Molten steel production method by electric furnace

Abstract

To provide a molten steel production method with which the molten steel can be produced at high dephosphorization ratio in a continuous operation using an electric furnace.SOLUTION: A molten steel production method using an electric furnace includes charging, as main raw materials, 20-70 mass% of pig iron, which is one or both of hot metal and cold metal, and 80-30 mass% of scrap with respect to the main raw materials into an electric furnace, charging an auxiliary raw material containing a refining material into the electric furnace to perform a refining treatment of molten steel in which the main raw materials are melted, tapping the refined molten steel from the electric furnace, and leaving at least a part of slag generated in a preceding operation in the electric furnace and carrying over the slag to a next operation. A ratio of the CaO concentration to the SiO2 concentration (CaO concentration / SiO2 concentration) in the slag carried over to the next operation is 2.00 or more and 3.50 or less, the P2O5 concentration is 1.00 mass% or less, and the amount of slag carried over to the next operation is 15 kg or more per ton of the main material charged into the electric arc furnace in the next operation.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing molten steel in an electric furnace. [Background technology]

[0002] There are two methods for producing steel: the blast furnace method and the electric furnace method. In the blast furnace method, pig iron is produced in a blast furnace (blast furnace) using coke made from iron ore and coal as raw materials, and then refined in a converter to adjust the composition to produce steel. In the electric furnace method, scrap iron is used, and the raw material iron scrap is heated and melted using electricity in an electric furnace (electric furnace), and steel is produced while adjusting the composition.

[0003] Compared to the blast furnace method, the electric furnace method can significantly reduce carbon dioxide emissions, but normal electric furnace operation uses scrap as its main raw material, and requires enormous amounts of electricity to melt the scrap. Patent Document 1 proposes a method for significantly reducing the amount of electricity required by charging molten iron produced in a blast furnace or the like as the raw material into an electric furnace, and melting scrap using the sensible heat of the molten iron and the heat of combustion of carbon and silicon in the molten iron. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-109408 Summary of the Invention [Problem to be solved by the invention]

[0005] The method disclosed in Patent Document 1 involves blending molten iron containing a large amount of P (phosphorus), a typical impurity in the steelmaking process, making it difficult to produce steel with a lower P content (sometimes referred to as "low-P steel" in this disclosure) than in normal electric furnace operation using scrap as the main raw material. Furthermore, compared to converters, which are typical reaction vessels used to refine molten iron in the blast furnace steelmaking process, dephosphorization is difficult in electric furnaces due to the vessel shape and equipment.

[0006] In view of the above circumstances, an object of the present disclosure is to provide a method for producing molten steel that can produce molten steel with a high dephosphorization rate in continuous operation using an electric furnace. [Means for solving the problem]

[0007] The above problems can be solved by the following means. <1> A method for producing molten steel by continuous operation in which main raw materials including scrap are charged into an electric furnace, molten steel is refined, and then the steel is tapped, charging, as the main raw materials, pig iron, which is one or both of hot metal and cold metal, in an amount of 20 to 70 mass % of the main raw materials and the scrap in an amount of 80 to 30 mass % of the main raw materials into the electric furnace; Adding auxiliary raw materials containing refining materials to the electric furnace and refining the molten steel in which the main raw materials are melted; tapping the refined molten steel from the electric furnace; leaving at least a portion of the slag generated in the previous operation in the electric furnace and carrying it over to the next operation; Including, The ratio of the CaO concentration to the SiO2 concentration in the slag carried over to the next operation (CaO concentration / SiO2 concentration) is 2.00 or more and 3.50 or less, and the P2O5 concentration is 1.00 mass% or less, The amount of slag carried over to the next operation is 15 kg or more per ton of the main raw materials charged into the electric furnace in the next operation. A method for producing molten steel using an electric furnace. <2> The amount of slag carried over to the next operation is set to 20 to 50 kg per ton of the main raw materials charged into the electric furnace in the next operation. <1> A method for producing molten steel using the electric furnace described in claim 1. <3> The pig iron charged into the electric furnace contains the molten pig iron. <1> or <2> A method for producing molten steel using the electric furnace described in claim 1. <4> The P concentration of the pig iron is 0.100% to 0.150%. <1> ~ <3> 1. A method for producing molten steel using an electric furnace according to claim 1. <5> The P concentration in the molten steel is adjusted to 0.025% or less by the refining process. <1> ~ <4> 1. A method for producing molten steel using an electric furnace according to claim 1. [Effects of the Invention]

[0008] According to the present disclosure, there is provided a method for producing molten steel that can produce molten steel with a high dephosphorization rate in continuous operation using an electric furnace. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment that is an example of the present disclosure will be described. In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. However, when the numerical values ​​written before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit. Unless otherwise specified, "%" for the content of an element or compound means "% by mass."

[0010] First, we will explain slag (composition, role, and control method).

[0011] Slag composition Slag refers to molten oxides that are generated and produced in refining vessels such as converters and electric furnaces. Oxides that make up slag in general converters and electric furnaces, which are the subject of this disclosure, include SiO2, MnO, PO5, and Al2O3, which are produced when impurities contained in molten pig iron are oxidized and removed by top-blown oxygen, CaO, which is added to promote the deP reaction, FeO, which is produced when molten iron is oxidized by top-blown oxygen, and MgO, which is produced when refractories in contact with the slag melt (an MgO source is often added in advance to suppress refractory meltdown).

[0012] ·role Slag is inevitably produced in oxidation refining, but controlling its composition appropriately can promote the desiliconization and dephosphorization reactions. Furthermore, covering the molten iron surface with a molten slag phase also helps prevent the reduction in iron yield due to the scattering and evaporation of molten iron caused by the decarburization reaction. On the other hand, a phenomenon known as "slag foaming" occurs, in which CO bubbles generated by the decarburization reaction cause the slag phase to foam. If this phenomenon occurs suddenly, large amounts of slag and molten iron will splash out of the furnace, resulting in "slopping," which can lead to reduced iron yield and operational interruptions. Therefore, controlling the slag composition and amount appropriately is extremely important for oxidation refining.

[0013] Control method The most important indicator of slag composition is the C / S ratio, which refers to the ratio of CaO concentration to SiO2 concentration. The amount of CaO added is determined based on the amount of SiO2 generated from the Si concentration in the molten iron. Because not all CaO dissolves in the slag, the ratio of the added CaO / SiO2 generated and the analytical C / S do not necessarily coincide, but a positive correlation is obtained. Therefore, to achieve the desired analytical C / S value, CaO is added by multiplying it by a certain coefficient depending on the Si concentration in the molten iron. A higher C / S ratio promotes desiliconization and dephosphorization reactions, but because CaO has a high melting point, slag with a high C / S ratio is difficult to dissolve. Therefore, a C / S ratio of around 2 to 5 is generally used at steelmaking temperatures of 1600 to 1700°C.

[0014] There are limited means for controlling the slag mass. When the composition of the molten iron is fixed, the following can be done: (A) increasing or decreasing the amount of CaO added, (B) increasing or decreasing the amount of FeO produced by top blowing, or (C) discharging the slag from the furnace by tilting the furnace body. (A) has little flexibility because it affects the refining reaction, and the same is true for (B), but since no highly accurate control methods have been established, (C) can be said to be the method that allows for the most independent control of slag volume. In converter refining, 90% of the main raw material is molten pig iron, so the amount of decarburization is large and it is relatively easy to discharge the slag by tilting the furnace. On the other hand, in electric furnaces, in order to efficiently discharge the slag, carbonaceous material is sometimes injected into the slag phase, causing a decarburization reaction with the FeO in the slag and foaming the slag.

[0015] The inventors of the present disclosure have discovered that in continuous operation of an electric furnace, by leaving refining slag from a charge (sometimes referred to as a "preceding charge" in this disclosure) immediately preceding a charge (sometimes referred to as the "relevant charge" in this disclosure) for producing low-P steel in the furnace, the deP reaction proceeds at a high speed from the early stage of the refining process due to the slag (sometimes referred to as "carried-over slag" in this disclosure) left in the electric furnace in the relevant charge. In other words, since a large amount of slag with a high P absorption capacity is present in the relevant charge (carried-over slag + slag produced in the relevant charge), the driving force for the deP reaction is high, and as a result, molten steel can be produced in the relevant charge with a high dephosphorization rate.

[0016] The method for producing molten steel using an electric furnace according to the present disclosure includes: A method for producing molten steel by continuous operation in which main raw materials including scrap are charged into an electric furnace, molten steel is refined, and then the steel is tapped, charging, as the main raw materials, pig iron, which is one or both of hot metal and cold metal, in an amount of 20 to 70 mass % of the main raw materials and the scrap in an amount of 80 to 30 mass % of the main raw materials into the electric furnace; Adding auxiliary raw materials containing refining materials to the electric furnace and refining the molten steel in which the main raw materials are melted; tapping the refined molten steel from the electric furnace; leaving at least a portion of the slag generated in the previous operation in the electric furnace and carrying it over to the next operation; Including, The ratio of the CaO concentration to the SiO2 concentration in the slag carried over to the next operation (CaO concentration / SiO2 concentration) is 2.00 or more and 3.50 or less, and the P2O5 concentration is 1.00 mass% or less, The amount of slag carried over to the next operation is set to 15 kg or more per ton of the main raw materials charged into the electric furnace in the next operation.

[0017] (Charging of main raw materials) As main raw materials, pig iron, which is either or both of hot metal and cold metal, is charged into an electric furnace in an amount of 20 to 70 mass % of the main raw materials, and scrap is charged in an amount of 80 to 30 mass % of the main raw materials.

[0018] The primary raw material is a material that is melted to become molten steel. In this disclosure, pig iron is also used as the primary raw material in addition to scrap used in general electric furnace operation. Note that "pig iron" used as the primary raw material in this disclosure refers to molten pig iron and cold pig iron produced in a blast furnace or a converter. Only molten pig iron may be used as the pig iron, only cold pig iron may be used, or both molten pig iron and cold pig iron may be used. The cold pig iron may be either mold pig iron (pig iron cast in a mold) or granule pig iron (granular pig iron). From the viewpoint of reducing the amount of electricity consumed by the electric furnace due to the latent heat of the pig iron, it is preferable that the pig iron contains molten pig iron, and it is preferable that the proportion of molten pig iron is higher than the proportion of cold pig iron.

[0019] If the P concentration of pig iron used as a main raw material is high, the P concentration in molten steel will also be high, so a low P concentration is preferable. On the other hand, the lower the P concentration in pig iron, the higher the deP cost in the converter used to produce pig iron. From this perspective, the P concentration in pig iron is preferably 0.100% to 0.150%.

[0020] In the present disclosure, the amount of pig iron charged into the electric furnace is 20 to 70 mass % of the main raw materials. If the pig iron charged into the electric furnace is 20% by mass or more of the main raw materials, the latent heat of the pig iron can be used to reduce the amount of electricity used by the electric furnace, and if it is 70% by mass or less, the scrap ratio in the main raw materials will be 30% by mass or more, which is one of the benefits of operating an electric furnace: recycling scrap and reducing material costs. From this perspective, the pig iron ratio in the main raw materials is 20 to 70% by mass, and preferably 20 to 50% by mass.

[0021] In the present disclosure, the remainder of the main raw materials charged into the electric furnace, i.e., 80 to 30% by mass, is scrap. Scrap that is used as a raw material for molten iron in normal electric furnace operation can be used. If the scrap ratio in the main raw materials is 30% by mass or more, one of the benefits of electric furnace operation, namely recycling of scrap and reduction of material costs, can be achieved. If the scrap ratio is 80% by mass or less, the latent heat of pig iron can be used to reduce the amount of electricity used in the electric furnace. From this perspective, it is preferable that the scrap ratio in the main raw materials is 50 to 80% by mass.

[0022] In the present disclosure, the order in which the main raw materials are charged into an electric furnace is not limited, and they may be charged in two or more batches. From the viewpoint of facilitating the main raw material charging operation, it is preferable to charge the solid main raw materials (scrap, cold pig iron) and then charge the liquid main raw materials (molten pig iron). For example, scrap and cold pig iron may be charged together and then the molten pig iron may be charged, or the scrap, cold pig iron, and molten pig iron may be charged in this order.

[0023] (Refining process) Sub-raw materials including refining materials are fed into an electric furnace, and the main raw materials are melted to produce molten steel, which is then refined. Refining materials are materials used to remove unnecessary components contained in molten steel or to add necessary components, and examples of such materials include CaO, iron oxide, MgO, etc. The timing for charging the refining materials into the electric furnace may be before or after charging the main raw materials. In the refining process, oxygen is sent to the main raw materials (hot metal, unmelted scrap, and cold iron) charged into the electric furnace from a lance to perform desiliconization, decarburization, and dephosphorization by oxidation refining, so that the molten steel components are, for example, C: 0.1% or less and P: 0.025% or less.

[0024] (steel tapping) The refined molten steel is tapped from the electric furnace. After checking the composition and temperature of the molten steel in the refined electric furnace, the steel is tapped from the bottom of the furnace.

[0025] (Slag carryover) In the method for producing molten steel using an electric furnace according to the present disclosure, a continuous operation is carried out in which a series of operations, such as charging of main raw materials, refining treatment, and tapping, are repeated. In particular, in an operation for producing low-P steel (preferably with a P content of 0.025% or less), at least a portion of the slag generated in the preceding operation (preceding charge), which is the operation immediately before, is left in the electric furnace and carried over to the next operation (current charge). From the viewpoint of the deP effect in the charge, the ratio of the CaO concentration to the SiO2 concentration in the slag of the preceding charge carried over to the charge (CaO concentration / SiO2 concentration, sometimes referred to as "C / S" in this disclosure) is set to 2.00 or more and 3.50 or less, and the P2O5 concentration is set to 1.00 mass% or less. For example, before or after tapping of the preceding charge, the furnace body is tilted toward the end of the oxidation refining process, and a portion of the slag is discharged from the top of the furnace body or from a slag removal port provided on the side of the top of the furnace body. Note that if the amount of slag generated in the preceding charge is small, the slag may be carried over to the next charge without being discharged from the furnace body.

[0026] The amount of slag from the preceding charge carried over to the current charge is 15 kg or more per ton of main raw materials in the current charge. Even in typical continuous operation, slag is not completely discharged from the electric furnace, and a certain amount remains in the electric furnace. In this disclosure, the slag generated in the preceding charge is intentionally left behind, and 15 kg or more of slag per ton of main raw materials in the current charge is carried over to the current charge. This carried-over slag can achieve a high dephosphorization effect in the current charge. From the perspective of suppressing slopping, it is preferable to carry over 20 to 50 kg of slag from the preceding charge per ton of main raw materials in the current charge (20 to 50 kg / t).

[0027] In this charge in which the slag from the preceding charge has been carried over, low-P steel can be obtained by carrying out the above-mentioned charging of the main raw materials, refining process, and tapping. The mechanism by which the method for producing molten steel using an electric furnace according to the present disclosure exhibits the effect of obtaining low-P steel is believed to be as follows. In the electric furnace operation that is the subject of this disclosure, the amount of molten iron blended is smaller than in the converter furnace, and the stirring is weaker in the electric furnace than in the converter furnace. Therefore, the refining reaction does not reach an apparent equilibrium and the process ends. As a result, the P concentration in the slag is lower than in the slag produced in a typical converter furnace. A lower P concentration in the slag means that there is more room for P absorption. Furthermore, in normal refining, it takes time for the added CaO to dissolve, so the deP reaction does not progress easily in the initial to first half of the process. However, the carried-over slag is already in a dissolved state in the preceding charge, so it can contribute to the refining reaction as molten slag from the early stage of the charge.

[0028] (An example of the flow of electric furnace operation in this disclosure) An example of the flow of electric furnace operation in the present disclosure will be specifically described. As an example to be described later, the inventors of the present disclosure conducted an experiment using a DC electric furnace with a tapping capacity of 100 t according to the following procedure. (1) First, 20 to 80 tons of scrap and pig iron are charged into the electric furnace. (2) About 30 to 40 percent of the scrap and pig iron is melted by combustion using electricity and oxygen supply. (3) Various auxiliary materials including CaO, iron oxide, and MgO are added as refining materials. (4) Molten iron is charged into the electric furnace from the center above the furnace body. (5) A water-cooled lance is inserted into the furnace from the operation port at the top of the furnace body, and oxygen necessary for desiliconization, decarburization, and dephosphorization is supplied to the molten iron and unmelted scrap at an oxygen supply rate of 5,000 to 7,000 Nm 3 / hr to carry out oxidation refining, so that the molten steel components after oxygen injection are C: 0.1% or less and P: 0.025% or less. (6) From the end of the oxidation refining process to the final stage, the furnace body is tilted, and the slag is foamed by a decarburization reaction caused by injecting carbon material into the slag phase, and the slag is discharged from a slag removal port provided in the furnace body. During the discharge process, the distance from the tip of the lance to the bath surface (apparent molten iron surface) is appropriately controlled within the range of 0.1 to 1.0 m. (7) Check the composition and temperature and tap the steel from the bottom of the hearth.

[0029] The method for producing molten steel using an electric furnace according to the present disclosure is based on continuous operation, in which the above steps (1) to (7) are repeated in the same refining vessel of the same furnace. In this disclosure, the refining process aimed at tapping low-P steel (preferably with a P content of 0.025% or less) is referred to as the "current charge," and the charge immediately preceding it is referred to as the "previous charge." In this disclosure, step (6) of the previous charge is omitted, or the inclination angle of the furnace body is reduced to reduce the amount of slag discharged (for example, to half the amount of slag in the furnace before discharge). Low-P steel can then be easily obtained by performing steps (1) to (7) in the current charge as usual. [Example]

[0030] Hereinafter, examples of the method for producing molten steel using an electric furnace according to the present disclosure will be described, but the method for producing molten steel using an electric furnace according to the present disclosure is not limited to the following examples.

[0031] <Comparative Examples 1 to 7, Examples 1 to 7> According to the above-mentioned steps (1) to (7), scrap and pig iron (hot metal and mold pig iron) were used as the raw materials (main raw materials) for molten steel, and molten steel was refined in an electric furnace so that the P concentration was 0.025% or less. In each example, the ratio of pig iron in the main raw materials, whether or not slag was left, the amount of slag carried over, etc. were changed, and the deP rate was evaluated. The operating conditions and evaluation results are shown in Table 1. In Table 1, underlined items indicate items outside the scope of this disclosure.

[0032] [Table 1]

[0033] The meanings of the items in Table 1 are as follows: HMR (%): The mass ratio of hot metal and mould iron to the total mass of the main raw materials in the charge. The main raw materials are hot metal, mould iron and scrap. C / S: The ratio of the CaO concentration to the SiO2 concentration in the slag. This is the ratio of the amount of CaO newly added as a refining material to the total amount of SiO2, which is the SiO2 produced by the combustion of Si in the molten iron and mold iron, and the SiO2 contained in other auxiliary materials. Here, it refers to the C / S in the charge immediately before the charge under evaluation in continuous operation. · (%P2O5): P2O5 mass % concentration in the slag (analytical value). Slag retention: If slag retention operation was performed, it was marked "Yes," and if not, it was marked "No." Slag retention operation refers to an operation in which the furnace is tilted to allow slag to flow out of the slag removal port in normal operation, but when slag retention is performed, the amount of slag that flows out of the slag removal port is reduced by, for example, halving the tilting angle or halving the tilting time. Pig iron [%P]: The mass percent concentration of P contained in the molten pig iron and mold pig iron. The P concentration in the molten pig iron was determined using a value analyzed in advance, and the P concentration in the mold pig iron was set to 0.120%, and the weighted average P concentration according to the amount of mold pig iron charged was used as the pig iron [%P]. Tapping [%P]: The P mass % concentration analysis value of a sample taken at the end of processing by rapidly cooling the molten steel. ·Pig iron removal rate: 100 × (pig iron [%P] - tapped steel [%P]) / pig iron [%P] Evaluation: If the P elimination rate was 90% or more, it was marked as "Good", and if it was less than 90%, it was marked as "Poor". · Tapped volume (t): The mass (tons) of molten steel tapped after refining. Carry-over slag amount (kg / t): The mass of slag (kg / t) carried over from the previous charge to the current charge due to residual slag per ton of the main raw material of the current charge.

[0034] In Comparative Examples 1 to 4, no slag was left behind, resulting in a small amount of carried-over slag and a low deP rate. For example, Comparative Example 3 had a low HMR, which was favorable for deP, but was unable to meet the deP rate standard.

[0035] In Comparative Example 5, although slag was left behind, the deP rate was low at 85.1%, which did not meet the standard. In addition, the C / S was low at 1.70, which is thought to be because the effect of leaving slag on improving the deP rate was small.

[0036] Although slag was left behind in Comparative Example 6, the deP rate was low at 83.7%, which did not meet the standard. This is thought to be because the C / S ratio was excessively high at 3.80, making it difficult for the carried-over slag to dissolve, and the effect of leaving the slag on improving the deP rate was small.

[0037] In Comparative Example 7, slag was left behind, and the C / S was 2.40, which was within the range considered suitable, but the deP rate did not meet the standard. This is thought to be because the HMR was excessive at 80%, meaning that a large amount of P had to be deP'd, but the (%P2O5) was high at 1.40, and the deP rate of the carried-over slag was low.

[0038] On the other hand, in Examples 1 to 7 in which the charging was carried out in a manner that satisfied the requirements of the present disclosure, the dephosphorization rate was 90.0% or more in all cases, and low-P steel could be easily produced compared to the comparative examples.

Claims

1. A method for producing molten steel by continuous operation in which main raw materials including scrap are charged into an electric furnace, molten steel is refined, and then the steel is tapped, charging, as the main raw materials, pig iron, which is one or both of molten pig iron and cold pig iron, in an amount of 20 to 70 mass % of the main raw materials and the scrap in an amount of 80 to 30 mass % of the main raw materials into the electric furnace; Adding auxiliary raw materials containing refining materials to the electric furnace and refining the molten steel in which the main raw materials are melted; tapping the refined molten steel from the electric furnace; leaving at least a portion of the slag generated in the previous operation in the electric furnace and carrying it over to the next operation; Including, SiO in the slag carried over to the next operation ２ The ratio of CaO concentration to SiO concentration (CaO concentration / SiO ２ density) is 2.00 or more and 3.50 or less, and P ２ O ５ The concentration is 1.00% by mass or less, The amount of slag carried over to the next operation is 15 kg or more per ton of the main raw materials charged into the electric furnace in the next operation. A method for producing molten steel using an electric furnace.

2. 2. The method for producing molten steel using an electric furnace according to claim 1, wherein the amount of slag carried over to the next operation is 20 to 50 kg per ton of the main raw materials charged into the electric furnace in the next operation.

3. 2. The method for producing molten steel using an electric furnace according to claim 1, wherein the pig iron charged into the electric furnace contains the molten pig iron.

4. 2. The method for producing molten steel using an electric furnace according to claim 1, wherein the P concentration of the pig iron is 0.100% to 0.150%.

5. 2. The method for producing molten steel using an electric furnace according to claim 1, wherein the P concentration in the molten steel is adjusted to 0.025% or less by the refining treatment.

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