JPH04308024A - Production of ultralow carbon steel - Google Patents

Abstract

PURPOSE:To produce an ultralow carbon steel by using a refining device in reduced pressure. CONSTITUTION:In the method for executing the refining in the reduced pressure by blowing the gas of inert gas, etc., into a molten steel 2 in a ladle 1 combined with a vacuum degassing vessel, pressure in the vacuum degassing vessel is reduced and also, decarbonizing treatment blowing the inert gas into the undeoxidizing molten steel 2 is executed in the range of surrounded with (a), (b), (c), (d) in the figure, and by blowing powder making developing nucleuses for gaseous CO of MgO, CaO, etc., in the range of <=50ppm carbon concn. in the molten steel, the decarbonizing promotion is executed.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing ultra-low carbon steel using a reduced pressure refining apparatus.

0002

[Prior Art] Conventionally, as a manufacturing method for ultra-low carbon steel,
Decarburization of molten steel has been carried out by blowing inert gas into undeoxidized molten steel and circulating the molten steel in a vacuum chamber using an RH reflux type vacuum degassing apparatus. When processing such molten steel, emphasis is placed on increasing the molten steel return flow rate by increasing the diameter of the immersion pipe and increasing the circulating Ar gas flow rate, as well as increasing the stirring power of the molten steel, in order to improve the decarburization reaction efficiency. As an example, a vacuum degassing apparatus described in Japanese Patent Application Laid-Open No. 110611/1983 is known. As shown in FIG.
This device is equipped with a gas outlet 11 located between the opening ends of tanks A and 9B, and the amount of molten steel (height) in the vacuum tank is 500 to 10
50mm from the gas outlet 11 under the condition of increasing the diameter to 00mm.
By blowing an inert gas such as Ar at 0 to 2000 Nl/min into the molten steel, the molten steel 2 is promoted to become droplets, the area exposed in the vacuum chamber is increased, and a neutral or oxidizing flux is injected. This is intended to effectively promote decarburization and the like. 1 in the figure is a ladle,
10 indicates an inert gas outlet.

0003

[Problems to be Solved by the Invention] However, in the conventional ultra-low carbon steel production equipment described above, in order to increase the decarburization efficiency (specifically, to shorten the decarburization treatment time and to achieve ultra-low carbon steel In this method, the reaction area of molten steel is increased by increasing the flow rate of gas blown from the gas outlet 11 under conditions where the amount of molten steel in the vacuum chamber is increased. However, in this method, there is an upper limit to the flow rate of the blown gas for promoting decarburization, and when the upper limit is exceeded, gas blow-through occurs, and the reaction area with molten steel is conversely reduced. This method of blowing inert gas into the bottom of (refluxing) molten steel and from the open end has a limit to improving decarburization efficiency because there is an upper limit to the gas flow rate that does not cause gas blow-through. Since the gas is blown toward the exhaust port, metal is scattered not only in the vacuum chamber but also in the gas cooler of the vacuum exhaust device, making operation virtually impossible.

The present invention seeks to advantageously overcome the drawbacks of the prior art as described above.

[0005]

[Means for Solving the Problems] The gist of the present invention is to reduce the pressure inside the vacuum degassing tank in a method for vacuum refining by blowing gas such as an inert gas into molten steel in a ladle combined with a vacuum degassing tank. At the same time, inert gas is blown into the undeoxidized molten steel within the range surrounded by A, B, C, and D in Fig. 1(b), and MgO, CaO
A method for producing ultra-low carbon steel, which is characterized by injecting powder that serves as a nucleus for generating CO gas.

That is, the present invention blows an inert gas from above into molten steel in a ladle placed in a vacuum chamber under a reduced pressure atmosphere to promote the formation of droplets of molten steel and increase the air-liquid interface area in the vacuum chamber. By blowing in powder such as MgO, CaO, etc., a CO gas generating nucleus for decarburization is created, thereby promoting the decarburization reaction. The details are described below.

[0007]

[Operations and Examples] In the present invention, so-called ordinary unoxidized steel is used as the molten steel. First, a ladle containing molten steel is combined with a vacuum degassing tank. In this case, the premise is that it is possible to blow inert gas and powder into the molten steel on the device in parallel with the vacuum degassing treatment.

The inside of the vacuum degassing tank first begins to be depressurized and reaches a normal vacuum state. Inert gas is blown into the molten steel substantially in parallel with these pressure reduction processes.

However, these methods do not simply involve blowing inert gas, but require specific conditions.

That is, the blown gas flow rate Q per ton of molten steel
It is more effective to increase the decarburization rate by increasing . However, as the flow rate of the blown gas is increased, the molten steel becomes more violently shaken and splashed, and metal is scattered not only in the vacuum chamber but also in the gas cooler of the vacuum evacuation device, making operation virtually impossible. Furthermore, if the lance immersion depth h is increased, the stirring force will be increased even with the same gas flow rate, so the time for uniform mixing of the molten steel in the ladle will be shortened, and although the decarburization rate will increase in the high carbon concentration region, the molten steel will not splash. The amount generated decreases, and in the so-called extremely low carbon concentration region of [C]≦30 ppm, the decarburization rate decreases significantly. In order to improve the decarburization rate in the extremely low carbon concentration region, it is essential to increase the amount of splash generated, but in order to prevent molten steel from flowing out from the ladle, for example, as disclosed in Utility Model Application No. 84597/1980, It is desirable to provide a splash prevention lid 7 as shown in FIG. 2 (where [C] is the carbon concentration in the molten steel in weight percent).

Therefore, the decarburization speed was investigated by varying the lance immersion depth h and the blown gas flow rate under the condition that the splash prevention lid 7 as shown in FIG. 2 was provided, and the upper limit of the blown gas flow rate and the lance immersion Depth conditions were determined. This is shown in Figure 1 (a
) and (b) are shown in conceptual diagrams. In the figure, 1 is a ladle, 2
3 is a vacuum interface, 4 is a ladle bottom, 5 is an inert gas and powder injection lance, and 6 is a gas and powder injection port of the lance.

[0012] Let h be the distance from the vacuum interface 3 to the gas inlet 6 of the lance, and let H be the distance from the vacuum interface 3 to the ladle bottom 4, then make it less than or equal to Q calculated by h/H = -0.117Q + 1.97. It is desirable that the area D in Figure (b) has excessive splash.

The lower limit of the blowing gas flow rate is 1.7 Nl/min·ts, which is necessary to prevent lance nozzle clogging.
The above is desirable, and the area C in Figure (b) is not possible. In addition, it is desirable to inject gas into a shallow position with small h/H in order to increase the gas-liquid interface area. Since the decarburization rate decreases and the treatment time increases, there is an appropriate range for actual operation. Furthermore, if the depth is too deep, the bricks on the bottom of the ladle 4 will be severely damaged. In order to avoid adversely affecting the paving bricks, h/H≦0.8. That is, considering the decarburization rate required for normal operation, h/H = -0.0208Q + 0.433, h/H
Figure 3 below Q found by =0.175Q-0.125
Areas B and F in the middle are not possible, and should be specified as area A in the figure.

Under the above operating conditions, the undeoxidized molten steel is decarburized and after [C] becomes 50 ppm, the inert gas and powder are blown into the lance using the inert gas blown in from the lance 5 as a carrier gas. From mouth 6, new C such as MgO, CaO, etc.
Inject powder that can become O gas generation nuclei.

[0015] The present invention has an improvement effect not only on decarburization but also on degassing such as dehydrogenation.

Next, FIG. 3 shows an example according to the present invention and a comparative example.
and shown in FIG. Figure 3 shows the decarburization rate in the case of 20 minutes of decarburization using an example of 300 t/ch of molten steel.
This figure shows a comparison between cases in which gO was blown in and cases in which gO was not blown in. It is recognized that the reaction efficiency is poor in the B and F regions, and the reaction efficiency is good in the A region.Moreover, in the case where MgO is injected, the case marked with ● in the figure reaches even lower [C] concentration and the reaction efficiency is lower. becomes good.

FIG. 4 shows the flow of molten steel due to excessive splash in the same example as described above. Looking from this, it can be seen that the area D has molten steel shaking and excessive splash, and the molten steel is shaken by blowing powder. It can be seen that there are no major changes in the motion and splash generation.

[0018]

[Effects of the Invention] According to the present invention, by operating under specific conditions, it becomes possible to stably produce ultra-low carbon steel with [C]≦25 ppm in molten steel.

[Brief explanation of drawings]

[Fig. 1] (a) and (b) are explanatory diagrams of the limiting conditions of the present invention;

FIG. 2 is an explanatory diagram of an embodiment of the present invention,

FIG. 3 is a diagram showing an embodiment of the present invention;

FIG. 4 is a diagram showing an embodiment of the present invention;

FIG. 5 is an explanatory diagram of an embodiment of a conventional method.

[Explanation of symbols]

1...Ladle
2...Undeoxidized molten steel 3...Vacuum interface
4... Ladle bottom 5... Inert gas and powder injection lance 6... Inert gas and powder injection lance outlet 7...
Splash prevention lid 8...RH vacuum chamber 9A...Suction pipe
9B...Downcomer pipe 10...Inert gas outlet
11...Inert gas outlet

Claims (1)

[Claims] Claim 1: In a method for vacuum refining by blowing a gas such as an inert gas into molten steel in a ladle combined with a vacuum degassing tank, the pressure in the vacuum degassing tank is reduced and the
Decarburize the undeoxidized molten steel by blowing inert gas within the range surrounded by a, b, c, and d in b), and generate CO gas such as MgO and CaO in the area where the carbon concentration in the molten steel is 50 ppm or less. A method for producing ultra-low carbon steel characterized by promoting decarburization by injecting core powder.