JP4770415B2 - High tensile steel plate excellent in weldability and method for producing the same - Google Patents

High tensile steel plate excellent in weldability and method for producing the same Download PDF

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JP4770415B2
JP4770415B2 JP2005330767A JP2005330767A JP4770415B2 JP 4770415 B2 JP4770415 B2 JP 4770415B2 JP 2005330767 A JP2005330767 A JP 2005330767A JP 2005330767 A JP2005330767 A JP 2005330767A JP 4770415 B2 JP4770415 B2 JP 4770415B2
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公宏 西村
眞司 三田尾
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JFE Steel Corp
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本発明は、橋梁、建築、造船、建設機械、産業機械、海洋構造物、ペンストック等に用いて好適な、高張力厚鋼板に関し、特に、780MPa以上の引張強さを有する高張力厚鋼板の溶接性改善に関する。なお、本発明にいう「溶接性」の改善とは、主として、溶接継手部、とくに溶接熱影響部の耐軟化特性の向上、および溶接割れ感受性の低下をいうものとする。また、「厚鋼板」とは板厚6mm以上の鋼板をいうものとする。   The present invention relates to a high-tensile steel plate suitable for use in bridges, buildings, shipbuilding, construction machinery, industrial machinery, offshore structures, penstock, etc., and in particular, a high-tensile steel plate having a tensile strength of 780 MPa or more. It relates to weldability improvement. The “improvement of weldability” referred to in the present invention mainly means an improvement in softening resistance of a welded joint portion, particularly a weld heat affected zone, and a reduction in weld crack sensitivity. Further, the “thick steel plate” means a steel plate having a thickness of 6 mm or more.

高張力鋼板は、従来から、強度を高める合金元素を添加しているが、とくに、引張強さ:780MPa級以上の高張力厚鋼板では、添加する合金元素を多くする必要があり、そのため、焼入れ性が増大して、溶接時にその熱影響部が硬化し、溶接割れを発生しやすくなる。溶接割れの発生を防止するためには、溶接施工時に予熱作業が必要となる。しかし、溶接時の予熱作業は、溶接作業全体の能率を低下させるという問題があり、予熱作業を省略あるいは軽減できる高張力鋼板が要望されてきた。   Conventionally, high-strength steel sheets have been added with alloying elements that increase the strength, but especially for high-strength thick steel sheets with a tensile strength of 780 MPa or higher, it is necessary to increase the amount of alloying elements, and hence quenching is required. As a result, the heat-affected zone is hardened during welding and weld cracking is likely to occur. In order to prevent the occurrence of weld cracking, preheating work is required at the time of welding construction. However, the preheating work at the time of welding has a problem that the efficiency of the entire welding work is lowered, and a high-tensile steel sheet that can omit or reduce the preheating work has been demanded.

このような要望に対して、溶接割れ感受性バラメータPcmを低く抑えた、引張強さ780MPa級の高張力鋼板が提案されている。
例えば、特許文献1には、Cを0.02〜0.08重量%に制限し、Si、Mn、Alを適正量に調整したうえ、さらにCr、Mo、Ti、Nbを適量添加し、かつPcmを0.21以下とした鋼スラブを、1000℃以下の温度に加熱して熱間圧延した後、直接焼入れして焼戻す、引張強さ80kgf/mm以上(780MPa級)の高張力鋼板の製造方法が提案されている。特許文献1に記載された技術では、C含有量を制限してPcmを低くし、溶接性を向上させている。しかし、特許文献1に記載された技術では、加熱温度が1000℃以下と低温になるため、圧延能率が低下するという問題がある。
In response to such a demand, a high-tensile steel sheet with a tensile strength of 780 MPa class, in which the weld crack sensitivity parameter Pcm is kept low, has been proposed.
For example, in Patent Document 1, C is limited to 0.02 to 0.08% by weight, Si, Mn, Al are adjusted to appropriate amounts, Cr, Mo, Ti, Nb are added in appropriate amounts, and Pcm is 0.21 or less. the steel slab was, after hot rolling by heating to a temperature of 1000 ° C. or less, tempered by direct quenching method for producing a high-tensile steel plate of a tensile strength of 80 kgf / mm 2 or more (780 MPa class) it is proposed ing. In the technique described in Patent Document 1, the C content is limited to lower Pcm, and the weldability is improved. However, the technique described in Patent Document 1 has a problem that the rolling efficiency is lowered because the heating temperature is as low as 1000 ° C. or lower.

また、特許文献2には、重量比で、Cを0.05〜0.11%に制限し、Si、Mn、P、S、Al、Nを適正量に調整したうえ、さらにMo、Vを適正量添加し、かつPcmを0.222以下とした鋼を、1000〜1250℃の温度範囲に再加熱して、1000℃以下の累積圧下量が50%以上となるように圧延したのち、直接焼入れし、ついで焼戻しを行う、引張強さ80kgf/mm以上(780MPa級)の高張力鋼板の製造方法が提案されている。特許文献2に記載された技術では、C含有量を制限してPcmを低くし溶接性を向上させ、Vの析出強化を利用して所望の高強度を確保するとしている。 In Patent Document 2, C is restricted to 0.05 to 0.11% by weight ratio, Si, Mn, P, S, Al, and N are adjusted to appropriate amounts, and Mo and V are added in appropriate amounts. In addition, the steel with a Pcm of 0.222 or less is reheated to a temperature range of 1000 to 1250 ° C, rolled to a cumulative reduction of 1000 ° C or less to 50% or more, then directly quenched and then tempered. A method for producing a high-tensile steel sheet having a tensile strength of 80 kgf / mm 2 or more (780 MPa class) is proposed. In the technique described in Patent Document 2, the C content is limited to lower Pcm to improve weldability, and the desired high strength is secured by utilizing precipitation strengthening of V.

また、特許文献3には、質量%で、Cを0.075〜0.094%に制限し、Si、Mn、P、S、Al、Nを適正量に調整したうえ、さらにNi、Cr、Nbを適正量添加し、さらにCu、V、Tiの1種または2種以上を含み、かつPcmを0.24以下、Ceqを0.45以上とし、実質的にBを含有しない組成に調整した鋼を、1000〜1250℃に加熱し、圧延仕上温度を所定の範囲内とする圧延を行なったのち、Ar変態点以上から直接焼入れし、ついで焼戻し処理する溶接性および音響異方性に優れた780N/mm級高張力鋼の製造方法が提案されている。 In Patent Document 3, C is limited to 0.075 to 0.094% by mass%, Si, Mn, P, S, Al, and N are adjusted to appropriate amounts, and further, Ni, Cr, and Nb are appropriate amounts. Added to the steel containing 1000% or 1250 ° C, which contains Cu, V or Ti, Pcm is 0.24 or less, Ceq is 0.45 or more, and the composition is substantially free of B. 780 N / mm class 2 high tension excellent in weldability and acoustic anisotropy after heating and rolling to bring the rolling finishing temperature within a predetermined range, followed by quenching directly from the Ar 3 transformation point and then tempering Steel manufacturing methods have been proposed.

また、特許文献4には、重量比で、Cを0.04〜0.08%に制限し、Si、Mn、P、S、Al、Nを適正量に調整したうえ、さらにCu、Ni、Mo、Nb、V、Tiを適正量添加し、かつPcmを0.28以下とし、実質的にBを含有しない組成に調整した鋼を、1000〜1250℃の温度範囲に再加熱して、1000℃以下の累積圧下量が20%以上となるように圧延したのち、ついで空冷するか、あるいは圧延後800℃以上の温度から直接焼入れし、さらに850〜950℃に再加熱し焼入れ、焼戻する、溶接性に優れた厚手引張強さ80kgf/mm級(780MPa級)の高張力鋼の製造方法が提案されている。特許文献4に記載された技術では、C含有量を制限してPcmを低くし溶接性を向上させ、Cuを0.9%以上添加し、Cuの析出強化を利用して、所望の強度を確保するとしている。 In Patent Document 4, C is limited to 0.04 to 0.08% by weight ratio, Si, Mn, P, S, Al, and N are adjusted to appropriate amounts, and further Cu, Ni, Mo, Nb, The steel is prepared by adding appropriate amounts of V and Ti, with Pcm of 0.28 or less, and adjusted to a composition that does not substantially contain B, and reheating it to a temperature range of 1000 to 1250 ° C. After rolling to 20% or more, then air-cooled, or directly quenched after rolling at a temperature of 800 ° C or higher, reheated to 850-950 ° C, quenched and tempered, excellent weldability A method for producing high-tensile steel with a thick tensile strength of 80 kgf / mm class 2 (780 MPa class) has been proposed. In the technique described in Patent Document 4, the C content is limited to lower Pcm to improve weldability, Cu is added by 0.9% or more, and the desired strength is ensured by utilizing Cu precipitation strengthening. It is said.

また、特許文献5には、質量%で、Cを0.010〜0.060%とかなり低く制限し、さらにMn、Cr、Mo、Bを含み、さらに、Cu、Nbを所定量以下含み、KP値が3.20以上を満足する組成とした溶接性に優れた780MPa級高張力鋼が提案されている。特許文献5に記載された技術では、Mn、Cr、Moを積極添加し、さらにBを添加して、溶接性に優れた高張力鋼板を得るとしている。
特開昭63-145711号公報 特許第3208495号公報 特開2002-180132号公報 特許第2500948号公報 特開2000-160281号公報
Further, Patent Document 5 restricts C by 0.010 to 0.060% by mass%, further includes Mn, Cr, Mo, B, further includes Cu and Nb below a predetermined amount, and has a KP value of 3.20. A 780 MPa class high-strength steel having a composition satisfying the above and excellent in weldability has been proposed. In the technique described in Patent Document 5, Mn, Cr and Mo are positively added, and B is further added to obtain a high-tensile steel plate having excellent weldability.
Japanese Unexamined Patent Publication No. 63-145711 Japanese Patent No. 3208495 JP 2002-180132 A Japanese Patent No. 2500948 Japanese Unexamined Patent Publication No. 2000-160281

上記したように、C量を規制し、Pcmを低くした鋼板は、確かに溶接割れ感受性が低く予熱作業が軽減され溶接作業全体の能率が向上する。Pcmを低くするには、C含有量を低減することが最も効果がある。しかし、このような鋼板は、焼入れ性が低いため、溶接熱影響部の軟化が大きく、継手部の機械的特性で重要な項目である溶接継手強度を確保するには不利であり、溶接入熱を大きくした場合には所望の継手強度を満足することができなくなるという問題があった。   As described above, a steel sheet in which the amount of C is regulated and Pcm is lowered is certainly less susceptible to weld cracking and reduces the preheating work, thereby improving the efficiency of the entire welding work. In order to lower Pcm, it is most effective to reduce the C content. However, such steel sheets have low hardenability, so the weld heat-affected zone is greatly softened, which is disadvantageous in ensuring weld joint strength, which is an important item in the mechanical properties of the joint. There is a problem that when the value of is increased, the desired joint strength cannot be satisfied.

このようなことから、従来は、C含有量を適正範囲内に低減するとともに、他の合金元素、例えばMn、Cu、Ni、Cr、Mo、Bなどの合金元素を含有させ、さらに製造プロセスを工夫して、所望の強度を有し、溶接性に優れた高張力厚鋼板を製造してきた。
しかし、合金元素として、例えばNiを多量に含有すると、鋼板の表面に疵が発生しやすくなり、鋼板の表面品質が低下するとともに、疵の除去等手入れ作業が増加し、生産性が低下するという問題を残していた。
Therefore, conventionally, the C content is reduced within an appropriate range, and other alloy elements such as alloy elements such as Mn, Cu, Ni, Cr, Mo, and B are contained, and the manufacturing process is further improved. A high strength thick steel plate having a desired strength and excellent weldability has been devised.
However, as an alloying element, for example, when Ni is contained in a large amount, flaws are likely to occur on the surface of the steel sheet, and the surface quality of the steel sheet is lowered, and maintenance work such as removal of flaws is increased, resulting in reduced productivity. I left a problem.

本発明は、上記した従来技術の問題を有利に解決し、溶接割れ感受性が低く、かつ溶接熱影響部の軟化が少なく、溶接熱影響部の耐軟化特性および溶接性に優れるとともに、表面品質にも優れた、引張強さ780MPa級の高張力厚鋼板およびその製造方法を提供することを目的とする。なお、本発明で目標とする鋼板強度は、降伏点:685MPa以上、引張強さ:780MPa以上であり、目標とする鋼板靭性は、試験温度:0℃でのシャルピー吸収エネルギーが70J以上とする。   The present invention advantageously solves the above-mentioned problems of the prior art, has low weld cracking sensitivity, little softening of the heat affected zone, excellent resistance to softening and weldability of the heat affected zone, and improved surface quality. Another object of the present invention is to provide a high-tensile steel plate having a tensile strength of 780 MPa, and a method for producing the same. In addition, the steel plate strength targeted in the present invention is a yield point: 685 MPa or more and a tensile strength: 780 MPa or more, and the target steel plate toughness is a Charpy absorbed energy at a test temperature of 0 ° C. of 70 J or more.

本発明者らは、上記した目的を達成するために、引張強さ780MPa級高張力厚鋼板の溶接割れ感受性、溶接熱影響部の耐軟化特性に及ぼす各種要因について鋭意検討した。その結果、溶接割れ感受性を低く抑えるためにはやはりC含有量の低減が必須となるが、それに伴う溶接熱影響部の軟化を抑制するためにNbの析出作用を最大限に利用することに想到した。そして、C含有量を低減してPcmを低下させ、また、従来のMoとNiの含有に代えて、1.8質量%以上とMn含有量を増量し、さらに、Nbの適正量(0.03質量%以上)を含有することにより、すなわちC含有量を低減し、MnとNbを複合含有させることにより、所望の母材強度が確保でき、さらに溶接割れ感受性の低下と溶接熱影響部の耐軟化特性の向上が同時に実現でき、溶接部の靭性を低下させることなく、所望の溶接継手部強度を確保することが可能となり、さらには表面品質も向上させることができ、生産性を向上させることができることを新たに見出した。
本発明は、上記した知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨は次のとおりである。
In order to achieve the above-described object, the present inventors diligently studied various factors affecting the weld crack sensitivity of the high strength thick steel plate having a tensile strength of 780 MPa class and the softening resistance of the weld heat affected zone. As a result, in order to keep weld cracking sensitivity low, it is essential to reduce the C content, but in order to suppress the accompanying softening of the heat affected zone, it is conceived that the Nb precipitation action will be used to the fullest extent. did. Then, the C content is reduced to lower the Pcm, and the Mn content is increased to 1.8% by mass or more instead of the conventional Mo and Ni content. Further, the appropriate amount of Nb (0.03% by mass or more) ), I.e., by reducing the C content and combining Mn and Nb, the desired strength of the base metal can be secured, and further, the weld crack sensitivity is lowered and the softening resistance of the weld heat affected zone is improved. The improvement can be realized at the same time, it is possible to ensure the desired weld joint strength without reducing the toughness of the welded part, and also the surface quality can be improved and the productivity can be improved. Newly found.
The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.

(1)質量%で、C:0.06〜0.09%、Si:0.05〜0.60%、Mn:1.8〜2.5%(但し、2.00%超を除く)、Al:0.01〜0.08%、Nb:0.03〜0.08%、Ti:0.008〜0.020%、B:0.0005〜0.0025%、P:0.020%以下、S:0.0040%以下、N:0.0050%以下を含有し、残部Feおよび不可避的不純物からなり、かつ次(1)式
Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B……(1)
(ここで、C、Si、Mn、Cu、Ni、Cr、Mo、V、B:各元素の含有量(質量%))
で定義されるPcmが0.22以下である組成を有し、引張強さが780MPa以上であることを特徴とする溶接性に優れた高張力厚鋼板。
(1) By mass%, C: 0.06 to 0.09%, Si: 0.05 to 0.60%, Mn: 1.8 to 2.5% (excluding over 2.00%) , Al: 0.01 to 0.08%, Nb: 0.03 to 0.08% , Ti: 0.008 to 0.020%, B: 0.0005 to 0.0025%, P: 0.020% or less, S: 0.0040% or less, N: 0.0050% or less, the balance being Fe and inevitable impurities, and the following (1) Formula Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content of each element (mass%))
A high-tensile thick steel plate excellent in weldability, characterized by having a composition with a Pcm of 0.22 or less as defined by, and a tensile strength of 780 MPa or more.

(2)(1)において、前記組成に加えてさらに、質量%で、Cu:0.2%以下、Ni:0.3%以下、Cr:0.5%以下、Mo:0.04%以下、V:0.08%以下のうちから選ばれた1種または2種以上を含有する組成とすることを特徴とする高張力厚鋼板。
(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:0.0030%以下、REM:0.02%以下のうちから選ばれた1種または2種を含有する組成とすることを特徴とする高張力厚鋼板。
(2) In (1), in addition to the above composition, by mass%, Cu: 0.2% or less, Ni: 0.3% or less, Cr: 0.5% or less, Mo: 0.04% or less, V: 0.08% or less A high-tensile thick steel plate characterized by having a composition containing one or more selected from the above.
(3) In (1) or (2), in addition to the above composition, the composition further contains one or two kinds selected from Ca: 0.0030% or less and REM: 0.02% or less by mass% A high-strength thick steel plate characterized by

(4)質量%で、C:0.06〜0.09%、Si:0.05〜0.60%、Mn:1.8〜2.5%、Al:0.01〜0.08%、Nb:0.03〜0.08%、Ti:0.008〜0.020%、B:0.0005〜0.0025%、P:0.020%以下、S:0.0040%以下、N:0.0050%以下を含有し、残部Feおよび不可避的不純物からなり、かつ次(1)式
Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B……(1)
(ここで、C、Si、Mn、Cu、Ni、Cr、Mo、V、B:各元素の含有量(質量%))
で定義されるPcmが0.22以下である組成を有する鋼素材に、1000〜1300℃に加熱したのち、圧延終了温度を800℃以上とする圧延を施して厚鋼板とする熱間圧延と、該熱間圧延に引続き、該厚鋼板に300℃以下まで水冷を行う加速冷却と、ついで、400℃以上Ac1変態点以下の温度に加熱する焼戻とを、順次施すことを特徴とする引張強さ780MPa以上で溶接性に優れた高張力厚鋼板の製造方法。
(4) By mass%, C: 0.06-0.09%, Si: 0.05-0.60%, Mn: 1.8-2.5%, Al: 0.01-0.08%, Nb: 0.03-0.08%, Ti: 0.008-0.020%, B : 0.0005 to 0.0025%, P: 0.020% or less, S: 0.0040% or less, N: 0.0050% or less, consisting of the balance Fe and inevitable impurities, and the following formula (1) Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content of each element (mass%))
Hot rolling to a steel material having a composition with Pcm of 0.22 or less defined by the above, heated to 1000 to 1300 ° C., and then rolled to a rolling end temperature of 800 ° C. or more to form a thick steel plate, and the heat Tensile strength, characterized in that, following the hot rolling, the steel plate is subjected to accelerated cooling that is water-cooled to 300 ° C. or lower, and then tempering that is heated to a temperature of 400 ° C. or higher and below the Ac 1 transformation point. A method of manufacturing high-tensile steel plates with excellent weldability at 780 MPa or higher.

(5)(4)において、前記組成に加えてさらに、質量%で、Cu:0.2%以下、Ni:0.3%以下、Cr:0.5%以下、Mo:0.04%以下、V:0.08%以下のうちから選ばれた1種または2種以上を含有する組成とすることを特徴とする高張力厚鋼板の製造方法。
(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:0.0030%以下、REM:0.02%以下のうちから選ばれた1種または2種を含有する組成とすることを特徴とする高張力厚鋼板の製造方法。
(5) In (4), in addition to the above composition, by mass%, Cu: 0.2% or less, Ni: 0.3% or less, Cr: 0.5% or less, Mo: 0.04% or less, V: 0.08% or less A method for producing a high-tensile thick steel sheet, characterized in that the composition contains one or more selected from the above.
(6) In (4) or (5), in addition to the above composition, the composition further contains one or two selected from Ca: 0.0030% or less and REM: 0.02% or less in mass%. A method for producing a high-tensile thick steel plate.

本発明によれば、溶接割れ感受性が低く、かつ溶接熱影響部の軟化が少なく、溶接熱影響部の耐軟化特性および溶接性に優れるとともに、表面品質にも優れた、引張強さ780MPa級の高張力厚鋼板を、容易にしかも安価に製造でき、産業上格段の効果を奏する。   According to the present invention, the weld cracking sensitivity is low, the welding heat-affected zone is less softened, the weld heat-affected zone is excellent in softening resistance and weldability, and has excellent surface quality, with a tensile strength of 780 MPa class. A high-tensile thick steel plate can be manufactured easily and inexpensively, and has a remarkable industrial effect.

まず、本発明の高張力厚鋼板の組成限定理由について説明する。以下、組成における質量%は、単に%と記す。
C:0.06〜0.09%
Cは、鋼の強度を向上する元素であり、本発明では、所望の強度を確保するために0.06%以上の含有を必要とする。一方、0.09%を超える含有は、Pcmの上昇を招き、溶接割れ感受性が高くなり、溶接性が低下する。このため、Cは0.06〜0.09%の範囲に限定した。なお、好ましくは0.07〜0.09%である。
First, the reasons for limiting the composition of the high-tensile thick steel plate of the present invention will be described. Hereinafter, the mass% in the composition is simply referred to as%.
C: 0.06-0.09%
C is an element that improves the strength of steel, and in the present invention, it is necessary to contain 0.06% or more in order to ensure a desired strength. On the other hand, a content exceeding 0.09% causes an increase in Pcm, increases the sensitivity to weld cracking, and decreases the weldability. For this reason, C was limited to the range of 0.06 to 0.09%. In addition, Preferably it is 0.07 to 0.09%.

Si:0.05〜0.60%
Siは、脱酸剤として作用する元素であり、本発明では、脱酸を有効に行うための製鋼上の要請から、0.05%以上の含有を必要とする。一方、0.60%を超えて含有すると、靭性が低下する。このため、Siは0.05〜0.60%の範囲に限定した。なお、好ましくは0.20〜0.45%である。
Si: 0.05-0.60%
Si is an element that acts as a deoxidizing agent, and in the present invention, it is necessary to contain 0.05% or more because of demands on steel making for effective deoxidation. On the other hand, if the content exceeds 0.60%, toughness decreases. For this reason, Si was limited to the range of 0.05 to 0.60%. In addition, Preferably it is 0.20 to 0.45%.

Mn:1.8〜2.5%
Mnは、鋼の強度を向上する元素であり、780MPa以上の引張強さを確保するためには、1.8%以上の含有を必要とする。一方、2.5%を超える含有は、Pcmの上昇を招き、溶接割れ感受性が高くなるとともに、溶接熱影響部の靭性を著しく低下させる。このようなことから、Mnは1.8〜2.5%の範囲に限定した。
Mn: 1.8-2.5%
Mn is an element that improves the strength of steel, and in order to ensure a tensile strength of 780 MPa or more, it needs to contain 1.8% or more. On the other hand, a content exceeding 2.5% causes an increase in Pcm, increases the sensitivity to weld cracking, and significantly reduces the toughness of the weld heat affected zone. For these reasons, Mn was limited to a range of 1.8 to 2.5%.

Al:0.01〜0.08%
Alは、脱酸剤として作用する元素であり、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。一方、0.08%を超える含有は、母材靭性を低下させるとともに、溶接した場合に、溶接金属部の靭性をも低下させる。このため、Alは0.01〜0.08%の範囲に限定した。なお、好ましくは、0.02〜0.04%である。
Al: 0.01-0.08%
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, the content of 0.01% or more is required. On the other hand, when the content exceeds 0.08%, the base metal toughness is lowered and, when welded, the toughness of the weld metal part is also lowered. For this reason, Al was limited to the range of 0.01 to 0.08%. In addition, Preferably, it is 0.02 to 0.04%.

Nb:0.03〜0.08%
Nbは、焼入れ性を高める作用を有し、母材の高強度化に寄与する元素である。780MPa以上の母材引張強さを確保するために、本発明では0.03%以上の含有を必要とする。また、Nbは、溶接熱影響部でNbCとして析出して溶接熱影響部の軟化を防止し、継手部強度の確保に寄与する。一方、0.08%を超える含有は、析出するNbCの粗大化を招き、析出強化の効果が低減するうえ、粗大化したNbCは、靭性に悪影響を及ぼす。このため、Nbは0.03〜0.08%の範囲に限定した、なお、好ましくは0.03%超〜0.06%以下、より好ましくは0.03〜0.05%である。
Nb: 0.03-0.08%
Nb is an element that has the effect of improving hardenability and contributes to the enhancement of the strength of the base material. In order to ensure a base material tensile strength of 780 MPa or more, the present invention requires a content of 0.03% or more. Further, Nb precipitates as NbC in the weld heat affected zone, prevents softening of the weld heat affected zone, and contributes to securing the joint strength. On the other hand, a content exceeding 0.08% leads to coarsening of the precipitated NbC, reducing the effect of precipitation strengthening, and coarsened NbC adversely affects toughness. Therefore, Nb is limited to the range of 0.03 to 0.08%, preferably more than 0.03% to 0.06% or less, more preferably 0.03 to 0.05%.

なお、Nbは、従来から溶接熱影響部の軟化を抑制する元素として知られていたが、引張強さ780MPa以上の高張力鋼板では0.03%以下の微量な添加に制限することが一般的であった。これは、過剰なNbの含有が溶接ボンド部付近の粗粒域の靭性を低下させるため、従来はNbの多量含有を避ける必要があったためである。本発明では、MoとNiの含有に代えて、Mn含有量を増量することにより、Nbを0.03%以上多量に含有しても、Nbの溶接熱影響部靭性への悪影響が軽減でき、溶接熱影響部の軟化抑制効果を発揮しながら所望の靭性を確保できる。   Nb has been conventionally known as an element that suppresses softening of the weld heat-affected zone, but it is generally limited to a small amount of 0.03% or less for high-tensile steel sheets with a tensile strength of 780 MPa or more. It was. This is because excessive Nb content lowers the toughness of the coarse grain region in the vicinity of the weld bond, and thus it has conventionally been necessary to avoid a large amount of Nb. In the present invention, instead of containing Mo and Ni, by increasing the Mn content, even if Nb is contained in a large amount of 0.03% or more, the adverse effect on the weld heat affected zone toughness of Nb can be reduced, welding heat Desired toughness can be secured while exhibiting the effect of suppressing the softening of the affected part.

溶接熱影響部の軟化は、最高温度:850〜700℃に再熱される部分が最も大きい。というのは、850℃以上又はAc3変態点を超えて再熱される部分は、完全にオーステナイト化した状態から冷却されるため、再変態してある程度の強度が確保されるためである。しかし、最高温度:850℃以下の温度に再熱される部分では、マルテンサイトあるいはベイナイトの軟化が起こるため、強度が低下する。本発明では、850〜700℃の温度域に再熱される領域の軟化を抑えるために、0.03%以上のNbを含有する。溶接熱影響部でNbの析出強化を有効に発揮させるためには、母材ではNbを固溶させた状態で存在させ、溶接による再熱で析出させる。これにより、溶接熱影響部の強度低下、すなわち軟化を抑制することができ、継手部強度でも780MPa以上を確保できることになる。 The softening of the heat affected zone is the largest at the maximum temperature: 850 to 700 ° C. This is because the portion that is reheated above 850 ° C. or beyond the Ac 3 transformation point is cooled from the fully austenitic state, and is thus retransformed to ensure a certain level of strength. However, in the portion that is reheated to the maximum temperature: 850 ° C. or lower, martensite or bainite softens, and the strength decreases. In this invention, in order to suppress softening of the area | region reheated to the temperature range of 850-700 degreeC, 0.03% or more of Nb is contained. In order to effectively exhibit the precipitation strengthening of Nb in the weld heat affected zone, the base metal is present in a state where Nb is dissolved, and is precipitated by reheating by welding. As a result, a decrease in strength of the weld heat affected zone, that is, softening can be suppressed, and a joint strength of 780 MPa or more can be secured.

Ti:0.008〜0.020%
Tiは、NをTiNとして固定して、BNの析出を抑制する作用を有し、Bの焼入れ性向上の作用を有効に発揮させるために、不可欠な元素である。このような効果は、0.008%以上の含有で顕著となる。一方、0.020%を超える含有は、靭性を低下させる。このため、Tiは0.008〜0.020%の範囲に限定した。なお、好ましくは0.010〜0.015%である。
Ti: 0.008-0.020%
Ti is an indispensable element in order to fix N as TiN and to suppress the precipitation of BN and to effectively exhibit the effect of improving the hardenability of B. Such an effect becomes remarkable when the content is 0.008% or more. On the other hand, the content exceeding 0.020% lowers toughness. For this reason, Ti was limited to the range of 0.008 to 0.020%. In addition, Preferably it is 0.010 to 0.015%.

B:0.0005〜0.0025%
Bは、オーステナイト粒界に偏析して、微量で焼入れ性を顕著に向上させる元素であり、本発明では必須の元素である。このような効果を得るためには、0.0005%以上の含有を必要とする。一方、0.0025%を超えて含有しても、効果が飽和するうえ、靭性にも悪影響を及ぼす。このため、Bは0.0005〜0.0025%の範囲に限定した。なお、好ましくは0.0006〜0.0015%である。
B: 0.0005-0.0025%
B is an element that segregates at the austenite grain boundaries and significantly improves the hardenability in a small amount, and is an essential element in the present invention. In order to acquire such an effect, 0.0005% or more needs to be contained. On the other hand, if the content exceeds 0.0025%, the effect is saturated and the toughness is also adversely affected. For this reason, B was limited to the range of 0.0005 to 0.0025%. In addition, Preferably it is 0.0006 to 0.0015%.

P:0.020%以下
Pは、不可避的不純物元素として含有されるが、0.020%を超えて含有すると、母材及び溶接部の靭性を低下させる。このため、本発明では、0.020%以下に限定した。なお、好ましくは0.015%以下である。
S:0.0040%以下
Sは、不可避的不純物元素として含有されるが、0.0040%を超えて含有すると、母材および溶接部の靭性を低下させる。このため、本発明では、0.0040%以下に限定した。なお、好ましくは0.0030%以下である。
P: 0.020% or less P is contained as an unavoidable impurity element, but if contained in excess of 0.020%, the toughness of the base material and the welded portion is lowered. For this reason, in this invention, it limited to 0.020% or less. In addition, Preferably it is 0.015% or less.
S: 0.0040% or less S is contained as an unavoidable impurity element, but if contained in excess of 0.0040%, the toughness of the base metal and the welded portion is lowered. For this reason, in this invention, it limited to 0.0040% or less. In addition, Preferably it is 0.0030% or less.

N:0.0050%以下
Nは、溶接部の靭性を低下させる元素であり、0.0050%を超えて含有すると、溶接部の靭性低下が著しくなる。このため、Nは0.0050%以下に限定した。
以上が基本組成であるが、本発明では、上記した基本組成に加えて、さらに、Cu:0.2%以下、Ni:0.3%以下、Cr:0.5%以下、Mo:0.04%以下、V:0.08%以下のうちから選ばれた1種または2種以上、および/または、Ca:0.0030%以下、REM:0.02%以下のうちから選ばれた1種または2種を含有できる。
N: 0.0050% or less N is an element that lowers the toughness of the welded portion. If it exceeds 0.0050%, the toughness of the welded portion is significantly reduced. For this reason, N was limited to 0.0050% or less.
The above is the basic composition. In the present invention, in addition to the above basic composition, Cu: 0.2% or less, Ni: 0.3% or less, Cr: 0.5% or less, Mo: 0.04% or less, V: 0.08% One or more selected from the following and / or one or two selected from Ca: 0.0030% or less and REM: 0.02% or less can be contained.

Cu:0.2%以下、Ni:0.3%以下、Cr:0.5%以下、Mo:0.04%以下、V:0.08%以下のうちから選ばれた1種または2種以上
Cu、Ni、Cr、Mo、Vは、いずれも鋼の強度を向上する元素であり、必要に応じて選択して含有できる。
Cuは、固溶強化により鋼の強度を向上させる元素であり、このような効果を得るためには、0.05%以上含有することが好ましいが、0.2%を超える含有は、靭性を低下させる。このため、含有する場合、Cuは0.2%以下に限定することが好ましい。
One or more selected from Cu: 0.2% or less, Ni: 0.3% or less, Cr: 0.5% or less, Mo: 0.04% or less, V: 0.08% or less
Cu, Ni, Cr, Mo, and V are all elements that improve the strength of steel, and can be selected and contained as necessary.
Cu is an element that improves the strength of the steel by solid solution strengthening. In order to obtain such an effect, it is preferable to contain 0.05% or more, but inclusion exceeding 0.2% lowers toughness. For this reason, when it contains, it is preferable to limit Cu to 0.2% or less.

Niは、母材靭性を確保しつつ強度を向上させる元素であり、このような効果を得るためには、0.05%以上含有することが好ましいが、0.3%を超えて含有すると、必須成分であるNbとの複合添加により、溶接熱影響部粗粒域の靭性を低下させる。また、Niは、鋼板表面に疵を発生させやすく、鋼板の表面品質を低下させる。このため、含有する場合、Niは、0.3%以下に限定することが好ましい。なお、なお、好ましくは0.05〜0.15%である。   Ni is an element that improves the strength while ensuring the toughness of the base metal. In order to obtain such effects, it is preferable to contain 0.05% or more, but if it exceeds 0.3%, it is an essential component The combined addition with Nb reduces the toughness of the weld heat-affected zone coarse grain region. Moreover, Ni tends to generate wrinkles on the surface of the steel sheet and lowers the surface quality of the steel sheet. For this reason, when it contains, it is preferable to limit Ni to 0.3% or less. In addition, Preferably it is 0.05 to 0.15%.

Crは、鋼の強度を向上させる元素であり、このような効果を得るためには、0.05%以上含有することが好ましいが、0.5%を超える含有は、溶接熱影響部の靭性を低下させる。このため、含有する場合、Crは、0.5%以下に限定することが好ましい。
Moは、焼入れ性の向上を介して、微量で鋼の強度を向上させる元素であり、このような効果を得るためには、0.01%以上含有することが好ましい。一方、0.04%を超える含有は、溶接熱影響部の粗粒域の靭性を低下させる。このため、含有する場合、Moは0.04%以下に限定することが好ましい。
Cr is an element that improves the strength of steel. In order to obtain such an effect, it is preferably contained in an amount of 0.05% or more. However, if it exceeds 0.5%, the toughness of the weld heat affected zone is lowered. For this reason, when contained, Cr is preferably limited to 0.5% or less.
Mo is an element that improves the strength of the steel in a small amount through improvement of hardenability. In order to obtain such an effect, it is preferable to contain 0.01% or more. On the other hand, the content exceeding 0.04% lowers the toughness of the coarse grain region of the weld heat affected zone. For this reason, when contained, Mo is preferably limited to 0.04% or less.

Vは、V(CN)として析出し、鋼の強度を向上させる析出強化元素であり、このような効果を得るためには、0.003%以上含有させることが好ましいが、0.08%を超える含有は、靭性を低下させる。このため、Vは0.08%以下に限定することが好ましい。
Ca:0.0030%以下、REM:0.02%以下のうちから選ばれた1種または2種
Ca、REMはいずれも、溶接熱影響部靭性を向上させる元素であり、必要に応じて選択して含有できる。
V is a precipitation strengthening element that precipitates as V (CN) and improves the strength of the steel. In order to obtain such an effect, it is preferably contained in an amount of 0.003% or more. Reduce toughness. For this reason, it is preferable to limit V to 0.08% or less.
One or two selected from Ca: 0.0030% or less, REM: 0.02% or less
Both Ca and REM are elements that improve the weld heat-affected zone toughness, and can be selected and contained as necessary.

Caは、0.0003%以上の含有で、介在物の形態制御によりS、Oとのバランスを適切に選択することで溶接熱影響部靭性を向上させる。このような効果を得るためには、0.0003%以上含有することが好ましいが、一方、0.0030%を超えて含有してもその効果が飽和する。このため、Caは0.0030%以下に限定することが好ましい。
REMは、REM (O、S)を形成して、溶接熱影響部靭性を向上させる元素であり、このような効果を得るためには、0.0003%以上含有することが好ましいが、0.02%を超えて含有しても、その効果が飽和し、含有量に見合う効果が期待できなくなる。このため、REMは0.02%以下に限定することが好ましい。因みに、REMは、希土類元素を意味し、代表的なものとしてはLa、Ce、Hfなどがある。
Ca is contained in an amount of 0.0003% or more, and the weld heat affected zone toughness is improved by appropriately selecting the balance with S and O by controlling the form of inclusions. In order to acquire such an effect, it is preferable to contain 0.0003% or more, but even if it contains exceeding 0.0030%, the effect will be saturated. For this reason, it is preferable to limit Ca to 0.0030% or less.
REM is an element that forms REM (O, S) and improves the toughness of the heat affected zone of the weld. In order to obtain such an effect, the content is preferably 0.0003% or more, but exceeds 0.02%. Even if contained, the effect is saturated and an effect commensurate with the content cannot be expected. For this reason, REM is preferably limited to 0.02% or less. Incidentally, REM means a rare earth element, and representative examples include La, Ce, and Hf.

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。
本発明では、上記した各成分の範囲で、かつ次(1)式
Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B……(1)
(ここで、C、Si、Mn、Cu、Ni、Cr、Mo、V、B:各元素の含有量(質量%))
で定義されるPcmが0.22以下となるように、各成分を調整する。
The balance other than the above components is Fe and inevitable impurities.
In the present invention, the range of each component described above, and the following formula (1) Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: content of each element (mass%))
Each component is adjusted so that Pcm defined in (1) is 0.22 or less.

(1)式で定義されるPcmは、溶接割れ感受性指数(パラメータ)であり、Pcmが0.22を超えて大きくなると、溶接割れ感受性が高くなり、溶接時に予熱作業を必要とするようになる。このため、本発明ではPcmは0.22以下に限定した。なお、好ましくは0.20以下である。
つぎに、本発明の高張力厚鋼板の好ましい製造方法について説明する。
Pcm defined by the equation (1) is a weld crack susceptibility index (parameter). If Pcm exceeds 0.22, the weld crack sensitivity becomes high, and preheating work is required during welding. For this reason, in the present invention, Pcm is limited to 0.22 or less. In addition, Preferably it is 0.20 or less.
Below, the preferable manufacturing method of the high-tensile steel plate of this invention is demonstrated.

上記した組成の溶鋼を、転炉等の溶製炉で溶製し、連続鋳造等により鋼素材(スラブ)とする。ついで、得られた鋼素材に、1000〜1300℃に加熱したのち、圧延終了温度を800℃以上とする圧延を施して厚鋼板とする熱間圧延と、該熱間圧延に引続き、該厚鋼板に300℃以下まで水冷を行う加速冷却と、ついで、400℃以上Ac1変態点以下の温度に加熱する焼戻とを、順次施すことが好ましい。 The molten steel having the above composition is melted in a melting furnace such as a converter, and is made into a steel material (slab) by continuous casting or the like. Next, after heating the obtained steel material to 1000 to 1300 ° C., rolling it to a rolling end temperature of 800 ° C. or higher to form a thick steel plate, following the hot rolling, the thick steel plate In addition, it is preferable to sequentially perform accelerated cooling in which water is cooled to 300 ° C. or lower, and then tempering in which the temperature is heated to 400 ° C. or higher and below the Ac 1 transformation point.

本発明では、鋼素材に含有するNbを、鋼素材(スラブ)の加熱段階で十分に固溶させる必要がある。このため、鋼素材の加熱温度は1000℃以上とすることが好ましい。一方、1300℃を超える高温では、オーステナイト粒が粗大化し、靭性の低下を招くばかりか、酸化ロスが顕著となり、歩留が低下する。このため、鋼素材の加熱温度は1000〜1300℃とすることが好ましい。   In the present invention, it is necessary to sufficiently dissolve Nb contained in the steel material at the heating stage of the steel material (slab). For this reason, it is preferable that the heating temperature of a steel raw material shall be 1000 degreeC or more. On the other hand, at a high temperature exceeding 1300 ° C., the austenite grains become coarse, leading to a decrease in toughness, and an oxidation loss becomes remarkable, resulting in a decrease in yield. For this reason, it is preferable that the heating temperature of a steel raw material shall be 1000-1300 degreeC.

熱間圧延は、厚鋼板の製造ラインを利用して行うことが好ましいが、圧延終了温度は800℃以上とすることが好ましい。圧延終了温度が800℃未満であっても鋼板の機械的特性に問題はない。しかし、圧延後の鋼板の形状が不良となり、とくに平坦度の低下が著しくなる。なお、本発明では、圧延終了温度の上限は特に規定する必要はないが、1000℃以下とするのが望ましい。   Hot rolling is preferably performed using a thick steel plate production line, but the rolling end temperature is preferably 800 ° C. or higher. Even if the rolling end temperature is less than 800 ° C., there is no problem in the mechanical properties of the steel sheet. However, the shape of the steel sheet after rolling becomes poor, and the flatness is particularly lowered. In the present invention, the upper limit of the rolling end temperature need not be specified, but is preferably 1000 ° C. or lower.

熱間圧延に引続き、変態強化により高強度化するために、厚鋼板に300℃以下までの水冷を行い、その後放冷する加速冷却を施すことが好ましい。加速冷却の冷却停止温度が300℃超えでは、変態が終了していないため、所望の引張強さを確保できない。また、本発明の組成範囲では、加速冷却の冷却速度は、5℃/s以上とすることが好ましい。冷却速度が空冷や、5℃/s未満では、所望の引張強さを確保できない。   Following hot rolling, in order to increase the strength by transformation strengthening, it is preferable to subject the thick steel plate to water cooling to 300 ° C. or less, and then to accelerated cooling for cooling. If the cooling stop temperature for accelerated cooling exceeds 300 ° C., the transformation has not been completed, and thus the desired tensile strength cannot be ensured. Moreover, in the composition range of this invention, it is preferable that the cooling rate of accelerated cooling shall be 5 degrees C / s or more. If the cooling rate is air cooling or less than 5 ° C./s, the desired tensile strength cannot be ensured.

加速冷却についで、厚鋼板を400℃以上Ac変態点以下の温度に加熱する焼戻を施すことが好ましい。焼戻により、引張強さ780MPa級厚鋼板としての強度と靭性が適切に付与される。焼戻のための加熱温度が400℃未満では、母材靭性が低下し、一方、Ac変態点を超えると、オーステナイト(γ)に再変態するため、所望の母材強度および靭性を確保できなくなる。なお、焼戻は、オンラインで行なってもオフラインで行なっても、どちらでもよい。 Following accelerated cooling, it is preferable to perform tempering in which the thick steel plate is heated to a temperature of 400 ° C. or higher and below the Ac 1 transformation point. By tempering, the strength and toughness of a tensile strength 780 MPa class thick steel sheet are appropriately imparted. If the heating temperature for tempering is less than 400 ° C, the toughness of the base metal decreases. On the other hand, if it exceeds the Ac 1 transformation point, it retransforms into austenite (γ), so that the desired base metal strength and toughness can be secured. Disappear. The tempering may be performed online or offline.

表1に示す組成の溶鋼を、転炉で溶製し、連続鋳造法で鋼素材(スラブ)(鋼素材厚:250mm)とした。これらスラブに、熱間圧延、加速冷却および焼戻を施した。熱間圧延では、スラブに表2に示す条件の熱間圧延を施し、板厚25mmの厚鋼板とした。熱間圧延に引続く加速冷却では、熱間圧延後、厚鋼板に表2に示す条件で加速冷却を施した。冷却後に厚鋼板に施す焼戻では、厚鋼板に、オフラインの熱処理炉で、表2に示す温度で焼戻しを行なった。   Molten steel having the composition shown in Table 1 was melted in a converter and made into a steel material (slab) (steel material thickness: 250 mm) by a continuous casting method. These slabs were hot rolled, accelerated cooled and tempered. In the hot rolling, the slab was hot rolled under the conditions shown in Table 2 to obtain a thick steel plate having a thickness of 25 mm. In the accelerated cooling following the hot rolling, the hot steel sheet was subjected to accelerated cooling under the conditions shown in Table 2 after the hot rolling. In tempering the thick steel plate after cooling, the thick steel plate was tempered at the temperature shown in Table 2 in an offline heat treatment furnace.

得られた厚鋼板について、引張試験、シャルピー衝撃試験、最高硬さ試験、溶接継手試験を実施した。試験方法は次のとおりである。
(1)引張試験
得られた厚鋼板から全厚のJIS 5号引張試験片を引張方向が圧延直角方向となるように採取し、JIS Z 2241の規定に準拠して引張試験を行い、引張特性(降伏点YS、引張強さTS)を求めた。
The obtained thick steel plate was subjected to a tensile test, a Charpy impact test, a maximum hardness test, and a welded joint test. The test method is as follows.
(1) Tensile test A full thickness JIS No. 5 tensile test piece is taken from the obtained thick steel plate so that the tensile direction is the direction perpendicular to the rolling direction, and a tensile test is performed in accordance with the provisions of JIS Z 2241. (Yield point YS, tensile strength TS) were determined.

(2)シャルピー衝撃試験
得られた厚鋼板の板厚1/4t部から圧延方向に、JIS Z 2202の規定に準拠してVノッチ試験片を採取し、JIS Z 2242の規定に準拠してシャルピー衝撃試験を実施し、試験温度:0℃での吸収エネルギーvE0を求めた。
(3)最高硬さ試験
得られた厚鋼板から試験材を採取して、JIS Z 3101の規定に準拠して溶接熱影響部の最高硬さを求めた。得られた溶接熱影響部の最高硬さから、鋼板の溶接割れ感受性を評価した。なお、最高硬さHvが350以下であれば、溶接割れ感受性が低く、耐低温割れ性に優れると判定した。
(2) Charpy impact test V-notch test specimens were collected in the rolling direction from the 1/4 ton thickness of the obtained thick steel plate in accordance with the provisions of JIS Z 2202, and Charpy in accordance with the provisions of JIS Z 2242. An impact test was performed, and the absorbed energy vE 0 at the test temperature: 0 ° C. was determined.
(3) Maximum hardness test The test material was extract | collected from the obtained thick steel plate, and the maximum hardness of the welding heat affected zone was calculated | required based on the prescription | regulation of JISZ3101. The weld crack sensitivity of the steel sheet was evaluated from the maximum hardness of the obtained weld heat affected zone. When the maximum hardness Hv was 350 or less, it was determined that the weld cracking sensitivity was low and the cold cracking resistance was excellent.

(4)溶接継手試験
得られた厚鋼板から試験材を採取して、X開先の多層サブマージアーク溶接継手(溶接入熱:45kJ/cm)を作製した。得られた溶接継手からJIS 1号継手引張試験片を採取し、継手強度を測定した。
得られた結果を表3に示す。
(4) Welded joint test A test material was collected from the obtained thick steel plate to produce a multi-layer submerged arc welded joint (welding heat input: 45 kJ / cm) with an X groove. From the obtained welded joint, a JIS No. 1 joint tensile specimen was collected and the joint strength was measured.
The obtained results are shown in Table 3.

本発明例はいずれも、目標の鋼板強度および靭性を満足し、しかも溶接割れ感受性が低く、かつ溶接継手強度も母材と同等以上の強度を示した。一方、本発明の範囲から外れる比較例は、強度(YS、TS)、靭性(vE0)、溶接熱影響部の最高硬さ、継手強度のどれかが目標値を満足できていない。 All of the examples of the present invention satisfied the target steel plate strength and toughness, had low weld cracking sensitivity, and the weld joint strength was equal to or higher than that of the base material. On the other hand, in the comparative example that is out of the scope of the present invention, any of the strength (YS, TS), toughness (vE 0 ), the maximum hardness of the weld heat affected zone, and the joint strength does not satisfy the target value.

Claims (6)

質量%で、
C:0.06〜0.09%、 Si:0.05〜0.60%,
Mn:1.8〜2.5%(但し、2.00%超を除く)、 Al:0.01〜0.08%、
Nb:0.03〜0.08%、 Ti:0.008〜0.020%、
B:0.0005〜0.0025%、 P:0.020%以下、
S:0.0040%以下、 N:0.0050%以下
を含有し、残部Feおよび不可避的不純物からなり、かつ下記(1)式で定義されるPcmが0.22以下である組成を有し、引張強さが780MPa以上であることを特徴とする溶接性に優れた高張力厚鋼板。

Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B…(1)
ここで、C、Si、Mn、Cu、Ni、Cr、Mo、V、B:各元素の含有量(質量%)
% By mass
C: 0.06 to 0.09%, Si: 0.05 to 0.60%,
Mn: 1.8 to 2.5% (excluding over 2.00%) , Al: 0.01 to 0.08%,
Nb: 0.03-0.08%, Ti: 0.008-0.020%,
B: 0.0005 to 0.0025%, P: 0.020% or less,
S: 0.0040% or less, N: 0.0050% or less, comprising Fe and unavoidable impurities, Pcm defined by the following formula (1) is 0.22 or less, and tensile strength is 780 MPa A high-tensile thick steel plate with excellent weldability characterized by the above.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%)
前記組成に加えてさらに、質量%で、Cu:0.2%以下、Ni:0.3%以下、Cr:0.5%以下、Mo:0.04%以下、V:0.08%以下のうちから選ばれた1種または2種以上を含有する組成とすることを特徴とする請求項1に記載の高張力厚鋼板。   In addition to the above composition, one or two selected from the following by mass: Cu: 0.2% or less, Ni: 0.3% or less, Cr: 0.5% or less, Mo: 0.04% or less, V: 0.08% or less The high-tensile thick steel plate according to claim 1, wherein the high-tensile steel plate has a composition containing seeds or more. 前記組成に加えてさらに、質量%で、Ca:0.0030%以下、REM:0.02%以下のうちから選ばれた1種または2種を含有する組成とすることを特徴とする請求項1または2に記載の高張力厚鋼板。   The composition according to claim 1 or 2, further comprising one or two kinds selected from Ca: 0.0030% or less and REM: 0.02% or less in mass% in addition to the composition. The high-tensile thick steel plate described. 質量%で、
C:0.06〜0.09%、 Si:0.05〜0.60%,
Mn:1.8〜2.5%、 Al:0.01〜0.08%、
Nb:0.03〜0.08%、 Ti:0.008〜0.020%、
B:0.0005〜0.0025%、 P:0.020%以下、
S:0.0040%以下、 N:0.0050%以下
を含有し、残部Feおよび不可避的不純物からなり、かつ下記(1)式で定義されるPcmが0.22以下である組成を有する鋼素材に、1000〜1300℃に加熱したのち、圧延終了温度を800℃以上とする圧延を施して厚鋼板とする熱間圧延と、該熱間圧延に引続き、該厚鋼板に300℃以下まで水冷を行う加速冷却と、ついで、400℃以上Ac1変態点以下の温度に加熱する焼戻とを、順次施すことを特徴とする引張強さ780MPa以上で溶接性に優れた高張力厚鋼板の製造方法。

Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B……(1)
ここで、C、Si、Mn、Cu、Ni、Cr、Mo、V、B:各元素の含有量(質量%)
% By mass
C: 0.06 to 0.09%, Si: 0.05 to 0.60%,
Mn: 1.8 to 2.5%, Al: 0.01 to 0.08%,
Nb: 0.03-0.08%, Ti: 0.008-0.020%,
B: 0.0005 to 0.0025%, P: 0.020% or less,
A steel material containing S: 0.0040% or less, N: 0.0050% or less, consisting of the balance Fe and inevitable impurities, and having a Pcm defined by the following formula (1) of 0.22 or less, is 1000-1300. After heating to ℃, hot rolling to give a rolling end temperature of 800 ℃ or more to be a thick steel plate, following the hot rolling, accelerated cooling to perform water cooling to 300 ℃ or less to the thick steel plate, Next, a method for producing a high-tensile thick steel plate having a tensile strength of 780 MPa or more and excellent weldability, characterized by sequentially performing tempering to 400 ° C. or higher and a temperature of Ac 1 transformation point or lower.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%)
前記組成に加えてさらに、質量%で、Cu:0.2%以下、Ni:0.3%以下、Cr:0.5%以下、Mo:0.04%以下、V:0.08%以下のうちから選ばれた1種または2種以上を含有する組成とすることを特徴とする請求項4に記載の高張力厚鋼板の製造方法。   In addition to the above composition, one or two selected from the following by mass: Cu: 0.2% or less, Ni: 0.3% or less, Cr: 0.5% or less, Mo: 0.04% or less, V: 0.08% or less The method for producing a high-tensile thick steel plate according to claim 4, wherein the composition contains a seed or more. 前記組成に加えてさらに、質量%で、Ca:0.0030%以下、REM:0.02%以下のうちから選ばれた1種または2種を含有する組成とすることを特徴とする請求項4または5に記載の高張力厚鋼板の製造方法。   The composition according to claim 4 or 5, further comprising one or two kinds selected from Ca: 0.0030% or less and REM: 0.02% or less in mass% in addition to the composition. The manufacturing method of the high-tensile thick steel plate of description.
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