JP4609330B2 - Continuous casting method of ultra-thick steel plates with excellent internal quality and slabs for ultra-thick steel plates - Google Patents

Continuous casting method of ultra-thick steel plates with excellent internal quality and slabs for ultra-thick steel plates Download PDF

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JP4609330B2
JP4609330B2 JP2006018183A JP2006018183A JP4609330B2 JP 4609330 B2 JP4609330 B2 JP 4609330B2 JP 2006018183 A JP2006018183 A JP 2006018183A JP 2006018183 A JP2006018183 A JP 2006018183A JP 4609330 B2 JP4609330 B2 JP 4609330B2
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JP2007196265A (en
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正 平城
章裕 山中
康弘 佐藤
孝浩 加茂
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Sumitomo Metal Industries Ltd
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本発明は、超音波探傷試験において欠陥が検出されない、内質に優れた極厚鋼板の製造方法およびその素材となる鋳片を溶鋼から連続的に製造する際に、鋳片の厚さ中心部に発生する中心ポロシティの体積が小さく、内質の良好な鋳片を鋳造する連続鋳造方法に関するものである。   The present invention provides a method for producing an ultra-thick steel plate excellent in internal quality, in which no defects are detected in an ultrasonic flaw detection test, and a slab that is the raw material, and a slab that is the raw material, when continuously produced from molten steel. The present invention relates to a continuous casting method for casting a slab having a small internal porosity and a good internal quality.

一般に、鋼板は、連続鋳造方法で鋳造された鋳片を素材として製造される。この鋳片の厚さ中心部には、溶鋼が凝固する際の凝固収縮や凝固後の冷却による熱収縮によって、最終凝固位置である厚さ中心付近に小さな空孔、いわゆる中心ポロシティが形成される。   In general, a steel plate is manufactured using a slab cast by a continuous casting method as a raw material. In the center of the thickness of this slab, small voids, so-called central porosity, are formed near the center of the thickness, which is the final solidification position, due to solidification shrinkage when the molten steel solidifies and heat shrinkage due to cooling after solidification. .

鋳片に中心ポロシティが存在すると、溶鋼中に溶解していた水素が凝固時に拡散して中心ポロシティに集積する。水素が中心ポロシティに集積した鋳片を熱間圧延すると、圧延によって中心ポロシティが圧着されても、中心ポロシティに集積した水素は鋼板中に再固溶して残留することになる。このような場合、残留する水素量が多いと鋼板に割れが発生する(以下、この現象を「水素割れ」という)。   When the center porosity exists in the slab, hydrogen dissolved in the molten steel diffuses during solidification and accumulates in the center porosity. When the slab in which hydrogen is accumulated in the central porosity is hot-rolled, even if the central porosity is pressed by rolling, the hydrogen accumulated in the central porosity is re-dissolved in the steel sheet and remains. In such a case, if the amount of remaining hydrogen is large, the steel sheet is cracked (hereinafter, this phenomenon is referred to as “hydrogen cracking”).

近年、金型や機械部品用の高炭素鋼(SC材)では、素材を鍛造品から圧延材に変えることによる低コスト化や、産業機械や建設機械向け鋼材、海洋構造物や各種圧力容器用鋼材に対しても、設備の大型化等に伴い、板厚が100mmを超える極厚鋼板が使用される機会が増加している。   In recent years, high carbon steel (SC materials) for molds and machine parts has been reduced in cost by changing the material from forged products to rolled materials, steel materials for industrial machines and construction machinery, marine structures and various pressure vessels. Also for steel materials, with the enlargement of facilities, etc., the opportunity to use extra-thick steel plates with a plate thickness exceeding 100 mm is increasing.

上記の極厚鋼板の製造に鋳片を用いる場合に、鋳片に中心ポロシティが存在すると、現在一般に使用されている厚板圧延機の能力では鋳片の中心部に生成した中心ポロシティを安定して圧着させることは困難である。したがって、極厚鋼板の超音波探傷試験(以下、「UST」という)を行った場合には、未圧着の中心ポロシティが欠陥(以下、「UST欠陥」という)として検出されることが多い。このため、鋳片に発生する中心ポロシティを解消することを目的として、種々の方法が提案されている。   When a slab is used for the production of the above ultra-thick steel plate, if the center porosity is present in the slab, the center porosity generated at the center of the slab is stabilized by the ability of the currently used thick plate mill. It is difficult to make them crimped. Therefore, when an ultrasonic flaw detection test (hereinafter referred to as “UST”) is performed on an extremely thick steel plate, the unbonded central porosity is often detected as a defect (hereinafter referred to as “UST defect”). For this reason, various methods have been proposed for the purpose of eliminating the central porosity generated in the slab.

例えば、特許文献1には、連続鋳造によって熱間圧延用鋳片を製造するに当たり、凝固率が85%以上99%以下の位置において、面部材によって1mm以上25mm以下の軽圧下を断続的に行うことにより、鋳片板厚中心部に偏析や中心ポロシティの無い連続鋳造鋳片の製造方法が開示されている。   For example, in Patent Document 1, when producing a slab for hot rolling by continuous casting, light reduction of 1 mm or more and 25 mm or less is intermittently performed by a surface member at a position where the solidification rate is 85% or more and 99% or less. Thus, a method for producing a continuous cast slab having no segregation or central porosity in the center part of the slab thickness is disclosed.

また、特許文献2には、鋳片の未凝固末端部を実質的に面を構成する部材を用いて圧下しつつ凝固させ、1パス当りの平均圧延真歪が0.2%以下で、かつ、累積圧下率が30〜99%の圧延を行うことにより、板厚中心部の靱性及び内質に優れた厚鋼板を製造する方法が開示されている。   Further, in Patent Document 2, an unsolidified end portion of a slab is solidified while being reduced using a member that substantially constitutes a surface, and an average rolling true strain per pass is 0.2% or less, and And the method of manufacturing the thick steel plate excellent in the toughness of a plate | board thickness center part and the internal quality by performing rolling whose cumulative reduction rate is 30 to 99% is disclosed.

しかしながら、特許文献1および2で開示される方法では、鋳片を圧下するために連続鋳造機に大規模な面圧下設備を設置しなければならず、設備面積が必要になるとともに設備費用を要するという問題がある。   However, in the methods disclosed in Patent Documents 1 and 2, in order to reduce the slab, it is necessary to install a large-scale surface reduction equipment in the continuous casting machine, which requires equipment area and equipment cost. There is a problem.

特許文献3には、鋼の連続鋳造に当り、鋳片横断面の中心部における固相率が0.3〜0.7の範囲において鋳片に5〜15mmの軽圧下を加えると共に、鋳片横断面の中心部における固相率が0.8〜1.0の範囲、或いは、凝固完了後鋳片横断面の中心温度が1200℃以上の範囲において、少なくとも一方向に一段当り圧下率で30%以上の圧下を加える鋳片内質改善方法が開示されている。   In Patent Document 3, in continuous casting of steel, a light pressure of 5 to 15 mm is applied to the slab in the range where the solid phase ratio in the center of the slab cross section is 0.3 to 0.7, and the slab In the range where the solid phase ratio in the central portion of the cross section is 0.8 to 1.0, or the central temperature of the cross section of the slab after solidification is 1200 ° C. or higher, the reduction ratio per stage in at least one direction is 30. A method for improving the quality of slabs by applying a reduction of at least% is disclosed.

また、特許文献4には、鋳片中心部の温度が固相率0.05〜0.7に相当する鋳片位置に少なくとも一対のロールを設置して鋳片を4〜20mm圧下すると共に、鋳片中心部の温度が固相率0.8以上の鋳片位置に少なくとも一対のロールを設置して5〜20%の圧下率で鋳片を圧下することで、中心ポロシティを軽減する連続鋳造方法が開示されている。   Further, in Patent Document 4, at least a pair of rolls is installed at a slab position where the temperature of the slab center corresponds to a solid phase ratio of 0.05 to 0.7, and the slab is reduced by 4 to 20 mm. Continuous casting that reduces the center porosity by installing at least a pair of rolls at the position of the slab where the temperature of the slab center is 0.8 or more and reducing the slab at a reduction rate of 5 to 20%. A method is disclosed.

しかし、特許文献3および4で開示される方法では、鋳片の凝固末期つまり厚さ中心部の固相率が0.8以上で、圧下率が5%以上という大圧下を行うため、幅広の鋼板を対象とした鋳片では圧下力が9.8×103kN(1000ton)以上にもなり、圧下ロールや圧下設備が大規模になり、設備面積が必要になるとともに設備費用も要するという問題がある。 However, in the methods disclosed in Patent Documents 3 and 4, a large reduction is performed in which the solidification rate at the end of solidification of the slab, that is, the central portion of the thickness is 0.8 or more and the reduction rate is 5% or more. The slab for steel plates has a reduction force of 9.8 × 10 3 kN (1000 ton) or more, and a reduction roll and a reduction facility become large-scale, which requires an equipment area and an equipment cost. There is.

特許文献5には、連続鋳造法を用いて極厚鋼板を製造する方法において、連続鋳造鋳片の厚み中心部における固相率が0.6以上となる領域において、未凝固厚みの1.1倍以上2.0倍以下の圧下を加えた連続鋳造鋳片を用いることにより板厚中心部の靱性及び内質に優れた極厚鋼板を製造する方法が開示されている。しかし、この方法は、未凝固厚さの定義が明確ではなく、必要な圧下量も明瞭ではないので、中心ポロシティを解消する技術としては不十分なものである。   In Patent Document 5, in a method for producing an extremely thick steel plate using a continuous casting method, an unsolidified thickness of 1.1 is obtained in a region where the solid phase ratio is 0.6 or more in the thickness center portion of a continuous cast slab. There is disclosed a method for producing an extremely thick steel plate having excellent toughness and inner quality at the center of the plate thickness by using a continuous cast slab to which a reduction of 2 to 2.0 times is applied. However, this method is insufficient as a technique for eliminating the center porosity because the definition of the unsolidified thickness is not clear and the required amount of reduction is not clear.

また、特許文献6には、C≦0.18質量%の溶鋼を連続鋳造し、その鋳片の凝固末期に鋳片の中心部の固相率が90〜98%の部分を、2〜5%の圧下加工率で1回圧下することにより、内部品質に優れた連続鋳造鋳片を製造する方法が開示されている。この方法では、圧下率が小さいため、連続鋳造機に大規模な圧下設備を設置する必要が無く、設備面積および設備費用を要しないが、圧下率が小さいことから、中心ポロシティを全て圧着することができず、それらが鋳片に残る場合があった。また、その後、鋳片を圧延する際においても残った中心ポロシティは、その大きさによっては圧着できずに、極厚鋼板に欠陥として残るという問題がある。   In Patent Document 6, a molten steel having C ≦ 0.18 mass% is continuously cast, and a portion having a solid phase ratio of 90 to 98% at the center of the slab at the end of solidification of the slab is 2 to 5%. A method for producing a continuous cast slab excellent in internal quality by reducing once at a reduction rate of% is disclosed. In this method, since the rolling reduction is small, it is not necessary to install a large-scale rolling reduction equipment in the continuous casting machine, and the equipment area and equipment cost are not required. However, since the rolling reduction is small, all the central porosity is crimped. In some cases, they may remain in the slab. In addition, there is a problem in that the center porosity remaining after rolling the slab cannot be crimped depending on the size, and remains as a defect in the extra-thick steel plate.

特許文献7には、鋳片をバルジングさせて鋳片の最大厚さを鋳型の短辺長さよりも20〜100mm厚くし、凝固完了直前にて少なくとも一対の圧下ロールによりその一対あたり20mm以上の圧下を与えて、バルジング量相当分を圧下することにより、変形抵抗の大きい鋳片両端部の圧下による塑性変形を不要とする連続鋳造方法が開示されている。しかし、この方法は、中心偏析の防止対策として行われている方法であり、中心ポロシティに対する記載がなく、その効果については不明である。また、バルジングさせて鋳片短辺(端部)を圧下しないようにしても、圧下量が大きいために凝固末期には大きな圧下力が必要となり、大規模な圧下設備および設備面積が必要になる。   In Patent Document 7, the slab is bulged so that the maximum thickness of the slab is 20 to 100 mm thicker than the short side length of the mold, and at least a pair of squeezing rolls is used to reduce the slab by 20 mm or more immediately before completion of solidification. A continuous casting method is disclosed in which plastic deformation due to the reduction of both ends of a cast slab having a large deformation resistance is unnecessary by reducing the amount corresponding to the bulging amount. However, this method is performed as a measure for preventing the center segregation, and there is no description about the center porosity, and the effect is unknown. Even if the slab short side (end) is not crushed by bulging, a large squeezing force is required at the end of solidification due to the large squeezing amount, which requires a large squeezing equipment and equipment area. .

また、特許文献8には、極厚鋼板で発生するUST欠陥を防止するために、中心ポロシティの厚さd0の鋳片を、900〜1300℃に加熱後、下記の(a)式を満足する圧下比rで圧延し、かつ最終圧延パスにおいて下記の(b)式の条件を満足させることにより、安定して内質の優れた極厚鋼板を製造する方法が開示されている。 In Patent Document 8, in order to prevent a UST defect that occurs in an extremely thick steel plate, a slab having a central porosity thickness of d 0 is heated to 900 to 1300 ° C., and the following equation (a) is satisfied. A method of stably producing an extremely thick steel plate having excellent internal quality by rolling at a rolling reduction ratio r and satisfying the condition of the following formula (b) in the final rolling pass is disclosed.

r≧0.2×d0+1.0 ・・・(a)
1.67×((t0−t1)×R)1/2/t0+0.5≧1.1・・・ (b)
ここで、t0は被圧延材の最終圧延パス前の厚さ(mm)、t1は被圧延材の最終圧延パス後の厚さ(mm)、Rは圧延ロール半径(mm)を示す。
r ≧ 0.2 × d 0 +1.0 (a)
1.67 × ((t 0 −t 1 ) × R) 1/2 / t 0 + 0.5 ≧ 1.1 (b)
Here, t 0 represents the thickness (mm) of the material to be rolled before the final rolling pass, t 1 represents the thickness (mm) of the material to be rolled after the final rolling pass, and R represents the rolling roll radius (mm).

特許文献8で開示されている方法は、鋳片に存在する大きな中心ポロシティを厚板圧延だけで圧着解消させようとするものである。この方法における(a)式のd0は、試験材の中心ポロシティ厚さの大きいものから5個を抽出し、その平均をd0としている。 The method disclosed in Patent Document 8 is intended to eliminate the large central porosity existing in the slab by pressure bonding only by thick plate rolling. In this method, d 0 in the formula (a) is obtained by extracting five from the specimens having a large central porosity thickness, and taking the average as d 0 .

しかしながら、本発明者らが詳細に調査した結果、大きな中心ポロシティほどその発生比率は低くなるために平均値と最大値がかけ離れてしまう場合があること、また、大きな中心ポロシティほど圧延時に圧着されにくいためにUST欠陥として残りやすいことが判明した。そのため、圧延だけで中心ポロシティを圧着解消させようとするこの方法では、特に高強度極厚製品ニーズに対して完全にUST欠陥を解消することはできない。   However, as a result of detailed investigations by the present inventors, the larger the center porosity, the lower the generation ratio, so that the average value may be far from the maximum value, and the larger the center porosity, the more difficult it is to press-bond during rolling. Therefore, it has been found that it is likely to remain as a UST defect. For this reason, this method that attempts to eliminate the central porosity by rolling alone cannot completely eliminate the UST defect particularly for the needs of high-strength ultra-thick products.

さらに、前記の(b)式で最終圧延パスでの板厚中心圧縮応力(σmax)を規定しているが、圧下量(t0−t1に相当)は、被圧延材の温度や幅、変形抵抗、圧延機の圧下能力により制限され、他方、ロール半径(R)を大きくするためには圧延機自体を大型化する必要があり、現実的な技術ではない。 Furthermore, the thickness center compressive stress (σ max ) in the final rolling pass is defined by the above formula (b), but the reduction amount (corresponding to t 0 -t 1 ) is the temperature and width of the material to be rolled. However, it is limited by the deformation resistance and the rolling ability of the rolling mill. On the other hand, in order to increase the roll radius (R), it is necessary to enlarge the rolling mill itself, which is not a practical technique.

このように、中心ポロシティ体積が小さい、内質の優れた連続鋳造鋳片および極厚鋼板を製造するためには、解決されねばならない問題が残されている。   Thus, in order to manufacture a continuous cast slab and a very thick steel plate having a small central porosity volume and excellent internal quality, there remains a problem that must be solved.

特開平07−276020号公報Japanese Patent Application Laid-Open No. 07-276020 特開平02−156022号公報Japanese Patent Laid-Open No. 02-156022 特開平05−69099号公報JP 05-69099 A 特開平10−58106号公報JP-A-10-58106 特開平06−106316号公報Japanese Patent Laid-Open No. 06-106316 特開平07−80615号公報Japanese Patent Application Laid-Open No. 07-80615 特開平09−57410号公報JP 09-57410 A 特開2000−288604号公報(特許請求の範囲および段落[0030])JP 2000-288604 A (Claims and paragraph [0030])

上述した通り、鋳片に発生する中心ポロシティの解消を目的とする従来技術には、大規模な圧下設備および設備面積が必要となり設備費用を要するというコスト面での問題や、既存の圧下設備を用いる場合に、鋳片を圧延する際に残った中心ポロシティが極厚鋼板に欠陥として残るなどの技術面での問題がある。   As described above, the conventional technology aiming to eliminate the central porosity generated in the slab requires a large-scale reduction equipment and equipment area, which necessitates equipment costs. When used, there is a technical problem that the central porosity remaining when rolling the slab remains as a defect in the extra-thick steel plate.

本発明は、上記の問題に鑑みてなされたものであり、既存の厚板用圧延機を用いて製造される、UST欠陥のない内質に優れた高強度極厚鋼板およびその素材となる鋳片を溶鋼から連続的に製造する際に、鋳片の厚さ中心部に発生する中心ポロシティの体積が小さい内質の良好な鋳片を鋳造する連続鋳造方法の提供を目的としている。   The present invention has been made in view of the above-mentioned problems, and is manufactured using an existing thick plate rolling mill, and is a high-strength extra-thick steel plate excellent in internal quality without UST defects and a casting serving as a material thereof. An object of the present invention is to provide a continuous casting method for casting a slab having a good internal quality in which the volume of the central porosity generated at the center of the slab thickness is small when the slab is continuously manufactured from molten steel.

本発明者らは、400MPa級鋼を対象として試験を重ねた結果、UST欠陥のない内質に優れた極厚鋼板を製造するためには、連続鋳造の際の圧下と熱間圧延の際の圧下の併用が有効であり、鋳片を凝固末期に一定の圧下条件で圧下しながら連続鋳造することにより、中心ポロシティの形成を事前に抑制できることを見出した。   As a result of repeated tests on 400 MPa class steels, the present inventors have produced a very thick steel plate excellent in internal quality without UST defects, in order to reduce rolling during continuous casting and during hot rolling. It has been found that the combined use of reduction is effective, and the formation of the center porosity can be suppressed in advance by continuously casting the slab while reducing the slab under a constant reduction condition at the end of solidification.

さらに、鋳片および極厚鋼板の単位質量当たりの中心ポロシティ体積を定量的に把握することが可能であることを見出し、500MPa級鋼および600MPa級鋼に関しても400MPa級鋼と同様の試験を行い、検討を重ねた結果、下記の(a)〜(c)の知見を得た。
(a)鋳片の厚さ中心部およびその他の部分の比重測定に基づいて、中心ポロシティを単位質量当たりの体積割合として求め、この値を評価することにより、極厚鋼板の内質向上の指標とすることができる。
(b)鋳片を凝固末期に一定の圧下条件で圧下しながら連続鋳造することにより、極厚鋼板の内質向上に要求される中心ポロシティの低減効果が得られる。
(c)上記(b)の効果は、400MPa級鋼のみならず、さらに高強度の、500MPa級鋼および600MPa級鋼においても有効に発揮される。
Furthermore, it has been found that it is possible to quantitatively grasp the central porosity volume per unit mass of the slab and the extra-thick steel plate, and the same test as the 400 MPa class steel is performed with respect to the 500 MPa class steel and the 600 MPa class steel, As a result of repeated studies, the following findings (a) to (c) were obtained.
(A) Based on the measurement of the specific gravity of the center part of the slab and other parts, the central porosity is obtained as a volume ratio per unit mass, and this value is evaluated to improve the quality of the extra heavy steel sheet. It can be.
(B) By continuously casting the slab while reducing the slab under a constant reduction condition at the end of solidification, the effect of reducing the central porosity required for improving the quality of the extra-thick steel sheet can be obtained.
(C) The effect of (b) is effectively exhibited not only in 400 MPa class steel but also in high strength 500 MPa class steel and 600 MPa class steel.

本発明は、上記の知見に基づいて完成されたものであり、下記(1)の極厚鋼板(2)の連続鋳造方法および(3)の極厚鋼板の製造方法を要旨としている。ただし、下記(1)の極厚鋼板は、本発明の参考例としての発明である。
(1)連続鋳造された鋳片を素材として、仕上げ圧延までの圧下比rが1.5〜4.0の条件で熱間圧延して得られ、引張強度Xのレベルが400MPa級乃至600MPa級である極厚鋼板であって、該鋼板の厚さ中心部の中心ポロシティ体積Vp(cm3/g)が、下記(1)式および(2)式の関係を満たすことを特徴とする内質に優れた極厚鋼板。
The present invention has been completed on the basis of the above findings, and the gist of the present invention is the following (1) extra heavy steel sheet , (2) continuous casting method and (3) extra heavy steel sheet manufacturing method . However, the extra-thick steel plate (1) below is an invention as a reference example of the present invention.
(1) Using a continuously cast slab as a raw material, it is obtained by hot rolling under a condition where the rolling reduction ratio r until finish rolling is 1.5 to 4.0, and the level of tensile strength X is 400 MPa class to 600 MPa class. An inner thickness characterized in that the central porosity volume Vp (cm 3 / g) at the thickness center of the steel sheet satisfies the relationship of the following formulas (1) and (2): Extremely thick steel plate.

Vp < Vp0/r ・・・(1)
Vp0=(0.020×X/10−0.085)×10-4 ・・・(2)
ここで、極厚鋼板の引張強度Xは、400MPa〜600MPaの範囲で選択される値であり、換算中心ポロシティ体積Vp0(cm3/g)の算出根拠となる。
(2)上記(1)に記載の極厚鋼板を熱間圧延により製造するための素材として用いる鋳片を連続鋳造する方法であって、該鋳片の厚さ中心部の固相率が0.8以上、1.0未満の範囲において、未凝固部を含む鋳片の幅中央部を、一対の圧下ロールにより3〜15mm圧下し、前記極厚鋼板の単位質量あたりの中心ポロシティ体積Vp(cm 3 /g)と前記鋳片の単位質量あたりの中心ポロシティ体積Vp 0 ’(cm 3 /g)を極厚鋼板の内質向上の指標とし、下記(a)式を満足するか否かを評価することを特徴とする極厚鋼板用鋳片の連続鋳造方法(以下、この連続鋳造方法を、「凝固末期圧下法」という場合がある)。
Vp < Vp 0 ’/r ・・・(a)
(3)上記(2)に記載の連続鋳造方法で製造された極厚鋼板用鋳片を、仕上げ圧延までの圧下比rが1.5〜4.0の条件で熱間圧延し、前記(a)式のVp 0 ’を前記(2)式のVp 0 に置き換えて得られる前記(1)式を極厚鋼板の内質向上の指標として、同式を満足するか否かを評価し、満足するものを内質に優れた極厚鋼板とすることを特徴とする極厚鋼板の製造方法。
Vp <Vp 0 / r (1)
Vp 0 = (0.020 × X / 10−0.085) × 10 −4 (2)
Here, the tensile strength X of the extra-thick steel plate is a value selected in the range of 400 MPa to 600 MPa, and is a basis for calculating the converted center porosity volume Vp 0 (cm 3 / g).
(2) A method of continuously casting a slab used as a raw material for manufacturing the extra-thick steel plate according to (1) above by hot rolling, wherein the solid phase ratio at the thickness center of the slab is 0 In the range of 0.8 or more and less than 1.0, the width central portion of the slab including the unsolidified portion is reduced by 3 to 15 mm with a pair of reduction rolls , and the central porosity volume Vp (per unit mass of the extra- thick steel plate) cm 3 / g) and the central porosity volume Vp 0 ′ (cm 3 / g) per unit mass of the slab as an index for improving the quality of the extra-thick steel sheet, whether or not the following formula (a) is satisfied A continuous casting method for slabs for extra-thick steel plates characterized by evaluation (hereinafter, this continuous casting method may be referred to as “end-of-solidification reduction method”).
Vp <Vp 0 '/ r (a)
(3) The slab for extra-thick steel plate produced by the continuous casting method described in (2) above is hot-rolled under a condition where the rolling reduction r until finish rolling is 1.5 to 4.0, a) Evaluating whether the equation (1) obtained by replacing Vp 0 ′ in the equation (2) with Vp 0 in the equation (2) is satisfied as an index for improving the quality of the extra heavy steel sheet, A method for producing an extra-thick steel sheet, characterized in that an extra-thick steel sheet having excellent internal quality is satisfied.

本発明において、「内質に優れた」とは、鋼板の未圧着の中心ポロシティの体積が、USTを行った場合に、欠陥として検出されない程度に小さいことを意味する。   In the present invention, “excellent in quality” means that the volume of the unbonded central porosity of the steel sheet is small enough not to be detected as a defect when UST is performed.

「極厚鋼板」とは、連続鋳造方法で鋳造された鋳片を圧延して得られる板厚80mm以上の鋼板を意味する。   The “extremely thick steel plate” means a steel plate having a thickness of 80 mm or more obtained by rolling a slab cast by a continuous casting method.

本発明の極厚鋼板は、既存の厚板用圧延機を用いて製造することが可能なUST欠陥のない内質に優れた極厚鋼板であり、低コストかつ高品質の鋼板として広い用途に使用できる。   The extra-thick steel plate of the present invention is an extra-thick steel plate with excellent internal quality that has no UST defects and can be manufactured using an existing thick plate rolling mill, and is widely used as a low-cost and high-quality steel plate. Can be used.

また、本発明の連続鋳造方法によれば、前記極厚鋼板の素材となる鋳片を溶鋼から連続的に製造する際に、比較的簡易な連続鋳造装置の圧下設備を用いて、中心ポロシティを低減した鋳片を鋳造することができるので、極厚鋼板の製造に大きく貢献できる。   Further, according to the continuous casting method of the present invention, when continuously producing the slab, which is the raw material of the extra-thick steel plate, from the molten steel, the central porosity is reduced by using a relatively simple rolling equipment. Since the reduced slab can be cast, it can greatly contribute to the production of extra-thick steel plates.

本発明の極厚鋼板は、上述したとおり、連続鋳造された鋳片を素材として、仕上げ圧延までの圧下比rが1.5〜4.0の条件で熱間圧延して得られ、引張強度Xのレベルが400MPa級乃至600MPa級である極厚鋼板であって、該鋼板の厚さ中心部の中心ポロシティ体積Vp(cm3/g)が、下記(1)式および(2)式の関係を満たすことを特徴とする。 As described above, the extra-thick steel plate of the present invention is obtained by hot rolling using a continuously cast slab as a raw material under a condition where the reduction ratio r until finish rolling is 1.5 to 4.0, and has a tensile strength. A very thick steel sheet having a level of X of 400 MPa class to 600 MPa class, and the central porosity volume Vp (cm 3 / g) at the center of the thickness of the steel sheet is represented by the following expressions (1) and (2): It is characterized by satisfying.

Vp < Vp0/r ・・・(1)
Vp0=(0.020×X/10−0.085)×10-4 ・・・(2)
ここで、極厚鋼板の引張強度Xは、400MPa〜600MPaの範囲で選択される値であり、換算中心ポロシティ体積Vp0(cm3/g)の算出根拠となる。以下に、本発明を上記のように規定した理由および好ましい範囲について説明する。
(1)極厚鋼板の内質向上の指標
前述の従来技術に示されたとおり、中心ポロシティを低減するために固相率や圧下条件を規定する提案は数多くなされているが、鋳片または圧延製品の単位質量当たりの中心ポロシティ体積を定量的に規定したものは見当たらない。本発明者らはUST欠陥のない内質の良好な極厚鋼板を製造するためには、極厚鋼板の内質向上の指標の設定が不可欠であると考え、鋳片および極厚鋼板の単位質量当たりの中心ポロシティ体積の定量化を検討した。
Vp <Vp 0 / r (1)
Vp 0 = (0.020 × X / 10−0.085) × 10 −4 (2)
Here, the tensile strength X of the extra-thick steel plate is a value selected in the range of 400 MPa to 600 MPa, and is a basis for calculating the converted center porosity volume Vp 0 (cm 3 / g). Below, the reason and preferable range which prescribed | regulated this invention as mentioned above are demonstrated.
(1) Indicators for improving the quality of extra heavy steel sheets As shown in the above-mentioned prior art, many proposals have been made to specify the solid phase ratio and reduction conditions in order to reduce the center porosity. Nothing quantitatively defines the central porosity volume per unit mass of the product. The present inventors consider that it is indispensable to set an index for improving the inner quality of the extra-thick steel plate in order to produce a super-thick steel plate having a good quality without UST defects. The quantification of the central porosity volume per mass was studied.

本発明者らは、鋼種を400MPa級鋼、500MPa級鋼および600MPa級鋼の3水準とし、連続鋳造において様々な圧下量で鋳片を圧下し、得られた鋳片の1/4厚み位置および厚さ中心部から試料の採取を行った。   The present inventors set the steel grades to three levels of 400 MPa class steel, 500 MPa class steel, and 600 MPa class steel, and squeeze the slab with various reduction amounts in continuous casting. A sample was collected from the center of the thickness.

中心ポロシティの評価は、中心ポロシティ発生がほとんどないと推定される鋳片の1/4厚み位置の平均比重を基準として、厚さ中心部の比重から算出した中心ポロシティの比体積で評価する手法をとった。すなわち、1/4厚み位置の平均比重ρ0と、厚み中心の平均比重ρから、下記の(3)式で定義する中心ポロシティ体積Vp(cm3/g)を求めた。 The evaluation of the central porosity is based on the specific volume of the central porosity calculated from the specific gravity at the center of the thickness, based on the average specific gravity at the 1/4 thickness position of the slab that is estimated to have little generation of central porosity. I took it. That is, the central porosity volume Vp (cm 3 / g) defined by the following equation (3) was determined from the average specific gravity ρ 0 at the 1/4 thickness position and the average specific gravity ρ at the thickness center.

Vp=1/ρ−1/ρ0 ・・・(3)
さらに、極厚鋼板の場合にも、鋳片の場合と同様の手法で中心ポロシティ評価を行った。
Vp = 1 / ρ−1 / ρ 0 (3)
Further, in the case of an extremely thick steel plate, the center porosity was evaluated in the same manner as in the case of a slab.

圧延後の極厚鋼板は、後述するUST評価方法にて、未圧着の中心ポロシティを評価した。測定された欠陥の個数、欠陥1個当たりの最大指示長さ、密集度、占積率等が当該JISに規定された値以下の場合に、その極厚鋼板は合格とし、UST欠陥はないものと判断した。   The ultra-thick steel sheet after rolling was evaluated for unbonded central porosity by the UST evaluation method described later. When the measured number of defects, maximum indicated length per defect, density, space factor, etc. are below the values specified in the JIS, the heavy steel plate is accepted and there is no UST defect It was judged.

本発明者らは、中心ポロシティは空洞であり、圧延圧下比が変わればその圧着量、体積量は変化すると考え、圧下比を考慮し、鋳片および極厚鋼板の中心ポロシティ体積の関連性を考察した。その結果、極厚鋼板の単位質量あたりの中心ポロシティ体積Vp(cm3/g)は、鋳片の単位質量当たりの中心ポロシティ体積Vp0’(cm3/g)の値から圧延圧下比rに反比例することを知得し、下記の(4)式を得た。 The present inventors consider that the central porosity is hollow, and that the amount of crimping and volume changes if the rolling reduction ratio changes, considering the reduction ratio, and the relationship between the central porosity volume of the slab and extra heavy steel sheet. Considered. As a result, the central porosity volume Vp (cm 3 / g) per unit mass of the extra-thick steel plate is changed from the value of the central porosity volume Vp 0 ′ (cm 3 / g) per unit mass of the slab to the rolling reduction ratio r. Knowing that it was inversely proportional, the following equation (4) was obtained.

Vp < Vp0’/r ・・・(4)
本発明者らは、さらに検討を重ね、本発明の凝固末期圧下法を適用した鋳片を素材とする極厚鋼板は、上記のUST評価において欠陥が検出されないこと、また同じ凝固末期圧下条件であっても強度レベルにより鋳片の中心ポロシティ低減への効果が異なることを知見した。
Vp <Vp 0 '/ r (4)
The present inventors have further studied, and the ultra-thick steel plate made of a slab to which the end-solid-phase reduction method of the present invention is applied has no defect detected in the above-mentioned UST evaluation, and under the same end-solid-state reduction conditions. Even if it exists, it turned out that the effect on the center porosity reduction of a slab changes with strength levels.

図1は、本発明の凝固末期圧下法を適用した鋳片の中心ポロシティ体積Vp0と鋼種の引張強度レベルの相関を示す図である。この関係を、引張強度レベルX(MPa)で近似的に表すことにより、極厚鋼板の内質向上に要求される鋳片の中心ポロシティ体積Vp0(cm3/g)を規定する下記の(2)式を得た。 FIG. 1 is a diagram showing the correlation between the central porosity volume Vp 0 of a slab to which the final solidification reduction method of the present invention is applied and the tensile strength level of a steel type. By expressing this relationship approximately by the tensile strength level X (MPa), the following (specify the central porosity volume Vp 0 (cm 3 / g) of the slab required for improving the quality of the extra-thick steel plate) 2) The formula was obtained.

Vp0=(0.020×(X/10)−0.085)×10-4 ・・・(2)
ここで、(2)式は400MPa〜600MPaの引張強度レベルで回帰分析して、極厚鋼板の引張強度Xと鋳片の中心ポロシティ体積Vp0の関係を表した式であるから、引張強度Xは同じ範囲内で選択される値を適用できる。
Vp 0 = (0.020 × (X / 10) −0.085) × 10 −4 (2)
Here, since the equation (2) is a regression analysis at a tensile strength level of 400 MPa to 600 MPa and represents the relationship between the tensile strength X of the extra-thick steel plate and the central porosity volume Vp 0 of the slab, the tensile strength X Can apply values selected within the same range.

上記の(2)式は、極厚鋼板の引張強度の値から、極厚鋼板の内質向上に要求される鋳片の中心ポロシティ体積を規定する式であり、(2)式から定まる体積を換算中心ポロシティ体積Vp0(cm3/g)と定義する。 The above formula (2) is a formula that defines the central porosity volume of the slab required for improving the quality of the extra-thick steel plate from the value of the tensile strength of the extra-thick steel plate, and the volume determined from the formula (2) It is defined as a reduced central porosity volume Vp 0 (cm 3 / g).

また、上記の(4)式のVp0’と換算中心ポロシティ体積Vp0を置き換えることにより極厚鋼板の内質向上に要求される極厚鋼板の中心ポロシティ体積を規定する(1)式を得た。 Further, by substituting Vp 0 ′ and the converted central porosity volume Vp 0 in the above formula (4), the formula (1) that defines the central porosity volume of the extra heavy steel sheet required for improving the quality of the extra heavy steel sheet is obtained. It was.

Vp < Vp0/r ・・・(1)
上記の(1)式および(2)式は極厚鋼板の内質向上の指標となり得るものであり、上記の(1)式を満足すれば、UST欠陥のない、内質に優れた極厚鋼板を得ることができる。
(2)圧下比1.5〜4.0
連続鋳造した鋳片を圧延する場合の圧下比rは、圧下比=(鋳造完了後鋳片厚さ/鋼板圧延仕上げ厚さ)で定義される。この圧下比は、通常の連続鋳造では、以下の理由により2.5〜4.0とされている。
Vp <Vp 0 / r (1)
The above formulas (1) and (2) can serve as an index for improving the inner quality of the extra-thick steel sheet. If the above formula (1) is satisfied, the thickness is excellent without any UST defects and excellent in the inner quality. A steel plate can be obtained.
(2) Reduction ratio 1.5 to 4.0
The rolling reduction ratio r in the case of rolling continuously cast slabs is defined by the rolling reduction ratio = (cast piece thickness after completion of casting / steel plate rolled finish thickness). In the normal continuous casting, this reduction ratio is set to 2.5 to 4.0 for the following reason.

圧下比が2.5未満では、鋳片に残った小さい中心ポロシティ(厚さ1mm未満程度)でも、圧延時に圧着、解消できない場合があり、製造された極厚鋼板にはUST欠陥が発見される場合がある。   If the reduction ratio is less than 2.5, even a small central porosity (thickness less than about 1 mm) remaining in the slab may not be crimped or eliminated during rolling, and a UST defect is found in the manufactured extra-thick steel plate. There is a case.

また、板厚80mm以上の極厚鋼板を製造しようとする場合に、圧下比が4.0を超える圧下を要求することは、鋳型の厚さを大きくした連続鋳造機により鋳片の厚みを確保しなければならないことを意味する。このため、機長の長い巨大な連続鋳造機が必要になり、大きな設備費用がかかる。あるいは、操業時の鋳造速度を極端に遅くしなければならず、生産性が極めて悪くなるという問題がある。   In addition, when an extremely thick steel sheet with a thickness of 80 mm or more is to be manufactured, the reduction ratio exceeding 4.0 is required to ensure the thickness of the slab by a continuous casting machine with a large mold thickness. Means you have to do. For this reason, a huge continuous casting machine with a long captain is required, and a large equipment cost is required. Alternatively, there is a problem that the casting speed during operation must be extremely slow, and the productivity becomes extremely poor.

本発明の極厚鋼板では、凝固末期圧下法の適用を前提とすることにより圧延圧下比を1.5〜4.0としている。   In the extra-thick steel plate of the present invention, the rolling reduction ratio is set to 1.5 to 4.0 on the premise that the end-solidification reduction method is applied.

図2は、圧下比が3.0の圧延圧下を行った場合の、本発明の凝固末期圧下法の適用の有無による鋳片と極厚鋼板の中心ポロシティ体積を示した図であり、同図(a)は極厚鋼板の引張強度レベルが500MPa級の場合を示す図であり、同図(b)は極厚鋼板の引張強度レベルが600MPa級の場合を示す図である。   FIG. 2 is a diagram showing the central porosity volume of the slab and the extra heavy steel plate with or without the application of the end-of-solidification reduction method of the present invention when rolling reduction with a reduction ratio of 3.0 is performed. (A) is a figure which shows the case where the tensile strength level of an ultra-thick steel plate is a 500 MPa class, (b) is a figure which shows the case where the tensile strength level of an ultra-thick steel plate is a 600 MPa class.

同図に示すように、本発明の凝固末期圧下法を適用し、中心ポロシティ形成を事前に抑制することにより、連続鋳造の際に本発明で実施している圧下ロールによる一括圧下をしていない鋳片(以下、「通常鋳片」という)を素材とする極厚鋼板よりも中心ポロシティ体積を約1/3に低減可能となる。   As shown in the figure, by applying the end-solid-state reduction method of the present invention and suppressing the central porosity formation in advance, no batch reduction is performed by the reduction roll used in the present invention during continuous casting. The central porosity volume can be reduced to about 1/3 that of a very thick steel plate made of a slab (hereinafter referred to as “normal slab”).

さらに、本発明の凝固末期圧下法を適用した鋳片および通常鋳片を素材とし、圧延によって同じ200mm厚の極厚鋼板を製造する場合の圧延後のポロシティ体積比較を表1に示す。   Further, Table 1 shows a comparison of porosity volumes after rolling in the case where a very thick steel plate having the same thickness of 200 mm is manufactured by rolling using a slab and a normal slab to which the final solidification reduction method of the present invention is applied.

300mm厚の通常鋳片から200mm厚の極厚鋼板を製造する比較材1では、圧下比は1.5となり、600mm厚の鋳片で圧下比を3.0まで上昇させた比較材2よりも中心ポロシティ体積は大きくなる。これに対して、本発明の凝固末期圧下法を適用した300mm厚の鋳片を素材とする極厚鋼板は、同じ圧下比の比較材1と比べ、中心ポロシティ体積が1/3以下に低減しているのみならず、さらには圧下比が3.0の比較材2と比べても中心ポロシティ体積が低減する結果となる。   In the comparative material 1 for producing a 200 mm-thick extra-thick steel plate from a 300 mm-thick normal slab, the reduction ratio is 1.5, which is higher than that of the comparative material 2 in which the reduction ratio is increased to 3.0 with a 600 mm-thick slab. The central porosity volume is increased. On the other hand, the ultra-thick steel plate made of 300 mm-thick slab to which the end-solidification reduction method of the present invention is applied has a central porosity volume reduced to 1/3 or less compared to the comparative material 1 having the same reduction ratio. In addition, the central porosity volume is reduced as compared with the comparative material 2 having a reduction ratio of 3.0.

すなわち、通常鋳片の圧延では、圧下比は2.5〜4.0とする必要があるが、本発明の極厚鋼板による圧延では、鋳片段階で従来の圧下比が3.0程度の中心ポロシティ低減効果が予め付与されている。このため、本発明で規定する圧下比が1.5〜4.0の条件は、通常鋳片の圧下比では4.5(=1.5×3)〜12.0(=4.0×3)に相当することになる。   That is, in the rolling of a normal slab, the rolling ratio needs to be 2.5 to 4.0, but in the rolling using the extra-thick steel plate of the present invention, the conventional rolling ratio is about 3.0 at the slab stage. The center porosity reduction effect is previously given. For this reason, the conditions of the rolling ratio specified in the present invention of 1.5 to 4.0 are 4.5 (= 1.5 × 3) to 12.0 (= 4.0 ×) in the rolling ratio of a normal slab. This corresponds to 3).

ただし、内質向上には中心ポロシティ低減だけでなく凝固組織の微細化も重要な因子であるため、本発明の圧下比としては、1.5を下限とする。   However, in order to improve the internal quality, not only the reduction of the central porosity but also the refinement of the solidified structure is an important factor, so the rolling ratio of the present invention is 1.5.

また、本発明は、連続鋳造法により鋳造した300mm厚程度の鋳片を素材とし、板厚80mm以上の極厚鋼板を製造するため、本発明の圧下比は、4.0を上限とする。   In addition, the present invention uses a slab of about 300 mm thickness cast by a continuous casting method as a raw material to produce an extremely thick steel plate having a plate thickness of 80 mm or more, so the rolling ratio of the present invention is 4.0.

本発明によれば、鋳片に残った小さい中心ポロシティ(厚さ1mm未満程度)が、圧下比1.5〜4.0の圧延により圧着、解消できるので、圧下比を確保できなかった極厚材の製品の製造が可能となり得る。   According to the present invention, the small center porosity (thickness of less than about 1 mm) remaining in the slab can be eliminated by pressure bonding by rolling at a reduction ratio of 1.5 to 4.0, so that the reduction ratio cannot be ensured. It may be possible to produce a product of wood.

さらに、表1に示すように、従来は200mm厚の極厚鋼板製造の場合に、圧下比が3程度必要なため、少なくとも600mm厚以上の鋳型厚が必要であったのに対して、本発明によればその半分程度の300mm厚の鋳型で実現が可能となる。すなわち、通常その極厚鋼板を鋳造するために必要な鋳型厚以下でその半分以上の厚みの鋳型を用いることで製造が可能となり、既存の連続鋳造機により製造可能な範囲に収めることができる。従って、インゴットからの製造や、極厚専用の連続鋳造機を必要とすることなく製造が可能となる。
(3)化学組成
本発明が対象とする鋼板は、化学組成を限定するものではないが、主に最終製品である厚鋼板としての機械特性、溶接性、溶接熱影響部特性等に応じて各合金元素を組合せることが望ましい場合があり、下記の化学組成を含有することができる。以下の説明において、「%」は「質量%」を表す。
Furthermore, as shown in Table 1, in the case of producing a 200 mm thick extra-thick steel sheet, a reduction ratio of about 3 is required in the prior art. According to the above, it is possible to realize with a mold having a thickness of about 300 mm. That is, it is possible to manufacture by using a mold that is less than the mold thickness necessary for casting the extra-thick steel plate and more than half that thickness, and can be within the range that can be manufactured by an existing continuous casting machine. Therefore, it is possible to manufacture without the need for manufacturing from an ingot or a continuous casting machine dedicated to extra-thickness.
(3) Chemical composition Although the steel composition which this invention makes object does not limit a chemical composition, according to the mechanical characteristic as a thick steel plate which is a final product, weldability, a welding heat affected zone characteristic, etc. It may be desirable to combine alloying elements and may contain the following chemical composition. In the following description, “%” represents “mass%”.

C:0.02〜0.56%
Cは強度を確保するのに有効な元素であるが、その効果を得るためには0.02%以上含有させることが望ましい。一方、構造材料等の母材や溶接部の靱性が要求されるものについては靱性確保の観点から0.18%以下(例えば、−20、−40℃の低温靱性の要求がある場合は0.09%以下)の含有量のものを用いることが望ましい。また、高硬度にして摩耗性等を向上させる用途には、必要な硬度を得るために0.56%以下の含有量のものを用いることが望ましい。
C: 0.02 to 0.56%
C is an element effective for securing strength, but in order to obtain the effect, it is desirable to contain 0.02% or more. On the other hand, the base material such as a structural material and the toughness of the welded portion are required to be 0.18% or less from the viewpoint of securing the toughness (e.g., 0.about.20 ° C when low temperature toughness is required). It is desirable to use those having a content of 09% or less. In addition, it is desirable to use a material having a content of 0.56% or less in order to obtain the required hardness for the purpose of improving the wearability and the like by increasing the hardness.

Si:0.04〜0.60%
Siは溶鋼の脱酸に必要な元素であり、その効果を得るためには0.04%以上が望ましい。しかし、0.60%を超えると溶接熱影響部靱性を劣化させるので、望ましくは0.35%以下である。
Si: 0.04 to 0.60%
Si is an element necessary for deoxidation of molten steel, and 0.04% or more is desirable to obtain the effect. However, if it exceeds 0.60%, the weld heat-affected zone toughness deteriorates, so it is preferably 0.35% or less.

Mn:0.50〜2.00%
Mnは、Cと同様、母材強度を確保するのに有効な元素であり、効果的に強度を得るために0.50%以上含有することが望ましい。しかし、Mn含有量が多すぎると中心偏析による母材や溶接熱影響部靱性の劣化が顕著となるため、2.00%以下の範囲で用いるのが望ましい。
Mn: 0.50 to 2.00%
Mn, like C, is an element effective for ensuring the strength of the base material, and is desirably contained in an amount of 0.50% or more in order to effectively obtain the strength. However, if the Mn content is too large, deterioration of the base metal and weld heat affected zone toughness due to center segregation becomes significant, so it is desirable to use within a range of 2.00% or less.

P:0.020%以下およびS:0.006%以下
PおよびSは鋼の靱性を著しく悪化させる元素であり、その含有量は少ない方が望ましいが、極端に低下させるためにはコストがかかるために、上記の範囲以下で用いる。
P: 0.020% or less and S: 0.006% or less P and S are elements that remarkably deteriorate the toughness of steel, and it is desirable that the content is small, but it is expensive to reduce it extremely. Therefore, it is used within the above range.

さらに、目的とする強度、硬度および溶接熱影響部靱性を確保するために、必要に応じて、Cu:0.1〜1.2%、Ni:0.1〜4.0%、Cr:0.1〜1.2%、Mo:0.01〜0.6%、Nb:0.01〜0.1%、V:0.01〜0.1%、Ti:0.01〜0.03%、B:0.0003〜0.003%、Al:0.003〜0.10%、およびN:0.001〜0.01%を単独、または、組み合わせて含有させることができる。これらの元素も上記の範囲を超えて含有すると逆に特性が悪化したり、含有による効果が合金コストに見合わなくなるものである。   Furthermore, in order to ensure the intended strength, hardness, and weld heat affected zone toughness, Cu: 0.1 to 1.2%, Ni: 0.1 to 4.0%, Cr: 0 as necessary. 0.1-1.2%, Mo: 0.01-0.6%, Nb: 0.01-0.1%, V: 0.01-0.1%, Ti: 0.01-0.03 %, B: 0.0003 to 0.003%, Al: 0.003 to 0.10%, and N: 0.001 to 0.01% can be contained alone or in combination. If these elements are contained in excess of the above range, the characteristics are adversely deteriorated, or the effects of the inclusion are not commensurate with the alloy cost.

また、特に溶接熱影響部靱性の性能向上を目的として、Ca、Mg、REMを1種以上含有させることができるが、その範囲はそれぞれ0.0005%以上、0.01%以下であることが望ましい。   In addition, for the purpose of improving the performance of the weld heat affected zone toughness, one or more of Ca, Mg, and REM can be contained, but the ranges are 0.0005% or more and 0.01% or less, respectively. desirable.

近年、首都高速道路に使用される橋脚は、設置場所の制約から特殊な形状を採用する場合が多く、厚肉化の傾向がみられる。また、既設橋脚においては疲労亀裂が問題となるため、溶接部のUSTが厳格化されてきている。このため、圧下比を大きくできない極厚鋼板は、連続鋳造による軽圧下に加えて強圧下圧延を行ってもポロシティ欠陥がわずかに残り、高感度USTにより欠陥として検出されるため、製造が困難と考えられていた。本発明の鋼板は、上記のような用途の鋼種に適用することが望ましい。   In recent years, piers used on the Metropolitan Expressway often adopt a special shape due to restrictions on installation location, and there is a tendency to increase the thickness. In addition, since fatigue cracks are a problem in existing piers, UST of welds has been tightened. For this reason, an extra-thick steel sheet that cannot have a large reduction ratio has a slight porosity defect even if it is subjected to strong reduction rolling in addition to light reduction by continuous casting, and it is detected as a defect by high-sensitivity UST, which makes it difficult to manufacture. It was thought. The steel plate of the present invention is desirably applied to the steel types for the above uses.

次に、本発明の連続鋳造方法、すなわち、凝固末期圧下法について説明する。   Next, the continuous casting method of the present invention, that is, the end-solidification reduction method will be described.

本発明の連続鋳造方法は、前記極厚鋼板を熱間圧延により製造するための素材として用いる鋳片を連続鋳造する方法であって、該鋳片の厚さ中心部の固相率が0.8以上、1.0未満の範囲において、未凝固部を含む鋳片の幅中央部を、一対の圧下ロールにより3〜15mm圧下し、前記極厚鋼板の単位質量あたりの中心ポロシティ体積Vp(cm 3 /g)と前記鋳片の単位質量あたりの中心ポロシティ体積Vp 0 ’(cm 3 /g)を極厚鋼板の内質向上の指標とし、下記(a)式を満足するか否かを評価することを特徴とする。本発明の凝固末期圧下法は、上述のとおり、中心ポロシティ形成を事前に抑制する効果を十分に発揮するものであるが、凝固末期圧下法を上記のように規定した理由および好ましい範囲について以下に説明する。
(4)圧下時期
本発明の連続鋳造方法では、中心固相率が0.8以上の凝固末期に圧下する、すなわち、中心固相率が0.8以上の時に圧下ロールによって圧下できるように、操業条件(鋳造速度、冷却水量等)を調整する。
The continuous casting method of the present invention is a method of continuously casting a slab used as a raw material for producing the extra-thick steel plate by hot rolling, wherein the solid phase ratio at the center of the thickness of the slab is 0. In the range of 8 or more and less than 1.0, the width center portion of the slab including the unsolidified portion is reduced by 3 to 15 mm with a pair of reduction rolls , and the central porosity volume Vp (cm) per unit mass of the extra- thick steel plate 3 / g) and the central porosity volume Vp 0 ′ (cm 3 / g) per unit mass of the slab are used as indices for improving the quality of the extra-thick steel sheet, and whether or not the following formula (a) is satisfied is evaluated. It is characterized by doing. As described above, the end-coagulation reduction method of the present invention sufficiently exhibits the effect of suppressing central porosity formation in advance, but the reasons and preferred ranges for defining the end-coagulation reduction method as described above are as follows. explain.
(4) Reduction time In the continuous casting method of the present invention, the central solid fraction is reduced to the end of solidification of 0.8 or more, that is, when the central solid fraction is 0.8 or more, it can be reduced by a reduction roll. Adjust operating conditions (casting speed, cooling water volume, etc.).

中心固相率が0.8未満では、鋳片の厚さ中心部には凝固末期の溶鋼がまだ比較的多く残っているために、大きな圧下を加えると鋳片中心部に残っている溶鋼が排出され、母溶鋼に向かって流動する。   If the center solid phase ratio is less than 0.8, a relatively large amount of molten steel at the end of solidification still remains in the center of the slab thickness. It is discharged and flows toward the mother molten steel.

このため、凝固の進行は必ずしも均一ではなく、冷却むら等により凝固シェルの厚さは不均一になるので、圧下時の中心固相率は鋳片の位置によって厳密には異なっている。   For this reason, the progress of solidification is not necessarily uniform, and the thickness of the solidified shell becomes non-uniform due to uneven cooling, etc., so the central solid phase ratio during rolling is strictly different depending on the position of the slab.

従って、中心固相率が0.6以上0.8未満の場合、鋳片の位置によっては、中心固相率が0.8以上となっている部分が存在する可能性がある。この時、圧下により排出された溶鋼が、中心固相率が0.8以上の部分で流動できなくなり、その結果、母溶鋼まで流動して混合することが困難となる。このため、中心ポロシティは低減するものの、排出された溶鋼がそのまま鋳片に偏析として残り、中心偏析状況は逆に悪化する。   Therefore, when the central solid phase ratio is 0.6 or more and less than 0.8, there may be a portion where the central solid phase ratio is 0.8 or more depending on the position of the slab. At this time, the molten steel discharged by reduction cannot flow at a portion where the central solid phase ratio is 0.8 or more, and as a result, it becomes difficult to flow and mix to the mother molten steel. For this reason, although the center porosity is reduced, the discharged molten steel remains as segregation in the slab as it is, and the center segregation situation is worsened.

さらに中心固相率が低くなって、中心固相率が0.6未満の場合は、鋳片の内部に溶鋼が非常に多く残っているため、この溶鋼を排出するためには圧下量を大きくしなければならない。このため、大きな圧下力が必要となり、大規模な圧下設備が必要になる。   Further, when the central solid fraction is lower and the central solid fraction is less than 0.6, a large amount of molten steel remains inside the slab, so a large amount of reduction is required to discharge this molten steel. Must. For this reason, a large reduction force is required, and a large-scale reduction facility is required.

これに対して、固相率が0.8以上の場合は、鋳片の内部に凝固末期の溶鋼が少なく、大きな圧下を加えても溶鋼はほとんど流動することがない。このため、中心偏析状況が悪化することはない。そこで、本発明では、凝固末期の中心固相率が0.8以上において圧下することとしている。   On the other hand, when the solid phase ratio is 0.8 or more, there is little molten steel at the end of solidification in the slab, and even when a large reduction is applied, the molten steel hardly flows. For this reason, the center segregation situation does not deteriorate. Therefore, in the present invention, the reduction is performed when the central solid phase ratio at the end of coagulation is 0.8 or more.

このように、中心固相率が0.8以上、すなわち0.8〜1.0の領域において圧下を加えれば、中心ポロシティの圧着に効果を有する。しかし、中心固相率が1.0、すなわち完全に凝固してからでは、鋳片の厚さ中心部の温度が低下するため、変形抵抗が急激に大きくなる。このため、中心固相率が1.0になった後に大きな圧下を加えたのでは、中心ポロシティが分布している鋳片の厚さ中心部が有効に圧下されず、大きな中心ポロシティはあまり小さくならない可能性がある。   Thus, if reduction is applied in a region where the central solid phase ratio is 0.8 or more, that is, 0.8 to 1.0, it has an effect on the compression of the central porosity. However, after the central solid phase ratio is 1.0, that is, after solidification is complete, the temperature at the central part of the thickness of the slab decreases, and the deformation resistance increases rapidly. For this reason, if a large reduction is applied after the central solid phase ratio reaches 1.0, the thickness center of the slab where the central porosity is distributed is not effectively reduced, and the large central porosity is too small. It may not be possible.

なお、中心固相率fsは、溶鋼の液相線温度TLと固相線温度TSと厚さ中心の温度Tから、fs=(TL−T)/(TL−TS)により求めることができる。鋳片の厚さ中心の温度Tが溶鋼の液相線温度TL以上の場合にはfs=0であり、前記厚さ中心の温度Tが溶鋼の固相線温度TSより小さい場合にはfs=1.0である。また、鋳片の厚さ中心の温度Tは、鋳造速度、鋳片の表面冷却、鋳造鋼種の物性等を考慮した鋳片内の非定常伝熱解析計算によって求めることができる。
(5)圧下量
本発明の連続鋳造方法において、鋳片の幅方向中央部における圧下量を3〜15mmとするのは、圧下量が3mm未満では、鋳片の中心ポロシティを確実に軽減することができないからである。つまり、圧下量が3mm未満の場合には、大きな中心ポロシティがわずかに小さくなるだけで、その後の圧延によっても圧着できずに欠陥として残るからである。
The central solid phase ratio fs is calculated from fs = (T L −T) / (T L −T S ) from the liquidus temperature T L and the solidus temperature T S of the molten steel and the temperature T at the thickness center. Can be sought. When the temperature T at the thickness center of the slab is equal to or higher than the liquidus temperature TL of the molten steel, fs = 0, and when the temperature T at the thickness center is lower than the solidus temperature T S of the molten steel. fs = 1.0. The temperature T at the center of the slab thickness can be obtained by unsteady heat transfer analysis calculation in the slab considering the casting speed, surface cooling of the slab, physical properties of the cast steel type, and the like.
(5) Amount of reduction In the continuous casting method of the present invention, the amount of reduction at the central portion in the width direction of the slab is 3 to 15 mm because the center porosity of the slab is surely reduced when the amount of reduction is less than 3 mm. It is because it is not possible. That is, when the amount of reduction is less than 3 mm, the large central porosity is only slightly reduced, and it cannot be crimped by subsequent rolling and remains as a defect.

一方、中心固相率が0.8以上において、圧下量が15mmを超えて大きくなるようにするには、非常に大きな圧下力が必要であり、それゆえに油圧設備などを含めて大規模な圧下設備が必要になるからである。
(6)圧下方法
本発明では、必要圧下量が3mm〜15mmであるので、少ない圧下容量で効率よく圧下できるように鋳片の上面と下面とを対称圧下することが望ましく、そのためには、圧下時に下ロールを鋳片の下側パスラインよりも上部に突出させることが望ましい。また、鋳片バルジング量は特に規定していないが、例えば、必要圧下量が3mmの場合、バルジング量を2mmとすると、圧下時の変形抵抗の大きい鋳片短辺部の圧下量は1mmで済むこととなるので必要に応じてバルジングを併用してもよい。
On the other hand, when the central solid phase ratio is 0.8 or more, a very large reduction force is required to increase the reduction amount exceeding 15 mm. Therefore, a large-scale reduction including a hydraulic facility is required. This is because equipment is required.
(6) Reduction method In the present invention, since the required reduction amount is 3 mm to 15 mm, it is desirable to symmetrically reduce the upper and lower surfaces of the slab so that the reduction can be efficiently performed with a small reduction capacity. Sometimes it is desirable to have the lower roll protrude above the lower pass line of the slab. The slab bulging amount is not particularly defined. For example, when the required reduction amount is 3 mm, if the bulging amount is 2 mm, the reduction amount of the short side portion of the slab having a large deformation resistance during reduction may be 1 mm. Therefore, bulging may be used in combination as necessary.

本発明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、得られた鋳片を極厚鋼板に圧延し、超音波探傷試験により極厚鋼板の製品評価を行った。
(試験方法)
1)鋳造方法
図3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を模式的に示した図である。試験に用いた鋳型は、厚さが311mm、幅は2300mmの大きさのものを使用した。以下の説明では、この鋳型で鋳造した鋳片を「300mm厚鋳片」と記す。
In order to confirm the effect of the present invention, the following continuous casting test was performed, and the obtained slab was rolled into an extra-thick steel plate, and product evaluation of the extra-thick steel plate was performed by an ultrasonic flaw detection test.
(Test method)
1) Casting method FIG. 3 is a diagram schematically showing a vertical bending die continuous casting apparatus used for testing the continuous casting method of the present invention. The mold used for the test had a thickness of 311 mm and a width of 2300 mm. In the following description, a slab cast with this mold is referred to as a “300 mm thick slab”.

対象とした鋼種は、400MPa級鋼、500MPa級鋼および600MPa級鋼の3水準であり、それぞれ下記の表2に示す化学組成とした。   The target steel types were three levels of 400 MPa class steel, 500 MPa class steel and 600 MPa class steel, and the chemical compositions shown in Table 2 below were used.

鋳造速度Vcは、圧下時の鋳片の中心固相率が、凝固伝熱計算により予め0.8以上となるように、Vc=0.61〜0.62(m/min)の範囲で種々変更した。また、二次冷却水量は0.62〜0.73L/kg−steelとした。   The casting speed Vc varies in a range of Vc = 0.61 to 0.62 (m / min) so that the center solid phase ratio of the slab during reduction is 0.8 or more in advance by solidification heat transfer calculation. changed. The amount of secondary cooling water was 0.62 to 0.73 L / kg-steel.

タンディッシュ(図示せず)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3及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が形成されて鋳片8となる。鋳片8の内部に未凝固部を保持したまま、鋳片8はガイドロール6群を経て圧下ロール7により引き抜かれる。   The molten steel 4 injected into the mold 3 from the tundish (not shown) through the immersion nozzle 1 is cooled by spray water sprayed from the mold 3 and a group of secondary cooling spray nozzles (not shown) below it, A solidified shell 5 is formed to become a slab 8. The slab 8 is pulled out by the reduction roll 7 through the guide roll 6 group while the unsolidified portion is held inside the slab 8.

圧下ロール7は、鋳型3の内部に形成される溶鋼湯面(メニスカス)2より21.5m下方の位置に1対設置した。圧下ロール7の径は450mmで、圧下力は最大5.88×103kN(600ton)とした。なお、試験に用いた連続鋳造機は垂直曲げ型連続鋳造機であるが、湾曲型連続鋳造機を使用しても良いことは言うまでもない。 A pair of rolling rolls 7 was installed at a position 21.5 m below the molten steel surface (meniscus) 2 formed inside the mold 3. The diameter of the reduction roll 7 was 450 mm, and the maximum reduction force was 5.88 × 10 3 kN (600 ton). Although the continuous casting machine used for the test is a vertical bending type continuous casting machine, it goes without saying that a curved type continuous casting machine may be used.

圧下時の中心固相率は、主に鋳造速度と、鋳片の幅中央部の厚さに合わせて、種々鋳造速度を変えて一次元の伝熱計算を行い、所定の固相率になる条件を求めた。   The central solid fraction during rolling is one-dimensional heat transfer calculation at various casting speeds according to the casting speed and the thickness of the central part of the slab. Conditions were sought.

また、タンディッシュ内の溶鋼温度は、ΔT(過熱度)=27℃〜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の差である。
2)鋳片および極厚鋼板の中心ポロシティ評価
得られた鋳片は、中心ポロシティの調査のために一部から試料を採取した後、950〜1170℃に加熱し、1050〜750℃の範囲で仕上げ圧延を行い極厚鋼板を製造した。使用した仕上げ圧延機のワークロール径は1040mm、最大圧下力は6.17×104kN(6300ton)であった。中心ポロシティの調査のために極厚鋼板の一部から試料を採取した。
Moreover, the molten steel temperature in the tundish was made substantially constant between ΔT (degree of superheat) = 27 ° C. to 50 ° C. ΔT is the difference between the molten steel temperature and the liquidus temperature.
2) Evaluation of central porosity of cast slab and extra-thick steel plate The obtained slab was sampled from a part for investigation of central porosity, then heated to 950-1170 ° C, and in the range of 1050-750 ° C. Finished rolling was performed to produce a very thick steel plate. The finish rolling mill used had a work roll diameter of 1040 mm and a maximum rolling force of 6.17 × 10 4 kN (6300 ton). Samples were taken from some of the thick steel plates for the investigation of central porosity.

鋳片については、鋳片の1/4厚み位置の幅方向7箇所、厚み中心の幅方向16箇所から試料を採取した。試料形状は、比重測定の精度を勘案し長さ50mm×幅100mm×厚さ7mmとし、面の加工精度はJISに基づく上仕上げ(三角記号▽▽▽:最大表面粗さ3.2μ)程度とした。   About the slab, the sample was extract | collected from the width direction 7 places of the 1/4 thickness position of a slab, and the width direction 16 places of the thickness center. The sample shape is 50 mm long x 100 mm wide x 7 mm thick considering the accuracy of specific gravity measurement, and the surface processing accuracy is about JIS-based finishing (triangle symbol ▽▽▽: maximum surface roughness 3.2μ) did.

中心ポロシティ発生がほとんどないとみられる鋳片の1/4厚み位置の平均比重を基準として、厚さ中心部の比重から算出した中心ポロシティの比体積で評価した。1/4厚み位置の平均比重ρ0と、厚み中心の平均比重ρから、下記の(3)式で定義する中心ポロシティ体積Vp(cm3/g)を求めた。 Evaluation was made based on the specific volume of the central porosity calculated from the specific gravity at the center of the thickness with reference to the average specific gravity at the 1/4 thickness position of the slab where the generation of the central porosity is considered to be almost absent. From the average specific gravity ρ 0 at the ¼ thickness position and the average specific gravity ρ at the thickness center, the central porosity volume Vp (cm 3 / g) defined by the following equation (3) was determined.

Vp=1/ρ−1/ρ0 ・・・(3)
また、極厚鋼板についても、上記と同じ条件で試料採取を行ったが、各極厚鋼板で圧下比が異なるため、試料形状の厚みは極厚鋼板の厚みの約1/20で統一した。
Vp = 1 / ρ−1 / ρ 0 (3)
The extra thick steel plates were also sampled under the same conditions as described above, but because the reduction ratio was different for each extra thick steel plate, the thickness of the sample shape was unified at about 1/20 of the thickness of the extra thick steel plate.

圧延後の極厚鋼板は、下記のUST評価方法により、未圧着の中心ポロシティを評価した。UST装置は、Aスコープ表示式探傷器で、振動子直径30mm、公称周波数2MHzの垂直探傷子を用いた。測定された欠陥の個数、欠陥1個当たりの最大指示長さ、密集度、占積率等が当該JISに規定された値以下の場合に、その極厚鋼板は合格とし、UST欠陥はないものと判断した。
(UST評価方法)
本発明はUST欠陥のない内質の優れた高強度極厚鋼板を製造することを目的としているため、JIS G0801(1993)で規定するUSTよりも厳格な試験方法でUST欠陥の評価を行った。本発明で採用する垂直超音波探傷法では、JIS G0801を応用し、縦または横100mmピッチの線上探傷で、きずエコー高さ(以下「F1」という)が25%超えでその欠陥指示長さが5mm超えの欠陥が1個/平方メートル以下と規定した。
The extremely thick steel sheet after rolling was evaluated for unbonded central porosity by the following UST evaluation method. The UST apparatus was an A scope display type flaw detector, and a vertical flaw detector having a transducer diameter of 30 mm and a nominal frequency of 2 MHz was used. When the measured number of defects, maximum indicated length per defect, density, space factor, etc. are below the values specified in the JIS, the heavy steel plate is accepted and there is no UST defect It was judged.
(UST evaluation method)
The purpose of the present invention is to produce a high-strength ultra-thick steel plate having no internal defects and excellent in quality. Therefore, the UST defects were evaluated by a test method stricter than the UST specified in JIS G0801 (1993). . In the vertical ultrasonic flaw detection method employed in the present invention, JIS G0801 is applied, and the flaw echo height (hereinafter referred to as “F 1 ”) exceeds 25% in a line flaw detection with a vertical or horizontal 100 mm pitch, and the defect indication length. Is defined as 1 piece / square meter or less.

JIS G0801は、圧力容器用鋼板に対する規定であるが、便宜的にこの方法を本鋼材にも適用した。ただし、その感度設定においては、JIS G0801のままでは不十分であり、JIS G0801に基づく探傷感度設定をした後、さらにその感度を+12dBアップ(約4倍の感度)し、それをAスコープ表示装置における0〜100%表示の中で評価した。デジタル式探傷器による場合は、これに準じた。   JIS G0801 is a regulation for steel plates for pressure vessels, but this method was also applied to this steel material for convenience. However, it is not sufficient to set the sensitivity as it is in JIS G0801, and after setting the flaw detection sensitivity based on JIS G0801, the sensitivity is further increased by +12 dB (approximately 4 times the sensitivity), and it is displayed on the A scope display device. It evaluated in 0-100% display in. In the case of using a digital flaw detector, this was followed.

通常のJIS G0801においては、主にF1又はF1/B1が50%超えの中欠陥及び重欠陥が判定の対象となり、F1又はF1/B1が25〜50%の〇欠陥は問題とされない場合が多いが、本発明で採用する垂直超音波探傷法の鋼板内部の評価にあっては、〇欠陥も評価の対象とした。しかも、本発明で採用する評価方法は、探傷感度を+12dBアップ(約4倍の感度)していることから、評価対象となる〇欠陥は6〜12.5%の微小な欠陥を意味し、そのような微小な欠陥が、鋼板内部でその欠陥指示長さが5mm超えの欠陥が1個/平方メートル以下という極めて厳しい鋼の内部健全性を要求するものである。
(試験結果)
表3に実施例の試験条件を、表4に試験結果をそれぞれ示す。本発明例の試験番号T1〜T4は、圧下量を12mmとして本発明の凝固末期圧下法を適用して製造した鋳片を圧延素材とした極厚鋼板についての試験であり、比較例の試験番号T5〜T8は、凝固末期圧下法を適用しなかった、すなわち圧下量を0mmとして製造した鋳片を圧延素材とした極厚鋼板についての試験である。
In normal JIS G0801, F 1 or F 1 / B 1 is mainly subject to medium and heavy defects exceeding 50%, and F 1 or F 1 / B 1 is 25 to 50%. In many cases, it is not regarded as a problem, but in the evaluation of the inside of the steel plate of the vertical ultrasonic flaw detection method adopted in the present invention, the defect was also evaluated. Moreover, since the evaluation method employed in the present invention increases the flaw detection sensitivity by +12 dB (approximately 4 times the sensitivity), the defect to be evaluated means a defect of 6 to 12.5%, Such a minute defect requires extremely strict internal soundness of steel in which the number of defects having a defect indication length exceeding 5 mm within the steel sheet is 1 piece / square meter or less.
(Test results)
Table 3 shows the test conditions of the examples, and Table 4 shows the test results. Test numbers T1 to T4 of the present invention example are tests on a very thick steel plate using a slab manufactured by applying the final solidification reduction method of the present invention with a reduction amount of 12 mm, and a test number of a comparative example. T5 to T8 are tests on a very thick steel plate that did not apply the end-solidification reduction method, that is, a slab manufactured with a reduction amount of 0 mm as a rolling material.

図4は、本発明例および比較例の500MPa級鋼鋳片(1/4厚みおよび1/2厚み)の比重測定結果を示す図である。同図に示すように、本発明の凝固末期圧下法を適用した本発明例の鋳片では、中心(1/2厚み)のポロシティが、凝固末期圧下法を適用しなかった比較例の鋳片よりも、明らかに改善されていた。   FIG. 4 is a diagram showing specific gravity measurement results of 500 MPa class steel slabs (1/4 thickness and 1/2 thickness) of the present invention and comparative examples. As shown in the figure, in the slab of the present invention to which the end-solidification reduction method of the present invention was applied, the center (1/2 thickness) porosity was a slab of the comparative example in which the end-solidification reduction method was not applied Than it was clearly improved.

他の鋼種についても同様に測定を行い、前記の(3)式により算出した単位質量あたりの中心ポロシティ体積を求めた。   The other steel types were measured in the same manner, and the central porosity volume per unit mass calculated by the above equation (3) was obtained.

図5は、本発明例および比較例の鋳片の単位質量当たりの中心ポロシティ体積を示す図である。同図に示すように、本発明の凝固末期圧下法を適用した場合は、凝固末期圧下法を適用しなかった場合に比べて、400MPa級鋼、500MPa級鋼および600MPa級鋼のいずれの鋼種においても中心ポロシティ体積を1/3以下に低減する効果があることが確認された。   FIG. 5 is a diagram showing the central porosity volume per unit mass of the slabs of the present invention example and the comparative example. As shown in the figure, when the end-solidification reduction method of the present invention is applied, in any steel type of 400 MPa class steel, 500 MPa class steel, and 600 MPa class steel, compared to the case where the end-solidification reduction method is not applied. It was also confirmed that there is an effect of reducing the central porosity volume to 1/3 or less.

図6は、横軸に圧下比rを、縦軸に単位質量当たりの中心ポロシティ体積をとり、本発明例および比較例の極厚鋼板の単位質量当たりの中心ポロシティ体積の分布を示す図である。同図に示すように、本発明の凝固末期圧下法を適用した場合は、中心ポロシティ体積が曲線Vp0/rよりも下側の領域に分布し、本発明で規定する前記の(1)式の関係を満足した。これに対し、比較例の場合は、中心ポロシティ体積が全て曲線Vp0/rよりも上側の領域に分布した。
さらに、本発明で規定する前記の(1)式の関係を満足した試験番号T1〜T4は、表4に示すように、上記のUST評価においても合格する良好な結果を得た。
FIG. 6 is a graph showing the distribution of the central porosity volume per unit mass of the extra-thick steel plates of the present invention and the comparative example, with the rolling ratio r on the horizontal axis and the central porosity volume per unit mass on the vertical axis. . As shown in the figure, when the end-coagulation reduction method of the present invention is applied, the central porosity volume is distributed in a region below the curve Vp 0 / r, and the above formula (1) defined in the present invention is used. Satisfied with the relationship. On the other hand, in the case of the comparative example, the central porosity volume was all distributed in the region above the curve Vp 0 / r.
Furthermore, as shown in Table 4, the test numbers T1 to T4 satisfying the relationship of the expression (1) defined in the present invention obtained good results that passed in the above-mentioned UST evaluation.

本発明の極厚鋼板は、既存の厚板用圧延機を用いて製造することが可能なUST欠陥のない内質に優れた極厚鋼板であり、低コストかつ高品質の鋼板として広い用途に使用できる。   The extra-thick steel plate of the present invention is an extra-thick steel plate with excellent internal quality that has no UST defects and can be manufactured using an existing thick plate rolling mill, and is widely used as a low-cost and high-quality steel plate. Can be used.

また、本発明の連続鋳造方法によれば、前記極厚鋼板の素材となる鋳片を溶鋼から連続的に製造する際に、比較的簡易な連続鋳造装置の圧下設備を用いて、中心ポロシティを低減した鋳片を鋳造することができるので、極厚鋼板の製造に大きく貢献できる。   Further, according to the continuous casting method of the present invention, when continuously producing the slab, which is the raw material of the extra-thick steel plate, from the molten steel, the central porosity is reduced by using a relatively simple rolling equipment. Since the reduced slab can be cast, it can greatly contribute to the production of extra-thick steel plates.

これにより、製造が困難と考えられていた極厚の高感度UST対象材の製造可能範囲を拡大することができるという利点がある。   Thereby, there exists an advantage that the manufacturable range of the ultra-thin high sensitivity UST object material considered that manufacture is difficult can be expanded.

本発明の凝固末期圧下法を適用した鋳片の中心ポロシティ体積Vp0と鋼種の引張強度レベルの相関を示す図である。Is a graph showing the correlation of the coagulation end reduction method of the slab to which the center porosity volume Vp 0 and steels of a tensile strength level of the present invention. 圧下比が3.0の圧延圧下を行った場合の、本発明の凝固末期圧下法の適用の有無による鋳片と極厚鋼板の中心ポロシティ体積を示した図であり、同図(a)は極厚鋼板の引張強度レベルが500MPa級の場合を示す図であり、同図(b)は極厚鋼板の引張強度レベルが600MPa級の場合を示す図である。It is the figure which showed the center porosity volume of the slab and the extra-thick steel plate by the presence or absence of application of the end-of-solidification reduction method of the present invention when rolling reduction of a reduction ratio of 3.0 was performed. It is a figure which shows the case where the tensile strength level of a very thick steel plate is a 500 MPa class, and the figure (b) is a figure which shows the case where the tensile strength level of a very thick steel sheet is a 600 MPa class. 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を模式的に示した図である。It is the figure which showed typically the perpendicular | vertical bending type continuous casting apparatus used in order to test the continuous casting method of this invention. 本発明例および比較例の500MPa級鋼鋳片(1/4厚みおよび1/2厚み)の比重測定結果を示す図である。It is a figure which shows the specific gravity measurement result of the 500 MPa class steel slab (1/4 thickness and 1/2 thickness) of the example of this invention and a comparative example. 本発明例および比較例の鋳片の単位質量当たりの中心ポロシティ体積を示す図である。It is a figure which shows the center porosity volume per unit mass of the slab of the example of this invention and a comparative example. 横軸に圧下比rを、縦軸に単位質量当たりの中心ポロシティ体積をとり、本発明例および比較例の極厚鋼板の単位質量当たりの中心ポロシティ体積の分布を示す図である。It is a figure which shows the reduction ratio r on a horizontal axis | shaft and the center porosity volume per unit mass on a vertical axis | shaft, and shows the distribution of the center porosity volume per unit mass of the ultra-thick steel plate of the example of this invention and a comparative example.

符号の説明Explanation of symbols

1.浸漬ノズル
2.溶鋼湯面(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.圧下ロール
8.鋳片
1. Immersion nozzle Molten steel surface (meniscus)
3. Mold 4. Molten steel 5. Solidified shell 6. Guide roll 7. Rolling roll 8. Slab

Claims (2)

仕上げ圧延までの圧下比rが1.5〜4.0の条件で素材を熱間圧延して得られ、引張強度のレベルが400MPa級乃至600MPa級である極厚鋼板を製造するために、前記素材として用いる鋳片を連続鋳造する方法であって、
該鋳片の厚さ中心部の固相率が0.8以上、1.0未満の範囲において、未凝固部を含む鋳片の幅中央部を、一対の圧下ロールにより3〜15mm圧下し、
前記極厚鋼板の単位質量あたりの中心ポロシティ体積Vp(cm 3 /g)と前記鋳片の単位質量あたりの中心ポロシティ体積Vp 0 ’(cm 3 /g)を極厚鋼板の内質向上の指標とし、下記(a)式を満足するか否かを評価することを特徴とする極厚鋼板用鋳片の連続鋳造方法。
Vp < Vp 0 ’/r ・・・(a)
Reduction ratio r to the finish rolling to obtain hot rolled materials under conditions of 1.5 to 4.0, in order to level the tensile strength is manufacturing a very thick steel plate is 400MPa class to 600MPa class, a method of continuous casting a slab to be used as the material,
In the range where the solid phase ratio of the thickness center portion of the slab is 0.8 or more and less than 1.0, the width center portion of the slab including the unsolidified portion is reduced by 3 to 15 mm by a pair of reduction rolls ,
The central porosity volume Vp (cm 3 / g) per unit mass of the extra-thick steel plate and the central porosity volume Vp 0 ′ (cm 3 / g) per unit mass of the slab are indicators for improving the quality of the extra-thick steel plate And evaluating whether or not the following expression (a) is satisfied .
Vp <Vp 0 '/ r (a)
請求項1に記載の連続鋳造方法で製造された極厚鋼板用鋳片を、仕上げ圧延までの圧下比rが1.5〜4.0の条件で熱間圧延し、前記(a)式のVp  The ultra-thick steel plate slab manufactured by the continuous casting method according to claim 1 is hot-rolled under a condition that the reduction ratio r until finish rolling is 1.5 to 4.0, and the formula (a) Vp 00 ’を下記(2)式のVp′ Is Vp in the following equation (2) 00 に置き換えて得られる下記(1)式を極厚鋼板の内質向上の指標として、同式を満足するか否かを評価し、満足するものを内質に優れた極厚鋼板とすることを特徴とする極厚鋼板の製造方法。Using the following formula (1) obtained by substituting as an index for improving the quality of extra-thick steel plates, whether or not the formula is satisfied is evaluated, and what is satisfied is to make the extra-thick steel plate excellent in inner quality. A method for producing a very heavy steel sheet.
Vp < Vp  Vp <Vp 00 /r ・・・(1)/ R (1)
Vp  Vp 00 =(0.020×X/10−0.085)×10= (0.020 * X / 10-0.085) * 10 -4-Four ・・・(2)      ... (2)
ここで、極厚鋼板の引張強度Xは、400MPa〜600MPaの範囲で選択される値であり、換算中心ポロシティ体積Vp  Here, the tensile strength X of the extra-thick steel plate is a value selected in the range of 400 MPa to 600 MPa, and the converted center porosity volume Vp. 00 (cm(Cm 3Three /g)の算出根拠となる/ G)
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PL3012044T3 (en) 2013-06-18 2018-09-28 Nippon Steel & Sumitomo Metal Corporation Continuous casting method for casting for extra thick steel sheet
JP6303617B2 (en) * 2014-03-06 2018-04-04 新日鐵住金株式会社 Slab continuous casting method
CN105548207B (en) * 2016-02-02 2018-06-08 攀钢集团攀枝花钢铁研究院有限公司 Continuous casting billet center porosity or the method for quantitatively determining of shrinkage cavity
KR102223119B1 (en) * 2018-12-19 2021-03-04 주식회사 포스코 Manufacturing method for very thick steel plate and casting slab for very thick steel plate
CN113198992B (en) * 2021-03-31 2022-08-09 武汉钢铁有限公司 Production method and device of crack sensitive steel continuous casting billet and application of ultrasonic wave

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS6342345A (en) * 1986-08-08 1988-02-23 Mitsui Alum Kogyo Kk Aluminum casting alloy for welding
JPH01289552A (en) * 1988-05-16 1989-11-21 Sumitomo Metal Ind Ltd Forging press apparatus for round shaped continuously cast billet
JPH0569099A (en) * 1991-09-12 1993-03-23 Nippon Steel Corp Method for improving internal quality in cast slab
JPH07276020A (en) * 1994-04-08 1995-10-24 Nippon Steel Corp Continuous casting method
JP2005177848A (en) * 2003-12-22 2005-07-07 Nippon Steel Corp Center defect reducing method of continuously cast steel slab
JP2005305516A (en) * 2004-04-22 2005-11-04 Sumitomo Metal Ind Ltd Continuous casting method, and extra-thick steel plate excellent in inner quality and producing method therefor
JP2006341297A (en) * 2005-06-10 2006-12-21 Sumitomo Metal Ind Ltd Continuous casting method, and continuously cast slab

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS6342345A (en) * 1986-08-08 1988-02-23 Mitsui Alum Kogyo Kk Aluminum casting alloy for welding
JPH01289552A (en) * 1988-05-16 1989-11-21 Sumitomo Metal Ind Ltd Forging press apparatus for round shaped continuously cast billet
JPH0569099A (en) * 1991-09-12 1993-03-23 Nippon Steel Corp Method for improving internal quality in cast slab
JPH07276020A (en) * 1994-04-08 1995-10-24 Nippon Steel Corp Continuous casting method
JP2005177848A (en) * 2003-12-22 2005-07-07 Nippon Steel Corp Center defect reducing method of continuously cast steel slab
JP2005305516A (en) * 2004-04-22 2005-11-04 Sumitomo Metal Ind Ltd Continuous casting method, and extra-thick steel plate excellent in inner quality and producing method therefor
JP2006341297A (en) * 2005-06-10 2006-12-21 Sumitomo Metal Ind Ltd Continuous casting method, and continuously cast slab

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