JP2005139492A - Reforming agent for inclusion in molten steel, and method for producing low carbon steel cast slab - Google Patents
Reforming agent for inclusion in molten steel, and method for producing low carbon steel cast slab Download PDFInfo
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本発明は、加工性、成形性に優れ、表面疵も発生し難い低炭素鋼鋳片の製造方法と溶鋼中介在物の改質剤に関するものである。 The present invention relates to a method for producing a low carbon steel slab that is excellent in workability and formability and hardly generates surface flaws, and a modifier for inclusions in molten steel.
転炉や真空処理容器で精錬された溶鋼中には、多量の溶存酸素が含まれており、この過剰酸素は酸素との親和力が強い強脱酸元素であるＡｌにより脱酸されるのが一般的である。しかし、Ａｌは脱酸によりアルミナ系介在物を生成し、これが凝集合体して数１００μｍ以上の粗大なアルミナクラスターとなる。このアルミナクラスターは鋼板製造時に表面疵発生の原因となり、薄鋼板の品質を大きく劣化させる。特に、炭素濃度が低く、精錬後の溶存酸素濃度が高い薄鋼板用素材である低炭素溶鋼では、アルミナクラスターの量が非常に多く、表面疵の発生率が極めて高く、アルミナ系介在物の低減対策は大きな課題となっている。 The molten steel refined in a converter or vacuum processing vessel contains a large amount of dissolved oxygen, and this excess oxygen is generally deoxidized by Al, which is a strong deoxidizing element with a strong affinity for oxygen. Is. However, Al produces alumina inclusions by deoxidation, which aggregate and coalesce into coarse alumina clusters of several hundreds of μm or more. This alumina cluster causes surface flaws during the production of the steel sheet and greatly deteriorates the quality of the thin steel sheet. In particular, low carbon molten steel, which is a material for thin steel sheets with a low carbon concentration and a high dissolved oxygen concentration after refining, has a very high amount of alumina clusters, a very high rate of surface defects, and a reduction in alumina inclusions. Countermeasures are a major issue.
これに対して、従来は（特許文献１）の様に介在物吸着用フラックスを溶鋼表面に添加してアルミナ系介在物を除去する方法、或いは（特許文献２）の様に注入流を利用してＣａＯフラックスを溶鋼中に添加し、これによりアルミナ系介在物を吸着除去する方法が提案、実施されてきた。一方、アルミナ系介在物を除去するのではなく、生成させない方法として、（特許文献３）の様に溶鋼をＭｇで脱酸し、Ａｌでは殆ど脱酸しない薄鋼板用溶鋼の溶製方法や、（特許文献４）の様にＡｌを添加せずにＴｉで脱酸した薄鋼板も開示されている。
しかしながら、上述の（特許文献１）及び（特許文献２）に開示されている様な、アルミ系介在物を除去する方法では、低炭素溶鋼中に多量に生成したアルミナ系介在物を表面疵が発生しない程度まで低減することは非常に難しい。
また、（特許文献３）に開示されている様な、アルミナ系介在物を全く生成しないＭｇ脱酸では、Ｍｇの蒸気圧が高く、溶鋼への歩留まりが非常に低いため、低炭素鋼のように溶存酸素濃度が高い溶鋼をＭｇだけで脱酸するには多量のＭｇを必要とし、製造コストを考えると実用的なプロセスとは言えない。
さらに、（特許文献４）に開示されている様な、Ｔｉで脱酸する場合には、溶鋼中に生成した固相状態のＴｉ酸化物が鍋ノズルやタンディッシュノズルの内面に地金と共に付着堆積し、通常のＡｌ脱酸よりもかえってノズル詰まりを助長するといった問題があった。
However, in the method of removing aluminum inclusions as disclosed in the above-mentioned (Patent Document 1) and (Patent Document 2), the surface flaws cause alumina inclusions produced in a large amount in low-carbon molten steel. It is very difficult to reduce it to such an extent that it does not occur.
In addition, Mg deoxidation that does not generate any alumina inclusions as disclosed in (Patent Document 3) has a high vapor pressure of Mg and a very low yield to molten steel. In order to deoxidize molten steel having a high dissolved oxygen concentration with only Mg, a large amount of Mg is required.
Furthermore, in the case of deoxidizing with Ti as disclosed in (Patent Document 4), the solid state Ti oxide generated in the molten steel adheres to the inner surface of the pan nozzle or tundish nozzle together with the metal. There was a problem that it deposited and promoted nozzle clogging rather than normal Al deoxidation.
これらの問題を鑑み、本発明は溶鋼中介在物の凝集合体とノズルへの付着を防止し、鋼板中に介在物を微細分散させることにより、確実に表面疵を防止できる、溶鋼中介在物の改質剤および低炭素鋼鋳片の製造方法を提供することを目的とする。 In view of these problems, the present invention prevents agglomeration and inclusion of inclusions in the molten steel and adhesion to the nozzle, and finely disperses the inclusions in the steel sheet, thereby reliably preventing surface flaws. It aims at providing the manufacturing method of a modifier and a low carbon steel slab.
上記課題を解決するために、本発明は以下の構成を要旨とする。即ち、
（１）少なくともＬａ、Ｎｄが５０〜１００質量％、Ｃｅが０〜１０質量％、Ｆｅが０〜５０質量％で、Ｓｉが０〜４質量％、残部が不可避的不純物元素からなる組成であることを特徴とする溶鋼中介在物の改質剤。
（２）脱炭処理後の溶鋼にＴｉを添加して脱酸し、その後、(１)記載の溶鋼中介在物の改質剤を添加した溶鋼を鋳造することを特徴とする低炭素鋼鋳片の製造方法。
（３）溶鋼の炭素濃度を０．０１質量％以下まで脱炭した後、該溶鋼にＡｌを添加して予備脱酸処理を行い、溶鋼中の溶存酸素濃度を０．０１質量％以上０．０４質量％以下とし、次いでＴｉを添加して脱酸し、その後、(１)記載の溶鋼中介在物の改質剤を添加した溶鋼を鋳造することを特徴とする低炭素鋼鋳片の製造方法。
（４）溶鋼の炭素濃度を０．０１質量％以下まで脱炭した後、該溶鋼にＡｌを添加し３分以上攪拌して予備脱酸処理を行い、溶鋼中の溶存酸素濃度を０．０１質量％以上０．０４質量％以下とし、次いでＴｉを添加して溶鋼中のＴｉ濃度を０．００３質量％以上０．４質量％以下とし、その後、(１)記載の溶鋼中介在物の改質剤を添加し、少なくともＬａ、Ｎｄの溶鋼中濃度を０．００１質量％以上０．０３質量％以下とした溶鋼を鋳造することを特徴とする低炭素鋼鋳片の製造方法。
（５）溶鋼の炭素濃度を０．０１質量％以下まで脱炭するに際し、真空脱ガス装置を用いることを特徴とする（２）〜（４）のいずれか１項に記載の低炭素鋼鋳片の製造方法。
（６）溶鋼を鋳造するに際し、電磁攪拌機能を有する鋳型で鋳造することを特徴とする（２）〜（５）のいずれか１項に記載の低炭素鋼鋳片の製造方法。
（７）溶鋼を鋳造するに際し、１３００℃における粘性が４ｐｏｉｓｅ以上のモールドフラックスを用いて鋳造することを特徴とする（２）〜（６）のいずれか１項に記載の低炭素鋼鋳片の製造方法。
（８）溶鋼を鋳造するに際し、連続鋳造することを特徴とする（２）〜（７）のいずれか１項に記載の低炭素鋼鋳片の製造方法。
In order to solve the above problems, the present invention is summarized as follows. That is,
(1) At least La, Nd is 50 to 100% by mass, Ce is 0 to 10% by mass, Fe is 0 to 50% by mass, Si is 0 to 4% by mass, and the balance is an inevitable impurity element. A modifier for inclusions in molten steel.
(2) Low carbon steel casting characterized by adding Ti to decarburized molten steel to deoxidize, and then casting molten steel to which the modifier for inclusions in molten steel is added (1) A manufacturing method of a piece.
(3) After decarburizing the molten steel to a carbon concentration of 0.01% by mass or less, Al is added to the molten steel to perform a preliminary deoxidation treatment. The production of a low carbon steel slab characterized by casting the molten steel to which the content of the inclusions in the molten steel described in (1) is added. Method.
(4) After decarburizing the carbon concentration of the molten steel to 0.01% by mass or less, Al is added to the molten steel and stirred for 3 minutes or more to perform a preliminary deoxidation treatment, so that the dissolved oxygen concentration in the molten steel is 0.01 The Ti concentration in the molten steel is made 0.003 mass% to 0.4 mass% by adding Ti, and then Ti is added, and then the inclusion in the molten steel described in (1) is modified. A method for producing a low-carbon steel slab, comprising adding a quality material and casting a molten steel having a concentration of at least La and Nd in the molten steel of 0.001 to 0.03% by mass.
(5) The low carbon steel casting according to any one of (2) to (4), wherein a vacuum degassing device is used when the carbon concentration of the molten steel is decarburized to 0.01% by mass or less. A manufacturing method of a piece.
(6) The method for producing a low-carbon steel slab according to any one of (2) to (5), wherein the molten steel is cast with a mold having an electromagnetic stirring function.
(7) When casting the molten steel, the low carbon steel slab according to any one of (2) to (6), wherein the molten steel is cast using a mold flux having a viscosity at 1300 ° C. of 4 poise or more. Production method.
(8) The method for producing a low-carbon steel slab according to any one of (2) to (7), wherein the molten steel is continuously cast when cast.
本発明によると、溶鋼中の介在物を微細分散させ、ノズル閉塞をも抑制することができるため、確実に表面疵を防止できる加工性、成形性に優れた低炭素鋼鋳片を製造することが可能となる。 According to the present invention, since inclusions in molten steel can be finely dispersed and nozzle clogging can be suppressed, a low carbon steel slab excellent in workability and formability that can reliably prevent surface flaws is produced. Is possible.
以下に本発明を詳細に説明する。
転炉や真空処理容器で脱炭処理された溶鋼中には、多量の溶存酸素が含まれており、この溶存酸素は通常Ａｌの添加により殆ど脱酸される（（１）式の反応）ため、多量のアルミナ系介在物を生成する。
２Ａｌ＋３Ｏ＝Ａｌ２Ｏ３ （１）
これらの介在物は脱酸直後からお互いに凝集合体し、数１００μｍ以上の粗大なアルミナクラスターとなり、鋼板製造時に表面欠陥の原因となる。
The present invention is described in detail below.
The molten steel decarburized in a converter or vacuum processing vessel contains a large amount of dissolved oxygen, and this dissolved oxygen is usually almost deoxidized by the addition of Al (reaction (1)). A large amount of alumina inclusions are produced.
2Al + 3O = Al 2 O 3 (1)
These inclusions aggregate and coalesce with each other immediately after deoxidation to form coarse alumina clusters of several hundred μm or more, which cause surface defects during the production of the steel sheet.
そこで、アルミナクラスターを生成させないために、脱炭処理後の溶存酸素をＡｌ以外の脱酸剤で脱酸することに着目した。本発明方法として、転炉や電気炉等の製鋼炉で精錬して、或いはさらに真空脱ガス処理等を行った溶鋼にＴｉを添加して脱酸し、その後少なくともＬａ、Ｎｄが５０〜１００質量％、Ｃｅが０〜１０質量％、Ｆｅが０〜５０質量％で、Ｓｉが０〜４質量％、残部が不可避的不純物元素からなる改質剤を添加した溶鋼を鋳造する方法を考案した。
ここで、少なくともＬａ、Ｎｄというのは、Ｌａ、Ｎｄの１種以上という意味である。以降も同様の意味で用いている。
Therefore, in order not to generate alumina clusters, attention was focused on deoxidizing dissolved oxygen after decarburization with a deoxidizer other than Al. As a method of the present invention, Ti is added to a molten steel that has been refined in a steelmaking furnace such as a converter or an electric furnace, or further subjected to vacuum degassing treatment, and then deoxidized, and then at least La and Nd are 50 to 100 masses. %, Ce is 0 to 10% by mass, Fe is 0 to 50% by mass, Si is 0 to 4% by mass, and the balance is devised to cast a molten steel to which an inevitable impurity element is added.
Here, at least La and Nd mean one or more of La and Nd. Hereinafter, the same meaning is used.
本発明者らは、溶鋼へ添加する脱酸剤として、ＡｌまたはＴｉや、これに少なくともＬａ、Ｃｅ、Ｎｄを添加したものを適宜組み合わせて、これらの介在物の凝集挙動を実験的に評価したところ、アルミナ系介在物、Ｔｉ酸化物系介在物、アルミナ系介在物とＴｉ酸化物系介在物からなる複合介在物、或いはアルミナと少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物（例えば、アルミナ−Ｌａ酸化物系複合介在物、アルミナ−Ｃｅ酸化物−Ｎｄ酸化物系複合介在物、アルミナ−Ｌａ酸化物−Ｃｅ酸化物−Ｎｄ酸化物系複合介在物 等）は比較的容易に凝集合体し、るつぼ壁にも付着し易いのに対し、Ｔｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物（例えば、Ｔｉ酸化物−Ｌａ酸化物系複合介在物、Ｔｉ酸化物−Ｃｅ酸化物−Ｎｄ酸化物系複合介在物、Ｔｉ酸化物−Ｌａ酸化物−Ｃｅ酸化物−Ｎｄ酸化物系複合介在物 等）は凝集合体し難く、且つるつぼ壁にも付着せず、溶鋼中に微細分散することを見いだした。この理由は、アルミナ系介在物、Ｔｉ酸化物系介在物、アルミナ系介在物とＴｉ酸化物系介在物からなる複合介在物、或いはアルミナと少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物に比べて、Ｔｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物の方が、介在物と溶鋼間の界面エネルギーの低下が大きく、介在物同士の凝集合体と耐火物への付着を抑制したためである。 The present inventors experimentally evaluated the agglomeration behavior of these inclusions by appropriately combining Al or Ti with at least La, Ce, or Nd added thereto as a deoxidizer to be added to molten steel. However, alumina inclusions, Ti oxide inclusions, composite inclusions comprising alumina inclusions and Ti oxide inclusions, or composite inclusions comprising alumina and at least La oxide, Ce oxide, and Nd oxide. (For example, alumina-La oxide composite inclusions, alumina-Ce oxide-Nd oxide composite inclusions, alumina-La oxide-Ce oxide-Nd oxide composite inclusions, etc.) are relatively It easily aggregates and coalesces, and easily adheres to the crucible wall. On the other hand, a composite inclusion consisting of Ti oxide and at least La oxide, Ce oxide, Nd oxide (for example, Ti oxide-La oxide system) Combined inclusions, Ti oxide-Ce oxide-Nd oxide composite inclusions, Ti oxide-La oxide-Ce oxide-Nd oxide composite inclusions, etc.) It was found that it was finely dispersed in the molten steel. This is because alumina inclusions, Ti oxide inclusions, composite inclusions consisting of alumina inclusions and Ti oxide inclusions, or alumina and at least La oxide, Ce oxide, Nd oxide. Compared to composite inclusions, composite inclusions composed of Ti oxide and at least La oxide, Ce oxide, and Nd oxide have a greater decrease in interfacial energy between inclusions and molten steel, and the inclusions are aggregated. This is because adhesion to coalescence and refractory was suppressed.
これらの知見を基に、溶鋼中の溶存酸素をＴｉで脱酸し、さらに少なくともＬａ、Ｃｅ、Ｎｄを添加することによりＴｉ酸化物系介在物をＴｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物に改質し、溶鋼中に介在物を微細に分散させることに成功した。
しかし、実機設備を用いて少なくともＬａ、Ｃｅ、Ｎｄからなる合金を溶鋼中に添加する試験を実施したところ、これらの合金をベルトコンベヤーでホッパーまで輸送する際、ホッパーに挿入する際、或いは溶鋼に添加する際に、これらの合金同士が衝突することによる発火が起こり易いことが判った。そこで、少なくともＬａ、Ｃｅ、Ｎｄの金属に、他の金属を含有させた介在物改質剤（以降、改質剤と記載する。）を種々作製し、ＪＩＳ−Ｇ−０５６６の火花試験法を用いて発火を抑制する成分を検討した。
Based on these findings, the dissolved oxygen in the molten steel is deoxidized with Ti, and at least La, Ce, and Nd are added to form Ti oxide inclusions as Ti oxide, at least La oxide, and Ce oxide. The composite inclusions made of Nd oxide were modified to successfully disperse the inclusions in the molten steel.
However, when a test was performed to add an alloy composed of at least La, Ce, and Nd into molten steel using actual equipment, these alloys were transported to the hopper by a belt conveyor, inserted into the hopper, or into the molten steel. It was found that when these alloys were added, ignition was likely to occur due to collision of these alloys. Therefore, various kinds of inclusion modifiers (hereinafter referred to as modifiers) containing at least La, Ce, and Nd metals with other metals were prepared, and the spark test method of JIS-G-0666 was conducted. Ingredients to suppress ignition were investigated.
本発明者らは、改質剤中のＣｅを１０質量％以下にすると火花の発生がより起こりにくくなること、ＬａやＮｄは含有率１００質量％でも殆ど発火しないことから、改質剤中のＣｅ含有率を低下させるほど、発火をより起こりにくくできる可能性があることを見いだした。よって、改質剤中のＣｅ含有率を１０質量％以下とする。また、改質剤中のＣｅ含有率の下限値は０質量％を含む。 The inventors of the present invention have made it difficult for sparks to occur when Ce in the modifier is 10% by mass or less, and La and Nd hardly ignite even at a content of 100% by mass. It has been found that there is a possibility that the lower the Ce content, the more difficult ignition can occur. Therefore, the Ce content in the modifier is 10% by mass or less. Moreover, the lower limit of Ce content rate in a modifier contains 0 mass%.
また、改質剤中の、少なくともＬａ、Ｎｄの含有率は５０〜１００質量％とする。これは、少なくともＬａ、Ｎｄの含有率が５０質量％未満では介在物改質剤の効果の低下が大きいためである。反対に少なくともＬａ、Ｎｄの含有率が増加するほど、改質効果は向上するため、少なくともＬａ、Ｎｄの含有率の上限は１００質量％とする。
さらに、介在物改質剤中のＦｅ含有率は０〜５０質量％とする。これは、改質剤中の、少なくともＬａ、Ｎｄの含有率が５０〜１００質量％であることにより、これに対応して決定される範囲である。
The content of at least La and Nd in the modifier is 50 to 100% by mass. This is because at least the contents of La and Nd are less than 50% by mass and the effect of the inclusion modifier is greatly reduced. On the other hand, as the content of at least La and Nd increases, the reforming effect improves, so the upper limit of the content of at least La and Nd is 100% by mass.
Furthermore, the Fe content in the inclusion modifier is 0 to 50% by mass. This is a range determined corresponding to the content of at least La and Nd in the modifier being 50 to 100% by mass.
介在物改質剤中の金属Ｓｉは発火を抑制する効果があるが、４質量％を超えて添加すると、改質剤を添加した後に溶鋼中のＳｉ濃度が増加し、材質が低下する。このため、改質剤中の金属Ｓｉ含有率は４質量％以下にする必要がある。勿論、金属Ｓｉ含有率が０質量％であっても、Ｃｅ含有率が１０質量％以下であれば、上述の通り、火花の発生はより起こりにくくすることができる。 Metal Si in the inclusion modifier has an effect of suppressing ignition, but if added in excess of 4 mass%, the Si concentration in the molten steel increases after the modifier is added, and the material deteriorates. For this reason, the metal Si content in the modifier must be 4% by mass or less. Of course, even if the metal Si content is 0% by mass, as long as the Ce content is 10% by mass or less, the occurrence of sparks can be made less likely as described above.
上記成分の介在物改質剤は、発火現象を抑制して、または全く発火することなく、溶鋼中に添加することが可能である。合わせて、改質剤中の少なくともＬａ、Ｎｄにより溶鋼中の介在物は十分に改質される。
従って、転炉や真空処理容器等で脱炭処理された溶鋼中には、多量の溶存酸素が含まれているものの、脱炭処理後の溶存酸素をＴｉで脱酸し、その後上記改質剤を添加することにより、製品鋼材の材質低下なく、溶鋼中に介在物を微細に分散させ、ノズル付着をも解消できる結果、確実に表面疵を防止できる。
The inclusion modifier of the above components can be added to molten steel while suppressing the ignition phenomenon or without igniting at all. In addition, the inclusions in the molten steel are sufficiently modified by at least La and Nd in the modifier.
Therefore, although the molten steel decarburized in a converter, vacuum processing vessel, etc. contains a large amount of dissolved oxygen, the dissolved oxygen after decarburization is deoxidized with Ti, and then the modifier As a result, inclusions can be finely dispersed in the molten steel without causing deterioration in the quality of the product steel material, and nozzle adhesion can be eliminated. As a result, surface flaws can be reliably prevented.
本発明方法の別の形態として、転炉や電気炉等の製鋼炉で精錬して、或いはさらに真空脱ガス処理等して、炭素濃度を０．０１質量％以下まで脱炭した溶鋼にＡｌを添加して予備脱酸処理を行い、溶鋼中の溶存酸素濃度を０．０１質量％以上０．０４質量％以下とし、次いでＴｉを添加して脱酸し、それから少なくともＬａ、Ｎｄが５０〜１００質量％、Ｃｅが０〜１０質量％、Ｆｅが０〜５０質量％で、Ｓｉが０〜４質量％、残部が不可避的不純物元素からなる改質剤を添加した溶鋼を鋳造する方法を考案した。 As another form of the method of the present invention, Al is added to molten steel that has been decarburized to a carbon concentration of 0.01% by mass or less by refining in a steelmaking furnace such as a converter or an electric furnace, or further by vacuum degassing. Addition to perform preliminary deoxidation treatment, the dissolved oxygen concentration in the molten steel is 0.01 mass% or more and 0.04 mass% or less, then Ti is added to deoxidize, and at least La and Nd are 50 to 100 We devised a method for casting molten steel to which a modifier containing a mass%, Ce of 0 to 10 mass%, Fe of 0 to 50 mass%, Si of 0 to 4 mass%, and the balance consisting of inevitable impurity elements is added. .
ここで、脱炭した溶鋼の炭素濃度については、特に、薄板用鋼板は、自動車用外板等の加工が厳しい用途に用いられるため、加工性を付加する必要性がある場合は、Ｃ濃度を０．０１質量％以下にすることが好ましい。Ｃ濃度の下限値は特に規定するものではない。
また、この形態では、製造コストの面からより実用的なプロセスを考え、脱炭処理後の溶存酸素を全部Ｔｉで脱酸するのではなく、溶存酸素を残すようにＡｌを添加して予備脱酸を行い、害にならない程度までアルミナ系介在物量を短時間で浮上除去し、その後改めてＡｌ以外の元素を用いて脱酸することを考案し、品質向上と製造コスト低減を両立させるものである。
Here, as for the carbon concentration of the decarburized molten steel, the steel plate for thin plates is used for applications where processing of the outer plate for automobiles and the like is particularly difficult. It is preferable to make it 0.01% by mass or less. The lower limit value of the C concentration is not particularly specified.
In addition, in this embodiment, a more practical process is considered from the viewpoint of manufacturing cost, and not all deoxygenated oxygen after decarburization is deoxidized with Ti, but Al is added so as to leave dissolved oxygen, and preliminary desorption is performed. In order to achieve both improvement in quality and reduction in production cost, an acid is devised, and the amount of alumina inclusions is lifted and removed in a short time until it does not cause harm, and then deoxidation is performed again using an element other than Al. .
上述したように、本発明者らは、Ｔｉ酸化物と少なくともＬａ酸化物、Ｎｄ酸化物からなる複合介在物は凝集合体し難く、るつぼ壁にも付着せず、溶鋼中に微細分散することを明らかにした。これらの知見を基に、脱炭処理後の溶存酸素をＴｉだけで脱酸するのではなく、溶存酸素の一部をまずＡｌで予備脱酸し、害にならない程度までアルミナ系介在物を短時間で攪拌等により浮上除去した後、改めて残った溶存酸素をＴｉで脱酸し、さらに前述した介在物改質剤を添加することにより、アルミナ系介在物を含まないＴｉ酸化物と少なくともＬａ酸化物、Ｎｄ酸化物からなる複合介在物を生成させ、溶鋼中に介在物を微細分散させることに成功した。このことで、溶鋼中介在物の凝集合体を防止し、鋼板中に介在物を微細分散させることにより、確実に表面疵を防止できる。ここで、上記記載のＡｌ予備脱酸後の害にならない程度のアルミナ系介在物濃度は、鋼板の表面疵を防止できれば特に規定するものではないが、通常は例えば全Ａｌ濃度で高々５０ｐｐｍ程度以下である。 As described above, the present inventors have found that composite inclusions composed of Ti oxide and at least La oxide and Nd oxide are difficult to aggregate and coalesce, do not adhere to the crucible wall, and are finely dispersed in the molten steel. Revealed. Based on these findings, rather than deoxidizing the dissolved oxygen after decarburization with Ti alone, a part of the dissolved oxygen is first pre-deoxidized with Al, and the alumina inclusions are shortened to the point where they do not harm. After floating and removing by stirring, etc. over time, the remaining dissolved oxygen is again deoxidized with Ti, and by adding the inclusion modifier described above, Ti oxide not containing alumina inclusions and at least La oxidation The composite inclusions composed of the Nd oxide and the Nd oxide were produced, and the inclusions were successfully finely dispersed in the molten steel. Thus, surface flaws can be reliably prevented by preventing aggregation and inclusion of inclusions in the molten steel and finely dispersing the inclusions in the steel sheet. Here, the concentration of alumina inclusions that does not cause harm after the Al preliminary deoxidation described above is not particularly specified as long as the surface flaws of the steel sheet can be prevented, but usually, for example, at most about 50 ppm or less at the total Al concentration. It is.
ＬａとＮｄはＴｉに比べて非常に脱酸能が高いため、Ｔｉ添加後に生成したＴｉ酸化物系介在物を少量のＬａもしくはＮｄで還元し、Ｔｉ酸化物と少なくともＬａ酸化物、Ｎｄ酸化物からなる複合介在物に改質することは容易である。しかし、Ａｌ予備脱酸後の溶存酸素が０．０４質量％を超えると、Ｔｉ添加後に多量のＴｉ酸化物系介在物が生成するため、少なくともＬａ、Ｎｄを添加しても一部未改質のＴｉ酸化物系介在物が残留し、粗大なチタニアクラスターとなりやすい。一方、Ａｌ添加量を増大させ予備脱酸後の溶存酸素濃度を低下させると、多量のアルミナ系介在物を生成するため、これを分離除去することが困難になる。そこで、粗大化し易いアルミナ系介在物をできるだけ低減する条件から、Ａｌ脱酸後の溶存酸素濃度は０．０１質量％以上にすることが好ましい。したがって、本発明では、Ａｌ予備脱酸後の溶存酸素濃度を０．０１質量％以上０．０４質量％以下の範囲に制御することが好ましい。 Since La and Nd have a very high deoxidation capacity compared to Ti, Ti oxide inclusions generated after addition of Ti are reduced with a small amount of La or Nd, and Ti oxide and at least La oxide and Nd oxide are reduced. It is easy to modify the composite inclusion consisting of However, if the dissolved oxygen after Al pre-deoxidation exceeds 0.04% by mass, a large amount of Ti oxide inclusions are formed after Ti addition, so even if at least La and Nd are added, it is partially unmodified Ti oxide-based inclusions remain and tend to be coarse titania clusters. On the other hand, if the amount of Al added is increased and the dissolved oxygen concentration after the preliminary deoxidation is lowered, a large amount of alumina inclusions are produced, which makes it difficult to separate and remove them. Therefore, it is preferable to set the dissolved oxygen concentration after Al deoxidation to 0.01% by mass or more from the condition of reducing alumina inclusions that are likely to be coarsened as much as possible. Therefore, in this invention, it is preferable to control the dissolved oxygen concentration after Al preliminary deoxidation to the range of 0.01 mass% or more and 0.04 mass% or less.
さらに、本発明方法の詳細な形態として、転炉や電気炉等の製鋼炉で精錬して、或いはさらに真空脱ガス処理等して、炭素濃度を０．０１質量％以下とした溶鋼にＡｌを添加し３分以上攪拌して予備脱酸処理を行い、溶鋼中の溶存酸素濃度を０．０１質量％以上０．０４質量％以下とし、次いでＴｉを添加して溶鋼中のＴｉ濃度を０．００３質量％以上０．４質量％以下とし、それから少なくともＬａ、Ｎｄが５０〜１００質量％、Ｃｅが０〜１０質量％、Ｆｅが０〜５０質量％で、Ｓｉが０〜４質量％、残部が不可避的不純物元素からなる改質剤を添加した溶鋼を鋳造する方法を考案した。 Furthermore, as a detailed form of the method of the present invention, Al is added to molten steel having a carbon concentration of 0.01% by mass or less by refining in a steelmaking furnace such as a converter or an electric furnace, or further by vacuum degassing. Add and stir for 3 minutes or longer to perform a pre-deoxidation treatment, so that the dissolved oxygen concentration in the molten steel is 0.01 mass% or more and 0.04 mass% or less, and then Ti is added to bring the Ti concentration in the molten steel to 0.0. 003 mass% to 0.4 mass%, and at least La and Nd are 50 to 100 mass%, Ce is 0 to 10 mass%, Fe is 0 to 50 mass%, Si is 0 to 4 mass%, and the balance Has devised a method for casting molten steel to which modifiers consisting of inevitable impurity elements are added.
実験的な検討では、予備脱酸におけるＡｌ添加後の溶存酸素濃度を０．０１質量％以上とし、且つＡｌ添加後の攪拌時間を３分以上確保することで、殆どのアルミナ系介在物を浮上除去できるため、攪拌時間を３分以上とすることが好ましい。特に、真空脱ガス装置を用いた場合は、Ａｌ添加後の攪拌方法として還流することが一般的であり、ここでの還流時間を３分以上とすることが好ましい。Ａｌ予備脱酸後の溶存酸素濃度の範囲については、前述と同様である。 In experimental studies, the dissolved oxygen concentration after Al addition in the preliminary deoxidation is 0.01% by mass or more, and the stirring time after Al addition is secured for 3 minutes or more, so that most alumina inclusions emerge. Since it can be removed, the stirring time is preferably 3 minutes or more. In particular, when a vacuum degassing apparatus is used, reflux is generally performed as a stirring method after addition of Al, and the reflux time here is preferably set to 3 minutes or more. The range of dissolved oxygen concentration after Al preliminary deoxidation is the same as described above.
次に、Ａｌ予備脱酸後に、少量のＴｉを添加して脱酸すると、ＴｉはＡｌ等に比べて脱酸力が弱いため、一部溶存酸素が溶鋼中に残存する。Ｃ濃度が０．０１質量％以下の薄鋼板用の溶鋼では、溶存酸素濃度が０．０２質量％を超えると鋳造時にＣＯ気泡が発生し易いことから、溶鋼中のＴｉ濃度は、鋳造に供給する溶鋼中の溶存酸素濃度が０．０２質量％以下になるように添加することが好ましく、平衡計算からＴｉ濃度を算出すると０．００３質量％以上となる。一方、Ｔｉは脱酸力が比較的弱い方であるが、それでも溶鋼中に多量に添加すると、溶鋼中の溶存酸素濃度の低下が大きくなるため、その後に改質剤を添加してもＴｉ酸化物と、少なくともＬａ酸化物、Ｎｄ酸化物からなる複合介在物に改質しにくくなり、本発明の介在物微細化効果が損なわれ易くなる。このため、Ｔｉ濃度は数ｐｐｍ程度の溶存酸素を残せるように添加することが好ましく、平衡計算からＴｉ濃度を算出すると、０．４質量％以下となる。
以上から、Ｔｉ濃度は０．００３質量％以上０．４質量％以下にすることが望ましい。
Next, when a small amount of Ti is added and deoxidized after Al preliminary deoxidation, since Ti has a weaker deoxidizing power than Al or the like, some dissolved oxygen remains in the molten steel. In molten steel for thin steel sheets with a C concentration of 0.01 mass% or less, if dissolved oxygen concentration exceeds 0.02 mass%, CO bubbles are likely to be generated during casting, so the Ti concentration in the molten steel is supplied to the casting. It is preferable to add so that the dissolved oxygen concentration in the molten steel is 0.02% by mass or less, and when the Ti concentration is calculated from the equilibrium calculation, it becomes 0.003% by mass or more. On the other hand, Ti has a relatively weak deoxidizing power, but if it is still added in a large amount in molten steel, the decrease in dissolved oxygen concentration in the molten steel becomes large. And the inclusion inclusions composed of at least La oxide and Nd oxide are difficult to be modified, and the inclusion refinement effect of the present invention is likely to be impaired. For this reason, it is preferable to add the Ti concentration so as to leave about several ppm of dissolved oxygen. When the Ti concentration is calculated from the equilibrium calculation, it becomes 0.4 mass% or less.
From the above, it is desirable that the Ti concentration be 0.003 mass% or more and 0.4 mass% or less.
改質剤を添加することは、介在物の微細化に効果的であるが、改質剤中のＬａ、Ｎｄは非常に強い脱酸材であるため、耐火物やモールドフラックスと反応して、溶鋼を汚染させると共に、耐火物やモールドフラックスを劣化させる。このため、改質剤の添加量は、生成したＴｉ酸化物系介在物を改質するに必要な量以上であって、且つＬａ、Ｎｄが耐火物やモールドフラックスと反応して溶鋼を汚染させない量以下とすることが好ましい。
実験的検討では、少なくともＬａ、Ｎｄの溶鋼中濃度の適正範囲は、０．００１質量％以上０．０３質量％以下とすることが好ましい。
Adding a modifier is effective for making inclusions finer, but La and Nd in the modifier are very strong deoxidizers, so they react with refractories and mold flux, It contaminates molten steel and degrades refractories and mold flux. For this reason, the addition amount of the modifier is more than the amount necessary for modifying the generated Ti oxide inclusions, and La and Nd do not react with the refractory or mold flux to contaminate the molten steel. It is preferable to set it to the amount or less.
In the experimental study, it is preferable that the appropriate range of at least La and Nd concentrations in the molten steel is 0.001 mass% or more and 0.03 mass% or less.
また、改質剤の添加は、必ずしも真空脱ガス装置内で添加する必要はなく、Ｔｉ添加後から鋳型内に流入するまでの間で添加すれば良く、例えばタンディッシュ内で添加することも可能である。
最近では、連続鋳造機に鋳型内電磁攪拌装置が装備されるようになっており、鋳造時に溶鋼を電磁攪拌すれば、鋳片表層の凝固シェルへの介在物捕捉を抑制できる。本発明では、溶鋼中の介在物は微細に分散しており、鋳片に捕捉されても表面欠陥にはならないが、例えばモールドフラックス、耐火物、スラグ等の外来系介在物が混入した場合には、電磁攪拌を行うことに、これらの外来系介在物の捕捉を確実に防止できる。このため、本発明の効果を安定的に得られると言う観点で、電磁攪拌を実施することが好ましい。
In addition, it is not always necessary to add the modifier in the vacuum degassing apparatus, it may be added after the addition of Ti until it flows into the mold. For example, it can be added in the tundish. It is.
Recently, a continuous casting machine is equipped with an in-mold electromagnetic stirrer. When molten steel is electromagnetically stirred at the time of casting, inclusion trapping in the solidified shell of the slab surface layer can be suppressed. In the present invention, the inclusions in the molten steel are finely dispersed and do not cause surface defects even when trapped by the slab, but when foreign inclusions such as mold flux, refractory, slag, etc. are mixed. Can reliably prevent these foreign inclusions from being trapped by performing electromagnetic stirring. For this reason, it is preferable to implement electromagnetic stirring from the viewpoint that the effects of the present invention can be stably obtained.
さらに、本発明の溶鋼を鋳造する場合、鋳造時間の経過と共にＴｉ酸化物と少なくともＬａ酸化物、Ｎｄ酸化物からなる複合介在物がモールドフラックス中に吸収され、それと共に、モールドフラックスの粘性が低下する可能性がある。モールドフラックスの粘性低下は、フラックス巻き込みを助長し、モールドフラックス起因の欠陥を引き起こす原因となる。このため、本発明の溶鋼を鋳造する場合、介在物吸収による粘性低下を考慮して、モールドフラックスの粘性を予め高めに設計しておくことが有効である。実験によれば、１３００℃におけるモールドフラックスの粘性を４ｐｏｉｓｅ以上にしておけば、モールドフラックス起因の欠陥はより発生しにくくなるため好ましい。モールドフラックスはモールドと鋳型間の潤滑機能を有しており、その機能が損なわれない程度であれば、特に粘性の上限値を規定するものではない。 Furthermore, when the molten steel of the present invention is cast, the composite inclusion composed of Ti oxide, at least La oxide, and Nd oxide is absorbed into the mold flux as the casting time elapses, and the viscosity of the mold flux decreases with it. there's a possibility that. The decrease in the viscosity of the mold flux promotes flux entrainment and causes defects due to the mold flux. For this reason, when casting the molten steel according to the present invention, it is effective to design the viscosity of the mold flux to be high in advance in consideration of viscosity reduction due to inclusion absorption. According to experiments, it is preferable to set the viscosity of the mold flux at 1300 ° C. to 4 poise or more because defects due to the mold flux are less likely to occur. The mold flux has a lubrication function between the mold and the mold, and the upper limit of the viscosity is not particularly defined as long as the function is not impaired.
本発明は、インゴット鋳造および連続鋳造でも可能であるが、連続鋳造であれば通常の２５０ｍｍ厚み程度のスラブ連続鋳造に適用されるだけでなく、連続鋳造機の鋳型厚みがそれより薄い、例えば１５０ｍｍ以下の薄スラブ連続鋳造に対しても十分な効果が発現し、極めて表面疵の少ない鋳片を得ることができる。 The present invention can be applied to ingot casting and continuous casting. However, in the case of continuous casting, the present invention is not only applied to slab continuous casting with a thickness of about 250 mm, but the mold thickness of the continuous casting machine is thinner, for example, 150 mm. Sufficient effects are exhibited even for the following thin slab continuous casting, and a slab having very few surface defects can be obtained.
以下に、実施例及び比較例を挙げて、本発明について説明する。
［実施例１］
転炉での精錬と環流式真空脱ガス装置での処理により炭素濃度を０．００３質量％とした３００ｔの取鍋内溶鋼をＴｉで脱酸し、その後少なくともＬａ、Ｎｄが８０質量％、Ｃｅが０．５質量％、Ｆｅが１９質量％で、Ｓｉが０．５質量％からなる改質剤（残部に微量の不可避的不純物元素を含む）を添加して、溶鋼中のＴｉ濃度を０．０５質量％、Ｌａ、Ｎｄの合計濃度を０．００８質量％とした。この溶鋼を連続鋳造法で厚み２５０ｍｍ、幅１８００ｍｍのスラブに鋳造した。鋳造の際に使用したモールドフラックスの１３００℃における粘性は６ｐｏｉｓｅであった。
Hereinafter, the present invention will be described with reference to examples and comparative examples.
[Example 1]
300 t of molten steel in a ladle with a carbon concentration of 0.003 mass% was deoxidized with Ti by refining in a converter and treatment in a reflux-type vacuum degassing apparatus, and then at least La and Nd were 80 mass%, Ce. Is added 0.5 mass%, Fe is 19 mass%, Si is 0.5 mass% modifier (the balance contains a trace amount of inevitable impurity elements), the Ti concentration in the molten steel is 0 0.05 mass%, and the total concentration of La and Nd was 0.008 mass%. This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm by a continuous casting method. The viscosity at 1300 ° C. of the mold flux used for casting was 6 poise.
タンディッシュノズルや鍋ノズルの閉塞はなく、安定鋳造であった。鋳造した鋳片は８５００ｍｍ長さに切断し、１コイル単位とした。このようにして得られたスラブは、常法により熱間圧延、冷間圧延し、最終的には０．７ｍｍ厚みで幅１８００ｍｍコイルの冷延鋼板とした。鋳片品質については、冷間圧延後の検査ラインで目視観察を行い、１コイル当たりに発生する表面欠陥の発生個数を評価した。その結果、表面欠陥は発生しなかった。 There was no blockage of the tundish nozzle and pan nozzle, and the casting was stable. The cast slab was cut to a length of 8500 mm to make one coil unit. The slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm. Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, no surface defects occurred.
［実施例２］
転炉での精錬と真空脱ガス装置での処理により炭素濃度を０．００３質量％とした３００ｔの取鍋内溶鋼に予備脱酸Ａｌを１００ｋｇ添加して３分間環流させ、溶存酸素濃度０．０２質量％の溶鋼とした。さらに、この溶鋼にＴｉを添加して３分間環流し、その後少なくともＬａ、Ｎｄが７０質量％、Ｃｅが８質量％、Ｆｅが２０質量％、Ｓｉが２質量％からなる改質剤（残部に微量の不可避的不純物元素を含む）を添加し、Ｔｉ濃度を０．０３質量％、Ｌａ、Ｎｄの合計濃度を０．００７質量％にした溶鋼を溶製した。この溶鋼を連続鋳造法で厚み２５０ｍｍ、幅１８００ｍｍのスラブに鋳造した。鋳造の際に使用したモールドフラックスの１３００℃における粘性は６ｐｏｉｓｅであった。タンディッシュノズルや鍋ノズルの閉塞はなく、安定鋳造であった。鋳造した鋳片は８５００ｍｍ長さに切断し、１コイル単位とした。このようにして得られたスラブは、常法により熱間圧延、冷間圧延し、最終的には０．７ｍｍ厚みで幅１８００ｍｍコイルの冷延鋼板とした。鋼板品質については、冷間圧延後の検査ラインで目視観察を行い、１コイル当たりに発生する表面欠陥の発生個数を評価した。その結果、表面欠陥は発生しなかった。
[Example 2]
100 kg of pre-deoxidized Al was added to 300 t of molten steel in a ladle with a carbon concentration of 0.003 mass% by refining in a converter and treatment in a vacuum degassing apparatus, and the resulting mixture was refluxed for 3 minutes. The molten steel was 02 mass%. Further, Ti was added to the molten steel and refluxed for 3 minutes, and then a modifier comprising at least 70% by mass of La and Nd, 8% by mass of Ce, 20% by mass of Fe, and 2% by mass of Si (with the balance) A small amount of inevitable impurity elements was added), and molten steel having a Ti concentration of 0.03% by mass and a total concentration of La and Nd of 0.007% by mass was melted. This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm by a continuous casting method. The viscosity at 1300 ° C. of the mold flux used for casting was 6 poise. There was no blockage of the tundish nozzle and pan nozzle, and the casting was stable. The cast slab was cut to a length of 8500 mm to make one coil unit. The slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm. Regarding the steel sheet quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, no surface defects occurred.
［実施例３］
転炉での精錬と真空脱ガス装置での処理により炭素濃度を０．００５質量％とした３００ｔの取鍋内溶鋼に予備脱酸Ａｌを１５０ｋｇ添加して５分間環流させ、溶存酸素濃度０．０１２質量％の溶鋼とした。さらに、この溶鋼にＴｉを添加して４分間環流し、その後少なくともＬａ、Ｎｄが１００質量％、Ｃｅが０質量％、Ｆｅが０質量％、Ｓｉが０質量％からなる改質剤（残部に微量の不可避的不純物元素を含む）を添加し、Ｔｉ濃度を０．０４５質量％で、Ｌａ、Ｎｄの合計濃度を０．０１質量％にした溶鋼を溶製した。この溶鋼を電磁攪拌機能を有する鋳型を用いて、連続鋳造法で厚み７０ｍｍ、幅１８００ｍｍの薄スラブに鋳造した。鋳造の際に使用したモールドフラックスの１３００℃における粘性は１５ｐｏｉｓｅであった。タンディッシュノズルや鍋ノズルの閉塞はなく、安定鋳造であった。このようにして得られた薄スラブは、常法により熱間圧延、冷間圧延し、最終的には０．７ｍｍ厚みで幅１８００ｍｍコイルの冷延鋼板とした。鋼板品質については、冷間圧延後の検査ラインで目視観察を行い、１コイル当たりに発生する表面欠陥の発生個数を評価した。その結果、表面欠陥は発生しなかった。
[Example 3]
150 kg of pre-deoxidized Al was added to 300 t of molten steel in a ladle with a carbon concentration of 0.005 mass% by refining in a converter and treatment in a vacuum degassing apparatus, and the mixture was refluxed for 5 minutes. It was set to 012 mass% molten steel. Further, Ti was added to the molten steel and refluxed for 4 minutes, and then a modifier comprising at least La and Nd of 100% by mass, Ce of 0% by mass, Fe of 0% by mass, and Si of 0% by mass (the balance) A small amount of unavoidable impurity elements was added), and molten steel having a Ti concentration of 0.045% by mass and a total concentration of La and Nd of 0.01% by mass was melted. This molten steel was cast into a thin slab having a thickness of 70 mm and a width of 1800 mm by a continuous casting method using a mold having an electromagnetic stirring function. The viscosity at 1300 ° C. of the mold flux used during casting was 15 poise. There was no blockage of the tundish nozzle and pan nozzle, and the casting was stable. The thin slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm. Regarding the steel sheet quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, no surface defects occurred.
［比較例１］
転炉での精錬と環流式真空脱ガス装置での処理により炭素濃度を０．００３質量％とした取鍋内溶鋼をＡｌで脱酸し、Ａｌ濃度０．０４質量％、溶存酸素濃度０．０００２質量％とした。この溶鋼を連続鋳造法で厚み２５０ｍｍ、幅１８００ｍｍのスラブに鋳造した。鋳造後半で、タンディッシュノズルが閉塞傾向となり、湯面変動が発生した。鋳造した鋳片は８５００ｍｍ長さに切断し、１コイル単位とした。このようにして得られたスラブは、常法により熱間圧延、冷間圧延し、最終的には０．７ｍｍ厚みで幅１８００ｍｍコイルの冷延鋼板とした。鋳片品質については、冷間圧延後の検査ラインで目視観察を行い、１コイル当たりに発生する表面欠陥の発生個数を評価した。その結果、スラブ平均で５個／コイルの表面欠陥が発生した。
[Comparative Example 1]
The molten steel in the ladle having a carbon concentration of 0.003% by mass by refining in a converter and treatment in a reflux-type vacuum degassing apparatus is deoxidized with Al, and the Al concentration is 0.04% by mass and the dissolved oxygen concentration is 0.00. It was 0002 mass%. This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm by a continuous casting method. In the second half of casting, the tundish nozzle tended to close and the molten metal surface level changed. The cast slab was cut to a length of 8500 mm to make one coil unit. The slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm. Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, surface defects of 5 pieces / coil were generated on average on the slab.
［比較例２］
転炉での精錬と真空脱ガス装置での処理により炭素濃度を０．００３質量％とした取鍋内溶鋼をＴｉで脱酸し、Ｔｉ濃度０．０４質量％、溶存酸素濃度０．００４質量％とした。この溶鋼を連続鋳造法で厚み２５０ｍｍ、幅１８００ｍｍのスラブに鋳造した。鋳造後半で、鍋ノズルが閉塞し、５０ｔ程度の溶鋼を鍋ごと転炉に返送した。鋳造した鋳片は８５００ｍｍ長さに切断し、１コイル単位とした。このようにして得られたスラブは、常法により熱間圧延、冷間圧延し、最終的には０．７ｍｍ厚みで幅１８００ｍｍコイルの冷延鋼板とした。鋳片品質については、冷間圧延後の検査ラインで目視観察を行い、１コイル当たりに発生する表面欠陥の発生個数を評価した。その結果、スラブ平均で７個／コイルの表面欠陥が発生した。
[Comparative Example 2]
The molten steel in the ladle with a carbon concentration of 0.003% by mass by refining in a converter and processing in a vacuum degassing device is deoxidized with Ti, and the Ti concentration is 0.04% by mass and the dissolved oxygen concentration is 0.004% by mass. %. This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm by a continuous casting method. In the latter half of casting, the pan nozzle was closed, and about 50t of molten steel was returned to the converter together with the pan. The cast slab was cut to a length of 8500 mm to make one coil unit. The slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm. Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, surface defects of 7 pieces / coil were generated on an average slab.
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| WO2007049824A1 (en) | 2005-10-27 | 2007-05-03 | Nippon Steel Corporation | Method for manufacture of ultra-low carbon steel slab |
| JP2007177296A (en) * | 2005-12-28 | 2007-07-12 | Jfe Steel Kk | Method for deoxidizing molten steel |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| WO2007049824A1 (en) | 2005-10-27 | 2007-05-03 | Nippon Steel Corporation | Method for manufacture of ultra-low carbon steel slab |
| JP2007177296A (en) * | 2005-12-28 | 2007-07-12 | Jfe Steel Kk | Method for deoxidizing molten steel |
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