Method for melting steel and method for manufacturing steel

Abstract

Proposed is a method for melting and refining low-nitrogen steel by suppressing infiltration of nitrogen from the atmosphere within an arc electric furnace into molten steel during melting and smelting of an iron-based raw material in the furnace. This melting and refining method is characterized in that, in a step for melting an iron-based raw material using an arc electric furnace to melt and refine molten steel, a nitrogen-free gas is supplied along the circumferential surface of an electrode in the electric furnace and from the proximal-end side toward the distal-end side of the electrode. Also provided is a method for manufacturing steel, the method including adjusting the composition of the melted and refined molten steel and then providing the molten steel for casting.

Description

Technical Field

[0001] The present invention relates to a method for melting steel, particularly a method for melting low-nitrogen steel having a low nitrogen content using an arc-type electric furnace, and a method for manufacturing steel using the molten steel obtained by this melting method. Low-nitrogen steel refers to steel having a component composition in which the nitrogen content in the molten steel is 40 mass ppm or less.

Background Art

[0002] In recent years, from the perspective of carbon neutrality, as a method for melting low-carbon iron-based raw materials, an electric furnace steelmaking method with less CO2 generation than the blast furnace-converter steelmaking method has attracted attention. In electric furnace steelmaking, the raw materials may be various types of scrap, and many of the conventional products by this electric furnace steelmaking method are so-called low-grade steels such as bars and sections. Therefore, in order to replace the blast furnace-converter steelmaking method with the electric furnace steelmaking method, it is necessary to be able to manufacture so-called high-grade steels such as cold-rolled steel sheets and surface-treated steel sheets.

[0003] However, the nitrogen concentration in the molten steel produced by the electric furnace steelmaking method, particularly the arc-type electric furnace steelmaking method, is generally about 50 to 100 mass ppm, which is a problem because it is higher than that of the molten steel produced by a converter. For example, in high-grade thin steel sheets such as cold-rolled steel sheets for deep drawing, it is required to reduce the nitrogen content to 40 mass ppm or less in order to suppress the aging phenomenon caused by nitrogen in the steel. Stably manufacturing steel with such a low nitrogen content is a major issue in the arc-type electric furnace steelmaking method.

[0004] Here, when comparing the arc-type electric furnace with a converter, there are significant differences shown in the following 1) to 3) in terms of equipment and operation. 1) The arc-type electric furnace is a substantially open system, and compared with a converter, the amount of air sucked into the furnace is large, that is, the nitrogen concentration in the furnace is high. 2) In the arc-type electric furnace, the in-furnace atmosphere gas in the arc is atomized, and at the arc spot, nitrogen in the in-furnace atmosphere gas is easily absorbed into the molten steel. 3) Compared to converters, arc-type electric furnaces produce less CO gas and less denitrification due to CO gas bubbles.

[0005] In response to the above situation, conventional technology has proposed actively promoting CO boiling during the decarburization reaction in the refining stage by adjusting the main raw materials or blowing in carbon material when producing low-nitrogen steel using the arc-type electric furnace steelmaking method. Specifically, by actively promoting this CO boiling, slag forming is promoted, which reduces the amount of nitrogen absorbed by the molten steel caused by the factors described in 1) and 2) above, and promotes denitrification from the molten steel by CO gas bubbles as described in 3) above.

[0006] For example, Patent Document 1 proposes a method to increase the amount of CO gas generated by using coke oven gas, blast furnace gas, and converter gas as carrier gases for carbon injection, thereby increasing the carbon concentration in the molten steel after meltdown.

[0007] Furthermore, Patent Documents 2 and 3 propose a method for maintaining sufficient slag forming by supplying high-purity oxygen gas to molten metal in which the carbon content is maintained at an appropriate value, thereby continuing CO boiling for a certain period of time.

[0008] Patent Document 4 proposes a method of performing decarburization by making the electrode hollow, supplying hydrocarbon gas as a carbon source from the hollow part at a flow rate above a certain level, and blowing oxygen gas onto it.

[0009] Furthermore, as a method to reduce nitrogen absorption caused by the above 1) and 2) without decarburization reactions, Patent Document 5 proposes a method in which the electrode is made hollow and an inert gas such as Ar (argon) and / or a reducing gas such as hydrocarbons are supplied from the hollow part to the arc part. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 3-28312 [Patent Document 2] Japanese Patent Application Publication No. 10-121123 [Patent Document 3] Japanese Patent Application Publication No. 11-12634 [Patent Document 4] Patent No. 6413710 [Patent Document 5] Japanese Patent Application Publication No. 52-147513 [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] The methods described in Patent Documents 1 to 4 increase the carbon concentration of molten iron using a solid or gaseous carbon source, and then increase the amount of CO gas generated by performing a decarburization treatment. However, increasing the amount of CO gas generated contradicts the reason for switching from the blast furnace-converter method to the electric furnace method, which is to reduce CO2 emissions.

[0012] If an inert gas is used according to the method described in Patent Document 5, no carbon source is used, and therefore the generation of CO gas is suppressed. However, because the gas is blown into the hollow part of the electrode, i.e., inside the arc, it is difficult to suppress the atomization of the furnace atmosphere gas mentioned in 2) above, and there were limitations to the nitrogen absorption prevention effect.

[0013] This invention has been made in view of these circumstances, and its objective is to propose a method for producing low-nitrogen steel by suppressing the intrusion of nitrogen from the furnace atmosphere into molten steel during the melting and refining of iron-based raw materials in an arc-type electric furnace. [Means for solving the problem]

[0014] The inventors earnestly studied ways to advantageously solve the above problems, and as a result, found that supplying a nitrogen-free gas around the electrodes of an electric furnace to block the arc discharge from the furnace atmosphere is effective in suppressing the intrusion of nitrogen from the furnace atmosphere into molten steel, and thus completed the present invention. That is, the gist of the present invention is as follows.

[0015] 1. A method for melting steel, characterized in that in the step of melting iron-based raw materials using an arc-type electric furnace to produce molten steel, a nitrogen-free gas is supplied along the peripheral surface of the electrode of the electric furnace and from the base end side to the tip end side of the electrode.

[0016] 2. The method for melting steel according to 1 above, wherein due to the supply of the gas, part or all of the outer peripheral surface of the arc at the tip of the electrode is covered with the gas.

[0017] 3. The method for melting steel according to 1 or 2 above, wherein after the electrode is immersed in the slag formed on the surface of the molten steel, the supply of the gas is stopped.

[0018] 4. The method for melting steel according to 3 above, wherein the point when the electrode is immersed in the slag is detected via an optical camera inserted into the electric furnace.

[0019] 5. The method for melting steel according to any one of 1 to 4 above, wherein the gas is any one or more selected from the group consisting of inert gases, hydrogen, and hydrocarbons.

[0020] 6. The method for melting steel according to any one of 1 to 5 above, wherein when the gas contains carbon, oxygen gas corresponding to the increase in the carbon concentration in the molten steel is blown into the molten steel in a separate system from the gas.

[0021] 7. A method for manufacturing steel, characterized in that after adjusting the components of the molten steel melted by the method according to any one of 1 to 6 above, casting is performed.

Effects of the Invention

[0022] According to the present invention, since the contact between the arc and the furnace atmosphere is blocked by a nitrogen-free gas, preventing nitrogen absorption into the molten steel, it becomes possible to produce low-nitrogen steel using an arc-type electric furnace.

Brief Description of the Drawings

[0023] [Figure 1] It is a cross-sectional view of a half part showing the electric furnace used in the present invention. [Figure 2] It is a view showing the structure of the gas injection nozzle. [Figure 3] It is a schematic view showing the gas distribution around the arc discharge part at the tip of the electrode. [Figure 4] It is a view for explaining how to obtain the coverage ratio R.

Embodiments for Carrying Out the Invention

[0024] Hereinafter, based on the drawings, the steel melting method of the present embodiment will be described. First, the arc-type electric furnace device used for carrying out the melting method of low-nitrogen steel according to the present embodiment will be described, and then the melting method of low-nitrogen steel of the present embodiment will be described.

[0025] [Arc-Type Electric Furnace Device] FIG. 1 shows a longitudinal section in the furnace radius direction of a half part with the central axis of the electric furnace used in the method of the present invention as a boundary. Note that the arc-type electric furnace may be a DC type or an AC type.

[0026] In FIG. 1, 1 is the furnace body, 2 is the furnace lid, 3 is the electrode, 4 is the molten steel, 5 is the slag, 6 is the gas injection nozzle, 7 is the camera for observing the inside of the furnace, 8 is the thin tubular gas injection nozzle, and 9 is the cylindrical gas injection nozzle.

[0027] Electrodes 3 are typically installed one or more times on the furnace body 1, and are capable of moving up and down vertically. By generating an arc discharge from the tip of these electrodes 3 and heating the inside of the furnace, the iron-based raw materials charged into the furnace can be melted. In the diagram, the tip of electrode 3 is rounded, but other shapes such as flat tips are also available, and the shape may change depending on the usage conditions.

[0028] Furthermore, the gas blowing nozzle 6 may be arranged in a manner in which multiple tubular gas blowing nozzles 8, six in the illustrated example, are arranged around the electrode 3, as shown in the plan view from the nozzle tip side in Figure 2(a). Alternatively, as shown in Figure 2(b), a cylindrical gas blowing nozzle 9 having an inner diameter larger than the outer diameter of the electrode 3 may be arranged with the same axis as the electrode 3.

[0029] Furthermore, when the tip of the electrode 3 is rounded, it is preferable to determine the number of nozzles, their horizontal position relative to the electrode 3, and their height position so that the area around the electrode 3 is covered with a nitrogen-free gas (hereinafter also simply referred to as "gas") at the position of the line segment connecting the shoulders of the electrode tip, i.e., line aa in Figure 1. Specifically, this is illustrated in Figure 2. If the tip of the electrode 3 is flat, the above conditions for the gas-blowing nozzles should be set so that the area around the electrode 3 is covered with gas at the position of the tip surface.

[0030] Furthermore, it is preferable that the gas blowing nozzle 6 is positioned along the electrode 3 while being configured to move up and down independently of the electrode 3. This configuration makes it possible to vary the relative position of the gas blowing nozzle 6 with respect to the electrode 3, thereby more effectively blocking contact between the arc generated from the electrode 3 and the furnace atmosphere.

[0031] The above-mentioned drawings are schematic and may differ from actual ones. Furthermore, this embodiment illustrates an apparatus and method for realizing the technical concept of the present invention, and its configuration is not limited to those described above and below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0032] [Method for melting low-nitrogen steel] Next, a preferred example of this embodiment of the method of the present invention will be described.

[0033] First, iron-based raw materials are charged into the arc-type electric furnace. At this time, the furnace body 1 can be either empty or contain molten steel from the previous processing. In addition to iron scrap, reduced iron, molten pig iron, and pig iron can be used as iron-based raw materials.

[0034] Subsequently, the electrode 3 and the gas blowing nozzle 6 are inserted into the furnace body 1, and the electrode 3 is energized while gas is blown downwards from the gas blowing nozzle 6 to begin the melting process. As described in 2) above, nitrogen in the furnace atmosphere atomizes when it comes into contact with the arc generated from the electrode 3, promoting nitrogen absorption in the molten steel. Therefore, it is crucial to suppress nitrogen absorption in the molten steel by reducing the contact area between the arc and the furnace atmosphere.

[0035] Therefore, gas is blown between the arc and the furnace atmosphere to block contact between them. Even if only a portion of the arc is blocked from contact with the furnace atmosphere, it can be expected to be effective in suppressing nitrogen absorption of molten steel. For example, as mentioned above, when blowing gas from gas blowing nozzles 8 or 9 arranged around the electrode 3 so as to surround the outer circumference of the arc, it may not be possible to cover the entire outer surface of the arc with gas if there are few nozzles or if the nozzles are partially clogged. However, even in such cases, a portion of the outer surface of the arc is covered with gas, resulting in a nitrogen absorption suppression effect compared to when no gas is blown. Of course, the nitrogen absorption suppression effect will increase as the contact blocking area increases, and the nitrogen absorption suppression effect will be maximized when the entire outer surface of the arc is covered with gas and contact with the furnace atmosphere is completely blocked.

[0036] The region in contact between this arc and the furnace atmosphere is below the tip of electrode 3. Therefore, if the gas is supplied so that part or all of the outer surface of the arc at the tip of electrode 3 is covered, the gas will spread as it moves downwards, and the effects of contact blocking and nitrogen absorption suppression described above will not decrease. Here, "the entire outer surface of the arc at the tip of electrode 3 is covered" means that the entire area around the arc generated from the tip of electrode 3 is surrounded by gas, as shown in Figure 3(a), which is a side view of electrode 3, and in Figure 3(b), which is the view from arrow b in Figure 3(a).

[0037] Here, it is preferable to supply the gas by blowing it according to the following conditions: It is preferable to ensure that the average gas flow velocity at the tip of electrode 3 is 5 m / s or more. If the average gas flow velocity at the tip of electrode is less than 5 m / s, depending on the conditions of the molten steel, slag, and furnace atmosphere inside the electric furnace, the gas may not reach the tip of the electrode, and the effect of isolating the arc from the furnace atmosphere may be reduced.

[0038] The gas injected from the gas injection nozzle 6 is not particularly limited as long as it does not contain nitrogen. Examples include noble gases (argon, helium, etc.), inert gases such as CO2, hydrogen, oxygen, CO, and hydrocarbon gases (propane, methane, etc.). Considering the effects on the furnace body 1 or electrode 3, and the reaction with molten steel 4 or slag 5, it is preferable to use one or more gases selected from the group consisting of inert gases, hydrogen gas, and hydrocarbon gases. In particular, hydrogen gas and hydrocarbon gases are thought to undergo dehydrogenation, where hydrogen atomizes in the arc and dissolves in the molten steel to a concentration higher than thermodynamic equilibrium, and then decreases to the thermodynamic equilibrium hydrogen concentration outside the arc. Since dehydrogenation is accompanied by the generation of hydrogen bubbles in the molten steel, a denitrification effect by these hydrogen bubbles can also be expected.

[0039] Generally, the height of the electrode 3 can be changed depending on the melting state of the iron-based raw material, but in that case, it is preferable to raise and lower the gas supply nozzle 6 at the same time.

[0040] As the iron-based raw materials melt, slag 5 is formed on the molten steel 4 by the gangue components in the iron-based raw materials and the flux added to adjust the P and S components in the molten steel 4. To stabilize the current, it is effective to cover the tip of the electrode 3 with slag 5, so it is preferable to actively immerse the electrode 3 in slag 5. When the electrode 3 is immersed in slag 5, the arc will no longer be in direct contact with the furnace atmosphere, so the gas supply from the gas blowing nozzle 6 may be stopped. While it is possible to determine the timing to some extent by the operator's experience, to determine more accurately, it is preferable to install a furnace observation camera 7 in the furnace body 1 and directly observe the inside of the furnace to determine whether the electrode 3 is immersed in slag 5. The furnace observation camera 7 is not particularly limited as long as it is one that is generally used for observing the inside of a furnace, but an optical camera is preferred.

[0041] After stopping gas blowing, it is preferable to raise the gas blowing nozzle 6 to the furnace lid 2 and blow gas at a flow rate that does not cause blockage due to molten iron splash, etc.

[0042] If a gas containing carbon, such as a hydrocarbon, is used as the gas injected from the gas injection nozzle 6, carbon will dissolve into the molten steel, increasing the carbon concentration. In this case, it is preferable to inject or blow oxygen gas into the molten steel from a lance or nozzle (not shown) separate from the gas injection nozzle 6 installed in the furnace body 1 to remove the increase in carbon concentration (acid supply). This supply of oxygen gas can also be expected to have the effect of promoting denitrification with CO bubbles generated by the decarburization reaction.

[0043] [Methods for manufacturing steel] The molten steel obtained by the above steel melting method is subjected to compositional adjustment as necessary and then used for casting. Here, although the molten steel produced by the low-nitrogen steel melting method according to the above embodiment is low-nitrogen molten steel, the subsequent compositional adjustment and casting of the molten steel are not particularly limited and can be carried out according to standard methods. Furthermore, since the steel manufacturing method of this embodiment uses low-nitrogen molten steel as the casting material for steel, low-nitrogen steel can be manufactured.

[0044] In other words, by using the molten steel produced by the steel melting method according to this embodiment as a material for steel products, low-nitrogen steel products can be obtained. From this technical standpoint, the applications of steel products produced by the steel manufacturing method according to this embodiment are not limited and can be broadly and suitably used. In particular, it is suitable for thin steel sheets produced by rolling and steel products produced therefrom.

[0045] Although the present invention has been described above with reference to embodiments, the present invention is not limited to the above embodiments. Various modifications to the structure and details of the present invention can be made, as can be understood by those skilled in the art within the technical scope of the present invention. [Examples]

[0046] Molten steel was produced by melting ferrous scrap in an arc-type electric furnace with the following equipment specifications. The equipment specifications of this electric furnace are shown below. Furnace body: Furnace diameter 7m, furnace height 5m Power: AC 50Hz Transformer capacity: 75 MVA Electrode diameter: 0.64 mm Number of electrodes: 3

[0047] 130 tons of iron-based scrap and 25 kg / t of slag-forming agent were charged into the electric furnace described above, and an arc was generated using electrodes (upper graphite electrodes) to heat and melt the iron-based scrap. The iron-based scrap used had a composition of 90% steel scrap and 10% cold iron.

[0048] Ar was supplied via gas spray nozzles from the start of the melting of the iron-based scrap (Examples 1-4 of the invention). The inner diameter of the gas spray nozzles was 3 mm, and the number of nozzles and nozzle height (distance between nozzle tip and electrode tip) were as shown in Table 1.

[0049] Furthermore, assuming that the injected gas spreads from the nozzle opening at a spray angle of 12 degrees, the ratio of the area where the gas hits the electrode to the entire circumference of the electrode was calculated. This ratio is shown in Table 1 as the coverage rate R of the injected gas on the outer surface of the arc at the tip of the electrode. This coverage rate can be calculated by the following equations (1) to (3). Equation (1) is the equation for determining the gas spreading radius r as shown in Figure 4(a). Next, as shown in Figure 4(b), first, θ' is calculated using equation (2) from the obtained r and electrode diameter, and then the coverage rate R is obtained by substituting this θ' into equation (3).

[0050]

number

[0051] However, in equation (3), R=1 when n×4θ'≧360°. Also, r: gas spread radius, h: nozzle height, D: electrode diameter, n: number of nozzles, R: coverage.

[0052] During operation, an optical camera was used as a furnace observation camera to monitor the melting process. Once it was confirmed that slag had formed after the scrap had melted down, the electrode was lowered to immerse it in the slag. The experiment also included conditions in which the gas supply was stopped after the electrode was immersed in the slag (Example 5). Furthermore, the experiment also included conditions in which hydrogen and propane gas were used as the blown-in gas (Examples 6 and 7).

[0053] Furthermore, when propane gas was injected, the yield of carbon into the molten steel under the same conditions was confirmed in advance, and conditions were also implemented in which oxygen gas sufficient to decarburize the retained carbon was injected from a separate lance (Example 8 of Invention). In Table 1, these conditions are indicated as "with acid injection".

[0054] Furthermore, for comparison, the experiment was also conducted under conditions without gas injection (comparative example).

[0055] After all the iron-based raw materials had melted, the electric current was continued until the molten steel reached the target temperature. After the electric current was stopped, a sample of molten iron was taken and the nitrogen concentration in the steel was analyzed. The nitrogen concentration in the steel after the electric current was stopped is shown in Table 1.

[0056] As shown in Table 1, by blowing gas to isolate the arc from the furnace atmosphere while the electric furnace was energized, it was possible to suppress the rise in nitrogen in the molten steel.

[0057] [Table 1] [Industrial applicability]

[0058] According to the present invention's method for melting low-nitrogen steel, nitrogen absorption into molten steel can be suppressed by isolating the arc from the furnace atmosphere with gas in an arc-type electric furnace. The present invention's method for manufacturing low-nitrogen steel is extremely useful in industries such as steelmaking because it can suppress nitrogen absorption of molten steel while an arc-type electric furnace is energized. [Explanation of Symbols]

[0059] 1 Furnace body 2 Hearth lid 3 electrodes 4 Molten steel 5 slags 6. Gas spray nozzle 7. Camera for observing inside the reactor 8. Thin-tube gas spray nozzle 9. Cylindrical gas blowing nozzle

Claims

1. A method for melting steel, characterized in that, in the process of melting iron-based raw materials to produce molten steel using an arc-type electric furnace, a nitrogen-free gas is continuously supplied from a gas-blowing nozzle positioned around the electrode, along the circumferential surface of the electrode in the electric furnace, and from the base end to the tip end of the electrode, so as the electrode is energized, surrounding the outer circumference of the arc, and the supply of the gas is stopped after the electrode is immersed in the slag generated on the surface of the molten steel.

2. The method for melting steel according to claim 1, wherein the supply of the gas covers part or all of the outer surface of the arc at the tip of the electrode with the gas.

3. The method for melting steel according to claim 1, wherein the point in time when the electrode is immersed in the slag is detected via an optical camera inserted into the electric furnace.

4. The method for melting steel according to claim 2, wherein the point in time when the electrode is immersed in the slag is detected via an optical camera inserted into the electric furnace.

5. The method for melting steel according to claim 1, wherein the gas is one or more selected from the group consisting of inert gases, hydrogen, and hydrocarbons.

6. The method for melting steel according to claim 2, wherein the gas is one or more selected from the group consisting of inert gases, hydrogen, and hydrocarbons.

7. The method for melting steel according to claim 3, wherein the gas is one or more selected from the group consisting of inert gases, hydrogen, and hydrocarbons.

8. The method for melting steel according to claim 4, wherein the gas is one or more selected from the group consisting of inert gases, hydrogen, and hydrocarbons.

9. The method for melting steel according to any one of claims 1 to 8, wherein if the gas contains carbon, oxygen gas corresponding to the increase in carbon concentration in the molten steel is blown into the molten steel via a separate system from the gas.

10. A method for producing steel, characterized by performing component adjustment on molten steel produced by the method described in any one of claims 1 to 8, and then casting it.

11. A method for producing steel, characterized by adjusting the composition of molten steel produced by the method described in claim 9, and then casting it.

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