JPH0649896B2 - Method of degassing and dephosphorizing molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is required when an extremely low phosphorus steel (for example, [P] <50 ppm) is produced by dephosphorization in a molten steel stage, or in terms of steel quality. When the molten steel [P] level is higher than the [P] level and the molten steel must be dephosphorized to the required quality level, the degree of vacuum in the degassing tank is adjusted according to the [C] level in the vacuum degassing equipment. The present invention relates to a method for efficiently performing molten steel dephosphorization by controlling the.

(Prior Art) As a conventional method for dephosphorizing molten steel using a vacuum degassing tank, there is a method described in JP-A-62-205221 (method for degassing and dephosphorizing molten steel). This is because the free oxygen is 100 to 8 through the powder blowing tuyere provided in the lower part of the vacuum degassing tank.
It is characterized by blowing powder dephosphorizing agent into molten steel of 00 ppm. However, according to this method, the decarburization reaction occurs simultaneously with the dephosphorization reaction due to the characteristics of the vacuum degassing equipment. That is, as shown below, since both the decarburization reaction and the dephosphorization reaction are oxidation reactions, there is a disadvantage that the free oxygen in the molten steel is deprived of each other in the degassing tank, and as a result, the dephosphorization reaction rate decreases. .

Decarburization reaction C + O = CO 2 Dephosphorization reaction 4CaO + 2P + 50 = 4CaO · P ₂ O ₅ (Problems to be solved by the invention) In contrast to the disadvantage that the dephosphorization rate is reduced by competing for oxygen, the decarburization reaction is controlled by controlling the vacuum degree in the degassing tank within a specific vacuum degree range according to the [C] level when the dephosphorizing agent is blown. It is overcome by preferentially performing dephosphorization under the condition that the and dephosphorization reactions do not compete with each other.

(Means for Solving the Problem) The gist of the present invention is that a dephosphorizing agent is blown into undeoxidized molten steel (free oxygen> 100 ppm) in a vacuum degassing tank to degas according to the [C] level. When performing degassing / dephosphorization treatment by controlling the degree of vacuum in the tank, the degree of vacuum in the degassing tank according to the [C] level is controlled within the following range. There is a phosphorus method.

[C] <40 ppm Vacuum degree> {307.7 x log [C] (ppm) -454.5} torr [C] ≥ 40 ppm Vacuum degree> {100.0 x log [C] (ppm) -120.0} torr The present invention has the above-described problems with the prior art. When performing degassing and dephosphorization of molten steel by injecting a dephosphorizing agent, the degassing tank during injecting the dephosphorizing agent according to the [C] level of the molten steel. The solution is to control the degree of vacuum in a certain range to avoid the competition between the decarburization reaction and the dephosphorization reaction and to improve the dephosphorization rate.

Next, an example of the present invention will be described in detail with reference to FIG.

FIG. 1 shows an example of the RH vacuum degassing equipment. 1 is a vacuum degassing tank, 2 is an exhaust hole, 3 is molten steel, 4 is a ladle, 5 is a powder dephosphorizing agent blowing tuyere, 6 Is a powder supply pipe, 7 is a dephosphorizing agent supply device, 8 is a carrier gas body, 9 is a degassing tank vacuum degree measuring device, and 10 is a vacuum exhaust facility.

According to the present invention, when the dephosphorizing agent is blown into the molten steel 3 from the powder blowing tuyere 5 from the dephosphorizing agent supply device 7 through the powder supply pipe 6 during the vacuum degassing process, The vacuum degree measured by the in-vessel vacuum degree measuring device 9 is controlled by turning on / off the vacuum exhaust equipment 10 (ejector etc.) according to the [C] level of the molten steel and adjusting the suction gas amount. is there. At this time, the free oxygen level is 100-
800 ppm is desirable from the viewpoint of reaction efficiency and prevention of refractory melting damage.

Fig. 2 shows the optimum control range for the [C] level and the degree of vacuum in the tank. When the control range is [C] <40 ppm, the degree of vacuum is {307.7 x log [C] ppm-454.5} torr [ C] ≧ 4
In the case of 0 ppm, the degree of vacuum is controlled to be {100.0 × log [C] ppm-120.0} torr. That is, at an extremely low carbon level of [C] <40 ppm, the decarburization reaction stagnates even in a high vacuum state, so that the dephosphorization reaction occurs efficiently even in a high vacuum state. However, in the range of 40 ppm ≦ [C], the decarburization reaction occurs even in a low vacuum state, so that the vacuum degree must be controlled to be low in order to efficiently perform the dephosphorization reaction. Here, for a high [C] material (for example, [C] = 0.15%), it is impossible to completely eliminate the decarburizing reaction even if the vacuum degree is controlled. I have no choice but to refuel. At this time, if the carburizing agent is added during the blowing of the dephosphorizing agent, the free oxygen level in the molten steel will be lowered, and the dephosphorizing rate will be lowered.

Therefore, for the high [C] material, the lower the degree of vacuum during dephosphorization is, the more preferable. On the other hand, if the powder blowing tuyere is not immersed in the molten steel, the blown powder floats on the surface of the molten steel or is sucked by the vacuum exhaust equipment and does not contribute to the dephosphorization reaction, so that the dephosphorization efficiency deteriorates. Therefore, the powder blowing tuyere arranged in the lower part of the degassing tank, the molten steel in the ladle rises through the dip pipe by evacuating the inside of the degassing tank and reaches the degassing tank, The minimum degree of vacuum is the degree of vacuum when the molten steel depth in the state where the tuyere is immersed is reached. Therefore, although there are some differences depending on the installation position of the tuyere,
The vacuum degree control range is expanded by setting the installation position downward.

(Example) An example comparing the dephosphorization rates of the conventional technique and the method of the present invention under the following conditions is shown in FIG. In this embodiment, 250t of molten steel with a quality requirement [C] level of 50ppm (free oxygen =
Dephosphorizing agent (65% CaO / 35% Ca) against 450 ppm
The dephosphorization behavior when F ₂ ) was blown in 4 kg / t at a rate of 100 kg / min with 100 Nm ³ / h of Ar gas was controlled depending on whether the vacuum degree was controlled to 1 torr or 80 torr based on the present invention. It is a comparison made twice each.

As shown in FIG. 3, by applying the present invention and controlling the degree of vacuum, it is possible to obtain a significant effect of improving the phosphorus removal rate. As a result, it becomes possible to dephosphorize stably even in a state where the initial [P] is higher than before, and the basic unit of dephosphorizing agent required to obtain the same dephosphorizing effect is reduced, and at the same time molten steel by the dephosphorizing agent is used. RH because the temperature drop of the
The processing time can be shortened, and the molten steel dephosphorization cost can be significantly reduced.

(Effect of the Invention) According to the present invention, when performing demelting of molten steel in a vacuum degassing apparatus, dephosphorization can be efficiently performed with a smaller dephosphorizing agent as compared with the conventional method. It is possible to reduce the basic unit and the accompanying decrease in molten steel temperature lowering cost and shorten the RH treatment time, and it is possible to significantly reduce the phosphorus removal treatment cost.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an example of an apparatus to which the present invention is applied, FIG. 2 is a diagram specifically showing a limited range of the present invention, and FIG. It is a comparison.

Claims (1)

[Claims] 1. A dephosphorizing agent is blown into undeoxidized molten steel (free oxygen> 100 ppm) in a vacuum degassing tank, and the degree of vacuum in the degassing tank is controlled according to the level of [C] to perform degassing and dephosphorization. A method for degassing and dephosphorizing molten steel, characterized in that the degree of vacuum in the degassing tank according to the [C] level is controlled within the range shown below when performing the treatment. [C] <40 ppm Vacuum degree> {307.7 x log [C] (ppm) -454.5} torr [C] ≥ 40 ppm Vacuum degree> {100.0 x log [C] (ppm) -120.0} torr