JP6825507B2 - Manufacturing method of low carbon steel thin wall slab and manufacturing method of low carbon steel thin wall slab and low carbon steel thin steel sheet - Google Patents
Manufacturing method of low carbon steel thin wall slab and manufacturing method of low carbon steel thin wall slab and low carbon steel thin steel sheet Download PDFInfo
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Description
本発明は、双ロール式連続鋳造方法により製造された加工性・成形性と清浄性に優れた低炭素鋼薄肉鋳片およびその製造方法、並びに低炭素鋼薄鋼板の製造方法に関するものである。 The present invention relates to a low carbon steel thin-walled slab having excellent workability, formability and cleanliness produced by a double-roll continuous casting method, a method for producing the same, and a method for producing a low carbon steel thin steel sheet.
省工程・省エネルギーの観点から、最終品に近い薄板を鋳造段階で製造する技術、すなわちニア・ネット・シェイプ連続鋳造の開発が行われている。この内、薄板系のニア・ネット・シェイプ連続鋳造で有力なものとして、双ロール式連続鋳造方法が特許文献1に開示されている。双ロール式連続鋳造装置を用いた薄肉鋳片の連続鋳造においては、図1に示すように互いに逆方向に回転する一対の冷却ロール1により区画された湯溜まり部2に、溶鋼3を浸漬ノズル4とその内部に設けたフィルター7を介してタンディッシュ5から供給することにより薄肉鋳片6を鋳造するようになっている。この双ロール式連続鋳造において表面欠陥や内部欠陥のない薄肉鋳片を安定的に鋳造するためには、湯溜まり内の溶鋼流動を整流化し、湯面変動を防止することが重要である。
From the viewpoint of process saving and energy saving, a technique for manufacturing a thin plate close to the final product at the casting stage, that is, near net shape continuous casting is being developed. Among these, a double-roll type continuous casting method is disclosed in Patent Document 1 as a promising thin plate type near net shape continuous casting. In the continuous casting of thin-walled slabs using a twin-roll type continuous casting device, as shown in FIG. 1, the molten steel 3 is immersed in a
これに対し、特許文献2には、浸漬ノズル内にフィルターを内蔵させ、ノズル全幅にわたって乱れのない吐出流を生成させる方法が、また特許文献3にはスリット状ノズルに整流多孔ノズルを内装させ、ノズル吐出流を整流化する方法が、それぞれ開示されている。また、Al脱酸溶鋼の双ロール式連続鋳造法では、ノズル詰まりに起因する吐出流の乱れが湯面変動を引き起こし、鋳造を不安定化させることが知られているが、溶鋼中にCaを添加してCaO−Al2O3の低融点介在物に改質することでノズル詰まりを防止する方法が特許文献4で提案されている。さらに、Al脱酸溶鋼では、ノズル詰まり防止やアルミナ介在物による表面欠陥の防止が難しいことから、CeとLaで脱酸すると共に溶存酸素を残す方法およびTiとCe、Laで複合脱酸する方法が特許文献5に開示されており、非アルミナ介在物に制御することでノズル付着の抑制に効果を発揮している。
On the other hand, in
上記の特許文献2〜3の方法は、双ロール式連続鋳造法で製造されるステンレス鋼(Al脱酸ではない)ではある程度の効果を発揮しているが、Al脱酸の低炭素鋼鋳造に際しては脱酸生成物であるアルミナ介在物が凝集合体により粗大化すると共に、浸漬ノズル吐出孔、浸漬ノズル内のフィルターや整流多孔ノズルにも付着するため、吐出流は乱れ、湯面変動に起因する介在物の再巻き込みにより内部欠陥が多発するといった問題を生じる。さらに、双ロール式連続鋳造法ではタンディッシュから注入された溶鋼は極めて短時間で凝固し、介在物の浮上時間を確保できないため、Al脱酸溶鋼では殆どの粗大なアルミナ介在物が薄肉鋳片内に捕捉され、湯面変動がない安定鋳造状態であっても内部欠陥が発生する可能性は高い。また、特許文献4のアルミナ介在物の改質方法は、浸漬ノズルの詰まりやフィルターの目詰まり防止には有効に作用するが、改質されたCaO−Al2O3介在物は液相のため取鍋、タンディッシュ、浸漬ノズル内で容易に合体して粗大化する。この粗大介在物は浸漬ノズル内のフィルターでは除去されないため、上述のように直ちに薄肉鋳片内に捕捉され、加工時に割れ発生(内部欠陥)の原因となる。また、特許文献5に記載のように、非アルミナ介在物にして凝集性を低下させる方法では、浸漬ノズルの詰まりに対してある程度の効果を発揮するものの、極低炭素濃度域まで脱炭した溶鋼中には非常に高濃度の溶存酸素(0.1質量%程度)が含まれており、これをAl以外の脱酸材で脱酸しても溶鋼中には多量の非アルミナ介在物が生成することになるため、鋳造時間が長くなるとノズル詰まり防止効果の低下は避けられない。さらに、特許文献5の方法で、脱酸後にも溶鋼中に溶存酸素を残すと鋼板の成形性・加工性が低下することが知られており、十分な材質を確保できないといった問題も生じる。
The methods of
さらに、双ロール式連続鋳造法で製造した薄鋼板では、加工時に異方性が現れ、例えば製缶時に深絞り加工を施すと缶の円周方向に山部と谷部が交互に続く、いわゆるイヤリングが発生する。このイヤリングが大きいと製缶の歩留まりが低下すると共に、イヤリング部が金型に接触し製缶トラブルにつながるため、双ロール式連続鋳造法で得た薄鋼板は高い成形性・加工性を要求される用途には適用できていないのが現状である。さらに言えば、本発明者らは、後述のように加工時に異方性を低減する凝固組織制御の方法を新たに見出しているが、その制御性は十分でなく全長に渡って安定的にイヤリング発生を防止できるまでには至っておらず、従来知見していない変動要因が存在するものと推定される。 Furthermore, in thin steel sheets manufactured by the double-roll continuous casting method, anisotropy appears during processing. For example, when deep drawing is performed during can manufacturing, peaks and valleys alternate in the circumferential direction of the can, so-called Earrings occur. If these earrings are large, the yield of can making will decrease, and the earrings will come into contact with the mold, leading to can making troubles. Therefore, the thin steel sheet obtained by the double-roll continuous casting method is required to have high formability and workability. At present, it cannot be applied to various applications. Furthermore, the present inventors have newly found a method for controlling the coagulation structure that reduces anisotropy during processing as described later, but the controllability is not sufficient and the earrings are stable over the entire length. It has not been possible to prevent the occurrence, and it is presumed that there are variable factors that have not been known so far.
本発明は、これらの現状を鑑み、溶鋼中の介在物を極力低下させた上で、ノズル詰まりと介在物粗大化が起こり難い介在物組成と、異方性が発現し難い凝固組織に安定的に制御できる双ロール式連続鋳造方法、およびそれを用いて鋳造した加工性・成形性に優れた低炭素鋼薄肉鋳片、並びに低炭素鋼薄鋼板の製造方法の提供を課題としている。 In view of these current conditions, the present invention is stable in the composition of inclusions in which nozzle clogging and coarsening of inclusions are unlikely to occur and the solidified structure in which anisotropy is difficult to develop, while reducing inclusions in molten steel as much as possible. It is an object of the present invention to provide a double-roll type continuous casting method that can be controlled, a low-carbon steel thin-walled slab having excellent workability and formability cast using the same, and a method for manufacturing a low-carbon steel thin steel sheet.
このような状況を鑑み、溶鋼中の介在物を極力低下させた上で、ノズル詰まりと介在物粗大化が起こり難い介在物組成と、異方性が発現し難い凝固組織に安定して制御できる双ロール式連続鋳造方法、およびそれを用いて鋳造した加工性・成形性に優れた低炭素鋼薄肉鋳片を提供するために、低炭素鋼の介在物低減方法、ノズル詰まりと介在物粗大化防止に有効な添加元素の解明と介在物改質方法、加工時の異方性発現機構の解明、その防止対策と効果の安定発揮に関して鋭意研究を重ね、得られた知見を双ロール式連続鋳造工程の中で最適に組み合わせてプロセス設計することにより本発明の完成に至った。 In view of this situation, after reducing the inclusions in the molten steel as much as possible, it is possible to stably control the inclusion composition in which nozzle clogging and inclusions are less likely to be coarsened and the solidified structure in which anisotropy is less likely to occur. In order to provide a double-roll continuous casting method and a low-carbon steel thin-walled slab with excellent workability and formability cast using the method, a method for reducing inclusions in low-carbon steel, nozzle clogging and coarsening of inclusions. Elucidation of additive elements effective for prevention, method of modifying inclusions, elucidation of anisotropy expression mechanism during processing, preventive measures and stable exertion of effect, repeated diligent research, and obtained findings are double-roll continuous casting The present invention was completed by designing the process by optimally combining them in the process.
その要旨は以下の通りである。すなわち、
(1)大気圧下での脱炭処理に引き続き減圧下での脱炭処理を行って、溶存酸素濃度を0.005〜0.035質量%とした溶鋼に、少なくともAl、Tiの1種または2種を添加して脱酸し、酸可溶Al濃度を0.05質量%以下、酸可溶Ti濃度を0.1質量%以下、かつ酸可溶Al濃度と酸可溶Ti濃度の合計を0%超に成分調整した後、さらにMgを0.0003〜0.01質量%、SeもしくはTeの少なくとも1種以上の合計を0.0002〜0.005質量%添加した溶鋼を双ロール式連続鋳造法で鋳造することを特徴とする低炭素鋼薄肉鋳片の製造方法。
(2)大気圧下での脱炭処理後の溶鋼中のC濃度を0.05質量%以上0.1質量%以下とし、減圧下での脱炭処理後のC濃度を0.01質量%以上0.05質量%未満とすることを特徴とする(1)記載の低炭素鋼薄肉鋳片の製造方法。
(3)大気圧下での脱炭処理を転炉で行い、減圧下の脱炭処理を真空脱ガス装置で行うことを特徴とする(1)または(2)記載の低炭素鋼薄肉鋳片の製造方法。
(4)少なくともAl、Tiの1種または2種を添加して脱酸し、酸可溶Al濃度を0.05質量%以下、酸可溶Ti濃度を0.1質量%以下に成分調整すると共に、3分以上攪拌を行った後、Mgを0.0003〜0.01質量%、SeもしくはTeの少なくとも1種以上の合計を0.0002〜0.005質量%添加した溶鋼を双ロール式連続鋳造法で鋳造することを特徴とする(1)〜(3)のいずれか1つに記載の低炭素鋼薄肉鋳片の製造方法。
The summary is as follows. That is,
(1) Decarburization under atmospheric pressure is followed by decarburization under reduced pressure to make molten steel with a dissolved oxygen concentration of 0.005 to 0.035% by mass, at least one of Al and Ti, or Two kinds are added and deoxidized, the acid-soluble Al concentration is 0.05% by mass or less, the acid-soluble Ti concentration is 0.1% by mass or less, and the total of the acid-soluble Al concentration and the acid-soluble Ti concentration. The composition of molten steel is adjusted to more than 0%, and then 0.0003 to 0.01% by mass of Mg and 0.0002 to 0.005% by mass of the total of at least one of Se or Te are added to the molten steel in a double roll type. A method for producing a low-carbon steel thin-walled slab, which is characterized by casting by a continuous casting method.
(2) The C concentration in the molten steel after decarburization under atmospheric pressure is 0.05% by mass or more and 0.1% by mass or less, and the C concentration after decarburization under reduced pressure is 0.01% by mass. The method for producing a low carbon steel thin-walled slab according to (1), wherein the content is less than 0.05% by mass.
(3) The low-carbon steel thin-walled slab according to (1) or (2), wherein the decarburization treatment under atmospheric pressure is performed in a converter and the decarburization treatment under reduced pressure is performed by a vacuum degassing device. Manufacturing method.
(4) At least one or two kinds of Al and Ti are added to deoxidize the components, and the acid-soluble Al concentration is adjusted to 0.05% by mass or less and the acid-soluble Ti concentration is adjusted to 0.1% by mass or less. After stirring for 3 minutes or more, molten steel containing 0.0003 to 0.01% by mass of Mg and 0.0002 to 0.005% by mass of the total of at least one type of Se or Te was added in a double roll type. The method for producing a low-carbon steel thin-walled slab according to any one of (1) to (3), which comprises casting by a continuous casting method.
(5)質量%で、C:0.01質量%以上0.05質量%未満、Si:0.005〜0.03質量%、Mn:0.6質量%以下、P:0.02質量%以下、S:0.01質量%以下、酸可溶Al:0.05質量%以下、酸可溶Ti:0.1質量%以下、かつ酸可溶Al濃度と酸可溶Ti濃度の合計が0質量%超、N:0.0005〜0.01質量%、Mg:0.0003〜0.01質量%、SeもしくはTeの少なくとも1種以上の合計:0.0002〜0.005質量%、全酸素濃度が0.002質量%以下であり、残部Fe及び不可避的不純物であり、直径30μm超の酸化物が5個/cm2未満であり、且つ等軸晶率が10%以上であることを特徴とする厚みが5mm以下の低炭素鋼薄肉鋳片。
(6)さらにNb:0.05質量%以下、V:0.03質量%以下、Mo:0.03質量%以下、Ni:0.05質量%以下の1種又は2種以上を含有することを特徴とする(5)に記載の低炭素鋼薄肉鋳片。
(5) In mass%, C: 0.01 mass% or more and less than 0.05 mass%, Si: 0.005 to 0.03 mass%, Mn: 0.6 mass% or less, P: 0.02 mass% Hereinafter, S: 0.01% by mass or less, acid-soluble Al: 0.05% by mass or less, acid-soluble Ti: 0.1% by mass or less, and the total of the acid-soluble Al concentration and the acid-soluble Ti concentration is More than 0% by mass, N: 0.0005 to 0.01% by mass, Mg: 0.0003 to 0.01% by mass, total of at least one of Se or Te: 0.0002 to 0.005% by mass, All the oxygen concentration is not more than 0.002 mass% it is the balance Fe and unavoidable impurities, is less than 5 / cm 2 oxide having a diameter of 30μm greater, is and isometric Akiraritsu 10% or more A low-carbon steel thin-walled slab having a thickness of 5 mm or less.
(6) Further, Nb: 0.05% by mass or less, V: 0.03% by mass or less, Mo: 0.03% by mass or less, Ni: 0.05% by mass or less, one or more. The low carbon steel thin-walled slab according to (5).
(7)(5)又は(6)記載の低炭素鋼薄肉鋳片に、冷間圧延、再結晶温度以上での連続焼鈍を行い、引き続き調質圧延を施すことを特徴とする低炭素鋼薄鋼板の製造方法。 (7) The low-carbon steel thin-walled slab according to (5) or (6) is cold-rolled, continuously annealed at a recrystallization temperature or higher, and then temper-rolled. Steel plate manufacturing method.
本発明によると、溶鋼の清浄性を極力高めた上で、ノズル詰まりと介在物粗大化を抑制でき、さらに凝固組織の異方性を低減できるため、加工性、成形性に優れた低炭素鋼薄鋼板を、双ロール式連続鋳造法を用いて安定的に製造することが可能となる。 According to the present invention, it is possible to suppress nozzle clogging and coarsening of inclusions while improving the cleanliness of the molten steel as much as possible, and further reduce the anisotropy of the solidified structure. Therefore, a low carbon steel having excellent workability and formability. It becomes possible to stably manufacture a thin steel sheet by using a double-roll continuous casting method.
以下に本発明を詳細に説明する。
一般に、低炭素鋼は転炉等の大気圧下で酸素を吹き付けて脱炭処理し、最終C濃度の溶鋼を溶製している。脱炭処理後の溶鋼中にはC濃度に応じて多量の溶存酸素が含まれており、多い場合には0.1質量%を超える場合もある。この溶存酸素は通常Alの添加により殆ど脱酸されるため、溶鋼中には溶存酸素量に相当する多量のアルミナ介在物が生成し、溶鋼の清浄性を大きく低下させる。また、溶鋼中の溶存酸素濃度が高くなると、同時に取鍋スラグのFeO、MnO等の低級酸化物濃度も上昇するため、脱酸後にスラグによる溶鋼再酸化が生じ、アルミナ介在物量が更に増大する。このアルミナ介在物は溶鋼中で凝集合体しながら浮上分離していくが、タンディッシュ内でも溶鋼中の介在物量は全酸素濃度で0.004質量%程度もあり、溶鋼中には凝集合体で生成した数百μm程度にも達する大型のアルミナ介在物(アルミナクラスター)も含まれている。この溶鋼を双ロール式連続鋳造法で鋳造すると凝固時間が非常に短いため、通常のスラブ用連続鋳造装置とは大きく異なり、鋳型内での介在物浮上分離は殆ど期待できない。また、双ロール式連続鋳造用浸漬ノズルは吐出流を整流化する目的で、整流多孔ノズルやフィルターを設ける等の複雑な構造となっているため、通常の連続鋳造用浸漬ノズルに比べて多量の介在物がノズル内壁、吐出孔やフィルターに付着する。ノズル閉塞が発生すると、浸漬ノズルからの吐出流が不安定となり、ロール間の湯溜まり部で湯面変動に起因する介在物の再巻き込みが生じる。このように、双ロール式連続鋳造法で低炭素鋼を鋳造すると、加工時に割れ発生の原因となる多量のアルミナクラスターが薄肉鋳片内に捕捉されるため、これまで高品質な低炭素鋼薄鋼板を双ロール式連続鋳造法で製造することは非常に難しかった。
The present invention will be described in detail below.
Generally, low carbon steel is decarburized by blowing oxygen under atmospheric pressure such as a converter to melt molten steel having a final C concentration. The molten steel after the decarburization treatment contains a large amount of dissolved oxygen depending on the C concentration, and in some cases, it may exceed 0.1% by mass. Since this dissolved oxygen is usually almost deoxidized by the addition of Al, a large amount of alumina inclusions corresponding to the amount of dissolved oxygen is generated in the molten steel, which greatly reduces the cleanliness of the molten steel. Further, when the dissolved oxygen concentration in the molten steel increases, the concentration of lower oxides such as FeO and MnO in the ladle slag also increases, so that the molten steel is reoxidized by the slag after deoxidation, and the amount of alumina inclusions further increases. These alumina inclusions float and separate while agglomerating and coalescing in the molten steel, but even in the tundish, the amount of inclusions in the molten steel is about 0.004% by mass in total oxygen concentration, and they are formed by agglomeration and coalescence in the molten steel. It also contains large alumina inclusions (alumina clusters) that reach several hundred μm. When this molten steel is cast by the double-roll type continuous casting method, the solidification time is very short, so that it is very different from a normal continuous casting device for slabs, and the floating separation of inclusions in the mold can hardly be expected. In addition, the double-roll type continuous casting immersion nozzle has a complicated structure such as providing a rectifying porous nozzle and a filter for the purpose of rectifying the discharge flow, so that the amount is larger than that of a normal continuous casting immersion nozzle. Inclusions adhere to the nozzle inner wall, discharge holes and filters. When the nozzle blockage occurs, the discharge flow from the immersion nozzle becomes unstable, and inclusions due to fluctuations in the molten metal level are re-engaged in the pool portion between the rolls. In this way, when low-carbon steel is cast by the double-roll continuous casting method, a large amount of alumina clusters that cause cracks during processing are trapped in the thin-walled slab, so that high-quality low-carbon steel has been used so far. It was very difficult to manufacture a steel sheet by a double-roll continuous casting method.
一方、双ロール式連続鋳造法で鋳造した低炭素鋼薄肉鋳片の凝固組織は、鋳片内部まで真っ直ぐに成長した柱状晶からなっている。凝固組織の形態は溶鋼中のC濃度と凝固時の固液界面の温度勾配に強く影響され、低炭素鋼のようにC濃度が0.1質量%以下で、双ロール鋳造のように温度勾配が大きくなると、柱状晶が極めて成長し易くなる。双ロール式連続鋳造法で製造された数mm厚の薄肉鋳片は、最終板厚まで冷間圧延されるが、従来の250mm厚程度で鋳造される連続鋳造鋳片とは異なり圧下率を大きく確保できない。その結果、凝固組織成長の方向性が最終薄鋼板にも残留し、加工時に異方性として現れ、例えば製缶時に深絞り加工を施すと缶の円周方向に山部と谷部が交互に続く、いわゆるイヤリングが発生することを本発明者らは知見している。 On the other hand, the solidified structure of the low carbon steel thin-walled slab cast by the twin-roll continuous casting method consists of columnar crystals that grow straight up to the inside of the slab. The morphology of the solidified structure is strongly influenced by the C concentration in the molten steel and the temperature gradient of the solid-liquid interface during solidification. The C concentration is 0.1% by mass or less like low carbon steel, and the temperature gradient is like double roll casting. The larger the value, the easier it is for columnar crystals to grow. A thin-walled slab with a thickness of several mm manufactured by the double-roll continuous casting method is cold-rolled to the final plate thickness, but unlike the conventional continuous-cast slab cast with a thickness of about 250 mm, the reduction ratio is large. Cannot be secured. As a result, the direction of solidification structure growth remains in the final thin steel sheet and appears as anisotropy during processing. For example, when deep drawing is performed during can manufacturing, peaks and valleys alternate in the circumferential direction of the can. The present inventors have found that the so-called earrings that follow occur.
以上の課題を踏まえて、本発明は、[1]低炭素溶鋼中の介在物低減方法、[2]ノズル詰まりと介在物粗大化防止に有効な添加元素の解明と介在物改質方法、[3]加工時の異方性発現機構に基づく凝固組織制御とその制御の安定性に関して鋭意研究を重ね、得られた知見を低炭素鋼の溶製工程から双ロール式連続鋳造工程までの中で最適に組み合わせてプロセス設計することにより完成させたものである。 Based on the above problems, the present invention relates to [1] a method for reducing inclusions in low carbon molten steel, [2] a method for clarifying additive elements effective for preventing nozzle clogging and inclusion coarsening, and a method for modifying inclusions. 3] We have conducted extensive research on solidification structure control based on the anisotropy expression mechanism during processing and the stability of that control, and based on the findings obtained from the melting process of low carbon steel to the double-roll continuous casting process. It was completed by optimally combining and designing the process.
まず、[1]の低炭素溶鋼中の介在物低減方法について、以下に述べる。この低炭素鋼製造の技術思想は、大気圧下で精錬してC濃度を最終成分値よりも高めに吹き止め、溶鋼中に過剰な炭素を残し、この溶鋼をさらに減圧下で脱炭処理することにより、溶存酸素濃度を極限まで低減し、高清浄鋼を溶製することにある。低炭素鋼は転炉等の大気圧下で酸素を吹き付けて脱炭処理するため、脱炭処理後の溶鋼中にはC濃度に応じた溶存酸素が含まれており、例えば最終C濃度0.04質量%の低炭素鋼(平均的な成分)では0.06質量%程度の溶存酸素を含んでいる。この溶存酸素は通常Alの添加により殆ど脱酸される(下記(1)式の反応)ため、溶鋼中では0.06質量%の全酸素濃度に相当する多量のアルミナ介在物を生成し、溶鋼清浄性を大きく低下させる。
2Al+3O=Al2O3 (1)
First, the method for reducing inclusions in the low-carbon molten steel of [1] will be described below. The technical idea of manufacturing this low carbon steel is to refine it under atmospheric pressure to blow off the C concentration higher than the final component value, leave excess carbon in the molten steel, and decarburize this molten steel under further reduced pressure. By doing so, the dissolved oxygen concentration is reduced to the utmost limit, and high-clean steel is melted. Since low carbon steel is decarburized by blowing oxygen under atmospheric pressure such as in a converter, the molten steel after decarburization contains dissolved oxygen according to the C concentration. For example, the final C concentration is 0. 04% by mass of low carbon steel (average component) contains about 0.06% by mass of dissolved oxygen. Since this dissolved oxygen is usually almost deoxidized by the addition of Al (reaction of the following formula (1)), a large amount of alumina inclusions corresponding to a total oxygen concentration of 0.06% by mass is generated in the molten steel, and the molten steel is produced. Greatly reduces cleanliness.
2Al + 3O = Al 2 O 3 (1)
これに対して、本発明では、大気圧下における脱炭処理によるC濃度を製品値よりも高め、すなわち0.05〜0.1質量%にして脱炭処理を終了するため、溶存酸素濃度は0.049〜0.024質量%程度となり、大気圧下における脱炭処理のみで平均的な最終C濃度(0.04質量%)まで脱炭した場合の溶存酸素濃度0.06質量%よりも低い。本発明の大気圧下での脱炭処理溶鋼は、続いて減圧下で脱ガス処理されるため、下記(2)式の脱炭反応がさらに進行し、C濃度は最終成分値(0.04質量%)まで低下すると共に、それに応じて溶存酸素もさらに減少させることができる。
C+O=CO (2)
On the other hand, in the present invention, the C concentration by the decarburization treatment under atmospheric pressure is made higher than the product value, that is, 0.05 to 0.1% by mass, and the decarburization treatment is completed. Therefore, the dissolved oxygen concentration is set. It is about 0.049 to 0.024% by mass, which is higher than the dissolved oxygen concentration of 0.06% by mass when decarburized to the average final C concentration (0.04% by mass) only by decarburization under atmospheric pressure. Low. Since the molten steel decarburized under atmospheric pressure of the present invention is subsequently degassed under reduced pressure, the decarburization reaction of the following formula (2) further proceeds, and the C concentration is the final component value (0.04). It can be reduced to (% by mass), and the dissolved oxygen can be further reduced accordingly.
C + O = CO (2)
大気圧下での脱炭処理によるC濃度を最も低い0.05質量%にした際には、減圧下での脱炭処理後の溶存酸素濃度が最も高くなるが、それでも0.035質量%程度に抑えることができる。また、大気圧下での脱炭処理終了後のC濃度を0.065質量%程度よりも高くすると、その後の減圧下での脱炭処理において溶存酸素が不足し、C濃度を最終成分値(0.04質量%)まで低下させることができない。その場合には、減圧下での脱炭処理後半の脱炭反応が停滞し始めた時点(溶存酸素濃度は0.005質量%程度)で外部から酸素を供給することが可能であり、供給した酸素は同様に(2)式により消費されるため、溶存酸素は外部酸素を供給し始めた時点の低い溶存酸素濃度を維持しつつ、最終C濃度に成分調整することができる。このため、減圧下での脱炭処理後の溶存酸素濃度は、0.035〜0.005質量%程度まで低減できる。この状態でAlを添加して脱酸しても、生成するアルミナ介在物量は、通常の大気圧下での脱炭処理のみで溶製したC濃度0.04質量%の低炭素鋼の全酸素濃度0.06質量%の場合に比べて非常に低い。また、取鍋スラグのFeO、MnO等の低級酸化物濃度も低下しているため、Al脱酸後のスラグによる溶鋼汚染も大きく減少する。 When the C concentration by the decarburization treatment under atmospheric pressure is set to the lowest 0.05% by mass, the dissolved oxygen concentration after the decarburization treatment under reduced pressure becomes the highest, but it is still about 0.035% by mass. Can be suppressed to. Further, when the C concentration after the completion of the decarburization treatment under atmospheric pressure is higher than about 0.065% by mass, the dissolved oxygen is insufficient in the subsequent decarburization treatment under reduced pressure, and the C concentration is set to the final component value ( It cannot be reduced to 0.04% by mass). In that case, oxygen can be supplied from the outside when the decarburization reaction in the latter half of the decarburization treatment under reduced pressure begins to stagnate (dissolved oxygen concentration is about 0.005% by mass), and the oxygen is supplied. Since oxygen is similarly consumed by the equation (2), the dissolved oxygen can be adjusted to the final C concentration while maintaining the low dissolved oxygen concentration at the time when the external oxygen is started to be supplied. Therefore, the dissolved oxygen concentration after the decarburization treatment under reduced pressure can be reduced to about 0.035 to 0.005% by mass. Even if Al is added and deoxidized in this state, the amount of alumina inclusions produced is the total oxygen of low carbon steel with a C concentration of 0.04% by mass, which is melted only by decarburization under normal atmospheric pressure. It is very low compared to the case where the concentration is 0.06% by mass. Further, since the concentration of lower oxides such as FeO and MnO in the ladle slag is also reduced, the molten steel contamination by the slag after Al deoxidation is also greatly reduced.
大気圧下での脱炭処理後の溶鋼中のC濃度を、好ましくは0.05質量%以上0.1質量%以下にした理由は、C濃度を0.1質量%超にすると減圧下での脱炭処理が長くなるため、またC濃度を0.05質量%未満にすると溶存酸素濃度が急激に高くなり、減圧下での脱炭処理で溶存酸素濃度を十分に低下できにくいためである。また、鋼中のC濃度は鋼板の伸びや強度に大きく影響するため、減圧下での脱炭処理後のC濃度は低炭素鋼としての材質が十分に得られる0.01質量%以上0.05質量%未満とするのが望ましい。脱炭処理後のC濃度は鋳片のC濃度と対応している。 The reason why the C concentration in the molten steel after decarburization under atmospheric pressure is preferably 0.05% by mass or more and 0.1% by mass or less is that when the C concentration is more than 0.1% by mass, the pressure is reduced. This is because the decarburization treatment is lengthened, and when the C concentration is less than 0.05% by mass, the dissolved oxygen concentration rapidly increases, and it is difficult to sufficiently reduce the dissolved oxygen concentration by the decarburization treatment under reduced pressure. .. Further, since the C concentration in the steel greatly affects the elongation and strength of the steel sheet, the C concentration after the decarburization treatment under reduced pressure is 0.01% by mass or more, which is enough to obtain the material as low carbon steel. It is desirable that it is less than 05% by mass. The C concentration after the decarburization treatment corresponds to the C concentration of the slab.
減圧下での脱炭処理後の溶鋼は、AlもしくはTiの1種または2種を添加して脱酸することができる。しかし、減圧下での脱炭処理後の溶存酸素濃度が0.035質量%を超えると、AlもしくはTiの1種または2種を添加して生成する介在物量が多くなり、後述するMgを適正量添加しても、アルミナ介在物やチタニア介在物を改質できず、凝集合体やノズルへの介在物付着を防止することができない。反対に減圧下での脱炭処理後の溶存酸素濃度をできるだけ低くすることは清浄性向上に有効であるが、減圧下であっても溶存酸素濃度を0.005質量%未満に低下させることはコストと処理時間の両面から極めて難しい。したがって、減圧下での脱炭処理後の溶存酸素濃度は0.005〜0.035質量%に制御する必要がある。ここで、減圧下とは大気圧未満の圧力をいう。 The molten steel after the decarburization treatment under reduced pressure can be deoxidized by adding one or two types of Al or Ti. However, if the dissolved oxygen concentration after the decarburization treatment under reduced pressure exceeds 0.035% by mass, the amount of inclusions generated by adding one or two types of Al or Ti increases, and Mg described later is appropriate. Even if the amount is added, the alumina inclusions and titania inclusions cannot be modified, and it is not possible to prevent the agglomeration and the inclusions from adhering to the nozzle. On the contrary, reducing the dissolved oxygen concentration after decarburization under reduced pressure as much as possible is effective for improving cleanliness, but reducing the dissolved oxygen concentration to less than 0.005% by mass even under reduced pressure is possible. Extremely difficult in terms of both cost and processing time. Therefore, it is necessary to control the dissolved oxygen concentration after the decarburization treatment under reduced pressure to 0.005 to 0.035% by mass. Here, under reduced pressure means a pressure below atmospheric pressure.
本発明においては、上記のように、AlもしくはTiの1種または2種を添加するが、添加後の酸可溶Al濃度を0.05質量%以下、酸可溶Ti濃度を0.1%質量以下とする。その理由は、これらを超える酸可溶Al濃度と酸可溶Ti濃度では、後述するように各々アルミナ介在物とチタニア介在物をマグネシア、或いはアルミナマグネシアスピネルに改質する反応が進まず、残存した多量のアルミナ介在物とチタニア介在物の凝集・合体により粗大化すると共に、等軸晶の核生成サイトが不足し、十分な等軸晶組織が得られないためである。 In the present invention, one or two types of Al or Ti are added as described above, but the acid-soluble Al concentration after the addition is 0.05% by mass or less, and the acid-soluble Ti concentration is 0.1%. Not more than mass. The reason is that when the acid-soluble Al concentration and the acid-soluble Ti concentration exceed these, the reaction of reforming the alumina inclusions and the titania inclusions into magnesia or alumina magnesia spinel, respectively, did not proceed and remained as described later. This is because a large amount of alumina inclusions and titania inclusions are agglomerated and coalesced to cause coarsening, and equiaxed crystal nucleation sites are insufficient, so that a sufficient equiaxed crystal structure cannot be obtained.
また、溶鋼成分のばらつきと材質劣化を防止する観点から、溶存酸素をAlまたはTiで十分に脱酸して、アルミナもしくはチタニア(酸化物)として固定する必要があり、そのためには、溶鋼中に溶存Alもしくは溶存Tiを残すことが重要である。従って、脱酸が十分に実施される要件から、AlもしくはTiの1種または2種を添加後の酸可溶(溶存)Al濃度と酸可溶(溶存)Ti濃度の合計は、少なくとも0質量%超であって、好ましくは0.005質量%以上、さらに好ましくは0.01質量%以上である。減圧下での脱炭処理後に溶存酸素濃度を測定し、その測定値から化学量論比にしたがって求めたAl量もしくはTi量よりも過剰なAlもしくはTiを添加することにより、上記好適な酸可溶Alもしくは酸可溶Tiを溶鋼中に残すことができる。また、酸可溶Al濃度、酸可溶Ti濃度とは、酸に溶解したAl量とTi量を測定したもので、溶存Alと溶存Tiは酸に溶解し、アルミナやチタニアは酸に溶解しないことを利用した分析方法である。ここで、酸とは、例えば塩酸1、硝酸1、水2の割合で混合した混酸である。 In addition, from the viewpoint of preventing variation in molten steel components and deterioration of materials, it is necessary to sufficiently deoxidize dissolved oxygen with Al or Ti and fix it as alumina or titania (oxide). It is important to leave dissolved Al or dissolved Ti. Therefore, from the requirement that deoxidation is sufficiently carried out, the total of the acid-soluble (dissolved) Al concentration and the acid-soluble (dissolved) Ti concentration after the addition of one or two types of Al or Ti is at least 0 mass. It is more than%, preferably 0.005% by mass or more, and more preferably 0.01% by mass or more. After the decarburization treatment under reduced pressure, the dissolved oxygen concentration is measured, and by adding Al or Ti in excess of the Al amount or Ti amount obtained from the measured value according to the chemical quantity theory ratio, the above-mentioned suitable acid is possible. Molten Al or acid-soluble Ti can be left in the molten steel. The acid-soluble Al concentration and the acid-soluble Ti concentration are obtained by measuring the amount of Al and Ti dissolved in the acid. Dissolved Al and dissolved Ti are dissolved in the acid, and alumina and titania are not dissolved in the acid. This is an analysis method that utilizes this. Here, the acid is, for example, a mixed acid in which 1 hydrochloric acid, 1 nitric acid, and 2 water are mixed.
本発明においては、AlやTiを添加して脱酸した後の溶鋼は、3分以上の攪拌時間を設けることが好ましい。これは、減圧下での脱炭処理により溶鋼の清浄性を向上できているが、さらに攪拌時間を取ることで効率的に介在物を除去でき、清浄性を一段と高めることができるためである。 In the present invention, it is preferable to provide a stirring time of 3 minutes or more for the molten steel after deoxidizing by adding Al or Ti. This is because the cleanliness of the molten steel can be improved by the decarburization treatment under reduced pressure, but the inclusions can be efficiently removed by further taking a stirring time, and the cleanliness can be further improved.
また、大気圧下での溶鋼の脱炭処理としては、転炉や電気炉などの製鋼炉が、続いて行う減圧下での脱炭処理としては真空脱ガス装置や減圧精錬装置等が、通常使用される。 Further, as the decarburization treatment of molten steel under atmospheric pressure, a steelmaking furnace such as a converter or an electric furnace is usually used, and as a subsequent decarburization treatment under reduced pressure, a vacuum degassing device or a vacuum refining device is usually used. used.
次に、上記方法で清浄性を高めた低炭素溶鋼中の介在物を、[2]ノズル詰まりと介在物粗大化が起こりにくい組成の介在物に改質する方法について述べる。減圧下での脱炭処理により高清浄化した溶鋼であっても、アルミナ介在物やチタニア介在物は非常に凝集合体し易いため、それ以降の取鍋やタンディッシュ内で介在物の凝集合体は徐々に進行し、また双ロール式連続鋳造法における浸漬ノズルの複雑な構造にも起因してノズル内壁、吐出孔やフィルターに介在物が付着し、ノズル閉塞を発生させる可能性がある。また、双ロール式連続鋳造方法は、非常に短時間で凝固を完了する急冷凝固プロセスであることが最大の特徴である。溶鋼中での凝集合体を防止して双ロール式連続鋳造機内に溶鋼を注入できれば、その特徴である急冷効果により通常のスラブ連続鋳造法に比べて介在物をより均一微細に分散させることも可能であり、加工時の割れ発生を最も効果的に防止できる。 Next, a method of modifying the inclusions in the low-carbon molten steel whose cleanliness has been improved by the above method into inclusions having a composition that is less likely to cause nozzle clogging and inclusion coarsening will be described. Even in molten steel that has been highly purified by decarburization under reduced pressure, alumina inclusions and titania inclusions are very easy to aggregate and coalesce, so the aggregation and coalescence of inclusions gradually occurs in the subsequent ladle and tundish. In addition, due to the complicated structure of the immersion nozzle in the double-roll continuous casting method, inclusions may adhere to the inner wall of the nozzle, the discharge hole and the filter, causing nozzle blockage. The biggest feature of the double-roll continuous casting method is that it is a quenching solidification process that completes solidification in a very short time. If molten steel can be injected into a twin-roll continuous casting machine by preventing agglutination in molten steel, it is possible to disperse inclusions more uniformly and finely than in a normal slab continuous casting method due to its characteristic quenching effect. Therefore, the occurrence of cracks during processing can be prevented most effectively.
そこで、本発明者らは、比較的清浄性の高い溶鋼中でアルミナ介在物やチタニア介在物を改質して、凝集合体やノズルへの介在物付着を抑制する添加元素を検討し、AlやTiに比べて強脱酸元素であるMgが効果的な凝集・付着防止元素になることを見いだした。
比較的清浄性を高めた溶鋼中に強脱酸元素のMgを添加すると、溶鋼中のアルミナ介在物やチタニア介在物の一部または全体が還元され、少なくとも介在物表層にマグネシア、またはアルミナマグネシアスピネルが生成する。この介在物組成は低炭素溶鋼との界面エネルギーを大きく低下させるため、介在物のノズルやフィルター耐火物への付着と介在物同士の凝集合体を同時に抑制する。ここで、介在物制御に適正なMg添加量は、0.0003〜0.01質量%である。これは、Mgの添加量が0.0003質量%未満では、特にチタニア介在物よりも安定なアルミナ介在物で表層部をマグネシアまたはアルミナマグネシアスピネルに改質できないためである。反対にMgの添加量が0.01質量%を超えると介在物表層がマグネシア、アルミナマグネシアスピネルに改質されていても、強脱酸元素であるMgが溶存酸素を更に低下させ介在物と溶鋼との界面エネルギーを上昇させ粗大化とノズル付着を進行させてしまうためである。ここで、Mgの添加量が0.01質量%以下であれば酸化物界面には0.001質量%強の吸着酸素が残存しているが、強脱酸元素のMg添加量が0.01質量%を超えると酸化物界面の吸着酸素までが還元除去され、界面エネルギーを大きく上昇させるものと考えられる。
Therefore, the present inventors have investigated additive elements that modify alumina inclusions and titania inclusions in molten steel with relatively high cleanliness to suppress agglomeration and inclusions and adhesion of inclusions to nozzles. It was found that Mg, which is a strong deoxidizing element, becomes an effective aggregation / adhesion prevention element compared to Ti.
When Mg, a strong deoxidizing element, is added to molten steel with relatively high cleanliness, some or all of the alumina inclusions and titania inclusions in the molten steel are reduced, and at least the surface layer of the inclusions is magnesia or alumina magnesia spinel. Is generated. Since this inclusion composition greatly reduces the interfacial energy with the low-carbon molten steel, it simultaneously suppresses the adhesion of inclusions to the nozzle and filter refractory and the aggregation and coalescence of inclusions. Here, the amount of Mg added appropriate for inclusion control is 0.0003 to 0.01% by mass. This is because if the amount of Mg added is less than 0.0003% by mass, the surface layer portion cannot be modified to magnesia or alumina magnesia spinel, especially with alumina inclusions that are more stable than titania inclusions. On the contrary, when the amount of Mg added exceeds 0.01% by mass, even if the surface layer of inclusions is modified to magnesia or alumina magnesia spinel, Mg, which is a strong deoxidizing element, further lowers the dissolved oxygen and the inclusions and molten steel. This is because the interfacial energy with and is increased to promote coarsening and nozzle adhesion. Here, if the amount of Mg added is 0.01% by mass or less, more than 0.001% by mass of adsorbed oxygen remains at the oxide interface, but the amount of Mg added as a strongly deoxidizing element is 0.01. If it exceeds mass%, even the adsorbed oxygen at the oxide interface is reduced and removed, which is considered to greatly increase the interface energy.
さらに、[3]加工時に異方性が発現しにくい凝固組織の制御方法について述べる。前述したように、双ロール式連続鋳造法で製造した薄鋼板で異方性が生じるのは、低炭素溶鋼を急冷凝固させることにより発達した柱状晶組織に起因することを知見している。本発明者らは、この異方性の発現機構に基づけば、低炭素鋼の凝固組織を等軸晶化することが異方性の低減に有効であることから、低炭素溶鋼中にMgを添加して少なくともアルミナ介在物やチタニア介在物の表層部をマグネシアまたはアルミナマグネシアスピネルに改質し、それらの介在物を等軸晶核生成サイトとして活用することにより双ロール式連続鋳造法で凝固組織を等軸晶化する方法を新たに考案した。本発明によれば、介在物をマグネシアまたはアルミナマグネシアスピネルに改質し、溶鋼と介在物間の界面エネルギーを低下させることができるため、Mg添加は[2]介在物の粗大化・ノズル付着防止と[3]凝固組織の等軸晶化の両方に有効に作用し、双ロール式連続鋳造法を用いた低炭素鋼薄鋼板の製造において極めて効果的な制御手段となる。 Further, [3] a method for controlling a coagulated structure in which anisotropy is unlikely to appear during processing will be described. As described above, it has been found that the anisotropy of the thin steel sheet produced by the twin-roll continuous casting method is caused by the columnar crystal structure developed by quenching and solidifying the low-carbon molten steel. Based on this anisotropy expression mechanism, the present inventors have found that equiaxed crystallization of the solidified structure of low carbon steel is effective in reducing anisotropy. Therefore, Mg is added to low carbon molten steel. By adding at least the surface layer of alumina inclusions and titania inclusions to magnesia or alumina magnesia spinel and utilizing those inclusions as equiaxed crystal nucleation sites, the solidified structure is formed by a biroll continuous casting method. We devised a new method for equiaxed crystallization. According to the present invention, inclusions can be modified to magnesia or alumina magnesia spinel to reduce the interfacial energy between molten steel and inclusions. Therefore, addition of Mg [2] coarsens inclusions and prevents nozzle adhesion. [3] It works effectively on both equiaxed crystallization of the solidified structure, and is an extremely effective control means in the production of low carbon steel thin steel sheets using the twin-roll continuous casting method.
本発明者らは、加工時の異方性を問題のないレベルまで解消するため、凝固組織の制御条件についても実験による詳細な検討を実施し、等軸晶率(等軸晶厚み/板厚×100(%))を10%以上確保する必要があることを明らかにした。これは、薄肉鋳片の等軸晶率が10%以上になると冷間圧延による変形が伝わり難く、凝固組織の異方性が残留し易い板厚中央部を安定的に等軸晶化できるためである。なお、ノズル詰まりと介在物粗大化を防止する介在物組成制御と同様に、薄肉鋳片の凝固組織の等軸晶化率を10%以上確保するためのMgの適正添加量は0.0003〜0.01質量%である。Mgの添加量が0.0003質量%未満では等軸晶核生成サイトとなるマグネシアまたはアルミナマグネシアスピネルの量が少なくなることにより、反対に0.01質量%を超えるとマグネシアまたはアルミナマグネシアスピネルが粗大化することにより、何れも凝固組織の等軸晶化率は10%未満となり異方性が発現してしまうためである。 In order to eliminate the anisotropy during processing to a level that does not cause any problems, the present inventors have conducted a detailed experimental study on the control conditions of the solidified structure, and equiaxed crystal ratio (equal crystal thickness / plate thickness). It was clarified that it is necessary to secure 10% or more of × 100 (%)). This is because when the equiaxed crystal ratio of the thin-walled slab is 10% or more, the deformation due to cold rolling is difficult to be transmitted, and the central portion of the plate thickness where the anisotropy of the solidified structure tends to remain can be stably equiaxed. Is. Similar to the inclusion composition control for preventing nozzle clogging and coarsening of inclusions, the appropriate amount of Mg added to secure the equiaxed crystallization rate of the solidified structure of the thin-walled slab is 0.0003 to 10% or more. It is 0.01% by mass. If the amount of Mg added is less than 0.0003% by mass, the amount of magnesia or alumina magnesia spinel that becomes equiaxed nucleation sites decreases, and conversely if it exceeds 0.01% by mass, magnesia or alumina magnesia spinel becomes coarse. This is because the equiaxed crystallization rate of the coagulated structure becomes less than 10% and anisotropy develops.
双ロール式連続鋳造では、通常の連続鋳造に比較して取鍋やタンディッシュの容量が小さいため溶鋼温度が低下しやすく、安定鋳造のために鍋溶鋼温度は高めに調整される。特に、取鍋溶鋼の注入初期には双ロール式連続鋳造機に注入される溶鋼の過熱度(溶鋼温度と液相線温度との差)は非常に高く、等軸晶核生成サイトとなるマグネシアまたはアルミナマグネシアスピネルが十分存在しても、等軸晶核が生成しにくい条件となる。さらに、取鍋内溶鋼をタンディッシュに注入し始める鋳造初期や連々鋳の継ぎ目部では、タンディッシュ内の溶鋼高さが低く、取鍋からの注入流はロングノズルに覆われることなく落下流となるため、空気やスラグの巻き込みに起因して溶鋼再酸化が生じ、タンディッシュ内の溶鋼中に新たなアルミナ介在物やチタニア介在物を生成する。タンディッシュ内のArシールをおこなう等、溶鋼再酸化対策を実施していればノズル詰まりや鋼板加工時の割れ発生の原因となる比較的大きな介在物への影響はない。しかしながら、完全に溶鋼の再酸化を防止するのは難しく、新たに生成した比較的小さく、且つ等軸晶核生成能の低いアルミナ系介在物やチタニア系介在物が等軸晶核生成サイトとなるマグネシアまたはアルミナマグネシアスピネルの表面に付着するため、等軸晶化能は大きく低下する。このように、鋳造初期や連々鋳継ぎ目部では、溶鋼過熱度の増大と溶鋼再酸化が同時に起こり、所定のMg量を添加しても殆ど等軸晶化しないことを本発明者らは知見している。 In the double-roll type continuous casting, the molten steel temperature tends to decrease because the capacity of the ladle and the tundish is smaller than that of the normal continuous casting, and the hot pot molten steel temperature is adjusted higher for stable casting. In particular, the superheat degree (difference between the molten steel temperature and the liquidus temperature) of the molten steel injected into the twin-roll continuous casting machine is extremely high at the initial stage of ladle molten steel injection, and magnesia becomes an equiaxed crystal nucleation site. Alternatively, even if a sufficient amount of alumina magnesia spinel is present, the condition is such that equiaxed crystal nuclei are difficult to be formed. Furthermore, the height of the molten steel in the tundish is low at the initial stage of casting when the molten steel in the ladle begins to be injected into the tundish and at the seams of continuous casting, and the injection flow from the ladle is a falling flow without being covered by the long nozzle. As a result, molten steel reoxidation occurs due to the entrainment of air and slag, and new alumina inclusions and titania inclusions are generated in the molten steel in the tundish. If measures against molten steel reoxidation such as Ar sealing in the tundish are taken, there is no effect on relatively large inclusions that cause nozzle clogging and cracking during steel sheet processing. However, it is difficult to completely prevent the reoxidation of molten steel, and newly generated alumina-based inclusions and titania-based inclusions with relatively small equiaxed nucleation ability become equiaxed crystal nucleation sites. Since it adheres to the surface of magnesia or alumina magnesia spinel, the equiaxed crystallization ability is greatly reduced. As described above, the present inventors have found that the molten steel superheat degree and the molten steel reoxidation occur at the same time at the initial stage of casting and at the continuous casting seam, and almost no equiaxed crystallization occurs even when a predetermined amount of Mg is added. ing.
本発明者らはまた、微量のSeおよびTeを溶鋼中に添加し、固液界面エネルギーを低下させ、柱状晶自体を微細・脆弱化させれば、双ロール内の湯溜まり部で等軸晶核が新たに生成し、溶鋼過熱度の上昇や溶鋼再酸化に起因する等軸晶核生成能の低下を補償できることを見出した。SeおよびTeの添加により等軸晶核が増殖する理由は、固液界面エネルギーの低下により冷却ロール側から溶鋼側に成長する柱状晶が微細・脆弱化され、この柱状晶が冷却ロールの回転に伴う圧縮力と溶鋼流の剪断力により分断され新たな等軸晶核となるためである。 The present inventors also add a small amount of Se and Te to the molten steel to reduce the solid-liquid interfacial energy and make the columnar crystals themselves fine and fragile, so that equiaxed crystals are formed in the pool portion in the twin rolls. It was found that new nuclei are generated, and it is possible to compensate for the decrease in equiaxed nucleation ability due to the increase in molten steel superheat and the reoxidation of molten steel. The reason why equiaxed crystal nuclei proliferate due to the addition of Se and Te is that the columnar crystals that grow from the cooling roll side to the molten steel side become fine and fragile due to the decrease in solid-liquid interface energy, and these columnar crystals become the rotation of the cooling roll. This is because it is divided by the accompanying compressive force and the shearing force of the molten steel flow to form a new equiaxed crystal nucleus.
そこで本発明において、前述のとおりに溶鋼中にMgを添加するとともに、SeとTeの一方又は両方を添加して、双ロール連続鋳造を行った。その結果、鋳造安定部のみならず、鋳造初期や連々鋳の継ぎ目部においても、鋳片に等軸晶が生成することが明らかになった。ここで、柱状晶微細化の効果は、SeもしくはTeの少なくとも1種以上を合計で0.0002質量%以上添加すれば十分であるが、0.005質量%を超えて添加すると鋼板が脆弱化し、鋳造後のリコイル時や冷間圧延時に端部に割れが発生する。このため、溶鋼中にはSeおよびTeの内から1種以上を合計で0.0002〜0.005質量%になるように添加すればよい。 Therefore, in the present invention, as described above, Mg was added to the molten steel, and one or both of Se and Te were added to perform double roll continuous casting. As a result, it was clarified that equiaxed crystals are formed in the slab not only in the casting stable part but also in the initial stage of casting and the joint part of continuous casting. Here, for the effect of columnar crystal refinement, it is sufficient to add at least one type of Se or Te in a total amount of 0.0002% by mass or more, but if it is added in excess of 0.005% by mass, the steel sheet becomes fragile. , Cracks occur at the ends during recoil after casting or cold rolling. Therefore, one or more of Se and Te may be added to the molten steel so as to have a total of 0.0002 to 0.005% by mass.
上記[1]、[2]および[3]の方法を組み合わせることにより、高清浄性を確保した上で、ノズル詰まりと介在物粗大化が起こり難い介在物組成と異方性が発現し難い凝固組織に制御した薄肉鋳片を、双ロール式連続鋳造法を用いて安定的に鋳造することができる。尚、本発明において、薄肉鋳片とは、厚み5mm以下の鋳片をいうものとする。 By combining the above methods [1], [2] and [3], while ensuring high cleanliness, nozzle clogging and inclusion coarsening are unlikely to occur, and inclusion composition and anisotropy are unlikely to develop. The structure-controlled thin-walled slab can be stably cast by using the double-roll continuous casting method. In the present invention, the thin-walled slab means a slab having a thickness of 5 mm or less.
本発明により得られた薄肉鋳片内の大型介在物の存在状態を評価したところ、30μmを超える大きな酸化物は5個/cm2未満しか存在せず、酸化物は微細化されていた。ここで、介在物の分散状態は、鋳片または鋼板の研磨面(C断面)を100倍の光学顕微鏡で観察し、単位面積内の介在物粒径分布を評価した。この介在物の粒径は、長径と短径を測定し、(長径×短径)0.5として求めた相当直径とした。さらに、本発明の薄肉鋳片の清浄性を全酸素濃度で評価したところ、0.002質量%以下であり非常に良好であった。ここで、全酸素濃度とは、鋳片に含まれる酸素(酸化物の酸素や溶存酸素をすべて含む)の総和であり、通常のガス分析装置により分析できる。また、本発明の鋳片は、等軸晶率10%以上で鋳片中央部の凝固組織は等軸晶化されている。このように鋳片の清浄性を高め、鋳片内の介在物を微細な酸化物として分散させると共に、鋳片中央部の凝固組織を等軸晶化することにより、加工時における鋼板の割れ発生と異方性を抑制できるため、加工性と成形性に優れた薄鋼板素材となる薄肉鋳片を提供できる。 When the state of existence of large inclusions in the thin-walled slab obtained by the present invention was evaluated, only 5 large oxides exceeding 30 μm / cm 2 were present, and the oxides were refined. Here, as for the dispersed state of inclusions, the polished surface (C cross section) of the slab or steel plate was observed with a 100x optical microscope, and the particle size distribution of inclusions within a unit area was evaluated. The particle size of this inclusion was the equivalent diameter obtained by measuring the major axis and the minor axis and determining (major axis x minor axis) 0.5 . Further, when the cleanliness of the thin-walled slab of the present invention was evaluated by the total oxygen concentration, it was 0.002% by mass or less, which was very good. Here, the total oxygen concentration is the total amount of oxygen contained in the slab (including all oxide oxygen and dissolved oxygen), and can be analyzed by a normal gas analyzer. Further, in the slab of the present invention, the equiaxed crystal ratio is 10% or more, and the solidified structure at the center of the slab is equiaxed. In this way, the cleanliness of the slab is improved, the inclusions in the slab are dispersed as fine oxides, and the solidified structure at the center of the slab is isotropically crystallized to generate cracks in the steel sheet during processing. Since the anisotropy can be suppressed, it is possible to provide a thin-walled slab which is a thin steel plate material having excellent workability and formability.
本発明により鋳造した薄肉鋳片は、通常の冷間圧延、再結晶温度以上での連続焼鈍を行い、引き続き調質圧延を施すことにより鋼板を製造できる。 The thin-walled slab cast according to the present invention can be produced as a steel sheet by subjecting it to normal cold rolling, continuous annealing at a recrystallization temperature or higher, and subsequent temper rolling.
最後に、本発明の薄肉鋳片の化学成分のうち、既に述べたC、酸可溶Al、酸可溶Ti、Mg、Se、Te以外の化学成分の作用について言及する。 Finally, among the chemical components of the thin-walled slab of the present invention, the actions of chemical components other than C, acid-soluble Al, acid-soluble Ti, Mg, Se, and Te described above will be mentioned.
Siは、0.005質量%以上0.03質量%以下であることが好ましい。Si濃度は0.005質量%未満では板の強度が不足するため、またSi濃度が0.03質量%超では板の加工性が低下するためである。 Si is preferably 0.005% by mass or more and 0.03% by mass or less. This is because if the Si concentration is less than 0.005% by mass, the strength of the plate is insufficient, and if the Si concentration is more than 0.03% by mass, the workability of the plate is lowered.
MnはC、Siとともに鋼板の強度向上に有効な元素であり、必要な場合には0.1質量%以上は含有させることが好ましいが、0.6質量%を超えて含有させると粗大なMnSが生成し延性を低下させる可能性があるため0.6質量%以下にすることが好ましい。Mnがなくても本発明を損なうことはないため、下限値は定めない。 Mn is an element effective for improving the strength of the steel sheet together with C and Si, and if necessary, it is preferably contained in an amount of 0.1% by mass or more, but if it is contained in excess of 0.6% by mass, coarse MnS Is generated and may reduce ductility, so it is preferably 0.6% by mass or less. Since the present invention is not impaired even if there is no Mn, the lower limit is not set.
Pは材質を脆くし、過度に含有すると結晶粒界に偏析して深絞り加工割れの原因となるため、実用上支障のないことが明確な0.02質量%以下にすることが好ましい。Pがなくても本発明を損なうことはないため、下限値は定めない。 P makes the material brittle, and if it is contained excessively, it segregates at the grain boundaries and causes deep drawing cracks. Therefore, it is preferable that P is 0.02% by mass or less, which is clear that there is no problem in practical use. Since the present invention is not impaired without P, the lower limit is not set.
Sは、粗大なMnSを生成して延性や成形性を劣化させるため、0.01質量%以下にすることが好ましい。Sを含有しなくても本発明を損なうことはないため、下限値は特に定めない。 Since S produces coarse MnS and deteriorates ductility and moldability, it is preferably 0.01% by mass or less. Since the present invention is not impaired even if S is not contained, the lower limit value is not particularly set.
Nは添加し過ぎると、微量なAlであっても粗大な析出物を生成し、加工性を劣化させるので、0.01質量%以下とすることが好ましい。一方、0.0005質量%未満とするにはコストがかかるので、0.0005質量%以上にすることが好ましい。 If N is added too much, even a small amount of Al will generate a coarse precipitate and deteriorate the processability. Therefore, the amount of N is preferably 0.01% by mass or less. On the other hand, it is costly to make it less than 0.0005% by mass, so it is preferably 0.0005% by mass or more.
本発明の主要な添加元素の効果を述べたが、それ以外に、Nb、V、Mo、Niなどの元素も、Nb:0.05質量%以下、V:0.03質量%以下、Mo:0.03質量%以下、Ni:0.05質量%以下の範囲であれば、加工性を劣化させないので添加可能である。この範囲内での各元素の添加により、Nbによって深絞り性が向上し、VとMoによって強度が向上し、Niによって耐食性が向上する。また、スクラップの利用による微量のCu、NiおよびCr等の不可避的不純物としての混入は、本発明を損なうものではない。 Although the effects of the main additive elements of the present invention have been described, other elements such as Nb, V, Mo and Ni also have Nb: 0.05% by mass or less, V: 0.03% by mass or less, Mo: If it is in the range of 0.03% by mass or less and Ni: 0.05% by mass or less, it can be added because it does not deteriorate the workability. By adding each element within this range, Nb improves the deep drawing property, V and Mo improve the strength, and Ni improves the corrosion resistance. Further, mixing of trace amounts of Cu, Ni, Cr and the like as unavoidable impurities by using scrap does not impair the present invention.
以下の表1、表2に、実施例及び比較例を挙げて、本発明について説明する。表1、表2において、本発明から外れる数値・項目にアンダーラインを付している。 The present invention will be described in Tables 1 and 2 below with reference to Examples and Comparative Examples. In Tables 1 and 2, numerical values and items that deviate from the present invention are underlined.
脱酸前溶存酸素濃度については固体電解質酸素センサーを用いて評価し、結果を表2に示した。試験番号14は転炉での脱炭処理を終了し、取鍋に出鋼した段階で評価した。また、全酸素濃度は鋳片において評価を実施した。 The dissolved oxygen concentration before deoxidation was evaluated using a solid electrolyte oxygen sensor, and the results are shown in Table 2. Test number 14 was evaluated at the stage when the decarburization treatment in the converter was completed and the steel was put out in the ladle. In addition, the total oxygen concentration was evaluated on the slab.
双ロール鋳造時のノズルへの介在物付着状況は、ノズル開度がほぼ一定であれば「なし」、ノズル開度が徐々に増加傾向であれば「あり」、鋳造末期に溶鋼がでない状態であれば「閉塞」として、表2に記載した。 The state of inclusions adhering to the nozzle during double-roll casting is "none" if the nozzle opening is almost constant, "yes" if the nozzle opening is gradually increasing, and there is no molten steel at the end of casting. If there is, it is listed in Table 2 as "occlusion".
30μm超の酸化物個数密度については、鋳片の研磨面(C断面)を100倍の光学顕微鏡で観察して表2に示した。介在物の粒径は、長径と短径を測定し、(長径×短径)0.5として求めた円相当直径である。 The oxide number density of more than 30 μm is shown in Table 2 by observing the polished surface (C cross section) of the slab with a 100 times optical microscope. The particle size of the inclusions is the equivalent diameter of a circle obtained by measuring the major axis and the minor axis and calculating (major axis x minor axis) 0.5 .
鋳片の等軸晶率は、鋳片C断面でピクリン酸エッチにより凝固組織を顕出し、鋳片厚みに対する等軸晶領域厚みの比とした。 The equiaxed crystal ratio of the slab was defined as the ratio of the equiaxed crystal region thickness to the slab thickness by revealing the solidified structure by picric acid etching on the slab C cross section.
製造した薄肉鋳片を冷間圧延した後、焼鈍温度700℃で連続焼鈍を行い、さらに調質圧延を行って、板厚0.16mmの冷延鋼板とした。なお、再結晶温度は650℃未満であるので、焼鈍温度700℃であれば確実に再結晶温度以上で連続焼鈍できている。 The produced thin-walled slab was cold-rolled, then subjected to continuous annealing at an annealing temperature of 700 ° C., and further temper-rolled to obtain a cold-rolled steel sheet having a plate thickness of 0.16 mm. Since the recrystallization temperature is less than 650 ° C., continuous annealing can be surely performed at an annealing temperature of 700 ° C. or higher.
実製缶機より割れ発生が1000倍程度高い難製缶条件とした2ピース缶用の製缶試験機で製缶した。製缶時の割れ発生率は、この製缶試験機で製缶した個数に対する割れ発生缶個数の比率を、さらに1/1000倍して求めた値として評価した。また、イヤリング高さは、缶円周方向の最大高さ(山部)と最小高さ(谷部)の差(mm)として評価し、結果を表2に示した。 Cans were made with a can-making test machine for two-piece cans, which had difficult can-making conditions in which cracking was about 1000 times higher than that of an actual can-making machine. The crack generation rate during can production was evaluated as a value obtained by further multiplying the ratio of the number of cracked cans to the number of cans produced by this can manufacturing tester by 1/1000. The earring height was evaluated as the difference (mm) between the maximum height (mountain part) and the minimum height (valley part) in the circumferential direction of the can, and the results are shown in Table 2.
本発明の実施例である試験番号1−7では、浸漬ノズルへの介在物付着はなく鋳造は安定しており、薄肉鋳片の全長に渡って高清浄化と介在物微細化も両立されていたため、製缶時の割れ発生率は1ppm以下であった。また、薄肉鋳片の板厚中央部における凝固組織も鋳片全長に渡って等軸晶率10%以上が確保され、その異方性を消失させることができたため、深絞り加工時のイヤリングは通常の連続鋳造材の1.5mmと同等あるいはさらに低下させることが可能となった。 In Test No. 1-7, which is an example of the present invention, there was no inclusions adhering to the immersion nozzle, casting was stable, and both high purification and miniaturization of inclusions were achieved over the entire length of the thin-walled slab. The cracking rate during can manufacturing was 1 ppm or less. In addition, the solidified structure at the center of the plate thickness of the thin-walled slab also secured an equiaxed crystal ratio of 10% or more over the entire length of the slab, and the anisotropy could be eliminated. It has become possible to reduce the thickness to the same level as or even lower than the 1.5 mm of a normal continuous cast material.
一方、比較例の試験番号8、9ではMg濃度が適正でなく、試験番号11、12ではAlやTi濃度が適正でないため、何れもノズル付着や介在物粗大化が生じ、加工時に割れが発生した。また、凝固組織も等軸晶化できなかったため、イヤリングが発生した。比較例の試験番号10では、Mgは適正に添加しているが、SeとTeの何れも添加しなかったため、浸漬ノズルへの介在物付着と介在物粗大化は抑制され製缶時の割れ発生率も1ppm以下であったが、鋳造温度が高く溶鋼再酸化の影響を受ける鋳造初期では十分な等軸晶率が得られずイヤリングが問題となった。比較例の試験番号13では、C濃度を0.01質量%以下に低下させるため、酸素を補う必要があり、真空脱ガス装置において酸素吹き込みを実施し、溶存酸素濃度を0.035質量%超に増大させてしまったため、介在物量が増加すると共に、Mgを添加しても介在物の凝集やノズル付着を抑制できず、浸漬ノズルは鋳造末期に完全に閉塞し、深絞り加工時に割れも多発した。勿論、凝固組織の異方性も解消できず、大きなイヤリングを発生させた。 On the other hand, in the test numbers 8 and 9 of the comparative example, the Mg concentration is not appropriate, and in the test numbers 11 and 12, the Al and Ti concentrations are not appropriate. Therefore, nozzle adhesion and coarsening of inclusions occur in each of the test numbers 11 and 12, and cracks occur during processing. did. In addition, since the solidified structure could not be equiaxed, earrings were generated. In Test No. 10 of the comparative example, Mg was added properly, but neither Se nor Te was added, so that the adhesion of inclusions to the immersion nozzle and the coarsening of inclusions were suppressed and cracks occurred during can making. The ratio was also 1 ppm or less, but the earrings became a problem because a sufficient equiaxed crystal ratio could not be obtained at the initial stage of casting due to the high casting temperature and the influence of molten steel reoxidation. In Test No. 13 of the comparative example, it is necessary to supplement oxygen in order to reduce the C concentration to 0.01% by mass or less, oxygen is blown in the vacuum degassing device, and the dissolved oxygen concentration exceeds 0.035% by mass. As the amount of inclusions increased, the aggregation of inclusions and nozzle adhesion could not be suppressed even if Mg was added, the immersion nozzle was completely closed at the end of casting, and cracks frequently occurred during deep drawing. did. Of course, the anisotropy of the coagulated tissue could not be eliminated, and large earrings were generated.
さらに、比較例の試験番号14の実験では、転炉のみで表1のC濃度まで脱炭処理した溶鋼にAlを添加して脱酸し、続いてMgを添加して最終表1の成分の溶鋼を溶製したため、溶存酸素濃度が0.035質量%超で過剰となり、介在物量が増加すると共に、Mgを添加しても介在物の凝集やノズル付着を抑制できず、浸漬ノズルは鋳造末期に完全に閉塞し、深絞り加工時に割れも多発した。板厚中央部の凝固組織の等軸晶化もできなかったため、大きなイヤリングが発生した。 Further, in the experiment of Test No. 14 of the comparative example, Al was added to deoxidize the molten steel that had been decarburized to the C concentration in Table 1 only in the converter, and then Mg was added to obtain the components of the final Table 1. Since the molten steel was melted, the dissolved oxygen concentration became excessive at more than 0.035% by mass, the amount of inclusions increased, and even if Mg was added, aggregation of inclusions and nozzle adhesion could not be suppressed, and the immersion nozzle was in the final stage of casting. It was completely closed and cracks occurred frequently during deep drawing. Large earrings were generated because the solidified structure in the center of the plate thickness could not be crystallized equiaxed.
1.冷却ロール
2.湯溜まり部
3.溶鋼
4.ノズル
5.タンディッシュ
6.薄肉鋳片
7.整流多孔ノズルまたはフィルター
1. 1.
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