JPH01191725A - Method for producing low-sulfur high-carbon molten iron from ferruginous cold charge - Google Patents

Abstract

PURPOSE:To effectively produce a low-sulfur high-carbon molten iron at a low cost by adjusting the carbon content in the molten iron and removing the sulfer in the molten iron via slag while stirring the molten iron with an inert gas in the end period of oxygen blowing of a ferruginous cold charge and carbonaceous material. CONSTITUTION:The solid ferruginous cold charge such as cold pig and scrap and the carbonaceous material such as coal are supplied to a ferruginous cold charge melting furnace in which a seed hot metal exists to execute oxygen blowing. The carbon content in the molten iron is adjusted to >=4% by such means as use of the carbonaceous material having a relatively coarse specific grain size only in the end period of the oxygen blowing at this time. The activity of the sulfur in the molten iron is thereby increased to put the molten iron into an easily desulfurizable state. Then, an inert gas is blown in the molten iron to impart agitating energy. The sulfur in the molten iron is thereby removed via the slag essentially consisting of the previously formed CaO, SiO2 and Al2O3, by which the low-sulfur high-carbon molten iron is obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides a method for efficiently producing low-sulfur, high-carbon molten iron by melting a large amount of solid iron-containing cold materials such as cold pig iron and scrap using carbonaceous material as a heat source. It is related to.

Furthermore, this provides a method for obtaining molten steel from solid iron-containing cold material at low cost.

[Conventional technology]

The converter steel manufacturing method, which currently dominates the steel manufacturing process, uses hot metal supplied from a blast furnace as the main raw material, adds slag material in the converter, and performs oxygen blowing. The method is to obtain molten steel with a composition and temperature of . However, in the usual method, the amount of iron-containing cold material used is at most 35% of the total amount of iron source charged due to restrictions due to the heat source, and it cannot be said that it is a process that can use a large amount of iron-containing cold material. This shows that the converter steel manufacturing method lacks flexibility in the amount of cold iron source used and production flexibility.

In order to overcome these weaknesses, the applicant has filed a patent application filed in 1983.
In No. 73997, we proposed a new process consisting of a melting-only converter that melts a cold iron source to produce high-carbon molten iron, and a refining-only converter that refines the high-carbon molten iron obtained in the melting-only converter.
We confirmed its superiority.

In other words, the gist of the new process is to obtain high-carbon molten iron by supplying a ferrous composite material, carbonaceous material, and oxygen to a converter exclusively for melting in which a seed metal exists, and to use this molten iron as a raw material for another converter exclusively for refining. In the converter steelmaking method in which molten steel with the required composition is obtained by oxygen blowing in a furnace, in the converter exclusively for melting, the total amount of the required refining amount in the converter exclusively for refining and the amount of seed metal required in the converter exclusively for melting. of high-carbon molten iron is obtained, and the required amount of high-carbon molten iron is sent from the above-mentioned melting-only converter to the above-mentioned refining-only converter for oxygen refining in one tap, while the remaining amount of high-carbon molten iron is used exclusively for melting. A converter steel manufacturing method characterized in that the hot water is left in the converter and used as a seed hot water for melting the iron-containing cold material.

This method provided a converter steel manufacturing method with high melting efficiency that can melt a large amount of iron-containing composite material in a short time.

Furthermore, this method eliminates the need for a container for storing seed water.
It is also possible to reduce the manufacturing cost of high carbon molten iron.

[Problem that the invention seeks to solve]

In the method disclosed in Japanese Patent Application No. 62-73997, carbonaceous material is used as a heat source for melting the iron-containing cold material. Inexpensive industrial coking coal is generally used as carbonaceous material, but the sulfur content in industrial coking coal is as high as 0.1% or more, which causes sulfur contamination from the carbonaceous material into the molten iron. The sulfur content increases. As a countermeasure to this problem, as detailed in Japanese Patent Application No. 1982-73997, an effective method is to add a desulfurizing agent to the molten iron in the ladle discharged from the melting converter, and to desulfurize it by mechanically stirring with an impeller, etc. It is true.

However, as a result of various tests, the patent application
The molten iron obtained by melting a large amount of iron-containing composite material while oxygen blowing using a carbonaceous material as a heat source in a converter exclusively for melting, such as No. 3997, has the following compositional characteristics. It turned out that ladle desulfurization of the molten iron was extremely difficult.

■ Since a large amount of carbonaceous material is used in proportion to the amount of dissolved iron-containing cold material, sulfur contamination from the carbonaceous material is severe, and the sulfur content in the molten iron becomes 0.05% or more.

- Because oxygen blowing is performed, the silicon in the molten iron, which is a component that promotes desulfurization, is oxidized and its content becomes 0.03% or less.

The present invention advantageously solves these problems and provides a method for efficiently producing low-sulfur, high-carbon molten iron in a melting-only converter.

[Means for solving problems]

That is, the gist of the present invention is that when producing high carbon molten iron by supplying a ferrous composite material and carbonaceous material to a ferrous cold material melting furnace in which a seed metal exists and performing oxygen blowing, By adjusting the carbon content in the molten iron to 4% or more and applying stirring energy by blowing an inert gas into the molten iron, 310g of CaO, IVzO A method for producing low-sulfur, high-carbon molten iron from an iron-containing composite material, characterized by removing sulfur from the molten iron.

The details of the present invention will be described below.

The present inventors have proposed a method for ladle desulfurization of molten iron obtained by melting a large amount of iron-containing composite material while oxygen blowing using carbonaceous materials as a heat source in a converter exclusively for melting, as disclosed in Japanese Patent Application No. 62-73997. In order to solve the problem, we investigated methods to reduce the sulfur content in molten iron and methods to suppress oxidation of silicon in molten iron.

However, as far as oxygen blowing is concerned, it is difficult to suppress the oxidation of silicon in molten iron, and the method of adding a ferrous alloy with high silicon content at the end of blowing also requires selection of the ferrous alloy. Therefore, we came to the conclusion that it is not realistic. On the other hand, as methods for reducing the sulfur content in molten iron, there are two possible methods: using a carbonaceous material with a low sulfur content, and desulfurizing the material in an iron-containing cold material melting furnace. However, using a carbonaceous material with a low sulfur content is not preferable because it leads to the use of an expensive carbonaceous material and increases costs. Therefore, it is considered most promising to develop a method for efficient desulfurization in a ferrous composite melting furnace.

Based on the above studies, the present inventors have invented a refining method for controlling the carbon content in molten iron as a method for efficiently desulfurizing in a cold melting furnace containing iron. Specifically, we focused on the fact that carbon in molten iron is a desulfurization-promoting component that increases the activity of sulfur in molten iron, and developed a highly efficient desulfurization method to control the carbon content in molten iron during melt-blowing of iron-containing composites. .

First, the reason why the carbon content is set to 4% or more will be described. As mentioned above, carbon in molten iron is a desulfurization-promoting component, but the appropriate content to stably perform desulfurization at a high level in high-sulfur, low-silicon molten iron is not clear. Therefore, we conducted a small-scale crucible experiment to determine the appropriate carbon content. As a result, as shown in FIG. 1, it was found that when the carbon content in the molten iron was 4% or more, the desulfurization rate was 60 to 80%, and the desulfurization rate was highly stable. Therefore, the appropriate carbon content is 4% or more.

The small-scale crucible experiment was conducted as follows.

2000 g of the melted base material with the composition shown in Table 1 was
After melting in a magnesia crucible with a size of
The temperature was maintained at 400°C, 100g of slag shown in Table 2 was added, and Ar gas was introduced at 1000 cc/mi through an alumina tube.
The effect of the carbon content in the molten iron on the desulfurization reaction was investigated while stirring the molten iron.

The components in the hot metal other than carbon were set to be high in sulfur and low in silicon, assuming a steel manufacturing method such as that disclosed in Japanese Patent Application No. 73997/1983. The slag composition was also targeted at the composition produced during normal operations in a steel manufacturing process such as that disclosed in Japanese Patent Application No. 62-73997.

Next, we will discuss the reason why the carbon content in the molten iron is set to 4% or more only at the final stage of blowing. In melting and blowing of iron-containing cold materials, if the carbon content in the molten iron is high, the utilization efficiency of the supplied carbon material decreases, which is undesirable.This is because when carbon material is added to high-carbon molten iron, the carbon material dissolves in the molten iron. This is because the speed decreases and the so-called atrium becomes a phenomenon and no longer contributes as a heat source. On the other hand, when the carbon content in the molten iron is low, the dissolution rate of the iron-containing composite material decreases significantly, which is not preferable. This is because the melting of the iron-containing composite is dominated by heat conduction from the molten iron to the iron-containing composite and carbon diffusion (carburization), and the carbon content in the molten iron is low and carbon diffusion is inhibited. Therefore, during melting and blowing of iron-containing cold materials, there is an appropriate carbon content in the molten iron, and the optimum carbon content is usually 3 to 4%. Therefore, it is most efficient to increase the carbon content in molten iron to 4% or more only at the final stage of blowing for desulfurization refining. There are no particular limitations on the method of increasing the carbon content to 4% or more, but from the perspective of preventing blow-throughs, a method of adding relatively coarse-grained carbonaceous material from above a converter dedicated to melting iron-containing cold materials is recommended. Efficient. In such an upward addition method of carbonaceous material, some of the carbonaceous material is mixed into the molten slag, but this also has the advantage that iron oxide in the slag is reduced and the desulfurization reaction becomes more favorable.

Next, we will discuss the reason for applying stirring energy to molten iron. The metallurgical reaction rate is improved by imparting stirring energy to molten iron. Also in the present invention, the desulfurization reaction rate is greatly improved by providing stirring energy, resulting in desulfurization refining in a short time. Therefore, the drop in temperature of molten iron during desulfurization and refining is small, contributing to saving on heat sources. Therefore, in the present invention, imparting stirring energy to molten iron is an essential requirement for improving the efficiency of desulfurization refining. The most efficient method for applying stirring energy is to blow inert gas through the tuyeres at the bottom of the furnace. The inert gas is generally nitrogen or argon, but a nitrogen-oxygen mixed gas containing 50% or less oxygen may also be used. When oxygen exceeds 50%, it promotes the production of FeO and inhibits the desulfurization reaction.

Next, the slag conditions will be explained in detail.

The present invention is based on desulfurization using slag. That is, sulfur in the molten iron is removed into the slag by the basic component contained in the slag. As the basic component, it is preferable to add CaO, which is inexpensive and does not adversely affect the refractory. In this case, the basic component system of the slag is CaO-5iOz-lV
It becomes 2O3 system. CaO is a component added for desulfurization, and 5i02 is Si oxidation in iron-containing cold materials and Sin contained in coal ash.

caused by. Furthermore, Ah03 is also 7V, O in coal ash.
.

is mixed in. In order to promote desulfurization, it is desirable to use molten slag, and therefore CaO should be added at the initial stage of blowing to form CaO-5iOz-AjtO+ type molten slag in advance. In addition, the slag composition is high Cab/5if=
However, if it is too high, the slag will not melt, so there is an appropriate range of CaO/5iO=1.2 to 2.0 according to the experience of the present inventors. Within this range, the desulfurization ability can be stabilized to a high level. Note that the IV and O contents are approximately 5 to 15%.

The present invention can also be applied to a method that does not use a seed bath, that is, a method that produces 100% iron-containing cold material or carbon molten iron coating material.

Examples and comparative examples of the present invention will be described below, and the effects of the present invention will be described.

(Example) Seed bath 8 with a carbon content of 3.5% and a sulfur content of 0.030%
29 tons of scrap and 1 ton of limestone were charged into a melting converter in the presence of 4 tons of molten slag and 2 tons of pre-charged molten slag. Subsequently, 6.5 tons of coal was supplied from the bottom tuyere using nitrogen as a carrier gas, and 550 ONra'' of oxygen was supplied from the bottom tuyere and top blowing lance to melt the scrap. Molten iron carbon content at the time of completion of melting is 3.6%, sulfur content is 0.055%, Ca043%, 5iOz33%,
4 tons of 7117tes 12% molten slag were produced. Next, the coal is poured into the melting converter from above the melting converter.
．． 8 tons were added, nitrogen was supplied from the bottom tuyere, and the molten iron was stirred. As a result, the carbon content of the molten iron increased to 4.1%, and the sulfur content decreased to 0.015%.

Further, the amount of scattered iron dust during this blowing was 1.7 tons, and there was no blow-through of coal.

(Comparative Example-1) Seed water 8 with a carbon content of 3.5% and a sulfur content of 0.028%
29 tons of scrap and 1 ton of limestone were charged into a melting converter in the presence of 4 tons of molten slag and 2 tons of pre-charged molten slag. Next, 6 tons of coal was supplied from the bottom tuyere using nitrogen as a carrier gas, and 550 ONm' of oxygen was supplied from the bottom tuyere and top blowing lance to melt the scrap. The carbon content after treatment in this blowing process was as low as 3.4%, the amount of scattered iron dust was 1.8 tons, and there was no coal blow-through.

However, the molten iron sulfur content after treatment was 0.056%, which was 0.041% higher than in the example.

(Comparative Example-2) Seed water 8 with a carbon content of 3.5% and a sulfur content of 0.031%
29 tons of scrap and 1 ton of graystone were charged into a melting converter in the presence of 4 tons of molten slag and 2 tons of pre-charge molten slag. Subsequently, 7.5 tons of coal was supplied from the bottom tuyere using nitrogen as a carrier gas, and 570 ONm3 of oxygen was supplied from the bottom tuyere and top blowing lance to melt the scrap. In this blowing, the amount of coal supplied was large, so the carbon content of the molten iron after treatment rose to 4.3%, but since the blowing process was not performed by blowing only nitrogen gas, the sulfur content was 0.03%.
It remained at %. In addition, the amount of scattered iron dust is as large as 2.5 tons.
A phenomenon was also observed in the coal stairwell.

(Effects of the Invention) As detailed above, the present invention makes it possible to efficiently produce low-sulfur, high-carbon molten iron by melting a large amount of solid iron-containing cold materials such as cold pig iron and scrap. As a result, a method for obtaining molten steel from solid iron-containing cold material at low cost has been established, which is extremely beneficial to the steel industry.

[Brief explanation of the drawing]

FIG. 1 is a drawing showing the relationship between the carbon content of molten iron and the desulfurization rate. Patent applicant Nippon Steel Corporation

Claims (2)

[Claims] (1) When producing high-carbon molten iron by supplying ferrous cold material and carbonaceous material to a ferrous cold material melting furnace in which a seed metal exists and performing oxygen blowing, the carbon content in the molten iron is reduced to 4% only at the final stage of blowing. % or more, and by blowing an inert gas into the molten iron and applying stirring energy, the pre-formed CaO, S
A method for producing low-sulfur, high-carbon molten iron from iron-containing cold material, characterized in that sulfur in molten iron is removed through slag containing iO_2 and Al_2O_3 as main components. (2) The method according to claim 1, which produces low-sulfur, high-carbon molten iron from iron-containing cold material without using a seed bath.