JPS635450B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improvement of the arc furnace steel manufacturing method, and in addition to particularly remarkable productivity improvement and energy saving effects,
The present invention provides a steel manufacturing method that effectively achieves high refining yield, reduced amount of waste, and further suppresses furnace wall wear. While the mainstay of steelmaking shifted from open-hearth furnaces to converter furnaces, arc furnace steelmaking continued to maintain its position. The biggest support for this is the significant improvement in production efficiency of arc furnace steelmaking. Compared to converter steelmaking, which operates using raw materials such as hot metal, arc furnace steelmaking has the disadvantage of starting operations from cold material such as scrap. It depends on how the time and energy required for melting can be saved. The main measures taken for this purpose were:
One is to increase the amount of power input per unit time.
This is so-called UHP electric furnace operation, and another method is the use of an auxiliary burner, which seeks an energy source other than electricity. Although these efforts can be said to have achieved some success, there are naturally limitations due to the equipment used, such as transformer capacity and burner capacity. Therefore, attempts were made to utilize oxygen gas to overcome this limit. That is, during the melting of the charge and at the end of melting, the maximum amount of oxygen gas is blown into the furnace within the range that the equipment allows, and after the smooth steel bath surface has been formed, the amount of oxygen that is greater than that required for decarburization is blown into the furnace. This operation involves injecting into molten steel while continuing to transmit electricity (arc heating). In addition, in equipment having an auxiliary combustion burner, oxygen in excess of the amount required for combustion of oil is sent through the burner. The present inventor calls such an operation an oxygen-enriched operation in arc furnace steelmaking. This oxygen-enriched operation
The reaction heat generated when silicon, iron, and other components in the charge of the arc furnace are oxidized by the input oxygen accelerates the dissolution of solid matter and the temperature rise of molten steel, and also removes unfinished material that adheres to the furnace wall. Since the molten solids are forcibly melted by oxygen and uneven melting in the furnace is eliminated, production efficiency is greatly improved. The benefits of such oxygen-enriched operations are not only time savings, but also significant reductions in the amount of power required for melting and refining in arc furnaces.
Because the heat input by the arc is performed from above the charge or the steel bath due to the structure of the furnace, it is unavoidable to some extent that energy is lost due to upward reflection of the arc. On the other hand, since the heat due to the reaction with oxygen is generated on the surface of the charge or in the steel bath, most of the energy is utilized. Therefore, when converting the amount of oxygen gas used into energy and adding it up to the energy of electricity and considering the basic unit of total input energy, the value increases as the amount of oxygen gas input increases (oxygen input amount of 10Nm ³ /charging ton more than 10%, 20Nm ³ /charging ton reaching 20%). As described above, although oxygen-enriched operation is an advantageous arc furnace steelmaking method, new difficulties were encountered in the process of pursuing its benefits. One of these is a decrease in the melting yield, which is the ratio of the weight of molten steel obtained to the weight of charged raw materials such as scrap, and the other, which is related to this, is the decrease in the amount of slag, which is the melting residue. This is an increase in In addition, even though oxygen-enriched operation enables a significant reduction in power consumption as mentioned above, the generation of reaction heat due to oxygen injection becomes effective after the melt is formed in the furnace. Since the charging raw materials are basically melted by arc heating, there is a limit to the reduction in electric power energy by adding oxygen. The present invention has been made against this background, and its purpose is to significantly improve productivity and save energy, especially power consumption, while utilizing the advantages of oxygen-enriched operation. The object of the present invention is to provide an electric arc furnace steelmaking method that can save a large amount of energy. In addition, another object of the present invention is to
By eliminating the above-mentioned difficulties in oxygen-enriched operation, we have achieved improvements in production efficiency and energy savings, while also improving melting yield, suppressing the increase in the amount of slag that becomes waste, and further suppressing wear and tear on the furnace walls. The objective is to provide a steel manufacturing method that can be used with high performance. In order to achieve these objects, the present invention is characterized by including the following steps. That is, in arc furnace steelmaking, (a) oxygen is blown into the furnace during the melting phase, oxidation phase, or both to reduce the carbon content;
obtaining a molten steel of less than 0.20% carbon, preferably less than 0.15%; to be charged. The first step (a) of the operation according to the invention may be carried out in accordance with known methods, for example by charging a raw material such as scrap into a given arc furnace and heating it under normal conditions. All you have to do is arc heating. Further, oxygen gas may be blown in using a normal lance pipe (6 in FIG. 1), and when an auxiliary combustion burner is used, oxygen gas may be blown in in excess of the amount required for combustion of the oil. Note that, as is well known, pure oxygen gas or oxygen-enriched air is preferably used as the oxygen gas to be blown. Moreover, if the situation in the second stage (b) is shown schematically,
As shown in Fig. 1, in the arc furnace 1 there are electrodes (usually three electrodes are used, but only one of them is shown here to simplify the explanation) 2. An arc is generated between the steel and the molten steel 3 to perform heating and refining, and the surface of the molten steel 3 is covered with slag 4. Further, a lance pipe 6 is inserted into the melt in the furnace through a small hole 5 provided at a slag outlet (operation port), and oxygen is blown into the melt through the lance pipe 6. Note that the position of the tip 6A of the lance pipe 6 is normally near the boundary between the molten steel 3 and the molten slag 4, but it may be moved up or down as appropriate depending on the amount of carbon that should be present in the steel and the stage of refining. and transferred into the molten steel 3 or into the slag 4. Furthermore, a refractory container 7 containing hot metal with a high carbon content of about 2% or more is provided on the side of the arc furnace 1, and the container 7 penetrates through the slag outlet (operating port) in the furnace wall. Hot metal is continuously charged into the arc furnace 1 at a predetermined ratio through a refractory gutter 8 that opens into the furnace. If hot metal is continuously charged while continuing arc heating and oxygen injection in this manner, the molten slag 4 will rise as described later. In order to better understand the significance of adopting the above configuration, it seems appropriate to explain the circumstances that led to the establishment of the present invention, so this will be described below. As is well known, the iron oxide content in the molten slag that exists on the molten steel is controlled by the oxygen content (more precisely, the activity) in the molten steel, and the higher the oxygen content in the melt, the more the slag The iron oxide content in it also increases. On the other hand, the oxygen content in molten steel is mainly determined by the carbon content in molten steel under conditions where oxygen supply is sufficient, such as in oxygen-enriched operations. The relationship between carbon content and oxygen content in molten steel has been studied for a long time, and a graph known as the so-called ``Butcher-Hamilton relationship'' is published in various handbooks. According to this, generally the higher the carbon content in molten steel, the lower the oxygen content. Therefore, the iron oxide content in the slag coexisting with the molten steel should also be low. Conversely, a low carbon content in molten steel means a high oxygen content, which results in a high iron oxide content in the slag. When the present inventor attempted to improve the melting yield in the process of exploring oxygen-enriched operations, the first measures he attempted were:
The aim was to adjust the molten steel composition based on the above understanding. In other words, by increasing the amount or selecting the type of carbon additive (pre-carburizer) charged into the arc furnace along with raw materials such as scrap, the carbon content in molten steel can be reduced at the end of oxygen enrichment operation. the oxygen content in the liquid steel at a relatively low level. Adjustment of the carbon content in molten steel using this pre-charged recarburizer is generally effective until the amount of oxygen fed into the furnace is around 5 to 6 Nm ³ /charging ton, and the benefits of oxygen-enriched operation are reduced to some extent. While enjoying the benefits, we were able to somewhat suppress the decrease in melting yield and increase in the amount of slag. However, to obtain the full benefits of oxygen-enriched operation, the oxygen input must be higher. However, when using large amounts of oxygen of 10 Nm ³ /tonne or more, it has been found that it is no longer possible to maintain the carbon content in the molten steel by pre-carburizing. This is because most of the carbon provided by normal charging means is burned away by oxygen, and there is no longer any carbon that can be dissolved in the molten steel. Even if you try to increase the amount of pre-carburized material to an extremely large extent, it may get mixed with the slag that is partially discharged during the refining process and come out of the furnace, or it may be retained in the molten material that sticks to the furnace wall and be used in the subsequent refining process. There is a risk that the metal may fall into the molten steel and interfere with the refining work. In view of this experience, the present invention has been devised to charge hot metal with a high carbon content. The novelty of this method of the present invention is that the carbon content in the molten steel is once lowered to a low level and then carbon is added again. In other words, when carrying out oxygen-enriched operations, we abandon the idea that was conventionally accepted in the art of maintaining the carbon content in molten steel based on the above-mentioned Butcher-Hamilton relationship. A sufficient amount of oxygen gas is injected into the furnace without worrying about the reduction of the carbon content in the iron or the oxidation of the iron content, and the iron oxide produced by combustion is later reduced with carbon to produce metallic iron. The significance of the present invention is that it presents an improvement plan for recovering wastewater as waste. Furthermore, since Hess's law naturally applies to the various chemical reactions that occur in the arc furnace, the heat generated by the oxidation of iron obtained during oxygen-enriched operation is offset by the reduction process of iron oxide. There may be concerns that this may result in When the same amount of oxygen reacts with iron and when it reacts with carbon, Fe+O=FeO+109.6Kcal/mol・O ₂ C+O=CO+52.8Kcal/mol・O _2The former generates more heat, but FeO+C→ Due to the endotherm of Fe+CO−56.8Kcal/mol・O ₂ , it seemed that only the heat of combustion of carbon could be used in the end. However, as is clear from the examples shown later, the energy saving effect brought about by the oxygen-enriched operation is not diminished by charging hot metal with a high carbon content under arc heating and oxygen injection. The main reason for this is considered to be the foaming of the slag mentioned above. In other words, by introducing carbon through hot metal, iron oxide is reduced and carbon monoxide is produced.
As a result of this foaming the slag with the injected oxygen, the upper part of the foam layer rises by 50cm to 1m when viewed from before the blowing and wraps around the tip of the electrode that is transmitting power, causing the arc to completely bury itself in the slag and generate heat. It is a phenomenon that is highly utilized. Due to the formation of a foam layer on the top of the slag based on this forming phenomenon, this slag foam envelops the arc and also helps keep the molten steel warm, improving the power consumption and shortening the required time. be. Moreover, since the arc is flying in the slag, it is possible to effectively suppress melting of the furnace wall due to the direct effects of the arc, arc noise, and radiant heat from the slag surface. be. In particular, in the case of the present invention, carbon is introduced into the molten steel and slag where the oxygen content is high due to the oxygen enrichment operation in the first stage, so a large amount of carbon monoxide gas is generated. , the above effects become even more pronounced. According to the inventor's experience, the thermal efficiency when heating by electric power without charging hot metal (introducing carbon) after oxygen-enriched operation is approximately 30%; This increases to about 60% when the child gets older. This efficiency increase is, for example, 300 ton in a nominal capacity 70 ton reactor.
This is sufficient to compensate for the endotherm due to the reduction of iron oxide in an example operation in which hot metal containing Kg of carbon is introduced for 5 minutes. Next, a supplementary explanation will be given regarding the configuration of the present invention. The carbon content in molten steel is reduced by blowing oxygen.
Below 0.20%, preferably below 0.15%, and in normal operation below 0.10%, is a condition reached as a result of oxygen-enriched operation rather than being essential to the arc furnace refining process itself. In other words, it is confirmed by the carbon content in the molten steel that a sufficient amount of oxygen has been blown to ensure the benefits of the oxygen-enriched operation. In terms of providing guidance for operations,
This condition has important significance. In terms of actual operation, the amount of oxygen blown to give the above carbon content in molten steel is usually 10 Nm ³ /charging ton or more. These conditions were found empirically by the present inventor in the process of exploring oxygen-enriched operation, and are also supported by the results of decarburization tests in a 70-ton arc furnace. FIG. 2 is a graph showing the relationship between the carbon content in molten steel and the decarburization rate when performing decarburization by oxygen blowing. As is clear from this graph, when the carbon content in molten steel becomes 0.20% or less, more specifically 0.15% or less, the decarburization reaction changes from oxygen supply rate-limiting to carbon diffusion rate-limiting; At this rate, the oxygen supplied into the furnace will primarily oxidize iron. However, according to the present invention, even if the iron content is lost due to oxidation, it is recovered by subsequent carbon injection, so there is no disadvantage at all even if the carbon content of molten steel is reduced to a low value, such as 0.10% or less. do not have. Therefore, oxygen can be blown to the extent that the benefits of oxygen-enriched operation can be sufficiently obtained without worrying about a decrease in carbon content. In addition, the hot metal that is continuously charged into the furnace according to the present invention must contain at least about 2.0% carbon, and if the carbon content is too low, it will not be effective in reducing iron oxide. Therefore, it is difficult to fully achieve the purpose of the present invention. In particular, in the present invention, hot metal produced in a blast furnace or cupola and having a high carbon content of about 3 to 4% or more is advantageously used. Since hot metal is produced in a blast furnace or cupola using coal energy, the overall power consumption in arc furnace steelmaking can be significantly reduced by using such hot metal. While using the carbon component in the hot metal to reduce iron oxide,
This is because the iron component, which is the main component of the hot metal, can be used as it is as the molten steel component of the arc furnace product, and this is a significant feature of the present invention. In other words, the aim of the present invention is to utilize the effect of killing two birds with one stone in that by using the carbon in the hot metal to be removed for reducing iron oxide in arc furnace steelmaking, the hot metal is also converted into molten steel at the same time. By doing so, it is possible to achieve not only a reduction in electric energy in the arc furnace but also a significant improvement in molten steel productivity. In addition, such hot metal should be used so that it will eventually account for 10 to 50% of the total charge (therefore, cold solid materials such as scrap will account for 50 to 90% of the total charge). ), and will be charged into the arc furnace. Further, it is desirable that the charging of the hot metal be carried out at a rate such that the amount of carbon supplied from the hot metal is 0.2 to 2.0 kg/min per ton of molten metal present in the arc furnace. If this charging speed is too slow, the efficiency will be poor, and if it is too fast, problems such as fluctuations in the quality of molten steel are likely to occur when melting is repeated. Furthermore, during such continuous charging of hot metal, arc heating by the electrode 2 continues,
In addition, oxygen gas continues to be blown through the lance pipe 6. Therefore, as the oxygen gas blown in this step (b), the same pure oxygen gas or oxygen-enriched air as in step (a) is used.
Through such arc heating and continuous charging of hot metal under oxygen gas blowing, the above-mentioned effective reduction of iron oxide and the formation of an effective foam layer by the generated carbon monoxide are achieved. By doing so, the excellent effects of the present invention can be achieved. In addition to the specific examples described above, various changes and modifications may be made to the present invention based on the knowledge of those skilled in the art without departing from the spirit of the present invention. Hereinafter, the present invention will be explained in more detail by comparing it with the prior art using an example of steel manufacturing using a UHP arc furnace with a nominal capacity of 70 tons. Comparative Example 1 (Conventional method) 32 tons of commercially purchased scrap, 21 tons of internally generated scrap
1 ton of coke powder and 2 tons of quicklime were charged into an arc furnace, and power transmission (arc heating) was started. After 33 minutes, power transmission was interrupted, and an additional 34 tons of commercially purchased scrap was loaded into the furnace, and power transmission resumed. At 64 minutes after the start of the first power transmission, a portion of the molten steel was collected as a sample for component analysis. The carbon content in the molten steel at this point was 0.25%. Thereafter, 120 Nm ³ of gaseous oxygen was blown into the molten steel using a lance for the purpose of reducing the carbon content in the molten steel (decarburization). After the blowing was completed, a sample for analysis was taken again. The carbon content in the molten steel at this point is
It was 0.08%. After completing the oxygen injection and confirming that the steel bath temperature had reached 1650°C by power transmission (arc heating), samples were taken for analysis again, and then the slag inside the furnace was discharged outside the furnace. The carbon content in the molten steel at this point is
It was 0.06%. According to the analysis of the furnace slag collected at the same time, the iron oxide content in the slag in the furnace at this time was 25%. After discharging the slag, 2.6 tons of elemental alloys were added into the furnace along with quicklime and fluorspar. After these additives were melted by power transmission, it was confirmed that the steel bath temperature had reached 1630℃, and the molten steel in the furnace was tapped into a ladle outside the furnace. The carbon content in the molten steel after tapping was 0.21%.
The weight of molten steel after tapping was 86,200 kg, and therefore the melting yield was 96.2%. The weight of the slag discharged outside the furnace after decarburization and heating was completed was 5.1 tons. The electricity intensity per ton of steel tapped was 510 KWH, which was calculated by dividing the amount of electricity used throughout the entire process by the weight of tapped steel. After tapping is completed, the inside of the arc furnace is repaired with refractories.
After charging the materials for the next melting, power transmission was started 121 minutes after the start of power transmission for the pre-melting. Comparative Example 2 (Oxygen-enriched operation) The same raw materials as in Comparative Example 1 were charged into an arc furnace, and power transmission was started. Ten minutes after the start of power transmission, gaseous oxygen was supplied into the furnace using a lance. 28 minutes after power transmission started, power and air transmission was stopped, 34 tons of commercially purchased scrap was loaded into the furnace, and power transmission resumed. Power transmission resumed 5
After several minutes, gaseous oxygen supply into the furnace was resumed. The operation continued as it was, and samples for analysis were taken when most of the solid charge had entered the steel bath and a smooth surface had been formed. Carbon content in molten steel at this point is 0.10%
It was hot. Thereafter, air was continued to be supplied to the steel bath, and electricity was also transmitted to raise the temperature of the steel bath. After confirming that the steel bath temperature had reached 1650°C, another sample was taken for analysis, and then the molten slag inside the furnace was discharged outside the furnace. The carbon content in the molten steel at this point was 0.04%. According to the analysis of the furnace slag sampled at the same time, the iron oxide content in the furnace slag at this time was 45%. After discharging the slag, the same operation as in Comparative Example 1 was performed to tap the steel into a ladle. The steel melting yield obtained through this operation was 94.5%.
It was hot. The amount of slag discharged outside the furnace was 7.2 tons. The electricity consumption rate was 455KWH/ton of molten steel, and the amount of gaseous oxygen used was ^1490Nm3 . One cycle time from the start of power transmission to the start of the next power transmission was 87 minutes. Example: 30 tons of commercially purchased scrap, 20 tons of internally generated scrap
A ton of coke was charged into an arc furnace along with 0.5 ton of coke powder and 2 tons of quicklime, and arc heating was started by power transmission. Approximately 10 minutes after the start of power transmission, gaseous oxygen is blown into the molten steel through a lance into the arc furnace, and the scrap in the furnace is completely melted and the temperature of the molten steel is approximately
After the temperature reached 1500°C, some of the molten steel and slag were collected from inside the furnace as samples for component analysis. At this point, the carbon content in the molten steel was 0.03%, and the iron oxide content in the slag was 53%. While continuing to transmit power and blow gaseous oxygen, 40 tons of molten pig iron at 1350°C with a carbon content of 2.95%, which had been melted and produced in Cupora, was pumped at 2/min.
It was continuously charged into the arc furnace at a speed of 100,000 tons. It took 20 minutes to complete the entire charging of the hot metal, and at the same time the blowing of gaseous oxygen was stopped. The gaseous oxygen required from the start to the end of hot metal charging was ^1100Nm3 . Further, shortly after the start of charging the hot metal, the arc noise caused by power transmission became significantly smaller, and the molten slag in the furnace was found to be bubbling and rising to about 1 m. Then, after stopping the oxygen pneumatic supply, the molten steel was heated to 1680°C under power supply, and an analysis sample was collected.
The slag inside the furnace was discharged outside the furnace. The carbon content in the molten steel immediately before slag discharge was 0.1%, and the iron oxide content in the slag was 22%. After discharging the slag, along with quicklime and fluorite, 2.5
After adding tons of element-added alloy into the furnace and melting these additives by power transmission, we confirmed that the molten steel temperature had reached 1630℃ and tapped the molten steel into a ladle outside the furnace. . The amount of molten steel tapped was 89,700 kg.
Therefore, the melting yield is 97% (=89700Kg/92500Kg
Kg). Furthermore, the slag discharged outside the furnace is
It weighed 2.4 tons. The time required from the start of power transmission to the start of power transmission for the next melting was 75 minutes. In addition, the electricity intensity required was
It was 265KWH/ton of molten steel. The above results can be summarized as follows. From this comparison, it can be seen that the arc furnace steelmaking method according to the present invention further significantly improves the level of oxygen-enriched operation in terms of production efficiency and energy cost, and
It is clear that the benefits are high melt yield and low slag emissions.

[Table] 1 for relative comparison.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram mainly showing a cross-sectional view of an arc furnace for explaining the second stage of the method of the present invention. FIG. 2 is a graph showing the relationship between carbon content in molten steel and decarburization rate when oxygen is blown in the first step of the method of the present invention. 1: Arc furnace, 2: Electrode, 3: Molten steel, 4: Molten slag, 6: Lance pipe, 7: Hot metal storage container,
8: Gutter.

Claims (1)

[Scope of Claims] 1. An arc furnace steelmaking method characterized by including the following steps. (a) blowing oxygen into the furnace during the melting phase and/or oxidation phase to obtain molten steel with a carbon content of 0.20% or less, and (b) while performing arc heating and blowing oxygen, Continuously charging hot metal containing at least about 2.0% carbon into the furnace. 2. The steel manufacturing method according to claim 1, wherein the oxygen injection in step (a) is carried out to obtain molten steel having a carbon content of 0.15% or less. 3 Charge the hot metal so that the amount of carbon supplied from the hot metal is 0.2 to 2.0 per ton of molten metal present in the furnace.
Kg/min.
Steel manufacturing method described in section. 4. Claim 1, wherein the hot metal is charged at a rate of 10 to 50% of the total charge.
Steel manufacturing method described in section. 5. The steel manufacturing method according to any one of claims 1 to 4, wherein the hot metal is produced in advance in a blast furnace or cupola. 6. The steel manufacturing method according to claim 1, wherein the oxygen injection in steps (a) and (b) is performed using pure oxygen gas or oxygen-enriched air. 7. The steel manufacturing method according to claim 1, wherein the amount of oxygen blown in step (a) is at least 10 Nm ³ /charging ton in terms of pure oxygen gas.