JP5811044B2 - Thick high-strength steel sheet excellent in weldability and weld heat-affected zone toughness and method for producing the same - Google Patents
Thick high-strength steel sheet excellent in weldability and weld heat-affected zone toughness and method for producing the same Download PDFInfo
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本発明は、海洋構造物等の高い安全性を要求される大型溶接構造物用として溶接性、溶接熱影響部靭性に優れる厚手高強度鋼板およびその製造方法に関するものである。 The present invention relates to a thick high-strength steel sheet excellent in weldability and weld heat-affected zone toughness for large-sized welded structures that require high safety such as offshore structures, and a method for producing the same.
近年、世界的に旺盛なエネルギー需要に呼応して、石油・天然ガス等海洋資源開発が活発化している。それとともに、掘削、生産の効率化や開発環境の苛酷化などにより、海洋構造物の大型化が指向され、鋼材に対しては厚手化、高強度化が求められてきている。加えて、洋上に設置される海洋構造物は破壊に対する高い安全性も求められ、鋼材に対し優れた溶接性、溶接熱影響部靭性が要求される。 In recent years, the development of marine resources such as oil and natural gas has been activated in response to the global demand for energy. At the same time, due to excavation and production efficiency and severe development environment, the size of offshore structures is increasing, and steel materials are required to be thicker and stronger. In addition, offshore structures installed on the ocean are also required to have high safety against destruction, and excellent weldability and weld heat affected zone toughness are required for steel materials.
一般に、鋼材の溶接性、溶接熱影響部靭性は、厚手、高強度になるほど不利になる傾向にある。強度確保上、高成分とならざるを得ないためである。溶接性というのは広い意味を持つが、狭義には溶接熱影響部の硬化性や溶接冷間割れ性を表し、各種の炭素当量Ｃｅｑや溶接割れ感受性組成ＰＣＭなど成分パラメータで表されることが多い。高成分ほどこれら指標は高くなり、溶接熱影響部の硬化性や溶接冷間割れ感受性が高まり、一般に溶接性が劣るとされる。溶接熱影響部靭性は、これら溶接性の指標の大小と必ずしも完全に一致するものではないが、高い相関があることは広く知られている。そのため、『溶接性』と言う場合、広義には溶接熱影響部靭性をも含まれるケースもある。 In general, the weldability and weld heat affected zone toughness of steel materials tend to be disadvantageous as they become thicker and higher in strength. This is because it has to be a high component for securing the strength. Although a broad sense because weldability, heat affected zone of the represent curability and welding cold cracking resistance, be represented by various component parameters such as carbon equivalent Ceq and weld cracking susceptibility composition P CM of narrowly There are many. The higher the component, the higher the index, and the hardenability of the weld heat-affected zone and the weld cold cracking sensitivity increase, and the weldability is generally inferior. It is well known that the weld heat-affected zone toughness does not necessarily completely match the magnitude of these weldability indices, but has a high correlation. Therefore, in the case of “weldability”, in a broad sense, there are cases in which the weld heat affected zone toughness is also included.
上述してきたように、通常、鋼の厚手化および／または高強度化は、溶接性、溶接熱影響部靭性を高める方向性とは相反し、これら相反する鋼材特性を両立させる成分設計、製造技術が課題となっていた。 As described above, steel thickening and / or high strength is usually contrary to the direction of improving weldability and weld heat affected zone toughness, and component design and manufacturing technology that achieves these conflicting steel properties. Was an issue.
溶接性を損ねず、換言すれば鋼成分を必要以上に高めることなく鋼の厚手化および／または高強度化を達成する手段としては、加工熱処理、すなわちＴＭＣＰ（Ｔｈｅｒｍｏ−ｍａｃｈａｎｉｃａｌ ｃｏｎｔｒｏｌ ｐｒｏｃｅｓｓ）やＢ（ボロン）添加鋼の調質処理（焼入−焼戻処理）があった（例えば、特許文献１参照）。しかし、それら手段によっても十分ではないこともまた事実である。 As a means for achieving thickening and / or high strength of steel without impairing the weldability, in other words, without increasing the steel components more than necessary, thermomechanical processing, that is, TMCP (Thermo-mechanical control process) or B ( There was a tempering treatment (quenching-tempering treatment) of boron-added steel (for example, see Patent Document 1). However, it is also true that these measures are not sufficient.
ＴＭＣＰは、加熱−圧延−冷却に至る鋼材製造プロセス全般を制御するもので、厚手材においては圧延後、加速冷却あるいは制御冷却とも呼ばれる水冷プロセスが高強度化に有効である。しかし、冷却は伝熱という物理現象のため、厚手材の板厚中心部は水冷によっても十分な冷却速度が得られず、厚手材で高強度を低成分で確保することは困難であった。 TMCP controls the entire steel manufacturing process from heating to rolling to cooling. For thick materials, a water cooling process called accelerated cooling or controlled cooling is effective for increasing strength after rolling. However, because cooling is a physical phenomenon of heat transfer, a sufficient cooling rate cannot be obtained even with water cooling at the center of the plate thickness of the thick material, and it has been difficult to ensure high strength and low components with the thick material.
一方、高強度調質鋼で用いられるＢ（ボロン）は、旧オーステナイト粒界に固溶状態で偏析することでｐｐｍオーダーの極微量でも鋼の焼入性を著しく高めることが知られ、高強度化に有効である。しかし、このことは同時に溶接熱影響部の硬化性を著しく高めることにもなる。とりわけ高い安全性（溶接熱影響部の高い破壊靭性）が求められる海洋構造物では、建造時の溶接入熱が比較的低く制限されており、その硬化性は一段と高まる。溶接熱影響部の硬化性は、前述したように溶接割れ感受性や溶接熱影響部靭性とも高い相関を有し、Ｂ（ボロン）を無条件に活用することには問題があった。また、Ｂ（ボロン）の高い焼入性を活用する場合、その効果はＢ（ボロン）が固溶状態で存在して初めて発揮するため、ボロン化合物の析出を制御する成分、プロセス制御が不可欠であり、ＴＭＣＰとの組み合わせでは、調質処理での知見がそのまま適用できないケースがあった。だからといって調質処理、すなわち焼入−焼戻処理で製造することは、熱処理の工期やコストの面でＴＭＣＰとの比較上不利である。さらに近年では、環境負荷、省エネルギーの観点からも、非調質すなわちＴＭＣＰ化が社会的要請となりつつあるのが実情である。 On the other hand, B (boron) used in high-strength tempered steel is known to significantly increase the hardenability of steel even in a very small amount on the order of ppm by segregating in the solid solution state at the prior austenite grain boundaries. It is effective for conversion. However, this also significantly increases the curability of the weld heat affected zone. In particular, in an offshore structure that requires high safety (high fracture toughness of the weld heat affected zone), the welding heat input during construction is limited to a relatively low level, and its curability is further enhanced. As described above, the curability of the weld heat affected zone has a high correlation with the weld crack sensitivity and the weld heat affected zone toughness, and there is a problem in unconditionally utilizing B (boron). In addition, when utilizing the high hardenability of B (boron), the effect is exhibited only when B (boron) exists in a solid solution state. Therefore, it is indispensable to control the precipitation of boron compounds and process control. In some cases, in combination with TMCP, knowledge in the tempering treatment cannot be applied as it is. However, manufacturing by tempering treatment, that is, quenching-tempering treatment, is disadvantageous in comparison with TMCP in terms of heat treatment period and cost. Furthermore, in recent years, from the viewpoint of environmental load and energy saving, non-tempering, ie, TMCP conversion, is becoming a social requirement.
特許文献１にＢ添加のＴＭＣＰ鋼が開示されているが、Ｃｕ、Ｎｉ量が低いことに加え、特性として溶接入熱５ｋＪ／ｍｍ相当の再現熱サイクル（しかも、実溶接継手における多層溶接を模擬したものか否かも不明）におけるシャルピー衝撃吸収エネルギー値が開示されているにすぎず、実溶接継手部に対してもより厳格なＣＴＯＤ特性が要求される海洋構造物用鋼板を対象としたものではないことは明らかである。 Patent Document 1 discloses B-added TMCP steel. In addition to its low Cu and Ni contents, it has a reproducible thermal cycle equivalent to a welding heat input of 5 kJ / mm (and simulates multi-layer welding in an actual weld joint). Only the Charpy impact absorption energy value is disclosed, and it is not intended for steel plates for offshore structures that require stricter CTOD characteristics for actual welded joints. Clearly not.
このような中で、後述する本願発明の主たるターゲットと同等の板厚、降伏強さを有する溶接継手部のき裂開口変位ＣＴＯＤ特性に優れる海洋構造物用鋼としては、例えば特許文献２に０．８％以上の比較的多いＣｕを含有するＣｕ析出型鋼にかかる発明が開示されている。しかし、Ｃｕは単独で多く添加すると、熱間圧延時にＣｕクラックが発生し、製造困難になるという問題がある。 Under such circumstances, as a steel for offshore structures having excellent crack opening displacement CTOD characteristics of a welded joint having a plate thickness and yield strength equivalent to the main target of the present invention described later, for example, Patent Document 2 discloses 0. An invention relating to a Cu precipitation steel containing a relatively large amount of Cu of 8% or more is disclosed. However, if a large amount of Cu is added alone, there is a problem that Cu cracks are generated during hot rolling, which makes manufacturing difficult.
本発明は、海洋構造物等の高い安全性を要求される大型溶接構造物用として溶接性、溶接熱影響部靭性に優れる厚手高強度鋼板およびその製造方法を提供するものである。 The present invention provides a thick high-strength steel sheet excellent in weldability and weld heat affected zone toughness for use in large welded structures that require high safety such as offshore structures, and a method for producing the same.
本発明の溶接性、溶接熱影響部靭性に優れる厚手高強度鋼板としての主たるターゲットは、板厚５０〜１００ｍｍ、引張強さ６００〜８００ＭＰａ、降伏強さ５００〜６５０ＭＰａ級鋼で、用途は溶接継手部（溶接熱影響部）のき裂開口変位ＣＴＯＤ（ｃｒａｃｋ−ｔｉｐ ｏｐｅｎｉｎｇ ｄｉｓｐｌａｃｅｍｅｎｔ）特性として最低ＣＴＯＤ値０．５０ｍｍ以上が要求される海洋構造物用鋼板である。用途は特に限定するものではなく、溶接熱影響部靭性評価としては、シャルピー衝撃特性と比較し、ＣＴＯＤ特性の方がより厳しい評価法と考え、海洋構造物用鋼を主たるターゲットとしたものである。したがって、本発明は、船舶、鉄骨、橋梁、各種のタンクなど広く溶接構造物用鋼として適用できることはいうまでもない。 The main target as a thick high-strength steel plate excellent in weldability and weld heat-affected zone toughness of the present invention is a steel plate having a thickness of 50 to 100 mm, a tensile strength of 600 to 800 MPa, and a yield strength of 500 to 650 MPa. It is a steel plate for marine structures that requires a minimum CTOD value of 0.50 mm or more as a crack opening displacement CTOD (crack-tip opening displacement) characteristic of the zone (welding heat affected zone). The application is not particularly limited, and the toughness evaluation of the weld heat affected zone is considered to be a more rigorous evaluation method for CTOD characteristics compared to Charpy impact characteristics, and the main target is steel for marine structures. . Therefore, it goes without saying that the present invention can be widely applied as steel for welded structures such as ships, steel frames, bridges, and various tanks.
背景技術で指摘した各種の問題点、課題を解決するため、ＴＭＣＰ前提で、Ｂ（ボロン）を有効に活用する方法を鋭意探索、検討し、溶接性を損ねず、溶接熱影響部靭性を向上させる最良の手段を考案した。主なポイントは、（ａ）固溶Ｂ（ボロン）確保のためのＢ−Ｎ−Ｔｉ量バランスを適正化、（ｂ）（固溶）Ｂによる溶接熱影響部の硬化性を緩和するための極低Ｃ化、（ｃ）強度と溶接性、溶接熱影響部靭性確保のためのＰＣＭ適正化、（ｄ）Ｂ（ボロン）添加鋼において溶接熱影響部の過剰な硬化性を緩和するためのＭｇとＣａによる複合脱酸技術を用いた結晶粒微細化、（ｅ）溶接熱影響部靭性確保のためのＡｌレス脱酸化、（ｆ）粗大酸化物抑制のためのＡｌレス下での低Ｏ（酸素）化などである。これらポイントは、独立事象ではなく互いに密接な関係があるため同時に達成することは容易ではなく、本発明者らの系統的で緻密な実験により初めて実現できたものである。 In order to solve the various problems and issues pointed out in the background art, we eagerly searched and examined methods for effectively using B (boron) under the TMCP premise, improving weld heat affected zone toughness without impairing weldability I devised the best way to make it happen. The main points are (a) optimizing the B—N—Ti amount balance for securing the solid solution B (boron), and (b) relaxing the curability of the weld heat affected zone by the (solid solution) B. ultra-low C content, (c) strength and weldability, P CM optimized for weld heat-affected zone toughness ensured, (d) B (boron) to mitigate excessive hardening of the HAZ in steels Grain refinement using composite deoxidation technology of Mg and Ca, (e) Al-less deoxidation for securing weld heat-affected zone toughness, (f) Low under Al-less for suppressing coarse oxide For example, O (oxygenation). Since these points are not independent events but are closely related to each other, it is not easy to achieve them at the same time, and can be realized for the first time by our systematic and precise experiments.
本発明の要旨は以下のとおりである。 The gist of the present invention is as follows.
（１） 質量％で、
Ｃ：０．０１５〜０．０４５％、
Ｓｉ：０．１５％以下、
Ｍｎ：１．８０〜２．２０％、
Ｐ：０．００８％以下、
Ｓ：０．００５％以下、
Ｃｕ：０．４０〜０．７０％、
Ｎｉ：０．８０〜１．８０％、
Ｎｂ：０.００５〜０．０１５％、
Ｍｏ：０．０５〜０．２５％、
Ｔｉ：０．００５〜０．０１５％、
Ｍｇ：０．０００３〜０．００３％、
Ｃａ：０．０００３〜０．００３％、
Ｂ：０．０００４〜０．００２０％、
Ｎ：０．００２０〜０．００６０％、
Ｏ：０．００１５〜０．００３５％、
Ａｌ：０．００４％以下、
を含有し、残部が鉄および不可避的不純物の成分からなり、
Ｍｇ＋Ｃａ≦０．００５％、
Ｎｉ／Ｃｕ＞２．０、
Ｎ−Ｔｉ／３．４≧０％、
Ｂ−０．８５（Ｎ−Ｔｉ／３．４）≧０．０００４％、
下記式（１）で示すＰＣＭが０．１８〜０．２３％
を満足することを特徴とする、引張強さ６００〜８００ＭＰａ、降伏強さ５００〜６５０ＭＰａ、ＢＳ（Ｂｒｉｔｉｓｈ Ｓｔａｎｄａｒｄｓ）規格５７６２に準拠した試験温度−２０℃でのＣＴＯＤ試験により測定された、溶接熱影響部のき裂開口変位の最低ＣＴＯＤ値０．５０ｍｍ以上の特性を有する溶接性、溶接熱影響部靭性に優れた厚手高強度鋼板。
ＰＣＭ＝Ｃ＋Ｓｉ／３０＋Ｍｎ／２０＋Ｃｕ／２０＋Ｎｉ／６０＋Ｍｏ／１５＋５Ｂ
・・・・ 式（１）
ここで、各元素は鋼中に含有されている質量％である。
(1) In mass%,
C: 0.015-0.045%,
Si: 0.15% or less,
Mn: 1.80 to 2.20%
P: 0.008% or less,
S: 0.005% or less,
Cu: 0.40 to 0.70%,
Ni: 0.80 to 1.80%,
Nb: 0.005 to 0.015%,
Mo: 0.05 to 0.25%,
Ti: 0.005 to 0.015%,
Mg: 0.0003 to 0.003%,
Ca: 0.0003 to 0.003%,
B: 0.0004 to 0.0020%,
N: 0.0020 to 0.0060%,
O: 0.0015 to 0.0035%,
Al: 0.004% or less,
The balance is composed of iron and inevitable impurities,
Mg + Ca ≦ 0.005%,
Ni / Cu> 2.0,
N-Ti / 3.4 ≧ 0%,
B-0.85 (N-Ti / 3.4) ≧ 0.0004%,
P CM represented by the following formula (1) is 0.18 to 0.23 percent
Welding heat effect measured by CTOD test at a test temperature of −20 ° C. in accordance with BS (British Standards) standard 5762, which has a tensile strength of 600 to 800 MPa, a yield strength of 500 to 650 MPa. Thick high-strength steel sheet with excellent weldability and weld heat-affected zone toughness having a minimum CTOD value of 0.50 mm or more for crack opening displacement at the joint.
P CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Mo / 15 + 5B
.... Formula (1)
Here, each element is the mass% contained in steel.
（２） 上記（１）に記載の鋼成分を有する鋼片または鋳片を、１０００〜１１００℃の温度に加熱後、９５０℃以上の温度での累積圧下量が３０％以上、７２０〜９５０℃の温度で累積圧下量が４０％以上で、累積総圧下量が６０％以上として７００〜７５０℃の温度で圧延を終了し、圧延終了後８０秒以内に水冷を開始して２８０℃以下まで冷却し、その後さらに４００〜５５０℃の温度範囲で焼戻しすることを特徴とする、引張強さ６００〜８００ＭＰａ、降伏強さ５００〜６５０ＭＰａ、ＢＳ（Ｂｒｉｔｉｓｈ Ｓｔａｎｄａｒｄｓ）規格５７６２に準拠した試験温度−２０℃でのＣＴＯＤ試験により測定された、溶接熱影響部のき裂開口変位の最低ＣＴＯＤ値０．５０ｍｍ以上の特性を有する溶接性、溶接熱影響部靭性に優れた厚手高強度鋼板の製造方法。 (2) After heating the steel slab or slab having the steel component described in (1) above to a temperature of 1000 to 1100 ° C, the cumulative reduction at a temperature of 950 ° C or higher is 30% or more, 720 to 950 ° C. Rolling is completed at a temperature of 700 to 750 ° C. with a cumulative reduction amount of 40% or more and a cumulative total reduction amount of 60% or more at a temperature of 80 ° C., and cooling is started to 80 ° C. within 80 seconds after the completion of rolling. And then tempering in a temperature range of 400 to 550 ° C., characterized by a tensile strength of 600 to 800 MPa, a yield strength of 500 to 650 MPa, and a test temperature of −20 ° C. according to BS (British Standards) standard 5762 thick that the measured by CTOD test, weldability with the lowest CTOD value 0.50mm or more characteristics of the weld heat affected zone of Crack opening displacement, excellent HAZ toughness Method of manufacturing strength steel sheet.
本発明によれば、溶接性、溶接熱影響部靭性に優れた厚手高強度鋼を安価に提供することができ、海洋構造物などの溶接構造物の大型化と同時に、安全性を一段と高めることが可能となった。 According to the present invention, thick high-strength steel excellent in weldability and weld heat-affected zone toughness can be provided at low cost, and at the same time as increasing the size of a welded structure such as an offshore structure, safety can be further enhanced. Became possible.
以下本発明を詳細に説明する。 The present invention will be described in detail below.
ます、本発明の溶接性、溶接熱影響部靭性に優れる厚手高強度鋼板の鋼成分の限定範囲と理由を述べる。ここで記載した％は質量％を意味する。 First, the limited range and reasons for the steel components of the thick high-strength steel sheet excellent in weldability and weld heat affected zone toughness of the present invention will be described. The% described here means mass%.
Ｃ：０．０１５〜０．０４５％
Ｂの高い焼入性を活用する本発明では、溶接熱影響部の過剰な硬化性を抑えるため、Ｃは比較的低く抑える必要がある。しかし、低過ぎると強度補償のため合金元素量を増やさざるを得ず、経済性を失する。合金コストを抑えつつ、本発明のターゲットである厚手材で降伏強さ５００〜５６０ＭＰａ級鋼（鋼種としての強度グレードであって、実際の降伏強さの範囲でない）としての強度を安定して得るために本発明では０．０１５％以上に限定する。経済性の観点からは、０．０２０％以上が好ましく、０．０２５％以上がより好ましい。一方、０．０４５％超では、Ｂ効果と相俟って溶接熱影響部の硬化性が過剰となって溶接熱影響部靭性を劣化させるため、０．０４５％を上限とする。
C: 0.015-0.045%
In the present invention utilizing the high hardenability of B, C must be kept relatively low in order to suppress excessive curability of the weld heat affected zone. However, if it is too low, the amount of alloy elements must be increased to compensate for the strength, and the economy is lost. While keeping the alloy cost low, yield strength 500 to 560 MPa class steel (strength grade as a steel type and not in the range of actual yield strength) is obtained with a thick material which is the target of the present invention. Therefore, in the present invention, it is limited to 0.015% or more. From the economical viewpoint, 0.020% or more is preferable, and 0.025% or more is more preferable. On the other hand, if over 0.045%, combined with the B effect, the curability of the weld heat affected zone becomes excessive and the weld heat affected zone toughness is deteriorated, so 0.045% is made the upper limit.
Ｓｉ：０．１５％以下
Ｓｉは、特に溶接熱影響部で硬くて脆いＭＡ（Ｍａｒｔｅｎｓｉｔｅ−Ａｕｓｔｅｎｉｔｅ）−ｃｏｎｓｔｉｔｕｅｎｔ（以下ＭＡと略記）生成を助長し、溶接熱影響部靭性を劣化させる。このため、Ｓｉは低いほど好ましいが、Ｃ量を比較的低い範囲に限定する本発明においては、０．１５％までの含有であればＭＡ生成量が少なく、溶接熱影響部靭性の観点から許容できる。しかし、溶接構造物用鋼としての多様な溶接条件を勘案すると少ない方が好ましいことは言うまでもなく、０．１２％以下、さらには０．１０％以下に制限することがより好ましい。
Si: 0.15% or less Si promotes the formation of hard and brittle MA (Martensite-Austenite) -constituent (hereinafter abbreviated as MA) in the weld heat affected zone, and degrades the weld heat affected zone toughness. For this reason, Si is preferably as low as possible. However, in the present invention in which the C content is limited to a relatively low range, if it is contained up to 0.15%, the amount of MA produced is small and acceptable from the viewpoint of weld heat affected zone toughness it can. However, it is needless to say that less is preferable in consideration of various welding conditions as steel for welded structures, and it is more preferable to limit the content to 0.12% or less, and further to 0.10% or less.
Ｍｎ：１．８０〜２．２０％
Ｍｎは比較的安価な元素であるが、強度向上効果が大きく、母材および溶接熱影響部の靭性への悪影響も比較的小さい。ＡｌレスＴｉ脱酸とする本発明では、溶接熱影響部靭性を向上させるため、溶接熱影響部においてＴｉ酸化物などを核とした粒内フェライト生成がポイントになるが、その際、Ｍｎも重要な役割を果たしている。それは、Ｔｉ酸化物にＭｎＳが析出し、その近傍にＭｎの希薄域が形成され、マトリックスより変態温度が高くなってフェライト変態を助長・促進するというものである。母材の強度−靭性、溶接熱影響部靭性、さらには合金コストなどを総合的に勘案し、本発明ではＭｎは１．８０％以上に限定する。この下限には冶金上、技術上の臨界的な意味合いはなく、本発明が目的とする優れた特性が発現される範囲内で、成分的な特徴を明確にするために限定したものである。上限については、安価な元素でもあり極力活用したいところであるが、Ｍｎ量が多すぎると連続鋳造スラブの中心偏析やミクロ偏析が助長され、局所的な脆化域が形成され母材あるいは溶接熱影響部靭性を損ねる可能性が高まるため、２．２０％以下に制限する。ただし、この上限の理由としたスラブの中心偏析やミクロ偏析は、今後、鋳造技術の進歩などにより緩和、解消される可能性もあり、冶金的な臨界意義を有する限界ではない。
Mn: 1.80 to 2.20%
Mn is a relatively inexpensive element, but has a large strength improvement effect, and has a relatively small adverse effect on the toughness of the base material and the weld heat affected zone. In the present invention where Al-less Ti deoxidation is used, in order to improve the toughness of the weld heat affected zone, the formation of intragranular ferrite with the Ti oxide as the nucleus in the weld heat affected zone is the point. Plays an important role. That is, MnS is precipitated in the Ti oxide, and a Mn dilute region is formed in the vicinity thereof, and the transformation temperature is higher than that of the matrix to promote and promote the ferrite transformation. Considering comprehensively the strength-toughness of the base metal, the weld heat affected zone toughness, and the alloy cost, Mn is limited to 1.80% or more in the present invention. This lower limit is not critical in terms of metallurgy and technology, and is limited to clarify the component characteristics within the range in which the excellent characteristics intended by the present invention are expressed. As for the upper limit, it is an inexpensive element and we would like to utilize it as much as possible. However, if the amount of Mn is too large, the center segregation and microsegregation of the continuous cast slab are promoted, and a local embrittlement region is formed, which affects the base metal or welding heat. Since possibility of impairing part toughness increases, it limits to 2.20% or less. However, the center segregation and microsegregation of the slab, which are the reasons for this upper limit, may be alleviated and eliminated by the advancement of casting technology and the like, and are not limits having metallurgical critical significance.
Ｐ：０．００８％以下、Ｓ：０．００５％以下
Ｐ、Ｓは、不可避的不純物として含有され、母材靭性、ＨＡＺ靭性からともに少ない方が良いが、工業生産的な制約もあり、それぞれ０．００８％、０．００５％を上限とした。より良好なＨＡＺ靭性を得るために、それぞれＰ：０．００５％以下、Ｓ：０．００３％以下が望ましい。
P: 0.008% or less, S: 0.005% or less P and S are contained as unavoidable impurities, and it is better to reduce both the base material toughness and the HAZ toughness, but there are also restrictions on industrial production. The upper limit was set to 0.008% and 0.005%. In order to obtain better HAZ toughness, P: 0.005% or less and S: 0.003% or less are desirable, respectively.
Ｃｕ：０．４０〜０．７０％
Ｃｕは、母材の強度を向上させる一方で、母材および溶接熱影響部の靭性の劣化程度は比較的小さいため、有用な元素である。本発明がターゲットとする高強度鋼においては、０．４０％以上の添加が好ましい。しかし、Ｃｕは、０．７０％を超えると析出硬化現象を示すようになり、鋼材の材質、特に強度が不連続的に大きく変化してしまう。このため、本発明では、強度変化が連続的で制御しやすい範囲として０．７０％以下に限定する。好ましくは０．６５％以下である。Ｃｕ量を０．７０％以下に限定することで、後述するＮｉ量とも相俟って熱間圧延時のＣｕクラック発生の危険性がほとんどなくなると言う効果も有する。
Cu: 0.40 to 0.70%
While Cu improves the strength of the base material, Cu is a useful element because the degree of deterioration of the toughness of the base material and the weld heat affected zone is relatively small. In the high-strength steel targeted by the present invention, addition of 0.40% or more is preferable. However, if Cu exceeds 0.70%, it will show a precipitation hardening phenomenon, and the material of the steel material, particularly the strength, will change greatly discontinuously. For this reason, in the present invention, the intensity change is limited to 0.70% or less as a continuous and easily controlled range. Preferably it is 0.65% or less. By limiting the amount of Cu to 0.70% or less, there is an effect that there is almost no risk of Cu crack generation during hot rolling in combination with the amount of Ni described later.
Ｎｉ：０．８０〜１．８０％、およびＮｉ／Ｃｕ＞２．０
Ｎｉは、高靭化元素として知られ、溶接熱影響部の靭性の劣化が少なく、母材の強度、靭性を向上させる効果がある。したがって、本発明のような高強度鋼においては、極めて有用な元素で、特に本発明のような極低Ｃでは、合金元素による強度補償が必須であり、少なくとも０．８０％以上含有させる必要がある。しかし、Ｎｉは高価な合金でもあり、含有量は強度、靭性等必要な特性が得られる最小限に抑えることが好ましい。本発明がターゲットとする強度および最大板厚（１００ｍｍ）を考慮した場合、最大１．８０％まで必要であり、これを上限とするが、特性あるいは冶金的な上限ではないことは言うまでもない。なお、前述したようにやや多いＣｕを含有する本発明鋼においては、鋳片のＣｕ割れを抑制するため、ＮｉはＣｕ量の２倍超を含有させることが有効であり、Ｎｉ／Ｃｕ＞２．０に限定する。
Ni: 0.80 to 1.80% and Ni / Cu> 2.0
Ni is known as a toughening element, has little effect on improving the strength and toughness of the base metal, with little deterioration in the toughness of the heat affected zone. Therefore, in high-strength steels such as the present invention, it is an extremely useful element, and particularly at extremely low C as in the present invention, strength compensation with an alloy element is essential, and it is necessary to contain at least 0.80% or more. is there. However, Ni is also an expensive alloy, and it is preferable that the content be kept to a minimum at which necessary characteristics such as strength and toughness can be obtained. In consideration of the target strength and maximum plate thickness (100 mm) of the present invention, a maximum of 1.80% is necessary, and this is the upper limit, but it goes without saying that it is not a characteristic or metallurgical upper limit. As described above, in the steel of the present invention containing a little more Cu, in order to suppress Cu cracking of the slab, it is effective that Ni contains more than twice the amount of Cu, and Ni / Cu> 2 Limited to 0.0.
Ｎｂ：０．００５〜０．０１５％
Ｎｂは、圧延工程でのオーステナイト未再結晶温度域を高温域に広げ、組織の微細化に有効な制御圧延効果を享受する上で有用な元素である。組織の微細化は、強度、靭性をともに向上させる有効な手段である。この効果を確実に享受する上で、少なくとも０．００５％の含有が必要である。このような母材には極めて有用な効果を発現するＮｂも、溶接熱影響部では硬化性を増大させ、ＭＡ生成を助長するなどその靭性には有害である。このため、上限は０．０１５％に抑えなければならない。好ましくは０．０１３％以下、さらに好ましくは０．０１１％以下とすべきである。
Nb: 0.005 to 0.015%
Nb is an element useful for expanding the austenite non-recrystallization temperature range in the rolling process to a high temperature range and enjoying a controlled rolling effect effective for refinement of the structure. Refinement of the structure is an effective means for improving both strength and toughness. In order to surely enjoy this effect, it is necessary to contain at least 0.005%. Nb, which exhibits a very useful effect for such a base material, is also harmful to its toughness, such as increasing the curability in the weld heat affected zone and promoting the formation of MA. For this reason, the upper limit must be suppressed to 0.015%. Preferably it should be 0.013% or less, more preferably 0.011% or less.
Ｍｏ：０．０５〜０．２５％
Ｍｏは、母材の強度向上の観点からはきわめて有効で、本発明のような厚手高強度鋼においては、不可欠の元素である。特に、Ｂを活用する本発明においては、両者を同時に含有することでより一層の焼入性向上効果を発現する。このようなＭｏの優れた効果を享受するためには、少なくとも０．０５％の含有が必要である。しかし、効果が大きいゆえに、多過ぎる添加は硬化性を著しく高め、ＭＡ生成も顕著に助長するため、０．２５％以下に制限する必要がある。０．２０％以下に抑えることがさらに好ましい。
Mo: 0.05-0.25%
Mo is extremely effective from the viewpoint of improving the strength of the base material, and is an indispensable element in the thick high-strength steel as in the present invention. In particular, in the present invention using B, a further effect of improving hardenability is exhibited by containing both at the same time. In order to enjoy such excellent effects of Mo, it is necessary to contain at least 0.05%. However, since the effect is great, too much addition remarkably increases curability and remarkably promotes the formation of MA, so it is necessary to limit it to 0.25% or less. More preferably, it is suppressed to 0.20% or less.
Ｔｉ：０．００５〜０．０１５％、およびＮ−Ｔｉ／３．４≧０％
Ｔｉは０．００５％以上含有することで、Ｔｉ窒化物を生成させミクロ組織を微細化させることにより靭性向上に大きく寄与する。しかし、含有量が多くなり化学量論的にＮに対して過剰になると、窒化物形成後の過剰なＴｉはＴｉＣを生成し、溶接熱影響部の靭性を劣化させる可能性が高まるため、０．０１５％を上限とする。また、それと同時にＴｉＣ生成を極力防止する観点から、請求項２において、Ｎとの化学量論的関係として、Ｎ過剰（Ｔｉ不足）を意味するＮ−Ｔｉ／３．４≧０％に限定する。なお、厳密には脱酸によるＴｉの消費も考慮すべきであるが、煩雑さを避けるとともに、実質的に大きな影響がないことを実験的に確認している。
Ti: 0.005-0.015% and N-Ti / 3.4 ≧ 0%
When Ti is contained in an amount of 0.005% or more, Ti nitride is generated and the microstructure is refined, thereby greatly contributing to improvement of toughness. However, when the content is increased and stoichiometrically excessive with respect to N, excessive Ti after the formation of nitrides generates TiC, which increases the possibility of degrading the toughness of the weld heat affected zone. The upper limit is .015%. At the same time, from the viewpoint of preventing TiC generation as much as possible, the stoichiometric relationship with N is limited to N-Ti / 3.4 ≧ 0%, which means N excess (Ti deficiency). . Strictly speaking, the consumption of Ti by deoxidation should be taken into consideration, but it has been experimentally confirmed that there is no substantial influence while avoiding complexity.
Ｍｇ：０．０００３〜０．００３％、
Ｍｇは本発明の主たる合金元素の一つである。Ｂ（ボロン）を添加し、その高い焼入性を活用する本発明においては、溶接熱影響部、特に溶融線近傍の過剰な硬化を緩和するため、オーステナイト粒を可能な限り小さくすることが不可欠である。そのためには、高温での安定な酸化物を微細に分散させ、結晶粒成長をピニングすることが有効であり、Ｍｇが重要な役割を担う。Ｍｇは、主に脱酸剤あるいは硫化物生成元素として添加されるが、０．００３％を越えて添加されると、粗大な酸化物あるいは硫化物が生成し易くなり、母材およびＨＡＺ靭性の低下をもたらす。しかしながら、０．０００３％未満の添加では、ピニング粒子として必要な酸化物の生成が十分に期待できなくなるため、その添加範囲を０．０００３〜０．００３％と限定する。
Mg: 0.0003 to 0.003%,
Mg is one of the main alloying elements of the present invention. In the present invention in which B (boron) is added and its high hardenability is utilized, it is indispensable to make the austenite grains as small as possible in order to alleviate excessive hardening near the weld heat affected zone, particularly in the vicinity of the melting line. It is. For that purpose, it is effective to finely disperse a stable oxide at a high temperature and pin the crystal grain growth, and Mg plays an important role. Mg is mainly added as a deoxidizer or sulfide-forming element, but if added over 0.003%, coarse oxides or sulfides are easily generated, and the base material and HAZ toughness are improved. Bring about a decline. However, since addition of less than 0.0003% makes it impossible to sufficiently generate oxides necessary as pinning particles, the addition range is limited to 0.0003 to 0.003%.
Ｃａ：０．０００３〜０．００３％、
Ｃａは硫化物を生成することにより伸長ＭｎＳの生成を抑制し、鋼材の板厚方向の特性、特に耐ララティアー性を改善する。さらに、ＣａはＭｇと同様な効果を有していることから、本発明の重要な元素である。Ｃａは０．０００３％未満では、十分な効果が得られないので下限値を０．０００３％にした。逆に、Ｃａが０．００３％を超えるとＣａの粗大酸化物個数が増加し、超微細な酸化物あるいは硫化物の個数が低下するため、その上限を０．００３％とする。
Ca: 0.0003 to 0.003%,
Ca suppresses the generation of stretched MnS by forming sulfides, and improves the properties in the thickness direction of the steel material, particularly the lalatier resistance. Furthermore, Ca is an important element of the present invention because it has the same effect as Mg. If Ca is less than 0.0003%, a sufficient effect cannot be obtained, so the lower limit was made 0.0003%. Conversely, if Ca exceeds 0.003%, the number of coarse oxides of Ca increases and the number of ultrafine oxides or sulfides decreases, so the upper limit is made 0.003%.
Ｍｇ＋Ｃａ≦０．００５％
ＭｇとＣａは同時に添加され、いずれも強力な脱酸元素であることから、粗大な介在物を生成する危険が大きく靭性が劣化するため、その合計量としては最大でも０．００５％とする必要がある。０．００４％以下に抑えることがより好ましい。
Mg + Ca ≦ 0.005%
Since Mg and Ca are added simultaneously and both are strong deoxidizing elements, the risk of generating coarse inclusions is great and the toughness deteriorates, so the total amount must be 0.005% at the maximum. There is. It is more preferable to suppress it to 0.004% or less.
Ｂ：０．０００４〜０．００２０％、およびＢ−０．８５（Ｎ−Ｔｉ／３．４）≧０．０００４％
Ｂは、本発明においてキーとなる元素の一つである。Ｂの焼入性向上効果はきわめて大きく、Ｂを活用することで合金元素を大幅に抑えることが可能となる。このためのＢの含有量は、少なくとも０．０００４％は必要である。しかし、単にＢ含有量だけを規定するだけでは不十分である。Ｂの焼入性を活用するためには、固溶状態で存在させる必要があるからである。Ｂは、窒化物を形成しやすく、Ｎとの化学量論的バランスも重要となる。ただし、窒化物形成能はＢよりＴｉの方がより高いため、それも勘案し、Ｂ−０．８５（Ｎ−Ｔｉ／３．４）≧０．０００４％に限定した。上限については、必要以上に含有させても効果が飽和するため、発明者らが鋼の特性に悪影響をおよぼさない範囲として実験的に確認した範囲で０．００２０％としたが、必ずしも臨界的意味合いを有するものではない。Ｂ−０．８５（Ｎ−Ｔｉ／３．４）の上限は特に限定しないが、各元素の限定範囲から自ずと限定されるものである。
B: 0.0004 to 0.0020%, and B-0.85 (N-Ti / 3.4) ≧ 0.0004%
B is one of the key elements in the present invention. The effect of improving the hardenability of B is extremely large, and the use of B makes it possible to greatly suppress alloy elements. For this purpose, the content of B must be at least 0.0004%. However, it is not sufficient to merely specify the B content. This is because in order to utilize the hardenability of B, it is necessary to exist in a solid solution state. B tends to form nitrides, and the stoichiometric balance with N is also important. However, since the nitride forming ability of Ti is higher than that of B, it is also taken into consideration and limited to B-0.85 (N-Ti / 3.4) ≧ 0.0004%. The upper limit is 0.0020% in the range that the inventors have experimentally confirmed as a range that does not adversely affect the properties of the steel because the effect is saturated even if contained more than necessary. It does not have a meaningful meaning. The upper limit of B-0.85 (N-Ti / 3.4) is not particularly limited, but is naturally limited from the limited range of each element.
Ｎ：０．００２〜０．００６％
Ｎは、製鋼上不可避的に含有するもので、必要以上に低減することは製鋼負荷が高く、工業生産上好ましくない。むしろＮは、Ｔｉを添加することで窒化物を形成し、しかもその窒化物は高温で安定であるため、鋼材の熱間圧延に先立つ加熱時あるいは溶接溶融線から若干離れた溶接熱影響部のオーステナイト粒の成長粗大化をピン止めする効果を有するため、０．００２％以上含有することが好ましい。しかし、多すぎる含有は、上述したようにＢと結合して窒化物を形成する可能性が高まり、Ｂの焼入性向上効果を減殺することになる。上述したＢ、Ｔｉの絶対量と化学量論的関係から、自ずと上限は制約されるが、それ以外にも０．００６％超では鋼片製造時に表面疵が発生するため、上限を０．００６％とした。好ましくは０．００５５％以下、さらに好ましくは０．００５％以下である。
N: 0.002 to 0.006%
N is inevitably contained in steelmaking, and reducing it more than necessary is not preferable for industrial production because of high steelmaking load. Rather, N forms a nitride by adding Ti, and the nitride is stable at a high temperature. Therefore, during heating prior to hot rolling of a steel material or a welding heat-affected zone slightly away from the weld melting line. Since it has the effect of pinning growth coarsening of austenite grains, it is preferable to contain 0.002% or more. However, when the content is too large, the possibility of forming a nitride by combining with B increases as described above, and the hardenability improving effect of B is reduced. The upper limit is naturally limited by the above-described absolute amounts of B and Ti and the stoichiometric relationship, but in addition to that, if over 0.006%, surface flaws occur during the production of steel slabs, the upper limit is set to 0.006. %. Preferably it is 0.0055% or less, More preferably, it is 0.005% or less.
Ｏ：０．００１５〜０．００３５％
Ｏは、溶接熱影響部での粒内フェライト生成核としてのＴｉ酸化物の生成性から０．００１５％以上が必須である。しかし、Ｏが多すぎると酸化物のサイズおよび個数が過大となって、むしろ脆性破壊の発生起点として作用する可能性が高まり、結果として靭性を劣化させることになるため、上限は０．００３５％を制限する必要がある。より良好で、安定した溶接熱影響部靭性を得るためには、０．００３０％以下、より好ましくは０．００２８％以下とすることが望ましい。
O: 0.0015 to 0.0035%
O is essential to be 0.0015% or more from the productivity of Ti oxide as intragranular ferrite formation nuclei in the weld heat affected zone. However, if the amount of O is too large, the size and number of oxides become excessive, and the possibility of acting as a starting point for brittle fracture increases. As a result, the toughness is deteriorated, so the upper limit is 0.0035%. Need to be restricted. In order to obtain better and stable weld heat-affected zone toughness, it is desirable that the content be 0.0030% or less, more preferably 0.0028% or less.
Ａｌ：０．００４％以下、
ＡｌレスＴｉ脱酸の本発明においては、不可避的不純物の一つである。請求項２において、あえて上限を限定するのは、不可避といえども含有量が０．００４％を超えると、酸化物の組成が変化し、粒内フェライトの核として機能しなくなる可能性が高まるため、０．００４％以下に限定する。
Al: 0.004% or less,
In the present invention of Al-less Ti deoxidation, it is one of inevitable impurities. In claim 2, although the upper limit is intentionally limited, if the content exceeds 0.004%, the composition of the oxide changes and the possibility of not functioning as a nucleus of intragranular ferrite increases. , 0.004% or less.
個々の元素を上記のように限定した上で、さらに総量規制とも言うべき下記式（１）のＰＣＭを適正範囲に限定する必要がある。なお、式（１）は溶接割れ感受性指数（ＰＣＭ）として公知の式である。各元素がすべて限定範囲であっても、すべて下限または上限の場合には、焼入性が不足または過剰となって、前者の場合は厚手で高強度化が達成できず、後者の場合は溶接熱影響部の硬化性、ＭＡ生成が過剰となって、靭性確保ができないためである。本発明のターゲットとする板厚で所定の強度を安定して確保し、かつ溶接熱影響部靭性も安定して確保するためには、ＰＣＭを０．１８〜０．２３％とする必要がある。
ＰＣＭ＝Ｃ＋Ｓｉ／３０＋Ｍｎ／２０＋Ｃｕ／２０＋Ｎｉ／６０＋Ｍｏ／１５＋５Ｂ ・・・・ （１）
The individual elements on which is limited as described above, it is necessary to limit further the following formula should be called a total amount control of P CM (1) to the appropriate range. Incidentally, the formula (1) are known formula as weld crack sensitivity index (P CM). Even if all the elements are in the limited range, if all of them are at the lower limit or upper limit, the hardenability is insufficient or excessive. In the former case, thick and high strength cannot be achieved, and in the latter case, welding is not possible. This is because the curability of the heat-affected zone and the formation of MA are excessive, and toughness cannot be ensured. In order to ensure a predetermined strength in plate thickness targeting of the present invention stably ensured, and stable even HAZ toughness, it is required to be 0.18 to 0.23% of P CM is there.
P CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Mo / 15 + 5B (1)
以上のように鋼の成分を限定した上で、厚手高強度鋼を安定して工業生産するためには、製造方法も限定する必要がある。 As described above, in order to stably produce thick high-strength steel industrially after limiting the components of steel, it is also necessary to limit the manufacturing method.
本発明鋼は工業的には連続鋳造法で製造することが好ましい。その理由は、溶鋼の凝固冷却速度が速く、スラブ中に微細なＴｉ酸化物とＴｉ窒化物を多量に生成することが可能なためである。 The steel of the present invention is industrially preferably produced by a continuous casting method. The reason for this is that the solidification cooling rate of the molten steel is fast, and a large amount of fine Ti oxide and Ti nitride can be generated in the slab.
スラブの圧延に際し、その再加熱温度は１０００〜１１００℃とする必要がある。再加熱温度が１１００℃を超えるとＴｉ窒化物が粗大化して母材の靭性劣化やＨＡＺ靭性改善効果が期待できないためである。また、１０００℃未満の再加熱温度では、圧延反力が大きくなって圧延負荷が高まり、生産性を阻害するためである。 In rolling the slab, the reheating temperature needs to be 1000 to 1100 ° C. This is because if the reheating temperature exceeds 1100 ° C., the Ti nitride becomes coarse, and the toughness deterioration of the base metal and the effect of improving the HAZ toughness cannot be expected. Further, at a reheating temperature of less than 1000 ° C., the rolling reaction force increases, the rolling load increases, and the productivity is hindered.
再加熱後は、ＴＭＣＰでの製造が必須である。まず、９５０℃以上の温度での累積圧下量が３０％以上の圧延をおこなう。高温域での圧延は、加熱ままの粗大なオーステナイトを整細粒化するためで、累積圧下量は多いほど好ましいが、鋳片厚およびその後の圧延条件により制約を受ける。高温状態の圧延組織は実際には把握すべくもないが、本発明者らの向上およびラボ実験では累積圧下量は３０％以上であれば、その後の圧延−冷却条件が適正範囲であれば特性が安定する。 After reheating, production with TMCP is essential. First, rolling is performed at a cumulative reduction amount of 30% or more at a temperature of 950 ° C. or higher. Rolling in a high temperature range is for fine graining of coarse austenite as it is heated. The larger the amount of rolling reduction, the better, but there are limitations due to the slab thickness and subsequent rolling conditions. Although it is not possible to actually grasp the rolling structure in a high temperature state, in the improvement and lab experiment of the present inventors, if the cumulative reduction amount is 30% or more, the characteristics are good if the subsequent rolling-cooling conditions are in an appropriate range. Stabilize.
次いで、７２０〜９５０℃の温度で累積圧下量が４０％以上で、累積総圧下量が６０％以上として７００〜７５０℃の温度で圧延を終了させる。これらの温度域は、概ねオーステナイトの未再結晶温度域である。しかし、厚手材では板厚方向に温度分布を有し、板厚中心部近傍は温度が高いため、未再結晶温度域圧延が不十分となるケースがある。そのため、本発明は、二段階に温度、累積圧下量を限定するものである。７２０〜９５０℃の温度で累積圧下量が４０％以上の圧延は、表裏面表層から概ね板厚１／４までの最低限必要なオーステナイト未再結晶圧延量である。さらに、累積総圧下量を６０％以上として７００〜７５０℃の温度で圧延を終了するのは、板厚中心部でも組織微細化できる程度にオーステナイト未再結晶域での圧下を付与するためである。板厚中心部は、相対的にオーステナイト未再結晶域での圧下量が少ないのは已むを得ないが、本発明に限定する比較的低い加熱温度、高温域での適正圧下と相俟って、良好な強度−靭性バランスを確保し得る程度に組織を微細化することが可能となる。これら限定範囲を逸脱する圧延条件では、特に、板厚中心部靭性が劣ることを実験的に確認している。 Next, rolling is finished at a temperature of 700 to 750 ° C. at a temperature of 720 to 950 ° C. with a cumulative reduction amount of 40% or more and a cumulative total reduction amount of 60% or more. These temperature ranges are generally austenite non-recrystallization temperature ranges. However, thick materials have a temperature distribution in the plate thickness direction, and the temperature near the plate thickness center is high, so there are cases where unrecrystallized temperature range rolling becomes insufficient. Therefore, the present invention limits the temperature and cumulative reduction amount in two stages. Rolling with a cumulative reduction amount of 40% or more at a temperature of 720 to 950 ° C. is the minimum required austenite non-recrystallization rolling amount from the front and back surface layers to approximately ¼ of the plate thickness. Further, the reason why rolling is completed at a temperature of 700 to 750 ° C. with the cumulative total reduction amount being 60% or more is to provide reduction in the austenite non-recrystallized region to such an extent that the structure can be refined even at the center of the plate thickness. . Although it is inevitable that the amount of reduction in the austenite non-recrystallized region is relatively small in the center of the plate thickness, this is coupled with the relatively low heating temperature and the appropriate reduction in the high temperature region that are limited to the present invention. Thus, the structure can be refined to such an extent that a good strength-toughness balance can be secured. Under rolling conditions that deviate from these limited ranges, it has been experimentally confirmed that the toughness at the center of the sheet thickness is particularly inferior.
さらに、圧延後の冷却は、圧延終了後８０秒以内に水冷を開始して２８０℃以下まで冷却する必要がある。圧延後は速やかに水冷を開始することが好ましいが、大型の実生産設備においては、圧延機端から冷却設備まである程度の搬送時間を要することは避けられない。その場合でも、圧延後冷却までの放冷間にフェライトが析出することは強度上も、また放冷での析出のためそのフェライトは粗大である可能性高く靭性上も好ましくない。このため、圧延終了後８０秒以内に水冷を開始する必要がある。好ましくは６０秒以内である。水冷は、伝熱ネックとなる板厚中心部でも完全に変態が完了するまで冷却する必要があるため、２８０℃以下までの冷却が必要である。なお、本発明がターゲットとする厚手材の板厚中心部でも加速冷却効果を享受するため、概ね１．２ｍ３／ｍ２／分以上の水量密度で冷却することが好ましい。 Furthermore, the cooling after rolling needs to start water cooling within 80 seconds after the end of rolling to cool to 280 ° C. or lower. Although it is preferable to start water cooling immediately after rolling, in a large-scale actual production facility, it is inevitable that a certain amount of conveyance time is required from the end of the rolling mill to the cooling facility. Even in such a case, it is not preferable in terms of strength and ferrite that the ferrite precipitates during the cooling from the rolling to the cooling after the rolling, and the ferrite is coarse because of the precipitation in the cooling. For this reason, it is necessary to start water cooling within 80 seconds after the end of rolling. Preferably, it is within 60 seconds. Water cooling requires cooling to 280 ° C. or lower because it is necessary to cool even the central portion of the plate thickness that becomes a heat transfer neck until transformation is completely completed. In addition, in order to enjoy an accelerated cooling effect also in the thickness center part of the thick material which this invention makes a target, it is preferable to cool with the water density of 1.2 m < 3 > / m < 2 > / min or more in general.
冷却後は、さらに４００〜５５０℃の温度範囲で焼戻ししなければならない。焼戻処理をおこなうことで、母材の強度−靭性バランスが改善するだけでなく、高精度に安定して所定の範囲に制御できる。さらに、冷却時の不均一性も緩和され、鋼材内の残留応力解消にも効果を有し、それらに起因した切断時の形状変化も抑制される。４００℃未満での焼戻しではそれらの効果が小さく、５５０℃を超える焼戻しでは、強度低下が大きく、本発明がターゲットとする高強度の確保が困難である。 After cooling, it must be tempered in the temperature range of 400 to 550 ° C. By performing the tempering process, not only the strength-toughness balance of the base material is improved, but also it can be stably controlled with high accuracy within a predetermined range. Furthermore, the non-uniformity at the time of cooling is relieved, and it has an effect also in the cancellation of the residual stress in the steel material, and the shape change at the time of cutting resulting from them is also suppressed. Those effects are small when tempering at less than 400 ° C., and when tempering at more than 550 ° C., the strength is greatly reduced, and it is difficult to ensure the high strength targeted by the present invention.
なお、上述した温度はいずれも鋼材表面温度である。 In addition, all the temperature mentioned above is steel material surface temperature.
以下、発明例及び比較例に基づいて本発明を説明する。 Hereinafter, the present invention will be described based on invention examples and comparative examples.
転炉−連続鋳造−厚板工程で種々の鋼成分の厚鋼板を製造し、母材特性ならびに溶接熱影響部の靭性を評価した。 Thick steel plates of various steel components were manufactured in the converter-continuous casting-thick plate process, and the base material characteristics and the toughness of the weld heat affected zone were evaluated.
溶接は、一般的に試験溶接として用いられている潜弧溶接（ＳＡＷ）法で、溶接溶け込み線（ＦＬ）が垂直になるようにレ型開先で溶接入熱は４．５ｋＪ／ｍｍの多層盛りとした。溶接熱影響部の靭性評価は、ＢＳ（Ｂｒｉｔｉｓｈ Ｓｔａｎｄａｒｄｓ）規格５７６２に準拠したＣＴＯＤ試験をおこなった。ノッチ位置はＧＣＨＡＺ（ｃｏａｒｓｅ−ｇｒａｉｎｅｄ ＨＡＺ）とよばれる溶接溶融線で、試験温度−２０℃でそれぞれ６本の試験を実施した。 Welding is a submerged arc welding (SAW) method that is generally used as test welding, and is a multi-layer with a welding die heat input of 4.5 kJ / mm so that the weld penetration line (FL) is vertical. It was prime. For the toughness evaluation of the weld heat affected zone, a CTOD test based on BS (British Standards) standard 5762 was performed. The notch position was a weld melt line called GCHAZ (coarse-grained HAZ), and six tests were performed at a test temperature of -20 ° C.
表１に鋼の化学成分を示し、表２に製造条件および母材特性、溶接熱影響部靭性（ＣＴＯＤ特性）を示す。本発明で製造した鋼板（本発明鋼：鋼成分Ｎｏ．１〜１２、１４、１５及び発明例Ｎｏ．Ａ〜Ｌ、Ｎ、Ｏ）は、降伏強さが５００〜６５０ＭＰａ、引張強さが６００〜７００ＭＰａ、母材靭性がｖＴｒｓ試験結果の鋼板１／４厚位置で−６４〜−７９℃、鋼板１／２厚位置で−３８〜−４７℃、−２０℃の最低ＣＴＯＤ値が０．７５〜０．９０ｍｍの良好な破壊靭性を示した。 Table 1 shows the chemical composition of the steel, and Table 2 shows the manufacturing conditions, base material characteristics, and weld heat affected zone toughness (CTOD characteristics). The steel plates manufactured according to the present invention (invention steel: steel components No. 1 to 12, 14, 15 and invention examples No. A to L , N, O) have a yield strength of 500 to 650 MPa and a tensile strength of 600. The minimum CTOD value of -700 MPa, the base metal toughness is -64 to -79 ° C at the steel plate 1/4 thickness position of the vTrs test result, -38 to -47 ° C and -20 ° C at the steel plate 1/2 thickness position is 0.75. Good fracture toughness of ˜0.90 mm was exhibited.
これに対し、鋼成分および／または製造方法が本発明の限定範囲を逸脱する比較例（鋼成分Ｎｏ．１６〜２８及び比較例Ｎｏ．ａ〜ｏ）は、母材強度が低かったり、母材靭性が劣っていたり、あるいは溶接熱影響部靭性が劣っている。 On the other hand, in the comparative examples (steel component Nos. 16 to 28 and comparative examples No. a to o) in which the steel components and / or the manufacturing method deviate from the limited range of the present invention, the base material strength is low, or the base material The toughness is inferior, or the weld heat affected zone toughness is inferior.
即ち、比較例ａ〜ｏは鋼成分が本発明範囲外で上記機械的性質が満足されるものではなかった。特にまた、鋼成分２１による比較例ｆは、Ｎｉ／Ｃｕ＞２．０を満足していないため、熱間圧延時にクラックが生じ、製造が困難となった。 That is, in Comparative Examples a to o, the steel components were outside the scope of the present invention, and the mechanical properties were not satisfied. In particular, since Comparative Example f using the steel component 21 does not satisfy Ni / Cu> 2.0, cracks were generated during hot rolling, making manufacture difficult.
また、比較鋼ｐ〜ｔは、製造条件が本願範囲から逸脱しているため母材靭性が劣っている。比較鋼ｕは、圧延終了〜冷却開始までの時間が長すぎたため降伏強さ、引張強さが共に低かった。比較鋼ｖは、焼き戻し温度が高すぎたため強度低下が大きくかった。比較鋼ｗは、焼き戻しを実施していないため母材靭性が低かった。 Further, the comparative steels p to t are inferior in the base metal toughness because the manufacturing conditions deviate from the scope of the present application. Since the time from the end of rolling to the start of cooling was too long, the comparative steel u had low yield strength and tensile strength. The comparative steel v had a large decrease in strength because the tempering temperature was too high. Since the comparative steel w was not tempered, the base metal toughness was low.
Claims (2)
Ｃ：０．０１５〜０．０４５％、
Ｓｉ：０．１５％以下、
Ｍｎ：１．８０〜２．２０％、
Ｐ：０．００８％以下、
Ｓ：０．００５％以下、
Ｃｕ：０．４０〜０．７０％、
Ｎｉ：０．８０〜１．８０％、
Ｎｂ：０.００５〜０．０１５％、
Ｍｏ：０．０５〜０．２５％、
Ｔｉ：０．００５〜０．０１５％、
Ｍｇ：０．０００３〜０．００３％、
Ｃａ：０．０００３〜０．００３％、
Ｂ：０．０００４〜０．００２０％、
Ｎ：０．００２０〜０．００６０％、
Ｏ：０．００１５〜０．００３５％、
Ａｌ：０．００４％以下、
を含有し、残部が鉄および不可避的不純物の成分からなり、
Ｍｇ＋Ｃａ≦０．００５％、
Ｎｉ／Ｃｕ＞２．０、
Ｎ−Ｔｉ／３．４≧０％、
Ｂ−０．８５（Ｎ−Ｔｉ／３．４）≧０．０００４％、
下記式（１）で示すＰＣＭが０．１８〜０．２３％
を満足することを特徴とする、引張強さ６００〜８００ＭＰａ、降伏強さ５００〜６５０ＭＰａ、ＢＳ（Ｂｒｉｔｉｓｈ Ｓｔａｎｄａｒｄｓ）規格５７６２に準拠した試験温度−２０℃でのＣＴＯＤ試験により測定された、溶接熱影響部のき裂開口変位の最低ＣＴＯＤ値０．５０ｍｍ以上の特性を有する溶接性、溶接熱影響部靭性に優れた厚手高強度鋼板。
ＰＣＭ＝Ｃ＋Ｓｉ／３０＋Ｍｎ／２０＋Ｃｕ／２０＋Ｎｉ／６０＋Ｍｏ／１５＋５Ｂ
・・・・ 式（１）
ここで、各元素は鋼中に含有されている質量％である。 % By mass
C: 0.015-0.045%,
Si: 0.15% or less,
Mn: 1.80 to 2.20%
P: 0.008% or less,
S: 0.005% or less,
Cu: 0.40 to 0.70%,
Ni: 0.80 to 1.80%,
Nb: 0.005 to 0.015%,
Mo: 0.05 to 0.25%,
Ti: 0.005 to 0.015%,
Mg: 0.0003 to 0.003%,
Ca: 0.0003 to 0.003%,
B: 0.0004 to 0.0020%,
N: 0.0020 to 0.0060%,
O: 0.0015 to 0.0035%,
Al: 0.004% or less,
The balance is composed of iron and inevitable impurities,
Mg + Ca ≦ 0.005%,
Ni / Cu> 2.0,
N-Ti / 3.4 ≧ 0%,
B-0.85 (N-Ti / 3.4) ≧ 0.0004%,
P CM represented by the following formula (1) is 0.18 to 0.23 percent
Welding heat effect measured by CTOD test at a test temperature of −20 ° C. in accordance with BS (British Standards) standard 5762, which has a tensile strength of 600 to 800 MPa, a yield strength of 500 to 650 MPa. Thick high-strength steel sheet with excellent weldability and weld heat-affected zone toughness having a minimum CTOD value of 0.50 mm or more for crack opening displacement at the joint.
P CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Mo / 15 + 5B
.... Formula (1)
Here, each element is the mass% contained in steel.
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