JP2005097740A - High-carbon hot-rolled steel sheet, and method for manufacturing the same - Google Patents
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本発明は、自動車の構造部品等に使用され、素材である熱延鋼板の強度において440MPa以上を有する伸びフランジ性に優れ、あるいはさらに延性にも優れた高炭素熱延鋼板およびその製造方法に関する。 The present invention relates to a high-carbon hot-rolled steel sheet that is used for structural parts of automobiles and the like and has excellent stretch flangeability having a strength of a hot-rolled steel sheet that is 440 MPa or more, and further excellent in ductility, and a method for producing the same.
工具あるいは自動車部品（ギア、ミッション）等に使用される高炭素鋼板は、打抜き、成形後、焼入れ焼戻し等の熱処理が施される。これらの部品加工を行うユーザの要求の一つに打抜き後の成形において、穴拡げ加工（バーリング）性の向上がある。この穴拡げ加工性は、プレス成形性としては伸びフランジ性で評価されている。そのため、伸びフランジ性の優れた材料が望まれている。また、複雑形状に成形する場合は、延性の指標である伸び特性が良好であることも要求される。 High carbon steel sheets used for tools or automobile parts (gears, missions) and the like are subjected to heat treatment such as quenching and tempering after punching and forming. One of the demands of users who process these parts is to improve the hole expansion process (burring) in forming after punching. This hole expansion workability is evaluated as stretch flangeability as press formability. Therefore, a material excellent in stretch flangeability is desired. In addition, when forming into a complicated shape, it is also required that the elongation characteristic, which is an index of ductility, is good.
このような、高炭素鋼板の伸びフランジ性の向上については、いくつかの技術が検討されている。例えば、特許文献１には、冷間圧延を経たプロセスにおいて、伸びフランジ性に優れた中・高炭素鋼板を作る方法が提案されている。この技術は、Ｃ：0.1〜0.8質量％を含有する鋼からなり、金属組織が実質的にフェライト＋パーライト組織であり、必要に応じて初析フェライト面積率がＣ含有量（質量％）により決まる所定の値以上、パーライトラメラ間隔が0.1μm以上の熱延鋼板に、15％以上の冷間圧延を施し、次いで、３段階又は２段階の温度範囲で長時間保持する３段階又は２段階焼鈍を施すというものである。 Several techniques have been studied for improving the stretch flangeability of such a high-carbon steel sheet. For example, Patent Document 1 proposes a method of producing a medium / high carbon steel sheet having excellent stretch flangeability in a process after cold rolling. This technique consists of steel containing C: 0.1 to 0.8% by mass, the metal structure is substantially a ferrite + pearlite structure, and the pro-eutectoid ferrite area ratio is determined by the C content (% by mass) as necessary. Hot rolling steel sheet with a pearlite lamella spacing of 0.1μm or more is subjected to cold rolling of 15% or more, and then subjected to three-stage or two-stage annealing for a long time in a three-stage or two-stage temperature range. It is to give.
また、特許文献２には、Ｃ：0.1〜0.8質量％を含有する鋼からなり、初析フェライト面積率（％）がＣ含有量により決まる所定値以上である、初析フェライト＋パーライト組織の熱延鋼板に、1段目の加熱保持と２段目の加熱保持を連続して行う焼鈍を施す、という技術が開示されている。 Patent Document 2 discloses that the heat of pro-eutectoid ferrite + pearlite structure is made of steel containing C: 0.1 to 0.8% by mass, and the pro-eutectoid ferrite area ratio (%) is not less than a predetermined value determined by the C content. A technique is disclosed in which a steel sheet is subjected to annealing in which the first stage of heat holding and the second stage of heat holding are continuously performed.
さらに特許文献３には、伸びフランジ性に優れた高炭素熱延鋼板が提案されている。これは、Ｃを0.2〜0.7質量％含有する鋼を、仕上温度 (Ar3変態点-20℃)以上で熱間圧延した後、冷却速度120℃/秒超かつ冷却停止温度650℃以下で冷却を行い、次いで巻取温度600℃以下で巻取り、酸洗後、焼鈍温度640℃以上Ac1変態点以下で焼鈍する技術である。金属組織については、炭化物平均粒径を0.1μm以上1.2μm未満、炭化物を含まないフェライト粒の体積率を10％以下に制御することを特徴としている。
Further,
また、特許文献４には、伸びフランジ性に優れた高炭素冷延鋼板が提案されている。これは、Ｃを0.2〜0.7質量％含有する鋼を、仕上温度(Ar3変態点-20℃)以上で熱間圧延した後、冷却速度120℃/秒超かつ冷却停止温度650℃以下で冷却を行い、次いで巻取温度600℃以下で巻取り、酸洗後、冷圧率30%以上で冷間圧延を行い、焼鈍温度600℃以上Ac1変態点以下で焼鈍する技術である。金属組織については、炭化物平均粒径を0.1μm以上2.0μm未満、炭化物を含まないフェライト粒の体積率を15％以下に制御することを特徴としている。
これら特許文献１、２記載の技術では、フェライト組織が初析フェライトからなり、炭化物を実質的に含まないため柔らかく延性に優れているが、伸びフランジ性は必ずしも良好ではない。それは、以下のように考えられる。すなわち、打抜き加工時に、打抜き端面の近傍で初析フェライトの部分が大きく変形するため、初析フェライトと球状化炭化物を含むフェライトでは変形量が大きく異なる。その結果、これら変形量が大きく異なる粒の粒界付近に応力が集中し、球状化組織とフェライトの界面にボイドが発生する。これがクラックに成長するため、結果的には伸びフランジ性を劣化させると考えられる。 In the techniques described in Patent Documents 1 and 2, since the ferrite structure is composed of pro-eutectoid ferrite and does not substantially contain carbide, it is soft and excellent in ductility, but stretch flangeability is not always good. This is considered as follows. That is, during the punching process, the pro-eutectoid ferrite portion is greatly deformed in the vicinity of the punched end face, and therefore the deformation amount differs greatly between the pro-eutectoid ferrite and the ferrite containing the spheroidized carbide. As a result, stress concentrates in the vicinity of the grain boundaries of the grains having greatly different deformation amounts, and voids are generated at the interface between the spheroidized structure and the ferrite. Since this grows into a crack, it is considered that the stretch flangeability is deteriorated as a result.
この対策として、球状化焼鈍を強化することにより、全体として軟質化させることが考えられる。しかし、その場合は球状化した炭化物が粗大化し、加工の際にボイド発生の起点となるとともに、加工後の熱処理段階で炭化物が溶解し難くなり、焼入強度の低下につながる。 As a countermeasure against this, it is conceivable to soften the whole by strengthening the spheroidizing annealing. However, in that case, the spheroidized carbides become coarse and become the starting point of void generation during processing, and the carbides are difficult to dissolve in the heat treatment stage after processing, leading to a decrease in quenching strength.
また、最近では従来にもまして、生産性向上の観点からの加工レベルに対する要求が厳しくなっている。そのため、高炭素鋼板の穴拡げ加工についても、加工度の増加等により、打抜き端面の割れが発生しやすくなっている。従って、高炭素鋼板にも高い伸びフランジ性が要求されている。 In recent years, demands for processing levels from the viewpoint of productivity improvement have become stricter than ever before. Therefore, also in the hole expanding process of the high carbon steel sheet, the punched end face is likely to be cracked due to an increase in the degree of processing. Therefore, a high stretch steel sheet is also required to have high stretch flangeability.
本発明者らは、かかる事情に鑑み、長時間を要する多段階焼鈍を用いることなく製造でき、打抜き端面の割れが発生しにくい伸びフランジ性に優れた高炭素鋼板を提供することを目的として、特許文献３、４記載の技術を開発した。これらの技術により、伸びフランジ性に優れた高炭素熱延鋼板あるいは高炭素冷延鋼板が製造できるようになった。
In view of such circumstances, the present inventors have been able to manufacture without using multi-stage annealing that takes a long time, and for the purpose of providing a high carbon steel sheet excellent in stretch flangeability that hardly causes cracking of the punched end face. The technologies described in
最近では、駆動系部品などの用途に対しては、高耐久・軽量化の観点から一体成形部品などで非熱処理部においても高強度化が進み、素材である鋼板の引張強度（ＴＳ）として440MPa以上の強度を要求されるようになってきている。このような要求と共に、部品の製造コスト低減のため、熱延鋼板で供給することが要求されている。 Recently, for applications such as driveline parts, the strength of non-heat treated parts has been increasing with integral molded parts from the viewpoint of high durability and light weight, and the tensile strength (TS) of the steel plate is 440 MPa. The above strength has been demanded. Along with such demands, it is required to supply hot-rolled steel sheets in order to reduce the manufacturing cost of parts.
また、一体成形においては、10数工程のプレス工程を有し、バーリング加工のみならず、張出し、曲げなどの成形モードが複雑に組み合わされて成形がなされているため、伸びフランジ性とさらには伸び性の両特性を同時に要求されるようになってきている。 In addition, in the integral molding, there are more than 10 pressing processes, and not only burring, but also molding is performed by complex combinations of molding modes such as overhanging and bending. Both characteristics of sex have been required at the same time.
しかしながら、上記特許文献３、４記載の技術では、ＴＳ≧440MPa（ＨＲＢ硬度換算で73ポイント以上）を達成しようとすると、十分な伸びフランジ性が必ずしも得られなかった。すなわち、伸びフランジ性は穴拡げ率（λ）により評価され、λ≧70％が望まれており、特に優れた伸びフランジ性を求められる場合にはλ≧85％が望まれているが、上記技術ではこのＴＳと伸びフランジ性の要望を、同時に安定して確保することができなかった。また、伸びについては言及してなかった。
However, with the techniques described in
本発明は、かかる事情に鑑み、長時間を要する多段階焼鈍を用いることなく製造でき、打抜き端面の割れが発生しにくく、440MPa以上の引張強度を有するとともに、λ≧70％、好ましくはλ≧85％を満足する伸びフランジ性に優れた高炭素熱延鋼板を提供することを目的とし、さらに特に優れた延性が要求される場合には、伸び35％以上をも満足する延性および伸びフランジ性に優れた高炭素熱延鋼板を提供することを目的とする。 In view of such circumstances, the present invention can be manufactured without using multi-stage annealing that requires a long time, is not easily cracked at the punched end face, has a tensile strength of 440 MPa or more, and λ ≧ 70%, preferably λ ≧ The purpose is to provide a high-carbon hot-rolled steel sheet with excellent stretch flangeability that satisfies 85%. When particularly excellent ductility is required, ductility and stretch flangeability that satisfy even 35% or more are required. An object of the present invention is to provide a high-carbon hot-rolled steel sheet having excellent resistance.
上記課題は、次の発明により解決される。その発明は、質量％で、Ｃ：0.20〜0.48％、Ｓｉ：0.1％以下、Ｍｎ：0.20〜0.60％、Ｐ：0.02％以下、Ｓ：0.01％以下、ｓｏｌ．Ａｌ：0.1％以下、Ｎ：0.005％以下、Ｔｉ：0.005〜0.05％、Ｂ：0.0005〜0.003％、Ｃｒ：0.05〜0.3％を含有し、Ｔｉ−(48/14)Ｎ≧0.005（式中の元素記号はそれぞれの元素の含有量の質量％を示す）を満足し、残部鉄および不可避的不純物である組成と、フェライト平均粒径が6μm以下、炭化物平均粒径が0.1μm以上1.20μm未満、炭化物を実質的に含まないフェライト粒の体積率が5％以下である組織を有することを特徴とする高炭素熱延鋼板である。 The above problem is solved by the following invention. The invention is, in mass%, C: 0.20 to 0.48%, Si: 0.1% or less, Mn: 0.20 to 0.60%, P: 0.02% or less, S: 0.01% or less, sol. Al: 0.1% or less, N: 0.005% or less, Ti: 0.005 to 0.05%, B: 0.0005 to 0.003%, Cr: 0.05 to 0.3%, Ti— (48/14) N ≧ 0.005 (in the formula Element symbol indicates the mass% of the content of each element), the composition of the balance iron and inevitable impurities, the ferrite average particle size is 6 μm or less, the carbide average particle size is 0.1 μm or more and less than 1.20 μm, A high-carbon hot-rolled steel sheet characterized by having a structure in which the volume fraction of ferrite grains substantially free of carbides is 5% or less.
上記高炭素熱延鋼板の発明においては、さらに、炭化物平均粒径が0.5μm以上1.20μm未満であることを特徴とする高炭素熱延鋼板とすることもできる。 In the invention of the high-carbon hot-rolled steel sheet, a high-carbon hot-rolled steel sheet characterized by having an average carbide particle size of 0.5 μm or more and less than 1.20 μm can be obtained.
製造方法の発明としては、上記組成を有する鋼を、(Ar3変態点-10℃)以上の仕上温度で熱間圧延した後、冷却速度120℃/秒超かつ冷却停止温度620℃以下として冷却を行い、次いで巻取温度600℃以下で巻取り熱延鋼板とした後、焼鈍温度640℃以上Ac1変態点以下で焼鈍することを特徴とする高炭素熱延鋼板の製造方法である。 As an invention of the manufacturing method, steel having the above composition is hot-rolled at a finishing temperature of (Ar 3 transformation point −10 ° C.) or higher, and then cooled at a cooling rate of over 120 ° C./second and a cooling stop temperature of 620 ° C. or lower. And then producing a rolled hot-rolled steel sheet at a coiling temperature of 600 ° C. or lower, followed by annealing at an annealing temperature of 640 ° C. or higher and an Ac 1 transformation point or lower.
製造方法の発明としては、前述の発明において、さらに、前記焼鈍を焼鈍温度680℃以上Ac1変態点以下で行うことを特徴とする高炭素熱延鋼板の製造方法とすることもできる。 As an invention of the production method, in the above-mentioned invention, it is also possible to provide a production method of a high carbon hot rolled steel sheet characterized in that the annealing is further performed at an annealing temperature of 680 ° C. or more and an Ac 1 transformation point or less.
また、製造方法の発明としては、前述の発明において、さらに、前記冷却停止温度600℃以下で冷却を行い、前記巻取温度500℃以下で巻取ることを特徴とする高炭素熱延鋼板の製造方法とすることもできる。 Further, as an invention of a manufacturing method, in the above-mentioned invention, further cooling is performed at the cooling stop temperature of 600 ° C. or lower, and winding is performed at the winding temperature of 500 ° C. or lower. It can also be a method.
これらの発明は、高炭素鋼板の伸びフランジ性および延性に及ぼす組成およびミクロ組織の影響について鋭意研究を進める中でなされた。その過程で、鋼板の伸びフランジ性および延性に影響を及ぼす因子は、組成や炭化物の形状および量のみならず、炭化物の分散状態も大きな影響を及ぼしていることを見出した。 These inventions were made in the course of diligent research on the effects of composition and microstructure on stretch flangeability and ductility of high carbon steel sheets. In the process, it has been found that factors affecting the stretch flangeability and ductility of the steel sheet have a great influence not only on the composition and shape and amount of carbide, but also on the dispersion state of carbide.
また、炭化物の形状としては炭化物平均粒径、炭化物の分散状態としては炭化物を実質的に含まないフェライト粒の体積率を、それぞれ制御することにより、高炭素熱延鋼板の伸びフランジ性が向上することがわかった。さらに、組成およびフェライト粒径を制御することにより、伸びフランジ性と強度を、安定してかつ高いレベルで両立でき、炭化物粒径をさらに規定し制御することで伸びを安定して高めることを見出した。この知見に基づき、上記の組織を制御するための製造方法を検討し、伸びフランジ性に優れ、あるいはさらに延性にも優れた高炭素熱延鋼板の製造方法を確立した。 Further, by controlling the carbide average particle size as the shape of carbide and the volume fraction of ferrite grains substantially free of carbide as the carbide dispersion state, the stretch flangeability of the high carbon hot rolled steel sheet is improved. I understood it. Furthermore, it has been found that by controlling the composition and ferrite particle size, it is possible to achieve both stable and high levels of stretch flangeability and strength, and to stably increase the elongation by further defining and controlling the carbide particle size. It was. Based on this knowledge, a production method for controlling the above structure was examined, and a production method of a high carbon hot-rolled steel sheet having excellent stretch flangeability or even ductility was established.
以下、本発明の構成要素について説明する。 Hereinafter, the components of the present invention will be described.
Ｃ含有量：0.20〜0.48％（質量％、以下同様）
Ｃは、炭化物を形成し、焼入後の硬度を付与する重要な元素である。しかし、Ｃ含有量が0.20％未満では、熱延後の組織において初析フェライトの生成が顕著となり、炭化物を実質的に含まないフェライト粒が多くなって、炭化物の分布が不均一となる。また、フェライト粒も粗大化する。さらにその場合、焼入後も、機械構造用部品として十分な強度が得られない。一方、Ｃ含有量が0.48％を超える場合、焼鈍後でも伸びフランジ性および延性が低い。従って、Ｃ含有量を0.20％以上0.48％以下とする。
C content: 0.20 to 0.48% (mass%, the same applies hereinafter)
C is an important element that forms carbides and imparts hardness after quenching. However, if the C content is less than 0.20%, the formation of pro-eutectoid ferrite becomes remarkable in the structure after hot rolling, the number of ferrite grains substantially not containing carbides increases, and the distribution of carbides becomes uneven. Further, the ferrite grains are also coarsened. Furthermore, in that case, sufficient strength cannot be obtained as a machine structural component even after quenching. On the other hand, when the C content exceeds 0.48%, stretch flangeability and ductility are low even after annealing. Therefore, the C content is set to 0.20% or more and 0.48% or less.
Ｓｉ：0.1％以下
Ｓｉは、焼入れ性を向上させるとともに固溶強化により素材強度を上昇させる元素であるため、0.005％以上含有することが好ましい。しかし、0.1％を超えて含有すると、初析フェライトが生成し易くなり、炭化物を実質的に含まないフェライト粒が多くなって、伸びフランジ性が劣化する。従って、Ｓｉ含有量を0.1％以下に制限する。
Si: 0.1% or less Since Si is an element that improves hardenability and increases the strength of the material by solid solution strengthening, it is preferably contained in an amount of 0.005% or more. However, if the content exceeds 0.1%, pro-eutectoid ferrite is likely to be generated, and ferrite grains that do not substantially contain carbides increase, and stretch flangeability deteriorates. Therefore, the Si content is limited to 0.1% or less.
Ｍｎ：0.20〜0.60％
Ｍｎは、Ｓｉと同様に焼入れ性を向上させるとともに固溶強化により素材強度を上昇させる元素である。また、ＳをＭｎＳとして固定し、スラブの熱間割れを防止する重要な元素である。そして、Ｍｎの含有量については、焼入性に大きな影響をおよぼすことが知られている。そこで、本発明のＢ、Ｃｒ、Ｔｉ添加鋼における焼入性におよぼすＭｎ量の影響について調査した。
Mn: 0.20 to 0.60%
Mn is an element that improves the hardenability and increases the strength of the material by solid solution strengthening as in the case of Si. Moreover, it is an important element which fixes S as MnS and prevents the hot crack of a slab. And it is known that the content of Mn has a great influence on the hardenability. Therefore, the influence of the amount of Mn on the hardenability in the B, Cr, Ti-added steel of the present invention was investigated.
Ｃ：0.36％、Ｓｉ：0.02％、Ｍｎ：0.10〜0.90％、Ｐ：0.01％、Ｓ：0.004％、ｓｏｌ.Ａｌ：0.04％、Ｎ：0.0035％、Ｔｉ：0.025％、Ｂ：0.0027％、Ｃｒ：0.23％からなる鋼を溶解後、加熱温度1250℃、熱延仕上温度880℃、巻取温度560℃で熱間圧延を行った。次いで、710℃で40ｈ保持の条件で焼鈍を行い、板厚5.0ｍｍの鋼板を作製した。得られた鋼板を50ｘ100ｍｍの大きさに切断後、加熱炉にて820℃に昇温し、60秒保持後に約60℃の油中へ焼入れた。焼入れ後の試験片における硬さをロックウェルＣスケール（ＨＲＣ）で試料表面を測定面として10点測定し、焼入れ性を評価した。評価は測定した10点の平均硬さ（ＨＲＣ）50以上を良好とした。得られた結果を図1に示す。 C: 0.36%, Si: 0.02%, Mn: 0.10-0.90%, P: 0.01%, S: 0.004%, sol. Al: 0.04%, N: 0.0035%, Ti: 0.025%, B: 0.0027%, Cr : After melting 0.23% steel, hot rolling was performed at a heating temperature of 1250 ° C, a hot rolling finishing temperature of 880 ° C, and a winding temperature of 560 ° C. Subsequently, annealing was performed at 710 ° C. for 40 hours, thereby producing a steel plate having a thickness of 5.0 mm. The obtained steel sheet was cut into a size of 50 × 100 mm, heated to 820 ° C. in a heating furnace, held for 60 seconds, and then quenched into oil at about 60 ° C. The hardness of the test piece after quenching was measured at 10 points using the Rockwell C scale (HRC) with the sample surface as the measurement surface, and the hardenability was evaluated. Evaluation made the average hardness (HRC) 50 or more of 10 points measured good. The obtained results are shown in FIG.
図1は、Ｍｎ量と焼入れ後の平均硬さとの関係を示す図である。図1より、Ｍｎ量が0.20％以上で平均硬さ（ＨＲＣ）50以上が確保され、さらにＭｎ量が0.35％以上で平均硬さ（ＨＲＣ）が55に達し、より高い焼入れ硬さが安定して得られることがわかる。 FIG. 1 is a graph showing the relationship between the amount of Mn and the average hardness after quenching. From Fig. 1, when the Mn content is 0.20% or more, an average hardness (HRC) of 50 or more is secured, and when the Mn content is 0.35% or more, the average hardness (HRC) reaches 55, and a higher quenching hardness is stabilized. It can be seen that
また、素材強度を上昇させ、ＳをＭｎＳとして固定し、スラブの熱間割れを防止する点から、Ｍｎ含有量が0.20％未満では、これらの効果が小さくなるとともに、初析フェライトの生成を助長し、フェライト粒を粗大化させる。 Also, since the strength of the material is increased, S is fixed as MnS, and hot cracking of the slab is prevented, when the Mn content is less than 0.20%, these effects are reduced and the formation of proeutectoid ferrite is promoted. And coarsening the ferrite grains.
一方、0.60％を超えると、引張強度は得られるが、偏析帯であるマンガンバンドの生成が顕著となり、伸びフランジ性および伸びが劣化する。 On the other hand, if it exceeds 0.60%, the tensile strength can be obtained, but the formation of a manganese band which is a segregation band becomes remarkable, and the stretch flangeability and elongation deteriorate.
以上より、Ｍｎ含有量は0.20％以上0.60％以下、好ましくは0.35％以上0.60％以下とする。 Accordingly, the Mn content is 0.20% or more and 0.60% or less, preferably 0.35% or more and 0.60% or less.
Ｐ：0.02％以下
Ｐは、粒界に偏析し、靭性を低下させるため、低減しなければならない元素である。しかし、Ｐの含有量が0.02％までは許容できるため、Ｐ含有量を0.02％以下に制限する。
P: 0.02% or less P is an element that must be reduced in order to segregate at grain boundaries and reduce toughness. However, since the P content is acceptable up to 0.02%, the P content is limited to 0.02% or less.
Ｓ：0.01％以下
Ｓは、ＭｎとＭｎＳを形成し伸びフランジ性を劣化させるため、低減しなければならない元素である。しかし、Ｓの含有量が0.01％までは許容できるため、Ｓ含有量を0.01％以下に制限する。
S: 0.01% or less S is an element that must be reduced in order to form Mn and MnS and degrade stretch flangeability. However, since the S content is acceptable up to 0.01%, the S content is limited to 0.01% or less.
ｓｏｌ.Ａｌ：0.1％以下
Ａｌは、脱酸剤として用い、鋼の清浄度を向上させるため、製鋼段階で添加し、鋼中には通常ｓｏｌ.Ａｌで概ね0.005％以上含有される。一方、ｓｏｌ.Ａｌ含有量が0.1％を超える程Ａｌを添加しても、清浄度を向上させるという効果が飽和しコスト増となる。従って、鋼中のｓｏｌ.Ａｌ含有量を0.1％以下とする。
sol.Al: 0.1% or less Al is used as a deoxidizer, and is added in the steelmaking stage to improve the cleanliness of the steel. Usually, the steel contains 0.005% or more of sol.Al. On the other hand, even if Al is added so that the sol.Al content exceeds 0.1%, the effect of improving the cleanliness is saturated and the cost increases. Therefore, the sol.Al content in the steel is set to 0.1% or less.
Ｎ：0.005％以下
Ｎは、ＴｉＮを形成して熱間圧延後の冷却中の初析フェライト生成に有効な固溶Ｔｉ量を減少させるので、Ｎの含有量が多いとＴｉ添加量を増加させる必要があり、コスト増を招くため極力低減しなければならない元素である。しかし、Ｎの含有量が0.005％までは許容できるため、Ｎ含有量を0.005％以下に制限する。より好ましくは0.0050％以下に制限する。
N: 0.005% or less N forms TiN and decreases the amount of solute Ti effective for proeutectoid ferrite formation during cooling after hot rolling. Therefore, if the content of N is large, the amount of Ti added is increased. It is an element that must be reduced as much as possible in order to increase costs. However, since the N content is acceptable up to 0.005%, the N content is limited to 0.005% or less. More preferably, it is limited to 0.0050% or less.
Ｔｉ：0.005〜0.05％
Ｔｉは、固溶状態において熱間圧延後の冷却中の初析フェライトの生成を抑制するので、伸びフランジ性の向上に有効な元素である。しかし、Ｔｉ含有量が0.005％未満では、Ｎが微量でも十分な効果が得られない。一方、0.05％を超える多量の含有では、焼鈍中に冷却過程でＴｉＣが析出して増加し、強度上昇が大きくなり伸びフランジ性および伸びが著しく劣化する。従って、Ｔｉ含有量を0.005％以上0.05％以下とする。
Ti: 0.005-0.05%
Ti is an element effective in improving stretch flangeability because it suppresses the formation of pro-eutectoid ferrite during cooling after hot rolling in a solid solution state. However, if the Ti content is less than 0.005%, a sufficient effect cannot be obtained even if the amount of N is very small. On the other hand, when the content exceeds 0.05%, TiC precipitates and increases during the cooling process during annealing, and the strength rises greatly, and the stretch flangeability and elongation deteriorate significantly. Therefore, the Ti content is set to 0.005% or more and 0.05% or less.
Ｂ：0.0005〜0.003％
Ｂは、熱間圧延後の冷却中の初析フェライトの生成を抑制し、伸びフランジ性を向上させると同時に、焼入性を高める重要な元素である。しかし、Ｂ含有量が0.0005％未満では、十分な効果が得られない。一方、0.003％を超えると、効果が飽和するとともに、熱間圧延の負荷が高くなり操業性が低下する。従って、Ｂ含有量を0.0005％以上0.003％とする。より好ましくは0.0005％以上0.0030％とする。
B: 0.0005-0.003%
B is an important element that suppresses the formation of pro-eutectoid ferrite during cooling after hot rolling, improves stretch flangeability, and at the same time enhances hardenability. However, if the B content is less than 0.0005%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 0.003%, the effect will be saturated and the hot rolling load will increase and the operability will decrease. Therefore, the B content is set to 0.0005% or more and 0.003%. More preferably, it is 0.0005% or more and 0.0030%.
Ｃｒ：0.05〜0.3％
Ｃｒは、Ｂと同様に熱間圧延後の冷却中の初析フェライトの生成を抑制し、伸びフランジ性を向上させると同時に、焼入性を高める重要な元素である。しかし、Ｃｒ含有量が0.05％未満では、十分な効果が得られない。一方、0.3％を超えて含有しても、焼入性は向上するが、初析フェライト生成の抑制効果が飽和するとともに、コスト増となる。従って、Ｃｒ含有量を0.05％以上0.3％以下とする。より好ましくは0.05％以上0.30％以下とする。
Cr: 0.05-0.3%
Cr, like B, is an important element that suppresses the formation of pro-eutectoid ferrite during cooling after hot rolling, improves stretch flangeability, and at the same time enhances hardenability. However, if the Cr content is less than 0.05%, a sufficient effect cannot be obtained. On the other hand, if the content exceeds 0.3%, the hardenability is improved, but the effect of suppressing the formation of pro-eutectoid ferrite is saturated and the cost is increased. Therefore, the Cr content is 0.05% or more and 0.3% or less. More preferably, it is 0.05% or more and 0.30% or less.
有効Ｔｉ：Ｔｉ−(48/14)Ｎ≧0.005（％）
有効Ｔｉは、熱間圧延冷却前の固溶状態のＴｉ量に相当し、Ｔｉ含有量からＴｉＮとして析出した量を差し引いた値［Ｔｉ−(48/14)Ｎ］で表すことができる。なお、該式中のＴｉはＴｉの含有量（質量％）、ＮはＮの含有量（質量％）である。有効Ｔｉは、前述のように熱間圧延後の冷却中の初析フェライトの生成を抑制し、伸びフランジ性を向上させる。しかし、有効Ｔｉ量が0.005％未満の場合は、この効果が得られない。従って、有効Ｔｉ量は0.005％以上とする。
Effective Ti: Ti- (48/14) N ≧ 0.005 (%)
Effective Ti corresponds to the amount of Ti in a solid solution state before hot rolling cooling, and can be represented by a value obtained by subtracting the amount precipitated as TiN from the Ti content [Ti- (48/14) N]. In the formula, Ti is the Ti content (mass%), and N is the N content (mass%). Effective Ti suppresses the formation of pro-eutectoid ferrite during cooling after hot rolling as described above, and improves stretch flangeability. However, this effect cannot be obtained when the effective Ti amount is less than 0.005%. Therefore, the effective Ti amount is 0.005% or more.
次に、本発明の鋼板の組織について説明する。 Next, the structure of the steel sheet of the present invention will be described.
フェライト平均粒径： 6μm以下
フェライト粒径は、伸びフランジ性と素材強度を支配する重要な因子であり、本発明において重要な要件である。フェライト粒を微細化することにより、伸びフランジ性を劣化させることなく強度を上昇させることが可能となる。すなわち、フェライト平均粒径を6μm以下とすることにより、素材の引張強度を440MPa以上に確保しつつ、優れた伸びフランジ性が得られる。一方、1.0μm未満の微細粒になると強度上昇が著しく、プレス加工時の負荷が増大する可能性があるため、下限は1.0μm以上とすることが好ましい。なお、フェライト粒径は、製造条件、特に熱間圧延の仕上温度、冷却停止温度により、制御することができる。
Average ferrite particle diameter: 6 μm or less The ferrite particle diameter is an important factor governing stretch flangeability and material strength, and is an important requirement in the present invention. By refining the ferrite grains, it is possible to increase the strength without deteriorating stretch flangeability. That is, by setting the average ferrite grain size to 6 μm or less, excellent stretch flangeability can be obtained while securing the tensile strength of the material to 440 MPa or more. On the other hand, if the particle size is less than 1.0 μm, the strength is remarkably increased and the load during pressing may increase, so the lower limit is preferably 1.0 μm or more. The ferrite grain size can be controlled by manufacturing conditions, particularly the hot rolling finishing temperature and the cooling stop temperature.
炭化物平均粒径： 0.1μm以上かつ1.20μm未満
炭化物粒径は、加工性一般、および穴拡げ加工におけるボイドの発生に大きく影響する。炭化物が微細になるとボイドの発生は抑制できるが、炭化物平均粒径が0.1μm未満になると、硬度の上昇に伴い延性が低下し、そのため伸びフランジ性も低下する。炭化物平均粒径の増加に伴い加工性一般は向上するが、1.20μm以上になると、穴拡げ加工におけるボイドの発生により伸びフランジ性が低下する。従って、炭化物平均粒径を0.1μm以上かつ1.20μm未満に制御する。より好ましくは0.1μm以上かつ1.2μm未満に制御する。さらに、炭化物平均粒径を0.5μm以上かつ1.20μm未満に制御することにより、強度上昇が抑えられると同時に、伸びが増大し優れた伸び特性が得られる。よって、好ましくは0.5μm以上かつ1.20μm未満とする。より好ましくは0.5μm以上かつ1.2μm未満とする。なお、炭化物平均粒径は、製造条件、特に熱間圧延後の冷却停止温度、巻取温度、およびその後の熱延板焼鈍処理における焼鈍温度により制御することができる。ここで、炭化物の粒径については、炭化物の長径と短径の平均を個々の炭化物の粒径とし、この個々の炭化物の粒径を平均した値を、炭化物平均粒径とする。
Carbide average particle size: 0.1 μm or more and less than 1.20 μm Carbide particle size greatly affects the workability in general and the generation of voids in hole expansion processing. When the carbide becomes finer, the generation of voids can be suppressed. However, when the carbide average particle size is less than 0.1 μm, the ductility decreases with an increase in hardness, and the stretch flangeability also decreases. Workability generally improves as the average carbide particle size increases, but if it exceeds 1.20 μm, stretch flangeability deteriorates due to the generation of voids in the hole expanding process. Therefore, the carbide average particle size is controlled to be 0.1 μm or more and less than 1.20 μm. More preferably, it is controlled to be 0.1 μm or more and less than 1.2 μm. Furthermore, by controlling the carbide average particle size to 0.5 μm or more and less than 1.20 μm, an increase in strength can be suppressed, and at the same time, elongation can be increased and excellent elongation characteristics can be obtained. Therefore, it is preferably 0.5 μm or more and less than 1.20 μm. More preferably, it is 0.5 μm or more and less than 1.2 μm. The average carbide particle size can be controlled by the production conditions, particularly the cooling stop temperature after hot rolling, the coiling temperature, and the annealing temperature in the subsequent hot-rolled sheet annealing treatment. Here, regarding the particle size of the carbide, the average of the major axis and the minor axis of the carbide is defined as the particle size of the individual carbide, and the average value of the particle size of the individual carbide is defined as the average particle size of the carbide.
炭化物の分散状態： 炭化物を実質的に含まないフェライト粒の体積率が5％以下
炭化物の分散状態を均一とすることにより、前述のように、穴拡げ加工の際の打抜き端面における応力集中が緩和され、ボイドの発生が抑制できる。炭化物を実質的に含まないフェライト粒を、体積率にして5％以下にすることにより、炭化物の分散状態を極めて均一にすることができ、伸びフランジ性が著しく向上する。従って、炭化物を実質的に含まないフェライト粒の体積率を5％以下とする。一方、本成分系が亜共析鋼であり、初析フェライトを完全に抑制することは困難であることを考慮すると、炭化物を実質的に含まないフェライト粒の体積率の下限は1％程度とするのが好ましい。なお、炭化物の分散状態、即ち炭化物を実質的に含まないフェライト粒の体積率は、製造条件、特に熱間圧延の仕上温度、熱間圧延後の冷却速度、冷却停止温度および巻取温度により、制御することができる。
Carbide dispersion: The volume fraction of ferrite grains that are substantially free of carbide is 5% or less. By making the carbide dispersion uniform, stress concentration at the punched end face is alleviated as described above. And generation of voids can be suppressed. By making ferrite particles substantially free of carbides 5% or less in volume ratio, the dispersion state of carbides can be made extremely uniform, and stretch flangeability is remarkably improved. Therefore, the volume fraction of ferrite grains substantially free of carbides is set to 5% or less. On the other hand, considering that this component system is hypoeutectoid steel and it is difficult to completely suppress pro-eutectoid ferrite, the lower limit of the volume fraction of ferrite grains substantially free of carbide is about 1%. It is preferable to do this. The carbide dispersion state, that is, the volume fraction of ferrite grains substantially free of carbide, depends on the manufacturing conditions, particularly the finishing temperature of hot rolling, the cooling rate after hot rolling, the cooling stop temperature and the winding temperature. Can be controlled.
ここで、炭化物を実質的に含まないフェライト粒とは、通常の光学顕微鏡による金属組織観察では炭化物が検出されないフェライト粒を意味し、走査型電子顕微鏡でも低倍率では炭化物が検出されないフェライト粒を意味する。すなわち、本発明における炭化物を実質的に含まないフェライト粒とは、鋼板試料の板厚断面を研磨し、ナイタルで腐食後、走査型電子顕微鏡で1000倍で観察しても炭化物が検出されないフェライト粒とする。このようなフェライト粒は、熱延後に初析フェライトとして生成した部分であり、焼鈍後の状態でも粒内に炭化物が観察されない、即ち炭化物を実質的に含まないフェライト粒と言える。 Here, the ferrite grain substantially free of carbide means a ferrite grain in which carbide is not detected by observation of a metal structure with a normal optical microscope, and means a ferrite grain in which carbide is not detected at a low magnification even with a scanning electron microscope. To do. That is, the ferrite grains substantially free of carbides in the present invention is a ferrite grain in which the thickness of a steel sheet sample is polished, corroded with a night, and no carbide is detected even when observed with a scanning electron microscope at 1000 times And Such a ferrite grain is a portion generated as pro-eutectoid ferrite after hot rolling, and it can be said that no carbide is observed in the grain even after annealing, that is, a ferrite grain substantially free of carbide.
次に、本発明の高炭素熱延鋼板を製造するに好適な製造条件の限定理由について述べる。 Next, the reason for limiting the production conditions suitable for producing the high carbon hot rolled steel sheet of the present invention will be described.
熱間圧延の仕上温度： (Ar3変態点-10℃)以上
鋼を熱間圧延する際の仕上温度が(Ar3変態点-10℃)未満では、一部でフェライト変態が進行するため炭化物を実質的に含まないフェライト粒が増加し、伸びフランジ性が劣化する。また、フェライト粒の粗大化が顕著となりフェライト平均粒径が6μmを超えるため、伸びフランジ性とともに強度が低下する。よって、熱間圧延の仕上圧延の仕上温度は(Ar3変態点-10℃)以上とする。これにより、組織の均一微細化を図ることができ、伸びフランジ性と強度の向上が図れる。一方、仕上温度の上限は特に規定しないが、1000℃を超えるような高温の場合、スケール性欠陥が発生し易くなるため、1000℃以下が好ましい。なお、Ar3変態点（℃）は次の式で算出することができる。
Hot rolling finishing temperature: (Ar 3 transformation point -10 ° C) or higher If the finishing temperature during hot rolling of steel is less than (Ar 3 transformation point -10 ° C), ferrite transformation proceeds in part, so carbide As a result, ferrite grains that do not contain substantially increase and stretch flangeability deteriorates. Further, the coarsening of the ferrite grains becomes remarkable and the average ferrite grain diameter exceeds 6 μm, so that the strength decreases with stretch flangeability. Therefore, the finishing temperature of the hot rolling finish rolling is set to (Ar 3 transformation point −10 ° C.) or higher. Thereby, uniform refinement | miniaturization of a structure | tissue can be achieved and the stretch flangeability and intensity | strength can be aimed at. On the other hand, the upper limit of the finishing temperature is not particularly defined. However, when the temperature is higher than 1000 ° C., a scale defect is likely to occur. The Ar 3 transformation point (° C.) can be calculated by the following formula.
Ar3=930.21-394.75C+54.99Si-14.40Mn+5.77Cr (1)
ここで、式中の元素記号はそれぞれの元素の含有量（質量％）を表す。
Ar 3 = 930.21-394.75C + 54.99Si-14.40Mn + 5.77Cr (1)
Here, the element symbol in a formula represents content (mass%) of each element.
熱間圧延後の冷却条件： 冷却速度＞120℃/秒
本発明では、変態後のフェライト粒の体積率の低減を図るため、熱間圧延後に急冷（冷却）を行う。熱間圧延後の冷却方法が徐冷であると、オーステナイトの過冷度が小さく初析フェライトが多く生成する。冷却速度が120℃/秒以下の場合、初析フェライトの生成が顕著となり、炭化物を実質的に含まないフェライト粒が5％超となり、伸びフランジ性が劣化する。従って、圧延後の冷却の冷却速度を120℃/秒超とする。一方、冷却速度の上限は、現在の設備上の能力からは700℃/秒程度である。
Cooling conditions after hot rolling: Cooling rate> 120 ° C./second In the present invention, rapid cooling (cooling) is performed after hot rolling in order to reduce the volume fraction of ferrite grains after transformation. If the cooling method after hot rolling is slow cooling, the degree of supercooling of austenite is small and a large amount of proeutectoid ferrite is generated. When the cooling rate is 120 ° C./second or less, pro-eutectoid ferrite is prominently produced, and ferrite grains substantially not containing carbide exceed 5%, and stretch flangeability deteriorates. Therefore, the cooling rate of cooling after rolling is set to more than 120 ° C./second. On the other hand, the upper limit of the cooling rate is about 700 ° C./second based on the current facility capacity.
ここで、冷却速度とは仕上圧延後の冷却開始から冷却停止までの平均冷却速度である。また、仕上圧延後、通常は3秒以内程度で冷却を開始するが、変態後のフェライト結晶粒やパーライト等をより微細化し、加工性をより一層向上させる点から、仕上圧延後、0.1秒を超え1.0秒未満の時間内で冷却を開始することが好ましい。 Here, the cooling rate is an average cooling rate from the start of cooling after finish rolling to the stop of cooling. In addition, after finishing rolling, cooling usually starts within about 3 seconds, but in order to further refine the ferrite crystal grains and pearlite after transformation and further improve workability, 0.1 seconds after finishing rolling. It is preferable to start the cooling within a time exceeding 1.0 seconds.
冷却停止温度： 620℃以下
熱間圧延後の冷却の冷却停止温度が高い場合、巻取りまでの冷却中にフェライトが生成するとともに、パーライトのコロニーおよびラメラ間隔が増大する。そのため、焼鈍後にフェライト粒が粗大化すると同時に微細炭化物が得られなくなり、強度が低下し、伸びフランジ性が劣化する。冷却停止温度が620℃より高い場合、炭化物を実質的に含まないフェライト粒が5％超となり、伸びフランジ性が劣化する。従って、圧延後の冷却の冷却停止温度を620℃以下とする。炭化物を実質的に含まないフェライト粒をさらに低減させる場合は、冷却停止温度を600℃以下とすることが好ましい。一方、冷却停止温度の下限は特に規定しないが、低温になるほど鋼板の形状が劣化するため、200℃以上とすることが好ましい。
Cooling stop temperature: 620 ° C. or less When the cooling stop temperature for cooling after hot rolling is high, ferrite is generated during cooling until winding, and pearlite colonies and lamella spacing increase. Therefore, the ferrite grains become coarse after annealing, and at the same time fine carbides cannot be obtained, the strength is lowered, and the stretch flangeability is deteriorated. When the cooling stop temperature is higher than 620 ° C., ferrite grains substantially not containing carbide exceed 5%, and stretch flangeability deteriorates. Therefore, the cooling stop temperature for cooling after rolling is set to 620 ° C. or lower. In order to further reduce the ferrite grains substantially not containing carbide, it is preferable to set the cooling stop temperature to 600 ° C. or lower. On the other hand, the lower limit of the cooling stop temperature is not particularly defined, but the shape of the steel sheet deteriorates as the temperature becomes lower, and therefore it is preferably set to 200 ° C. or higher.
巻取温度： 600℃以下
冷却停止後は鋼板を巻き取るが、巻取温度が高いほどパーライトのラメラ間隔が大きくなる。そのため、焼鈍後の炭化物が粗大化し、巻取温度が600℃を超えると伸びフランジ性が劣化する。従って、巻取温度を600℃以下とする。さらに、巻取温度を500℃以下とすることにより、炭化物の分散状態が一層均一化し、極めて優れた伸びフランジ性が得られるため、500℃以下とすることが好ましい。一方、巻取温度の下限は特に規定しないが、低温になるほど鋼板の形状が劣化するため、200℃以上とすることが好ましい。
Winding temperature: 600 ° C or less The steel sheet is wound after cooling is stopped. The higher the winding temperature, the larger the pearlite lamella spacing. Therefore, the carbide after annealing becomes coarse, and when the coiling temperature exceeds 600 ° C., the stretch flangeability deteriorates. Accordingly, the coiling temperature is set to 600 ° C. or lower. Furthermore, by setting the coiling temperature to 500 ° C. or lower, the dispersion state of carbides becomes even more uniform and extremely excellent stretch flangeability can be obtained. On the other hand, the lower limit of the coiling temperature is not particularly defined, but the shape of the steel sheet deteriorates as the temperature is lowered, and therefore it is preferably set to 200 ° C. or higher.
なお、炭化物の分散をさらに均一化し、優れた伸びフランジ性を得るためには、冷却停止温度を600℃以下として冷却するとともに、巻取温度500℃以下で巻取ることが特に好ましい。 In order to make the dispersion of carbides more uniform and to obtain excellent stretch flangeability, it is particularly preferable to cool at a cooling stop temperature of 600 ° C. or lower and to wind at a winding temperature of 500 ° C. or lower.
焼鈍温度： 640℃以上Ac1変態点以下
上記のようにして得た熱延鋼板について、炭化物を球状化するために焼鈍を行う。焼鈍温度が640℃未満の場合、炭化物の球状化が不十分あるいは炭化物平均粒径が0.1μm未満となり、伸びフランジ性が劣化する。一方、焼鈍温度がAc1変態点を超える場合、一部がオーステナイト化し、冷却中に再度パーライトを生成するため、やはり、伸びフランジ性が劣化する。また、伸びも低下する。以上より、焼鈍温度は640℃以上Ac1変態点以下とする。さらに、焼鈍温度を680℃以上とすることにより、炭化物平均粒径が0.5μm以上となり、高い伸び特性が得られる。よって、好ましくは680℃以上Ac1変態点以下である。なお、Ac1変態点（℃）は次の式で算出することができる。
Annealing temperature: 640 ° C. or higher and Ac 1 transformation point or lower The hot-rolled steel sheet obtained as described above is annealed to spheroidize carbides. When the annealing temperature is less than 640 ° C., the spheroidization of the carbide is insufficient or the average particle size of the carbide is less than 0.1 μm, and the stretch flangeability deteriorates. On the other hand, when the annealing temperature exceeds the Ac 1 transformation point, a part is austenitized and pearlite is generated again during cooling, so that the stretch flangeability is deteriorated. Also, the elongation decreases. From the above, the annealing temperature is set to 640 ° C. or more and Ac 1 transformation point or less. Furthermore, by setting the annealing temperature to 680 ° C. or higher, the average carbide particle size becomes 0.5 μm or higher, and high elongation characteristics can be obtained. Therefore, it is preferably 680 ° C. or higher and Ac 1 transformation point or lower. The Ac 1 transformation point (° C.) can be calculated by the following formula.
Ac1=754.83-32.25C+23.32Si-17.76Mn+17.13Cr (2)
ここで、式中の元素記号はそれぞれの元素の含有量（質量％）を表す。
Ac 1 = 754.83-32.25C + 23.32Si-17.76Mn + 17.13Cr (2)
Here, the element symbol in a formula represents content (mass%) of each element.
この発明は、伸びフランジ性の向上を図るに当たって、成分組成および製造条件の制御のみならず、フェライト粒径、炭化物粒径、および炭化物の分散状態をも制御することで、打抜き時の端面におけるボイドの発生を抑制し、穴拡げ加工におけるクラックの成長を遅くすることができる。その結果、440MPa以上の引張強度を有するとともに極めて伸びフランジ性に優れ、あるいはさらに極めて延性にも優れた高炭素熱延鋼板が提供可能となる。 In the present invention, in order to improve stretch flangeability, not only the control of the component composition and manufacturing conditions but also the control of the ferrite particle size, carbide particle size, and carbide dispersion state, voids at the end face during punching are achieved. The generation of cracks can be suppressed and the growth of cracks in the hole expanding process can be slowed. As a result, it is possible to provide a high carbon hot-rolled steel sheet having a tensile strength of 440 MPa or more and extremely excellent stretch flangeability, or even extremely excellent ductility.
本発明の高炭素鋼の成分調製には、転炉あるいは電気炉のどちらでも使用可能である。このように成分調製された高炭素鋼を、造塊−分塊圧延または連続鋳造により鋼スラブとする。この鋼スラブについて熱間圧延を行うが、その際、スラブ加熱温度は、スケール発生による表面状態の劣化を避けるため1300℃以下とすることが好ましい。 Either a converter or an electric furnace can be used for preparing the components of the high carbon steel of the present invention. The high carbon steel whose components are prepared in this way is made into a steel slab by ingot-bundling rolling or continuous casting. The steel slab is hot-rolled. At this time, the slab heating temperature is preferably 1300 ° C. or lower in order to avoid deterioration of the surface state due to generation of scale.
なお、熱間圧延時に粗圧延を省略して仕上圧延を行ってもよく、連続鋳造スラブをそのまま又は温度低下を抑制する目的で保熱しつつ圧延する直送圧延を行ってもよい。また、仕上温度確保のため、熱間圧延中にバーヒータ等の加熱手段により圧延材の加熱を行ってもよい。なお、球状化促進あるいは硬度低減のため、巻取後にコイルを徐冷カバー等の手段で保温してもよい。 In addition, rough rolling may be omitted during hot rolling and finish rolling may be performed, or direct feed rolling may be performed in which a continuously cast slab is rolled as it is or for the purpose of suppressing temperature reduction. In order to secure the finishing temperature, the rolled material may be heated by a heating means such as a bar heater during hot rolling. In order to promote spheroidization or reduce hardness, the coil may be kept warm by means such as a slow cooling cover after winding.
巻取を行い熱延鋼板とした後、好ましくは常法に従い酸洗した後に焼鈍を行う。焼鈍については、箱焼鈍、連続焼鈍のいずれでもよい。その後、必要に応じて調質圧延を行う。この調質圧延については焼入れ性には影響を及ぼさないことから、その条件に対して特に制限はない。 After winding and making a hot-rolled steel sheet, it is preferably annealed after pickling according to a conventional method. As for annealing, either box annealing or continuous annealing may be used. Thereafter, temper rolling is performed as necessary. Since this temper rolling does not affect the hardenability, there is no particular limitation on the conditions.
以上より、伸びフランジ性に優れ、あるいはさらに延性にも優れた高炭素熱延鋼板が得られる。なお、上記は本発明の一実施態様を示すものであり、これに限定されるものではない。 As described above, a high carbon hot rolled steel sheet having excellent stretch flangeability or further excellent ductility can be obtained. The above shows one embodiment of the present invention, and the present invention is not limited to this.
このようにして得られた高炭素熱延鋼板が、優れた伸びフランジ性を有する理由は次のように考えられる。伸びフランジ性には、打抜き端面の部分の内部組織が大きく影響する。特に、炭化物を実質的に含まないフェライト粒（熱延後の初析フェライト）が多い場合、球状化組織の部分との粒界からクラックが発生することが、確認されている。 The reason why the high carbon hot-rolled steel sheet thus obtained has excellent stretch flangeability is considered as follows. Stretch flangeability is greatly affected by the internal structure of the punched end face. In particular, it has been confirmed that cracks are generated from the grain boundary with the portion of the spheroidized structure when there are many ferrite grains (pre-deposited ferrite after hot rolling) containing substantially no carbide.
ミクロ組織の挙動を見ると、打抜き加工時には炭化物の界面に、応力集中によるボイドの発生が顕著となる。この応力集中は、炭化物の寸法が大きいほど、また、炭化物を実質的に含まないフェライト粒が多いほど大きくなる。穴拡げ加工の際は、これらのボイドが連結しクラックとなる。 Looking at the behavior of the microstructure, voids due to stress concentration become prominent at the carbide interface during punching. This stress concentration increases as the size of the carbide increases and as the number of ferrite grains substantially free of carbide increases. During the hole expanding process, these voids are connected to form a crack.
このように、製造条件の制御のみならず、炭化物平均粒径、および炭化物を実質的に含まないフェライト粒の占める割合を制御することにより、応力集中を小さくし、ボイドの発生を低減することができる。 Thus, by controlling not only the production conditions but also the average particle size of carbides and the proportion of ferrite grains substantially free of carbides, stress concentration can be reduced and void generation can be reduced. it can.
表１に示す化学成分を有する鋼の連続鋳造スラブを、加熱温度1250℃、熱延仕上温度880℃、仕上げ圧延後冷却開始までの時間0.7秒、熱延後冷却速度150℃／秒、冷却停止温度610℃、巻取温度560℃で熱間圧延を行い熱延鋼板とした。その後、酸洗し、680℃で40hの焼鈍を行い、板厚5.0mmの鋼板を製造した。ここで、鋼No.A〜Eは化学成分（組成）が本発明範囲内の本発明例であり、鋼No.F〜0は組成が本発明範囲を外れた比較例である。 Continuous casting slab of steel with chemical components shown in Table 1 is heated at 1250 ° C, hot-rolled finish temperature of 880 ° C, time after finish rolling to start cooling 0.7 seconds, cooling rate after hot-rolling 150 ° C / second, cooling stopped Hot rolling was performed at a temperature of 610 ° C. and a winding temperature of 560 ° C. to obtain a hot-rolled steel sheet. Thereafter, pickling and annealing at 680 ° C. for 40 hours were performed to produce a steel plate having a thickness of 5.0 mm. Here, Steel Nos. A to E are examples of the present invention whose chemical components (compositions) are within the scope of the present invention, and Steel Nos. F to 0 are comparative examples whose compositions are outside the scope of the present invention.
これらの鋼板からサンプルを採取し、フェライト平均粒径、炭化物平均粒径ならびに炭化物の分散状態の測定、伸びフランジ性評価、および引張試験を行った。それぞれの試験・測定の方法および条件について以下に示す。 Samples were taken from these steel plates, and the ferrite average particle size, carbide average particle size and carbide dispersion state measurement, stretch flangeability evaluation, and tensile test were performed. Each test and measurement method and conditions are shown below.
(i) フェライト平均粒径および炭化物平均粒径およびその分散状態
サンプルの板厚断面を研磨・ナイタル腐食後、走査型電子顕微鏡にてミクロ組織を撮影し、標記の特性値を測定した。
(i) Average ferrite particle diameter, carbide average particle diameter and dispersion state After the plate thickness cross section of the sample was polished and subjected to night corrosion, the microstructure was photographed with a scanning electron microscope, and the characteristic values shown were measured.
まず、フェライト平均粒径については、上記走査型電子顕微鏡で1000倍で撮影した組織写真について、JIS規格G0552に規定されているフェライト結晶粒度試験方法の中の切断法に準拠して測定した。 First, the average ferrite grain size was measured according to the cutting method in the ferrite grain size test method defined in JIS standard G0552, with respect to the structure photograph taken at 1000 times with the scanning electron microscope.
炭化物平均粒径については、同様に3000倍で撮影した組織写真を用い、実面積0.01mm2の範囲で、板厚方向に100mmの線分20本を引き、これらの線分と交差した炭化物についてその長径と短径を測定し、両者の平均値をその炭化物の粒径とし、さらに測定した全炭化物の粒径の平均を求め炭化物平均粒径とした。 The average carbide grain size, using tissue photographs were similarly taken at 3000 times, the range of actual area 0.01 mm 2, pull the twenty segments of 100mm in the thickness direction, the carbides cross the line segments The major axis and minor axis were measured, the average value of both was taken as the grain size of the carbide, and the average of the grain sizes of all the measured carbides was determined as the average grain size of the carbide.
また、炭化物の分散状態については、上記1000倍で撮影した組織写真について、炭化物が観察されないフェライト粒の面積率を測定し、これをもって炭化物を実質的に含まないフェライト粒の体積率とし、炭化物の分散状態の指標とした。 As for the dispersion state of carbide, for the structure photograph taken at a magnification of 1000 times, the area ratio of ferrite grains in which carbide is not observed is measured, and this is used as the volume ratio of ferrite grains substantially free of carbide. It was used as an indicator of the dispersion state.
(ii) 伸びフランジ性評価
サンプルを、ポンチ径d0=10mm、ダイス径12mm（クリアランス20％）の打抜き工具を用いて打抜き後、穴拡げ試験を実施した。穴拡げ試験は、円筒平底ポンチ（50mmφ、5R(肩半径5mm)）にて押し上げる方法で行い、穴縁に板厚貫通クラックが発生した時点での穴径db（mm）を測定して、次式で定義される穴拡げ率λ(％)を求めた。
(ii) Stretch flangeability evaluation After punching the sample with a punching tool having a punch diameter d 0 = 10 mm and a die diameter 12 mm (
λ＝100×(db-d0)/d0 (3)．
(iii) 引張試験
圧延方向に対し、90゜方向（C方向）に沿ってJIS５号試験片を採取し、引張速度10mm/minで引張試験を行い、引張強度および伸びを測定した。
λ = 100 × (db-d 0 ) / d 0 (3).
(iii) Tensile test A JIS No. 5 test piece was taken along the 90 ° direction (C direction) with respect to the rolling direction, a tensile test was performed at a tensile speed of 10 mm / min, and tensile strength and elongation were measured.
以上の試験結果より得られた、フェライト平均粒径、炭化物平均粒径、炭化物の分散状態、伸びフランジ性、伸びおよび引張強度を表２に示す。ここで、伸びフランジ性は上記式(3)の穴拡げ率λで評価した。なお、本発明では、引張強度TSについては440MPa以上、穴拡げ率λについては70％以上（板厚5.0mm）をそれぞれ目標とする。また、優れた延性を要求される場合の伸びとして、35％以上を目標とする。 Table 2 shows the ferrite average particle size, carbide average particle size, carbide dispersion state, stretch flangeability, elongation, and tensile strength obtained from the above test results. Here, the stretch flangeability was evaluated by the hole expansion ratio λ of the above formula (3). In the present invention, the tensile strength TS is set to 440 MPa or more, and the hole expansion rate λ is set to 70% or more (plate thickness 5.0 mm). In addition, when the excellent ductility is required, the target is 35% or more.
表２では、鋼No.A〜Eは、化学成分（組成）が本発明範囲内であり、フェライト平均粒径が6μm以下、炭化物平均粒径が0.1μm以上かつ1.20μm未満、炭化物を実質的に含まないフェライト粒の体積率が5％以下の発明例である。これらは、引張強度（TS）が440MPa以上、穴拡げ率λが70％以上という本発明の目標を達成しており、λについては85％以上であり、特に優れた伸びフランジ性を有している。また、炭化物平均粒径が0.5μm以上であり、伸びも35％以上達成している。 In Table 2, steel Nos. A to E have chemical components (compositions) within the scope of the present invention, an average ferrite particle size of 6 μm or less, an average carbide particle size of 0.1 μm or more and less than 1.20 μm, and substantially free of carbides. This is an invention example in which the volume fraction of ferrite grains not contained in the steel is 5% or less. These have achieved the objectives of the present invention with a tensile strength (TS) of 440 MPa or more and a hole expansion ratio λ of 70% or more, and λ is 85% or more, and has particularly excellent stretch flangeability. Yes. Further, the carbide average particle size is 0.5 μm or more, and the elongation is 35% or more.
これに対して、表２の鋼No.F〜Oは、化学成分（組成）が本発明範囲を外れた比較例である。鋼No.FはＣが低く、フェライト平均粒径、炭化物平均粒径、炭化物を実質的に含まないフェライト粒の体積率が本発明範囲を超えており、引張強度が440MPa未満で、穴拡げ率も目標より低い。鋼No.GはＣが高く、組織は本発明範囲となったものの、穴拡げ率が目標より低い。また、伸びも低い。鋼No.HはＳｉとＰが高く、鋼No.L,M,NはＢ，Ｃｒ，有効Ｔｉがそれぞれ低いため、いずれも初析フェライトが多量に生成し、炭化物を実質的に含まないフェライト粒の体積率が本発明範囲の上限5％を超えており、穴拡げ率が目標より低い。 On the other hand, Steel Nos. F to O in Table 2 are comparative examples in which chemical components (compositions) are out of the scope of the present invention. Steel No. F has low C, ferrite average particle size, carbide average particle size, volume fraction of ferrite grains substantially free of carbides is beyond the scope of the present invention, tensile strength is less than 440 MPa, hole expansion rate Is lower than the target. Steel No. G has a high C and the structure is within the scope of the present invention, but the hole expansion rate is lower than the target. Also, the elongation is low. Steel No. H is high in Si and P, and steel No. L, M, and N are low in B, Cr, and effective Ti, respectively. The volume ratio of the grains exceeds the upper limit of 5% of the present invention range, and the hole expansion ratio is lower than the target.
比較例の鋼No.IはＭｎが低いため、初析フェライトが多量に生成し、炭化物を実質的に含まないフェライト粒の体積率が本発明範囲より高く、さらにフェライト平均粒径が6μmを超えており、強度および穴拡げ率が目標より低い。鋼No.JはＭｎが高く、バンド組織が発生し、炭化物を実質的に含まないフェライト粒の体積率も本発明範囲より少し高いため、穴拡げ率が目標より低い。また、伸びも低い。鋼No.KはＳが高く、ＭｎＳが増大して、炭化物を実質的に含まないフェライト粒の体積率も本発明範囲より少し高いため、穴拡げ率が大幅に低下している。鋼No.Oは、Ｔｉが高く、引張強度（TS）が著しく高くなり、穴拡げ率が低下している。また、伸びも低い。 Steel No. I in the comparative example has a low Mn, so that a large amount of pro-eutectoid ferrite is generated, the volume fraction of ferrite grains substantially free of carbides is higher than the range of the present invention, and the average ferrite grain size exceeds 6 μm. Strength and hole expansion rate are lower than target. Steel No. J has a high Mn, a band structure is generated, and the volume ratio of ferrite grains substantially free of carbides is slightly higher than the range of the present invention, so that the hole expansion ratio is lower than the target. Also, the elongation is low. Steel No. K has a high S, an increase in MnS, and the volume fraction of ferrite grains substantially free of carbides is slightly higher than the range of the present invention, so that the hole expansion ratio is greatly reduced. Steel No. O has a high Ti, a markedly high tensile strength (TS), and a hole expansion rate is reduced. Also, the elongation is low.
前掲の表１に示した鋼の内、本発明例の鋼No.A,Cの連続鋳造スラブを1250℃に加熱した後、表３に示す条件にて熱間圧延を行い熱延鋼板とし、次いで酸洗、焼鈍を行い、板厚5.0mmの鋼板を製造した。ここで、鋼板No.1〜8は、製造条件が本発明範囲内の本発明例であり、鋼板No.9〜16は製造条件が本発明範囲を外れた比較例である。 Among the steels shown in Table 1 above, the steel Nos. A and C continuous cast slabs of the inventive examples were heated to 1250 ° C. and then hot rolled under the conditions shown in Table 3 to obtain hot rolled steel sheets. Next, pickling and annealing were performed to produce a steel plate having a thickness of 5.0 mm. Here, steel plates Nos. 1 to 8 are examples of the present invention in which the manufacturing conditions are within the scope of the present invention, and steel plates Nos. 9 to 16 are comparative examples in which the manufacturing conditions are outside the scope of the present invention.
これらの鋼板からサンプルを採取し、実施例１と同様に、フェライト平均粒径、炭化物平均粒径ならびに炭化物の分散状態の測定、伸びフランジ性測定、および引張試験を行った。結果を表４に示す。 Samples were collected from these steel plates, and in the same manner as in Example 1, measurements of ferrite average particle size, carbide average particle size and carbide dispersion state, stretch flangeability measurement, and tensile test were performed. The results are shown in Table 4.
表４では、製造条件が本発明範囲内の鋼板No.1〜8は、フェライト平均粒径が6μm以下、炭化物平均粒径が0.1μm以上かつ1.20μm未満、炭化物を実質的に含まないフェライト粒の体積率が5％以下となっており、本発明例の鋼板である。 In Table 4, steel plates Nos. 1 to 8 whose manufacturing conditions are within the scope of the present invention are ferrite grains having an average ferrite grain size of 6 μm or less, an average carbide grain size of 0.1 μm or more and less than 1.20 μm, and substantially free of carbides. Is a steel sheet of the present invention example.
これらの本発明例の鋼板は、引張強度（TS）が440MPa以上、穴拡げ率λが70％以上という本発明の目標を達成しており、λについては85％以上であり、特に優れた伸びフランジ性を有している。その中でも、鋼板Ｎｏ1,3,5,7は焼鈍温度が680℃以上で本発明の製造条件の好ましい範囲であり、炭化物平均粒径が0.5μｍ以上で高い伸び（35％以上）が得られている。特に鋼板No.3,7は、冷却停止温度が600℃以下、巻取温度が500℃以下、焼鈍温度が680℃以上で、本発明の製造条件の特に好ましい範囲内であり、炭化物を実質的に含まないフェライト粒の体積率が2％以下と低く抑制され、炭化物平均粒径が0.5μm以上で、極めて高い穴拡げ率（90％以上）と同時に高い伸び（35％以上）が得られている。 These steel sheets of the present invention achieved the objectives of the present invention with a tensile strength (TS) of 440 MPa or more and a hole expansion ratio λ of 70% or more, and λ is 85% or more, and particularly excellent elongation. Has flangeability. Among them, steel plates No. 1, 3, 5, and 7 are in a preferable range of the production conditions of the present invention at an annealing temperature of 680 ° C. or higher, and a high elongation (35% or higher) is obtained with a carbide average particle size of 0.5 μm or more. Yes. In particular, the steel plates No. 3 and 7 have a cooling stop temperature of 600 ° C. or lower, a coiling temperature of 500 ° C. or lower, and an annealing temperature of 680 ° C. or higher, which are within the particularly preferable range of the manufacturing conditions of the present invention, and substantially reduce carbide The volume fraction of ferrite grains not contained in steel is suppressed to a low level of 2% or less, the average carbide grain size is 0.5μm or more, and a high elongation (35% or more) is obtained at the same time as an extremely high hole expansion ratio (90% or more). Yes.
これに対して、表４の鋼板No.9〜16は製造条件（表３）が本発明範囲を外れた比較例である。鋼板No.9,13は、圧延終了温度が本発明範囲より低く、フェライト平均粒径、炭化物を実質的に含まないフェライト粒の体積率が本発明範囲の上限を超えており、引張強度および穴拡げ率が目標より低い。鋼板No.10,14は、熱間圧延後の冷却速度が本発明範囲より低く、炭化物を実質的に含まないフェライト粒の体積率が本発明範囲の上限を超えており、穴拡げ率が目標より低い。 On the other hand, steel plates Nos. 9 to 16 in Table 4 are comparative examples in which the production conditions (Table 3) are out of the scope of the present invention. Steel plates No. 9 and 13 have a rolling end temperature lower than the range of the present invention, the ferrite average particle size, the volume fraction of ferrite grains substantially free of carbides exceeds the upper limit of the range of the present invention, the tensile strength and the hole Expansion rate is lower than target. Steel plates Nos. 10 and 14 have a cooling rate after hot rolling lower than the range of the present invention, the volume fraction of ferrite grains substantially not containing carbide exceeds the upper limit of the range of the present invention, and the hole expansion rate is the target. Lower.
比較例の鋼板No.11,15は、冷却停止温度と巻取温度が本発明範囲より高く、フェライト平均粒径、炭化物平均粒径、炭化物を実質的に含まないフェライト粒の体積率も本発明範囲の上限を超えており、引張強度および穴拡げ率が目標より低い。鋼板No.12は、焼鈍温度が本発明範囲より高く、炭化物平均粒径、炭化物を実質的に含まないフェライト粒の体積率も本発明範囲の上限を超えており、穴拡げ率が目標より低い。また、伸びも低い。鋼板No.16は、焼鈍温度が本発明範囲より低く、炭化物の球状化が不十分で正確な粒径測定が不可能であるが、炭化物平均粒径は明らかに1.2μmを超えており、穴拡げ率が大幅に低下している。また、伸びも低い。 Steel plates No. 11 and 15 of the comparative examples have a cooling stop temperature and a coiling temperature higher than the scope of the present invention, and the ferrite average particle diameter, carbide average particle diameter, and the volume fraction of ferrite grains substantially free of carbides are also the present invention. The upper limit of the range is exceeded, and the tensile strength and hole expansion rate are lower than the target. Steel plate No. 12 has an annealing temperature higher than the range of the present invention, the average particle size of carbide, the volume fraction of ferrite grains substantially free of carbides also exceeds the upper limit of the range of the present invention, and the hole expansion rate is lower than the target . Also, the elongation is low. Steel plate No. 16 has an annealing temperature lower than the scope of the present invention, and carbide spheroidization is insufficient and accurate particle size measurement is impossible, but the average particle size of carbide clearly exceeds 1.2 μm, The expansion rate has dropped significantly. Also, the elongation is low.
本発明の高炭素熱延鋼板を用いることにより、ギアに代表される変速機部品等の加工において加工度を高くとることができ、その結果、製造工程を省略して低コストで部品等を製造することが可能となる。 By using the high carbon hot-rolled steel sheet of the present invention, it is possible to increase the degree of processing in processing of transmission parts and the like typified by gears. As a result, the manufacturing process is omitted and parts and the like are manufactured at low cost. It becomes possible to do.
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