JP7248153B2 - Method for manufacturing slab - Google Patents

Method for manufacturing slab Download PDF

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JP7248153B2
JP7248153B2 JP2021576544A JP2021576544A JP7248153B2 JP 7248153 B2 JP7248153 B2 JP 7248153B2 JP 2021576544 A JP2021576544 A JP 2021576544A JP 2021576544 A JP2021576544 A JP 2021576544A JP 7248153 B2 JP7248153 B2 JP 7248153B2
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molten steel
vacuum degassing
titanium
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steel
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JPWO2022065511A1 (en
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由枝 中井
裕計 近藤
晃史 原田
章敏 松井
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JFE Steel Corp
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D1/00—Treatment of fused masses in the ladle or the supply runners before casting
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/10—Handling in a vacuum
    • 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

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Treatment Of Steel In Its Molten State (AREA)

Description

本発明は、鋳片の製造方法に関する。 The present invention relates to a method for producing cast slabs.

自動車のベアリングやクランクシャフトに用いられる鋼材は、繰り返し応力にさらされるため、金属疲労破壊の起点となる非金属介在物が極めて少ない所謂「高清浄鋼」であることが要求される。このため、鋼材の疲労特性の改善を目的として、製鋼工程では鋼中の酸化物系非金属介在物を低減するために種々の手法が採られている。
例えば、量産鋼においては、真空脱ガス処理によって酸化物系非金属介在物の浮上・分離を促進させる方法や、取鍋内の溶鋼上に存在するスラグに金属Ａｌなどの脱酸剤を添加してスラグを還元し、スラグと溶鋼中のＡｌとの反応により生成するＡｌ２Ｏ３の生成を抑制する方法、連続鋳造機の鋳型或いは鋳型直下に磁場発生装置を配置して鋳型内での酸化物系非金属介在物の浮上・分離を促進させる方法が用いられている。Steel materials used for automobile bearings and crankshafts are subjected to repeated stress, so they are required to be so-called "highly clean steel" with extremely few non-metallic inclusions, which are the starting points of metal fatigue fractures. For this reason, in order to improve the fatigue properties of steel materials, various techniques have been adopted in the steelmaking process to reduce oxide-based nonmetallic inclusions in steel.
For example, in mass-produced steel, there is a method of promoting the floating and separation of oxide-based non-metallic inclusions by vacuum degassing, and a method of adding a deoxidizing agent such as metal Al to the slag existing on the molten steel in the ladle. A method of suppressing the formation of Al 2 O 3 produced by the reaction between the slag and Al in molten steel by reducing the slag with a A method of promoting floating and separation of non-metallic inclusions is used.

例えば、特許文献１には、凝集粗大化して疲労寿命に悪影響を及ぼすＡｌ２Ｏ３酸化物に着目し、Ｓｉ、Ｍｎ及びＡｌを含有し、Ｃ含有量が０．２質量％以上の溶鋼に、Ｍｇ含有量が０．５質量％超且つ３０質量％以下のＭｇ合金を添加する方法が開示されている。特許文献１によれば、Ｍｇ合金の添加によって溶鋼中のＡｌ２Ｏ３がＡｌ２Ｏ３－ＭｇＯに変化することで、介在物の粗大化を抑制することができる。For example, Patent Document 1 focuses on Al 2 O 3 oxides that aggregate and coarsen to adversely affect fatigue life, and contains Si, Mn and Al, and has a C content of 0.2% by mass or more. , a method of adding a Mg alloy with a Mg content of more than 0.5 mass % and less than or equal to 30 mass %. According to Patent Document 1, addition of Mg alloy changes Al 2 O 3 in molten steel to Al 2 O 3 —MgO, thereby suppressing coarsening of inclusions.

また、特許文献２には、高清浄度軸受鋼の製造方法として以下のプロセスが開示されている。まず、電気炉で酸素富化操業を行って、溶鋼中の溶存酸素濃度を２５０ｐｐｍ以上の過酸化状態として原料中のチタンなどを酸化させてスラグ中に移行させる。次いで、このスラグを取鍋から排出した後に、Ｓｉ脱酸及びＡｌ脱酸を順次実施して溶鋼及びスラグを脱酸する。さらに、取鍋精錬炉、ＲＨ真空脱ガス装置で順次精錬する。
また、更により高い信頼性や性能の要求される軸受鋼を製造する方法として、特許文献３には、母材としてＭｎを０．２質量％を超えて含まない軸受鋼を用い、該母材を電子ビーム溶解法によって再溶解する超清浄度軸受鋼の製造方法が開示されている。Further, Patent Document 2 discloses the following process as a method for manufacturing high-cleanliness bearing steel. First, an oxygen enrichment operation is performed in an electric furnace to make the dissolved oxygen concentration in the molten steel a peroxide state of 250 ppm or more, thereby oxidizing titanium and the like in the raw material and transferring it to the slag. Then, after the slag is discharged from the ladle, the molten steel and slag are deoxidized by successively performing Si deoxidation and Al deoxidation. Further, the steel is successively refined in a ladle refining furnace and an RH vacuum degassing unit.
Further, as a method for producing bearing steel that requires even higher reliability and performance, Patent Document 3 discloses that a bearing steel containing no more than 0.2% by mass of Mn is used as a base material, and the base material is A method for producing ultra-clean bearing steel is disclosed in which the is remelted by an electron beam melting process.

特開平５－３１１２２５号公報JP-A-5-311225 特開平６－１４５８８３号公報JP-A-6-145883 特開平７－１０９５４１号公報JP-A-7-109541

上述した従来の技術は、酸化物や硫化物の個数又は最大径を低減して転動疲労寿命の向上を図るものである。ただし、軸受材料の長寿命化と圧砕強度とのさらなる向上を目指すには、酸化物系介在物量の減少や酸化物系介在物量の最大径の低減はもちろんのこと、窒化物系介在物の低減も必要になってくる。ここで問題となる窒化物系介在物は、溶鋼中チタンと溶鋼中に溶存するＮとの反応により凝固時に生成する微細なＴｉＮであり、チタン濃度とＮ濃度との積、すなわちチタンとＮとの濃度積が晶出限を超えると晶出する。従って、ＴｉＮの生成抑制のためには、生成起因となる溶鋼中のチタン及びＮの両方の濃度を低下させるか、チタン及びＮの少なくとも一方の濃度を極めてゼロに近い値まで低下させる必要がある。 The conventional techniques described above aim to improve the rolling contact fatigue life by reducing the number or maximum diameter of oxides and sulfides. However, in order to extend the life of the bearing material and further improve its crushing strength, it is necessary to reduce the amount of oxide inclusions, reduce the maximum diameter of oxide inclusions, and reduce nitride inclusions. will also become necessary. The nitride-based inclusions in question here are fine TiN formed during solidification due to the reaction between titanium in the molten steel and N dissolved in the molten steel. crystallizes when the concentration product of exceeds the crystallization limit. Therefore, in order to suppress the formation of TiN, it is necessary to reduce the concentrations of both titanium and N in the molten steel that cause formation, or to reduce the concentration of at least one of titanium and N to a value very close to zero. .

ここで、Ｎは、真空脱ガス精錬工程で脱Ｎが行われることで除去されるが、大気中に含まれる窒素からの溶鋼への吸Ｎを完全に防止することは困難なため、溶鋼中Ｎ濃度の低減には限界がある。一方、チタンは、意図的に添加されずとも、精錬段階で不可避的に混入（ピックアップ）することがあるため、溶鋼中チタン濃度を常時低位に維持することは困難であった。その結果、鋳造時のＴｉＮの生成が完全に抑制できず、これを素材として製造される軸受鋼などの高疲労強度が要求される鋼の疲労寿命をさらに向上させることが困難であった。 Here, N is removed by denitrification in the vacuum degassing refining process. There is a limit to reducing the N concentration. On the other hand, even if titanium is not added intentionally, it may inevitably be mixed (picked up) during the refining stage, so it has been difficult to always maintain the titanium concentration in molten steel at a low level. As a result, the formation of TiN during casting cannot be completely suppressed, and it has been difficult to further improve the fatigue life of steel that requires high fatigue strength, such as bearing steel manufactured using TiN as a raw material.

そこで、本発明は、上記課題に鑑みてなされたものであって、その目的は、鋼の精錬時に溶鋼中のチタン含有量を低位に維持して、鋳造時にＴｉＮが生成することを抑制し、清浄度の高い鋳片を製造する方法を提供することにある。なお、溶鋼中のチタン等の特定元素の含有量は、単位質量あたりの溶鋼に含有される特定元素の質量であり、溶鋼質量に対する溶鋼中の特定元素の質量の比（質量％）で示される。つまり、溶鋼中の特定元素の含有量は、溶鋼中の特定元素の濃度と同義である。 Therefore, the present invention has been made in view of the above problems, and its object is to keep the titanium content in molten steel at a low level during steel refining, suppress the formation of TiN during casting, To provide a method for manufacturing a cast slab with high cleanliness. The content of a specific element such as titanium in molten steel is the mass of the specific element contained in the molten steel per unit mass, and is indicated by the ratio (mass%) of the mass of the specific element in the molten steel to the mass of the molten steel. . That is, the content of the specific element in molten steel is synonymous with the concentration of the specific element in molten steel.

本発明の一態様に係る鋳片の製造方法によれば、転炉又は電気炉から出鋼され、取鍋に受鋼した上記溶鋼に、少なくとも真空脱ガス精錬処理を施して得られる精錬後溶鋼を鋳造する、鋳片の製造方法であって、上記鋳片を製造する対象チャージで使用する取鍋を、少なくとも上記対象チャージの受鋼直前に上記取鍋で保持していた溶鋼中のチタン含有量により制限し、上記対象チャージの出鋼後の溶鋼中のチタン含有量を０．００２質量％以下とし、さらに、上記対象チャージについて上記真空脱ガス精錬処理を行う真空脱ガス設備を、少なくとも上記対象チャージの上記真空脱ガス精錬処理の直前に上記真空脱ガス設備で処理した溶鋼中の上記真空脱ガス設備での処理終了時のチタン含有量により制限する、鋳片の製造方法が提供される。 According to the method for producing a cast slab according to one aspect of the present invention, the molten steel after refining obtained by subjecting the molten steel tapped from a converter or an electric furnace and received in a ladle to at least vacuum degassing refining treatment. A method for producing a slab, wherein the ladle used in the target charge for producing the slab is held in the ladle at least immediately before receiving the target charge. Titanium content in the molten steel The amount of titanium in the molten steel after tapping of the target charge is set to 0.002% by mass or less, and the vacuum degassing equipment for performing the vacuum degassing refining treatment on the target charge includes at least the above Provided is a method for producing cast slabs, limited by the titanium content at the end of treatment in the vacuum degassing facility in molten steel processed in the vacuum degassing facility immediately prior to the vacuum degassing refining treatment of a target charge. .

本発明の一態様によれば、鋼の精錬時に溶鋼中のチタン含有量を低位に維持して鋳造時にＴｉＮが生成することを抑制し、清浄度の高い鋳片を製造する方法が提供される。 According to one aspect of the present invention, there is provided a method of manufacturing a cast slab with high cleanliness by maintaining a low titanium content in molten steel during steel refining to suppress the formation of TiN during casting. .

以下の詳細な説明では、図面を参照して、本発明の実施形態を説明する。図面の記載において、同一又は類似の部分には同一又は類似の符号を付し、重複する説明を省略する。各図面は模式的なものであり、現実のものとは異なる場合が含まれる。また、以下に示す実施形態は、本発明の技術的思想を具体化するための装置や方法を例示するものであって、本発明の技術的思想は、構成部品の材質、構造、配置等を下記のものに特定するものでない。本発明の技術的思想は、特許請求の範囲に記載された請求項が規定する技術的範囲内において種々の変更を加えることができる。 The following detailed description describes embodiments of the invention with reference to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals, and overlapping descriptions are omitted. Each drawing is schematic and may differ from the actual one. In addition, the embodiments shown below are examples of apparatuses and methods for embodying the technical idea of the present invention, and the technical idea of the present invention is based on the material, structure, arrangement, etc. of component parts. It is not specific to the following. Various modifications can be made to the technical idea of the present invention within the technical scope defined by the claims.

本発明者らは、高疲労強度鋼の精錬にあたり、その精錬段階で溶鋼へのチタン混入が生じ、鋳造時にＴｉＮが生成する要因として、下記（Ａ）～（Ｃ）の要因に注目した。そして、（Ａ）～（Ｃ）の成分をそれぞれ抽出し、チタン混入に与える影響について順次検討した。
（Ａ）転炉又は電気炉の精錬で生成したスラグ（転炉スラグ又は電気炉スラグ）に含まれるＴｉＯ２
（Ｂ）転炉又は電気炉からの出鋼後、真空脱ガス精錬処理終了までの期間に溶鋼に添加される合金中のチタン
（Ｃ）取鍋及び真空脱ガス設備の精錬槽に付着した地金に含まれるチタンThe present inventors paid attention to the following factors (A) to (C) as factors that cause titanium to be mixed into the molten steel in the refining stage and generate TiN during casting when refining high fatigue strength steel. Then, the components (A) to (C) were extracted, respectively, and their effects on titanium contamination were examined in sequence.
(A) TiO 2 contained in slag produced by refining in a converter or electric furnace (converter slag or electric furnace slag)
(B) Titanium in the alloy added to the molten steel during the period from tapping from the converter or electric furnace until the end of the vacuum degassing refining process. titanium in gold

（Ａ）転炉スラグ又は電気炉スラグ
高炉から出銑された溶銑は、脱硫処理や脱燐処理等の溶銑予備処理が施された後、転炉での酸素の上吹きや底吹きによって脱炭精錬が施される。転炉で発生したスラグには種々の酸化物が含有されており、主に溶銑に含まれるチタンに起因したチタン酸化物も含有される。また、スクラップ等の冷鉄源を電気炉で溶解して溶鋼を得る場合もある。電気炉で発生したスラグにも種々の酸化物が含有されており、スクラップ中にチタン含有鋼などが配合されると、これに起因したチタン酸化物も含有される。この転炉スラグ又は電気炉スラグの少なくとも一部が、二次精錬工程（加熱撹拌精錬及び真空脱ガス精錬）へ持ち越され、二次精錬工程でスラグ中のチタン酸化物が還元可能な条件になると、溶鋼中にチタンが混入する可能性がある。ただし、通常の精錬条件ではそのような条件になることは少なく、実際に転炉スラグ又は電気炉スラグ起因のチタン混入が認められることは稀であった。従って、本実施形態においては、転炉スラグ又は電気炉スラグからのチタン混入防止対策を施すことは必須ではなく、他の要因による溶鋼へのチタン混入量が多いと予想される場合などに、後述する転炉スラグ又は電気炉スラグの二次精錬工程への持ち越し量を低減する対策を採ることが好ましい。(A) Converter slag or electric furnace slag Hot metal tapped from a blast furnace is subjected to hot metal pretreatment such as desulfurization and dephosphorization, and then decarburized by oxygen top blowing or bottom blowing in the converter. Refining is applied. Slag generated in a converter contains various oxides, including titanium oxides mainly derived from titanium contained in hot metal. Further, molten steel may be obtained by melting a cold iron source such as scrap in an electric furnace. Slag generated in an electric furnace also contains various oxides, and when titanium-containing steel or the like is blended into scrap, titanium oxides are also contained due to this. At least part of this converter slag or electric furnace slag is carried over to the secondary refining process (heating-stirring refining and vacuum degassing refining), and in the secondary refining process, the titanium oxide in the slag becomes a condition that can be reduced. , titanium may be mixed in the molten steel. However, under ordinary refining conditions, such conditions are rare, and in fact, titanium contamination due to converter slag or electric furnace slag is rarely observed. Therefore, in the present embodiment, it is not essential to take measures to prevent titanium contamination from converter slag or electric furnace slag. It is preferable to take measures to reduce the amount of converter slag or electric furnace slag carried over to the secondary refining process.

（Ｂ）溶鋼に添加される合金
チタンを含有する合金としては、例えばＦｅＳｉ合金が挙げられる。本発明者は、高清浄度鋼を溶製する際に、真空脱ガス精錬で用いる添加合金の種類を変化させて溶鋼中のチタン濃度を調査した。調査の結果、溶鋼中のチタン濃度は添加した合金のチタン純分量（合金添加量×合金のチタン濃度）に応じて確実に増加することがわかった。すなわち、精錬段階での溶鋼中のチタン濃度を常時低位に維持するには、添加合金からのチタン混入は避けられないとの前提のもと、添加する合金のチタン純分量を見込んだ上で、他の要因によるチタン混入量を極力低減する必要がある。もちろん、添加する合金からのチタン混入量が少なくなるよう、含有するチタンの濃度の低い合金を使用することが望ましい。(B) Alloys Added to Molten Steel Examples of alloys containing titanium include FeSi alloys. The present inventor investigated the concentration of titanium in molten steel by changing the types of additive alloys used in vacuum degassing refining when melting high-cleanliness steel. As a result of the investigation, it was found that the concentration of titanium in the molten steel certainly increases according to the pure amount of titanium in the added alloy (amount of alloy added x titanium concentration of alloy). In other words, in order to keep the titanium concentration in the molten steel at the refining stage always at a low level, on the premise that titanium contamination from the additive alloy cannot be avoided, and after considering the pure titanium content of the alloy to be added, It is necessary to reduce the amount of titanium mixed due to other factors as much as possible. Of course, it is desirable to use an alloy with a low concentration of titanium so that the amount of titanium mixed in from the alloy to be added is small.

（Ｃ）取鍋及び真空脱ガス設備の精錬槽に付着した地金
鉄鋼生産工程で工業的に用いられる取鍋は、鋼製の鉄皮の内面に耐火物が設けられた溶鋼保持容器であり、生産工程において繰り返し使用される。生産工程においては通常、様々な種類の鋼が反復的に生産される為、取鍋は、その使用開始から終了まで、数十回から百数十回の溶鋼の受け入れ（受鋼）及び排出を繰り返す。この際、溶鋼が排出された取鍋には、取鍋から完全に排出されなかった前回の溶鋼やスラグの一部が、取鍋内の耐火物表面に凝固して残留する。これらの残留した溶鋼及びスラグ（以下、両者をあわせて地金と称する）は、不純物元素或いはその酸化物を含んでいる。この地金は、次チャージの溶鋼を取鍋に受鋼した際、この溶鋼の熱量で再溶解されるため、地金に含まれる不純物或いはその酸化物は、溶鋼中に溶解又は混入することとなる。従って、高疲労強度鋼向け溶鋼（高疲労強度鋼用の溶鋼）の精錬に使用する取鍋に、前チャージの地金が付着しており、その地金にチタンが含まれていると、溶鋼中の含有チタン濃度の上昇を引き起こすとともに鋳片にＴｉＮ介在物が生成する原因となると考えられる。ここでチャージ（製鋼チャージ）とは、溶鋼の精錬処理における処理単位を示すものである。また、次チャージとは、対象とするチャージ（対象チャージ）の一つ後のチャージであり、対象チャージの処理直後のチャージである。さらに、前チャージとは、対象チャージの一つ前のチャージであり、対象チャージの処理直前のチャージである。(C) Ingots attached to ladles and refining tanks of vacuum degassing equipment Ladles used industrially in the steel production process are molten steel holding containers with refractories on the inner surface of the steel shell. , are used repeatedly in the production process. In the production process, various types of steel are usually produced repeatedly, so the ladle receives and discharges molten steel from tens to hundreds of times from the start of use to the end of use. repeat. At this time, part of the previous molten steel and slag that was not completely discharged from the ladle solidifies and remains on the surface of the refractory in the ladle. These remaining molten steel and slag (both collectively referred to as base metal hereinafter) contain impurity elements or their oxides. This metal is remelted by the heat of the molten steel when it is received in the ladle of molten steel for the next charge, so impurities or their oxides contained in the metal are dissolved or mixed in the molten steel. Become. Therefore, if the ladle used for refining molten steel for high fatigue strength steel (molten steel for high fatigue strength steel) has pre-charged metal attached to it, and the metal contains titanium, the molten steel It is thought that this causes an increase in the concentration of titanium contained in the steel and causes formation of TiN inclusions in the slab. Here, the charge (steelmaking charge) indicates a processing unit in the molten steel refining process. Further, the next charge is the charge one after the target charge (target charge), and is the charge immediately after the target charge is processed. Furthermore, the pre-charge is the charge immediately before the target charge, and is the charge immediately before the target charge is processed.

本発明者は、高疲労強度鋼向け溶鋼を精錬するに際し、溶鋼中のチタン濃度の変化の挙動を詳細に調査した。その結果、溶鋼中のチタン濃度は、真空脱ガス精錬処理前から真空脱ガス精錬処理後までの期間に増加することが多いことがわかった。また、真空脱ガス精錬処理前後でのチタン濃度の増加量は、高疲労強度鋼向け溶鋼の精錬に用いた取鍋が、高疲労強度鋼向け溶鋼の精錬（以下、対象精錬ともいう。）の前チャージでチタン含有量が高い溶鋼を受鋼していた場合、及び対象精錬で使用した真空脱ガス設備が、対象精錬の前チャージでチタン含有量が高い溶鋼を処理していた場合に大きくなることがわかった。このことは、取鍋に付着している地金及び真空脱ガス設備の槽内に付着している地金が、高疲労強度鋼向け溶鋼の精錬中に溶解し、地金中のチタンが溶鋼に混入したことを示すものと考えられる。そこで高疲労強度鋼向け溶鋼の精錬を行なう際に、その前の処理で精錬する溶鋼にチタン含有量が低いものを指定するように製造順を調整したところ、対象精錬の溶鋼中のチタン濃度の増加量が少なくなり、鋳片に生成するＴｉＮ介在物も顕著に減少していた。 The present inventor investigated in detail the behavior of changes in titanium concentration in molten steel when refining molten steel for high fatigue strength steel. As a result, it was found that the concentration of titanium in molten steel often increased during the period from before vacuum degassing refining to after vacuum degassing refining. In addition, the amount of increase in titanium concentration before and after the vacuum degassing refining process is that the ladle used for refining the molten steel for high fatigue strength steel is the same as the refining of molten steel for high fatigue strength steel (hereinafter also referred to as target refining). Increases when molten steel with a high titanium content was received in the previous charge, and when the vacuum degassing equipment used in the target refining was processing molten steel with a high titanium content in the previous charge of the target refining. I understand. This means that the metal adhering to the ladle and the metal adhering to the tank of the vacuum degassing equipment melt during the refining of molten steel for high fatigue strength steel, and the titanium in the metal is dissolved in the molten steel. It is thought that this indicates that the Therefore, when refining molten steel for high fatigue strength steel, we adjusted the production order so that the molten steel to be refined in the previous process had a low titanium content. The amount of increase decreased, and the amount of TiN inclusions generated in the slab was also significantly reduced.

すなわち、鋳片に生成するＴｉＮ介在物を低減するためには、精錬段階での溶鋼中のチタン含有量を常時低位に維持することが重要である。しかし、単に溶鋼中のチタン含有量を低位に維持するだけでなく、取鍋に付着した地金及び真空脱ガス設備の槽内に付着した地金からのチタン混入を低減すると、鋳片に生成するＴｉＮ介在物の低減効果が大きくなる。この理由は明らかではないが、本発明者は以下の可能性があると考えている。すなわち、チタン源となる上記の地金は、多くの場合、その表面は一旦大気に曝される。このため、地金表面には鉄及びチタンを含む合金成分の酸化物が生成しており、地金が溶解すると、これらの酸化物が溶鋼中に懸濁して溶鋼の清浄性が低下する。溶鋼中の懸濁した酸化物のうち、チタンを含む酸化物が還元されると、溶鋼中のチタン含有量が増加する可能性があるのはもちろん、チタンを含まない化合物であっても鋳造中にＴｉＮ介在物生成の核となり、ＴｉＮ介在物の核生成頻度が増加したりすることが考えられる。そして、このような現象の抑制のため、高疲労強度鋼向け溶鋼の精錬に際し、対象精錬の前チャージに受鋼した溶鋼中のチタン含有量が低い取鍋を使用すること、及び対象精錬の前チャージに処理した溶鋼中のチタン含有量が低い真空脱ガス設備を使用すること、が必要であることを知見した。 That is, in order to reduce the amount of TiN inclusions formed in the slab, it is important to always keep the titanium content in the molten steel at the refining stage at a low level. However, if the titanium content in the molten steel is not simply maintained at a low level, but if the titanium contamination from the metal adhering to the ladle and the metal adhering to the tank of the vacuum degassing equipment is reduced, The effect of reducing TiN inclusions is increased. Although the reason for this is not clear, the present inventor believes that there is the following possibility. That is, in many cases, the surface of the base metal, which is the source of titanium, is once exposed to the atmosphere. Therefore, oxides of alloy components including iron and titanium are formed on the surface of the base metal, and when the base metal is melted, these oxides are suspended in the molten steel, reducing the cleanliness of the molten steel. Among the suspended oxides in the molten steel, if the titanium-containing oxides are reduced, the titanium content in the molten steel can increase, and even compounds that do not contain titanium can be reduced during casting. It is conceivable that the nucleation frequency of the TiN inclusions increases. In order to suppress such a phenomenon, when refining molten steel for high fatigue strength steel, use a ladle with a low titanium content in the molten steel received before the target refining, and before the target refining It has been found that it is necessary to use vacuum degassing equipment where the titanium content in the molten steel processed into the charge is low.

以上のことから、高疲労強度鋼向けの溶鋼においてチタンの混入を低減し、鋳造時のＴｉＮの生成を抑制するには、上記（Ａ）の検証による、転炉スラグ又は電気炉スラグの二次精錬工程への持ち越し量を低減、上記（Ｂ）の検証による、含有チタン濃度の低い合金の使用、上記（Ｃ）の検証による、取鍋及び真空脱ガス設備の精錬槽に付着した地金からのチタン混入の低減が有効であることが知見された。 From the above, in order to reduce the contamination of titanium in molten steel for high fatigue strength steel and suppress the formation of TiN during casting, secondary Reducing carry-over to the refining process, using alloys with low titanium content based on the verification of (B) above, and removing the metal adhering to the ladle and the refining tank of the vacuum degassing equipment based on the verification of (C) above. It was found that the reduction of titanium contamination in the steel is effective.

＜鋳片の製造方法＞
以下に本発明の具体的な実施の形態について、転炉溶鋼から鋳片を製造する場合を例として説明する。
まず、高炉から出銑された溶銑を溶銑鍋やトーピードカー等の溶銑保持・搬送用容器で受銑し、次工程の転炉精錬を行う転炉に搬送する（搬送工程）。搬送工程では、必要に応じて、この搬送途中で溶銑に対して脱硫処理や脱燐処理等の溶銑予備処理を施してもよい。<Method for manufacturing slab>
A specific embodiment of the present invention will be described below by taking as an example a case of producing a cast slab from molten steel in a converter.
First, molten iron tapped from a blast furnace is received in a molten iron ladle, a torpedo car or other vessel for holding and transporting molten iron, and then transported to a converter for refining in the next process (transporting process). In the transporting step, hot metal pretreatment such as desulfurization or dephosphorization may be performed on the hot metal during the transportation, if necessary.

搬送工程の後、転炉にて溶銑に転炉精錬を施す（一次精錬工程）。一次精錬工程では、必要に応じて少量の生石灰等を媒溶剤として用い、酸素を上吹き又は底吹きして溶銑の脱炭精錬を行う。転炉内で発生したスラグには種々の酸化物が含有されており、チタン酸化物も含有されている。転炉スラグの次工程への持ち越しと持ち越されたスラグからのチタンの混入とを低減するためには、転炉出鋼時のスラグ流出防止手段及びスラグカット手段の少なくとも一方を用いることが好ましい。転炉出鋼時のスラグ流出防止手段としては、赤外線等を用いたスラグ流出検知やダーツ型ストッパーの炉内投入等の方法を用いてもよい。また、スラグカット手段としては、転炉の出鋼口に栓やスライディングノズル式のスラグストッパー等を設け、出鋼を終了させる際にこれらを使用してもよい。さらに、それでも流出した転炉スラグは、転炉から取鍋への出鋼後、掻き板等を用いて取鍋からスラグを掻き出す処理である除滓をすることによって、さらに除去することができる。次工程に持ち越される転炉スラグの量は、少ない程望ましいが、３２ｋｇ－ｓｌａｇ／ｔ－ｓｔｅｅｌ以下であることが望ましく、２５ｋｇ－ｓｌａｇ／ｔ－ｓｔｅｅｌ以下であると更によい。これは、流出スラグ中に含有されるチタン酸化物からの溶鋼へのチタン混入の影響量の影響を最小化できるためである。なお、本実施形態の説明において、「ｔ－ｓｔｅｅｌ」は対象チャージの溶鋼重量であり、「ｋｇ－ｓｌａｇ」は対象チャージのスラグ重量であり、ｔ（ｔｏｎ）はＭｅｔｒｉｃＴｏｎ（ＭＴ）である。 After the transfer process, the molten iron is subjected to converter refining in a converter (primary refining process). In the primary refining process, if necessary, a small amount of quicklime or the like is used as a solvent, and oxygen is top-blown or bottom-blown to decarburize the hot metal. Slag generated in a converter contains various oxides, including titanium oxide. In order to reduce the carry-over of converter slag to the next process and the contamination of titanium from the carried-over slag, it is preferable to use at least one of slag outflow prevention means and slag cutting means at the time of steel tapping from the converter. As a means for preventing slag outflow at the time of steel tapping from the converter, methods such as detection of slag outflow using infrared rays or the like and introduction of a dart-type stopper into the furnace may be used. As the slag cutting means, a plug, a sliding nozzle type slag stopper, or the like may be provided at the tapping port of the converter, and these may be used when tapping is finished. Furthermore, the converter slag that still flows out can be further removed by slag removal, which is a process of scraping the slag from the ladle using a scraper or the like after tapping from the converter to the ladle. The amount of converter slag carried over to the next step is preferably as small as possible, preferably 32 kg-slag/t-steel or less, more preferably 25 kg-slag/t-steel or less. This is because it is possible to minimize the influence of titanium mixing into molten steel from titanium oxides contained in the outflow slag. In the description of the present embodiment, "t-steel" is the molten steel weight of the target charge, "kg-slag" is the slag weight of the target charge, and t (ton) is MetricTon (MT).

上述の通り、転炉精錬を施された溶鋼は、溶鋼保持容器である取鍋に出鋼される。本実施形態では、使用する取鍋に制限を設け、条件を満足する取鍋を使用する。取鍋に対する制限の条件は、対象精錬を行うチャージ（以下、対象チャージともいう。）で使用する前記取鍋を、少なくとも対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量により制限し、さらに対象チャージにおいては出鋼後の溶鋼中のチタン含有量を０．００２質量％以下とするものである。具体的には、対象チャージで使用する前記取鍋について、少なくとも対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量の上限を設定し、少なくとも対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量が上限以下となる取鍋を高疲労強度鋼向けの溶鋼の製造に用い、そして対象チャージの出鋼後の溶鋼中のチタン含有量を０．００２質量％以下とする。 As described above, molten steel subjected to converter refining is tapped into a ladle, which is a molten steel holding container. In this embodiment, the ladle to be used is limited, and a ladle that satisfies the conditions is used. The condition for limiting the ladle is that the ladle used in the charge that performs the target refining (hereinafter also referred to as the target charge) is limited by the titanium content in the molten steel that was held immediately before receiving the target charge. Furthermore, in the target charge, the content of titanium in the molten steel after tapping is set to 0.002% by mass or less. Specifically, for the ladle used in the target charge, the upper limit of the titanium content in the molten steel held at least immediately before receiving the steel of the target charge is set, and the content is held at least immediately before the steel receiving of the target charge. The ladle in which the titanium content in the molten steel is below the upper limit is used for the production of molten steel for high fatigue strength steel, and the titanium content in the molten steel after tapping of the target charge is 0.002% by mass or less. .

さらに、対象チャージの受鋼直前に保持していた溶鋼中のアルミニウム含有量を制限すると良い。具体的には、対象チャージの受鋼直前に保持していた溶鋼中のアルミニウム含有量の上限を０．０５質量％以下に設定すると良い。これは、溶鋼中のアルミニウム含有量が０．０５質量％より多くなると、溶鋼上に存在しているスラグ中に含有されるチタンの酸化物（ＴｉＯ２）が溶鋼中のアルミニウムによって還元され、溶鋼のチタン濃度を増加させてしまうからである。In addition, it is advisable to limit the aluminum content in the molten steel held just prior to receiving the target charge. Specifically, it is preferable to set the upper limit of the aluminum content in the molten steel held immediately before receiving the target charge to 0.05% by mass or less. This is because when the aluminum content in the molten steel exceeds 0.05% by mass, titanium oxide (TiO 2 ) contained in the slag present on the molten steel is reduced by the aluminum in the molten steel, This is because it increases the titanium concentration of the

対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量の上限としては、製造する高疲労強度鋼向け溶鋼の成分規格上限の６倍程度かそれ以下とすることが好ましく、取鍋の通常の地金付着量を考慮すると、対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量を０．０２０質量％以下とすることがより好ましい。対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量の上限をこの範囲とすることで、より確実に溶鋼へのチタン混入を防止できる。なお、対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量の上限値を設けたが、溶鋼中のチタン濃度は低いほどチタンの混入を防ぐことができるため対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量に下限を設ける必要はない。 The upper limit of the titanium content in the molten steel held immediately before receiving the target charge is preferably about 6 times or less than the upper limit of the chemical composition standard for molten steel for high fatigue strength steel to be manufactured. Considering the normal base metal adhesion amount, it is more preferable to set the titanium content in the molten steel held immediately before receiving the target charge to 0.020% by mass or less. By setting the upper limit of the titanium content in the molten steel held immediately before receiving the target charge within this range, it is possible to more reliably prevent titanium from being mixed into the molten steel. In addition, although the upper limit of the titanium content in the molten steel held immediately before receiving the target charge was set, the lower the titanium concentration in the molten steel, the more titanium can be prevented from being mixed in. There is no need to set a lower limit on the titanium content in the molten steel held at .

さらに、対象チャージの受鋼直前チャージである前チャージで保持していた溶鋼中のチタン含有量のみならず、前チャージのさらにもう１つ前のチャージ（対象チャージの２つ前のチャージ）である、対象チャージの前々チャージで保持していた溶鋼中のチタン含有量が高すぎると影響を受ける場合がある。従って、使用する取鍋は、対象チャージの前チャージで受鋼した溶鋼中のチタン含有量で制限することに加え、対象チャージの前々チャージで受鋼した溶鋼中のチタン含有量でも制限を行なうとなおよい。この場合においても、前々チャージで受鋼した溶鋼中のチタン含有量を０．０２０質量％以下とすることが好ましいが、前チャージで受鋼した溶鋼に比べてチタン混入に与える影響は小さいため、上限を０．０２０質量％より高い含有量としてもよい。 Furthermore, it is not only the titanium content in the molten steel held in the previous charge, which is the charge immediately before receiving steel of the target charge, but also the charge that is one more before the previous charge (the charge that is two charges before the target charge). , it may be affected if the titanium content in the molten steel held in the charge before the target charge is too high. Therefore, in addition to limiting the ladle to be used by the titanium content in the molten steel received in the pre-charge of the target charge, the titanium content in the molten steel received in the pre-charge of the target charge is also restricted. And even better. Even in this case, it is preferable that the titanium content in the molten steel received in the pre-charge is 0.020% by mass or less, but the effect on titanium contamination is smaller than that in the molten steel received in the pre-charge. , the upper limit may be higher than 0.020% by mass.

ところで、取鍋に付着している地金量（取鍋の付着地金量）と、取鍋に付着している地金（取鍋の付着地金）のチタン含有量（質量％）とがわかれば、取鍋に付着した地金からのチタン混入量をより正確に見積もることができる。ただし、通常は取鍋に付着している地金量（取鍋の付着地金量）と、取鍋に付着している地金（取鍋の付着地金）のチタン含有量（質量％）を正確に把握することは難しいので、本発明では、対象チャージで使用する前記取鍋を、少なくとも対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量により制限するものとする。 By the way, the amount of metal adhering to the ladle (the amount of metal adhering to the ladle) and the titanium content (% by mass) of the metal adhering to the ladle (the amount of metal adhering to the ladle) If known, the amount of titanium mixed from the bare metal adhering to the ladle can be estimated more accurately. However, usually, the amount of metal adhering to the ladle (adhesive metal amount of the ladle) and the titanium content (mass%) of the metal adhering to the ladle (adhesive metal of the ladle) Therefore, in the present invention, the ladle used in the target charge is limited at least by the titanium content in the molten steel held immediately before receiving the target charge.

なお、取鍋の対象チャージ、つまり高疲労強度鋼向けの溶鋼の製造チャージの以前の処理において付着していた地金量を予め測定するなどして把握された、およその地金の量が求められるならば、その値を取鍋の付着地金量として用いても良い。または、前回の処理前後の取鍋の重量をロードセルなどで実測し、付着している地金量を測定できるならば、その値を用いてもよい。ここで、前回の処理前の取鍋の重量とは、前回の処理（対象チャージに用いられる取鍋における前回の処理）において転炉から出鋼された溶鋼を受鋼する前の取鍋の重量である。また、前回の処理後の取鍋の重量とは、前回の処理において鋳造後に溶鋼を排出した後の取鍋の重量である。さらに、溶鋼を排出した後に、取鍋内の付着地金を除去するなど整備を行った場合は、その整備後の重量を、前回の処理後の取鍋の重量としてもよい。つまり、前回の処理後の取鍋の重量とは、今回の処理の溶鋼を転炉から受鋼する前の取鍋の重量である。 In addition, the approximate amount of metal that was grasped by measuring in advance the amount of metal that had adhered in the previous processing of the target charge of the ladle, that is, the production charge of molten steel for high fatigue strength steel, was obtained. If available, that value may be used as the amount of bare metal deposited on the ladle. Alternatively, if the weight of the ladle before and after the previous treatment can be measured using a load cell or the like, and the amount of metal adhered can be measured, that value may be used. Here, the weight of the ladle before the previous treatment is the weight of the ladle before receiving the molten steel tapped from the converter in the previous treatment (the previous treatment in the ladle used for the target charge). is. Also, the weight of the ladle after the previous treatment is the weight of the ladle after the molten steel has been discharged after casting in the previous treatment. Furthermore, when maintenance such as removal of metal deposits in the ladle is performed after the molten steel is discharged, the weight after the maintenance may be used as the weight of the ladle after the previous treatment. That is, the weight of the ladle after the previous treatment is the weight of the ladle before receiving the molten steel for the current treatment from the converter.

また、取鍋に付着している地金量（取鍋の付着地金量）と、取鍋に付着している地金（取鍋の付着地金）のチタン含有量（質量％）とから、取鍋の付着地金からのチタン混入量を見積もる際に用いる取鍋の付着地金のチタン含有量は、少なくとも対象チャージの受鋼直前（前チャージ）に保持していた溶鋼中のチタン含有量（質量％）としてもよい。使用される取鍋の条件としては、取鍋の付着地金のチタン含有量を、製造する高疲労強度鋼向け溶鋼の成分規格上限の６倍程度かそれ以下とすることが好ましく、取鍋の通常の地金付着量を考慮すると、取鍋の付着地金のチタン含有量を０．０２０質量％以下とすることがより好ましい。 In addition, from the amount of metal adhering to the ladle (amount of metal adhering to the ladle) and the titanium content (mass%) of the metal adhering to the ladle (adhesion metal to the ladle) , The content of titanium in the adhering metal of the ladle used when estimating the amount of titanium mixed in from the adhering metal of the ladle is at least the titanium content in the molten steel held immediately before the target charge (pre-charge). It is good also as an amount (mass %). As for the conditions of the ladle to be used, it is preferable that the titanium content of the metal deposited on the ladle is about 6 times or less than the upper limit of the chemical composition standard of molten steel for high fatigue strength steel to be manufactured. Considering the normal amount of base metal deposited, it is more preferable to set the titanium content of the base metal deposited on the ladle to 0.020% by mass or less.

次いで、この溶鋼を取鍋精錬炉に搬送して加熱撹拌精錬処理を実施し、その後、溶鋼を真空脱ガス設備に搬送し真空脱ガス精錬処理を実施する（二次精錬工程）。加熱撹拌精錬処理では、不活性ガス雰囲気下で、加熱や介在物除去、脱酸、成分調整などの精錬が行われる。真空脱ガス設備としては、ＲＨ真空脱ガス装置やＤＨ真空脱ガス装置、ＶＯＤ炉等を用いることができる。なお、以下の説明では、代表的な真空脱ガス設備としてＲＨ真空脱ガス装置を用いて真空脱ガス精錬処理をする例について説明する。また、二次精錬工程を経た溶鋼、つまり真空脱ガス精錬処理が施された溶鋼を精錬後溶鋼ともいう。 Next, the molten steel is conveyed to a ladle refining furnace and subjected to heating, stirring, and refining treatment, and thereafter, the molten steel is conveyed to vacuum degassing equipment and subjected to vacuum degassing refining treatment (secondary refining step). In the heat stirring refining process, refining such as heating, removal of inclusions, deoxidation, and component adjustment is performed in an inert gas atmosphere. As the vacuum degassing equipment, an RH vacuum degassing device, a DH vacuum degassing device, a VOD furnace, or the like can be used. In the following description, an example of performing a vacuum degassing refining process using an RH vacuum degassing apparatus as a typical vacuum degassing facility will be described. Molten steel that has undergone a secondary refining process, that is, molten steel that has undergone a vacuum degassing refining process is also referred to as post-refining molten steel.

本実施形態では、使用するＲＨ真空脱ガス装置に制限を設け、条件を満足した状態のＲＨ真空脱ガス装置を使用する。ＲＨ真空脱ガス装置に対する制限の条件は、少なくとも対象チャージの処理直前に処理した溶鋼中のチタン含有量（質量％）により制限するものである。
具体的には、対象チャージの処理直前にチタン含有量が低い溶鋼を処理していたＲＨ真空脱ガス装置を高疲労強度鋼向けの溶鋼の製造に用いる。対象チャージの処理直前に処理していた溶鋼のチタン含有量の上限としては、製造する高疲労強度鋼向け溶鋼の成分規格上限の６倍程度かそれ以下とするが好ましく、ＲＨ真空脱ガス装置の通常の地金付着量を考慮すると、対象チャージの真空脱ガス精錬処理の直前に当該真空脱ガス設備で処理した溶鋼中のチタン含有量を０．０２０質量％以下とすることがより好ましい。この範囲とすることで、より確実に溶鋼へのチタン混入を防止できる。なお、対象チャージの処理直前に処理した溶鋼中のチタン含有量の値には、例えば、対象チャージの処理直前にＲＨ真空脱ガス装置で処理した前チャージの溶鋼中のＲＨ真空脱ガス装置での処理終了時のチタン含有量を用いることが出来る。In this embodiment, the RH vacuum degassing device to be used is limited, and the RH vacuum degassing device that satisfies the conditions is used. The limiting condition for the RH vacuum degasser is at least by the titanium content (mass %) in the molten steel processed just before the target charge is processed.
Specifically, the RH vacuum degasser, which was processing molten steel with a low titanium content just prior to processing the target charge, is used to produce molten steel for high fatigue strength steel. The upper limit of the titanium content of the molten steel processed immediately before the target charge is processed is preferably about 6 times or less than the upper limit of the chemical composition standard of the molten steel for high fatigue strength steel to be manufactured, and the RH vacuum degassing device Considering the normal amount of ingot deposited, it is more preferable that the content of titanium in the molten steel processed by the vacuum degassing equipment immediately before the vacuum degassing refining treatment of the target charge is 0.020% by mass or less. By setting the content within this range, it is possible to more reliably prevent titanium from being mixed into the molten steel. In addition, the value of the titanium content in the molten steel processed immediately before the processing of the target charge includes, for example, The titanium content at the end of processing can be used.

さらに、対象チャージのＲＨ真空脱ガス装置での処理直前のチャージである前チャージの溶鋼中のチタン含有量のみならず、前チャージのさらにもう１つ前のチャージ（対象チャージの２つ前のチャージ）である、対象チャージの前々チャージの溶鋼のチタン含有量が高すぎると影響を受ける場合がある。従って、前々チャージで処理した溶鋼中のチタン含有量でも制限を行なうとなおよい。この場合においても、前々チャージで処理した溶鋼中のチタン含有量の条件を前チャージと同様とすることが好ましい。しかし、前々チャージで処理した溶鋼は、前チャージで処理した溶鋼に比べてチタン混入に与える影響は小さいため、前チャージで処理した溶鋼高い含有量としてもよい。 Furthermore, not only the titanium content in the molten steel of the previous charge, which is the charge immediately before the treatment in the RH vacuum degasser of the target charge, but also the charge before the previous charge (the charge two before the target charge) ), it may be affected by too high a titanium content in the molten steel of the charge before the target charge. Therefore, it is more preferable to limit the titanium content in the molten steel processed by the pre-charge. Also in this case, it is preferable that the condition of the titanium content in the molten steel processed by the pre-charge is the same as that of the pre-charge. However, since the molten steel processed by pre-charging has a smaller influence on the mixing of titanium than the molten steel processed by pre-charging, the molten steel processed by pre-charging may have a higher content.

さらに、対象チャージの処理直前に処理していた溶鋼のアルミニウム含有量を制限すると良い。具体的には、対象チャージの処理直前に処理していた溶鋼中のアルミニウム含有量の上限を０．０５質量％以下に設定すると良い。これは、溶鋼中のアルミニウム濃度が０．０５質量％より多くなると、溶鋼上に存在しているスラグ中に含有されるチタンの酸化物（ＴｉＯ２）が溶鋼中のアルミニウムによって還元され、溶鋼のチタン含有量を増加させてしまうからである。なお、対象チャージの処理直前に処理した溶鋼中のアルミニウム含有量の値には、例えば、対象チャージの処理直前にＲＨ真空脱ガス装置で処理した前チャージの溶鋼中のＲＨ真空脱ガス装置での処理終了時のアルミニウム含有量を用いることが出来る。In addition, it is advisable to limit the aluminum content of the molten steel that was being processed immediately prior to processing the target charge. Specifically, it is preferable to set the upper limit of the aluminum content in the molten steel processed immediately before the target charge is processed to 0.05% by mass or less. This is because when the aluminum concentration in the molten steel exceeds 0.05% by mass, titanium oxide (TiO 2 ) contained in the slag present on the molten steel is reduced by the aluminum in the molten steel, resulting in This is because the titanium content is increased. In addition, the value of the aluminum content in the molten steel processed immediately before the processing of the target charge includes, for example, The aluminum content at the end of processing can be used.

ところで、ＲＨ真空脱ガス装置の精錬槽に付着している地金量（装置の付着地金量）と、ＲＨ真空脱ガス装置の精錬槽に付着している地金（装置の付着地金）のチタン含有量（質量％）とがわかれば、装置に付着した地金からのチタン混入量をより正確に見積もることができる。ただし、通常は装置に付着している地金量と、装置の付着地金のチタン含有量（質量％）を正確に把握することは難しいので、本発明では、対象チャージで使用する前記ＲＨ真空脱ガス装置を、少なくとも対象チャージの処理直前に処理した溶鋼中のチタン含有量により制限するものとする。 By the way, the amount of metal adhering to the refining tank of the RH vacuum degassing device (the amount of metal adhering to the device) and the metal adhering to the refining tank of the RH vacuum degassing device (adhering metal to the device) If the titanium content (% by mass) is known, the amount of titanium mixed from the base metal adhering to the apparatus can be estimated more accurately. However, since it is usually difficult to accurately grasp the amount of metal adhering to the device and the titanium content (% by mass) of the metal adhering to the device, in the present invention, the RH vacuum used in the target charge The degasser shall be limited by the titanium content in the molten steel processed at least immediately prior to processing the charge of interest.

なお、真空脱ガス精錬処理においては、以前の処理において付着していた地金量を予め測定するなどして把握された、およその地金の量が求められるならば、装置の付着地金量として用いても良い。または、前回の処理前後のＲＨ真空脱ガス装置の精錬槽（真空槽及び浸漬管）の重量をロードセルなどで実測し、付着している地金量を測定できるならば、その値を用いてもよい。 In the vacuum degassing refining process, if the approximate amount of metal that has been grasped by measuring the amount of metal that had adhered in the previous process can be obtained, the amount of metal that has adhered to the equipment You can use it as Alternatively, if the weight of the smelting tank (vacuum tank and immersion tube) of the RH vacuum degassing apparatus before and after the previous treatment can be measured using a load cell or the like, and the amount of metal adhered can be measured, that value can be used. good.

また、装置の付着地金量と、装置の付着地金のチタン含有量（質量％）とから、装置に付着した地金からのチタン混入量を見積もる際に用いる装置の付着地金のチタン濃度は、前回の真空脱ガス精錬処理における、真空脱ガス精錬処理終了時の溶鋼中のチタン濃度とすることができる。使用されるＲＨ真空脱ガス装置の条件としては、装置の付着地金のチタン含有量を、製造する高疲労強度鋼向け溶鋼の成分規格上限の６倍程度かそれ以下とするが好ましく、ＲＨ真空脱ガス装置の通常の地金付着量を考慮すると、装置の付着地金のチタン含有量を０．０２０質量％以下とすることがより好ましい。 In addition, the titanium concentration of the deposited metal of the device used when estimating the amount of titanium mixed from the deposited metal of the device from the amount of deposited metal of the device and the titanium content (% by mass) of the deposited metal of the device can be the titanium concentration in the molten steel at the end of the vacuum degassing refining process in the previous vacuum degassing refining process. As for the conditions of the RH vacuum degassing device to be used, it is preferable that the titanium content of the base metal deposited on the device is about 6 times or less than the upper limit of the chemical composition standard of molten steel for high fatigue strength steel to be manufactured. Considering the normal amount of base metal deposited in the degassing device, it is more preferable to set the titanium content of the deposited metal in the device to 0.020% by mass or less.

二次精錬工程の後、精錬処理された溶鋼（精錬後溶鋼）を連続鋳造することで、ブルームやビレット、スラブ等の中間製品である鋳片が製造される（鋳造工程）。なお、鋳造工程における鋳造方法は、連続鋳造に限らず、造塊法による鋳造が用いられてもよい。
以上の工程を経ることで、高疲労強度鋼の素材となる鋳片が製造される。After the secondary refining process, the refined molten steel (refined molten steel) is continuously cast to produce slabs, which are intermediate products such as blooms, billets, and slabs (casting process). The casting method in the casting process is not limited to continuous casting, and ingot casting may be used.
Through the above steps, a cast slab that is a material for high fatigue strength steel is manufactured.

＜変形例＞
以上で、特定の実施形態を参照して本発明を説明したが、これら説明によって発明を限定することを意図するものではない。本発明の説明を参照することにより、当業者には、開示された実施形態とともに種々の変形例を含む本発明の別の実施形態も明らかである。従って、特許請求の範囲に記載された発明の実施形態には、本明細書に記載したこれらの変形例を単独または組み合わせて含む実施形態も網羅すると解すべきである。<Modification>
Although the invention has been described with reference to particular embodiments, it is not intended that the invention be limited by these descriptions. Along with the disclosed embodiments, other embodiments of the invention, including various modifications, will be apparent to persons skilled in the relevant art(s) upon reference to the description of the invention. Therefore, the embodiments of the invention set forth in the claims should be construed to cover the embodiments that include these variations described herein singly or in combination.

例えば、上記実施形態では、チタン混入を低減するため、取鍋の制限、真空脱ガス設備の精錬槽の制限を行ったが、本発明はかかる例に限定されない。取鍋の制限、真空脱ガス設備の精錬槽の制限に加え、少なくとも添加される合金鉄の制限、つまり、出鋼から二次精錬工程の終了において添加する合金鉄が上記（Ｂ）を満足するようにすることが好ましい。 For example, in the above embodiment, the ladle and the smelting tank of the vacuum degassing equipment are restricted in order to reduce titanium contamination, but the present invention is not limited to such examples. In addition to the limitations of the ladle and the limitations of the refining tank of the vacuum degassing equipment, at least the limitations of the ferroalloy to be added, that is, the ferroalloy added at the end of the secondary refining process from tapping satisfies the above (B). It is preferable to

また、上記実施形態において、添加する合金に含有されるチタンを考慮して、対象チャージについて、出鋼後から真空脱ガス精錬処理終了までの期間において、溶鋼中のチタン濃度の増加量が下記の（１）式を満たすようにするとより好適である。この場合、対象チャージの出鋼直前の溶鋼中のＴｉ含有量は分析限界以下となるゼロであり、溶鋼が転炉から出鋼される場合には、送酸処理が完了した転炉吹止めの溶鋼中のＴｉ含有量がゼロ（分析限界以下）である。
Δ［Ｔｉ］＝［Ｔｉ］ＬＤ＋Σ（Ｗｔ×ηｔ／１００）＋Δ［Ｔｉ］Ｍ≦Ｔ１×１０ ・・・（１）In addition, in the above embodiment, considering the titanium contained in the alloy to be added, for the target charge, the amount of increase in titanium concentration in the molten steel during the period from after tapping to the end of the vacuum degassing refining process is as follows. It is more preferable to satisfy the formula (1). In this case, the Ti content in the molten steel immediately before tapping of the target charge is zero, which is below the analysis limit. The Ti content in the molten steel is zero (below the analytical limit).
Δ[Ti]=[Ti] LD +Σ( Wt × ηt /100)+Δ[Ti] M ≦ T1 ×10 (1)

なお、（１）式において、Δ［Ｔｉ］は出鋼後から真空脱ガス精錬処理終了までの期間における溶鋼中チタン含有量の増加量（ｋｇ－Ｔｉ／ｔ－ｓｔｅｅｌ）、［Ｔｉ］ＬＤは出鋼後の溶鋼中のチタン含有量（ｋｇ－Ｔｉ／ｔ－ｓｔｅｅｌ）、Ｗｔは対象チャージにおいて、出鋼から真空脱ガス精錬処理終了までに添加する合金の添加量（ｋｇ－ａｌｌｏｙ／ｔ－ｓｔｅｅｌ）、ηｔは対象チャージにおいて、出鋼から真空脱ガス精錬処理終了までに添加する合金のチタン含有量（質量％）、Δ［Ｔｉ］Ｍは合金以外からピックアップした溶鋼中チタン含有量の増加量（ｋｇ－Ｔｉ／ｔ－ｓｔｅｅｌ）、Ｔ１は溶製対象の鋼のチタン成分規格上限値（質量％）をそれぞれ示す。なお、チタンを含有する合金を複数種使用する場合もあるが、（１）式では、各合金の添加量Ｗｔとチタン含有量ηｔとを乗じた値が足しあわされる。
ここで、チタンを含有する合金としては、ＦｅＳｉ合金が挙げられる。ＦｅＳｉ合金を添加する場合には、転炉での出鋼後に取鍋精錬炉及びＲＨ真空脱ガス装置にて添加されるＦｅＳｉ合金の添加量と、溶鋼の目標チタン含有量とに応じて、ＦｅＳｉ合金の品位（チタン含有量）を選択するとよい。In formula (1), Δ[Ti] is the amount of increase in titanium content in molten steel (kg-Ti/t-steel) during the period from tapping to the end of the vacuum degassing refining process, and [Ti] LD is Titanium content in molten steel after tapping (kg-Ti/t-steel), W t is the amount of alloy added from tapping to the end of vacuum degassing refining treatment (kg-alloy/t −steel), η t is the titanium content (mass%) of the alloy added from tapping to the end of the vacuum degassing refining process in the target charge, and Δ [Ti] M is the titanium content in the molten steel picked up from other than the alloy. The amount of increase in (kg-Ti/t-steel) and T 1 indicate the standard upper limit value (mass%) of the titanium composition of the steel to be smelted. In some cases, a plurality of titanium-containing alloys are used, but in the formula (1), the values obtained by multiplying the added amount W t of each alloy by the titanium content η t are added together.
Here, an FeSi alloy is mentioned as an alloy containing titanium. When FeSi alloy is added, depending on the amount of FeSi alloy added in the ladle refining furnace and RH vacuum degassing device after tapping in the converter and the target titanium content of molten steel, FeSi The alloy grade (titanium content) should be selected.

また、上記実施形態では、転炉精錬を行う一次精錬工程、加熱撹拌精錬処理と真空脱ガス精錬処理とを行う二次精錬を行うとしたが、本発明はかかる例に限定されない。例えば、一次精錬工程では、精錬炉として転炉ではなく電気炉等の設備で溶鋼を製造してもよい。また、二次精錬工程では、取鍋精錬炉での処理を行わず、真空脱ガス精錬処理のみを行うようにしてもよい。 In the above embodiment, the primary refining step of converter refining and the secondary refining step of heat stirring refining and vacuum degassing refining are performed, but the present invention is not limited to such an example. For example, in the primary refining process, the molten steel may be produced using equipment such as an electric furnace instead of a converter as the refining furnace. Further, in the secondary refining process, only the vacuum degassing refining process may be performed without performing the process in the ladle refining furnace.

なお、高疲労強度鋼の一例としては、ＪＩＳ（日本産業規格）Ｇ４８０５でＳＵＪ２として規定された軸受鋼が挙げられる。その成分範囲は、炭素濃度が０．９５質量％以上１．１０質量％以下、珪素濃度が０．１５質量％以上０．３５質量％以下、マンガン濃度が０．５０質量％以下、燐濃度が０．０２５質量％以下、硫黄濃度が０．０２５質量％以下、クロム濃度が１．３０質量％以上１．６０質量％以下、モリブデン濃度が０．０８質量％以下、ニッケル濃度が０．２５質量％以下、銅濃度が０．２５質量％以下である。 An example of high fatigue strength steel is a bearing steel specified as SUJ2 in JIS (Japanese Industrial Standards) G4805. The component ranges are carbon concentration of 0.95% by mass or more and 1.10% by mass or less, silicon concentration of 0.15% by mass or more and 0.35% by mass or less, manganese concentration of 0.50% by mass or less, and phosphorus concentration of 0.025% by mass or less, sulfur concentration of 0.025% by mass or less, chromium concentration of 1.30% by mass or more and 1.60% by mass or less, molybdenum concentration of 0.08% by mass or less, nickel concentration of 0.25% by mass % or less, and the copper concentration is 0.25% by mass or less.

その他の規格で規定された軸受鋼としては、ＩＳＯ（国際標準化機構）６８３－１７規格で１００Ｃｒ６として規定された軸受鋼、ＧＢ（中国国家標準規格）でＧＣｒ１５として規定された軸受鋼、ＡＳＴＭ（米国試験材料協会）Ａ２９５規格で５２１００として規定された軸受鋼、及び、ＤＩＮ（ドイツ規格協会）規格で１００Ｃｒ６として規定された軸受鋼などが存在する。因みに、ＩＳＯ（国際標準化機構）６８３－１７規格で１００Ｃｒ６として規定された軸受鋼の成分範囲は、炭素濃度が０．９５質量％以上１．１０質量％以下、珪素濃度が０．１５質量％以上０．３５質量％以下、マンガン濃度が０．２５質量％以上０．４５質量％以下、燐濃度が０．０３０質量％以下、硫黄濃度が０．０２５質量％以下、クロム濃度が１．３５質量％以上１．６５質量％以下である。 Bearing steel specified by other standards includes bearing steel specified as 100Cr6 in ISO (International Organization for Standardization) 683-17 standard, bearing steel specified as GCr15 in GB (China National Standard), ASTM (US There are bearing steels specified as 52100 in the Society for Testing Materials A295 standard and bearing steels specified as 100Cr6 in the DIN (German Standards Institute) standard. Incidentally, the composition range of bearing steel specified as 100Cr6 in ISO (International Organization for Standardization) 683-17 standard has a carbon concentration of 0.95% by mass or more and 1.10% by mass or less, and a silicon concentration of 0.15% by mass or more. 0.35% by mass or less, manganese concentration of 0.25% by mass or more and 0.45% by mass or less, phosphorus concentration of 0.030% by mass or less, sulfur concentration of 0.025% by mass or less, chromium concentration of 1.35% by mass % or more and 1.65 mass % or less.

また、ＪＩＳ Ｇ４０５３で規定されるクロムモリブデン鋼鋼材（ＳＣＭ材）も、用途により疲労強度が要求される。ＳＣＭ材の例としてＳＣＭ４２０として規定されたクロムモリブデン鋼材の成分は、炭素濃度が０．１８～０．２３質量％、珪素濃度が０．１５～０．３５質量％、マンガン濃度が０．６０～０．９０質量％、燐濃度が０．０３０質量％以下、硫黄濃度が０．０３０質量％以下、ニッケル濃度が０．２５質量％以下、クロム濃度が０．９０～１．２０質量％、モリブデン濃度が０．１５～０．２５質量％である。 In addition, the chromium-molybdenum steel material (SCM material) specified by JIS G4053 is also required to have fatigue strength depending on the application. The composition of the chromium molybdenum steel material specified as SCM420 as an example of the SCM material has a carbon concentration of 0.18 to 0.23% by mass, a silicon concentration of 0.15 to 0.35% by mass, and a manganese concentration of 0.60 to 0.90% by mass, phosphorus concentration of 0.030% by mass or less, sulfur concentration of 0.030% by mass or less, nickel concentration of 0.25% by mass or less, chromium concentration of 0.90 to 1.20% by mass, molybdenum The concentration is 0.15-0.25 mass %.

その他の規格で規定されたクロムモリブデン鋼材としては、ＩＳＯ（国際標準化機構）規格，ＥＮ（欧州標準化委員会）規格，およびＤＩＮ（ドイツ規格協会）規格等で２５ＣｒＭｏ４として規定された鋼材、ＧＢ（中国国家標準規格）で３０ＣｒＭｎとして規定された鋼材、ＡＳＴＭ（米国試験材料協会）Ａ２９規格で４１３０として規定された鋼材などが存在する。 Chromium molybdenum steel materials specified by other standards include steel materials specified as 25CrMo4 by ISO (International Organization for Standardization) standards, EN (European Committee for Standardization) standards, and DIN (German Standards Institute) standards, GB (China There are steel materials specified as 30CrMn in the National Standards) and steel materials specified as 4130 in the ASTM (American Society for Testing and Materials) A29 standard.

その他、ＪＩＳ Ｇ ４０５１で規定される機械構造用炭素鋼鋼材（ＳＣ材）も、用途により疲労強度が要求される。ＳＣ材の例としてＳ５３Ｃとして規定されたクロムモリブデン鋼材の成分は、炭素濃度が０．５０～０．５６質量％、珪素濃度が０．１５～０．３５質量％、マンガン濃度が０．６０～０．９０質量％、燐濃度が０．０３０質量％以下、硫黄濃度が０．０３５質量％以下、である。 In addition, carbon steel materials for machine structural use (SC materials) specified in JIS G 4051 are also required to have fatigue strength depending on the application. The composition of the chromium molybdenum steel material specified as S53C as an example of the SC material has a carbon concentration of 0.50 to 0.56% by mass, a silicon concentration of 0.15 to 0.35% by mass, and a manganese concentration of 0.60 to 0.60%. 0.90% by mass, a phosphorus concentration of 0.030% by mass or less, and a sulfur concentration of 0.035% by mass or less.

＜実施形態の効果＞
（１）本発明の一態様に係る鋳片の製造方法は、転炉又は電気炉から出鋼され、取鍋に受鋼した溶鋼に、少なくとも真空脱ガス精錬処理を施して得られる精錬後溶鋼を鋳造する、鋳片の製造方法であって、鋳片を製造する対象チャージで使用する取鍋を、少なくとも対象チャージの受鋼直前に取鍋で保持していた溶鋼中のチタン含有量により制限し、対象チャージの出鋼後の溶鋼中のチタン含有量を０．００２質量％以下とし、さらに、対象チャージについて真空脱ガス精錬処理を行う真空脱ガス設備を、少なくとも対象チャージの真空脱ガス精錬処理の直前に真空脱ガス設備で処理した溶鋼中の上記真空脱ガス設備での処理終了時のチタン含有量により制限する。<Effects of Embodiment>
(1) A method for producing a cast slab according to an aspect of the present invention is a molten steel after refining obtained by subjecting molten steel tapped from a converter or an electric furnace and received in a ladle to at least vacuum degassing refining treatment. A method for producing a slab, in which the ladle used in the target charge for producing the slab is limited by the titanium content in the molten steel held in the ladle at least immediately before receiving the target charge The titanium content in the molten steel after tapping of the target charge is set to 0.002% by mass or less, and the vacuum degassing equipment for performing vacuum degassing refining treatment for the target charge is set to at least vacuum degassing refining of the target charge It is limited by the titanium content at the end of the treatment with the vacuum degassing equipment in the molten steel treated with the vacuum degassing equipment immediately before the treatment.

上記（１）の構成によれば、取鍋や真空脱ガス設備に付着した地金からのチタン混入に加え、地金表面の酸化物の混入が抑えられるため、精錬時に溶鋼中のチタン含有量を低位に維持できるとともに溶鋼の清浄度も保たれるのでＴｉＮの生成が抑制され、清浄度の高い鋳片の製造が可能になる。 According to the above configuration (1), in addition to the contamination of titanium from the bare metal adhering to the ladle and the vacuum degassing equipment, the contamination of oxides on the surface of the bare metal is suppressed, so the titanium content in the molten steel during refining can be maintained at a low level and the cleanliness of the molten steel is also maintained, the generation of TiN is suppressed, and it is possible to manufacture a cast slab with a high degree of cleanliness.

（２）上記（１）の構成において、転炉又は電気炉から出鋼され、取鍋に受鋼した溶鋼に、加熱撹拌精錬処理及び真空脱ガス精錬処理をこの順に施して得られる精錬後溶鋼を鋳造する。
上記（２）の構成によれば、出鋼後に精錬加熱撹拌精錬処理及び真空脱ガス精錬処理を行うプロセスにおいても、上記（１）と同様の効果が得られる。(2) In the configuration of (1) above, molten steel after refining obtained by subjecting molten steel tapped from a converter or an electric furnace and received in a ladle to heat stirring refining treatment and vacuum degassing refining treatment in this order. to cast.
According to the configuration of (2) above, the same effect as that of (1) above can be obtained even in the process of performing the refining heating stirring refining treatment and the vacuum degassing refining treatment after tapping.

（３）上記（１）又は（２）の構成において、対象チャージで使用する取鍋について、対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量を０．０２０質量％以下とする。
上記（３）の構成によれば、取鍋の付着地金からのチタン混入を抑制することができ、清浄度の高い鋳片を製造することができる。(3) In the configuration of (1) or (2) above, for the ladle used in the target charge, the titanium content in the molten steel held immediately before receiving the target charge is set to 0.020% by mass or less. .
According to the above configuration (3), it is possible to suppress the contamination of titanium from the base metal adhered to the ladle, and it is possible to produce a cast slab with a high degree of cleanliness.

（４）上記（３）の構成において、対象チャージで使用する取鍋について、対象チャージの受鋼直前に保持していた溶鋼中のアルミニウム含有量を０．０５０質量％以下とする。
上記（４）の構成によれば、取鍋の付着地金に含まれるチタン酸化物のアルミニウムによる還元が抑制されるため、取鍋の付着地金からのチタン混入をさらに抑制することができ、清浄度の高い鋳片を製造することができる。(4) In the configuration of (3) above, for the ladle used in the target charge, the aluminum content in the molten steel held immediately before receiving the target charge is set to 0.050% by mass or less.
According to the configuration of (4) above, since the reduction of the titanium oxide contained in the metal adhered to the ladle by aluminum is suppressed, it is possible to further suppress the contamination of titanium from the metal adhered to the ladle. A cast slab having a high degree of cleanliness can be produced.

（５）上記（１）～（４）のいずれか一つの構成において、対象チャージの真空脱ガス精錬処理を行う真空脱ガス設備について、対象チャージの真空脱ガス精錬処理の直前に真空脱ガス設備で処理した溶鋼中の真空脱ガス設備での処理終了時のチタン含有量を０．０２０質量％以下とする。
上記（５）の構成によれば、真空脱ガス設備の精錬槽の付着地金からのチタン混入を抑制することができ、清浄度の高い鋳片を製造することができる。(5) In any one of the above configurations (1) to (4), the vacuum degassing equipment for performing the vacuum degassing refining process of the target charge immediately before the vacuum degassing refining process of the target charge. The titanium content at the end of the treatment in the vacuum degassing equipment in the molten steel treated with is 0.020% by mass or less.
According to the configuration (5) above, it is possible to suppress the contamination of titanium from the adhered bare metal in the refining tank of the vacuum degassing equipment, and to produce a cast slab with a high degree of cleanliness.

（６）上記（５）の構成において、対象チャージの真空脱ガス精錬処理を行う真空脱ガス設備について、対象チャージの真空脱ガス精錬処理の直前に真空脱ガス設備で処理した溶鋼中の真空脱ガス設備での処理終了時のアルミニウム含有量を０．０５０質量％以下とする。
上記（６）の構成によれば、真空脱ガス設備の精錬槽の付着地金に含まれるチタン酸化物のアルミニウムによる還元が抑制されるため、真空脱ガス設備の精錬槽の付着地金からのチタン混入をさらに抑制することができ、清浄度の高い鋳片を製造することができる。(6) In the configuration of (5) above, for the vacuum degassing equipment that performs the vacuum degassing refining treatment of the target charge, vacuum degassing in the molten steel processed by the vacuum degassing equipment immediately before the vacuum degassing refining treatment of the target charge. The aluminum content at the end of treatment in the gas facility is set to 0.050% by mass or less.
According to the above configuration (6), since the reduction by aluminum of the titanium oxide contained in the deposited metal in the refining tank of the vacuum degassing equipment is suppressed, the deposited metal in the refining tank of the vacuum degassing equipment is reduced. Titanium contamination can be further suppressed, and slabs with high cleanliness can be produced.

（７）上記（１）～（６）のいずれか一つの構成において、対象チャージの出鋼直前の前記溶鋼中のＴｉ含有量は分析限界以下となるゼロであり、出鋼後から前記真空脱ガス精錬処理終了までの期間において、溶鋼中のチタン含有量の増加量が下記の（１）式を満たす。
Δ［Ｔｉ］＝［Ｔｉ］ＬＤ＋Σ（Ｗｔ×ηｔ／１００）＋Δ［Ｔｉ］Ｍ≦Ｔ１×１０ ・・・（１）
但し、（１）式において、
Δ［Ｔｉ］：出鋼後から真空脱ガス精錬処理終了までの期間における溶鋼中チタン含有量の増加量（ｋｇ－Ｔｉ／ｔ－ｓｔｅｅｌ）
［Ｔｉ］ＬＤ：出鋼後の溶鋼中のチタン含有量（ｋｇ－Ｔｉ／ｔ－ｓｔｅｅｌ）
Ｗｔ：対象チャージにおいて、出鋼から真空脱ガス精錬処理終了までに添加する合金の添加量（ｋｇ－ａｌｌｏｙ／ｔ－ｓｔｅｅｌ）
ηｔ：対象チャージにおいて、出鋼から真空脱ガス精錬処理終了までに添加する合金のチタン含有量（質量％）
Δ［Ｔｉ］Ｍ：合金以外から混入した溶鋼中チタン含有量の増加量（ｋｇ－Ｔｉ／ｔ－ｓｔｅｅｌ）
Ｔ１：溶製対象の鋼のチタン成分規格上限値（質量％）
である。
上記（７）の構成によれば、添加される合金からのチタン混入も抑えられるため、精錬時に溶鋼中のチタン含有量を低位に維持できるのでＴｉＮの生成が抑制され、清浄度の高い鋳片の製造が可能になる。(7) In any one of the above configurations (1) to (6), the Ti content in the molten steel immediately before tapping of the target charge is zero, which is below the analysis limit, and the vacuum desorption is performed after tapping. During the period until the end of the gas refining process, the amount of increase in titanium content in the molten steel satisfies the following formula (1).
Δ[Ti]=[Ti] LD +Σ( Wt × ηt /100)+Δ[Ti] M ≦ T1 ×10 (1)
However, in formula (1),
Δ [Ti]: Increase in titanium content in molten steel during the period from tapping to completion of vacuum degassing refining (kg-Ti/t-steel)
[Ti] LD : Titanium content in molten steel after tapping (kg-Ti/t-steel)
W t : Addition amount of alloy added from tapping to completion of vacuum degassing refining treatment in target charge (kg-alloy/t-steel)
η t : Titanium content (% by mass) of the alloy added from tapping to completion of vacuum degassing refining treatment in the target charge
Δ[Ti] M : Increase in titanium content in molten steel mixed from sources other than alloys (kg-Ti/t-steel)
T 1 : the upper limit of the standard titanium content of the steel to be smelted (% by mass)
is.
According to the above configuration (7), since the contamination of titanium from the alloy to be added is also suppressed, the titanium content in the molten steel can be maintained at a low level during refining, so the generation of TiN is suppressed and the cast slab with high cleanliness. manufacturing becomes possible.

（８）上記（２）～（７）のいずれか一つの構成において、対象チャージにおいて、転炉又は電気炉からの出鋼時に取鍋内に流出したスラグの溶鋼重量に対する量を、加熱撹拌精錬処理前の時点で２５ｋｇ－ｓｌａｇ／ｔ－ｓｔｅｅｌ以下とする。
上記（８）の構成によれば、一次精錬処理工程にて発生したスラグの取鍋への流出を十分に抑えられることから、このスラグに含まれるチタンによるチタン混入を抑制することができる。
（９）上記（１）～（８）のいずれか一つの構成において、連続鋳造される精錬後溶鋼中のチタン含有量が０．００２０質量％以下である。
なお、本明細書では高疲労強度が要求される鋼（高疲労強度鋼）の精錬を例として説明したが、本発明は、高疲労強度鋼に限らず、溶鋼中へのチタンの混入が望ましくない鋼種の精錬においても適用可能である。(8) In any one of the above configurations (2) to (7), in the target charge, the amount of slag flowing out into the ladle at the time of tapping from the converter or electric furnace relative to the weight of the molten steel is 25 kg-slag/t-steel or less before treatment.
According to the above configuration (8), it is possible to sufficiently suppress the outflow of slag generated in the primary refining process to the ladle, so that the titanium contained in the slag can be suppressed from mixing with titanium.
(9) In any one of the above (1) to (8), the content of titanium in the continuously cast molten steel after refining is 0.0020% by mass or less.
In this specification, the refining of steel that requires high fatigue strength (high fatigue strength steel) has been described as an example, but the present invention is not limited to high fatigue strength steel, and it is desirable to mix titanium in molten steel. It can also be applied to the refining of steel grades that do not have

１チャージの溶鋼量が約２００ｔｏｎの規模の実機にて、上記実施形態と同様に転炉での一次精錬工程、取鍋精錬炉及びＲＨ真空脱ガス装置での二次精錬工程、連続鋳造機による鋳造工程を行い、高疲労強度鋼である軸受鋼の素材となるブルーム鋳片の製造を行なった。軸受鋼の成分組成は、炭素濃度が０．９０質量％以上１．１０質量％以下、ケイ素濃度が０．１５質量％以上０．２５質量％以下、マンガン濃度が０．４５質量％以下、リン濃度が０．００２０質量％以下、硫黄濃度が０．００５０質量％以下、アルミニウム濃度が０．０３０質量％以下、クロム濃度が１．４質量％以上１．７質量％以下、窒素濃度が０．００５０質量％以下、チタン濃度が０．００２０質量％以下、残部は鉄及び不可避的不純物である。 In an actual machine with a scale of about 200 tons of molten steel per charge, as in the above embodiment, the primary refining process in the converter, the secondary refining process in the ladle refining furnace and the RH vacuum degasser, and the continuous casting machine A casting process was performed to produce bloom slabs, which are materials for bearing steel, which is high fatigue strength steel. The chemical composition of the bearing steel is such that the carbon concentration is 0.90% by mass or more and 1.10% by mass or less, the silicon concentration is 0.15% by mass or more and 0.25% by mass or less, the manganese concentration is 0.45% by mass or less, and the phosphorus concentration is 0.45% by mass or less. concentration of 0.0020% by mass or less, sulfur concentration of 0.0050% by mass or less, aluminum concentration of 0.030% by mass or less, chromium concentration of 1.4% by mass or more and 1.7% by mass or less, nitrogen concentration of 0.005% by mass or less; 0050% by mass or less, the titanium concentration is 0.0020% by mass or less, and the balance is iron and unavoidable impurities.

転炉から取鍋への溶鋼の出鋼時には、加炭材や合金、石灰を溶鋼に添加した。また、出鋼時には、赤外線等を用いてスラグ流出検知を行い、ダーツ型ストッパーを用いて転炉からのスラグ流出を防止した。次いで、加熱撹拌精錬処理では、所定のフラックスを用いて精錬を行い、Ａｌを添加して脱酸を行った。さらに、ＲＨ真空脱ガス装置へ溶鋼を搬送し、ＲＨ真空脱ガス装置で、溶鋼温度や成分などの調整を行った。その後、連続鋳造によりブルーム鋳片（３００ｍｍ×４００ｍｍ断面）を製造した。次いで、ブルーム鋳片に対して、熱処理を施した後、直径２１５ｍｍのビレットに圧延した。さらに、このビレットを熱間圧延することで直径６０ｍｍの棒鋼とし、焼鈍処理を経て、製品丸棒とした。 When the molten steel was tapped from the converter to the ladle, recarburizers, alloys, and lime were added to the molten steel. At the time of tapping, the outflow of slag was detected using infrared rays, etc., and the outflow of slag from the converter was prevented using a dart type stopper. Next, in the heat stirring refining treatment, refining was performed using a predetermined flux, and deoxidation was performed by adding Al. Further, the molten steel was transported to the RH vacuum degassing device, and the molten steel temperature and composition were adjusted in the RH vacuum degassing device. After that, bloom slabs (300 mm×400 mm cross section) were produced by continuous casting. Then, the bloom slab was heat-treated and then rolled into a billet with a diameter of 215 mm. Furthermore, this billet was hot-rolled to obtain a steel bar with a diameter of 60 mm, which was then annealed to obtain a product round bar.

実施例では、転炉からの溶鋼を受鋼する取鍋は、受鋼前にその重量を測定し、付着している地金量を測定した。さらに、付着地金のチタン含有量は、前回の処理における鋳造時の溶鋼中のチタン含有量として求めた。また、ＲＨ真空脱ガス装置に付着している地金量を、前回の真空脱ガス精錬処理を行う前後のＲＨ真空脱ガス装置の真空槽及び浸漬管の重量を測定することにより測定した。また、付着している地金のチタン含有量は、前回の処理における真空脱ガス精錬処理終了時の溶鋼中のチタン含有量として求めた。この際、取鍋における前回処理とＲＨ真空脱ガス装置における前回処理とにおいては、同じ鋼種であるとは限らず、それぞれが違った前回処理を行っていることが多い。 In the examples, the ladle receiving the molten steel from the converter was weighed before receiving the steel, and the amount of metal adhered was measured. Furthermore, the titanium content of the deposited ingot was determined as the titanium content in the molten steel during casting in the previous treatment. Also, the amount of ingot adhering to the RH vacuum degassing device was measured by measuring the weight of the vacuum chamber and the immersion tube of the RH vacuum degassing device before and after the previous vacuum degassing refining treatment. Also, the titanium content of the adhering base metal was obtained as the titanium content in the molten steel at the end of the vacuum degassing refining treatment in the previous treatment. At this time, the previous treatment in the ladle and the previous treatment in the RH vacuum degassing apparatus are not necessarily the same steel type, and different pretreatments are often performed for each.

さらに、加熱撹拌精錬処理及び真空脱ガス精錬処理を行う二次精錬工程にて添加したＦｅＳｉ合金の添加量と含有チタン含有量とを考慮した。実施例では、各試験操業における取鍋の付着地金量及び前回処理の鋼種、ＲＨ真空脱ガス装置の付着地金量及び前回処理の鋼種、並びに二次精錬工程にて添加したＦｅＳｉ合金の添加量及びチタン含有量を変化させて試験を行った。それぞれの条件及び試験結果を表１に示す。なお、真空脱ガス精錬処理にて添加したＦｅＳｉ合金中チタンの溶鋼への歩留は、１００％と仮定した。また、表１において、取鍋地金（取鍋の付着地金）及びＲＨ地金（装置の付着地金）のチタン含有量はそれぞれ、直前に受鋼していたチャージにおける鋳造時の溶鋼中のチタン含有量及び直前のチャージにおける真空脱ガス精錬処理終了後の溶鋼中のチタン含有量である。 Furthermore, the addition amount of the FeSi alloy added in the secondary refining process including the heat-stirring refining process and the vacuum degassing refining process and the contained titanium content were considered. In the examples, the amount of metal deposited on the ladle and the steel type of the previous treatment in each test operation, the amount of metal deposited on the RH vacuum degasser and the steel type of the previous treatment, and the addition of the FeSi alloy added in the secondary refining process Tests were conducted with varying amount and titanium content. Table 1 shows the respective conditions and test results. The yield of titanium in the FeSi alloy added in the vacuum degassing refining process to the molten steel was assumed to be 100%. In addition, in Table 1, the titanium content of the ladle base metal (ladle base metal) and RH base metal (apparatus base metal) are and the titanium content in the molten steel after the vacuum degassing refining treatment in the immediately preceding charge.

それぞれの試験溶製時において得られた製品丸棒の１／４厚部及び１／２厚部における圧延方向の縦断面を、検鏡法により観察した。被検面積は３０００ｍｍ２とした。検鏡法での介在物測定と併せて、ＳＥＭ（走査型電子顕微鏡）及びＥＤＸ（エネルギー分散型Ｘ線分光法）により介在物組成を特定し、ＴｉＮ系介在物の大きさと個数を測定して、清浄度を評価した。Vertical cross-sections in the rolling direction at the 1/4 thick part and 1/2 thick part of the product round bars obtained at the time of each test melting were observed by microscopy. The test area was 3000 mm 2 . In addition to measuring inclusions by spectroscopy, SEM (scanning electron microscope) and EDX (energy dispersive X-ray spectroscopy) were used to identify the composition of inclusions and measure the size and number of TiN inclusions. , to evaluate cleanliness.

Figure 0007248153000001
Figure 0007248153000001

本発明例１，２では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金として、チタン含有量が０．０５質量％以下の高品位のものを用いた。また、本発明例１，２では、取鍋及びＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０４０質量％以下の鋼種にともに規制して二次精錬工程を行った。
本発明例３～５では、取鍋及びＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０２０質量％以下の鋼種にともに規制して二次精錬工程を行った。また、本発明例３～５では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金として、チタン含有量が約０．２０質量％の通常品位のものを用いた。In Examples 1 and 2 of the present invention, a high-grade FeSi alloy with a titanium content of 0.05% by mass or less was used as the FeSi alloy used in the RH vacuum degassing apparatus. In Examples 1 and 2 of the present invention, the molten steel processed before one charge of the ladle and the RH vacuum degassing device was regulated to a steel type with a titanium content standard of 0.040% by mass or less, and the secondary refining process was performed. did
In Examples 3 to 5 of the present invention, the molten steel processed before one charge of the ladle and the RH vacuum degassing device was subjected to the secondary refining process while restricting both the steel type to a steel type with a titanium content standard of 0.020% by mass or less. rice field. Further, in Examples 3 to 5 of the present invention, the FeSi alloy used in the RH vacuum degassing apparatus was of normal grade with a titanium content of about 0.20% by mass.

本発明例１１、１２では、本発明例３～５と同じように、取鍋及びＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０２０質量％以下の鋼種にともに規制して二次精錬工程を行った。また、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金として、チタン含有量が約０．２０質量％の通常品位のものを用いた。ただし、取鍋またはＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼のアルミニウム成分規格が０．０５０質量％より高い鋼種で二次精錬工程を行った。なお、本発明例１１，１２以外の実施例においては、取鍋及びＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼のアルミニウム成分規格が０．０５０質量％以下の鋼種に規制して二次精錬工程を行っている。 In Inventive Examples 11 and 12, in the same way as Inventive Examples 3 to 5, the molten steel treated before one charge of the ladle and the RH vacuum degassing device has a titanium content standard of 0.020% by mass or less. The secondary refining process was carried out by controlling the As the FeSi alloy used in the RH vacuum degassing apparatus, a normal grade FeSi alloy with a titanium content of about 0.20% by mass was used. However, the secondary refining process was performed with a steel type having a higher aluminum component standard than 0.050% by mass in the molten steel processed before one charge of the ladle or the RH vacuum degasser. In addition, in Examples other than Examples 11 and 12 of the present invention, the molten steel treated before one charge of the ladle and the RH vacuum degassing device was regulated to a steel type with an aluminum content standard of 0.050% by mass or less. undergoing the refining process.

本発明例６では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金として、チタン含有量が０．０５質量％以下の高品位のものを用いた。また、本発明例６では、取鍋及びＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０１０質量％以下とより厳しい鋼種にともに規制して二次精錬工程を行った。
本発明例７では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金として、チタン含有量が０．０５質量％以下の高品位のものを用いた。また、本発明例７では、取鍋及びＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０２０質量％以下及び０．０１０質量％以下の鋼種にそれぞれ規制して二次精錬工程を行った。In Example 6 of the present invention, a high-grade FeSi alloy with a titanium content of 0.05% by mass or less was used as the FeSi alloy used in the RH vacuum degassing apparatus. In addition, in Example 6 of the present invention, the molten steel processed before one charge of the ladle and the RH vacuum degassing device was regulated to a steel type with a stricter titanium content standard of 0.010% by mass or less, and the secondary refining process was performed. did
In Example 7 of the present invention, a high-grade FeSi alloy with a titanium content of 0.05% by mass or less was used as the FeSi alloy used in the RH vacuum degassing apparatus. In addition, in Example 7 of the present invention, the molten steel processed before one charge of the ladle and the RH vacuum degassing device was regulated to steel types with titanium composition standards of 0.020% by mass or less and 0.010% by mass or less, respectively. The secondary refining process was carried out.

本発明例８では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金として、チタン含有量が０．０５質量％以下の高品位のものを用いた。また、本発明例８では、取鍋及びＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０１０質量％以下及び０．０２０質量％以下の鋼種にそれぞれ規制して二次精錬工程を行った。
本発明例９では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金として、チタン含有量が０．２０質量％以下の通常品位のものを用いた。また、本発明例９では、取鍋及びＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０１０質量％以下とより厳しい鋼種にともに規制して二次精錬工程を行った。In Example 8 of the present invention, a high-grade FeSi alloy with a titanium content of 0.05% by mass or less was used as the FeSi alloy used in the RH vacuum degassing apparatus. In addition, in Example 8 of the present invention, the molten steel treated before one charge of the ladle and the RH vacuum degassing device was regulated to steel types with a titanium composition standard of 0.010% by mass or less and 0.020% by mass or less, respectively. The secondary refining process was carried out.
In Example 9 of the present invention, a normal grade FeSi alloy with a titanium content of 0.20% by mass or less was used as the FeSi alloy used in the RH vacuum degassing apparatus. In addition, in Example 9 of the present invention, the molten steel processed before one charge of the ladle and the RH vacuum degassing device was regulated to a steel type with a stricter titanium content standard of 0.010% by mass or less, and the secondary refining process was performed. did

本発明例１０では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金として、チタン含有量が０．０５質量％以下の高品位のものを用いた。また、本発明例１０では、取鍋及びＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０１０質量％以下とより厳しい鋼種にともに規制して二次精錬工程を行った。ただし、本発明例１０では、原料溶銑中のチタン含有量が高かったため、出鋼後のチタン含有量は０．０１９質量％と他の発明例より高くなった。そのため、転炉から取鍋への出鋼後、除滓をすることによって、スラグ量が確実に２５ｋｇ－ｓｌａｇ／ｔ－ｓｔｅｅｌ以下となるようにした。 In Inventive Example 10, a high-grade FeSi alloy with a titanium content of 0.05% by mass or less was used as the FeSi alloy used in the RH vacuum degassing apparatus. In addition, in Example 10 of the present invention, the molten steel processed before one charge of the ladle and the RH vacuum degassing device was regulated to a steel type with a stricter titanium content standard of 0.010% by mass or less, and the secondary refining process was performed. did However, in Inventive Example 10, the titanium content in the raw material hot metal was high, so the titanium content after tapping was 0.019% by mass, which was higher than in the other Inventive Examples. Therefore, after tapping from the converter to the ladle, the slag was removed to ensure that the amount of slag was 25 kg-slag/t-steel or less.

比較例１では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金としてチタン含有量が約０．２０質量％の通常品位のものを用い、取鍋の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０２０質量％以下の鋼種に規制して二次精錬工程を行った。一方、ＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼については、チタンの成分規格については規制せずに、チタン含有量が０．１５質量％程度のものとした。 In Comparative Example 1, a normal grade FeSi alloy with a titanium content of about 0.20% by mass was used as the FeSi alloy used in the RH vacuum degassing device, and the molten steel processed before one charge of the ladle was subjected to the titanium composition standard. The secondary refining process was performed by restricting steel grades to 0.020% by mass or less. On the other hand, regarding the molten steel processed before one charge of the RH vacuum degassing device, the content of titanium was set to about 0.15% by mass without any restriction on the standard of titanium composition.

比較例２では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金としてチタン含有量が約０．２０質量％の通常品位のものを用い、ＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０２０質量％以下の鋼種に規制して二次精錬工程を行った。一方、取鍋の１チャージ前に処理した溶鋼については、チタンの成分規格については規制せずに、チタン含有量が０．０８質量％程度のものとした。 In Comparative Example 2, a normal grade FeSi alloy with a titanium content of about 0.20% by mass was used as the FeSi alloy used in the RH vacuum degasser. The secondary refining process was carried out by restricting the steel grades to those with a chemical standard of 0.020% by mass or less. On the other hand, regarding the molten steel processed before one charge of the ladle, the content of titanium was set to about 0.08% by mass without restricting the standard of titanium composition.

比較例３では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金としてチタン含有量が約０．０５質量％の高品位のものを用い、取鍋の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０２０質量％以下の鋼種に規制して二次精錬工程を行った。一方、ＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼については、チタンの成分規格については規制せずに、チタン含有量が０．１５質量％程度のものとした。 In Comparative Example 3, a high-grade FeSi alloy with a titanium content of about 0.05% by mass was used as the FeSi alloy used in the RH vacuum degassing device, and the molten steel processed before one charge of the ladle was used. The secondary refining process was performed by restricting steel grades to 0.020% by mass or less. On the other hand, regarding the molten steel processed before one charge of the RH vacuum degassing device, the content of titanium was set to about 0.15% by mass without any restriction on the standard of titanium composition.

比較例４では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金としてチタン含有量が約０．０５質量％の高品位のものを用い、ＲＨ真空脱ガス装置の１チャージ前に処理した溶鋼について、チタンの成分規格が０．０２０質量％以下の鋼種に規制して二次精錬工程を行った。一方、取鍋の１チャージ前に処理した溶鋼については、チタンの成分規格については規制せずに、チタン含有量が０．０８質量％程度のものとした。 In Comparative Example 4, a high-grade FeSi alloy with a titanium content of about 0.05% by mass was used as the FeSi alloy used in the RH vacuum degasser, and the molten steel treated before one charge of the RH vacuum degasser was treated with titanium. The secondary refining process was carried out by restricting the steel grades to those with a chemical standard of 0.020% by mass or less. On the other hand, regarding the molten steel processed before one charge of the ladle, the content of titanium was set to about 0.08% by mass without restricting the standard of titanium composition.

比較例５では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金としてチタン含有量が約０．０５質量％の高品位のものを用いた。また、取鍋及びＲＨ真空脱ガス装置の１チャージ前の処理した溶鋼については、チタンの成分規格が０．０４０質量％以下の鋼種にともに規制して二次精錬工程を行った。しかし、副原料中のチタン含有量が高かったため、出鋼後の溶鋼のチタン含有量が０．００２６質量％と高くなり、成分規格の上限を外れた。 In Comparative Example 5, a high-grade FeSi alloy with a titanium content of about 0.05% by mass was used in the RH vacuum degassing apparatus. In addition, the secondary refining process was carried out for the molten steel treated before one charge of the ladle and the RH vacuum degassing device, both of which were restricted to steel grades with a titanium component standard of 0.040% by mass or less. However, since the content of titanium in the auxiliary raw material was high, the content of titanium in the molten steel after tapping was as high as 0.0026% by mass, falling outside the upper limit of the composition standard.

比較例６，７では、ＲＨ真空脱ガス装置で用いるＦｅＳｉ合金としてチタン含有量が約０．０５質量％の高品位のものを用いた。しかし、取鍋及びＲＨ真空脱ガス装置の１チャージ前の処理した溶鋼については、チタンの成分規格をともに規制せずに二次精錬工程を行った。
なお、表１の「（１）式を満たすか」の項目において、「○」は条件が（１）式を満たすことを示し、「×」は条件が（１）式を満たさないことを示す。In Comparative Examples 6 and 7, a high-grade FeSi alloy with a titanium content of about 0.05% by mass was used as the FeSi alloy used in the RH vacuum degassing apparatus. However, the secondary refining process was carried out for the molten steel processed before one charge of the ladle and the RH vacuum degassing unit without restricting the specification of titanium composition.
In addition, in the item "whether expression (1) is satisfied" in Table 1, "○" indicates that the condition satisfies expression (1), and "×" indicates that the condition does not satisfy expression (1). .

本発明例１～１０では、対象チャージで使用する取鍋を、少なくとも対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量により制限し、出鋼後の溶鋼中のチタン含有量を０．００２質量％以下とし、さらに、対象チャージの真空脱ガス精錬処理を行うＲＨ真空脱ガス装置を、少なくとも対象チャージの処理の直前のチャージで処理した溶鋼中の真空脱ガス精錬処理終了時のチタン含有量により制限した。この結果、本発明例においては、真空脱ガス精錬処理終了後に鋳造される溶鋼中のチタン含有量が溶製対象のチタンの成分規格上限以下に制御できており、窒化物系の介在物個数が５個以下／１００ｍｍ２となり、１５μｍ以上のＴｉＮ系介在物も発見されなかった。In Examples 1 to 10 of the present invention, the ladle used in the target charge is limited by the titanium content in the molten steel held immediately before receiving the target charge, and the titanium content in the molten steel after tapping is limited. 0.002% by mass or less, and further, the RH vacuum degassing apparatus for performing vacuum degassing refining treatment of the target charge is used at least at the time of completion of the vacuum degassing refining treatment in molten steel processed with the charge immediately before the target charge treatment Limited by titanium content. As a result, in the example of the present invention, the titanium content in the molten steel cast after the completion of the vacuum degassing refining process can be controlled below the upper limit of the composition standard of titanium to be melted, and the number of nitride-based inclusions is 5 or less/100 mm 2 , and no TiN-based inclusions of 15 μm or more were found.

特に、取鍋の地金及びＲＨ真空脱ガス装置の地金について規制を行うことで、窒化物系の介在物個数を低減する効果が大きい傾向が見られた。ただし、本発明例１１、１２では、対象チャージで使用する取鍋を、少なくとも対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量により制限し、さらに、対象チャージの真空脱ガス精錬処理を行うＲＨ真空脱ガス装置を、少なくとも対象チャージの真空脱ガス精錬処理の直前のチャージで処理した溶鋼中の真空脱ガス精錬処理時のチタン含有量により制限したが、いずれかの溶鋼中のアルミニウム濃度が０．０５０質量％より高かった。この結果、取鍋の地金及びＲＨ真空脱ガス装置の地金について規制を行った割には窒化物系の介在物低減効果がやや少なかった。
一方、比較例においては、鋳造される溶鋼中のチタン含有量が溶製対象のチタンの成分規格上限をオーバーするものが多く、いずれの場合も、窒化物系の介在物個数は１０個以上／１００ｍｍ２と高位であり、１５μｍ以上のＴｉＮ系介在物も発見され、清浄性が悪かった。In particular, there was a tendency that the effect of reducing the number of nitride-based inclusions was large by regulating the base metal of the ladle and the base metal of the RH vacuum degassing device. However, in Examples 11 and 12 of the present invention, the ladle used in the target charge was limited by the titanium content in the molten steel held immediately before receiving the target charge, and furthermore, the vacuum degassing refining of the target charge was performed. The RH vacuum degassing device that performs the treatment was limited by the titanium content during the vacuum degassing refining treatment in the molten steel that was treated at least in the charge immediately before the vacuum degassing refining treatment of the target charge. The aluminum concentration was higher than 0.050 wt%. As a result, although the base metal of the ladle and the base metal of the RH vacuum degassing device were regulated, the effect of reducing nitride-based inclusions was somewhat small.
On the other hand, in many of the comparative examples, the titanium content in the molten steel to be cast exceeded the upper limit of the composition standard of titanium to be melted, and in all cases, the number of nitride-based inclusions was 10 or more/ It was as high as 100 mm 2 , and TiN-based inclusions of 15 µm or more were also found, indicating poor cleanliness.

Claims (7)

転炉又は電気炉から出鋼され、取鍋に受鋼した溶鋼に、少なくとも真空脱ガス精錬処理を施して得られる精錬後溶鋼を鋳造する、鋳片の製造方法であって、
前記鋳片を製造する対象チャージで使用する取鍋を、少なくとも前記対象チャージの受鋼直前に前記取鍋で保持していた溶鋼中のチタン含有量により制限し、
前記対象チャージの出鋼後の溶鋼中のチタン含有量を０．００２質量％以下とし、さらに、
前記対象チャージについて前記真空脱ガス精錬処理を行う真空脱ガス設備を、少なくとも前記対象チャージの前記真空脱ガス精錬処理の直前に前記真空脱ガス設備で処理した溶鋼中の前記真空脱ガス設備での処理終了時のチタン含有量により制限し、
前記対象チャージで使用する取鍋について、前記対象チャージの受鋼直前に保持していた溶鋼中のチタン含有量を０．０２０質量％以下とし、前記対象チャージの受鋼直前に保持していた溶鋼中のアルミニウム含有量を０．０５０質量％以下とする、鋳片の製造方法。 A method for producing a cast slab, wherein the molten steel tapped from a converter or an electric furnace and received in a ladle is subjected to at least vacuum degassing refining treatment to cast refined molten steel obtained,
The ladle used in the target charge for producing the slab is limited by the titanium content in the molten steel held in the ladle at least immediately before receiving the target charge,
The titanium content in the molten steel after tapping of the target charge is set to 0.002% by mass or less, and
Vacuum degassing equipment for performing the vacuum degassing refining treatment for the target charge, and at least the molten steel processed by the vacuum degassing equipment immediately before the vacuum degassing refining treatment for the target charge in the vacuum degassing equipment Limited by titanium content at the end of treatment ,
Regarding the ladle used in the target charge, the titanium content in the molten steel held immediately before receiving the target charge is 0.020% by mass or less, and the molten steel held immediately before receiving the target charge A method for producing a cast slab, wherein the aluminum content in the cast slab is 0.050% by mass or less . 前記転炉又は前記電気炉から出鋼され、前記取鍋に受鋼した前記溶鋼に、加熱撹拌精錬処理及び前記真空脱ガス精錬処理をこの順に施して得られる前記精錬後溶鋼を鋳造する、請求項１に記載の鋳片の製造方法。 Casting the molten steel after refining obtained by subjecting the molten steel tapped from the converter or the electric furnace and received in the ladle to the heating stirring refining treatment and the vacuum degassing refining treatment in this order. Item 1. A method for producing a cast slab according to item 1. 前記対象チャージにおいて、前記転炉又は前記電気炉からの出鋼時に前記取鍋内に流出したスラグの溶鋼重量に対する量を、前記加熱撹拌精錬処理前の時点で２５ｋｇ－ｓｌａｇ／ｔ－ｓｔｅｅｌ以下とする、請求項２に記載の鋳片の製造方法。 In the target charge, the amount of slag flowing out into the ladle at the time of tapping from the converter or the electric furnace relative to the weight of the molten steel is 25 kg-slag/t-steel or less before the heating and stirring refining treatment. The method for producing a slab according to claim 2, wherein 前記対象チャージの前記真空脱ガス精錬処理を行う前記真空脱ガス設備について、前記対象チャージの前記真空脱ガス精錬処理の直前に前記真空脱ガス設備で処理した溶鋼中の前記真空脱ガス設備での処理終了時のチタン含有量を０．０２０質量％以下とする、請求項１～３のいずれか１項に記載の鋳片の製造方法。 With respect to the vacuum degassing equipment for performing the vacuum degassing refining treatment of the target charge, in the vacuum degassing equipment in the molten steel processed by the vacuum degassing equipment immediately before the vacuum degassing refining treatment of the target charge The method for producing a cast slab according to any one of claims 1 to 3 , wherein the titanium content at the end of treatment is 0.020% by mass or less. 前記対象チャージの前記真空脱ガス精錬処理を行う前記真空脱ガス設備について、前記対象チャージの前記真空脱ガス精錬処理の直前に前記真空脱ガス設備で処理した溶鋼中の前記真空脱ガス設備での処理終了時のアルミニウム含有量を０．０５０質量％以下とする、請求項４に記載の鋳片の製造方法。 With respect to the vacuum degassing equipment for performing the vacuum degassing refining treatment of the target charge, in the vacuum degassing equipment in the molten steel processed by the vacuum degassing equipment immediately before the vacuum degassing refining treatment of the target charge 5. The method for producing a cast slab according to claim 4 , wherein the aluminum content at the end of treatment is 0.050% by mass or less. 前記転炉又は前記電気炉から出鋼される直前の前記対象チャージの溶鋼中のＴｉ含有量は分析限界以下となるゼロであり、
前記対象チャージの出鋼後から前記真空脱ガス精錬処理終了までの期間において、溶鋼中のチタン含有量の増加量が下記の（１）式を満たす、請求項１～５のいずれか１項に記載の鋳片の製造方法。
Δ［Ｔｉ］＝［Ｔｉ］LD＋Σ（Ｗt×ηt／１００）＋Δ［Ｔｉ］M≦Ｔ1×１０ ・・・（１）
但し、（１）式において、
Δ［Ｔｉ］：出鋼後から真空脱ガス精錬処理終了までの期間における溶鋼中チタン含有量の増加量（ｋｇ－Ｔｉ／ｔ－ｓｔｅｅｌ）
［Ｔｉ］LD：出鋼後の溶鋼中のチタン含有量（ｋｇ－Ｔｉ／ｔ－ｓｔｅｅｌ）
Ｗt：対象チャージにおいて、出鋼から真空脱ガス精錬処理終了までに添加する合金の添加量（ｋｇ－ａｌｌｏｙ／ｔ－ｓｔｅｅｌ）
ηt：対象チャージにおいて、出鋼から真空脱ガス精錬処理終了までに添加する合金のチタン含有量（質量％）
Δ［Ｔｉ］Ｍ：合金以外から混入した溶鋼中チタン含有量の増加量（ｋｇ－Ｔｉ／ｔ－ｓｔｅｅｌ）
Ｔ1：溶製対象の鋼のチタン濃度規格上限値（質量％）
である。 The Ti content in the molten steel of the target charge immediately before being tapped from the converter or the electric furnace is zero, which is below the analysis limit,
6. The method according to any one of claims 1 to 5 , wherein the amount of increase in the titanium content in the molten steel satisfies the following formula (1) during the period from the tapping of the target charge to the end of the vacuum degassing refining process. A method for producing the described slab.
Δ[Ti]=[Ti] LD +Σ( Wt × ηt /100)+Δ[Ti] M ≦ T1 ×10 (1)
However, in formula (1),
Δ [Ti]: Increase in titanium content in molten steel during the period from tapping to completion of vacuum degassing refining (kg-Ti/t-steel)
[Ti] LD : Titanium content in molten steel after tapping (kg-Ti/t-steel)
W t : Addition amount of alloy added from tapping to completion of vacuum degassing refining treatment in target charge (kg-alloy/t-steel)
η t : Titanium content (% by mass) of the alloy added from tapping to completion of vacuum degassing refining treatment in the target charge
Δ [Ti] M: Increase in titanium content in molten steel mixed from other than alloy (kg-Ti/t-steel)
T 1 : Standard upper limit of titanium concentration of steel to be smelted (% by mass)
is. 連続鋳造される前記精錬後溶鋼中のチタン含有量が０．００２０質量％以下である、請求項１～６のいずれか１項に記載の鋳片の製造方法。
The method for producing a slab according to any one of claims 1 to 6 , wherein the content of titanium in the molten steel after refining which is continuously cast is 0.0020% by mass or less.
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JP2008303406A (en) 2007-06-05 2008-12-18 Kobe Steel Ltd Method for manufacturing high-cleanliness steel
JP2009197285A (en) 2008-02-22 2009-09-03 Nippon Steel Corp Method for producing high clean steel slab
CN107312906A (en) 2017-07-10 2017-11-03 西王金属科技有限公司 A kind of smelting process of inexpensive ultrapure low titanium bearing steel

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JP2008303406A (en) 2007-06-05 2008-12-18 Kobe Steel Ltd Method for manufacturing high-cleanliness steel
JP2009197285A (en) 2008-02-22 2009-09-03 Nippon Steel Corp Method for producing high clean steel slab
CN107312906A (en) 2017-07-10 2017-11-03 西王金属科技有限公司 A kind of smelting process of inexpensive ultrapure low titanium bearing steel

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