JPH0225251A - Rh vacuum degassing treatment - Google Patents

Abstract

PURPOSE:To prevent the suction of converter slag and to obtain a clean molten steel by adjusting the immersion length of a riser to such a length at which the numerical value related by a molten steel density, molten steel reflux rate, the inside diameter of the riser and the immersion length of the riser attains a prescribed value or below. CONSTITUTION:The relation F=K.B is obtd. between the slug inclusion index B to the riser and the F expressed by the equation. The above-mentioned immersion length l is so adjusted that the value of the F of the equation related by said index B, the molten steel density rho(kg/cm<3>), the molten steel reflux rate Q(m<3>/s), the inside diameter D(m) of the riser and the immersion length l(m) of the riser attains <=4000. The slag inclusion during the treatment with the RH vacuum degassing device is prevented in this way and the steel having the high cleanlines is obtd. The yield of a deoxidizing agent is improved in this way and the fluctuation of the quality in production is effectively suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field 1] The present invention relates to an R11 vacuum degassing treatment method. The present invention relates to a method for RH vacuum degassing treatment in which the immersion length and the like are adjusted.

[Prior art l The R1111 empty degassing treatment method involves immersing two or more immersion tubes provided at the bottom of a cylindrical vacuum degassing tank into molten steel in a ladle, and placing one of the immersion tubes (this one above) into the molten steel in a ladle. In the process of sucking up molten steel from the immersion pipe (called the S1 pipe) by the gas lift effect and discharging it into the ladle from the other immersion pipe (called the downcomer pipe), gas components in the molten steel are removed or non-metallic inclusions are removed. This is a scouring method that removes by floating.

The molten steel rises from the rising pipe and descends from the downcomer pipe, which is the so-called reflux motion.The speed of the molten steel is called the reflux velocity Q (tn'/s
), and in order to increase the degassing efficiency and promote the cleaning of molten steel, the diameter of the immersion tube is increased and the reflux rate is increased.For example, the inner diameter of the immersion tube is 30 to 40 cm. However, the diameter has increased to 55 to 70 cm, and the circulation speed has also increased to 5.
0~70t/min to 120~l 50t/
It has increased to min. Further, as another method, for example, as disclosed in Japanese Patent Laid-Open No. 61-226143, a method has been proposed in which the immersion length of the downcomer pipe is adjusted in order to sequentially circulate the entire amount of molten steel into the membrane gas Dti.

[Problem to be solved by the invention J Normally, when steel is tapped from a converter to a ladle, it is unavoidable that converter slag gets mixed in. The T and Fe of converter slag in a top-bottom blowing converter are 10
What is the weight percent? In the case of carbon steel with [C] = 0.04 weight percent or less, the converter slag is oxidizing steel slag with T and Fe content of about 15 weight percent.

The RH vacuum deoxidation treatment is a method in which a required amount of deoxidizer is introduced before or during the RH treatment, and nonmetallic inclusions are floated and removed by the reflux action of the RH device. However, in the above method, if the immersion length of the upper tube is short, oxidizing converter slag is sucked in from the ladle steel bath surface near the riser tube, contaminates the steel, and reduces the amount of total oxygen ( (hereinafter abbreviated as T and O)
The current situation that does not go down will occur. Moreover, the addition yield of the deoxidizing agent becomes worse.

The present invention was made in order to solve the above problems, and it is possible to prevent converter slag from being sucked in and to obtain clean molten steel.
The purpose of the present invention is to provide a vacuum degassing treatment method.

[Means to solve the problem]

The factors that affect the suction of converter slag into the riser pipe are considered to be the reflux velocity and the immersion length of the upper limit pipe. When we investigated the effect of suction on the riser pipe, we found that F = (8/)Q3/ (KO2)']
/123, the slab entrainment index B into the upper tube
As shown in Fig. 1, a relationship of -H is obtained for F=, and it was found that it can be rearranged by the following equation.

[8ρQ3/ (KO2)2]/g3≦-B...
(1) Here, B: ρ: Q: D22: To: Slab entrainment into the riser pipe Exponential melt 'M density (kg/tr+') Circulation rate (Kg/tr+')
s) Immersion pipe inner diameter (m) Immersion length (m) Constant.

The slag entrainment index B value represents an index value of the area ratio occupied by slag-based inclusions when a molten steel sample taken from a ladle is observed under a microscope. B value and 1′ in steel! ! As shown in FIG. 2, the elements are in a nearly proportional relationship. T and O are 20ppm
In the following, inclusions other than slag-based inclusions are considered to be the main components.

Oxygen in steel after RH treatment is reduced by reducing B value, but B
= 20 or less, there is almost no difference.

To prevent the influence of slag entrainment on oxygen in steel, B≦
It is desirable to set it to 20, and at the same time T.

This satisfies 0≦30ppm. Therefore, B≦2
From the condition of 0, as shown in Figure 1, [8ρQ3/(π
D')2]/β3≦4000 (2) It was found that oxygen in steel can be reduced if the following relationship is satisfied.

[Effect J] The above formula (2) can be satisfied by decreasing the circulation velocity Q, increasing the inner diameter of the riser tube, or increasing the length 2 of the immersion tube.

Decreasing the reflux rate Q will lengthen the processing time and be inefficient. Increasing the inner diameter of the upper μ tube has a limit due to equipment problems. Therefore, the immersion length of the immersion tube in the steel It is most desirable to adjust the value of 2, and by this adjustment, steel with high purity can be obtained.

[Example] Deoxidation treatment of molten steel tapped into a ladle from a 250 ton converter in an RH degassing device was performed using a conventional method and a method of the present invention, which will be described below.

FIG. 3 is a longitudinal cross-sectional view showing the bottom of the RH degassing device l installed on the ladle 2.
The inside is vacuumed by a vacuum pump (not shown), and is removed from the rising pipe 5! The molten fl13 in 42 is sucked up and degassed under vacuum, and then descended from the downcomer pipe 6 and circulated. fEll 7M gas 9 is supplied into the upper four pipes 5. Slag 4 is on the upper surface of the molten steel in the drawer A2, and the molten steel 3 generates a vortex 7 in the rising pipe, and the molten steel in the immersion pipe (upper pipe 5 and descending pipe 6) flows into the molten steel. If the immersion length (β) 8 is insufficient, there is a risk that the slag 4 will be sucked into the riser pipe.

Using a device in which the riser pipe 5 has an inner diameter of 550 mm.

Reflux gas ill 50ONβ/min, reflux flow rate 10
The treatment was carried out at 0 ton/min (14 rrl'/min) for 10 minutes. First, the immersion length is 400 m in the above RH device I.
It was treated at m.

At this time, F=13,000, significantly exceeding 4,000. At this time, the slag entrainment index into the upper tube is B
=35. Moreover, T and O were 30 ppm.

However, T before RH treatment.

O was 450 ppm.

Therefore, under the same operating conditions as above (excluding the immersion length), the calculated value of [8ρQ3/(π[)2)2]/fi3 is 40
00 or less, the height of the ladle was lowered and the immersion length was changed to 600 mm.

At this time, F=3900, which is less than 4000,
The index of slag entrainment into the riser pipe was B=19. T,
The O content was revised to 25 ppm. However, T and O before III treatment were 440 ppm.

[Effects of the Invention] As described above, according to the present invention, the slag entrainment index into the riser pipe, the molten steel density, the reflux velocity, the inner diameter of the riser pipe, and the immersion length of the riser pipe are used to Since the immersion length of the riser pipe is controlled so that the calculated value is less than the value that does not involve slag, slag inclusion during processing in the RH vacuum degasser is prevented and highly clean steel is obtained. At the same time, it is highly effective in increasing the yield of the deoxidizing agent and suppressing variations in manufacturing quality.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between RH degassing operation factors and slag entrainment, Figure 2 is a graph showing the relationship between slag entrainment index and T, O, and Figure 3 is a longitudinal section explaining R)I degassing treatment. It is a front view. 1-RH degassing device 2... Ladle 3... Molten steel 4... Slag 5... Rising pipe 6... Descending pipe 7... Vortex 8... Immersion length/T, O >30PPm

Claims (1)

[Claims] 1. In the RH vacuum degassing method, the molten steel density ρ (kg/
m^3), molten steel circulation velocity Q (m^3/s), riser pipe inner diameter D (m), and riser pipe immersion length l (m), the value of F in the following formula is 4000 or less. R characterized in that at least the immersion length l of the riser pipe is adjusted so that
H Vacuum degassing treatment method. F=[8ρQ^3/(πD^2)2]/l^3