JP2003129120A - Preliminary treatment method for molten pig iron - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a preliminary treatment method for molten pig iron, which can efficiently desulphurise molten pig iron at a lower cost than before. SOLUTION: The preliminary treatment method for the molten pig iron, which dephoshorizes and desulfurizes the molten pig iron to be supplied for decarbonization refining, comprises desiliconizing and dephosphorizing the above molten pig iron with an oxygen gas and a smelting agent mainly consisting of iron oxide and CaO, charging a desulfurizing agent onto a bath surface of the molten pig iron which has been discharged into the cupola pot, blowing hydrogen gas or gas containing hydrocarbon gas which generates hydrogen gas after decomposition, onto the above bath surface in a flow rate of 12 liters (in a standard condition)/min or more in hydrogen gas terms, per 1 ton of the molten pig iron, and desulphurising the molten pig iron while mechanically stirring it with stirring blades immersed in it.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot metal pretreatment method, and in particular, before decarburizing the hot metal in a steelmaking furnace such as a converter to obtain molten steel, silicon is preliminarily charged, phosphorus,
The present invention relates to a technology for reducing the refining load of a steelmaking furnace by removing impurity elements such as sulfur.

[0002]

2. Description of the Related Art In recent years, the characteristics of steel materials are becoming more demanding, and the load of reducing impurity elements in the steelmaking process is increasing. The impurity elements to be reduced are:
Examples include silicon, phosphorus, sulfur and oxygen. At present, it is popular to remove phosphorus and sulfur among these impurity elements at the stage of hot metal before charging into a steelmaking furnace (this is referred to as hot metal pretreatment).

In order to desulfurize the hot metal in this hot metal pretreatment, CaO or soda ash-based flux is blown into the hot metal held in the hot metal cart (this is called the injection method), or the hot metal held in the hot metal ladle of the reaction vessel is used. A method is used in which a desulfurizing agent is added to the mixture to perform mechanical stirring (mechanical stirring type), or metal Mg is injected as a desulfurizing agent. As the desulfurizing agent, it is desirable to use a CaO-based flux that is inexpensive and has less problems of slag treatment after the fact. In addition, among these, the mechanical stirring type hot metal desulfurization method causes the CaO-based flux added above to be entrained in the hot metal by the rotating force of the stirring blade to increase the reaction interfacial area in which desulfurization occurs, resulting in a low value of 10 ppm or less. It enables desulfurization up to the sulfur concentration range.

On the other hand, as a dephosphorization method of hot metal in the pretreatment, there are a converter type, a hot metal ladle type, a toped type, etc. when classified according to the reaction vessel used, and the dephosphorizing agent is used as an oxygen source. In addition to oxygen gas and iron oxide, CaO is mainly used.

By the way, Japanese Patent Laid-Open Publication No. 55-76005 discloses a technique in which, in the desulfurization of hot metal by mechanical stirring, a hydrocarbon gas is blown into the hot metal through a stirring device to accelerate the desulfurization reaction. However, according to a detailed investigation by the present inventors, the hydrocarbon gas blown into the hot metal through the stirring device (hollow rotating shaft) installed at the center of the hot metal ladle is It was found that the effect of accelerating the desulfurization reaction was insufficient because it floated preferentially in the center part and prevented the desulfurization agent added above from being caught in the hot metal from the center part by the rotating force of the stirring blade. . Further, since it is necessary to provide a gas flow path in the rotating shaft of the stirring device, there are problems that the structure of the stirring blade becomes complicated and the life is shortened. Further, Japanese Patent Laid-Open No. 2001-20006 discloses a mechanical stirring type desulfurization method in which 3 liters (standard state) / m of hydrocarbon gas is added per ton of hot metal from above.
Disclosed is a method of spraying in or more or adding a solid or liquid which generates a hydrocarbon gas. However, as a result of detailed investigations by the present inventors, the effect of accelerating the desulfurization reaction changes depending on the spraying time.
It was found that spraying at a constant flow rate during desulfurization reduces the efficiency of addition, that is, there is no reaction that corresponds to the amount of solid or liquid added.

[0006] In general, thermodynamics teach that the higher the desulfurization reaction of hot metal and the lower the dephosphorization reaction, the more advantageous. Therefore, in the conventional hot metal pretreatment, in consideration of the temperature drop during the treatment, the desulfurization reaction is generally performed first, and then the dephosphorization treatment is subsequently performed. However, in the dephosphorization treatment to be performed later, the sulfur contained in the iron oxide used as the dephosphorizing agent used as a dephosphorizing agent, as well as residual debris slag in the previous step,
A large amount of sulfur is picked up from the metal that adheres to the inside of the converter, and a so-called “resulfurization” phenomenon occurs in which the S concentration in the hot metal desulfurized first is increased during the subsequent dephosphorization treatment. Since this dephosphorization process and the converter in the post-process mainly perform oxidation reaction, desulfurization does not proceed, so when melting low-sulfur steel, secondary refining (molten steel discharged from the converter is performed in a vacuum degassing tank or It is necessary to desulfurize molten steel in a refining process that uses a ladle, further removes impurities and gases, and adjusts the composition by adding alloys. In other words, the desulfurization load in the secondary refining increases, leading to an increase in the cost of low-sulfur steel and a hindrance to productivity.

[0007]

SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide a hot metal pretreatment method capable of desulfurizing molten iron more inexpensively and efficiently than ever before.

[0008]

In order to achieve the above-mentioned object, the inventor has conducted diligent research focusing on making the order of performing desulfurization treatment and dephosphorization treatment reverse to the conventional one, and achieved the result of the present invention. Embodied in.

That is, the present invention relates to a hot metal pretreatment method for dephosphorizing and desulfurizing hot metal used for decarburizing and refining, wherein the hot metal is desiliconized with a refining agent mainly containing oxygen gas, iron oxide and CaO. After dephosphorization, a desulfurizing agent is charged onto the bath surface of the hot metal discharged into the hot metal ladle, and a gas containing hydrogen gas or a hydrocarbon gas that decomposes to generate hydrogen gas is placed on the bath surface. A hot metal pretreatment method characterized by spraying at a flow rate of 12 liters (standard state) per minute of hot metal per ton of hot metal (standard state) / min, and mechanically stirring with a stirring blade immersed in hot metal to desulfurize. is there. In this case, it is preferable that the gas flow rate of the gas is increased in the latter half of the desulfurization.

In the present invention, the order of performing the desulfurization treatment and the dephosphorization treatment is reversed from the conventional one, and the hydrogen gas conventionally blown into the hot metal or the gas containing the hydrocarbon gas which decomposes to generate hydrogen gas is used. Since the amount of hydrogen gas converted per 1 ton of hot metal is 12 liters (standard state) / min or more on the bath surface of the hot metal, the slag formed by the previous dephosphorization (called dephosphorization slag) ) Will be cooled quickly. As a result, even if desulfurization is performed, re-phosphorization from the dephosphorization slag to the hot metal does not occur, and desulfurization can be efficiently performed from the hot metal. Here, the hydrogen gas conversion amount means the amount of hydrogen gas for hydrogen gas or hydrogen-containing gas, and the amount of hydrogen gas generated by decomposition of hydrocarbon gas for hydrocarbon gas or hydrocarbon-containing gas. And

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below along with the background of the invention.

In the present invention, mechanical agitation is used for desulfurization. The desulfurizing agent added on the hot metal bath surface is rolled into the hot metal by the rotating force of the stirring blade (impeller), and the reaction interfacial area between the hot metal and the desulfurizing agent is increased to achieve rapid desulfurization treatment. It is possible. As the desulfurizing agent, CaO type,
Although soda ash-based, Mg-based, etc. are used, CaO-based desulfurizing agents are the mainstream in terms of cost and environment.

The desulfurization reaction by the CaO desulfurizing agent is generally represented by the formula (1).

[S] + CaO → (CaS) + [O] (1) The equation (1) is a reduction reaction, and the desulfurization reaction is further promoted when the reaction interface is in a reducing atmosphere. Further, since it is an exothermic reaction, the desulfurization reaction proceeds at higher temperatures. Here, [] means a component in the hot metal (sulfur in this case), and () means a component in the slag.

Further, the hydrocarbon-based gas blown into the hot metal in the above-mentioned prior art is, as shown by the following equation (2),
It decomposes completely at about 300 ° C to generate hydrogen gas. The hydrogen gas is oxygen gas in the atmosphere, [O] in the hot metal,
It reacts as shown in the formulas (3) and (4). Therefore, when the system is set in a reducing atmosphere, the desulfurization reaction of the above formula (1) is promoted.

CnHm → nC + m / 2H ₂ (2) 1 / 2O ₂ + H ₂ → + H ₂ O (3) [O] + H ₂ → + H ₂ O (4) On the other hand, The dephosphorization reaction of hot metal occurs as shown in equation (5).

2 [P] + 5 / 2O ₂ + 3CaO → 3CaO · P ₂ O ₅ (5) This dephosphorization reaction is an endothermic reaction, and the reaction proceeds at lower temperatures. Further, since it is an oxidation reaction, the dephosphorization reaction proceeds in an oxidizing atmosphere. As described above, the dephosphorizing agent is
Oxygen sources such as oxygen gas and iron oxide and CaO are the main components.

By the way, as described above, in order to produce low-sulfur steel efficiently and at low cost, low sulfurization at the hot metal stage, not at the decarburizing stage in the steelmaking furnace, is desirable. Therefore, the present inventors performed a desulfurization treatment first as in the conventional case, and then tried a dephosphorization treatment subsequently. As a result, according to this conventional method, in the latter half of the dephosphorization treatment, the slag produced in the first half of the desulfurization treatment is brought in, the iron oxide used as the dephosphorization agent, and the S from the adhered metal in the container during the dephosphorization treatment. It was confirmed that the desulfurization efficiency was poor due to the S-reversion reaction at the pickup. Therefore, the present inventors decided to conduct an experiment in which the treatment order of the dephosphorization treatment and the desulfurization treatment was changed. The experimental apparatus used was a dephosphorization apparatus for a converter-type vessel as shown in FIG. 1 and a desulfurization apparatus having a mechanical stirring means in a hot metal ladle as shown in FIG. 2, and the experimental conditions are shown in Table 1. As shown in.

[0019]

[Table 1]

The experimental results are collectively shown in Table 2. In Table 2, the order of performing desulfurization → desulfurization is Level 1 and the order of performing desulfurization → desulfurization is Level 2. From Table 2, it is clear that in Level 1 in which desulfurization is performed first, S reduced in the desulfurization treatment is restored S during the dephosphorization treatment. On the other hand, in level 2 in which dephosphorization is first performed, P reduced by the dephosphorization process was restored to P during the desulfurization process, but the amount was small.

[0021]

[Table 2]

Further, when the EPMA analysis of the slag after the treatment was carried out at each level, in the level 1, S existing around the CaO-based spherical slag after the desulfurization treatment is restored to S during the dephosphorization treatment. On the other hand, in Level 2, it was found that the slag that had been almost melted after the dephosphorization treatment was surrounded by the slag mainly composed of CaO by the desulfurization treatment and formed into a spherical shape, and there was little reversion P. Comparing the two, the reached S concentration is lower in Level 2 than in Level 1. It is considered that this is because the level 2 reaction is thermodynamically disadvantageous under the processing temperature condition, but the P and S reactions are small, so that such a result is obtained.
At the same time, the oxygen partial pressure in the hot metal was measured with an oxygen sensor before each level of desulfurization treatment. In the case of level 1, the oxygen activity was 1 ppm, whereas in the case of level 2, it was 4 ppm.
It was a high value. In the case of Level 2, this is S in the hot metal.
It is considered that i is extremely small and the iron oxide concentration in the slag after the dephosphorization treatment is high. After each treatment, the slag was removed with a slag dragger, but it could not be completely removed.

From the above experimental results, the present inventors believe that as a pretreatment method for hot metal, it is promising to perform dephosphorization treatment first and then desulfurization as in Level 2, and the present invention relates to the present invention. It was decided to make it one requirement for the hot metal pretreatment method.

Next, in the above level 2 experiment, a slight amount of reconstitution of 0.003 mass% was found during the desulfurization treatment. However, even with this amount of phosphorus replenishment, in order to prevent the phosphorus content of the final product steel from falling outside the standard or to prevent it, the hot metal after the hot metal pretreatment is converted to molten steel in a converter or the like. The refining load of performing dephosphorization treatment at the same time as decarburizing and refining (increasing the amount of refining flux such as CaO and fluorite, extending the decarburizing and refining time to increase FeO in the slag, oxygen unit consumption) It is desirable to reduce the amount of re-phosphorization as much as possible because the increase in the amount of phosphorus and the increase in damage to the refractory material of the converter due to highly reactive slag.

Further, as described above, the slag generated by the dephosphorization treatment cannot be completely removed even if it is removed by the slag dragger, and it floats on the hot metal bath surface in the hot metal ladle used as a desulfurization vessel. ing. Furthermore, the slag that is entrained in the hot metal together with the desulfurization flux added during mechanical stirring is surrounded by CaO, which is the main component of the desulfurization flux, and does not cause rephosphorization, but is not entrained in the hot metal. Resuspended slag may cause rephosphorization.

Therefore, the inventors of the present invention considered to maintain the phosphorus absorption capacity of the floating slag at a high level, and included a gas containing hydrogen gas having high thermal conductivity and a hydrocarbon gas which decomposes to generate hydrogen gas. It was conceived that the floating slag was cooled by spraying it onto the surface of the hot metal bath instead of blowing it into the bath. In particular, hydrocarbon gas such as propane absorbs heat when decomposed, and is more effective for cooling the slag. Further, cooling the dephosphorization slag is more advantageous in the dephosphorization reaction at lower temperatures, so that the phosphorus absorption capacity of the dephosphorization slag is maintained at a high level, and the cooled slag increases in viscosity, or It was thought that it would be convenient for prevention of re-phosphorus since it solidifies and its reactivity with the hot metal decreases.

Then, in order to reduce re-phosphorus in the level 2 desulfurization treatment, propane gas was selected as the hydrocarbon gas and the top blowing experiment was conducted. The experimental conditions are shown in Table 3 and the experimental results are shown in FIG.

[0028]

[Table 3]

From FIG. 3, it is apparent that re-phosphorization hardly occurs in the range of 12 liters (standard state) / min / t or more of propane gas in terms of hydrogen gas conversion. Further, the relationship between the flow rate of propane gas and the desulfurization rate was also investigated, and the relationship shown in FIG. 4 was obtained. Here, the phosphorus recovery rate is defined by the following equation (6), [% P] is the phosphorus concentration in the hot metal,
The desulfurization rate is defined by the following equation (7), and [% S] is the sulfur concentration in the hot metal. The subscripts i and f represent before and after the treatment of recondensation or desulfurization, respectively.

Phosphorus reversion rate (%) = ([% P] _i − [% P] _f ) / [% P] _i × 100 (6) Desulfurization rate (%) = ([% S] _i − [ % S] _f ) / [% S] _i × 100 (7) Further, from FIG. 4, propane gas accelerates the desulfurization reaction at a flow rate of hydrogen gas equivalent of 12 liters (standard state) / min / t or more. The effect is recognized. The oxygen partial pressure of the hot metal before and after desulfurization is from 4 ppm before the treatment to 0.1 to 0.
It was reduced to 5 ppm. From this, it can be seen that the partial pressure of oxygen after the treatment is reduced by blowing the propane gas, and the reducing atmosphere is advantageous for the desulfurization reaction. It was also found that it is better that the tip of the lance for blowing propane gas is closer to the center of the container on the molten metal surface.

Therefore, the inventors of the present invention did not blow hydrogen gas or a gas containing hydrocarbon gas which decomposes to generate hydrogen gas into the bath, but instead, on the surface of the hot metal bath in terms of hydrogen gas.
The present invention has been completed by making the second requirement to be mechanically stirred by spraying at a flow rate of 2 liters (standard state) / min / t or more.

In the present invention, the blowing gas is
Not only lopan gas, but also C gas and H ₂Gas, LNG, etc.
H₂Containing or decomposed to H₂Gas that produces
Anything will do as long as it is. In addition, before the desulfurization process
The desiliconization and dephosphorization process, which is about the same, is not limited to the converter type
Any processing method such as a dough type or a hot metal ladle type may be used.

Next, the inventors of the present invention considered that there is an appropriate spraying time of propane gas for desulfurization efficiency, and conducted experiments in which the flow rate of propane gas was variously increased after the addition of the desulfurizing agent. Table 4 shows an example of the experimental conditions and results.

[0034]

[Table 4]

According to Table 4, when the flow rate of propane gas is increased after a certain time has passed since the desulfurizing agent was added (here, 4 minutes), desulfurization can be performed even with a relatively small amount of propane gas used (basic unit). It was clarified that the effect of promoting the reaction was large. According to the observation during the experiment, it was confirmed that the desulfurization agent after being added was caught in the hot metal, and the propane gas frame was often caught in the hot metal from around the stirrer shaft from the time when the hot metal appeared on the molten metal surface. did it.

Therefore, the present inventors decided to add to the present invention an increase in the amount of hydrogen generating gas added according to the elapsed time.

[0037]

EXAMPLE Using hot metal from a blast furnace, a sulfur content of 10
A large amount of low-sulfur steel with a concentration of ppm or less was melt-produced. that time,
The hot metal before being fed to the converter was pretreated in the order of dephosphorization and desulfurization as shown in Table 5. The conditions for the dephosphorization and desulfurization treatment are collectively shown in Table 6. The equipment used is
The one shown in FIGS. 1 and 2 was adopted. Bropan gas was adopted as the hydrogen-evolving gas, and the blowing conditions were the same as those shown in Table 3. A large number of charges were melted under such conditions, and the results were evaluated by the average values of S and P of the obtained molten steel, as shown in Table 7.

It is clear from Table 7 that the hot metal pretreatment method according to the method of the present invention makes it possible to reduce the P content and S content of the hot metal with high efficiency.

[0039]

[Table 5]

[0040]

[Table 6]

[0041]

[Table 7]

[0042]

As described above, according to the present invention, the efficiency of the dephosphorization reaction and the desulfurization reaction of hot metal is improved, and the reduction of P and S is promoted. As a result, the refining load in the converter and secondary refining is reduced, and inexpensive low-phosphorus sulfur steel can be melted.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a converter type experimental apparatus used for dephosphorization.

FIG. 2 is a schematic diagram showing an experimental apparatus used for mechanical stirring type desulfurization.

FIG. 3 is a diagram showing a relationship between a re-phosphorization rate of hot metal and a flow rate of sprayed propane gas.

FIG. 4 is a diagram showing a relationship between a desulfurization rate of hot metal and a flow rate of propane gas sprayed.

[Explanation of symbols]

1 Top blowing lance of oxygen gas 2 converter 3 slag 4 hot metal 5 Bottom blowing gas (nitrogen gas) 6 Stirring motor 7 Stirring blade 8 Stirrer shaft cover 9 Lance for blowing hydrogen generation gas 10 hood 11 Desulfurizing agent 12 Hot metal pot

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Takashi Yamauchi             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Chiba Steel Works, Ltd. (72) Inventor Yoshihisa Kitano             1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Made in Kawasaki             Chiba Steel Works, Ltd. (72) Inventor Masayuki Manzono             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works (72) Inventor Kenji Ando             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works F-term (reference) 4K014 AA02 AA03 AB00 AB03 AC03                       AC08 AC14 AC17 AD00 AD23

Claims (2)

[Claims] 1. A method for pretreatment of hot metal for preliminarily dephosphorizing and desulfurizing hot metal used for decarburization refining, wherein the hot metal is desiliconized and dephosphorized with a refining agent mainly containing oxygen gas, iron oxide and CaO. Then, while desulfurizing agent is put on the bath surface of the hot metal discharged to the hot metal ladle, hydrogen gas or gas containing hydrocarbon gas that decomposes to generate hydrogen gas is added to the hot metal surface per ton of hot metal. A pretreatment method for hot metal, characterized by spraying at a flow rate of 12 liters (standard state) / min or more in terms of hydrogen gas, and mechanically stirring with a stirring blade immersed in hot metal to desulfurize. 2. The hot metal pretreatment method according to claim 1, wherein the gas flow rate of the gas is increased in the latter half of the first half of desulfurization.