JP3699633B2 - Steel material excellent in toughness of heat affected zone and its manufacturing method - Google Patents

Description

Ｏ−０．１７×ＲＥＭ−０．４×Ｃａ−０．６６×Ｍｇ−０．８９×Ａｌ＜０の場合、
有効Ｔｉ量＝Ｔｉ−３．４×Ｎ ・ ・ ・（２）
【００２７】
（１）式および（２）式の各元素の係数は想定される酸化物および窒化物から化学量論的に決定された値である。１４００℃を超えるような溶融線近傍ＨＡＺでは、ＴｉとＮの存在形態はさらに複雑である。その理由は、溶接加熱時にＴｉＣとＴｉＮの多くが地鉄中に一旦固溶し、固溶したＴｉ、Ｎ、Ｃは溶接冷却時にＴｉＮあるいはＴｉＣとして再析出するとともに、一部は固溶のまま存在するからである。このようなＴｉとＮの存在形態を制御してＨＡＺ靭性の向上を目指すためには、ＴｉとＮの各々の量を規定するとともに、有効Ｔｉの概念を用いて他の成分とのバランスを図ることが重要である。図４は溶接入熱量が５０ｋＪ／ｍｍの場合をシミュレートした１４００℃加熱再現ＨＡＺ靭性に及ぼす有効Ｔｉ量の影響を示す。有効Ｔｉ濃度が−０．０１％〜＋０．００５％の範囲で良好な靭性を示す。すなわち、この範囲がＴｉＣの析出脆化とＮの固溶脆化の両方を回避できる適正な成分範囲であることを示している。有効Ｔｉ量ｇ−０．０１％未満の婆は固溶Ｎ量が過剰となり、有効Ｔｉ量が＋０．００５％を超える場合にはＴｉＣ析出量が過剰となり、ＨＡＺ靭性が劣化する。
【００２８】
このように有効Ｔｉを考慮することにより、さらに良好なＨＡＺ靭性が得られる。
【００２９】
Ｍｇは本発明の特徴的な元素であり、最も重要な役割を有する。Ｍｇを適量含有することで本発明における酸化物の分散状態を達成することができる。Ｍｇが０．００１％以下の場合、Ｍｇは０．５〜５μｍの酸化物に消費され、ＴｉＮの析出核であるＭｇとＡｌとから成る酸化物の個数が不足する。一方、酸化物として消費されるＭｇは０．００６％あれば十分であり、これを超えるＭｇが金属的に何ら効果をもたらさない。Ｍｇは蒸気圧が高くて酸化力が強い非常に活性な元素であることから、必要以上に鋼中に含有させることは製造コストの上昇を招き好ましくない。
【００３０】
Ｏは、ＴｉＮの析出核であるＭｇとＡｌとから成る酸化物の個数を確保する上で必要である。Ｏが０．００１％未満の場合、酸化物の個数が不足し、ＨＡＺ靭性が劣化する。一方、Ｏが０．００４％を超える場合、鋼の清浄度が低下して機械的性質が劣化する。
【００３１】
Ｎは、ピンニング粒子である複合析出ＴｉＮの個数を確保する上で重要であり、有効Ｔｉ量の適正範囲と相俟って狭い範囲に限定されなければならない。Ｎが０．００２５％未満の場合、ＴｉＮの個数が確保できない。一方、Ｎが０．００６％を超える場合、有効Ｔｉ量が適正範囲内にあっても実質的に固溶Ｎが過剰となり、ＨＡＺ靭性が低下する。
【００３２】
続いて、Ｃｕ、Ｎｉ、Ｍｏ、Ｃｒ、Ｎｂ、Ｖ、Ｂ、Ｃａ、ＲＥＭを添加する理由について説明する。
【００３３】
Ｃｕ、Ｎｉは溶接性およびＨＡＺ靭性に悪影響を及ぼすことなく母材の強度、靭性を向上させる。しかし、１．５％を超えると溶接性およびＨＡＺ靭性が劣化する。
【００３４】
Ｍｏ、Ｃｒは母材の強度、靭性を向上させる。しかし、１％を超えると母材の靭性、溶接性およびＨＡＺ靭性が劣化する。
【００３５】
Ｎｂは母材組織の微細化に有効な元素であり、母材の機械的性質を控除させる。しかし、０．０５％を超えるとＨＡＺ靭性が劣化する。
【００３６】
Ｖは母材の靭性を向上させる。しかし０．０５％を超えると溶接性およびＨＡＺ靭性が劣化する。
【００３７】
Ｂは焼き入れ正を高めて母材やＨＡＺの機械的性質を向上させる。しかし、０．００２％を超えて添加するとはＺ靭性や溶接性が劣化する。
【００３８】
ＣａとＲＥＭは酸化物や硫化物を形成して材質を改善する。ここで、ＲＥＭとは、Ｌａ、Ｃｅなどの希土類金属元素を示す。Ｃａを０．００４％を超えて添加しても材質改善効果が飽和する。ＲＥＭを０．００３％を超えて添加しても同様に材質改善効果が飽和する。必要以上に添加することは製造コストの増加を招き好ましくない。 ＣａとＲＥＭの両方を添加しても効果は同等である。
【００３９】
本発明鋼は、鉄鋼業の製鋼工程において所定の化学成分に調整し、連続鋳造を行い、鋳片を再加熱した後に圧延によって形状と母材材質を付与することで製造される。必要に応じ、鋼材に各種の熱処理を施して母材の材質を制御することも行われる。鋳片を再加熱することなく、ホットチャージ圧延することも可能である。
【００４０】
本発明で規定した酸化物の分散状態は、例えば、以下にような方法で定量的に測定される。０．０１以上０．５μｍ未満のＭｇとＡｌとから成る酸化物とＴｉＮの複合析出物の分散状態は、母材鋼材の任意の場所から抽出レプリカ試料を作製し、これを透過電子顕微鏡（ＴＥＭ）を用いて１００００〜５００００倍の倍率で少なくとも１０００μｍ²以上の面積にわたって観察し、対象となる大きさの複合析出物の個数を測定し、単位面積当たりの個数に換算する。この時、ＭｇとＡｌとから成る酸化物とＴｉＮの同定は、ＴＥＭに付属のエネルギー分散型Ｘ線分光法（ＥＤＳ）による組成分析と、ＴＥＭによる電子線回折像の結晶構造解析によって行われる。このような同定を測定するすべての複合析出物に対して行うことが煩雑な場合、簡易的に次の手順による。まず、四角い形状の析出物をＴｉＮとみなし、対象となる大きさのＴｉＮ中に酸化物が複合しているものの個数を上記の要領で測定する。次のこのような方法で個数を測定した複合析出物の内少なくとも１０個以上について上記の要領で同定を行い、ＭｇとＡｌとから成る酸化物とＴｉＮが複合的に存在している割合を算出する。そして、はじめに測定された複合析出物の個数にこの割合を掛け合わせる。鋼中の炭化物が以上のＴＥＭ観察を邪魔する場合、５００℃以下の熱処理によって炭化物を凝集・粗大化させ、対象となる複合析出物の観察を容易にすることができる。
【００４１】
０．５〜５μｍの酸化物の組成の測定例を次に示す。母材鋼材の任意の場所から小片試料を切り出し、これを１４００〜１４５０℃で１０分間以上保持することで酸化物以外の０．５〜５μｍの介在物を溶体化させ、その後水冷する。これを鏡面研磨し、光学顕微鏡を用いて１０００倍の倍率で少なくとも１ｍｍ²以上の面積にわたって観察する。対象となる酸化物の内少なくとも１０個以上についてＸ線マイクロアナライザー（ＥＰＭＡ）に付属の波長分散型分光法（ＷＤＳ）を用いて組成を分析し、酸化物の平均組成におけるＭｇとＡｌの含有量を質量％で求める。介在物の時、酸化物の分析値に地鉄のＦｅが検出される場合は、分析値からＦｅを除外して酸化物の平均組成を求める。
【００４２】
ＭｇとＣａは酸素との親和力が強く、蒸気圧も高いため、酸化され、酸化物として溶鋼中から除去されたり、蒸発してロスする。そのため添加歩留まりが低い。歩留まりを向上させるためには、酸化ロスと蒸発ロスを極力抑制することが重要である。
【００４３】
酸化ロスを小さくするためには、ＭｇやＣａ添加前の溶鋼中の酸素やスラグ中のＦｅＯ濃度とＭｎＯ濃度を低減することが重要である。本発明の鋼材には、Ｓｉ、Ｍｎ、Ａｌ、Ｔｉなどの脱酸元素が含まれており、これらの元素を添加した後にＭｇやＣａを添加することによって、酸化ロスを小さくすることができる。すなわち、ＭｇやＣａ以外の元素を添加し、溶鋼中の酸素濃度を低下させるため、ＭｇやＣａの酸化ロスが低減する。
【００４４】
スラグからの酸素供給によってＭｇやＣａが酸化ロスするのを抑制するため、スラグ中のＦｅＯ濃度とＭｎＯ濃度を低減することが有効である。ＭｇやＣａの添加前のスラグ中のＴ．Ｆｅ＋ＭｎＯを質量％で１０％を超えるとＭｇやＣａの歩留まりが著しく低下する。したがって、Ｔ．Ｆｅ＋ＭｎＯを１０％以下とする。この値は小さいほど、Ｍｇの酸化ロス防止には有効であり、５％以下が望ましい。
【００４５】
ＭｇやＣａの蒸発ロスを抑制するため、できるだけ精錬工程の末期に添加することが有利である。したがって、精錬工程で他の元素を添加したのちに、添加するのがよい。これは上述のように酸化ロスを抑制することからも有利である。ただし、成分の微調整のため、Ｍｇ、Ｃａ添加後に、Ｍｇ、Ｃａ以外の元素を少量添加しても構わない。
【００４６】
Ｍｇを溶鋼に添加するには、Ｍｇ含有合金、ＭｇＯ含有酸化物の１種もしくは、２種以上を用いる。
【００４７】
Ｍｇ含有合金、ＭｇＯ含有酸化物を溶鋼に添加する方法は、粉状にしたＭｇ合金、ＭｇＯ含有酸化物を不活性ガスを搬送ガスとして取鍋内の溶鋼中に吹き込む方法、塊状のものを取鍋内溶鋼、ＲＨ、ＤＨ等の真空槽内溶鋼に上方添加する方法、粉状のものを例えば鉄で被覆しワイヤ状にしたものを取鍋内溶鋼または／およびタンディッシュ内溶鋼または／およびモールド内溶鋼に添加する方法が考えられる。これらのいずれの方法を用いてもよく、その効果は同等である。さらに、これらの方法を組み合わせてもよい。
【００４８】
ＣａはＣａを含有する合金であれば何を用いても構わない。一般的にはＣａ−Ｓｉ合金が用いられる。
【００４９】
Ｍｇの添加時期は、Ｃａ添加前、Ｃａ添加と同時、Ｃａ添加後のいずれか、または、これらの組み合わせのいずれでもよい。
【００５０】
ＭｇとＣａを同時に添加する場合は、Ｍｇ含有合金または／およびＭｇＯ含有酸化物をＣａ含有合金と混合して添加する方法、ＭｇとＣａの両方を含有する合金を添加する方法のいずれの方法でもよく、その効果は同等である。
【００５１】
【実施例】
（実施例１）
表１に鋼材の化学成分と介在物の分散状態を、表２に鋼材の製造条件と機械的性質を示す。
【００５２】
表１のピンニング粒子の個数の測定は、鋼材母材の板厚中心部から抽出レプリカ試料を作製し、これを、３００００倍の倍率で２０００μｍ²の面積にわたってＴＥＭ観察することで行った。また、表１の０．５〜５μｍの大きさの酸化物の個数の測定は、同じく、鋼材母材の板厚中心部から小片を切り出して１４００℃で２０分間保定した後に水冷し、鏡面研磨面を１０００倍の倍率で４ｍｍ²の面積にわたって光学顕微鏡観察することで行った。さらに、ＥＰＭＡ−ＷＤＳによって、０．５〜５μｍの２０個の酸化物について組成を分析し、地鉄（Ｆｅ）の分析値を差し引いて平均組成を求め、Ｍｇ＋Ａｌの値を求めた。
【００５３】
本発明鋼は溶接入熱量が２０〜１００ｋＪ／ｍｍのエレクトロガス溶接部あるいはエレクトロスラグ溶接部の溶融線において従来にない良好なＨＡＺ靭性を有する。本発明鋼は、Ａｌ、Ｔｉ、Ｍｇ、Ｏ、Ｎの量を厳密に制御し、有効Ｔｉ量なる概念を用いてＨＡＺにおけるＴｉとＮの存在形態を適正化し、さらに、γ粒成長抑制に有効な酸化物の分散状態を有することで大入熱溶接においても良好なＨＡＺ靭性を達成している。一方、比較鋼は化学成分や酸化物の分散状態が適正でないため、母材およびＨＡＺの機械的性質が劣っている。
【００５４】
鋼１２は、Ｃの量が低すぎるために、鋼１３はＣ量が高すぎるために、母材およびＨＡＺの靭性が劣る。鋼１４は、Ｓｉ量が高すぎるためにＨＡＺ靭性が劣る。鋼１５はＭｎ量が低すぎるために、鋼１６はＭｎ量が高すぎるために、母材およびＨＡＺの靭性が劣る。鋼１７はＰ量が高すぎるために、母材およびＨＡＺの靭性が劣る。鋼１８は、Ｓ量が高すぎるために、母材およびＨＡＺの靭性が劣る。鋼１９はＡｌ量が低すぎるために０．５〜５μｍの酸化物中のＭｇ＋Ａｌが低く、ピンニング粒子の個数が少ないため、ＨＡＺ靭性が劣る。鋼２０はＴｉ量が低すぎるため、ピンニング粒子であるＴｉＮの個数が少なく、ＨＡＺ組織が著しく粗大化してＨＡＺ靭性が劣る。鋼２１はＴｉ量が高すぎるため、有効Ｔｉ量が適正範囲から外れ、ＴｉＣ析出脆化によってＨＡＺ靭性が劣る。鋼２２はＭｇ量が低すぎるため、ＴｉＮの析出核であるＭｇとＡｌとから成る酸化物の個数が少なく、γ粒が粗大化してＨＡＺ靭性が劣る。鋼２３は、Ｏ量が低すぎるため、ＭｇとＡｌとから成る酸化物の個数が少なく、γ粒が粗大化してＨＡＺ靭性が劣る。鋼２４はＯ量が高すぎるため、鋼の清浄度が悪くなり、破壊起点が増えてＨＡＺ靭性が劣る。鋼２５はＮ量が低すぎるためピンニング粒子であるＴｉＮの個数が少なく、ＨＡＺ組織が著しく粗大化してＨＡＺ靭性が劣る。鋼２６はＮ量が高すぎるため、有効Ｔｉ量の適正範囲から外れ、固溶Ｎが過剰となりＨＡＺ靭性が劣る。鋼２７と鋼２８は各々の元素は適正範囲にあるが、有効Ｔｉ量が不適当であるため、ＴｉＣ析出脆化あるいは固溶Ｎ脆化によりＨＡＺ靭性が劣る。
【００５５】
【表１】

【００５６】
【表２】

【００５７】
（実施例２）
表１の本発明鋼１の組成の鋼を溶製するに際して、Ｍｇ添加前の取鍋スラグ中のＴ．Ｆｅ＋ＭｎＯ濃度を種々変化させた。その時の成品におけるＭｇの歩留まりを図５に示す。Ｍｇの歩留まりは、Ｔ．Ｆｅ＋ＭｎＯ濃度が低いほど高い。Ｔ．Ｆｅ＋ＭｎＯ濃度を質量％で１０％以下望ましくは５％以下にすることでＭｇ歩留まりは著しく向上する。
【００５８】
（実施例３）
表１の本発明鋼１の組成の鋼を溶製するに際して、Ｓｉ、Ｍｎ、Ｔｉ、Ａｌ、Ｍｇ、Ｃａの添加時期を変化させた。その時の成品におけるＭｇとＣａの歩留まりを比較した結果を表３に示す。Ｍｇ添加前のスラグ中Ｔ．Ｆｅ＋ＭｎＯ濃度はいずれも２％であった。
【００５９】
Ｍｇ、Ｃａ以外の元素を添加した後に、ＭｇやＣａを添加した場合には、ＭｇとＣａの両方の歩留まりが１０％以上で良好であるのに対して、Ｍｇ、Ｃａ以外の元素をＭｇやＣａの添加後に添加した場合には、Ｍｇ、Ｃａのいずれかまたは、両方の歩留まりが低い。
【００６０】
【表３】

【００６１】
【発明の効果】
本発明により、大入熱溶接においても良好なＨＡＺ靭性を有する鋼材の製造が可能となり、各種の溶接構造物の安全性が格段に向上した。また、本発明鋼を使用することで高能率溶接の適用範囲が広がり、溶接施工コストを大幅に低減することが可能となった。
【図面の簡単な説明】
【図１】ＨＡＺ靭性に及ぼすγ粒径の影響を示す図である。
【図２】１４００℃加熱γ粒径に及ぼすピンニング粒子個数の影響を示す図である。
【図３】ピンニング粒子個数に及ぼす０．５〜５μｍの大きさの酸化物中Ｍｇ＋Ａｌの影響を示す図である。
【図４】１４００℃加熱ＨＡＺ靭性に及ぼす有効Ｔｉ量の影響を示す図である。
【図５】Ｍｇの添加歩留まりに及ぼすスラグ中のＴ．Ｆｅ＋ＭｎＯ濃度の影響を示す図である。[0001]
[Field of the Invention]
The present invention relates to a steel material having excellent heat-affected zone (HAZ) toughness. Since the steel material of the present invention has good HAZ toughness in a wide range of welding conditions from small heat input welding to ultra-high heat input welding, various steels such as buildings, bridges, shipbuilding, line pipes, construction machinery, offshore structures, tanks, etc. Used for welded steel structures.
[0002]
[Prior art]
In HAZ, the closer to the melting line, the higher the heating temperature at the time of welding. Particularly in the region heated to 1400 ° C. or more near the melting line, the heated austenite (γ) becomes extremely coarse, and the HAZ structure after cooling becomes It becomes coarse and deteriorates toughness. This tendency becomes more prominent as the welding heat input increases.
[0003]
As means for solving such problems, JP-A-60-245768, JP-A-60-152626, JP-A-63-1210235, JP-A-63-1210235, JP-A-2 JP-A-250917 and JP-A-1-73320 actively generate intragranular transformed ferrite with a core of Ti oxide or a composite precipitate of TiN and MnS inside coarse γ grains, and HAZ toughness. Has been trying to improve. However, it has been difficult to obtain sufficient toughness in the high heat input welding HAZ in which the heat input by welding exceeds 20 kJ / mm in the steel manufactured by these techniques.
[0004]
[Problems to be solved by the invention]
The problem to be solved by the present invention is to provide a steel material having good HAZ toughness and a method for producing the same even in high heat input welding in which the welding heat input exceeds 20 kJ / mm.
[0005]
[Means for Solving the Problems]
With the aim of improving high heat input welding HAZ toughness with a welding heat input exceeding 20 kJ / mm, the present inventors have made extensive studies on (1) suppression of heated γ grain growth and (2) appropriate forms of Ti and N. The present inventors have found a new metallurgical effect and have arrived at the present invention.
[0006]
The gist of the present invention is as follows.
[0007]
(1) In mass%,
C: 0.03% to 0.2%,
Si: 0.4% or less,
Mn: 0.5-2%
P: 0.015% or less,
S: 0.006% or less,
Al: more than 0.01% to 0.03% or less,
Ti: 0.007% to 0.02%,
Mg: more than 0.001% to 0.006% or less,
O: 0.001 to 0.004%,
N: 0.0025 to 0.006% is contained,
Furthermore, it contains either one or both of Ca: 0.004% or less, REM: 0.003% or less,
Balance has a chemical composition consisting of Fe and unavoidable impurities, TiN of 0.01 or more and less than 0.5μm enclosing the oxide composed of Mg and Al are present 10000 / mm ² or more, further, 0.5 The average value of the sum of the Mg content and the Al content in the oxide having a size of ˜5 μm is 30% or more by mass% , and further using the mass%, the following formula (1) or (2) A steel material having an excellent weld heat-affected zone toughness, characterized in that the effective Ti amount calculated in (1) is in the range of -0.01% to + 0.005% .
When O-0.17 × REM-0.4 × Ca-0.66 × Mg-0.89 × Al ≧ 0,
Effective Ti amount = Ti-2 × (O−0.17 × REM−0.4 × Ca−0.66 × Mg−0.89 × Al) −3.4 × N (1)
In the case of O−0.17 × REM−0.4 × Ca−0.66 × Mg−0.89 × Al <0,
Effective Ti amount = Ti-3.4 × N (2)
[0008]
(2) In mass%,
Cu: 1.5% or less,
Ni: 1.5% or less,
Mo: 1% or less,
Cr: 1% or less,
Nb: 0.05% or less,
V: 0.05% or less,
B: A steel material having excellent weld heat affected zone toughness as described in the above item (1), comprising one or more of 0.002% or less.
[0011]
( 3 ) T. in slag before addition of Mg and Ca. Fe + MnO is 10 mass% or less, The manufacturing method of the steel material excellent in the weld heat affected zone toughness as described in said (1) or (2) characterized by the above-mentioned .
[0012]
( 4 ) The method for producing a steel material having excellent weld heat affected zone toughness according to (1) or (2) above , wherein elements other than Mg and Ca are added and then Mg and Ca are added. "
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The new metallurgical effect discovered by this invention is demonstrated below.
[0014]
First, suppression of heated γ grain growth will be described. Because the heating temperature of the HAZ near the weld line reaches 1400 ° C, the ability to pin the movement of the γ grain boundary is significantly reduced by the dissolution and coarsening of carbides and nitrides, and the growth of γ grains can be avoided. There wasn't. Therefore, it was studied to suppress γ grain growth by pinning with an oxide that is thermally stable even at a high temperature of 1400 ° C. or higher. As a result, it has been found that by containing a small amount of Mg and Al in the steel, a large amount of an oxide composed of extremely fine Mg and Al having a size of 0.01 to 0.1 μm is produced in a large amount. It was. Furthermore, fine TiN with a size of 0.01 or more and less than 0.5 μm is deposited on the oxide of Mg and Al, and exhibits a very strong pinning force at a high temperature of 1400 ° C. or higher. Clarified what to do. The TiN composite particles are preferably more than 0.01 and 0.2 μm or less.
[0015]
Since this Mg and Al oxide has good lattice matching with TiN, it effectively acts as a TiN precipitation nucleus. And , when TiN is combined with an oxide composed of 0.01 to 0.1 μm of Mg and Al , the surface area is increased, and a stronger pinning force is expressed. FIG. 1 shows the influence of γ grain size on the HAZ toughness when the cooling time from 800 ° C. to 500 ° C. during welding cooling is 330 s. This cooling time corresponds to a case where electroslag welding is performed on a steel material having a plate thickness of 80 mm with a welding heat input of about 70 kJ / mm. As shown in FIG. 1, the HAZ toughness is improved as the γ grains are refined. This is because the grain boundary ferrite and ferrite side plate transformed from the γ grain boundary become smaller as the γ grain becomes finer, and the HAZ structure is refined. Such an effect is remarkable when the γ particle size is 150 μm or less. FIG. 2 shows the influence of the number of composite precipitated TiN of 0.01 μm or more and less than 0.5 μm on the γ grains when held at 1400 ° C. for 30 s. This heating condition corresponds to the vicinity of the melt line HAZ when electroslag welding is performed on a steel material having a plate thickness of 80 mm with a welding heat input of about 70 kJ / mm. From FIG. 2, when the number of composite precipitated TiN is less than 10,000 / mm ² , the γ grain size becomes 150 μm or more, and the HAZ structure is not sufficiently refined, so that good toughness cannot be obtained. Such a dispersion state of the composite precipitated TiN effective for suppressing the γ grain growth can be achieved by controlling the amounts of Mg, Al, Ti, O, and N within the range of the present invention.
[0016]
Next, the relationship between the oxide composition of 0.5 to 5 μm and the number of composite precipitated TiN of 0.01 μm or more and less than 0.5 μm will be described. FIG. 3 shows the sum of Mg content and Al content expressed in mass% in the oxide composition of 0.5 to 5 μm, and Mg + Al and the number of composite precipitated TiN of 0.01 μm or more and less than 0.5 μm. Show the relationship. When Mg + Al is 30% or more, the number of composite precipitated TiN is 10,000 pieces / mm ² or more. When Al and Mg in the molten steel are within the scope of the present invention, increasing Mg + Al in the oxide of 0.5 to 5 μm increases the Mg in the oxide of this size. This is because the number of oxides composed of Mg and Al of .about.0.5 .mu.m increases.
[0017]
Next, the reasons for limiting each chemical component will be described.
[0018]
The lower limit of C is 0.03% of the minimum amount for securing the strength and toughness of the base material and the welded portion. However, too much C lowers the toughness of the base metal and the HAZ and degrades the weldability, so the upper limit is made 0.2%.
[0019]
Si is contained in steel for deoxidation, but if it is too much, weldability and HAZ toughness deteriorate, so the upper limit is made 0.4%. The deoxidation of the present invention can be sufficiently performed with Ti alone, and in order to obtain good HAZ toughness, it is desirable to make Si 0.3% or less.
[0020]
Mn is indispensable for ensuring the strength and toughness of the base material and the welded portion, and the lower limit is 0.5%. However, if there is too much Mn, the upper limit is made 2% in order to deteriorate the HAZ toughness, promote the center segregation of the slab, and deteriorate the weldability.
[0021]
P is an impurity element in the steel of the present invention, and is made 0.015% or less. The reduction of P improves the mechanical properties of the base metal and the HAZ through the reduction of slab center segregation, and further suppresses HAZ grain boundary fracture.
[0022]
If the amount of S is too large, center segregation is promoted or a large amount of stretched MnS is generated, so that the mechanical properties of the base material and the HAZ are deteriorated. Therefore, the upper limit is made 0.006%.
[0023]
Al is important in controlling the number of oxides composed of 0.01 to 0.1 μm Mg and Al, which are precipitation nuclei of composite precipitated TiN that is pinning particles for γ grain growth. When Al is less than 0.01%, Mg + Al in the oxide of 0.5 to 5 μm is less than 30%, and the number of oxides of 0.01 to 0.1 μm of Mg and Al is 10,000 / mm. ^Since the number of composite precipitated TiN is insufficient, the γ grains are not sufficiently finely divided, and good HAZ toughness cannot be obtained. On the other hand, even if Al is added over 0.03%, the effect is saturated. Therefore, Al is more than 0.01% and 0.03% or less.
[0024]
Ti is important in controlling the dispersion state of the composite precipitated TiN as pinning particles, and must be limited to a narrow range in combination with an appropriate range of effective Ti concentration described later. When Ti is less than 0.007%, the number of TiN compositely deposited on the oxide composed of Mg and Al is less than 10,000 / mm ^2, and the effect of suppressing the growth of γ grains necessary for improving the HAZ toughness cannot be obtained. . On the other hand, when Ti exceeds 0.02%, even if the effective Ti is within an appropriate range, TiC is substantially excessively generated, and the HAZ toughness is lowered. TiN contributes to improving the strength and toughness of steel by refining the base metal structure through the suppression of γ grain growth during slab heating in thick plate rolling.
[0025]
Here, an appropriate form of Ti and N will be described. Ti in the steel combines with O to form an oxide, the remaining Ti combines with N to form TiN, and if there is any remaining Ti, it combines with C to form TiC, TiC causes precipitation embrittlement. On the other hand, if all Ti in the steel is consumed as an oxide and TiN, excess N that could not be combined with Ti is dissolved in the ground iron, but the solid solution N also causes embrittlement. Thus, the presence forms of Ti and N differ depending on whether or not the remaining Ti consumed as oxides and nitrides exists, and this greatly affects the toughness. In the present invention, the remaining Ti amount consumed as oxides and nitrides is defined by the formulas (1) and (2) using the mass% as the “effective Ti amount”.
[0026]
When O-0.17 × REM-0.4 × Ca-0.66 × Mg-0.89 × Al ≧ 0,

In the case of O−0.17 × REM−0.4 × Ca−0.66 × Mg−0.89 × Al <0,
Effective Ti amount = Ti-3.4 × N (2)
[0027]
The coefficient of each element in the formulas (1) and (2) is a value determined stoichiometrically from the assumed oxide and nitride. In the vicinity of the melting line HAZ exceeding 1400 ° C., the existence form of Ti and N is further complicated. The reason for this is that most of TiC and TiN are once dissolved in the ground iron during welding and heating, and Ti, N, and C that are dissolved again reprecipitate as TiN or TiC during welding cooling, and some of them remain in solid solution. Because it exists. In order to improve the HAZ toughness by controlling the existence forms of Ti and N, the amounts of Ti and N are specified, and the concept of effective Ti is used to balance with other components. This is very important. FIG. 4 shows the effect of the effective Ti amount on 1400 ° C. heating reproduction HAZ toughness simulating the case where the welding heat input is 50 kJ / mm. Good toughness is exhibited when the effective Ti concentration is in the range of -0.01% to + 0.005%. That is, this range is an appropriate component range that can avoid both precipitation embrittlement of TiC and solid solution embrittlement of N. When the effective Ti amount is less than g-0.01%, the amount of solid solution N is excessive, and when the effective Ti amount exceeds + 0.005%, the TiC precipitation amount is excessive and the HAZ toughness is deteriorated.
[0028]
Thus, by considering the effective Ti, further better HAZ toughness can be obtained.
[0029]
Mg is a characteristic element of the present invention and has the most important role. By containing an appropriate amount of Mg, the oxide dispersion state in the present invention can be achieved. When Mg is 0.001% or less, Mg is consumed in an oxide of 0.5 to 5 μm, and the number of oxides composed of Mg and Al, which are TiN precipitation nuclei, is insufficient. On the other hand, 0.006% of Mg consumed as an oxide is sufficient, and Mg exceeding this has no metallic effect. Since Mg is a very active element having a high vapor pressure and strong oxidizing power, inclusion in the steel more than necessary causes an increase in manufacturing costs and is not preferable.
[0030]
O is necessary for securing the number of oxides composed of Mg and Al, which are TiN precipitation nuclei. When O is less than 0.001%, the number of oxides is insufficient, and the HAZ toughness deteriorates. On the other hand, when O exceeds 0.004%, the cleanliness of the steel decreases and the mechanical properties deteriorate.
[0031]
N is important in securing the number of composite precipitated TiNs that are pinning particles, and must be limited to a narrow range in combination with an appropriate range of effective Ti amount. When N is less than 0.0025%, the number of TiN cannot be secured. On the other hand, when N exceeds 0.006%, even if the effective Ti amount is within an appropriate range, the solid solution N is substantially excessive, and the HAZ toughness is lowered.
[0032]
Next, the reason for adding Cu, Ni, Mo, Cr, Nb, V, B, Ca, and REM will be described.
[0033]
Cu and Ni improve the strength and toughness of the base material without adversely affecting the weldability and the HAZ toughness. However, if it exceeds 1.5%, weldability and HAZ toughness deteriorate.
[0034]
Mo and Cr improve the strength and toughness of the base material. However, if it exceeds 1%, the toughness, weldability and HAZ toughness of the base metal deteriorate.
[0035]
Nb is an effective element for refining the base material structure, and deducts the mechanical properties of the base material. However, if it exceeds 0.05%, the HAZ toughness deteriorates.
[0036]
V improves the toughness of the base material. However, if it exceeds 0.05%, weldability and HAZ toughness deteriorate.
[0037]
B enhances the quenching positive and improves the mechanical properties of the base material and HAZ. However, if added over 0.002%, the Z toughness and weldability deteriorate.
[0038]
Ca and REM improve the material by forming oxides and sulfides. Here, REM indicates a rare earth metal element such as La or Ce. Even if Ca is added in excess of 0.004%, the material improvement effect is saturated. Even if REM is added in excess of 0.003%, the material improvement effect is saturated similarly. Adding more than necessary is undesirable because it increases the production cost. The effect is equivalent even when both Ca and REM are added.
[0039]
The steel of the present invention is manufactured by adjusting the chemical composition to a predetermined chemical component in the steelmaking process of the steel industry, performing continuous casting, reheating the slab, and then giving the shape and base material by rolling. If necessary, various heat treatments are performed on the steel material to control the material of the base material. Hot charge rolling is also possible without reheating the slab.
[0040]
The oxide dispersion state defined in the present invention is quantitatively measured, for example, by the following method. The dispersion state of the composite precipitates of oxide and TiN composed of Mg and Al of 0.01 or more and less than 0.5 μm is obtained by preparing an extracted replica sample from an arbitrary place of the base steel, and using a transmission electron microscope (TEM). ) Is observed over an area of at least 1000 μm ² at a magnification of 10,000 to 50,000 times, and the number of composite precipitates of a target size is measured and converted to the number per unit area. At this time, the identification of oxide and TiN composed of Mg and Al is performed by composition analysis by energy dispersive X-ray spectroscopy (EDS) attached to TEM and crystal structure analysis of electron diffraction image by TEM. When it is complicated to perform such identification on all the composite precipitates to be measured, the following procedure is simply performed. First, a square-shaped precipitate is regarded as TiN, and the number of oxides in a target size of TiN is measured as described above. At least 10 or more of the composite precipitates whose number was measured by the following method are identified as described above, and the ratio of the composite of Mg and Al oxide and TiN is calculated. To do. Then, this ratio is multiplied by the number of composite precipitates measured first. When the carbide in steel interferes with the above TEM observation, the carbide can be aggregated and coarsened by a heat treatment at 500 ° C. or less, and the target composite precipitate can be easily observed.
[0041]
An example of measuring the composition of an oxide of 0.5 to 5 μm is shown below. A small piece sample is cut out from an arbitrary place of the base steel material, and this is held at 1400 to 1450 ° C. for 10 minutes or more to form 0.5 to 5 μm inclusions other than oxides, and then water-cooled. This is mirror-polished and observed over an area of at least 1 mm ² at a magnification of 1000 using an optical microscope. At least 10 or more of the target oxides are analyzed for composition using wavelength dispersion spectroscopy (WDS) attached to an X-ray microanalyzer (EPMA), and the contents of Mg and Al in the average composition of the oxides Is determined by mass%. In the case of inclusions, when Fe of iron is detected in the analysis value of the oxide, the average composition of the oxide is obtained by excluding Fe from the analysis value.
[0042]
Since Mg and Ca have a strong affinity for oxygen and a high vapor pressure, they are oxidized and removed as oxides from the molten steel, or evaporated and lost. Therefore, the addition yield is low. In order to improve the yield, it is important to suppress oxidation loss and evaporation loss as much as possible.
[0043]
In order to reduce the oxidation loss, it is important to reduce the oxygen in the molten steel before the addition of Mg and Ca and the FeO concentration and the MnO concentration in the slag. The steel material of the present invention contains deoxidizing elements such as Si, Mn, Al, and Ti. By adding these elements and then adding Mg and Ca, the oxidation loss can be reduced. That is, since elements other than Mg and Ca are added to lower the oxygen concentration in the molten steel, the oxidation loss of Mg and Ca is reduced.
[0044]
It is effective to reduce the FeO concentration and the MnO concentration in the slag in order to suppress the oxidation loss of Mg and Ca due to the oxygen supply from the slag. T. in slag before addition of Mg or Ca. If the Fe + MnO content exceeds 10% by mass, the yield of Mg or Ca is significantly reduced. Therefore, T.W. Fe + MnO is 10% or less. The smaller this value is, the more effective it is for preventing oxidation loss of Mg, and 5% or less is desirable.
[0045]
In order to suppress the evaporation loss of Mg and Ca, it is advantageous to add as much as possible at the end of the refining process. Therefore, it is preferable to add after adding other elements in the refining process. This is also advantageous from suppressing oxidation loss as described above. However, a small amount of elements other than Mg and Ca may be added after adding Mg and Ca for fine adjustment of the components.
[0046]
In order to add Mg to molten steel, one or more of Mg-containing alloys and MgO-containing oxides are used.
[0047]
The Mg-containing alloy and MgO-containing oxide are added to the molten steel by a powdered Mg alloy, MgO-containing oxide being blown into the molten steel in the ladle using an inert gas as a carrier gas, Method of adding upward to the molten steel in the vacuum chamber, such as molten steel in the pan, RH, DH, etc., the powdered material coated with iron, for example, in the form of a wire, the molten steel in the pan and / or the molten steel in the tundish or / and the mold A method of adding to the inner molten steel is conceivable. Any of these methods may be used, and the effects are equivalent. Furthermore, these methods may be combined.
[0048]
As long as Ca is an alloy containing Ca, anything may be used. Generally, a Ca—Si alloy is used.
[0049]
The timing of adding Mg may be before Ca addition, at the same time as Ca addition, after Ca addition, or any combination thereof.
[0050]
When adding Mg and Ca at the same time, either a method of adding an Mg-containing alloy or / and an MgO-containing oxide mixed with a Ca-containing alloy or a method of adding an alloy containing both Mg and Ca Well, the effect is equivalent.
[0051]
【Example】
(Example 1)
Table 1 shows the chemical composition of steel materials and the dispersion state of inclusions, and Table 2 shows the manufacturing conditions and mechanical properties of the steel materials.
[0052]
The number of pinning particles shown in Table 1 was measured by preparing an extracted replica sample from the center of the plate thickness of the steel base material, and observing this with a 30,000 magnification over an area of 2000 μm ² . In addition, the number of oxides having a size of 0.5 to 5 μm in Table 1 is similarly measured by cutting out a small piece from the center part of the thickness of the steel base material and holding it at 1400 ° C. for 20 minutes, followed by water cooling and mirror polishing. The surface was observed by optical microscope observation over an area of 4 mm ² at a magnification of 1000 times. Further, the composition of 20 oxides of 0.5 to 5 μm was analyzed by EPMA-WDS, and the average composition was determined by subtracting the analysis value of ground iron (Fe), and the value of Mg + Al was determined.
[0053]
The steel according to the present invention has a good HAZ toughness that has not been seen in the past in the melt line of an electrogas weld or electroslag weld having a welding heat input of 20 to 100 kJ / mm. The steel of the present invention strictly controls the amount of Al, Ti, Mg, O, N, optimizes the existence form of Ti and N in HAZ using the concept of effective Ti amount, and is also effective in suppressing γ grain growth As a result of having a stable oxide dispersion, good HAZ toughness is achieved even in high heat input welding. On the other hand, since the comparative steel is not suitable for chemical components and oxide dispersion, the mechanical properties of the base material and HAZ are inferior.
[0054]
Since the amount of C in the steel 12 is too low and the amount of C in the steel 13 is too high, the toughness of the base material and the HAZ is inferior. Steel 14 is inferior in HAZ toughness because the amount of Si is too high. Steel 15 has an excessively low Mn content, and steel 16 has an excessively high Mn content, so that the toughness of the base material and HAZ is inferior. Since the steel 17 has an excessively high P content, the toughness of the base material and the HAZ is inferior. Steel 18 is inferior in the toughness of the base material and HAZ because the amount of S is too high. Since the steel 19 has an Al amount that is too low, Mg + Al in the oxide of 0.5 to 5 μm is low, and the number of pinning particles is small, so that the HAZ toughness is inferior. Since the amount of Ti in the steel 20 is too low, the number of TiN as pinning particles is small, the HAZ structure becomes extremely coarse, and the HAZ toughness is inferior. Since the steel 21 has an excessively high amount of Ti, the effective amount of Ti deviates from the appropriate range, and the HAZ toughness is inferior due to TiC precipitation embrittlement. Since the amount of Mg in the steel 22 is too low , the number of oxides composed of Mg and Al, which are TiN precipitation nuclei, is small, and the γ grains become coarse and the HAZ toughness is inferior. Since the amount of O in the steel 23 is too low , the number of oxides composed of Mg and Al is small, the γ grains are coarsened, and the HAZ toughness is inferior. Since the amount of O in the steel 24 is too high, the cleanliness of the steel is deteriorated, the fracture starting point is increased, and the HAZ toughness is inferior. In Steel 25, since the amount of N is too low, the number of TiN as pinning particles is small, the HAZ structure becomes extremely coarse, and the HAZ toughness is inferior. Since the amount of N in the steel 26 is too high, it deviates from the appropriate range of the effective Ti amount, so that the solute N becomes excessive and the HAZ toughness is inferior. In Steel 27 and Steel 28, each element is in an appropriate range, but since the effective Ti amount is inappropriate, the HAZ toughness is inferior due to TiC precipitation embrittlement or solute N embrittlement.
[0055]
[Table 1]

[0056]
[Table 2]

[0057]
(Example 2)
When melting the steel of the composition of the present invention steel 1 in Table 1, T.O. in the ladle slag before addition of Mg. Various changes were made to the Fe + MnO concentration. The yield of Mg in the product at that time is shown in FIG. The yield of Mg is T.W. The lower the Fe + MnO concentration, the higher. T.A. When the Fe + MnO concentration is 10% or less by mass%, desirably 5% or less, the Mg yield is remarkably improved.
[0058]
(Example 3)
When melting steel of the composition of the present invention steel 1 in Table 1, the addition time of Si, Mn, Ti, Al, Mg, Ca was changed. Table 3 shows the result of comparing the yields of Mg and Ca in the product at that time. T. in slag before Mg addition. The Fe + MnO concentration was 2% in all cases.
[0059]
When elements other than Mg and Ca are added and then Mg and Ca are added, the yield of both Mg and Ca is good at 10% or more, whereas elements other than Mg and Ca are Mg or Ca. When added after the addition of Ca, the yield of either or both of Mg and Ca is low.
[0060]
[Table 3]

[0061]
【The invention's effect】
According to the present invention, it is possible to produce a steel material having good HAZ toughness even in high heat input welding, and the safety of various welded structures is greatly improved. Further, the use of the steel of the present invention broadens the application range of high-efficiency welding, and can greatly reduce the welding construction cost.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of γ particle size on HAZ toughness.
FIG. 2 is a diagram showing the influence of the number of pinning particles on the 1400 ° C. heated γ particle size.
FIG. 3 is a diagram showing the influence of Mg + Al in an oxide having a size of 0.5 to 5 μm on the number of pinning particles.
FIG. 4 is a diagram showing the effect of effective Ti amount on 1400 ° C. heated HAZ toughness.
FIG. 5 shows the effect of T. in slag on the yield of addition of Mg. It is a figure which shows the influence of Fe + MnO density | concentration.

Claims (4)

% By mass
C: 0.03% to 0.2%,
Si: 0.4% or less,
Mn: 0.5-2%
P: 0.015% or less,
S: 0.006% or less,
Al: more than 0.01% to 0.03% or less,
Ti: 0.007% to 0.02%,
Mg: more than 0.001% to 0.006% or less,
O: 0.001 to 0.004%,
N: 0.0025 to 0.006% is contained,
Furthermore, it contains either one or both of Ca: 0.004% or less, REM: 0.003% or less,
Balance has a chemical composition consisting of Fe and unavoidable impurities, TiN of 0.01 or more and less than 0.5μm enclosing the oxide composed of Mg and Al are present 10000 / mm ² or more, further, 0.5 The average value of the sum of the Mg content and the Al content in the oxide having a size of ˜5 μm is 30% or more by mass% , and further, using the mass%, the following formula (1) or formula (2) A steel material having an excellent weld heat-affected zone toughness, characterized in that the effective Ti amount calculated in (1) is in the range of -0.01% to + 0.005% .
When O-0.17 × REM-0.4 × Ca-0.66 × Mg-0.89 × Al ≧ 0,
Effective Ti amount = Ti-2 × (O−0.17 × REM−0.4 × Ca−0.66 × Mg−0.89 × Al) −3.4 × N (1)
In the case of O−0.17 × REM−0.4 × Ca−0.66 × Mg−0.89 × Al <0,
Effective Ti amount = Ti-3.4 × N (2)   In addition, Cu: 1.5% or less, Ni: 1.5% or less, Mo: 1% or less, Cr: 1% or less, Nb: 0.05% or less, V: 0.05% or less, B: The steel material excellent in weld heat affected zone toughness according to claim 1, characterized by containing one or more of 0.002% or less. T. in slag before addition of Mg and Ca. Fe + MnO is 10 mass% or less, The manufacturing method of the steel material excellent in the weld heat affected zone toughness of Claim 1 or 2 characterized by the above-mentioned. The method for producing a steel material having excellent weld heat affected zone toughness according to claim 1 or 2 , wherein Mg and Ca are added after adding elements other than Mg and Ca.

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