JP2013087334A - Steel sheet having excellent toughness in weld heat affected zone and method for manufacturing the same - Google Patents

Steel sheet having excellent toughness in weld heat affected zone and method for manufacturing the same Download PDF

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JP2013087334A
JP2013087334A JP2011229422A JP2011229422A JP2013087334A JP 2013087334 A JP2013087334 A JP 2013087334A JP 2011229422 A JP2011229422 A JP 2011229422A JP 2011229422 A JP2011229422 A JP 2011229422A JP 2013087334 A JP2013087334 A JP 2013087334A
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Yoshiaki Shintaku
祥晃 新宅
Takahiro Kamo
孝浩 加茂
Genki Inokari
玄樹 猪狩
Shuichi Suzuki
秀一 鈴木
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet having excellent toughness in a weld heat affected zone (HAZ) and a method for manufacturing the steel sheet.SOLUTION: The steel sheet having excellent toughness in a weld heat affected zone has a chemical composition which includes, by mass%, 0.02-0.2% of C, 0.03-0.5% of Si, 0.5-2.0% of Mn, 0.02% or less of P, less than 0.002% of S, 0.005-0.08% of Al, 0.003-0.02% of Ti, 0.002-0.009% of N, 0.001-0.0035% of O, and 0.0003-0.0045% of Ca with the balance being Fe and unavoidable impurities, and satisfies the following formulas (1), (2) and (3). In a microstructure in the weld heat affected zone, an area ratio of island martensite is less than 1.0%. An inclusion including Al and Ca and having a particle size of 5.0 μm or less is present in steel, and an aspect ratio of the inclusion is 1.9 or less. Further, the steel sheet may include one or more selected from among Cu, Ni, Cr, Mo, V, Nb and B. 0.50≤Ca/O≤1.30 (1), Pcm*≤0.23 (2) and Pcm*-0.75C≥0.1 (3).

Description

本発明は、造船、建築、橋梁、海洋構造物およびラインパイプなどに使用される溶接熱影響部(Heat Affected Zone:以下、「HAZ」と称す。)の靭性に優れた鋼板及びその製造方法に関するものである。特に、近年要求の高まっている入熱25kJ/mmを超える大入熱溶接を施工した場合にも、HAZにおいて優れた靭性を有する鋼板に関するものである。   The present invention relates to a steel plate excellent in toughness of a welded heat affected zone (hereinafter referred to as “HAZ”) used for shipbuilding, construction, bridges, offshore structures, line pipes, and the like, and a method for producing the same. Is. In particular, the present invention relates to a steel sheet having excellent toughness in HAZ even when a large heat input welding exceeding 25 kJ / mm, which has been increasingly demanded in recent years, is applied.

造船、建築、橋梁、海洋構造物およびラインパイプなどの大型構造物に用いられる溶接構造用鋼板は、溶接部の破壊に対する安全性および信頼性を高める観点から、靱性に対する要求が年々厳しさを増しており、母材鋼板の靭性と同様に、HAZにおいてもより優れた靱性を確保することが要求されている。例えば、HAZ靭性が母材靭性と比較して大幅に劣る場合、構造物が衝撃荷重を受けた場合にHAZからき裂が発生し、大規模な構造物の破壊に進展するといった危険性があるからである。   Steel plates for welded structures used in large structures such as shipbuilding, architecture, bridges, offshore structures and line pipes are becoming increasingly demanding of toughness from the viewpoint of improving safety and reliability against fracture of welds. In addition, as in the case of the toughness of the base steel sheet, it is required to ensure better toughness in the HAZ. For example, if the HAZ toughness is significantly inferior to the base metal toughness, there is a risk that when the structure is subjected to an impact load, a crack is generated from the HAZ and progresses to the destruction of a large-scale structure. It is.

一方で、この種の大型鋼構造物の建造コストに占める溶接施工コストの割合は大きく、溶接施工コストを低減するために、高能率の溶接法が用いられるようになった。溶接施工コストを低下させるために最も有効な方法は、溶接パス数を減らすことであり、このためには溶接入熱を大きくした高能率溶接法を用いて大入熱溶接施工を行うことが望ましい。しかし、大入熱溶接を行った場合、HAZ靭性が低下することは避けられない。したがって、靭性の要求が厳しい構造物に対しては、入熱を制限して溶接パス数を増やし、能率と経済性を犠牲にして溶接施工せざるを得ないという問題点があった。   On the other hand, the ratio of the welding construction cost to the construction cost of this type of large steel structure is large, and a highly efficient welding method has been used to reduce the welding construction cost. The most effective way to reduce the welding cost is to reduce the number of welding passes. For this purpose, it is desirable to perform high heat input welding using a high efficiency welding method with increased welding heat input. . However, when high heat input welding is performed, the HAZ toughness is inevitably lowered. Therefore, there has been a problem that a structure requiring severe toughness must be welded at the expense of efficiency and economy by limiting the heat input to increase the number of welding passes.

これらの問題を解決するため、これまでにも大入熱溶接HAZ靱性を改善するための種々の対策が実施されてきた。   In order to solve these problems, various countermeasures for improving the high heat input welding HAZ toughness have been implemented so far.

例えば、特許文献1には、鋼材中のCa,O,Sの含有量を調整し、大入熱溶接をした際に、溶接熱影響部を微細な組織とすることにより、優れたHAZ靭性を有する大入熱溶接用鋼材に関する発明が記載されている。加えて、特許文献1には、鋼板を溶製する際の凝固段階で晶出したCaSの表面上にMnSを析出させ、さらにMnS上には、TiN、BN、AlN、VN等のフェライト生成核を析出させることにより、大入熱溶接時の高温下でも溶解しないフェライト変態生成核を微細に分散させ、HAZ組織を微細なフェライト-パーライト組織として高靭性化を達成させる発明が記載されている。   For example, Patent Document 1 describes excellent HAZ toughness by adjusting the content of Ca, O, and S in a steel material and making the weld heat affected zone a fine structure when performing high heat input welding. The invention relating to the steel material for large heat input welding having is described. In addition, Patent Document 1 discloses that MnS is precipitated on the surface of CaS crystallized in the solidification stage when the steel sheet is melted, and that ferrite nuclei such as TiN, BN, AlN, and VN are further formed on MnS. Describes that the ferrite transformation nuclei that do not dissolve even at high temperatures during high heat input welding are finely dispersed by precipitating steel, and the HAZ structure is made fine ferrite-pearlite structure to achieve high toughness.

また、特許文献2には、製鋼における脱酸材をTi、Al、Caの順に添加し、さらにAlを添加することで酸化物を微細に分散させ、溶接ボンド部近傍で1400℃以上に加熱される領域のオーステナイト細粒化、粒内フェライト生成を同時に達成する溶接継手部靱性の優れた鋼材の製造方法に関する発明が記載されている。加えて、特許文献2では、Ca、Al、またはCa、Al、Tiのいずれかを含有する酸化物の微細分散、個数増加を図り、オーステナイト粒細粒化や微細フェライト生成によって優れたHAZ靭性を有する鋼材が得られるとの発明が記載されている。   Further, in Patent Document 2, a deoxidizing material in steel making is added in the order of Ti, Al, and Ca, and further, by adding Al, oxides are finely dispersed and heated to 1400 ° C. or more in the vicinity of the weld bond portion. Invention relating to a method for producing a steel material having excellent weld joint toughness that simultaneously achieves austenite refinement and intragranular ferrite formation in a region. In addition, Patent Document 2 aims to achieve fine HAZ toughness by austenite grain refinement and fine ferrite generation by finely dispersing and increasing the number of oxides containing Ca, Al, or Ca, Al, or Ti. The invention that the steel material which has is obtained is described.

特開2002-256379号公報JP 2002-256379 A 特開2001-288509号公報JP 2001-288509 A

しかし、これらの発明におけるCa含有酸化物やCa硫化物は、その一部は溶鋼中において微細に分散されるものの、凝集粗大化して介在物として存在する。これら粗大介在物が鋼中に残存すると大入熱溶接HAZにおいて金属組織を微細化したとしても靱性の低下が避けられない。また、微細に分散した介在物も、凝固後の圧延過程において破砕、延伸されることで扁平した形状となり、破壊の起点として作用するため粗大介在物と同様にHAZ靱性の低下を招く原因となる。   However, some of the Ca-containing oxides and Ca sulfides in these inventions are finely dispersed in the molten steel, but are agglomerated and coarsened and exist as inclusions. If these coarse inclusions remain in the steel, a reduction in toughness is inevitable even if the metal structure is refined in high heat input welding HAZ. In addition, the finely dispersed inclusions are flattened by being crushed and stretched in the rolling process after solidification, and act as a starting point of fracture, which causes a reduction in HAZ toughness like coarse inclusions. .

よって、上述のCa含有酸化物やCa硫化物による靭性改善技術は、粗大介在物や扁平した介在物の存在によりHAZ靭性改善効果には自ずと限界があった。   Therefore, the above-described toughness improvement technology using Ca-containing oxides or Ca sulfides has a limit in its HAZ toughness improvement effect due to the presence of coarse inclusions or flattened inclusions.

前述の通り、近年、鋼構造物に対する安全性および信頼性の確保に対する要求はますます高まっている。鋼板の靱性評価手法として一般的に用いられているシャルピー衝撃試験においても、従来であれば目的とする温度でくり返し試験をした場合の吸収エネルギの平均値が規定値を上回れば規格を満足するとされてきた。しかし、最近ではより安定的な高靭性が求められ、大入熱溶接HAZにおけるシャルピー衝撃試験においても、くり返し試験をした場合の吸収エネルギの最低値まで問題にされるようになってきた。   As described above, in recent years, there has been an increasing demand for ensuring safety and reliability of steel structures. Even in the Charpy impact test, which is generally used as a method for evaluating the toughness of steel sheets, if the average value of absorbed energy in repeated testing at the target temperature exceeds the specified value, the standard is satisfied. I came. Recently, however, more stable and high toughness has been demanded, and even in the Charpy impact test in the high heat input welding HAZ, there has been a problem up to the minimum value of the absorbed energy when the repeated test is performed.

その背景は、次の(i)および(ii)の2点にある。   The background is in the following two points (i) and (ii).

(i) シャルピー衝撃試験において吸収エネルギの平均値が高くても、個々の値にバラツキがあるということは、構造物の中に靱性の低い領域が存在していることを示しており、こうした特定の脆弱な部位で破壊が発生すると構造物全体に甚大な被害を及ぼす可能性が高いと考えられるようになってきたこと。   (i) Even if the average value of absorbed energy is high in the Charpy impact test, the variation in individual values indicates that there is a region with low toughness in the structure. It has been considered that there is a high possibility that the entire structure will be seriously damaged if a fragile part of the site is damaged.

(ii) 重要な鋼構造物に対しては、安全性確認のために、例えば溶接長100m当り1回のシャルピー衝撃試験を行い合格することを求められる場合があり、このような場合には、シャルピー吸収エネルギの個々の値で安定した高靭性が得られなければ、何度も繰り返される試験にすべて合格することが困難であること。   (ii) For important steel structures, it may be required to pass a Charpy impact test once per 100 m of weld length, for example, to confirm safety. If stable and high toughness cannot be obtained with individual values of Charpy absorbed energy, it is difficult to pass all repeated tests.

以上の背景から、鋼板中に粗大な介在物や扁平した介在物が多数存在する場合、大入熱溶接HAZにおけるシャルピー衝撃試験時の破壊起点として作用する確率が高くなるため、安定して高靱性を得ることができず、重要構造物への適用は困難であった。   From the above background, when there are many coarse inclusions and flattened inclusions in the steel sheet, the probability of acting as a fracture starting point in the Charpy impact test in the high heat input welding HAZ is increased, so that the high toughness is stable. Therefore, it was difficult to apply to important structures.

このため本発明の目的は、溶接熱影響部(HAZ)の靱性に優れた鋼板およびその製造方法を提供することにある。特に、重要構造物に用いることができ、入熱25kJ/mm以上の大入熱溶接でのHAZにおけるシャルピー衝撃試験で安定して高い吸収エネルギを示す靱性に優れた鋼板およびその製造方法を提供することにある。   For this reason, the objective of this invention is providing the steel plate excellent in the toughness of a welding heat affected zone (HAZ), and its manufacturing method. In particular, the present invention provides a steel sheet excellent in toughness that can be used for an important structure and stably exhibits high absorbed energy in a Charpy impact test in HAZ in a high heat input welding with a heat input of 25 kJ / mm or more, and a manufacturing method thereof. There is.

本発明者等は、大入熱溶接熱影響部においても安定した高靭性を有する鋼板を提供するべく、種々の検討と実験を行った。その結果、次の(a)〜(e)に示すとおり、適正な介在物制御と硬質第二相組織制御の技術を組合せることによって、新たな金属学的効果が得られるとの知見を得た。   The present inventors conducted various examinations and experiments in order to provide a steel sheet having stable high toughness even in the heat input zone with high heat input welding. As a result, as shown in the following (a) to (e), we obtained the knowledge that a new metallurgical effect can be obtained by combining appropriate inclusion control and hard second phase structure control technology. It was.

(a) 一般に、溶接熱影響部に高靭性が要求される重要構造物には、Al添加により鋼中の酸素を固定したAlキルド鋼が用いられる。ところが、大入熱溶接を適用する場合にはAlキルド鋼では、充分な靱性を確保することが困難である。そこで上述した通り、Alに加えてCaを添加し、Caを含む介在物を利用した大入熱溶接HAZ靱性改善技術の開発が進められてきた。しかし、これらのCa添加鋼板は、溶接熱影響部においてシャルピー衝撃試験をくり返し実施した場合の個々の値のばらつきが大きく、溶接継手中に脆弱な部位が存在すると考えられる。したがって、溶接構造物の破壊安全性をさらに高めることができる鋼板開発が求められている。   (a) In general, Al killed steel in which oxygen in steel is fixed by adding Al is used for important structures that require high toughness in the heat affected zone of welding. However, when applying high heat input welding, it is difficult to secure sufficient toughness with Al killed steel. Therefore, as described above, development of a high heat input welding HAZ toughness improving technique using Ca in addition to Al and using inclusions containing Ca has been advanced. However, these Ca-added steel plates have a large variation in individual values when the Charpy impact test is repeatedly performed in the weld heat-affected zone, and it is considered that weak portions exist in the welded joint. Accordingly, there is a demand for the development of steel sheets that can further enhance the fracture safety of welded structures.

(b) 発明者らは、シャルピー衝撃試験の吸収エネルギ個々の値のばらつきについて鋭意研究し、鋼中の粗大介在物や圧延で延伸された介在物、または圧延により破砕され点列状に連なった介在物の集合体が、溶接熱影響部のシャルピー衝撃試験時に、き裂の発生と伝播を助長し、吸収エネルギの低値発生とばらつきの原因となっていることを見出した。また、従来より、HAZ靱性の改善に有効とされてきた金属組織の微細化だけでは、これらの低値発生を抑えられないことが判明した。なお、鋼中の粗大介在物とは、主としてAlとCaを含む酸硫化物のことである。そして、これらの介在物の内、比較的小さいサイズのものであっても圧延によって延伸、破砕され、点列状に連なった介在物群が生じることが判明した。   (b) The inventors diligently studied the variation in the individual values of the absorbed energy in the Charpy impact test, and the coarse inclusions in the steel, inclusions stretched by rolling, or crushing by rolling were connected in a sequence of dots. It was found that the aggregate of inclusions promotes crack generation and propagation during the Charpy impact test of the weld heat affected zone, causing low values and variations in absorbed energy. Further, it has been found that the generation of these low values cannot be suppressed only by the refinement of the metal structure, which has conventionally been effective for improving the HAZ toughness. In addition, the coarse inclusion in steel is an oxysulfide mainly containing Al and Ca. And even if it was a comparatively small size among these inclusions, it turned out that the inclusion group which was extended | stretched and crushed by rolling and continued in the shape of a point sequence was produced.

(c) このような粗大介在物ないし延伸された介在物、または点列状の介在物群が、シャルピー衝撃試験片のノッチ近傍に存在すれば、その周囲に発生する応力集中効果により、へき開破壊の発生およびき裂の伝播が容易となり、シャルピー衝撃試験の吸収エネルギは著しく低下する。したがって、鋼板中の介在物サイズおよび形態を制御してこれらの有害な介在物の生成を防止することが、吸収エネルギの個々の値のばらつきを防止するための重要な要素となる。このためには、AlとCaを含む酸硫化物を微細分散させ、かつ圧延中にも延伸、破砕しないようにすることが肝要である。AlとCaを溶鋼中に添加した場合、ある特定の組成の酸化物では融点が低下し、溶鋼中で液体となり、その表面積を最小にするために球状化する性質があることが判明した。また、この球状化した介在物は凝固後の圧延過程においても延伸、破砕が生じないことを見出した。この効果を得るためにはCaを酸素で除した値であるCa/Oが0.5以上、1.30以下であることが必要であり、この条件を満たす介在物が分散した鋼は、介在物の長径を短径で除した値、つまり介在物のアスペクト比が1.9以下となり、球状に近い介在物が得られるとともに、大入熱溶接HAZにおいてくり返しシャルピー衝撃試験を行った場合にも安定して高い吸収エネルギ値が得られた。しかし、溶鋼中の酸素量が著しく高い場合には、鋼中の介在物の絶対量が増加するため、上述のCa/Oの値を満足しても粗大介在物の生成を避けることはできない。これを防止するためには、溶鋼中の酸素を制御する必要があるが、溶鋼中のAl含有量が0.005〜0.08質量%の範囲となるようにAlを添加して脱酸し、さらに脱ガス装置で15分以上処理して溶鋼の酸素量を充分に下げた後、溶鋼温度を1600±70℃に保った状態でCaを添加することで粗大介在物の生成を防止し、大入熱溶接HAZ靱性のばらつきを抑制することができることを見出した。   (c) If such coarse inclusions, elongated inclusions, or dot-like inclusion groups are present in the vicinity of the notch of the Charpy impact test piece, cleavage fracture occurs due to the stress concentration effect generated around it. Generation and crack propagation are facilitated, and the absorbed energy in the Charpy impact test is significantly reduced. Therefore, controlling the size and form of inclusions in the steel sheet to prevent the formation of these harmful inclusions is an important factor for preventing variations in individual values of absorbed energy. For this purpose, it is important to finely disperse oxysulfide containing Al and Ca and not to stretch or crush even during rolling. When Al and Ca are added to the molten steel, it has been found that an oxide having a specific composition has a melting point that is reduced to a liquid in the molten steel and spheroidizes to minimize its surface area. It was also found that the spheroidized inclusions do not stretch or crush even in the rolling process after solidification. In order to obtain this effect, Ca / O, which is a value obtained by dividing Ca by oxygen, needs to be 0.5 or more and 1.30 or less. The value obtained by dividing the major axis of the object by the minor axis, that is, the aspect ratio of the inclusion is 1.9 or less, and an inclusion that is nearly spherical is obtained, and also when the repeated Charpy impact test is performed in the high heat input welding HAZ A stable and high absorbed energy value was obtained. However, when the amount of oxygen in the molten steel is extremely high, the absolute amount of inclusions in the steel increases, so that the formation of coarse inclusions cannot be avoided even if the above Ca / O value is satisfied. In order to prevent this, it is necessary to control oxygen in the molten steel, but deoxidization is performed by adding Al so that the Al content in the molten steel is in the range of 0.005 to 0.08 mass%. In addition, after sufficiently reducing the oxygen content of the molten steel by treating with a degassing apparatus for 15 minutes or more, the formation of coarse inclusions is prevented by adding Ca with the molten steel temperature maintained at 1600 ± 70 ° C., It has been found that variation in high heat input welding HAZ toughness can be suppressed.

(d) しかし、一方で、入熱25kJ/mmを超える大入熱溶接HAZではこれら介在物影響のみならず、硬質第二相組織の生成もまた、へき開破壊の発生およびき裂の伝播を助長することが判明した。つまり硬質第二相組織が生成すると介在物サイズならびに形態制御によるシャルピー吸収エネルギの改善効果が失われてしまうことが新たに判明した。   (d) On the other hand, in the high heat input welding HAZ with a heat input exceeding 25 kJ / mm, not only the influence of these inclusions but also the formation of a hard second phase structure promotes the occurrence of cleavage fracture and crack propagation. Turned out to be. In other words, it was newly found that the effect of improving Charpy absorbed energy by inclusion size and morphology control is lost when a hard second phase structure is formed.

(e) その理由は、これらHAZにて生成する硬質第二相組織をつぶさに観察した結果から、マルテンサイト組織とオーステナイト相とが複合した島状マルテンサイト組織(Martensite−Austenite constituent:以下「MA」と称す。)が生成したためであると判明した。このようなMAは母相と比較して非常に硬度が高いため、介在物と同じくシャルピー衝撃試験片のノッチ近傍に存在すれば、その周囲に発生する応力集中効果により、へき開破壊の発生およびき裂の伝播が容易となり、シャルピー衝撃試験の吸収エネルギを著しく低下させる原因となる。したがって、入熱25kJ/mmを超える大入熱溶接HAZの靭性を改善するためには、鋼板中の介在物形態制御に加えて、MAの生成を防止し、その生成量を大入熱HAZにおいて1%未満とする必要がある。発明者等が鋭意検討した結果、このためにはCを始めとする合金元素の制御が必要で、その組成は溶接低温割れ感受性組成であるPcm*を用いて簡便に整理することができ、Pcm*≦0.23およびPcm*−0.75C≧0.1の2つの式を同時に満足することで大入熱溶接HAZにおけるMAの生成量を面積率で1%未満に制御できることを見出した。ここで、Pcm*=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/3+Nb/2+23{B−10.8/14.1(N−Ti/3.4)}と表される。ただし、B−10.8/14.1(N−Ti/3.4)≦0のとき、B−10.8/14.1(N−Ti/3.4)=0として取り扱うこととする。また各式の元素記号はその元素の含有量(質量%)を示す。   (e) The reason for this is that, based on the result of observing the hard second phase structure generated in these HAZs, an island-like martensite structure (Martensite-Austenite constituent: hereinafter referred to as “MA”) in which the martensite structure and the austenite phase are combined. It was proved that it was produced. Since such MA is extremely harder than the parent phase, if it exists in the vicinity of the notch of the Charpy impact test piece as in the case of inclusions, the occurrence of cleavage fracture and cracking will occur due to the stress concentration effect around it. Propagation of cracks is facilitated, causing a significant decrease in absorbed energy in the Charpy impact test. Therefore, in order to improve the toughness of the high heat input welding HAZ exceeding 25 kJ / mm heat input, in addition to controlling the form of inclusions in the steel sheet, the formation of MA is prevented, and the generated amount is reduced in the high heat input HAZ. Must be less than 1%. As a result of intensive studies by the inventors, it is necessary to control alloy elements such as C for this purpose, and the composition can be easily arranged using Pcm *, which is a weld cold cracking sensitive composition. It has been found that the amount of MA produced in the high heat input welding HAZ can be controlled to less than 1% in terms of area ratio by simultaneously satisfying the two formulas of * ≦ 0.23 and Pcm * −0.75C ≧ 0.1. Here, Pcm * = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 3 + Nb / 2 + 23 {B-10.8 / 14.1 (N-Ti / 3.4)}. However, when B-10.8 / 14.1 (N-Ti / 3.4) ≦ 0, it is assumed that B-10.8 / 14.1 (N-Ti / 3.4) = 0. . Moreover, the element symbol of each formula shows the content (mass%) of the element.

本発明は、このような知見に基づいて完成したものであり、その要旨は、下記の(1)および(2)の溶接熱影響部の靱性に優れた鋼板並びに(3)溶接熱影響部の靱性に優れた鋼板の製造方法にある。   The present invention has been completed on the basis of such findings, the gist of which is the following (1) and (2) steel sheet excellent in toughness of the weld heat affected zone and (3) the weld heat affected zone. It exists in the manufacturing method of the steel plate excellent in toughness.

(1) 質量%で
C:0.02〜0.2%,
Si:0.03〜0.5%,
Mn:0.5〜2.0%,
P:0.02%以下,
S:0.002%未満,
Al:0.005〜0.08%,
Ti:0.003〜0.02%,
N:0.002〜0.009%,
O:0.001〜0.0035%,
Ca:0.0003〜0.0045%
を含有し、残部はFe及び不純物からなり、下記の(1)式、(2)式および(3)式を満足する化学組成を有する鋼板であって、
溶接熱影響部のミクロ組織において、島状マルテンサイトの面積率が1.0%未満であり、かつ鋼中にAlとCaを含む粒径5.0μm以下の介在物が存在し、その介在物のアスペクト比が1.9以下であることを特徴とする溶接熱影響部の靱性に優れた鋼板。

0.50≦Ca/O≦1.30 ・・・・・(1)、
Pcm*≦0.23 ・・・・・(2)、
Pcm*−0.75C≧0.1 ・・・・・(3)
ここで、
Pcm*=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/3+Nb/2+23{B−10.8/14.1(N−Ti/3.4)}
ただし、B−10.8/14.1(N−Ti/3.4)≦0のとき、B−10.8/14.1(N−Ti/3.4)=0として取り扱う。
また、(1)式、(2)式および(3)式の元素記号はその元素の含有量(質量%)を示し、アスペクト比とは、鋼板の圧延方向に平行な断面で観察される介在物の長径を短径で除した値を意味する。
(1) C: 0.02 to 0.2% by mass%,
Si: 0.03 to 0.5%,
Mn: 0.5 to 2.0%,
P: 0.02% or less,
S: less than 0.002%,
Al: 0.005 to 0.08%,
Ti: 0.003 to 0.02%,
N: 0.002 to 0.009%,
O: 0.001 to 0.0035%,
Ca: 0.0003 to 0.0045%
The balance is made of Fe and impurities, and has a chemical composition satisfying the following formulas (1), (2) and (3):
In the microstructure of the weld heat-affected zone, the area ratio of island martensite is less than 1.0%, and inclusions having a particle size of 5.0 μm or less containing Al and Ca are present in the steel. A steel sheet excellent in toughness of a heat-affected zone of welding, characterized by having an aspect ratio of 1.9 or less.
0.50 ≦ Ca / O ≦ 1.30 (1),
Pcm * ≦ 0.23 (2),
Pcm * −0.75C ≧ 0.1 (3)
here,
Pcm * = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 3 + Nb / 2 + 23 {B-10.8 / 14.1 (N-Ti / 3.4)}
However, when B-10.8 / 14.1 (N-Ti / 3.4) ≤0, it is handled as B-10.8 / 14.1 (N-Ti / 3.4) = 0.
The element symbols in the formulas (1), (2) and (3) indicate the content (% by mass) of the element, and the aspect ratio is an intervening observed in a cross section parallel to the rolling direction of the steel sheet. It means the value obtained by dividing the major axis of the product by the minor axis.

(2) さらにFeの一部に代えて、質量%で
Cu:1.5%以下,
Ni:6.0%以下,
Cr:1.0%以下,
Mo:0.8%以下,
V:0.1%以下,
Nb:0.05%以下及び
B:0.005%以下
の中から選んだ1種または2種以上を含有することを特徴とする、上記(1)の溶接熱影響部の靱性に優れた鋼板。
(2) Further, instead of part of Fe, Cu: 1.5% or less in mass%,
Ni: 6.0% or less,
Cr: 1.0% or less,
Mo: 0.8% or less,
V: 0.1% or less,
Nb: 0.05% or less and B: One or more selected from 0.005% or less, and a steel plate excellent in toughness of the heat affected zone of (1) above .

(3) 溶鋼中のAl含有量が0.005〜0.08質量%の範囲となるようにAlを添加して脱酸し、さらに脱ガス装置で15分以上処理した後、溶鋼温度1530〜1670℃で溶鋼中に下記(4)式を満足する量のCaを添加し、鋳造してなる上記(1)または(2)の化学組成を有するスラブを熱間圧延することを特徴とする、溶接熱影響部の靱性に優れた鋼板の製造方法。

21+2.52・O−5≦CaA≦21+2.52・O+5・・・・・(4)
ただし、Caは溶鋼1トン当たりのCa添加量(g)を、そしてOは溶鋼中の酸素量(ppm)を、それぞれ表す。
(3) Al is added so that the Al content in the molten steel is in the range of 0.005 to 0.08% by mass, deoxidation is performed, and further, the degassing apparatus is used for 15 minutes or longer, and then the molten steel temperature is 1530 to A quantity of Ca satisfying the following formula (4) is added to molten steel at 1670 ° C., and the slab having the chemical composition of (1) or (2) formed by casting is hot-rolled: A method for producing a steel sheet having excellent toughness of the heat affected zone.
Serial 21 + 2.52 · O M -5 ≦ CaA ≦ 21 + 2.52 · O M +5 ····· (4)
However, Ca A is Ca addition amount per molten steel one ton of (g), and O M is the amount of oxygen in the molten steel (ppm), expressed respectively.

本発明によれば、溶接熱影響部(HAZ)の靱性に優れた鋼板およびその製造方法を提供することができる。特に、重要構造物に用いることができ、入熱25kJ/mm以上の大入熱溶接でのHAZにおけるシャルピー衝撃試験で安定して高い吸収エネルギを示す靱性に優れた鋼板およびその製造方法を提供することができる。   ADVANTAGE OF THE INVENTION According to this invention, the steel plate excellent in the toughness of a welding heat affected zone (HAZ) and its manufacturing method can be provided. In particular, the present invention provides a steel sheet excellent in toughness that can be used for an important structure and stably exhibits high absorbed energy in a Charpy impact test in HAZ in a high heat input welding with a heat input of 25 kJ / mm or more, and a manufacturing method thereof. be able to.

1.鋼板の化学組成について
以下、本発明に係る鋼板の化学組成について説明する。なお、含有量に関する「%」および「ppm」は、いずれも質量割合を意味する。
1. About the chemical composition of a steel plate Hereinafter, the chemical composition of the steel plate concerning this invention is demonstrated. “%” And “ppm” relating to the content both mean mass percentages.

C:0.02〜0.2%
Cは、母材及び溶接部の強度、靭性を確保するため、0.02%以上含有させる必要がある。しかし、Cが多すぎると島状マルテンサイトの生成を助長しHAZ靭性を低下させるとともに溶接性を劣化させるため、その上限を0.2%とする。したがって、Cの含有量は0.02〜0.2%とする。
C: 0.02-0.2%
C needs to be contained by 0.02% or more in order to ensure the strength and toughness of the base material and the weld. However, if C is too much, the formation of island martensite is promoted, the HAZ toughness is lowered and the weldability is deteriorated, so the upper limit is made 0.2%. Therefore, the C content is 0.02 to 0.2%.

Si:0.03〜0.5%
Siは、脱酸作用を有し、鋼の予備脱酸によって鋼中に含有される。また、母材の強度確保に有効である。これらの効果を得るために、0.03%以上含有させる。しかし、過剰に含有させると島状マルテンサイトの生成を助長しHAZ靭性を劣化させるため、上限を0.5%とする。したがって、Siの含有量は0.03〜0.5%とする。なお、良好なHAZ靭性を得るために、Siの含有量の上限を0.4%以下にするのが望ましい。
Si: 0.03-0.5%
Si has a deoxidizing action and is contained in the steel by preliminary deoxidation of the steel. It is also effective for securing the strength of the base material. In order to acquire these effects, it is made to contain 0.03% or more. However, if excessively contained, the formation of island martensite is promoted and the HAZ toughness is deteriorated, so the upper limit is made 0.5%. Therefore, the Si content is 0.03 to 0.5%. In order to obtain good HAZ toughness, the upper limit of the Si content is desirably 0.4% or less.

Mn:0.5〜2.0%
Mnは母材及びHAZ部の強度、靭性の確保に不可欠であり、そのためにMnを0.5%以上含有させる。しかし、Mnの含有量が多すぎると、HAZ靭性の劣化や、スラブの中心偏析助長による溶接性劣化などが起こるため、Mnの含有量の上限を2.0%とする。したがって、Mnの含有量は0.5〜2.0%とする。
Mn: 0.5 to 2.0%
Mn is indispensable for ensuring the strength and toughness of the base material and the HAZ part. For this purpose, Mn is contained in an amount of 0.5% or more. However, if the Mn content is too large, HAZ toughness deterioration and weldability deterioration due to the center segregation promotion of the slab occur, so the upper limit of the Mn content is set to 2.0%. Therefore, the Mn content is set to 0.5 to 2.0%.

P:0.02%以下
Pは本発明においては不純物元素である。Pの含有量が0.02%を超えると、スラブ中心の偏析が大きくなり、母材及びHAZの機械的性質を低下させ、更にはHAZの粒界破壊が起こるおそれがある。したがって、Pの含有量を0.02%以下とする。
P: 0.02% or less P is an impurity element in the present invention. When the content of P exceeds 0.02%, segregation at the center of the slab becomes large, the mechanical properties of the base material and the HAZ are deteriorated, and there is a possibility that the grain boundary fracture of the HAZ occurs. Therefore, the P content is 0.02% or less.

S:0.002%未満
Sは不純物として存在し、多すぎると板厚中心部で延伸したMnSが多量に生成するため、母材及びHAZの靭性が劣化する。また、SはCaとの親和力が大きくCaSを生成して、適正な複合酸化物の生成を阻害する。したがって、Sの含有量を0.002%未満とする。
S: Less than 0.002% S is present as an impurity, and if it is too much, a large amount of MnS stretched at the center of the plate thickness is generated, so that the toughness of the base material and HAZ deteriorates. In addition, S has a large affinity with Ca and produces CaS, thereby inhibiting the production of an appropriate composite oxide. Therefore, the S content is less than 0.002%.

Al:0.005〜0.08%
Alは本発明において重要な元素の一つである。Alを溶鋼中に添加した場合、脱酸剤として作用し、Alを生成する。Alは溶鋼中にてクラスターを形成し、圧延を施した場合にはこれらのクラスターが分離し、点列状につらなって鋼板中に分散することとなる。この場合、点状につらなったAlはシャルピー試験時のき裂の発生起点となり、母材の靭性を劣化させる。また、Alは安定な酸化物であるため溶接によっても変化せず、最終的にHAZに残留するため、HAZ靭性をも劣化させる。
Al: 0.005 to 0.08%
Al is one of the important elements in the present invention. When Al is added to molten steel, it acts as a deoxidizer and produces Al 2 O 3 . Al 2 O 3 forms clusters in the molten steel, and when rolling is performed, these clusters are separated and dispersed in the steel sheet in the form of dotted lines. In this case, the Al 2 O 3 gathered in the form of dots serves as a starting point of crack generation during the Charpy test, and deteriorates the toughness of the base material. Moreover, since Al 2 O 3 is a stable oxide, it does not change even by welding, and finally remains in the HAZ, so that the HAZ toughness is also deteriorated.

しかし、本発明ではAlとともにCaを添加することにより、鋼中にAlとCaを含む介在物を生成せしめ、破壊起点として作用することを防止する。したがって、Alは0.005%以上含有させる必要がある。一方、大量に添加すると、鋼中に固溶するAlが増加し、溶接冷却過程において残留オーステナイトのセメンタイトへの分解反応を抑制して島状マルテンサイトを増加させ、溶接部の靭性を低下させる。したがって、Al含有量の上限を0.08%とする。したがって、Alの含有量は0.005〜0.08%とする。なお、Alは0.02%以上含有させるのが好ましい。   However, in the present invention, by adding Ca together with Al, inclusions containing Al and Ca are generated in the steel and are prevented from acting as a fracture starting point. Therefore, Al needs to be contained by 0.005% or more. On the other hand, when added in a large amount, the amount of Al dissolved in the steel increases, and the decomposition reaction of retained austenite to cementite is suppressed in the welding cooling process to increase island martensite and toughness of the welded portion. Therefore, the upper limit of the Al content is 0.08%. Therefore, the Al content is set to 0.005 to 0.08%. In addition, it is preferable to contain Al 0.02% or more.

Ti:0.003〜0.02%
TiはTiNとして析出し、HAZでのオーステナイトの粗大化抑制効果を有する。また、フェライト変態の核として作用し、その粒内組織微細化効果のために、HAZ靭性が向上する。この効果を得るには、Tiを0.003%以上含有させる必要がある。一方、Tiの含有量が多くなると固溶Tiが増加し、HAZ靭性が低下するため、Tiの含有量の上限を0.02%とする。したがって、Tiの含有量は0.003〜0.02%とする。
Ti: 0.003-0.02%
Ti precipitates as TiN and has the effect of suppressing austenite coarsening in HAZ. Further, it acts as a nucleus of ferrite transformation, and the HAZ toughness is improved due to the effect of refining the intragranular structure. In order to obtain this effect, it is necessary to contain 0.003% or more of Ti. On the other hand, when the Ti content increases, the solid solution Ti increases and the HAZ toughness decreases, so the upper limit of the Ti content is set to 0.02%. Therefore, the Ti content is set to 0.003 to 0.02%.

N:0.002〜0.009%
NはTiNの析出に極めて重要な元素の1つであり、Nの含有量が0.002%未満ではTiNの析出量が不足し、冷却時に有害なTi炭化物が生成するため、Nを0.002%以上含有させる必要がある。一方、含有量が多くなると、固溶Nが過剰となりTiNが粗大化するため、0.009%以下とする。したがって、Nの含有量は0.002〜0.009%とする。なお、Nは0.004%を超えて含有させるのが好ましい。
N: 0.002 to 0.009%
N is one of the elements extremely important for TiN precipitation. If the N content is less than 0.002%, the amount of TiN precipitation is insufficient, and harmful Ti carbides are generated during cooling. It is necessary to contain 002% or more. On the other hand, when the content increases, the solute N becomes excessive and the TiN becomes coarse, so the content is made 0.009% or less. Therefore, the N content is 0.002 to 0.009%. In addition, it is preferable to contain N exceeding 0.004%.

O:0.001〜0.0035%
O(酸素)は、本発明において最も重要な元素の1つであり、介在物の球状化のみならず、分散個数や介在物サイズとも直接的に関わるため、厳密に制御されなければならない。本発明においては、Oの含有量が少なければ少ないほど好ましいが、0.001%未満にすることは工業的に困難であり、コスト上昇に見合った効果が得られない。よってその下限を0.001%とした。一方、0.0035%を超える過剰なOは、粗大な酸化物を形成するとともに、介在物個数を必要以上に増加させ、鋼板の清浄性を劣化させるため母材およびHAZの靭性に悪影響を及ぼす。よって、その上限を0.0035%とした。したがって、Oの含有量は0.001〜0.0035%とする。
O: 0.001 to 0.0035%
O (oxygen) is one of the most important elements in the present invention, and not only spheroidization of inclusions but also directly relates to the number of dispersed particles and inclusion size, and must be strictly controlled. In the present invention, the smaller the O content, the better. However, it is industrially difficult to make it less than 0.001%, and an effect commensurate with the cost increase cannot be obtained. Therefore, the lower limit was made 0.001%. On the other hand, excessive O exceeding 0.0035% adversely affects the toughness of the base material and the HAZ because it forms coarse oxides, increases the number of inclusions more than necessary, and deteriorates the cleanliness of the steel sheet. . Therefore, the upper limit was made 0.0035%. Therefore, the content of O is set to 0.001 to 0.0035%.

Ca:0.0003〜0.0045%
Caは、本発明において最も重要な元素の1つであり、介在物の球状化を達成するためには、AlおよびOとともに厳密に制御する必要がある。Caは脱酸材として作用するとともに、鋼中にAlとCaを含む酸化物を形成せしめ、介在物形態を制御するためにも必要な元素である。したがって、Caを0.0003%以上含有させる必要がある。しかし、大量に添加すると鋼の清浄性を低下させるとともに、Alを含まないCaO主体の酸化物を生成し、圧延で破砕され点列状に連なりやすい介在物へと変化するため、HAZの靭性を劣化させる。このため、Caの含有量を0.0045%以下とする。したがって、Caの含有量は0.0003〜0.0045%とする。
Ca: 0.0003 to 0.0045%
Ca is one of the most important elements in the present invention, and in order to achieve inclusion spheroidization, it is necessary to strictly control it together with Al and O. Ca acts as a deoxidizing material and is an element necessary for forming an oxide containing Al and Ca in steel and controlling the form of inclusions. Therefore, it is necessary to contain 0.0003% or more of Ca. However, when added in a large amount, the cleanliness of the steel is lowered, and an oxide mainly composed of CaO that does not contain Al is generated, and it is changed to inclusions that are crushed by rolling and easily connected in a sequence of dots. Deteriorate. For this reason, content of Ca shall be 0.0045% or less. Therefore, the content of Ca is set to 0.0003 to 0.0045%.

Ca/O:0.50〜1.30
溶鋼中で生成されるAlとCaを含む酸化物において、CaOとAlがほぼ1:1で共存した場合、酸化物の融点は溶鋼温度以下に低下し液化する。この時、酸化物には表面張力が作用し球状となる。この作用を利用してAlとCaを含む物の形態制御をするためには、Ca/Oを0.50〜1.30とする必要がある。Ca/Oが1.30を超えるとCaO主体の酸化物になり、また、Ca/Oが0.50未満であるとAl主体の酸化物となって、何れの場合も酸化物の融点が溶鋼温度を超えることとなり、介在物の球状化は困難となる。なお、より球状化を促進するためには、Ca/Oを0.63〜1.13とすることが望ましい。
Ca / O: 0.50 to 1.30
In the oxide containing Al and Ca produced in the molten steel, when CaO and Al 2 O 3 coexist at a ratio of 1: 1, the melting point of the oxide is lowered below the molten steel temperature and liquefied. At this time, the surface tension acts on the oxide and becomes spherical. In order to control the form of the substance containing Al and Ca using this action, it is necessary to set Ca / O to 0.50 to 1.30. When Ca / O exceeds 1.30, it becomes an oxide mainly composed of CaO, and when Ca / O is less than 0.50, it becomes an oxide mainly composed of Al 2 O 3 . Since the melting point exceeds the molten steel temperature, it becomes difficult to spheroidize the inclusions. In order to further promote spheroidization, it is desirable to set Ca / O to 0.63 to 1.13.

Pcm*≦0.23かつPcm*−0.75C≧0.1
大入熱溶接HAZで生成される島状マルテンサイトは、Cのみならず他の合金元素が増加することによっても生成が助長される。Pcm*は、元来TMCP鋼の溶接低温割れ防止のためのパラメーターとして開発されたものである。しかし、このパラメーターはHAZの硬度と良い相関を持つことが知られており、HAZの硬度が高くなるとフェライト主体の組織からベイナイト主体の組織へと変化し、島状マルテンサイトも増加する。そこで本発明ではPcm*を島状マルテンサイト生成防止のための指標とするとともに、元来の溶接低温割れ特性を改善する指標とすることから、Pcm*を0.23以下と規定した。
一方、靭性もPcm*と関連する。ただし、靭性は鋼中のCが大きく影響する。よって、Pcm*にCの影響度を考慮し、Pcm*−0.75Cを靭性の指標とした。そして、靭性の確保を可能にするために、Pcm*−0.75C≧0.1を満足することが必要である。
Pcm * ≦ 0.23 and Pcm * −0.75C ≧ 0.1
The generation of island martensite generated by high heat input welding HAZ is facilitated not only by the increase of C but also other alloy elements. Pcm * was originally developed as a parameter for preventing weld cold cracking of TMCP steel. However, this parameter is known to have a good correlation with the hardness of HAZ. When the hardness of HAZ increases, the structure changes from a ferrite-based structure to a bainite-based structure, and island martensite also increases. Therefore, in the present invention, Pcm * is defined as 0.23 or less because Pcm * is used as an index for preventing the formation of island martensite and is used as an index for improving the original weld cold cracking characteristics.
On the other hand, toughness is also related to Pcm *. However, the toughness is greatly influenced by C in the steel. Therefore, considering the degree of influence of C on Pcm *, Pcm * −0.75C was used as an index of toughness. In order to ensure toughness, it is necessary to satisfy Pcm * −0.75C ≧ 0.1.

本発明に係る鋼板は、上記の元素を含有し、残部がFeおよび不純物からなる。ここで、不純物とは、鋼板を工業的に製造する際に鉱石やスクラップ等のような原料をはじめとして製造工程の種々の要因によって混入する成分であって、本発明に悪影響を与えない範囲で許容されるものを意味する。   The steel sheet according to the present invention contains the above elements, with the balance being Fe and impurities. Here, the impurity is a component that is mixed due to various factors in the manufacturing process including raw materials such as ore and scrap when industrially manufacturing a steel sheet, and does not adversely affect the present invention. It means what is allowed.

本発明に係る鋼板には、必要に応じて、次の元素の中から選んだ1種または2種以上を含有させることができる。   The steel plate according to the present invention may contain one or more selected from the following elements as necessary.

Cu:1.5%以下
Cuは、必要に応じて含有させることができる。Cuを含有させると、靭性を劣化させずに強度を上昇させることができる。しかし、その含有量が1.5%を超えると、鋼の焼き入れ性を過度に高め、HAZ靱性を損なう傾向が強くなる。したがって、Cuの含有量は1.5%以下とする。なお、Cuによる効果を得たい場合には、Cuを0.1%以上含有させることが好ましい。
Cu: 1.5% or less Cu can be contained as necessary. When Cu is contained, the strength can be increased without degrading toughness. However, if the content exceeds 1.5%, the hardenability of the steel is excessively increased and the tendency to deteriorate the HAZ toughness becomes strong. Therefore, the Cu content is 1.5% or less. In addition, when acquiring the effect by Cu, it is preferable to contain Cu 0.1% or more.

Ni:6.0%以下
Niは、必要に応じて含有させることができる。Niの適正量を添加することによって、溶接性およびHAZ靱性に悪影響を及ぼすこともなく、母材の強度、靱性を向上させることができる。しかし、その含有量が6.0%を超えると、構造用鋼板として極めて高価になって経済性を失うので、Ni含有量は6.0%以下とする。なお、Niによるこれらの効果を得たい場合には、Niを0.1%以上含有させることが望ましい。Niによる焼入性向上効果を得たい場合には、Niを0.1%以上含有させることが望ましい。特に、Cuを共存させる場合は圧延時のひび割れ(Cuチェッキング)を防止するために、0.1%以上のNiを含有させるのが望ましい。
Ni: 6.0% or less Ni can be contained as necessary. By adding an appropriate amount of Ni, the strength and toughness of the base material can be improved without adversely affecting the weldability and the HAZ toughness. However, if its content exceeds 6.0%, it becomes extremely expensive as a structural steel plate and loses its economic efficiency, so the Ni content is 6.0% or less. In addition, when obtaining these effects by Ni, it is desirable to contain Ni 0.1% or more. When it is desired to obtain the effect of improving hardenability with Ni, it is desirable to contain 0.1% or more of Ni. In particular, when Cu coexists, it is desirable to contain 0.1% or more of Ni in order to prevent cracking (Cu checking) during rolling.

Cr:1.0%以下
Crは、必要に応じて含有させることができる。Crの適正量を含有させることによって、焼入性を高めることができる。一方、Crの含有量が1.0%を超えると、他の成分条件を満足させても、HAZ靭性が劣化するので、Cr含有量は1.0%以下とする。なお、Crのこの様な効果を得たい場合には、Crの含有量を0.05%以上とするのが好ましい。
Cr: 1.0% or less Cr can be contained as necessary. By containing an appropriate amount of Cr, hardenability can be improved. On the other hand, if the Cr content exceeds 1.0%, the HAZ toughness deteriorates even if other component conditions are satisfied, so the Cr content is 1.0% or less. In order to obtain such an effect of Cr, the Cr content is preferably 0.05% or more.

Mo:0.8%以下
Moは、必要に応じて含有させることができる。Moを含有させると、母材の強度と靱性を向上させる効果がある。一方、Moの含有量が0.8%を超えると、特にHAZの硬度が高まり靱性を損なうので、Mo含有量は0.8%以下とする。なお、Moによる効果を得たい場合には、Moを0.05%以上含有させるのが好ましい。
Mo: 0.8% or less Mo can be contained as necessary. Inclusion of Mo has an effect of improving the strength and toughness of the base material. On the other hand, if the Mo content exceeds 0.8%, the hardness of the HAZ increases and the toughness is impaired, so the Mo content is set to 0.8% or less. In addition, when obtaining the effect by Mo, it is preferable to contain Mo 0.05% or more.

V:0.1%以下
Vは、必要に応じて含有させることができる。Vを含有させると、主に焼戻し時の炭窒化物析出により母材の強度を向上させる効果がある。一方、Vの含有量が0.1%を超えると、母材の性能向上効果が飽和し、靱性劣化を招くので、Vの含有量は0.1%以下とする。なお、Vによる効果を得たい場合には、Vを0.005%以上含有させるのが好ましい。
V: 0.1% or less V can be contained as necessary. Inclusion of V has an effect of improving the strength of the base material mainly due to carbonitride precipitation during tempering. On the other hand, if the V content exceeds 0.1%, the performance improvement effect of the base material is saturated and the toughness is deteriorated, so the V content is 0.1% or less. In addition, when obtaining the effect by V, it is preferable to contain V 0.005% or more.

Nb:0.05%以下
Nb母材組織の微細化に有効であり、母材の機械的性質を向上させる効果がある。一方、Nbの含有量が0.05%を超えると、母材ならびにHAZの靭性が劣化するので、Nbの含有量は0.05%以下とする。なお、Nbによる効果を得たい場合には、0.0040%以上含有させることが好ましい。
Nb: 0.05% or less Effective for refining the Nb base material structure, and has the effect of improving the mechanical properties of the base material. On the other hand, if the Nb content exceeds 0.05%, the base material and the toughness of the HAZ deteriorate, so the Nb content is set to 0.05% or less. In addition, when obtaining the effect by Nb, it is preferable to make it contain 0.0040% or more.

B:0.005%以下
Bは、必要に応じて含有させることができる。Bを含有させると焼入れ性を高めて母材やHAZの機械的性質を向上させる。一方、Bの含有量が0.005%を超えると、HAZ靭性や溶接性が劣化するので、Bの含有量は0.005%以下とする。なお、Bによる効果を得たい場合には、0.0003%以上含有させることが好ましい。
B: 0.005% or less B can be contained if necessary. When B is contained, the hardenability is increased and the mechanical properties of the base material and HAZ are improved. On the other hand, if the B content exceeds 0.005%, the HAZ toughness and weldability deteriorate, so the B content is set to 0.005% or less. In addition, when obtaining the effect by B, it is preferable to make it contain 0.0003% or more.

2.島状マルテンサイトについて
溶接熱影響部のミクロ組織において、島状マルテンサイトの面積率を1.0%未満とする。ここで、溶接熱影響部は入熱25kJ/mm以上の1層溶接により接合されたときの溶接熱影響部であり、島状マルテンサイトが1.0%以上であると、HAZ靭性が低下する。
2. Regarding the island martensite, the area ratio of the island martensite is less than 1.0% in the microstructure of the weld heat affected zone. Here, the weld heat-affected zone is a weld heat-affected zone when joined by one-layer welding with a heat input of 25 kJ / mm or more. If the island martensite is 1.0% or more, the HAZ toughness is lowered. .

3.AlとCaを含む介在物について
溶鋼中にAlおよびCaを添加した場合、AlおよびCaOの生成は避けられないが、このような介在物が生成すると母材靭性またはHAZ靭性の低下を招く。したがって、適切に介在物を制御する必要がある。以下で述べる介在物では、点列状につらなった介在物を一つの延伸した介在物と見なして差し支えない。
3. Inclusions containing Al and Ca When Al and Ca are added to the molten steel, the formation of Al 2 O 3 and CaO is inevitable, but if such inclusions are generated, the base metal toughness or HAZ toughness is reduced. Invite. Therefore, it is necessary to control the inclusion appropriately. In the inclusions described below, the inclusions arranged in the form of a dot line may be regarded as one extended inclusion.

粒径:5.0μm以下
介在物の大きさが粗大となると、シャルピー衝撃試験において破壊の起点となるため、介在物の粒径は5.0μm以下とする。粒径が小さいほど、破壊の起点として作用しにくくなるため、介在物の粒径の下限は規定しない。
Particle size: 5.0 μm or less When the size of the inclusion becomes coarse, it becomes the starting point of destruction in the Charpy impact test, so the particle size of the inclusion is 5.0 μm or less. Since the smaller the particle size, the less likely it is to act as a starting point for destruction, the lower limit of the particle size of inclusions is not specified.

アスペクト比:1.9以下
介在物が球状化し、アスペクト比(長径/短径)が1に近い場合、シャルピー試験時の同介在物および周辺組織への応力集中が緩和されるため、靭性が向上し、安定化する。一方でアスペクト比の大きい長径化した介在物がシャルピー試験片のノッチ近傍に存在する場合、応力集中源となり、そこから発生するき裂の伝播によって、靭性が著しく低下する。このため、アスペクト比を1.9以下とする。
Aspect ratio: 1.9 or less When inclusions are spheroidized and the aspect ratio (major axis / minor axis) is close to 1, stress concentration on the inclusions and surrounding tissues during the Charpy test is reduced, improving toughness. And stabilize. On the other hand, when an inclusion with a large aspect ratio and an elongated diameter is present in the vicinity of the notch of the Charpy specimen, it becomes a stress concentration source, and the toughness is remarkably reduced due to the propagation of cracks generated therefrom. Therefore, the aspect ratio is set to 1.9 or less.

本発明にかかるAlとCaを含む介在物の粒径およびアスペクト比は、以下のような方法で定量的に測定すればよい。すなわち、鋼板の圧延方向に対し平行な断面、好ましくは断面中心部から観察用試料を作成し、走査型電子顕微鏡(SEM)を用いて3000〜10000倍の倍率で、少なくとも100個以上のAlとCaを含む酸化物を観察すればよい。具体的には、画像処理により、粒径は介在物の投影面積を同じ面積の円と見立てた場合の円の直径(いわゆる円相当径)から求めればよく、アスペクト比は介在物の長径を短径で除した値の平均値を算出すればよい。なお、このときに測定した長径の平均値を、介在物の粒径として使用することに何ら問題はない。   The particle size and aspect ratio of the inclusion containing Al and Ca according to the present invention may be quantitatively measured by the following method. That is, a cross section parallel to the rolling direction of the steel sheet, preferably a sample for observation is prepared from the central portion of the cross section, and at least 100 Al or more at a magnification of 3000 to 10,000 times using a scanning electron microscope (SEM). What is necessary is just to observe the oxide containing Ca. Specifically, by image processing, the particle diameter can be obtained from the diameter of a circle (so-called equivalent circle diameter) when the projected area of the inclusion is regarded as a circle of the same area, and the aspect ratio is the short diameter of the inclusion. What is necessary is just to calculate the average value of the value divided by the diameter. Note that there is no problem in using the average value of the major axis measured at this time as the particle size of the inclusions.

AlおよびCaOの介在物は、Ca/Oが0.50〜1.30の範囲であれば、溶鋼中で球状化し、またこの組成の介在物は圧延によって破砕や延伸されることがないため、そのアスペクト比は小さくなる傾向がある。しかし、Ca/Oが0.50を下回るか、または1.30を超える場合には、酸化物は溶鋼中で完全に球状化せず、圧延中に破砕され点列状につらなった形状となり、アスペクト比が大きくなる傾向がある。 Inclusions of Al 2 O 3 and CaO may be spheroidized in molten steel if Ca / O is in the range of 0.50 to 1.30, and inclusions of this composition may be crushed or stretched by rolling. Therefore, the aspect ratio tends to be small. However, when Ca / O is less than 0.50 or more than 1.30, the oxide does not completely spheroidize in the molten steel, but becomes a shape that is crushed during rolling and pin-shaped. The aspect ratio tends to increase.

AlおよびCaOの介在物は、上述のAl含有量、Ca含有量およびO含有量の上限とCa/Oの条件を満たすように組成制御するとともに、後述するような製造方法により所定の粒径、アスペクト比を有する介在物とすることができる。 The inclusions of Al 2 O 3 and CaO are controlled in composition so as to satisfy the above-described Al content, upper limit of Ca content and O content, and Ca / O conditions. Inclusions having a particle size and an aspect ratio can be obtained.

4.鋼板の製造方法について
本発明に係る鋼板は、たとえば、その製鋼段階に特徴をもたせることによって製造することができる。すなわち、溶鋼中のAlが0.005〜0.08%となるようにAlを添加して脱酸し、さらに脱ガス装置(RH)にて15分以上還流処理した後、溶鋼温度を1530〜1670℃に保った状態でCaを添加してスラブを鋳造し、圧延することによって、本発明に係る鋼板の製造方法とすることができる。
4). About the manufacturing method of a steel plate The steel plate concerning the present invention can be manufactured by giving a feature to the steel-making stage, for example. That is, Al is added so that Al in the molten steel is 0.005 to 0.08% for deoxidation, and after refluxing for 15 minutes or more in a degassing apparatus (RH), the molten steel temperature is set to 1530 to A method for producing a steel sheet according to the present invention can be obtained by adding Ca and casting a slab in a state maintained at 1670 ° C. and rolling.

最初に添加するAlは脱酸力が強いため、溶鋼中の固溶酸素と結合し、Alを生成する。この時、Alは、溶鋼中のAlが0.005〜0.08%となるようにAlを添加する。溶鋼中のAlが0.005%未満の場合には、Alによる脱酸が不十分となり、Tiの酸化物が鋼中に生成してしまい、十分に鋼中にTiNを形成することができない。また、溶鋼中のAlが0.08%を超える場合には、余分なAlが鋼中に固溶Alとして残留し、母材、ならびにHAZの靭性が劣化する。 Since Al added first has a strong deoxidizing power, it combines with solute oxygen in the molten steel to produce Al 2 O 3 . At this time, Al is added so that Al in the molten steel is 0.005 to 0.08%. When Al in molten steel is less than 0.005%, deoxidation by Al becomes insufficient, Ti oxides are generated in the steel, and TiN cannot be sufficiently formed in the steel. Moreover, when Al in molten steel exceeds 0.08%, excess Al remains as solid solution Al in steel, and the toughness of a base material and HAZ deteriorates.

続いて脱ガス装置(RH)にて15分以上還流することにより、粗大なAlを浮上分離させる。ここで「脱ガス装置で15分以上処理」とは、投入したAlが溶鋼中に均一に混合した後に15分以上処理することを意味する。 Subsequently, by refluxing for 15 minutes or more in a degassing apparatus (RH), coarse Al 2 O 3 is floated and separated. Here, “treating for 15 minutes or more with a degassing apparatus” means that after the introduced Al is uniformly mixed in the molten steel, the treatment is performed for 15 minutes or more.

還流処理によりAlは基本的に除去されるが、一部溶鋼中に残留する。このような溶鋼にCaを添加すると、Al介在物が一部還元され、AlとCaを含む酸化物が形成される。このとき、溶鋼の温度を1530〜1670℃の範囲に制御することにより酸化物は液化が促進され、表面張力が作用するために、同介在物は球状化する。Caを添加する際の溶鋼の温度は、1580〜1620℃の範囲に制御することが望ましい。 Al 2 O 3 is basically removed by the reflux treatment, but partially remains in the molten steel. When Ca is added to such molten steel, a part of Al 2 O 3 inclusions is reduced, and an oxide containing Al and Ca is formed. At this time, by controlling the temperature of the molten steel in the range of 1530 to 1670 ° C., the liquefaction of the oxide is promoted and the surface tension acts, so that the inclusions are spheroidized. As for the temperature of the molten steel at the time of adding Ca, it is desirable to control in the range of 1580-1620 degreeC.

また、形成されるAlとCaを含む介在物の形態は、Caの添加量に依存する。介在物の粒径およびアスペクトを調整するために、Caは次の(4)式を満足するように添加する。なお、溶鋼中の酸素量Oはその場測定を行えばよい。
21+2.52・O−5≦CaA≦21+2.52・O+5・・・・・(4)
ただし、Caは溶鋼1トン当たりのCa添加量(g)を、そしてOは溶鋼中の酸素量(ppm)を、それぞれ表す。
Moreover, the form of the inclusion containing Al and Ca to be formed depends on the amount of Ca added. In order to adjust the particle size and aspect of the inclusion, Ca is added so as to satisfy the following formula (4). The oxygen content O M in the molten steel may be carried out in situ measurements.
21 + 2.52 · O M -5 ≦ CaA ≦ 21 + 2.52 · O M +5 ····· (4)
However, Ca A is Ca addition amount per molten steel one ton of (g), and O M is the amount of oxygen in the molten steel (ppm), expressed respectively.

さらにこの後は、鋳造工程、圧延工程を経て溶接熱影響部靭性に優れた溶接構造用鋼板を製造する。本製造方法においては、製鋼段階後の鋳造および圧延は、通常の方法により行うことが出来る。圧延以後のプロセスとして、通常圧延まま、制御圧延、さらにこれと制御冷却と焼もどしの組合せ、及び焼入れ・焼もどしの組合せなどを行っても、酸化物の形態、組成は変化しないため、靭性への影響にはなんら変化はないため差し支えない。   Thereafter, a steel sheet for welded structure having excellent weld heat affected zone toughness is produced through a casting process and a rolling process. In this production method, casting and rolling after the steel making stage can be performed by ordinary methods. As the process after rolling, the form and composition of the oxide does not change even if the normal rolling is used as it is, the controlled rolling, the combination of this with controlled cooling and tempering, and the combination of quenching and tempering. Since there is no change in the influence of, there is no problem.

表1に示す43種類の化学組成を有する試験鋼を、150kg真空溶解炉を用いて溶製した。このとき、溶鋼中にAlを添加して脱酸し、さらに脱ガス装置(RH)にて還流処理した後、Caを添加して、各150kgのスラブを鋳造した。このときの溶鋼中のAl含有量(質量%)、脱ガス処理時間(分)、Caを添加する際の溶鋼温度(℃)、溶鋼中の酸素量O(ppm)および溶鋼1トン当たりのCa添加量Ca(g)を表2に示す。 Test steels having 43 kinds of chemical compositions shown in Table 1 were melted using a 150 kg vacuum melting furnace. At this time, Al was added to the molten steel for deoxidization, and after refluxing with a degasser (RH), Ca was added to cast 150 kg of slabs. At this time, the Al content (% by mass) in the molten steel, the degassing time (minutes), the molten steel temperature (° C.) when adding Ca, the oxygen content O M (ppm) in the molten steel, and the ton of molten steel per ton Table 2 shows the Ca addition amount Ca A (g).

そして、各鋼塊を鍛造して、厚さ160mmの鋼片とした。次いで、各鋼片を表3に示す温度(℃)に加熱して熱間圧延し、各仕上温度で仕上げた後に冷却した。その後、表3に示す温度(℃)で1時間保持して焼戻し熱処理を施し、板厚25〜60mmの鋼板とした。   Each steel ingot was forged into a steel piece having a thickness of 160 mm. Next, each steel slab was heated to the temperature (° C.) shown in Table 3 and hot-rolled. After finishing at each finishing temperature, it was cooled. Then, it kept at the temperature (degreeC) shown in Table 3 for 1 hour, and performed the tempering heat processing, and it was set as the steel plate of plate | board thickness 25-60mm.

このようにして得た各鋼板について、開先加工した鋼板突き合わせ部に25〜95kJ/mmのエレクトロガスアーク溶接を実施した。また、それぞれの溶接熱影響部(HAZ)において小型試験片を採取し、鏡面に研磨した後、光学顕微鏡を用いて観察し、島状マルテンサイトの面積率およびAlとCaを含む粒径0.5〜5μmの介在物のアスペクト比を測定した。さらに、それぞれのHAZから、JIS4号シャルピー衝撃試験片を圧延方向と平行な方向に採取し、シャルピー衝撃試験を3回実施し、試験温度0℃での吸収エネルギーを測定した。これらの測定結果を合わせて表3に示す。なお、HAZにおけるシャルピー衝撃試験の目標値は試験温度0℃での吸収エネルギー値で100J以上である。   About each obtained steel plate, electrogas arc welding of 25-95 kJ / mm was implemented in the steel plate butt | matching part which carried out the groove process. In addition, a small test piece was collected at each welding heat affected zone (HAZ), polished to a mirror surface, and then observed using an optical microscope, and the area ratio of island martensite and the particle size containing Al and Ca were 0. The aspect ratio of inclusions of 5-5 μm was measured. Furthermore, from each HAZ, a JIS No. 4 Charpy impact test piece was taken in a direction parallel to the rolling direction, the Charpy impact test was performed three times, and the absorbed energy at a test temperature of 0 ° C. was measured. These measurement results are shown together in Table 3. In addition, the target value of the Charpy impact test in HAZ is 100 J or more as an absorbed energy value at a test temperature of 0 ° C.

なお、島状マルテンサイトの面積率およびAlとCaを含む粒径0.5〜5μmの介在物のアスペクト比に関しては、以下のようにして測定をした。   The area ratio of island martensite and the aspect ratio of inclusions containing Al and Ca having a particle size of 0.5 to 5 μm were measured as follows.

島状マルテンサイトの面積率については、レペラ腐食法を用いて現出した組織を倍率1000倍で光学顕微鏡観察し、得られた像を画像解析した。そして、20視野観察分の全面積に対する島状マルテンサイトの全面積割合を算出し、その値が1.0%未満か否かを判断した。   Regarding the area ratio of the island-like martensite, the structure revealed using the repeller corrosion method was observed with an optical microscope at a magnification of 1000 times, and the obtained image was subjected to image analysis. And the total area ratio of the island-like martensite with respect to the total area for 20 visual field observation was computed, and it was judged whether the value was less than 1.0%.

AlとCaを含む介在物については、倍率1000倍での光学顕微鏡観察によって得られた像を画像解析し、その中で粒径0.5〜5μmのもの(粒径を算出する場合には、介在物の面積から求めた円相当径を粒径とした。)に関して、そのアスペクト比(長径/短径の比)を、20視野観察して算出した。そして、個々のAlとCaを含む粒径0.5〜5μmの介在物のアスペクト比について、算術平均した値が1.9以下か否かを判断した。   For inclusions containing Al and Ca, image analysis was performed on an image obtained by observation with an optical microscope at a magnification of 1000 times, and a particle size of 0.5 to 5 μm among them ( The aspect ratio (ratio of major axis / minor axis) was calculated by observing 20 visual fields. And it was judged whether the value which carried out arithmetic average about the aspect-ratio of the inclusion of the particle size of 0.5-5 micrometers containing each Al and Ca is 1.9 or less.

表3において、鋼の化学組成(Ca/O比、Pcm*、Pcm*−0.75Cの規定を含む)並びに溶接熱影響部(HAZ)における島状マルテンサイトの面積率およびAlとCaを含む粒径0.5〜5μmの介在物のアスペクト比が本発明で規定する範囲内にある鋼No.1〜30については、いずれも入熱量25kJ/mm以上という大入熱溶接であるにもかかわらず、HAZにおけるシャルピー衝撃試験の目標値(試験温度0℃での吸収エネルギー値:100J以上)を満足しており、HAZの靱性に優れていることが分かる。   In Table 3, the chemical composition of the steel (including Ca / O ratio, Pcm *, Pcm * -0.75C) and the martensite area ratio in the weld heat affected zone (HAZ) and Al and Ca are included. Steel Nos. 1 to 30 in which the aspect ratio of inclusions having a particle size of 0.5 to 5 μm is within the range specified by the present invention are all high heat input welding with a heat input of 25 kJ / mm or more. In other words, it satisfies the target value of Charpy impact test in HAZ (absorbed energy value at a test temperature of 0 ° C .: 100 J or more), which indicates that the HAZ is excellent in toughness.

これに対して、鋼No.31〜39は鋼の化学組成(Ca/O比、Pcm*、Pcm*−0.75Cの規定を含む)が本発明鋼の規定から外れるため、HAZにおけるシャルピー衝撃試験の目標値(試験温度0℃での吸収エネルギー値:100J以上)に達せず、HAZの靱性が劣ることが分かる。   On the other hand, steel Nos. 31 to 39 have a Charpy impact in HAZ because the chemical composition of the steel (including Ca / O ratio, Pcm *, and Pcm * -0.75C) is out of the definition of the steel of the present invention. It can be seen that the target value of the test (absorbed energy value at a test temperature of 0 ° C .: 100 J or more) is not reached, and the toughness of the HAZ is inferior.

そして、鋼No.40〜43は鋼の化学組成(Ca/O比、Pcm*、Pcm*−0.75Cの規定を含む)は本発明鋼で規定する範囲内にあるが、HAZにおける島状マルテンサイトの面積率およびAlとCaを含む粒径0.5〜5μmの介在物のアスペクト比が本発明鋼の規定から外れるため、HAZにおけるシャルピー衝撃試験の目標値(試験温度0℃での吸収エネルギー値:100J以上)に達せず、HAZの靱性が劣ることが分かる。   Steel Nos. 40 to 43 have steel chemical compositions (Ca / O ratio, Pcm *, including Pcm * -0.75C) within the range defined by the steel of the present invention, but are in the form of islands in HAZ. Since the area ratio of martensite and the aspect ratio of inclusions containing Al and Ca and having a particle size of 0.5 to 5 μm are not included in the steel of the present invention, the target value of the Charpy impact test in HAZ (absorption at a test temperature of 0 ° C.) It can be seen that the toughness of HAZ is inferior.

本発明に係る鋼板は、入熱25kJ/mm以上の大入熱溶接を経ても溶接熱影響部(HAZ)の靱性に優れているので、重要構造物に用いることができる。   Since the steel plate according to the present invention is excellent in the toughness of the weld heat affected zone (HAZ) even after large heat input welding with a heat input of 25 kJ / mm or more, it can be used for an important structure.

Claims (3)

質量%で
C:0.02〜0.2%,
Si:0.03〜0.5%,
Mn:0.5〜2.0%,
P:0.02%以下,
S:0.002%未満,
Al:0.005〜0.08%,
Ti:0.003〜0.02%,
N:0.002〜0.009%,
O:0.001〜0.0035%,
Ca:0.0003〜0.0045%
を含有し、残部はFe及び不純物からなり、下記の(1)式、(2)式および(3)式を満足する化学組成を有する鋼板であって、溶接熱影響部のミクロ組織において、島状マルテンサイトの面積率が1.0%未満であり、かつ鋼中にAlとCaを含む粒径5.0μm以下の介在物が存在し、その介在物のアスペクト比が1.9以下であることを特徴とする溶接熱影響部の靱性に優れた鋼板。

0.50≦Ca/O≦1.30 ・・・・・(1)、
Pcm*≦0.23 ・・・・・(2)、
Pcm*−0.75C≧0.1 ・・・・・(3)
ここで、
Pcm*=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/3+Nb/2+23{B−10.8/14.1(N−Ti/3.4)}
ただし、B−10.8/14.1(N−Ti/3.4)≦0のとき、B−10.8/14.1(N−Ti/3.4)=0として取り扱う。
また、(1)式、(2)式および(3)式の元素記号はその元素の含有量(質量%)を示し、アスペクト比とは、鋼板の圧延方向に平行な断面で観察される介在物の長径を短径で除した値を意味する。
C: 0.02 to 0.2% by mass%,
Si: 0.03 to 0.5%,
Mn: 0.5 to 2.0%,
P: 0.02% or less,
S: less than 0.002%,
Al: 0.005 to 0.08%,
Ti: 0.003 to 0.02%,
N: 0.002 to 0.009%,
O: 0.001 to 0.0035%,
Ca: 0.0003 to 0.0045%
In which the balance is composed of Fe and impurities and has a chemical composition satisfying the following formulas (1), (2) and (3), in the microstructure of the weld heat affected zone: The area ratio of the martensite is less than 1.0%, and there are inclusions having a particle size of 5.0 μm or less containing Al and Ca in the steel, and the aspect ratio of the inclusions is 1.9 or less. A steel sheet excellent in toughness of the weld heat affected zone.
0.50 ≦ Ca / O ≦ 1.30 (1),
Pcm * ≦ 0.23 (2),
Pcm * −0.75C ≧ 0.1 (3)
here,
Pcm * = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 3 + Nb / 2 + 23 {B-10.8 / 14.1 (N-Ti / 3.4)}
However, when B-10.8 / 14.1 (N-Ti / 3.4) ≤0, it is handled as B-10.8 / 14.1 (N-Ti / 3.4) = 0.
The element symbols in the formulas (1), (2) and (3) indicate the content (% by mass) of the element, and the aspect ratio is an intervening observed in a cross section parallel to the rolling direction of the steel sheet. It means the value obtained by dividing the major axis of the product by the minor axis.
さらにFeの一部に代えて、質量%で
Cu:1.5%以下,
Ni:6.0%以下,
Cr:1.0%以下,
Mo:0.8%以下,
V:0.1%以下,
Nb:0.05%以下及び
B:0.005%以下
の中から選んだ1種または2種以上を含有することを特徴とする、請求項1に記載の溶接熱影響部の靱性に優れた鋼板。
Furthermore, instead of a part of Fe, Cu: 1.5% or less in mass%,
Ni: 6.0% or less,
Cr: 1.0% or less,
Mo: 0.8% or less,
V: 0.1% or less,
It is excellent in the toughness of the weld heat affected zone according to claim 1, characterized by containing one or more selected from Nb: 0.05% or less and B: 0.005% or less. steel sheet.
溶鋼中のAl含有量が0.005〜0.08質量%の範囲となるようにAlを添加して脱酸し、さらに脱ガス装置で15分以上処理した後、溶鋼温度1530〜1670℃で溶鋼中に下記(4)式を満足する量のCaを添加し、鋳造してなる請求項1または2の化学組成を有するスラブを熱間圧延することを特徴とする、溶接熱影響部の靱性に優れた鋼板の製造方法。

21+2.52・O−5≦Ca≦21+2.52・O+5・・・・・(4)
ただし、Caは溶鋼1トン当たりのCa添加量(g)を、そしてOは溶鋼中の酸素量(ppm)を、それぞれ表す。
Al is added so that the Al content in the molten steel is in the range of 0.005 to 0.08 mass% for deoxidation, and after further treatment with a degassing apparatus for 15 minutes or more, the molten steel temperature is 1530 to 1670 ° C. The toughness of the heat affected zone of a weld, characterized by hot-rolling a slab having the chemical composition according to claim 1 or 2 formed by adding Ca in an amount satisfying the following formula (4) into molten steel and casting: Steel sheet manufacturing method with excellent performance.
Serial 21 + 2.52 · O M -5 ≦ Ca A ≦ 21 + 2.52 · O M +5 ····· (4)
However, Ca A is Ca addition amount per molten steel one ton of (g), and O M is the amount of oxygen in the molten steel (ppm), expressed respectively.
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CN113272451B (en) * 2019-01-11 2022-07-05 日本制铁株式会社 Steel material
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