JP2007046096A - Method for producing thick high strength steel plate having excellent toughness, and thick high strength steel plate having excellent toughness - Google Patents

Method for producing thick high strength steel plate having excellent toughness, and thick high strength steel plate having excellent toughness Download PDF

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JP2007046096A
JP2007046096A JP2005230595A JP2005230595A JP2007046096A JP 2007046096 A JP2007046096 A JP 2007046096A JP 2005230595 A JP2005230595 A JP 2005230595A JP 2005230595 A JP2005230595 A JP 2005230595A JP 2007046096 A JP2007046096 A JP 2007046096A
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toughness
strength steel
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JP4660315B2 (en
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Akihiko Kojima
明彦 児島
Kazutoshi Ichikawa
和利 市川
Yoichi Tanaka
洋一 田中
Masanori Minagawa
昌紀 皆川
Yukitaka Masuda
晋宇 益田
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Nippon Steel Corp
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<P>PROBLEM TO BE SOLVED: To simultaneously satisfy a sheet thickness of 50 to 80 mm, a yield strength in a class of 390 to 460 MPa, toughness in the central part of the sheet thickness, high heat input weld HAZ (Heat Affected Zone) toughness and high rolling efficiency. <P>SOLUTION: A continuously cast slab having a composition in which the contents of C, Si, Mn, P, S, Al, Ti, N and O are controlled to prescribed ranges, the content of B is controlled to 0.0003 to 0.003%, the content of Mo is controlled to 0.01 to 0.2%, one or more selected from Ca and Mg are comprised, an effective B content obtained by prescribed formula is controlled to ≥0.0003%, a carbon equivalent Ceq is controlled to 0.30 to 0.40%, and the balance iron is cooled to Ar<SB>3</SB>-200°C, is thereafter reheated at 1,000 to 1,250°C, is hot-rolled so as to be finished at 800 to 900°C, and is successively subjected to accelerated cooling from Ar<SB>3</SB>or higher. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

本発明は、大入熱溶接の熱影響部（Ｈｅａｔ Ａｆｆｅｃｔｅｄ Ｚｏｎｅ ： ＨＡＺ）靭性と母材靭性に優れた厚手高強度鋼板を高い圧延能率で製造する方法と厚手高強度鋼板に係るものであり、本発明は鉄鋼業における厚鋼板の製造現場に適用され、本発明によって製造される厚手鋼板は、大型コンテナ船などの造船向けとして主に使用されるが、建築、橋梁、タンク、海洋構造物など、その他の溶接構造物に広く使用することも可能である。   The present invention relates to a method for producing a thick high strength steel sheet with high rolling efficiency and a thick high strength steel sheet excellent in heat-affected zone (HAZ) toughness and base metal toughness of high heat input welding, The present invention is applied to the production site of thick steel plates in the steel industry, and the thick steel plates produced by the present invention are mainly used for shipbuilding such as large container ships, but are used for construction, bridges, tanks, offshore structures, etc. It can also be widely used for other welded structures.

造船に代表される溶接構造物の近年のニーズは、構造物の大型化、破壊に対する高い安全性、建造における溶接の高能率化、鋼材の安定供給と経済性、である。このような動向を受け、溶接構造物に使われる厚手鋼板に対して、（１）高い降伏強度、（２）良好な板厚中心部靭性、（３）良好な大入熱溶接ＨＡＺ靭性、（４）高い圧延能率、のニーズが高まりつつある。
具体的には、大型コンテナ船に用いられる板厚５０〜８０ｍｍの鋼板に対して、（１）降伏強度：３９０〜４６０ＭＰａ級、（２）板厚中心部靭性：ｖＴｒｓ≦−６０℃、（３）溶接入熱量≧２０ｋＪ／ｍｍのＨＡＺ靭性：ｖＥ（−４０℃）≧４７Ｊ、（４）圧延能率：圧延終了温度≧８００℃、を同時に満たすことが要求される。 The recent needs for welded structures such as shipbuilding are large size structures, high safety against breakage, high efficiency of welding in construction, stable supply of steel materials and economic efficiency. In response to such trends, for thick steel plates used in welded structures, (1) high yield strength, (2) good thickness center toughness, (3) good high heat input HAZ toughness, ( 4) The need for high rolling efficiency is increasing.
Specifically, for a steel plate having a thickness of 50 to 80 mm used for a large container ship, (1) yield strength: 390 to 460 MPa class, (2) thickness center toughness: vTrs ≦ −60 ° C., (3 ) HAZ toughness of welding heat input ≧ 20 kJ / mm: vE (−40 ° C.) ≧ 47 J, (4) rolling efficiency: rolling end temperature ≧ 800 ° C.

特許文献１は造船向け厚手高強度鋼板に関する技術の一例であり、この特許文献１には板厚５０〜７０ｍｍを有しつつ、上記（１）、（３）、（４）のニーズを部分的に満足することができる技術が開示されている。しかしながら、特許文献１の実施例の記載からわかるように、上記（２）のニーズを満足することは示されていない。
次に、非特許文献１の図９に示されるように、厚手鋼板では板厚１／２部の靭性が板厚１／４部に比べて低いのが一般的であるから、厚手鋼板を用いた溶接構造物の安全性を考える場合は、鋼板の板厚１／２部靭性を重視する必要がある。例えば、船舶の安全性においては、溶接部から発生した脆性破壊が母材部に伝播して停止すること（アレスト性）が重要である。そして、板厚の厚手化に伴ってアレスト性は板厚１／２部靭性に支配される傾向を強める。従って、大型船舶の安全性向上のためには、厚手鋼板の板厚１／２部靭性を高めることが有効である。特許文献１に示されるように、現状の大型船舶向け厚手鋼板は板厚１／４部靭性が−４０℃を満足するレベルであるが、今後の安全性を重視した大型船舶に対しては、板厚１／２部靭性が−６０℃を満足するレベルの厚手鋼板が望まれる。 Patent Document 1 is an example of a technology related to a thick high-strength steel sheet for shipbuilding. This Patent Document 1 has a thickness of 50 to 70 mm and partially satisfies the needs of (1), (3), and (4). A technique that can satisfy the requirements is disclosed. However, as can be seen from the description of the example in Patent Document 1, it is not shown that the above-mentioned need (2) is satisfied.
Next, as shown in FIG. 9 of Non-Patent Document 1, a thick steel plate generally has a toughness of 1/2 plate thickness lower than that of a 1/4 thickness portion. When considering the safety of the welded structure, it is necessary to place an emphasis on the toughness of the 1/2 thickness part of the steel sheet. For example, in ship safety, it is important that brittle fracture generated from a welded portion propagates to the base material and stops (arrestability). As the plate thickness increases, the arrestability increases the tendency to be dominated by the plate thickness 1/2 part toughness. Therefore, in order to improve the safety of large vessels, it is effective to increase the plate thickness 1/2 part toughness of the thick steel plate. As shown in Patent Document 1, the current thick steel plate for large ships is at a level where the ¼ part thickness toughness satisfies −40 ° C., but for large ships with an emphasis on future safety, A thick steel plate having a level of 1/2 thickness toughness satisfying −60 ° C. is desired.

従来から、母材靭性の向上には、ＴＭＣＰ（Ｔｈｅｒｍｏ Ｍｅｃｈａｎｉｃａｌ Ｃｏｎｔｒｏｌ Ｐｒｏｃｅｓｓ）を前提として圧延終了温度を低く制御して、鋼板の金属組織を微細化することが有効であることが知られている。前記非特許文献１の図９や図１２では、７００〜７８０℃の範囲で圧延終了温度（圧延仕上温度）を低くするほど板厚１／２部靭性が高まることが示されており、−５０〜−７０℃程度のｖＴｒｓ（板厚中心部靭性）が達成されている。
また、非特許文献２の図４に示す例では、圧延終了温度（圧延仕上温度）を７００℃程度まで低くすることで−６０℃程度のｖＴｒｓが達成されている。このように、従来のＴＭＣＰ技術では、圧延終了温度を８００℃未満の低い温度に制御することで厚手鋼板の良好な靭性が達成されてきた。しかしながら、このような低温圧延では圧延途中の温度待ち時間が長くなって圧延能率が著しく低下し、生産性の低下によって製造原価が上昇する問題がある。また、短期間に多量の鋼板供給を求められる場合には、生産能力が逼迫して供給不可能に陥る問題がある。従って、圧延終了温度を８００℃以上に高めて圧延能率を阻害することなく厚手鋼板の板厚１／２部靭性を確保する技術が望まれる。 Conventionally, it is known that to improve the base metal toughness, it is effective to refine the metal structure of the steel sheet by controlling the rolling end temperature low on the premise of TMCP (Thermo Mechanical Control Process). 9 and 12 of Non-Patent Document 1 show that the plate thickness 1/2 part toughness increases as the rolling end temperature (rolling finishing temperature) is lowered in the range of 700 to 780 ° C., and −50 VTrs (plate thickness center part toughness) of about -70 degreeC is achieved.
In the example shown in FIG. 4 of Non-Patent Document 2, vTrs of about −60 ° C. is achieved by lowering the rolling end temperature (rolling finishing temperature) to about 700 ° C. Thus, with the conventional TMCP technology, good toughness of thick steel plates has been achieved by controlling the rolling end temperature to a low temperature of less than 800 ° C. However, in such low temperature rolling, there is a problem that the temperature waiting time in the middle of rolling becomes long, the rolling efficiency is remarkably lowered, and the manufacturing cost is increased due to the decrease in productivity. In addition, when a large amount of steel sheet supply is required in a short period of time, there is a problem that the production capacity becomes tight and the supply becomes impossible. Therefore, there is a demand for a technique for securing the plate thickness ½ part toughness of the thick steel plate without increasing the rolling end temperature to 800 ° C. or higher and inhibiting the rolling efficiency.

ＴＭＣＰによって製造される厚手鋼板では、従来からボロン（Ｂ）添加による高強度化が行われてきた。Ｂの効果は、圧延後の加速冷却において、γ粒界に偏析したＢが変態時の焼入性を高める効果と理解されている。特許文献１では、ＢにＮｂを複合添加することを特徴として高強度化を図っているが、実施例に示されるように、この場合の圧延終了温度は９３０〜１０００℃と高いので、板厚５５〜７０ｍｍの鋼板において良好な板厚１／２部靭性が達成できることは示されておらず、板厚１／２部で−６０℃以下のｖＴｒｓを安定的に達成できる保証はない。   Conventionally, thick steel sheets manufactured by TMCP have been strengthened by adding boron (B). The effect of B is understood as an effect that B segregated at the γ grain boundary enhances the hardenability at the time of transformation in accelerated cooling after rolling. In Patent Literature 1, Nb is added to B in combination to increase the strength, but as shown in the examples, the rolling end temperature in this case is as high as 930 to 1000 ° C. It has not been shown that a good thickness 1/2 part toughness can be achieved in a 55 to 70 mm steel sheet, and there is no guarantee that a vTrs of −60 ° C. or lower can be stably achieved at a thickness 1/2 part.

非特許文献３では、板厚２０ｍｍの薄手鋼板ではあるが、ＢにＭｏを複合添加することで効果的に焼入性が高まり、８００〜８８０℃の高い圧延終了温度のもとで良好な強度と靭性を同時に達成できることが示されている。ただしこの場合、Ｃが０．０１５％と低く、Ｍｏが０．３５％と多い。加えて、板厚が薄いことに由来して、圧延によるγ微細化効果が大きく、８００℃から５００℃への平均冷却速度も２０℃／ｓと大きいことから、良好な強度と靭性を達成することは容易であると、考えられる。   In Non-Patent Document 3, although it is a thin steel plate having a thickness of 20 mm, hardenability is effectively increased by adding Mo to B in combination, and good strength is obtained under a high rolling end temperature of 800 to 880 ° C. And toughness can be achieved simultaneously. However, in this case, C is as low as 0.015% and Mo is as high as 0.35%. In addition, due to the fact that the plate thickness is thin, the effect of γ refinement by rolling is large, and the average cooling rate from 800 ° C. to 500 ° C. is also as large as 20 ° C./s, so that good strength and toughness are achieved. It is considered easy.

更に、薄手鋼板に大入熱溶接は適用されることはないから、非特許文献３の技術では大入熱溶接ＨＡＺの局所脆化相ＭＡ（Ｍａｒｔｅｎｓｉｔｅ Ａｕｓｔｅｎｉｔｅ ｃｏｎｓｔｉｔｕｅｎｔ）の増加に対するＭｏの有害性を気にすることなく、０．３５％もの多量のＭｏを利用できる。従って、ＴＭＣＰ型のＢ添加厚手鋼板において、高い圧延能率のもとで良好な板厚１／２部靭性を確保することや、良好な大入熱溶接ＨＡＺ靭性を確保する前提のもとでＭｏを有効利用する技術が望まれる。
特許第３５９９５５６号公報 製鉄研究第326号（1987）、pp.55-61 Ｒ＆Ｄ神戸製鋼技報、Vol.52、No.1(Apr.2002)、pp.20-24 ISIJ International， Vol.44(2004)，No.8，pp.1431-1440 Furthermore, since high heat input welding is not applied to thin steel plates, the technique of Non-Patent Document 3 is concerned with the harmfulness of Mo to the increase in the local embrittlement phase (MA) of high heat input welding HAZ. As much as 0.35% Mo can be used without making it. Therefore, in the TMCP type B-added thick steel plate, Mo is secured on the premise of ensuring good plate thickness 1/2 part toughness under high rolling efficiency and ensuring high heat input HAZ toughness. Technology that makes effective use of is desired.
Japanese Patent No. 3599556 Steel Research No.326 (1987), pp.55-61 R & D Kobe Steel Engineering Reports, Vol.52, No.1 (Apr.2002), pp.20-24 ISIJ International, Vol.44 (2004), No.8, pp.1431-1440

以上説明のように、ＴＭＣＰ型Ｂ添加厚手鋼板において、母材強度、母材靭性、大入熱溶接ＨＡＺ靭性、圧延能率、を本発明が狙う高い次元で同時に満足する技術の確立が望まれている。
本発明の課題は、板厚５０〜８０ｍｍの厚手鋼板において、（１）高い降伏強度（３９０〜４６０ＭＰａ級）、（２）良好な板厚中心部靭性（ｖＴｒｓ≦−６０℃）、（３）良好な大入熱溶接ＨＡＺ靭性（溶接入熱量≧２０ｋＪ／ｍｍ、ｖＥ（−４０℃）≧４７Ｊ）、（４）高い圧延能率（圧延終了温度≧８００℃）、を同時に満足することである。 As described above, in the TMCP type B-added thick steel sheet, it is desired to establish a technology that satisfies the base material strength, base material toughness, high heat input welding HAZ toughness, and rolling efficiency at the same high level that the present invention aims at. Yes.
The problems of the present invention are as follows: (1) high yield strength (390-460 MPa class), (2) good center thickness toughness (vTrs ≦ −60 ° C.), (3) Good high heat input welding HAZ toughness (welding heat input ≧ 20 kJ / mm, vE (−40 ° C.) ≧ 47 J) and (4) high rolling efficiency (rolling end temperature ≧ 800 ° C.) are satisfied at the same time.

上記課題を解決するための本発明の要旨は、下記のとおりである。
「１」 本発明の靭性に優れた厚手高強度鋼板の製造方法は、質量％で、Ｃ ：０．０３〜０．０７％、Ｓｉ：０．２％以下、Ｍｎ：１．２〜２．０％、Ｐ：０．０１５％以下、Ｓ：０．０００５〜０．００５％、Ｂ：０．０００３〜０．００３％、Ｍｏ：０．０１〜０．２％、Ａｌ：０．００１〜０．０３４％、Ｔｉ：０．００５〜０．０２％、Ｎ：０．００１〜０．００８％、Ｏ：０．００１〜０．００４％を含有し、さらにＣａ：０．０００３〜０．００４％、Ｍｇ：０．０００３〜０．００４％のうち１種以上を含有し、下式（１）と（２）を満たし、
有効Ｂ量（％）≧０．０００３％ （１）
０．３０％≦Ｃｅｑ（％）≦０．４０％ （２）
残部が鉄および不可避的不純物によって化学成分が構成される連続鋳造スラブを、Ａｒ３が下式（４）で計算されるとき、連続鋳造後にＡｒ３−２００℃以下まで冷却した後に１０００〜１２５０℃に再加熱し、その後に下式（３）を満たす熱間圧延を８００℃〜９００℃で終了し、その後にＡｒ３以上から加速冷却することを特徴とする。
ただし、Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５ の関係とし、熱間圧延の圧下について、スラブ厚み（ｍｍ）／圧延終了時鋼板厚み（ｍｍ）≧２．５ （３）の関係とし、Ａｒ３について、Ａｒ３（℃）＝９１０−３１０Ｃ−８０Ｍｎ−２０Ｃｕ−５５Ｎｉ−８０Ｍｏ （４）の関係とし、有効Ｂ量とは、変態前のγ（オーステナイト）素地に固溶するＢ量を意味する。 The gist of the present invention for solving the above problems is as follows.
"1" The manufacturing method of the thick high-strength steel plate excellent in toughness of the present invention is mass%, C: 0.03-0.07%, Si: 0.2% or less, Mn: 1.2-2. 0%, P: 0.015% or less, S: 0.0005-0.005%, B: 0.0003-0.003%, Mo: 0.01-0.2%, Al: 0.001- 0.034%, Ti: 0.005-0.02%, N: 0.001-0.008%, O: 0.001-0.004%, and Ca: 0.0003-0. 004%, Mg: containing one or more of 0.0003 to 0.004%, satisfying the following formulas (1) and (2),
Effective B amount (%) ≧ 0.0003% (1)
0.30% ≦ Ceq (%) ≦ 0.40% (2)
Balance iron and unavoidable chemical component configured continuously cast slab by impurities, when Ar 3 is calculated by the following formula (4), after cooling to Ar 3 -200 ° C. or less after continuous casting 1000 to 1250 ° C. Then, the hot rolling satisfying the following formula (3) is finished at 800 ° C. to 900 ° C., and then accelerated cooling from Ar 3 or higher is performed.
However, the relationship of Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 is satisfied, and the reduction of hot rolling is as follows: slab thickness (mm) / steel plate thickness at the end of rolling (mm) ≧ 2.5 (3) In relation to Ar 3 , Ar 3 (° C.) = 910-310C-80Mn-20Cu-55Ni-80Mo (4), and the effective B amount is B dissolved in the γ (austenite) substrate before transformation. Means quantity.

「２」 本発明の靭性に優れた厚手高強度鋼板の製造方法は、前記有効Ｂ量において、
（ａ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ≦０のとき
（ａ１） Ｎ−０．２９Ｔｉ＞０ならば、
有効Ｂ量＝Ｂ−０．７７（Ｎ−０．２９Ｔｉ）
（ａ２） Ｎ−０．２９Ｔｉ≦０ならば、
有効Ｂ量＝Ｂ
（ｂ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ＞０、かつ、Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）≧０．００５のとき、
（ｂ１）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕＞０ならば、
有効Ｂ量＝Ｂ−０．７７｛Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕｝
（ｂ２）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕≦０ならば、有効Ｂ量＝Ｂ
とすることを特徴とする。 “2” The method for producing a thick high-strength steel sheet having excellent toughness according to the present invention is as follows.
(A) When O-0.4Ca-0.66Mg-0.89Al ≦ 0 (a1) If N-0.29Ti> 0,
Effective B amount = B−0.77 (N−0.29Ti)
(A2) If N−0.29Ti ≦ 0,
Effective B amount = B
(B) When O-0.4Ca-0.66Mg-0.89Al> 0 and Ti-2 (O-0.4Ca-0.66Mg-0.89Al) ≧ 0.005,
(B1) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.89Al)]> 0,
Effective B amount = B−0.77 {N−0.29 [Ti-2 (O−0.4Ca−0.66Mg−0.89Al)]}
(B2) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.89Al)] ≤0, effective B amount = B
It is characterized by.

「３」 本発明の靭性に優れた厚手高強度鋼板の製造方法は、前記の組成に加えて、さらに、質量％でＣｕ：０．０１〜１％、Ｎｉ：０．０１〜３％、Ｃｒ：０．０１〜０．５％、Ｎｂ：０．００１〜０．０２％、Ｖ：０．００１〜０．１％の中から１種以上を含有する化学成分組成の連続鋳造スラブを用いることを特徴とする。 [3] In addition to the above composition, the method for producing a thick high-strength steel sheet having excellent toughness according to the present invention further includes Cu: 0.01 to 1%, Ni: 0.01 to 3%, Cr in mass%. : 0.01-0.5%, Nb: 0.001-0.02%, V: Use a continuous casting slab having a chemical composition containing at least one of 0.001-0.1%. It is characterized by.

「４」 本発明の靭性に優れた厚手高強度鋼板の製造方法は、前記の組成に加えて、さらに、質量％でＲＥＭ：０．０００３〜０．０２％、Ｚｒ：０．０００３〜０．０２％のうち１種以上を含有し、（１）式に代わって下式（５）を満たすことを特徴とする。
ここで、有効Ｂについて、
有効Ｂ量（％）≧０．０００３％ （５）
ただし、
（ｃ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ≦０のとき
（ｃ１）Ｎ−０．２９Ｔｉ＞０ならば、
有効Ｂ量＝Ｂ−０．７７（Ｎ−０．２９Ｔｉ）
（ｃ２）Ｎ−０．２９Ｔｉ≦０ならば、
有効Ｂ量＝Ｂ
（ｄ）ただし、Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ＞０、かつ、Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）≧０．００５のとき、
（ｄ１）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕＞０ならば、
有効Ｂ量＝Ｂ−０．７７｛Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕｝
（ｄ２）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕≦０ならば、
有効Ｂ量＝Ｂ “4” In addition to the above composition, the method for producing a thick high-strength steel sheet having excellent toughness according to the present invention, in addition to REM: 0.0003 to 0.02%, Zr: 0.0003 to 0.00. It contains at least one of 02% and satisfies the following formula (5) instead of the formula (1).
Here, for effective B,
Effective B amount (%) ≧ 0.0003% (5)
However,
(C) O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al≤0 (c1) If N-0.29Ti> 0,
Effective B amount = B−0.77 (N−0.29Ti)
(C2) If N−0.29Ti ≦ 0,
Effective B amount = B
(D) However, O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al> 0 and Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0 .35Zr-0.89Al) ≧ 0.005,
(D1) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)]> 0,
Effective B amount = B-0.77 {N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)]}
(D2) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)] ≤0,
Effective B amount = B

「５」 本発明の靭性に優れた厚手高強度鋼板は、Ｃ：０．０３〜０．０７％（質量％、以下同じ）、Ｓｉ：０．２％以下、Ｍｎ：１．２〜２．０％、Ｐ：０．０１５％以下、Ｓ：０．０００５〜０．００５％、Ｂ：０．０００３〜０．００３％、Ｍｏ：０．０１〜０．２％、Ａｌ：０．００１〜０．０３４％、Ｔｉ：０．００５〜０．０２％、Ｎ：０．００１〜０．００８％、Ｏ：０．００１〜０．００４％、を含有し、さらにＣａ：０．０００３〜０．００４％、Ｍｇ：０．０００３〜０．００４％、のうち１種以上を含有し、残部が鉄および不可避的不純物からなり、板厚：５０〜８０ｍｍ、降伏強度：３９０〜４６０ＭＰａ級、板厚中心部靭性：ｖＴｒｓ：−６０℃以下、溶接入熱量≧２０ｋＪ／ｍｍ、のＨＡＺ靭性：ｖＥ（−４０℃）≧４７Ｊの条件を満足することを特徴とする。 “5” Thick high-strength steel sheets with excellent toughness according to the present invention are: C: 0.03 to 0.07% (mass%, the same shall apply hereinafter), Si: 0.2% or less, Mn: 1.2-2. 0%, P: 0.015% or less, S: 0.0005-0.005%, B: 0.0003-0.003%, Mo: 0.01-0.2%, Al: 0.001- 0.034%, Ti: 0.005-0.02%, N: 0.001-0.008%, O: 0.001-0.004%, and Ca: 0.0003-0 0.004%, Mg: 0.0003 to 0.004%, one or more of them, the balance being iron and inevitable impurities, plate thickness: 50 to 80 mm, yield strength: 390 to 460 MPa class, plate Center thickness toughness: vTrs: −60 ° C. or less, welding heat input ≧ 20 kJ / mm, HAZ toughness: vE (−40 ) And satisfies the condition of ≧ 47J.

「６」 本発明は前記の靭性に優れた厚手高強度鋼板において、下式（１）と（２）を満たすことを特徴とする。
有効Ｂ量（％）≧０．０００３％ （１）
０．３０％≦Ｃｅｑ（％）≦０．４０％ （２）
ただし、有効Ｂ量とは、変態前のγ（オーステナイト）素地に固溶するＢ量を意味し、Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５とする。 “6” The present invention is characterized in that the thick high-strength steel sheet having excellent toughness satisfies the following expressions (1) and (2).
Effective B amount (%) ≧ 0.0003% (1)
0.30% ≦ Ceq (%) ≦ 0.40% (2)
However, the effective B amount means the amount of B dissolved in the γ (austenite) substrate before transformation, and Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15.

「７」 本発明は前記の靭性に優れた厚手高強度鋼板において、前記（１）式の有効Ｂ量において、
（ａ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ≦０のとき
（ａ１） Ｎ−０．２９Ｔｉ＞０ならば、
有効Ｂ量＝Ｂ−０．７７（Ｎ−０．２９Ｔｉ）
（ａ２） Ｎ−０．２９Ｔｉ≦０ならば、
有効Ｂ量＝Ｂ
（ｂ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ＞０、かつ、Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）≧０．００５のとき、
（ｂ１）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕＞０ならば、
有効Ｂ量＝Ｂ−０．７７｛Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕｝
（ｂ２）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕≦０ならば、有効Ｂ量＝Ｂとされてなることを特徴とする。 "7" The present invention is the above-described thick high-strength steel sheet excellent in toughness, and in the effective B amount of the formula (1),
(A) When O-0.4Ca-0.66Mg-0.89Al ≦ 0 (a1) If N-0.29Ti> 0,
Effective B amount = B−0.77 (N−0.29Ti)
(A2) If N−0.29Ti ≦ 0,
Effective B amount = B
(B) When O-0.4Ca-0.66Mg-0.89Al> 0 and Ti-2 (O-0.4Ca-0.66Mg-0.89Al) ≧ 0.005,
(B1) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.89Al)]> 0,
Effective B amount = B−0.77 {N−0.29 [Ti-2 (O−0.4Ca−0.66Mg−0.89Al)]}
(B2) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.89Al)] ≤0, the effective B amount is set to B.

「８」 本発明は前記の靭性に優れた厚手高強度鋼板において、前記（１）式の有効Ｂ量において、さらに、質量％でＲＥＭ：０．０００３〜０．０２％、Ｚｒ：０．０００３〜０．０２％のうち１種以上を含有し、（１）式に代わって下式（５）を満たすことを特徴とする。
有効Ｂ量（％）≧０．０００３％ （５）
ただし、
（ｃ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ≦０のとき
（ｃ１）Ｎ−０．２９Ｔｉ＞０ならば、
有効Ｂ量＝Ｂ−０．７７（Ｎ−０．２９Ｔｉ）
（ｃ２）Ｎ−０．２９Ｔｉ≦０ならば、
有効Ｂ量＝Ｂ
（ｄ）ただし、Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ＞０、かつ、Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）≧０．００５のとき、
（ｄ１）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕＞０ならば、
有効Ｂ量＝Ｂ−０．７７｛Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕｝
（ｄ２）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕≦０ならば、
有効Ｂ量＝Ｂ “8” The present invention is the above-described thick high-strength steel sheet excellent in toughness, and in the effective B amount of the formula (1), REM: 0.0003 to 0.02% by mass%, Zr: 0.0003 It contains at least one of -0.02%, and satisfies the following formula (5) instead of the formula (1).
Effective B amount (%) ≧ 0.0003% (5)
However,
(C) O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al≤0 (c1) If N-0.29Ti> 0,
Effective B amount = B−0.77 (N−0.29Ti)
(C2) If N−0.29Ti ≦ 0,
Effective B amount = B
(D) However, O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al> 0 and Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0 .35Zr-0.89Al) ≧ 0.005,
(D1) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)]> 0,
Effective B amount = B-0.77 {N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)]}
(D2) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)] ≤0,
Effective B amount = B

本発明によれば、板厚が５０〜８０ｍｍの厚手鋼板において、（１）高い降伏強度（３９０〜４６０ＭＰａ級）、（２）良好な板厚中心部靭性（ｖＴｒｓ≦−６０℃）、（３）良好な大入熱溶接ＨＡＺ靭性（溶接入熱量≧２０ｋＪ／ｍｍ、ｖＥ（−４０℃）≧４７Ｊ）、（４）高い圧延能率（圧延終了温度≧８００℃）、を同時に満足できる。
従って、本発明によって製造される厚手鋼板が造船をはじめとする各種の溶接構造物に使用されることで、構造物の大型化、建造における溶接の高能率化、破壊に対する高い安全性、鋼材の安定供給と経済性、が同時に満たされ、産業上の効果は計り知れない。 According to the present invention, in a thick steel plate having a thickness of 50 to 80 mm, (1) high yield strength (390 to 460 MPa class), (2) good center thickness toughness (vTrs ≦ −60 ° C.), (3 ) Good high heat input welding HAZ toughness (welding heat input ≧ 20 kJ / mm, vE (−40 ° C.) ≧ 47 J), (4) high rolling efficiency (rolling end temperature ≧ 800 ° C.) can be satisfied at the same time.
Therefore, the thick steel plate manufactured by the present invention is used for various welded structures including shipbuilding, so that the size of the structure, the efficiency of welding in construction, the high safety against destruction, Stable supply and economy are satisfied at the same time, and industrial effects are immeasurable.

本発明は、ＴＭＣＰ型のＢ添加厚手鋼板において、母材強度、母材靭性、大入熱溶接ＨＡＺ靭性、圧延能率、を同時に高める技術である。これら複数の特性を高い次元でバランスさせるために本発明者が種々研究した結果、緩冷条件で特徴的に発現されるＢと、ごく微量のＭｏの組合せよる焼入性の向上効果を見つけ出し、緩冷条件となる厚手母材と大入熱溶接ＨＡＺの両方で生成する上部ベイナイトの靭性を高める技術を発明した。   The present invention is a technique for simultaneously improving the base material strength, base material toughness, high heat input welding HAZ toughness, and rolling efficiency in a TMCP type B-added thick steel plate. As a result of various studies conducted by the inventor in order to balance these multiple characteristics at a high level, the present inventors have found out the effect of improving hardenability by combining B that is characteristically expressed under slow cooling conditions and a very small amount of Mo. The inventors have invented a technique for increasing the toughness of upper bainite produced by both a thick base metal and a large heat input welding HAZ that are under slow cooling conditions.

まず第一に、母材強度と大入熱ＨＡＺ靭性が相反することの解決を目指した。
板厚５０〜８０ｍｍでの加速冷却時の冷却速度は、現有設備による通常の冷却条件においては、平均冷却速度で３〜１０℃／ｓと小さく、板厚１／２部ではさらに緩冷となる。このような緩冷条件のもとで十分な焼入性を確保して３９０〜４６０ＭＰａ級の降伏強度を達成するためには、単純には合金元素を多量に添加すればよいが、そうすると大入熱溶接ＨＡＺの焼入性も高まってＨＡＺが硬化して脆化する。そこで、ＨＡＺ硬さの目安である炭素当量（Ｃｅｑ）をできるだけ低減しつつ、母材強度を確保する技術として、ＢとＭｏの複合添加に着眼し、ＴＭＣＰ型厚手鋼板を前提に、母材強度、大入熱溶接ＨＡＺ靭性、母材靭性、圧延能率を同時に満足する最適な鋼の化学成分と鋼板製造条件を見出した。
まず、母材でＢの焼入性を引き出すためにＭｏに着眼したが、Ｍｏは非常に高価であり、大入熱溶接ＨＡＺでＭＡ生成（マルテンサイト−オーステナイト混合相生成：ＭＡ：Ｍａｒｔｅｎｓｉｔｅ− Ａｕｓｔｅｎｉｔｅ ｃｏｎｓｔｉｔｕｅｎｔ）を助長する有害性があることから、従来技術に対してＭｏの添加量を可能な限り抑えつつ、その効果を有効に引き出す工夫が不可欠となった。そのためには、Ｂに複合させる微量Ｍｏの効果と、その効果が発揮される条件を定量的に明確化することが肝心であると考えた。 First of all, we aimed to solve the conflict between the base metal strength and the high heat input HAZ toughness.
The cooling rate at the time of accelerated cooling with a plate thickness of 50 to 80 mm is as low as 3 to 10 ° C./s at an average cooling rate under normal cooling conditions with existing equipment, and is further cooled at a plate thickness of ½ part. . In order to ensure sufficient hardenability under such slow cooling conditions and achieve a yield strength of 390 to 460 MPa, a large amount of alloy elements may be simply added. The hardenability of the heat-welded HAZ is also increased, and the HAZ hardens and becomes brittle. Therefore, as a technology to ensure the strength of the base metal while reducing the carbon equivalent (Ceq), which is a measure of the HAZ hardness, as much as possible, we focused on the combined addition of B and Mo, and based on the TMCP type thick steel plate, In addition, the present inventors have found the optimum steel chemical composition and steel plate production conditions that simultaneously satisfy high heat input welding HAZ toughness, base metal toughness, and rolling efficiency.
First, Mo was focused on in order to bring out the hardenability of B with the base material, but Mo is very expensive, and MA is generated by high heat input welding HAZ (Martensite-austenite mixed phase generation: MA: Martensite-Austenite). Therefore, it has become indispensable to devise the effect effectively while suppressing the addition amount of Mo as much as possible with respect to the prior art. For that purpose, it was considered important to clarify quantitatively the effect of the trace amount Mo combined with B and the conditions under which the effect is exhibited.

そこで、Ｂ添加を前提にＭｏ低減を系統的に検討する中で次の知見を見い出した。
図１は１０ｐｐｍＢを含む鋼材（ベース組成：０．０５％Ｃ−０．１％Ｓｉ−１．５％Ｍｎ−０．０１％Ａｌ−０．０１％Ｔｉ−０．００３％Ｎ−０．００２％Ｏ−０．００１％Ｃａ）にわずか０．０８％のＭｏを複合することによる変態開始温度の低下代と冷却速度の関係を示す。
図１に示す結果から、Ｂにわずか０．０８％のＭｏを複合して鋼材に添加すると、１０℃／ｓ以下（例えば０．５℃／ｓ〜１０℃／ｓの範囲）の緩冷条件において鋼材の焼入性が効果的に向上する効果を見出した。しかも、冷却速度が小さくなるほどその効果が増大する現象を発見した。 Then, the following knowledge was found in examining Mo reduction systematically on the assumption of B addition.
FIG. 1 shows a steel material containing 10 ppm B (base composition: 0.05% C-0.1% Si-1.5% Mn-0.01% Al-0.01% Ti-0.003% N-0.002). % O-0.001% Ca) shows a relationship between a cooling allowance and a reduction margin of the transformation start temperature due to the composite of only 0.08% Mo.
From the result shown in FIG. 1, when only 0.08% of Mo is combined with B and added to the steel material, a slow cooling condition of 10 ° C./s or less (for example, a range of 0.5 ° C./s to 10 ° C./s). The effect of improving the hardenability of the steel material was found. Moreover, they discovered a phenomenon in which the effect increases as the cooling rate decreases.

つまり、加速冷却される板厚５０〜８０ｍｍの鋼板（平均冷却速度：３〜１０℃／ｓ）において、従来知見に対してごく微量のＭｏをＢと複合添加することで焼入性の向上効果が現れ、その効果は冷却速度が不足する厚手材や板厚内部ほど大きくなるという特徴を見出した。この定量的な新規の知見に基づき、本発明ではＢと微量Ｍｏの複合効果をＴＭＣＰ型厚手鋼板へ利用し、母材組織を均一な上部ベイナイト主体に制御することで所定の降伏強度を達成する。このとき、有効Ｂ量を０．０００３％以上確保し、Ｃｅｑを０．３０％以上確保し、スラブ再加熱温度を１０００〜１２５０℃に制御し、圧延後にＡｒ３以上から加速冷却（平均冷却速度３〜１０℃／ｓで冷却することを意味する。）を行い、このときに加速冷却が可能な手段の一例として水量密度を０．３ｍ３／ｍ２／ｍｉｎ以上確保することが好ましい。これは、高い焼入性を確保して高い降伏強度を安定的に達成するためである。
鋼の化学成分で決まる有効Ｂ量が０．０００３％より少ないと固溶Ｂが不足してＢの焼入性が不足する。Ｃｅｑが０．３０％未満ではＢ以外の化学成分の焼入性が不足する。 In other words, in a steel plate with an average thickness of 50 to 80 mm (average cooling rate: 3 to 10 ° C./s), the effect of improving hardenability is obtained by adding a very small amount of Mo in combination with B to the conventional knowledge. As a result, the effect was found to increase as the thickness of the thick material and the thickness inside the plate decreased. Based on this quantitative new knowledge, in the present invention, the composite effect of B and trace Mo is used for the TMCP type thick steel plate, and the predetermined yield strength is achieved by controlling the base material structure to be mainly the upper bainite. . At this time, the effective B amount is secured 0.0003% or more, Ceq is secured 0.30% or more, the slab reheating temperature is controlled to 1000 to 1250 ° C., and accelerated cooling (average cooling rate) from Ar 3 or more after rolling. It is preferable to secure a water density of 0.3 m 3 / m 2 / min or more as an example of means capable of accelerated cooling at this time. This is to ensure high hardenability and stably achieve high yield strength.
If the effective B amount determined by the chemical composition of the steel is less than 0.0003%, the solid solution B is insufficient and the hardenability of B is insufficient. If Ceq is less than 0.30%, the hardenability of chemical components other than B is insufficient.

また、スラブ再加熱温度が１０００℃未満だとＢ炭化物が溶解しないので、有効Ｂ量が十分でも実質的な固溶Ｂが不足してＢの焼入性が不足する。スラブ再加熱温度が１２５０℃を超えるとＴｉＮが溶解して固溶Ｎが増加し、圧延中あるいは圧延後にＢＮが析出するので、有効Ｂ量が十分でも実質的な固溶Ｂが不足しＢの焼入性が不足する。このとき同時に、ピン止め効果が低減してγ粒の粗大化が生じる。
圧延後の加速冷却をＡｒ３未満から行うと、フェライト分率が増加して降伏強度が大きく低下する。加速冷却を行う場合の水量密度が０．３ｍ３／ｍ２／ｍｉｎ未満だと冷却速度が不足して焼入性が不足する。 Moreover, since B carbide | carbonized_material does not melt | dissolve when slab reheating temperature is less than 1000 degreeC, even if effective B amount is enough, substantial solid solution B is insufficient and the hardenability of B is insufficient. When the slab reheating temperature exceeds 1250 ° C., TiN dissolves and solute N increases, and BN precipitates during or after rolling. Therefore, even if the effective B amount is sufficient, substantial solute B is insufficient and B Insufficient hardenability. At the same time, the pinning effect is reduced and coarsening of γ grains occurs.
When accelerated cooling after rolling is performed from less than Ar 3 , the ferrite fraction increases and the yield strength decreases significantly. If the water density in the case of accelerated cooling is less than 0.3 m 3 / m 2 / min, the cooling rate is insufficient and the hardenability is insufficient.

次に、大入熱溶接ＨＡＺ靭性でのＭｏ添加の有害性、つまり、ＭＡの増加を抑えることが本発明の大きな課題である。単純にＢと微量Ｍｏを複合添加すると大入熱溶接ＨＡＺでの変態温度が低下し、ＭＡを多く含む上部ベイナイトが生成し、靭性が不安定となる問題が生じる。そこで、Ｂ−微量Ｍｏ成分を前提に大入熱溶接ＨＡＺでのベイナイト変態温度を高め、冷却中のＭＡ分解を促進することを検討した。
その検討の結果、適正にＣとＳｉを低減すれば、ＭＡをごく少量しか含まない、あるいはＭＡを全く含まない上部ベイナイトが高温で生成することを見出した。このように、Ｂ−微量Ｍｏ成分に低Ｃ−低Ｓｉ成分を組み合わせると、脆化相であるＭＡが減少することに加えて、脆化相であるセメンタイトが少量かつ微細に分散し、硬さも低下する効果を得られることが判明した。このような大入熱溶接ＨＡＺの上部ベイナイト制御を主眼とした成分最適化により、ＭＡ減少、セメンタイト少量微細化、硬さ低減、の三つの効果が重畳し、Ｂ−微量Ｍｏ成分でも良好な大入熱溶接ＨＡＺ靭性を達成できる見込みが得られた。 Next, it is a big problem of the present invention to suppress the harmful effect of Mo addition in high heat input welding HAZ toughness, that is, to suppress the increase of MA. If B and a trace amount of Mo are simply added together, the transformation temperature in the high heat input welding HAZ is lowered, upper bainite containing a large amount of MA is generated, and the toughness becomes unstable. Therefore, it was studied to increase the bainite transformation temperature in the high heat input welding HAZ on the premise of the B-trace Mo component and promote the MA decomposition during cooling.
As a result of the study, it was found that if C and Si are appropriately reduced, upper bainite containing only a small amount of MA or not containing MA at all is formed at a high temperature. As described above, when the low C-low Si component is combined with the B-trace Mo component, the embrittlement phase MA is decreased, and the embrittlement phase cementite is dispersed in a small amount and finely. It turned out that the effect to reduce can be acquired. By optimizing the components focusing on the upper bainite control of such high heat input welding HAZ, the three effects of MA reduction, cementite small-scale refinement, and hardness reduction are superimposed, and the B-trace Mo component is also good in size. The prospect of achieving heat input welding HAZ toughness was obtained.

さらに、ＨＡＺ靭性を安定化させるために、ＣａやＭｇの適正添加によって微細な酸化物や硫化物を多数分散させ、γ粒成長抑制技術（ピン止め）を組合せることを検討した。本発明のようにＨＡＺが上部ベイナイトに制御される場合、脆性破壊における破面単位はγ粒径にほぼ一致する。このような場合、破面単位の微細化を通じて靭性を向上する手段はγ細粒化しかない、と思われる。従って、本発明のＨＡＺ靭性に対しては、従来よりもピン止め効果の寄与は大きいことに着眼した。換言すると、従来鋼のＨＡＺはフェライトとベイナイトが混在し、しかも粒内変態を利用できる余地があるから、本発明よりピン止め効果の寄与は小さい。
このような理由から、本発明の大入熱溶接ＨＡＺではピン止め効果を積極的に利用する。このとき、Ｃｅｑを０．４０％以下に制御する必要がある。Ｃｅｑが０．４０％を超えると、例え低Ｃ−低Ｓｉ−微量Ｍｏ−Ｂ成分でも、上部ベイナイトにおけるＭＡ増加と硬さ上昇を回避することが難しく、γ細粒化を図っても良好なＨＡＺ靭性を得ることは容易ではない。 Furthermore, in order to stabilize the HAZ toughness, it was investigated to disperse a large number of fine oxides and sulfides by appropriate addition of Ca and Mg, and to combine γ grain growth suppression technology (pinning). When HAZ is controlled to upper bainite as in the present invention, the fracture surface unit in brittle fracture substantially matches the γ grain size. In such a case, it seems that the means for improving toughness through the refinement of the fracture surface unit is only γ-fine graining. Therefore, it was noted that the contribution of the pinning effect to the HAZ toughness of the present invention is greater than in the past. In other words, the HAZ of the conventional steel contains ferrite and bainite, and there is room for utilizing the intragranular transformation, so the contribution of the pinning effect is smaller than that of the present invention.
For this reason, the high heat input welding HAZ of the present invention positively uses the pinning effect. At this time, it is necessary to control Ceq to 0.40% or less. When Ceq exceeds 0.40%, it is difficult to avoid the increase in MA and hardness in the upper bainite, even with a low C-low Si-trace Mo-B component, and good γ grain refinement Obtaining HAZ toughness is not easy.

上述の理由から、本発明の基本成分は低Ｃ−低Ｓｉ−微量Ｍｏ−Ｂに絞り込むこととした。続いて、母材靭性と圧延能率が相反することの解決を目指した。８００℃以上の高温圧延を前提に、板厚１／２部靭性を高めることが課題である。
本発明の母材組織は上部ベイナイト主体に制御されるから、上部ベイナイトの脆性破壊における金属学的要因を、本成分系のもとで具体的に検討した。
その結果、i）γ粒微細化、ii）ＭＡ低減、iii）セメンタイト少量微細化、が板厚１／２部靭性の向上にとって極めて重要であるとの結論に達し、先述した大入熱溶接ＨＡＺ靭性への対策が有効利用できることが判明した。 For the reasons described above, the basic components of the present invention are narrowed down to low C-low Si-trace Mo-B. Subsequently, we aimed to solve the conflict between base material toughness and rolling efficiency. On the premise of high temperature rolling at 800 ° C. or higher, it is a problem to increase the plate thickness ½ part toughness.
Since the base metal structure of the present invention is controlled mainly by upper bainite, metallurgical factors in the brittle fracture of upper bainite were specifically examined under this component system.
As a result, it was concluded that i) refinement of γ grains, ii) reduction of MA, and iii) refinement of a small amount of cementite were extremely important for improving the toughness of the ½ part thickness, and the high heat input welding HAZ described above. It has been found that measures against toughness can be used effectively.

以下にi）γ粒微細化、ii）ＭＡ低減、iii）セメンタイト少量微細化を制御する手段を説明する。
i）γ粒微細化のために、スラブ再加熱時のγ粒を細粒化する。そのために、連続鋳造後にスラブをＡｒ３−２００℃以下に冷却してγ→α変態させ、その後に１２５０℃以下に再加熱することでα→γ変態させる必要がある。スラブをＡｒ３−２００℃以上から再加熱すると、スラブ内部でγ→α変態が未完了のうちに再加熱されて鋳造時の粗大γが残存する。本成分系では、鋳造時のγ粒が従来鋼より大きくなる欠点があり、この欠点を克服するために確実にγ→α→γ変態を経由してスラブを再加熱することが重要である。
本発明では、スラブ冷却時や圧延後水冷までの空冷時を対象に以下の式（４）を用いてＡｒ３を計算する。これらの場合、冷却速度が極めて小さいので、Ｂの焼入性は実質的に無視できるほど小さく、下式（４）をＡｒ３の目安として使って実用上問題ない。
Ａｒ３（℃）＝９１０−３１０Ｃ−８０Ｍｎ−２０Ｃｕ−５５Ｎｉ−８０Ｍｏ （４） Hereinafter, means for controlling i) γ grain refinement, ii) MA reduction, and iii) small cementite refinement will be described.
i) To refine γ grains, refine γ grains during slab reheating. Therefore, after continuous casting, it is necessary to cool the slab to Ar 3 −200 ° C. or lower and perform γ → α transformation, and then reheat to 1250 ° C. or lower to perform α → γ transformation. When the slab is reheated from Ar 3 −200 ° C. or higher, the γ → α transformation is not yet completed inside the slab, and coarse γ during casting remains. In this component system, there is a drawback that the γ grains at the time of casting become larger than that of conventional steel, and in order to overcome this disadvantage, it is important to reliably reheat the slab via the γ → α → γ transformation.
In the present invention, Ar 3 is calculated using the following equation (4) for slab cooling and air cooling until rolling and water cooling. In these cases, since the cooling rate is extremely low, the hardenability of B is so small that it can be substantially ignored, and there is no practical problem using the following formula (4) as a measure of Ar 3 .
Ar 3 (° C.) = 910-310C-80Mn-20Cu-55Ni-80Mo (4)

また、再加熱温度が１２５０℃を超えるとγ粒成長が著しくなり粗大γが生成する。更にi）γ粒微細化のために、圧延終了温度が８００〜９００℃のもとで圧延再結晶を繰り返す。そのために、スラブ厚み／圧延終了時鋼板厚みが２．５以上となる十分な累積圧下量を確保する必要がある。累積圧下量がこれより少ないと、圧延再結晶γが十分には微細化しない。また、圧延終了温度が９００℃を超えると、圧延途中や圧延終了後に再結晶γが成長して粗大化する。これは、Ｂと微量Ｍｏの複合添加による再結晶γ粒の成長抑制効果が９００℃を超えるとほとんど効かなくなるからである。Ｎｂを添加して８００〜９００℃で圧延を終了すると、γの一部あるいは全部が未再結晶γとなるから、より一層のγ微細化が容易であるが、Ｂ−微量Ｍｏ成分にＮｂを添加すると大入熱溶接ＨＡＺでＭＡ生成が助長される有害性がある。
従って、本発明では大入熱溶接ＨＡＺ靭性の観点からできるだけＮｂ添加を控えつつ、母材靭性を高めなければならない制約がある。 On the other hand, when the reheating temperature exceeds 1250 ° C., the growth of γ grains becomes remarkable and coarse γ is generated. Further, i) rolling recrystallization is repeated at a rolling end temperature of 800 to 900 ° C. for the refinement of γ grains. Therefore, it is necessary to ensure a sufficient cumulative reduction amount that the slab thickness / steel plate thickness at the end of rolling is 2.5 or more. If the cumulative reduction amount is less than this, the rolling recrystallization γ is not sufficiently refined. When the rolling end temperature exceeds 900 ° C., recrystallized γ grows and becomes coarse during and after rolling. This is because the effect of suppressing the growth of recrystallized γ grains due to the combined addition of B and a small amount of Mo becomes almost ineffective when it exceeds 900 ° C. When Nb is added and rolling is completed at 800 to 900 ° C., part or all of γ becomes non-recrystallized γ, so that further γ refinement is easy. However, Nb is added to the B-trace Mo component. If added, there is a danger that MA formation is promoted by high heat input welding HAZ.
Therefore, in the present invention, from the viewpoint of high heat input welding HAZ toughness, there is a restriction that the base metal toughness must be improved while adding Nb as much as possible.

ii）ＭＡ低減とiii）セメンタイト少量微細化のためには、基本的には大入熱溶接ＨＡＺの場合と同様の成分最適化が有効である。つまり、Ｂ−微量Ｍｏ成分を前提にＢの焼入性を適正に確保したうえで、Ｃ低減、Ｓｉ低減、Ｃｅｑ上限規制、Ｎｂ低減を図り、ＭＡ生成を抑えてセメンタイトを少量微細化した上部ベイナイトに制御する。ＭＡ減少には必要に応じて水冷停止温度の高温化や６５０℃以下の焼戻し処理を適用することが有効であるが、母材強度が低下するので所定の強度を下回らない条件で適用する必要がある。なお、焼戻し処理はオフライン作業となるので、圧延ラインにおける圧延能率を低下させる原因にならない。
以上説明したように、本発明の要点は、ＴＭＣＰ型厚手母材と大入熱溶接ＨＡＺに共通する小さな冷却速度のもとで、Ｂと微量Ｍｏの組合せによる焼入性の向上効果を利用し、ＭＡやセメンタイトなどの脆化相が少量かつ微細に分散した靭性の良好な上部ベイナイト組織を活用することである。 In order to reduce ii) MA and iii) refine cementite in small quantities, the same component optimization as in the case of high heat input welding HAZ is basically effective. In other words, on top of the B-trace Mo component, after ensuring the hardenability of B properly, C reduction, Si reduction, Ceq upper limit regulation, Nb reduction is aimed at, MA formation is suppressed and a small amount of cementite is refined Control to bainite. In order to reduce MA, it is effective to increase the water cooling stop temperature or tempering at 650 ° C. or less as necessary. However, since the strength of the base material decreases, it is necessary to apply it under conditions that do not fall below a predetermined strength. is there. In addition, since the tempering process is an off-line operation, it does not cause a reduction in rolling efficiency in the rolling line.
As described above, the main point of the present invention is that the effect of improving the hardenability by the combination of B and a very small amount of Mo is used under a small cooling rate common to the TMCP type thick base metal and the high heat input welding HAZ. It is to utilize an upper bainite structure having good toughness in which a brittle phase such as MA or cementite is dispersed in a small amount and finely.

以下に本発明における鋼の化学成分について限定理由を説明する。
Ｃは厚手母材で高い強度を確保するためにＢ−微量Ｍｏ添加と相俟って０．０３％以上必要である。Ｂ−微量Ｍｏ成分のもとで良好な大入熱溶接ＨＡＺ靭性と良好な板厚１／２部靭性を確保するために、Ｃを０．０７％以下に抑える必要がある。
Ｓｉは脱酸作用を有するが、Ｂ−微量Ｍｏ成分のもとで良好な大入熱溶接ＨＡＺ靭性と良好な板厚１／２部靭性を確保するために、Ｓｉを０．２％以下に抑える必要がある。
Ｍｎは脱酸作用を有すると同時に、厚手母材で高い強度を経済的に確保するために１．２％以上必要である。ただし、２．０％を超えてＭｎを添加すると、スラブの中心偏析の有害性が顕著となるため、板厚１／２部の母材靭性や大入熱溶接ＨＡＺ靭性が劣化するためこれが上限である。
Ｐは不純物元素であり、板厚１／２部靭性と大入熱溶接ＨＡＺ靭性を安定的に確保するために、０．０１５％以下に低減する必要がある。
Ｓは必要な元素であり、後述する大入熱溶接ＨＡＺでのピン止め効果のために０．０００５％以上添加する。Ｓは適正に添加されたＣａやＭｇと結合して、微細な硫化物を数多く形成してγ細粒化をもたらす。しかし、Ｓが０．００５％を超えると硫化物が粗大化してピン止め効果が低下すると同時に、破壊起点しての有害性も顕著となるから、大入熱溶接ＨＡＺ靭性が劣化する。 Below, the reason for limitation is demonstrated about the chemical component of the steel in this invention.
C is a thick base material and needs to be 0.03% or more in combination with B-trace Mo addition in order to ensure high strength. In order to ensure good large heat input welding HAZ toughness and good thickness 1/2 part toughness under the B-trace Mo component, it is necessary to suppress C to 0.07% or less.
Si has a deoxidizing action, but in order to ensure good large heat input HAZ toughness and good plate thickness 1/2 part toughness under the B-trace Mo component, Si should be 0.2% or less. It is necessary to suppress.
Mn has a deoxidizing action, and at the same time, 1.2% or more is necessary to economically ensure a high strength with a thick base material. However, if Mn is added in excess of 2.0%, the hazard of central segregation of the slab becomes significant, so that the base metal toughness and the high heat input weld HAZ toughness of the plate thickness 1/2 part deteriorate, so this is the upper limit. It is.
P is an impurity element, and needs to be reduced to 0.015% or less in order to stably secure the plate thickness 1/2 part toughness and the high heat input welded HAZ toughness.
S is a necessary element, and is added in an amount of 0.0005% or more for the pinning effect in high heat input welding HAZ described later. S combines with properly added Ca and Mg to form a large number of fine sulfides, resulting in γ-fine graining. However, if S exceeds 0.005%, the sulfide becomes coarse and the pinning effect decreases, and at the same time, the harmfulness from the start of fracture becomes significant, so the high heat input HAZ toughness deteriorates.

ＢとＭｏは本発明の特徴的な元素であり、最も重要な元素である。これらを本発明の低いＣと低いＳｉのもとで複合的に添加すると、厚手母材の大入熱溶接ＨＡＺの小さな冷却条件で靭性の良好な上部ベイナイトが得られ、厚手母材と大入熱溶接ＨＡＺの材質を高い次元でバランスさせることができる。そのためには、Ｂを０．０００３％以上、Ｍｏを０．０１％以上添加する必要がある。ただし、０．００３％を超えてＢを添加すると焼入性が低下すると同時に、Ｂ系の粗大析出物が生成して厚手母材と大入熱溶接ＨＡＺの両方で靭性が劣化するため、これが上限である。また、０．２％を超えてＭｏを添加すると、大入熱溶接ＨＡＺのみならず厚手母材においてもＭＡが増加して靭性が劣化する。その上、Ｍｏは非常に高価なので０．２％以上の多量添加は工業製品としての経済性を著しく失う。従って、Ｍｏの上限は０．２％である。大入熱溶接ＨＡＺ靭性と経済性の両面を考慮すると、０．１５％以下のＭｏ添加が好ましい。このようなごく微量のＭｏであっても、小さな冷却速度のもとではＢの焼入性を有効に高める作用が本発明で定量的に明らかになった。   B and Mo are characteristic elements of the present invention and are the most important elements. When these are added together under the low C and low Si of the present invention, an upper bainite with good toughness can be obtained under the small cooling conditions of the high heat input welding HAZ of the thick base material. The material of the heat-welded HAZ can be balanced at a high level. For that purpose, it is necessary to add 0.0003% or more of B and 0.01% or more of Mo. However, when B is added in excess of 0.003%, the hardenability deteriorates, and at the same time, B-based coarse precipitates are generated and the toughness deteriorates in both the thick base metal and the high heat input weld HAZ. It is an upper limit. Moreover, when Mo is added exceeding 0.2%, MA increases not only in large heat input welding HAZ but also in a thick base material, and toughness deteriorates. Moreover, since Mo is very expensive, the addition of a large amount of 0.2% or more remarkably loses economic efficiency as an industrial product. Therefore, the upper limit of Mo is 0.2%. In consideration of both high heat input welding HAZ toughness and economic efficiency, 0.15% or less of Mo addition is preferable. Even with such a very small amount of Mo, the effect of effectively improving the hardenability of B under a small cooling rate has been quantitatively clarified by the present invention.

Ａｌは脱酸を担いＯを低減するために必要である。Ａｌ以外のＳｉ、Ｍｎ、Ｔｉ、Ｃａ、Ｍｇなども脱酸作用があるが、例えこれらの元素が添加される場合でも、０．００１％以上のＡｌがないと安定的にＯを０．００４％以下に抑えることは難しい。ただし、Ａｌが０．０３４％を超えると、アルミナ系の粗大酸化物がクラスター化する傾向を強め、破壊起点として有害化が顕著となるため、これが上限である。
ＴｉはＮと結合してＴｉＮを形成し固溶Ｎを低減する。その結果、添加されたＢがＢＮを形成することを抑え、γ中の固溶Ｂを確保することでＢの焼入性を確保する効果がある。同時に、ＴｉＮはスラブ再加熱時と大入熱溶接ＨＡＺでピン止め効果に貢献しγ細粒化に寄与する。このような二つの効果を同時に発揮するために、Ｔｉを０．００５〜０．０２％、Ｎを０．００１〜０．００８％、有効Ｂ量を０．０００３％以上とする必要がある。ＴｉとＮがそれぞれ０．００５％、０．００１％に満たないとＴｉＮによるピン止め効果が十分に発揮されず、厚手母材と大入熱溶接ＨＡＺ靭性が劣化する。ＴｉとＮがそれぞれ０．０２％、０．００８％を超えると、ＴｉＣ析出や固溶Ｎ増加によって厚手母材と大入熱溶接ＨＡＺ靭性が劣化する。さらに、ＴｉとＮが適正範囲にあっても、有効Ｂ量が０．０００３％未満であると、γ中の固溶Ｂの量が不足して焼入性を確保できないから、厚手母材で上部ベイナイト主体の組織制御が困難となり、強度、靭性を確保できなくなる。 Al is necessary for carrying out deoxidation and reducing O. Si, Mn, Ti, Ca, Mg, etc. other than Al also have a deoxidizing action, but even when these elements are added, if 0.001% or more Al is not present, O is stably added to 0.004. It is difficult to keep it below%. However, if Al exceeds 0.034%, the tendency of the alumina-based coarse oxide to cluster increases, and the detrimental effect becomes significant as a starting point of fracture, so this is the upper limit.
Ti combines with N to form TiN and reduce solid solution N. As a result, there is an effect of suppressing the formation of BN by the added B and ensuring the hardenability of B by securing the solid solution B in γ. At the same time, TiN contributes to the pinning effect during slab reheating and high heat input welding HAZ, and contributes to γ refinement. In order to exhibit such two effects simultaneously, it is necessary to make Ti 0.005-0.02%, N 0.001-0.008%, and the effective B amount 0.0003% or more. When Ti and N are less than 0.005% and 0.001%, respectively, the pinning effect by TiN is not sufficiently exhibited, and the thick base metal and the high heat input welding HAZ toughness deteriorate. If Ti and N exceed 0.02% and 0.008%, respectively, the thick base metal and the high heat input weld HAZ toughness deteriorate due to TiC precipitation and solute N increase. Furthermore, even if Ti and N are in an appropriate range, if the amount of effective B is less than 0.0003%, the amount of solid solution B in γ is insufficient and hardenability cannot be secured. It becomes difficult to control the structure mainly composed of upper bainite, and the strength and toughness cannot be secured.

以下に、有効Ｂ量の考え方を説明する。
化学成分として添加されたＴｉは、溶鋼中の脱酸で消費される場合があり（低Ａｌの場合に起こりやすい）、脱酸後に残ったＴｉが凝固後のγ中でＴｉＮを形成する。このとき、Ｔｉに対してＮが過剰であると、ＴｉＮを形成した後に残ったＮがＢの一部と結合してＢＮを形成する。そして、ＢＮを形成した残りのＢが固溶Ｂとして焼入性に寄与する。この焼入性に寄与する固溶Ｂ量を本発明では有効Ｂ量として扱う。
各元素の添加量、熱力学的な反応順序、生成物質の化学量論組成、に基づいた有効Ｂ量の計算方法を以下に説明する。まず、脱酸力の高い順にＣａ、Ｍｇ、ＲＥＭ（希土類元素）、Ｚｒ、ＡｌがＯと結合すると仮定する。このときの脱酸生成物としてＣａＯ、ＭｇＯ、ＲＥＭ２Ｏ３、ＺｒＯ２、Ａｌ２Ｏ３を仮定して、脱酸されるＯ量を計算する。 The concept of the effective B amount will be described below.
Ti added as a chemical component may be consumed by deoxidation in molten steel (prone to occur in the case of low Al), and Ti remaining after deoxidation forms TiN in γ after solidification. At this time, if N is excessive with respect to Ti, N remaining after forming TiN combines with part of B to form BN. And the remaining B which formed BN contributes to hardenability as the solid solution B. In the present invention, the amount of dissolved B that contributes to the hardenability is treated as the effective amount of B.
A method for calculating the effective B amount based on the addition amount of each element, the thermodynamic reaction sequence, and the stoichiometric composition of the product will be described below. First, it is assumed that Ca, Mg, REM (rare earth element), Zr, and Al are combined with O in descending order of deoxidizing power. Assuming CaO, MgO, REM 2 O 3 , ZrO 2 , and Al 2 O 3 as deoxidation products at this time, the amount of O to be deoxidized is calculated.

Ｔｉよりも脱酸力の強いこれらの元素によって脱酸が完了しない場合、下式を満たす。
Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ＞０
この場合、残ったＯをＴｉが脱酸することになる。そこで、Ｔｉ２Ｏ３を仮定して、脱酸で消費されるＴｉを差し引いた残りのＴｉは下式で計算され、この値が０．００５％以上になる必要がある。
ここで、脱酸で消費されるＴｉを差し引いた残りのＴｉが０．００５％以上必要なのは、先の段落に記載するように、本願に必要なＴｉＮを確保するためである。脱酸で消費されるＴｉを差し引いた残りのＴｉが０．００５％未満であると、ＴｉＮによるピン止め効果が十分に発揮されず、厚手母材と大入熱溶接ＨＡＺ靭性が劣化する。 When deoxidation is not completed by these elements having stronger deoxidizing power than Ti, the following formula is satisfied.
O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al> 0
In this case, Ti will deoxidize the remaining O. Therefore, assuming Ti 2 O 3 , the remaining Ti after subtracting Ti consumed in deoxidation is calculated by the following formula, and this value needs to be 0.005% or more.
Here, the reason why 0.005% or more of the remaining Ti after subtracting Ti consumed in deoxidation is necessary is to secure TiN necessary for the present application as described in the previous paragraph. If the remaining Ti after subtracting Ti consumed by deoxidation is less than 0.005%, the pinning effect by TiN is not sufficiently exhibited, and the thick base material and the high heat input weld HAZ toughness deteriorate.

Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）≧０．００５
脱酸で残った０．００５％以上のＴｉがＴｉＮを形成し、Ｎが残る場合は下式が正の値、Ｎが残らない場合は下式が０または負の値になる。
Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕＞０ ：Ｎが残る場合
Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕≦０ ：Ｎが残らない場合
上式が正の値となってＮが残る場合は、Ｂの一部がＢＮとして消費されるから、下式によって有効Ｂ量が計算される。
有効Ｂ量＝Ｂ−０．７７｛Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕｝
上式が０または負の値となってＮが残らない場合は、下式によって有効Ｂ量が計算される。
有効Ｂ量＝Ｂ Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al) ≧ 0.005
The 0.005% or more of Ti remaining after deoxidation forms TiN, and when N remains, the following expression becomes a positive value, and when N does not remain, the following expression becomes 0 or a negative value.
N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)]> 0: When N remains N-0.29 [Ti-2 ( O−0.4Ca−0.66Mg−0.17REM−0.35Zr−0.89Al)] ≦ 0: N does not remain If the above expression is a positive value and N remains, part of B Is consumed as BN, and the effective B amount is calculated by the following equation.
Effective B amount = B-0.77 {N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)]}
When the above formula is 0 or a negative value and N does not remain, the effective B amount is calculated by the following formula.
Effective B amount = B

前記の式において、Ｃａ、Ｍｇ、ＲＥＭ、Ｚｒ、Ａｌの係数について述べると、溶鋼中での脱酸反応（酸化反応）による生成物（酸化物）としてＣａＯ、ＭｇＯ、ＲＥＭ２Ｏ３、ＺｒＯ２、Ａｌ２Ｏ３を仮定し、これらの酸化物として存在するＯ量を質量％で計算する。
例えば、ＣａＯの場合、原子量はＣａが４０でＯが１６であるから、Ｃａの質量％に対して１６／４０＝０．４のＯが結合する。Ａｌ２Ｏ３であれば原子量はＡｌが２７でＯが１６であるから、Ａｌの質量％に対して（１６×３）／（２７×２）＝０．８９のＯが結合する。以下同様の計算概念として前述の式の各元素の係数（０．６６：Ｍｇ、０．１７：ＲＥＭ、０．３５：Ｚｒ）を規定した。 In the above formula, the coefficients of Ca, Mg, REM, Zr, and Al are described. As products (oxides) by deoxidation reaction (oxidation reaction) in molten steel, CaO, MgO, REM 2 O 3 , ZrO 2 are used. Assuming Al 2 O 3 , the amount of O present as these oxides is calculated in mass%.
For example, in the case of CaO, since the atomic weight is 40 for Ca and 16 for O, O of 16/40 = 0.4 is bonded to the mass% of Ca. In the case of Al 2 O 3 , since the atomic weight is 27 for Al and 16 for O, O of (16 × 3) / (27 × 2) = 0.89 is bonded to the mass% of Al. Hereinafter, coefficients (0.66: Mg, 0.17: REM, 0.35: Zr) of each element of the above-described formula were defined as the same calculation concept.

また、有効Ｂ量の導出式概念を低温側から高温側に遡って示すと下記のようになる。
有効Ｂ量＝成分Ｂ量−ＢａｓＢＮ
→Ｂ ａｓ ＢＮ＝０．７７（Ｎ−Ｎ ａｓ ＴｉＮ）
→Ｎ ａｓ ＴｉＮ＝０．２９（Ｔｉ−Ｔｉ ａｓ Ｔｉ２Ｏ３）
→Ｔｉ ａｓ Ｔｉ２Ｏ３＝２（Ｏ−Ｏ ａｓ ＣａＯ−Ｏ ａｓ ＭｇＯ−Ｏ ａｓ ＲＥＭ２Ｏ３−Ｏ ａｓ ＺｒＯ２−Ｏ ａｓ Ａｌ２Ｏ３）
→Ｏ ａｓ ＣａＯ＝０．４Ｃａ
Ｏ ａｓ ＭｇＯ＝０．６６Ｍｇ
Ｏ ａｓ ＲＥＭ２Ｏ３＝０．１７ＲＥＭ
Ｏ ａｓ ＺｒＯ２＝０．３５Ｚｒ
Ｏ ａｓ Ａｌ２Ｏ３＝０．８９Ａｌ Further, the concept of the derivation formula for the effective B amount is shown as follows from the low temperature side to the high temperature side.
Effective B amount = component B amount−BasBN
→ B as BN = 0.77 (N-N as TiN)
→ N as TiN = 0.29 (Ti—Ti as Ti 2 O 3 )
→ Ti as Ti 2 O 3 = 2 (O—O as CaO—O as MgO—O as REM 2 O 3 —O as ZrO 2 —O as Al 2 O 3 )
→ O as CaO = 0.4Ca
O as MgO = 0.66Mg
O as REM 2 O 3 = 0.17 REM
O as ZrO 2 = 0.35Zr
O as Al 2 O 3 = 0.89Al

次に、有効Ｂ量の導出式概念を高温側から低温側への反応順に示すと下記のようになる。製鋼での精錬→凝固工程で下記の順に反応する。
１）液相（溶鋼中）での脱酸反応（１６００℃付近）
Ｏとの化学的親和力の強い順にＣａＯ→ＭｇＯ→ＲＥＭ２Ｏ３→ＺｒＯ２→Ａｌ２Ｏ３の反応が生じて溶鋼中の溶存Ｏが減少していく。これで脱酸が完了する場合は式（ａ）と（ｃ）で表現される。脱酸が完了せずに溶存Ｏが残る場合は、式（ｂ）、（ｄ）で表現され、Ａｌより弱脱酸元素であるＴｉがＴｉ２Ｏ３として脱酸に寄与し、成分Ｔｉから脱酸で消費されたＴｉａｓＴｉ２Ｏ３を差し引いた残りのＴｉが０．００５％以上となる。 Next, the derivation concept of the effective B amount is shown as follows in the order of reaction from the high temperature side to the low temperature side. Refining in steel making → Reaction in the following order in the solidification process.
1) Deoxidation reaction in liquid phase (in molten steel) (around 1600 ° C)
Dissolved O reactions in the molten steel occurring in strong order of chemical affinity between O CaO → MgO → REM 2 O 3 → ZrO 2 → Al 2 O 3 decreases. When deoxidation is completed by this, it represents with Formula (a) and (c). When dissolved O remains without completion of deoxidation, Ti, which is expressed by the formulas (b) and (d), is weaker deoxidizing element than Al contributes to deoxidation as Ti 2 O 3 , and from component Ti The remaining Ti after subtracting TiasTi 2 O 3 consumed by deoxidation is 0.005% or more.

２）固相（凝固γ中）での脱窒反応（１３００℃付近〜８００℃付近）
Ｎとの化学的親和力の強い順にＴｉＮ→ＢＮ→ＡｌＮの反応が生じて固相γ中の固溶Ｎが減少していく。まず、脱酸で消費された残りのＴｉが脱窒反応を生じる。これで脱窒が完了する場合は式（ａ２）、（ｂ２）、（ｃ２）、（ｄ２）で表現され、γ中に固溶Ｎが存在しないからＢはＢＮを形成せずに全てが固溶Ｂとして存在する。一方、Ｔｉによって脱窒が完了せず固溶Ｎが残る場合は式（ａ１）、（ｂ１）、（ｃ１）、（ｄ１）で表現され、Ｂの一部がＢＮを生成し残りが固溶Ｂとなる。 2) Denitrification reaction in solid phase (in solidification γ) (around 1300 ° C to 800 ° C)
The reaction of TiN → BN → AlN occurs in descending order of chemical affinity with N, and the solid solution N in the solid phase γ decreases. First, the remaining Ti consumed by deoxidation causes a denitrification reaction. When denitrification is completed in this way, it is expressed by the formulas (a2), (b2), (c2), and (d2), and since solute N does not exist in γ, B does not form BN and is all solid. Present as dissolved B. On the other hand, when denitrification is not completed by Ti and solid solution N remains, it is expressed by the formulas (a1), (b1), (c1), and (d1), and a part of B generates BN and the remaining is solid solution. B.

一方、Ｔｉよりも脱酸力の強い元素によって脱酸が完了する場合、下式を満たす。
Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ≦０
この場合、Ｔｉは脱酸で消費されない。ＴｉがＴｉＮを形成し、Ｎが残る場合は下式を満たす。
Ｎ−０．２９Ｔｉ＞０
このときの有効Ｂ量は下式で計算される。
有効Ｂ量＝Ｂ−０．７７（Ｎ−０．２９Ｔｉ）
ＴｉがＴｉＮを形成し、Ｎが残らない場合は下式を満たす。
Ｎ−０．２９Ｔｉ≦０
このときの有効Ｂ量は下式で計算される。
有効Ｂ量＝Ｂ On the other hand, when deoxidation is completed by an element having a stronger deoxidizing power than Ti, the following formula is satisfied.
O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al ≦ 0
In this case, Ti is not consumed by deoxidation. When Ti forms TiN and N remains, the following equation is satisfied.
N-0.29Ti> 0
The effective B amount at this time is calculated by the following equation.
Effective B amount = B−0.77 (N−0.29Ti)
When Ti forms TiN and N does not remain, the following formula is satisfied.
N−0.29Ti ≦ 0
The effective B amount at this time is calculated by the following equation.
Effective B amount = B

前記の式において、Ｎ−０．２９Ｔｉにおける０．２９ＴｉはＮａｓＴｉＮを意味する。原子量はＴｉが４８でＮが１４であるから、Ｔｉ（正確には脱酸で消費されたＴｉを差し引いた残りのＴｉ）の質量％に対して１４／４８＝０．２９のＮが結合する。Ｎ−０．２９Ｔｉ≦０ならば、Ｎは全てＴｉＮで固定され、γ（オーステナイト）素地中に固溶Ｎは存在しない。一方、Ｎ−０．２９Ｔｉ＞０ならば、γ素地中にはＴｉＮのほかに固溶Ｎが存在するので、この固溶ＮはＢと結合してＢＮを生成し、有効Ｂ量を減少させる。   In the above formula, 0.29Ti in N-0.29Ti means NasTiN. Since the atomic weight is 48 for Ti and 14 for N, N of 14/48 = 0.29 is bonded to the mass% of Ti (exactly Ti remaining after subtracting Ti consumed in deoxidation). . If N−0.29Ti ≦ 0, all N is fixed by TiN, and no solid solution N exists in the γ (austenite) substrate. On the other hand, if N−0.29Ti> 0, solid solution N exists in the γ substrate in addition to TiN. Therefore, this solid solution N combines with B to produce BN, reducing the effective B amount. .

Ｏは０．００４％以下に抑える必要がある。Ｏが０．００４％を超えると、酸化物の一部が粗大化して破壊起点として有害性をもたらし、厚手母材と大入熱溶接ＨＡＺ靭性が劣化する。一方で、Ｏは０．００１％以上確保する必要がある。その理由は、大入熱溶接ＨＡＺの溶融線近傍において、ＨＡＺ靭性を安定化させるために、後述するＣａやＭｇの適正添加によって微細な酸化物を多数分散させ、ピン止め効果を強化してγ細粒化をはかるためである。Ｏが０．００１％未満では、酸化物個数が不足して十分なピン止め効果が得られない。
Ｃａ、Ｍｇは溶鋼への添加順序を考慮しつつ一方あるいは両方を０．０００３％以上添加することで、ＣａやＭｇを含有する１０〜５００ｎｍの酸化物や硫化物を１０００個／ｍｍ２以上確保することができる。ＣａやＭｇが０．０００３％未満では、ピン止め粒子である酸化物や硫化物の個数が不足する。しかし、それぞれ０．００４％以上添加すると、酸化物や硫化物が粗大化してピン止め粒子の個数が不足すると同時に、破壊起点としての有害性も顕著となって、大入熱溶接ＨＡＺ靭性が劣化する。 O needs to be suppressed to 0.004% or less. When O exceeds 0.004%, a part of the oxide is coarsened to cause harmfulness as a fracture starting point, and the thick base material and the high heat input welding HAZ toughness are deteriorated. On the other hand, it is necessary to secure O of 0.001% or more. The reason is that in order to stabilize the HAZ toughness in the vicinity of the melting line of the high heat input welding HAZ, a large amount of fine oxides are dispersed by appropriate addition of Ca and Mg, which will be described later, and the pinning effect is strengthened. This is to make the particles finer. If O is less than 0.001%, the number of oxides is insufficient and a sufficient pinning effect cannot be obtained.
Ca or Mg is added in an amount of 0.0003% or more in consideration of the order of addition to the molten steel, thereby ensuring at least 1000 pieces / mm 2 of 10-500 nm oxides and sulfides containing Ca and Mg. can do. When Ca and Mg are less than 0.0003%, the number of oxides and sulfides that are pinning particles is insufficient. However, when 0.004% or more of each is added, the oxides and sulfides become coarse and the number of pinning particles becomes insufficient. At the same time, the hazard as a failure starting point becomes remarkable, and the high heat input welding HAZ toughness deteriorates. To do.

次に、鋼の化学成分における選択元素の限定理由を説明する。
Ｃｕ、Ｎｉ、Ｃｒ、Ｎｂ、Ｖは厚手母材の強度と靭性を確保するために有効であるが、いずれの元素も大入熱溶接ＨＡＺのＭＡ生成を助長するので、各元素の利害得失を慎重に判断して添加量を吟味する必要がある。特に、Ｂ−微量Ｍｏ成分へのＮｂは細心の注意が必要である。これらの元素が厚手母材の材質改善に効果を発揮する下限は、Ｃｕ、Ｎｉ、Ｃｒでは０．０１％であり、Ｎｂ、Ｖでは０．００１％である。一方、大入熱溶接ＨＡＺ靭性を劣化させる観点から、各元素の有害性を反映して、Ｃｕ、Ｎｉ、Ｃｒ、Ｎｂ、Ｖの上限はそれぞれ１％、３％、０．５％、０．０２％、０．１％である。本発明ではＮｂの有害性が大きい。
ＲＥＭ、Ｚｒは脱酸と脱硫に関与するが、板厚１／２部の粗大な延伸ＭｎＳの生成を抑えて硫化物を球状無害化し、厚手母材と大入熱溶接ＨＡＺ靭性を改善する。これらの効果を発揮するためには、ＲＥＭとＺｒの下限はともに０．０００３％である。ただし、これらの添加量を増やしても効果は飽和するため、経済性の観点からＲＥＭとＺｒの上限はともに０．０２％である。ここでのＲＥＭとはＬａやＣｅなどのランタノイド系元素のことである。 Next, the reasons for limiting the selected elements in the chemical composition of steel will be described.
Cu, Ni, Cr, Nb, and V are effective for securing the strength and toughness of the thick base material, but any element promotes the MA formation of the high heat input welding HAZ, so the interests of each element can be reduced. Judgment should be made with careful judgment. In particular, Nb to the B-trace Mo component requires careful attention. The lower limit at which these elements are effective for improving the material quality of the thick base material is 0.01% for Cu, Ni and Cr, and 0.001% for Nb and V. On the other hand, from the viewpoint of degrading high heat input welding HAZ toughness, the upper limits of Cu, Ni, Cr, Nb, and V are 1%, 3%, 0.5%,. 02% and 0.1%. In the present invention, Nb is highly harmful.
REM and Zr are involved in deoxidation and desulfurization, but suppress the formation of coarse stretched MnS having a thickness of 1/2 part to make the sulfide spherical harmless, and improve the thick base metal and high heat input welding HAZ toughness. In order to exert these effects, the lower limits of REM and Zr are both 0.0003%. However, since the effect is saturated even if the addition amount of these is increased, the upper limits of REM and Zr are both 0.02% from the viewpoint of economy. Here, REM is a lanthanoid element such as La or Ce.

製鋼工程で溶鋼の脱酸・脱硫と化学成分を制御して連続鋳造によってスラブを作製し、これを再加熱して厚板圧延することで板厚５０〜８０ｍｍの鋼板を作製し、加速冷却を行った。さらに、必要に応じてオフラインでの焼戻し処理を行った。
表１に鋼の化学成分を示す。表２と表３に鋼板製造条件を示す。表４と表５に鋼板の機械的性質を示す。母材の機械的性質は、板厚１／２部−圧延長手（Ｌ）方向から試験片を採取した。継手のＨＡＺ靭性は、突合せ開先をエレクトロガス溶接（ＥＧＷ）による１パス溶接を行ない、板厚１／２部ＨＡＺの溶融線から１ｍｍ離れたＨＡＺにノッチを入れて調べた。−４０℃で３本のシャルピー衝撃試験を行ない、平均の吸収エネルギー値を評価した。 In the steelmaking process, deoxidation and desulfurization of the molten steel and chemical components are controlled to produce a slab by continuous casting, and this is reheated and rolled into a thick plate to produce a steel plate having a thickness of 50 to 80 mm, and accelerated cooling is performed. went. Furthermore, offline tempering was performed as necessary.
Table 1 shows the chemical composition of the steel. Tables 2 and 3 show the steel sheet manufacturing conditions. Tables 4 and 5 show the mechanical properties of the steel sheets. As for the mechanical properties of the base material, test pieces were collected from the plate thickness 1/2 part-rolling longitudinal (L) direction. The HAZ toughness of the joint was examined by performing a one-pass welding with electrogas welding (EGW) on the butt groove, and notching a HAZ 1 mm away from the melt line of 1/2 thickness of the HAZ. Three Charpy impact tests were conducted at −40 ° C. to evaluate the average absorbed energy value.

Figure 2007046096
Figure 2007046096

Figure 2007046096
Figure 2007046096

Figure 2007046096
Figure 2007046096

Figure 2007046096
Figure 2007046096

Figure 2007046096
Figure 2007046096

本発明鋼１〜１３は、鋼の化学成分を適正化し、圧延能率を重視した鋼板製造条件を適正化することで、厚手にも関わらず３９０〜４６０ＭＰａ級の高い降伏強度と、ｖＴｒｓが−６０℃以下である良好な板厚１／２部靭性と、大入熱溶接にも関わらず−４０℃で良好なＨＡＺ靭性を、同時に満足している。また、本発明鋼２３はＡ式：＜０、Ｂ式：＜０の場合で有効Ｂ量が０．０００３の場合の例であり、いずれの特性も良好となった。
一方、比較鋼１４〜２２は鋼の化学成分が適正でないため、比較鋼１Ａ〜１Ｈは鋼板製造条件が適正でないため、降伏強度、板厚１／２部靭性、大入熱溶接ＨＡＺ靭性、圧延能率、のいずれかが劣り、本発明のようにこれら複数の要求特性を同時に満足することができない。比較鋼２４は化学成分は好ましい範囲であるが有効Ｂ量が少ない例である。 Inventive steels 1 to 13 have a high yield strength of 390 to 460 MPa class and a vTrs of −60 by optimizing the chemical composition of the steel and optimizing the steel sheet production conditions with an emphasis on rolling efficiency. A satisfactory plate thickness ½ part toughness of ℃ or lower and a good HAZ toughness at -40 ℃ despite high heat input welding are simultaneously satisfied. In addition, the inventive steel 23 is an example in the case of A formula: <0, B formula: <0, and the effective B amount is 0.0003, and all the characteristics are good.
On the other hand, since the comparative steels 14 to 22 are not suitable for the chemical components of the steels, the comparative steels 1A to 1H are not suitable for the steel plate production conditions, so the yield strength, the plate thickness 1/2 part toughness, the high heat input welding HAZ toughness, rolling Any of the efficiency is inferior, and the plurality of required characteristics cannot be satisfied simultaneously as in the present invention. The comparative steel 24 is an example in which the amount of effective B is small although the chemical composition is in a preferable range.

比較鋼１４はＣとＣｅｑが低いため、比較鋼１７はＢと有効Ｂ量が低いため、比較鋼２１と比較鋼２２は有効Ｂ量が低いため、適正な焼入性を確保できず、適正な母材組織制御が困難となり、降伏強度と板厚１／２部靭性の両方が劣る。さらに、大入熱溶接ＨＡＺでの組織制御も難しくなってその靭性が劣る。比較鋼１５はＣが高いために、比較鋼１６はＳｉが高ために、比較鋼１８はＭｏが高いために、比較鋼１９はＮｂが高いために、比較鋼２０はＣｅｑが低いために、大入熱溶接ＨＡＺでの焼入性が過多となって硬化したり、ＭＡ生成が助長されたり、セメンタイトが多量粗大化したりすることで、その靭性が劣る。比較鋼１５と比較鋼１８は母材の板厚１／２部においてもＭＡが多く存在し、その靭性が劣る。
比較鋼１Ａはスラブ再加熱の開始温度が高いため、比較鋼１Ｃはスラブ再加熱温度が高いため、スラブ再加熱時のγ粒が粗大化して板厚１／２部靭性が劣る。比較鋼１Ｄは圧下比が小さいため、比較鋼１Ｅは圧延終了温度が高いため、圧延終了時のγ粒が粗大化して板厚１／２部靭性が劣る。比較鋼１Ｂはスラブ再加熱温度が低いため、比較鋼１Ｃはスラブ再加熱温度が高いため、鋼の化学成分できまる有効Ｂ量が十分であっても実質的な固溶Ｂが不足し、焼入性を確保できず降伏強度が劣る。この焼入性の低下は、板厚１／２部靭性の劣化要因にもなっている。比較鋼１Ｆは圧延終了温度が低いため、圧延時間が長くなる。圧延途中の温度待ちにおいて、鋼板温度が８００℃未満になるとなかなか冷えないため、待ち時間が長くなって圧延能率が大幅に劣化する。比較鋼１Ｇは水冷開始温度がＡｒ３未満と低いため、比較鋼１Ｈは水量密度が小さいため、フェライト生成が促され、セメンタイトも粗大化するため、降伏強度と板厚１／２部靭性が劣る。 Since the comparative steel 14 has a low C and Ceq, the comparative steel 17 has a low amount of B and effective B, and the comparative steel 21 and the comparative steel 22 have a low effective B amount, so that appropriate hardenability cannot be ensured. Therefore, it is difficult to control the base metal structure, and both the yield strength and the toughness of 1/2 part thickness are inferior. Furthermore, the structure control in the high heat input welding HAZ becomes difficult and the toughness is inferior. Since the comparative steel 15 is high in C, the comparative steel 16 is high in Si, the comparative steel 18 is high in Mo, the comparative steel 19 is high in Nb, and the comparative steel 20 is low in Ceq, The hardenability of the high heat input welding HAZ becomes excessive and hardens, the formation of MA is promoted, and a large amount of cementite is coarsened, resulting in poor toughness. The comparative steel 15 and the comparative steel 18 have a large amount of MA even at a thickness of 1/2 part of the base metal, and their toughness is inferior.
Since the comparative steel 1A has a high slab reheating start temperature and the comparative steel 1C has a high slab reheating temperature, the γ grains at the time of slab reheating become coarse and the plate thickness ½ part toughness is inferior. Since comparative steel 1D has a small rolling reduction, comparative steel 1E has a high rolling end temperature, so that the γ grains at the end of rolling become coarse and the plate thickness ½ part toughness is inferior. Since the comparative steel 1B has a low slab reheating temperature, the comparative steel 1C has a high slab reheating temperature. Impairment cannot be secured and yield strength is inferior. This decrease in hardenability is also a factor of deterioration in the toughness of the plate thickness 1/2 part. Since the comparison steel 1F has a low rolling end temperature, the rolling time becomes long. In the temperature waiting in the middle of rolling, since the steel sheet temperature is less than 800 ° C., it does not cool easily, so the waiting time becomes longer and the rolling efficiency is greatly deteriorated. Since Comparative Steel 1G has a low water cooling start temperature of less than Ar 3 , Comparative Steel 1H has a low water content density, so that ferrite formation is promoted and cementite is also coarsened, resulting in inferior yield strength and sheet thickness 1/2 part toughness. .

図１は１０ｐｐｍＢを含む鋼材に０．０８％のＭｏを複合することによる変態開始温度の低下代と冷却速度の関係を示す。FIG. 1 shows a relationship between a cooling margin and a reduction margin of a transformation start temperature by combining 0.08% Mo with a steel material containing 10 ppmB.

Claims (8)

質量％で、
Ｃ ：０．０３〜０．０７％
Ｓｉ：０．２％以下
Ｍｎ：１．２〜２．０％
Ｐ ：０．０１５％以下
Ｓ ：０．０００５〜０．００５％
Ｂ ：０．０００３〜０．００３％
Ｍｏ：０．０１〜０．２％
Ａｌ：０．００１〜０．０３４％
Ｔｉ：０．００５〜０．０２％
Ｎ ：０．００１〜０．００８％
Ｏ ：０．００１〜０．００４％
を含有し、さらに
Ｃａ：０．０００３〜０．００４％
Ｍｇ：０．０００３〜０．００４％
のうち１種以上を含有し、
下式（１）と（２）を満たし、
有効Ｂ量（％）≧０．０００３％ （１）
０．３０％≦Ｃｅｑ（％）≦０．４０％ （２）
残部が鉄および不可避的不純物によって化学成分が構成される連続鋳造スラブを、Ａｒ３が下式（４）で計算されるとき、連続鋳造後にＡｒ３−２００℃以下まで冷却した後に１０００〜１２５０℃に再加熱し、その後に下式（３）を満たす熱間圧延を８００℃〜９００℃で終了し、その後にＡｒ３以上から加速冷却することを特徴とする、靭性に優れた厚手高強度鋼板の製造方法。
ただし、Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５ の関係とし、熱間圧延の圧下について、
スラブ厚み（ｍｍ）／圧延終了時鋼板厚み（ｍｍ）≧２．５ （３）の関係とし、
Ａｒ３について、
Ａｒ３（℃）＝９１０−３１０Ｃ−８０Ｍｎ−２０Ｃｕ−５５Ｎｉ−８０Ｍｏ （４）の関係とし、有効Ｂ量とは、変態前のγ（オーステナイト）素地に固溶するＢ量を意味する。 % By mass
C: 0.03-0.07%
Si: 0.2% or less Mn: 1.2-2.0%
P: 0.015% or less S: 0.0005-0.005%
B: 0.0003 to 0.003%
Mo: 0.01 to 0.2%
Al: 0.001 to 0.034%
Ti: 0.005-0.02%
N: 0.001 to 0.008%
O: 0.001 to 0.004%
And Ca: 0.0003 to 0.004%
Mg: 0.0003 to 0.004%
Containing one or more of
The following formulas (1) and (2) are satisfied,
Effective B amount (%) ≧ 0.0003% (1)
0.30% ≦ Ceq (%) ≦ 0.40% (2)
Balance iron and unavoidable chemical component configured continuously cast slab by impurities, when Ar 3 is calculated by the following formula (4), after cooling to Ar 3 -200 ° C. or less after continuous casting 1000 to 1250 ° C. A thick high-strength steel sheet having excellent toughness, characterized in that hot rolling that satisfies the following formula (3) is finished at 800 ° C. to 900 ° C., and thereafter accelerated cooling is performed from Ar 3 or higher. Manufacturing method.
However, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15
Slab thickness (mm) / steel plate thickness at the end of rolling (mm) ≧ 2.5 (3)
About Ar 3
Ar 3 (° C.) = 910-310C-80Mn-20Cu-55Ni-80Mo (4) The effective B amount means the amount of B dissolved in the γ (austenite) substrate before transformation. 前記有効Ｂ量において、
（ａ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ≦０のとき
（ａ１） Ｎ−０．２９Ｔｉ＞０ならば、
有効Ｂ量＝Ｂ−０．７７（Ｎ−０．２９Ｔｉ）
（ａ２） Ｎ−０．２９Ｔｉ≦０ならば、
有効Ｂ量＝Ｂ
（ｂ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ＞０、かつ、Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）≧０．００５のとき、
（ｂ１）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕＞０ならば、
有効Ｂ量＝Ｂ−０．７７｛Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕｝
（ｂ２）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕≦０ならば、有効Ｂ量＝Ｂ
とすることを特徴とする請求項１に記載の靭性に優れた厚手高強度鋼板の製造方法。 In the effective B amount,
(A) When O-0.4Ca-0.66Mg-0.89Al ≦ 0 (a1) If N-0.29Ti> 0,
Effective B amount = B−0.77 (N−0.29Ti)
(A2) If N−0.29Ti ≦ 0,
Effective B amount = B
(B) When O-0.4Ca-0.66Mg-0.89Al> 0 and Ti-2 (O-0.4Ca-0.66Mg-0.89Al) ≧ 0.005,
(B1) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.89Al)]> 0,
Effective B amount = B−0.77 {N−0.29 [Ti-2 (O−0.4Ca−0.66Mg−0.89Al)]}
(B2) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.89Al)] ≤0, effective B amount = B
The method for producing a thick high strength steel plate excellent in toughness according to claim 1. さらに、質量％で
Ｃｕ：０．０１〜１％
Ｎｉ：０．０１〜３％
Ｃｒ：０．０１〜０．５％
Ｎｂ：０．００１〜０．０２％
Ｖ ： ０．００１〜０．１％
の中から１種以上を含有することを特徴とする、請求項１または２に記載の靭性に優れた厚手高強度鋼板の製造方法。 Furthermore, Cu: 0.01 to 1% by mass%
Ni: 0.01 to 3%
Cr: 0.01 to 0.5%
Nb: 0.001 to 0.02%
V: 0.001 to 0.1%
The manufacturing method of the thick high strength steel plate excellent in toughness of Claim 1 or 2 characterized by containing 1 or more types from among. さらに、質量％で
ＲＥＭ：０．０００３〜０．０２％
Ｚｒ：０．０００３〜０．０２％
のうち１種以上を含有し、（１）式に代わって下式（５）を満たすことを特徴とする、請求項１〜３のいずれかに記載の靭性に優れた厚手高強度鋼板の製造方法。
ここで、有効Ｂについて、
有効Ｂ量（％）≧０．０００３％ （５）
ただし、
（ｃ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ≦０のとき
（ｃ１）Ｎ−０．２９Ｔｉ＞０ならば、
有効Ｂ量＝Ｂ−０．７７（Ｎ−０．２９Ｔｉ）
（ｃ２）Ｎ−０．２９Ｔｉ≦０ならば、
有効Ｂ量＝Ｂ
（ｄ）ただし、Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ＞０、かつ、Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）≧０．００５のとき、
（ｄ１）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕＞０ならば、
有効Ｂ量＝Ｂ−０．７７｛Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕｝
（ｄ２）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕≦０ならば、
有効Ｂ量＝Ｂ Further, REM in mass%: 0.0003 to 0.02%
Zr: 0.0003 to 0.02%
The production of a thick high-strength steel sheet excellent in toughness according to any one of claims 1 to 3, characterized by containing at least one of the above and satisfying the following formula (5) instead of the formula (1): Method.
Here, for effective B,
Effective B amount (%) ≧ 0.0003% (5)
However,
(C) O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al≤0 (c1) If N-0.29Ti> 0,
Effective B amount = B−0.77 (N−0.29Ti)
(C2) If N−0.29Ti ≦ 0,
Effective B amount = B
(D) However, O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al> 0 and Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0 .35Zr-0.89Al) ≧ 0.005,
(D1) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)]> 0,
Effective B amount = B-0.77 {N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)]}
(D2) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)] ≤0,
Effective B amount = B Ｃ ：０．０３〜０．０７％（質量％、以下同じ）、
Ｓｉ：０．２％以下、
Ｍｎ：１．２〜２．０％、
Ｐ：０．０１５％以下、
Ｓ ：０．０００５〜０．００５％、
Ｂ ：０．０００３〜０．００３％、
Ｍｏ：０．０１〜０．２％、
Ａｌ：０．００１〜０．０３４％、
Ｔｉ：０．００５〜０．０２％、
Ｎ：０．００１〜０．００８％、
Ｏ ：０．００１〜０．００４％、
を含有し、さらに
Ｃａ：０．０００３〜０．００４％
Ｍｇ：０．０００３〜０．００４％
のうち１種以上を含有し、残部が鉄および不可避的不純物からなり、
板厚：５０〜８０ｍｍ、降伏強度：３９０〜４６０ＭＰａ級、板厚中心部靭性：ｖＴｒｓ：−６０℃以下、溶接入熱量≧２０ｋＪ／ｍｍ、のＨＡＺ靭性：ｖＥ（−４０℃）≧４７Ｊの条件を満足する靭性に優れた厚手高強度鋼板。 C: 0.03 to 0.07% (mass%, the same shall apply hereinafter)
Si: 0.2% or less,
Mn: 1.2 to 2.0%,
P: 0.015% or less,
S: 0.0005 to 0.005%,
B: 0.0003 to 0.003%,
Mo: 0.01 to 0.2%,
Al: 0.001 to 0.034%,
Ti: 0.005 to 0.02%,
N: 0.001 to 0.008%,
O: 0.001 to 0.004%,
And Ca: 0.0003 to 0.004%
Mg: 0.0003 to 0.004%
One or more of them, the balance consisting of iron and inevitable impurities,
Sheet thickness: 50 to 80 mm, yield strength: 390 to 460 MPa class, sheet thickness center toughness: vTrs: −60 ° C. or less, welding heat input ≧ 20 kJ / mm, HAZ toughness: vE (−40 ° C.) ≧ 47 J Thick high-strength steel sheet with excellent toughness that satisfies 下式（１）と（２）を満たすことを特徴とする請求項５に記載の靭性に優れた厚手高強度鋼板。
有効Ｂ量（％）≧０．０００３％ （１）
０．３０％≦Ｃｅｑ（％）≦０．４０％ （２）
ただし、有効Ｂ量とは、変態前のγ（オーステナイト）素地に固溶するＢ量を意味し、Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５とする。 The thick high-strength steel sheet excellent in toughness according to claim 5, wherein the following expressions (1) and (2) are satisfied.
Effective B amount (%) ≧ 0.0003% (1)
0.30% ≦ Ceq (%) ≦ 0.40% (2)
However, the effective B amount means the amount of B dissolved in the γ (austenite) substrate before transformation, and Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15. 前記（１）式の有効Ｂ量において、
（ａ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ≦０のとき
（ａ１） Ｎ−０．２９Ｔｉ＞０ならば、
有効Ｂ量＝Ｂ−０．７７（Ｎ−０．２９Ｔｉ）
（ａ２） Ｎ−０．２９Ｔｉ≦０ならば、
有効Ｂ量＝Ｂ
（ｂ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ＞０、かつ、Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）≧０．００５のとき、
（ｂ１）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕＞０ならば、
有効Ｂ量＝Ｂ−０．７７｛Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕｝
（ｂ２）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．８９Ａｌ）〕≦０ならば、有効Ｂ量＝Ｂ
とされてなることを特徴とする請求項６に記載の靭性に優れた厚手高強度鋼板。 In the effective B amount of the formula (1),
(A) When O-0.4Ca-0.66Mg-0.89Al ≦ 0 (a1) If N-0.29Ti> 0,
Effective B amount = B−0.77 (N−0.29Ti)
(A2) If N−0.29Ti ≦ 0,
Effective B amount = B
(B) When O-0.4Ca-0.66Mg-0.89Al> 0 and Ti-2 (O-0.4Ca-0.66Mg-0.89Al) ≧ 0.005,
(B1) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.89Al)]> 0,
Effective B amount = B−0.77 {N−0.29 [Ti-2 (O−0.4Ca−0.66Mg−0.89Al)]}
(B2) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.89Al)] ≤0, effective B amount = B
The thick high-strength steel sheet having excellent toughness according to claim 6. さらに、質量％で
ＲＥＭ：０．０００３〜０．０２％
Ｚｒ：０．０００３〜０．０２％
のうち１種以上を含有し、（１）式に代わって下式（５）を満たすことを特徴とする、請求項６に記載の靭性に優れた厚手高強度鋼板。
有効Ｂ量（％）≧０．０００３％ （５）
ただし、
（ｃ）Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ≦０のとき
（ｃ１）Ｎ−０．２９Ｔｉ＞０ならば、
有効Ｂ量＝Ｂ−０．７７（Ｎ−０．２９Ｔｉ）
（ｃ２）Ｎ−０．２９Ｔｉ≦０ならば、
有効Ｂ量＝Ｂ
（ｄ）ただし、Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ＞０、かつ、Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）≧０．００５のとき、
（ｄ１）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕＞０ならば、
有効Ｂ量＝Ｂ−０．７７｛Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕｝
（ｄ２）Ｎ−０．２９〔Ｔｉ−２（Ｏ−０．４Ｃａ−０．６６Ｍｇ−０．１７ＲＥＭ−０．３５Ｚｒ−０．８９Ａｌ）〕≦０ならば、
有効Ｂ量＝Ｂ
Further, REM in mass%: 0.0003 to 0.02%
Zr: 0.0003 to 0.02%
The thick high-strength steel sheet having excellent toughness according to claim 6, wherein the steel sheet contains at least one of the above and satisfies the following formula (5) instead of the formula (1).
Effective B amount (%) ≧ 0.0003% (5)
However,
(C) O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al≤0 (c1) If N-0.29Ti> 0,
Effective B amount = B−0.77 (N−0.29Ti)
(C2) If N−0.29Ti ≦ 0,
Effective B amount = B
(D) However, O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al> 0 and Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0 .35Zr-0.89Al) ≧ 0.005,
(D1) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)]> 0,
Effective B amount = B-0.77 {N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)]}
(D2) If N-0.29 [Ti-2 (O-0.4Ca-0.66Mg-0.17REM-0.35Zr-0.89Al)] ≤0,
Effective B amount = B
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JP2013256699A (en) * 2012-06-13 2013-12-26 Nippon Steel & Sumitomo Metal Corp Thick high strength steel plate having excellent weldability and toughness of weld heat-affected zone and method for producing the same
JP2017172010A (en) * 2016-03-24 2017-09-28 新日鐵住金株式会社 Sour-resistant steel sheet and sour- resistant steel tube
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