JP2017080787A - Continuous casting method of steel - Google Patents
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本発明は、鋼の連続鋳造方法に関するものであり、特に鋳片厚み中心部の中心偏析の悪影響を低減することのできる、鋼の連続鋳造方法に関するものである。 The present invention relates to a continuous casting method of steel, and more particularly to a continuous casting method of steel that can reduce the adverse effect of center segregation at the center of the slab thickness.
鋼の連続鋳造中、鋳片内では主にデンドライト凝固が進行する。デンドライト凝固の固液界面では、平衡分配係数が小さい元素については、固相側で低濃度、液相側で高濃度となり、デンドライト樹芯で負偏析、隣接するデンドライト間隙部（樹間）で正偏析する。このようにデンドライト樹間で発生する成分偏析をミクロ偏析という。 During continuous casting of steel, dendrite solidification mainly proceeds in the slab. At the solid-liquid interface for dendritic solidification, elements with a small equilibrium partition coefficient are low in the solid phase and high in the liquid phase, negative segregation in the dendrite core, and positive in the adjacent dendrite gap (between trees). Segregate. Such component segregation occurring between dendrite trees is called microsegregation.
鋳片の厚み中心部において、上面側と下面側からのデンドライト凝固が接触すると、鋳片厚み中心部の固相率（中心部固相率）が０以上の有限の数字となる。その後、凝固が進行するとともに中心部固相率が増大し、中心部固相率が１となった時点で鋳片の凝固が完了する。鋳片厚み中心部の凝固が開始してから中心部固相率が１となるまでの区間については、鋳片厚み中心部は固液共存領域であり、デンドライト樹間の液相部（未凝固溶鋼部）には成分のミクロ偏析が形成されている。このような状況で、鋳片がロール間バルジングし、あるいは鋳片の凝固収縮に伴って未凝固溶鋼が流動して中心部に集積すると、鋳片厚み中心部は特に成分偏析が激しくなり、マクロ偏析である中心偏析が発生する。中心偏析部に濃化する成分としては、Ｃ、Ｍｎ、Ｐ、Ｓなどが挙げられる。この中心偏析は、鋼板における例えば溶接熱影響部（ＨＡＺ部）の靭性の低下や水素誘起割れの原因となり、特に極厚鋼板の場合に問題となる。 When the dendrite solidification from the upper surface side and the lower surface side comes into contact with each other at the thickness center of the slab, the solid phase ratio (center solid phase ratio) at the center of the slab thickness becomes a finite number of 0 or more. Thereafter, as the solidification proceeds, the solid fraction of the central part increases, and the solidification of the slab is completed when the solid fraction of the central part becomes 1. For the section from the start of solidification of the slab thickness center until the central solid phase ratio becomes 1, the center of the slab thickness is a solid-liquid coexistence region, and the liquid phase part between the dendrite trees (unsolidified) In the molten steel part), component microsegregation is formed. In such a situation, when the slab bulges between rolls, or when unsolidified molten steel flows and accumulates in the center with the solidification shrinkage of the slab, the segregation of the center of the slab thickness becomes particularly severe, and the macro Central segregation, which is segregation, occurs. Examples of the component concentrated in the central segregation part include C, Mn, P, and S. This center segregation causes, for example, a decrease in the toughness of the heat affected zone (HAZ portion) and hydrogen induced cracks in the steel sheet, and particularly in the case of an extremely thick steel sheet.
この中心偏析の抑制法として、鋳片厚み中心部の凝固末期の段階において鋳片を軽圧下することにより、凝固末期における凝固収縮による体積収縮分を補償して濃化溶鋼の流動を抑制することが有効である。このため、従来から圧下に関する様々な方法が提案されてきた。 As a method of suppressing this center segregation, the slab is lightly reduced at the end of solidification at the center of the slab thickness to compensate for the volume shrinkage due to solidification shrinkage at the end of solidification, thereby suppressing the flow of concentrated molten steel. Is effective. For this reason, various methods related to reduction have been proposed.
特許文献１には、鋳片厚み中心部の固相率が０．４以下の時点から鋳片の軽圧下を開始して、鋳片厚み中心部が凝固完了するまで軽圧下を継続し、かつ軽圧下しつつ鋳片厚み中心部が凝固完了するまで鋳片表面を冷却して、この冷却による鋳片の熱収縮速度を０．２５〜１．０ｍｍ／ｍｉｎの範囲に制御することを特徴とする連続鋳造鋳片の中心偏析の低減技術が提案されている。 In Patent Document 1, light reduction of the slab is started from the time when the solid phase ratio of the slab thickness center portion is 0.4 or less, and continues to light reduction until the slab thickness center portion is completely solidified, and The slab surface is cooled until solidification of the slab thickness center is completed while lightly reducing, and the heat shrinkage rate of the slab by this cooling is controlled in the range of 0.25 to 1.0 mm / min. A technique for reducing the center segregation of continuously cast slabs has been proposed.
特許文献２には、連続鋳造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ことを特徴とする連続鋳造鋳片の偏析調整方法が開示されている。また、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、あるいは低くしていくような手法が有効であるとしている。 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 method for adjusting segregation of a continuous cast slab characterized by changing a solid phase ratio of a strand at a forging point by adjusting a speed or a cooling speed is disclosed. In addition, the allowable range of the solid fraction at the slab pressure lowering point is preferably 0.5 to 0.95, and the solid fraction is gradually increased or lowered as the casting time elapses. Is said to be effective.
特許文献３には、炭素量が０．２〜０．５質量％であるスラブ鋳片を圧下ロールによって圧下しつつ連続鋳造し、その後の熱間圧延によって所定の関係を満足する鋼材を得るスラブ鋳片を製造するための連続鋳造設備であって、連続鋳造時におけるスラブ鋳片の圧下に際して、スラブ鋳片の中心部固相率が０．７となる位置から下流側５ｍまでの範囲を圧下領域とすると共に、この圧下有効領域にはロール径が５３０ｍｍ以上で軸方向に一体的に形成された圧下ロールを１対以上配置し、鋳片への総圧下量が１３ｍｍ以上となるように構成したものであることを特徴とする連続鋳造設備に関する技術が記載されている。しかしながら、炭素量が０．２質量％未満であるスラブ鋳片に関する記述はなく、他の成分についての記述もない。 Patent Document 3 discloses a slab in which a slab slab having a carbon content of 0.2 to 0.5 mass% is continuously cast while being reduced by a reduction roll, and a steel material satisfying a predetermined relationship is obtained by subsequent hot rolling. This is a continuous casting facility for producing a slab, and when the slab slab is reduced during continuous casting, the slab slab is reduced from the position where the solid fraction of the central portion of the slab slab becomes 0.7 to the downstream 5 m. In this effective reduction area, one or more pairs of reduction rolls integrally formed in the axial direction with a roll diameter of 530 mm or more are arranged, and the total reduction amount to the slab is 13 mm or more. The technique regarding the continuous casting equipment characterized by being made is described. However, there is no description about a slab slab whose carbon content is less than 0.2% by mass, and there is no description about other components.
鋳片における成分のミクロ偏析やマクロ偏析が、その後の圧延段階まで残存していると、前述のとおり、鋳片の靭性の低下や水素誘起割れの原因となり特に極厚鋼板の場合に問題となる。熱間圧延前に鋳片を加熱する加熱時に、偏析成分が拡散で解消できれば、偏析の問題は解決する。しかし、中心偏析などのマクロ偏析は、偏析部の径が大きいため、熱間圧延前の加熱で解消することは困難である。また、特許文献４に記載のように、鋳片厚み中心部以外の部分に形成されるデンドライト樹間のミクロ偏析についても、通常の熱間圧延前の加熱温度及び時間の範囲では、拡散によって解消することができず、その後の熱間圧延工程及び冷間圧延工程を経ても偏析が残存することが明らかとなった。 If the microsegregation or macrosegregation of the components in the slab remains until the subsequent rolling stage, as described above, it causes a decrease in the toughness of the slab and hydrogen-induced cracking, which is a problem particularly in the case of very thick steel plates. . If the segregation component can be eliminated by diffusion during heating in which the slab is heated before hot rolling, the problem of segregation is solved. However, macrosegregation such as center segregation is difficult to eliminate by heating before hot rolling because the diameter of the segregation part is large. In addition, as described in Patent Document 4, microsegregation between dendrite trees formed in portions other than the center part of the slab thickness is also eliminated by diffusion in the range of heating temperature and time before normal hot rolling. Thus, it became clear that segregation remained even after the subsequent hot rolling process and cold rolling process.
また、鋳片の凝固組織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、冷却速度を速くすればデンドライト１次アーム間隔を小さくすることができる。しかし、極厚鋼板用鋳片のように鋳片の厚さが大きくなると、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速めることができない。このため、鋳片表面の冷却速度を速くして製造した極厚鋼板では、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１次アーム間隔が大きくなり、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１次アーム間隔の大きさの差が著しく、中心近傍ではデンドライト樹間のミクロ偏析も増大するので、加熱処理および熱間圧延工程を経てもこ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。デンドライト樹間のミクロ偏析が増大することに呼応し、中心偏析部のマクロ偏析度合も増大することとなる。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。 Further, since the solidified structure of the slab depends on the cooling rate, when the steel components are the same, the primary arm spacing of the dendrite can be reduced by increasing the cooling rate. However, when the thickness of the slab increases as in the case of an extremely thick steel plate slab, the solidified shell itself becomes the rate of heat conduction, and the cooling rate inside the slab cannot be increased. For this reason, in the ultra-thick steel plate manufactured by increasing the cooling rate of the slab surface, the dendrite on the slab surface layer is small, but the dendrite primary arm interval increases toward the center in the thickness direction of the slab, and the casting Near the center in the thickness direction of the piece, the primary arm spacing of the dendrite may reach several mm. In this way, the difference in the size of the dendrite primary arm spacing is remarkable between the surface layer portion of the slab and the vicinity of the center, and microsegregation between dendritic trees also increases near the center, so that the heat treatment and the hot rolling process are performed. However, it is impossible to eliminate the uneven concentration of the solute element due to the influence of this difference. Corresponding to the increase in microsegregation between dendritic trees, the degree of macrosegregation in the central segregation part also increases. In the case of slabs for extra heavy steel plates, the difference in the size of such dendrites is particularly remarkable. Become.
特許文献４に記載の発明は、溶鋼中に界面活性元素であるＢｉを０．０００１％以上０．０５％以下の範囲で含有することにより、溶鋼とデンドライトとの固液界面エネルギーを低減させ、連続鋳造鋳片の表層から１０ｍｍの範囲内におけるデンドライトの１次アームの間隔が３００μｍ以下とすること、その結果として、デンドライトの１次アームの樹間のＭｎ含有率と鋳片の平均Ｍｎ含有率の比を２．５以下とすることを特徴とする高強度鋼板用の連続鋳造鋳片の製造技術が提案されている。Ｍｎはデンドライトアームの間隙に濃化するが、連続鋳造後の加熱工程で拡散によりアーム間隔のＭｎ濃度は低下する。しかしながら、鋳片厚み中心部の凝固組織を微細化するためには厚み中心部のみに必要な濃度のＢｉを添加すればよいが、特許文献４の技術は溶鋼全体に均一に添加することを想定している。 The invention described in Patent Document 4 reduces the solid-liquid interfacial energy between molten steel and dendrites by containing Bi, which is a surface active element, in the range of 0.0001% to 0.05% in the molten steel, The distance between the primary arms of the dendrite within the range of 10 mm from the surface layer of the continuous cast slab is 300 μm or less. As a result, the Mn content between the dendrite primary arms and the average Mn content of the slab A technique for producing a continuous cast slab for a high-strength steel sheet, characterized in that the ratio is 2.5 or less, has been proposed. Mn is concentrated in the gap between the dendrite arms, but the Mn concentration in the arm interval is lowered by diffusion in the heating process after continuous casting. However, in order to refine the solidification structure at the thickness center of the slab, Bi of a necessary concentration may be added only to the thickness center, but the technique of Patent Document 4 assumes that it is uniformly added to the entire molten steel. doing.
前述のように、連続鋳造鋳片は厚み中心部に中心偏析が存在するとともに、この領域の冷却速度が小さいことから凝固組織であるデンドライトのサイズ（１次アーム間隔）も大きくなり、ミクロ偏析も顕著になる。従来の技術では、中心偏析を抑制するため連続鋳造工程で軽圧下が行われてきた。ただし、この軽圧下では中心偏析は抑制されるものの、鋳片厚み中心部には粗大なデンドライトが残存したままである。 As described above, the continuous cast slab has central segregation at the center of thickness, and the cooling rate in this region is small, so the size of the dendrites (primary arm spacing), which is a solidified structure, increases, and microsegregation also occurs. Become prominent. In the prior art, light reduction has been performed in a continuous casting process in order to suppress center segregation. However, although center segregation is suppressed under this light pressure, coarse dendrite remains at the center of the slab thickness.
本発明は、軽圧下を行える設備を用いて連続鋳造を行う場合に、特許文献４の場合ほどＢｉを添加しなくても、鋳片厚み中心部のデンドライト１次アーム間隔を微細化し、中心偏析をも低減して、鋼の靱性をはじめとする品質を向上することのできる鋼の連続鋳造方法を提供することを目的とする。 In the present invention, when continuous casting is performed using equipment capable of light reduction, the primary arm spacing of the dendrite at the center of the slab thickness is made finer and the center segregation can be achieved without adding Bi as in Patent Document 4. An object of the present invention is to provide a continuous casting method of steel that can improve quality such as toughness of steel by reducing the toughness of steel.
即ち、本発明の要旨とするところは以下のとおりである。
（１）質量％で、Ｃ：０．０４％〜０．３５％、Ｓｉ：０．００５％〜３．０％、Ｍｎ：０．１％〜３．５％、Ｐ：０．０２％以下、Ｓ：０．０００２〜０．００２％、Ａｌ：０．０００５〜１．０％、Ｎ：０．００２〜０．０１０％、Ｏ：０．０００１〜０．０１％を含有し、Ｂｉ濃度が０．０００１％以上０．００１０％未満であり、残部がＦｅおよび不純物からなる鋼の連続鋳造方法であって、鋳片厚み中心部の固相率（以下「中心部固相率」という。）が０．３未満の時点で軽圧下を開始して、中心部固相率が０．３以上０．９６以下の範囲で軽圧下を完了することを特徴とする鋼の連続鋳造方法。
（２）鋼成分は、Ｆｅの一部に代えて、質量％で、Ｔｉ：０．００５〜０．０３％、Ｃｕ：０．０５〜１．５％、Ｎｉ：０．０５〜５．０％、Ｃｒ：０．０２〜１．０％、Ｍｏ：０．０２〜１．０％、Ｎｂ：０．００５〜０．０５％、Ｖ：０．００５〜０．１％およびＢ：０．０００４〜０．００４％のうち１種以上を含有することを特徴とする上記（１）に記載の鋼の連続鋳造方法。
（３）鋳片厚み中心部付近のＭｎ最大偏析部における平均Ｂｉ濃度が０．００１５％以上であることを特徴とする上記（１）又は（２）に記載の鋼の連続鋳造方法。
That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.04% to 0.35%, Si: 0.005% to 3.0%, Mn: 0.1% to 3.5%, P: 0.02% or less , S: 0.0002 to 0.002%, Al: 0.0005 to 1.0%, N: 0.002 to 0.010%, O: 0.0001 to 0.01%, Bi concentration Is a continuous casting method of steel in which the balance is 0.0001% or more and less than 0.0010%, and the balance is Fe and impurities, and is referred to as the solid phase ratio at the center of the slab thickness (hereinafter referred to as “center solid phase ratio”). ) Is less than 0.3, and light rolling is started, and light rolling is completed when the solid fraction in the center is in the range of 0.3 to 0.96.
(2) Steel component is replaced by a part of Fe in mass%, Ti: 0.005 to 0.03%, Cu: 0.05 to 1.5%, Ni: 0.05 to 5.0 %, Cr: 0.02-1.0%, Mo: 0.02-1.0%, Nb: 0.005-0.05%, V: 0.005-0.1% and B: 0.00. The continuous casting method for steel as described in (1) above, comprising one or more of 0004 to 0.004%.
(3) The steel continuous casting method as described in (1) or (2) above, wherein the average Bi concentration in the maximum Mn segregation portion in the vicinity of the center portion of the slab thickness is 0.0015% or more.
本発明の鋼の連続鋳造方法は、溶鋼中に添加するＢｉ濃度を０．０００１％から０．００１０％未満とし、中心部固相率が０．３未満の時点で軽圧下を開始して、中心部固相率が０．３以上０．９６以下の範囲で軽圧下を完了することにより、中心偏析部におけるＢｉ濃度を０．００１５％以上として、中心偏析部付近のデンドライト１次アーム間隔を微細化して偏析を軽減し、この鋳片から製造した製品の靱性を向上することができる。 In the continuous casting method of the steel of the present invention, the Bi concentration to be added to the molten steel is 0.0001% to less than 0.0010%, and light reduction is started when the central solid fraction is less than 0.3, By completing the light reduction in the range where the solid fraction at the center is 0.3 or more and 0.96 or less, the Bi concentration in the center segregation part is set to 0.0015% or more, and the dendrite primary arm interval near the center segregation part is The segregation can be reduced by miniaturization, and the toughness of a product manufactured from this slab can be improved.
特許文献４に記載のように、界面活性元素であるＢｉを０．０００１％以上０．０５％以下の範囲で溶鋼中に添加することにより、連続鋳造鋳片の表層から１０ｍｍの範囲内におけるデンドライトの１次アームの間隔を３００μｍ以下とすることができる。一方、Ｂｉ添加によって鋳片厚み中心部のデンドライト１次アーム間隔を微細化し、さらに中心偏析を軽減するためには、鋳片厚み中心部におけるＢｉ濃度を０．００１５質量％以上とすることが必要であると判明した。しかし、Ｂｉは高価であり、鋳片厚み方向全体にわたってＢｉ濃度が０．００１５質量％となるようにＢｉを添加すると、鋳片製造コストの増大を来すこととなる。 As described in Patent Document 4, by adding Bi, which is a surface active element, to the molten steel in a range of 0.0001% to 0.05%, a dendrite within a range of 10 mm from the surface layer of the continuous cast slab The distance between the primary arms can be 300 μm or less. On the other hand, in order to refine the dendrite primary arm interval at the center of the slab thickness by adding Bi and further reduce the center segregation, the Bi concentration at the center of the slab thickness needs to be 0.0015% by mass or more. Turned out to be. However, Bi is expensive, and if Bi is added so that the Bi concentration is 0.0015% by mass over the entire slab thickness direction, the slab manufacturing cost increases.
連続鋳造鋳片の中心偏析を低減するために、鋳片厚み中心部に固液が共存する領域で軽圧下を行うことは有効であり、本発明の鋼の連続鋳造方法においても軽圧下の実施を前提とする。軽圧下の効果を十分に発揮して中心偏析部の偏析を低減するためには、鋳片厚み中心部の固液共存領域から完全凝固部に至るまでを軽圧下する必要がある。Ｂｉも偏析元素であり、中心偏析部にＢｉも正偏析する。軽圧下を行うことにより、鋳片厚み中心部のＢｉ偏析も低減することができる。これに対して本発明では、完全凝固部よりも手前側で軽圧下を終了して中心偏析低減効果を不十分なものとすることにより、意図的に鋳片厚み中心部にＢｉを濃化させることを着想した。鋳片厚み中心部のデンドライト１次アーム間隔を微細化し、中心偏析を軽減するためには、主に中心偏析が生成する領域においてＢｉ濃度を０．００１５質量％以上とすればいいのであるから、軽圧下を意図的に不十分に実施して中心偏析領域のＢｉ濃度を０．００１５質量％以上に濃化させることができれば、本発明の目的を達成することとなる。 In order to reduce the center segregation of the continuous cast slab, it is effective to perform light reduction in the region where the solid and liquid coexist in the center of the slab thickness. Assuming In order to fully exhibit the effect of light pressure reduction and reduce segregation at the center segregation portion, it is necessary to lightly reduce from the solid-liquid coexistence region at the center of the slab thickness to the complete solidification portion. Bi is also a segregating element, and Bi is also segregated positively at the center segregating portion. By performing light reduction, Bi segregation at the center of the slab thickness can also be reduced. On the other hand, in the present invention, Bi is intentionally concentrated at the center portion of the slab thickness by finishing the light reduction on the near side of the completely solidified portion and making the center segregation reduction effect insufficient. I was inspired by that. In order to reduce the dendrite primary arm interval at the center of the slab thickness and reduce the center segregation, the Bi concentration should be 0.0015% by mass or more mainly in the region where the center segregation is generated. If light pressure is intentionally insufficiently performed and the Bi concentration in the central segregation region can be increased to 0.0015% by mass or more, the object of the present invention can be achieved.
そこで、溶鋼中のＢｉ濃度を０．００１０質量％未満の範囲とした上で、軽圧下の条件を種々変更して、鋳片厚み中心部におけるＢｉ濃度と、同じく鋳片厚み中心部のデンドライト１次アーム間隔及びＭｎの偏析状況について評価した。その結果、軽圧下を終了する位置の中心部固相率が０．９６を超えて下流側まで至ると、軽圧下の効果が効き過ぎるため、中心偏析部のＢｉ濃度を０．００１５質量％以上とすることが困難であった一方、軽圧下を終了する位置の中心部固相率が０．３以上０．９６以下であれば、軽圧下の効果が不十分となり、中心偏析部のＢｉ濃度を０．００１５質量％以上とすることができた。 Then, after making Bi concentration in molten steel into the range of less than 0.0010 mass%, the conditions under light pressure are variously changed, Bi concentration in the slab thickness center part, and dendrite 1 of slab thickness center part similarly. Next arm spacing and Mn segregation were evaluated. As a result, if the central solid phase ratio at the position where light reduction is finished exceeds 0.96 and reaches the downstream side, the effect of light reduction is too effective, so the Bi concentration in the central segregation part is 0.0015% by mass or more. On the other hand, if the center solid phase ratio at the position where the light reduction is finished is 0.3 or more and 0.96 or less, the effect of the light reduction becomes insufficient, and the Bi concentration in the center segregation part Can be made 0.0015 mass% or more.
上記条件で軽圧下を行うことにより、十分に軽圧下を行った場合に比較すると軽圧下効果は低下するが、中心偏析部のＢｉ濃度が０．００１５質量％以上に濃化することによる効果（中心偏析部におけるデンドライト１次アーム間隔の低減と偏析の低減）が勝り、鋳片厚み中心部における凝固組織の微細化を実現できる。凝固組織が微細化するので、熱間圧延工程における加熱処理中と熱間圧延中における偏析元素の拡散促進効果が高まるため、総合的に鋳片の品質を向上することができる。 By performing the light reduction under the above conditions, the light reduction effect is reduced as compared with the case where the light reduction is sufficiently performed. However, the effect of increasing the Bi concentration of the central segregation part to 0.0015% by mass or more ( The dendrite primary arm spacing in the center segregation part and the segregation reduction) are excellent, and the solidification structure in the center part of the slab thickness can be made finer. Since the solidified structure is refined, the effect of promoting the diffusion of segregating elements during the heat treatment and hot rolling in the hot rolling process is enhanced, so that the quality of the slab can be improved comprehensively.
軽圧下を開始する位置における中心部固相率が高すぎると、軽圧下が不十分になりすぎ、たとえ中心偏析部にＢｉ濃度を濃化させたとしても、中心偏析の悪影響を軽減することができなくなる。本発明において、軽圧下を開始する位置における中心部固相率が０．３未満であれば、本発明にとって必要な範囲で十分な軽圧下効果を発揮することができる。軽圧下を開始する位置における中心部固相率が０．２以下であるとより好ましい。軽圧下を開始する位置の中心部固相率が０．３未満であれば、中心部固相率が０を超えて増大し始めた以降のいずれであってもよい。厚み中心部が未凝固である時点から軽圧下を開始しても良い。軽圧下を開始する位置の中心部固相率が０．３以上となると、この領域内で生成した固相が、これらの間隙に残存する液相の流動性の低下により相互に影響を及ぼし合う段階に達しているため、濃化した液相が流動し難くなり、軽圧下の効果を発揮させるためには開始するのが遅い。 If the solid fraction at the center at the position where light reduction starts is too high, light reduction will be insufficient, and even if the Bi concentration is concentrated in the central segregation part, the adverse effect of center segregation can be reduced. become unable. In the present invention, if the central part solid phase ratio at the position where the light reduction is started is less than 0.3, a sufficient light reduction effect can be exhibited within a range necessary for the present invention. It is more preferable that the solid fraction at the center at the position where light pressure starts is 0.2 or less. As long as the center part solid phase ratio at the position where light pressure starts is less than 0.3, it may be any after the center part solid ratio exceeds 0 and starts to increase. Light pressure reduction may be started from the time when the thickness center portion is unsolidified. When the solid phase ratio at the center where light pressure starts is 0.3 or more, the solid phase generated in this region affects each other due to the decrease in fluidity of the liquid phase remaining in these gaps. Since the stage has been reached, the concentrated liquid phase is less likely to flow and is slow to start in order to exert the effect of light pressure.
中心部固相率が０．３以上０．９６以下の範囲で軽圧下を終了する。軽圧下終了時の中心部固相率が０．３以上であれば、軽圧下の開始時点からの偏析低減効果が維持される。軽圧下終了時の中心部固相率が０．５以上であるとより好ましい。また、軽圧下終了時の中心部固相率が増えて０．９６を超えると、中心偏析により濃化したＢｉの濃度が、本来の軽圧下の効果により減少し、中心偏析部のＢｉ濃度を０．００１５質量％以上とすることが困難となるため、軽圧下終了時の中心部固相率の上限を０．９６とした。軽圧下終了時の中心部固相率が０．９０以下であるとより好ましい。 The light reduction is completed when the solid fraction at the center is in the range of 0.3 to 0.96. If the solid fraction at the center at the end of light pressure is 0.3 or more, the segregation reduction effect from the start time of light pressure is maintained. More preferably, the solid fraction at the center at the end of light pressure is 0.5 or more. In addition, when the solid fraction at the center at the end of light pressure increases and exceeds 0.96, the concentration of Bi concentrated by the center segregation decreases due to the effect of the original light pressure, and the Bi concentration in the center segregation part is reduced. Since it becomes difficult to make it 0.0015% by mass or more, the upper limit of the solid fraction at the center at the end of light pressure reduction is set to 0.96. More preferably, the solid fraction at the center at the end of light pressure is 0.90 or less.
軽圧下条件が本発明で規定する条件よりも不十分である場合、即ち、軽圧下開始時の中心部固相率が０．３以上、あるいは軽圧下終了時の中心部固相率が０．３未満である場合には、鋳片厚み中心部付近のＢｉ濃度変動が増大することがある。本発明で規定する軽圧下を行うことにより、Ｂｉ濃度変動を抑制し、鋳片厚み中心部付近のＭｎ最大偏析部における平均Ｂｉ濃度を安定して０．００１５％以上とすることができる。 When the light reduction condition is insufficient than the conditions specified in the present invention, that is, the central solid phase ratio at the start of the light reduction is 0.3 or more, or the central solid ratio at the end of the light reduction is 0. If it is less than 3, Bi concentration fluctuations near the center of the slab thickness may increase. By performing the light reduction defined in the present invention, the Bi concentration fluctuation can be suppressed, and the average Bi concentration in the Mn maximum segregation portion in the vicinity of the center portion of the slab thickness can be stably made 0.0015% or more.
軽圧下に際しては、軽圧下中の圧下勾配を０．２〜２．０ｍｍ／ｍｉｎの範囲とすることにより、鋳片の凝固収縮に見合った圧下を行うことができる。このような軽圧下を行ったとき、軽圧下前後の鋳片厚みの変化を軽圧下前鋳片厚みで除した圧下率は、２〜４０％となる。これにより、鋳片の凝固収縮に見合った圧下を行うことができ、鋳片の厚み中心部において未凝固溶鋼の流動とそれに伴う成分の濃化を防止することができる。 In the light reduction, the reduction corresponding to the solidification shrinkage of the slab can be performed by setting the reduction gradient during the light reduction in the range of 0.2 to 2.0 mm / min. When such light reduction is performed, the reduction ratio obtained by dividing the change in slab thickness before and after light reduction by the thickness of the slab before light reduction is 2 to 40%. Thereby, the reduction corresponding to the solidification shrinkage of the slab can be performed, and the flow of unsolidified molten steel and the accompanying concentration of components can be prevented at the center of the thickness of the slab.
ここで、連続鋳造中の鋳造長手方向における中心部固相率の算出方法について説明する。鋳片の伝熱シミュレーションプログラムを用いて、予めミクロ偏析による溶質元素の濃化を考慮した凝固解析を行い、鋳片厚み中心部の温度と固相率の関係を求め、さらに操業中においても測定可能な鋳片の表面温度と鋳片厚み中央の固相率の関係を求めることができる。この計算で鋳造長手方向位置と中心部固相率の関係を算出することができる。ここでは、鋳造中の鋳片表面温度を実測し、実測した鋳片表面温度を代入して計算機にて凝固解析を行い、鋳造長手方向各部位の中心部固相率を算出した。ここで、密度ρは、ρ＝７．２７＋０．２５×固相率（ｇ／ｃｍ3）とした。（鉄と鋼、ｖｏｌ．９４（２００８）、ｐ．５０７：水上英夫、山中章裕） Here, the calculation method of the center part solid-phase rate in the casting longitudinal direction during continuous casting is demonstrated. Using a heat transfer simulation program for slabs, solidification analysis that takes into account the concentration of solute elements due to microsegregation is performed in advance to determine the relationship between the temperature at the center of the slab thickness and the solid fraction, and measurements are also made during operation. The relationship between the surface temperature of a possible slab and the solid phase ratio at the center of the slab thickness can be obtained. With this calculation, it is possible to calculate the relationship between the position in the casting longitudinal direction and the solid fraction at the center. Here, the slab surface temperature during casting was measured, and the measured slab surface temperature was substituted and solidification analysis was performed by a computer to calculate the solid fraction at the center of each part in the casting longitudinal direction. Here, the density ρ was ρ = 7.27 + 0.25 × solid phase rate (g / cm 3 ). (Iron and Steel, vol. 94 (2008), p. 507: Hideo Mizukami, Akihiro Yamanaka)
中心偏析部におけるＢｉ濃度の測定方法について説明する。まず、鋳片の厚み断面において、厚み中心部付近のＭｎ偏析状況を評価する。ＥＰＭＡなどの手段を用い、厚み方向のＭｎ分布状況を測定し、Ｍｎ濃度が最も増大した部分を中心偏析部と判定する。次に、判定した中心偏析部において、５ｍｍφのドリルで穿孔してドリルサンプルを採取する。このドリルサンプルで評価したＢｉ濃度を、「鋳片厚み中心部付近のＭｎ最大偏析部における平均Ｂｉ濃度」とする。 A method for measuring the Bi concentration in the center segregation part will be described. First, the Mn segregation situation in the vicinity of the thickness center is evaluated in the thickness cross section of the slab. Using means such as EPMA, the Mn distribution state in the thickness direction is measured, and the portion where the Mn concentration has increased most is determined as the central segregation portion. Next, a drill sample is taken by drilling with a 5 mmφ drill at the determined center segregation part. The Bi concentration evaluated with this drill sample is defined as “the average Bi concentration in the Mn maximum segregation portion in the vicinity of the center portion of the slab thickness”.
鋳片厚み中心部におけるデンドライト１次アーム間隔の相対評価方法について説明する。同一の鋳造条件において、Ｂｉを添加しない鋳造とＢｉを添加した鋳造を行う。それぞれについて、鋳片厚み中心部におけるデンドライト１次アーム間隔λを計測する。Ｂｉを添加しない場合の値λ1 0を基準として、Ｂｉを添加したときの値λ1との比λ1／λ1 0が０．８以下であれば、厚み中心部のデンドライト微細化効果があったと評価した。中心偏析部のＢｉ濃度を０．００１５％以上とすることにより、λ1／λ1 0を０．８以下とすることができる。なお、デンドライト組織の顕出は、エッチング液には８０℃に加熱したピクリン酸飽和溶液を用い、６０秒間試料を浸漬した。この組織を、２５倍の光学顕微鏡で観察し、デンドライト１次アーム間隔を測定した。 A relative evaluation method of the dendrite primary arm interval at the center of the slab thickness will be described. Under the same casting conditions, casting without adding Bi and casting with Bi added are performed. About each, the dendrite primary arm space | interval (lambda) in slab thickness center part is measured. As a reference value lambda 1 0 without the addition of Bi, if the ratio λ 1 / λ 1 0 and the value lambda 1 upon addition of Bi is 0.8 or less, the dendrite refining effect of the thickness center Evaluated that there was. The Bi concentration of the center segregation area by 0.0015% or more, the λ 1 / λ 1 0 can be set to 0.8 or less. In order to reveal the dendrite structure, a saturated picric acid solution heated to 80 ° C. was used as an etching solution, and the sample was immersed for 60 seconds. This structure was observed with a 25 × optical microscope, and the dendrite primary arm interval was measured.
溶鋼中に添加するＢｉ濃度を０．０００１％から０．００１０％未満とする。これに加え、さらに鋳造中の軽圧下パターンを上記本発明のパターンとすることにより、鋳片の中心偏析部におけるＢｉの濃度を０．００１５％以上にすることができる。 The Bi concentration added to the molten steel is set to 0.0001% to less than 0.0010%. In addition, by making the light reduction pattern during casting the pattern of the present invention, the concentration of Bi in the center segregation portion of the slab can be made 0.0015% or more.
本発明の効果を奏するためには、上記のように中心偏析部のデンドライト１次アーム間隔が微細化するとともに、中心偏析部の偏析元素（この場合はＭｎ）の偏析度合が十分に低いことも必要である。中心偏析部について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の最大濃度を求める。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する。本発明の鋳片と、Ｂｉを添加せず軽圧下も行わずに鋳造した鋳片とでそれぞれＭｎの偏析比を求め、（本発明材のＭｎ偏析比）／（Ｂｉ無添加・非軽圧下材のＭｎ偏析比）の比を「偏析比指数」とした。偏析比指数が０．６５以下であれば、上記λ1／λ1 0を０．８以下とすることとあいまって、製品品質を向上し、特に製品鋼板の溶接熱影響部（ＨＡＺ部）の破壊靱性を向上することができる。本発明においては、中心部固相率が０．３未満で軽圧下を開始し、中心部固相率が０．３以上０．９６以下の範囲で軽圧下を完了パターンで軽圧下を行うことと、鋳片厚み中心部付近のＭｎ最大偏析部における平均Ｂｉ濃度を０．００１５％以上とすることがあいまって、上記偏析比指数を０．６５以下とすることができる。 In order to achieve the effect of the present invention, the dendrite primary arm interval of the center segregation part is made finer as described above, and the segregation degree of the segregation element (Mn in this case) of the center segregation part is sufficiently low. is necessary. The central segregation part is subjected to line analysis 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 is obtained. 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 is defined as the segregation ratio. The Mn segregation ratio was determined for each of the slab of the present invention and the slab cast without adding Bi and without light reduction. (Mn segregation ratio of the present invention material) / (Bi non-added / non-light reduced) The ratio of Mn segregation ratio of the material was defined as the “segregation ratio index”. If the segregation ratio index is 0.65 or less, combined with the above-mentioned λ 1 / λ 1 0 being 0.8 or less, the product quality is improved, especially in the weld heat affected zone (HAZ part) of the product steel plate. Fracture toughness can be improved. In the present invention, light reduction is started when the central solid fraction is less than 0.3, and light reduction is performed in a complete pattern when the central solid fraction is in the range of 0.3 to 0.96. In addition, the average Bi concentration in the Mn maximum segregation part in the vicinity of the center part of the slab thickness is 0.0015% or more, so that the segregation ratio index can be 0.65 or less.
次に、本発明で必須とする鋼成分について説明する。以下、成分の記載において、％は質量％を意味する。 Next, the steel components essential in the present invention will be described. Hereinafter, in description of a component,% means the mass%.
Ｃ：０．０４〜０．３５％
Ｃは、強度および靱性を確保するために有効な元素である。その含有量が０．０４％未満では、上記の効果が充分に得られず、一方、その含有量が０．３５％を超えて高くなると母材およびＨＡＺ部の靭性が低下する。そこで、Ｃの適正範囲を０．０４〜０．３５％とした。
C: 0.04 to 0.35%
C is an element effective for securing strength and toughness. If the content is less than 0.04%, the above effects cannot be obtained sufficiently. On the other hand, if the content exceeds 0.35%, the toughness of the base material and the HAZ part decreases. Therefore, the appropriate range of C is set to 0.04 to 0.35%.
Ｓｉ：０．００５〜３．０％
Ｓｉは、０．００５％未満では母材の強度を確保できないので下限を０．００５％とした。また、３．０％を超えると溶接性が低下するため上限を３．０％とした。上記の理由から、その適正範囲を０．００５〜３．０％とした。
Si: 0.005 to 3.0%
If Si is less than 0.005%, the strength of the base material cannot be secured, so the lower limit was made 0.005%. Moreover, since weldability will fall when it exceeds 3.0%, an upper limit was made into 3.0%. For the above reason, the appropriate range is 0.005 to 3.0%.
Ｍｎ：０．１〜３．５％
Ｍｎは、鋼板の高強度化と靱性の確保のために有効な元素である。上記の効果を得るためには、その含有量を０．１％以上とする必要がある。一方、その含有量が３．５％を超えて高くなると、靱性が損なわれる。このため、Ｍｎ含有量の適正範囲を０．１〜３．５％とした。
Mn: 0.1 to 3.5%
Mn is an element effective for increasing the strength of the steel sheet and ensuring toughness. In order to acquire said effect, it is necessary to make the content into 0.1% or more. On the other hand, if the content exceeds 3.5%, the toughness is impaired. For this reason, the suitable range of Mn content was made into 0.1 to 3.5%.
Ｐ：０．０２％以下
Ｐは、鋼板の延性および靱性および加工性を劣化させる元素であることから、その含有量を０．０２％以下に制限する。
P: 0.02% or less P is an element that deteriorates the ductility, toughness, and workability of the steel sheet, so the content is limited to 0.02% or less.
Ｓ：０．０００２〜０．００２％
Ｓは、Ｍｎと反応して介在物ＭｎＳを形成して鋼材の延性を低下させるが、結晶粒内にフェライトの生成を促進する効果がある。０．０００２％未満ではフェライトの生成する効果がほとんど無いため、０．０００２％を下限とした。ただし、０．００２％を超えると鋼板の延性を低下させるため、０．００２％を上限とした。上記の理由から、Ｓ含有量の適正範囲を０．０００２〜０．００２％とした。
S: 0.0002 to 0.002%
S reacts with Mn to form inclusion MnS to lower the ductility of the steel material, but has the effect of promoting the formation of ferrite in the crystal grains. If it is less than 0.0002%, there is almost no effect of forming ferrite, so 0.0002% was made the lower limit. However, if over 0.002%, the ductility of the steel sheet is lowered, so 0.002% was made the upper limit. For the above reason, the appropriate range of the S content is set to 0.0002 to 0.002%.
Ａｌ：０．０００５％〜１．０％
Ａｌは、鋼を脱酸させるために添加される元素である。０．０００５％未満ではその効果が認められず、１．０％を超えると、酸化物系介在物のサイズが大きくなるため、鋼板の表面性状も劣化する。これらのことから、本発明では、Ａｌ含有率の適正範囲を０．０００５％〜１．０％とすることが好ましい。
Al: 0.0005% to 1.0%
Al is an element added to deoxidize steel. If the content is less than 0.0005%, the effect is not recognized. If the content exceeds 1.0%, the size of the oxide inclusions increases, and the surface properties of the steel sheet also deteriorate. For these reasons, in the present invention, the appropriate range of the Al content is preferably 0.0005% to 1.0%.
Ｎ：０．００２〜０．０１０％
Ｎは、鋼に不可避的に含有される不純物であり、鋼板の曲げ性の観点からは、含有率は低いほど好ましいが、窒化物を活用するには０．００２％以上必要である。そのため、本発明では、Ｎ含有率を０．００２〜０．０１０％とすることが好ましい。
N: 0.002 to 0.010%
N is an impurity inevitably contained in the steel, and from the viewpoint of the bendability of the steel sheet, the content is preferably as low as possible, but 0.002% or more is necessary to utilize the nitride. Therefore, in this invention, it is preferable that N content rate shall be 0.002-0.010%.
Ｏ：０．０００１〜０．０１％
Ｏは、鋼に不可避的に含有される不純物であり、鋼中に粗大な介在物を形成して鋼の靭性を低下させるため、含有率は低いほど好ましいが、酸化物を活用するには０．０００１％以上必要である。そのため、本発明では、Ｏ含有率を０．０００１〜０．０１％とすることが好ましい。
O: 0.0001 to 0.01%
O is an impurity inevitably contained in the steel, and since it forms coarse inclusions in the steel and lowers the toughness of the steel, the lower the content, the better. 0.001% or more is necessary. Therefore, in the present invention, the O content is preferably 0.0001 to 0.01%.
Ｂｉ：０．０００１％以上０．００１０％未満
以上説明したように、本発明においてＢｉは最重要な元素である。鋳片の中心偏析部におけるＢｉの濃度を０．００１５％以上にするために、上記範囲で溶鋼中にＢｉを含有させる。溶鋼中Ｂｉ濃度が０．０００１％未満では、中心偏析部のＢｉ濃度を０．００１５％以上とすることが難しくなる。一方、中心偏析部のＢｉ濃度を０．００１５％以上とするためには、溶鋼中Ｂｉ濃度を０．００１０％未満とすれば十分であり、それ以上に溶鋼中にＢｉを含有しても、Ｂｉは高価であるから、鋼の製造コストが増大するばかりで不利となる。
Bi: 0.0001% or more and less than 0.0010% As described above, Bi is the most important element in the present invention. In order to make the concentration of Bi in the center segregation part of the slab 0.0015% or more, Bi is contained in the molten steel in the above range. If the Bi concentration in molten steel is less than 0.0001%, it is difficult to make the Bi concentration in the central segregation part 0.0015% or more. On the other hand, in order to make the Bi concentration in the central segregation part 0.0015% or more, it is sufficient that the Bi concentration in the molten steel is less than 0.0010%, and even if Bi is contained in the molten steel, Since Bi is expensive, it is disadvantageous as it only increases the manufacturing cost of steel.
ここで、溶鋼中へのＢｉの添加方法について説明する。Ｂｉの沸点は１５６４℃であり、溶鋼の温度はそれ以上であるため、Ｂｉを無駄なく溶鋼中に添加する上では注意が必要である。ＲＨ真空脱ガス処理を行い、脱酸が終了したＲＨ真空脱ガス処理の末期に、真空槽内においてＢｉを添加することができる。また、ＲＨ真空脱ガス処理が終了した後、ワイヤーに被覆したＢｉを鍋内溶鋼中に添加することができる。タンディッシュ内の溶鋼中に、ワイヤーに被覆したＢｉを添加することとしても良い。 Here, the addition method of Bi in molten steel is demonstrated. Since the boiling point of Bi is 1564 ° C. and the temperature of the molten steel is higher than that, care must be taken when adding Bi to the molten steel without waste. Bi can be added in the vacuum chamber at the end of the RH vacuum degassing process after the RH vacuum degassing process is completed. Moreover, after the RH vacuum degassing process is completed, Bi coated on the wire can be added to the molten steel in the pan. It is good also as adding Bi coat | covered with the wire in the molten steel in a tundish.
本発明は、さらに必要に応じて下記元素を含有することとしても良い。 The present invention may further contain the following elements as necessary.
Ｔｉ：０．００５〜０．０３％
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母材強度の向上に寄与する有効な元素である。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母材強度を向上させる効果が充分ではなく、一方、Ｔｉ含有率が０．０３％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、鋼の靭性を低下させる。上記の理由から、Ｔｉ含有率の適正範囲を０．００５〜０．０３％とした。
Ti: 0.005 to 0.03%
Ti is an effective element that mainly precipitates carbonitrides and contributes to improvement of the strength of the base metal by its precipitation strengthening action. If the Ti content is less than 0.005%, the effect of improving the strength of the base metal by the precipitation strengthening action of carbonitride is not sufficient, while if the Ti content exceeds 0.03%, Coarse precipitates and inclusions are formed to reduce the toughness of the steel. For the above reason, the appropriate range of Ti content is set to 0.005 to 0.03%.
Ｃｕ：０．０５〜１．５％
Ｃｕは、含有させれば焼入れ性の向上および析出強化に有効な作用を有する元素である。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無い。一方、Ｃｕ含有率が１．５％を超えて高くなると、鋼の熱間加工性が低下する。上記の理由から、Ｃｕを含有させる場合のＣｕ含有率の範囲を０．０５〜１．５％とした。
Cu: 0.05 to 1.5%
If Cu is contained, it is an element having an effect effective in improving hardenability and precipitation strengthening. However, when the Cu content is less than 0.05%, there is no effect of improving hardenability and effect of precipitation strengthening. On the other hand, when the Cu content is higher than 1.5%, the hot workability of steel is lowered. For the above reason, the range of the Cu content when Cu is contained is set to 0.05 to 1.5%.
Ｎｉ：０．０５〜５．０％
Ｎｉは、含有させれば母材の靭性を向上させる作用を有する元素である。しかし、Ｎｉ含有率が０．０５%未満では、母材の靭性を向上させる効果が無い。一方、Ｎｉ含有率が５．０％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。そこで、Ｎｉを含有させる場合のＮｉ含有率の範囲を０．０５〜５．０％とした。
Ni: 0.05-5.0%
Ni is an element having an action of improving the toughness of the base material if contained. However, when the Ni content is less than 0.05%, there is no effect of improving the toughness of the base material. On the other hand, if the Ni content exceeds 5.0%, the hardenability becomes excessive and adversely affects the toughness of the steel. Therefore, the range of Ni content when Ni is included is set to 0.05 to 5.0%.
Ｃｒ：０．０２〜１．０％
Ｃｒは、含有させれば焼入れ性の向上、および析出強化による母材強度の向上に有効な作用を発揮する元素である。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無い。一方、Ｃｒ含有率が１．０％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。そこで、Ｃｒを含有させる場合のＣｒ含有率の範囲を０．０２〜１．０％とした。
Cr: 0.02-1.0%
When Cr is contained, it is an element that exhibits an effective action for improving the hardenability and for improving the strength of the base metal by precipitation strengthening. However, if the Cr content is less than 0.02%, there is no effect of improving hardenability and effect of precipitation strengthening. On the other hand, when the Cr content is higher than 1.0%, there is a tendency for the toughness and weldability of steel to deteriorate. Therefore, the range of Cr content when Cr is contained is set to 0.02 to 1.0%.
Ｍｏ：０．０２〜１．０％
Ｍｏは、含有させれば焼入れ性の向上および強度の向上に有効な作用を発揮する元素である。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１．０％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。そこで、Ｍｏを含有させる場合のＭｏ含有率の範囲を０．０２〜１．０％とした。
Mo: 0.02-1.0%
Mo is an element that exhibits an effective action for improving hardenability and strength when contained. 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 exceeds 1.0%, the toughness and ductility of the steel and the weldability deteriorate. Therefore, the range of the Mo content when Mo is contained is set to 0.02 to 1.0%.
Ｎｂ：０．００５〜０．０５％
Ｎｂは、含有させれば炭化物や窒化物を生成して鋼の強度を向上させる作用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０．０５％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、逆に靭性を低下させる。上記の理由から、Ｎｂを含有させる場合のＮｂ含有率の範囲を０．００５〜０．０５％とした。
Nb: 0.005 to 0.05%
Nb is an element that has the effect of improving the strength of steel by forming carbides and nitrides when contained. 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 is higher than 0.05%, coarse carbides and nitrides are formed in the steel, so that the toughness is reduced. For the above reason, the range of Nb content when Nb is contained is set to 0.005 to 0.05%.
Ｖ：０．００５〜０．１％
Ｖは、含有させれば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０．１％を超えて高くなると、鋼の靭性を低下させる。上記の理由から、Ｖを含有させる場合のＶ含有率の範囲を０．００５〜０．１％とした。
V: 0.005 to 0.1%
V is an element that has the effect of improving the strength of steel by forming carbides and nitrides when contained. 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, when the V content exceeds 0.1%, the toughness of the steel is reduced. For the above reason, the range of the V content when V is contained is set to 0.005 to 0.1%.
Ｂ：０．０００４〜０．００４％
Ｂは、含有させれば焼入れ性を増大させるとともに、ＢＮを生成することで固溶Ｎの含有率を低下させ、ＨＡＺの靭性を向上させる効果がある。ただし、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無い。しかしながら、Ｂ含有率が０．００４％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劣化する。上記の理由から、Ｂを含有させる場合のＢ含有率の範囲を０．０００４〜０．００４％とした。
B: 0.0004 to 0.004%
When B is contained, it has the effect of increasing the hardenability and reducing the content of solid solution N by generating BN and improving the toughness of HAZ. 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. However, if the B content exceeds 0.004%, coarse borides precipitate in the steel, which deteriorates the toughness of the steel. For the above reason, the range of the B content when B is contained is set to 0.0004 to 0.004%.
本発明の鋳片の連続鋳造方法の効果を確認するため、転炉にて表１に示す成分含有量の溶鋼を溶製し、垂直曲げ型のスラブ連続鋳造装置によって連続鋳造した。今回は、軽圧下設備を有する連続鋳造装置を用いた評価のみを行った。Ｂｉについては、脱酸が終了したＲＨ真空脱ガス処理の末期に、真空槽内において添加した。 In order to confirm the effect of the continuous casting method of the slab of the present invention, molten steel having the component contents shown in Table 1 was melted in a converter and continuously cast by a vertical bending type slab continuous casting apparatus. This time, only evaluation using a continuous casting machine with light reduction equipment was performed. Bi was added in the vacuum chamber at the end of the RH vacuum degassing process after deoxidation.
連続鋳造において、タンディッシュ内溶鋼温度は１５７０℃、鋳型サイズ：幅１６００ｍｍ×厚さ２５０ｍｍ、鋳造速度：０．８〜１．６ｍ／分、添加したＢｉ濃度：０．００００２〜０．００５％（表１、２参照）、軽圧下の範囲：鋳片厚み中心部の固相率が０．２〜０．９９（表２参照）、軽圧下時の圧下速度：０．２〜２．０ｍｍ／ｍｉｎとした。 In continuous casting, the molten steel temperature in the tundish is 1570 ° C., mold size: width 1600 mm × thickness 250 mm, casting speed: 0.8 to 1.6 m / min, added Bi concentration: 0.00002 to 0.005% ( Tables 1 and 2), light reduction range: solid fraction at the center of the slab thickness is 0.2 to 0.99 (see Table 2), reduction speed during light reduction: 0.2 to 2.0 mm / It was set to min.
中心偏析部におけるＢｉ濃度は、前述の方法で評価した。即ち、鋳片の厚み断面において、ＥＰＭＡなどの手段を用い、厚み中心部付近の厚み方向のＭｎ分布状況を測定し、Ｍｎ濃度が最も増大した部分を中心偏析部（Ｍｎ最大偏析部）と判定した。次に、判定した中心偏析部において、５ｍｍφのドリルで穿孔してドリルサンプルを採取し、このドリルサンプルで評価したＢｉ濃度を、「鋳片厚み中心部付近のＭｎ最大偏析部における平均Ｂｉ濃度」として、表２の「鋳片厚み中心部品質」の「Ｂｉ濃度」欄に記載した。 The Bi concentration in the center segregation part was evaluated by the method described above. That is, in the thickness cross section of the slab, the Mn distribution state in the thickness direction near the thickness center portion is measured using means such as EPMA, and the portion where the Mn concentration has increased the most is determined as the center segregation portion (Mn maximum segregation portion). did. Next, in the determined center segregation portion, a drill sample was taken by drilling with a 5 mmφ drill, and the Bi concentration evaluated with this drill sample was determined as “average Bi concentration in Mn maximum segregation portion near the center of slab thickness” As shown in the “Bi concentration” column of “Slab Thickness Center Quality” in Table 2.
連続鋳造方法の効果の評価は、鋳片の組織観察によって行った。組織観察用の試験片は、鋳片幅中央で、厚み中心部±２５ｍｍ、鋳造方向に５０ｍｍの試料を採取し、凝固組織であるデンドライト１次アーム間隔λを測定した。デンドライトの微細化効果の評価は、Ｂｉを添加しない場合（表２の試験番号１１）の値λ1 0を基準として、各試験水準における値λ1との比λ1／λ1 0で評価し、表２に記載した。デンドライト組織の顕出は、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、エッチング液には８０℃に加熱したピクリン酸飽和溶液を用い、６０秒間試料を浸漬した。この組織を、２５倍の光学顕微鏡で観察し、デンドライト１次アーム間隔を測定した。λ1／λ1 0：０．８以下を良好とした。 The effect of the continuous casting method was evaluated by observing the structure of the slab. As a specimen for observing the structure, a sample having a thickness center of ± 25 mm at the center of the slab width and 50 mm in the casting direction was taken, and the dendrite primary arm interval λ, which is a solidified structure, was measured. Evaluation of effect of refining dendrite, based on the value lambda 1 0 if (Test No. 11 in Table 2) without the addition of Bi, and evaluated by the ratio λ 1 / λ 1 0 and the value lambda 1 in each test level The results are shown in Table 2. The dendritic structure is revealed by polishing the sample with emery paper and abrasives (diamond grains of 6 μm and 1 μm in diameter) in order, and using an etching solution of Picrin heated to 80 ° C. The sample was immersed for 60 seconds using an acid saturated solution. This structure was observed with a 25 × optical microscope, and the dendrite primary arm interval was measured. λ 1 / λ 1 0 : 0.8 or less was considered good.
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厚み中心部で厚み方向の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。その上で、Ｂｉを添加せず軽圧下も行わない場合（表２の試験番号１１）のＭｎの偏析比を基準として、各試験水準におけるＭｎの偏析比との比を「偏析比指数」として表２に記載した。偏析比指数０．６５以下を良好とした。 The solute element to be measured for segregation was Mn. A line analysis was performed using EPMA with a beam diameter of 50 μm, and the Mn concentration distribution in the thickness direction was measured at the center of the thickness of the sample to obtain the maximum concentration of Mn in the measurement range. 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. In addition, based on the segregation ratio of Mn when Bi is not added and light pressure is not applied (test number 11 in Table 2), the ratio of the segregation ratio of Mn at each test level is referred to as a “segregation ratio index”. It described in Table 2. A segregation ratio index of 0.65 or less was considered good.
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、制御圧延・制御冷却法、焼入れ・焼戻し法、および直接焼入れ・焼戻し法のいずれかの製造方法によって厚み５０ｍｍの鋼板を製造した。試料の形状は、縦１０ｍｍ、横１０ｍｍ、長さ５０ｍｍの角柱状とした。鋼板の厚み方向を試料の長手方向とし、鋼板の厚み中心部を試料の長手中央部とし、当該中央部をノッチ位置とした。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。 Samples for toughness measurement were prepared by performing heat treatment for 90 minutes at 1250 ° C on a continuous cast slab produced under the above conditions, followed by controlled rolling / controlled cooling method, quenching / tempering method, and direct quenching / tempering method. A steel plate having a thickness of 50 mm was produced by any one of the production methods. The shape of the sample was a prismatic shape having a length of 10 mm, a width of 10 mm, and a length of 50 mm. The thickness direction of the steel plate was taken as the longitudinal direction of the sample, the thickness center portion of the steel plate was taken as the longitudinal central portion of the sample, and the central portion was taken as the notch position. 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１４００℃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 1400 ° C. for 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。Ｂｉを添加せず軽圧下を実施しない場合（表２の試験番号１１）の吸収エネルギーを基準として、各試験水準の吸収エネルギー比を靭性指数と定義した。靱性指数１．２０以上を良好とした。 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. The absorption energy ratio at each test level was defined as the toughness index based on the absorption energy when Bi was not added and light reduction was not performed (test number 11 in Table 2). A toughness index of 1.20 or higher was considered good.
溶鋼へのＢｉ添加量の多寡による製造コストの増減を参考までにコスト指数として指標化し、試験番号１１をコスト指数１とした。 The increase or decrease in production cost due to the amount of Bi added to the molten steel was indexed as a cost index for reference, and test number 11 was designated as cost index 1.
表２に試験結果を示す。表１、表２において、本発明範囲から外れる数値にアンダーラインを付している。また表２において、品質指標が本発明の好適範囲から外れる数値に＊を付している。 Table 2 shows the test results. In Tables 1 and 2, numerical values that fall outside the scope of the present invention are underlined. Further, in Table 2, * is attached to a numerical value whose quality index is out of the preferred range of the present invention.
試験番号１〜１０の本発明例１〜１０のように、鋳片厚み中心部の固相率が０．３未満の時点で軽圧下を開始して、固相率が０．３以上０．９６以下の範囲で軽圧下を完了することで、鋳片厚み中心部の中心偏析部におけるＢｉ濃度が０．０１５％以上となり、デンドライト１次アーム間隔が微細化してλ1／λ1 0を０．８以下とすることができ、同時に偏析比指数が０．６５以下に低減することで、靭性指数が増大している。 As in Test Examples 1 to 10 of Invention Examples 1 to 10, when the solid fraction at the center of the slab thickness is less than 0.3, light reduction is started, and the solid fraction is 0.3 or more and 0.00. By completing the light reduction in the range of 96 or less, the Bi concentration in the central segregation portion of the slab thickness center portion becomes 0.015% or more, the dendrite primary arm interval becomes finer, and λ 1 / λ 1 0 becomes 0 .8 or less, and at the same time the segregation ratio index is reduced to 0.65 or less, the toughness index is increased.
試験番号１１の比較例１はＢｉ添加なし、凝固後の軽圧下なしの比較例であり、λ1／λ1 0、偏析比指数、靱性指数、コスト指数はいずれも１．０である。試験番号１２の比較例２は、Ｂｉ以外の成分が相違するのみで他は試験番号１１と同様の条件であり、成績も試験番号１１と同様であった。 Comparative example 1 of test number 11 is a comparative example without Bi addition and without light reduction after solidification, and λ 1 / λ 1 0 , segregation ratio index, toughness index, and cost index are all 1.0. Comparative example 2 of test number 12 was the same as test number 11 except that the components other than Bi were different, and the results were the same as test number 11.
試験番号１３の比較例３は、Ｂｉを添加したものの軽圧下を行っておらず、中心偏析が過大であって靱性を改善することができなかった。 Although the comparative example 3 of the test number 13 did not perform light pressure reduction although Bi was added, center segregation was excessive and toughness was not able to be improved.
試験番号１４の比較例４は、軽圧下終了時の中心部固相率が０．９９と本発明の上限を超え、軽圧下による中心偏析改善効果が過剰であって中心偏析部におけるＢｉ濃度が不足し、デンドライト微細化指標であるλ1／λ1 0の低減効果が不十分であり、靱性指数が改善しなかった。試験番号１５の比較例５は、軽圧下開始時の中心部固相率が０．４０と本発明範囲から外れ、軽圧下効果が不十分で中心偏析部の偏析比指数が高く、靱性指数が改善しなかった。 In Comparative Example 4 of Test No. 14, the solid fraction at the center at the end of light pressure was 0.99, exceeding the upper limit of the present invention, the effect of improving center segregation by light pressure was excessive, and the Bi concentration in the center segregation part was missing, reducing the effect of λ 1 / λ 1 0 is a dendrite refinement index is insufficient, toughness index did not improve. In Comparative Example 5 of Test No. 15, the central solid fraction at the start of light reduction is 0.40, which is out of the scope of the present invention, the effect of light reduction is insufficient, the segregation ratio index of the central segregation part is high, and the toughness index is It did not improve.
試験番号１６の比較例６は溶鋼中Ｂｉ濃度が下限を外れ、試験番号１７の比較例７は溶鋼中にＢｉを添加せず、いずれも中心偏析部のＢｉ濃度が不十分となり、λ1／λ1 0の低減効果が不十分であり、靱性指数が改善しなかった。 In Comparative Example 6 of Test No. 16, the Bi concentration in the molten steel deviated from the lower limit, and in Comparative Example 7 of Test No. 17, Bi was not added to the molten steel, and in both cases, the Bi concentration in the center segregation part was insufficient, and λ 1 / the effect of reducing the λ 1 0 is insufficient, toughness index did not improve.
試験番号１８の比較例８は、溶鋼中Ｂｉ濃度が上限を外れ、靱性指数は良好であるが、若干コストが上昇する結果となった。 In Comparative Example 8 of Test No. 18, the Bi concentration in the molten steel deviated from the upper limit and the toughness index was good, but the cost increased slightly.
試験番号１９の参考例は、軽圧下を行わない場合であって溶鋼中Ｂｉ濃度を高めにすることによって靱性指数を良好としたもの（特許文献４の技術）であり、溶鋼中Ｂｉ濃度が本発明の上限を外れている。 The reference example of Test No. 19 is a case where light reduction is not performed, and the toughness index is improved by increasing the Bi concentration in the molten steel (the technique of Patent Document 4). It is outside the upper limit of the invention.
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| JPH0550201A (en) * | 1991-08-20 | 1993-03-02 | Nippon Steel Corp | Light rolling reduction method in continuous casting |
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| JPH0550201A (en) * | 1991-08-20 | 1993-03-02 | Nippon Steel Corp | Light rolling reduction method in continuous casting |
| JP2010158719A (en) * | 2008-12-10 | 2010-07-22 | Jfe Steel Corp | Method for manufacturing continuously cast slab |
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