JP4830330B2 - Manufacturing method of thick-walled low yield ratio high-tensile steel sheet - Google Patents

Manufacturing method of thick-walled low yield ratio high-tensile steel sheet Download PDF

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JP4830330B2
JP4830330B2 JP2005089312A JP2005089312A JP4830330B2 JP 4830330 B2 JP4830330 B2 JP 4830330B2 JP 2005089312 A JP2005089312 A JP 2005089312A JP 2005089312 A JP2005089312 A JP 2005089312A JP 4830330 B2 JP4830330 B2 JP 4830330B2
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謙次 林
康宏 室田
照輝 貞末
秀和 高橋
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JFE Steel Corp
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Description

本発明は、橋梁、土木、建築などの分野で使用される、耐震性に優れた引張強さが５５０ＭＰａ以上の低降伏比厚肉高張力鋼板の製造方法に関する。   The present invention relates to a method for producing a low yield ratio thick-walled high-tensile steel plate having a tensile strength of 550 MPa or more and excellent in earthquake resistance, which is used in fields such as bridges, civil engineering, and architecture.

橋梁、土木、建築分野で使用される鋼材は、耐震性の観点から、高い靱性とともに、低降伏比が要求されている。しかし、一般的に鋼材の強度が高くなると降伏比も高くなる傾向にあり、たとえば５９０ＭＰａ級以上の高強度鋼では、低降伏比を確保するために、特殊な熱処理などを必要としてきた。これは、フェライトとマルテンサイトあるいはフェライトとベイナイトなどの軟質相と硬質相の２相組織とすることにより高強度かつ低降伏比を得ることを目的としたプロセスである。   Steel materials used in the bridge, civil engineering, and construction fields are required to have a low yield ratio as well as high toughness from the viewpoint of earthquake resistance. However, generally, the yield ratio tends to increase as the strength of the steel material increases. For example, a high-strength steel of 590 MPa class or higher has required special heat treatment or the like to ensure a low yield ratio. This is a process aimed at obtaining a high strength and low yield ratio by forming a two-phase structure of a soft phase and a hard phase such as ferrite and martensite or ferrite and bainite.

例えば、非特許文献１に記載されるとおり、圧延後、焼入れ（Ｑ）を行い、その後、２相域に加熱し焼入れ（Ｑ’）を行い焼戻し（Ｔ）を行うＱ-Ｑ’-Ｔプロセスが一般的であり、５９０ＭＰａ級の強度と８０％以下の低降伏比を両立させている。しかし、この文献に示されている製造プロセスでは、熱処理工程が多く複雑であり、工期や熱処理コストの点でも熱処理工程の省略が望まれていた。   For example, as described in Non-Patent Document 1, Q-Q'-T process in which quenching (Q) is performed after rolling, and then the two-phase region is heated and quenched (Q ') and tempered (T). The 590 MPa class strength and the low yield ratio of 80% or less are compatible. However, the manufacturing process shown in this document has many heat treatment steps and is complicated, and it has been desired to omit the heat treatment step from the viewpoint of construction period and heat treatment cost.

そこで、特許文献１、２に記載されるごとく、Ｑを省略して、圧延後直接焼入れ（ＤＱ）を行い、その後、Ｑ’、Ｔを行うＤＱ-Ｑ’-Ｔプロセスや圧延後空冷したのちＱ’-Ｔを行うプロセスなどが開発され、低降伏比高張力鋼板の一般的な製造法となっている。しかし、現状、安定して高強度と低降伏比および高靭性を得るには、Ｑ’熱処理が不可欠となっているのが実情である。   Therefore, as described in Patent Documents 1 and 2, after Q is omitted, direct quenching (DQ) is performed after rolling, and then Q ′ and T are performed, or after air cooling after rolling. A process for performing Q′-T has been developed, and has become a general production method of low-yield ratio high-tensile steel sheets. However, at present, in order to stably obtain high strength, a low yield ratio, and high toughness, the Q 'heat treatment is indispensable.

Ｑ’を必要としない熱処理方法として、特許文献３、４には、圧延後、空冷を行い、２相域から冷却するプロセスが提案されている。しかし、この技術では、一部フェライト変態させた後焼入れることによりフェライト−ベイナイト（またはマルテンサイト）の２相組織が得られ、高強度かつ低降伏比が達成されるが、圧延を低温域で行う必要があることや圧延後の空冷に時間を要することによる製造性の低下や、空冷の時間管理・温度管理が重要となり、安定して高強度・低降伏比の得られるプロセスとは言えない。このような圧延後直接焼入れまたは加速冷却まま、あるいは冷却後焼戻しを行うプロセスにおいて、上記のように高強度かつ低降伏比が得られるとされているが、Ｑ’−Ｔプロセスに比べると安定して高強度・低降伏比の得られるプロセスとは言えない。   As heat treatment methods that do not require Q ', Patent Documents 3 and 4 propose processes in which air cooling is performed after rolling and cooling is performed from a two-phase region. However, in this technique, a two-phase structure of ferrite-bainite (or martensite) is obtained by quenching after partial ferrite transformation, and high strength and a low yield ratio are achieved. It is not a process that can stably obtain high strength and low yield ratio because it is important to reduce the manufacturability due to the necessity to perform and the time required for air cooling after rolling, and the time management and temperature management of air cooling are important. . In such a process of direct quenching or accelerated cooling after rolling or tempering after cooling, it is said that high strength and a low yield ratio can be obtained as described above, but it is more stable than the Q′-T process. Therefore, it cannot be said that it is a process that can achieve high strength and low yield ratio.

また、これらの製造プロセスでの冷却では、特に厚肉材の板厚方向の特性差が考慮されておらず、板厚の１／４ｔ位置（鋼板の表面から板厚（ｔ）の１／４の位置）と１／２ｔ位置（鋼板の表面から板厚（ｔ）の１／２の位置、すなわち板厚中央位置）の特性差が大きく、１／４ｔ位置で低降伏比を満足させると、１／２ｔ位置の強度が不足する可能性が高いことや、１／２ｔの強度を確保すると１／４ｔ位置の降伏比（ＹＲ）が８０％を超える可能性が高いなど、１／４ｔ位置と１／２ｔ位置の両方の特性を安定して満足させることは非常に困難である。これは、通常、厚肉の鋼板を表面および裏面から水冷やミスト冷却などにより冷却した場合、その冷却速度は板厚表面に近いほど速く、板厚中央（１／２ｔ）位置では遅いことにより、変態組織が変化するためである。Ｑ’熱処理を施さないプロセスにおいては特にその傾向が強く、１／４ｔ位置と１／２ｔ位置の特性差は大きくなる傾向にある。
特開平１−１５６４２１号公報 特開平３−２０７８１４号公報 特開昭６３−２２３１２３号公報 特開昭６４−５５３３５号公報 日本鋼管技報No.122（1988年）５〜１０ページ Further, in the cooling in these manufacturing processes, the characteristic difference in the plate thickness direction of the thick material is not particularly taken into consideration, and the 1/4 t position of the plate thickness (from the surface of the steel plate to 1/4 of the plate thickness (t)). ) And 1 / 2t position (1/2 position of the sheet thickness (t) from the surface of the steel sheet, that is, the center position of the sheet thickness), and when the low yield ratio is satisfied at the 1 / 4t position, There is a high possibility that the strength at the 1 / 2t position is insufficient, and when the strength at 1 / 2t is secured, the yield ratio (YR) at the 1 / 4t position is likely to exceed 80%. It is very difficult to stably satisfy both characteristics at the 1 / 2t position. This is usually because when the thick steel plate is cooled from the front and back by water cooling or mist cooling, the cooling rate is faster as it is closer to the plate thickness surface, and slow at the plate thickness center (1 / 2t) position. This is because the transformation organization changes. This tendency is particularly strong in the process in which the Q ′ heat treatment is not performed, and the characteristic difference between the 1 / 4t position and the 1 / 2t position tends to increase.
JP-A-1-156421 Japanese Patent Laid-Open No. 3-207814 JP 63-223123 A JP-A 64-55335 Nippon Steel Pipe Technical Report No.122 (1988) 5-10 pages

本発明はかかる事情に鑑みてなされたものであって、４０ｍｍを超える引張強さが５５０ＭＰａ以上の厚肉高張力鋼板において、Ｑ’およびＴプロセスを必要とせずに、板厚方向の特性差が小さく１／４ｔ位置と１／２ｔ位置の双方において低降伏比かつ高強度を達成することができる低降伏比高張力鋼板の製造方法を提供することを目的とする。   The present invention has been made in view of such circumstances, and in a thick high-tensile steel sheet having a tensile strength exceeding 40 mm and having a tensile strength of 550 MPa or more, there is no need for Q ′ and T processes, and there is a difference in characteristics in the thickness direction. It is an object of the present invention to provide a method for producing a low yield ratio high tensile strength steel sheet that can achieve a low yield ratio and high strength at both a small 1/4 t position and a 1/2 t position.

上記課題を解決するため、本発明は、スラブを１０００〜１３００℃に加熱し、熱間圧延した後、５００℃以下の温度まで水冷により加速冷却して、質量％で、Ｃ：０．０１〜０．１８％、Ｓｉ：０．０１〜０．５％、Ｍｎ：０．３〜２．５％を含有し、残部がＦｅおよび不可避不純物からなる板厚４０ｍｍ以上の厚肉高張力鋼板を製造するにあたり、加速冷却時の板厚方向１／４ｔ位置と板厚方向１／２ｔ位置の少なくとも７００〜５００℃の温度帯の冷却速度の差が１５％以内となるように鋼板の表面からの冷却を冷却初期よりも冷却後期のほうが強冷却に制御することを特徴とする厚肉低降伏比高張力鋼板の製造方法を提供する。 In order to solve the above-mentioned problems, the present invention heats a slab to 1000 to 1300 ° C., hot-rolls it, and then accelerates and cools it to a temperature of 500 ° C. or less by water cooling. Manufactures thick high-strength steel sheets containing 0.18%, Si: 0.01-0.5%, Mn: 0.3-2.5%, the balance being Fe and inevitable impurities and having a thickness of 40 mm or more In this case, the cooling from the surface of the steel sheet is performed so that the difference in the cooling rate in the temperature range of at least 700 to 500 ° C. between the position of 1/4 t in the plate thickness direction and the position of 1/2 t in the plate thickness direction during accelerated cooling is within 15%. The present invention provides a method for producing a thick-walled, low-yield ratio, high-tensile steel sheet, characterized in that the cooling is controlled to be stronger in the later stage than in the early stage .

また、本発明は、スラブを１０００〜１３００℃に加熱し、熱間圧延した後、５００℃以下の温度まで水冷により加速冷却して、質量％で、Ｃ：０．０１〜０．１８％、Ｓｉ：０．０１〜０．５％、Ｍｎ：０．３〜２．５％を含有し、さらに、Ｃｕ：０．０３〜１％、Ｎｉ：０．０３〜２％、Ｃｒ：０．０３〜０．５％、Ｍｏ：０．０３〜０．５％、Ｖ：０．０２〜０．１５％、Ｔｉ：０．００５〜０．１％、Ｎｂ：０．００５〜０．０５％、Ｂ：０．０００４〜０．００２％、Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００５〜０．００５％、ＲＥＭ：０．０００５〜０．０２％を1種または２種以上含有し、残部がＦｅおよび不可避不純物からなる板厚４０ｍｍ以上の厚肉高張力鋼板を製造するにあたり、加速冷却時の板厚方向１／４ｔ位置と板厚方向１／２ｔ位置の少なくとも７００〜５００℃の温度帯の冷却速度の差が１５％以内となるように鋼板の表面からの冷却を冷却初期よりも冷却後期のほうが強冷却に制御することを特徴とする厚肉低降伏比高張力鋼板の製造方法を提供する。 Further, in the present invention, the slab is heated to 1000 to 1300 ° C. and hot-rolled, and then accelerated and cooled by water cooling to a temperature of 500 ° C. or less, and in mass%, C: 0.01 to 0.18%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.5%, Cu: 0.03 to 1%, Ni: 0.03 to 2%, Cr: 0.03 -0.5%, Mo: 0.03-0.5%, V: 0.02-0.15%, Ti: 0.005-0.1%, Nb: 0.005-0.05%, B: 0.0004 to 0.002%, Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005%, REM: 0.0005 to 0.02%, one or more In the production of a thick high-tensile steel plate having a thickness of 40 mm or more, the balance being Fe and inevitable impurities , the thickness direction 1 during accelerated cooling is 1 The cooling from the surface of the steel sheet is stronger in the later stage of cooling than in the early stage of cooling so that the difference in the cooling rate at the temperature zone of at least 700 to 500 ° C. between the / 4t position and the 1 / 2t position in the thickness direction is within 15%. The present invention provides a method for producing a thick-walled, low-yield-ratio, high-tensile steel sheet, characterized by being controlled.

さらに、本発明は、上記いずれかの発明において、前記加速冷却は、冷却水量密度を調整することにより、または水冷と非水冷の繰り返しの時間間隔を調整することにより、鋼板表面からの冷却を冷却初期よりも冷却後期のほうが強冷却に制御することを特徴とする厚肉低降伏比高張力鋼板の製造方法を提供する。 Further, according to the present invention, in any one of the above-described inventions, the accelerated cooling is performed by cooling the cooling from the steel sheet surface by adjusting a cooling water amount density or by adjusting a time interval between water cooling and non-water cooling. Provided is a method for producing a thick-walled, low-yield ratio, high-tensile steel sheet, characterized in that the latter stage of cooling is controlled to be stronger cooling than the initial stage .

本発明で最も重要な点は、４０ｍｍを超える厚肉鋼板の板厚１／４ｔ位置と１／２ｔ位置の特性差を小さくするために、冷却を制御して、特性を左右する特定の温度域における１／４ｔ位置と１／２ｔ位置の冷却速度を制御する点である。また、それに加えて、圧延・冷却条件等を調整することにより、高強度かつ低降伏比を双方の板厚位置で達成することができ、さらには高靱性を達成することができる。さらに、適切な成分調整を行うことにより所望の強度および降伏比を得ることができ、さらには靱性を高めることができる。さらにまた、冷却条件を制約することにより、表層の硬さの低減を図り、板厚方向の特性差をさらに小さくすることができる。   In the present invention, the most important point is that a specific temperature range that affects the characteristics by controlling the cooling in order to reduce the difference in characteristics between the 1/4 t position and 1/2 t position of the thick steel plate exceeding 40 mm. This is a point for controlling the cooling rate at the 1 / 4t position and the 1 / 2t position. In addition, by adjusting the rolling / cooling conditions and the like, high strength and low yield ratio can be achieved at both plate thickness positions, and high toughness can be achieved. Further, by performing appropriate component adjustment, a desired strength and yield ratio can be obtained, and further, toughness can be increased. Furthermore, by limiting the cooling conditions, the hardness of the surface layer can be reduced, and the characteristic difference in the plate thickness direction can be further reduced.

本発明によれば、所定条件で熱間圧延、加速冷却および焼戻しを行って厚鋼板を製造するにあたり、圧延・冷却条件の加速冷却時の板厚方向１／４ｔ位置と板厚方向１／２ｔ位置の少なくとも７００〜５００℃の温度帯の冷却速度の差が１５％以内となるように鋼板の表面からの冷却を制御するので、Ｑ’プロセスを用いることなく、板厚方向の特性差が小さく１／４ｔ位置と１／２ｔ位置の双方において低降伏比かつ高強度を達成する板厚４０ｍｍを超える引張強さが５５０ＭＰａ以上の厚肉高張力鋼板を得ることができる。   According to the present invention, in producing a thick steel sheet by performing hot rolling, accelerated cooling and tempering under predetermined conditions, the position in the thickness direction 1 / 4t and the thickness direction 1 / 2t at the time of accelerated cooling under the rolling and cooling conditions. Since the cooling from the surface of the steel sheet is controlled so that the difference in the cooling rate in the temperature range of at least 700 to 500 ° C. is within 15%, the characteristic difference in the thickness direction is small without using the Q ′ process. A thick high-tensile steel sheet having a tensile strength exceeding 550 MPa and a sheet thickness of 40 mm that achieves a low yield ratio and high strength at both the 1 / 4t position and the 1 / 2t position can be obtained.

以下、本発明についてより具体的に説明する。
図１に、鋼の成分組成と圧延・冷却開始・停止条件を一定としたときの引張特性に及ぼす冷却速度の影響を示す。ここでは、成分組成を０．０８Ｃ−０．２５Ｓｉ−１．５８Ｍｎ−０．２８Ｃｕ−０．３０Ｎｉ−０．２９Ｃｒ−０．０４Ｖ−０．０１５Ｔｉ−０．００１４Ｂ（数字は質量％）とし、製造条件を１１００℃加熱−８００℃圧延仕上げ−加速冷却４５０℃停止とした。この図に示すように、冷却速度が２．５℃／ｓｅｃ以下では、強度が目標（本試験での目標は降伏強度ＹＳ≧３８５ＭＰａ）を満足せず、また、冷却速度が１０℃／ｓｅｃ以上では、降伏比ＹＲが８０％を超え、強度と降伏比を両立させるには最適な冷却速度範囲（本条件の場合２．５〜１０℃／ｓｅｃ）が存在する。この最適冷却速度範囲は、フェライト＋ベイナイトの最適な組織の得られる範囲であり、強度および降伏比は、冷却速度以外にも成分や圧延条件、焼戻し条件などによって変化し、低い冷却速度の条件であっても、成分調整などで、目標性能を満足させることが可能である。 Hereinafter, the present invention will be described more specifically.
FIG. 1 shows the effect of the cooling rate on the tensile properties when the steel component composition and rolling / cooling start / stop conditions are constant. Here, the component composition is 0.08C-0.25Si-1.58Mn-0.28Cu-0.30Ni-0.29Cr-0.04V-0.015Ti-0.0014B (numbers are mass%), and manufactured. The conditions were 1100 ° C. heating-800 ° C. rolling finish-accelerated cooling 450 ° C. stop. As shown in this figure, when the cooling rate is 2.5 ° C./sec or less, the strength does not satisfy the target (the target in this test is the yield strength YS ≧ 385 MPa), and the cooling rate is 10 ° C./sec or more. Then, the yield ratio YR exceeds 80%, and there is an optimum cooling rate range (2.5 to 10 ° C./sec in the case of this condition) in order to achieve both strength and yield ratio. This optimum cooling rate range is the range where the optimum structure of ferrite + bainite can be obtained, and the strength and yield ratio change depending on the components, rolling conditions, tempering conditions, etc. in addition to the cooling rate. Even so, the target performance can be satisfied by adjusting the components.

しかし、板厚４０ｍｍを超える厚肉鋼板では、通常の冷却条件（例えば、鋼板の表面および裏面から連続して冷却水あるいはミストを吹き付けることにより冷却する方法）では１／４ｔ位置と１／２ｔ位置の冷却速度が異なり、１／４ｔ位置と１／２ｔ位置の特性を両立させることは難しい。   However, in the case of a thick steel plate having a thickness of more than 40 mm, the 1/4 t position and the 1/2 t position under normal cooling conditions (for example, cooling by spraying cooling water or mist continuously from the front and back surfaces of the steel plate). It is difficult to make the characteristics of the 1 / 4t position and the 1 / 2t position compatible with each other.

そこで、本発明では、板厚４０ｍｍ以上の厚肉高張力鋼板を製造するにあたり、スラブを加熱し、熱間圧延した後、水冷により加速冷却し、その後焼戻しを行わない製造プロセスを採用し、所定温度範囲において加速冷却時の１／４ｔ位置と１／２ｔ位置の冷却速度の差が１５％以内となるように、鋼板表面からの冷却を制御する。これにより１／４ｔ位置と１／２ｔ位置の特性差を小さくし、板厚１／４ｔ位置と１／２ｔ位置の双方において高強度かつ低降伏比を両立させることに成功した。 Therefore, in the present invention, in manufacturing a thick high-tensile steel sheet having a thickness of 40 mm or more, a slab is heated, hot-rolled, accelerated by water cooling, and then subjected to a manufacturing process in which tempering is not performed. Cooling from the steel sheet surface is controlled so that the difference in cooling rate between the 1 / 4t position and the 1 / 2t position during accelerated cooling is within 15% in the temperature range. As a result, the characteristic difference between the 1 / 4t position and the 1 / 2t position was reduced, and both a high strength and a low yield ratio were achieved at both the 1 / 4t position and the 1 / 2t position.

このように、１／４ｔ位置と１／２ｔ位置の冷却速度差を小さくすることができる冷却速度の制御としては、鋼板の表面および裏面から水冷またはミスト冷却によって水冷する方法などで、冷却初期段階と冷却後期段階での冷却条件を変え、初期を弱冷却、後期を強冷却とすることが挙げられる。   As described above, the cooling rate control capable of reducing the cooling rate difference between the 1 / 4t position and the 1 / 2t position is a cooling initial stage by a method of water cooling by water cooling or mist cooling from the front and back surfaces of the steel sheet. In other words, the cooling conditions in the later stage of cooling are changed so that the initial stage is weakly cooled and the latter stage is strongly cooled.

通常、厚肉鋼板を鋼板の表面および裏面から一定の冷却条件で冷却した場合には、板厚方向に温度勾配持ちながら冷却され、１／４ｔ位置と１／２ｔ位置の冷却速度は１／４ｔ位置の方が速く、さらに１／４ｔ位置の方が早く低温まで冷却される。５５０ＭＰａ級鋼や５９０ＭＰａ級鋼などの高強度鋼材の特性に大きく影響するのは、本鋼種の本製造方法では７００〜５００℃の温度帯の冷却速度であるが、厚肉材の１／４ｔ位置および１／２ｔ位置が７００〜５００℃の温度帯を通過する時間は１／４ｔ位置の方が早く、１／２ｔ位置は遅れてこの温度帯を通過する。   Normally, when a thick steel plate is cooled from the front and back surfaces of the steel plate under a constant cooling condition, it is cooled while having a temperature gradient in the thickness direction, and the cooling rate at the 1/4 t position and 1/2 t position is 1/4 t. The position is faster, and the 1/4 t position is cooled to a lower temperature more quickly. It is the cooling rate in the temperature range of 700 to 500 ° C. in this production method of this steel type that greatly affects the properties of high-strength steel materials such as 550 MPa class steel and 590 MPa class steel. The time at which the 1 / 2t position passes through the temperature zone of 700 to 500 ° C. is earlier at the 1 / 4t position, and the 1 / 2t position passes through this temperature zone with a delay.

そこで、冷却初期段階と冷却後期段階での冷却条件を変え、初期の弱冷却、後期を強冷却とすることにより、７００〜５００℃の温度帯における１／４ｔ位置と１／２ｔ位置の冷却速度差を小さくすることが可能となる。冷却の強弱は、冷却水量密度などで調整する方法や、水冷と非水冷（空冷）の繰り返しの時間間隔を調整することなどにより可能である。なお、冷却速度は少なくとも７００〜５００℃の温度帯での冷却速度であればよく、熱間圧延の終了温度が８００℃超えであれば、８００〜５００℃の温度帯の冷却速度としてもよい。   Therefore, by changing the cooling conditions in the initial cooling stage and the late cooling stage and setting the initial weak cooling and the late cooling to strong cooling, the cooling rates at the 1/4 t position and 1/2 t position in the temperature range of 700 to 500 ° C. The difference can be reduced. The strength of the cooling can be adjusted by adjusting the cooling water density or the like, or by adjusting the time interval between water cooling and non-water cooling (air cooling). In addition, the cooling rate should just be a cooling rate in the temperature range of at least 700-500 degreeC, and if the completion | finish temperature of hot rolling exceeds 800 degreeC, it is good also as a cooling rate in the temperature range of 800-500 degreeC.

上述したように、本発明では、高強度かつ低降伏比を満足し、さらに高靱性を達成するために、スラブを１０００〜１３００℃に加熱し、熱間圧延した後、５００℃以下の温度まで水冷により加速冷却する製造プロセスを採用する。
以下、これらの限定理由について説明する。 As described above, in the present invention, the slab is heated to 1000 to 1300 ° C. and hot-rolled to a temperature of 500 ° C. or lower in order to satisfy high strength and low yield ratio and achieve high toughness. A manufacturing process that uses accelerated cooling with water cooling will be adopted.
Hereinafter, these reasons for limitation will be described.

圧延加熱温度：１０００〜１３００℃
圧延加熱温度は、強度確保の観点から１０００℃以上とするが、加熱温度が高くなりすぎると靱性が劣化するため、上限を１３００℃とする。好ましくは１０５０〜１２００℃である。 Rolling heating temperature: 1000-1300 ° C
The rolling heating temperature is set to 1000 ° C. or more from the viewpoint of securing the strength, but if the heating temperature becomes too high, the toughness deteriorates, so the upper limit is set to 1300 ° C. Preferably it is 1050-1200 degreeC.

冷却停止温度：５００℃以下
冷却停止温度を５００℃より高くすると、１／２ｔ位置の冷却速度が遅くなり１／２ｔの強度が低下する。そのため５００℃以下とする。 Cooling stop temperature: 500 ° C. or less When the cooling stop temperature is higher than 500 ° C., the cooling rate at the 1 / 2t position is slowed and the strength at 1 / 2t is lowered. Therefore, it shall be 500 degrees C or less.

圧延温度、圧延仕上げ温度は特に規定しないが、適度な結晶粒径とし靱性を確保するため、圧延仕上げ温度は１０００℃から７５０℃とすることが好ましい。また、加速冷却の冷却速度や冷却方法についても特に規定しないが、冷却速度は、板厚と冷却方法によってその上限が制約される範囲の中で、成分組成や圧延条件により、最適な冷却条件を選択することが好ましい。溶接性などを考慮すると、冷却速度は設定できる最速値に設定し、成分や圧延条件を調整して、目標とする強度を確保することが望ましい。冷却方法につては、鋼板の表裏面からの水冷またはミストによる冷却が考えられるが、鋼板の冷却後の歪みや板内の均一性の確保の観点から、高い水量密度で鋼板表面が核沸騰状態のみとでの冷却が望ましく、冷却能の制御は、水量によって調整することも可能であるが、強水冷と非水冷（空冷）の繰り返しにより冷却を制御する方法が望ましい。   Although the rolling temperature and the rolling finishing temperature are not particularly defined, the rolling finishing temperature is preferably set to 1000 ° C. to 750 ° C. in order to secure an appropriate crystal grain size and ensure toughness. In addition, the cooling rate and cooling method for accelerated cooling are not particularly specified, but the cooling rate is set within the range in which the upper limit is restricted by the plate thickness and cooling method, depending on the component composition and rolling conditions. It is preferable to select. In consideration of weldability and the like, it is desirable to set the cooling rate to the fastest possible value and adjust the components and rolling conditions to ensure the target strength. As for the cooling method, water cooling from the front and back surfaces of the steel sheet or cooling by mist can be considered, but from the viewpoint of ensuring distortion after cooling of the steel sheet and uniformity within the sheet, the surface of the steel sheet is in a nucleate boiling state with a high water density. Cooling only with cooling is desirable, and the cooling capacity can be controlled by the amount of water, but a method of controlling cooling by repeating strong water cooling and non-water cooling (air cooling) is desirable.

この際の冷却制御は、具体的には図２に示すような装置で行うことが可能である。この装置においては、複数個のテーブルロール２上を矢印Ｘ方向に連続的に移送される高温の鋼板１の上面および下面に向け、板幅方向にスリットノズル４，４が設けられており、これによって第１冷却ブロックが構成されている。この第１冷却ブロックに続き、鋼板１の上面および下面に向け設けられた複数個のスプレーノズル５，５によって、第２〜第２０冷却ブロックが構成されている。なお、３はスリットノズル４およびスプレーノズル５に冷却水を供給するヘッダー管である。このような各冷却ブロックは、各々テーブルロール２と、テーブルロール２の上方に、鋼板１を間に挟んで設けられた水切りロール６とによって、隣接する冷却ブロックに冷却水が進入しないように区画されている。スリットノズル４およびスプレーノズル５の各々には、遮断弁７が取り付けられており、この遮断弁７によって所定の冷却ブロックの冷却水をオフにすることにより、水冷と非水冷とを１回以上繰り返す種々のパターンで冷却制御することができる。   The cooling control at this time can be specifically performed by an apparatus as shown in FIG. In this apparatus, slit nozzles 4 and 4 are provided in the plate width direction toward the upper and lower surfaces of the high-temperature steel plate 1 continuously transferred in the direction of the arrow X on the plurality of table rolls 2. The 1st cooling block is comprised by these. Subsequent to the first cooling block, the plurality of spray nozzles 5 and 5 provided toward the upper and lower surfaces of the steel plate 1 constitute second to twentieth cooling blocks. Reference numeral 3 denotes a header pipe that supplies cooling water to the slit nozzle 4 and the spray nozzle 5. Each such cooling block is partitioned so that the cooling water does not enter the adjacent cooling block by the table roll 2 and the draining roll 6 provided above the table roll 2 with the steel plate 1 interposed therebetween. Has been. Each of the slit nozzle 4 and the spray nozzle 5 is provided with a shutoff valve 7. By turning off the cooling water of a predetermined cooling block by the shutoff valve 7, water cooling and non-water cooling are repeated one or more times. Cooling control can be performed in various patterns.

以上の製造条件を満足すれば、鋼板の成分組成は通常のものでよいが、高強度かつ低降伏比、さらには高靱性を得るためには、質量％で、Ｃ：０．０１〜０．１８％、Ｓｉ：０．０１〜０．５％、Ｍｎ：０．３〜２．５％を含有し、残部が実質的にＦｅおよび不可避的不純物からなるものであることが好ましい。さらに、種々の特性を向上させる観点から、Ｃｕ：０．０３〜１％、Ｎｉ：０．０３〜２％、Ｃｒ：０．０３〜０．５％、Ｍｏ：０．０３〜０．５％、Ｖ：０．０２〜０．１５％、Ｔｉ：０．００５〜０．１％、Ｎｂ：０．００５〜０．０５％、Ｂ：０．０００４〜０．００２％、Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００５〜０．００５％、ＲＥＭ：０．０００５〜０．０２％を1種または２種以上を含有させることが好ましい。
以下、各添加元素の含有量の限定理由を説明する。 If the above production conditions are satisfied, the component composition of the steel sheet may be normal, but in order to obtain high strength, low yield ratio, and high toughness, C: 0.01-0. It is preferable that it contains 18%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.5%, with the balance being substantially made of Fe and inevitable impurities. Furthermore, from the viewpoint of improving various properties, Cu: 0.03 to 1%, Ni: 0.03 to 2%, Cr: 0.03 to 0.5%, Mo: 0.03 to 0.5% , V: 0.02-0.15%, Ti: 0.005-0.1%, Nb: 0.005-0.05%, B: 0.0004-0.002%, Ca: 0.0005 ˜0.005%, Mg: 0.0005 to 0.005%, REM: 0.0005 to 0.02% are preferably contained in one kind or two or more kinds.
Hereinafter, the reason for limiting the content of each additive element will be described.

Ｃ：０．０１〜０．１８％
Ｃは鋼の常温強度、高温強度を安定して確保するための有効な元素であるが、Ｃ含有量が高くなると靱性や溶接性を劣化させる。このため、Ｃ含有量を０．０１〜０．１８％とした。 C: 0.01 to 0.18%
C is an effective element for stably securing the normal temperature strength and high temperature strength of steel, but when the C content increases, the toughness and weldability deteriorate. For this reason, C content was made into 0.01 to 0.18%.

Ｓｉ：０．０１〜０．５％
Ｓｉは脱酸元素として有効な元素であり、そのために少なくとも０．０１％以上の添加が必要である。また、Ｓｉは固溶強化に対しても有効な元素であるが、０．５％を超えると延靭性が低下することや、常温強度を必要以上に上げてしまう。このため、Ｓｉ含有量を０．０１〜０．５％とした。 Si: 0.01 to 0.5%
Si is an effective element as a deoxidizing element, and therefore, it is necessary to add at least 0.01% or more. Further, Si is an element effective for solid solution strengthening, but if it exceeds 0.5%, the ductility is lowered and the room temperature strength is increased more than necessary. For this reason, Si content was made into 0.01 to 0.5%.

Ｍｎ：０．３〜２．５％
Ｍｎは強度確保の上で有効な元素であり、そのために０．３％以上の添加が必要である。また、２．５％を超えると溶接性が劣化するとともに常温強度を必要以上に高める。このため、Ｍｎ含有量を０．３〜２．５％とした。 Mn: 0.3 to 2.5%
Mn is an element effective in securing strength, and for that purpose, addition of 0.3% or more is necessary. On the other hand, if it exceeds 2.5%, the weldability deteriorates and the room temperature strength is increased more than necessary. For this reason, Mn content was made into 0.3 to 2.5%.

Ｃｕ：０．０３〜１％
Ｃｕは固溶強化に対し有効な元素であるから、必要に応じて添加することができる。しかし、０．０３％未満ではこのような効果が小さく、１％を超えるとコスト上昇に加えて、鋼板の表面疵の問題があるため、添加する場合は０．０３〜１％の範囲であることが好ましい。 Cu: 0.03 to 1%
Since Cu is an element effective for solid solution strengthening, it can be added as necessary. However, if it is less than 0.03%, such an effect is small, and if it exceeds 1%, there is a problem of surface flaws of the steel sheet in addition to an increase in cost. It is preferable.

Ｎｉ：０．０３〜２％
Ｎｉは低温靭性の向上や焼入性の向上を通して強度の向上に有効な元素であるから必要に応じて添加することができる。しかし、０．０３％未満ではこのような効果が小さく、またＮｉは高価な元素であり、２％を超えるとコストが上昇するため、添加する場合は０．０３〜２％の範囲であることが好ましい。 Ni: 0.03 to 2%
Ni is an element effective in improving strength through improvement of low temperature toughness and hardenability, and therefore can be added as necessary. However, if it is less than 0.03%, such an effect is small, and Ni is an expensive element, and if it exceeds 2%, the cost increases, so when it is added, it should be in the range of 0.03 to 2%. Is preferred.

Ｃｒ：０．０３〜０．５％
Ｃｒは固溶強化により強度を上昇させるのに有効な元素であるから必要に応じて添加することができる。しかし、０．０３％未満ではこのような効果が小さく、また、０．５％を超えて添加すると溶接性を劣化させるので、添加する場合は０．０３〜０．３％の範囲であることが好ましい。 Cr: 0.03-0.5%
Since Cr is an element effective for increasing the strength by solid solution strengthening, it can be added as necessary. However, if it is less than 0.03%, such an effect is small, and if added over 0.5%, the weldability deteriorates, so when added, it should be in the range of 0.03-0.3%. Is preferred.

Ｍｏ：０．０３〜０．５％
Ｍｏは焼入性の向上、析出強化等により鋼の強度を上昇させる有効な元素であるから必要に応じて添加することができる。しかし、０．０３％未満ではこのような効果が小さく、また、０．５％を超えて添加すると、コスト高になる上に溶接性も劣化させ、さらにＹＲが高くなる傾向にあるため、添加する場合は０．０３〜０．５％の範囲であることが好ましい。 Mo: 0.03-0.5%
Mo is an effective element that increases the strength of the steel by improving hardenability, precipitation strengthening, and the like, so that it can be added as necessary. However, if it is less than 0.03%, such an effect is small, and if added over 0.5%, the cost is increased, weldability is deteriorated, and YR tends to be higher. When it is, it is preferable that it is 0.03 to 0.5% of range.

Ｖ：０．０２〜０．１５％
Ｖは焼入性および析出強化により強度を上昇させるのに有効であるから必要に応じて添加することができる。しかし、０．０２％未満ではこのような効果が小さく、また、０．１５％を超えて添加すると溶接性が劣化するため、添加する場合は０．０２〜０．１５％の範囲であることが好ましい。 V: 0.02-0.15%
V is effective in increasing the strength by hardenability and precipitation strengthening, and therefore can be added as necessary. However, if it is less than 0.02%, such an effect is small, and if it exceeds 0.15%, weldability deteriorates, so when it is added, it should be in the range of 0.02 to 0.15%. Is preferred.

Ｔｉ：０．００５〜０．１％
Ｔｉは結晶粒の微細化に有効であり靱性を向上させ、また、ＮをＴｉＮとして固定する効果を有するため、必要に応じて添加することができる。そして、このような効果は、Ｍｏ，Ｎｂ（および／またはＶ）と複合添加することによりさらに大きくなる。しかし、０．００５％未満ではこのような効果が小さく、０．１％を超えると溶接性が劣化するため、添加する場合は０．００５〜０．１％の範囲であることが好ましい。 Ti: 0.005 to 0.1%
Ti is effective for refining crystal grains, improves toughness, and has an effect of fixing N as TiN, so it can be added as necessary. And such an effect becomes still larger by adding together with Mo and Nb (and / or V). However, if it is less than 0.005%, such an effect is small, and if it exceeds 0.1%, the weldability deteriorates. Therefore, when it is added, the content is preferably in the range of 0.005 to 0.1%.

Ｎｂ：０．００５〜０．０５％
Ｎｂは結晶粒の微細化に有効であり靱性を向上させ、また、焼入性の向上、析出強化により強度を上昇させるのに有効であるから必要に応じて添加することができる。しかし、０．００５％未満ではこのような効果が小さく、また、０．０５％を超えて添加すると溶接性を劣化させるとともにＹＲを高めるため、添加する場合は０．００５〜０．０５％の範囲であることが好ましい。 Nb: 0.005 to 0.05%
Nb is effective for refining crystal grains and improves toughness, and it is effective for increasing the strength by improving hardenability and precipitation strengthening, so Nb can be added as necessary. However, if the amount is less than 0.005%, such an effect is small, and if added over 0.05%, the weldability is deteriorated and the YR is increased. A range is preferable.

Ｂ：０．０００３〜０．００２％
Ｂは、焼入性の向上に有効な元素であり、強度を向上させるため、必要に応じて添加することができる。しかし、０．０００３％未満ではその効果が小さく、また、０．００２％を超えて添加すると溶接性を劣化させるため、添加する場合は０．０００３〜０．００２％の範囲であることが好ましい。 B: 0.0003 to 0.002%
B is an element effective for improving hardenability, and can be added as necessary in order to improve the strength. However, if it is less than 0.0003%, the effect is small, and if added over 0.002%, the weldability is deteriorated. Therefore, when added, the content is preferably in the range of 0.0003 to 0.002%. .

Ｃａ：０．０００５〜０．００５％
Ｍｇ：０．０００５〜０．００５％
ＲＥＭ：０．０００５〜０．０２％
Ｃａ、Ｍｇ、ＲＥＭは介在物の形態制御やＳ等の不純物元素の固定を通して靭性の向上などに有効であるから必要に応じて添加することができる。そして、これらを添加する場合は、このような特性を有効に発揮することができる観点から、Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００５〜０．００５％、ＲＥＭ：０．０００５〜０．０２％の範囲であることが好ましい。 Ca: 0.0005 to 0.005%
Mg: 0.0005 to 0.005%
REM: 0.0005 to 0.02%
Ca, Mg, and REM can be added as necessary because they are effective in controlling the form of inclusions and improving the toughness through fixing of impurity elements such as S. And when adding these, from a viewpoint which can exhibit such a characteristic effectively, Ca: 0.0005-0.005%, Mg: 0.0005-0.005%, REM: 0.00. A range of 0005 to 0.02% is preferred.

残部は実質的にＦｅであり、不可避的に混入する不純物元素や、他の微量添加元素は許容される。例えば、Ｐ、Ｓは不純物元素であり、延靭性の低下、加工性、溶接性の低下等の問題の原因となる元素であり、できるだけ低減することが望ましい。しかしながら、著しく低減するのはコストの上昇を招くため、大きな材質劣化を及ぼさない範囲として、通常０．０３％以下としている。また、Ａｌは脱酸元素であり、ｓｏｌ.ＡｌはＡｌＮとして鋼中に析出し、結晶粒の微細化に有効であるが、過剰に添加すると介在物が多くなり延靭性が劣化するため、通常０．０７％以下としている。さらに、ＮはＡｌＮとして析出し結晶粒の微細化に有効であるが、大量添加では溶接部の靭性が劣化するため、通常０．００１０〜０．０２０％としている。   The balance is substantially Fe, and impurity elements inevitably mixed in and other trace additive elements are allowed. For example, P and S are impurity elements that cause problems such as a reduction in ductility, workability, and weldability, and it is desirable to reduce them as much as possible. However, a significant reduction leads to an increase in cost. Therefore, the range that does not cause significant material deterioration is usually 0.03% or less. In addition, Al is a deoxidizing element, and sol.Al is precipitated in the steel as AlN and is effective in refining crystal grains. However, if added excessively, inclusions increase and ductility deteriorates. 0.07% or less. Further, N precipitates as AlN and is effective for refining crystal grains. However, when added in a large amount, the toughness of the welded portion deteriorates, so the content is usually 0.0010 to 0.020%.

以下、本発明の実施例について説明する。
表１に実施例の供試鋼の成分組成を示す。鋼１は合金元素としてＳｉ、Ｍｎ以外を添加しない成分系であり、鋼２〜７は合金元素としてＳｉ、Ｍｎの他、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｖ、Ｎｂ、Ｔｉ、Ｂ、Ｃａ、Ｍｇ、ＲＥＭの１種または２種以上を添加した成分系である。 Examples of the present invention will be described below.
Table 1 shows the component compositions of the test steels of the examples. Steel 1 is a component system that does not add other than Si and Mn as alloy elements, and Steels 2 to 7 have Cu, Ni, Cr, Mo, V, Nb, Ti, B, Ca, Si, Mn as alloy elements. It is a component system to which one or more of Mg and REM are added.

これら供試鋼について表２に示す製造条件で種々の板厚の鋼板を製造した。その際の機械的性質を合わせて表２に示す。また、表３に、各板厚で冷却条件を変えたときの板厚１／４ｔ位置と１／２ｔ位置の冷却速度の関係を示す。表３の符号Ａ、Ｃは通常の冷却パターンであり、符号Ｂ１，Ｂ２，Ｄが１／４ｔ位置と１／２ｔ位置の冷却速度の差が小さくなるように制御した本発明のプロセスである。表３に示すように、冷却パターンＡ、Ｃでは、１／４ｔと１／２ｔの冷却速度比が１．１５以上（差が１５％以上）であるのに対し、本発明の冷却パターンであるＢ１，Ｂ２，Ｄでは冷却速度比が１．１５以下（差が１５％以下）となっている。表２の冷却パターンの符号がこの表３のＡ〜Ｄに対応し、表２には鋼１〜７にＡ〜Ｄの冷却パターンを適用した場合の機械的性質を示している。なお、表２の符号は、１〜７の鋼番の末尾に表３の冷却パターンの符号Ａ〜Ｄをつけたものである。   For these test steels, steel sheets having various thicknesses were manufactured under the manufacturing conditions shown in Table 2. The mechanical properties at that time are shown together in Table 2. Table 3 shows the relationship between the cooling speeds at the 1/4 t position and 1/2 t position when the cooling condition is changed for each thickness. Symbols A and C in Table 3 are normal cooling patterns, and symbols B1, B2, and D are processes according to the present invention in which the difference between the cooling rates at the 1 / 4t position and the 1 / 2t position is controlled to be small. As shown in Table 3, in the cooling patterns A and C, the cooling rate ratio of 1 / 4t and 1 / 2t is 1.15 or more (the difference is 15% or more), whereas the cooling pattern of the present invention. In B1, B2, and D, the cooling rate ratio is 1.15 or less (the difference is 15% or less). The symbols of the cooling patterns in Table 2 correspond to AD in Table 3, and Table 2 shows mechanical properties when the cooling patterns A to D are applied to steels 1 to 7. In addition, the code | symbol of Table 2 attaches | subjects code | symbol AD of the cooling pattern of Table 3 to the end of the steel numbers of 1-7.

ここでは、５５０ＭＰａ級鋼（ＹＳ≧３８５ＭＰａ、ＴＳ≧５５０ＭＰａ）および５９０ＭＰａ級鋼（ＹＳ≧４４０ＭＰａ、ＴＳ≧５９０ＭＰａ）を対象としており、表２では、５５０ＭＰａ級鋼または５９０ＭＰａ級鋼の目標性能（強度下限値は５５０ＭＰａ級鋼、強度上限値は５９０ＭＰａ級鋼）で性能の評価を行っている。   Here, 550 MPa grade steel (YS ≧ 385 MPa, TS ≧ 550 MPa) and 590 MPa grade steel (YS ≧ 440 MPa, TS ≧ 590 MPa) are targeted, and in Table 2, target performance (lower strength limit) of 550 MPa grade steel or 590 MPa grade steel The performance is evaluated with a value of 550 MPa class steel and a strength upper limit value of 590 MPa class steel).

表２に示すように、鋼板１Ａは、鋼１を用いて通常の冷却条件Ａで冷却した場合であり、１／２ｔの性能は目標を満足しているが、１／４ｔではＹＲが高くなっており、１／４ｔと１／２ｔの性能の両立ができていない。一方、鋼板１Ｂ１および１Ｂ２は、１／４ｔ位置と１／２ｔ位置の冷却速度差を小さくする冷却制御を行っており、１／４ｔ、１／２ｔのいずれにおいても目標性能を満足している。また、鋼１Ｂ２は水量調整により冷却速度を制御しているのに対し、鋼１Ｂ１は水冷と非水冷を繰り返すことにより冷却速度を制御しており、鋼１Ｂ１の方が、表層と１／２ｔ位置の硬さの差（△ＨＶ１０）がより小さくなっており、より板厚方向特性のばらつきが小さくなっている。   As shown in Table 2, the steel plate 1A is a case where the steel 1 is cooled under the normal cooling condition A, and the performance of 1 / 2t satisfies the target, but the YR becomes high at 1 / 4t. In other words, the performances of 1 / 4t and 1 / 2t are not compatible. On the other hand, the steel plates 1B1 and 1B2 perform cooling control to reduce the cooling rate difference between the 1 / 4t position and the 1 / 2t position, and satisfy the target performance at both 1 / 4t and 1 / 2t. Steel 1B2 controls the cooling rate by adjusting the amount of water, whereas Steel 1B1 controls the cooling rate by repeating water cooling and non-water cooling. The difference in hardness (ΔHV10) is smaller, and the variation in the thickness direction characteristics is smaller.

鋼板２Ｄ、３Ｄ、４Ｄ、７Ｄは、冷却パターンがＤであり、成分・板厚に応じて冷却速度の制御を行っており、１／４ｔ、１／２ｔともに目標性能を満足している。   The steel plates 2D, 3D, 4D, and 7D have a cooling pattern D, and the cooling rate is controlled according to the component and the plate thickness, and both 1 / 4t and 1 / 2t satisfy the target performance.

鋼板５Ａは、鋼５を用いて通常の冷却条件Ａで冷却した場合であり、１／２ｔの性能は目標を満足しているが、１／４ｔではＹＳおよびＹＲが高くなっており、１／４ｔと１／２ｔの性能の両立ができていない。一方、鋼板５Ｂ１は、１／４ｔ位置と１／２ｔ位置の冷却速度差を小さくする冷却制御を行っており、１／４ｔ、１／２ｔいずれにおいても目標性能を満足している。   The steel plate 5A is a case where the steel 5 is cooled under the normal cooling condition A, and the performance of 1 / 2t satisfies the target, but YS and YR are high at 1 / 4t. The performance of 4t and 1 / 2t is not compatible. On the other hand, the steel plate 5B1 performs cooling control to reduce the cooling rate difference between the 1 / 4t position and the 1 / 2t position, and satisfies the target performance at both 1 / 4t and 1 / 2t.

鋼板６Ｃは、鋼６を用いて通常の冷却条件Ｃで冷却した場合であり、１／２ｔの性能は目標を満足しているが、１／４ｔではＹＲが高くなっており、１／４ｔと１／２ｔの性能の両立ができていない。一方、鋼板６Ｄは、１／４ｔ位置と１／２ｔ位置の冷却速度差を小さくする冷却制御を行っており、１／４ｔ、１／２ｔいずれにおいても目標性能を満足している。   The steel plate 6C is a case where the steel 6 is cooled under the normal cooling condition C, and the performance of 1 / 2t satisfies the target, but the YR is high at 1 / 4t, and 1 / 4t 1 / 2t of performance is not compatible. On the other hand, the steel plate 6D performs cooling control to reduce the cooling rate difference between the 1 / 4t position and the 1 / 2t position, and satisfies the target performance at both 1 / 4t and 1 / 2t.

Figure 0004830330
Figure 0004830330

Figure 0004830330
Figure 0004830330

Figure 0004830330
Figure 0004830330

本発明によれば、板厚方向の特性差が小さく１／４ｔ位置と１／２ｔ位置の双方において低降伏比かつ高強度を達成することができる板厚４０ｍｍ以上の低降伏比高張力鋼板が得られるので、橋梁用、土木用、建築用の鋼材として有効であり、工業的に利用価値が高い。   According to the present invention, there is provided a low-yield-ratio high-tensile steel plate having a thickness of 40 mm or more that has a small characteristic difference in the thickness direction and can achieve a low yield ratio and high strength at both the 1 / 4t position and the 1 / 2t position. Since it is obtained, it is effective as a steel material for bridges, civil engineering, and construction, and its industrial utility value is high.

冷却速度を変化させた場合の、鋼板の降伏比ＹＲ、引張強度ＴＳ、降伏強度ＹＳを示す図。The figure which shows the yield ratio YR, tensile strength TS, and yield strength YS of a steel plate at the time of changing a cooling rate. 冷却制御を行う装置を示す図。The figure which shows the apparatus which performs cooling control.

Claims (3)

スラブを１０００〜１３００℃に加熱し、熱間圧延した後、５００℃以下の温度まで水冷により加速冷却して、質量％で、Ｃ：０．０１〜０．１８％、Ｓｉ：０．０１〜０．５％、Ｍｎ：０．３〜２．５％を含有し、残部がＦｅおよび不可避不純物からなる板厚４０ｍｍ以上の厚肉高張力鋼板を製造するにあたり、加速冷却時の板厚方向１／４ｔ位置と板厚方向１／２ｔ位置の少なくとも７００〜５００℃の温度帯の冷却速度の差が１５％以内となるように鋼板の表面からの冷却を冷却初期よりも冷却後期のほうが強冷却に制御することを特徴とする厚肉低降伏比高張力鋼板の製造方法。 The slab is heated to 1000 to 1300 ° C., hot-rolled, and then accelerated and cooled by water cooling to a temperature of 500 ° C. or lower. In mass%, C: 0.01 to 0.18%, Si: 0.01 to 0.5%, Mn: 0.3-2.5% content, the thickness direction of accelerated cooling 1 in the production of a thick high-tensile steel plate having a thickness of 40 mm or more consisting of Fe and inevitable impurities. The cooling from the surface of the steel sheet is stronger in the later stage of cooling than in the early stage of cooling so that the difference in the cooling rate at the temperature zone of at least 700 to 500 ° C. between the / 4t position and the 1 / 2t position in the thickness direction is within 15%. A method for producing a thick-walled, low-yield-ratio, high-tensile steel sheet, characterized in that スラブを１０００〜１３００℃に加熱し、熱間圧延した後、５００℃以下の温度まで水冷により加速冷却して、質量％で、Ｃ：０．０１〜０．１８％、Ｓｉ：０．０１〜０．５％、Ｍｎ：０．３〜２．５％を含有し、さらに、Ｃｕ：０．０３〜１％、Ｎｉ：０．０３〜２％、Ｃｒ：０．０３〜０．５％、Ｍｏ：０．０３〜０．５％、Ｖ：０．０２〜０．１５％、Ｔｉ：０．００５〜０．１％、Ｎｂ：０．００５〜０．０５％、Ｂ：０．０００４〜０．００２％、Ｃａ：０．０００５〜０．００５％、Ｍｇ：０．０００５〜０．００５％、ＲＥＭ：０．０００５〜０．０２％を1種または２種以上含有し、残部がＦｅおよび不可避不純物からなる板厚４０ｍｍ以上の厚肉高張力鋼板を製造するにあたり、加速冷却時の板厚方向１／４ｔ位置と板厚方向１／２ｔ位置の少なくとも７００〜５００℃の温度帯の冷却速度の差が１５％以内となるように鋼板の表面からの冷却を冷却初期よりも冷却後期のほうが強冷却に制御することを特徴とする厚肉低降伏比高張力鋼板の製造方法。 The slab is heated to 1000 to 1300 ° C., hot-rolled, and then accelerated and cooled by water cooling to a temperature of 500 ° C. or lower. In mass%, C: 0.01 to 0.18%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.5%, further Cu: 0.03 to 1%, Ni: 0.03 to 2%, Cr: 0.03 to 0.5%, Mo: 0.03-0.5%, V: 0.02-0.15%, Ti: 0.005-0.1%, Nb: 0.005-0.05%, B: 0.0004- 0.002%, Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005%, REM: 0.0005 to 0.02%, or one or more, and the balance is Fe When manufacturing a thick high-tensile steel plate with an inevitable impurity thickness of 40 mm or more, the thickness direction 1 / 4t position and plate thickness during accelerated cooling The cooling from the surface of the steel sheet is controlled to be stronger in the later stage of cooling than in the early stage of cooling so that the difference in the cooling rate in the temperature zone of at least 700 to 500 ° C. in the direction 1 / 2t position is within 15%. A method of manufacturing a thick-walled low yield ratio high-tensile steel sheet. 前記加速冷却は、冷却水量密度を調整することにより、または水冷と非水冷の繰り返しの時間間隔を調整することにより、鋼板表面からの冷却を冷却初期よりも冷却後期のほうが強冷却に制御することを特徴とする請求項１または請求項２に記載の厚肉低降伏比高張力鋼板の製造方法。 In the accelerated cooling, the cooling from the steel sheet surface is controlled to be stronger in the late cooling stage than in the early cooling stage by adjusting the cooling water density or by adjusting the time interval between repeated water cooling and non-water cooling. The manufacturing method of the thick-walled low yield ratio high-tensile steel sheet according to claim 1 or 2 .
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