JP6579147B2 - Manufacturing method of high cleanliness steel - Google Patents
Manufacturing method of high cleanliness steel Download PDFInfo
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- JP6579147B2 JP6579147B2 JP2017068809A JP2017068809A JP6579147B2 JP 6579147 B2 JP6579147 B2 JP 6579147B2 JP 2017068809 A JP2017068809 A JP 2017068809A JP 2017068809 A JP2017068809 A JP 2017068809A JP 6579147 B2 JP6579147 B2 JP 6579147B2
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Description
本発明は、酸化物系非金属介在物量が少ない鋼、すなわち高清浄度鋼の製造方法に関するものであり、特にカルシウム添加鋼の製造方法に関する。 The present invention relates to a method for producing a steel having a small amount of oxide-based nonmetallic inclusions, that is, a high cleanliness steel, and particularly to a method for producing a calcium-added steel.
製品特性の厳格化やより高機能な材料の要求から、鋼材中の酸化物系非金属介在物量をより低下させた高清浄度鋼に対する要請が高まっている。また、油井管などの用途で使用される高強度鋼管は、腐食性ガスの硫化水素を含む酸性化した厳しい環境（サワー環境）下で使用されることから、耐水素誘起割れ性（耐HIC性）、及び耐硫化物応力腐食割れ性（耐SSC性）に優れることが求められる。 Due to stricter product characteristics and demands for more sophisticated materials, there is an increasing demand for high cleanliness steels with a reduced amount of oxide-based nonmetallic inclusions in steel. In addition, high-strength steel pipes used in applications such as oil well pipes are used in acidified and harsh environments (sour environments) containing the hydrogen sulfide corrosive gas, so hydrogen-induced crack resistance (HIC resistance) ) And sulfide stress corrosion cracking resistance (SSC resistance).
耐HIC性及び耐SSC性の改善に対しては、溶鋼段階で酸化物系非金属介在物を低減することだけでなく、溶鋼の凝固時に析出、晶出してくるMnSに代表される硫化物を低減及び無害化することが必要となる。特にMnSは伸延性が高く、その後の鋼を圧延する際に伸延し、水素吸蔵サイトになることから、耐HIC性及び耐SSC性に対しては有害であることが知られている。 In order to improve HIC resistance and SSC resistance, not only reducing oxide-based non-metallic inclusions in the molten steel stage, but also sulfides typified by MnS that precipitate and crystallize during solidification of the molten steel. Reduction and detoxification are necessary. In particular, MnS has a high ductility, and it is known that it is detrimental to HIC resistance and SSC resistance because it is stretched during subsequent rolling of the steel and becomes a hydrogen storage site.
この対策としては、溶鋼段階でCa含有金属を添加することでMnSをCaSにすることが有効であることが一般的に知られている。このCa含有金属の添加方法および添加量について、以下の技術が知られている。 As a countermeasure, it is generally known that it is effective to change MnS to CaS by adding a Ca-containing metal at the molten steel stage. The following techniques are known for the addition method and addition amount of the Ca-containing metal.
特許文献１には、転炉出鋼後から、鋳造までの間に溶鋼にCaまたはCa含有物質を添加し、溶鋼中にCaを0.0005〜0.005質量％以上を含有させるとともに、鋼中Ｓ、Al、CaおよびTotal Ｏ（酸素）が下記式を満足するように制御することを特徴とする、耐硫化物応力腐食割れ性に優れた油井用鋼の製造方法が記載されている。
-0.005≦(Ca/40-S/32)×sol.Al×TotalＯ×1000000≦0.0042
In
-0.005 ≦ (Ca / 40-S / 32) × sol.Al × TotalO × 1000000 ≦ 0.0042
特許文献２には、二次精錬終了後に溶鋼のT．[O]を測定し、該溶鋼を連続鋳造機のタンディッシュへと注入を開始する前に、その測定値に基づいて計算した添加量のCaを添加して介在物の制御を行う高強度・高耐食性油井管用鋼材の溶製方法が記載されている。 In Patent Document 2, T. of molten steel after the secondary refining is completed. [O] is measured, and before starting to inject the molten steel into the tundish of a continuous caster, the added amount of Ca calculated based on the measured value is added to control inclusions. A method for melting a steel material for high corrosion resistance oil well pipes is described.
特許文献３には、転炉から取鍋への出鋼時または出鋼後に溶鋼にAlを添加して溶鋼を脱酸し、先ず、この取鍋内の溶鋼にCaOを含有するフラックスを添加して脱硫処理を施すとともに、この脱硫処理時にCa含有金属を添加し、次いで、取鍋内の溶鋼に真空脱ガス処理を施し、更に、真空脱ガス処理後の取鍋内の溶鋼にCa含有金属を添加し、その後、該溶鋼を鋳造するプロセスにおいて、前記脱硫処理時におけるCa含有金属のCa純分の添加量を、溶鋼中のAl濃度及びトータル酸素濃度に応じて調整することを特徴とする、耐硫化物腐食割れ性に優れた清浄鋼の製造方法が記載されている。
In
溶鋼にCa含有金属を添加することにより、前述したようにMnSの生成を抑制できるだけでなく、Al2O3系介在物をCaO-Al2O3系介在物に形態を変化させることが可能となる。特許文献１〜３の技術は、この観点で耐HIC性及び耐SSC性を改善することを目的に、Ca含有金属の添加量を規定するものである。すなわち、特許文献１〜３の技術は、Ca添加前にはAl2O3系介在物のみが存在するとみなして、このAl2O3系介在物にCaが反応することで、適正なCaO-Al2O3系介在物に変化するという考え方に基づいて、添加方法や添加量を規定する技術である。
By adding a Ca-containing metal to the molten steel, it is possible not only to suppress the formation of MnS as described above, but also to change the form of Al 2 O 3 inclusions to CaO-Al 2 O 3 inclusions. Become. The techniques of
しかしながら、本発明者らの検討によると、このような考え方に基づくCa添加では、より厳格な耐HIC性、耐SSC性の要求には応えられないことが判明した。 However, according to the study by the present inventors, it has been found that addition of Ca based on such a concept cannot meet the more strict requirements for HIC resistance and SSC resistance.
そこで本発明は、上記課題に鑑み、より優れた耐水素誘起割れ性（耐HIC性）及び耐硫化物応力腐食割れ性（耐SSC性）を有する高清浄度鋼の製造方法を提供することを目的とする。 Therefore, in view of the above problems, the present invention provides a method for producing a high cleanliness steel having superior hydrogen-induced crack resistance (HIC resistance) and sulfide stress corrosion cracking resistance (SSC resistance). Objective.
本発明者らは、サワー環境で使用される高強度シームレスパイプ用鋼などの介在物組成を詳細に調査した。この鋼には一般的に極低S、P成分、低O成分が要求されるため、以下のプロセスで製造されるのが一般的である。まず、転炉から取鍋への出鋼時又は出鋼後に溶鋼にAlを添加して、脱酸処理を行う。次に、取鍋内の溶鋼にCaOを含有するフラックスを添加して、レードルファーネス（LF）による取鍋精錬工程（脱硫処理）を行う。次に、RH真空脱ガス装置による真空脱ガス処理を行う。次に、溶鋼にCa含有金属を添加する（本明細書において、単に「Ca添加」とも称する。）。その後、溶鋼を取鍋からタンディッシュに移し、連続鋳造を行って鋳片とする。 The inventors investigated in detail the composition of inclusions such as steel for high-strength seamless pipes used in sour environments. Since this steel generally requires extremely low S, P component, and low O component, it is generally produced by the following process. First, Al is added to molten steel at the time of steel extraction from the converter to the ladle or after steel extraction, and deoxidation treatment is performed. Next, a flux containing CaO is added to the molten steel in the ladle, and a ladle refining process (desulfurization treatment) using a ladle furnace (LF) is performed. Next, a vacuum degassing process using an RH vacuum degassing apparatus is performed. Next, a Ca-containing metal is added to the molten steel (also simply referred to as “Ca addition” in the present specification). Then, the molten steel is transferred from the ladle to the tundish, and continuous casting is performed to obtain a slab.
ここで、溶鋼中の介在物に関して、脱酸処理の直後はAl2O3系介在物が主体である。しかし、取鍋精錬LF終了時の介在物組成は、Al2O3系介在物単体でなくCaO，MgOを含有したCaO-MgO-Al2O3系介在物に変化している場合が多くみられることを確認した。その理由としては、第一に、脱硫を目的に添加するCaO-Al2O3-SiO2系フラックスとMgO-C組成の耐火物との反応により、溶鋼中にMgが侵入すること、第二に、Si含有量が0.1％以上の組成では、FeSi合金の添加が行われるが、このFeSi合金中のCa成分が溶鋼中に侵入すること、が考えられる。 Here, regarding inclusions in the molten steel, Al 2 O 3 inclusions are mainly used immediately after the deoxidation treatment. However, the composition of inclusions at the end of the ladle refining LF is often changed to CaO-MgO-Al 2 O 3 inclusions containing CaO and MgO instead of Al 2 O 3 inclusions alone. It was confirmed that The reason for this is that, first, Mg penetrates into the molten steel due to the reaction between the CaO—Al 2 O 3 —SiO 2 flux added for the purpose of desulfurization and the refractory having the MgO—C composition. In addition, when the Si content is 0.1% or more, FeSi alloy is added, and it is conceivable that the Ca component in the FeSi alloy penetrates into the molten steel.
そして、本発明者らの検討によると、１チャージ内でCa添加前の複数の介在物の組成においてMgO濃度及びCaO濃度がばらついている場合には、所定量のCa含有金属を添加しても、タンディッシュ段階の最終的な介在物の組成にもばらつきが生じ、その結果、タンディッシュ段階の最終的な介在物の平均組成を1600℃液相範囲に制御することができないことが判明した。そして、上述した介在物の組成のばらつきが生じた場合には、より厳格なHIC性、耐SSC性の要求には応えられないことが判明した。 According to the study by the present inventors, when MgO concentration and CaO concentration vary in the composition of a plurality of inclusions before addition of Ca within one charge, a predetermined amount of Ca-containing metal may be added. As a result, it was found that the composition of the final inclusions in the tundish stage varied, and as a result, the average composition of the final inclusions in the tundish stage could not be controlled in the 1600 ° C. liquid phase range. And when the dispersion | variation in the composition of the inclusion mentioned above arises, it turned out that it cannot respond to the request | requirement of stricter HIC property and SSC resistance.
そこで、本発明では、Ca添加前の溶鋼における介在物の組成をばらつかせないこと、具体的には、Ca添加前の溶鋼におけるCaO-MgO-Al2O3系介在物の平均組成を下記の（I）又は（II）とすることが重要である。
記
MgO：10〜30質量％、CaO：0〜10質量％、Al2O3：残部 ・・・（I）
MgO：0〜5質量％、CaO：20〜35質量％、Al2O3：残部 ・・・（II）
Therefore, in the present invention, the composition of inclusions in the molten steel before Ca addition does not vary, specifically, the average composition of CaO-MgO-Al 2 O 3 inclusions in the molten steel before Ca addition is as follows: (I) or (II) is important.
Record
MgO: 10 to 30% by mass, CaO: 0 to 10% by mass, Al 2 O 3 : balance (I)
MgO: 0 to 5 wt%, CaO: 20 to 35 wt%, Al 2 O 3: balance · · · (II)
さらに本発明者らの検討によれば、当該平均組成が（I）か（II）かに応じて、より優れた耐HIC性及び耐SSC性を実現するためのCa添加量が異なることを見出した。 Furthermore, according to the study by the present inventors, it has been found that the amount of Ca added to achieve better HIC resistance and SSC resistance differs depending on whether the average composition is (I) or (II). It was.
本発明は、上記の知見に基づき完成されたものであり、その要旨構成は、以下のとおりである。
［１］転炉から取鍋への出鋼時又は出鋼後に溶鋼にAlを添加して、前記溶鋼に脱酸処理を施す工程と、
前記取鍋内の前記溶鋼にCaOを含有するフラックスを添加して、前記溶鋼に脱硫処理を施す取鍋精錬工程と、
その後、真空脱ガス装置により前記溶鋼に真空脱ガス処理を施す工程と、
その後、前記溶鋼にCa含有金属を添加する工程と、
その後、前記溶鋼を連続鋳造する工程と、
を有する高清浄度鋼の製造方法であって、
前記真空脱ガス処理後かつ前記Ca含有金属の添加前の前記溶鋼におけるCaO-MgO-Al2O3系介在物の平均組成を下記の（I）又は（II）とし、
（I）の場合には、前記Ca含有金属の添加後の前記溶鋼中のCa濃度［％Ca］が下記の（1）式及び（2）式を満たすように、（II）の場合には、前記Ca含有金属の添加後の前記溶鋼中のCa濃度［％Ca］が下記の（1）式及び（3）式を満たすように、前記Ca含有金属の添加量を設定することを特徴とする高清浄度鋼の製造方法。
記
MgO：10〜30質量％、CaO：0〜10質量％、Al2O3：残部 ・・・（I）
MgO：0〜5質量％、CaO：20〜35質量％、Al2O3：残部 ・・・（II）
［％Ca］≧1.25×［％S］ ・・・（1）
0.62×［％T.O］≦［％Ca］≦0.91×［％T.O］ ・・・（2）
0.27×［％T.O］≦［％Ca］≦0.49×［％T.O］ ・・・（3）
ここで、
［％Ca］：Ca含有金属の添加後の溶鋼中のCa濃度（質量％）
［％S］：真空脱ガス処理後かつCa含有金属の添加前の溶鋼中のS濃度（質量％）
［％T.O］：真空脱ガス処理後かつCa含有金属の添加前の溶鋼中の全酸素濃度（質量％）
The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
[1] A step of adding Al to the molten steel at the time of or after the steel from the converter to the ladle and subjecting the molten steel to a deoxidation treatment;
A ladle refining step of adding a flux containing CaO to the molten steel in the ladle and subjecting the molten steel to a desulfurization treatment,
Then, a step of vacuum degassing the molten steel with a vacuum degassing device,
Then, adding a Ca-containing metal to the molten steel,
Then, the step of continuously casting the molten steel,
A method for producing a high cleanliness steel having
The average composition of the CaO—MgO—Al 2 O 3 inclusions in the molten steel after the vacuum degassing treatment and before the addition of the Ca-containing metal is (I) or (II) below,
In the case of (I), the Ca concentration [% Ca] in the molten steel after the addition of the Ca-containing metal satisfies the following formulas (1) and (2). The addition amount of the Ca-containing metal is set so that the Ca concentration [% Ca] in the molten steel after the addition of the Ca-containing metal satisfies the following formulas (1) and (3): To produce high cleanliness steel.
Record
MgO: 10 to 30% by mass, CaO: 0 to 10% by mass, Al 2 O 3 : balance (I)
MgO: 0 to 5 wt%, CaO: 20 to 35 wt%, Al 2 O 3: balance · · · (II)
[% Ca] ≧ 1.25 × [% S] (1)
0.62 x [% TO] ≤ [% Ca] ≤ 0.91 x [% TO] (2)
0.27 × [% TO] ≦ [% Ca] ≦ 0.49 × [% TO] (3)
here,
[% Ca]: Ca concentration (mass%) in molten steel after addition of Ca-containing metal
[% S]: S concentration (% by mass) in molten steel after vacuum degassing and before addition of Ca-containing metal
[% TO]: Total oxygen concentration (% by mass) in molten steel after vacuum degassing and before addition of Ca-containing metal
［２］前記取鍋精錬工程の直後の前記取鍋内の前記溶鋼におけるCaO-MgO-Al2O3系介在物の平均組成を求め、
当該平均組成が前記（I）及び（II）のいずれにも該当しない場合には、前記取鍋内の前記溶鋼にCa含有金属又はMg含有金属を添加することによって、前記真空脱ガス処理後かつ前記Ca含有金属の添加前の前記溶鋼におけるCaO-MgO-Al2O3系介在物の平均組成を前記（I）又は（II）とする、上記［１］に記載の高清浄度鋼の製造方法。
[2] Obtain an average composition of CaO—MgO—Al 2 O 3 inclusions in the molten steel in the ladle immediately after the ladle refining step,
When the average composition does not correspond to any of (I) and (II), by adding a Ca-containing metal or an Mg-containing metal to the molten steel in the ladle, after the vacuum degassing treatment and Production of high cleanliness steel according to [1] above, wherein the average composition of CaO—MgO—Al 2 O 3 inclusions in the molten steel before addition of the Ca-containing metal is the (I) or (II). Method.
本発明によれば、より優れた耐水素誘起割れ性（耐HIC性）及び耐硫化物応力腐食割れ性（耐SSC性）を有する高清浄度鋼を製造することが可能となる。 According to the present invention, it is possible to produce high cleanliness steel having more excellent hydrogen-induced crack resistance (HIC resistance) and sulfide stress corrosion crack resistance (SSC resistance).
本発明の一実施形態による高清浄度鋼の製造方法は、転炉から取鍋への出鋼時又は出鋼後に溶鋼にAlを添加して、前記溶鋼に脱酸処理を施す工程と、前記取鍋内の前記溶鋼にCaOを含有するフラックスを添加して、前記溶鋼に脱硫処理を施す取鍋精錬工程と、その後、真空脱ガス装置により前記溶鋼に真空脱ガス処理を施す工程と、その後、前記溶鋼にCa含有金属を添加する工程と、その後、前記溶鋼を連続鋳造する工程と、を有する。 The method for producing a high cleanliness steel according to an embodiment of the present invention includes adding Al to the molten steel at the time of steel extraction from the converter to the ladle or after steel output, and subjecting the molten steel to deoxidation treatment, and Adding a flux containing CaO to the molten steel in the ladle, performing a ladle refining process for desulfurizing the molten steel, and then performing a vacuum degassing process on the molten steel with a vacuum degassing apparatus; And a step of adding a Ca-containing metal to the molten steel, and then a step of continuously casting the molten steel.
脱酸処理は、溶鋼にAlを添加する一般的な方法により行うことができる。脱酸処理によって形成される脱酸生成物はAl2O3（アルミナ）である。 The deoxidation treatment can be performed by a general method of adding Al to molten steel. The deoxidation product formed by the deoxidation treatment is Al 2 O 3 (alumina).
取鍋精錬工程は、レードルファーネス（LF）を用いて、溶鋼をアーク放電で加熱しつつ、溶鋼内にガスを導入する加熱撹拌処理を含む。溶鋼にはCaOを含有するフラックスを添加して、脱硫処理を行う。フラックスとしては、生石灰（CaO）単独、或いは、生石灰と、CaOの滓化促進剤であるAl2O3又はSiO2との混合物などを用いることができる。真空脱ガス処理は、例えば、RH真空脱ガス装置などの一般的な装置を用いて行うことができる。 The ladle refining process includes a heating and stirring process of introducing gas into the molten steel while heating the molten steel by arc discharge using a ladle furnace (LF). A flux containing CaO is added to the molten steel and desulfurized. As the flux, quick lime (CaO) alone or a mixture of quick lime and Al 2 O 3 or SiO 2 which is a CaO hatching accelerator can be used. The vacuum degassing process can be performed using, for example, a general apparatus such as an RH vacuum degassing apparatus.
本発明者らは、このような製造プロセスに対して溶鋼サンプルを各プロセスで採取し、溶鋼成分、介在物量及び介在物組成の調査を実施した。図１は、あるチャージにおいて、RH処理後かつCa添加前に採取した溶鋼サンプルにおけるCaO-MgO-Al2O3系介在物の組成の調査結果であり、SEM（走査型電子顕微鏡）により直径5μｍ以上のサイズの介在物を特定し、EDX（エネルギー分散型X線分光法）により介在物組成を測定して、CaO-MgO-Al2O3の3元系状態図上にプロットしたものである。図１より、RH処理後かつCa添加前の溶鋼における介在物の組成は、Al2O3系ではなく、MgO、CaO成分をある程度含んだCaO-MgO-Al2O3系介在物へと変化していることが確認できた。図１中には、直径5μm以上の全介在物の平均組成も示した。 The inventors of the present invention collected molten steel samples in each process and investigated the molten steel composition, the amount of inclusions, and the composition of inclusions. Fig. 1 shows the results of investigating the composition of CaO-MgO-Al 2 O 3 inclusions in molten steel samples taken after RH treatment and before Ca addition at a certain charge. The diameter was 5 µm by SEM (scanning electron microscope). identify inclusions or more sizes, by measuring the composition of inclusions by EDX (energy dispersive X-ray spectroscopy), is plotted on a ternary phase diagram of CaO-MgO-Al 2 O 3 . From Fig. 1, the composition of inclusions in the molten steel after RH treatment and before addition of Ca is not Al 2 O 3 type, but changes to CaO-MgO-Al 2 O 3 type inclusions containing some MgO and CaO components. I was able to confirm. FIG. 1 also shows the average composition of all inclusions having a diameter of 5 μm or more.
また、介在物の組成は、MgO・Al2O3スピネル組成〜CaO・2Al2O3組成の間でばらつきが大きくみられることが確認できた。このように、１チャージ内で複数の介在物間で組成のばらつきが生じる理由は、後述するように、取鍋精錬時に溶鋼にCa,Mgが侵入することが考えられた。 In addition, it was confirmed that the composition of inclusions showed a large variation between the MgO · Al 2 O 3 spinel composition to the CaO · 2Al 2 O 3 composition. As described above, the reason why the composition variation occurs among a plurality of inclusions within one charge is considered to be that Ca and Mg enter the molten steel during ladle refining as described later.
さらに本発明者らは、直径5μm以上の全介在物の平均組成に関して、RH処理後かつCa添加前の溶鋼と、Ca添加後の溶鋼との比較を行った。図２（Ａ）は、RH処理後かつCa添加前に採取した溶鋼サンプルにおけるCaO-MgO-Al2O3系介在物の平均組成を複数のチャージで調査した結果であり、図２（Ｂ）は、図２（Ａ）の各チャージにおいて、Ca添加後にタンディッシュにて採取した溶鋼サンプルにおけるCaO-MgO-Al2O3系介在物の平均組成を調査した結果である。 Furthermore, the present inventors compared the molten steel after the RH treatment and before the Ca addition with the molten steel after the Ca addition with respect to the average composition of all inclusions having a diameter of 5 μm or more. Fig. 2 (A) is the result of investigating the average composition of CaO-MgO-Al 2 O 3 inclusions in molten steel samples taken after RH treatment and before Ca addition with multiple charges, Fig. 2 (B) These are the results of investigating the average composition of CaO—MgO—Al 2 O 3 inclusions in molten steel samples collected by tundish after addition of Ca in each charge of FIG.
いずれのチャージでも、Ca添加後の介在物組成をタンディッシュ段階で1600℃液相範囲になるようにCa添加量を決めてCa添加を実施した。しかし、図２（Ａ），（Ｂ）に示すように、結果としてはCa添加前の平均組成が、MgO・Al2O3スピネル主体の場合（○印）とCaO・2Al2O3組成が主体の場合（△印）には、タンディッシュでの介在物の平均組成を1600℃液相範囲に制御することができたが、Ca添加前の平均組成がMgO・Al2O3スピネル〜CaO・2Al2O3組成にばらつきが大きい場合（◆印）には、CaSの析出を伴う高CaO濃度の介在物の生成を十分に抑制することができず、結果として、タンディッシュでの介在物の平均組成を1600℃液相範囲に制御することができなかった。 In any charge, the addition of Ca was carried out by determining the amount of Ca added so that the inclusion composition after addition of Ca would be in the liquid phase range at 1600 ° C. in the tundish stage. However, as shown in FIGS. 2 (A) and 2 (B), as a result, the average composition before Ca addition is the same as that of the MgO · Al 2 O 3 spinel (circle) and CaO · 2Al 2 O 3 composition. In the case of the main body (marked with Δ), the average composition of inclusions in the tundish could be controlled to 1600 ° C liquid phase range, but the average composition before Ca addition was MgO · Al 2 O 3 spinel to CaO.・ When the variation in 2Al 2 O 3 composition is large (marked with ◆), the formation of inclusions with high CaO concentration accompanied by the precipitation of CaS cannot be sufficiently suppressed, resulting in inclusions in the tundish. It was not possible to control the average composition of the liquid to the 1600 ° C. liquid phase range.
ここで、タンディッシュ段階の介在物の平均組成を1600℃液相範囲にする目的は、以下のとおりである。
（１）図２の◆のように、タンディッシュ段階でCaS析出を伴う高CaO濃度のCaO-Al2O3介在物（3CaO・Al2O3〜CaO+CaS）となった場合、タンディッシュ〜鋳型での浸漬ノズルにおいて温度低下時にCaS起因のノズル閉塞が発生しやすく、さらには凝集合体により巨大化した介在物がノズル付着箇所より脱落して鋳片に取り込まれ耐HIC性及び耐SSC性の劣化が顕著となる。
（２）タンディッシュでの介在物の平均組成が1600℃液相範囲よりも低CaO濃度のCaO-Al2O3介在物組成（特にCaO・6Al2O3〜CaO・2Al2O3）となった場合にも、ノズル閉塞が生じやすくなり、凝固時に有害なMnSが析出しやすくなり耐HIC性及び耐SSC性の劣化が顕著となる。
したがってCaO・Al2O3〜3CaO・Al2O3の介在物組成、好ましくは12CaO・7Al2O3介在物組成に制御することが重要である。
Here, the purpose of setting the average composition of inclusions in the tundish stage to the 1600 ° C. liquid phase range is as follows.
(1) As shown in ◆ in Fig. 2, when the CaO-Al 2 O 3 inclusions (3CaO · Al 2 O 3 to CaO + CaS) with high CaO concentration accompanied by CaS precipitation in the tundish stage, the tundish 〜Causes of nozzle clogging due to CaS are likely to occur when the temperature drops in the immersion nozzle in the mold, and further, the inclusions that have become enormous due to agglomeration fall off from the location where the nozzle adheres and are taken into the slab and are resistant to HIC and SSC Degradation becomes remarkable.
(2) CaO-Al 2 O 3 inclusion composition (especially CaO · 6Al 2 O 3 to CaO · 2Al 2 O 3 ) with an average composition of inclusions in the tundish with a lower CaO concentration than the 1600 ° C liquid phase range Even in such a case, nozzle clogging is likely to occur, and harmful MnS is liable to precipitate during solidification, and the deterioration of HIC resistance and SSC resistance becomes significant.
Therefore, it is important to control the inclusion composition of CaO · Al 2 O 3 to 3CaO · Al 2 O 3 , preferably 12CaO · 7Al 2 O 3 inclusion composition.
本発明者らは、本結果を基にCa添加前の介在物組成がAl2O3介在物主体でなくCaO-MgO-Al2O3系介在物で組成にバラツキが生じる理由を鋭意検討した。一般にLF処理での溶鋼脱硫を極低Sレベルまで実施させる場合には、CaO-Al2O3-SiO2系フラックスとMgO-C組成の耐火物との反応により、溶鋼中にMgが侵入し、このMgが介在物と反応してMgO・Al2O3スピネル介在物を形成する。MgO・Al2O3スピネル形成に必要なT.Mg濃度は1〜2ppmと低位であり、反応時間が長いほど安定してMgO・Al2O3スピネルが生成されることが知られている。 Based on these results, the present inventors diligently investigated why the inclusion composition before Ca addition was not Al 2 O 3 inclusions but CaO-MgO-Al 2 O 3 inclusions. . Generally, when molten steel desulfurization by LF treatment is performed to an extremely low S level, Mg penetrates into the molten steel due to the reaction between the CaO-Al 2 O 3 -SiO 2 system flux and the refractory with MgO-C composition. The Mg reacts with the inclusions to form MgO.Al 2 O 3 spinel inclusions. It is known that the T.Mg concentration necessary for forming the MgO · Al 2 O 3 spinel is as low as 1 to 2 ppm, and that the MgO · Al 2 O 3 spinel is more stably generated as the reaction time is longer.
またCaO系の脱硫フラックスによるスラグが溶鋼に介在物として巻き込まれたり、溶鋼中のAl2O3介在物と反応することでT.Ca値としてCa=0〜2ppm程度が検出されるようになる。しかしながらCa＝0ppm以上3ppm未満、T.Mg＝1〜2ppmの場合には、MgO・Al2O3スピネル介在物の組成はほぼバラツキが少なく（I）の領域にある。
（I）領域； MgO：10〜30質量％、CaO：0〜10質量％、Al2O3:残部
In addition, slag caused by CaO-based desulfurization flux is caught in molten steel as inclusions, or reacts with Al 2 O 3 inclusions in molten steel, so that Ca = 0 to 2 ppm is detected as a T.Ca value. . However, when Ca = 0 ppm or more and less than 3 ppm and T.Mg = 1 to 2 ppm, the composition of the MgO · Al 2 O 3 spinel inclusion is almost inconsistent and is in the region (I).
(I) region; MgO: 10 to 30% by mass, CaO: 0 to 10% by mass, Al 2 O 3 : balance
またSi濃度が0.1％以上の鋼種では転炉出鋼後もしくはLF処理中にFeSi合金添加によりSi濃度の調整を行うことが一般的である。FeSi合金にはCa成分が1％程度含有しているものが多く、この場合FeSi合金添加によりT.Ca濃度がさらに2〜3ppm程度高くなる。T.Ca＞5ppm、T.Mg=1〜2ppmの場合には、（II）に示した通り、介在物組成はCaO・2Al2O3組成になり、CaO-Al2O3系介在物として安定する。
（II）領域； MgO：0〜5質量％、CaO：20〜35質量％、Al2O3:残部
For steel grades with an Si concentration of 0.1% or more, it is common to adjust the Si concentration by adding FeSi alloy after converter steelmaking or during LF treatment. Many FeSi alloys contain about 1% of the Ca component. In this case, the addition of the FeSi alloy further increases the T.Ca concentration by about 2 to 3 ppm. In the case of T.Ca> 5 ppm and T.Mg = 1 to 2 ppm, the inclusion composition becomes CaO · 2Al 2 O 3 composition as shown in (II), and as a CaO—Al 2 O 3 inclusion Stabilize.
(II) region; MgO: 0 to 5% by mass, CaO: 20 to 35% by mass, Al 2 O 3 : balance
しかしながら、T.Ca＝3〜5ppm、T.Mg=0〜3ppmでは、介在物は、MgO・Al2O3スピネル組成〜CaO・2Al2O3組成の間にばらついた組成分布をとり、結果として、介在物の平均組成は、（I）の領域にも（II）の領域にも入らなくなる。理由としては全体としてのCa量が不十分なため、Caと十分に反応した一部の介在物ではCaO・2Al2O3介在物に変化するが、大半がMgO・Al2O3スピネル組成付近のものとしてほとんど反応していないためと考えられる。 However, when T.Ca = 3 to 5 ppm and T.Mg = 0 to 3 ppm, the inclusion takes a composition distribution that varies between the MgO · Al 2 O 3 spinel composition and the CaO · 2Al 2 O 3 composition, and the result As a result, the average composition of inclusions does not enter the region (I) or the region (II). The reason is that the amount of Ca as a whole is insufficient, so some inclusions that react sufficiently with Ca change to CaO · 2Al 2 O 3 inclusions, but most of them are in the vicinity of MgO · Al 2 O 3 spinel composition. This is thought to be due to the fact that there was almost no reaction.
このように、本発明では、真空脱ガス処理後かつCa含有金属の添加前の溶鋼におけるCaO-MgO-Al2O3系介在物の平均組成を以下の（I）又は（II）とすることが肝要である。
MgO：10〜30質量％、CaO：0〜10質量％、Al2O3：残部 ・・・（I）
MgO：0〜5質量％、CaO：20〜35質量％、Al2O3：残部 ・・・（II）
平均組成が上記（I）又は（II）にあることは、各介在物の組成が図１とは異なってばらついていないことを意味している。この場合、Ca含有金属の添加量を適切に設定することで、Ca添加後のタンディッシュ段階での溶鋼中の介在物の平均組成を1600℃液相範囲とすることができ、その結果、より優れた耐HIC性及び耐SSC性を実現でき、また、タンディッシュ〜鋳型での浸漬ノズルの閉塞も抑制できる。
Thus, in the present invention, the average composition of CaO—MgO—Al 2 O 3 inclusions in the molten steel after vacuum degassing treatment and before addition of the Ca-containing metal is defined as (I) or (II) below. Is essential.
MgO: 10 to 30% by mass, CaO: 0 to 10% by mass, Al 2 O 3 : balance (I)
MgO: 0 to 5% by mass, CaO: 20 to 35% by mass, Al 2 O 3 : balance (II)
The fact that the average composition is in the above (I) or (II) means that the composition of each inclusion does not vary unlike FIG. In this case, by appropriately setting the addition amount of the Ca-containing metal, the average composition of inclusions in the molten steel in the tundish stage after the addition of Ca can be in the 1600 ° C liquid phase range, as a result, more Excellent HIC resistance and SSC resistance can be realized, and blockage of the immersion nozzle from tundish to mold can be suppressed.
ここで、「真空脱ガス処理後かつCa含有金属の添加前の溶鋼におけるCaO-MgO-Al2O3系介在物の平均組成」は、以下の方法で測定することにより、求めることができる。まず、溶鋼サンプルを浴面から2ｍ以上の深さ位置から採取し、樹脂埋め込み・研磨を実施して、SEM観察用試料を作製する。その試料をSEM観察に供し、15×15ｍｍの視野中の介在物径が5μｍ以上の全ての介在物について、EDXで組成を求め、その平均を算出する。なお、介在物断面形状が異方性を持つ場合には、その断面を囲む楕円の長径と短径の積の平方根を介在物径とする。 Here, “the average composition of CaO—MgO—Al 2 O 3 inclusions in the molten steel after the vacuum degassing treatment and before the addition of the Ca-containing metal” can be determined by measuring by the following method. First, a molten steel sample is taken from a depth of 2 m or more from the bath surface, and resin embedding and polishing are performed to prepare a sample for SEM observation. The sample is subjected to SEM observation, and the composition is determined by EDX for all inclusions having an inclusion diameter of 5 μm or more in a 15 × 15 mm visual field, and the average is calculated. When the inclusion cross-sectional shape has anisotropy, the square diameter of the product of the major axis and the minor axis of the ellipse surrounding the section is defined as the inclusion diameter.
上述のように、RH処理後かつCa添加前介在物の平均組成が（I）と（II）間にある場合、すなわち、介在物組成がばらついた場合には、単純にCa添加量を低減しただけでは、タンディッシュ段階でバラツキの少ない介在物組成に制御して、タンディッシュ段階の最終的な介在物の平均組成を1600℃液相範囲に制御することは難しい。 As described above, when the average composition of inclusions after RH treatment and before Ca addition is between (I) and (II), that is, when the inclusion composition varies, the amount of Ca addition is simply reduced. Alone, it is difficult to control the inclusion composition with little variation in the tundish stage and to control the average composition of the final inclusion in the tundish stage to the 1600 ° C. liquid phase range.
そこで、RH処理後かつCa添加前の介在物の平均組成を確実に（I）又は（II）の範囲にするための方法が検討された。具体的には、図３に示すように、取鍋精錬工程（LFプロセス）の直後の取鍋内の溶鋼におけるCaO-MgO-Al2O3系介在物の平均組成を求め、当該平均組成が前記（I）及び（II）のいずれにも該当しない場合には、取鍋内の溶鋼にCa含有金属又はMg含有金属を添加する。 Therefore, a method for ensuring that the average composition of inclusions after RH treatment and before Ca addition is in the range of (I) or (II) was studied. Specifically, as shown in FIG. 3, an average composition of the CaO-MgO-Al 2 O 3 inclusions in the molten steel in the ladle just after the ladle refining process (LF process), is the average composition When neither of the above (I) and (II) applies, a Ca-containing metal or an Mg-containing metal is added to the molten steel in the ladle.
すなわち、LFプロセス直後の介在物の平均組成が（I）と（II）の間にある場合には、Mg合金を微量添加して、Mg＝1〜2ppm、Ca=0〜3ppmに調整することで、RH処理後かつCa添加前には、（I）範囲のMgO・Al2O3スピネル組成に安定して制御可能となることを確認した。また、同様に微量のCa添加を実施してCa≧5ppm、Mg＝1〜2ppmとすると、RH処理後かつCa添加前には、（II）範囲のCaO・2Al2O3介在物組成に制御することが可能となる。 That is, if the average composition of inclusions immediately after the LF process is between (I) and (II), add a small amount of Mg alloy to adjust Mg = 1 to 2 ppm and Ca = 0 to 3 ppm. Thus, after the RH treatment and before the addition of Ca, it was confirmed that the MgO · Al 2 O 3 spinel composition in the range (I) can be stably controlled. Similarly, if a small amount of Ca is added and Ca ≥ 5 ppm and Mg = 1 to 2 ppm, the CaO · 2Al 2 O 3 inclusion composition within the range (II) is controlled after RH treatment and before Ca addition. It becomes possible to do.
Mg、Ca添加はLF終了後に実施するのが望ましい。すなわち処理プロセスの構成としては、図３に示した転炉〜タンディッシュまでのプロセスで実施するのが最適である。LFプロセスとRHプロセスの処理終了直前に溶鋼成分の分析を実施し、迅速分析結果から微量なCa,Mg添加量及びCa処理時のCa合金添加条件を決定する方法が有効である。 It is desirable to add Mg and Ca after completion of LF. That is, it is optimal to implement the processing process from the converter to the tundish shown in FIG. An effective method is to analyze the molten steel components immediately before the end of the LF process and RH process, and to determine the trace amount of Ca and Mg added and the Ca alloy addition conditions during Ca treatment from the results of rapid analysis.
なお、LF処理後のタイミングでMg,Ca合金を添加する理由は、その後のRH処理の強攪拌ならびに真空処理での蒸発で、Ca、Mg成分が均一に数ppmの上昇にとどまるため、ほぼ目標値への制御がしやすいためである。 The reason why Mg and Ca alloys are added at the timing after LF treatment is that the target of Ca and Mg components is only a few ppm increase due to subsequent strong RH treatment and evaporation during vacuum treatment. This is because it is easy to control the value.
またCa添加方法は特に規定しないが、含有量がCa：70質量％、Si：30質量％の塊状合金や、それをFeフープでくるんだワイヤーを溶鋼中に添加する方法が一般的に用いられている。Ca合金は溶鋼と激しく反応するため、添加時に溶鋼再酸化物を生成しやすく、添加時のアルゴンシールを完全にすることが好ましい。また、Mg添加法についてもCa添加法と同様の方法で、FeフープにMg、SiMg、MgSiCa合金などをくるんだワイヤーや塊状の合金を添加することで溶鋼中にMgを投入することができ、添加速度、歩留りなどはここでは特に規定しない。 The Ca addition method is not particularly specified, but a method of adding a massive alloy containing Ca: 70% by mass and Si: 30% by mass or a wire wrapped with Fe hoop into molten steel is generally used. ing. Since Ca alloy reacts violently with molten steel, it is easy to form molten steel re-oxide when added, and it is preferable that the argon seal is completely added. Also, the Mg addition method is the same method as the Ca addition method, and Mg can be introduced into the molten steel by adding a wire or block alloy wrapped with Mg, SiMg, MgSiCa alloy, etc. to the Fe hoop, The addition speed, yield, etc. are not particularly specified here.
そして、本発明者らは、真空脱ガス後のCa添加の際のCa添加量に関して、RH処理後かつCa添加前の介在物の平均組成が（I）の領域にあるか、(II)の領域にあるかに応じて、より優れた耐HIC性及び耐SSC性を実現するためのCa添加量が異なることを見出した。 And, regarding the amount of Ca added at the time of Ca addition after vacuum degassing, the present inventors determine whether the average composition of inclusions after RH treatment and before Ca addition is in the region (I) or (II) It was found that the amount of Ca added to achieve better HIC resistance and SSC resistance was different depending on whether it was in the region.
図４（Ａ）に、Ca合金添加前の全酸素量T.Oと、Ca合金添加後のタンディッシュ段階でのCa濃度との関係をまとめた結果を示す。図中の○、△印は、タンディッシュ段階での介在物の平均組成を目標の1600℃液相範囲とすることができたものであり、×印は1600℃液相範囲から外れた場合を示す。 FIG. 4A shows the results of summarizing the relationship between the total oxygen amount T.O before the addition of the Ca alloy and the Ca concentration in the tundish stage after the addition of the Ca alloy. ○ and △ in the figure indicate that the average composition of inclusions in the tundish stage could be the target 1600 ° C liquid phase range, and × indicates that the inclusion was outside the 1600 ° C liquid phase range. Show.
最終製品においてはMnS介在物が生成するとHIC欠陥の発生が問題となる。図４（Ａ）で、1600℃液相範囲に介在物を制御した場合でも、△印の場合は、凝固時にMnS介在物が生成するため適正でないことを本発明者らは経験的に確認した。
さらにはMnS生成を抑制するためには、下記のCa,S量の条件を満足することが必要となる。
［％Ca］≧1.25×［％S］ ・・・（1）
ここで、
［％Ca］：Ca含有金属の添加後の溶鋼中のCa濃度（質量％）
［％S］：真空脱ガス処理後かつCa含有金属の添加前の溶鋼中のS濃度（質量％）
である。
In the final product, generation of HIC defects becomes a problem when MnS inclusions are formed. In FIG. 4A, even when inclusions are controlled within the liquid phase range of 1600 ° C., the inventors have empirically confirmed that in the case of Δ, MnS inclusions are generated during solidification, which is not appropriate. .
Furthermore, in order to suppress MnS production, it is necessary to satisfy the following conditions for Ca and S content.
[% Ca] ≧ 1.25 × [% S] (1)
here,
[% Ca]: Ca concentration (mass%) in molten steel after addition of Ca-containing metal
[% S]: S concentration (% by mass) in molten steel after vacuum degassing and before addition of Ca-containing metal
It is.
そして、図４（Ａ）に示すように、Ca添加前の初期介在物平均組成を（I）範囲に制御した場合には上記の（1）式に加えて下記の（2）式を満たすように、（II）範囲に制御した場合には、上記の（1）式に加えて下記の（3）式を満たすように、Ca含有金属の添加量を設定することによって、タンディッシュ段階での介在物の平均組成を目標の1600℃液相範囲とすることができた。
0.62×［％T.O］≦［％Ca］≦0.91×［％T.O］ ・・・（2）
0.27×［％T.O］≦［％Ca］≦0.49×［％T.O］ ・・・（3）
ここで、
［％Ca］：Ca含有金属の添加後の溶鋼中のCa濃度（質量％）
［％T.O］：真空脱ガス処理後かつCa含有金属の添加前の溶鋼中の全酸素濃度（質量％）
である。
以上の結果に基づいて、Ca添加量の目標値を決定すればよい。なお、真空脱ガス後のCa添加は、RH真空脱ガス装置の取鍋内で、真空脱ガス処理に引き続き行ってもよいが、別途Ca処理専用の取鍋に溶鋼を移した後、当該取鍋内で溶鋼にしてCa添加を行うことが好ましい。
As shown in FIG. 4A, when the initial inclusion average composition before Ca addition is controlled within the range (I), the following formula (2) is satisfied in addition to the above formula (1). In addition, when controlled to the range (II), in addition to the above formula (1), by setting the addition amount of the Ca-containing metal so as to satisfy the following formula (3), in the tundish stage The average composition of inclusions could be the target 1600 ° C liquid phase range.
0.62 x [% TO] ≤ [% Ca] ≤ 0.91 x [% TO] (2)
0.27 × [% TO] ≦ [% Ca] ≦ 0.49 × [% TO] (3)
here,
[% Ca]: Ca concentration (mass%) in molten steel after addition of Ca-containing metal
[% TO]: Total oxygen concentration (% by mass) in molten steel after vacuum degassing and before addition of Ca-containing metal
It is.
Based on the above results, the target value of the Ca addition amount may be determined. The addition of Ca after vacuum degassing may be continued following the vacuum degassing treatment in the ladle of the RH vacuum degassing device. It is preferable to add Ca to molten steel in the pan.
また、図５には初期介在物平均組成が（I）範囲の場合にMgO・Al2O3、（II）範囲の場合 CaO・6Al2O3組成として熱力学計算により、Ca添加量と平衡する介在物組成の計算例を示した。1600℃液相領域となる組成は（I）範囲と(II)範囲とで異なる結果を示しており、本結果からも図４（Ａ），（Ｂ）に示した適正範囲が存在することが裏付けられた。 In addition, Fig. 5 shows MgO · Al 2 O 3 when the initial inclusion average composition is in the range (I), and CaO · 6Al 2 O 3 composition when it is in the range (II). An example of the inclusion composition calculation is shown. The composition that becomes the 1600 ° C liquid phase region shows different results in the (I) range and (II) range. From these results, the proper range shown in FIGS. 4 (A) and (B) may exist. It was supported.
以上説明した本発明によれば、Ca添加前のMgO-CaO-Al2O3系介在物の組成を考慮してCa添加量を決定するため、その後の酸化物組成及び硫化物組成をより精度よく制御可能となる。また、タンディッシュ浸漬ノズルの介在物閉塞防止、および耐HIC,SSC特性に対して有害な酸化物や硫化物などの介在物生成を十分に抑制することが可能となる。本発明の適用により、浸漬ノズルでの介在物閉塞無しに耐HIC性、耐SSC性に優れた鋼管の製造が可能となり、製造コスト削減及び歩止り安定化を達成できる。 According to the present invention described above, since the Ca addition amount is determined in consideration of the composition of the MgO-CaO-Al 2 O 3 inclusions before the Ca addition, the subsequent oxide composition and sulfide composition are more accurate. It can be controlled well. In addition, it becomes possible to prevent inclusions such as oxides and sulfides that are harmful to the HIC and SSC resistance, and to prevent inclusions from clogging inclusions in the tundish immersion nozzle. By applying the present invention, it becomes possible to manufacture a steel pipe excellent in HIC resistance and SSC resistance without obstruction of inclusions in the immersion nozzle, and it is possible to achieve manufacturing cost reduction and yield stabilization.
タンディッシュでの溶鋼の化学組成C：0.2-0.3％、Si：0.22-0.27%、Mn：0.4-0.6％、P：0.005-0.009%、S：0.0005-0.002％、sol.Al：0.03-0.05%、Ca：0.0001-0.003%、O：0.0010-0.0020％、残部：Fe及び不可避的不純物の鋼を溶製し、鋳片サイズ210Φの丸ビレット連鋳機にて鋳造を実施した。 Chemical composition of molten steel in tundish C: 0.2-0.3%, Si: 0.22-0.27%, Mn: 0.4-0.6%, P: 0.005-0.009%, S: 0.0005-0.002%, sol.Al: 0.03-0.05 %, Ca: 0.0001-0.003%, O: 0.0010-0.0020%, balance: Fe and steel with inevitable impurities were melted and cast in a round billet continuous caster with a slab size of 210Φ.
具体的には、まず、転炉から取鍋への出鋼後に溶鋼にAlを添加して、脱酸処理を行った。次に、取鍋内の溶鋼にCaO-Al2O3-SiO2系フラックスを添加して、LFによる取鍋精錬工程（脱硫処理）を行った。次に、RH真空脱ガス装置による真空脱ガス処理を行った。次に、別の取鍋に溶鋼を移し、溶鋼にCa添加を行った。その後、溶鋼を取鍋からタンディッシュに移し、連続鋳造を行って鋳片とした。 Specifically, first, Al was added to the molten steel after the steel from the converter to the ladle was subjected to deoxidation treatment. Next, a CaO—Al 2 O 3 —SiO 2 flux was added to the molten steel in the ladle, and a ladle refining process (desulfurization treatment) using LF was performed. Next, the vacuum degassing process by RH vacuum degassing apparatus was performed. Next, the molten steel was transferred to another ladle, and Ca was added to the molten steel. Thereafter, the molten steel was transferred from the ladle to the tundish, and continuous casting was performed to obtain a slab.
ここで、RH処理後Ca添加前の溶鋼を採取し、既述の方法で介在物の平均組成を求めた。当該平均組成が（I）又は（II）を充足するか否かと合わせて、表１に示す。また、当該溶鋼中のS濃度［％S］及び全酸素濃度［％T.O］も測定し、結果を表１に示す。測定した［％S］及び［％T.O］を用いて、（1）〜（3）式に基づいて、タンディッシュ段階での溶鋼の目標Ca濃度範囲を算出し、結果を表１に示す。 Here, the molten steel after the RH treatment and before the addition of Ca was collected, and the average composition of inclusions was determined by the method described above. Table 1 shows whether the average composition satisfies (I) or (II). Further, S concentration [% S] and total oxygen concentration [% T.O] in the molten steel were also measured, and the results are shown in Table 1. Using the measured [% S] and [% T.O], the target Ca concentration range of the molten steel at the tundish stage is calculated based on the equations (1) to (3), and the results are shown in Table 1.
また、各発明例・比較例において、タンディッシュ段階での溶鋼のCa濃度を測定し、目標Ca濃度範囲を充足するか否かと合わせて、表１に示した。 Moreover, in each invention example and comparative example, the Ca concentration of the molten steel in the tundish stage was measured and shown in Table 1 together with whether or not the target Ca concentration range was satisfied.
＜特性評価＞
各発明例・比較例において、浸漬ノズル閉塞有無の評価試験、耐HIC性の評価試験、及び耐SSC性の評価試験を行い、結果を表１に示した。浸漬ノズルの閉塞有無の評価試験については、浸漬ノズルのスライディングノズル開度を目標の溶鋼量を供給するのに必要な値より大きくするか開度100％にした場合でも鋳型への溶鋼供給が間に合わず、鋳型湯面が大きく低下した場合を×とした。耐HIC試験は、シームレス鋼管サンプルより採取した試験片に対してNACE試験を行うことにより実施した。また、耐SSC試験は、1気圧の硫化水素が飽和したNACE試験液中で最小降伏応力の85％の応力を付与して単軸引張試験を720時間実施した。なお、HIC試験、SSC試験に供試したサンプルは熱処理により硬度をHRC=27にそろえた。表中の○×記号は、耐HIC試験では各水準で16本のサンプルを試験し、耐SSC試験では各水準で3本のサンプルを試験し、1つでもサンプルに破断が生じた場合を×とした。
<Characteristic evaluation>
In each of the inventive examples and comparative examples, an evaluation test on whether or not the immersion nozzle was blocked, an HIC resistance evaluation test, and an SSC resistance evaluation test were performed, and the results are shown in Table 1. As for the evaluation test of whether or not the immersion nozzle is clogged, even if the sliding nozzle opening of the immersion nozzle is larger than the value required to supply the target molten steel amount or the opening is 100%, the molten steel supply to the mold is in time. The case where the mold surface level was greatly reduced was marked with x. The HIC resistance test was carried out by performing a NACE test on test pieces collected from seamless steel pipe samples. In addition, the SSC resistance test was conducted for 720 hours by applying a stress of 85% of the minimum yield stress in a NACE test solution saturated with 1 atm of hydrogen sulfide. The samples subjected to the HIC test and SSC test were adjusted to a hardness of HRC = 27 by heat treatment. The XX symbol in the table indicates that 16 samples were tested at each level in the HIC resistance test, and 3 samples were tested at each level in the SSC resistance test. It was.
タンディッシュでの溶鋼のCa濃度が目標Ca濃度範囲になるようにCa添加を実施した本発明例では、ノズル閉塞やHIC、SSC破断が全く発生しなかった。これに対して、比較例ではノズル閉塞や耐HIC,SSC性の合格率の低下が確認された。また、鋳造後の浸漬ノズル調査よりノズル閉塞付着物の組成は高融点介在物の3CaO・Al2O3＋CaS又はCaO/6Al2O3が主体であることを確認した。 In the present invention example in which Ca was added so that the Ca concentration of the molten steel in the tundish was within the target Ca concentration range, nozzle clogging, HIC, and SSC fracture did not occur at all. On the other hand, in the comparative example, it was confirmed that the nozzle clogging and the HIC and SSC resistance pass rate declines. Moreover, it was confirmed from the investigation of the immersion nozzle after casting that the composition of the nozzle clogging deposit was mainly composed of high melting point inclusions 3CaO.Al 2 O 3 + CaS or CaO / 6Al 2 O 3 .
本発明によれば、より優れた耐HIC性及び耐SSC性を有する高清浄度鋼を製造することが可能となる。 According to the present invention, it is possible to produce a high cleanliness steel having better HIC resistance and SSC resistance.
Claims (2)
前記取鍋内の前記溶鋼にCaOを含有するフラックスを添加して、前記溶鋼に脱硫処理を施す取鍋精錬工程と、
その後、真空脱ガス装置により前記溶鋼に真空脱ガス処理を施す工程と、
その後、前記溶鋼にCa含有金属を添加する工程と、
その後、前記溶鋼を連続鋳造する工程と、
を有する高清浄度鋼の製造方法であって、
前記真空脱ガス処理後かつ前記Ca含有金属の添加前の前記溶鋼におけるCaO-MgO-Al2O3系介在物の平均組成を下記の（I）又は（II）とし、
（I）の場合には、前記Ca含有金属の添加後の前記溶鋼中のCa濃度［％Ca］が下記の（1）式及び（2）式を満たすように、（II）の場合には、前記Ca含有金属の添加後の前記溶鋼中のCa濃度［％Ca］が下記の（1）式及び（3）式を満たすように、前記Ca含有金属の添加量を設定することを特徴とする高清浄度鋼の製造方法。
記
MgO：10〜30質量％、CaO：0〜10質量％、Al2O3：残部 ・・・（I）
MgO：0〜5質量％、CaO：20〜35質量％、Al2O3：残部 ・・・（II）
［％Ca］≧1.25×［％S］ ・・・（1）
0.62×［％T.O］≦［％Ca］≦0.91×［％T.O］ ・・・（2）
0.27×［％T.O］≦［％Ca］≦0.49×［％T.O］ ・・・（3）
ここで、
［％Ca］：Ca含有金属の添加後の溶鋼中のCa濃度（質量％）
［％S］：真空脱ガス処理後かつCa含有金属の添加前の溶鋼中のS濃度（質量％）
［％T.O］：真空脱ガス処理後かつCa含有金属の添加前の溶鋼中の全酸素濃度（質量％） Adding Al to the molten steel at the time of or after the steel from the converter to the ladle, and a step of deoxidizing the molten steel;
A ladle refining step of adding a flux containing CaO to the molten steel in the ladle and subjecting the molten steel to a desulfurization treatment,
Then, a step of vacuum degassing the molten steel with a vacuum degassing device,
Then, adding a Ca-containing metal to the molten steel,
Then, the step of continuously casting the molten steel,
A method for producing a high cleanliness steel having
The average composition of the CaO—MgO—Al 2 O 3 inclusions in the molten steel after the vacuum degassing treatment and before the addition of the Ca-containing metal is (I) or (II) below,
In the case of (I), the Ca concentration [% Ca] in the molten steel after the addition of the Ca-containing metal satisfies the following formulas (1) and (2). The addition amount of the Ca-containing metal is set so that the Ca concentration [% Ca] in the molten steel after the addition of the Ca-containing metal satisfies the following formulas (1) and (3): To produce high cleanliness steel.
Record
MgO: 10 to 30 wt%, CaO: 0 wt%, Al 2 O 3: balance · · · (I)
MgO: 0 to 5% by mass, CaO: 20 to 35% by mass, Al 2 O 3 : balance (II)
[% Ca] ≧ 1.25 × [% S] (1)
0.62 x [% TO] ≤ [% Ca] ≤ 0.91 x [% TO] (2)
0.27 × [% TO] ≦ [% Ca] ≦ 0.49 × [% TO] (3)
here,
[% Ca]: Ca concentration (mass%) in molten steel after addition of Ca-containing metal
[% S]: S concentration (% by mass) in molten steel after vacuum degassing and before addition of Ca-containing metal
[% TO]: Total oxygen concentration (% by mass) in molten steel after vacuum degassing and before addition of Ca-containing metal
当該平均組成が前記（I）及び（II）のいずれにも該当しない場合には、前記取鍋内の前記溶鋼にCa含有金属又はMg含有金属を添加することによって、前記真空脱ガス処理後かつ前記Ca含有金属の添加前の前記溶鋼におけるCaO-MgO-Al2O3系介在物の平均組成を前記（I）又は（II）とする、請求項１に記載の高清浄度鋼の製造方法。 Obtain the average composition of CaO-MgO-Al 2 O 3 inclusions in the molten steel in the ladle immediately after the ladle refining step,
When the average composition does not correspond to any of (I) and (II), by adding a Ca-containing metal or an Mg-containing metal to the molten steel in the ladle, after the vacuum degassing treatment and The method for producing a high cleanliness steel according to claim 1, wherein an average composition of CaO-MgO-Al 2 O 3 inclusions in the molten steel before addition of the Ca-containing metal is the (I) or (II). .
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