JP2006265604A - Method for producing soft-quality high workability high carbon hot-rolled steel sheet excellent in stretch-flanging - Google Patents
Method for producing soft-quality high workability high carbon hot-rolled steel sheet excellent in stretch-flanging Download PDFInfo
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Abstract
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本発明は、Cを0.2〜0.7質量%含有する伸びフランジ性に優れた軟質高加工性高炭素熱延鋼板の製造方法に関する。 The present invention relates to a method for producing a soft, highly workable, high carbon hot-rolled steel sheet excellent in stretch flangeability containing 0.2 to 0.7 mass% of C.
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1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。 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 requirements of users who perform these parts processing is to improve the hole expansion (burring) property in the molding after punching. This hole expansion workability is evaluated as stretch flangeability as press formability. Therefore, a material excellent in stretch flangeability is desired.
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、Cを0.2〜0.7質量%含有する鋼に対し、熱間圧延後、120℃/秒超の冷却速度で冷却停止温度650℃以下まで冷却し、次いで巻取温度600℃以下として炭化物制御を行い、酸洗後、焼鈍を行うという技術が開示されている。 Several techniques have been studied for improving the stretch flangeability of such a high-carbon steel sheet. For example, in Patent Document 1, for steel containing 0.2 to 0.7% by mass of C, after hot rolling, the steel is cooled to a cooling stop temperature of 650 ° C. or lower at a cooling rate exceeding 120 ° C./second, A technique is disclosed in which carbide control is performed at a coiling temperature of 600 ° C. or less, and annealing is performed after pickling.
このような特許文献1の技術では、炭化物制御により、球状化焼鈍を強化し、打ち抜き加工時の球状化組織とフェライトの界面におけるボイド発生を抑制することにより、クラック発生を抑え、高い伸びフランジ性を確保している。
最近では、従来にもまして、生産性向上の観点から伸びフランジ性とともに軟質化に対する要求も厳しくなっているが、上記特許文献1の技術では、熱間圧延後の冷却速度を120℃/秒超とし、かつ冷却停止温度を620℃以下としているため、フェライト粒径が微細となり、軟質化することが困難であり、結果的に加工時のプレス負荷を増大させてしまうという問題がある。特許文献1の技術でも、酸洗後の焼鈍を長時間化すること、または高温焼鈍することにより軟質化を達成できはするが、エネルギー経済性および生産性の観点から非現実的である。 Recently, from the viewpoint of productivity improvement, the demand for softening as well as stretch flangeability has become stricter than before. However, in the technique of Patent Document 1, the cooling rate after hot rolling exceeds 120 ° C./second. In addition, since the cooling stop temperature is set to 620 ° C. or less, the ferrite grain size becomes fine, and it is difficult to soften, and as a result, there is a problem that the press load during processing increases. Even with the technique of Patent Document 1, softening can be achieved by increasing the annealing after pickling for a long time or by high-temperature annealing, but it is unrealistic from the viewpoint of energy economy and productivity.
本発明はかかる事情に鑑みてなされたものであって、長時間または高温での焼鈍を伴うことなく、容易に、高い伸びフランジ性を有しかつ軟質な高炭素熱延鋼板を製造することができる製造方法を提供することを目的とする。 The present invention has been made in view of such circumstances, and it is possible to easily produce a soft, high-carbon hot-rolled steel sheet having high stretch flangeability and without accompanying annealing for a long time or at a high temperature. It aims at providing the manufacturing method which can be performed.
本発明者らは、Cを0.2〜0.7質量%含有する高炭素鋼に対し熱間圧延後急速冷却を実施して伸びフランジ性を向上させる技術において、容易に鋼の軟質化を図ることができる条件を得るべく、軟質化の要因について詳細に調査した。その結果、熱間圧延時の仕上圧延温度を制御すれば酸洗後の焼鈍の負荷を低減させることができ、容易に軟質化を図れることを見出した。 The inventors of the present invention can easily soften steel in a technique for improving stretch flangeability by performing rapid cooling after hot rolling on high carbon steel containing 0.2 to 0.7 mass% of C. In order to obtain conditions that can be achieved, the factors of softening were investigated in detail. As a result, it has been found that if the finish rolling temperature during hot rolling is controlled, the annealing load after pickling can be reduced, and softening can be easily achieved.
一般的に、仕上圧延後に120℃/秒超の急速冷却を実施するとベイナイトまたは微細なフェライト/パーライトを主体とする組織となり、この後焼鈍を施してもフェライトの粒径を粗大化させて鋼を軟質化することは困難であるとされていた。これに対して、本発明者らは、熱間圧延時の仕上温度を従来よりも高くすることにより、冷却中に生じるフェライトの粒径を粗大化させることができ、酸洗後の焼鈍の負荷を上昇させることなく容易に高い伸びフランジ性と軟質性を兼ね備えた鋼を得ることができることを知見し、本発明を完成するに至った。 Generally, rapid cooling exceeding 120 ° C./second after finish rolling results in a structure mainly composed of bainite or fine ferrite / pearlite. It was considered difficult to soften. On the other hand, the present inventors can increase the grain size of the ferrite generated during cooling by increasing the finishing temperature during hot rolling than before, and the annealing load after pickling. It has been found that a steel having both high stretch flangeability and softness can be easily obtained without increasing the temperature, and the present invention has been completed.
すなわち、本発明は、Cを0.2〜0.7質量%含有する鋼を、仕上温度(Ar3変態点+60℃)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℃以下で冷却し、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ことを特徴とする伸びフランジ性に優れた軟質高加工性高炭素熱延鋼板の製造方法を提供する。 That is, in the present invention, a steel containing 0.2 to 0.7 mass% of C is hot-rolled at a finishing temperature (Ar 3 transformation point + 60 ° C.) or higher, and then the cooling rate exceeds 120 ° C./second and the cooling is stopped. Cooling at a temperature of 650 ° C. or lower, winding at a coiling temperature of 600 ° C. or lower, pickling, and annealing at an annealing temperature of 640 ° C. or higher and an Ac 1 transformation point or lower. A method for producing a heat-resistant high carbon hot-rolled steel sheet is provided.
また、本発明は、Cを0.2〜0.7質量%含有する鋼を、仕上温度(Ar3変態点+60℃)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℃以下で冷却し、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変態点以下で焼鈍し、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する伸びフランジ性に優れた軟質高加工性高炭素熱延鋼板の製造方法を提供する。 Further, the present invention provides a steel containing 0.2 to 0.7% by mass of C after hot rolling at a finishing temperature (Ar 3 transformation point + 60 ° C.) or higher, and then cooling rate is over 120 ° C./second and cooling is stopped. Cooling at a temperature of 650 ° C. or lower, winding at a coiling temperature of 600 ° C. or lower, pickling, annealing at an annealing temperature of 640 ° C. or higher and an Ac 1 transformation point or lower, and an average particle size of carbide of 0.1 μm or higher and lower than 1.2 μm The present invention provides a method for producing a soft, high-workability, high-carbon hot-rolled steel sheet excellent in stretch flangeability, characterized by controlling the volume fraction of ferrite grains not containing carbides to 10% or less.
本発明によれば、Cを0.2〜0.7質量%含有する高炭素鋼に対し熱間圧延後急速冷却を実施して伸びフランジ性を向上させる技術において、酸洗後の焼鈍の負荷を上昇させることなく容易に高い伸びフランジ性と軟質性を兼ね備えた鋼を得ることができる。したがって、ギアに代表される変速機部品等に適用した場合に、加工において加工度を高く、金型寿命を長くすることができ、製造工程を簡略化して低コストで部品等を製造することが可能となる。 According to the present invention, in a technique for improving the stretch flangeability by performing rapid cooling after hot rolling on a high carbon steel containing 0.2 to 0.7% by mass of C, the load of annealing after pickling Steel having both high stretch flangeability and softness can be easily obtained without raising. Therefore, when applied to transmission parts typified by gears, it is possible to increase the degree of processing and prolong the life of the mold, simplify the manufacturing process, and manufacture parts and the like at low cost. It becomes possible.
以下、本発明について詳細に説明する。
本発明に係る高炭素熱延鋼板の製造方法は、Cを0.2〜0.7質量%含有する鋼を、仕上温度(Ar3変態点+60℃)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℃以下で冷却し、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ものである。以下、構成要件毎に説明する。
Hereinafter, the present invention will be described in detail.
In the method for producing a high carbon hot rolled steel sheet according to the present invention, a steel containing 0.2 to 0.7% by mass of C is hot-rolled at a finishing temperature (Ar 3 transformation point + 60 ° C.) or higher, and then cooled. Cooling is performed at a temperature exceeding 120 ° C./second and at a cooling stop temperature of 650 ° C. or lower, and then winding is performed at a coiling temperature of 600 ° C. or lower. After pickling, annealing is performed at an annealing temperature of 640 ° C. or higher and an Ac 1 transformation point or lower. Hereinafter, each component requirement will be described.
C含有量:0.2〜0.7質量%
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。C含有量が0.2質量%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、炭化物の分布が不均一となる。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0.7質量%を超えると、焼鈍後でも十分な加工性が得られない。また、その場合、熱延後の鋼板の硬度が高く脆いため取扱いに不便であり、焼入後の強度も飽和する。したがって、C含有量を0.2〜0.7質量%に規定する。
C content: 0.2-0.7 mass%
C is an important element that forms carbides and imparts hardness after quenching. When the C content is less than 0.2% by mass, proeutectoid ferrite is prominently formed in the structure after hot rolling, and the distribution of carbides is nonuniform. Furthermore, in that case, sufficient strength cannot be obtained as a machine structural component even after quenching. On the other hand, if the C content exceeds 0.7% by mass, sufficient workability cannot be obtained even after annealing. Moreover, in that case, the hardness of the steel sheet after hot rolling is high and brittle, which is inconvenient to handle, and the strength after quenching is saturated. Therefore, the C content is specified to be 0.2 to 0.7 mass%.
仕上温度:(Ar3変態点+60℃)以上
上記特許文献1では、熱間圧延の仕上温度を(Ar3変態点−20℃)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として、(Ar3変態点−20℃)以上の仕上温度で仕上圧延しており、これにより、組織の均一化を図り、伸びフランジ性の向上を図っている。しかし、本発明者らの検討結果により、(Ar3変態点−20℃)以上であっても、特許文献1の実施例に存在する上限である(Ar3変態点+40℃)まででは酸洗後の焼鈍の負荷を上昇させることなく軟質化することは困難であることが判明した。
Finishing temperature: (Ar 3 transformation point + 60 ° C.) or more In the above-mentioned Patent Document 1, if the finishing temperature of hot rolling is less than (Ar 3 transformation point−20 ° C.), ferrite transformation proceeds in part, and thus carbide is not included. As the ferrite grains increase and stretch flangeability deteriorates, finish rolling is performed at a finishing temperature of (Ar 3 transformation point-20 ° C) or higher, thereby making the structure uniform and improving stretch flangeability I am trying. However, according to the results of the study by the present inventors, even if (Ar 3 transformation point −20 ° C.) or higher, it is pickled up to the upper limit (Ar 3 transformation point + 40 ° C.) existing in the examples of Patent Document 1. It turned out to be difficult to soften without increasing the subsequent annealing load.
これに対して、仕上温度を(Ar3変態点+60℃)以上とした場合、仕上圧延直後のオーステナイト粒の粗大化が進行し、その結果、圧延後の冷却中にフェライト変態した際の初期フェライト粒径が大きくなり酸洗後の焼鈍の負荷を上昇させることなく容易に軟質化を図ることができる。したがって、仕上温度を(Ar3変態点+60℃)以上とする。 On the other hand, when the finishing temperature is (Ar 3 transformation point + 60 ° C.) or higher, the austenite grains are coarsened immediately after finish rolling, and as a result, the initial ferrite when ferrite transformation occurs during cooling after rolling. Softening can be easily achieved without increasing the particle size and increasing the annealing load after pickling. Therefore, the finishing temperature is set to (Ar 3 transformation point + 60 ° C.) or higher.
このことを実験結果に基づいて説明する。
質量%で、C=0.036%、Si=0.19%、Mn=0.77%、P=0.014%、S=0.002%の鋼を用い、熱間圧延の仕上温度を(Ar3変態点+10℃)〜(Ar3変態点+100℃)の間で変化させ、熱間圧延後の冷却において冷却速度200℃/秒、冷却停止温度580℃とし、次いで巻取温度530℃で巻取り、酸洗後、680℃×20時間の焼鈍を行った。図1に、このようにして得られたサンプルの仕上温度と硬度との関係を示す。図1の横軸は、仕上温度:(Ar3+X)(℃)のXの値である。この図に示すように、Xが60℃以上、つまり仕上温度が(Ar3変態点+60℃)以上の場合には、焼鈍負荷を軽減しても容易に軟質化を達成していることがわかる。
This will be described based on experimental results.
Using steel with C = 0.036%, Si = 0.19%, Mn = 0.77%, P = 0.014%, S = 0.002% in mass%, the hot rolling finishing temperature is The temperature was changed between (Ar 3 transformation point + 10 ° C.) to (Ar 3 transformation point + 100 ° C.), the cooling rate was 200 ° C./second in the cooling after hot rolling, the cooling stop temperature was 580 ° C., and the coiling temperature was 530 ° C. And after pickling, annealing was performed at 680 ° C. for 20 hours. FIG. 1 shows the relationship between the finishing temperature and hardness of the sample thus obtained. The horizontal axis in FIG. 1 is the value of X at the finishing temperature: (Ar 3 + X) (° C.). As shown in this figure, when X is 60 ° C. or higher, that is, when the finishing temperature is (Ar 3 transformation point + 60 ° C.) or higher, softening is easily achieved even if the annealing load is reduced. .
圧延後の冷却における冷却速度:120℃/秒超
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、圧延後に急冷(冷却)を行う。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生成する。具体的には、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、炭化物を含まないフェライト粒が10%超となり、伸びフランジ性が劣化する。したがって、圧延後の冷却の冷却速度を120℃/秒超とする。
Cooling rate in cooling after rolling: Over 120 ° C./second In the present invention, rapid cooling (cooling) is performed after rolling in order to reduce the volume fraction of ferrite grains after transformation. When the cooling method is slow cooling, the degree of supercooling of austenite is small and proeutectoid ferrite is generated. Specifically, when the cooling rate is 120 ° C./second or less, the formation of pro-eutectoid ferrite becomes remarkable, the ferrite grains not containing carbide exceed 10%, and the stretch flangeability deteriorates. Therefore, the cooling rate of cooling after rolling is set to more than 120 ° C./second.
なお、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望ましい。この場合には、変態後のフェライト結晶粒やパーライト等の析出物をより微細化でき、加工性をより一層向上させることができる。 In addition, it is desirable to start cooling within the time exceeding 0.1 second and less than 1.0 second after finish rolling. In this case, precipitates such as ferrite crystal grains and pearlite after transformation can be further refined, and workability can be further improved.
圧延後の冷却における冷却停止温度:650℃以下
圧延後の冷却の冷却停止温度が高い場合、巻取までの冷却中にフェライトが生成するとともに、パーライトのラメラ間隔が粗大化する。そのため、焼鈍後に微細炭化物が得られなくなり、伸びフランジ性が劣化する。具体的には、冷却停止温度が650℃より高い場合、炭化物を含まないフェライト粒が10%超となり、伸びフランジ性が劣化する。したがって、圧延後の冷却の冷却停止温度を650℃以下とする。さらに、炭化物を含まないフェライト粒を5%以下とする場合は、冷却停止温度を600℃以下とする。
Cooling stop temperature in cooling after rolling: 650 ° C. or less When the cooling stop temperature in cooling after rolling is high, ferrite is generated during cooling up to winding and the lamella spacing of pearlite becomes coarse. Therefore, fine carbide cannot be obtained after annealing, and stretch flangeability deteriorates. Specifically, when the cooling stop temperature is higher than 650 ° C., ferrite grains not containing carbides exceed 10%, and stretch flangeability deteriorates. Therefore, the cooling stop temperature for cooling after rolling is set to 650 ° C. or lower. Furthermore, when making the ferrite grain which does not contain a carbide | carbonized_material 5% or less, a cooling stop temperature shall be 600 degrees C or less.
巻取温度:600℃以下
冷却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。したがって、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。なお、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 Although the steel sheet is wound after cooling, the lamella spacing of pearlite increases as the winding temperature increases. For this reason, the carbide after annealing becomes coarse, and when the coiling temperature exceeds 600 ° C., the stretch flangeability deteriorates. Therefore, the coiling temperature is set to 600 ° C. or less. Furthermore, by setting the coiling temperature to 500 ° C. or less, the dispersion state of the carbides becomes more uniform, and extremely excellent stretch flangeability can be obtained. In addition, although the minimum in particular of coiling temperature is not prescribed | regulated, since the shape of a steel plate deteriorates, so that it becomes low temperature, it is preferable to set it as 200 degreeC or more.
焼鈍温度:640℃以上Ac1変態点以下
熱延鋼板を酸洗した後、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。なお、優れた伸びフランジ性を得るには、焼鈍温度を680℃以上とすることが好ましい。
Annealing temperature: 640 ° C. or more and Ac 1 transformation point or less After pickling the hot-rolled steel sheet, annealing is performed to spheroidize the carbide. 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. In order to obtain excellent stretch flangeability, the annealing temperature is preferably 680 ° C. or higher.
なお、本発明においては、上述のように焼鈍温度を極端に高くすることなく鋼の軟質化を図ることができるが、焼鈍時間についても極端に長い時間は不要であり、30時間以下、例えば20時間で十分である。この点において、本発明は軟質化のために長時間または高温焼鈍が必要な上記特許文献1の技術よりも有利である。ただし、焼鈍時間が5時間未満では十分な軟質化を達成することは困難であるため、5時間以上とすることが望ましい。 In the present invention, as described above, the steel can be softened without extremely increasing the annealing temperature. However, an extremely long time is not required for the annealing time, and 30 hours or less, for example, 20 Time is enough. In this respect, the present invention is more advantageous than the technique of Patent Document 1 that requires a long time or high temperature annealing for softening. However, since it is difficult to achieve sufficient softening if the annealing time is less than 5 hours, it is desirable that the annealing time be 5 hours or more.
以上のような方法により、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ができる。 By the method as described above, the carbide average particle size can be controlled to 0.1 μm or more and less than 1.2 μm, and the volume fraction of ferrite grains not containing carbide can be controlled to 10% or less.
炭化物平均粒径:0.1μm以上かつ1.2μm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、1.2μm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。したがって、炭化物平均粒径を0.1μm以上かつ1.2μm未満に制御する。なお、炭化物平均粒径は上記製造条件、特に冷却停止温度、巻取温度、および焼鈍温度により制御することができる。
Carbide average particle size: 0.1 μm or more and less than 1.2 μm The carbide particle size greatly affects the workability in general and the generation of voids in the hole expanding process. When the carbide becomes fine, the generation of voids can be suppressed. However, when the average particle size of the carbide is less than 0.1 μm, the ductility decreases as the hardness increases, and the stretch flangeability also decreases. The workability in general improves with an increase in the average carbide particle diameter, but when it becomes 1.2 μm or more, the 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 1.2 μm. The carbide average particle size can be controlled by the above production conditions, particularly the cooling stop temperature, the coiling temperature, and the annealing temperature.
炭化物の分散状態: 炭化物を含まないフェライト粒の体積率が10%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、ボイドの発生を抑制することができる。炭化物を含まないフェライト粒を、体積率にして10%以下にすることにより、炭化物の分散状態が均一化され、伸びフランジ性が著しく向上する。したがって、炭化物を含まないフェライト粒の体積率を10%以下とする。さらに、炭化物を含まないフェライト粒を、体積率にして5%以下にすることで、炭化物の分散状態を一層均一化し、極めて優れた伸びフランジ性が得られる。
Dispersion state of carbide: The volume fraction of ferrite grains not containing carbide is 10% or less. By making the dispersion state of carbide uniform, stress concentration at the punched end face during hole expansion processing is reduced as described above, and voids are formed. Can be suppressed. By setting the ferrite grains not containing carbide to a volume ratio of 10% or less, the dispersion state of carbide is made uniform, and stretch flangeability is remarkably improved. Therefore, the volume fraction of ferrite grains not containing carbide is set to 10% or less. Furthermore, by making ferrite particles not containing carbide 5% or less in volume ratio, the dispersion state of carbides can be made more uniform, and extremely excellent stretch flangeability can be obtained.
以上の説明において、炭化物を含まないというのは、通常の金属組織観察(光学顕微鏡)では炭化物が検出されないという意味である。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の炭化物が実質的に見られない。また、炭化物を含まないフェライト粒の体積率が10%以下であれば、機械的性質(硬度)への影響も無視できる。なお、炭化物の分散状態は前述のように製造条件、特に仕上温度、圧延後の冷却の冷却速度、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。 In the above description, the fact that carbides are not included means that carbides are not detected by normal metallographic observation (optical microscope). Such ferrite grains are portions generated as pro-eutectoid ferrite after hot rolling, and carbides in the grains are not substantially seen even after annealing. Further, if the volume fraction of ferrite grains not containing carbide is 10% or less, the influence on mechanical properties (hardness) can be ignored. As described above, the dispersion state of the carbides can be controlled by the manufacturing conditions, particularly the finishing temperature, the cooling rate of cooling after rolling, the cooling stop temperature, and the winding temperature.
本発明において、Cを0.2〜0.7質量%含有し、金属組織が前述の炭化物平均粒径および炭化物の分散状態を満たしさえすれば、原理的に他の成分は特に規定する必要はなく、Mn、Si、P、S、Al、Nなどの一般的な元素が通常の範囲で含有されているものであれば問題ない。 In the present invention, if C is contained in an amount of 0.2 to 0.7% by mass and the metal structure satisfies the above-mentioned carbide average particle size and carbide dispersion state, in principle, the other components need to be specified in particular. There is no problem as long as general elements such as Mn, Si, P, S, Al, and N are contained in a normal range.
ただし、好ましい鋼組成としては、C:0.2〜0.7質量%の他、Si:2質量%以下、Mn:2質量%以下、P:0.03質量%以下、S:0.03質量%以下、sol.Al:0.08質量%以下、N:0.01質量%以下含有するものが挙げられる。 However, as a preferable steel composition, C: 0.2 to 0.7 mass%, Si: 2 mass% or less, Mn: 2 mass% or less, P: 0.03 mass% or less, S: 0.03 % By mass or less, sol. Examples include Al: 0.08% by mass or less and N: 0.01% by mass or less.
以下、これらについて説明する。まず、Siについては、炭化物を黒鉛化し、焼入性を阻害する傾向があるので、2質量%以下とするのが望ましい。Mnについては、過剰の添加は延性の低下を引き起こす傾向があるので、2質量%以下とするのが望ましい。P,Sについては、過剰に含有すると延性が低下し、またクラックも生成しやすくなるのでともに0.03質量%以下であることが望ましい。また、Alについては、過剰に添加するとAlNが多量に析出し焼入性を低下させるので、sol.Alとして0.08質量%以下とするのが望ましい。Nについても、過剰に含有している場合は延性の低下をもたらすため、0.01%以下であることが望ましい。 Hereinafter, these will be described. First, about Si, it is desirable to make it 2 mass% or less because it tends to graphitize carbides and inhibit hardenability. About Mn, since excessive addition tends to cause reduction in ductility, it is desirable to make it 2% by mass or less. About P and S, when it contains excessively, ductility will fall and it will become easy to produce | generate a crack, Therefore It is desirable that both are 0.03 mass% or less. As for Al, if added excessively, a large amount of AlN precipitates and lowers the hardenability. The Al content is preferably 0.08% by mass or less. N is also preferably contained in an amount of 0.01% or less because it causes a decrease in ductility when it is excessively contained.
さらに、目的に応じて、通常添加される範囲でB,Cr,Cu,Ni,Mo,Ti,Nb,W,V,Zr等の各種元素を必要に応じて添加してもよい。これらの元素は、本発明の効果には特に影響を及ぼさずに所望の効果を得ることができる。具体的には、上記組成にさらに、B:0.005質量%以下、Cr:1.5質量%以下、Cu:1.0質量%以下、Ni:1.0質量%以下、Mo:0.5質量%以下、Ti:0.5質量%以下、Nb:0.5質量%以下、W:0.5質量%以下、V:0.5質量%以下、Zr:0.5質量%以下の1種または2種以上を含有してもよい。B,Cr,Cu,Ni,Mo,Ti,Nb,Wについては、添加することにより熱延冷却時の初析フェライト生成の抑制、焼入れ性の向上を図ることができるが、上記含有量を超えると効果が飽和し、コスト増となる。また、V,Zrについては、添加することにより強度強化を図ることができるが、上記含有量を超えると効果が飽和し、コスト増となる。 Furthermore, various elements such as B, Cr, Cu, Ni, Mo, Ti, Nb, W, V, and Zr may be added as necessary according to the purpose. These elements can obtain a desired effect without particularly affecting the effect of the present invention. Specifically, in addition to the above composition, B: 0.005 mass% or less, Cr: 1.5 mass% or less, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Mo: 0.0. 5 mass% or less, Ti: 0.5 mass% or less, Nb: 0.5 mass% or less, W: 0.5 mass% or less, V: 0.5 mass% or less, Zr: 0.5 mass% or less You may contain 1 type, or 2 or more types. B, Cr, Cu, Ni, Mo, Ti, Nb, and W can be added to suppress the formation of pro-eutectoid ferrite during hot rolling cooling and improve hardenability, but exceed the above contents. The effect is saturated and the cost increases. Moreover, about V and Zr, although intensity | strength reinforcement can be aimed at by adding, if the said content is exceeded, an effect will be saturated and cost will increase.
これら元素および残部のFeの他、製造過程でSn,Pb等の各種不純物元素が混入するが、このような不可避的な不純物元素は本発明の効果に特に影響を及ぼすものではなく、許容される。 In addition to these elements and the remaining Fe, various impurity elements such as Sn and Pb are mixed in the manufacturing process, but such inevitable impurity elements do not particularly affect the effects of the present invention and are allowed. .
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。また、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、また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。 Either a converter or an electric furnace can be used for preparing the components of the high carbon steel of the present invention. Further, rough rolling may be omitted during hot rolling, and finish rolling may be performed, or direct casting rolling in which continuous cast slabs are rolled as they are for the purpose of suppressing temperature decrease may be performed.
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スラブとする。この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1280℃以下とすることが好ましい。 The high carbon steel whose components are prepared in this way is made into a slab by ingot-bundling rolling or continuous casting. The slab is hot-rolled. At this time, the slab heating temperature is preferably set to 1280 ° C. or less in order to avoid deterioration of the surface state due to generation of scale.
なお、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。また、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 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.
熱間圧延後の焼鈍については、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。 The annealing after hot rolling may be either box annealing or continuous annealing. Thereafter, temper rolling is performed as necessary. Since this temper rolling does not affect the hardenability, there is no particular limitation on the conditions.
表1に示す化学成分を有する鋼の連続鋳造スラブを1250℃に加熱し、表2に示す条件にて熱間圧延および焼鈍を行い、板厚5.0mmの鋼板を製造した。ここで、鋼板No.1〜17は製造条件が本発明範囲内の本発明例であり、鋼板No.18〜27は製造条件が本発明範囲から外れる比較例である。 A steel continuous casting slab having chemical components shown in Table 1 was heated to 1250 ° C., and hot rolled and annealed under the conditions shown in Table 2 to produce a steel plate having a thickness of 5.0 mm. Here, the steel plate No. Nos. 1 to 17 are examples of the present invention whose manufacturing conditions are within the scope of the present invention. 18 to 27 are comparative examples in which the production conditions deviate from the scope of the present invention.
これらの鋼板からサンプルを採取し、炭化物平均粒径ならびに炭化物の分散状態の測定、硬度測定、および伸びフランジ性測定を行った。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 average particle size of carbides and the dispersion state of carbides, hardness, and stretch flangeability were measured. Each test and measurement method and conditions are shown below.
(1)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・腐食後、走査型電子顕微鏡にてミクロ組織を撮影し、0.01mm2の範囲で炭化物粒径およびその分散状態(炭化物を含まないフェライト粒の体積率)の測定を行った。
(1) Carbide average particle size and its dispersion state After polishing and corrosion of the plate thickness section of the sample, the microstructure was photographed with a scanning electron microscope, and the carbide particle size and its dispersion state (carbide in the range of 0.01 mm 2 ). (Volume ratio of ferrite grains not containing) was measured.
(2)伸びフランジ性測定
サンプルを、ポンチ径d0=10mm、ダイス径11mm(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)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を測定して、以下の(1)式で定義される穴拡げ率:λ(%)を求めた。
λ=100×(db-d0)/d0 (1)
(2) Stretch flangeability measurement The sample was punched with a punching tool having a punch diameter d 0 = 10 mm and a die diameter 11 mm (
λ = 100 × (db−d 0 ) / d 0 (1)
以上の測定結果より得られた、炭化物平均粒径、炭化物の分散状態、硬度および伸びフランジ性を表2に併記する。ここで、伸びフランジ性は上記(1)式の穴拡げ率λで評価した。 The carbide average particle size, carbide dispersion state, hardness and stretch flangeability obtained from the above measurement results are also shown in Table 2. Here, the stretch flangeability was evaluated by the hole expansion rate λ of the above formula (1).
表2に示すように、本発明例である鋼板No.1〜17は、各C含有量に応じて十分軟化しており、かつ高い伸びフランジ性が得られることが確認された。これに対して比較例である鋼板No.18〜27では、各C含有量に対して、硬度が高いか、伸びフランジ性に劣っているかのいずれかであった。すなわち、鋼板No.18,20,22,24,26は、仕上温度が低いため、硬度HRBが高くなっている。一方、鋼板No.19,21,23,25,27については、鋼板No.19,23は冷却速度が小さいため、炭化物を含まないフェライト粒の体積率が10%超であり、鋼板No.21は、冷却終了温度が高いため、フェライト粒の体積率が10%超かつ炭化物平均粒径が1.2μm以上であり、鋼板No.25は巻取温度が600℃を超えているため、炭化物の平均粒径が1.2μm以上であり、鋼板No.27は、焼鈍温度が640℃未満であるため、炭化物の平均粒径が0.1μm未満であり、いずれも伸びフランジ性に劣っていた。 As shown in Table 2, steel plate No. which is an example of the present invention. It was confirmed that Nos. 1 to 17 were sufficiently softened according to each C content and high stretch flangeability was obtained. On the other hand, steel plate No. which is a comparative example. In 18 to 27, either the hardness was high or the stretch flangeability was poor with respect to each C content. That is, the steel plate No. Since 18,20,22,24,26 has a low finishing temperature, the hardness HRB is high. On the other hand, steel plate No. For steel sheets 19, 21, 23, 25, and 27, steel plate no. Nos. 19 and 23 have a low cooling rate, so that the volume fraction of ferrite grains not containing carbide exceeds 10%. No. 21 has a high cooling end temperature, so that the volume fraction of ferrite grains exceeds 10% and the average carbide grain size is 1.2 μm or more. No. 25 has a coiling temperature exceeding 600 ° C., so the average particle size of the carbide is 1.2 μm or more. In No. 27, since the annealing temperature was less than 640 ° C., the average particle size of the carbide was less than 0.1 μm, and all had poor stretch flangeability.
本発明は、高い伸びフランジ性を有しかつ軟質な高炭素熱延鋼板を製造することができる製造方法を提供するものであり、このようにして得られた鋼板は、工具あるいは自動車部品(ギア、ミッション)等に好適である。 The present invention provides a production method capable of producing a soft, high-carbon hot-rolled steel sheet having high stretch flangeability, and the steel sheet thus obtained can be used as a tool or an automobile part (gear). , Mission) and the like.
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