JP2018099704A - Continuous casting method for steel - Google Patents
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本発明は、鋼の連続鋳造において鋳片の表面割れを防止し表面性状の良好な鋳片を製造する方法に関する。 The present invention relates to a method for preventing a surface crack of a slab in continuous casting of steel and producing a slab having a good surface property.
鋼の靭性向上のために、鋼中にＮｉを添加することが一般的に行われている。Ｎｉ添加鋼を連続鋳造すると、鋳片表面および表皮下にオーステナイト粒界に沿った割れ（以降、本願で「表面割れ」と呼称）が発生することがある。表面割れが発生した場合には、鋳造後の工程でこれを除去する必要があり、Ｎｉ添加鋼を製造する上で、上記の鋳片表面割れを防止することが課題となっている。 In order to improve the toughness of steel, it is a common practice to add Ni to the steel. When Ni-added steel is continuously cast, cracks along the austenite grain boundaries (hereinafter referred to as “surface cracks” in the present application) may occur on the surface of the slab and in the epidermis. When surface cracks occur, it is necessary to remove them in the post-casting process, and it is a problem to prevent the above-described slab surface cracks when manufacturing Ni-added steel.
このようなオーステナイト粒界（γ粒界）割れは、連続鋳造の２次冷却帯で鋳片の高温延性が低下するγ→α変態温度付近（約７５０〜８５０℃）（以下「脆化温度域」ともいう。）で、鋳片の曲げ歪、曲げ戻し矯正歪で発生すると言われている（例えば特許文献１参照）。以下、曲げ戻し矯正を単に「矯正」ともいう。この対策として一般的に、曲げ時や矯正時において、鋳片表面温度をこの脆化温度域よりも低温側もしくは高温側に回避する温度に維持しながら鋳造する方法が採用されている（特許文献１、非特許文献１）。 Such austenite grain boundary (γ grain boundary) cracks occur in the vicinity of the γ → α transformation temperature (about 750 to 850 ° C.) (hereinafter referred to as “embrittlement temperature range”) in which the hot ductility of the slab decreases in the secondary cooling zone of continuous casting. In other words, it is said that the slab is caused by bending strain and bending correction strain (see, for example, Patent Document 1). Hereinafter, the bending back correction is also simply referred to as “correction”. In general, as a countermeasure, a method of casting while maintaining the slab surface temperature at a temperature avoiding the lower temperature side or the higher temperature side than the embrittlement temperature range is adopted during bending or straightening (Patent Document). 1, Non-Patent Document 1).
Ｎｉを含有する鋼では、曲げ部、矯正部で脆化温度域を低温側に回避しようとすると、鋳片の冷却不均一が生じること、及びＮｉ含有鋼は脆化温度域の下限が低温側に拡大することにより、脆化温度域を低温側に回避しての表面品質改善が難しいといわれている。一方、曲げ部、矯正部で単純に脆化温度域を高温側に回避するのみでは、表面割れを十分に改善することができない（特許文献１）。 In steels containing Ni, when trying to avoid the embrittlement temperature range at the low temperature side at the bending part and the straightening part, the cooling of the slab becomes uneven, and the Ni-containing steel has a lower limit of the embrittlement temperature range at the low temperature side. It is said that it is difficult to improve the surface quality by avoiding the embrittlement temperature range to the low temperature side. On the other hand, the surface crack cannot be sufficiently improved by simply avoiding the embrittlement temperature region to the high temperature side at the bent portion and the straightening portion (Patent Document 1).
特許文献１においては、Ｎｉを含有する鋼の連続鋳造において、鋳片全面の表面温度を鋳型を出てから多くとも２分以内の間に一旦６００℃〜Ａr3点温度まで低下させ、曲げ部と矯正部における鋳片表面温度が８５０℃以上になるように２次冷却を行う方法が開示されている。６００℃〜Ａr3点温度まで冷却することにより、γ粒界が不明瞭な組織となり、割れ感受性が低い組織とすることができるとしている。 In Patent Document 1, in continuous casting of steel containing Ni, the surface temperature of the entire surface of the slab is temporarily lowered from 600 ° C. to the Ar 3 point temperature within 2 minutes after leaving the mold. A method is disclosed in which secondary cooling is performed so that the slab surface temperature in the straightening portion is 850 ° C. or higher. By cooling to 600 ° C. to Ar 3 point temperature, the γ grain boundary becomes an unclear structure, and a structure with low cracking sensitivity can be obtained.
また特許文献２には、Ｎｉ含有鋼の割れを解決する手段として、鋳型内メニスカスから鋳型下端までの引き抜き時間を１分以内とし、鋳型から引き抜いた後２次冷却を行い、１分以内に鋳片表面温度をＡ３変態以下にまで冷却することを特徴とする表面割れの防止方法、さらには鋳片をＡ３変態温度以下に冷却したあと復熱させ、曲げ点、矯正点における鋳片表面温度を８５０℃以上とし、２０分以内に鋳片の矯正を終了することを趣旨とする表面割れ防止方法が開示されている。１分以内にＡ３変態温度以下まで急速に冷却すれば、γ粒界が不明瞭となるとしている。 In Patent Document 2, as a means of solving cracks in Ni-containing steel, the drawing time from the meniscus in the mold to the lower end of the mold is set to within 1 minute, and after the drawing from the mold, the secondary cooling is performed. A method for preventing surface cracking, characterized by cooling the surface temperature of the single piece to the A3 transformation or lower, and further cooling the cast piece to the A3 transformation temperature or lower and then reheating it to determine the slab surface temperature at the bending point and the correction point. A method for preventing surface cracking is disclosed in which the correction is made at 850 ° C. or higher and the slab correction is finished within 20 minutes. It is said that the γ grain boundary becomes unclear if it is rapidly cooled to below the A3 transformation temperature within 1 minute.
さらに特許文献３においては、Ｎｉ含有鋼の表面割れをさらに抑制するために、鋳片表面温度を５５０℃以下に冷却した後、８５０℃以上に復熱させる方法が開示されている。一旦５５０℃以下まで冷却してフェライトを生成させてから復熱し、再度オーステナイト化することにより、オーステナイト粒を微細化し、結果として鋳片表面付近のフェライト粒径を３０μｍ以下の微細組織とし、割れ感受性の低減を図っている。 Further, Patent Document 3 discloses a method of cooling the slab surface temperature to 550 ° C. or lower and then reheating it to 850 ° C. or higher in order to further suppress the surface cracking of the Ni-containing steel. Once cooled to 550 ° C or lower, ferrite is generated, reheated, and austenitized again to refine the austenite grains. As a result, the ferrite grain size near the slab surface is reduced to a microstructure of 30 µm or less, and is susceptible to cracking. We are trying to reduce it.
特許文献１〜３に記載の発明により、Ｎｉ添加鋼を連続鋳造するに際しての鋳片表面割れの低減を図っているが、割れ発生を十分に低減するには至っていない。Ｎｉ添加鋼における割れ防止技術について連続鋳造機において実験を行い、鋳片の表面割れの発生について調査したところ、鋳片表面において割れが存在しない部分と残存する部分があった。 The inventions described in Patent Documents 1 to 3 attempt to reduce the slab surface cracks when continuously casting the Ni-added steel, but have not yet sufficiently reduced the occurrence of cracks. Experiments on crack prevention technology in Ni-added steel were carried out in a continuous casting machine, and the occurrence of surface cracks in the slab was investigated. As a result, there were portions where cracks did not exist and portions that remained on the slab surface.
本発明は、Ｎｉ等の合金元素を含む鋼を連続鋳造するに際して、鋳片表面の一部に割れが発生するという不均一な現象の原因を解明し、安定的に鋳片の表面割れを防止する鋼の連続鋳造方法を提供することを目的とする。 The present invention elucidates the cause of the non-uniform phenomenon that a part of the slab surface cracks during continuous casting of steel containing alloy elements such as Ni, and stably prevents the slab surface cracks. An object of the present invention is to provide a continuous casting method for steel.
本発明者らは、Ｎｉ添加鋼における割れ防止技術について連続鋳造機において実験を行い、鋳片の表面割れの発生について調査した。その結果、曲げ部と矯正部で鋳片表面温度が脆化温度域を高温側に回避する鋳造を行ったときでも、前述のように、鋳片表面において割れが存在しない部分と残存する部分があった。その原因として鋳片表面における連続鋳造用パウダーの残存により、２次冷却帯における熱伝達にばらつきが生じていることが一因であるとの認識に至った。 The present inventors conducted an experiment in a continuous casting machine on crack prevention technology in Ni-added steel, and investigated the occurrence of surface cracks in the slab. As a result, even when casting is performed in which the slab surface temperature avoids the embrittlement temperature range to the high temperature side at the bending part and the straightening part, as described above, there are no cracked parts and remaining parts on the slab surface. there were. As a cause of this, it has been recognized that one of the reasons is that variation in heat transfer in the secondary cooling zone is caused by the residual powder for continuous casting on the surface of the slab.
鋼の連続鋳造においては、鋳型内における鋳型壁と凝固シェルとの間の潤滑剤として連続鋳造用パウダーが用いられる。これはＣａＯ−ＳｉＯ2−Ｆ−Ｎａ2Ｏ−Ａｌ2Ｏ3−ＭｇＯなどから成る多元系の人工パウダーである。この連続鋳造用パウダーが鋳型内溶鋼湯面に投入されると、溶鋼からの熱で溶融して溶融スラグとなり、パウダーフィルムとして鋳型壁と鋳片凝固シェルの間に流入し、鋳型・鋳片間の潤滑や伝熱制御媒体として重要な役割を果たす。流入したパウダーフィルムは、凝固シェルとともに鋳型内を下方に移動し、鋳型下端から排出される。 In continuous casting of steel, powder for continuous casting is used as a lubricant between the mold wall and the solidified shell in the mold. This is a multi-component artificial powder made of CaO—SiO 2 —F—Na 2 O—Al 2 O 3 —MgO or the like. When this continuous casting powder is poured into the molten steel surface in the mold, it is melted by the heat from the molten steel to form molten slag, which flows as a powder film between the mold wall and the slab solidified shell, and between the mold and slab. It plays an important role as a lubrication and heat transfer control medium. The powder film that flows in moves downward in the mold together with the solidified shell, and is discharged from the lower end of the mold.
詳細は後述するが、鋳型下端から鋳片が引き出された時点において、鋳片表面に存在するパウダーフィルムが鋳片表面で生成したスケール（酸化鉄）と反応することで、著しく鋳片から剥離しにくくなることがわかった。またＮｉを添加した鋼片では特にその傾向が顕著であった。そして、連続鋳造において鋳型下端を通過した鋳片表面に付着したパウダーフィルムが鋳片表面でスケールと反応しないうちに、鋳片表面に高圧で水を吹き付け、パウダーフィルムを鋳片表面から剥離することにより、その後の鋳片２次冷却における冷却挙動を均一化し、曲げ部、矯正部での鋳片表面割れ発生を解消できることが明らかとなった。 Although details will be described later, when the slab is pulled out from the lower end of the mold, the powder film existing on the surface of the slab reacts with the scale (iron oxide) generated on the surface of the slab, so that it is remarkably separated from the slab. I found it difficult. The tendency was particularly remarkable in the steel pieces to which Ni was added. And before the powder film adhering to the slab surface that has passed through the lower end of the mold in continuous casting does not react with the scale on the slab surface, water is sprayed on the slab surface at high pressure to peel the powder film from the slab surface. Thus, it became clear that the cooling behavior in the subsequent secondary cooling of the slab can be made uniform, and the occurrence of cracks on the slab surface at the bent part and the straightened part can be eliminated.
本発明は、上記知見に基づいてなされたものであり、その要旨は以下のとおりである。
（１）質量％で、Ｃ＝０．０６〜０．２０％、Ｓｉ＝０．０５〜０．４％、Ｍｎ＝０．４〜２％、Ｎｉ＝０．１〜２％を含有する鋼の鋳片を、鋼の連続鋳造用パウダーを用いて連続鋳造機で製造するに際して、鋳型通過後の鋳型と１本目のロール間に設置した水冷ノズルもしくは１本目と２本目のロール間に設置した水冷ノズルにおいて、平均水量密度７５０〜２５００Ｌ／ｍｉｎ／ｍ2、衝突圧５〜１５ｇｆ／ｃｍ2で冷却水を吹き付け、その後は平均水量密度０〜３０Ｌ／ｍｉｎ／ｍ2とすることを特徴とする鋼の連続鋳造方法。
（２）鋼の連続鋳造パウダー中のＣａＯ、ＳｉＯ2の質量％で定義される塩基度Ｂ＝ＣａＯ％／ＳｉＯ2％が、Ｂ＝１．１〜１．８であることを特徴する、（１）に記載の鋼の連続鋳造方法
This invention is made | formed based on the said knowledge, The summary is as follows.
(1) Steel containing, by mass%, C = 0.06-0.20%, Si = 0.05-0.4%, Mn = 0.4-2%, Ni = 0.1-2% When the slab was manufactured with a continuous casting machine using powder for continuous casting of steel, a water-cooled nozzle installed between the mold after passing through the mold and the first roll or between the first and second rolls in water-cooled nozzle, the average water flow rate 750~2500L / min / m 2, spraying the cooling water impact pressure 5~15gf / cm 2, then is characterized by an average water flow rate 0~30L / min / m 2 Steel continuous casting method.
(2) The basicity B = CaO% / SiO 2 % defined by the mass% of CaO and SiO 2 in the continuous casting powder of steel is characterized in that B = 1.1 to 1.8. Method for continuous casting of steel as described in 1)
本発明の鋼の連続鋳造方法では、Ｎｉ含有鋼で発生する鋳片表面の割れを発生させることなく、表面性状に優れた鋳片を得ることができる。 In the continuous casting method of steel of the present invention, a slab excellent in surface properties can be obtained without causing cracks on the slab surface generated in Ni-containing steel.
本発明が対象とする鋼は、鋳片表面にオーステナイト粒界割れが発生しやすい鋼種である。一般的に、Ｃ＝０．０６〜０．２０％、Ｎｉ＝０．１〜２％含有する鋼の鋳片ではオーステナイト粒界割れが発生しやすく、加えて、Ｎｂ、Ｖなどが含まれるとさらに割れが発生しやすい。 The steel targeted by the present invention is a steel type in which austenite grain boundary cracks are likely to occur on the slab surface. In general, a steel slab containing C = 0.06-0.20% and Ni = 0.1-2% is susceptible to austenite grain boundary cracking, and in addition, Nb, V, etc. are included. Furthermore, cracking is likely to occur.
Ｎｉ等の合金元素を含む鋼を連続鋳造すると、前述のように、鋳片表面において割れが存在しない部分と残存する部分があった。Ｎｉ添加鋼の鋳片の表面全面において割れを防止し、安定的に鋳造するためには、鋳片表面全面においてより均一な熱履歴を与える必要があるが、鋳片表面に付着したスケールや連続鋳造用パウダーを考慮すると、必ずしも容易ではない。 When steel containing an alloy element such as Ni was continuously cast, there were portions where no cracks existed and portions where cracks remained on the slab surface as described above. In order to prevent cracking over the entire surface of the Ni-added steel slab and to stably cast it, it is necessary to give a more uniform thermal history over the entire slab surface. Considering the powder for casting, it is not always easy.
本発明は、Ｎｉ等の合金元素を含む鋼を連続鋳造するに際して、鋳型内の潤滑剤として用いる鋼の連続鋳造用パウダー（パウダーフィルム）を鋳型直下で鋳片の表面から均一に剥離すると同時に鋳片表面を強冷却し、さらに復熱を行った上で曲げ部と矯正部を通過させることにより、安定的に鋳片の表面割れを防止する。 In the present invention, when steel containing an alloy element such as Ni is continuously cast, the powder (powder film) for continuous casting of steel used as a lubricant in the mold is uniformly peeled from the surface of the slab immediately under the mold and cast. The surface of the slab is stably prevented by allowing the surface of the slab to be strongly cooled and reheated and then passing through the bent portion and the straightening portion.
本発明者らは、連続鋳造用パウダーフィルムの鋼片からの剥離性について実験的な検討（オフライン実験）を進めた。鋼片の表面に連続鋳造用パウダーを載せた上で１２００℃まで急速に加熱して、鋼片表面に溶融パウダーフィルム層を形成し、一定時間保持した後、表面にスプレー水を噴射して冷却する実験を行った。その結果、１２００℃に到達した後３０秒以内にスプレー水を噴射した場合にはパウダーフィルムは均一に剥離するが、３０秒以上経過すると、パウダーフィルムが剥離している部分と剥離していない部分が不均一に混在することがわかった。剥離していない部分を観察すると、溶融したパウダーフィルムがスケールと反応し、鋼片と強固に密着していることが判明した。以上のオフライン実験の結果から、鋳片表面に存在するパウダーフィルムが、時間経過とともに鋳片表面で生成したスケール（酸化鉄）と反応することで、著しく鋳片から剥離しにくくなることがわかった。またＮｉを添加した鋼片では特にその傾向が顕著であった。したがって、連続鋳造において鋳型下端を通過した鋳片表面に付着したパウダーフィルムが鋳片表面でスケールと反応しないうちに、鋳片に高圧で水を吹き付け、パウダーフィルムを鋳片表面から剥離することが有効であると推定された。 The inventors proceeded with an experimental study (offline experiment) on the peelability of a continuous casting powder film from a steel piece. The powder for continuous casting is placed on the surface of the steel slab and heated rapidly up to 1200 ° C to form a molten powder film layer on the surface of the steel slab and held for a certain period of time. An experiment was conducted. As a result, when spray water is sprayed within 30 seconds after reaching 1200 ° C., the powder film peels uniformly, but after 30 seconds or more, the part where the powder film is peeled and the part where it is not peeled Was found to be unevenly mixed. Observation of the unexfoliated part revealed that the melted powder film reacted with the scale and was firmly adhered to the steel piece. From the results of the above offline experiments, it was found that the powder film present on the slab surface reacts with the scale (iron oxide) generated on the slab surface over time, making it extremely difficult to peel from the slab. . The tendency was particularly remarkable in the steel pieces to which Ni was added. Therefore, before the powder film adhering to the slab surface that has passed through the lower end of the mold in continuous casting does not react with the scale on the slab surface, water can be sprayed on the slab at high pressure to peel the powder film from the slab surface. Estimated to be effective.
そこで上記オフライン実験において、鋼片に吹き付ける冷却水の衝突圧力とパウダーフィルム剥離性との関係についても調査した。鋼片の表面に連続鋳造用パウダーを載せた上で１２００℃まで急速に加熱して鋼片表面に溶融パウダーフィルム層を形成し、０．５分保持した後、表面に種々の衝突圧力でスプレー水を噴射して冷却する実験を行った。その後、鋼片表面のうちでパウダーフィルム層が剥離した部分の面積率をパウダー剥離率（％）として評価した。鋼片上に載置する連続鋳造用パウダーとして、塩基度（ＣａＯ質量％／ＳｉＯ2質量％）が１．０から１．８の４種類のものを用いた。結果を図１に示す。 Therefore, in the above offline experiment, the relationship between the collision pressure of the cooling water sprayed on the steel slab and the powder film peelability was also investigated. The powder for continuous casting is placed on the surface of the steel slab, heated rapidly to 1200 ° C to form a molten powder film layer on the surface of the steel slab, held for 0.5 minutes, and then sprayed with various collision pressures on the surface. An experiment was conducted in which water was injected to cool. Then, the area ratio of the part from which the powder film layer peeled in the steel piece surface was evaluated as the powder peeling ratio (%). As the powder for continuous casting placed on the steel piece, four types of basicity (CaO mass% / SiO 2 mass%) of 1.0 to 1.8 were used. The results are shown in FIG.
パウダーフィルムとスケールが反応する前の時間であれば、図１に示すように、衝突圧力を５ｇｆ／ｃｍ2以上となるように設定すれば、いずれのパウダー塩基度であっても、パウダーフィルムの剥離が促進されることが明らかになった。 As long as the time before the powder film reacts with the scale, as shown in FIG. 1, if the collision pressure is set to be 5 gf / cm 2 or more, the powder film has any basicity. It became clear that peeling was promoted.
加えて種々のパウダーについて、鋳片表面からのパウダーフィルムの剥離性について検討した結果、図１に示すように、塩基度が高いほどパウダーが剥離しやすいことが確認できた。 In addition, as a result of examining the peelability of the powder film from the slab surface for various powders, as shown in FIG. 1, it was confirmed that the higher the basicity, the easier the powder was peeled.
そこで、Ｎｉ含有鋼の連続鋳造の鋳造試験を繰り返し実施し、鋳片表面からパウダーフィルムを確実に剥離することのできる２次冷却条件について検討を行った。その結果、連続鋳造の鋳型通過後の鋳型と１本目のロール間に設置した水冷ノズルもしくは１本目と２本目のロール間に設置した１本の水冷ノズル（以下「強冷却水冷ノズル」ともいう。）によって、衝突圧５ｇｆ／ｃｍ2以上で冷却水を吹き付けることにより、鋳造を完了した連続鋳造鋳片の表面において、不均一な割れの発生（割れの発生した部分と発生しない部分が共存）を防止できることがわかった。このことから、パウダーフィルムが確実に剥離されることが確認された。衝突圧については、ノズルの中心軸方向において、ノズルと鋳片表面との距離と等距離に受圧センサーを配置し、水を吹き付けつつ圧力を測定することによって計測することができる。 Then, the casting test of the continuous casting of Ni-containing steel was repeatedly performed, and the secondary cooling conditions that can surely peel the powder film from the slab surface were studied. As a result, it is also referred to as a water-cooled nozzle installed between the mold after passing through the continuous casting mold and the first roll or one water-cooled nozzle (hereinafter referred to as “strongly cooled water-cooled nozzle”) installed between the first and second rolls. ), By spraying cooling water at a collision pressure of 5 gf / cm 2 or more, non-uniform cracking occurs on the surface of the continuously cast slab after the casting is completed (the part where cracking occurs and the part where it does not occur). I found that it can be prevented. From this, it was confirmed that the powder film was peeled off reliably. The collision pressure can be measured by arranging a pressure receiving sensor at the same distance as the distance between the nozzle and the slab surface in the central axis direction of the nozzle and measuring the pressure while spraying water.
鋳型の下端と２本目のロール間の距離は６００ｍｍ以下である。従って、鋳造速度１．２ｍ／ｍｉｎで鋳造を行う一般的な場合についてみると、本発明の強冷却水冷ノズルによる冷却は、鋳片が鋳型下端を出てから０．５分以内に行われることになる。従って、本発明の強冷却水冷ノズルを用いて衝突圧５ｇｆ／ｃｍ2以上で冷却水を吹き付けることによってパウダーフィルムを剥離できる点については、前記オフライン実験において、１２００℃に到達した後３０秒以内にスプレー水を噴射した場合にパウダーフィルムが均一に剥離するという結果と符合することがわかる。 The distance between the lower end of the mold and the second roll is 600 mm or less. Therefore, in the general case where casting is performed at a casting speed of 1.2 m / min, cooling by the strong cooling water cooling nozzle of the present invention is performed within 0.5 minutes after the slab leaves the lower end of the mold. become. Accordingly, the point that the powder film can be peeled off by spraying cooling water at a collision pressure of 5 gf / cm 2 or more using the strong cooling water cooling nozzle of the present invention is within 30 seconds after reaching 1200 ° C. in the offline experiment. It can be seen that the result agrees with the result that the powder film peels uniformly when spray water is sprayed.
本発明においては、鋳型下端から引き出された直後の鋳片表面において上記のようにパウダーフィルムを確実に剥離すると同時に、鋳片の表面温度をＡ３変態点以下まで冷却し、その後復熱して温度を上昇し、曲げ部及び矯正部の鋳片表面温度を脆化温度域の高温側に回避することにより、Ｎｉ含有鋼における鋳片表面割れを確実に防止する。 In the present invention, the powder film is surely peeled off as described above on the surface of the slab immediately after being drawn from the lower end of the mold, and at the same time, the surface temperature of the slab is cooled to the A3 transformation point or less, and then the temperature is recovered by reheating. The slab surface temperature of the Ni-containing steel is reliably prevented by rising and avoiding the slab surface temperature of the bent part and the straightened part to the high temperature side of the embrittlement temperature range.
鋳型下端から引き出された直後の鋳片表面において、鋳片の表面温度を安定的にＡ３変態点以下に冷却するためには、鋳型通過後の鋳型と１本目のロール間に設置した水冷ノズルもしくは１本目と２本目のロール間に設置した１本の水冷ノズルによって、衝突圧５ｇｆ／ｃｍ2以上とすると同時に、水量密度が７５０Ｌ／ｍｉｎ／ｍ2以上とする必要があることがわかった。なお、強冷却水冷ノズルの水量密度については、ノズルの水量（Ｌ／ｍｉｎ）を、ロール間隔×鋳片幅の面積（ｍ2）で除することにより算出する。 In order to stably cool the surface temperature of the slab below the A3 transformation point on the surface of the slab immediately after being drawn from the lower end of the mold, a water-cooled nozzle installed between the mold after passing through the mold and the first roll or It was found that the water pressure density should be 750 L / min / m 2 or more at the same time as the collision pressure of 5 gf / cm 2 or more by one water-cooled nozzle installed between the first and second rolls. In addition, about the water amount density of a strong cooling water cooling nozzle, it calculates by dividing the water amount (L / min) of a nozzle by the area (m < 2 >) of roll space | interval x slab width.
以上のように、鋳型下端直下において強冷却水冷ノズルを用いて行う、鋳造方向に１本のノズルからの鋳片の吹き付けは、パウダーフィルムを鋳片表面から確実に剥離することと、鋳片の表面温度をＡ３変態点以下にまですること、両方の目的で行われる。Ａ３変態点以下にするだけが目的であれば、鋳造方向に複数のロール間の複数のノズルで冷却水を吹き付ければよいが、パウダーの完全な剥離も含めて目的を実現するためには、鋳造方向に１本のノズルで高い水量密度、高い衝突圧力での冷却水の吹き付けが必要である。１本に限定する理由は、冷却設備の増強等なく、冷却設備能力を効率的に利用し、高い水量密度、高い衝突圧力を確保するためである。 As described above, the blasting of the slab from one nozzle in the casting direction performed using the strong cooling water cooling nozzle directly below the lower end of the mold is to surely peel off the powder film from the surface of the slab, It is performed for both purposes to bring the surface temperature below the A3 transformation point. If it is only aimed to be below the A3 transformation point, it is sufficient to spray cooling water with a plurality of nozzles between a plurality of rolls in the casting direction, but in order to realize the purpose including complete peeling of the powder, It is necessary to blow cooling water at a high water density and high collision pressure with one nozzle in the casting direction. The reason for limiting to one is to efficiently utilize the cooling facility capacity without increasing the cooling facility, and to ensure high water density and high collision pressure.
さらに、鋳型と１本目のロール間に設置した水冷ノズルもしくは１本目と２本目のロール間に設置した１本の水冷ノズル（強冷却水冷ノズル）によって鋳片表面温度をＡ３変態温度以下に冷却した後、２次冷却パターンを調整して鋳片表面温度を復熱させる。この過程では、特にＮｉを含む鋼では、垂直曲げ連続鋳造機の曲げ応力および矯正応力だけでなく、熱応力によっても割れが発生するため、可能な限りの緩冷却、すなわち水量密度の低減が必要である。本発明では、強冷却水冷ノズルによって冷却水を吹き付けた後、連続鋳造機の機端に至るまでの２次冷却における平均水量密度０〜３０Ｌ／ｍｉｎ／ｍ2とする。これにより、曲げ部と矯正部における鋳片表面温度が脆化温度域を高温側に回避可能であるとともに、２次冷却起因の熱応力をも低減し、割れの発生を防止することができる。これらの対策をとることで、鋳片表面割れを安定的に防止できる。 Furthermore, the surface temperature of the slab was cooled below the A3 transformation temperature by a water-cooled nozzle installed between the mold and the first roll or a single water-cooled nozzle (strongly cooled water-cooled nozzle) installed between the first and second rolls. Thereafter, the secondary cooling pattern is adjusted to reheat the slab surface temperature. In this process, especially in steels containing Ni, cracks are generated not only by the bending stress and straightening stress of the vertical bending continuous casting machine but also by thermal stress, so it is necessary to cool as slowly as possible, that is, to reduce the water density. It is. In this invention, after spraying cooling water with a strong cooling water cooling nozzle, it is set as the average water quantity density of 0-30 L / min / m < 2 > in secondary cooling until it reaches the end of a continuous casting machine. As a result, the slab surface temperature at the bent portion and the straightened portion can avoid the embrittlement temperature range to the high temperature side, and the thermal stress due to secondary cooling can also be reduced, and the occurrence of cracks can be prevented. By taking these measures, slab surface cracks can be stably prevented.
以上のように、本発明の鋼の連続鋳造においては、鋳型内の潤滑剤として連続鋳造用パウダーを均一に剥離するため、鋳片表面に生成したスケールとの反応が進行しないうちに、パウダーを表面から除去する必要があるので、鋳型下端と１本目のロール間、もしくは１本目と２本目のロールとの間に設置した水冷ノズルから大量の水を鋳片に吹き付ける。そのときの衝突圧力はオフラインで水冷ノズルから所定量の冷却水・エアを噴霧しながら受圧センサーで測定することが可能である。また簡易的には、以下の式で見積もることができる。
ＰC＝１０-2×Ｗ0.81×Ｖａ0.5／Ｈ0.2／(ＡＷ)0.263（ｇ／ｃｍ2）
ここで、Ｗ：水量密度（Ｌ／ｍｉｎ／ｍ2）、Ｖａ：圧空吐出流速（ｍ／ｓ）、Ｈ：噴射距離（ｍ）、ＡＷ：気水比＝（単位時間当たりに供給される空気の重量）／（単位時間当たりに供給される水の重量）である。
As described above, in the continuous casting of the steel of the present invention, the powder for continuous casting is uniformly peeled off as a lubricant in the mold. Since it needs to be removed from the surface, a large amount of water is sprayed onto the slab from a water-cooled nozzle installed between the lower end of the mold and the first roll or between the first and second rolls. The collision pressure at that time can be measured with a pressure sensor while spraying a predetermined amount of cooling water / air from a water-cooled nozzle off-line. Moreover, it can estimate simply with the following formula | equation.
P C = 10 −2 × W 0.81 × Va 0.5 / H 0.2 / (AW) 0.263 (g / cm 2 )
Where W: water density (L / min / m 2 ), Va: compressed air discharge flow velocity (m / s), H: injection distance (m), AW: air / water ratio = (air supplied per unit time) Weight) / (weight of water supplied per unit time).
パウダーを確実に剥離させるには、ＰCが５ｇｆ／ｃｍ2以上である必要がある。それより小さいとパウダーは一部剥がれるが、残存する部分が残り、冷却のばらつきをもたらす。一方、鋳型下端と１本目のロール間で水冷ノズルから大量の水を鋳片に吹き付ける場合、衝突圧が高すぎると鋳型内の鋳片と鋳型との隙間に冷却水が入り込む可能性がある。そのため本発明では、上限の衝突圧力を１５ｇｆ／ｃｍ2としている。 To reliably separated powder, it is necessary P C is 5 gf / cm 2 or more. If it is smaller than that, the powder will be partly peeled off, but the remaining part will remain, resulting in cooling variations. On the other hand, when a large amount of water is sprayed from the water-cooled nozzle to the slab between the lower end of the mold and the first roll, if the collision pressure is too high, cooling water may enter the gap between the slab and the mold in the mold. Therefore, in the present invention, the upper limit collision pressure is set to 15 gf / cm 2 .
なお、ロールは軸受を有する分割ロールよりも、一本ロールが望ましい。吹き付けた冷却水がロールの両端に排出され、冷却水が下流側に流出しないからである。 The roll is preferably a single roll rather than a split roll having a bearing. This is because the sprayed cooling water is discharged to both ends of the roll and the cooling water does not flow downstream.
鋳型直下の強冷却における水量密度は、前述のとおり、７５０Ｌ／ｍｉｎ／ｍ2以上である。この水量密度とすることで、鋳型直下にておよそ１０００〜１２００℃であった鋳片の表面温度はＡ３変態温度以下にまでなる。これにより、鋳片表面のγ粒界を不明瞭な組織とし、割れ感受性を低減することができる。さらに、鋳型直下の強冷却における水量密度を１２００Ｌ／ｍｉｎ／ｍ2以上とすることで、鋳片表面温度を５５０℃以下にまで冷却することができる。これにより、鋳片表面の組織を非常に微細な組織とすることができ、割れ感受性をさらに低減することができる。これを復熱させることにより、鋳片表面は不明瞭なγ粒界となり、さらには逆変態して割れにくい組織が形成される。一方水量密度の上限は２５００Ｌ／ｍｉｎ／ｍ2であるが、これを超える水量密度とすると鋳片の温度が大きく低下し、その後の冷却を緩冷却化しても復熱が不十分となる。 As described above, the water density in the strong cooling just below the mold is 750 L / min / m 2 or more. By setting it as this water density, the surface temperature of the slab which was about 1000-1200 degreeC just under a casting_mold | template will be below A3 transformation temperature. Thereby, the γ grain boundary on the surface of the slab can be made an unclear structure, and the cracking sensitivity can be reduced. Furthermore, the slab surface temperature can be cooled to 550 ° C. or less by setting the water density in strong cooling immediately below the mold to 1200 L / min / m 2 or more. Thereby, the structure of the surface of a slab can be made into a very fine structure | tissue, and a crack sensitivity can further be reduced. When this is reheated, the surface of the slab becomes an unclear γ grain boundary, and further, a structure that is hard to crack by reverse transformation is formed. On the other hand, the upper limit of the water density is 2500 L / min / m 2 , but if the water density exceeds this, the temperature of the slab is greatly lowered, and recuperation is insufficient even if the subsequent cooling is slow cooling.
鋳型と１本目のロール間に設置した水冷ノズルもしくは１本目と２本目のロール間に設置した１本の水冷ノズル（強冷却水冷ノズル）によって鋳片表面を強冷却した後は、２次冷却における平均水量密度を３０Ｌ／ｍｉｎ／ｍ2以下として、鋳片表面を復熱させる。Ｎｉを含む鋼は非常に脆化しやすく、連続鋳造機のマシンプロフィールによる曲げ応力、矯正応力だけでなく熱応力によっても割れが発生するため、この水量密度の値を超えないようにする必要がある。また、この水量密度を採用することにより、曲げ部、矯正部における鋳片表面温度が脆化温度域を高温側（８５０℃以上）に回避することができる。好ましくは強冷却水冷ノズルによって鋳片表面を強冷却した後は冷却水をかけることなく鋳造することで、さらに鋳片の表面割れを安定的に防止することができる。なお、垂直曲げ型の連続鋳造機を用いる場合は、鋳型直下強冷却部より下方の機端に至るまでの平均水量密度を上記のように３０Ｌ／ｍｉｎ／ｍ2以下とするとともに、強冷却部より下方の曲げ部に至るまでの平均水量密度を８０Ｌ／ｍｉｎ／ｍ2以下とすることにより、曲げ部における鋳片表面温度を確実に脆化温度域の高温側に回避することができるので好ましい。また、強冷却部より下方の矯正部（曲げ戻し矯正部）に至るまでの平均水量密度を２５Ｌ／ｍｉｎ／ｍ2以下とすることにより、矯正部における鋳片表面温度を確実に脆化温度域の高温側に回避することができるので好ましい。 After the slab surface is strongly cooled by a water-cooled nozzle installed between the mold and the first roll or by one water-cooled nozzle (strongly cooled water-cooled nozzle) installed between the first and second rolls, secondary cooling is performed. The slab surface is reheated at an average water density of 30 L / min / m 2 or less. Steel containing Ni is very brittle, and cracks are generated not only by bending stress and straightening stress but also by thermal stress due to the machine profile of a continuous casting machine, so it is necessary not to exceed this water density value. . Moreover, by adopting this water density, the slab surface temperature at the bent part and the straightened part can avoid the embrittlement temperature range from the high temperature side (850 ° C. or higher). Preferably, after the surface of the slab is strongly cooled by the strong cooling water cooling nozzle, the surface cracking of the slab can be stably prevented by casting without applying cooling water. In the case of using a vertical bending type continuous casting machine, the average water density from the strong cooling part directly below the mold to the machine end below is set to 30 L / min / m 2 or less as described above, and the strong cooling part By setting the average water density up to the lower bent part to be 80 L / min / m 2 or less, the slab surface temperature in the bent part can be surely avoided on the high temperature side of the embrittlement temperature range, which is preferable. . Moreover, the slab surface temperature in a straightening part is reliably made into an embrittlement temperature range by setting the average water amount density to 25 L / min / m < 2 > or less from the strong cooling part to the straightening part (bending return straightening part) below. This is preferable because it can be avoided on the high temperature side.
また、前述の図１の結果から明らかなように、連続鋳造パウダーの成分によっても鋳片表面からのパウダーの剥離性は変化し、パウダー中のＣａＯとＳｉＯ2の比で表わされる塩基度Ｂが、１．１〜１．８の範囲であると、パウダーフィルムの剥離性が向上するので好ましい。１．１未満の場合には、冷却水の衝突圧力を高くしても、パウダーが剥離しにくくなる。一方塩基度が１．８を超えると、パウダーの凝固温度が高くなり、鋳型・鋳片間の潤滑が確保できず、ブレークアウトを起こす懸念があるので好ましくない。 Further, as is apparent from the results of FIG. 1 described above, the peelability of the powder from the surface of the slab also varies depending on the components of the continuous casting powder, and the basicity B represented by the ratio of CaO to SiO 2 in the powder is 1.1 to 1.8 is preferable because the peelability of the powder film is improved. If it is less than 1.1, the powder is difficult to peel off even if the collision pressure of the cooling water is increased. On the other hand, when the basicity exceeds 1.8, the solidification temperature of the powder becomes high, lubrication between the mold and the slab cannot be secured, and there is a concern that breakout may occur, which is not preferable.
本発明が対象とする鋼の成分は、質量％で、Ｃ：０．０６〜０．２０％、Ｓｉ：０．０５〜０．４％、Ｍｎ：０．４〜２％、Ｎｉ：０．１〜２％を含有する。
Ｃ：０．０６〜０．２０％
Ｃは、鋼の強度を向上させるのに欠かせない元素であり、０．０６％以上含有する。一方、含有量が多いと靭性、溶接性を悪化させるため、上限を０．２０％とする。
Ｓｉ：０．０５〜０．４％
Ｓｉは脱酸に有効な元素であり、０．０５％以上含有する。一方、含有量が高いと靭性を悪化するため、好ましくは０．４％を上限とする。０．０１％以上とすると好ましい。
Ｍｎ：０．４〜２％
Ｍｎは強度を増加させるため、好ましくは０．４％以上添加するが、溶接性を悪化させるため、２％以下とする。
Ｎｉ：０．１〜２％以下
Ｎｉを添加すると、強度と靭性が向上する。本発明は、Ｎｉ含有鋼に特有の連続鋳造時の表面割れを対象とする。Ｎｉ含有量が０．１％以上において特有の割れが問題となるので、下限を０．１％とした。ただし多量に添加するとコストが増大するので、上限を２％とした。
The components of the steel targeted by the present invention are mass%, C: 0.06 to 0.20%, Si: 0.05 to 0.4%, Mn: 0.4 to 2%, Ni: 0.00. Contains 1-2%.
C: 0.06-0.20%
C is an element indispensable for improving the strength of steel, and is contained by 0.06% or more. On the other hand, if the content is large, the toughness and weldability are deteriorated, so the upper limit is made 0.20%.
Si: 0.05-0.4%
Si is an element effective for deoxidation, and is contained at 0.05% or more. On the other hand, if the content is high, the toughness deteriorates, so 0.4% is preferably set as the upper limit. It is preferable to be 0.01% or more.
Mn: 0.4-2%
Mn is preferably added in an amount of 0.4% or more in order to increase the strength, but is made 2% or less in order to deteriorate the weldability.
Ni: 0.1 to 2% or less When Ni is added, strength and toughness are improved. The present invention is directed to surface cracks during continuous casting unique to Ni-containing steels. Since a specific crack becomes a problem when the Ni content is 0.1% or more, the lower limit is set to 0.1%. However, since the cost increases when added in a large amount, the upper limit was made 2%.
本発明においては、必要に応じて下記の元素を添加することが望ましい。
Ａｌ：０．０３％以下
Ａｌは脱酸元素として使用される。Ａｌは強脱酸元素であり、あまり多く添加しすぎると鋼材の靭性を低下させるので、０．０３％以下とする。
Ｔｉ：０．０５％以下
Ｔｉは脱酸元素として、また材質の向上に有効である。ただし多く添加するとＴｉＮが過度に析出するので、０．０５％以下とする。
Ｎｂ：０．０５％以下
Ｎｂは強度を向上させるが、Ｎｂが０．０５％を超えると粗大なＮｂの炭窒化物が生成し靭性が悪化する。
Ｃｕ：０．５％以下
Ｃｕを添加すると、強度と靭性が向上するため、上限を０．５％として含有させることができる。
不純物元素については、以下のように制限すると好ましい。
Ｐ：０．０２％以下
Ｐは靭性を悪化させるため０．０２％以下とすると好ましい。
Ｓ：０．００３％以下
Ｓは材質の悪化を招くため、０．００３％以下とすると好ましい。
Ｎ：０．００５％以下
Ｎ含有量が多いと靭性を悪化させるので、０．００５％以下とすると好ましい。
In the present invention, it is desirable to add the following elements as necessary.
Al: 0.03% or less Al is used as a deoxidizing element. Al is a strong deoxidizing element. If too much is added, the toughness of the steel material is lowered, so the content is made 0.03% or less.
Ti: 0.05% or less Ti is effective as a deoxidizing element and for improving the material. However, if TiN is added in a large amount, TiN will precipitate excessively, so 0.05% or less.
Nb: 0.05% or less Nb improves the strength, but if Nb exceeds 0.05%, coarse Nb carbonitrides are produced and the toughness deteriorates.
Cu: 0.5% or less Since addition of Cu improves strength and toughness, the upper limit can be made 0.5%.
The impurity element is preferably limited as follows.
P: 0.02% or less P is preferably made 0.02% or less because it deteriorates toughness.
S: 0.003% or less Since S causes deterioration of the material, 0.003% or less is preferable.
N: 0.005% or less Since a large N content deteriorates toughness, it is preferably 0.005% or less.
表１に示す成分を含有する鋼のスラブ（２０００ｍｍ幅×３００ｍｍ厚）を垂直曲げ型連続鋳造機を用いて鋳造した。表１の鋼Ｎｏ．と表２の鋼Ｎｏ．が対応している。 A steel slab (2000 mm width × 300 mm thickness) containing the components shown in Table 1 was cast using a vertical bending type continuous casting machine. Steel No. 1 in Table 1 And steel No. in Table 2. Corresponds.
本発明例及び比較例では、鋳型通過後の鋳型と１本目のロール間に設置した強冷却水冷ノズル（表１の設置位置「Ａ」）、もしくは１本目と２本目のロール間に設置した強冷却水冷ノズル（表１の設置位置「Ｂ」）において、水量密度、衝突圧を増減し、さらにそれに引き続く下流の２次冷却帯の平均水量密度を変更して、鋳造を行った。なお強冷却水冷ノズルは、鋳造方向に１本、幅方向に９本設置した。設置位置「Ｂ」の場合は鋳型下端と１本目の間では、水をかけなかった。従来例Ｎｏ．１においては、鋳型下端から３本目までの各ロール間に強冷却用水冷ノズルを配置し（表１の設置位置「Ｃ」）、従来例Ｎｏ．２においては、鋳型下端から８本目までの各ロール間に強冷却用水冷ノズルを配置した（表１の設置位置「Ｄ」）。 In the present invention example and the comparative example, the strong cooling water cooling nozzle (installation position “A” in Table 1) installed between the mold after passing through the mold and the first roll, or the strong installed between the first roll and the second roll. In the cooling water cooling nozzle (installation position “B” in Table 1), the water density and the collision pressure were increased / decreased, and the average water density in the downstream secondary cooling zone was changed to perform casting. One strong cooling water cooling nozzle was installed in the casting direction and nine in the width direction. In the case of the installation position “B”, no water was applied between the lower end of the mold and the first one. Conventional Example No. 1, a strong cooling water-cooling nozzle is disposed between each roll from the lower end of the mold to the third roll (installation position “C” in Table 1). In No. 2, a water cooling nozzle for strong cooling was arranged between each roll from the lower end of the mold to the eighth roll (installation position “D” in Table 1).
また連続鋳造用パウダーの成分（塩基度）も変更した。鋳造速度は１．２ｍ／ｍｉｎとした。 The component (basicity) of the powder for continuous casting was also changed. The casting speed was 1.2 m / min.
鋳造後鋳片の表面を酸洗し、割れと割れ発生の面内均一性状況を観察した。さらに鋳片を切断し、鋳造方向に垂直な断面において割れと結晶組織を観察し、割れの程度を指数化した鋳片割れ指数で評価をおこなった。鋳片幅方向に１０カ所から幅方向５０ｍｍ長さの顕微鏡観察用サンプルを切り出し、割れの個数を観察した。１０サンプルの割れ個数の総数を鋳片割れ指数と定義した。 After casting, the surface of the slab was pickled and observed for in-plane uniformity of cracking and cracking. Further, the slab was cut, the crack and the crystal structure were observed in a cross section perpendicular to the casting direction, and the evaluation was performed using the slab crack index in which the degree of cracking was indexed. Samples for microscopic observation having a length of 50 mm in length from 10 locations in the slab width direction were cut out, and the number of cracks was observed. The total number of cracks in 10 samples was defined as the slab cracking index.
表１の実施例に、本発明例を比較例とともに示す。本発明例Ｎｏ．１〜１０では、割れが発生していない。また、鋳片表面直下の結晶組織を観察したところ、本発明例Ｎｏ．１、２はγ粒界が不明瞭であり、本発明例Ｎｏ．３〜６は非常に微細な結晶組織が見られた。以上の結果より、本発明例においては、鋳型直下においてパウダーフィルムを良好に剥離できており、その結果としてスケールが均一かつ良好に剥離できているものと推定される。また、表面直下の結晶組織から、鋳型直下での強冷却によって鋳片表面温度がＡ３変態点以下、さらには５５０℃以下まで低下していたと推定できる。さらに鋳片表面に割れが発生していなかったことから、曲げ部、矯正部における鋳片表面温度が脆化温度域を高温側に回避できていたことがわかる。 Examples of the present invention are shown in Table 1 together with comparative examples. Invention Example No. In 1-10, the crack has not generate | occur | produced. Further, when the crystal structure immediately below the surface of the slab was observed, Example No. of the present invention. 1 and 2 have unclear γ grain boundaries. In 3-6, a very fine crystal structure was observed. From the above results, in the present invention example, it is presumed that the powder film can be peeled off right under the mold, and as a result, the scale can be peeled off uniformly and well. Moreover, it can be presumed from the crystal structure immediately below the surface that the slab surface temperature was lowered to not more than the A3 transformation point and further to not more than 550 ° C. due to strong cooling immediately below the mold. Furthermore, since no crack was generated on the slab surface, it can be seen that the slab surface temperature at the bent part and the straightened part was able to avoid the embrittlement temperature region on the high temperature side.
これに対して、比較例１〜３ではノズルからの衝突圧力、水量密度ともに不十分であり、この場合には鋳片に割れが発生した。特に衝突圧力および水量密度ともに低い比較例２、３では、大きな表面割れが発生した。鋳片表面において、割れ発生の面内分布は不均一であり、スケール剥離が不均一であったと推定される。また、鋳片表面直下の結晶組織はγ粒界が明瞭であった。 On the other hand, in Comparative Examples 1-3, the collision pressure from the nozzle and the water density were insufficient, and in this case, cracks occurred in the slab. Particularly in Comparative Examples 2 and 3 where both the collision pressure and the water density were low, large surface cracks occurred. On the slab surface, the in-plane distribution of crack generation is non-uniform, and it is presumed that the scale peeling was non-uniform. Further, the crystal structure immediately below the slab surface had a clear γ grain boundary.
比較例４、５では、水量密度は十分に高かったが衝突圧力が不十分であり、このような条件でも割れを防止することはできなかった。鋳片表面において、割れ発生の面内分布は不均一であり、鋳片表面直下の結晶組織はγ粒界が不明瞭であった。 In Comparative Examples 4 and 5, the water density was sufficiently high, but the collision pressure was insufficient, and cracking could not be prevented even under such conditions. On the surface of the slab, the in-plane distribution of crack generation was uneven, and the γ grain boundary was unclear in the crystal structure immediately below the surface of the slab.
比較例６は強冷却水冷ノズルの衝突圧力、水量密度は本発明の範囲内であるが、その後の２次冷却水量密度が高く、結果的に鋳片の深い表面割れが発生した。曲げ部、矯正部における鋳片表面温度が脆化温度域を高温側に回避できていなかったためと推定される。 In Comparative Example 6, the collision pressure and water density of the strong cooling water cooling nozzle were within the range of the present invention, but the secondary cooling water density thereafter was high, resulting in deep surface cracks in the slab. It is presumed that the slab surface temperature at the bent part and the straightened part could not avoid the embrittlement temperature range on the high temperature side.
本発明例Ｎｏ．７〜１０は、連続鋳造用パウダーの塩基度を変更して鋳造を行った。本発明例Ｎｏ．９、１０は塩基度がＢ＝１．０のパウダーを使用したが、冷却条件は本発明の範囲内であるものの、パウダーの剥離がやや不均一であったために、割れが鋳片表面の一部に発生したが、軽微であり問題なかった。 Invention Example No. 7-10 performed casting by changing the basicity of the powder for continuous casting. Invention Example No. Nos. 9 and 10 used powders having a basicity of B = 1.0, but although the cooling conditions were within the scope of the present invention, the peeling of the powder was slightly non-uniform, so cracks were observed on the surface of the slab. However, it was minor and no problem.
従来例Ｎｏ．１、２については、前述のように、鋳型の下方において鋳造方向に広い範囲で強冷却を行っている。鋳片表面には割れが発生しており、割れ発生の面内分布は不均一であった。鋳片表面直下の凝固組織はγ粒界が不明瞭であり、強冷却時の鋳片表面温度はＡ３温度以下まで低下したことが推定できるが、鋳造方向に広い範囲で冷却したため、衝突圧が低くかつ平均水量密度は低くなり、鋳型直下でのパウダーフィルム剥離が不十分であってスケールを均一に剥離することができなかったことから、割れが不均一に発生したものと推定される。 Conventional Example No. As described above, for 1 and 2, strong cooling is performed in a wide range in the casting direction below the mold. Cracks were generated on the surface of the slab, and the in-plane distribution of crack generation was uneven. The γ grain boundary is unclear in the solidified structure immediately below the slab surface, and it can be estimated that the slab surface temperature during strong cooling decreased to A3 temperature or less, but because the cooling was performed in a wide range in the casting direction, the impact pressure was It is presumed that cracks were unevenly generated because the powder density was low and the average water density was low, and the powder film was not sufficiently peeled directly under the mold and the scale could not be peeled uniformly.
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