JP5849982B2 - Continuous casting method of steel with excellent hydrogen-induced cracking resistance - Google Patents
Continuous casting method of steel with excellent hydrogen-induced cracking resistance Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 64
- 239000010959 steel Substances 0.000 title claims description 64
- 238000009749 continuous casting Methods 0.000 title claims description 12
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 title claims description 11
- 239000001257 hydrogen Substances 0.000 title claims description 11
- 229910052739 hydrogen Inorganic materials 0.000 title claims description 11
- 238000000034 method Methods 0.000 title claims description 11
- 238000005336 cracking Methods 0.000 title description 12
- 239000007789 gas Substances 0.000 claims description 32
- 229910004709 CaSi Inorganic materials 0.000 description 11
- 230000007547 defect Effects 0.000 description 10
- 239000011575 calcium Substances 0.000 description 9
- 230000000694 effects Effects 0.000 description 8
- 238000007664 blowing Methods 0.000 description 6
- 238000005266 casting Methods 0.000 description 6
- 229910000882 Ca alloy Inorganic materials 0.000 description 5
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 5
- 238000007667 floating Methods 0.000 description 5
- 239000001301 oxygen Substances 0.000 description 5
- 229910052760 oxygen Inorganic materials 0.000 description 5
- XKRFYHLGVUSROY-UHFFFAOYSA-N Argon Chemical compound [Ar] XKRFYHLGVUSROY-UHFFFAOYSA-N 0.000 description 4
- 239000011261 inert gas Substances 0.000 description 4
- 150000004767 nitrides Chemical class 0.000 description 4
- 239000002344 surface layer Substances 0.000 description 4
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 description 3
- 229910052791 calcium Inorganic materials 0.000 description 3
- 238000001816 cooling Methods 0.000 description 3
- QTBSBXVTEAMEQO-UHFFFAOYSA-N Acetic acid Chemical compound CC(O)=O QTBSBXVTEAMEQO-UHFFFAOYSA-N 0.000 description 2
- 229910000838 Al alloy Inorganic materials 0.000 description 2
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 2
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 2
- 229910052786 argon Inorganic materials 0.000 description 2
- 230000000052 comparative effect Effects 0.000 description 2
- 238000001514 detection method Methods 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 238000007654 immersion Methods 0.000 description 2
- 238000005096 rolling process Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 238000009864 tensile test Methods 0.000 description 2
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000005098 hot rolling Methods 0.000 description 1
- 229910000037 hydrogen sulfide Inorganic materials 0.000 description 1
- 238000005339 levitation Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 239000011148 porous material Substances 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 239000011780 sodium chloride Substances 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 238000010998 test method Methods 0.000 description 1
- 238000009849 vacuum degassing Methods 0.000 description 1
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Description
本発明は、耐水素誘起割れ特性に優れた鋼の連続鋳造方法に関するものである。 The present invention relates to a continuous casting method for steel having excellent resistance to hydrogen-induced cracking.
水素誘起割れ(Hydrogen Induced Cracking,以下HICという)しない耐HIC鋼の連続鋳造に関しては、従来種々の提案がなされている。
例えば、特許文献１においては、溶鋼中に吹き込まれたアルゴンガス等の不活性ガスの一部が、気泡として鋳片内に残留したものがHICの発生原因であるとして、アルゴンガス等の不活性ガスの吹き込みをしないようにしている。そして、溶鋼中の[Ca]を10〜50ppmとすることにより、Al2O3等の非金属介在物が生成せず、ノズルに付着し難い組成にすることで、浸漬ノズル等の閉塞を抑制できるとしている。
Various proposals have been made regarding continuous casting of HIC-resistant steel that does not cause hydrogen induced cracking (hereinafter referred to as HIC).
For example, in Patent Document 1, it is assumed that a part of an inert gas such as argon gas blown into molten steel remains in the slab as bubbles is a cause of generation of HIC. The gas is not blown. And by making [Ca] in molten steel 10-50ppm, non-metallic inclusions such as Al 2 O 3 are not generated, and it is difficult to adhere to the nozzle, thereby suppressing clogging of the immersion nozzle etc. I can do it.
また、特許文献２においては、溶鋼を連続鋳造鋳型に注入する際に、従来のArガスに代え、浸漬ノズルもしくはタンディッシュノズルから溶鋼中に、可溶性のガスであるN2ガスを吹き込むことで、介在物の浮上作用を促進できるとともに、吹き込みガスの気泡に起因するHIC割れの発生も抑制できるとしている。また、特許文献２では、窒化物の固溶障害を排除することを目的として、N2ガスの流量を25L/min以下にするとしている。 Moreover, in patent document 2, when injecting molten steel into a continuous casting mold, instead of the conventional Ar gas, by blowing N 2 gas, which is a soluble gas, into the molten steel from an immersion nozzle or a tundish nozzle, It is said that the floating action of inclusions can be promoted and the occurrence of HIC cracks due to the bubbles of the blown gas can be suppressed. In Patent Document 2, the flow rate of N 2 gas is set to 25 L / min or less for the purpose of eliminating the solid solution obstacle of nitride.
さらに、特許文献３においては、「溶鋼が脱酸される前の前記溶鋼のフリ−酸素含有量が６００ｐｐｍ以下となるように前記溶鋼の前記転炉における終点炭素含有量を制御し、前記溶鋼を脱酸し、そして前記溶鋼に前記カルシウムを添加した後、前記溶鋼を不活性ガスによって２分以上攪拌し、次いで前記溶鋼を前記取鍋内で３０分以上静置することを特徴とする、耐水素誘起割れ用鋼材の製造方法。」（特許文献３の請求項１参照）が提案されている。 Furthermore, in Patent Document 3, the end point carbon content in the converter of the molten steel is controlled so that the free oxygen content of the molten steel before the molten steel is deoxidized is 600 ppm or less, and the molten steel is After deoxidizing and adding the calcium to the molten steel, the molten steel is stirred with an inert gas for 2 minutes or more, and then the molten steel is allowed to stand in the ladle for 30 minutes or more. A method for producing a steel material for hydrogen-induced cracking "(see claim 1 of Patent Document 3) has been proposed.
特許文献１の方法では、アルゴンガス等の不活性ガスの吹き込みをしないことにより、気泡原因での耐HIC割れに対する効果は期待できるが、介在物浮上作用が抑制されるために、介在物に起因する欠陥に対しては効果が期待できない。 In the method of Patent Document 1, an effect on anti-HIC cracking caused by bubbles can be expected by not blowing an inert gas such as argon gas. However, since the levitation action of inclusions is suppressed, it is caused by inclusions. No effect can be expected against defects that occur.
また、特許文献２の方法では、上ノズルからN2の吹き込みを行うことにより、介在物に起因する欠陥に対する効果が期待できるが、それでも介在物に起因する超音波探傷欠陥（「UST欠陥」）の内部無欠陥率は依然として90%のレベルにとどまっている。 Further, in the method of Patent Document 2, an effect on defects caused by inclusions can be expected by blowing N 2 from the upper nozzle. Nevertheless, ultrasonic flaw detection defects (“UST defects”) caused by inclusions can be expected. The internal defect-free rate remains at 90%.
また、特許文献３の方法では、脱酸後の溶鋼にカルシウムを添加した後、溶鋼を不活性ガスによって２分以上攪拌するとしているが、気泡に起因する耐HIC割れに対する効果については明記されていない。 In the method of Patent Document 3, after adding calcium to the deoxidized molten steel, the molten steel is stirred for 2 minutes or more with an inert gas, but the effect on HIC cracking resistance caused by bubbles is clearly specified. Absent.
本発明は、かかる課題を解決するためになされたものであり、介在物及び気泡に起因する耐HIC割れに対する効果が期待でき、かつ窒化物増による鋼の靭性劣化の問題もない耐水素誘起割れ特性に優れた鋼の連続鋳造方法を提供することを目的としている。 The present invention has been made to solve such problems, and can be expected to have an effect on HIC cracking resistance caused by inclusions and bubbles, and also has no problem of steel toughness deterioration due to increased nitrides. It aims at providing the continuous casting method of the steel excellent in the characteristic.
（１）本発明に係る耐水素誘起割れ鋼の連続鋳造方法は、1.08≦T.Ca/T.O≦1.40を満たすようにCaを添加し、上ノズルから2NL/min≦N2ガス≦25NL/minの流量で吹き込むことを特徴とするものである。 (1) In the continuous casting method of hydrogen-resistant cracked steel according to the present invention, Ca is added so as to satisfy 1.08 ≦ T.Ca / TO ≦ 1.40, and 2 NL / min ≦ N 2 gas ≦ 25 NL / min from the upper nozzle. It is characterized by blowing at a flow rate of.
（２）また、上記（１）に記載のものにおいて、ポーラス部気孔率≧21%の上ノズルを用いてN2ガスを吹き込むことを特徴とするものである。 (2) Further, in the above (1), N 2 gas is blown using an upper nozzle having a porosity of porous part ≧ 21%.
本発明においては、耐水素誘起割れ鋼を連続鋳造する方法において、1.0≦T.Ca/T.O≦1.5を満たすようにCaを添加し、上ノズルからN2ガスを2NL/min≦N2ガス≦25NL/minの流量で吹き込むようにしたので、気泡起因割れと介在物起因割れの両方の耐水素誘起割れ特性に優れ、また上ノズルからN2ガス流量を少なく設定しているので、窒化物増による鋼の靭性劣化の問題もない。 In the present invention, in the method for continuously casting hydrogen-resistant cracked steel, Ca is added so as to satisfy 1.0 ≦ T.Ca / TO ≦ 1.5, and N 2 gas is supplied from the upper nozzle to 2 NL / min ≦ N 2 gas ≦ Since it was blown at a flow rate of 25 NL / min, it has excellent resistance to hydrogen-induced cracking, both bubble-induced cracking and inclusion-induced cracking, and the N 2 gas flow rate is set low from the upper nozzle, increasing the number of nitrides. There is no problem of deterioration of toughness of steel due to.
本実施の形態に係る耐水素誘起割れ鋼の連続鋳造方法は、溶鋼に1.0≦T.Ca/T.O≦1.5を満たすようにCaを添加し、上ノズルからN2ガスを2NL/min≦N2ガス≦25NL/minの流量で吹き込むことを特徴とするものである。
Caの溶鋼に対する添加量を1.0≦T.Ca/T.O≦1.5とし、N2ガスの吹込み流量を2NL/min≦N2ガス≦25NL/minにした理由を以下に説明する。
In the continuous casting method of hydrogen-resistant cracked steel according to the present embodiment, Ca is added to molten steel so as to satisfy 1.0 ≦ T.Ca / TO ≦ 1.5, and N 2 gas is supplied from the upper nozzle to 2 NL / min ≦ N 2. The gas is blown at a flow rate of gas ≦ 25 NL / min.
The amount to Ca molten steel and 1.0 ≦ T.Ca / TO ≦ 1.5, explaining the reason why the blowing flow rate of N 2 gas to 2 NL / min ≦ N 2 gas ≦ 25 NL / min below.
＜1.0≦T.Ca/T.O≦1.5の理由＞
発明者らは、CaO介在物浮上を目的として、CaSi投入後から鋳造開始までの静置時間を管理した。図１はCaSi投入量（kg/ch）(ch:タンディッシュへの溶鋼のチャージの意味である)を縦軸に、横軸を静置時間（CaSi投入後から鋳造開始までの時間）を示す。図１に示すように、静置時間を50分以上にすることでHIC試験における表層割れ表面率（CAR）を低減できることがわかる。しかしながら、静置時間を50分以上に管理したとしても、表層割れ表面率（CAR）が4%超えの不合格が発生していた。
<Reason for 1.0 ≦ T.Ca / TO ≦ 1.5>
The inventors managed the standing time from the introduction of CaSi to the start of casting for the purpose of floating CaO inclusions. Fig. 1 shows the amount of CaSi input (kg / ch) (ch: meaning the charge of molten steel to the tundish) on the vertical axis, and the horizontal axis shows the standing time (time from the start of CaSi to the start of casting). . As shown in FIG. 1, it is understood that the surface crack surface ratio (CAR) in the HIC test can be reduced by setting the standing time to 50 minutes or more. However, even if the standing time was controlled to be 50 minutes or more, the surface crack surface ratio (CAR) failed to exceed 4%.
この原因を調査するため、素鋼T.OとCaSi投入量（kg/ch）との関係で、割れ表面率（CAR）を整理した。なお、溶鋼は、溶鋼中に金属Alが添加されて脱酸処理され、脱酸生成物としてアルミナが形成される。本明細書では、このアルミナなどの酸化物として溶鋼中に存在する酸素と、溶存酸素とを合計したものを「T.O」（トータル酸素）と記載している。 In order to investigate this cause, the crack surface area (CAR) was arranged in relation to the base steel T.O and CaSi input (kg / ch). The molten steel is deoxidized by adding metal Al to the molten steel, and alumina is formed as a deoxidized product. In the present specification, the total of oxygen present in molten steel as an oxide such as alumina and dissolved oxygen is described as “T.O” (total oxygen).
図２のグラフは素鋼T.O（ppm）とCaSi投入量（kg/ch）との関係を示すものであり、縦軸が素鋼T.O（ppm）を示し、横軸がCaSi投入量（kg/ch）を示している。図２のグラフから、素鋼T.Oに対してCaSi投入量が増えると割れ表面率（CAR）が高くなる傾向があるとの知見を得た。
そこで、素鋼T.Ca/素鋼T.Oと割れ表面率（CAR）との関係調査した。調査結果を、図３に示す。図３（ａ）は、縦軸が表層割れ表面率（CAR）を示し、横軸が素鋼T.Ca/素鋼T.Oを示している。また、図３（ｂ）は縦軸が1/2t（板厚中央部）割れ表面率(CAR)を示し、横軸が素鋼T.Ca/素鋼T.Oを示している。本明細書では、素鋼中の全Ca濃度（ppm）を「T.Ca」（トータルカルシウム）と記載している。
図３（ａ）のグラフから、素鋼T.Ca/素鋼T.Oを1.5以下にすることで、割れ表面率（CAR）を低減できることがわかる。また、図３（ｂ）のグラフから素鋼T.Ca/素鋼T.Oが1.0未満ではＭｎＳ起因による中心偏析による1/2t（板厚中央部）割れ表面率(CAR)が悪化し、素鋼T.Ca/素鋼T.O≧1.0以上が好ましいことが分かる。
The graph in Fig. 2 shows the relationship between the steel TO (ppm) and the CaSi input (kg / ch). The vertical axis shows the steel TO (ppm) and the horizontal axis shows the CaSi input (kg / ch). ch). From the graph in FIG. 2, it was found that the crack surface area (CAR) tends to increase as the amount of CaSi input increases with respect to the steel TO.
Therefore, the relationship between steel T.Ca/steel TO and crack surface area (CAR) was investigated. The survey results are shown in FIG. In FIG. 3A, the vertical axis represents the surface crack surface ratio (CAR), and the horizontal axis represents the base steel T.Ca/base steel TO. In FIG. 3 (b), the vertical axis indicates 1 / 2t (plate thickness center) crack surface area (CAR), and the horizontal axis indicates steel T.Ca/steel TO. In this specification, the total Ca concentration (ppm) in the steel is described as “T.Ca” (total calcium).
From the graph of FIG. 3 (a), it can be seen that the crack surface ratio (CAR) can be reduced by setting the steel T.Ca/steel TO to 1.5 or less. Also, from the graph of Fig. 3 (b), when the steel T.Ca/steel TO is less than 1.0, the 1 / 2t (plate thickness center) crack surface area (CAR) due to center segregation due to MnS deteriorates, and the steel It can be seen that T.Ca/element steel TO ≧ 1.0 or more is preferable.
＜2.0NL/min≦N2ガス≦25NL/minの理由＞
図４は、UST欠陥不良率（％）とN2ガス(NL/min)との関係を示している。図４に示すように、N2ガスの吹込み量が2.0NL/min未満では、介在物浮上が不十分となり、UST欠陥不良率が高くなる。他方、N2ガスの吹込み量が25NLL/minを越えると、窒化物固溶により靱性が悪化する。以上の理由から、N2ガスの吹込み量は、2.0NL/min≦N2ガス≦25NL/minが好ましい。
<Reason for 2.0NL / min ≦ N 2 gas ≦ 25NL / min>
FIG. 4 shows the relationship between the UST defect defect rate (%) and N 2 gas (NL / min). As shown in FIG. 4, when the N 2 gas blowing rate is less than 2.0 NL / min, the inclusion floating is insufficient and the UST defect defect rate is increased. On the other hand, when the amount of N 2 gas blown exceeds 25 NLL / min, toughness deteriorates due to nitride solid solution. For the above reasons, the amount of N 2 gas blown is preferably 2.0 NL / min ≦ N 2 gas ≦ 25 NL / min.
また、N2ガスを吹き込む上ノズルにおけるポーラス部気孔率を、21%以上にするのが好ましい。
表層HIC割れの起点として気泡が原因となることを受けて、N2ガスを吹き込む上ノズルについても検討した。具体的には、上ノズルポーラス部気孔率と表層HIC割れと関係を調査した。調査結果を図５に示す。図５においては、縦軸が表層CAR(％)(最大値)を示し、横軸が上ノズル部ポーラス部の気孔率（％）を示している。
図５より、気孔率を21％以上とすることで、表層CARが低減することがわかる。この理由は、気孔率が低いとガス気泡が小さく、介在物浮上作用が小さくなり、逆に気孔率が高いとガス気泡が大きく、介在物浮上作用が大きくなると考えられる。そして、気孔率を21％以上とすることで表層CAR低減に対する効果が表れると推察される。
In addition, the porosity of the porous portion in the upper nozzle into which N 2 gas is blown is preferably 21% or more.
In consideration of the cause of air bubbles as a starting point of surface HIC cracking, an upper nozzle that blows N 2 gas was also studied. Specifically, the relationship between the porosity of the upper nozzle porous part and the surface HIC cracking was investigated. The survey results are shown in FIG. In FIG. 5, the vertical axis indicates the surface layer CAR (%) (maximum value), and the horizontal axis indicates the porosity (%) of the upper nozzle portion porous portion.
FIG. 5 shows that the surface CAR is reduced by setting the porosity to 21% or more. The reason is that if the porosity is low, the gas bubbles are small and the inclusion floating action is small. Conversely, if the porosity is high, the gas bubbles are large and the inclusion floating action is large. And it is guessed that the effect with respect to surface CAR reduction appears by making porosity into 21% or more.
上記実施の形態の効果を確認するための実験を行ったので、これについて以下の実施例において説明する。 An experiment for confirming the effect of the above embodiment was performed, and this will be described in the following examples.
（１）操業条件
転炉で約250トンの溶鋼を酸素吹錬した後、取鍋に出鋼し、ＲＨ真空脱ガス装置に搬送した。ＲＨ真空脱ガス装置では、成分調整等の必要に応じた精錬とともに、Al合金を所定量添加し、脱酸処理をした。Al合金添加後、溶鋼サンプルを採取し、T.Ca濃度とT.Oを分析した。
ＲＨ処理後、取鍋をCa合金ワイヤ投入設備へ搬送し、Ca合金ワイヤ投入設備において、Ca合金ワイヤを投入した。Ca合金ワイヤの投入は、T.Ca/T.Oを変化させてT.Ca/T.Oの影響を確認した。具体的には、表１を参照。
(1) Operation conditions About 250 tons of molten steel was blown with oxygen in a converter, then the steel was taken out into a ladle and transferred to an RH vacuum degasser. In the RH vacuum degassing apparatus, a predetermined amount of Al alloy was added and deoxidation treatment was performed along with refining as needed for component adjustment and the like. After adding the Al alloy, molten steel samples were collected and analyzed for T.Ca concentration and TO.
After the RH treatment, the ladle was transported to the Ca alloy wire charging facility, and the Ca alloy wire was charged in the Ca alloy wire charging facility. The introduction of Ca alloy wire changed the T.Ca/TO and confirmed the effect of T.Ca/TO. See Table 1 for details.
Ca合金ワイヤ投入後、取鍋を連続鋳造機上に設置されたタンディッシュ上へ搬送し、タンディッシュへ溶鋼を注入した。また上ノズル（２段のポーラス部）よりN2ガスを吹き込んだ。
尚、ポーラス部の平均気孔径は３０μｍであった。取鍋内溶鋼の約50%を注入した時点で、タンディッシュ内から溶鋼サンプルを採取し、T.Ca,T.O濃度を分析した。
After introducing the Ca alloy wire, the ladle was transported onto the tundish installed on the continuous casting machine, and molten steel was injected into the tundish. Further, N 2 gas was blown from the upper nozzle (two-stage porous portion).
The average pore diameter of the porous part was 30 μm. When about 50% of the molten steel in the ladle was poured, a molten steel sample was taken from the tundish and analyzed for T.Ca and TO concentrations.
タンディッシュに注入した溶鋼を連続鋳造法によりスラブを鋳造速度1.1〜1.4m/minで鋳造した。鋳造したスラブよりブロックサンプルを採取し、そこからスラブ厚み断面の全体に渡ってサンプルを採取し、介在物組成、個数を分析した。
前記スラブを用いて板厚31.8mm及び36.9mmの厚鋼板を製造した。
加熱したスラブを熱間圧延により圧延し、その後、加速冷却を施して所定の強度とした。この時のスラブ加熱温度は1050℃、圧延終了温度は800〜840℃、加速冷却開始温度は760〜800℃、加速冷却停止温度は450〜550℃とした。得られた鋼板の強度はいずれもAPIX65を満足するものであり、引張強度は570〜630ＭＰａであった。鋼板の引張特性については、圧延垂直方向の全厚試験片を引張試験片として引張試験を行い、引張強度を測定した。
The molten steel poured into the tundish was cast at a casting speed of 1.1 to 1.4 m / min by continuous casting. A block sample was taken from the cast slab, and a sample was taken over the entire slab thickness section, and the inclusion composition and number were analyzed.
Thick steel plates having thicknesses of 31.8 mm and 36.9 mm were manufactured using the slab.
The heated slab was rolled by hot rolling and then subjected to accelerated cooling to a predetermined strength. The slab heating temperature at this time was 1050 ° C., the rolling end temperature was 800-840 ° C., the accelerated cooling start temperature was 760-800 ° C., and the accelerated cooling stop temperature was 450-550 ° C. The strength of the obtained steel sheet satisfied APIX65, and the tensile strength was 570 to 630 MPa. Regarding the tensile properties of the steel sheet, a tensile test was performed using a full thickness test piece in the rolling vertical direction as a tensile test piece, and the tensile strength was measured.
（２）試験方法
これらの鋼板について、複数の位置から各10〜15個のHIC試験片を採取し、耐HIC特性を調査した。耐HIC特性は、pHが約３の硫化水素を飽和させた5%NaCl+0.5%CH3COOH水溶液（通常のNACE溶液）中に試験片を96時間浸漬した後、超音波探傷により試験片全面の割れの有無を調査し、割れ面積率（CAR）で評価した。ここで、それぞれの試験片の割れ面積率３％以下を合格とした。
表１に各HIC試験片の成分濃度、及びHIC試験成績を示す。
(2) Test method About these steel plates, 10-15 HIC test pieces were sampled from a plurality of positions, and the HIC resistance characteristics were investigated. The HIC resistance is that the specimen is immersed in a 5% NaCl + 0.5% CH3COOH aqueous solution (normal NACE solution) saturated with hydrogen sulfide with a pH of about 3 for 96 hours, and then the entire specimen is cracked by ultrasonic flaw detection. The crack area ratio (CAR) was evaluated. Here, a crack area ratio of 3% or less for each test piece was regarded as acceptable.
Table 1 shows the component concentration of each HIC test piece and the HIC test results.
表１に示すように、T.Ca/T.Oが本発明の範囲を満たすもの（表中の本発明例１〜4）は、表層CAR(％)が、1.00%以下、1/2t（板厚中央部）割れ表面率(CAR)(％)が、0.00％であり、耐HIC性能が極めて良好であった。
他方、T.Ca/T.Oが本発明の範囲よりも小さい比較例１，２では、1/2t（板厚中央部）割れ表面率(CAR)(％)が大きく、また、T.Ca/T.Oが本発明の範囲よりも大きい比較例３〜５では、表層CAR(％)が大きくなっている。
このことから、T.Ca/T.Oを本発明の範囲にすることで、耐HIC性能を向上できることが実証された。
As shown in Table 1, when T.Ca/TO satisfies the scope of the present invention (Invention Examples 1 to 4 in the table), the surface layer CAR (%) is 1.00% or less, 1 / 2t (plate thickness) The center part) crack surface area (CAR) (%) was 0.00%, and the HIC resistance was very good.
On the other hand, in Comparative Examples 1 and 2 where T.Ca/TO is smaller than the range of the present invention, the 1 / 2t (plate thickness center) crack surface area (CAR) (%) is large, and T.Ca/TO In Comparative Examples 3 to 5, which are larger than the range of the present invention, the surface layer CAR (%) is large.
From this, it was demonstrated that the HIC resistance can be improved by setting T.Ca/TO within the scope of the present invention.
Claims (2)
The continuous casting method for hydrogen-resistant cracked steel according to claim 1, wherein N 2 gas is blown using an upper nozzle having a porosity of porous part ≧ 21%.
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