JP3820686B2 - Melting method of low nitrogen stainless steel - Google Patents

Description

【００１７】
表１から明らかなように、転炉での予備脱炭で希釈ガスを窒素ガスとしたことにより、転炉から出鋼した溶鋼中の窒素は、１７００ｐｐｍと従来より高い値を示している。しかしながら、その後に行う仕上げ脱炭に使用する溶鋼中の硫黄濃度［Ｓ］を０．０１０ｗｔ％以下、炭素濃度［Ｃ］を０．１０ｗｔ％以上とすることで、最終の溶鋼中窒素を２００ｐｐｍ以下とすることができた。また、溶製時間は、従来と同程度であり、高い生産性を維持して安価に低窒素ステンレス鋼が製造できるようになった。
【００１８】
【発明の効果】
以上述べたように、本発明により、低窒素ステンレス鋼が、高価なＡｒガスを使用せずに、Ｃｒ歩留高く、高生産性を維持して安価に溶製できるようになった。
【図面の簡単な説明】
【図１】真空脱炭前の溶鋼中硫黄濃度［Ｓ］と、真空脱炭時における脱窒速度との関係を示す図である。
【図２】真空脱炭前の溶鋼中硫黄濃度［Ｓ］と、真空脱炭後に到達した溶鋼中の窒素濃度［Ｎ］との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for melting low nitrogen stainless steel, and more particularly, preliminary decarburization refining under atmospheric pressure, finish decarburization refining under reduced pressure, and low nitrogen stainless steel at low cost while ensuring high productivity. Is a technology to melt.
[0002]
[Prior art]
High chromium steels such as ferritic stainless steels have improved corrosion resistance, toughness, workability and weldability by reducing the content of impurity elements such as carbon and nitrogen. However, since chromium (hereinafter referred to as Cr) has a property of increasing the solubility of nitrogen in molten iron, denitrification from high Cr steel is extremely difficult.
[0003]
Conventionally, in general, in order to melt low nitrogen stainless steel, first, the preliminary decarburization and refining at atmospheric pressure is used to achieve a relatively low nitrogen concentration, and then final decarburization and refining are performed to obtain the final target nitrogen. A method of denitrification to the content is taken. Specifically, a high-carbon Cr-containing molten metal is preliminarily decarburized and refined with oxygen using a converter or a so-called AOD (argon oxygen decarburization) furnace, and the resulting crude molten steel is treated with RH (reflux type) or VOD ( Oxygen blowing decarburization is performed under reduced pressure in a vacuum degassing apparatus such as vacuum oxygen decarburization. However, in recent years, in order to reduce the melting cost of stainless steel, various attempts have been made to prioritize decarburization by suppressing the oxidation loss of Cr, and “a method of decarburizing and refining using a mixed gas of oxygen and inert gas” ”,“ Method of decarburizing by strengthening the stirring of molten metal by blowing inert gas or oxygen from the tuyeres provided at the bottom of the furnace ”, and“ Method of using these together ”have been developed and put into practical use. .
[0004]
However, in the improved decarburization refining method in these converters and AOD furnaces, a large amount of inert gas is used, and in addition, in order to prevent nitrogen absorption when melting low nitrogen stainless steel, Therefore, a large amount of expensive Ar gas is used. Therefore, the problem that refining cost increases significantly arises.
Therefore, various countermeasures have been taken, for example, as disclosed in Japanese Patent Publication No. 56-17405, after decarburizing and refining without using an inert gas in the atmosphere, without performing slag reduction treatment. A method of realizing a low nitrogen concentration by transferring the molten steel to a vacuum degassing apparatus and performing degassing treatment there is also conceivable. However, in this method, Cr oxide generated during decarburization in the atmosphere is produced without reduction, so it was necessary to recover Cr in a vacuum degassing apparatus. Therefore, there is a disadvantage that the processing time in the vacuum degassing apparatus becomes long, which is disadvantageous from the viewpoint of productivity. In addition, the amount of Cr oxide transferred into the slag is oxidatively refined using almost no inert gas, so that the amount of Cr oxide is increased, resulting in a decrease in Cr yield due to scattering of slag generated during blowing outside the system, or slag. There were other disadvantages such as an increase in Cr recovery cost due to reduction.
[0005]
[Problems to be solved by the invention]
In view of such circumstances, the present invention provides a method for melting low nitrogen stainless steel capable of decarburization and denitrification in a short time while maintaining high productivity while suppressing chromium oxidation loss from molten steel. The purpose is to do.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the inventor has eagerly improved the conventional technique that combines the pre-decarburization and the vacuum decarburization treatment under the atmospheric pressure without using an expensive argon gas as an inert gas. Made an effort. In particular, the present invention was completed by focusing on improving the denitrification rate during the latter vacuum decarburization treatment.
[0007]
That is, the present invention melts low-nitrogen stainless steel by spraying oxygen gas and inert gas under atmospheric pressure or preliminarily blowing decarburization to a crude stainless steel melt, followed by finish decarburization under reduced pressure. At the time of manufacturing, the above stainless steel crude melt with [S] ≧ 0.019 wt% or more is used, nitrogen gas is used as an inert gas in the preliminary decarburization, and after the preliminary decarburization, finish decarburization Before the reduction treatment of slag is performed and the sulfur concentration in molten steel [S] is accelerated , the sulfur and carbon concentrations in the molten metal before finishing decarburization are reduced to [S] ≦ 0.010 wt% and [ C] is 0.10 to 0.20 wt%, and is a method for melting low nitrogen stainless steel.
[0009]
According to the present invention, the sulfur and carbon concentrations in the molten steel at the end of preliminary decarburization are limited to a range that ensures a high denitrification rate by finish decarburization, so that denitrification by finish decarburization is sufficiently performed. Will come to be. In addition, since the slag reduction treatment is performed after preliminary decarburization and before finish decarburization, denitrification can be performed in a state where Cr oxidation loss is suppressed. As a result, low nitrogen stainless steel can be manufactured at low cost with a high Cr yield. Hereinafter, embodiments of the present invention will be described in detail including the background to the invention.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The stainless steel to be used in the present invention is a low nitrogen stainless steel containing 5 wt% or more, preferably 11 wt% or more of Cr, particularly having low nitrogen as an impurity element and N ≦ 200 ppm. Moreover, as a refining vessel used in the present invention, a converter or an AOD furnace is preferably used for preliminary decarburization under atmospheric pressure, but a ladle or a lance that can be easily decarburized by placing a hood. Alternatively, an electric furnace provided with a tuyere and provided with an oxygen blowing function may be used. The apparatus used in the final decarburization is a vacuum degassing apparatus, which is mainly a VOD apparatus, but a known vacuum degassing apparatus such as an RH or DH method may be used. At that time, in order to oxidize and remove carbon in the molten steel, it is desirable to provide blowing means such as a lance or tuyere and supply oxygen gas or oxide.
[0011]
First, the inventor conducted various experiments using the above-described apparatus for the finish decarburization, that is, the denitrification rate during the vacuum decarburization treatment. As a result, the relationship shown in FIG. 1 was obtained between the denitrification rate and the components in the molten steel before treatment. That is, it has been found that the denitrification rate during the vacuum decarburization treatment is greatly influenced by the sulfur concentration [S] in the molten steel before the vacuum decarburization treatment and is greatly improved in the region of S ≦ 0.010%. . This is because sulfur in the molten steel is a surface active element, and when it is at a high concentration, the sulfur element collects around the CO gas bubbles generated during decarburization in the molten steel, and the nitrogen element is contained in the bubbles. It seems to be caused by a phenomenon that prevents entry.
[0012]
Therefore, in order to promote denitrification, the inventor needs to secure a certain amount of CO bubble generation in the decarburization reaction, reduce the sulfur concentration in the molten steel, and sufficiently incorporate nitrogen element into the CO bubble. An experiment was conducted to confirm this idea. The result is shown in FIG. From FIG. 2, it was found that the denitrification is more effectively performed as the sulfur concentration [S] in the molten steel before the vacuum decarburization treatment is lower. Even when the sulfur concentration [S] in the molten steel is as low as 0.010 wt% or less, if the carbon concentration [C] in the molten steel is less than 0.10 wt%, low nitrogen of 200 ppm or less is stabilized. It became clear that it was not achieved. This proves the above idea that denitrification requires a certain amount of CO bubbles.
[0013]
The present invention embodies the experimental results described above. The sulfur concentration [S] and the shoreline concentration [C] in the molten steel before the vacuum decarburization treatment are set to S ≦ 0.010 wt%, C is set to 0.0. 10 to 0.20 wt%. The carbon concentration [C] in the molten steel before the vacuum decarburization process may be 0.10 wt% or more from the above experimental results . However, and the carbon concentration in the molten steel during the vacuum decarburization [C] is too much high, but the certainty that the low nitrogen, economical, such taking too long to vacuum decarburization itself. For this reason, in the present invention, the carbon concentration in the vacuum decarburization before the molten steel [C] shall be the 0.10wt% ~0.20wt%. Et al is, in the present invention, as a method to 0.010 wt% sulfur concentration [S] in the molten steel during the preliminary decarburization after the end of tapping, was charged with a reducing agent such as Fe-Si in the furnace, the slag The reduction was done. This not only promotes the reduction of the sulfur concentration in the molten steel, but also reduces and recovers the oxidation loss in the slag into the molten steel, which is advantageous in terms of Cr yield. At that time, in order to improve the so-called slag / metal reaction, stirring with an inert gas is generally performed, but in the present invention, nitrogen gas is also used as the inert gas. This is because denitrification is sufficiently possible by subsequent vacuum decarburization without using Ar gas.
[0014]
In the present invention, nitrogen in the molten steel after completion of preliminary decarburization shows a very high value exceeding 1000 ppm by making all inert gas used in preliminary decarburization under atmospheric pressure nitrogen gas instead of Ar gas. . However, CO boiling during decarburization under reduced pressure is ensured by setting the sulfur concentration [S] in the molten steel before the vacuum degassing process to be 0.010 wt% or less and ensuring the carbon concentration to be 0.10 wt% or more. The maximum denitrification capacity is achieved, and the purpose can be achieved. Therefore, there is no need to reduce the nitrogen concentration [N] in the molten steel after decarburization under atmospheric pressure and to suppress the nitrogen absorption during the steel output as has been done in the prior art. Moreover, since high Cr yield is ensured by slag reduction | restoration and productivity can be maintained at the high level, low nitrogen stainless steel can be manufactured cheaply conventionally. At the time of decarburization, Cr tends to be oxidized as the carbon in steel is lowered. Therefore, it is preferable to reduce the ratio of oxygen gas / nitrogen gas as the decarburization reaction proceeds. This is the same as the method of sequentially reducing the ratio of oxygen gas / Ar gas which is normally performed by refining using a stainless steel AOD furnace or converter.
[0015]
【Example】
When melting stainless steel containing 16 wt% Cr, 0.06 to 0.07 wt% carbon, and 0.0200 wt% or less nitrogen, it was preparatory under atmospheric pressure using an upper bottom blowing converter. All dilution gases for decarburization were nitrogen gas. The obtained crude molten steel was taken out in a ladle and subjected to oxygen blowing under reduced pressure using a VOD type vacuum degassing apparatus. The implementation conditions and results are shown in Table 1 together with those according to the conventional method.
[0016]
[Table 1]

[0017]
As is clear from Table 1, by using nitrogen gas as the dilution gas in the preliminary decarburization in the converter, the nitrogen in the molten steel discharged from the converter is 1700 ppm, which is higher than the conventional value. However, the sulfur concentration [S] in the molten steel used in the subsequent decarburization performed thereafter is 0.010 wt% or less, and the carbon concentration [C] is 0.10 wt% or more, so that the final nitrogen in the molten steel is 200 ppm or less. And was able to. In addition, the melting time is about the same as in the past, and low nitrogen stainless steel can be manufactured at low cost while maintaining high productivity.
[0018]
【The invention's effect】
As described above, according to the present invention, low nitrogen stainless steel can be melted at low cost while maintaining high productivity and high yield without using expensive Ar gas.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the sulfur concentration [S] in molten steel before vacuum decarburization and the denitrification rate during vacuum decarburization.
FIG. 2 is a diagram showing a relationship between sulfur concentration [S] in molten steel before vacuum decarburization and nitrogen concentration [N] in molten steel reached after vacuum decarburization.

Claims (1)

When a low-nitrogen stainless steel is melted by blowing oxygen gas and inert gas under atmospheric pressure or pre-decarburizing by blowing into a stainless steel crude molten metal, followed by finish decarburization under reduced pressure.
[S] ≧ 0.019 wt% or more is used as the stainless steel crude melt, and nitrogen gas is used as an inert gas in the preliminary decarburization. After the preliminary decarburization, before the final decarburization, slag is used. By reducing the sulfur concentration [S] in the molten steel, the sulfur and carbon concentrations in the molten metal before finishing decarburization are reduced to [S] ≦ 0.010 wt% and [C], respectively . A method for melting low nitrogen stainless steel, characterized by being 0.10 to 0.20 wt%.

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