JP4057711B2 - Rolled steel material excellent in weather resistance and fatigue resistance and method for producing the same - Google Patents

Description

【００１７】
図２に、Ｍｏ、Ｃｒ、Ｍｏ＋Ｃｒの添加量を変化させた場合における、各々の合金添加量と粒界酸化の粒界総長との関係を示す。（試料表面での断面長さ６０ｍｍ中に存在する粒界酸化部の長さの合計。）また、図３ａにＣｒフリー（Ｃｒ無添加）鋼の断面組織写真を、また、図３ｂにＣｒ：０．２０％添加鋼の断面組織写真をそれぞれ示した。この両者の断面組織写真から分かるように、Ｃｒ：０．１〜０．５％添加によって粒界酸化が顕著に抑制されていることが明らかである。一方、Ｍｏは、図２からも分かるように粒界酸化を促進する傾向がある。
【００１８】
更に、本発明者らは、Ｍｏ：０．２０％、Ｃｒ：０．２％、Ｍｏ：０．１％＋Ｃｒ：０．１％をそれぞれ添加した鋼についてＣＭＡ解析を行ったところ、Ｍｏはスケール中に酸化物として分散しているのに対し、Ｃｒは内部酸化層内にＣｒ酸化物として分散していることが判明した。この傾向は、ＭｏとＣｒを複合添加した場合においては極めて顕著になり、Ｍｏはスケール中と内部酸化層の表面とに、Ｃｒは内部酸化層中にのみ存在することも分かった。更に、Ｃｒ：０．２０％添加鋼のＣＭＡ解析した同一部位についての、Ｃｒと〔Ｏ〕の複合濃度分布を調査した結果、〔Ｏ〕の閾値レベルを下げていくと、Ｃｒ酸化物の分布領域がスケール／内部酸化層界面付近から内部の方に拡がっており、Ｃｒ酸化物中のＯ／Ｃｒ比が低減する傾向が認められることも分かった。更に、上記の鋼と同一試料の内部酸化層の深さ方向中央部についてＳＥＭ解析を行ったところ、Ｍｏ：０．２０％鋼の粒界酸化層の先端部では、ファイヤライト（２ＦｅＯ・ＳｉＯ₂ ）と推定されるＳｉとＯが検出され、内部酸化層中の酸化物粒子からはＳｉとＯに加え、Ｍｎが検出された。一方、Ｃｒ：０．２０％添加鋼では、内部酸化層中の酸化物粒子にはＳｉとＯに加えてＣｒも検出された。
【００１９】
そこで、耐候性を向上させるための種々の要因を検討し、前述のＣｒ添加による粒界酸化層の生成を抑制する機構が以下の要因に起因するものと考えられる。
▲１▼ 酸素は、表面からγ粒界をパスに内方拡散するが、ＣｒはＦｅより酸化し易いために直ちにＣｒ酸化物を生成するため、粒界酸化層を形成しない。
▲２▼ Ｃｒ₂ Ｏ₃ と、ＦｅＯとは容易にＦｅＯ・Ｃｒ₂ Ｏ₃ スピネルを生成し、このスピネルには、多量の陽イオン空孔を要すると考えられ、この陽イオン空孔を介して拡散するＣｒおよびＦｅイオンとγ粒界を経て内方拡散してくる酸素とが化合し、酸化物を形成するために、酸素の粒界拡散が阻害される。
【００２０】
３）ＦｅＯ・Ｃｒ₂ Ｏ₃ スピネルを生成することにより、低融点のファイヤライトの生成が抑制され、粒界酸化層を形成しない。
このように、本発明においては、上述のファイヤライト生成の原因となるＳｉを極力低減させ、内部酸化層を極端に薄くし、更に、Ｍｎ量の低減により、孔食の起点となり耐候性を著しく阻害するＭｎＳの生成を少なくすることで、耐孔食性および耐候性に優れた高張力Ｈ形鋼が得られる。また、本発明においては含有Ｓ量の低減に加え、Ｃａ，Ｍｇを添加することで硫化物生成により固溶Ｓ量も併せて低減可能になるものである。
【００２１】
更に、本発明においては、前述の耐候性向上の要因を製造プロセスの観点から探索し、Ｎｉ，Ｃｕ，Ｍｏが添加された高張力Ｈ形鋼の場合には、厚み２μｍ以下の内部酸化層上にＮｉ，Ｃｕ，Ｍｏの濃化層が形成され、その濃化層形成量がスラブ加熱温度の高低に非常に左右されることを知見し、特に、スラブ加熱が１１００℃〜１３００℃、好ましくは１３００℃で４．５時間、という高温で行われる場合には図４ｂに示すように、前述のＮｉ，Ｃｕ，Ｍｏの濃化層が２μｍ以上の厚みで形成されていることも知見した。一方、従来のような１１００℃以下という低温スラブ加熱の場合では、前記濃化層は生成されないか、生成しても極めて薄い濃化層であることが分かり、このために。腐食および孔食深さも抑制され、安定錆の生成速度上昇効果による 耐候性向上が図れるものである。
【００２２】
一方、耐疲労強度という観点からみると、前述したように、鉄（ＦｅＯ）より酸化し易いＳｉ、Ｍｎのそれぞれの量を低減させることによって腐食を起点として作用する内部酸化層の生成を著しく抑制することにより、内部酸化層の生成に伴う軟化層・粒界酸化層による疲労強度低下を防止することができる。なお、前記粒界酸化層はノッチ効果による応力集中を生じ、同様に疲労強度低下させる原因ともなっている。また、Ｓｉ量を低減させることによって、粒界酸化ファイヤライト層の生成抑制作用から疲労強度が上昇させることができる。更に、前述したような１１００℃〜１３００℃、好ましくは１３００℃で４．５時間、という高温スラブ加熱により、酸化による内部酸化層上へのＮｉ，Ｃｕ，Ｍｏの濃化層が２μｍ以下の厚みで形成されるため、表面層内部酸化層の軟化抑制効果によって疲労強度が上昇する。また、この疲労強度は、降伏強度および引張強度とほぼ直線的な関係にあるため、降伏強度および引張強度の上昇に伴い疲労強度も上昇することになる。
【００２３】
次に、本発明による耐候性および耐疲労特性に優れた圧延鋼材の合金成分範囲とその製造方法について詳細に説明する。
炭素（Ｃ）は、４０〜７０kgf 級のＨ形鋼の母材の降伏強度および引張強度を確保するために、０．０２〜０．２０％の範囲で添加する。
珪素（Ｓｉ）は、母材の強度確保、溶鋼の予備脱酸などに必要であるが、０．１％以上の添加は、ＭｎＳｉ・Ｏを形成し、内部酸化層増加、および粒界酸化を促す２ＳｉＯ ₂ＦｅＯを形成する傾向を強めることになるので少ない程好ましく、上限を０．１％とする。
【００２４】
マンガン（Ｍｎ）は、母材の強度確保に必要な元素であるが、母材および溶接部の靱性および割れ性に対する許容濃度、およびＭｎＳを生成し、孔食の起点となり耐候性を著しく阻害するため、その上限を２．０％とする必要がある。
クロム（Ｃｒ）は、本発明においては重要な元素であり、ＦｅＯ・Ｃｒ₂ Ｏ₃ スピネルを生成することにより、低融点のファイヤライトを生成が抑制して粒界酸化層を形成しないために、また母材強度上昇の意味からも、少なくとも０．１％以上は必要であるが０．５％を超える過剰な添加は、Ｃｒ・Ｏとなって内部酸化層を形成して腐食の起点となるため、その上限を０．５％とする。
【００２５】
アルミニウム（Ａｌ）は、強力な脱酸元素であり、脱酸と鋼の清浄化およびＡｌＮを析出させ固溶Ｎを固定し、靱性を向上させるために０．１％を上限として添加される。しかし、Ｃａ，Ｍｇ，ＲＥＭ等を添加し、これらの微細酸化物を積極的に利用する場合には、多量のＡｌ量添加ではＣａ，Ｍｇ，ＲＥＭ等の微細酸化物形成を阻害するために、できるだけ少ない方が好ましい。
【００２６】
チタン（Ｔｉ）は、ＴｉＮを析出し、固溶Ｎを低減することにより島状マルテンサイトの生成を抑制し、微細析出したＴｉＮはγ相の微細化に寄与する。これらのＴｉの作用により組織を微細化し強度・靱性を向上させためにも０．００５％は必要である。しかし、０．０２５％以上の過剰な添加は、ＴｉＣを析出し、その析出効果により母材および溶接熱影響部の靱性を劣化させるので上限を０．０２５％とした。
【００２７】
次に、本発明ではＮｉ、Ｃｕ、Ｍｏの添加が必須となる。これらの元素は共に高強度化元素として、いずれも母材の靱性を高め、しかも内部酸化層上に２μｍ以上のＮｉ、Ｃｕ、Ｍｏを濃化層を形成する重要な元素である。Ｎｉの添加量は、０．３〜３．０％、Ｃｕは０．３〜１．５％の範囲で添加される。Ｍｏは母材強度および高温強度確保に有効な元素であるが、過剰な添加はＭｏ炭化物を析出して固溶Ｍｏとして焼き入れ性向上効果が飽和するので０．１〜０．７％の範囲で添加する必要がある。
【００２８】
マグネシウム、Ｃａは孔食の起点となり耐候性を低下させるＭｎＳの生成を防止する目的で、より高温安定性の高いＭｇ，Ｃａの硫化物を形成させイオウを固定するために添加するものである。
また、マグネシウム（Ｍｇ）は、合金化によりＭｇ含有濃度を低減し、溶鋼への添加時の脱酸反応を抑制し、添加時の安全確保とＭｇの歩留まりを向上させ、更にＭｇＯの微細酸化物を生成させ、これらを微細分散させることにより鋼の強度および靱性向上に寄与させる目的で０．０００５〜０．０１０％添加する。また、Ｃａはスラブ割れ防止の目的から０．０００５〜０．００５％の範囲で添加される。
【００２９】
ニオブ（Ｎｂ）およびバナジウム（Ｖ）は、焼き入性を上昇させ、強度を増加させる目的から、Ｎｂ：０．００５〜０．１０％、Ｖ：０．０１〜０．２０％がそれぞれ添加される。しかし、Ｎｂの場合には０．００５％、Ｖの場合には０．２０％を超えるとＮｂ炭窒化物或いはＶ炭窒化物の析出量が増加し、固溶Ｎｂ或いは固溶Ｖとしての効果が飽和するためＮｂ：０．１０％、Ｖ：０．２０％を上限とし、また、焼き入れ性、母材の強度確保の点からは下限をＮｂ：０．００５％、Ｖ：０．０１％とした。
【００３０】
ボロン（Ｂ）は、鋼材の焼き入れ性に重要な元素であり、０．０００３〜０．００３０％添加される。
窒素（Ｎ）は、窒化物を形成し、γ粒の結晶化に寄与するが、過剰な固溶Ｎは靱性を劣化させるのでＮの含有量は０．００１〜０．０１０％添加される。
Ｎｉ／Ｃｕ≦０．８にする理由は、Ｃｕ添加鋼の高温加熱による表面割れを防止するためである。この割れは、１１００℃以上の高温加熱により内部酸化層上にＣｕが濃縮し、溶融Ｃｕがγ粒界に侵入しＣｕ溶融割れを生じる。この防止には、１１００℃以下の低温加熱をするか、Ｎｉ／Ｃｕ≦０．８のＮｉ添加し高融点化することにより防止できる。
【００３１】
鋼材表面の内部酸化層の厚さ≦２μｍとする理由は、実際に、２０μｍ厚さの内部酸化層存在はおよそ２０倍の２００μｍ深さまで表面軟化層を形成させる。内部酸化層厚さ２μｍでは表面軟化層深さ２０μｍとなり疲労および腐食の防止には限界の厚さであることから内部酸化層２μｍ以下とした。
Ｎｉ，Ｃｕ，Ｍｏの濃化層の厚さ≧２μｍとする理由は、ＥＰＭＡでの測定結果から、Ｎｉ，Ｃｕ，Ｍｏ濃化層厚さが２μｍ以下では耐候性効果が小さいことが塩水噴霧試験により確認された。
【００３２】
元素濃度の総量４．０重量％以上とする理由は、１２５０℃の加熱実験によると、内部酸化層上へのＣｕ，Ｎｉの濃化度は、およそ５〜１０倍であり、Ｍｏは２〜５倍であった。しかも、これらの濃度の総和が４．０重量％以下では目標の耐候性・疲労特性が達成できないためである。
次に、本発明における製造方法について説明する。
【００３３】
本発明において重要なプロセスは、スラブ加熱温度を１１００〜１３００℃の高温スラブ加熱を行う必要がある。これは、前述の高温スラブ加熱において、高温加熱酸化により内部酸化層上へのＮｉ，Ｃｕ，Ｍｏの濃化層を２μｍ以上の厚さで形成させるものである。
高温加熱酸化において、内部酸化層上へＮｉ，Ｃｕ，Ｍｏが濃化する理由は、これら金属の酸化物の生成エネルギーは鉄酸化物（ＦｅＯ）より高いため、酸化物を生成できず内部酸化層上に取り残され濃化するためである。
【００３４】
１２５０℃加熱結果では、Ｎｉ，Ｃｕ，Ｍｏの濃化層が、およそ３０μｍ厚さほど形成される。これが圧延により延伸され、延伸比に対応しほぼ比例して薄くなる。すなわち、厚さが１／１０になった場合は、ほぼその厚さは３μｍとなる。
更に、前述のように、高温で加熱されたスラブは熱間圧延に付されるが、この熱間圧延においては、９５０℃以下での累積圧下率が４０％以上となる圧延を行う必要がある。
【００３５】
９５０℃以下での累積圧下率が４０％以上で熱延するのは、制御圧延により組織微細化を達成するには、オーステナイトの再結晶・未再結晶温度域において、４０％以上の圧下を加える必要があるためである。
【００３６】
【実施例】
＜実施例１＞
試作Ｈ形鋼として、表１に示す本発明鋼と比較鋼についての化学成分値を有する鋼を転炉溶製し、合金を添加後、予備脱酸処理を行い、溶鋼の酸素濃度を調整後、Ｃａ，Ｍｇ合金を添加し、連続鋳造により２５０〜３００ｍｍ厚鋳片に鋳造した。
【００３７】
【表２】

【００３８】
鋳片の冷却はモールド下方の二次冷却帯の水量と鋳片の引き抜き速度の選択により制御した。このようにして得た鋳片を１２８０℃の高温で加熱し、粗圧延工程を経て図５に示すユニバーサル圧延装置列でＨ形鋼に圧延した。この時の圧延・加速冷却条件を表２に示した。
【００３９】
【表３】

【００４０】
この圧延で得られたＨ形鋼の機械的特性を表３に示した。
【００４１】
【表４】

【００４２】
また、疲労特性を図６に示した。図７にＨ形鋼の断面形状および機械試験片の採取位置を示した。図７において、フランジ２の板厚ｔ₂ の中心部（１／２ｔ₂ ）でフランジ幅全長（Ｂ）の１／４幅（１／４Ｂ）から採取した試験片を用い前述の機械的特性を求めた。これらの部位について機械的特性を求めた理由は、フランジ１／４Ｆ部はＨ形鋼の平均的な機械的特性を示し、Ｈ形鋼の機械的特性を代表できると判断したものである。
【００４３】
このように、本発明による鋼組成と製造方法の両者の条件が全て満足された時に表３に示されるＨ形鋼、すなわち、本発明鋼Ａ〜Ｄのように、耐候性・耐疲労性能にすぐれた、高い耐久性を有する高張力圧延形鋼の生産が可能になる。
なお、本発明が対象とする圧延形鋼は、上記実施例のＨ形鋼に限らずＩ形鋼、山形鋼、溝形鋼、不等辺不等厚山形鋼等のフランジを有する形鋼にも適用できることは勿論である。
【００４４】
【発明の効果】
以上述べたように、本発明は、海塩粒子の飛散による鋼の腐食および継手部疲労が懸念される海浜および融雪塩使用地区に施設される橋梁、鉄塔などの鋼構造物部材として使用される耐候性および耐疲労特性に優れた圧延鋼材を低コストで、しかも簡易な製造方法で提供できることが可能になる。
【図面の簡単な説明】
【図１】日本における炭素鋼および耐候性鋼の大気暴露試験の結果を示す図。
【図２】粒界酸化に及ぼすＭｏ，Ｃｒの影響を示す図。
【図３】ａは、従来のＣｒフリー鋼の断面組織図、ｂは，本発明によるＣｒ：０．２０％添加鋼の断面組織図。
【図４】ａは、従来の形鋼におけるＮｉ，Ｃｕ，Ｍｏの濃化層の生成状態を示す図、ｂ，Ｃは本発明によるＮｉ，Ｃｕ，Ｍｏの濃化層の生成状態を示す図。
【図５】本発明において使用されるユニバーサル圧延装置列を示す図。
【図６】引張強さと疲労限の関係を示す図。
【図７】Ｈ形鋼の断面形状および機械試験片の採取位置を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to weather resistance and fatigue resistance used as steel structural members such as bridges and steel towers installed in beaches and snowmelt salt use areas where corrosion of steel due to scattering of sea salt particles and joint fatigue are a concern. The present invention relates to a rolled steel material excellent in and a manufacturing method thereof.
[0002]
[Prior art]
The useful life of steel structures such as bridges and steel towers is determined by the corrosion and fatigue of the steel, but it is possible to significantly extend the service life due to corrosion prevention and fatigue. However, even in the current weather-resistant steel, it is difficult to prevent corrosion without clothing in areas with high chlorine concentration near the beach or in areas where snow melting salt is used, and it is essential to perform anti-corrosion treatment such as regular painting and plating. It has become. In addition, there is a problem in that metal fatigue occurs at joints such as welded joints due to vibration during long-term vehicle travel, and large-scale repair work is required.
[0003]
Fig. 1 shows the results of atmospheric exposure tests of carbon steel and weathering steel in Japan. This data is the atmospheric exposure test results, especially in large coastal industrial zone corrosion, in long-term study period of 10 years, with increasing SO _X concentration in the atmosphere, the thickness which is a measure of the the amount of corrosion In the case of carbon steel, the reduction amount has reached a thickness reduction of 0.5 mm per side, whereas in the weathering steel, it shows an excellent result of 0.2 mm or less. There is an increasing need for various types of steel, and there is a need for further improvements.
[0004]
Various proposals have been made to solve these problems. As typical examples thereof, Japanese Patent Application Laid-Open Nos. 8-134987 and 9-165647 contain C: 0.15% or less, and further contain strengthening elements such as Mn, Ni, and Mo to add Ni + 3Mo. Steel for welded structure excellent in weather resistance adjusted to ≧ 1.2% or Ni + Cu + 3Mo ≧ 1.2% and Ceq: 0.5 or less is disclosed. Japanese Patent Application Laid-Open No. 8-277439 discloses a steel made of lath-like ferrite and cementite, which has a high fatigue strength by forming a metal structure containing martensite with an area ratio of 0.5% or more and 5% or less. A welding heat-affected zone is disclosed. Furthermore, in Japanese Patent Laid-Open No. 9-249915, a proper amount of Mn, Ti and B is added to make the structure a single phase of bainite without depending on the cooling rate. This is used for precipitation and solid solution strengthening to increase tensile strength and improve fatigue resistance, and further, rolling at a reduction rate of 30% or more in a non-recrystallized low temperature range or a two-phase temperature range. It is disclosed that the fatigue limit is increased.
[0005]
However, none of these prior arts can withstand uncovered use in areas close to the beach with high chlorine concentration or in areas where snow melting salt is used. Metal fatigue was caused by vibration during running, and regular large-scale repair work was required.
[0006]
[Problems to be solved by the invention]
The present invention has been made to solve the above-mentioned problems, and steel structures such as bridges and steel towers installed in beaches and in areas where snow melting salt is used, where corrosion of steel due to scattering of sea salt particles and fatigue of joints are a concern. An object of the present invention is to provide a rolled steel material excellent in weather resistance and fatigue resistance in a steel material used as a member, and a manufacturing method thereof.
[0007]
[Means for Solving the Problems]
The present invention relates to a steel material used as a steel structure member such as a bridge or a steel tower installed in a coastal area where snow corrosion and joint fatigue are caused by the scattering of the sea salt particles described above and in the area where snow melting salt is used. In order to suppress the formation of an internal oxide layer that acts as a starting point by adding Cr, to prevent grain boundary oxidation, a small amount of Ni, Cu, Mo is added, the concentration ratio of Ni / Cu is adjusted, By controlling the thickness of the internal oxide layer on the steel surface, the thickness of the concentrated layer of Ni, Cu, and Mo formed on the internal oxide layer, and the total amount of these element concentrations, the weather resistance and fatigue resistance were excellent. The company has succeeded in developing rolled steel, and the summary is as follows.
[0008]
1)% by mass, C: 0.02 to 0.20%,
Mn: 0.4 to 2.0%,
Si: ≦ 0.1%,
Cr: 0.1 to 0.5%,
Al: 0.001 to 0.10%,
Ti: 0.005 to 0.025%,
Ni: 0.3 to 3.0%,
Cu: 0.3 to 1.5%,
Mo: 0.1 to 0.7%,
N: 0.001 to 0.010%,
P: ≦ 0.1%,
S: ≦ 0.006%,
Mg: 0.0005 to 0.010%,
And the Ni / Cu concentration ratio is 0.8 or more, the balance is Fe and inevitable impurities, the inner oxide layer on the steel surface is 2 μm or less, and the Ni oxide has a thickness of 2 μm or more on the inner oxide layer. A rolled steel material excellent in weather resistance and fatigue resistance, characterized by having a concentrated layer of Cu, Mo, and a total amount of these element concentrations of 4.0% by weight or more.
[0010]
2) By mass%, Nb: 0.005 to 0.10%, V: 0.01 to 0.20%, B: 0.0003 to 0.0030% , Ca: 0.0005 to 0.0050 % Rolled rolled steel material excellent in weather resistance and fatigue resistance as described in 2) above.
3) % by mass, C: 0.02 to 0.20%,
Mn: 0.4 to 2.0%,
Si: ≦ 0.1%,
Cr: 0.1 to 0.5%,
Al: 0.001 to 0.10%,
Ti: 0.005 to 0.025%,
Ni: 0.3 to 3.0%,
Cu: 0.3 to 1.5%,
Mo: 0.1 to 0.7%,
N: 0.001 to 0.010%,
P: ≦ 0.1%,
S: ≦ 0.006%,
Mg: 0.0005 to 0.010%,
The Ni / Cu concentration ratio is 0.8 or more, and the slab comprising the balance of Fe and inevitable impurities is reheated to a temperature range of 1100 to 1300 ° C., then rolling is started, and 950 ° C. The steel sheet is hot rolled so that the cumulative rolling reduction is 40% or more, the inner oxide layer on the steel surface is 2 μm or less, and a concentrated layer of Ni, Cu, or Mo having a thickness of 2 μm or more is provided on the inner oxide layer. And the manufacturing method of the rolled steel material excellent in the weather resistance and fatigue resistance characterized by the total amount of these element concentrations being 4.0 weight% or more.
[0011]
5)% by mass, Nb: 0.005 to 0.10%, V: 0.01 to 0.20%, B: 0.0003 to 0.0030% , Ca: 0.0005 to 0.0050 any one or the 4) method for producing a rolled steel with excellent weather resistance and fatigue resistance according to characterized in that it contains two or more%.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
As a result of intensive studies on the mechanism of grain boundary oxidation of 400-700 MPa class H-section steel, the present inventors have found that the formation of the internal oxide layer is closely related to the seam ridge generated on the flange inner surface of the high-strength H-section steel. This seam wrinkle acts as a starting point for corrosion and pitting corrosion and significantly impairs the weather resistance. It was also clarified that this seam wrinkle was generated by the formation of wrinkles at the flange inner surface strain concentration due to slab edging and this folding. As a measure for preventing the occurrence of seam wrinkles, the present inventors. Research was conducted on the formation of grain boundary oxide layer on the slab surface by addition of trace elements that contribute to the suppression of soot formation and its influence, and the suppression of grain boundary oxide layer formation.
[0013]
It has been found that the generation of the internal oxide layer described above and trace elements such as Cr, Ni, Cu, and Mo added as strengthening elements are greatly affected. That is, the internal oxide layer formed on the surface layer of the iron base is formed of a single, complex oxide of Si, Mn, Fe, that is, a dealloyed layer in which Fe and particles such as MnO, SiO ₂ are mixed. These elements are combined with oxygen in the air to form firelite (2SiO ₂ FeO), which is the starting point of corrosion, causing grain boundary oxidation, and the presence of Mn causes MnS to form. It has also been found that it is a starting point for pitting corrosion and significantly impairs the weather resistance.
[0014]
Then, the formation of the grain boundary oxide layer can be suppressed by adding Cr, and corrosion and pitting depth expansion can be suppressed. Further, the grain amount can be reduced by reducing the amount of Si. Suppression of the formation of interfacial oxidized firelite also enabled the suppression of corrosion and pitting corrosion depth expansion. Furthermore, corrosion and pitting depth expansion could be suppressed by preventing the formation of MnS. This can also reduce the amount of S by reducing the amount of solid solution S, and reduces the amount of solid solution S by the sulfide production | generation by Ca and Mg .
[0015]
The present inventors conducted experiments using various steel types on Ni and Cu-added steels with remarkable grain boundary oxidation. As shown in Table 1, a trace amount of Mo. Cr is added and the vacuum-melted ingot is cut in half, heated in a reheating furnace at a temperature within 1300 ° C. for about 4.5 hours, and by grain observation and CMA and SEM analysis, grain boundaries due to these added elements are observed. The influence on oxidation behavior was investigated.
[0016]
[Table 1]

[0017]
FIG. 2 shows the relationship between each alloy addition amount and the total grain boundary length of grain boundary oxidation when the addition amount of Mo, Cr, and Mo + Cr is changed. (The total length of the grain boundary oxidation portion existing in the cross-sectional length of 60 mm on the sample surface.) FIG. 3a shows a cross-sectional structure photograph of Cr-free (Cr-free) steel, and FIG. 3b shows Cr: The cross-sectional structure photographs of 0.20% added steel are shown. As can be seen from the cross-sectional structure photographs of both, it is clear that the grain boundary oxidation is remarkably suppressed by adding Cr: 0.1 to 0.5%. On the other hand, Mo tends to promote grain boundary oxidation as can be seen from FIG.
[0018]
Furthermore, when the present inventors performed CMA analysis about the steel which added Mo: 0.20%, Cr: 0.2%, Mo: 0.1% + Cr: 0.1%, respectively, Mo is a scale. It was found that Cr was dispersed as Cr oxide in the internal oxide layer, whereas it was dispersed as oxide. This tendency becomes very remarkable when Mo and Cr are added in combination, and it has also been found that Mo exists in the scale and on the surface of the internal oxide layer, and Cr exists only in the internal oxide layer. Furthermore, as a result of investigating the composite concentration distribution of Cr and [O] in the same part analyzed by CMA of Cr: 0.20% added steel, the distribution of Cr oxides is reduced when the threshold level of [O] is lowered. It was also found that the region expanded from the vicinity of the scale / internal oxide layer interface toward the inside, and a tendency for the O / Cr ratio in the Cr oxide to decrease was observed. Further, when SEM analysis was performed on the central portion in the depth direction of the internal oxide layer of the same sample as the above steel, at the tip of the grain boundary oxide layer of Mo: 0.20% steel, firelite (2FeO · SiO ₂ In addition to Si and O, Mn was detected from the oxide particles in the internal oxide layer. On the other hand, in addition to Si and O, Cr was also detected in the oxide particles in the internal oxide layer in the Cr: 0.20% added steel.
[0019]
Therefore, various factors for improving the weather resistance are examined, and the mechanism for suppressing the formation of the grain boundary oxide layer due to the above-mentioned Cr addition is considered to be caused by the following factors.
{Circle around (1)} Oxygen diffuses inward from the surface through the γ grain boundary, but since Cr is easier to oxidize than Fe, a Cr oxide is immediately formed, so a grain boundary oxide layer is not formed.
(2) Cr ₂ O ₃ and FeO easily produce FeO · Cr ₂ O ₃ spinel, and it is considered that this spinel requires a large amount of cation vacancies. Since the diffusing Cr and Fe ions combine with oxygen diffused inward via the γ grain boundary to form an oxide, the grain boundary diffusion of oxygen is inhibited.
[0020]
3) By generating FeO · Cr ₂ O ₃ spinel, generation of low melting point firelite is suppressed and a grain boundary oxide layer is not formed.
As described above, in the present invention, Si that causes the above-mentioned generation of firelite is reduced as much as possible, the internal oxide layer is extremely thinned, and further, the Mn content is reduced, thereby causing pitting corrosion and significantly improving the weather resistance. By reducing the generation of MnS to be inhibited, a high-tensile H-section steel excellent in pitting corrosion resistance and weather resistance can be obtained. Further, in the present invention, in addition to the reduction of the content of S, addition of Ca and Mg makes it possible to reduce the amount of solid solution S by the formation of sulfide.
[0021]
Furthermore, in the present invention, the above-mentioned factors for improving the weather resistance are searched from the viewpoint of the manufacturing process, and in the case of a high-tensile H-section steel to which Ni, Cu, and Mo are added, on the internal oxide layer having a thickness of 2 μm or less. It is found that a concentrated layer of Ni, Cu, and Mo is formed, and the amount of the concentrated layer formed is greatly influenced by the slab heating temperature. In particular, the slab heating is 1100 ° C. to 1300 ° C., preferably It has also been found that the Ni, Cu, and Mo enriched layer described above is formed with a thickness of 2 μm or more as shown in FIG. 4B when performed at a high temperature of 1300 ° C. for 4.5 hours. On the other hand, in the case of the conventional low-temperature slab heating of 1100 ° C. or lower, it can be seen that the concentrated layer is not generated or is an extremely thin concentrated layer even if generated. Corrosion and pitting depth are also suppressed, and weather resistance can be improved by increasing the rate of stable rust formation.
[0022]
On the other hand, from the viewpoint of fatigue strength, as described above, the generation of an internal oxide layer that acts as a starting point for corrosion is remarkably suppressed by reducing the amounts of Si and Mn that are more easily oxidized than iron (FeO). By doing so, it is possible to prevent a decrease in fatigue strength due to the softened layer / grain boundary oxide layer accompanying the generation of the internal oxide layer. Note that the grain boundary oxide layer causes stress concentration due to the notch effect, and also causes a decrease in fatigue strength. Further, by reducing the amount of Si, the fatigue strength can be increased due to the effect of suppressing the formation of the grain boundary oxidized firelite layer. Further, by the high temperature slab heating of 1100 ° C. to 1300 ° C., preferably 1300 ° C. for 4.5 hours as described above, the Ni, Cu, Mo concentrated layer on the internal oxide layer by oxidation has a thickness of 2 μm or less. Therefore, the fatigue strength is increased by the effect of suppressing the softening of the internal oxide layer in the surface layer. Moreover, since this fatigue strength has a substantially linear relationship with the yield strength and the tensile strength, the fatigue strength increases with an increase in the yield strength and the tensile strength.
[0023]
Next, the alloy component range of the rolled steel material excellent in weather resistance and fatigue resistance according to the present invention and the production method thereof will be described in detail.
Carbon (C) is added in the range of 0.02 to 0.20% in order to ensure the yield strength and tensile strength of the 40-70 kgf class H-shaped steel base material.
Silicon (Si) is necessary for securing the strength of the base metal and preliminary deoxidation of the molten steel. However, addition of 0.1% or more forms MnSi.O, increases the internal oxide layer, and causes intergranular oxidation. Since the tendency to promote 2SiO ₂ FeO is strengthened, it is preferably as small as possible, and the upper limit is set to 0.1%.
[0024]
Manganese (Mn) is an element necessary for ensuring the strength of the base metal, but it generates an allowable concentration for the toughness and cracking of the base metal and the welded part, and MnS, which becomes the starting point of pitting corrosion and significantly impairs the weather resistance. Therefore, the upper limit needs to be 2.0%.
Chromium (Cr) is an important element in the present invention. By generating FeO · Cr ₂ O ₃ spinel, generation of low melting point firelite is suppressed and a grain boundary oxide layer is not formed. Also, from the viewpoint of increasing the strength of the base material, at least 0.1% is necessary, but excessive addition exceeding 0.5% becomes Cr.O to form an internal oxide layer and become a starting point of corrosion. Therefore, the upper limit is made 0.5%.
[0025]
Aluminum (Al) is a strong deoxidizing element, and is added to the upper limit of 0.1% in order to deoxidize, clean steel, precipitate AlN, fix solid solution N, and improve toughness. However, when Ca, Mg, REM, etc. are added and these fine oxides are actively used, the addition of a large amount of Al inhibits the formation of fine oxides such as Ca, Mg, REM, etc. It is preferable to have as few as possible.
[0026]
Titanium (Ti) precipitates TiN and reduces the solid solution N to suppress the formation of island martensite, and the finely precipitated TiN contributes to the refinement of the γ phase. In order to refine the structure and improve the strength and toughness by the action of Ti, 0.005% is necessary. However, excessive addition of 0.025% or more causes TiC to precipitate and deteriorates the toughness of the base metal and the weld heat affected zone due to the precipitation effect, so the upper limit was made 0.025%.
[0027]
Next, in the present invention, addition of Ni, Cu, and Mo is essential. Both of these elements are high-strength elements, and are all important elements that enhance the toughness of the base material and form a concentrated layer of Ni, Cu, and Mo of 2 μm or more on the internal oxide layer. Ni is added in an amount of 0.3 to 3.0%, and Cu is added in a range of 0.3 to 1.5%. Mo is an element effective for securing the base material strength and high temperature strength, but excessive addition causes Mo carbides to precipitate and saturates the effect of improving hardenability as solid solution Mo, so the range of 0.1 to 0.7% Need to be added.
[0028]
Magnesium and Ca are added to fix sulfur by forming Mg and Ca sulfides with higher stability at higher temperatures in order to prevent the formation of MnS, which is a starting point of pitting corrosion and lowers the weather resistance.
Magnesium (Mg) reduces the Mg content concentration by alloying, suppresses deoxidation reaction when added to molten steel, improves safety during addition and improves the yield of Mg, and further MgO fine oxide In order to contribute to improving the strength and toughness of the steel by finely dispersing them. Ca is added in the range of 0.0005 to 0.005% for the purpose of preventing slab cracking.
[0029]
Niobium (Nb) and vanadium (V) are added in amounts of Nb: 0.005 to 0.10% and V: 0.01 to 0.20% for the purpose of increasing hardenability and increasing strength, respectively. The However, if Nb exceeds 0.005% and V exceeds 0.20%, the amount of Nb carbonitride or V carbonitride deposited increases, and the effect as solid solution Nb or solid solution V is increased. Is saturated with Nb: 0.10% and V: 0.20% as upper limits, and the lower limits are Nb: 0.005% and V: 0.01 from the viewpoint of ensuring hardenability and strength of the base material. %.
[0030]
Boron (B) is an element important for the hardenability of the steel material, and is added in an amount of 0.0003 to 0.0030%.
Nitrogen (N) forms nitrides and contributes to crystallization of γ grains, but excessive solute N deteriorates toughness, so the N content is added in an amount of 0.001 to 0.010%.
The reason why Ni / Cu ≦ 0.8 is to prevent surface cracking due to high-temperature heating of the Cu-added steel. This crack is caused by Cu being concentrated on the internal oxide layer by high-temperature heating at 1100 ° C. or higher, and the molten Cu enters the γ grain boundary to cause Cu melt crack. This can be prevented by heating at a low temperature of 1100 ° C. or lower, or by adding Ni with Ni / Cu ≦ 0.8 to increase the melting point.
[0031]
The reason why the thickness of the internal oxide layer on the steel surface is ≦ 2 μm is that the presence of the internal oxide layer having a thickness of 20 μm actually forms a surface softened layer to a depth of 200 μm, which is approximately 20 times. When the thickness of the internal oxide layer is 2 μm, the surface softened layer has a depth of 20 μm, which is the limit for preventing fatigue and corrosion.
The reason why Ni, Cu, Mo concentrated layer thickness ≧ 2 μm is that the salt spray test shows that the weather resistance effect is small when the Ni, Cu, Mo concentrated layer thickness is 2 μm or less based on the results of EPMA measurement. Confirmed by
[0032]
The reason why the total element concentration is 4.0% by weight or more is that, according to a heating experiment at 1250 ° C., the concentration of Cu and Ni on the internal oxide layer is about 5 to 10 times, and Mo is 2 to 2%. It was 5 times. In addition, if the sum of these concentrations is 4.0% by weight or less, the target weather resistance / fatigue characteristics cannot be achieved.
Next, the manufacturing method in this invention is demonstrated.
[0033]
An important process in the present invention is to perform high-temperature slab heating at a slab heating temperature of 1100 to 1300 ° C. In the above-described high-temperature slab heating, a concentrated layer of Ni, Cu, and Mo is formed on the internal oxide layer with a thickness of 2 μm or more by high-temperature heat oxidation.
The reason why Ni, Cu, and Mo are concentrated on the internal oxide layer in the high-temperature heat oxidation is that the formation energy of these metal oxides is higher than that of iron oxide (FeO), so the oxide cannot be generated and the internal oxide layer. This is because it is left behind and thickens.
[0034]
As a result of heating at 1250 ° C., a concentrated layer of Ni, Cu, and Mo is formed with a thickness of about 30 μm. This is stretched by rolling and thins in proportion to the stretch ratio. That is, when the thickness becomes 1/10, the thickness is approximately 3 μm.
Furthermore, as described above, the slab heated at a high temperature is subjected to hot rolling, and in this hot rolling, it is necessary to perform rolling at a cumulative reduction ratio of 950 ° C. or lower of 40% or higher. .
[0035]
The reason why hot rolling at 950 ° C. or lower with a cumulative rolling reduction of 40% or more is that in order to achieve finer structure by controlled rolling, a rolling of 40% or more is applied in the recrystallization / non-recrystallization temperature range of austenite. This is necessary.
[0036]
【Example】
<Example 1>
As a prototype H-section steel, steels having the chemical composition values of the present invention steel and the comparative steel shown in Table 1 are melted in a converter, added with an alloy, preliminarily deoxidized, and after adjusting the oxygen concentration of the molten steel Ca, Mg alloy was added, and cast into a 250 to 300 mm thick slab by continuous casting.
[0037]
[Table 2]

[0038]
The cooling of the slab was controlled by selecting the amount of water in the secondary cooling zone below the mold and the drawing speed of the slab. The slab thus obtained was heated at a high temperature of 1280 ° C., and was rolled into an H-section steel by a universal rolling apparatus array shown in FIG. 5 through a rough rolling process. The rolling / accelerated cooling conditions at this time are shown in Table 2.
[0039]
[Table 3]

[0040]
Table 3 shows the mechanical properties of the H-shaped steel obtained by this rolling.
[0041]
[Table 4]

[0042]
Further, the fatigue characteristics are shown in FIG. FIG. 7 shows the cross-sectional shape of the H-section steel and the sampling position of the mechanical test piece. In FIG. 7, the above-mentioned mechanical characteristics are obtained by using a test piece taken from a quarter width (1 / 4B) of the flange width overall length (B) at the center portion (1 / 2t ₂ ) of the plate thickness t ₂ of the flange 2. Asked. The reason why the mechanical characteristics were obtained for these parts is that the flange 1 / 4F portion shows the average mechanical characteristics of the H-section steel and is judged to be representative of the mechanical characteristics of the H-section steel.
[0043]
As described above, when both the conditions of the steel composition and the manufacturing method according to the present invention are satisfied, the H-shaped steel shown in Table 3, that is, the inventive steels A to D, is improved in weather resistance and fatigue resistance performance. It becomes possible to produce excellent high-strength rolled sections with high durability.
Note that the rolled shape steel targeted by the present invention is not limited to the H-shape steel of the above-described embodiment, but is also a shape steel having a flange such as an I-shape steel, an angle steel, a groove shape steel, an unequal side unequal thickness angle steel. Of course, it can be applied.
[0044]
【The invention's effect】
As described above, the present invention is used as a steel structure member such as a bridge or a steel tower installed in a beach or in a snow melting salt use area where there is a concern about corrosion of steel due to scattering of sea salt particles and fatigue of a joint. It becomes possible to provide a rolled steel material excellent in weather resistance and fatigue resistance at a low cost and with a simple manufacturing method.
[Brief description of the drawings]
FIG. 1 is a diagram showing the results of atmospheric exposure tests of carbon steel and weathering steel in Japan.
FIG. 2 is a diagram showing the influence of Mo and Cr on grain boundary oxidation.
FIG. 3 a is a cross-sectional structure diagram of conventional Cr-free steel, and b is a cross-sectional structure diagram of Cr: 0.20% added steel according to the present invention.
FIG. 4A is a diagram showing a state of formation of a concentrated layer of Ni, Cu, and Mo in a conventional shape steel, and b and C are diagrams showing a state of formation of a concentrated layer of Ni, Cu, and Mo according to the present invention. .
FIG. 5 is a diagram showing a universal rolling device row used in the present invention.
FIG. 6 is a diagram showing the relationship between tensile strength and fatigue limit.
FIG. 7 is a view showing a cross-sectional shape of an H-section steel and a sampling position of a mechanical test piece.

Claims (4)

% By mass, C: 0.02 to 0.20%,
Mn: 0.4 to 2.0%,
Si: ≦ 0.1%,
Cr: 0.1 to 0.5%,
Al: 0.001 to 0.10%,
Ti: 0.005 to 0.025%,
Ni: 0.3 to 3.0%,
Cu: 0.3 to 1.5%,
Mo: 0.1 to 0.7%,
N: 0.001 to 0.010%,
P: ≦ 0.1%,
S: ≦ 0.006%,
Mg: 0.0005 to 0.010%,
And the Ni / Cu concentration ratio is 0.8 or more, the balance is Fe and inevitable impurities, the inner oxide layer on the steel surface is 2 μm or less, and the Ni oxide has a thickness of 2 μm or more on the inner oxide layer. A rolled steel material excellent in weather resistance and fatigue resistance, characterized by having a concentrated layer of Cu, Mo, and a total amount of these element concentrations of 4.0% by weight or more. Further, Nb: 0.005 to 0.10%, V: 0.01 to 0.20%, B: 0.0003 to 0.0030% , Ca: 0.0005 to 0.0050% The rolled steel material having excellent weather resistance and fatigue resistance according to claim 1, which contains any one or more of them. % By mass, C: 0.02 to 0.20%,
Mn: 0.4 to 2.0%,
Si: ≦ 0.1%,
Cr: 0.1 to 0.5%,
Al: 0.001 to 0.10%,
Ti: 0.005 to 0.025%,
Ni: 0.3 to 3.0%,
Cu: 0.3 to 1.5%,
Mo: 0.1 to 0.7%,
N: 0.001 to 0.010%,
P: ≦ 0.1%,
S: ≦ 0.006%,
Mg: 0.0005 to 0.010%,
The Ni / Cu concentration ratio is 0.8 or more, and the slab comprising the balance of Fe and inevitable impurities is reheated to a temperature range of 1100 to 1300 ° C., then rolling is started, and 950 ° C. The steel sheet is hot rolled so that the cumulative rolling reduction is 40% or more, the inner oxide layer on the steel surface is 2 μm or less, and a concentrated layer of Ni, Cu, or Mo having a thickness of 2 μm or more is provided on the inner oxide layer. And the manufacturing method of the rolled steel material excellent in the weather resistance and fatigue resistance characterized by the total amount of these element concentrations being 4.0 weight% or more. Further, Nb: 0.005 to 0.10%, V: 0.01 to 0.20%, B: 0.0003 to 0.0030% , Ca: 0.0005 to 0.0050% The method for producing a rolled steel material having excellent weather resistance and fatigue resistance according to claim 3, comprising any one kind or two or more kinds.

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