。4 is a graph showing a particle size distribution in a plate thickness direction in Examples. 実施例における引張強さと伸びの関係を示すグラフである。It is a graph which shows the relationship between tensile strength and elongation in an example. 実施例における引張強さと疲労限の関係を示すグラフである。It is a graph which shows the relationship between tensile strength and a fatigue limit in an example.

符号の説明Explanation of reference numerals

1 鋼板
2 粗圧延機列
3 仕上圧延機列
3a ワークロール
3b バックアップロール
4 第一の冷却装置
5 レベラ
6 第二の冷却装置
7 巻取装置
DESCRIPTION OF SYMBOLS 1 Steel plate 2 Rough rolling mill row 3 Finish rolling mill row 3a Work roll 3b Backup roll 4 First cooling device 5 Leveler 6 Second cooling device 7 Winding device

Claims (9)

mass%で、
C:0.05〜0.3 %、 Si:0.01〜2.5 %、
Mn:0.5 〜3.0 %
を含み、残部Feおよび不可避的不純物からなる組成と、主相としてポリゴナルフェライトを体積分率で70%以上、ポリゴナルフェライト以外の第二相を体積分率で5%以上含み、さらに、前記ポリゴナルフェライトの平均結晶粒径が板厚中心から表層に向かい漸次小さくなる結晶粒径傾斜組織を有することを特徴とする耐疲労特性に優れ、かつ強度−延性バランスに優れた高強度熱延鋼板。
mass%
C: 0.05-0.3%, Si: 0.01-2.5%,
Mn: 0.5 to 3.0%
And a composition comprising the balance of Fe and unavoidable impurities, and 70% or more by volume fraction of polygonal ferrite as a main phase, and 5% or more by volume fraction of a second phase other than polygonal ferrite. A high-strength hot-rolled steel sheet with excellent fatigue resistance and an excellent balance of strength-ductility, characterized by having a crystal grain size gradient structure in which the average crystal grain size of polygonal ferrite gradually decreases from the center of the sheet thickness toward the surface layer. .
前記強度−延性バランスが、21000MPa%以上であることを特徴とする請求項1に記載の高強度熱延鋼板。   The high-strength hot-rolled steel sheet according to claim 1, wherein the strength-ductility balance is 21000 MPa% or more. 前記ポリゴナルフェライトの表層における平均結晶粒径が1μm以下、前記ポリゴナルフェライトの板厚全域を対象とする板厚平均結晶粒径が1.8μm以下であることを特徴とする請求項1または2に記載の高強度熱延鋼板。   The average crystal grain size in the surface layer of the polygonal ferrite is 1 μm or less, and the plate thickness average crystal grain size covering the entire thickness of the polygonal ferrite is 1.8 μm or less. The high-strength hot-rolled steel sheet as described. 前記組成に加えてさらに、mass%で、Ti:0.005 〜0.25%、Nb:0.005 〜0.2 %のうちから選ばれた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3のいずれかに記載の高強度熱延鋼板。   4. The method according to claim 1, further comprising one or two selected from Ti: 0.005 to 0.25% and Nb: 0.005 to 0.2% in mass% in addition to the composition. A high-strength hot-rolled steel sheet according to crab. 前記組成に加えてさらに、mass%で、Cu:0.2 %以下、Ni:0.2 %以下、Cr:0.2 %以下、Mo:0.2 %以下、B:0.2 %以下、V:0.2 %以下のうちから選ばれた1種または2種以上を、合計で1.0 %以下含有することを特徴とする請求項1ないし4のいずれかに記載の高強度熱延鋼板。   In addition to the above composition, in mass%, Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, B: 0.2% or less, V: 0.2% or less The high-strength hot-rolled steel sheet according to any one of claims 1 to 4, further comprising 1.0% or less of the selected one or more kinds. mass%で、
C:0.05〜0.3 %、 Si:0.01〜2.5 %、
Mn:0.5 〜3.0 %
を含み、残部Feおよび不可避的不純物からなる組成の鋼素材を加熱し、 仕上圧延を含む熱間圧延を施す熱間圧延工程と、該仕上圧延後の熱延鋼板に5℃/s以上500 ℃/s以下の冷却速度で650 ℃以上950 ℃以下の温度域まで冷却する一次冷却工程と、前記温度域まで冷却された熱延鋼板にレベラによる繰返し曲げ・曲げ戻し加工を施し、熱延鋼板最表部に0.05以上2.0 以下の歪を付与する歪付与工程と、前記歪付与工程を経た熱延鋼板に10℃/s以上300 ℃/s以下の冷却速度で650 ℃以下の温度まで冷却する二次冷却工程と、を順次施すことを特徴とする、耐疲労特性に優れ、かつ強度−延性バランスに優れた高強度熱延鋼板の製造方法。
mass%
C: 0.05-0.3%, Si: 0.01-2.5%,
Mn: 0.5 to 3.0%
A hot rolling step of heating a steel material having a composition comprising the balance of Fe and unavoidable impurities and performing hot rolling including finish rolling, and applying a hot rolled steel sheet having the finish rolling of 5 ° C./s to 500 ° C. / S at a cooling rate of not more than 650 ° C and not more than 950 ° C, and a hot rolled steel sheet cooled to the above temperature range is subjected to repeated bending and unbending by a leveler. A strain imparting step of imparting a strain of 0.05 to 2.0 to the surface part, and cooling the hot-rolled steel sheet having passed through the strain imparting step to a temperature of 650 ° C. or less at a cooling rate of 10 ° C./s to 300 ° C./s. A method for producing a high-strength hot-rolled steel sheet having excellent fatigue resistance and an excellent balance of strength and ductility, characterized by sequentially performing the following cooling step.
前記一次冷却工程における冷却を停止する温度域を800℃以下とし、前記歪付与工程における歪を1.0超とすることを特徴とする請求項6に記載の高強度熱延鋼板の製造方法。   The method for producing a high-strength hot-rolled steel sheet according to claim 6, wherein the temperature range in which cooling in the primary cooling step is stopped is set to 800 ° C or less, and the strain in the strain applying step is set to more than 1.0. 前記組成に加えてさらに、mass%で、Ti:0.005 〜0.25%、Nb:0.005 〜0.2 %のうちから選ばれた1種または2種を含有することを特徴とする請求項6または7に記載の高強度熱延鋼板の製造方法。   8. The composition according to claim 6, further comprising one or two selected from Ti: 0.005 to 0.25% and Nb: 0.005 to 0.2% in mass% in addition to the composition. Production method of high-strength hot-rolled steel sheet. 前記組成に加えてさらに、mass%で、Cu:0.2 %以下、Ni:0.2 %以下、Cr:0.2 %以下、Mo:0.2 %以下、B:0.2 %以下、V:0.2 %以下のうちから選ばれた1種または2種以上を、合計で1.0 %以下含有することを特徴とする請求項6ないし8のいずかに記載の高強度熱延鋼板の製造方法。   In addition to the above composition, in mass%, Cu: 0.2% or less, Ni: 0.2% or less, Cr: 0.2% or less, Mo: 0.2% or less, B: 0.2% or less, V: 0.2% or less The method for producing a high-strength hot-rolled steel sheet according to any one of claims 6 to 8, wherein one or more of the selected ones are contained in a total of 1.0% or less.
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