JP2015183279A - Thick steel sheet for marine vessel, for marine structure and for hydraulic pressure steel pipe excellent in brittle crack arrest property - Google Patents
Thick steel sheet for marine vessel, for marine structure and for hydraulic pressure steel pipe excellent in brittle crack arrest property Download PDFInfo
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Abstract
Description
本発明は、船舶や海洋構造物、水圧鉄管等に用いられる、引張強さが５７０ＭＰａ以上、７２０ＭＰａ以下で、脆性亀裂伝播停止特性、特にＮＲＬ落重試験における無延性遷移温度（ＴＮＤＴ）に優れる厚鋼板とその製造方法に関するものである。なお、本発明の「厚鋼板」とは、板厚が６ｍｍ以上１００ｍｍ以下の熱延鋼板をいう。 The present invention is used for ships, offshore structures, hydraulic iron pipes, etc., and has a tensile strength of 570 MPa or more and 720 MPa or less, and is excellent in brittle crack propagation stop characteristics, particularly in the non-ductile transition temperature (T NDT ) in the NRL drop weight test. The present invention relates to a thick steel plate and a manufacturing method thereof. In addition, the “thick steel plate” of the present invention refers to a hot rolled steel plate having a thickness of 6 mm or more and 100 mm or less.
船舶や海洋構造物、水圧鉄管等に用いられる鋼板には、強度特性や低温靱性に優れることの他に、良好な溶接性を有することが要求される。特に近年では、エネルギー需要の拡大等によって、従来よりも低温環境で使用されるようになってきており、上記特性に優れる材料の開発が強く望まれるようになってきている。 Steel sheets used for ships, offshore structures, hydraulic iron pipes and the like are required to have good weldability in addition to excellent strength characteristics and low temperature toughness. Particularly in recent years, due to the expansion of energy demand and the like, it has come to be used in a lower temperature environment than before, and development of a material having excellent characteristics has been strongly desired.
上記低温靭性の評価基準としては、一般的にはシャルピー衝撃試験が用いられているが、ＡＳＴＭでは、さらに脆性亀裂伝播停止特性を評価する基準として、ＮＲＬ落重試験が規定されている。特に近年では、使用温度の低温化に伴い、上記ＮＲＬ落重試験における要求温度の低温化が進んでおり、鋼板に対するＮＲＬ落重特性改善要求も高まってきている。 The Charpy impact test is generally used as the evaluation standard for the low temperature toughness, but the NRL drop weight test is specified as a standard for further evaluating the brittle crack propagation stop characteristic in ASTM. Particularly in recent years, as the service temperature is lowered, the required temperature in the NRL drop weight test has been lowered, and the demand for improving the NRL drop weight characteristic for steel sheets has also increased.
ところで、低温靭性を改善するには、従来からＮｉの添加が有効であることが知られている。しかし、Ｎｉは高価な元素であり、原料コストの上昇を招くため、Ｎｉ添加量をできるだけ低く抑えた上で、必要なＮＲＬ落重特性を満たすことが望まれている。 By the way, in order to improve the low-temperature toughness, it has been conventionally known that addition of Ni is effective. However, since Ni is an expensive element and causes an increase in raw material cost, it is desired to satisfy the necessary NRL drop weight characteristics while keeping the amount of Ni added as low as possible.
引張強さＴＳが５７０ＭＰａ以上、７２０ＭＰａ以下で、かつ、Ｎｉを多量に含まない、ＮＲＬ落重特性に優れた調質型高強度鋼板としては、例えば、特許文献１には、Ｃ：０．０８〜０．１６ｍａｓｓ％、Ｎｉ：０．１５〜０．６０ｍａｓｓ％、ＮｂおよびＢを必須添加元素とし、ミクロ組織を下部ベイナイト組織とすることで、ＮＲＬ落重特性を向上させた板厚４０ｍｍ〜８０ｍｍの鋼板が開示されている。
As a tempered high-strength steel sheet having a tensile strength TS of 570 MPa or more and 720 MPa or less and not containing a large amount of Ni and having excellent NRL drop weight characteristics, for example, Patent Document 1 discloses C: 0.08. ~ 0.16 mass%, Ni: 0.15 to 0.60 mass%, Nb and B are essential additive elements, and the microstructure is a lower bainite structure, thereby improving the NRL
また、特許文献２には、Ｃ：０．０４〜０．０８ｍａｓｓ％、Ｎｉ：０．０５〜０．６０ｍａｓｓ％、ＮｂおよびＢを実質的に無添加とした調質型６００ＭＰａ級鋼板において、ミクロ組織を焼戻しベイナイトおよび／または焼戻しマルテンサイトとすることで、応力除去焼鈍（ＰＷＨＴ：Post Weld Heat Treatment）後の落重特性を向上させた板厚８０ｍｍ以下の鋼板が開示されている。 In Patent Document 2, C: 0.04 to 0.08 mass%, Ni: 0.05 to 0.60 mass%, a tempered 600 MPa grade steel plate substantially free of Nb and B, A steel sheet having a thickness of 80 mm or less is disclosed in which the drop weight characteristic after post-weld heat treatment (PWHT) is improved by using tempered bainite and / or tempered martensite as the structure.
また、特許文献３には、Ｃ：０．１〜０．１６ｍａｓｓ％、Ｎｉ：０．６ｍａｓｓ％以下、Ｂ：０．０００５〜０．００２ｍａｓｓ％を添加し、Ｆ＝９．４×［Ｍｏ］＋８．１×［Ｖ］＋４．７×［Ｃｒ］からなるＦ値を３．２０以上４．５０以下とし、１５°以上の大角粒界で囲まれた結晶粒の平均円相当径が４μｍ以下の焼戻しベイナイト組織とすることで落重特性を向上させた板厚８０ｍｍ以上の鋼板が開示されている。 Further, Patent Document 3 includes C: 0.1 to 0.16 mass%, Ni: 0.6 mass% or less, and B: 0.0005 to 0.002 mass%, and F = 9.4 × [Mo]. The F value consisting of + 8.1 × [V] + 4.7 × [Cr] is 3.20 or more and 4.50 or less, and the average equivalent circle diameter of crystal grains surrounded by a large-angle grain boundary of 15 ° or more is 4 μm or less. A steel sheet having a thickness of 80 mm or more with improved drop weight characteristics by using a tempered bainite structure is disclosed.
また、特許文献４には、Ｃ：０．０３〜０．１５０ｍａｓｓ％、Ｎｉ：２ｍａｓｓ％以下を含有し、ミクロ組織を、ベイナイトの面積率が９０％以上で、ベイナイトのラス幅の平均値が３．５μｍ以下、ベイナイト中の島状マルテンサイトの円相当径の最大値が３．０μｍ以下とすることで、落重特性を向上させた板厚５０ｍｍ以上の鋼板が開示されている。 Patent Document 4 contains C: 0.03 to 0.150 mass%, Ni: 2 mass% or less, and has a microstructure with an area ratio of bainite of 90% or more and an average value of bainite lath width. A steel sheet having a thickness of 50 mm or more with improved drop weight characteristics is disclosed by setting the maximum equivalent circle diameter of island martensite in bainite to 3.5 μm or less and 3.0 μm or less.
しかしながら、上記特許文献１および２に開示の技術は、記載されている落重特性ＴＮＤＴは、−３０℃〜−５０℃程度であり、近年の低温化したＮＲＬ落重特性に対する要求（ＴＮＤＴ≦−６５℃）を満足することはできない。また、特許文献３に開示の技術は、板厚８０ｍｍ以上の厚肉材を対象としており、焼入時の冷却速度が遅い厚肉材においては、良好な落重特性が得られるが、Ｃ含有量が０．１０ｍａｓｓ％以上と高いために、板厚４０〜６０ｍｍ程度の鋼板に適用した場合には、マルテンサイト主体の組織となり、落重特性の低下が懸念される。また、特許文献４に開示の技術は、板厚５０ｍｍ以上、特に１００ｍｍの鋼板を対象としていることから、鋼板のミクロ組織は島状マルテンサイトを含む上部ベイナイト組織を主体とした組織となるため、その上部ベイナイト中の島状マルテンサイトを小さくするために、焼戻しを２回実施することが必須となっているため、製造コストが高いという問題がある。 However, in the techniques disclosed in Patent Documents 1 and 2, the drop weight characteristic T NDT described is about −30 ° C. to −50 ° C., and the recent demand for NRL drop weight characteristics (T NDT) ≦ −65 ° C.) cannot be satisfied. In addition, the technique disclosed in Patent Document 3 is intended for thick materials with a plate thickness of 80 mm or more, and in thick materials with a slow cooling rate during quenching, good drop weight characteristics can be obtained. Since the amount is as high as 0.10 mass% or more, when it is applied to a steel plate having a thickness of about 40 to 60 mm, it becomes a martensite-based structure, and there is a concern that the drop weight characteristic may be deteriorated. In addition, since the technique disclosed in Patent Document 4 is intended for a steel sheet having a thickness of 50 mm or more, particularly 100 mm, the microstructure of the steel sheet is a structure mainly composed of an upper bainite structure including island martensite. In order to reduce the island-like martensite in the upper bainite, it is essential to perform tempering twice, so that there is a problem that the manufacturing cost is high.
また、近年では、設計温度の低温化により、ＮＲＬ落重試験のＴＮＤＴとして−６５℃以下が要求されるようになってきている。さらに、鋼板中心部での靭性確保も重要であり、板厚１／２位置でのＮＲＬ落重特性の向上も求められている。そのため、従来技術では、焼入−焼戻処理を施す調質型６００ＭＰａ級で、板厚１００ｍｍ以下の厚鋼板において、多量のＮｉ添加を必要とすることなく、脆性亀裂伝播停止特性、即ち、ＮＲＬ落重試験のＴＮＤＴを−６５℃以下とすることは難しいのが実情であった。
In recent years, by lowering the design temperature, it is -65 ° C. or less has come to be required as T NDT of NRL drop weight test. Furthermore, securing toughness at the center of the steel sheet is also important, and improvement of the NRL drop weight characteristic at the position of 1/2 the sheet thickness is also required. Therefore, in the prior art, a
本発明は、従来技術が抱える上記問題点に鑑みてなされたものであり、その目的は、Ｎｉを多量に添加することなく、引張強さＴＳが５７０ＭＰａ以上、７２０ＭＰａ以下、かつ、ＮＲＬ落重試験のＴＮＤＴが−６５℃以下で、板厚１００ｍｍ以下の厚鋼板を提供するとともに、その有利な製造方法を提案することにある。 The present invention has been made in view of the above-described problems of the prior art, and the purpose thereof is to add a large amount of Ni, and have a tensile strength TS of 570 MPa or more and 720 MPa or less, and an NRL drop weight test. In addition to providing a thick steel plate having a TNDT of −65 ° C. or less and a plate thickness of 100 mm or less, an advantageous manufacturing method thereof is proposed.
発明者らは、上記課題の解決に向けて鋭意研究を重ねた。その結果、所定の成分組成を有する鋼素材（スラブ）を、適正条件下で１００ｍｍ以下に制御圧延・制御冷却する熱間圧延した後、焼入れ−焼戻し処理して厚鋼板を製造する際、上記制御圧延・制御冷却により、板厚１／２位置において｛１１１｝＜１１０＞集合組織を発達させるとともに、焼入後の引張強さＴＳを８５０ＭＰａ以上、１１００ＭＰａ以下に制御した後、適切な温度で焼戻し、ミクロ組織を焼戻し下部ベイナイトおよび焼戻しマルテンサイトとすることで、引張強さＴＳが５７０ＭＰａ以上、７２０ＭＰａ以下で、かつ、ＮＲＬ落重試験のＴＮＤＴが−６５℃以下の脆性亀裂伝播停止特性に優れた厚鋼板を得ることができることを見出し、本発明を開発するに至った。 The inventors have intensively studied to solve the above problems. As a result, a steel material (slab) having a predetermined component composition is hot-rolled under controlled conditions and controlled to 100 mm or less under appropriate conditions, and then quenched and tempered to produce a thick steel plate. By rolling and controlled cooling, the {111} <110> texture is developed at the 1/2 position of the plate thickness, and the tensile strength TS after quenching is controlled to 850 MPa or more and 1100 MPa or less, and then tempered at an appropriate temperature. , with the microstructure tempered lower bainite and tempered martensite, the tensile strength TS is more than 570 MPa, below 720 MPa, and, T NDT of NRL drop weight test excellent brittle crack propagation stop characteristics of -65 ° C. or less The present inventors have found that a thick steel plate can be obtained and developed the present invention.
上記知見に基づく本発明は、制御圧延、制御冷却した後、再加熱−焼入れ−焼戻し処理した厚鋼板であって、Ｃ：０．０５〜０．１５ｍａｓｓ％、Ｓｉ：０．０５〜０．６０ｍａｓｓ％、Ｍｎ：０．８〜１．８ｍａｓｓ％、Ｐ：０．０２０ｍａｓｓ％以下、Ｓ：０．００５ｍａｓｓ％以下、Ａｌ：０．００５〜０．０８０ｍａｓｓ％、Ｎ：０．０００５〜０．００５０ｍａｓｓ％を含有し、さらに、Ｃｕ：０．５０ｍａｓｓ％以下、Ｎｉ：１．５０ｍａｓｓ％以下、Ｃｒ：０．５０ｍａｓｓ％以下、Ｍｏ：０．４０ｍａｓｓ％以下、Ｖ：０．０７ｍａｓｓ％以下およびＢ：０．０００３〜０．００２０ｍａｓｓ％のうちから選ばれる１種または２種以上を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有し、下記（１）式；
Ｃｅｑ（ｍａｓｓ％）＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５ ・・・（１）
で定義されるＣ当量Ｃｅｑ（ｍａｓｓ％）が０．３６〜０．５５で、下記（２）式；
ＤＩ＊＝８．２×（Ｃ）０．５×（０．７Ｓｉ＋１）×ｆＭｎ×（０．３５Ｃｕ＋１）×（０．３６Ｎｉ＋１）×（２．１６Ｃｒ＋１）×（３．００Ｍｏ＋１）×（１．７５Ｖ＋１）×ｆＢ ・・・（２）
ただし、ｆＭｎ＝３．３３Ｍｎ＋１ （Ｍｎ≦１．２ｍａｓｓ％の場合）
ｆＭｎ＝（Ｍｎ−１．２）×５．１＋５ （Ｍｎ＞１．２ｍａｓｓ％の場合）
ｆＢ＝１．３ （Ｂ≧０．０００３ｍａｓｓ％の場合）
ｆＢ＝１．０ （Ｂ＜０．０００３ｍａｓｓ％の場合）
で定義されるＤＩ＊値が６５以上であり、さらに、板厚１／２位置におけるミクロ組織が焼戻し下部ベイナイトおよび焼戻しマルテンサイト組織からなり、板厚１／２ｔにおける｛１１１｝＜１１０＞方位の集積度が１．８以上で、かつ、板厚１／２位置におけるＮＲＬ落重試験のＴＮＤＴが−６５℃以下であることを特徴とする船舶用、海洋構造物用および水圧鉄管用厚鋼板である。
The present invention based on the above findings is a steel plate that has been subjected to re-rolling, quenching and tempering after controlled rolling and controlled cooling, and C: 0.05 to 0.15 mass%, Si: 0.05 to 0.60 mass. %, Mn: 0.8 to 1.8 mass%, P: 0.020 mass% or less, S: 0.005 mass% or less, Al: 0.005 to 0.080 mass%, N: 0.0005 to 0.0050 mass% Cu: 0.50 mass% or less, Ni: 1.50 mass% or less, Cr: 0.50 mass% or less, Mo: 0.40 mass% or less, V: 0.07 mass% or less, and B: 0.00. 1 type or 2 types or more chosen from 0003-0.0020mass% are contained, and the remainder has the component composition which consists of Fe and an unavoidable impurity, following formula (1);
C eq (mass%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
The C equivalent C eq (mass%) defined by the formula is 0.36 to 0.55, and the following formula (2):
DI * = 8.2 × (C) 0.5 × (0.7Si + 1) × fMn × (0.35Cu + 1) × (0.36Ni + 1) × (2.16Cr + 1) × (3.00Mo + 1) × (1.75V + 1) ) × fB (2)
However, fMn = 3.33Mn + 1 (when Mn ≦ 1.2 mass%)
fMn = (Mn−1.2) × 5.1 + 5 (in the case of Mn> 1.2 mass%)
fB = 1.3 (when B ≧ 0.0003 mass%)
fB = 1.0 (when B <0.0003 mass%)
In defined as DI * value is not less than 65, further, the microstructure in the sheet thickness 1/2 position consists tempered lower bainite and tempered martensite structure, {111} in the sheet thickness 1 / 2t <110> orientation of the in integration of 1.8 or more and, marine, offshore structures and for penstock for steel plate, wherein the T NDT of NRL drop weight test in the sheet thickness 1/2 position is -65 ° C. or less It is.
本発明の上記厚鋼板は、上記成分組成に加えてさらに、Ｎｂ：０．０３０ｍａｓｓ％以下、Ｔｉ：０．０３０ｍａｓｓ％以下、Ｃａ：０．００４０ｍａｓｓ％以下、ＲＥＭ：０．０１０ｍａｓｓ％以下、Ｗ：０．４０ｍａｓｓ％以下、Ｃｏ：０．５０ｍａｓｓ％以下、Ｍｇ：０．００５ｍａｓｓ％以下およびＺｒ：０．０２ｍａｓｓ％以下のうちから選ばれる１種または２種以上を含有することを特徴とする。 In addition to the above component composition, the thick steel plate of the present invention further includes Nb: 0.030 mass% or less, Ti: 0.030 mass% or less, Ca: 0.0040 mass% or less, REM: 0.010 mass% or less, W: It contains one or more selected from 0.40 mass% or less, Co: 0.50 mass% or less, Mg: 0.005 mass% or less, and Zr: 0.02 mass% or less.
また、本発明の上記厚鋼板は、板厚１／２位置における焼入れままの引張強さが８５０以上１１００ＭＰａ以下の厚鋼板を焼戻ししたものであることを特徴とする。 The thick steel plate of the present invention is characterized by being tempered from a thick steel plate having a tensile strength of 850 or more and 1100 MPa or less as-quenched at a position of 1/2 the plate thickness.
また、本発明の上記厚鋼板は、板厚１／２位置における方位差角が１５度以上の大角粒界で囲まれた結晶粒の円相当径の平均値が５μｍ以下であり、旧オーステナイト粒の平均粒径が３０μｍ以下であることを特徴とする。 In the thick steel plate of the present invention, the average equivalent circle diameter of crystal grains surrounded by a large-angle grain boundary having a misorientation angle of 15 degrees or more at a half-thickness position is 5 μm or less, and prior austenite grains The average particle size is 30 μm or less.
また、本発明は、上記のいずれかに記載の成分組成からなるスラブを１０００〜１２００℃の温度に再加熱した後、再結晶温度域で２０〜６０％の累積圧下率で圧延し、引き続き、Ａｒ３変態点以上の未再結晶温度域で３０〜６０％の累積圧下率で圧延し、直ちに５℃／ｓ以上の冷却速度で水冷する制御圧延・制御冷却し、その後、Ａｃ３変態点〜（Ａｃ３変態点＋７０℃）の温度に再加熱後、焼入れする焼入処理を１回以上施してから、５５０〜７００℃の温度で焼戻処理を施すことを特徴とする船舶用、海洋構造物用および水圧鉄管用厚鋼板の製造方法を提案する。 In addition, the present invention re-heats a slab comprising any of the above-described component compositions to a temperature of 1000 to 1200 ° C., then rolls the slab at a cumulative reduction of 20 to 60% in the recrystallization temperature range, Control rolling and controlled cooling are performed by rolling at a cumulative reduction rate of 30 to 60% in the non-recrystallization temperature range above the Ar 3 transformation point, followed by water cooling at a cooling rate of 5 ° C./s or more, and then the Ac 3 transformation point to after reheating to a temperature of (Ac 3 transformation point + 70 ° C.), the hardening process of hardening the subjected one or more times, for vessels characterized by applying tempering treatment at a temperature of 550 to 700 ° C., marine structures We propose a method for manufacturing thick steel plates for materials and hydraulic iron pipes.
本発明の上記厚鋼板の製造方法は、板厚１／２位置における焼入れままの引張強さを８５０ＭＰａ以上１１００ＭＰａ以下とすることを特徴とする。 The method for producing a thick steel plate according to the present invention is characterized in that the tensile strength as-quenched at a position of 1/2 the plate thickness is 850 MPa to 1100 MPa.
本発明によれば、引張強さが５７０ＭＰａ以上、７２０ＭＰａ以下の高い強度を有するとともに、ＮＲＬ落重試験におけるＴＮＤＴが−６５℃以下の脆性亀裂伝播停止特性に優れる船舶用、海洋構造物用および水圧鉄管用の厚鋼板を安定して供給することが可能となる。 According to the present invention, the tensile strength of more than 570 MPa, which has the following high strength 720 MPa, for vessels T NDT in NRL drop weight test is excellent in brittle crack propagation stop characteristics of -65 ° C. or less, and for marine structures It is possible to stably supply a thick steel plate for a hydraulic iron pipe.
まず、本発明が開発目標とする厚鋼板について説明する。
本発明は、調質型６００ＭＰａ級鋼に対応する、引張強さＴＳが５７０ＭＰａ以上、７２０ＭＰａ以下で、かつ、船級規格などで求められる昨今における設計温度の低温化に対応し、ＮＲＬ落重試験における無延性遷移温度ＴＮＤＴが−６５℃以下の低温靭性特性（脆性亀裂伝播停止特性）を有する厚鋼板の開発を目標とする。ここで、本発明における上記ＴＮＤＴとは、ＡＳＴＭ Ｅ２０８に規定されたＮＲＬ落重試験における無延性遷移温度（ＮＤＴ温度：nil-ductility transition temperature）のことを意味する。
First, the thick steel plate that is the development target of the present invention will be described.
The present invention corresponds to a tempered 600 MPa class steel, has a tensile strength TS of 570 MPa or more and 720 MPa or less, and corresponds to the recent lowering of the design temperature required by ship class standards, etc. The goal is to develop a thick steel plate having low-temperature toughness characteristics (brittle crack propagation stopping characteristics) with a non-ductile transition temperature T NDT of −65 ° C. or less. Here, the above-mentioned T NDT in the present invention, non-ductile transition temperature in NRL drop weight test as specified in ASTM E208 (NDT Temperature: nil-ductility transition temperature) means that.
次に、本発明を開発するに至った実験について説明する。
発明者らは、上記の高強度で優れた脆性亀裂伝播停止特性を有する鋼を開発するため、Ｃ：０．０４〜０．２０ｍａｓｓ％、Ｎｉ：１．５０ｍａｓｓ％以下を含有する鋼に、さらに、Ｓｉ，Ｍｎ，Ｃｕ，Ｃｒ，ＭｏおよびＶを添加し、下記（１）式；
Ｃｅｑ（ｍａｓｓ％）＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５ ・・・（１）
で定義されるＣ当量Ｃｅｑ（ｍａｓｓ％）を、０．３５〜０．５５ｍａｓｓ％の範囲に揃えた各種成分組成の鋼を実験室的に溶製し、種々の条件で制御圧延・制御冷却して厚さが２５ｍｍの厚鋼板とした後、再加熱し、２℃／ｓ以上の種々の冷却速度で焼入れ処理した。この焼入後の鋼板について、引張試験を行い、引張強さＴＳを測定するとともに、光学顕微鏡および走査型電子顕微鏡を用いてミクロ組織を観察した。
次いで、上記焼入後の厚鋼板を、５５０〜７００℃の温度で焼き戻し処理し、焼き戻し後の引張強さＴＳが５７０〜７２０ＭＰａの調質型６００ＭＰａ級厚鋼板を得た。
斯くして得た厚鋼板について、ＡＳＴＭ規格 Ａ３７０に準拠し、Ｃ方向に試験片を採取し、Ａ３７０に準拠してシャルピー衝撃試験を行い、破面遷移温度ｖＴｒｓを測定するとともに、ＡＳＴＭ Ｅ２０８に準拠して、Ｐ−３試験片（厚さ１６ｍｍ×幅５０ｍｍ×長さ１３０ｍｍ）をＣ方向に採取し、落重試験を行い、無延性遷移温度ＴＮＤＴを求めた。
Next, the experiment that led to the development of the present invention will be described.
In order to develop a steel having the above-described high strength and excellent brittle crack propagation stopping characteristics, the inventors further added a steel containing C: 0.04 to 0.20 mass%, Ni: 1.50 mass% or less, , Si, Mn, Cu, Cr, Mo and V are added, and the following formula (1):
C eq (mass%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
In the laboratory, steels with various compositional compositions with C equivalent C eq (mass%) defined in the above are in the range of 0.35 to 0.55 mass%, and controlled rolling / cooling under various conditions. Then, after making a steel plate having a thickness of 25 mm, it was reheated and quenched at various cooling rates of 2 ° C./s or more. The quenched steel sheet was subjected to a tensile test to measure the tensile strength TS, and the microstructure was observed using an optical microscope and a scanning electron microscope.
Subsequently, the quenched steel plate was tempered at a temperature of 550 to 700 ° C. to obtain a tempered 600 MPa class thick steel plate having a tensile strength TS of 570 to 720 MPa after tempering.
About the thick steel plate thus obtained, in accordance with ASTM standard A370, a test piece is taken in the C direction, a Charpy impact test is performed in accordance with A370, the fracture surface transition temperature vTrs is measured, and in accordance with ASTM E208. Then, a P-3 test piece (thickness 16 mm × width 50 mm × length 130 mm) was sampled in the C direction, a drop weight test was performed, and a non-ductile transition temperature T NDT was obtained.
図１は、焼入れ後の引張強さＴＳおよびミクロ組織と、焼き戻し後の落重試験におけるＴＮＤＴとの関係を模式的に示したものである。この図から、焼き入れ性を高めて焼入れ後の引張強さＴＳを８５０ＭＰａ以上、１１００ＭＰａ以下、鋼のミクロ組織を下部ベイナイトおよびマルテンサイトとした後、焼戻して引張強さＴＳを５７０ＭＰａ以上、７２０ＭＰａ以下とした場合にのみ、焼戻し後の落重試験における無延性遷移温度ＴＮＤＴを−６５℃以下にすることができることがわかる。
本発明は、上記の新規な知見に基づいて開発したものである。
Figure 1 differs from that of the tensile strength TS and microstructure after quenching, the relationship between T NDT in drop weight test after tempering is shown schematically. From this figure, the hardenability is increased to a tensile strength TS after quenching of 850 MPa or more and 1100 MPa or less, and the steel microstructure is changed to lower bainite and martensite, and then tempered to obtain a tensile strength TS of 570 MPa or more and 720 MPa or less. It can be seen that only in this case, the non-ductile transition temperature T NDT in the drop weight test after tempering can be set to −65 ° C. or lower.
The present invention has been developed based on the above novel findings.
次に、本発明の厚鋼板の成分組成を限定する理由について説明する。
Ｃ：０．０５〜０．１５ｍａｓｓ％
Ｃは、鋼の焼入性および強度を確保するために必須の元素であり、Ｃｅｑが０．５０以下の成分組成で、焼入後の引張強さを８５０ＭＰａ以上とするためには、０．０５ｍａｓｓ％以上含有させる必要がある。一方、０．１５ｍａｓｓ％を超えて添加すると、母材と溶接熱影響部の靭性低下、および、溶接性の低下を招く。よって、Ｃの含有量は０．０５〜０．１５ｍａｓｓ％の範囲とする。好ましくは０．０７〜０．１２ｍａｓｓ％の範囲であり、より好ましくは０．０８〜０．１０ｍａｓｓ％の範囲である。
Next, the reason which limits the component composition of the thick steel plate of this invention is demonstrated.
C: 0.05-0.15 mass%
C is an essential element for ensuring the hardenability and strength of the steel. The component composition of C eq is 0.50 or less, and in order to set the tensile strength after quenching to 850 MPa or more, 0 .05 mass% or more must be contained. On the other hand, if added in excess of 0.15 mass%, the toughness of the base metal and the weld heat-affected zone and the weldability are reduced. Therefore, the C content is in the range of 0.05 to 0.15 mass%. Preferably it is the range of 0.07-0.12 mass%, More preferably, it is the range of 0.08-0.10 mass%.
Ｓｉ：０．０５〜０．６０ｍａｓｓ％
Ｓｉは、鋼の脱酸剤として添加される元素である。また、固溶強化によって鋼の強度を高めるのに有効な元素でもある。上記の効果を得るためには、０．０５ｍａｓｓ％以上含有させる必要がある。一方、０．６０ｍａｓｓ％を超えて添加すると、溶接熱影響部の靭性が低下する。よって、Ｓｉの含有量は０．０５〜０．６０ｍａｓｓ％の範囲とする。好ましくは０．１０〜０．４０ｍａｓｓ％の範囲である。
Si: 0.05-0.60 mass%
Si is an element added as a deoxidizer for steel. It is also an effective element for increasing the strength of steel by solid solution strengthening. In order to acquire said effect, it is necessary to contain 0.05 mass% or more. On the other hand, if added over 0.60 mass%, the toughness of the weld heat affected zone decreases. Therefore, the Si content is in the range of 0.05 to 0.60 mass%. Preferably it is the range of 0.10-0.40 mass%.
Ｍｎ：０．８〜１．８ｍａｓｓ％
Ｍｎは、鋼の脱酸剤として添加される元素である。また、鋼の焼入れ性を向上し、強度を高めるのに有効な元素でもある。上記効果を得るためには、０．８ｍａｓｓ％以上含有させる必要がある。一方、１．８ｍａｓｓ％を超えて添加すると、溶接性や溶接熱影響部の靭性を低下させる。よって、Ｍｎの含有量は０．８〜１．８ｍａｓｓ％の範囲とする。好ましくは１．２〜１．６ｍａｓｓ％の範囲である。
Mn: 0.8 to 1.8 mass%
Mn is an element added as a deoxidizer for steel. It is also an element effective in improving the hardenability of steel and increasing its strength. In order to acquire the said effect, it is necessary to make it contain 0.8 mass% or more. On the other hand, if added over 1.8 mass%, the weldability and the toughness of the heat affected zone are reduced. Therefore, the Mn content is in the range of 0.8 to 1.8 mass%. Preferably it is the range of 1.2-1.6 mass%.
Ｐ：０．０２０ｍａｓｓ％以下
Ｐは、鋼中に不可避的に混入し、焼戻し後の冷却中や、溶接後の応力除去焼鈍（ＰＷＨＴ）の徐冷過程で、旧オーステナイト粒界に偏析して粒界脆化を助長し、落重特性を低下させる。そのため、Ｐの含有量は極力低減することが望ましいが、０．０２０ｍａｓｓ％以下であれば、粒界脆化を防止することができる。よって、本発明では、Ｐの含有量を０．０２０ｍａｓｓ％以下に制限する。好ましくは０．０１５ｍａｓｓ％以下である。より好ましくは０．０１０ｍａｓｓ％以下である。
P: 0.020 mass% or less P is inevitably mixed in steel and segregates in the prior austenite grain boundaries during cooling after tempering or in the annealing process of stress relief annealing (PWHT) after welding. It promotes interfacial embrittlement and reduces the falling weight characteristics. Therefore, it is desirable to reduce the content of P as much as possible, but if it is 0.020 mass% or less, grain boundary embrittlement can be prevented. Therefore, in the present invention, the P content is limited to 0.020 mass% or less. Preferably it is 0.015 mass% or less. More preferably, it is 0.010 mass% or less.
Ｓ：０．００５ｍａｓｓ％以下
Ｓは、鋼中に不可避的に混入し、鋼中にＭｎＳなどの硫化物系介在物として存在し、靱性を低下させる元素であるため、極力低減することが望ましい。そこで、本発明においては、落重特性を確保するため、Ｓの含有量を０．００５ｍａｓｓ％以下に制限する。好ましくは０．００３ｍａｓｓ％以下である。より好ましくは０．００２ｍａｓｓ％以下である。
S: 0.005 mass% or less S is an element that is inevitably mixed in steel and exists as sulfide inclusions such as MnS in the steel, and is an element that lowers toughness. Therefore, it is desirable to reduce it as much as possible. Therefore, in the present invention, the S content is limited to 0.005 mass% or less in order to ensure the falling weight characteristic. Preferably it is 0.003 mass% or less. More preferably, it is 0.002 mass% or less.
Ａｌ：０．００５〜０．０８０ｍａｓｓ％
Ａｌは、脱酸元素として添加される元素であり、また、ＡｌＮによる焼入れ加熱時の結晶粒の微細化を通じて靭性向上に寄与する有用な元素である。これらの効果を発現させるためには、０．００５ｍａｓｓ％以上含有させる必要がある。一方、０．０８０ｍａｓｓ％を超えて添加すると、ＡｌＮの粗大化による結晶粒粗大化に起因した母材靭性低下や溶接熱部の靭性低下という問題を引き起こす。よって、Ａｌの含有量は０．００５〜０．０８０ｍａｓｓ％の範囲とする。好ましくは０．０２０〜０．０６０ｍａｓｓ％の範囲である。
Al: 0.005-0.080 mass%
Al is an element added as a deoxidizing element, and is a useful element that contributes to improving toughness through refinement of crystal grains during quenching heating with AlN. In order to express these effects, it is necessary to contain 0.005 mass% or more. On the other hand, if it is added in excess of 0.080 mass%, it causes a problem that the base metal toughness is reduced due to the coarsening of the crystal grains due to the coarsening of AlN and the toughness of the welded hot zone is reduced. Therefore, the Al content is in the range of 0.005 to 0.080 mass%. Preferably it is the range of 0.020-0.060mass%.
Ｎ：０．０００５〜０．００５０ｍａｓｓ％
Ｎは、ＡｌとＡｌＮを形成し、焼入れ加熱時のオーステナイト粒を微細化することによって、鋼の靱性向上に有効に寄与する。上記効果を得るためには０．０００５ｍａｓｓ％以上含有させることが必要である。しかし、０．００５０ｍａｓｓ％を超えて添加すると、溶接部の靱性低下を招く。よって、Ｎの含有量は０．０００５〜０．００５０ｍａｓｓ％の範囲とする。好ましくは０．００１０〜０．００４０ｍａｓｓ％の範囲である。
N: 0.0005 to 0.0050 mass%
N contributes effectively to improving the toughness of the steel by forming Al and AlN and refining the austenite grains during quenching heating. In order to acquire the said effect, it is necessary to contain 0.0005 mass% or more. However, if added over 0.0050 mass%, the toughness of the welded portion is reduced. Therefore, the N content is in the range of 0.0005 to 0.0050 mass%. Preferably it is the range of 0.0010-0.0040 mass%.
本発明の厚鋼板は、上記の成分に加えて、下記Ｃｕ，Ｎｉ，Ｃｒ，Ｍｏ，ＶおよびＢのうちから選ばれる１種または２種以上の含有を必要とする。
Ｃｕ：０．５０ｍａｓｓ％以下
Ｃｕは、固溶強化により鋼の強度を高めるのに有用な元素である。しかし、０．５０ｍａｓｓ％を超えて添加すると、熱間圧延時のＣｕ割れを起こすおそれがある。よって、Ｃｕの含有量は０．５０ｍａｓｓ％以下とする。好ましくは０．０１〜０．４０ｍａｓｓ％の範囲である。
In addition to the above components, the thick steel plate of the present invention needs to contain one or more selected from the following Cu, Ni, Cr, Mo, V and B.
Cu: 0.50 mass% or less Cu is an element useful for increasing the strength of steel by solid solution strengthening. However, if added over 0.50 mass%, Cu cracks may occur during hot rolling. Therefore, the Cu content is 0.50 mass% or less. Preferably it is the range of 0.01-0.40 mass%.
Ｎｉ：１．５０ｍａｓｓ％以下
Ｎｉは、Ｃｕと同様、固溶強化元素として有用な元素である他、低温靱性の向上にも有用な元素である。しかし、１．５０ｍａｓｓ％を超えて添加すると、原料コストの上昇を招いたり、スラブ割れを起こして生産性を阻害したりする。よって、Ｎｉの含有量は１．５０ｍａｓｓ％以下とする。好ましくは０．０１〜１．２０ｍａｓｓ％、より好ましくは０．０１〜１．００ｍａｓｓ％の範囲である。
Ni: 1.50 mass% or less Ni, as well as Cu, is an element useful as a solid solution strengthening element and an element useful for improving low-temperature toughness. However, if it is added in excess of 1.50 mass%, the raw material cost is increased, or slab cracking is caused to hinder productivity. Therefore, the Ni content is 1.50 mass% or less. Preferably it is 0.01-1.20 mass%, More preferably, it is the range of 0.01-1.00 mass%.
Ｃｒ：０．５０ｍａｓｓ％以下
Ｃｒは、焼入れ性を向上して鋼の強度を高めるとともに、焼戻しやＰＷＨＴ後の強度を確保するために有用な元素である。しかし、０．５０ｍａｓｓ％を超えて添加すると、溶接性を低下させるとともに、ＰＷＨＴ後の靱性が著しく低下する。よって、Ｃｒの含有量は０．５０ｍａｓｓ％以下の範囲とする。好ましくは０．０１〜０．３０ｍａｓｓ％の範囲である。
Cr: 0.50 mass% or less Cr is an element useful for improving the hardenability and increasing the strength of the steel, and for ensuring the strength after tempering or PWHT. However, if added over 0.50 mass%, the weldability is lowered and the toughness after PWHT is significantly lowered. Therefore, the Cr content is set to a range of 0.50 mass% or less. Preferably it is the range of 0.01-0.30 mass%.
Ｍｏ：０．４０ｍａｓｓ％以下
Ｍｏは、焼入れ性を向上して鋼の強度を高めるとともに、靱性を確保するのに有用な元素である。また、Ｃｒと同様、焼戻しやＰＷＨＴ後の強度確保に有用な元素である。しかし、０．４０ｍａｓｓ％を超えて添加すると、溶接性を低下させるとともに、原料コストの上昇を招く。よって、Ｍｏの含有量は０．４０ｍａｓｓ％以下の範囲とする。好ましくは０．０１〜０．３０ｍａｓｓ％の範囲である。
Mo: 0.40 mass% or less Mo is an element useful for improving the hardenability and increasing the strength of the steel and ensuring toughness. Moreover, like Cr, it is an element useful for ensuring the strength after tempering or PWHT. However, if added over 0.40 mass%, the weldability is lowered and the raw material cost is increased. Therefore, the Mo content is set to a range of 0.40 mass% or less. Preferably it is the range of 0.01-0.30 mass%.
Ｖ：０．０７ｍａｓｓ％以下
Ｖは、鋼の焼入れ性を向上させる他、Ｃ，Ｎと炭窒化物を形成し、焼戻しやＰＷＨＴ後の強度の確保に有用な元素である。しかし、０．０７ｍａｓｓ％を超えて添加すると、溶接性を低下させるとともに、炭窒化物析出による母材靱性の低下を招く。よって、Ｖの含有量は０．０７ｍａｓｓ％とする。好ましくは０．００３〜０．０６ｍａｓｓ％の範囲である。
V: 0.07 mass% or less V is an element useful for improving the hardenability of steel, forming carbon and nitrides with C, N, and ensuring strength after tempering and PWHT. However, if added over 0.07 mass%, the weldability is lowered and the base material toughness is lowered due to carbonitride precipitation. Therefore, the content of V is set to 0.07 mass%. Preferably it is the range of 0.003-0.06 mass%.
Ｂ：０．０００３〜０．００２０ｍａｓｓ％
Ｂは、極微量の添加量で焼入れ性を高め、ミクロ組織を下部ベイナイトやマルテンサイト組織にして、鋼の強度向上に有効に寄与する元素である。上記焼入れ性の向上効果を得るためには、０．０００３ｍａｓｓ％以上の添加を必要とする。一方、０．００２０ｍａｓｓ％を超えて添加しても、上記効果は飽和するだけである。よって、Ｂは０．０００３〜０．００２０ｍａｓｓ％の範囲で添加する。好ましくは０．０００５〜０．００１５ｍａｓｓ％の範囲である。
B: 0.0003 to 0.0020 mass%
B is an element that enhances the hardenability with a very small amount of addition and makes the microstructure lower bainite or martensite structure and contributes effectively to improving the strength of steel. In order to obtain the effect of improving the hardenability, addition of 0.0003 mass% or more is required. On the other hand, even if added over 0.0020 mass%, the above effect is only saturated. Therefore, B is added in the range of 0.0003 to 0.0020 mass%. Preferably it is the range of 0.0005-0.0015 mass%.
また、本発明の厚鋼板は、上記の成分組成を満たすことに加えて、下記（１）式；
Ｃｅｑ（ｍａｓｓ％）＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５ ・・・（１）
で定義されるＣ当量Ｃｅｑ（ｍａｓｓ％）が、０．３６〜０．５５ｍａｓｓ％の範囲であることが必要である。
Ｃｅｑが、０．３６ｍａｓｓ％以下では、焼入時の焼入性を確保できない上に、焼戻し後に６００ＭＰａ級の強度を得ることが困難となる。一方、０．５５ｍａｓｓ％を超えると、母材および溶接熱影響部の靭性低下を招くからである。好ましくは０．４０〜０．５３ｍａｓｓ％の範囲である。
Moreover, in addition to satisfy | filling said component composition, the thick steel plate of this invention has the following (1) formula;
C eq (mass%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
The C equivalent C eq (mass%) defined by the above is required to be in the range of 0.36 to 0.55 mass%.
When C eq is 0.36 mass% or less, hardenability at the time of quenching cannot be secured and it becomes difficult to obtain a strength of 600 MPa after tempering. On the other hand, if it exceeds 0.55 mass%, the toughness of the base metal and the weld heat affected zone will be reduced. Preferably it is the range of 0.40-0.53 mass%.
また、本発明の厚鋼板は、上記成分組成を満たすことに加えて、下記（２）式；
ＤＩ＊＝８．２×（Ｃ）０．５×（０．７Ｓｉ＋１）×ｆＭｎ×（０．３５Ｃｕ＋１）×（０．３６Ｎｉ＋１）×（２．１６Ｃｒ＋１）×（３．００Ｍｏ＋１）×（１．７５Ｖ＋１）×ｆＢ ・・・（２）
ただし、ｆＭｎ＝３．３３Ｍｎ＋１ （Ｍｎ≦１．２ｍａｓｓ％の場合）
ｆＭｎ＝（Ｍｎ−１．２）×５．１＋５ （Ｍｎ＞１．２ｍａｓｓ％の場合）
ｆＢ＝１．３ （Ｂ≧０．０００３ｍａｓｓ％の場合）
ｆＢ＝１．０ （Ｂ＜０．０００３ｍａｓｓ％の場合）
で定義されるＤＩ＊値が６５以上であることが必要である。
ここで、上記ＤＩ＊値は、Ｃ当量Ｃｅｑと同様、焼入性を表す指標であり、文献（例えば、熱処理Vol.27, No.3, p.136）に記載されているＤＩ式を用いて、実験結果から係数をフィッティングして求めた。ＤＩ＊値が６５未満では、焼入性を確保できないため、焼入後の引張強さが８５０ＭＰａ以上とならない上に、所望の下部ベイナイトおよびマルテンサイト組織とすることができないからである。
Moreover, in addition to satisfy | filling the said component composition, the thick steel plate of this invention has the following (2) formula;
DI * = 8.2 × (C) 0.5 × (0.7Si + 1) × fMn × (0.35Cu + 1) × (0.36Ni + 1) × (2.16Cr + 1) × (3.00Mo + 1) × (1.75V + 1) ) × fB (2)
However, fMn = 3.33Mn + 1 (when Mn ≦ 1.2 mass%)
fMn = (Mn−1.2) × 5.1 + 5 (in the case of Mn> 1.2 mass%)
fB = 1.3 (when B ≧ 0.0003 mass%)
fB = 1.0 (when B <0.0003 mass%)
It is necessary that the DI * value defined by is 65 or more.
Here, the above DI * value is an index representing hardenability like the C equivalent C eq, and the DI formula described in the literature (for example, heat treatment Vol. 27, No. 3, p. 136) is used. It was obtained by fitting the coefficient from the experimental result. If the DI * value is less than 65, the hardenability cannot be ensured, the tensile strength after quenching does not become 850 MPa or more, and the desired lower bainite and martensite structure cannot be obtained.
本発明の厚鋼板は、上述した必須とする成分以外の残部は、Ｆｅおよび不可避的不純物であるが、鋼の強度や低温靭性を高めるため、上記成分に加えて、Ｎｂ，Ｔｉ，Ｃａ，ＲＥＭ，Ｗ，Ｃｏ，ＭｇおよびＺｒのうちから選ばれる１種または２種以上を含有することができる。
Ｎｂ：０．０３０ｍａｓｓ％以下
Ｎｂは、Ｖと同様、Ｃ，Ｎと炭窒化物を形成し、鋼の強度を高める元素である。また、焼入れ時のオーステナイト粒微細化を通じて靭性の向上にも有効に寄与する元素でもある。しかし、ＰＷＨＴを施す場合には、Ｎｂ炭窒化物の析出による脆化が大きく、特に溶接熱影響部の靭性低下が著しいため、０．０３０ｍａｓｓ％を上限として添加するのが好ましい。
In the thick steel plate of the present invention, the balance other than the essential components described above is Fe and inevitable impurities, but in order to increase the strength and low temperature toughness of the steel, in addition to the above components, Nb, Ti, Ca, REM , W, Co, Mg, and Zr can be included.
Nb: 0.030 mass% or less Nb, like V, is an element that forms carbonitrides with C and N and increases the strength of steel. It is also an element that contributes effectively to improving toughness through austenite grain refinement during quenching. However, when PWHT is applied, embrittlement due to precipitation of Nb carbonitride is large, and particularly the toughness of the weld heat-affected zone is markedly reduced. Therefore, it is preferable to add 0.030 mass% as the upper limit.
Ｔｉ：０．０３０ｍａｓｓ％以下
Ｔｉは、Ｃ，Ｎと炭窒化物を形成し、オーステナイト粒の微細化を通じて靭性を向上するのに有効な元素であるので添加することができる。しかし、０．０３０ｍａｓｓ％を超えて添加すると、析出脆化を起こすようになるため、０．０３０ｍａｓｓ％以下として添加するのが好ましい。
Ti: 0.030 mass% or less Ti is an element that forms carbonitride with C and N, and is effective in improving toughness through refinement of austenite grains, and therefore can be added. However, if added over 0.030 mass%, precipitation embrittlement occurs. Therefore, it is preferable to add 0.030 mass% or less.
Ｃａ：０．００４０ｍａｓｓ％以下
Ｃａは、硫化物（ＭｎＳ）の形態を制御し、母材の低温靱性や異方性の改善、耐水素誘起割れ性の向上ならびに再熱割れ（応力除去焼鈍時に発生する割れ）の抑制に効果を発揮する有用な元素である。しかし、０．００４０ｍａｓｓ％を超える添加は、Ｃａオキシサルファイド（介在物）を生成し、低温靱性や清浄度を害するようになる。よって、Ｃａの含有量は０．００４０ｍａｓｓ％以下の範囲で添加するのが好ましい。
Ca: 0.0040 mass% or less Ca controls the form of sulfide (MnS), improves the low-temperature toughness and anisotropy of the base material, improves resistance to hydrogen-induced cracking, and reheat cracking (occurs during stress relief annealing) It is a useful element that is effective in suppressing cracking. However, addition exceeding 0.0040 mass% produces Ca oxysulfide (inclusions), and harms low temperature toughness and cleanliness. Therefore, the Ca content is preferably added in the range of 0.0040 mass% or less.
ＲＥＭ：０．０１０ｍａｓｓ％
ＲＥＭは、Ｃａと同様、硫化物（ＭｎＳ）の形態制御、母材の低温靱性や異方性の改善、耐水素誘起割れ性の向上ならびに再熱割れ抑制に有効な元素である。しかし、０．０１０ｍａｓｓ％を超えて添加すると、低温靱性や清浄度が低下する。よって、ＲＥＭの添加量は０．０１０ｍａｓｓ％以下とするのが好ましい。
REM: 0.010 mass%
REM, like Ca, is an element effective for controlling the form of sulfide (MnS), improving low-temperature toughness and anisotropy of the base material, improving hydrogen-induced crack resistance, and suppressing reheat cracking. However, if added over 0.010 mass%, the low temperature toughness and cleanliness are reduced. Therefore, the amount of REM added is preferably 0.010 mass% or less.
Ｗ：０．４０ｍａｓｓ％以下
Ｗは、焼入れ性を向上して鋼の強度を高める元素であるが、０．４０ｍａｓｓ％を超えて添加すると、母材靭性を低下させる。よって、Ｗの添加量は０．４０ｍａｓｓ％以下とするのが好ましい。
W: 0.40 mass% or less W is an element that improves the hardenability and increases the strength of the steel, but if added over 0.40 mass%, the base metal toughness is reduced. Therefore, the amount of W added is preferably 0.40 mass% or less.
Ｃｏ：０．５０ｍａｓｓ％以下
Ｃｏは、焼入れ性を向上して鋼の強度を高める元素であるが、０．５０ｍａｓｓ％を超えて添加すると母材靭性を低下させる。よって、Ｃｏの添加量は０．５０ｍａｓｓ％以下とするのが好ましい。
Co: 0.50 mass% or less Co is an element that improves the hardenability and increases the strength of the steel, but if added in excess of 0.50 mass%, the base metal toughness is reduced. Therefore, the amount of Co added is preferably 0.50 mass% or less.
Ｍｇ：０．００５ｍａｓｓ％以下
Ｍｇは、酸化物が分散することによって、溶接熱影響部の靭性改善に寄与する元素であるが、０．００５ｍａｓｓ％を超えて添加すると、却って母材靭性や溶接熱影響部靭性を低下させる。よって、Ｍｇの添加量は０．００５ｍａｓｓ％以下とするのが好ましい。
Mg: 0.005 mass% or less Mg is an element that contributes to improving the toughness of the weld heat-affected zone by dispersing the oxide. However, if added over 0.005 mass%, the base metal toughness and welding heat are conversely added. Reduces affected area toughness. Therefore, the amount of Mg added is preferably 0.005 mass% or less.
Ｚｒ：０．０２ｍａｓｓ％以下
Ｚｒは、酸化物が分散することによって、溶接熱影響部の靭性改善に寄与する元素であるが、０．０２ｍａｓｓ％を超えて添加すると、却って母材靭性や溶接熱影響部靭性を低下させる。よって、Ｚｒの添加量は０．０２ｍａｓｓ％以下とするのが好ましい。
Zr: 0.02 mass% or less Zr is an element that contributes to the improvement of toughness of the weld heat affected zone due to the dispersion of oxides, but if added over 0.02 mass%, the base metal toughness and welding heat Reduces affected area toughness. Therefore, the amount of Zr added is preferably 0.02 mass% or less.
次に、本発明の厚鋼板の焼入れ−焼戻し後のミクロ組織について説明する。
本発明の厚鋼板の焼入れ−焼戻し後のミクロ組織は、焼戻し下部ベイナイトおよび焼戻しマルテンサイトであることが必要である。上記ミクロ組織とする理由は、低温靭性、特に、ＮＲＬ落重特性を向上させるためには、脆性亀裂伝播における亀裂進展の抵抗となる有効結晶粒径（劈開破壊の破面単位）を微細化することが重要であるからである。なお、上記焼戻し下部ベイナイトおよび焼戻しマルテンサイト以外の他の組織は、本発明の作用効果を損なわない範囲、具体的には、１０ｖｏｌ％以下であれば混在しても構わない。また、焼戻し後の本発明の厚鋼板では、島状マルテンサイトはセメンタイトに分解するため、基本的には存在してないが、０．５ｖｏｌ％以下であれば許容できる。
Next, the microstructure after quenching and tempering of the thick steel plate of the present invention will be described.
The microstructure after quenching and tempering of the thick steel plate of the present invention needs to be tempered lower bainite and tempered martensite. The reason for the microstructure is to refine the effective crystal grain size (fracture surface unit of cleavage fracture) that becomes resistance to crack propagation in brittle crack propagation in order to improve low-temperature toughness, especially NRL drop weight characteristics. Because it is important. In addition, the structure other than the tempered lower bainite and the tempered martensite may be mixed so long as the effect of the present invention is not impaired, specifically, 10 vol% or less. Moreover, in the thick steel plate of the present invention after tempering, island-like martensite is decomposed into cementite, and thus basically does not exist, but it is acceptable if it is 0.5 vol% or less.
また、本発明の厚鋼板の焼入れ−焼戻し後のミクロ組織は、電子後方散乱パターン（ＥＢＳＰ）で測定した１５度以上の大角粒界で囲まれた結晶粒の円相当径の平均値が５μｍ以下であることが好ましい。ここで、上記１５度以上の大角粒界とは、ＥＢＳＰにより測定した方位差マッピンッグを基に、隣り合った結晶粒の粒界の方位差角を求め、その値が１５度以上の粒界を意味する。ＥＢＳＰで測定した１５度以上の大角粒界で囲まれた結晶粒が小さいほど、結晶粒界への応力集中を低減することができるので、破壊に対する抵抗が高まり、低温靭性や落重特性が向上する。すなわち、上記１５度以上の大角粒界で囲まれた結晶粒の平均粒径が５μｍを超えると、粗大な粒を起点として破壊が生じ、靭性が低下するが、５μｍ以下にすることで、優れた落重特性（ＴＮＤＴ≦−６５℃）が得られる。 Further, the microstructure after quenching and tempering of the thick steel plate of the present invention has an average equivalent circle diameter of 5 μm or less of crystal grains surrounded by large-angle grain boundaries of 15 degrees or more as measured by an electron backscattering pattern (EBSP). It is preferable that Here, the above-mentioned large-angle grain boundary of 15 degrees or more is based on the orientation mapping measured by EBSP, and the grain boundary angle between adjacent crystal grains is obtained, and the grain boundary whose value is 15 degrees or more is obtained. means. The smaller the crystal grain surrounded by a large-angle grain boundary of 15 degrees or more measured by EBSP, the more the stress concentration at the crystal grain boundary can be reduced, so the resistance to fracture increases and the low-temperature toughness and drop weight characteristics improve. To do. That is, when the average grain size of the crystal grains surrounded by the large-angle grain boundaries of 15 degrees or more exceeds 5 μm, breakage occurs starting from coarse grains and the toughness is reduced. The falling weight characteristic (T NDT ≦ −65 ° C.) is obtained.
さらに、旧オーステナイト粒径は、平均粒径を３０μｍ以下とすることが必要である。旧オーステナイト粒径を３０μｍよりも小さくすることによって、下部ベイナイトおよびマルテンサイト組織のＥＢＳＰで測定した方位差角が１５度以上の大角粒界で囲まれた結晶粒径を微細化することができ、落重特性が向上する。しかし、旧オーステナイト粒径が３０μｍを超えると、粗大なオーステナイト粒から変態した下部ベイナイトやマルテンサイトが粗大となり、それに伴って、有効結晶粒径も粗大となるので、落重特性が低下する。
ここで、上記粒界で囲まれた結晶粒の円相当径とは、上記粒界をトレースし、画像解析して求めた結晶粒の円相当径をいう。
また、旧オーステナイト粒径は、適切な腐食液を用いて、粒界を現出した組織を光学顕微鏡にて撮影した後、粒界をトレースし、画像解析して求めた円相当径をいう。
Furthermore, the prior austenite particle size needs to have an average particle size of 30 μm or less. By making the prior austenite grain size smaller than 30 μm, it is possible to refine the crystal grain size surrounded by large-angle grain boundaries with a misorientation angle measured by EBSP of the lower bainite and martensite structure of 15 degrees or more, Drop weight characteristics are improved. However, when the prior austenite grain size exceeds 30 μm, the lower bainite and martensite transformed from the coarse austenite grains become coarse, and the effective crystal grain size becomes coarse in association with it, so that the drop weight characteristic is lowered.
Here, the equivalent circle diameter of the crystal grain surrounded by the grain boundary means the equivalent circle diameter of the crystal grain obtained by tracing the grain boundary and analyzing the image.
The prior austenite grain size refers to a circle-equivalent diameter obtained by photographing a structure in which a grain boundary appears with an appropriate microscope using an optical microscope, tracing the grain boundary, and analyzing the image.
さらに、本発明の厚鋼板は、焼入れ−焼戻し後の鋼板の板厚１／２位置における集合組織の｛１１１｝＜１１０＞方位の集積度が１．８以上であることが好ましい。｛１１１｝＜１１０＞方位の集積度を１．８以上とすることで、劈開面｛１００｝を圧延方向および圧延直角方向に対して斜めに配向させることができるので、圧延方向や圧延直角方向での亀裂伝播抵抗を高めることができるからである。
なお、上記｛１１１｝＜１１０＞方位の集積度は、板厚１／２位置で、板面に平行な面について試験片厚１ｍｍの試験片を採取し、Ｘ線回折して、（１１０）、（２００）、（２１１）正極点図を測定し、これらを用いて３次元結晶方位密度関数を計算し、Φ２＝４５°断面における集積度から求めた。また、上記集合組織の制御は、後述する制御圧延・制御冷却を行うことで達成することができる。
Furthermore, it is preferable that the thick steel plate of the present invention has a {111} <110> orientation accumulation degree of the texture at the position of half the thickness of the steel plate after quenching and tempering is 1.8 or more. By setting the degree of integration of {111} <110> orientation to 1.8 or more, the cleavage plane {100} can be oriented obliquely with respect to the rolling direction and the direction perpendicular to the rolling direction. This is because the crack propagation resistance can be increased.
The degree of integration in the {111} <110> orientation is as follows: a test piece having a thickness of 1 mm and a specimen having a thickness of 1 mm is taken from a plane parallel to the plate at a position of 1/2 the plate thickness, , (200), (211) Positive electrode dot diagrams were measured, and using these, a three-dimensional crystal orientation density function was calculated, and obtained from the degree of integration in the Φ2 = 45 ° cross section. Moreover, the control of the texture can be achieved by performing controlled rolling / controlled cooling described later.
次に、本発明の厚鋼板の製造方法について説明する。
本発明の厚鋼板は、成分組成を前述した範囲に調整した鋼を溶製し、連続鋳造し、または、造塊後、分塊圧延して鋼素材（スラブ）とした後、再加熱し、制御圧延・制御冷却して厚さが６〜１００ｍｍの厚鋼板とし、その後、焼入れ−焼戻し処理を施して製造する。
ここで、上記スラブの再加熱温度は、１０００〜１２００℃の範囲とする必要がある。１０００℃未満ではオーステナイト化が不十分であり、一方、１２００℃を超えるとオーステナイト粒が粗大となるからである。好ましくは１０００〜１１５０℃の範囲である。
Next, the manufacturing method of the thick steel plate of this invention is demonstrated.
The steel plate of the present invention is a steel material (slab) that has been prepared by melting steel whose component composition has been adjusted to the above-mentioned range, continuously casting, or ingoting, and then rolling it into a steel material (slab), and then reheating, Control rolling and control cooling are performed to obtain a thick steel plate having a thickness of 6 to 100 mm, and then subjected to quenching-tempering treatment.
Here, the reheating temperature of the slab needs to be in the range of 1000 to 1200 ° C. When the temperature is lower than 1000 ° C., austenite formation is insufficient. On the other hand, when the temperature exceeds 1200 ° C., the austenite grains become coarse. Preferably it is the range of 1000-1150 degreeC.
続く熱間圧延は、再結晶温度域で２０〜６０％の累積圧下率で圧延し、引き続き、Ａｒ３変態点以上の未再結晶温度域で３０〜６０％の累積圧下率で制御圧延した後、直ちに５℃／ｓ以上の冷却速度で冷却する制御圧延・制御冷却を施すことが必要である。
上記制御圧延により、特にオーステナイト未再結晶温度域での制御圧延によって｛１１１｝＜１１０＞方位の集積度が１．８以上となり、劈開面｛１００｝を圧延方向および圧延直角方向に対して斜めに配向させることができるので、亀裂伝播抵抗を高めることができる。
In the subsequent hot rolling, after rolling with a cumulative reduction of 20 to 60% in the recrystallization temperature range, and subsequently controlled rolling with a cumulative reduction of 30 to 60% in the non-recrystallization temperature range above the Ar 3 transformation point. It is necessary to perform controlled rolling / controlled cooling that immediately cools at a cooling rate of 5 ° C./s or more.
By the controlled rolling described above, the degree of accumulation of the {111} <110> orientation becomes 1.8 or more by the controlled rolling particularly in the austenite non-recrystallization temperature range, and the cleavage plane {100} is inclined with respect to the rolling direction and the perpendicular direction of the rolling. Therefore, the crack propagation resistance can be increased.
また、制御冷却時の冷却速度を５℃／ｓ以上とすることで、圧延冷却後の組織をベイナイトおよびマルテンサイト組織からなるものとすることができる。オーステナイト未再結晶温度域での圧延後に、ベイナイト変態およびマルテンサイト変態させることで、変態時にバリアント選択が生じ、｛１１１｝＜１１０＞方位の集積度を向上させることができる。しかし、冷却速度が５℃／ｓ未満では、冷却時の相変態が拡散型のフェライト変態が主体となるため、｛１１１｝＜１１０＞方位の集積度を向上させることが難しくなる。 Moreover, the structure after rolling cooling can consist of a bainite and a martensitic structure by making the cooling rate at the time of control cooling into 5 degrees C / s or more. By performing bainite transformation and martensite transformation after rolling in the austenite non-recrystallization temperature range, variant selection occurs during transformation, and the degree of accumulation of {111} <110> orientation can be improved. However, when the cooling rate is less than 5 ° C./s, it is difficult to improve the degree of integration of the {111} <110> orientation because the phase transformation during cooling is mainly diffusion type ferrite transformation.
なお、上記Ａｒ３変態点の温度は、実測して求めてもよいが、次式で計算した値を用いることができる。
Ａｒ３（℃）＝９１０-２７３Ｃ−７４Ｍｎ−５６Ｎｉ−１６Ｃｒ−９Ｍｏ−５Ｃｕ
（ただし、上記式中の各元素記号は、それぞれの元素の含有量（ｍａｓｓ％））
The temperature at the Ar 3 transformation point may be obtained by actual measurement, but a value calculated by the following equation can be used.
Ar 3 (° C.) = 910-273C-74Mn-56Ni-16Cr-9Mo-5Cu
(However, each element symbol in the above formula is the content of each element (mass%))
上記の制御圧延・制御冷却して得た厚鋼板は、Ａｃ３変態点〜（Ａｃ３変態点＋７０℃）の温度域に再加熱した後、焼入れする焼入処理を１回以上施すことが必要である。
上記熱間圧延して得た厚鋼板は、その後、Ａｃ３変態点〜（Ａｃ３変態点＋７０℃）の温度域に再加熱後、焼入れする焼入処理を１回以上行うことが必要である。
焼入れ温度が、Ａｃ３変態点未満では、再加熱時に（オーステナイト＋フェライト）の２相域になる上、オーステナイト粒径が小さくなって焼入れが不十分となるため、所定の強度を確保できない。一方、（Ａｃ３変態点＋７０℃）を超えて加熱した場合には、オーステナイト粒径が粗大化し、母材の靭性低下が著しく、落重特性も低下するからである。
The steel plate obtained by the above controlled rolling / cooling needs to be re-heated to a temperature range of Ac 3 transformation point to (Ac 3 transformation point + 70 ° C.) and then subjected to quenching treatment for quenching at least once. It is.
The thick steel plate obtained by the above hot rolling needs to be subjected to quenching once or more after reheating to a temperature range of Ac 3 transformation point to (Ac 3 transformation point + 70 ° C.). .
When the quenching temperature is less than the Ac 3 transformation point, a two-phase region of (austenite + ferrite) is obtained during reheating, and the austenite grain size becomes small and quenching becomes insufficient, so that a predetermined strength cannot be ensured. On the other hand, when heating is performed exceeding (Ac 3 transformation point + 70 ° C.), the austenite grain size becomes coarse, the toughness of the base material is remarkably lowered, and the drop weight characteristic is also lowered.
なお、上記Ａｃ１変態点、Ａｃ３変態点の温度は、実測して求めてもよいが、次式で計算した値を用いることができる。
Ａｃ１（℃）＝７５１−２７×Ｃ＋１８×Ｓｉ−１２×Ｍｎ−１６９×Ａｌ
−２３×Ｃｕ−２３×Ｎｉ＋２４×Ｃｒ＋２３×Ｍｏ−４０×Ｖ
＋２３３×Ｎｂ−６×Ｔｉ−８９５×Ｂ
Ａｃ３（℃）＝９３７−４７７×Ｃ＋５６×Ｓｉ−２０×Ｍｎ＋１９８×Ａｌ
−１６×Ｃｕ−２７×Ｎｉ−５×Ｃｒ＋３８×Ｍｏ＋１２５×Ｖ−１９×Ｎｂ
＋１３６×Ｔｉ＋３３１５×Ｂ
（ただし、上記式中の各元素記号は、それぞれの元素の含有量（ｍａｓｓ％））
Incidentally, the Ac 1 transformation point, the temperature of the Ac 3 transformation point can be determined by actually measuring it, it is possible to use a value determined by the following equation.
Ac 1 (° C.) = 751-27 × C + 18 × Si-12 × Mn-169 × Al
−23 × Cu-23 × Ni + 24 × Cr + 23 × Mo-40 × V
+ 233 × Nb-6 × Ti-895 × B
Ac 3 (° C.) = 937-477 × C + 56 × Si-20 × Mn + 198 × Al
-16 * Cu-27 * Ni-5 * Cr + 38 * Mo + 125 * V-19 * Nb
+ 136 × Ti + 3315 × B
(However, each element symbol in the above formula is the content of each element (mass%))
また、上記温度に再加熱した後の焼入れは、５℃／ｓ以上の冷却速度で水冷するのが好ましい。冷却速度が５℃／ｓ未満では、焼入性が確保できず、所望の下部ベイナイトおよびマルテンサイト組織とすることができず、落重特性が低下するからである。
さらに、焼入れ処理の回数については、通常は１回で十分であるが、２回以上行い、オーステナイトへの変態とフェライトへの変態を繰り返すことで、焼入れ時のオーステナイト粒径が細粒化、整粒化するので、母材靭性を向上させる効果を得ることができるので好ましい。
The quenching after reheating to the above temperature is preferably water-cooled at a cooling rate of 5 ° C./s or more. If the cooling rate is less than 5 ° C./s, the hardenability cannot be secured, the desired lower bainite and martensite structure cannot be obtained, and the drop weight characteristic is lowered.
Furthermore, the number of times of quenching is usually sufficient, but it is performed twice or more, and by repeating the transformation to austenite and transformation to ferrite, the austenite grain size at the time of quenching is refined and adjusted. Since it granulates, since the effect which improves a base material toughness can be acquired, it is preferable.
上記焼入処理を施した厚鋼板は、その後、ＰＷＨＴを実施する場合の強度低下を極力抑えることを考慮し、比較的高温の５５０〜７００℃の温度で焼戻処理を施すことが必要である。焼戻し温度が５５０℃未満では、５５０℃以上温度でＰＷＨＴを行う場合の強度低下が大きく、所望の強度（ＴＳ：５７０ＭＰａ以上、７２０ＭＰａ以下）を安定して確保することができない。一方、７００℃超えでは、焼戻し後に、上記所望の強度を確保することが難しくなるからである。なお、焼戻処理後の冷却は、空冷でも構わない。 The steel plate subjected to the above quenching treatment needs to be subjected to a tempering treatment at a relatively high temperature of 550 to 700 ° C. in consideration of minimizing the strength reduction when PWHT is performed thereafter. . When the tempering temperature is less than 550 ° C., the strength is greatly reduced when PWHT is performed at a temperature of 550 ° C. or higher, and a desired strength (TS: 570 MPa or higher, 720 MPa or lower) cannot be secured stably. On the other hand, when the temperature exceeds 700 ° C., it is difficult to ensure the desired strength after tempering. The cooling after the tempering process may be air cooling.
ここで、本発明において重要なことは、焼入れままの鋼板の板厚１／２位置での引張強さを８５０ＭＰａ以上、１１００ＭＰａ以下に制御することである。上述した成分組成を有する板厚１００ｍｍ以下の鋼板において、焼入後の板厚１／２位置の組織を下部ベイナイトおよびマルテンサイト組織とし、有効結晶粒径の微細化を通じて、落重特性を向上させるためには、焼入後の引張強度を８５０ＭＰａ以上１１００ＭＰａ以下に制御することが必要である。８５０ＭＰａ未満では、焼入性が不十分で、上部ベイナイトやフェライトなどの組織が混在することになり、母材靭性、特に、落重特性が大きく低下する。一方、１１００ＭＰａを超えると落重特性が劣化するからである。焼入れままの引張強さは、より好ましくは９００〜１０５０ＭＰａの範囲である。 Here, what is important in the present invention is to control the tensile strength of the as-quenched steel sheet at the position of half the plate thickness to 850 MPa or more and 1100 MPa or less. In a steel sheet having a thickness of 100 mm or less having the above-described component composition, the structure at the half thickness position after quenching is made to be a lower bainite and martensite structure, and the drop weight characteristics are improved through refinement of the effective crystal grain size. Therefore, it is necessary to control the tensile strength after quenching to 850 MPa or more and 1100 MPa or less. If it is less than 850 MPa, hardenability is inadequate and structures, such as an upper bainite and a ferrite, will be mixed, and base material toughness, especially a drop weight characteristic will fall large. On the other hand, when the pressure exceeds 1100 MPa, the drop weight characteristic deteriorates. The tensile strength as quenched is more preferably in the range of 900 to 1050 MPa.
なお、本発明は、板厚１００ｍｍ以下の厚鋼板を適用の対象としているが、特に船舶や、海洋構造物、水圧鉄管等に用いられる厚鋼板の主要板厚である３０〜８０ｍｍ、中でも板厚が４０〜７０ｍｍの厚鋼板において、優れた効果が得られる。 In addition, although this invention makes the object of application the thick steel plate of 100 mm or less in thickness, it is 30-80 mm which is the main plate thickness of the thick steel plate especially used for a ship, a marine structure, a hydraulic iron pipe, etc. In a thick steel plate having a thickness of 40 to 70 mm, an excellent effect is obtained.
表１に示した各種成分組成を有するＮｏ．１〜３２の鋼を溶製し、連続鋳造して厚さが３１０ｍｍまたは２１０ｍｍの鋼素材（スラブ）とした後、表２に示す種々の条件で制御圧延・制御冷却した後、表２に示した板厚の厚鋼板（Ｎｏ．１〜３７）とした。
次いで、上記厚鋼板を、同じく表２に示す種々の条件で、加熱炉で再加熱した後、水冷して焼入れし、引き続き、加熱炉で再加熱した後、空冷する焼戻し処理を施した。
No. having various component compositions shown in Table 1. 1 to 32 are melted and continuously cast into a steel material (slab) having a thickness of 310 mm or 210 mm, and then subjected to controlled rolling and controlled cooling under various conditions shown in Table 2, and then shown in Table 2. It was set as the thick steel plate (No. 1-37).
Next, the thick steel plate was reheated in a heating furnace under various conditions, which are also shown in Table 2, and then water-cooled and quenched, and then reheated in a heating furnace and then air-cooled.
斯くして得た厚鋼板について、下記の試験に供した。
＜ミクロ組織＞
上記厚鋼板の圧延方向（Ｌ方向）から採取したサンプルの断面を研磨し、ナイタール液で腐食した後、板厚１／２位置において、光学顕微鏡および走査型電子顕微鏡を用いて組織観察を行い、金属組織相を特定した。
また、上記サンプルの同じ板厚１／２位置をＥＢＳＰで測定した方位差マッピンッグから、隣り合った結晶粒の粒界方位差が１５度以上の粒界によって囲まれた結晶粒の粒界をトレースし、画像解析により円相当径を求め、平均結晶粒径を算出した。
＜集合組織＞
｛１１０｝＜１１０＞方位への集積度は、板厚１/２位置から、板面に平行な面について試験片厚１ｍｍの試験片を採取し、Ｘ線回折して（１１０）、（２００）、（２１１）正極点図を測定し、これらを用いて３次元結晶方位密度関数を計算し、Φ２＝４５°断面における集積度から求めた。
＜引張試験＞
上記厚鋼板および焼入れ後の鋼板板厚１／２位置から引張方向が圧延方向に直角な方向（Ｃ方向）となるようにＡＳＴＭ Ａ３７０に規定された直径が１２．７ｍｍ、ＧＬが５０．８ｍｍの引張試験片を採取し、Ａ３７０に準拠して引張試験を行い、降伏応力ＹＳおよび引張強さＴＳを測定した。
＜靭性の評価＞
上記厚鋼板の板厚１／２位置から圧延方向に直角（Ｃ方向）となるように、ＡＳＴＭ Ａ３７０に規定された２ｍｍＶノッチシャルピー衝撃試験片を採取し、Ａ３７０に準拠してシャルピー衝撃試験を実施し、破面遷移温度（ｖＴｒｓ）を求めた。
＜落重特性＞
上記厚鋼板の板厚１／２位置から、ＡＳＴＭ Ｅ２０８に準拠して、圧延方向に直角（Ｃ方向）となるようにＰ−３試験片（厚さ１６ｍｍ×幅５０ｍｍ×長さ１３０ｍｍ）を採取し、落重試験を行い、無延性遷移温度ＴＮＤＴを求めた。
The thick steel plate thus obtained was subjected to the following test.
<Microstructure>
After polishing the cross section of the sample taken from the rolling direction of the thick steel plate (L direction) and corroding with the nital liquid, at the plate thickness 1/2 position, the structure is observed using an optical microscope and a scanning electron microscope, A metallographic phase was identified.
In addition, from the orientation difference mapping obtained by measuring the same plate thickness 1/2 position of the above sample with EBSP, the grain boundary of the crystal grain surrounded by the grain boundary where the grain boundary orientation difference between adjacent crystal grains is 15 degrees or more is traced. The equivalent circle diameter was determined by image analysis, and the average crystal grain size was calculated.
<Group organization>
The degree of integration in the {110} <110> orientation is determined by taking a test piece having a thickness of 1 mm from a position parallel to the plate surface from a position of 1/2 the plate thickness, and performing X-ray diffraction (110), (200 ), (211) The positive electrode dot diagram was measured, and using these, the three-dimensional crystal orientation density function was calculated, and obtained from the degree of integration in the Φ2 = 45 ° cross section.
<Tensile test>
The diameter specified in ASTM A370 is 12.7 mm and GL is 50.8 mm so that the tensile direction is a direction perpendicular to the rolling direction (C direction) from the thick steel plate and the steel plate thickness 1/2 position after quenching. Tensile test specimens were collected and subjected to a tensile test according to A370, and yield stress YS and tensile strength TS were measured.
<Evaluation of toughness>
A 2 mm V notch Charpy impact test piece specified in ASTM A370 is sampled so that it is perpendicular to the rolling direction (C direction) from the position of half the thickness of the steel plate, and a Charpy impact test is conducted according to A370. The fracture surface transition temperature (vTrs) was obtained.
<Drop weight characteristics>
P-3 specimen (thickness 16 mm x width 50 mm x length 130 mm) is taken from the position of 1/2 the thickness of the steel plate so as to be perpendicular to the rolling direction (C direction) in accordance with ASTM E208. and performs drop weight test was determined no ductile transition temperature T NDT.
上記測定の結果を表２に併記した。これらの結果から、本発明に適合するＮｏ．１〜１８の厚鋼板は、焼入後の引張強さが８５０ＭＰａ以上、１１００ＭＰａ以下であり、焼戻し後の引張強さＴＳが５７０〜７２０ＭＰａの範囲にあり、かつ、落重試験におけるＴＮＤＴが−６５℃以下という優れた靭性特性を有していることがわかる。
これに対して、本発明の条件を満たさないＮｏ．１９〜３７の厚鋼板は、引張強さＴＳおよび落重試験におけるＴＮＤＴのいずれか１つ以上の特性が、上記条件を満たしていない。
なお、参考として、図１に示した焼入れ後の引張強さと焼戻し後の無延性遷移温度ＴＮＤＴとの関係図の上に、上記発明例および比較例の結果をプロットした結果を、図２に示した。
The results of the above measurements are also shown in Table 2. From these results, No. 1 suitable for the present invention. Steel plate of 1 to 18, baked tensile strength of after turning at least 850 MPa, or less 1100 MPa, tensile strength TS after tempering is in the range of 570～720MPa, and the T NDT in drop weight test - It turns out that it has the outstanding toughness characteristic of 65 degrees C or less.
On the other hand, No. which does not satisfy the conditions of the present invention. Steel plate of 19 to 37 may be any one or more characteristics of T NDT in tensile strength TS and drop weight test, it does not satisfy the above condition.
For reference, the results of plotting the results of the above-described invention examples and comparative examples on the relationship diagram between the tensile strength after quenching shown in FIG. 1 and the non-ductile transition temperature T NDT after tempering are shown in FIG. Indicated.
Claims (6)
Ｃ：０．０５〜０．１５ｍａｓｓ％、
Ｓｉ：０．０５〜０．６０ｍａｓｓ％、
Ｍｎ：０．８〜１．８ｍａｓｓ％、
Ｐ：０．０２０ｍａｓｓ％以下、
Ｓ：０．００５ｍａｓｓ％以下、
Ａｌ：０．００５〜０．０８０ｍａｓｓ％、
Ｎ：０．０００５〜０．００５０ｍａｓｓ％を含有し、さらに、
Ｃｕ：０．５０ｍａｓｓ％以下、
Ｎｉ：１．５０ｍａｓｓ％以下、
Ｃｒ：０．５０ｍａｓｓ％以下、
Ｍｏ：０．４０ｍａｓｓ％以下、
Ｖ：０．０７ｍａｓｓ％以下および
Ｂ：０．０００３〜０．００２０ｍａｓｓ％のうちから選ばれる１種または２種以上を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有し、
下記（１）式で定義されるＣ当量Ｃｅｑ（ｍａｓｓ％）が０．３６〜０．５５で、
下記（２）式で定義されるＤＩ＊値が６５以上であり、さらに、
板厚１／２位置におけるミクロ組織が焼戻し下部ベイナイトおよび焼戻しマルテンサイト組織からなり、板厚１／２位置における｛１１１｝＜１１０＞方位の集積度が１．８以上で、かつ、板厚１／２位置におけるＮＲＬ落重試験のＴＮＤＴが−６５℃以下であることを特徴とする船舶用、海洋構造物用および水圧鉄管用厚鋼板。
記
Ｃｅｑ（ｍａｓｓ％）＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５ ・・・（１）
ＤＩ＊＝８．２×（Ｃ）０．５×（０．７Ｓｉ＋１）×ｆＭｎ×（０．３５Ｃｕ＋１）×（０．３６Ｎｉ＋１）×（２．１６Ｃｒ＋１）×（３．００Ｍｏ＋１）×（１．７５Ｖ＋１）×ｆＢ ・・・（２）
ただし、ｆＭｎ＝３．３３Ｍｎ＋１ （Ｍｎ≦１．２ｍａｓｓ％の場合）
ｆＭｎ＝（Ｍｎ−１．２）×５．１＋５ （Ｍｎ＞１．２ｍａｓｓ％の場合）
ｆＢ＝１．３ （Ｂ≧０．０００３ｍａｓｓ％の場合）
ｆＢ＝１．０ （Ｂ＜０．０００３ｍａｓｓ％の場合） It is a steel plate that has been subjected to re-rolling, quenching and tempering after controlled rolling and controlled cooling,
C: 0.05-0.15 mass%,
Si: 0.05-0.60 mass%,
Mn: 0.8 to 1.8 mass%,
P: 0.020 mass% or less,
S: 0.005 mass% or less,
Al: 0.005-0.080 mass%,
N: 0.0005 to 0.0050 mass%,
Cu: 0.50 mass% or less,
Ni: 1.50 mass% or less,
Cr: 0.50 mass% or less,
Mo: 0.40 mass% or less,
V: 0.07 mass% or less and B: One or more selected from 0.0003 to 0.0020 mass%, with the balance being composed of Fe and inevitable impurities,
C equivalent C eq (mass%) defined by the following formula (1) is 0.36 to 0.55,
The DI * value defined by the following formula (2) is 65 or more, and
The microstructure at the plate thickness 1/2 position is composed of a tempered lower bainite and a tempered martensite structure, the accumulation degree of {111} <110> orientation at the plate thickness 1/2 position is 1.8 or more, and the plate thickness 1 / 2 marine where T NDT of NRL drop weight test in position is characterized in that at -65 ° C. or less, offshore structures and for penstock for steel plate.
C eq (mass%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (1)
DI * = 8.2 × (C) 0.5 × (0.7Si + 1) × fMn × (0.35Cu + 1) × (0.36Ni + 1) × (2.16Cr + 1) × (3.00Mo + 1) × (1.75V + 1) ) × fB (2)
However, fMn = 3.33Mn + 1 (when Mn ≦ 1.2 mass%)
fMn = (Mn−1.2) × 5.1 + 5 (in the case of Mn> 1.2 mass%)
fB = 1.3 (when B ≧ 0.0003 mass%)
fB = 1.0 (when B <0.0003 mass%)
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