JPH02290929A - Vacuum refining method for molten metal - Google Patents

Abstract

PURPOSE:To generate fine bubbles from the innermost in a bath and to improve refining efficiency by controlling the atmosphere in a furnace to the prescribed pressure at the time of removing inclusions by allowing the inclusions to be entrapped by the bubbles generated by evacuating a molten metal in which gas is dissolved. CONSTITUTION:Evacuation is carried out simultaneously with or after the dissolution of gas in a molten metal, by which fine gas bubbles are generated in the above molten metal. Subsequently, inclusions existing in the molten metal are removed by allowing the inclusions to be entrapped by the bubbles. At this time, the pressure Pa of the atmosphere in the furnace in which the above treatment is exerted is controlled so that the value of Px-Pa (where Px means the equilibrium of gaseous partial pressure the soluble gas in the molten metal) is regulated to >=0.05atm.

Description

[Detailed description of the invention]

[Industrial Application Field] This invention relates to a vacuum refining method for removing impurities and inclusions in molten metal. [Prior Art] The present inventors have proposed the following in order to improve the efficiency of removing inclusions from molten metal. That is, a gas soluble in the molten metal is bubbled into the molten metal to dissolve it, and fine gas bubbles are generated in the molten metal by reducing the pressure. According to this method, even minute inclusions in the molten metal are trapped by the bubbles and floated to the surface, resulting in extremely high inclusion removal ability. On the other hand, the present inventors got a hint from the bubble floating state in the above-mentioned inclusion removal, and developed the application of this to a degassing refining method or a slag refining method. That is, according to the above procedure, fine gas bubbles are generated in the molten metal, and their floating causes the interface between the molten metal and the atmosphere or the interface between the molten metal and molten slag to flutter, thereby increasing the reaction interface area. It is. Since these gas bubbles are generated from the entire area of the molten metal, the flutter at the interface extends over the entire area of the interface. Moreover, since a large number of fine gas bubbles are generated, the increase in the reaction interface area becomes remarkable, and the ability to remove impurities by degassing refining or slab refining becomes extremely high. [Problems to be solved by the invention] Both refining methods are very efficient methods for removing inclusions and impurities in molten metal and obtaining ultra-clean molten metal, but the decompression processing time still takes 20 minutes. There was a request to further shorten this time and improve efficiency. The present invention was devised in view of the above problems, and aims to optimize the reduced pressure processing conditions in the vacuum refining method described above.
This is intended to shorten the processing time. [Means for solving the problem] Therefore, the present invention provides the above-mentioned method for removing inclusions and
The basic feature of the impurity removal method is that the atmospheric pressure Pa in the processing furnace is controlled under the conditions shown in the following formula with respect to the equilibrium gas partial pressure Px of the soluble gas component in the molten metal during pressure reduction. Px? a≧0. 0 5 (atm) The following was the basis for creating this configuration. In other words, the inclusion trapping efficiency by fine gas bubbles becomes better as the bubble diameter becomes smaller and the bubbles are generated from deeper in the bath, provided that the amount of fine gas bubbles generated is the same. Also, when attempting to increase the reaction interface area by flapping the interface with the molten slag by floating gas bubbles, the smaller the bubble diameter, the better from the standpoint of reaction efficiency. On the other hand, as a bubble generation condition, for example, when soluble gas X is bubbled into molten metal (1) and dissolved as shown in Fig. 2, the gas component (X) is generated as fine gas bubbles (2). The conditions for this must satisfy the following equation for the equilibrium gas partial pressure Px. 2 σ Px ≧ Pa +ρgh+- r Pa: Atmospheric pressure in the furnace during degassing g: Gravitational acceleration h: Bubble generation depth σ: Surface tension of the molten metal r: Critical bubble radius As is clear from the equation, the atmosphere inside the furnace The lower the pressure Pa becomes during depressurization, the more likely fine gas bubbles (2) are generated. Also, air bubbles (2
) can be generated, and furthermore, the diameter of the bubbles (2) itself becomes smaller. Therefore, the present inventors conducted tests in the examples described below, and from the test results, the molten metal (
1) To what extent should the atmospheric pressure Pa in the furnace be lowered relative to the equilibrium gas partial pressure Px of the soluble gas component (X) in the mixture?
We clarified whether this method is effective in removing impurities. In other words, by controlling the pressure so that Px-Pa≧0.05 atm during the depressurization process, it is possible to generate a large amount of fine gas bubbles from deep within the bath, thereby greatly increasing the refining efficiency. We have discovered this and proposed the above configuration as the present invention. [Example] Specific examples of the present invention will be described above. The present inventors installed T, (0)=
= 25 ppl1, (N) = 50 tons of molten steel of 260 ppm and heated at 1660°C to 650 to
10 with a porous plug from the bottom of the ladle while keeping it at rr.
6N113 Nt gas was injected over a period of minutes. Afterwards, the pressure was rapidly reduced and the specimen was left as it was. At the same time, Ar gas was bubbled from the bottom of the ladle at a rate of 3 Q N Q /win to stir the molten steel. Figure 1 shows that when the furnace gas pressure Pa is lowered below the equilibrium gas partial pressure Px of N8 gas in molten steel, T

This is a graph of the standing time (deoxidation time) required after the start of decompression for the molten steel, which had 25 ppm of [0], to become 6 ppm or less. Note that the X-axis coordinate in the figure is the average of the difference (Px-Pa) between the equilibrium gas partial pressure Px and the furnace gas pressure Pa during the pressure reduction process. As is clear from the figure, the average (Px-Pa) is 0,0
The standing time required for deoxidation is significantly shorter after 5 atm, so the furnace pressure Pa is controlled so that this difference is 0.05 atm or more (preferably 0.1atII1 or more) when depressurizing. It has become clear that by doing so, the overall processing time can be shortened. In addition, in degassing refining and slag refining, the dehydration reaction and slag refining can be achieved by controlling the furnace pressure Pa so that the pressure difference is within the above range during the depressurization treatment after once injecting N2 gas into the molten steel. It has been confirmed that the desulfurization reaction progresses significantly and is effective in shortening processing time. Note that if the atmospheric pressure Pa in the furnace is suddenly lowered during depressurization, only fine gas bubbles will be generated in large quantities and will float up and escape without trapping inclusions, so soluble gas components (
The atmospheric pressure Pa in the furnace is appropriately controlled while observing or predicting the degassing behavior of X), and as a result, the equilibrium gas partial pressure Px
It is preferable that the average difference between the two values falls within the above range.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the correlation between deoxidation time and average (Px-Pa), and FIG. 2 is an explanatory diagram showing the conditions for generating fine gas bubbles in the gas dissolved in the molten metal. In the figure, (1) shows the molten metal, and (2) shows the fine gas bubbles. Deoxidation time (min)

Claims (1)

[Claims] 1. Dissolve a gas soluble in the molten metal, reduce the pressure at the same time or after that, generate fine gas bubbles in the molten metal, trap the gas in the molten metal, and melt the metal. In a vacuum refining method for molten metal to remove inclusions in the metal, the atmospheric pressure Pa in the processing furnace is expressed by the following formula with respect to the equilibrium gas partial pressure Px of the soluble gas component in the molten metal when the pressure is reduced. A vacuum refining method for molten metal characterized by controlled conditions. Px-Pa≧0.05 (atm) 2. Dissolve a soluble gas in the molten metal, and reduce the pressure at the same time or after that to generate fine gas bubbles in the molten metal, and In addition to removing inclusions in the molten metal by trapping them, the floating of the bubbles causes the interface between the molten metal and the atmosphere and/or the interface between the molten metal and molten slag to flutter, increasing the reaction interface area, and removing the molten metal. In the vacuum refining method of molten metal to remove impurities in the molten metal, the atmospheric pressure Pa in the processing furnace is set under the conditions shown in the following formula with respect to the equilibrium gas partial pressure Px of the soluble gas component in the molten metal when the pressure is reduced. A vacuum refining method for molten metal characterized by control. Px-Pa≧0.05 (atm)