JPS59208011A - Method for desulfurizing molten steel - Google Patents

Abstract

PURPOSE:To desulfurize molten steel while degassing it by blowing an inert gas for causing the reflux of the molten steel and a powdered desulfurizing agent from plural tuyeres each having a specified inside diameter or below when the molten steel is vacuum-degassed. CONSTITUTION:Two pipes 2, 3 extended downward from the bottom of a vacuum vessel 1 are immersed in molten steel 5 in a ladle 4, and an inert gas is blown into the pipe 2 to vaccum-degas the molten steel 5 while circulating it in the ladle 4 and the vessel 1. At this time, plural tuyeres 7 each having <=6mm. inside diameter are attached to the pipe 2, and a powdered desulfurizing agent suspended in the inert gas is blown from the tuyeres 7 to carry out desulfurization as well as vacuum degassing.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for performing gas treatment on molten steel and simultaneously desulfurizing the molten steel using the RH1152 gas method.

Recently, with the aim of improving the material properties of steel materials, there has been an increasing demand for ultra-low sulfur steel with a sulfur concentration of about 10 ppIIl or less, and for this reason, technology has been developed to industrially desulfurize molten steel to an extremely low S concentration at low cost. There is a strong demand for the development of Conventionally, as a method for desulfurizing molten steel to a low concentration range, the molten steel discharged from a refining furnace such as a converter into a ladle is treated with fine powder such as CaO powder in the molten a in the ladle. A so-called "trickling injection method" is known in which desulfurization is carried out by injecting a desulfurizing agent with a carrier gas together with an inert gas as a carrier gas, and with this method, it is possible to reduce only the sulfur concentration to a considerable extent. However, for ultra-low sulfur steel, it is often required not only to have a low sulfur concentration, but also to have low nitrogen and hydrogen concentrations, but the ladle injection method described above requires a denitrification system. and dehydrogenation are not carried out at the same time, and on the contrary, the nitrogen concentration and hydrogen concentration in the molten steel often rise due to atmospheric entrainment and desulfurization agents during the injection process. Therefore, after undergoing desulfurization treatment using the ladle injection method, the molten steel is subjected to vacuum degassing treatment.
It is necessary to reduce nitrogen and hydrogen concentrations.

As mentioned above, in order to obtain molten steel with an extremely low sulfur concentration, low visitor element concentration, and low hydrogen concentration, conventionally desulfurization treatment and degassing treatment had to be performed in separate processes, resulting in low production efficiency. There was a problem. In addition, molten steel discharged from a refining furnace such as a converter into a ladle must be held for two long periods for desulfurization and descaling treatment. It became necessary to raise the temperature to 1,700°C or higher, which caused moldy damage to the refractories in the refractory furnace, which caused major problems in terms of production costs.

As one method that may potentially solve the above problems, a method has been proposed in which vacuum degassing treatment and desulfurization treatment are performed at the same time, as illustrated in, for example, Japanese Patent Publication No. 45-22204@. There is. In this method, a finely powdered desulfurizing agent is injected together with inert gas that is blown into the riser pipe in order to return the molten steel between C9 and the vacuum tank at the RH degassing treatment neck. Desulfurization treatment can be performed at the same time. However, the method proposed above does not take into account desulfurization down to a low detection area of about 10 pI) III or less, and in fact, the inventor of the present invention used an ordinary RH degassing device to desulfurize the tuyere of the riser pipe. When an experiment was conducted in which fine powder of a desulfurizing agent was injected together with an inert gas, it was difficult to desulfurize stably down to the extremely low sulfur range of about 1 oppm or less. Therefore, it is difficult to industrially apply the proposed method as it is to the production of extremely low sulfur content.

This invention was made in view of the above circumstances, and is an improvement on the method proposed in Japanese Patent Publication No. 45-22204, and the sulfur resistance in the wire is 101JllI! The object of the present invention is to provide a method capable of industrially stably producing ultra-low sulfur steel in one step, which has an extremely low concentration of about 1 or less, and also has a low nitrogen concentration and low hydrogen density.

The inventors improved the method of the above-mentioned Japanese Patent Publication No. 45-22204 and conducted various experiments and experiments in order to find a method for desulfurizing molten steel to an extremely low sulfur concentration, such as less than the ioppm level of sulfur pickling.
As a result of repeated studies, it was discovered that the inner diameter of the tuyeres and the number of tuyere for injecting the desulfurizing agent fine powder into the riser pipe together with an inert gas did not affect the desulfurization rate, and this invention was achieved. .

In other words, the desulfurization method of this invention consists of two pipes extending downward from the lower surface of the vacuum inlet into the chain, and an inert gas injected into one of the pipes (i.e., the upper pipe). Blow it in and melt it
When vacuum treating molten steel while circulating it in a vacuum (between ∆), the desulfurization treatment of the molten steel is carried out by suspending and injecting a powdered desulfurizing agent into the above-mentioned non-toxic gas. In the method carried out simultaneously with gas treatment, a tuyere with an inner diameter of 6 j'
It is characterized by the simultaneous use of a plurality of fence tuyeres of a size smaller than 1.5 mm.

The method of this invention will be explained in more detail below.

FIG. 1 schematically shows an example of a situation in which the method of the present invention is implemented, and shows the lower portions of two pipes extending downward from the bottom surface of the vacuum nago 1, that is, the ascending pipe 2 and the descending pipe 3. is included in the melt 635 of ladle 4. Further, an exhaust port 6 is formed in the upper part of the vacuum chamber 1;
Then, by reducing the pressure inside the vacuum chamber 1 with a pressure exhaust device (not shown), the melt (] 5 in the tube 4 is sucked up into the vacuum chamber 1. In addition, in the middle part of the riser pipe 2, G2
As explained in detail in 2007, the inner diameter (several cypress blade lowers made of stainless steel of 31111ii or less) is inserted.From these blades D7 to the old S eye in the riser pipe 2 When active gas is injected, the density immediately decreases by an order of magnitude (because the boarding in the riser pipe 2 contains air bubbles) and starts an upward movement, thereby increasing W q to Ii;! The molten steel 84'q + 〒flow rises from the ladle 4 through the riser pipe 2 and returns from the vacuum chamber 1 to the ladle 4 via the downcomer pipe 2. At this time, the degassing treatment is carried out by adding a desulfurizing agent such as CaO to an inert gas and blowing it into the molten steel from the lower of the 20 risers. At the same time, desulfurization treatment is performed.In addition, in the lower part of Figure 1,
? Individual conveyance pipes 10A to 100 connected to the powder supply device 9 independently for each vane lower are used as conveyance tubes for conveying the powder desulfurization agent 8 from the powder supply device 9 to each vane lower. It is being

In the method of this invention, as described above, as a means for injecting the fine powder desulfurization agent into the molten steel in the riser pipe 2 together with the inert gas, the inner diameter is 6n+m or less, preferably 4mm.
I use several of the following.

The reason for this determination will be explained below based on experiments conducted by the inventors.

90% Ca O-
10% CaF2 powder desulfurization agent (44νn
When injecting particles with the following diameters into the riser pipe together with Δr gas for degassing and desulfurization, the inner diameter and number of the blowing blades made of stainless steel pipes were varied. We investigated the effects of the inner diameter and number of linings on the shedding rate. However, the injection of desulfurization agent a is 20k (l) per 11 to 100 m of molten steel, and for molten steel with a Heishin S concentration of 30 to 4 ppm at the start of treatment,
80 to 120 k
L/w: h, hr gas 1800-2200 N 1
/ rth, and the S resistance in the molten steel after treatment is G.
It was fixed at 1. The results are shown in Table 1.

From Table 1, it is clear that the desulfurization rate can be improved by decreasing the inner diameter of the tuyere and increasing the number of tuyeres. In particular, 30 to 40 pp11 is considered to be the lowest S concentration that can be produced economically by hot metal pre-treatment and converter refining.
In order to desulfurize the molten steel No. 1 to about ioppm or less,
It is necessary to use a tuyere with an inner diameter of 3 mr (H or less, preferably 4 mm or less).

The reason why the tuyere inner diameter and water number affect the desulfurization rate as described above is considered to be as follows. In other words, if the number of tuyeres is small, the inert gas and desulfurization agent will be unevenly distributed in the riser pipe and rise, resulting in less contact and mixing between the molten steel and the desulfurization agent.
If the inside diameter of the slats is large, the air bubbles become large and the ladle-shaped desulfurizing agent also rises in the rising pipe in a two-odor state, resulting in sufficient diffusion and mixing of the desulfurizing agent into the molten steel. It is thought that this decreases the desulfurization rate. In addition, if the number of tuyeres is small and the tuyere inner diameter is large, the bubbles will rise in a crowded manner as described above, creating a relative air bubble, but in this case, the time the bubbles stay in the rising tube will also be shortened. If the gas injection rate is constant, i? in the riser pipe at a certain mouth opening. l! Desulfurization is also caused by the fact that the air bubbles present in the molten steel become smaller and the action of the air lift pump becomes smaller, reducing the molten steel circulation TM flow rate and reducing the contact and mixing of molten steel with the desulfurization agent. This is thought to be one reason for the rate decline.

In addition, when using a large-diameter tuyere for simple degassing without desulfurization, it is necessary to increase the inert gas flow rate per tuyere to prevent tuyere erosion. However, if the gas flow is made extremely large, the cost increases due to the increase in the amount of inert gas used, and problems arise such as splashes of molten steel landing on the girder walls due to the large amount of gas. Therefore, in the case of large-diameter tuyere, the number of tuyere is actually limited, and the number of tuyere cannot be increased too much. As a result, the desulfurization rate is inferior.

Furthermore, although the arrangement of the large number of tuyere in the middle part of the upper tube is arbitrary, for example, an appropriate number of tuyeres, two or more, are arranged at regular intervals in the circumferential direction of the riser tube in the same horizontal plane, Moreover, it is sufficient to arrange them in an appropriate number of stages by using a certain number of gates in the vertical direction as well.

Next, a practical example of this invention will be described.

For 100 tons of melt kn in Example
The inner diameter of the lowering cost is 450L1! l! Using a vacuum degassing device, 14 stainless steel ξZ tuyeres with an inner diameter of 4 m11 were installed in the middle of the riser pipe to carry out degassing and desulfurization. The melting temperature at the start of treatment is 1670℃, ? 86'A
Composition is C; 0.15 wt%, SI 0.23 wt%
, M* 1.23wt 9/l, P 0.01hyt%
, 30.0032wt%, △Q O,058! Yt%,
The remainder was essentially Fe. At the start of the treatment, only r gas was applied from the above-mentioned surface to 1.8 to 2.3 Nm'/ml.
9096cao-
It has a composition of io%ca F2 and under 44JJNX (
7) Finely powdered desulfurization agent with diameter particles of 96% or more
A total of 3200 kq blown lυ over 25 minutes at a speed of 10-150 kg/m+n. Thereafter, the blowing of the desulfurizing agent was stopped, and only Δr gas was blown for 5 minutes to complete the treatment. The S permeability in the molten circle after completion of the treatment was 7 ppm, indicating that extremely low sulfur steel of 310 ppm or less could be reliably obtained.

As is clear from the above explanation, according to the method of this invention,
In order to generate a flow of molten steel during the vacuum descaling process of melting, an inert gas is blown into the riser pipe, and a powdered desulfurization agent is suspended in the inert gas and blown in.
The inner diameter is 6! By injecting inert gas and powdered desulfurization agent using multiple fence tuyeres of 11 or less, it is possible to reliably and stably desulfurize the molten steel to a constant low sulfurization of less than SlilOppm. However, IOP
A significant effect can be obtained that allows industrially stable production of low-sulfurized silk with low nitrogen and low hydrogen as well as below pm.

[Brief explanation of the drawing]

FIG. 1 is a 8N schematic diagram showing an example of a situation in which the method of the present invention is put into practice. 1... Real fence, 2... Ascending pipe, 3... Descending pipe, 5... Molten steel, 7... Wind. Applicant Kawamoe Steel Co., Ltd. Agent Patent attorney Takehisa Toyoda (and one other person)

Claims (1)

[Claims] Two tubes extending downward from the bottom of the vacuum chamber are immersed in the molten steel in a ladle, and an inert gas is blown into one of the tubes to treat the molten steel.The molten steel is vacuum treated while being circulated within the vacuum chamber. In this method, the inert gas and the powder desulfurization agent are provided in one of the pipes, in the method of simultaneously performing the desulfurization treatment of molten steel by suspending and blowing a powder desulfurization agent into the inert gas. A method for desulfurizing molten steel characterized by injecting it from multiple tuyeres with an inner diameter of 6 mm or less.