【０００１】
【発明の属する技術分野】
本発明は、連鋳片の中心偏析軽減方法に係わり、特に、溶鋼を連続鋳造して得られる鋳片に形成される化学成分に関する中心偏析を軽減する技術に関する。
【０００２】
【従来の技術】
一般に、溶融金属を鋳型に注入して冷却凝固させ、その金属の鋳片を製造すると、該鋳片の中心部分には不純物成分が濃化する。つまり、鋳片は、その凝固過程での体積収縮やふくれ等に起因し、溶融金属が含有している種々の不純物元素（化学成分）が全体で均一に分布せず、中心部分に成分偏析（以下、これを中心偏析という）を有するものになる。例えば、溶鋼を連続鋳造して得た鋳片（以下、連鋳片という）では、該鋳片の最終凝固部分の中心に、炭素（記号Ｃ）、マンガン（Ｍｎ）、燐（Ｐ）、硫黄（Ｓ）等の成分が濃化した中心偏析が出現する。この中心偏析が存在すると、その後、該鋳片に圧延等の加工を施して鋼板、条鋼等の製品とした際に、製品の品質欠陥の一つになるので、中心偏析は、鋳片の製造過程においてできるだけ軽減しておくのが望ましい。そのため、この中心偏析の軽減対策については従来より多くの技術が開示され、実用されているものも多い。
【０００３】
例えば、本出願人は、図１に示すように、溶鋼１を注入する鋳型２と、該鋳型２より下方へ引き抜いた半凝固状態の鋳片３を各種ローラ４等でさらに下流へ連続的に搬送しながら水冷する二次冷却帯５と、搬送途上の鋳片３を上下で挟み、鍛圧する一対の金型６を備えた垂直曲げ型又は湾曲型連続鋳造機において、製造する鋳片３の中心偏析を軽減するため、鋳型２、搬送工程の途中及び／又は金型６の直前の３ケ所に電磁撹拌装置７〜９を設け、未凝固状態の溶鋼１を電磁撹拌すると共に、凝固完了前鋳片の未凝固部分厚みが８０ｍｍ以下になったら鍛圧を行う技術を提案している（特許文献１参照）。
【０００４】
【特許文献１】
特開平７−４００１９号公報（３頁、左欄２２行〜４頁、右欄５行）
この技術は、不完全凝固領域を鍛圧して大きな圧下を加えた際に、搾り出された未凝固溶鋼中のＣ，Ｍｎ，Ｐ，Ｓ等の各成分濃度を電磁撹拌により均一分布するようにしながら凝固させることで、中心偏析を軽減させるものである。そして、前記３ケ所に設けた電磁撹拌装置７〜９の使用組み合わせ、及びそれぞれに印加する磁束密度を種々変更して中心偏析の軽減効果を向上させることも図っている。
【０００５】
【発明が解決しようとする課題】
本発明は、溶融金属の連続鋳造で得た連鋳片の中心偏析を従来よりさらに軽減可能な連鋳片の中心偏析軽減方法を提供することを目的としている。
【０００６】
【課題を解決するための手段】
発明者は、上記目的を達成するため鋭意研究し、その成果を本発明に具現化した。
【０００７】
すなわち、本発明は、溶融金属を注入する鋳型、該鋳型より下方へ引き抜かれ、まだ未凝固溶融金属を内包する半凝固状態の鋳片をさらに下流側へローラで連続的に搬送しながら水冷する二次冷却帯、搬送途上の前記半凝固状態の鋳片を上下で挟み、圧下する一対の金型及び鋳型から該金型までの間に設けられ、前記半凝固状態の鋳片が内包する未凝固溶融金属を撹拌する電磁撹拌装置を備えた垂直曲げ型又は湾曲型連続鋳造機を用い、溶融金属の連鋳片を製造するに際して、前記半凝固状態にある鋳片で未凝固部分厚みが１０ｍｍ以下になった鋳片に対して、前記金型により圧下量１０ｍｍ以下で圧下することを特徴とする連鋳片の中心偏析軽減方法である。この場合、前記溶融金属が溶鋼であることが好ましい。
【０００８】
本発明では、垂直曲げ型又は湾曲型連続鋳造機を用い、溶融金属を鋳造するに際し、鋳片の圧下開始時期及び圧下量を適切にしたので、得られた連鋳片に生じる中心偏析が従来より大幅に軽減されるようになる。
【０００９】
【発明の実施の形態】
以下、発明をなすに至った経緯をまじえ、本発明の実施の形態を説明する。
【００１０】
まず、発明者は、上記した特許文献１記載の従来技術の問題点を検討した。そして、中心偏析が所望通り軽減しない原因は、圧下を開始する際の未凝固溶鋼が多いこと、及び圧下量が大き過ぎるためと考えた。なぜならば、未凝固溶鋼が多いと、圧下によって鋳片の上方に搾りだされる溶鋼の量が増えるばかりでなく、不純物成分も増えるからである。また、圧下量が５０〜８０ｍｍと大きくなると、未凝固部分と凝固部分との界面での引っ張り応力が大きくなり、内部割れが生じ易くなるからである。
【００１１】
そこで、発明者は、圧下を開始する未凝固溶鋼量の中心偏析に及ぼす影響、及び圧下量の内部割れに及ぼす影響について、断面サイズが４００ｍｍ×５６０ｍｍの鋳片を得る場合でさらなる検討を行った。その結果、未凝固溶鋼量が１０ｍｍ程度以下にまで凝固が進んだ状態であれば、上方の鋳片へ搾り出される溶鋼量が従来に比べてかなり少なくなり、且つ偏析する不純物成分量も少なくなることを見出した。なお、未凝固溶鋼量が１０ｍｍとなる位置は、鋳込み条件（鋳造速度、鋳造温度、冷却条件等）により見出される。また、内部割れについても、圧下量が１０ｍｍ程度以下であれば、前記界面に作用する応力が小さく、内部割れの発生が少ないことがわかった。そして、これらの知見を要件にして、半凝固状態にある鋳片の未凝固部分厚みが１０ｍｍ以下の鋳片に対して、前記金型により圧下量１０ｍｍ以下で圧下する本発明を完成させたのである。
【００１２】
なお、上記結果は、垂直曲げ型連続鋳造機を用い、図１に示した鋳型、搬送工程の途中及び／又は金型直前の３ケ所に設けた電磁撹拌装置を種々組み合わせ、又それぞれに印加する磁束密度を種々変更し、溶鋼を炭素鋼として得たものである。しかしながら、溶鋼以外の金属（例えば、Ｃｕ，Ａｌ，Ｚｎ，それらの合金等について同様の調査を行ったところ、それら金属の連続鋳造においても、半凝固状態にある鋳片の未凝固部分厚みが１０ｍｍ以下になったら、前記金型により圧下量１０ｍｍで軽圧下すれば、中心偏析が軽減できることが確認されたので、本発明は、溶鋼以外の金属に対しても適用できる。
【００１３】
【実施例】
（従来例）
Ｃ：０．８０ｍａｓｓ％，Ｓｉ：０．８０ｍａｓｓ％，Ｍｎ：０．５０ｍａｓｓ％，Ｐ：０．００５ｍａｓｓ％，Ｓ：０．００８ｍａｓｓ％，Ａｌ：０．０４ｍａｓｓ％を含有し、残部は鉄及び不可避不純物からなる硬鋼線材用鋼を、図１に示した垂直曲げ型連続鋳造機で鋳造し、断面サイズが４００ｍｍ×５６０ｍｍの鋳片（この場合、ブルーム）を得た。その際の鋳造速度は、１．０ｍ／ｍｉｎである。また、電磁撹拌装置としては、図１に示した３ケ所のうちのストランド途中に配置したもの８及び金型より９ｍ上流側にあるもの９を使用し、それらの磁束密度はそれぞれ１０００ガウス、５００ガウスとした。そして、鋳造中の鋳片には、未凝固溶鋼の厚みが８０ｍｍの位置で、金型を用いて圧下量１１０ｍｍの圧下を加えた。
（本発明例）
上記従来例と溶鋼、連続鋳造機、鋳片サイズ、鋳造速度及び電磁撹拌条件を同一にし、圧下だけを本発明に係る条件、つまり未凝固溶鋼の厚みが８ｍｍの位置で、金型を用いて圧下量１０ｍｍの圧下を加えた。
【００１４】
これらの操業で得た鋳片の中心偏析状況を図２に示すが、本発明によれば、中心偏析を評価する指数である中心偏析比の値が従来に比べて大幅に低下していることが明らかである。なお、中心偏析比は、圧下後の鋳片軸芯部におけるある成分（例えば、炭素）の濃度Ｃと、鋳型に注入した溶鋼の前記成分（ここでは、炭素）の濃度Ｃｏとの比（Ｃ／Ｃｏ）で表される。
【００１５】
次に、上記結果を踏まえ、多種の鋼種について鋳造速度を変更して、上記同様に鋳造を多数チャージ行い、Ｃ，Ｍｎ，Ｐ，Ｓ等についての中心偏析状況を調査した。そして、中心偏析比が０．９５以上を製品としての合格基準として定め、合格率を求めた。その結果を表１に示す。表１より、本発明を実施した場合は、すべてについて合格し、本発明が非常に優れていることが確認できた。
【００１６】
【表１】
【００１７】
【発明の効果】
以上述べたように、本発明により、垂直曲げ型又は湾曲型連続鋳造機を用い、溶融金属を鋳造するに際し、得られた連鋳片に生じる中心偏析が従来より大幅に軽減されるようになる。
【図面の簡単な説明】
【図１】本発明に係る連鋳片の中心偏析軽減方法を適用する垂直曲げ型連続鋳造機を示す縦断面図である。
【図２】本発明に係る中心偏析軽減方法及び従来法で溶鋼を鋳造した際に得られた連鋳片の中心偏析状況を比較した図である。
【符号の説明】
１ 溶融金属（溶鋼）
２ 鋳型
３ 鋳片
４ 各種ローラ
５ 二次冷却帯
６ 金型
７ 電磁撹拌装置１
８ 電磁撹拌装置２
９ 電磁撹拌装置３[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for reducing center segregation of continuous cast slabs, and more particularly to a technique for reducing center segregation relating to chemical components formed in cast slabs obtained by continuously casting molten steel.
[0002]
[Prior art]
Generally, when a molten metal is poured into a mold and cooled and solidified to produce a slab of the metal, an impurity component is concentrated in a central portion of the slab. In other words, in the cast slab, various impurity elements (chemical components) contained in the molten metal are not uniformly distributed as a whole due to volume shrinkage or blistering in the solidification process, and component segregation ( Hereinafter, this is referred to as center segregation). For example, in a slab obtained by continuously casting molten steel (hereinafter, referred to as a continuous slab), carbon (symbol C), manganese (Mn), phosphorus (P), sulfur (S) is provided at the center of the final solidified portion of the slab. Center segregation in which components such as (S) are concentrated appears. When this center segregation is present, when the slab is subjected to a process such as rolling to produce a product such as a steel plate or a bar, it becomes one of the quality defects of the product. It is desirable to reduce as much as possible in the process. For this reason, many techniques for reducing this center segregation have been disclosed and many of them have been put to practical use.
[0003]
For example, as shown in FIG. 1, the present applicant continuously casts a mold 2 into which molten steel 1 is poured and a slab 3 in a semi-solid state drawn downward from the mold 2 by various rollers 4 and the like. In a vertical bending or curved continuous casting machine having a secondary cooling zone 5 for water cooling while being conveyed, and a pair of dies 6 for sandwiching the slab 3 being conveyed up and down and forging, the slab 3 to be manufactured is In order to reduce center segregation, electromagnetic stirrers 7 to 9 are provided at three places in the mold 2, in the middle of the transporting process and / or immediately before the mold 6, to electromagnetically stir the unsolidified molten steel 1 and before solidification is completed. A technique has been proposed in which forging is performed when the thickness of an unsolidified portion of a slab becomes 80 mm or less (see Patent Document 1).
[0004]
[Patent Document 1]
JP-A No. 7-40019 (page 3, left column, line 22 to page 4, right column, line 5)
In this technology, when a large reduction is applied by forging the incompletely solidified region, the concentration of each component such as C, Mn, P, and S in the squeezed unsolidified molten steel is uniformly distributed by electromagnetic stirring. The solidification while reducing the center segregation is reduced. The use of the electromagnetic stirrers 7 to 9 provided at the three locations and the magnetic flux density applied to each are variously changed to improve the effect of reducing center segregation.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a method of reducing the segregation of the center of a continuous cast piece that can further reduce the center segregation of a continuous cast piece obtained by continuous casting of a molten metal.
[0006]
[Means for Solving the Problems]
The inventor has conducted intensive studies to achieve the above object, and has embodied the results in the present invention.
[0007]
In other words, the present invention provides a mold for injecting molten metal, which is drawn downward from the mold, and water-cooled while continuously transporting a semi-solid state slab still containing unsolidified molten metal further downstream with rollers. A secondary cooling zone, which is provided between a pair of molds and molds that sandwich the semi-solidified slabs in the middle of transportation and which are in the process of being conveyed, and from the mold to the molds. Using a vertical bending or curved continuous casting machine equipped with an electromagnetic stirrer for stirring the solidified molten metal, when manufacturing a continuous cast piece of molten metal, the unsolidified portion thickness of the semi-solidified cast piece is 10 mm A method for reducing segregation in the center of continuous cast slabs, wherein the cast slab is reduced with a reduction amount of 10 mm or less using the mold. In this case, the molten metal is preferably molten steel.
[0008]
In the present invention, when a molten metal is cast by using a vertical bending type or curved type continuous casting machine, the rolling start time and the rolling reduction of the slab are appropriately set, so that the center segregation occurring in the obtained continuous slab is conventionally known. It will be much more reduced.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described, taking into account the circumstances that led to the invention.
[0010]
First, the inventor studied the problems of the conventional technology described in Patent Document 1 described above. The reason why the center segregation is not reduced as desired is considered to be that there is a large amount of unsolidified molten steel at the time of starting rolling and that the amount of rolling is too large. This is because if the amount of unsolidified molten steel is large, not only does the amount of molten steel squeezed out above the slab due to the reduction increase, but also the amount of impurity components increases. Also, when the rolling reduction is as large as 50 to 80 mm, the tensile stress at the interface between the unsolidified portion and the solidified portion increases, and internal cracks are easily generated.
[0011]
Therefore, the inventor further studied the effect of the amount of unsolidified molten steel on the center segregation at which the reduction starts, and the effect of the amount of reduction on the internal cracks when obtaining a slab having a cross-sectional size of 400 mm × 560 mm. . As a result, if the unsolidified molten steel amount is solidified to about 10 mm or less, the amount of molten steel squeezed into the upper slab becomes considerably smaller than in the past, and the amount of impurity components segregated is also reduced. I found that. The position where the amount of unsolidified molten steel is 10 mm is found by casting conditions (casting speed, casting temperature, cooling conditions, etc.). Also, regarding internal cracks, it was found that when the rolling reduction was about 10 mm or less, the stress acting on the interface was small, and the occurrence of internal cracks was small. Based on these findings, the present invention has been completed in which the unsolidified portion thickness of the slab in the semi-solid state is reduced to 10 mm or less by the mold with a reduction amount of 10 mm or less. is there.
[0012]
The above results were obtained by using a vertical bending type continuous casting machine and variously combining the mold shown in FIG. 1 and the electromagnetic stirring devices provided at three places in the middle of the conveying process and / or immediately before the mold, and applying them to each. The magnetic flux density was changed variously, and molten steel was obtained as carbon steel. However, similar investigations were conducted on metals other than molten steel (for example, Cu, Al, Zn, their alloys, and the like). In continuous casting of these metals, the unsolidified portion thickness of a slab in a semi-solid state was 10 mm. In the following cases, it was confirmed that the center segregation could be reduced by lightly reducing the amount of reduction by 10 mm using the mold, so that the present invention can be applied to metals other than molten steel.
[0013]
【Example】
(Conventional example)
C: 0.80% by mass, Si: 0.80% by mass, Mn: 0.50% by mass, P: 0.005% by mass, S: 0.008% by mass, Al: 0.04% by mass, the balance being iron and inevitable The steel for a hard steel wire made of impurities was cast by the vertical bending type continuous casting machine shown in FIG. 1 to obtain a slab (in this case, bloom) having a cross-sectional size of 400 mm × 560 mm. The casting speed at that time is 1.0 m / min. As the electromagnetic stirrer, one of the three places shown in FIG. 1 that is arranged in the middle of the strand 8 and one that is 9 m upstream from the mold 9 are used, and their magnetic flux densities are 1000 Gauss and 500 Gauss, respectively. Gaussian. Then, a reduction of 110 mm was applied to the slab during casting using a mold at a position where the thickness of the unsolidified molten steel was 80 mm.
(Example of the present invention)
Molten steel, continuous caster, slab size, casting speed and electromagnetic stirring conditions are the same as the above conventional example, and only the reduction is the condition according to the present invention, that is, the thickness of the unsolidified molten steel is 8 mm, using a mold. A 10 mm reduction was applied.
[0014]
FIG. 2 shows the state of center segregation of the slabs obtained in these operations. According to the present invention, the value of the center segregation ratio, which is an index for evaluating center segregation, is significantly lower than in the past. Is evident. The center segregation ratio is determined by the ratio (C) between the concentration C of a certain component (for example, carbon) in the core of the cast slab after rolling and the concentration Co of the component (here, carbon) of the molten steel injected into the mold. / Co).
[0015]
Next, based on the above results, the casting speed was changed for various types of steel, and a large number of castings were charged in the same manner as described above, and the state of center segregation of C, Mn, P, S, and the like was investigated. Then, a center segregation ratio of 0.95 or more was determined as a passing criterion as a product, and a passing rate was determined. Table 1 shows the results. From Table 1, when the present invention was implemented, all passed, and it was confirmed that the present invention was very excellent.
[0016]
[Table 1]
[0017]
【The invention's effect】
As described above, according to the present invention, when a molten metal is cast using a vertical bending type or curved type continuous casting machine, the center segregation occurring in the obtained continuous cast piece is significantly reduced as compared with the related art. .
[Brief description of the drawings]
FIG. 1 is a vertical cross-sectional view showing a vertical bending type continuous casting machine to which a method of reducing center segregation of a continuous cast piece according to the present invention is applied.
FIG. 2 is a diagram comparing a center segregation state of a continuous cast piece obtained when a molten steel is cast by a method of reducing center segregation according to the present invention and a conventional method.
[Explanation of symbols]
1 molten metal (molten steel)
2 mold 3 slab 4 various rollers 5 secondary cooling zone 6 mold 7 electromagnetic stirring device 1
8 Electromagnetic stirrer 2
9 Electromagnetic stirrer 3
