JP3630136B2 - Method for producing ultra-low carbon steel for automobiles - Google Patents

Description

【００１７】
ただし、
Ｎ_ＦＭ：ＲＨ処理終了後の取鍋スラグ中の(ＦｅＯ)＋(ＭｎＯ)濃度(質量％)、
Ｖ_Ｌ ：メニスカスからの連続鋳造機の垂直部長さ(m)、
Ｗ ：タンディッシュ容量(トン)、
ｈ ：タンディッシュ浴深さ(m)、
Ｔ_Ｐ ：スループット(トン/min)、
Ｖ_Ｐ ：介在物浮上速度 (m/min)、
α ：介在物除去速度定数(m^−１ )
【００１８】
【発明の実施の形態】
次に、本発明の基となるスラブの介在物濃度と種々の操業条件との関係を表す式（１） の内容を説明する。
【００１９】
ａ．極低炭素鋼の製造においては、転炉で精錬した溶鋼を取鍋に出鋼し、これをＲＨ真空脱ガス装置にて脱炭し、脱酸などの成分調整をした後、溶鋼を取鍋からタンディシュに移し、連続的にモールド内に注湯してスラブに鋳造する。
【００２０】
上記製鋼工程において、真空脱ガス処理（以下、ＲＨ処理とも記す）後の取鍋内スラグ中の低級酸化物濃度は、溶鋼をタンディシュに移した際のタンディッシュ内溶鋼の全酸素濃度に大きく影響する。
【００２１】
図１は、本発明者の研究結果による、上記低級酸化物濃度と、溶鋼の全酸素濃度（図１では、「Ｔ−［Ｏ］」と記す）との関係を示すグラフである。図１からわかるように、両者の間には強い正の相関関係がある。
【００２２】
図１に示す取鍋内スラグ中の低級酸化物濃度とタンディシュ内溶鋼の全酸素濃度との関係は、下記式（２） で示すことができる。
Ｎ_ＴＤ＝３Ｎ_ＦＭ ＋１０ （２）
ただし、Ｎ_ＴＤ：タンディッシュ内溶鋼の全酸素濃度（ 単位はｐｐｍ）、
Ｎ_ＦＭ：ＲＨ処理終了後の取鍋内スラグ中の （ＦｅＯ） ＋ （ＭｎＯ） 濃度（単位は質量％）。
【００２３】
このように鋼に含有される介在物の量は鋼の全酸素濃度と良好な相関関係を有する。従って鋼の全酸素濃度は、鋼の清浄度を表す指標として用いることができるので、本発明では、溶鋼中およびスラブ中ともに全酸素濃度を介在物量を表す指標とした。
【００２４】
ｂ．溶鋼はタンディシュから連続的にモールドに注湯される。注湯の際に、タンディッシュ内からモールド内に持ち込まれる介在物の量は、タンディッシュの形状やスループットなどの操業条件で規定される。
【００２５】
すなわち、タンディッシュ内で介在物を浮上させることで、モールド内の介在物量を減少させることができるが、介在物の浮上除去効率はタンディッシュ内での溶鋼の滞留時間が長いほど向上する。また、タンディシュ浴の深さが浅いほど介在物は容易に浮上する。
【００２６】
図２は、本発明者の研究結果による、タンディッシュ内での介在物の浮上に対する操業条件の影響を示すグラフである。モールド内溶鋼の全酸素濃度をＮ_ＭＤ（単位はｐｐｍ）、タンディッシュ内溶鋼の全酸素濃度をＮ_ＴＤ（単位はｐｐｍ）とすると、タンディシュ内で介在物が浮上するにつれて、Ｎ_ＭＤ／Ｎ_ＴＤは減少する。この意味で、図２の縦軸の、Ｎ_ＭＤ／Ｎ_ＴＤはタンディシュ内溶鋼における介在物の浮上度合いを表すものである。
【００２７】
図２の横軸は、介在物の浮上の容易さを表す介在物浮上指数である。これは、介在物浮上速度をＶ_Ｐ （単位はｍ／ｍｉｎ）、タンディッシュ、つまりタンディッシュ内溶鋼量をＷ（単位はトン）、スループットをＴ_Ｐ （単位はトン／ｍｉｎ）、タンディッシュ、つまりタンディッシュ内溶鋼の深さをｈ（単位はｍ）としたとき、（Ｖ_Ｐ ・Ｗ／Ｔ_Ｐ ・ｈ）で表される。
【００２８】
介在物浮上指数においてＷ／Ｔ_Ｐ はタンディッシュ内での介在物の滞留時間である。図２に示すように、滞留時間が長く、タンディッシュ深さが浅いほどモールド内溶鋼の全酸素濃度の低下が大きく、介在物の浮上が促進されることがわかる。
【００２９】
図２から、モールド内溶鋼の全酸素濃度は、下記式（３） で示すことができる。
【００３０】
【数３】

【００３１】
ここで、Ｖ_Ｐ は介在物浮上速度 （単位はｍ／ｍｉｎ）を意味し、その値は０．４ｍ／ｍｉｎで一定と考えてよい。
ｃ．スラブ内に介在物を残存させないために、連続鋳造機のモールド内に介在物の浮上分離に有効な垂直部を設けるのが有効である。
【００３２】
図３は、本発明者の研究結果による、モールド内での介在物の浮上分離に対する上記垂直部の影響を示すグラフである。図３の縦軸は、モールド内溶鋼の全酸素濃度（Ｎ_ＭＤ）に対するスラブ中の全酸素濃度（Ｎ_ＳＬ、単位はｐｐｍ ）の比（Ｎ_ＳＬ／Ｎ_ＭＤ）であり、Ｎ_ＳＬ／Ｎ_ＭＤが小さくなることは、モールド内での介在物の浮上除去が促進されていることを意味する。
【００３３】
図３に示すように、メニスカスからの連続鋳造機の垂直部の （ほぼモールド＋連続鋳造機の垂直部に等しい） 長さが大きくなるにつれてモールド内での介在物の浮上分離が促進される。この関係は、図３から、下記式（４） で表すことができる。
【００３４】
Ｎ_ＳＬ＝Ｎ_ＭＤ×α（ −０．０３１ ×Ｖ_Ｌ ＋０．９１） （４）
ただし、Ｖ_Ｌ はメニスカスからの連続鋳造機の垂直部長さ（単位はｍ ）、αは介在物除去速度定数 （単位は ｍ^−１）を意味し、その値は１．０ｍ^−１で一定と考えてよい。
【００３５】
ｄ．上記式 （２）〜（４） を連立させて解くことにより、下記式（５） を得ることができ。
【００３６】
【数４】

【００３７】
式（５） によれば、操業条件からスラブの全酸素濃度を予測し、スラブの介在物量を推定することができる。なお、連続鋳造機の垂直部長さはすでに当業者には明らかなところ、要するに鋳込まれた溶鋼内を介在物が上昇する領域の垂直方向長さである。
【００３８】
ｅ．スラブ中の介在物が増すにつれて、得られる製品の不良率が高くなる。ここでの製品不良率は、上記スラブを熱延鋼板、冷延鋼板あるいは、これらを母材とする各種のめっき鋼板などの最終製品に加工した場合に、材料原因の庇があるために、切下げ不良が発生するが、その不良率を指数化したものである。製品不良率が高いと、歩留悪化によるコスト損失が大きくなる。
【００３９】
図４は、本発明者の調査結果による、スラブの全酸素濃度（Ｎ_ＳＬ）が製品不良率に及ぼす影響を示すグラフである。図４では製品不良率は指標化して、製品不良指数として示した。図４に示すように、スラブの全酸素濃度が増すにつれて製品不良指数が大きくなり、特に２７ｐｐｍ を超えるとその増加が著しい。
【００４０】
本発明者はさらに、種々の操業条件について鋼の精錬から連続鋳造までの製造段階で介在物減少対策に要する種々の費用（製鋼ロスコスト）と、その結果生じる製品検査段階での切下げ発生による費用損失（切り下げコスト）を求め、スラブの全酸素濃度とトータルのコスト損失（製鋼ロスコスト＋切り下げコスト）との関係を調査した。
【００４１】
図５は、上記の研究結果から得られた、スラブの全酸素濃度がトータルのコスト損失に及ぼす影響を示すグラフである。図５において、縦軸のトータルロスコスト指数は、製鋼ロスコスト、切下ロスコストともに一般的な操業条件の場合を基準にしてトータルロスコストを指標化したものである。
【００４２】
図５に示すように、スラブの全酸素濃度が２７ｐｐｍ の場合にロスコストが最小となる。トータルロスコスト指数は、スラブの全酸素濃度が２７ｐｐｍ よりも小さくなるにつれて製鋼ロスコストが増すために大きくなり、２７ｐｐｍ を超えて大きくなるにつれて切下コストが増すために大きくなる。
【００４３】
製品不良指数を低く抑制し、介在物が少なく健全な製品を安定して得るには、スラブの全酸素濃度が２７ｐｐｍ 以下になる条件で製造するのがよく、そのためには、式（１） の左辺で計算される値が２７以下、好ましくは２５以下になるように、製鋼工程における操業条件、特に溶鋼の製造条件および／または鋳造条件を調整するのが好適である。
【００４４】
操業条件の調整方法は、現実的には、連続鋳造設備レイアウトおよびタンディッシュ形状は、決まっている場合がほとんどであるため、鋳造中のスループットとスラグ中低級酸化物濃度を、式（１） を満たす範囲で連続鋳造設備毎に設定し、操業するのが望ましい。
【００４５】
【実施例】
転炉で精錬した溶鋼を取鍋に出鋼し、これをＲＨ真空脱ガス装置にて脱炭処理し、引き続いて脱酸処理して製造された溶鋼を、３種類の連続鋳造設備を使用して、極低炭素鋼スラブを製造し、コイルの形状の最終製品に加工し、出荷検査において生じた製品格落ち率を調査した。合わせて、それぞれの場合の鋼の精錬から連続鋳造までの間の操業コストを調査し、連続鋳造機の仕様毎に、従来の操業条件の場合の操業コストを基準とするコスト指数を計算した。
【００４６】
表１に連続鋳造機の設備仕様を示し、表２に操業条件と式（１） で計算されるスラブ中の介在物濃度およびそれぞれのコスト指数を示す。
【００４７】
【表１】

【００４８】
【表２】

【００４９】
ケース１は、連続鋳造設備Ａにより、通常の条件で製造した場合である。この場合、式（１） の左辺の計算値は目標の２７以下を達成しておらず、製品不良指数が高く、製品採取が安定しなかった。ケース１の場合の精錬から連続鋳造までの間の操業コストを連続鋳造機Ａの場合の操業コストの基準とした。
【００５０】
ケース２として、ケース１に比較してスループットを小さくして鋳込み速度（スラブ引抜き速度）を低下させた場合を、ケース３として、スラグ改質材を多量に使用してスラグ中の低級酸化物濃度を低下させた場合を評価した。いずれの場合とも式（１） の左辺の計算値は目標の２７以下となり、製品不良指数が低く、製品採取が安定して良好であった。両ケースとも操業コストはケース１に比較して高くなったが、スラグ中の低級酸化物濃度を低下させたケース３の場合は、前述したようにスラグ改質材のコストが高いので、ケース２よりもコスト指数が高くなった。このことは、生産能率に問題のない場合はケース２のように、スループットを低下させ、タンディッシュ内の滞留時間を延長させたほうが効果的である。
【００５１】
ケース４はスラグ改質をさらに強力に行なってスラグ中低級酸化物濃度をケース３以上に低下させ、鋳込み速度をケース２同様に低下させた場合である。この場合は式（１） の左辺の計算値はさらに小さくなるが、製品不良指数の改善効果は飽和に近いためにさほどは向上しなかった。他方、操業コストの上昇が大きく、コスト面での不利が大きくなった。このことは、必要以上に清浄度を向上させる必要はないことを意味する。
【００５２】
ケース５は、ケース１と同一条件で精錬し、真空脱ガス処理を施した溶鋼を連続鋳造設備Ｂで鋳造した場合の例である。連続鋳造設備Ｂはタンディッシュ容量が少なく、タンディッシュ内滞留時間が短いため、介在物の除去には不利な連続鋳造設備である。このため、式（１） の左辺の計算値は、連続鋳造設備Ａの場合に比較して高く、製品不良指数が大きかった。ケース６またはケース７に示すように、スループットを小さくして鋳込み速度を低下させた場合、あるいは、スラグ改質材を多量に使用してスラグ中の低級酸化物濃度を低下させた場合には、目標とするスラブが得られ、最終製品の製品不良指数も小さく良好であった。操業コストはケース７よりもケース６の方が有利であることは、連続鋳造機Ａの場合と同様であった。
【００５３】
ケース８およびケース９に示した連続鋳造設備Ｃは、垂直部を有さない連続鋳造設備であるため、連続鋳造設備Ｂ以上に介在物減少対策が必要であった。
【００５４】
【発明の効果】
本発明の規定する式（１） を満足するように操業条件を調整することで、所望の品質要求レベルを満たす極低炭素鋼を安定して、かつ、効率よく製造することができる。
【図面の簡単な説明】
【図１】ＲＨ処理後のスラグ中の低級酸化物濃度と、溶鋼の全酸素濃度との関係を示すグラフである。
【図２】タンディッシュ内での介在物の浮上に対する操業条件の影響を示すグラフである。
【図３】モールド内での介在物の浮上分離に対するモールド垂直部の影響を示すグラフである。
【図４】スラブの全酸素濃度（Ｎ_ＳＬ）が製品不良率に及ぼす影響を示すグラフである。
【図５】スラブの全酸素濃度がトータルのコスト損失に及ぼす影響を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a converter, an RH vacuum degassing apparatus, and a method for producing ultra-low carbon steel using a continuous casting facility.
[0002]
[Prior art]
In the continuous casting method combined with vacuum degassing treatment, the molten steel refined in the converter is taken out into a ladle, and after decarburizing with a RH vacuum degassing device, components such as deoxidation are adjusted, and then The molten steel is transferred from the ladle to a tundish (hereinafter also referred to as “T / D”), and continuously poured from the tundish into a mold of a continuous casting machine to be cast into a slab.
[0003]
For ultra-low carbon steels mainly used for automobiles, the level of quality requirements is increasing year by year. For this reason, measures for reducing inclusions are an important issue in the steel making process as described above.
[0004]
Measures to reduce inclusions include slag reforming treatment using a ladle of molten steel smelted in a converter, promotion of separation in a vacuum degasser, promotion of floating in a tundish, promotion of floating in a mold of a continuous casting machine Various methods have been studied.
[0005]
For example, as measures to reduce inclusions after refining, as disclosed in JP-A-10-298629, JP-A-11-158537, JP-A-2000-119732, JP-A-06-256837, etc. Many methods for reducing the amount of reoxidation of molten steel by reducing the lower oxide concentration in the pan slag by various means have been disclosed.
[0006]
In JP-A-08-141708 and JP-A-09-38753, in order to prevent the inclusions in the molten steel remaining in the refining stage from being brought into the slab, that is, in the product stage, the residence time of the molten steel in the tundish is extended. Therefore, a means for improving the floating separation efficiency of inclusions in the tundish has been proposed.
[0007]
Japanese Patent Application Laid-Open No. 11-33687 proposes a method of reducing the amount of inclusions brought into the slab by providing a vertical portion below the meniscus of molten steel in a continuous casting facility and through an electromagnetic stirrer.
[0008]
[Problems to be solved by the invention]
However, although the above-mentioned various methods have the effect of reducing the inclusion of slabs and improving the cleanliness, they are not only accompanied by an increase in manufacturing cost but also a large amount of modifications such as tundish remodeling and continuous casting equipment remodeling. All that needs investment.
[0009]
For example, when slag reforming is performed as a method of reducing the lower oxide concentration in the ladle slag, the cost increases because a deoxidizer containing metal Al 2 is usually used. In addition, the method of extending the residence time in the tundish can be realized by changing the shape of the tundish or reducing the throughput (hot water supply speed from the tundish to the mold). If the throughput is reduced, the production efficiency decreases and the cost increases due to the temperature loss accompanying the extension of the casting time. Providing a vertical part in a continuous casting machine requires enormous capital investment for remodeling equipment, and is not an easy method as long as it operates with existing equipment.
[0010]
In continuous casting equipment that already has a vertical part in a continuous casting machine and uses a large tundish, the inclusion removal capability is high, so measures to reduce the amount of inclusions at the molten steel stage are mild. Even the quality requirement level can be satisfied. However, as described above, the specifications and capacity of the equipment used and various operating conditions affect the amount of inclusions remaining in the slab continuously cast from ultra-low carbon steel. Further, these inclusion floating removal means have various effects on the manufacturing cost and productivity of the slab. For this reason, it has not been easy to obtain a slab stably containing few inclusions at a minimum manufacturing cost.
[0011]
An object of the present invention is to provide a method for efficiently and stably producing a healthy ultra-low carbon steel that can satisfy a severe quality requirement level without defects caused by inclusions.
[0012]
[Means for Solving the Problems]
When continuously casting an extremely low carbon steel slab, various operating conditions affect the amount of inclusions remaining in the slab. In addition, various types of inclusion floating removal means have been considered, but there are various effects on the effects, manufacturing costs, and productivity of each method. Moreover, these differ depending on the specifications and capabilities of the equipment used.
[0013]
Therefore, if the present inventor can clearly grasp the influence of various operating factors from the steel refining stage to casting on the inclusions remaining in the slab, the inventor shall always select the optimal operating conditions. Thus, the idea was obtained that ultra-low carbon steel having a desired quality can be stably obtained according to the specifications and capabilities of the equipment used.
[0014]
In particular, if the operating cost from refining to continuous casting of steel under various operating conditions, especially the cost required for the removal of floating inclusions, can be grasped, the desired quality can be obtained by combining this with the above inclusion reduction method. It has been found that products having a low cost can be manufactured efficiently and stably.
[0015]
The present invention has been completed on the basis of the knowledge obtained as a result of various studies based on these ideas. The gist of the present invention is that the molten steel refined in the converter is put into a ladle, which is In the method for producing ultra-low carbon steel for automobiles , which is decarburized by an RH vacuum degassing apparatus and subsequently deoxidized, and then cast using a continuous casting facility, the following formula (1) is satisfied. Thus, it is the manufacturing method of the ultra-low carbon steel for motor vehicles characterized by adjusting the processing conditions and / or casting conditions of molten steel.
[0016]
[Expression 2]

[0017]
However,
N _FM: RH treatment after completion of ladle slag of (FeO) + (MnO) concentration (wt%),
V _L : Vertical length of continuous casting machine from meniscus (m),
W: Tundish capacity (tons),
h: tundish bath depth (m),
T _P : Throughput (ton / min),
_VP : Inclusion ascent rate (m / min),
α: Inclusion removal rate constant (m ⁻¹ )
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Next, the contents of the equation (1) representing the relationship between the inclusion concentration of the slab, which is the basis of the present invention, and various operating conditions will be described.
[0019]
a. In the production of ultra-low carbon steel, the molten steel refined in the converter is put into a ladle, decarburized with an RH vacuum degassing device, adjusted for components such as deoxidation, and then the molten steel ladle To the tundish, continuously poured into the mold and cast into a slab.
[0020]
In the steelmaking process, the lower oxide concentration in the slag in the ladle after vacuum degassing (hereinafter also referred to as RH treatment) greatly affects the total oxygen concentration of the molten steel in the tundish when the molten steel is transferred to the tundish. To do.
[0021]
FIG. 1 is a graph showing the relationship between the lower oxide concentration and the total oxygen concentration of molten steel (indicated as “T- [O]” in FIG. 1) based on the results of research by the present inventors. As can be seen from FIG. 1, there is a strong positive correlation between the two.
[0022]
The relationship between the lower oxide concentration in the ladle slag shown in FIG. 1 and the total oxygen concentration in the molten steel in the tundish can be expressed by the following formula (2).
N _TD = 3N _FM +10 (2)
However, _NTD : total oxygen concentration (unit: ppm) of molten steel in tundish,
N _FM : (FeO) + (MnO) concentration (unit: mass%) in the slag in the ladle after completion of the RH treatment.
[0023]
Thus, the amount of inclusions contained in the steel has a good correlation with the total oxygen concentration of the steel. Therefore, since the total oxygen concentration of steel can be used as an index representing the cleanliness of steel, in the present invention, the total oxygen concentration is used as an index representing the amount of inclusions in both the molten steel and the slab.
[0024]
b. Molten steel is poured continuously from the tundish into the mold. During pouring, the amount of inclusions brought into the mold from the tundish is defined by operating conditions such as the tundish shape and throughput.
[0025]
That is, by raising the inclusions in the tundish, the amount of inclusions in the mold can be reduced, but the floating removal efficiency of inclusions increases as the residence time of the molten steel in the tundish increases. In addition, inclusions float more easily as the depth of the tundish bath is shallower.
[0026]
FIG. 2 is a graph showing the influence of operating conditions on the floating of inclusions in a tundish, according to the results of the inventor's research. Assuming that the total oxygen concentration of the molten steel in the mold is N _MD (unit: ppm) and the total oxygen concentration of the molten steel in the tundish is N _TD (unit: ppm), N _MD / N _TD as inclusions float in the tundish. Decrease. In this sense, N _MD / N _{TD on} the vertical axis in FIG. 2 represents the degree of floating of inclusions in the molten steel in tundish.
[0027]
The horizontal axis of FIG. 2 is an inclusion floating index representing the ease of floating of inclusions. This is because the inclusion flotation speed is _VP (unit is m / min), tundish, that is, the amount of molten steel in the tundish is W (unit is ton), throughput is T _P (unit is ton / min) That is, when the depth of the molten steel in the tundish is h (unit is m), it is expressed by (V _P · W / T _P · h).
[0028]
W / T _P in inclusions floating index is the residence time of the inclusions in the tundish. As shown in FIG. 2, it can be seen that the longer the residence time and the shallower the tundish depth, the greater the decrease in the total oxygen concentration of the molten steel in the mold, and the more the inclusions are promoted.
[0029]
From FIG. 2, the total oxygen concentration of the molten steel in the mold can be expressed by the following formula (3).
[0030]
[Equation 3]

[0031]
Here, _VP means the inclusion floating speed (unit: m / min), and the value may be considered constant at 0.4 m / min.
c. In order not to leave inclusions in the slab, it is effective to provide a vertical portion effective for floating separation of inclusions in the mold of the continuous casting machine.
[0032]
FIG. 3 is a graph showing the influence of the vertical portion on the floating separation of inclusions in the mold according to the research results of the present inventors. The vertical axis in FIG. 3 is the ratio (N _SL / N _MD ) of the total oxygen concentration (N _SL , unit: ppm) in the slab to the total oxygen concentration (N _MD ) of the molten steel in the mold, and N _SL / N _MD Smaller means that the floating removal of inclusions in the mold is promoted.
[0033]
As shown in FIG. 3, as the length of the vertical part of the continuous casting machine from the meniscus (approximately equal to the vertical part of the mold + continuous casting machine) increases, the floating separation of inclusions in the mold is promoted. This relationship can be expressed by the following formula (4) from FIG.
[0034]
N _SL = N _MD × α (−0.031 × V _L +0.91) (4)
Where V _L is the length of the vertical portion of the continuous casting machine from the meniscus (unit is m), α is the inclusion removal rate constant (unit is m ⁻¹ ), and the value is constant at 1.0 m ^−1. You can think about it.
[0035]
d. The following formula (5) can be obtained by solving the above formulas (2) to (4) simultaneously.
[0036]
[Expression 4]

[0037]
According to Equation (5), the total oxygen concentration of the slab can be predicted from the operating conditions, and the amount of inclusions in the slab can be estimated. The length of the vertical portion of the continuous casting machine is already apparent to those skilled in the art. In short, it is the length in the vertical direction of the region in which the inclusion rises in the cast molten steel.
[0038]
e. As the inclusions in the slab increase, the defect rate of the resulting product increases. The product defect rate here is rounded down due to material flaws when the slab is processed into final products such as hot-rolled steel sheets, cold-rolled steel sheets, or various plated steel sheets using these as base materials. Defects occur, but the defect rate is indexed. When the product defect rate is high, the cost loss due to the deterioration of yield increases.
[0039]
FIG. 4 is a graph showing the influence of the total oxygen concentration (N _SL ) of the slab on the product defect rate according to the results of the inventor's investigation. In FIG. 4, the product defect rate is indexed and shown as a product defect index. As shown in FIG. 4, the product defect index increases as the total oxygen concentration of the slab increases, and the increase is particularly remarkable when it exceeds 27 ppm.
[0040]
Furthermore, the present inventor further explains various expenses (steel making loss cost) required for inclusion reduction measures in the manufacturing stage from steel refining to continuous casting for various operating conditions, and resulting cost loss due to devaluation in the product inspection stage. (Devaluation cost) was obtained, and the relationship between the total oxygen concentration of the slab and the total cost loss (steel making loss cost + devaluation cost) was investigated.
[0041]
FIG. 5 is a graph showing the effect of the total oxygen concentration of the slab on the total cost loss obtained from the above research results. In FIG. 5, the total loss cost index on the vertical axis is an index of the total loss cost based on the case of general operating conditions for both the steelmaking loss cost and the cut-off loss cost.
[0042]
As shown in FIG. 5, the loss cost is minimized when the total oxygen concentration of the slab is 27 ppm. The total loss cost index increases because the steelmaking loss cost increases as the total oxygen concentration of the slab becomes smaller than 27 ppm, and increases as the cut-off cost increases as it exceeds 27 ppm.
[0043]
In order to suppress the product defect index low and to stably obtain a healthy product with few inclusions, it is preferable to manufacture under the condition that the total oxygen concentration of the slab is 27 ppm or less. It is preferable to adjust the operating conditions in the steelmaking process, particularly the manufacturing conditions and / or casting conditions of the molten steel so that the value calculated on the left side is 27 or less, preferably 25 or less.
[0044]
As for the adjustment method of the operating conditions, the continuous casting equipment layout and the tundish shape are practically determined in most cases. Therefore, the throughput during casting and the lower oxide concentration in the slag are expressed by Equation (1). It is desirable to set and operate each continuous casting facility within the range to satisfy.
[0045]
【Example】
Using 3 types of continuous casting equipment, the molten steel refined in the converter is put into a ladle, decarburized with an RH vacuum degasser, and subsequently deoxidized. Then, an ultra-low carbon steel slab was manufactured, processed into a final product in the shape of a coil, and the rate of product degradation that occurred in the shipping inspection was investigated. At the same time, the operation cost from refining steel to continuous casting in each case was investigated, and the cost index based on the operation cost under the conventional operating conditions was calculated for each specification of the continuous casting machine.
[0046]
Table 1 shows the equipment specifications of the continuous casting machine, and Table 2 shows the operating conditions, the inclusion concentration in the slab calculated by Equation (1), and the cost index for each.
[0047]
[Table 1]

[0048]
[Table 2]

[0049]
Case 1 is a case where the continuous casting equipment A is manufactured under normal conditions. In this case, the calculated value on the left side of Equation (1) did not achieve the target of 27 or less, the product defect index was high, and product collection was not stable. The operating cost from refining to continuous casting in case 1 was used as the standard for operating cost in continuous casting machine A.
[0050]
Case 2 has a lower throughput than Case 1 and the casting speed (slab drawing speed) is reduced. Case 3 has a lower oxide concentration in the slag using a large amount of slag modifier. The case of lowering was evaluated. In either case, the calculated value on the left side of Equation (1) was 27 or less of the target, the product defect index was low, and product sampling was stable and good. In both cases, the operation cost was higher than in case 1, but in case 3 where the lower oxide concentration in the slag was lowered, the cost of the slag modifier was high as described above, so case 2 The cost index was higher than. When there is no problem in production efficiency, it is more effective to lower the throughput and extend the residence time in the tundish as in Case 2.
[0051]
Case 4 is a case where slag reforming is performed more strongly so that the lower oxide concentration in the slag is lowered to Case 3 or higher, and the casting speed is lowered as in Case 2. In this case, the calculated value on the left side of Equation (1) is further reduced, but the improvement effect of the product defect index is not so improved because it is close to saturation. On the other hand, the operating cost has increased significantly, and the cost disadvantage has increased. This means that it is not necessary to improve the cleanliness more than necessary.
[0052]
Case 5 is an example in which molten steel refined under the same conditions as Case 1 and subjected to vacuum degassing treatment is cast in continuous casting equipment B. Since the continuous casting facility B has a small tundish capacity and a short residence time in the tundish, it is a disadvantageous continuous casting facility for removing inclusions. For this reason, the calculated value on the left side of the formula (1) was higher than that of the continuous casting equipment A, and the product defect index was large. As shown in Case 6 or Case 7, when the casting speed is reduced by reducing the throughput, or when the lower oxide concentration in the slag is reduced by using a large amount of the slag modifier, The target slab was obtained, and the product defect index of the final product was small and good. As in the case of the continuous casting machine A, the operating cost is more advantageous in case 6 than in case 7.
[0053]
Since the continuous casting equipment C shown in the case 8 and the case 9 is a continuous casting equipment that does not have a vertical portion, measures for reducing inclusions are required more than the continuous casting equipment B.
[0054]
【The invention's effect】
By adjusting the operating conditions so as to satisfy the formula (1) defined by the present invention, it is possible to stably and efficiently produce an ultra-low carbon steel that satisfies a desired quality requirement level.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the lower oxide concentration in slag after RH treatment and the total oxygen concentration of molten steel.
FIG. 2 is a graph showing the influence of operating conditions on the floating of inclusions in a tundish.
FIG. 3 is a graph showing the influence of the mold vertical part on the floating separation of inclusions in the mold.
FIG. 4 is a graph showing the influence of the total oxygen concentration (N _SL ) of a slab on the product defect rate.
FIG. 5 is a graph showing the influence of the total oxygen concentration of the slab on the total cost loss.

Claims (1)

The electrode for automobiles that uses the continuous casting equipment to cast the molten steel smelted in the converter into a ladle, decarburize it with an RH vacuum degasser, and subsequently deoxidize it. A method for producing ultra-low carbon steel for automobiles, characterized in that in the method for producing low carbon steel, the treatment conditions and / or casting conditions of the molten steel are adjusted so as to satisfy the following formula (1).

However,
N _FM: RH treatment after completion of ladle slag of (FeO) + (MnO) concentration (wt%),
V _L : Vertical length (m) of continuous casting machine from meniscus,
W: Tundish capacity (tons),
h: tundish bath depth (m),
T _P : Throughput (ton / min),
_VP : Inclusion ascent rate (m / min),
α: Inclusion removal rate constant (m ⁻¹ )

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