JP5157598B2 - Ni-containing steel slab and method for continuously casting Ni-containing steel - Google Patents
Ni-containing steel slab and method for continuously casting Ni-containing steel Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 74
- 239000010959 steel Substances 0.000 title claims description 74
- 238000005266 casting Methods 0.000 title claims description 27
- 238000000034 method Methods 0.000 title claims description 25
- 238000007711 solidification Methods 0.000 claims description 66
- 230000008023 solidification Effects 0.000 claims description 66
- 239000000843 powder Substances 0.000 claims description 31
- 238000009749 continuous casting Methods 0.000 claims description 29
- 229910052698 phosphorus Inorganic materials 0.000 claims description 10
- 229910052759 nickel Inorganic materials 0.000 claims description 9
- 229910052717 sulfur Inorganic materials 0.000 claims description 8
- 229910052799 carbon Inorganic materials 0.000 claims description 5
- 229910052796 boron Inorganic materials 0.000 claims description 4
- 229910052791 calcium Inorganic materials 0.000 claims description 4
- 229910052804 chromium Inorganic materials 0.000 claims description 4
- 229910052750 molybdenum Inorganic materials 0.000 claims description 4
- 229910052758 niobium Inorganic materials 0.000 claims description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 229910052719 titanium Inorganic materials 0.000 claims description 4
- 229910052720 vanadium Inorganic materials 0.000 claims description 4
- 229910052782 aluminium Inorganic materials 0.000 claims description 3
- 239000012535 impurity Substances 0.000 claims description 3
- 229910052748 manganese Inorganic materials 0.000 claims description 3
- 239000000203 mixture Substances 0.000 claims description 3
- 229910052760 oxygen Inorganic materials 0.000 claims description 3
- 229910052710 silicon Inorganic materials 0.000 claims description 3
- 229910052742 iron Inorganic materials 0.000 claims description 2
- PXHVJJICTQNCMI-UHFFFAOYSA-N nickel Substances [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 65
- 210000004027 cell Anatomy 0.000 description 26
- 238000012360 testing method Methods 0.000 description 22
- 230000001965 increasing effect Effects 0.000 description 19
- 239000000463 material Substances 0.000 description 17
- 230000007423 decrease Effects 0.000 description 16
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- 238000001816 cooling Methods 0.000 description 14
- 238000005336 cracking Methods 0.000 description 14
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- 239000010949 copper Substances 0.000 description 5
- 238000007670 refining Methods 0.000 description 5
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 4
- 239000011651 chromium Substances 0.000 description 4
- 229910052802 copper Inorganic materials 0.000 description 4
- 238000002844 melting Methods 0.000 description 4
- 239000010955 niobium Substances 0.000 description 4
- 238000009628 steelmaking Methods 0.000 description 4
- 239000010936 titanium Substances 0.000 description 4
- 238000012937 correction Methods 0.000 description 3
- 239000007791 liquid phase Substances 0.000 description 3
- 239000011572 manganese Substances 0.000 description 3
- 238000004519 manufacturing process Methods 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 2
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 2
- 229910000746 Structural steel Inorganic materials 0.000 description 2
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 2
- 230000003247 decreasing effect Effects 0.000 description 2
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- 238000001514 detection method Methods 0.000 description 2
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- 229910052751 metal Inorganic materials 0.000 description 2
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- 230000002265 prevention Effects 0.000 description 2
- 238000005204 segregation Methods 0.000 description 2
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- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- 229910052582 BN Inorganic materials 0.000 description 1
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 1
- PZNSFCLAULLKQX-UHFFFAOYSA-N Boron nitride Chemical compound N#B PZNSFCLAULLKQX-UHFFFAOYSA-N 0.000 description 1
- 229910004261 CaF 2 Inorganic materials 0.000 description 1
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 1
- 229910004298 SiO 2 Inorganic materials 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 239000003963 antioxidant agent Substances 0.000 description 1
- 230000003078 antioxidant effect Effects 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 239000003795 chemical substances by application Substances 0.000 description 1
- 230000015271 coagulation Effects 0.000 description 1
- 238000005345 coagulation Methods 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000006477 desulfuration reaction Methods 0.000 description 1
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- 238000011156 evaluation Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
- 239000000314 lubricant Substances 0.000 description 1
- 239000011733 molybdenum Substances 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 1
- 230000006911 nucleation Effects 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- OXNIZHLAWKMVMX-UHFFFAOYSA-N picric acid Chemical compound OC1=C([N+]([O-])=O)C=C([N+]([O-])=O)C=C1[N+]([O-])=O OXNIZHLAWKMVMX-UHFFFAOYSA-N 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 238000010791 quenching Methods 0.000 description 1
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- 239000010703 silicon Substances 0.000 description 1
- 238000009849 vacuum degassing Methods 0.000 description 1
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 description 1
Classifications
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Description
本発明は、Ｎｉ（ニッケル）を７．５〜１０質量％含有するＮｉ含有鋼の鋳片、並びに、該Ｎｉ含有鋼鋳片を得るための連続鋳造方法に関するものである。 The present invention relates to a slab of Ni-containing steel containing 7.5 to 10% by mass of Ni (nickel) and a continuous casting method for obtaining the Ni-containing steel slab.
鋼にＮｉを添加すると低温靭性が向上することが知られており、Ｎｉを２〜１０質量％程度含有する鋼が低温用鋼材として広く使用されている。なかでも９質量％前後のＮｉを含有する、所謂、９％Ｎｉ鋼は、−１６０℃以下での使用にも耐えることから、低温用の溶接構造用鋼として、ＬＮＧタンクなどに広く使用されている。しかしながら、９％Ｎｉ鋼をはじめとして、Ｎｉを７．５〜１０質量％含有するＮｉ含有鋼は、表面疵が発生しやすいことが知られており、鋳造後の鋳片の段階で、すでに表面及び表面近傍に多数の割れが存在する。 It is known that low temperature toughness is improved when Ni is added to steel, and steel containing about 2 to 10% by mass of Ni is widely used as a low temperature steel material. Among them, so-called 9% Ni steel containing about 9% by mass of Ni can withstand use at −160 ° C. or less, and is widely used in LNG tanks and the like as low-temperature welded structural steel. Yes. However, it is known that Ni-containing steel containing 7.5 to 10% by mass of Ni, including 9% Ni steel, is prone to surface flaws. And there are many cracks near the surface.
この連続鋳造工程におけるＮｉ含有鋼鋳片の表面割れの発生機構としては、凝固時に結晶粒界に偏析したＳ（硫黄）やＰ（燐）などが結晶粒界を脆化させ、連続鋳造機の二次冷却帯での鋳片の曲げ矯正による応力や冷却による熱応力などによって結晶粒界が破壊され、割れに至ると考えられてきた。特に、６００〜８００℃の温度範囲（「高温脆化温度域」という）において低い歪速度（１０-3／ｓ程度）で曲げ矯正などの加工を加えると、多数の表面割れが発生すると考えられてきた。 As a mechanism for generating surface cracks in the Ni-containing steel slab in this continuous casting process, S (sulfur), P (phosphorus), etc. segregated at the grain boundaries during solidification embrittle the grain boundaries, It has been thought that the grain boundaries are destroyed and cracked due to stress due to bending correction of the slab in the secondary cooling zone or thermal stress due to cooling. In particular, when surface processing such as bending correction is applied at a low strain rate (about 10 −3 / s) in a temperature range of 600 to 800 ° C. (referred to as “high temperature embrittlement temperature range”), it is considered that many surface cracks occur. I came.
このように、Ｎｉ含有鋼を連続鋳造によって製造する際には、鋳片の表面割れの発生を防止することが極めて重要であり、また、上記のように、Ｎｉ含有鋼の連続鋳造時の表面割れを防止するには高温脆化温度域での矯正を避けることが有効であるとして、鋳片表面温度の管理を目的とした多数の二次冷却の制御方法が、Ｎｉ含有鋼鋳片の表面割れ防止方法として提案されてきた。 Thus, when producing Ni-containing steel by continuous casting, it is extremely important to prevent the occurrence of surface cracks in the slab, and as described above, the surface of Ni-containing steel during continuous casting In order to prevent cracking, it is effective to avoid straightening in the high temperature embrittlement temperature range. It has been proposed as a crack prevention method.
例えば、特許文献１には、Ｎｉを５〜１０質量％含有する溶鋼を連続鋳造するに際して、溶鋼のＰ濃度を０．００６質量％以下、Ｓ濃度を０．００３質量％以下に調整し、且つ、二次冷却帯においては、鋳片の液相線温度から１０５０℃までの温度領域は３．０℃／秒以上の冷却速度で冷却するとともに、鋳片の矯正点通過時の表面温度を８５０℃以上に維持する連続鋳造方法が提案されている。 For example, in Patent Document 1, when continuously casting molten steel containing 5 to 10% by mass of Ni, the P concentration of the molten steel is adjusted to 0.006% by mass or less, and the S concentration is adjusted to 0.003% by mass or less, and In the secondary cooling zone, the temperature range from the liquidus temperature of the slab to 1050 ° C. is cooled at a cooling rate of 3.0 ° C./second or more, and the surface temperature when the slab passes through the correction point is 850. There has been proposed a continuous casting method that maintains the temperature at or above C.
また、特許文献２には、Ｎｉを８〜１０質量％含有する高Ｎｉ鋼を垂直−曲げ型連続鋳造機によって連続鋳造する際に、鋳型内での鋳片のコーナー部表面温度の極小値（Ｔ１）、鋳片が鋳型を出てから上部矯正帯に至るまでの時間（ｔ）、及び鋳型を出てから上部矯正帯に至るまでの鋳片のコーナー部表面温度の平均値（Ｔ２）を算出し、算出した極小値（Ｔ１）、時間（ｔ）、平均値（Ｔ２）、及び予め測定した当該高Ｎｉ鋼の絞り値から、当該鋳造時における絞り値を推定し、この絞り値が５０％以上になるように、鋳造速度及び二次冷却強度を制御する連続鋳造方法が提案されている。
上記のように、表面割れを抑制するべく、Ｎｉを７．５〜１０質量％含有するＮｉ含有鋼の連続鋳造方法が多数提案されているが、いまだ表面割れの発生を完全に防止することはできないのが実状である。そのために、グラインダーなどを用いた鋳片表面の研削による割れ除去処理を余儀なくされ、生産性の低下や製造コストの上昇をもたらしていた。 As described above, in order to suppress surface cracking, many continuous casting methods of Ni-containing steel containing 7.5 to 10% by mass of Ni have been proposed, but it is still possible to completely prevent the occurrence of surface cracking. The actual situation is not possible. Therefore, the crack removal process by grinding of the slab surface using a grinder etc. was forced, resulting in a decrease in productivity and an increase in manufacturing cost.
本発明は上記事情に鑑みてなされたもので、その目的とするところは、生産性の低下や製造コストの上昇をもたらしていた表面割れの発生の少ない、Ｎｉを７．５〜１０質量％含有するＮｉ含有鋼鋳片を提供するとともに、該Ｎｉ含有鋼鋳片を鋳造するための連続鋳造方法を提供することである。 The present invention has been made in view of the above circumstances, and the object is to contain Ni in an amount of 7.5 to 10% by mass with less generation of surface cracks that caused a decrease in productivity and an increase in manufacturing cost. An Ni-containing steel slab is provided, and a continuous casting method for casting the Ni-containing steel slab is provided.
上記課題を解決するための第１の発明に係るＮｉ含有鋼鋳片は、Ｎｉを７．５〜１０．０質量％含有し、連続鋳造によって鋳造されたＮｉ含有鋼鋳片であって、鋳片表面での凝固核発生密度が０．６個／ｍｍ2以上であることを特徴とするものである。 The Ni-containing steel slab according to the first invention for solving the above-mentioned problem is a Ni-containing steel slab containing Ni of 7.5 to 10.0% by mass and cast by continuous casting, The solidification nucleus generation density on one surface is 0.6 pieces / mm 2 or more.
第２の発明に係るＮｉ含有鋼鋳片は、質量％で、Ｃ：０．０３〜０．１０％、Ｓｉ：０．０１〜０．５％、Ｍｎ：０．３〜１．０％、Ｐ：０．００１〜０．０１０％、Ｓ：０．０００１〜０．００５％以下、Ｎｉ：７．５〜１０．０％、Ａｌ：０．０１０〜０．０８０％、Ｎ：０．００１０〜０．００５０％、Ｏ：０．０００５〜０．００４０％を含有し、残部がＦｅ及び不可避的不純物からなる、連続鋳造によって鋳造されたＮｉ含有鋼鋳片であって、鋳片表面での凝固核発生密度が０．６個／ｍｍ2以上であることを特徴とするものである。 The Ni-containing steel slab according to the second invention is in mass%, C: 0.03-0.10%, Si: 0.01-0.5%, Mn: 0.3-1.0%, P: 0.001 to 0.010%, S: 0.0001 to 0.005% or less, Ni: 7.5 to 10.0%, Al: 0.010 to 0.080%, N: 0.0010 A Ni-containing steel slab cast by continuous casting, containing ~ 0.0050%, O: 0.0005-0.0040%, the balance consisting of Fe and inevitable impurities, The solidification nucleus generation density is 0.6 pieces / mm 2 or more.
第３の発明に係るＮｉ含有鋼鋳片は、第２の発明において、前記Ｎｉ含有鋼鋳片は、鋼組成として、更に、質量％で、Ｃｕ：０．０３〜１．５％、Ｃｒ：０．０３〜１．０％、Ｍｏ：０．０２〜１．０％、Ｎｂ：０．００３〜０．１０％、Ｖ：０．００３〜０．１０％、Ｔｉ：０．００５〜０．０２％、Ｂ：０．０００２〜０．００２５％、Ｃａ：０．０００５〜０．００５％のなかの１種または２種以上を含有することを特徴とするものである。 The Ni-containing steel slab according to the third invention is the Ni-containing steel slab according to the second invention, wherein the Ni-containing steel slab further has a steel composition in mass%, Cu: 0.03 to 1.5%, Cr: 0.03-1.0%, Mo: 0.02-1.0%, Nb: 0.003-0.10%, V: 0.003-0.10%, Ti: 0.005-0. One or more of 02%, B: 0.0002 to 0.0025%, and Ca: 0.0005 to 0.005% are contained.
第４の発明に係るＮｉ含有鋼の連続鋳造方法は、第１ないし第３の発明の何れか１つに記載のＮｉ含有鋼鋳片を鋳造するための連続鋳造方法であって、１３００℃における粘度が１．０Ｐａ・ｓ（１０ポアズ）以上のモールドパウダーを鋳型内に添加して鋳造することを特徴とするものである。 A Ni-containing steel continuous casting method according to a fourth invention is a continuous casting method for casting the Ni-containing steel slab according to any one of the first to third inventions, at 1300 ° C. A mold powder having a viscosity of 1.0 Pa · s (10 poise) or more is added into a mold and cast.
第５の発明に係るＮｉ含有鋼の連続鋳造方法は、第１ないし第３の発明の何れか１つに記載のＮｉ含有鋼鋳片を鋳造するための連続鋳造方法であって、鋳型を毎分８０サイクル以上の振動数で振動させて鋳造することを特徴とするものである。 A Ni-containing steel continuous casting method according to a fifth invention is a continuous casting method for casting the Ni-containing steel slab according to any one of the first to third inventions, wherein It is characterized by casting at a frequency of 80 cycles or more.
第６の発明に係るＮｉ含有鋼の連続鋳造方法は、第１ないし第３の発明の何れか１つに記載のＮｉ含有鋼鋳片を鋳造するための連続鋳造方法であって、１３００℃における粘度が１．０Ｐａ・ｓ（１０ポアズ）以上のモールドパウダーを鋳型内に添加して鋳造するとともに、鋳型を毎分８０サイクル以上の振動数で振動させて鋳造することを特徴とするものである。 A Ni-containing steel continuous casting method according to a sixth invention is a continuous casting method for casting the Ni-containing steel slab according to any one of the first to third inventions, at 1300 ° C. The molding powder is characterized in that a mold powder having a viscosity of 1.0 Pa · s (10 poise) or more is added to the mold and cast, and the mold is cast at a frequency of 80 cycles / min or more. .
本発明によれば、鋳片表面での凝固核発生密度が０．６個／ｍｍ2以上と多いので、凝固セルのサイズが小さくなって、凝固セル界面でのＳ及びＰの偏析が軽減して凝固セル界面での脆化が少なくなり、且つ、凝固セル界面に作用する応力も分散され、これにより、凝固セル界面における凝固割れが抑制され、鋳片表面における割れの発生が低減する。その結果、鋳片表面の割れを除去するための表面手入れ工程が軽減され、生産性の向上及び製造コストの低減が可能となり、工業上有益な効果がもたらされる。 According to the present invention, the solidification nucleus generation density on the slab surface is as high as 0.6 pieces / mm 2 or more, so the size of the solidification cell is reduced and the segregation of S and P at the solidification cell interface is reduced. Thus, embrittlement at the solidification cell interface is reduced, and stress acting on the solidification cell interface is also dispersed, thereby suppressing solidification cracking at the solidification cell interface and reducing occurrence of cracks on the slab surface. As a result, the surface care process for removing cracks on the surface of the slab is reduced, the productivity can be improved and the manufacturing cost can be reduced, and an industrially beneficial effect is brought about.
以下、本発明を具体的に説明する。 Hereinafter, the present invention will be specifically described.
本発明の基本的な考え方は、鋳片表面の初期凝固を制御することにより、具体的には多数の凝固核を発生させることにより、凝固セル界面へのＰ（燐）やＳ（硫黄）などの不純物元素及びＣ（炭素）の濃化を低減し、凝固セル界面での凝固割れを防止し、これにより、鋳片表面に発生する割れを防止するというものである。 The basic idea of the present invention is that by controlling the initial solidification of the slab surface, specifically by generating a large number of solidification nuclei, P (phosphorus), S (sulfur), etc. at the solidification cell interface Concentration of the impurity element and C (carbon) is reduced, and solidification cracks at the solidification cell interface are prevented, thereby preventing cracks generated on the surface of the slab.
従来から、Ｎｉ（ニッケル）を７．５〜１０質量％含有するＮｉ含有鋼鋳片の表面割れは、粗大な凝固組織の結晶粒界に沿って発生していることが知られていた。また、この割れの発生原因は、Ｎｉ含有鋼の延性が低下する６００〜９００℃の範囲で、矯正応力、バルジング応力、熱応力などによる引張り応力が鋳片表面に加えられ、この引張り応力によってＳやＰなどが濃化して脆化した結晶粒界が破壊するからであると考えられてきた。尚、本発明において、Ｎｉ含有鋼とは、７．５〜１０質量％のＮｉを含有する鋼の総称である。 Conventionally, it has been known that surface cracks in Ni-containing steel slabs containing 7.5 to 10% by mass of Ni (nickel) occur along the grain boundaries of a coarse solidified structure. In addition, the cause of this crack is the range of 600 to 900 ° C. at which the ductility of the Ni-containing steel decreases, and tensile stress due to straightening stress, bulging stress, thermal stress, etc. is applied to the slab surface. It has been thought that this is because the grain boundaries that have become brittle due to the concentration of P and P are destroyed. In addition, in this invention, Ni containing steel is a general term for the steel containing 7.5-10 mass% Ni.
しかしながら、本発明者らが実際のＮｉ含有鋼鋳片の表面割れを詳細に調査したところ、割れ破面には、６００〜９００℃の延性低下温度領域で破壊したと考えられる細かいディンプルからなる破面の他に、極めて平坦な面が存在することを発見した。この面は、その形状から、凝固の際に最終凝固部にＣ、Ｓ、Ｐなどが濃化して融点が低下し、最終凝固部に低融点の液相が存在している状態で、周囲の既に凝固の完了した部分が収縮するために発生する、一種の凝固割れであると考えられた。 However, when the present inventors investigated in detail the surface cracks of actual Ni-containing steel slabs, the cracked fracture surface was a fracture consisting of fine dimples that were considered to have broken in the ductile temperature range of 600 to 900 ° C. In addition to the surface, we found that there is a very flat surface. This surface has a shape in which C, S, P, etc. are concentrated in the final solidified part during the solidification, the melting point is lowered, and a liquid phase having a low melting point is present in the final solidified part. It was thought to be a kind of solidification cracking that occurs because the already solidified part contracts.
即ち、Ｎｉ含有鋼鋳片の表面割れは、従来考えられていた連続鋳造機の二次冷却帯で初めて割れが発生するのではなく、鋳型内で凝固シェルが成長する際に、凝固シェルを構成する凝固セルと凝固セルとの境界に、Ｃ、Ｓ、Ｐなどの溶質元素が濃化して低融点の液相が生じ、この低融点の液相が生じることにより凝固割れが発生し、この凝固割れを起点として、二次冷却帯での熱応力や曲げ矯正による応力などによって更に割れが進展するものと考えられた。 That is, surface cracks in Ni-containing steel slabs do not occur for the first time in the secondary cooling zone of a continuous casting machine that has been considered in the past, but constitute solidified shells when they grow in the mold. At the boundary between the solidifying cell and the solidifying cell, solute elements such as C, S, and P are concentrated to form a low-melting liquid phase. This low-melting liquid phase causes solidification cracks. It was thought that the cracks progressed further due to the thermal stress in the secondary cooling zone and the stress due to bending straightening.
従って、割れを防止するには、従来実施されていた二次冷却帯での熱応力や曲げ矯正応力の緩和だけでは十分ではなく、むしろ、鋳型での初期凝固における凝固割れを防止することが重要且つ必要であることが分かった。 Therefore, in order to prevent cracking, it is not sufficient to relax the thermal stress and bending straightening stress in the secondary cooling zone that has been implemented in the past, but rather it is important to prevent solidification cracking during initial solidification in the mold. And it turned out to be necessary.
凝固割れは、最終凝固部での溶質元素の濃化が少ないほど、また、最終凝固部に作用する熱応力が小さいほど発生しにくい。従って、凝固セルのサイズを小さくすることが凝固割れの防止に有効である。これは、凝固セルのサイズが小さくなると、自ずと冷却速度が速くなって溶質元素の濃化が抑制され、また、凝固セルのサイズが小さいと熱応力が分散されて個々の凝固セル界面に作用する熱応力が小さくなるからである。 The solidification crack is less likely to occur as the concentration of the solute element in the final solidified portion is smaller and the thermal stress acting on the final solidified portion is smaller. Therefore, reducing the size of the solidification cell is effective in preventing solidification cracking. This is because when the size of the solidification cell is reduced, the cooling rate is naturally increased and the concentration of the solute element is suppressed, and when the size of the solidification cell is small, the thermal stress is dispersed and acts on each solidification cell interface. This is because the thermal stress is reduced.
凝固セルのサイズを小さくするには、溶鋼が鋳型と接触する部分（正確には溶鋼と鋳型との間にはモールドパウダーの極薄い層が存在するので、溶鋼の凝固は、モールドパウダー層の表面で発生する）における凝固核発生密度を大きくすることが必要となる。一般的に、凝固セルの個数と凝固核の個数とは相関関係があり、凝固セルのサイズは、凝固核発生密度が大きくなるほど小さくなることが知られている。 In order to reduce the size of the solidification cell, the part where the molten steel comes into contact with the mold (exactly, there is an extremely thin layer of mold powder between the molten steel and the mold, so solidification of the molten steel takes place on the surface of the mold powder layer. It is necessary to increase the solidification nucleus generation density in Generally, there is a correlation between the number of solidification cells and the number of solidification nuclei, and it is known that the size of the solidification cells decreases as the solidification nucleus generation density increases.
本発明者らは、実際のＮｉ含有鋼鋳片を詳細に調査した結果、表面割れ防止のためには、凝固核の発生個数は多いほど望ましいが、鋳片表面での凝固核発生密度が０．６個／ｍｍ2以上であれば、初期凝固シェルにおける凝固割れを減少させ、鋳片の表面割れを防止する効果が発現されることを確認した。 As a result of detailed investigation of actual Ni-containing steel slabs, the present inventors found that the number of solidified nuclei generated is desirable in order to prevent surface cracking, but the solidification nucleation density on the slab surface is 0. It was confirmed that the effect of reducing the solidification cracking in the initial solidification shell and preventing the surface cracking of the cast slab was exhibited when the number was 6 pieces / mm 2 or more.
本発明は、上記知見に基づきなされたものであり、本発明に係るＮｉ含有鋼鋳片は、Ｎｉを７．５〜１０．０質量％含有し、連続鋳造によって鋳造されたＮｉ含有鋼鋳片であって、鋳片表面での凝固核発生密度が０．６個／ｍｍ2以上であることを特徴とする。 This invention is made | formed based on the said knowledge, Ni containing steel slab which concerns on this invention contains 7.5-10.0 mass% of Ni, and Ni containing steel slab cast by continuous casting The solidification nucleus generation density on the slab surface is 0.6 pieces / mm 2 or more.
Ｎｉ含有鋼鋳片に限らず、連続鋳造鋳片表面での凝固核発生密度を高くする方法の１つとして、初期凝固時の冷却を強くすること、つまり、鋳型内での冷却を強化することが挙げられる。しかしながら、Ｎｉ含有鋼鋳片の連続鋳造では、鋳型内溶鋼湯面上に、酸化防止剤、保温剤、鋳型と凝固シェルとの潤滑剤などとして機能するモールドパウダーを添加しており、このモールドパウダーは凝固シェルと鋳型との間隙に流入することから、溶鋼と鋳型とは直接接触せず、溶鋼はモールドパウダーの流入層を介して鋳型によって間接的に冷却されている。 As one of the methods for increasing the density of solidification nuclei on the surface of continuous cast slabs, not limited to Ni-containing steel slabs, increasing the cooling during initial solidification, that is, enhancing the cooling in the mold Is mentioned. However, in continuous casting of Ni-containing steel slabs, mold powder that functions as an antioxidant, a heat retaining agent, a lubricant between the mold and the solidified shell, etc. is added onto the molten steel surface in the mold. Flows into the gap between the solidified shell and the mold, so that the molten steel and the mold do not directly contact each other, and the molten steel is indirectly cooled by the mold through the mold powder inflow layer.
モールドパウダーは、ＣａＯ、ＳｉＯ2、Ｎａ2Ｏ、ＣａＦ2、Ａｌ2Ｏ3などで構成されており、その熱伝導度は、金属である溶鋼及び連続鋳造用鋳型を構成する銅のそれよりも著しく小さい。そのために、溶鋼から鋳型への抜熱は、モールドパウダーの流入層厚みに左右される。つまり、モールドパウダーの流入層厚みが薄くなれば鋳型による冷却が高くなり、逆に、厚みが厚くなれば冷却が低下する。モールドパウダーの流入層厚みは、モールドパウダーの消費量から推定でき、通常、０．１〜０．３ｍｍ程度と算出されている。 The mold powder is composed of CaO, SiO 2 , Na 2 O, CaF 2 , Al 2 O 3, etc., and its thermal conductivity is higher than that of molten metal that is a metal and copper that constitutes a continuous casting mold. Remarkably small. Therefore, the heat removal from the molten steel to the mold depends on the inflow layer thickness of the mold powder. That is, when the inflow layer thickness of the mold powder is reduced, the cooling by the mold is increased, and conversely, when the thickness is increased, the cooling is decreased. The inflow layer thickness of the mold powder can be estimated from the consumption of the mold powder, and is usually calculated to be about 0.1 to 0.3 mm.
モールドパウダーの流入層厚みを薄くする方法は、粘度の高いモールドパウダーを使用することである。モールドパウダーの粘度が高いと、凝固シェルと鋳型との間隙に流れ込みにくくなり、モールドパウダーの流入層厚みが薄くなり、溶鋼から鋳型への抜熱が大きくなる。 A method for reducing the thickness of the inflow layer of the mold powder is to use a mold powder having a high viscosity. When the viscosity of the mold powder is high, it becomes difficult to flow into the gap between the solidified shell and the mold, the thickness of the inflow layer of the mold powder becomes thin, and heat removal from the molten steel to the mold increases.
また、鋳型の振動数（オシレーション・サイクル）を高めることも、鋳片表面での凝固核発生密度を高めることに有効である。鋳型振動により凝固途中のデンドライト樹枝状晶の一部が解離してモールドパウダー流入層の表面に付着し、そこから凝固が発生することがあるからである。 Increasing the frequency (oscillation cycle) of the mold is also effective in increasing the density of solidification nuclei on the slab surface. This is because part of the dendrite dendrites in the middle of solidification dissociate due to mold vibration and adhere to the surface of the mold powder inflow layer, and solidification may occur from there.
本発明者らは、鋳型の振動数（オシレーション・サイクル）を毎分６０回の一定としてモールドパウダーの粘度を変更した試験結果から、１３００℃における粘度が１．０Ｐａ・ｓ（１０ポアズ）以上のモールドパウダーを使用することにより、Ｎｉ含有鋼鋳片表面での凝固核発生密度が０．６個／ｍｍ2以上となって、鋳片の表面割れが減少することを確認した。 The present inventors have found that the viscosity at 1300 ° C. is not less than 1.0 Pa · s (10 poise) based on the test result of changing the viscosity of the mold powder with the mold frequency (oscillation cycle) constant at 60 times per minute. By using this mold powder, it was confirmed that the solidification nucleus generation density on the surface of the Ni-containing steel slab became 0.6 pieces / mm 2 or more, and the surface cracks of the slab were reduced.
また、本発明者らは、モールドパウダーとして１３００℃における粘度が０．２Ｐａ・ｓ（２ポアズ）のモールドパウダーを使用し、鋳型の振動数（オシレーション・サイクル）を変更した試験結果から、鋳型の振動数を毎分８０回以上とすることにより、Ｎｉ含有鋼鋳片表面での凝固核発生密度が０．６個／ｍｍ2以上となって、鋳片の表面割れが減少することを確認した。また更に、１３００℃における粘度が１．０Ｐａ・ｓ（１０ポアズ）以上のモールドパウダーを使用し、且つ、鋳型の振動数を毎分８０回以上とすることにより、Ｎｉ含有鋼鋳片表面での凝固核発生密度が０．６個／ｍｍ2以上となって、鋳片の表面割れが大幅に減少することも確認した。 In addition, the present inventors used a mold powder having a viscosity at 1300 ° C. of 0.2 Pa · s (2 poise) as the mold powder, and changed the mold frequency (oscillation cycle). It is confirmed that the solidification nucleus generation density on the Ni-containing steel slab surface becomes 0.6 pieces / mm 2 or more by reducing the vibration frequency of the steel to 80 times or more per minute and the surface cracks of the slab are reduced. did. Furthermore, by using a mold powder having a viscosity at 1300 ° C. of 1.0 Pa · s (10 poise) or more and making the frequency of the mold 80 times or more per minute, It was also confirmed that the solidification nucleus generation density was 0.6 pieces / mm 2 or more, and the surface cracks of the slab were greatly reduced.
本発明は、Ｎｉを７．５〜１０．０質量％含有するＮｉ含有鋼に適用されるものであり、Ｎｉ以外の化学成分を限定することは必須ではないが、本発明を適用する上で好適な低温用の溶接構造用鋼としてのＮｉ含有鋼の化学組成を限定したので、その理由を以下に説明する。 The present invention is applied to Ni-containing steel containing 7.5 to 10.0% by mass of Ni, and it is not essential to limit chemical components other than Ni, but in applying the present invention. The reason is described below because the chemical composition of Ni-containing steel as a suitable low-temperature welded structural steel is limited.
Ｃは、母材の強度確保に必須の元素である。０．０３質量％（以下、単に「％」と記す）未満では、母材強度が確保できないので、０．０３％を下限とした。また、Ｃを過剰に添加すると、脆性破壊の起点となるセメンタイトや島状マルテンサイトが増加し、好適な靭性が得られない。０．１０％を超えると靭性低下が顕著となるので、これを上限値とした。 C is an element essential for ensuring the strength of the base material. If it is less than 0.03% by mass (hereinafter simply referred to as “%”), the strength of the base material cannot be secured, so 0.03% was made the lower limit. Moreover, when C is added excessively, the cementite and island martensite which become the starting point of a brittle fracture will increase, and suitable toughness will not be obtained. When the content exceeds 0.10%, the toughness is significantly reduced.
Ｓｉ（珪素）は、脱酸に有効であり、また母材強度上昇に有効な元素であるが、過剰に添加すると、溶接熱影響部（ＨＡＺ）の組織中に島状マルテンサイトが生成し、良好なＨＡＺ靭性が得られない。従って、ＨＡＺ靭性確保のために、上限値を０．５％とした。また、脱酸の効果を得るためには、０．０１％以上の添加が必要であり、下限を０．０１％とした。 Si (silicon) is an element effective for deoxidation and effective for increasing the strength of the base material, but when added excessively, island martensite is generated in the structure of the weld heat affected zone (HAZ), Good HAZ toughness cannot be obtained. Therefore, the upper limit is set to 0.5% in order to ensure HAZ toughness. Moreover, in order to acquire the effect of deoxidation, addition of 0.01% or more is required, and the minimum was made into 0.01%.
Ｍｎ（マンガン）は、母材強度上昇に有効な元素である。０．３％未満では、この効果が得られないので、下限値を０．３％とした。逆に、１．０％を超えて含有すると、母材及びＨＡＺの良好な靭性が得られない。従って、上限を１．０％とした。 Mn (manganese) is an effective element for increasing the strength of the base material. If less than 0.3%, this effect cannot be obtained, so the lower limit was set to 0.3%. On the other hand, if the content exceeds 1.0%, good toughness of the base material and HAZ cannot be obtained. Therefore, the upper limit was made 1.0%.
Ｐは、粒界脆化をもたらし、表面割れの発生を促進させ、また、母材及びＨＡＺの靭性を低下させるため、低い方が望ましい。０．０１０％を超えて含有すると、表面割れが顕著となるので、０．０１０％を上限とした。また、０．００１％未満へのＰ含有量の低下は、製鋼工程における脱燐精錬の負荷が増し、コスト上昇を招くので、下限値を０．００１％とした。 P is desirable to be low because it causes grain boundary embrittlement, promotes the generation of surface cracks, and lowers the toughness of the base material and HAZ. If the content exceeds 0.010%, surface cracking becomes remarkable, so 0.010% was made the upper limit. Moreover, since the reduction | decrease of P content to less than 0.001% increases the load of the dephosphorization refining in a steelmaking process, and causes a cost rise, the lower limit was made into 0.001%.
Ｓも、粒界脆化をもたらし、表面割れの発生を促進させ、また、ＭｎＳなどの介在物として靭性を低下させるため、低い方が望ましい。０．００５％を超えて含有すると、表面割れが顕著となるので、０．００５％を上限とした。また、０．０００１％未満へのＳ含有量の低下は、製鋼工程における脱硫精錬の負荷が増し、コスト上昇を招くので、下限値を０．０００１％とした。 S is also preferable to be low because S also causes grain boundary embrittlement, promotes the generation of surface cracks, and reduces toughness as inclusions such as MnS. If the content exceeds 0.005%, surface cracks become prominent, so 0.005% was made the upper limit. Moreover, since the reduction | decrease of S content to less than 0.0001% increases the load of desulfurization refining in a steelmaking process, and causes a cost rise, the lower limit was made 0.0001%.
Ｎｉは、低温靭性の確保に必須の元素であり、７．５％未満ではこの効果が十分に得られないので、下限値を７．５％とした。一方、Ｎｉは高価な元素であり、１０．０％を超えて含有すると、経済性を損なうので、上限値を１０．０％とした。 Ni is an element essential for ensuring low temperature toughness, and if it is less than 7.5%, this effect cannot be sufficiently obtained, so the lower limit was set to 7.5%. On the other hand, Ni is an expensive element, and if it is contained in excess of 10.0%, the economic efficiency is impaired, so the upper limit was made 10.0%.
Ａｌ（アルミニウム）は、鋼の脱酸に必要な元素であり、０．０１０％未満の含有量では、この効果が得られないので、下限値を０．０１０％とした。また、０．０８０％を超えて含有すると、粗大なＡｌＮにより母材及びＨＡＺの靭性が低下する。従って、上限値を０．０８０％とした。 Al (aluminum) is an element necessary for deoxidation of steel, and if the content is less than 0.010%, this effect cannot be obtained, so the lower limit was made 0.010%. Moreover, when it contains exceeding 0.080%, the toughness of a base material and HAZ will fall with coarse AlN. Therefore, the upper limit is set to 0.080%.
Ｎ（窒素）は、母材及びＨＡＺの靭性を低下させる元素であり、低いほど望ましい。０．００５０％を超えて含有すると、粗大なＡｌＮを生成し、母材及びＨＡＺの良好な靭性が得られない。従って、上限値を０．００５０％とした。一方、０．００１０％未満へのＮ含有量の低下は、製鋼工程における脱窒処理及び吸窒防止処理の負荷が増し、コスト上昇を招くので、下限値を０．００１０％とした。 N (nitrogen) is an element that lowers the toughness of the base material and the HAZ, and is preferably as low as possible. If the content exceeds 0.0050%, coarse AlN is generated, and good toughness of the base material and HAZ cannot be obtained. Therefore, the upper limit is set to 0.0050%. On the other hand, a decrease in the N content to less than 0.0010% increases the load of denitrification treatment and nitrogen absorption prevention treatment in the steel making process, leading to an increase in cost, so the lower limit was set to 0.0010%.
Ｏ（酸素）は、介在物を形成して靭性に有害な元素であり、低い方が望ましい。０．００４０％を超えて含有すると、靭性低下が顕著となるので、０．００４０％を上限とした。一方、０．０００５％未満へのＯ含有量の低下は、製鋼工程における介在物除去処理の負荷が増し、コスト上昇を招くので、下限値を０．０００５％とした。 O (oxygen) is an element that forms inclusions and is harmful to toughness, and is preferably low. When the content exceeds 0.0040%, a decrease in toughness becomes remarkable, so 0.0040% was made the upper limit. On the other hand, a decrease in the O content to less than 0.0005% increases the load of the inclusion removal process in the steel making process and causes an increase in cost, so the lower limit was set to 0.0005%.
更に、上記の合金元素の他に、母材や継手の強度及び靭性の向上のために、Ｃｕ、Ｃｒ、Ｍｏ、Ｎｂ、Ｖ、Ｔｉ、Ｂ、Ｃａのなかの１種または２種以上を含有させることが好ましく、以下にその成分の限定理由を説明する。 Further, in addition to the above alloy elements, one or more of Cu, Cr, Mo, Nb, V, Ti, B, and Ca are contained in order to improve the strength and toughness of the base material and joint. The reasons for limiting the components are described below.
Ｃｕ（銅）は、母材の強度を上昇させる。０．０３％未満ではこの効果が得られないので、添加する場合の下限値を０．０３％とした。逆に、１．５％を超えるとＨＡＺの靭性が低下するので、上限値を１．５％とした。 Cu (copper) increases the strength of the base material. If less than 0.03%, this effect cannot be obtained, so the lower limit for addition is set to 0.03%. On the other hand, if it exceeds 1.5%, the toughness of the HAZ decreases, so the upper limit was set to 1.5%.
Ｃｒ（クロム）は、母材の強度を上昇させる。０．０３％未満ではこの効果が得られないので、添加する場合の下限値を０．０３％とした。逆に、１．０％を超えるとＨＡＺの靭性が低下するので、上限値を１．０％とした。 Cr (chromium) increases the strength of the base material. If less than 0.03%, this effect cannot be obtained, so the lower limit for addition is set to 0.03%. On the contrary, if it exceeds 1.0%, the toughness of HAZ decreases, so the upper limit value was made 1.0%.
Ｍｏ（モリブデン）は、母材の強度を上昇させる。０．０２％未満ではこの効果が得られないので、添加する場合の下限値を０．０２％とした。逆に、１．０％を超えるとＨＡＺの靭性が低下するので、上限値を１．０％とした。 Mo (molybdenum) increases the strength of the base material. If less than 0.02%, this effect cannot be obtained, so the lower limit for addition is set to 0.02%. On the contrary, if it exceeds 1.0%, the toughness of HAZ decreases, so the upper limit value was made 1.0%.
Ｎｂ（ニオブ）は、母材の強度上昇及び細粒化に効果がある。０．００３％未満ではこれらの効果が得られないので、添加する場合の下限値を０．００３％とした。逆に、０．１０％を超えるとＨＡＺの靭性が低下するので、上限値を０．１０％とした。 Nb (niobium) is effective in increasing the strength and refining of the base material. If less than 0.003%, these effects cannot be obtained, so the lower limit for addition is set to 0.003%. On the other hand, if it exceeds 0.10%, the toughness of the HAZ decreases, so the upper limit was made 0.10%.
Ｖ（バナジウム）は、母材の強度上昇及び細粒化に効果がある。０．００３％未満ではこれらの効果が得られないので、添加する場合の下限値を０．００３％とした。逆に、０．１０％を超えるとＨＡＺの靭性が低下するので、上限値を０．１０％とした。 V (vanadium) is effective in increasing the strength and refining of the base material. If less than 0.003%, these effects cannot be obtained, so the lower limit for addition is set to 0.003%. On the other hand, if it exceeds 0.10%, the toughness of the HAZ decreases, so the upper limit was made 0.10%.
Ｔｉ（チタン）は、母材の強度上昇及び細粒化に効果がある。０．００５％未満ではこれらの効果が得られないので、添加する場合の下限値を０．００５％とした。逆に、０．０２％を超えると粗大なＴｉＮを生成し、これが破壊の起点となり、ＨＡＺの良好な靭性が得られないので、上限値を０．０２％とした。 Ti (titanium) is effective in increasing the strength and refining of the base material. If less than 0.005%, these effects cannot be obtained, so the lower limit for addition is 0.005%. On the other hand, if it exceeds 0.02%, coarse TiN is generated, which becomes the starting point of fracture, and good toughness of HAZ cannot be obtained, so the upper limit was made 0.02%.
Ｂ（ボロン）は、極微量の添加で焼入れ性を向上させるので、制御冷却及び焼入れ熱処理を施す場合に、顕著な強度上昇をもたらす。０．０００２％未満では強度上昇効果が得られないので、添加する場合の下限値を０．０００２％とした。逆に、０．００２５％を超えると粗大なボロン窒化物や炭硼化物を析出し、これが破壊の起点となり、ＨＡＺの靭性が低下するので、上限値を０．００２５％とした。 B (boron) improves the hardenability when added in a very small amount, and therefore, when subjected to controlled cooling and quenching heat treatment, the strength is significantly increased. If it is less than 0.0002%, the effect of increasing the strength cannot be obtained. Therefore, the lower limit for addition is set to 0.0002%. On the other hand, if it exceeds 0.0025%, coarse boron nitride or carboboride precipitates, which becomes the starting point of fracture and decreases the toughness of the HAZ, so the upper limit value was made 0.0025%.
Ｃａ（カルシウム）は、介在物の形態制御を担い、これによる靭性向上に有効な他、ＳをＣａＳとして強力に固定するため、粒界での延性低下割れを防止して、表面割れの低減に効果のある元素である。０．０００５％未満ではこれらの効果が得られないので、添加する場合の下限値を０．０００５％とした。逆に、０．００５％を超えると粗大なＣａ含有介在物が生成し、これが破壊の起点となり、ＨＡＺの靭性が低下するので、上限値を０．００５％とした。 Ca (calcium) is responsible for controlling the form of inclusions, and is effective in improving toughness, and in order to strongly fix S as CaS, it prevents ductile drop at grain boundaries and reduces surface cracks. It is an effective element. If less than 0.0005%, these effects cannot be obtained, so the lower limit for addition is set to 0.0005%. On the other hand, if it exceeds 0.005%, coarse Ca-containing inclusions are generated, which becomes the starting point of fracture, and the toughness of HAZ decreases, so the upper limit value was made 0.005%.
以上説明したように、本発明に係るＮｉ含有鋼鋳片では、鋳片表面での凝固核発生密度が０．６個／ｍｍ2以上と多いので、凝固セルのサイズが小さくなり、凝固セル界面でのＳ及びＰの偏析が軽減して凝固セル界面での脆化が少なくなり、且つ、凝固セル界面に作用する応力も分散されることにより、凝固セル界面における凝固割れが抑制されて、鋳片表面における割れの発生が低減する。 As described above, in the Ni-containing steel slab according to the present invention, the solidification nucleus generation density on the slab surface is as large as 0.6 pieces / mm 2 or more, so the size of the solidification cell is reduced and the solidification cell interface is reduced. Segregation of S and P at the surface is reduced, embrittlement at the solidification cell interface is reduced, and stress acting on the solidification cell interface is also dispersed, so that solidification cracking at the solidification cell interface is suppressed, and Generation of cracks on one surface is reduced.
転炉及びＲＨ真空脱ガス装置を用いてＮｉ含有鋼を溶製し、この溶鋼を厚みが２５０ｍｍ、幅が１７００ｍｍの垂直曲げ型スラブ連続鋳造機で鋳造する試験を合計４ヒート実施（試験Ｎo.１〜４）した。表１に、試験Ｎo.１〜４のＮｉ含有鋼の化学成分を示し、また、表２に、試験Ｎo.１〜４の連続鋳造機での鋳造条件を示す。 Using a converter and RH vacuum degassing apparatus, a Ni-containing steel was melted, and a test in which this molten steel was cast with a vertical bending slab continuous casting machine having a thickness of 250 mm and a width of 1700 mm was conducted in total 4 heats (Test No. 1-4). Table 1 shows the chemical components of the Ni-containing steels of Test Nos. 1 to 4, and Table 2 shows the casting conditions in the continuous casting machine of Tests No. 1 to 4.
試験Ｎo.１の鋳造条件は、鋳造速度が０．８ｍ／ｍｉｎ、鋳型オシレーションの振幅が８ｍｍ、振動数が６０ｃｐｍであり、使用するモールドパウダーの１３００℃における粘度は０．２Ｐａ・ｓであり、一般的な鋳造条件である。 The casting conditions of the test No. 1 are a casting speed of 0.8 m / min, a mold oscillation amplitude of 8 mm, a vibration frequency of 60 cpm, and the mold powder used has a viscosity at 1300 ° C. of 0.2 Pa · s. It is a general casting condition.
試験Ｎo.２の鋳造条件は、鋳造速度が０．８ｍ／ｍｉｎ、鋳型オシレーションの振幅が８ｍｍ、振動数が６０ｃｐｍであり、使用するモールドパウダーの１３００℃における粘度は４．０Ｐａ・ｓであり、試験Ｎo.１に対して、使用するモールドパウダーの粘度を高めた試験である。試験Ｎo.３の鋳造条件は、鋳造速度が０．８ｍ／ｍｉｎ、鋳型オシレーションの振幅が８ｍｍ、振動数が１００ｃｐｍであり、使用するモールドパウダーの１３００℃における粘度は０．２Ｐａ・ｓであり、試験Ｎo.１に対して、鋳型オシレーションの振動数を高めた試験である。試験Ｎo.４の鋳造条件は、鋳造速度が０．８ｍ／ｍｉｎ、鋳型オシレーションの振幅が８ｍｍ、振動数が１００ｃｐｍであり、使用するモールドパウダーの１３００℃における粘度は４．０Ｐａ・ｓであり、試験Ｎo.１に対して、使用するモールドパウダーの粘度を高め且つ鋳型オシレーションの振動数を高めた試験である。 The casting conditions of Test No. 2 are a casting speed of 0.8 m / min, a mold oscillation amplitude of 8 mm, a vibration frequency of 60 cpm, and the viscosity at 1300 ° C. of the mold powder used is 4.0 Pa · s. In this test, the viscosity of the mold powder used was increased with respect to test No. 1. The casting conditions for test No. 3 are: casting speed is 0.8 m / min, mold oscillation amplitude is 8 mm, vibration frequency is 100 cpm, and the viscosity at 1300 ° C. of the mold powder used is 0.2 Pa · s. This is a test in which the oscillation frequency of the mold oscillation is increased with respect to test No. 1. The casting conditions of test No. 4 were as follows: casting speed was 0.8 m / min, mold oscillation amplitude was 8 mm, vibration frequency was 100 cpm, and the viscosity at 1300 ° C. of the mold powder used was 4.0 Pa · s. The test No. 1 is a test in which the viscosity of the mold powder used is increased and the vibration frequency of the mold oscillation is increased.
鋳造後の鋳片を厚みが１９０ｍｍになるまで圧延し、次いで、表面割れの評価のための３００ｍｍ長さ×全幅の試料を切り出した。この試料の表面をショットブラスト処理して表面の酸化膜を除去し、その後、浸透探傷試験により表面割れを判別し、割れの長さ及び個数を測定した。また、表面割れの深さを調べるために、表面から３ｍｍ位置、６ｍｍ位置、９ｍｍ位置で研削し、それぞれの面で浸透探傷試験により表面割れを判別し、割れの長さ及び個数を測定した。 The cast slab was rolled until the thickness reached 190 mm, and then a sample of 300 mm length × full width for evaluation of surface cracking was cut out. The surface of this sample was shot blasted to remove the oxide film on the surface, and then surface cracks were identified by a penetrant flaw detection test, and the length and number of cracks were measured. In addition, in order to investigate the depth of surface cracks, grinding was performed at 3 mm position, 6 mm position, and 9 mm position from the surface, surface cracks were determined on each surface by a penetrant flaw detection test, and the length and number of cracks were measured.
また、鋳片表面の凝固核発生密度は、以下の方法により測定した。即ち、鋳片表面から試料を採取し、鋳片表面の酸化膜を除去して鏡面研磨し、ピクリン酸で腐食して凝固組織を現出させ、凝固組織の写真を撮影した。写真を観察して、デンドライト樹枝がほぼ同じ方向を向いている塊（凝固セル、或いはデンドライトセル）を一つの凝固核から成長したものとみなし、凝固核発生密度を算出した。つまり、凝固組織の写真から凝固セルの個数を数え、その凝固セルの占めている面積で除して凝固核発生密度とした。この場合、凝固セルの大きさはオシレーションマークの近くでは小さく、オシレーションマークから離れると大きくなるので、凝固セルを数える範囲は、オシレーションマークからオシレーションマークまでとして、平均的な値を求めている。 Moreover, the solidification nucleus generation density on the surface of the slab was measured by the following method. Specifically, a sample was taken from the surface of the slab, the oxide film on the surface of the slab was removed and mirror-polished, corroded with picric acid to reveal a solidified structure, and a photograph of the solidified structure was taken. By observing the photograph, the mass (solidification cell or dendrite cell) in which the dendritic tree branches are oriented in substantially the same direction was regarded as having grown from one solidification nucleus, and the solidification nucleus generation density was calculated. That is, the number of solidification cells was counted from a photograph of the solidification structure and divided by the area occupied by the solidification cells to obtain solidification nucleus generation density. In this case, the size of the coagulation cell is small near the oscillation mark and increases as it moves away from the oscillation mark. Therefore, the average number of solidification cells is calculated from the oscillation mark to the oscillation mark. ing.
表３に、各試験における凝固核発生密度、及び、割れ総長さ（＝割れ長さ×割れ個数）の調査結果を示す。 Table 3 shows the results of investigation of solidification nucleus generation density and total crack length (= crack length × number of cracks) in each test.
凝固核発生密度が０．４個／ｍｍ2の試験Ｎo.１では、多くの表面割れが発生し、しかも、表面から６ｍｍ位置でも割れが発生していた。 In the test No. 1 in which the solidification nucleus generation density was 0.4 / mm 2 , many surface cracks occurred, and cracks also occurred at a position of 6 mm from the surface.
これに対して、試験Ｎo.２、３、４では、凝固核発生密度が０．６個／ｍｍ2よりも多くなっており、凝固核発生密度が多くなるに応じて、表面割れの発生が減少することが確認できた。特に、高粘度のモールドパウダーを使用し、且つ鋳型オシレーションの振動数を増加させた試験Ｎo.４では、大幅な表面割れの減少が確認できた。尚、表３の備考欄には、本発明の範囲内の試験を「本発明例」と表示し、それ以外を「比較例」と表示している。 On the other hand, in tests No. 2, 3, and 4, the solidification nucleus generation density is higher than 0.6 / mm 2, and as the solidification nucleus generation density increases, surface cracks are generated. It was confirmed that it decreased. In particular, in test No. 4 in which a high-viscosity mold powder was used and the frequency of mold oscillation was increased, a significant reduction in surface cracks was confirmed. In the remarks column of Table 3, tests within the scope of the present invention are displayed as “examples of the present invention”, and the others are displayed as “comparative examples”.
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