JP5391842B2 - Melting method of high clean steel - Google Patents
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Description
本発明は、高清浄鋼の溶製方法に関し、詳しくは、二次精錬工程で脱硫のために投入したフラックス粒子の残存を抑制した高清浄鋼の溶製方法に関するものである。 The present invention relates to a smelting method for high clean steel, and more particularly to a smelting method for high clean steel in which residual flux particles introduced for desulfurization in a secondary refining process are suppressed.
加工性の良好な高張力鋼では、鋼の不純物としてのＳ濃度を極力低くすることが望まれている。このために、鋼製造時の二次精錬工程(例えばRH設備)で溶鋼の脱硫が行なわれている。この脱硫法に関しては、例えば、特許文献１には、ＣａＯを含有し、溶鋼温度で固液２相となるようなフラックスの使用が記載されている。固液２相を得るための方法として、例えばＣａＦ２をＣａＯに添加することも記載されている。 In high-tensile steel with good workability, it is desired to reduce the S concentration as an impurity of the steel as much as possible. For this reason, molten steel is desulfurized in a secondary refining process (for example, RH equipment) during steel production. Regarding this desulfurization method, for example, Patent Document 1 describes the use of a flux containing CaO and having a solid-liquid two phase at the molten steel temperature. As a method for obtaining a solid-liquid two phase, for example, addition of CaF 2 to CaO is also described.
しかしながら、この方法では、脱硫後にも溶鋼中にＣａＯとＣａＦ２を含むフラックス粒子が残存し、この粒子のＣａＦ２が高い領域が鋼の圧延時に粒子の前後で伸びて、両側が伸びた針状の介在物となる恐れがある。この介在物が多量にあると、良加工性高張力鋼の加工性評価法の一種である穴拡げ試験での穴拡げ性が悪化する。このような現象は、ＣａＦ２でなくても低融点の酸化物を含むＣａＯ系フラックスであれば、生じる恐れがある。 However, in this method, the flux particles containing CaO and CaF 2 remain in the molten steel even after desulfurization, and the high CaF 2 region of the particles extends before and after the particles during the rolling of the steel, and both needles extend on both sides. There is a risk of inclusion. If there are a large amount of these inclusions, the hole expandability in a hole expansion test, which is a kind of workability evaluation method for good workability high-tensile steel, deteriorates. Such a phenomenon may occur if it is a CaO-based flux containing a low melting point oxide even if it is not CaF 2 .
穴拡げ性に優れた熱延鋼板の製造方法として、特許文献２や特許文献３で、ＣａＦ２を含む脱硫フラックスを用いた製造方法が記載されているが、脱硫後に溶鋼に残存するフラックス粒子が穴拡げ性に悪影響を与える観点からの説明はない。 As a method for producing a hot-rolled steel sheet having excellent hole expansibility, Patent Document 2 and Patent Document 3 describe a production method using a desulfurized flux containing CaF 2 , but the flux particles remaining in the molten steel after desulfurization are There is no explanation from the viewpoint of adversely affecting the hole expandability.
溶鋼に残存したフラックス粒子を浮上させる手段として、脱硫フラックス吹き込み終了後も続けて減圧下での溶鋼還流を行ない、粒子を浮上させる方法があるが、粒子数を確実に大幅に低減させるためには長時間の還流が必要であり、生産性向上の観点からは不適当である。 As a means of floating the flux particles remaining in the molten steel, there is a method in which the molten steel is refluxed under reduced pressure after the desulfurization flux blowing is completed, and the particles are floated. A long-time reflux is required, which is inappropriate from the viewpoint of improving productivity.
加工性の良好な高張力鋼では、鋼の不純物としてのＳ濃度を極力低くすることが望まれており、鋼製造時の二次精錬工程(例えばRH設備)で溶鋼の脱硫が行なわれているが、その際に脱硫材として溶鋼中に吹き込んだCaF2のような低融点酸化物を含有するＣａＯ系フラックスが、溶鋼中に残留し、圧延時に伸びて、加工時の割れの起点となってしまう場合がある。 In high-tensile steel with good workability, it is desired to reduce the S concentration as an impurity of steel as much as possible, and molten steel is desulfurized in the secondary refining process (for example, RH equipment) at the time of steel production. However, a CaO-based flux containing a low melting point oxide such as CaF 2 blown into the molten steel as a desulfurization material at that time remains in the molten steel and extends during rolling, and becomes a starting point of cracking during processing. May end up.
そこで、本発明は、上述した問題点を鑑みて案出したものであり、低融点酸化物含有フラックスを溶鋼中に極力残存させないための高清浄鋼溶製方法を提供するものである。 Therefore, the present invention has been devised in view of the above-described problems, and provides a highly clean steel melting method for preventing a low melting point oxide-containing flux from remaining in molten steel as much as possible.
本発明は上記課題を解決するためになされたものであり、その手段は、以下の通りである。
第１の発明に係る高清浄鋼の溶製方法は、二次精錬設備で、主成分がＣａＯで、融点が１５００℃以下の酸化物またはフッ化物を一種以上含む脱硫フラックスで溶鋼を脱硫した後にＡｌを溶鋼中に、０．０２〜０．０４ｍａｓｓ％分添加し、その後に溶鋼中に酸素ガスを、溶鋼１ｔあたり０．１〜０．２Ｎｍ３吹き込むまたは吹き付けることを特徴とする。
第２の発明に係る高清浄鋼の溶製方法は、第１の発明において、酸素ガスの吹き込み終了後または吹き付け終了後２分以上還流することを特徴とする。
The present invention has been made to solve the above-mentioned problems, and the means thereof is as follows.
The method for melting highly clean steel according to the first invention is a secondary refining facility, after desulfurizing molten steel with a desulfurization flux containing at least one oxide or fluoride having a main component of CaO and a melting point of 1500 ° C. or less. Al is added to the molten steel in an amount of 0.02 to 0.04 mass%, and thereafter oxygen gas is blown into or blown into the molten steel in an amount of 0.1 to 0.2 Nm 3 per ton of molten steel.
The method for melting high-clean steel according to the second invention is characterized in that, in the first invention, the high-refining steel is refluxed for 2 minutes or more after completion of blowing of oxygen gas or after completion of blowing.
本発明により、溶鋼中に懸濁する脱硫フラックスの問題を気にすることなく、高い生産性を確保しながら高い脱硫能を持つ、ＣａＯを主成分とし、融点が１５００℃以下の酸化物またはフッ化物を一種以上含む脱硫フラックスを用いてＳ濃度の非常に低い加工性の良好な高張力鋼を製造することが可能となる。 According to the present invention, without worrying about the problem of the desulfurization flux suspended in the molten steel, it has high desulfurization ability while ensuring high productivity, and it is mainly composed of CaO and having an melting point of 1500 ° C. or less. It becomes possible to produce a high-tensile steel having a very low S concentration and good workability using a desulfurization flux containing one or more compounds.
発明者らは、溶鋼に介在物として残存したＣａＦ２含有脱硫フラックスを浮上させる方法を検討した。溶鋼中の介在物を浮上させる方法は、一般的に溶鋼をガスで攪拌して介在物の浮上を促進する方法が行なわれており、この観点からは、脱酸完了または脱硫フラックス吹き込み終了時点からの攪拌時間を長くすることが有効である。しかしながら攪拌時間を長くすることは生産性の低下を引き起こす問題がある。 The inventors examined a method for levitating the CaF 2 -containing desulfurization flux remaining as inclusions in the molten steel. As a method for floating inclusions in molten steel, generally, a method in which molten steel is stirred with a gas to promote the floating of inclusions is performed. From this viewpoint, deoxidation is completed or desulfurization flux blowing is completed. It is effective to lengthen the stirring time. However, increasing the stirring time has a problem of causing a decrease in productivity.
そこで、溶鋼中に懸濁した脱硫フラックスを短時間で浮上させる方法として、溶鋼中に多量にＡｌを添加し、そこに酸素ガスを吹き込む方法を着想した。溶鋼中にＡｌを添加し、酸素ガスを吹き込む方法自体は、Ａｌ昇熱法として知られており、溶鋼温度が低下した場合に、その温度を上げる目的で使われている。しかしながら、その場合はＡｌ添加と酸素ガス吹込みを同時に行なっているので、すぐにアルミナ（Ａｌ２Ｏ３）が生成し、脱酸に関する溶解度積、すなわち（ｍａｓｓ％Ａｌ）2×（ｍａｓｓ％Ｏ）３の値が大きくならない。この結果、多量の介在物が生成しにくくので、後述するような生成した介在物が溶鋼中に懸濁している脱硫フラックスと凝集合体して、浮上を促進する効果が期待できない。 Therefore, as a method of floating the desulfurization flux suspended in the molten steel in a short time, the idea of adding a large amount of Al into the molten steel and blowing oxygen gas there was conceived. The method itself of adding Al into molten steel and blowing oxygen gas is known as an Al heating method, and is used for the purpose of raising the temperature when the molten steel temperature is lowered. However, in that case, since Al addition and oxygen gas blowing are simultaneously performed, alumina (Al 2 O 3 ) is immediately formed, and the solubility product relating to deoxidation, that is, (mass% Al) 2 × (mass% O ) The value of 3 does not increase. As a result, it is difficult to generate a large amount of inclusions, so that the effect of accelerating the floating cannot be expected because the generated inclusions described later are aggregated and coalesced with the desulfurization flux suspended in the molten steel.
本発明では、溶鋼中のＡｌが吹き込まれた酸素と反応してアルミナが生成する際に、既に溶鋼中に懸濁していたＣａＦ２のような低融点酸化物を含む脱硫フラックス粒子を核として核生成したアルミナが、それらどうしや直接均質核生成したアルミナと凝集合体し、粒径が大きくなって浮上することを狙いとしている。この場合には、溶解度積（ｍａｓｓ％Ａｌ）2×（ｍａｓｓ％Ｏ）３を増大させるために、酸素ガス吹き込みより先にＡｌを添加してＡｌ濃度（ｍａｓｓ％Ａｌ）を大きくすることが重要である。 In the present invention, when alumina is produced by reacting with oxygen in which Al in the molten steel is blown, desulfurized flux particles containing a low-melting point oxide such as CaF 2 already suspended in the molten steel are used as nuclei. The aim is for the produced alumina to agglomerate and coalesce with each other or directly nucleated alumina to increase the particle size and float. In this case, in order to increase the solubility product (mass% Al) 2 × (mass% O) 3 , it is important to increase the Al concentration (mass% Al) by adding Al prior to the oxygen gas blowing. It is.
そこで、どのようなＡｌ濃度、酸素ガス吹き込み量で、溶鋼中の懸濁介在物が浮上し易いか、実験室規模での実験を行なった。実験の鋼成分（ｍａｓｓ％）を表１に、実験条件を表２に示す。介在物浮上率の定義として、まず、溶鋼を所定の成分に調整してフラックスをインジェクションして溶鋼脱硫し、2分経過後、そのまま冷却・凝固させたインゴット中の介在物個数をベースとし、Ａｌ濃度や酸素吹き付け量を変化させた水準のインゴットの介在物量をベース個数で割り、１００分率化した。酸素吹き付け時間は2分であり、脱硫後の経過時間がベース水準と同じ２分間となるようにした。Ａｌ濃度に関しては、まず、溶鋼脱硫を行なう前には、脱酸が必要なので、所定量のＡｌを脱硫前に添加した。脱硫を行なった後に更にＡｌを添加したが、この時に添加するＡｌ量を変化させた。実験結果を図１に示すが、脱硫後に添加したＡｌが濃度として０．０２〜０．０４ｍａｓｓ％の範囲で、かつ吹き込み酸素量が溶鋼１ｔあたり０．１〜０．２Ｎｍ３の範囲である場合に、溶鋼に懸濁した介在物の浮上率が８０％以上と高いことが判った。 Therefore, an experiment on a laboratory scale was conducted to determine what kind of Al concentration and oxygen gas blowing amount the suspended inclusions in the molten steel easily float. The experimental steel components (mass%) are shown in Table 1, and the experimental conditions are shown in Table 2. The definition of the inclusion floating rate is as follows. First, the molten steel is adjusted to the prescribed components, the flux is injected and the molten steel is desulfurized. Dividing the amount of inclusions of the ingot at the level where the concentration and the amount of oxygen sprayed were changed by the number of bases, the fraction was made 100 minutes. The oxygen blowing time was 2 minutes, and the elapsed time after desulfurization was set to 2 minutes, the same as the base level. Regarding the Al concentration, first, since deoxidation is necessary before performing molten steel desulfurization, a predetermined amount of Al was added before desulfurization. Al was further added after desulfurization, but the amount of Al added at this time was changed. The experimental results are shown in FIG. 1, where the Al added after desulfurization is in the range of 0.02 to 0.04 mass% and the amount of oxygen blown is in the range of 0.1 to 0.2 Nm 3 per ton of molten steel. Furthermore, it was found that the floating rate of inclusions suspended in the molten steel was as high as 80% or more.
以下に本発明の作用を記す。まず、二次精錬での溶鋼脱流は、脱硫前に溶鋼酸素を極力下げておくことが前提であり、一般にはＡｌ添加で脱酸が行なわれている。したがって、この時点で溶鋼中の酸素濃度は非常に低い。次に脱硫を行なうために、ＣａＯ系脱硫フラックスを溶鋼中に吹き込む。このときの脱硫フラックスは、良好な脱硫能を得るために、ＣａＯを主成分とし、融点が１５００℃以下の酸化物またはフッ化物を一種以上含む脱硫フラックスである。このような脱硫フラックスは、脱硫能は非常に良いが、溶鋼中に懸濁して残った場合には、圧延で伸ばされるため、鋼板の穴拡げ試験で低値が出やすくなる。溶鋼脱硫処理の間は溶鋼の攪拌も同時に行なわれる。脱硫が終了すると、溶鋼中には浮上し切れなかった脱硫フラックスが懸濁している状態となる。 The operation of the present invention will be described below. First, molten steel deflowing in secondary refining is based on the premise that molten steel oxygen is lowered as much as possible before desulfurization, and deoxidation is generally performed by adding Al. Therefore, at this time, the oxygen concentration in the molten steel is very low. Next, in order to perform desulfurization, a CaO-based desulfurization flux is blown into the molten steel. The desulfurization flux at this time is a desulfurization flux containing CaO as a main component and one or more oxides or fluorides having a melting point of 1500 ° C. or lower in order to obtain a good desulfurization ability. Such a desulfurization flux has a very good desulfurization ability, but if it remains suspended in molten steel, it is stretched by rolling, so that a low value is likely to appear in a steel sheet hole expansion test. During the molten steel desulfurization process, the molten steel is simultaneously stirred. When the desulfurization is completed, the desulfurization flux that has not been floated in the molten steel is suspended.
ここからが本発明の重要な点であり、まず、溶鋼攪拌は継続したままでＡｌを０．０２〜０．０４ｍａｓｓ％の範囲で添加する。より好ましくは０．０３〜０．０４ｍａｓｓ％の範囲で添加する。Ａｌは、脱酸のために、脱硫前にすでに添加されているが、本発明では脱硫後に新たに添加する。Ａｌの添加が終了した後に酸素ガスを溶鋼１ｔあたり０．１〜０．２Ｎｍ３の範囲で溶鋼中に吹き込む。より好ましくは溶鋼１ｔあたり０．１５〜０．２Ｎｍ３の範囲で溶鋼中に吹き込む。この操作により、多量のアルミナが生成する。 This is the important point of the present invention. First, Al is added in the range of 0.02 to 0.04 mass% while stirring the molten steel is continued. More preferably, it adds in 0.03-0.04 mass%. Al is already added before desulfurization for deoxidation. In the present invention, Al is newly added after desulfurization. After the addition of Al is completed, oxygen gas is blown into the molten steel in the range of 0.1 to 0.2 Nm 3 per ton of molten steel. More preferably, the molten steel is blown into the molten steel in the range of 0.15 to 0.2 Nm 3 per ton of molten steel. This operation generates a large amount of alumina.
このアルミナの生成の仕方は二通りあり、一方は懸濁している脱硫フラックスを核として、その周りに生成し、脱硫フラックス粒子と一体の粒子となる。他方は、直接溶鋼中に単独粒子の状態で均質核生成する。Ａｌ濃度と酸素濃度の積が所定の値以上になっていれば、双方の粒子は凝集合体し、サイズの大きな粒子となって溶鋼中を浮上する。 There are two ways of producing this alumina, one of which is produced around the desulfurization flux that is suspended as a core, and becomes an integral particle with the desulfurization flux particles. The other is homogeneous nucleation in the form of single particles directly in molten steel. If the product of the Al concentration and the oxygen concentration is equal to or higher than a predetermined value, both particles aggregate and coalesce and float in the molten steel as large size particles.
脱硫後、酸素吹き込みまたは吹き付け前に添加するＡｌ量を、濃度で０．０２〜０．０４ｍａｓｓ％の範囲と規定したのは、ラボ実験の結果で介在物浮上に効果があったＡｌ濃度に基づき決定した。添加量が０．０２よりも低い場合には、アルミナ介在物の生成量が少ないため、溶鋼中に懸濁している脱硫フラックス粒子を多量に浮上させる事が出来ない。また、添加量の上限を０．０４ｍａｓｓ％としたのは、Ａｌ濃度が高すぎると、多量にアルミナが生成するために、今度は浮上し切れなかったアルミナそのものが逆に製品特性に悪影響を与えるためである。 After desulfurization, the amount of Al added before oxygen blowing or before blowing was defined as a concentration range of 0.02 to 0.04 mass% based on the Al concentration that was effective for inclusion flotation as a result of laboratory experiments. Were determined. When the addition amount is lower than 0.02, the amount of alumina inclusions produced is small, so that a large amount of desulfurization flux particles suspended in the molten steel cannot be levitated. In addition, the upper limit of the addition amount is set to 0.04 mass%. If the Al concentration is too high, a large amount of alumina is generated, so that the alumina that has not been lifted this time adversely affects the product characteristics. Because.
吹き込み酸素量が溶鋼１ｔあたり０．１〜０．２Ｎｍ３の範囲と規定したのも、ラボ実験の結果で介在物浮上に効果があった酸素量に基づき決定した。酸素量が溶鋼１ｔあたり０．１よりも低い場合には、アルミナ介在物の生成量が少ないため、溶鋼中に懸濁している脱硫フラックス粒子を多量に浮上させる事が出来ない。また、酸素量の上限を０．２Ｎｍ3としたのは、Ａｌの場合と同様に、多量にアルミナが生成するために、今度は浮上し切れなかったアルミナそのものが逆に製品特性に悪影響を与えるためである。 The reason why the amount of oxygen blown was defined to be in the range of 0.1 to 0.2 Nm 3 per 1 ton of molten steel was determined based on the amount of oxygen that had an effect on the floating of inclusions as a result of laboratory experiments. When the amount of oxygen is lower than 0.1 per 1 ton of molten steel, a large amount of desulfurized flux particles suspended in the molten steel cannot be levitated because the amount of alumina inclusions produced is small. Also, the upper limit of the oxygen amount is set to 0.2 Nm 3 , as in the case of Al, because a large amount of alumina is produced, the alumina that has not been lifted this time adversely affects the product characteristics. Because.
このようにして、懸濁していた脱硫フラックス粒子は浮上し、溶鋼外に排出される。更に、フラックス粒子の浮上を確実なものにするには、酸素ガス吹き込み終了後の還流時間を２分以上確保することが望ましい。なお、本発明を実施すると、Ａｌの酸化発熱のために溶鋼温度が上昇する。しかしながら、一般に溶鋼脱硫時には温度低下を危惧する場合が多いので、本発明は脱硫処理において、良い方向に作用することになる。 In this way, the suspended desulfurized flux particles float up and are discharged out of the molten steel. Furthermore, in order to ensure the floating of the flux particles, it is desirable to secure a reflux time of 2 minutes or more after the completion of the oxygen gas blowing. When the present invention is carried out, the molten steel temperature rises due to the oxidation heat of Al. However, in general, since there are many cases where there is a concern about a temperature drop during molten steel desulfurization, the present invention works in a good direction in the desulfurization treatment.
４００ｔ容量の転炉を用いて溶製した溶鋼を、RH真空脱ガス装置を用いて脱硫した。脱硫後、条件を変えてＡｌ添加と酸素ガス供給を行ない、その後連続鋳造して熱延を行ない、熱延板の介在物調査を行なった。併せて穴拡げ試験で評価した。鋼の組成（ｍａｓｓ％）を表３に、使用した脱硫フラックス組成（ｍａｓｓ％）を表４に、試験条件を表５に示す。酸化物・フッ化物の融点： ＣａＯ： 約２５７０℃、ＣａＦ２： １４２３℃、Ｎａ２Ｏ： １１３３℃である。 The molten steel melted using a 400 t capacity converter was desulfurized using an RH vacuum degasser. After desulfurization, conditions were changed, Al addition and oxygen gas supply were performed, and then continuous casting was performed to perform hot rolling, and inclusions in the hot rolled sheet were investigated. In addition, it was evaluated by a hole expansion test. Table 3 shows the steel composition (mass%), Table 4 shows the desulfurization flux composition (mass%) used, and Table 5 shows the test conditions. Melting point of oxide / fluoride: CaO: about 2570 ° C., CaF 2 : 1423 ° C., Na 2 O: 1133 ° C.
結果を表６に示すが、水準１〜５は本発明の場合の条件を満たすものであり、熱延板の介在物個数が減少し、特に伸びた介在物が著しく減少した。また、穴拡げ性も良好な結果を得た。 The results are shown in Table 6. Levels 1 to 5 satisfy the conditions of the present invention, and the number of inclusions in the hot-rolled sheet decreased, and in particular, the elongated inclusions decreased remarkably. Moreover, the hole expansibility was also good.
一方、水準６〜１１のいずれの比較例においても、熱延板の介在物個数、特に伸びた介在物の個数が多く、穴拡げ性も不良であった。すなわち、水準６では、追加添加分のＡｌ濃度が低いために、水準７および８では、吹き込み酸素ガス量が少ないために、また水準９および１０では、追加添加分のＡｌ濃度が多すぎるために、本発明条件を満足できず、評価結果が不良であった。また水準１１は、酸素ガス吹込みを除けば本発明の条件を満たす水準３と同じ条件であるが、Ａｌ添加と酸素ガス吹込みが同時なので効果がなかった。 On the other hand, in any of the comparative examples of levels 6 to 11, the number of inclusions in the hot-rolled sheet, particularly the number of inclusions extended, was large, and the hole expandability was poor. That is, since the Al concentration of the additional addition is low at level 6, the injected oxygen gas amount is low at levels 7 and 8, and the Al concentration of the additional addition is too high at levels 9 and 10. The conditions of the present invention could not be satisfied, and the evaluation results were poor. Level 11 is the same as Level 3 that satisfies the conditions of the present invention except for oxygen gas blowing, but was ineffective because Al addition and oxygen gas blowing were simultaneous.
特に、自動車足回り部品として用いられる高張力鋼で、複雑な形状の部品製造のために高度な成形性・加工性が要求される鋼である良加工性高張力鋼の加工性を更に高めるための高清浄鋼溶製技術として利用されるものである。 In particular, to further improve the workability of high-strength steel with good workability, which is a high-strength steel used for automobile undercarriage parts, and requires high formability and workability for the manufacture of parts with complex shapes. It is used as a highly clean steel melting technology.
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