JP2007211334A - High-tensile hot-rolled steel sheet and manufacturing method thereof - Google Patents
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Abstract
【課題】引張強さ５９０ＭＰａ以上、延性、曲げ加工性、疲労特性、表面性状にすぐれた高張力熱延鋼板を提供する。
【解決手段】質量％で、Ｃ：0.01％超0.25％以下、Ｓｉ：0.2％超1.0％未満、Ｍｎ：0.5〜2.5％、Ｐ：0.005％以上0.03％未満、Ｃｕ：0.005％以上0.045％未満、Ｓ：0.02％以下、Ａｌ：0.005〜1.0％およびＮ：0.01％以下、Ｏ：0.0010％以上0.0100％未満、Ｃｕ、ＰおよびＯの含有量が下記式（１）を満足し、残部Ｆｅ不純物の鋼組成を有し、鋼組織が、50〜95面積％のフェライトと残部第２相からなり、フェライトの平均結晶粒径３〜20μｍ、第２相の平均粒径1.0〜８μｍかつ平均粒子間隔２〜10μｍ、第２相が鋼組織全体を基準とした面積率で５〜50％のマルテンサイトとからなり、鋼板表面における最大長さ５ｍｍ以上の島状スケール疵が面積率で10％以下である。
０．４≦Ｃｕ／（Ｐ＋Ｏ）≦３．０ （１）A high-tensile hot-rolled steel sheet having a tensile strength of 590 MPa or more, excellent ductility, bending workability, fatigue characteristics, and surface properties is provided.
SOLUTION: In mass%, C: more than 0.01% and less than 0.25%, Si: more than 0.2% and less than 1.0%, Mn: 0.5 to 2.5%, P: 0.005% to less than 0.03%, Cu: 0.005% to less than 0.045% , S: 0.02% or less, Al: 0.005-1.0% and N: 0.01% or less, O: 0.0010% or more and less than 0.0100%, the contents of Cu, P and O satisfy the following formula (1), and the remaining Fe impurities The steel structure is composed of 50 to 95 area% ferrite and the remaining second phase, the average crystal grain size of ferrite is 3 to 20 μm, the average grain size of the second phase is 1.0 to 8 μm, and the average grain spacing is 2 to 10 μm, the second phase is composed of martensite with an area ratio of 5 to 50% based on the entire steel structure, and the island-like scale ridges with a maximum length of 5 mm or more on the steel sheet surface are 10% or less in area ratio. is there.
0.4 ≦ Cu / (P + O) ≦ 3.0 (1)
Description
本発明は、高張力熱延鋼板およびその製造方法に関する。特に、本発明は、延性、曲げ加工性ならびに疲労特性に優れるとともに表面性状にも優れた５９０ＭＰａ以上の引張強度を有する高張力熱延鋼板およびその製造方法に関する。本発明にかかる鋼板は自動車や各種の産業機械に用いられる構造部材の素材、なかでも自動車の足廻り部品やバンパー等の補強材に代表される構造部材の素材、またはホイール用の素材として好適である。 The present invention relates to a high-tensile hot-rolled steel sheet and a method for producing the same. In particular, the present invention relates to a high-tensile hot-rolled steel sheet having a tensile strength of 590 MPa or more, which is excellent in ductility, bending workability and fatigue characteristics and has excellent surface properties, and a method for producing the same. The steel sheet according to the present invention is suitable as a material for structural members used in automobiles and various industrial machines, especially as a material for structural members typified by reinforcing materials for automobile undercarriage parts and bumpers, or a material for wheels. is there.
高張力鋼板は、自動車を初めとする輸送用機械や各種産業機械の構造部材の素材として広く使用されており、経済性の観点からプレス加工等の成形加工によって所定の形状に加工されることが多い。このため、高張力鋼板には優れた加工性が要求される。 High-tensile steel plates are widely used as materials for structural members of transportation machines such as automobiles and various industrial machines. From the viewpoint of economy, high-tensile steel sheets can be processed into a predetermined shape by molding such as pressing. Many. For this reason, the high-tensile steel sheet is required to have excellent workability.
一方、近年、特に地球環境の保護という観点から、自動車の各種部材を高強度・薄肉化して車体重量を軽減し、燃費を向上させたり炭酸ガス等の排出を規制することが検討されている。自動車の各種部材の中でも大型部材であるフレームや足廻り部品、バンパー等の補強材等を軽量化することによって、車体重量の軽減が極めて有効に行える。このフレームや足廻り部品またはホイールリムなどに求められる鋼板特性には、強度、延性、曲げ加工性、疲労特性がある。 On the other hand, in recent years, particularly from the viewpoint of protecting the global environment, it has been studied to reduce the weight of a vehicle body by reducing the strength and thickness of various members of an automobile, to improve fuel consumption, and to regulate emissions of carbon dioxide and the like. By reducing the weight of reinforcing members such as frames, undercarriage parts, and bumpers, which are large members among various members of automobiles, the weight of the vehicle body can be reduced extremely effectively. The steel plate characteristics required for the frame, undercarriage parts or wheel rim include strength, ductility, bending workability, and fatigue characteristics.
従来技術の高い静的強度と高延性とを兼備する材料として、例えば特許文献１あるいは特許文献２に記載されているようなＤｕａｌ−Ｐｈａｓｅ鋼が知られている。このＤｕａｌ−Ｐｈａｓｅ鋼は、フェライト相とマルテンサイト相の二相混合組織であり、フェライト単相鋼、ベイナイト単相鋼、フェライト＋ベイナイト組織鋼に比べて、降伏比（降伏強さと引張強さの比）が低く、延性が高く、優れた強度延性バランスを示すことを特徴としている。 As a conventional material having both high static strength and high ductility, for example, Dual-Phase steel as described in Patent Document 1 or Patent Document 2 is known. This dual-phase steel is a two-phase mixed structure of ferrite and martensite phases, and has a yield ratio (yield strength and tensile strength) as compared to ferrite single-phase steel, bainite single-phase steel, and ferrite + bainite structure steel. Ratio) is low, the ductility is high, and an excellent balance of strength and ductility is exhibited.
さらに、特許文献３や特許文献４に記載されているようにＤｕａｌ−Ｐｈａｓｅ鋼は、疲労特性が優れていることが開示されている。
しかしながら、Ｄｕａｌ−Ｐｈａｓｅ鋼は、フェライト単相鋼、ベイナイト単相鋼、フェライト＋ベイナイト組織鋼に比べて、一般に高強度で延性に優れているが、曲げ加工性が劣化する欠点がある。これは、曲げ加工は、局所的な加工であり、軟質なフェライトと硬質なマルテンサイト組織の硬度差により、その界面から加工初期にクラックが入るためである。そのため、曲げ加工性に優れた熱延鋼板は、特許文献５に開示されるようにベイナイト単相鋼のような単相組織が主流であり、従来技術の中に、曲げ加工性に優れたＤｕａｌ−Ｐｈａｓｅ鋼の開示がなされていないのが現状である。
Furthermore, as described in Patent Document 3 and Patent Document 4, it is disclosed that Dual-Phase steel has excellent fatigue characteristics.
However, dual-phase steel is generally higher in strength and ductility than ferritic single-phase steel, bainite single-phase steel, and ferrite + bainite structure steel, but has a drawback that bending workability deteriorates. This is because the bending process is a local process, and cracks are generated from the interface in the initial stage due to the hardness difference between the soft ferrite and the hard martensite structure. Therefore, the hot-rolled steel sheet excellent in bending workability has a single-phase structure like bainite single-phase steel as disclosed in Patent Document 5, and the conventional technique has dual bending workability. -There is no disclosure of Phase steel.
一方、Ｄｕａｌ−Ｐｈａｓｅ鋼は加工性向上に好ましいフェライトを安定して生成させるために、多量のＳｉを含有させる必要がある。しかしながら、Ｓｉ含有量が０．２％以上の高Ｓｉ含有鋼では、スラブの加熱中にスラブ地金と生じたスケールとの界面に生成したファイアライト（Ｆｅ２ＳｉＯ４）がスケールの地金への密着性を向上させるためにデスケーリング不良が発生し易く、島状スケールと呼ばれるスケール疵が発生し易い。また酸洗後の鋼板表面を荒らしたりスケール疵が残ったりするため、足廻り部品やホイール用など外に現われる部材に適用することは好ましくない。 On the other hand, the Dual-Phase steel needs to contain a large amount of Si in order to stably generate ferrite preferable for improving workability. However, in a high Si content steel having a Si content of 0.2% or more, the firelite (Fe 2 SiO 4 ) generated at the interface between the slab metal and the generated scale during the heating of the slab becomes the scale metal. Descaling failure is likely to occur in order to improve the adhesion, and scale wrinkles called island scales are likely to occur. Moreover, since the steel plate surface after pickling is roughened or scale wrinkles remain, it is not preferable to apply it to parts that appear outside such as undercarriage parts and wheels.
デスケーリング不良に起因したスケール疵の発生を防止するための従来技術として以下が挙げられる。 The following are examples of conventional techniques for preventing the occurrence of scale wrinkles due to a descaling failure.
例えば、特許文献６には仕上圧延前の粗バーに対して高圧水デスケーリングを強化して行うことによりスケール疵の発生を防止する発明が、特許文献７にはスラブの加熱温度又は粗バーの表面温度をファイアライトの生成が急増する共晶点である１１７３℃以下に抑制してファイアライトの生成を抑制しながら高圧水によりデスケーリングを行うことによりスケール疵の発生を防止する発明が、特許文献８には粗圧延開始前に高圧水デスケーリングを行い、［１１７３−４２０×（Ｐ％／Ｓｉ％）］℃以上で粗圧延を終了することによりスケール疵の発生を防止する発明が、それぞれ開示されているが、これらの発明によっても、Ｓｉ含有量が０．２％以上の高Ｓｉ含有鋼からなる熱延鋼板を製造する際に、スケール疵の発生を確実に防止して、優れた表面性状を有する熱延鋼板を製造することは難しい。 For example, Patent Document 6 discloses an invention that prevents generation of scale flaws by strengthening high pressure water descaling with respect to a rough bar before finish rolling, and Patent Document 7 describes the heating temperature of the slab or the rough bar. An invention that prevents the generation of scale flaws by suppressing the surface temperature to 1173 ° C. or lower, which is the eutectic point at which the production of firelite rapidly increases, while suppressing the production of firelite while performing descaling with high-pressure water. Document 8 discloses an invention in which high-pressure water descaling is performed before the start of rough rolling, and generation of scale flaws is prevented by finishing rough rolling at [1173-420 × (P% / Si%)] ° C. or more, respectively. Although disclosed, these inventions also reliably prevent the occurrence of scale flaws when producing hot-rolled steel sheets made of high-Si steel with a Si content of 0.2% or more. , It is difficult to manufacture a hot-rolled steel sheet having excellent surface properties.
さらに、Ｓｉ含有量が０．２％以上の熱延酸洗板では、デスケーリング不良に起因したスケール疵の発生ならびに鋼板面内の局所的なスケール除去の不足により、鋼板の表面粗度が大きくなってしまう。そのため、曲げ加工特性や疲労特性の劣化が懸念される。Ｄｕａｌ−Ｐｈａｓｅ鋼の鋼板表面粗度と疲労特性については特許文献９に開示されている。しかし、金属組織における第２相の分散に関して記載されておらず、優れた疲労特性をもつ鋼板とは言いがたい。また、曲げ加工性については、一切記載されていない。
本発明の目的は、５９０ＭＰａ以上の高い引張強さを持ちつつ優れた延性を保ち、さらに優れた曲げ加工性と疲労特性ならびに優れた表面性状を有する高張力熱延鋼板とその製造方法を提供することにある。 An object of the present invention is to provide a high-tensile hot-rolled steel sheet having a high tensile strength of 590 MPa or more while maintaining excellent ductility, and having excellent bending workability and fatigue characteristics and excellent surface properties, and a method for producing the same. There is.
本発明者等は、上記の目的を達成すべく、高強度で優れた延性をもつＳｉ含有量が０．２％超であるＤｕａｌ−Ｐｈａｓｅ鋼の金属組織ならびに鋼板の表面性状を最適化することにより、高強度で高延性を保ちつつ、曲げ加工性と疲労特性ならびに表面性状に優れた熱延鋼板が得られることを知見した。そして、さらに研究を続けたところ、更なる曲げ加工性の改善と曲げ疲労特性の改善にはＣｕ／（Ｐ＋Ｏ）の比を規定することが有効であることを知り、ここに本発明を完成したものである。 In order to achieve the above object, the present inventors optimize the metal structure of dual-phase steel having a high strength and excellent ductility and a Si content of more than 0.2% and the surface properties of the steel sheet. Thus, it was found that a hot-rolled steel sheet excellent in bending workability, fatigue characteristics and surface properties can be obtained while maintaining high strength and high ductility. As a result of further research, it was found that it was effective to define the ratio of Cu / (P + O) for further improvement of bending workability and bending fatigue characteristics, and the present invention was completed here. Is.
本発明は、高張力熱延鋼板及びその製造方法に係るものであって、次の（１）から（６）までのいずれかに記載の高張力熱延鋼板に係るものと、（７）から（８）までのいずれかに記載の高張力熱延鋼板の製造方法に係るものである。以下、それぞれ、本発明（１）から本発明（８）という。なお、高加工性熱延鋼板に係る本発明（１）から本発明（６）と、高加工性熱延鋼板の製造方法に係る本発明（７）から本発明（８）を総称して、本発明ということがある。 The present invention relates to a high-tensile hot-rolled steel sheet and a method for producing the same, and relates to the high-tensile hot-rolled steel sheet according to any one of (1) to (6) below; This relates to the method for producing a high-tensile hot-rolled steel sheet according to any one of up to (8). Hereinafter, the present invention (1) to the present invention (8), respectively. The present invention (1) to the present invention (6) related to the high workability hot-rolled steel sheet and the present invention (7) to the present invention (8) related to the method for producing the high workability hot-rolled steel sheet are collectively referred to as Sometimes referred to as the present invention.
これらをまとめて示すと以下の通りである。
（１）鋼組成が、質量％で、Ｃ：０．０１％超０．２５％以下、Ｓｉ：０．２％超１．０％未満、Ｍｎ：０．５〜２．５％、Ｐ：０．００５％以上０．０３０％未満、Ｃｕ：０．００５％以上０．０４５％未満、Ｓ：０．０２％以下、Ａｌ：０．００５〜１．０％、Ｎ：０．０１％以下およびＯ：０．００１０％以上０．０１００％未満を含有し、Ｃｕ、ＰおよびＯの含有量が下記式（１）を満足し、残部がＦｅおよび不純物からなり、鋼組織が、５０〜９５面積％のフェライトと残部第２相とからなり、前記フェライトの平均結晶粒径が３〜２０μｍ、前記第２相の平均粒径が１．０〜８μｍかつ平均粒子間隔が２〜１０μｍ、前記第２相が鋼組織全体を基準とした面積率で５〜５０％のマルテンサイトと０〜５％未満のマルテンサイト以外の第２相とからなり、表面性状が、鋼板表面における最大長さ５ｍｍ以上の島状スケール疵が面積率で１０％以下であり、機械特性が、引張強度：５９０ＭＰａ以上、曲げ性：密着〜２．０ｔ、曲げ疲労限度耐久比：０．４８以上であることを特徴とする高張力熱延鋼板。
These are summarized as follows.
(1) Steel composition is mass%, C: more than 0.01% and 0.25% or less, Si: more than 0.2% and less than 1.0%, Mn: 0.5 to 2.5%, P: 0.005% or more and less than 0.030%, Cu: 0.005% or more and less than 0.045%, S: 0.02% or less, Al: 0.005-1.0%, N: 0.01% or less And O: 0.0010% or more and less than 0.0100%, the contents of Cu, P and O satisfy the following formula (1), the balance consists of Fe and impurities, and the steel structure is 50 to 95 The ferrite comprises an area% ferrite and the remaining second phase, the ferrite has an average crystal grain size of 3 to 20 μm, the second phase has an average grain size of 1.0 to 8 μm, and an average grain spacing of 2 to 10 μm. Other than martensite of 5-50% and martensite of less than 0-5% in terms of area ratio based on the entire steel structure It consists of a second phase, and the surface property is 10% or less in terms of area ratio of island-like scale ridges having a maximum length of 5 mm or more on the steel sheet surface, and the mechanical properties are tensile strength: 590 MPa or more, bendability: adhesion-2 A high-tensile hot-rolled steel sheet characterized by 0.0 t, bending fatigue limit durability ratio: 0.48 or more.
０．４≦Ｃｕ／（Ｐ＋Ｏ）≦３．０ （１）
ここで、式中の各元素記号は各元素の含有量（単位：質量％）を表す。
（２）前記鋼組成におけるＡｌ含有量が、質量％で、０．１％超１．０％以下であることを特徴とする前記（１）に記載の高張力熱延鋼板。
0.4 ≦ Cu / (P + O) ≦ 3.0 (1)
Here, each element symbol in the formula represents the content (unit: mass%) of each element.
(2) The high-tensile hot-rolled steel sheet according to (1), wherein an Al content in the steel composition is greater than 0.1% and less than or equal to 1.0% by mass.
（３）前記鋼組成が、Ｆｅの一部に代えて、質量％で、Ｔｉ：０．２％以下、Ｎｂ：０．１％以下、Ｖ：０．５％以下およびＷ：０．５％以下からなる群から選ばれる１種または２種以上を含有することを特徴とする前記（１）または（２）に記載の高張力熱延鋼板。 (3) The steel composition is mass% in place of part of Fe, Ti: 0.2% or less, Nb: 0.1% or less, V: 0.5% or less, and W: 0.5% The high-tensile hot-rolled steel sheet according to (1) or (2) above, which contains one or more selected from the group consisting of:
（４）前記鋼組成が、Ｆｅの一部に代えて、質量％で、Ｃｒ：１．０％以下、Ｍｏ：１．０％以下、Ｎｉ：１．０％以下およびＢ：０．０１％以下からなる群から選ばれる１種または２種以上を含有することを特徴とする前記（１）〜（３）のいずれかに記載の高張力熱延鋼板。 (4) The steel composition is mass% instead of part of Fe, Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, and B: 0.01% The high-tensile hot-rolled steel sheet according to any one of the above (1) to (3), comprising one or more selected from the group consisting of:
（５）前記鋼組成が、Ｆｅの一部に代えて、ＲＥＭ：０．１％以下、Ｍｇ：０．０１％以下およびＣａ：０．０１％以下からなる群から選ばれる１種または２種以上を含有することを特徴とする前記（１）〜（４）のいずれかに記載の高張力熱延鋼板。 (5) The steel composition is one or two selected from the group consisting of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less, instead of part of Fe. The high-tensile hot-rolled steel sheet according to any one of (1) to (4), which contains the above.
（６）酸洗後の鋼板の平均表面粗さＲａが１．２μｍ以下、鋼板表面から鋼板表面からの深さが０．０３ｍｍの位置までの鋼板表層部におけるフェライトの平均結晶粒径をαｓ、鋼板表面からの深さが０．０３ｍｍ超の位置から板厚中心までの鋼板中心部におけるフェライトの平均結晶粒径をαｂとした時、αｓ≧αｂ×１．１であることを特徴とする前記（１）〜（５）のいずれかに記載の高張力熱延鋼板。 (6) The average crystal grain size of ferrite in the surface layer portion of the steel sheet from the steel sheet surface to the position where the average surface roughness Ra of the steel sheet after pickling is 1.2 μm or less and the depth from the steel sheet surface to 0.03 mm is α s. When the average crystal grain size of ferrite in the center of the steel sheet from the position where the depth from the steel sheet surface exceeds 0.03 mm to the center of the sheet thickness is α b , α s ≧ α b × 1.1 The high-tensile hot-rolled steel sheet according to any one of (1) to (5), which is characterized in that
（７）前記（１）〜（５）のいずれかに記載の鋼組成を有するスラブを１１００℃以上として粗熱間圧延を施して粗バーとなし、得られた粗バーを下記式（２）で定められる限界温度Ｔ以上としてデスケーリングを行なった後に、Ａｒ３点〜Ａｒ３＋１５０℃で圧延を完了する仕上熱間圧延を施して熱延鋼板とし、前記仕上熱間圧延の完了後３秒以内に冷却を開始して平均冷却速度２０〜２００℃／秒で７６０〜６００℃の温度域の所定温度まで冷却する１次冷却と、前記１次冷却後２〜２０秒間の中間空冷と、前記中間空冷後１０℃／秒以上の平均冷却速度で冷却する２次冷却とを前記熱延鋼板に施して、次いで２５０℃以下で巻き取ることを特徴とする高張力熱延鋼板の製造方法。 (7) The slab having the steel composition described in any one of (1) to (5) above is subjected to rough hot rolling at 1100 ° C. or higher to form a rough bar, and the obtained rough bar is represented by the following formula (2). After the descaling is performed at a temperature not less than the limit temperature T defined by the above, finish hot rolling is performed to complete the rolling at Ar 3 points to Ar 3 + 150 ° C. to obtain a hot-rolled steel sheet, and 3 seconds after the completion of the finish hot rolling Cooling within a range of 760 to 600 ° C. at an average cooling rate of 20 to 200 ° C./second, intermediate air cooling for 2 to 20 seconds after the primary cooling, A method for producing a high-tensile hot-rolled steel sheet, comprising subjecting the hot-rolled steel sheet to secondary cooling that is cooled at an average cooling rate of 10 ° C./second or more after intermediate air cooling, and then winding it at 250 ° C. or less.
限界温度Ｔ（℃）＝１６８．１５×（（５×Ｐ＋Ａｌ）／Ｓｉ）２−２４５．１２×（５×Ｐ＋Ａｌ）／Ｓｉ＋１１７０（℃） （２）
ここで、式中の各元素記号は各元素の含有量（単位：質量％）を表す。
Limit temperature T (° C.) = 168.15 × ((5 × P + Al) / Si) 2 −245.12 × (5 × P + Al) / Si + 1170 (° C.) (2)
Here, each element symbol in the formula represents the content (unit: mass%) of each element.
（８）前記（１）〜（５）のいずれかに記載の鋼組成の溶鋼を、スラブ表面から下記式（３）で定められるスラブ内部位置Ｓｔまでの液相線温度〜固相線温度の冷却速度を１０℃／秒以上として連続鋳造法によりスラブとなし、得られたスラブを１１００℃以上として粗熱間圧延を施して粗バーとなし、前記粗バーを下記式（２）で定められる限界温度Ｔ以上としてデスケーリングを行なった後に、Ａｒ３点〜Ａｒ３＋１５０℃で圧延を完了する仕上熱間圧延を施して熱延鋼板とし、前記仕上熱間圧延の完了後３秒以内に冷却を開始して平均冷却速度２０〜１５０℃／秒で７６０〜６００℃の温度域の所定温度まで冷却する１次冷却と、前記１次冷却後２〜２０秒間の中間空冷と、前記中間空冷後１０℃／秒以上の平均冷却速度で冷却する２次冷却とを前記熱延鋼帯に施して、２５０℃以下で巻き取り、その後酸洗することを特徴とする高張力熱延鋼板の製造方法。 (8) The molten steel having the steel composition according to any one of (1) to (5) above having a liquidus temperature to a solidus temperature from a slab surface to a slab internal position St defined by the following formula (3): A slab is formed by a continuous casting method at a cooling rate of 10 ° C./sec or more, and the obtained slab is subjected to rough hot rolling at 1100 ° C. or more to form a rough bar. The rough bar is defined by the following formula (2). After descaling above the limit temperature T, finish hot rolling to complete rolling at Ar 3 points to Ar 3 + 150 ° C. is performed to form a hot-rolled steel sheet, which is cooled within 3 seconds after completion of the finish hot rolling. Starting to cool to a predetermined temperature in the temperature range of 760 to 600 ° C. at an average cooling rate of 20 to 150 ° C./second, intermediate air cooling for 2 to 20 seconds after the primary cooling, and after the intermediate air cooling Cool at an average cooling rate of 10 ° C / second or more Is subjected to the following cooling in the hot-rolled steel strip, wound at 250 ° C. or less, the method of producing a high tensile hot-rolled steel sheet, characterized by then pickling.
限界温度Ｔ（℃）＝１６８．１５×（（５×Ｐ＋Ａｌ）／Ｓｉ）２−２４５．１２×（５×Ｐ＋Ａｌ）／Ｓｉ＋１１７０（℃） （２）
ここで、式中の各元素記号は各元素の含有量（単位：質量％）を表す。
Limit temperature T (° C.) = 168.15 × ((5 × P + Al) / Si) 2 −245.12 × (5 × P + Al) / Si + 1170 (° C.) (2)
Here, each element symbol in the formula represents the content (unit: mass%) of each element.
Ｓｔ（ｍｍ）＝［Ｂ（ｍｍ）］／Ａ（ｍｍ）］×０．０３ｍｍ （３）
ここで、Ｂ（ｍｍ）：スラブ厚、Ａ（ｍｍ）：鋼板の板厚である。
St (mm) = [B (mm)] / A (mm)] × 0.03 mm (3)
Here, B (mm): slab thickness, A (mm): plate thickness of the steel plate.
本発明の鋼板は、高強度で加工性を確保しつつ、曲げ加工性及び耐疲労特性、表面性状に優れている。そのため、自動車や各種の産業機械に用いられる構造部材の素材、特に自動車のメンバーや足廻り部品に代表される構造部材の素材として最適である。また安価に製造できるので産業上格段の効果を奏する。 The steel sheet of the present invention is excellent in bending workability, fatigue resistance, and surface properties while ensuring high strength and workability. Therefore, it is optimal as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile members and undercarriage parts. Moreover, since it can be manufactured at a low cost, it has a remarkable industrial effect.
本発明に係る鋼板の鋼組成について説明する。本明細書において鋼組成を示す「％」は「質量％」である。
Ｃ：０．０１％超０．２５％以下
Ｃは、鋼板の強度を高める元素であり、延性に優れた高強度鋼を製造するためには特に重要な元素である。すなわち、Ｃの含有量が、０．０１％以下では、十分なマルテンサイトの量が確保できず、５９０ＭＰａ以上の強度を有する高強度鋼板が製造できなくなる。一方、０．２５％を超えると溶接性が低下する。したがって、Ｃの含有量を０．０１％を超え０．２５％以下とした。なお、容易に５９０ＭＰａ以上の高強度を得るには、Ｃを０．０３％以上含有させることが望ましく、そして、７８０ＭＰａ以上の高強度を得るには、Ｃを０．０４％以上含有させることが望ましい。
The steel composition of the steel sheet according to the present invention will be described. In this specification, “%” indicating the steel composition is “mass%”.
C: More than 0.01% and 0.25% or less C is an element that increases the strength of the steel sheet, and is an especially important element for producing high-strength steel excellent in ductility. That is, when the C content is 0.01% or less, a sufficient amount of martensite cannot be secured, and a high-strength steel sheet having a strength of 590 MPa or more cannot be manufactured. On the other hand, if it exceeds 0.25%, the weldability decreases. Therefore, the C content is more than 0.01% and not more than 0.25%. In order to easily obtain a high strength of 590 MPa or more, it is desirable to contain 0.03% or more of C, and in order to obtain a high strength of 780 MPa or more, it is desirable to contain 0.04% or more of C. desirable.
Ｓｉ：０．２％超１．０％未満
Ｓｉは固溶強化によってフェライト相を強化できるだけでなく、フェライトの生成を促進し、未変態オーステナイト中にＣを濃縮させ、容易に第２相をマルテンサイトにさせることができる。そのため、Ｓｉの含有は、高強度で高延性であるＤｕａｌ−Ｐｈａｓｅ鋼をつくるには重要であり、その効果を得るには、Ｓｉの含有量を０．２％超とする必要がある。特に、高強度で高延性型のＤｕａｌ−Ｐｈａｓｅ鋼とするにはＳｉの含有量を０．４％以上とすることが望ましい。
Si: more than 0.2% and less than 1.0% Si not only strengthens the ferrite phase by solid solution strengthening, but also promotes the formation of ferrite, concentrates C in the untransformed austenite, and easily converts the second phase into martensite. You can let the site. Therefore, the Si content is important for producing a dual-phase steel having high strength and high ductility, and in order to obtain the effect, the Si content needs to be more than 0.2%. In particular, in order to obtain a high strength and high ductility type Dual-Phase steel, the Si content is preferably 0.4% or more.
一方、Ｓｉ含有量が１．０％超であるＳｉ含有鋼は、熱間圧延前の鋳片又は鋼片の加熱段階で液体のスケールが生成し、地鉄内部へくさび状に生成する。加熱後デスケーリングする際に鋳片又は鋼片が冷却して、このくさび状スケールが固化して剥離性の悪い、いわゆる「Ｓｉスケール」が発生する。このＳｉスケールは完全にはデスケーリングすることが困難であり、Ｓｉスケールが残存した状態で熱間圧延が行われるので、酸洗前の熱延板表面には島状スケールが顕著に発生し、圧延後の鋼板または酸洗後の表面には島状スケール模様が残り美観が損なわれ且つ表面粗度も損なわれて曲げ加工性や表面品質が劣化する。 On the other hand, in the Si-containing steel having a Si content exceeding 1.0%, a liquid scale is generated in the heating stage of the slab or the steel slab before hot rolling, and is generated in a wedge shape inside the base iron. When descaling after heating, the slab or steel slab cools, and this wedge-shaped scale is solidified to generate a so-called “Si scale” having poor peelability. This Si scale is difficult to completely descal, and since hot rolling is performed with the Si scale remaining, an island-like scale is noticeably generated on the surface of the hot rolled sheet before pickling. An island-like scale pattern remains on the rolled steel plate or the surface after pickling, and the aesthetic appearance is impaired and the surface roughness is also impaired, so that the bending workability and the surface quality are deteriorated.
Ｍｎ０．５〜２．５％
Ｍｎは、鋼の焼入性を高め強度を上昇させるのに有効な元素であるが、その含有量が０．５％未満では、マルテンサイトを生成させることができず、十分な強度と延性を得ることができない。
Mn 0.5-2.5%
Mn is an element effective for increasing the hardenability of steel and increasing the strength. However, if its content is less than 0.5%, martensite cannot be generated, and sufficient strength and ductility are achieved. Can't get.
一方、２．５％を超えて、Ｍｎを含有させてもその効果は飽和する。
容易にマルテンサイトを生成させるための焼き入れ性を確保するには、Ｍｎを１．０％以上含有させることが望ましい。
On the other hand, even if it exceeds 2.5% and Mn is contained, the effect is saturated.
In order to ensure hardenability for easily generating martensite, it is desirable to contain Mn at 1.0% or more.
Ｐ：０．００５％以上０．０３０％未満
Ｐは固溶強化として働く元素であり、高強度化のために有効である。また、酸洗性を高め熱間圧延後の熱延鋼板の脱スケール性を高める効果も有する。しかし、その含有量が０．００５％未満では上記の効果が得難い。一方、Ｐは偏析し易い元素であるため多量に添加した場合には、溶接性の低下を招き、特に、その含有量が０．０３０％以上になると偏析が著しくなって溶接性の低下が極めて大きくなる。また、酸洗時に粒界が優先的にエッチングされることにより、曲げ加工性や耐疲労特性を劣化させる。
P: 0.005% or more and less than 0.030% P is an element that works as a solid solution strengthening, and is effective for increasing the strength. Moreover, it has the effect of improving pickling property and improving the descaling property of the hot-rolled steel sheet after hot rolling. However, if the content is less than 0.005%, it is difficult to obtain the above effect. On the other hand, since P is an element that easily segregates, when added in a large amount, the weldability is deteriorated. In particular, when the content is 0.030% or more, the segregation becomes remarkable and the weldability is extremely decreased. growing. Further, the grain boundaries are preferentially etched during pickling, thereby deteriorating bending workability and fatigue resistance.
したがって、Ｐの含有量を０．００５％以上０．０３０％未満とした。好ましい下限は、熱延鋼板の脱スケール性を高める観点から０．０１０％以上であり、好ましい上限は、酸洗時の粒界の優先的エッチングを抑制する観点から０．０２％以下である。 Therefore, the P content is set to be 0.005% or more and less than 0.030%. The preferable lower limit is 0.010% or more from the viewpoint of enhancing the descalability of the hot-rolled steel sheet, and the preferable upper limit is 0.02% or less from the viewpoint of suppressing preferential etching of the grain boundaries during pickling.
さらに、本願発明にあっては後述するようにＣｕ／（Ｐ＋Ｏ）の比を規定することで、酸洗性を高めることにより良好な表面性状を確保するとともに曲げ加工性および耐疲労特性の劣化を抑制する効果を有し、そのためにも、Ｐは上記範囲に限定される。 Furthermore, in the present invention, by defining the ratio of Cu / (P + O) as will be described later, it is possible to ensure good surface properties by increasing the pickling property and to reduce bending workability and fatigue resistance characteristics. For this reason, P is limited to the above range.
Ｃｕ：０．００５％以上０．０４５％未満
Ｃｕは、Ｐとは反対に酸洗性を低下させる性質を有し、酸洗時に粒界が優先的にエッチングされるのを抑制したり、酸化物系介在物の脱落を抑制したりする作用を有する。しかし、その含有量が０．０５％未満では上記効果が得難い。一方、その含有量が０．０４５％以上になると、酸洗性の低下により生産性が低下したり、コストの増加を招いたりする。
Cu: 0.005% or more and less than 0.045% Cu has the property of reducing pickling properties contrary to P, and suppresses the preferential etching of grain boundaries during pickling or oxidation. It has the effect of suppressing the falling of physical inclusions. However, if the content is less than 0.05%, it is difficult to obtain the above effect. On the other hand, when the content is 0.045% or more, productivity is lowered due to a drop in pickling properties, or cost is increased.
したがって、Ｃｕ含有量を０．００５％以上０．０４５％未満とする。好ましくは０．０１０％以上０．０３０％以下である。
特に、本発明ではＣｕ／（Ｐ＋Ｏ）の比を０．４〜３．０の範囲に規定するが、これはＰおよびＯによる酸洗性向上とＣｕによる酸洗性抑制の機構を利用して曲げ加工性および曲げ疲労特性を改善するためである。
Therefore, the Cu content is set to 0.005% or more and less than 0.045%. Preferably they are 0.010% or more and 0.030% or less.
In particular, in the present invention, the ratio of Cu / (P + O) is specified in the range of 0.4 to 3.0, which utilizes a mechanism of pickling improvement by P and O and suppression of pickling by Cu. This is to improve bending workability and bending fatigue characteristics.
Ｓ：０．０２％以下
Ｓは、曲げ加工性を低下させる硫化物を生成するため、可能な限り低減する必要のある不純物である。本発明においては、他の成分元素添加による曲げ加工性の向上度合と製鋼コストを考慮して、その含有量の上限を０．０２％とした。望ましくは、０．０１％以下である。
S: 0.02% or less S is an impurity that needs to be reduced as much as possible in order to produce a sulfide that lowers bending workability. In the present invention, the upper limit of the content is set to 0.02% in consideration of the degree of improvement in bending workability by adding other component elements and the steelmaking cost. Desirably, it is 0.01% or less.
Ａｌ：０．００５〜１．０％
Ａｌは、鋼の脱酸に有用な元素である。その効果を得るには、少なくとも０．００５％の含有量が必要である。一方、その含有量が１．０％を超えると、粗大なアルミナ系介在物が増加して、延性と曲げ加工性が著しく低下する。したがって、Ａｌの含有量を０．００５〜１．０％とした。また、Ａｌを０．１％超含有させることにより、フェライトの生成が促進され、加工性ならびに曲げ加工性が向上する。さらに、ＦｅＯ／Ｆｅ２ＳｉＯ４の共晶温度が低下するため、脱スケール性が向上し、島状スケール疵が減少する。
Al: 0.005 to 1.0%
Al is an element useful for deoxidation of steel. In order to obtain the effect, a content of at least 0.005% is necessary. On the other hand, when the content exceeds 1.0%, coarse alumina inclusions increase, and ductility and bending workability are remarkably lowered. Therefore, the Al content is set to 0.005 to 1.0%. In addition, when Al is contained in an amount exceeding 0.1%, the formation of ferrite is promoted, and the workability and bending workability are improved. Furthermore, since the eutectic temperature of FeO / Fe 2 SiO 4 is lowered, descaling property is improved, and island-like scale defects are reduced.
Ｎ：０．０１％以下
Ｎは、ＡｌやＴｉと結合し、窒化物を形成する。窒化物は延性を劣化させる傾向を有するため、できるだけ低減するのが望ましい。０．０１％以下であれば、無害化できる。そのため、上限を０．０１％とした。Ｎ低減のためのコストと材質の改善度合との兼ね合いでＮ含有量の上限は０．００５０％とするのが好ましい。また製造コストの観点から下限は０．０００５％以上とすることが好ましい。
N: 0.01% or less N combines with Al and Ti to form a nitride. Since nitride has a tendency to degrade ductility, it is desirable to reduce it as much as possible. If it is 0.01% or less, it can be rendered harmless. Therefore, the upper limit was made 0.01%. The upper limit of the N content is preferably 0.0050% in consideration of the cost for reducing N and the improvement degree of the material. From the viewpoint of production cost, the lower limit is preferably 0.0005% or more.
Ｏ：０．００１０％以上０．０１００％未満
Ｏは、鋼板中にＭｎＯ等の酸化物系介在物を形成し、酸洗時にこれらの酸化物系介在物が脱落してボイドの形成が促進されるため、曲げ加工や繰り返し疲労時の割れや亀裂の起点となる。一方、酸化物系介在物が多いと酸化物系介在物と接触する鋼部分との表面積が増加し、結果的にＰと同様の効果を生じ、酸洗性の向上をもたらす。このため、Ｏ含有量を０．０１００％未満とする。Ｏ含有量の下限は、コストの観点から０．００１０％以上とする。
O: 0.0010% or more and less than 0.0100% O forms oxide inclusions such as MnO in the steel sheet, and these oxide inclusions drop off during pickling to promote the formation of voids. Therefore, it becomes the starting point of cracks and cracks during bending and repeated fatigue. On the other hand, if there are many oxide inclusions, the surface area with the steel part in contact with the oxide inclusions will increase, resulting in the same effect as P, resulting in improved pickling. For this reason, the O content is less than 0.0100%. The lower limit of the O content is set to 0.0010% or more from the viewpoint of cost.
Ｃｕ／（Ｐ＋Ｏ）：０．４〜３．０
本発明の効果を得るために、Ｃｕ、Ｐ、Ｏを関連づけて規定する。
ここに、上記式の意義は次の通りである。
Cu / (P + O): 0.4 to 3.0
In order to obtain the effect of the present invention, Cu, P, and O are defined in association with each other.
Here, the significance of the above formula is as follows.
すなわち、上述したようにＰおよびＯは、酸洗性を高める作用を有し熱延鋼板の脱スケール性を高めて良好な表面性状を確保するのに有効であるが、過剰に含有させると、酸洗時に粒界が優先的にエッチングされたり酸化物系介在物の脱落に起因してボイドの生成が促進されたりして曲げ加工性や耐疲労特性を劣化させる。 That is, as described above, P and O are effective to increase the descaling property of the hot-rolled steel sheet and to ensure good surface properties by increasing the pickling property. The grain boundaries are preferentially etched during pickling or the formation of voids is promoted due to detachment of oxide inclusions, thereby degrading bending workability and fatigue resistance.
一方、Ｃｕは酸洗性を低下させる作用を有し、酸洗時に粒界が優先的にエッチングされたり酸化物系介在物が脱落したりするのを抑制する。このため、Ｃｕと（Ｐ＋Ｏ）とをバランスさせることにより、ＰおよびＯによる表面性状向上作用を享受しつつ、曲げ加工性劣化作用や耐疲労特性劣化作用をＣｕにより減ずるのである。
Ｃｕ／（Ｐ＋Ｏ）が３．０超では、材質は硬質化し曲げ加工性が劣化する。Ｃｕ／（Ｐ＋Ｏ）が０．４未満では曲げ加工性や耐疲労特性が劣化する。
On the other hand, Cu has the effect of reducing the pickling property, and suppresses the grain boundary from being preferentially etched or the oxide inclusions falling off during pickling. For this reason, by balancing Cu and (P + O), the bending property deterioration effect and the fatigue resistance deterioration effect are reduced by Cu while enjoying the surface property improvement effect by P and O.
If Cu / (P + O) exceeds 3.0, the material becomes hard and bending workability deteriorates. When Cu / (P + O) is less than 0.4, bending workability and fatigue resistance are deteriorated.
Ｔｉ：０．２％以下、Ｎｂ：０．１％以下、Ｖ：０．５％以下およびＷ：０．５％以下からなる群から選ばれる１種または２種以上
本発明において、Ｔｉ、Ｎｂ、Ｖ、Ｗはいずれも析出強化によって強度を高める元素であり、強度を一層高める作用を有し、２種以上添加しても、それぞれの作用は失われない。その作用は、Ｔｉ：０．２％、Ｎｂ：０．１％、Ｖ：０．５％およびＷ：０．５％をそれぞれ超えて含有させても飽和し、コストがかさむばかりである。そのため、含有量の上限をＴｉ：０．２％以下、Ｎｂ：０．１％以下、Ｖ：０．５％以下およびＷ：０．５％以下のうちの１種または２種以上とした。またその効果は、Ｔｉ：０．０１％以上ならびにＮｂ：０．００５％以上、Ｖ：０．０１％以上、Ｗ：０．０１％以上の含有により有効に作用するので、それを下限とするのが好ましい。
In the present invention, one or more selected from the group consisting of Ti: 0.2% or less, Nb: 0.1% or less, V: 0.5% or less, and W: 0.5% or less , in the present invention, Ti, Nb , V and W are elements that increase the strength by precipitation strengthening, have the effect of further increasing the strength, and even when two or more types are added, the respective functions are not lost. Even if the action exceeds Ti: 0.2%, Nb: 0.1%, V: 0.5%, and W: 0.5%, the effect is saturated and the cost is increased. Therefore, the upper limit of the content is set to one or more of Ti: 0.2% or less, Nb: 0.1% or less, V: 0.5% or less, and W: 0.5% or less. Moreover, since the effect acts effectively by containing Ti: 0.01% or more and Nb: 0.005% or more, V: 0.01% or more, W: 0.01% or more, the lower limit is set. Is preferred.
Ｃｒ：１．０％以下、Ｍｏ：１．０％以下、Ｎｉ：１．０％以下およびＢ：０．０１％以下からなる群から選ばれる１種または２種以上
本発明において、Ｃｒ、Ｍｏ、ＮｉおよびＢはいずれも固溶強化によって強度を高める元素であり、強度を一層高める作用を有し、２種以上添加しても、それぞれの作用は失われない。その作用は、Ｃｒ：１．０％、Ｍｏ：１．０％、Ｃｕ：１．０％、Ｎｉ：１．０％およびＢ：０．０１％をそれぞれ超えて含有させても飽和し、コストがかさむばかりである。
In the present invention, one or more selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, and B: 0.01% or less In the present invention, Cr, Mo , Ni and B are elements that increase the strength by solid solution strengthening, have the effect of further increasing the strength, and even if two or more types are added, the respective effects are not lost. The effect is saturated even if Cr: 1.0%, Mo: 1.0%, Cu: 1.0%, Ni: 1.0% and B: 0.01% are contained, respectively, and the cost It's just going to be bulky.
そのため、含有量の上限をＣｒ：１．０％以下、Ｍｏ：１．０％以下、Ｃｕ：１．０％以下、Ｎｉ：１．０％以下およびＢ：０．０１％以下とした。またその効果は、Ｃｒ：０．０５％以上ならびにＭｏ：０．０５％以上、Ｎｉ：０．０５％以上およびＢ：０．０００２％以上の含有により有効に作用するので、それを下限とするのが好ましい。 Therefore, the upper limit of the content was set to Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B: 0.01% or less. Moreover, since the effect works effectively by inclusion of Cr: 0.05% or more and Mo: 0.05% or more, Ni: 0.05% or more and B: 0.0002% or more, the lower limit is set. Is preferred.
ＲＥＭ：０．１％以下、Ｍｇ：０．０１％以下およびＣａ：０．０１％以下からなる群から選ばれる１種または２種以上
本発明において、ＲＥＭ、ＭｇおよびＣａはいずれも硫化物、酸化物などの介在物を球状化し無害化させることができ、２種以上添加しても、それぞれの作用は失われない。その作用は、ＲＥＭ：０．１％、Ｍｇ：０．０１％、Ｃａ：０．０１％をそれぞれ超えて含有させても飽和し、コストがかさむばかりである。そのため、含有量の上限をＲＥＭ：０．１％以下、Ｍｇ：０．０１％以下およびＣａ：０．０１％以下とした。またその効果は、ＲＥＭ：０．００５％以上、Ｍｇ：０．０００５％以上およびＣａ：０．０００５％以上の含有により有効に作用するので、それを下限とするのが好ましい。
REM: 0.1% or less, Mg: 0.01% or less, and Ca: one or more selected from the group consisting of 0.01% or less In the present invention, REM, Mg, and Ca are all sulfides. Inclusions such as oxides can be spheroidized and detoxified, and even when two or more inclusions are added, each action is not lost. Even if the action exceeds REM: 0.1%, Mg: 0.01%, and Ca: 0.01%, the action is saturated and the cost is increased. Therefore, the upper limit of the content was set to REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less. Moreover, since the effect acts effectively by inclusion of REM: 0.005% or more, Mg: 0.0005% or more, and Ca: 0.0005% or more, it is preferable to set the lower limit.
ここで、ＲＥＭとは、Ｓｃ、Ｙ及びランタノイドの合計１７元素を指し、ランタノイドの場合、工業的にはミッシュメタルの形で添加される。なお、本発明では、ＲＥＭの含有量はこれらの元素の合計含有量を指す。 Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In the present invention, the content of REM refers to the total content of these elements.
本発明に係る鋼板の金属組織について説明すれば次の通りである。
引張強度が５９０ＭＰａ以上の領域で、良好な延性と曲げ加工性ならびに耐疲労特性を得るためには、金属組織において、平均結晶粒径３〜２０μｍのフェライトが面積率で５０〜９５％、第２相が平均粒径１〜８μｍで平均粒子間隔２〜１０μｍで存在し、その第２相はマルテンサイトが面積率で５〜５０％である。マルテンサイト以外の第２相は面積率で５％未満まで存在することが許容される。
The metal structure of the steel sheet according to the present invention will be described as follows.
In order to obtain good ductility, bending workability and fatigue resistance in a region where the tensile strength is 590 MPa or more, ferrite having an average crystal grain size of 3 to 20 μm is 50 to 95% in area ratio in the metal structure. The phase is present with an average particle size of 1 to 8 μm and an average particle interval of 2 to 10 μm, and the second phase has a martensite area ratio of 5 to 50%. The second phase other than martensite is allowed to be present in an area ratio of less than 5%.
フェライト
フェライトの面積率が５０％未満であると、フェライト粒が少ないため、曲げ加工性が劣化する。一方、フェライトの面積率が９５％を超えると、フェライト粒が多いため、曲げ疲労限度耐久比が低下する。フェライトの平均結晶粒径は３〜２０μｍとする必要がある。
If the area ratio of ferrite ferrite is less than 50%, bending workability deteriorates because there are few ferrite grains. On the other hand, when the area ratio of ferrite exceeds 95%, since there are many ferrite grains, the bending fatigue limit durability ratio decreases. The average crystal grain size of ferrite needs to be 3 to 20 μm.
フェライトの平均結晶粒径が３μｍ未満であると、降伏点が上昇し、加工性が劣化する。また、フェライトの平均結晶粒径が２０μｍ超であると、曲げ疲労限度耐久比が低下してしまう。疲労き裂は、軟質であるフェライトの粒内を積極的に進展する。そのため、き裂の進展を抑制するフェライトの粒界が少なくなると早期にき裂が進展してしまう。 If the average crystal grain size of ferrite is less than 3 μm, the yield point increases and the workability deteriorates. Further, if the average crystal grain size of ferrite is more than 20 μm, the bending fatigue limit durability ratio is lowered. Fatigue cracks actively propagate in the ferrite grains, which are soft. Therefore, if the ferrite grain boundary that suppresses the crack growth decreases, the crack progresses at an early stage.
第２相
第２相は、マルテンサイトが面積率で５〜５０％である。マルテンサイト以外の第２相の存在は面積率で５％未満までは許容される。
Second phase The second phase has a martensite area ratio of 5 to 50%. The presence of the second phase other than martensite is allowed up to an area ratio of less than 5%.
マルテンサイトが面積率で５％未満であると曲げ疲労限度耐久比が低下する。マルテンサイトは硬質であるため、軟質なフェライト粒を進展してくる疲労き裂進展を遅延させる効果がある。一方、マルテンサイトの面積率が５０％超であると、延性と曲げ加工性が劣化する。マルテンサイトは硬質であるため、それ自体延性や曲げ加工時における局部延性が乏しい。 If the martensite is less than 5% in terms of area ratio, the bending fatigue limit durability ratio decreases. Since martensite is hard, it has the effect of delaying fatigue crack growth that propagates soft ferrite grains. On the other hand, when the area ratio of martensite exceeds 50%, ductility and bending workability deteriorate. Since martensite is hard, it itself has poor ductility and local ductility during bending.
マルテンサイト以外の第２相を含む場合は、その面積率は、５％未満である。マルテンサイト以外の第２相とは、ベイナイト組織、パーライト組織、セメンタイト、残留γである。マルテンサイト以外の第２相の面積率が５％以上であると、フェライトとマルテンサイトの硬度差によって向上する均一伸びが減少し、結果的に延性が劣化してしまう。また、残留γを含む場合には、均一伸びは向上するが、曲げ性が劣化するので同様に５％未満である。 When the second phase other than martensite is included, the area ratio is less than 5%. The second phase other than martensite is a bainite structure, a pearlite structure, cementite, and residual γ. When the area ratio of the second phase other than martensite is 5% or more, the uniform elongation that is improved by the hardness difference between ferrite and martensite decreases, and as a result, ductility deteriorates. Further, when the residual γ is included, the uniform elongation is improved, but the bendability is deteriorated.
第２相の平均粒径が１μｍ未満であると、曲げ疲労限度耐久比が低下する。第２相は、疲労き裂進展を抑制する効果がある。平均粒径１μｍ以上の第２相が存在すると、フェライト粒内を進展してきた疲労き裂は、その第２相を迂回するようにき裂が進展する。第２相を迂回時、き裂を進展させてきた応力とき裂進展の方向が変わるので、き裂進展が遅延するようになるからである。 Bending fatigue limit durability ratio falls that the average particle diameter of a 2nd phase is less than 1 micrometer. The second phase has the effect of suppressing fatigue crack growth. When the second phase having an average particle size of 1 μm or more exists, the fatigue crack that has propagated in the ferrite grains progresses so as to bypass the second phase. This is because, when the second phase is bypassed, the direction of crack propagation changes when the stress has propagated the crack, so that the crack propagation is delayed.
一方、第２相の平均粒径が８μｍ超であると曲げ加工性が劣化する。曲げ加工の場合、フェライトと硬質な第２相の界面からき裂が入る。第２相の大きさが８μｍ超であると、フェライトと第２相の界面の応力集中が大きくなり、曲げ加工の早期にき裂が入る。 On the other hand, if the average particle size of the second phase is more than 8 μm, the bending workability deteriorates. In the case of bending, a crack enters from the interface between the ferrite and the hard second phase. If the size of the second phase is more than 8 μm, the stress concentration at the interface between the ferrite and the second phase becomes large, and a crack occurs early in the bending process.
第２相の平均粒子間隔は２〜１０μｍである。第２相の平均粒径が１〜８μｍであっても、第２相の平均粒子間隔が２μｍ未満であると、曲げ加工性が劣化する。これは、平均間隔が２μｍ未満であると、フェライトと第２相の界面から発生したき裂が早期に連結するためである。また平均間隔粒子が１０μｍ超であると、曲げ疲労限度耐久比が劣化してしまう。これは、第２相の間隔が広いと第２相による疲労き裂の進展抑制効果が減少してしまうからである。 The average particle interval of the second phase is 2 to 10 μm. Even if the average particle size of the second phase is 1 to 8 μm, if the average particle interval of the second phase is less than 2 μm, the bending workability deteriorates. This is because cracks generated from the interface between the ferrite and the second phase are connected early when the average interval is less than 2 μm. Further, if the average interval particle exceeds 10 μm, the bending fatigue limit durability ratio deteriorates. This is because if the interval between the second phases is wide, the effect of suppressing the growth of fatigue cracks by the second phase is reduced.
本発明にかかる鋼板の表面性状は次の通りである。
鋼板表面において最大長さ５ｍｍ以上の島状スケール疵が面積率で１０％以下が必要である。島状スケール疵の面積率の基準は、鋼板面積１．５ｍ２あたりとする。通常使用される自動車の足廻り部品やバンパー等の補強材に代表される構造部材の素材、またはホイール用の素材としての鋼板の面積は１．５ｍ２のものが多いためである。
The surface properties of the steel sheet according to the present invention are as follows.
On the steel plate surface, an island-like scale ridge having a maximum length of 5 mm or more is required to have an area ratio of 10% or less. The standard for the area ratio of the island scale ridges is about 1.5 m 2 of the steel plate area. This is because the area of a steel plate as a material of a structural member typified by a reinforcing member such as an automobile undercarriage part or a bumper or a material for a wheel or a wheel material is often 1.5 m 2 .
鋼板表面において、最大長さ５ｍｍ以上の島状スケール疵が面積率で１０％超の場合、外観が美麗でないばかりか、鋼板表面の粗さが大きくなる。そのため、曲げ加工性が劣化する。最大長さが５ｍｍ未満の島状スケール疵ならびに最大長さ５ｍｍ以上の島状スケール疵の面積率が１０％未満ならばその悪影響が無い。 When the island-shaped scale ridges having a maximum length of 5 mm or more on the steel sheet surface have an area ratio of more than 10%, not only the appearance is not beautiful but also the roughness of the steel sheet surface becomes large. Therefore, bending workability deteriorates. If the area ratio of the island scale ridges having a maximum length of less than 5 mm and the island scale ridges having a maximum length of 5 mm or more is less than 10%, there is no adverse effect.
本発明にかかる鋼板の特性について説明すると次の通りである。
本発明の鋼板は、上記の成分からなり、かつ上記の金属組織を呈した鋼板であるので、引張り強度が５９０ＭＰａ以上、１８０度曲げにおける限界曲げ半径が板厚の２倍以下であり、かつ曲げ疲労限度耐久比が０．４８以上であるものが得られる。なお、強度７８０ＭＰａ以上の鋼板とすることによって、特に部材の薄肉化に効力が発揮される。
The characteristics of the steel sheet according to the present invention will be described as follows.
Since the steel plate of the present invention is a steel plate comprising the above components and exhibiting the above metal structure, the tensile strength is not less than 590 MPa, the limit bending radius in 180-degree bending is not more than twice the plate thickness, and the bending A fatigue limit durability ratio of 0.48 or more is obtained. In addition, especially by making it the steel plate of intensity | strength 780 Mpa or more, an effect is exhibited for thickness reduction of a member.
酸洗板
酸洗板の場合には、酸洗後の鋼板の平均表面粗さＲａが１．２μｍ以下、鋼板表面から鋼板表面からの深さが０．０３ｍｍの位置までの鋼板表層部におけるフェライトの平均結晶粒径をαｓ、鋼板表面からの深さが０．０３ｍｍ超の位置から板厚中心までにおける鋼板中心部におけるフェライトの平均結晶粒径をαｂとした時、フェライトの平均結晶粒径３〜２０μｍの範囲で、αＳ≧αｂ×１．１であることが必要である。
In the case of the pickled plate, the ferrite in the steel sheet surface layer portion where the average surface roughness Ra of the pickled steel plate is 1.2 μm or less and the depth from the steel plate surface to the depth of the steel plate surface is 0.03 mm. The average crystal grain size of ferrite is α s , and the average crystal grain size of ferrite in the center of the steel sheet from the position where the depth from the steel sheet surface exceeds 0.03 mm to the center of the sheet thickness is α b. It is necessary that α S ≧ α b × 1.1 in the diameter range of 3 to 20 μm.
酸洗板の場合には、表面スケール付着のままの鋼板と異なり、酸洗により表面粗さが大きくなる。その結果、曲げ加工性が劣化する。表面粗さＲａが１．２μｍ超であると、曲げ加工時、鋼板表面での応力集中が過大となり、曲げ加工初期に割れが発生する。 In the case of a pickled plate, the surface roughness is increased by pickling, unlike a steel plate with a surface scale attached. As a result, bending workability deteriorates. When the surface roughness Ra is more than 1.2 μm, stress concentration on the steel sheet surface becomes excessive during bending, and cracks occur at the initial stage of bending.
したがって、鋼板の表面粗さは、Ｒａで１．２μｍ以下とする必要がある。加えて、酸洗板において優れた曲げ加工性を保つためには、鋼板表面から鋼板内部０．０３ｍｍにおけるフェライトの平均粒径を大きくさせる必要がある。曲げ加工時、鋼板表面部では、鋼板内部に比べて、曲げ応力が大きくなるためで、鋼板表層部のフェライトの結晶粒径を大きくすることにより、鋼板表面部での加工性を向上させ、結果、曲げ加工性を向上させることができる。 Therefore, the surface roughness of the steel sheet needs to be 1.2 μm or less in terms of Ra. In addition, in order to maintain excellent bending workability in the pickled plate, it is necessary to increase the average grain size of ferrite from the steel plate surface to 0.03 mm inside the steel plate. When bending, because the bending stress is larger at the steel plate surface than at the inside of the steel plate, the workability at the steel plate surface is improved by increasing the crystal grain size of ferrite in the steel plate surface layer. , Bending workability can be improved.
鋼板表層から鋼板内部０．０３ｍｍ位置までのフェライトの平均粒径をαＳ、鋼板内部０．０３ｍｍ超から板厚中心部までにおけるフェライトの平均粒径をαｂとした時、αＳ＜αｂ×１．１の場合、鋼板表面におけるフェライト粒径が内部に比べ小さいため、曲げ加工性は向上しない。 When the average grain diameter of ferrite from the steel sheet surface layer to the 0.03 mm position inside the steel sheet is α S , and the average grain diameter of ferrite from the steel sheet inside 0.03 mm to the center of the plate thickness is α b , α S <α b In the case of x1.1, since the ferrite grain size on the steel sheet surface is smaller than that inside, the bending workability is not improved.
次に、本発明にかかる上述のような鋼板を製造するための製造方法について説明する。
本発明の鋼板を得るためには、スラブを１１００℃以上として粗熱間圧延を施して粗バーとなし、前記粗バーを下記式（１）で仕上げ圧延前に限界温度Ｔ以上で粗バーを加熱した後、デスケーリングを実施し、前記粗バーを下記式（１）で定められる限界温度Ｔ以上としてデスケーリングした後にＡｒ３点〜Ａｒ３＋１５０℃で圧延を完了する仕上熱間圧延を施して熱延鋼板とし、前記仕上熱間圧延の完了後３秒以内に冷却を開始して平均冷却速度２０〜２００℃／秒で７６０〜６００℃の温度域の所定温度まで冷却する１次冷却と、前記１次冷却後２〜２０秒間の中間空冷と、前記中間空冷後１０℃／秒以上の平均冷却速度で冷却する２次冷却とを前記熱延鋼板に施して、２５０℃以下で巻き取ることが必要である。
Next, the manufacturing method for manufacturing the above steel plates concerning this invention is demonstrated.
In order to obtain the steel sheet of the present invention, the slab is subjected to rough hot rolling at 1100 ° C. or more to form a rough bar, and the rough bar is formed by the following formula (1) before the final rolling at a limit temperature T or higher. After heating, descaling is performed, and after the descaling is performed with the rough bar set to a temperature equal to or higher than the limit temperature T defined by the following formula (1), finish hot rolling is performed to complete rolling at Ar 3 points to Ar 3 + 150 ° C. Primary cooling for forming a hot-rolled steel sheet, starting cooling within 3 seconds after completion of the finish hot rolling, and cooling to a predetermined temperature in the temperature range of 760 to 600 ° C. at an average cooling rate of 20 to 200 ° C./second; The hot-rolled steel sheet is subjected to intermediate air cooling for 2 to 20 seconds after the primary cooling and secondary cooling to be cooled at an average cooling rate of 10 ° C./second or more after the intermediate air cooling and wound at 250 ° C. or lower. It is necessary.
限界温度Ｔ（℃）＝１６８．１５×（（５×Ｐ＋Ａｌ）／Ｓｉ）２−２４５．１２×（５×Ｐ＋Ａｌ）／Ｓｉ＋１１７０（℃） （１）
ここで、式中の各元素記号は各元素の含有量（単位：質量％）を表す。
Limit temperature T (° C.) = 168.15 × ((5 × P + Al) / Si) 2 −245.12 × (5 × P + Al) / Si + 1170 (° C.) (1)
Here, each element symbol in the formula represents the content (unit: mass%) of each element.
スラブ加熱
粗熱間圧延に供する際にスラブ温度を１１００℃以上とする。スラブ温度が、１１００℃未満であると、スラブ中に存在する粗大な析出物や硫化物、窒化物が再固溶せず、圧延後の鋼板に残存し、著しく、延性、曲げ加工性、曲げ疲労耐久比を劣化させる。また、オーステナイトが粗大化しないため、フェライトの生成が過剰となり、所望のフェライト面積率が得られなくなる。スラブ温度の上限は１３００℃が望ましい。１３００℃超であると、スラブが自重で変形し、圧延トラブルに繋がる危険性がある。
Slab temperature shall be 1100 degreeC or more when using for slab heating rough hot rolling. When the slab temperature is less than 1100 ° C., coarse precipitates, sulfides and nitrides present in the slab do not re-dissolve and remain in the steel sheet after rolling, and remarkably ductility, bending workability, bending Degradation of fatigue durability ratio. Further, since austenite is not coarsened, the generation of ferrite becomes excessive, and a desired ferrite area ratio cannot be obtained. The upper limit of the slab temperature is preferably 1300 ° C. If it exceeds 1300 ° C, the slab is deformed by its own weight, which may lead to rolling trouble.
なお、本発明においては、粗熱間圧延に供するスラブ温度が上記温度域にあればよく、１１００℃未満の温度となったスラブを加熱する場合のみならず、連続鋳造により得られたスラブを１１００℃未満の温度に低下させることなく粗熱間圧延に供する場合も含まれる。 In the present invention, the slab temperature used for rough hot rolling only needs to be in the above temperature range, and not only when the slab having a temperature of less than 1100 ° C. is heated, but also the slab obtained by continuous casting is 1100. The case where it is subjected to rough hot rolling without lowering to a temperature of less than 0 ° C is also included.
粗バー温度
粗熱間圧延を行ない、得られた粗バーを粗バーの表面に生成するＦｅＯ／Ｆｅ２ＳｉＯ４の共晶温度以上、具体的には、Ｔ（℃）＝１６８．１５×（（５×Ｐ＋Ａｌ）／Ｓｉ）２−２４５．１２×（５×Ｐ＋Ａｌ）／Ｓｉ＋１１７０（℃）として規定される限界温度Ｔ（℃）以上に加熱した後、デスケーリングを実施する。
Coarse bar temperature rough hot rolling is performed, and the obtained coarse bar is not less than the eutectic temperature of FeO / Fe 2 SiO 4 that forms the surface of the coarse bar. Specifically, T (° C.) = 168.15 × ( Descaling is performed after heating to a temperature not lower than the limit temperature T (° C.) defined as (5 × P + Al) / Si) 2 −245.12 × (5 × P + Al) / Si + 1170 (° C.).
デスケーリング前の粗バーの温度がＴ（℃）未満であると、粗バーへの加熱が不十分であり、粗バー表面に生成するスケール量が少ない。そのため粗バーへのデスケーリング性が悪化し、島状スケール疵が発生する。デスケーリングの悪化理由としては、スケールは、その生成量が大きくなるほど、スケールの内部に圧縮応力が発生するとともに、粗バーとスケールとの界面に生成するボイドの生成量も増加する。 When the temperature of the coarse bar before descaling is less than T (° C.), the coarse bar is not sufficiently heated, and the amount of scale generated on the coarse bar surface is small. For this reason, the descaling property to the coarse bar is deteriorated, and the island scale flaws are generated. As the reason for the deterioration of descaling, as the scale generation amount increases, compressive stress is generated inside the scale, and the generation amount of voids generated at the interface between the coarse bar and the scale increases.
発生した圧縮応力及び生成したボイドの相互作用により、粗圧延を終了した時からデスケーリングを開始する時までにおける、スケールが進行するほど、粗バーの表面に生成するスケールは剥離し易いものとなる。 Due to the interaction between the generated compressive stress and the generated void, the scale generated on the surface of the coarse bar becomes easier to peel as the scale progresses from the time when the rough rolling is completed until the time when descaling is started. .
しかしながら、Ｓｉ含有量が０．２％以上であるＳｉ含有鋼は、高温かつ長時間のスラブ加熱によってＳｉ酸化物（Ｆｅ２ＳｉＯ４）が母材及びスケールの界面に濃化することによってスケール（ＦｅＯ）の生成を抑制している。ここで、一般的に、スケールの生成量を増加するには鋼板温度を高く設定すればよいが、Ｓｉ含有鋼ではこの酸化抑制効果がかなり大きいために、鋼板温度を多少高めた程度ではスケールの生成量はあまり増加しない。 However, a Si-containing steel having a Si content of 0.2% or more has a scale (Si (Fe 2 SiO 4 ) concentration at the interface between the base material and the scale by high-temperature and long-time slab heating. Generation of FeO) is suppressed. Here, in general, the steel plate temperature should be set high in order to increase the amount of scale generated. However, since this oxidation-suppressing effect is considerably large in the Si-containing steel, the scale of The production amount does not increase much.
そのため、ＦｅＯ／Ｆｅ２ＳｉＯ４の共晶温度である約１１７７℃以上に維持されておれば、Ｓｉ含有鋼のＳｉ量が高くとも、あるいはＳｉが界面に濃化していようとも、Ｆｅ２ＳｉＯ４が溶融化するためにスケール生成に対する抑制効果はなくなり、スケールの生成が進行する。ＦｅＯ／Ｆｅ２ＳｉＯ４の共晶温度は粗バーの組成、特にＰ、Ａｌを含有するとこの共晶温度は低下し、Ｔ（℃）＝１６８．１５×（（５×Ｐ＋Ａｌ）／Ｓｉ）２−２４５．１２×（５×Ｐ＋Ａｌ）／Ｓｉ＋１１７０（℃）以上の温度であれば、ＦｅＯ／Ｆｅ２ＳｉＯ４の共晶温度以上となり、その温度加熱後にデスケーリングを実施すれば、鋼板表面のスケール疵は減少する。 Therefore, if the eutectic temperature of FeO / Fe 2 SiO 4 is maintained at about 1177 ° C. or higher, even if the Si content of the Si-containing steel is high or Si is concentrated at the interface, Fe 2 SiO 4 Since the material melts, the effect of suppressing the scale generation disappears, and scale generation proceeds. The eutectic temperature of FeO / Fe 2 SiO 4 decreases with the composition of the coarse bar, particularly when P and Al are contained. T (° C.) = 168.15 × ((5 × P + Al) / Si) 2 −245.12 × (5 × P + Al) / Si + 1170 (° C.) or higher eutectic temperature of FeO / Fe 2 SiO 4 or higher, and if descaling is performed after the temperature heating, the scale of the steel sheet surface疵 decreases.
一方、上限は、１２３０℃以下が望ましい。１２３０℃超であると粗圧延で細粒化されたオーステナイトが再度粗大化し、その後のフェライトやマルテンサイトを細粒化させることができず、所望の金属組織を得ることができない欠点がある。好ましい上限は、Ｔ＋５０℃以下で行うのが良い。 On the other hand, the upper limit is desirably 1230 ° C. or lower. If it exceeds 1230 ° C., austenite refined by rough rolling is coarsened again, and subsequent ferrite and martensite cannot be refined, and a desired metal structure cannot be obtained. The upper limit is preferably T + 50 ° C. or less.
なお、本発明においては、粗バーの温度が上記限界温度Ｔ以上であればよいので、粗熱間圧延機と仕上熱間圧延機の間に誘導加熱等による粗バー加熱装置を配して、前記粗バー加熱装置によりＴ以上とする場合のみならず、粗圧延完了温度をＴ以上としてもよい。一般的には、粗圧延完了温度をＴ以上とするには粗熱間圧延に供する鋳片の温度を高温とする必要が生じてコスト的に不利となることから、上述したように粗バー加熱装置を配して加熱することが好ましい。 In the present invention, since the temperature of the coarse bar only needs to be equal to or higher than the above limit temperature T, a coarse bar heating device such as induction heating is arranged between the coarse hot rolling mill and the finish hot rolling mill, Not only when the temperature is set to T or more by the rough bar heating apparatus, the rough rolling completion temperature may be set to T or more. Generally, since it is necessary to raise the temperature of the slab to be used for rough hot rolling at a high temperature in order to set the rough rolling completion temperature to T or higher, the rough bar heating is performed as described above. It is preferable to arrange and heat the apparatus.
デスケーリング
デスケーリング装置は、公知のデスケーリング装置であればよく、本実施の形態では、粗バーの幅方向へ粗バーの表面へ高圧水を、高圧水吐出圧：１０ＭＰａ以上１００ＭＰａ以下及び粗バー単位幅当たり流量：０．０１ｍ３／秒／ｍ以上０．４ｍ３／秒／ｍ以下の条件で噴射するための噴射用ノズルを複数個配置されたデスケーリング装置を用いた。また、スケール除去時の粗バーの移動速度は０．１ｍ／秒以上２．５ｍ／秒以下とした。なお、仕上圧延前にデスケーリングを行う際の粗バーの温度も特に限定を要さない。
The descaling descaling apparatus may be a known descaling apparatus. In the present embodiment, high pressure water is supplied to the surface of the coarse bar in the width direction of the coarse bar, the high pressure water discharge pressure is 10 MPa to 100 MPa, and the coarse bar Flow rate per unit width: A descaling device in which a plurality of injection nozzles for injection under conditions of 0.01 m 3 / second / m or more and 0.4 m 3 / second / m or less was used was used. The moving speed of the coarse bar during scale removal was set to 0.1 m / second or more and 2.5 m / second or less. In addition, the temperature of the rough bar at the time of descaling before finish rolling is not particularly limited.
仕上熱間圧延
仕上熱間圧延は、Ａｒ３点〜（Ａｒ３点＋１５０℃）の温度範囲で行う。 仕上げ圧延温度がＡｒ３点以下の場合、フェライト域圧延となり加工フェライトが生成し、加工性が劣化する。酷い場合には圧延時体積膨張が起こり、圧延トラブルが発生する。Ａｒ３＋１５０℃超では、フェライトの生成が抑制され、フェライトの面積率が５０％未満となる。
Finish hot rolling Finish hot rolling is performed in the temperature range of Ar 3 points to (Ar 3 points + 150 ° C.). When the finish rolling temperature is 3 points or less of Ar, the ferrite region is rolled and processed ferrite is generated, and the workability deteriorates. In severe cases, volume expansion occurs during rolling, and rolling trouble occurs. If it exceeds Ar 3 + 150 ° C., the formation of ferrite is suppressed, and the area ratio of ferrite is less than 50%.
また仕上げ圧延後、３秒以内に冷却を開始して平均冷却速度２０〜１５０℃／秒で７６０〜６００℃の温度域の所定温度まで冷却する１次冷却と、前記１次冷却後２〜２０秒間の中間空冷と、前記中間空冷後１０℃／秒以上の平均冷却速度で冷却する２次冷却とを前記熱延鋼帯に施して、２５０℃以下で巻き取ることにより、所望の金属組織を得ることができる。 In addition, after the finish rolling, cooling is started within 3 seconds and is cooled to a predetermined temperature in a temperature range of 760 to 600 ° C. at an average cooling rate of 20 to 150 ° C./second, and 2 to 20 after the primary cooling. The intermediate air cooling for 2 seconds and the secondary cooling to be cooled at an average cooling rate of 10 ° C./second or more after the intermediate air cooling are performed on the hot-rolled steel strip and wound up at 250 ° C. or less to obtain a desired metal structure. Obtainable.
仕上熱間圧延後の冷却開始時間が３秒超の場合、または、冷却開始時間が３秒以内であっても１次冷却の冷却速度が、２０℃／秒未満であると、フェライトが微細化せず、フェライトの平均結晶粒径が２０μｍ超になり、加えて、第２相の平均結晶粒径が８μｍ超となる。 If the cooling start time after finish hot rolling exceeds 3 seconds, or if the cooling rate of primary cooling is less than 20 ° C / second even if the cooling start time is within 3 seconds, the ferrite becomes finer Without, the average crystal grain size of ferrite exceeds 20 μm, and in addition, the average crystal grain size of the second phase exceeds 8 μm.
一方、冷却速度が２００℃／秒超の場合、フェライト粒が成長せず、フェライトの平均結晶粒径が３μｍ未満となり、加えて第２相の平均結晶粒径が、１μｍ未満となる。１次冷却停止温度が７６０℃超の場合、フェライトの生成が促進されフェライトの面積率が９５％超となり、第２相の面積率が５％未満となる。 On the other hand, when the cooling rate exceeds 200 ° C./second, ferrite grains do not grow, the average crystal grain size of ferrite is less than 3 μm, and in addition, the average crystal grain size of the second phase is less than 1 μm. When the primary cooling stop temperature is higher than 760 ° C., the formation of ferrite is promoted, the area ratio of ferrite is more than 95%, and the area ratio of the second phase is less than 5%.
逆に１次冷却停止温度が６００℃未満であると、冷却が過大であるため、フェライト面積率が５０％未満となる。更に中間空冷時間を２〜２０秒とすることで、第２相の平均粒子間隔を２〜１０μｍにすることができる。中間空冷時間が２秒未満であるとフェライトからオーステナイトへのカーボンの拡散が不十分であり、フェライト粒の周辺に多くの第２相が生成し、第２相の平均粒子間隔が２μｍ未満となる。 Conversely, if the primary cooling stop temperature is less than 600 ° C., the cooling is excessive, and the ferrite area ratio becomes less than 50%. Furthermore, by setting the intermediate air cooling time to 2 to 20 seconds, the average particle interval of the second phase can be set to 2 to 10 μm. When the intermediate air cooling time is less than 2 seconds, the diffusion of carbon from ferrite to austenite is insufficient, many second phases are generated around the ferrite grains, and the average particle spacing of the second phase is less than 2 μm. .
逆に、中間空冷時間が２０秒超になると、カーボンの拡散が過剰になり、第２相は、フェライト粒の３重点のみに生成しやすくなる。その結果、第２相の平均粒子間隔が１０μｍ超となる。 On the other hand, when the intermediate air cooling time exceeds 20 seconds, the diffusion of carbon becomes excessive, and the second phase is likely to be generated only at the triple point of the ferrite grains. As a result, the average particle interval of the second phase is more than 10 μm.
中間空冷後、１０℃／秒以上で冷却後、２５０℃以下で巻き取ることにより、第２相をマルテンサイト面積率で５〜５０％かつマルテンサイト以外の第２相の面積率を５％未満にすることができる。中間空冷後１０℃／秒未満の冷却速度の場合、または、巻き取り温度が２５０℃超であると、冷却が不十分であり、第２相において、マルテンサイトの面積率５％以上を確保することができない。 After the intermediate air cooling, after cooling at 10 ° C./sec or more and winding up at 250 ° C. or less, the second phase has a martensite area ratio of 5 to 50% and the area ratio of the second phase other than martensite is less than 5%. Can be. In the case of a cooling rate of less than 10 ° C./second after the intermediate air cooling, or when the coiling temperature is higher than 250 ° C., the cooling is insufficient, and a martensite area ratio of 5% or more is ensured in the second phase. I can't.
酸洗板の製造方法
酸洗後の鋼板の平均表面粗さＲａを１．２μｍ以下にするには、まず、スラブを連続鋳造する際、スラブ表面から下記式（２）で定められるスラブ内部位置Ｓｔまでの液相線温度〜固相線温度の冷却速度を１０℃／秒以上としたスラブを用いる。
Manufacturing method of pickling plate To make the average surface roughness Ra of the pickled steel plate 1.2 μm or less, first, when continuously casting the slab, the slab internal position defined by the following formula (2) from the slab surface A slab having a cooling rate from the liquidus temperature to the solidus temperature up to St of 10 ° C./second or more is used.
Ｓｔ（ｍｍ）＝［Ｂ（ｍｍ）／Ａ（ｍｍ）］×０．０３ｍｍ （２）
ここで、Ｂ（ｍｍ）：スラブ厚、Ａ（ｍｍ）：鋼板の板厚である。
表面粗さは、鋼板表面に濃化するＳｉやＭｎの偏析のばらつきに大きく影響される。その理由としては、鋼板表面においてＳｉやＭｎの偏析にばらつきが生じていると、酸洗時の酸による鋼板の溶解が不均一であるためそれに準じて、表面粗さが粗くなる。また、Ｓｉ偏析のばらつきが多いと、スケールの生成も不均一であるため、酸によるスケ−ル除去時に鋼板の粗さを増加させる要因となる。
St (mm) = [B (mm) / A (mm)] × 0.03 mm (2)
Here, B (mm): slab thickness, A (mm): plate thickness of the steel plate.
The surface roughness is greatly influenced by variations in segregation of Si and Mn concentrated on the steel sheet surface. The reason for this is that if the segregation of Si or Mn is uneven on the surface of the steel sheet, the steel sheet is not evenly dissolved by the acid during pickling, and the surface roughness is accordingly increased. In addition, when there is a large variation in Si segregation, scale generation is also non-uniform, which causes an increase in the roughness of the steel sheet during scale removal by acid.
そのため、スラブ表面での偏析を軽減する必要がある。スラブ表面からスラブ内部位置Ｓｔ（ｍｍ）までの液相線温度〜固相線温度の冷却速度を１０℃／秒以上にすることにより、スラブ表面近傍のＳｉやＭｎの偏析が軽減される。加えて本発明で実施する仕上げ圧延前の粗バーの加熱、その後のデスケーリングを併用することにより、酸洗後の鋼板表面粗さＲａを１．２μｍ以下とすることができる。 Therefore, it is necessary to reduce segregation on the slab surface. By setting the cooling rate of the liquidus temperature to the solidus temperature from the slab surface to the slab internal position St (mm) to 10 ° C./second or more, segregation of Si and Mn near the slab surface is reduced. In addition, by using together the heating of the rough bar before finish rolling and the subsequent descaling performed in the present invention, the steel plate surface roughness Ra after pickling can be reduced to 1.2 μm or less.
酸洗は、常法で構わない。また、酸洗前にスキンパスで平坦矯正をおこなってもその効果は失われない。
さらに、本発明のスラブを用いた場合、鋼板表面から鋼板表面からの深さが０．０３ｍｍの位置までのフェライト組織は、Ｍｎの偏析が軽減されているため生成が促進される傾向がある。さらに仕上熱間圧延後、３秒以内に冷却速度２０〜１５０℃／秒とすることで、鋼板表面から鋼板表面からの深さが０．０３ｍｍの位置までのフェライトの平均結晶粒径をαＳ、鋼板表面からの深さが０．０３ｍｍ超の位置から板厚中心までの鋼板中心部におけるフェライトの平均粒径をαｂとした時、αＳ≧αｂ×１．１とすることができる。
Pickling may be performed by a conventional method. Even if flattening is performed with a skin pass before pickling, the effect is not lost.
Furthermore, when the slab of the present invention is used, the ferrite structure from the steel sheet surface to the position where the depth from the steel sheet surface is 0.03 mm tends to be promoted because Mn segregation is reduced. Furthermore, after finishing hot rolling, by setting the cooling rate to 20 to 150 ° C./second within 3 seconds, the average crystal grain size of ferrite from the steel sheet surface to the position where the depth from the steel sheet surface is 0.03 mm is α S When the average grain diameter of ferrite in the center of the steel sheet from the position where the depth from the steel sheet surface exceeds 0.03 mm to the center of the sheet thickness is α b , α S ≧ α b × 1.1 can be established. .
表１に示す化学成分を有する鋼を転炉で溶製し、試験連続鋳造機にて連続鋳造を実施し、巾１０００ｍｍで厚み２５０ｍｍのスラブとした。スラブ表面近傍における液相線温度から固相線温度までの平均冷却速度の変更は、鋳型の水量にて調整した。 Steel having the chemical components shown in Table 1 was melted in a converter and continuously cast by a test continuous casting machine to obtain a slab having a width of 1000 mm and a thickness of 250 mm. The change in the average cooling rate from the liquidus temperature to the solidus temperature near the slab surface was adjusted by the amount of water in the mold.
試験圧延装置を用いて、得られたスラブを表２に示す条件にて加熱した後、粗圧延を実施し、厚み３５ｍｍの粗バーとし、誘導加熱装置で粗バーを加熱した。その後、デスケーリングを行い、仕上げ圧延、鋼板冷却を実施した。その後、酸洗を実施した。 The obtained slab was heated under the conditions shown in Table 2 using a test rolling device, and then rough rolling was performed to obtain a rough bar having a thickness of 35 mm, and the coarse bar was heated with an induction heating device. Thereafter, descaling was performed, and finish rolling and steel plate cooling were performed. Thereafter, pickling was performed.
評価方法
＜スラブ平均冷却速度＞
得られたスラブの断面をピクリン酸にてエッチングし、０．５ｍｍピッチでデンドライト２次アーム間隔λ（μｍ）を測定し、次式に基づいて、その値からスラブの液相線〜固相線内の冷却速度Ａ（℃／秒）を算出した。なお、平均冷却速度は、スラブ表面から厚方向のスラブＳｔ位置まで０．５ｍｍピッチで測定した冷却速度の算術計算での平均値とした。
Evaluation method <Slab average cooling rate>
The cross section of the obtained slab is etched with picric acid, the dendrite secondary arm interval λ (μm) is measured at a pitch of 0.5 mm, and the slab liquidus to solidus is calculated from the value based on the following formula: The cooling rate A (° C./second) was calculated. The average cooling rate was an average value in arithmetic calculation of the cooling rate measured at a pitch of 0.5 mm from the slab surface to the slab St position in the thickness direction.
λ＝７１０×Ａ−０．３９
＜金属組織の評価＞
鋼板の圧延方向に平行な断面について、光学顕微鏡または走査型電子顕微鏡を用いて、ＪＩＳ Ｇ ０５５２に準拠してフェライトの平均結晶粒径を測定した。フェライトの面積率は、画像処理にてもとめた。
λ = 710 × A −0.39
<Evaluation of metal structure>
About the cross section parallel to the rolling direction of the steel sheet, the average crystal grain size of ferrite was measured according to JIS G 0552 using an optical microscope or a scanning electron microscope. The area ratio of ferrite was also determined by image processing.
第２相の同定ならびに平均結晶粒径、平均粒子間隔は、走査型電子顕微鏡を用いて調査した。平均粒子間隔は、個々の第２相について最近接距離を測定し、その算術計算の平均値とした。 The identification of the second phase, the average crystal grain size, and the average particle spacing were investigated using a scanning electron microscope. For the average particle spacing, the closest distance was measured for each second phase, and the average value of the arithmetic calculation was used.
＜鋼板の表面性状の評価＞
鋼板表面における最大長さ５ｍｍ以上の島状スケールの面積率の算出は、得られた鋼板の外観写真を撮影し、画像処理にて面積率をもとめた。
<Evaluation of surface properties of steel sheet>
The area ratio of the island-shaped scale having a maximum length of 5 mm or more on the surface of the steel sheet was obtained by taking an appearance photograph of the obtained steel sheet and obtaining the area ratio by image processing.
＜引張試験＞
各鋼板の圧延直角方向からＪＩＳ ５号引張試験を採取した。試験方法はＪＩＳ Ｚ２２４１に準じた。降伏点ＹＰ、引張強さＴＳ、伸びＥｌを測定した。
<Tensile test>
A JIS No. 5 tensile test was taken from the direction perpendicular to the rolling of each steel plate. The test method conformed to JIS Z2241. Yield point YP, tensile strength TS, and elongation El were measured.
＜限界曲げ試験＞
各鋼板の圧延直角方向から巾４０ｍｍ、長さ２００ｍｍの試験片を採取した。試験形状ならびに試験方法はＪＩＳ Ｚ２２４８に準じた。曲げ半径は、密着から板厚の１倍、２倍、３倍、４倍にて実施し、その割れが発生しない板厚に対する曲げ半径を限界曲げ半径とした。
<Limit bending test>
A test piece having a width of 40 mm and a length of 200 mm was taken from the direction perpendicular to the rolling direction of each steel plate. The test shape and test method conformed to JIS Z2248. The bending radius was 1 to 2, 3 times, and 4 times the plate thickness from close contact, and the bending radius with respect to the plate thickness at which no cracking occurred was defined as the critical bending radius.
＜平面曲げ疲労試験＞
各鋼板からＪＩＳ Ｚ２２７５に記載されている形状にて長さ９０ｍｍ、巾４０ｍｍの試験片を採取した。試験方法は、ＪＩＳ Ｚ２２７５に準じた。両振り平面曲げ疲労（応力比：−１）にて実施し、１０７乗回の繰り返し数にて破断しない応力振幅値を疲労限界とし、次式により、ＴＳとの算術計算から耐久比をもとめた。
<Plane bending fatigue test>
A test piece having a length of 90 mm and a width of 40 mm was collected from each steel plate in the shape described in JIS Z2275. The test method conformed to JIS Z2275. Performed in double-bending plane bending fatigue (stress ratio: -1), the stress amplitude value that does not break at the number of repetitions of 10 7 times as the fatigue limit, and calculated the durability ratio from the arithmetic calculation with TS by the following formula It was.
耐久比＝１０７乗回で破断した応力振幅値／ＴＳ
鋼板の特性結果を表３、表４に示した。
Endurance ratio = 10 7th power of stress amplitude / TS broken
The characteristic results of the steel plates are shown in Tables 3 and 4.
表３、４からも分かるように、本発明である材質Ｎｏ．Ｂ１〜Ｂ８は、強度が５９０ＭＰａ以上で限界曲げ半径が「密着〜２．０ｔ」であり、曲げ疲労限度耐久比が０．４８以上であった。そのため加工性（伸び）、曲げ加工性、耐疲労特性に優れていた。加えて、島状スケール疵面積率が２％以内であり鋼板の表面性状にも優れていた。なお、ここで、ｔは板厚を表わし、２．０ｔとは、板厚の２倍を意味する。 As can be seen from Tables 3 and 4, the material no. B1 to B8 had a strength of 590 MPa or more, a critical bending radius of “adhesion to 2.0 t”, and a bending fatigue limit durability ratio of 0.48 or more. Therefore, it was excellent in workability (elongation), bending workability, and fatigue resistance. In addition, the area ratio of the island-like scale wrinkles was within 2%, and the surface properties of the steel sheet were excellent. Here, t represents a plate thickness, and 2.0 t means twice the plate thickness.
これに対して、材質Ｎｏ．Ｂ９〜Ｂ１３は、Ｐ、Ｃｕ、Ｏ、Ｃｕ／（Ｐ＋Ｏ）のいずれかが本発明外であるため、曲げ加工性が３．０ｔと劣化した。加えて曲げ疲労限度耐久比が０．４７以下で、曲げ疲労限度耐久比が劣化したものもあった。 On the other hand, the material No. In B9 to B13, any of P, Cu, O, and Cu / (P + O) is outside the scope of the present invention, so that the bending workability deteriorated to 3.0 t. In addition, the bending fatigue limit durability ratio was 0.47 or less, and the bending fatigue limit durability ratio was also deteriorated.
Claims (8)
０．４≦Ｃｕ／（Ｐ＋Ｏ）≦３．０ （１）
ここで、式中の各元素記号は各元素の含有量（単位：質量％）を表す。 Steel composition is mass%, C: more than 0.01% and 0.25% or less, Si: more than 0.2% and less than 1.0%, Mn: 0.5 to 2.5%, P: 0.005 %: Less than 0.030%, Cu: 0.005% or more and less than 0.045%, S: 0.02% or less, Al: 0.005 to 1.0%, N: 0.01% or less, and O: 0.0010% or more and less than 0.0100%, the contents of Cu, P and O satisfy the following formula (1), the balance is composed of Fe and impurities, and the steel structure is 50 to 95 area%. It consists of ferrite and the remaining second phase, the average crystal grain size of the ferrite is 3 to 20 μm, the average grain size of the second phase is 1.0 to 8 μm, the average grain spacing is 2 to 10 μm, and the second phase is Second, other than martensite of 5-50% and martensite of less than 0-5% in terms of area ratio based on the entire steel structure And the surface properties are 10% or less in terms of area ratio of island-like scale ridges having a maximum length of 5 mm or more on the steel sheet surface, and mechanical properties are tensile strength: 590 MPa or more, bendability: adhesion to 2.0 t, Bending fatigue limit durability ratio: A high-tensile hot-rolled steel sheet characterized by being 0.48 or more.
0.4 ≦ Cu / (P + O) ≦ 3.0 (1)
Here, each element symbol in the formula represents the content (unit: mass%) of each element.
限界温度Ｔ（℃）＝１６８．１５×（（５×Ｐ＋Ａｌ）／Ｓｉ）２−２４５．１２×（５×Ｐ＋Ａｌ）／Ｓｉ＋１１７０（℃） （２）
ここで、式中の各元素記号は各元素の含有量（単位：質量％）を表す。 The slab having the steel composition according to any one of claims 1 to 5 is subjected to rough hot rolling at 1100 ° C or higher to form a rough bar, and the obtained rough bar is defined as a limit temperature T defined by the following formula (2). After descaling as described above, finish hot rolling is performed to complete rolling at Ar 3 points to (Ar 3 points + 150 ° C.) to form a hot-rolled steel sheet, which is cooled within 3 seconds after completion of the finish hot rolling. Is started to cool to a predetermined temperature in the temperature range of 760 to 600 ° C. at an average cooling rate of 20 to 200 ° C./second, intermediate air cooling for 2 to 20 seconds after the primary cooling, and after the intermediate air cooling A method for producing a high-tensile hot-rolled steel sheet, wherein the hot-rolled steel sheet is subjected to secondary cooling that is cooled at an average cooling rate of 10 ° C / second or more, and then wound at 250 ° C or less.
Limit temperature T (° C.) = 168.15 × ((5 × P + Al) / Si) 2 −245.12 × (5 × P + Al) / Si + 1170 (° C.) (2)
Here, each element symbol in the formula represents the content (unit: mass%) of each element.
限界温度Ｔ（℃）＝１６８．１５×（（５×Ｐ＋Ａｌ）／Ｓｉ）２−２４５．１２×（５×Ｐ＋Ａｌ）／Ｓｉ＋１１７０（℃） （２）
ここで、式中の各元素記号は各元素の含有量（単位：質量％）を表す。
Ｓｔ（ｍｍ）＝［Ｂ（ｍｍ）］／Ａ（ｍｍ）］×０．０３ｍｍ （３）
ここで、Ｂ（ｍｍ）：スラブ厚、Ａ（ｍｍ）：鋼板の板厚である。 The molten steel having the steel composition according to any one of claims 1 to 5, wherein the cooling rate from the liquidus temperature to the solidus temperature from the slab surface to the slab internal position St defined by the following formula (3) is 10 ° C / More than a second, a slab is formed by a continuous casting method, the slab is subjected to rough hot rolling at 1100 ° C. or more to form a rough bar, and the obtained rough bar is set to a limit temperature T or more determined by the following formula (2). After scaling, finish hot rolling to complete rolling at Ar 3 point to (Ar 3 point + 150 ° C.) is made into a hot-rolled steel sheet, and cooling is started within 3 seconds after completion of the finish hot rolling. Primary cooling for cooling to a predetermined temperature in the temperature range of 760 to 600 ° C. at an average cooling rate of 20 to 150 ° C./second, intermediate air cooling for 2 to 20 seconds after the primary cooling, and 10 ° C./after the intermediate air cooling. Secondary cooling with an average cooling rate of more than 1 second Preparative subjected to the hot-rolled steel strip, wound at 250 ° C. or less, the method of producing a high tensile hot-rolled steel sheet, characterized by then pickling.
Limit temperature T (° C.) = 168.15 × ((5 × P + Al) / Si) 2 −245.12 × (5 × P + Al) / Si + 1170 (° C.) (2)
Here, each element symbol in the formula represents the content (unit: mass%) of each element.
St (mm) = [B (mm)] / A (mm)] × 0.03 mm (3)
Here, B (mm): slab thickness, A (mm): plate thickness of the steel plate.
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