JPH0741837A - Rh vacuum degassing treatment - Google Patents

Abstract

PURPOSE:To accelerate the decarburization speed of molten steel and to steady smelt an low carbon steel at a low cost by controlling the depth of the molten steel in a vacuum vessel in corresponding to the threshold value of the depth of the molten steel at which gaseous CO bubbles are generatable in the prescribed period within degassing treatment time. CONSTITUTION:The lower parts of immersion pipes 2, 3 of the vacuum vessel 1 are immersed into the molten steel 10 in ladle 5. The pressure in the vacuum vessel 1 is reduced to rise the molten steel 10 and gaseous Ar is supplied from the wall of the immersion pipe 2 to induce circulating flow. As a result, the gaseous CO formed by reaction of carbon and oxygen in the molten steel 10 is removed to decarbonize the molten steel 10. The degassing treatment is executed by controlling the depth h1 of the molten steel in the vacuum vessel 1 to alphaH in the period of the degassing treatment time satisfying alphaH>=15cm in the RH degassing treatment described above, where 0.5<=alpha<1.0, H=gamma(PCO-PV-beta), (PCO: pressure (atom) of the gaseous CO balanced to the carbon concn. and oxygen concn. in the molten steel, PV: the pressure (atm) in the vacuum vessel, beta: constant 0.02, gamma: coefft. 140, H: the threshold value (cm) of the depth of the molten steel at which the gaseous CO bubbles are generatable).

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to RH degassing of molten steel.

[0002]

As a method for removing carbon in molten steel, R
It is common to carry out H 2 degassing treatment,
Recently, demands for shortening the degassing treatment time of low carbon steels and demands for manufacturing ultra-low carbon steels have increased, and further improvement of the decarburization rate is required. Conventionally, in order to accelerate the decarburization rate in the RH degassing process, a method of increasing the amount of gas injected from the rising pipe or the diameter of the dipping pipe to increase the amount of molten steel reflux, and blowing oxygen gas into the molten steel in the vacuum chamber Hiraoka Teruyoshi: The 143rd Nishiyama Memorial Technical Lecture, 1992, p. 107-135 [Japan Iron and Steel Institute] (Reference 1).

[0003]

The method disclosed in Document 1 is effective in improving the decarburization rate, but has a limited decarburization rate, and increases the amount of gas blown from the riser pipe and the vacuum chamber. Oxygen blowing to the molten steel inside and the increase of the immersion pipe diameter are accompanied by an increase in operating costs and equipment costs. For these reasons, a method for further promoting decarburization rate at low cost is desired.

[0004]

According to the present invention, in the RH degassing process, when carbon in molten steel is removed by reaction with oxygen under reduced pressure, during the degassing process time period satisfying αH ≧ 15 cm. , RH vacuum treatment characterized by increasing the decarburization reaction rate by promoting the generation of CO gas bubbles at the bottom of the vacuum chamber by controlling the molten steel depth in the vacuum chamber to αH for degassing Is the way. However, the coefficient α is a value of 0.5 ≦ α <1.0, and H is a value (cm) obtained from the equation (1).

H = γ (P _CO −P _V −β) (1) Here, P _CO is the pressure (atm) of CO gas in equilibrium with the carbon concentration and oxygen concentration in the molten steel, and P _V is the RH vacuum chamber. The internal pressure (atm) and β are constants having a value of 0.02, and γ is a coefficient having a value of 140.

[0006]

FUNCTION FIG. 1 is a diagram schematically showing the RH degassing treatment method. As the RH degassing treatment method, the RH vacuum chamber 1
When the dipping tubes 2 and 3 installed at the bottom of the ladle are immersed in the molten steel 10 in the ladle 5 and the vacuum tank 6 is decompressed, the molten steel in the ladle is removed according to the difference between the atmospheric pressure and the pressure in the vacuum tank. Enter the vacuum chamber. When Ar gas is blown from the wall of the immersion pipe 2, a rising force acts on the molten steel by the action of the buoyancy of the Ar gas, the molten steel is sucked from the ladle into the immersion pipe 2 and rises into the vacuum tank, and the molten steel in the vacuum tank 9 descends from the dip tube 3 and reflux occurs.

In the RH degassing process, mainly
Interface between blown Ar gas bubbles and molten steel, molten steel surface in vacuum tank, splash of molten steel in vacuum tank (molten steel drop)
At the interface between the molten steel and carbon in the molten steel, CO gas becomes CO gas and is degassed to the gas phase side to cause decarburization. At a non-uniform interface or the like, CO gas bubbles are generated by the reaction between carbon and oxygen in the molten steel, and decarburization occurs while the bubbles float in the molten steel. In order to increase the decarburization rate, it is important to promote the reaction in various places such as these, but in the RH degassing process, when the carbon concentration and oxygen concentration in the molten steel are relatively high, Decarburization by CO gas bubbles is active at the refractory interface in the vacuum chamber.

This CO gas generation is likely to occur at the non-uniform interface between the refractory and the molten steel, and tends to occur as the static pressure of the molten steel decreases, that is, the depth of the molten steel decreases and the pressure in the vacuum chamber decreases. In addition, the larger the area of contact with the refractory material, the greater the number of locations where CO bubbles are generated and the greater the amount of CO gas generated.

Limit value H of molten steel depth at which CO bubbles can be generated
(Cm) can be described by the following equation (2) obtained by considering that the bubble can grow when the pressure inside the bubble is larger than the sum of the static pressure of molten steel and the supersaturation pressure when the bubble is generated.

H = γ (P _CO −P _V −β) (2) Here, P _CO is the pressure (atm) of CO gas in equilibrium with the carbon concentration and oxygen concentration in the molten steel, and P _V is the RH vacuum chamber. The internal pressure (atm), β is a constant related to the supersaturation pressure when bubbles are generated, which is a value of about 0.02, and γ is a coefficient of 140.

At the interface between the refractory in which the CO gas bubbles are generated and the molten steel, there is an inner wall 8 and a bottom portion 7 in the vacuum chamber in FIG. 1, but the molten steel depth h ₁ in the vacuum chamber is set to the above (2). When the depth is made deeper than the depth H described by the formula), CO gas bubbles are generated only in a part of the inner wall 8 and are not generated from the bottom portion 7. Therefore, in order to increase the decarburization rate, it is necessary to make the molten steel depth h ₁ of the vacuum chamber shallower than H and perform degassing operation. However, if the depth of molten steel in the vacuum tank is made too shallow, the flow of molten steel will be hindered, the amount of reflux will decrease, and the decarburization rate will decrease, so there is a lower limit for the depth of molten steel. Regarding the molten steel depth h ₁ in the vacuum tank, the molten steel height (h ₂ ) from the ladle inner surface to the vacuum tank inner molten surface is mainly controlled by the pressure in the vacuum tank and the molten steel recirculation amount. It is possible to control the depth of molten steel to the desired level by moving the position of the pan up and down.

FIG. 2 is a diagram schematically showing changes with time in the degassing treatment time with respect to the pressure P _{CO of the} CO gas and the pressure P _V in the vacuum chamber, which are in equilibrium with the carbon concentration and the oxygen concentration in the molten steel. is there. (2) from the equation, the limit value H of the molten steel depth CO gas bubbles can be generated increases as the difference between P _CO (P _V-beta) is large, CO gas bubbles are seen to be generated in a deeper position . On the other hand, from FIG. 2, the difference between P _CO (P _V -β) becomes a positive value, the time value of H is a value that does not cause trouble in the reflux of molten steel, except for the initial and end of treatment If the molten steel depth in the vacuum chamber is controlled to be sufficiently shallow during this period, CO gas bubbles will also be generated from the bottom of the vacuum chamber and the decarburization rate will increase.

[0013]

[Example] Using an RH vacuum chamber with an inner diameter of the vacuum tank of 2.4 m and an inner diameter of the dip tube of 0.65 m, the molten steel (30
The degassing treatment experiment of 0t) was performed. The carbon concentration in the molten steel before the treatment was 300 ppm and the oxygen concentration was 400 ppm, and the molten steel in the ladle was sampled at regular intervals during the treatment time, and the change with time of the molten steel composition was measured. Molten steel depth in vacuum chamber (αH)
By moving the ladle up and down, the above (1)
The molten steel depth was controlled so that the value of H in the equation was multiplied by a coefficient α to obtain αH. P _CO in equation (1) is _calculated from equation (3),
Using the measured values in P _V.

[0014] In _{P CO = K · C C ·} C O ... (3) where, C _C is a carbon concentration _{(mass%), C C =} C C0
-Expressed by exp (-k _C · T), C _O is the oxygen concentration (m
%). As the value of the oxygen concentration, the average value of the measured values was used because the change during the degassing process is relatively small. K is the equilibrium constant of the CO reaction, C _C0 is the initial carbon concentration (m
%), T is T = t−t ₀ , t is a degassing treatment time (min), t ₀ is a constant (min), and k _C is a decarburization rate constant (1 / min). Constant t ₀ and constant k
The value of _C can be determined so that the calculated value best matches the measured value of the change in carbon concentration over time. In the RH degassing treatment, the carbon concentration is apparently substantially constant during the time t ₀ from the start of the treatment, and after the time t ₀ , the carbon concentration begins to decrease. In this experiment, the value of the constant t ₀ was about 2.5 minutes.
Further, the value of k _C in each experiment was obtained, and as a result of changing the molten steel depth of the vacuum chamber, it was about 0.17 to 0.27 (1 / mi
The value was in the range of n).

In one decarburization experiment, the coefficient α was kept constant and the value of α was changed for each experiment. The experiment was conducted with the operating conditions other than the molten steel depth in the vacuum chamber being kept constant. For comparison, an experiment was also conducted in which the molten steel depth during processing was kept constant at 50 cm. Further, if the molten steel depth is made too shallow, the molten steel recirculation amount decreases and the decarburization rate decreases, so the value of αH is 1
In the period of 5 cm or less, the molten steel depth is 50 cm
It was fixed at. For comparison, an experiment was also conducted in which the molten steel height was controlled to αH during αH ≧ 10 cm.

As a result of the experiment, the depth of molten steel in the vacuum chamber was set to 50 cm.
In the case where the treatment was carried out at the same temperature as above, the carbon concentration at the treatment time of 15 minutes was 25 ppm. On the other hand, according to the present invention, the processing time when the molten steel depth is controlled to αH is 15
The carbon concentration in minutes is shown in FIG. When the molten steel depth is controlled in the period of αH ≧ 15 cm, the coefficient α is 0.5 ≦
In the range of α <1.0, the carbon concentration is lower than 25 ppm when the molten steel depth is not changed, and there is an effect of the method of shallowing the molten steel depth during the treatment according to the present invention. I understand. On the other hand, when the molten steel depth is controlled for a period of αH ≧ 10 cm, the molten steel depth becomes 15 cm or less and the reflux of the molten steel deteriorates. Higher than 25 ppm of
It is disadvantageous for decarburization.

[0017]

According to the present invention, the decarburization rate of molten steel can be easily promoted, and low-carbon steel can be stably produced at low cost.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of an RH degassing treatment method.

FIG. 2 is a diagram showing a control period of molten steel depth in a vacuum chamber during a degassing process.

FIG. 3 is a diagram showing a carbon concentration in molten steel after 15 minutes of degassing.

[Explanation of symbols]

1 ... Vacuum tank 2 ... Immersion pipe (upward pipe) 3 ... Immersion pipe (downward pipe) 4 ... Ar gas injection position 5 ... Ladle 6 ... Vacuum tank inside 7 ... Vacuum tank bottom 8 ... Vacuum tank inner wall 9 ... Vacuum tank molten steel 10 ... from the molten steel 11 ... slag h ₁ ... vacuum tank molten steel depth h ₂ ... ladle molten steel surface in the ladle to a vacuum vessel molten steel surface level of the inner (molten steel head)

Claims (1)

[Claims] 1. In RH degassing treatment, when carbon in molten steel is removed by reaction with oxygen under reduced pressure, αH ≧ 15
The degassing treatment is performed by controlling the molten steel depth in the vacuum chamber to αH during the degassing treatment time that satisfies the cm, thereby promoting the generation of CO gas bubbles at the bottom of the vacuum chamber and performing the decarburization reaction. RH vacuum treatment method characterized by increasing speed However, the coefficient α is a value of 0.5 ≦ α <1.0, and H is a value (cm) obtained from the equation (1) H = γ (P _CO -P _V -β) (1) P _CO : Pressure of CO gas in equilibrium with carbon concentration and oxygen concentration in molten steel (atm) P _V : Pressure in RH vacuum chamber (atm) β: 0.02 in constant Value γ: 140 value as a coefficient