JP5304297B2 - Continuous casting method for steel slabs - Google Patents
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- 229910052760 oxygen Inorganic materials 0.000 claims abstract description 34
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Abstract
Description
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、鋳片の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. During continuous casting of molten steel, this alumina cluster flows into the mold together with the molten steel from the tundish and is trapped by the solidified shell of the slab, resulting in a surface defect of the slab, and extremely low carbon that requires strict quality. The quality of steel slabs is significantly reduced.
従って、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 there is a surface defect in the cast slab, a work for removing the 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、鋳片の品質を向上させるために、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, in order to improve the quality of the slab, non-metallic inclusions such as alumina clusters adhering to the solidified shell can be obtained by applying a flow velocity to the molten steel in front of the solidified shell using an electromagnetic stirring device installed on the back of the mold. Many methods have been proposed for cleaning and thereby reducing non-metallic inclusions such as alumina clusters on the surface of the slab (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。 However, Patent Document 2 and Patent Document 3 do not discuss the trapping of alumina clusters in the solidified shell. In addition, although it is suggested that the trapping of bubbles in the solidified shell depends on the molten steel components, the relationship between the trapping of bubbles and the flow velocity of molten steel at the solidification interface has not been clarified. Cannot be grasped. This is because in the actual mold, simultaneously with the surface tension due to the concentration distribution of C, S, N, O (= capturing force to the solidified shell), the drag due to the molten steel flow velocity also works, This is because when considering capturing non-metallic inclusions, it is necessary to consider the drag due to the molten steel flow velocity.
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 require clean quality, such as automotive steel plates, with few defects due to non-metallic inclusions such as alumina clusters in the surface layer of the slabs. 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、２４９８９×［質量％Ｔｉ］と３８６１４７×［質量％Ｓ］と８５３３５３０×［質量％Ｏ］と１１８１７３×［質量％Ｓｂ］との和が４０００を超える場合は、鋳片の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。 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 24899 × [mass% Ti], 386147 × [mass% S], 8533530 × [mass% O], and 118173 × [mass% Sb] in the molten steel component exceeds 4000, The molten steel flow rate at the front surface of the solidified shell of the piece is controlled so as to be within the range of the following formula (1), and is cast.
(0.4-2.2V)×(6.7×[T.O])≦1/(24989×[Ti]+386147×[S]+8533530×[O]+118173×[Sb]-4000) …(1)
但し、（１）式において、Ｖは、凝固殻前面での溶鋼流速（ｍ／ｓ）、［Ｔ．Ｏ］は、溶鋼中のＯ（溶存酸素）と溶鋼中に酸化物として存在する酸素との合計濃度（質量％）、［Ｔｉ］は、溶鋼中のＴｉ濃度（質量％）、［Ｓ］は、溶鋼中のＳ濃度（質量％）、［Ｏ］は、溶鋼中のＯ（溶存酸素）濃度（質量％）、［Ｓｂ］は、溶鋼中のＳｂ濃度（質量％）である。
(0.4-2.2V) × (6.7 × [TO]) ≦ 1 / (24989 × [Ti] + 386147 × [S] + 8533530 × [O] + 118173 × [Sb] -4000)… (1)
However, in Formula (1), V is the molten steel flow velocity (m / s) in front of the solidified shell, [T. O] is the total concentration (mass%) of O (dissolved oxygen) in the molten steel and oxygen present as an oxide in the molten steel, [Ti] is the Ti concentration (mass%) in the molten steel, and [S] is , S concentration (mass%) in molten steel, [O] is O (dissolved oxygen) concentration (mass%) in molten steel, and [Sb] is Sb concentration (mass%) in 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、前記凝固殻前面での溶鋼流速は、鋳型内溶鋼湯面から鋳造方向に１００ｍｍ離れた位置付近における凝固殻前面での溶鋼流速であ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 molten steel flow velocity at the front surface of the solidified shell is measured at the front surface of the solidified shell near the position 100 mm away from the molten steel surface in the mold in the casting direction. The molten steel flow velocity is characterized by the following.
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％、Ｓｂ：０．０００５〜０．０１５質量％、Ｔｉ：０．００５〜０．０５質量％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 third invention, in the first or second invention, the ultra-low carbon steel contains, as a chemical component other than C, Si: 0.05% by mass or less, Mn: 1 0.0 mass% or less, P: 0.05 mass% or less, S: 0.015 mass% or less, Al: 0.010 to 0.075 mass%, Sb: 0.0005 to 0.015 mass%, Ti: It contains 0.005 to 0.05% by mass, and the balance consists of Fe and inevitable impurities.
第４の発明に係る鋼鋳片の連続鋳造方法は、第３の発明において、化学成分として、更に、Ｎｂ：０．００５〜０．０５質量％を含有することを特徴とするものである。 The continuous casting method for steel slabs according to the fourth invention is characterized in that, in the third invention, Nb: 0.005 to 0.05% by mass is further contained as a chemical component.
第５の発明に係る鋼鋳片の連続鋳造方法は、第１ないし第４の発明の何れかにおいて、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 a fifth aspect of the present invention, in any one of the first to fourth aspects, the molten steel flow velocity at the front surface of the solidified shell is controlled by an AC moving magnetic field applying device disposed at the rear surface of the mold. It is characterized by this.
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。このアルミナなどの酸化物として溶鋼中に存在する酸素と、溶存酸素とを合計したものを、トータル酸素（「Ｔ．Ｏ」とも記す）と称している。 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 if much 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. A total of oxygen present in molten steel as oxide such as alumina and dissolved oxygen is referred to as total oxygen (also referred to as “TO”).
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鋳片の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 is captured by the solidified shell of the slab, 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、「溶鋼成分における、２４９８９×［質量％Ｔｉ］と３８６１４７×［質量％Ｓ］と８５３３５３０×［質量％Ｏ］と１１８１７３×［質量％Ｓｂ］との和が４０００を超える場合は、鋳片の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。
(0.4-2.2V)×(6.7×[T.O])≦1/(24989×[Ti]+386147×[S]+8533530×[O]+118173×[Sb]-4000) …(1)
但し、（１）式において、Ｖは、凝固殻前面での溶鋼流速（ｍ／ｓ）、［Ｔ．Ｏ］は、溶鋼中のＯ（溶存酸素）と溶鋼中に酸化物として存在する酸素との合計濃度（質量％）、［Ｔｉ］は、溶鋼中のＴｉ濃度（質量％）、［Ｓ］は、溶鋼中のＳ濃度（質量％）、［Ｏ］は、溶鋼中のＯ（溶存酸素）濃度（質量％）、［Ｓｂ］は、溶鋼中のＳｂ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, in the case where the sum of 24899 × [mass% Ti], 386147 × [mass% S], 8533530 × [mass% O], and 118173 × [mass% Sb] in the molten steel component exceeds 4000, The molten steel flow velocity at the front of the solidified shell is controlled so as to be within the range of the following formula (1).
(0.4-2.2V) × (6.7 × [TO]) ≦ 1 / (24989 × [Ti] + 386147 × [S] + 8533530 × [O] + 118173 × [Sb] -4000)… (1)
However, in Formula (1), V is the molten steel flow velocity (m / s) in front of the solidified shell, [T. O] is the total concentration (mass%) of O (dissolved oxygen) in the molten steel and oxygen present as an oxide in the molten steel, [Ti] is the Ti concentration (mass%) in the molten steel, and [S] is , S concentration (mass%) in molten steel, [O] is O (dissolved oxygen) concentration (mass%) in molten steel, and [Sb] is Sb concentration (mass%) in molten steel.
ここで、（１）式における「(24989×[Ti]+386147×[S]+8533530×[O]+118173×[Sb]-4000)」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。また、本発明は、凝固殻へのアルミナクラスターの捕捉を防止することを目的としており、従って、（１）式における凝固殻前面での溶鋼流速Ｖは、凝固殻へのアルミナクラスターの捕捉が発生しやすい、鋳型内溶鋼湯面から鋳造方向に１００ｍｍ離れた位置付近における凝固殻前面での溶鋼流速を対象とすることが好ましい。 Here, “(24989 × [Ti] + 386147 × [S] + 8533530 × [O] + 118173 × [Sb] −4000)” in the formula (1) is formed on the front surface of the solidified shell during continuous casting. This is a measure of the attractive force in the direction of the solidified shell due to the gradient of the interfacial tension acting on the alumina cluster that has penetrated into the concentration boundary layer of the solute element (hereinafter also simply referred to as “solute”). Further, the present invention aims to prevent the capture of alumina clusters in the solidified shell. Therefore, the molten steel flow velocity V at the front of the solidified shell in the equation (1) causes the capture of alumina clusters in the solidified shell. It is preferable to target the molten steel flow velocity on the front surface of the solidified shell in the vicinity of a position 100 mm away from the molten steel surface in the mold in the casting direction.
以下、（１）式の導出方法について説明する。 Hereinafter, a method for deriving the expression (1) will be described.
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（２）式で示される。
F＝-(8/3)×πR2K…(2)
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。
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 inclusions in the direction of the solidified shell is expressed by the following equation (2).
F =-(8/3) × πR 2 K… (2)
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 ).
この界面張力勾配Ｋは、下記の（３）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。
K＝dσ/dx＝(dσ/dc)×(dc/dx)…(3)
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）であり、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。
The 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 (3).
K = dσ / dx = (dσ / dc) × (dc / dx) (3)
Here, σ is the interfacial tension (N / m) of the molten steel, x is the distance (m) from the solidification interface, and dσ / dc is the change in interfacial tension due to the solute concentration (N / m · mass%), dc / dx is a concentration gradient (mass% / m) of the component.
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（４）式で表される。
dc/dx＝-C0×(1-K0)×(Vs/D)×exp[-Vs×(x-δ)/D]…(4)
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／ｓ）、Ｄは溶鋼中での溶質の拡散係数（ｍ2／ｓ）、δは濃度境界層の厚み（ｍ）である。
From the solidification theory, the concentration gradient dc / dx of the component under the condition where the molten steel flow rate exists in the mold is expressed by the following equation (4).
dc / dx = -C 0 × (1-K 0 ) × (V s / D) × exp [-V s × (x-δ) / D] ... (4)
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 / s), and D is the diffusion of the solute in the molten steel. The coefficient (m 2 / s), δ is the thickness (m) of the concentration boundary layer.
（４）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（５）式で求められる。 If x = δ is substituted in the equation (4), the concentration gradient (dc / dx) at x = δ can be obtained by the following equation (5).
ｄc/dx＝-Ci×(1-K0)×(Vs/D)…(5)
（５）式を（３）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（６）式により求めることができる。
dc / dx = −Ci × (1-K 0 ) × (V s / D) (5)
By substituting the equation (5) into the equation (3), 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 (6).
K＝(dσ/dc)×[-Ci×(1-K0)×(Vs/D)]…(6)
（６）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）、Ｓｂ（アンチモン）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)]...(6)
The dσ / dc shown in the equation (6) 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), O (oxygen = dissolved oxygen), and Sb (antimony). The value of the interfacial tension gradient K calculated using only these elements is used. However, it was found that there was no problem in considering the trapping of alumina clusters 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（６）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、Ｏ及びＳｂ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「(24989×[Ti]+386147×[S]+8533530×[O]+118173×[Sb])」を得ることができる。 Therefore, 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 (6), in the concentration boundary layer As the attractive force in the direction of the solidified shell due to the interfacial tension gradient due to Ti, S, O and Sb acting on the alumina cluster, it is shown in the formula (1) “(24989 × [Ti] + 386147 × [S] + 8533530 × [O] + 118173 × [Sb]) ”.
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凝固殻へ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「(24989×[Ti]+386147×[S]+8533530×[O]+118173×[Sb])」の値と、凝固殻に捕捉されるアルミナクラスターの捕捉率とは、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捕捉率とは、下記の（７）式に示すように、凝固殻でのアルミナクラスター指数を溶鋼中でのアルミナクラスター指数で除算した値であり、単位アルミナクラスター濃度あたりの捕捉の頻度を示す値である。
α＝I/A…(7)
ここで、αはアルミナクラスターの捕捉率、Ｉは凝固殻でのアルミナクラスター指数（−）、Ａは溶鋼中でのアルミナクラスター指数（−）である。
In addition, the present inventors investigated the frequency of trapping alumina clusters in the solidified shell using molten steel having various compositions. As a result, as shown in FIG. 1, the value of “(24989 × [Ti] + 386147 × [S] + 8533530 × [O] + 118173 × [Sb])” shown in the equation (1) It has been found that there is a proportional relationship with the capture rate of the captured alumina clusters. Here, the capture rate of the alumina cluster is a value obtained by dividing the alumina cluster index in the solidified shell by the alumina cluster index in the molten steel, as shown in the following formula (7). It is a value indicating the frequency of capture.
α ＝ I / A… (7)
Here, α is the capture rate of alumina clusters, I is the alumina cluster index (−) in the solidified shell, and A is the alumina cluster index (−) in the molten steel.
尚、アルミナクラスター指数（−）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て、楕円体としての面積を算出し、観測されたアルミナクラスターの面積を総和した値を、測定面積で除算した値であり、単位測定面積中にアルミナクラスターがどの程度含まれているかを示す指標である。溶鋼中でのアルミナクラスター指数は、溶鋼から採取した試料中のアルミナクラスターを測定することで算出できる。 The alumina cluster index (-) is a value obtained by measuring the major axis and minor axis of an alumina cluster with an optical microscope, calculating the area as an ellipsoid, and summing the areas of the observed alumina clusters. The value divided by the area is an index indicating how much alumina cluster is included in the unit measurement area. The alumina cluster index in molten steel can be calculated by measuring alumina clusters in a sample collected from molten steel.
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、「(24989×[Ti]+386147×[S]+8533530×[O]+118173×[Sb])」の値が４０００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、「(24989×[Ti]+386147×[S]+8533530×[O]+118173×[Sb])」の値と、凝固殻に捕捉されるアルミナクラスターの捕捉率との比例定数は、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 clusters in the concentration boundary layer due to the interfacial tension gradient. However, as shown in FIG. 1, “(24989 × [Ti] + 386147 × [[ It was found that when the value of “S] + 8533530 × [O] + 118173 × [Sb])” was 4000 or less, alumina clusters were not trapped in the solidified shell. Furthermore, as shown in FIG. 1, the value of “(24989 × [Ti] + 386147 × [S] + 8533530 × [O] + 118173 × [Sb])” and the capture of alumina clusters captured in the solidified shell It was found that the proportionality constant with the rate changes depending on 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アルミナクラスター捕捉率の、「(24989×[Ti]+386147×[S]+8533530×[O]+118173×[Sb])」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。
f(V)＝0.0004-0.0022V…(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／ｓ）である。
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 proportion of alumina cluster trapped by the solidified shell is proportional to the value of “(24989 × [Ti] + 386147 × [S] + 8533530 × [O] + 118173 × [Sb])” It was found that the constant is a function f (V) of the molten steel flow velocity V in front of the solidification interface, and the function f (V) is expressed by a regression equation shown in the following equation (8).
f (V) = 0.004-0.0022V… (8)
However, V in Formula (8) is the molten steel flow velocity (m / s) 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、Ｓｂによる界面張力勾配の総和の４０００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アルミナクラスターの捕捉率αを求めることができる。
(0.0004-0.0022V)×(24989×[Ti]+386147×[S]+8533530×[O]+118173×[Sb]-4000)＝α…(9)
しかしながら、実操業中に溶鋼中でのアルミナクラスター指数Ａをリアルタイムで溶鋼採取試料から求めることは測定時間を考えれば極めて困難である。
Therefore, as shown in the following equation (9), the sum of the interfacial tension gradients due to Ti, S, O, and Sb in the molten steel exceeding 4000 is multiplied by a proportionality constant determined by the molten steel flow velocity in front of the solidification interface. Together, the capture rate α of the alumina clusters captured by the solidified shell can be determined.
(0.0004-0.0022V) × (24989 × [Ti] + 386147 × [S] + 8533530 × [O] + 118173 × [Sb] -4000) = α… (9)
However, it is extremely difficult to obtain the alumina cluster index A in the molten steel from the molten steel sample in real time during actual operation in consideration of the measurement time.
そこで、本発明者らは研究を重ね、図３に示すように、溶鋼中でのアルミナクラスター指数Ａは溶鋼のトータル酸素濃度に比例することを見出し、下記の（１０）式に示す回帰式を得た。
A＝6.7×[T.O]…(10)
ここで、［Ｔ．Ｏ］は、溶鋼中のトータル酸素濃度（質量％）である。
Accordingly, the present inventors have conducted research and found that the alumina cluster index A in the molten steel is proportional to the total oxygen concentration of the molten steel, as shown in FIG. 3, and the regression equation shown in the following equation (10) is obtained. Obtained.
A = 6.7 × [TO]… (10)
Here, [T. O] is the total oxygen concentration (mass%) in the molten steel.
また、自動車用極低炭素鋼において、鋳片表面のアルミナクラスター指数（「凝固殻でのアルミナクラスター指数Ｉ」に相当）が０．００１を超えると、表面欠陥が発生することが分かっている。即ち、（９）式の右辺のαに（７）式及び（１０）式を代入し、且つ、凝固殻でのアルミナクラスター指数Ｉの範囲を０．００１以下として、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。 Further, in the ultra-low carbon steel for automobiles, it has been found that when the alumina cluster index on the slab surface (corresponding to “alumina cluster index I in the solidified shell”) exceeds 0.001, surface defects occur. That is, by substituting Eqs. (7) and (10) for α on the right side of Eq. (9) and setting the range of the alumina cluster index I in the solidified shell to 0.001 or less, the molten steel flow velocity V in front of the solidified shell is V. The equation for obtaining the range is the equation (1) described above.
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Ｖを制御する範囲は、鋳片の表層部に相当する範囲であり、前述したように、鋳型内溶鋼湯面から鋳造方向に１００ｍｍ離れた位置付近における凝固殻前面での溶鋼流速とすることが好ましい。具体的には、鋳型内溶鋼湯面から鋳造方向に５０ｍｍないし１５０ｍｍ離れた位置の凝固殻前面での溶鋼流速を制御対象とすればよい。当然ながら、更に鋳造方向下方の範囲までを制御対象としても構わない。 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 V on the front surface of the solidified shell is a range corresponding to the surface layer portion of the slab, and as described above, at the position in front of the solidified shell near the position 100 mm away from the molten steel surface in the mold in the casting direction. It is preferable to use a molten steel flow rate. Specifically, the molten steel flow velocity at the front surface of the solidified shell at a position 50 mm to 150 mm away from the molten steel surface in the mold in the casting direction may be controlled. As a matter of course, a control range up to a lower range in the casting direction may be used.
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（学振１９委、Ｎo.10977）に基づけば、鋳型内溶鋼湯面の流速が０．４８ｍ／ｓ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０．４８ｍ／ｓ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. If the flow rate of the molten steel surface in the mold is 0.48 m / s or less based on known literature (Gakushin 19 Committee, No. 10977), the mold powder will not be entrained. What is necessary is just to control the molten steel flow velocity of the solidification interface front surface so that it may become in the range below 0.48 m / s.
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％、Ｓｂ：０．０００５〜０．０１５質量％、Ｔｉ：０．００５〜０．０５質量％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、上記化学成分に加えて、更に、Ｎｂ：０．００５〜０．０５質量％を含有する鋼にも効果が著しい。
以下に、成分を規定する理由を説明する。
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 %, Sb: 0.0005 to 0.015 mass%, Ti: 0.005 to 0.05 mass%, and the effect is particularly remarkable when the steel is composed of Fe and inevitable impurities. . Further, in addition to the above chemical components, the effect is also remarkable in steel containing Nb: 0.005 to 0.05 mass%.
The reason for defining the components will be described below.
Ｃは、その含有量が高くなると薄鋼板の加工性を劣化させる。それゆえ、ＴｉやＮｂなどの炭化物形成元素を添加したときにＩＦ鋼（Interstitial-Free steel）として優れた伸び及び深絞り性を得ることのできる０．００３質量％を上限とした。 C deteriorates the workability of the thin steel sheet when its content increases. Therefore, the upper limit is set to 0.003 mass% at which excellent elongation and deep drawability can be obtained as IF steel (Interstitial-Free steel) when carbide forming elements such as Ti and Nb are added.
Ｓｉは、固溶強化元素であり、含有量が多いと薄鋼板の加工性が劣化する。また、表面処理への影響も考慮し、０．０５質量％を上限とした。 Si is a solid solution strengthening element, and if the content is large, the workability of the thin steel sheet deteriorates. In consideration of the influence on the surface treatment, 0.05 mass% was made the upper limit.
Ｍｎは、固溶強化元素であり、鋼の強度を増加させるが、本発明は軟鋼を想定しており、加工性を優先する。従って、上限を１．０質量％とした。 Mn is a solid solution strengthening element and increases the strength of steel. However, the present invention assumes mild steel and gives priority to workability. Therefore, the upper limit was set to 1.0 mass%.
Ｐは、固溶強化元素であり、鋼の強度を増加させる。しかし、含有量が０．２質量％を超えると加工性や溶接性が劣化するため、上限を０．０５質量％とした。 P is a solid solution strengthening element and increases the strength of steel. However, if the content exceeds 0.2% by mass, workability and weldability deteriorate, so the upper limit was made 0.05% by mass.
Ｓは熱間圧延時に割れの原因となり、また、薄鋼板の加工性を低下させるＡ系介在物を生成するので、可能な限りその含有量を低減する必要がある。そこで、本発明では上限を０．０１５質量％とした。 S causes cracking during hot rolling and generates A-based inclusions that lower the workability of the thin steel sheet. Therefore, the content thereof needs to be reduced as much as possible. Therefore, in the present invention, the upper limit is set to 0.015% by mass.
Ａｌは脱酸剤として機能し、脱酸効果を得るためには、０．００５質量％含有される必要がある。また、必要以上のＡｌ添加はコストアップの増加を招く。そこで、本発明ではＡｌ含有量の範囲を０．０１０〜０．０７５質量％とした。 Al functions as a deoxidizing agent, and in order to obtain a deoxidizing effect, it is necessary to contain 0.005% by mass. Moreover, adding more Al than necessary causes an increase in cost. Therefore, in the present invention, the range of Al content is set to 0.010 to 0.075% by mass.
Ｔｉは、鋼中のＣ、Ｎ、Ｓを析出物として固定し、加工性や深絞り性を向上させる。しかし、含有量が０．００５質量％未満では、その効果が乏しく、また一方で、析出強化元素でもあるため、含有量が０．０５質量％を超えると鋼板が硬くなり、加工性が劣化する。そこで、本発明ではＴｉ含有量の範囲を０．００５〜０．０５質量％とした。 Ti fixes C, N, and S in the steel as precipitates and improves workability and deep drawability. However, if the content is less than 0.005% by mass, the effect is poor, and on the other hand, since it is also a precipitation strengthening element, if the content exceeds 0.05% by mass, the steel sheet becomes hard and the workability deteriorates. . Therefore, in the present invention, the range of Ti content is set to 0.005 to 0.05 mass%.
Ｓｂは、０．０１５質量％以下であれば加工性に悪影響を及ぼすことはない。Ｓｂは冷延板の表面の窒化を防止する効果があり、表面めっきの美麗さに寄与する。この理由は明確でないが、Ｓｂが冷延板の表面に濃化することに起因しているとされている。上記の効果を発揮させるために、Ｓｂ含有量の範囲を０．０００５〜０．０１５質量％とした。 If Sb is 0.015 mass% or less, workability will not be adversely affected. Sb has an effect of preventing nitriding of the surface of the cold-rolled plate, and contributes to the beauty of surface plating. Although this reason is not clear, it is said that it originates in Sb concentrating on the surface of a cold-rolled sheet. In order to exhibit the above effects, the Sb content range was set to 0.0005 to 0.015 mass%.
Ｎｂは、Ｔｉと同様に鋼中のＣ、Ｎ、Ｓを析出物として固定し、加工性や深絞り性を向上させる。しかし、含有量が０．００５％未満では、その効果が乏しく、また一方で析出強化元素であるため、含有量が０．０５質量％を超えると鋼板が硬くなり、加工性の劣化が生じる。そこで、本発明ではＮｂ含有量の範囲を０．００５〜０．０５質量％とした。 Nb, like Ti, fixes C, N, and S in steel as precipitates, and improves workability and deep drawability. However, if the content is less than 0.005%, the effect is poor, and on the other hand, since it is a precipitation strengthening element, if the content exceeds 0.05% by mass, the steel sheet becomes hard and workability deteriorates. Therefore, in the present invention, the range of Nb content is set to 0.005 to 0.05 mass%.
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２００ｍｍ、幅が１４４０ｍｍのスラブ鋳片に、鋳造速度を１．７６ｍ／ｍｉｎ、溶鋼鋳造量を３．７５トン／ｍｉｎ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、鋳型内溶鋼湯面から約１００ｍｍ鋳造方向に離れた位置での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に調整して鋳造した。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。（１）式を算出するにあたり、溶鋼の化学成分は、ＲＨ真空脱ガス装置での精錬終了時に溶鋼から採取した試料の分析値を用い、溶鋼のトータル酸素濃度は、鋳型への注入開始前にタンディッシュ内の溶鋼から試料を採取し、この試料の化学分析値を使用した。 8 charges (test No. 1-8) of ultra-low carbon steel with a charge of about 200 tons, cast into a slab slab having a thickness of 200 mm and a width of 1440 mm, a casting speed of 1.76 m / min, and a cast amount of molten steel Was cast at 3.75 tons / min. Table 1 shows the chemical composition of the molten steel for each test charge. In the slab continuous casting machine, the molten steel flow rate at the front surface of the solidification interface at a position away from the molten steel surface in the mold by about 100 mm in the casting direction satisfies the condition of the above-described formula (1), 1) Casting was performed under conditions that did not satisfy the range of the equation. That is, Table 1 shows the necessary minimum flow velocity obtained from the equation (1). In the 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. In calculating the formula (1), the chemical composition of the molten steel is the analysis value of the sample taken from the molten steel at the end of refining in the RH vacuum degassing unit. The total oxygen concentration of the molten steel is calculated before the injection into the mold. A sample was taken from the molten steel in the tundish, and the chemical analysis value of this sample was used.
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、鋳型内溶鋼湯面から約１００ｍｍ鋳造方向に離れた位置近傍における凝固界面前面での溶鋼流速を０．０５ｍ／ｓ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／ｓ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／ｓ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／ｓ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 in the vicinity of a position about 100 mm away from the molten steel surface in the mold in the casting direction is 0.05 m / s, the decelerating magnetic field having a magnetic flux density of 0.10 Tesla. In the case where the molten steel flow velocity at the solidification interface front is 0.10 m / s, the magnetic flux density is 0.08 Tesla, and the molten steel flow velocity at the solidification interface is 0.15 m / s. In this case, a decelerating magnetic field with a magnetic flux density of 0.05 Tesla was applied, and when the molten steel flow velocity at the front of the solidification interface was 0.20 m / s, 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、鋳造後の鋳片から全厚（２００ｍｍ）×全幅（１４４０ｍｍ）の試料を採取し、この試料を鏡面仕上げした後に酸で腐食して凝固組織を現出させ、図４に示す６箇所の位置において凝固組織のデンドライト樹枝状晶の傾き角度を測定し、測定した傾き角度から、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。尚、予め測定した鋳型内での凝固定数は１４ｍｍ／ｍｉｎ1/2であったので、鋳型内溶鋼湯面から鋳造方向に１００ｍｍ離れた位置での凝固殻厚みは約３．５ｍｍとなることから、鋳片表面から３．５ｍｍ内部におけるデンドライト樹枝状晶の傾き角度を測定した。 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 (200 mm) × full width (1440 mm) was taken from the cast slab, and after the sample was mirror-finished, it was corroded with acid to reveal a solidified structure. Measure the tilt angle of dendritic dendrites in the solidified structure at the position, and calculate the flow velocity of molten steel from the measured tilt angle using the formula of Okano et al. Obtained and confirmed from the average value of 6 locations. Since the solidification constant in the mold measured in advance was 14 mm / min 1/2 , the thickness of the solidified shell at a position 100 mm away from the molten steel surface in the mold in the casting direction is about 3.5 mm. The inclination angle of the dendrite dendrites in the inside of 3.5 mm from the slab surface was measured.
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、鋳片表面から５ｍｍ内部の位置までの範囲に存在する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て鋳片でのアルミナクラスター指数を算出した。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。 Further, after collecting the inclusion investigation sample from the vicinity of the solidification structure investigation sample, mirroring the collected inclusion investigation sample, the range from the slab surface to a position within 5 mm using an optical microscope The alumina cluster index in the slab was calculated by counting the number of alumina clusters present in the sample and measuring the major and minor axes of the alumina clusters. 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、鋳片でのアルミナクラスター指数は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、鋳片でのアルミナクラスター指数は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 alumina cluster index in the slab is 0.001 or less, and surface defects have occurred even in the rolled steel sheet. There wasn't. On the other hand, in test Nos. 5 to 8, which are comparative examples, the alumina cluster index in the slab exceeded 0.001, and surface defects could be confirmed even in the rolled steel sheet.
Claims (5)
(0.4-2.2V)×(6.7×[T.O])≦1/(24989×[Ti]+386147×[S]+8533530×[O]+118173×[Sb]-4000) …(1)
但し、（１）式において、Ｖは、凝固殻前面での溶鋼流速（ｍ／ｓ）、［Ｔ．Ｏ］は、溶鋼中のＯ（溶存酸素）と溶鋼中に酸化物として存在する酸素との合計濃度（質量％）、［Ｔｉ］は、溶鋼中のＴｉ濃度（質量％）、［Ｓ］は、溶鋼中のＳ濃度（質量％）、［Ｏ］は、溶鋼中のＯ（溶存酸素）濃度（質量％）、［Ｓｂ］は、溶鋼中のＳｂ濃度（質量％）である。 This is a continuous casting method for ultra-low carbon steel slab containing 0.003% by mass or less of C, and in the molten steel component, 24899 × [mass% Ti], 386147 × [mass% S], and 8533530 × [mass% O ] And 118173 × [mass% Sb] exceeds 4000, the molten steel flow velocity on the front surface of the solidified shell of the slab is controlled so as to be within the range of the following formula (1). A method for continuous casting of steel slabs.
(0.4-2.2V) × (6.7 × [TO]) ≦ 1 / (24989 × [Ti] + 386147 × [S] + 8533530 × [O] + 118173 × [Sb] -4000)… (1)
However, in Formula (1), V is the molten steel flow velocity (m / s) in front of the solidified shell, [T. O] is the total concentration (mass%) of O (dissolved oxygen) in the molten steel and oxygen present as an oxide in the molten steel, [Ti] is the Ti concentration (mass%) in the molten steel, and [S] is , S concentration (mass%) in molten steel, [O] is O (dissolved oxygen) concentration (mass%) in molten steel, and [Sb] is Sb concentration (mass%) in molten steel.
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