JP2004204346A

JP2004204346A - Method for manufacturing thick steel plate of tensile strength of 400 n/mm2 class with excellent scale property

Info

Publication number: JP2004204346A
Application number: JP2003190808A
Authority: JP
Inventors: Yoshiyuki Watabe; 義之渡部; Rikio Chijiiwa; 力雄千々岩; Katsuto Tomonaga; 克仁朝永; Yasushi Mizutani; 泰水谷
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2002-10-28
Filing date: 2003-07-03
Publication date: 2004-07-22

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a thick steel plate of tensile strength of 400 N/mm<SP>2</SP>class with excellent scale property. <P>SOLUTION: A steel slab or a cast slab in which the steel composition expressed by mass % is ≤ 0.40% in terms of carbon equivalent Ceq defined as "Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14" is reheated to the temperature of ≤ 1,150°C, the rolling thereof is completed at the roll-in temperature of a final rolling pass of 870-980°C, the water cooling is started within two minutes after the rolling from the temperature of ≥ 720°C, the cooling is performed for ≥ 50°C from the rolling start temperature, and the water cooling is stopped at the temperature of ≥ 600°C. In addition, in at least two final passes of the rolling, preferably, descaling with high-pressure water is not performed. When a thick steel slab or cast slab is subjected to laser cutting, C amount is preferably 0.15%. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、スケール性状の良好な引張強さ４００Ｎ／ｍｍ^２級厚鋼板の製造方法に関するものである。本発明により得られる厚鋼板は、スケールの部分剥離や色調などの外観上優れることはもとより、各種冷間加工時のスケール剥離が少なく抑えられたり、あるいは、近年、急速に普及しつつあるレーザー切断用鋼として良好な切断性・切断面品位を発現するなど、付加価値を高めるものである。
【０００２】
【従来の技術】
本発明が称するところの「スケール」とは、鋼板表面に生成した高温酸化皮膜、すなわち酸化鉄のことで、一般に工業生産される熱間圧延鋼材ではその生成は避けられない。スケール性状は、加熱温度、圧延温度、全圧下率および各パス圧下率、デスケーリング条件、冷却条件などにより異なることは定性的には知られているものの、必ずしも定量的な因果関係は明確になっているとは言えない。特に、引張強さ４００Ｎ／ｍｍ^２級の厚鋼板は、通常、加熱・圧延温度とも特段の規制なく圧延されることが多いため、スケール性状制御の不明確さもあって、良好なスケール性状はもとより、安定したスケール性状を得ることは困難であった。
【０００３】
また、引張強さ４００Ｎ／ｍｍ^２級の厚鋼板は、その製造プロセスとしては、一般に、製造が容易かつ生産性も良好な圧延ままで製造され、圧延後水冷（加速冷却）されることはほとんどない。逆に、通常、圧延ままで製造されるため、鋼成分上も圧延ままで４００Ｎ／ｍｍ^２級の引張強さが得られるような成分設計となっているため、圧延後の過度の水冷（加速冷却）は、機械的性質を大きく変化させるため、好ましくないケースもある。
【０００４】
各種冷間加工時のスケール剥離を抑えるため、生成するスケールを極力薄くすることが考えられ、圧延パス間で頻繁に高圧水によるデスケーリングを行う方法がある（例えば、特許文献１参照）。また、スケールと地鉄との密着性を高めることも有効と考えられ、ホットストリップ・ミルを対象に、スケールのマグネタイト化を促進する目的で巻き取り温度を制御する方法がある（例えば、特許文献２及び３参照）。しかし、この方法を現状の一般的な厚板ミルに適用することは、大規模な設備改造を伴うため極めて困難と考えられる。
【０００５】
【特許文献１】
特開２０００−２９０７２７号公報
【特許文献２】
特開昭６３−１１１１２５号公報
【特許文献３】
特開２００１−１９８６０４号公報
【０００６】
【発明が解決しようとする課題】
本発明は、スケールの部分剥離が少なく、スケールが青〜黒色に見えるなど外観・色調にも優れ、その結果として各種冷間加工時のスケール剥離が少なく抑えられたり、あるいは、レーザー切断にも好適なスケール性状の良好な４００Ｎ／ｍｍ^２級厚鋼板を工業的に安定かつ大量に供給可能な方法を提供するものである。
【０００７】
【課題を解決するための手段】
本発明のポイントは、スケール性状の良好な４００Ｎ／ｍｍ^２級厚鋼板を得るために、加熱、圧延、デスケーリング、冷却にわたる製造方法を特定条件に限定することにあり、その要旨は以下に示す通りである。
【０００８】
（１）質量％で表される鋼成分が、「Ｃｅｑ＝Ｃ＋Ｓｉ／２４＋Ｍｎ／６＋Ｎｉ／４０＋Ｃｒ／５＋Ｍｏ／４＋Ｖ／１４」と定義する炭素当量Ｃｅｑで０．４０％以下となる鋼片または鋳片を１１５０℃以下の温度に再加熱後、最終圧延パスの噛込温度が８７０〜９８０℃の温度で圧延を終了し、圧延後２分以内かつ７２０℃以上の温度から水冷を開始して水冷開始温度から５０℃以上冷却、かつ６００℃以上の温度で水冷を停止し、その後放冷することを特徴とするスケール性状の良好な引張強さ４００Ｎ／ｍｍ^２級厚鋼板の製造方法。
【０００９】
（２）質量％で表したＣ量が０．１５％以下であることを特徴とする（１）に記載のスケール性状の良好な引張強さ４００Ｎ／ｍｍ^２級厚鋼板の製造方法。
【００１０】
（３）圧延の少なくとも最終２パスにおいては高圧水によるデスケーリングを行わないことを特徴とする（１）または（２）に記載のスケール性状の良好な引張強さ４００Ｎ／ｍｍ^２級厚鋼板の製造方法。
【００１１】
【発明の実施の形態】
以下に、本発明を詳細に説明する。
【００１２】
まず、鋼成分については、スケール性状制御の観点からはＣ、Ｓｉ、Ｍｎなど個々の元素の添加範囲を特に限定する必要はない。これは、スケール性状の制御が目的であっても、第一義的には、鋼材の機械的性質（特に強度）が所定の値を満足することが重要であるためで、鋼成分は自ずと制約されるものであるからである。加えて、引張強さ４００Ｎ／ｍｍ^２級鋼は、従来から最も広く使用されており、その鋼成分は、鋼板製造者毎に多少の違いはあるものの、すでに確立したものがあり、鋼成分上は特段の特徴を有するものではなく、規定する必要はない。したがって、本発明者らのスタンスは、スケール性状に及ぼす鋼成分の影響は否定できないものの、まずは鋼成分ありきで考えるべきとの立場に立つものである。ただし、後述するように、圧延後水冷することが本発明の重要な構成要素となるため、水冷により機械的性質が大幅に変わることがないよう、鋼成分トータル量としての炭素当量（Ｃｅｑ）を０．４０％以下に限定するに止めた。この上限値は、本来、機械的性質は製造条件はもとより、板厚によっても変化するため、必ずしも臨界的な意味合いはないが、現時点では板厚毎の限界値が明確ではないため、本発明者らにより、広い板厚範囲にわたって安定して４００Ｎ／ｍｍ^２級鋼の機械的性質を満足することが確認された値とした。下限についても同様であるが、４００Ｎ／ｍｍ^２級鋼と謳う本発明においては、特に限定しなくとも、強度上自ずと制約される性質のものである。
【００１３】
なお、後述するように、製造方法を本発明の通り限定することで良好なスケール性状が得られ、その結果として、優れたレーザー切断性を発現することができる。この理由は後述するが、このとき、板厚が厚い場合（概ね２２ｍｍ超）では、Ｃ量は０．１５％以下とすることが好ましい。レーザー切断はレーザー照射と同時にアシストガスとして酸素を吹き付けるが、鋼中のＣがＣＯガスとなって酸素アシストガスの純度を下げるため、板厚が厚くなると、良好な切断面（特に変下面側）が得にくくなる傾向にある。これが本発明の請求項２にかかるＣ量限定の理由である。
【００１４】
次に、製造条件の限定理由について説明する。
【００１５】
上述したＣｅｑで０．４０％以下の鋼成分を有する鋼片または鋳片の熱間圧延に先立つ再加熱温度は１１５０℃以下に限定する。スケールは、高温ほど生成しやすく、その量も多くなる。また、それら加熱時のような高温で生成するスケールは、多孔質（ポーラス）かつ亀裂を多く含み脆い傾向にある。一般に、加熱炉抽出後や圧延パス間では高圧水によるデスケーリングが施されたり、そもそも圧延時には粉砕されたりし、一旦は除去されるが、高温状態ではすぐさま再生成する。これらの観点から、酸化雰囲気中での高温滞留を極力抑えるため、鋼片または鋳片の再加熱温度を１１５０℃以下に限定したものであるが、低いほど好ましいことは言うまでもない。なお、下限については、後述する最終圧延パスの噛込温度確保の観点から自ずと制限されるものであるが、圧延パススケジュールや板厚によっても変わるため、あえて温度を限定することはしない。
【００１６】
圧延条件については、既に述べたように引張強さ４００Ｎ／ｍｍ^２級の厚鋼板は、通常、加熱・圧延温度とも特段の規制なく圧延されることが多く、本発明においても機械的性質の観点では規制するものではない。最終圧延パスの噛込温度を８７０〜９８０℃とした理由は、スケール性状、特にスケール厚の観点からである。本発明に当たっての発明者らの実験によれば、スケールの厚みは最終圧延パスの噛込温度と高い相関を有し、その温度が低いほど薄くなる傾向にある。このような傾向の中で、最終圧延パスの噛込温度が９８０℃を超えるとスケールが厚くなりすぎ、厚さの不均一性が目立つようになるばかりでなく、圧延温度が高いと圧延後に再生成するスケールは上述したように脆いために部分的な剥離を生じやすく、外観を著しく損なうケースが多発する。逆に最終圧延パスの噛込温度が８７０℃を下回るとスケールが薄くなって、この場合も厚さの不均一性が目立つようになる。また、最終圧延パスの噛込温度が低くなると、圧延後の鋼板表面（スケール）の色調が明らかに赤みを帯びてくるようになり、この観点からも好ましくなく、下限温度を８７０℃に限定した。最終圧延パスの噛込温度を本発明の通り制御し、後述する圧延後の水冷を行った場合、スケール厚は概ね３０μｍ前後となる。
【００１７】
圧延後は、２分以内かつ７２０℃以上の温度から水冷することが本発明の特徴の一つである。要は、圧延後速やかに水冷することが基本的な思想で、スケールの再生成、成長を極力抑えるという観点に基づくものである。とはいうものの、圧延後水冷設備までの搬送にはある程度の時間を要するため、本発明においては水冷までの時間と水冷開始の温度を上記のように限定した。いずれも、本発明者らによる実生産設備（厚板ミル）での系統的な試験圧延結果に基づくもので、圧延後は極力短時間、かつ高温から水冷することが好ましいことは言うまでもなく、前記時間ないしは温度規定を逸脱する場合は、スケール厚、スケールの緻密性などスケール性状制御が不十分となる。
【００１８】
水冷は、上記水冷開始温度から５０℃以上の水冷で、かつ６００℃以上の温度で停止する必要がある。水冷開始温度から５０℃未満の水冷では、水冷時間がごく短時間となって冷却の均一性確保が困難となるばかりでなく、本来の目的であるスケールの再生成、成長を極力抑えるといったスケール性状制御の実質的効果は小さい。一方、水冷停止温度は、６００℃を下回ると機械的性質、特に強度への影響が顕著となるため、６００℃以上に限定した。鋼成分をも適正にすれば、水冷停止温度については本発明の限定範囲の限りではないが、本来、圧延ままでも必要特性（強度）を満足する既存の成分体系の中でスケール制御するという本発明のスタンスで、上記のように限定したものである。このような製造法による鋼板では、スケールが緻密でスケール中のボイドが少なく、かつ地鉄との密着性も良好となるばかりでなく、色調も明らかに黒く良好な外観も呈する。
【００１９】
ここで、上記スケール性状制御により、レーザー切断性が優れる理由について簡単に説明する。前述したようにレーザー切断は、レーザー照射と同時に酸素アシストガスを吹き付ける。このときスケール性状が不良、例えば厚みの不均質やスケール中にボイドが多かったり、密着性に劣る場合、レーザー切断時の熱歪やスケール中ボイド内のガスの膨張などによってスケールの剥離・破砕が生じ、酸素アシストガスの気流を乱し、切断性、切断面性状を著しく劣化させる原因となる。逆に、スケールが緻密でスケール中のボイドが少なく、かつ地鉄との密着性が良好な鋼板は、レーザー切断時にスケールが剥離することなく、安定して優れた切断性および切断面品質を示す。
【００２０】
なお、鋼板内におけるスケール性状の均一性を確保する上では、さらに圧延の少なくとも最終２パスにおいては高圧水によるデスケーリングを行わないことが極めて有効である。本発明の請求項２にかかる発明は、スケール性状に対する要求が厳しい場合に適用すべきものとして限定する。この限定理由は、デスケーリングを行うことで、むしろスケールの不均一性を助長することを避けることにある。デスケーリング設備が常に十分にメンテナンスされた状態であれば問題となることはほとんどないが、工業的には必ずしも常に最高の状態ばかりとは言えず、デスケーリングの不均一性（むら）の危険性は常に孕んでいる。圧延の最終パス近くでのデスケーリングの不均一性は、最終製品にまでその影響が及ぶことが往々にしてあるため、本発明では圧延の少なくとも最終２パスにおいては高圧水によるデスケーリングを行わないこととした。
【００２１】
【実施例】
種々の鋼成分を有する鋼片または鋳片を各種の方法で板厚６〜４０ｍｍの鋼板に製造し、その機械的性質、スケール性状を調査した。
【００２２】
表１に比較例とともに本発明例の鋼成分のうち基本となるＣ、Ｓｉ、Ｍｎ量およびＣｅｑを、表２に鋼板の製造条件、機械的性質およびスケール性状の調査結果を示す。
【００２３】
本発明法に則った成分および製造方法による鋼板（本発明例）は、引張強さ４００Ｎ／ｍｍ^２級鋼としてまったく問題ない特性を有し、かつ本発明のポイントであるスケール性状も良好である。これに対し、鋼成分や製造条件が本発明の限定範囲を逸脱する比較例は、スケール性状が明らかに劣るばかりでなく、引張強さ４００Ｎ／ｍｍ^２級鋼として機械的性質を逸脱するケースが見られる。
【００２４】
すなわち、比較例８は、Ｃｅｑが高いのに加え水冷停止温度も低いため、引張強さが４００Ｎ／ｍｍ^２級鋼の規格強度を超過している。また最終圧延２パス内でデスケーリングを行っているため、外観に劣る。比較例９は、加熱温度高く、圧延後水冷までの時間、水冷開始温度など水冷条件も適正でないためスケール性状、外観に劣る。比較例１０は、最終圧延パスの噛込温度が低いため、スケール性状ならびに外観に劣る。比較例１１では、圧延後水冷が施されておらず、また、最終圧延２パス内でデスケーリングを行っているため、スケール性状、外観に劣る。
【００２５】
なお、比較例１２は、製造条件自体は本発明の通り実施したため、スケール性状や外観には優れるものの、レーザー切断を行った結果、板厚が厚く（２５ｍｍ）、Ｃ量が高いため、切断面下面近傍にドロスが付着し、切断面性状に劣ることを確認している。
【００２６】
【表１】

【００２７】
【表２】

【００２８】
【発明の効果】
本発明により、引張強さ４００Ｎ／ｍｍ^２級の基本特性を満足しつつ、スケール性状の良好な厚鋼板を、生産性を損なうことなく安定的かつ大量に供給することが可能となった。このような厚鋼板は、外観上優れるばかりでなく、各種冷間加工時にもスケールの剥離が少なく、作業環境改善に資することができるようになった。また、レーザー切断性もきわめて安定となり、切断の夜間無人操業も可能となった。[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to a method for producing a ^second- class thick steel plate having a good tensile strength of 400 N / mm in scale properties. The steel plate obtained by the present invention is not only excellent in appearance such as partial peeling of the scale and color tone, but also the scale peeling at the time of various cold working is suppressed to be small, or in recent years, the laser cutting which is rapidly spreading widely It enhances added value, such as developing good cutting properties and cut surface quality as steel for use.
[0002]
[Prior art]
The term "scale" as used in the present invention refers to a high-temperature oxide film formed on the surface of a steel sheet, that is, iron oxide, and its formation is unavoidable in hot rolled steel materials generally produced industrially. Although it is known qualitatively that the scale properties vary depending on the heating temperature, rolling temperature, total rolling reduction and each rolling reduction, descaling conditions, cooling conditions, etc., the quantitative consequences are clearly clarified. I can't say that. In particular, the tensile strength of 400 N / mm ² grade thick steel plate, usually, for both heating and rolling temperature often be rolled without special restriction, there is also ambiguity scale property control, good scale properties as well It was difficult to obtain stable scale properties.
[0003]
The tensile strength of 400 N / mm ² grade thick steel plate, the manufacturing process, in general, is easy to manufacture and is produced while also good rolling productivity, unlikely to be rolled after cooled (accelerated cooling) Absent. On the other hand, since it is usually manufactured as-rolled, the steel component is designed so that a tensile strength of 400 N / mm ² class can be obtained as-rolled, so that excessive water cooling after rolling (acceleration) Cooling) is undesirable in some cases because it significantly changes the mechanical properties.
[0004]
In order to suppress scale exfoliation during various types of cold working, it is conceivable to make the generated scale as thin as possible, and there is a method of frequently performing descaling with high-pressure water between rolling passes (for example, see Patent Document 1). It is also considered effective to increase the adhesion between the scale and the base steel, and there is a method of controlling the winding temperature of a hot strip mill for the purpose of promoting the conversion of the scale to magnetite (for example, Patent Document 1). 2 and 3). However, it is considered extremely difficult to apply this method to existing general plate mills because of the large-scale equipment modification.
[0005]
[Patent Document 1]
JP 2000-290727 A [Patent Document 2]
JP-A-63-111125 [Patent Document 3]
Japanese Patent Application Laid-Open No. 2001-198604
[Problems to be solved by the invention]
The present invention has less partial peeling of the scale and is excellent in appearance and color tone such that the scale looks blue to black, and as a result, scale peeling during various cold working is suppressed to be small, or suitable for laser cutting. An object of the present invention is to provide a method capable of industrially supplying a 400N / mm ^second- grade thick steel plate having excellent scale properties and a large amount in a stable manner.
[0007]
[Means for Solving the Problems]
The point of the present invention is to limit the manufacturing method including heating, rolling, descaling, and cooling to specific conditions in order to obtain a 400 N / mm class ² thick steel sheet having good scale properties, and the gist thereof is described below. It is on the street.
[0008]
(1) A steel slab or a slab having a steel component represented by mass% of 0.40% or less in carbon equivalent Ceq defined as “Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14”. After reheating to a temperature of 1150 ° C. or less, the rolling is finished at a biting temperature of the final rolling pass of 870 to 980 ° C., and water cooling is started within 2 minutes after the rolling and at a temperature of 720 ° C. or more to start water cooling. A method for producing a ^second- grade thick steel plate having good tensile strength of 400 N / mm, characterized in that water cooling is stopped at a temperature of not less than 50 ° C and at a temperature of not less than 600 ° C and then left to cool.
[0009]
(2) The method for producing a ^second- class thick steel plate having good tensile strength of 400 N / mm as described in (1), wherein the C content in mass% is 0.15% or less.
[0010]
(3) At least in the final two passes of rolling, descaling by high-pressure water is not performed. (1) or (2) a good tensile strength of 400 N / mm of a ^second- class thick steel plate having good tensile properties. Production method.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0012]
First, regarding the steel component, it is not necessary to particularly limit the addition range of each element such as C, Si, and Mn from the viewpoint of scale property control. This is because even if the purpose is to control the scale properties, it is important that the mechanical properties (especially the strength) of the steel material satisfy a predetermined value in the first place. It is because it is done. In addition, the 400N / mm class ² tensile strength steel has been most widely used in the past, and its steel composition is already established, although there are some differences among steel plate manufacturers. Has no special features and need not be specified. Therefore, although the stance of the present inventors cannot deny the influence of the steel component on the scale properties, it is from a standpoint that the steel component should be considered first. However, since water cooling after rolling is an important component of the present invention as described later, the carbon equivalent (Ceq) as the total amount of steel components is set so that the mechanical properties are not significantly changed by water cooling. It was limited to 0.40% or less. This upper limit does not necessarily have a critical meaning because the mechanical properties vary not only with the manufacturing conditions but also with the plate thickness, but the limit value for each plate thickness is not clear at present. These values were determined to satisfy the mechanical properties of the 400 N / mm class ² steel stably over a wide range of plate thickness. The same applies to the lower limit, but in the present invention, which claims to be 400 N / mm class ² steel, the strength is naturally restricted without any particular limitation.
[0013]
In addition, as described later, good scale properties can be obtained by limiting the production method according to the present invention, and as a result, excellent laser cutting properties can be exhibited. The reason for this will be described later. At this time, when the plate thickness is large (generally more than 22 mm), the C content is preferably set to 0.15% or less. In laser cutting, oxygen is blown as an assist gas at the same time as laser irradiation. However, C in the steel becomes CO gas and lowers the purity of the oxygen assist gas. Tends to be difficult to obtain. This is the reason for limiting the amount of C according to claim 2 of the present invention.
[0014]
Next, the reasons for limiting the manufacturing conditions will be described.
[0015]
The reheating temperature prior to hot rolling of a slab or a slab having a steel component of 0.40% or less in Ceq described above is limited to 1150 ° C or less. The higher the temperature, the easier the scale is formed and the larger the scale. Further, the scale generated at a high temperature such as during heating tends to be porous (porous), contains many cracks, and is brittle. In general, after extraction in a heating furnace or between rolling passes, descaling is performed with high-pressure water, or crushed during rolling in the first place, and once removed, but immediately regenerated in a high temperature state. From these viewpoints, the reheating temperature of the steel slab or the cast slab is limited to 1150 ° C. or lower in order to minimize high-temperature stagnation in an oxidizing atmosphere, but it goes without saying that the lower the temperature, the better. Note that the lower limit is naturally limited from the viewpoint of ensuring the biting temperature of the final rolling pass described later, but the temperature is not limited because it varies depending on the rolling pass schedule and the sheet thickness.
[0016]
The rolling conditions, the tensile strength of 400 N / mm ² grade steel plate, as already mentioned, usually with heating and rolling temperature can be rolled without special regulations often of mechanical properties in the present invention in view It is not a regulation. The reason why the biting temperature in the final rolling pass is set to 870 to 980 ° C. is from the viewpoint of scale properties, particularly scale thickness. According to experiments performed by the inventors of the present invention, the thickness of the scale has a high correlation with the biting temperature of the final rolling pass, and the smaller the temperature, the smaller the thickness. In such a tendency, when the biting temperature of the final rolling pass exceeds 980 ° C., the scale becomes too thick and unevenness of the thickness becomes noticeable, and when the rolling temperature is high, the scale is regenerated after rolling. As described above, the formed scale is fragile as described above, so that partial peeling is likely to occur, and in many cases, the appearance is significantly impaired. Conversely, when the biting temperature of the final rolling pass is lower than 870 ° C., the scale becomes thinner, and in this case, the thickness non-uniformity becomes conspicuous. Further, when the biting temperature in the final rolling pass is low, the color tone of the steel sheet surface (scale) after rolling becomes clearly reddish, which is not preferable from this viewpoint, and the lower limit temperature is limited to 870 ° C. . When the biting temperature of the final rolling pass is controlled according to the present invention and water cooling after rolling described below is performed, the scale thickness is about 30 μm.
[0017]
One of the features of the present invention is that after rolling, water cooling is performed within 2 minutes and at a temperature of 720 ° C. or more. The point is that water cooling immediately after rolling is a basic idea, and is based on the viewpoint of minimizing regeneration and growth of scale. Nevertheless, since it takes a certain amount of time to convey to the water cooling facility after rolling, in the present invention, the time until water cooling and the temperature at which water cooling is started are limited as described above. Both are based on the systematic test rolling results in actual production equipment (thick plate mill) by the present inventors, and it is needless to say that it is preferable to perform water cooling from a high temperature in a short time after rolling and from a high temperature. If the time or temperature is out of the specified range, the control of the scale properties such as the scale thickness and the denseness of the scale becomes insufficient.
[0018]
Water cooling needs to be stopped at a temperature of 50 ° C. or more from the above-mentioned water cooling start temperature and at a temperature of 600 ° C. or more. With water cooling of less than 50 ° C from the water cooling start temperature, the water cooling time becomes extremely short, making it difficult to ensure uniformity of cooling. In addition, the scale properties such as minimizing the original purpose of scale regeneration and growth are minimized. The net effect of the control is small. On the other hand, when the water cooling stop temperature is lower than 600 ° C., the mechanical properties, particularly the strength, are significantly affected. If the steel components are also appropriate, the water cooling stop temperature is not limited to the scope of the present invention, but the scale control is performed in the existing component system that originally satisfies the required properties (strength) even as it is rolled. The stance of the invention is limited as described above. In the steel sheet manufactured by such a method, not only the scale is dense and the voids in the scale are small, and the adhesion to the base steel is good, but also the color tone is clearly black and a good appearance is exhibited.
[0019]
Here, the reason why the laser cutting property is excellent by controlling the scale property will be briefly described. As described above, in laser cutting, oxygen assist gas is blown simultaneously with laser irradiation. At this time, if the scale properties are poor, for example, if the thickness is uneven or there are many voids in the scale, or if the adhesion is poor, peeling and crushing of the scale due to thermal distortion during laser cutting or expansion of gas in the voids in the scale will occur. This causes the air flow of the oxygen assist gas to be disturbed, which causes the cutting properties and cut surface properties to be significantly deteriorated. Conversely, a steel sheet that is dense and has few voids in the scale, and has good adhesion to the base iron, shows stable and excellent cutability and cut surface quality without peeling off the scale during laser cutting. .
[0020]
In order to ensure the uniformity of the scale properties in the steel sheet, it is extremely effective not to perform descaling with high-pressure water in at least the last two passes of rolling. The invention according to claim 2 of the present invention is limited to those to be applied when demands on scale properties are severe. The reason for this limitation is to avoid de-scaling, rather promoting the non-uniformity of the scale. There is almost no problem if the descaling equipment is always in good condition, but it is not always the best condition industrially, and there is a risk of non-uniform descaling (unevenness). Is always pregnant. In the present invention, high-pressure water descaling is not performed in at least the last two passes of rolling, since non-uniformity of descaling near the final pass of rolling often affects the final product. I decided.
[0021]
【Example】
Steel slabs or cast slabs having various steel components were manufactured into steel plates having a thickness of 6 to 40 mm by various methods, and their mechanical properties and scale properties were investigated.
[0022]
Table 1 shows the basic amounts of C, Si, Mn and Ceq among the steel components of the present invention along with the comparative examples, and Table 2 shows the results of investigations on the manufacturing conditions, mechanical properties, and scale properties of the steel sheets.
[0023]
A steel sheet (invention example) having a composition and a production method in accordance with the method of the present invention has no problem at all as a tensile strength of 400 N / mm class ² steel, and also has good scale properties, which is the point of the present invention. . On the other hand, in the comparative examples in which the steel composition and the production conditions deviate from the limited range of the present invention, not only the scale properties are clearly inferior, but also the case where the tensile strength is 400 N / mm and the mechanical properties are deviated as the ^second grade steel. Can be seen.
[0024]
That is, in Comparative Example 8, since the Ceq is high and the water-cooling stop temperature is low, the tensile strength exceeds the standard strength of 400 N / mm grade ² steel. In addition, since the descaling is performed within the two final rolling passes, the appearance is poor. Comparative Example 9 is inferior in scale properties and appearance because the heating temperature is high and the water cooling conditions such as the time from rolling to water cooling and the water cooling start temperature are not appropriate. Comparative Example 10 is inferior in scale properties and appearance because the biting temperature in the final rolling pass is low. In Comparative Example 11, since water cooling was not performed after the rolling and the descaling was performed in the two final rolling passes, the scale properties and appearance were inferior.
[0025]
In Comparative Example 12, although the production conditions were carried out in accordance with the present invention, the scale properties and appearance were excellent, but as a result of laser cutting, the plate thickness was large (25 mm) and the amount of C was high. It has been confirmed that dross adheres to the vicinity of the lower surface and the cut surface properties are inferior.
[0026]
[Table 1]

[0027]
[Table 2]

[0028]
【The invention's effect】
The present invention, while satisfying the tensile strength of 400 N / mm ² class basic characteristics of a good steel plate of the scale properties, it becomes possible to stably and in large quantities supplied without impairing the productivity. Such a thick steel plate is not only excellent in appearance, but also has little scale peeling during various cold workings, which can contribute to improvement of the working environment. In addition, the laser cutting properties became extremely stable, and unmanned cutting operation at night became possible.

Claims

A steel slab or a slab having a steel component represented by mass% of 0.40% or less by a carbon equivalent Ceq defined as “Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14” is 1150 ° C. or less. After reheating to a temperature of, the rolling is completed at a biting temperature of the final rolling pass of 870 to 980 ° C, and water cooling is started within 2 minutes after the rolling and at a temperature of 720 ° C or higher, and 50 ° C from the water cooling starting temperature. A method for producing a ^second- grade thick steel plate having good scale properties and a tensile strength of 400 N / mm, wherein cooling is stopped, water cooling is stopped at a temperature of 600 ° C. or more, and then cooling is performed.

The method for producing a ^second- class thick steel plate having a good tensile strength of 400 N / mm according to claim 1, wherein the C content represented by mass% is 0.15% or less.

3. The method for producing a ^second- class thick steel plate having good scale properties and a tensile strength of 400 N / mm according to claim 1, wherein descaling with high-pressure water is not performed in at least the last two passes of rolling.