JP4548231B2 - Steel continuous casting method and continuous cast slab - Google Patents

Steel continuous casting method and continuous cast slab Download PDF

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JP4548231B2
JP4548231B2 JP2005171102A JP2005171102A JP4548231B2 JP 4548231 B2 JP4548231 B2 JP 4548231B2 JP 2005171102 A JP2005171102 A JP 2005171102A JP 2005171102 A JP2005171102 A JP 2005171102A JP 4548231 B2 JP4548231 B2 JP 4548231B2
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章裕 山中
善久 白井
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Sumitomo Metal Industries Ltd
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本発明は、例えば橋梁や建築部材等の極厚鋼板用の鋼を連続鋳造する方法、及び、この連続鋳造方法を用いて製造される連続鋳造鋳片に関するものである。なお、本明細書において、極厚鋼板とは厚さが70mm以上、120mm以下の鋼板を言う。   The present invention relates to a method of continuously casting steel for an extremely thick steel plate such as a bridge or a building member, and a continuous cast slab manufactured using this continuous casting method. In the present specification, the extremely thick steel plate refers to a steel plate having a thickness of 70 mm or more and 120 mm or less.

例えば極厚鋼板の製造において、連続鋳造スラブ鋳片を圧延する場合、圧延比が大きくとれないために、鋳造欠陥であるポロシティが十分に圧着されずに残り、製品欠陥となる問題がある。
特開平2−156022号公報
For example, in the production of a very thick steel plate, when a continuous cast slab slab is rolled, since the rolling ratio cannot be increased, the porosity that is a casting defect remains uncompressed enough to cause a product defect.
Japanese Patent Laid-Open No. 2-156022

この問題を解決すべく、圧下比を大きくとるために、大断面鋳片の連続鋳造を想定した場合、連続鋳造機の長さ限界から低速で鋳造せざるを得ないので、能率が非常に悪くなる。また、連続鋳造しないで通常の造塊法で大径の鋳塊を鋳造することも考えられるが、連続鋳造法に較べ能率がさらに悪くなるのは言うまでもない。   In order to solve this problem, assuming continuous casting of large-section slabs in order to increase the reduction ratio, the efficiency is very poor because casting must be performed at a low speed due to the length limit of the continuous casting machine. Become. Further, it is conceivable to cast a large-diameter ingot by a normal ingot forming method without continuous casting, but it goes without saying that the efficiency is further deteriorated as compared with the continuous casting method.

そこで、発明者らは、特願2004−127054号において、上記問題を解決すべく、連続鋳造方法において鋳片を圧下する前に2mm〜20mmバルジングさせた後、鋳片の厚さ中心の固相率が0.80以上のときに、鋳片の幅中央部を3mm〜15mm圧下することを特徴とする連続鋳造方法を提案した。   Therefore, in order to solve the above-mentioned problem, the inventors of the present invention disclosed in Japanese Patent Application No. 2004-127054 that the slab is bulged by 2 mm to 20 mm before the slab is crushed in the continuous casting method, and then a solid phase centered on the thickness of the slab. A continuous casting method was proposed in which when the rate is 0.80 or more, the width central portion of the slab is reduced by 3 mm to 15 mm.

しかしながら、前記提案した連続鋳造方法では、その実施例に記載した程度の鋳造速度でも最終の鋼板における超音波探傷試験検査(以下、USTと称する。)結果に満足な結果が得られない場合があることが判明した。   However, in the proposed continuous casting method, a satisfactory result may not be obtained in the ultrasonic flaw detection test (hereinafter referred to as UST) result in the final steel plate even at the casting speed described in the examples. It has been found.

そして、その鋳片を詳細に調査した結果、鋳片の全幅方向でポロシティの一部に粗大な部分が存在する事が確認され、それがUSTの結果に反映されている事が判明した。   As a result of detailed investigation of the slab, it was confirmed that there was a coarse part of the porosity in the entire width direction of the slab, and this was reflected in the result of UST.

本発明が解決しようとする問題点は、従来の連続鋳造では、最終の鋼板におけるUST結果に満足な結果が得られない場合があるという点である。   The problem to be solved by the present invention is that conventional continuous casting may not provide satisfactory results for the UST results in the final steel sheet.

本発明の連続鋳造方法は、
鋳造条件によらないで、鋳片全幅に亘る厚み中心部のポロシティを低減し、これを圧延加工した極厚鋼板の内部品質を確実に保証するために、
鋳片をバルジングさせた後、未凝固部を含む鋳片を圧下する連続鋳造方法であって、
バルジング量を2mm〜20mmとし、
鋳片幅方向の平均中心固相率が0.85以上、1.0未満である凝末期の鋳片を、
少なくとも一対の圧下ロール対を用いて圧下する際に、
圧下直前に鋳片中心部が等軸晶で満たされて鋳片厚み中心部から上半面側の等軸晶厚み5mm以上の場合に、鋳片厚み方向の圧下率が2%以上となる条件で強圧下することを最も主要な特徴としている。
The continuous casting method of the present invention comprises:
To reduce the porosity at the center of the thickness over the entire width of the slab, regardless of the casting conditions, and to ensure the internal quality of the extra-thick steel sheet that has been rolled,
After bulging the slab, a continuous casting method of rolling down the slab including the unsolidified portion,
The bulging amount is 2mm ~ 20mm,
Slab width direction of the mean center solid phase ratio is 0.85 or more, a solid end coagulation Ru der less than 1.0 cast slab,
When rolling down using at least one pair of rolling rolls,
The condition that the reduction ratio in the slab thickness direction is 2% or more when the center part of the slab is filled with equiaxed crystal immediately before the reduction and the equiaxed crystal thickness from the center part of the slab to the upper half surface is 5 mm or more. The main feature is that it is overwhelmed.

また、本発明の鋼の連続鋳造鋳片は、
前記本発明の連続鋳造方法を用いて得られる鋳片であって、
鋳片の全幅方向において鋳片厚み中心部のポロシティ体積が2×10−4(cm/g)以下であることを最も主要な特徴としている。
Moreover, the continuous cast slab of the steel of the present invention is
A slab obtained using the continuous casting method of the present invention,
The most important feature is that the porosity volume at the center of the slab thickness is 2 × 10 −4 (cm 3 / g) or less in the entire width direction of the slab.

本発明によれば、ポロシティの著しく低減されたスラブ鋳片の製造が可能となるので、この鋳片を用いることにより低圧延比でも高品質の極厚鋼板を製造できるようになる。   According to the present invention, it is possible to manufacture a slab slab having a significantly reduced porosity. Therefore, by using this slab, it is possible to manufacture a high quality extra heavy steel plate even at a low rolling ratio.

以下、本発明を実施するための最良の形態について、発明成立に至るまでの過程と共に詳細に説明する。
発明者らの研究の結果、以下の3点についての知見が得られた。
Hereinafter, the best mode for carrying out the present invention will be described in detail together with the process up to the establishment of the invention.
As a result of the inventors' research, knowledge about the following three points was obtained.

(1)スラブ鋳片の凝固末期に圧下を加えてポロシティを低減しようとする場合、圧下する直前の鋳片中心部の組織形態によって、その低減の度合いが異なることが判明した。すなわち圧下する直前に鋳片中心部が等軸晶で満たされている場合に、ポロシティの低減効果が向上する事が判明した。 (1) When reducing the porosity by applying reduction at the end of solidification of the slab slab, it was found that the degree of reduction differs depending on the structure of the center of the slab immediately before reduction. That is, it has been found that the porosity reduction effect is improved when the center part of the slab is filled with equiaxed crystals immediately before the reduction.

(2)一方、湾曲型あるいは垂直曲げ型のような湾曲部を有し未凝固で水平状態に矯正して鋳造する通常のスラブの連続鋳造において、鋳造速度を増加していくと鋳片中心部の等軸晶がスラブの幅方向に必ずしも十分に充填されない場合がある事が判明した。この場合、鋳片中心部の等軸晶を十分に充填させるべく、この鋳片を凝固末期で強圧下することで、鋳片全幅方向のポロシティが向上した。 (2) On the other hand, in the continuous casting of a normal slab that has a curved part such as a curved type or a vertical bending type and is cast in an unsolidified and straightened state, when the casting speed is increased, the center part of the slab It has been found that the equiaxed crystal may not be sufficiently filled in the width direction of the slab. In this case, the porosity in the full width direction of the slab was improved by squeezing the slab at the end of solidification to sufficiently fill the equiaxed crystal at the center of the slab.

(3)鋳片中心部の内質は、後に述べる密度測定を応用したポロシティ体積で指標化でき、この指標と圧延加工された鋼板のUST成績とは良く合致する。従って、全幅でこの指標の限界値を規定した鋳片を供給することで、良好な鋼板を供給できる。 (3) The inner quality of the slab center can be indexed by the porosity volume to which density measurement described later is applied, and this index agrees well with the UST results of the rolled steel sheet. Therefore, a good steel plate can be supplied by supplying a slab that defines the limit value of this index over the entire width.

本発明は、以上の知見に基づいてなされたもので、
鋳片をバルジングさせた後、未凝固部を含む鋳片を圧下する連続鋳造方法であって、
バルジング量を2mm〜20mmとし、
鋳片幅方向の平均中心固相率が0.85以上、1.0未満である凝末期の鋳片を、
少なくとも一対の圧下ロール対を用いて圧下する際に、
圧下直前に鋳片中心部が等軸晶で満たされて鋳片厚み中心部から上半面側の等軸晶厚み5mm以上の場合に、鋳片厚み方向の圧下率が2%以上となる条件で強圧下する鋼の連続鋳造方法である。
The present invention has been made based on the above findings,
After bulging the slab, a continuous casting method of rolling down the slab including the unsolidified portion,
The bulging amount is 2mm ~ 20mm,
Slab width direction of the mean center solid phase ratio is 0.85 or more, a solid end coagulation Ru der less than 1.0 cast slab,
When rolling down using at least one pair of rolling rolls,
The condition that the reduction ratio in the slab thickness direction is 2% or more when the center part of the slab is filled with equiaxed crystals immediately before the reduction and the equiaxed crystal thickness from the center part of the slab to the upper half surface is 5 mm or more. This is a continuous casting method for steel that is strongly squeezed at.

本発明の鋼の連続鋳造方法において、鋳片を一旦2mm〜20mmバルジングさせて圧下する理由は、完全に凝固しておりかつ低温で変形抵抗の高い鋳片端部の圧下を回避し、鋳片の幅方向端部から中央部寄りの部分を強圧下するためである。   In the continuous casting method of the steel of the present invention, the reason why the slab is bulged once by 2 mm to 20 mm is to reduce the slab by avoiding the reduction of the slab end which is completely solidified and has high deformation resistance at a low temperature. This is because the part closer to the center part is strongly reduced from the end in the width direction.

また、バルジング量の下限を2mmとするのは、2mmは、鋳片の幅方向端部である短辺部の圧下を回避し、幅方向中央寄りの部分の圧下が進む(少なくとも圧下率が2%となる)最低の値だからである。また、バルジング量の上限を20mmとするのは、20mmを超えると圧下の進行効果が飽和するからである。   Also, the lower limit of the bulging amount is set to 2 mm. 2 mm avoids the reduction of the short side portion that is the end portion in the width direction of the slab, and the reduction of the portion near the center in the width direction proceeds (at least the reduction rate is 2). Because it is the lowest value. Moreover, the upper limit of the bulging amount is set to 20 mm because if it exceeds 20 mm, the progressive effect of reduction is saturated.

また、本発明において、鋳片厚み中心部から上半面側の等軸晶厚みを5mm以上とする理由は、前記のようなポロシティ低減効果の知見に基づくものである。   In the present invention, the reason why the equiaxed crystal thickness from the center portion of the slab thickness to the upper half surface side is set to 5 mm or more is based on the knowledge of the porosity reduction effect as described above.

一般に等軸晶は溶鋼に較べて比重が大きいので、鋳造中の未凝固溶鋼の中でゆっくりと沈降していく。そして、湾曲部を有する連続鋳造機の場合、鋳造速度を大きくすると水平部での凝固時間が長くなり、等軸晶は下面側に集積しやすくなる。その際、スラブ鋳造のように幅が大きな鋳片を鋳造した場合、幅方向の溶鋼内の等軸晶の浮遊数にバラツキがあるので、必ずしも幅方向で等軸晶の充填が均一でない。また、等軸晶の絶対量が少ない場合、鋳片の厚み中心部で等軸晶がすべて充填される訳ではない。   In general, equiaxed crystals have a higher specific gravity than molten steel, so they settle slowly in the unsolidified molten steel being cast. In the case of a continuous casting machine having a curved portion, if the casting speed is increased, the solidification time in the horizontal portion becomes longer, and the equiaxed crystals tend to accumulate on the lower surface side. At that time, when a slab having a large width is cast as in slab casting, the number of equiaxed crystals in the molten steel in the width direction varies, so that the equiaxed crystal filling is not necessarily uniform in the width direction. Further, when the absolute amount of equiaxed crystals is small, not all equiaxed crystals are filled at the center of the slab thickness.

従って、これらの鋳片を圧下しても鋳片の幅方向でポロシティの低減にバラツキが生じる。また、鋳片の厚み中心の等軸晶の量もある程度必要で、発明者らの研究によれば、その下限は鋳片厚み中心部から上側で少なくとも5mmは必要である事が判明した。なお、等軸晶を多く確保するためには低温鋳込み、電磁撹拌(以下、EMSと称する。)の適用等の手段が考えられる。   Therefore, even if these slabs are squeezed, variations occur in the reduction of porosity in the width direction of the slabs. In addition, the amount of equiaxed crystals at the center of the slab thickness is also required to some extent, and according to the study by the inventors, it has been found that the lower limit is at least 5 mm above the center of the slab thickness. In order to secure a large number of equiaxed crystals, means such as low-temperature casting and electromagnetic stirring (hereinafter referred to as EMS) can be considered.

次に、鋳片の凝固末期の鋳片幅方向の平均中心固相率が0.85以上、1.0未満の間に、少なくとも1対の圧下ロールを用いて、鋳片厚み方向に2%以上の圧下を加える理由について述べる。   Next, when the average center solid phase ratio in the slab width direction at the end of solidification of the slab is 0.85 or more and less than 1.0, at least one pair of reduction rolls is used and 2% in the slab thickness direction. The reason for applying the above reduction will be described.

鋳片内部に効率的に圧下を加えるためには鋳片中心と表面の温度差ができるだけ大きい方が望ましく、かつ、凝固末期のポロシティ形成時期(中心fs=0.6〜1)に圧下を加えるのが得策である。従って、内部が未凝固の状態で圧下を与えるのが有効である。   In order to efficiently reduce the inside of the slab, it is desirable that the temperature difference between the center of the slab and the surface is as large as possible, and the reduction is applied to the porosity formation time (center fs = 0.6 to 1) at the end of solidification. Is a good idea. Therefore, it is effective to apply the reduction while the inside is not solidified.

一方、圧下時の中心固相率が小さな場合は、C、P、S他の偏析し易い元素を多数含むような鋼種によっては、偏析による濃化溶鋼が圧下によって鋳造方向とは反対方向に逆流し、濃化溶鋼の塊まりとなって圧下ロール上流に滞留する。   On the other hand, when the central solid phase ratio at the time of rolling is small, depending on the type of steel containing many elements that easily segregate such as C, P, S, etc., the concentrated molten steel due to segregation flows backward in the direction opposite to the casting direction due to rolling. Then, it becomes a lump of concentrated molten steel and stays upstream of the reduction roll.

この濃化溶鋼の塊まりは、鋳造引き抜きによって圧下ロールまで再度到達する。この際、圧下ロールによって完全に上流側に排出されれば問題は生じないが、この塊まりの存在による凝固の不均一により、圧下時、鋳片の幅方向で内部の効き目にバラツキが生じる。   This lump of concentrated molten steel reaches the reduction roll again by casting. At this time, there is no problem if it is completely discharged to the upstream side by the rolling roll, but due to the non-uniformity of solidification due to the presence of this lump, the internal effect varies in the width direction of the slab during rolling.

すなわち、鋳片幅方向における未凝固の大きな部分に濃化溶鋼の塊まりが存在すると、上下凝固シェルによる圧搾が不十分となり、濃化溶鋼が完全に排出されずにそのまま鋳片内に残り偏析欠陥となる場合がある。   That is, if there is a lump of concentrated molten steel in a large unsolidified portion in the width direction of the slab, pressing by the upper and lower solidified shells becomes insufficient, and the concentrated molten steel remains in the slab as it is without being completely discharged. May be defective.

鋳片幅方向の凝固の不均一は、元来から鋳片表面のスプレー等による冷却の不均一や鋳型内の凝固シェルの接触状況の不均一、溶鋼内流動の不均一によって生じるとされているが、上述したように等軸晶の堆積についてもその均一性が必ずしも保証されている訳ではない。   The non-uniform solidification in the slab width direction is originally caused by non-uniform cooling by spraying the slab surface, non-uniform contact state of the solidified shell in the mold, and non-uniform flow in the molten steel. However, as described above, the uniformity of the equiaxed crystal deposition is not necessarily guaranteed.

従って、鋳片幅方向のある一点で仮に中心固相率が0.8であったとしても他の部分では凝固が遅れ、中心固相率が0.5程度であることもあり得る。
一般に中心固相率が0.8以上になると、溶鋼はデンドライト樹間に閉じ込められた状態で流動性はなくなるとされているが、これを圧下開始の基準ととると鋳片幅方向の不均一性を考えた場合、甚だ危険である。
Therefore, even if the central solid fraction is 0.8 at a certain point in the slab width direction, solidification is delayed in other portions, and the central solid fraction may be about 0.5.
Generally, when the central solid fraction is 0.8 or more, the molten steel is said to lose its fluidity in a state of being confined between dendrite trees. When considering sex, it is extremely dangerous.

発明者らが、本発明の範囲において種々試験を繰り返した結果、鋳片の幅方向端部の完全凝固部を除く未凝固部分の中心固相率の平均で0.85を基準とすれば、鋳片幅方向のすべての位置で濃化溶鋼の逆流によると考えられる偏析欠陥を防止できる事が判明した。   As a result of repeating various tests by the inventors within the scope of the present invention, if the average of the central solid phase ratio of the unsolidified portion excluding the completely solidified portion at the end in the width direction of the slab is 0.85 as a reference, It was found that segregation defects, which are thought to be caused by the backflow of concentrated molten steel, can be prevented at all positions in the slab width direction.

また、中心固相率を1未満とした理由は、未凝固で凝固潜熱が十分に残っており中心の温度が十分に高い状況が内部への圧下浸透上極めて有効だからである。当然ながら幅方向の凝固の不均一を考えると一部で凝固完了部もあると言えるが、後述するように圧下量を大きめに設定しておけば良い。   The reason why the central solid phase ratio is less than 1 is that a state in which the solidification latent heat is not sufficiently solid and the central temperature is sufficiently high and the center temperature is sufficiently high is extremely effective in reducing the penetration into the inside. Of course, considering the non-uniformity of the solidification in the width direction, it can be said that there is a solidification completion part in part, but the reduction amount may be set larger as described later.

中心固相率fsは、溶鋼の液相線温度TLと固相線温度Tsと鋳片の厚さ中心温度Tから、fs=(TL−T)/(TL−Ts)で求めることができる。
鋳片の厚さ中心温度Tが溶鋼の液相線温度TL以上の場合はfs=0であり、前記厚さ中心温度Tが溶鋼の固相線温度Tsより小さい場合はfs=1.0である。また、鋳片の厚さ中心温度Tは、鋳造速度、鋳片の表面冷却、鋳造鋼種の物性等を考慮した非定常伝熱解析計算によって求めることができる。
The central solid phase ratio fs can be obtained by fs = (TL−T) / (TL−Ts) from the liquidus temperature TL of the molten steel, the solidus temperature Ts, and the thickness center temperature T of the slab.
When the thickness center temperature T of the slab is equal to or higher than the liquidus temperature TL of the molten steel, fs = 0, and when the thickness center temperature T is lower than the solidus temperature Ts of the molten steel, fs = 1.0. is there. Further, the thickness center temperature T of the slab can be obtained by unsteady heat transfer analysis calculation considering the casting speed, the surface cooling of the slab, the physical properties of the cast steel type, and the like.

鋳片幅方向の中心固相率は、鋳片厚み方向と幅方向の2次元の非定常伝熱解析によって求められる。この解析の精度は、未凝固鋳片への打鋲試験や表面温度測定等で通常行われている方法で確認した。これらの試験と伝熱解析から、鋳片幅方向の平均中心固相率は、鋳片の幅中央と幅の1/4の両位置、両端より鋳造鋳片厚相当の距離+10cmの位置の合計5点の平均でほぼ全幅の平均に等しい事が判明した。   The center solid phase ratio in the slab width direction is obtained by two-dimensional unsteady heat transfer analysis in the slab thickness direction and width direction. The accuracy of this analysis was confirmed by a method usually performed in a hammering test or surface temperature measurement on an unsolidified slab. From these tests and heat transfer analysis, the average center solid phase ratio in the slab width direction is the sum of the center of the slab width and ¼ of the width, the distance corresponding to the cast slab thickness + 10 cm from both ends. It was found that the average of 5 points was almost equal to the average of the entire width.

次に圧下量についてであるが、鋳片の中心には前記中心固相率範囲においても、固液共存相が鋳片厚み方向にある程度の幅をもって存在している。この中にポロシティが形成される訳であるが、この厚み方向の固液共存相幅の大きさは鋳造する鋳片の厚みと相関があり、鋳片厚が厚くなれば、これに応じてその幅も大きくなる。   Next, regarding the amount of reduction, a solid-liquid coexisting phase exists at a certain width in the thickness direction of the slab even in the central solid phase ratio range at the center of the slab. Although the porosity is formed in this, the size of the solid-liquid coexistence phase width in the thickness direction has a correlation with the thickness of the cast slab to be cast. The width also increases.

また、前述のように不均一凝固によりその幅の大きさは鋳片幅方向で分布しており、鋳片幅方向すべての位置のポロシティを低減するためには、鋳片の厚みに応じて最低限の圧下率を設定しておく必要がある。ここで言う圧下率とは、{(圧下前の鋳片厚−圧下後の鋳片厚)/圧下後の鋳片厚}×100(%)である。   In addition, as described above, the width is distributed in the slab width direction due to non-uniform solidification, and in order to reduce the porosity at all positions in the slab width direction, the minimum value depends on the thickness of the slab. It is necessary to set a limit reduction ratio. The reduction ratio referred to here is {(cast slab thickness before reduction-slab thickness after reduction) / slab thickness after reduction} × 100 (%).

発明者らが種々試験を繰り返した結果、その最低の圧下率を2%とすれば良い事が判った。圧下率を求めるには、ロールの変形が無い場合は、例えば圧下前の鋳片厚として、圧下ロールより上流の上下ロール間隔、圧下後の鋳片厚として圧下ロールの上下ロール間隔としておけば良い。   As a result of repeating various tests by the inventors, it was found that the minimum rolling reduction should be 2%. In order to obtain the reduction ratio, when there is no deformation of the roll, for example, as the slab thickness before the reduction, the upper and lower roll intervals upstream of the reduction roll, and the upper and lower roll interval of the reduction roll as the slab thickness after reduction .

前記のように鋳造した鋳片を詳細に調べると、鋳片圧下開始時に凝固が完了していた両端部を除く鋳片全幅方向において、鋳片厚み中心部のポロシティ体積が2×10−4(cm/g)以下であり、この鋳片圧延加工した極厚鋼USTを行った場合には、未圧着のポロシティ欠陥が発見されなかった。これが本発明の鋼の連続鋳造鋳片である。 When the slab cast as described above is examined in detail, the porosity volume at the center of the slab thickness is 2 × 10 −4 (in the full width direction of the slab excluding both ends where solidification has been completed at the start of slab reduction. cm 3 / g) or less, and when this slab-rolled extra heavy steel UST was performed, no unbonded porosity defect was found. This is the continuous cast slab of steel of the present invention.

ここで、ポロシティ体積(以下、Pvとする。)は、同じ鋳片の1/4厚み部の代表サンプルの密度をρ0とし、中心部のサンプル密度をρとすると、Pv=(1/ρ)−(1/ρ0)(cm/g)として求まる。 Here, the porosity volume (hereinafter referred to as Pv) is Pv = (1 / ρ), where ρ0 is the density of the representative sample of the 1/4 thickness part of the same slab and ρ is the sample density of the center part. -(1 / ρ0) (cm 3 / g)

この場合、サンプルの大きさは30mm×30mm×30mm以内とするのが良い。サンプルをあまりに大きくすると、元来、ポロシティのほとんどない鋳片の厚み中心から外れた部分まで入ってしまい、中心部のポロシティの検出感度が鈍くなってしまうからである。   In this case, the size of the sample is preferably within 30 mm × 30 mm × 30 mm. This is because if the sample is made too large, it will enter the part of the slab where there is almost no porosity from the center of the thickness, and the sensitivity for detecting the porosity at the center will be reduced.

同じ鋳片の1/4厚み部の代表サンプルの密度ρ0を基準として選定した理由は、この部分は、ほとんどポロシティが検出されない部分(例えば顕微鏡での検査等により)であり、素材本来の密度に等しいとして置き換えることができるからである。   The reason for selecting the standard sample density ρ0 of the 1/4 thickness part of the same slab as the standard is that this part is a part where porosity is hardly detected (for example, by inspection with a microscope, etc.). This is because they can be replaced as equal.

なお、圧延後の極厚鋼板は、JIS G 0801−1993「圧力容器用鋼板の超音波探傷検査方法」に基づいて、未圧着のポロシティを評価した。UST装置は、Aスコープ表示式探傷器で、振動子直径30mm、公称周波数2MHzの垂直探傷子を用いた。   In addition, the extra-thickness steel plate after rolling evaluated the non-pressure bonding porosity based on JIS G 0801-1993 “Ultrasonic flaw detection inspection method of steel plate for pressure vessels”. The UST apparatus was an A scope display type flaw detector, and a vertical flaw detector having a transducer diameter of 30 mm and a nominal frequency of 2 MHz was used.

以下、本発明を検証するために行った試験について説明する。
図1は試験に用いた垂直曲げ型の連続鋳造機を示したものである。試験に用いた鋳型3は出側鋳片厚さが、310mm、230mm、幅は双方とも2300mmの大きさのものを使用した。試験に用いた鋼種は0.15質量%Cの40kg級鋼である。
Hereinafter, tests conducted for verifying the present invention will be described.
FIG. 1 shows a vertical bending type continuous casting machine used in the test. The mold 3 used in the test had a casting slab thickness of 310 mm, 230 mm, and a width of 2300 mm. The steel type used for the test is 40 kg grade steel with 0.15 mass% C.

鋳造速度と二次冷却強度を変更して圧下前のそれぞれの鋳片厚(バルジングを含む)における中心固相率を調整した。二次冷却水量は0.75〜0.85L/kg−steelとした。   The central solid fraction in each slab thickness (including bulging) before reduction was adjusted by changing the casting speed and the secondary cooling strength. The amount of secondary cooling water was 0.75 to 0.85 L / kg-steel.

タンディッシュ(図示せず)から浸漬ノズル1を経て鋳型3に注入された溶鋼4は、鋳型3及びその下方の二次冷却スプレーノズル群(図示せず)から噴射されるスプレー水によって冷却され、凝固シェル5が形成される。凝固シェル5の内部に未凝固部10を保持したまま、圧下ロール7で強圧下され未凝固部10が排除された鋳片8は、ガイドロール6群を経てピンチロール11により引き抜かれる。   The molten steel 4 injected into the mold 3 from the tundish (not shown) through the immersion nozzle 1 is cooled by spray water sprayed from the mold 3 and a group of secondary cooling spray nozzles (not shown) below it, A solidified shell 5 is formed. The slab 8 from which the unsolidified portion 10 is removed by being strongly squeezed by the rolling roll 7 while the unsolidified portion 10 is held inside the solidified shell 5 is pulled out by the pinch roll 11 through the group of guide rolls 6.

圧下ロール7は、鋳型3のメニスカス2より21m下方の位置に設置した。圧下ロール7の径は450mmで、圧下力は最大600tonとした。なお、試験に用いた連続鋳造機は垂直曲げ型連続鋳造機であるが、湾曲型連続鋳造機を使用しても良いことは言うまでもない。   The reduction roll 7 was installed at a position 21 m below the meniscus 2 of the mold 3. The diameter of the rolling roll 7 was 450 mm, and the rolling force was 600 tons at maximum. Although the continuous casting machine used for the test is a vertical bending type continuous casting machine, it goes without saying that a curved type continuous casting machine may be used.

ガイドロール6群は、鋳片の厚み方向の間隔を一定値に制御できるように配置されており、鋳片の内部に未凝固部10が存在するとき(B1〜B2の間)にバルジングさせ、鋳片の短辺長さより幅中央部の厚さを大きくして、その後の圧下ロール7により該幅中央〜端部を圧下する。   The guide rolls 6 group are arranged so that the interval in the thickness direction of the slab can be controlled to a constant value, and when the unsolidified portion 10 is present inside the slab (between B1 and B2), The thickness of the central portion of the width is made larger than the short side length of the slab, and the central portion to the end portion of the width is reduced by the subsequent reduction roll 7.

バルジング量は、鋳片の短辺長さ方向に配置された対を成すガイドロール6群の厚み方向の間隔を鋳型出側の鋳片の厚みより大きく調整することにより調節できる。先に述べた圧下率とはバルジングした鋳片と圧下後の鋳片の厚み、すなわち前述のようにそれぞれのロール間隔から求めた。   The amount of bulging can be adjusted by adjusting the distance in the thickness direction of the pair of guide rolls 6 arranged in the short side length direction of the slab to be larger than the thickness of the slab on the mold exit side. The above-mentioned rolling reduction was obtained from the thickness of the slab bulged and the slab after rolling, that is, from the respective roll intervals as described above.

圧下時の中心固相率は、主に鋳造速度と、鋳片の幅中央部の厚さ、すなわち鋳片バルジング量によって定まるため、鋳片バルジング量に合わせて、種々鋳造速度を変えて2元の伝熱計算を行い、前記の方法で幅方向の平均中心固相率fsを求めた。   The central solid fraction during rolling is mainly determined by the casting speed and the thickness of the center of the slab width, that is, the slab bulging amount. The average central solid fraction fs in the width direction was determined by the above method.

また、タンディッシュ内の溶鋼温度は、△T=40℃〜50℃の間でほぼ一定とした。なお、ΔTは溶鋼温度と液相線温度の差である。   Moreover, the molten steel temperature in the tundish was made substantially constant between ΔT = 40 ° C. and 50 ° C. ΔT is the difference between the molten steel temperature and the liquidus temperature.

さらに図1に示したように中心部の等軸晶を確保するために連続鋳造機内に電磁撹拌装置9を設置して、EMSを実施した。EMSの条件としては電流値900Aで1.5Hz、鋳片中心の磁束密度を400ガウス、30秒正転−5秒停止−30秒反転の交番撹拌とした。鋳片の等軸晶の厚みは下記の横断面サンプルを研磨、酸腐食により確認し厚み中心の上側を+、下側を−として数値標記した。   Further, as shown in FIG. 1, in order to ensure equiaxed crystals at the center, an electromagnetic stirring device 9 was installed in the continuous casting machine, and EMS was performed. The EMS conditions were as follows: alternating current stirring of 1.5 Hz at a current value of 900 A, a magnetic flux density at the center of the slab of 400 gauss, 30 seconds normal rotation, 5 seconds stopped, and 30 seconds reversed. The thickness of the equiaxed crystal of the slab was confirmed by polishing the following cross-sectional sample and acid corrosion, and numerically marking the upper side of the thickness center as + and the lower side as-.

得られた鋳片は、ポロシティと組織調査のために一部からサンプルを採取した後、950〜1170℃に加熱し、1050〜750℃の範囲で仕上げ圧延を行い極厚材を製造した。   The obtained slab was sampled from a portion for porosity and structure investigation, heated to 950 to 1170 ° C., and finish-rolled in the range of 1050 to 750 ° C. to produce a very thick material.

鋳型出側サイズで、310mmの厚鋳片からは100mm厚の、また、230mmの厚鋳片からは75mm厚の鋼板とした。使用した仕上げ圧延機のワークロール径は1040mm、最大圧下力は6300tonである。   The mold exit side size was a steel plate having a thickness of 100 mm from a 310 mm thick cast, and a steel plate having a thickness of 75 mm from a 230 mm thick cast. The finish rolling mill used has a work roll diameter of 1040 mm and a maximum rolling force of 6300 tons.

ポロシティ体積(Pv)調査用のサンプルは定常部の鋳片横断面ブロックより幅方向に均等に15点、中心部から採取した。サンプルの大きさは横断面に平行な面を30mm×30mmとして厚みを20mmとした。同様に基準密度(ρ)測定用のサンプルを同サイズで鋳片幅中央の1/4厚み位置から採取した。また、同時に鋳片横断面からマクロ板を採取し、研磨後、10質量%ピクリン酸水溶液で研磨面を腐食して排出濃化溶鋼の偏析部の有無の調査を行った。   The sample for examining the porosity volume (Pv) was collected from the center part at 15 points evenly in the width direction from the slab cross section block of the stationary part. As for the size of the sample, the plane parallel to the transverse section was 30 mm × 30 mm, and the thickness was 20 mm. Similarly, a sample for measuring the reference density (ρ) was taken from the 1/4 thickness position at the center of the slab width with the same size. At the same time, a macro plate was sampled from the cross section of the slab, and after polishing, the polished surface was corroded with a 10% by mass picric acid aqueous solution to investigate the presence or absence of segregation in the discharged concentrated molten steel.

密度はそれぞれの重量と体積から算出した。体積は水中にサンプルを浸漬し、水中での重量を測定することで浮力を求め水の密度とから算出した。これらより幅方向のPvを求めその最大値を記した。   The density was calculated from each weight and volume. The volume was calculated from the density of water by immersing the sample in water and measuring the weight in water to obtain buoyancy. From these, Pv in the width direction was determined and the maximum value was noted.

圧延後の極厚鋼板は、上述のUST装置を用いて検査した。測定された欠陥の個数、欠陥1個の最大指示長さ、密集度、占積率等が当該JISに規定された値以下の場合に、その極厚鋼板は合格とし、UST欠陥はないものと判断した。
下記表1に本発明の効果を確認するために実施した一連の試験条件を、下記表2にその結果を示す。
The ultra-thick steel plate after rolling was inspected using the above-mentioned UST apparatus. When the measured number of defects, maximum indicated length of one defect, density, space factor, etc. are below the values specified in the JIS, the heavy steel plate is accepted and there is no UST defect. It was judged.
Table 1 below shows a series of test conditions carried out to confirm the effect of the present invention, and Table 2 below shows the results.

実施例1〜7は本発明の要件をすべて満たしており、その鋳片の中央部のポロシティの状況は大変良好であり、中心部ポロシティ体積は鋳片の全幅位置で2×10-4cm 3 /g)以下となっていた。この鋳片を圧延して厚鋼板を製造し、UST検査を実施したところすべて合格した。 Examples 1 to 7 satisfy all the requirements of the present invention, the state of porosity at the center of the slab is very good, and the central porosity volume is 2 × 10 −4 ( cm 2) at the full width position of the slab. 3 / g ) or less. This slab was rolled to produce a thick steel plate, and when the UST inspection was conducted, all passed.

一方、詳細は後述するが、排出濃化溶鋼による偏析が残存し、圧延を中止した比較例7、8および13以外の比較例では、ポロシティの状況は悪くスラブ幅方向に2×10-4cm 3 /g)を超える部分が存在した。その鋳片を圧延し厚鋼板にしたところ、USTはすべて不合格であった。以下に、各比較例について詳細に説明する。 On the other hand, although details will be described later, in the comparative examples other than Comparative Examples 7, 8 and 13 in which the segregation due to the discharged concentrated molten steel remained and the rolling was stopped, the situation of porosity was bad and 2 × 10 −4 (in the slab width direction). There was a part exceeding cm 3 / g ). When the slab was rolled into a thick steel plate, all the USTs failed. Below, each comparative example is demonstrated in detail.

比較例1と比較例2は、それぞれ実施例1と実施例2からEMSのみOFFにして鋳造したものである。その結果、上面側充填等軸晶の厚みは4mmと2mmとなりポロシティが悪くなった。   Comparative Example 1 and Comparative Example 2 are cast from Example 1 and Example 2 with only EMS turned OFF, respectively. As a result, the thickness of the equiaxed crystal on the upper surface side was 4 mm and 2 mm, and the porosity was poor.

同じく比較例9と比較例10は、実施例5と実施例6からEMSのみOFFにして鋳造したものである。この場合も、やはり上面側充填等軸晶の厚みは−2mmと1mmと極めて小さくなり、その結果、ポロシティも悪化した。   Similarly, Comparative Example 9 and Comparative Example 10 were cast from Example 5 and Example 6 with only EMS turned OFF. Also in this case, the thickness of the equiaxed crystal on the upper surface side became extremely small, -2 mm and 1 mm, and as a result, the porosity deteriorated.

比較例3と比較例11は実施例2と実施例5からバルジング量のみを1mmと小さくしたものであるが、その分、圧下量そのものがそれぞれ1mm低下し、その結果、鋳片圧下率が2%以下に低下してポロシティが悪化した。   In Comparative Example 3 and Comparative Example 11, only the bulging amount was reduced to 1 mm from Example 2 and Example 5, but the reduction amount itself was reduced by 1 mm, and as a result, the slab reduction ratio was 2 %, The porosity deteriorated.

比較例4、比較例5及び比較例12は、それぞれ実施例1、実施例4、実施例7から圧下量のみを低下させて圧下率を2%以下に低減したものであるが、その結果、やはりポロシティは悪くなった。   Comparative Example 4, Comparative Example 5 and Comparative Example 12 are those in which only the amount of reduction was reduced from Example 1, Example 4 and Example 7, respectively, and the reduction rate was reduced to 2% or less. After all, porosity became worse.

比較例6と比較例14は、それぞれ実施例3と実施例5と鋳造速度を少々低下させて鋳造した。その結果、圧下ロールの時点で、鋳片は完全に凝固(fs=1.0)しておりポロシティの改善効果は小さくなった。   Comparative Example 6 and Comparative Example 14 were cast at a slightly lower casting speed than in Examples 3 and 5, respectively. As a result, at the time of the rolling roll, the slab was completely solidified (fs = 1.0), and the effect of improving the porosity was reduced.

比較例7、比較例8、及び比較例13は、それぞれ実施例1、実施例2、実施例7から鋳造速度を多少増加させた。その結果、圧下時の中心固相率は小さくなり0.85未満の値となった。その鋳片の横断面を調査したところ、多数の排出濃化溶鋼の偏析が見られたので、極厚鋼板への圧延は取り止めた。
以上より、本発明の効果は明らかである。
In Comparative Example 7, Comparative Example 8, and Comparative Example 13, the casting speed was slightly increased from Example 1, Example 2, and Example 7, respectively. As a result, the central solid fraction at the time of reduction was reduced to a value less than 0.85. When the cross section of the slab was examined, a large number of exhausted concentrated molten steel was segregated.
From the above, the effects of the present invention are clear.

本発明は上記の例に限らず、各請求項に記載された技術的思想の範囲内で、適宜実施の形態を変更しても良いことは言うまでもない。   The present invention is not limited to the above example, and it goes without saying that the embodiment may be appropriately changed within the scope of the technical idea described in each claim.

本発明は、実施例に示したような中炭素鋼鋳片のみならず低炭素鋼や高炭素鋼などの連続鋳造にも適用できる。   The present invention can be applied not only to medium carbon steel slabs as shown in the examples, but also to continuous casting of low carbon steel and high carbon steel.

試験に用いた垂直曲げ型の連続鋳造機の模式図である。It is a schematic diagram of the vertical bending type continuous casting machine used for the test.

符号の説明Explanation of symbols

1 浸漬ノズル
2 メニスカス
3 鋳型
4 溶鋼
5 凝固シェル
6 ガイドロール
7 圧下ロール
9 電磁撹拌装置
10 未凝固溶鋼
11 ピンチロール
B1〜B2 バルジングゾーン
DESCRIPTION OF SYMBOLS 1 Immersion nozzle 2 Meniscus 3 Mold 4 Molten steel 5 Solidified shell 6 Guide roll 7 Rolling-down roll 9 Electromagnetic stirrer 10 Unsolidified molten steel 11 Pinch roll B1-B2 Bulging zone

Claims (2)

鋳片をバルジングさせた後、未凝固部を含む鋳片を圧下する連続鋳造方法であって、
バルジング量を2mm〜20mmとし、
鋳片幅方向の平均中心固相率が0.85以上、1.0未満である凝末期の鋳片を、
少なくとも一対の圧下ロール対を用いて圧下する際に、
圧下直前に鋳片中心部が等軸晶で満たされて鋳片厚み中心部から上半面側の等軸晶厚み5mm以上の場合に、鋳片厚み方向の圧下率が2%以上となる条件で強圧下することを特徴とする鋼の連続鋳造方法。
After bulging the slab, a continuous casting method of rolling down the slab including the unsolidified portion,
The bulging amount is 2mm ~ 20mm,
Slab width direction of the mean center solid phase ratio is 0.85 or more, a solid end coagulation Ru der less than 1.0 cast slab,
When rolling down using at least one pair of rolling rolls,
The condition that the reduction ratio in the slab thickness direction is 2% or more when the center part of the slab is filled with equiaxed crystal immediately before the reduction and the equiaxed crystal thickness from the center part of the slab to the upper half surface is 5 mm or more. A continuous casting method of steel, characterized in that the steel is strongly squeezed at a low temperature.
請求項1に記載の方法を用いて得られる鋳片であって、
鋳片の全幅方向において鋳片厚み中心部のポロシティ体積が2×10-4(cm3/g)以下であることを特徴とする鋼の連続鋳造鋳片。
A slab obtained using the method according to claim 1,
A continuous cast slab of steel, wherein the porosity volume at the center of the slab thickness is 2 × 10 −4 (cm 3 / g) or less in the entire width direction of the slab.
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JP4508087B2 (en) * 2005-11-17 2010-07-21 住友金属工業株式会社 Continuous casting method and continuous cast slab
JP4609330B2 (en) * 2006-01-26 2011-01-12 住友金属工業株式会社 Continuous casting method of ultra-thick steel plates with excellent internal quality and slabs for ultra-thick steel plates
JP4997983B2 (en) * 2007-01-16 2012-08-15 住友金属工業株式会社 Continuous casting method for slabs for extra heavy steel plates
JP5073356B2 (en) * 2007-04-24 2012-11-14 株式会社神戸製鋼所 Continuous cast slab slab for hot rolling for thick plate products with final product thickness Df [mm] of 90 or more
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