JPH0154409B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) We present here the results of research and development on a method that enables economical operation with simple equipment regarding decarburization of crude molten stainless steel using a converter or similar refining vessel. do. (Conventional technology) Stainless steel melting is detailed in the Iron and Steel Handbook, Volume 2, Pigmaking and Steelmaking, pages 693 to 728, edited by the Japan Iron and Steel Institute, and representative examples include converter furnaces and AOD furnaces. In this method, crude molten stainless steel melted in an electric furnace is charged into a reaction vessel having a bottom blowing tuyere, and oxygen or a mixed gas of oxygen and argon is blown into the bottom tuyere to decarburize. On the other hand, recently, as disclosed in Japanese Unexamined Patent Publication No. 58-130216, a top blowing There is also a method of blowing a mixed gas of oxygen and argon or oxygen and nitrogen, and in Japanese Patent Publication No. 59-21367, there is a method of blowing oxygen and an inert gas under the bath surface of a chromium-containing steel bath to cause a decarburization reaction. At the same time, pure oxygen or an oxygen-containing gas containing oxygen is blown onto the bath surface.
A method is also disclosed. The above method requires that oxygen for decarburization be injected through the bottom tuyeres of the furnace. This hearth bottom tuyere usually has a double pipe structure, and the AOD
In the furnace, a mixed gas of argon and oxygen is supplied from the inner tube, argon is supplied from the gap between the inner tube and the outer tube, and in the case of a bottom blowing converter, hydrocarbon gas is supplied from the gap between the inner tube and the outer tube. (Problems to be Solved by the Invention) In the conventional method described above, since oxygen is supplied from the bottom tuyere, the tuyere receives heat from oxidation of the molten steel and reaches a high temperature, resulting in melting. For this reason, the life of the tuyere is a major factor in determining the life of the furnace body, which hinders productivity and increases the amount of refractories used. On the other hand, it is also possible to supply pure oxygen from a top-blown lance, but the above-mentioned "Steel Handbook" p709-711
As described in , Cr oxidation is large and a large amount of highly viscous slag is generated, so it is not practical and has not been developed into a process. Here, we present a method for achieving economical decarburization of stainless steel using simple equipment by overcoming the above-mentioned disadvantages caused by the lifespan of the furnace body due to tuyere melting without problems such as losses caused by Cr oxidation. This is the purpose of this invention. (Means for Solving the Problems) This invention relates to a decarburization refining method for crude molten stainless steel, in which a non-oxidizing gas is introduced below the bath surface of the molten stainless steel to stir the molten steel, and at the same time, a non-oxidizing gas is introduced below the bath surface. The key to solving the above problem is to spray a mixed gas of oxygen and non-oxidizing gas. Here, rather than keeping the ratio of non-oxidizing gas to oxygen in the mixed gas blown onto the bath surface constant, it should be increased (increasing the amount of non-oxidizing gas) as the carbon concentration in the molten steel decreases. is more desirable. According to this invention, the corrosion rate of the bottom tuyere is lower than in the conventional method, so the furnace life is extended, and Cr oxidation can be suppressed to the same level as the oxygen bottom blowing method, so it is a simple method with low equipment and operating costs. You can get an economical process. When carrying out this invention, a reaction vessel 1 as shown in FIG. 1 is suitable, and it is necessary to equip a normal top-blowing converter with equipment capable of bottom-blowing gas. In addition, from the top blow lance 2, there are piping systems 3 and 4 that can blow a mixed gas of oxygen and non-oxidizing gas.
It is desirable to provide a device that can control the flow rate of each gas, such as an automatic flow rate control valve. Note that the piping systems 5 and 6 were installed in order to carry out a comparative example to be described later, and are not necessary in the present invention. Describe the specific details of the operating method. First, crude molten stainless steel is charged into a top-bottom blowing converter having the above-mentioned functions. At this time, non-oxidizing gas is allowed to flow through the bottom tuyere. Next, a top blowing lance is inserted into the furnace to blow oxygen or a mixed gas of oxygen and non-oxidizing gas onto the bath surface. The gas supplied from the top blowing lance depends on the carbon concentration in the steel, but blowing is started with only oxygen, and when the carbon concentration has been decarburized to around 1%, it is switched to a mixed gas of oxygen and non-oxidizing gas. If the blowing is continued, the amount of non-oxidizing gas used can be reduced, which is advantageous. Since there is an appropriate range at which the gas type is switched depending on the oxygen supply rate and the molten steel temperature, the above carbon concentration of around 1% is only an approximate value. As the decarburization reaction progresses, it is practical to reduce the ratio of oxygen to non-oxidizing gas to create conditions that thermodynamically facilitate preferential decarburization. Indicated by The flow rate of the non-oxidizing gas supplied from the bottom tuyere during blowing may be constant, but in the high carbon concentration region where decarburization takes priority, it should be kept relatively small, so that the flow rate of the non-oxidizing gas supplied from the bottom tuyeres during blowing is relatively small. The total amount of non-oxidizing gas used can be reduced by supplying a large amount of non-oxidizing gas once the concentration is reached. Specifically, after the carbon concentration reaches the point where Cr begins to oxidize, the
It is desirable to supply 0.2 Nm ³ /min or more; if the amount of bottom-blown gas is less than this, oxidation of chromium tends to increase and is not economical. However, if it exceeds 1.0 Nm ³ /min per ton of molten steel, the effect of stirring will not change significantly and the amount of non-oxidizing gas used will increase, which is also not economical. Therefore, considering economic efficiency, 0.2 to 1.0Nm ³ /
min is appropriate. The auxiliary raw materials added before and during blowing are the same as in the conventional method, such as quicklime, dolomite, or a coolant. In addition, instead of so-called stainless crude molten steel,
It is also possible to charge hot metal that has been dephosphorized in advance and then introduce a chromium source such as ferrochrome or chromium-containing ore into the furnace, and such cases are also simply included in the expression of crude stainless steel. . Of course, the chromium oxide in the slag is removed after decarburization and refining.
Similar to the conventional method, reduction with a reducing agent such as Fe-Si and simultaneous desulfurization are performed. In the above explanation, the non-oxidizing gas may be any gas that does not oxidize the chromium content, but in consideration of economic efficiency and handling, it is recommended to use argon, nitrogen, hydrogen, hydrocarbon gas, etc. alone or in combination. Be realistic. It is also possible to use nitrogen for the first half of the blowing and argon for the second half. On the other hand, the non-oxidizing gas supplied from the top blowing lance may be the same or different from the bottom blowing non-oxidizing gas. (Function) In the method of the present invention, a non-oxidizing gas which solely controls the stirring of the steel bath is used below the bath surface of the crude molten stainless steel, so damage to the bottom tuyere and further damage to the bottom refractory is greatly reduced. few. On the other hand, since a mixed gas is blown onto the bath surface for the purpose of preferential decarburization, the amount of Cr loss is also the same as in the conventional method. (Example) Example 1 Using a top-bottom blowing converter as shown in FIG. 55−
Blowing was attempted in each case of Comparative Example 2, which is described in Japanese Patent No. 115914, in which a small amount of non-oxidizing gas was blown from the bottom blowing tuyere and only oxygen was blown from the top blowing lance. Crude stainless steel is SUS304 melted in an electric furnace.
Five tons of (18% Cr-8% Ni-2.5% C) was poured into a top-bottom blowing converter as shown in Figure 1. All non-oxidizing gases supplied were argon. In blowing using a mixed gas, the mixing ratio with oxygen was gradually changed according to the calculated carbon concentration in the molten steel, as shown in Table 1. Further, when carrying out the present invention, only argon from the top blowing lance was used in calculating the mixed gas ratio in Table 1, without taking into consideration argon for bottom blowing.

[Table] Table 2 shows the blowing conditions and results. The charging temperature of the crude molten stainless steel was in the range of 1425 to 1450°C. In addition, quicklime was added to adjust the basicity of the slag, and the CaO/SiO ₂ ratio was adjusted to approximately 2.5. Comparing the amount of chromium oxidation at the same carbon concentration, the second
As shown in the figure. As is clear from the figure, according to the present invention, the amount of chromium oxidation is smaller than the blowing using only top-blown pure O ₂ in Comparative Example 2, and is equivalent to the mixed gas bottom-blowing method. Furthermore, the amount of erosion of the bottom tuyeres during these blowing processes is shown in comparison in FIG. In the case of this invention, the erosion rate is 1/3 to 1/4 that of the bottom blowing method, and the furnace life is estimated to be 3 to 4 times longer. The rightmost column of Table 2 shows the stirring time after Fe-Si injection and the sulfur concentration after the treatment when reductive desulfurization treatment with Fe-Si was performed after decarburization. As in this invention, even when the amount of bottom-blown gas is 1/2 to 1/5 of that of the conventional bottom-blowing method, the reduction of chromium oxide is sufficiently advanced, and therefore the desulfurization is progressing.

【table】

[Table] Example 2 When melting 16% Cr stainless steel by the blowing method according to the present invention, 4 tons of hot metal whose phosphorus concentration had been dephosphorized to 0.020% was charged, and pure O ₂ was blown from a top blowing lance. Coke lumps were added at the same time.
At the same time as coke, a total of 1.6 tons of high carbon ferrochrome was added in small amounts to produce 16% Cr stainless steel. The carbon concentration at this time was 3.9%. Continued decarburization blowing using pure _O2 . When the carbon concentration reached 2.5%, blowing of the mixed gas from the top blowing lance was started according to Table 1. However, nitrogen was used instead of argon for both mixed gas and bottom blowing. When the carbon concentration reached 0.25%, the gas was changed from nitrogen to argon, and blowing with the top-blown mixed gas was continued. Table 3 shows the components and temperatures of the molten steel during and at the end of the above blowing process. From this table, it can be seen that this invention can produce stainless steel equivalent to the conventional bottom blowing method even when dephosphorized hot metal is used or when blowing is performed by switching from nitrogen to argon as the non-oxidizing gas. is obvious. Note that the nitrogen concentration of the final product was 210 ppm.

[Table] Effects of the Invention The present invention provides a method for refining conventional stainless steel.
Unlike AOD furnaces and bottom-blowing converters, there is no need to blow oxygen through the bottom tuyeres, so there are the following effects. (1) Since the tuyere erosion rate is low and the furnace life is extended, the cost of refractories can be significantly reduced, and the furnace repair interval is lengthened, which improves the productivity of molten steel production. (2) Since the equipment is simple, construction costs and operating costs are low, making it economical. Although this invention has been described above for decarburization refining of stainless steel, it can of course also be applied to decarburization refining of high alloy steel containing easily oxidized elements such as chromium and manganese.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of a converter in which blowing was carried out to demonstrate the superiority of this invention. Figure 2 shows the carbon concentration and chromium oxidation cross-section after blowing when decarburizing stainless steel. FIG. 3 is a chart showing the melting rate of the bottom tuyere in comparison with the conventional method. 1...Top-bottom blowing converter, 2...Top-blowing lance, 3
...Piping for top-blown argon and nitrogen, 4... Piping for top-blowing oxygen, 5... Piping for bottom-blowing oxygen, 6...
Piping for bottom-blown argon and nitrogen.

Claims (1)

[Claims] 1. When decarburizing crude molten stainless steel, a non-oxidizing gas is introduced below the bath surface of the molten steel to stir the molten steel, and at the same time, oxygen and non-oxidizing gas are mixed onto the bath surface. A stainless steel melting method characterized by blowing gas. 2. The method according to 1, wherein the ratio of non-oxidizing gas to oxygen in the mixed gas blown onto the bath surface is increased as the carbon concentration in the molten steel decreases.