JP2015006680A - Continuous casting method of cast piece and continuous casting cast piece - Google Patents
Continuous casting method of cast piece and continuous casting cast piece Download PDFInfo
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本発明は、凝固組織および析出物が微細な鋳片の連続鋳造方法に関し、特に、極厚鋼板に用いるために、中心偏析の生成抑制を図った鋳片の連続鋳造方法、およびその連続鋳造鋳片に関する。 The present invention relates to a continuous casting method of a slab having a fine solidification structure and fine precipitates, and in particular, a continuous casting method of a slab that suppresses the generation of center segregation for use in an extremely thick steel plate, and the continuous casting of the slab. Regarding the piece.
近年、構造物の大型化や高強度化への要求に応えるため、構造物の素材となる極厚鋼板に対する品質の向上が、コストの低減とともに課題となっている。 In recent years, in order to meet the demands for increasing the size and strength of structures, improving the quality of extra-thick steel sheets that are the materials of the structures has become a challenge as well as reducing costs.
従来、極厚鋼板は、インゴット鋳造により製造された大型鋼塊を分塊圧延することで分塊スラブを作製し、これを圧延することで製造されている。しかし、この分塊スラブを用いる場合には、大型鋼塊の上部に存在する押し湯部を除去したり、底部に生成した偏析や引け巣を除去したり必要があるため、歩留まりが低いという問題がある。また、鋼板を製造するには分塊圧延工程が必要であり、製造コストが大幅に増大するとともに製造期間も長くなり、生産効率の低下を招く。 Conventionally, an extra-thick steel plate is manufactured by producing a block slab by rolling a large steel ingot manufactured by ingot casting and rolling the slab. However, when using this slab, it is necessary to remove the hot-water part that exists at the top of the large steel ingot, or to remove the segregation and shrinkage generated at the bottom, so that the yield is low. There is. In addition, in order to manufacture a steel sheet, a block rolling process is required, which greatly increases the manufacturing cost and lengthens the manufacturing period, leading to a decrease in production efficiency.
これらの問題を解決するため、近年では、極厚鋼板用の素材として連続鋳造法で製造された鋳片が適用されるようになり、歩留まりの向上および生産効率の向上が図られてきている。 In order to solve these problems, in recent years, slabs manufactured by a continuous casting method have been applied as a material for extra-thick steel sheets, and improvements in yield and production efficiency have been attempted.
しかし、その場合、連続鋳造鋳片も極厚化するため、鋳片内部、特に中心部における凝固組織が粗大化する。これにともなって、ミクロ偏析およびマクロ偏析、特に鋳片の中心部に中心偏析が生成し、品質向上を阻害するという新たな問題が顕在化する。 However, in that case, since the continuous cast slab is also made extremely thick, the solidified structure inside the slab, particularly in the center, becomes coarse. Along with this, microsegregation and macrosegregation, in particular, center segregation is generated at the center of the slab, and a new problem of hindering quality improvement becomes apparent.
そのため、連続鋳造鋳片において中心偏析の生成を抑制する技術について、従来から多くの提案がなされている。 Therefore, many proposals have conventionally been made on techniques for suppressing the generation of center segregation in continuous cast slabs.
例えば、特許文献１には、鋳片の厚さ方向中心部の固相率が０．４以下の時点から鋳片の軽圧下を開始して、鋳片の厚さ方向中心部が凝固完了するまで軽圧下を継続し、且つ、軽圧下しつつ鋳片の厚さ方向中心部が凝固完了するまで鋳片表面を強冷却して、この冷却による鋳片の熱収縮速度を所定の範囲に制御する連続鋳造方法が開示されている。この連続鋳造方法では、熱収縮速度と軽圧下速度の和も所定の範囲に抑制することで、中心偏析低減効果を一層発揮させることができるとされている。 For example, in Patent Document 1, light reduction of the slab is started from the time when the solid phase ratio in the center part in the thickness direction of the slab is 0.4 or less, and the center part in the thickness direction of the slab is solidified. The slab surface is strongly cooled until the solidification of the center of the slab in the thickness direction is completed, and the heat shrinkage rate of the slab is controlled within a predetermined range. A continuous casting method is disclosed. In this continuous casting method, it is said that the center segregation reduction effect can be further exerted by suppressing the sum of the heat shrinkage rate and the light reduction rate within a predetermined range.
また、特許文献２には、連続鋳造用鋳型より引き抜いた鋳片ストランドの引き抜き移動中に、それを両側に挟む金敷にて該鋳片ストランドの凝固完了点近傍に鍛圧加工を施すに当たり、鋳片ストランドの鋳造速度もしくは冷却速度を調整して鍛圧加工点におけるストランドの固相率を変化させる連続鋳造方法が開示されている。この連続鋳造方法では、鋳片圧下点における固相率の許容範囲は０．５〜０．９５が好適であり、鋳造時間の経過とともに固相率を徐々に高めていくか、または低くしていくような手法が有効であるとされている。 Further, in Patent Document 2, during the drawing movement of a slab strand drawn from a continuous casting mold, a forging process is performed in the vicinity of a solidification completion point of the slab strand with an anvil sandwiching the slab strand. A continuous casting method is disclosed in which the solid casting rate of the strand at the forging point is changed by adjusting the casting speed or cooling speed of the strand. In this continuous casting method, the allowable range of the solid fraction at the slab reduction point is preferably 0.5 to 0.95, and the solid fraction is gradually increased or lowered as the casting time elapses. It is said that the following methods are effective.
しかしながら、特許文献１および２に開示された連続鋳造方法は、いずれも鋳片の未凝固領域が残存する部分を圧下する方法である。鋳片の未凝固領域が残存する部分を圧下すると未凝固領域の溶質濃度が変化し、それとともに固相率も変化するため、最適とされる固相率に制御して圧下することは操業上難しい。 However, the continuous casting methods disclosed in Patent Documents 1 and 2 are both methods of reducing the portion of the slab where the unsolidified region remains. When the part of the slab where the unsolidified region remains is reduced, the solute concentration in the unsolidified region changes, and the solid phase ratio also changes accordingly. difficult.
特許文献３には、動的再結晶に着目し、連続鋳造鋳片を中心部まで凝固させた後、鋳片の厚さ方向中心部の温度と鋳片の表面温度との差を４００℃以上とし、歪速度を１×１０-3ｓ-1〜１×１０-2ｓ-1として圧下することにより、微細組織を有し機械的性質に優れた連続鋳造鋳片を製造する方法が開示されている。この連続鋳造方法によれば、合金元素を添加することなく凝固２次組織が微細化した連続鋳造鋳片を安価に製造することができるとされている。この連続鋳造鋳片を用いることにより、鋳片から極厚鋼板等を低圧下比で製造する際の特別な設備や複雑な製造工程は不要となる。また、この連続鋳造鋳片を熱間圧延加工することにより、厚さ方向中心部の材質特性に優れた鋼材を得ることが可能である。 Patent Document 3 focuses on dynamic recrystallization, solidifies a continuous cast slab to the center, and then sets the difference between the temperature at the center of the slab in the thickness direction and the surface temperature of the slab at 400 ° C. or more. And a strain rate of 1 × 10 −3 s −1 to 1 × 10 −2 s −1 , and a method for producing a continuous cast slab having a fine structure and excellent mechanical properties is disclosed. ing. According to this continuous casting method, it is said that a continuous cast slab with a refined solidified secondary structure can be manufactured at low cost without adding an alloy element. By using this continuous cast slab, special equipment and a complicated manufacturing process are not required when a very thick steel plate or the like is manufactured from the slab at a low pressure ratio. Further, by subjecting this continuous cast slab to hot rolling, it is possible to obtain a steel material having excellent material properties in the central portion in the thickness direction.
しかしながら、上述のとおり、極厚鋼板を製造するための連続鋳造鋳片は極厚であることから、鋳片の中心近傍の冷却速度は小さく、凝固組織が粗大となるため、凝固組織の形成に伴って生成するミクロ偏析も著しくなる。さらに、ミクロ偏析に起因してマクロ偏析が生じ、鋳片の厚さ方向での溶質濃度が変化し、中心偏析となる。中心偏析が生成した場合、所望の機械的特性に優れた鋼材を製造することは困難である。 However, as described above, since the continuous cast slab for producing an extremely thick steel plate is extremely thick, the cooling rate near the center of the slab is small and the solidified structure becomes coarse, so that the solidified structure is formed. Accompanying microsegregation also becomes significant. Furthermore, macrosegregation occurs due to microsegregation, the solute concentration in the thickness direction of the slab changes, and central segregation occurs. When center segregation is generated, it is difficult to produce a steel material having excellent desired mechanical properties.
本発明は、上記の問題に鑑みてなされたものであり、中心偏析の生成を抑制することが可能で極厚鋼板の製造に適した鋳片の連続鋳造方法およびこの連続鋳造方法による鋳片を提供することを目的とする。 The present invention has been made in view of the above-described problems. A continuous casting method of a slab that can suppress the generation of center segregation and is suitable for manufacturing an extremely thick steel plate, and a slab produced by the continuous casting method. The purpose is to provide.
大型構造物の素材となる極厚鋼板の機械的特性を確保するには、組成が鋼板内で均一であることが前提となる。連続鋳造法で製造され、極厚鋼板用の素材となる鋳片には、厚さ方向の中心近傍で粗大なデンドライトを含む凝固組織が形成されており、このデンドライトの凝固に伴ってミクロ偏析が生じる。そのため、デンドライトおよびデンドライトの間隙において、デンドライトの成長方向に溶質元素の濃度が大きく変化する。 In order to ensure the mechanical properties of the ultra-thick steel plate that is the material of the large structure, it is assumed that the composition is uniform within the steel plate. The cast slab produced by the continuous casting method and used as the material for the extra-thick steel sheet has a solidified structure containing coarse dendrites near the center in the thickness direction, and microsegregation occurs as the dendrites solidify. Arise. Therefore, the concentration of the solute element greatly changes in the dendrite growth direction in the gap between the dendrite and the dendrite.
また、ミクロ偏析は、中心偏析および粗大な析出物が発生する原因となる。これは、ミクロ偏析が元になって、鋳片の厚さ方向中心部においてマクロ偏析が生じ、このマクロ偏析が顕在化して中心偏析が生成し、これにともない粗大な析出物が生成することによる。 Microsegregation also causes central segregation and coarse precipitates. This is due to the occurrence of macro-segregation at the center of the slab in the thickness direction due to micro-segregation, and the macro-segregation is manifested to produce center segregation, resulting in the formation of coarse precipitates. .
通常の操業範囲内の熱処理では、このようにデンドライトおよびデンドライトの間隙において、デンドライトの成長方向に濃度が大きく変化した溶質元素を拡散させ、組成を均一とすることが困難である。そのため、この鋳片を熱間圧延しても、残存したデンドライトを含む凝固組織が単に圧延されるだけで、デンドライトの間隙には濃化した溶質元素が残存したままとなる。 In the heat treatment within the normal operating range, it is difficult to make the composition uniform by diffusing the solute element whose concentration greatly changes in the dendrite growth direction in the gap between the dendrites and the dendrites. Therefore, even if this slab is hot-rolled, the solidified structure containing the remaining dendrite is simply rolled, and the concentrated solute element remains in the gap between the dendrites.
また、鋳片の凝固組織の大きさは冷却速度に依存するため、鋼の成分が同一である場合には、冷却速度を高めればデンドライトを小さくすることができる。しかし、極厚鋼板用の鋳片のように鋳片の厚さが厚くなると、凝固シェル自体が熱伝導律速となり、鋳片内部の冷却速度を高めることができない。 Moreover, since the size of the solidified structure of the slab depends on the cooling rate, if the steel components are the same, the dendrite can be reduced by increasing the cooling rate. However, when the thickness of the slab becomes thick like a slab for extra-thick steel plates, the solidified shell itself becomes the rate of heat conduction, and the cooling rate inside the slab cannot be increased.
このため、冷却速度を高めて製造した極厚鋼板用の鋳片では、鋳片表層のデンドライトは小さいものの、鋳片の厚さ方向の中心に向かってデンドライトが大きくなり、鋳片の厚さ方向の中心近傍ではデンドライトの１次アーム間隔が数ｍｍに達する場合もある。このように、鋳片の表層部と中心近傍とではデンドライトの大きさの差が著しく、熱処理および熱間圧延工程を経てもこのデンドライトの大きさの差の影響による溶質元素の濃度の不均一さを解消することができない。 For this reason, in slabs for ultra-thick steel sheets manufactured with an increased cooling rate, the dendrite on the slab surface layer is small, but the dendrite increases toward the center of the slab thickness direction, and the slab thickness direction In the vicinity of the center of the dendrite, the primary arm spacing of the dendrite may reach several mm. Thus, the difference in dendrite size between the surface layer of the slab and the vicinity of the center is significant, and the concentration of solute elements is not uniform due to the difference in size of the dendrite even after heat treatment and hot rolling. Can not be resolved.
極厚鋼板用の鋳片の場合、このようなデンドライトの大きさの差が特に顕著であり、この鋳片から得られた極厚鋼板は、部位によって機械的特性が異なり、不均一な状態となる。極厚鋼板は、単にそのまま鋼板として使用される場合は少なく、大入熱溶接によって構造物を構築したり、切削加工によって鋼板の厚さ方向のいずれかの部位が表面に露出したりする。 In the case of slabs for extra-thick steel plates, the difference in the size of such dendrites is particularly remarkable, and the extra-thick steel plates obtained from these slabs have different mechanical properties depending on the part, and are in a non-uniform state. Become. Extremely thick steel plates are rarely used as steel plates as they are, and structures are constructed by high heat input welding, or any part in the thickness direction of the steel plates is exposed on the surface by cutting.
前述のように、特許文献３では、鋼板の厚さ方向中心部の凝固２次組織を微細化させ、機械的特性を向上させる技術が開示されている。しかし、同文献では、凝固組織であるデンドライトや析出物については考慮されておらず、デンドライトの間隙で溶質元素が濃化し、溶質元素の濃度が低いデンドライトと濃度が高い間隙からなる凝固組織が形成されることについては検討されていない。 As described above, Patent Document 3 discloses a technique for refining the solidified secondary structure in the central portion in the thickness direction of a steel sheet and improving the mechanical characteristics. However, this document does not consider dendrites and precipitates, which are solidified structures, and solute elements are concentrated in the gaps between dendrites, forming a solidified structure consisting of dendrites with low solute element concentrations and gaps with high concentrations. It has not been studied about what will be done.
つまり、従来の技術では、凝固組織は組成が均一であることが前提とされており、デンドライトを含む凝固組織のミクロ偏析や析出物を伴う場合については検討されていない。 In other words, in the conventional technology, it is assumed that the solidified structure has a uniform composition, and the case where microsegregation or precipitates of the solidified structure including dendrite is not considered.
そこで、鋳片の凝固組織であるデンドライトの間隙に生じるミクロ偏析およびマクロ偏析と熱処理との関連について検討した結果、以下の２点が明らかとなった。
・ミクロ偏析およびマクロ偏析の大きさは隣接するデンドライトの間隔の大きさ（１次アーム間隔や２次アーム間隔の大きさ）に依存する。
・隣接するデンドライトの間隔が狭いほど熱処理を行う際の溶質元素の拡散が進行し易い。
Then, as a result of examining the microsegregation generated in the gap between the dendrites which is the solidification structure of the slab and the relationship between the macrosegregation and the heat treatment, the following two points were clarified.
The size of micro-segregation and macro-segregation depends on the size of the interval between adjacent dendrites (primary arm interval and secondary arm interval).
-The narrower the distance between adjacent dendrites, the easier the diffusion of solute elements during heat treatment.
溶質元素の拡散が進行し易いと、連続鋳造した鋳片における溶質元素の濃度が均一化しやすく、最終製品の機械的特性の向上が図れる。 If the diffusion of the solute element is easy to proceed, the concentration of the solute element in the continuously cast slab is easily made uniform, and the mechanical properties of the final product can be improved.
さらに、本発明者らは、後述する基礎実験等の検討を行い、下記（ａ）〜（ｄ）の知見を得た。 Furthermore, the present inventors have studied basic experiments and the like described later, and obtained the following findings (a) to (d).
（ａ）本発明者らは、厚さ方向の組成が均一ではない鋳片に対して、熱処理および圧延を施して、厚さ方向の凝固組織を微細化することにより、厚さ方向の組成が均一であり、中心偏析のない鋼板とすることができることを知見した。鋼板、特に極厚鋼板の機械的特性を向上させるには、鋼板の厚さ方向の組成を均一にする必要がある。厚さ方向の組成は、連続鋳造鋳片において均一でない場合でも、最終製品である鋼板において均一であれば鋼板の機械的特性を向上させることができる。従来の文献には、連続鋳造鋳片の厚さ方向の組成が均一ではない場合に加熱工程および圧延工程によって鋼板の厚さ方向の組成を均一化させるという概念は見当たらない。 (A) The present inventors perform heat treatment and rolling on a slab whose composition in the thickness direction is not uniform, thereby refining the solidified structure in the thickness direction, whereby the composition in the thickness direction is reduced. It was found that the steel sheet can be made uniform and free of center segregation. In order to improve the mechanical properties of a steel plate, particularly an extra-thick steel plate, it is necessary to make the composition in the thickness direction of the steel plate uniform. Even if the composition in the thickness direction is not uniform in the continuous cast slab, the mechanical properties of the steel sheet can be improved if it is uniform in the steel sheet as the final product. In the conventional literature, when the composition in the thickness direction of the continuous cast slab is not uniform, there is no concept of making the composition in the thickness direction of the steel sheet uniform by the heating process and the rolling process.
（ｂ）本発明者らは、連続鋳造鋳片の凝固組織を微細化するには、凝固が完了した後で鋳片の圧下を行うことが有効であることを知見した。特に、鋳片の厚さ方向中心が凝固した直後に圧下を行うと、鋳片の厚さ方向中心部において圧下量が大きくなり、凝固組織の変形量が大きくなるため、凝固組織の微細化により有効であることを知見した。 (B) The present inventors have found that, in order to refine the solidification structure of a continuously cast slab, it is effective to reduce the slab after solidification is completed. In particular, if the reduction is performed immediately after the center of the slab in the thickness direction is solidified, the amount of reduction in the center of the slab in the thickness direction increases, and the amount of deformation of the solidified structure increases. It was found to be effective.
（ｃ）本発明者らは、凝固過程における析出物は、凝固組織であるデンドライトの間隙で生成する場合が多く、また、粗大化しやすいことを知見した。これはデンドライトの間隙において溶質元素が濃化し、析出物の溶解度積が上限を超えるためである。このようにデンドライトの間隙で生成する析出物は、溶質濃度の高い領域で生成することから、デンドライトの間隙の広さで析出物自体の大きさが決まる。逆に、析出物を微細化するには、デンドライトの間隙を狭くすればよい。デンドライトの間隙を狭くするには、鋳片を圧下すればよい。圧下によりデンドライトの間隙を強制的に狭くすることができる。鋳片の圧下は、鋳片の厚さ方向中心が凝固した直後に行うのが、デンドライトの間隙を狭くするのにより効果的である。 (C) The present inventors have found that precipitates in the solidification process are often generated in the gaps between dendrites, which are solidified structures, and are easily coarsened. This is because the solute element is concentrated in the gap between the dendrites, and the solubility product of the precipitate exceeds the upper limit. Since precipitates generated in the dendrite gap are generated in a region having a high solute concentration, the size of the precipitate itself is determined by the width of the dendrite gap. On the contrary, in order to refine the precipitate, the gap between the dendrites may be narrowed. To narrow the gap between the dendrites, the slab may be reduced. The dendrite gap can be forcibly narrowed by the reduction. The reduction of the slab is performed immediately after the center of the slab in the thickness direction is solidified, so that the dendrite gap is narrowed.
（ｄ）本発明者らは、凝固組織であるデンドライトを微細化するとともに析出物を微細化するには、鋳片の厚さ方向中心が凝固した直後における圧下の他に、界面活性元素（Ｂｉ、ＳｎおよびＴｅ）を溶鋼中に添加して、鋳片を連続鋳造することが効果的であることを知見した。 (D) In order to refine the dendrite, which is a solidified structure, and to refine the precipitates, the inventors of the present invention include a surface active element (Bi) in addition to the reduction immediately after the center of the slab in the thickness direction is solidified. , Sn and Te) were found to be effective in continuously casting the slab by adding it to the molten steel.
本発明は、これらの知見に基づいてなされたものであり、その要旨は、下記の連続鋳造方法およびこの連続鋳造方法によって製造された連続鋳造鋳片にある。 This invention is made | formed based on these knowledge, The summary exists in the following continuous casting method and the continuous cast slab manufactured by this continuous casting method.
質量％で、Ｃ：０．０１％〜０．２０％、Ｓｉ：０．０２％〜０．５０％、Ｍｎ：０．６％〜３．０％、Ｐ：０．０２％以下、Ｓ：０．００２〜０．０３０％、Ａｌ：０．０００５〜０．０５００％、Ｔｉ：０．００５〜０．０３０％、Ｎ：０．００２〜０．０１０％およびＯ：０．０００１〜０．０１５０％を含有し、残部がＦｅおよび不純物からなる鋳片の連続鋳造方法であって、圧下を行うことなく鋳造した場合の鋳片の厚さ方向中心におけるデンドライト１次アーム間隔λ0を基準とし、鋳片の厚さ方向中心におけるデンドライト１次アーム間隔λと前記λ0の比の値λ／λ0が０．１〜０．９となるように、鋳片の厚さ方向中心が凝固した直後に圧下を行うことを特徴とする鋳片の連続鋳造方法。 In mass%, C: 0.01% to 0.20%, Si: 0.02% to 0.50%, Mn: 0.6% to 3.0%, P: 0.02% or less, S: 0.002-0.030%, Al: 0.0005-0.0500%, Ti: 0.005-0.030%, N: 0.002-0.010%, and O: 0.0001-0. This is a continuous casting method of a slab containing 0150%, the balance being Fe and impurities, and based on the dendrite primary arm interval λ 0 at the center of the slab in the thickness direction when cast without reduction. The center of the slab in the thickness direction was solidified so that the ratio λ / λ 0 of the dendrite primary arm interval λ and the λ 0 in the thickness direction center of the slab was 0.1 to 0.9. A method for continuously casting a slab, wherein the reduction is performed immediately after.
本発明の連続鋳造方法では、鋳片が、Ｆｅの一部に代えて、質量％で、Ｂｉ、ＳｎおよびＴｅのうちの１種以上を合計で０．０００１〜０．０３００％を含有することが好ましい。 In the continuous casting method of the present invention, the slab contains 0.0001 to 0.0300% in total of at least one of Bi, Sn, and Te in mass% instead of a part of Fe. Is preferred.
また、圧下を行うことなく鋳造した場合の鋳片の厚さ方向中心における析出物の直径ｄ0を基準とし、鋳片の厚さ方向中心における析出物の直径ｄと前記ｄ0の比の値ｄ／ｄ0が０．１〜０．９となるように、鋳片の厚さ方向中心が凝固した直後に圧下を行うことが好ましい。 Further, the diameter d 0 of the precipitates in the thickness direction center of the slab in the case of casting without performing reduction as a reference, the ratio of the value of the d 0 the diameter d of precipitates in the thickness direction center of the slab It is preferable to perform the reduction immediately after the center of the slab in the thickness direction is solidified so that d / d 0 is 0.1 to 0.9.
デンドライトの大きさや形状を特徴付ける寸法として１次アーム間隔の他に２次アーム間隔がある。一般的には１次アーム間隔と２次アーム間隔の間には比例関係があることから、本発明で指標とする１次アーム間隔は、２次アーム間隔から算出することもできる。 In addition to the primary arm interval, there is a secondary arm interval as a characteristic characterizing the size and shape of the dendrite. In general, since there is a proportional relationship between the primary arm interval and the secondary arm interval, the primary arm interval as an index in the present invention can also be calculated from the secondary arm interval.
以下の説明では、鋼の成分組成についての「質量％」を、単に「％」と表記する。 In the following description, “mass%” for the composition of steel is simply expressed as “%”.
本発明の連続鋳造方法は、鋳片の厚さによらず適用が可能であるとともに、鋳片の凝固が完了した直後に圧下を行うため、鋳片の未凝固領域の固相率の制御を行う必要がなく、容易に実施することができる。 The continuous casting method of the present invention can be applied regardless of the thickness of the slab, and since the reduction is performed immediately after the solidification of the slab is completed, the solid phase ratio of the unsolidified region of the slab is controlled. It is not necessary to do so and can be carried out easily.
また、本発明の連続鋳造方法で製造された鋳片、すなわち本発明の連続鋳造鋳片は、凝固組織が微細かつ均一であり、析出物も微細であり、中心偏析の生成が抑制され、組成が均一であるため、機械的特性が良好であり、大型構造物に用いられる極厚鋼板用の素材として適する。 In addition, the slab produced by the continuous casting method of the present invention, that is, the continuous cast slab of the present invention has a solidified structure that is fine and uniform, and the precipitates are also fine, the generation of center segregation is suppressed, and the composition Is uniform, it has good mechanical properties and is suitable as a material for extra-thick steel plates used for large structures.
本発明の連続鋳造方法は、Ｃ：０．０１％〜０．２０％、Ｓｉ：０．０２％〜０．５０％、Ｍｎ：０．６％〜３．０％、Ｐ：０．０２％以下、Ｓ：０．００２〜０．０３０％、Ａｌ：０．０００５〜０．０５００％、Ｔｉ：０．００５〜０．０３０％、Ｎ：０．００２〜０．０１０％およびＯ：０．０００１〜０．０１５０％を含有し、残部がＦｅおよび不純物からなる鋳片の連続鋳造方法であって、圧下を行うことなく鋳造した場合の鋳片の厚さ方向中心におけるデンドライト１次アーム間隔λ0を基準とし、鋳片の厚さ方向中心におけるデンドライト１次アーム間隔λと前記λ0の比の値λ／λ0が０．１〜０．９となるように、鋳片の厚さ方向中心が凝固した直後に圧下を行うことを特徴とする。 The continuous casting method of the present invention is as follows: C: 0.01% to 0.20%, Si: 0.02% to 0.50%, Mn: 0.6% to 3.0%, P: 0.02% Hereinafter, S: 0.002-0.030%, Al: 0.0005-0.0500%, Ti: 0.005-0.030%, N: 0.002-0.010%, and O: 0.00. It is a continuous casting method of a slab containing 0001 to 0.0150%, the balance being Fe and impurities, and the dendrite primary arm interval λ at the center in the thickness direction of the slab when cast without reduction With reference to 0 , the thickness direction of the slab is such that the ratio λ / λ 0 of the dendrite primary arm interval λ and the λ 0 at the center of the slab thickness direction is 0.1 to 0.9. The rolling is performed immediately after the center is solidified.
１．基礎実験（１）
本発明者らは、既に、特許文献４において、鋼中に界面活性元素を添加することで、凝固組織であるデンドライト組織を微細化可能とする技術を開示している。この技術によれば、デンドライトの間隙に濃化した溶質元素の拡散が促進され、凝固後の組織の均一化が図れることがわかっている。界面活性元素としては、Ｂｉ、ＳｎおよびＴｅが挙げられる。
1. Basic experiment (1)
In the patent document 4, the present inventors have already disclosed a technique that makes it possible to refine a dendrite structure, which is a solidified structure, by adding a surface active element to steel. According to this technique, it is known that diffusion of solute elements concentrated in the gaps between dendrites is promoted, and the structure after solidification can be made uniform. Examples of the surface active element include Bi, Sn, and Te.
しかし、本発明者らは、まず、界面活性元素を含有しない鋼の鋳片に熱処理および圧延を施して得られた鋼板の組織について調査を行った。 However, the present inventors first investigated the structure of a steel sheet obtained by subjecting a steel slab containing no surface active element to heat treatment and rolling.
１−１．実験条件
連続鋳造機内での圧下を行わずに０．１１％Ｃ−１．５％Ｍｎ鋼の連続鋳造を行い、得られた厚さ２４０ｍｍ、幅１２００ｍｍの連続鋳造鋳片から、長さ８０００ｍｍのスラブを複数採取した。
1-1. Experimental conditions The 0.11% C-1.5% Mn steel was continuously cast without being reduced in the continuous casting machine. From the obtained continuous cast slab having a thickness of 240 mm and a width of 1200 mm, the length was 8000 mm. Multiple slabs were collected.
熱処理時間の影響および熱処理温度の影響を調査することを目的とし、各スラブについて異なる条件で熱処理を施した後、熱間圧延して鋼板とし、これを試片とした。また、熱処理を施さず、圧延もしなかった鋳造ままのスラブを基準試片とした。 In order to investigate the influence of the heat treatment time and the influence of the heat treatment temperature, each slab was heat-treated under different conditions and then hot-rolled into a steel plate, which was used as a specimen. Further, a slab as cast without being heat-treated and not rolled was used as a reference specimen.
熱処理時間の影響の調査は、加熱温度を１２５０℃、加熱時間を２時間、５時間、１０時間および２４時間とし、圧延により厚さを２００ｍｍ、１５０ｍｍおよび５０ｍｍとした鋼板を試片として用いた。 In the investigation of the influence of the heat treatment time, a steel plate having a heating temperature of 1250 ° C., a heating time of 2, 5, 10 and 24 hours and a thickness of 200 mm, 150 mm and 50 mm by rolling was used as a specimen.
熱処理温度の影響の調査は、加熱温度を１１００℃、１２００℃および１２５０℃、加熱時間を１０時間とし、圧延により厚さを１５０ｍｍとした鋼板を試片として用いた。 In the investigation of the influence of the heat treatment temperature, a steel plate having a heating temperature of 1100 ° C., 1200 ° C. and 1250 ° C., a heating time of 10 hours, and a thickness of 150 mm by rolling was used as a specimen.
各試片から、厚さ方向中心を含む、３０ｍｍ角の組織観察用の試料を採取し観察面を研磨した。組織観察用試料の凝固組織の顕出にはピクリン酸飽和溶液を用いた。 A sample for 30 mm square tissue observation including the center in the thickness direction was collected from each specimen, and the observation surface was polished. A picric acid saturated solution was used to reveal the coagulated tissue of the tissue observation sample.
組織観察後の試料を用いて、析出物の析出位置の確認を行った。観察の対象はＭｎＳ析出物とし、試料を再度研磨して鏡面とした面について、ＳＥＭ−ＥＤＡＸ（エネルギー分散型Ｘ線分光走査型電子顕微鏡）を用いて倍率１００００倍で観察した。 Using the sample after the structure observation, the deposit position of the deposit was confirmed. The object of observation was a MnS precipitate, and the surface which was polished again to be a mirror surface was observed at a magnification of 10,000 using a SEM-EDAX (energy dispersive X-ray spectroscopic scanning electron microscope).
また、組織観察後の試料を用いて、溶質元素の偏析の測定を行った。測定の対象とした溶質元素はＭｎとした。鋳片の圧延方向のＭｎ濃度の分析はＥＰＭＡ（電子線マイクロアナライザ）を用い、ビーム径５０μｍで厚さ方向に線分析を行って、鋼板内のＭｎ濃度分布を測定し、測定範囲でのＭｎの最大濃度を求めた。Ｍｎの最大濃度の値を溶鋼段階の化学分析から求めたＭｎの初期含有率で割った値を偏析比とした。 Moreover, the segregation of the solute element was measured using the sample after the structure observation. The solute element to be measured was Mn. For the analysis of the Mn concentration in the rolling direction of the slab, an EPMA (electron beam microanalyzer) is used, the line analysis is performed in the thickness direction with a beam diameter of 50 μm, the Mn concentration distribution in the steel sheet is measured, and the Mn in the measurement range is measured. The maximum concentration of was determined. The value obtained by dividing the value of the maximum concentration of Mn by the initial content of Mn obtained from the chemical analysis at the molten steel stage was taken as the segregation ratio.
１−２．実験結果
上記条件で作製した試片について、「析出物の径比ｄ／ｄ0」および「偏析比指数」を評価項目として評価を行い、その結果を表１および２に示した。表１には熱処理時間の影響の調査結果を示し、表２には熱処理温度の影響の調査結果を表２に示した。
1-2. Experimental Results The specimens prepared under the above conditions were evaluated using “precipitate diameter ratio d / d 0 ” and “segregation ratio index” as evaluation items, and the results are shown in Tables 1 and 2. Table 1 shows the results of the investigation of the influence of the heat treatment time, and Table 2 shows the results of the investigation of the influence of the heat treatment temperature.
「析出物の径比ｄ／ｄ0」および「偏析比指数」は、それぞれ「試片の厚さ方向中心における析出物の大きさｄ」および「偏析比」について、基準試片（鋳造ままのスラブ）の値に対する各試片（鋼板）での値の比の値である。 The “diameter ratio d / d 0 of precipitates” and the “segregation ratio index” are the reference specimens (as cast as for the “size d of precipitates in the center of specimen thickness direction” and “segregation ratio”, respectively). It is the value of the ratio of the value at each specimen (steel plate) to the value of slab.
表１から、スラブの加熱時間が長いほど、また試片の厚さが薄いほど、ｄ／ｄ0および偏析比指数のいずれも小さいことがわかる。また、表２から、加熱温度が高いほど、ｄ／ｄ0および偏析比指数のいずれも小さいことがわかる。 Table 1 shows that both the d / d 0 and the segregation ratio index are smaller as the heating time of the slab is longer and the thickness of the specimen is thinner. Table 2 also shows that both the d / d 0 and the segregation ratio index are smaller as the heating temperature is higher.
ＳＥＭ―ＥＤＡＸによる観察の結果、凝固組織はデンドライト形状であること、ＭｎＳの析出は主にデンドライトの間隙において生じていたこと、およびデンドライトの間隙ではＭｎＳ析出物が粗大化しやすいことがわかった。また、ＥＰＭＡによる分析の結果、鋳片および鋼板ともに、厚さ方向のＭｎ濃度は均一ではなかった。 As a result of observation by SEM-EDAX, it was found that the solidified structure was in a dendrite shape, MnS precipitation was mainly generated in the dendrite gap, and that the MnS precipitate was easily coarsened in the dendrite gap. Further, as a result of analysis by EPMA, the Mn concentration in the thickness direction was not uniform for both the slab and the steel plate.
１−３．まとめ
以上の結果について検討したところ、鋳片の熱処理および熱間圧延だけでは、鋼板の厚さ方向の組成を十分に均一にすることはできないことがわかった。最終製品である鋼板の厚さ方向の組成を均一にし、かつ析出物を微細化するには、まず鋳片の段階で凝固組織を微細化する必要があると考えた。さらに、デンドライトの間隙に濃化した溶質元素を熱処理によって拡散させることで、鋼板の厚さ方向の組成をより均一にし、析出物をより微細化できると考えた。
1-3. Summary When the above results were examined, it was found that the composition in the thickness direction of the steel sheet could not be made sufficiently uniform only by heat treatment and hot rolling of the slab. In order to make the composition in the thickness direction of the steel sheet, which is the final product, uniform and to refine the precipitates, it was first thought that it was necessary to refine the solidification structure at the stage of the slab. Furthermore, the solute elements concentrated in the gaps between dendrites were diffused by heat treatment, so that the composition in the thickness direction of the steel sheet could be made more uniform and the precipitates could be made finer.
２．基礎実験（２）
本発明者らは、基礎実験（１）の結果を踏まえ、基礎実験（１）に用いた鋳片に加えて、Ｂｉを含有する鋳片を用いた実験を行った。
2. Basic experiment (2)
Based on the result of the basic experiment (1), the present inventors conducted an experiment using a slab containing Bi in addition to the slab used in the basic experiment (1).
２−１．実験条件
連続鋳造鋳片がＢｉを０．００１０％含有する点以外は基礎実験（１）と同様の条件で、実験を行った。Ｂｉは、溶鋼に添加して含有させた。厚さ２４０ｍｍ、幅１２００ｍｍの連続鋳造鋳片から採取した、長さ８０００ｍｍのスラブに対して、１２００℃、２時間の熱処理を施した後、厚さが５０ｍｍとなるように熱間圧延し、鋼板の試片（以下「Ｂｉ添加鋼板」という。）を作製した。
2-1. Experimental conditions The experiment was performed under the same conditions as in the basic experiment (1) except that the continuous cast slab contained 0.0010% Bi. Bi was added to the molten steel. A 8000 mm long slab sampled from a continuous cast slab having a thickness of 240 mm and a width of 1200 mm was subjected to heat treatment at 1200 ° C. for 2 hours, and then hot-rolled to a thickness of 50 mm. A specimen (hereinafter referred to as “Bi-added steel sheet”) was prepared.
比較対象として、このＢｉ添加鋼板と同様の熱処理条件および圧下条件で、基礎実験（１）において作製したＢｉを添加しない鋼板の試片（以下「Ｂｉ無添加鋼板」という。）を用いた。Ｂｉ無添加鋼板では、Ｂｉの含有率は測定限界以下であった。 As a comparison object, a specimen (hereinafter referred to as “Bi-free steel sheet”) of a steel sheet not added with Bi prepared in the basic experiment (1) was used under the same heat treatment conditions and rolling conditions as those of the Bi-added steel sheet. In the Bi-free steel sheet, the Bi content was less than the measurement limit.
２−２．実験結果
組織観察の結果、Ｂｉ無添加鋼板では厚さ方向中心部の凝固組織のデンドライトの１次アームの間隔が１１００±１２０μｍであったのに対して、Ｂｉ添加鋼板では５４０±３０μｍであった。このように、Ｂｉ添加鋼板は、Ｂｉ無添加鋼板と比べて凝固組織（デンドライト）が微細であった。また、ＥＰＭＡによる分析の結果、Ｂｉ添加鋼板では、Ｂｉ無添加鋼板と比べて厚さ方向のＭｎ濃度は均一化されていた。
2-2. Experimental results As a result of structural observation, the Bi-added steel sheet had a primary arm spacing of 1100 ± 120 μm in the solidified structure dendrite in the central portion in the thickness direction, whereas the Bi-added steel sheet had 540 ± 30 μm. . Thus, the Bi-added steel sheet had a finer solidification structure (dendrites) than the Bi-free steel sheet. As a result of analysis by EPMA, the Bi-added steel sheet had a uniform Mn concentration in the thickness direction as compared with the Bi-free steel sheet.
２−３．まとめ
基礎実験（２）の結果から、界面活性元素の添加の有無に関わらず、連続鋳造鋳片を、熱処理後、熱間圧延することによって、デンドライトの１次アームの間隔を縮小し、凝固組織の微細化を実現でき、この熱間圧延により得られた鋼板の厚さ方向の溶質濃度を均一化する効果が得られることがわかった。また、界面活性元素を添加することにより、その効果が大きくなることがわかった。
2-3. Summary Based on the results of the basic experiment (2), regardless of the presence or absence of the addition of surface active elements, the continuous cast slab is heat-rolled and then hot-rolled to reduce the interval between the primary arms of the dendrite and to solidify the structure. It was found that the effect of homogenizing the solute concentration in the thickness direction of the steel sheet obtained by this hot rolling can be obtained. It was also found that the effect is increased by adding a surface active element.
本発明者らは、さらに検討を行い、連続鋳造鋳片の凝固組織を微細化するには、凝固が完了した後で鋳片の圧下を行うことが有効であることを知見した。特に、鋳片の厚さ方向中心が凝固した直後に圧下を行うと、鋳片の厚さ方向中心部において圧下量が大きくなり、凝固組織の変形量が大きくなるため、凝固組織の微細化により有効であることを知見した。 The present inventors have further studied and found that it is effective to reduce the slab after the solidification is completed in order to refine the solidified structure of the continuously cast slab. In particular, if the reduction is performed immediately after the center of the slab in the thickness direction is solidified, the amount of reduction in the center of the slab in the thickness direction increases, and the amount of deformation of the solidified structure increases. It was found to be effective.
これは、鋳片の表層から厚さ方向中心に向かって温度が高くなることによると考えられる。汎用の熱応力解析プログラムで解析および計算を行ったところ、鋳片の厚さ方向中心が凝固完了した直後の温度分布での圧下では、鋳片の厚さ方向中心部における圧下量が大きかったのに対して、鋳片の表層から厚さ方向中心に向かって温度勾配がなく、鋳片の断面全体の温度が均一である場合の圧下では、鋳片の厚さ方向中心部よりも、鋳片の表層近傍における圧下量が大きかったことから、この知見の裏付けが得られた。 This is considered to be due to the temperature increasing from the surface layer of the slab toward the center in the thickness direction. Analysis and calculation using a general-purpose thermal stress analysis program revealed that the amount of reduction at the center of the slab in the thickness direction was large when the slab was pressed at the temperature distribution immediately after the completion of solidification. On the other hand, when there is no temperature gradient from the surface layer of the slab toward the center in the thickness direction and the temperature of the entire cross-section of the slab is uniform, the slab is smaller than the center part in the thickness direction of the slab. This finding was supported by the large amount of reduction in the vicinity of the surface layer.
本発明は、以上の基礎実験（１）、（２）および検討の結果から得られた上述の（ａ）〜（ｄ）の知見に基づいて完成された。 The present invention has been completed based on the above findings (a) to (d) obtained from the basic experiments (1) and (2) and the results of the examination.
３．鋳片の組成の範囲および限定理由
次に、本発明における鋳片の組成の限定理由について説明する。
3. Range of slab composition and reason for limitation Next, the reason for limitation of the slab composition in the present invention will be described.
３−１．必須元素
Ｃ：０．０１％〜０．２０％
Ｃは、鋼の強度向上に寄与する元素である。極厚鋼板を大型構造物用として十分な強度にするには、Ｃ含有率を０．０１％以上とする必要がある。しかし、Ｃ含有率が０．２０％を超えると、鋼の溶接性が不十分となる。これらのことから、本発明では、Ｃ含有率を０．０１％〜０．２０％とする。
3-1. Essential element C: 0.01% to 0.20%
C is an element that contributes to improving the strength of steel. In order to make the extra-thick steel plate sufficiently strong for large structures, the C content needs to be 0.01% or more. However, if the C content exceeds 0.20%, the weldability of the steel becomes insufficient. Therefore, in the present invention, the C content is set to 0.01% to 0.20%.
Ｓｉ：０．０２％〜０．５０％
Ｓｉ含有率が、０．０２％未満では鋼の強度を確保できない。また、Ｓｉ含有率が０．５０％を超えて高くなると溶接性が不十分となる。これらのことから、本発明では、Ｓｉ含有率を０．０２〜０．５０％とする。
Si: 0.02% to 0.50%
If the Si content is less than 0.02%, the strength of the steel cannot be secured. Further, when the Si content exceeds 0.50%, the weldability becomes insufficient. From these things, in this invention, Si content rate shall be 0.02-0.50%.
Ｍｎ：０．６〜３．０％
Ｍｎは、鋼板の高強度化と靭性の確保のために有効な元素である。鋼板を高強度化し、靭性を確保するには、Ｍｎ含有率を０．６％以上とする必要がある。一方、Ｍｎ含有率が３．０％を超えて高くなると靭性が不十分となる。これらのことから、本発明では、Ｍｎ含有率を０．６〜３．０％とする。
Mn: 0.6 to 3.0%
Mn is an effective element for increasing the strength of the steel sheet and ensuring toughness. In order to increase the strength of the steel sheet and ensure toughness, the Mn content must be 0.6% or more. On the other hand, if the Mn content exceeds 3.0%, the toughness becomes insufficient. For these reasons, in the present invention, the Mn content is set to 0.6 to 3.0%.
Ｐ：０．０２％以下
Ｐは、鋼板の延性、靭性および加工性を劣化させる元素である。特に、Ｐ含有率が０．０２％を超えると、鋼板のこれらの特性が不十分となる。そのため、本発明では、Ｐ含有率を０．０２％以下とする。
P: 0.02% or less P is an element that deteriorates the ductility, toughness, and workability of the steel sheet. In particular, when the P content exceeds 0.02%, these properties of the steel sheet become insufficient. Therefore, in the present invention, the P content is set to 0.02% or less.
Ｓ：０．００２〜０．０３０％
Ｓは、ＭｎＳ等の介在物を形成して結晶粒内にフェライトの生成を促進する効果がある。しかし、Ｓ含有率が０．００２％未満ではフェライトを生成する効果がほとんど無い。一方、Ｓ含有率が０．０３０％を超えて高くなると鋼板の延性が不十分となる。これらのことから、本発明では、Ｓ含有率を０．００２〜０．０３０％とする。
S: 0.002 to 0.030%
S has the effect of promoting the formation of ferrite in the crystal grains by forming inclusions such as MnS. However, when the S content is less than 0.002%, there is almost no effect of generating ferrite. On the other hand, if the S content exceeds 0.030%, the ductility of the steel sheet becomes insufficient. From these things, in this invention, S content rate shall be 0.002-0.030%.
Ａｌ：０．０００５〜０．０５００％
Ａｌは、溶鋼からの脱酸をするための元素であり、Ｔｉ含有酸化物の生成を抑制するため含有率は低い方が望ましい。しかし、溶鋼中の酸素をある程度残して鋼の靭性を確保するために、Ａｌ含有率の下限は０．０００５％とする。一方、Ａｌ含有率が０．０５００％を超えて高くなると、Ｔｉ含有酸化物の生成量（任意元素として後述するＭｇを含有させた場合はＭｇ含有酸化物の生成量も）が不十分となる。これらのことから、本発明では、Ａｌ含有率を０．０００５〜０．０５００％とする。
Al: 0.0005 to 0.0500%
Al is an element for deoxidizing molten steel, and the content is preferably low in order to suppress the formation of Ti-containing oxides. However, in order to leave some oxygen in the molten steel and ensure the toughness of the steel, the lower limit of the Al content is set to 0.0005%. On the other hand, when the Al content is higher than 0.0500%, the amount of Ti-containing oxide generated (when Mg described later as an optional element is included, the amount of Mg-containing oxide generated is insufficient). . From these things, in this invention, Al content rate shall be 0.0005 to 0.0500%.
Ｔｉ：０．００５〜０．０３０％
Ｔｉは、主として炭窒化物を析出し、その析出強化作用により鋼の強度の向上に寄与する有効な元素である。しかし、Ｔｉ含有率が０．００５％未満では、炭窒化物の析出強化作用により鋼の強度を向上させる効果が不十分である。一方、Ｔｉ含有率が０．０３０％を超えて高くなると、鋼中に粗大な析出物や介在物を形成して、鋼の靭性が不十分となる。これらのことから、本発明では、Ｔｉ含有率を０．００５〜０．０３０％とする。
Ti: 0.005-0.030%
Ti is an effective element that mainly precipitates carbonitrides and contributes to improving the strength of the steel by its precipitation strengthening action. However, if the Ti content is less than 0.005%, the effect of improving the strength of the steel by the precipitation strengthening action of carbonitride is insufficient. On the other hand, when the Ti content exceeds 0.030%, coarse precipitates and inclusions are formed in the steel, and the toughness of the steel becomes insufficient. From these things, in this invention, Ti content rate shall be 0.005-0.030%.
Ｎ：０．００２〜０．０１０％
Ｎは、Ｔｉと反応してＴｉＮを析出させるために必要な元素である。しかし、Ｎ含有率が０．０１０％を超えて高くなると、鋼の靭性が低下する。そのため、Ｎ含有率の上限は余裕をもって０．０１０％とする。一方、工業的にＮを完全に鋼から除去することは不可能である。そのため、実操業において低減可能な範囲を考慮し、Ｎ含有率の下限を０．００２％とする。Ｎ含有率は、０．００２〜０．００８％とすることが好ましい。
N: 0.002 to 0.010%
N is an element necessary for reacting with Ti to precipitate TiN. However, if the N content exceeds 0.010%, the toughness of the steel decreases. Therefore, the upper limit of the N content is set to 0.010% with a margin. On the other hand, it is impossible to remove N from steel completely industrially. Therefore, considering the range that can be reduced in actual operation, the lower limit of the N content is set to 0.002%. The N content is preferably 0.002 to 0.008%.
Ｏ：０．０００１〜０．０１５０％
Ｏは酸化物を生成させるために必要な元素である。Ｏ含有率が０．０００１％未満では酸化物の個数が不足する。また、Ｏ含有率が０．０１５０％を超えて高くなると、酸化物が多くなり過ぎて鋼の靭性が不十分となる。これらのことから、本発明では、Ｏ含有率を０．０００１〜０．０１５０％とする。
O: 0.0001 to 0.0150%
O is an element necessary for forming an oxide. If the O content is less than 0.0001%, the number of oxides is insufficient. On the other hand, if the O content exceeds 0.0150%, the amount of oxide increases and the toughness of the steel becomes insufficient. Therefore, in the present invention, the O content is set to 0.0001 to 0.0150%.
上述の成分以外の残部は、Ｆｅおよび不純物である。 The balance other than the above components is Fe and impurities.
３−２．任意元素
Ｆｅの一部に代えて、以下の任意元素を含有させてもよい。
3-2. Arbitrary Elements The following optional elements may be included instead of a part of Fe.
３−２−１．界面活性元素
Ｂｉ、ＳｎおよびＴｅのうち１種以上：合計で０．０００１％〜０．０３００％
Ｂｉ、ＳｎおよびＴｅは、いずれも鋼の凝固過程において界面活性元素として作用し、デンドライト状の凝固組織を微細化する効果を有する元素である。これらの元素のうちの１種を含有させるだけでもこの微細化効果を得ることができる。この微細化効果を十分に得るには、これらの元素の含有率を合計で０．０００１％以上とする必要がある。また、これらの元素の含有率が合計で０．０３００％を超えると、これらの元素の粗大な酸化物が生成し、鋼の靱性が不十分となる。以上のことから、本発明では、Ｂｉ、ＳｎおよびＴｅの１種以上の含有率を、合計で０．０００１％〜０．０３００％とすることが好ましい。
3-2-1. Surface active elements One or more of Bi, Sn and Te: 0.0001% to 0.0300% in total
Bi, Sn, and Te are all elements that act as surface active elements in the solidification process of steel and have the effect of refining the dendritic solidification structure. This refinement effect can be obtained only by including one of these elements. In order to sufficiently obtain this fine effect, the content of these elements needs to be 0.0001% or more in total. Moreover, when the content rate of these elements exceeds 0.0300% in total, the coarse oxide of these elements will produce | generate and the toughness of steel will become inadequate. From the above, in the present invention, the content of one or more of Bi, Sn, and Te is preferably set to 0.0001% to 0.0300% in total.
３−２−２．その他の任意元素
Ｃｕ：０．０５〜１．５０％
Ｃｕは、焼入れ性の向上および析出強化に有効な元素である。しかし、Ｃｕ含有率が０．０５％未満では、焼入れ性向上効果および析出強化効果が得られない。一方、Ｃｕ含有率が１．５０％を超えて高くなると、鋼の熱間加工性が不十分となる。これらの理由から、Ｃｕ含有率は０．０５〜１．５０％とすることが好ましい。
3-2-2. Other optional elements Cu: 0.05 to 1.50%
Cu is an element effective for improving hardenability and precipitation strengthening. However, if the Cu content is less than 0.05%, the hardenability improving effect and the precipitation strengthening effect cannot be obtained. On the other hand, when the Cu content is higher than 1.50%, the hot workability of steel becomes insufficient. For these reasons, the Cu content is preferably 0.05 to 1.50%.
Ｎｉ：０．０５〜５．００％
Ｎｉは、鋼の靭性を向上させる効果を有する元素である。しかし、Ｎｉ含有率が０．０５%未満では、鋼の靭性を向上させる効果が得られない。一方、Ｎｉ含有率が５．００％を超えて高くなると、焼入れ性が過剰となり、鋼の靭性に悪影響を及ぼす。これらの理由から、Ｎｉ含有率は０．０５〜５．００％とすることが好ましい。
Ni: 0.05-5.00%
Ni is an element having an effect of improving the toughness of steel. However, if the Ni content is less than 0.05%, the effect of improving the toughness of the steel cannot be obtained. On the other hand, if the Ni content exceeds 5.00%, the hardenability becomes excessive and adversely affects the toughness of the steel. For these reasons, the Ni content is preferably 0.05 to 5.00%.
Ｃｒ：０．０２〜１．００％
Ｃｒは、焼入れ性の向上、および析出強化による鋼の強度の向上に有効な元素である。しかし、Ｃｒ含有率が０．０２%未満では、焼入れ性向上効果および析出強化効果が得られない。一方、Ｃｒ含有率が１．００％を超えて高くなると、鋼の靭性および溶接性が劣化する傾向が認められる。これらの理由から、Ｃｒ含有率は０．０２〜１．００％とすることが好ましい。
Cr: 0.02 to 1.00%
Cr is an element effective in improving hardenability and improving the strength of steel by precipitation strengthening. However, if the Cr content is less than 0.02%, the hardenability improving effect and the precipitation strengthening effect cannot be obtained. On the other hand, when the Cr content is higher than 1.00%, the tendency of the steel toughness and weldability to deteriorate is recognized. For these reasons, the Cr content is preferably 0.02 to 1.00%.
Ｍｏ：０．０２〜１．００％
Ｍｏは、焼入れ性の向上および強度の向上に有効な元素である。しかし、Ｍｏ含有率が０．０２%未満では、焼入れ性向上効果および強度向上効果が明確ではない。一方、Ｍｏ含有率が１．００％を超えて高くなると、鋼の靭性および延性の低下ならびに溶接性の劣化が顕在化する。これらの理由から、Ｍｏ含有率は０．０２〜１．００％とすることが好ましい。
Mo: 0.02 to 1.00%
Mo is an element effective for improving hardenability and strength. However, if the Mo content is less than 0.02%, the hardenability improving effect and the strength improving effect are not clear. On the other hand, when the Mo content is higher than 1.00%, a reduction in the toughness and ductility of steel and a deterioration in weldability become apparent. For these reasons, the Mo content is preferably 0.02 to 1.00%.
Ｎｂ：０．００５〜０．０５０％
Ｎｂは、炭化物や窒化物を生成して鋼の強度を向上させる効果を有する元素である。しかし、Ｎｂ含有率が０．００５％未満では、炭化物や窒化物の生成による鋼の強度向上効果が明確ではない。一方、Ｎｂ含有率が０．０５０％を超えて高くなると、鋼中に粗大な炭化物や窒化物を形成するため、逆に靭性が不十分となる。これらの理由から、Ｎｂ含有率は０．００５〜０．０５０％とすることが好ましい。
Nb: 0.005 to 0.050%
Nb is an element that has the effect of generating carbides and nitrides and improving the strength of the steel. However, if the Nb content is less than 0.005%, the effect of improving the strength of steel due to the formation of carbides and nitrides is not clear. On the other hand, when the Nb content exceeds 0.050%, coarse carbides and nitrides are formed in the steel, so that the toughness is insufficient. For these reasons, the Nb content is preferably 0.005 to 0.050%.
Ｖ：０．００５〜０．１００％
Ｖは、炭化物や窒化物を生成して鋼の強度を向上させる効果を有する元素である。しかし、Ｖ含有率が０．００５％未満では、炭化物や窒化物の生成による鋼の強度向上効果が明確ではない。一方、Ｖ含有率が０．１００％を超えて高くなると、鋼の靭性が不十分となる。これらの理由から、Ｖ含有率は０．００５〜０．１００％とすることが好ましい。
V: 0.005-0.100%
V is an element having an effect of improving the strength of steel by generating carbides and nitrides. However, if the V content is less than 0.005%, the effect of improving the strength of steel due to the formation of carbides and nitrides is not clear. On the other hand, if the V content exceeds 0.100%, the toughness of the steel becomes insufficient. For these reasons, the V content is preferably 0.005 to 0.100%.
Ｂ：０．０００４〜０．００４０％
Ｂは、焼入れ性を増大させるとともに、Ｎと反応してＢＮを生成することで固溶Ｎの含有率を低下させ、ＨＡＺ（熱影響部）の靭性を向上させる効果を有する元素である。しかし、Ｂ含有率が０．０００４％未満では、焼入れ性の増大効果およびＨＡＺの靭性向上効果が明確ではない。一方、Ｂ含有率が０．００４０％を超えて高くなると、鋼中に粗大な硼化物が析出し、これにより鋼の靭性が不十分となる。これらの理由から、Ｂ含有率は０．０００４〜０．００４０％とすることが好ましい。
B: 0.0004 to 0.0040%
B is an element that has the effect of increasing hardenability and reducing the content of solid solution N by reacting with N to produce BN and improving the toughness of HAZ (heat affected zone). However, if the B content is less than 0.0004%, the effect of increasing hardenability and the effect of improving the toughness of HAZ are not clear. On the other hand, if the B content exceeds 0.0040%, coarse borides precipitate in the steel, which results in insufficient steel toughness. For these reasons, the B content is preferably 0.0004 to 0.0040%.
３−３．鋼組成の限定の効果
連続鋳造鋳片の鋼組成を上述の範囲とすることにより、連続鋳造鋳片の凝固組織を一定の範囲で微細化することができる。この組成の鋳片を、以下の方法で圧下して歪を加えることにより、さらに凝固組織を微細化することができる。
3-3. Effect of limiting steel composition By setting the steel composition of the continuous cast slab to the above range, the solidification structure of the continuous cast slab can be refined within a certain range. The solidified structure can be further refined by squeezing the slab of this composition and applying strain by the following method.
４．鋳片の連続鋳造方法（圧下方法）
上述の組成であり、通常の方法で、圧下を行うことなく連続鋳造した場合の鋳片の、厚さ方向中心におけるデンドライト１次アーム間隔をλ0とする。本発明の鋳片の連続鋳造方法では、通常の方法で連続鋳造した上述の組成の鋳片について、このλ0を基準とし、鋳片の厚さ方向中心におけるデンドライト１次アーム間隔λとλ0の比の値λ／λ0が０．１〜０．９となるように、鋳片の厚さ方向中心が完全に凝固した直後に圧下を行う。圧下には圧下用ロール対を使用し、圧下の際の歪速度を２×１０-3ｓ-1〜１×１０-1ｓ-1の範囲とすることにより、λ／λ0を０．１〜０．９とすることができる。
4). Continuous casting method of slab (reduction method)
The dendrite primary arm interval at the center in the thickness direction of the slab of the above-described composition, which is continuously cast without reducing by the usual method, is λ 0 . In the continuous casting method of the slab according to the present invention, the slabs of the above-described composition continuously cast by a normal method are based on this λ 0 , and the dendrite primary arm intervals λ and λ 0 at the center in the thickness direction of the slab. The reduction is performed immediately after the center of the slab in the thickness direction is completely solidified so that the ratio value λ / λ 0 is 0.1 to 0.9. A reduction roll pair is used for reduction, and the strain rate during reduction is in the range of 2 × 10 −3 s −1 to 1 × 10 −1 s −1 so that λ / λ 0 is 0.1. It can be set to -0.9.
このように連続鋳造鋳片の厚さ方向中心が完全に凝固した直後に圧下を行って歪を加えることにより、鋳片の凝固組織を微細化し、均一化することができる。また、この連続鋳造鋳片に熱間圧延を施して得られた鋼板は、鋼板の厚さ方向の組成が均一であるとともに、内部組織も均一であるため大型構造物用の素材として適している。 In this way, the solidification structure of the slab can be refined and uniformed by applying the strain immediately after the center in the thickness direction of the continuous cast slab is completely solidified and applying strain. In addition, the steel sheet obtained by hot rolling the continuous cast slab is suitable as a material for large structures because the composition in the thickness direction of the steel sheet is uniform and the internal structure is uniform. .
また、上述の組成であり、通常の方法で、圧下を行うことなく連続鋳造した場合の鋳片の厚さ方向中心における析出物の直径をｄ0とする。本発明の鋳片の連続鋳造方法では、通常の方法で連続鋳造した上述の組成の鋳片について、λ／λ0が０．１〜０．９となるとともに、このｄ0を基準とし、鋳片の厚さ方向中心における析出物の直径ｄとｄ0の比の値ｄ／ｄ0が０．１〜０．９となるように、鋳片の厚さ方向中心が完全に凝固した直後に圧下を行うことが好ましい。 Also, the diameter of the precipitate at the center in the thickness direction of the slab when the composition is the above-described composition and is continuously cast without performing reduction by a normal method is defined as d 0 . In the continuous casting method of the slab of the present invention, λ / λ 0 is 0.1 to 0.9 for the slab of the above composition continuously cast by a normal method, and this s 0 is used as a reference. Immediately after the center in the thickness direction of the slab is completely solidified so that the value d / d 0 of the ratio of the diameters d and d 0 of the precipitates at the center in the thickness direction of the slab is 0.1 to 0.9. It is preferable to perform the reduction.
本発明の鋳片の連続鋳造方法の効果を確認するため、以下に示す試験を実施して、その結果を評価した。 In order to confirm the effect of the continuous casting method of the slab of the present invention, the following tests were performed and the results were evaluated.
１．試験条件
１−１．鋳造条件
溶鋼成分：上述の必須元素（Ｃ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ｔｉ、Ｎ、ＡｌおよびＯ）が後述する表３に記載された組成に調製された溶鋼を使用し、任意元素である界面活性元素（Ｂｉ、ＳｎおよびＴｅ）（以下、「添加金属」ともいう。）については下記の添加方法により添加して表３に示される組成に調製
溶鋼温度：１５７０℃（タンディッシュ内溶鋼温度）
鋳型サイズ：幅１２００ｍｍ×厚さ２４０ｍｍ
鋳造速度：１．０ｍ／分
添加金属の添加方法：添加金属を充填した直径３ｍｍの鉄被ワイヤーを溶鋼に添加
添加金属の添加位置：レードル内
圧下用ロール径：直径８００ｍｍ
鋳片の厚さ方向中心が凝固した直後の圧下後の鋳片厚さ：１２０ｍｍ、１４０ｍｍ、１７０ｍｍ
鋳片の熱処理条件：１２５０℃、９０分
1. Test conditions 1-1. Casting conditions Molten steel component: The above-mentioned essential elements (C, Si, Mn, P, S, Ti, N, Al, and O) are prepared using the molten steel prepared in the composition described in Table 3 described later. Certain surface active elements (Bi, Sn and Te) (hereinafter also referred to as “additional metals”) are added by the following addition method to prepare the compositions shown in Table 3. Molten steel temperature: 1570 ° C. (molten steel in tundish) temperature)
Mold size: 1200mm width x 240mm thickness
Casting speed: 1.0 m / min Addition metal addition method: Addition of 3 mm diameter iron covered wire filled with addition metal to molten steel Addition metal addition position: inside ladle Roll diameter for rolling: 800 mm diameter
Thickness of slab after reduction immediately after the thickness direction center of the slab solidifies: 120 mm, 140 mm, 170 mm
Heat treatment condition of slab: 1250 ° C, 90 minutes
本試験では、溶鋼成分および鋳片の厚さ方向中心が凝固した直後の圧下の条件を変化させて連続鋳造を行い、連続鋳造鋳片を作製した。本発明例の試験において鋳造された溶鋼の成分組成および圧下条件を表３および表４中の試験番号１〜６の欄に示し、比較例の試験において鋳造された溶鋼の成分組成および圧下条件を同表中の試験番号７の欄に示した。表４に示す「圧下比」は、圧下直前の鋳片の厚さを圧下直後の鋳片の厚さで割った値である。比較例では、鋳片の連続鋳造鋳片の圧下を行わなかったため、鋳片の厚さは２４０ｍｍであり、圧下比は１．００である。表３において「−」はその元素の含有率が測定限界以下であることを示し、以下、元素について含有率が測定限界以下であることを「含まない」ともいう。 In this test, continuous casting was performed by changing the conditions of reduction immediately after the molten steel component and the thickness direction center of the slab were solidified to produce a continuous cast slab. The composition of the molten steel cast in the test of the present invention example and the rolling conditions are shown in the columns of test numbers 1 to 6 in Table 3 and Table 4, and the composition of the molten steel cast in the test of the comparative example and the rolling condition are shown. This is shown in the column of test number 7 in the same table. The “reduction ratio” shown in Table 4 is a value obtained by dividing the thickness of the slab immediately before reduction by the thickness of the slab immediately after reduction. In the comparative example, since the reduction of the continuous casting slab was not performed, the thickness of the slab was 240 mm and the reduction ratio was 1.00. In Table 3, “-” indicates that the content of the element is below the measurement limit, and hereinafter, the content of the element is below the measurement limit is also referred to as “not included”.
試験番号１〜６は、いずれも上述の必須元素（試験番号１〜３では必須元素および界面活性元素）を全て本発明の規定範囲内で含有し、デンドライト１次アーム間隔の比の値（λ／λ0）が本発明の規定範囲内である、本発明例である。 Test numbers 1 to 6 all contain the above-mentioned essential elements (essential elements and surface active elements in test numbers 1 to 3) within the specified range of the present invention, and the value of the ratio of the dendrite primary arm interval (λ / Λ 0 ) is an example of the present invention within the specified range of the present invention.
試験番号７は、必須元素の含有率が本発明の規定範囲内であり、界面活性元素を含まず（すなわち界面活性元素の含有率が測定限界以下の０．００００１％未満である。）、また、鋳片の圧下を行わなかった比較例である。 In Test No. 7, the content of essential elements is within the specified range of the present invention, and no surface active elements are included (that is, the content of surface active elements is less than 0.00001% below the measurement limit). This is a comparative example in which the slab was not reduced.
１−２．評価条件
本発明の連続鋳造方法の効果の評価は、鋳片の組織観察および偏析の測定によって行った。また、参考用の評価として、靭性の測定も行った
1-2. Evaluation Conditions The effect of the continuous casting method of the present invention was evaluated by observation of the structure of the slab and measurement of segregation. In addition, as a reference evaluation, we also measured toughness.
組織観察および偏析の測定用の試料は、上記条件で作製した連続鋳造鋳片の横断面の中心部から採取した。この試料を用いて、デンドライト１次アーム間隔、析出物の大きさ、および偏析の測定を行った。 Samples for texture observation and segregation measurement were collected from the center of the cross section of the continuous cast slab produced under the above conditions. Using this sample, dendrite primary arm spacing, precipitate size, and segregation were measured.
試料は、観察面をエメリー・ペーパーおよび研磨剤（粒径が６μｍおよび１μｍのダイヤモンドの砥粒）を順に使用して研磨した。研磨面の組織の顕出に用いた溶液は、デンドライト組織を観察する場合にはピクリン酸飽和溶液とした。 The sample was polished using the emery paper and an abrasive (diamond grains having a particle diameter of 6 μm and 1 μm) in order on the observation surface. The solution used for revealing the structure of the polished surface was a picric acid saturated solution when observing the dendrite structure.
デンドライト１次アーム間隔は、光学顕微鏡を用いて倍率１０倍で試料を観察して測定し、観察視野（３０ｍｍ×３０ｍｍ）内での測定結果の平均値をその試料の値とした。 The dendrite primary arm interval was measured by observing the sample at a magnification of 10 using an optical microscope, and the average value of the measurement results in the observation field (30 mm × 30 mm) was taken as the value of the sample.
大きさの測定対象とする析出物はＭｎＳとした。ＭｎＳの直径は、ＳＥＭ−ＥＤＡＸを用いて倍率１００００倍で試料を観察して測定した。ＭｎＳ粒子の形状が球ではない場合には、そのＭｎＳ粒子の最長の長さを直径とした。１００個のＭｎＳの直径を測定し、その平均値を各試験番号の鋳片の析出物の直径とした。 The precipitate to be measured for size was MnS. The diameter of MnS was measured by observing a sample at a magnification of 10,000 times using SEM-EDAX. When the shape of the MnS particles was not a sphere, the longest length of the MnS particles was taken as the diameter. The diameters of 100 MnS were measured, and the average value was taken as the diameter of the precipitate of the slab of each test number.
偏析の測定対象とする溶質元素はＭｎとした。ＥＰＭＡを用いてビーム径を５０μｍとして線分析を行って試料のＭｎ濃度分布を測定し、測定範囲でのＭｎの最大濃度を求めた。Ｍｎの最大濃度の値を溶鋼段階の化学分析から求めたＭｎの初期含有率で割った値を偏析比とした。 The solute element to be measured for segregation was Mn. A line analysis was performed using EPMA with a beam diameter of 50 μm to measure the Mn concentration distribution of the sample, and the maximum concentration of Mn in the measurement range was determined. The value obtained by dividing the value of the maximum concentration of Mn by the initial content of Mn obtained from the chemical analysis at the molten steel stage was taken as the segregation ratio.
靭性の測定用の試料は、上記条件で作製した連続鋳造鋳片に、１２５０℃で９０分保持する熱処理を行った後、その表面から厚さ方向に１／２の位置から採取した。試料の形状は、縦１０ｍｍ、横１０ｍｍ、長さ１００ｍｍの角柱状とした。この試料を用いて再現ＨＡＺ試験およびシャルピー試験を行なった。 A sample for toughness measurement was sampled from a position ½ in the thickness direction from the surface of the continuous cast slab produced under the above conditions after heat treatment at 90 ° C. for 90 minutes. The shape of the sample was a prismatic shape having a length of 10 mm, a width of 10 mm, and a length of 100 mm. A reproducible HAZ test and a Charpy test were performed using this sample.
再現ＨＡＺ試験は、高周波誘導加熱装置を用いてＡｒガス雰囲気中で行い、試料の長さ方向の中心の幅１０ｍｍの領域を加熱した。加熱は室温から１４５０℃まで３０秒間で加熱し、６０秒間保持した後、Ｈｅガスを用いて加熱部を急速冷却した。 The reproduction HAZ test was performed in an Ar gas atmosphere using a high-frequency induction heating apparatus, and a region having a width of 10 mm at the center in the length direction of the sample was heated. Heating was performed from room temperature to 1450 ° C. in 30 seconds, held for 60 seconds, and then the heated portion was rapidly cooled using He gas.
再現ＨＡＺ試験を行った試験片の長さ方向の中心部にノッチを入れ、温度０℃の雰囲気中においてシャルピー試験を行い、吸収エネルギーを求めた。 A notch was made in the central portion in the length direction of the test piece subjected to the reproduction HAZ test, and a Charpy test was performed in an atmosphere at a temperature of 0 ° C. to obtain the absorbed energy.
２．試験結果
上記条件で作製した連続鋳造鋳片について、「デンドライト１次アーム間隔比λ／λ0」、「析出物の径比ｄ／ｄ0」および「偏析比指数」を評価項目として評価を行い、その結果を前記表４に示した。また、「靭性指数」を参考用の評価項目として評価を行い、併せて前記表４に示した。
2. Test results The continuous cast slabs produced under the above conditions were evaluated using “dendritic primary arm spacing ratio λ / λ 0 ”, “precipitate diameter ratio d / d 0 ” and “segregation ratio index” as evaluation items. The results are shown in Table 4 above. Further, the “toughness index” was evaluated as a reference evaluation item, and the results are shown in Table 4 above.
「デンドライト１次アーム間隔比λ／λ0」、「析出物の径比ｄ／ｄ0」、「偏析比指数」および「靭性指数」は、それぞれ「連続鋳造鋳片の厚さ方向中心におけるデンドライト１次アーム間隔λ」、「連続鋳造鋳片の厚さ方向中心における析出物の大きさｄ」、「偏析比」および「吸収エネルギー」について、比較例である試験番号７での値に対する各試験番号での値の比の値である。靭性指数は、大きいほど吸収エネルギーが高く靭性が良好であることを示す。 “Dendrite primary arm spacing ratio λ / λ 0 ”, “precipitate diameter ratio d / d 0 ”, “segregation ratio index” and “toughness index” are respectively “dendrites at the center in the thickness direction of the continuous cast slab. Each test with respect to the values in Test No. 7, which is a comparative example, for “primary arm interval λ”, “size d of the precipitate at the center in the thickness direction of the continuous cast slab”, “segregation ratio” and “absorbed energy” It is the value of the ratio of values with numbers. The larger the toughness index, the higher the absorbed energy and the better the toughness.
試験番号７では、界面活性元素の含有率の合計が０．０００１％未満であり、連続鋳造鋳片を圧下しなかったため、「デンドライト１次アーム間隔比λ／λ0」、「析出物の径比ｄ／ｄ0」、「偏析比指数」および「靭性指数」の基準とした。 In Test No. 7, the total content of the surface active elements was less than 0.0001%, and the continuous cast slab was not squeezed. Therefore, “dendritic primary arm spacing ratio λ / λ 0 ”, “precipitate diameter” The ratio d / d 0 , “segregation ratio index” and “toughness index” were used as standards.
表４に示すように、試験番号１〜６の本発明例では、λ／λ0は０．５〜０．８、ｄ／ｄ0は０．５０〜０．８０、偏析比指数は０．４〜０．８であり、いずれも試験番号７の比較例よりも小さい値であった。 As shown in Table 4, in the inventive examples of test numbers 1 to 6, λ / λ 0 is 0.5 to 0.8, d / d 0 is 0.50 to 0.80, and the segregation ratio index is 0.00. 4 to 0.8, all of which were smaller than the comparative example of test number 7.
このことから、本発明の連続鋳造方法によれば、鋳片の圧下を行わない場合よりも優れたデンドライト組織の微細化効果および析出物の微細化効果を得ることができるとともに、偏析の発生を抑制できることがわかる。すなわち、連続鋳造鋳片の厚さ方向中心が凝固した直後に圧下を行うと、デンドライト組織および析出物のいずれも微細化することが可能であり、さらに中心偏析の生成を抑制できることがわかる。 From this, according to the continuous casting method of the present invention, it is possible to obtain a dendrite structure refinement effect and a precipitate refinement effect superior to those in the case where slab reduction is not performed, and the occurrence of segregation. It turns out that it can suppress. That is, it can be seen that if the reduction is performed immediately after the center in the thickness direction of the continuous cast slab is solidified, both the dendrite structure and the precipitate can be refined, and the generation of center segregation can be suppressed.
表４で、試験番号１〜３と試験番号４〜６を比較すると、λ／λ0、ｄ／ｄ0および偏析比指数のいずれも、試験番号１〜３の方が小さい値であった。このことから、連続鋳造鋳片の厚さ方向中心が凝固した直後に圧下する場合、溶鋼に界面活性元素を添加することにより、さらにデンドライト組織および析出物を微細化するとともに中心偏析の生成を抑制できることがわかる。 In Table 4, when test numbers 1 to 3 and test numbers 4 to 6 were compared, all of λ / λ 0 , d / d 0, and segregation ratio index were smaller in test numbers 1 to 3. From this, when rolling down immediately after the thickness direction center of the continuous cast slab solidifies, adding a surface active element to the molten steel further refines the dendrite structure and precipitates and suppresses the generation of center segregation. I understand that I can do it.
また、靭性指数は、本発明例では１．５以上と良好な値であり、製品としての許容範囲内であった。このことから、本発明の連続鋳造方法によれば、圧下を行わない場合よりも靭性が良好な鋳片が得られることがわかる。さらに、溶鋼に界面活性元素を添加することにより、靭性指数が２．１以上となり、より靭性が良好な鋳片が得られることがわかる。 Further, the toughness index was a good value of 1.5 or more in the examples of the present invention, and was within an allowable range as a product. From this, it can be seen that according to the continuous casting method of the present invention, a slab having better toughness can be obtained than when no reduction is performed. Furthermore, it can be seen that by adding a surface active element to the molten steel, the toughness index becomes 2.1 or more, and a slab with better toughness can be obtained.
本発明の連続鋳造方法は、鋳片の厚さによらず適用が可能であるとともに、鋳片の凝固が完了した直後に圧下を行うため、鋳片の未凝固領域の固相率の制御を行う場合と比べて容易に実施することができる。 The continuous casting method of the present invention can be applied regardless of the thickness of the slab, and since the reduction is performed immediately after the solidification of the slab is completed, the solid phase ratio of the unsolidified region of the slab is controlled. Compared with the case where it carries out, it can carry out easily.
また、本発明の連続鋳造方法で製造された鋳片、すなわち本発明の連続鋳造鋳片は、凝固組織が微細かつ均一であり、析出物も微細であり、中心偏析の生成が抑制され、組成が均一であるため、機械的特性が良好であり、大型構造物に用いられる極厚鋼板用の素材として適する。 In addition, the slab produced by the continuous casting method of the present invention, that is, the continuous cast slab of the present invention has a solidified structure that is fine and uniform, and the precipitates are also fine, the generation of center segregation is suppressed, and the composition Is uniform, it has good mechanical properties and is suitable as a material for extra-thick steel plates used for large structures.
Claims (4)
圧下を行うことなく鋳造した場合の鋳片の厚さ方向中心におけるデンドライト１次アーム間隔λ0を基準とし、
鋳片の厚さ方向中心におけるデンドライト１次アーム間隔λと前記λ0の比の値λ／λ0が０．１〜０．９となるように、鋳片の厚さ方向中心が凝固した直後に圧下を行うことを特徴とする鋳片の連続鋳造方法。 In mass%, C: 0.01% to 0.20%, Si: 0.02% to 0.50%, Mn: 0.6% to 3.0%, P: 0.02% or less, S: 0.002-0.030%, Al: 0.0005-0.0500%, Ti: 0.005-0.030%, N: 0.002-0.010%, and O: 0.0001-0. A method for continuously casting a slab containing 0150%, the balance being Fe and impurities,
Based on the dendrite primary arm interval λ 0 at the center in the thickness direction of the slab when cast without reduction,
Immediately after the center of the slab in the thickness direction is solidified so that the ratio λ / λ 0 of the dendrite primary arm interval λ and the λ 0 at the center of the slab in the thickness direction is 0.1 to 0.9. A method for continuously casting a slab, wherein the slab is reduced.
鋳片の厚さ方向中心における析出物の直径ｄと前記ｄ0の比の値ｄ／ｄ0が０．１〜０．９となるように、鋳片の厚さ方向中心が凝固した直後に圧下を行うことを特徴とする請求項１または２に記載の鋳片の連続鋳造方法。 Based on the diameter d 0 of the precipitate at the center in the thickness direction of the slab when cast without reducing,
As the diameter d of precipitates in the thickness direction center of the slab d value d / d 0 ratio of 0 is 0.1-0.9, immediately after the thickness direction center of the slab has solidified 3. The method for continuously casting a slab according to claim 1, wherein the reduction is performed.
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| JP2018058107A (en) * | 2016-10-04 | 2018-04-12 | Ｊｆｅスチール株式会社 | Continuously cast slab, method for producing continuously cast slab, and high tensile strength steel plate |
| EP3382051A4 (en) * | 2015-11-27 | 2019-06-19 | Nippon Steel & Sumitomo Metal Corporation | Steel, carburized steel component, and carburized steel component production method |
| EP3521470A4 (en) * | 2016-09-30 | 2020-03-18 | Nippon Steel Corporation | Steel for cold forging and production method thereof |
| WO2020100729A1 (en) * | 2018-11-14 | 2020-05-22 | 日本製鉄株式会社 | Apparatus for manufacturing thin steel sheet, and method for manufacturing thin steel sheet |
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| JP2008280566A (en) * | 2007-05-09 | 2008-11-20 | Sumitomo Metal Ind Ltd | High-strength steel material having precipitates finely dispersed therein, and method for continuously casting slab of high-strength steel material |
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Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP3382051A4 (en) * | 2015-11-27 | 2019-06-19 | Nippon Steel & Sumitomo Metal Corporation | Steel, carburized steel component, and carburized steel component production method |
| EP3521470A4 (en) * | 2016-09-30 | 2020-03-18 | Nippon Steel Corporation | Steel for cold forging and production method thereof |
| US11111568B2 (en) | 2016-09-30 | 2021-09-07 | Nippon Steel Corporation | Steel for cold forging and manufacturing method thereof |
| JP2018058107A (en) * | 2016-10-04 | 2018-04-12 | Ｊｆｅスチール株式会社 | Continuously cast slab, method for producing continuously cast slab, and high tensile strength steel plate |
| WO2020100729A1 (en) * | 2018-11-14 | 2020-05-22 | 日本製鉄株式会社 | Apparatus for manufacturing thin steel sheet, and method for manufacturing thin steel sheet |
| KR20210076107A (en) | 2018-11-14 | 2021-06-23 | 닛폰세이테츠 가부시키가이샤 | Apparatus for manufacturing thin steel sheet and method for manufacturing thin sheet steel |
| JPWO2020100729A1 (en) * | 2018-11-14 | 2021-09-30 | 日本製鉄株式会社 | Thin steel sheet manufacturing equipment and thin steel sheet manufacturing method |
| JP7095748B2 (en) | 2018-11-14 | 2022-07-05 | 日本製鉄株式会社 | Manufacturing method of thin sheet metal |
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