本発明を実施するための継目無鋼管製管用丸鋳片の連続鋳造設備における冷却帯とその配置を示す概念図である。It is a conceptual diagram which shows the cooling zone and arrangement | positioning in the continuous casting installation of the round slab for seamless steel pipe pipe making for implementing this invention.
高Ｃｒ鋼継目無鋼管製管用丸鋳片に現れる内部欠陥の模式的説明図である。（ａ）Ａタイプ割れ、（ｂ）Ｂタイプ割れ、（ｃ）Ｃタイプ割れIt is a typical explanatory view of an internal defect which appears in a round slab for pipe making of high Cr steel seamless steel pipe. (A) A type crack, (b) B type crack, (c) C type crack
本発明において適用する水量密度Ｑ１，Ｑ２をストランドの軸心部の凝固状態ｆｓ及び軸心部温度（Ｔｓ−Ｘ）℃を横軸として模式的に示した説明図である。It is explanatory drawing which showed typically water quantity density Q1, Q2 applied in this invention by making the horizontal axis the solidification state fs of the axial center part of a strand, and axial center part temperature (Ts-X) degreeC.
Ａタイプ割れとその割れ長さの説明図である。It is explanatory drawing of A type crack and its crack length.
丸鋳片の反りの量の測定方法についての説明図である。It is explanatory drawing about the measuring method of the quantity of curvature of a round slab.
軸心部凝固後期強制冷却における適用水量密度と軸心部における圧縮応力−引張応力転換点との関係図（ａ図）及び製品丸鋳片に現れる反りとの関係図（ｂ図）である。It is a relationship figure (a figure) of the applied water quantity density in the axial center solidification late forced cooling and the compression stress-tensile stress conversion point in an axis part, and a relation figure (b figure) with the curvature which appears in a product round cast piece.
本発明に係る軸心部凝固前期強制冷却及び軸心部凝固後期強制冷却の凝固状態−適用水量密度の適用パターンを示す。The application pattern of the solidification state-applied water amount density of the axial center solidification early forced cooling and axial center solidification late forced cooling which concern on this invention is shown.
表２は、図１に示す形式の内径２１０ｍｍの水冷銅鋳型を備える垂直曲げ型の連続鋳造設備を用い、表１に示す組成の１３Ｃｒ鋼の溶鋼を連続鋳造したときの、軸心部凝固前期強制冷却の適用時期と１３Ｃｒ鋼継目無鋼管製管用丸鋳片に現れるＡタイプ軸心部割れ長さ及びＢタイプ軸心部割れ有無との関係を示す。表２において、条件(a)は、軸心部固相率ｆｓが０．２〜０．５の区間に亘って、上記水量密度Ｑ１を３０〜９０Ｌ／ｍ２／ｍｉｎの範囲とする軸心部凝固前期強制冷却を行い、その状態を軸心部固相率ｆｓが０．７になるまで継続した後、水量密度を低下させて、ストランドの軸心部温度が（Ｔｓ−１４５）℃となるまでの区間に亘って軸心部凝固後期強制冷却を行った場合であり、軸心部凝固前期強制冷却が適正に行われた場合に当たる。一方、条件(b)，(c)は、軸心部凝固前期強制冷却の適用時期をそれぞれ、軸心部固相率ｆｓが０．３〜０．５，０．１〜０．４の区間に亘って行い、その後、水量密度を低下させて、ストランドの軸心部温度が（Ｔｓ−１４５）℃となるまでの区間に亘って軸心部凝固後期強制冷却を行った場合であり、それぞれ、軸心部凝固前期強制冷却の開始が遅れた場合、早期に終了した場合に当たる。これに対し、条件(d)は、二次冷却後に軸心部凝固前期強制冷却をまったく行わなかった場合である。なお、軸心部凝固後期強制冷却は、いずれの場合についても、図３の領域（B）の範囲を満たすように行った。
Table 2 shows the first stage of solidification of the axial center when a molten steel of 13Cr steel having the composition shown in Table 1 is continuously cast using a vertical bending type continuous casting equipment having a water-cooled copper mold having an inner diameter of 210 mm of the type shown in FIG. The relationship between the application time of forced cooling and the A type shaft center part crack length and B type shaft center part crack presence which appear in the round cast slab for 13Cr steel seamless steel pipe production is shown. In Table 2, the condition (a) indicates that the axial center part has the water density Q1 in the range of 30 to 90 L / m2 / min over the section where the axial center solid fraction fs is 0.2 to 0.5. After the solidification forcibly cooled in the first stage, the state is continued until the shaft center solid fraction fs becomes 0.7, and then the water density is lowered so that the shaft core temperature becomes (Ts-145) ° C. This corresponds to the case where the axial center solidification late-stage forced cooling is performed over the interval up to and including the case where the axial center solidification early-stage forced cooling is appropriately performed. On the other hand, the conditions (b) and (c) are the periods in which the axial solid phase ratio fs is 0.3 to 0.5 and 0.1 to 0.4, respectively. And then the water density is decreased and the axial center solidification late forced cooling is performed over the interval until the axial center temperature of the strand reaches (Ts-145) ° C., respectively. This corresponds to the case where the start of forced cooling in the early stage of solidification of the axial center is delayed, or is terminated early. On the other hand, the condition (d) is a case where no forced cooling is performed at all before the axial solidification after the secondary cooling. In addition, the axial center solidification late forced cooling was performed so that the range of the area | region (B) of FIG. 3 might be satisfy | filled in any case.
得られた製品丸鋳片のＡタイプ軸心部割れ（復熱時Ａタイプ割れを含む）及びCタイプ割れの平均長ささらには、鋳片曲がりの程度を評価した。評価結果は表４に併せて示す。なお、Ａタイプ割れの長さとは、図４に示すように、収縮孔から延びる割れの長さ（ｍｍ）をいい、評価は、多数の丸鋳片の試験片断面に観察されるＡタイプ割れの長さの平均値によって行い、上記軸心部割れ長さが５ｍｍ以下の場合を合格とした。これは、本発明者の知見によれば、造管条件により多少の差は生じるものの、ビレット軸心部の割れ長さが５mm以下に抑制できれば、造管後の製品の内面カブレ欠陥は大幅に低減できることが経験的に確認されているためである。表４において、本発明例では割れ長さが５mm以下に抑制できており、また、ビレット曲がりも発生していないことが確認できている。Cタイプ割れについては、図２（ｃ）に示すＣタイプ割れの有無及びその差渡し長さによって評価した。鋳片曲がりについては、図５に示す曲がり量によって評価した。
The average length of A-type axial center cracks (including A-type cracks during recuperation) and C-type cracks of the obtained product round cast slab, and the degree of slab bending were evaluated. The evaluation results are also shown in Table 4. In addition, the length of A type crack means the length (mm) of the crack extended from a shrink hole as shown in FIG. 4 , and evaluation is A type crack observed in the test piece cross section of many round cast pieces. The average value of the lengths of the shafts was determined, and a case where the axial center crack length was 5 mm or less was regarded as acceptable. According to the knowledge of the present inventor, although there is a slight difference depending on the pipe making conditions, if the crack length of the billet shaft center part can be suppressed to 5 mm or less, the inner surface blurring defect of the product after pipe making is greatly reduced. This is because it has been empirically confirmed that this can be reduced. In Table 4, it can be confirmed that in the example of the present invention, the crack length can be suppressed to 5 mm or less, and no billet bending occurs. About the C type crack, it evaluated by the presence or absence of the C type crack shown in FIG.2 (c), and its passing length. The slab bending was evaluated based on the bending amount shown in FIG.