JP3917901B2 - Heating method of plain steel slab to obtain hot rolled sheet with less surface flaws - Google Patents
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Description
【0001】
【発明の属する技術分野】
本発明は、熱間圧延前の普通鋼スラブの加熱時における炉内雰囲気条件を制御することで、疵の起点となる粒界酸化の顕在化を防ぎ、熱間圧延時に発生する線状のスケール疵及び模様系欠陥(めっき時ムラ)の発生を少なくするための普通鋼スラブの加熱方法に関する。
【0002】
【従来の技術】
普通鋼薄板材で熱間圧延後顕在化する表面欠陥として線状のスケール疵やめっき材の模様系欠陥がある。この欠陥部には加熱炉で生成するスケールが残存し、線状のスケール疵もしくはめっきにて合金化処理時にめっきムラすなわち表面模様欠陥として顕在化し問題となる。お客様の表面品質に対する要求は厳格化する傾向を示しており、表面欠陥による歩留低下は大幅なコストアップを引き起こしている。
【0003】
上述の表面欠陥の原因となるスケールの残存原因としては次のように考えられている。まず加熱炉内で進行した粒界酸化がスラブコーナーで顕著な場合は、圧延時にそれを起点とし割れを発生させる。その後の圧延で伸ばされ線状のスケール疵となるか、または粒界酸化部に濃化したSiが粗圧延でのデスケーリング後も残存し、これが圧延され表層に引きのばされて鋼板表面を覆うことで合金化挙動に影響し、模様として表面に顕在化することが原因と考えられる。
【0004】
この疵発生の起点となる粒界酸化の抑制方法として、ステンレスの場合については、例えば特開昭62−13527号公報に酸素濃度をコントロールしてスケールオフ量をコントロールし改善する技術が、また特開平1−122606号公報にはオーステナイト系ステンレスについてスラブ加熱温度と酸素濃度及び圧下率をコントロールすることで粒界酸化を起因とする疵を低減する技術が、または特開平9−279316号公報にはフェライト系ステンレスにおいて異常ノジュール形成を抑制する技術などが報告されている。
【0005】
しかしながら、これらには積極的に粒界酸化の生じやすい成分であるCrやNiが多い場合にのみに限定されて研究されているという特徴があり、Cr、Niが不可避的な量しか含まれない普通鋼に関して疵につながる粒界酸化を低減し、その発生を抑制する技術について最適域が求められていないのが現状である。
【0006】
また、Siを含有している熱間圧延鋼材に関しては、例えば特開昭53−140219号公報にスケール剥離性を向上させるためにスラブ加熱温度と酸素濃度をコントロールする技術が開示されているが、粒界酸化しやすいSiを含んでいることや、粒界酸化そのものが起点となり疵になるのとは異なり加熱炉内で生成されたスケールが剥離しにくいことによってスケール疵が発生するという点で本発明が課題とする疵とは発生メカニズムが異なっていることから、そのまま適用することは困難なのが現状である。
【0007】
【発明が解決しようとする課題】
本発明は、今後も品質の厳格化が進むことが予想される普通鋼用スラブにおいて、熱延加熱炉内にてスラブ表面生じる粒界酸化を抑制し、歩留低下によって大幅なコストアップを招いている線状スケール疵やめっき材のめっきムラすなわち表面模様欠陥を防止するための普通鋼スラブの加熱炉雰囲気条件を提供することを目的とする
【0008】
【課題を解決するための手段】
本発明者らは、普通鋼スラブにおいて加熱炉内の雰囲気変動と粒界酸化挙動の関係を実験により明らかにした。併せて、粒界酸化と熱延後表面疵発生の関係を明確にし、疵につながる粒界酸化を抑制でき、実機熱延操業に反映可能な雰囲気条件を見出した。本発明はこれらの知見に基づき完成させるに至った。
【0009】
すなわち本発明の要旨は、以下のとおりである。
【0010】
(1) 熱間圧延の素材として質量%でC:0.20%以下、Si:0.1%以下、Mn:1.0%以下、P:0.050%以下、S:0.02%以下であり、かつTi:0.04%以下及びNb:0.04%以下を以下の式(1)の条件で含み、かつ、Ni:0.10%以下、Cu:0.10%以下、Sn:0.10%以下を以下の式(2)の条件で含み、その他Fe及び不可避的元素からなる普通鋼スラブを、空気比で1.0〜1.2の雰囲気で、炉内雰囲気中CO濃度が0.5%以下になるように制御し、スラブ加熱温度が1050℃〜1250℃の範囲内で加熱することを特徴とする表面疵の少ない熱延板を得るための普通鋼スラブの加熱方法。
[C]−12/48[Ti]−12/93[Nb]≦0 ・・・・(1)
(但し、[C]、[Ti]、[Nb]は、各元素の含有量(質量%))
[Ni]+[Cu]+[Sn]≦0.20% ・・・・(2)
(但し、[Ni]、[Cu]、[Sn]は、各元素の含有量(質量%))
【0013】
(2)空気比とスラブ加熱温度の関係として
疵発生指数=2290.4×(空気比−1)2+6.02×10−4×(スラブ加熱温度(℃)−1060)2≦45
の関係を満たすことを特徴とする上記(1)に記載の表面疵の少ない熱延板を得るための普通鋼スラブの加熱方法。
【0014】
(3)スラブ加熱時間を350分以内となるよう制御することを特徴とする上記(1)または(2)に記載の表面疵の少ない熱延板を得るための普通鋼スラブの加熱方法。
【0016】
【発明の実施の形態】
本発明者らは、熱間圧延に起因すると思われる普通鋼の線状スケール疵及びめっき模様系欠陥について多くの調査をした結果、次のような現象を見出した。
【0017】
(1) 線状スケール疵は、コイルのコーナー側に発生しやすい。また、疵部の断面調査ではスケールの上にメタルが被さったスケール疵となっている。
【0018】
(2) 模様系欠陥部を調査すると、めっきと鋼板の界面にスケールが存在する。そのスケール部には、EPMAやSIMSによる調査結果、SiやNiのような元素が確認できる。
【0019】
(3) スケール疵やめっき表面模様欠陥を発生しやすい普通鋼スラブの熱延加熱直後のスケール性状は、地鉄/スケール界面にSiの濃化層が存在し、Siを含んだスケールをもつ粒界酸化部が数多く存在する。
【0020】
(4) 粒界酸化がコーナー側に存在する場合、その後の粗圧延での幅方向の圧下により、割れを生ずることがある。また、コーナーにない場合では、粒界酸化の深さが深い場合は粒界酸化のみがデスケでも取り除かれないことがある。
【0021】
(5) 上記した幅圧下による割れが存在しない場合、製品にて線状スケール疵は発生しない。また、デスケにて粒界酸化が消失している場合、めっき材にて模様系欠陥が発生しない。
【0022】
このような現象から、普通鋼の線状スケール疵及びめっき模様系欠陥は、加熱炉にて生成した粒界酸化部が熱間圧延時幅圧下するために割れに結びついたり、粒界酸化部のみがスケールを噛み込んだまま引きのばされたりしたため、合金化時にムラとなり模様系の欠陥となることを見出した。
【0023】
そこで本発明者らは、実機における通常のスラブ加熱温度である1100℃〜1250℃の条件で、普通鋼として一般的なアルミキルド鋼について、粒界酸化を顕在化させない加熱炉内雰囲気と加熱条件を調査した。
【0024】
厚さ30mm、幅100mm、長さ100mmの表1に示す成分のサンプルを、LNGガス雰囲気でスラブ加熱温度と空気比を変更して加熱した。その後サンプルの断面検鏡を行い、サンプル界面の割れや模様系欠陥の原因となる粒界酸化の個数を調査した。その結果を図1に示す。
【0025】
この結果、粒界酸化は図1に示すように空気比が低下すると深さ及び個数は低下して品位改善されるが、空気比が1.0未満になると粒界酸化の品位は悪化する。加えて、高温加熱を実施した場合は空気比に関係なく総じて悪化することが判明した。これより、粒界酸化を起点とする表面疵を防止するにはスラブ加熱温度及び空気比に適正範囲が存在することを知見し、これらを制御するに至ったのである。
【0026】
次に本発明が対象とする鋼成分の限定理由について説明する。C、Mn、P、Sに関しては、普通鋼の一般的成分として使用されている範囲として、それぞれ質量%でC:0.20%以下、Mn:1.0%以下、P:0.050%以下、S:0.02%以下に限定したものである。
【0027】
Siに関しては、0.1%以上の場合は加熱炉雰囲気に関係なくスケール生成時に地鉄スケール界面に濃化し、割れやすくデスケさせにくいスケールを含んだ粒界酸化部を生成するため限定する。
【0028】
また、鋼中のCを無害化して加工性を向上するために、Ti:0.04%以下及びNb:0.04%以下を含有させる。このときの含有量としては、Ti及びNbを、Cとの関係で、[C]−12/48[Ti]−12/93[Nb]≦0
(但し、[C]、[Ti]、[Nb]は、各元素の含有量(質量%))
のように含有させる。但し、多すぎても効果が飽和するばかりか、鋼の強度が低下するなどの弊害があるため、Ti及びNbのそれぞれの上限を0.04質量%以下に制限する
【0029】
さらに、不可避的不純物もしくは特性向上を狙って添加される元素のうち、Ni、Cu、SnのようなFeよりも酸化されにくくスケール生成時に地鉄スケール界面に濃化する元素も、粒界酸化を顕在化させる。それぞれの元素が単独で0.10%超、もしくはNi、Cu、Snの合計で0.20%超存在した場合には、加熱炉雰囲気と関係なく表面疵が発生する。よって、この範囲にて規制する。
【0030】
このような成分構成の普通鋼スラブの加熱温度を1050℃〜1250℃の範囲内とする。1050℃より低温にて加熱した場合は、圧延可能な温度まで均一にスラブを昇温するためには長時間保定が必要となるため、生産性を阻害される。また高温で加熱した場合は、空気比つまりは酸素濃度を制御しても、生成スケールが非常に厚くなるため地鉄スケール界面にSiが多量に濃化する。これが極度に粒界酸化を進行させ、疵発生率を上昇させる問題が発生するためこの範囲で規制する。
【0031】
また加熱時間は、加熱炉総在炉時間を100分以上かつ350分以下の範囲内にて加熱する。100分未満ではスラブを均一に加熱できないためスラブコーナー部とセンター部で変形応力が大きく異なるため疵発生率は高くなり、また熱間圧延自体も困難となる。また350分を超えると、空気比つまりは酸素濃度を制御した場合も生成スケールが厚くなるためスケール界面にSiが多量に濃化する。この結果、図2に示すように疵発生率が高くなる傾向を示す。
【0032】
このスラブ加熱温度及び時間条件のもとLNG、LPG、COG燃焼ガス中で加熱する。このときの燃焼ガスと空気の空気比は、1.0〜1.2の範囲として、CO濃度が0.5Vol%以下となる条件に調整する。このときの酸素濃度はおよそ0.0〜4.0Vol%程度となる。空気比を制御することにより、粒界酸化を疵発生に影響しない個数・深さまで低減できる。COが0.5Vol%を超えると、図3に示すように疵発生個数が大幅に増加する。
【0033】
空気比が1.0未満でCO濃度が0.5Vol%超存在する状態となると、スケールオフ量が少なくなるため地鉄界面に濃化するSi量は少なくなるものと予想されてきたが、調査結果ではこの予想に反しSiが多量に濃化し、粒界酸化が顕著となった。EPMAによるSi定量分析の結果、鋼中のスケールオフによってスケール/地鉄界面に濃化する量が、想定されるよりも明らかに多いことが判明した。
【0034】
本発明者は多くの調査を実施した結果、加熱炉内に存在するスケールや耐火物等のSi酸化物が雰囲気中のCOガスと、以下の反応式
SiO2+CO→CO2+SiO(g)
で示される反応を起こし、Si酸化物が還元され、雰囲気中にSiOガスとして存在することを知見した。さらに調査の結果、還元されたSiOガスがスラブ表層に析出することでスケール生成時に地鉄/スケール界面に濃化するために、粒界酸化が顕在化したことを突き止めた。よって空気比は、必ずCO濃度が0.5Vol%以下の領域、つまり空気比1.0以上にする必要がある。
【0035】
また空気比1.2超となる場合は、スケール生成量が多くなる。このため地鉄スケール界面にSiが濃化し、粒界酸化は顕在化する。加えて実操業にて高酸素濃度で操業した場合は、排ガス中のNOxの濃度が高くなるため、実操業面からも空気比は下げるほうが好ましい。
【0036】
以上のように得られた空気比及びスラブ加熱温度と表面疵の発生量との関係についてさらに調査した結果、空気比及びスラブ加熱温度の関係について、以下の式で示される疵発生指数
疵発生指数=2290.4×(空気比−1)2+6.02×10-4×(スラブ加熱温度(℃)−1060)2
を定義し、この疵発生指標で45以下となる場合は疵が発生しないことを知見した。
【0037】
以上のような成分かつ加熱炉雰囲気条件にて熱間圧延された場合、線状のスケール疵及び模様系欠陥(めっき時ムラ)の発生は、非常に少なくすることができる。
【0038】
【実施例】
(実施例1)
本発明の実施例を以下に示す。
【0039】
厚さ30mm、幅100mm、長さ100mmの表1に示す成分組成(質量%)のサンプルを、LNG燃料ガス雰囲気でスラブ加熱温度及び空気比を変更して加熱した。このときの加熱時間は200分とした。
【0040】
加熱後サンプルの断面検鏡を行い、割れや模様系欠陥につながる粒界酸化の個数を調査した。その結果を図1に示す。本発明範囲においては、粒界酸化個数は低位安定することが判る。
【0041】
【表1】
【0042】
(実施例2)
厚さ30mm、幅100mm、長さ100mmの表1の鋼1に示す成分組成(質量%)のサンプルを、LNG燃料ガス雰囲気でスラブ加熱温度1230℃、空気比1.1として、加熱時間を種々変更して加熱した。
【0043】
加熱後サンプルの断面検鏡を行い、割れや模様系欠陥につながる粒界酸化の個数を調査した。その結果を図2に示す。本発明範囲においては、粒界酸化個数は低位安定することが判る。
【0044】
(実施例3)
厚さ30mm、幅100mm、長さ100mmの表1の鋼1に示す成分組成(質量%)のサンプルを、LNG燃料ガス雰囲気でスラブ加熱温度1230℃、CO濃度が0〜4%となるよう、空気比を変更して加熱した。このときの加熱時間は200分とした。
【0045】
加熱後サンプルの断面検鏡を行い、割れや模様系欠陥につながる粒界酸化の個数を調査した。その結果を図3に示す。本発明範囲においては、粒界酸化個数は低位安定することが判る。
【0046】
(実施例4)
厚み252mm、幅1000mm、長さ6000mmの表2に示す成分組成(質量%)の普通鋼スラブを、LNG燃料ガス雰囲気でスラブ加熱温度及び空気比を変更して加熱した。このスラブを200分間加熱した後に熱間にて圧延を実施、3mmの熱延鋼板を得た。さらに、7〜10%かつ70〜90℃のHClで酸洗を50〜250sec実施した。その後、めっきを施し線状スケール疵や表面模様の有無を評価した。評価結果を表3に示す。本発明条件を適用すると、疵発生もなく表面外観良好な製品を製造できることが判る。
【0047】
【表2】
【0048】
【表3】
【0049】
【発明の効果】
本発明は、熱間圧延前の普通鋼スラブの加熱時における炉内雰囲気条件を制御することで、疵の起点となる粒界酸化の顕在化を防ぎ、熱間圧延時に発生する線状のスケール疵及び模様系欠陥(めっき時ムラ)の発生を低減でき、表面外観良好な製品を製造できる。これにより、疵や模様発生に伴い生じていた歩留ロスを改善できるため、工業的価値は非常に大きい。
【図面の簡単な説明】
【図1】図1はスラブ加熱温度、空気比と粒界酸化の発生数との関係を示す図である。
【図2】図2はスラブ加熱時間と疵発生の関係を示す図である。
【図3】図3はCO濃度と疵発生率の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention controls the atmospheric conditions in the furnace during heating of the plain steel slab before hot rolling, thereby preventing the manifestation of grain boundary oxidation, which is the starting point of soot, and the linear scale generated during hot rolling. The present invention relates to a method for heating ordinary steel slabs to reduce the occurrence of defects and pattern defects (unevenness during plating).
[0002]
[Prior art]
As surface defects that become obvious after hot rolling in a thin sheet steel, there are linear scale defects and pattern defects in plating materials. Scales generated in the heating furnace remain in the defect portion, and are manifested as a plating unevenness, that is, a surface pattern defect at the time of alloying treatment with a linear scale flaw or plating. Customers' demands for surface quality are becoming stricter, and the yield reduction due to surface defects has caused a significant cost increase.
[0003]
The remaining cause of the scale causing the surface defects is considered as follows. First, when the grain boundary oxidation that has progressed in the heating furnace is remarkable at the slab corner, cracks are generated starting from that during rolling. Si that has been stretched by subsequent rolling to become linear scale wrinkles or concentrated in the grain boundary oxidation part remains even after descaling in rough rolling, and this is rolled and stretched to the surface layer to surface the steel sheet surface. The reason for this is that the covering influences the alloying behavior and appears on the surface as a pattern.
[0004]
As a method for suppressing grain boundary oxidation, which is the starting point for the generation of soot, for stainless steel, for example, Japanese Patent Application Laid-Open No. 62-13527 discloses a technique for controlling and improving the scale-off amount by controlling the oxygen concentration. Kaihei 1-122606 discloses a technique for reducing wrinkles caused by grain boundary oxidation by controlling the slab heating temperature, oxygen concentration, and rolling reduction of austenitic stainless steel, or Japanese Patent Laid-Open No. 9-279316. Techniques for suppressing abnormal nodule formation in ferritic stainless steel have been reported.
[0005]
However, these are characterized in that they are actively studied only when there are a large amount of Cr and Ni, which are components that are prone to grain boundary oxidation, and contain only inevitable amounts of Cr and Ni. The current situation is that there is no need for an optimum range of technology for reducing the occurrence of grain boundary oxidation that leads to defects in ordinary steel and suppressing its occurrence.
[0006]
As for hot-rolled steel materials containing Si, for example, JP-A-53-140219 discloses a technique for controlling the slab heating temperature and the oxygen concentration in order to improve the scale peelability. Unlike the fact that it contains Si, which easily undergoes grain boundary oxidation, and that the grain boundary oxidation itself becomes the starting point, it becomes difficult to peel off the scale generated in the heating furnace. The present situation is that it is difficult to apply as it is because the generation mechanism is different from that of the soot as the subject of the invention.
[0007]
[Problems to be solved by the invention]
The present invention suppresses grain boundary oxidation occurring on the surface of a slab in a hot-rolling heating furnace in a slab for ordinary steel, which is expected to be stricter in quality in the future, and causes a significant cost increase due to a decrease in yield. An object of the present invention is to provide a heating furnace atmosphere condition of a normal steel slab to prevent plating scale irregularities or plating pattern irregularities of the plating material, that is, surface pattern defects.
[Means for Solving the Problems]
The present inventors have clarified the relationship between the change in atmosphere in the heating furnace and the grain boundary oxidation behavior in an ordinary steel slab by experiments. In addition, the relationship between grain boundary oxidation and surface soot generation after hot rolling was clarified, and the grain boundary oxidation that leads to soot could be suppressed, and atmospheric conditions that could be reflected in the actual hot rolling operation were found. The present invention has been completed based on these findings.
[0009]
That is, the gist of the present invention is as follows.
[0010]
(1) C: 0.20% or less, Si: 0.1% or less, Mn: 1.0% or less, P: 0.050% or less, S: 0.02% by mass% as a raw material for hot rolling And Ti: 0.04% or less and Nb: 0.04% or less under the condition of the following formula (1), Ni: 0.10% or less, Cu: 0.10% or less, Sn: 0.10% or less under the condition of the following formula (2), and other steel and slabs composed of inevitable elements, in an atmosphere of 1.0 to 1.2 in the atmosphere in the furnace An ordinary steel slab for obtaining a hot-rolled sheet with less surface flaws, characterized in that the CO concentration is controlled to be 0.5% or less and the slab heating temperature is heated within a range of 1050 ° C to 1250 ° C. Heating method.
[C] -12/48 [Ti] -12/93 [Nb] ≦ 0 (1)
(However, [C], [Ti] and [Nb] are the contents of each element (mass%))
[Ni] + [Cu] + [Sn] ≦ 0.20% (2)
(However, [Ni], [Cu], and [Sn] are the contents of each element (mass%))
[0013]
( 2 ) As the relationship between the air ratio and the slab heating temperature, soot generation index = 2290.4 × (air ratio−1) 2 + 6.02 × 10 −4 × (slab heating temperature (° C.) − 1060) 2 ≦ 45
Heating method of ordinary steel slab to obtain a small hot-rolled sheet of the surface defects according to the above (1) to satisfy the relationship.
[0014]
( 3 ) The method for heating a plain steel slab for obtaining a hot-rolled sheet having a small surface flaw as described in (1) or (2) above, wherein the slab heating time is controlled to be within 350 minutes.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
As a result of many investigations on the linear scale defects and plating pattern system defects of ordinary steel which are considered to be caused by hot rolling, the present inventors have found the following phenomenon.
[0017]
(1) Linear scale wrinkles are likely to occur on the corner side of the coil. In addition, in the cross-sectional survey of the heel part, it is a scale ridge with metal on the scale.
[0018]
(2) When a pattern-type defect portion is investigated, a scale exists at the interface between the plating and the steel plate. In the scale portion, elements such as Si and Ni can be confirmed as a result of investigation by EPMA and SIMS.
[0019]
(3) The scale properties immediately after hot-rolling heating of plain steel slabs that are prone to scale defects and plating surface pattern defects are grains with a Si-enriched layer at the base metal / scale interface and scales containing Si. There are many boundary oxidation parts.
[0020]
(4) When grain boundary oxidation exists on the corner side, cracks may occur due to the rolling in the subsequent rough rolling in the width direction. In addition, when there is no corner, if the depth of grain boundary oxidation is deep, only the grain boundary oxidation may not be removed even by deske.
[0021]
(5) When there is no crack due to the above-described width reduction, no linear scale wrinkles occur in the product. In addition, when grain boundary oxidation disappears due to deske, pattern defects do not occur in the plating material.
[0022]
From such a phenomenon, the linear scale defects and plating pattern defects of ordinary steel lead to cracking because the grain boundary oxidation part generated in the heating furnace reduces the width during hot rolling, or only the grain boundary oxidation part Was pulled out with the scale biting in, resulting in unevenness during alloying and pattern defects.
[0023]
Therefore, the present inventors set the atmosphere in the furnace and the heating conditions that do not reveal the grain boundary oxidation for the aluminum killed steel, which is a general steel, under the conditions of 1100 ° C. to 1250 ° C., which is the normal slab heating temperature in the actual machine. investigated.
[0024]
Samples of the components shown in Table 1 having a thickness of 30 mm, a width of 100 mm, and a length of 100 mm were heated by changing the slab heating temperature and the air ratio in an LNG gas atmosphere. Thereafter, the sample was subjected to cross-sectional microscopy to investigate the number of grain boundary oxidations that cause cracks in the sample interface and pattern defects. The result is shown in FIG.
[0025]
As a result, as shown in FIG. 1, the grain boundary oxidation is improved in quality by reducing the depth and number when the air ratio is lowered, but the quality of grain boundary oxidation is deteriorated when the air ratio is less than 1.0. In addition, it has been found that when high-temperature heating is carried out, it generally deteriorates regardless of the air ratio. From this, in order to prevent surface flaws starting from grain boundary oxidation, it has been found that there are appropriate ranges for the slab heating temperature and the air ratio, and these have been controlled.
[0026]
Next, the reasons for limiting the steel components targeted by the present invention will be described. Regarding C, Mn, P, and S, the ranges used as general components of ordinary steel are C: 0.20% or less, Mn: 1.0% or less, and P: 0.050%, respectively, in mass%. Hereinafter, S is limited to 0.02% or less.
[0027]
Regarding Si, when it is 0.1% or more, it is limited because it concentrates at the scale scale interface at the time of scale generation regardless of the heating furnace atmosphere, and generates a grain boundary oxidation part including a scale that is easy to crack and is difficult to squeeze.
[0028]
Further, in order to render C in the steel harmless and improve workability, Ti: 0.04% or less and Nb: 0.04% or less are contained . As content at this time, in relation to C, Ti and Nb are [C] -12/48 [Ti] -12/93 [Nb] ≦ 0.
(However, [C], [Ti] and [Nb] are the contents of each element (mass%))
Make content as. However, if the amount is too large, not only the effect is saturated, but also the strength of the steel is lowered. Therefore, the upper limit of each of Ti and Nb is limited to 0.04% by mass or less.
Furthermore, among elements added for the purpose of improving unavoidable impurities or characteristics, elements such as Ni, Cu, and Sn that are less oxidized than Fe and concentrate at the scale scale interface during scale generation also undergo grain boundary oxidation. Make it manifest. When each element alone exceeds 0.10% or the total of Ni, Cu, and Sn exceeds 0.20%, surface flaws occur regardless of the heating furnace atmosphere. Therefore, it regulates in this range.
[0030]
The heating temperature of the ordinary steel slab having such a component structure is set within a range of 1050 ° C to 1250 ° C. When heated at a temperature lower than 1050 ° C., it is necessary to maintain for a long time in order to raise the temperature of the slab uniformly to a temperature at which rolling is possible, so that productivity is hindered. Further, when heated at a high temperature, even if the air ratio, that is, the oxygen concentration is controlled, the generated scale becomes very thick, so that a large amount of Si is concentrated at the interface of the scale. This causes a problem that the grain boundary oxidation progresses extremely and raises the soot generation rate.
[0031]
Moreover, the heating time is heated within the range of 100 minutes or more and 350 minutes or less of the total furnace time. If it is less than 100 minutes, the slab cannot be heated uniformly, so that the deformation stress is greatly different between the slab corner portion and the center portion, so that the rate of occurrence of wrinkles becomes high, and hot rolling itself becomes difficult. If the air ratio exceeds 350 minutes, the generated scale becomes thick even when the air ratio, that is, the oxygen concentration is controlled, so that Si is concentrated in a large amount at the scale interface. As a result, as shown in FIG. 2, the wrinkle occurrence rate tends to increase.
[0032]
It heats in LNG, LPG, and COG combustion gas under this slab heating temperature and time conditions. The air ratio of the combustion gas and air at this time is adjusted to a condition in which the CO concentration is 0.5 Vol% or less within a range of 1.0 to 1.2. At this time, the oxygen concentration is about 0.0 to 4.0 Vol%. By controlling the air ratio, grain boundary oxidation can be reduced to the number and depth that do not affect soot generation. When CO exceeds 0.5 Vol%, the number of soot generation increases significantly as shown in FIG.
[0033]
When the air ratio is less than 1.0 and the CO concentration exceeds 0.5 Vol%, it has been expected that the amount of Si concentrated at the iron-iron interface will decrease because the amount of scale-off decreases. In the results, contrary to this expectation, a large amount of Si was concentrated and grain boundary oxidation became remarkable. As a result of the quantitative analysis of Si by EPMA, it was found that the amount concentrated at the scale / base metal interface due to the scale-off in the steel was clearly larger than expected.
[0034]
As a result of many investigations by the present inventors, Si oxides such as scales and refractories existing in the heating furnace are CO gas in the atmosphere, and the following reaction formula: SiO 2 + CO → CO 2 + SiO (g)
It was found that Si oxide was reduced and existed as SiO gas in the atmosphere. Furthermore, as a result of the investigation, it was found that grain boundary oxidation became apparent because the reduced SiO gas was deposited on the surface of the slab and concentrated at the base metal / scale interface during scale generation. Therefore, the air ratio must be in the region where the CO concentration is 0.5 Vol% or less, that is, the air ratio is 1.0 or more.
[0035]
Moreover, when the air ratio exceeds 1.2, the amount of scale generation increases. For this reason, Si concentrates on the scale scale interface, and grain boundary oxidation becomes obvious. In addition, when operating at a high oxygen concentration in actual operation, the concentration of NO x in the exhaust gas increases, so it is preferable to reduce the air ratio also from the aspect of actual operation.
[0036]
As a result of further investigation on the relationship between the air ratio and slab heating temperature obtained as described above and the generation amount of surface soot, the relationship between the air ratio and slab heating temperature is shown by the following formula: = 2290.4 × (air ratio−1) 2 + 6.02 × 10 −4 × (slab heating temperature (° C.) − 1060) 2
It was found that no wrinkle occurs when the wrinkle generation index is 45 or less.
[0037]
When hot rolling is performed under the above-described components and heating furnace atmosphere conditions, the occurrence of linear scale wrinkles and pattern system defects (unevenness during plating) can be greatly reduced.
[0038]
【Example】
Example 1
Examples of the present invention are shown below.
[0039]
A sample having a component composition (mass%) shown in Table 1 having a thickness of 30 mm, a width of 100 mm, and a length of 100 mm was heated by changing the slab heating temperature and the air ratio in an LNG fuel gas atmosphere. The heating time at this time was 200 minutes.
[0040]
After heating, the sample was subjected to cross-sectional microscopy to investigate the number of intergranular oxidation leading to cracks and pattern defects. The result is shown in FIG. In the scope of the present invention, it can be seen that the number of grain boundary oxidations is low.
[0041]
[Table 1]
[0042]
(Example 2)
A sample having a component composition (mass%) shown in Table 1 of steel 1 having a thickness of 30 mm, a width of 100 mm, and a length of 100 mm is set to a slab heating temperature of 1230 ° C. and an air ratio of 1.1 in an LNG fuel gas atmosphere, and the heating time is various. Changed and heated.
[0043]
After heating, the sample was subjected to cross-sectional microscopy to investigate the number of intergranular oxidation leading to cracks and pattern defects. The result is shown in FIG. In the scope of the present invention, it can be seen that the number of grain boundary oxidations is low.
[0044]
(Example 3)
A sample having a component composition (mass%) shown in Steel 1 of Table 1 having a thickness of 30 mm, a width of 100 mm, and a length of 100 mm is set to a slab heating temperature of 1230 ° C. and a CO concentration of 0 to 4% in an LNG fuel gas atmosphere. The air ratio was changed and heated. The heating time at this time was 200 minutes.
[0045]
After heating, the sample was subjected to cross-sectional microscopy to investigate the number of intergranular oxidation leading to cracks and pattern defects. The result is shown in FIG. In the scope of the present invention, it can be seen that the number of grain boundary oxidations is low.
[0046]
Example 4
A plain steel slab having a composition (mass%) shown in Table 2 having a thickness of 252 mm, a width of 1000 mm, and a length of 6000 mm was heated by changing the slab heating temperature and air ratio in an LNG fuel gas atmosphere. The slab was heated for 200 minutes and then rolled hot to obtain a 3 mm hot-rolled steel sheet. Further, pickling with HCl of 7 to 10% and 70 to 90 ° C. was performed for 50 to 250 seconds. Thereafter, plating was performed to evaluate the presence or absence of linear scale wrinkles and surface patterns. The evaluation results are shown in Table 3. It can be seen that when the conditions of the present invention are applied, a product with good surface appearance can be produced without wrinkles.
[0047]
[Table 2]
[0048]
[Table 3]
[0049]
【The invention's effect】
The present invention controls the atmospheric conditions in the furnace during heating of the plain steel slab before hot rolling, thereby preventing the manifestation of grain boundary oxidation, which is the starting point of soot, and the linear scale generated during hot rolling. Occurrence of wrinkles and pattern defects (unevenness during plating) can be reduced, and a product with a good surface appearance can be produced. As a result, the yield loss caused by the occurrence of wrinkles and patterns can be improved, so the industrial value is very large.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between slab heating temperature, air ratio and the number of occurrences of grain boundary oxidation.
FIG. 2 is a diagram showing the relationship between slab heating time and wrinkle generation.
FIG. 3 is a graph showing the relationship between CO concentration and soot generation rate.
Claims (3)
[C]−12/48[Ti]−12/93[Nb]≦0 ・・・・(1)
(但し、[C]、[Ti]、[Nb]は、各元素の含有量(質量%))
[Ni]+[Cu]+[Sn]≦0.20% ・・・・(2)
(但し、[Ni]、[Cu]、[Sn]は、各元素の含有量(質量%)) As a raw material for hot rolling, C: 0.20% or less, Si: 0.1% or less, Mn: 1.0% or less, P: 0.050% or less, S: 0.02% or less . And Ti: 0.04% or less and Nb: 0.04% or less under the conditions of the following formula (1), Ni: 0.10% or less, Cu: 0.10% or less, Sn: 0 The ordinary steel slab containing 10% or less under the condition of the following formula (2) and other Fe and unavoidable elements in an atmosphere with an air ratio of 1.0 to 1.2, and the CO concentration in the furnace atmosphere is A method for heating a plain steel slab for obtaining a hot-rolled sheet with less surface flaws, characterized in that the slab heating temperature is controlled to be 0.5% or less and the slab heating temperature is within a range of 1050 ° C to 1250 ° C.
[C] -12/48 [Ti] -12/93 [Nb] ≦ 0 (1)
(However, [C], [Ti] and [Nb] are the contents of each element (mass%))
[Ni] + [Cu] + [Sn] ≦ 0.20% (2)
(However, [Ni], [Cu], and [Sn] are the contents of each element (mass%))
疵発生指数=2290.4×(空気比−1)2+6.02×10−4×(スラブ加熱温度(℃)−1060)2≦45
の関係を満たすことを特徴とする請求項1に記載の表面疵の少ない熱延板を得るための普通鋼スラブの加熱方法。As a relationship between the air ratio and the slab heating temperature, soot generation index = 2290.4 × (air ratio−1) 2 + 6.02 × 10 −4 × (slab heating temperature (° C.) − 1060) 2 ≦ 45
The method for heating a plain steel slab for obtaining a hot-rolled sheet with less surface flaws according to claim 1, wherein:
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