Steel smelting method and steel manufacturing method

Abstract

The present invention proposes a method for smelting low-nitrogen steel by suppressing the infiltration of nitrogen from the furnace atmosphere into molten steel in the melting, neutralizing and refining of an iron-based raw material in an electric arc electric furnace. The melting method is characterized in that, in a step of melting molten steel by melting an iron-based raw material using an electric arc electric furnace, a gas that does not contain nitrogen is supplied along the circumferential surface of an electrode of the electric furnace from the base end side to the tip side of the electrode. In addition, the present invention also provides a method for manufacturing steel, which comprises adjusting the components of the molten steel that has been melted and then casting the molten steel.

Description

Technical Field

[0001] The present invention relates to a method for melting steel, and particularly to a method for melting low-nitrogen steel with 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 composition with a nitrogen content in the molten steel of 40 mass ppm or less. Background Art

[0002] In recent years, from the viewpoint of carbon neutrality, as a method for melting low-carbon iron-based raw materials, an electric arc furnace steelmaking method with less CO2 generation than the blast furnace-converter steelmaking method has attracted attention. In electric arc furnace steelmaking, the raw materials are sometimes various scraps, and most of the existing products of this electric arc furnace steelmaking method are so-called low-grade steels such as bar steel and section steel. Therefore, in order to replace the blast furnace-converter steelmaking method with the electric arc 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 problem is that the nitrogen concentration in the molten steel produced by the electric arc furnace steelmaking method, especially the arc-type electric furnace steelmaking method, is generally about 50 to 100 mass ppm, which is higher than that of the molten steel produced by the converter. For example, in high-grade thin steel sheets such as cold-rolled steel sheets for drawing, a nitrogen content of 40 mass ppm or less is required 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 the converter, the following major differences 1) to 3) can be cited in terms of equipment and operation.

[0005] 1) The arc-type electric furnace is essentially an open system, and compared with the converter, the amount of atmospheric air inhaled into the furnace is large, that is, the nitrogen concentration in the furnace is high.

[0006] 2) In the arc-type electric furnace, the in-furnace atmosphere gas in the arc is atomized, and it is easy for nitrogen in the in-furnace atmosphere gas to be absorbed into the molten steel at the arc point.

[0007] 3) In the arc-type electric furnace, the amount of CO gas generated is less than that of the converter, and the amount of denitrification caused by CO gas bubbles is small.

[0008] In view of the above situation, in the prior art, when manufacturing low-nitrogen steel using the arc-type electric furnace steelmaking method, it has been proposed to actively carry out CO boiling accompanying the decarburization reaction during the refining period by adjusting the main raw materials or blowing in carbon materials, etc. Specifically, by actively carrying out this CO boiling, the foaming of the slag can be promoted, the nitrogen absorption amount of the molten steel caused by the contents described in the above 1) and 2) can be reduced, and the denitrification of the molten steel using CO gas bubbles described in the above 3) can be promoted.

[0009] For example, Patent Document 1 proposed the following method: In order to increase the amount of CO gas generated, coke oven gas, blast furnace gas, and converter gas are used as the carrier gas for blowing in carbon, thereby increasing the carbon concentration in the molten steel after penetration.

[0010] In addition, Patent Documents 2 and 3 proposed the following method: By supplying high-purity oxygen to a melt in which the C content rate is maintained at an appropriate value, CO boiling is continued for a certain period of time, thereby maintaining sufficient slag foaming.

[0011] Patent Document 4 proposed the following method: The electrode is set to be hollow, and hydrocarbon gas is supplied as a carbon source from the hollow part at a flow rate of a certain value or more, and oxygen is blown to perform decarburization treatment.

[0012] In addition, as a method for reducing the amount of nitrogen absorption caused by the above 1) and 2) that does not depend on the decarburization reaction, Patent Document 5 proposed a method of setting the electrode to be hollow and supplying an inert gas such as Ar (argon) and / or a reducing gas such as hydrocarbon from the hollow part to the arc part.

[0013] Prior art documents

[0014] Patent documents

[0015] Patent Document 1: Japanese Patent Laid-Open No. 3-28312

[0016] Patent Document 2: Japanese Patent Laid-Open No. 10-121123

[0017] Patent Document 3: Japanese Patent Laid-Open No. 11-12634

[0018] Patent Document 4: Japanese Patent No. 6413710

[0019] Patent Document 5: Japanese Patent Laid-Open No. 52-147513 Summary of the invention

[0020] In the methods described in Patent Documents 1 to 4, by using a solid or gaseous carbon source to increase the carbon concentration of hot metal, and then performing decarburization treatment, the amount of CO gas generated is increased. However, increasing the amount of CO gas generated is contrary to the reason for the conversion from the blast furnace-converter process to the electric furnace process, that is, the reduction of CO2 generation.

[0021] If an inert gas is used according to the method described in Patent Document 5, the generation of CO gas is suppressed because no carbon source is used. However, since the gas is blown into the hollow part of the electrode, that is, the inside of the arc, it is difficult to suppress the atomization of the furnace atmosphere gas mentioned in the above 2), and the effect of preventing nitrogen absorption is limited.

[0022] The present invention has been completed in view of such circumstances, and its object is to provide a method for smelting low-nitrogen steel by suppressing the intrusion of nitrogen from the furnace atmosphere into the molten steel during the melting and refining of iron-based raw materials in an arc-type electric furnace.

[0023] The inventors have conducted in-depth research on methods for advantageously solving the above problems, and as a result, it has been 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 the molten steel, thus completing the present invention. That is, the gist of the present invention is as follows.

[0024] 1. A method for smelting steel, characterized in that, in the step of melting molten steel by using an arc-type electric furnace to melt iron-based raw materials, a nitrogen-free gas is supplied along the circumferential surface of the electrodes of the electric furnace and from the base end side to the front end side of the electrodes.

[0025] 2. The method for smelting steel according to item 1 above, characterized in that, by supplying the gas, a part or all of the outer peripheral surface of the arc at the front end of the electrode is covered by the gas.

[0026] 3. The method for smelting steel according to item 1 or 2 above, characterized in that, after the electrodes are immersed in the molten slag generated on the surface of the molten steel, the supply of the gas is stopped.

[0027] 4. The method for smelting steel according to item 3 above, characterized in that the moment when the electrodes are immersed in the molten slag is detected by an optical camera inserted into the electric furnace.

[0028] 5. The method for smelting steel according to any one of items 1 to 4 above, characterized in that the gas is any one or more selected from inert gases, hydrogen, and hydrocarbons.

[0029] 6. The method for smelting steel according to any one of items 1 to 5 above, characterized in that, when the gas contains carbon, oxygen corresponding to the increase in the carbon concentration in the molten steel is blown into the molten steel through a system different from the gas.

[0030] 7. A method for manufacturing steel, characterized in that the molten steel smelted by the method according to any one of items 1 to 6 above is subjected to composition adjustment and then cast.

[0031] According to the present invention, the contact between the arc and the furnace atmosphere is blocked by a nitrogen-free gas, preventing nitrogen absorption into the molten steel, so that low-nitrogen steel can be manufactured using an arc-type electric furnace. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a half-sectional view of an electric furnace used in the present invention.

[0033] Figure 2 This is a structural diagram of a nozzle for gas blowing.

[0034] Figure 3 This is a schematic diagram showing the gas distribution around the arc discharge part at the front end of the electrode.

[0035] Figure 4 This is a diagram for explaining the method of obtaining the coverage rate R. Specific embodiments

[0036] Hereinafter, the method for melting steel according to this embodiment will be described with reference to the accompanying drawings. First, an arc furnace device for implementing the method for melting low-nitrogen steel according to this embodiment will be described, and then the method for melting low-nitrogen steel according to this embodiment will be described.

[0037] [Arc furnace device]

[0038] Figure 1 This is a diagram showing a longitudinal section in the furnace radius direction of a half part of the electric furnace for the method of the present invention with the central axis as the boundary. It should be noted that as the arc furnace method, it can be a DC type or an AC type.

[0039] In Figure 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 nozzle for gas blowing, 7 is the camera for observing inside the furnace, 8 is the slender tubular nozzle for gas blowing, and 9 is the cylindrical nozzle for gas blowing.

[0040] The electrode 3 is usually provided with one or more than two electrodes that can move up and down relative to the furnace body 1. By generating an arc discharge from the front end of the electrode 3 and heating the inside of the furnace, the melting of the iron-based raw materials charged into the furnace can be carried out. Here, in the figure, the front end of the electrode 3 is circular, but the shape of the electrode front end can also be a flat shape, etc., and in addition, its shape also changes according to the usage conditions.

[0041] In addition, as shown in Figure 2 (a), the top view observed from the nozzle front end side, a plurality of, in the illustrated example, 6 slender tubular nozzles 8 for gas blowing can be arranged around the electrode 3. Or as shown in Figure 2 (b), a cylindrical nozzle 9 for gas blowing having an inner diameter larger than the outer diameter of the electrode 3 can be arranged in the same manner as the axis of the electrode 3.

[0042] It should be noted that when the nozzle 6 for gas blowing is circular at the front end of the electrode 3, it is preferably on the line connecting the shoulders of the electrode front end, that is, in Figure 1At the position of the a-a line, the number of nozzles, the horizontal and vertical positions relative to the electrode 3 are determined such that the periphery of the electrode 3 is preferably covered with a nitrogen-free gas (hereinafter also simply referred to as "gas") over the entire circumference. Specifically, as exemplified in Figure 2 . In addition, when the tip of the electrode 3 is flat, the above conditions can be set for the gas blowing nozzle such that the periphery of the electrode 3 is covered with gas at the position of the front end face.

[0043] In addition, the gas blowing nozzle 6 is preferably configured to be arranged along the electrode 3 and on the other hand, capable of lifting and lowering independently of the electrode 3. According to this configuration, the relative position between the gas blowing nozzle 6 and the electrode 3 can be changed, so that the contact between the arc generated from the electrode 3 and the furnace atmosphere can be effectively blocked.

[0044] It should be noted that the above drawings are schematic drawings and there are cases different from the actual situation. In addition, this embodiment is an example showing a device and a method for embodying the technical idea of the present invention, and these configurations are not limited to the above and below. That is, various changes can be made to the technical idea of the present invention within the technical scope described in the claims.

[0045] [Smelting method of low nitrogen steel]

[0046] Next, a preferred example of this embodiment of the method of the present invention will be described.

[0047] First, an iron-based raw material is charged into the arc furnace. At this time, the inside of the furnace body 1 can be in an empty state or in a state where molten steel remains from the previous treatment. Here, as the iron-based raw material, in addition to iron scrap, reduced iron, hot metal, and pig iron can also be used.

[0048] Then, the electrode 3 and the gas blowing nozzle 6 are inserted into the furnace body 1, the electrode 3 is energized, and at the same time, gas is blown downward from the gas blowing nozzle 6, and the melting operation is started. As shown in the above 2), nitrogen in the furnace atmosphere is atomized by contacting the arc generated by the electrode 3 and absorbs nitrogen in the molten steel. Therefore, it is important to suppress the nitrogen absorption of the molten steel by reducing the contact area between the arc and the furnace atmosphere.

[0049] Therefore, gas is blown between the arc and the furnace atmosphere to block their contact. Even if only a part of the area where the contact between the arc and the furnace atmosphere is blocked exists, an effect of suppressing nitrogen absorption by the molten steel can be expected. For example, as described above, when gas is blown from the gas blowing nozzles 8 or 9 arranged around the electrode 3 so as to surround the outer peripheral portion of the arc, there may be a case where the entire outer peripheral surface of the arc cannot be covered by the gas, such as when the number of nozzles is small or some nozzles are blocked. However, even in such a case, since a part of the outer peripheral surface of the arc is covered by the gas, a nitrogen absorption suppression effect can be brought compared with the case where no gas is blown. Of course, the greater the above-mentioned contact blocking area, the greater the nitrogen absorption suppression effect. By covering the entire outer peripheral surface of the arc with gas to completely block the contact with the furnace atmosphere, the nitrogen absorption suppression effect is maximized.

[0050] The area where the arc contacts the furnace atmosphere is the area below the front end of the electrode 3. Therefore, if the gas can be blown and supplied in such a way that a part or all of the outer peripheral surface of the arc at the front end of the electrode 3 is covered, the gas diffuses more as it progresses downward, so the above-mentioned contact blocking and nitrogen absorption suppression effects will not weaken. Here, "covering the entire outer peripheral surface of the arc at the front end of the electrode 3" means, as shown in the side view of the electrode 3 in Figure 3 (a) and as shown by the arrow b in the figure (a) in Figure 3 (b), a state where the entire periphery of the arc generated at the front end of the electrode 3 is surrounded by gas.

[0051] Here, the blowing and supply of the gas are preferably carried out according to the following conditions. It is preferable to ensure that the average flow velocity of the gas at the front end position of the electrode 3 is 5 m / s or more. When the average flow velocity of the gas at the front end position of the electrode is less than 5 m / s, depending on the conditions of the molten steel, slag, and furnace atmosphere in the electric furnace, there is a possibility that the gas does not reach the front end of the electrode and the effect of blocking the arc from the furnace atmosphere is reduced.

[0052] It should be noted that the gas blown in from the gas blowing nozzle 6 is not particularly limited as long as it does not contain nitrogen. For example, noble gases (argon, helium, etc.); non-reactive gases such as CO2; or hydrogen, oxygen, CO, hydrocarbon gases (propane, methane, etc.). Considering the influence on the furnace body 1 or the electrode 3, the reaction with the molten steel 4 or the slag 5, etc., it is preferably any one or a mixture of two or more selected from non-reactive gases, hydrogen gas, and hydrocarbon gases. In particular, it is considered that hydrogen gas and hydrocarbon gases, like the nitrogen in the above 2), are atomized in the arc and dissolved in the molten steel until a concentration higher than the thermodynamic equilibrium, and are reduced to the thermodynamic equilibrium hydrogen concentration outside the arc, that is, dehydrogenation occurs. Since dehydrogenation is accompanied by the generation of hydrogen bubbles in the molten steel, a denitrification effect brought by these hydrogen bubbles can also be expected.

[0053] Generally, the height of the electrode 3 can be changed according to the melting state of the iron-based raw material. In this case, it is preferable to raise and lower the gas supply nozzle 6 together.

[0054] As the melting of the iron-based raw material progresses, a slag 5 is formed on the molten steel 4 by the flux added to adjust components such as the gangue components in the iron-based raw material and P and S in the molten steel 4. To stabilize the energizing current, it is effective to cover the tip of the electrode 3 with the slag 5. Therefore, the electrode 3 can be actively immersed in the slag 5. If the electrode 3 is immersed in the slag 5, the arc will not come into direct contact with the furnace atmosphere. Thus, the gas supply from the gas blowing nozzle 6 can be stopped. Although it is considered that the determination of this timing can rely on the operator's experience to some extent, for more accurate determination, it is preferable to provide an in-furnace observation camera 7 on the furnace body 1 to directly observe the inside of the furnace to determine whether the electrode 3 has been immersed in the slag 5. As the in-furnace observation camera 7, there is no particular limitation as long as it is a camera generally used for in-furnace observation, but an optical camera is preferred.

[0055] After the gas blowing is stopped, to prevent blockage of the gas blowing nozzle 6 due to splashing of hot metal, etc., it is preferable to raise the gas blowing nozzle to the furnace lid 2 and blow in gas at a flow rate that does not cause blockage.

[0056] If a carbon-containing gas such as a hydrocarbon is used for the gas blown from the gas blowing nozzle 6, carbon dissolves in the molten steel and the carbon concentration increases. At this time, it is preferable to blow or inject oxygen (oxygen supply) in an amount required for decarburizing the above-mentioned increase in carbon concentration into the molten steel from a lance or nozzle (not shown) different from the gas blowing nozzle 6 provided on the furnace body 1. The supply of this oxygen can also be expected to have the effect of denitrification using CO bubbles generated by the decarburization reaction.

[0057] [Steel manufacturing method]

[0058] The molten steel obtained by the above steel melting method is supplied for casting after composition adjustment as needed. Here, the molten steel melted by the low-nitrogen steel melting method of the above embodiment is low-nitrogen molten steel, but the subsequent composition adjustment and casting of the molten steel are not particularly limited and can be carried out according to conventional methods. In addition, since the steel manufacturing method of the present embodiment uses low-nitrogen molten steel as the casting material of steel, low-nitrogen steel can be manufactured.

[0059] That is, by using the molten steel melted by the steel melting method of the present embodiment as the material of steel products, low-nitrogen steel products can be obtained. From such a technical viewpoint, the uses of the steel products manufactured by the steel manufacturing method of the present embodiment are not limited and can be widely applied. Among them, it is particularly suitable for thin steel plates manufactured by rolling treatment and steel products manufactured therefrom.

[0060] As described above, the invention of the present application has been described with reference to the embodiments, but the invention of the present application is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the invention of the present application within the technical scope of the invention of the present application.

[0061] Example

[0062] In an arc-type electric furnace with the following equipment specifications, iron-based waste is melted to produce molten steel. The equipment specifications of this electric furnace equipment are shown below.

[0063] Furnace body: Furnace diameter 7m, furnace height 5m

[0064] Electric power: AC 50Hz

[0065] Transformer capacity: 75MVA

[0066] Electrode diameter: 0.64m

[0067] Number of electrodes: 3

[0068] 130 tons of iron-based waste and 25 kg / t of slag-forming agent are charged into the above electric furnace, and an electric arc is generated by the electrodes (graphite electrodes on the upper part), heating the above iron-based waste to melt it. As the above iron-based waste, an iron-based waste with a composition of 90% by mass of steel waste and 10% by mass of cold pig iron is used.

[0069] Starting from the start of melting of this iron-based waste, Ar is blown and supplied through a gas blowing nozzle (Inventive Examples 1 to 4). The inner diameter of the gas blowing nozzle at this time is 3 mm, and the number and nozzle height (distance between the nozzle tip and the electrode tip) are the respective conditions shown in Table 1.

[0070] In addition, the gas blown and supplied diffuses from the nozzle opening at a spray angle of 12 degrees, and the ratio of the area where the gas faces the electrode to the entire circumference of the electrode is calculated. This ratio is shown as the coverage rate R of the arc outer peripheral surface of the electrode tip by the blown gas in Table 1. This coverage rate can be calculated according to the following formulas (1) to (3). Formula (1) is a formula for obtaining the gas diffusion radius r as shown in Figure 4 (a). Then, as shown in Figure 4 (b), first, θ″ is calculated using the obtained r and the electrode diameter with Formula (2), and this θ″ is substituted into Formula (3) to thereby obtain the coverage rate R.

[0071] [Mathematical formula 1]

[0072] r = tan(6°) × h…(1)

[0073]

[0074] However, in formula (3), when n×4θ′′≥360°, R = 1. Additionally, r is the gas diffusion radius, h is the nozzle height, D is the electrode diameter, n is the number of nozzles, and R is the coverage rate.

[0075] During the operation, an optical camera is used as the in-furnace observation camera to monitor the melting condition. When it is confirmed that slag is formed after the scrap is burned through, the electrode is lowered to immerse in the slag. Additionally, the condition of stopping the gas supply after the electrode is immersed in the slag was also implemented (Inventive Example 5). Moreover, the condition of setting the blown gas to hydrogen or propane gas was also implemented (Inventive Examples 6 and 7).

[0076] Furthermore, when propane gas is blown in, the yield rate of carbon in the molten steel under the same conditions when propane gas is blown in is confirmed in advance, and the condition of blowing oxygen in an amount capable of decarburizing the carbon with the yield rate from other spray guns was also implemented (Inventive Example 8). In Table 1, this condition is expressed as oxygen supply "yes".

[0077] And, for comparison, the condition of not performing gas blowing was also implemented (Comparative Example).

[0078] After all the iron-based raw materials are melted, power is continuously supplied until the temperature of the molten steel reaches the target temperature. After the power supply is completed, a molten iron sample is collected and the nitrogen concentration in the steel is analyzed. The nitrogen concentration in the steel after the power supply is completed is also listed in Table 1.

[0079] As shown in Table 1, it can be seen that by blowing gas to block the arc from the furnace atmosphere during the power supply of the electric arc furnace, the increase of nitrogen in the molten steel can be suppressed.

[0080]

[0081] Industrial applicability

[0082] According to the method for melting low-nitrogen steel of the present invention, in an arc-type electric furnace, by using gas to block the arc from the furnace atmosphere, the absorption of nitrogen by the molten steel can be suppressed. The method for manufacturing low-nitrogen steel of the present invention can suppress the nitrogen absorption of the molten steel during the power supply of the arc-type electric furnace, and thus is extremely useful in industries such as the steel manufacturing industry.

[0083] Symbol description

[0084] 1 Furnace body

[0085] 2 Furnace lid

[0086] 3 Electrode

[0087] 4 Molten steel

[0088] 5 Slag

[0089] 6 Gas blowing nozzle

[0090] 7 Furnace observation camera

[0091] 8 Fine tubular gas blowing nozzle

[0092] 9 Cylindrical gas blowing nozzle

Claims

1. A method for smelting steel, characterized in that, In the process of melting iron-based raw materials using an arc furnace to melt molten steel, a nitrogen-free gas is supplied along the circumferential surface of the electrode of the electric furnace from the base end side to the front end side of the electrode.

2. The steel melting method according to claim 1, characterized in that, By the supply of the gas, a part or all of the outer peripheral surface of the arc at the front end of the electrode is covered by the gas.

3. The steel melting method according to claim 1 or 2, characterized in that, After the electrode is immersed in the slag generated on the surface of the molten steel, the supply of the gas is stopped.

4. The steel melting method according to claim 3, characterized in that, The moment when the electrode is immersed in the slag is detected by an optical camera inserted into the electric furnace.

5. The steel melting method according to any one of claims 1 to 4, characterized in that, The gas is any one or more selected from inert gases, hydrogen, and hydrocarbons.

6. The steel melting method according to any one of claims 1 to 5, characterized in that, When the gas contains carbon, oxygen corresponding to the increase in the carbon concentration in the molten steel is blown into the molten steel through a system different from the gas.

7. A method for manufacturing steel, characterized in that, After adjusting the composition of the molten steel melted by the method according to any one of claims 1 to 6, casting is performed.

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