JP4463701B2 - Decarburization method for molten stainless steel and method for producing ultra-low carbon stainless steel - Google Patents
Decarburization method for molten stainless steel and method for producing ultra-low carbon stainless steel Download PDFInfo
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- 238000005261 decarburization Methods 0.000 title claims description 94
- 238000000034 method Methods 0.000 title claims description 37
- 229910001220 stainless steel Inorganic materials 0.000 title claims description 27
- 239000010935 stainless steel Substances 0.000 title claims description 23
- 229910052799 carbon Inorganic materials 0.000 title claims description 12
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 title claims description 11
- 238000004519 manufacturing process Methods 0.000 title claims description 8
- 239000002893 slag Substances 0.000 claims description 78
- 229910052760 oxygen Inorganic materials 0.000 claims description 75
- 239000001301 oxygen Substances 0.000 claims description 75
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 74
- 238000007664 blowing Methods 0.000 claims description 49
- 229910000831 Steel Inorganic materials 0.000 claims description 40
- 239000010959 steel Substances 0.000 claims description 40
- 238000003756 stirring Methods 0.000 claims description 25
- 230000000694 effects Effects 0.000 claims description 16
- 239000011261 inert gas Substances 0.000 claims description 14
- 229910017082 Fe-Si Inorganic materials 0.000 claims 1
- 229910017133 Fe—Si Inorganic materials 0.000 claims 1
- 239000003610 charcoal Substances 0.000 claims 1
- 230000007423 decrease Effects 0.000 description 11
- 239000007789 gas Substances 0.000 description 10
- 238000006243 chemical reaction Methods 0.000 description 9
- 230000003647 oxidation Effects 0.000 description 9
- 238000007254 oxidation reaction Methods 0.000 description 9
- 230000008569 process Effects 0.000 description 8
- 238000007670 refining Methods 0.000 description 6
- 229910000519 Ferrosilicon Inorganic materials 0.000 description 5
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 5
- 238000004458 analytical method Methods 0.000 description 5
- MCMNRKCIXSYSNV-UHFFFAOYSA-N Zirconium dioxide Chemical compound O=[Zr]=O MCMNRKCIXSYSNV-UHFFFAOYSA-N 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- 238000011088 calibration curve Methods 0.000 description 3
- 230000000052 comparative effect Effects 0.000 description 3
- 238000009749 continuous casting Methods 0.000 description 3
- 239000002184 metal Substances 0.000 description 3
- 229910052751 metal Inorganic materials 0.000 description 3
- 239000007787 solid Substances 0.000 description 3
- 238000003723 Smelting Methods 0.000 description 2
- 239000003795 chemical substances by application Substances 0.000 description 2
- 238000007796 conventional method Methods 0.000 description 2
- 230000000875 corresponding effect Effects 0.000 description 2
- 238000007429 general method Methods 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 238000012545 processing Methods 0.000 description 2
- 238000005070 sampling Methods 0.000 description 2
- 238000009628 steelmaking Methods 0.000 description 2
- 229910004261 CaF 2 Inorganic materials 0.000 description 1
- 229910004298 SiO 2 Inorganic materials 0.000 description 1
- 239000000654 additive Substances 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 230000008901 benefit Effects 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 239000003638 chemical reducing agent Substances 0.000 description 1
- 230000002596 correlated effect Effects 0.000 description 1
- 238000007872 degassing Methods 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 230000004907 flux Effects 0.000 description 1
- 238000004868 gas analysis Methods 0.000 description 1
- 230000002401 inhibitory effect Effects 0.000 description 1
- 229910044991 metal oxide Inorganic materials 0.000 description 1
- 150000004706 metal oxides Chemical class 0.000 description 1
- 238000012544 monitoring process Methods 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 239000007784 solid electrolyte Substances 0.000 description 1
- 239000002436 steel type Substances 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 230000001629 suppression Effects 0.000 description 1
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Description
本発明は、ステンレス溶鋼の精錬における脱炭方法であって、特に極低炭素鋼の製造に適した効率的な脱炭方法、およびその方法を用いた極低炭素ステンレス鋼の製造法に関する。 The present invention relates to a decarburization method in refining molten stainless steel, and more particularly to an efficient decarburization method suitable for production of extremely low carbon steel, and a method of producing ultralow carbon stainless steel using the method.
ステンレス溶鋼の精錬においては、AOD法、VOD法などにより仕上げ脱炭処理が行われている。このうちVOD法は真空下での酸素吹錬によって脱炭処理を行うものであり、真空下でCO分圧を下げ、溶鋼中のCrの酸化を抑制しながら脱炭精錬ができるという長所を有する。VOD法を用いたステンレス鋼の製鋼プロセスでは、電気炉で溶製したステンレス溶銑を転炉などの脱炭炉で酸素吹錬により粗脱炭したのち、真空精錬用の取鍋に出鋼し、その取鍋内の溶鋼をVOD工程で成分調整したのち、連続鋳造法などで鋳片とする。 In the refining of molten stainless steel, finish decarburization processing is performed by AOD method, VOD method or the like. Among these, the VOD method performs decarburization processing by oxygen blowing under vacuum, and has an advantage that decarburization refining can be performed while lowering the CO partial pressure under vacuum and suppressing oxidation of Cr in molten steel. . In the steelmaking process of stainless steel using the VOD method, stainless steel melted in an electric furnace is roughly decarburized by oxygen blowing in a decarburization furnace such as a converter, and then steeled in a ladle for vacuum refining. After adjusting the components of the molten steel in the ladle in the VOD process, it is made into a slab by a continuous casting method or the like.
VOD工程では、粗脱炭終了後の溶鋼を収容した取鍋が真空排気装置につながる密閉容器内にセットされると、排気操作が開始され、所定の真空度に達した段階で上吹きランスから溶鋼中に酸素が吹き込まれ、真空酸素吹錬による脱炭が行われる(真空酸素吹錬期)。容器内の真空度は徐々に上昇し最終的には10Torr(1333Pa)以下まで達する。通常、この段階以降に酸素吹錬が終了される。酸素吹錬終了時点の代表的な決定方法では、排ガス中のCO、CO2濃度を測定して脱炭量を経時的に算出し、その算出値を溶鋼中のC含有量に変換し、所定のC含有量となった時点で酸素吹錬を終了する。その後、真空酸素吹錬によらない脱酸、すなわち、真空中で溶鋼を底吹きArガスなどで攪拌することによる脱炭処理が行われ、更なる脱炭が進められる(真空脱炭期)。 In the VOD process, when the ladle containing the molten steel after the completion of rough decarburization is set in a sealed container connected to the vacuum exhaust device, the exhaust operation is started, and when the predetermined vacuum degree is reached, the upper blow lance is used. Oxygen is blown into the molten steel, and decarburization is performed by vacuum oxygen blowing (vacuum oxygen blowing period). The degree of vacuum in the container gradually increases and finally reaches 10 Torr (1333 Pa) or less. Usually, oxygen blowing is finished after this stage. In a typical determination method at the end of oxygen blowing, the CO and CO 2 concentrations in the exhaust gas are measured and the decarburization amount is calculated over time, and the calculated value is converted into the C content in the molten steel. When the C content is reached, the oxygen blowing is finished. Thereafter, deoxidation without vacuum oxygen blowing, that is, decarburization treatment is performed by stirring the molten steel with a bottom-blown Ar gas or the like in vacuum, and further decarburization proceeds (vacuum decarburization period).
真空酸素吹錬期においては、脱炭が進んで低炭素域になると脱炭速度が低下し、排ガス成分としてのCO、CO2濃度が著しく低下する。このため酸素吹錬終了時点を適切に判断することが難しくなる。しかも種々の外乱によって終点制御の精度が低下する。また、低炭素域では酸素吹錬によるCrの酸化が避けられない。生成したCr酸化物はスラグ中に取り込まれ、スラグの流動性低下を引き起こす。スラグの流動性が低下すると、その後の真空脱炭期で脱炭効率が低下してしまう。 In the vacuum oxygen blowing period, when the decarburization progresses and becomes a low carbon region, the decarburization rate decreases, and the CO and CO 2 concentrations as exhaust gas components significantly decrease. For this reason, it is difficult to properly determine the end point of oxygen blowing. Moreover, the accuracy of the end point control decreases due to various disturbances. In the low carbon region, oxidation of Cr by oxygen blowing is inevitable. The produced Cr oxide is taken into the slag, causing a decrease in the fluidity of the slag. When the fluidity of slag decreases, the decarburization efficiency decreases in the subsequent vacuum decarburization period.
したがってVOD工程では、スラグ中のCr酸化物濃度をできるだけ増大させないようにしながら所定のC含有量に精度良く調整することが重要であるが、これらの要件を満足するように真空酸素吹錬の終了時点をコントロールすることは容易でない。特に極低炭素鋼を溶製する場合はなおさらである。所定のC含有量に未達の場合は再度の真空酸素吹錬が必要になり、生産性の低下を招く。一方、過度の真空酸素吹錬を行うと過剰の酸素供給に起因してCrの酸化が増大し、スラグの流動性を悪化させ真空脱炭期における脱炭を阻害する。 Therefore, in the VOD process, it is important to accurately adjust to a predetermined C content while minimizing the Cr oxide concentration in the slag, but the vacuum oxygen blowing is completed to satisfy these requirements. It is not easy to control the time. This is especially true when ultra-low carbon steel is melted. If the predetermined C content is not achieved, re-vacuum oxygen blowing is required, resulting in a decrease in productivity. On the other hand, if excessive vacuum oxygen blowing is performed, the oxidation of Cr increases due to the excessive supply of oxygen, and the fluidity of the slag is deteriorated, thereby inhibiting decarburization during the vacuum decarburization period.
ステンレス溶鋼の脱炭方法として、特許文献1には、取鍋中の溶鋼に浸漬管を浸漬させ、浸漬管内を減圧して底部から不活性ガスを供給して攪拌しながら酸素吹錬を行い、次いで減圧下で脱ガス処理を行う方法が記載されている。この方法では浸漬管内の真空度と酸素吹錬後の溶鋼中C量を規定することにより、Crの酸化を抑制した脱炭ができるという。しかし、この方法でCrの酸化を抑制するためには酸素吹錬が過度にならないよう制御する必要があり、その結果、酸素吹錬にてCを極低域まで低減することは難しく、酸素吹錬終了後の脱炭に委ねる負荷が大きくなりやすい。このため、精錬時間の増大や、目標C量未達を招くケースが生じやすい。 As a decarburizing method for molten stainless steel, Patent Document 1 discloses that the dip tube is immersed in the molten steel in the ladle, the inside of the dip tube is depressurized, an inert gas is supplied from the bottom, and oxygen blowing is performed while stirring. Next, a method for performing degassing under reduced pressure is described. According to this method, decarburization with suppressed oxidation of Cr can be achieved by defining the degree of vacuum in the dip tube and the amount of C in the molten steel after oxygen blowing. However, in order to suppress the oxidation of Cr by this method, it is necessary to control oxygen blowing so that it does not become excessive. As a result, it is difficult to reduce C to an extremely low range by oxygen blowing, The load entrusted to decarburization after smelting tends to increase. For this reason, the case where the refining time increases and the target C amount is not achieved easily occurs.
特許文献2には真空精錬炉で高Cr鋼を精錬する際に、種々のフラックスを添加してスラグ中の過剰Cr酸化物を還元することによりスラグの流動性を改善し、スラグ/メタル接触攪拌を確保して真空脱炭期における脱炭反応を促進させる方法が開示されている。しかし、このようなスラグ成分の適正化を精度良く行うにはスラグのサンプリング・分析が不可欠であり、操業時間の増大を招く。分析を行わない場合はスラグ組成を推定値に頼らざるを得ず、C含有量を精度良く安定的にコントロールすることは難しい。 In Patent Document 2, when high-Cr steel is refined in a vacuum smelting furnace, various fluxes are added to reduce excess Cr oxide in the slag, thereby improving the slag fluidity and slag / metal contact stirring. Has been disclosed to promote the decarburization reaction in the vacuum decarburization period. However, sampling and analysis of slag is indispensable for optimizing such slag components with high accuracy, leading to an increase in operation time. If the analysis is not performed, the slag composition must be relied on the estimated value, and it is difficult to control the C content accurately and stably.
特許文献3にはVOD工程において酸素吹錬前に予めスラグの塩基度(%CaO)/(%SiO2)を1.5〜3.5に調整しておき、更に酸素吹錬時に吹き込み酸素のトータル量と排ガス中酸素のトータル量をモニターしてその差からスラグ中Cr酸化物量を推定し、その推定Cr酸化物量が40%未満の段階で酸素吹錬を終了する終点制御を行うことで、真空脱炭期における脱炭効率を向上させる方法が開示されている。しかし、この方法ではスラグ塩基度の調整操作に手間がかかる。またスラグ中のCr酸化物量が増大しすぎない段階で酸素吹錬を止める厳密な終点制御が必要となり、終点時期がずれると目標脱炭量が未達となったり、真空脱炭効率の低下やSi等の脱酸元素の過剰消費による成分変動が生じたりしやすいという問題がある。 In Patent Document 3, the basicity (% CaO) / (% SiO 2 ) of slag is adjusted to 1.5 to 3.5 in advance before oxygen blowing in the VOD process. By monitoring the total amount and the total amount of oxygen in the exhaust gas, estimating the amount of Cr oxide in the slag from the difference, and performing end point control to end oxygen blowing when the estimated amount of Cr oxide is less than 40%, A method for improving the decarburization efficiency in the vacuum decarburization period is disclosed. However, this method requires time and effort for adjusting the slag basicity. In addition, strict end point control is required to stop oxygen blowing when the amount of Cr oxide in the slag does not increase too much. If the end point time is shifted, the target decarburization amount may not be achieved, There is a problem that component fluctuations easily occur due to excessive consumption of deoxidizing elements such as Si.
以上のように、VOD工程でステンレス溶鋼の脱炭を効率的に行うには、真空酸素吹錬期ではCrの酸化をできるだけ抑えながらC含有量をできるだけ低減することが重要であり、真空脱炭期ではCr酸化物濃度が高すぎない流動性の良好なスラグ状態において不活性ガス攪拌による脱炭を進行させることが重要である。しかしながら、Crの酸化抑制と十分な脱炭を両立させる酸素吹錬終点制御は難しく、結局、後の真空脱炭期あるいはさらに最終的な成分調整において負荷を増大させることになりやすい。真空脱炭効率は溶鋼の攪拌を強化することである程度改善できるが、そのためにはArの使用量増大や設備増強などにコストがかかる。また、攪拌時間を長くすればそれに伴って脱炭も進行するが、生産性が低下するので得策ではない。 As described above, in order to efficiently decarburize molten stainless steel in the VOD process, it is important to reduce the C content as much as possible while suppressing the oxidation of Cr as much as possible during the vacuum oxygen blowing process. In the period, it is important to advance decarburization by stirring with inert gas in a slag state with good fluidity where the Cr oxide concentration is not too high. However, it is difficult to control the end point of oxygen blowing to achieve both the suppression of oxidation of Cr and sufficient decarburization. Eventually, the load tends to increase in the subsequent vacuum decarburization period or further final component adjustment. The vacuum decarburization efficiency can be improved to some extent by strengthening the stirring of the molten steel, but this requires a cost for increasing the amount of Ar used and facility enhancement. Further, if the stirring time is lengthened, decarburization proceeds along with this, but this is not a good idea because the productivity is lowered.
さらに、このような問題に加え、いわゆる「Cピックアップ」の問題がある。Cピックアップは、取鍋のスラグラインに付着している地金やスラグ塊に高濃度のCが含まれていることがあり、真空酸素吹錬および攪拌による真空脱炭を行った後、成分調整段階の前後でそれらの地金やスラグ塊が溶解し、溶鋼中のC含有量が不用意に上昇する現象であると考えられている。Cピックアップの問題は極低炭素鋼の溶製において時として深刻な打撃を与える。従来の精錬方法ではCピックアップの問題は未解決であった。 In addition to this problem, there is a so-called “C pickup” problem. C pickup may contain high concentration of C in the bullion or slag lump adhering to the slag line of the ladle. After the vacuum decarburization by vacuum oxygen blowing and stirring, the component adjustment It is considered that the metal and slag lump are melted before and after the stage, and the C content in the molten steel is inadvertently increased. The problem of C pick-up can sometimes be a serious blow in the melting of ultra-low carbon steel. The conventional refining method has not solved the problem of C pickup.
本発明は、真空酸素吹錬の特殊な終点制御を行うことなく、真空脱炭期における脱炭効率の向上を図り、かつCピックアップにも対処できる簡便でかつ生産性の良いステンレス溶鋼の脱炭方法を提供しようというものである。 The present invention improves the decarburization efficiency in the vacuum decarburization period without performing special end point control of vacuum oxygen blowing, and is a simple and highly productive decarburization of stainless steel that can cope with C pickup. Is to provide a method.
発明者らは種々検討の結果、真空脱炭期に使用するスラグの成分調整を真空酸素吹錬の終点制御に委ねるのではなく、真空酸素吹錬終了後の還元操作によって行うことが作業効率、コスト、生産性の面で非常に有利であることを見出した。そして、真空酸素吹錬後に溶鋼中の酸素活量を測定することにより、スラグ中のCr酸化物量を推定することが可能であり、それに基づいて還元剤(脱酸剤)の添加量を精度良く決定できることがわかった。本発明ではこのようにしてスラグ中のCr酸化物量を適正化した状態で効率的な真空脱炭を行うのである。 As a result of various studies, the inventors do not leave the adjustment of the slag components used in the vacuum decarburization period to the end point control of the vacuum oxygen blowing, but work efficiency by performing the reduction operation after the completion of the vacuum oxygen blowing, We found it very advantageous in terms of cost and productivity. Then, by measuring the oxygen activity in the molten steel after vacuum oxygen blowing, it is possible to estimate the amount of Cr oxide in the slag, and based on that, the amount of addition of the reducing agent (deoxidizing agent) can be accurately determined. I found out that I could decide. In the present invention, efficient vacuum decarburization is performed in such a state that the amount of Cr oxide in the slag is optimized.
すなわち本発明で提供するステンレス溶鋼の脱炭方法は、真空容器中で酸素吹錬終了後に不活性ガス攪拌による脱炭を行うステンレス溶鋼の脱炭処理において、「前記不活性ガス攪拌による脱炭の開始前または開始後に溶鋼中の酸素活量を測定し、予め求めてある溶鋼中の酸素活量とスラグ中のCr酸化物濃度との相関関係からスラグ中のCr酸化物濃度を推定し、スラグ中のCr酸化物濃度が10〜30質量%の範囲になるように脱酸剤を溶鋼に添加する」、というスラグ成分調整操作を1回以上行うことにより、スラグ中のCr酸化物濃度が10〜30質量%の状態で例えば10Torr(1333Pa)以下の真空下において不活性ガス攪拌による脱炭を進行させるというものである。 That is, the decarburization method of molten stainless steel provided in the present invention is a decarburization process of molten stainless steel in which decarburization is performed by stirring with inert gas after completion of oxygen blowing in a vacuum vessel. The oxygen activity in the molten steel was measured before or after the start, and the Cr oxide concentration in the slag was estimated from the correlation between the oxygen activity in the molten steel and the Cr oxide concentration in the slag that had been obtained in advance. The deoxidizer is added to the molten steel so that the Cr oxide concentration in the range of 10 to 30% by mass ”is performed at least once, so that the Cr oxide concentration in the slag is 10 For example, decarburization by stirring with an inert gas is allowed to proceed under a vacuum of 10 Torr (1333 Pa) or less in a state of ˜30 mass%.
真空容器は内部が真空状態に維持された容器であり、「真空」とはここでは概ね120Torr(約16000Pa)以下の減圧状態をいう。前記脱酸剤としてはAl、Fe−Si(フェロシリコン)の1種または2種を使用することができる。Fe−SiはSiを概ね60〜80質量%含む合金であり、一般的に製鋼でのSi原料として使用されるものである。 The vacuum container is a container whose inside is maintained in a vacuum state, and “vacuum” here means a reduced pressure state of approximately 120 Torr (about 16000 Pa) or less. As the deoxidizer, one or two of Al and Fe-Si (ferrosilicon) can be used. Fe-Si is an alloy containing approximately 60 to 80% by mass of Si, and is generally used as a Si raw material in steelmaking.
また本発明では、このような脱炭方法によってC含有量を0.010質量%以下に低減させる極低炭素ステンレス鋼(例えばフェライト系ステンレス鋼)の製造法が提供される。 Moreover, in this invention, the manufacturing method of the ultra-low carbon stainless steel (for example, ferritic stainless steel) which reduces C content to 0.010 mass% or less by such a decarburization method is provided.
本発明によれば、ステンレス溶鋼を「真空酸素吹錬→攪拌による真空脱炭」の工程で脱炭処理する際、真空脱炭期に使用するスラグの成分調整を真空酸素吹錬終了後に脱酸剤を添加することによって行うことができる。このため、特殊な終点制御を採用する必要はない。酸素吹錬前にスラグ塩基度を特段調整する必要もない。また、酸素吹錬を多少過剰に行ってもその後のスラグ成分調整操作で十分適正化できる。さらに、Cピックアップも営業生産で問題ないレベルに改善される。したがって本発明は、迅速・簡便な手法で極低炭素ステンレス鋼の製造を可能にするものである。 According to the present invention, when the molten stainless steel is decarburized in the process of “vacuum oxygen blowing → vacuum decarburization by stirring”, the deoxidation is performed after the vacuum oxygen blowing is completed after adjusting the components of the slag used in the vacuum decarburization period. This can be done by adding an agent. For this reason, it is not necessary to employ special end point control. There is no need to adjust slag basicity before oxygen blowing. Moreover, even if oxygen blowing is performed somewhat excessively, it can be sufficiently optimized by the subsequent slag component adjustment operation. Furthermore, the C pickup will also be improved to a level where there is no problem in commercial production. Therefore, the present invention enables the production of ultra-low carbon stainless steel by a quick and simple method.
ステンレス溶鋼を真空酸素吹錬処理するとき、低炭素域になるに従って脱炭反応よりもCrの酸化が優先的に生じやすくなり、その結果スラグ中のCr酸化物の濃度が増加する。スラグ中のCr酸化物濃度は操業条件にもよるが、脱炭反応が停滞してきた場合に徐々に増大する傾向を示す。したがって脱炭反応が停滞してきた後は、それ以上に酸素吹錬を継続しても効率よく脱炭反応が進まない。またスラグ中のCr酸化物濃度の増大はスラグの流動性低下を招き、真空脱炭期における脱炭効率を著しく低下させる。本発明では後述のようにスラグ中に増大したCr酸化物を脱酸剤の添加によって還元するので、真空酸素吹錬の終点制御は従来一般的な方法で行えばよい。すなわち、排ガス中のCO、CO2濃度を測定して脱炭速度を経時的にモニターし、例えばCr含有量11〜30質量%、Cの最終目標値0.010質量%以下の極低炭素ステンレス鋼を溶製する場合だと、脱炭速度が10〜25ppm/minになった時点で酸素吹錬を終了すればよい。 When stainless steel molten steel is vacuum-oxygen blown, the oxidation of Cr is more likely to occur preferentially than the decarburization reaction as the carbon content decreases, and as a result, the concentration of Cr oxide in the slag increases. Although the Cr oxide concentration in the slag depends on the operating conditions, it tends to gradually increase when the decarburization reaction is stagnant. Therefore, after the decarburization reaction has stagnated, the decarburization reaction does not proceed efficiently even if oxygen blowing is continued further. Moreover, the increase in the Cr oxide concentration in the slag causes a decrease in the fluidity of the slag, and the decarburization efficiency in the vacuum decarburization period is remarkably lowered. In the present invention, as described later, Cr oxide increased in the slag is reduced by the addition of a deoxidizer, so that the end point control of the vacuum oxygen blowing may be performed by a conventional method. That is, the CO and CO 2 concentrations in the exhaust gas are measured and the decarburization rate is monitored over time. For example, an extremely low carbon stainless steel having a Cr content of 11 to 30% by mass and a final target value of C of 0.010% by mass or less. In the case of melting steel, the oxygen blowing may be terminated when the decarburization rate reaches 10-25 ppm / min.
真空脱炭期においては、溶鋼中のCは主としてスラグ中に固相状態で含まれる酸素(以下「固体酸素」という)と反応することで脱炭が進行する。固体酸素は主として金属酸化物の形で存在しており、Cr酸化物も脱炭反応における重要な酸素供給源の1つとなる。すなわち、真空脱炭期のスラグ中にはCr酸化物がある程度含まれていることが有効である。種々検討の結果、スラグ中のCr酸化物濃度が10質量%未満では真空脱炭期の脱炭反応が十分促進しないことがわかった。 In the vacuum decarburization period, decarburization proceeds by C in the molten steel mainly reacting with oxygen contained in the solid state in the slag (hereinafter referred to as “solid oxygen”). Solid oxygen exists mainly in the form of metal oxide, and Cr oxide is one of important oxygen sources in the decarburization reaction. That is, it is effective that the slag in the vacuum decarburization period contains Cr oxide to some extent. As a result of various studies, it has been found that when the Cr oxide concentration in the slag is less than 10% by mass, the decarburization reaction in the vacuum decarburization period is not sufficiently accelerated.
一方、スラグ中のCr酸化物濃度が高すぎると上述のようにスラグの流動性が低下し、脱炭反応効率が低下する。従来一般的な方法で真空酸素吹錬を行った場合、真空酸素吹錬終了後のスラグにおけるCr酸化物濃度は40質量%以上となるケースもある。発明者らの検討によると、真空脱炭期においてはスラグ中のCr酸化物濃度を30質量%以下にすることが脱炭効率の向上に極めて有効であることがわかった。 On the other hand, if the Cr oxide concentration in the slag is too high, the fluidity of the slag decreases as described above, and the decarburization reaction efficiency decreases. When vacuum oxygen blowing is performed by a conventional general method, the Cr oxide concentration in the slag after the completion of vacuum oxygen blowing may be 40% by mass or more. According to the study by the inventors, it has been found that it is extremely effective to improve the decarburization efficiency when the Cr oxide concentration in the slag is 30% by mass or less in the vacuum decarburization period.
したがって本発明では、真空脱炭期において、Cr酸化物濃度が10〜30質量%に調整されたスラグを溶鋼と接触させた状態で、溶鋼の攪拌を行い、脱炭を進行させる。スラグ中のCr酸化物濃度を調整するにはAlやFe−Si等の脱酸剤を溶鋼に添加する。その際、スラグ成分調整操作は以下のようにして行うことができる。 Therefore, in the present invention, in the vacuum decarburization period, the molten steel is stirred in a state where the slag whose Cr oxide concentration is adjusted to 10 to 30% by mass is brought into contact with the molten steel, and the decarburization proceeds. In order to adjust the Cr oxide concentration in the slag, a deoxidizer such as Al or Fe-Si is added to the molten steel. In this case, the slag component adjustment operation can be performed as follows.
まず、不活性ガス攪拌による脱炭の開始前または開始後に溶鋼中の酸素活量を酸素センサーを用いて測定する。具体的な測定時期として、真空酸素吹錬の終了時点が好適である。一般に溶鋼中の酸素活量とスラグ中の酸素活量には相関関係があるが、発明者らの詳細な検討の結果、溶鋼中の酸素活量とスラグ中のCr酸化物濃度との間にも相関関係があることがわかった。したがって、予め当該組成の溶鋼について溶鋼中の酸素活量とスラグ中のCr酸化物濃度との関係を示す検量線を作っておけば、溶鋼中の酸素活量測定値からスラグ中のCr酸化物濃度を迅速に推定することができる。酸素センサーはジルコニア等の固体電解質を用いることができる。 First, before or after the start of decarburization by stirring with an inert gas, the oxygen activity in the molten steel is measured using an oxygen sensor. As a specific measurement time, the end point of vacuum oxygen blowing is suitable. In general, there is a correlation between the oxygen activity in the molten steel and the oxygen activity in the slag, but as a result of detailed studies by the inventors, there is a difference between the oxygen activity in the molten steel and the Cr oxide concentration in the slag. Was also found to be correlated. Therefore, if a calibration curve showing the relationship between the oxygen activity in the molten steel and the Cr oxide concentration in the slag is prepared in advance for the molten steel of the composition, the Cr oxide in the slag is determined from the measured oxygen activity in the molten steel. The concentration can be estimated quickly. The oxygen sensor can use a solid electrolyte such as zirconia.
次に、溶鋼中の酸素活量の測定値から上記の検量線を用いてスラグ中のCr酸化物濃度を推定し、その推定値と目標値(10〜30質量%の範囲)との差分に相当する量のCr酸化物を還元するに足る脱酸剤の量を算出する。そして、その量の脱酸剤を溶鋼中に添加する。添加された脱酸剤は鋼中に溶解し、溶解した脱酸剤成分とスラグ中のCr酸化物が反応し、Crが還元されてスラグは所定のCr酸化物濃度となる。この適正化されたスラグと溶鋼とを接触させた状態でAr等の不活性ガス攪拌を続け、効率的に極低炭素域での脱炭を進行させるのである。不活性ガスの吹き込みは底吹きとする。以上のような一連のスラグ成分調整操作とそれに続く不活性ガス攪拌による脱炭を複数回行ってもよい。 Next, the Cr oxide concentration in the slag is estimated from the measured value of the oxygen activity in the molten steel using the above calibration curve, and the difference between the estimated value and the target value (range of 10 to 30% by mass) is obtained. The amount of deoxidizer sufficient to reduce the corresponding amount of Cr oxide is calculated. Then, that amount of deoxidizer is added to the molten steel. The added deoxidizer dissolves in the steel, the dissolved deoxidizer component reacts with the Cr oxide in the slag, Cr is reduced, and the slag has a predetermined Cr oxide concentration. In this state where the optimized slag and molten steel are in contact with each other, stirring of an inert gas such as Ar is continued, and decarburization in an extremely low carbon region is efficiently advanced. The inert gas is blown at the bottom. A series of slag component adjustment operations as described above and subsequent decarburization by inert gas stirring may be performed a plurality of times.
スラグ中のCr酸化物濃度を調整するために添加する脱酸剤としてはAl、Fe−Si等の強脱酸剤を使用する。脱酸剤添加時にはスラグ中のCr酸化物濃度が10〜30%を外れない範囲でCaOやCaF2等の造滓剤を添加してもよい。このスラグ成分調整操作を行うと、スラグの流動性が向上すること、および脱酸剤が添加されることに起因して取鍋スラグラインに付着した地金やスラグ塊も溶解し一旦Cピックアップが起きるが、スラグ中のCr酸化物濃度が適正化されているため真空脱炭期のスラグ/メタル反応が活性化し、特別な強攪拌あるいは長時間の攪拌を必要とすることなく、溶鋼中C含有量は目標の極低レベルに到達する。つまり本発明では、Cピックアップの原因となる付着物をむしろ積極的に溶解させることでCピックアップの問題を解消するのである。なお、真空脱炭期の終了時点については、目標C濃度によっても異なるが、従来の一般的な方法、例えば排ガス中のCO、CO2濃度の分析値から算出される脱炭速度などにより決定することができる。 A strong deoxidizer such as Al or Fe-Si is used as a deoxidizer added to adjust the Cr oxide concentration in the slag. When the deoxidizer is added, an additive such as CaO or CaF 2 may be added as long as the Cr oxide concentration in the slag does not deviate from 10 to 30%. When this slag component adjustment operation is carried out, the fluidity of the slag is improved, and the bullion and slag lump attached to the ladle slag line due to the addition of the deoxidizer are also dissolved, and once the C pickup is However, since the Cr oxide concentration in the slag is optimized, the slag / metal reaction in the vacuum decarburization stage is activated, and C is contained in the molten steel without requiring special strong stirring or long-time stirring. The quantity reaches the target very low level. That is, in the present invention, the problem of the C pickup is solved by rather positively dissolving the deposits that cause the C pickup. The end point of the vacuum decarburization period varies depending on the target C concentration, but is determined by a conventional general method, for example, a decarburization rate calculated from analysis values of CO and CO 2 concentration in exhaust gas. be able to.
表1に示すCr含有量のフェライト系ステンレス溶銑約80トンを電気炉で溶製し、続いて転炉で酸素吹錬を行って粗脱炭し、成分組成を確認したのち取鍋に出鋼した。このステンレス溶鋼が収容された取鍋をVOD装置の密閉容器内にセットし、真空排気を開始した。真空排気開始後約5分で密閉容器内の圧力が120Torr(約16000Pa)に達した。この時点で上吹きランスを使用して酸素吹錬を開始した。ほぼ同時期に取鍋底のポーラスノズルからArを吹き込み、溶鋼のガス攪拌を開始した。真空酸素吹錬期においては排気ガス中のCO、CO2濃度を測定することにより、従来一般的な方法で脱炭速度の変化をモニターした。いずれのチャージにおいても脱炭速度が10〜25ppm/minになった時点で真空酸素吹錬を終了した。この時の真空度は10Torr(1333Pa)以下であった。その後も真空引きとArの底吹き攪拌を継続した。
なお、表1に示した各チャージの最終成分の化学分析値は表2に示すとおりであった。
About 80 tons of ferritic stainless steel with Cr content shown in Table 1 is melted in an electric furnace, followed by rough decarburization by oxygen blowing in a converter, and after confirming the composition, the steel is put into a ladle. did. The ladle in which the molten stainless steel was accommodated was set in a sealed container of the VOD device, and evacuation was started. About 5 minutes after the start of evacuation, the pressure in the sealed container reached 120 Torr (about 16000 Pa). At this point, oxygen blowing was started using the top blowing lance. At about the same time, Ar was blown from the porous nozzle at the bottom of the ladle, and gas stirring of the molten steel was started. In the vacuum oxygen blowing period, the change in the decarburization rate was monitored by a conventional method by measuring the CO and CO 2 concentrations in the exhaust gas. In any charge, vacuum oxygen blowing was completed when the decarburization rate reached 10-25 ppm / min. The degree of vacuum at this time was 10 Torr (1333 Pa) or less. After that, vacuuming and Ar bottom stirring were continued.
The chemical analysis values of the final components of each charge shown in Table 1 are as shown in Table 2.
チャージNo.1〜6(本発明例)については、真空酸素吹錬の終了直後にジルコニア酸素センサーを用いて溶鋼中の酸素活量を測定した。結果を表1中に示してある。各鋼種について別途実験により予め求めてある「溶鋼中の酸素活量とスラグ中のCr酸化物濃度」の相関関係を表す検量線に基づいて、前記酸素活量の測定値からスラグ中のCr酸化物濃度を推定した。その結果も表1に示してある。真空脱炭期のスラグ中Cr酸化物濃度の目標値をそれぞれ表1に記載の値に設定し、Cr酸化物濃度の前記推定値と目標値の差分に相当する量のCr酸化物を還元するに足る脱酸剤の添加量を表1のとおり算出した。そしてその脱酸剤を溶鋼中に添加し、Arガスによる攪拌を継続した。なお、真空脱炭期におけるArガス導入量は300L/min以上とした。脱酸剤添加後、約5分後にスラグ組成を確認するためにスラグのサンプリングを行った。そのスラグサンプルの分析結果は表1に示すようにほぼ目標値に近いものであった。つまり、これらのチャージではスラグ中のCr酸化物濃度が10〜30質量%の状態で不活性ガス攪拌による脱炭を行ったことになる。脱酸剤添加後の脱炭時間(攪拌継続時間)は表1に示してある。この脱炭時間は排ガス分析値から計算される脱炭速度が一定値以下になる点を終点時期とする方法により定めた。真空脱炭処理後には容器を大気に開放したのち、最終成分調整を行い、連続鋳造でスラブとした。これら本発明例では比較的短時間で効率的に極低炭素フェライト系ステンレス鋼を溶製でき、CピックアップによるC濃度変動もなく、最終目標C値にほぼ一致したC含有量の極低炭素鋼が安定して製造できた。 About charge No. 1-6 (example of this invention), the oxygen activity in molten steel was measured using the zirconia oxygen sensor immediately after completion | finish of vacuum oxygen blowing. The results are shown in Table 1. Based on a calibration curve representing the correlation between “oxygen activity in molten steel and Cr oxide concentration in slag” obtained in advance by separate experiments for each steel type, Cr oxidation in slag is determined from the measured oxygen activity. The object concentration was estimated. The results are also shown in Table 1. The target values of the Cr oxide concentration in the slag during the vacuum decarburization period are set to the values shown in Table 1, respectively, and an amount of Cr oxide corresponding to the difference between the estimated value of the Cr oxide concentration and the target value is reduced. As shown in Table 1, the amount of the deoxidizer added was sufficient. Then, the deoxidizer was added to the molten steel, and stirring with Ar gas was continued. The amount of Ar gas introduced during the vacuum decarburization period was 300 L / min or more. About 5 minutes after the addition of the deoxidizer, slag was sampled to confirm the slag composition. The analysis result of the slag sample was almost close to the target value as shown in Table 1. That is, in these charges, decarburization was performed by stirring with an inert gas while the Cr oxide concentration in the slag was 10 to 30% by mass. Table 1 shows the decarburization time (stirring duration) after adding the deoxidizer. This decarburization time was determined by a method in which the end point time is a point at which the decarburization rate calculated from the exhaust gas analysis value becomes a certain value or less. After the vacuum decarburization treatment, the container was opened to the atmosphere, the final components were adjusted, and a slab was formed by continuous casting. In these examples of the present invention, ultra-low carbon ferritic stainless steel can be efficiently melted in a relatively short time, and there is no C concentration fluctuation due to C pick-up, and the ultra-low carbon steel having a C content almost coincident with the final target C value. Could be manufactured stably.
一方、チャージNo.7〜12(比較例)では、真空酸素吹錬を終了したのち、酸素活量の測定や、スラグ中のCr酸化物濃度を調整するための脱酸剤添加は行わず、酸素吹錬終了時点のスラグをそのまま用いて真空脱炭を行った。脱炭時間は前記本発明例の場合と同様の方法により定めた。なお、酸素吹錬終了後約5分経過後にスラグのサンプリングを行い、そのスラグサンプルを分析した結果、表1に示すようにいずれもCr酸化物濃度が30質量%を超えていた。つまり、これらのチャージではスラグ中のCr酸化物濃度が30質量%を超える状態で不活性ガス攪拌による脱炭を行ったことになる。真空脱炭処理後には容器を大気に開放したのち、最終成分調整を行い、連続鋳造でスラブとした。 On the other hand, in the charge Nos. 7 to 12 (comparative examples), after the vacuum oxygen blowing was completed, the oxygen activity measurement and the deoxidizer addition for adjusting the Cr oxide concentration in the slag were not performed. Vacuum decarburization was performed using the slag at the end of oxygen blowing. The decarburization time was determined by the same method as in the case of the present invention. In addition, as a result of sampling slag about 5 minutes after completion | finish of oxygen blowing and analyzing the slag sample, as shown in Table 1, all had Cr oxide concentration over 30 mass%. That is, in these charges, decarburization was performed by stirring with an inert gas in a state where the Cr oxide concentration in the slag exceeds 30% by mass. After the vacuum decarburization treatment, the container was opened to the atmosphere, the final components were adjusted, and a slab was formed by continuous casting.
比較例7、8では真空脱炭時間を前記本発明例と同程度の時間として脱炭を行ったが、スラグ中のCr2O3濃度が30%を超えた状態であったため、目標C値まで到達することができなかった。また、比較例9〜12では前記本発明例より長時間の脱炭を行ったにもかかわらず目標C値まで到達せず、極低炭素鋼の製造ができなかった。CピックアップによるC量の変動も目標未達の要因になっているものと推察される。 In Comparative Examples 7 and 8, decarburization was performed with the vacuum decarburization time being the same as that of the present invention example, but the Cr 2 O 3 concentration in the slag exceeded 30%, so the target C value Could not reach up to. Moreover, in Comparative Examples 9-12, although it decarburized for a long time than the said invention example, it did not reach | attain target C value but was not able to manufacture ultra-low carbon steel. It can be inferred that the fluctuation of the C amount due to the C pickup is also a factor of not achieving the target.
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