JP2018015794A - Manufacturing method of low carbon steel thin slab, low carbon steel thin slab, and manufacturing method of low carbon steel thin steel sheet - Google Patents
Manufacturing method of low carbon steel thin slab, low carbon steel thin slab, and manufacturing method of low carbon steel thin steel sheet Download PDFInfo
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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 excellent in workability, formability and cleanliness produced by a twin-roll continuous casting method, a method for producing the same, and a method for producing a low-carbon steel thin steel plate.
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・シェイプ連続鋳造として有力なものとして、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、湯溜まり部２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 technology for producing a thin plate close to the final product at the casting stage, that is, a near net shape continuous casting has been developed. Of these, a twin-roll type continuous casting method is disclosed in
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、ノズル詰まりの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。
In contrast, Patent Document 2 discloses a method in which a filter is built in an immersion nozzle to generate an undisturbed discharge flow over the entire width of the nozzle. In Patent Document 3, a slit nozzle is provided with a rectifying porous nozzle, Methods for rectifying the nozzle discharge flow are disclosed. In addition, in the twin roll continuous casting method of Al deoxidized molten steel, it is known that the turbulence of the discharge flow caused by nozzle clogging causes fluctuations in the molten metal surface and destabilizes the casting. Patent Document 4 proposes a method for preventing nozzle clogging by adding and reforming to a low melting point inclusion of CaO—Al 2 O 3 . Furthermore, since it is difficult to prevent clogging of nozzles and surface defects due to alumina inclusions in Al deoxidized molten steel, a method of deoxidizing with Ce and La and leaving dissolved oxygen and combined deoxidation with Ti, Ce and La The method is disclosed 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 Patent Documents 2 to 3 described above show some effects in stainless steel (not Al deoxidation) produced by a twin-roll type continuous casting method. As the alumina inclusions, which are deoxidation products, become coarse due to agglomeration, they also adhere to the submerged nozzle discharge holes, the filter in the submerged nozzle, and the rectifying porous nozzle. There is a problem that internal defects frequently occur due to re-entrainment of inclusions. Furthermore, in the twin roll type continuous casting method, the molten steel injected from the tundish solidifies in a very short time, and the floating time of inclusions cannot be secured, so in Al deoxidized molten steel, most coarse alumina inclusions are thin-walled slabs. There is a high possibility that internal defects will occur even in a stable casting state that is trapped inside and does not fluctuate. In addition, the modification method of alumina inclusions of Patent Document 4 works effectively for preventing clogging of the immersion nozzle and clogging of the filter, but the modified CaO—Al 2 O 3 inclusion is a liquid phase. Easily coalesces and coarsens in ladle, tundish, immersion nozzle. Since this coarse inclusion is not removed by the filter in the submerged nozzle, it is immediately captured in the thin-walled slab as described above, and causes cracks (internal defects) during processing. Further, as described in
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。 Furthermore, in a thin steel plate manufactured by a twin roll type continuous casting method, anisotropy appears during processing, for example, when deep drawing is performed during can making, so-called crests and troughs continue alternately in the circumferential direction of the can. Earrings occur. If these earrings are large, the yield of cans will decrease, and the earrings will come into contact with the mold and lead to troubles in can making. Therefore, thin steel sheets obtained by the twin-roll continuous casting method are required to have high formability and workability. The current situation is that it cannot be applied to other applications.
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 the present situation, the present invention reduces the inclusions in the molten steel as much as possible, and can control the inclusion composition and the solidified structure in which anisotropy is unlikely to occur and the inclusion composition and the inclusion coarsening hardly occur. An object of the present invention is to provide a roll-type continuous casting method, a low-carbon steel thin-walled slab excellent in workability and formability cast using the same, and a method for producing 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とその防止対策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 such a situation, the twin roll continuous that can be controlled to a solidified structure in which the inclusion composition and anisotropy are less likely to develop anisotropy in which the clogging of the nozzle and the inclusion coarsening hardly occur after the inclusion in the molten steel is reduced as much as possible. In order to provide a casting method, and a low-carbon steel thin-walled slab excellent in workability and formability cast using the same, it is effective in the inclusion reduction method of low-carbon steel, prevention of nozzle clogging and inclusion coarsening Elucidation of additive elements, modification of inclusions, elucidation of anisotropy mechanism during processing and countermeasures to prevent it, and repeated researches, combining the obtained knowledge optimally in the twin roll continuous casting process The design has led to the completion of the present invention.
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、且つ酸可溶Ａｌ濃度と酸可溶Ｔｉ濃度の合計を０質量％超に成分調整した後、さらにＣｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上の合計を０．０００３〜０．０３質量％添加して介在物制御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）に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）に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、且つ酸可溶Ａｌ濃度と酸可溶Ｔｉ濃度の合計を０質量％超に成分調整すると共に、３分以上攪拌を行った後、Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上の合計を０．０００３〜０．０３質量％添加して介在物制御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。
（５）Ｃ：０．０１〜０．０５質量％、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、酸可溶Ａｌ＋酸可溶Ｔｉ：０質量％超、Ｎ：０．０００５〜０．０１質量％、Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上の合計：０．０００３〜０．０３質量％、全酸素濃度が０．００２質量％以下、残部が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。
（７）（５）又は（６）に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。
The summary is as follows. That is,
(1) Decarburization treatment under reduced pressure following decarburization treatment under atmospheric pressure, and molten steel having a dissolved oxygen concentration of 0.005 to 0.035 mass%, at least one of Al and Ti 2 types are added and deacidified, acid-soluble Al concentration is 0.05 mass% or less, acid-soluble Ti concentration is 0.1 mass% or less, and the sum of acid-soluble Al concentration and acid-soluble Ti concentration After adjusting the component to more than 0% by mass, a molten steel in which inclusions are controlled by adding 0.0003 to 0.03% by mass of at least one of Ce, La, Nd or Pr is added to the twin roll type. A method for producing a low-carbon steel thin-walled slab characterized by casting by a continuous casting method.
(2) The C concentration in the molten steel after decarburization treatment under atmospheric pressure is 0.05 mass% or more and 0.1 mass% or less, and the C concentration after decarburization treatment under reduced pressure is 0.01 mass%. More than 0.05 mass%, The manufacturing method of the low carbon steel thin-walled slab as described in (1) characterized by the above-mentioned.
(3) Low carbon steel thin-wall casting according to (1) or (2), wherein decarburization treatment under atmospheric pressure is performed in a converter and decarburization treatment under reduced pressure is performed with a vacuum degassing apparatus. A manufacturing method of a piece.
(4) At least one or two of Al and Ti are added for deoxidation, 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 acid is acceptable. The total amount of dissolved Al concentration and acid-soluble Ti concentration is adjusted to more than 0% by mass, and after stirring for 3 minutes or more, at least one of Ce, La, Nd, or Pr is added to a total of 0.00. The low-carbon steel thin-wall casting according to any one of (1) to (3), wherein molten steel with inclusions controlled by adding 0003 to 0.03% by mass is cast by a twin-roll continuous casting method A manufacturing method of a piece.
(5) C: 0.01 to 0.05 mass%, Si: 0.005 to 0.03 mass%, Mn: 0.6 mass% or less, P: 0.02 mass% or less, 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, acid-soluble Al + acid-soluble Ti: more than 0% by mass, N: 0.0005-0. 01% by mass, a total of at least one of Ce, La, Nd, or Pr: 0.0003 to 0.03% by mass, total oxygen concentration is 0.002% by mass or less, the balance being Fe and inevitable impurities A low-carbon steel thin-walled slab having a thickness of 5 mm or less, wherein the number of oxides having a diameter of more than 30 μm is less than 5 / cm 2 and the equiaxed crystal ratio is 10% or more.
(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, or one or more of them are contained. The low-carbon steel thin-walled slab as described in (5),
(7) Low carbon steel characterized by subjecting the low-carbon steel thin-walled slab described in (5) or (6) to cold rolling, continuous annealing at a recrystallization temperature or higher, and subsequent temper rolling. Manufacturing method of thin steel sheet.
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, the cleanliness of molten steel is enhanced as much as possible, nozzle clogging and inclusion coarsening can be suppressed, and the anisotropy of the solidified structure can be reduced. A thin steel plate can be stably manufactured using a twin roll type continuous casting method.
以下に本発明を詳細に説明する。 The present invention is described in detail below.
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。 In general, low carbon steel is decarburized by blowing oxygen under atmospheric pressure such as a converter to produce molten steel having a final C concentration. The molten steel after the decarburization treatment contains a large amount of dissolved oxygen according to the C concentration, and if it is large, 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, and the cleanliness of the molten steel is greatly reduced. In addition, 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 slag after deoxidation, and the amount of alumina inclusions further increases. This alumina inclusion floats and separates while agglomerated and coalesced in the molten steel, but the amount of inclusions in the molten steel is about 0.004% by mass in the tundish, and is produced by agglomeration in the molten steel. The large-sized alumina inclusion (alumina cluster) which reaches about several hundred μm is also included. When this molten steel is cast by the twin-roll type continuous casting method, the solidification time is very short. Therefore, unlike the conventional continuous casting apparatus for slab, inclusion floating in the mold is hardly expected. In addition, the twin roll type continuous casting immersion nozzle has a complicated structure such as a flow straightening nozzle and a filter for the purpose of rectifying the discharge flow, so it has a larger amount than a normal continuous casting immersion nozzle. Inclusions adhere to the nozzle inner wall, discharge holes and filter. When the nozzle clogging occurs, the discharge flow from the immersion nozzle becomes unstable, and inclusions are re-entrained due to the fluctuation of the molten metal surface in the hot water pool between the rolls. In this way, when casting low-carbon steel by the twin roll continuous casting method, a large amount of alumina clusters that cause cracks during processing are trapped in the thin-walled slab. It was very difficult to produce a steel plate by a twin 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 solidification structure of the low-carbon steel thin-walled slab cast by the twin-roll type 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 at the solid-liquid interface during solidification. The C concentration is 0.1% by mass or less as in low carbon steel, and the temperature gradient as in twin roll casting. When becomes large, the columnar crystals are extremely easy to grow. Thin cast slabs with a thickness of several millimeters manufactured by the twin-roll type continuous casting method are cold-rolled to the final plate thickness, but unlike the conventional continuous cast slabs cast at a thickness of about 250 mm, the rolling reduction is increased. It cannot be secured. As a result, the direction of solidification structure growth also remains in the final thin steel sheet and appears as anisotropy during processing.For example, when deep drawing is performed during canning, peaks and troughs alternate in the circumferential direction of the can. The present inventors have found that so-called earrings are generated.
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。 In light of the above problems, the present invention provides [1] a method for reducing inclusions in low carbon molten steel, [2] a method for elucidating additive elements effective for preventing nozzle clogging and inclusion coarsening, and a method for modifying inclusions, [ 3] Process design by combining earnest research on solidification structure control based on anisotropy mechanism during processing and optimally combining the obtained knowledge from low-carbon steel melting process to twin-roll continuous casting process It was completed by doing.
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 inclusion reduction method in the low carbon molten steel of [1] will be described below. The technical idea of producing this low carbon steel is that it is refined under atmospheric pressure to blow the C concentration higher than the final component value, leaving excess carbon in the molten steel, and further decarburizing the molten steel under reduced pressure. This is to reduce the dissolved oxygen concentration to the limit and to produce a highly clean steel. 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 corresponding to the C concentration. 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 formed in the molten steel, and the molten steel 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、続いて減圧下で脱炭処理されるため、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 decarburization treatment under atmospheric pressure is higher than the product value, that is, 0.05 to 0.1% by mass, and the decarburization treatment is terminated. It becomes about 0.049-0.024 mass%, and the dissolved oxygen concentration is 0.06 mass% when decarburized to an average final C concentration (0.04 mass%) only by decarburization treatment under atmospheric pressure. Low. Since the decarburized molten steel under atmospheric pressure of the present invention is subsequently decarburized under reduced pressure, the decarburization reaction of the following formula (2) further proceeds, and the C concentration is the final component value (0.04). In addition, 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、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％に比べて非常に小さい。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 decarburization under atmospheric pressure is the lowest 0.05% by mass, the dissolved oxygen concentration after decarburization under reduced pressure is the highest, but still about 0.035% by mass Can be suppressed. Further, if the C concentration after the decarburization process under atmospheric pressure is higher than about 0.065% by mass, dissolved oxygen is insufficient in the subsequent decarburization process under reduced pressure, and the C concentration is reduced to the final component value ( 0.04% by mass). In that case, it is possible to supply oxygen from the outside when the decarburization reaction in the latter half of the decarburization process begins to stagnate (the dissolved oxygen concentration is about 0.005% by mass). Since it is consumed by the equation (2), dissolved oxygen can be adjusted to the final C concentration while maintaining a low dissolved oxygen concentration at the time when the external oxygen starts to be supplied. For this reason, the dissolved oxygen concentration after the decarburization process under reduced pressure can be reduced to about 0.035 to 0.005 mass%. Even if Al is added and deoxidized in this state, the amount of alumina inclusions produced is the total oxygen of the low carbon steel having a C concentration of 0.04% by mass produced only by decarburization under normal atmospheric pressure. It is very small compared to the concentration of 0.06% by mass. In addition, since the concentration of lower oxides such as FeO and MnO in the ladle slag is also reduced, the contamination of the molten steel by the slag after Al deoxidation is greatly reduced.
大気圧下でのＣ濃度を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 under atmospheric pressure is 0.05% by mass or more and 0.1% by mass or less is that when the C concentration exceeds 0.1% by mass, the decarburization treatment under reduced pressure becomes longer. This is because when the 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 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 decarburization treatment under reduced pressure is 0.01 mass% or more, which is sufficient to obtain a material as a low carbon steel. It is desirable to be less than 05% by mass. The C concentration after decarburization 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Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上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 decarburization treatment under reduced pressure can be deoxidized by adding one or two of Al or Ti. However, when the dissolved oxygen concentration after decarburization treatment under reduced pressure exceeds 0.035% by mass, the amount of inclusions generated by adding one or two of Al or Ti increases, and Ce, La described later Even if an appropriate amount of at least one of Nd and Pr is added, the alumina inclusions and titania inclusions cannot be modified, and the inclusions cannot be prevented from agglomerating and adhering to the nozzle. Conversely, 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. Extremely difficult in terms of both cost and processing time. Therefore, it is necessary to control the dissolved oxygen concentration after decarburization treatment under reduced pressure to 0.005 to 0.035% by mass. Here, under reduced pressure means a pressure lower than 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、Ｃｅ酸化物、Ｌａ酸化物、Ｎｄ酸化物、Ｐｒ酸化物、Ｃｅ酸硫化物、Ｌａ酸硫化物、Ｎｄ酸硫化物、Ｐｒ酸硫化物、或いはこれらの複合酸化物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, as described above, one or two of Al or Ti are added. The acid-soluble Al concentration after the addition is 0.05% by mass or less, and the acid-soluble Ti concentration is 0.1%. Less than mass. The reason for this is that, in the acid-soluble Al concentration and the acid-soluble Ti concentration exceeding these, as described later, alumina inclusions and titania inclusions are replaced with Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or complex oxides of these oxides do not proceed, and a large amount of remaining alumina inclusions and titania inclusions are aggregated and combined. This is because the nucleation sites of equiaxed crystals are insufficient and 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 variations in molten steel components and material deterioration, 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, the total of the acid-soluble (dissolved) Al concentration and the acid-soluble (dissolved) Ti concentration after addition of one or two of Al or Ti is at least 0 mass because of the requirement for sufficient deoxidation. %, Preferably 0.005% by mass or more, more preferably 0.01% by mass or more. By measuring the dissolved oxygen concentration after decarburization under reduced pressure and adding Al or Ti in excess of the amount of Al or Ti determined according to the stoichiometric ratio from the measured value, the above-mentioned preferred acidity can be obtained. 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 an acid. Dissolved Al and dissolved Ti dissolve in an acid, and alumina and titania do not dissolve in an acid. This is an analysis method that uses this. Here, the acid is a mixed acid mixed at a ratio of
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 that the molten steel after deoxidation by adding Al or Ti is provided with a stirring time of 3 minutes or more. This is because the decarburization treatment under reduced pressure can improve the cleanliness of the molten steel, but the inclusions can be efficiently removed by further taking the stirring time, and the cleanliness can be further enhanced.
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。 In addition, as a 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 a reduced pressure, a vacuum degassing device, a vacuum refining device, or the like 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、非常に単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 description will be given of a method for modifying the inclusions in the low-carbon molten steel whose cleanliness has been improved by the above method into [2] inclusions having a composition in which nozzle clogging and inclusion coarsening hardly occur. Even in molten steel that has been highly purified by decarburization treatment under reduced pressure, alumina inclusions and titania inclusions are very easy to agglomerate and coalesce. In addition, due to the complicated structure of the submerged nozzle in the twin-roll continuous casting method, there is a possibility that inclusions adhere to the inner wall of the nozzle, the discharge holes and the filter, and the nozzle is blocked. The twin roll type continuous casting method is characterized by a rapid solidification process that completes solidification in a very single hour. If molten steel can be injected into a twin-roll type continuous casting machine while preventing agglomeration in the molten steel, inclusions can be dispersed more uniformly and finely than the conventional slab continuous casting method due to its rapid cooling effect. And the most effective prevention of cracking during processing.
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Ｃｅ、Ｌａ、ＮｄもしくはＰｒ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Ｃｅ、Ｌａ、ＮｄもしくはＰｒ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Ｃｅ酸化物、Ｌａ酸化物、Ｎｄ酸化物、Ｐｒ酸化物、Ｃｅ酸硫化物、Ｌａ酸硫化物、Ｎｄ酸硫化物、Ｐｒ酸硫化物、或いはこれらの複合酸化物が生成する。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、介在物制御に適切なＣｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上の合計添加量は、０．０００３〜０．０３質量％である。これは、Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上の合計添加量が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Ｃｅ酸化物、Ｌａ酸化物、Ｎｄ酸化物、Ｐｒ酸化物、Ｃｅ酸硫化物、Ｌａ酸硫化物、Ｎｄ酸硫化物、Ｐｒ酸硫化物、或いはこれらの複合酸化物に改質できないためである。反対にＣｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上の合計添加量が０．０３質量％を超えると少なくとも介在物表層がＣｅ酸化物、Ｌａ酸化物、Ｎｄ酸化物、Ｐｒ酸化物、Ｃｅ酸硫化物、Ｌａ酸硫化物、Ｎｄ酸硫化物、Ｐｒ酸硫化物、或いはこれらの複合酸化物に改質されていても、強脱酸元素であるＣｅ、Ｌａ、ＮｄもしくはＰｒ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、Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上の合計添加量が０．０３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、これら強脱酸元素の合計添加量が０．０３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。なお、Ｃｅ、Ｌａ、Ｎｄ及びＰｒは互いに近似した特性を有しているため、本発明に係る効果に関してはＣｅ、Ｌａ、Ｎｄ及びＰｒの合計添加量で決定することができる。 Therefore, the present inventors have modified the alumina inclusions and titania inclusions in molten steel with relatively high cleanliness, and studied additive elements that suppress inclusion adhesion to aggregated coalescence and nozzles. It has been found that Ce, La, Nd, or Pr, which is a strong deoxidizing element compared to Ti, is an effective element for preventing aggregation and adhesion. When the strong deoxidizing element Ce, La, Nd or Pr is added to the molten steel with relatively high cleanliness, part or all of the alumina inclusions and titania inclusions in the molten steel is reduced, and at least on the inclusion surface layer Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or a composite oxide thereof is formed. Since this inclusion composition greatly reduces the interfacial energy with the low carbon molten steel, the inclusions of the inclusions on the nozzle and the filter refractory and the aggregate coalescence of the inclusions are suppressed at the same time. Here, the total addition amount of at least one of Ce, La, Nd, or Pr suitable for inclusion control is 0.0003 to 0.03% by mass. This is because, when the total addition amount of at least one of Ce, La, Nd, or Pr is less than 0.0003 mass%, the surface layer portion is made of an alumina inclusion that is more stable than the titania inclusion, and the surface layer portion is made of Ce oxide, La This is because the oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or a composite oxide thereof cannot be modified. On the other hand, if the total addition amount of at least one of Ce, La, Nd or Pr exceeds 0.03% by mass, at least the inclusion surface layer is Ce oxide, La oxide, Nd oxide, Pr oxide, Even if it is modified to Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or complex oxides thereof, strong deoxidation element Ce, La, Nd or Pr is dissolved oxygen This is because the interfacial energy between the inclusions and the molten steel is further increased to increase the coarsening and the nozzle adhesion. Here, if the total addition amount of at least one of Ce, La, Nd, or Pr is 0.03% by mass or less, an adsorbed oxygen of more than 0.001% by mass remains at the oxide interface. However, when the total amount of these strong deoxidizing elements exceeds 0.03% by mass, it is considered that even the adsorbed oxygen at the oxide interface is reduced and removed, and the interface energy is greatly increased. Since Ce, La, Nd, and Pr have characteristics that are close to each other, the effect according to the present invention can be determined by the total amount of Ce, La, Nd, and Pr.
さらに、[３]加工時に異方性が発現しにくい凝固組織制御の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Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上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Ｃｅ酸化物、Ｌａ酸化物、Ｎｄ酸化物、Ｐｒ酸化物、Ｃｅ酸硫化物、Ｌａ酸硫化物、Ｎｄ酸硫化物、Ｐｒ酸硫化物、或いはこれらの複合酸化物に改質し、それらの介在物を等軸晶生成の核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Ｃｅ酸化物、Ｌａ酸化物、Ｎｄ酸化物、Ｐｒ酸化物、Ｃｅ酸硫化物、Ｌａ酸硫化物、Ｎｄ酸硫化物、Ｐｒ酸硫化物、或いはこれらの複合酸化物に改質し、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、Ｃｅ、Ｌａ、ＮｄもしくはＰｒの添加は、[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。 Furthermore, [3] a method for controlling a solidified structure in which anisotropy is unlikely to occur during processing will be described. As described above, it has been found that the anisotropy occurs in the thin steel plate produced by the twin roll continuous casting method is due to the columnar crystal structure developed by rapidly solidifying the low carbon molten steel. Based on this anisotropy mechanism, the inventors of the present invention are effective in reducing the anisotropy by equiaxing the solidified structure of the low carbon steel. At least one of La, Nd and Pr is added, and at least the surface layer of alumina inclusions and titania inclusions is added to Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La Solidified structure by twin-roll continuous casting method by modifying to oxysulfide, Nd oxysulfide, Pr oxysulfide, or complex oxides of these, and utilizing these inclusions as nuclei for the formation of equiaxed crystals A new method for equiaxed crystallization was devised. According to the present invention, the inclusion is Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or a composite oxidation thereof. It is possible to reduce the interfacial energy between molten steel and inclusions, so the addition of Ce, La, Nd, or Pr is [2] coarsening of inclusions, prevention of nozzle adhesion and [3] solidification. It effectively acts on both the equiaxed crystallization of the structure, and becomes a very effective control means in the production of a low carbon steel sheet using a twin-roll continuous casting method.
上記本方法の制御条件は、Ｃ：０．０３質量％、Ｓｉ：０．０２質量％、Ｍｎ：０．４質量％、Ｐ：０．０１質量％、Ｓ：０．００５質量％、酸可溶Ａｌ：０．０３５質量％、Ｎ：０．００６質量％、Ｃｅ＋Ｌａ＋Ｎｄ＋Ｐｒ：０〜０．０２３質量％の低炭素鋼薄鋼板を双ロール式連続鋳造機を用いて製造し、その鋼板を深絞り加工することにより鋳造組織とイヤリング高さとの関係を評価して求めた。なお、Ｃｅ、Ｌａ、ＮｄもしくはＰｒの添加には、４５質量％Ｃｅ−３５質量％Ｌａ−１０質量％Ｎｄ−１０質量％Ｐｒ合金を用いた。 The control conditions of the above method are: C: 0.03% by mass, Si: 0.02% by mass, Mn: 0.4% by mass, P: 0.01% by mass, S: 0.005% by mass, acid OK A low carbon steel thin steel sheet of molten Al: 0.035% by mass, N: 0.006% by mass, Ce + La + Nd + Pr: 0-0.023% by mass is produced using a twin-roll continuous casting machine, and the steel plate is deep-drawn. The relationship between the cast structure and the earring height was evaluated by processing. For addition of Ce, La, Nd or Pr, a 45 mass% Ce-35 mass% La-10 mass% Nd-10 mass% Pr alloy was used.
Ｃｅ＋Ｌａ＋Ｎｄ＋Ｐｒを０．０００３質量％以上添加することにより、薄肉鋳片の等軸晶率（等軸晶厚み／板厚×１００（％））は１０％以上となり、イヤリング高さ（缶円周方向の最大高さ（山部）と最小高さ（谷部）の差（ｍｍ））はＣｅ＋Ｌａ＋Ｎｄ＋Ｐｒを含有せず、それ以外の成分が上記と同成分である通常の連続鋳造材（イヤリング高さで１．５ｍｍ程度）のそれと同等以下に低減した。さらに、Ｃｅ＋Ｌａ＋Ｎｄ＋Ｐｒ濃度０．００２質量％、等軸晶率２５％でイヤリング高さは０．８ｍｍ程度まで低減でき、通常の連続鋳造材よりも良好となったが、等軸晶化の効果はそこで飽和した。 By adding Ce + La + Nd + Pr 0.0003% by mass or more, the equiaxed crystal ratio (equal crystal thickness / plate thickness × 100 (%)) of the thin cast slab becomes 10% or more, and the earring height (in the circumferential direction of the can) The difference between the maximum height (mountain portion) and the minimum height (valley portion (mm)) does not contain Ce + La + Nd + Pr, and the other components are the same as those described above. About 5 mm) or less. Furthermore, the Ce + La + Nd + Pr concentration was 0.002% by mass, the equiaxed crystal ratio was 25%, and the height of the earrings could be reduced to about 0.8 mm, which was better than ordinary continuous cast material, but the effect of equiaxed crystallization was there. Saturated.
等軸晶率の増加と共にイヤリング高さが低下するのは、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にＣｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上の添加により等軸晶組織が形成されるためであり、さらに等軸晶化によるイヤリング高さ低減の効果が飽和するのは、２５％の等軸晶率で凝固組織起因の異方性が全て解消したためだと考えられる。したがって、異方性改善にはＣｅ＋Ｌａ＋Ｎｄ＋Ｐｒを０．０００３質量％以上、好ましくは０．００２質量％以上に制御することが有効である。 As the equiaxed crystal ratio increases, the earring height decreases because the deformation due to cold rolling is difficult to be transmitted and the anisotropy of the solidified structure is likely to remain in Ce, La, Nd or Pr, This is because an equiaxed crystal structure is formed by the addition of at least one kind, and the effect of reducing the earring height by equiaxed crystallization is saturated at an equiaxed crystal ratio of 25%. This is thought to be due to the elimination of all the directivity. Therefore, for improving the anisotropy, it is effective to control Ce + La + Nd + Pr to 0.0003 mass% or more, preferably 0.002 mass% or more.
一方、前述したようにＣｅ＋Ｌａ＋Ｎｄ＋Ｐｒを、０．０３質量％を超えて添加すると、Ｃｅ酸化物、Ｌａ酸化物、Ｎｄ酸化物、Ｐｒ酸化物、Ｃｅ酸硫化物、Ｌａ酸硫化物、Ｎｄ酸硫化物、Ｐｒ酸硫化物、或いはこれらの複合酸化物に改質しても凝集合体により粗大化することから、等軸晶の核生成サイトも不足し、異方性は改善されない。よって、Ｃｅ＋Ｌａ＋Ｎｄ＋Ｐｒ濃度は０．０３質量％以下にする必要がある。 On the other hand, as described above, when Ce + La + Nd + Pr is added in an amount exceeding 0.03% by mass, Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide Even if it is modified to Pr oxysulfide or a composite oxide thereof, it is coarsened by agglomeration, so that equiaxed nucleation sites are insufficient and anisotropy is not improved. Therefore, the Ce + La + Nd + Pr concentration needs to be 0.03% by mass or less.
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 methods [1], [2] and [3] above, while ensuring high cleanliness, it is difficult to cause clogging of nozzles and coarsening of inclusions, and solidification that hardly causes anisotropy. A thin-walled slab controlled to a texture can be cast using a twin-roll continuous casting method. In the present invention, the thin slab refers to 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、本発明の薄肉鋳片においても、材質劣化防止の観点から溶鋼と同様に、酸可溶Ａｌ濃度と酸可溶Ｔｉ濃度の合計が、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。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 presence of large inclusions in the thin cast slab obtained by the present invention was evaluated, there were less than 5 / cm 2 of large oxides exceeding 30 μm, and the oxides were refined. Here, the dispersion state of the inclusions was evaluated by observing the polished surface (C cross section) of the slab with a 100 × optical microscope and evaluating the inclusion particle size distribution within the unit area. The particle diameter of the inclusions was determined by measuring the major axis and the minor axis, and taking the equivalent diameter as 0.5 (major axis x minor axis). Furthermore, when the cleanliness of the thin cast slab of the present invention was evaluated by the total oxygen concentration, it was 0.002% by mass or less and was very good. Here, the total oxygen concentration is the sum of oxygen contained in the slab (including all oxide oxygen and dissolved oxygen) and can be analyzed by a normal gas analyzer. Also in the thin cast slab of the present invention, the total of the acid-soluble Al concentration and the acid-soluble Ti concentration is at least more than 0% by mass, preferably from 0. It is 005 mass% or more, More preferably, it is 0.01 mass% or more. The slab of the present invention has an equiaxed crystal ratio of 10% or more and the solidified structure in the center of the slab is equiaxed. In this way, the cleanliness of the slab is increased, the inclusions in the slab are dispersed as fine oxides, and the solidification structure in the center of the slab is equiaxed to generate cracks in the steel sheet during processing. Therefore, it is possible to provide a thin cast slab that becomes a thin steel plate material excellent in 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 by performing 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、Ｃｅ、Ｌａ、Ｎｄ、Ｐｒ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 cast slab of the present invention, mention will be made of the action of chemical components other than C, acid-soluble Al, acid-soluble Ti, Ce, La, Nd, and Pr already described.
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。 It is preferable that Si is 0.005 mass% or more and 0.03 mass% or less. This is because if the Si concentration is less than 0.005 mass%, the strength of the plate is insufficient, and if the Si concentration exceeds 0.03 mass%, the workability of the plate decreases.
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 is an element effective for improving the strength of the steel sheet together with C and Si. If necessary, Mn is preferably contained in an amount of 0.1% by mass or more, but if it exceeds 0.6% by mass, coarse MnS is contained. It is preferable to make it 0.6% by mass or less because there is a possibility of reducing the ductility.
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 contained excessively, it segregates at the grain boundaries and causes deep drawing cracks, so it is preferable to make it 0.02% by mass or less, which is clearly not problematic for practical use. Even if P is not present, the present invention is not impaired, so a lower limit is not determined.
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 formability, it is preferable to be 0.01% by mass or less. Even if it does not contain S, the present invention is not impaired, so the lower limit is not particularly defined.
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 if it is a trace amount of Al, a coarse precipitate is generated and the workability is deteriorated. Therefore, the N content is preferably 0.01% by mass or less. On the other hand, since it takes cost to make it less than 0.0005 mass%, it is preferable to make it 0.0005 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％、Ｖ≦０．０３質量％、Ｍｏ≦０．０３質量％、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 mass%, V ≦ 0.03 mass%, and Mo ≦ 0.03. If it is the range of mass% and Ni <= 0.05 mass% or less, since workability is not deteriorated, it can be added. By adding each element within this range, deep drawability is improved by Nb, strength is improved by V and Mo, and corrosion resistance is improved by Ni. In addition, the incorporation of trace amounts of Cu, Ni, Cr and other inevitable impurities due to the use of scrap does not impair the present invention.
以下に、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にアンダーラインを付している。 Hereinafter, the present invention will be described with reference to examples and comparative examples. In Tables 1 and 2, numerical values deviating from the present invention are underlined.
表１、表２の試験番号１〜１２については、転炉での脱炭処理によりＣ濃度を０．０５５質量％まで低下させ、続いて真空脱ガス装置により表１のＣ濃度まで脱炭処理し溶鋼にＡｌまたはＴｉを添加して脱酸し、表２に示す脱酸後攪拌時間で攪拌を実施した後、Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上を添加して表１成分の溶鋼１００ｔを溶製した（試験番号１−１２）。試験番号１３と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Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上を添加して最終表１の成分の溶鋼を溶製した。これらの溶鋼を、図１に示すような双ロール式連続鋳造法を用いて、厚み２．５ｍｍ、幅１２００ｍｍの薄肉鋳片に鋳造した。
For
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を行い、結果を表２に示した。試験番号１〜１２は真空脱ガス処理中の脱酸前の段階で評価を行い、試験番号１３、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を行った。また、全酸素濃度は鋳片において評価を行った。 The dissolved oxygen concentration before deoxidation was evaluated using a solid electrolyte oxygen sensor, and the results are shown in Table 2. Test Nos. 1 to 12 are evaluated at the stage before deoxidation during vacuum degassing, and Test Nos. 13 and 14 are evaluated at the stage where the decarburization process in the converter is finished and the steel is put out in a ladle. It was. Further, the total oxygen concentration was evaluated in the slab.
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評価して表２に示した。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。 The oxide number density of more than 30 μm was evaluated by observing the polished surface (C cross section) of the slab with a 100 × optical microscope and shown in Table 2. The particle diameter of the inclusions was determined by measuring the major axis and the minor axis, and taking the equivalent diameter as 0.5 (major axis x minor axis).
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の厚みの比として評価した。 The equiaxed crystal ratio of the slab was evaluated as a ratio of the thickness of the equiaxed crystal region to the thickness of the slab by revealing a solidified structure by picric acid etching in the cross section of the slab C.
連続鋳造時における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」、鋳造末期に溶鋼が出ない状態であれば「閉塞」とし、表２に示した。 During continuous casting, the state of inclusion adhesion to the nozzle is “None” if the nozzle opening is almost constant, “Yes” if the nozzle opening is gradually increasing, and no molten steel is produced at the end of casting. If there is any, “blocking” is shown in Table 2.
製造した薄肉鋳片について冷間圧延を行い、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 manufactured thin cast slab was cold-rolled, continuously annealed at an annealing temperature of 700 ° C., and further temper-rolled to obtain a cold-rolled steel sheet having a thickness of 0.16 mm. In addition, since the recrystallization temperature is less than 650 ° C., if the annealing temperature is 700 ° C., the continuous annealing can be surely performed at the recrystallization temperature 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を行った。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、更に１／１０００倍して求めた値として評価し、またイヤリング高さを缶円周方向の最大高さ（山部）と最小高さ（谷部）の差（ｍｍ）とし評価し、結果を表２に示した。 Cans were made with a can-making tester for two-piece cans, which had difficult-to-make conditions where cracking was about 1000 times higher than actual cans. The rate of occurrence of cracks during can making was evaluated as a value obtained by further multiplying the ratio of the number of cracked cans with respect to the number of cans made with this can making tester, and the earring height was measured in the circumferential direction of the can. The difference between the maximum height (peak) and the minimum height (valley) was evaluated (mm), 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、薄肉鋳片の高清浄化と介在物の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等軸晶化され、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よりも低下させることが可能となった。 In Test Nos. 1-7, which are examples of the present invention, there was no inclusion adhesion to the immersion nozzle, casting was stable, and both high cleaning of thin-walled slabs and refinement of inclusions were compatible. The crack occurrence rate at that time was 1 ppm or less. In addition, the solidification structure in the center of the thickness of the thin cast slab was also equiaxed and the anisotropy could be eliminated. Therefore, the earrings during deep drawing were more than 1.5 mm of normal continuous cast material. It became possible to reduce.
一方、比較例の試験番号８、９ではＣｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上の合計濃度が適正でなく、試験番号１０、１１では酸可溶Ａｌや酸可溶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１２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、脱酸前溶存酸素濃度を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、Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上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 examples, the total concentration of at least one of Ce, La, Nd, or Pr is not appropriate, and in test numbers 10 and 11, the acid-soluble Al and acid-soluble Ti concentrations are low. Since they were not appropriate, both nozzle adhesion and inclusion coarsening occurred, and cracks occurred during processing. Further, since the solidified structure could not be equiaxed, earrings were generated. In the test number 12 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 was blown in a vacuum degassing apparatus, and the dissolved oxygen concentration before deoxidation was set to 0.035. Since the amount of inclusions has increased, the amount of inclusions has increased, and even if at least one of Ce, La, Nd, or Pr is added, aggregation of inclusions and nozzle adhesion cannot be suppressed. Closed completely at the end of casting and cracked frequently during deep drawing. Of course, the anisotropy of the solidified structure could not be eliminated and a large earring was generated.
さらに、比較例の試験番号１３と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Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上を添加して最終表１の成分の溶鋼を溶製したため、脱酸前溶存酸素濃度が０．０３５質量％超で過剰となり、介在物量が増加すると共に、Ｃｅ、Ｌａ、ＮｄもしくはＰｒの内、少なくとも１種以上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。 Furthermore, in the experiments of test numbers 13 and 14 of the comparative examples, Al was added to the decarburized molten steel that was decarburized to the C concentration shown in Table 1 only by the converter, and then, within Ce, La, Nd, or Pr In addition, since at least one kind was added to melt the molten steel of the components in the final table 1, the dissolved oxygen concentration before deoxidation exceeded 0.035% by mass, the amount of inclusions increased, and Ce, La, Nd Alternatively, even if at least one of Pr was added, inclusion aggregation and nozzle adhesion could not be suppressed, and the immersion nozzle was completely closed at the end of casting, and many cracks occurred during deep drawing. Since the equiaxed crystallization of the solidified structure at the center of the plate thickness could not be performed, large earrings were generated.
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. Cooling roll 2. Hot water pool Molten steel 4.
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