JP3881465B2 - High-tensile hot-rolled steel sheet with good surface quality - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、自動車、産業機械等における高強度部材用鋼板として好適な材質と熱延工程における優れたデスケーリング性を有する表面品質の良好な高張力熱延鋼板に関するものである。
【０００２】
【従来の技術】
一般に鋼板の強度が増加すると加工性が低下することは否めないが、それでも高強度と高加工性が両立する鋼板として鉄鋼便覧第３版、第４巻、２３６〜２３７頁に開示されているように、２相組織鋼（Ｄｕａｌ Ｐｈａｓｅ鋼、以下ＤＰ鋼と表す）がある。ＤＰ鋼は延性に優れたフェライト相中に強度の高いマルテンサイト相が分散した鋼であり各相の長所を併せ持ち、疲労強度にも優れた特長を有する。２相組織化してＤＰ鋼を製造するためにはＳｉを０．３質量％以上（以下、質量％を％と表す）含有させることが必要である。Ｓｉ濃度が高いと、熱延前の鋳片又は鋼片の加熱段階で液体のスケールが生成し、地鉄内部へくさび状に生成する。加熱後デスケーリングする際に鋳片又は鋼片が冷却して、このくさび状スケールが固化して剥離性の悪い、いわゆる「Ｓｉスケール」が発生する。このＳｉスケールは、完全にデスケーリングできないまま熱間圧延を行うので酸洗前の熱延板表面には残存スケールによる赤さびが発生する場合がある。また酸洗後の表面にはスケール模様が残り美観が損なわれ且つ表面粗度も損なわれて表面品質が劣化する場合がある。自動車、産業機械等の部材として加工後、塗装しても塗装面にスケール模様が現れる場合がある。ＤＰ鋼の持つ優れた材質を維持したまま、表面品質の良好な高張力鋼板が望まれている。
【０００３】
従来、上記のような高張力鋼板の製造方法としては、
▲１▼熱延工程での超高圧水ジェット法（例えば、特開平４−１８７３１７号公報に開示されているように１３ＭＰａ以上）、▲２▼鋼へのＰ添加法（例えば、鉄と鋼、第８３巻、第５号、１９９７年、３０５〜３１０頁）、▲３▼鋼へのＳ添加法（例えば、鉄と鋼、第８１巻、第５号、１９９５年、５５９〜５６３頁）が知られている。しかしながら、▲１▼の超高圧水ジェット法は高価な設備を必要とすること、また▲２▼、▲３▼の鋼へのＰ、Ｓ添加法はいずれも鋼材の機械的性質を著しく損なう場合がありうるという問題点がある。
【０００４】
【本発明が解決しようとする課題】
本発明は上記のような問題点を有利に解消するために、ミクロ組織と合金含有量の組合せを適正にすることにより熱延工程における優れたデスケーリング性を有する表面品質の良好な高張力熱延鋼板を提供することを目的とするものである。
【０００５】
【課題を解決するための手段】
本発明者らは上記の課題を解決するために種々検討を行った結果、高価な特別な設備なしでデスケーリング性に優れた高張力熱延鋼板の製造に成功した。その要旨を下記に示す。
【０００６】
（１） 質量％で、
Ｃ ：０．０１〜０．２５％、
Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．１〜２．２％、
Ｐ ：≦０．０５％、
Ｓ ：≦０．０５％、
Ｎ ：０．０００５〜０．０１％、
Ａｌ：０．１２０〜０．６０５％
を含有し、残部Ｆｅ及び不可避不純物からなる鋼であって、Ａｌ含有量が下記（Ａ）式を満足し、且つフェライト相と低温変態組織のマルテンサイト相の２相もしくはフェライト相と低温変態組織のマルテンサイト相およびベーナイト相の３相組織であることを特徴とする熱延工程における優れたデスケーリング性を有する表面品質の良好な高張力熱延鋼板。
０．２７９×［％Ｓｉ］−０．２６６×［％Ｓｉ］２ ＋０．０８３５×［％Ｓｉ］３−０．０４２２×［％Ｍｎ］≦［％Ａｌ］ −−−−−（Ａ）
【０００７】
（２） 質量％で、
Ｃ ：０．０１〜０．２５％、
Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．１〜２．２％、
Ｐ ：≦０．０５％、
Ｓ ：≦０．０５％、
Ｎ ：０．０００５〜０．０１％、
Ａｌ：０．１２０〜０．６０５％
を基本成分して含有し、更に選択成分として、
Ｎｂ：０．０１〜０．１％、
Ｃｒ：０．０１〜０．２％、
Ｔｉ：０．０１〜０．１％、
Ｖ：０．０１〜０．１％
のいづれか１種もしくは２種以上を含有し、残部Ｆｅ及び不可避不純物からなる鋼であって、Ａｌ含有量が下記（Ａ）式を満足し、且つフェライト相と低温変態組織のマルテンサイト相の２相もしくはフェライト相と低温変態組織のマルテンサイト相およびベーナイト相の３相組織であることを特徴とする熱延工程における優れたデスケーリング性を有する表面品質の良好な高張力熱延鋼板。
０．２７９×［％Ｓｉ］−０．２６６×［％Ｓｉ］２＋０．０８３５×［％Ｓｉ］３−０．０４２２×［％Ｍｎ］≦［％Ａｌ］−−−−−（Ａ）
【００１２】
上記の内容の構成としたのは、以下に述べるようにスケール剥離性に関する添加元素に関して種々検討を行い見出した新たな知見に基づくものである。通常、鋳片又は鋼片を熱延する前には予め加熱炉で１２００〜１３００℃程度に加熱する。このとき鋼がＳｉを０．０１％以上含有する場合には、鋳片又は鋼片表面にＦｅＯ−ＳｉＯ2を主成分とする低融点液体酸化物が生成し、スケールの融点が低下する。加熱中に液体スケールが多量に生成すると地鉄中に酸素が侵入して地鉄内部までくさび状にスケールが生成する。加熱後デスケーリングを行う際に、鋳片又は鋼片の表面温度が低下するので、このくさび状スケールは固化し地鉄に食いついてしまう。このいわゆるアンカリング効果によりスケールが剥離しがたく、デスケーリング性が阻害され、前記の如き課題があった。
【００１３】
本発明者らは検討を重ねた結果、鋼中にＡｌをある関係で含有せしめるとと、スケール中に融点の高い酸化物（Ａｌ 2 Ｏ 3 酸化物）を生成したスケールが地鉄界面に生成することにより、ＦｅＯ−ＳｉＯ2の低融点スケールの生成を抑制して地鉄界面近傍のスケールの融点を上昇させること、同時に酸素の地鉄中への過剰な侵入も防止し、特定ミクロ組織の形成とともにくさび状スケールの発生や成長を抑制し、その結果、地鉄に入り込むくさび状スケールを無くしてスケールの剥離性を向上させてデスケーリング性を改善し、酸洗後等の高張力熱延鋼板の表面品質を改善することが可能であることを本発明者らは新たに見出して、本発明を成し遂げたものである。
【００１４】
本発明者らが、鋼中のＡｌに着眼した理由は以下のとおりである。Ｓｉを０．０１％以上含有する鋼の場合、通常の加熱炉では酸性酸化物のＳｉＯ2と塩基性酸化物のＦｅＯの生成を完全に無くすことは極めて困難である。そこでこれらの酸化物と共存する条件でスケールの融点を上昇させる酸化物は原理的に両性酸化物である可能性が高いと本発明者らは考えた。このような両性酸化物を作る元素のうちＦｅＯとＳｉＯ2よりも安定に酸化物を生成させる必要があるので強酸化元素であるＡｌを選択した。なお前述の公知技術であるＰ添加法はスケール中に強酸性酸化物であるＰ2Ｏ5を生成させることでスケールの融点を一層低下させてデスケーリング性を改善する方法であるから、本発明はこれとは全く逆の発想に基づくものである。
【００１５】
【発明の実施の形態】
次に、本発明の構成要件のそれぞれについて詳述し、またその限定理由について述べる。
【００１６】
Ｃ：強度を確保するために最低限０．０１％が必要である。これ未満の濃度では強度が出ない。しかし、０．２５％を超えると伸びが劣化するため０．２５％を上限とした。
【００１７】
Ｓｉ：フェライト相の延性を損なわずにフェライト相を強化するために添加した。高張力熱延鋼板（例えばＤＰ鋼板）として最低限要求される強度を発現させるために下限を０．０１％以上とした。２．０％を超えると延性の低下が顕著になり靱性も低下するので２．０％を上限とした。
【００１８】
Ｍｎ：強化元素として添加した。強度を確保するために０．１％を下限とした。溶製上のコストから２．２％を上限とした。
【００１９】
Ｐ、Ｓ：Ｐ及びＳは鋼の製造工程から不可避的に含有される不純物元素であって、許容できる上限はそれぞれ０．０５％である。そのためＰの上限を０．０５％、Ｓの上限を０．０５％とした。
【００２０】
Ｎｂ：Ｎｂは炭化物を形成し強度を確保する元素であり、０．０１％未満では、強度が不足し、０．１％超では、効果が飽和し、コスト的に不利であるので、含有範囲は０．０１％〜０．１％とした。
【００２１】
Ｔｉ：Ｔｉは炭化物を形成し強度を確保する元素であり、また、ＮをＴｉＮとして固定して時効硬化性を抑制する元素でもある。０．０１％未満では、強度が不足し、０．１％超では、強度が上昇しすぎて延性が劣化するため、０．１％を上限とした．
【００２２】
Ｃｒ：固溶強化と２相組織の安定化のために添加した。０．０１％以上で強度の増加と安定化効果が発現する。溶製上のコストから０．２％を上限とした。
【００２３】
Ｖ：Ｖは炭化物を形成し強度を確保する元素であり、０．０１％未満では、強度が不足し、０．１％超では、前記効果が飽和し、コスト的に不利である。更に、Ｖの過剰添加はＶＮ等の粒界偏析で本組織を有する高張力熱延鋼板の靱性を劣化させるので０．１％を上限とした。
【００２４】
Ｎ ：Ｎは、０．０００５％未満では製鋼コストが飛躍的に上昇するのでこれを下限とした。また、０．０１％超では、Ｎの時効が大きくなり、また、ＴｉとＴｉＮを形成し、有効Ｔｉを低減するのでこの値を上限とした。
【００２５】
Ａｌ：前述のとおりデスケーリング性を向上し、高張力熱延鋼板の表面品質改善のために非常に重要で０．１２０％以上必要であるが、下記（Ａ）式のとおりＡｌ濃度の必要下限はＳｉ濃度とＭｎ濃度の関数であり、上限は溶製コストを考慮として０．６０５％とした。
０．２７９×［％Ｓｉ］−０．２６６×［％Ｓｉ］2＋０．０８３５×［％Ｓｉ］3
−０．０４２２×［％Ｍｎ］≦［％Ａｌ］ −−−−−（Ａ）
【００２６】
この式を得た根拠は以下のとおりである。種々検討した結果、Ｓｉ濃度が高いと必要Ａｌ濃度が増すこと、Ｍｎ濃度が高いと必要Ａｌ濃度が減少することがわかった。実験室の小型溶解した熱延鋼板や実炉溶製した各種高張力熱延鋼板において、熱延工程でのデスケーリング性や該鋼板の表面品質を調査して、前記の（Ａ）式を得た。また、（Ａ）式を求める際には、実施例１や実施例２の結果も用いた。
【００３１】
ミクロ組織について述べる。フェライト相と低温変態組織の２相又は３相組織である鋼板とは、通常の加工性と高強度付与に好適な、例えば、ＤＰ鋼板（フェライトとマルテンサイトの２相組織からなるデュアルフェイス）鋼板、ＴＰ鋼板（例えば、フェライトとマルテンサイトとベーナイトの３相組織からなるトリフェイス鋼板）がある。残留オーステナイトは、低温変態組織ではないが、残留オーステナイト量が５％以下では、加工性、高強度、スケール剥離性を阻害しないので、残留オーステナイト量が５％以下であれば本願発明を逸脱せずに、実質的に、フェライト相と低温変態組織の２相又は３相組織である。残留オーステナイト量は、例えば、鋼板の板厚方向の１／４の位置でγの（２００）、（２２０）、（３１１）の３面についてＸ線強度を測定し、標準資料と比較して求めることが出来る。
【００３２】
鋼中にＡｌを含有させることで、スケール中に１３００℃以上の融点の高いＡｌ 2 Ｏ 3 を生成させることでＦｅＯ−ＳｉＯ2の低融点スケールの生成を抑制してスケールの融点を上昇させる。同時に酸素の地鉄中への過剰な侵入も防止する。その結果、前記のミクロ組織の生成過程においても、地鉄に入り込むくさび状スケールを無くしてスケールの剥離性を向上させてデスケーリング性を改善するものである。
【００３３】
当然ながら、本発明の高張力熱延鋼板は、表面処理（例えば、亜鉛めっき、亜鉛系めっき、クロムめっき、錫めっき、ニッケルめっき等）用の素材としても有効であり、本発明を逸脱するものではない。
【００３４】
また、前記のミクロ組織を含有するものであれば、高張力鋼板の引張り強さは限定されるものではなく、引張強さは、３４０Ｎ／ｍｍ2以上でも、５９０Ｎ／ｍｍ2以上でも、また７８０Ｎ／ｍｍ2以上でも、９５０Ｎ／ｍｍ2以上でもかまわない。
【００３５】
更に、製造条件も限定されるものではなく、鋳造後加熱炉で加熱することなく、熱間圧延を施して本願発明の鋼板を製造しても良い。また、熱間圧延の際の、水圧デスケーリングは、通常の１〜２ＭＰａでも良いが、高圧４〜１０ＭＰａでも良く、１０ＭＰａ超でもかまわない。
【００３６】
【実施例】
本発明の実施例と比較例を表１、２に示す。
【００３７】
（実施例１）
３０ｋｇの小型溶解材を実験室で鋳造し、通常の実機加熱炉と同じガス燃焼環境で１２５０℃で４時間加熱後、熱間圧延機で板厚２ｍｍの熱延板を製造した。
【００３８】
表１に熱延板の化学組成、スケール剥離性とくさび状スケールの発生状況を示す。
【００３９】
スケール剥離性は、熱延板の１８０度折り曲げ密着曲げを行い曲げ部にセロハンテープ（幅１６ｍｍ×長さ４０ｍｍ）を接着した後に、はがしてテープに付着したスケール量から剥離性を評価した。テープへのスケール付着面積率が５０％未満を○、５０％以上を×とした。加熱後に急速冷却して、熱延前の鋳片の表層断面部（圧延方向と直角方向の断面）を光学顕微鏡で観察（倍率５０倍）し、くさび状スケール発生状況を測定した。
【００４０】
図１に熱延鋼板のくさび状スケールの断面概略図を示す。
【００４１】
図１に示すように、各酸洗前熱延鋼板１で、酸洗前熱延鋼板表層部２の表層断面部長さ１ｃｍ当たりのスケール３を観察し、地鉄内への深さ２０μｍ以上のくさび状スケール４の発生頻度が１箇所未満であれば○、１箇所以上であれば×とした。尚、鋼板Ｎｏ．１〜１１は、いずれも前記ＤＰ鋼板又はＴＰ鋼板であり、引張強さは３４０〜１０００Ｎ／ｍｍ2である。表１に示すように、本実施例は、いずれもくさび状スケールの発生頻度が○評価であった。
【００４２】
【表１】
（実施例２）
実炉で２７０ｔｏｎの溶鋼を連続鋳造し、厚さ２５０ｍｍ、幅１３００ｍｍのスラブを製造した。該スラブを１２５０〜１３００℃の加熱炉で加熱した。加熱後に１００Ｋｇｆ／ｃｍ2の通常水圧でデスケーリングを行い、板厚４ｍｍまで熱間圧延した。
【００４４】
表２に熱延板の化学組成、くさび状スケールの有無、スケール模様、及び酸洗後板粗度を示す。
【００４５】
酸洗前の熱延鋼板の表層断面部（圧延方向と直角方向の断面）を光学顕微鏡で観察（倍率５０倍）し、くさび状スケールの有無を調査した。深さ２０μｍ以上のくさび状スケールが１ｃｍ当たり１個以上あれば×、１個未満であれば〇とした。また、酸洗後の高張力熱延鋼板の表面スケール模様を、優、良、不可の３段階で目視判定した。更に、酸洗後の表面粗度（Ｒmax）はＪＩＳ Ｂ ０６０１に基づいて判定し、Ｒmaxが３０μｍ未満を〇、３０μｍ超を×とした。尚、鋼板Ｎｏ．１〜１２は、いずれも前記ＤＰ鋼板又はＴＰ鋼板であり、引張強さは３４０〜１０００Ｎ／ｍｍ2である。表２に示すように本実施例は、くさび状スケール及び酸洗後の表面粗度についての評価はいずれも○で、スケール模様は優又は良であった。
【００４６】
【表２】
【発明の効果】
本発明によれば、Ａｌの添加範囲を特定することによりフェライト相と低温変態組織の２相もしくは３相組織である高張力熱延鋼板の熱延工程でのデスケーリング性が著しく向上することが判明した。前記の成分とミクロ組織の特定により、設備制約無しに、高張力で高加工性を有し、表面品質の良好な高張力熱延鋼板の製造が可能となり産業上極めて大きな効果を有する。
【図面の簡単な説明】
【図１】熱延鋼板のくさび状スケールの断面概略図である。
【符号の説明】
１ 酸洗前熱延鋼板
２ 酸洗前熱延鋼板表層部
３ スケール
４ くさび状スケール[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength hot-rolled steel sheet having a good surface quality and a material suitable as a steel sheet for high-strength members in automobiles, industrial machines and the like and an excellent descaling property in a hot-rolling process.
[0002]
[Prior art]
In general, it is undeniable that the workability decreases when the strength of the steel sheet increases, but it is still disclosed in Steel Handbook 3rd Edition, Volume 4, Pages 236 to 237 as a steel sheet that achieves both high strength and high workability. There is a dual phase steel (Dual Phase steel, hereinafter referred to as DP steel). DP steel is a steel in which a high-tensile martensite phase is dispersed in a ferrite phase having excellent ductility. It has the advantages of each phase and has excellent fatigue strength. In order to produce DP steel by forming a two-phase structure, it is necessary to contain Si by 0.3 mass % or more (hereinafter, mass % is expressed as%). When the Si concentration is high, a liquid scale is generated in the heating stage of the slab or steel slab before hot rolling, and is generated in a wedge shape inside the base iron. When descaling after heating, the slab or steel slab cools, and this wedge-shaped scale is solidified to generate a so-called “Si scale” having poor peelability. Since this Si scale is hot-rolled without being completely descalable, red rust due to residual scale may occur on the surface of the hot rolled sheet before pickling. In addition, a scale pattern may remain on the surface after pickling, and the aesthetics may be impaired and the surface roughness may be impaired to deteriorate the surface quality. A scale pattern may appear on the painted surface even if it is painted after being processed as a member of an automobile or industrial machine. A high-tensile steel sheet with good surface quality is desired while maintaining the excellent material of DP steel.
[0003]
Conventionally, as a manufacturing method of the above high-tensile steel plate,
(1) Ultra-high pressure water jet method in the hot rolling process (for example, 13 MPa or more as disclosed in JP-A-4-187317), (2) P addition method to steel (for example, iron and steel, 83, No. 5, 1997, pp. 305-310), (3) S addition method to steel (for example, iron and steel, Vol. 81, No. 5, 1995, pp. 559-563) Are known. However, (1) the ultra-high pressure water jet method requires expensive equipment, and (2) and (3) methods of adding P and S to the steel significantly impair the mechanical properties of the steel. There is a problem that there may be.
[0004]
[Problems to be solved by the present invention]
In order to advantageously eliminate the above-mentioned problems, the present invention has an excellent descaling property in the hot rolling process by optimizing the combination of the microstructure and the alloy content , and has a high surface quality and high tension heat. The object is to provide a rolled steel sheet.
[0005]
[Means for Solving the Problems]
As a result of various studies to solve the above-mentioned problems, the present inventors have succeeded in producing a high-tensile hot-rolled steel sheet excellent in descaling properties without expensive special equipment. The summary is shown below.
[0006]
(1) In mass%,
C: 0.01 to 0.25%
Si: 0.01 to 2.0%,
Mn: 0.1-2.2%
P: ≦ 0.05%,
S: ≦ 0.05%,
N: 0.0005 to 0.01%,
Al: 0.120 to 0.605%
And the balance Fe and inevitable impurities, the Al content satisfies the following formula (A), and the two phases of ferrite phase and low-temperature transformation structure or ferrite phase and low-temperature transformation structure A high-strength hot-rolled steel sheet with excellent surface quality having excellent descaling properties in a hot-rolling process, characterized by a three-phase structure of a martensite phase and a bainite phase .
0.279 × [% Si] −0.266 × [% Si] 2 + 0.0835 × [% Si] 3 −0.0422 × [% Mn] ≦ [% Al] −−−−− (A)
[0007]
(2) By mass%
C: 0.01 to 0.25%
Si: 0.01 to 2.0%,
Mn: 0.1-2.2%
P: ≦ 0.05%,
S: ≦ 0.05%,
N: 0.0005 to 0.01%,
Al: 0.120 to 0.605%
As a basic component, and as an optional component,
Nb: 0.01 to 0.1%,
Cr: 0.01 to 0.2%,
Ti: 0.01 to 0.1%,
V: 0.01 to 0.1%
Is a steel containing one or more of the above, the balance being Fe and inevitable impurities, the Al content satisfying the following formula (A), and the ferrite phase and the martensitic phase of the low temperature transformation structure 2 A high-strength hot-rolled steel sheet with excellent surface quality having excellent descaling properties in a hot-rolling process, characterized by a three-phase structure of a martensite phase and a bainite phase of a phase or ferrite phase and a low-temperature transformation structure .
0.279 × [% Si] −0.266 × [% Si] 2 + 0.0835 × [% Si] 3 −0.0422 × [% Mn] ≦ [% Al] −−−−− (A)
[0012]
The configuration described above is based on new findings found by various studies on additive elements related to scale peelability as described below. Usually, before hot-rolling a slab or a steel slab, it is preliminarily heated to about 1200 to 1300 ° C. in a heating furnace. At this time, when the steel contains 0.01% or more of Si, a low melting point liquid oxide mainly composed of FeO—SiO 2 is formed on the surface of the cast slab or the slab, and the melting point of the scale is lowered. When a large amount of liquid scale is generated during heating, oxygen enters the ground iron and a scale is formed in a wedge shape inside the ground iron. When the descaling is performed after heating, the surface temperature of the slab or the steel slab is lowered, so that the wedge-shaped scale is solidified and bites into the steel. Due to this so-called anchoring effect, the scale is difficult to peel off, and the descaling property is hindered.
[0013]
As a result of repeated investigations by the inventors, when Al is contained in the steel in a certain relationship, a scale in which an oxide having a high melting point ( Al 2 O 3 oxide ) is generated in the scale is generated at the iron-iron interface. By suppressing the generation of the low melting point scale of FeO-SiO 2 , the melting point of the scale near the iron interface is raised, and at the same time, excessive penetration of oxygen into the steel is prevented, and the specific microstructure Suppresses the generation and growth of wedge-shaped scales along with the formation, and as a result, eliminates the wedge-shaped scales that enter the base iron, improving the scale peeling property and improving the descaling property, and high-tensile hot rolling after pickling, etc. The present inventors have newly found that it is possible to improve the surface quality of a steel sheet and have accomplished the present invention.
[0014]
The reason why the inventors focused on Al in steel is as follows. In the case of steel containing 0.01% or more of Si, it is extremely difficult to completely eliminate the generation of acidic oxide SiO 2 and basic oxide FeO in a normal heating furnace. Therefore, the present inventors considered that an oxide that raises the melting point of the scale under the condition of coexisting with these oxides is likely to be an amphoteric oxide in principle. Of the elements that make such amphoteric oxides, Al, which is a strong oxidizing element, was selected because it is necessary to generate oxides more stably than FeO and SiO 2 . The P addition method, which is the above-mentioned known technique, is a method for further improving the descaling property by further reducing the melting point of the scale by generating P 2 O 5 which is a strongly acidic oxide in the scale. Is based on the opposite idea.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Next, each of the components of the present invention will be described in detail, and the reasons for limitation will be described.
[0016]
C: A minimum of 0.01% is necessary to ensure strength. If the concentration is lower than this, no strength is obtained. However, if it exceeds 0.25%, the elongation deteriorates, so 0.25% was made the upper limit.
[0017]
Si: Added to strengthen the ferrite phase without impairing the ductility of the ferrite phase. In order to develop the minimum required strength as a high-tensile hot-rolled steel sheet (for example, DP steel sheet), the lower limit was made 0.01% or more. If it exceeds 2.0%, the ductility is significantly lowered and the toughness is also lowered, so 2.0% was made the upper limit.
[0018]
Mn: added as a strengthening element. In order to ensure strength, 0.1% was made the lower limit. The upper limit was set at 2.2% from the cost of melting.
[0019]
P, S: P and S are impurity elements inevitably contained from the steel manufacturing process, and the allowable upper limit is 0.05%. Therefore, the upper limit of P is set to 0.05% and the upper limit of S is set to 0.05%.
[0020]
Nb: Nb is an element that forms carbides and ensures strength. If the content is less than 0.01%, the strength is insufficient, and if it exceeds 0.1%, the effect is saturated and disadvantageous in terms of cost. Was from 0.01% to 0.1%.
[0021]
Ti: Ti is an element that forms carbides and ensures strength, and is also an element that suppresses age hardening by fixing N as TiN. If it is less than 0.01%, the strength is insufficient, and if it exceeds 0.1%, the strength increases so much that the ductility deteriorates, so 0.1% was made the upper limit.
[0022]
Cr: Added for solid solution strengthening and stabilization of the two-phase structure. An increase in strength and a stabilizing effect are manifested at 0.01% or more. The upper limit was made 0.2% from the cost of melting.
[0023]
V: V is an element that forms carbides and ensures strength. If the content is less than 0.01%, the strength is insufficient, and if it exceeds 0.1%, the above-described effect is saturated, which is disadvantageous in terms of cost. Further, excessive addition of V deteriorates the toughness of the high-tensile hot-rolled steel sheet having this structure due to segregation of grain boundaries such as VN, so the upper limit was made 0.1%.
[0024]
N: When N is less than 0.0005%, the steelmaking cost increases dramatically, so this was made the lower limit. If it exceeds 0.01%, the aging of N becomes large, and Ti and TiN are formed to reduce the effective Ti, so this value was made the upper limit.
[0025]
Al: As described above, the descalability is improved, and it is very important for improving the surface quality of the high-tensile hot-rolled steel sheet. 0.120% or more is necessary, but the lower limit of the Al concentration is required as shown in the following formula (A). Is a function of the Si concentration and the Mn concentration, and the upper limit is set to 0.605 % in consideration of the melting cost.
0.279 × [% Si] −0.266 × [% Si] 2 + 0.0835 × [% Si] 3
−0.0422 × [% Mn] ≦ [% Al] −−−−− (A)
[0026]
The basis for obtaining this formula is as follows. As a result of various studies, it was found that the required Al concentration increases when the Si concentration is high, and the required Al concentration decreases when the Mn concentration is high. In the laboratory hot-rolled steel sheet and various high-strength hot-rolled steel sheets melted in the actual furnace, the descaling property in the hot-rolling process and the surface quality of the steel sheet were investigated, and the above formula (A) was obtained. It was. Moreover, when calculating | requiring (A) Formula, the result of Example 1 or Example 2 was also used.
[0031]
Describe the microstructure. A steel sheet having a two-phase or three-phase structure of a ferrite phase and a low temperature transformation structure is suitable for imparting normal workability and high strength, for example, a DP steel sheet (dual face composed of two phases of ferrite and martensite) steel sheet And TP steel plates (for example, triface steel plates made of a three-phase structure of ferrite, martensite, and bainite). Residual austenite is not a low-temperature transformation structure, but if the amount of retained austenite is 5% or less, workability, high strength, and scale peelability are not hindered. Therefore, if the amount of retained austenite is 5% or less, it does not depart from the present invention. Further, it is substantially a two-phase or three-phase structure of a ferrite phase and a low temperature transformation structure. The amount of retained austenite is obtained, for example, by measuring the X-ray intensity of three surfaces (γ) (200), (220), and (311) at ¼ position in the plate thickness direction of the steel sheet and comparing it with the standard material. I can do it.
[0032]
By containing Al in the steel, Al 2 O 3 having a high melting point of 1300 ° C. or higher is generated in the scale, thereby suppressing the generation of a low melting point scale of FeO—SiO 2 and increasing the melting point of the scale. At the same time, it prevents excessive penetration of oxygen into the steel. As a result, even in the generation process of the microstructure described above, the wedge-like scale that enters the ground iron is eliminated, and the scale peelability is improved to improve the descaling property.
[0033]
Naturally, the high-tensile hot-rolled steel sheet of the present invention is also effective as a material for surface treatment (for example, zinc plating, zinc-based plating, chromium plating, tin plating, nickel plating, etc.) and departs from the present invention. is not.
[0034]
Further, as long as it contains the microstructure, the tensile strength of the high-tensile steel plate is not limited, tensile strength, even 340 N / mm 2 or more, even at 590N / mm 2 or more, 780N / Mm 2 or more, or 950 N / mm 2 or more.
[0035]
Further, the production conditions are not limited, and the steel sheet of the present invention may be produced by hot rolling without heating in a heating furnace after casting. In addition, the hydraulic descaling at the time of hot rolling may be normal 1 to 2 MPa, but may be high pressure 4 to 10 MPa, or higher than 10 MPa.
[0036]
【Example】
Examples and comparative examples of the present invention are shown in Tables 1 and 2.
[0037]
Example 1
A 30 kg small molten material was cast in a laboratory, heated at 1250 ° C. for 4 hours in the same gas combustion environment as that of a normal actual heating furnace, and then a hot-rolled sheet having a thickness of 2 mm was manufactured with a hot rolling mill.
[0038]
Table 1 shows the chemical composition of the hot-rolled sheet, the scale peelability, and the state of occurrence of the wedge-shaped scale.
[0039]
The scale peelability was evaluated by evaluating the peelability based on the amount of scale attached to the tape after the cellophane tape (width 16 mm × length 40 mm) was adhered to the bent portion by bending and bonding the hot rolled plate 180 degrees. The scale adhesion area ratio to the tape was less than 50%, and 50% or more was rated as x. It was rapidly cooled after heating, and the surface layer cross section (cross section perpendicular to the rolling direction) of the slab before hot rolling was observed with an optical microscope (magnification 50 times) to measure the state of wedge scale generation.
[0040]
FIG. 1 shows a schematic cross-sectional view of a wedge-shaped scale of a hot-rolled steel sheet.
[0041]
As shown in FIG. 1, in each hot-rolled steel sheet 1 before pickling, the
[0042]
[Table 1]
(Example 2)
A 270 ton molten steel was continuously cast in an actual furnace to produce a slab having a thickness of 250 mm and a width of 1300 mm. The slab was heated in a heating furnace at 1250 to 1300 ° C. After heating, descaling was performed at a normal water pressure of 100 kgf / cm 2 and hot rolling was performed to a plate thickness of 4 mm.
[0044]
Table 2 shows the chemical composition of the hot-rolled sheet, the presence or absence of a wedge-shaped scale, the scale pattern, and the board roughness after pickling.
[0045]
The surface layer cross section (cross section perpendicular to the rolling direction) of the hot-rolled steel sheet before pickling was observed with an optical microscope (magnification 50 times), and the presence or absence of a wedge-shaped scale was investigated. When the number of wedge-shaped scales having a depth of 20 μm or more is 1 or more per 1 cm, it is evaluated as x when the number is 1 or less. Further, the surface scale pattern of the high-tensile hot-rolled steel sheet after pickling was visually determined in three stages: excellent, good, and impossible. Furthermore, the surface roughness (R max ) after pickling was determined based on JIS B 0601. R max was less than 30 μm, and x was more than 30 μm. Steel plate No. 1 to 12 are all the DP steel plate or TP steel plate, and the tensile strength is 340 to 1000 N / mm 2 . As shown in Table 2, in this example, the evaluation of the wedge-shaped scale and the surface roughness after pickling was both “good”, and the scale pattern was excellent or good.
[0046]
[Table 2]
【The invention's effect】
According to the present invention, by specifying the addition range of Al, the descalability in the hot rolling process of a high-tensile hot-rolled steel sheet that is a two-phase or three-phase structure of a ferrite phase and a low-temperature transformation structure can be significantly improved. found. By specifying the above components and microstructure, it is possible to produce a high-tensile hot-rolled steel sheet having high tension, high workability, and good surface quality without equipment restrictions, and has a very large industrial effect.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a wedge-shaped scale of a hot-rolled steel sheet.
[Explanation of symbols]
1 Hot-rolled steel sheet before pickling 2 Surface layer of hot-rolled steel sheet before pickling 3 Scale 4 Wedge scale
Claims (2)
Ｃ ：０．０１〜０．２５％、
Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．１〜２．２％、
Ｐ ：≦０．０５％、
Ｓ ：≦０．０５％、
Ｎ ：０．０００５〜０．０１％、
Ａｌ：０．１２０〜０．６０５％
を含有し、残部Ｆｅ及び不可避不純物からなる鋼であって、Ａｌ含有量が下記（Ａ）式を満足し、且つフェライト相と低温変態組織のマルテンサイト相の２相もしくはフェライト相と低温変態組織のマルテンサイト相およびベーナイト相の３相組織であることを特徴とする熱延工程における優れたデスケーリング性を有する表面品質の良好な高張力熱延鋼板。
０．２７９×［％Ｓｉ］−０．２６６×［％Ｓｉ］２＋０．０８３５×［％Ｓｉ］３−０．０４２２×［％Ｍｎ］≦［％Ａｌ］ −−−−−（Ａ）% By mass
C: 0.01 to 0.25%
Si: 0.01 to 2.0%,
Mn: 0.1-2.2%
P: ≦ 0.05%,
S: ≦ 0.05%,
N: 0.0005 to 0.01%,
Al: 0.120 to 0.605%
And the balance Fe and inevitable impurities, the Al content satisfies the following formula (A), and the two phases of ferrite phase and low-temperature transformation structure or ferrite phase and low-temperature transformation structure A high-strength hot-rolled steel sheet with excellent surface quality having excellent descaling properties in a hot-rolling process, characterized by a three-phase structure of a martensite phase and a bainite phase .
0.279 × [% Si] −0.266 × [% Si] 2 + 0.0835 × [% Si] 3 −0.0422 × [% Mn] ≦ [% Al] −−−−− (A)
Ｃ ：０．０１〜０．２５％、
Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．１〜２．２％、
Ｐ ：≦０．０５％、
Ｓ ：≦０．０５％、
Ｎ ：０．０００５〜０．０１％、
Ａｌ：０．１２０〜０．６０５％
を基本成分して含有し、更に選択成分として、
Ｎｂ：０．０１〜０．１％、
Ｃｒ：０．０１〜０．２％、
Ｔｉ：０．０１〜０．１％、
Ｖ：０．０１〜０．１％
のいづれか１種もしくは２種以上を含有し、残部Ｆｅ及び不可避不純物からなる鋼であって、Ａｌ含有量が下記（Ａ）式を満足し、且つフェライト相と低温変態組織のマルテンサイト相の２相もしくはフェライト相と低温変態組織のマルテンサイト相およびベーナイト相の３相組織であることを特徴とする熱延工程における優れたデスケーリング性を有する表面品質の良好な高張力熱延鋼板。
０．２７９×［％Ｓｉ］−０．２６６×［％Ｓｉ］２＋０．０８３５×［％Ｓｉ］３−０．０４２２×［％Ｍｎ］≦［％Ａｌ］−−−−−（Ａ）% By mass
C: 0.01 to 0.25%
Si: 0.01 to 2.0%,
Mn: 0.1-2.2%
P: ≦ 0.05%,
S: ≦ 0.05%,
N: 0.0005 to 0.01%,
Al: 0.120 to 0.605%
As a basic component, and as an optional component,
Nb: 0.01 to 0.1%,
Cr: 0.01 to 0.2%,
Ti: 0.01 to 0.1%,
V: 0.01 to 0.1%
Is a steel containing one or more of the above, the balance being Fe and inevitable impurities, the Al content satisfying the following formula (A), and the ferrite phase and the martensitic phase of the low temperature transformation structure 2 A high-strength hot-rolled steel sheet with excellent surface quality having excellent descaling properties in a hot-rolling process, characterized by a three-phase structure of a martensite phase and a bainite phase of a phase or ferrite phase and a low-temperature transformation structure .
0.279 × [% Si] −0.266 × [% Si] 2 + 0.0835 × [% Si] 3 −0.0422 × [% Mn] ≦ [% Al] −−−−− (A)
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