JP3562483B2

JP3562483B2 - Method for manufacturing high-strength thick steel plate with excellent constraint weld cracking resistance

Info

Publication number: JP3562483B2
Application number: JP2001117970A
Authority: JP
Inventors: 章裕山中; 純古川
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 2001-04-17
Filing date: 2001-04-17
Publication date: 2004-09-08
Anticipated expiration: 2021-04-17
Also published as: JP2002316240A

Description

【０００１】
【発明の属する技術分野】
本発明は、耐拘束溶接割れ性に優れた高強度厚鋼板の製造方法に関する。
【０００２】
【従来の技術】
産業機械、圧力容器などに用いられる高強度の厚鋼板の熱間圧延用の素材として、連続鋳造した鋳片が用いられている。ところで、近年、連続鋳造における生産性の向上の要望が高まり、鋳片の厚さを厚くし、速い速度で鋳造することが指向されている。鋳片の厚さが厚く、鋳造速度が速くなると、鋳片内部の凝固界面が鋳造中に破断して、鋳片内部に割れが発生しやすい。
【０００３】
鋳造後の鋳片を切断して調査すると、破断した凝固界面である割れの部分が空孔として観察されることはまれで、切断面を腐食したり、サルファプリントした後に、偏析線として確認できることが多い。このような偏析線とは、凝固界面の破断した部分に偏析成分が濃化した溶鋼が吸引され、そのまま凝固して形成されたものである。
【０００４】
実際に割れている内部割れと、上記のような破断した凝固界面である空孔に偏析成分が濃化した偏析線を、総称して、一般に鋳片の内部割れと呼んでいる（以下、このような偏析線を、単に内部割れと記す場合がある）。
【０００５】
このような内部割れの発生した鋳片を熱間圧延して厚鋼板とすると、鋳片の内部割れは、厚鋼板においてＣ、Ｍｎ、Ｐ、Ｓなどの成分が濃化した偏析部分として残存する。この厚鋼板の偏析部分は、その周りの組織より硬化している。その硬化の程度が著しいと、厚鋼板を溶接する際、または曲げ加工などの際に、その硬化した偏析部分を起点として割れが発生しやすい。とくに、厚鋼板の厚さが厚い程、鋳片の内部割れが厚鋼板に偏析部分として残存しやすくなる。
【０００６】
このような鋳片の内部割れの発生機構は、つぎのように考えられている。すなわち、横断面形状が長方形である、いわゆるスラブの連続鋳造においては、鋳片の非金属介在物対策などの観点から、垂直曲げ型連続鋳造機を用いた鋳造が主流となっており、このような連続鋳造機では、鋳造中の鋳片にさまざまな応力が作用する。たとえば、内部に未凝固部を含む鋳片が引き抜かれて垂直部から湾曲部に進む際に、鋳片は円弧状に曲げられるので、円弧の外側に相当する鋳片内部の凝固殻の凝固界面には、引張り応力が作用して引張り歪が発生する。さらに、内部に未凝固部を含む鋳片が湾曲部から水平部に進む際に、鋳片は矯正されて真っ直ぐになるので、円弧の内側の鋳片の凝固殻の凝固界面には、引張り応力が作用して引張り歪が発生する。
【０００７】
また、未凝固部を含む鋳片が支持用ガイドロールで支持されつつ引き抜かれる際、溶鋼静圧によって鋳片はガイドロール間でバルジングし、バルジングした部分は、その後ガイドロールで圧下される。そのため、圧下された近傍の鋳片内部の凝固殻の凝固界面には、引張り応力が作用して引張り歪が発生する。鋳片は複数の支持用ガイドロールを通過するので、繰り返してバルジングおよび圧下の変形を受けることになり、繰り返して引張り歪みが発生する。さらに、支持用ガイドロールが所定のパスラインよりずれている場合、そのずれに相当する厚さだけ、鋳片のバルジング量が大きくなり、さらに、圧下量も大きくなって、凝固界面に作用する引張り歪が大きくなる。
鋳片の冷却過程で、ＺＳＴ（抗張力発現温度）からＺＤＴ（延性発現温度）までの間に鋳片内部の凝固殻に作用する引張り歪量の総和が、鋼に固有の限界値（内部割れ発生限界歪）を超えると、鋳片の内部割れが発生することが知られている。近年指向されているように、鋳片の厚さが厚くなり、鋳造速度が速くなると、鋳片の抜熱が相対的に低下し、上記のＺＳＴからＺＤＴまでの温度領域に相当する鋳造方向の間隔が長くなり、その間に凝固殻に作用する引張り歪量の総和が多くなる。したがって、鋳片の内部割れの発生の危険性が飛躍的に大きくなるのである。とくに鋳片の厚さが２００ｍｍを超えると、これら内部割れが著しく発生しやすくなる。
【０００８】
ところで、産業機械、圧力容器などに用いられる高強度厚鋼板には、通常、Ｂ含有鋼が用いられている。Ｂを含有させるのは、厚鋼板の焼入れ性を向上させ、厚さ方向に均一な高強度厚鋼板を得るためである。
【０００９】
しかし、鋳片の連続鋳造において、Ｂを含有すると鋳片では、割れ感受性が高くなり、鋳片の内部割れ発生の危険性が高くなる。
【００１０】
このように、鋳片の厚さを厚くし、鋳造速度を速くする近年の連続鋳造では、破断した凝固界面である空孔およびその空孔に偏析成分が濃化した偏析線である鋳片の内部割れが発生しやすく、さらに、Ｂを含有する鋳片では、内部割れがより発生しやすい。このような内部割れの発生した鋳片を素材として熱間圧延した厚鋼板では、溶接時または曲げ加工時などの際に、割れが発生しやすい。
【００１１】
【発明が解決しようとする課題】
本発明は、鋳片の厚さを厚くし、鋳造速度を速くする条件で連続鋳造した鋳片を素材として熱間圧延することを特徴とする、耐拘束溶接割れ性に優れた高強度厚鋼板の製造方法の提供を目的とする。
【００１２】
【課題を解決するための手段】
本発明の要旨は、質量％で、Ｃ：０．０５〜０．２％、Ｓｉ：０．０５〜０．４％、Ｍｎ：０．２〜２％、Ｐ：０．０２％以下、Ｓ：０．００５％以下、Ｂ：０．０００８〜０．００３％を含み、Ｍｎ／Ｓの値が３００以上の炭素鋼または低合金鋼からなる、厚さが６〜１５０ｍｍの高強度厚鋼板の製造方法であって、厚さ２００〜３５０ｍｍの横断面形状が長方形の鋳片を、速度０．７〜２．５ｍ／分で鋳造し、次いで上記鋳片を素材として熱間圧延することを特徴とする、耐拘束溶接割れ性に優れた高強度厚鋼板の製造方法にある。
【００１３】
本発明で規定する「炭素鋼または低合金鋼」とは、上記Ｃ、Ｓｉ、Ｍｎ、Ｐ、ＳおよびＢ以外に、必要に応じて、質量％で、Ａｌ；０．１％以下、Ｃｒ；１．５％以下、Ｍｏ；１．５％以下、Ｎｉ；１．５％以下、Ｃｕ；１．５％以下、Ｔｉ；０．１％以下、Ｎｂ；０．１％以下およびＶ；０．１％以下のうちの１種類または２種類以上を含有し、残部がＦｅおよび不純物からなる鋼を意味する。
【００１４】
破断した凝固界面である空孔、およびその空孔に偏析成分が濃化した偏析線である鋳片の内部割れの発生には、鋳片の厚さ、鋳造速度および鋼の化学組成の条件が影響する。
【００１５】
内部割れに対する鋳片の厚さおよび鋳造速度の影響は、下記のとおりである。すなわち、とくに、０．７〜２．５ｍ／分の高速で鋳造した２００〜３５０ｍｍの厚さの横断面形状が長方形の鋳片には、内部割れが発生しやすい。鋳造速度が０．７〜２．５ｍ／分程度に速くなると、メニスカスからの距離が同じ位置の未凝固部を含む鋳片において、凝固殻の厚さが相対的に薄くなるので、凝固界面で発生するバルジング歪み、曲げ歪み、矯正歪みなどが大きくなり、歪量の総和が大きくなって、鋼に固有の限界値を超え、凝固界面が破断しやすくなる。さらに、鋳片の厚さが２００〜３５０ｍｍ程度に厚くなると、未凝固部を含む鋳片からの抜熱が相対的に低下するので、ＺＳＴ（抗張力発現温度）からＺＤＴ（延性発現温度）までの間隔が長くなり、内部割れが発生しやすくなるのは、前述のとおりである。
【００１６】
つぎに、内部割れに対する鋼の化学組成の影響は、下記のとおりである。すなわち、産業機械、圧力容器などに用いられる高強度厚鋼板には、詳細後述するように、鋼の強度確保のためにＣを、鋼の焼入れ性の向上のためにＢを、それぞれ鋼中に含有させる。また、ＰおよびＳは不純物として鋼中に含有される。しかし、これらＣ、Ｐ、ＳおよびＢは、平衡分配係数が１よりかなり小さいので、鋳片において偏析しやすい。これらが偏析した凝固殻は、割れ感受性が高くなるが、とくにＢを含有する場合に、凝固殻の割れ感受性がより高くなる。さらに、Ｂは、凝固殻と未凝固の溶鋼とのぬれ性を促進させる。このぬれ性が促進されると、未凝固の溶鋼と接する凝固殻の界面、すなわち、凝固界面の割れの原因となる液膜脆化を助長するので、内部割れが発生しやすくなる。ここでいう液膜脆化の現象とは、既に凝固した結晶粒全体が未凝固の溶鋼で液膜状に覆われ、結晶粒同士の接合性が妨げら、凝固組織が脆くなって、内部割れが発生しやすくなる現象のことである。
【００１７】
さらに、本発明者らは、不純物元素であり、偏析しやすいＳの作用を詳細検討した結果、Ｓは凝固殻と未凝固の溶鋼とのぬれ性を促進させ、Ｂと同様に内部割れを発生しやすくする作用があるばかりでなく、ＳおよびＢが同時に溶鋼中に存在すると、上記の液膜脆化の現象が、相乗的に大きく現れることを知見した。つまり、凝固界面近傍にある凝固直前の未凝固の溶鋼中に偏析したＳおよびＢは、その未凝固の溶鋼の融点を、それらＳまたはＢが単独で偏析した場合よりも大きく低下させる。そのため、凝固直前の未凝固の溶鋼の過熱度が上昇し、凝固殻と未凝固の溶鋼とのぬれ性が相乗的に促進し、相乗的に内部割れが発生しやすくなる。
【００１８】
そこで、本発明の高強度厚鋼板では、まず、鋳片の内部割れの発生の要因となるＳおよびＢの含有率をともに低下させる。具体的には、Ｓ含有率は０．００５質量％以下と低くし、Ｂ含有率は０．０００８〜０．００３質量％と低くする。さらに、液膜脆化に対するＢとＳの相乗的な作用を、より確実に低減させるため、ＳおよびＢの含有率を、それぞれ上記範囲内に低くすることに加えて、凝固界面近傍の凝固直前の未凝固の溶鋼中に偏析したＳをＭｎＳとして固定することが効果的であることがわかった。その際、Ｓ含有率に対するＭｎ含有率の比、Ｍｎ／Ｓの値を３００以上とすることが効果的である。
【００１９】
ところで、未凝固の溶鋼中に偏析したＢをＮで固定することも考えられるが、Ｎ含有率を多くすると、鋳片表面に横割れが発生しやすくなるという問題があり、ＢをＮで固定することは効果的な方法ではない。このように、ＳおよびＢの含有率を低くすることに加えて、Ｓ含有率に対するＭｎ含有率の比、Ｍｎ／Ｓの値を高くすることにより、液膜脆化に対するＢとＳの相乗的な作用を低減でき、厚鋼板が速度０．７〜２．５ｍ／分で鋳造した厚さ２００〜３５０ｍｍの横断面形状が長方形の鋳片を素材として熱間圧延した厚鋼板であっても、鋳片の内部割れの発生を防止できるので、鋳片の内部割れに起因する厚鋼板の溶接時または曲げ加工時における割れの発生も防止できる。
【００２０】
【発明の実施の形態】
まず、厚鋼板の化学組成および厚さを以下に説明する。なお、以下の％表示は質量％を意味する。
Ｃ：０．０５〜０．２％
Ｃは、厚鋼板の強度を確保する上で安価で有用な元素であり、所要の強度などの機械的特性による成分設計に基づいて含有率を決めればよい。その効果を発揮させるためには、その下限は０．０５％とする。一方、０．２％を超えて含有させると、厚鋼板の溶接性および加工性を悪化させることから、その上限は０．２％とする。また、Ｃは、鋳片において偏析しやすい元素であり、Ｃが偏析した凝固界面の割れ感受性が高くなるが、上記の範囲内の含有率であれば、とくに影響はない。したがって、Ｃ含有率は、０．０５〜０．２％とする。
【００２１】
Ｓｉ：０．０５〜０．４％
Ｓｉは、通常、溶鋼の脱酸と厚鋼板の強度確保のために含有されるが、厚鋼板の加工性を劣化することなく強度を高めることができる元素であり、その効果を発揮させるためには、その下限は０．０５％とする。０．４％を超えて含有させると厚鋼板の化成処理性が悪化するので、その上限は０．４％とする。したがって、Ｓｉ含有率は、０．０５〜０．４％とする。
【００２２】
Ｍｎ：０．２〜２％で、かつ、Ｍｎ／Ｓの値が３００以上
Ｍｎは、厚鋼板の強度を高める上で有用な元素であり、その効果を発揮させるためには、その下限は０．２％とする。しかし、鋳片において偏析しやすい元素であるので、その上限は２％とする。鋳片の厚さ中心部にＭｎが偏析すると、その鋳片の中心偏析が厚鋼板の厚さ中心部に硬化層として残存し、厚鋼板の曲げ加工時に、その硬化層を起点に割れが発生しやすい。したがって、Ｍｎ含有率は、０．２〜２％とする。Ｍｎは偏析しやすい元素であるが、Ｓ含有率に対してＭｎを適宜多く含有させることにより、鋳片の内部割れの発生を防止できる。すなわち、ＳはＭｎＳとしてＭｎに固定されるので、凝固殻と未凝固の溶鋼とのぬれ性の上昇が抑制され、鋳片の内部割れの発生が抑制される。したがって、Ｓ含有率に対するＭｎ含有率の比であるＭｎ／Ｓの値は３００以上とする。また、この比、Ｍｎ／Ｓの値は１０００以下が望ましい。Ｍｎ／Ｓの値が１０００を超えると、高価なＭｎ合金鉄を多く添加したり、またはＳを大きく低下させるために、製造コストが高くなる。
【００２３】
Ｐ：０．０２％以下
Ｐは、不純物元素であり、また、鋳片において偏析しやすく、Ｐが偏析した凝固界面の割れ感受性は高くなる。しかし、０．０２％以下の範囲内の含有率であれば、とくに影響はない。したがって、Ｐ含有率は、０．０２％以下とする。
【００２４】
Ｓ：０．００５％以下
Ｓは、不純物元素であり、また、鋳片において偏析しやすく、Ｓが偏析した凝固界面の割れ感受性は高くなる。さらに、Ｓは未凝固の溶鋼のぬれ性を上昇させる元素であり、鋳片の内部割れが発生しやすくなるが、前述のとおり、Ｓ含有率に対するＭｎ含有率の比であるＭｎ／Ｓの値は３００以上とするので、上記Ｓの範囲内の含有率であれば、とくに鋳片の内部割れに対する影響はない。したがって、Ｓ含有率は、０．００５％以下とする。
【００２５】
Ｂ：０．０００８〜０．００３％
Ｂは、微量含有させるだけで厚鋼板の焼入れ性を向上させることができる。その効果を発揮させるためには、その下限は０．０００８％とする。一方、Ｂは、鋳片において偏析しやすく、Ｂが偏析した凝固界面の割れ感受性は高くなり、さらに、未凝固の溶鋼のぬれ性を上昇させ、鋳片に内部割れが発生しやすくなるので、その上限は０．００３％とする。したがって、Ｂ含有率は、０．０００８〜０．００３％とする。
【００２６】
本発明の炭素鋼または低合金鋼からなる高強度厚鋼板は、上記Ｃ、Ｓｉ、Ｍｎ、Ｐ、ＳおよびＢ以外に、必要に応じて、Ａｌ；０．１％以下、Ｃｒ；１．５％以下、Ｍｏ；１．５％以下、Ｎｉ；１．５％以下、Ｃｕ；１．５％以下、Ｔｉ；０．１％以下、Ｎｂ；０．１％以下およびＶ；０．１％以下のうちの１種類または２種類以上を含有し、残部がＦｅおよび不純物からなる鋼とする。
【００２７】
これらＡｌ、Ｃｒ、Ｍｏ、Ｎｉ、Ｃｕ、Ｔｉ、ＮｂおよびＶの元素を含有させると、厚鋼板の強度、靱性などの機械的特性が改善される。また、これらの元素は、上記の範囲内の含有率であれば、鋳片の内部割れの発生への影響はない。
【００２８】
本発明の高強度厚鋼板の厚さは６〜１５０ｍｍとする。産業機械、圧力容器などに用いられる厚鋼板の製品用途から、上記厚さとする。さらに、１５０ｍｍを超える厚さの厚鋼板では、後述する厚さの鋳片を素材として熱間圧延した場合に、所要の厚鋼板の強度、靱性などの機械的特性が得られにくい。
【００２９】
つぎに、高強度厚鋼板の熱間圧延用の素材である鋳片の鋳造方法を説明する。断面形状が長方形で厚さ２００〜３５０ｍｍの鋳片を速度０．７〜２．５ｍ／分の条件で鋳造する。
【００３０】
鋳片の厚さが２００ｍｍ未満では、上記鋳造速度の範囲内において、鋳片の生産性が低い。また、鋳片の厚さが３５０ｍｍを超えると、鋳片の内部割れが発生しやすい。また、連続鋳造機が大型になる。したがって、鋳片の厚さは２００〜３５０ｍｍとする。
【００３１】
鋳片の厚さを厚鋼板の厚さで除した圧下比（鋳片の厚さ／厚鋼板の厚さ）が１．５以上となるように、厚鋼板の厚さに応じた鋳片の厚さを確保するのが望ましい。上記圧下比が１．５未満では、鋳片の凝固組織が厚鋼板にまで残存し、厚鋼板の強度、靱性などに影響するからである。
【００３２】
鋳造速度が０．７ｍ／分未満では、鋳片の内部割れの発生を抑制できるが、上記鋳片の厚さの範囲内において、鋳片の生産性が低い。また、２．５ｍ／分を超えると、鋳片表面に割れが発生しやすく、また内部割れが発生しやすい。したがって、鋳造速度は０．７〜２．５ｍ／分とする。
【００３３】
厚さ２００〜３５０ｍｍの鋳片を、０．７〜２．５ｍ／分の速度で鋳造する場合に、前述の化学組成とすることにより、鋳片の内部割れの発生を防止することができる。
【００３４】
鋳片を熱間圧延して高強度厚鋼板を得る際に、圧延前の鋳片の加熱温度、加熱時間などの加熱条件、および圧延温度、仕上温度などの圧延条件は、鋼に応じた通常の条件とすることができる。
【００３５】
【実施例】
垂直部長さ３ｍ、円弧半径１０ｍ、５点曲げ４点矯正、機長３０ｍの垂直曲げ型連続鋳造機を用い、厚さ２００ｍｍまたは２５０ｍｍ、幅１８００ｍｍの鋳片を鋳造する試験を実施した。連続鋳造機のガイドロールの軸心間距離は、垂直部で２５０ｍｍ、湾曲部で２５０〜４００ｍｍ、水平部で４００〜４５０ｍｍである。鋳片の二次冷却の比水量は１〜２リットル／ｋｇ−鋼とした。
【００３６】
用いた鋼は、後述する表１および表２に示すように、Ｃ含有率が０．０７〜０．２０質量％、Ｂ含有率が０．０００９〜０．００３５質量％である低炭素鋼または中炭素鋼である。後述する試験Ｎｏ．１およびＮｏ．８以外の試験では、Ａｌ、Ｔｉ、ＮｂおよびＶを、１種または２種含有させた鋼を用いた。また鋳造速度は、厚さ２００ｍｍの鋳片の鋳造時には２．０ｍ／分、厚さ２５０ｍｍの鋳片の鋳造時には１．０ｍ／分とした。なお、以下に記載する％は、質量％を意味する。
【００３７】
各試験で得られた鋳片から鋳造方向に長さ１００ｍｍの全幅の横断サンプルを採取し、その横断面をサルファプリントした後、破断した凝固界面である空孔およびその空孔に偏析成分が濃化した偏析線である鋳片の内部割れの発生の有無を目視により調査した。
【００３８】
得られた鋳片を熱間圧延し、厚さ１０〜１２０ｍｍの厚鋼板とした。その際、鋳片を１０００〜１３００℃の温度範囲内で加熱し、７５０〜１０００℃で圧延を終了した。得られた厚鋼板の厚さ中心部相当の位置から、ＪＩＳＺ２２０１で規定される４号または５号試験片を圧延方向に垂直な方向に採取し、引張試験を行った。
【００３９】
また、得られた厚鋼板から溶接試験用サンプルを採取し、拘束溶接割れ試験を行い、熱影響部において割れが発生するかどうかを調査した。
図１は、拘束溶接割れ試験に用いる試験片の形状、寸法を示す図である。図１（ａ）は開先の横断面を示す側面図、図１（ｂ）は開先を上から見た平面図である。図中の符号ａは厚鋼板の厚さを示し、１０〜１２０ｍｍの範囲内の厚さである。符号ｂは拘束板の厚さを示し、用いる厚鋼板の厚さに対応して、６０〜２００ｍｍとした。厚鋼板および拘束板の縦横寸法である符号ｆおよびｇの寸法は、それぞれ５００ｍｍとした。厚鋼板と拘束板を重ね、開先と反対側の下端部を溶接試験前に事前に溶接した。符号ｈが、その溶接部である。この溶接部の符号ｃで示す寸法は約１０ｍｍ、符号ｄで示す寸法は約２０ｍｍとした。開先角度は９０゜とし、開先深さｅは２０ｍｍとした。
【００４０】
被覆アーク溶接棒を用い、下向姿勢で溶接を行い、溶接ビードは符号ｇで示す長さとした。溶接後、図１（ｂ）中に示すＡ１−Ａ２線部分を機械加工により切断し、その横断面の溶接熱影響部の割れの発生の有無をダイチェックして観察した。試験条件および試験結果を、表１および表２に示す。
【００４１】
【表１】

【表２】

本発明例の試験Ｎｏ．１では、厚さ２００ｍｍの鋳片を速度２．０ｍ／分で鋳造した。用いた鋼は中炭素鋼で、Ｍｎ含有率１．１５％、Ｓ含有率０．００２０％で、Ｍｎ／Ｓの値は５７５である。得られた鋳片を熱間圧延し、厚さ１０ｍｍの厚鋼板とした。これら化学組成、鋳片の厚さ、鋳造速度および厚鋼板の厚さの各条件は、本発明で規定する条件の範囲内である。この試験では、鋳片の内部割れは発生せず、また、厚鋼板の拘束溶接割れ試験において、熱影響部に割れは発生しなかった。得られた厚鋼板の引張強さは６７４ＭＰａで、高強度の厚鋼板であった。
【００４２】
本発明例の試験Ｎｏ．２では、Ａｌを含有させた中炭素鋼を用いたこと以外は、試験Ｎｏ．１とほぼ同じ条件で試験した。化学組成、鋳片の厚さ、鋳造速度および厚鋼板の厚さの各条件は、本発明で規定する条件の範囲内である。この試験では、鋳片の内部割れは発生しなかった。また、厚鋼板の拘束溶接割れ試験において、熱影響部に割れは発生しなかった。得られた厚鋼板の引張強さは６６９ＭＰａで、高強度の厚鋼板であった。
【００４３】
本発明例の試験Ｎｏ．３では、ＡｌおよびＴｉを含有させた低炭素鋼を用いたこと以外は、試験Ｎｏ．１とほぼ同じ条件で試験した。化学組成、鋳片の厚さ、鋳造速度および厚鋼板の厚さの各条件は、本発明で規定する条件の範囲内である。この試験では、鋳片の内部割れは発生しなかった。また、厚鋼板の拘束溶接割れ試験において、熱影響部に割れは発生しなかった。得られた厚鋼板の引張強さは、試験Ｎｏ．１に比べてＣ含有率が低いために、Ｎｏ．１より低い５４５ＭＰａであった。
【００４４】
本発明例の試験Ｎｏ．４では、Ｎｂを含有させた中炭素鋼を用いたこと以外は、試験Ｎｏ．２とほぼ同じ条件で試験した。化学組成、鋳片の厚さ、鋳造速度および厚鋼板の厚さの各条件は、本発明で規定する条件の範囲内である。この試験では、鋳片の内部割れは発生しなかった。また、厚鋼板の拘束溶接割れ試験において、熱影響部に割れは発生しなかった。得られた厚鋼板の引張強さは６８４ＭＰａで、試験Ｎｏ．２の厚鋼板より高強度であった。
【００４５】
本発明例の試験Ｎｏ．５では、Ｖを含有させた中炭素鋼を用いたこと、厚鋼板の厚さを５０ｍｍとしたこと以外は、試験Ｎｏ．２とほぼ同じ条件で試験した。化学組成、鋳片の厚さ、鋳造速度および厚鋼板の厚さの各条件は、本発明で規定する条件の範囲内である。この試験では、鋳片の内部割れは発生しなかった。また、厚鋼板の拘束溶接割れ試験において、熱影響部に割れは発生しなかった。さらに、得られた厚鋼板の引張強さは６３２ＭＰａで、鋳片の厚さを厚鋼板の厚さで除した圧下比が４であり、試験Ｎｏ．２の圧下比２０に比べて小さいことから、試験Ｎｏ．２より低い強度であった。
【００４６】
本発明例の試験Ｎｏ．６では、厚さ２５０ｍｍの鋳片を速度１．０ｍ／分で鋳造したこと、厚鋼板の厚さを７０ｍｍとしたこと以外は、試験Ｎｏ．５とほぼ同じ条件で試験した。化学組成、鋳片の厚さ、鋳造速度および厚鋼板の厚さの各条件は、本発明で規定する条件の範囲内である。この試験では、鋳片の内部割れは発生しなかった。また、厚鋼板の拘束溶接割れ試験において、熱影響部に割れは発生しなかった。さらに、得られた厚鋼板の引張強さは６３８ＭＰａで、鋳片の厚さを厚鋼板の厚さで除した圧下比が３．６であり、試験Ｎｏ．５の圧下比４より小さい圧下比であるが、Ｂ含有率がＮｏ．５より高いので、試験Ｎｏ．５とほぼ同じ程度の高強度であった。
【００４７】
本発明例の試験Ｎｏ．７では、ＴｉおよびＮｂを含有させた中炭素鋼を用いたこと、厚鋼板の厚さを１２０ｍｍとしたこと以外は、試験Ｎｏ．６とほぼ同じ条件で試験した。化学組成、鋳片の厚さ、鋳造速度および厚鋼板の厚さの各条件は、本発明で規定する条件の範囲内である。この試験では、鋳片の内部割れは発生しなかった。また、厚鋼板の拘束溶接割れ試験において、熱影響部に割れは発生しなかった。さらに、得られた厚鋼板の引張強さは６２５ＭＰａで、鋳片の厚さを厚鋼板の厚さで除した圧下比が２．１であり、試験Ｎｏ．６の圧下比３．６に比べて小さく、試験Ｎｏ．６より低い強度であった。
【００４８】
比較例の試験Ｎｏ．８では、Ｓ含有率を０．００４０％と高くし、Ｍｎ／Ｓの値を２８８と低くすること以外は、試験Ｎｏ．１とほぼ同じ条件で試験した。Ｍｎ／Ｓの値が、本発明で規定する条件を外れて低い値である。この試験では、鋳片に著しい偏析線である内部割れが発生した。また、得られた厚鋼板の引張強さは６８２ＭＰａで、試験Ｎｏ．１とほぼ同じ高強度であったが、厚鋼板の拘束溶接割れ試験において、熱影響部に割れが発生した。
【００４９】
比較例の試験Ｎｏ．９では、Ｓ含有率を０．００４３％と高くし、Ｍｎ／Ｓの値を２８６と低くすること以外は、試験Ｎｏ．３とほぼ同じ条件で試験した。Ｍｎ／Ｓの値が、本発明で規定する条件を外れて低い値である。この試験では、鋳片に著しい偏析線である内部割れが発生した。また、得られた厚鋼板の引張強さは５４９ＭＰａで、試験Ｎｏ．３とほぼ同じ強度であったが、厚鋼板の拘束溶接割れ試験において、熱影響部に割れが発生した。
【００５０】
比較例の試験Ｎｏ．１０では、Ｓ含有率を０．００４５％と高くし、Ｍｎ／Ｓの値を２７８と低くすること以外は、試験Ｎｏ．５とほぼ同じ条件で試験した。Ｍｎ／Ｓの値が、本発明で規定する条件を外れて低い値である。この試験では、鋳片に著しい偏析線である内部割れが発生した。また、得られた厚鋼板の引張強さは６２５ＭＰａで、試験Ｎｏ．５とほぼ同じ高強度であったが、厚鋼板の拘束溶接割れ試験において、熱影響部に割れが発生した。
【００５１】
比較例の試験Ｎｏ．１１では、Ｍｎ含有率を１．４０％と高くしたが、Ｓ含有率も０．００４８％と高くし、Ｍｎ／Ｓの値を２９２と低くすること以外は、試験Ｎｏ．６とほぼ同じ条件で試験した。Ｍｎ／Ｓの値が、本発明で規定する条件を外れて低い値である。この試験では、鋳片に著しい偏析線である内部割れが発生した。得られた厚鋼板の引張強さは、Ｂ含有率が試験Ｎｏ．６より低いので、試験Ｎｏ．６より低い６２９ＭＰａであった。また、厚鋼板の拘束溶接割れ試験において、熱影響部に割れが発生した。
【００５２】
比較例の試験Ｎｏ．１２では、Ｓ含有率を０．００２４％と低くしたが、Ｍｎ含有率も０．７０％と低くし、Ｍｎ／Ｓの値を２９２と低くすること以外は、試験Ｎｏ．７とほぼ同じ条件で試験した。Ｍｎ／Ｓの値が、本発明で規定する条件を外れて低い値である。この試験では、鋳片に著しい偏析線である内部割れが発生した。また、得られた厚鋼板の引張強さは６２８ＭＰａで、試験Ｎｏ．７とほぼ同じ高強度であったが、厚鋼板の拘束溶接割れ試験において、熱影響部に割れが発生した。
【００５３】
比較例の試験Ｎｏ．１３では、Ｂ含有率を０．００３２％と高くしたこと以外は、試験Ｎｏ．６とほぼ同じ条件で試験した。Ｂ含有率が、本発明で規定する条件を外れて高い値である。この試験では、鋳片に著しい偏析線である内部割れが発生した。また、得られた厚鋼板の引張強さは、試験Ｎｏ．６よりＢ含有率が高いので、試験Ｎｏ．６より高い６５４ＭＰａであったが、厚鋼板の拘束溶接割れ試験において、熱影響部に割れが発生した。
【００５４】
比較例の試験Ｎｏ．１４では、Ｂ含有率を０．００３５％と高くしたこと以外は、試験Ｎｏ．７とほぼ同じ条件で試験した。Ｂ含有率が、本発明で規定する条件を外れて高い値である。この試験では、鋳片に著しい偏析線である内部割れが発生した。また、得られた厚鋼板の引張強さは、試験Ｎｏ．７よりＢ含有率が高いので、試験Ｎｏ．７より高い６３８ＭＰａであったが、厚鋼板の拘束溶接割れ試験において、熱影響部に割れが発生した。
【００５５】
【発明の効果】
本発明の適用により、鋳片の厚さを厚くし、鋳造速度を速くする条件で鋳造した鋳片を素材として熱間圧延することによって、耐拘束溶接割れ性に優れた高強度厚鋼板を得ることができる。
【図面の簡単な説明】
【図１】拘束溶接割れ試験に用いる試験片の形状、寸法を示す図である。
【符号の説明】
ａ：厚鋼板の厚さｂ：拘束板の厚さ
ｃ：溶接部の寸法ｄ：溶接部の寸法
ｅ：開先深さｆ：厚鋼板および拘束板の縦寸法
ｇ：厚鋼板および拘束板の横寸法ｈ：厚鋼板を拘束するための溶接部[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention Excellent constraint weld cracking resistance The present invention relates to a method for manufacturing a high-strength steel plate.
[0002]
[Prior art]
BACKGROUND ART Continuously cast slabs are used as a material for hot rolling of high-strength steel plates used for industrial machines, pressure vessels, and the like. In recent years, there has been an increasing demand for improvement in productivity in continuous casting, and it has been directed to increase the thickness of cast slabs and perform casting at a high speed. When the thickness of the slab is large and the casting speed is high, the solidification interface inside the slab breaks during casting, and cracks easily occur inside the slab.
[0003]
When investigating the cast slab after cutting it, it is rare that the cracked part, which is the broken solidification interface, is rarely observed as a hole, and it can be confirmed as a segregation line after corroding the cut surface or sulfur printing. There are many. Such a segregation line is formed by sucking molten steel in which a segregation component is concentrated at a broken portion of a solidification interface and solidifying the molten steel as it is.
[0004]
The internal cracks that are actually cracked and the segregation lines in which the segregation component is concentrated in the pores, which are the solidification interfaces that have been broken as described above, are generally referred to as internal cracks of the slab (hereinafter, referred to as the internal cracks). Such a segregation line may be simply referred to as an internal crack).
[0005]
When a slab having such internal cracks is hot-rolled into a thick steel plate, the internal cracks of the slab remain as segregated portions in which components such as C, Mn, P, and S are concentrated in the thick steel plate. . The segregated portion of this thick steel plate is hardened by the structure around it. If the degree of hardening is remarkable, cracks are likely to occur from the hardened segregated portion when welding a thick steel plate or bending. In particular, as the thickness of the thick steel plate increases, internal cracks in the slab tend to remain as segregated portions in the thick steel plate.
[0006]
The mechanism of occurrence of such internal cracks in the slab is considered as follows. That is, in the continuous casting of a so-called slab having a rectangular cross-sectional shape, from the viewpoint of measures against non-metallic inclusions in the slab, casting using a vertical bending type continuous casting machine has become mainstream, In a continuous caster, various stresses act on the slab during casting. For example, when a slab including an unsolidified portion is pulled out and proceeds from a vertical portion to a curved portion, the slab is bent into an arc shape, so that a solidification interface of a solidified shell inside the slab corresponding to the outside of the arc is formed. , A tensile stress acts to generate tensile strain. Furthermore, when a slab including an unsolidified portion inside goes from a curved portion to a horizontal portion, the slab is straightened and straightened, so that the solidification interface of the solidified shell of the slab inside the arc has a tensile stress. Acts to generate tensile strain.
[0007]
In addition, when the slab including the unsolidified portion is pulled out while being supported by the supporting guide rolls, the slab bulges between the guide rolls due to the molten steel static pressure, and the bulged portion is thereafter reduced by the guide rolls. Therefore, a tensile stress acts on the solidification interface of the solidified shell inside the cast slab in the vicinity of the reduced area, causing tensile strain. Since the slab passes through the plurality of supporting guide rolls, it is repeatedly subjected to bulging and deformation under pressure, and tensile strain is repeatedly generated. Further, when the supporting guide roll is displaced from the predetermined pass line, the bulging amount of the slab is increased by a thickness corresponding to the deviation, and the rolling reduction is also increased, so that the tension acting on the solidification interface is increased. The distortion increases.
In the process of cooling the slab, the sum of the amount of tensile strain acting on the solidified shell inside the slab from ZST (tensile strength onset) to ZDT (ductile onset temperature) is a critical value (internal cracking It is known that when the critical strain is exceeded, internal cracks of the slab occur. As directed in recent years, when the thickness of the slab is increased and the casting speed is increased, the heat removal of the slab relatively decreases, and the casting direction corresponding to the temperature range from ZST to ZDT described above. The interval increases, during which the total amount of tensile strain acting on the solidified shell increases. Therefore, the danger of the occurrence of internal cracks in the slab is greatly increased. In particular, when the thickness of the slab exceeds 200 mm, these internal cracks are liable to occur remarkably.
[0008]
Incidentally, B-containing steel is usually used for high-strength steel plates used for industrial machines, pressure vessels, and the like. B is contained in order to improve the hardenability of the thick steel plate and to obtain a uniform high-strength steel plate in the thickness direction.
[0009]
However, in the continuous casting of cast slabs, if B is contained, the cast slabs have high cracking susceptibility, and the risk of occurrence of internal cracks in the cast slabs is increased.
[0010]
As described above, in recent continuous casting in which the thickness of the slab is increased and the casting speed is increased, the slab which is a segregation line in which a segregation component is concentrated in a vacancy which is a broken solidification interface and a segregation component is concentrated in the vacancy. Internal cracks are apt to occur, and in the case of slabs containing B, internal cracks are more likely to occur. In a thick steel plate hot-rolled using a slab having such internal cracks as a raw material, cracks are likely to occur at the time of welding or bending.
[0011]
[Problems to be solved by the invention]
The present invention is to hot-roll a slab continuously cast under the condition of increasing the thickness of the slab and increasing the casting speed. Excellent in resistance to weld cracking resistance It is intended to provide a method for manufacturing a high-strength steel plate.
[0012]
[Means for Solving the Problems]
The gist of the present invention is that, in mass%, C: 0.05 to 0.2%, Si: 0.05 to 0.4%, Mn: 0.2 to 2%, P: 0.02% or less, S: : A high-strength steel plate having a thickness of 6 to 150 mm, comprising 0.005% or less, B: 0.0008 to 0.003%, and a carbon steel or a low alloy steel having a Mn / S value of 300 or more. A manufacturing method, in which a slab having a thickness of 200 to 350 mm and a rectangular cross section is cast at a speed of 0.7 to 2.5 m / min, and then hot-rolled using the slab as a raw material. Characterized by excellent resistance to constraint welding cracking It is in a method of manufacturing a high-strength steel plate.
[0013]
The “carbon steel or low alloy steel” defined in the present invention means, in addition to the above C, Si, Mn, P, S and B, if necessary, in mass%, Al; 0.1% or less, Cr; 1.5% or less, Mo: 1.5% or less, Ni: 1.5% or less, Cu: 1.5% or less, Ti: 0.1% or less, Nb: 0.1% or less, and V: 0% or less. It means steel containing one or two or more of 1% or less, with the balance being Fe and impurities.
[0014]
The conditions of the slab thickness, casting speed, and steel chemical composition are necessary for the occurrence of internal cracks in the vacancies, which are the fractured solidification interface, and in the slabs, which are segregation lines in which segregation components are concentrated in the vacancies. Affect.
[0015]
The effects of slab thickness and casting speed on internal cracking are as follows. That is, internal cracks are likely to occur particularly in a slab having a thickness of 200 to 350 mm and a rectangular cross section that is cast at a high speed of 0.7 to 2.5 m / min. When the casting speed is increased to about 0.7 to 2.5 m / min, the thickness of the solidified shell becomes relatively thin in the cast piece including the unsolidified portion at the same distance from the meniscus, The generated bulging strain, bending strain, correction strain, and the like increase, the total amount of strain increases, exceeds the limit value inherent to steel, and the solidification interface easily breaks. Further, when the thickness of the cast slab is increased to about 200 to 350 mm, the heat removal from the cast slab including the unsolidified portion relatively decreases, so that the temperature from ZST (tensile strength developing temperature) to ZDT (ductility developing temperature) is reduced. As described above, the interval becomes longer and internal cracks are more likely to occur.
[0016]
Next, the effect of the chemical composition of steel on internal cracking is as follows. That is, as described in detail later, C for securing the strength of steel and B for improving the hardenability of the steel are added to the high-strength steel plates used for industrial machines and pressure vessels, respectively, as described in detail later. To be included. P and S are contained in steel as impurities. However, since C, P, S, and B have an equilibrium distribution coefficient much smaller than 1, segregation is likely to occur in a slab. The segregated solidified shell has a higher cracking susceptibility, but particularly when B is contained, the solidified shell has a higher cracking susceptibility. Further, B promotes the wettability between the solidified shell and the unsolidified molten steel. When the wettability is promoted, the interface of the solidified shell in contact with the unsolidified molten steel, that is, the liquid film embrittlement which causes the crack of the solidified interface is promoted, so that the internal crack is easily generated. The phenomenon of liquid film embrittlement referred to here is that the entire solidified crystal grains are covered in liquid film with unsolidified molten steel, the bonding between the crystal grains is hindered, the solidified structure becomes brittle, and internal cracking occurs. Is a phenomenon that tends to occur.
[0017]
Furthermore, the present inventors have studied in detail the action of S, which is an impurity element and easily segregated. As a result, S promotes the wettability between the solidified shell and the unsolidified molten steel, and internal cracks occur similarly to B. It has been found that, in addition to the effect of facilitating the embrittlement, if S and B are simultaneously present in the molten steel, the above-mentioned phenomenon of liquid film embrittlement appears synergistically. That is, S and B segregated in the unsolidified molten steel immediately before solidification near the solidification interface lower the melting point of the unsolidified molten steel more greatly than when S or B segregates alone. Therefore, the degree of superheat of the unsolidified molten steel immediately before solidification increases, so that the wettability between the solidified shell and the unsolidified molten steel is synergistically promoted, and internal cracks are easily generated synergistically.
[0018]
Therefore, in the high-strength steel plate of the present invention, first, both the contents of S and B, which cause the occurrence of internal cracks in the slab, are reduced. Specifically, the S content is made as low as 0.005% by mass or less, and the B content is made as low as 0.0008 to 0.003% by mass. Furthermore, in order to more reliably reduce the synergistic effect of B and S on liquid film embrittlement, in addition to lowering the contents of S and B within the above ranges, respectively, It was found that fixing S segregated in unsolidified molten steel as MnS was effective. At this time, it is effective to set the ratio of the Mn content to the S content and the value of Mn / S to 300 or more.
[0019]
By the way, it is conceivable to fix B segregated in unsolidified molten steel with N. However, when the N content is increased, there is a problem that lateral cracks are easily generated on the slab surface, and B is fixed with N. Doing so is not an effective method. As described above, by increasing the ratio of the Mn content to the S content and the value of Mn / S in addition to decreasing the content of S and B, the synergistic effect of B and S on liquid film embrittlement is obtained. Can be reduced, and even if the steel plate is a hot-rolled steel plate having a thickness of 200 to 350 mm and a rectangular slab cast at a speed of 0.7 to 2.5 m / min and a rectangular slab as a raw material, Since the occurrence of internal cracks in the slab can be prevented, the occurrence of cracks during welding or bending of a thick steel plate due to the internal cracks in the slab can also be prevented.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the chemical composition and thickness of the thick steel plate will be described below. In addition, the following% display means mass%.
C: 0.05-0.2%
C is an inexpensive and useful element for securing the strength of a thick steel plate, and its content may be determined based on a component design based on mechanical properties such as required strength. In order to exhibit the effect, the lower limit is set to 0.05%. On the other hand, if the content exceeds 0.2%, the weldability and workability of the thick steel plate are deteriorated, so the upper limit is made 0.2%. Further, C is an element that is easily segregated in the slab, and the susceptibility of the solidified interface where C is segregated to cracking increases. However, if the content is within the above range, there is no particular effect. Therefore, the C content is set to 0.05 to 0.2%.
[0021]
Si: 0.05 to 0.4%
Si is usually contained for the purpose of deoxidizing molten steel and securing the strength of a thick steel plate, but is an element that can increase the strength without deteriorating the workability of the thick steel plate. , The lower limit is 0.05%. If the content exceeds 0.4%, the chemical conversion property of the thick steel plate deteriorates, so the upper limit is made 0.4%. Therefore, the Si content is set to 0.05 to 0.4%.
[0022]
Mn: 0.2 to 2%, and the value of Mn / S is 300 or more
Mn is a useful element for increasing the strength of a thick steel plate, and its lower limit is set to 0.2% in order to exert its effect. However, since the element is easily segregated in the slab, its upper limit is set to 2%. When Mn segregates in the center of the thickness of the slab, the center segregation of the slab remains as a hardened layer in the center of the thickness of the thick steel plate, and cracks occur from the hardened layer at the time of bending of the thick steel plate. It's easy to do. Therefore, the Mn content is set to 0.2 to 2%. Mn is an element that is easily segregated, but by appropriately adding Mn to the S content, the occurrence of internal cracks in the slab can be prevented. That is, since S is fixed to Mn as MnS, an increase in wettability between the solidified shell and the unsolidified molten steel is suppressed, and the occurrence of internal cracks in the slab is suppressed. Therefore, the value of Mn / S, which is the ratio of the Mn content to the S content, is set to 300 or more. Also, the ratio and the value of Mn / S are desirably 1000 or less. If the value of Mn / S exceeds 1000, a large amount of expensive Mn alloy iron is added, or the amount of S is significantly reduced, so that the production cost increases.
[0023]
P: 0.02% or less
P is an impurity element, and tends to segregate in a cast slab, and the solidification interface where P segregates has a high crack sensitivity. However, there is no particular effect if the content is within the range of 0.02% or less. Therefore, the P content is set to 0.02% or less.
[0024]
S: 0.005% or less
S is an impurity element, and is easily segregated in a slab, and the solidification interface where S is segregated has high cracking susceptibility. Further, S is an element that increases the wettability of the unsolidified molten steel, and the internal cracks of the slab tend to occur. As described above, the value of Mn / S, which is the ratio of the Mn content to the S content, is described above. Is 300 or more, so long as the content is within the above range of S, there is no particular effect on the internal cracks of the slab. Therefore, the S content is set to 0.005% or less.
[0025]
B: 0.0008 to 0.003%
B can improve the hardenability of a thick steel plate only by adding a small amount of B. In order to exhibit the effect, the lower limit is 0.0008%. On the other hand, B is easily segregated in the slab, the cracking susceptibility of the solidified interface where B is segregated is increased, and further, the wettability of the unsolidified molten steel is increased, and the internal slab is easily generated in the slab. The upper limit is 0.003%. Therefore, the B content is 0.0008 to 0.003%.
[0026]
The high-strength steel sheet made of carbon steel or low-alloy steel of the present invention may contain, as necessary, Al: 0.1% or less, Cr: 1.5 % Or less, Mo; 1.5% or less, Ni: 1.5% or less, Cu: 1.5% or less, Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less Steel containing one or more of the above, and the balance being Fe and impurities.
[0027]
When these elements of Al, Cr, Mo, Ni, Cu, Ti, Nb and V are contained, mechanical properties such as strength and toughness of the thick steel plate are improved. In addition, as long as the content of these elements is within the above range, there is no effect on the occurrence of internal cracks in the slab.
[0028]
The thickness of the high-strength steel plate of the present invention is 6 to 150 mm. The thickness is set to the above-mentioned thickness from the product use of the thick steel plate used for industrial machines and pressure vessels. Further, in the case of a thick steel plate having a thickness exceeding 150 mm, when hot rolling is performed using a slab having a thickness described later as a raw material, it is difficult to obtain required mechanical properties such as strength and toughness of the thick steel plate.
[0029]
Next, a method of casting a slab, which is a raw material for hot rolling a high-strength steel plate, will be described. A slab having a rectangular cross section and a thickness of 200 to 350 mm is cast at a speed of 0.7 to 2.5 m / min.
[0030]
When the thickness of the slab is less than 200 mm, the productivity of the slab is low within the range of the casting speed. Further, when the thickness of the slab exceeds 350 mm, internal cracks in the slab tend to occur. In addition, the size of the continuous casting machine becomes large. Therefore, the thickness of the slab is 200 to 350 mm.
[0031]
In order to reduce the slab thickness by the thickness of the thick steel plate, the reduction ratio (thickness of the slab / thickness of the thick steel plate) is 1.5 or more. It is desirable to secure the thickness. If the reduction ratio is less than 1.5, the solidified structure of the slab remains in the thick steel plate, which affects the strength and toughness of the thick steel plate.
[0032]
When the casting speed is less than 0.7 m / min, the occurrence of internal cracks in the slab can be suppressed, but the productivity of the slab is low within the range of the thickness of the slab. On the other hand, if it exceeds 2.5 m / min, cracks tend to occur on the slab surface and internal cracks tend to occur. Therefore, the casting speed is set to 0.7 to 2.5 m / min.
[0033]
When casting a slab having a thickness of 200 to 350 mm at a speed of 0.7 to 2.5 m / min, the above-described chemical composition can prevent the occurrence of internal cracks in the slab.
[0034]
When hot rolling a slab to obtain a high-strength steel plate, the heating conditions such as the heating temperature of the slab before rolling, the heating time, and the rolling conditions such as the rolling temperature and the finishing temperature are usually determined according to the steel. Condition.
[0035]
【Example】
Using a vertical bending type continuous casting machine having a vertical part length of 3 m, an arc radius of 10 m, five-point bending and four-point correction, and a machine length of 30 m, a test of casting a slab having a thickness of 200 mm or 250 mm and a width of 1800 mm was performed. The distance between the axes of the guide rolls of the continuous casting machine is 250 mm in the vertical part, 250 to 400 mm in the curved part, and 400 to 450 mm in the horizontal part. The specific water volume for the secondary cooling of the slab was 1-2 liters / kg-steel.
[0036]
As shown in Tables 1 and 2 below, the steel used was a low-carbon steel having a C content of 0.07 to 0.20 mass% and a B content of 0.0009 to 0.0035 mass%. Medium carbon steel. Test No. described later. 1 and No. 1 In tests other than 8, a steel containing one or two types of Al, Ti, Nb, and V was used. The casting speed was 2.0 m / min when casting a 200 mm thick slab, and 1.0 m / min when casting a 250 mm thick slab. In addition,% described below means mass%.
[0037]
From the slab obtained in each test, a transverse sample having a full width of 100 mm in the casting direction was taken, and the transverse section thereof was sulfa-printed. The presence or absence of the occurrence of internal cracks in the cast slab, which was a segregated line, was visually inspected.
[0038]
The obtained slab was hot-rolled to obtain a thick steel plate having a thickness of 10 to 120 mm. At that time, the slab was heated within a temperature range of 1000 to 1300 ° C, and the rolling was completed at 750 to 1000 ° C. A No. 4 or No. 5 test piece specified in JIS Z 2201 was sampled in a direction perpendicular to the rolling direction from a position corresponding to the center of the thickness of the obtained thick steel plate, and a tensile test was performed.
[0039]
In addition, a sample for a welding test was taken from the obtained thick steel plate, and a constraint welding crack test was performed to investigate whether cracks occurred in the heat-affected zone.
FIG. 1 is a diagram showing the shape and dimensions of a test piece used for a constraint welding crack test. FIG. 1A is a side view showing a cross section of a groove, and FIG. 1B is a plan view of the groove as viewed from above. The symbol a in the figure indicates the thickness of the thick steel plate, which is within the range of 10 to 120 mm. Symbol b indicates the thickness of the restraint plate, and is set to 60 to 200 mm according to the thickness of the thick steel plate to be used. The dimensions of the symbols f and g, which are the vertical and horizontal dimensions of the thick steel plate and the restraint plate, were each 500 mm. The thick steel plate and the restraint plate were overlapped, and the lower end opposite to the groove was welded before the welding test. The symbol h is the weld. The dimension indicated by the symbol c of this welded portion was about 10 mm, and the dimension indicated by the symbol d was about 20 mm. The groove angle was 90 ° and the groove depth e was 20 mm.
[0040]
Welding was performed in a downward position using a covered arc welding rod, and the weld bead had a length indicated by a symbol g. After welding, the line A1-A2 shown in FIG. 1 (b) was cut by machining, and the cross-section of the cross-section was inspected by die checking for the occurrence of cracks in the heat affected zone. Tables 1 and 2 show the test conditions and test results.
[0041]
[Table 1]

[Table 2]

Test No. of the present invention example. In No. 1, a slab having a thickness of 200 mm was cast at a speed of 2.0 m / min. The steel used was a medium carbon steel with a Mn content of 1.15%, an S content of 0.0020% and a Mn / S value of 575. The obtained slab was hot-rolled to obtain a thick steel plate having a thickness of 10 mm. Each of these conditions of the chemical composition, the thickness of the slab, the casting speed, and the thickness of the steel plate is within the range defined by the present invention. In this test, no internal cracks occurred in the slab, and no cracks occurred in the heat-affected zone in the restrained welding crack test of the thick steel plate. The obtained steel plate had a tensile strength of 674 MPa, and was a high-strength steel plate.
[0042]
Test No. of the present invention example. In Test No. 2, except that medium carbon steel containing Al was used. The test was conducted under almost the same conditions as in Example 1. Each condition of the chemical composition, the thickness of the slab, the casting speed, and the thickness of the steel plate is within the range defined by the present invention. In this test, no internal cracks occurred in the slab. Further, in the constraint welding crack test of the thick steel plate, no crack occurred in the heat-affected zone. The obtained steel plate had a tensile strength of 669 MPa and was a high-strength steel plate.
[0043]
Test No. of the present invention example. In Test No. 3, except that a low carbon steel containing Al and Ti was used. The test was conducted under almost the same conditions as in Example 1. Each condition of the chemical composition, the thickness of the slab, the casting speed, and the thickness of the thick steel plate is within the range defined by the present invention. In this test, no internal cracks occurred in the slab. Further, in the constraint welding crack test of the thick steel plate, no crack occurred in the heat-affected zone. The tensile strength of the obtained thick steel plate was measured in Test No. Since the C content was lower than that of No. 1, It was 545 MPa lower than 1.
[0044]
Test No. of the present invention example. In Test No. 4, except that medium carbon steel containing Nb was used. The test was performed under almost the same conditions as in Example 2. Each condition of the chemical composition, the thickness of the slab, the casting speed, and the thickness of the steel plate is within the range defined by the present invention. In this test, no internal cracks occurred in the slab. Further, in the constraint welding crack test of the thick steel plate, no crack occurred in the heat-affected zone. The tensile strength of the obtained steel plate was 684 MPa. 2 had higher strength than the thick steel plate.
[0045]
Test No. of the present invention example. In Test No. 5, except that the medium carbon steel containing V was used and the thickness of the thick steel plate was set to 50 mm. The test was performed under almost the same conditions as in Example 2. Each condition of the chemical composition, the thickness of the slab, the casting speed, and the thickness of the steel plate is within the range defined by the present invention. In this test, no internal cracks occurred in the slab. Further, in the constraint welding crack test of the thick steel plate, no crack occurred in the heat-affected zone. Further, the tensile strength of the obtained thick steel plate was 632 MPa, and the reduction ratio obtained by dividing the thickness of the slab by the thickness of the thick steel plate was 4, and Test No. Test No. 2 was smaller than the reduction ratio 20 of Test No. 2. The strength was lower than 2.
[0046]
Test No. of the present invention example. In Test No. 6, except that a 250 mm thick slab was cast at a speed of 1.0 m / min and the thickness of the thick steel plate was 70 mm. The test was performed under almost the same conditions as in Example 5. Each condition of the chemical composition, the thickness of the slab, the casting speed, and the thickness of the steel plate is within the range defined by the present invention. In this test, no internal cracks occurred in the slab. Further, in the constraint welding crack test of the thick steel plate, no crack occurred in the heat-affected zone. Further, the tensile strength of the obtained thick steel plate was 638 MPa, and the reduction ratio obtained by dividing the thickness of the slab by the thickness of the thick steel plate was 3.6. Although the reduction ratio was smaller than the reduction ratio of 4, the B content was no. Test No. 5 The strength was as high as about 5.
[0047]
Test No. of the present invention example. In Test No. 7, except that the medium carbon steel containing Ti and Nb was used and the thickness of the thick steel plate was 120 mm. The test was conducted under almost the same conditions as in Example 6. Each condition of the chemical composition, the thickness of the slab, the casting speed, and the thickness of the steel plate is within the range defined by the present invention. In this test, no internal cracks occurred in the slab. Further, in the constraint welding crack test of the thick steel plate, no crack occurred in the heat-affected zone. Further, the tensile strength of the obtained thick steel plate was 625 MPa, and the reduction ratio obtained by dividing the thickness of the slab by the thickness of the thick steel plate was 2.1. Test No. 6 was smaller than the reduction ratio of 3.6. The strength was lower than 6.
[0048]
Test No. of the comparative example. In Test No. 8, except that the S content was as high as 0.0040% and the value of Mn / S was as low as 288. The test was conducted under almost the same conditions as in Example 1. The value of Mn / S is a low value outside the conditions defined in the present invention. In this test, internal cracks, which are remarkable segregation lines, occurred in the slab. In addition, the tensile strength of the obtained thick steel plate was 682 MPa. Although the high strength was almost the same as that of No. 1, a crack occurred in the heat-affected zone in a constraint welding crack test of a thick steel plate.
[0049]
Test No. of the comparative example. In Test No. 9, except that the S content was as high as 0.0043% and the value of Mn / S was as low as 286. The test was performed under almost the same conditions as in Example 3. The value of Mn / S is a low value outside the conditions defined in the present invention. In this test, internal cracks, which are remarkable segregation lines, occurred in the slab. In addition, the tensile strength of the obtained thick steel plate was 549 MPa. Although the strength was almost the same as that of No. 3, a crack occurred in the heat-affected zone in a constraint welding crack test of the thick steel plate.
[0050]
Test No. of the comparative example. In Test No. 10, except that the S content was as high as 0.0045% and the value of Mn / S was as low as 278. The test was performed under almost the same conditions as in Example 5. The value of Mn / S is a low value outside the conditions defined in the present invention. In this test, internal cracks, which are remarkable segregation lines, occurred in the slab. The tensile strength of the obtained thick steel plate was 625 MPa. Although the high strength was almost the same as that of No. 5, cracks occurred in the heat-affected zone in a constraint welding crack test of the thick steel plate.
[0051]
Test No. of the comparative example. In Test No. 11, the Mn content was increased to 1.40%, but the S content was also increased to 0.0048% and the value of Mn / S was decreased to 292, except for Test No. 11. The test was conducted under almost the same conditions as in Example 6. The value of Mn / S is a low value outside the conditions defined in the present invention. In this test, internal cracks, which are remarkable segregation lines, occurred in the slab. As for the tensile strength of the obtained thick steel plate, the B content was the same as in Test No. Test No. 6 It was 629 MPa lower than 6. In addition, cracks occurred in the heat-affected zone in a constraint welding crack test of a thick steel plate.
[0052]
Test No. of the comparative example. In Test No. 12, the S content was reduced to 0.0024%, but the Mn content was also reduced to 0.70% and the Mn / S value was reduced to 292, except for Test No. 12. 7 was tested under almost the same conditions. The value of Mn / S is a low value outside the conditions defined in the present invention. In this test, internal cracks, which are remarkable segregation lines, occurred in the slab. In addition, the tensile strength of the obtained thick steel plate was 628 MPa. Although the high strength was almost the same as that of No. 7, a crack occurred in the heat-affected zone in a constraint welding crack test of the thick steel plate.
[0053]
Test No. of the comparative example. In Test No. 13, except that the B content was increased to 0.0032%. The test was conducted under almost the same conditions as in Example 6. The B content is a high value outside the conditions defined in the present invention. In this test, internal cracks, which are remarkable segregation lines, occurred in the slab. In addition, the tensile strength of the obtained thick steel plate was the same as that of Test No. Since the B content is higher than that of Test No. 6, Although it was 654 MPa, which was higher than 6, cracking occurred in the heat-affected zone in a constraint welding crack test of a thick steel plate.
[0054]
Test No. of the comparative example. In Test No. 14, except that the B content was increased to 0.0035%. 7 was tested under almost the same conditions. The B content is a high value outside the conditions defined in the present invention. In this test, internal cracks, which are remarkable segregation lines, occurred in the slab. In addition, the tensile strength of the obtained thick steel plate was the same as that of Test No. Since the B content is higher than that of Test No. 7, Although it was 638 MPa higher than 7, cracks occurred in the heat-affected zone in a constraint welding crack test of a thick steel plate.
[0055]
【The invention's effect】
By applying the present invention, the thickness of the slab is increased, and the slab cast under the condition of increasing the casting speed is hot-rolled as a raw material. By doing so, it has excellent restraint weld cracking resistance High strength thick steel plate can be obtained.
[Brief description of the drawings]
FIG. 1 is a view showing the shape and dimensions of a test piece used for a constraint welding crack test.
[Explanation of symbols]
a: Thickness of thick steel plate b: Thickness of restraint plate
c: Dimension of welded part d: Dimension of welded part
e: groove depth f: vertical dimension of thick steel plate and restraint plate
g: lateral dimension of thick steel plate and restraint plate h: welded part for restraining thick steel plate

Claims

質量％で、Ｃ：０．０５〜０．２％、Ｓｉ：０．０５〜０．４％、Ｍｎ：０．２〜２％、Ｐ：０．０２％以下、Ｓ：０．００５％以下、Ｂ：０．０００８〜０．００３％を含み、Ｍｎ／Ｓの値が３００以上の炭素鋼または低合金鋼からなる、厚さが６〜１５０ｍｍの高強度厚鋼板の製造方法であって、厚さ２００〜３５０ｍｍの横断面形状が長方形の鋳片を、速度０．７〜２．５ｍ／分で鋳造し、次いで上記鋳片を素材として熱間圧延することを特徴とする、耐拘束溶接割れ性に優れた高強度厚鋼板の製造方法。In mass%, C: 0.05 to 0.2%, Si: 0.05 to 0.4%, Mn: 0.2 to 2%, P: 0.02% or less, S: 0.005% or less , B: a method for producing a high-strength steel plate having a thickness of 6 to 150 mm, comprising 0.0008 to 0.003%, a carbon steel or a low alloy steel having a Mn / S value of 300 or more, the thickness of cross-sectional shape is rectangular slab of 200-350 mm, and the casting at a rate 0.7～2.5M / min, and then characterized by hot rolling the slab as a material, resistant restraint weld A method for manufacturing high-strength steel plates with excellent cracking properties .