JP2004223572A - Method for continuous casting and cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for continuous casting capable of manufacturing a bloom cast slab, which stably improves segregation and prevents occurrence of internal cracks, and the bloom cast slab. <P>SOLUTION: The method for continuous casting is so constituted that a non-solidified part of the cast slab, which has a value (the length of the longer side/the length of the short side) of 1-2.5 at the cross section of the cast slab, is reduced after buldging. The buldging is executed with the conditions in which a buldging amount δ and a distortion amount ε on the solidification interface during buldging satisfy the following formula (1) and formula (2). After that, a part, to which the buldging is executed, of the cast slab is reduced in the range below the buldging amount by using at least a pair of reduction rolls in the range in which the maximum reduction gradient in the casting direction is less than 1.40 mm/m. Here, when the C content of the cast slab is < 0.70 mass %, the formula (1) is δ≤20 mm and ε<2.00 %. When the C content of the cast slab is ≥ 0.70 mass %, the formula (2) is δ≤10 mm and ε<1.50 %. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

表１に、各試験の試験条件およびを試験結果を示す。
【００４０】
【表１】

【００４１】
表１に示す試験では、圧下セグメントとしては連続する２〜４セグメント、すなわち、メニスカスからの距離では約１７〜２６．６ｍ、圧下距離では２．４〜９．６ｍの区間において圧下するセグメントの組み合わせ条件を変えた。
【００４２】
鋼種としてＳ４５Ｃを用いた比較例の試験番号１〜４では、バルジング量が大きすぎたため、バルジング時に凝固シェルに内部割れが発生した。また、バルジング量に相当する圧下量を確保するために圧下量を増加した場合は、中心偏析を改善する以前にすでに圧下による内部割れが発生し、極めて劣った性状の鋳片となった。
【００４３】
これに対して、バルジング量は適正範囲にまで低減させたがバルジング後に圧下しなかった比較例の試験番号５では、凝固シェルの内部割れは発生しなかったが、中心偏析は改善されなかった。
また、バルジング時の凝固界面の歪が前記（１）式で表される量を超える試験番号７、および最大圧下勾配が１．４０ｍｍ／ｍ以上である試験番号８では、内部割れが発生し、劣った性状の鋳片となった。
【００４４】
これに対して、本発明例の試験番号６および９〜１１は、バルジング量、バルジング時の凝固界面の歪、バルジング後の最大圧下勾配および圧下量がともに本発明で規定する範囲内にあり、その結果、内部割れの発生が無く、中心偏析も改善されている。とくに、本発明例のうちで、バルジング後の圧下領域における鋳片中心部固相率の平均値が０．３〜０．６である試験番号９〜１１は、内部割れの発生がないことはもちろんのこと、中心偏析の改善効果についても一段と良好な結果が得られた。
【００４５】
鋳片のＣ含有量が増加すると割れ感受性は高くなる。そこで、Ｓ４５Ｃ材とＳ７０Ｃ材の場合とについて比較した。
【００４６】
図６は、Ｓ４５Ｃ材およびＳ７０Ｃ材についてバルジング量と歪との関係を示す図である。Ｃ含有量が０．７０質量％のＳ７０Ｃ材では、歪が１．５％未満となる総バルジング量１０ｍｍ以下では内部割れの発生は軽微であった。
【００４７】
そこで、総バルジング量を８ｍｍの条件で試験番号１２〜１５の試験を行った。その結果、表１に示されるとおり、圧下量がバルジング量を超えず、中心部固相率の平均値が０．６の本発明例である試験番号１２および１３では、内部割れは発生せず、また偏析も改善されて良好な品質の鋳片が得られた。
【００４８】
総バルジング量が８ｍｍであっても、圧下量が総バルジング量を超える８．４２ｍｍであり、最大圧下勾配が１．４０ｍｍ／ｍである比較例の試験番号１４では、偏析成分の濃化した溶鋼がデンドライト樹枝間に封じ込まれて搾り出し効果が現れず、中心偏析の改善が見られなかった。また、最大圧下勾配が大きいため、内部割れが発生した。圧下勾配の好ましい範囲は、１．２ｍｍ／ｍ以下である。
【００４９】
バルジング後に圧下しない比較例の試験番号１５では、Ｓ４５Ｃの比較例である試験番号５の場合と同様に、内部割れは発生しないものの、中心偏析の改善は認められなかった。
【００５０】
総バルジング量が１１ｍｍでバルジング時の歪が大きい比較例の試験番号１６では、バルジング時に内部割れが発生し、性状の劣る鋳片となった。
【００５１】
Ｓ４５Ｃ材の場合は、図６および表１の結果に見られるとおり、バルジング量が２２．５ｍｍにバルジング時の内部割れが発生しており、バルジング量が１６．５ｍｍでは、内部割れは発生せずに良好であった。また、バルジング量が１６．５ｍｍ〜２０ｍｍの間では、割れは軽微であった。そこで、バルジング量の適正範囲を１０ｍｍ以下、かつ、バルジング時の凝固界面における歪の適正範囲を２％未満とした。
【００５２】
Ｓ４５Ｃ材とＳ７０Ｃ材とで、適正なバルジング圧下条件は相違するが、適正条件を選択して制御することにより、鋳片中心部の未凝固部における濃化溶鋼が効率よく搾り出される。
【００５３】
図７は、本発明法による偏析の改善状況を示す図である。炭素含有量の比Ｃ／Ｃ_０が１．１０以下となり、鋳片内部品質および鋳片形状が改善された良好な鋳片が得られた。
【００５４】
以上の結果から、材質により、バルジング量およびバルジング時の凝固界面歪量のそれぞれの適正値を区分し、前記（１）式および（２）式を得た。
【００５５】
また、圧下量はバルジング量以下の範囲が適正であり、圧下勾配は１．４ｍｍ／ｍ以下が適正範囲である。鋳造方向の圧下領域における中心部固相率の平均値は０．３〜０．６が好ましい。
（実施例２）
鋳片幅が４２０ｍｍおよび５００ｍｍのＳ４５Ｃ材を用いた場合について、バルジング開始位置とバルジング量の関係について説明する。
【００５６】
図８は、鋳片サイズおよび鋳造速度が異なる場合についてのバルジング開始位置とバブジング量との関係を示す図であり、前記の各種試験結果結果に基いて計算により求められたものである。なお、バルジング開始位置はメニスカスからの距離で表し、バルジング開始位置とは、ロールキャビティのを開放を開始する位置をいう。
【００５７】
図８で示すＣＡＳＥ１は、鋳片幅が４２０ｍｍの鋳片を鋳造速度が０．８ｍ／ｍｉｎの条件で鋳造し、メニスカスからの距離が６ｍの位置で上側サポートロールのロールキャビティを全開放とした場合である。同図によれば、バルジング量は約６ｍｍである。
同ＣＡＳＥ２は、鋳片幅が５００ｍｍの鋳片を鋳造速度０．８ｍ／ｍｉｎで鋳造し、メニスカスからの距離が６ｍの位置で同様にロールキャビティを全開放とした場合であり、バルジング量は約４５ｍｍに増大する。
【００５８】
同ＣＡＳＥ３のように、鋳造速度を０．５５ｍ／ｍｉｎに低下させると、バルジング量は約１８ｍｍに低減し、前記の実施例１における結果と同程度の値とすることができる。しかし、この場合は、鋳造速度が変化しているため、バルジング後の圧下領域を変更する必要がある。また、鋳造速度を過度に低下させると、メニスカスでの湯面皮張りなどが発生して鋳造が不可能となることから、一定の速度以上の鋳造速度に調整する必要がある。
【００５９】
従って、鋳片幅が５００ｍｍの鋳片を鋳造速度０．８ｍ／ｍｉｎのままで鋳造する場合は、同図中のＣＡＳＥ２の曲線において、ハッチングを施した領域においてバルジング量が２０ｍｍ以上となるため、少なくともこの区間においては、サポートロールのロールキャビティを２０ｍｍ未満に設定してバルジング量を制御する必要がある。ＣＡＳＥ２の場合、少なくともメニスカスから６〜１０ｍの区間については、ロールキャビティを２０ｍｍ未満とする制御が必要となる。
【００６０】
図１０は、バルジング領域の一部の区間においてロールキャビティ制御により鋳片をサポートし、バルジング量を制御する状況を模式的に示している。前記のＣＡＳＥ２の場合は、同図に示されるように、前半の一部の区間で鋳片をサポートするロールキャビティ制御を行い、それ以降の区間では前記表１に記載されたＳ４５Ｃ材の適正バルジングおよび圧下条件にて鋳造すればよい。
【００６１】
このように、ブルームの連続鋳造においては、ロールキャビティを全開放としたままバルジング量を制御できる場合と、鋳片をサポートしてバルジング量の過度な増加を抑制すべく制御する必要のある場合とがある。例えば鋳片断面の長辺長さが長くなるなど、ある程度ブルームサイズが大きくなると、上下サポートロールのロールキャビティ制御を考慮する必要がある。本発明者らの行った試験結果に基づけば、鋳片幅５００ｍｍ以上では積極的なロールキャビティ制御を行うことが好ましい。連続鋳造装置の仕様、鋳造するブルームのサイズがわかれば、鋳造速度と必要なバルジング量に応じて上下ロールキャビティ量の制御の要否は決定することができる。
【００６２】
鋳片幅が５００ｍｍ未満の鋳片のバルジング量の制御においては、バルジング開始位置に加えて、溶鋼温度、鋳片の二次冷却条件および鋳造速度の制御を併用することが好ましい。
【００６３】
なお、前記の図１に示されるように、バルジング領域全域についてロールキャビティを制御できる機能を有する設備であれば好ましいことは言うまでも無い。
【００６４】
【発明の効果】
本発明の方法によれば、バルジングさせにくいブルーム鋳片などの未凝固部をバルジングさせ、凝固完了前に前記バルジング量以下の範囲で圧下する鋳造方法において、安定した偏析改善効果を得るとともに内部割れの発生を防止し、健全な鋳片を製造できる。また、本発明の方法により製造されたブルーム鋳片は、偏析および内部割れのない極めて良好な品質を有する。このように、本発明は、ブルーム鋳造分野の発展に大きく寄与できる価値ある発明である。
【図面の簡単な説明】
【図１】特許文献３および４に開示された異径ロールによる圧下方法を模式的に示す図であり、（ａ）は圧下前の状態を、（ｂ）は圧下の状態を、そして（ｃ）は圧下後の状態をそれぞれ表す。
【図２】本発明のバルジングさせた後に圧下する方法を模式的に示す図であり、（ａ）は圧下前の状態を、（ｂ）は圧下の状態を、そして（ｃ）は圧下後の状態をそれぞれ表す。
【図３】バルジングさせた後に圧下する従来の一般的方法を模式的に示す図である。
【図４】メニスカスからの距離とバルジング量との関係を示す図である。
【図５】バルジング量と歪との関係を導出するための概念を示す図であり、同図（ａ）は未凝固溶鋼を含む鋳片の横断面を、同図（ｂ）は同図（ａ）のコの字形状骨組によるモデル化を、同図（ｃ）はバルジング後の鋳片の横断面を、同図（ｄ）は同図（ｃ）のコの字形状骨組によるモデル化をそれぞれ表す。
【図６】バルジング量と歪との関係を示す図である。
【図７】本発明法による偏析の改善状況を示す図である。
【図８】鋳片サイズなどが異なる場合のバルジング開始位置とバルジング量との関係を示す図である。
【図９】ロールキャビティを全開放としたときのバルジング制御の状況を模式的に示す図である。
【図１０】バルジングの一部をロールキャビティ制御により行うときの状況を模式的に示す図である。
【符号の説明】
１：浸漬ノズル、
２：鋳型内の湯面（メニスカス）、
３：鋳型、
４：溶鋼、
５：凝固シェル、
６：サポートロール、
７、７１：圧下ロール、
８、８１：圧下前の鋳片、
９、９１：圧下後の鋳片、
１０、１０１：未凝固溶鋼、[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for reducing segregation occurring in the center of a slab in continuous casting and preventing internal cracks in the slab, and more particularly, to a continuous casting method in which bulging is performed and then rolling down, and a method for manufacturing the same. Slab.
[0002]
[Prior art]
In order to improve the center segregation of steel, a method of rolling down the slab during continuous casting has been implemented.In this method, when parallel rolls are used, the reduction of the unsolidified portion in the target slab width direction is performed. Therefore, the solidified portions at both ends in the slab width direction, which originally do not need to be reduced, are also reduced, so that there is a problem that an unnecessarily large reduction force is required.
[0003]
On the other hand, there is a method in which the slab is reduced by rolls having different diameters in the axial direction of the roll in order to selectively reduce only the unsolidified portion in the width direction. However, in this method, a concave portion or an indentation remains at the center in the slab width direction.
[0004]
On the other hand, in a casting method in which the bulging is performed after the bulging is performed (hereinafter, referred to as “bulging reduction”), a ratio of a cross section (long side length / short side length) like a slab (hereinafter, referred to as “aspect ratio”) When the aspect ratio of the cross section is small, such as bloom, there is a problem that bulging is difficult. Therefore, it is necessary to determine an appropriate range of the aspect ratio of the cross section.
The above-mentioned center segregation is a phenomenon in which solute components such as C, S, P and Mn are concentrated at the center in the thickness direction of the slab, which is the final solidified portion of the slab, to cause positive segregation. This is a serious problem particularly in cord wire products and the like, because it causes a positive drop and causes a product defect such as a break in wire drawing. The following technology is disclosed as a method for preventing this center segregation.
[0005]
Patent Document 1 discloses that a bulging force is applied to a solidified shell between a mold and a liquidus crater end of a slab to increase the thickness of an unsolidified layer. A continuous casting method is disclosed in which a slab between a solidus crater end and a slab is reduced to reduce the occurrence of center segregation. However, in the method disclosed here, the reduction range is too wide, and the effect of reducing segregation is not stable.
Patent Document 2 discloses that in continuous casting of a slab having an aspect ratio of 1.6 or less, the interval between a plurality of pairs of rolls is widened from the inner thickness of the lower end of the mold, and bulging is performed in the slab thickness direction. Discloses a method for producing a high quality cast slab in which the slab is reduced by 0.04 to 10% with another pair of rolls. In this method, since the range of the reduction gradient is wide, cracks may be generated inside the slab depending on the conditions of the reduction gradient.
Patent Literatures 3 and 4 disclose a method for preventing a reduction in both ends in a slab width direction having a large deformation resistance and efficiently applying a rolling force to an unsolidified portion. A continuous casting method is disclosed in which a solidified portion is locally reduced by a stepped different-diameter roll provided with a protruding portion at the center of a large-diameter roll called a camel roll.
[0006]
FIGS. 1A and 1B are diagrams schematically showing a method of rolling down using rolls of different diameters disclosed in Patent Documents 3 and 4, wherein FIG. 1A shows a state before rolling down, FIG. 1B shows a state before rolling down, and FIG. ) Indicates the state after the reduction.
The slab 81 before rolling including the unsolidified molten steel 101 is locally reduced at the center in the slab width direction by the rolls 71 of different diameters, and becomes the slab 91 after rolling. In the methods disclosed in these publications, since the roll is locally reduced by the stepped roll, a concave portion is formed on the surface of the slab, which remains even after the subsequent rolling step, and causes dimensional defects and poor flatness.
In Patent Document 5, bulging is caused at a position where the solid phase ratio at the center of the slab is 0.1 or less, the maximum thickness of the slab is made 20 to 100 mm thicker than the short side length of the slab, and solidification is performed. A continuous casting method is disclosed in which a reduction of at least 20 mm per pair is given by at least one pair of reduction rolls immediately before the completion point to reduce the bulging amount. However, even in this method, in the case of a bloom having a small aspect ratio, if the relationship between the amount of bulging and the amount of reduction is not appropriate, a slight crack may occur at the solidification interface or the effect of improving center segregation may be reduced. .
[0007]
As described above, in casting a slab having a small aspect ratio, reduction in center segregation can be achieved by using either a method of rolling down by a roll of different diameter or a method of rolling down after bulging a slab including an unsolidified portion. Problems still remain in terms of securing flatness.
[Patent Document 1]
JP-A-60-6254 (Claims)
[Patent Document 2]
JP-A-60-21150 (claims, page 2, lower right column, lines 6-17)
[Patent Document 3]
JP-A-61-132247 (claims, page 14, upper right column, lines 14 to 19)
[Patent Document 4]
JP-A-60-184455 (Claims)
[Patent Document 5]
JP-A-9-57410 (Claims)
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a method of bulging an unsolidified portion such as a bloom slab which is difficult to bulge and reducing the bulging within a range corresponding to the bulging amount before completion of solidification, to obtain a stable segregation improvement effect and to prevent internal cracking. An object of the present invention is to provide a continuous casting method for producing sound slabs while preventing occurrence of cracks.
[0009]
[Means for Solving the Problems]
The present inventors have studied the appropriate bulging reduction method for bloom cast slabs and the like based on the conventional problems described above in order to solve the above-described problems, and found that the unsolidified portion was bulged. After rolling, it is appropriate to use a flat roll to reduce the roll back to the rectangular cross-sectional shape before bulging. Also, by selecting a support roll at the appropriate position in the casting direction to start bulging, it is possible to control the appropriate amount of reduction It was found that
[0010]
The present invention has been completed based on the above findings, and the gist of the present invention resides in a continuous casting method shown in the following (1) to (5) and a slab shown in (6).
[0011]
(1) A continuous casting method in which the unsolidified portion of a slab having a ratio of the long side length of the cross section divided by the short side length of 1 to 2.5 is bulged and then reduced in pressure, and the bulging amount is The bulging is performed under the condition that δ and the strain ε at the solidification interface at the time of bulging satisfy the following expressions (1) and (2), and then the bulging portion of the slab is subjected to the maximum reduction in the casting direction. A continuous casting method in which a gradient is less than 1.40 mm / m and a reduction is performed using at least one pair of reduction rolls in a range of a bulging amount or less.
When the C content of the slab is less than 0.70% by mass, δ ≦ 20 mm and ε <2.00% (1)
When the C content of the slab is 0.70% by mass or more, δ ≦ 10 mm and ε <1.50% (2)
(2) In the continuous casting method according to the above (1), after bulging the unsolidified portion, the region where the average value of the solid portion ratio at the center of the reduction section is 0.3 to 0.6 may be reduced. preferable.
[0012]
(3) In the continuous casting method according to the above (1) or (2), the bulging amount of a slab having a maximum length of a long side or a short side of a cross section of less than 500 mm is controlled by the following (1) and (2). It is preferable to carry out by adjusting either or both of 2).
[0013]
(1) Molten steel temperature, slab secondary cooling conditions and casting speed
(2) Bulging start position
(4) In the continuous casting method according to the above (1) or (2), the control of the bulging amount of the slab having the maximum length of the long side or the short side of the cross section of 500 mm or more is performed by setting the bulging amount equal to or more than the target bulging amount. It is preferable to adjust the roll cavity of the support roll corresponding to the region in the casting direction.
(5) In the continuous casting method according to any one of the above (1) to (4), it is preferable that the unsolidified portion of the slab is electromagnetically stirred before the slab is reduced.
[0014]
(6) A slab cast by the continuous casting method according to any one of the above (1) to (5), wherein a carbon content C (mass%) at a central portion in a thickness direction of the slab is determined by a central portion. Content C of base metal excluding carbon ₀ (% By mass) C / C ₀ Having a value of 1.10 or less.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors have conducted repeated studies to extend and apply the bulging reduction technology developed for the purpose of improving the segregation of the center of the slab to the continuous casting of bloom, and as a result, the amount of bagging, which is an issue in continuous casting of bloom, has been studied. Control and proper reduction conditions were found. In addition, regarding the bulging control of a bloom slab having a small aspect ratio of the cross section, by selecting a support roll at an appropriate position in the casting direction to start bulging, it becomes possible to control an appropriate bulging amount, thereby reducing equipment costs. It has been found that the bulging pressure of the unsolidified slab can be efficiently reduced.
[0016]
When the unsolidified portion of the bloom slab is rolled down, as described above, if the slab is rolled down with rolls of different diameters, the required rolling force can be reduced, but a slab having an accurate rectangular cross-sectional shape cannot be obtained. Without sacrificing the cross-sectional shape of the slab to be cast, as a result of examining a method that can secure a good rectangular shape and also reduce the rolling force, bulging the unsolidified part of the slab, and then using a flat roll It has been found that a method of reducing the pressure to return to a rectangular cross-sectional shape before bulging is appropriate.
[0017]
2A and 2B are diagrams schematically illustrating a bulging reduction method of the present invention, wherein FIG. 2A illustrates a state before reduction, FIG. 2B illustrates a state after reduction, and FIG. 2C illustrates a state after reduction. . The cast slab 8 before rolling including the unsolidified molten steel 10 is reduced by the flat roll 7 to become the cast slab 9 after rolling. According to the method shown in FIG. 2, a recess is not formed on the slab surface as in the case of rolling down by a roll having a different diameter, and good flatness can be ensured.
[0018]
In the blooming slab bulging reduction technology, it is important to grasp the optimum conditions of the bulging amount and the reduction amount and to secure a sufficient bulging amount. In casting a slab having a large cross-sectional aspect ratio, the deformation of the long side surface due to bulging is large. Therefore, control of the roll cavity is indispensable for controlling the bulging amount. On the other hand, in the casting of a bloom slab having a relatively small aspect ratio, the bulging amount is small. As a countermeasure, the casting conditions are controlled so that the thickness of the solidified shell is reduced, and the solidified shell is easily deformed by the static pressure of the molten steel, so that a sufficient bulging amount can be secured.
[0019]
FIG. 3 is a view schematically showing a bulging reduction method in continuous bloom casting using a conventional general vertical bending type continuous casting apparatus.
[0020]
The molten steel 4 injected into the mold 3 through the immersion nozzle 1 forms a solidified shell 5 from the surface below the meniscus 2 that contacts the mold. The slab that has formed the solidified shell is pulled out below the mold, and bulges while being supported by the support roll 6 whose roll cavity is enlarged and adjusted in the bulging region. Further, the slab 8 including the unsolidified molten steel 10 is reduced in the rolling region. In the process, the slab 9 is formed by being reduced by the reduction roll 7.
Based on the conventional casting method shown in the figure, we obtained a stable segregation improvement effect and prevented the occurrence of internal cracks, and studied an appropriate bloom bulging reduction method to produce sound slabs. (A) to (e) were obtained.
[0021]
(A) Since the thickness of the solidified shell changes depending on the casting speed, the bulging amount determined by the balance between the solidified shell thickness and the static pressure of molten steel below the meniscus changes. For bloom cast slabs with a small aspect ratio, the bulging amount selects and determines the roll segment corresponding to the bulging start position in the casting direction, and opens the roll cavity or adjusts the roll cavity within a range that supports the slab. By doing so, it can be controlled.
[0022]
(B) In (a) above, an appropriate amount of bulging that can prevent the occurrence of internal cracks due to bulging has been found.
[0023]
(C) By reducing the bulging area of the slab with an appropriate reduction gradient, the squeezing of the component-enriched molten steel present in the unsolidified portion at the center of the slab at the end of solidification proceeds sufficiently, and the center segregation is significantly improved. Is done.
(D) By controlling the amount of reduction so that the sectional shape of the slab returns to the rectangular shape before bulging, the sectional shape of the slab becomes a rectangular shape with high accuracy.
(E) No rolling force is required for rolling down the widthwise end of the slab, and equipment costs can be reduced.
[0024]
【Example】
In order to confirm the effects of the present invention, the following continuous casting test was performed.
(Example 1)
〔Test method〕
A slab having a thickness of 310 mm and a width of 420 mm was cast by a vertical bending-type bloom continuous casting apparatus, and a test was performed in which the slab was reduced while controlling the bulging amount. The material of the bloom is S45C (in mass%, C: 0.45%, Si: 0.20%, Mn: 0.74%, P: 0.025% and S: 0.008%), and A high carbon steel of S70C (C: 0.70%, Si: 0.20%, Mn: 0.45%, P: 0.010% and S: 0.008%) was used.
[0025]
Casting tests were carried out, mainly changing the casting speed and the position of the roll segment at which the opening of the roll cavity starts (ie, the distance from the meniscus in the mold), and the following results were obtained. In addition, the bulging amount and the amount of reduction under each test condition are calculated from the roll cavity, and the thickness of the slab is obtained by collecting both samples of the as-bulged slab and the slab after bulging and reducing the slab. The difference was calculated, and the calculation was performed by any of the methods of correcting the coagulation shrinkage.
[0026]
The bulging amount is determined by the deformation amount of the solidified shell determined by the static pressure of the molten steel and the deformation resistance of the solidified shell. The present inventors arranged the actual bulging amount in the case of steels having a C content of 0.45% by mass and 0.70% by mass as a relationship with the distance from the meniscus.
[0027]
FIG. 4 is a diagram showing the relationship between the distance from the meniscus and the bulging amount when the S45C material is used. According to the figure, since the solidification progresses as the distance from the meniscus increases, the roll cavity is opened (the roll cavity is enlarged so that the roll does not abut the slab) to start the bulging. The further the position (hereinafter also referred to as "bulging start segment") from the meniscus, the greater the deformation resistance of the solidified shell and therefore the less bulging. In addition, as the casting speed increases, the progress of solidification at a position at the same distance from the meniscus is delayed, so that the bulging amount increases.
[0028]
For example, when bulging is started by opening the roll cavity at a distance of 3.3 m from the meniscus when the casting speed is 0.9 m / min, the bulging amount is about 52 mm, and the same casting speed is used. When the bulging start position is shifted to a position at a distance of 8.6 m from the meniscus further rearward at 0.9 m / min, the bulging amount becomes about 14 mm. When bulging is started at a position at a distance of 8.6 m when the casting speed is 0.75 m / min, the bulging amount is about 8 mm.
[0029]
Therefore, in order to obtain the necessary bulging amount, first, the casting speed is selected, the bulging start segment corresponding to the distance from the meniscus is determined from the figure, and the roll cavity of the roll pair from that segment to the start of the reduction is determined. It can be seen that it should be open.
[0030]
FIG. 9 is a diagram schematically illustrating a bulging situation when the roll cavity is fully opened in the bulging region. As in the case of FIG. 3, the molten steel 4 injected into the mold 3 through the immersion nozzle 1 forms a solidified shell 5 from a contact surface of the meniscus 2 below the mold with the mold. The cast slab having the solidified shell is pulled out below the mold, bulged by a bulging force while passing through a bulging region in which the roll cavity of the support roll 6 is fully open, and further cast with unsolidified molten steel 10. The piece 8 is reduced by a reduction roll 7 in a reduction region to form a cast piece 9.
In order to form an equiaxed crystal and disperse the center segregation, it is preferable to electromagnetically stir the molten metal in the unsolidified portion of the slab before rolling.
[0031]
(Strain of solidification interface, and method of calculating the average value of the central solid fraction)
The distortion of the solidification interface during bulging was calculated by applying a U-shaped deformation model, assuming that the deformation of the slab due to bulging was a deformation of the “U-shaped” frame structure.
[0032]
FIG. 5 is a diagram showing a concept for deriving the relationship between the bulging amount and the strain. FIG. 5A shows a cross section of a slab containing unsolidified molten steel, and FIG. (a) Modeling with a U-shaped skeleton of the left half, FIG. (c) shows the cross section of the slab after bulging, and FIG. (d) shows the model of the left half of the figure (c). Modeling with a character-shaped skeleton is shown.
[0033]
As shown in FIG. 2B, the deformation due to bulging is represented by the deformation when a load W is applied to the other end A of the U-shaped frame structure having one end D as a fixed fulcrum and the other end A as a movement fulcrum. And calculate the distortion. The hatched portions in FIG. 2B represent the distribution of the bending moment generated in the frame structure. The bulging of the slab shown in FIG. (C) is modeled as a deformation of the skeleton structure as shown in FIG. (D).
[0034]
The solidification interface on the short side of the slab section is located at (1/2) d from the neutral surface of the solidified shell to the inside of the slab, where d is the thickness of the solidified shell on the short side. Therefore, the strain at the solidification interface on the short side of the slab section was calculated as the strain on the inside (right side in the figure) surface of the beam BC in FIG.
ε = d / (2 · r + d) × 100 (a)
r = [3 · δ / {Lw (2 · Lw + 3 · Lt)}] ^-1 ... (b)
Lw = Lwo-d (C)
Lt = Lto−2 · d (d)
here,
ε: strain (%) at the solidification interface of the slab short-side solidification shell during bulging;
δ: bagging amount (mm)
d: thickness of solidified shell at the time of bulging (mm),
Lwo: slab width (mm) before bulging,
Lto: slab thickness (mm) before bulging,
r: radius of curvature (mm) of the outer surface of the slab short-side solidified shell.
[0035]
The average value of the solid phase ratio at the center of the slab is calculated, for example, by calculating the temperature distribution and the solid phase formation ratio inside the slab by heat transfer calculation taking solidification into consideration. By averaging over
[0036]
[Evaluation method of slab internal quality]
A macro plate sample having a thickness of about 15 mm was sampled from a plane perpendicular to the casting direction of the cast bloom slab and subjected to sulfur printing.
[0037]
The state of center segregation was evaluated based on the results of analysis of Mn content by sulfur print and mapping analyzer, and the results of analysis of C content by chemical analysis of chips collected in the thickness direction. In the analysis by the mapping analyzer, a visual field of 40 mm × 120 mm was analyzed with a beam diameter of 100 μm.
[0038]
Internal cracks were evaluated based on the results of sulfur print observation.
The comprehensive evaluation was made by combining the results of the segregation investigation and the results of the internal crack investigation.
[0039]
The evaluation criteria are as follows (indicated by symbols in Table 1).

Table 1 shows test conditions and test results of each test.
[0040]
[Table 1]

[0041]
In the test shown in Table 1, a combination of two or four continuous segments as a rolling segment, that is, a combination of segments that are reduced in a section of about 17 to 26.6 m from the meniscus and 2.4 to 9.6 m in a rolling distance. The conditions were changed.
[0042]
In Test Nos. 1 to 4 of Comparative Examples using S45C as the steel type, the bulging amount was too large, so that an internal crack occurred in the solidified shell during bulging. Further, when the rolling reduction was increased in order to secure the rolling reduction corresponding to the bulging amount, internal cracks were already generated by the rolling before the center segregation was improved, resulting in an extremely inferior slab.
[0043]
On the other hand, in Test No. 5 of the comparative example in which the bulging amount was reduced to an appropriate range but the bulging was not reduced after bulging, no internal crack of the solidified shell occurred, but the center segregation was not improved.
In Test No. 7 in which the strain at the solidification interface during bulging exceeds the amount represented by the above formula (1), and in Test No. 8 in which the maximum rolling gradient is 1.40 mm / m or more, internal cracks occur. A slab of inferior properties was obtained.
[0044]
On the other hand, the test numbers 6 and 9 to 11 of the examples of the present invention are such that the bulging amount, the strain of the solidification interface during bulging, the maximum reduction gradient and the reduction amount after bulging are both within the range specified by the present invention, As a result, there is no occurrence of internal cracks, and the center segregation is also improved. In particular, among the examples of the present invention, Test Nos. 9 to 11 in which the average value of the slab central portion solid phase ratio in the rolling region after bulging is 0.3 to 0.6 indicate that there is no occurrence of internal cracks. Needless to say, even better results were obtained with respect to the effect of improving center segregation.
[0045]
The crack susceptibility increases as the C content of the slab increases. Then, the S45C material and the S70C material were compared.
[0046]
FIG. 6 is a diagram showing the relationship between the bulging amount and the strain for the S45C material and the S70C material. In the case of the S70C material having a C content of 0.70% by mass, the occurrence of internal cracks was slight when the total bulging amount was 10 mm or less where the strain was less than 1.5%.
[0047]
Therefore, tests Nos. 12 to 15 were performed under the condition that the total bulging amount was 8 mm. As a result, as shown in Table 1, in Test Nos. 12 and 13, which are examples of the present invention in which the amount of reduction did not exceed the bulging amount and the average value of the solid fraction in the center was 0.6, no internal cracks occurred In addition, segregation was improved, and a slab of good quality was obtained.
[0048]
Even if the total bulging amount is 8 mm, in the test number 14 of the comparative example in which the reduction amount is 8.42 mm exceeding the total bulging amount and the maximum reduction gradient is 1.40 mm / m, the molten steel in which the segregation component is concentrated is Was sealed between the dendrite trees, and no squeezing effect appeared, and no improvement in center segregation was observed. In addition, internal cracks occurred because the maximum draft was large. The preferred range of the rolling gradient is 1.2 mm / m or less.
[0049]
In Test No. 15 of the comparative example in which no reduction was made after bulging, as in Test No. 5 which was a comparative example of S45C, no internal cracking occurred, but no improvement in center segregation was observed.
[0050]
In Test No. 16 of Comparative Example, in which the total bulging amount was 11 mm and the distortion during bulging was large, internal cracks occurred during bulging, resulting in inferior slab.
[0051]
In the case of the S45C material, as can be seen from the results in FIG. 6 and Table 1, internal cracking occurs during bulging at a bulging amount of 22.5 mm, and no internal cracking occurs at a bulging amount of 16.5 mm. Was good. When the bulging amount was between 16.5 mm and 20 mm, cracking was slight. Therefore, the appropriate range of the bulging amount is set to 10 mm or less, and the appropriate range of the strain at the solidification interface during bulging is set to less than 2%.
[0052]
The appropriate bulging reduction conditions are different between the S45C material and the S70C material, but by selecting and controlling the appropriate conditions, the concentrated molten steel in the unsolidified portion at the center of the slab is efficiently squeezed out.
[0053]
FIG. 7 is a diagram showing the state of improvement of segregation by the method of the present invention. Carbon content ratio C / C ₀ Was 1.10 or less, and a good slab having improved slab internal quality and slab shape was obtained.
[0054]
Based on the above results, appropriate values of the bulging amount and the solidification interface strain amount during bulging were classified according to the material, and the above-mentioned expressions (1) and (2) were obtained.
[0055]
Further, the reduction amount is appropriately in the range of not more than the bulging amount, and the reduction gradient is appropriately 1.4 mm / m or less. The average value of the center solid phase ratio in the rolling region in the casting direction is preferably 0.3 to 0.6.
(Example 2)
A description will be given of the relationship between the bulging start position and the bulging amount in the case of using the S45C material having a slab width of 420 mm and 500 mm.
[0056]
FIG. 8 is a diagram showing the relationship between the bulging start position and the amount of bubbling when the slab size and the casting speed are different, and were obtained by calculation based on the results of the various tests described above. The bulging start position is represented by a distance from the meniscus, and the bulging start position is a position at which opening of the roll cavity is started.
[0057]
In the case 1 shown in FIG. 8, a slab having a slab width of 420 mm was cast at a casting speed of 0.8 m / min, and the roll cavity of the upper support roll was fully opened at a distance of 6 m from the meniscus. Is the case. According to the figure, the bulging amount is about 6 mm.
The CASE 2 is a case where a slab having a slab width of 500 mm is cast at a casting speed of 0.8 m / min, and the roll cavity is similarly fully opened at a distance of 6 m from the meniscus. Increase to 45 mm.
[0058]
When the casting speed is reduced to 0.55 m / min as in CASE 3, the bulging amount is reduced to about 18 mm, which can be set to a value similar to the result in the first embodiment. However, in this case, since the casting speed has changed, it is necessary to change the reduction region after bulging. If the casting speed is excessively reduced, the casting surface cannot be cast due to, for example, hot-skinning at the meniscus. Therefore, it is necessary to adjust the casting speed to a certain speed or higher.
[0059]
Therefore, when casting a slab having a slab width of 500 mm at a casting speed of 0.8 m / min, the bulging amount is 20 mm or more in the hatched region in the curve of CASE2 in FIG. At least in this section, it is necessary to control the bulging amount by setting the roll cavity of the support roll to less than 20 mm. In the case of CASE2, it is necessary to control the roll cavity to be less than 20 mm at least in the section from 6 to 10 m from the meniscus.
[0060]
FIG. 10 schematically shows a situation in which a slab is supported by roll cavity control in a part of the bulging region to control the bulging amount. In the case of CASE2, as shown in the figure, the roll cavity control for supporting the slab is performed in a part of the first half, and the appropriate bulging of the S45C material described in Table 1 is performed in the subsequent sections. The casting may be performed under the conditions of reduction and reduction.
[0061]
Thus, in the continuous casting of bloom, the case where the amount of bulging can be controlled while the roll cavity is fully opened, the case where it is necessary to control the slab to control the excessive increase of the bulging amount, and There is. For example, when the bloom size is increased to some extent, for example, the length of the long side of the slab section is increased, it is necessary to consider the roll cavity control of the upper and lower support rolls. Based on the test results performed by the present inventors, it is preferable to actively control the roll cavity when the slab width is 500 mm or more. If the specifications of the continuous casting apparatus and the size of the bloom to be cast are known, the necessity of controlling the upper and lower roll cavity amounts can be determined according to the casting speed and the required bulging amount.
[0062]
In controlling the bulging amount of a slab having a slab width of less than 500 mm, it is preferable to control the molten steel temperature, the secondary cooling condition of the slab, and the casting speed in addition to the bulging start position.
[0063]
Needless to say, as shown in FIG. 1 described above, any equipment having a function of controlling the roll cavity in the entire bulging region is preferable.
[0064]
【The invention's effect】
According to the method of the present invention, in a casting method in which an unsolidified portion such as a bloom slab that is difficult to be bulged is bulged, and before the solidification is completed, a reduction is performed within the bulging amount or less. Generation of sound slab can be prevented. Also, bloom slabs produced by the method of the present invention have extremely good quality without segregation and internal cracks. As described above, the present invention is a valuable invention that can greatly contribute to the development of the bloom casting field.
[Brief description of the drawings]
FIGS. 1A and 1B are diagrams schematically showing a rolling method using different diameter rolls disclosed in Patent Documents 3 and 4, wherein FIG. 1A shows a state before rolling, FIG. 1B shows a state before rolling, and FIG. ) Indicates the state after the reduction.
FIGS. 2A and 2B are diagrams schematically illustrating a method of reducing the pressure after bulging according to the present invention, wherein FIG. 2A illustrates a state before the reduction, FIG. 2B illustrates a state after the reduction, and FIG. Represents each state.
FIG. 3 is a view schematically showing a conventional general method of reducing the pressure after bulging.
FIG. 4 is a diagram illustrating a relationship between a distance from a meniscus and a bulging amount.
5A and 5B are diagrams showing a concept for deriving a relationship between a bulging amount and a strain. FIG. 5A is a cross-sectional view of a slab including unsolidified molten steel, and FIG. (a) Modeling with a U-shaped frame, FIG. (c) shows a cross section of a slab after bulging, and (d) shows modeling with a U-shaped frame in (c). Respectively.
FIG. 6 is a diagram illustrating a relationship between a bulging amount and distortion.
FIG. 7 is a diagram showing an improved state of segregation by the method of the present invention.
FIG. 8 is a diagram showing a relationship between a bulging start position and a bulging amount when a slab size and the like are different.
FIG. 9 is a diagram schematically illustrating a state of bulging control when the roll cavity is fully opened.
FIG. 10 is a diagram schematically showing a situation when part of bulging is performed by roll cavity control.
[Explanation of symbols]
1: immersion nozzle,
2: Hot surface (meniscus) in mold
3: mold,
4: molten steel
5: solidified shell,
6: Support roll,
7, 71: reduction roll,
8, 81: slab before reduction
9, 91: slab after rolling,
10, 101: unsolidified molten steel,

Claims (6)

A continuous casting method in which an unsolidified portion of a slab having a ratio of a long side length of a cross section divided by a short side length of 1 to 2.5 is bulged and then rolled down. The bulging is performed under the condition that the strain amount ε of the solidification interface at the time satisfies the relationship expressed by the following formulas (1) and (2), and then the bulging portion of the slab has a maximum reduction gradient of 1 in the casting direction. A continuous casting method characterized in that at least one pair of reduction rolls is used in a range of less than 40 mm / m to reduce the bulging amount or less.
When the C content of the slab is less than 0.70% by mass, δ ≦ 20 mm and ε <2.00% (1)
When the C content of the slab is 0.70% by mass or more, δ ≦ 10 mm and ε <1.50% (2) 2. The continuous casting method according to claim 1, wherein after the bulging of the unsolidified portion of the slab, the region where the average value of the solid phase ratio at the center of the reduction section is 0.3 to 0.6 is reduced. 3. . The control of the bulging amount of the slab having a maximum length of the long side or the short side of the cross section of less than 500 mm is performed by adjusting one or both of the following (1) and (2). Item 3. The continuous casting method according to item 1 or 2.
(1) Molten steel temperature, slab secondary cooling condition and casting speed (2) Bulging start position in casting direction The control of the bulging amount of the slab having a maximum length of 500 mm or more of the long side or the short side of the cross section is performed by adjusting the roll cavity of the support roll corresponding to a region in the casting direction where the bulging amount is equal to or more than the target bulging amount. The continuous casting method according to claim 1 or 2, wherein: The continuous casting method according to any one of claims 1 to 4, wherein the unsolidified portion of the slab is electromagnetically stirred before the slab is reduced. A slab cast by the continuous casting method according to any one of claims 1 to 5, wherein a carbon content C (mass%) at a center in a thickness direction of the slab is a carbon of a base material excluding a center. A slab, wherein the value of the ratio C / C ₀ divided by the content C ₀ (% by mass) is 1.10.

Images