Steel manufacturing method
By strategically adding cold iron with high carbon content during specific stages of the melting process, the method effectively reduces nitrogen content in steel produced using electric furnaces, addressing inefficiencies in conventional carbon addition and slag penetration.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- NIPPON STEEL CORPORATION
- Filing Date
- 2024-11-22
- Publication Date
- 2026-06-03
AI Technical Summary
Existing steel manufacturing methods using electric furnaces face challenges in reducing nitrogen content due to higher nitrogen absorption from the atmosphere, especially when continuously feeding raw materials, and conventional carbon addition methods are inefficient in penetrating slag.
The method involves adding cold iron with a high carbon content during specific stages of the melting process to generate CO gas and release nitrogen, while controlling the timing and amount to ensure efficient denitrification and dephosphorization, using molded iron with a high specific gravity to penetrate slag effectively.
Stable reduction of nitrogen content in steel is achieved by optimizing carbon addition through cold iron, enhancing denitrification efficiency and minimizing phosphorus impurities.
Smart Images
Figure 2026091012000001
Abstract
Description
Technical Field
[0001] The present invention relates to a method for manufacturing steel.
Background Art
[0002] In recent years, due to the increasing interest in environmental issues, attention has been focused on operations using electric furnaces. However, when producing molten steel using an electric furnace, the nitrogen content in the molten steel tends to be higher compared to when producing it using a blast furnace and a converter. Therefore, even when using an electric furnace, there is a demand for establishing a steel manufacturing method capable of reducing the nitrogen content.
[0003] For example, in Patent Document 1, attempts have been made to reduce the nitrogen concentration by supplying high-purity oxygen gas and carbon to molten iron during the refining period.
Prior Art Documents
Patent Documents
[0004]
Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0008] (1) A method for manufacturing steel using an arc-type electric furnace, The process includes a melting step in which raw materials are continuously fed into a furnace and melted to form molten steel, and a refining step in which the molten steel is refined after the feeding of the raw materials is complete. In the melting process, cold iron is added in an amount that increases the carbon content of the steel by 0.12 mass% or more until the amount of raw material added reaches 70% to 100% by weight. A method for manufacturing steel.
[0009] (2) In the melting process, cold iron is added in an amount that increases the carbon content of the steel by 0.12 mass% or more until the amount of raw material added reaches 70% to 90% by weight. The method for manufacturing steel as described in (1) above. [Effects of the Invention]
[0010] According to the present invention, it is possible to stably reduce the nitrogen content in steel. [Modes for carrying out the invention]
[0011] As a result of diligent research conducted by the inventors to solve the above problems, we have obtained the following findings.
[0012] To stably reduce the nitrogen content in steel manufactured using electric furnaces, it is necessary to reduce the amount of nitrogen absorbed from the atmosphere during the manufacturing process, as well as to efficiently perform denitrification.
[0013] In the melting of steel using electric furnaces, a method is often employed in which raw materials such as scrap are transported by conveyor and continuously fed into the furnace. This method has the advantage of suppressing the absorption of nitrogen from the atmosphere, as slag is always present above the molten steel.
[0014] Furthermore, when melting steel using an electric furnace, oxygen is blown in and carbon is added. The oxygen blowing is for the purpose of slag forming and stirring the molten steel, and the carbon addition is for the purpose of reducing FeO generated by slag forming and oxygen blowing. As a result, carbon and oxygen react in the molten steel, generating CO gas, and nitrogen is released from the molten steel along with the CO gas bubbles, thus denitrification occurs.
[0015] A common method for adding carbon to molten steel in a furnace is to supply charcoal powder or lumps of charcoal material. However, when using a method that continuously feeds raw materials into the furnace, as mentioned above, slag is always present at the top of the molten steel. Therefore, it is necessary to supply the carbon to the steel by penetrating the slag, and there is a problem in that it is difficult to stably add carbon to the molten steel using only charcoal material with a low specific gravity.
[0016] Therefore, in this embodiment, carbon is added by mixing cold iron with a high carbon content into the raw material. Because cold iron has a high specific gravity, carbon can be added stably without being affected by slag.
[0017] There are no particular restrictions on the shape and size of the cold iron, but molded iron or granular iron can be used. Of these, molded iron is preferred from the viewpoint of ease of penetration into the slag. Molded iron is cold iron formed in a mold. There are no particular restrictions on the size of the molded iron, but for example, molded iron weighing 5 to 30 kg / piece can be used, and considering the ease of transport and loading, it is more preferable to use molded iron weighing 5 to 20 kg / piece.
[0018] Since cold iron contains many impurity elements such as phosphorus, it is desirable to allow time for dephosphorization when using cold iron as a raw material. Therefore, from the above perspective, it is advisable to introduce cold iron into the furnace at an early stage of the melting process.
[0019] However, as a result of the inventors' study, it was found that when pig iron is charged at an early stage, denitrification can only be carried out on a part of the raw materials, so it is difficult to carry out denitrification efficiently. And it was found that by charging a predetermined amount or more of pig iron in the latter stage of the melting period when the amount of molten steel increases, denitrification can be carried out efficiently and the nitrogen content in the steel can be stably reduced.
[0020] The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.
[0021] 1. Melting method The method for manufacturing steel according to the present embodiment is a melting method using an arc-type electric furnace, which includes a melting step of continuously charging raw materials into the furnace and melting them into molten steel, and a refining step of refining the molten steel after the melting of the raw materials is completed. In an electric furnace in which raw materials are continuously charged, it is common to leave about 25 to 35% of the molten steel produced in the furnace without tapping it and use it as the seed melt for the next melting. That is, the tapping amount is about 65 to 75% of the molten steel melted in the furnace. Hereinafter, each step of the melting step and the refining step will be described.
[0022] 2. Melting step In the melting step of the present embodiment, raw materials are continuously charged into the furnace. In this specification, the raw materials refer to iron source raw materials. As described above, in the present embodiment, pig iron is used as the carbon supply source. Pig iron is pig iron produced in a blast furnace that has cooled and solidified. Therefore, at least pig iron is used as the raw material. As raw materials other than pig iron, one or more selected from scrap, reduced iron, etc. may be used.
[0023] The raw materials are fed one by one in sequence, for example, by a conveyor or the like. Also, the raw materials do not necessarily need to be fed at regular intervals, and after feeding a batch of raw materials, the time can be segmented, and the operation of feeding a batch of raw materials again can be repeated. In this specification, in addition to the case where the raw materials are fed one by one in sequence, even in the case where a batch of raw materials is intermittently fed, when the feeding is repeated 10 times or more in one melting process, it shall be determined that the feeding is continuous.
[0024] In the melting process of this embodiment, pig iron with an amount such that the carbon content of the steel increases by 0.12 mass% or more is fed while the degree of raw material feeding is from 70% to 100% on a weight basis. Here, the degree of raw material feeding is an index with the start time of the melting process being 0% and the end time of the melting process when all raw materials are fed being 100%. For example, a degree of raw material feeding of 70% means the time point when raw materials with a weight of 70% of all raw materials are fed.
[0025] Also, feeding pig iron with an amount such that the carbon content of the steel increases by 0.12 mass% or more means that, with respect to the total amount of molten steel in the furnace after the refining is completed and before tapping, the amount of carbon contained in the fed pig iron is 0.12% or more in mass%. The total amount of molten steel is the amount obtained by adding the amount of molten steel remaining as seed metal to the amount of molten steel formed by melting the raw materials fed in the melting process.
[0026] As described above, the added carbon reacts with the oxygen dissolved in the molten steel, generating CO gas, and nitrogen is discharged from the molten steel together with the bubbles of CO gas. In the initial stage of the melting process, since the amount of raw materials fed is small and the amount of molten steel is also small, even if a large amount of carbon is supplied at that time, denitrification cannot be efficiently performed. On the other hand, in the later stage of the melting process, specifically, by supplying a predetermined amount or more of carbon while the degree of raw material feeding is from 70% to 100% on a weight basis, it becomes possible to generate CO gas and perform denitrification associated therewith for a large amount of molten steel.
[0027] Furthermore, the amount of cold iron to be added should be such that the carbon content of the steel increases by 0.12 mass% or more when the amount of raw material added reaches 70% to 100% by weight, while the remaining cold iron can be added when the amount of raw material added reaches 0% to 70% by weight. From the viewpoint of forming the slag immediately after the start of power supply, it is preferable to add an amount of cold iron that increases the carbon content by 0.12% or more when the amount of raw material added reaches 0% to 20% by weight. Moreover, it is even more preferable to add an amount of cold iron that increases the carbon content by 0.20% or more when the amount of raw material added reaches 0% to 20% by weight.
[0028] There is no particular upper limit on the amount of cold iron to be added, but the amount of cold iron added when the amount of raw material added increases from 70% to 100% by weight is preferably an amount that increases the carbon content of the steel by 0.45 mass% or less, more preferably an amount that increases it by 0.20 mass% or less, and even more preferably an amount that increases it by 0.15 mass% or less. Furthermore, the amount of cold iron added when the amount of raw material added increases from 0% to 100% by weight is preferably an amount that increases the carbon content of the steel by 0.60 mass% or less, more preferably an amount that increases it by 0.50 mass% or less, and even more preferably an amount that increases it by 0.45 mass% or less.
[0029] Furthermore, since cold iron contains many impurity elements such as phosphorus, it is preferable to add an amount of cold iron that increases the carbon content of the steel by 0.12 mass% or more while the amount of raw material added reaches 70% to 90% by weight, in order to ensure sufficient time for dephosphorization.
[0030] For similar reasons, the risk of poor dephosphorization increases when a large amount of cold iron is added at the very end of the melting stage by weight, i.e., at 90-100%. Therefore, the amount of cold iron added between 90% and 100% by weight of the raw materials is preferably such that it increases the carbon content of the steel by 0.19 mass% or less, more preferably by 0.10 mass% or less, more preferably by less than 0.04 mass% or less, and even more preferably by 0.02 mass% or less.
[0031] For example, when using a continuous horizontal charging electric furnace in which raw materials are transported to the electric furnace by a conveyor and continuously fed in, it is possible to feed in cold iron at the appropriate timing by adjusting the order in which the raw materials are loaded onto the conveyor.
[0032] Furthermore, it is not possible to feed in cold iron at a rate exceeding the scrap feeding rate specified by the equipment capacity. In addition, if only cold iron is fed in as raw material, the furnace bottom will be damaged when the cold iron is fed into the furnace. For this reason, it is preferable to feed in cold iron at the same time as plate-shaped scrap. Specifically, it is preferable that the proportion of cold iron in the raw material when cold iron is fed in be less than 70%.
[0033] In the dissolution process, oxygen may be blown onto the material, and carbon may be added using charcoal powder or lumpy charcoal material for the purpose of slag forming. This makes it possible to suppress the absorption of nitrogen from the atmosphere.
[0034] There are no particular restrictions on the time required for the melting process; it is determined according to the settings of the electric furnace's processing capacity (tons / ch), raw material input rate (tons / min), electric furnace capacity (W), remaining amount of starter molten metal, and raw material input rate. Generally, the melting process and the refining process described later take approximately 45 to 70 minutes in total.
[0035] 3. Refining process In the refining process of this embodiment, the molten steel is refined after the raw materials have been added. In the refining process, any remaining undissolved raw materials are completely dissolved, and the temperature and chemical composition are adjusted. In the refining process, the process can be carried out under any other known conditions. Furthermore, in the refining process, by supplying oxygen and carbon to the molten steel following the melting process, denitrification associated with the generation of CO gas can be further promoted.
[0036] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. [Examples]
[0037] Using a continuous horizontal charging electric furnace with a molten metal capacity of 170 tons, 105 tons of scrap metal, including cold iron, were continuously fed into the furnace over a period of 45 to 70 minutes, starting with 65 tons of seed molten metal, to produce 170 tons of molten steel. During this process, the cold iron was fed under the conditions shown in Table 1. The weight of the cold iron fed was 15 tons in all cases, and molds of approximately 10 kg / piece were used.
[0038] In addition, oxygen was blown in from the wall lances and door lances, and carbon was supplied. During the refining process after the melting process, the carbon concentration and temperature were adjusted to the predetermined levels over 5 to 15 minutes, and 105 tons of molten steel were tapped, leaving 65 tons of seed molten steel in the furnace. The operating conditions, such as the amount of oxygen and carbon supplied from the lances, the amount of auxiliary materials added, and the energization pattern, were kept the same in all examples.
[0039] [Table 1]
[0040] The results are shown in Table 1. In Table 1, Test No. 1 is the baseline process, in which the entire amount of cold iron was added between 0% and 30% of the raw material input. Note that in Table 1, the amount of cold iron added is shown as an increase in carbon content (mass %).
[0041] In this example, the nitrogen content in the molten steel was compared to that of Test No. 1, and a reduction of 5.0 ppm or more was deemed satisfactory. Furthermore, the phosphorus content in the molten steel was checked to evaluate whether dephosphorization was successful. A circle (○) indicated no problems, while a triangle (△) indicated slightly elevated levels.
[0042] In Test No. 2, the entire amount of cold iron was added during the raw material input stage, which ranged from 0% to 50%. Compared to Test No. 1, the charging rate of cold iron per unit time was reduced, resulting in the supply of carbon later in the melting process. As a result, the nitrogen content in the steel was slightly reduced, but it did not pass the test.
[0043] In Test No. 3, the amount of cold iron added initially was reduced, and cold iron was added in an amount that increased the carbon content by 0.09% between 70% and 100% of the raw material input. Because cold iron was added in the later stages of the melting process, the nitrogen content was reduced, but the amount added in the later stages was insufficient, so it did not pass the test.
[0044] In Test No. 4, the amount of cold iron added initially was reduced, and cold iron was added in an amount that increased the carbon content by 0.12% when the raw material input rate was between 60% and 90%. Because the cold iron was added in the later stages of the melting process, the nitrogen content was reduced, but the timing of the addition was too early, so it did not pass the test.
[0045] In Test No. 5, the amount of cold iron added initially was reduced, and cold iron was added in an amount that increased the carbon content by 0.11% between 50% and 100% of the raw material input. Because cold iron was added in the later stages of the melting process, the nitrogen content was reduced, but the amount added in the later stages was insufficient, and the timing was too early, so it did not pass the test.
[0046] In Test No. 6, the amount of cold iron initially added was reduced, and an amount of cold iron was added between 70% and 100% of the raw material input, resulting in a 0.12% increase in carbon content. The nitrogen content in the steel decreased by 5.1 ppm compared to Test No. 1, and the test passed. On the other hand, adding cold iron at the end of the melting process resulted in a higher phosphorus content in the steel.
[0047] In Test No. 7, the amount of cold iron initially added was reduced, and an amount of cold iron was added that increased the carbon content by 0.15% when the raw material input rate reached 70-80%. The nitrogen content in the steel decreased by 5.0 ppm compared to Test No. 1, and the test was deemed successful.
[0048] In Test No. 8, the amount of cold iron added initially was reduced, and an amount of cold iron was added between 90% and 100% of the raw material input, resulting in a 0.12% increase in carbon content. The nitrogen content in the steel decreased by 6.0 ppm compared to Test No. 1, and the test passed. On the other hand, adding cold iron at the end of the melting process resulted in a higher phosphorus content in the steel.
[0049] In Test No. 9, the amount of cold iron initially added was reduced, and an amount of cold iron was added that increased the carbon content by 0.12% when the raw material input rate reached 80-90%. The nitrogen content in the steel decreased by 5.2 ppm compared to Test No. 1, and the test was deemed successful. [Industrial applicability]
[0050] As described above, the present invention makes it possible to stably reduce the nitrogen content in steel.
Claims
1. A method for manufacturing steel using an arc-type electric furnace, The process includes a melting step in which raw materials are continuously fed into a furnace and melted to form molten steel, and a refining step in which the molten steel is refined after the feeding of the raw materials is complete. In the melting process, cold iron is added in an amount that increases the carbon content of the steel by 0.12 mass% or more until the amount of raw material added reaches 70% to 100% by weight. A method for manufacturing steel.
2. In the melting process, cold iron is added in an amount that increases the carbon content of the steel by 0.12 mass% or more until the amount of raw material added reaches 70% to 90% by weight. The method for manufacturing steel according to claim 1.