Method of operating a molten steel processing furnace and auxiliary materials for the molten steel processing furnace

Abstract

To prevent nitrogen absorption from the atmosphere into molten steel by a gas shield while minimizing the amount of charcoal applied to the molten steel.SOLUTION: An operation method of a molten steel treatment furnace includes the steps of feeding an auxiliary raw material having a volatile content greater than 20 mass% into the furnace where the molten steel is stored, supplying oxygen to the auxiliary raw material fed into the furnace, and forming a gas shield on the hot water surface of the molten steel with the gas generated by the reaction of the volatile of the auxiliary raw material with the oxygen.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for operating a molten steel processing furnace and to auxiliary materials for a molten steel processing furnace. [Background technology]

[0002] In the refining process of molten steel, it is necessary to reduce the nitrogen content in the molten steel depending on the steel type. For example, Patent Document 1 describes a technology in which a gas jetting machine that jets out a gas that does not contain nitrogen gas is provided around the tapping hole of a converter in order to prevent nitrogen absorption from the atmosphere in molten Cr steel tapped from a refining furnace. This makes it possible to shield the tapping flow of molten Cr steel tapped from the tapping hole of the converter into a ladle by jetting gas from the gas jetting machine, thereby preventing nitrogen absorption from the atmosphere. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-138446 Summary of the Invention [Problem to be solved by the invention]

[0004] The problem of nitrogen absorption by molten steel from the atmosphere can also occur in processing furnaces such as electric furnaces and converters. Therefore, in electric furnaces, for example, the molten steel being processed is pre-carburized. This forms a gas shield on the molten steel surface with CO and CO2, which are generated as oxygen is supplied to the molten steel, preventing nitrogen absorption from the atmosphere. Converters also prevent nitrogen absorption by using a gas shield of CO and CO2. However, as the carbon concentration in the molten steel decreases, for example, at the end of the blow, the molten steel may be recarburized to prevent nitrogen absorption. However, in both cases, the carbon concentration of the molten steel must ultimately be reduced to a predetermined value by decarburization, so recarburizing the molten steel to prevent nitrogen absorption is not efficient.

[0005] Therefore, an object of the present invention is to provide a method for operating a molten steel processing furnace and an auxiliary material for a molten steel processing furnace that can prevent nitrogen absorption from the atmosphere into the molten steel by using a gas shield while minimizing the amount of carburization of the molten steel. [Means for solving the problem]

[0006] [1] A method for operating a molten steel processing furnace, comprising the steps of: charging auxiliary materials having a volatile content of more than 20 mass% into a furnace storing molten steel; supplying oxygen to the auxiliary materials charged into the furnace; and forming a gas shield on the surface of the molten steel with gas generated by a reaction between the volatile content of the auxiliary materials and the oxygen. [2] The method for operating a molten steel processing furnace according to [1], wherein the step of charging the auxiliary materials is carried out continuously, and the step of supplying oxygen is carried out continuously at least while the step of charging the auxiliary materials is carried out continuously. [3] The method for operating a molten steel processing furnace according to [1] or [2], wherein the auxiliary raw materials have a sulfur content of 0.02% or less and an ash content of 5% or less. [4] A method for operating a molten steel processing furnace according to any one of [1] to [3], wherein the auxiliary material is a carbonized material obtained by carbonizing waste plastic or biomass, or waste plastic that has not been carbonized. [5] A secondary raw material for a molten steel processing furnace having a volatile matter content of more than 20% by mass, a sulfur content of 0.02% or less, and an ash content of 5% or less. [Effects of the Invention]

[0007] According to the above configuration, auxiliary materials having a volatile content of more than 20 mass % are charged into the furnace and reacted with oxygen to efficiently generate gas, so that the amount of carburization of the molten steel can be minimized while the gas shield prevents nitrogen absorption from the atmosphere into the molten steel. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 2 is a diagram showing a method of operating a processing furnace according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant description will be omitted.

[0010] FIG. 1 is a diagram illustrating a method for operating a processing furnace according to one embodiment of the present invention. In the illustrated example, molten steel 2 is stored in a processing furnace 1. The processing furnace 1 may be, for example, an electric furnace that melts iron scrap to produce molten steel, or a converter that blows molten iron to produce molten steel. In either case, the molten steel 2 is temporarily stored in the processing furnace 1 for processing such as blowing. Note that parts of the processing furnace 1 that are not related to the operation method described below are not shown in the illustration.

[0011] In this embodiment, auxiliary materials 3 are charged into a treatment furnace 1 in which molten steel 2 is temporarily stored. An oxygen-containing gas is supplied (oxygen supply) to the molten steel 2 in the furnace using an oxygen supply lance 4. The oxygen supplied by the oxygen supply reacts not only with the molten steel 2 but also with the auxiliary materials 3, and CO, CO2, and H2O gases are generated by the vaporization, thermal decomposition, and combustion of the auxiliary materials 3. These gases spread along the surface 5 of the molten steel 2, forming a gas shield 6. The gas shield 6 prevents nitrogen from being absorbed into the molten steel 2 from the atmosphere in the furnace.

[0012] Here, the auxiliary material 3 has a volatile content of more than 20% by mass, a sulfur content of 0.02% or less, and an ash content of 5% or less. The volatile content and ash content are defined in JIS M0104 "Terminology for Coal Utilization" and can be quantified in accordance with JIS M8812 "Coals and Cokes - Methods of Proximate Analysis." The sulfur content is defined as total sulfur in JIS M0104 and can be quantified in accordance with JIS M8813 "Coals and Cokes - Methods of Elemental Analysis." In this specification, the content is defined as the dry mass of the entire auxiliary material.

[0013] The above definitions generally apply to carbonaceous materials such as coals, cokes, charcoal, and briquettes, but in this embodiment, even materials that are not necessarily called carbonaceous materials but contain the volatile matter, ash content, and sulfur content defined and quantified as above can be used as the auxiliary material 3. For example, the auxiliary material 3 may be a charcoal obtained by carbonizing waste plastic or biomass, or waste plastic that has not been carbonized.

[0014] As will be explained below, by making the volatile content of the auxiliary materials 3 greater than 20% by mass, gas can be generated efficiently and quickly relative to the amount of auxiliary materials 3 charged, forming a gas shield 6 and preventing nitrogen absorption in the molten steel 2. Since it is not necessary to carburize the molten steel 2 to prevent nitrogen absorption, an extra decarburization process is not required. Although heat removal can occur when fixed carbon other than the volatile content contained in the auxiliary materials 3 dissolves in the molten steel 2, by making the volatile content greater than 20% by mass, the heat removal can be compensated for by radiant heat due to the combustion reaction between the volatile content and oxygen, preventing a drop in the temperature of the molten steel 2.

[0015] (Gas generation amount relative to the amount of auxiliary raw material input) The fixed carbon content of the auxiliary raw material is α, the volatile content is β, and the remainder is ash. The volatile content is a carbon-based linear chain (CH2) n In this case, the amount of gas generated per unit input of auxiliary material can be calculated as follows. First, fixed carbon dissolves in molten steel, then reacts with iron oxide to generate CO gas. The reaction at this time is C + 1 / 2O2 → CO, and CO is generated at a rate of (10 6 On the other hand, as soon as the volatiles are vaporized and thermally decomposed, they react with the supplied oxygen to generate CO2 and H2O gas. The reaction at this time is (CH2) n +O2 → n·CO2 + n·H2O, and the total amount of CO2 and H2O is (10 6 × β) ÷ 14 × 2 mol is generated. Therefore, the gas generation volume V per unit input of auxiliary raw material is unitcan be calculated as shown in equation (1).

[0016]

number

[0017] In formula (1), the denominator of β is smaller than the denominator of α. Therefore, increasing the volatile matter content β in the auxiliary material is more effective than increasing the fixed carbon content α in the auxiliary material because it reduces the gas generation volume V unit It can be seen that increasing β has a significant effect. Furthermore, as explained above, fixed carbon first dissolves in molten steel and then reacts with iron oxide to generate gas, whereas volatile matter reacts with oxygen to generate gas as soon as it is vaporized and thermally decomposed. Therefore, gas derived from volatile matter is generated more quickly after the addition of auxiliary materials. Therefore, it can be seen that increasing the volatile matter content β in the auxiliary materials is effective not only for increasing the amount of gas generated to form the gas shield, but also for quickly generating gas.

[0018] (Heat removal compensation for fixed carbon dissolution) As mentioned above, volatile matter vaporizes and burns immediately after the addition of auxiliary materials, both of which are exothermic reactions. In contrast, fixed carbon dissolves (carburizes) in the molten steel before reacting with the iron oxide. This carburization is an endothermic reaction. Furthermore, the ash contained in the auxiliary materials is heated above the molten steel and melts, which is also an endothermic reaction. Therefore, in order to stabilize the molten steel processing and avoid a drop in the molten steel temperature, it is preferable to set the contents of volatile matter, fixed carbon, and ash so that the following relationship holds: [amount of heat generated by the vaporization and combustion of volatile matter] ≥ [amount of heat absorbed by the carburization of fixed carbon] + [amount of heat absorbed by the temperature rise of ash]. Therefore, the conditions shown in Table 1 below were set and the optimum contents of each component were calculated.

[0019] [Table 1]

[0020] Among the conditions listed in Table 1, the heat of SiO2 decomposition (b2) was set as a constant-pressure specific heat of 4.42 J / g·k × 1500 K rise in temperature, based on the paper "Development of a Continuous Solidification Process for Steelmaking Slag Suitable for Sensible Heat Recovery" in "Iron and Steel," Vol. 99, No. 12, 2013. The ash content (γ) of 0.03 is the average ash content in biomass char. The heat transfer rate (ε) of combustion heat to molten steel is the rate at which the combustion heat of volatile matter is transferred to the molten steel. The remaining combustion heat becomes the high-temperature furnace atmosphere and is recovered, for example, as converter gas (LDG). The volatile matter combustion rate (μ) is the rate at which vaporized volatile matter is burned. Unburned volatile matter is recovered as LDG. In the above example, the combustion rate was estimated conservatively, with μ = 0.60.

[0021] The reaction heat per kg of added auxiliary material can be calculated as follows: Endothermic reaction Carburizing heat = a × α Heat of decomposition of volatile matter = b1 × β SiO2 decomposition heat = b2 × γ Exothermic reaction Heat of CO formation = c×β×(36 / 44) Heat of formation of H2O = e × β × (8 / 44)

[0022] Therefore, the conditions under which the heat quantity of the exothermic reaction exceeds the heat quantity of the endothermic reaction are as follows: a×α+b1×β+b2×γ <c×β×(36 / 44)+e×β×(8 / 44) Substituting the values ​​in Table 1 for a to e, γ, ε, and μ, and further eliminating α from the above equation since α + β + γ = 1 and γ = 0.03, results in β > approximately 0.2. Therefore, from the perspective of compensating for the heat removal during the dissolution of fixed carbon, it is preferable that the volatile content β of the auxiliary material is greater than 0.2, that is, the volatile content is greater than 20 mass%.

[0023] Furthermore, from the viewpoint of reducing the load of the subsequent desulfurization treatment of molten steel, it is preferable that the sulfur content of the auxiliary materials be 0.02 mass% or less. When the sulfur content is within this range, the sulfur content does not dissolve in the molten steel but is absorbed in the slag or volatilizes, and the impact on the desulfurization treatment is small. Similarly, from the viewpoint of reducing the load of the desiliconization treatment, it is preferable that the ash content of the auxiliary materials be 5% or less. When the ash content is within this range, the silicon content contained in the ash does not dissolve in the molten steel but is absorbed in the slag or volatilizes, and the impact on the desiliconization treatment is small.

[0024] (Maintaining the gas shield) In the embodiment of the present invention described above, the formed gas shield can be maintained by continuously feeding the auxiliary materials into the furnace. In this specification, "continuously" includes repeatedly performing the process at predetermined intervals. Furthermore, while the process of feeding the auxiliary materials is continuously performed, the process of supplying oxygen to the auxiliary materials by oxygen supply is also continuously performed. Below, we verify this point by calculating the thickness of the gas shield per hour formed when an auxiliary material with a volatile content β is continuously fed into a furnace with a diameter of 5 m at a rate of 0.1 t / min.

[0025] Experimental results show that the volatile content of auxiliary materials added onto the surface of molten steel vaporizes almost entirely within about one minute. n C k ·Si m " and after addition, the volatile component (CH2) n Assuming that is vaporized and half of it is burned, the reaction that generates gas can be expressed as follows. In this case, the gases generated are CO and H2O when the volatiles are vaporized and burned, and C3H8 when the volatiles are only vaporized and not burned. Note that the volatiles that are thermally decomposed become gases with various carbon numbers, but here we assume that they are propane (C3H8), which has an average of 3 carbon atoms. (CH2) n+O2→(1 / 2){n(CO)+H2O}+(1 / 2)(1 / 3)(C3H8)

[0026] Since the auxiliary material is fed at 0.1 t / min, the value of n in the above reaction formula is β × 0.1 ÷ 14 × 10 per minute. 6 The amount of gas generated is as follows: CO and HO: 1 / 2 × 2 × β × 0.1 ÷ 14 × 10 6 mol / min C3H8:1 / 2×1 / 3×β×0.1÷14×10 6 mol / min Total: (2 + 1 / 3)(1 / 2) × β × 0.1 ÷ 14 × 10 6 mol / min

[0027] Furthermore, the generated gas is heated to approximately 1500°C by the heat of combustion, and its volume expands. Taking this volume expansion into account, the volume of the generated gas, V [m 3 / min] is calculated as follows: V=(2+1 / 3)(1 / 2)×β×0.1÷14×10 6 ×(22.4×10 -3 )×{(273+1500)÷273} In the above formula, if the volatile content β = 0.2, V = 242 m 3 If we assume that this volume V of gas spreads uniformly inside a furnace with a diameter of 5 m, the thickness t of the gas shield that is formed is as follows: t=242÷π×5 2 =3.09[m / min]≒5[cm / sec] In the above example, by continuously feeding auxiliary materials with a volatile content of β=0.2 into a furnace with a diameter of 5 m at a rate of 0.1 t / min, a new gas shield approximately 5 cm thick can be formed per second. This compensates for the amount of generated gas that diffuses into the furnace atmosphere, and maintains the gas shield.

[0028] On the other hand, if the volatile content β = 0.05 in the above formula, the thickness t of the formed gas shield is 0.77 [m / min] ≒ 1 [cm / sec]. In this case, it is difficult to adequately compensate for the generated gas diffusing into the furnace atmosphere, and the gas shield is not maintained. As already mentioned, by adding an auxiliary material with a volatile content greater than 20 mass% (β > 0.2), gas is efficiently generated while compensating for the heat removal due to the dissolution of fixed carbon, and the gas shield formed by continuous addition can be maintained. Note that the upper limit of the volatile content of the auxiliary material is not particularly limited, but is, for example, 80 mass% or 90 mass%.

[0029] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications or alterations within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Explanation of symbols]

[0030] 1...treatment furnace, 2...molten steel, 3...auxiliary material, 4...oxygen supply lance, 5...molten steel surface, 6...gas shield

Claims

1. charging an auxiliary material having a volatile content of more than 20% by mass, a sulfur content of 0.02% or less, and an ash content of 5% or less into a furnace storing molten steel; supplying oxygen to the auxiliary materials introduced into the furnace; forming a gas shield on the surface of the molten steel with a gas generated by the reaction between the volatile content of the auxiliary materials and the oxygen; Including, The step of adding the auxiliary material is continuously performed, The step of supplying oxygen is continuously performed at least while the step of introducing the auxiliary material is continuously performed. A method for operating a molten steel processing furnace that prevents nitrogen absorption in molten steel.

2. 2. The method for operating a molten steel processing furnace according to claim 1, wherein the auxiliary material is a carbide obtained by carbonizing waste plastic or biomass, or waste plastic that has not been carbonized.

3. An auxiliary material for a molten steel processing furnace, used in the method for operating a molten steel processing furnace that prevents nitrogen absorption in molten steel according to claim 1 or claim 2, comprising: A secondary raw material for a molten steel processing furnace, having a volatile matter content of more than 20 mass%, a sulfur content of 0.02% or less, and an ash content of 5% or less.

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