JP4259374B2 - High strength steel sheet with excellent low temperature toughness and weld heat affected zone toughness and method for producing the same - Google Patents

High strength steel sheet with excellent low temperature toughness and weld heat affected zone toughness and method for producing the same Download PDF

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JP4259374B2
JP4259374B2 JP2004099882A JP2004099882A JP4259374B2 JP 4259374 B2 JP4259374 B2 JP 4259374B2 JP 2004099882 A JP2004099882 A JP 2004099882A JP 2004099882 A JP2004099882 A JP 2004099882A JP 4259374 B2 JP4259374 B2 JP 4259374B2
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純二 嶋村
隆二 村岡
伸一 鈴木
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JFE Steel Corp
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本発明は、ラインパイプ、船舶、海洋構造物、建築機械、建築構造物、橋梁、タンク、ペンストックなどの溶接鋼構造物に用いて好適な、高強度鋼板に係り、とくに母材低温靭性および溶接熱影響部(HAZ)靭性の改善に関する。なお、「低温靭性に優れた」とは、シャルピー衝撃試験における-40℃での吸収エネルギーが200J以上の母材靭性を有する場合をいうものとする。また、「溶接熱影響部靭性に優れた」とは、溶接入熱:100kJ/cm以下のSAW溶接の溶接熱影響部のシャルピー衝撃試験における-20℃での吸収エネルギーが80J以上の靭性を有する場合をいうものとする。   The present invention relates to a high-strength steel plate suitable for use in welded steel structures such as line pipes, ships, offshore structures, building machines, building structures, bridges, tanks, and penstocks, It relates to improvement of weld heat affected zone (HAZ) toughness. Note that “excellent in low temperature toughness” means a case where the absorbed energy at −40 ° C. in the Charpy impact test has a base material toughness of 200 J or more. “Excellent welding heat-affected zone toughness” means toughness with an absorbed energy at −20 ° C. of 80 J or more in the Charpy impact test of the welding heat-affected zone of SAW welding with a heat input of 100 kJ / cm or less. It shall be a case.

一般に、鋼板の強度が増加するにしたがい、低温靭性は低下する傾向にある。このため、とくに、引張強さが900MPa以上の高強度鋼板において、良好な低温靭性を具備させることは容易ではない。このため、引張強さが900MPa以上の高強度鋼板の低温靭性を向上させるために、種々の提案がなされている。   Generally, as the strength of the steel sheet increases, the low temperature toughness tends to decrease. For this reason, it is not easy to provide good low-temperature toughness particularly in a high-strength steel sheet having a tensile strength of 900 MPa or more. For this reason, various proposals have been made in order to improve the low temperature toughness of high strength steel sheets having a tensile strength of 900 MPa or more.

例えば、特許文献1には、C:0.03〜0.10%、Mn:1.7〜3.0%、Mo:0.1〜0.8%、Nb0.01〜0.10%、Ti:0.005〜0.03%と、低C−高Mn−Mo−Nb−Ti系を主成分として、Si含有量を0.8%以上と高くし、組織を下部ベイナイトを主体とする組織とし、引張強さが900MPa以上で低温靭性および溶接熱影響部靭性に優れた超高強度鋼板が開示されている。しかし、特許文献1に記載された技術では、Cが0.03〜0.10%と高く、HAZ靭性の改善には自ずと限界があった。   For example, Patent Document 1 discloses that C: 0.03-0.10%, Mn: 1.7-3.0%, Mo: 0.1-0.8%, Nb 0.01-0.10%, Ti: 0.005-0.03%, low C-high Mn- The main component is Mo-Nb-Ti, Si content is as high as 0.8% or more, the structure is mainly composed of lower bainite, the tensile strength is 900MPa or more, and low temperature toughness and weld heat affected zone toughness are excellent. An ultra-high strength steel sheet is disclosed. However, in the technique described in Patent Document 1, C is as high as 0.03 to 0.10%, and there has been a limit in improving HAZ toughness.

また、特許文献2には、C:0.01〜0.2%、Ti:0.003〜0.05%、Mg:0.0001〜0.01%を含み、粒子径が0.2〜5μmのMg含有酸化物を核にして、硫化物および窒化物の一方が単独であるいは両方が複合して析出した複合粒子と、粒子径が0.005〜0.2μmのMg含有酸化物を核にして、硫化物および窒化物の一方が単独で析出した複合粒子を鋼中に分散させ、溶接熱影響部でのベイナイト・マルテンサイトの混合組織が組織全体に対する比率で80%以上である溶接熱影響部靭性に優れた900MPa以上の引張強さを有する超高強度ラインパイプ鋼管が開示されている。しかし、特許文献2に記載された技術は、厳格な製鋼プロセスの管理が必要であり、製造コストが高くなるという問題があった。   Patent Document 2 includes C: 0.01 to 0.2%, Ti: 0.003 to 0.05%, Mg: 0.0001 to 0.01%, and an Mg-containing oxide having a particle size of 0.2 to 5 μm as a nucleus. Composite particles in which one of the nitrides is deposited alone or in combination with both, and a composite particle in which one of the sulfides and nitrides is deposited alone with the Mg-containing oxide having a particle size of 0.005 to 0.2 μm as the core Is dispersed in steel, and the mixed structure of bainite and martensite in the heat affected zone is 80% or more of the whole structure. A line pipe steel pipe is disclosed. However, the technique described in Patent Document 2 requires a strict management of the steel making process, and has a problem that the manufacturing cost increases.

また、特許文献3には、C:0.03〜0.12%、少なくともNi:1.0%を含み、さらにMo、Nb、Cu、Ti、Al、N量を適正量含み、組織を細粒低ベイナイト、細粒ラスマルテンサイト、微粒状ベイナイト、10体積%までの残留オーステナイトを含むミクロ組織とした、優れた極低温靭性を持つ超高強度のオースエイジング処理された鋼が提案されている。しかし、特許文献3に記載された技術では、C量が高く、優れた溶接熱影響部靭性を得るために2.0%以上の高Ni 系とする必要があり、材料コストが高くなるという問題がある。
特開2002-146471号公報 特開2001-303191号公報 特表2002-534601号公報
Patent Document 3 includes C: 0.03 to 0.12%, at least Ni: 1.0%, and further includes appropriate amounts of Mo, Nb, Cu, Ti, Al, and N, and has a fine-grained low bainite and fine-grained structure. An ultra-high-strength austenized steel with excellent cryogenic toughness that has a microstructure containing lath martensite, fine-grained bainite, and residual austenite up to 10% by volume has been proposed. However, the technique described in Patent Document 3 has a problem that the amount of C is high, and in order to obtain excellent weld heat-affected zone toughness, it is necessary to use a high Ni system of 2.0% or more, which increases the material cost. .
JP 2002-146471 A JP 2001-303191 A Special table 2002-534601 gazette

本発明は、上記した従来技術の問題を解決し、引張強さ900MPa以上の高強度を有し、かつ母材の低温靭性に優れ、かつ溶接熱影響部靭性にも優れた高強度鋼板およびその製造方法を提供することを目的とする。   The present invention solves the above-mentioned problems of the prior art, has a high strength of a tensile strength of 900 MPa or more, is excellent in the low temperature toughness of the base material, and is excellent in the weld heat affected zone toughness, and its An object is to provide a manufacturing method.

本発明者らは、上記した課題を達成するために、引張強さ900MPa以上の強度レベルにおいて、母材靭性および溶接熱影響部(以下、HAZともいう)靭性に及ぼす各種要因について鋭意検討した。その結果、
C:0.030%以下、Mn:1.5%以上、Ni:1.6%未満、B:2〜15ppmを含有する極低C-高Mn-低Ni-微量Bの成分系とし、かつ、炭素当量Ceqが0.54〜0.63を満足するように成分を最適化し、さらに制御圧延後の水冷を、450℃〜300℃の温度域で停止し、その後空冷することにより、ラス間、ラス内の炭化物の析出量がきわめて少ないベイニティックフェライト主体の組織が多量に生成され、母材の強度−靭性バランスが向上すること、および良好なHAZ靭性が得られることを見いだした。これにより、900MPa以上の引張強さを有するとともに母材の低温靭性に優れ、さらに入熱50kJ/cm相当溶接のHAZ靭性(CGHAZおよびICCGHAZ)に優れた、低温靭性およびHAZ靭性に優れた高強度鋼板が製造可能であるという知見を得た。
In order to achieve the above-described problems, the present inventors diligently studied various factors affecting the base material toughness and the weld heat affected zone (hereinafter also referred to as HAZ) toughness at a tensile strength of 900 MPa or more. as a result,
C: 0.030% or less, Mn: 1.5% or more, Ni: less than 1.6%, B: 2 to 15 ppm, extremely low C-high Mn-low Ni-trace B component system, and carbon equivalent Ceq is 0.54 The components are optimized to satisfy ~ 0.63, and water cooling after controlled rolling is stopped in the temperature range of 450 ° C to 300 ° C, and then air-cooled, so that the amount of carbide precipitated in the lath is extremely high. It was found that a large amount of a bainitic ferrite-based structure was produced in a large amount, the strength-toughness balance of the base material was improved, and good HAZ toughness was obtained. As a result, it has a tensile strength of 900MPa or more, excellent low-temperature toughness of the base metal, and excellent HAZ toughness (CGHAZ and ICCGHAZ) equivalent to a heat input equivalent to 50kJ / cm. High strength with excellent low-temperature toughness and HAZ toughness The knowledge that a steel plate can be manufactured was acquired.

本発明は、このような知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨はつぎのとおりである。
(1)質量%で、C : 0.005〜0.030%、Si : 0.01〜0.60%、Mn : 1.50〜3.50%、P : 0.020%以下、S : 0.0050%以下、Al : 0.001〜0.100%、Ni: 0.10%以上1.60%未満、Cr:0.60%以下、Cu: 1.00%以下、Mo:0.01〜0.60%、Nb:0.01〜0.10%、Ti:0.005〜0.03%、B:0.0002〜0.0015%、O:0.0005〜0.0100%、N:0.001〜0.010%を、次(1)式
Ceq = C + Mn/6 + (Cu+Ni)/15 + (Cr+Mo+V)/5 ………(1)
(ここで、Ceq(%):炭素当量、C、Mn、Cu、Ni、Cr、Mo、V:各元素の含有量(質量%))
で定義される炭素当量Ceqが0.50〜0.68%を満足するように含有し、残部がFeおよび不可避的不純物からなる組成と、体積率で90%以上のベイニティックフェライト相を含む組織とを有し、引張強さ:900MPa以上を有することを特徴とする低温靭性および溶接熱影響部靭性に優れた高強度鋼板。
(2)(1)において、前記組成に加えてさらに、質量%で、V:0.005〜0.100%を含有することを特徴とする高強度鋼板。
(3)(1)または(2)において、前記組成に加えてさらに、質量%で、Ca : 0.010%以下、REM : 0.020%以下のうちから選ばれた1種又は2種を含有することを特徴とする高強度鋼板。
(4)質量%で、C : 0.005〜0.030%、Si : 0.01〜0.60%、Mn : 1.50〜3.50%、P : 0.020%以下、S : 0.0050%以下、Al : 0.001〜0.100%、Ni: 0.10%以上1.60%未満、Cr:0.60%以下、Cu: 1.00%以下、Mo:0.01〜0.60%、Nb:0.01〜0.10%、Ti:0.005〜0.03%、B:0.0002〜0.0015%、O:0.0005〜0.0100%、N:0.001〜0.010%を、次(1)式
Ceq = C + Mn/6 + (Cu+Ni)/15 + (Cr+Mo+V)/5 ………(1)
(ここで、Ceq(%):炭素当量、C、Mn、Cu、Ni、Cr、Mo、V:各元素の含有量(質量%))
で定義される炭素当量Ceqが0.50〜0.68%を満足するように含有し、好ましくは残部Feおよび不可避的不純物からなる組成を有する鋼素材を、1000〜1250℃に加熱し、次いで750℃以上での累積圧下率が50%以上となる熱間圧延を施し、750℃以上で熱間圧延を終了し、ついで700℃以上の温度域から10℃/s以上の冷却速度で450℃〜300℃の温度域の冷却停止温度まで冷却した後、ついで室温まで空冷することを特徴とする低温靭性および溶接熱影響部靭性に優れた高強度鋼板の製造方法。
(5)(4)において、前記組成に加えてさらに、質量%で、V:0.005〜0.100%を含有することを特徴とする高強度鋼板の製造方法。
(6)(4)または(5)において、前記組成に加えてさらに、質量%で、Ca : 0.010%以下、REM : 0.020%以下のうちから選ばれた1種又は2種を含有することを特徴とする高強度鋼板。
The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.005 to 0.030%, Si: 0.01 to 0.60%, Mn: 1.50 to 3.50%, P: 0.020% or less, S: 0.0050% or less, Al: 0.001 to 0.100%, Ni: 0.10 %: Less than 1.60%, Cr: 0.60% or less, Cu: 1.00% or less, Mo: 0.01 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.03%, B: 0.0002 to 0.0015%, O: 0.0005 to 0.0100%, N: 0.001 to 0.010%, the following equation (1): Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ……… (1)
(Where Ceq (%): carbon equivalent, C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by the formula is contained so as to satisfy 0.50 to 0.68%, the balance is composed of Fe and inevitable impurities, and the structure contains a bainitic ferrite phase of 90% or more by volume. And high strength steel sheet with excellent low temperature toughness and weld heat affected zone toughness, characterized by having a tensile strength of 900 MPa or more.
(2) In (1), in addition to the said composition, V: 0.005-0.100% is further contained by the mass%, The high strength steel plate characterized by the above-mentioned.
(3) In (1) or (2), in addition to the above-mentioned composition, it further contains, by mass%, one or two selected from Ca: 0.010% or less and REM: 0.020% or less High-strength steel sheet characterized.
(4) By mass%, C: 0.005 to 0.030%, Si: 0.01 to 0.60%, Mn: 1.50 to 3.50%, P: 0.020% or less, S: 0.0050% or less, Al: 0.001 to 0.100%, Ni: 0.10 %: Less than 1.60%, Cr: 0.60% or less, Cu: 1.00% or less, Mo: 0.01 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.03%, B: 0.0002 to 0.0015%, O: 0.0005 to 0.0100%, N: 0.001 to 0.010%, the following equation (1): Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ……… (1)
(Where Ceq (%): carbon equivalent, C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
A steel material having a carbon equivalent Ceq defined by the formula of 0.50 to 0.68%, preferably composed of the balance Fe and inevitable impurities, is heated to 1000 to 1250 ° C, and then at 750 ° C or higher. The hot rolling is performed so that the cumulative rolling reduction of 50% or more is finished, the hot rolling is finished at 750 ° C or higher, and then the temperature range from 700 ° C or higher to 450 ° C to 300 ° C at a cooling rate of 10 ° C / s or higher A method for producing a high-strength steel sheet excellent in low temperature toughness and weld heat affected zone toughness, characterized by cooling to a cooling stop temperature in a temperature range and then air cooling to room temperature.
(5) In (4), in addition to the said composition, V: 0.005-0.100% is further contained by the mass%, The manufacturing method of the high strength steel plate characterized by the above-mentioned.
(6) In (4) or (5), in addition to the above-mentioned composition, it further contains, by mass%, one or two selected from Ca: 0.010% or less and REM: 0.020% or less High-strength steel sheet characterized.

本発明によれば、引張強さ900MPa以上の高強度を有し、かつ母材の低温靭性および溶接熱影響部靭性に優れる高強度鋼板を、高能率でかつ安価に製造でき、産業上格段の効果を奏する。   According to the present invention, a high-strength steel plate having a high tensile strength of 900 MPa or more and excellent in low-temperature toughness and weld heat-affected zone toughness of a base material can be produced at a high efficiency and at a low cost. There is an effect.

本発明鋼板の組成限定理由についてまず説明する。   The reason for limiting the composition of the steel sheet of the present invention will be described first.

C : 0.005〜0.030%
Cは、鋼の強度を増加する元素であり、所望の高強度とするためには、0.005%以上の含有を必要とする。一方、0.030%を超えて含有すると900MPa以上の引張強さを有する鋼板では溶接性が劣化し、溶接割れが生じやすくなるとともに、母材靭性およびHAZ靭性が低下する。このため、Cは0.005〜0.030%の範囲に限定した。
C: 0.005-0.030%
C is an element that increases the strength of the steel, and in order to obtain a desired high strength, the content of 0.005% or more is required. On the other hand, if the content exceeds 0.030%, a steel sheet having a tensile strength of 900 MPa or more deteriorates weldability, easily causes weld cracking, and lowers the base metal toughness and HAZ toughness. For this reason, C was limited to the range of 0.005 to 0.030%.

Si : 0.01〜0.60%
Siは、脱酸剤として作用し、さらに固溶強化により鋼材の強度を増加させる元素である。このような効果を得るためには、0.01%以上の含有を必要とするが、0.60%を超える含有は、HAZ靭性を著しく劣化させる。このため、Siは0.01〜0.60%の範囲とした。
Si: 0.01-0.60%
Si is an element that acts as a deoxidizing agent and further increases the strength of the steel material by solid solution strengthening. In order to obtain such an effect, a content of 0.01% or more is required, but a content exceeding 0.60% significantly deteriorates the HAZ toughness. For this reason, Si was made into the range of 0.01 to 0.60%.

Mn : 1.50〜3.50%
Mnは、鋼の焼入れ性を高めるとともに、靭性を向上させる作用を有する元素であり、本発明では、1.50%以上の含有を必要とするが、3.50%を超える含有は、溶接性を劣化させる恐れがある。このため、本発明では、Mnは1.50〜3.50%の範囲に限定した。
Mn: 1.50 to 3.50%
Mn is an element that has the effect of improving the hardenability of steel and improving toughness. In the present invention, it is necessary to contain 1.50% or more, but if it exceeds 3.50%, the weldability may be deteriorated. There is. For this reason, in this invention, Mn was limited to 1.50 to 3.50% of range.

P : 0.020%以下
Pは、固溶強化により強度を増加させる元素であるが、靭性、溶接性を劣化させるため、本発明ではできるだけ低減することが好ましいが、0.020%までの含有は許容できる。このため、Pは0.020%以下に限定した。
P: 0.020% or less P is an element that increases the strength by solid solution strengthening. However, in order to deteriorate toughness and weldability, P is preferably reduced as much as possible in the present invention, but up to 0.020% is acceptable. For this reason, P was limited to 0.020% or less.

S : 0.0050%以下
Sは、鋼中では硫化物として存在し、延性を低下させる作用を示す。このため、Sはできるだけ低減することが望ましいが、0.0050%までは許容できる。
S: 0.0050% or less S is present as a sulfide in steel and exhibits an effect of reducing ductility. For this reason, it is desirable to reduce S as much as possible, but it is acceptable up to 0.0050%.

Al : 0.001〜0.100%
Alは、製鋼過程では脱酸剤として作用し、本発明では、0.001%以上の含有を必要とするが、0.100%を超える含有は、靭性の低下を招く。このため、Alは0.001〜0.100%の範囲に限定した。
Al: 0.001 to 0.100%
Al acts as a deoxidizer in the steelmaking process, and in the present invention, it needs to be contained in an amount of 0.001% or more. However, if it exceeds 0.100%, the toughness is lowered. For this reason, Al was limited to the range of 0.001 to 0.100%.

Ni: 0.10%以上1.60%未満
Niは、鋼の焼入れ性をより向上させるとともに、靭性をも向上させる作用を有する。このような効果は、0.10%以上の含有で認められるが、1.60%を超える含有は、製造コスト上昇を招く傾向になる。このため、Niは0.10〜1.60%の範囲に限定した。
Ni: 0.10% to less than 1.60%
Ni has the effect of improving the hardenability of the steel and improving the toughness. Such an effect is recognized at a content of 0.10% or more, but a content exceeding 1.60% tends to cause an increase in production cost. For this reason, Ni was limited to the range of 0.10 to 1.60%.

Cr:0.60%以下
Crは、鋼の焼入れ性を向上させる安価な元素である。このような効果は0.1%以上の含有で認められるが、0.60%を超える含有は、溶接性および靭性を劣化させる。このため、Crは0.60%以下に限定した。
Cr: 0.60% or less
Cr is an inexpensive element that improves the hardenability of steel. Such an effect is recognized at a content of 0.1% or more, but a content exceeding 0.60% deteriorates weldability and toughness. For this reason, Cr was limited to 0.60% or less.

Cu: 1.00%以下
Cuは、焼入れ性を向上させる作用を有し、本発明では0.1%以上含有することが好ましい。一方、1.00%を超える含有は熱間脆性を引き起こす危険性が増大する。このため、Cuは1.00%以下に限定した。
Cu: 1.00% or less
Cu has an effect of improving hardenability, and is preferably contained in an amount of 0.1% or more in the present invention. On the other hand, the content exceeding 1.00% increases the risk of causing hot brittleness. For this reason, Cu was limited to 1.00% or less.

Mo:0.01〜0.60%
Moは、鋼の焼入れ性を向上させる作用を有する元素であり、このような効果は0.01%以上の含有で認められるが、0.60%を超える含有は、溶接性および靭性を劣化させる。このため、Moは0.01〜0.60%の範囲に限定した。
Mo: 0.01 ~ 0.60%
Mo is an element having an effect of improving the hardenability of steel, and such an effect is recognized with a content of 0.01% or more. However, a content exceeding 0.60% deteriorates weldability and toughness. For this reason, Mo was limited to the range of 0.01 to 0.60%.

Nb:0.01〜0.10%
Nbは、熱間圧延過程でオーステナイト粒の再結晶を抑制して、熱間圧延によるオーステナイト粒の展伸を容易にし、水冷後の変態組織を微細化させて強度、靭性を向上させる作用を有する。このような効果は、0.01%以上の含有で顕著となるが、0.10%を超える含有は、溶接性およびHAZ靭性を劣化させる。このため、Nbは0.01%以上0.10%以下の範囲に限定した。
Nb: 0.01-0.10%
Nb has the effect of suppressing the recrystallization of austenite grains in the hot rolling process, facilitating the expansion of austenite grains by hot rolling, and refining the transformation structure after water cooling to improve strength and toughness. . Such an effect becomes remarkable when the content is 0.01% or more, but when the content exceeds 0.10%, the weldability and the HAZ toughness deteriorate. For this reason, Nb was limited to a range of 0.01% to 0.10%.

Ti:0.005〜0.03%
Tiは、鋼中のNと結合しTiNを形成し、結晶粒の粗大化を抑制してHAZ靭性の向上に寄与するとともに、固溶Nを減少させ、Bの焼入れ性向上効果を確保する作用を有する。このような効果は、0.005%以上の含有で認められるが、0.03%を超えて含有すると、TiNが粗大化し、γ粒の微細化効果が消滅し、靭性が劣化する。このため、Tiは0.005〜0.03%の範囲に限定した。
Ti: 0.005-0.03%
Ti combines with N in the steel to form TiN, suppresses the coarsening of crystal grains and contributes to the improvement of HAZ toughness, and also reduces the solid solution N and ensures the effect of improving the hardenability of B. Have Such an effect is recognized when the content is 0.005% or more. However, when the content exceeds 0.03%, TiN becomes coarse, the effect of refining γ grains disappears, and toughness deteriorates. For this reason, Ti was limited to the range of 0.005 to 0.03%.

B:0.0002〜0.0015%
Bは、少量で鋼の焼入れ性を向上させる元素であり、このような効果は0.0002%以上の含有で認められるが、0.0015%を超えて含有すると、溶接性および靭性を劣化させる。このため、Bは0.0002〜0.0015%の範囲に限定した。
B: 0.0002-0.0015%
B is an element that improves the hardenability of the steel in a small amount, and such an effect is recognized with a content of 0.0002% or more, but when it exceeds 0.0015%, the weldability and toughness are deteriorated. For this reason, B was limited to the range of 0.0002 to 0.0015%.

O:0.0005〜0.0100%
Oは、Ti系酸化物やSiO2-MnO系酸化物を生成し、HAZのオーステナイト粒径の粗大化を防止する作用を有する。このような効果は、0.0005%以上の含有で顕著となるが、0.0100%を超えて含有すると粗大な酸化物系の介在物が多量に生成するため、その酸化物が破壊発生の起点となり母材靭性およびHAZ靭性が劣化する。このため、Oは0.0005〜0.0100%の範囲に限定した。
O: 0.0005 to 0.0100%
O generates Ti-based oxides and SiO 2 -MnO-based oxides and has an action of preventing the austenite grain size of HAZ from becoming coarse. Such an effect becomes remarkable when the content is 0.0005% or more. However, if the content exceeds 0.0100%, a large amount of coarse oxide inclusions are generated. Toughness and HAZ toughness deteriorate. For this reason, O was limited to 0.0005 to 0.0100% of range.

N:0.001〜0.010%
Nは、TiとともにTiNを生成し、HAZのオーステナイト粒径の粗大化を抑制してフェライト生成を促進させることによりHAZ靭性を向上させる効果を有する元素である。このような効果は、0.001%以上の含有で認められる。一方、0.010%を超えて含有すると、靭性に有害な固溶N量が増加し、靭性が劣化する。このため、Nは0.001〜0.010%の範囲に限定した。
N: 0.001 to 0.010%
N is an element that produces TiN together with Ti and has the effect of improving HAZ toughness by suppressing the coarsening of the austenite grain size of HAZ and promoting ferrite formation. Such an effect is recognized when the content is 0.001% or more. On the other hand, if the content exceeds 0.010%, the amount of solute N harmful to toughness increases and toughness deteriorates. For this reason, N was limited to the range of 0.001 to 0.010%.

上記した基本成分に加えて、さらにV: 0.005〜0.100%、および/または、Ca : 0.010%以下、REM : 0.020%以下のうちから選ばれた1種又は2種を含有できる。   In addition to the above basic components, V: 0.005 to 0.100%, and / or Ca: 0.010% or less, REM: 0.020% or less can be included.

V: 0.005〜0.100%
Vは、C、Nと結合し炭化物または窒化物として析出し、析出硬化により鋼の強度を増加させる作用を有する。このような効果は0.005%以上の含有で認められるが、0.100%を超えて含有すると、溶接性が劣化する。このため、Vは0.005〜0.100%の範囲に限定することが好ましい。
V: 0.005-0.100%
V combines with C and N and precipitates as carbide or nitride, and has the effect of increasing the strength of the steel by precipitation hardening. Such an effect is recognized at a content of 0.005% or more, but if it exceeds 0.100%, the weldability deteriorates. For this reason, it is preferable to limit V to 0.005 to 0.100% of range.

Ca : 0.010%以下、REM : 0.020%以下のうちから選ばれた1種又は2種
Ca、REMはいずれも、硫化物形成元素であり、硫化物を球状化し鋼の延性を向上させる元素である。しかし、Ca:0.010%、REM:0.020%を超える含有は靭性を劣化させる。このため、Ca:0.010%以下、REM:0.020%以下に限定した。
One or two selected from Ca: 0.010% or less, REM: 0.020% or less
Both Ca and REM are sulfide forming elements, and are elements that spheroidize sulfides and improve the ductility of steel. However, the content exceeding Ca: 0.010% and REM: 0.020% deteriorates toughness. For this reason, it was limited to Ca: 0.010% or less and REM: 0.020% or less.

本発明では、上記した成分を上記した範囲で、かつ次(1)式
Ceq = C + Mn/6 + (Cu+Ni)/15 + (Cr+Mo+V)/5 ………(1)
(ここで、Ceq(%):炭素当量、C、Mn、Cu、Ni、Cr、Mo、V:各元素の含有量(質量%))
で定義される炭素当量Ceqが0.50〜0.68%を満足するように含有する。
In the present invention, the above-described components are within the above-mentioned range, and the following formula (1): Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
(Where Ceq (%): carbon equivalent, C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by is contained so as to satisfy 0.50 to 0.68%.

Ceqが0.50%未満では、図2に示すように、引張強さ900MPa以上の母材強度を確保できない。一方、Ceqが0.68%を超えると、図3に示すように、母材靭性を確保できない。このため、Ceqは0.50〜0.68%の範囲に限定した。   When Ceq is less than 0.50%, as shown in FIG. 2, it is not possible to secure a base material strength with a tensile strength of 900 MPa or more. On the other hand, if Ceq exceeds 0.68%, the base material toughness cannot be secured as shown in FIG. For this reason, Ceq was limited to the range of 0.50 to 0.68%.

上記した成分以外の残部は、Feおよび不可避的不純物である。   The balance other than the above components is Fe and inevitable impurities.

本発明鋼板は、上記した組成を有し、ベイニティックフェライトを主体とする組織を有する。ベイニティックフェライトを主体とする組織は、板厚方向位置に拠らず、体積率で90%以上のベイニティックフェライトを含む組織をいう。ベイニティックフェライトの組織分率が90%未満では、マルテンサイトが増加するので900MPa以上の高強度を有し、優れた母材低温靭性とHAZ靭性をともに有することが困難となる。ベイニティックフェライト以外の相としては、体積率で10%以下の上部ベイナイトやマルテンサイト、フェライトの混在が許容できる。   The steel sheet of the present invention has the above-described composition and has a structure mainly composed of bainitic ferrite. The structure mainly composed of bainitic ferrite refers to a structure containing bainitic ferrite having a volume ratio of 90% or more regardless of the position in the plate thickness direction. When the structural fraction of bainitic ferrite is less than 90%, martensite increases, so it has a high strength of 900 MPa or more, and it is difficult to have both excellent base material low temperature toughness and HAZ toughness. As phases other than bainitic ferrite, mixing of upper bainite, martensite and ferrite with a volume ratio of 10% or less is acceptable.

ベイニティックフェライトは、ラス内にセメンタイトの実質上析出しない組織であり、C:0.03%以下とすることにより、図5に示すように板厚中心部においても、ベイニティックフェライトを主体とする組織とすることができる。これにより、優れたCGHAZ(Coarse-grain HAZ)靭性およびICCGHAZ(Intercritically-reheated coarse-grain HAZ)靭性を得ることが可能となる。一方、Cが0.03%を超えて高くなると、板厚中心部の母材組織は、図4に示すように、ラス内にセメンタイトの析出した下部ベイナイトを主体とする組織となり、CGHAZおよびICCGHAZ靭性が劣化する。なお、この組織の違いは板厚中心部のみに限定されない。   Bainitic ferrite is a structure in which cementite does not substantially precipitate in the lath, and by making C: 0.03% or less, as shown in FIG. It can be an organization. This makes it possible to obtain excellent CGHAZ (Coarse-grain HAZ) toughness and ICCGHAZ (Intercritically-reheated coarse-grain HAZ) toughness. On the other hand, when C exceeds 0.03%, the base metal structure at the center of the plate thickness becomes a structure mainly composed of lower bainite in which cementite is precipitated in the lath, as shown in FIG. 4, and CGHAZ and ICCGHAZ toughness to degrade. Note that this difference in structure is not limited to the center portion of the plate thickness.

つぎに、本発明鋼板の製造方法について説明する。   Below, the manufacturing method of this invention steel plate is demonstrated.

上記した組成を有する溶鋼を、転炉、電気炉等の通常の溶製手段で溶製し、連続鋳造法または造塊-分塊法等の通常の鋳造法で、鋼片やスラブ等の鋼素材とすることが好ましい。なお、溶製方法、鋳造法については上記した方法に限定されるものではない。   The molten steel having the above composition is melted by a normal melting means such as a converter or an electric furnace, and steel such as a steel slab or slab is obtained by a normal casting method such as a continuous casting method or an ingot-bundling method. It is preferable to use a raw material. The melting method and the casting method are not limited to the methods described above.

上記した組成を有する鋼素材を1000〜1250℃に加熱したのち、熱間圧延を施す。   A steel material having the above composition is heated to 1000 to 1250 ° C. and then hot-rolled.

加熱温度が1000℃未満では、熱間変形抵抗が高すぎて1回あたりの圧下率を高く採れず、熱間圧延の生産性が低下する。V、Nb等の析出物形成元素を含有する場合には、これら元素が十分にオーステナイト中に固溶せず、これら元素の効果を十分に発揮することが困難となる。一方、加熱温度が1250℃を超えると、結晶粒が粗大化するとともに、酸化ロス量の増加や炉の改修頻度の増加を招く。このため、鋼素材の加熱温度は1000〜1250℃の範囲に限定した。   If the heating temperature is less than 1000 ° C., the hot deformation resistance is too high to obtain a high rolling reduction per time, and the hot rolling productivity decreases. When a precipitate-forming element such as V or Nb is contained, these elements are not sufficiently dissolved in austenite, and it is difficult to sufficiently exhibit the effects of these elements. On the other hand, when the heating temperature exceeds 1250 ° C., the crystal grains become coarse, and the amount of oxidation loss increases and the frequency of furnace repairs increases. For this reason, the heating temperature of the steel material was limited to a range of 1000 to 1250 ° C.

ついで、加熱された鋼素材は、圧延終了温度を750℃以上とする熱間圧延を施され、厚鋼板とされる。   Next, the heated steel material is subjected to hot rolling at a rolling end temperature of 750 ° C. or higher to obtain a thick steel plate.

圧延終了温度が750℃未満では、圧延中にフェライトが析出し、その後に冷却処理を行っても所望の組織が得られず、所望の強度を確保できなくなる。なお、750℃以上、好ましくは950℃以下の温度域での累積圧下率を50%以上とする。累積圧下率が50%未満では、十分なオーステナイト粒の微細化が達成できないため、母材靭性を確保できなくなる。   When the rolling end temperature is less than 750 ° C., ferrite precipitates during rolling, and a desired structure cannot be obtained even if a cooling treatment is performed thereafter, and a desired strength cannot be ensured. The cumulative rolling reduction in the temperature range of 750 ° C. or higher, preferably 950 ° C. or lower is set to 50% or higher. If the cumulative rolling reduction is less than 50%, sufficient austenite grain refinement cannot be achieved, and thus the base material toughness cannot be secured.

熱間圧延終了後、厚鋼板は、700℃以上の温度域から10℃/s以上の冷却速度で冷却される。冷却の開始温度が、700℃未満では、冷却開始時にすでにフェライトが増加しすぎるため、冷却後に所望の強度を確保することができなくなる。また、冷却速度が、10℃/s未満では、ベイニティックフェライト以外の組織が生成し、ベイニティックフェライトの組織分率が低下するため、所望の強度を確保できない。このため、冷却速度は10℃/s以上とすることが好ましい。   After the hot rolling, the thick steel plate is cooled at a cooling rate of 10 ° C./s or more from a temperature range of 700 ° C. or more. When the cooling start temperature is less than 700 ° C., ferrite already increases too much at the start of cooling, and thus it becomes impossible to secure a desired strength after cooling. Further, when the cooling rate is less than 10 ° C./s, a structure other than bainitic ferrite is generated, and the structural fraction of bainitic ferrite is lowered, so that a desired strength cannot be ensured. For this reason, it is preferable that a cooling rate shall be 10 degrees C / s or more.

上記した冷却速度で厚鋼板を、450℃以下、300℃以上の温度域の冷却停止温度まで冷却する。   The thick steel plate is cooled to a cooling stop temperature in a temperature range of 450 ° C. or lower and 300 ° C. or higher at the above cooling rate.

冷却停止温度が、450℃を超える場合には、ベイニティックフェライトの粗大化および転位密度の極端な低下を招き、図1に示すように、900MPa以上の母材の引張強さを確保できなくなる。一方、冷却停止温度が300℃未満では、拡散性水素および鋼板内残留応力が過剰となり、母材靭性、HAZ靭性が劣化する。このようなことから、冷却停止温度は450℃以下300℃以上の温度域の温度に限定することが好ましい。なお、より好ましくは、450℃以下350℃以上である。   If the cooling stop temperature exceeds 450 ° C, it will cause coarsening of bainitic ferrite and an extremely low dislocation density, and as shown in Fig. 1, it will not be possible to secure the tensile strength of the base material of 900 MPa or more. . On the other hand, if the cooling stop temperature is less than 300 ° C., diffusible hydrogen and residual stress in the steel sheet become excessive, and the base metal toughness and HAZ toughness deteriorate. For this reason, the cooling stop temperature is preferably limited to a temperature in the temperature range of 450 ° C. or lower and 300 ° C. or higher. More preferably, it is 450 ° C. or lower and 350 ° C. or higher.

なお、実操業においては、鋼板の温度管理は、鋼板表面温度により行われ、リアルタイムで板厚方向全体の平均温度を計算して、この平均温度に基づいて温度制御や速度制御を行うのが一般的である。このため、本発明でいう「温度」は鋼板全体の平均温度、「冷却速度」は鋼板全体の平均冷却速度、「昇温速度」は鋼板全体の平均昇温速度を意味するものとする。   In actual operation, the temperature of the steel sheet is generally controlled by the surface temperature of the steel sheet, and the average temperature in the entire thickness direction is calculated in real time, and the temperature control and speed control are generally performed based on this average temperature. Is. Therefore, “temperature” in the present invention means the average temperature of the entire steel sheet, “cooling rate” means the average cooling rate of the entire steel sheet, and “temperature increase rate” means the average temperature increase rate of the entire steel sheet.

表1に示す組成の溶鋼を真空溶解炉で溶製し、連続鋳造法により100mm厚のスラブ(鋼素材)とした。ついで、これら鋼素材に、表2に示す条件の熱間圧延を施し、引き続き、表2に示す条件の冷却処理を施して、厚鋼板(板厚:16〜20mm)とした。   Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace and made into a 100 mm thick slab (steel material) by a continuous casting method. Subsequently, these steel materials were subjected to hot rolling under the conditions shown in Table 2 and subsequently subjected to cooling treatment under the conditions shown in Table 2 to obtain thick steel plates (plate thickness: 16 to 20 mm).

また、得られた厚鋼板の板厚方向1/2t位置から、組織観察用試験片を採取し、走査型電子顕微鏡および透過型電子顕微鏡により組織観察を行い、組織の同定、および組織分率を求めた。組織分率は、走査型電子顕微鏡を用いて線分法により平均オーステナイト(γ)粒径を測定し、その平均的なγ粒径の粒をランダムに10個選び、そのγ粒内の各組について断面面積率として求め、10個の断面面積率の平均値をその鋼板各位置の各相の組織分率として、体積率に換算した。   In addition, from the position of 1 / 2t in the thickness direction of the obtained thick steel plate, a specimen for structure observation is collected, and the structure is observed with a scanning electron microscope and a transmission electron microscope to identify the structure and determine the structure fraction. Asked. The structure fraction is determined by measuring the average austenite (γ) grain size by the line segment method using a scanning electron microscope, randomly selecting 10 grains having the average γ grain size, and setting each group in the γ grain. Was obtained as a cross-sectional area ratio, and the average value of the ten cross-sectional area ratios was converted into a volume ratio as a structure fraction of each phase at each position of the steel sheet.

また得られた厚鋼板の板厚方向1/2tの位置からJIS Z 2201の規定に準拠して、4号引張試験片を採取して、JIS Z 2241の規定に準拠して引張試験を実施し、0.2%耐力YSおよび引張強さTSを求めた。   In addition, a No. 4 tensile test specimen was taken from the position of the obtained steel plate in the thickness direction 1 / 2t in accordance with the provisions of JIS Z 2201, and a tensile test was conducted in accordance with the provisions of JIS Z 2241. 0.2% yield strength YS and tensile strength TS were determined.

また、得られた厚鋼板の板厚方向1/2tの位置からJIS Z 2202の規定に準拠して、Vノッチ標準寸法のシャルピー衝撃試験片を採取して、JIS Z 2242の規定に準拠してシャルピー衝撃試験を実施し、-40℃での吸収エネルギーvE−40(J)を求めた。 In addition, a Charpy impact test specimen with a V-notch standard dimension was taken from the position of the obtained thick steel plate in the thickness direction 1 / 2t according to JIS Z 2202, and conformed to JIS Z 2242. A Charpy impact test was performed to determine the absorbed energy vE- 40 (J) at -40 ° C.

また、得られた鋼板から再現熱サイクル試験片を採取し、最高加熱温度:1400℃、800〜500℃の冷却時間t800-500=57sの1サイクルの再現熱サイクルを付与する再現溶接熱サイクル試験(溶接入熱50kJ/cmのボンド部相当の再現熱サイクルを付与)(TP1)と、同様の再現熱サイクルを付与後、さらに最高加熱温度:800℃、800〜500℃の冷却時間t800-500=57sの再現熱サイクルを付与する、2サイクルの再現熱サイクルを付与する再現溶接熱サイクル試験(TP2)とを行った。これら再現熱サイクルを付与された試験片から、JIS Z 2202の規定に準拠して、Vノッチ標準寸法のシャルピー衝撃試験片を採取して、JIS Z 2242の規定に準拠してシャルピー衝撃試験を実施し、-20℃での吸収エネルギーvE−20(J)を求め、HAZ靭性を評価した。 In addition, a reproducible heat cycle test piece is taken from the obtained steel plate, and a reproducible welding heat cycle is applied that gives a reproducible heat cycle of 1 cycle of maximum heating temperature: 1400 ° C, cooling time of 800-500 ° C t 800-500 = 57s. After applying test (applying reproducible thermal cycle equivalent to bond part with welding heat input of 50kJ / cm) (TP1) and similar reproducible thermal cycle, maximum heating temperature: 800 ° C, cooling time of 800-500 ° C t 800 A reproducible welding thermal cycle test (TP2) was performed, which gave a reproducible thermal cycle of −500 = 57 s and gave a reproducible thermal cycle of 2 cycles. Charpy impact test specimens with V-notch standard dimensions are collected from these test pieces that have been given a reproducible thermal cycle in accordance with JIS Z 2202, and Charpy impact tests are conducted in accordance with JIS Z 2242 regulations. Then, the absorbed energy vE- 20 (J) at -20 ° C was obtained, and the HAZ toughness was evaluated.

得られた結果を表3に示す。   The obtained results are shown in Table 3.

なお、表2中の鋼板温度、冷却速度は、平均温度、平均冷却速度を用いて表示した。   In addition, the steel plate temperature and cooling rate in Table 2 were displayed using average temperature and average cooling rate.

本発明例はいずれも、引張強さ:900MPa以上の高強度と、vE−40:200J以上の優れた母材靭性、vE−20:80J以上の優れたHAZ靭性を有する鋼板となっている。これに対して、本発明の範囲を外れる比較例は、いずれかの特性が劣化している。 Each of the examples of the present invention is a steel sheet having high tensile strength: 900 MPa or more, excellent base material toughness of vE- 40 : 200 J or more, and excellent HAZ toughness of vE- 20 : 80 J or more. On the other hand, in the comparative example that is out of the scope of the present invention, one of the characteristics is degraded.

例えば、B含有量が本発明の範囲を外れる比較例(鋼板No.8)では、1サイクルの熱サイクルを付与後の再現HAZ靭性(Tp1)が劣化している。また、さらにB含有量が本発明の範囲を高く外れる比較例(鋼板No.9)では、2サイクルの熱サイクルを付与された後の再現HAZ靭性が劣化している。また、B含有量及びCeqが本発明の範囲を外れる比較例(鋼板No.10)では、母材強度、および1サイクル、2サイクルの熱サイクルを付与後の再現HAZ靭性が全て劣化している。   For example, in the comparative example (steel plate No. 8) in which the B content is outside the scope of the present invention, the reproduced HAZ toughness (Tp1) after applying one thermal cycle is deteriorated. Further, in the comparative example (steel plate No. 9) in which the B content deviates from the range of the present invention, the reproduced HAZ toughness after being given two cycles of heat cycle is deteriorated. Further, in the comparative example (steel plate No. 10) in which the B content and Ceq are out of the scope of the present invention, the base metal strength and the reproduced HAZ toughness after applying the 1-cycle and 2-cycle thermal cycles are all deteriorated. .

また、C含有量が本発明の範囲を外れる比較例(鋼板No.11)では、ミクロ組織が本発明の範囲を外れており、2サイクルの熱サイクル付与後の再現HAZ靭性が劣化している。また、C及びB含有量が本発明の範囲を外れる比較例(鋼板No.12)では、ミクロ組織が本発明の範囲を外れており、母材強度、および1サイクル、2サイクルの熱サイクルを付与後の再現HAZ靭性が全て劣化している。   Moreover, in the comparative example (steel plate No. 11) in which the C content is outside the scope of the present invention, the microstructure is outside the scope of the present invention, and the reproduced HAZ toughness after two cycles of thermal cycling is deteriorated. . Moreover, in the comparative example (steel plate No. 12) in which the C and B contents are out of the range of the present invention, the microstructure is out of the range of the present invention, and the base material strength and the heat cycle of 1 cycle or 2 cycles are reduced. All of the reproduced HAZ toughness after application has deteriorated.

また、750℃〜950℃での累積圧下率が低く、組織が本発明の範囲を外れる比較例(鋼板No.13)、冷却速度が本発明の好適範囲から外れ、組織が本発明の範囲を外れる比較例(鋼板No.14)、冷却停止温度が本発明の好適範囲から高く外れ、組織が本発明の範囲を外れる比較例(鋼板No.15)では、母材強度あるいはHAZ靭性が劣化している。また、冷却停止温度が本発明の好適範囲から低く外れ、組織が本発明の範囲を外れる比較例(鋼板No.16)では、母材靭性が劣化している。   In addition, a comparative example (steel plate No. 13) in which the cumulative rolling reduction at 750 ° C. to 950 ° C. is low and the structure deviates from the scope of the present invention, the cooling rate deviates from the preferred range of the present invention, and the structure departs from the scope of the present invention. In the comparative example (steel plate No. 14) that deviates, and in the comparative example (steel plate No. 15) in which the cooling stop temperature deviates from the preferred range of the present invention and the structure deviates from the range of the present invention, the base metal strength or HAZ toughness deteriorates. ing. Further, in the comparative example (steel plate No. 16) in which the cooling stop temperature deviates from the preferred range of the present invention and the structure deviates from the range of the present invention, the base material toughness is deteriorated.

母材引張特性に及ぼす冷却停止温度の影響を示すグラフである。It is a graph which shows the influence of the cooling stop temperature which acts on a base material tensile characteristic. 冷却停止温度を450℃以下として製造された鋼板の炭素当量Ceqと母材引張強さの関係を示すグラフである。4 is a graph showing the relationship between the carbon equivalent Ceq and the base metal tensile strength of a steel plate produced with a cooling stop temperature of 450 ° C. or lower. 冷却停止温度を450℃以下として製造された鋼板の炭素当量Ceqと母材靭性の関係を示すグラフである。It is a graph which shows the relationship between the carbon equivalent Ceq and base material toughness of the steel plate manufactured by making cooling stop temperature into 450 degrees C or less. 比較例の板厚中心部の母材組織の一例を示す走査型電子顕微鏡組織写真である。It is a scanning electron microscope structure photograph which shows an example of the base material structure | tissue of the plate | board thickness center part of a comparative example. 本発明例の板厚中心部の母材組織を示す走査型電子顕微鏡組織写真である。It is a scanning electron microscope structure photograph which shows the base material structure | tissue of the plate | board thickness center part of the example of this invention.

Claims (4)

質量%で、
C : 0.005〜0.030%、 Si : 0.01〜0.60%、
Mn : 1.50〜3.50%、 P : 0.020%以下、
S : 0.0050%以下、 Al : 0.001〜0.100%、
Ni: 0.10%以上1.60%未満、 Cr:0.60%以下、
Cu: 1.00%以下、 Mo:0.01〜0.60%、
Nb:0.01〜0.10%、 Ti:0.005〜0.03%、
B:0.0002〜0.0015%、 O:0.0005〜0.0100%、
N:0.001〜0.010%
を、下記(1)式で定義される炭素当量Ceqが0.50〜0.68%を満足するように含有し、残部がFeおよび不可避的不純物からなる組成と、体積率で90%以上のベイニティックフェライト相を含む組織とを有し、引張強さ:900MPa以上を有することを特徴とする低温靭性および溶接熱影響部靭性に優れた高強度鋼板。

Ceq = C + Mn/6 + (Cu+Ni)/15 + (Cr+Mo+V)/5 ………(1)
ここで、Ceq(%):炭素当量、
C、Mn、Cu、Ni、Cr、Mo、V:各元素の含有量(質量%)
% By mass
C: 0.005 to 0.030%, Si: 0.01 to 0.60%,
Mn: 1.50 to 3.50%, P: 0.020% or less,
S: 0.0050% or less, Al: 0.001 to 0.100%,
Ni: 0.10% or more and less than 1.60%, Cr: 0.60% or less,
Cu: 1.00% or less, Mo: 0.01-0.60%,
Nb: 0.01-0.10%, Ti: 0.005-0.03%,
B: 0.0002 to 0.0015%, O: 0.0005 to 0.0100%,
N: 0.001 to 0.010%
Is contained so that the carbon equivalent Ceq defined by the following formula (1) satisfies 0.50 to 0.68%, and the balance is composed of Fe and inevitable impurities, and bainitic ferrite having a volume ratio of 90% or more A high-strength steel sheet excellent in low-temperature toughness and weld heat-affected zone toughness, characterized by having a structure including a phase and a tensile strength of 900 MPa or more.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ……… (1)
Where Ceq (%): carbon equivalent,
C, Mn, Cu, Ni, Cr, Mo, V: Content of each element (% by mass)
前記組成に加えてさらに、質量%で、V:0.005〜0.100%を含有することを特徴とする請求項1に記載の高強度鋼板。   The high-strength steel sheet according to claim 1, further comprising, in addition to the composition, V: 0.005 to 0.100% by mass. 前記組成に加えてさらに、質量%で、Ca : 0.010%以下、REM : 0.020%以下のうちから選ばれた1種又は2種を含有することを特徴とする請求項1または2に記載の高強度鋼板。   The high content according to claim 1 or 2, further comprising one or two selected from Ca: 0.010% or less and REM: 0.020% or less in mass% in addition to the composition. Strength steel plate. 質量%で、
C : 0.005〜0.030%、 Si : 0.01〜0.60%、
Mn : 1.50〜3.50%、 P : 0.020%以下、
S : 0.0050%以下、 Al : 0.001〜0.100%以下、
Ni: 0.10%以上1.60%未満、 Cr:0.60%以下、
Cu: 1.00%以下、 Mo:0.01〜0.60%、
Nb:0.01〜0.10%、 Ti:0.005〜0.03%、
B:0.0002〜0.0015%、 O:0.0005〜0.0100%、
N:0.001〜0.010%
を、下記(1)式で定義される炭素当量Ceqが0.50〜0.68%を満足するように含有する組成の鋼素材を1000〜1250℃に加熱し、次いで750℃以上での累積圧下率が50%以上となる熱間圧延を施し、750℃以上で熱間圧延を終了し、ついで700℃以上の温度域から10℃/s以上の冷却速度で450℃〜300℃の温度域の冷却停止温度まで冷却した後、室温まで空冷することを特徴とする低温靭性および溶接熱影響部靭性に優れた高強度鋼板の製造方法。

Ceq = C + Mn/6 + (Cu+Ni)/15 + (Cr+Mo+V)/5 ………(1)
ここで、Ceq(%):炭素当量、
C、Mn、Cu、Ni、Cr、Mo、V:各元素の含有量(質量%)
% By mass
C: 0.005 to 0.030%, Si: 0.01 to 0.60%,
Mn: 1.50 to 3.50%, P: 0.020% or less,
S: 0.0050% or less, Al: 0.001 to 0.100% or less,
Ni: 0.10% or more and less than 1.60%, Cr: 0.60% or less,
Cu: 1.00% or less, Mo: 0.01-0.60%,
Nb: 0.01-0.10%, Ti: 0.005-0.03%,
B: 0.0002 to 0.0015%, O: 0.0005 to 0.0100%,
N: 0.001 to 0.010%
Is heated to 1000 to 1250 ° C., and the cumulative rolling reduction at 750 ° C. or higher is 50. The steel material having a carbon equivalent Ceq defined by the following formula (1) satisfies 0.50 to 0.68%. % Hot rolling is completed at 750 ° C or higher, and then the cooling stop temperature in the temperature range of 450 ° C to 300 ° C at a cooling rate of 10 ° C / s or higher from the temperature range of 700 ° C or higher. A method for producing a high strength steel sheet excellent in low temperature toughness and weld heat affected zone toughness, characterized by cooling to room temperature and then air cooling to room temperature.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 ……… (1)
Where Ceq (%): carbon equivalent,
C, Mn, Cu, Ni, Cr, Mo, V: Content of each element (% by mass)
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