JPS61136613A - Vacuum decarburization method of molten steel - Google Patents

Abstract

PURPOSE:To decarburize efficiently a molten steel and to produce economically the low-carbon molten steel by evacuating the inside of an immersion pipe immersed into the molten steel and blowing gaseous oxygen and powder contg. iron oxide, etc., from a top blowing lance to the molten steel risen in the pipe. CONSTITUTION:The immersion pipe 2 which has the inside diameter of preferably about >=30% of the inside diameter of a ladle 1 contg. the molten steel A such as rough molten high-chrome steel subjected to primary decarburization under atmospheric pressure and is coated with refractories on the inside surface is inserted into said ladle 1 and is partly immersed into the molten steel A. The pipe 2 is connected to a vacuum pump and the inside thereof is evacuated to suck up the molten steel A, at the same time, the powder 6. contg. iron oxide such as iron ore powder or nickel oxide such as NiO powder is blown together with the gaseous oxygen by the top blowing lance 4 into the molten steel A in the pipe. Gaseous Ar is further blown through a bottom blowing porous plug 3 provided in the bottom of the ladle 1 to stir the molten steel A. The molten steel A is thus efficiently decarburized and refined.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention is for efficiently decarburizing molten steel, for example, high chromium molten steel, and economically producing low carbon molten steel such as stainless steel in a mass production process. Concerning vacuum decarburization method (Prior technology) When attempting to decarburize molten steel containing 10 or more chromium to a low carbon range by blowing acid under normal conditions at atmospheric pressure, the chromium content tends to oxidize. . Methods to suppress chromium oxidation and preferentially decarburize down to the lower order range include a method in which oxygen gas is injected into molten steel together with a diluting gas such as Ar (AOD method, etc.), and a method in which oxygen gas is injected into molten steel together with a diluting gas such as Ar (AOD method, etc.), and a method in which oxygen gas is injected into molten steel in a vacuum (under reduced pressure). There is a method of blowing or blowing gas into molten steel (Steel Handbook ■, 3rd edition, p. 718 onwards).

Among these methods, methods (such as the VOD method) in which oxygen gas is top-blown into molten steel in a ladle under reduced pressure suppress chromium oxidation down to the low carbon range, making it possible to produce ultra-low carbon steel.

It is widely used as mass production process equipment because it is easy to perform and does not require 7erosilicon as a slag reducing agent.

The problem with this method is that decarburization progresses depending on the rate of oxygen supply.
It is difficult to increase the decarburization rate in the region (9≧0.21). In other words, if an attempt is made to increase the amount of oxygen gas supplied as an oxygen source, the amount and height of scattering of the snails produced by the f-strain will increase, so the amount of oxygen gas supplied will be restricted. Even if it is possible to increase the absolute amount of blown acid by reducing the amount of slag and slag scattered by improving the shape of the blowing acid nozzle, the decarburization reaction occurring on the molten steel surface alone will not increase the stirring intensity, i.e., the reaction. Although chromium oxidation tends to occur due to the restriction on the amount of movement of 9 to the site, the decarburization rate is also restricted.

When the decarburization speed of the final decarburization process is restricted, and the molten steel supply pitch is defined from a subsequent process (for example, a casting process), the final decarburization process has no choice but to lower the C before the process. As a result, the end point of the primary decarburization step, which is normally carried out under atmospheric pressure, has to be lowered, which increases the amount of chromium silane, which poses a problem as an integrated process.

Two of these inventors previously developed a ``vacuum refining method for molten steel in which molten steel in a ladle is sucked up into a vacuum tank through an immersion tube, and fermenting gas is blown through an upper lance'' (special Gansho 5'9-1
57598), but even if you use that method,
This is insufficient for the purpose of obtaining the required decarburization rate.

(Problems to be solved by the invention) This invention is a method for increasing the decarburization rate under reduced pressure, and as a result, it is possible to perform secondary decarburization under atmospheric pressure and final decarburization under reduced pressure. This makes it possible to optimize the decarburization load with coal.

(Means for Solving the Problems) The present invention is characterized by immersing a part of a pipe whose inner surface is coated with a refractory material into molten steel in a ladle, and sucking up the molten steel by reducing the pressure inside the immersed pipe. At the same time, the molten steel in the immersion tube can be sprayed with powder containing iron oxide or nickel oxide using a top blowing lance, or in addition to the above method, by winding an induction coil around the immersion tube and applying electric power, the molten steel in the immersion tube can be The method is to induce stirring.

(Function) An example of equipment used to carry out the present invention is shown in FIG.

In FIG. 1, 1 is a ladle, and 2 is an immersion tube, which are disposed so that their open lower ends are immersed in molten steel A in the ladle l. The immersion tube 2 has a circular surface lined with a refractory material, and the other opening is connected to a vacuum inlet. Reference numeral 3 denotes a porous plug, which in this embodiment is arranged at the center of the bottom of the ladle 1 and is connected to Ar, ? The bubbles 5 are used to stir the molten steel by blowing into the ladle self-molten steel A. Reference numeral 4 denotes a top blowing lance, which functions to blow oxygen-containing gas and metal oxide 6 toward the surface of the molten steel rising into the immersion tube 2.

Insert into the ladle K containing the high chromium crude molten steel that has been primary decarburized under atmospheric pressure, and insert the immersion tube whose inner diameter is 30% or more of the ladle inner diameter (the average diameter of the bottom of the molten steel and the surface of the molten steel). . If the inner diameter of the immersion tube is smaller than this, uniform mixing of the molten steel in the ladle will be inhibited. Further, as will be described later, when powder containing iron oxide or the like is sprayed to promote the decarburization reaction, the inner refractory material of the immersion tube is likely to be damaged due to its influence, which is not preferable. The immersion tube is connected to a vacuum evacuation pump, and the pressure inside the immersion tube can be reduced. Further, the molten steel in the ladle is stirred by, for example, blowing Ar gas into the molten steel through a porous plug. The molten steel in the ladle is sucked up into the immersion pipe depending on the degree of pressure reduction. For this molten steel surface,
A powder containing fluorine f-sulfur and iron oxide or nickel oxide is sprayed from above the immersion tube through a top blowing lance inserted into the tube. Powder containing iron oxide refers to iron ore powder, sintered ore powder, iron-chromium ore powder, iron-manganese ore powder, and the like. Further, the powder containing nickel oxide refers to NIO powder and the like. When these are sprayed onto the molten steel surface, they penetrate into the molten steel and react with C in the molten steel to generate CO gas.

Through these processes, the stirring of molten steel in the immersion tube is promoted,
In addition, most of the supplied powder is iron oxide or nickel oxide, which is used as a raw material source for decarburization.
Even if the blowing acid rate is constant, the decarburization rate can be increased accordingly.

Since the gangue content in the powder forms slag, it is undesirable to have too much gangue from the viewpoint of corrosion of the lining refractories, etc.
Since the reduction rate of chromium oxide is lower than that of iron oxide, the ratio of unreduced steel floating to the surface tends to increase especially when the molten steel becomes low. Therefore, the most preferred are iron ore, sintered ore powder, or NIO powder with a low gangue content. Nickel oxide is effectively used only when melting molten steel containing nickel (for example, 18'1Cr-8%Ni stainless steel).

These oxides are supplied through a top blowing lance using oxygen gas or an inert gas such as Ar as a carrier.

The gas flow velocity at the tip of the lance is set to be 80 m/aec or more, preferably 130 m/+se or more. It is desirable that the lance be perpendicular to the surface of the molten steel, rather than being diagonal. The reasons for this are: (1) abrasion of the inner surface of the top blowing lance due to powder can be significantly reduced; (11) local damage to the refractory lining of the immersion pipe can be prevented;
iii) This is because it is easy to obtain effects such as reducing molten steel scattering and promoting molten steel stirring by powder spraying, as described below.

The effects of using powder containing iron oxide or nickel oxide as an oxygen source for decarburizing molten steel are as follows.

(1) Even if the amount of gaseous oxygen supplied is constant, the decarburization rate can be increased as the amount of oxygen supplied from the oxide increases. This is because the amount of gaseous oxygen supplied causes a splash,
This is an effective solution when constrained by other factors.

(2) When the decarburization rate becomes high, (C1) before treatment is high, and the decarburization width can be increased. As shown in FIG. 2, when only gaseous oxygen is used, the larger the width of decarburization, the higher the temperature of molten steel after decarburization. On the other hand, the temperature of molten steel after treatment can be adjusted by changing the ratio of the amount of oxygen for decarburization supplied from the oxide.

(3) When powder is sprayed onto the molten steel surface, the amount of evaporation of metal components can be reduced as shown in FIG. 3, especially by adjusting the temperature rise at the hot spot.

It has a high vapor pressure, especially compared to iron. It is effective when decarburizing molten steel containing a large amount of chromium and manganese under reduced pressure.

(4) The biggest drawback of the method of refining molten steel by sucking it up into a dipping tube is that uniform mixing of the molten steel is likely to be hindered.

When only gaseous oxygen is used as oxygen for decarburization, since the decarburization reaction is an exothermic reaction, the temperature of the molten steel at the top of the immersion tube is higher than that of the bulk, and mixing by convection is inhibited. On the other hand, when a metal oxide is used as an oxygen source, since the decarburization reaction is an endothermic reaction, mixing is promoted by convection as decarburization progresses.

(5) When oxidizing gas is sprayed onto high-chromium molten steel, a chromium oxide coating is formed, which reacts with carbon in the molten steel, causing a decarburization reaction.

If the rate at which the oxide film is engulfed into the molten steel is slow, the oxides will accumulate and form agglomerates, making it increasingly difficult to decarburize the molten steel. On the other hand, when oxides are sprayed, the oxides enter the molten steel due to inertial force, but the oxides generated on the surface of the molten steel are also knocked into the molten steel, and the decarburization reaction occurs in the molten steel. Due to the synergistic effect of the CO gas generated by the stirring of molten steel,
The decarburization reaction can proceed efficiently without accumulation of re-oxides.

(6) In the method of refining in a dipping tube, it is possible that metal particles may adhere to the inside of the dipping tube due to scattered metal. Figure 4 shows the ratio of oxygen supplied from oxides for decarburizing molten steel (F・0 or N
(assuming that IO is involved in decarburization) and the index of the amount of metal deposited inside the immersion pipe. It can be seen that the vacuum decarburization method combined with oxide spraying according to the present invention is highly effective in reducing metal build-up.

The optimum conditions for spraying the brewing material are determined as follows. As shown in FIGS. 3 and 4, when the ratio of oxygen supplied from the oxide as decarburization oxygen is 8 or more, the various effects described above become more noticeable. The upper limit of the oxide supply amount is determined by the drop in molten steel temperature accompanying decarburization. In other words, this value is the molten steel temperature EC before treatment) 1.
Although it varies depending on the amount of heat dissipated during processing (that is, the scale of the molten steel), as shown in FIG. 2, there is an upper limit to the amount of oxide used when there is no external heat supply. A method for removing this thermal restriction is to wind the induction heating coil 7 around the immersion tube 2 and to inductively heat the molten steel in the immersion tube (FIG. 5). In FIG. 5, 7 is an induction coil, and 8 is a heat shield.

This method of induction stirring (simultaneously induction heating) the molten steel sucked up has the following advantages: (i) Unlike other heating methods, there is no need to use a special ladle; (ii) the immersion tube has an inward-facing (io) Since the molten steel in the immersion tube is stirred by induction, cracks and joint openings are less likely to occur, and there is no risk of molten steel seeping in and causing problems with the induction coil. It has the advantage of being able to promote reactions with

By using induction heating in combination, thermal and reaction rate constraints can be removed, and the ratio of the sulfur source supplied as an oxide can be increased to an arbitrary value.

During refining, temperatures are measured and samples are taken from the molten steel surface in contact with atmospheric pressure in the ladle, and depending on the situation, the blowing acid rate and powder supply rate (when using an induction coil) It is possible to adjust the induction power load) and perform vacuum decarburization under optimal conditions.

(Example) (1) 18°Cr-0,796C molten steel (1650°C) produced in a converter was received in a ladle, and 0□ and iron oxidation were carried out using the equipment shown in Figure 1. The powder containing solids was decarburized under reduced pressure by top blowing. The powder containing iron oxide used was 1 m to 0,
Scale ground to l wm (T, Fe: 74%)
Then, a double pipe lance consisting of an inner tube and an outer tube was used to carry the meter from the inner tube (tip gas flow rate 130 m/sea).
) onto the molten steel surface. The outer tube supplies oxygen gas.

While decarburizing, sampling the molten steel from inside the ladle and measuring its temperature, the amount of blowing tip, atmospheric pressure in the immersion pipe, and scale powder injection speed are adjusted as follows to achieve a specified low carbon content. Decarburization was carried out up to the area.

The time required for decarburization is 20 minutes. Molten steel temperature after decarburization is 16
The temperature was 10°C. The composition was adjusted and cast.

(ii) 1 B'1iCr -7,79 melted in a converter
11Nl -1,0C molten steel (1630℃) was received in a ladle, and using the equipment shown in Fig. 5, the molten steel was stirred by induction, containing 0□, equipment oxides, and nickel oxides. The powder was decarburized under reduced pressure by top blowing. In FIG. 5, the same reference numerals as in FIG. 1 indicate the same parts as in FIG. 1, 7 is an induction heating coil, and 8 is a heat shield.

As the nickel oxide, 0.5 μm or less of NIO powder was used. The other methods are the same as those described in (i).

Decarburization to c:o, os% in 18 minutes, 1605
C. 18% Cr-7,99JNI molten steel was obtained. The composition was adjusted and cast.

Although the decarburization of high-chromium molten steel has been described above as an example, the present invention can be directly applied to vacuum decarburization of molten steel with other components.

(Effects of the Invention) This invention solves the problems of the vacuum decarburization method for molten steel, especially high-chromium molten steel, in which the pressure inside the immersion tube is reduced, the molten steel is sucked up, and the molten steel is refined in the immersion tube. This makes it possible to expand into other countries, and has a large economic effect.

[Brief explanation of drawings]

Figure 111W is an explanatory diagram showing an example of equipment used to carry out the present invention, and Figure 2 is an explanatory diagram showing an example of equipment used to carry out the present invention. Figure 2 shows that oxygen as iron oxide (calculated as F@O) accounts for the oxygen supplied for decarburization. A diagram showing an example of the relationship between the ratio and the temperature change of molten steel due to decarburization. Figure 3 shows how oxygen in the form of iron oxides (calculated as F 0) accounts for the oxygen supplied for decarburization. Figure 4 shows an example of the relationship between the ratio and the amount of evaporation (relative value) of chromium in molten steel.
A diagram showing an example of the relationship between the proportion occupied by oxygen (calculated as F. It is an explanatory view showing an example of the case. Patent issued by Nippon Steel Corporation Fig. 1 E Ar-bottom soft bolus aluminum Fig. 2 For decarburization 1 Occupancy 2 Hifu Figure 3 Figure 4

Claims (2)

[Claims] (1) A part of the tube whose inner surface is coated with refractory is immersed in the molten steel in the ladle, the inside of the immersed tube is depressurized and the molten steel is sucked up, and the molten steel inside the tube is exposed to oxygen gas and iron oxidation using a top blowing lance. A vacuum decarburization method for molten steel characterized by spraying powder containing nickel oxide or nickel oxide. (2) The method according to claim (1), characterized in that the molten steel in the immersion tube is heated by induction.