JP4508087B2 - Continuous casting method and continuous cast slab - Google Patents

Continuous casting method and continuous cast slab Download PDF

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JP4508087B2
JP4508087B2 JP2005332768A JP2005332768A JP4508087B2 JP 4508087 B2 JP4508087 B2 JP 4508087B2 JP 2005332768 A JP2005332768 A JP 2005332768A JP 2005332768 A JP2005332768 A JP 2005332768A JP 4508087 B2 JP4508087 B2 JP 4508087B2
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章裕 山中
正 平城
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Sumitomo Metal Industries Ltd
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本発明は、耐水素誘起割れ性能に優れた鋼板製造用鋳片の連続鋳造方法およびその連続鋳造方法により鋳造される鋼板製造用の連続鋳造鋳片に関する。   TECHNICAL FIELD The present invention relates to a continuous casting method for steel plate production slabs excellent in hydrogen-induced cracking resistance and a continuous casting slab for steel plate production cast by the continuous casting method.

鋼の連続鋳造鋳片において、鋳片厚さ方向の中心部にC、S、P、Mnといった不純物成分や合金成分が濃縮した偏析帯である中心偏析、または上記の成分が等軸晶間に濃縮して存在する粒状の偏析は、厚板製品における機械特性低下の原因となる重大な鋳造欠陥の一つである。特に、硫化水素を含む原油や天然ガスなどの輸送に使用されるラインパイプなどでは、このような偏析が残っていると水素集積のサイトとなり、しばしば水素誘起割れ(以降、「HIC」とも称する)の起点となる。これらの欠陥は、鋳造末期の未凝固相における残溶鋼が凝固するときに収縮して負圧状態となり、デンドライト樹間に微細に濃化したミクロ偏析をともなう溶鋼が吸い出されてデンドライト樹間から流出し、局所的に凝固組織が凝着した閉空間内に集積して凝固することにより、マクロ的な偏析となるものである。   In continuous cast slabs of steel, central segregation, which is a segregation band in which impurity components such as C, S, P, and Mn and alloy components are concentrated in the center of the slab thickness direction, or the above components are between equiaxed crystals Concentrated granular segregation is one of the serious casting defects that cause deterioration of mechanical properties in thick plate products. In particular, in the case of line pipes used for transportation of crude oil and natural gas containing hydrogen sulfide, if such segregation remains, it becomes a site of hydrogen accumulation, often hydrogen induced cracking (hereinafter also referred to as “HIC”). Is the starting point. These defects are caused by shrinkage when the residual molten steel in the unsolidified phase at the end of casting solidifies, resulting in a negative pressure state. Molten steel with fine segregation between the dendritic trees is sucked out from between the dendritic trees. It flows out and accumulates in the closed space where the solidified tissue locally adheres and solidifies, resulting in macroscopic segregation.

従来より、鋳片の凝固組織の制御や凝固末期に鋳片の表面から厚さ方向に機械的な圧下を与えることにより、これらのマクロ偏析による欠陥を低減しようとする方法が開示されている。   Conventionally, there has been disclosed a method for reducing defects due to macrosegregation by controlling the solidification structure of the slab and applying mechanical reduction in the thickness direction from the surface of the slab at the end of solidification.

例えば、特許文献1においては、モールドと鋳片の液相線クレータエンドとの凝固シェルに積極的にバルジング力を作用させて鋳片内未凝固層の厚さを増大させ、次いで液相線クレータエンドと固相線クレータエンドとの間の鋳片に圧下を加え中心偏析の発生を低減させることを特徴とする連続鋳造方法が開示されている。圧下を加える理由は、鋳片内部の凝固収縮に対して、外部より圧下を加えることで、上述したマクロ偏析の根本的な原因を除去しようというものである。鋳片内部の凝固収縮を補償する程度の圧下を加える鋳片の未凝固圧下法は、「軽圧下法」と称され、連続鋳造分野において広く適用されている。   For example, in Patent Document 1, a bulging force is positively applied to a solidified shell between a mold and a liquid phase crater end of a slab to increase the thickness of an unsolidified layer in the slab, and then the liquid phase crater A continuous casting method characterized by reducing the occurrence of central segregation by reducing the slab between the end and the solidus crater end is disclosed. The reason for applying the reduction is to remove the root cause of the macrosegregation described above by applying the reduction from the outside to the solidification shrinkage inside the slab. The unsolidified reduction method of a slab that applies a reduction that compensates for solidification shrinkage inside the slab is called a “light reduction method” and is widely applied in the field of continuous casting.

マクロ偏析欠陥の低減に対しては、上述したような従来の軽圧下方法によって、少なからぬ効果が得られたが、昨今の製品品質への高度な要求を考慮すると、従来の軽圧下方法では、その効果は不十分と言わざるを得ない。   For the reduction of macro segregation defects, a considerable effect was obtained by the conventional light reduction method as described above, but considering the recent high demands on product quality, in the conventional light reduction method, It must be said that the effect is insufficient.

近年、ますます過酷なサワー環境に耐えられる高強度および高靱性を有する耐HIC(耐水素誘起割れ)性能に優れた鋼板の製造が要求されつつある。従来、HICの主な原因は、中心偏析などのマクロ偏析およびセミマクロ偏析とされ、これらの偏析を低減することにより耐HIC性能の向上が図られると考えられてきた。しかしながら、前記のような高性能の耐HIC性能を備えた鋼板製造の要求に応じるためには、鋳片の中心部において発生するセンターポロシティをも低減する必要のあることが明らかとなってきた。すなわち、センターポロシティの程度が大きい場合には、鋳片を圧延してもセンターポロシティの痕跡を完全に消し去ることができずに、サワー環境下においてその痕跡部分が水素の集積サイトとなりやすいからである。   In recent years, there has been a demand for the production of steel sheets having high strength and high toughness that can withstand increasingly severe sour environments and excellent in HIC (hydrogen induced cracking resistance) performance. Conventionally, the main causes of HIC are macro-segregation such as center segregation and semi-macro segregation, and it has been considered that the anti-HIC performance can be improved by reducing these segregation. However, it has become clear that it is necessary to reduce the center porosity generated in the center portion of the slab in order to meet the demand for manufacturing a steel plate having such high-performance HIC resistance as described above. That is, when the degree of center porosity is large, the trace of the center porosity cannot be completely erased even if the slab is rolled, and the trace portion tends to be a hydrogen accumulation site in a sour environment. is there.

上記の水素の集積サイトの形成は、具体的には下記の機構によると推察される。つまり、圧延を経た後においてもセンターポロシティが完全には圧着されずに鋼板内で微小な空隙となって残存し、これが水素の集積サイトになると推察される。あるいは、鋳片の凝固途上でポロシティの形成時に界面張力のバランスにより非金属介在物粒子がポロシティの界面に点状に突出した形態で集積し、そのまま鋳片が凝固することにより鋼板に至るまでこの形態が残存することから、この集積介在物とマトリックスとの境界が水素の集積サイトになるとも推察される。   The formation of the hydrogen accumulation site is speculated to be specifically due to the following mechanism. That is, it is assumed that even after rolling, the center porosity is not completely pressed and remains as fine voids in the steel sheet, which becomes a hydrogen accumulation site. Alternatively, during the solidification of the slab, non-metallic inclusion particles are accumulated in a form protruding in the form of dots at the interface of the porosity due to the balance of interfacial tension when the porosity is formed. Since the form remains, it is assumed that the boundary between the accumulation inclusion and the matrix becomes a hydrogen accumulation site.

上記のセンターポロシティは、鋳片の中心部が完全に凝固するときの凝固収縮により形成される。しかしながら、この凝固収縮量を補償する程度の圧下量しか付与しない軽圧下法のみでは、センターポロシティ改善効果が極めて小さく、したがって、高性能の耐HIC性能を備えた鋼板製造の要求に応じることはできなかった。   The center porosity is formed by solidification shrinkage when the center portion of the slab is completely solidified. However, only the light reduction method that provides only a reduction amount that compensates for the amount of solidification shrinkage has a very small effect on improving the center porosity. Therefore, it is not possible to meet the demand for manufacturing a steel plate with high-performance HIC resistance. There wasn't.

特公昭 62−34461号公報(特許請求の範囲および第2欄23行〜第3欄4行)Japanese Patent Publication No. 62-34461 (Claims and column 2, line 23 to column 3, line 4)

本発明は、上記の問題を解決するためになされたものであり、その課題は、センターポロシティならびにマクロ偏析およびセミマクロ偏析の発生を同時に低減した高性能の耐HIC特性を有する鋼板製造用スラブの連続鋳造方法およびその方法を用いた連続鋳造鋳片を提供することにある。   The present invention has been made to solve the above-mentioned problems, and the problem is that a slab for manufacturing a steel sheet having high-performance HIC resistance that simultaneously reduces the center porosity and the occurrence of macro-segregation and semi-macro segregation. It is providing the casting method and the continuous casting slab using the method.

本発明者は、上述の課題を解決するために、従来の問題点を踏まえて、高性能の耐HIC鋼板製造用スラブの連続鋳造方法および連続鋳造鋳片について調査および検討を行い、下記の(a)〜(e)の知見を得て、本発明を完成させた。   In order to solve the above-mentioned problems, the present inventors have investigated and studied a continuous casting method and a continuous cast slab of a slab for producing a high-performance HIC-resistant steel sheet based on the conventional problems. The knowledge of a) to (e) was obtained and the present invention was completed.

(a)鋳片厚さを鋳造方向に対してテーパ状に圧下する方法によりマクロ偏析を改善することによって、鋼板のHICの発生率を極めて低い値とすることはできるが、なお、若干のHICが発生することは避けられない。   (A) By improving the macro segregation by a method of reducing the thickness of the slab in a taper shape with respect to the casting direction, the HIC generation rate of the steel sheet can be reduced to a very low value. It is inevitable that this will occur.

(b)鋳片のセンターポロシティの発生程度を一定の水準以下まで低下させることにより、上記(a)にて述べた若干のHICの発生率を明確に低下させることができる。   (B) By reducing the generation degree of the center porosity of the slab to a certain level or less, the slight HIC generation rate described in the above (a) can be clearly reduced.

(c)センターポロシティは、鋳片のマクロ偏析をある程度改善した後、鋳片中心部が完全に凝固する直前にその厚さ方向に数mmの圧下を与えることにより、改善することができる。鋳片のマクロ偏析改善のためのテーパ状圧下と完全凝固直前の圧下には、相互に関連する適正範囲が存在する。   (C) The center porosity can be improved by reducing the macro segregation of the slab to some extent and then applying a reduction of several millimeters in the thickness direction immediately before the center of the slab is completely solidified. There is a proper range related to taper reduction for improving macrosegregation of a slab and reduction immediately before complete solidification.

(d)すなわち、鋳片凝固シェル厚さが60mmになるまでの間に鋳片厚さ方向に3〜10mmのバルジングをさせた後、中心固相率が0.8未満の位置において、鋳片をテーパ状のロールアライメントにより鋳片厚さ方向に圧下し、中心固相率が0.8〜0.95の範囲内の位置において、1段の圧下ロール対を用いて鋳片厚さ方向に5〜10mmの範囲内で、段差状に圧下することにより、鋳片のセンターポロシティを低減できる。   (D) That is, after bulging 3 to 10 mm in the slab thickness direction until the slab solidified shell thickness reaches 60 mm, the slab is at a position where the central solid phase ratio is less than 0.8. Is reduced in the slab thickness direction by taper roll alignment, and in the slab thickness direction using a single-stage reduction roll pair at a position where the central solid phase ratio is in the range of 0.8 to 0.95. The center porosity of the slab can be reduced by reducing the stepped shape within a range of 5 to 10 mm.

(e)上記(d)の方法によって、鋳片の厚さ中心部のポロシティ体積を2×10-4(cm3/g)以下に減じることにより、当該鋳片から得られた鋼板の耐HIC性能は格段に向上する。 (E) By reducing the porosity volume at the center of the slab thickness to 2 × 10 −4 (cm 3 / g) or less by the method of (d) above, the HIC resistance of the steel sheet obtained from the slab Performance is greatly improved.

本発明は、上記の知見に基づいて完成されたものであり、その要旨は、下記の(1)に示す連続鋳造方法および(2)に示す鋳片にある。   The present invention has been completed based on the above findings, and the gist thereof resides in the continuous casting method shown in the following (1) and the slab shown in (2).

(1)鋳片の片側の凝固シェル厚さが60mm以下の鋳片位置において、鋳片厚さを鋳片幅中央部で3〜10mmの範囲内でバルジングさせた後、該バルジングさせた状態を維持しつつ、厚さ中心部に固相が生成する鋳片位置から、厚さ中心部における中心固相率が0.8未満の鋳片位置までの間の鋳片を、バルジングさせた厚さの範囲内で鋳片の鋳造方向に対してテーパ状に圧下し、引き続き、厚さ中心部における中心固相率が0.8〜0.95の範囲内の鋳片位置において、鋳片厚さ方向に5〜10mmの範囲内で、1段の圧下ロール対を用いて段差状に圧下することにより、厚さ中心部におけるポロシティ体積が0.8×10 -4 〜2×10 -4 (cm 3 /g)で、かつ厚さ中心部におけるMnの偏析比が1.10〜1.25である鋳片を製造することを特徴とする耐水素誘起割れ性能に優れた鋼板製造用鋳片の連続鋳造方法(以下、「第1発明」とも記す)。
(1) At the slab position where the solidified shell thickness on one side of the slab is 60 mm or less, the slab thickness is bulged within a range of 3 to 10 mm at the center of the slab width, and then the bulging state is performed. Thickness of bulging the slab from the slab position where the solid phase is generated in the thickness center to the slab position where the central solid fraction is less than 0.8 in the thickness center while maintaining The slab thickness is reduced in a taper direction with respect to the casting direction of the slab within the range of slab, and subsequently, at the slab position where the central solid fraction in the thickness center is in the range of 0.8 to 0.95. The porosity volume at the center of the thickness is 0.8 × 10 −4 to 2 × 10 −4 (cm) by using a pair of rolling rolls in a step within a range of 5 to 10 mm in the direction. in 3 / g), and the slab segregation ratio is 1.10 to 1.25 of Mn in the thickness center portion Resistance to hydrogen induced cracking performance superior continuous casting method of manufacturing a steel sheet for insert piece, characterized by forming (hereinafter, referred to as "first invention").

(2)前記(1)に記載の連続鋳造方法により鋳造された鋳片であって、鋳片の厚さ中心部におけるポロシティ体積が0.8×10 -4 2×10-4(cm3/g)で、かつ鋳片の厚さ中心部におけるMnの偏析比が1.10〜1.25であり、NACE T0284に規定された試験法に準拠して、圧延後の鋼板における水素誘起割れの面積率が0.5%以下であることを特徴とする鋼板製造用の連続鋳造鋳片(以下、「第2発明」とも記す)。
(2) A slab cast by the continuous casting method according to (1), wherein a porosity volume at a thickness central portion of the slab is 0.8 × 10 −4 to 2 × 10 −4 (cm 3). / G) and the segregation ratio of Mn at the center of the slab thickness is 1.10 to 1.25, and in accordance with the test method specified in NACE T0284, hydrogen-induced cracking in the steel sheet after rolling A continuous cast slab for producing a steel sheet (hereinafter, also referred to as “second invention”) , wherein the area ratio is 0.5% or less .

本発明において、「中心固相率」とは、鋳片中心部において、固相および液相が占める全領域に対して固相が占める領域の分率をいう。   In the present invention, the “center solid phase ratio” means the fraction of the area occupied by the solid phase with respect to the entire area occupied by the solid phase and the liquid phase in the center of the slab.

「テーパ状に圧下する」とは、ロール対のロールギャプが鋳造方向に対して所定の勾配を有して減少するように配置された複数のロール対により鋳片を圧下することを意味する。   “To taper down” means to roll down the slab by a plurality of roll pairs arranged so that the roll gap of the roll pair decreases with a predetermined gradient with respect to the casting direction.

また、「1段の圧下ロール対」とは、鋳造方向に対して1組のロール対を有する圧下ロール対をいい、「段差状に圧下する」とは、上記の1組のロール対を用いて鋳片厚さが1段のステップ状に減少するように圧下することを意味する。   In addition, “one-stage reduction roll pair” refers to a reduction roll pair having one pair of roll pairs in the casting direction, and “to reduce in a stepped manner” uses the one set of roll pairs described above. This means that the slab thickness is reduced so as to decrease in a single step.

そして、「ポロシティ体積」とは、後に詳述する方法により求められる鋳片単位質量当たりのポロシティの体積のうち、その最大値を意味する。   And "porosity volume" means the maximum value among the volumes of porosity per slab unit mass determined by the method described in detail later.

本発明の連続鋳造方法によれば、センターポロシティならびにマクロ偏析およびセミマクロ偏析を著しく低減したスラブ鋳片を鋳造することができ、前記鋳片を素材とする耐HIC性能に優れた鋼板を製造することが可能となる。また、本発明の連続鋳造鋳片は、ラインパイプなどに用いられる耐HIC性能に優れた鋼板の製造に最適である。   According to the continuous casting method of the present invention, it is possible to cast a slab slab with significantly reduced center porosity, macro segregation and semi-macro segregation, and to produce a steel sheet having excellent HIC resistance using the slab as a raw material. Is possible. Moreover, the continuous cast slab of the present invention is most suitable for the production of a steel plate having excellent HIC resistance used for a line pipe or the like.

本発明は、前記のとおり、片側の凝固シェル厚さが60mm以下の鋳片位置において、鋳片厚さを3〜10mmの範囲内でバルジングさせた後、その状態を維持しつつ、厚さ中心部に固相が生成する位置から、厚さ中心部における中心固相率が0.8未満の位置までの間の鋳片を、バルジング厚さの範囲内で鋳造方向にテーパ状に圧下し、中心固相率が0.8〜0.95の範囲内の位置において、鋳片厚さ方向に5〜10mmの範囲内で、1段の圧下ロール対を用いて段差状に圧下する鋼板製造用鋳片の連続鋳造方法、および前記の連続鋳造方法により鋳造された鋳片であって、厚さ中心部におけるポロシティ体積が2×10-4(cm3/g)以下である連続鋳造鋳片である。以下に、本発明を前記の範囲に限定した理由および好ましい範囲について述べる。 As described above, in the present invention, the slab thickness of 60 mm or less on one side is bulged within a range of 3 to 10 mm, and the thickness center is maintained while maintaining the state. The slab between the position where the solid phase is generated in the part and the position where the central solid fraction in the thickness center is less than 0.8 is tapered in the casting direction within the range of the bulging thickness, For steel sheet production where the center solid phase ratio is reduced to a step using a pair of reduction rolls within a range of 5 to 10 mm in the slab thickness direction at a position within the range of 0.8 to 0.95. A slab continuous casting method, and a slab cast by the above-described continuous casting method, wherein the porosity volume at the center of the thickness is 2 × 10 −4 (cm 3 / g) or less. is there. The reason why the present invention is limited to the above range and the preferable range will be described below.

(1)バルジングの付与位置およびバルジング量
鋳片にバルジングを付与する理由は、前記特許文献1と同じく、スラブ鋳片の幅方向端部の圧下を回避し、端部以外の未凝固部を含む鋳片幅方向全体を効果的にテーパ状に圧下、あるいは完全凝固直前に段差状に圧下するためである。
(1) Bulging application position and bulging amount The reason for applying bulging to the slab is to avoid the reduction of the width direction end of the slab slab, including the unsolidified portion other than the end, as in the above-mentioned Patent Document 1. This is because the entire slab width direction is effectively reduced in a taper shape or in a step shape just before complete solidification.

鋳片の凝固シェル厚さが鋳片の片側で60mmを超えた条件でバルジングを行わせると、鋳片の短辺側の凝固シェル界面において、バルジングによる曲げ歪により微細な内部割れ(すなわち、凝固界面が一旦割れ、その割れた部分にミクロ偏析を伴った溶鋼を吸い込み、凝固した組織)が発生するので、凝固シェル厚さが60mmに達するまでの間にバルジングを開始させる必要があることが判明した。また、この微細な内部割れの存在も、鋼板のHICの発生原因になることが判明した。そこで、鋳片の片側の凝固シェル厚さが60mmになるまでの間にバルジングを開始させることとした。   When bulging is performed under the condition that the solidified shell thickness of the slab exceeds 60 mm on one side of the slab, fine internal cracks (that is, solidification) occur due to bending strain due to bulging at the solidified shell interface on the short side of the slab. It was found that the bulging needs to be started before the solidified shell thickness reaches 60 mm because the interface cracks once, and molten steel with microsegregation is sucked into the cracked part and a solidified structure is generated. did. It has also been found that the presence of such fine internal cracks causes the generation of HIC in the steel sheet. Therefore, bulging was started before the solidified shell thickness on one side of the slab reached 60 mm.

バルジング量の適正範囲を3〜10mmとした理由は、バルジング量が3mm未満では、バルジングした鋳片を圧下することによる効果が十分ではなく、一方、バルジング量が10mmを超えて大きくなると、凝固シェル厚みが60mm以下の場合であっても、鋳片の短辺側凝固シェル界面における内部割れ発生の危険性が増すからである。   The reason why the appropriate range of the bulging amount is 3 to 10 mm is that if the bulging amount is less than 3 mm, the effect of reducing the bulging slab is not sufficient, whereas if the bulging amount exceeds 10 mm, the solidified shell This is because even when the thickness is 60 mm or less, the risk of occurrence of internal cracks at the short-side solidified shell interface of the slab increases.

(2)テーパ状圧下の鋳片位置および圧下量
テーパ状圧下は、一般に行われているように、鋳片中心部が凝固し始める鋳片位置、すなわち、中心部に固相が生成する鋳片位置(または時期)から完全に凝固する鋳片位置(または時期)までの間を連続して微小なテーパにより実施するのがよい。そこで、テーパ状圧下の適正開始位置を、中心部に固相が生成する鋳片位置と規定した。さらに、鋳片の中心固相率が0.02程度になると、鋳片の上側および下側の凝固シェルがブリッジングを開始し始め、凝固収縮の影響が顕著になってくるので、この鋳片位置以降からテーパ状圧下を開始するのが好ましい。また、ブリッジングが大きくなる、中心固相率が0.3以上の鋳片位置からテーパ状圧下を開始すれば、さらに一層好ましい。
(2) Cast slab position and amount of taper reduction Taper reduction is a slab position where the center of the slab begins to solidify, that is, a slab where a solid phase is generated at the center, as is generally done. It is preferable to carry out continuously from the position (or time) to the position (or time) of the slab where it completely solidifies with a small taper. Therefore, the appropriate starting position under the taper-shaped reduction is defined as the slab position where a solid phase is generated at the center. Further, when the center solid phase ratio of the slab becomes about 0.02, the solidified shells on the upper side and the lower side of the slab start to start bridging, and the influence of solidification shrinkage becomes remarkable. It is preferable to start the tapered reduction from the position onward. Further, it is even more preferable that the taper-shaped reduction is started from a slab position where bridging is large and the central solid phase ratio is 0.3 or more.

テーパ状圧下の終了位置は、後述する鋳片の完全凝固直前における段差状圧下の適正範囲を考慮して、中心固相率で0.8未満の鋳片位置とした。溶鋼が流動できる最大の固相率は0.8と考えられており、この固相率では凝固収縮を駆動力とする偏析成分の濃化した残溶鋼の流動も、ほぼ停止状態となる。そこで、さらに下流側における段差状圧下の領域の確保も考慮して、テーパ状圧下の終了位置を中心固相率が0.8未満の位置と規定した。テーパ状圧下の終了位置をさらに中心固相率の低い位置としても偏析改善は可能であるが、少なくとも中心固相率が0.6程度の鋳片位置まではテーパ状圧下を継続するのが好ましい。   The end position of the taper-shaped reduction was set to a slab position having a central solid phase ratio of less than 0.8 in consideration of an appropriate range of step-like reduction immediately before complete solidification of the slab described later. The maximum solid phase rate at which the molten steel can flow is considered to be 0.8. At this solid phase rate, the flow of the residual molten steel enriched with segregation components whose solidification shrinkage is the driving force is almost stopped. Therefore, in consideration of securing a step-like reduction region on the downstream side, the end position of the taper-like reduction is defined as a position where the central solid fraction is less than 0.8. Although the segregation can be improved even if the end position of the taper-shaped reduction is further reduced to a position where the central solid phase ratio is low, it is preferable to continue the taper-shaped reduction at least until the slab position where the central solid ratio is about 0.6. .

テーパ状圧下におけるテーパ量は、鋳造方向1m当たり、0.8〜2.0mmとするのが好ましい。テーパ量を鋳造方向1m当たり0.8mm未満にすると、凝固収縮量の補償が不十分となり、偏析が悪化するおそれがある。一方、テーパ量が鋳造方向1m当たり2.0mmを超えて大きくなると、圧下量が凝固収縮量を超えて大きくなり、偏析成分の濃化した残溶鋼の逆流が起こり、逆V偏析を生じるおそれがある。   The taper amount under the taper pressure is preferably 0.8 to 2.0 mm per 1 m in the casting direction. When the taper amount is less than 0.8 mm per 1 m in the casting direction, compensation of the solidification shrinkage amount becomes insufficient and segregation may be deteriorated. On the other hand, if the taper amount becomes larger than 2.0 mm per 1 m in the casting direction, the reduction amount becomes larger than the solidification shrinkage amount, and the reverse flow of the residual molten steel concentrated in the segregation component may occur, which may cause reverse V segregation. is there.

(3)段差状圧下の鋳片位置および圧下量
テーパ状圧下に引き続き、一対のロール対を用いて、中心固相率が0.8〜0.95の範囲内の鋳片位置において、段差状に圧下量5〜10mmの範囲内で圧下を加える。その理由は下記のとおりである。すなわち、上記(2)のテーパ状圧下を行っても、鋳片の凝固収縮量を完全には補償できず、その結果、凝固末期に、凝固収縮による収縮孔としてセンターポロシティが形成される。このタイミングにおいて、鋳片に、一本の圧下ロール当たりやや大きめの圧下量を与えて、鋳片内部に形成されつつある収縮孔を圧着させるのが極めて有効だからである。つまり、この段階では、鋳片中心部に凝固潜熱が残っているために、中心部の温度は融点近くの高温に維持されている。そのため、鋳片表層部と中心部との温度差が大きく、相対的に表層部よりも内部の方が柔らかいので、圧下による変形は鋳片の中心部の方に集中し易いからである。
(3) Casting slab position and amount of step-like reduction Step following the taper-like reduction, using a pair of rolls, at the slab position where the central solid fraction is in the range of 0.8 to 0.95 The reduction is applied within the range of the reduction amount of 5 to 10 mm. The reason is as follows. That is, even if the taper-shaped reduction of (2) is performed, the solidification shrinkage amount of the slab cannot be completely compensated, and as a result, a center porosity is formed as a contraction hole due to solidification shrinkage at the end of solidification. This is because, at this timing, it is extremely effective to give the slab a slightly larger amount of reduction per one squeeze roll and press the shrinkage hole formed inside the slab. That is, at this stage, since solidification latent heat remains in the center part of the slab, the temperature of the center part is maintained at a high temperature near the melting point. Therefore, the temperature difference between the slab surface layer portion and the center portion is large, and the inside is relatively softer than the surface layer portion, so that deformation due to rolling is likely to concentrate on the center portion of the slab.

ただし、中心固相率が0.8未満の場合に、段差状圧下量が大きすぎると、偏析成分の濃化した流動性を保有する残溶鋼を圧下ロール対の上流側に逆流させることになる。この場合、残溶鋼の排出による逆流量は、前記した逆V偏析発生の原因となる過度のテーパ状圧下による残溶鋼の逆流量よりも多量となり、ロール対の上流側に数mm以上の塊状の偏析が一旦形成され、場合によっては、これが完全凝固まで残存ずる可能性がある。一方、中心固相が0.95を超えて大きい鋳片位置では、凝固完了にともないセンターポロシティの形成も終了し、圧下の効果は低減する。そこで、段差状圧下を行う鋳片の適正位置を、中心固相率が0.8〜0.95の鋳片位置とした。   However, when the central solid phase ratio is less than 0.8, if the step-like reduction amount is too large, the residual molten steel having fluidity concentrated in the segregation component is caused to flow backward to the upstream side of the reduction roll pair. . In this case, the reverse flow rate due to the discharge of the residual molten steel is larger than the reverse flow rate of the residual molten steel due to excessive taper reduction that causes the occurrence of reverse V segregation, and a lump of several mm or more is formed upstream of the roll pair. Segregation is formed once, and in some cases, it may remain until complete solidification. On the other hand, at the slab position where the central solid phase exceeds 0.95 and the solidification is completed, the formation of the center porosity is completed and the reduction effect is reduced. Therefore, the appropriate position of the slab where the step-like reduction is performed is the slab position having a central solid phase ratio of 0.8 to 0.95.

段差状圧下による圧下量が5mm以上において、センターポロシティの低減効果が得られ、圧下量の増加にともなってセンターポロシティの低減効果は増大する。しかし、圧下量が10mmを超える範囲では、その低減効果はほぼ飽和する。そこで、圧下量の適正範囲を5〜10mmとした。   When the reduction amount due to the step-like reduction is 5 mm or more, a center porosity reduction effect is obtained, and the center porosity reduction effect increases as the reduction amount increases. However, in the range where the amount of reduction exceeds 10 mm, the reduction effect is almost saturated. Therefore, the appropriate range of the amount of reduction was set to 5 to 10 mm.

なお、鋳片の圧下時における中心固相率およびバルジング開始時の凝固シェル厚さは、非定常伝熱解析によって求めることができる。この解析方法が本発明の実施に十分に使用できる精度を有するものであることは、未凝固鋳片内部への打鋲試験や鋳片表面温度の測定などの通常用いられる方法により確認を行った。   In addition, the center solid phase ratio at the time of slab reduction and the solidified shell thickness at the start of bulging can be obtained by unsteady heat transfer analysis. That this analysis method has sufficient accuracy to be used in the practice of the present invention was confirmed by a commonly used method such as a hammering test into an unsolidified slab and measurement of the slab surface temperature. .

(4)ポロシティ体積
上記のようにして得られた鋳片は偏析が低減されており、かつ鋳片のセンターポロシティも極めて低減されているが、偏析状態が良好であっても、センターポロシティが低減されずに不良な場合には、その鋳片を素材として製造された鋼板においてHICが発生することが判明した。鋳片の厚さ中心部のポロシティを下記のとおり定量化した結果、中心部のポロシティ体積を2×10-4(cm3/g)以下に減じることにより、鋼板のHICの発生を抑制して低位に維持できることが判明した。そこで、第2発明では、鋳片の厚さ中心部のポロシティ体積を2×10-4(cm3/g)以下と規定した。
(4) Porosity volume Although the slab obtained as described above has reduced segregation and the center porosity of the slab is also extremely reduced, the center porosity is reduced even if the segregation state is good. It was proved that HIC was generated in a steel plate manufactured using the cast slab as a raw material when it was not good. As a result of quantifying the porosity of the center part of the slab as follows, the porosity volume of the center part is reduced to 2 × 10 −4 (cm 3 / g) or less, thereby suppressing the occurrence of HIC in the steel sheet. It was found that it can be maintained at a low level. Therefore, in the second invention, the porosity volume at the center of the slab thickness is defined as 2 × 10 −4 (cm 3 / g) or less.

ここで、ポロシティ体積は下記の方法により求める。ポロシティの体積をPv(cm3/g)、同じ鋳片の1/4厚さ部(鋳片の厚さ方向1/4の位置の部分)の代表サンプルの密度をρ0、鋳片中心部のサンプル密度をρとすると、ポロシティの体積は、Pv=1/ρ−1/ρ0(cm3/g)により算出することができる。本発明では、後述するとおり鋳片の複数箇所から採取したサンプルについて、上記の方法によりポロシティの体積(Pv)を求め、これらの最大値を「ポロシティ体積(Pvmax)」として使用した。 Here, the porosity volume is determined by the following method. The volume of the porosity is Pv (cm 3 / g), the density of a representative sample of a ¼ thickness portion of the same slab (portion at a ¼ thickness direction of the slab) is ρ 0 , and the center portion of the slab When the sample density of ρ is ρ, the volume of porosity can be calculated by Pv = 1 / ρ−1 / ρ 0 (cm 3 / g). In the present invention, as described later, the porosity (Pv) of the porosity was obtained by the above method for samples collected from a plurality of locations of the slab, and these maximum values were used as “porosity volume (Pvmax)”.

この場合、サンプルの大きさは、30mm×30mm×30mm以内の立方体とするのがよい。サンプルサイズが大きすぎると、元来、ポロシティのほとんど含まれない、鋳片中心部から外れた部分までがサンプル中に含まれ、中心部のポロシティの検出感度が鈍くなってしまう。同じ鋳片の1/4厚さ部の代表サンプルの密度ρ0を基準として選定した理由は、例えば顕微鏡による検査などによれば、当該部分ではほとんどポロシティは検出されず、したがって、この部分の鋳片の密度は、素材本来の密度に等しいとすることができるからである。 In this case, the size of the sample is preferably a cube within 30 mm × 30 mm × 30 mm. If the sample size is too large, the sample originally includes almost no porosity, and even a portion off the center of the slab is included in the sample, so that the sensitivity of detecting the porosity at the center becomes dull. The reason why the density ρ 0 of the representative sample of the ¼ thickness portion of the same slab is selected as a reference is that, for example, according to inspection by a microscope, the porosity is hardly detected in the portion. This is because the density of the pieces can be equal to the original density of the material.

本発明の連続鋳造方法の効果を確認するため、下記の連続鋳造試験を行うとともに、得られた鋳片を鋼板に圧延し、その耐水素誘起割れ性能を評価した。   In order to confirm the effect of the continuous casting method of the present invention, the following continuous casting test was performed, and the obtained slab was rolled into a steel plate to evaluate its hydrogen-induced crack resistance.

(試験方法)
1)鋳造方法
図1は、本発明の連続鋳造方法を試験するために用いた垂直曲げ型の連続鋳造装置の例を示したものである。試験に用いた鋳型は、出側鋳片厚さが310mm、鋳片幅が2300mmの大きさのものを使用した。対象とした鋼種は、鋼成分組成が質量%にて、C:0.06〜0.08%、Si:0.18〜0.25%、Mn:1.5〜1.55%、P:0.007〜0.009%、S:0.0004〜0.0006%、Ti:0.015〜0.02%、Nb:0.03〜0.04%、Ca:0.002〜0.0025%の耐HIC鋼である。鋳造速度は0.60〜0.65m/minの範囲で種々変更した。また、二次冷却比水量は0.8〜1.5L/kg−steelとした。
(Test method)
1) Casting Method FIG. 1 shows an example of a vertical bending die continuous casting apparatus used for testing the continuous casting method of the present invention. As the mold used for the test, a mold having a casting slab thickness of 310 mm and a slab width of 2300 mm was used. The target steel types are steel components in mass%, C: 0.06 to 0.08%, Si: 0.18 to 0.25%, Mn: 1.5 to 1.55%, P: 0.007-0.009%, S: 0.0004-0.0006%, Ti: 0.015-0.02%, Nb: 0.03-0.04%, Ca: 0.002-0. 0025% HIC steel. The casting speed was variously changed in the range of 0.60 to 0.65 m / min. The secondary cooling specific water amount was 0.8 to 1.5 L / kg-steel.

また、バルジング開始時の凝固シェル厚さおよび圧下前の中心固相率は、鋳造速度および二次冷却比水量を変更することにより調整した。   Further, the thickness of the solidified shell at the start of bulging and the central solid phase ratio before the reduction were adjusted by changing the casting speed and the secondary cooling specific water amount.

タンディッシュから浸漬ノズル1を経て鋳型3に注入された溶鋼4は、鋳型4およびその下方の二次冷却スプレーノズル群(図示せず)から噴射されるスプレー水によって冷却され、凝固シェル5を形成して鋳片8となる。鋳片は、内部に未凝固部9を保持したまま、ガイドロール群6を経てピンチロール10により引き抜かれる。
完全凝固直前の段差状圧下に用いる強圧下用ロール対(以下、「圧下ロール対」または「圧下ロール」とも記す)7は、鋳型内の溶鋼メニスカス2より21m下流の位置に設置した。上記の圧下ロール対7を構成するロールの直径は450mmであり、圧下力は最大で5.88×106Nとした。また、この圧下ロールよりも上流のガイドロール群6を用いてテーパ状圧下を実施した。なお、試験に用いた連続鋳造装置は垂直曲げ型連続鋳造装置であるが、湾曲型連続鋳造装置を使用しても良いことは言うまでもない。
The molten steel 4 injected into the mold 3 from the tundish through the immersion nozzle 1 is cooled by spray water sprayed from the mold 4 and a group of secondary cooling spray nozzles (not shown) below the mold 4 to form a solidified shell 5. Thus, the slab 8 is obtained. The slab is pulled out by the pinch roll 10 through the guide roll group 6 while holding the unsolidified portion 9 inside.
A strong reduction roll pair (hereinafter also referred to as “reduction roll pair” or “reduction roll”) 7 used for stepwise reduction immediately before complete solidification was installed at a position 21 m downstream from the molten steel meniscus 2 in the mold. The diameter of the rolls constituting the above-mentioned rolling roll pair 7 was 450 mm, and the rolling force was 5.88 × 10 6 N at the maximum. Moreover, taper-shaped reduction was implemented using the guide roll group 6 upstream from this reduction roll. In addition, although the continuous casting apparatus used for the test is a vertical bending type continuous casting apparatus, it goes without saying that a curved type continuous casting apparatus may be used.

図1中の矢印B1−B2により示されるバルジングおよびテーパ状圧下ゾーンにおいては、ガイドロール群6は、鋳片の厚み方向の間隔を所定値に制御できるように配置されており、バルジングおよびテーパ状圧下のパスラインを付与することができる。   In the bulging and taper reduction zone indicated by arrows B1-B2 in FIG. 1, the guide roll group 6 is arranged so that the interval in the thickness direction of the slab can be controlled to a predetermined value. A rolling pass line can be provided.

圧下時の中心固相率は、主として鋳造速度、二次冷却強度(すなわち、二次冷却比水量)および鋳片幅中央部の厚さ(すなわち、鋳片のバルジング量)によって定まることから、鋳片のバルジング量に対して、鋳造速度を種々に変更して伝熱計算を行うことにより中心固相率を求めた。   The central solid fraction during rolling is mainly determined by the casting speed, the secondary cooling strength (ie, secondary cooling specific water amount) and the thickness of the center of the slab width (ie, the bulging amount of the slab). The central solid fraction was determined by performing heat transfer calculation with various casting speeds for the bulging amount of the piece.

また、タンディッシュ内の溶鋼の過熱度(ΔT)は、40℃〜50℃の間でほぼ一定とした。なお、ΔTは、溶鋼温度と液相線温度との差である。テーパ状圧下の総圧下量は、鋳造方向の鋳片圧下範囲とテーパ量により決定されるが、本発明の圧下範囲およびテーパ量の場合、最大で10mm程度である。   Moreover, the superheat degree ((DELTA) T) of the molten steel in a tundish was made substantially constant between 40 degreeC-50 degreeC. ΔT is the difference between the molten steel temperature and the liquidus temperature. The total amount of taper reduction is determined by the slab reduction range and taper amount in the casting direction, but in the case of the reduction range and taper amount of the present invention, it is about 10 mm at the maximum.

2)鋼板の耐HIC性能試験ならびにポロシティ体積および偏析比調査方法
長さ8mの鋳片を採取し、加熱炉にて1150℃に加熱した後、約800℃から圧延を開始し、約500℃にて圧延を完了する一般的な圧延条件により、厚さ20mm、幅2400mmの鋼板に圧延した。
2) HIC resistance test of steel plate and porosity volume and segregation ratio investigation method An 8 m long slab was collected and heated to 1150 ° C in a heating furnace, then rolled from about 800 ° C to about 500 ° C. The steel sheet was rolled into a steel plate having a thickness of 20 mm and a width of 2400 mm under general rolling conditions for completing the rolling.

得られた各鋼板の長手方向を4等分した各板の幅方向端部から1/4の位置(以下、「1/4幅の位置」とも記す)および幅方向中央部の位置の計3箇所の位置から、縦100mm×横100mm×厚さ20mmの全板厚試験片を採取し、NACE T0284に規定されたHIC試験法に準拠して、5質量%NaCl+0.5質量%CH3COOH+1気圧H2S飽和で温度25℃のNACE TM0177溶液中に96時間浸漬した。浸漬後の試験片に発生したHICによる割れの面積を超音波によるCスキャンにより測定して、試験片の全面積に占めるHICによる割れの面積率(以下、「CAR」とも記す)を求め、上記3箇所の平均値を算出した。 A total of 3 positions (hereinafter also referred to as “1/4 width position”) and a position in the center in the width direction from the width direction end of each plate obtained by dividing the longitudinal direction of each steel plate into four equal parts. From the position of the part, a total thickness test piece of length 100 mm × width 100 mm × thickness 20 mm was collected, and 5 mass% NaCl + 0.5 mass% CH 3 COOH + 1 atm in accordance with the HIC test method defined in NACE T0284 It was immersed for 96 hours in a NACE TM0177 solution at a temperature of 25 ° C. with H 2 S saturation. The area of cracks due to HIC generated in the test piece after immersion was measured by ultrasonic C-scan to determine the area ratio of cracks due to HIC (hereinafter also referred to as “CAR”) in the total area of the test piece. The average value of 3 places was calculated.

また、バルジング時の内部割れの影響を調査するために、各板の幅方向端部から板幅の1/10の位置(以下、「1/10幅の位置」とも記す)の2箇所においても上記CARを求め、これら2箇所の平均値を算出した。   In addition, in order to investigate the influence of internal cracks during bulging, it is also possible at two locations from the width direction end of each plate to 1/10 of the plate width (hereinafter also referred to as “1/10 width position”). The above CAR was obtained, and the average value of these two locations was calculated.

さらに、上記の鋳片からポロシティ体積および偏析比調査のためにサンプルを下記の方法により採取した。ポロシティ体積(Pvmax)調査用のサンプルは、連続鋳造の定常部の鋳片における横断面ブロックの厚さ中心部において鋳片幅方向に均等に15箇所の位置から採取した。サンプルの大きさは、横断面に平行な面を30mm×30mmとし、厚さ(鋳造方向)を20mmとした。同様に、基準密度(ρ0)測定用のサンプルとして、鋳片の幅方向中央の1/4厚さの位置から同サイズのサンプルを採取した。 Furthermore, a sample was taken from the above slab by the following method for examining the porosity volume and the segregation ratio. Samples for investigating the porosity volume (Pvmax) were collected from 15 positions evenly in the width direction of the slab at the center of the thickness of the cross section block in the slab of the continuous part of continuous casting. As for the size of the sample, the plane parallel to the transverse section was 30 mm × 30 mm, and the thickness (casting direction) was 20 mm. Similarly, as a sample for measuring the reference density (ρ 0 ), a sample of the same size was taken from the position of the 1/4 thickness at the center in the width direction of the slab.

密度は、それぞれのサンプルの質量と体積とから算出した。体積は、水中にサンプルを浸漬し、水中での重量を測定することにより浮力を求め、この浮力と水の密度とから算出した。これらの結果を用いて前記(4)にて述べた方法により、鋳片幅方向のポロシティの体積(Pv)を求め、さらに、その最大値であるポロシティ体積(Pvmax)を求めた。   The density was calculated from the mass and volume of each sample. The volume was calculated from the buoyancy and the density of water by immersing the sample in water and measuring the weight in water to obtain the buoyancy. Using these results, the volume (Pv) of porosity in the slab width direction was determined by the method described in (4) above, and the porosity volume (Pvmax), which was the maximum value, was determined.

偏析比の調査については、下記の方法により行った。鋳片横断面ブロックの厚さ中心部より、鋳片幅方向に100mmピッチで22箇所について、直径5mm×深さ3mmのドリル穴を開け、得られた切粉サンプルを用いてMn分析を行った。上記Mn分析値の算術平均値(C)を求め、母材濃度(鍋中濃度:C0)で除すことにより、偏析比(C/C0)を求めた。 The segregation ratio was investigated by the following method. Drill holes with a diameter of 5 mm and a depth of 3 mm were drilled at 22 locations at a pitch of 100 mm in the width direction of the slab from the thickness center of the slab cross-sectional block, and Mn analysis was performed using the obtained chip sample. . The segregation ratio (C / C 0 ) was obtained by obtaining the arithmetic average value (C) of the Mn analysis value and dividing by the base material concentration (concentration in the pan: C 0 ).

(試験結果)
表1および表2に、本発明の効果を確認するために行った一連の試験条件および試験結果を示した。
(Test results)
Tables 1 and 2 show a series of test conditions and test results performed to confirm the effects of the present invention.

同表において、試験番号H1〜H10は、本発明で規定する条件を全て満足する本発明例についての試験である。
上記の本発明例では、3種類の鋳造速度条件で、バルジング量(同表中に「BA」で示す)、バルジング時の凝固シェル厚さ(同表中に「BS」で示す)、段差状圧下時の中心固相率(fs)、および段差状圧下量を第1発明で規定する範囲内で変化させて鋳造を行い、得られた鋳片をさらに鋼板に圧延する試験を行った。また、このときのテーパ状圧下の圧下位置の範囲(中心固相率(fs)の範囲)、圧下テーパ量およびテーパ状圧下による総圧下量を併せて示した。
In the same table, test numbers H1 to H10 are tests for the inventive examples that satisfy all the conditions defined in the present invention.
In the above example of the present invention, the bulging amount (indicated by “BA” in the same table), the solidified shell thickness during bulging (indicated by “BS” in the same table), and stepped shape at three casting speed conditions Casting was performed by changing the central solid phase ratio (fs) during the reduction and the step-like reduction amount within the range specified in the first invention, and a test was performed in which the obtained slab was further rolled into a steel plate. In addition, the range of the reduction position under taper reduction (the range of the central solid phase ratio (fs)), the reduction taper amount, and the total reduction amount due to the taper reduction are also shown.

同表の結果によれば、第1発明で規定する条件を満足する試験番号H1〜H10では、いずれも、鋳片の厚さ中心部のポロシティ体積(Pvmax)が2.0×10-4(cm3/g)以下と低く、Mnの偏析比(C/C0)も1.25以下の範囲で低位安定しており、かつ、バルジング時の鋳片短辺部の内部割れの発生も見られない良好な品質の鋳片が得られた。さらに、圧延後の鋼板におけるHICの面積率(CAR)も0.5%以下と極めて低く、高性能の耐HIC特性を有する鋼板が得られたことが確認された。 According to the results in the table, in the test numbers H1 to H10 that satisfy the conditions specified in the first invention, the porosity volume (Pvmax) at the center of the slab thickness is 2.0 × 10 −4 ( cm 3 / g) or less, the Mn segregation ratio (C / C 0 ) is stable at a low level in the range of 1.25 or less, and the occurrence of internal cracks in the short side of the slab during bulging is also observed. An unsatisfactory slab of good quality was obtained. Furthermore, the area ratio (CAR) of HIC in the steel sheet after rolling was as extremely low as 0.5% or less, and it was confirmed that a steel sheet having high-performance HIC resistance was obtained.

これらに対して、試験番号C1〜C22は、第1発明で規定する条件のうち少なくとも1つを満足しない比較例についての試験である。   On the other hand, test numbers C1 to C22 are tests for comparative examples that do not satisfy at least one of the conditions defined in the first invention.

試験番号C1は、テーパ状圧下および凝固末期の段差状圧下以外は、試験番号H1と同じ条件とし、テーパ状圧下および凝固末期の段差状圧下をいずれも行わなかった比較例についての試験である。Mn偏析比および中心部のポロシティ体積の値はともに非常に高く、鋳片品質は劣ったものとなっている。その結果、鋼板のHIC面積率も15.3%と極度に高く、耐HIC性能の極めて劣った鋼板となった。   Test No. C1 is a test for a comparative example in which the same conditions as in Test No. H1 except for the taper reduction and the stepwise reduction at the end of coagulation were performed under the same conditions as in Test No. H1. Both the Mn segregation ratio and the porosity volume at the center are very high, and the slab quality is inferior. As a result, the HIC area ratio of the steel plate was extremely high at 15.3%, and the steel plate was extremely inferior in HIC resistance.

試験番号C2、C4およびC6は、試験番号H2、H4およびH8と同様の条件で、テーパ状圧下のみを行わなかった比較例についての試験である。ポロシティ体積の値はいずれも低く良好であったが、Mn偏析比の値が高く、鋼板の耐HIC性能も劣ったものとなった。   Test numbers C2, C4, and C6 are tests for a comparative example in which only taper reduction was not performed under the same conditions as test numbers H2, H4, and H8. Although the values of the porosity volume were all low and good, the value of the Mn segregation ratio was high, and the HIC resistance performance of the steel sheet was inferior.

試験番号C3、C5およびC7は、試験番号H1、H4およびH8と同様の条件で、凝固末期の段差状圧下を行わなかった比較例についての試験である。Mn偏析比の値は比較的低く良好であるが、ポロシティ体積の値が非常に高く、その結果、鋼板の耐HIC性能は劣ったものとなった。   Test numbers C3, C5, and C7 are tests for a comparative example in which stepwise reduction at the end of coagulation was not performed under the same conditions as test numbers H1, H4, and H8. The value of the Mn segregation ratio was relatively low and good, but the value of the porosity volume was very high. As a result, the HIC resistance of the steel sheet was inferior.

試験番号C8、C9およびC10は、試験番号H1、H5およびH9と同様の条件で、凝固末期の段差状圧下量を5mm未満とした比較例である。その結果、ポロシティ体積の値は2.0×10-4(cm3/g)よりも大きくなっており、鋼板の耐HIC性能はやや不良であった。 Test numbers C8, C9, and C10 are comparative examples in which the stepwise reduction amount at the end of coagulation is less than 5 mm under the same conditions as test numbers H1, H5, and H9. As a result, the value of the porosity volume was larger than 2.0 × 10 −4 (cm 3 / g), and the HIC resistance performance of the steel sheet was somewhat poor.

試験番号C11〜C16は、試験番号H1、H5およびH9と同様の条件で、凝固末期の段差状圧下のタイミング(鋳片の中心固相率)のみを変更した比較例についての試験である。試験番号C11、C13およびC15では、いずれも鋳片の中心固相率が0.8未満の鋳片位置において段差状圧下を行った。その結果、ポロシティ体積の値は低く良好であったが、偏析状況の悪い部分が散見され、Mn偏析比の値が高くなり、鋼板の耐HIC性能も不良となった。これは、偏析成分の濃化した残溶鋼が排出して上流側に逆流し、成分偏析となって残存したことによると推察される。   Test numbers C11 to C16 are tests for a comparative example in which only the timing of the step-like pressure reduction at the end of solidification (center solid phase ratio) is changed under the same conditions as in test numbers H1, H5, and H9. In test numbers C11, C13, and C15, stepwise reduction was performed at the slab position where the center solid phase ratio of the slab was less than 0.8. As a result, the value of the porosity volume was low and good, but there were some parts with poor segregation status, the value of Mn segregation ratio was high, and the HIC resistance performance of the steel sheet was also poor. This is presumably because the residual molten steel enriched with segregation components was discharged and flowed back upstream, and remained as component segregation.

一方、試験番号C12、C14およびC16では、いずれも鋳片の中心固相率が0.95を超えた鋳片位置において段差状圧下を行った。その結果、ポロシティ体積の値は2.0×10-4(cm3/g)よりも大きくなり、鋼板の耐HIC性能はやや不良であった。これは、先に述べたように、このように凝固が進行した状況下においては段差状圧下の効果が低減したためである。 On the other hand, in test numbers C12, C14, and C16, stepwise reduction was performed at the slab position where the center solid phase ratio of the slab exceeded 0.95. As a result, the value of the porosity volume was larger than 2.0 × 10 −4 (cm 3 / g), and the HIC resistance performance of the steel sheet was somewhat poor. As described above, this is because the effect of the step-like reduction is reduced in the situation where the solidification has progressed in this way.

試験番号C17〜C22は、バルジングの条件を変更した比較例についての試験である。試験番号C17およびC18では、試験番号H1と同様の条件下で、それぞれバルジング時の凝固シェル厚さおよびバルジング量を第1発明で規定する範囲を超えて大きく変更させた。同じく、試験番号C19およびC20では、試験番号H6およびH8と同様の条件で、それぞれバルジング時の凝固シェル厚さおよびバルジング量を第1発明で規定する範囲を超えて大きく変更させた。その結果、いずれの場合においても、バルジング時に発生したと推察される内部割れが発生しており、鋼板の耐HIC性能は特に1/10幅端部において劣っていた。   Test numbers C17 to C22 are tests for comparative examples in which the bulging conditions are changed. In test numbers C17 and C18, under the same conditions as in test number H1, the thickness of the solidified shell and the amount of bulging during bulging were largely changed beyond the range specified in the first invention. Similarly, in test numbers C19 and C20, the thickness of the solidified shell and the amount of bulging during bulging were largely changed beyond the ranges specified in the first invention under the same conditions as in test numbers H6 and H8. As a result, in all cases, internal cracks presumed to have occurred during bulging occurred, and the HIC resistance of the steel sheet was inferior particularly at the 1/10 width end.

試験番号C21およびC22では、試験番号H1およびとH4と同様の条件で、バルジング量のみを第1発明で規定する範囲の下限である3mm未満として鋳造を行った。その結果、Mn偏析比の値が若干上昇するとともに、ポロシティ体積は2.0×10-4(cm3/g)よりも高くなり、鋼板の耐HIC性能もやや不良となった。 In test numbers C21 and C22, casting was performed under the same conditions as in test numbers H1 and H4 with only the bulging amount being less than 3 mm, which is the lower limit of the range defined in the first invention. As a result, the value of the Mn segregation ratio slightly increased, the porosity volume became higher than 2.0 × 10 −4 (cm 3 / g), and the HIC resistance performance of the steel sheet also became slightly poor.

以上の本発明例および比較例についてのそれぞれの試験結果の比較から、本発明の優位性が立証された。   The superiority of the present invention was proved from the comparison of the respective test results of the present invention examples and comparative examples.

本発明の連続鋳造方法によれば、センターポロシティならびにマクロ偏析およびセミマクロ偏析を著しく低減したスラブ鋳片を鋳造することができ、前記鋳片を素材とする耐HIC性能に優れた鋼板を製造することが可能となる。また、本発明の連続鋳造鋳片は、ラインパイプなどに用いられる耐HIC性能に優れた鋼板の製造に最適である。したがって、本発明の連続鋳造方法および連続鋳造鋳片は、高性能の耐HIC性を要求される鋼板製造技術分野において広範に適用できる。   According to the continuous casting method of the present invention, it is possible to cast a slab slab with significantly reduced center porosity, macro segregation and semi-macro segregation, and to produce a steel sheet having excellent HIC resistance using the slab as a raw material. Is possible. Moreover, the continuous cast slab of the present invention is most suitable for the production of a steel plate having excellent HIC resistance used for a line pipe or the like. Therefore, the continuous casting method and continuous cast slab of the present invention can be widely applied in the field of steel sheet manufacturing technology that requires high performance HIC resistance.

本発明の連続鋳造方法を実施するための連続鋳造装置の例を示す図である。It is a figure which shows the example of the continuous casting apparatus for enforcing the continuous casting method of this invention.

符号の説明Explanation of symbols

1:浸漬ノズル、 2:溶鋼メニスカス、 3:鋳型、 4:溶鋼、
5:凝固シェル、 6:ガイドロール、 7:強圧下用ロール対、 8:鋳片、
9:未凝固溶鋼、 10:ピンチロール、
B1−B2:バルジングおよびテーパ状圧下ゾーン
1: immersion nozzle, 2: molten steel meniscus, 3: mold, 4: molten steel,
5: Solidified shell, 6: Guide roll, 7: Roll pair for high pressure reduction, 8: Slab,
9: Unsolidified molten steel, 10: Pinch roll,
B1-B2: bulging and tapered reduction zones

Claims (2)

鋳片の片側の凝固シェル厚さが60mm以下の鋳片位置において、鋳片厚さを鋳片幅中央部で3〜10mmの範囲内でバルジングさせた後、
該バルジングさせた状態を維持しつつ、厚さ中心部に固相が生成する鋳片位置から、厚さ中心部における中心固相率が0.8未満の鋳片位置までの間の鋳片を、バルジングさせた厚さの範囲内で鋳片の鋳造方向に対してテーパ状に圧下し、
引き続き、厚さ中心部における中心固相率が0.8〜0.95の範囲内の鋳片位置において、鋳片厚さ方向に5〜10mmの範囲内で、1段の圧下ロール対を用いて段差状に圧下することにより、
厚さ中心部におけるポロシティ体積が0.8×10 -4 〜2×10 -4 (cm 3 /g)で、かつ厚さ中心部におけるMnの偏析比が1.10〜1.25である鋳片を製造することを特徴とする耐水素誘起割れ性能に優れた鋼板製造用鋳片の連続鋳造方法。
At the slab position where the solidified shell thickness on one side of the slab is 60 mm or less, the slab thickness is bulged within a range of 3 to 10 mm at the center of the slab width,
While maintaining the bulging state, the slab between the slab position where the solid phase is generated in the center of the thickness and the slab position where the central solid fraction in the center of the thickness is less than 0.8 , Squeezed down in a taper direction with respect to the casting direction of the slab within the bulging thickness
Subsequently, at the slab position where the central solid phase ratio in the central portion of the thickness is in the range of 0.8 to 0.95, a single rolling roll pair is used within the range of 5 to 10 mm in the slab thickness direction. By rolling down into steps ,
Casting having a porosity volume of 0.8 × 10 −4 to 2 × 10 −4 (cm 3 / g) in the thickness center and a Mn segregation ratio of 1.10 to 1.25 in the thickness center. A continuous casting method for a steel plate production slab excellent in hydrogen-induced cracking resistance, characterized by producing a piece.
請求項1に記載の連続鋳造方法により鋳造された鋳片であって、
鋳片の厚さ中心部におけるポロシティ体積が0.8×10 -4 2×10-4(cm3/g)で、かつ鋳片の厚さ中心部におけるMnの偏析比が1.10〜1.25であり、
NACE T0284に規定された試験法に準拠して、圧延後の鋼板における水素誘起割れの面積率が0.5%以下であることを特徴とする鋼板製造用の連続鋳造鋳片。
A slab cast by the continuous casting method according to claim 1,
The porosity volume at the center of the slab thickness is 0.8 × 10 −4 to 2 × 10 −4 (cm 3 / g) , and the segregation ratio of Mn at the center of the slab thickness is 1.10. 1.25,
A continuous cast slab for producing a steel sheet, wherein the area ratio of hydrogen-induced cracking in the steel sheet after rolling is 0.5% or less in accordance with a test method defined in NACE T0284 .
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