JP3979059B2 - Manufacturing method of clean steel - Google Patents

Description

【００２２】
Ｓ≦５×10⁴ P/kg-steel
Ｄ：オリフィス径
ｘ：介在物粒径
【００２３】
【発明の実施の形態】
本発明は、ESZ(Electric Sensing Zone ）法によって溶鋼中介在物を「その場測定」し、溶鋼の清浄度をモニタしながら清浄鋼を連続鋳造するプロセスにおいて、粗大介在物の径および／または介在物数の予め決めた各しきい値をそれぞれ越えることのない品質を確保した清浄鋼とその製造方法を提供するもので、以下おいてその具体的内容を詳述する。
【００２４】
図１は、ESZ 法による介在物検出法の原理の説明図である。
図中、オリフィスの中を非導電性微粒子、つまり介在物粒子が１→２→３→４→５と順次通過すると、オリフィスの対向面に設けた電極間に一種のパルス電流が生じる。これをもって介在物の通過を知り、電位の高さ( ΔＶ) からその大きさを計算するのである。
【００２５】
図２は、かかる原理に基づいてプローブを溶鋼に浸漬したときにプローブに設けたオリフィスを通過する介在物粒子の大きさおよび量を計測するシステムを説明するものである。
【００２６】
すなわち、上述のようにして計測されたΔＶは、プレアンプおよびログアンプを経てP.H.A.( パルスハイトアナライザ) によって出力される。プレアンプおよびログアンプの間には、演算処理システムが介在してノイズ分離回路を構成する。
【００２７】
ここに、本発明によれば、連続鋳造における取鍋、タンディッシュ、トラフつまりストリップキャスティング等で用いられる小さなタンディッシュ、または鋳型内で連続的あるいは適当な間隔を置いて間歇的に介在物センサーで溶鋼中の介在物濃度、粒度を「その場測定」する。適用するプローブのオリフィス径は、清浄鋼の仕様によって決まる粗大介在物の径のしきい値を考慮して、100 μｍから2000μｍの範囲から選択する。
【００２８】
取鍋、タンディッシュ、および鋳型内のいずれかにおける非金属介在物の排除すべき粒径をＸμm 以上としたとき、前記オリフィスの直径を2.5 Ｘとし、溶鋼中での計測時にオリフィスの詰まりが生じた場合をもって当該溶鋼のリジェクトを決定するか、あるいは精錬〜タンディッシュ段階での介在物低減対策を指示する警告を発する。
【００２９】
本発明者の研究によれば、オリフィス径の40％以上の直径の介在物はオリフィスからプローブ内に吸引される際に完全には流線に乗らずに、オリフィス内面に付着する。ここにさらに他の介在物が付着成長し、オリフィスを詰まらせるという問題がある。従って、この問題を逆用して、本発明の一つの実施形態においては当該鋼種の製品品質上有害となる単独介在物最小直径の2.5 倍のオリフィス径を用いる。
【００３０】
このように、単独有害介在物の許容される最大直径の2.5 倍の直径のオリフィスを用いた場合、許容される単独有害介在物最大直径以上の相当径を持つ介在物は、該オリフィスに捕捉され、オリフィス詰まりを生ずる。
【００３１】
これによりその後の溶鋼の該プローブ内への吸引が不可能となり、電流値のベースラインが不安定となる。このことは介在物センサーシステムのオシロスコープあるいはこれに相当する電位差パルスの時間経過を観察できるシステム上で明確に判別され、当該溶鋼が、必要とされる最終製品の品質を満足しないことを瞬時に判定することができる。
【００３２】
ここに、本発明は、最も広義には、溶鋼の連続鋳造に際して、溶鋼内に浸漬するプローブに設けたオリフィス内を通過する溶鋼に含有される介在物を該オリフィスの対向する面の間の電位差パルスに基づいてカウントするＥＳＺ法によって溶鋼の清浄度を「その場測定」しつつ鋳造する清浄鋼の製造方法である。
【００３３】
その具体的態様によれば、本発明にかかる清浄鋼の製造方法は、次の各段階を有する。(i)溶鋼の鋼種および用途に応じて、許容される介在物の最大径のしきい値を決定する段階:これは、予め経験的にあるいは規格によって、介在物の許容最大径を決定しておいて、それをしきい値とするのである。これは後述するように、そのような径の介在物が計測されたときには、その用途への適用は不可となることから、かなり厳格に規定する必要がある。例えば、許容最大径120 μm というようにして決定される。
【００３４】
(ii) 上記介在物の最大径に関するしきい値に基づいて、前記プローブに用いるオリフィス径を決定する段階:
上述のように許容できる介在物の径、つまり最大径のしきい値が決まれば、例えば前述のようにそれを計測できるオリフィス径も決まってくる。
【００３５】
例えば、上記許容介在物径がＸ以下の場合、オリフィス径を2.5 Ｘ以下とすることで、溶鋼中にＸ径の介在物が存在する場合は、オリフィスの閉塞をもたらし、そのときをもって規格外れと判断できるのである。
【００３６】
(iii)取鍋、タンディッシュ、または鋳型内の溶鋼に前記介在物センサーを浸漬する段階:この段階ではじめて介在物センサーを溶鋼内に浸漬するのである。オリフィスの湯面よりの浸漬深さを３００ｍｍ以上とする。このときの介在物の検出場所は特に制限はなく、取鍋、タンディッシュ、鋳型内のいずれであってもよい。
【００３７】
(iv) 前記しきい値を越えた介在物によってオリフィスが閉塞したことが感知されたとき、精錬〜連続鋳造の段階での介在物低減対策を命じる警告を発するか、得られた製品の仕向け変更を行うか、等の対策を直ちにその場でとる。
【００３８】
本発明の場合、そのような閉塞が見られるときは、確率的に、溶鋼１kg当たりかなりの数の粗大介在物が存在することになり、直ちに上述のような対策を取る必要がある。
【００３９】
本発明のさらに別の態様によれば、本発明にかかる清浄鋼の製造方法は、次の各段階を有する。
(i)鋼種および用途に応じて、介在物の許容される粒径の範囲と、さらに必要によりその範囲における介在物の個数との各しきい値を決定する段階:
これは、予め経験的にあるいは規格によって、介在物の許容粒径範囲および許容介在物の数を予め決定しておいて、それをしきい値とするのである。
【００４０】
具体的には、例えば、許容粒径範囲を20〜120 μm とし、その介在物径の範囲において許容介在物個数50000 個/kg 、というのである。
(ii) 上記介在物の許容される粒径の範囲のしきい値に基づいて、使用する介在物センサープローブに用いるオリフィス径を決定する段階:
上述のように介在物の許容粒径範囲のしきい値が決まれば、例えば前述のようにそれを計測できるオリフィス径も決まってくる。なお、実用上からは、その決定されたオリフィス径の値に近い大きさの径のオリフィスを用いることになる。
【００４１】
例えば、上記許容粒径範囲の最大径がＸの場合、オリフィス径を2.5 Ｘ以下とすることで、溶鋼中にＸ以下の径の介在物について、その大きさ、個数が容易に計測できるのである。
【００４２】
(iii)取鍋、タンディッシュ、または鋳型内の溶鋼に前記センサープローブを浸漬して介在物径と、さらに必要によりそのような介在物の数を算出するデータを検出する段階:
介在物の検出場所は特に制限はなく、取鍋、タンディッシュ、鋳型内のいずれであってもよいが、介在物低減対策を講じることができることを考慮した場合、タンディッシュで検出することが好ましい。各しきい値を超える場合ような異常が検出されたならば、その場で、例えば介在物浮上処理をとることもでき、取鍋における処理操作を変更することもできるからである。鋳型内では鋳型内溶鋼についてはこれ以上変更はできず、異常が検出されたときはフィードバックすることで、かろうじてタンディッシュ内溶鋼についてそのような処理操作を行うことができるにすぎない。
【００４３】
(iv) 検出されたデータから介在物径および介在物数を算出する段階:
これはすでに公知のESZ 法による計測によればよい。
連続式プローブの場合には鋳造中連続して測定し、判定すれば良い。
【００４４】
また、コスト的に安価なワンショット方式のプローブを用いて断続的な測定を行う場合には、各ヒートの連鋳定常部、非定常部 (連々スタート部、連々停止部、連々つなぎ部) をあらかじめ特定しておきその代表的な部位での測定値をそれぞれの鋳片の代表値とみなすことができる。
【００４５】
(v)対策をとる段階:
得られた介在物径または介在物径および介在物数のいずれかが、前記しきい値を超えるとき、精錬〜連続鋳造の段階、例えば転炉精錬の場合、転炉〜タンディッシュの段階での介在物低減対策を命じる警告を発するか、得られた製品の仕向け変更を行うか、等の対策を直ちにその場でとる必要がある。
【００４６】
このときの介在物低減対策としては、転炉精錬の場合、例えば次のような手段 (i)〜(iii) が考えられる。
(i) 転炉では、ａ. 吹き下げの防止、ｂ. 出鋼等スラグ流出量の低減、ｃ. 出鋼時脱酸強化等
(ii)炉外精錬ではａ. 処理時間の延長 (介在物浮上、分離促進) 、b.スラグ改質、c.かくはん動力の増大等
(iii) タンディッシュでは、ａ.Ar シールの強化、ｂ. 取鍋〜タンディッシュ間スラグ混入防止、ｃ. 介在物浮上促進のためのArバブリング強化等がある。
【００４７】
一方、ワンショット方式のセンサープローブを使って測定を行う場合に、各ヒートの連鋳定常部、非定常部 (連々鋳スタート部、連々鋳停止部、連々鋳つなぎ部) をあらかじめ特定しておき、その代表的な部位での測定値を、それぞれの鋳片の代表値とみなすことができる。
【００４８】
このように、オリフィス径は、製品品質毎に求められている単独での有害介在物の許容最大径によって規定される。
ここに、耐高圧材料に用いる高清浄鋼を本発明にしたがって製造する場合を説明すると次の通りである。
【００４９】
耐高圧材料において通常、製品を評価する際、疲労強度が重要な項目である。一般に介在物に対する疲労限度の評価式は以下のように表される。¹⁾
・表面の介在物
σ_w ＝1.43（Ｈ_V ＋120 ）×（√area）^-1/6 ・・・・・・・・(2)
・表面に接する介在物
σ_w ＝1.41（Ｈ_V ＋120 ）×（√area）^-1/6 ・・・・・・・・(3)
・表面から離れた内部の介在物
σ_w ＝1.56（Ｈ_V ＋120 ）×（√area）^-1/6 ・・・・・・・・(4)
註１）村上敬宜：「金属疲労 微小欠陥と介在物の影響」(1993)、p.90、養賢堂
本発明によれば、製品の所要品質上要求されるσ_w をあたえれば、上記(2) 、(3) 、(4) 式から、当該製品にとって単体で有害となる介在物の最小径√areaが求まる。
【００５０】
この最小径√areaの2.5倍以上の直径を持つオリフィスを用いて当該溶鋼中の介在物の粒度分布を測定することができる。その際検出される介在物の最大径から、当該溶鋼を製品にした場合の品質、あるいはσ_w の推定も可能となる。その上で、当該溶鋼の製品振り当ての判定をすることが可能となる。なお、この最小径√ area の 2.5 倍の直径のオリフィスを用いて、該プローブが詰まるようなことがあれば、この溶鋼あるいは鋳造部分（該当する鋳片）は当該製品には適用できないことが瞬時に判明する。
【００５１】
上記のことを検鏡で行なおうとすれば、時間がかかるが、介在物センサーを使用する本発明によれば、瞬時に判定できる。
次に、介在物の最大粒径も問題となるが総個数も問題となる品種の場合の判定方法について述べる。
【００５２】
上述のように、安定して測定可能な介在物の最大径はオリフィス直径の40％である。よって、計測範囲の上限はオリフィス直径の40％とする。通常、この実施形態ではオリフィス詰まりが生じ難いように、当該鋼種の製品品質上有害となる単独介在物最小径の2.5 倍以上のオリフィス径を用いるが、この径近傍のオリフィスを用いると有害単独介在物最小径のみならず、介在物の個数からも品質判定が可能である。
【００５３】
すなわち、介在物の最大粒径も問題となるが、総個数も問題となる品種の場合の判定にも有用である。
本発明者の研究によれば、有害単独介在物最小径の(2.5±0.1)倍のオリフィス径のプローブを用いた場合に清浄鋼としての基本特性を満足するのは10μＶ以上のΔＶ₂ を与える介在物径から、該有害単独介在物最小径の範囲の径をもつ介在物が溶鋼１kg当たり５×10⁴ 以下の場合である。
【００５４】
すなわち、本発明の好適実施態様は、一般的なシステムの場合、簡易的に例えばオリフィス径の下限６ないし10％から上限40％の介在物粒径範囲の粒子に関して、タンディッシュ内あるいはトラフ内溶鋼について介在物個数Ｓが
Ｓ≦５×10⁴ 個／kg-steel
の範囲にあることを確認しつつ鋳造することを特徴とする清浄鋼の製造方法である。
【００５５】
この場合、前述のごとく、オリフィス径の40％以上の径を検知( 計測) しない。図１において、微小粒子１がオリフィス２を通過する際の電気抵抗値の変化ΔR は、球形粒子、円筒オリフィスを仮定して、
ΔＲ＝（４ρ_e ｄ³ ）／（πＤ⁴ ） ・・・・・・・・(5)
で与えられる。ここにρ_e は流体の比抵抗、ｄは粒子直径、D はオリフィスの直径である。実際には(5) 式は次式(6) で表される補正係数F(ｄ／D)を必要とする。
【００５６】
F （ｄ／D ）＝｛１−0.8(d/D)³ ｝^-1 ・・・・・・・・・・(6)
結局ΔR は
ΔＲ＝（４ρ_e ｄ³ ）／（πＤ⁴ ）×｛１−0.8(d/D)³ ｝^-1 ・・・(7)
で与えられる。詰まりもなく安定的に測定できる（ｄ／Ｄ）がほとんど0.3 以下と考えられるので、F （ｄ／D ）は１とおくことができる。よって、ΔR は
ΔＲ＝（４ρ_e ｄ³ ）／（πＤ⁴ ）・・・・・・・・・・・(8)
一方、基本的な計測システムの一例として、図３に掲げる回路がある。
【００５７】
非導電性の球形粒子がオリフィスを通過する時の電位差パルスΔＶ₂ は次式で表される。
ΔＶ₂ ＝Ｖ_E ×Ｒ₃ ×ΔＲ／｛（Ｒ₂ ＋Ｒ₃ ＋ΔＲ）（Ｒ₂ ＋Ｒ₃ ）｝
・・・・(9)
ここでΔＲは（Ｒ₂ ＋Ｒ₃ ）に比して十分に小さいので、結局、
ΔＶ₂ ≒Ｖ_E ×Ｒ₃ ×ΔＲ／（Ｒ₂ ＋Ｒ₃ ）² ・・・・・・(10)
【００５８】
あるいは、式(10)に（Ｒ₂ ＋Ｒ₃ ）＝Ｖ_E ／Ｉを代入して
ΔＶ₂ ＝Ｒ₃ ×Ｉ×ΔＲ／（Ｒ₂ ＋Ｒ₃ ）・・・・・・・・(11)
あるいは、
ΔＶ₂ ＝Ｉ² ×Ｒ₃ ×ΔＲ／Ｖ_E ・・・・・・・・(12)
ここに、Ｉは測定される電流値である。式(8) を式(11)に代入して電位差パルスΔＶ₂ を求めると次式(13)で表される。
【００５９】
ΔＶ₂ ＝Ｒ₃ ×Ｉ×（４ρ_e ｄ³ ）／｛（πＤ⁴ ）（Ｒ₂ ＋Ｒ₃ ）｝・・(13)ここに、ノイズレベルは通常１０μV 以下で測定するので、この場合の検出限界ｄ_min は、
ΔＶ₂ ＝１０μＶとして、
ｄ_min ＝１８μｍ
となる。
【００６０】
これは、式(12)におけるΔＶ₂ が少なくとも10μV 以上の信号を介在物によるものとして検出しカウントするものである。
一般に製鋼工場でこのシステムを用いようとすると、ノイズの影響を受ける。そのため、フィルターや波形の傾き等をノイズと識別する条件として用いる様々な信号処理が行われる。しかしながら、大きな強度のノイズについては上記の対応である程度弁別可能であるが、強度の小さなノイズやごく微小な介在物そのものによる小さな電位差パルスによる信号についてはベースラインとの弁別が困難である。
【００６１】
本発明者の研究によれば、大量生産の製鋼工場でノイズの影響を10μＶ以下にすることは困難である。したがって、この場合、介在物の粒度分布を精度良く測定できる範囲は、ΔＶ₂ ＞10μＶの強度の信号ということができる。この場合例えばオリフィス径が300 μm の場合６％以上の径の介在物の測定が可能と言うことである。
【００６２】
すなわち、このオリフィスを用いた場合の介在物センサーのダイナミックレンジはオリフィス径の６％から40％ということができる。
この有効ダイナミックレンジにおいてあらかじめ製品品質と介在物センサーの測定データとの対応に基づいて各測定値についてしきい値を決めておけば、鋳片の清浄度が瞬時に判定できる。
【００６３】
即ち、オリフィス直径はまず測定対象となる製品品質に対して単体での有害介在物の最小径の2.5 倍以上とする。
図３のシステムで300 μｍのオリフィスを用いた場合には、ΔＶ₂ ＞10μＶとなるのはｄ＞18μｍだから、介在物径の検出範囲は小さくとも18μｍから最大120 μｍということになる。この範囲での介在物粒度分布あるいは濃度（個数／ｋｇ）についてそれぞれしきい値を決めておいて品質判定に用いる。
【００６４】
すなわち、単体での有害介在物があればオリフィス詰まりで判定可能であり、総個数（介在物濃度）や粒度分布についても、P.H.A.の出力から判定できる。これにより、それぞれのしきい値を超えたときをもって当該鋳片または／および溶鋼をリジェクトしたり、用途変更したりすることができる。
【００６５】
さらにこれらのデータをプロセス制御（精錬時間の延長、攪拌強化、タンディッシュ誘導加熱強化、鋳込み速度低下等々）に反映することが可能となる。
本発明者の研究によれば、オリフィス径が100 μｍ未満であると、通常の商業生産プロセスにおいてはオリフィスの詰まりを生じ、安定した測定ができない。この詰まりは、測定しようとする介在物そのものであったり、溶鋼がプローブ本体に冷却されてオリフィス内で凝固することによって起こるものである。また、通常クォーツ等で製作されるプローブに直径100 μｍ未満の細孔を空けるのはコスト的にも割高である。
【００６６】
一方、オリフィス径が2000μｍを超えるとこのオリフィスを通って、プローブ内へ溶鋼が進入する際、流入速度が大きいためにプローブ内で大きな流れを生じ、電流値のベースラインが安定せず、正確な測定ができない。
【００６７】
さらには、一般に介在物の検出可能範囲は前述の式(12)におけるΔＶ₂ ＞10μＶとなる直径の介在物であり、このとき例えば2000μｍのオリフィス径を持つプローブによる最小介在物検出限界は226 μｍとなり、これより微小な径の介在物を問題とする清浄鋼の測定に際して実用的でない。
【００６８】
以上の知見より、本発明者は連続鋳造機周辺での測定に際して実用に供しうる該プローブオリフィス径を100 μｍ以上2000μｍ以下としたものである。
【００６９】
【実施例】
[実施例１]
本例では図１に示すシステムを用い、直径300 μm のオリフィスを具備する連続測定方式およびワンショット方式のセンサープローブを用いて測定を行った。測定はブルーム連鋳機のタンディッシュ内の溶鋼について実施した。溶鋼にセンサープローブを浸漬し、オリフィスを通して内部に1550℃の溶鋼を吸引して測定を行った。
【００７０】
本例では、粗大介在物の許容最大径を100 μm とし、介在物の個数のしきい値を50000 個/kg とした。
センサーの浸漬深さをオリフィス位置を湯面下400mm として測定を行った。
【００７１】
約１kgの溶鋼を測定中に検出された電位差パルスは、ベースラインも安定しており、滑らかな波形であることがわかる。
図３には、本例で採用した測定システムの一例を示す。
【００７２】
ログアンプへの入力ΔＶ_inは、プリアンプのゲインをＧとして、
ΔＶ_in＝G ×ΔＶ₂ ・・・・・・・・・・・（14）
ログアンプの出力ΔＶ_out は、この場合
ΔＶ_out ＝10/3×｛log₁₀(ΔＶ_in）＋２｝・・・・・・（15）
このP.H.A.は８Ｖの出力範囲を512 チャンネルに分割しており、チャンネルナンバーをChとすれば、
Ch＝（512 ／８）×ΔＶ_out ・・・・・・・（16）
となる。ここにΔＶ_out は式(15)で表されるところのログアンプの出力である。ゲインＧは100 であった。
【００７３】
250Ch 近傍で最大径の出力があり、Ｉ＝20Ａ、Ｖ_E ＝6.4 Ｖ、Ｒ₃ ＝0.125 Ωのとき、ρ_e ＝1.4 ×10^-6Ωｍとして、前述の式(8) 、(12)、(14)、(15)、(16)よりこの最大介在物の径は95μｍであると推定される。この最大径95μｍは、オリフィス径の40％よりも十分に小さいので、オリフィス詰まりを生ずること無く介在物の粒度分布と個数の測定が可能であった。
【００７４】
さらに、この溶鋼を連続鋳造中にΔＶ₂ が10μＶ以上の介在物即ち、該オリフィスを用いて測定し、直径18μｍ以上の介在物が50000 個／kgを超える場合には向け先変更を行い、50000 個／kg以下の場合には正規の向け先に振り当てた。
【００７５】
この鋼種の場合には、介在物の最大径とともに総数が問題となり、経験的に、最大径が120 μｍ以下で且つ18μｍ以上の介在物が50000 個／kg以下の場合には、その特性の実質的な影響を与えることがなかったため、これらをしきい値とした。具体的には、連々鋳のつなぎ部位 (取鍋交換部位) の鋳片にこのしきい値を超えるものが多い。
【００７６】
また、この許容有害単独介在物最小径のしきい値は、鋼種ならびに向け先・用途によって、経験的に得られるものであり、さらには、介在物の単位鋼試料当たりの個数の閾値は、式(17)によって規定される。
【００７７】
【数３】

【００７８】
Ｓ≦５×10⁴P/kg-steel
Ｄ: オリフィス径
ｘ: 介在物粒径
ここに、図４は、式(17)の内容をグラフで示すものであり、オリフィス径に比例する介在物径( ｘ) と疲労特性( Ｎ) との関係: Ｎ＝F(x)を示すグラフであり、これを0.06Ｄないし0.4 Ｄまで積分したものがＳ、つまり介在物個数である。
【００７９】
いずれにせよ本発明により、鋼種、向け先・用途ごとに適切なオリフィス径を選定し、介在物の最大径または／および単位鋼試料当たりの個数のしきい値を以って、当該溶鋼の品質を「その場」で判定し、用途の振り当てをすることが可能となった。
【００８０】
ここに、本発明の対象となる鋼組成は、代表例として次の例を挙げることができる。
Ｃ：1.2 ％以下、Si：3.0 ％以下、Mn：1.15％以下、
Ｐ：0.025 ％以下、Ｓ：0.2 ％以下、Al：1.0 ％以下、
Cr、Mo等の元素については必要により適宜含有してもよい。
【００８１】
[実施例２]
図３に示すシステムを用い、直径100 μｍのオリフィスを具備するセンサープローブを用いて測定を行った。
【００８２】
本例では、粗大介在物の許容最大径を40μm とし、これを介在物径のしきい値とし、また介在物の個数のしきい値を50000 個/kg とした。
測定はブルーム連鋳機のタンディッシュ内の溶鋼について実施した。1545℃の溶鋼を吸引して測定を行った。溶鋼を吸引して約10秒後にオリフィスの詰まりを生じ、測定を中断した。
【００８３】
オリフィスの詰まりは、オシロスコープ上のベースラインの乱れで判定した。センサープローブを溶鋼から引き上げてみると、実際にプローブの先端が閉塞しているのが観察された。このときの溶鋼過熱度は33℃であった。測定開始から10秒以前の信号を解析したところ、式(8) 、式(12)より、40μｍ以上の径の介在物が散見された。
【００８４】
このことから、この溶鋼は40μｍ以上の径の介在物を含有していることが即座に判定され、オリフィス詰まりが単体で有害な介在物の存在を判断する条件となることを示す。
【００８５】
また、この溶鋼を鋳造して得られたブルームを圧延したビレットから採取したサンプルを顕微鏡観察したところ、40μｍ以上の径の介在物が観察された。
この材質の許容介在物最大径は40μｍであったため、製品は当初の用途には振り当てなかった。
【００８６】
このビレット段階での介在物の顕微鏡観察結果が出たのは、鋳造完了後７日後であった。このように、従来法では介在物の判定に７日間かかっていたものが、本発明を適用すれば、溶鋼段階での判定が可能となった。
【００８７】
[実施例３]
直径200 μｍのオリフィスを具備するセンサープローブを用いて測定を行った。測定はブルーム連鋳機のタンディッシュ内の溶鋼について実施した。
【００８８】
1552℃の溶鋼を吸引して測定を行った。
本例では、粗大介在物の許容最大径を25μm とし、これを介在物径のしきい値とし、また10μｍ〜25μｍの範囲の介在物の個数を50000 個/kg としてこれをしきい値とした。
【００８９】
測定の結果、計測された介在物の最大径の範囲は25μｍから30μｍに存在し、この粒径範囲における介在物粒子の個数は溶鋼１kg当たり約300 個と推定された。
【００９０】
当該鋼種は高圧条件下に於いて繰り返し疲労を受ける用途に使われ、経験的に本発明による測定による許容介在物最大径と、製品に要求される疲労強度に基づいて式(2) 、(3) 、(4) から求められる許容介在物最大径は25μｍであった。また、10〜30μm の範囲の介在物個数は5.5 ×10⁴ 個/kg であった。本測定結果をもとに、当該溶鋼から鋳造された鋳片は鋳造直後に判定され、当初の用途に振り向けられなかった。
【００９１】
また、該測定結果に基づいて、その直後、２次精錬工程での攪拌保持時間の延長と、取鍋におけるスラグ改質を強化した結果、直後の別の鋳造トライにおいて同様の測定を実施した結果、連続鋳造タンディッシュ内溶鋼中の介在物の最大径は15μｍから20μｍの範囲に縮小し、さらに10〜20μm の範囲の介在物個数も2.5 ×10⁴ 個/kg であった。当該鋳片を当初の用途に振り当てることが可能となった。製品上も全く異常が無かった。
【００９２】
これらのプロセス改善手続きを数十分から数時間以内に実行することが可能となり、従来の製品での顕微鏡観察結果を待って判定し改善する手法の鋳造後１週間から10日間の時間に比して格段の迅速化が図れ、顧客の納期要望に十分に応えることが出来るようになった。
【００９３】
【発明の効果】
本発明によれば、介在物分布を特定できる清浄鋼の製造が可能となり、品質の信頼性は格段に改善され、しかもそのような高品質の清浄鋼を安定して製造できるなど実用上の意義は大きい。
【図面の簡単な説明】
【図１】介在物検出の原理の説明図である。
【図２】基本的計測システムの概念図である。
【図３】本発明を実施するためのシステムの説明図である。
【図４】実施例におけるＮ−Ｘとの関係を説明するグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing clean steel, and more particularly to a method for producing clean steel in which casting is performed while monitoring inclusions in molten steel during continuous casting.
[0002]
[Prior art]
Conventionally, inclusions in steel are measured by cutting out small pieces from a solidified slab or product and observing them with a microscope, or by dissolving them with an acid to remove insoluble nonmetallic inclusions. There is a method of extracting and analyzing the particle size and number. All of these methods require a period of several days to a week or more for analysis after casting, and are not in time for quality judgment of the slab before hot rolling.
[0003]
Therefore, the relationship between operating conditions and slab cleanliness is usually determined empirically, and if the operating conditions exceed a certain range, the slab is regarded as an abnormal product and applied to scrap or other applications. Used. Also, in steel grades that are sensitive to product degradation rates due to inclusions such as DI cans and deep-drawn steel for automobiles, at the start of casting, the connecting part between ladle and ladle (continuous connecting part), Measures such as making slabs on the premise of scrapping or diverting to other uses are taken.
[0004]
Furthermore, in the case of high-pressure resistant materials such as fuel injection pump materials for diesel engines, inclusions are generally evaluated and judged by observing the obtained product under a microscope. Needless to say, it takes time. Moreover, there is no guarantee that inclusions with a low frequency of appearance can be reliably measured.
[0005]
[Problems to be solved by the invention]
By the way, as steel becomes highly functional, the need for clean steel is becoming increasingly strict. On the other hand, there is a demand for cost reduction at the same time. Here, stricter clean steel refers to accurately grasping and managing other chemical composition, molten steel temperature, level of molten metal in mold, detection of ladle brewing, etc. On the other hand, in terms of cost, in order to increase the yield, it is required to perform “in-situ measurement” and feed back the result to the operation condition in real time.
[0006]
In recent years, especially in the so-called near net shape casting such as steelmaking to direct rolling operation, thin slab casting, and strip casting, the processing speed from steelmaking to products is remarkably fast. If it flows downstream, it will be a large loss in terms of economy, and an immediate judgment technique for slab (product) quality is indispensable.
[0007]
Here, an object of the present invention is to provide a method for producing a highly reliable clean steel that employs an inclusion detection method that enables “in-situ measurement” in continuous casting of clean steel, for example, near-net shape casting. Is to provide.
[0008]
[Means for Solving the Problems]
Various studies have been made to achieve this problem, and the present inventors have come up with the idea of adopting an inclusion detection method based on the so-called ESZ method, investigated various problems in its application, and found out how to solve it. Thus, the present invention has been completed by finding that an unexpected effect can be obtained.
[0009]
That is, the present inventors previously developed an inclusion sensor for molten steel of the continuous type and the one-shot type by the ESZ method. Patents 02594681 and 02591253. However, these patents did not disclose the specific quality determination method described above. Moreover, no method for producing clean steel has been disclosed.
[0010]
In subsequent studies, by applying this ESZ method to the steel refining stage or continuous casting stage, the present inventor is able to perform quality judgment on the spot while manufacturing clean steel from a completely new viewpoint. It was possible, and according to this, it was found that reliable and clean steel could be manufactured reliably and inexpensively.
[0011]
In other words, the ESZ method is theoretically an excellent method that can accurately measure the number of inclusions, but it is necessary and sufficient on the spot as, for example, a practically practical technique for near net shape casting. It was unclear whether a certain level of data could be obtained. Therefore, as a result of further research by the present inventor, threshold values are set for the size and number of inclusions, and the threshold values, particularly the inclusion size threshold values, are taken into consideration. Knowing that such an object can be easily realized by determining the orifice diameter provided in the probe constituting the object sensor, the present invention has been completed.
[0012]
That is, when the present inventor repeated a series of experiments, sometimes measurement was impossible. The orifice was clogged with inclusions. When the correlation between the size of the inclusion and the orifice diameter at that time was examined, it was found that there was a certain correlation between the orifice diameter and the minimum value of the diameter of the inclusion that clogs the orifice diameter. Therefore, the present inventors can use it for the determination of coarse inclusions due to clogging of the orifice if such a correlation is known in advance, and if the orifice diameter is regulated to a predetermined value thereby. it can.
[0013]
By the way, as in the past, the number N of inclusions per unit area obtained by microscopic observation_AIs simply the number of inclusions per unit volume N_VIs converted by the following equation. Ie
N_V= N_A ^3/2・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (1)
For example, 100mm, which is usually applied in extreme value statistics²One inclusion counted in the field of view corresponds to a count of 4054 inclusions per kg. However, such a measurement under a microscope lacks reliability in determining the influence of coarse inclusions having a low appearance frequency. In other words, relatively large inclusions do not occur evenly and may not be detected. Moreover, such a measurement under a microscope is performed after the fact and is not merely a confirmation area.
[0014]
Here, regarding the number of inclusions counted by the ESZ method, especially coarse inclusions, if even one such coarse inclusion is detected, the product is immediately redirected and used as a clean steel. Reliability can be ensured. Therefore, if the critical diameter of the coarse inclusion is defined by the orifice diameter as described above, the use of a probe having a predetermined orifice diameter will immediately cause the quality to fall out of specification when the blockage is experienced. I can judge.
[0015]
Furthermore, when the accurate distribution and size (number) of inclusions are measured by the ESZ method, it is determined whether or not the inclusion is out of the target range. By taking this countermeasure, it is possible to stably produce clean steel having only inclusions within the target range. In such a case, measurement is performed with a tundish. If it is found that the distribution of inclusions is deviated and coarse inclusions are present, for example, an increase in the current (force) applied to the tundish heater Immediately take measures such as casting speed reduction and out-of-furnace refining treatment time extension. And even when the amount of inclusions is large, for example, immediately increase the power applied to the tundish heater, decrease the casting speed, strengthen the sealing seal between the ladle and the tundish, and enhance the refining slag reforming outside the furnace. Take. Of course, it is also possible to record the location where such measurement data is obtained or the molten steel portion and reject it as a product. The inclusion control as described above is particularly effective as a countermeasure for “in-situ measurement” as in the present invention because of its very high responsiveness.
[0016]
That is, the present invention is as follows.
  (1) During continuous casting of molten steel, inclusions contained in the molten steel passing through the orifice provided in the probe constituting the inclusion sensor immersed in the molten steel are determined based on the potential difference pulse between the opposed surfaces of the orifice. In the manufacturing method of the clean steel that casts while performing the "in-situ measurement" of the cleanliness of the molten steel by the ESZ method to count
Determining the maximum diameter threshold of inclusions allowed, depending on the steel type and application of the molten steel;
Determining an orifice diameter to be used for the probe based on a threshold regarding the maximum diameter of the inclusion;
Immersing the inclusion sensor in a ladle, tundish, or molten steel in a mold; and
When it is detected that the orifice has been blocked by inclusions exceeding the above threshold, whether to issue a warning instructing measures to reduce inclusions in the refining to continuous casting stage, or whether to change the destination of the resulting product , Etc., immediately taking measures on the spot;
A method for producing clean steel.
[0017]
  (2) During continuous casting of molten steel, inclusions contained in the molten steel passing through the orifice provided in the probe constituting the inclusion sensor immersed in the molten steel are determined based on the potential difference pulse between the opposed surfaces of the orifice. In the manufacturing method of the clean steel that casts while performing the "in-situ measurement" of the cleanliness of the molten steel by the ESZ method to count
Depending on the steel type and application of the molten steel, determining the threshold value of the allowable particle size range of inclusions and, if necessary, the number of inclusions in that range;
Determining an orifice diameter to be used for the probe based on a threshold value of an acceptable particle size range of the inclusion;
Detecting the data for calculating the diameter of inclusions or the diameter of inclusions and the number of inclusions by immersing the inclusion sensor in a ladle, tundish, or molten steel in a mold;
Calculating the diameter of inclusions and, if necessary, the number of inclusions from the detected data; and
When calculating the resulting inclusion diameter or inclusion diameter, if either the inclusion diameter or the number of inclusions exceeds the above threshold, take measures to reduce inclusions at the refining to continuous casting stage. Immediately taking on-the-spot actions such as issuing a warning to command or changing the destination of the resulting product;
A method for producing clean steel.
[0018]
  (3) When performing continuous measurement using a continuous measurement type inclusion sensor, the immersion depth from the molten metal surface of the orifice is 300 mm or more, and the data is detected and collected, (1) or (2) The manufacturing method of the clean steel of description.
[0019]
  (4) When intermittent measurement is performed using a one-shot type inclusion sensor, the data is detected and collected with the immersion depth from the molten metal surface of the orifice being 300 mm or more. 2) The manufacturing method of the clean steel as described.
[0020]
  (5) The method for producing clean steel according to (2) above, wherein the threshold value for the number of inclusions is 50,000 or less per kg of molten steel determined by the following formula for molten steel in a tundish or trough.
[0021]
[Expression 2]

[0022]
S ≦ 5 × 10^Four P / kg-steel
D: Orifice diameter
x: Inclusion particle size
[0023]
DETAILED DESCRIPTION OF THE INVENTION
The present invention “in-situ measurement” of inclusions in molten steel by the ESZ (Electric Sensing Zone) method, and in the process of continuously casting clean steel while monitoring the cleanliness of the molten steel, the diameter and / or inclusions of coarse inclusions The present invention provides a clean steel having a quality that does not exceed a predetermined threshold value of the number of objects and a method for producing the same, and the specific contents thereof will be described in detail below.
[0024]
FIG. 1 is an explanatory diagram of the principle of the inclusion detection method by the ESZ method.
In the figure, when non-conductive fine particles, that is, inclusion particles, sequentially pass through the orifice in the order of 1 → 2 → 3 → 4 → 5, a kind of pulse current is generated between the electrodes provided on the opposed surface of the orifice. With this, the passage of inclusions is known, and the magnitude is calculated from the potential height (ΔV).
[0025]
FIG. 2 illustrates a system for measuring the size and amount of inclusion particles that pass through an orifice provided in a probe when the probe is immersed in molten steel based on this principle.
[0026]
That is, ΔV measured as described above is output by a P.H.A. (pulse height analyzer) through a preamplifier and a log amplifier. An arithmetic processing system is interposed between the preamplifier and the log amplifier to constitute a noise separation circuit.
[0027]
Here, according to the present invention, a ladle, tundish, trough or strip casting used in continuous casting, or a small tundish used in strip casting, etc., or an inclusion sensor continuously or at an appropriate interval in a mold. “In-situ measurement” of inclusion concentration and particle size in molten steel. The probe orifice diameter to be applied is selected from the range of 100 μm to 2000 μm in consideration of the coarse inclusion diameter threshold determined by the specifications of the clean steel.
[0028]
When the particle size to be excluded of non-metallic inclusions in any of the ladle, tundish, and mold is set to Xμm or more, the diameter of the orifice is set to 2.5 X, and the orifice is clogged during measurement in molten steel. If it is necessary to determine the rejection of the molten steel, a warning is issued instructing measures to reduce inclusions in the refining to tundish stages.
[0029]
  According to the inventor's research, inclusions having a diameter of 40% or more of the orifice diameter adhere to the inner surface of the orifice without being completely on the streamline when sucked into the probe from the orifice. Another problem here is that other inclusions grow and stick to the orifice. Therefore,By reversing this problem,Of the present inventionOne embodimentIn this case, the minimum diameter of a single inclusion that is harmful to the product quality of the steel grade2.5 DoubleUse the orifice diameter.
[0030]
  in this way, Single harmful inclusion is acceptablemaximumWhen using an orifice with a diameter of 2.5 times the diameter, an acceptable single harmful inclusionmaximumInclusions having an equivalent diameter greater than or equal to the diameter are trapped in the orifice and cause orifice clogging.
[0031]
As a result, it becomes impossible to suck the molten steel into the probe, and the baseline of the current value becomes unstable. This is clearly discriminated on the inclusion sensor system oscilloscope or a system that can observe the time course of the corresponding potential difference pulse, and instantly determines that the molten steel does not meet the required quality of the final product. can do.
[0032]
Here, in the broadest sense, the present invention, in the continuous casting of molten steel, the inclusions contained in the molten steel passing through the orifice provided in the probe immersed in the molten steel, the potential difference between the opposing surfaces of the orifice This is a method for producing clean steel, in which the cleanliness of the molten steel is cast “in situ” by the ESZ method that counts based on pulses.
[0033]
  According to the specific aspect, the manufacturing method of the clean steel concerning this invention has the following each step. (i) Depending on the steel type and application of the molten steel,Determining the maximum diameter threshold of inclusions allowed: This is because the maximum allowable diameter of the inclusion is determined in advance empirically or according to the standard, and this is used as a threshold value. As will be described later, when inclusions having such a diameter are measured, it is impossible to apply the inclusion to the intended use. For example, the maximum allowable diameter is 120 μm.
[0034]
 (ii) determining an orifice diameter to be used for the probe based on a threshold concerning the maximum diameter of the inclusions:
When the allowable inclusion diameter, that is, the maximum diameter threshold value is determined as described above, the orifice diameter that can be measured is determined as described above, for example.
[0035]
For example, when the above-mentioned allowable inclusion diameter is X or less, the orifice diameter is set to 2.5 X or less. When inclusions with an X diameter are present in the molten steel, the orifice is blocked, and at that time it is out of specification. It can be judged.
[0036]
  (iii) Step of immersing the inclusion sensor in molten steel in a ladle, tundish, or mold: The inclusion sensor is immersed in the molten steel only at this stage.The immersion depth from the molten metal surface of the orifice is set to 300 mm or more.There are no particular restrictions on the location of the inclusions detected at this time, and any of them may be a ladle, a tundish, or a mold.
[0037]
 (iv) When it is detected that the orifice has been blocked by inclusions exceeding the above threshold, a warning is issued to order measures to reduce inclusions in the refining to continuous casting stage, or the resulting product is changed Or take immediate measures on the spot.
[0038]
In the case of the present invention, when such a blockage is observed, a considerable number of coarse inclusions are probabilistically present per 1 kg of molten steel, and it is necessary to immediately take the measures described above.
[0039]
According to still another aspect of the present invention, a method for producing clean steel according to the present invention includes the following steps.
 (i) determining the threshold value of the allowable particle size range of inclusions and, if necessary, the number of inclusions in the range depending on the steel type and application:
This is because the allowable particle diameter range of inclusions and the number of allowable inclusions are determined in advance empirically or according to specifications, and these are used as threshold values.
[0040]
Specifically, for example, the allowable particle diameter range is 20 to 120 μm, and the allowable inclusion number is 50,000 / kg in the inclusion diameter range.
 (ii) determining the orifice diameter to be used for the inclusion sensor probe to be used based on a threshold value of the acceptable particle size range of the inclusions:
If the threshold value of the allowable particle size range of the inclusion is determined as described above, the orifice diameter that can be measured is determined as described above, for example. From a practical point of view, an orifice having a diameter close to the determined orifice diameter value is used.
[0041]
For example, when the maximum diameter of the allowable particle size range is X, by setting the orifice diameter to 2.5 X or less, the size and number of inclusions having a diameter of X or less in the molten steel can be easily measured. .
[0042]
 (iii) detecting the data for calculating the diameter of inclusions and, if necessary, the number of such inclusions by immersing the sensor probe in a ladle, tundish, or molten steel in a mold:
There are no particular restrictions on the location of inclusions, which may be any of a ladle, tundish, or mold, but it is preferable to detect with inclusions when considering that inclusion reduction measures can be taken. . This is because if an abnormality such as exceeding each threshold is detected, the inclusion floating process can be performed on the spot, and the processing operation in the ladle can be changed. In the mold, the molten steel in the mold cannot be changed any more, and when an abnormality is detected, such a processing operation is barely possible for the molten steel in the tundish by feeding back.
[0043]
 (iv) Calculate the inclusion diameter and the number of inclusions from the detected data:
This may be measured by the already known ESZ method.
In the case of a continuous probe, it may be determined by measuring continuously during casting.
[0044]
In addition, when performing intermittent measurement using an inexpensive one-shot probe, the continuous casting and unsteady parts (continuous start part, continuous stop part, continuous joint part) of each heat are used. It is possible to identify the measured values at the representative parts that have been specified in advance as the representative values of each slab.
[0045]
 (v) Steps to take measures:
When any of the obtained inclusion diameter or inclusion diameter and the number of inclusions exceeds the threshold value, in the refining to continuous casting stage, for example, in the case of converter refining, in the converter to tundish stage. It is necessary to immediately take measures such as issuing warnings for measures to reduce inclusions, or changing the destination of the obtained products.
[0046]
As measures for reducing inclusions at this time, in the case of converter refining, for example, the following means (i) to (iii) can be considered.
(i) In the converter, a. Prevention of blow-down, b. Reduction of slag outflow, etc.
(ii) Out-of-furnace refining a. Extension of processing time (inclusion flotation, separation promotion), b. Slag reforming, c. Increase of stirring power, etc.
(iii) In tundish, there are a.Strengthening of Ar seal, b.Preventing slag mixing between ladle and tundish, and c.Strengthening of Ar bubbling to promote inclusion floating.
[0047]
On the other hand, when performing measurement using a one-shot sensor probe, the continuous casting steady part and non-steady part (continuous casting start part, continuous casting stop part, continuous casting joint part) of each heat must be specified in advance. The measured value at the representative part can be regarded as the representative value of each slab.
[0048]
As described above, the orifice diameter is defined by the maximum allowable diameter of the harmful inclusions required for each product quality.
Here, the case where the high clean steel used for the high pressure resistant material is manufactured according to the present invention will be described.
[0049]
In high pressure resistant materials, fatigue strength is usually an important item when evaluating products. In general, the evaluation formula of the fatigue limit for inclusions is expressed as follows.¹⁾
・ Surface inclusions
σ_w= 1.43 (H_V+120) x (√area)^-1/6    ... (2)
-Inclusions in contact with the surface
σ_w= 1.41 (H_V+120) x (√area)^-1/6    ... (3)
・ Internal inclusions away from the surface
σ_w= 1.56 (H_V+120) x (√area)^-1/6    ········(Four)
1) Takayoshi Murakami: “Metal Fatigue Effect of Micro Defects and Inclusions” (1993), p.90, Yokendo
According to the present invention, σ required for the required quality of the product_wFrom the above formulas (2), (3), and (4), the minimum diameter √area of inclusions that are harmful to the product alone can be obtained.
[0050]
  thisThe particle size distribution of inclusions in the molten steel can be measured using an orifice having a diameter 2.5 times or more than the minimum diameter √area. From the maximum diameter of inclusions detected at that time, the quality when the molten steel is made into a product, or σ_w Can also be estimated. In addition, it is possible to determine the product allocation of the molten steel.This minimum diameter √ area of 2.5 If the probe sometimes gets clogged using a double-diameter orifice, it immediately becomes clear that this molten steel or cast part (corresponding slab) is not applicable to the product..
[0051]
If it is going to do the above with a speculum, it will take time, but according to this invention using an inclusion sensor, it can judge instantaneously.
Next, a determination method will be described in the case where the maximum particle size of inclusions is a problem but the total number is also a problem.
[0052]
  As mentioned above, the maximum diameter of inclusions that can be measured stably is 40% of the orifice diameter. Therefore, the upper limit of the measurement range is 40% of the orifice diameter. Normal,In this embodiment, orifice clogging is unlikely to occur., Use an orifice diameter that is at least 2.5 times the minimum diameter of single inclusions, which is harmful to the product quality of the steel grade.If an orifice near this diameter is used, not only the minimum diameter of harmful inclusions but also the number of inclusions is used. Quality judgment is possible.
[0053]
That is, the maximum particle size of inclusions is also a problem, but the total number is also useful for determination in the case of varieties that have a problem.
According to the inventor's research, when using a probe having an orifice diameter (2.5 ± 0.1) times the minimum diameter of harmful single inclusions, it is ΔV of 10 μV or more that satisfies the basic characteristics as a clean steel.₂Inclusions having a diameter in the range of the minimum harmful inclusion inclusion diameter from the inclusion diameter giving 5% per 10 kg of molten steel^FourThis is the case.
[0054]
That is, in the case of a general system, the preferred embodiment of the present invention is simply applied to molten steel in a tundish or trough, for example, for particles having an inclusion particle size range of a lower limit of 6 to 10% to an upper limit of 40%. The number of inclusions S is
S ≦ 5 × 10^FourPiece / kg-steel
It is the manufacturing method of the clean steel characterized by casting, confirming that it exists in the range of this.
[0055]
  In this case, as described above, a diameter of 40% or more of the orifice diameter is detected (measured).do not do. FIG.The change ΔR in the electric resistance value when the microparticle 1 passes through the orifice 2 is assumed to be a spherical particle or a cylindrical orifice.
ΔR = (4ρ_e d^Three ) / (ΠD^Four ) ········(Five)
Given in. Where ρ_e Is the specific resistance of the fluid, d is the particle diameter, and D is the diameter of the orifice. In practice, equation (5) requires a correction coefficient F (d / D) expressed by the following equation (6).
[0056]
  F (d / D) = {1-0.8 (d / D)^Three}^-1    (6)
After all ΔR is
ΔR = (4ρ_ed^Three) / (ΠD^Four) X {1-0.8 (d / D)^Three}^-1  ... (7)
Given in. F (d / D) can be set to 1 because (d / D) that can be stably measured without clogging is considered to be almost 0.3 or less. Therefore, ΔR is
ΔR = (4ρ_ed^Three) / (ΠD^Four(8)
On the other hand, as an example of a basic measurement system, there is a circuit shown in FIG.
[0057]
Potential difference pulse ΔV when non-conductive spherical particles pass through the orifice₂Is expressed by the following equation.
ΔV₂= V_E× R_Three× ΔR / {(R₂+ R_Three+ ΔR) (R₂+ R_Three)}
.... (9)
Where ΔR is (R₂+ R_Three) Is sufficiently small compared to
ΔV₂≒ V_E× R_Three× ΔR / (R₂+ R_Three)²······(Ten)
[0058]
Alternatively, in formula (10) (R₂+ R_Three) = V_ESubstituting / I
ΔV₂= R_Three× I × ΔR / (R₂+ R_Three(11)
Or
ΔV₂= I²× R_Three× ΔR / V_E... (12)
Here, I is a current value to be measured. Substituting equation (8) into equation (11), potential difference pulse ΔV₂Is expressed by the following equation (13).
[0059]
ΔV₂= R_Three× I × (4ρ_ed^Three) / {(ΠD^Four) (R₂+ R_Three)} ··· (13) Here, the noise level is usually measured at 10 μV or less, so the detection limit d in this case_minIs
ΔV₂= 10 μV,
d_min= 18μm
It becomes.
[0060]
This is ΔV in equation (12).₂Detects and counts signals of at least 10 μV or more as being caused by inclusions.
In general, when this system is used in a steel factory, it is affected by noise. For this reason, various signal processing is performed that uses a filter, a slope of a waveform, and the like as a condition for identifying noise. However, although high intensity noise can be distinguished to some extent by the above-mentioned correspondence, it is difficult to discriminate it from the baseline for low intensity noise or a signal due to a small potential difference pulse due to a very small inclusion itself.
[0061]
According to the inventor's research, it is difficult to reduce the influence of noise to 10 μV or less in a mass-produced steel factory. Therefore, in this case, the range in which the particle size distribution of inclusions can be measured with high accuracy is ΔV.₂It can be said that the signal has an intensity of> 10 μV. In this case, for example, when the orifice diameter is 300 μm, inclusions with a diameter of 6% or more can be measured.
[0062]
That is, the dynamic range of the inclusion sensor when this orifice is used can be 6% to 40% of the orifice diameter.
If the threshold value is determined for each measured value based on the correspondence between the product quality and the measurement data of the inclusion sensor in this effective dynamic range, the cleanness of the slab can be determined instantaneously.
[0063]
That is, the orifice diameter should be at least 2.5 times the minimum diameter of a single harmful inclusion for the product quality to be measured.
When using a 300 μm orifice in the system of FIG.₂Since> 10 μV is d> 18 μm, the detection range of the inclusion diameter is 18 μm to a maximum of 120 μm even if it is small. Threshold values are determined for the inclusion particle size distribution or concentration (number / kg) in this range and used for quality determination.
[0064]
That is, if there is a single harmful inclusion, it can be determined by orifice clogging, and the total number (inclusion concentration) and particle size distribution can also be determined from the output of P.H.A. Thereby, when each threshold value is exceeded, the said slab or / and molten steel can be rejected, or a use change can be carried out.
[0065]
Furthermore, these data can be reflected in process control (extended refining time, enhanced stirring, enhanced tundish induction heating, reduced casting speed, etc.).
According to the study by the present inventor, when the orifice diameter is less than 100 μm, the orifice is clogged in a normal commercial production process, and stable measurement cannot be performed. This clogging is caused by the inclusion itself to be measured or when molten steel is cooled by the probe body and solidifies in the orifice. Moreover, it is expensive in terms of cost to make a pore having a diameter of less than 100 μm in a probe usually made of quartz or the like.
[0066]
On the other hand, when the diameter of the orifice exceeds 2000 μm, when the molten steel enters the probe through the orifice, a large flow occurs in the probe due to the large inflow velocity, and the current value baseline is not stable and accurate. Measurement is not possible.
[0067]
Furthermore, in general, the detectable range of inclusions is ΔV in the above equation (12).₂> 10μV diameter inclusions. At this time, for example, the minimum inclusion detection limit with a probe having an orifice diameter of 2000 μm is 226 μm. Not right.
[0068]
Based on the above knowledge, the present inventor has set the probe orifice diameter that can be put to practical use in the measurement around the continuous casting machine to 100 μm or more and 2000 μm or less.
[0069]
【Example】
 [Example 1]
In this example, the system shown in FIG. 1 was used, and measurement was performed using a continuous measurement method and a one-shot method sensor probe having an orifice having a diameter of 300 μm. The measurement was performed on molten steel in the tundish of the bloom caster. The measurement was performed by immersing the sensor probe in molten steel and sucking 1550 ° C molten steel through the orifice.
[0070]
In this example, the allowable maximum diameter of coarse inclusions was 100 μm, and the threshold value for the number of inclusions was 50000 pieces / kg.
The immersion depth of the sensor was measured with the orifice position 400mm below the surface of the molten metal.
[0071]
It can be seen that the potential difference pulse detected during measurement of about 1 kg of molten steel has a stable baseline and a smooth waveform.
FIG. 3 shows an example of the measurement system employed in this example.
[0072]
Input to log amp ΔV_inIs the preamplifier gain G,
ΔV_in= G x ΔV₂···········(14)
Log amp output ΔV_outIn this case
ΔV_out= 10/3 x {log_Ten(ΔV_in) +2} (15)
This P.H.A. divides the 8V output range into 512 channels, and if the channel number is Ch,
Ch = (512/8) × ΔV_out      .... (16)
It becomes. Where ΔV_outIs the output of the log amp as expressed by equation (15). The gain G was 100.
[0073]
There is a maximum diameter output near 250Ch, I = 20A, V_E= 6.4 V, R_Three= 0.125 Ω, ρ_e= 1.4 × 10^-6As Ωm, the diameter of the maximum inclusion is estimated to be 95 μm from the above formulas (8), (12), (14), (15), and (16). This maximum diameter of 95 μm was sufficiently smaller than 40% of the orifice diameter, so that the particle size distribution and number of inclusions could be measured without causing orifice clogging.
[0074]
Furthermore, this molten steel is subjected to ΔV during continuous casting.₂If the number of inclusions exceeding 100000V is measured using the orifice, and the number of inclusions with a diameter of 18μm or more exceeds 50000 pieces / kg, change the destination, and if it is 50000 pieces / kg or less I assigned it first.
[0075]
In the case of this steel type, the total number becomes a problem along with the maximum diameter of inclusions, and empirically, if the maximum diameter is 120 μm or less and 18 μm or more inclusions are 50,000 pieces / kg or less, the actual characteristics These were used as threshold values. Specifically, many slabs in the continuous casting joint (ladder replacement site) exceed this threshold.
[0076]
In addition, the threshold value for the minimum diameter of the allowable harmful single inclusion is empirically obtained depending on the steel type and the destination / application. Furthermore, the threshold value for the number of inclusions per unit steel sample is expressed by the formula Defined by (17).
[0077]
[Equation 3]

[0078]
S ≦ 5 × 10^FourP / kg-steel
D: Orifice diameter
x: Inclusion particle size
FIG. 4 is a graph showing the contents of the equation (17), and shows the relationship between the inclusion diameter (x) proportional to the orifice diameter and the fatigue characteristics (N): N = F (x) It is a graph, and the result obtained by integrating this to 0.06D to 0.4D is S, that is, the number of inclusions.
[0079]
In any case, according to the present invention, an appropriate orifice diameter is selected for each steel type, destination and application, and the quality of the molten steel is determined by using the maximum diameter of inclusions and / or the threshold number of units per unit steel sample. Can be determined “on the spot” and used for allocation.
[0080]
Here, the steel composition used as the object of the present invention can include the following examples as typical examples.
C: 1.2% or less, Si: 3.0% or less, Mn: 1.15% or less,
P: 0.025% or less, S: 0.2% or less, Al: 1.0% or less,
  Elements such as Cr and Mo may be appropriately contained if necessary.
[0081]
 [Example 2]
Using the system shown in FIG. 3, measurement was performed using a sensor probe having an orifice having a diameter of 100 μm.
[0082]
In this example, the allowable maximum diameter of coarse inclusions was set to 40 μm, which was used as a threshold value for inclusion diameter, and the threshold value for the number of inclusions was set to 50000 pieces / kg.
The measurement was performed on molten steel in the tundish of the bloom caster. Measurement was performed by sucking molten steel at 1545 ° C. About 10 seconds after sucking the molten steel, the orifice was clogged, and the measurement was interrupted.
[0083]
The orifice clogging was judged by the baseline disturbance on the oscilloscope. When the sensor probe was pulled up from the molten steel, it was observed that the tip of the probe was actually blocked. The degree of superheated molten steel at this time was 33 ° C. When the signal 10 seconds before the start of measurement was analyzed, inclusions with a diameter of 40 μm or more were found from Equation (8) and Equation (12).
[0084]
From this, it is immediately determined that this molten steel contains inclusions having a diameter of 40 μm or more, and it indicates that orifice clogging is a condition for judging the presence of harmful inclusions alone.
[0085]
Further, when a sample collected from a billet obtained by rolling a bloom obtained by casting the molten steel was observed with a microscope, inclusions having a diameter of 40 μm or more were observed.
Since the maximum allowable inclusion diameter of this material was 40 μm, the product was not allocated to the original use.
[0086]
The result of microscopic observation of the inclusions at this billet stage was obtained 7 days after the completion of casting. As described above, in the conventional method, it took 7 days for the inclusion to be determined, but if the present invention is applied, the determination at the molten steel stage becomes possible.
[0087]
 [Example 3]
Measurement was performed using a sensor probe having an orifice having a diameter of 200 μm. The measurement was performed on molten steel in the tundish of the bloom caster.
[0088]
Measurement was performed by sucking molten steel at 1552 ° C.
In this example, the allowable maximum diameter of coarse inclusions is set to 25 μm, which is used as a threshold value for inclusion diameters, and the number of inclusions in the range of 10 μm to 25 μm is set to 50000 pieces / kg. .
[0089]
As a result of the measurement, the range of the maximum diameter of the inclusions measured was 25 μm to 30 μm, and the number of inclusion particles in this particle size range was estimated to be about 300 per 1 kg of molten steel.
[0090]
This steel type is used for applications subject to repeated fatigue under high pressure conditions, and empirically, based on the maximum allowable inclusion diameter measured by the present invention and the fatigue strength required for the product (2), (3 ), (4) The maximum allowable inclusion diameter determined from (4) was 25 μm. The number of inclusions in the range of 10 to 30 μm is 5.5 × 10^FourPieces / kg. Based on this measurement result, the slab cast from the molten steel was judged immediately after casting and was not turned to the original use.
[0091]
Also, based on the measurement results, immediately after that, the result of carrying out the same measurement in another casting try immediately after the extension of the stirring holding time in the secondary refining process and the slag reforming in the ladle The maximum diameter of inclusions in molten steel in continuous cast tundish is reduced from 15μm to 20μm, and the number of inclusions in the range of 10-20μm is 2.5 × 10.^FourPieces / kg. It has become possible to allocate the slab to the original application. There was no abnormality on the product.
[0092]
These process improvement procedures can be carried out within tens of minutes to hours, compared with the time from 1 week to 10 days after casting, which is a method of judging and improving after waiting for the microscopic observation results of conventional products. As a result, the speed has been greatly improved, and it has become possible to fully meet customer demands for delivery.
[0093]
【The invention's effect】
According to the present invention, it is possible to produce clean steel capable of specifying the inclusion distribution, the reliability of the quality is remarkably improved, and such high-quality clean steel can be produced stably and practically. Is big.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of the principle of inclusion detection.
FIG. 2 is a conceptual diagram of a basic measurement system.
FIG. 3 is an explanatory diagram of a system for carrying out the present invention.
FIG. 4 is a graph for explaining a relationship with NX in an example.

Claims (5)

ESZ which counts inclusions contained in molten steel passing through an orifice provided in a probe constituting an inclusion sensor immersed in molten steel based on a potential difference pulse between opposed surfaces of the molten steel during continuous casting of molten steel In the manufacturing method of clean steel that casts while cleanliness of molten steel is "in-situ measurement" by the method,
Determining the maximum diameter threshold of inclusions allowed, depending on the steel type and application of the molten steel;
Determining an orifice diameter to be used for the probe based on a threshold regarding the maximum diameter of the inclusion;
Immersing the inclusion sensor in a ladle, tundish, or molten steel in a mold; and when it is sensed that the orifice has been blocked by inclusions exceeding the threshold, during the refining to continuous casting stage To immediately take measures such as issuing warnings to reduce the amount of inclusions, or changing the destination of the obtained products;
A method for producing clean steel. ESZ which counts inclusions contained in molten steel passing through an orifice provided in a probe constituting an inclusion sensor immersed in molten steel based on a potential difference pulse between opposed surfaces of the molten steel during continuous casting of molten steel In the manufacturing method of clean steel that casts while cleanliness of molten steel is "in-situ measurement" by the method,
Depending on the steel type and application of the molten steel, determining the threshold value of the allowable particle size range of inclusions and, if necessary, the number of inclusions in that range;
Determining an orifice diameter to be used for the probe based on a threshold value of an acceptable particle size range of the inclusion;
Detecting the data for calculating the diameter of inclusions or the diameter of inclusions and the number of inclusions by immersing the inclusion sensor in a ladle, tundish, or molten steel in a mold;
Calculating the diameter of inclusions and, if necessary, the number of inclusions from the detected data;
And when calculating the obtained inclusion diameter or the number of inclusions, if any of the inclusion diameter and the number of inclusions exceeds the above threshold, take measures to reduce inclusions in the refining to continuous casting stage. Immediately taking on-the-spot actions such as issuing a warning to command or changing the destination of the resulting product;
A method for producing clean steel. 3. The production of clean steel according to claim 1 or 2, wherein, when continuous measurement is performed using an inclusion sensor of a continuous measurement method, the data is detected and collected with the immersion depth from the molten metal surface of the orifice being 300 mm or more. Method. The clean steel according to claim 1 or 2, wherein, when intermittent measurement is performed using a one-shot type inclusion sensor, the data is detected and collected by setting the immersion depth from the molten metal surface of the orifice to 300 mm or more. Manufacturing method. The method for producing a clean steel according to claim 2, wherein a threshold value of the number of inclusions is 50000 or less per kg of molten steel determined by the following formula for molten steel in a tundish or trough.

S ≦ 5 × 10 ⁴ P / kg-steel
D: Orifice diameter x: Inclusion particle diameter

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