JP4736374B2 - Steel material with super large heat input welding characteristics - Google Patents
Steel material with super large heat input welding characteristics Download PDFInfo
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- JP4736374B2 JP4736374B2 JP2004230046A JP2004230046A JP4736374B2 JP 4736374 B2 JP4736374 B2 JP 4736374B2 JP 2004230046 A JP2004230046 A JP 2004230046A JP 2004230046 A JP2004230046 A JP 2004230046A JP 4736374 B2 JP4736374 B2 JP 4736374B2
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- 239000000463 material Substances 0.000 title claims description 37
- 229910000831 Steel Inorganic materials 0.000 title claims description 33
- 239000010959 steel Substances 0.000 title claims description 33
- 238000003466 welding Methods 0.000 title claims description 17
- 229910001563 bainite Inorganic materials 0.000 claims description 9
- 229910000734 martensite Inorganic materials 0.000 claims description 9
- 229910000859 α-Fe Inorganic materials 0.000 claims description 9
- 229910052760 oxygen Inorganic materials 0.000 claims description 5
- 229910052758 niobium Inorganic materials 0.000 claims description 4
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 3
- 229910052804 chromium Inorganic materials 0.000 claims description 3
- 239000012535 impurity Substances 0.000 claims description 3
- 229910052748 manganese Inorganic materials 0.000 claims description 3
- 239000002184 metal Substances 0.000 claims description 3
- 229910052751 metal Inorganic materials 0.000 claims description 3
- 229910052750 molybdenum Inorganic materials 0.000 claims description 3
- 229910052759 nickel Inorganic materials 0.000 claims description 3
- 239000001301 oxygen Substances 0.000 claims description 3
- 229910052720 vanadium Inorganic materials 0.000 claims description 3
- 229910052802 copper Inorganic materials 0.000 claims description 2
- 229910052742 iron Inorganic materials 0.000 claims description 2
- 230000000694 effects Effects 0.000 description 13
- 238000009863 impact test Methods 0.000 description 9
- 238000010276 construction Methods 0.000 description 4
- 239000002244 precipitate Substances 0.000 description 4
- 238000005096 rolling process Methods 0.000 description 4
- 238000001816 cooling Methods 0.000 description 3
- 238000010438 heat treatment Methods 0.000 description 3
- 238000000034 method Methods 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- 238000001556 precipitation Methods 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 2
- 230000032683 aging Effects 0.000 description 2
- 229910001566 austenite Inorganic materials 0.000 description 2
- 229910001567 cementite Inorganic materials 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 230000000052 comparative effect Effects 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- IOVCWXUNBOPUCH-UHFFFAOYSA-M Nitrite anion Chemical compound [O-]N=O IOVCWXUNBOPUCH-UHFFFAOYSA-M 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 239000010953 base metal Substances 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 238000000151 deposition Methods 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 230000008520 organization Effects 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 238000001953 recrystallisation Methods 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 238000010583 slow cooling Methods 0.000 description 1
- 230000035882 stress Effects 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
- 238000009864 tensile test Methods 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
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- Heat Treatment Of Steel (AREA)
Description
本発明は、建築、土木、建設機械、造船、パイプ、タンクおよび海洋構造物などの溶接構造物として使用される超大入熱溶接用鋼材に関するものである。 The present invention relates to a steel material for super large heat input welding used as a welded structure such as a building, civil engineering, construction machine, shipbuilding, pipe, tank and offshore structure.
厚鋼板に代表される溶接構造用鋼材は、上記種々の分野で使用され、高強度化や高靭性化などの特性の改善が図られてきた。 Steel materials for welded structures represented by thick steel plates have been used in the various fields described above, and have been improved in properties such as higher strength and higher toughness.
特許文献１には、主にラインパイプに用いる鋼としてＣを０．０３％以下に低減しかつ微量のＢを添加した極低Ｃ−Ｂ添加鋼が開示されおり、Ｃ量の低減により島状マルテンサイトの低減が図られ、溶接部の靱性が向上すること、微量のＢの添加により母材組織をベイナイトとし高強度、高靱性な母材が得られることが示されている。 Patent Document 1 discloses an ultra-low CB-added steel in which C is reduced to 0.03% or less as a steel mainly used for a line pipe and a small amount of B is added. It has been shown that martensite is reduced, the toughness of the welded portion is improved, and that a high strength and high toughness base material can be obtained by adding a trace amount of B to the base material structure as bainite.
特許文献２には、材質ばらつきの小さなＭｎ、Ｎｂ量を調整した極低Ｃ−Ｂ添加ベイナイト鋼とその製造方法が開示されている。 Patent Document 2 discloses an ultra-low C—B-added bainitic steel in which the amount of Mn and Nb with small material variations is adjusted, and a method for manufacturing the same.
特許文献３には、材質のばらつきが少なく、かつ耐疲労特性に優れたＣｕ添加による析出強化を利用した極低Ｃ−Ｂ添加高靭性鋼材とその製造方法が開示されている。 Patent Document 3 discloses an extremely low CB-added high toughness steel material using precipitation strengthening by Cu addition with little variation in material and excellent fatigue resistance, and a manufacturing method thereof.
溶接構造物の製作にあたっては溶接施工コスト削減の観点から大入熱溶接の適用が望ましい。上記各公報には、極低Ｃ−Ｂ添加鋼において低Ｃ化による島状マルテンサイトの低減は、溶接部靱性の向上に効果があることが示されているが、いずれも対象は溶接入熱量１００ｋＪ／ｃｍ未満の比較的入熱量の少ない領域に限られており、施工コスト低減に対する寄与は小さいという問題があった。 In producing a welded structure, it is desirable to apply high heat input welding from the viewpoint of reducing welding construction costs. In each of the above publications, it is shown that the reduction of island martensite by reducing C in an extremely low CB-added steel is effective in improving weld toughness. There is a problem that it is limited to a region with a relatively small amount of heat input of less than 100 kJ / cm, and that the contribution to reduction in construction cost is small.
本発明の課題は、大入熱溶接においても靱性劣化が少なく、溶接構造物の施工コスト低減に寄与できる鋼材を提供することにあり、具体的には母材の特性として降伏応力が４５０ＭＰａ以上または引張り強さが５７０ＭＰａ以上、JIS Z 220２ に規定の幅１０ｍｍのＶノッチシャルピー衝撃試験片を用いた衝撃試験における破面遷移温度（ＶＴＳ）が−４０℃以下、溶接ボンドでの吸収エネルギーが−１０℃において１００Ｊ以上の鋼材を提供することにある。 An object of the present invention is to provide a steel material that has little toughness deterioration even in high heat input welding and can contribute to a reduction in the construction cost of a welded structure. Specifically, the yield stress is 450 MPa or more as a property of the base material. Fracture surface transition temperature ( V T S ) in an impact test using a V-notch Charpy impact test piece having a tensile strength of 570 MPa or more and a width of 10 mm as defined in JIS Z 2202 is −40 ° C. It is to provide a steel material of 100 J or more at −10 ° C.
本発明者らは、上記課題を解決するため種々の実験と検討をおこなった結果、下記の知見を得た。 As a result of various experiments and studies to solve the above problems, the present inventors have obtained the following knowledge.
母材の引張り強さを５７０ＭＰａ以上の強度とし、かつ大入熱溶接した場合の溶接ボンド部の靭性を確保するにはＣｕを０．８％を超えて含有させ析出強化を図るのが有効で、かつフェライト組織分率を５０％以下にするのがよい。 In order to ensure the tensile strength of the base material to be 570 MPa or more and to secure the toughness of the weld bond part when high heat input welding is performed, it is effective to contain Cu exceeding 0.8% to enhance the precipitation. In addition, the ferrite structure fraction is preferably 50% or less.
本発明は、このような知見に基づきなされたもので、その要旨は下記に示す超大入熱溶接性に優れた鋼材にある。 The present invention has been made on the basis of such knowledge, and the gist thereof is a steel material excellent in super-high heat input weldability shown below.
（１）質量％で、Ｃ：０．０３〜０．１％、Ｓｉ：０．０１〜０．５％以下、Ｍｎ：０．５〜２％、Ｃｕ：０．８超〜２％、Ｔｉ：０．００５〜０．０２５％、Ｎ：０．００２〜０．００８％、sol.Ａｌ：０．００２〜０．０５％、Ｏ（酸素）：０．００３５％以下を含有し、残部はＦｅおよび不純物からなり、金属組織がベイナイトまたはマルテンサイト組織の５０％以上とフェライト組織の５０％以下からなるものであることを特徴とする、５７０ＭＰａ以上の引張強度を有する超大入熱溶接特性に優れた鋼材。 (1) By mass%, C: 0.03 to 0.1%, Si: 0.01 to 0.5% or less, Mn: 0.5 to 2%, Cu: more than 0.8 to 2%, Ti : 0.005-0.025%, N: 0.002-0.008%, sol.Al: 0.002-0.05%, O (oxygen): 0.0035% or less, the balance being It is composed of Fe and impurities, and the metal structure is composed of 50% or more of bainite or martensite structure and 50% or less of ferrite structure , and is excellent in super large heat input welding characteristics having a tensile strength of 570 MPa or more. Steel material.
（２）Ｆｅの一部に代えて、質量％で、Ｎｉ：０．２〜２％、Ｃｒ：０．０５〜１％、Ｍｏ：０．０５〜１％、Ｖ：０．０１〜０．１％、Ｎｂ：０．００５〜０．０７％のうちの１種以上を含有することを特徴とする、上記（１）に記載の５７０ＭＰａ以上の引張強度を有する超大入熱溶接特性に優れた鋼材。
(2) Instead of a part of Fe, in mass%, Ni: 0.2-2%, Cr: 0.05-1%, Mo: 0.05-1%, V: 0.01-0. 1%, Nb: It contains one or more of 0.005 to 0.07%, and is excellent in super large heat input welding characteristics having a tensile strength of 570 MPa or more as described in (1) above Steel material.
本発明によれば、ＴＳが５７０ＭＰａ以上、ＹＳが４５０ＭＰａ以上、JIS Z 2202 に規定の１０ｍｍ幅、Ｖノッチシャルピー衝撃試験片を用いた衝撃試験でのｖＴｓが−４０℃以下で、３００ｋＪ／ｍｍでの溶接時の−１０℃での吸収エネルギーが１００Ｊ以上の鋼が得られ、種々の溶接構造物において優れた効果を発揮する。 According to the present invention, TS is 570 MPa or more, YS is 450 MPa or more, 10 mm width as defined in JIS Z 2202, vTs in an impact test using a V-notch Charpy impact test piece is −40 ° C. or less, and 300 kJ / mm. Steel having an absorption energy at −10 ° C. during welding of 100 J or more is obtained, and exhibits excellent effects in various welded structures.
以下、本発明の実施の形態について詳しく説明する。なお、各元素の含有量の「％」表示は全て「質量％」とする。 Hereinafter, embodiments of the present invention will be described in detail. The “%” display of the content of each element is all “mass%”.
Ｃ：０．０３〜０．１％
Ｃは、母材の強度確保を目的に含有させる。０．０３％未満では５７０ＭＰａ以上の強度を確保することができない。一方、０．１％を超えると溶融線近傍の溶接熱影響部の靱性が劣化する。したがって、Ｃの含有量は０．０３〜０．１％とした。なお、好ましくは０．０８％以下、さらに望ましくは０．０６％以下である。望ましい下限は０．０４％である。
C: 0.03-0.1%
C is contained for the purpose of securing the strength of the base material. If it is less than 0.03%, a strength of 570 MPa or more cannot be secured. On the other hand, if it exceeds 0.1%, the toughness of the weld heat-affected zone near the melting line deteriorates. Therefore, the content of C is set to 0.03 to 0.1%. In addition, Preferably it is 0.08% or less, More desirably, it is 0.06% or less. A desirable lower limit is 0.04%.
Ｓｉ：０．０１〜０．５％
Ｓｉは、脱酸剤として添加する元素である。その効果を得るためには、０．０１％以上含有させる必要がある。一方、含有量が増加すると溶接冷却過程において残留γがセメンタイトへ分解する反応を抑制し島状マルテンサイトを増加させるので、溶接部靱性の確保の観点からは含有量は少ない方が望ましく、上限を０．５％とした。好ましい上限は０．３％、さらに好ましくは０．１５％以下である。
Si: 0.01 to 0.5%
Si is an element added as a deoxidizer. In order to acquire the effect, it is necessary to contain 0.01% or more. On the other hand, when the content is increased, the reaction of residual γ is decomposed into cementite in the welding cooling process and the island-like martensite is increased. 0.5%. The upper limit is preferably 0.3%, more preferably 0.15% or less.
Ｍｎ：０．５〜２％
Ｍｎは、母材の強度確保、溶接熱影響部の焼入性の確保に有効で、また脱酸元素としても有効である。０．５％未満では、溶接熱影響部にフェライトが生成し靱性が劣化するので０．５％以上含有させる必要がある。一方、２％を超えると中心偏析による板厚方向での母材特性の不均一や靱性の劣化をもたらす。したがって、Ｍｎの上限は２％とした。
Mn: 0.5-2%
Mn is effective for ensuring the strength of the base material, ensuring the hardenability of the weld heat affected zone, and is also effective as a deoxidizing element. If it is less than 0.5%, ferrite is generated in the weld heat-affected zone and the toughness deteriorates, so it is necessary to contain 0.5% or more. On the other hand, if it exceeds 2%, the base material properties are not uniform in the thickness direction due to center segregation and the toughness is deteriorated. Therefore, the upper limit of Mn is 2%.
Ｃｕ：０．８超〜２％
Ｃｕは、ε析出物として単独で析出して母材の強度を向上させる効果がある。鋼材に時効処理や冷却途中からの徐冷などの処理を施すことにより前記析出物を析出させることができる。析出したＣｕは母材では強度を向上させ、また溶接熱影響部ではＣｕが再溶解するので硬度過剰となりがちな溶接部の硬度を低減し溶接部の靱性を向上させる効果がある。ε析出物を析出させることによって、効果的に強度を向上させるためには、０．８％を超えるＣｕ量が必要である。しかしながら、２％を超えて含有させると、溶接熱影響部で未固溶の析出物が増加し、靭性が低下するため２％以下とする必要がある。
Cu: more than 0.8 to 2%
Cu has the effect of improving the strength of the base material by depositing alone as an ε precipitate. The precipitate can be precipitated by subjecting the steel material to a treatment such as an aging treatment or a slow cooling from the middle of cooling. Precipitated Cu has the effect of improving the strength of the base metal and reducing the hardness of the welded portion, which tends to be excessively hard, and improving the toughness of the welded portion because Cu is remelted in the weld heat affected zone. In order to effectively improve the strength by precipitating ε precipitates, an amount of Cu exceeding 0.8% is required. However, if the content exceeds 2%, undissolved precipitates increase in the weld heat-affected zone and the toughness decreases, so it is necessary to make it 2% or less.
Ｔｉ：０．００５〜０．０２５％
Ｔｉは、母材の強度を向上させると同時に、連続鋳造スラブの横ひび割れを防止するのに有効である。また、固溶Ｎと結合することによって形成されるＴｉＮは、加熱時のγ結晶粒の粗大化を抑制し、母材および溶接部の靱性を向上させる効果がある。これらの効果を発揮させるには０．００５％以上含有させる必要がある。しかしながら、０．０２５％を超えて含有させると母材の靱性が劣化するため上限は０．０２５％とした。
Ti: 0.005-0.025%
Ti is effective for improving the strength of the base material and at the same time preventing lateral cracking of the continuously cast slab. Moreover, TiN formed by combining with solute N has an effect of suppressing the coarsening of γ crystal grains during heating and improving the toughness of the base material and the welded portion. In order to exert these effects, it is necessary to contain 0.005% or more. However, if the content exceeds 0.025%, the toughness of the base material deteriorates, so the upper limit was made 0.025%.
Ｎ：０．００２〜０．００８％
Ｎは、Ｔｉと結合してＴｉＮを形成し、加熱時のγ粒の粗大化を抑制し、母材および溶接熱影響部の靱性を改善する。このたには、０．００２％以上含有させることが必要である。しかしながら、Ｂを含有させない場合Ｎが０．００８％を超えると、過剰のＮが組織中に存在し靱性を劣化させる。γ粒の細粒化の観点からは、Ｎ量はＴｉ量の４．３分の１よりもやや多く含有されることが望ましい。
N: 0.002 to 0.008%
N combines with Ti to form TiN, suppresses the coarsening of γ grains during heating, and improves the toughness of the base material and the weld heat affected zone. For this purpose, it is necessary to contain 0.002% or more. However, when N is not contained when N exceeds 0.008%, excess N is present in the structure and the toughness is deteriorated. From the viewpoint of finer γ grains, it is desirable that the N content is slightly larger than 4.3 times the Ti content.
sol.Ａｌ：０．００２〜０．０５％
Ａｌは、脱酸を目的で添加する。その効果を得るには、０．００２％以上含有させる必要がある。一方、Ａｌ含有が増加するとＳｉと同様、溶接冷却過程において残留オーステナイトがセメンタイトへ分解する反応を抑制し島状マルテンサイトを増加させる。したがって、溶接熱影響部の靱性を確保する観点から含有は少ないほうが望ましが、０．０５％までの量であれば問題がない。望ましい上限は０．０３％、さらに望ましくは０．０１％である。
sol.Al: 0.002 to 0.05%
Al is added for the purpose of deoxidation. In order to acquire the effect, it is necessary to contain 0.002% or more. On the other hand, when Al content increases, the reaction of residual austenite to decompose to cementite in the welding cooling process is suppressed as in the case of Si, and island martensite is increased. Therefore, it is desirable that the content is less from the viewpoint of ensuring the toughness of the weld heat affected zone, but there is no problem if the amount is up to 0.05%. A desirable upper limit is 0.03%, more desirably 0.01%.
Ｏ：０．００３５％以下
酸素は、必要により含有させてもよい元素で、含有させれば酸化物を形成することにより溶接熱影響部の組織を微細化する作用がある。この効果は不純物量を超える量でも効果があるが、含有させる場合は０．００１％を超えた量とするのが好ましい。一方、過剰な添加は粗大な酸化物の形成から靱性に悪影響を及ぼすため含有させる場合の上限を０．００３５％とした。
O: 0.0035% or less Oxygen is an element that may be contained if necessary, and if contained, forms an oxide and has the effect of refining the structure of the weld heat affected zone. This effect is effective even if the amount exceeds the amount of impurities, but when it is contained, the amount is preferably over 0.001%. On the other hand, excessive addition has an adverse effect on toughness due to the formation of coarse oxides, so the upper limit for inclusion is set to 0.0035%.
上記の化学組成のＣｕ含有鋼材において、さらに母材の強度を高めたい場合には各鋼材に、Ｎｉ、Ｃｒ、Ｍｏ、ＶおよびＮｂのうちの１種以上を含有させるのが有効である。以下、各元素毎に説明する。 In the Cu-containing steel material having the above chemical composition, it is effective that each steel material contains at least one of Ni, Cr, Mo, V, and Nb when it is desired to further increase the strength of the base material. Hereinafter, each element will be described.
Ｎｉ：０．２〜２％
Ｎｉは、靱性劣化を最小限に抑え母材の強度を上昇させるのに有効で、０．２％以上の添加が好ましい。一方、２％を超えて含有させてもコストアップに見合う強度、靱性の改善効果が見られないため、上限を２％とした。
Ni: 0.2-2%
Ni is effective in minimizing toughness deterioration and increasing the strength of the base material, and is preferably added in an amount of 0.2% or more. On the other hand, even if the content exceeds 2%, the effect of improving the strength and toughness commensurate with the cost increase is not seen.
Ｃｒ：０．０５〜１％
Ｃｒは、母材の強度確保を目的として含有させる。そのためには、少なくとも０．０５％含有させる必要がある。一方、１％以上含有させると靱性を劣化させるため、その上限は１％とした。好ましい上限は０．５％以下である。
Cr: 0.05 to 1%
Cr is contained for the purpose of securing the strength of the base material. For that purpose, it is necessary to contain at least 0.05%. On the other hand, if the content is 1% or more, the toughness deteriorates, so the upper limit was made 1%. A preferable upper limit is 0.5% or less.
Ｍｏ：０．０５〜１％
Ｍｏは、母材の強度確保を目的として含有させる。そのためには少なくとも０．０５％含有させる必要がある。一方、１％を超えて含有させた場合は靱性を劣化させるので上限を１％とした。好ましい上限は０．５％である。
Mo: 0.05 to 1%
Mo is contained for the purpose of securing the strength of the base material. For that purpose, it is necessary to contain at least 0.05%. On the other hand, if the content exceeds 1%, the toughness deteriorates, so the upper limit was made 1%. A preferable upper limit is 0.5%.
Ｖ：０．０１〜０．１％
Ｖは、母材の強度確保を目的として含有させる。そのためには少なくとも０．０１％含有させる必要がある。一方、０．１％を超えて含有させた場合は靱性を劣化させるため、その上限を０．１％とした。好ましい上限は０．０５％以下、さらに好ましくは０．０３％以下である。
V: 0.01 to 0.1%
V is contained for the purpose of securing the strength of the base material. For that purpose, it is necessary to contain at least 0.01%. On the other hand, if the content exceeds 0.1%, the toughness deteriorates, so the upper limit was made 0.1%. The upper limit is preferably 0.05% or less, more preferably 0.03% or less.
Ｎｂ：０．００５〜０．０７％
Ｎｂは母材の強度を上昇させると同時に組織の微細化を通して母材の低温靱性を改善する効果がある。これらの効果を得るには０．００５％以上含有させる必要がある。一方、０．０７％を超えて過剰に含有させると粗大な炭化物、窒化物を形成して靱性を低下させる。したがって、上限は０．０７％とした。強度と靱性のバランスの観点より、好ましい上限は０．０５％、さらに好ましくは０．０３％である。
Nb: 0.005 to 0.07%
Nb has the effect of increasing the strength of the base material and improving the low temperature toughness of the base material through refinement of the structure. In order to obtain these effects, it is necessary to contain 0.005% or more. On the other hand, if the content exceeds 0.07%, coarse carbides and nitrides are formed and the toughness is lowered. Therefore, the upper limit was made 0.07%. From the viewpoint of balance between strength and toughness, the preferable upper limit is 0.05%, more preferably 0.03%.
フェライト組織分率：５０％以下
上述したように、本発明鋼において５７０ＭＰａ以上の強度を確保するためには、上記のような化学組成とし、さらに適切なフェライト組織分率を確保する必要がある。時効析出による強化を図ることができるためにフェライト組織分率は５０％以下でよい。
Ferrite structure fraction: 50% or less As described above, in order to ensure the strength of 570 MPa or more in the steel of the present invention, it is necessary to have the above-described chemical composition and further ensure an appropriate ferrite structure fraction. Since strengthening by aging precipitation can be achieved, the ferrite structure fraction may be 50% or less.
本発明の鋼材は、金属組織がフェライト組織以外は、ベイナイトまたはマルテンサイト組織である。この場合、靱性改善の理由からベイナイト組織のラスの平均長さは５０μm以下であるのが好ましい。靱性改善の観点からは、ベイナイトラスの長さは短ければ短いほど良く、それを実現するためには一般的には、例えばオーステナイトの未再結晶温度域で強圧下圧延をすればよい。しかしながら、本発明鋼材のように比較的焼入性の低い鋼では、ベイナイトの平均ラス長さを１５μm以下にしようとすると、強圧下圧延によって、同時に初析αの生成頻度を高めることとなり、ベイナイトとマルテンサイトの組織分率が不足する。ベイナイトとマルテンサイトの組織分率が不足すると所望の強度が確保されないため、ベイナイトの平均ラス長さは１５μm以上に調整するのがよい。 The steel material of the present invention has a bainite or martensite structure except that the metal structure is a ferrite structure. In this case, for the purpose of improving toughness, the average length of the lath of the bainite structure is preferably 50 μm or less. From the viewpoint of improving toughness, the shorter the length of the bainite lath, the better. In order to achieve this, in general, rolling under high pressure may be performed in a non-recrystallization temperature range of austenite, for example. However, in a steel with a relatively low hardenability like the steel of the present invention, if the average lath length of bainite is set to 15 μm or less, the generation frequency of pro-eutectoid α is simultaneously increased by rolling under strong rolling. And the martensite organization fraction is insufficient. If the bainite and martensite structure fractions are insufficient, the desired strength cannot be ensured. Therefore, the average lath length of bainite is preferably adjusted to 15 μm or more.
表１に示す化学組成を有する３３種の鋼を１８０ｋｇ真空溶解炉を用いて溶製した。表中記号１〜１２は本発明例 、記号Ｘ１〜Ｘ７は比較例である。 33 types of steel having chemical compositions shown in Table 1 were melted using a 180 kg vacuum melting furnace. In the table, symbols 1 to 12 are examples of the present invention, and symbols X1 to X7 are comparative examples.
これらの各１８０ｋｇ鋼塊を鍛造して厚さ１６０ｍｍの鋼片とした。次いで、表２に示す各温度に加熱して熱間圧延して、各温度で仕上げて冷却した。その後、これらの表に示す６００〜６３０℃の温度範囲で１時間保持して焼戻し熱処理を施し、板厚４０ｍｍの鋼板とした。 These 180 kg steel ingots were forged into steel pieces having a thickness of 160 mm. Subsequently, it heated to each temperature shown in Table 2, hot-rolled, finished at each temperature, and cooled. Thereafter, the steel plate was held at a temperature range of 600 to 630 ° C. shown in these tables for 1 hour and subjected to tempering heat treatment to obtain a steel plate having a thickness of 40 mm.
このようにして得た各鋼板の板厚中心部から、ＪＩＳ４号引張試験片とJIS Z 2202に規定の幅１０ｍｍのＶノッチシャルピー衝撃試験片をそれぞれ圧延方向と平行な方向で採取し、母材の機械的性質を調査した。また、各鋼板についてナイタルで腐食して組織を現出させた後、光学顕微鏡により２０視野を観察して面積率を求め、フェライト組織分率を調べた。 JIS No. 4 tensile test pieces and V-notch Charpy impact test pieces with a width of 10 mm specified in JIS Z 2202 were sampled in the direction parallel to the rolling direction from the center of the thickness of each steel plate thus obtained. The mechanical properties of were investigated. Moreover, after corroding each steel plate with the nitrite to reveal the structure, the area ratio was obtained by observing 20 fields of view with an optical microscope, and the ferrite structure fraction was examined.
さらに、各鋼板を大入熱溶接した場合の靭性を調べるため、入熱３００ｋＪ／ｍｍの条件にてエレクトロガスアーク溶接をおこない、溶接ボンド部にノッチを成形できるように上記と同じシャルピー衝撃試験片採取し、溶接部のボンドの吸収エネルギーを測定した。 Furthermore, in order to investigate the toughness when each steel plate is subjected to high heat input welding, the same Charpy impact test piece as above was taken so that electrogas arc welding was performed under conditions of heat input of 300 kJ / mm and a notch could be formed in the weld bond part. Then, the absorbed energy of the bond in the weld was measured.
表２に試験結果をまとめて示す。 Table 2 summarizes the test results.
なお、母材の強度と靭性の目標はそれぞれＹＳで４５０ＭＰａ以上、ＴＳで５７０ＭＰａ以上、ｖＴｓを−４０℃以下とした。また、シャルピー衝撃試験において溶接ボンド部の吸収エネルギーの目標値は−１０℃で１００Ｊ以上とした。 The base material strength and toughness were set to 450 MPa or higher for YS, 570 MPa or higher for TS, and vTs of −40 ° C. or lower. Further, in the Charpy impact test, the target value of the absorbed energy of the weld bond portion was set to 100 J or more at −10 ° C.
表２から明らかなように、本発明例１〜１２は全てＴＳで５７０ＭＰａ以上、ＹＳで４５０ＭＰａ以上の強度と−４０℃以下のｖＴｓが得られている。また３００ｋＪ／ｍｍの溶接にて−１０℃でのエネルギーが１００Ｊ以上となった。
成分のいずれかが本発明で規定する範囲から外れた比較例の記号Ｘ１〜Ｘ７は、強度、靭性、溶接性の少なくとも１つが目標に達していなかった。
As is clear from Table 2, Examples 1 to 12 of the present invention all have strengths of 570 MPa or more in TS, 450 MPa or more in YS, and vTs of −40 ° C. or less. Further, the energy at −10 ° C. was 100 J or more by welding at 300 kJ / mm.
As for the symbols X1 to X7 of Comparative Examples in which any of the components deviated from the range defined in the present invention, at least one of strength, toughness, and weldability did not reach the target.
ＴＳが５７０ＭＰａ以上、ＹＳが４５０ＭＰａ以上、JIS Z 2202 に規定の１０ｍｍ幅、Ｖノッチシャルピー衝撃試験片を用いた衝撃試験でのｖＴｓが−４０℃以下で、３００ｋＪ／ｍｍでの溶接時の−１０℃での吸収エネルギーが１００Ｊ以上の鋼が得られ、種々の溶接構造物に用いることができる。
TS of 570 MPa or more, YS of 450 MPa or more, 10 mm width as defined in JIS Z 2202, vTs in an impact test using a V-notch Charpy impact test piece is −40 ° C. or less, and −10 when welding at 300 kJ / mm Steel having an absorption energy at 100 ° C. of 100 J or more can be obtained and used for various welded structures.
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