JP4160763B2 - Steel plate with low welding distortion - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、造船、海洋構造物、建築、橋梁、土木等に用いられる鋼板の溶接作業時に発生する溶接歪みの少ない鋼板、および溶接歪みを軽減もしくは溶接歪み発生防止を可能とする溶接構造用高降伏点型高張力鋼に関する。
【０００２】
【従来の技術】
従来、各種鋼構造物における鋼材の溶接時には溶接金属の凝固収縮およびその後の冷却と相変態による収縮、膨張により、例えば、隅肉溶接の継ぎ手形状の場合には、角変形と呼ばれる面外変形（残留変形）が発生する。この残留変形は、圧縮荷重が負荷されると巣座屈強度の低下をもたらし、構造強度を低下させる。また、この残留変形を拘束治具により強制的に防止しようとする場合には過大な残留応力の発生、寸法精度の低下という現象が生じることとなる。そこで、例えば、溶接学会誌１９８８年第５２巻第４〜９号の掲載されている「溶接変形の発生とその防止」に見られるように、溶接時に発生した残留応力を局所的な加熱により矯正する手法が経験的に多数提案され実施されている。しかし、溶接部の再加熱によって鋼材の材質劣化を生じること、矯正作業に多大の時間と費用は実質的に重大な障害となっており、これを軽減もしくは省略することが可能な鋼材の開発が切望されている。
【０００３】
溶接時に発生する残留応力や変形の発生機構については、「溶接構造要覧」１９８８（黒木出版）や、“Analysis of Welded Structures”1988, PERGAMON PRESS 、に詳細に述べられている。しかし、溶接変形は主として溶接時の入熱に対する部材の幾何学的形状によって決定されるというように、使用される鋼材が具備する特性に注目したものでない。また、溶接学会誌１９７６年第４５巻第１号に掲載されている「構造用材料の溶接残留応力・溶接変形に及ぼす溶接条件の影響」に見られるように、溶接入熱の小さな場合、溶接変形の因子として変態膨張より寧ろ降伏応力が考えられることが列挙されており、例として、ＨＴ８０と９％Ｎｉ鋼では降伏応力の小さい９％Ｎｉ鋼の方が溶接変形は大きいことが開示されている。しかし、通常、鋼構造物に使用される普通鋼材の成分、組織および降伏強度に対してそのまま適用できるという知見ではなく、更に、変形に対する材料の強度をＣｒ，Ｍｏ，Ｖ，Ｎｂ等の合金元素を添加することによって推測されることが特開平４−２２５９７号公報に開示されてはいるが実際に確認されている訳ではなく、更に、溶接変形に対して鋼材の成分および組織との関係で注目されたものでもない。
【０００４】
そこで、上記問題点を解決するための一つの技術として、ミクロ組織に所定量以上のベイナイトを含ませることで降伏強度を高め、溶接歪みを低減できる鋼板が特開平６−１７２９２１号公報に開示されているが、構成組織のサイズや炭窒化物の状態については何ら言及しておらず、必ずしも実用上十分な材質特性、および溶接歪み低減効果を得るに至っていないことを本発明者らは知見した。
【０００５】
【発明が解決しようとする課題】
本発明は、鋼構造物の溶接において溶接変形を低減し、また、溶接変形防止作業および形状矯正作業の多大な費用と労力の低減を可能とする溶接歪みの少ない鋼板を提供する。
【０００６】
【課題を解決するための手段】
本発明は、上記課題を解決するためになされたもので、その要旨は以下の通りである。
（１）質量％で、Ｃ：０．０５〜０．２０％、Ｓｉ：０．０５〜１．０％、Ｍｎ：０．６〜２．０％、Ｐ：≦０．０２５％、Ｓ：≦０．０１０％、Ａｌ：０．００５〜０．１０％、Ｎ：０．００１０〜０．００８０％を含み、残部Ｆｅおよび不可避的不純物からなり、ミクロ組織が、平均粒径２０μｍ以下のベイナイト及び／又はマルテンサイトを面積％で２０％以上、平均粒径２０μｍ以下のフェライトおよび／又はパーライト組織からなり、更に、平均粒径０．２μｍ以下の炭窒化物を面積％で０．１〜１０％含むことを特徴とする溶接歪みの少ない鋼板。
（２）前記ベイナイト及び／又はマルテンサイトのミクロ組織を面積％で５０％以上を含み降伏強度４５０Ｎ／mm2 以上を有することを特徴とする（１）記載の溶接歪みの少ない鋼板。
（３）前記鋼板が、質量％で、さらに、Ｎｂ：０．００３〜０．０５０％、Ｍｏ：０．０５〜０．５０％、Ｖ：０．００５〜０．１００％、Ｗ：０．０５〜０．５０％、Ｔａ：０．０５〜０．５０％、Ｃｕ：０．０５〜１．５％、Ｎｉ：０．０５〜３．５％、Ｃｒ：０．０５〜１．０％、Ｔｉ：０．００５〜０．１０％、Ｂ：０．０００２〜０．００３０％、Ｃａ：０．０００３〜０．００５０％、ＲＥＭ：０．０００５〜０．００６０％の１種または２種以上を含むことを特徴とする（１）又は（２）記載の溶接歪みの少ない鋼板。
【０００７】
【発明の実施の形態】
本発明における溶接歪みの少ない鋼板のミクロ組織と高降伏強度を得るための化学組成の添加量とその限定理由を以下に説明する。
【０００８】
Ｃは、ベイナイトまたはマルテンサイト組織化および強度確保のために添加し、その効果の限界から下限を０．０５％とし、また母材靭性への悪影響、溶接性の劣化、高炭素島状マルテンサイトの生成による溶接継手靭性の劣化を防止するために０．２０％を上限とした。
【０００９】
Ｓｉは、脱酸上必要な元素であり、更に強度を高める上で有効な元素であるので０．０５％を下限とし、溶接性、溶接継手靭性の劣化を防止するために１．０％を上限とした。
【００１０】
Ｍｎは、ベイナイトまたはマルテンサイト組織化および強度と靭性を確保のために０．６％を下限として添加し、多量の添加は焼入れ性を増加させ硬化組織を生成させ、また溶接性を劣化させるので２．０％を上限とする。
【００１１】
不純物であるＰおよびＳは、それぞれ母材および溶接継手靭性を所望のレベルに維持するため、Ｐ≦０．０２５％、Ｓ≦０．０１０％とした。
【００１２】
Ａｌは、脱酸上必要な元素であるので０．００５％を下限とし、多量の添加は鋼の清浄度を損なうので０．１０％を上限とした。
【００１３】
Ｎは、Ａｌを結合し、鋼材の結晶粒を微細化し、靭性を高めるのに有効な０．００１０％を下限とし、多量に添加すると鋼材の靭性を損なうので０．００８０％を上限とした。
【００１４】
更に、上記元素に加え、Ｎｂ，Ｍｏ，Ｖ，Ｗ，Ｔａ，Ｃｕ，Ｎｉ，Ｃｒ，Ｔｉ，Ｂ，Ｃａ，ＲＥＭの１種または２種以上を含有することで、本発明による溶接歪みの少ない鋼板のミクロ組織と高降伏強度を得ることができる。その各添加元素の添加理由を説明する。
【００１５】
Ｎｂは、ベイナイトまたはマルテンサイト組織化、組織微細化、炭窒化物形成により降伏強度を高めるのに有効な元素であるので、０．００３％を下限とし、多量の添加は溶接継手靭性を損なうので上限を０．０５０％とした。
【００１６】
Ｍｏ，Ｖ，Ｗ，Ｔａは、ベイナイトまたはマルテンサイト組織化、炭窒化物形成により降伏強度を高めるのに有効な元素であるので、Ｍｏ，Ｗ，Ｔａについては０．０５％、Ｖについては０．００５をそれぞれ下限とし、多量の添加は溶接性、溶接継手靭性を損なうので上限をＭｏ，Ｗ，Ｔａについて０．５０％、Ｖについては０．１００％をそれぞれ上限とした。
【００１７】
Ｃｕ及びＣｒは、ベイナイトまたはマルテンサイト組織化とともに強度を高めるのに有効な元素であるので、０．０５％をそれぞれ下限とし、多量の添加は溶接継手靭性を損なうので、Ｃｕについては１．５％、Ｃｒについては１．０％を上限とした。
【００１８】
Ｎｉは、靭性を損なうことなくベイナイトまたはマルテンサイト組織化するのに有効な元素であるのが高価な元素であるので経済性の点から０．０５〜３．５％の範囲で添加する。
【００１９】
Ｔｉは、溶接熱影響部の靭性確保に有効な元素であるため０．００５％を下限とし、更に過剰な添加による靭性の劣化を防止するために０．１０％を上限とする。
【００２０】
Ｂは、ベイナイトまたはマルテンサイト組織化とともに鋼材の強度を高め、かつ溶接熱影響部の結晶粒微細化に有効な元素であるが過剰な添加は靭性を劣化させるで０．０００２〜０．００３０％の範囲に限定した。
【００２１】
Ｃａは、硫化物の形態制御に有効な元素であるが多量の添加は鋼の清浄度を損なうので０．０００３〜０．００５０％の範囲に限定した。また、ＲＥＭは、溶接熱影響部の組織を微細化し、靭性を高めるのに有効な元素であるが、多量の添加は鋼の清浄度を損なうので０．０００５〜０．００６０％の範囲に限定した。
【００２２】
次に、本発明において規定した鋼のミクロ組織の限定理由について説明する。一般に溶接時には、ビードに近い位置は高温にさらされるため、熱応力が降伏強度を直ちに超えて塑性変形が進む。この部分は冷却時に収縮するために、変形を生じる。一方、溶接ビードから離れた位置では鋼板温度があまり上昇しないため、ビードの位置よりかなり遅れて熱応力が降伏強度を超える。このときの降伏強度が十分高ければ、溶接ビードに近い位置が収縮して変形を起こそうとしても、その変形の大きな抵抗となる。そこで本発明者らは、種々の条件で実験および計算機シミュレーションを行い、溶接変形を最小限に抑えるためには、４００〜６００℃という中温域における降伏強度を高めることが有効であるとの知見を得た。
【００２３】
一般に常温強度は、結晶粒径、ベイナイトやマルテンサイト等の低温変態相の分率、合金元素固溶量、炭窒化物に代表される分散・析出粒子のサイズおよび量に支配される。一方、６００℃超の高温強度は、組織因子の寄与が小さくなり、析出物粒子の状態（サイズ、量、分布状態、安定性）によって概ね決まってしまう。したがって４００〜６００℃の中温降伏強度は、組織の影響を受けつつ、析出物の寄与が大きくなってくる領域であり、中温強度を高めるためには、構成組織の粒径を微細に保ち、ベイナイトまたはマルテンサイトを一定量以上確保した上で、微細な分散・析出粒子を多量に存在せしめる必要があることを知見し、本発明をなすに至った。ここで、結晶粒径というのは、組織がフェライト及び／又はパーライトの場合はフェライト、パーライトの平均粒径、ベイナイトまたはマルテンサイトの場合は、旧オーステナイト粒径ではなく、結晶方位のほぼ等しいパケットやブロックと呼ばれる領域の平均粒径である。また、分散・析出粒子に関しては、必ずしも常温で存在している必要はなく、溶接の影響で中温域に加熱されたときにフェライト中、ベイナイトまたはマルテンサイト中に析出してくるものも中温強度向上に寄与する。
【００２４】
ベイナイトまたはマルテンサイトの面積分率が２０％未満、あるいは平均粒径が２０μｍ超であると、炭窒化物が存在していても中温域における転位の運動を抑えることができず、降伏強度が低下すると同時に、靭性も劣化してしまう。その結果、図１に示したように、溶接角変形δが大きくなってしまう。フェライト、パーライトの平均粒径を２０μｍ以下と規定したのも同様の理由である。また、組織が微細であっても炭窒化物の平均サイズが０．２μｍを超えていたり、その分率が０．１％未満であると、転位の運動の障害となりえず、やはり中温降伏強度が低下してしまい、図２に示したように溶接角変形δを抑えることができない。一方、炭窒化物の面積分率が１０％超である場合には、炭窒化物自体が粗大化していることもあり、強度低下を引き起こすとともに靭性も顕著に劣化してしまう。炭窒化物の種類としては、Ｆｅ主体のセメンタイトでも、ＮｂやＭｏなどの合金炭窒化物でもよい。炭窒化物の面積率は、炭窒化物のみの面積率であるが、炭窒化物は、ミクロ組織中に分散しているので、マルテンサイト、ベイナイト、フェライト、及び、パーライトの面積率は、炭窒化物を含むマルテンサイト、ベイナイト、フェライト、及び、パーライトの面積率である。
【００２５】
上記のように、組織の平均サイズ・分率、ならびに炭窒化物のサイズ・分率を規定することにより、降伏強度は概ね３２０Ｎ／mm2 以上となり、溶接変形は実用上さほど問題にならない程度に低減できる。さらに変形量を小さくするためには、ベイナイト、マルテンサイトの面積率を５０％以上確保し、降伏強度を４５０Ｎ／mm2 以上とすることが有効である。
〔実施例〕
表１に示した化学成分で試作を実施した。試作鋼は転炉溶製し、連続鋳造、再加熱、圧延、冷却を行い、一部については熱処理も実施した。
【００２６】
表２には、鋼板母材の組織、機械的性質、溶接による角変形量を示す。溶接角変形量δの測定に用いたＴ形隅肉溶接試験体を図３に示す。評価対象材を図のように配置し、４ヶ所仮付溶接して、立板を拘束したまま、表３に示す溶接条件で両側１パス溶接した。溶接角変形量δは図４に示したように、ｗとｄの測定値から算出した。
【００２７】
表２から明らかなように、本発明例１〜８は、本発明の範囲内の組織、炭窒化物を有するために、４００℃における降伏強度が３００Ｎ／mm2 超であり、溶接角変形量δは０．５×１０−２radian以下と極めて小さくなっている。
【００２８】
一方、比較例１〜８は、４００℃降伏強度が低下するために、いずれも溶接角変形量δが大きくなっている。これらの原因は、比較例１〜４、７はベイナイト及びマルテンサイトまたは炭窒化物の分率が本発明の下限を外れ、あるいは構成組織の平均粒径が本発明の上限を外れたために、中温（４００℃）強度が低下したことによる。比較例８ではＣ，Ｍｎの含有量が本発明の下限を外れたために本発明の範囲のベイナイト及びマルテンサイト分率が得られず、中温強度が低下した。また、比較例５，６は組織の粒径や分率は本発明の範囲内にあるものの、炭窒化物の粒径、または分率が本発明の上限を外れたために、やはり中温強度低下、靭性劣化を引き起こし、溶接角変形量δが大きくなってしまった。
【００２９】
【表１】
【表２】
【表３】
【発明の効果】
以上述べたように、本発明により造船、海洋構造物、建築、橋梁、土木等に用いられる鋼板の溶接作業時に発生する溶接変形量が低減でき、歪取り作業或いは溶接歪み発生防止のための作業を軽減もしくは省力することが可能となり、多大の労力と費用の削減が可能となる溶接構造用高降伏点型高張力鋼を提供しうる。
【図面の簡単な説明】
【図１】ベイナイト分率と溶接角変形量との関係を示す図。
【図２】粒径０．２μｍ以下の炭窒化物分率と溶接角変形量との関係を示す図。
【図３】隅肉溶接継手の施工方法の説明図。
【図４】溶接角変形量の算定方法の説明図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel plate with less welding distortion that occurs during welding work of steel plates used in shipbuilding, offshore structures, architecture, bridges, civil engineering, and the like, and a high for welding structure that can reduce welding distortion or prevent the occurrence of welding distortion. Related to yield point type high strength steel.
[0002]
[Prior art]
Conventionally, during welding of steel materials in various steel structures, due to solidification shrinkage of the weld metal and subsequent shrinkage and expansion due to cooling and phase transformation, for example, in the case of the joint shape of fillet welds, out-of-plane deformation called angular deformation ( Residual deformation) occurs. This residual deformation causes a decrease in the nest buckling strength when a compressive load is applied, thereby reducing the structural strength. In addition, when the residual deformation is forcibly prevented by a restraining jig, a phenomenon that excessive residual stress is generated and dimensional accuracy is lowered occurs. Therefore, for example, as seen in “Generation and prevention of welding deformation” published in Journal of Welding Society, Vol. 52, Nos. 4-9, 1988, residual stress generated during welding is corrected by local heating. Many methods have been proposed and implemented empirically. However, reheating of the weld causes deterioration of the steel material, and a great deal of time and cost for the correction work is a substantial obstacle. Longed for.
[0003]
The residual stress generated during welding and the mechanism of deformation are described in detail in “Welding Structure Manual” 1988 (Kuroki Publishing) and “Analysis of Welded Structures” 1988, PERGAMON PRESS. However, the welding deformation is not focused on the characteristics of the steel material used, as determined mainly by the geometrical shape of the member with respect to heat input during welding. In addition, as shown in “Effects of welding conditions on welding residual stress and welding deformation of structural materials” published in Journal of Welding Society, Vol. It is listed that the yield stress is considered as a factor of deformation rather than transformation expansion, and as an example, it is disclosed that in HT80 and 9% Ni steel, 9% Ni steel having a lower yield stress has a larger weld deformation. Yes. However, it is not the knowledge that it can be directly applied to the composition, structure and yield strength of ordinary steel used for steel structures, and the strength of the material against deformation is further changed to alloy elements such as Cr, Mo, V and Nb. Although it is disclosed in Japanese Patent Application Laid-Open No. 4-22597, it is not actually confirmed that it is presumed to be added by the addition of steel, and further, in relation to the composition and structure of the steel material against welding deformation. It is not something that has been noticed.
[0004]
Therefore, as one technique for solving the above-mentioned problems, a steel sheet capable of increasing yield strength and reducing welding distortion by including a predetermined amount or more of bainite in the microstructure is disclosed in JP-A-6-172921. However, the present inventors have found that nothing is mentioned about the size of the structural structure or the state of carbonitride, and it has not necessarily resulted in practically sufficient material properties and a weld distortion reduction effect. .
[0005]
[Problems to be solved by the invention]
The present invention provides a steel plate with less welding distortion that can reduce welding deformation in welding of a steel structure, and can reduce the great cost and labor of welding deformation prevention work and shape correction work.
[0006]
[Means for Solving the Problems]
The present invention has been made to solve the above problems, and the gist thereof is as follows.
(1) By mass%, C: 0.05-0.20%, Si: 0.05-1.0%, Mn: 0.6-2.0%, P: ≦ 0.025%, S: ≦ 0.010%, Al: 0.005 to 0.10%, N: 0.0010 to 0.0080%, the balance is Fe and inevitable impurities, and the microstructure is bainite with an average particle size of 20 μm or less And / or martensite is composed of ferrite and / or a pearlite structure having an area percentage of 20% or more and an average grain size of 20 μm or less, and carbonitride having an average grain diameter of 0.2 μm or less is 0.1 to 10 in area%. % Steel sheet with low welding distortion.
(2) The steel sheet with low welding distortion according to (1), wherein the microstructure of the bainite and / or martensite includes an area% of 50% or more and has a yield strength of 450 N / mm 2 or more.
(3) The said steel plate is mass%, Furthermore, Nb: 0.003-0.050%, Mo: 0.05-0.50%, V: 0.005-0.100%, W: 0.00. 05-0.50%, Ta: 0.05-0.50%, Cu: 0.05-1.5%, Ni: 0.05-3.5%, Cr: 0.05-1.0% , Ti: 0.005 to 0.10%, B: 0.0002 to 0.0030%, Ca: 0.0003 to 0.0050%, REM: 0.0005 to 0.0060% The steel plate with little welding distortion as described in (1) or (2) above, comprising the above.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The addition amount of the chemical composition for obtaining the microstructure and high yield strength of the steel sheet with less welding distortion in the present invention and the reason for the limitation will be described below.
[0008]
C is added for bainite or martensite organization and securing strength, and the lower limit is set to 0.05% from the limit of the effect, and adverse effects on the base metal toughness, deterioration of weldability, high carbon island martensite In order to prevent deterioration of the toughness of welded joints due to the generation of 0.20%, the upper limit was made.
[0009]
Si is an element necessary for deoxidation and is an element effective for further increasing the strength, so 0.05% is set as the lower limit, and 1.0% is added to prevent deterioration of weldability and weld joint toughness. The upper limit.
[0010]
Mn is added to the bainite or martensite structure and 0.6% as a lower limit to ensure strength and toughness, and a large amount of addition increases hardenability and produces a hardened structure, and also deteriorates weldability. The upper limit is 2.0%.
[0011]
Impurities P and S are respectively set to P ≦ 0.025% and S ≦ 0.010% in order to maintain the base metal and weld joint toughness at desired levels, respectively.
[0012]
Al is an element necessary for deoxidation, so 0.005% is set as the lower limit, and addition of a large amount impairs the cleanliness of the steel, so 0.10% was set as the upper limit.
[0013]
N binds Al, refines the crystal grains of the steel material, and lowers 0.0010%, which is effective for increasing the toughness. If added in a large amount, the toughness of the steel material is impaired, so 0.0080% was made the upper limit.
[0014]
Furthermore, in addition to the above elements, one or more of Nb, Mo, V, W, Ta, Cu, Ni, Cr, Ti, B, Ca, and REM are contained, thereby reducing welding distortion according to the present invention. The microstructure and high yield strength of the steel sheet can be obtained. The reason for the addition of each additive element will be described.
[0015]
Nb is an element effective for increasing the yield strength by bainite or martensite organization, structure refinement, and carbonitride formation, so 0.003% is the lower limit, and a large amount of addition impairs weld joint toughness. The upper limit was made 0.050%.
[0016]
Mo, V, W and Ta are effective elements for increasing the yield strength by bainite or martensite organization and carbonitride formation, so 0.05% for Mo, W and Ta and 0 for V 0.005 was set as the lower limit, and addition of a large amount impairs weldability and weld joint toughness, so the upper limits were set to 0.50% for Mo, W, and Ta, and 0.100% for V.
[0017]
Since Cu and Cr are effective elements for increasing the strength together with bainite or martensite organization, 0.05% is set as the lower limit, and a large amount of addition impairs the weld joint toughness. For% and Cr, the upper limit was 1.0%.
[0018]
Ni is an expensive element that is effective for forming a bainite or martensite structure without impairing toughness. Therefore, Ni is added in an amount of 0.05 to 3.5% from the viewpoint of economy.
[0019]
Ti is an element effective for securing the toughness of the weld heat affected zone, so 0.005% is set as the lower limit, and 0.10% is set as the upper limit in order to prevent toughness deterioration due to excessive addition.
[0020]
B is an element effective for improving the strength of steel materials together with bainite or martensite structure and for refining the crystal grains in the weld heat affected zone. However, excessive addition deteriorates toughness, so 0.0002 to 0.0030%. It was limited to the range.
[0021]
Ca is an effective element for controlling the form of sulfides, but addition of a large amount impairs the cleanliness of the steel, so it was limited to the range of 0.0003 to 0.0050%. REM is an element effective for refining the structure of the heat affected zone and enhancing toughness, but adding a large amount impairs the cleanliness of the steel, so it is limited to a range of 0.0005 to 0.0060%. did.
[0022]
Next, the reason for limiting the microstructure of the steel defined in the present invention will be described. In general, at the time of welding, a position close to the bead is exposed to a high temperature, so that the thermal stress immediately exceeds the yield strength, and plastic deformation proceeds. Since this portion shrinks during cooling, deformation occurs. On the other hand, since the steel plate temperature does not rise so much at a position away from the weld bead, the thermal stress exceeds the yield strength considerably later than the position of the bead. If the yield strength at this time is sufficiently high, even if the position close to the weld bead contracts to cause deformation, a large resistance to deformation occurs. Therefore, the present inventors conducted experiments and computer simulations under various conditions, and found that it is effective to increase the yield strength in the middle temperature range of 400 to 600 ° C. in order to minimize welding deformation. Obtained.
[0023]
In general, the normal temperature strength is governed by the crystal grain size, the fraction of low-temperature transformation phase such as bainite and martensite, the amount of alloy element solid solution, and the size and amount of dispersed / precipitated particles represented by carbonitride. On the other hand, the high-temperature strength exceeding 600 ° C. has a small contribution of the tissue factor, and is generally determined by the state (size, amount, distribution state, stability) of the precipitate particles. Accordingly, the medium temperature yield strength at 400 to 600 ° C. is a region where the contribution of precipitates is increased while being affected by the structure. In order to increase the medium temperature strength, the grain size of the constituent structure is kept fine, and bainite. Alternatively, the inventors have found that it is necessary to make a large amount of fine dispersed / precipitated particles after securing a certain amount or more of martensite, and have made the present invention. Here, the crystal grain size is the ferrite when the structure is ferrite and / or pearlite, the average grain size of pearlite, and the bainite or martensite is not a prior austenite grain size but a packet with almost the same crystal orientation. It is the average particle size of a region called a block. Dispersed / precipitated particles do not necessarily exist at room temperature, and those that precipitate in ferrite, bainite, or martensite when heated to an intermediate temperature range due to welding are also improved in intermediate temperature strength. Contribute to.
[0024]
If the area fraction of bainite or martensite is less than 20% or the average particle size is more than 20 μm, the dislocation movement in the middle temperature range cannot be suppressed even if carbonitride is present, and the yield strength decreases. At the same time, the toughness deteriorates. As a result, as shown in FIG. 1, the welding angle deformation δ increases. For the same reason, the average particle size of ferrite and pearlite is regulated to 20 μm or less. Also, even if the structure is fine, if the average size of carbonitride exceeds 0.2 μm or the fraction is less than 0.1%, it cannot be an obstacle to the movement of dislocations. As a result, the welding angle deformation δ cannot be suppressed as shown in FIG. On the other hand, when the area fraction of carbonitride is more than 10%, the carbonitride itself may be coarsened, which causes a decrease in strength and significantly deteriorates toughness. The type of carbonitride may be Fe-based cementite or an alloy carbonitride such as Nb or Mo. The area ratio of carbonitride is the area ratio of carbonitride only, but since carbonitride is dispersed in the microstructure, the area ratio of martensite, bainite, ferrite and pearlite is It is the area ratio of martensite containing nitride, bainite, ferrite, and pearlite.
[0025]
As described above, by specifying the average size / fraction of the structure and the size / fraction of the carbonitride, the yield strength becomes approximately 320 N / mm 2 or more, and the welding deformation is not so much a problem in practical use. Can be reduced. In order to further reduce the amount of deformation, it is effective to secure an area ratio of bainite and martensite of 50% or more and a yield strength of 450 N / mm 2 or more.
〔Example〕
Trial manufacture was carried out with the chemical components shown in Table 1. The prototype steel was melted in the converter, subjected to continuous casting, reheating, rolling and cooling, and some were heat treated.
[0026]
Table 2 shows the structure of the steel plate base material, the mechanical properties, and the amount of angular deformation caused by welding. A T-shaped fillet weld specimen used for measuring the welding angle deformation δ is shown in FIG. The material to be evaluated was placed as shown in the figure, and was tack-welded at four locations, and one-pass welding was performed on both sides under the welding conditions shown in Table 3 while restraining the standing plate. As shown in FIG. 4, the welding angle deformation amount δ was calculated from the measured values of w and d.
[0027]
As is clear from Table 2, Examples 1 to 8 of the present invention have a structure and carbonitride within the scope of the present invention, so the yield strength at 400 ° C. is more than 300 N / mm 2 , and the welding angle deformation amount δ is extremely small, 0.5 × 10 −2 radius or less.
[0028]
On the other hand, in Comparative Examples 1 to 8, since the 400 ° C. yield strength is lowered, the welding angle deformation amount δ is large in all cases. These causes are that in Comparative Examples 1 to 4 and 7, since the fraction of bainite and martensite or carbonitride is outside the lower limit of the present invention, or the average particle size of the constituent structure is out of the upper limit of the present invention, (400 ° C.) Due to the decrease in strength. In Comparative Example 8, because the C and Mn contents deviated from the lower limit of the present invention, the bainite and martensite fractions within the range of the present invention could not be obtained, and the intermediate temperature strength was lowered. Further, in Comparative Examples 5 and 6, although the grain size and fraction of the structure are within the scope of the present invention, since the grain size or fraction of the carbonitride is outside the upper limit of the present invention, the medium temperature strength is decreased. It caused toughness deterioration and the welding angle deformation amount δ increased.
[0029]
[Table 1]
[Table 2]
[Table 3]
【The invention's effect】
As described above, according to the present invention, it is possible to reduce the amount of welding deformation that occurs during welding work of steel plates used in shipbuilding, offshore structures, architecture, bridges, civil engineering, etc. It is possible to provide a high-yield point type high-strength steel for welded structures that can reduce or save labor, and can reduce a great deal of labor and cost.
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between a bainite fraction and a welding angle deformation amount.
FIG. 2 is a graph showing the relationship between the carbonitride fraction with a grain size of 0.2 μm or less and the welding angle deformation.
FIG. 3 is an explanatory diagram of a method for constructing a fillet welded joint.
FIG. 4 is an explanatory diagram of a method for calculating a welding angle deformation amount.
Claims (3)
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| JP2002067163A JP4160763B2 (en) | 2002-03-12 | 2002-03-12 | Steel plate with low welding distortion |
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| JP2002067163A JP4160763B2 (en) | 2002-03-12 | 2002-03-12 | Steel plate with low welding distortion |
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| JP4733950B2 (en) * | 2004-09-21 | 2011-07-27 | 新日本製鐵株式会社 | Linear heating deformation method of steel sheet |
| KR101568530B1 (en) | 2013-12-24 | 2015-11-11 | 주식회사 포스코 | Thick steel sheet having excellent high temperature yield strength and low-temperature toughness, and method for manufacturing the same |
| KR101568531B1 (en) | 2013-12-24 | 2015-11-11 | 주식회사 포스코 | Thick steel sheet having excellent high temperature yield strength and low-temperature toughness, and method for manufacturing the same |
| KR101585730B1 (en) | 2013-12-24 | 2016-01-14 | 주식회사 포스코 | Thick steel sheet having excellent high temperature yield strength and low-temperature toughness, and method for manufacturing the same |
| KR101568532B1 (en) | 2013-12-24 | 2015-11-11 | 주식회사 포스코 | Thick steel sheet having excellent high temperature yield strength and low-temperature toughness, and method for manufacturing the same |
| CN103981441B (en) * | 2014-05-30 | 2016-02-03 | 武汉钢铁(集团)公司 | Yield strength >=490MPa construction(al)steel and production method |
| KR102209581B1 (en) * | 2018-11-29 | 2021-01-28 | 주식회사 포스코 | The steel plate having excellent heat affected zone toughness and method for manufacturing thereof |
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