JP5883369B2 - Thick steel plate with excellent toughness in weld heat affected zone - Google Patents
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Description
本発明は、橋梁や高層建造物、船舶、ラインパイプなどの溶接構造物に適用される厚鋼板に関し、より詳しくは、大入熱後の熱影響部(以下、HAZとも述べる。)の靭性に優れた厚鋼板に関するものである。 The present invention relates to a thick steel plate applied to a welded structure such as a bridge, a high-rise building, a ship, or a line pipe. More specifically, the present invention relates to the toughness of a heat affected zone (hereinafter also referred to as HAZ) after large heat input. It relates to an excellent thick steel plate.
近年、橋梁や高層建造物、船舶、ラインパイプなどの溶接構造物の大型化に伴い、このような溶接構造物には50mm以上の板厚の厚鋼板が適用されることが多くなってきており、50mm以上の板厚の厚鋼板の溶接が不可避となっている。以上のような実情もあり、溶接施工効率向上を目的とした大入熱溶接が求められている。 In recent years, with the increase in the size of welded structures such as bridges, high-rise buildings, ships, and line pipes, thick steel plates with a thickness of 50 mm or more are often applied to such welded structures. The welding of thick steel plates with a thickness of 50 mm or more is inevitable. In view of the above circumstances, high heat input welding for the purpose of improving welding construction efficiency is required.
しかしながら、大入熱溶接時のHAZは、加熱によって高温のオーステナイト(γ)領域に長時間保持された後、徐冷されるため、加熱時におけるγ粒の成長、冷却過程における粗大フェライト(α)粒の生成に代表されるような組織の粗大化がもたらされやすく、それが大入熱溶接時のHAZの靭性低下の原因となっている。そのため、大入熱溶接時におけるHAZの靭性(以下、HAZ靭性とも述べる。)を安定して高い水準に保つ技術を開発することが、必要課題となっている。 However, since HAZ during high heat input welding is kept in a high temperature austenite (γ) region by heating for a long time and then gradually cooled, γ grains grow during heating, and coarse ferrite (α) during the cooling process. The coarsening of the structure as typified by the formation of grains is likely to be caused, which causes a reduction in the toughness of the HAZ during high heat input welding. Therefore, it is necessary to develop a technique for stably maintaining the HAZ toughness (hereinafter also referred to as HAZ toughness) at the time of high heat input welding at a high level.
HAZ靭性を確保するための手段としては、酸化物、窒化物、硫化物等の介在物粒子によるγ粒成長ピン止め、介在物粒子を起点とする粒内α生成による組織の微細化に関する技術等が提案されている。こうした技術の提案例として、特許文献1や特許文献2に記載の技術があり、鋼材中に微細なTi窒化物をγ粒成長ピン止め粒子として分散析出させることで、大入熱溶接時のHAZで生じるオーステナイト粒の粗大化を抑制し、HAZ靭性の劣化を抑えることが開示されている。しかしながら、Ti窒化物は、溶接入熱を増大させると消失しやすく、安定したHAZ靭性を得るためには、特別な工夫が必要となる。 Means for ensuring HAZ toughness include pinning gamma grain growth by inclusion particles such as oxides, nitrides, and sulfides, and techniques for refining the structure by intragranular α formation starting from inclusion particles. Has been proposed. As a proposal example of such a technique, there are techniques described in Patent Document 1 and Patent Document 2, and by dispersing and depositing fine Ti nitride as γ-growth growth pinning particles in a steel material, HAZ at the time of high heat input welding It suppresses the coarsening of the austenite grain which arises by this, and suppresses deterioration of HAZ toughness. However, Ti nitride tends to disappear when the welding heat input is increased, and special measures are required to obtain stable HAZ toughness.
発明者らも、特許文献3において、微細Ti窒化物のサイズおよび個数を精密に制御することで、大入熱HAZ靭性を改善する技術を提案している。しかしながら、想定する入熱は55kJ/mmにとどまっており、更なる溶接入熱の増大に対応するためには、いっそうの改善が必要である。 Inventors have also proposed a technique for improving the high heat input HAZ toughness by precisely controlling the size and number of fine Ti nitrides in Patent Document 3. However, the assumed heat input is only 55 kJ / mm, and further improvement is necessary to cope with further increase in welding heat input.
また、特許文献4〜7では、高温で安定な酸化物系介在物をγ粒成長ピン止め粒子として利用する技術が提案されている。しかしながら、酸化物系介在物はTi含有窒化物に比べて数が少なく、十分なピン止め効果を得ることができないため、大入熱溶接に対して対応することが十分にはできず、尚一層の改善が必要である。 Patent Documents 4 to 7 propose a technique of using oxide inclusions that are stable at high temperatures as γ-grown pinning particles. However, the number of oxide inclusions is smaller than that of Ti-containing nitrides, and a sufficient pinning effect cannot be obtained. Improvement is necessary.
すなわち、特許文献4には、REMやZrを含む酸化物を存在させることによって良好なHAZ特性が得られると記載されてはいるものの、想定した入熱は低い水準にとどまっており、必ずしも大入熱溶接で良好なHAZ特性が得られるとはいいえない。また、特許文献5には、特許文献4と同様にREMやZrを含む酸化物を利用する技術が記載されており、HAZ靭性としてシャルピー吸収エネルギーを評価しているものの、材料の信頼性という観点では、平均値のみならずその最小値も高い水準に保障する必要があると考えられる。 That is, although Patent Document 4 describes that good HAZ characteristics can be obtained by the presence of an oxide containing REM or Zr, the assumed heat input is only at a low level, and it is not necessarily large input. No good HAZ properties can be obtained by thermal welding. Patent Document 5 describes a technique that uses an oxide containing REM or Zr as in Patent Document 4, and evaluates Charpy absorbed energy as HAZ toughness, but it is a viewpoint of material reliability. Therefore, it is considered necessary to guarantee not only the average value but also its minimum value to a high level.
更には、特許文献6には、酸化物系介在物とTi含有介在物の両方をγ粒成長ピン止め粒子として利用することで、高いHAZ靭性を得る技術が記載されている。しかしながら、特許文献6では、大入熱溶接を模擬した熱サイクル試験にてHAZ靭性の評価を行っているが、最高加熱温度が1400℃と一部のTi含有窒化物が残存する温度にて行っている。ところが、HAZの最高加熱温度が部分的に1450℃を上回る高熱となり、Ti含有窒化物の消失がいっそう促進される。よって、大入熱HAZ靭性を正確に評価するためには、実際に大入熱溶接試験を行うことが望ましい。また、発明者らは特許文献7で、微細酸化物系介在物のγ粒成長ピン止め効果を活用した技術を提案しているが、この技術は微細Mn硫化物の再析出抑制を併用した技術であり、溶存酸素量、溶存硫黄量に基づき合金添加量を決定するという煩雑な制御を必要としている。 Furthermore, Patent Document 6 describes a technique for obtaining high HAZ toughness by using both oxide inclusions and Ti-containing inclusions as γ grain growth pinning particles. However, in Patent Document 6, the HAZ toughness is evaluated by a thermal cycle test that simulates high heat input welding, but the maximum heating temperature is 1400 ° C. and a temperature at which some Ti-containing nitride remains. ing. However, the highest heating temperature of the HAZ is partly higher than 1450 ° C., and the disappearance of the Ti-containing nitride is further promoted. Therefore, in order to accurately evaluate the high heat input HAZ toughness, it is desirable to actually perform a high heat input welding test. In addition, the inventors have proposed a technique that utilizes the gamma grain growth pinning effect of fine oxide inclusions in Patent Document 7, but this technique is a technique that also uses reprecipitation suppression of fine Mn sulfide. Therefore, the complicated control of determining the alloy addition amount based on the dissolved oxygen amount and the dissolved sulfur amount is required.
また、介在物粒子を起点とする粒内α生成による組織の微細化に関する技術としては、特許文献8に記載のTiやREMを含む複合酸化物とMnSを利用した技術が提案されているほか、発明者らは、特許文献9で介在物形状を制御することで、粒内α生成を促進する技術を提案している。これらの技術は、粒内α生成に対し、(粒内α/介在物)界面エネルギーの低い介在物が有効との前提で構築されているものである。しかしながら、粒内αの生成に際しては、(粒内α/γ)界面エネルギーの寄与も大きく、単に(粒内α/介在物)界面エネルギーを下げるだけでは、十分な粒内αの生成を得ることができないため、大入熱HAZ靭性を十分に保障するまでには至っていない。 In addition, as a technique related to the refinement of the structure by intragranular α generation starting from inclusion particles, a technique using a composite oxide containing Ti and REM described in Patent Document 8 and MnS is proposed, Inventors have proposed the technique which accelerates | stimulates intragranular alpha production | generation by controlling the inclusion shape by patent document 9. FIG. These techniques are constructed on the premise that inclusions with low interfacial energy are effective for intragranular α production (intragranular α / inclusions). However, in the production of intra-granular α, the contribution of (intra-granular α / γ) interfacial energy is also large, and by simply reducing (intra-granular α / inclusion) interfacial energy, sufficient intra-granular α can be obtained. Therefore, sufficient heat input HAZ toughness has not been sufficiently ensured.
更に、発明者らは、酸硫化物起点の粒内α生成を活用した高HAZ靭性技術を構築し、特許文献10として提案している。しかしながら、代償として2μm以上の比較的サイズの大きい酸硫化物粒子を一定数分散させる必要があるため、この技術でも、大入熱HAZ靭性を十分に保障するまでには至っていない。すなわち、特許文献8記載の技術では、想定する入熱量自体が小さく、また、特許文献9や特許文献10に記載の技術においても、シャルピー吸収エネルギーの平均値こそ高いものの、最小値には改善の余地があるのが現状である。 Furthermore, the inventors have constructed a high HAZ toughness technique utilizing intragranular α generation starting from an oxysulfide, and have proposed it as Patent Document 10. However, since it is necessary to disperse a certain number of relatively large oxysulfide particles having a size of 2 μm or more as a compensation, even this technique has not yet sufficiently secured the high heat input HAZ toughness. That is, in the technique described in Patent Document 8, the assumed heat input itself is small, and even in the techniques described in Patent Document 9 and Patent Document 10, although the average value of Charpy absorbed energy is high, the minimum value is not improved. There is room for it now.
加えて、発明者らは、組織を制御した酸化物を分散させることで、高いHAZ靭性を得ることができる技術を、特許文献11および特許文献12として提案している。これらの技術により溶接熱影響部の靭性に優れた厚鋼板を実現することができたが、製造上において、まだ改善すべき課題が残っていた。 In addition, the inventors have proposed, as Patent Document 11 and Patent Document 12, a technique capable of obtaining high HAZ toughness by dispersing an oxide whose structure is controlled. Although these techniques enabled the realization of a thick steel plate with excellent weld heat affected zone toughness, problems still need to be improved in manufacturing.
特許文献11記載の技術では、所定の酸化物形態を実現するために、Ca添加前の溶存酸素量に基づいてCa添加量を制御しているが、同時にTi添加からCa添加までの時間を3〜20分に収める必要があるため、作業者の負担が増すことが懸念される。一方、特許文献12記載の技術では、Ca添加から鋳込み開始まで40分〜90分保持する必要があるため、生産性に改善点が残っている。 In the technique described in Patent Document 11, the amount of Ca added is controlled based on the amount of dissolved oxygen before the addition of Ca in order to realize a predetermined oxide form, but at the same time, the time from Ti addition to Ca addition is 3 There is a concern that the burden on the operator will increase because it is necessary to keep it within -20 minutes. On the other hand, in the technique described in Patent Document 12, since it is necessary to hold for 40 to 90 minutes from the addition of Ca to the start of casting, an improvement point remains in productivity.
本発明は、上記従来の実情を鑑みてなされたもので、大入熱溶接を行った場合であっても、HAZ靭性の平均値は勿論のこと、その最小値をも向上させることができ、溶接熱影響部の靭性に優れ、更には生産性にも優れた、厚鋼板を提供することを課題とするものである。 The present invention was made in view of the above-described conventional situation, and even when high heat input welding is performed, the average value of HAZ toughness can be improved as well as the minimum value, An object of the present invention is to provide a thick steel plate that is excellent in the toughness of the weld heat-affected zone and is also excellent in productivity.
請求項1記載の発明は、質量%で、C:0.03〜0.12%、Si:0.10〜0.25%、Mn:1.00〜2.00%、P:0.030%以下(0%を含まない)、S:0.015%以下(0%を含まない)、Al:0.004〜0.050%、Ti:0.010〜0.050%、REM:0.0003〜0.0200%、Zr:0.0003〜0.0200%、Ca:0.0005〜0.0100%、N:0.0020〜0.0100%を含有し、残部が鉄および不可避的不純物であり、酸素を除く構成元素が、質量%で、2%<Ti<40%、5%<Al<30%、5%<Ca<40%、5%<REM<50%、2%<Zr<30%、1.0≦REM/Zrを満たす酸化物を含有し、且つ、前記酸化物のうち、円相当径が0.2μm超2μm未満の酸化物が300個/mm2以上、円相当径が2μm以上の酸化物が100個/mm2以下、存在すると共に、含有されるTi窒化物のうち、円相当径が1μm以上のTi窒化物が7個/mm2以下、円相当径が20nm以上のTi窒化物が1.0×106個/mm2以上、存在し、更に、前記円相当径が20nm以上のTi窒化物の大きさを、円相当径が20nmのものから小さい順に5nm置きの領域に区切って各領域毎の円相当径の範囲を(di−5)以上di未満とし、前記各領域内に存在するTi窒化物の個数密度が最多であった領域の前記diをdfとしたときの前記dfと、円相当径が20nm以上500nm未満のTi窒化物の平均円相当径daが、|da−df|/da≦0.35という関係式を満たす厚鋼板であって、前記厚鋼板から、溶接継手用試験片を採取し、V先加工を施した後、入熱量:50kJ/mmにてエレクトロガス溶接を実施し、その表面から深さt/4の位置の溶接線近傍のHAZに切欠きを加工したシャルピー衝撃試験片を、3本採取し、−40℃でシャルピー衝撃試験を行った時の吸収エネルギー:vE −40 の測定値の、平均値が180Jを超え、最小値が120Jを超え、前記厚鋼板から、溶接継手用試験片を採取し、V先加工を施した後、入熱量:60kJ/mmにてエレクトロガス溶接を実施し、その表面から深さt/4の位置の溶接線近傍のHAZに切欠きを加工したシャルピー衝撃試験片を、3本採取し、−40℃でシャルピー衝撃試験を行った時の吸収エネルギー:vE −40 の測定値の平均値が120Jを超えることを特徴とする溶接熱影響部の靭性に優れた厚鋼板である。但し、前記したtは板厚、シャルピー衝撃試験片はJIS Z 2242のVノッチ試験片である。 Invention of claim 1, wherein, in mass%, C: 0.03~0.12%, Si : 0.10~0.25%, Mn: 1.00 ~ 2.00%, P: 0.030 % Or less (excluding 0%), S: 0.015% or less (not including 0%), Al: 0.004 to 0.050 %, Ti: 0.010 to 0.050 %, REM: 0 .0003~ 0.0200%, Zr: 0.0003~ 0.0200 %, Ca: 0.0005~ 0.0100%, N: 0.0020 contains ~ 0.0100%, the balance being iron and unavoidable Constituent elements excluding oxygen, which are impurities, are 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <% by mass. Contains an oxide satisfying Zr <30%, 1.0 ≦ REM / Zr, and is equivalent to a circle of the oxides There 0.2μm oxides and less than 2μm is 300 / mm 2 or more, circle oxides of equivalent diameter above 2μm have 100 / mm 2 or less, with present, among Ti nitrides contained equivalent circle Ti nitride having a diameter of 1 μm or more is 7 pieces / mm 2 or less, Ti nitride having a circle equivalent diameter of 20 nm or more is 1.0 × 10 6 pieces / mm 2 or more, and the equivalent circle diameter is 20 nm. The size of the above-mentioned Ti nitride is divided into regions every 5 nm in ascending order of the equivalent circle diameter from 20 nm, and the range of equivalent circle diameter for each region is set to (di-5) or more and less than di. When the di in the region where the number density of Ti nitrides existing in the region is the largest is df, the average equivalent circle diameter da of the Ti nitride having an equivalent circle diameter of 20 nm to less than 500 nm is | The relationship da−df | /da≦0.35 A thick steel sheet satisfying, from the steel plate, it was taken test pieces for welded joints, after performing V destination processing, heat input: the electro-gas welding performed at 50 kJ / mm, the depth from the surface of Three Charpy impact test pieces with notches formed in the HAZ near the weld line at the t / 4 position were collected and the absorbed energy when the Charpy impact test was conducted at -40 ° C: measured value of vE- 40 The average value exceeds 180 J, the minimum value exceeds 120 J, and specimens for welded joints are collected from the thick steel plate, subjected to V pre-processing, and then subjected to electrogas welding at a heat input of 60 kJ / mm. Then, three Charpy impact test pieces in which notches were machined in the HAZ near the weld line at a depth t / 4 from the surface were collected, and the absorbed energy when the Charpy impact test was performed at −40 ° C .: flat of the measured values of vE -40 Value is an excellent thick steel sheet toughness of the heat affected zone, characterized in that more than 120 J. However, the above-mentioned t is the plate thickness, and the Charpy impact test piece is a V-notch test piece of JIS Z 2242.
尚、上記記載を含め、本発明で説明する円相当径とは、酸化物およびTi窒化物の大きさに着目して、その面積が等しくなるように想定した円の直径を求めたもので、透過型電子顕微鏡(TEM)や走査型電子顕微鏡(SEM)で観察することで求めることができる。 In addition, including the above description, the equivalent circle diameter described in the present invention is the diameter of a circle assumed to have the same area, focusing on the size of the oxide and Ti nitride. It can obtain | require by observing with a transmission electron microscope (TEM) or a scanning electron microscope (SEM).
請求項2記載の発明は、更に、酸素を除く構成元素が、質量%で、2%<Ti<40%、5%<Al<30%、5%<Ca<40%、5%<REM<50%、2%<Zr<30%、1.5≦REM/Zrを満たし、且つ、円相当径が0.2μm超2μm未満の酸化物が300個/mm2以上存在することを特徴とする請求項1記載の溶接熱影響部の靭性に優れた厚鋼板である。 According to the second aspect of the present invention, the constituent elements excluding oxygen are, in mass%, 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <. 50%, 2% <Zr < 30%, meets the 1.5 ≦ REM / Zr, and a feature that the circle equivalent diameter oxides of less than 0.2μm ultra 2μm is present 300 / mm 2 or more It is a thick steel plate excellent in the toughness of the welding heat affected zone of Claim 1.
請求項3記載の発明は、更に、質量%で、Ni:0.05〜1.50%、Cu:0.05〜1.50%、Cr:0.05〜1.50%、Mo:0.05〜1.50%よりなる群から選ばれる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記載の溶接熱影響部の靭性に優れた厚鋼板である。 The invention according to claim 3 further includes, in mass%, Ni: 0.05 to 1.50%, Cu: 0.05 to 1.50%, Cr: 0.05 to 1.50%, Mo: 0. It is a thick steel plate excellent in the toughness of the welding heat affected zone of Claim 1 or 2 characterized by containing 1 or more types chosen from the group which consists of 0.05 to 1.50%.
請求項4記載の発明は、更に、質量%で、Nb:0.002〜0.100%および/またはV:0.002〜0.100%を含有することを特徴とする請求項1乃至3のいずれかに記載の溶接熱影響部の靭性に優れた厚鋼板である。 The invention according to claim 4 further contains Nb: 0.002 to 0.100 % and / or V: 0.002 to 0.100 % by mass%. It is a thick steel plate excellent in the toughness of the welding heat affected zone in any one of.
請求項5記載の発明は、更に、B:0.0005〜0.0050%を含有することを特徴とする請求項1乃至4のいずれかに記載の溶接熱影響部の靭性に優れた厚鋼板である。 The invention according to claim 5 further comprises B: 0.0005 to 0.0050%, and the thick steel plate having excellent toughness of the weld heat affected zone according to any one of claims 1 to 4 It is.
本発明によると、小〜中入熱溶接は勿論のこと、大入熱溶接を行った場合であっても、HAZ靭性の平均値および最小値を向上させることができ、溶接熱影響部の靭性に優れ、更には生産性にも優れた、厚鋼板を得ることができる。 According to the present invention, the average value and the minimum value of the HAZ toughness can be improved not only for small to medium heat input welding but also for large heat input welding. In addition, it is possible to obtain a thick steel plate having excellent productivity.
本発明者らは、比較的生産性の高い製造条件下で厚鋼板の大入熱HAZ靭性を改善する手段を探索した。その結果、酸化物起点の粒内αの生成を確保すると共に、HAZ靭性阻害因子である粗大Ti窒化物の生成を抑制し、Ti窒化物分散形態を適切に制御することで、厚鋼板の生産性と大入熱HAZ靭性を両立できることを見出した。すなわち、酸化物組成を適切に制御することで、粒内αの生成を確保できると共に、Ti窒化物のサイズ、個数を適切に制御し、旧γ粒の粗大化を抑制することで、旧γ粒界に生成する粒界フェライトを微細化できるため、優れた大入熱HAZ靭性を有する厚鋼板を得ることができることを知見した。 The inventors searched for a means for improving the high heat input HAZ toughness of a thick steel plate under production conditions with relatively high productivity. As a result, it is possible to produce thick steel sheets by ensuring the formation of intra-granular α at the oxide origin, suppressing the formation of coarse Ti nitride, which is a HAZ toughness inhibiting factor, and appropriately controlling the Ti nitride dispersion form. And high heat input HAZ toughness were found to be compatible. That is, by appropriately controlling the oxide composition, it is possible to ensure the formation of α within the grains, and also to appropriately control the size and number of Ti nitrides and suppress the coarsening of the old γ grains. It has been found that a thick steel plate having excellent high heat input HAZ toughness can be obtained because the grain boundary ferrite generated at the grain boundary can be refined.
より詳しくは、これら酸化物のうち、円相当径が2μm未満の酸化物を300個/mm2以上分散させると共に、円相当径が2μm以上の酸化物は100個/mm2以下に抑制することなどで、優れたHAZ靭性が得られることを確認した。 More specifically, among these oxides, oxides having an equivalent circle diameter of less than 2 μm are dispersed at 300 / mm 2 or more, and oxides having an equivalent circle diameter of 2 μm or more are suppressed to 100 / mm 2 or less. As a result, it was confirmed that excellent HAZ toughness was obtained.
以上説明したような知見を基に、本発明を完成したものであるが、各構成要件を規定した理由は下記に示す通りである。 The present invention has been completed on the basis of the knowledge described above. The reasons for defining each constituent element are as follows.
(酸素を除く構成元素が、質量%で、2%<Ti<40%、5%<Al<30%、5%<Ca<40%、5%<REM<50%、2%<Zr<30%、1.0≦REM/Zrを満たし、円相当径が2μm未満の酸化物が300個/mm2以上)
酸化物の円相当径を2μm未満とすることで、粒内α促進によってHAZ靭性を促進することができる。酸化物の円相当径が2μm以上になると、粗大Ti窒化物が晶出する際の障壁エネルギーが低下し、粗大Ti窒化物の生成量が増加してしまう。また、酸化物の組成が、質量%で、2%<Ti<40%、5%<Al<30%、5%<Ca<40%、5%<REM<50%、2%<Zr<30%、1.0≦REM/Zrという範囲から外れると十分な粒内α生成が得られなくなる。尚、酸化物中のREM/Zr比(質量%)を1.5以上とすることで、溶鋼中において酸化物の表面に生成する粗大晶出Ti窒化物量が更に減少し、いっそう優れたHAZ靭性が実現される。
(Constituent elements excluding oxygen are in mass%, 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30. %, 1.0 ≦ REM / Zr and the equivalent circle diameter is less than 2 μm, 300 oxides / mm 2 or more)
By setting the equivalent circle diameter of the oxide to less than 2 μm, HAZ toughness can be promoted by promoting intragranular α. When the equivalent circle diameter of the oxide is 2 μm or more, the barrier energy when the coarse Ti nitride is crystallized decreases, and the amount of coarse Ti nitride generated increases. Further, the composition of the oxide is 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30 in mass%. %, 1.0 ≦ REM / Zr, a sufficient intragranular α formation cannot be obtained. By setting the REM / Zr ratio (mass%) in the oxide to 1.5 or more, the amount of coarse crystallization Ti nitride formed on the surface of the oxide in the molten steel is further reduced, and the HAZ toughness is further improved. Is realized.
(円相当径が2μm以上の酸化物が100個/mm2以下)
上記した組成を満足する酸化物のうち、円相当径が2μm以上の酸化物は、脆性破壊を助長し、HAZ靭性を劣化させるので、できるだけ少ないことが好ましい。こうした観点から本発明では、円相当径が2μm以上の酸化物は100個/mm2以下と規定した。
(Equivalent circle diameter of 2 μm or more is 100 oxides / mm 2 or less)
Of the oxides satisfying the above composition, an oxide having an equivalent circle diameter of 2 μm or more promotes brittle fracture and deteriorates HAZ toughness, so that it is preferably as small as possible. From this point of view, in the present invention, the oxide having an equivalent circle diameter of 2 μm or more is defined as 100 / mm 2 or less.
本発明では、Ti窒化物の形態についても詳細に規定した。Ti窒化物は、HAZ高温加熱時のγ粒粗大化を抑制し、冷却時に生成する粒界フェライトのサイズを低減することでHAZ靭性改善に寄与する。γ粒粗大化を十分抑制するためには、当然ながらTi窒化物の粒子を数多く分散させる必要があるが、本発明者らは、更にTi窒化物粒子のサイズが均一に近いほど、HAZ高温加熱時のTi窒化物の溶解速度が低下することを見出し、Ti窒化物のサイズおよび個数を適切に制御することで、大入熱溶接であってもγ粒粗大化抑制効果が得られることを明らかにした。具体的には、次の2つの条件を満足することで、高い大入熱HAZ靭性が実現される。 In the present invention, the form of Ti nitride is also defined in detail. Ti nitride contributes to HAZ toughness improvement by suppressing the coarsening of γ grains during high-temperature heating of HAZ and reducing the size of grain boundary ferrite generated during cooling. In order to sufficiently suppress the coarsening of γ grains, it is of course necessary to disperse a large number of Ti nitride particles, but the present inventors have further increased the HAZ high-temperature heating as the size of the Ti nitride particles becomes more uniform. That the dissolution rate of Ti nitride decreases at the same time, and by controlling the size and number of Ti nitrides appropriately, it is clear that the effect of suppressing the coarsening of γ grains can be obtained even with high heat input welding I made it. Specifically, high large heat input HAZ toughness is realized by satisfying the following two conditions.
(円相当径が1μm以上のTi窒化物が7個/mm2以下)
円相当径が1μm以上のTi窒化物の個数が7個/mm2を超えると、脆性破壊を助長し、HAZ靭性を劣化させてしまう。このようなTi窒化物は、直方体形状を有することに加えて、鋼に比べて著しく硬度が高いため、応力集中によりHAZ靭性を著しく劣化させるという特性を有する。よって、粗大Ti窒化物は粗大酸化物より厳密に制御する必要がある。
(Ti nitride with equivalent circle diameter of 1 μm or more is 7 pieces / mm 2 or less)
When the number of Ti nitrides having an equivalent circle diameter of 1 μm or more exceeds 7 / mm 2 , brittle fracture is promoted and HAZ toughness is deteriorated. In addition to having a rectangular parallelepiped shape, such a Ti nitride has a characteristic that the HAZ toughness is remarkably deteriorated due to stress concentration because the hardness is significantly higher than that of steel. Therefore, coarse Ti nitride needs to be controlled more strictly than coarse oxide.
(円相当径が20nm以上のTi窒化物が1.0×106個/mm2以上)
円相当径が20nm以上のTi窒化物が1.0×106個/mm2を下回ると、γ粒粗大化抑制に必要なTi窒化物粒子が確保されない。尚、円相当径20nm未満の極微細Ti窒化物粒子は、大入熱溶接時の高温加熱において、短時間で消失し、γ粒粗大化抑制に殆ど寄与しないため、特に制御は必要としない。
(Ti nitride with an equivalent circle diameter of 20 nm or more is 1.0 × 10 6 pieces / mm 2 or more)
When the number of Ti nitrides having an equivalent circle diameter of 20 nm or more is less than 1.0 × 10 6 pieces / mm 2 , Ti nitride particles necessary for suppressing γ grain coarsening cannot be secured. The ultrafine Ti nitride particles having an equivalent circle diameter of less than 20 nm disappear in a short time during high-temperature heating during large heat input welding, and hardly contribute to the suppression of γ grain coarsening.
(|da−df|/da≦0.35)
Ti窒化物粒子は、サイズが小さいものほどエネルギー的に不安定であり、具体的には、全粒子の平均サイズに比べ小さい粒子ほど、HAZ高温加熱時に消失しやすくなる。よって、平均サイズより大きい、或いは、平均サイズより小さくても、平均サイズに比較的近いサイズのTi窒化物粒子数が多いほど、γ粒粗大化抑制に寄与する実質的な粒子数は増加する。
(| Da-df | /da≦0.35)
The smaller the size of Ti nitride particles, the more unstable the energy, and more specifically, the smaller the average size of all the particles, the easier it is to disappear during HAZ high-temperature heating. Therefore, even if it is larger than the average size or smaller than the average size, as the number of Ti nitride particles having a size relatively close to the average size increases, the number of substantial particles contributing to the suppression of γ grain coarsening increases.
本発明では、Ti窒化物のサイズ−個数ヒストグラムにおいて、最多のTi窒化物個数が記録されたサイズと平均サイズとの差が小さくなるよう制御することで、この実質的なTi窒化物粒子数が増加し、高いγ粒粗大化抑制効果が実現されることを見出した。 In the present invention, in the Ti-nitride size-number histogram, the substantial number of Ti nitride particles is controlled by controlling the difference between the recorded size and the average size of the largest number of Ti nitrides. As a result, it was found that a high γ grain coarsening suppressing effect was realized.
詳しく説明すると、円相当径が20nm以上のTi窒化物の大きさを、円相当径が20nmのものから小さい順に5nm置きの領域に区切って各領域毎の円相当径の範囲を(di−5)以上di未満とし、それら各領域内に存在するTi窒化物の個数密度が最多であった領域の前記diをdfとしたときの前記dfと、円相当径が500nm未満のTi窒化物の平均円相当径daとの差が、小さくなるよう制御する。この制御により、実質的なTi窒化物粒子数が増加し、高いγ粒粗大化抑制効果が実現される。 More specifically, the size of the Ti nitride having an equivalent circle diameter of 20 nm or more is divided into regions every 5 nm in ascending order from the equivalent circle diameter of 20 nm to the range of equivalent circle diameter for each region (di-5). ) And less than di, and the average of Ti nitride having an equivalent circle diameter of less than 500 nm when the di of the region where the number density of Ti nitride existing in each region is the largest is df The difference from the equivalent circle diameter da is controlled to be small. By this control, the substantial number of Ti nitride particles is increased, and a high γ grain coarsening suppressing effect is realized.
尚、Ti窒化物の平均円相当径の算出に際しては、実施例の欄で説明する後述の条件により、透過型電子顕微鏡(TEM)観察を行い、画像解析によって、この観察視野中の各Ti窒化物の面積を測定し、その面積から各Ti窒化物の円相当径を算出した後、円相当径が20nm以上500nm未満のTi窒化物について、円相当径の算術平均を求めた。 In calculating the average equivalent circular diameter of Ti nitride, transmission electron microscope (TEM) observation is performed under the conditions described later in the example section, and each Ti nitride in the observation field is analyzed by image analysis. After measuring the area of the product and calculating the equivalent circle diameter of each Ti nitride from the area, the arithmetic average of the equivalent circle diameter was obtained for the Ti nitride having an equivalent circle diameter of 20 nm or more and less than 500 nm.
具体的には、|da−df|/daから求めた値が0.35を超えてしまうと、たとえTi窒化物粒子数が多くても、γ粒粗大化が十分に抑制されず、高い大入熱HAZ靭性が得られなくなる。 Specifically, if the value obtained from | da−df | / da exceeds 0.35, even if the number of Ti nitride particles is large, γ grain coarsening is not sufficiently suppressed, and the high Heat input HAZ toughness cannot be obtained.
(製造方法)
上記した要件を満足する本発明の厚鋼板、すなわち、酸素を除く構成元素が、質量%で、2%<Ti<40%、5%<Al<30%、5%<Ca<40%、5%<REM<50%、2%<Zr<30%、1.0≦REM/Zrを満たす酸化物を含有し、且つ、前記酸化物のうち、円相当径が2μm未満の酸化物が300個/mm2以上、円相当径が2μm以上の酸化物が100個/mm2以下、存在すると共に、含有されるTi窒化物のうち、円相当径が1μm以上のTi窒化物が7個/mm2以下、円相当径が20nm以上のTi窒化物が1.0×106個/mm2以上、存在し、更に、|da−df|/da≦0.35という関係式を満たす厚鋼板を製造するためには、以下の製造要件を満足するようにして、厚鋼板を製造する必要がある。
(Production method)
The thick steel plate of the present invention that satisfies the above-mentioned requirements, that is, the constituent elements excluding oxygen are 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% by mass. % <REM <50%, 2% <Zr <30%, 1.0 ≦ REM / Zr satisfying oxide, and among the oxides, 300 oxides having an equivalent circle diameter of less than 2 μm / mm 2 or more, circle oxides of equivalent diameter above 2μm have 100 / mm 2 or less, with present, among Ti nitrides contained, Ti nitride circle equivalent diameter of more than 1μm is 7 pieces / mm 2, circles Ti nitrides of equivalent diameter than 20nm is 1.0 × 10 6 cells / mm 2 or more, there, further, | da-df | that /da≦0.35 a steel plate satisfying the relation In order to manufacture, it is necessary to manufacture thick steel plates that satisfy the following manufacturing requirements. The
その製造要件は、溶製時において、Mn、Si等を用いた脱酸により溶鋼中の溶存酸素量を、質量%で、0.002〜0.01%とした後、Al→Ti→(REM、Zr)→Caの順に、REMまたはZrの添加からCa添加までの時間t1が5分以上となるようにして制御しつつ、各元素を添加し、更に、鋳造時における1500〜1450℃の温度範囲での冷却時間t2を300秒以内とすると共に、鋳造時における1300〜1200℃の温度範囲での冷却時間t3を680秒以内とすれば良い。また、REM添加量[REM]とZr添加量[Zr]の質量比である[REM]/[Zr]を1.8以上、且つt1を10分以上とすることで、より適正な酸化物形態が実現され、溶鋼中において酸化物の表面に生成する粗大晶出Ti窒化物が減少することで、いっそう優れたHAZ靭性が得られる。次に、これらの製造要件の規定理由について詳しく説明する。 The manufacturing requirement is that at the time of melting, the amount of dissolved oxygen in the molten steel is 0.002 to 0.01% by mass% by deoxidation using Mn, Si, etc., then Al → Ti → (REM , Zr) → Ca in the order of REM or Zr to Ca addition, while controlling the time t1 to be 5 minutes or more, each element was added, and the temperature at 1500 to 1450 ° C. at the time of casting The cooling time t2 in the range may be set within 300 seconds, and the cooling time t3 in the temperature range of 1300 to 1200 ° C. during casting may be set within 680 seconds. Further, by setting [REM] / [Zr], which is a mass ratio of the REM addition amount [REM] and the Zr addition amount [Zr], to 1.8 or more and t1 to 10 minutes or more, a more appropriate oxide form Is realized, and the coarse crystallized Ti nitride formed on the surface of the oxide in the molten steel is reduced, so that more excellent HAZ toughness can be obtained. Next, the reasons for defining these manufacturing requirements will be described in detail.
・Mn、Si等を用いた脱酸により溶鋼中の溶存酸素量を、0.002〜0.01%
溶存酸素量が0.002%を下回ると、粒内α生成の起点となる適切な組成を有する酸化物を必要量確保できなくなる。また、溶存酸素量が0.01%を超えると、円相当径が2μm以上の粗大酸化物が増加し、HAZ靭性を劣化させてしまう。
-The amount of dissolved oxygen in molten steel is 0.002 to 0.01% by deoxidation using Mn, Si, etc.
When the amount of dissolved oxygen is less than 0.002%, a required amount of oxide having an appropriate composition that becomes the starting point of intragranular α formation cannot be secured. On the other hand, if the amount of dissolved oxygen exceeds 0.01%, coarse oxides having an equivalent circle diameter of 2 μm or more increase, and the HAZ toughness is deteriorated.
・REMまたはZrの添加からCa添加までの時間t1を5分以上
本発明で規定した酸化物は、粒内αの生成促進作用を有すると共に、粗大Ti窒化物の晶出起点として機能し難いという特徴を有する。特に、酸化物中のREM/Zr比(質量%)を1.0以上とするためには、強脱酸元素であるCaの添加に先立ち、REMまたはZrの酸化物形成反応を十分に進行させる必要がある。具体的には、REMまたはZrの添加からCa添加までの時間t1を5分以上に制御することで、所定の個数密度のREM/Zr≧1.0を満たす酸化物を得ることができる。REMまたはZrの添加からCa添加までの時間t1が10分未満であると、REM/Zr≧1.0を満たす酸化物が不足することになる。また、これに加えて、REM添加量[REM]とZr添加量[Zr]の質量比である[REM]/[Zr]を1.8以上かつt1を10分以上とすることで、REM/Zr≧1.5を満たす酸化物を、所定の個数密度で得ることができる。
The time t1 from the addition of REM or Zr to the addition of Ca is 5 minutes or more. The oxide defined in the present invention has an action of promoting the formation of intra-granular α and is difficult to function as a crystallization starting point of coarse Ti nitride. Has characteristics. In particular, in order to set the REM / Zr ratio (mass%) in the oxide to 1.0 or more, the REM or Zr oxide formation reaction is sufficiently advanced prior to the addition of Ca, which is a strong deoxidizing element. There is a need. Specifically, an oxide satisfying REM / Zr ≧ 1.0 having a predetermined number density can be obtained by controlling the time t1 from the addition of REM or Zr to the addition of Ca to 5 minutes or more. When the time t1 from the addition of REM or Zr to the addition of Ca is less than 10 minutes, the oxide satisfying REM / Zr ≧ 1.0 is insufficient. In addition to this, by setting [REM] / [Zr], which is a mass ratio of the REM addition amount [REM] and the Zr addition amount [Zr], to 1.8 or more and t1 to 10 minutes or more, REM / An oxide satisfying Zr ≧ 1.5 can be obtained at a predetermined number density.
尚、溶製時において、Al→Ti→(REM、Zr)→Caの順に添加する理由は、この添加順序以外の順序で各元素を添加すると、粒内α生成の起点となる適切な組成を有する酸化物を必要数確保できなくなるからである。特に、Caは脱酸力が極めて強い強脱酸元素であるため、TiやAlに先立って添加すると、TiやAlと結びつく酸素が著しく少なくなる。 At the time of melting, the reason for adding Al → Ti → (REM, Zr) → Ca in this order is that when each element is added in an order other than this addition order, an appropriate composition that becomes the starting point of intragranular α formation is obtained. This is because the required number of oxides cannot be secured. In particular, since Ca is a strong deoxidizing element having a very strong deoxidizing power, if it is added prior to Ti or Al, oxygen associated with Ti or Al is remarkably reduced.
・鋳造時の1500〜1450℃における冷却時間t2を300秒以内
鋳造時の1500〜1450℃における冷却時間t2が300秒を超えると、粗大な酸化物が増加する。或いは、凝固時の成分偏析により粗大Ti窒化物が晶出し、HAZ靭性が劣化することになる。
-Cooling time t2 at 1500-1450 ° C during casting is within 300 seconds When cooling time t2 at 1500-1450 ° C during casting exceeds 300 seconds, coarse oxides increase. Alternatively, coarse Ti nitrides crystallize due to component segregation during solidification, and HAZ toughness deteriorates.
・鋳造時における1300〜1200℃の温度範囲での冷却時間t3を680秒以内
鋳造時における1300〜1200℃の温度範囲での冷却時間t3が680秒を超えると、|da−df|/da≦0.35という関係式を満たすことができない。その原因は、次の通りと考えることができる。
The cooling time t3 in the temperature range of 1300 to 1200 ° C. during casting is within 680 seconds. When the cooling time t3 in the temperature range of 1300 to 1200 ° C. during casting exceeds 680 seconds, | da−df | / da ≦ The relational expression of 0.35 cannot be satisfied. The cause can be considered as follows.
鋳造時に生成するTi窒化物には、A.溶鋼中で晶出するTi窒化物、B.凝固した鋼の凝固偏析部で生成するTi窒化物、C.凝固した鋼の非凝固偏析部で生成するTi窒化物があり、A→B→Cの順に生成し、その大きさ(粒子径)は、A>B>Cの順である。一方、粒子数は、A<B<Cの順であり、前記df相当の大きさのTi窒化物の大部分は、CのTi窒化物である。また、AのTi窒化物は、B,CのTi窒化物に比べて粒子数が少ないため、Ti窒化物の平均円相当径daに殆ど影響を及ぼすことがない。よって、|da−df|/daを所定の範囲に収めるには、B,CのTi窒化物の生成を制御する必要があるといえる。鋳造時における1300〜1200℃の温度範囲での冷却時間t3が680秒を超えると、CのTi窒化物の生成に先立ち、BのTi窒化物が成長するため、Ti窒化物の平均円相当径daが大きくなってしまい、|da−df|/daが0.35を超えてしまうと考えられる。 Ti nitrides produced during casting include A.I. Ti nitride crystallizing in molten steel; Ti nitride formed in a solidified segregation part of solidified steel, C.I. There is Ti nitride generated in the non-solidified segregation part of the solidified steel, which is generated in the order of A → B → C, and the size (particle diameter) is in the order of A> B> C. On the other hand, the number of particles is in the order of A <B <C, and most of the Ti nitride having a size corresponding to the df is C Ti nitride. Further, since the Ti nitride of A has a smaller number of particles than the Ti nitrides of B and C, the average equivalent circular diameter da of the Ti nitride is hardly affected. Therefore, it can be said that it is necessary to control the formation of B and C Ti nitrides in order to keep | da-df | / da within a predetermined range. When the cooling time t3 in the temperature range of 1300 to 1200 ° C. during casting exceeds 680 seconds, B Ti nitride grows prior to the formation of C Ti nitride, so the average equivalent circle diameter of Ti nitride It is considered that da becomes large and | da−df | / da exceeds 0.35.
(化学成分組成)
次に、本発明の厚鋼板における化学成分組成について説明する。本発明の厚鋼板は、先に説明した酸化物の分散状態等が適切であっても、夫々の化学成分(元素)の含有量が適正範囲内でなければ、母材(厚鋼板)の特性とHAZを良好にすることができない。従って、本発明の厚鋼板では、夫々の化学成分の含有量が、以下に説明する範囲内にあることも併せて要件とする。これらの化学成分のうち、酸化物を構成するAl、Ca、Ti等の含有量は、その作用効果から明らかなように、酸化物を構成する量を含めたものである。尚、下記の化学成分の含有量(%)は全て質量%を示す。
(Chemical composition)
Next, the chemical component composition in the thick steel plate of the present invention will be described. The steel plate of the present invention has characteristics of the base material (thick steel plate) if the content of each chemical component (element) is not within the proper range even if the oxide dispersion state described above is appropriate. And HAZ cannot be improved. Therefore, in the thick steel plate of the present invention, it is also a requirement that the content of each chemical component is within the range described below. Among these chemical components, the content of Al, Ca, Ti, etc. constituting the oxide includes the amount constituting the oxide, as is apparent from its action and effect. In addition, all the content (%) of the following chemical component shows the mass%.
C:0.03〜0.12%
Cは、鋼板の強度を確保するための必須元素である。Cの含有量が0.03%より低い場合は、必要な強度を確保できなくなる。一方で、Cの含有量が過剰になると、硬質な島状マルテンサイト(MA)が多く生成して母材の靭性劣化を招くことになる。従って、Cの含有量は0.12%以下とする必要がある。Cの含有量の好ましい下限は0.04%、好ましい上限は0.10%である。
C: 0.03-0.12%
C is an essential element for ensuring the strength of the steel sheet. If the C content is lower than 0.03%, the required strength cannot be ensured. On the other hand, when the C content is excessive, a large amount of hard island martensite (MA) is generated, leading to deterioration of the toughness of the base material. Therefore, the C content needs to be 0.12% or less. The minimum with preferable content of C is 0.04%, and a preferable upper limit is 0.10%.
Si:0.10〜0.25%
Siは、Tiの活量を向上させる元素であり、所定のTi窒化物形態を実現するために、適切に添加する必要がある。添加量が0.10%を下回ると、円相当径が20nm以上のTi窒化物の個数密度が1.0×106個/mm2を確保できなくなる。また、0.25%を上回ると、粗大なTi窒化物が生成しやすくなるのに加え、硬質なMA組織が形成されるようになり、所定のHAZ靭性が得られない。好ましい下限は0.12%、より好ましい下限は0.14%、好ましい上限は0.22%、より好ましい上限は0.20%である。
Si: 0.10 to 0.25%
Si is an element that improves the activity of Ti, and needs to be added appropriately in order to realize a predetermined Ti nitride form. If the added amount is less than 0.10%, the number density of Ti nitride having an equivalent circle diameter of 20 nm or more cannot be secured at 1.0 × 10 6 pieces / mm 2 . On the other hand, if it exceeds 0.25%, coarse Ti nitride is likely to be formed, and a hard MA structure is formed, and a predetermined HAZ toughness cannot be obtained. A preferred lower limit is 0.12%, a more preferred lower limit is 0.14%, a preferred upper limit is 0.22%, and a more preferred upper limit is 0.20%.
Mn:1.0〜2.0%
Mnは、鋼板の強度を確保するのに有用な元素である。こうした効果を有効に発揮させるには1.0%以上含有させる必要がある。しかし、2.0%を超えて過剰に含有させるとHAZの強度が上昇しすぎて靭性が劣化するので、Mnの含有量は2.0%以下とする。Mnの含有量の好ましい下限は1.4%、好ましい上限は1.8%である。
Mn: 1.0-2.0%
Mn is an element useful for ensuring the strength of the steel sheet. In order to exhibit such an effect effectively, it is necessary to contain 1.0% or more. However, if the content exceeds 2.0% excessively, the strength of the HAZ increases excessively and the toughness deteriorates, so the Mn content is set to 2.0% or less. The preferable lower limit of the Mn content is 1.4%, and the preferable upper limit is 1.8%.
P:0.03%以下(0%を含まない)
Pは、粒界破壊を起こし易く靭性に悪影響を及ぼす不純物元素であるので、その含有量はできるだけ少ないことが好ましい。HAZ靭性を確保するという観点からして、Pの含有量は0.03%以下に抑制する必要があり、好ましくは0.02%以下とする。しかし、工業的に鋼中のPを0%にすることは困難である。
P: 0.03% or less (excluding 0%)
Since P is an impurity element that easily causes grain boundary fracture and adversely affects toughness, its content is preferably as small as possible. From the viewpoint of securing HAZ toughness, the P content needs to be suppressed to 0.03% or less, and preferably 0.02% or less. However, it is difficult to make P in steel 0% industrially.
S:0.015%以下(0%を含まない)
Sは、HAZにおいて、旧オーステナイト粒界にMn硫化物を形成して、HAZ靭性を劣化させる元素であるので、その含有量はできるだけ少ないことが好ましい。HAZ靭性を確保するという観点からして、Sの含有量は0.015%以下に抑制する必要があり、好ましくは0.010%以下とする。しかし、工業的に鋼中のSを0%にすることは困難である。
S: 0.015% or less (excluding 0%)
Since S is an element that degrades the HAZ toughness by forming Mn sulfide at the prior austenite grain boundaries in the HAZ, its content is preferably as small as possible. From the viewpoint of ensuring HAZ toughness, the S content must be suppressed to 0.015% or less, and preferably 0.010% or less. However, it is difficult to industrially make S in steel 0%.
Al:0.004〜0.05%
Alは、粒内αの起点となる酸化物を形成する元素である。その含有量が0.004%未満であると、所定の酸化物形態が得られなくなり、粒内変態が十分に促進されなくなるため、HAZ靭性が劣化する。一方、含有量が過剰であると、粗大酸化物が生成してHAZ靭性が劣化するので、0.05%以下に抑える必要がある。Alの含有量の好ましい下限は0.007%、好ましい上限は0.04%である。
Al: 0.004 to 0.05%
Al is an element that forms an oxide serving as a starting point of the intra-grain α. When the content is less than 0.004%, a predetermined oxide form cannot be obtained, and the intragranular transformation is not sufficiently promoted, so that the HAZ toughness is deteriorated. On the other hand, if the content is excessive, a coarse oxide is generated and the HAZ toughness is deteriorated, so it is necessary to suppress it to 0.05% or less. The preferable lower limit of the Al content is 0.007%, and the preferable upper limit is 0.04%.
Ti:0.010〜0.050%
Tiは、Ti窒化物を形成する元素であると共に、REM、Zr、Caに先立ち添加することによって、粒内αの生成促進作用を有する酸化物の微細分散が可能となる。所定のTi窒化物、酸化物形態を実現するためには、Tiを0.010%以上含有させる必要がある。しかしながら、その含有量が過剰であると粗大Ti窒化物が多く晶出してHAZ靭性を劣化させるので、0.050%以下に抑える必要がある。Tiの含有量の好ましい下限は0.012%、好ましい上限は0.035%、より好ましい上限は0.025%である。
Ti: 0.010 to 0.050%
Ti is an element that forms Ti nitride, and by adding it prior to REM, Zr, and Ca, it is possible to finely disperse an oxide having an action of promoting the formation of intragranular α. In order to realize a predetermined Ti nitride and oxide form, it is necessary to contain 0.010% or more of Ti. However, if the content is excessive, a large amount of coarse Ti nitride is crystallized and the HAZ toughness is deteriorated, so it is necessary to suppress it to 0.050% or less. The preferable lower limit of the Ti content is 0.012%, the preferable upper limit is 0.035%, and the more preferable upper limit is 0.025%.
REM:0.0003〜0.02%、Zr:0.0003〜0.02%
REM(希土類元素)およびZrは、Tiの添加後、Caの添加に先立って添加することで、粒内αの生成に有効な酸化物を形成する。これら酸化物は、Ti窒化物が複合析出することでより好適な粒内α生成サイトとなる。こうした効果は、それらの含有量が増加するにつれて増大するが、こうした効果を有効に発揮させるためには、いずれも0.0003%以上含有させる必要がある。しかし、これらを過剰に含有させると、酸化物が粗大になってHAZ靭性を劣化させるため、いずれも0.02%以下に抑えるべきである。これらの含有量のより好ましい下限は0.0005%、より好ましい上限は0.015%である。
REM: 0.0003-0.02%, Zr: 0.0003-0.02%
REM (rare earth element) and Zr are added prior to the addition of Ca after the addition of Ti, thereby forming an oxide effective for the generation of intragranular α. These oxides become a more preferable intra-granular α-generation site by complex precipitation of Ti nitride. Such effects increase as their content increases, but in order to effectively exhibit these effects, it is necessary to contain 0.0003% or more of all. However, if these are excessively contained, the oxide becomes coarse and deteriorates the HAZ toughness, so both should be suppressed to 0.02% or less. The more preferable lower limit of these contents is 0.0005%, and the more preferable upper limit is 0.015%.
Ca:0.0005〜0.010%
Caは、Ti、REM、Zrの添加から10分以上後に添加することによって、粒内αの生成に有効で、且つ粗大Ti窒化物の晶出を抑制する酸化物を形成する。こうした効果を有効に発揮させるためには、0.0005%以上含有させる必要がある。しかしながら、その含有量が過剰であると粗大酸化物が生成してHAZ靭性が劣化するので0.010%以下に抑える必要がある。Caの含有量の好ましい下限は0.0008%、好ましい上限は0.008%である。
Ca: 0.0005 to 0.010%
When Ca is added 10 minutes or more after the addition of Ti, REM, and Zr, it forms an oxide that is effective in generating intragranular α and suppresses crystallization of coarse Ti nitride. In order to exhibit such an effect effectively, it is necessary to contain 0.0005% or more. However, if the content is excessive, a coarse oxide is generated and the HAZ toughness is deteriorated, so it is necessary to suppress it to 0.010% or less. The preferable lower limit of the Ca content is 0.0008%, and the preferable upper limit is 0.008%.
N:0.002〜0.010%
Nは、微細なTi窒化物を形成することによって、HAZの靭性を確保する上で有用な元素である。その含有量を0.002%以上とすることで、所望のTi窒化物を確保することができる。しかし、その含有量が過剰になると、粗大Ti窒化物の晶出が助長されるので0.010%以下に抑える必要がある。Nの含有量の好ましい下限は0.003%、好ましい上限は0.008%である。
N: 0.002 to 0.010%
N is an element useful for securing the toughness of HAZ by forming fine Ti nitride. By making the content 0.002% or more, desired Ti nitride can be secured. However, if the content is excessive, crystallization of coarse Ti nitride is promoted, so it is necessary to suppress it to 0.010% or less. The preferable lower limit of the N content is 0.003%, and the preferable upper limit is 0.008%.
以上が本発明で規定する必須の含有元素であって、残部は鉄および不可避的不純物である。不可避的不純物としては、原料、資材、製造設備等の状況によって持ち込まれるSn、As、Pb等の元素の混入が許容される。また、更に以下に示す元素を積極的に含有させることも有効であり、含有される化学成分(元素)の種類によって厚鋼板の特性が更に改善される。 The above are the essential elements specified in the present invention, and the balance is iron and inevitable impurities. As an inevitable impurity, mixing of elements such as Sn, As, and Pb brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. Further, it is also effective to positively contain the following elements, and the characteristics of the thick steel plate are further improved depending on the kind of chemical components (elements) contained.
Ni:0.05〜1.50%、Cu::0.05〜1.50%、Cr:0.05〜1.50%、Mo:0.05〜1.50%よりなる群から選ばれる1種以上
Ni、Cu、Cr、およびMoは、いずれもが鋼板の高強度化に有効な元素であり、その効果はそれらの含有量が増加するにつれて増大する。こうした効果を有効に発揮させるためには、いずれも0.05%以上含有させることが好ましい。しかし、それらを過剰に含有させると、強度の過大な上昇を招き、HAZ靭性を劣化させるため、いずれも1.50%以下に抑えることが好ましい。それらの含有量のより好ましい下限は0.10%、より好ましい上限は1.20%である。
Selected from the group consisting of Ni: 0.05 to 1.50%, Cu :: 0.05 to 1.50%, Cr: 0.05 to 1.50%, Mo: 0.05 to 1.50%. One or more types Ni, Cu, Cr, and Mo are all effective elements for increasing the strength of the steel sheet, and the effect thereof increases as the content thereof increases. In order to exhibit such an effect effectively, it is preferable to contain 0.05% or more of all. However, if they are contained excessively, the strength is excessively increased, and the HAZ toughness is deteriorated. The more preferable lower limit of the content thereof is 0.10%, and the more preferable upper limit is 1.20%.
Nb:0.002〜0.10%および/またはV:0.002〜0.10%
NbおよびVは、炭窒化物として析出し、γ粒の粗大化を抑制することで、母材靭性を良好にするのに有効な元素である。その効果はそれらの含有量が増加するにつれて増大するが、こうした効果を有効に発揮させるためには、いずれも0.002%以上含有させることが好ましい。しかし、それらを過剰に含有させると、HAZ組織の粗大化を招き、HAZ靭性を劣化させるため、いずれも0.10%以下に抑えることが好ましい。それらの含有量のより好ましい下限は0.005%、より好ましい上限は0.08%である。
Nb: 0.002-0.10% and / or V: 0.002-0.10%
Nb and V precipitate as carbonitrides and are effective elements for improving the base material toughness by suppressing the coarsening of γ grains. Although the effect increases as the content thereof increases, in order to effectively exhibit such an effect, it is preferable that the content is 0.002% or more. However, if they are contained excessively, the HAZ structure is coarsened and the HAZ toughness is deteriorated. The more preferable lower limit of the content thereof is 0.005%, and the more preferable upper limit is 0.08%.
B:0.0005〜0.0050%
Bは、粗大な粒界αの生成を抑制することで、HAZ靭性を向上させる効果を有する。こうした効果は、含有量が増加するにつれて増大するが、効果的に発揮させるためには、0.0005%以上含有させることが好ましい。より好ましくは0.0010%以上、更に好ましくは0.0015%以上である。しかし、B含有量が過剰となると、旧オーステナイト粒界からの粗大ベイナイトバケットが促進され、HAZ靭性はかえって低下する。好ましい上限は0.0050%、より好ましい上限は0.0040%、更に好ましい上限は0.0035%である。
B: 0.0005 to 0.0050%
B has the effect of improving the HAZ toughness by suppressing the formation of coarse grain boundaries α. Such an effect increases as the content increases, but in order to exert the effect effectively, the content is preferably 0.0005% or more. More preferably, it is 0.0010% or more, More preferably, it is 0.0015% or more. However, when the B content is excessive, coarse bainite buckets from the prior austenite grain boundaries are promoted, and the HAZ toughness is reduced. A preferable upper limit is 0.0050%, a more preferable upper limit is 0.0040%, and a still more preferable upper limit is 0.0035%.
また、化学成分組成の説明で、Ni、Cu、Cr、Moよりなる群から選ばれる1種以上を含有することが有効であることを説明したが、その場合、それらの含有量(質量%)が、[Ni]+[Cu]+[Cr]+[Mo]<2.5%を満足することが好ましい(但し、前式で[ ]は各元素の含有量(質量%)を示す。)。 In addition, in the description of the chemical component composition, it has been explained that it is effective to contain one or more selected from the group consisting of Ni, Cu, Cr, and Mo. In that case, their content (mass%) However, it is preferable that [Ni] + [Cu] + [Cr] + [Mo] <2.5% is satisfied (however, [] in the preceding formula indicates the content (% by mass) of each element). .
粗大Ti窒化物は、溶鋼の凝固段階において、凝固偏析によりTi、Nが濃化した液相に晶出する。[Ni]+[Cu]+[Cr]+[Mo]が2.5%を超えると、凝固温度が低温化し、粗大Ti窒化物晶出の駆動力が大きくなる低温まで液相が残存するようになるため、粗大Ti窒化物の生成量が増加してしまう。 Coarse Ti nitride crystallizes in a liquid phase in which Ti and N are concentrated by solidification segregation in the solidification stage of the molten steel. When [Ni] + [Cu] + [Cr] + [Mo] exceeds 2.5%, the solidification temperature is lowered, and the liquid phase remains until a low temperature at which the driving force of coarse Ti nitride crystallization becomes large. Therefore, the amount of coarse Ti nitride produced increases.
(|da−df|/da≦0.35)
|da−df|/daは、HAZ高温加熱時のγ粒粗大化抑制に寄与するTi窒化物数と関連するパラメータである。0.35を超えると、γ粒の粗大化が十分抑制されず、所定のHAZ靭性が確保できない。好ましい上限は0.30、更に好ましい上限は0.25である。
(| Da-df | /da≦0.35)
| Da-df | / da is a parameter related to the number of Ti nitrides contributing to the suppression of γ grain coarsening during high-temperature heating of the HAZ. If it exceeds 0.35, the coarsening of the γ grains is not sufficiently suppressed, and the predetermined HAZ toughness cannot be ensured. A preferable upper limit is 0.30, and a more preferable upper limit is 0.25.
本発明は厚鋼板に関する発明であるが、一般に厚鋼板とは、JISで定義されるように、板厚が3.0mm以上の鋼板のことを示す。一方、本発明の厚鋼板は、50mm以上の板厚の厚鋼板の溶接を対象として発明されたものであり、対象とする鋼板は、板厚が50mm以上の鋼板であるということができると思われるが、これらは単に好ましい態様に過ぎず、本発明を50mm未満の板厚の厚鋼板へ適用することを排除するものではない。 Although this invention is invention regarding a thick steel plate, generally a thick steel plate shows the steel plate whose plate | board thickness is 3.0 mm or more as defined by JIS. On the other hand, the thick steel plate of the present invention was invented for welding thick steel plates having a thickness of 50 mm or more, and the target steel plate can be said to be a steel plate having a thickness of 50 mm or more. However, these are merely preferred embodiments and do not exclude application of the present invention to thick steel plates having a thickness of less than 50 mm.
以下、実施例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、本発明の趣旨に適合し得る範囲で適宜変更を加えて実施することも可能であり、それらは何れも本発明の技術的範囲に含まれる。 EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.
本発明の実施例では、まず、表1および表2に示す各成分組成の鋼を、真空溶解炉(VIF:150kg)によって溶製した後、その溶鋼を用いて鋳片(断面形状:150mm×250mm)を鋳造し、更にその鋳片を用いて熱間圧延を行うことで、板厚80mmの熱間圧延板を得た。尚、熱間圧延条件は、圧延前加熱:1100℃×3時間、仕上げ圧延温度:780℃以上、450℃までの平均冷却速度:6℃/s、冷却停止温度:450℃とした。 In the examples of the present invention, first, steels having respective component compositions shown in Tables 1 and 2 were melted in a vacuum melting furnace (VIF: 150 kg), and then cast into slabs (cross-sectional shape: 150 mm × 250 mm) was cast, and hot rolling was further performed using the slab, thereby obtaining a hot rolled sheet having a thickness of 80 mm. The hot rolling conditions were heating before rolling: 1100 ° C. × 3 hours, finish rolling temperature: 780 ° C. or higher, average cooling rate up to 450 ° C .: 6 ° C./s, and cooling stop temperature: 450 ° C.
この熱間圧延板(厚鋼板)を製造するにあたり、制御した各条件を表3および表4に示す。その条件は、Al(Ti)添加前の溶鋼中の溶存酸素量[Of](質量%)、Al,Ti,REM,Zr,Caの添加順序、REMまたはZr添加からCa添加までの時間t1、REM添加量[REM]とZr添加量[Zr]の質量比:[REM]/[Zr](表にはREM/Zrと記載)、鋳造時の1500〜1450℃における冷却時間t2、鋳造時の1300〜1200℃における冷却時間t3である。 Tables 3 and 4 show the controlled conditions in producing this hot-rolled sheet (thick steel sheet). The conditions are as follows: dissolved oxygen amount [Of] (% by mass) in molten steel before addition of Al (Ti), addition order of Al, Ti, REM, Zr, Ca, time t1 from addition of REM or Zr to addition of Ca, Mass ratio of REM addition amount [REM] and Zr addition amount [Zr]: [REM] / [Zr] (described as REM / Zr in the table), cooling time t2 at 1500 to 1450 ° C. during casting, This is the cooling time t3 at 1300 to 1200 ° C.
尚、表1および表2において、REMは、質量%で、Ceを50%程度とLaを25%程度含有するミッシュメタルの形態で添加した。また、表1および表2で、「−」は該当元素を添加していないことを示す。 In Tables 1 and 2, REM was added in the form of a misch metal containing, by mass%, about 50% Ce and about 25% La. In Tables 1 and 2, “-” indicates that the corresponding element is not added.
また、表3および表4において、Al,Ti,REM,Zr,Caの添加順序は、Al→Ti→(REM、Zr)→Caの順序のときを「○」、それ以外の順序のときを「×」で示す。 In Tables 3 and 4, the order of addition of Al, Ti, REM, Zr, and Ca is “◯” when Al → Ti → (REM, Zr) → Ca, and other orders. Indicated by “x”.
以上の要件で製造した各熱間圧延板(厚鋼板)を用いて、円相当径が2μm未満かつTi、Al、Ca、REM、Zrを所定の濃度範囲で含有し、[REM]/[Zr]≧1.0である酸化物の個数密度N1、円相当径が2μm未満かつTi、Al、Ca、REM、Zrを所定の濃度範囲で含有し、[REM]/[Zr]≧1.5である酸化物の個数密度NA、円相当径が2μm以上の酸化物の個数密度N2、円相当径が1μm以上のTi窒化物の個数密度N3、円相当径が20nm以上のTi窒化物の個数密度N4、|da−df|/da、およびHAZ靭性を下記する測定により求め出した。これらの測定結果を表5および表6に示す。 Using each hot-rolled plate (thick steel plate) manufactured according to the above requirements, the equivalent circle diameter is less than 2 μm, and Ti, Al, Ca, REM, and Zr are contained in a predetermined concentration range, and [REM] / [Zr ] The number density N1 of the oxide satisfying ≧ 1.0, the equivalent circle diameter is less than 2 μm, and Ti, Al, Ca, REM, and Zr are contained in a predetermined concentration range, and [REM] / [Zr] ≧ 1.5 The number density NA of oxides, the number density N2 of oxides with an equivalent circle diameter of 2 μm or more, the number density N3 of Ti nitrides with an equivalent circle diameter of 1 μm or more, and the number of Ti nitrides with an equivalent circle diameter of 20 nm or more Density N4, | da-df | / da, and HAZ toughness were determined by the following measurements. These measurement results are shown in Tables 5 and 6.
(円相当径が2μm未満の酸化物の個数密度の測定)
各厚鋼板の表面から深さt/4(t:板厚)の位置から試験片を切り出し(試験片の軸心がt/4の位置を通るように採取)、圧延方向および板厚方向に平行な断面を、Carl Zeiss社製の電界放射式走査型電子顕微鏡「SUPRA35(商品名)」(以下、FE−SEMと呼ぶ)を用いて観察した。その観察条件は、倍率:5000倍、観察面積:0.048mm2とした。画像解析によって、この観察視野中の各酸化物の面積を測定し、その面積から各酸化物の円相当径を算出した。尚、各酸化物が上記した成分組成を満足するものであることは、EDX(エネルギー分散型X線検出器)によって確認した。EDXによる成分組成測定時の加速電圧は15kV、測定時間は100秒である。そして、円相当径が2μm未満となる酸化物の個数(N1、NA)を1mm2相当の個数密度に換算して求めた。但し、円相当径が0.2μm以下となる酸化物については、EDXの信頼性が十分でないため、解析から除外した。
(Measurement of number density of oxides with equivalent circle diameter less than 2 μm)
A test piece is cut out from the surface of each thick steel plate at a depth of t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. The parallel cross section was observed using a field emission scanning electron microscope “SUPRA35 (trade name)” (hereinafter referred to as FE-SEM) manufactured by Carl Zeiss. The observation conditions were as follows: magnification: 5000 times, observation area: 0.048 mm 2 . The area of each oxide in this observation field was measured by image analysis, and the equivalent circle diameter of each oxide was calculated from the area. In addition, it was confirmed by EDX (energy dispersive X-ray detector) that each oxide satisfies the above-described component composition. The acceleration voltage at the time of component composition measurement by EDX is 15 kV, and the measurement time is 100 seconds. Then, the number of oxides (N1, NA) having an equivalent circle diameter of less than 2 μm was determined by converting into a number density equivalent to 1 mm 2 . However, oxides having an equivalent circle diameter of 0.2 μm or less were excluded from the analysis because the reliability of EDX was not sufficient.
(円相当径が2μm以上の酸化物の個数密度の測定)
各厚鋼板の表面から深さt/4(t:板厚)の位置から試験片を切り出し(試験片の軸心がt/4の位置を通るように採取)、圧延方向および板厚方向に平行な断面を、FE−SEMを用いて観察した。その観察条件は、倍率:1000倍、観察視野:0.06mm2、観察箇所:20箇所とした。画像解析によって、この観察視野中の各酸化物の面積を測定し、その面積から各酸化物の円相当径を算出した。尚、各酸化物が上記した成分組成を満足するものであることは、EDX(エネルギー分散型X線検出器)によって確認した。EDXによる成分組成測定時の加速電圧は15kV、測定時間は100秒である。そして、円相当径が2μm以上となる酸化物の個数(N2)を1mm2相当の個数密度に換算して求めた。
(Measurement of number density of oxides with equivalent circle diameter of 2 μm or more)
A test piece is cut out from the surface of each thick steel plate at a depth of t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. Parallel cross sections were observed using FE-SEM. The observation conditions were as follows: magnification: 1000 times, observation visual field: 0.06 mm 2 , observation location: 20 locations. The area of each oxide in this observation field was measured by image analysis, and the equivalent circle diameter of each oxide was calculated from the area. In addition, it was confirmed by EDX (energy dispersive X-ray detector) that each oxide satisfies the above-described component composition. The acceleration voltage at the time of component composition measurement by EDX is 15 kV, and the measurement time is 100 seconds. Then, the number (N2) of oxides having an equivalent circle diameter of 2 μm or more was determined by converting into a number density equivalent to 1 mm 2 .
(円相当径が1μm以上のTi窒化物の個数密度の測定)
各厚鋼板の表面から深さt/4(t:板厚)の位置から試験片を切り出し(試験片の軸心がt/4の位置を通るように採取)、圧延方向および板厚方向に平行な断面を、光学顕微鏡を用いて倍率:200倍で20視野撮影し、の粗大Ti窒化物の個数をカウントし、1mm2相当の個数密度(N3)に換算して求めた。測定画像の面積は、1視野あたり0.148mm2、1試料あたり2.96mm2である。Ti窒化物の同定は形状および色に基づいて行い、角ばった形状且つ鮮やかなオレンジ色の介在物をTi窒化物と見なした。また、Ti窒化物の円相当径は解析ソフトにより算出した。尚、粗大Ti窒化物は、酸化物を起点として晶出することが多いが、その場合、内部の酸化物は円相当径の計測の対象から除外した。
(Measurement of number density of Ti nitride with equivalent circle diameter of 1 μm or more)
A test piece is cut out from the surface of each thick steel plate at a depth of t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. A parallel cross section was photographed with 20 optical fields using an optical microscope at a magnification of 200 ×, and the number of coarse Ti nitrides was counted and calculated by converting the number density (N3) corresponding to 1 mm 2 . Area of the measurement image is one field per 0.148 mm 2, per sample 2.96 mm 2. Ti nitrides were identified based on shape and color, and square-shaped and bright orange inclusions were considered Ti nitrides. The equivalent circle diameter of Ti nitride was calculated by analysis software. Coarse Ti nitride is often crystallized starting from an oxide. In that case, the internal oxide was excluded from the measurement of the equivalent circle diameter.
(円相当径が20nm以上のTi窒化物の個数密度の測定、および|da−df|/daの算出)
各厚鋼板のt/4位置から試験片を切り出し、圧延方向および板厚方向に平行な断面からレプリカTEM試験片を作製し、透過型電子顕微鏡(TEM)で、観察倍率15000倍、観察視野6.84μm×8.05μmの条件で4視野観察したうえで、EDX(エネルギー分散型蛍光X線分析装置)によってTi、Nを含む粒子を判別してTi含有窒化物とした。更に画像解析によって、その視野中のTi含有窒化物の面積を測定し、円相当径に換算して20nm以上のTi含有窒化物の個数を計測し、1mm2あたりに換算して個数密度(N4)を求めた。加えて、得られたデータから、円相当径が20nm以上のTi窒化物の大きさを、円相当径が20nmものから小さい順に5nm置きの領域に区切って各領域毎の円相当径の範囲を(di−5)以上di未満とし、前記各領域内に存在するTi窒化物の個数密度が最多であった領域の前記diをdfとしたときの前記dfと、円相当径が20nm以上500nm未満のTi窒化物の平均円相当径daを求め、|da−df|/daを算出した。
(Measurement of number density of Ti nitride having equivalent circle diameter of 20 nm or more and calculation of | da-df | / da)
A test piece is cut out from the t / 4 position of each thick steel plate, a replica TEM test piece is produced from a cross section parallel to the rolling direction and the plate thickness direction, and observed with a transmission electron microscope (TEM) at an observation magnification of 15000 times and an observation field of view 6 After observing four visual fields under the condition of .84 μm × 8.05 μm, particles containing Ti and N were discriminated by EDX (energy dispersive X-ray fluorescence analyzer) to obtain a Ti-containing nitride. Additionally the image analysis, the area of the Ti-containing nitride in the visual field was measured, in terms of circle-equivalent diameter is measured the number of the above Ti-containing nitride 20 nm, number density in terms of per 1 mm 2 (N4 ) In addition, from the obtained data, the size of the Ti nitride having an equivalent circle diameter of 20 nm or more is divided into regions every 5 nm in ascending order from the equivalent circle diameter of 20 nm to the range of equivalent circle diameter for each region. (Di-5) or more and less than di, the df when the di of the region where the number density of Ti nitride existing in each region is the most is df, and the equivalent circle diameter is 20 nm or more and less than 500 nm The average equivalent circle diameter da of Ti nitride was obtained, and | da-df | / da was calculated.
(HAZ靭性の評価)
各厚鋼板から、溶接継手用試験片を採取し、V先加工を施した後、入熱量:50kJ/mmにてエレクトロガスアーク溶接を実施した。これら試験片から、各厚鋼板の表面から深さt/4(t:板厚)の位置の溶接線(ボンド)近傍のHAZに切欠きを加工したシャルピー衝撃試験片(JIS Z 2242のVノッチ試験片)を3本ずつ採取し、−40℃でシャルピー衝撃試験を行い、吸収エネルギー(vE−40)を測定し、それらの平均値と最小値を求めた。この測定結果から、vE−40の平均値が180Jを超え、最小値が120Jを超えるものを、HAZ靭性に優れると評価した。
(Evaluation of HAZ toughness)
Test specimens for welded joints were collected from each thick steel plate, subjected to V pre-processing, and then subjected to electrogas arc welding at a heat input of 50 kJ / mm. From these test pieces, Charpy impact test pieces (V notches of JIS Z 2242) in which notches were formed in the HAZ near the weld line (bond) at a depth t / 4 (t: thickness) from the surface of each thick steel plate. Three test specimens) were collected and subjected to a Charpy impact test at −40 ° C., the absorbed energy (vE −40 ) was measured, and the average value and the minimum value thereof were obtained. From this measurement result, an average value of vE- 40 exceeding 180 J and a minimum value exceeding 120 J were evaluated as excellent in HAZ toughness.
また、入熱量:60kJ/mmにてエレクトロガスアーク溶接を実施する以外は全て上記した条件と同じ条件でもシャルピー衝撃試験を行い、3本の試験片の吸収エネルギー(vE−40)を測定して、その平均値を求めた。この測定結果から、vE−40の平均値が120Jを超えるものを、HAZ靭性に優れると評価した。また、vE−40の平均値が150Jを超えるものを、特にHAZ靭性に優れると評価した。 In addition, the Charpy impact test was performed under the same conditions as described above except that electrogas arc welding was performed at a heat input of 60 kJ / mm, and the absorbed energy (vE- 40 ) of three test pieces was measured. The average value was obtained. From this measurement result, those having an average value of vE- 40 exceeding 120 J were evaluated as having excellent HAZ toughness. Moreover, it evaluated that the average value of vE- 40 exceeds 150J especially excellent in HAZ toughness.
No.1〜35は、本発明の要件を満足する発明例であり、化学成分組成、酸化物、Ti窒化物の分散等が適切になされており、入熱量を50kJ/mmにした場合のHAZ靭性(平均値および最小値)、並びに入熱量を60kJ/mmにした場合のHAZ靭性(平均値)が優れていることが分かる。すなわち、No.1〜35は、溶接熱影響部の靭性に優れた厚鋼板であるということができる。 No. 1 to 35 are examples of the invention that satisfy the requirements of the present invention, in which chemical composition, oxide, Ti nitride dispersion, etc. are appropriately performed, and the HAZ toughness when the heat input is 50 kJ / mm ( It can be seen that the HAZ toughness (average value) is excellent when the average value and the minimum value) and the heat input amount are 60 kJ / mm. That is, no. It can be said that 1-35 are thick steel plates excellent in the toughness of the weld heat affected zone.
更には、請求項2記載の要件を満足する厚鋼板は、vE−40の平均値が150Jを超え、特に溶接熱影響部の靭性に優れた厚鋼板であると評価することができる。 Furthermore, the thick steel plate that satisfies the requirements of claim 2 can be evaluated as a thick steel plate having an average value of vE- 40 of more than 150 J and particularly excellent in toughness of the weld heat affected zone.
これに対し、No.36〜55は、本発明の要件のうちいずれかの要件を満足しない比較例であり、入熱量を50kJ/mmにした場合のHAZ靭性(平均値および最小値)、並びに入熱量を60kJ/mmにした場合のHAZ靭性(平均値)のいずれかが、評価基準を満足していないことが分かる。 In contrast, no. 36 to 55 are comparative examples that do not satisfy any of the requirements of the present invention, and the HAZ toughness (average value and minimum value) when the heat input is 50 kJ / mm, and the heat input is 60 kJ / mm. It can be seen that any of the HAZ toughness (average value) does not satisfy the evaluation criteria.
Claims (5)
酸素を除く構成元素が、質量%で、2%<Ti<40%、5%<Al<30%、5%<Ca<40%、5%<REM<50%、2%<Zr<30%、1.0≦REM/Zrを満たす酸化物を含有し、且つ、前記酸化物のうち、円相当径が0.2μm超2μm未満の酸化物が300個/mm2以上、円相当径が2μm以上の酸化物が100個/mm2以下、存在すると共に、
含有されるTi窒化物のうち、円相当径が1μm以上のTi窒化物が7個/mm2以下、円相当径が20nm以上のTi窒化物が1.0×106個/mm2以上、存在し、
更に、前記円相当径が20nm以上のTi窒化物の大きさを、円相当径が20nmのものから小さい順に5nm置きの領域に区切って各領域毎の円相当径の範囲を(di−5)以上di未満とし、前記各領域内に存在するTi窒化物の個数密度が最多であった領域の前記diをdfとしたときの前記dfと、
円相当径が20nm以上500nm未満のTi窒化物の平均円相当径daが、
|da−df|/da≦0.35という関係式を満たす厚鋼板であって、
前記厚鋼板から、溶接継手用試験片を採取し、V先加工を施した後、入熱量:50kJ/mmにてエレクトロガス溶接を実施し、その表面から深さt/4の位置の溶接線近傍のHAZに切欠きを加工したシャルピー衝撃試験片を、3本採取し、−40℃でシャルピー衝撃試験を行った時の吸収エネルギー:vE −40 の測定値の、平均値が180Jを超え、最小値が120Jを超え、
前記厚鋼板から、溶接継手用試験片を採取し、V先加工を施した後、入熱量:60kJ/mmにてエレクトロガス溶接を実施し、その表面から深さt/4の位置の溶接線近傍のHAZに切欠きを加工したシャルピー衝撃試験片を、3本採取し、−40℃でシャルピー衝撃試験を行った時の吸収エネルギー:vE −40 の測定値の平均値が120Jを超えることを特徴とする溶接熱影響部の靭性に優れた厚鋼板。
但し、前記したtは板厚、シャルピー衝撃試験片はJIS Z 2242のVノッチ試験片である。 In mass%, C: 0.03 to 0.12%, Si: 0.10 to 0.25%, Mn: 1.00 to 2.00 %, P: 0.030 % or less (excluding 0%) ), S: 0.015% or less (excluding 0%), Al: 0.004 to 0.050 %, Ti: 0.010 to 0.050 %, REM: 0.0003 to 0.0200 %, Zr: 0.0003~ 0.0200%, Ca: 0.0005~ 0.0100%, N: 0.0020 contains ~ 0.0100%, balance being iron and unavoidable impurities,
Constituent elements excluding oxygen are 2% <Ti <40%, 5% <Al <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30% in mass%. contains an oxide satisfying 1.0 ≦ REM / Zr, and, among the oxide, circles oxide equivalent diameter of less than 0.2μm ultra 2μm 300 pieces / mm 2 or more, the equivalent circle diameter 2μm There are 100 or more oxides / mm 2 or more of the above oxides,
Among Ti nitrides contained, Ti nitride with an equivalent circle diameter of 1 μm or more is 7 pieces / mm 2 or less, Ti nitride with an equivalent circle diameter of 20 nm or more is 1.0 × 10 6 pieces / mm 2 or more, Exists,
Further, the size of the Ti nitride having an equivalent circle diameter of 20 nm or more is divided into regions every 5 nm in ascending order from the equivalent circle diameter of 20 nm to the range of equivalent circle diameter for each region (di-5). The df when the di is df in the region where the number density of Ti nitrides present in each region is the largest, and less than di,
The average equivalent circle diameter da of the Ti nitride having an equivalent circle diameter of 20 nm or more and less than 500 nm is:
A thick steel plate satisfying the relational expression | da-df | /da≦0.35 ,
A specimen for a welded joint is collected from the thick steel plate, subjected to V-preprocessing, and then subjected to electrogas welding at a heat input of 50 kJ / mm, and a weld line at a depth t / 4 from the surface. Three samples of Charpy impact test pieces in which notches were machined in the nearby HAZ were collected , and the average value of the measured value of absorbed energy: vE- 40 when the Charpy impact test was performed at −40 ° C. exceeded 180 J. Minimum value exceeds 120J,
A specimen for a welded joint is collected from the thick steel plate and subjected to V-tip machining, and then electrogas welding is performed at a heat input of 60 kJ / mm, and a weld line at a depth t / 4 from the surface. Three Charpy impact test pieces with notches formed in nearby HAZ were sampled and the average value of measured values of absorbed energy: vE- 40 exceeded 120 J when Charpy impact test was conducted at -40 ° C. A thick steel plate with excellent toughness in the heat affected zone.
However, the above-mentioned t is the plate thickness, and the Charpy impact test piece is a V-notch test piece of JIS Z 2242.
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