KR100426852B1 - A method for increasing terminal carbon in electric furnace - Google Patents

Abstract

The present invention relates to a method of raising the end point carbon in the electric furnace operation during the production of medium-carbon steel by the mini-mill process, in the operating method for producing a medium carbon steel in the mini-mill process using the electric furnace, in order to raise the end point carbon Production of medium-carbon steel by adding 1.5-2.5% by weight of carbon into an electric furnace to raise charged carbon or by inserting 0.07-0.09% by weight of carbon into an electric furnace immediately before tapping to produce carbon in the steel. It is possible to increase productivity by improving the quality of molten steel, reducing refractory and aluminum units, and improving the yield rate by inserting a foaming agent by using a lance before charging or tapping the steel, etc. It is.

Description

A METHOD FOR INCREASING TERMINAL CARBON IN ELECTRIC FURNACE}

The present invention relates to a method of raising the end point carbon in the furnace operation during the production of medium-carbon steel by the mini-mill process, in particular, the raw unit of aluminum required for deoxidation by raising the end point carbon in the electric furnace, which is an upper step of the mini mill process, which mainly uses scrap. The present invention relates to a method of elevating the end point carbon in an electric furnace that improves productivity and improves cleanliness of molten steel through reduction and increased yield rate.

Recently, the steel industry has introduced a mini mill process directly connected to an electric furnace, a thin slab player, and a rolling mill, which has advantages such as lower equipment investment cost, much lower energy consumption for production products, and flexibility of operation compared to an integrated steel mill. Doing.

This is a very groundbreaking process considering that the electric furnace industry has produced only low-grade products such as rebar, section steel, and thick plates, and thus it is possible to produce hot rolled products and cold-rolled products, which are high-value steels, in the electric furnace.

This means that not only has the requirements become more demanding than those of conventional electric furnace products, but also new operation technology in the electric furnace, the upper process, has to be preceded.

A great feature of the mini mill process is the thin slab continuous casting method.

The thin slab continuous casting method is an innovative continuous casting process for making bar casts having a bar thickness of 50 mm to 100 mm, which is much thinner than conventional slabs having a thickness of about 150 mm to 300 mm.

Therefore, the thin slab continuous casting method has to maintain a high casting speed of 4.0 m / min or more in order to maintain comparable productivity compared with the conventional casting method. This is considered that the casting speed of the conventional continuous casting method is 0.8 to 1.5 m / min. Very high.

For stable operation under such high casting speed, the most important thing is productivity and quality. In other words, in comparison with the conventional continuous casting method, the steelmaking process, which is the upper part of the mini mill process, in particular, an electric furnace providing a source of molten steel quality is a very important process.

Steel grades produced in the mini mill process are subject to steel grade limitations due to the process characteristics of high speed casting mentioned above. In other words, metallurgy is known to be difficult to cast in a range of 0.065 to 0.14% by weight of carbon, which is a delta phase in the section where the molten steel in the liquid state solidifies to a solid state. The phase transformation of ferrite to gamma-austenite occurs with a volume reduction of approximately 0.38%.

In high speed casting, volume reduction in this area causes air gap between mold and coagulation cell, which decreases heat transfer between mold and coagulation cell, resulting in uneven thickness of coagulation cell and increasing stress on the surface. A fatal duty-free operation in the car will cause a burst (break-out).

Due to this condensation, steel produced in the mini mill process is mainly made up of low carbon steel with less than 0.06% of carbon and medium carbon steel with more than 0.16% of carbon.

In general, the electric furnace, which is the upper part of the mini-mill process, has much handicap in product quality and operation compared to the furnace of the blast furnace mill using a large amount of high-quality molten iron by using scrap as a main raw material.

In other words, the furnace operation of blast furnace mill uses 80% or more of high-quality molten iron as an iron source, so that there are very few tramp elements such as copper (Cu), nickel (Ni), and chromium (Cr). Do.

On the other hand, in the electric furnace operation, as the total amount of iron source depends on the scrap, the quality variation in the scrap itself is severe and the inclusion of the tramp element is inevitable, which acts as a limiting factor in the production of the product.

Oxidation removal In the furnace refining reaction of blast furnace, impurities are oxidized and deoxidized in a short time by the reaction of slag and metal at the interface between the oxygen jet blown at supersonic and molten iron. By these reactions, the bath is stirred and heated, and the slag goods are promoted by dissolving the raw materials such as quicklime, iron ore, light dolomite, and fluorspar in the solid state.

In comparison, an electric furnace is a facility for dissolving and refining an iron source by converting electrical energy and chemical energy into heat to maximize the efficiency of input energy.

In general, the operation pattern of the electric furnace can be divided into a dissolving device for melting scrap iron and a refiner for oxidative removal of impurities. The dissolver is mainly subjected to electrical energy in the form of a low arc, high voltage, long arc, which is applied to the scrap. The thermal energy applied is determined by the applied power energy and the arc shape.

In addition, after the melting down (Melt Down), which is almost the point at which the melting operation is completed, the slag forming material (hereinafter, the forming material) in the form of oxygen and powdered coke through the lance is blown in and the slag forming operation is performed. At this time, the main reaction is mainly an oxidation reaction to remove impurities in the furnace, and the heating operation is performed in parallel with a short arc operation of high current and low voltage through the upper electrode.

As described above, the operation of the electric furnace, which is an upper step in the mini-mill process, is not only complicated compared with the converter operation of the blast furnace mill using a large amount of molten iron but also requires a new operation technology that is incomparable with the operation of a general electric furnace.

In other words, as described above, the mini-mill process is an on-line integrated process connected by electric furnace-thin slab continuous casting-rolling, which is different from the conventional process. do.

In particular, due to the nature of the mini mill called high-speed casting, it is possible to produce only low carbon steel with less than 0.06 wt% carbon and medium carbon steel with more than 0.14 wt% carbon.

In the mini-mill process, it is produced by the same method as the conventional method for manufacturing medium carbon steel, but has the following problems.

In other words, the conventional method limits the end point carbon at the end of the furnace to 0.03 to 0.04% by weight, thereby satisfying the conditions within the carbon range of the low carbon steel. As the content is very high as 35 ~ 45% level, this method of operation has the following problems when applied to the same medium carbon steel as it is.

First, there is a problem in that the slag fluidity is rapidly increased and the viscosity is weak, so that the slag forming operation deteriorates after the melter, and thus the power unit is increased and the steelmaking time is increased accordingly.

Second, there is a problem that productivity decreases due to a decrease in the error rate of products due to severe iron loss in molten steel due to slag peroxidation.

Third, iron and manganese, which are valuable metals in molten steel, are excessively oxidized, and iron oxide (FeO) and manganese oxide (MnO) increase in the slag in the form of oxides, resulting in excessive loss of furnace refractory and oxygen in molten steel. There is a problem in that the use of expensive aluminum as a deoxidizer in nature increases.

Fourth, oxides produced by reacting with dissolved oxygen in ferroalloy and molten steel remain as non-metallic inclusions and cause deterioration of the quality of molten steel, and cause productivity due to nozzle clogging during secondary refining and high speed casting processes. Of course, the deterioration of workability, there is a problem causing various defects in the product.

The present invention has been invented to solve various defects and problems in the production of medium-carbon steel in the electric furnace of the conventional mini-mill process in consideration of the above-described situation, by increasing the end carbon of the electric furnace to improve the slag forming property to lower the power unit and deoxidation. It provides a method to raise the end point carbon in the electric furnace that can improve the workability of the continuous casting process and improve the quality of the final product by reducing the expensive aluminum raw materials required for the construction, increasing the room yield and improving the cleanliness of molten steel. There is a purpose.

1 is a structural diagram of an electric furnace,

2 is a schematic diagram of an electric furnace operation process,

3 is an equilibrium relationship diagram between oxygen and carbon in molten steel in an electric furnace;

4 is a T-T change diagram according to the charging carbon in the electric furnace,

5 is a change of the end point carbon according to the charging carbon in the electric furnace,

6 is a conceptual view of raising the endpoint carbon by the input of the foaming agent according to the present invention,

Figure 7 is a comparison between the end point oxygen and operation results in the furnace according to the conventional method and the present invention

All.

<Explanation of symbols for main parts of drawing>

1: electric furnace 2: electric furnace roof

3: upper electrode 4: molten steel

5: exit door 6: slag door

7: oxygen injection pipe 8: carbon injection pipe

9: lance body

In order to achieve the above object in the present invention, the method of raising the end point carbon in the operation method for manufacturing the medium carbon steel in the mini mill process using the electric furnace, 1.5 to 2.5% by weight at the time of loading the raw material to raise the end point carbon Injecting carbon into the electric furnace to increase the charged carbon or 0.07 to 0.09% by weight of carbon in the electric furnace immediately before the tapping is characterized in that the production of medium carbon steel by raising the carbon in the steel.

Hereinafter, with reference to the accompanying drawings will be described in detail a method for raising the end point carbon in the electric furnace of the present invention.

As shown in FIG. 1, one to three upper electrodes 3, which are electrical conductors, are provided in the center of the furnace roof 2 located in the upper part of the furnace body 1, as shown in FIG. 1. After the raw material is introduced into the furnace, the arc is generated between the upper electrode 3 and the raw material, thereby dissolving the raw material by heat.

The slag door 6 is configured to blow oxygen and carbon into the oxygen inlet pipe 7 and the carbon inlet pipe 8 so as to perform cutting (melting machine) and slag forming operation (heater) of scrap metal. . Reference numeral 4 denotes a molten steel, 5 denotes a tap hole, and 9 denotes a lance body.

The general furnace process has a series of operation modes of charging, melting, heating, tapping, as shown in FIG. 2, and the period of using oxygen and carbon is mainly a dissolver and a heater.

Electric furnace operation in mini-mill producing thin slab is possible in the continuous casting operation area of carbon in less than 0.06% and more than 0.14%, so in the furnace, the amount of carbon rising by heating up during refining (generally 0.01 ~ 0.02% Taking into account the rise, the end carbon (carbon contained in the molten steel immediately before tapping) is controlled.

To this end, the amount of oxygen in the furnace is excessively input, and as a result, the oxygen in the molten steel increases, thereby increasing the refractory erosion due to the production of iron oxide, the decrease in missed yield and the increased slag fluidity.

In addition, oxygen contained in a large amount of steel is a harmful component to the steel to remove it during the deoxidation treatment during tapping, at this time, the amount of deoxidizer (aluminum, silicon, etc.) increases the raw unit increases.

The present invention is a method of eliminating the above-mentioned problems by reducing the oxygen in the molten steel by increasing the carbon in the molten steel by adding carbon to the steel in the furnace using a lance and a basket.

In general, the equilibrium relationship between oxygen and carbon in steel is inversely proportional to that shown in FIG. 3.

As can be seen in Figure 3, the amount of oxygen decreases as carbon increases in the steel. Oxygen in steel reacts with Fe and produces oxides to reduce the real rate, reacts with electric furnaces and ladle refractory to raise raw units, and increases the amount of deoxidizer.

The present invention is to increase the end carbon higher than the existing value in order to lower the oxygen in the steel during the manufacture of medium-carbon steel to charge the raw material with high carbon content, such as lump coke or carbonaceous material into the basket, or using a lance immediately before the tapping It is a method of reducing the amount of aluminum used in post-processing, improving the yield of tapping, and reducing the unit of refractory unit by adding charcoal (forming material input).

The medium carbon steels are subjected to continuous casting under conditions of 0.14 to 0.18% of carbon standards, and the carbon behavior of each process is compared with those of the existing operations and the present invention.

Carbon Trends by Process of Heavy Carbon division Electric furnace Refining furnace Continuous casting Existing operation 0.03-0.04% 0.04-0.18% 0.14 ~ 0.18% Invention 0.06-0.09% 0.09-0.18% 0.14 ~ 0.18%

As shown in Table 1, the carbon content at the end of the electric furnace is increased to 0.06 to 0.09 wt%, which is more than twice the existing 0.03 to 0.04 wt%, to reduce the oxygen in the steel, and to operate the medium carbon steel carbon (0.14 ~ 0.18% by weight) is adjusted by adding charcoal to the refinery.

The present invention proposes two methods for upgrading the endpoint carbon with electricity.

First, the terminal carbon is raised by raising the charged carbon (carbon contained in the iron source charged into the electric furnace).

Since the scrap used in the electric furnace contains carbon in the furnace, the furnace always has a certain amount of charged carbon, which is 1.0 to 1.5% by weight of the total charged amount. In general electric furnace, it is common to blow scrap metal and slag coarse lime as a raw material before melting, but the present invention also adds high carbon raw material, coke coke, to increase charged carbon.

The charge carbon reaches the target end point carbon by oxygen introduced by the lance during operation, and the charge carbon affects the end point carbon. Under the same operating conditions, the terminal carbon increases as the charged carbon increases, and the appropriate amount of charged carbon increases the arc heat transfer efficiency to molten steel due to good slag forming operation due to the proper reaction with oxygen during the operation. 2.5 wt% or more) increases the amount of carbon in the furnace to reduce slag, but the temperature decreases and TT rapidly increases due to the endothermic reaction during reduction, and the boiling phenomenon in molten steel due to the rapid reaction with oxygen. Overflow occurs), and the furnace molten steel flows out of the furnace (slag door) to reduce the error rate. Too low charged carbon (1.5 wt% or less) increases the oxidation rate of the molten steel, reducing the error rate and forming the slag viscosity. This becomes poor and the heat transfer efficiency of the arc is reduced.

Therefore, in the electric furnace, charged carbon is suitable in the range of 1.5 to 2.5% by weight, and for this purpose, a cold carbon or molten iron (carbon content of 4% or more) is used as a raw material having high carbon content. Use of raw materials such as peat ash (carbon content of 80% or more) is required. Table 2 shows the change in the real rate according to the charged carbon.

Comparison of real rates by charged carbon Charging Carbon Less than 1.5% 1.5% or more % Real 91.2% 91.8%

The T-T change according to the charged carbon is shown in FIG. 4.

As shown in Fig. 4, it is good to operate the charged carbon in the appropriate charged carbon region (ⓑ), and additional raw materials such as Goke coke are added to raise the existing charged carbon.

Table 3 below shows the change amount of the carbon charging furnace according to the amount of Goke coke usage.

Changes in Charged Carbon According to the Input of Goke Coke Coke coke amount [C] change (%) Charge amount (kg) Remarks 0.2% 352 -Loading amount 150 tons / Ch-Go-coke carbon grade 85% 0.4% 704 0.6% 1,056 0.8% 1,410 1.0% 1.760

In order to raise the charging carbon compared to the existing operation (1% level), 1,500 ~ 2,000kg of goose coke was charged by using basket or hopper when charging (1st or more charging).

It was expected that the amount of oxygen introduced as the charged carbon increased, but the amount of oxygen decreased due to the decrease in the amount of oxygen introduced as the T-T decreased.

The following is a method of using the carbon input pipe (8) in the lance to raise the end carbon before the electric furnace tap.

In general, in the furnace operation, oxygen and carbon are used to dissolve scrap metal using oxygen before the collapsed phase (80% molten iron; Melt Down) and slag forming operation using oxygen and carbon simultaneously after the collapsed phase. Do it.

The slag forming machine has a state where the charged carbon existing in the scrap metal reaches the final target level (0.04% level), and the injected carbon and oxygen are reactively balanced, so that there is almost no change in carbon in the molten steel.

After the slag forming operation, when the molten steel reaches a temperature at which the steel can be pulled out, the carbon (forming material) is introduced into the molten steel using the carbon injection tube 8 attached to the lance to increase the carbon content in the molten steel. At this time, oxygen is not used to shorten the carburizing time of molten steel by the carbon injected.

In FIG. 4, section ⓐ is a section in which oxygen and carbon are blown at the same time by the lance 9, and section ⓑ is a section in which only carbon is added alone. Electric furnace endpoint carbon up method is to increase the content of carbon contained in the molten steel by injecting carbon into the molten steel using the carbon input pipe (8) in the operating section ⓑ.

6 is a conceptual diagram of a method of raising the end point carbon by the input of the foaming agent according to the present invention, wherein the input amount of the foaming agent for raising the carbon of the molten steel is shown in Table 4.

Changes in Carbon in Steel According to Forming Material Input Forming agent amount [C] change (%) Input (kg) 0.02% 33 0.04% 67 0.06% 100 0.08% 134 0.10% 167

Table 4 shows that the carbon content of the foaming agent is 90%, and was performed in a 130 ton electric furnace (150 tons of steel).

Through these two methods, oxygen in the molten steel in the furnace was reduced by 500 ~ 700ppm, the end point carbon was increased by 0.04 ~ 0.05%.

Figure 7 shows a comparison of the results of the operation according to the present invention and the conventional method, (A) is the performance in the conventional method of manufacturing a medium carbon steel, (B) shows the operation results according to the present invention.

Operational results as shown in the following table 5 were obtained due to the decrease of the end point oxygen due to the uplink end point of the electric furnace.

Item Conventional method Invention Terminal Carbon (Electric) 0.030-0.040% 0.070-0.090% End oxygen (electric furnace) 900-1500 ppm 300 ~ 600ppm Attendance rate 91.2% 91.8% Al unit 4.5kg / ts 3.3kg / ts Refractory unit 17.2kg / ts 11.8kg / ts Sogang Oxygen (Tundish) 31 ppm 29 ppm

As shown in Table 5, in the present invention, the degree of oxidation (oxygen) in molten steel and slag is reduced compared to the conventional method of operating an electric furnace, thereby reducing Al and refractory raw units, and improving dissolved steel quality by decreasing dissolved oxygen that promotes inclusions. In addition, the error rate was increased, which greatly improved productivity.

As described above, according to the method of elevating the end point carbon in the electric furnace of the present invention, the quality of the molten steel is improved by inputting a forming agent using a lance before charging or tapping the lump coke or cold wire during the production of medium carbon steel. There is an advantage that can increase the productivity by reducing the refractory and aluminum unit and improving the room yield.

Claims (1)

In the operation method for manufacturing medium-carbon steel in the mini mill process using an electric furnace, in order to raise the end point carbon, 1.5-2.5% by weight of carbon is introduced into the electric furnace when charging raw materials to raise the charged carbon or 0.07 to Method of raising the end point carbon in the electric furnace, characterized in that to produce a medium-carbon steel by adding 0.09% by weight of carbon in the electric furnace to raise the carbon in the steel.