JP2010099704A - Continuous casting method for steel cast slab - Google Patents
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本発明は、鋼鋳片の連続鋳造方法に関し、詳しくは、鋳型内で鋳片表層部に捕捉されるアルミナクラスターの少ない鋼鋳片を鋳造するための連続鋳造方法に関するものである。 The present invention relates to a continuous casting method of a steel slab, and more particularly, to a continuous casting method for casting a steel slab having a small number of alumina clusters captured by a slab surface layer in a mold.
自動車用鋼板などの極低炭素鋼を製造する場合、溶鋼をＡｌで脱酸処理することから、精錬終了時に溶鋼中へのアルミナ（Ａｌ2Ｏ3）の混入は避けられず、脱酸生成物として生成した溶鋼中のアルミナは、輸送用容器内や連続鋳造設備のタンディッシュ内で凝集し、アルミナのクラスラーを形成する。このアルミナクラスターは、溶鋼の連続鋳造の際に、溶鋼とともにタンディッシュから鋳型内に流入し、凝固殻に捕捉されて鋳片の表面欠陥となり、厳格な品質が要求される極低炭素鋼鋳片の品質を著しく低下させる。 When manufacturing ultra-low carbon steel such as steel sheets for automobiles, the molten steel is deoxidized with Al, so mixing of alumina (Al 2 O 3 ) into the molten steel is inevitable at the end of refining. As a result, the alumina in the molten steel is agglomerated in a transport container or a tundish of a continuous casting facility to form an alumina clasler. This alumina cluster flows into the mold from the tundish together with the molten steel during the continuous casting of molten steel, and is trapped by the solidified shell and becomes a surface defect of the slab, which requires extremely strict quality. Significantly reduces the quality of
そこで、鋳造後の鋳片に表面欠陥が存在する場合には、表面欠陥の存在する部位を溶削して除去する作業、所謂「手入れ作業」が行われている。しかしながら、この手入れ作業では、鋼歩留りの低下によるコスト上昇や作業処理費によるコスト上昇が生ずるのみならず、製造工程が延長されて効率的な生産体制が阻害されるという問題も発生する。 Therefore, when a surface defect exists in the cast slab, a work for removing a portion where the surface defect exists by welding, a so-called “care work” is performed. However, this maintenance work not only causes an increase in cost due to a decrease in steel yield and an increase in work processing cost, but also causes a problem that the manufacturing process is extended and the efficient production system is hindered.
そこで、鋳型背面に設置した電磁攪拌装置により、凝固殻前面の溶鋼に流速を付与するなどして、凝固殻に付着するアルミナクラスターなどの非金属介在物を洗浄し、それにより、鋳片表層部のアルミナクラスターなどの非金属介在物を低減する方法が多数提案されている（例えば、特許文献１など）。 Therefore, non-metallic inclusions such as alumina clusters adhering to the solidified shell are washed by applying a flow velocity to the molten steel in front of the solidified shell using an electromagnetic stirrer installed on the back of the mold. Many methods for reducing non-metallic inclusions such as alumina clusters have been proposed (for example, Patent Document 1).
しかしながら、電磁攪拌装置によって凝固殻前面の溶鋼に流速を付与する方法では、必要以上の流速を溶鋼に付与する場合が発生し、このような場合には、鋳型内溶鋼湯面上に添加したモールドパウダーの巻き込みが発生し、却って鋳片表層部の品質を劣化させるのみならず、電気消費量の不要な増大によるエネルギー浪費を招くという問題が発生する。 However, in the method of applying a flow rate to the molten steel in front of the solidified shell using an electromagnetic stirrer, a flow rate higher than necessary may be applied to the molten steel. In such a case, the mold added on the molten steel surface in the mold The entrainment of powder occurs, and on the contrary, the quality of the slab surface layer is deteriorated, and there is a problem that energy is wasted due to an unnecessary increase in electric consumption.
その他の対策として、特許文献２及び特許文献３には、凝固殻前面での溶鋼中のＣ、Ｓ、Ｎ、Ｏの濃度勾配による表面張力を制御することにより、気泡の凝固殻への捕捉を抑制する方法、つまり、表面張力が所定値以下になるように、溶鋼中のＣ、Ｓ、Ｎ、Ｏの濃度を予め調整してから連続鋳造する方法が提案されている。 As other countermeasures, Patent Document 2 and Patent Document 3 capture bubbles in the solidified shell by controlling the surface tension due to the concentration gradient of C, S, N, and O in the molten steel in front of the solidified shell. A method of suppressing casting, that is, a method of continuous casting after adjusting the concentrations of C, S, N, and O in the molten steel in advance so that the surface tension becomes a predetermined value or less has been proposed.
しかしながら、特許文献２及び特許文献３では、アルミナクラスターの凝固殻への捕捉に関しては検討していない。また、溶鋼成分に応じて気泡の凝固殻への捕捉が左右されることを示唆するものの、気泡の捕捉と凝固界面での溶鋼流速との関係が明らかになっておらず、気泡の捕捉を定量的に把握することができない。これは、実際の鋳型内においては、Ｃ、Ｓ、Ｎ、Ｏの濃度分布による表面張力（＝凝固殻への捕捉力）と同時に、溶鋼流速による抗力もはたらいており、凝固殻への気泡や非金属介在物の捕捉を検討する場合には、溶鋼流速による抗力も考慮しなければならないからである。
上記説明のように、自動車用鋼板などの厳格な品質が要求される鋼板の素材となる鋳片を、生産性を損なわずに且つ安価に製造することが切望されているにも拘わらず、従来、有効な手段はなく、鋳片の表層部にはアルミナクラスターによる欠陥が発生し、やむなくスカーファーなどを用いて溶削して欠陥を除去しており、製造コストの上昇をもたらしていた。 As described above, in spite of the desire to produce a slab, which is a raw material of a steel plate that requires strict quality such as a steel plate for automobiles, at low cost without impairing productivity, the conventional However, there was no effective means, and defects due to alumina clusters occurred in the surface layer portion of the slab, and the defects were inevitably removed by scouring with a scarfer, resulting in an increase in manufacturing cost.
本発明は上記事情に鑑みてなされたもので、その目的とするところは、鋳片の表層部にアルミナクラスターなどの非金属介在物による欠陥が少なく、清浄で高品質の鋳片を、生産性を損なわずに、安価に且つ安定して製造することのできる、鋼鋳片の連続鋳造方法を提供することである。 The present invention has been made in view of the above circumstances. The object of the present invention is to produce a clean, high-quality slab with less defects due to non-metallic inclusions such as alumina clusters in the surface portion of the slab. It is an object to provide a continuous casting method of a steel slab that can be manufactured inexpensively and stably without impairing the above.
本発明者らは、上記課題を解決すべく、鋭意研究・検討を行った。その結果、鋳片の表層部にアルミナクラスターなどの非金属介在物による欠陥が少なく、清浄で高品質な、自動車用鋼板などの厳格な品質が要求される鋼板の素材となる鋳片を、生産性を損なわずに、安価に且つ安定して製造するためには、電磁攪拌装置を利用する或いは浸漬ノズルの吐出孔から吐出される吐出流を利用するなどして、凝固界面の溶鋼に流速を与え、アルミナクラスターを洗浄することを第１の条件とした上で、モールドパウダーの巻き込みなどを防止するために、それぞれの鋼種の化学成分に応じた適切な溶鋼流速を付与することが必要であるとの知見が得られた。 In order to solve the above-mentioned problems, the present inventors have intensively studied and studied. As a result, it produces slabs that are made of steel plate materials that are free from defects due to non-metallic inclusions such as alumina clusters in the surface layer of slabs, and that require strict quality such as automotive steel plates that are clean and high quality. In order to manufacture stably and inexpensively without impairing the properties, the flow rate of the molten steel at the solidification interface is reduced by using an electromagnetic stirrer or using the discharge flow discharged from the discharge hole of the immersion nozzle. Given that the first condition is to clean the alumina cluster, it is necessary to provide an appropriate molten steel flow rate according to the chemical composition of each steel type in order to prevent entrainment of mold powder and the like. And the knowledge was obtained.
本発明は、上記知見に基づいてなされたものであり、第１の発明に係る鋼鋳片の連続鋳造方法は、Ｃを０．００３質量％以下含有する極低炭素鋼鋳片の連続鋳造方法であって、溶鋼成分における、４４８５３×［質量％Ｔｉ］と２６６１７５０×［質量％Ｓ］と４７０９８６３×［質量％Ｏ］との和が２５０００を超える場合は、凝固殻前面での溶鋼流速が下記の（１）式の範囲内となるように制御して鋳造することを特徴とするものである。
(1/41)×[7-10000/(44853×[Ti]+2661750×[S]+4709863×[O]-25000)]≦V…(1)
但し、（１）式において、Ｖは、凝固殻前面での溶鋼流速（ｍ／秒）、［Ｔｉ］は、溶鋼のＴｉ濃度（質量％）、［Ｓ］は、溶鋼のＳ濃度（質量％）、［Ｏ］は、溶鋼のＯ（溶存酸素）濃度（質量％）である。
The present invention has been made on the basis of the above knowledge, and the continuous casting method of a steel slab according to the first invention is a continuous casting method of an ultra-low carbon steel slab containing 0.003% by mass or less of C. When the sum of 44853 × [mass% Ti], 2661750 × [mass% S] and 4709863 × [mass% O] in the molten steel component exceeds 25000, the molten steel flow velocity at the front of the solidified shell is as follows: The casting is controlled so as to be within the range of the formula (1).
(1/41) × [7-10000 / (44853 × [Ti] + 2661750 × [S] + 4709863 × [O] -25000)] ≦ V… (1)
However, in the formula (1), V is a molten steel flow velocity (m / sec) in front of the solidified shell, [Ti] is a Ti concentration (mass%) of the molten steel, and [S] is an S concentration (mass%) of the molten steel. ), [O] is the O (dissolved oxygen) concentration (mass%) of the molten steel.
第２の発明に係る鋼鋳片の連続鋳造方法は、第１の発明において、前記極低炭素鋼は、Ｃ以外の化学成分として、Ｓｉ：０．０５質量％以下、Ｍｎ：１．０質量％以下、Ｐ：０．０５質量％以下、Ｓ：０．０１５質量％以下、Ａｌ：０．０１０〜０．０７５質量％、Ｔｉ：０．０５質量％以下を含有し、残部がＦｅ及び不可避的不純物からなることを特徴とするものである。 In the continuous casting method of the steel slab according to the second invention, in the first invention, the ultra-low carbon steel contains, as a chemical component other than C, Si: 0.05 mass% or less, Mn: 1.0 mass. %: P: 0.05% by mass or less, S: 0.015% by mass or less, Al: 0.010 to 0.075% by mass, Ti: 0.05% by mass or less, with the balance being Fe and inevitable It is characterized by comprising an impurity.
第３の発明に係る鋼鋳片の連続鋳造方法は、第１または第２の発明において、前記凝固殻前面での溶鋼流速を、鋳型背面に配置した交流移動磁場印加装置によって制御することを特徴とするものである。 In the continuous casting method of a steel slab according to the third invention, in the first or second invention, the molten steel flow velocity at the front surface of the solidified shell is controlled by an AC moving magnetic field applying device arranged at the back surface of the mold. It is what.
本発明によれば、凝固殻前面の溶鋼流速を溶鋼成分に応じた適切な流速に制御するので、モールドパウダーの巻き込みも発生せず、アルミナクラスターなどの非金属介在物による表面欠陥が極めて少なく、清浄で高品質の鋳片を、生産性を損なわずに、安価に且つ安定して製造することが達成される。 According to the present invention, the molten steel flow velocity in front of the solidified shell is controlled to an appropriate flow velocity according to the molten steel component, so that no entrainment of mold powder occurs, and surface defects due to non-metallic inclusions such as alumina clusters are extremely small. A clean and high-quality slab can be produced inexpensively and stably without impairing productivity.
以下、本発明を説明する。 The present invention will be described below.
Ｃの含有量が０．００３質量％以下である極低炭素鋼は、転炉における大気下での脱炭精錬と、ＲＨ真空脱ガス装置などの真空脱ガス設備における減圧下での脱炭精錬（「真空脱炭精錬」という）との二回の脱炭精錬により、溶銑から溶製される。脱炭精錬は溶鋼中の溶存酸素濃度が或る程度高くならないと進行せず、従って、脱炭精錬終了時には溶鋼中に多くの溶存酸素（「フリー酸素」ともいう）が残留する。多くの溶存酸素が残留したままでは鋼の清浄性が劣化するので、極低炭素鋼の溶製工程においては、真空脱炭精錬が終了した後に溶鋼中に金属Ａｌが添加され、溶鋼は脱酸処理される。この脱酸処理により、溶鋼中の溶存酸素濃度は急激に低下し、脱酸生成物としてアルミナが形成される。尚、アルミナ中の酸素はＡｌと化学結合しており、アルミナが溶鋼中に懸濁していても、アルミナ中の酸素は溶存酸素とはいわない。 An ultra-low carbon steel having a C content of 0.003% by mass or less is decarburized and refined under atmospheric pressure in a converter and decarburized and refined under reduced pressure in a vacuum degassing facility such as an RH vacuum degassing apparatus. It is made from hot metal by decarburizing and refining twice ("vacuum decarburizing and refining"). Decarburization refining does not proceed unless the concentration of dissolved oxygen in the molten steel is increased to some extent. Therefore, a large amount of dissolved oxygen (also referred to as “free oxygen”) remains in the molten steel at the end of decarburization refining. Since the cleanliness of steel deteriorates when a large amount of dissolved oxygen remains, in the melting process of ultra-low carbon steel, metal Al is added to the molten steel after vacuum decarburization refining, and the molten steel is deoxidized. It is processed. By this deoxidation treatment, the dissolved oxygen concentration in the molten steel is rapidly lowered, and alumina is formed as a deoxidation product. Note that oxygen in alumina is chemically bonded to Al, and even if alumina is suspended in molten steel, oxygen in alumina is not called dissolved oxygen.
脱酸生成物として生成したアルミナは、溶鋼が、真空脱ガス設備から連続鋳造設備に搬送される期間及びタンディッシュに注入された後に鋳型内に注入されるまでの期間、時間の経過とともに凝集してアルミナクラスターを形成する。このアルミナクラスターが溶鋼とともに鋳型内に注入されて凝固殻に捕捉されると、極低炭素鋼鋳片の表面欠陥となり、鋳片の品質が低下する。 The alumina produced as a deoxidation product agglomerates with the passage of time for the period during which molten steel is transported from the vacuum degassing equipment to the continuous casting equipment and the time it is injected into the mold after being injected into the tundish. To form an alumina cluster. When this alumina cluster is injected into the mold together with molten steel and trapped in the solidified shell, it becomes a surface defect of the ultra-low carbon steel slab, and the quality of the slab deteriorates.
本発明者らは、アルミナクラスターの凝固殻への捕捉に及ぼす溶鋼の化学成分及び凝固界面での溶鋼流速の影響について研究を重ね、その結果、以下の手段によって上記課題を解決できるとの知見を得た。即ち、「Ｃを０．００３質量％以下含有する極低炭素鋼鋳片を連続鋳造する際に、溶鋼成分における、４４８５３×［質量％Ｔｉ］と２６６１７５０×［質量％Ｓ］と４７０９８６３×［質量％Ｏ］との和が２５０００を超える場合は、凝固殻前面での溶鋼流速が下記の（１）式の範囲内となるように制御して鋳造する」という方法である。
(1/41)×[7-10000/(44853×[Ti]+2661750×[S]+4709863×[O]-25000)]≦V…(1)
但し、（１）式において、Ｖは、凝固殻前面での溶鋼流速（ｍ／秒）、［Ｔｉ］は、溶鋼のＴｉ濃度（質量％）、［Ｓ］は、溶鋼のＳ濃度（質量％）、［Ｏ］は、溶鋼のＯ（溶存酸素）濃度（質量％）である。
The present inventors have repeatedly studied the influence of the chemical composition of molten steel on the trapping of alumina clusters in the solidified shell and the molten steel flow velocity at the solidification interface, and as a result, have found that the above problems can be solved by the following means. Obtained. That is, “when continuously casting an ultra-low carbon steel slab containing 0.003% by mass or less of C, 44853 × [mass% Ti], 2661750 × [mass% S] and 4709863 × [mass of the molten steel component When the sum of% O] exceeds 25000, the molten steel flow velocity at the front surface of the solidified shell is controlled so as to be within the range of the following formula (1).
(1/41) × [7-10000 / (44853 × [Ti] + 2661750 × [S] + 4709863 × [O] -25000)] ≦ V… (1)
However, in the formula (1), V is a molten steel flow velocity (m / sec) in front of the solidified shell, [Ti] is a Ti concentration (mass%) of the molten steel, and [S] is an S concentration (mass%) of the molten steel. ), [O] is the O (dissolved oxygen) concentration (mass%) of the molten steel.
ここで、（１）式における「44853×[Ti]+2661750×[S]+4709863×[O]-25000」は、連続鋳造中の凝固殻前面に形成される溶質元素（以下、単に「溶質」とも記す）の濃度境界層に侵入したアルミナクラスターに働く、界面張力勾配による凝固殻方向への引力の尺度を示している。 Here, “44853 × [Ti] + 2661750 × [S] + 4709863 × [O] -25000” in the formula (1) is a solute element (hereinafter simply referred to as “solute” formed on the front surface of the solidified shell during continuous casting. It shows a measure of the attractive force in the direction of the solidified shell due to the interfacial tension gradient acting on the alumina cluster that has entered the concentration boundary layer.
刊行物：鉄と鋼（80(1994)p.527）に示されるように、凝固界面前面の濃度境界層中の界面張力勾配Ｋ、即ちｄσ／ｄｘ（σ：界面張力、ｘ：距離）によって介在物が凝固殻方向に受ける力Ｆは、下記の（３）式で示される。 Publication: As shown in iron and steel (80 (1994) p. 527), depending on the interfacial tension gradient K in the concentration boundary layer in front of the solidification interface, ie dσ / dx (σ: interfacial tension, x: distance) The force F received by the inclusions in the direction of the solidified shell is expressed by the following equation (3).
F＝-(8/3)×πR2K…(3)
ここで、Ｆは介在物の受ける力（Ｎ）、πは円周率、Ｒは介在物の半径（ｍ）、Ｋは界面張力勾配（Ｎ／ｍ2）である。この界面張力勾配Ｋは、下記の（４）式に示すように、界面張力の溶質濃度による変化と成分の濃度勾配との積である。
F =-(8/3) × πR 2 K… (3)
Here, F is the force (N) received by the inclusions, π is the circumference, R is the radius (m) of the inclusions, and K is the interfacial tension gradient (N / m 2 ). This interfacial tension gradient K is the product of the change in interfacial tension due to the solute concentration and the component concentration gradient, as shown in the following equation (4).
K＝dσ/dx＝(dσ/dc)×(dc/dx)…(4)
ここで、σは溶鋼の界面張力（Ｎ／ｍ）、ｘは凝固界面からの距離（ｍ）である。また、ｄσ／ｄｃは界面張力の溶質濃度による変化（Ｎ／ｍ・質量％）、ｄｃ／ｄｘは成分の濃度勾配（質量％／ｍ）である。
K = dσ / dx = (dσ / dc) × (dc / dx) (4)
Here, σ is the interfacial tension (N / m) of the molten steel, and x is the distance (m) from the solidification interface. Dσ / dc is the change in interfacial tension due to the solute concentration (N / m · mass%), and dc / dx is the concentration gradient (mass% / m) of the components.
凝固理論から、鋳型内のような溶鋼流速が存在する条件下での成分の濃度勾配ｄｃ／ｄｘは下記の（５）式で表される。 From the solidification theory, the concentration gradient dc / dx of the component under the condition where the molten steel flow velocity exists in the mold is expressed by the following equation (5).
dc/dx＝-C0×(1-K0)×(Vs/D)×exp[-Vs×(x-δ)/D]…(5)
ここで、Ｃ0は鋳造前の溶鋼中の溶質濃度（質量％）、Ｋ0は溶質の分配係数（−）、Ｖsは凝固速度（ｍ／秒）、Ｄは溶鋼中での溶質の拡散係数（ｍ2／秒）、δは濃度境界層の厚み（ｍ）である。
dc / dx = -C 0 × (1-K 0 ) × (V s / D) × exp [-V s × (x-δ) / D] ... (5)
Here, C 0 is the solute concentration (mass%) in the molten steel before casting, K 0 is the solute distribution coefficient (−), V s is the solidification rate (m / sec), and D is the diffusion of the solute in the molten steel. The coefficient (m 2 / sec), δ is the thickness (m) of the concentration boundary layer.
（５）式において、ｘ＝δを代入すると、ｘ＝δでの濃度勾配（ｄｃ／ｄｘ）は下記の（６）式で求められる。 In the equation (5), when x = δ is substituted, the concentration gradient (dc / dx) at x = δ is obtained by the following equation (6).
ｄc/dx＝-Ci×(1-K0)×(Vs/D)…(6)
（６）式を（４）式に代入することにより、アルミナクラスターが濃度境界層に侵入した直後に作用する力の尺度を示す界面張力勾配Ｋを下記の（７）式により求めることができる。
dc / dx = −Ci × (1-K 0 ) × (V s / D) (6)
By substituting the equation (6) into the equation (4), the interfacial tension gradient K indicating the scale of the force acting immediately after the alumina cluster enters the concentration boundary layer can be obtained by the following equation (7).
K＝(dσ/dc)×[-Ci×(1-K0)×(Vs/D)]…(7)
（７）式に示すｄσ／ｄｃは、刊行物：溶鉄と溶滓の物性値便覧（日本鉄鋼協会編）などに示されており、極低炭素鋼の化学成分元素のなかで界面張力勾配Ｋの値に大きな影響を及ぼす元素は、Ｔｉ（チタン）、Ｓ（硫黄）、Ｏ（酸素＝溶存酸素）であり、これらの元素だけで計算した界面張力勾配Ｋの値を用いても、アルミナクラスターの凝固殻への捕捉を検討する上で問題ないことが分かった。また、各溶質の分配係数Ｋ0や拡散係数Ｄは、刊行物：金属データブック（日本金属学会編）などに示されており、凝固速度Ｖsは、伝熱計算から求めることができる。
K = (dσ / dc) x [-Ci x (1-K 0 ) x (V s / D)] (7)
The dσ / dc shown in the equation (7) is shown in publications: Handbook of physical properties of molten iron and hot metal (edited by the Japan Iron and Steel Institute), etc., and the interfacial tension gradient K among the chemical constituent elements of extremely low carbon steel. Elements that greatly affect the value of Ti are Ti (titanium), S (sulfur), and O (oxygen = dissolved oxygen). Even if the value of the interfacial tension gradient K calculated only with these elements is used, the alumina cluster It was found that there is no problem in considering the trapping of the solid in the solidified shell. Further, the partition coefficient K 0 and the diffusion coefficient D of each solute are shown in the publication: Metal Data Book (edited by the Japan Institute of Metals), and the solidification rate V s can be obtained from heat transfer calculation.
従って、それぞれの元素の界面張力の溶質濃度による変化ｄσ／ｄｃ、分配係数Ｋ0、拡散係数Ｄ、及び、鋳型内における凝固速度Ｖsを（７）式に代入することにより、濃度境界層においてアルミナクラスターに働く、Ｔｉ、Ｓ及びＯによる界面張力勾配による凝固殻方向への引力として、（１）式に示す「44853×[Ti]+2661750×[S]+4709863×[O]」を得ることができる。 Accordingly, by substituting the change dσ / dc of the interfacial tension of each element due to the solute concentration, the distribution coefficient K 0 , the diffusion coefficient D, and the solidification rate V s in the mold into the equation (7), in the concentration boundary layer “44853 × [Ti] + 2661750 × [S] + 4709863 × [O]” shown in Equation (1) is obtained as the attractive force in the direction of the solidified shell due to the interfacial tension gradient due to Ti, S and O acting on the alumina cluster. be able to.
また、本発明者らは、種々の組成の溶鋼を使用してアルミナクラスターの捕捉の頻度を調査した。その結果、図１に示すように、（１）式に示す「44853×[Ti]+2661750×[S]+4709863×[O]」の値と、凝固殻に捕捉される単位面積あたりのアルミナクラスターの面積とは、比例関係にあることを見出した。ここで、アルミナクラスターの面積とは、アルミナクラスターの長軸及び短軸を光学顕微鏡で測定し、楕円体としての面積を算出し、このようにして測定されたアルミナクラスターの面積を総和した値である。 In addition, the present inventors investigated the frequency of capture of alumina clusters using molten steel having various compositions. As a result, as shown in FIG. 1, the value of “44853 × [Ti] + 2661750 × [S] + 4709863 × [O]” shown in equation (1) and the alumina per unit area captured by the solidified shell It was found that there is a proportional relationship with the area of the cluster. Here, the area of the alumina cluster is a value obtained by measuring the major axis and the minor axis of the alumina cluster with an optical microscope, calculating the area as an ellipsoid, and summing up the areas of the alumina cluster thus measured. is there.
また、濃度境界層中のアルミナクラスターには界面張力勾配によって凝固界面側に向いた引力が働くが、溶鋼流の抗力により、図１に示すように、「44853×[Ti]+2661750×[S]+4709863×[O]」の値が２５０００以下であると、凝固殻にアルミナクラスターが捕捉されないということを見出した。更に、図１に示すように、「44853×[Ti]+2661750×[S]+4709863×[O]」の値と、凝固殻に捕捉される単位面積あたりのアルミナクラスターの面積との比例定数は、凝固界面前面における溶鋼流速によって変化することが分かった。 In addition, an attractive force directed to the solidification interface side acts on the alumina cluster in the concentration boundary layer due to the interfacial tension gradient, but due to the drag of the molten steel flow, as shown in FIG. 1, “44853 × [Ti] + 2661750 × [S It was found that when the value of “] + 4709863 × [O]” was 25000 or less, alumina clusters were not trapped in the solidified shell. Further, as shown in FIG. 1, a proportional constant between the value of “44853 × [Ti] + 2661750 × [S] + 4709863 × [O]” and the area of the alumina cluster per unit area trapped in the solidified shell. Was found to vary with the molten steel flow velocity in front of the solidification interface.
図２に、図１における直線の傾き、つまり比例定数と、凝固界面前面における溶鋼流速との関係を示す。図２に示すように、凝固殻に捕捉される単位面積あたりのアルミナクラスターの面積の、「44853×[Ti]+2661750×[S]+4709863×[O]」の値に対する比例定数は、凝固界面前面における溶鋼流速（Ｖ）の関数ｆ（Ｖ）であり、関数ｆ（Ｖ）は下記の（８）式に示す回帰式で表されることを見出した。 FIG. 2 shows the relationship between the slope of the straight line in FIG. 1, that is, the proportionality constant, and the molten steel flow velocity in front of the solidification interface. As shown in Fig. 2, the proportionality constant of the area of the alumina cluster per unit area trapped in the solidified shell to the value of "44853 x [Ti] + 2661750 x [S] + 4709863 x [O]" It was a function f (V) of the molten steel flow velocity (V) in front of the interface, and the function f (V) was found to be represented by a regression equation shown in the following equation (8).
f(V)＝7×10-7-41×10-7V＝10-7×(7-41V)…(8)
但し、（８）式におけるＶは凝固殻前面における溶鋼流速（ｍ／秒）である。
f (V) = 7 × 10 -7 -41 × 10 -7 V = 10 -7 × (7-41V)… (8)
However, V in Formula (8) is a molten steel flow velocity (m / sec) in front of the solidified shell.
従って、下記の（９）式に示すように、溶鋼中のＴｉ、Ｓ、Ｏによる界面張力勾配の総和の２５０００を超えた分に、凝固界面前面の溶鋼流速によって決定する比例定数を掛け合わせれば、凝固殻に捕捉される単位面積あたりのアルミナクラスターの面積Ｉ（ｍｍ2／ｍｍ2）を求めることができる。 Therefore, as shown in the following formula (9), if the sum of the interfacial tension gradients due to Ti, S, and O in the molten steel exceeds 25,000, the proportional constant determined by the molten steel flow velocity in front of the solidification interface is multiplied. The area I (mm 2 / mm 2 ) of the alumina cluster per unit area captured by the solidified shell can be determined.
I＝10-7×(7-41V)×(44853×[Ti]+2661750×[S]+4709863×[O]-25000)…(9)
また、自動車用極低炭素鋼において、単位面積あたりのアルミナクラスターの面積Ｉ（ｍｍ2／ｍｍ2）が０．００１を超えると、表面欠陥が発生することが分かった。即ち、（９）式の左辺のＩの範囲を０．００１以下とし、凝固殻前面の溶鋼流速Ｖの範囲を求めた式が、前述した（１）式である。凝固殻前面つまり凝固界面前面の溶鋼流速Ｖを（１）式の範囲内に制御することで、アルミナクラスターの凝固殻への捕捉が防止される。凝固殻前面の溶鋼流速を制御する範囲は、鋳片の表層部に相当する範囲であり、具体的には、鋳型内溶鋼湯面の位置から凝固殻が１０ｍｍないし１５ｍｍ程度となる位置までの範囲で十分である。当然ながら、更に鋳造方向下方の範囲までとしても構わない。
I = 10 -7 × (7-41V) × (44853 × [Ti] + 2661750 × [S] + 4709863 × [O] -25000)… (9)
Further, in automotive ultra low carbon steel, the area of the alumina clusters per unit area I (mm 2 / mm 2) is more than 0.001, surface defects were found to occur. That is, the equation (1) described above is obtained by setting the range of I on the left side of the equation (9) to 0.001 or less and obtaining the range of the molten steel flow velocity V in front of the solidified shell. By controlling the flow velocity V of the molten steel in front of the solidified shell, that is, in front of the solidified interface, within the range of the formula (1), it is possible to prevent the alumina cluster from being trapped in the solidified shell. The range for controlling the molten steel flow velocity on the front surface of the solidified shell is a range corresponding to the surface layer portion of the slab, and specifically, the range from the position of the molten steel surface in the mold to the position where the solidified shell is about 10 mm to 15 mm. Is enough. Of course, it may be further up to a range below the casting direction.
凝固界面前面の溶鋼流速を制御する方法としては、タンディッシュ内の溶鋼を鋳型内に注入するための浸漬ノズルの吐出孔の大きさ、角度、浸漬深さなどを調整し、吐出孔から吐出される溶鋼の吐出流を利用する方法や、鋳型背面に配置した磁場印加装置から磁場を印加し、磁場と溶鋼流とで形成される電磁力を利用する方法などを用いることができる。磁場発生装置としては、交流移動印加装置と直流磁場（静磁場）印加装置とがあるが、鋳造速度が変更されても、凝固殻前面の溶鋼流速を任意に調整することができることから、交流移動磁場印加装置を用いることが好ましい。特に、鋳型長辺の背面全幅に配置した交流移動磁場印加装置によって制御することが好ましい。 As a method of controlling the molten steel flow velocity in front of the solidification interface, the size, angle, immersion depth, etc. of the discharge hole of the immersion nozzle for injecting the molten steel in the tundish into the mold are adjusted and discharged from the discharge hole. A method using a discharge flow of molten steel, a method using a magnetic field applied from a magnetic field application device arranged on the back of a mold, and using an electromagnetic force formed by the magnetic field and the molten steel flow can be used. There are two types of magnetic field generators: an alternating current movement application device and a direct current magnetic field (static magnetic field) application device. Even if the casting speed is changed, the molten steel flow velocity in front of the solidified shell can be adjusted arbitrarily. It is preferable to use a magnetic field application device. In particular, it is preferable to control by an alternating-current moving magnetic field applying device arranged over the entire back surface of the long side of the mold.
スラブ連続鋳造機の鋳型長辺背面全幅に鋳片を挟んで相対させて交流移動磁場印加装置を配置し、この交流移動磁場印加装置から印加する移動磁場の移動方向を、相対する磁場印加装置ともに鋳型短辺側から浸漬ノズル側に向かう方向とすることで、浸漬ノズルから吐出される溶鋼の吐出流は減速され、これに伴って凝固界面前面の溶鋼流速が減速（「減速磁場印加」と称す）し、逆に、交流移動磁場印加装置から印加する移動磁場の移動方向を、相対する磁場印加装置ともに浸漬ノズル側から鋳型短辺側に向かう方向とすることで、浸漬ノズルから吐出される溶鋼の吐出流は加速され、これに伴って凝固界面前面の溶鋼流速が増速（「加速磁場印加」と称す）する。更に、一方の鋳型長辺の背面に配置した交流移動磁場印加装置から印加する移動磁場の移動方向を同一方向とし、且つ、鋳片を挟んで相対する交流移動磁場印加装置から印加する移動磁場の移動方向をこれとは逆方向とすることで、鋳型内の溶鋼は水平方向に回転するように攪拌され、これに伴って凝固界面前面の溶鋼流速が増速（「旋回磁場印加」と称す）する。 An AC moving magnetic field application device is placed across the entire width of the back side of the long side of the mold of the slab continuous casting machine, and the moving direction of the moving magnetic field applied from this AC moving magnetic field application device is set to the opposite magnetic field application device. By setting the direction from the mold short side to the immersion nozzle side, the discharge flow of the molten steel discharged from the immersion nozzle is decelerated, and the molten steel flow velocity in front of the solidification interface is reduced accordingly (referred to as “deceleration magnetic field application”). Conversely, the molten steel discharged from the immersion nozzle is set so that the moving magnetic field applied from the AC moving magnetic field application device is in the direction from the immersion nozzle side to the mold short side with the opposite magnetic field application device. As a result, the molten steel flow velocity in front of the solidification interface is increased (referred to as “acceleration magnetic field application”). Further, the moving magnetic field applied from the AC moving magnetic field applying device arranged on the back side of one long side of the mold is set to the same direction, and the moving magnetic field applied from the AC moving magnetic field applying device opposite to the slab is sandwiched. By moving the moving direction in the opposite direction, the molten steel in the mold is agitated so as to rotate in the horizontal direction, and the molten steel flow velocity in front of the solidification interface is increased accordingly (referred to as “swirl magnetic field application”). To do.
このように、鋳型長辺の背面全幅に配置した交流移動磁場印加装置により、鋳造速度に応じて適宜選択した３種類の磁場印加パターンで磁場を印加することで、凝固界面前面の溶鋼流速を減速或いは加速することができ、鋳造速度の如何に拘わらず、凝固界面前面の溶鋼流速を任意の流速に制御することが可能となる。 In this way, the flow velocity of the molten steel at the front of the solidification interface is reduced by applying a magnetic field with three types of magnetic field application patterns appropriately selected according to the casting speed by the AC moving magnetic field application device arranged at the entire back surface of the mold long side. Alternatively, it can be accelerated, and the molten steel flow velocity in front of the solidification interface can be controlled to an arbitrary flow velocity regardless of the casting speed.
直流磁場印加装置の場合は、磁場印加装置をスラブ連続鋳造機の鋳型長辺背面に鋳片を挟んで相対させて配置し、鋳型の厚み方向に貫通する磁場を印加することで、移動する溶鋼に制動力が付与されて、凝固界面前面の溶鋼流速が制御される。直流磁場印加装置の場合、溶鋼は減速されるだけではなく、浸漬ノズルからの溶鋼吐出流は直流磁場印加装置を迂回するように流れるので、直流磁場印加装置の設置位置によっては、凝固界面前面の溶鋼流速はかえって増加することも発生する。 In the case of a direct current magnetic field application device, the magnetic field application device is placed opposite to the back of the long side of the mold of the slab continuous casting machine with the slab interposed therebetween, and the molten steel moves by applying a magnetic field penetrating in the mold thickness direction. Is applied with a braking force to control the molten steel flow velocity in front of the solidification interface. In the case of a DC magnetic field application device, the molten steel is not only decelerated, but the molten steel discharge flow from the immersion nozzle flows so as to bypass the DC magnetic field application device, so depending on the installation position of the DC magnetic field application device, The molten steel flow velocity may also increase.
但し、攪拌強度が強くなりすぎるなどして凝固界面前面の溶鋼流速が速くなりすぎると、それに応じて鋳型内溶鋼湯面の溶鋼流が強くなり、鋳型内溶鋼湯面上に添加したモールドパウダーの巻き込みが発生するので、モールドパウダーの巻き込みが発生しない範囲内で、凝固界面前面の溶鋼流速を制御することが好ましい。公知文献に基づけば、鋳型内溶鋼湯面の流速が０．５ｍ／秒以下であれば、モールドパウダーの巻き込みが発生しないことから、鋳型内溶鋼湯面の流速が０．５ｍ／秒以下の範囲内となるように、凝固界面前面の溶鋼流速を制御すればよい。 However, if the molten steel flow velocity at the front of the solidification interface becomes too high due to excessively strong stirring strength, the molten steel flow on the molten steel surface in the mold will increase accordingly, and the mold powder added on the molten steel surface in the mold will Since entrainment occurs, it is preferable to control the molten steel flow velocity in front of the solidification interface within a range in which entrainment of mold powder does not occur. Based on the known literature, if the flow rate of the molten steel surface in the mold is 0.5 m / second or less, the mold powder does not entrain, so the flow rate of the molten steel surface in the mold is in the range of 0.5 m / second or less. What is necessary is just to control the molten steel flow velocity of the solidification interface front surface so that it may become inside.
本発明は、Ｃの含有量が０．００３質量％以下である極低炭素鋼である限り、鋼種を問わずに適用できることは勿論であるが、得られる効果の点からすれば、Ｃ以外の化学成分として、Ｓｉ：０．０５質量％以下、Ｍｎ：１．０質量％以下、Ｐ：０．０５質量％以下、Ｓ：０．０１５質量％以下、Ａｌ：０．０１０〜０．０７５質量％、Ｔｉ：０．０５質量％以下を含有し、残部がＦｅ及び不可避的不純物からなる鋼を対象としたときに、特に効果が著しい。また、Ｎｂを０．０３０質量％以下及び／またはＳｂを０．０１５質量％以下含有する鋼にも効果が著しい。 The present invention can be applied to any steel type as long as it is an ultra-low carbon steel having a C content of 0.003% by mass or less. As chemical components, Si: 0.05 mass% or less, Mn: 1.0 mass% or less, P: 0.05 mass% or less, S: 0.015 mass% or less, Al: 0.010 to 0.075 mass %, Ti: 0.05% by mass or less, and the effect is particularly remarkable when the steel is made of Fe and inevitable impurities. Moreover, the effect is remarkable also in steel containing 0.030 mass% or less of Nb and / or 0.015 mass% or less of Sb.
以上説明したように、本発明によれば、凝固殻前面の溶鋼流速を溶鋼成分に応じた適切な流速に制御するので、モールドパウダーの巻き込みも発生せず、アルミナクラスターなどの非金属介在物による表面欠陥が少なく、清浄で高品質の鋳片を、生産性を損なわずに、安価に且つ安定して製造することが可能となる。 As described above, according to the present invention, the molten steel flow velocity on the front surface of the solidified shell is controlled to an appropriate flow velocity according to the molten steel component, so that no entrainment of mold powder occurs, and non-metallic inclusions such as alumina clusters. A clean and high-quality slab with few surface defects can be produced inexpensively and stably without impairing productivity.
以下、スラブ連続鋳造機で実施した８チャージの試験鋳造結果を説明する。 Hereinafter, the test casting result of 8 charges performed with the slab continuous casting machine will be described.
１チャージ約２００トンの８チャージ（試験Ｎo.１〜８）の極低炭素鋼の溶鋼を、厚みが３００ｍｍ、幅が１５６０ｍｍのスラブ鋳片に、溶鋼鋳造量を３．７５トン／分として鋳造した。各試験チャージの溶鋼の化学成分を表１に示す。スラブ連続鋳造機では、これらの溶鋼を、凝固界面前面での溶鋼流速が、前述した（１）式の範囲を満たす条件と、（１）式の範囲を満たさない条件とで鋳造した。つまり、表１に（１）式から求めた必要最低流速を示しているが、試験Ｎo.１〜４では（１）式の範囲を満たす条件（本発明例）とし、試験Ｎo.５〜８では（１）式の範囲を満たさない条件（比較例）とした。 Casting an ultra-low carbon steel of 8 charges (test No. 1-8) of about 200 tons per charge into a slab slab having a thickness of 300 mm and a width of 1560 mm with a molten steel casting amount of 3.75 tons / min. did. Table 1 shows the chemical composition of the molten steel for each test charge. In the slab continuous casting machine, these molten steels were cast under the conditions that the molten steel flow velocity at the front surface of the solidification interface satisfies the range of the formula (1) and the conditions that do not satisfy the range of the formula (1). That is, Table 1 shows the necessary minimum flow velocity obtained from the equation (1). In Test Nos. 1 to 4, the conditions satisfying the range of the equation (1) (examples of the present invention) are used, and the test Nos. 5 to 8 are performed. Then, it was set as the conditions (comparative example) which do not satisfy | fill the range of (1) Formula.
凝固界面前面での溶鋼流速は、鋳片を挟んで鋳型長辺の背面全幅に配置した交流移動磁場印加装置を用いて制御した。具体的には、凝固界面前面での溶鋼流速を０．０５ｍ／秒とする場合には、磁束密度が０．１０テスラの減速磁場印加とし、凝固界面前面での溶鋼流速を０．１０ｍ／秒とする場合には、磁束密度が０．０８テスラの減速磁場印加とし、凝固界面前面での溶鋼流速を０．１５ｍ／秒とする場合には、磁束密度が０．０５テスラの減速磁場印加とし、凝固界面前面での溶鋼流速を０．２０ｍ／秒とする場合には、磁束密度が０．０２テスラの減速磁場印加とした。 The molten steel flow velocity at the front surface of the solidification interface was controlled by using an AC moving magnetic field application device disposed across the entire width of the back surface of the mold long side with the slab interposed therebetween. Specifically, when the molten steel flow velocity at the front of the solidification interface is 0.05 m / second, a deceleration magnetic field with a magnetic flux density of 0.10 Tesla is applied, and the molten steel flow velocity at the front of the solidification interface is 0.10 m / second. If the magnetic flux density is 0.08 Tesla, the decelerating magnetic field application is 0.08 Tesla, and if the molten steel flow velocity at the solidification interface is 0.15 m / sec, the magnetic flux density is 0.05 Tesla decelerating magnetic field application. When the molten steel flow velocity at the front surface of the solidification interface was 0.20 m / sec, a decelerating magnetic field with a magnetic flux density of 0.02 Tesla was applied.
凝固界面前面での溶鋼流速は、鋳造後の鋳片から試料を採取し、その試料の凝固組織から確認した。即ち、鋳造後の鋳片から鋳造方向長さが３００ｍｍの全厚（３００ｍｍ）×全幅（１５６０ｍｍ）の試料を採取し、この試料から図３に示す６箇所の位置から検鏡用試料を切り出し、鏡面仕上げした後に酸で腐食し、凝固組織を現出させ、凝固組織のデンドライト樹枝状晶の傾き角度から、岡野らの式（刊行物：鉄と鋼（61(1975)p.69）参照）を用いて溶鋼流速を求め、６箇所の平均値から確認した。 The molten steel flow velocity at the front of the solidification interface was confirmed from the solidification structure of the sample taken from the cast slab. That is, a sample having a total thickness (300 mm) × full width (1560 mm) having a casting direction length of 300 mm is taken from a cast slab, and a spectroscopic sample is cut out from the six positions shown in FIG. After mirror finishing, it corrodes with acid to reveal a solidified structure, and from the inclination angle of dendritic dendrites of the solidified structure, Okano et al. (See publication: Iron and Steel (61 (1975) p.69)) The flow rate of molten steel was determined using, and confirmed from the average value of 6 locations.
また、前記凝固組織調査用試料の近傍から介在物調査用試料を採取し、採取した介在物調査用試料を鏡面仕上げした後、光学顕微鏡を用いて表面から２０ｍｍまでの範囲のアルミナクラスターの個数をカウントするとともに、アルミナクラスターの長軸及び短軸を測定した。また、鋳片を薄鋼板に圧延後、薄鋼板における表面欠陥の有無についても調査した。鋳片及び薄鋼板での調査結果を表２に示す。 In addition, an inclusion investigation sample is collected from the vicinity of the solidification structure investigation sample, and after the collected inclusion investigation sample is mirror-finished, the number of alumina clusters in the range from the surface to 20 mm is measured using an optical microscope. While counting, the major axis and minor axis of the alumina cluster were measured. Further, after the slab was rolled into a thin steel plate, the presence or absence of surface defects in the thin steel plate was also investigated. Table 2 shows the results of investigations on slabs and thin steel sheets.
表２に示すように、本発明例である試験Ｎo.１〜４では、鋳片単位面積あたりのアルミナクラスターの面積は０．００１以下になっており、圧延後の薄鋼板においても表面欠陥が発生していなかった。これに対して、比較例である試験Ｎo.５〜８では、鋳片単位面積あたりのアルミナクラスターの面積は０．００１を越えており、圧延後の薄鋼板においても表面欠陥が確認できた。 As shown in Table 2, in Test Nos. 1 to 4 which are examples of the present invention, the area of the alumina cluster per slab unit area is 0.001 or less, and surface defects are also present in the rolled steel sheet. It did not occur. On the other hand, in test No. 5-8 which is a comparative example, the area of the alumina cluster per slab unit area exceeded 0.001, and the surface defect was able to be confirmed also in the thin steel plate after rolling.
Claims (3)
(1/41)×[7-10000/(44853×[Ti]+2661750×[S]+4709863×[O]-25000)]≦V…(1)
但し、（１）式において、Ｖは、凝固殻前面での溶鋼流速（ｍ／秒）、［Ｔｉ］は、溶鋼のＴｉ濃度（質量％）、［Ｓ］は、溶鋼のＳ濃度（質量％）、［Ｏ］は、溶鋼のＯ（溶存酸素）濃度（質量％）である。 It is a continuous casting method of ultra-low carbon steel slab containing 0.003% by mass or less of C, and in the molten steel component, 44853 × [mass% Ti], 2661750 × [mass% S], 4709863 × [mass% O ] Is more than 25000, the molten steel flow rate at the front surface of the solidified shell is controlled so as to be within the range of the following formula (1), and casting is performed, .
(1/41) × [7-10000 / (44853 × [Ti] + 2661750 × [S] + 4709863 × [O] -25000)] ≦ V… (1)
However, in the formula (1), V is a molten steel flow velocity (m / sec) in front of the solidified shell, [Ti] is a Ti concentration (mass%) of the molten steel, and [S] is an S concentration (mass%) of the molten steel. ), [O] is the O (dissolved oxygen) concentration (mass%) of the molten steel.
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